BIOGRAFIJA Dr. Dušan V. Tripković je rođen 28.04.1979. godine u ...

BIOGRAFIJA
Dr. Dušan V. Tripković je rođen 28.04.1979. godine u Beogradu. Završio je osovnu
školu “Vladislav Ribnikar” i “Prvu beogradsku gimnaziju” u Beogradu, sa odličnim uspehom.
Tehnološko metalurški fakultet upisao je 1998. godine (smer hemijsko inženjerstvo), a
diplomirao je 26.11.2003. godine sa prosečnom ocenom 8,53 i ocenom diplomskog rada 10.
Diplomski rad je uradio u Razvojno-istraživačkom centru “Haldor Topsoe” u Kopenhagenu,
Danska. Magistraske studije upisao je na Tehnološko metalurškom fakultetu 2003. godine.
Nakon položenih ispita sa prosečnom ocenom 10, eksperimentalni deo teze je uradio na
Institute of Catalysis and Surface Chemistry, Polish Academy of Science, Krakov, Poljska.
Zvanje Magistar tehničkih nauka stekao je 2005. godine odbranivši magistarski rad na temu
“Karakterizacija katalizatora dobijenih modifikovanjem staklastog ugljenika platinom” na
Tehnološko metalurškom fakultetu. Doktorsku disertaciju “Uticaj morfologije površine
platinskih materijala na elektrokatalitičku aktivnost u gorivim spregovima” uradio je u
Nacionalnoj laboratoriji Argonne, Čikago, SAD, a odbranio je na Tehnološko metalurškom
fakultetu u Beogradu 2008. godine i time stekao zvanje Doktor tehničkih nauka
Od 1 januara 2003. godine zaposlen je na Institutu za hemiju, tehnologiju i
metalurgiju, Centar za elektrohemiju, u Beogradu. Trenutno zvanje Kandidata je viši naučni
saradnik.
Od 2003 god angažovan je na projektima osnovnih istraživanja iz hemije finansiranih
od strane Ministarstva
U okviru međunarodne saradnje učestvovao je i učestvuje na dva bilateralna projekta
sa Poljskom i četiri američka projekta.
U toku 2004/2005 godine bio je na studijskom boravku u Institute of Catalysis and
Surface Chemistry, Polish Academy of Science, Krakov, Poljska. Na doktorskim studijama je
bio 2005/2007 godine u Argonne National Laboratory, Čikago, SAD. U istoj insituciji
boravio je kao post doktor u periodu 2009/2011 god.
Dobitnik je nagrade ”Panta Tutundžić” za izuzetan uspeh na poslednjoj godini studija
na TMF-u i nagrade Nacionalne laboratorije „Argonne“ za prijavljen patent (oktobar 2012.)
Govori tečno engleski jezik i ima školsko znanje francuskog jezika.

BIBLIOGRAFIJA
Dr. Dušan V. Tripković je izuzetno uspešan u karakterizaciji površine
katalitičkih materijala za primenu u gorivim spregovima korišćenjem spektroskopskih
(FTIR) i mikroskopskih tehnika (STM i AFM). Osmišljenom pripremom površina i
primenom ovih tehnika Dr. Dušan Tripković je prvi u svetu detektovao površinska
ostrva kao aktivna mesta na platinskim monokristalnim katalizatorima, čime je
otvoren put dizajniranju superiornih katalizatora.
Naučni doprinos Dr. Dušana V. Tripkovića pored pronalaska aktivnih mesta
na površini monokristala je i sinteza i karakterizacija katalizatora na bazi Pt sa
klasterima Ni(OH)2 koji poseduju za red veličine veću aktivnost i značajno
unapređenu stabilnost od čiste Pt koja se trenutno koristi za reakciju izdvajanja
vodonika u alkalnim i hlor-alkalnim elektrolizerima. Ovaj pronalazak potencijalno
može dovesti do uštede pri proizvodnji vodonika u vrednosti od više stotina miliona
dolara na godišnjoj bazi i to samo u SAD (prijavljen patent).
Po pitanju gorivih spregova, koji i predstavljaju glavnu oblast njegovog
interesovanja, Dr. Dušan Tripković je pokušao da pronađe rešenje za dva ključna
problema komercjalizacije ovih uređaja; stablinost u agresivnim uslovima pod kojima
rade i aktivnost katalitičkog materijala za reakciju redukcije kiseonika. Sa tim u vezi
kandidat je predstavio i sasvim novi metod zaštite katalitizatora, na bazi orijentisanog
sloja calih[4]arene molekula, od extremnih uslova u kojima se mogu naći tokom
puštanja u pogon/gašenja gorivih spregova. Takođe je i dizajnirao Pt-bimetalni
katalizator u vidu tankog filma nanesenog na nosač koji pokazuje za red veličine veću
aktivnost i značajno unapređenu stabilnost od aktuelnog Pt/C kazalizatora za reakciju
redukcije kiseonika.
Dr. Dušan V. Tripković je objavio 30 naučnih radova od kojih: 18 radova u
prvoj kategoriji vrhunskih međunarodnih časopisa (M21) sa izuzetno visokim impakt
faktorima, 1 rad u istaknutim časopisima međunarodnog značaja (M22), 10 radova u
međunarodnim časopisima (M23) i 1 rad u nacionalnom časopisu (M51). Ukupni
IF=215.335. Kandidat takođe ima i 26 saopštenja na međunarodnim (M34) i 4 na
nacionalnim naučnim skupovima (M61).
Od izbora u zvanje viši naučni saradnik objavio je: 8 radova kategorije M21,
1 rad kategorije M22 i 5 radova kategorije M23. Ukupni IF=172.876. Kandidat ima
16 saopštenja na međunarodnim skupovima iz kategorija M33 i M34.
Dr. Dušan V. Tripković je učestvovao i aktivno učestvuje na šest
međunarodnih projekata, dva u okviru bilateralne saradnje IHTM-a i Poljske
akademije nauka, tri Američka projekta finansirana od strane; University of Chicago
and Argonne National laboratory, US Department of Energy i Basic Energy Science i
projekat koji predstavlja saradnju američke Nacionalne laboratorije Argonne i
razvojnog centra Toyote.
Februara 2009. godine po pozivu je održao predavanje na Technical
University of Denmark, Kopenhagen, Danska.
Citiranost Dr. Dušana V. Tripkovića prema izveštaju servisa Scopus iznosi
358, odnosno 330 bez autocitata, na dan 25 januar 2013.
Dr. Dušan V. Tripković ima i jedan prijavljen patent za koji je dobio nagradu
američke Nacionalne laboratorije „Argonne“.

III. SPISAK NAUČNIH RADOVA I SAOPŠTENJA OD IZBORA U ZVANJE
VIŠI NAUČNI SARADNIK
M 21 – Radovi objavljeni u vrhunskim časopisima međunarodnog značaja (8x8=64)
M21/1. Bostjan Genorio, Dusan Strmcnik, Ram Subbaraman, Dusan Tripkovic,
Goran Karapetrov, Vojislav R. Stamenkovic, Stane Pejovnik and Nenad M.
Markovic “Selective catalyst for the hydrogen oxidation and oxygen
reduction reactions by patterning of platinum with calix[4]arene
molecules” Nature Materials (12), 998-1003, (2010)
[IF: 29.920 (2010); oblast: Chemistry, Physical, 1/127]
M21/2 Bostjan Genorio, Ram Subbaraman, Dusan Strmcnik, Dusan Tripkovic,
Vojislav R. Stamenkovic, and Nenad M. Markovic “ Tailoring the selectivity
and stability of chemically modified platinum nanocatalysts to design
highly durable anodes for PEM fuel cells” Angewante Chemie, 50(24),
5468-72, (2011)
[IF: 13.455 (2011); oblast: Chemistry, Multidisciplinary, 7/154
M21/3 Chao Wang, Miaofang Chi, Dongguo Li, Dusan Strmcnik, Dennis van der
Vliet,Guofeng Wang,Vladimir Komanicky, Kee-Chul Chang, Arvydas P.
Paulikas, Dusan Tripkovic, John Pearson,Karren L. More, Nenad M.
Markovic, and Vojislav R. Stamenkovic ” Design and Synthesis of
Bimetallic Electrocatalyst with Multilayered Pt-Skin Surfaces” Journal of
the American Chemical Society,133(36),14396-403, (2011)
[IF: 9.907 (2011); oblast: Chemistry, Multidisciplinary, 11/154]
M21/4 Ram Subbaraman, Dusan Tripkovic, Dusan Strmcnik, Kee-Chul Chang,
Masanobu Uchimura, Arvydas P. Paulikas, Vojislav Stamenkovic, Nenad M.
Markovic “Enhancing Hydrogen Evolution Activity in Water Splitting by
Tailoring Li + -Ni(OH)2-Pt Interfaces”, Science, vol. 334 no. 6060 pp. 1256-
1260, (2011)
[IF: 31.201 (2011); oblast: Multidisciplinary Sciences, 2/56]
M21/5 Dennis van der Vliet, Chao Wang, Dongguo Li, Arvydas P. Paulikas,Jeffrey
Greely,Rees B. Rankin, Dusan Strmcnik, Dusan Tripkovic, Nenad M.
Markovic, and Vojislav R. Stamenkovic “Unique Electrochemical
Adsorption Properties of Pt-skin Surfaces”, Angewante Chemie,Vol 51,
Issue 13, 3139–3142, (2012)
[IF: 13.455 (2011); oblast: Chemistry, Multidisciplinary, 7/154]

M21/6 Ram Subbaraman, Dusan Tripkovic,Kee-Chul Chang,Dusan Strmcnik,
Arvydas P.Paulikas,Pussana Hirunsit, Maria Chan, Jeff Greeley, Vojislav
Stamenkovic and Nenad M.Markovic” Trends in activity for the water
electrolyser reactions on 3dM (Ni,Co,Fe,Mn) hydr(oxy)oxide
catalysts”,Nature Materials 11, (6), 550-557 (2012) DOI: 10.1038/nmat3313
[IF: 32.841 (2011); oblast: Chemistry, Physical, 1/134]
M21/7 Dennis van der Vliet, Chao Wang, Dusan Tripkovic, Dusan Strmcnik, Xiao
Zhang, Mark Debe, Radoslav Atanososki, Nenad Markovic and Vojislav
Stamenkovic “Mesostructured Thin Films as Electrocatalysts with
Tunable Composition and Surface Morphology”, Nature Materials, (2012),
DOI: 10.1038/nmat3457
[IF: 32.841 (2011); oblast: Chemistry, Physical, 1/134]
M21/8 Ram Subbaraman, N. Danilovic, P. P. Lopes, D. Tripkovic, D. Strmcnik, V.
R. Stamenkovic, and N. M. Markovic “Origin of Anomalous Activities for
Electrocatalysts in Alkaline Electrolytes”, J. Phys. Chem. C, 116 (42), pp
22231–22237 (2012), DOI: 10.1021/jp3075783
[IF: 4.805 (2011); oblast: Chemistry, Physical, 26/134]
M22 – Radovi objavljeni u istaknutim časopisima međunarodnog značaja (1x5=5)
M22/1 Stevanovic I. Sanja, Tripkovic V. Dusan, Rogan R. Jelena, Popovic Dj.
Ksenija, Lovic D. Jelena, Tripkovic V. Amalija and Jovanovic M Vladislava,
“Microwave-assisted polyol synthesis of carbon-supported platinumbased
bimetallic catalysts for ethanol oxidation”, Journal of Solid State
Electrochemistry, vol. 16 br. 10, str. 3147-3157, (2012)
[IF: 2.234 (2010); oblast: Electrochemistry, 13/26]
M23 – Radovi objavljeni u časopisima međunarodnog značaja (3x3=9)
M23/1 S. Stevanovic, D. Tripkovic, J. Rogan, D. Minic, A. Gavrilovic, A. Tripkovic
and V. M. Jovanovic: “Enhanced Activity in Ethanol Oxidation of Pt3Sn
Electrocatalysts Synthesized by Microwave Irradiation”, Russ. J. Phys.
Chem., 85(13),2299-2304, (2011)
[IF=0.459 (2011); oblast: Chemistry, Physical, 125/134]

M23/2 S. Stevanovic, D. Tripkovic, D. Poleti, J. Rogan, A. Tripkovic and
V.M.Jovanovic: “Microwave Synthesis and Characterization of PT and
Pt-Rh-Sn Electrocatalyst for Ethanol Oxidation” J. Serb. Chem.Soc. 76
(12), 1673-1685, (2011)
[IF= 0.879 (2011); oblast: Chemistry – multidisciplinary, 103/154]
M23/3 Jelena D.Lovic, Dusan V. Tripkovic, Ksenija DJ.Popovic, Vladislava M.
Jovanovic and Amalija Tripkovic “Electrocatalytic Properties of Pt-Bi
Electrodes Towards the Electrooxidation of Formic Acid”, J. Serb. Chem.
Soc., DOI:10.2298/JSC121012138L
[IF= 0.879 (2011); oblast: Chemistry – multidisciplinary, 103/154]
M23/4 Dongguo Li, Chao Wang, Dusan Tripkovic, Shouheng Sun, Nenad M.
Markovic, and Vojislav R. Stamenkovic “Surfactant Removal for Colloidal
Nanoparticles from Solution Synthesis: The Effect on Catalytic
Performance”, ACS Catalysis, 2 (7), pp 1358–1362 (2012),
DOI:10.1021/cs300219j
[IF dobija 2013 god.; oblast: Chemistry-Physical, 130/134]
M23/5 J.D.Lovic, M.D.Obradovic, D.V.Tripkovic, K,Dj.Popovic,V.M.Jovanovic,
S.LJ. Gojkovic, A.V.Tripkovic “High Activity and Stability of Pt2Bi
Catalyst in Formic Acid Oxidation”, Springer Electrocatalysis, (2012),
DOI: 10.1007/sl12678-012-0099-9
[IF dobija 2013 god.: oblast: Electrochemistry]
M33- Radovi saopšteni na skupu međunarodnog značaja štampani u celini (1x1 = 1 )
M33/1 S. Stevanović, D. Tripković, J. Rogan, D. Minić, A. Gavrilović, A.Tripković,
V.M. Jovanović: ″Microvawe assisted sinthesys of Pt and Pt3Sn
electrocatalysts for ethanol oxidation″ , 10th International Conference on
Fundamental and Applied Aspects of Physical Chemistry, Proceedings E-O-2,
Belgrade, Serbia, 21-24 September 2010.

M34- Radovi saopšteni na skupu međunarodnog značaja štampani u izvodu
(15x0.5=7.5 )
M34/1 Dusan Strmcnik, Dusan Tripkovic, Dennis Van der Vliet, Jeffrey P. Greeley,
Alexander Brownrigg, Christopher Lucas, Goran Karapetrov, Vojislav
Stamenkovic, and Nenad Markovic „Active Sites for PEM Fuel Cell
Reactions“, 216th ECS Meeting, Abstract #868, Vienna, Austria, Oct 6 2009
M34/2 C. Wang, D. Li, D. van der Vliet, D. Strmcnik, D. Tripkovic, N. Markovic,
and V. Stamenkovic: ”Advanced Electrocatalysts: - From Extended to
Nanoscale Surfaces”, 220th ECS Abstract #947, Boston, MA, Oct 11 2011
M34/3 Dusan Strmcnik, Kensaku Kodama, Dusan Tripkovic,Dennis Vander
Vliet,Chao Wang,Vojislav Stamenkovic,and Nenad M. Markovic: “Design
Catalytic Properties of Electrochemical Interfaces”, 217 th ECS Meeting
Abstract #1755, Vancouver, Canada, Apr 27 2010
M34/4 Dusan Tripkovic,Dusan Strmcnik,Dennis F. Van der Vliet,Chao Wang,Nenad
M. Markovic,and Vojislav R. Stamenkovic: “In-situ Infrared Spectroscopy
at Solid-Liquid Interfaces as a Tool for Evaluation of Nanoscale Surface
Morphology”, 221 th ECS Meeting Abstract #1570, Seattle, WA, May 9 2012
M34/5 Chao Wang,Dusan Strmcnik,Dusan Tripkovic,Dennis Van der Vliet,Nenad
Markovic,and Vojislav R. Stamenkovic ” Electrocatalysis on Well-Defined
Solid-Liquid Interfaces”, 219 th ECS Meeting Abstract 1924, Montreal, QC,
Canada, May 2 2011
M34/6 Dusan Strmcnik,Ramachandran Subbaraman,Dusan Tripkovic,Kee-Chul
Chang,Nemanja Danilovic,Dennis F. Van der Vliet,Pietro Lopes,Arvydas Paul
Paulikas,Vojislav R. Stamenkovic,and Nenad M. Markovic: ” Controlling
Reactivity of Electrochemical Interfaces by Tuning Non-covalent
Interactions”, 221 th ECS Meeting Abstract #1560, Seattle, WA, May 7 2012
M34/7 Dusan Strmcnik,Dusan Tripkovic, Ramachandran Subbaraman, Nemanja
Danilovic, Dennis F. Van der Vliet, Arvydas Paul Paulikas, Vojislav R.
Stamenkovic and Nenad M. Markovic” Fundamental investigations of
precious metal stability in energy conversion systems”, 221 th ECS Meeting
Abstract #1532, Seattle, WA, May 10 2012
M34/8 Ramachandran Subbaraman, Dusan Tripkovic, Dusan Strmcnik, Gustav K.
Wiberg, Jakub S. Jirkovsky, Chao Wang, Vojislav R. Stamenkovic and Nenad
M. Markovic: ” Electrochemical Interfaces for Energy Conversion and
Storage”, 221 th ECS Meeting Abstract #549, Seattle, WA, May 7 2012

M34/9 S. Stevanović, D. Tripković, V. Tripković, D. Minić, A.Gavrilović, K.
Popović, A. Tripković and V.M. Jovanović: ″Insight of Sn influence on
formic acid oxidation at Pt based catalysts″, 63rd Meeting of International
Society of Electrochemistry, Prague (Czech Republic), Book of Abstracts S05-
027, August 2012
M34/10 J.D. Lović, M.D. Obradović, D.V. Tripković, K.Dj. Popović, V.M.
Jovanović, S.Lj. Gojković and A.V. Tripković: ″High Activity and Stability
of Pt2Bi Catalyst in Formic Acid Oxidation″, 63rd Meeting of International
Society of Electrochemistry, Prague (Czech Republic), Book of Abstracts S05-
063, August 2012
M34/11 S. Stevanović, D. Tripković, J. Rogan, J. Lović, K. Popović, A. Tripković
and V.M. Jovanović: ″Ehanol oxidation on carbon supported platinum
based bimetallic catalysts synthesized by microwave assisted polyol
procedure″, 63rd Meeting of International Society of Electrochemistry,
Prague ( Czech Republic ), Book of Abstracts S05-052, August 2012
M34/12 Chao Wang, Dennis Van der Vliet, Dusan Tripkovic, Dusan Strmcnik,
Dongguo Li, Nenad M. Markovic, and Vojislav R. Stamenkovic: ”Advanced
Electrocatalysts for PEM Fuel Cells”, Abstract 1650, PRiME 2012.
Honolulu, Hawaii October 7-12, 2012
M34/13 Nemanja Danilovic, Ramachandran Subbaraman, Dusan Strmcnik, Dusan
Tripkovic, Kee-Chul Chang, Arvydas Paul Paulikas, Vojislav R.
Stamenkovic, Debbie J. Myers, and Nenad M. Markovic „Electrocatalysts for
the Oxygen Evolution Reaction“, 221 th ECS Meeting, Abstract #1510,
Seattle, WA, May 9 2012
M33/14 R. Subbaraman, D. Tripkovic, G.K. Wiberg, J.S. Jirkovsky and N. M.
Markovic: ” Li-Air- Just a Dream or Future Reality”,DOE-Chine Meeting-
Beyond Li-ion systems, Boston, MS, September 2012, (Invited)
M33/15 D. Tripkovic D. Strmcnik, R. Subbaraman, D. V. Stamenkovic and N. M.
Markovic: “Tailored Nanomaterials for Clean Energy Conversion and
Storage International Symposium”, Tsukuba, Japan, March 2012 (Invited)

III. ANALIZA RADOVA
Rad M21/1 U radu je prikazan novi pristup dizajniranju katalizatora za gorive ćelija
sa polimernom membranom koji mora biti vođen sa dva jednako važna principa:
optimizacija katalitičke aktivnosti i dugoročna stabilnost metalnih katalizatora i
njihovih nosača u radnoj sredini (elektrolitu). Postoji širok spektar metoda koje se
koriste za unapređivanje katalitičke aktivnosti, međutim, metode za poboljšanje
stabilnosti ugljeničnog nosača i katalizatora su ograničene, a naročito tokom gašenja i
paljenja gorivog sprega. Pod ovim okolsnostima na stabilnost katodnog materijala
(oksidacije ugljenika) snažno utiče redukcija kiseonika (Orr) na anodnoj strani. U
ovom radu pokazano je da hemijski modifikovana platina sa samo orijentisanim
monoslojem calix[4]arene molekula može biti primenjena kao selektivni anodi
materijal koji efektivno suzbija Orr a da pri tome ne inhibira reakciju oksidacije
vodonika. Ovako dizajnirani katalitički materijal ispoljava izuzetno veliku stabilnost
čak i pri najekstremnijim uslovima u kojima se može naći tokom rada gorivog sprega.
Rad M21/2 U ovom radu nastavljeno je ispitivanje efekta hemijskog modifikovanja
platine monoslojem calix[4]arene molekula koje daje vrlo stabilnu anodu u gorivom
spregu usled mogućnosti da efektivno suzbija reakciju oksidacije kiseonika, a da pri
tome ne utiče na maksimalnu aktivnost za reakciju oksidacije vodonika. Najveći
doprinos ovog rada je to sto je pokazano da je pomenuti efekat univerzalan i da se
proteže od visoko uređenih monokristalnih površina pa sve do komercijalnih
nanokatalizatora.
Rad M21/3 Napredak u dizajniranju heterogenih katalizatora oslanja se na sposobnost
modifikovanja strukture materijala na nanoskali. U ovom radu dizajniran je i
napravljen katalizator na bazi platine sa višeslojnom bimetalnom strukturom koji
pokazuje izuzetnu elektrokatalitičku aktivnost u reakciji redukcije kiseonika (Orr).
Ovakva struktura prvo je testirana na tankoslojnim površinama sa tačno određenim
sastavom, a zatim implementirana u izradu nanokatalizatora korišćenjem organske
sinteze. Elektrokemijska istraživanja za Orr su pokazala da nakon dužeg izlaganja
reakcijskim uslovima, Pt-bimetalni katalizator sa višeslojnom strukturom pokazuje za
red veličine veću aktivnost, dok je pri tome i stabilnost katalizatora značajno
unapredjena u odnosu na aktivnosti i stabilnost konvencionalnog Pt katalizatora.
Znatno poboljšane katalitičke aktivnosti i stabilnosti pokazuje veliki potencijal pri
izradi superiornih komercijalnih nanokatalizatora za Orr.
Rad M21/4 Poboljšanje spore kinetike elektrokemijske redukcije vode do
molekularnog vodonika u alkalnim sredinama je ključni faktor za smanjenje visokih
nadpotencijala i sa njima povezanih energetskih gubitaka u alkalnim i hlor-alkalnim
elektrolizerima. U ovom radu pokazano je da kontrolisanim raspoređivanjem nanoklastera
Ni(OH)2 po površini platine manifestuje povećanja katalitičke aktivnosti za
faktor 8 kod vodonične reakcije u poređenju sa najmodernijim metalnim i metaloksidnim
katalizatorima. Kako bi se objasnila dobijena aktivnost postavljena je nova
teorija da bifunkcionalnim mehanizmom, ivice Ni(OH)2 klastera podstiču disocijaciju
vode i nastanak intermedijera koji se adsorbuju na obližnje atome platine i nakon
rekombinacije daju konačan proizvod - molekularni vodonik. Pokazano je i da
nastanak intermedijera može dodatno biti favorizovano putem Li + indukovane
destabilizacije HO-H veze, što na kraju dovodi do ukupnog povećanja aktivnosti za
faktor 10.

Rad M21/5 Legiranjem platine sa drugim ne plemenitim metalom i formiranjem Ptskin
strukture dolazi do značajne promene adsorpcionih svojstava u odnosu na čistu
platinu. Naime, potencijal adsorpcije vodonika i oksidnih vrsta je pomeren i smanjen
u odnosu na čistu Pt u istoj potencijalnoj regiji. Činjenica da je količina naelektrisanja
dobijena oksidacijom punog monosloja CO približno ista na aniliranim Pt3M (Mmetal)
površinama kao kod čiste Pt potvrđuje da se površina sastoji od čiste Pt.
Smanjena adsorpcija vodonika (Hupd) na površini se može koristiti kao metoda za
potvrdu Pt-skin strukture. Sa druge strane proračun na osnovu količine adsorbovanog
vodonika predstavlja problem pri određivanju elektrohemijski aktivne površine na
nanočesticama sa Pt-skin strukturom. U ovom radu, kako bi se izbegla greška
prilikom proračuna korišćena je i metoda oksidacije monosloja CO uporedo sa
metodom proračuna aktivne površine Pt na osnovu količine adsorbovanog vodonika.
Rad M21/6 Dizajn i sinteza materijala za efikasanu elektrohemijsku transformaciju
vode do molekularnog vodonika i hidroksilnih jona do kiseonika u alkalnim
sredinama je od neprocenjive važnosti za smanjenje gubitaka energije u alkalnim
elektrolizerima. U ovom radu, korišćenjem 3D-M hydr(oksi)oksida, strogo
definisanih u pogledu stehiometrije i morfologije u reakcijama izdvajanja vodonika
(HER) i redukcije kiseonika (OER), određene su ukupne katalitičke aktivnosti za ove
reakcije kao funkcija fundamentalne zavisnosti, deskriptora, OH-M2 + δ jačina veze
(0 ≤ δ ≤ 1,5). Ovaj odnos pokazuje trendove u reaktivnosti (Mn

Rad M22/1 Katalizatori na bazi Pt, PtRh i PtSn nanetih na ugljenik razvijene
površine sintetisani su pomoć u mikrotalasne-poliol metode i testirani za elektrooksidaciju
etanola. Katalizatori su okarakterisani korišćenjem: termogravimetrijske
analize (TGA), X-ray difrakcije (XRD), skenirajuće tunelirajuće mikroskopije (STM),
TEM, i EDX tehnika. Rezultati su pokazali da je PtSn/C približno tri puta aktivniji
dok je PtRh/C pokazao samo neznatno poboljšanje u odnosu na Pt/C katalizator.
Potencijal početka oksidacije etanola na PtSn katalizatoru predtsavlja najniži
zabeleženi potencijal u literaturi. Hronoamerometrijska merenja su pokazala da je
PtSn/C znatno stabilniji od Pt/C katalizatora.
Rad M23/1. U ovom radu sintetisani su katalizatori na bazi Pt i Pt3Sn korišćenjem
mikrotalasnog zračenja. Nakon sinteze katalizatori su naneti na nosač od ugljenika
razvijene površine i ispitani u reakciji elektro-oksidacije etanola. Katalizatori su
dobijeni primenom izmenjene poliol metode u rastvoru etilen glikola i okarakterisani
u pogledu strukture, morfologije i sastava korišćenjem XED, STM i EDX tehnika.
Pokazano je da je Pt3Sn / C katalizator vrlo aktivan za oksidaciju etanola sa početnim
potencijalom pomerenim za ~ 150 mV negativnije i strujom i ~ 2 puta većom u
odnosu na Pt/C katalizator.
Rad M23/2 Pt i Pt-Rh-Sn katalizatori sintetisani su korišćenjem mikrotalasne-poliol
metode u rastvoru etilen glikola i testirani u reakciji elektro-oksidacije etanola.
Katalizatori su okarakterisani u pogledu strukture, morfologije i sastava korišćenjem
XRD, STM i EDX tehnika. Male veličine i homogena distribucija veličina čestica oba
katalizatora je rezultat korišćenja mikrotalasnog zračenja. Pokazano je da je Pt-Rh-
Sn/C katalizatora je vrlo aktivan za elektro-oksidaciju etanola sa početnim
potencijallom oksidacije pomerenim za više od 150 mV negativnije i strujom
oksidacije gotovo pet puta većom u odnosu na Pt/C katalizator. Unapređena aktivnost
je posledica bifuncionalnog i elektronskog efekta. Ispitana je i stabilnosti katalizatora
korišćenjem hronoamerimetrijskih merenja koja je pokazala da je Pt-Rh-Sn stabilniji
od Pt/C katalizatora.
Rad M23/2 U ovom radu oksidacija mravlje kiselina ispitivana je na dva Pt-Bi
katalizatora; Pt2Bi i polikristalnj Pt modifikovanoj ireverzibilno adsorbovanim Bi (Pt /
Biirr) kako bi se uspostavila razlika između dejstva Biirr i Bi u obliku legure. Rezultati
su predstavljeni u odnosu na čistu Pt. Utvrđeno je da su oba bimetalna katalizatora
aktivnija od čiste Pt, sa početnim potencijalima pomerenim ka negativnijim
vrednostima i strujom većom do dva reda veličina (na 0,0 V vs SCE pod stacionarnim
uslovima). Poreklo visoke aktivnosti i stabilnosti Pt2Bi katalizatora je povećana
selektivnost prema direktnom putu, dehidrogenaciji mravlje kiseline uzrokovane
ensemble efektom i elektroniskom modifikacijom koja dovodi do suzbijanja
izluživanja Bi sa površine tokom reakcije. Međutim, iako Pt / Biirr takođe ispoljava
izuzetnu početnu aktivnost u odnosu na čistu Pt, izluživanje Bi nije inhibirano pa je
trovanje površine katalizatora izazvano dehidratacijom prisutno. Poređenjem
inicijalnog kvazi-stabilnog stanja i potenciodinamičkih rezultata za ova dva Pt-Bi
katalizatora može se zaključiti da elektronski efekat, prisutan kod legure, doprinosi
ranijem početku reakcije, dok je maksimalna gustina struje određena ensemble
efektom.

Rad M23/4 Koloidne nanočestice pripremljene korišćenjem metode sinteze iz
rastvora sa preciznom kontrolom nad parametrima kao što su veličina čestica, oblik,
sastav i struktura su pokazali veliki potencijal za katalitičke aplikacije. Međutim,
ovakve koloidne nanočestice su obično prekrivene sa organskim ligandima i ne mogu
se direktno koristiti kao katalizatori. U ovom radu ispitan je efekat površinskog
uklanjanja hemijski vezanih organskih materija na elektrokatalitička svojstva Pt
nanočestica dobijenih organskom sintezom iz rastvora. Korišćene su raznovrsne
metode za uklanjanje oleylamine surfaktanta, kao što su: žarenje, pranje sirćetnom
kiselinom i UV- zračenje. Ovako tretirane nanočestice su testirane kao katalizatori za
reakciju izdvajanja kiseonika (Orr). Utvrđeno je da su katalitičke performanse koje
podrazumevaju elektrohemijski aktivnu površinu i katalitičku aktivnost direktno
zavisne od pred tretmana. Od metoda koje su u ovom radu korišćene žarenje do
temperature od 185 ° C na vazduhu se pokazalo kao najefikasnije za čišćenje površina
bez izazivanja promena u pogledu veličine i morfologije čestica.
Rad M23/5 U ovom radu oksidacija mravlje kiseline ispitivana je na sveže
pripremljenom Pt2Bi katalizatoru koji je okarakterisan korišćenjem difrakcije X-zraka
(fazni sastav), skenirajuće tunelirajuće mikroskopije (STM) (morfologija poršine) i
oksidacijom adsorbovanog sloja CO (sastav površine). Rezultati karakterizacije su
pokazali da se Pt2Bi katalizatora sastoji od dve faze-55% PtBi legure i 45% Pt. Pt2Bi
katalizator poseduje visoku aktivnost i stabilnost u reakciji oksidacije mravlje kiseline
favorizovanjem direktnog puta,dehidrogenacije, kao posledice ensemble efekta.
Visoka stabilnost Pt2Bi površine je posledica suzbijanja izluživanja Bi što se vidi na
osnovu minimalne promene površinske morfologije prikazanimna STM slikama pre i
posle elektrohemijskih tretmana u mravljoj kiselini. Pt2Bi se pokazao kao moćan
katalizator sa gustinama struje većim za dva reda veličina na 0,0 V i potencijalom
pomerenim za ~ 0,2 V negativnije u odnosu na čistu Pt.

IV. PRILOG
Prilog 1. Kopije separata radova i saopštenja
Prilog 2. kopija potvrde o citiranosti naučnih radova
Prilog 3.
- potvrda o prijavljenom patentu
- poziv za predavanje
- dokaz o rukovođenju podprojektom
- dokaz o učestvovanju na izradi magistarskih teza i doktorskih disertacija
Prilog 4. Predlog rezimea izveštaja o kandidatu za sticanje naučnog zvanja
naučni savetnik

Др. Душан Трипковић
СПИСАК НАУЧНИХ РАДОВА И САОПШТЕЊА
(2005. – 2012.)
(радови и саопштења од предходног избора у звање су означени *)
РАДОВИ ОБЈАВЉЕНИ У НАУЧНИМ ЧАСОПИСИМА МЕЂУНАРОДНОГ
ЗНАЧАЈА (М20):
Радови у врхунским међународним часописима (М21):
1. J.D. Lović, A.V. Tripković, S.Lj. Gojković, K.Đ. Popović, D.V. Tripković, P.
Olszewski, A. Kowal, "Kinetic study of formic acid oxidation on carbon-supported
platinum electrocatayst", J. Electroanal. Chem., 581 (2005) 294.
[IF=2.223 (2005); oblast: Chemistry,analytical, 17/70]
2. Vladislava M. Jovanovic, Dusan Tripkovic, Amalija Tripkovic, Andrzej Kowal, Jerzy
Stoch: “Oxidation of formic acid at platinum electrodeposited on polished and
oxidized glassy carbon”, Electrochem. Comm., 7, 1039-1044, (2005)
[IF: 3.388 (2005); oblast: Electrochemistry, 2/21]
3. A. Tripković, S. Gojković, K. Popović, J.D. Lović, A. Kowal, "Study of the kinetics
and influence of Biir on formic acid oxidation on Pt2Ru3 / C", Electrochim. Acta,53
(2007) 887.
[IF: 2.848 (2007); oblast: Electrochemistry, 7/23]
4. Strmcnik D. S., Rebec P., Gaberscek M., Tripkovic D., Stamenkovic, V., Lucas C. and
Markovic N. M.: “Relationship between the Surface Coverage of Spectator Species
and the Rate of Electrocatalytic Reactions“, J. Phys Chem.C. 111, 18672-18678,
(2008)
[IF: 3.396 (2008); oblast: Chemistry, Physical, 28/113]

5. D. Tripković, S. Stevanović, A. Tripković, A. Kowal and V.M. Jovanović: “Strustural
effect in electrocatalysis: Formic acid oxidation on Pt electrodeposited on glassy
carbon support“, J. Electrochem. Soc., 155, B281-289, (2008)
[IF: 2.387 (2006); oblast: Electrochemistry, 6/22]
6. D. V. Tripkovic, D. Strmcnik, D. van der Vliet, V. Stamenkovic and N. M. Markovic:
“The role of anions in surface electrochemistry“, Faraday Discuss., 140 (2008) 25-40
[IF: 4.604 (2008); oblast: Chemistry, Physical, 19/113]
7. D. Strmcnik, D. Tripkovic, D. van der Vliet, V. Stamenkovic and N.M. Markovic:
“Adsorption of hydrogen on Pt(1 1 1) and Pt(1 0 0) surfaces and its role in the HOR“,
Electrochem. Communications, 10, 1602-1605, (2008)
[IF: 4.194 (2008); oblast: Electrochemistry, 2/22]
8. D. Strmcnik, D. Tripkovic, D. van der Vliet, K. C. Chang, V. Komanicky, H. You, G.
Karapetrov, J. Greeley, V. Stamenkovic and N. M. Markovic: “Unique activity of
platinum ad-islands in the CO electrooxidation reaction“, J. American Chem. Soc.,
130 (46), 15332–15339, (2008)
[IF: 8.091 (2008); oblast: Chemistry, Multidisciplinary, 7/127]
9. Sanja Stevanović, Vladimir Panić, Dušan Tripković, and Vladislava M. Jovanović:
''Promoting effect of carbon functional groups in methanol oxidation on supported Pt
catalyst'', Electrochem. Comm., 11, 18-21, (2009)
[IF: 4.243 (2009); oblast: Electrochemistry, 2/24]
10. A.V. Tripković, K.Dj. Popović, J.D. Lović, V.M. Jovanović, S.I. Stevanović, D.V.
Tripković and A. Kowal " Promotional effect of Snad on the ethanol oxidation at
Pt3Sn/C catalyst" Electrochem. Comm., 11 (2009) 1030.
[IF: 4.243 (2009); oblast: Electrochemistry, 2/24]

11. *Bostjan Genorio, Dusan Strmcnik, Ram Subbaraman, Dusan Tripkovic, Goran
Karapetrov, Vojislav R. Stamenkovic, Stane Pejovnik and Nenad M. Markovic
“Selective catalyst for the hydrogen oxidation and oxygen reduction reactions by
patterning of platinum with calix[4]arene molecules” Nature Materials (12), 998-
1003, (2010)
[IF: 29.920 (2010); oblast: Chemistry, Physical, 1/127]
12. *Bostjan Genorio, Ram Subbaraman, Dusan Strmcnik, Dusan Tripkovic, Vojislav R.
Stamenkovic, and Nenad M. Markovic “ Tailoring the selectivity and stability of
chemically modified platinum nanocatalysts to design highly durable anodes for PEM
fuel cells” Angewandte Chemie, 50(24), 5468-72, (2011)
[IF: 13.455 (2011); oblast: Chemistry, Multidisciplinary, 7/154
13. *Chao Wang, Miaofang Chi, Dongguo Li, Dusan Strmcnik, Dennis van der
Vliet,Guofeng Wang,Vladimir Komanicky, Kee-Chul Chang, Arvydas P. Paulikas,
Dusan Tripkovic, John Pearson,Karren L. More, Nenad M. Markovic, and Vojislav R.
Stamenkovic ” Design and Synthesis of Bimetallic Electrocatalyst with Multilayered
Pt-Skin Surfaces” Journal of the American Chemical Society,133(36),14396-403,
(2011)
[IF: 9.907 (2011); oblast: Chemistry, Multidisciplinary, 11/154]
14. *Ram Subbaraman, Dusan Tripkovic, Dusan Strmcnik, Kee-Chul Chang, Masanobu
Uchimura, Arvydas P. Paulikas, Vojislav Stamenkovic, Nenad M. Markovic
“Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring Li + -
Ni(OH)2-Pt Interfaces”, Science, vol. 334 no. 6060 pp. 1256-1260, (2011)
[IF: 31.201 (2011); oblast: Multidisciplinary Sciences, 2/56]
15. *Dennis van der Vliet, Chao Wang, Dongguo Li, Arvydas P. Paulikas,Jeffrey
Greely,Rees B. Rankin, Dusan Strmcnik, Dusan Tripkovic, Nenad M. Markovic, and
Vojislav R. Stamenkovic “Unique Electrochemical Adsorption Properties of Pt-skin
Surfaces”, Angewandte Chemie,Vol 51, Issue 13, 3139–3142, (2012)
[IF: 13.455 (2011); oblast: Chemistry, Multidisciplinary, 7/154
16. *Ram Subbaraman, Dusan Tripkovic,Kee-Chul Chang,Dusan Strmcnik, Arvydas
P.Paulikas,Pussana Hirunsit, Maria Chan, Jeff Greeley, Vojislav Stamenkovic and
Nenad M.Markovic” Trends in activity for the water electrolyser reactions on 3dM
(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts”,Nature Materials 11, (6), 550-557 (2012)
DOI: 10.1038/nmat3313
[IF: 32.841 (2011); oblast: Chemistry, Physical, 1/134]

17. *Dennis van der Vliet, Chao Wang, Dusan Tripkovic, Dusan Strmcnik, Xiao Zhang,
Mark Debe, Radoslav Atanososki, Nenad Markovic and Vojislav Stamenkovic
“Mesostructured Thin Films as Electrocatalysts with Tunable Composition and
Surface Morphology”, Nature Materials, (2012), DOI: 10.1038/nmat3457
[IF: 32.841 (2011); oblast: Chemistry, Physical, 1/134]
18. *Ram Subbaraman, N. Danilovic, P. P. Lopes, D. Tripkovic, D. Strmcnik, V. R.
Stamenkovic, and N. M. Markovic “Origin of Anomalous Activities for
Electrocatalysts in Alkaline Electrolytes”, J. Phys. Chem. C, 116 (42), pp 22231–
22237 (2012), DOI: 10.1021/jp3075783
Радови у истакнутим међународним часописима (М22):
[IF: 4.805 (2011); oblast: Chemistry, Physical, 26/134]
1. *Stevanovic I. Sanja, Tripkovic V. Dusan, Rogan R. Jelena, Popovic Dj. Ksenija,
Lovic D. Jelena, Tripkovic V. Amalija and Jovanovic M Vladislava, “Microwaveassisted
polyol synthesis of carbon-supported platinum-based bimetallic catalysts for
ethanol oxidation”, Journal of Solid State Electrochemistry, vol. 16 br. 10, str. 3147-
3157, (2012)
Радови у међународним часописима (М23):
[IF: 2.234 (2010); oblast: Electrochemistry, 13/26]
1. D. Tripković, V. Radojević, R. Aleksić: “Factors affecting the microstructure of porous
ceramics“, J. Serb. Chem. Soc, 71 (3), 277-284, (2006)
[IF=0.522 (2004); oblast: Chemistry – multidisciplinary, 85/124]
2. A. Kowal, P. Olszewski, D. Tripković, R. Stevanović: “Nanoscale topography of
GC/Pt-C and GC/Pt-Ru-C electrodes studied by means of STM, AFM and XRD
methods“, Material Science Forum (RECENT DEVELOPMENTS IN ADVANCED
MATERIALS AND PROCESSES), 518, 271-275, (2006)
[IF=0.498 (2004); oblast: Materials Science – multidisciplinary, 137/178]

3. J.D.Lović, S. Lj. Gojković, K.Đ. Popović, D.V. Tripković, A.V. Tripković, "Structural
effects in electrocatalysis: formic acid oxidation at model and real Pt catalysts"
Mat.Sci.Forum, 518, (2006) 259.
[IF=0.498 (2004); oblast: Materials Science – multidisciplinary, 137/178]
4. Sanja Terzić, Dušan Tripković, Vladislava M. Jovanović, Amalija Tripković and
Andrzej Kowal: “Effect of glassy carbon properties on electrochemical deposition of
platinum nano-catalyst and its activity for methanol oxidation”, J. Serb. Chem. Soc., 72
(2), 165-181, (2007)
[IF=0.536 (2007); oblast: Chemistry – multidisciplinary, 95/127]
5. S. Stevanović, D. Tripković, A. Kowal, D. Minić, V. M. Jovanović and A. Tripković:
“Influence of surface morphology on methanol oxidation at glassy carbon supported Pt
catalyst”, J. Serb. Chem. Soc., 73, 845-859, (2008)
[IF=0.611 (2009); oblast: Chemistry – multidisciplinary, 91/127]
6. *S. Stevanovic, D. Tripkovic, J. Rogan, D. Minic, A. Gavrilovic, A. Tripkovic and V.
M. Jovanovic: “Enhanced Activity in Ethanol Oxidation of Pt3Sn Electrocatalysts
Synthesized by Microwave Irradiation”, Russ. J. Phys. Chem., 85 (13), 2299-2304,
(2011)
[IF=0.459 (2011); oblast: Chemistry, Physical, 125/134]
7. *S. Stevanovic, D. Tripkovic, D. Poleti, J. Rogan, A. Tripkovic and V.M.Jovanovic:
“Microwave Synthesis and Characterization of PT and Pt-Rh-Sn Electrocatalyst for
Ethanol Oxidation”, J. Serb. Chem. Soc., 76 (12), 1673-1685, (2011)
[IF= 0.879 (2011); oblast: Chemistry – multidisciplinary, 103/154]
8. *J.D.Lović, D.V. Tripković, K.Dj. Popović, V.M. Jovanović, A.V. Tripković,
“Electrocatalytic properties of Pt-Bi electrodes towards the electrooxidation of formic
acid”, J.Serb.Chem.Soc., DOI: 10.2298/JSC121012138L
[IF=0.879 (2011); oblast: Chemistry – multidisciplinary, 102/152]

9. *Dongguo Li, Chao Wang, Dusan Tripkovic, Shouheng Sun, Nenad M. Markovic, and
Vojislav R. Stamenkovic “Surfactant Removal for Colloidal Nanoparticles from
Solution Synthesis: The Effect on Catalytic Performance”, ACS Catalysis, 2 (7), pp
1358–1362 DOI:10.1021/cs300219j, (2012)
[IF dobija 2013; oblast: Chemistry - Physical, 130/134]
10. *J.D. Lović, M.D. Obradović, D.V. Tripković, K.Dj. Popović, V.M. Jovanović,
S.Lj. Gojković, A.V. Tripković “High activity and stability of Pt2Bi catalyst in formic
acid oxidation”, Springer Electrocatalysis, DOI: 10.1007/s12678-012-0099-9
[IF dobija 2013; oblast: Electrochemistry]
УКУПНИ М=205.5 УКУПНИ ИФ=215.335
ЗБОРНИЦИ МЕЂУНАРОДНИХ НАУЧНИХ СКУПОВА (М30):
Саопштење са међународног скупа штампано у целини (М33):
1. * S. Stevanović, D. Tripković, J. Rogan, D. Minić, A. Gavrilović, A.Tripković and V.M.
Jovanović: „Microwave assisted synthesis of Pt and Pt3Sn electrocatalysts for
ethanol oxidation“ , 10th International Conference on Fundamental and Applied
Aspects of Physical Chemistry, Proceedings E-O-2, Belgrade, Serbia, 21-24 September
2010.

Саопштење са међународног скупа штампано у изводу (М34):
1. D.Tripković, S.Terzić, V. M. Jovanović and A. Kowal: ″ Characterisation of
Platinum nanoparticles electrochemically deposited on glassy carbon″, The seventh
Yugoslav Matherials Research Conference, The Book of Abreacts, P.S.A.8, page 94,
September 2005, Herceg-Novi, Yugoslavia
2. D. Tripković, V. Radojević, R. Aleksić : “Factors affecting the microstructure of
porous ceramics“, The seventh Yugoslav Materials Research Conference, The Book of
Absreacts, P.S.A.8, page 71, September 2005, Herceg-Novi, Yugoslavia
3. S. Terzić, D. Tripković, A. Tripković, A. Kowal and V. M. Jovanović: “Methanol
oxidation at glassy carbon supported Pt catalyst in acid and alkaline solution –
effect of support activation”, 56th Meeting of International Society of
Electrochemistry, Bussan (Korea), September 2005
4. S. Terzić, D. Tripković, A. Tripković, A. Kowal and V. M. Jovanović: “The effect of
electrochemically treated glassy carbon on the activity of supported Pt catalyst”,
56th Meeting of International Society of Electrochemistry, Bussan (Korea), September
2005
5. V. M. Jovanović, D. Tripković and A. Tripković: “Formic acid oxidation on glassy
carbon supported platinum nanoparticles – structural effect“, 57th Meeting of
International Society of Electrochemistry, Edinburgh (United Kingdom), August 2006,
Book of Abstracts S10-P-24
6. D. Tripković, S. Terzić, V. M. Jovanović and A. Kowal: “Electrocatalytic activity of
platinum nanoparticles on oxidized support“, 57th Meeting of International Society
of Electrochemistry, Edinburgh (United Kingdom), August 2006, Book of Abstracts S4-
P-48
7. V. Stamenkovic, C. Lucas, D. Tripkovic, D. Strmcnik and N. M. Markovic“In-Situ
SXS/STM Characterization of Temperature Controlled ElectrifiedSolid-Liquid
Interfaces” 210th Int. Meeting of the Electrochemical Society, November 2006,
Cancun, Mexico

8. V. Stamenkovic, C. A. Lucas, D. Tripkovic, D. Strmcnik, K. C. Chang, H. You, N. M.
Markovic “In-Situ characterization Temperature Controlled Electrified Solid-
Liquid Interfaces”233rd American Chemical Society National Meeting, March 2007,
Chicago, IL, USA
9. D. Strmcnik, D. Tripkovic, D.van der Vliet, J. Greeley, V. Stamenkovic, N. M.
Markovic “Active Sites for Fuel Cell reactions: Experiments and Theory” 11th
International Conference on Electrified Interfaces, June 2007, Sapporo, Japan.
10. V. Stamenkovic, D. Strmcnik, D. Tripkovic, D. van der Vliet, H.You, N.
Markovic“Nanocatalysts Engineering on Extended and Nanoscale Surfaces”9th
International Conf. of the Yugoslav Materials Research Society, September 2007,
Herceg Novi, Montenegro.
11. D. Tripkovic, D. Strmcnik, D. van der Vliet, V. Stamenkovic and N. M. Markovic
“Active sites in the CO oxidation on Pt(111)” 59th Annual Meeting of the
International Society of Electrochemistry, September 2008, Seville, Spain
12. D. Tripković, S. Stevanović, Amalija Tripković, A. Kowal and V. M. Jovanović
“Structural effect in electrocatalysis:Formic acid oxidation on Pt electrodeposited
on glassy carbon support“, First Regional Symposium on Electrochemistry of South-
East Europe, Crveni Otok, Rovinj, Istria, Croatia, May 4-8, 2008.
13. D.Tripković “The influence of surface morphology of platinum materials on
electrocatalytic activity toward CO oxidation” Invited lecture at Center for Atomic
Scale Materials Designee, Technical University of Denmark, Department of Physics,
February 2009, Copenhagen, Denmark
14. *Dusan Strmcnik, Dusan Tripkovic, Dennis Van der Vliet, Jeffrey P. Greeley,
Alexander Brownrigg, Christopher Lucas, Goran Karapetrov, Vojislav Stamenkovic and
Nenad Markovic „Active Sites for PEM Fuel Cell Reactions“, 216th ECS Meeting,
Abstract #868, Vienna, Austria, Oct 6 2009

15. *Dusan Strmcnik, Kensaku Kodama, Dusan Tripkovic, Dennis Vander Vliet, Chao
Wang, Vojislav Stamenkovic and Nenad M. Markovic: “Design Catalytic Properties
of Electrochemical Interfaces”, 217 th ECS Meeting, Abstract #1755, Vancouver,
Canada, Apr 27 2010
16. *Chao Wang, Dusan Strmcnik, Dusan Tripkovic, Dennis Van der Vliet, Nenad
Markovic and Vojislav R. Stamenkovic ” Electrocatalysis on Well-Defined Solid-
Liquid Interfaces”, 219 th ECS Meeting, Abstract #1924, Montreal, QC, Canada, May 2
2011
17. *C. Wang, D. Li, D. van der Vliet, D. Strmcnik, D. Tripkovic, N. Markovic and V.
Stamenkovic: ”Advanced Electrocatalysts: - From Extended to Nanoscale
Surfaces”, 220th ECS Meeting, Abstract #947, Boston, MA, Oct 11 2011
18. *S. Stevanović, D. Tripković, V. Tripković, D. Minić, A.Gavrilović, K. Popović, A.
Tripković and V.M. Jovanović: „Insight of Sn influence on formic acid oxidation at
Pt based catalysts“, 63rd Meeting of International Society of Electrochemistry, Prague
(Czech Republic), August 2012, Book of Abstracts S05-027
19. *J. D. Lović, M. D. Obradović, D.V. Tripković, K. Dj. Popović, V. M. Jovanović, S
.Lj. Gojković and A.V. Tripković: „High Activity and Stability of Pt2Bi Catalyst in
Formic Acid Oxidation“, 63rd Meeting of International Society of Electrochemistry,
Prague (Czech Republic), August 2012, Book of Abstracts S05-063
20. *S. Stevanović, D. Tripković, J. Rogan, J. Lović, K. Popović, A. Tripković and V.M.
Jovanović: „Ethanol oxidation on carbon supported platinum based bimetallic
catalysts synthesized by microwave assisted polyol procedure“, 63rd Meeting of
International Society of Electrochemistry, Prague (Czech Republic), August 2012, Book
of Abstracts S05-052
21. *Dusan Strmcnik, Ramachandran Subbaraman, Dusan Tripkovic, Kee-Chul Chang,
Nemanja Danilovic, Dennis F. Van der Vliet, Pietro Lopes, Arvydas Paul Paulikas,
Vojislav R. Stamenkovic and Nenad M. Markovic: ” Controlling Reactivity of
Electrochemical Interfaces by Tuning Non-covalent Interactions”, 221 th ECS
Meeting, Abstract #1560, Seattle, WA, May 7 2012

22. *Dusan Strmcnik, Dusan Tripkovic, Ramachandran Subbaraman, Nemanja Danilovic,
Dennis F. Van der Vliet, Arvydas Paul Paulikas, Vojislav R. Stamenkovic and Nenad
M. Markovic” Fundamental investigations of precious metal stability in energy
conversion systems”, 221 th ECS Meeting, Abstract #1532, Seattle, WA, May 10 2012
23. *Ramachandran Subbaraman, Dusan Tripkovic, Dusan Strmcnik, Gustav K. Wiberg,
Jakub S. Jirkovsky, Chao Wang, Vojislav R. Stamenkovic and Nenad M. Markovic: ”
Electrochemical Interfaces for Energy Conversion and Storage”, 221 th ECS
Meeting, Abstract #549, Seattle, WA, May 7 2012
24. *Nemanja Danilovic, Ramachandran Subbaraman, Dusan Strmcnik, Dusan Tripkovic,
Kee-Chul Chang, Arvydas Paul Paulikas, Vojislav R. Stamenkovic, Debbie J. Myers
and Nenad M. Markovic „Electrocatalysts for the Oxygen Evolution Reaction“, 221 th
ECS Meeting, Abstract #1510, Seattle, WA, May 9 2012
25. *Dusan Tripkovic, Dusan Strmcnik, Dennis F. Van der Vliet, Chao Wang, Nenad M.
Markovic and Vojislav R. Stamenkovic: “In-situ Infrared Spectroscopy at Solid-
Liquid Interfaces as a Tool for Evaluation of Nanoscale Surface Morphology”,
221 th ECS Meeting, Abstract #1570, Seattle, WA, May 9 2012
26. *Chao Wang, Dennis Van der Vliet, Dusan Tripkovic, Dusan Strmcnik, Dongguo Li,
Nenad M. Markovic and Vojislav R. Stamenkovic: ”Advanced Electrocatalysts for
PEM Fuel Cells”, PRiME 2012., Abstract 1650 Honolulu, Hawaii October 7-12, 2012
27. *R. Subbaraman, D. Tripkovic, G. K. Wiberg, J. S. Jirkovsky and N. M. Markovic:”
Li-Air- Just a Dream or Future Reality”, DOE-Chine Meeting-Beyond Li-ion systems,
Boston, MS, September 2012, (Invited)
28. *D. Tripkovic, D. Strmcnik, R. Subbaraman, D. V. Stamenkovic and N. M. Markovic:
“Tailored Nanomaterials for Clean Energy Conversion and Storage”, International
Symposium, Tsukuba, Japan, March 2012 (Invited)

РАДОВИ ОБЈАВЉЕНИ У ЧАСОПИСИМА НАЦИОНАЛНОГ ЗНАЧАЈА (М50):
Радови у водећим часописима националног значаја (М51)
1. S. Terzić, V. M. Jovanović, D.Tripković, A. Kowal, J. Stoch: “Elektrohemijska,
mikroskopska i spektroskopska karakterizacija platine nataložene na staklasti ugljenik“,
Hemijska industrija, 61 (3), 135-141, (2007)
[IF=0.177 (2009); oblast: Engineering, Chemical,118/127]
ЗБОРНИЦИ СКУПОВА НАЦИОНАЛНОГ ЗНАЧАЈА (М60):
Саопштење на скупу националног значаја штампано у целини (М63):
Остварени
М
коефицјент
М коефицјент од избора у звање
виши научни сарадник
М20 M21: 8 x 8 = 64
M22: 1 x 5 = 5
M23: 3 x 5 = 15
М30 M33: 1 x 1 = 1
M34: 15 x 0,5 = 7,5
Укупни М коефицјент
M21: 18 x 8 = 144
M22: 1 x 5 = 5
M23: 10 x 3 = 30
M33: 1 x 1 = 1
M34: 28 x 0,5 = 14
М50 M51: 1 x 2 = 2
М60 M63: 1 x 0,5 = 0.5
М70 М71: 1 x 6 = 6
М72: 1 x 3 = 3
Укупно: 92,5 Укупно: 205,5

PIB: 100160355
Matični broj: 07805497
Šifra delatnosti: 073101
UNIVERZITET U BEOGRADU
IHTM INSTITUT ZA HEMIJU TEHNOLOGIJU I METALURGIJU
CEH CENTAR ZA ELEKTROHEMIJU
Njegoševa 12, 11001 Beograd, Srbija
Kao komentor magistarske teze Mr. Sanje Stevanović (Terzić) pod nazivom
"Oksidacija metanola na platinskim nano-česticama elektrohemijski nataloženim na
staklasti ugljenik" odbranjene 2007. god. na Fakultetu za fizičku hemiju u Beogradu i kao
rukovodilac podprojekta u okviru koga se izvodi doktorska disertacija koleginice Stevanović
na temu ″Sinteza i karakterizacija platinskih legura za anodne reakciju u gorivim
spregovima″ prihvaćena i odobrena 2009. god. (odbrana se oćekuje do leta 2013. god.)
potvrdjujem da je Dr. Dušan Tripković učestvovao u rukovodjenju i izradi delova obe teze
koji se odnose na karakterizacije površina ispitivanih katalizatora (čestica) koriščenjem AFM
i STM tehnika kao i oksidacije CO. Zajednički radovi proizašli iz magistarske teze i
doktorske disertacije su:
1. Sanja Terzić, Dušan Tripković, Vladislava M. Jovanović, Amalija Tripković and
Andrzej Kowal: “Effect of glassy carbon properties on electrochemical deposition of
platinum nano-catalyst and its activity for methanol oxidation”, J.Serb.Chem.Soc.,
72 (2) (2007) 165-181
2. S. Stevanović, D. Tripković, A. Kowal, D. Minić, V.M. Jovanović and A.Tripković:
“Influence of surface morphology on methanol oxidation at glassy carbon
supported Pt catalyst”, J.Serb.Chem.Soc., 73 (8-9) (2008) 845-859
3. Sanja Stevanović, Vladimir Panić, Dušan Tripković, and Vladislava M. Jovanović:
“Promoting effect of carbon functional groups in methanol oxidation on supported
Pt catalyst“, Electrochem .Comm., 11 (2009) 18-21
4. A.V. Tripković, K.Dj. Popović, J.D. Lović, V.M. Jovanović, S.I. Stevanović, D.V.
Tripković and A. Kowal: “Promotional effect of Snad on the ethanol oxidation at
Pt3Sn/C catalyst“, Electrochem. Comm. 11 (5) (2009) 1030-1033.
5. S. Stevanović, D. Tripković, D. Poleti, J. Rogan, A. Tripković and V.M. Jovanović:
“Microwave synthesis and characterization of Pt and Pt–Rh–Sn electrocatalysts
for ethanol oxidation“, J. Serb. Chem. Soc. 76 (12) (2011)1673–1685
6. S. Stevanović, D. Tripković, J. Rogan, D. Minić, A. Gavrilović, A. Tripković and V. M.
Jovanović: “Enhanced Activity in Ethanol Oxidation of Pt3Sn Electrocatalysts
Synthesized by Microwave Irradiation“, Russian Journal of Physical Chemistry A, 85
(13) (2011) 2299–2304
7. S. Stevanović, D. Tripković, J. Rogan, K. Popović and J. Lović, A. Tripkovic and
V.M.Jovanovic: "Microwave-assisted polyol synthesis of carbon-supported
platinum-based bimetallic catalysts for ethanol oxidation", J. Solid State
Electrochemistry. 16 (2012) 3147-3157
Beograd, 14.12.2012. god.
Tel/Fax: (+38111 ) 3370-389, 3370-390
Dr. Vladislava M. Jovanović
Naucni savetnik IHTM-CEH

~
Univerzitet uBeogradu
Fakultet zafizicku hemiju
Sanja Terzic
OKSIDACIJA MET ANOLA NA PLATINSKIM
NANO-CESTICAMA ELEKTROHEMIJSKI
NATALOZENIM NA STAKLASTI UGLJENIK
magistarska teza
Beograd, jun 2007.

~
Mentor:
Dr. Dragica Minie, red. Prof.,
Fakultet za fizicku hemiju, Beograd
Komentor: Dr. Vladislava Jovanovic,
naucni savetnik IHTM-a
Komisija: Dr. Nikola Vukeli6, van. Prof,
Fakultet za fizicku hemiju, Beograd
Datum odbrane: jun. 2007.

~
{66//._._..
. /!J J::
/17- '. ,'t. (IJ. . Jv\
Na osnovu clana 227. stay 2. Statuta Univerzitet u Beogradu - Fakulteta za fizicku
hemiju, Nastavno-naucno vece Fakulteta, na I redovnoj sednici, odrzanoj 19.10.2006. godine,
donosi sledecu
1.- Prihvata se pozitivni izvestaj 0 predlogu teme za izradu magistarske teze i odobrava se
tema za izradu magistarske teze studenta Sanje Terzic, diplomiranog fizikohemicara,
istrafivaca-pripravnika, IHTM - Centar za elektrohemiju, pod naslovom: «Oksidacija
metanola na platinskim nano-cesticama elektrohemijski natalozenim na staklasti ugljenik»,
Komisije u sastavu:
]) dr Dragica Minic,redovni profesor, Fakultet za fizicku hemiju,
(!J dr Vladislava Jovanovic, naucni savetnik, IHTM - Centar za elektrohemiju,
3) dr Nikola Vukeli6, docent, Fakultet za fizicku hemiju.
2.- Na zahtev studenta, za mentora za izradu eve magistarske teze odreduju se
1) dr Dragica Minie, redovni profesor, i 2) dr Vladislava Jovanovic, naucni savetnik.
3.- Student maze braniti magistarsku tezu u roku od tri godine od danaodobrenja teme
magistarske teze, a ovaj rok, na zahtev studenta, maze biti produzen, najduzeza pet godina od
dana stupanja na snagu Zakonao visokom obrazovanju, odnosno do 11.9.2010. godine. Odluku 0
produzenju, na predlog mentora, donosi Nastavno-naucno vece.
Po uradenoj magistarskoj tezi, student podnosi Nastavno-naucnom vecu zahtev za
odbranu teze idostavlja primerak teze.
Odluku dostaviti:
- studentu,
- mentoru,
- Arhivi Fakulteta.
.,',
. )
ODLUKU
'"
,)

Dr. Nenad M Markovic
Argonne National Laboratory
Materials Science Division
Phone: (630) 252-5181
Email: nmmarkovic@anl.gov
Dear Madam/Sir,
December 14 , 2012
This letter is to confirm that Dr. Dusan Tripkovic, employed at ICTM-Department of
Electrochemistry, was visiting Argonne National Laboratory first as a student (2005-2007) and
then as a postdoctoral fellow from 2009 to 2011. During his stay in my group Dr. Tripkovic was
working on the development and design of surface sensitive probes (scanning tunneling
microscopy, STM) and vibrational spectroscopy (infraread spectroscopy, IR) capable to provide
information at electrochemical interfaces at atomic and molecular levels. In addition to doing his
own experiments Dr. Tripkovic was actively involved in designing STM and IR experiments for
both Dusan Tripovic and Dennis Van der Vliet. As outlined below, both Strmcinik and Van der
Vliet used these results in their thesis to explain structure function relations in surface chemistry
of CO oxidation reaction.
1. In Strmcnik’s thesis, entitled “Active Sites for PEM Fuel Cell Reactions Model and Real
Systems”, STM and IR results were used to find correlations between the number of active sites
on Pt single crystal surfaces and surface structure of CO. Dr. Tripkovic’s role was to help
Strmcing in designing, collecting and interpreting the STM and IR results.
2. In Van der Vliet’s thesis, entitled “Oxygen Reduction on Pt-based Nanoparticle Catalyst – the
experimental STM and IR results van der Vliet used to find relationships between the structure
of Pt single crystal and the size of naoparticles in the case of the CO oxidation reaction. As in the
case with Strimcnik, Dr. Tripkovic was carrying out the measurements related to both the
determination of the surface morphology of the catalysts, using STM technique as well as the IR
measurements which explained anomalous dependence of the rate of CO oxidation and the size
of Pt nanoparticles. Dr. Tripkovic was also actively involved in the discussions with both
Strmcnik and van der Vliet, giving important suggestions in finalizing the final draft of papers

which are produced based on their experimental results. From these collaboration 4 papers were
published in high impact journals including:
1. Strmcnik D. S., Rebec P., Gaberscek M., Tripkovic D., Stamenkovic, V., Lucas C. and
Markovic N. M.: “Relationship between the Surface Coverage of Spectator Species and
the Rate of Electrocatalytic Reactions“, J. Phys Chem.C. 111, 18672-18678, (2008)
2. D. V. Tripkovic, D. Strmcnik, D. van der Vliet, V. Stamenkovic and N. M. Markovic: “The
role of anions in surface electrochemistry“, Faraday Discuss., 140 (2008) 25-40
3. D. Strmcnik, D. Tripkovic, D. van der Vliet, V. Stamenkovic and N.M. Markovic:
“Adsorption of hydrogen on Pt(1 1 1) and Pt(1 0 0) surfaces and its role in the HOR“,
Electrochem. Communications, 10, 1602-1605, (2008)
4. D. Strmcnik, D. Tripkovic, D. van der Vliet, K. C. Chang, V. Komanicky, H. You, G.
Karapetrov, J. Greeley, V. Stamenkovic and N. M. Markovic: “Unique activity of platinum
ad-islands in the CO electrooxidation reaction“, J. American Chem. Soc., 130 (46), 15332–
15339, (2008)
Sincerely,
Dr. Nenad M. Markovic
Senior Staff Scientist

Univerza v Ljubljani
Fakulteta za kemijo kemijsko tehnologijo
I
b .~r.;.t
I. :~:I\-1";j:::,~
ACTIVE,SITES FOR PEM FUEL CELL
REACTIONS IN MODEL AND REAL
SYSTEMS
AKTIVNA MESTA ZA REAKCIJE V PEM GORIVNIH
CELICAH V MODELNIH IN REALNIH SISTEMIH
Ph.D. DISSERTATION
DOKTORSKA DISER T ACIJA
Dusan Strmcnik

~
Ljubljana, October 2007
My work was performed under mentorship of:
Dr. Nenad Markovic,
Argonne National Laboratory, Chicago, Illipois, USA
and
Doc.Dr. Miran Gaberscek
National Institute of Chemistry, Ljubljana, Slovenia

~
Fuel Cell Electrocatalysis
Oxygen Reductional" .Pt-based Nanoparticle
Catalysts
Proefscllrift
ter verkrijging van
de graad van Doctor aan de Universiteit Leiden,
op gezag van Rector Magnificus Prof. Mr. P.F. van der Heijden,
volgens besluit van bet College veer Promoties
te verdedigen op dinsdag 21. september 2010
klokke 16.15 Our
door
Dennis Franciscus van de}" Vliet
geboren te Tilbmg in ] 98]

~
Promotiecommissie:
Promotor:
CoproJnotor:
Overige Lcden;
Prof. Dr. M. T. M. Koper
Dr. N. M. Markovic (Argonne National Lab, USA)
Prof. Dr. J. Brouwer
Prof. Dr. B. E. Nieuwel11mys
Prof. Dr, ], W, N, Frenken
Prof. Dr. G. A. Attard (University of Cardiff, UK)
Prof. Dr. J. A. R. van Veen (Shell)
Dr. N. P. Lebedeva (ECN)

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Author: Tripkovi?, Du?an V.
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Enhancing Hydrogen Evolution Activity in Water Splitting by
Tailoring Li + -Ni(OH) 2-Pt
Interfaces
Ram Subbaraman,
et al.
Science 334,
1256 (2011);
DOI: 10.1126/science.1211934
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http://www.sciencemag.org/content/suppl/2011/11/30/334.6060.1256.DC1.html
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REPORTS
1256
(aL † aR † |vac〉) given an expected mean number of
events m 0 = d 2 Np s consistent with zero concurrence,
where ps =1.0×10 −3 is the probability
of generating a Stokes heralding photon and N
is the number of experimental runs. Our results
(X =3,N =1.9×10 14 ,andm 0 =9.1T 0.9) indicate
positive concurrence at a 98 T 1% confidence level.
Therefore, based on this detection of entanglement
between Stokes and anti-Stokes modes, we
can infer entanglement between the phonon modes
of two macroscopic solids at room temperature.
Finally, we examine the quality of entanglement
generated between the diamonds by neglecting
the vacuum component in Fig. 3, which
is only caused by inefficiencies in coupling, detection,
and readout of the anti-Stokes mode. To
do this, we performed quantum state tomography
(25) on the joint polarization state of the
Stokes and anti-Stokes modes, postselecting on
the detection of both photons. The reconstructed
state is shown in Fig. 4, and we have subtracted
accidental coincidences calculated from the Stokes
and anti-Stokes singles rates. These results provide
a more complete estimate of the coherence between
the two modes than the interference fringes
in Fig. 2. The concurrence of this subspace, 0.85,
provides an estimate of the achievable entanglement
between the two diamond phonon modes
as the readout efficiency, coupling, and detector
efficiencies approach unity (i.e., p00 → 0). Further,
the fidelity to the nearest Bell state ½jHV 〉 þ
jVH〉Š= ffiffi p
2 is 0.91. However, in the presence of
real coupling and detection losses, the existence
of entanglement can only be inferred from the
density matrix in Fig. 3 (22).
In our experiment, short-lived quantum correlations
were revealed by combining an ultrafast
interferometric pump-probe scheme with
photon-counting techniques. The large optical
bandwidth enabled the resolution of extremely
fast dynamics in the solids, and also operation
at high data rates, providing sufficient statistics
to establish entanglement even in the presence
of losses. This approach lays the foundation for
future studies of quantum phenomena in manybody,
strongly interacting systems coupled to strongly
decohering environments and points toward a
novel platform for ultrafast quantum information
processing at room temperature.
References and Notes
1. R. Penrose, in Mathematical Physics, A. Fokas,
T. W. B. Kibble, A. Grigoriou, B. Zegarlinski, Eds.
(Imperial College Press, London, 2000), pp. 266–282.
2. D. P. DiVincenzo, Fortschr. Phys. 48, 771 (2000).
3. S. Gigan et al., Nature 444, 67 (2006).
4. J. D. Thompson et al., Nature 452, 900 (2008).
5. B. Abbott et al., New J. Phys. 11, 073032 (2009).
6. S. Gerlich et al., Nat Commun 2, 263 (2011).
7. A. D. O’Connell et al., Nature 464, 697 (2010).
8. J. D. Teufel et al., Nature 475, 359 (2011).
9. J. Chan et al., Nature 478, 89 (2011).
10. G. Panitchayangkoon et al., Proc. Natl. Acad. Sci. U.S.A.
107, 12766 (2010).
Enhancing Hydrogen Evolution Activity
in Water Splitting by Tailoring
Li + -Ni(OH) 2-Pt Interfaces
Ram Subbaraman, 1,2 Dusan Tripkovic, 1 Dusan Strmcnik, 1 Kee-Chul Chang, 1
Masanobu Uchimura, 1,3 Arvydas P. Paulikas, 1 Vojislav Stamenkovic, 1 Nenad M. Markovic 1 *
Improving the sluggish kinetics for the electrochemical reduction of water to molecular hydrogen in
alkaline environments is one key to reducing the high overpotentials and associated energy losses in
water-alkali and chlor-alkali electrolyzers. We found that a controlled arrangement of nanometer-scale Ni
(OH)2 clusters on platinum electrode surfaces manifests a factor of 8 activity increase in catalyzing the
hydrogen evolution reaction relative to state-of-the-art metal and metal-oxide catalysts. In a bifunctional
effect, the edges of the Ni(OH)2 clusters promoted the dissociation of water and the production of
hydrogen intermediates that then adsorbed on the nearby Pt surfaces and recombined into molecular
hydrogen. The generation of these hydrogen intermediates could be further enhanced via Li + -induced
destabilization of the HO–H bond, resulting in a factor of 10 total increase in activity.
Electrocatalysis of the hydrogen evolution
reaction (HER) is critical to the operation
of water-alkali electrolyzers (1–6), in which
hydrogen is the main product, and chlor-alkali
electrolyzers (5, 6), in which it is a side product.
1
Materials Science Division, Argonne National Laboratory,
Lemont, IL 60559, USA. 2 Nuclear Engineering Division,
Argonne National Laboratory, Lemont, IL 60559, USA. 3 Advanced
Materials Laboratory, Nissan Research Center, Kanagawa
237-8523, Japan.
*To whom correspondence should be addressed. E-mail:
nmmarkovic@anl.gov
These two technologies are highly energy-intensive
and are known to account for ~25 to 30% (87,600
to 92,000 GWh/year) of the total electrical energy
consumption by industrial processes in the United
States (3, 7). The HER is also an electrochemical
reaction of fundamental scientific importance; the
basic laws of electrode kinetics, as well as many
modern concepts in electrocatalysis, were developed
and verified by examining the reaction mechanisms
related to the charge transfer–induced
conversion of protons (in acid solutions) and water
(in alkaline solutions) to molecular hydrogen.
2 DECEMBER 2011 VOL 334 SCIENCE www.sciencemag.org
11. E. Collini et al., Nature 463, 644 (2010).
12. L.-M. Duan, M. D. Lukin, J. I. Cirac, P. Zoller, Nature 414,
413 (2001).
13. T. Chanelière et al., Nature 438, 833 (2005).
14. K. S. Choi, H. Deng, J. Laurat, H. J. Kimble, Nature 452,
67 (2008).
15. D. N. Matsukevich, A. Kuzmich, Science 306, 663 (2004).
16. C. W. Chou et al., Nature 438, 828 (2005).
17. G. Milburn, J. Opt. Soc. Am. B 24, 167 (2007).
18. See supplementary information on Science Online.
19. K. C. Lee et al., Diam. Relat. Mater. 19, 1289 (2010).
20. A. Greentree, B. Fairchild, F. Hossain, S. Prawer,
Mater. Today 11, 22 (2008).
21. L. Prechtel et al., Nano Lett. 11, 269 (2011).
22. S. J. van Enk, Phys. Rev. A 75, 052318 (2007).
23. W. Wootters, Phys. Rev. Lett. 80, 2245 (1998).
24. G. J. Feldman, R. D. Cousins, Phys. Rev. D Part. Fields 57,
3873 (1998).
25. D. F. V. James, P. G. Kwiat, W. J. Munro, A. G. White,
Phys. Rev. A 64, 052312 (2001).
Acknowledgments: We thank V. Vedral, A. Datta, and
L. Zhang for valuable insights. This work was supported by the
Royal Society, Engineering and Physical Sciences Research
Council (grant GR/S82176/01), EU IP Q-ESSENCE (grant
248095), EU ITN FASTQUAST, U.S. European Office of
Aerospace Research and Development (grant 093020),
Clarendon Fund, St. Edmund Hall, and Natural Sciences and
Engineering Research Council of Canada.
Supporting Online Material
www.sciencemag.org/cgi/content/full/334/6060/1253/DC1
Materials and Methods
SOM Text
References (26–36)
29 July 2011; accepted 27 October 2011
10.1126/science.1211914
Although previous studies have helped to rationalize
which surface properties govern the
variations in reactivity among catalysts (8–12),
many key questions concerning the HER remain
unanswered. For example, it is not clear why the
rate of the HER is ~2 to 3 orders of magnitude
lower at pH = 13 than at pH = 1, nor why the
reaction is sensitive to the catalyst surface structure
in alkaline media but largely insensitive in acids
(13–17). A practical implication of the slow kinetics
in alkaline solution is the lower energy efficiency
for both water-alkali and chlor-alkali electrolyzers.
For water-alkali electrolyzers, the high overpotentials
for the oxygen evolution reaction (OER) at
the anode also contribute significantly overall
energy losses (18). This has led to various approaches
to identify catalysts for both the OER
and HER. However, such design strategies
have rarely been based on molecular-level
understanding of the reaction pathways. In
addition, the influence of noncovalent (van der
Waals–type) interactions on the overall kinetics
of the HER has been underexplored, particularly
in light of recent studies highlighting the
impact of noncovalent interactions on the rates
of many electrochemical reactions such as the
oxygen reduction reaction, together with CO
and methanol oxidation reactions (19–22).
Currently, various combinations of metals (Pt,
Pd,Ir,Ru,Ag,Ni),metalalloys(Ni-Co,Ni-Mn,
Ni-Mo), metal oxides (RuO 2), and Ni sulfides
and phosphides are used to catalyze the conver-
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sion of H2O toH2 (2, 10–12, 23–27). Although
Pt and Pt-based systems offer the highest activity
and stability of all materials used for the HER,
the benefits have not, to date, warranted the high
cost associated with these materials. As a result,
conventional electrolyzers generally use high–
surface area Raney Ni and Ni alloys as the HER
catalysts (2, 23, 24, 28). Several engineering approaches
have been used to improve these nonnoble
catalyst materials, such as enhancing the
surface area, changing the alloy composition, and
using higher catalyst loadings (~25 to 40 times
the equivalent for Pt) (2). Although these approaches
have offered small performance gains,
key problems with the use of such non-noble materials
remain, including the decrease in activity
arising from both the formation of hydrides and
the oxidative dissolution of the catalyst during
intermittent operation (2). These materials limitations
suggest that superior performance might
be achieved if lower-cost Pt-based cathode materials
could be developed. Indeed, by substantially
increasing the activity of Pt and by also
decreasing the Pt loading through the use of
Pt-shell nanomaterials with non-noble cores
(29, 30), it would be possible to envision the use
of highly active, durable, and low-cost Pt-based
HER electrodes for alkaline electrolyzers.
Limitations in the catalytic activities of Pt and
Pt-group metals arise from the fact that although
most of these materials are good catalysts for the
adsorption and recombination of the reactive hydrogen
intermediates (H ad), they are generally
inefficient in the prior step of water dissociation.
Conversely, metal oxides (and in some cases
other compounds such as sulfides), although effective
for cleaving the HO–H bond, are poor at
converting the resulting Had intermediates to H2
(31–33). Hence, optimal HER catalyst design
will depend on combining the catalytic proficiencies
of metals and metal oxides by creating
new bifunctional metal oxide–metal systems
(metal oxides deposited on metal substrates)
(34–37).
Here, we report the design and performance
of composite materials to facilitate different parts
of the overall multistep HER process in alkaline
environments: an oxide to provide the active sites
for dissociation of water, and a metal to facilitate
adsorption of the atomic hydrogen produced and
its subsequent association to form H2 from these
intermediates. This involved growth of conductive
ultrathin Ni(OH)2 clusters (height 0.7 nm,
width 8 to 10 nm) on both pristine Pt singlecrystal
surfaces and Pt surfaces modified by twodimensional
(2D) Pt ad-islands [Pt-islands/Pt(111)].
We found that, relative to the corresponding Pt
single-crystal surfaces, the most active Ni(OH) 2/Ptislands/Pt(111)
electrodes in KOH solutions are
more active for the HER by a factor of ~8 at an
overpotential of –0.1 V. Further enhancement
of water dissociation is achieved by the intro-
Fig. 1. (A to C) STM images (60 nm by 60 nm) and CV traces for (A) Pt(111), (B) Pt(111) with 2D Pt
islands, and (C) Pt(111) modified with 3D Ni(OH)2 clusters in 0.1 M KOH electrolyte. (D) HER activities for
Pt(111), Pt-islands/Pt(111), and Ni(OH)2/Pt(111) surfaces in alkaline solution (a, b, and c, respectively).
Corresponding HER activities for Pt(111) and Pt-islands/Pt(111) electrodes in acid solutions (a´ and b´) are
shown to emphasize large initial differences between kinetics of the HER in alkaline versus acid solutions.
The inset shows XANES spectra for Ni(OH)2 on Pt(111) shown for three different potentials: HER (–0.1 V),
Hupd (0.1 V), and near OER (1.2 V). Also shown is the reference spectrum for Ni(OH)2. No shift in the edge
energy in XANES spectra between HER and H upd regionsisobserved.
REPORTS
duction of solvated Li + ions into the compact
portion of the double layer, resulting in a factor
of 10 total increase in activity. Finally, we demonstrate
that the knowledge attained by studying
single-crystal surfaces can be used for the design
of prospective commercial nanocatalysts for alkaline
electrolyzers.
As a starting point, to develop more complete
structure-function relationships for the HER, we
used scanning tunneling microscopy (STM) to
compare the atomic structures of Pt(111) and
Pt(111) modified by electrochemically deposited
Pt islands, referred as Pt-islands/Pt(111), (38). In
agreement with prior reports (39, 40), the image
of Pt(111) in Fig. 1A displays the presence of
a few randomly distributed mono-atomic steps
and 2D Pt islands with diameters of 1 to 2 nm and
monoatomic height. Considering that the shape
of the current-potential curve in both the underpotentially
deposited hydrogen (Hupd) region
[defined as the state of hydrogen adsorbed at a
potential that is positive of the Nernst potentialforthehydrogenreaction(33)],
between
0.05 to 0.35 V, and the region of reversible adsorption
of hydroxyl (OH ad) species, above 0.6 V,
is consistent with earlier reports for a perfect
Pt(111) surface, we conclude that these defects
are invisible in cyclic voltammetry (CV) traces.
In the STM image of the islands shown in Fig.
1B, however, the Pt adatoms can be clearly
resolved as 2D features with diameters of ~1 to
3 nm and a height of 1 atomic layer. The CV
trace of such a surface encompasses two sharp
Hupd peaks centered at 0.23 V and 0.4 V (Fig.
1B). On the basis of prior studies (39, 40), we
associate these peaks with hydrogen adsorption
at the (111)-(111) and (111)-(100) terrace-step
sites, respectively. Consistent with the higher
oxophilicity of low-coordinated Pt sites, the
onset of OH adsorption starts at more negative
potentials on the Pt island–covered electrode
than on pristine Pt(111), whereas the OHad
peaks are less reversible on the former surface.
After 50 potential cycles between –0.3 V and
+0.3 V, the STM images and the CV traces
remain the same, indicating that within this
potential range, the morphologies of the Pt(111)
and Pt-islands/Pt(111) surfaces are stable. Therefore,
such well-defined surfaces offer a unique
opportunity to correlate the kinetic rates of the
HER with a truly atomically resolved surface
structure.
The mechanism of the HER in alkaline media
is typically treated as a combination of three elementary
steps: the Volmer step—water dissociation
and formation of a reactive intermediate Had
(2H2O+M+2e – ⇆ 2M-Had +2OH – )—followed
by either the Heyrovsky step (H 2O+H ad-M + e – ⇆
M+H2 +OH – ) or the Tafel recombination step
(2M-H ad ⇆ 2M + H 2). Adsorbed hydrogen species
Had formed at potentials negative of the Nernst
reversible potential for the HER are also referred
to as overpotentially deposited hydrogen (Hopd).
To distinguish the different states of adsorbed
hydrogen, we use a thermodynamic notation,
www.sciencemag.org SCIENCE VOL 334 2 DECEMBER 2011 1257
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REPORTS
1258
referring to Hupd as the strongly adsorbed state
and H ad (i.e., H opd) as a weakly adsorbed state.
Any rigorous kinetic analysis of the HER lies
beyond the scope of the present discussion. Rather,
we focus mainly on the design of interfaces
for efficient electrochemical conversion of H 2O
to H2. For example, Fig. 1D shows that in alkaline
solution, relative to the corresponding pristine
Pt(111) surface, the Pt-islands/Pt(111)
surface is ~5 to 6 times as active for the HER.
Figure 1D also shows that in acid solution, the
HER on the Pt-islands/Pt(111) electrode is improved
by a factor of only ~1.5. In turn, this
strong pH effect indicates that the low-coordinated
Pt atoms may have a major effect on the ratedetermining
step of the HER in alkaline solutions.
Because the major difference between the
reaction pathways in alkaline and acid solutions
is that the hydrogen in alkaline solutions is discharged
from water instead of from hydronium
ions (H3O + )(13–15), we propose that the large
promoting effect of low-coordinated Pt atoms in
alkaline solution is due to more facile dissociative
adsorption of water. In turn, this would be
consistent with the Volmer reaction being the
rate-determining step for the HER in alkaline
electrolytes. The role of edge-step sites in accelerating
dissociative adsorption of water on metal
surfaces is well documented in ultrahigh-vacuum
(UHV) environments (33). We therefore conclude
that for materials with near-optimal M-H ad energetics
(such as Pt), surface reactivity for the
HER can be further improved by tailoring the active
sites for more efficient dissociative adsorption
of water molecules.
To fulfill this requirement, we modified Pt(111)
and Pt-island/Pt(111) surfaces by depositing
Ni-(hydr)oxide clusters (38), as the 3d-transition
metal oxides might be even more active for water
dissociation than Pt defect sites (31). The facile
water dissociation properties of Ni(hydroxy)
oxides relative to other transition metal oxides
have been well established (26–28), motivating
the use of Ni(OH)2 for this work. The local symmetry,
the oxidation state of Ni atoms, and the
number and identities of nearest-neighbor atoms
and the distances between them were determined
by in situ x-ray absorption spectroscopy (XAS)
measurements (41, 42). For example, from the
analysis of the x-ray absorption near-edge structure
(XANES) and extended x-ray absorption
fine structure (EXAFS) of the XAS spectra (inset
of Fig. 1D; see also fig. S3), we found Ni-O and
Ni-Ni bond distances of 2.05 T 0.01 Å and 3.08 T
0.01 Å, and we also determined that Ni remains
mostly in the +2 valence state, even after multiple
hours of holding the electrode potential at –0.1 V.
Furthermore, from the edge shift (defined as the
half-height energy of the normalized XANES
edge step), we concluded that between –0.1 Vand
+0.8 V, the change in the oxidation state of Ni is
less than 0.5. These results suggest that stable
Ni(OH)2 clusters are the predominant hydr(oxide)
form on the Pt(111) and Pt-islands/Pt(111) surfaces,
especially in the HER potential region.
Because the octahedral symmetry of the a and/or
b phases of Ni(OH) 2 prevents p-d hybridization,
the prominent pre-edge from 1s → 3d transitions
implies that the Ni(OH) 2 species are rich in defects
that, according to prior reports (31–33), are
particularly active for dissociative adsorption
of water molecules.
The surface morphologies of Ni(OH) 2/Pt(111)
and Ni(OH)2/Pt-islands/Pt(111) were probed by
STM and in situ surface x-ray crystal truncation
rod (CTR) measurements (43, 44)(seefig.S4for
the CTR data). Although atomic resolution could
not be obtained, the STM image in Fig. 1C clearly
shows that the Ni(OH) 2 clusters are randomly
distributed across the (111) terraces. All Ni(OH)2
clusters exhibited hemisphere-like shapes, with
characteristic diameters of ~8 to 10 nm and heights
of ~0.7 nm, the latter corresponding to two layers
of Ni(OH)2. This result indicates that the oxide
exhibits Volmer-Weber (VW)–type growth
whereby 3D clusters of Ni(OH)2 grow even at
the lowest coverages (45). VW growth, in turn, is
possible if the heat of adsorption of Ni(OH)2 on
Pt is lower than the cohesive energy of Ni(OH) 2.
Fig. 2. (A) STM image (60 nm by 60 nm) and CV trace of the Ni(OH)2/Pt-islands/Pt(111) surface. Clusters
of Ni(OH) 2 in the STM image appear ellipsoidal with particle sizes between 4 and 12 nm. (B)Comparison
of HER activities with Pt(111) as the substrate. Incremental improvements in activities for the HER in 0.1 M
KOH from the unmodified Pt(111) surface are shown for the hierarchical materials [ad-islands, Ni(OH)2,
and their combination] as well as the double layer (addition of Li + cations). The activity for the unmodified
Pt(111) surface in 0.1 M HClO 4 is shown for reference. Dashed arrow shows the activity trend.
Fig. 3. Schematic representation of water dissociation, formation of M-H ad intermediates, and subsequent
recombination of two Had atoms to form H2 (magenta arrow) as well as OH – desorption from
the Ni(OH)2 domains (red arrows) followed by adsorption of another water molecule on the same site
(blue arrows). Water adsorption requires concerted interaction of O atoms with Ni(OH) 2 (broken orange
spikes) and H atoms with Pt (broken magenta spikes) at the boundary between Ni(OH)2 and Pt domains.
The Ni(OH)2-induced stabilization of hydrated cations (AC + ) (broken dark blue spikes) likely occurs through
noncovalent (van der Waals–type) interactions. Hydrated AC + can further interact with water molecules
(broken yellow spikes), altering the orientation of water as well as the nature and strength of interaction
of the oxide with water.
2 DECEMBER 2011 VOL 334 SCIENCE www.sciencemag.org
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The surface coverage of Ni(OH)2 on Pt(111) is
estimated from the STM image by measuring
the area covered by the particles on the Pt(111)
substrate. According to such analysis, the cluster
density reached a maximum at a surface coverage
of ~35%. Upon comparing STM images of
Pt(111) and Ni(OH)2/Pt(111) surfaces (Fig. 1, A
and C), however, it was not clear whether isolated
defects or flat Pt(111) terraces were the
preferred nucleation sites for Ni(OH) 2. This information
was more accessible by comparing
STM images of the Ni(OH) 2-free (Fig. 1B) and
Ni(OH)2-covered Pt-island/Pt(111) (Fig. 2A)
electrodes. The STM image in Fig. 2A was acquired
after deposition of Ni(OH)2 on a Pt(111)
surface modified by ~0.2 monolayer (ML) of
Pt islands. We observed formation of both 3D
Ni(OH)2 clusters (having a predominantly ellipsoidal
shape) and oxide-free terraces. The
clusters had approximately constant heights of
~0.8 nm but diameters ranging from 4 to 12 nm.
The same STM image, however, revealed no
visible presence of 2D Pt islands, which suggests
that Ni(OH)2 preferentially nucleates on the Pt
surface defects and that most of the Pt islands
are covered by Ni(OH)2.
Having established the nature and morphology
of the Ni(OH)2 clusters, we briefly summarize
the role of the Ni(OH) 2 clusters in the formation
of Hupd and OHad adlayers on Pt(111) and Ptislands/Pt(111)
electrodes. Addition of Ni(OH) 2
on the surface of Pt(111) as well as on the Pt(111)
surface covered with Pt islands led to a decrease
(~35%) in the coverage of Hupd. This finding
suggests that the Ni(OH) 2 clusters selectively
block the Pt sites corresponding to Hupd. Furthermore,
the two sharp H upd peaks characteristic
of hydrogen adsorption/desorption on the Pt(111)
electrode modified by the 2D Pt islands are completely
suppressed on the surface covered by the
3D Ni(OH)2 clusters (Fig. 2A). This is additional
evidence, consistent with the STM results,
that defects serve as the nucleation centers for
electrodeposition of Ni(OH)2. We therefore conclude
that Pt islands are predominantly covered
by Ni(OH)2. In contrast to the Hupd potential re-
Fig. 4. (A) STM image (50 nm by 50 nm) and CV trace for Ni(OH)2/Pt-islands/Pt(110) single-crystal
surface. STM image shows Ni(OH) 2 randomly distributed on inherently rough Pt(110). . (B)Comparisonof
HER activities with Pt(110) as the substrate. Incremental improvements in activities for the HER in 0.1 M
KOH from the unmodified Pt(110) surface are shown for the hierarchical materials [Ni(OH)2 and the
combination of ad-islands with Ni(OH)2] as well as the double layer (addition of Li + cations). The activity
for the unmodified Pt(110) surface in 0.1 M HClO 4 is shown for reference. Dashed arrow shows the activity
trend. (C) Transmission electron micrograph image (50 nm by 50 nm) and corresponding CV trace of
Ni(OH)2-free Pt-nano catalysts (TKK) with an average particle size of 5 nm. (D) Comparison of HER activities
with commercial nanocatalyst Pt/C (TKK) as the substrate. Incremental improvements in activities for the
HER in 0.1 M KOH from unmodified Pt/C are shown for the hierarchical materials [surface covered with
Ni(OH)2] as well as the double layer (addition of Li + cations). The activity for the unmodified Pt/C surface
in 0.1 M HClO4 is shown for reference. Dashed arrow shows the activity trend. All the current densities for
the TKK catalyst system are normalized by the geometric surface area of the glassy carbon substrate.
REPORTS
gion, an enhanced adsorption of OHad, whichis
accompanied by irreversible reduction of OH ad
on the negative-going sweep, is observed on both
electrodes, arising from the higher oxophilicity
of the surface elements.
Although in the presence of Ni(OH) 2 clusters
there are 35% fewer Pt sites available for
the HER than on the bare Pt(111) substrate, the
Ni(OH)2/Pt(111) electrode is ~7 times as active
for the HER relative to Pt(111) (Fig. 1D). Moreover,
Fig. 2B shows that the activity is further enhanced
[by a factor of ~8 relative to bare Pt(111)]
on the Ni(OH)2/Pt-island/Pt(111) surface; at
6 mA/cm 2 , the difference in overpotential between
the HER in alkaline and acid solutions is
reduced to only 100 mV. Clearly, then, on both
surfaces, Ni(OH)2 must play a promoting role in
the dissociation of water and thereby enhances
therateofformationofHad intermediates on the
metal surface. As schematically depicted in Fig.
3, we propose that water adsorption requires
concerted interaction of O atoms with Ni(OH)2
and H atoms with Pt at the boundary between
Ni(OH)2 and Pt domains. Water adsorption is
then followed by water dissociation and hydrogen
adsorption (Had) on the nearby vacant Pt
sites. Finally, two H ad atoms on the Pt surface
recombine to form H2 (H2 desorption step) and
OH – desorbs from the Ni(OH) 2 domains, followed
by adsorption of another water molecule
on the same site.
From a surface reactivity standpoint, fruitful
kinetic synergy (bifunctionality) between Ni(OH) 2
and Pt appears to be the key to maximizing the rate
of the HER. As shown in Fig. 2B, this bifunctionality
in turn brings the activity of the HER in
alkaline solutions very close to the activity of Pt
in acid solutions. To verify this conclusion, we
have also compared the HER on Au(111) and
Ni(OH)2/Au(111) in alkaline solution. The relatively
weak interaction between Au and H ad offsets
the benefit of the enhanced water dissociation
at the Au/Ni(OH) 2 interface (38). As a result, the
rate of the HER on the Ni(OH)2/Au(111) surface
is much lower than on the Pt(111)/Ni(OH) 2 surface.
This further emphasizes the importance of
choosing the correct metal oxide–metal pairs in
optimizing the kinetics of the HER. In what
follows, we demonstrate that the nature of interactions
in double layer is equally important, and
the rate-determining Volmer step can be further
enhanced by tuning the double-layer properties.
Several recent studies have unambiguously
shown that the rate of electrochemical reactions
on Pt in alkaline solutions is controlled by the
presence of alkali-metal cations (AC + )(20, 21).
However, these effects have been entirely restricted
to the potential region of a critical OH ad
coverage (for electrode potential E > 0.6 V),
the latter species serving to stabilize hydrated
cations in the compact part of the double layer
through noncovalent (van der Waals–type) interactions.
This stabilization leads to the formation
of OH ad-AC + (H 2O) x complexes that can
either decrease the reactivity of Pt by blocking
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REPORTS
1260
the active sites for adsorption of reactants such
as O 2,H 2, and CH 3OH (20, 21) or, as in the
case of the CO oxidation reaction, improve
the reactivity of Pt via enhanced adsorption of
OHad (22). For the present reaction, the key questions
are whether hydrated alkali cations on the
Ni(OH)2/Pt-islands/Pt(111) surface can be stabilized
through interactions with OH species in the
Ni(OH)2 cluster and, if so, whether these hydrated
cations could affect the alkaline HER kinetics.
For these purposes, the effect of hydrated
Li cations was probed mainly because, in alkaline
environments, Li + is known to interact
with H 2O and OH ad more strongly than K + .In
line with the previous electrochemical report
(20), we found that Li + cations have no effect on
the HER on Pt(111) surfaces. However, the
results in Fig. 2B revealed that the HER on
Ni(OH)2/Pt-islands/Pt(111) is enhanced by almost
a factor of 2 in the presence of Li + cations.
This increase in activity has substantially narrowed
the gap between the rates of HER on Pt
in acid and alkaline solution; more specifically,
inspection of Fig. 2B shows that, at 5 mA/cm 2 ,
the difference in overpotential between acid and
alkaline environments is narrowed to only 35 mV.
The fact that the activity of the HER is affected
by the nature of alkali metal cations strongly
suggests that Ni(OH) 2-Li + -OH-H complexes are
present in the compact portion of the double
layer. The presence of this complex does not completely
explain the factor of 2 increase in HER activity.
It is likely that the probability of water
dissociation is enhanced via possible Li + -induced
steric and/or electronic effects on the interfacial
water structure and reactivity, as shown schematically
in Fig. 3. Thus, Ni(OH) 2 plays a dual role:
In addition to assisting with water dissociation, it
also provides an anchor to hold the beneficial Li +
ions in the compact portion of the double layer.
Having established the methodology of tuning
interfacial activity for the HER on Pt(111),
we demonstrate that the very same guiding principles
for accelerating the Volmer reaction step
in alkaline solutions are equally applicable to
Pt(110). For simplicity, only the CV and STM
data for the most active system, Ni(OH) 2/Ptislands/Pt(110),
are shown in Fig. 4A. The general
characteristics (both structural and electrochemical)
are similar to what was observed for the
corresponding Pt(111) systems. The current denities
for the HER on Pt(110) and Pt-nanocatalyst
systems are presented in the logarithmic Tafel
form (Fig. 4, B and D) to offer the most condensed
representation of surface characterization
and polarization curves. As expected, the
systematic modification of Pt(110), first with Pt
islands and then with Ni(OH) 2, exhibits an HER
activity trend (Fig. 4B) with the same order as the
driving force for dissociative adsorption of water
molecules, as discussed above for the Pt(111) systems:
Pt(110) < Ni(OH) 2/Pt(110)

Surface Chemistry
DOI: 10.1002/anie.201107668
Unique Electrochemical Adsorption Properties of Pt-Skin Surfaces**
Dennis F. van der Vliet, Chao Wang, Dongguo Li, Arvydas P. Paulikas, Jeffrey Greeley,
Rees B. Rankin, Dusan Strmcnik, Dusan Tripkovic, Nenad M. Markovic, and
Vojislav R. Stamenkovic*
PtM alloys (M = Co, Ni, Fe, etc.) have been extensively
studied for their use in fuel cells, both in well-defined
extended surfaces, [1] as well as in nanoparticles. [2] After the
report about exceptional activity of Pt 3Ni(111)-skin surface [1a]
for the oxygen reduction reaction (ORR) a lot of efforts have
been made to mimic this catalytic behavior at the nanoscale.
It has been shown that a Pt 3Ni(111) crystal annealed in ultrahigh
vacuum (UHV) shows an oscillating segregation profile,
with the outermost layer consisting of pure platinum while the
second layer is enriched in nickel compared to the bulk
composition. [1a,3] Such a surface we termed Pt skin, and owing
to the presence of the non-noble metal in the subsurface layer
it has altered electronic properties compared to the monometallic
Pt single crystal with the same orientation. Accordingly,
altered electronic properties induce a change in
adsorption behavior, specifically a shift of surface-oxide
formation to higher potentials. [1a,b] This adsorption behavior
is believed to be the origin of the high activity for the ORR.
On the opposite side of the potential scale, the adsorption of
hydrogenated species, denoted as underpotentially adsorbed
hydrogen (H upd), is also largely affected on Pt-skin surfaces. [4]
Despite numerous efforts dedicated to synthesize nanocatalysts
with Pt-skin-type surfaces, [2c,5] it still remains a challenge
to claim their existence at the nanoscale. To systematically
resolve this issue, we attempt to provide fundamental insight
into the adsorption properties of well-defined Pt-skin surfaces
under relevant electrochemical conditions and to transfer that
knowledge to corresponding nanocatalysts.
For that reason, we first examine the formation and
composition of Pt-skin surfaces by low-energy ion scattering
(LEIS) and scanning tunneling microscopy (STM) in UHV,
[*] Dr. D. F. van der Vliet, Dr. C. Wang, D. Li, A. P. Paulikas,
Dr. D. Strmcnik, Dr. D. Tripkovic, Dr. N. M. Markovic,
Dr. V. R. Stamenkovic
Materials Science Division, Argonne National Lab
9700 S. Cass Ave, Argonne IL (USA)
E-mail: vrstamenkovic@anl.gov
vrstamenkovic@anl.gov
D. Li
Brown University
1 Prospect St., Providence, RI, 12912, USA
Dr. J. Greeley, Dr. R. B. Rankin
Center for Nanoscale Materials, Argonne National Lab
9700 S. Cass Ave, Argonne IL (USA)
[**] This work was supported by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences, under contract No. DE-
AC02-06CH11357.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107668.
and second we study the composition of the surfaces in an
electrochemical environment to establish their adsorption
properties. We demonstrate by cyclic voltammetry that the
surface coverage of H upd on Pt skin is about half of that found
on Pt(111), whereas the surface coverage of a saturated
monolayer of carbon monoxide is similar for both surfaces.
This is an important finding, which provides a link towards
accurate determination of the electrochemically active surface
area of nanoscale catalysts. The developed methodology
provides additional evidence for the existence of Pt-skin
surfaces on Pt-bimetallic nanocatalysts and can substantially
diminish errors in the evaluation of the real surface area and
catalytic activity.
A thorough examination of the Pt-skin surfaces was
performed in view of their importance in electrocatalysis as
well as in response to recent questions and doubts in the
Figure 1. Surface characterization of Pt 3M(111) surfaces in UHV: STM
images of A) sputtered and B) annealed Pt 3Ni(111) surfaces. The
brightness in colors is a measure of the depth profile, with each color
change marking a single atomic step. Low-energy ion scattering
spectra of C) sputtered (blue traces) versus annealed (red traces)
surfaces of Pt 3Ni(111) and Pt 3Co(111) crystals. D) Successive LEIS
spectra of Pt 3Ni(111) surface during Ne-sputtering. The graphs in (C)
and (D) are shifted, and the scale bars indicate the intensities in
arbitrary units. E/E 0 is the ratio of the energy of the scattered ion
beam (E) and the energy of the incident ion beam (E 0).
Angew. Chem. Int. Ed. 2012, 51, 3139 –3142 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angewandte
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Angewandte .
Communications
literature [6] on if the state of the Pt-skin surface formed over
the Pt 3M systems in UHV and in an electrochemical environment
consists of pure Pt. Figure 1C shows the LEIS results
obtained by a 1 keV Ne + ion beam for the Pt 3Ni(111) and
Pt 3Co(111) crystals before and after thermal annealing in
UHV. Based on these LEIS spectra it is obvious that after
consecutive annealing/sputtering cycles, both Pt and Ni (or
Co) are present on the surface, while the STM image in
Figure 1A illustrates the roughness of the sputtered surface.
However, the LEIS signal for Ni and/or Co is diminished if
annealing is the final step, which is indicative for the
formation of a Pt skin. The morphology of this surface is
revealed by STM (Figure 1B), which displays the smooth and
ordered formation that is typical for single-crystalline systems.
To investigate the stability of these surfaces in UHV,
more energy was applied through the monochromatic Ne + ion
beam, causing a sputtering effect to take place on the topmost
Pt surface layer. Consequently, the subsurface nickel and/or
cobalt atoms become exposed in consecutive LEIS spectra
(shown for Pt 3Ni(111) in Figure 1D; the result for Pt 3Co(111)
was identical), indicating a change in surface composition
from Pt-skin (100% Pt) into Pt-rich. These combined LEIS
and STM results unambiguously confirm that both crystals
exhibit substantial transition of surface composition/morphology
upon annealing in UHV owing to complete segregation of
Pt to the surface, thereby forming a full Pt skin.
Cyclic voltammetry is used to examine electrochemical
adsorption properties of Pt-skin surfaces. [9] As shown in
Figure 2A, the onset of H upd adsorption on Pt 3Ni(111) skin is
shifted towards lower potentials when compared to Pt(111).
The onset is approximately 150 mV lower and shows a reversible
pre-wave ahead of the main H upd adsorption owing to
surface steps, which are inevitably present on Pt 3Ni(111).
Moreover, the formation of surface oxides OH ad at potentials
greater than 0.6 V is delayed by 100 mV versus Pt(111). [1a]
Furthermore, from the integrated H upd regions it was revealed
that charges are considerably different (see Table 1), i.e., in
the same potential range, Pt-skin surfaces have about half of
the value obtained for Pt(111).
It is important to emphasize that Pt(111) and Pt 3M(111)skin
surfaces have essentially identical geometric surface
areas, surface compositions, and surface structures, albeit the
interatomic distances may be altered by the composition of
the second layer. However, the electrochemical adsorption
properties between them are quite different. This is a consequence
of the electronically modified structure of Pt for Ptskin
surfaces by Ni or Co subsurface atoms, which leads to
Table 1: Integrated charges for Pt(111), Pt 3Ni(111), Pt 3Co (111), and
polycrystalline-Pt (Pt(poly)) extended surfaces obtained from cyclic
voltammetry curves for H upd (Q H), CO stripping (Q CO), and the ratio
between measured charges. Analysis of the charges can be found in the
Supporting Information.
Catalyst Q H
[mCcm 2 ]
Q CO
[mCcm 2 ]
Q CO/2Q H
Pt(111) 152 315 1.04
Pt 3Ni(111) 98 304 1.55
Pt 3Co(111) 91 283 1.55
Pt(poly) 190 386 1.02
Figure 2. Electrochemical characterization of Pt(111) (black traces)
and Pt 3Ni(111)-skin surfaces (red traces) by using a rotating disc
electrode (RDE) in 0.1m HClO 4: A) cyclic voltammograms; B) electrooxidation
of an adsorbed monolayer of CO on Pt(111) and on
Pt 3Ni(111) skin. RHE = reversible hydrogen electrode.
weakened interactions between Pt and adsorbates such as
H upd and OH ad. The H upd integrated charge is used as
conventional approach in the estimation of the electrochemical
surface area; however, the discrepancy that is coming
from altered electronic properties of Pt can substantially
affect the accurate estimation of the real surface area of Ptalloy
catalysts.
Another surface-sensitive reaction that is commonly used
to probe surface properties is the electro-oxidation of
adsorbed carbon monoxide, well-known as CO stripping. In
this reaction, the surface is first saturated with a CO adlayer
at the negative potential limit. Owing to a strong Pt–CO ad
interaction, CO molecules stay adsorbed on Pt surface atoms,
while the electrolyte is purged with argon gas. The consecutive
step is electro-oxidation of CO by sweeping the
potential towards the positive limit. The obtained CO
stripping voltammetric profiles of surfaces prepared in
UHV are shown in Figure 2B. The onset of CO stripping on
Pt 3Ni(111) skin is 150 mV lower than that on Pt(111), while
the shape of the stripping peak is much broader, and the main
oxidation peak is also shifted by about 100 mV. The oxidation
of adsorbed CO ad proceeds at lower potentials on Pt-skin
surfaces owing to a weaker interaction of the Pt surface atoms
with CO, caused by the modified electronic properties, but the
3140 www.angewandte.org 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2012, 51, 3139 –3142

similar charge of CO oxidation points to an equal coverage of
CO. [7] This hypothesis is supported by our density functional
theory calculations, which indicate that CO binding to
a Pt 3Ni(111)-skin surface, with a 1:1 Pt/Ni ratio in the
subsurface layer, is approximately 0.27 eV weaker than the
corresponding binding on Pt(111). [10] For that reason, the
reduced binding of surface oxides (OH ad) onPt 3Ni (111) skin
does not cause an increase in onset potential for CO
electrooxidation. In fact, since CO ad and OH ad are competing
for the same Pt adsorption sites, the reduced binding energy
between Pt 3Ni (111) skin and CO ad leads to a lower onset
potential as the blocking effect of CO ad is diminished (see the
Supporting Information for additional computational details).
It is important to mention that it was demonstrated that CO ad
can modify the surface composition and/or morphology; [8]
such an effect would imply a stronger Pt–CO interaction,
whereas we clearly note a weakened adsorption, convincingly
disproving such an interaction in the Pt-skin case. This was
additionally confirmed by unchanged cyclic voltammetry
after CO electrooxidation.
Based on the results depicted in Figure 2B and Table 1 the
onset, shape, and main oxidation peak of the CO stripping
curves differ between Pt 3M(111)-skin surfaces and Pt(111),
while the integrated charges are nearly equal. The similar CO
stripping charges add to the LEIS evidence that the outer
layer of the Pt 3M(111) skin consists of only Pt atoms. Moreover,
the ratio between the charges of CO stripping versus
H upd was calculated to visualize the difference in adsorption
properties for all surfaces (see Table 1). Both H upd and CO
stripping are surface-specific probes for Pt atoms, so in case of
pure Pt systems, such as Pt(111) and polycrystalline Pt, this
charge ratio is identical and close to one. However, the ratio
obtained for Pt 3Ni(111)- and Pt 3Co(111)-skin surfaces is 1.55,
which confirms that the surface coverage of CO ad is not
affected by altered electronic properties of Pt-skin surfaces.
As a consequence of these results, it is obvious that the
integrated H upd region cannot be solely used in estimation of
the electrochemical surface area in case of Pt-skin surfaces.
For that reason, special care must be taken when analyzing
annealed PtM alloys in the form of high-surface-area nanoscale
catalysts.
As a case in point, in Table 2 the charges for H upd and CO
oxidation are compared for three different catalysts of equally
sized nanoparticles (NPs): monometallic Pt/C, PtNi/C, and
annealed PtNi/C NPs with a Pt skin. Detailed information on
the particles size distribution and composition is presented in
the Supporting Information. Transmission electron microscopy
(TEM) results show that the particles size of about 5 nm
and shape are not affected by thermal annealing, while
energy-dispersive X-ray (EDX) line scan analyses revealed
the elemental distribution across the particles. In Figure S1 in
the Supporting Information, a uniform distribution of Pt and
Ni is confirmed for as-prepared particles. On the other hand,
after the acid leaching and annealing treatments, a Pt-overlayer
has emerged on PtNi nanoparticles. Representative
voltammetric curves for skin-type PtNi NPs compared to Pt/C
can be seen in Figure 3. The CO stripping curves in Figure 3B
closely match the results obtained on single crystals (Figure
2B). For the non-annealed NPs, the ratio between the
Table 2: Integrated charges and calculated surface areas (ECSAs) for
H upd (Q H) and CO stripping (Q CO) obtained from cyclic voltammetries of
Pt/C, acid-treated PtNi/C, and annealed PtNi/C catalysts. The ratio
between the integrated charges for H upd and CO stripping demonstrates
the discrepancy in ECSAs and the underestimation of the real surface
area if H upd is used in case of Pt-skin surfaces.
Catalyst Q H
[mC]
ECSA H
[cm 2 ]
Q CO
[mC]
ECSA CO
[cm 2 ]
Q CO/2Q H
Pt/C 279 1.47 545 1.41 0.98
PtNi/C 292 1.54 615 1.60 1.05
PtNi skin/C 210 1.10 595 1.54 1.42
charges for H upd and CO stripping is similar to Pt/C (and
Pt(poly)), and thus the surface-area estimation based on the
H upd charge is reasonable. However, the adsorption properties
of annealed PtNi particles with a Pt-skin-type surface
resemble those previously established on well-defined skintype
bulk crystals of Pt 3Ni(111) and Pt 3Co(111), as judged by
the suppression of H upd adsorption (measured as charge).
This suppression of H upd adsorption confirms that the
current focus in nanoparticle research on skin-type or core–
shell structures [2c,5] can potentially suffer from substantial
underestimation of the electrochemically active surface area
if only the integrated H upd region is used. To be certain that
the electrochemical surface area is not overlooked for skintype
alloy catalysts, the estimated H upd charge always has to be
Figure 3. Electrochemical surface characterization of catalysts consisting
of Pt/C (black dashed traces) and PtNi/C with Pt skin (red traces)
by using a RDE in 0.1m HClO 4: A) cyclic voltammograms; B) CO
stripping curves.
Angewandte
Chemie
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Angewandte .
Communications
compared to the CO stripping charge. Furthermore, the
observed discrepancy between surface-area estimations based
on H upd and CO stripping can give an indication of the
formation of a skin-type nanocatalyst alloy, because acid
leached skeleton-type surfaces and nanoparticles do not show
such behavior.
In summary, we have demonstrated that by alloying
platinum with a non-noble second metal and inducing a Ptskin-type
structure, the adsorption properties of the resulting
PtM alloys are significantly altered. Specifically, the adsorption
of hydrogen and oxide species is shifted in potential and
reduced in magnitude compared to monometallic Pt in the
same potential region. Added to LEIS evidence, the fact that
the charge of the oxidation of a complete monolayer of CO is
similar on annealed Pt 3M surfaces as on monometallic Pt
confirms the surface consists of only platinum. The suppression
of H upd adsorptionon these surfaces is both a method to
confirm Pt-skin formation, but also prompts a difficulty in
determining the electrochemically active surface area on Ptskin-type
nanoparticles. To avoid underestimation of the
surface area owing to suppression of H upd adsorption, and
hence overrating of specific activity, we demonstrate that CO
stripping has to be used alongside the H upd charge for the
determination of the electrochemically active surface area of
Pt-alloy catalysts.
Experimental Section
Methods: After annealing cycles in UHV, the Pt 3Ni(111) crystal was
transferred to the electrochemical cell in hanging meniscus mode in
which only the (111) surface is exposed to the electrolyte. The crystal
was protected from the airborne impurities by a drop of hydrogensaturated
Milli-Q water during transfer to the electrolyte, which was
deoxygenated 0.1m HClO 4. All gases are of research grade (5N5 or
higher).
After voltammetry confirmed a clean and stable surface, CO was
inserted into the cell for one minute, while the crystal was rotated at
1600 rpm. Consecutively, rotation was stopped and argon was
bubbled through the cell for 45 min to remove any trace of dissolved
CO. After CO was purged from the solution, two cycles were
recorded: the first one being the CO stripping voltammetry, the
second to verify the absence of residual CO in solution. All
electrochemical measurements were performed with a scan rate of
50 mVs 1 in 0.1m HClO 4 at room temperature.
The electrochemical glass cell was a standard 3-electrode cell, as
used in previous experiments, [1a,b] with a Pt counter and a calomel
reference electrode. All potentials in this article are given versus the
reversible hydrogen electrode (RHE).
Nanoparticle RDE electrodes were prepared by the solvothermal
method described previously. [2b] Ni(acetate) 2·4H 2O and [Pt(acac) 2]
(acac = acetylacetonate) were the precursors for the PtNi nanoparticles
and added in a proper ratio, to ensure a 1:1 PtNi alloy is
formed. PtNi acid-treated and PtNi acid-treated/annealed nanoparticles
with the average particle size of 5 nm were supported on
high-surface-area carbon and their electrochemical properties were
compared to Tanaka Pt/C with similar particle size (total metal
content for all catalysts was 40 %). The catalysts were deposited on
6 mm glassy carbon electrode and the loading in all cases was adjusted
to be 12 mg Pt/cm disc.
Received: October 31, 2011
Revised: December 21, 2011
Published online: February 20, 2012
. Keywords: adsorption · alloys · electrochemistry ·
nanoparticles · platinum
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3142 www.angewandte.org 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2012, 51, 3139 –3142

© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
REPRINT

Reprint
Heterogeneous Catalysis
Tailoring the Selectivity and Stability of
Chemically Modified Platinum
Nanocatalysts To Design Highly Durable
Anodes for PEM Fuel Cells
Big and small: Chemically modifying
platinum with calix[4]arene yields a highly
stable anode catalyst that effectively suppresses
the oxidation reduction reaction
without altering the maximum activity for
the hydrogen oxidation reaction (see
picture, Pt blue, C gray, O red, S yellow).
This behavior extends from long-rangeordered
stepped single-crystal surfaces to
nanocatalysts.
B. Genorio, R. Subbaraman, D. Strmcnik,
D. Tripkovic, V. R. Stamenkovic,
N. M. Markovic* 5468 – 5472
Keywords: calixarenes · electrochemistry ·
heterogeneous catalysis ·
oxygen reduction reaction · platinum
2011 – 50/24
WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Communications
Heterogeneous Catalysis
Angewandte
Chemie
DOI: 10.1002/anie.201100744
Tailoring the Selectivity and Stability of Chemically
Modified Platinum Nanocatalysts To Design Highly
Durable Anodes for PEM Fuel Cells**
Bostjan Genorio, Ram Subbaraman, Dusan Strmcnik, Dusan Tripkovic,
Vojislav R. Stamenkovic, and Nenad M. Markovic*
5468 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2011, 50, 5468 –5472

An ever-changing energy landscape and the global drive
toward greener energy technologies have made fuel cells a
focal point of numerous research initiatives. In their current
form, proton-exchange-membrane fuel cells (PEMFCs) have
been shown to perform well under operating conditions for
both automotive and stationary applications. In order for
PEMFCs to reach commercial implementation, issues such as
durability under both normal and startup and shutdown
conditions have to be tackled effectively. Reactivity, selectivity,
and stability are the quintessential properties that need to
be tailored to develop catalysts that can tackle the durability
issues arising during startup and shutdown. [1] One approach to
accomplish this task is to design an anode catalyst that can
efficiently suppress the undesired oxygen reduction reaction
(ORR; imparting selectivity) and preserve the platinum-like
hydrogen oxidation reaction (HOR) activity (imparting
reactivity), while remaining stable under operating conditions
(imparting stability). Such an approach not only reduces the
overpotential on the cathode side owing to negligible ORR
currents on the anode but also prevents formation of
detrimental products such as hydrogen peroxide, which can
be formed under “normal” anode startup and shutdown
conditions. We have shown that chemically modified electrodes
(CME) consisting of self-assembled monolayers (SAMs)
of calix[4]arene molecules on extended platinum singlecrystal
surfaces can selectively block the ORR without
affecting the HOR activities and kinetics. [2] Usually, the
lessons learned from such extended surfaces have helped in
the understanding of nanocatalysts that mimic the reactivity
and catalytic behavior of the extended surfaces. [3] Seldom,
however, can the behavior of extended surfaces be completely
translated down to the nanocatalysts.
Herein, we show that the platinum modified with
calix[4]arene (calix) is, in fact, one of these rare examples in
which the modified nanocatalyst system behaves in line with
the corresponding extended-surface system. First, we demonstrate
high selectivity of the HOR on calix-modified
Pt(1099){10(111) ” (100)}and Pt(1 10){2(111) ” (100)} step
[*] Dr. B. Genorio, Dr. D. Strmcnik, Dr. D. Tripkovic,
Dr. V. R. Stamenkovic, Dr. N. M. Markovic
Materials Science Division, Argonne National Laboratory
Argonne, IL 60439 (USA)
E-mail: nmmarkovic@anl.gov
Dr. B. Genorio
Faculty of Chemistry and Chemical Technology
University of Ljubljana (Slovenia)
Dr. R. Subbaraman
Nuclear Engineering Division
Argonne National Laboratory (USA)
[**] This work was supported by the Director, Office of Science, Office of
Basic Energy Sciences, Division of Materials Sciences, US Department
of Energy under Contract No. DE-AC03-76SF00098 and the
Center of Excellence Low Carbon Technologies Slovenia (CO NOT),
Center of Excellence Advanced Materials and Technologies for the
Future Slovenia (CO NAMASTE). R.S. is grateful for financial
support from an Argonne postdoctoral fellowship. PEM = Protonexchange
membrane.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100744.
surfaces. Then, we developed a methodology to form highly
selective and stable SAMs of calix molecules on commercial
nanocatalysts (3M nanostructured thin film (NSTF) [4] and
Tanaka 5 nm Pt/C (TKK) catalysts). We find that if the
synthesis is precisely controlled, the selectivity of nanoparticles
for the ORR in the presence of hydrogen under
conditions relevant to PEMFC operations is almost 100 %.
We start with the electrochemical characteristics of
calix[4]arene-decorated Pt(1 10) and Pt(1099). As summarized
in Figure 1, both stepped surfaces show characteristic
cyclic voltammograms; the under-potentially deposited (H upd)
hydrogen (0–0.4 V) is followed first by a double-layer region
and then at E > 0.6 V by reversible (OH ad) and irreversible
oxide formation. [5] On the calix-covered surfaces, however,
both the H upd and OH ad regions are significantly suppressed.
In line with Ref. [2], on the highly covered surfaces the
number of “free” Pt sites (determined from the H upd charge) is
extremely low (ca. 2–3 %). However, on the same surface the
HOR is similar to calix-free Pt, thus confirming that the
turnover frequency (TOF) of the hydrogen reaction is
extremely high [6] and that the Pt–H 2 energetics are not
affected by the adsorbed calix molecules.
More importantly, we find that at E > 0.6 V, while the
ORR is almost completely inhibited (these modifications are
not accompanied by undesired peroxide production), [2] the
HOR is under pure diffusion control. This unique selectivity is
attributed to very strong ensemble effects in which the
number of bare Pt sites available for adsorption of O 2 is much
smaller than that for the adsorption of H 2 and the subsequent
HOR. [2] We conclude therefore that the selectivity achieved
using calix-modified electrodes is not affected by the presence
of steps. This result is very important because it provides
evidence that such behavior can be successfully translated to
the most commonly used forms of nanocatalysts, which are
known to contain a vast majority of such sites (steps and
short-range terraces).
Having established the behavior of well-defined surfaces,
we move on to the most relevant electrocatalyst systems:
nanocatalysts. To encompass a wide range of electrocatalyst
designs and properties, we provide an analysis for the two
most commonly used commercial electrocatalysts. The TKK
catalyst and the 3M NSTF catalyst were both studied
(Figure 2). The TKK catalyst represents supported nanocatalysts,
where platinum nanoparticles 2–10 nm in diameter
are supported on amorphous carbon black. NSTF catalysts,
comprised of a unique catalyst structure which is free of
carbon support, are usually applied directly to the membrane
to provide a compact membrane electrode assembly structure
(Figure 2). Aqueous electrochemical experiments conducted
using the RDE/RRDE (RDE = rotating disk electrode,
RRDE = rotating ring disk electrode) methods for these
nanocatalysts are well-established [7] and have been shown to
correlate very well with operating fuel-cell systems.
We present herein results obtained from the RDE study
that should be relevant for operating fuel-cell systems.
Various modifications of the calix molecules were studied,
including the thiolated derivatives of calix[6]arenes and
calix[8]arenes (see the Supporting Information for the synthesis),
but only the derivatives of the calix[4]arene family
Angew. Chem. Int. Ed. 2011, 50, 5468 –5472 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org
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Communications
Figure 1. Electrochemical characteristics of calix-modified stepped surfaces: a) Pt(1099), b) Pt(110). Potentials of interest during startup and
shutdown are shown in the shaded region. [1,2]
Figure 2. a) SEM image of an unmodified Pt nanowhisker. b) TEM
morphology of typical TKK nanocatalysts. Calix molecules are not
visible by electron microscopy. Based on our previous STM study, [2] we
present a schematic representation of calix-modified nanocatalysts:
c) Model morphology of calix[4]arene (yellow-gray-red)-modified Pt
NSTF nanowhisker (blue). d) Model for TKK nanocatalyst chemically
modified with calix[4]arene.
proved to be effective in achieving the selectivity, so only the
results pertaining to the latter are presented. As can be seen
in Figure 3, the calix[4]arene molecules are found to suppress
the H upd region (0.05–0.4 V) for both NSTF and TKK
catalysts. The relative coverages for similar methods of
preparation are slightly different, but the net results appear
to be the same: an exceptional selectivity for the HOR versus
ORR. As for stepped surfaces discussed above, the diffusion-
limiting currents for the HOR are observed at potentials
above 0.1 V and the activities below 0.1 V are, within the
experimental limits, almost identical.
Furthermore, the ORR polarization curves show limited
or insignificant currents in the potential region of interest for
the anode-side catalyst. As was shown in the earlier study with
Pt(111), [2] the peroxide yield on all extended and nanoparticle
Pt–calix systems is negligible above 0.6 V, and the overall
ORR behavior of these surfaces mimic ORR on uncovered or
partially covered patches. [6] All of these observations suggest
that SAMs of calix molecule can be used to tailor the
selectivity of the nanocatalyst toward ORR while preserving
the HOR activity, the goal for an ideal anode catalyst. It is
also important that the established selectivity was possible
only because the required number of active sites for maximal
rates of the HOR is, in fact, extremely small but is sufficient to
provide enough sites for the diffusion-limiting currents. [2]
In addition to selectivity of CME, both thermal and
electrochemical stability of these electrodes are important
properties that need to be addressed to evaluate the anode
catalyst s applicability to PEMFC. In order to study the
stability of calix-modified electrodes, we tested a Pt–calix
system in an oxygen-rich environment at 0.8 V for approximately
14 h in solution at 608C. These conditions are
expected to be harsher than those experienced by the
electrode in a real fuel-cell system. The exposure of the
anode catalyst to high potentials (E < 0.8 V for anode) in an
air (oxygen)-rich atmosphere during startup and shutdown is
expected to last between tens of seconds and a few minutes a
day. The temperatures are expected to be similar to those
used in our test conditions.
Figure 4 shows the current–time relationship for the CME
held at 0.8 V in an oxygen-rich atmosphere. The ORR current
5470 www.angewandte.org 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2011, 50, 5468 –5472

Figure 3. Electrochemical characteristics of calix-modified Pt nanocatalysts: a) NSTF, b) TKK (50% Pt loading). Catalyst loadings were
approximately 14–16 mgcm 2 ; lower loadings were also used to mimic anode catalyst performance, yielding similar qualitative results. All current
densities are given with respect to the disk geometric area.
actually shows a small decay, thus suggesting that there is no
loss of the calix molecules from the surface owing to
oxidation. (Removal of the molecules by oxidation or
desorption would increase the reduction current.) A similar
experiment was also performed for the nanocatalysts (TKK)
modified with calix[4]arene molecules, which show qualitatively
similar results. This finding suggests that the calix[4]arene-modified
electrodes are stable under these operating
conditions. Moreover, during the long-term experiments, the
HOR (results not shown) is not affected at all.
In conclusion, our CMEs prepared by modifying Pt with
calix[4]arene molecules are highly stable and can effectively
Figure 4. Stability of the Pt–calix system at 60 8C inO 2-saturated 0.1m
HClO 4 at 0.8 V. Inset: ORR curves for unmodified surface and Pt–calix
surface both before and after the stability test. Note: the HOR remains
unchanged for the duration of the experiment.
tune the selectivity of anode catalysts for ORR without
altering the maximum activity of the HOR. This behavior is
highly transformational, extending from long-range-ordered
stepped single-crystal surfaces to nanocatalysts. The CME
approach is not restricted to a Pt–calix system, and we
envision it to provide many applications in analytical,
synthetic, and materials chemistry as well as in chemical
energy conversion and storage.
Experimental Section
Synthesis of the thiolated derivative of calix[4]arene: Adsorption of
organic groups on noble-metal surfaces has been well-established for
various groups (-S, -CN, -A, where A denotes the anchoring group). [8]
The driving force for ordering of such large molecules is presumable
governed by a synergy between the strong chemical bond between the
anchoring groups and Pt surface atoms and the local steric interaction
between adsorbed molecules. The calix[4]arene molecules anchoring
groups are usually of the form S(R) where (R) is used to cap the thiol
group on the quadrupole anchoring groups. A detailed description of
the synthesis procedure as well as molecular designs considered are
presented in the Supporting Information.
Preparation of Pt(1099), Pt(110), and Pt(polycrystalline) surfaces
and self-assembly: Pt electrodes were prepared by inductive
heating for 10 min at approximately 1100 K in an argon–hydrogen
flow (3 % hydrogen). The annealed specimen was cooled slowly to
room temperature in this flow stream and immediately covered by a
droplet of water. The electrode was then immersed in a THF solution
of calix[4]arene for 24 h, allowing the formation of a calix[4]arene
SAM. The concentration of calix[4]arene in THF was 600 mm to
obtain samples with very high coverages of calix on Pt surface.
Coverages were estimated from the H upd measurements. The effect of
coverage on ORR and HOR was previously presented. [3] The
coverages can be modified by either varying the concentrations of
the calix/THF solution or the exposure time to the high-concentration
solution. After SAM preparation, the crystals were washed thor-
Angew. Chem. Int. Ed. 2011, 50, 5468 –5472 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org
5471

Communications
oughly with deionized water before assembly and immersion in the
electrochemical cell.
Preparation of NSTF and TKK catalyst electrodes and their selfassembly:
The catalysts were mixed with water at a concentration of
1mgmL 1 . This dispersion was then ultrasonically mixed for one
hour, after which a stable suspension was obtained. A glassy carbon
disk (6 mm diameter) was then mechanically polished. Known
volumes of the suspensions were then added using a micropipette
onto the glassy carbon disk electrode. The electrode was dried at 60 8C
in an inert atmosphere. The suspension was applied so that it coated
the surface of the electrode very uniformly. Once dry, these electrodes
were washed with water to verify the good adhesion of particles to the
glassy carbon substrate. Subsequently, the electrodes were immersed
in 1000 mm solution of calix in THF. We chose to use a high
concentration of calix owing to the larger surface area of Pt compared
to the disk electrodes. The systems were equilibrated for 24 h.
Another method involved assembly of the disk electrode in a hanging
meniscus arrangement with subsequent immersion of the electrode in
the calix solution with rotation (600 rpm) for 4 h. Both of these
methods yielded similar coverages. After equilibration, the samples
were washed thoroughly with water before being immersed in the
electrochemical cell. For a discussion on the relative coverages
obtained for the same conditions for TKK and NSTF, please refer to
the Supporting Information.
RDE method, electrolytes, and electrochemical setup: After
extensive rinsing, the electrode was embedded into the rotating-disk
electrode (RDE) and transferred into a standard three-compartment
electrochemical cell containing 0.1m HClO 4 (Sigma–Aldrich). In each
experiment, the electrode was immersed at 0.07 V in solution
saturated with Ar. After obtaining a stable voltammogram between
0.07 and 0.7 V the polarization curve for the ORR was recorded on
the disk on the disk electrode. Subsequently, oxygen was purged out
of the solution and replaced with hydrogen, and HOR polarization
curves were measured. Finally, the voltammetric response was again
recorded in argon-purged solution to confirm that calix coverages had
not changed significantly.
All gases were 5N5 quality purchased from Airgas Inc. The sweep
rate for all measurements was 50 mVs 1 ; for the ORR measurements,
the electrode was rotated at 1600 rpm. Electrode potentials are given
versus the reversible hydrogen electrode (RHE).
Received: January 29, 2011
Published online: May 12, 2011
. Keywords:
calixarenes · electrochemistry ·
heterogeneous catalysis · oxygen reduction reaction · platinum
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5472 www.angewandte.org 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2011, 50, 5468 –5472

Surfactant Removal for Colloidal Nanoparticles from Solution
Synthesis: The Effect on Catalytic Performance
Dongguo Li, †,‡ Chao Wang, † Dusan Tripkovic, † Shouheng Sun,* ,‡ Nenad M. Markovic, †
and Vojislav R. Stamenkovic* ,†
† Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
‡ Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States
*S Supporting Information
ABSTRACT: Colloidal nanoparticles prepared by solution
synthesis with robust control over particle size, shape,
composition, and structure have shown great potential for
catalytic applications. However, such colloidal nanoparticles are
usually capped with organic ligands (as surfactants) and cannot
be directly used as catalyst. We have studied the effect of
surfactant removal on the electrocatalytic performance of Pt
nanoparticles made by organic solution synthesis. Various
methods were applied to remove the oleylamine surfactant,
which included thermal annealing, acetic acid washing, and UV-Ozone irradiation, and the treated nanoparticles were applied as
electrocatalysts for the oxygen reduction reaction. It was found that the electrocatalytic performance, including electrochemically
active surface area and catalytic activity, was strongly dependent on the pretreatment. Among the methods studied here, lowtemperature
thermal annealing (∼185 °C) in air was found to be the most effective for surface cleaning without inducing particle
size and morphology changes.
KEYWORDS: nanoparticles, organic solution synthesis, surfactant removal, catalysis, oxygen reduction reaction
Recently colloidal nanoparticles from aqueous 1−4 or
organic, 5−7 solution synthesis have attracted increasing
interest for the development of advanced catalysts. Through
controlled nucleation and growth in a homogeneous solution,
this method has been demonstrated to be powerful in
controlling nanoparticle (NP) size, 6,8−10 shape, 1,11,12 composition
13−15 and structure, 16−19 which has enabled systematic
studies of these parameters in catalysis. 5,10,18−21 However, the
NPs obtained by solution synthesis are usually capped by
organic surfactants and cannot be directly applied as catalysts.
For example, in the organic solution synthesis of Pt and Ptbased
alloy NPs, oleylamine and/or oleic acid are usually used
to stabilize the NPs and control NP sizes and shapes. 11,18,22,23
While these ligands are indispensible in the NP synthesis, they
are detrimental for catalysis as they block the access of reactant
molecules to the surface atoms. Therefore, it is critical to
establish a proper procedure of surfactant removal and surface
cleaning, without inducing particle size and morphology
changes, for application of colloidal NPs in catalysis.
Here we report a systematical investigation of the effect of
surfactant removal on the electrocatalytic performance of Pt
NPs prepared by organic solution synthesis. Various methods
were applied to remove the oleylamine surfactant, including
thermal annealing, acetic acid washing, and UV-Ozone
irradiation, and the efficiency of surface cleaning was examined
by electrocatalytic studies of the treated catalysts for the oxygen
reduction reaction (ORR). The removal of organic surfactants
from the NP surface was further confirmed by thermogravi-
metric analysis (TGA) and infrared adsorption spectroscopy
(IRAS) studies.
■ METHODS
Synthesis. For the synthesis of Pt NPs, 50 mg of platinum
acetylacetonate (Pt(acac) 2) was dissolved in 10 mL of
oleylamine at 100 °C under Ar flow. After 20 min, a mixture
of 0.2 g of borane tributylamine (Aldrich) and 5 mL of
oleylamine was added into the solution. The solution
temperature was raised to 120 °C and kept at this temperature
for an hour. The solution was then cooled to room
temperature, and 30 mL of ethanol was added. The product
was collected by centrifugation (8,000 rpm, 6 min). The
obtained precipitation was dispersed in hexane for further
experiments.
Catalyst Preparation and Surfactant Removal. The assynthesized
Pt NPs were washed by ethanol and then dried in
vacuum. After that, they were mixed with carbon black
(Tanaka, ∼900 m 2 /g) with a mass ratio of 1:1 in chloroform
(CHCl 3) by sonication. The solvent was then evaporated at
room temperature in air. The obtained powder was denoted as
“untreated” catalyst.
The following treatments were applied to the untreated
catalyst for surfactant removal: (1) Thermal annealing. The
Received: January 24, 2012
Published: May 21, 2012
Letter
pubs.acs.org/acscatalysis
© 2012 American Chemical Society 1358 dx.doi.org/10.1021/cs300219j | ACS Catal. 2012, 2, 1358−1362

ACS Catalysis Letter
Figure 1. TEM images of (a) as-synthesized Pt NPs, (b) Pt NPs loaded on carbon black, and (c) Pt NPs on carbon black after 185 °C annealing.
Figure 2. (a) Cyclic voltammograms of various Pt/C catalysts. (b) ORR polarization curves of the annealed, the UV-ozone treated and the untreated
samples. (c) Tafel plots of Pt/C catalysts at 20 mV/s, 1600 rpm, 20 °C. (d) Specific activity at 0.90 V and specific surface area.
catalyst was heated in a tube furnace for 5 h at 185 °C in air;
(2) Chemical washing. The catalyst was dispersed in pure acetic
acid (HAc) with vigorous stirring at 75 °C for 10 h, and then
collected by adding ethanol and centrifugation; (3) UV-Ozone
treatment. An aqueous suspension of the untreated catalyst was
deposited onto a glassy carbon electrode. After drying in air, the
electrode was placed in a UV-Ozone chamber and subject to
UV irradiation for 30 min.
Characterizations. Transmission electron microscopy
(TEM) images were collected on a Philips EM 30 (200 kV).
X-ray diffraction (XRD) patterns were collected on a
PANalytical X’pert PRO diffractometer using Cu Kα radiation
at room temperature. Thermogravimetric analysis for the Pt
NPs was done with a Pyris 1 TGA/HT Lab System
(PerkinElmer). Fourier transformed infrared (FTIR) spectra
were collected on a Nicolet Nexus 8700 spectrometer with a
MCT detector cooled by liquid nitrogen and in situ electrode
potential control, following the previous reported procedures. 24
1359
Electrochemical Studies. All electrochemical measurements
were performed in a three-compartment electrochemical
cell in 0.1 M perchloric acid at room temperature. A Ag/AgCl
electrode was used as the reference electrode, and a platinum
wire as the counter electrode. A mirror-polished glassy carbon
disk (6 mm in diameter) was used as the working electrode.
The catalyst was dispersed in deionized water by sonication and
30−40 μL of the suspension was deposited onto the glassy
carbon electrode by pipet. The loading of Pt was controlled to
be ∼20 μg/cm 2 disk. A solution of Nafion (0.1 wt %, 15 μL) was
added on top of the catalyst and then dried in air. Cyclic
voltammograms (CVs) were recorded in Ar saturated electrolytes
with a scan rate of 50 mV/s. Polarization curves for the
ORR were recorded in an oxygen saturated electrolytes at 20
mV/s with an electrode rotation speed of 1600 rpm. All the
potentials given were calibrated versus reversible hydrogen
electrode (RHE).
dx.doi.org/10.1021/cs300219j | ACS Catal. 2012, 2, 1358−1362

■ RESULTS AND DISCUSSION
Monodisperse Pt NPs were synthesized by reduction of
Pt(acac) 2 with borane tributylamine in an oleylamine solution
(see the Methods). Here oleylamine served as both solvent and
surfactant. Figure 1a shows a representative TEM image of the
as-synthesized Pt NPs with a particle size of 2.8 ± 0.4 nm
(Supporting Information, Figure S1a). The synthesized NPs
were supported on high-surface-area carbon (Tanaka, ∼900
m2 /g) and various treatments were applied to remove the
organic surfactants and clean the surface. These include mildtemperature
heating (160−200 °C) in air, 25 chemical washing
by acetic acid, 26 and UV-Ozone irradiation. 27 No significant
change in particle size or morphology was found after these
treatments, as shown by the TEM images presented in Figure 1,
for example, for the 185 °C thermal treatment in air (Figure
1b,c and Supporting Information, Figure S1b). X-ray powder
diffraction patterns of the as-synthesized Pt NPs, 185 °C
annealed and HAc treated catalysts were presented in
Supporting Information, Figure S2, which shows no significant
particle size change or coarsening after these treatments.
Catalytic performance of the various treated catalysts was
examined by the RDE method. Figure 2 summarizes the results
of electrochemical studies. CVs show typical Pt like features
with under potential deposited hydrogen (Hupd) peaks at E <
0.4 V (Figure 2a). Among the various treated catalysts, the 185
°C annealed one shows more pronounced Hupd peaks than the
others. As the catalyst loading was consistently controlled to be
∼20 μg/cm2 disk, this difference directly corresponds to the
variation in specific surface area, namely, the electrochemically
active surface area (ECSA, estimated from the Hupd charges by
ECSA = QHupd/210 μC/cm2 ) normalized by the mass of Pt in
the catalyst. As shown in Figure 2d, the 185 °C annealed
catalyst gave a specific surface area of ∼330 cm2 /gPt, versus
∼270 cm2 /gPt for the HAc washed, 240 cm2 /gPt for the UV-
Ozone treated, and 190 cm2 /gPt for the untreated catalyst.
Moreover, the electrocatalytic activity was found to be also
strongly dependent on the methods of surfactant removal. Both
the polarization curves (Figure 2b) and Tafel plots (Figure 2c)
show that the 185 °C annealed catalyst has the highest ORR
activity, reaching 0.64 mA/cm2 at 0.9 V, compared to 0.49 mA/
cm2 for the HAc washed, 0.29 mA/cm2 for the UV-Ozone
treated, and 0.22 mA/cm2 total loss in weight, ∼45%, corresponds to the weight ratio of
surfactants in the NPs. On the basis of the TGA studies, we can
conclude that the thermal annealing at 185 °C in air is efficient
for removing the organic surfactants from the Pt NP surface. It
is also important to point out that the employed carbon black is
stable in the surfactant removal process, as no weight change
was observed in TGA for the carbon support at 185 °C.
After surfactant removal, surface atoms on the Pt NPs were
exposed and became electrochemically active. This was further
confirmed by in situ electrochemical IRAS studies, in which
surface conditions of the NPs as well as the CO-Pt interaction
for the untreated catalyst. Both the
results of specific surface areas and specific activities indicate
that the efficiency of surfactant removal follows the trend 185
°C annealing > HAc washing > UV-Ozone (Figure 2d). It has
to be pointed out that potential cycling can bring additional
cleaning for the catalysts. For example, after one hundred
potential cycles, the HAc washed Pt/C shows comparable
electrocatalytic performance as the thermal annealed one
(Supporting Information, Figure S3). However, such potential
cycling had very limited effect on the UV-Ozone treated and
untreated catalysts. In the following discussion, we will focus on
the study of surfactant removal by thermal annealing to confirm
the effectiveness of surfactant removal.
Figure 3 shows the weight loss curves for the process of
surfactant removal by 185 °C annealing in air for the assynthesized
Pt NPs and carbon support. It was found that a
weight loss of up to 44% took place during the isothermal
process at 185 °C for the NPs, because of the removal of
organic surfactants. Not much organic substances were left after
the 185 °C stage, as further increase of the temperature up to
900 °C did not lead to substantial loss in addition (∼1%). The
24
was examined. Figure 4a shows the IR spectra of the annealed
and untreated NPs collected at 0.1 V in 0.1 M HClO4 electrolyte. The insets of closeup view highlight the difference
between the two spectra in the region from 2800 cm−1 to 3000
cm−1 . The bands at 2850 cm−1 and 2920 cm−1 observed in the
untreated NPs can be associated to the C−H stretching
vibrations in oleylamine molecules. 28 After the thermal
annealing, the C−H bands disappeared, suggesting successful
removal of surfactants from the particle surface. Figure 4b
shows the differentiation spectra obtained by subtracting the IR
spectra recorded at 0.1 V with the spectra at 0.9 V (see
Supporting Information, Figure S4 for the primary spectra).
Considering that the adsorbed CO (COad) is oxidized at high
potentials (E = 0.9 V) while at low potentials CO is strongly
adsorbed on Pt, the differentiation spectra are thus able to
exclusively tell the situation of CO adsorption on the particle
surface. The untreated NPs do not exhibit any observable
feature in the wavenumber range from 1800 cm−1 to 2800 cm−1 because of the blocked CO adsorption on the particle surface.
The annealed NPs show a sharp peak at 2060 cm−1 which can
be assigned to atop-bonded CO on Pt, 29 indicating that the
surface atoms were activated by the thermal annealing and
became accessible for CO adsorption. The positive peak around
2340 cm−1 can be associated with CO2 which was the product
of CO oxidation. Consistent with the observations from
electrochemical and TGA studies (Figure 2 and 3), the IRAS
results demonstrate that organic surfactants present on the NP
surface, which block Hupd and CO adsorption and ORR, can be
sufficiently removed by the thermal annealing at a mild
temperature in air.
■ CONCLUSION
ACS Catalysis Letter
Figure 3. TGA of the as-synthesized Pt NPs and carbon support.
We have studied the effect of surfactant removal on the
electrocatalytic performance of Pt nanoparticles made by
1360
dx.doi.org/10.1021/cs300219j | ACS Catal. 2012, 2, 1358−1362

ACS Catalysis Letter
Figure 4. (a) FTIR spectra of adsorbed CO on untreated and annealed Pt NPs. The spectra were recorded after CO adsorption at 0.1 V. The insets
show the region of the spectra that is typical for surfactant molecules. (b) Subtracted IR spectra at 0.9 and 0.1 V for annealed and untreated samples.
organic solution synthesis. Various methods existing in the
literature were applied to remove the oleylamine surfactant, and
the dependence of electrocatalytic performance for the ORR on
the treatment was found to follow the trend: 185 °C annealing
in air > HAc washing > UV-Ozone. The effectiveness of
surfactant removal and surface cleaning by thermal annealing in
air was further confirmed by TGA and IRAS studies. Our work
revealed the importance of pretreatment in catalyst preparation
and would have great implication for the development of
advanced catalysts with colloidal nanoparticles from solution
■synthesis.
ASSOCIATED CONTENT
*S Supporting Information
Further details are given in Figures S1−S4. This material is
available
■
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ssun@brown.edu (S.S.), vrstamenkovic@anl.gov
(V.R.S.).
Funding
This work was conducted at Argonne National Laboratory, a
U.S. Department of Energy, Office of Science Laboratory,
operated by UChicago Argonne, LLC, under contract no. DE-
AC02−06CH11357. This research was sponsored by the U.S.
Department of Energy, Office of Energy Efficiency and
Renewable Energy, Fuel Cell Technologies Program. Microscopy
research was conducted at the Electron Microscopy
Center for Materials Research at Argonne.
1361
Notes
The authors declare no competing financial interest.
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Electrocatalysis
DOI 10.1007/s12678-012-0099-9
High Activity and Stability of Pt2Bi Catalyst in Formic
Acid Oxidation
J. D. Lović & M. D. Obradović & D. V. Tripković &
K. Dj. Popović & V. M. Jovanović & S. Lj. Gojković &
A. V. Tripković
# Springer Science+Business Media, LLC 2012
Abstract Formic acid oxidation was studied on a new
prepared Pt2Bi characterized by X-ray diffraction spectroscopy
(phase composition), scanning tunneling microscopy
(STM) (surface morphology), and CO ads stripping voltammetry
(surface composition). Bulk composition of Pt2Bi
revealed two phases—55% PtBi alloy and 45% Pt. Estimated
contribution of pure Pt on the Pt 2Bi surface (43.5%)
determined by COads stripping voltammetry corresponds
closely to bulk composition. Pt2Bi reveals high activity
and stability in formic acid oxidation. High activity originates
from the fact that formic acid oxidation proceeds
completely through dehydrogenation path based on an ensemble
effect. The high stability of Pt 2Bi surface is induced
by the suppression of Bi leaching as it was evidenced by
insignificant changes of surface morphology and surface
roughness shown by STM images before and after electrochemical
treatment in formic acid containing solution. Pt2Bi
is found to be powerful catalyst exhibiting up to two orders
of magnitude larger current densities at 0.0 V and onset
potential shifted for ∼0.2 V to more negative value relative
to Pt under steady-state condition.
Keywords Formic acid . Electrochemical oxidation . Pt2Bi
catalyst . Fuel cell
J. D. Lović : M. D. Obradović : D. V. Tripković : K. D. Popović :
V. M. Jovanović : A. V. Tripković (*)
ICTM-Institute of Electrochemistry, University of Belgrade,
Njegoševa 12, P.O.Box 473, 11000 Belgrade, Serbia
e-mail: amalija@tmf.bg.ac.rs
S. L. Gojković
Faculty of Technology and Metallurgy, University of Belgrade,
Karnegijeva 4, P.O.Box 3503, 11000 Belgrade, Serbia
Introduction
Polymer electrolyte membrane fuel cell using formic acid as
a fuel, i.e., direct formic acid fuel cell has been attracting a
lot of attention in the last few years due to its advantages
over direct methanol fuel cell such as less positive onset
potential of the reaction and lower crossover trough the
polymer membrane [1, 2].
It has been widely accepted that formic acid on Pt is
oxidized to CO 2 via a dual path mechanism [3] involving
a reactive intermediate in the main path–dehydrogenation
and adsorbed COads in the parallel path–dehydration. Since
potential of CO ads formation is lower than potential of formic
acid dehydrogenation, COads remains on the Pt surface
until it can be oxidized at high positive potentials (∼0.65 V
vs. saturated calomel electrode (SCE)). This susceptibility
of Pt to poisoning species significantly reduces its catalytic
performance at low potentials [4]. To solve this problem, Ptbased
bimetallic catalysts were used. Although bifunctional
and electronic effects influence oxidative removal of COads
from the Pt sites, the approach recently proposed based on
the ensemble effect which increases selectivity of Pt toward
dehydrogenation path seems to be a better solution [5].
Intermetallic PtBi [6–10] as well as PtBi alloy [11, 12]
were proposed as powerful catalysts for formic acid oxidation.
The origin of their catalytic activity was related to the
electronic effect [13, 14], to enhanced tolerance to CO
poisoning compared with Pt [15], and to bifunctional effect
[11]. However, they are not stable in acidic media. It has
been reported that instability of Pt-Bi bimetallic catalysts is
attributed to the leaching/dissolution of Bi from the alloy
matrix [16]. This leaching occurs through the oxidation of
the less-noble metal generating a Pt rich surface. The onset
potential of Bi leaching from PtBi corresponds closely to
that predicted from the Pourbaix diagram for elemental Bi

and proceeds all along anodic potentials. Moreover, the adsorption
of previously dissolved Bi could be playing a role as
well. High activity of PtBi alloy relative to polycrystalline Pt
electrode in formic acid oxidation was also discussed in terms
of electrochemically detected underpotential phenomenon of
readsorbed Bi on Pt [11]. However, leaching of Bi causes
instability of PtBi catalysts which is the main problem for
practical application of any Pt-Bi catalysts.
In this work, activity and stability of new prepared specific
Pt2Bi catalyst consisting of two phases (45% pure Pt
and 55% PtBi alloy) in formic acid oxidation was studied.
Experimental
Pt2Bi Alloy Preparation
Pt2Bi was obtained by the induction melting of the constituents
in quartz container under purified Argon atmosphere.
According to Bi–Pt phase diagram [17], Pt starts to dissolve
in liquid Bi already at 271.4° C. At about 500 ° C, fast
dissolution of Pt in liquid Bi was observed. Temperature
was increased slowly up to 1,000 ° C at which clear alloy
meniscus was formed. The meniscus size and shape varied
upon changes of electromagnetic field, attesting intensive
mixing of liquid alloy at 1,000–1,100 ° C. Finally, the alloy
was cooled and stored in Ar atmosphere.
Electrode Characterization
The Pt2Bi alloy was characterized by X-ray diffraction
(XRD). XRD measurements were carried out with a
Bruker/AXS diffractometer using CuKα source operating
at 30 mA and 45 kV and a graphite monochromator. The
polished sample was placed in a Plexiglas holder. Data were
collected over the range 10–135° 2θ using CuKα radiation
with a 0.04° step size and counting time of 2 s.
The quantitative analysis of phase content and crystallite
size calculations were carried out by multiphase Rietveld
refinement using Topas 2.1 software [18].
Surface morphology of Pt2Bi electrode before and after
formic acid oxidation was examined by scanning tunneling
microscopy (STM) using a NanoScope III D (Veeco, USA)
microscope. The STM images were obtained in the height
mode using a Pt–Ir tip (current set-point, it—1 nA; bias
voltage, Vb—100 mV). Fifteen different parts at each electrode
surface were analyzed.
Electrochemical Measurements
Pt2Bi alloy (A00.42 cm 2 ) and polycrystalline Pt discs
(A00.196 cm 2 ) were used in a rotating disc assembly. The
experiments were carried out at the rotation rate of 2,000 rpm.
Prior to each experiment, the electrodes were mirror-polished
(1–0.05 μm Buehler alumina). The surfaces were rinsed with
Millipore water, sonicated for 2–3 min, and rinsed again with
high purity water.
All experiments were carried out on as-prepared catalysts.
Three-compartment electrochemical glass cells with Pt wire as
the counter electrode and SCE as the reference electrode was
used. All the potentials are expressed on the scale of SCE. The
electrolyte contained 0.1 M H2SO4 as a supporting electrolyte
and 0.125 M HCOOH prepared with high purity water (Millipore,
18 MΩ cm resistivity) and p.a. grade chemicals (Merck).
The electrolyte was deaerated by bubbling of nitrogen. Upon
addition of HCOOH at –0.2 V, potentiodynamic (ν050 mV s –1 ),
quasi steady-state measurements (ν01 mVs –1 ), or chronoamperometric
measurements were carried out. The experiments
were conducted at 295±0.5 K. A VoltaLab PGZ 402 (Radiometer
Analytical, Lyon, France) was employed.
For the CO stripping measurements, pure CO was bubbled
through the electrolyte for 20 min while keeping the electrode
potential at –0.2 V vs. SCE. After purging the electrolyte by
N2 for 30 min to eliminate the dissolved CO, the adsorbed CO
wasoxidizedinananodicscanat50mVs −1 . Two subsequent
voltammograms were recorded to verify the completeness of
the CO oxidation. Real surface area of the Pt electrode was
calculated from the charge of CO stripping corrected for
background currents. Assuming charge of 420 μC cm −2 for
the CO monolayer adsorption, roughness factor of 1.4±0.1
was estimated. The real surface area of Pt 2Bi electrode was
estimated assuming the same roughness factor as for Pt electrode,
which is to be expected since both electrodes were
polished in the same way. The specific activity of Pt and Pt 2Bi
electrodes for formic acid oxidation are normalized using
these values of the surface area.
The CO stripping charge was also determined for Pt 2Bi
electrode and corrected for the background currents to eliminate
the contribution of the double layer charge, as well as
Bi oxidation charge. Since CO does not adsorb at PtBi (1:1)
alloy [9, 11, 14, 19], the charge under the CO stripping peak
on Pt2Bi surface corresponds to the surface area of Pt phase
and may serve for determination of the surface composition.
Results and Discussion
Electrode Characterization
Phase Composition
Electrocatalysis
XRD characterization of the Pt 2Bi alloy was performed to
determine its phase composition (Fig. 1).
Diffraction pattern for Pt2Bi sample reveals two crystal
phases: platinum (fcc, cubic system, space group Fm-3 m,
no. 225) and platinum bismuth PtBi (hcp, hexagonal system,

Electrocatalysis
Fig. 1 XRD patterns of Pt 2Bi, PtBi, and polycrystalline Pt catalysts.
Markers represent position of peaks for pure Pt and PtBi alloy
space group P63/mmc, no. 194, ICSD 58845). No other
crystalline phases were found.
The phase composition of the sample was calculated using
the Rietveld refinement as 45% and 55% for Pt and PtBi
phases, respectively. The slight difference between intended
(nominal) and calculated composition can be ascribed to the
sample texture which influences the Rietveld analysis and is
within usual errors for this method. The lattice constants of Pt
fcc phase were refined as a03.9279(2) and for PtBi hcp phase
as a04.3375(7) and c05.5107(3).
As revealed by the XRD analysis, phase composition of the
Pt2Bi sample is consistent with the Bi–Pt phase diagram
(Pt2Bi → Pt+PtBi) [17]. Accordingly, the diffraction patterns
for polycrystalline Pt and PtBi alloy are presented in Fig. 1 to
support qualitatively two-phase composition of Pt2Bi catalyst.
CO ads Stripping Voltammetry
COads stripping voltammetry recorded at Pt2Bi and Pt are
displayed in Fig. 2. The onset potential of CO ads oxidation
as well as the peak position at Pt2Bi is slightly shifted to more
Fig. 2 CO stripping voltammograms on Pt 2Bi and Pt catalysts (first
positive-going sweeps) in 0.1 M H2SO4 solution corrected for background
current. Inset:basevoltammogramofPt 2Bi with CO ads stripping
voltammogram and the voltammogram recorded upon holding the potential
at −0.2 V for 20 min in CO-free solution. Scan rate, 50 mV s –1
negative potentials relative to Pt, indicating the presence of
some electronic modification of Pt surface atoms, capable for
CO adsorption, by Bi. This is in accordance with a small shift
of oxide reduction peak to the same direction at the base
voltammogram for Pt2Bi regarding Pt (not shown). Since
CO does not adsorb at Bi [20] and PtBi [14, 19] alloys,
oxidation of CO occurs only on Pt domains. The sharpness
and the symmetry of the COads stripping peak generally
reflects the uniformity of Pt surface [21]. The shoulder on
CO stripping voltammogram recorded at Pt2Bi could indicate
the existence of two states of Pt atoms in the Pt phase on this
electrode surface. Sites on the edge of Pt domains being in
contact with PtBi alloy phase binds COads stronger than other
Pt sites. Stronger adsorption of CO and OH species on Pt
atoms in contact with PtBi sites arises from the some electronic
modification of Pt surface atoms by Bi [22, 23].
Since CO adsorbs only on Pt domains of Pt2Bi electrode,
the charge under the COads stripping peak can be used for
determining the surface area of pure Pt. Comparing this surface
area to the estimated surface area of Pt2Bi, contribution of
43.5% of Pt domains on the surface is obtained, which is close
to 45% Pt calculated from XRD measurement.
The fact that the methods for determination of phase and
surface composition (XRD and COads stripping voltammetry,
respectively), resulting approximately the same contribution
of Pt, demonstrates clearly that adsorbed CO prevents leaching
of Bi which should lead to Pt enriched surface. This
statement can be recognized by contrasting the base voltammogram
of Pt 2Bi and the voltammogram recorded upon holding
the potential at –0.2 V for 20 min in CO-free solution with
COads stripping voltammogram (inset in Fig. 2). The increase
of the currents at potentials E>0.1 Vobserved in forward scan
of the voltammogram recorded after the potential was kept in

hydrogen region without CO in solution compared with base
voltammogram is a consequence of additional oxidation of
leached Bi. On the contrary, COads stripping voltammogram
recorded in the potential region before COads oxidation commences
coincides with the base voltammogram demonstrating
that adsorbed CO prevents leaching of Bi. Effect of surface
stabilization with COads enables the accurate estimation of the
real surface area by calculation of CO ads stripping charge
which is the only way to do it since the hydrogen region at
Pt2Bi base voltammogram is featureless.
Oxidation of Formic Acid
Potentiodynamic Measurements
Cyclic voltammograms for formic acid oxidation on Pt2Bi and
Pt are presented in Fig. 3. Potentiodynamic profile of Pt shows
well-established feature of HCOOH oxidation on Pt [24]. In the
forward scan, small currents reach a plateau at ∼0.3 V followed
by ascending currents starting at ∼0.45 V and the maximum
at ∼0.6 V. Oxidation of formic acid on Pt proceeds through two
parallel pathways, i.e., dehydrogenation and dehydration, both
generating CO2 as the final reaction product. The dehydrogenation,
assigned as the direct path, is based on the oxidation of
formate [25], the active intermediate containing two oxygen
atoms, which does not need adsorbed water or other oxygencontaining
species [5, 26] to be oxidized to CO2. On the other
hand, dehydration assumes forming COads, well-known as
poisoning species at low potentials, which can be oxidized to
CO2 only by oxygen-containing species formed at higher
potentials. The currents below 0.45 V at the voltammogram
for formic acid oxidation on Pt are related to dehydrogenation
path on the surface partially covered by COads produced by
dehydration of formic acid molecules. At higher potentials,
Fig. 3 Cyclic voltammograms for the oxidation of 0.125 M HCOOH
in 0.1 M H 2SO 4 solution on Pt 2Bi and Pt catalysts. Scan rate,
50 mV s –1
formic acid is oxidized on Pt sites being released upon COads
oxidation with oxygen-containing species. The current peak
at ∼0.6 V is a result of inactive Pt-oxide formation which finally
stops the reaction at ∼0.8 V. In the backward scan, currents
begin to rise at ∼0.7 V, simultaneously with the onset of Pt
oxide reduction, and then decreases at potential more negative
than ∼0.45 V since COads cannot be further oxidized due to a
lack of oxygen-containing species.
On Pt2Bi, catalyst formic acid oxidation commences
at ∼–0.2 V, i.e., more than 0.2 V earlier than on Pt. The
currents increase reaching a peak at ∼0.35 V which is ∼30
times higher than a plateau on Pt and then diminishes up
to positive potential limit. The onset of oxide reduction
at ∼0.6 V in the backward scan (voltammogram is given in
the inset in Fig. 2) causes an increase of formic acid oxidation
currents attaining the maximum at ∼0.35 V.
As Bi does not adsorb formic acid [11, 27], oxidation of
formic acid occurs on pure Pt domains and on Pt atoms on
PtBi domains. Bell-shaped voltammogram for formic acid
oxidation suggests that the reaction on Pt2Bi proceeds through
dehydrogenation path with the dehydration path completely
suppressed. This statement is supported by a remarkable increase
of current bellow 0.35 V, where the activity of Pt
electrode is poor because of high surface coverage with COads,
as well as by the absence of the peak at ∼0.6 V appearing on
Pt. Decrease of the activity at potential higher than 0.35 V is
induced by blocking the Pt sites, needed for decomposition of
adsorbed formate, with oxygen-containing species [5, 26].
Generally, high activity of Pt2Bi catalyst as bimetallic
catalyst could be explained by bifunctional effect, ensemble
effect, and electronic effect. However, bearing in mind all
the data mentioned above, it is reasonable to assume that
increased selectivity toward dehydrogenation path on Pt2Bi
compared with Pt is caused by ensemble effect originating
from the interruption of continuous Pt sites by Bi atoms. In
favor of this statement is the fact that the voltammetric
profile of the formic acid oxidation on Pt 2Bi (Fig. 3) does
not indicate any adsorption or oxidation of CO, although
Pt2Bi is capable of adsorbing CO (Fig. 2).
The electronic modification of Pt by Bi, based on charge
redistribution in a PtBi phase of Pt2Bi catalyst, enhances the
affinity towards formic acid adsorption thus increasing interaction
of formic acid molecules with the catalyst surface
[14], which results in lowering the onset potential of the
reaction compared with Pt.
As formic acid oxidation on Pt 2Bi proceeds through
dehydrogenation path, bifunctional mechanism is not relevant
for interpretation of high catalyst activity.
Chronoamperometric Measurements
Electrocatalysis
Current density at the constant potential of 0.2 V was recorded
over 30 min on Pt2Bi and Pt catalysts and presented in Fig. 4.

Electrocatalysis
Fig. 4 Chronoamperometric curves for the oxidation of 0.125 M
HCOOH at 0.2 V in 0.1 M H2SO4 solution on Pt2Bi and Pt catalysts
The higher initial current density on Pt2Bi than on Pt is in
accordance with potentiodynamic measurements (Fig. 3). The
initial current on Pt2Bi decreases slightly, and at the end of
experiment, its value is about 20 times higher than the current
on Pt. This confirms high activity and stability of Pt2Bi
catalyst also under the steady-state conditions.
Quasi-steady-State Measurements
Kinetics of HCOOH oxidation on Pt2Bi and Pt under the quasisteady-state
conditions is presented in Fig. 5 in a form of the
Tafel plots. The order of activity for the catalysts is the same as
under the potentiodynamic condition. Pt2Bi exhibits up to two
orders of magnitude larger current densities at 0.0 V than Pt.
Fig. 5 Tafel plots for oxidation of 0.125 M HCOOH in 0.1 M H 2SO 4
solution on Pt 2Bi and Pt catalysts. Scan rate, 1 mV s –1
Fig. 6 Cyclic voltammograms for the HCOOH oxidation (first and
20th sweep) on Pt 2Bi catalyst in 0.1 M H 2SO 4 solution. Inset:
corresponding basic voltammograms. Scan rate, 50 mV s –1
Fig. 7 Cyclic voltammograms of Pt recorded before (dashed line) and
after (solid line) replacement of Pt 2Bi in 0.1 M H 2SO 4 solution (a) and
in 0.125 M HCOOH solution (b). Scan rate, 50 mV s –1

Fig. 8 STM images of Pt 2Bi catalyst (300×300×50 nm) before and after formic acid oxidation
The Tafel slope of 120 mV dec –1 indicates that HCOOH
oxidation proceeds on CO ads free surfaces through dehydrogenation
path. Larger Tafel slope of ∼150 mV dec −1 obtained on
Pt suggests that HCOOH oxidation takes place through dehydrogenation
path as on Pt 2Bi but on the surface partially
coveredbyCOads produced in dehydration path occurring in
parallel. The same value of Tafel slope was obtained for
HCOOH oxidation on Pt/C [28]. On both catalysts, first electron
transfer is the rate-determining step in the reaction.
Stability of Pt2Bi
Potentiodynamic Measurements Cyclic voltammograms
(first and 20th sweeps) recorded on Pt2Bi catalyst in a formicacid-containing
solution are shown in Fig. 6. Activity of Pt2Bi
electrode continuously decreases with cycling up to 0.8 V
during initial five to seven sweeps but remains unchanged with
further cycling. On the contrary, cycling of Pt2Bi in base electrolyte
leads to enhancement of currents related to oxidation of
Bi species, indicating some surface decomposition caused by Bi
leaching/dissolution process (inset in Fig. 6). It appears, consequently,
that stability of Pt 2Bi during oxidation of formic acid
could be induced by the presence of formic acid in the electrolyte.
To test this assumption, Pt2Bi electrode was subjected to
potential cycling in base electrolyte, and after 20 cycles, the
electrode was replaced by Pt electrode (experiment 1).
The same procedure was repeated in the electrolytecontaining
formic acid (experiment 2). The voltammetric profiles
of Pt electrode after the experiments 1 and 2, along with
Electrocatalysis
the voltammograms of Pt electrode in fresh base and formicacid-containing
electrolytes are presented in Fig. 7. Results of
the experiment 1 (Fig. 7a) show slightly reduced charge of
hydrogen adsorption/desorption, indicating underpotential deposition
of previously leached/dissolved Bi. The voltammogram
of formic acid oxidation recorded on Pt in experiment 2
almost retraces the characteristic profile of pure Pt, suggesting
that leaching of Bi is suppressed in the presence of formic acid
(Fig. 7b). It should be noted that the experiment performed
after replacing of PtBi electrode with Pt [11] revealedsignificant
Bi leaching under the same experimental conditions,
meaning that Pt2Bi is more resistant to Bi leaching than PtBi.
STM Measurements In order to test whether the surface morphology
of Pt2Bi changes during formic acid oxidation, STM
measurements were carried out before and after the reaction.
Typical STM images of the electrode before and after the
experiment are presented in Fig. 8. The image of polished
surface pointes out inhomogeneity of the material structure
illustrated by larger and smaller domains separated by both
wide and narrow cavities, as a consequence of catalyst
preparation. A similar image is recorded for the electrode
after the oxidation of formic acid. Moreover, roughness
analysis of 15 different parts at the electrode surface, both
before and after the experiment, reveals comparable surface
roughness values (Rms). The mean Rms value for the polished
electrode before formic acid oxidation is 2.72 with
standard deviation of ±0.28, while the mean Rms for the
electrode after 20 cycles in HCOOH-containing sulfuric

Electrocatalysis
acid is 3.13 with standard deviation of ±0.46. Thus, based
on this small difference in Rms, it could be reasonable to
assume that no significant leaching of Bi occurred during
the formic acid oxidation.
These STM analyses, which refer to insignificant change
of surface morphology and roughness, are in accordance
with previously determined suppression of Bi leaching during
formic acid oxidation. It appears, consequently, that
Pt2Bi surface becomes kinetically stabilized due to the competition
between the oxidation of formic acid at the electrode/solution
interface and Bi leaching, i.e., corrosion/
oxidation process of the electrode surface itself [29]. Accordingly,
the main reasons for high stability of Pt2Bi catalyst
is the suppression of Bi leaching, as well as inhibition of
dehydration path in the reaction of formic acid oxidation.
Conclusion
Bulk composition of Pt 2Bi characterized by XRD revealed two
phases—55% PtBi alloy and 45% Pt. Estimated contribution
of pure Pt on the Pt2Bi surface (43.5%) determined by COads
stripping voltammetry corresponding closely to bulk composition
indicates that adsorbed CO prevents leaching of Bi.
Pt2Bi exhibits high activity and stability in formic acid
oxidation. High activity is caused by the fact that formic
acid oxidation proceeds completely through dehydrogenation
path. Increased selectivity toward dehydrogenation is
caused by an ensemble effect.
Stability of Pt2Bi surface in formic acid oxidation is
induced by preventing Bi leaching/dissolution during formic
acid oxidation, as well as by the absence of surface poisoning
by COads species due to inhibition of dehydration path.
Pt2Bi is powerful catalyst for formic acid oxidation exhibiting
up to two orders of magnitude larger current densities
at 0.0 V and onset potential negatively shifted for ∼0.2 V
compared with Pt.
Acknowledgments This work was financially supported by the Ministry
of Education and Science, Republic of Serbia, contract no. H-172060.
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Received: May 24, 2011
Published: July 19, 2011
ARTICLE
pubs.acs.org/JACS
Design and Synthesis of Bimetallic Electrocatalyst with Multilayered
Pt-Skin Surfaces
Chao Wang, † Miaofang Chi, ‡ Dongguo Li, †,§ Dusan Strmcnik, † Dennis van der Vliet, † Guofeng Wang, ||
Vladimir Komanicky, †,z Kee-Chul Chang, † Arvydas P. Paulikas, † Dusan Tripkovic, † John Pearson, †
KarrenL.More, ‡ Nenad M. Markovic, † and Vojislav R. Stamenkovic* ,†
†
Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
‡
Division of Material Science and Technology, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
§
Department of Chemistry, Brown University, Providence, RI 02912, United States
Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
z
Safarik University, Faculty of Science, Kosice 04154, Slovakia
)
bS Supporting Information
ABSTRACT: Advancement in heterogeneous catalysis relies
on the capability of altering material structures at the nanoscale,
and that is particularly important for the development of highly
active electrocatalysts with uncompromised durability. Here, we
report the design and synthesis of a Pt-bimetallic catalyst with
multilayered Pt-skin surface, which shows superior electrocatalytic
performance for the oxygen reduction reaction (ORR).
This novel structure was first established on thin film extended
surfaces with tailored composition profiles and then implemented
in nanocatalysts by organic solution synthesis. Electrochemical
studies for the ORR demonstrated that after prolonged
exposure to reaction conditions, the Pt-bimetallic catalyst with
multilayered Pt-skin surface exhibited an improvement factor of more than 1 order of magnitude in activity versus conventional Pt
catalysts. The substantially enhanced catalytic activity and durability indicate great potential for improving the material properties by
fine-tuning of the nanoscale architecture.
’ INTRODUCTION
The foreground of sustainable energy is built up on a renewable
and environmentally compatible scheme of chemical-electrical
energy conversion. One of the key processes for such energy
conversion is the electrocatalytic reduction of oxygen, the cathode
reaction in fuel cells 1 and metal air batteries, 2 where an electrocatalyst
is used to accelerate the course of ORR. Current electrocatalysts
used for this reaction are typically in the form of dispersed Pt
nanoparticles (NPs) on amorphous high-surface-area carbon. Considering
the high cost and limited resource of Pt, large-scale applications
of these renewable energy technologies demand substantial
improvement of the catalyst performance so that the amount of Pt
needed can be significantly reduced. For example, a 5-fold improvement
of catalytic activity for the ORR is required for the commercial
implementation of fuel cell technology in transportation. 3
Our recent work on well-defined extended surfaces has shown
that high catalytic activity for the ORR can be achieved on
Pt bimetallic alloys (Pt 3M, M = Fe, Co, Ni, etc.), due to the altered
electronic structure of the Pt topmost layer and hence reduced
adsorption of oxygenated spectator species (e.g., OH )onthe
surface. 4 It was also found that in an acidic electrochemical environment
the non-noble 3d transition metals are dissolved from the near-
surface layers, which leads to the formation of Pt-skeleton surfaces.
Moreover, the thermal treatment of Pt3M alloys in ultra high vacuum
(UHV) has been shown to induce segregation of Pt and formation of
a distinctive topmost layer that was termed Pt-skin surface. However,
the same treatment did not cause Pt to segregate over PtM alloys
with high content (g50%) of non-Pt elements. 4b,5 Recently, we
further demonstrated the surfacing of an ordered Pt(111)-skin over
Pt3Ni(111) single crystal with 50% of Ni in the subsurface layer. This
unique nanosegregated composition profile was found to be responsibleforthedramatically
enhanced ORR activity. 6
On the basis of these findings, it could be envisioned that the
most advantageous nanoscale architecture for a bimetallic electrocatalyst
would correspond to the segregated Pt-skin composition
profile established on extended surfaces. A lot of effort has
thus been dedicated, 7 but it still remains elusive, to finely tune the
Pt-bimetallic nanostructure to achieve this desirable surface
structure and composition profile. Major obstacles reside not
only in the difficulty to manipulate elemental distribution at the
nanoscale, but also in the fundamental differences in atomic
r 2011 American Chemical Society 14396 dx.doi.org/10.1021/ja2047655 | J. Am. Chem. Soc. 2011, 133, 14396–14403

Journal of the American Chemical Society ARTICLE
structures, electronic properties, and catalytic performance between
extended surfaces and confined nanomaterials. For example, in
an attempt to induce surface segregation, high-temperature (>600 °C)
annealing is typically applied for Pt-based alloy nanocatalysts.
While improvement in specific activity is obtained, such treatment
usually causes particle sintering and loss of electrochemical
surface area (ECSA). 7b,8 Besides that, the surface coordination
of nanomaterials is quite different from that of bulk materials;
that is, the surface of NPs is rich in corner and edge sites, which
have a smaller coordination number than the atoms on longrange
ordered terraces of extended surfaces. 9 These low-coordination
surface atoms are considered as preferential sites for the
adsorption of oxygenated spectator species (e.g., OH ) 10 and
thus become blocked for adsorption of molecular oxygen and
inactive for the ORR. 9b,11 Additionally, due to strong Pt O
interaction, these sites are more vulnerable for migration and
dissolution, resulting in poor durability and fast decay of the
catalyst. 12 The latter effect is even more pronounced in Pt bimetallic
systems, considering that more undercoordinated atoms
are present on the skeleton surfaces formed after the depletion of
nonprecious metals from near-surface regions. 4a,13 Therefore, a
systematic approach with all of these factors integrally considered
becomes necessary to pursue the design and synthesis of
advanced bimetallic catalysts.
Our focus in this study has been placed on the fine-tuning of
Pt bimetallic nanostructure aiming to achieve the advantageous
Pt-skin surface structure and composition profile established on
extended surfaces. We started with Pt thin films of controlled
thickness deposited over PtNi substrate to explore the correlation
between the surface composition profile and catalytic performance.
These findings were then applied for guiding the synthesis of
nanocatalysts with the optimized structure. The outcome of such
effort is an advanced Pt bimetallic catalyst with altered nanoscale
architecture that is highly active and durable for the ORR.
’ EXPERIMENTAL SECTION
Thin Film Preparation. Pt films were deposited at room temperature
on PtNi substrates (6 mm in diameter), which were set 125 mm
away from DC sputter magnetrons in 4 mTorr argon gas (base vacuum
1 10 7 Torr). The Pt source rate (0.32 Å/s) was determined by quartz
crystal microbalance, and an exposure duration of 7.0 s was calibrated for
the nominal thickness of 2.2 2.3 Å for a monolayer of Pt. The film
thickness was derived from the exposure time of computer-controlled
shutters during sputtering.
NP and Catalyst Synthesis. In a typical synthesis of PtNi NPs,
0.67 mmol of Ni(ac)2 3 4H2O was dissolved in 20 mL of diphenyl either
in the presence of 0.4 mL of oleylamine and 0.4 mL of oleic acid.
0.33 mmol of 1,2-tetradodecanediol was added, and the formed solution
washeatedto80°C under Ar flow. After a transparent solution formed, the
temperature was further raised to 200 °C, where 0.33 mmol of Pt(acac)2
dissolved in 1.5 mL of dichlorobenzene was injected. The solution was
kept at this temperature for 1 h and then cooled to room temperature.
An amount of 60 mL of ethanol was added to precipitate the NPs, and
the product was collected by centrifuge (6000 rpm, 6 min). The
obtained NPs were further washed by ethanol two times and then
dispersed in hexane. The as-synthesized PtNi NPs were deposited on
high surface area carbon (∼900 m 2 /g) by mixing the NPs with carbon
black (Tanaka, KK) in hexane or chloroform with a 1:1 ratio in mass.
This mixture was sonicated for 1 h and then dried under nitrogen flow.
The organic surfactants were removed by thermal treatment at
150 200 °C in an oxygenated atmosphere. The obtained catalyst is
denoted as “as-prepared PtNi/C”. For the acid treatment, ∼10 mg of the
as-prepared PtNi/C catalyst was mixed with 20 mL of 0.1 M HClO4 that
has been used as electrolyte in electrochemical measurements. After
overnight exposure to the acidic environment, the product was collected
by centrifuge and washed three times by deionized water. Such NPs are
named as “acid treated PtNi/C”. The acid treated PtNi/C was further
annealed at 400 °C to reduce low-coordinated surface sites, and the
obtained catalyst is termed as “acid treated/annealed PtNi/C”.
Microscopic Characterization. TEM images were collected on a
Philips EM 30 (200 kV) equipped with EDX functionality. XRD patterns
were collected on a Rigaku RTP 300 RC machine. STEM and elemental
analysis were carried out on a JEOL 2200FS TEM/STEM with a CEOS
aberration (probe) corrector. The microscope was operated at 200 kV in
HAADF-STEM mode equipped with a Bruker-AXS X-Flash 5030 silicon
drift detector. The probe size was ∼0.7 Å and the probe current was
∼30 pA during HAADF-STEM imaging. When accumulating EDX data,
the probe current was increased to ∼280 pA and the probe size was
∼2 Å. The presented EDX data were confirmed to be from “e-beam
damage-free” particles by comparing STEM images before and after
EDX acquisition.
X-ray Absorption Spectroscopy. X-ray fluorescence spectra of
at the Ni K and Pt L 3 edges were acquired at bending magnet beamline
12-BM-B at the Advanced Photon Source (APS), Argonne National
Laboratory. The incident radiation was filtered by a Si(111) doublecrystal
monochromator (energy resolution ΔE/E = 14.1 10 5 ) with a
double mirror system for focusing and harmonic rejection. 14 All of the
data were taken in fluorescence mode using a 13-element Germanium
array detector (Canberra), which was aligned with the polarization of the
X-ray beam to minimize the elastic scattering intensity. Co and Ge filters
(of 6 absorption length in thickness) were applied in front of the
detector to further reduce the elastic scattering intensity for the Ni K and
Pt L 3 edges, respectively. The Ni K and Pt L 3 edge spectra were
calibrated by defining the zero crossing point of the second derivative
of the XANES spectra for Ni and Pt reference foils as 8333 and 11564 eV,
respectively. The background was subtracted using the AUTOBK
algorithm, 15 and data reduction was performed using Athena from the
IFEFFIT software suite. 16 A scheme of the homemade in situ electrochemical
cell and setup at beamline was shown in the Supporting
Information, Figure S9.
Electrochemical Characterization. The electrochemical measurements
were conducted in a three-compartment electrochemical cell
with a rotational disk electrode (RDE, 6 mm in diameter) setup (Pine)
and a Autolab 302 potentiostat. A saturated Ag/AgCl electrode and a Pt
wire were used as reference and counter electrodes, respectively. 0.1 M
HClO 4 was used as electrolyte. The catalysts were deposited on glassy
carbon electrode substrate and dried in Ar atmosphere without using any
ionomer. The loading was controlled to be 12 μgPt/cm 2 disk for PtNi/C
nanocatalysts. All of the potentials given in the discussion were against
reversible hydrogen electrode (RHE), and the readout currents were
recorded with ohmic iR drop correction during the measurements. 17
’ RESULTS AND DISCUSSION
Pt films of various thicknesses, that is, 1 7 atomic monolayers
(ML), were deposited in a vacuum by sputtering on PtNi
(Pt:Ni = 1:1) substrate and then transferred to an electrochemical
cell for further characterizations (see the Experimental
Section). The as-sputtered Pt films consist of randomly distributed
Pt nanoclusters (

Journal of the American Chemical Society ARTICLE
Figure 1. Electrochemical studies on the Pt thin films deposited over PtNi substrate by RDE: (a) cyclic voltammograms, (b) polarization curves,
and (c) summary of specific activities and corresponding improvement factors (vs polycrystalline Pt surface) for the Pt films of various
thicknesses. Cyclic voltammograms were recorded in Ar saturated 0.1 M HClO4 electrolyte with a sweeping rate of 50 mV/s. Polarization curves
were recorded in the same electrolyte under O2 saturation with a sweep rate of 20 mV/s. Specific activities were presented as kinetic currents
normalized by ECSAs obtained from integrated Hupd, except that for the annealed 3 ML Pt/PtNi surface which was based on COad stripping
polarization curve.
which had confirmed the superior catalytic properties of
systems with 50% of Ni in subsurface layers. Figure 1 summarizes
the results of electrochemical studies for these thin films by
rotating disk electrode (RDE). Cyclic voltammograms (CVs,
Figure 1a) of the as-sputtered films correspond to polycrystalline
Pt (poly-Pt) with similar, but slightly enlarged, underpotentially
deposited hydrogen (Hupd) regions (E < 0.4 V) due to the
rougher surfaces. Consistent with our previous findings, 6 the
onset of Pt OH ad formation has anodic shifts for most of the Pt
films (e5 ML) as compared to poly-Pt (more visible in the CVs
shown in the Supporting Information, Figure S1, with currents
normalized by the electrochemical surface area (ECSA) obtained
from integrated Hupd region). Correspondingly, similar positive
shifts are also present in the polarization curves for the ORR
(Figure 1b). The largest shift of ∼30 mV was obtained for the Pt
films with thicknesses of three atomic layers. Measured specific
activities at 0.95 V, expressed as kinetic current normalized by the
ECSA, show that the thinner films (e3 ML) have improvement
factors of ∼2.5 versus poly-Pt surface, which is in line with the
previous results on polycrystalline Pt3M bulk alloys with the
skeleton type of surfaces. 4b Reduced enhancement was observed
for thicker Pt films, for example, improvement factor of 1.7 for
5MLofPt,whilethespecific activity measured for the 7 ML
film was close to that of poly-Pt. It should be noted here that
for the as-sputtered films,1MLofPtmaynotbeableto
entirely cover Ni atoms in the alloy substrate and protect
them from dissolution, whereas addition of a second and/or
third layer can effectively diminish this process. Along the
same lines, this may also be the reason that the surfaces with 2 or
3 ML of Pt were found to be more active than that with 1 ML.
These findings revealed that bimetallic systems with Pt-skeleton
near-surface formation of up to three atomic layers in thickness
are also capable of efficiently harvesting the beneficial properties
of bimetallic alloys, while protecting the subsurface Ni from
leaching out.
Because the as-sputtered skeleton type of surfaces have abundant
low-coordination sites 4a that are detrimental to the ORR, we have
applied thermal treatment to investigate potential surface restructuring
and further catalytic improvement. A moderate temperature
of ∼400 °C was chosen as it was determined to be optimal for
Pt bimetallic nanocatalysts. 8a In Figure 1a, the CV of annealed
3 ML Pt/PtNi surface is also shown. The suppressed Hupd region
and an even larger positive shift of the Pt OHad peak (Figure S1)
indicates the formation of Pt-skin type of surface, which is smoother
and less oxophilic with significantly reduced number of lowcoordination
surface atoms. 5,6 Additional proof of the transition
toward Pt-skin is provided by the measured boost in specific
activity for the ORR (Figure 1c), reaching an improvement factor
of more than 5 with respect to poly-Pt (Figure 1d). Moreover,
this high catalytic activity was based on the ECSA estimated from
CO stripping polarization curves, not Hupd. The ECSA estimated
from integrated Hupd charge was found to be substantially smaller
than that obtained from the electrochemical oxidation of adsorbed
CO monolayer (Figure S2), which was not observed on unannealed
Pt-skeleton surfaces. Such a difference can only be interpreted
in terms of the altered electronic properties of the Pt-skin surface
14398 dx.doi.org/10.1021/ja2047655 |J. Am. Chem. Soc. 2011, 133, 14396–14403

Journal of the American Chemical Society ARTICLE
Figure 2. Representative transmission electron microscopy (TEM)
images for (a,b) the as-synthesized PtNi NPs, (c) the as-prepared, and
(d) the acid treated/annealed PtNi/C catalysts. (d) X-ray diffraction
(XRD) patterns for the PtNi/C catalysts in comparison with commercial
Pt/C (∼6 nm in particle size).
that have affected the adsorption of hydrogenated species, but
not the Pt COad interaction. 6
The above studies of Pt thin films over PtNi substrate, as well
as the knowledge previously acquired from poly-/single-crystalline
surfaces 4 6 and nanocatalysts, 8a,11b,13 have led us to a novel
approach toward the design and synthesis of Pt bimetallic
catalysts with Pt terminated surfaces. The initial step in this
approach should involve the synthesis of monodisperse and
homogeneous PtNi NPs followed by intentional depletion of
Ni from the surface, producing a skeleton type of surface
structure. The final step is supposed to be the thermal treatment
aimed to induce the transition from Pt-skeleton into Pt-skin type
of structure by surface relaxation and restructuring. For that
purpose, PtNi NPs were synthesized by simultaneous reduction of
platinum acetylacetonate, Pt(acac)2, and nickel acetate, Ni(ac)2,
in an organic solution (see the Experimental Section). 13,18
Figure 2a and b shows representative transmission electron
microscopy (TEM) images of the as-synthesized PtNi NPs
prepared with a molar ratio of 1:2 between the Pt and Ni
precursors. The NP size was controlled to be ∼5 nm 11b with a
very narrow size distribution, as evidenced by the formation of
various types of super lattices after drying the nanoparticle
suspension (in hexane) under ambient conditions. 19 The final
composition was characterized by energy-dispersive X-ray spectroscopy
(EDX), which confirmed an atomic ratio of Pt/Ni ≈ 1/1
(Figure S3). The as-synthesized NPs were incorporated into
carbon black (∼900 m 2 /g) via a colloidal-deposition method,
and the organic surfactants were efficiently removed by thermal
treatment. 11b Such as-prepared PtNi/C catalyst was first treated by
acid to dissolve the surface Ni atoms 13 (Figure S3) and then
annealed at 400 °C. These consecutive treatments were expected
to bring on the Pt-skin type of surface over the substrate with 50%
of Ni, which otherwise would not be possible because complete
segregation of Pt only takes place in Pt3M systems. 5 TEM images
of the acid treated/annealed catalyst do not show notable
changes in morphology (Figure 2c and d), except a slight decrease
(∼0.3 nm) in average particle size (Figures S4). Additionally,
X-ray diffraction (XRD) analysis was used to characterize the
crystal structure of the NPs. As compared to the commercial Pt/C
catalyst (Tanaka, ∼6 nm), both the as-prepared and the acid
treated/annealed PtNi/C systems show a face-centered cubic (fcc)
pattern with noticeable shifts (e.g., ∼1° for (111) peak) toward
high angle (Figure 2e), corresponding to a decrease of lattice
constant due to alloying between Pt and Ni. 20 The XRD pattern of
the acid treated/annealed NPs has sharper peaks as compared to
the as-prepared one, which indicates the increased crystallinity
after annealing. These observations, in addition to the absence of
peaks for separate Pt or Ni phases, show that the bimetallic catalyst
preserved the alloy properties after the applied treatments.
The nanostructures and composition profiles of the PtNi/C
catalysts were characterized by atomically resolved aberrationcorrected
high angle annular dark field-scanning transmission
electron microscopy (HAADF-STEM) in combination with energy
dispersive X-ray spectroscopy (EDX). Figure 3a shows representative
HAADF-STEM images taken along the Æ110æ zone axis of the
as-prepared (left), acid treated (middle), and acid treated/annealed
(right) PtNi/C catalysts, with the intensity profiles along Æ001æ
directions across the single particles shown in Figure 3b. As
compared to the benchmark intensity profiles calculated for ideal
octahedral alloy NPs of the same size and orientation (see the
Supporting Information), NP exposed to acid shows 3 4peakson
each side stretching above the standards, indicating the formation of
a Pt-rich overlayer. This feature was preserved after annealing, but
with 2 3 Pt-rich peaks on each side, corresponding to a reduced Pt
overlayer thickness due to restructuring and smoothing (Figure 3b).
These findings were further confirmed by EDX analysis. By
scanning the e-beam across the particle while simultaneously
analyzing the generated X-rays, composition line profiles were
obtained for the NPs (Figure 3c). It can be seen that the distribution
of Pt and Ni in the as-prepared catalyst was highly intermixed and
the sketched trend lines were almost identical, indicating a homogeneous
alloy nature of the catalyst particles. The treated catalysts
have substantially broader distribution of Pt than Ni, with a
difference of ∼1 nm (at the half-maximum of the trend lines) for
the acid treated and ∼0.6 nm for the acid treated/annealed catalyst.
Hence, both the intensity and the composition line profiles show
that multilayered Pt-rich surface structure was formed by acid
treatment and preserved after annealing.
The microscopic characterizations strongly point toward surface
restructuring in the bimetallic catalyst upon annealing. This was
additionally depicted by atomistic simulation of the nanostructure
evolution subject to the acid and annealing treatments (Figure 3d
for overviews and Figure 3e for cross-section views; see the
Supporting Information for more details). It shows that removing
Ni atoms from the surface led to the formation of a Pt-skeleton
overlayer with a thickness of up to 3 atomic layers. Further relaxation
of low-coordination surface atoms resulted in a multilayered Pt-skin
surface, whereas the PtNi core was barely affected. It is important to
mention that the relaxation process is expected to induce preferential
formation of highly active (111) surface 6 (labeled by arrows in
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Journal of the American Chemical Society ARTICLE
Figure 3. Microscopic characterization and theoretical simulation of nanostructure evolution in the PtNi/C catalysts: (a) Representative high-angle
annular dark-field scanning transmission electron microscopy (HAADF-STEM) images taken along the zone axis Æ110æ, as confirmed by the fast Fourier
transfer (FFT) patterns of the STEM images (shown as insets); (b) background subtracted, normalized intensity line profiles extracted for the regions
marked in (a); (c) composition line profiles (normalized for Pt L peaks) obtained by energy-dispersive X-ray spectroscopy (EDX) with an electron
beam (∼2 Å in spot size) scanning across individual catalyst particles; (d) overview; and (e) cross-section views of the nanostructures depicted by
atomistic particle simulation. The figure is also organized in columns for the as-prepared (left), acid treated (middle), and acid treated/annealed (right)
PtNi/C catalysts, respectively.
Figure 3e), due to the higher atomic coordination, that is, lower
surface energy, of this facet as compared to others.
To gain more insights into the nanostructure evolution,
especially the correlation of surface structures to their electrochemical
properties, we have carried out in situ X-ray absorption
near edge structure (XANES) studies of the nanocatalysts (see the
Experimental Section and Supporting Information for details).
Figure 4a and b shows the normalized XANES spectra collected
at Ni K and Pt L3 edges, under the ORR-relevant conditions
(∼1.0 V). As compared to the spectra of reference foils, Pt and Ni
edge positions were found to correspond to the bulk oxidation
state of zero for both elements in the treated catalysts. 21 It is
intriguing to see that the acid treated catalyst shows higher white
line intensity than does the acid treated/annealed catalyst at the
Ni edge, which is caused by the presence of a small amount of
NiO underlying the highly corrugated Pt-skeleton surface
morphology in the acid treated catalyst, 22 whereas subsurface
Ni in the acid treated/annealed catalyst was well protected.
The distinction in surface structure between the two treated
catalysts is even more visible at the Pt edge, where a slightly lower
white line intensity for the acid treated/annealed catalyst
corresponds to a reduced amount of platinum oxides under the
same conditions, and, more fundamentally, less oxophilic surface
with larger average surface coordination number. 5,22a,23 The
findings from XANES provide additional evidence for the
formation of surface relaxed multilayered Pt-skin in the acid
treated/annealed catalyst and its superiority in protecting the
inner Ni from leaching out.
On the basis of these results, we have managed to achieve the
desirable nanoscale architecture established on PtNi supported
Pt films, that is, multilayered Pt-skin over a particle core with 50%
of Ni. Considering what was revealed from the studies on
extended surfaces, the obtained nanocatalyst should show superior
catalytic performance for the ORR, which was examined by RDE
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Journal of the American Chemical Society ARTICLE
Figure 4. In situ X-ray absorption and electrochemical studies of the PtNi/C catalysts. (a,b) XANES spectra for the PtNi/C catalysts recorded at Ni K
and Pt L3 edges with an electrode potential of 1.0 V, in comparison with standard spectra of Ni, NiO, and Pt. (c) Cyclic voltammograms, (d) polarization
curves, and (e) Tafel plots with the specific activity (jk, kinetic current density) as a function of electrode potential, in comparison with the commercial
Pt/C catalyst. Estimation of ECSA was based on integrated H upd for the Pt/C and acid treated PtNi/C catalysts, and CO ad stripping polarization curve
for the acid treated/annealed PtNi/C catalyst.
measurements (see the Experimental Section). Figure 4c e
summarizes the electrochemical studies for the three types of
nanocatalysts. It can be seen from the voltammograms (Figure 4c)
that the Hupd region (E < 0.4 V) of the acid treated/annealed
catalyst is largely suppressed versus the acid treated sample. On the
positive side of the potential scale, the onset of oxide formation
for the acid treated/annealed catalyst is shifted positively by
about 20 mV versus that for the acid treated catalyst, and more
than 50 mV with respect to Pt/C. Similar shifts are also seen for
the reduction peaks in the cathodic scans. Such peak shifts are
representative of a less oxophilic catalyst surface due to the
formation of multilayered Pt-skin structure, and further corresponding
to remarkable enhancement in the ORR activity as
evidenced by the polarization curves shown in Figure 4d and the
Tafel plots, Figure 4e. These findings are reminiscent of those on
extended surfaces (Figure 1) and from in situ XANES studies
(Figure 4b). At 0.95 V, the specific activity of the acid treated/
annealed PtNi/C reaches 0.85 mA/cm 2 , as compared to
0.35 mA/cm 2 for the acid treated specimen and 0.13 mA/cm 2
for Pt/C. This translates into improvement factors versus Pt/C
of 3 and over 6 for the acid treated and acid treated/annealed
PtNi/C catalysts, respectively, which is also in line with the
results obtained on extended surfaces (Figure 1c). Therefore,
the electrochemical studies of nanocatalysts validated that the
scheme of the near surface architecture established on extended
surfaces had been successfully applied to nanocatalysts by forming
a multilayered Pt-skin surface. Remarkably, the ECSA of this
catalyst obtained from integrated H upd region was over 30%
lower than that from CO stripping (Figure S2), which also confirms
the formation of Pt-skin type of surface in the nanocatalyst.
Moreover, the developed Pt bimetallic catalyst with the
unique nanoscale architecture does not only show enhanced
catalytic activity, but also improved catalyst durability for the
ORR. Figure 5 summarizes the electrochemical results for the
PtNi/C catalysts before and after 4000 potential cycles between
0.6 and 1.1 V at 60 °C. Both the acid treated and the acid treated/
annealed PtNi/C catalysts had minor losses (∼10%) in ECSA
after cycling, in comparison to a substantial drop (∼40%) for Pt/
C (Figure 5a). An additional observation was that the acid
treated/annealed PtNi/C had only 15% loss in specific activity,
in contrast to 57% for the acid treated catalyst and 38% for Pt/C
(Figure 5b). We have also applied in situ XANES to monitor the
catalyst structures in the durability studies (Figure 5d and e, and
more details in the Supporting Information). Not surprisingly,
the acid treated/annealed PtNi/C does not show visible changes,
whereas reduction of absorption at the Ni edge was observed for
the acid treated PtNi/C during and after potential cycling. These
findings are in line with the elemental analysis of the PtNi/C
catalysts after the durability experiments, which indicate no loss
for the Ni content in the acid treated/annealed catalyst in
contrast to the significant loss of Ni in the acid treated catalyst
(Figures S5). It is thus assured that the multilayered Pt-skin
formation has indeed provided complete protection of the Ni
inside the catalyst and enabled the sustained high catalytic
activity under fuel cell operating conditions. On the basis of
that, in addition to diminished number of vulnerable undercoordinated
Pt surface atoms after annealing, multilayered
Pt-skin formation is also thick enough to protect subsurface
Ni from dissolution that otherwise occurs through the placeexchange
mechanism 12a (Figures 5 and S5). At the same time,
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Journal of the American Chemical Society ARTICLE
Figure 5. Summary of electrochemical durability studies obtained by RDE before and after 4000 potential cycles between 0.6 and 1.1 V for the Pt/C
and PtNi/C catalysts in 0.1 M HClO4 at 0.95 V and 60 °C: (a) specific surface area, (b) specific activity, and (c) mass activity. Activity improvement
factors versus Pt/C before and after cycling were also shown for specific and mass activities in (b) and (c). Parts (d) and (e) show the XANES spectra
recorded for the acid treated and acid treated/annealed PtNi/C catalysts at Ni K edge, at 1.0 V before and after potential cycling. Estimation of ECSA
was based on integrated Hupd for the Pt/C and acid treated PtNi/C catalysts, and COad stripping polarization curve for the acid treated/annealed
PtNi/C catalyst.
the multilayered Pt-skin is thin enough to maintain typical skin-like
properties (discussed above), which originates from altered
electronic structures due to the presence of a desirable amount of
subsurface Ni. As a result, the PtNi/C catalyst with multilayered
Pt-skin surfaces exhibited improvement factors in mass activity of
more than 1 order of magnitude after elongated potential cycling
versus the Pt/C catalyst (Figure 5c).
’ SUMMARY
We have demonstrated the design and synthesis of an
advanced Pt bimetallic catalyst, which simultaneously achieves
high catalytic activity and superior durability for the ORR. The
developed catalyst contains a unique nanoscale architecture with
a PtNi core of 50 at% Ni and a multilayered Pt-skin surfaces. This
structure was built up through synergistic studies of extended
surfaces and nanocatalysts, with critical parameters such as
particle size, thermal treatment, particle sintering, alloy composition,
and elemental composition profile integrally designed and
optimized. Delicate structure function correlation in the bimetallic
electrocatalysts with composite nanostructures has been
comprehensively resolved by employing state-of-the-art electron
microscopy and in situ X-ray spectroscopy characterization. Our
findings have immense implications for the development of
heterogeneous catalysts and nanostructure engineering toward
advanced functional materials.
’ ASSOCIATED CONTENT
bS Supporting Information. Additional material characterization
and theoretical analysis. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
vrstamenkovic@anl.gov
’ ACKNOWLEDGMENT
This work was conducted at Argonne National Laboratory, a
U.S. Department of Energy, Office of Science Laboratory,
operated by UChicago Argonne, LLC, under contract no. DE-
AC02-06CH11357. It was sponsored by the U.S. Department of
Energy, Office of Energy Efficiency and Renewable Energy, Fuel
Cell Technologies Program. Microscopy research was conducted
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Journal of the American Chemical Society ARTICLE
at the Electron Microscopy Center for Materials Research at
Argonne, and ORNL’s SHaRE User Facility sponsored by the
Scientific User Facilities Division, Office of Basic Energy
Sciences, the U.S. Department of Energy. XANES were accomplished
at the Advanced Photon Source at Argonne. We thank
Dr. Cindy Chaffee for the help on setup at APS, and Dr. Sonke
Seifert and Byeongdu Lee from APS for valuable discussion on
X-ray absorption experiments.
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1 Design and Synthesis of Bimetallic Electrocatalyst with Multilayered
2 Pt-Skin Surfaces
3 Chao Wang, † Miaofang Chi, ‡ Dongguo Li, †,§ Dusan Strmcnik, † Dennis van der Vliet, † Guofeng Wang, ||
4 Vladimir Komanicky, † Kee-Chul Chang, † Arvydas P. Paulikas, † Dusan Tripkovic, † John Pearson, †
5 KarrenL.More, ‡ Nenad M. Markovic, † and Vojislav R. Stamenkovic* ,†
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†
Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
‡
Division of Material Science and Technology, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
§
Department of Chemistry, Brown University, Providence, RI 02912, United States
Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
)
10 bS Supporting Information
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ABSTRACT: Advancement in heterogeneous catalysis relies
on the capability of altering material structures at the nanoscale,
and that is particularly important for the development of highly
active electrocatalysts with uncompromised durability. Here, we
report the design and synthesis of a Pt-bimetallic catalyst with
multilayered Pt-skin surface, which shows superior electrocatalytic
performance for the oxygen reduction reaction (ORR).
This novel structure was first established on thin film extended
surfaces with tailored composition profiles and then implemented
in nanocatalysts by organic solution synthesis. Electrochemical
studies for the ORR demonstrated that after prolonged
exposure to reaction conditions, the Pt-bimetallic catalyst with
multilayered Pt-skin surface exhibited an improvement factor of more than 1 order of magnitude in activity versus conventional Pt
catalysts. The substantially enhanced catalytic activity and durability indicate great potential for improving the material properties by
fine-tuning of the nanoscale architecture.
27 ’ INTRODUCTION
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28 The foreground of sustainable energy is built up on a renew-
29 able and environmentally compatible scheme of chemical-electrical
30 energy conversion. One of the key processes for such energy
31 conversion is the electrocatalytic reduction of oxygen, the
32 cathode reaction in fuel cells 1 and metal air batteries, 2 where
33 an electrocatalyst is used to accelerate the course of ORR.
34 Current electrocatalysts used for this reaction are typically in
35 the form of dispersed Pt nanoparticles (NPs) on amorphous
36 high-surface-area carbon. Considering the high cost and limited
37 resource of Pt, large-scale applications of these renewable energy
38 technologies demand substantial improvement of the catalyst
39 performance so that the amount of Pt needed can be significantly
40 reduced. For example, a 5-fold improvement of catalytic activity
41 for the ORR is required for the commercial implementation of
42 fuel cell technology in transportation. 3
43 Our recent work on well-defined extended surfaces has shown
44 that high catalytic activity for the ORR can be achieved on
45 Pt bimetallic alloys (Pt 3M, M = Fe, Co, Ni, etc.), due to the
46 altered electronic structure of the Pt topmost layer and hence
47 reduced adsorption of oxygenated spectator species (e.g., OH )
48 on the surface. 4 It was also found that in an acidic electrochemical
49 environment the non-noble 3d transition metals are dissolved
from the near-surface layers, which leads to the formation of
Pt-skeleton surfaces. Moreover, the thermal treatment of Pt3M alloys in ultra high vacuum (UHV) has been shown to induce
segregation of Pt and formation of a distinctive topmost layer
that was termed Pt-skin surface. However, the same treatment
did not cause Pt to segregate over PtM alloys with high content
(g50%) of non-Pt elements. 4b,5 Recently, we further demonstrated
the surfacing of an ordered Pt(111)-skin over Pt3Ni(111) single crystal with 50% of Ni in the subsurface layer. This unique
nanosegregated composition profile was found to be responsible
for the dramatically enhanced ORR activity. 6
On the basis of these findings, it could be envisioned that the
most advantageous nanoscale architecture for a bimetallic electrocatalyst
would correspond to the segregated Pt-skin composition
profile established on extended surfaces. A lot of effort has
thus been dedicated, 7 but it still remains elusive, to finely tune the
Pt-bimetallic nanostructure to achieve this desirable surface
structure and composition profile. Major obstacles reside not
only in the difficulty to manipulate elemental distribution at the
nanoscale, but also in the fundamental differences in atomic
Received: May 24, 2011
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70 structures, electronic properties, and catalytic performance between
71 extended surfaces and confined nanomaterials. For example, in
72 an attempt to induce surface segregation, high-temperature (>600 °C)
73 annealing is typically applied for Pt-based alloy nanocatalysts.
74 While improvement in specific activity is obtained, such treatment
75 usually causes particle sintering and loss of electrochemical
76 surface area (ECSA). 7b,8 Besides that, the surface coordination
77 of nanomaterials is quite different from that of bulk materials;
78 that is, the surface of NPs is rich in corner and edge sites, which
79 have a smaller coordination number than the atoms on long-
80 range ordered terraces of extended surfaces. 9 These low-coordi-
81 nation surface atoms are considered as preferential sites for the
82 adsorption of oxygenated spectator species (e.g., OH ) 10 and
83 thus become blocked for adsorption of molecular oxygen and
84 inactive for the ORR. 9b,11 Additionally, due to strong Pt O
85 interaction, these sites are more vulnerable for migration and
86 dissolution, resulting in poor durability and fast decay of the
87 catalyst. 12 The latter effect is even more pronounced in Pt bi-
88 metallic systems, considering that more undercoordinated atoms
89 are present on the skeleton surfaces formed after the depletion of
90 nonprecious metals from near-surface regions. 4a,13 Therefore, a
91 systematic approach with all of these factors integrally considered
92 becomes necessary to pursue the design and synthesis of
93 advanced bimetallic catalysts.
94 Our focus in this study has been placed on the fine-tuning of
95 Pt bimetallic nanostructure aiming to achieve the advantageous
96 Pt-skin surface structure and composition profile established on
97 extended surfaces. We started with Pt thin films of controlled
98 thickness deposited over PtNi substrate to explore the correlation
99 between the surface composition profile and catalytic performance.
100 These findings were then applied for guiding the synthesis of
101 nanocatalysts with the optimized structure. The outcome of such
102 effort is an advanced Pt bimetallic catalyst with altered nano-
103 scale architecture that is highly active and durable for the ORR.
104 ’ EXPERIMENTAL SECTION
105 Thin Film Preparation. Pt films were deposited at room tempera-
106 ture on PtNi substrates (6 mm in diameter), which were set 125 mm
107 away from DC sputter magnetrons in 4 mTorr argon gas (base vacuum
108 1 10 7 Torr). The Pt source rate (0.32 Å/s) was determined by quartz
109 crystal microbalance, and an exposure duration of 7.0 s was calibrated for
110 the nominal thickness of 2.2 2.3 Å for a monolayer of Pt. The film
111 thickness was derived from the exposure time of computer-controlled
112 shutters during sputtering.
113 NP and Catalyst Synthesis. In a typical synthesis of PtNi NPs,
114 0.67 mmol of Ni(ac)2 3 4H2O was dissolved in 20 mL of diphenyl either
115 in the presence of 0.4 mL of oleylamine and 0.4 mL of oleic acid.
116 0.33 mmol of 1,2-tetradodecanediol was added, and the formed solution
117 washeatedto80°C under Ar flow. After a transparent solution formed, the
118 temperature was further raised to 200 °C, where 0.33 mmol of Pt(acac)2
119 dissolved in 1.5 mL of dichlorobenzene was injected. The solution was
120 kept at this temperature for 1 h and then cooled to room temperature.
121 An amount of 60 mL of ethanol was added to precipitate the NPs, and
122 the product was collected by centrifuge (6000 rpm, 6 min). The
123 obtained NPs were further washed by ethanol two times and then
124 dispersed in hexane. The as-synthesized PtNi NPs were deposited on
125 high surface area carbon (∼900 m 2 /g) by mixing the NPs with carbon
126 black (Tanaka, KK) in hexane or chloroform with a 1:1 ratio in mass.
127 This mixture was sonicated for 1 h and then dried under nitrogen flow.
128 The organic surfactants were removed by thermal treatment at
129 150 200 °C in an oxygenated atmosphere. The obtained catalyst is
130 denoted as “as-prepared PtNi/C”. For the acid treatment, ∼10 mg of the
as-prepared PtNi/C catalyst was mixed with 20 mL of 0.1 M HClO4 that
has been used as electrolyte in electrochemical measurements. After
overnight exposure to the acidic environment, the product was collected
by centrifuge and washed three times by deionized water. Such NPs are
named as “acid treated PtNi/C”. The acid treated PtNi/C was further
annealed at 400 °C to reduce low-coordinated surface sites, and the
obtained catalyst is termed as “acid treated/annealed PtNi/C”.
Microscopic Characterization. TEM images were collected on a
Philips EM 30 (200 kV) equipped with EDX functionality. XRD patterns
were collected on a Rigaku RTP 300 RC machine. STEM and elemental
analysis were carried out on a JEOL 2200FS TEM/STEM with a CEOS
aberration (probe) corrector. The microscope was operated at 200 kV in
HAADF-STEM mode equipped with a Bruker-AXS X-Flash 5030 silicon
drift detector. The probe size was ∼0.7 Å and the probe current was
∼30 pA during HAADF-STEM imaging. When accumulating EDX data,
the probe current was increased to ∼280 pA and the probe size was
∼2 Å. The presented EDX data were confirmed to be from “e-beam
damage-free” particles by comparing STEM images before and after
EDX acquisition.
X-ray Absorption Spectroscopy. X-ray fluorescence spectra of
at the Ni K and Pt L3 edges were acquired at bending magnet beamline
12-BM-B at the Advanced Photon Source (APS), Argonne National
Laboratory. The incident radiation was filtered by a Si(111) doublecrystal
monochromator (energy resolution ΔE/E = 14.1 10 5 ) with a
double mirror system for focusing and harmonic rejection. 14 All of the
data were taken in fluorescence mode using a 13-element Germanium
array detector (Canberra), which was aligned with the polarization of the
X-ray beam to minimize the elastic scattering intensity. Co and Ge filters
(of 6 absorption length in thickness) were applied in front of the
detector to further reduce the elastic scattering intensity for the Ni K and
Pt L3 edges, respectively. The Ni K and Pt L3 edge spectra were
calibrated by defining the zero crossing point of the second derivative
of the XANES spectra for Ni and Pt reference foils as 8333 and 11564 eV,
respectively. The background was subtracted using the AUTOBK
algorithm, 15 and data reduction was performed using Athena from the
IFEFFIT software suite. 16 A scheme of the homemade in situ electrochemical
cell and setup at beamline was shown in the Supporting
Information, Figure S9.
Electrochemical Characterization. The electrochemical measurements
were conducted in a three-compartment electrochemical cell
with a rotational disk electrode (RDE, 6 mm in diameter) setup (Pine)
and a Autolab 302 potentiostat. A saturated Ag/AgCl electrode and a Pt
wire were used as reference and counter electrodes, respectively. 0.1 M
HClO4 was used as electrolyte. The catalysts were deposited on glassy
carbon electrode substrate and dried in Ar atmosphere without using any
ionomer. The loading was controlled to be 12 μgPt/cm 2 disk for PtNi/C
nanocatalysts. All of the potentials given in the discussion were against
reversible hydrogen electrode (RHE), and the readout currents were
recorded with ohmic iR drop correction during the measurements. 17
’ RESULTS AND DISCUSSION
Pt films of various thicknesses, that is, 1 7 atomic monolayers
(ML), were deposited in a vacuum by sputtering on PtNi
(Pt:Ni = 1:1) substrate and then transferred to an electrochemical
cell for further characterizations (see the Experimental
Section). The as-sputtered Pt films consist of randomly distributed
Pt nanoclusters (

Journal of the American Chemical Society ARTICLE
Figure 1. Electrochemical studies on the Pt thin films deposited over PtNi substrate by RDE: (a) cyclic voltammograms, (b) polarization curves,
and (c) summary of specific activities and corresponding improvement factors (vs polycrystalline Pt surface) for the Pt films of various
thicknesses. Cyclic voltammograms were recorded in Ar saturated 0.1 M HClO4 electrolyte with a sweeping rate of 50 mV/s. Polarization curves
were recorded in the same electrolyte under O2 saturation with a sweep rate of 20 mV/s. Specific activities were presented as kinetic currents
normalized by ECSAs obtained from integrated Hupd, except that for the annealed 3 ML Pt/PtNi surface which was based on COad stripping
polarization curve.
193 which had confirmed the superior catalytic properties of
F1 194 systems with 50% of Ni in subsurface layers. Figure 1 summarizes
195 the results of electrochemical studies for these thin films by
196 rotating disk electrode (RDE). Cyclic voltammograms (CVs,
197 Figure 1a) of the as-sputtered films correspond to polycrystalline
198 Pt (poly-Pt) with similar, but slightly enlarged, underpotentially
199
200
deposited hydrogen (Hupd) regions (E < 0.4 V) due to the
rougher surfaces. Consistent with our previous findings, 6 the
201
202
onset of Pt OHad formation has anodic shifts for most of the Pt
films (e5 ML) as compared to poly-Pt (more visible in the CVs
203 shown in the Supporting Information, Figure S1, with currents
204 normalized by the electrochemical surface area (ECSA) obtained
205
206
from integrated Hupd region). Correspondingly, similar positive
shifts are also present in the polarization curves for the ORR
207 (Figure 1b). The largest shift of ∼30 mV was obtained for the Pt
208 films with thicknesses of three atomic layers. Measured specific
209 activities at 0.95 V, expressed as kinetic current normalized by the
210 ECSA, show that the thinner films (e3 ML) have improvement
211 factors of ∼2.5 versus poly-Pt surface, which is in line with the
212
213
previous results on polycrystalline Pt3M bulk alloys with the
skeleton type of surfaces. 4b Reduced enhancement was observed
214 for thicker Pt films, for example, improvement factor of 1.7 for
215 5MLofPt,whilethespecificactivity measured for the 7 ML
216 film was close to that of poly-Pt. It should be noted here that
217 for the as-sputtered films,1MLofPtmaynotbeableto
218 entirely cover Ni atoms in the alloy substrate and protect
219 them from dissolution, whereas addition of a second and/or
220 third layer can effectively diminish this process. Along the
same lines, this may also be the reason that the surfaces with 2 or
3 ML of Pt were found to be more active than that with 1 ML.
These findings revealed that bimetallic systems with Pt-skeleton
near-surface formation of up to three atomic layers in thickness
are also capable of efficiently harvesting the beneficial properties
of bimetallic alloys, while protecting the subsurface Ni from
leaching out.
Because the as-sputtered skeleton type of surfaces have abundant
low-coordination sites 4a that are detrimental to the ORR, we have
applied thermal treatment to investigate potential surface restructuring
and further catalytic improvement. A moderate temperature
of ∼400 °C was chosen as it was determined to be optimal for
Pt bimetallic nanocatalysts. 8a In Figure 1a, the CV of annealed
3 ML Pt/PtNi surface is also shown. The suppressed Hupd region
and an even larger positive shift of the Pt OHad peak (Figure S1)
indicates the formation of Pt-skin type of surface, which is smoother
and less oxophilic with significantly reduced number of lowcoordination
surface atoms. 5,6 Additional proof of the transition
toward Pt-skin is provided by the measured boost in specific
activity for the ORR (Figure 1c), reaching an improvement factor
of more than 5 with respect to poly-Pt (Figure 1d). Moreover,
this high catalytic activity was based on the ECSA estimated from
CO stripping polarization curves, not Hupd. The ECSA estimated
from integrated Hupd charge was found to be substantially smaller
than that obtained from the electrochemical oxidation of adsorbed
CO monolayer (Figure S2), which was not observed on unannealed
Pt-skeleton surfaces. Such a difference can only be interpreted
in terms of the altered electronic properties of the Pt-skin surface
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Journal of the American Chemical Society ARTICLE
Figure 2. Representative transmission electron microscopy (TEM)
images for (a,b) the as-synthesized PtNi NPs, (c) the as-prepared, and
(d) the acid treated/annealed PtNi/C catalysts. (d) X-ray diffraction
(XRD) patterns for the PtNi/C catalysts in comparison with commercial
Pt/C (∼6 nm in particle size).
249 that have affected the adsorption of hydrogenated species, but
250 not the Pt CO ad interaction. 6
251 The above studies of Pt thin films over PtNi substrate, as well
252
253
as the knowledge previously acquired from poly-/single-crystalline
surfaces 4 6 and nanocatalysts, 8a,11b,13 have led us to a novel
254 approach toward the design and synthesis of Pt bimetallic
255 catalysts with Pt terminated surfaces. The initial step in this
256 approach should involve the synthesis of monodisperse and
257 homogeneous PtNi NPs followed by intentional depletion of
258 Ni from the surface, producing a skeleton type of surface
259 structure. The final step is supposed to be the thermal treatment
260 aimed to induce the transition from Pt-skeleton into Pt-skin type
261 of structure by surface relaxation and restructuring. For that
262 purpose, PtNi NPs were synthesized by simultaneous reduction of
263
264
platinum acetylacetonate, Pt(acac) 2, and nickel acetate, Ni(ac) 2,
in an organic solution (see the Experimental Section). 13,18
F2 265 Figure 2a
and b shows representative transmission electron
266 microscopy (TEM) images of the as-synthesized PtNi NPs
267
268
prepared with a molar ratio of 1:2 between the Pt and Ni
precursors. The NP size was controlled to be ∼5 nm 11b with a
269 very narrow size distribution, as evidenced by the formation of
270
271
various types of super lattices after drying the nanoparticle
suspension (in hexane) under ambient conditions. 19 The final
272 composition was characterized by energy-dispersive X-ray spec-
273 troscopy (EDX), which confirmed an atomic ratio of Pt/Ni ≈ 1/1
274
275
(Figure S3). The as-synthesized NPs were incorporated into
carbon black (∼900 m 2 /g) via a colloidal-deposition method,
276
277
and the organic surfactants were efficiently removed by thermal
treatment. 11b 278
Such as-prepared PtNi/C catalyst was first treated
by acid to dissolve the surface Ni atoms 13 (Figure S3) and then
279 annealed at 400 °C. These consecutive treatments were expected
to bring on the Pt-skin type of surface over the substrate with 280
50% of Ni, which otherwise would not be possible because
complete segregation of Pt only takes place in Pt3M systems.
281
282
5
TEM images of the acid treated/annealed catalyst do not show 283
notable changes in morphology (Figure 2c and d), except a slight 284
decrease (∼0.3 nm) in average particle size (Figures S4). 285
Additionally, X-ray diffraction (XRD) analysis was used to 286
characterize the crystal structure of the NPs. As compared to 287
the commercial Pt/C catalyst (Tanaka, ∼6 nm), both the as- 288
prepared and the acid treated/annealed PtNi/C systems show a 289
face-centered cubic (fcc) pattern with noticeable shifts (e.g., ∼1° 290
for (111) peak) toward high angle (Figure 2e), corresponding to
a decrease of lattice constant due to alloying between Pt and Ni.
291
292
20
The XRD pattern of the acid treated/annealed NPs has sharper 293
peaks as compared to the as-prepared one, which indicates the 294
increased crystallinity after annealing. These observations, in 295
addition to the absence of peaks for separate Pt or Ni phases, 296
show that the bimetallic catalyst preserved the alloy properties 297
after the applied treatments.
298
The nanostructures and composition profiles of the PtNi/C 299
catalysts were characterized by atomically resolved aberration- 300
corrected high angle annular dark field-scanning transmission 301
electron microscopy (HAADF-STEM) in combination with 302
energy dispersive X-ray spectroscopy (EDX). Figure 3a shows 303 F3
representative HAADF-STEM images taken along the Æ110æ 304
zone axis of the as-prepared (left), acid treated (middle), and 305
acid treated/annealed (right) PtNi/C catalysts, with the intensity 306
profiles along Æ001æ directions across the single particles shown 307
in Figure 3b. As compared to the benchmark intensity profiles 308
calculated for ideal octahedral alloy NPs of the same size and 309
orientation (see the Supporting Information), NP exposed to 310
acid shows 3 4 peaks on each side stretching above the 311
standards, indicating the formation of a Pt-rich overlayer. This 312
feature was preserved after annealing, but with 2 3 Pt-rich peaks 313
on each side, corresponding to a reduced Pt overlayer thickness 314
due to restructuring and smoothing (Figure 3b). These findings 315
were further confirmed by EDX analysis. By scanning the e-beam 316
across the particle while simultaneously analyzing the generated 317
X-rays, composition line profiles were obtained for the NPs 318
(Figure 3c). It can be seen that the distribution of Pt and Ni in the 319
as-prepared catalyst was highly intermixed and the sketched 320
trend lines were almost identical, indicating a homogeneous 321
alloy nature of the catalyst particles. The treated catalysts have 322
substantially broader distribution of Pt than Ni, with a difference 323
of ∼1 nm (at the half-maximum of the trend lines) for the acid 324
treated and ∼0.6 nm for the acid treated/annealed catalyst. 325
Hence, both the intensity and the composition line profiles show 326
that multilayered Pt-rich surface structure was formed by acid 327
treatment and preserved after annealing.
328
The microscopic characterizations strongly point toward sur- 329
face restructuring in the bimetallic catalyst upon annealing. This 330
was additionally depicted by atomistic simulation of the nano- 331
structure evolution subject to the acid and annealing treatments 332
(Figure 3d for overviews and Figure 3e for cross-section views; 333
see the Supporting Information for more details). It shows 334
that removing Ni atoms from the surface led to the formation 335
of a Pt-skeleton overlayer with a thickness of up to 3 atomic 336
layers. Further relaxation of low-coordination surface atoms 337
resulted in a multilayered Pt-skin surface, whereas the PtNi core 338
was barely affected. It is important to mention that the relaxation 339
process is expected to induce preferential formation of highly
active (111) surface
340
341
6 (labeled by arrows in Figure 3e), due to the
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Journal of the American Chemical Society ARTICLE
Figure 3. Microscopic characterization and theoretical simulation of nanostructure evolution in the PtNi/C catalysts: (a) Representative high-angle
annular dark-field scanning transmission electron microscopy (HAADF-STEM) images taken along the zone axis Æ110æ, as confirmed by the fast Fourier
transfer (FFT) patterns of the STEM images (shown as insets); (b) background subtracted, normalized intensity line profiles extracted for the regions
marked in (a); (c) composition line profiles (normalized for Pt L peaks) obtained by energy-dispersive X-ray spectroscopy (EDX) with an electron
beam (∼2 Å in spot size) scanning across individual catalyst particles; (d) overview; and (e) cross-section views of the nanostructures depicted by
atomistic particle simulation. The figure is also organized in columns for the as-prepared (left), acid treated (middle), and acid treated/annealed (right)
PtNi/C catalysts, respectively.
342 higher atomic coordination, that is, lower surface energy, of this
343 facet as compared to others.
344 To gain more insights into the nanostructure evolution,
345 especially the correlation of surface structures to their electro-
346 chemical properties, we have carried out in situ X-ray absorption
347 near edge structure (XANES) studies of the nanocatalysts (see the
348 Experimental Section and Supporting Information for details).
F4 349 Figure 4a
and b shows the normalized XANES spectra collected
350
351
at Ni K and Pt L3 edges, under the ORR-relevant conditions
(∼1.0 V). As compared to the spectra of reference foils, Pt and Ni
352
353
edge positions were found to correspond to the bulk oxidation
state of zero for both elements in the treated catalysts. 21 It is
354 intriguing to see that the acid treated catalyst shows higher white
355 line intensity than does the acid treated/annealed catalyst at the
356 Ni edge, which is caused by the presence of a small amount of
357
358
NiO underlying the highly corrugated Pt-skeleton surface
morphology in the acid treated catalyst, 22 whereas subsurface
Ni in the acid treated/annealed catalyst was well protected.
The distinction in surface structure between the two treated
catalysts is even more visible at the Pt edge, where a slightly lower
white line intensity for the acid treated/annealed catalyst
corresponds to a reduced amount of platinum oxides under the
same conditions, and, more fundamentally, less oxophilic surface
with larger average surface coordination number. 5,22a,23 The
findings from XANES provide additional evidence for the
formation of surface relaxed multilayered Pt-skin in the acid
treated/annealed catalyst and its superiority in protecting the
inner Ni from leaching out.
On the basis of these results, we have managed to achieve the
desirable nanoscale architecture established on PtNi supported
Pt films, that is, multilayered Pt-skin over a particle core with 50%
of Ni. Considering what was revealed from the studies on
extended surfaces, the obtained nanocatalyst should show superior
catalytic performance for the ORR, which was examined by RDE
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Journal of the American Chemical Society ARTICLE
Figure 4. In situ X-ray absorption and electrochemical studies of the PtNi/C catalysts. (a,b) XANES spectra for the PtNi/C catalysts recorded at Ni K
and Pt L3 edges with an electrode potential of 1.0 V, in comparison with standard spectra of Ni, NiO, and Pt. (c) Cyclic voltammograms, (d) polarization
curves, and (e) Tafel plots with the specific activity (jk, kinetic current density) as a function of electrode potential, in comparison with the commercial
Pt/C catalyst. Estimation of ECSA was based on integrated H upd for the Pt/C and acid treated PtNi/C catalysts, and CO ad stripping polarization curve
for the acid treated/annealed PtNi/C catalyst.
376 measurements (see the Experimental Section). Figure 4c e
377 summarizes the electrochemical studies for the three types of
378 nanocatalysts. It can be seen from the voltammograms (Figure 4c)
379 that the Hupd region (E < 0.4 V) of the acid treated/annealed
380 catalyst is largely suppressed versus the acid treated sample. On the
381 positive side of the potential scale, the onset of oxide formation
382 for the acid treated/annealed catalyst is shifted positively by
383 about 20 mV versus that for the acid treated catalyst, and more
384 than 50 mV with respect to Pt/C. Similar shifts are also seen for
385 the reduction peaks in the cathodic scans. Such peak shifts are
386 representative of a less oxophilic catalyst surface due to the
387 formation of multilayered Pt-skin structure, and further corre-
388 sponding to remarkable enhancement in the ORR activity as
389 evidenced by the polarization curves shown in Figure 4d and the
390 Tafel plots, Figure 4e. These findings are reminiscent of those on
391 extended surfaces (Figure 1) and from in situ XANES studies
392 (Figure 4b). At 0.95 V, the specific activity of the acid treated/
393 annealed PtNi/C reaches 0.85 mA/cm 2 , as compared to
394 0.35 mA/cm 2 for the acid treated specimen and 0.13 mA/cm 2
395 for Pt/C. This translates into improvement factors versus Pt/C
396 of 3 and over 6 for the acid treated and acid treated/annealed
397 PtNi/C catalysts, respectively, which is also in line with the
398 results obtained on extended surfaces (Figure 1c). Therefore,
399 the electrochemical studies of nanocatalysts validated that the
400 scheme of the near surface architecture established on extended
401 surfaces had been successfully applied to nanocatalysts by form-
402 ing a multilayered Pt-skin surface. Remarkably, the ECSA of this
403 catalyst obtained from integrated H upd region was over 30%
404 lower than that from CO stripping (Figure S2), which also con-
405 firms the formation of Pt-skin type of surface in the nanocatalyst.
Moreover, the developed Pt bimetallic catalyst with the 406
unique nanoscale architecture does not only show enhanced 407
catalytic activity, but also improved catalyst durability for the 408
ORR. Figure 5 summarizes the electrochemical results for the 409 F5
PtNi/C catalysts before and after 4000 potential cycles between 410
0.6 and 1.1 V at 60 °C. Both the acid treated and the acid treated/ 411
annealed PtNi/C catalysts had minor losses (∼10%) in ECSA 412
after cycling, in comparison to a substantial drop (∼40%) for Pt/ 413
C (Figure 5a). An additional observation was that the acid 414
treated/annealed PtNi/C had only 15% loss in specific activity, 415
in contrast to 57% for the acid treated catalyst and 38% for Pt/C 416
(Figure 5b). We have also applied in situ XANES to monitor the 417
catalyst structures in the durability studies (Figure 5d and e, and 418
more details in the Supporting Information). Not surprisingly, 419
the acid treated/annealed PtNi/C does not show visible changes, 420
whereas reduction of absorption at the Ni edge was observed for 421
the acid treated PtNi/C during and after potential cycling. These 422
findings are in line with the elemental analysis of the PtNi/C 423
catalysts after the durability experiments, which indicate no loss 424
for the Ni content in the acid treated/annealed catalyst in 425
contrast to the significant loss of Ni in the acid treated catalyst 426
(Figures S5). It is thus assured that the multilayered Pt-skin 427
formation has indeed provided complete protection of the Ni 428
inside the catalyst and enabled the sustained high catalytic 429
activity under fuel cell operating conditions. On the basis of 430
that, in addition to diminished number of vulnerable under- 431
coordinated Pt surface atoms after annealing, multilayered 432
Pt-skin formation is also thick enough to protect subsurface 433
Ni from dissolution that otherwise occurs through the placeexchange
mechanism
434
435
12a (Figures 5 and S5). At the same time,
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Figure 5. Summary of electrochemical durability studies obtained by RDE before and after 4000 potential cycles between 0.6 and 1.1 V for the Pt/C
and PtNi/C catalysts in 0.1 M HClO4 at 0.95 V and 60 °C: (a) specific surface area, (b) specific activity, and (c) mass activity. Activity improvement
factors versus Pt/C before and after cycling were also shown for specific and mass activities in (b) and (c). Parts (d) and (e) show the XANES spectra
recorded for the acid treated and acid treated/annealed PtNi/C catalysts at Ni K edge, at 1.0 V before and after potential cycling. Estimation of ECSA
was based on integrated Hupd for the Pt/C and acid treated PtNi/C catalysts, and COad stripping polarization curve for the acid treated/annealed
PtNi/C catalyst.
436 the multilayered Pt-skin is thin enough to maintain typical skin-like
437 properties (discussed above), which originates from altered
438 electronic structures due to the presence of a desirable amount of
439 subsurface Ni. As a result, the PtNi/C catalyst with multilayered
440 Pt-skin surfaces exhibited improvement factors in mass activity of
441 more than 1 order of magnitude after elongated potential cycling
442 versus the Pt/C catalyst (Figure 5c).
443 ’ SUMMARY
444 We have demonstrated the design and synthesis of an
445 advanced Pt bimetallic catalyst, which simultaneously achieves
446 high catalytic activity and superior durability for the ORR. The
447 developed catalyst contains a unique nanoscale architecture with
448 a PtNi core of 50 at% Ni and a multilayered Pt-skin surfaces. This
449 structure was built up through synergistic studies of extended
450 surfaces and nanocatalysts, with critical parameters such as
451 particle size, thermal treatment, particle sintering, alloy composi-
452 tion, and elemental composition profile integrally designed and
453 optimized. Delicate structure function correlation in the bime-
454 tallic electrocatalysts with composite nanostructures has been
455 comprehensively resolved by employing state-of-the-art electron
456 microscopy and in situ X-ray spectroscopy characterization. Our
457 findings have immense implications for the development of
heterogeneous catalysts and nanostructure engineering toward
advanced functional materials.
’ ASSOCIATED CONTENT
bS Supporting Information. Additional material characterization
and theoretical analysis. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
vrstamenkovic@anl.gov
’ ACKNOWLEDGMENT
This work was conducted at Argonne National Laboratory, a
U.S. Department of Energy, Office of Science Laboratory,
operated by UChicago Argonne, LLC, under contract no. DE-
AC02-06CH11357. It was sponsored by the U.S. Department of
Energy, Office of Energy Efficiency and Renewable Energy, Fuel
Cell Technologies Program. Microscopy research was conducted
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474 at the Electron Microscopy Center for Materials Research at
475 Argonne, and ORNL’s SHaRE User Facility sponsored by the
476 Scientific User Facilities Division, Office of Basic Energy
477 Sciences, the U.S. Department of Energy. XANES were accom-
478 plished at the Advanced Photon Source at Argonne. We thank
479 Dr. Cindy Chaffee for the help on setup at APS, and Dr. Sonke
480 Seifert and Byeongdu Lee from APS for valuable discussion on
481 X-ray absorption experiments.
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Origin of Anomalous Activities for Electrocatalysts in Alkaline
Electrolytes
Ram Subbaraman, † N. Danilovic, † P. P. Lopes, †,‡ D. Tripkovic, † D. Strmcnik, † V. R. Stamenkovic, †
and N. M. Markovic* ,†
† Materials Science Division, Argonne National Laboratory, Lemont, Illinois 60439 United States
‡ Instituto de Química de Saõ Carlos/USP, C.P.780, CEP 13560-970, Saõ Carlos, SP, Brazil
*S Supporting Information
ABSTRACT: Pt extended surfaces and nanoparticle electrodes
are used to understand the origin of anomalous activities
for electrocatalytic reactions in alkaline electrolytes as a
function of cycling/time. Scanning tunneling microscopy
(STM) of the surfaces before and after cycling in alkaline
electrolytes was used to understand the morphology of the
impurities and their impact on the catalytic sites. The nature of
the contaminant species is identified as 3d-transition metal
cations, and the formation of hydr(oxy)oxides of these
elements is established as the main reason for the observed
behavior. We find that, while for the oxygen reduction reaction
(ORR) and the hydrogen oxidation reaction (HOR) the blocking of the sites by the undesired 3d-transition metal
hydr(oxy)oxide species leads to deactivation of the reaction activities, the CO oxidation reaction and the hydrogen evolution
reaction (HER) can have beneficial effects from the same impurities, the latter being dependent on the exact nature of the
adsorbing species. These results show the significance of impurities present in real electrolytes and their impact on
electrocatalysis.
■ INTRODUCTION
Alkaline energy conversion devices (alkaline fuel cells, AFCs)
and fuel generation devices (alkaline electrolyzers, AEs) have
garnered significant attention lately due to the applicability of
low-cost materials as catalysts in comparison to their acid
counterparts. For example, while in PEM fuel cells Pt-based
catalysts are required for efficient transformation of chemical
energy of hydrogen and oxygen into electricity, 1,2 in alkaline
electrolytes efficient energy conversion in AFCs can be
achieved with much cheaper nonprecious materials, including
Ag and abundantly available transition metal elements. 2−4 A
major limiting factor thus has been the development of robust
alkaline anion-exchange membranes, for which some significant
progress is being made. 5,6 The same is true for the fuel
production reactions, where the hydrogen evolution reaction
(HER) and the oxygen evolution reaction (OER) can
efficiently be carried out on metal surfaces modified with
inexpensive 3d transition metal (3d-TM) oxides, especially for
the HER or on 3d-TM compounds, such as oxides and
sulfides. 7−9 In addition to such “material factors” (e.g., strong
covalent metal−adsorbate interactions), the reactivity of
interfaces in alkaline environments is also governed via rather
weak noncovalent interactions between hydrated cations
(located in the double layer at ca. 3.5 Å 10 (ORR),
) and covalently
bound oxygenated species. This issue has been recently
discussed especially for understanding the role of the
noncovalent interactions in the oxygen reduction reaction
11 the oxidation of small organic molecules, 11,12 the
hydrogen oxidation reaction (HOR), 13,11 and the HER. 13,14
The influence of noncovalent reactions on kinetic rates can be
either catalytic (as in a case of the HER and CO oxidation
reaction) or inhibiting (e.g., ORR, HOR, and the oxidation of
methanol), a fact that significantly contributes to the richness of
methods that can be used to control the activity of
electrochemical interfaces in alkaline environments.
A central complication involved with experimentally
exploring the role of covalent and noncovalent interactions in
alkaline solutions, however, is that the measured rates are
influenced substantially (or even predominantly) by the
presence of varying levels of impurities. The puzzling role of
impurities in alkaline electrochemical measurements has been a
topic of interest for over 25 years. Conway 15−17 addressed
some of the issues employing recrystallization protocols to
purify electrolytes to remove some transition elements, while
Spendelow et al. 18 demonstrated the use of sacrificial adsorbers ,
in particular for Fe. However, the impact of the impurity on the
electrocatalytic activities, particularly for O2 and H2 reactions,
was seldom studied. Recently, effort has been focused on
identifying the role of impurities from the reaction of the glass
components of the electrochemical cell and how they affect the
Received: July 31, 2012
Revised: September 5, 2012
Published: September 19, 2012
Article
pubs.acs.org/JPCC
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The Journal of Physical Chemistry C Article
electrochemical behavior of polycrystalline Pt electrodes. 19
While the use of non-glass-based cells have addressed some of
the inconsistencies, it has been found that depending on the
prehistory of electrolyte preparation, temperature of electrolyte,
duration of experiment, scan rates, and applied potential
windows, adsorption properties and the reaction activities may
vary substantially even if experiments were performed in
fluoroethylenepolymer-based electrochemical cells. Consequently,
unraveling both the true nature of these impurities
as well as how they may affect the key electrochemical
processes in alkaline solutions should be of paramount
importance for the development of technologies that can
efficiently transform and store energy at electrochemical
interfaces at high pH values.
Here, in order to probe the impurities effect on the surface
electrochemistry in alkaline electrolytes, we employ the use of
well-defined metal single-crystal surfaces in a glass-free cell
(made with fluoropolymer). The use of single crystal surfaces
for “detecting” impurities in electrochemical environments is a
well-established tactic, as has been demonstrated where they
are used for monitoring the effects of trace levels of halides,
nitrates, and other anions on adsorption and catalytic properties
of metal−electrolyte interfaces in acidic media. 1 Combining
scanning tunneling microscopy (STM) with traditional electrochemical
methods, applied to the Pt(111) surface, we identify
the signature for the presence of the impurities and establish
the role of these impurities present in the alkaline electrolyte on
both the adsorption and reaction properties of the electrode for
the various reactions. Furthermore, intentionally adding
compounds of the 3d elements (Ni, Co, Cr, Fe), we further
establish the nature of the common impurities present in the
alkaline electrolyte as that of the 3d-TMs. Lastly, it is also
shown that the formation of 3d-TM hydr(oxy)oxides is mainly
responsible for the anomalous behavior observed in alkaline
electrocatalytic measurements, and depending on the nature of
these elements, we can observe beneficial or poisoning effects
for
■
the various reactions.
EXPERIMENTAL SECTION
A standard three-electrode cell made from fluoroethylenepolymer
(FEP) was used for the experiments. The chemical
solutions were prepared from KOH obtained from different
sources (GFS, Sigma Aldrich, Fluka, and Alfa Aesar) and Milli-
Q deionized (DI) water. The perchlorate salts of Ni, Co, Fe,
Mn, and Cr were all purchased from Sigma Aldrich at the
highest purity levels available and were made into 0.01 M
solution with the DI water. Small volumes of this solution were
added to the electrochemical cell to study the effect of the
concentrations. Six millimeter disk electrodes were used for all
experiments. The electrodes were prepared by radio frequency
(RF) annealing at ∼1100 °C in a 3% H2−Ar gas mixture for 7
min. The samples were transferred into the electrochemical cell
with the surface protected with a drop of DI water and
immersed under potential control at 0.05 V vs reversible
hydrogen electrode (RHE). The Pt/C catalyst obtained from
Tanaka (TKK) was mixed with water in the concentration of 1
mg/mL. This dispersion was then ultrasonically mixed for 1 h,
following which a stable suspension was obtained. A glassy
carbon disk (6 mm diameter) was then mechanically polished.
Known volumes of the suspensions were then added using a
micro pipet onto the glassy carbon disk electrode. The
electrode was dried at 60 °C in an inert atmosphere. The
suspension was applied so that it coats the surface of the
22232
electrode very uniformly. Once dry, these electrodes were
washed with water to ensure/verify the good adhesion of
particles to the glassy carbon substrate, after which they were
introduced into the alkaline cell. During the alkaline ORR,
HOR measurements, in order to ensure that particles had not
been dislodged during the experiments, the underpotentially
deposited hydrogen based charge was measured both before
and after in 0.1 M HClO4. Given that the 3d-TM hydr(oxy)oxides
are very soluble in acid solutions, we were able to obtain
accurate measures of the surface area.
A standard rotating disk electrode (RDE) setup with Ag/
AgCl reference (−0.96 V vs RHE), was used for electrochemical
measurements. All the results reported in the
manuscript are versus the RHE. A Pt counter electrode was
used for all the experiments. The sweep rates used in the cyclic
voltammetry (CV) experiments were 50 mV s−1 , while the
rotation rate was either 1600 or 2500 rpm. Typical experiments
were conducted at this sweep rate. We also tried slower sweep
rates, which showed similar behavior at much smaller number
of cycles and are not reported here. Most experiments were
performed at room temperature (RT) except for those
mentioned in the Results and Discussion section conducted
at 60 °C. For HOR and HER experiments, the potential was
swept in the cathodic direction from the hold potential; the
data presented is taken from first sweep curves. First scans were
used for deriving baselines for clean conditions. The first scans
are always obtained within 1−2 min of introduction of the
electrode into the electrolyte under potential control. This was
helpful to protect the electrode from electrolyte impurities.
Ohmic resistaances 20 were corrected for all the data reported
here using the Autolab PGSTAT 302N potentiostat. Electrolyte
resistance was also measured with AC impedance spectroscopy.
The gases used were research grade (5N) Ar and H2.CO cylinders were purchased from Airgas at research plus grade
(aluminum container, to avoid Fe contamination due to
stainless
■
steel materials).
RESULTS AND DISCUSSION
We begin by presenting the CVs obtained for Pt(111) under
three different experimental conditions (Figure 1a): the first
and 25th sweep in “clean” 0.1 M KOH at RT, the 15th sweep in
1 M KOH, and the 10th sweep for 1 M KOH at 60 °C. The CV
recorded on Pt(111) (see corresponding STM image) during
the first sweep shows all the expected characteristics of a wellordered
surface in “clean” KOH environments: reversible
adsorption of hydrogen (Hupd: 0.05 < E < 0.4), formation of the
double layer between 0.4 and 0.6 V, and adsorption of hydroxyl
species (OHad: 0.6 < E < 0.95 V). However, after 25 cycles
within the same potential region, three distinctive CV features
are observed: (i) the Hupd potential region is suppressed; (ii)
OHad formation starts at more negative potential; and (iii)
initial adsorption of the OHad becomes highly irreversible.
Similar distortions are observed for the other two cases
considered here as well. In the past, such variation in the OH
region has often been associated with an irreversible oxide
formed on the Pt surface with unknown stoichiometries. 21,22
Importantly, Figure 1b reveals that the degree of Hupd and
OHad distortion is strongly dependent on both the concentration
as well as temperature of the electrolyte. For example, in
1 M KOH, only 15 cycles were sufficient to produce a similar
degree of changes as previously observed after 25 potential
cycles in dilute electrolyte solutions. The temperature of the
electrolyte plays an even bigger role, i.e., at 60 °C, merely 10
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The Journal of Physical Chemistry C Article
Figure 1. Comparison of STM and cyclic voltammograms for Pt(111)
electrode. (a) STM image of as-prepared Pt(111) surface after the first
sweep at RT. The CV for this surface is also shown. Also shown are
the CVs after 25 sweeps in 0.1 M KOH at RT, 10 sweeps in 0.1 M
KOH at 60 °C, and 15 sweeps in 1 M KOH at RT. CVs exhibit
distortion of adsorption properties of both H upd and OH ad. Note the
scan window was shortened for the 1 M KOH in the H upd region due
to reference electrode correction. (b) STM image of Pt(111) electrode
cycled in 0.1 M KOH with 25 ppm of Co 2+ cations. Also shown are the
CVs after 25 sweeps in 0.1 M KOH in solutions containing different
cations such as Ni 2+ ,Co 2+ , and Cr 2+ . Fluka KOH electrolyte, ultra high
purity, was used for these spiking measurements (c) STM image of the
cycled electrode (Figure 1b) after cycling in CO saturated solution.
Also shown are the polarization curves for the CO oxidation reaction
for the fresh as-prepared electrode as well as the consecutive cycles of
the “contaminated” electrode. CV at the end of “cleaning” protocol is
shown in Figure S3. We note in passing that, holding the contaminated
electrode in CO-saturated solution at 0.05 V does not show any
22233
Figure 1. continued
change in the surface coverage, indicative of the absence of a “simple
CO displacement” mechanism. Enhanced CO oxidation activities are
observed in the presence of 3d-TM hydr(oxy)oxides on the surface.
potential cycles were sufficient to completely transform the
perfect to rather anomalous pseudocapacitive features of H upd
and OH ad. Interestingly, with the exception of the CV, which is
recorded during the very first potential sweep, the anomalous
shapes display a close resemblance to current−potential curves
recorded on Pt(111) modified with 3d-TM hydr(oxy)oxide
clusters. 23 The chemical nature of such species as a function of
potential was also established previously for different 3dmetals.
23 In turn, this was the first indication that the observed
changes might be due to the adsorption of 3d-TM elements,
which are known to be present in alkaline electrolytes at ppm−
ppb levels even in the most clean alkaline salts.
Here, the existence of trace levels (0.5−20 μg/L depending
on the supplier and batch of the electrolyte salt; averages for all
the cations and their levels are shown in Table S1, Supporting
Information) of 3d-TM elements in dilute alkaline electrolytes
was confirmed by using inductively coupled plasma mass
spectrometry (ICPMS). To further confirm that the 3d-TM
impurities are the main contributors to the observed anomalous
behavior of Pt(111) in Figure 1a, we intentionally “spiked” the
electrolyte with small concentrations (25 ppm) of 3d-TM
cations. As in ref 23, CV/STM results are used to establish the
correlation between the coverage of 3d-TM hydr(oxy)oxide
clusters and their effects on H upd and OH ad formation.
Inspection of Figure 1b indicates several features of interest
in this regard. First, the STM results for the Pt(111)/Co 2+
system (used as a representative of the 3d-TMs consider in this
work) reveals that after potential cycling in solution containing
ppm-levels of Co 2+ , nanoclusters (ca. 2 atomic layers thick) are
uniformly distributed across the (111) terrace. Second, the
adsorption of OH ad (position of irreversible peaks) follows the
trends in oxophilicity of the 3d elements, consistent with what
was reported previously. 23 To further probe the nature of
clusters, we use the CO oxidation reaction, which is known to
be strongly dependent on a nature and surface coverage by
hydroxyl species. A key observation in Figure 1c is that the
most active surface is covered with relatively high coverage of
Co-hydr(oxy)oxide clusters (see STM image in Figure 1b and
the polarization curve in Figure 1c). Moreover, there is STM
evidence that deactivation in the subsequent sweeps is closely
related to the disappearance of the very same clusters (Figure
1c); as evident from the 15th sweep recorded in CO saturated
solution. It is therefore plausible that, during the CO oxidation,
OH species associated with the 3d-TM clusters are consumed
and, as a result, the remaining Co species become
thermodynamically unstable and dissolve into the solution.
We notice in passing, after ∼30 potential cycles, that Co(OH) 2
clusters are barely present on the surface (see Figure S3). The
underlying CO oxidation mechanism can also be extended to
Pt−Co bimetallic systems; e.g., rather than affecting segregation
of Pt to the surface in Pt−Co bimetallic alloys, 24 a key role of
CO is simply to “clean” the Pt−Co surface of the Cohydr(oxy)oxides
species, which are formed when non-noble Co
metal atoms are exposed to alkaline electrolytes and positive
potentials. We conclude, therefore, that the anomalous behavior
of CVs in the “butterfly” potential region (0.6 < E < 0.9 V)
appears to be due to the presence of 3d-TM hydr(oxy)oxides
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The Journal of Physical Chemistry C Article
Figure 2. Polarization curves corresponding to the ORR and HOR on Pt surfaces. (a) Effect of cycling on the ORR and HOR activities of the
Pt(111) surface in 0.1 M KOH as a function of cycling at room temperature. Activities are found to decrease with cycling, consistent with
contamination of the surface by the adsorption of 3d-TM hydr(oxy)oxides. (b) Comparison of the ORR activity stability at 60 °C for Pt-poly
surfaces. Pt(111) surface exhibits significant deactivation within few cycles at such high temperatures. Inset shows the voltammetric features’
evolution with cycling for the Pt-poly electrode at RT. Pt-poly exhibits deactivation of ∼200 mV for the half-wave potential with cycling at high
temperature, which is higher, compared to ∼120 mV at 30 °C.
rather than the formation of Pt-oxide with yet unknown
stoichiometry, as has been considered previously in the
literature. 21,22 Given that these experiments were conducted
in a glass-free cell, the main source of contamination is the
electrolyte itself, and the role of glass components is secondary
in nature, contrary to what was thought previously. 19,25 We
would like to point out that the CO oxidation was studied here
primarily as a probe reaction, and our results indicate that CO
oxidation reaction is the least affected experiment with cycling
in alkaline solutions. In fact, CO oxidation or cycling appears to
be the possible protocol for cleaning the electrodes in alkaline
solutions.
It is important to note that the time required for the
complete distortion of the CVs is also a function of
hydrodynamic conditions such as stirring of electrolyte, i.e.,
the number of CVs required for complete distortion in “clean”
0.1 M KOH is less than ∼12 cycles. A similar effect is also
observed with decreasing the sweep rates and/or increasing the
potential hold times before experiments. The use of rotating
disk electrode (RDE) enhances the mass transport of both
reactants (H 2,O 2,H 2O) and the 3d-TM impurities present in
the bulk of the electrolyte. As we demonstrate further below,
interplay between these two mass-transport-dependent processes
will, in turn, govern the rate HOR/HER and ORR. In
general, in alkaline solutions, the rate of electrochemical
reaction (current density i) at a constant electrode potential
(E) can be governed by the following rate expression: 11
i = nFK c [1 − Θ − Θ ] exp
@E1 1 r cov noncov
*
− Δ
(
G
)
kT
where n is the number of electrons, K 1 is a constant that
incorporates all constant variables and the rate constants for a
particular reaction, F is Faraday’s constant, c r is the
concentration of reactant species in a solution (H 2,O 2 and
H 2O), Θ cov and Θ noncov represent the fraction of the surface
masked by site-blocking covalently and noncovalently bound
“spectator” species. The exponential ΔG* term (ΔG = ΔG* cov
+ ΔG* noncov) corresponds to the standard Gibb’s free energy
change required to form products from the reactants and the
active intermediates.
(1)
22234
In the following, we use this rate expression to discuss the
puzzling cycling-/temperature-induced activity variations in
alkaline solutions at constant rotation rates. As shown in Figure
2a, the activities for the HOR and the ORR are found to
decrease with potential cycling. Notice that the difference in
activity between the first and tenth potential cycles is massive,
consistent with 3d-TM impurity species blocking the sites
required for H 2 and O 2 adsorption; thus affecting the reaction
through a (1 − Θ cov − Θ noncov) term. The deactivation for the
reactions is large enough to prevent achieving the mass
transport limited currents in some cases for both the HOR and
the ORR.
Because the activities of the ORR and the HOR on Pt(111)
at high temperatures change dramatically (even ∼3−4 cycles
exhibit complete deactivation), the temperature effects will be
presented only for the Pt-poly electrodes, which we found to be
less sensitive toward poisoning effects by the impurities in the
electrolyte. The behavior exhibited by the Pt-poly surface at 60
°C is shown in Figure 2b. The half wave potential (potential at
half the maximum current) is found to shift toward higher
overpotentials much faster at higher temperatures: after 50
cycles, ∼100 mV at 30 °C compared to ∼200 mV at 60 °C.
Thus, from the high-temperature ORR experiments in Figure
2b, we conclude that one should be very careful in deriving
activation energies from Arrhenius plots in alkaline solutions.
With the exception of the CO oxidation reaction, the main
contribution of the 3d-TM impurities on surface activity
between 0.05 to 1.0 V (the HOR and ORR) appears to be via a
(1 − Θ cov − Θ noncov) term. So far, the role of the impurities has
been found to be generally independent of the chemical nature
of the elements (except for the CO oxidation reaction). Also,
the potential regions of interest, particularly for the ORR and
the HOR, are CV visible and hence can be correlated with the
observed CV behaviors of the cycled electrodes.
However, the situation below 0.05 V is rather different, as
revealed in Figure 3 for the HER in electrolytes with intrinsic
differences in the content and nature of transition metal
impurities. In particular, while in the case of one electrolyte
with a higher content of Co and Ni, the HER activity is found
to enhanced after extensive potential cycling, under the same
experimental conditions the HER is substantially deactivated in
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The Journal of Physical Chemistry C Article
Figure 3. Polarization curves comparing the HER activities for the
Pt(111) surface cycled in two different batches of 0.1 M KOH. The
first one is rich in Fe 2+ and as shown, the activity for the HER
decreases with cycling, and the second electrolyte was found to have a
higher content of Co 2+ compared to other TM cations and was found
to exhibit higher activities for the HER. This explains the onset of the
anomalous activities observed for the HER in alkaline solutions.
the other electrolyte, which had a higher concentration of Fe.
Such behavior is not uncommon, and has been discussed in
literature, for example, Petrii and Tsirilna, 26 as well as others
who have reported the deactivation of the HER activities as was
compiled by Trasatti. 9 Nevertheless, in our experiments, we
found that solution used in the first experiment is rich in the
concentrations of Co and Ni impurities, while in the second
experiment with higher concentrations of Fe. As was shown
recently, 23 the nature of the transition element and hence its
hydr(oxy)oxide on the surface plays a significant role in
determining the activity of the surface for the HER. It was
shown that the presence of less oxophilic hydroxides such as
Co(OH) 2 and Ni(OH) 2 on the surface of the Pt can optimize
the turnover frequency (TOF) of the water dissociation step
and indeed to catalyze the HER. 14,23 On the other hand, the
TOF is attenuated on highly oxophilic Fe or Mn hydr(oxy)oxides.
It must be noted that while these two cases presented
here exhibit qualitatively distinct features, it is often difficult to
predict the trends in the HER with cycling. This is often related
to the variations in the relative concentrations of the cations.
Therefore, significant precautions are necessary while measuring
the HER activities in alkaline solutions. For fundamental
studies, in order to avoid the contributions of the impurities,
activities for the HER must be either derived from the very first
potential scan, or the electrolyte purity must be significantly
enhanced. For practical applications, however, stable activities
are seldom established (in fact, it is customary to see
deactivation of the HER), signaling that alkaline electrolytes
are predominantly contaminated with undesired Fe-/Mn-type
impurities.
So far, we have demonstrated using the Pt extended surfaces,
that it is indeed possible to understand the origin of anomalous
activities in alkaline electrolytes. The relatively high sensitivity
of these surfaces aids in the detection and characterization of
the electrochemical signatures for the adsorbed species
responsible. In order to apply these principles and to
demonstrate that the behavior exhibited by such surfaces is
completely transformational across various length scales, we
compare the results for cycling of Pt nanoparticles (see
Experimental Section for preparation) in alkaline electrolytes.
Figure 4a,b shows the effect of cycling on both formation of
adsorbates as well as the role of CO oxidation reaction based
“cleaning” on recovering the original voltammogram. Unlike
the case of extended surfaces, the surface contamination
process is slower, due to the lower sensitivity and lack of welldefined
adsorption sites; however, within the time frame of a
reasonable long-term experiment (e.g., 100 cycles), the features
similar to those in Figure 1 appear: (i) suppression of the H upd
and (ii) irreversible peaks in the oxide (hydroxide) region (E >
0.6 V). On subjecting this surface to the CO-cycling protocol
discussed briefly in Figure 1c, we once again find that the
surface adsorbed species are removed, restoring the original
voltammogram. In order to confirm the difference between
oxidation of the CO molecules through the hydroxyl groups on
these 3d-hydr(oxy)oxides versus CO displacement of such
species, we have shown the CO stripping data for the
Figure 4. (a) Comparison of pristine Pt/C electrode and the one that was cycled for 100 cycles between 0.05 and 1.0 V at 50 mV/s. Slower scan
rates (20 mV/s) were also tried, and similar behavior was obtained in fewer than 100 cycles. Also shown is the CV for the “cleaned” electrode, which
was obtained after cycling in CO for 30 cycles at 50 mV/s. The surface essentially resembles that of the pristine electrode with small signatures for
the contamination. (b) CO stripping curves for both pristine electrode and the cycled electrode recorded at 20 mV/s (corresponding CV in 4a). The
cycled electrode clearly exhibits a prepeak at 0.45 V. The reduction curve exhibits the signature for the 3d-hydr(oxy)oxides present on the surface,
indicating that CO does not displace these species from the surface, and the removal involves reaction of the OH groups present on these oxides
with the adsorbed CO molecules. CV for the pristine electrode is also shown.
22235
dx.doi.org/10.1021/jp3075783 | J. Phys. Chem. C 2012, 116, 22231−22237

The Journal of Physical Chemistry C Article
nanoparticles in Figure 4b. Also shown is the comparison of
CO stripping for “clean” (uncycled) Pt nanoparticle electrode.
If the effect of addition of CO was to purely displace the oxide
species, one would not expect any difference between the
“clean” and the cycled electrodes for CO stripping. The
existence of a prepeak in this region is a clear indication that
CO oxidation proceeds through reaction with hydroxyl species
adsorbed on the adsorbates on the surface. Furthermore, the
preservation of the 3d-hydr(oxy)oxide peak after CO-stripping
voltammetry (at 0.5 V in Figure 4b) is another clear indication
that CO cleaning does not operate via a simple displacement of
adsorbed species on the surface. This procedure also provides
insight into the so-called CO-annealing in alkaline environments
24 where the surface segregation of Pt in the Pt-M alloy
nanoparticles was assumed to lead to the formation of Pt-rich
surfaces. Given the higher oxophilicity of 3d elements as well as
the similarity in the voltammetric features exhibited by these
alloying elements to the ones in Figure 4a, for example, it is
plausible that these alloying elements are removed from the Ptalloy
nanoparticles through destabilization by CO molecules.
The qualitative behavior of these nanocatalysts for the ORR,
HOR, and the HER mimic that of the extended surfaces, where
the former reactions are affected by a third-body effect of
poisoning, and the latter depends on the nature of contaminant
species present in the electrolyte.
In conclusion, the anomalous adsorption and catalytic
behavior in alkaline environments arise from the presence of
3d-TM impurities in the electrolyte. While using the FEP-based
cells helps alleviate the problems that arise from reaction of the
electrolyte with glass components of electrochemical cells, the
intrinsic transition metal impurities present in the electrolyte
play a bigger role in determining the electrochemical
characteristics of metal electrodes. The use of Pt(111) as a
probe was found to be very advantageous in determining the
nature of 3d-TM impurities even at very low levels. We found
that the ORR and HOR in KOH electrolytes are affected by a
third-body effect where, irrespective of the nature of the 3d-TM
elements, impurities simply act as spectators and block the
surface sites (the 1 − Θ term). On the other hand, there are
also desirable type of 3d-TM impurities, as in the case of
Co(OH) 2/Ni(OH) 2 acting catalytically (the ΔG*) on the CO
oxidation reaction and the HER. Lastly, this behavior is not
restricted to Pt extended surfaces, and is found to affect the Pt
nanoparticles system as well, albeit to a different degree. Taken
together, our results demonstrated that extensive care must be
taken while determining the adsorption and catalytic properties
of metal surfaces in alkaline electrolytes, further necessitating
the development of strategies to purify the alkaline electrolytes
for a “real system”, and/or careful design of experimental
protocols.
■ ASSOCIATED CONTENT
*S Supporting Information
ICPMS results for various alkaline electrolytes, cleanliness of
ultra-high purity electrolytes, the use of first scans for “control”
for the alkaline measurements, and CO oxidation-induced
cleaning of Pt(111) surfaces. This material is available free of
charge via the Internet at http://pubs.acs.org.
■ AUTHOR INFORMATION
Corresponding Author
*E-mail: nmmarkovic@anl.gov.
22236
Author Contributions
R.S. and N.M. designed the experiments and wrote the
manuscript. R.S., N.D., P.P.L., and D.S. did the electrochemical
experiments. R.S. and D.T. prepared the samples and
performed the STM measurements. R.S., N.M., and V.S.
discussed the results. R.S. and N.M. prepared the manuscript.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This work was supported by the Office of Science, Office of
Basic Energy Sciences, Division of Materials Science, U.S.
Department of Energy, under contract DE-AC02-06CH11357.
N.D. would like to thank the Chemical Sciences and
Engineering Division at Argonne National Laboratory for
funding. P.P.L. would like to thanks CAPES and FAPESP for
financial support.
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ACCEPTED MANUSCRIPT
This is an early electronic version of an as-received manuscript that has been
accepted for publication in the Journal of the Serbian Chemical Society but has
not yet been subjected to the editing process and publishing procedure applied by
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Please cite this article as: J. D. Lović, D. V. Tripković, K. Đ. Popović, V. M.
Jovanović, A. V. Tripković, J. Serb. Chem. Soc. (2012), doi:
10.2298/JSC121012138L
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J. Serb. Chem. Soc. 77 (0) 1–18 (2012) UDC
JSCS–5508 Original scientific paper
Electrocatalytic properties of Pt–Bi electrodes towards the
electrooxidation of formic acid
JELENA D. LOVIĆ # , DUŠAN V. TRIPKOVIĆ # , KSENIJA Đ. POPOVIĆ # , VLADISLAVA
M. JOVANOVIĆ # and AMALIJA V. TRIPKOVIĆ* #
ICTM – Institute of Electrochemistry, University of Belgrade, Njegoševa 12, P.O.Box 473,
11000 Belgrade, Serbia
(Received 12 October, revised 16 November 2012)
Abstract: Formic acid oxidation was studied on two Pt-Bi catalysts, Pt 2Bi and
polycrystalline Pt modified by irreversible adsorbed Bi (Pt/Bi irr) in order to
establish the difference between the effects of Bi irr and Bi in alloyed state. The
results were compared to pure Pt. It was found that both bimetallic catalysts
were more active than Pt with the onset potentials shifted to more negative
values and the currents at 0.0 V vs. SCE (under steady state conditions)
improved up to two order of magnitude. The origin of Pt 2Bi high activity and
stability is increased selectivity toward formic acid dehydrogenation caused by
the ensemble and electronic effect and suppression of Bi leaching from the
surface during formic acid oxidation. However, although Pt/Bi irr also shows
remarkable initial activity compared to pure Pt, dissolution of Bi is not
suppressed and the poisoning of the electrode surface induced by dehydration
path is observed. Comparison of the initial quasi-steady state and
potentiodynamic results obtained for these two Pt-Bi catalysts revealed that the
electronic effect, existing only in the alloy, contributes earlier start of the
reaction, while the maximum current density is determined by the ensemble
effect.
Keywords: formic acid; electrochemical oxidation; Pt 2Bi catalyst; Pt/Bi irr
catalyst; fuel cell.
INTRODUCTION
Accepted Manuscript
The electrocatalytic oxidation of small organic molecules, such as methanol,
ethanol and formic acid has been extensively studied because such molecules can
potentially be used as fuel in fuel cell applications. 1 Pt is an excellent catalyst for
dehydrogenation of small organic molecules but, on the other hand, has several
significant disadvantages: high cost and extreme susceptibility to poisoning by
CO. To improve its catalyst performance and minimize its quantity in the
* Corresponding author. E-mail: amalija@tmf.bg.ac.rs
# Serbian Chemical Society member.
doi: 10.2298/JSC121012138L
1

2 FORMIC ACID OXIDATION ON Pt–Bi ELECTRODES
catalyst, Pt was modified by the addition of other metals such as Ru, Rh, Pd, Sn,
Pb, etc. 2–5
Electrochemical oxidation of formic acid has been comprehensively
investigated as the anodic reaction in direct formic acid fuel cell (DFAFC) as
well as a model reaction important for understanding electrooxidation of other
small organic molecules. 6,7 Mechanistic studies of the formic acid
electrooxidation put forward that the reaction on Pt proceeds through two parallel
paths: 8 dehydrogenation with direct oxidation to CO2 and the path consisting of a
dehydration step, to yield water and adsorbed CO (COad) as a poisoning
intermediate, and the subsequent oxidation of COad to CO2. Since the potential of
COad formation is lower than the dehydrogenation potential, the poison remains
on electrode surfaces until it is oxidized by oxygen containing species at positive
potentials. 9 Thus, Pt atoms covered with poison are not available for
dehydrogenation and the catalytic efficiency of the Pt surface decreases.
Therefore, practical application of Pt for formic acid oxidation requires some
modification of the surface. This is commonly done by alloying or by alteration
of the Pt surface with adsorbed foreign metals in the amount less than a full
monolayer. As the COad poison needs an ensemble of Pt surface atoms to adsorb
on, modification of the local distribution of surface domains is achieved by
adsorption of different adatoms, such as Bi. Irreversibly adsorbed Bi. 10–13 inhibits
poison formation simultaneously enhancing dehydrogenation, 13 i.e. this
modification is an efficient way to hinder the dehydration path (CO-intermediate
pathway) in favor of direct path. 14 This increased selectivity for dehydrogenation
has been proposed as an “ensemble effect” 15,16 in which the adsorbed Bi divides
the Pt surface into small domains where only dehydrogenation can occur. A
correlation between ensemble size and formic acid oxidation activity were also
established. 17 According to literature data the activity of Pt catalyst modified with
Bi depends on the shape of Pt nanocrystals, 18 vary with the size of particles 19 and
Pt catalyst loading. 14
Ordered intermetallic PtBi or PtBi2 alloys 20–23 and PtBi alloy nanoparticles 24–
29 were proposed as good catalysts for formic acid oxidation. The onset potential
of the reaction is significantly shifted to more negative values (by over 300 mV)
and the current density is remarkably enhanced in whole potential range
Accepted Manuscript
compared to Pt. 30 Moreover, the PtBi surface appears to have a considerably
lower sensitivity to poisoning by CO according to DEMS, 22,30 FTIR 22,31 and DFT
calculations. 22,23,32 The origin of its catalytic activity was related to electronic
effects. The formation of the PtBi ordered intermetallic phase results in a charge
redistribution enhancing the affinity of PtBi for formic acid adsorption and
producing surface oxides at low potentials, 25,33 as well as to geometric effects
reducing the affinity for CO poisoning. In addition, the excellent catalytic

QUALITY OF MOLECULAR STRUCTURE DESCRIPTORS 3
properties of PtBi nanoparticles can be attributed to the occurrence of the
ensemble effect at the nanoscale level. 29
In our previous studies, the oxidation of formic acid was investigated on
single phase PtBi alloy, 34 as well as on Pt2Bi catalyst, a two-phase material
consisting of PtBi alloy and pure Pt. 35 A huge increase in catalytic activity on
PtBi electrode relative to polycrystalline Pt was observed and discussed in terms
of electrochemically detected UPD phenomena of Bi readsorbed on Pt.
Additionally, on the basis of the analysis of X-ray photoelectron spectra, it was
proposed that bifunctional action of hydroxylated Bi species may contribute to
the enhanced activity of PtBi alloys. 34 Pt2Bi, is found to be powerful catalyst for
formic acid oxidation exhibiting high activity and stability. High activity
originates from the fact that formic acid oxidation proceeds completely through
dehydrogenation path, while the high stability of Pt2Bi surface is induced by the
suppression of Bi leaching in the presence of formic acid. 35 It is difficult to
separate the contributions of the ensemble and the electronic effect since both of
them can be present in the Pt–Bi surfaces.
In this work formic acid oxidation was studied on two types Pt-Bi surfaces:
polycrystalline Pt modified by irreversible adsorbed Bi (designated as Pt/Biirr)
and on Pt2Bi catalyst. In order to investigate the promotional role of Bi in formic
acid oxidation, as well as the difference between the effect of irreversibly
adsorbed Bi and Bi in alloyed state, Pt was modified with ~ 30% Biirr which
correspond to the nominal content of Bi in Pt2Bi catalyst. Comparative
investigation based on the effects influencing the catalytic properties of these
electrodes enables better understanding of different activities between Pt2Bi
catalyst and Biirr modified Pt.
EXPERIMENTAL
Polycrystalline Pt and Pt2Bi electrodes in the form of disc were used in this study. Pt2Bi catalyst was prepared and characterized at Institute of Catalysis and Surface Chemistry, Polish
Academy of Sciences, Krakow, Poland. 35 Briefly, the catalyst was fabricated by melting of the
pure elements in inert atmosphere in the proportion of Bi to Pt = 1:2 and characterized by Xray
diffraction (XRD). Diffraction pattern for Pt2Bi sample reveals two crystal phases:
platinum (fcc) and platinum bismuth PtBi (hcp). The phase composition of the sample was
calculated using the Rietveld refinement as 45% and 55 % for Pt and PtBi phases,
respectively.
Accepted Manuscript
Prior to each experiment, the electrodes were mirror polished (1–0.05 µm Buehler
alumina). The surfaces were rinsed with high purity water (Millipore, 18 M� cm resistivity),
sonicated for 2-3 min and rinsed again with ultrapure water.
All experiments were carried out on as-prepared catalysts. Three-compartment
electrochemical glass cells with Pt wire as the counter electrode and saturated calomel
electrode (SCE) as the reference electrode was used. All the potentials are expressed on the
scale of SCE. The electrolyte containing 0.1 M H 2SO 4 as a supporting electrolyte and 0.125 M
HCOOH was prepared with high purity water and p.a. grade chemicals (Merck). The
electrolyte was deaerated by bubbling of nitrogen. Upon addition of HCOOH at -0.2 V,

4 FORMIC ACID OXIDATION ON Pt–Bi ELECTRODES
potentiodynamic (� = 50 mV s -1 ), quasi steady-state measurements (� = 1 mV s -1 ) or
chronoamperometric measurements were carried out. The electrode was rotating at 2000 rpm
in all the experiments.
Modification of the Pt electrode was performed from 1 x 10 -5 M Bi 2O 3 in 0.1 M H 2SO 4 at
open circuit potential during 45 s. After modification, the electrode was rinsed with water and
transferred into a cell containing supporting electrolyte. Fraction of sites covered by Bi
(denoted Pt/Bi irr) was estimated by decrease in the charge for desorption of hydrogen,
assuming a charge of 210 �C cm −2 for the hydrogen monolayer adsorption.
The real surface area of all as-prepared catalysts was calculated from CO stripping
voltammetry. For the CO stripping measurements, pure CO was bubbled through the
electrolyte for 20 min while keeping the electrode potential at -0.2 V vs. SCE. After purging
the electrolyte by N 2 for 30 min to eliminate the dissolved CO, the adsorbed CO was oxidized
in an anodic scan at 50 mV s −1 . Two subsequent voltammograms were recorded to verify the
completeness of the CO oxidation. Real surface area of all electrodes used was estimated by
the calculation of the charge from CO ad stripping voltammograms corrected for background
currents. Assuming charge of 420 μC cm −2 for the CO monolayer adsorption on the Pt
electrode, roughness factor of 1.4 � 0.1 was estimated. The real surface area of Pt 2Bi electrode
was estimated assuming the same roughness factor as for Pt electrode, which is to be expected
since both electrodes were polished in the same way. The specific activity of Pt and Pt 2Bi
electrodes for formic acid oxidation are normalized using these values of the surface area.
The experiments were conducted at 295 ± 0.5 K. A VoltaLab PGZ 402 (Radiometer
Analytical, Lyon, France) was employed.
Electrochemical characterization
RESULTS AND DISCUSSION
Initial voltammograms of as-prepared Pt and Pt/Biirr catalysts are presented
in Fig. 1. Cyclic voltammogram for polycrystalline Pt electrode is characterized
by defined region of hydrogen adsorption/desorption (E < 0.05 V), separated by
double layer from the region of surface oxide formation (E > 0.45 V). The
absence of well developed peaks at Pt polycrystalline in hydrogen
adsorption/desorption region is caused by preparation procedure used.
Figure 1 compares typical voltammograms recorded in supporting
electrolyte before and after Pt modification with adsorbed bismuth. Distinctive
characteristics for the presence of bismuth ad-atoms on the platinum surface are
the diminution of the hydrogen adsorption/desorption charge due to the fact that
hydrogen does not adsorb on Bi 36 and the appearance of peaks a and a’, as a
Accepted Manuscript
result of the oxidation/reduction of Bi irreversibly adsorbed onto the Pt surface,
which superimpose to those corresponding to Pt oxide formation/reduction. Since
only the Pt sites, unblocked by Bi are available for hydrogen adsorption, the
fractional coverage by Bi ad-atoms, evaluated from the decrease in charge
involved in the hydrogen desorption before and after adsorption of Bi on the Pt
electrode surface, was set to be about 30%, corresponding the nominal content of
Bi in Pt2Bi catalyst.

Oxidation of pre-adsorbed CO
QUALITY OF MOLECULAR STRUCTURE DESCRIPTORS 5
Oxidation of pre-adsorbed CO is examined on Pt, Pt2Bi and Pt/Biirr
electrodes and the stripping voltammograms after the subtraction of the
background current are given in Fig. 2. As one can see, the oxidation of COad
starts earlier on Pt2Bi catalyst than on pure Pt. The difference in onset and peak
potential of COad oxidation could be ascribed to the some electronic modification
of Pt surface atoms by Bi 10,17,37,38 resulting in weaker bonding of COad. This
statement is consistent with literature data based on theoretical calculations. 39,40
However, the difference in onset and peak potentials between Pt/Biirr and Pt
catalysts is insignificant.
The CO stripping charge was also determined for Pt2Bi electrode and
corrected for the background currents to eliminate the contribution of the double
layer charge, as well as Bi oxidation charge. Since Bi 37 Error! Bookmark not
defined. and PtBi 23,30 are inactive for CO adsorption, the oxidation of CO occurs
only on Pt domains. Therefore, the charge under the COad peak at Pt2Bi reflects a
process at Pt parts and can be used for determining the contribution of pure Pt in
the surface composition of Pt2Bi catalyst. 35
Oxidation of formic acid
Potentiodynamic measurements
Activity of catalysts. The activities of Pt2Bi, Pt/Biirr and Pt electrodes (first
sweeps) towards formic acid oxidation are compared in Fig. 3. Cyclic
voltammogram for Pt electrode shows well establish feature for the formic acid
oxidation. 8 In the positive scan current slowly increases reaching a plateau at ~
0.25 V followed by ascending current starting at 0.5 V which attains a maximum
at ~ 0.62 V. The explanation for this behavior could be described considering
dual path mechanism, i.e. dehydrogenation assigned as the direct path, based on
the oxidation of formate, 41,42 and dehydration, indirect path, assumes formation of
COad, both generates CO2 as the final reaction product. At low potentials
HCOOH oxidizes through the direct path with the simultaneous formation of
COad. Increasing coverage with COad reduces the Pt sites available for the direct
path and current slowly increases reaching a plateau. Subsequent formation of
oxygen-containing species on Pt enables the oxidative removal of COad, more Pt
Accepted Manuscript
sites become available for HCOOH oxidation and current increases until Pt
oxide, inactive for HCOOH oxidation, is formed,which results in the current peak
at ~0.62 V. In the backward scan the sharp increase of HCOOH oxidation current
coincides with reduction of Pt oxide. The currents are much higher than in the
forward sweep, because Pt surface is freed of COad.
The polarization curves for bimetallic surfaces indicate quite different
behavior. Figure 3 shows that the onset potential for the reaction on Pt/Biirr is
about 0.1 V less positive than on Pt electrode. The current raises up to 0.35V and

6 FORMIC ACID OXIDATION ON Pt–Bi ELECTRODES
reaches a peak which corresponds to the oxidation of HCOOH to CO2 in the
direct path, occurring on Pt sites that are not blocked by COad. On the descending
part of the curve the shoulder appears almost at the same potential as the peak on
bare Pt electrode. The currents recorded in the backward direction are slightly
higher, so the difference between forward and backward scan is not as large as on
bare Pt. Since the peak at ~ 0.62 V arises from the HCOOH oxidation on the Pt
sites being released by COad oxidation, its height is an indication of the degree of
the Pt poisoning at lower potentials. Accordingly, the amount of COad formed in
the indirect path on Pt/Biirr is much lower than on pure Pt electrode.
The oxidation of formic acid on Pt2Bi electrode starts at ~ -0.2 V, which is
0.1 V less positive than on Pt/Biirr and ~0.2 V less positive than on Pt electrode.
The current raises up to 0.35 V and reaches a peak about 15 times higher than the
plateau on Pt. At more positive potentials reaction currents decrease due to
surface oxide formation. The absence of the shoulder on the desceding part of the
curve, recorded on the Pt/Biirr, is indicating no poisoning of the alloy by COads.
Bell-shaped voltammogram clearly suggests that oxidation of HCOOH on
Pt2Bi proceeds through dehydrogenation path. 35 Error! Bookmark not defined.
Since Bi does not adsorb HCOOH 34,40 oxidation of HCOOH occurs on Pt sites in
pure Pt and Bi alloyed Pt domains. As it was shown in Fig. 3 lower activity and
appearance of shoulder on Pt/Biirr compared to Pt2Bi could suggest a low
coverage by COad at Pt sites on Pt/Biirr, i.e. incomplete suppression of the
dehydration path although there is almost the same (nominal) amount of Bi on
these two electrodes. Thus, HCOOH oxidation on Pt/Biirr proceeds
predominantly by dehydrogenation path with some minor degree of
dehydratation that takes place as well. Increased selectivity toward
dehydrogenation path on Pt2Bi as well as on Pt/Biirr compared to Pt is caused
mainly by an ensemble effect originating from diminishing of continuous Pt sites
(capable for dehydration) by Bi. However, the ensemble effect on Pt/Biirr catalyst
is enabled by the adsorbed Bi with practically no influence on the neighboring
free Pt atoms. When Pt2Bi alloy is in question, the ensembles are created by
alloyed Bi atoms incorporated in Pt lattice causing the shift in d-band center of
adjacent Pt atoms. Therefore, Bi in the alloy also exhibits electronic effect that
can be the reason for better performance of this catalyst resulting in higher
Accepted Manuscript
currents and lower onset potential.
Stability of catalysts. Cyclic voltammograms (first and 20 th sweeps) recorded
on Pt2Bi and Pt/Biirr catalysts in formic acid containing solution, are shown in
Fig. 4. Over the potential cycling up to 0.8 V the activity of Pt2Bi electrode (Fig.
4a) slowly decreases during first 5-7 cycles reaching the value of 90% and 82%
of the initial currents at the potential of the maximum current and at the potential
of 0.0 V in the low current region, respectively. After these first few sweeps the
currents remain unchanged with further cycling (Inset in Fig. 4a). Since cycling

QUALITY OF MOLECULAR STRUCTURE DESCRIPTORS 7
of Pt2Bi in supporting electrolyte leads to enhancement of currents related to
oxidation of Bi species indicating some surface decomposition caused by Bi
leaching/dissolution process, 35 the stability of Pt2Bi during oxidation of formic
acid could be induced by the presence of HCOOH in the electrolyte. Abruna and
co-workers 43 explained that the stability of Pt-Bi intermetallic surface originates
from the competition between oxidation of formic acid at the electrode/solution
interface and Bi leaching, i.e. corrosion/oxidation processes of the electrode
surface itself. Accordingly, the main reason for high stability of formic acid
oxidation current on Pt2Bi catalyst is the inhibition of dehydration path, as well
as, suppression of Bi leaching. This statement was confirmed by STM imaging
before and after electrochemical treatment in formic acid containing solution,
which did not indicate any significant change of surface morphology and
roughness after that procedure. 35
Contrary to Pt2Bi alloy, as can be seen in Fig. 4(b), Pt/Biirr electrode shows
significant changes with continuous cycling in the solution containing formic
acid (Inset in Fig. 4b). Repetitive cycling up to 0.8 V shifts the onset potential for
formic acid oxidation to more positive values, decreases the reaction currents,
while anodic peak diminishes and a new peak starts to emerge and grow at ~ 0.6
V. This transformation of cyclic voltammograms indicates continuous Bi
dissolution and modification of the surface composition. Upon prolonged
cycling, electrode surface becomes enriched in platinum and exhibits a Pt-like
electrochemical behavior in acid electrolyte containing formic acid. Apparently
re-adsorption of Bi species from the solution is rather low, so the initial
voltammogram was never restored, which is in accordance with results obtained
for formic acid oxidation on bismuth-coated mesoporous Pt microelectrodes. 44
Finally, it can be concluded that initial activity at two Bi-Pt catalysts
originates from the ensemble effect, but the activity of the Pt/Biirr in the reaction
is diminished due to leaching of Bi. An electronic effect contributes to the early
start of the reaction on alloy.
Chronoamperometric measurements
Chronoamperometric experiments were performed to prove activity and
stability of the catalysts investigated. The current density of formic acid
Accepted Manuscript
oxidation was recorded as a function of time at 0.2V over 30 min (Fig. 5). The
highest initial current density at 0.2 V on Pt2Bi related to the other two catalysts
is in accordance with the potentiodynamic measurements (Fig. 3). The initial
currents at Pt2Bi electrode decreases slightly and in observed period of time
attain a value which is about 3.5 times higher than on Pt/Biirr catalyst and almost
20 times higher than at Pt electrode. Chronoamperometic test confirmed high
activity and stability of the Pt2Bi catalyst.

8 FORMIC ACID OXIDATION ON Pt–Bi ELECTRODES
Quasi-steady state measurements
The quasi-steady state measurements for formic acid oxidation at all
investigated electrodes are presented in Fig. 6. The data obtained under the slow
sweep conditions corroborated the difference in the activities of pure Pt, Pt
modified by Bi and Pt2Bi alloy that was found under the potentiodynamic
measurements.
Tafel slope on Pt2Bi electrode is about 120 mV dec -1 , indicating that the first
electron transfer is the rate-determining step, with transfer coefficient being
about 0.5. This means that C–H bond cleavage, to form COOHad, is the slow step
and determines the rate of formic acid oxidation on Pt2Bi electrodes.
The Tafel slope of about 150 mV dec -1 obtained during formic acid oxidation
on bare Pt indicate that formed CO was adsorbed and collected on the surface
slowing down the reaction rate. The same value of Tafel slope was obtained for
formic acid oxidation on carbon supported high surface area platinum. 45 Tafel
slope of 135 mV dec −1 was found at Pt/Biirr and imply moderate surface coverage
by COad.
Comparing the activities of electrodes investigated at 0.0 V it can be seen
that the current densities are enhanced 15 times at Pt/Biirr and up two orders of
magnitude at Pt2Bi catalyst in respect to Pt electrode.
CONCLUSION
Pt2Bi and Pt/Biirr electrodes were investigated in formic acid oxidation and
the results were compared to Pt electrode. The results presented indicate that Bi
in alloy and irreversibly adsorbed Bi exhibit different effects on the catalytic
activity.
Pt2Bi is highly active for the formic acid oxidation because the
dehydrogenation path is predominant in the overall reaction. Bi in the alloy not
only that facilitates ensemble effect but also has electronic effect that can be the
reason for better performance of this catalyst resulting in higher currents and
lower onset potential. The main reason for high stability of Pt2Bi catalyst is the
inhibition of dehydration path in the reaction, as well as suppression of Bi
leaching indicated by insignificant change of surface morphology and roughness.
On the contrary, Pt/Biirr electrode is not stabilized by formic acid oxidation,
Accepted Manuscript
since the desorption of Bi is not suppressed in the presence of formic acid, as
well as poisoning effect induced by dehydration path. Since Biirr do not provoke
any significant modification of electronic environment, modified Pt catalysts is
less active than corresponding alloy.
Comparing the results obtained for these two Pt-Bi catalysts, the role of the
ensemble effect and electronic effect in the oxidation of formic acid on Pt-Bi
electrodes can be distinguished. The electronic effect, existing only on the alloy,
contributes earlier start of the reaction, while the maximum current density is

QUALITY OF MOLECULAR STRUCTURE DESCRIPTORS 9
determined by the ensemble effect. During potential cycling of Pt/Biirr electrode
Bi is leached form the electrode surface and the ensemble effect is reduced over
time, or lost. Insight into the chronoamperometric curves confirms an advantage
of alloys, i.e. necessity of alloying Pt with Bi to obtain a corrosion stable catalyst.
Acknowledgement. This work was financially supported by the Ministry of Education
and Science, Republic of Serbia, Contract No. H-172060.
И З В О Д
ЕЛЕКТРОКАТАЛИТИЧКЕ ОСОБИНЕ Pt–Bi ЕЛЕКТРОДА У ОКСИДАЦИЈИ
МРАВЉЕ КИСЕЛИНЕ
ЈЕЛЕНА Д. ЛОВИЋ, ДУШАН В. ТРИПКОВИЋ, КСЕНИЈА Ђ. ПОПОВИЋ, ВЛАДИСЛАВА М. ЈОВАНОВИЋ
и АМАЛИЈА В. ТРИПКОВИЋ
ИХТМ – Центар за електрохемију, Универзитет у Београду,Његошева 12, п. пр. 473, 11000 Београд
Оксидација мравље киселине испитивана је на два типа Pt–Bi катализатора: Pt2Bi
електроди и на поликристалној Pt електроди модификованој иреверзибилно
адсорбованим Bi (Pt/Biirr). Активности су упређене са резултатима добијеним на чистој
поликристалној Pt електроди. Циљ је био да се објасни разлика у деловању
иреверзибилно адсорбованог Bi (Biirr) и Bi у легираном стању. Показано је да су оба
биметална катализатора активнија од поликристалне Pt, почетак реакције је померен ка
негативнијим вредностима и у поређењу са чистом Pt при стационарним условима
добијене су до два реда величине веће густине струје. Разлог за велику активност и
стабилност Pt2Bi електроде у оксидацији мравље киселине је одигравање реакције по
главном реакционом путу (дехидроганација мравље киселине), што је изазвано ефектом
трећег тела и електронским ефектом, као и спречавање излуживања Bi из елецтроде. С
друге стране, иако Pt/Biirr показује значајну почетну активност у односу на Pt, ова
електрода није стабилна током реакције оксидације HCOOH због континуалног
растварања Bi са површине елецтроде, као и тровања површине изазваног током
реакције по индиректном, дехидратационом путу. Поређењем резултата добијених на
ове две Pt-Bi електроде може се објаснити улога ефекта трећег тела и електронског
ефекта у оксидацији HCOOH. Наиме, електронски ефекат, који постоји само код легуре,
доприноси ранијем почетку реакције, док је максимална струја одређена ефектом
трећег тела. Током циклизирања Pt/Biirr електроде Bi одлази са површине и ефекат
трећег тела се губи током времана. Хроноамперометријска метења указују на предност
легуре, односно неопходност легирања Bi са Pt да би се добио корозионо стабилан
катализатор.
(Примљено 12. октобра, ревидирано 16 новембра 2012)
Accepted Manuscript
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Accepted Manuscript

12 FORMIC ACID OXIDATION ON Pt–Bi ELECTRODES
Figure captions
Fig. 1. Initial basic voltammograms for Pt/Biirr and Pt catalysts in 0.1 M
H2SO4 solution. Scan rate: 50 mV s –1 .
Fig. 2. CO stripping voltammograms on Pt2Bi, Pt/Biirr and Pt catalysts
(first positive going sweeps) in 0.1 M H2SO4 solution corrected for
background current. Scan rate: 50 mV s –1 .
Fig. 3. Cyclic voltammograms for the oxidation of 0.125 M HCOOH in
0.1 M H2SO4 solution on Pt, Pt2Bi and Pt/Biirr catalysts. Scan rate:
50 mV s –1 .
Fig. 4. First and 20 th sweep for the oxidation of 0.125 M HCOOH in 0.1 M
H2SO4 solution on (a) Pt2Bi and (b) Pt/Biirr catalysts. Insets: Effect
of cycling -Plots of current density vs. number of cycles. Scan rate
50 mV s –1 .
Fig. 5. Chronoamperometric curves for the oxidation of 0.125 M HCOOH
at 0.2 V in 0.1 M H2SO4 solution on Pt2Bi, Pt/Biirr and Pt catalysts.
Fig. 6. Tafel plots for oxidation of 0.125 M HCOOH in 0.1 M H2SO4
solution on Pt2Bi, Pt/Biirr and Pt catalysts. Scan rate: 1 mV s –1 .
Accepted Manuscript

QUALITY OF MOLECULAR STRUCTURE DESCRIPTORS 13
Fig. 1
Accepted Manuscript
Fig. 2

14 FORMIC ACID OXIDATION ON Pt–Bi ELECTRODES
Fig. 3
Accepted Manuscript

QUALITY OF MOLECULAR STRUCTURE DESCRIPTORS 15
Accepted Manuscript
Fig. 4

16 FORMIC ACID OXIDATION ON Pt–Bi ELECTRODES
Fig. 5
Accepted Manuscript
Fig. 6

J. Serb. Chem. Soc. 76 (12) 1673–1685 (2011) UDC 542.913–732:546.92’97’811–
JSCS–4239 44:547.262+66.094.3
Original scientific paper
Microwave synthesis and characterization of Pt and
Pt–Rh–Sn electrocatalysts for ethanol oxidation
SANJA STEVANOVIĆ 1 * # , DUŠAN TRIPKOVIĆ 2# , DEJAN POLETI 3# , JELENA
ROGAN 3# , AMALIJA TRIPKOVIĆ 1# and VLADISLAVA M. JOVANOVIĆ 1
1 Department of Electrochemistry, ICTM, University of Belgrade, Njegoševa 12, Belgrade,
Serbia, 2 Materials Science Division, Argonne National Laboratory, Argonne, IL 60439,
USA and 3 Faculty of Technology and Metallurgy, University of Belgrade,
Karnegijeva 4, Belgrade, Serbia
(Received 5, revised 29 April 2011)
Abstract: Carbon-supported Pt and Pt–Rh–Sn catalysts were synthesized by the
microwave-polyol method in ethylene glycol solution and were investigated in
the ethanol electro-oxidation reaction. The catalysts were characterized in
terms of structure, morphology and composition employing the X-ray diffraction
(XRD), scanning tunneling microscopy and energy-dispersive X-ray spectroscopy
techniques. The STM analysis indicated rather uniform particles and
particle sizes below 2 nm for both catalysts. The XRD analysis of the Pt/C catalyst
revealed two phases, one with the main characteristic peaks of the face-
-centered cubic crystal structure (fcc) of platinum and the other related to the
graphite-like structure of the carbon support, Vulcan XC-72R. However, in the
XRD pattern of the Pt–Rh–Sn/C catalyst, diffraction peaks for Pt, Rh or Sn
could not be resolved, indicating extremely low crystallinity. The small particle
sizes and homogeneous size distributions of both catalysts could be attributed
to the advantages of the microwave-assisted modified polyol process in ethylene
glycol solution. The Pt–Rh–Sn/C catalyst was highly active for ethanol
oxidation with the onset potential shifted by more than 150 mV to more negative
values and with currents nearly 5 times higher in comparison to the Pt/C
catalyst. The stability tests of the catalysts, as studied by chronoamperometric
experiments, revealed that the Pt–Rh–Sn/C catalyst was evidently less poisoned
than the Pt/C catalyst. The increased activity of Pt–Rh–Sn/C in comparison
to Pt/C catalyst was most probably promoted by the bi-functional mechanism
and the electronic effect of the alloyed metals.
Keywords: Pt–Rh–Sn catalyst; ethanol oxidation; polyol synthesis; microwave
irradiation; STM.
* Corresponding author. E-mail: sanjat@tmf.bg.ac.rs
# Serbian Chemical Society member.
doi: 10.2298/JSC110405166S
1673
Available online at www.shd.org.rs/JSCS
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2011 Copyright (CC) SCS

1674 STEVANOVIĆ et al.
INTRODUCTION
High surface area carbon-supported Pt and Pt-alloys are widely proposed as
promising anode catalysts in direct alcohol fuel cells (DAFCs). 1 Platinum is an
excellent catalyst for the adsorption and dissociation of small organic molecules.
However, platinum itself is known to be rapidly poisoned by reaction intermediates,
such as CO, that are formed by dehydrogenation of the alcohol molecule 2
and limited ability for cleavage of C−C bonds. The most common fuel besides
methanol is ethanol, which is less toxic and can be easily produced by fermentation
of sugar-containing biomass. The oxidation mechanism of ethanol in acid
solution may be summarized in the following schema of parallel reactions: 1
CH3CH2OH→[CH3CH2OH]ad → C1ad, C2ad → CO2 (total oxidation) (1)
CH3CH2OH→[CH3CH2OH]ad → CH3CHO → CH3COOH (partial oxidation) (2)
Differential mass spectrometry (DEMS) and in situ infrared spectroscopy
(FTIR) determined acetaldehyde (CH3CHO) and acetic acid (CH3COOH) as the
main products of the oxidation of ethanol in acidic solution, with carbon dioxide
(CO2) appearing at very high positive potentials. 3,4 The efficiency of C−C bond
cleavage is the key to enable this reaction to be useful in fuel cell applications
and thus, the major challenge is to achieve total oxidation of ethanol to CO2 at
low overpotentials. Efforts in this sense have been focused on the addition of cocatalysts,
such as Ru, Rh, W, Pd, Sn, etc. 1,5–10 The addition of metals such as Sn
or Ru, even though beneficial for the overall electrochemical activity of the
catalysts for ethanol oxidation, does not enhance the yield of CO2, i.e., C−C bond
breakage. 4,11 On the other hand, the addition of Rh to Pt improves the activation
for C−C bond dissociation, but does not enhance the overall electrochemical reaction.
9,10 Thus, a good electrocatalyst for ethanol oxidation should have, besides
Pt, both kind of metals that would improve dehydrogenation, C−C bond dissociation
and CO–O coupling. 9 Different ternary catalysts have been described in
the literature1 (and references therein), but although PtSn appears to enhance
ethanol oxidation better than other bimetallic catalysts and presence of Rh significantly
increases C−C bond breakage, there are only a few data on ternary Pt
catalyst with Sn and Rh. 12–14 Colmati et al. 12 investigated the activity for
ethanol oxidation of Pt–Rh–Sn catalysts and found that at potentials higher than
0.45 V vs. RHE, the alloy possessed the highest activity for this reaction, while at
potentials more negative than 0.45 V, the activity was lower than that of the binary
PtSn catalyst. Kowal et al. 13 indicated that Pt–Rh–SnO2 exhibited higher
activity than PtSnO2 even at lower potentials. The extent of activity of such
catalyst depends strongly on the SnO2 content. 14
In general, metal catalytic activity is considerably dependent on the particle
shape, size and particle size distribution. 15 A variety of methods can be used for
nanocatalyst preparation, such as wet impregnation, sonochemical method, che-
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2011 Copyright (CC) SCS

MICROWAVE SYNTHESIS OF Pt AND Pt–Rh–Sn ELECTROCATALYSTS 1675
mical reduction of metal precursors, etc. In the last decade, Pt or Pt-based nanoclusters
with small particle size and narrow size distribution have often been synthesized
by the polyol method. 16 This procedure, as most of the other conventional
methods, requires longer treatment of metal precursors at a high temperature.
To overcome the arduous processes, in recent years, microwave irradiation
has been widely used for the preparation of nanomaterials. Compared with conventional
preparation methods, microwave synthesis has the advantages of very
short heating time and uniform heating of the substance, leading to a small particle
size, narrow particle size distribution and high purity. Yu et al. 17 suggested
that these advantages could be attributed to fast homogenous nucleation and
growth of metal particles.
The goal of this work was to examine ethanol oxidation on a carbon-supported
Pt–Rh–Sn catalyst synthesized by the microwave-assisted polyol method.
This procedure for the preparation of the previously mentioned catalyst, to the
best of our knowledge, has not hitherto been described in the literature.
EXPERIMENTAL
Preparation of Pt and Pt–Rh–Sn/C electocatalysts
To prepare Pt–Rh–Sn catalyst, a mixture of 0.5 ml of 0.05 M H2PtCl6, 0.5 ml of 0.1 M
SnCl2 solution and 0.5 ml 0.05M of RhCl3 was mixed with 25 ml of ethylene glycol (EG) in a
100 ml beaker under magnetic stirring. Then 0.8 M NaOH was added drop wise to adjust the
pH to ≈12. The same procedure was used to synthesize the Pt catalyst. In each case, the beaker
was placed in the center of a domestic microwave oven and heated 60 s for the Pt and 90 s for
Pt–Rh–Sn catalyst at 700 W. After microwave heating, the mixture was uniformly mixed with
20 ml of an aqueous suspension of Vulcan XC-72 carbon (containing 20 mg of carbon in the
case of Pt catalyst and 53.5 mg of carbon in the case of Pt–Rh–Sn catalyst) and 150 ml of 2 M
H2SO4 solution for 3 h under magnetic stirring. The resulting suspension was filtered and the
residue was washed with high purity water. The solid product was dried at 160 °C for 3 h
under a N2 atmosphere. The metal loading for both catalysts should have been ≈20 wt. %.
Thermogravimetric analysis (TGA) confirmed 19 wt. % for Pt/C, while for Pt–Rh–Sn/C, a
lower loading of metal (≈11 wt. %) was found.
Characterization of the Pt/C and Pt–Rh–Sn/C electrocatalysts
Thermogravimetry (TG) and differential thermal analyses (DTA) were performed simultaneously
(30–800 °C temperature range) on a SDT Q600 TGA/DSC instrument (TA Instruments).
The heating rate was 20 °C min-1 and the sample mass was less than 10 mg. The
furnace atmosphere consisted of air at a flow rate of 100 cm3 min-1 .
The unsupported Pt and Pt–Rh–Sn nanoparticles were characterized by scanning tunneling
microscopy (STM). Samples were prepared by applying a few drops of a diluted colloidal
solution of catalyst on a hot HOPG plate. The STM characterizations were realized
using a NanoScope III A (Veeco, USA) microscope. The images were obtained in the height
mode using a Pt–Ir tip (set-point current, It, from 1 to 2 nA, bias voltage, Vb = –300 mV). The
mean particle size and size distribution were acquired from several randomly chosen areas of
the STM images containing about 50 particles.
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1676 STEVANOVIĆ et al.
X-Ray diffraction (XRD) patterns of the powder catalysts were recorded with an Ital
Structure APD2000 X-ray diffractometer in Bragg–Brentano geometry using CuK α radiation
(λ = 0.15418 nm) in the step-scan mode (range: 15−85° 2θ, step-time: 2.50 s, step-width:
0.02°). The program PowderCell [18] was used for phase analysis and calculation of the unit
cell parameters.
Microstructural examination was performed by scanning electron microscopy (SEM). An
XL 30 environmental scanning microscope with a field emission gun (ESEM–FEG) (manufactured
by FEI, The Netherlands) equipped with an energy dispersive X-ray (EDX) spectrometer
was used. The samples were inspected using 5, 10 and 20 kV acceleration voltages at
magnifications of 20000× and 10000×.
The electrocatalytic activity of the catalysts was investigated by potentiodynamic and
chronoamperometric tests using an Autolab potentiostat/galvanostat (ECO Chemie, The
Netherlands) and a three-electrode compartment cell at room temperature. The working
electrode was a thin layer of Nafion-impregnated Pt/C or Pt–Rh–Sn/C catalyst applied on a
glassy carbon disk electrode with a loading of 10 μg cm -2 of the catalyst counted on metal
content. The thin layer was obtained from a suspension of 2 mg of the Pt–Rh–Sn/C or 1 mg of
Pt/C catalyst in a mixture of 1 ml water and 50 µl of 5 % aqueous Nafion solution, prepared in
an ultrasonic bath, placed onto the substrate and dried at room temperature. A Pt wire and a
saturated calomel electrode (SCE) were used as the counter and reference electrode, respectively.
The electrocatalytic activity of the as-prepared Pt/C and Pt–Rh–Sn/C was studied in 0.1
M HClO 4 + 0.5 M C 2H 5OH solution. The electrolyte was prepared with high purity water and
deaerated with N 2. Ethanol was added to the supporting electrolyte solution while holding the
electrode potential at –0.2 V. The potential was then cycled up to 0.3 V, i.e., the potential
range of technical interest (E < 0.4 V), at sweep rate of 20 mV s -1 . Current–time transient
curves were recorded after immersion of the freshly prepared electrode in the solution at –0.2 V
for 2 s followed by stepping the potential to 0.2 V and holding the electrode at this potential
for 30 min.
RESULTS AND DISCUSSION
Catalysts characterizations
The particle size and surface morphology of the unsupported Pt and Pt–Rh–Sn
catalysts were characterized by STM. As observed from the top view of the STM
images (Fig. 1), both catalysts had rather uniform particles of small diameter.
Most of particles were spherical in shape. Cross section analysis (Fig. 1) confirmed
particle sizes of < 1.7 nm for both catalysts (Table I).
The Pt/C and Pt–Rh–Sn/C catalysts were characterized by X-ray powder diffraction
analysis. The XRD patterns of the carbon-supported catalysts are shown
in Fig. 2. Two phases were identified in the Pt/C pattern, one with the main
characteristic peaks of the face-centered cubic crystal structure (fcc) of platinum
(111, 200, 220 and 311) and the other with a diffraction peak at around 2θ 25°
related to the graphite-like structure of the Vulcan XC-72R carbon support. The
XRD peaks of the Pt–Rh–Sn/C catalyst were rather broad and diffraction peaks
for Pt, Rh and Sn in the catalyst could not be separately resolved, indicating a
small metal content and a very low crystallinity or an amorphous form of the catalyst.
Since TGA analysis revealed the presence of only 11 wt. % of the metals
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MICROWAVE SYNTHESIS OF Pt AND Pt–Rh–Sn ELECTROCATALYSTS 1677
in the supported catalyst and the STM analysis showed very small particle size,
the low metal fraction and low crystallinity could be the reason for the obtained
XRD pattern of the Pt–Rh–Sn/C catalyst. The mean particle size for the Pt/C catalyst
calculated by the Scherrer formula 19 was larger than that obtained by STM
(Table I), possibly because unsupported catalysts were used for the STM analysis.
Still, the agreement can be described as good because the values calculated
by the Scherrer formula also account for a very probable lattice stress.
Fig. 1. STM Images and height profiles of the Pt catalyst (50×50 nm 2 ×4 nm)
and Pt–Rh–Sn catalyst (30×30 nm 2 ×4 nm).
The small particle sizes and homogeneous size distributions of both catalysts
could be attributed to the advantages of the microwave-assisted modified polyol
process in which ethylene glycol (EG) and hydroxide are used as stabilizers. The
metal salts and hydroxide react to form colloidal metal hydroxide particles,
which are reduced to metal nanoclusters by EG. 16 In this process, pH value of the
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1678 STEVANOVIĆ et al.
TABLE I. Characteristics of the Pt/C and Pt–Rh–Sn/C catalysts obtained by STM, XRD and
EDX analysis
Catalyst
Particle size, nm Unit cell parameter
STM XRD (XRD), a / nm
Elemental composition (Pt:Rh:Sn), at. %
Nominal EDX
Pt/C 1.7±0.3 2.5 0.3944
Pt–Rh–Sn/C 1.2±0.3 – –
25:25:50 46:32:22
Fig. 2. XRD Patterns of the Pt /C and Pt–Rh–Sn/C
catalysts.
solution is very important for obtaining stable metal particles. Hydroxide/metal
molar ratio depends on the metal in question and is characteristic of electrostatic
stabilization of metal colloids by the adsorbed anions. 16 EG acting as both reaction
and dispersion media can efficiently adsorb and stabilize the surface of the
particles 20 and favor the production of monodispersed metal particles with good
dispersivity. 17 The high viscosity of this compound also helps in preventing agglomeration
of the nanoparticles and for this reason, the water content in the reaction
solution effects this process as well as the reaction temperature in influencing
the particle size and size distribution. 21 Since EG has a large permanent
dipole, it is very susceptible to microwave irradiation, which can absorb the
energy from the microwave field and the polar reaction solution is heated up to a
high temperature instantaneously. 22 Nevertheless, the heating rate of EG dispersion
system could be affected by parameters of microwave operation, such as
irradiation time, amount of dielectric and additives. 23 The fast and uniform microwave
heating reduces the temperature and concentration gradients, thus accele-
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MICROWAVE SYNTHESIS OF Pt AND Pt–Rh–Sn ELECTROCATALYSTS 1679
rating the reduction of the metal ions and the formation of metallic nuclei. 24–27
In processes with plurality of pathways and activation barriers, according to Rao
et al. 26 , microwaves might promote the pathway with the lowest activation barrier.
EDX Analysis of the Pt–Rh–Sn/C surface composition gave 48 at. % Pt and
32 at. % Rh and 20 at. % Sn for the Pt–Rh–Sn alloy (Table I), which deviates
from the nominal composition in the initial mixture (25:25:50).
Bearing in mind the aforesaid, the irradiation time in the microwave heating
together with NaOH/metal and EG/water ratios used in the synthesis of both
catalyst were probably the reason for the so small particle size obtained, and also
for the yield of the metal and the catalyst composition.
Electrochemical performances
Ethanol oxidation was studied at the as-prepared Pt/C and Pt–Rh–Sn/C catalysts.
The cyclic voltammogram for Pt–Rh–Sn/C after only two cycles to characterize
roughly the surface but to avoid significant dissolution of Rh and Sn is
shown in Fig. 3. The cyclic voltammograms for Pt/C are given as steady state and
after two cycles for comparison. The steady state CV for Pt/C was similar to
those for polycrystalline platinum or other Pt catalysts supported on high surface
area carbon, with a well-defined region of hydrogen adsorption/desorption, separated
by a double layer from the region of surface oxide formation. These regions
were not well-defined at the beginning of cycling since the surface was still not
fully reconstructed. The CV for Pt–Rh–Sn/C was similar to the CV for PtRh/C10 i / mA
0.3
0.2
0.1
0.0
-0.1
-0.2
Pt-Rh-Sn/C 2 nd cycle
Pt/C
Pt/C 2 nd cycle
-0.3
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
E / V vs. SCE
Fig. 3. Basic voltammograms of the Pt/C and Pt–Rh–Sn/C catalysts in
0.1 M HClO 4, v = 50 mV s -1 .
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1680 STEVANOVIĆ et al.
and is characterized by a large single peak in the hydrogen adsorption/desorption
region, which Lima et al. 10 associated with adsorption/desorption of hydrogen on
the intermetallic phase of PtRh. The shift of the reduction peak for Pt–Rh–Sn/C
to more a negative potential value in comparison to Pt/C could be an indication
of alloyed Pt and Rh. The larger double layer in the case of Pt–Rh–Sn/C compared
to Pt/C was due to the lower metal content (11 mass %) of this catalyst in
comparison to Pt/C (20 mass %).
The electrocatalytic activities of the as-prepared catalysts were studied in 0.1
M HClO4 + 0.5 M C2H5OH solution and the positive scan voltammetric curves
are presented in Fig. 4. The Pt–Rh–Sn/C catalyst was highly active in ethanol
oxidation with the onset potential at approximately –0.15 V (shifted by ≈0.15 V
towards more negative potentials compared to Pt/C) and rapid kinetics. The hydrogen
adsorption/desorption peaks were clearly suppressed because the ethanol
adsorption displaced the adsorbed hydrogen from the interface. The current densities,
calculated on Pt content, throughout the studied potential region were five
times higher for the Pt–Rh–Sn/C catalyst in comparison to the Pt/C catalyst. The
stability of the catalysts was studied in chronoamperometric experiments and the
results are presented in Fig. 5. The higher initial current density at 0.2 V on the
Pt–Rh–Sn/C catalyst in comparison to the Pt/C catalyst is in accordance with the
-1
j / mA mg Pt
100
50
0
Pt/C
Pt-Rh-Sn/C
-0.2 -0.1 0.0 0.1 0.2 0.3
E / V (SCE)
Available online at www.shd.org.rs/JSCS
Fig. 4. Potentiodynamic curves for
the oxidation of 0.5 M C 2H 5OH at
the Pt/C and Pt–Rh–Sn/C catalysts in
0.1 M HClO 4, v = 20 mV s -1 .
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MICROWAVE SYNTHESIS OF Pt AND Pt–Rh–Sn ELECTROCATALYSTS 1681
potentiodynamic measurements. The initial current at Pt–Rh–Sn/C stabilizes at a
value that was significantly higher than that for the Pt/C catalyst. The Pt–Rh–
–Sn/C catalyst is evidently more active and less poisoned than the Pt/C catalyst.
-1
j / mA mg Pt
50
40
30
20
10
Pt/C
Pt-Rh-Sn/C
0
0 300 600 900 1200 1500 1800
t / s
Fig. 5. Chronoamperometric curves for the oxidation of 0.5M C2H5OH at 0.2 V
at the Pt/C and Pt–Rh–Sn/C catalysts in 0.1 M HClO4. The better activity for the EOR of the ternary Pt–Rh–Sn catalyst in comparison
to Pt/C catalyst must be related to the formation of the ternary alloy. Thus,
the presence of Sn and Rh in the catalyst can promote ethanol oxidation by an
electronic effect in the Pt-based electrode material affecting the adsorption properties
of the surface, making this system less prone to poisoning by organic
species than pure Pt. Sn or its oxides can supply surface oxygen-containing species
at lower potentials by activation of the interfacial water molecule necessary
to complete the oxidation of the adsorbed reaction intermediates leading to
carbon dioxide, in the situation that the C−C bond was broken, or to the formation
of acetic acid. 12,29 This oxidative removal of CO-like species strongly
adsorbed on adjacent Pt active sites proceeds through the so-called bi-functional
mechanism. According to density functional theory (DFT) calculations, 14 the role
of Rh was to adsorb and stabilize the key intermediate CH2CH2O in this route,
which leads to the cleavage of C−C bonds. A back donation from the Rh d-band
electrons to Pt was proposed. Thus, the presence of Pt could modify the electronic
structure of Rh by partially emptying its d-band states enabling its strong
bonding to CH2CH2O, while the activity of Pt was lowered, thereby preventing
the partial oxidation of ethanol on the Pt sites. 14
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1682 STEVANOVIĆ et al.
In comparison with other Pt–Rh–Sn/C catalysts described in the literature 12–
14 in which the atomic fraction of Sn was equal or higher than that of Pt, while
the atomic fraction of Rh was much smaller than both Pt and Sn, the presently
studied Pt–Rh–Sn/C catalyst with highest fraction of Pt and lowest fraction of Sn
exhibited a comparably high activity for ethanol oxidation.
Colmati et al. 12 studied ethanol oxidation at Pt–Rh–Sn/C catalysts with
molar ratios 1:1:1 and 1:0.3:1, prepared by formic acid oxidation. The ternary
catalyst with the lower fraction of Rh was found to have a higher activity, but
both the Pt–Rh–Sn/C catalysts exhibited a significantly lower activity in comparison
with PtSn/C catalyst at lower potentials (< 0.45 V RHE) and the opposite
at higher potential values. The higher activity of PtSn/C was ascribed to the presence
of SnO2 that could supply oxygen-containing species for the oxidative
removal of CO and CH3CO adsorbed on adjacent Pt active sites. The introduction
of Rh changes the geometry (Pt−Pt bond distances) and electronic (Pt d-band
vacancy) structure of PtSn catalysts, which could improve the adsorption of ethanol
and the cleavage of C−C bonds, increasing in this way the activity for ethanol
oxidation at higher potentials. 12 For ethanol oxidation, Kowal et al. 13 and Li et
al. 14 used Pt–Rh–SnO2/C catalysts prepared by the polyol method with conventional
heating. XRD Analysis of these catalysts showed the presence of two
phases, Pt–Rh alloy and SnO2. The catalysts were highly active at lower potentials
(< 0.5 V RHE). The catalysts were synthesized with different ratios of the
components (Pt:Rh:Sn from 3:1:2 to 3:1:6) and the highest activity for ethanol
oxidation and capability to split C−C bonds, as revealed by infrared reflectionadsorption
spectroscopy (IRRAS), was achieved with Pt–Rh–Sn 3:1:4. 14
Although XRD analysis of the present catalyst did not give any information
except for its very low crystallinity, some reasonable assumptions can be made
based on literature data. It can be assumed that, similarly to other Pt–Rh–Sn
catalysts, the Pt was alloyed with Rh since, according to the Pt–Rh phase diagram,
29 these two metals form a solid solution at any ratio. XPS and XRD analyses
of bimetallic PtSn catalysts prepared by microwave or conventional heating
of ethylene glycol solutions of H2PtCl6 and SnCl2 salts indicated significant
amounts of SnO2 and rather low degree of alloying. 25,30 Thus, it can be assumed
that the Sn in the present Pt–Rh–Sn catalyst existed mainly as SnO2. Both assumptions
mean that the prepared catalyst should be similar to the catalysts
obtained by Li et al. 14 but with a larger fraction of Rh and a lower fraction of Sn,
but still exhibiting a comparable performance. It was shown that the addition of a
small quantity of Sn greatly enhanced the electrooxidation of ethanol at low
potentials. 28,31 On the other hand, a low amount of Rh added to Pt only slightly
improved the CO2 yield in ethanol oxidation, while the optimum catalyst composition
for C−C bond breakage and CO2 formation was Pt:Rh 1:1 or even better
3:1. 9,10 Hence, the high activity for ethanol oxidation of the present Pt–Rh–Sn/C
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MICROWAVE SYNTHESIS OF Pt AND Pt–Rh–Sn ELECTROCATALYSTS 1683
catalyst can be explained by a balanced action and well-tuned content of all three
components. In addition, the particle size effect cannot be ignored since very
small particles of Pt–Rh–Sn/C catalyst contribute to an increase of the active
surface area of the catalyst.
CONCLUSIONS
A microwave-assisted polyol method was used to prepare carbon supported
Pt and Pt–Rh–Sn nanoparticles with high electrocatalytic activities for the ethanol
electrooxidation reaction.
The structural (XRD) and surface characterization (STM) of the catalysts
revealed that catalysts with small particles and a rather uniform size distribution
were synthesized by this method. This could be attributed to the advantages of
the microwave-assisted modified polyol process in ethylene glycol solution.
The electrochemical measurements revealed a high activity of the prepared
Pt–Rh–Sn/C catalyst for ethanol oxidation. This catalyst had five times higher
oxidation currents and significantly lower reaction onset potential than the Pt/C
catalyst. Chronoamperometric measurements confirmed notably less poisoning of
the Pt–Rh–Sn/C catalyst than of the Pt/C catalyst. Although with significantly
higher fraction of Rh and lower fraction of Sn in comparison to other Pt–Rh–Sn
catalysts described in the literature, the catalyst prepared in the present study
exhibited a similar shift of onset potential to negative values as well as lower
poisoning. The increased activity of Pt–Rh–Sn/C catalyst in comparison to Pt/C
catalyst was most probably promoted by the bi-functional mechanism and the
electronic effect of the alloyed metals.
Acknowledgements. This work was financially supported by the Ministry of Education
and Science of the Republic of Serbia, Contract No. 172060.
ИЗВОД
МИКРОТАЛАСНА СИНТЕЗА И КАРАКТЕРИЗАЦИЈА Pt И Pt–Rh–Sn КАТАЛИЗАТОРА
ЗА ОКСИДАЦИЈУ ЕТАНОЛА
САЊА СТЕВАНОВИЋ 1 , ДУШАН ТРИПКОВИЋ 2 , ДЕЈАН ПОЛЕТИ 3 , ЈЕЛЕНА РОГАН 3 , АМАЛИЈА ТРИПКОВИЋ 1
И ВЛАДИСЛАВА М. ЈОВАНОВИЋ 1
1 2
IHTM – Centar za elektrohemiju, Wego{eva 12, Beograd, Materials Science Division, Argonne National
Laboratory, Argonne, IL 60439, USA i 3Tehnolo{ko–metalur{ki fakultet,
Univerzitet u Beogradu, Karnegijeva 4, Beograd
Pt и Pt–Rh–Sn катализатори на угљенику развијене површине су синтетизовани полиол-
-микроталасним поступком у раствору етиленгликола и испитивани за реакцију елетрохемијске
оксидације етанола у киселој средини. Катализатори су окарактерисани структурно,
морфолошки и по саставу коришћењем XRD, STM и EDX техника. STM анализа је потврдила
да су Pt и Pt–Rh–Sn честице униформне величине и пречника мањег од 2 nm. XRD анализа
Pt/C катализатора показала је присуство две фазе, једне са главним карактеристичним
пиковима за пљосно-центрирану кубну кристалну структуру платине (111, 200, 220 и 311) и
друге са дифракционим пиком на 2θ око 25° карактеристичним за хексагоналну структуру
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1684 STEVANOVIĆ et al.
вулкана XC-72R (угљеничног носача). XRD анализа Pt–Rh–Sn/C катализатора није показала
карактеристичне пикове, што је индикација веома мале кристаличности катализатора. Активност
катализатора испитивана је потенциодинамичким и хроноамперометријским мерењима.
Pt–Rh–Sn/C катализатор је веома активан за оксидацију етанола са почетком реакције
на потенцијалима за око 150 mV помереним ка негативнијим вредностима и струјама које су
око пет пута веће у поређењу са Pt/C катализатором. Стабилност катализатора испитивана
хроноамперометријски показала је да се Pt–Rh–Sn/C катализатор мање трује од Pt/C катализатора.
Мала величина и хомогена дистрибуција честица могу се приписати предностима
микроталасне синтезе и модификованог полиол поступка у раствору етиленгликола. Већа активност
Pt–Rh–Sn/C катализатора у поређењу са Pt/C катализатором последица је би-функционалног
механизма и електронског (лиганд) ефекта метала у синтетизованој легури.
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Microwave-assisted polyol synthesis
of carbon-supported platinum-based
bimetallic catalysts for ethanol oxidation
S. Stevanović, D. Tripković, J. Rogan,
K. Popović, J. Lović, A. Tripković &
V. M. Jovanović
Journal of Solid State
Electrochemistry
Current Research and Development in
Science and Technology
ISSN 1432-8488
Volume 16
Number 10
J Solid State Electrochem (2012)
16:3147-3157
DOI 10.1007/s10008-012-1755-y
1 23

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1 23

J Solid State Electrochem (2012) 16:3147–3157
DOI 10.1007/s10008-012-1755-y
ORIGINAL PAPER
Microwave-assisted polyol synthesis of carbon-supported
platinum-based bimetallic catalysts for ethanol oxidation
S. Stevanović & D. Tripković & J. Rogan & K. Popović &
J. Lović & A. Tripković & V. M. Jovanović
Received: 10 October 2011 /Revised: 18 April 2012 /Accepted: 18 April 2012 /Published online: 5 May 2012
# Springer-Verlag 2012
Abstract High surface area carbon-supported Pt, PtRh, and
PtSn catalysts were synthesized by microwave-assisted polyol
procedure and tested for ethanol oxidation in perchloric acid.
The catalysts were characterized by thermogravimetric analysis
(TGA), X-ray diffraction (XRD), scanning tunnelling microscopy
(STM), TEM, and EDX techniques. STM analysis of
unsupported catalysts shows that small particles (∼2 nm)with
a narrow size distribution are obtained. TEM and XRD examinations
of supported catalysts revealed an increase in particle
size upon deposition on carbon support (diameter∼3nm).The
diffraction peaks of the bimetallic catalysts in X-ray diffraction
patterns are slightly shifted to lower (PtSn/C) or higher
(PtRh/C) 2θ values with respect to the corresponding peaks at
Pt/C catalyst as a consequence of alloy formation. Oxidation of
ethanol is significantly improved at PtSn/C with the onset
potential shifted for∼150 mV to more negative values and the
increase of activity for approximately three times in comparison
to Pt/C catalyst. This is the lowest onset potential found for
ethanol oxidation at PtSn catalysts with a similar composition.
Chronoamperometric measurements confirmed that PtSn/C is
notably less poisoned than Pt/C catalyst. PtRh/C catalyst
exhibited mild enhancement of overall electrochemical reaction
in comparison to Pt/C.
S. Stevanović : D. Tripković : K. Popović : J. Lović :
A. Tripković : V. M. Jovanović (*)
ICTM, Department of Electrochemistry, University of Belgrade,
Njegoševa 12,
Belgrade, Serbia
e-mail: vlad@tmf.bg.ac.rs
J. Rogan
Faculty of Technology and Metallurgy, University of Belgrade,
Karnegijeva 4,
Belgrade, Serbia
Author's personal copy
Keywords PtSn/C . PtRh/C . Ethanol oxidation . Polyol
synthesis . Microwave irradiation
Introduction
Limited sources of fossil fuels as well as the request for
reduction of environmental pollution led to the development
of alternative energy sources, such as fuel cells. Although the
best performance, so far, has been achieved using hydrogen as
fuel, its storage, handling, and distribution appear to be important
barriers for direct and widespread application. Direct use of
liquid fuels, such as methanol, ethanol, and formic acid, has
been studied as a convenient alternative. Ethanol is receiving
increasing attention due to its low toxicity and large quantity
production from the fermentation of biomass as a renewable
biofuel [1]. However, to oxidize ethanol efficiently, it is necessary
to develop a catalyst capable for converting it completely
to CO2. Pt is an excellent catalyst for dehydrogenation of small
organic molecules but, on the other hand, has several significant
drawbacks: high cost, extreme susceptibility to poisoning
by CO, and, in the case of ethanol, the main products of its
oxidation are acetaldehyde and acetic acid, while CO2 is produced
at high potentials [2]. Efforts to improve catalyst performance
and minimize quantity of Pt in the catalyst have been
centered on the addition of metals such as Ru, Rh, W, Pd, Sn,
etc. [1–10].
PtSn is one of the extensively studied and among the most
active bimetallic catalysts for ethanol oxidation. The activity
of PtSn is attributed to a bifunctional mechanism in which
alcohol is adsorbed and dehydrogenated at Pt, while added
metal supplies OH at significantly lower potentials compared
to Pt, providing oxidation of adsorbed reaction intermediates
(CO-like species strongly adsorbed on adjacent Pt active sites)
leading to carbon dioxide, in the situation that the C–C bond

was broken and the formation of acetaldehyde and acetic acid
was suppressed [11, 12]. Besides, the electronic interaction
between Pt and alloyed metal results in a weaker bond of
adsorbed species on Pt and contributes to enhanced activity of
these catalysts [12–16]. While there is no doubt in the beneficial
role of the bifunctional mechanism in increased activity
of PtSn catalyst for ethanol oxidation, this view on alloying
effect is rather controversial. According to some authors,
higher alloying degree leads to a larger increase in activity
of the PtSn catalyst [14, 15], whereas for others, unchanged
lattice parameter of Pt in nonalloyed Pt-SnO x catalysts enables
remarkable promotion of ethanol oxidation [17–19]. However,
although affecting the overall electrochemical activity in ethanol
oxidation, addition of Sn does not enhance the yield of
CO2, i.e., C–C bond braking as revealed by differential mass
spectrometry (DEMS) and in situ infrared spectroscopy (FTIR)
[2, 7, 13].
On the other hand, addition of Rh to Pt leads to increased
ratio of CO2/acetaldehyde and CO2/acetic acid as evidenced by
DEMS and FTIR techniques, which is ascribed to improved
activation of C–C bond dissociation [9, 20]. At the same time,
the overall electrochemical reaction is not enhanced by the
PtRh/C catalyst possibly because of a stronger CO–Rh bond
and/or slower dehydrogenation of ethanol at Rh in comparison
to Pt [21].
In general, electrocatalytic performance of the Pt-based
catalysts considerably depends not only on the nature of the
added metal but also on a variety of conditions of the
synthesis procedures which determine surface composition
and morphology of synthesized materials. In the last decade,
polyol method has been often used for the preparation of Pt
or Pt-based nanoclusters with small particle size and narrow
size distribution [22]. In this procedure, ethylene glycol
(EG) and hydroxide are used as stabilizers. EG acting as
both reaction and dispersion media can efficiently adsorb
and stabilize the surface of the particles [23] and favor
formation of metal particles with good dispersivity [24,
25]. Since EG has high permanent dipole, it is very susceptible
to microwave irradiation, which can take up the energy
from the microwave field and get the polar reaction solution
heated up to high temperature instantaneously [26]. The fast
and uniform microwave heating reduces the temperature and
concentration gradients, thus accelerating the reduction of
the metal ions and enabling homogeneous nucleation and
shorter crystallization time leading to formation of small
uniform metal particles [27–30].
In this work, as-prepared carbon-supported nanoparticles,
Pt, PtRh, and PtSn catalysts, synthesized by microwaveassisted
polyol method, were characterized for ethanol electrooxidation
reaction. Although microwave-assisted synthesis
of Pt/C and PtSn/C catalyst has been described in
literature [28, 31], to our knowledge, only recently have
such catalysts been studied for ethanol oxidation [32]. This
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3148 J Solid State Electrochem (2012) 16:3147–3157
reaction has not been examined by PtRh/C catalyst prepared
by microwave irradiation so far.
Experimental
Preparation of the catalysts
Stabile Pt, PtRh, and PtSn nanoparticles were prepared by
polyol method. Briefly, equal volumes of 0.05 M water solutions
of required metal precursor salts (H 2PtCl 6 alone or
either with SnCl2 or RhCl3) were mixed with EG, and NaOH
was added dropwise. The solutions mixture was constantly
stirred. In this way, EG/water ratio was 20/1, NaOH/metal
ratio 8/1, and pH of the solutions adjusted to over 12. The
prepared solutions were heated in a microwave oven at 700 W
for 60 s (Pt) or 90 s (bimetallic). After microwave heating,
colloidal solutions were uniformly mixed with water suspension
of high-area carbon (Vulcan XC-72) and 2 M H2SO4
solution and stirred for 3 h. The resulting suspension was
filtered, and the residue was rinsed with high purity water.
The solid product was dried at 160 °C for 3 h in N2 atmosphere.
In all cases, the amount of metal and carbon was
adjusted to the loading of 20 mass% of the catalyst.
Characterization of the catalysts
The thermogravimetric (TGA) and differential thermal (DTA)
analyses were performed simultaneously (30–800 °C range)
on a SDT Q600 TGA/DSC instrument (TA Instruments). The
heating rates were 20 °C min −1 , and the sample mass was less
than 10 mg. The furnace atmosphere consisted of air at a flow
rate of 100 cm 3 min −1 .
Unsupported Pt, PtRh, and PtSn nanoparticles were characterizedbyscanningtunnellingmicroscopy(STM).Samples
were prepared by applying a few drops of a diluted colloidal
solution of a catalyst on a hot HOPG plate. STM characterizations
were performed using a NanoScope III A (Veeco,
USA) microscope. The images were obtained in the height
mode using a Pt-Ir tip (set-point current, it, from1to2nA,
bias voltage, Vb0−300 mV). The mean particle size and size
distribution were acquired from a few randomly chosen areas
in the STM images containing about 100 particles.
The X-ray diffraction (XRD) patterns of the powder catalysts
were recorded with an Ital Structure APD2000 X-ray
diffractometer in a Bragg–Brentano geometry using CuKα
radiation (λ01.5418 Å) and step-scan mode (range: 15−85 °
2θ, step-time: 2.50 s, step-width: 0.02 °). The program PowderCell
[33] was used for phase analysis and calculation of
unit cell parameters.
Structural examination of the catalysts was performed by
EDX technique coupled with scanning electron microscopy
usingJEOLJSM-6610(USA)instrumentwithX-Max

(silicon drift) detector and super atmospheric thin window
(SATW) applying 20 kV. The measurements were performed
at ten different regions of each sample.
TEM analysis of the supported catalysts was carried out
using a Philips CM12 TEM microscope (The Netherlands)
operated at 100 kV. The samples were prepared by ultrasonically
dispersing the catalysts powders in ethanol and applying
a drop of the suspension onto the carbon-coated copper
grid. The size distribution of the catalysts particles were
created based on 450 particles from a few different areas
of the sample for each catalyst.
Electrochemical measurements
All of the electrochemical experiments were performed at
room temperature in a three-electrode compartment electrochemical
cell with a Pt wire as the counter electrode and a
bridged saturated calomel electrode (SCE) as reference. The
working electrode was a thin layer of Nafion-impregnated
Pt/C, PtRh/C, or PtSn/C catalysts applied on a polished
glassy carbon disk electrode with 20 μg/cm 2 loading of
the catalyst. The thin layer was obtained from a suspension
of 2 mg of the respective catalyst in a mixture of 1 ml water
and 50 μl of 5 % aqueous Nafion solution, prepared in an
ultrasonic bath, placed onto the substrate and dried at room
temperature.
The electrocatalytic activity of as-prepared Pt/C, PtRh/C,
and PtSn/C was studied in 0.1 M HClO4+0.5 M C2H5OH
solution. Ethanol was added to the supporting electrolyte
solution while holding the electrode potential at −0.2 V. The
potential was then cycled up to 0.3 V, i.e., the potential
range of technical interest (E

Fig. 1 STM images, crosssection
analysis, and
corresponding particle size distribution
diagrams of Pt, PtRh,
and PtSn unsupported catalysts
(50×50×4 nm)
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3150 J Solid State Electrochem (2012) 16:3147–3157
Pt
PtRh
PtSn
N/N total (%)
N/N total (%)
N/N total (%)
10
8
6
4
2
d = 1.4 ± 0.3
0
0 1 2 3
d (nm)
10
8
6
4
2
d = 2.5 ± 0.3
0
1 2 3 4
d (nm)
20
16
12
8
4
0
0 1 2 3
d (nm)
d = 1.1 ± 0.4

Fig. 2 XRD patterns of supported Pt/C, PtRh/C, and PtSn/C catalysts
shift of diffraction peaks of PtSn/C and PtRh/C catalysts with
respect to Pt/C and the values of their lattice parameters should
indicate the alloy formation between Pt and added metals but
in low degree. The average crystallite size of each catalyst is
calculated by Scherrer formula [36], and they are mutually
similar, but for each catalyst larger than obtained by STM
(Table 1). The reason for this difference is most probably
because unsupported catalysts have been used for STM analysis,
while XRD examinations have been performed with
supported catalysts. Since the process of particle deposition
onto the carbon support involves thermal treatment, it is
possible that sintering of the particles occurred. Still, the
agreement can be described as considerably good.
Typical TEM images of supported Pt-based catalysts (Pt/
C, PtRh/C, and PtSn/C) and corresponding histograms of
particle size distribution are displayed in Fig. 3. Observation
of these images points out that catalyst particles are uniformly
distributed on the carbon support. From the particle
size histograms, it is obvious that the supported catalysts
have similar particle size of ~3 nm which is in a very good
agreement with the mean particles size evaluated from the
Table 1 Pt/C, PtRh/C, and PtSn/C catalyst parameters obtained by
STM, XRD, TEM, and EDX analyses
Catalyst Pt/C PtRh/C PtSn/C
STM a Particle size (nm) 1.4±0.3 2.5±0.3 1.1±0.4
XRD Lattice parameter
a (nm)
0.3944 0.3934 0.3963
XRD Particle size
(nm)
2.5 2.6 2.6
TEM Particle size (nm) 2.7±0.8 3.1±0.7 2.4±0.7
Elemental composition
Nominal (at.%)
50:50 50:50
Elemental composition
EDX (at.%)
60.16:39.84 53.18:46.82
a STM analysis of unsupported catalyst particles
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XRD patterns. This confirms sintering of the metal particles
obtained by microwave irradiation upon their deposition
onto the carbon support and explains the difference with
STM analysis of unsupported catalysts.
Although no peaks for metallic Rh or Sn or their oxides
were detected by XRD, their presence should not be excluded,
since they might be present in amorphous state or have a
smaller particle size. Moreover, they should be present in a
significant amount because EDX and TGA analyses of the
catalysts have indicated practically no loss in the catalysts
components. TGA revealed 20, 18, and 16 wt.% for Pt/C,
PtSn/C, and PtRh/C catalyst, respectively. EDX compositions
of bimetallic PtSn/C and PtRh/C catalysts (Table 1)
slightly deviate from the nominal values.
Electrochemical performances
Ethanol oxidation was studied at as-prepared Pt/C,
PtRh/C, and PtSn/C catalysts. Therefore, the initial voltammograms,
i.e., only 1 cycle for each of the catalysts
have been recorded and presented in Fig. 4. Steady state
cyclic voltammogram for Pt/C is given in the insert to
illustrate similarity of this catalyst with polycrystalline
platinum or other Pt catalysts supported on high area
carbon [8, 13]. Cyclic voltammograms for Pt/C and
PtSn/C are characterized by a defined region of hydrogen
adsorption/desorption, separated by a double layer
from the region of surface oxide formation. In the
hydrogen adsorption/desorption region, the cyclic voltammogram
for PtSn/C catalyst is similar to the voltammogram
for Pt/C but with smaller currents due to a
smaller amount of Pt sites on the surface. There is no
clear boundary between hydrogen adsorption/desorption,
double layer, and surface oxide regions in the cyclic
voltammogram for the PtRh/C catalyst. The voltammogram
reminds us of the one for such catalyst obtained
by Lima et al. [9, 21], characterized by a large single
peak in hydrogen adsorption/desorption region which
the authors associated to adsorption/desorption of hydrogen
on a PtRh intermetallic phase. Shift of reduction
peak for PtRh/C to a more negative potential value in
comparison to Pt/C should be an indication of more
stable oxides of this catalyst [21].
The real surface area of the as-prepared catalysts was
evaluated from CO stripping measurements (Fig. 5), assuming
linear bonding of CO ads [2, 9, 37]. This enabled calculation
of particle size of Pt/C and PtRh/C catalysts
supposing homogenous distribution of spherical-shaped particles
from the equation [38]:
d ¼ 6ooo
p S
where S (square meter per gram) is specific surface area and ρ

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3152 J Solid State Electrochem (2012) 16:3147–3157
Fig. 3 TEM images and particle size distributions of supported Pt/C, PtRh/C, and PtSn/C catalysts
N/N total (%)
N/N total (%)
N/N total (%)
30
20
10
0
50
40
30
20
10
d = 2.7 ± 0.8
1 2 3
d (nm)
4 5 6
Pt/C
d = 3.1 ± 0.7
0
0 1 2 3
d (nm)
4 5 6
50
40
30
20
10
PtRh/C
d = 2.4 ± 0.7
0
0 1 2 3
d (nm)
4 5 6
PtSn/C

Fig. 4 Initial basic voltammograms of as-prepared Pt/C, PtRh/C, and
PtSn/C catalysts in 0.1 M HClO 4, v050 mV/s. Insert shows steady
state CV of Pt/C catalyst and high-area carbon (Vulcan XC-72)
is the density of platinum or alloy. The density of alloy was
calculated according to Chen et al. [39] from the equation:
1
¼
ρPtM cPt þ
ρPt cM ρM where χ is the mass fraction of the metal in alloy.
Since CO does not adsorb at Sn [40], the real surface area
for the PtSn/C catalyst refers only to Pt, and therefore,
particle size for this catalyst cannot be evaluated. On the
contrary, as CO adsorbs at Rh, the calculated real surface
area of the PtRh/C catalyst then correlates to whole catalyst
surface and enables particle size calculation. The data are
presented in Table 2. The values obtained for the real surface
area of the catalysts confirm that the surface composition
corresponds to the bulk composition. So, for PtSn/C catalyst,
the real surface area, which refers only to Pt, is 52 % of
the real surface area of the Pt/C catalyst that coincides well
with EDX result on the bulk composition of the PtSn/C
catalyst. On the other hand, the values obtained for the
particle size, being slightly higher than those obtained by
Fig. 5 CO ads stripping curves for as-prepared Pt/C, PtRh/C, and PtSn/
C catalysts in 0.1 M HClO 4, v050 mV/s
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Table 2 Particle size, real and specific surface areas of Pt/C, PtRh/C,
and PtSn/C catalysts calculated from CO ads stripping and TEM
analyses
Catalyst Pt/C PtRh/C PtSn/C
Real surface area (cm 2 ) 3.43 3.74 1.79
Mean particle size (nm) 3.3 3.4 n/a
Specific surface area (m 2 /g) 85.8 93.5 81.4 a
Specific surface area (TEM) b (m 2 /g) 88.3 91.7 155.6
a
Calculated only on Pt content
b
S was calculated taking into account particle size and their fractions,
i.e., size distribution
TEM, confirm agglomeration and sintering of the initial
particles during deposition onto the carbon support. Also,
a good agreement between values for the specific surface
areas for Pt/C and PtRh/C, calculated using particle size
obtained from TEM analysis and from the values for real
surface area obtained by CO stripping, points out mainly
monodispersion of the catalyst particles over the substrate.
The large difference between these values for PtSn/C catalyst,
is because the real surface area from CO stripping,
refers only to Pt.
Oxidation of adsorbed CO monolayer enables not only
the estimation of catalyst active surface area but also reveals
the catalyst tolerance to CO. Observing the curves presented
in Fig. 5, it can be noticed that at Pt/C, oxidation of CO
commences somewhat before 0.4 V versus SCE and proceeds
through a single sharp peak with current maximum at
0.55 V. The onset potential for CO oxidation at PtSn/C is
shifted to lower values for more than 0.3 V, and the oxidation
curve is significantly broadened as well as split into two
peaks (shoulders). The latter one is centered at the potential
value very close to the value of Pt/C, while the former is
shifted for about 100 mV more negative and corresponds to
CO oxidation peaks at Pt-Sn catalysts [12, 15]. Negative
shift of the onset potential of CO oxidation at PtSn/C catalyst
is attributed to the presence of oxygen-containing species
at Sn at lower potentials in comparison to Pt as well as
to the electronic effect of Sn on Pt [12, 15]. Two signals in
the potential peak during oxidation of CO can be explained
by low alloying degree of this catalyst and thus, part of
nonalloyed Pt. Contrary to the case of PtSn/C, CO oxidation
at PtRh/C commences at practically the same potential as at
Pt/C. Since CO adsorption energy for Pt-CO is 125 kJ/mol
and for Rh-CO is 134 kJ/mol [21], the addition of Rh to Pt
in the PtRh catalyst might increase CO bond energy at PtRh,
and although OH adsorbs at Rh at lower potentials in comparison
to Pt, the oxidation of CO at PtRh is retarded.
The activity of the as-prepared Pt/C, PtRh/C, and PtSn/C
catalysts for ethanol oxidation was evaluated from potentiodynamic
and chronoamperometric measurements in 0.1 M

HClO4+0.5 M C2H5OH solution. Potentiodynamic measurements
were carried out in the potential range of technical
interest, i.e., up to 0.3 V. This is also the range where no
leaching of the second metal (Sn or Rh) can occur [5]. One
anodic sweep at each catalyst was recorded in the extended
potential region up to 0.8 V as well, in order to follow
completeness of ethanol oxidation. The first anodic sweep
recorded at each catalyst in low potential region is presented
in Fig. 6. While PtSn/C exhibits significant activity related
to Pt/C, there is almost no difference in electrocatalytic
activity between PtRh/C and Pt/C catalysts. Ethanol oxidation
commences at −0.15 V at as-prepared PtSn/C and
proceeds with approximately three times higher currents in
comparison to Pt/C. At higher potentials in the extended
potential region (Fig. 7), PtSn/C remains more active than
Pt/C, whereas PtRh/C losses its activity and becomes inferior
in comparison to Pt/C. It should be mentioned here that
Pt/C alone with onset potential for ethanol oxidation as well
as the potential of anodic peak located at values lower for
more than 0.1 V exhibits better performance compared to Pt/
C catalysts described in literature [11, 32, 41, 42]. Analysis
of the voltammetric curves in Figs. 6 and 7 point out that the
Fig. 6 Potentiodynamic curves in low potential region for the oxidation
of 0.5 M C 2H 5OH at as-prepared Pt/C, PtRh/C, and PtSn/C
catalysts in 0.1 M HClO 4, v020 mV/s
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3154 J Solid State Electrochem (2012) 16:3147–3157
Fig. 7 Potentiodynamic curves for the oxidation of 0.5 M C 2H 5OH at
as-prepared Pt/C, PtRh/C, and PtSn/C catalysts in 0.1 M HClO 4, v0
20 mV/s
addition of Sn in PtSn/C catalyst leads to a significant
increase in activity in comparison to Pt/C specially at potentials
of technical interest (lower than 0.3 V). The catalyst
retains higher activity even in the extended potential region,
but the current density increases less with the increase in
potential. The initial potential for ethanol oxidation at asprepared
PtSn/C surface and the potential of anodic peak at
0.6 V are remarkably shifted to lower values in comparison
to other PtSn catalysts with similar composition [5, 6, 8, 11,
32, 42, 43].
As already mentioned, the presence of Sn can promote
ethanol oxidation by an electronic effect and/or by facilitating
a bifunctional mechanism. Activation of interfacial water
molecules leading to formation of OH species required
for the oxidation of adsorbates generated by dehydrogenation
of ethanol at adjusted Pt sites proceeds in Sn at lower
potentials in comparison to Pt. In this way, by oxidation of
C1,ad fragments (mostly COad), formed by splitting of C–C
bond, to CO2 and C2,ad species with the intact C–C bond,
i.e., acetaldehyde to acetic acid, a beneficial effect of Sn is in
refreshing active Pt surface and supplying enough Pt active
sites for adsorption and dissociation of ethanol [11, 32, 42].

Fig. 8 Chronoamperometric curves for the oxidation of 0.5 M
C 2H 5OH at as-prepared Pt/C, PtRh/C, and PtSn/C catalysts in 0.1 M
HClO 4 at 0.2 V versus SCE
Further, addition of Sn to Pt results in expansion of lattice
parameter and elongation of bonding structure that changes
geometric environment. The extended lattice parameter may
enable C–C bond cleavage and thus, improve the catalytic
activity [42]. Regarding the electronic effect, since Sn has
four valence electrons available for modification of unfilled
d band states of Pt atoms, this charge transfer from Sn atoms
to neighboring Pt atoms may induce a weaker bond between
adsorbates and Pt atoms. In the case of adsorbates with
carbon atom, the poisoning of Pt should be reduced and
adsorption of ethanol could also be diminished [42]. However,
this charge transfer leads to easier formation of Sn
oxide species, and these oxygen-containing species can
facilitate electrooxidation of adsorbates on Pt sites at lower
potentials.
XRD analysis of our PtSn/C catalyst reveals that the
addition of Sn induced only slight extension of Pt–Pt distances
resulting in rather low alloying degree. Since TG and
Fig. 9 di/dt versus time plots (from t00tot0120 s) for as-prepared Pt/
C, PtRh/C, and PtSn/C catalysts
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J Solid State Electrochem (2012) 16:3147–3157 3155
Fig. 10 Long-term stability of PtSn/C and PtRh/C catalysts in 0.1 M
HClO 4+0.5 M C 2H 5OH versus number of scans, v020 mV/s (current
values are at 0.3 V)
EDX analyses indicated negligible loss of catalyst components,
we could reasonably assume that significant quantity
of nonalloyed Sn is present in the catalyst and on its surface.
XPS and XRD analyses of bimetallic PtSn catalysts prepared
by microwave-assisted polyol method identify a significant
amount of SnO2 and rather low alloying degree [28,
31, 32]. Thus, it is very likely that Sn in our PtSn/C catalyst
is present mainly as SnO2. The increased activity of our
PtSn/C in comparison to Pt/C catalyst, therefore, is most
probably enabled by a balanced ratio of Pt ensembles for
ethanol dehydrogenation and C–C bond splitting and tin
oxide for OH formation, i.e., by bifunctional mechanism
although the electronic effect of alloyed Sn could play some
role as well. In addition, high activity of as-prepared PtSn/C,
and also Pt/C, catalyst could be supported by the presence of
Pt adislands as well. These Pt adislands, i.e., highly undercoordinated
Pt atoms on the catalyst surface are detected by
STM on Pt single crystals and their remarkable activity in
the oxidation of CO at lower potentials confirmed by FTIR
[44]. Since polycrystalline Pt should have a number of such
undercoordinated Pt atoms, they might as well contribute to
the high activity of the catalysts.
Contrary to PtSn/C, PtRh/C catalyst exhibits similar activity
for ethanol oxidation as Pt/C at potentials of technical
interest, i.e., up to 0.3 V (Fig. 6). At higher potentials, the
faradeic currents at PtRh/C are lower in comparison to Pt/C
(Fig. 7). Such behavior of the catalyst is in accordance with
literature data [9, 20, 21] for PtRh catalysts of similar Pt to
Rh molar ratio, with the exception for onset potential, since
the oxidation of ethanol commences at our PtRh/C at lower
values. According to Bergamaski et al. [20, 21] Rh is less
efficient for dehydrogenation of ethanol than Pt, and the
high-energy barrier for dehydrogenation can lower reaction
rate at surfaces containing Rh. Another reason for a low

eaction rate at PtRh catalysts might be difficulty in electrooxidation
of CO on Rh because of stronger O–Rh bonding
in comparison to Pt–O that can result in higher activation
energy for CO–O coupling and thus hinder CO oxidation
[20]. In this sense, small particle size plays an important
role, since small particles are more oxophylic. Still, DEMS
and FTIR studies revealed improved CO2 and decreased
acetaldehyde yields at PtRh surfaces that should mean that
the role of Rh in PtRh catalysts is more likely to increase C–
C bond splitting and not provide oxygen more readily for
CO electrooxidation [20].
To examine the poisoning tolerance of our Pt/C, PtRh/C, and
PtSn/C catalysts during ethanol oxidation, chronoamperometric
experiments have been performed, and the results are presented
in Fig. 8. The PtSn/C catalyst exhibits higher initial current
density at 0.2 V, while Pt/C and PtRh/C demonstrate similar
behavior with lower currents, which is in accordance with
potentiodynamic measurements (Fig. 6). In Pt/C and PtRh/C
catalysts, contrary to PtSn/C, current decay rapidly and reaches
low steady state values in a few minutes. Current decreases
slowly at PtSn/C and stabilizes at the value which is significantly
higher than for two other catalysts in the experimental
time period. This proves that PtSn/C is considerably less poisoned
than Pt/C or PtRh/C. Lower poisoning of PtSn/C is
confirmed by di/dt as a function of t for short period of time
(t from0to2min.)(Fig.9). A smaller value of the slope means
lower initial poisoning of the electrode surface [45]. Thus, it
appears that the addition of Sn to Pt facilitates lower poisoning
compared to Pt/C or PtRh/C, since the experimental slope is
lower by factors 1.29 and 1.33, respectively.
In addition, long-term stability of PtSn/C and PtRh/C
catalysts was tested for ethanol oxidation in 0.1 M HClO4
during 150 cycles in the potential range of technical interest,
i.e., between −0.2 V and 0.3 V. The results are presented in
Fig. 10 as the current value at 0.3 V versus number of the
cycle. For both catalysts after initial decrease during the first
5 cycles, decline in current values is significantly retarded
and for PtSn/C at the end of the test is about 70 % lower in
comparison to the fifth cycle, while for PtRh/C, it is about
50 %. This loss in catalyst activity may be due to the
poisoning of the surface or to leaching of the added nonnoble
metal. To test leaching of Sn and Rh, the PtSn/C and
PtRh/C electrodes were examined by EDX after the experiments
and the results are shown in Fig. 10 as well. These
analyses revealed minor leaching of both metals from bimetallic
catalysts and confirmed their stability.
Conclusions
Ethanol oxidation reaction was studied at carbon-supported
Pt/C, PtRh/C, and PtSn/C catalysts prepared by microwaveassisted
polyol procedure. According to lattice constants
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3156 J Solid State Electrochem (2012) 16:3147–3157
obtained from XRD analysis, low alloyed PtSn and PtRh
catalysts were obtained. STM examinations revealed that
unsupported catalysts have rather uniform small particles of
approximately 2 nm. This should be attributed to the advantages
of microwave-assisted polyol process in EG solution.
During deposition of catalyst particles onto the carbon substrate
and thermal treatment, sintering and agglomeration
occur, and supported catalysts as determined by TEM have
somewhat larger particles of approximately 3 nm.
It is found that the activity of Pt-based bimetallic catalyst
for ethanol oxidation greatly depends on secondary metal
and the electrode potential. While addition of Sn to Pt leads
to the significant enhancement of ethanol oxidation and
lower poisoning of electrode surface, addition of Rh only
modestly changes overall electrochemical reaction. The onset
potential for ethanol oxidation at our PtSn/C catalyst is
remarkably shifted to lower values in comparison to other
PtSn catalysts with similar composition. The increased activity
of PtSn/C catalyst is mainly due to a bifunctional
mechanism, enabled most probably by nonalloyed Sn
(SnO2), although an electronic effect of low alloyed PtSn
could play some role as well.
Acknowledgments This work was financially supported by the Ministry
of Education and Science, Republic of Serbia, Contract No.
172060.
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ARTICLES
PUBLISHED ONLINE: 6 MAY 2012 | DOI: 10.1038/NMAT3313
Trends in activity for the water electrolyser
reactions on 3d M(Ni,Co,Fe,Mn)
hydr(oxy)oxide catalysts
Ram Subbaraman 1,2 , Dusan Tripkovic 1 , Kee-Chul Chang 1 , Dusan Strmcnik 1 , Arvydas P. Paulikas 1 ,
Pussana Hirunsit 3 , Maria Chan 3 , Jeff Greeley 3 , Vojislav Stamenkovic 1 and Nenad M. Markovic 1 *
Design and synthesis of materials for efficient electrochemical transformation of water to molecular hydrogen and of hydroxyl
ions to oxygen in alkaline environments is of paramount importance in reducing energy losses in water–alkali electrolysers.
Here, using 3d-M hydr(oxy)oxides, with distinct stoichiometries and morphologies in the hydrogen evolution reaction (HER)
and the oxygen evolution reaction (OER) regions, we establish the overall catalytic activities for these reaction as a function of
a more fundamental property, a descriptor, OH–M 2+δ bond strength (0 ≤ δ ≤ 1.5). This relationship exhibits trends in reactivity
(Mn

NATURE MATERIALS DOI: 10.1038/NMAT3313 ARTICLES
a
Normalized absorption (a.u.)
c
1.5
1.0
0.5
0
5
i (mA cm –2 ) 10
0
0 20
Energy shift (eV)
Pt(111)
Pt(111) + CoOOH
25 nm 0.8 E (V) 1.7
E = ¬0.1 V
Co(OH) 2 ref
E = +1.4 V
CoOOH ref
40
OHad (μC cm–2 θ
)
300
200
100
150
i (μA cm –2 )
0
–150
Pt(111)
Pt(111) + Co(OH) 2
0 0.5 1.0
15 nm
E (V)
i (μA cm –2 )
300
200
100
M2+ δO δ(OH)
2-δ
M = Mn
M = Fe
M = Ni
Pt(111)
0
Ni Co
0
Fe
0.5 E (V) 1.0
0
Mn
Figure 1 | Characterization of M 2+δ O δ (OH)2−δ/Pt(111) systems using XANES, CV and STM measurements. a, Sample XAS spectra for Co 2+δ O δ (OH)2−δ
on Pt(111) surface for two different potentials E = −0.1 V and E = +1.4 V. Comparisons of reference samples at these potentials are also shown. The
comparison reveals that δ = 0 at −0.1 V and δ = 1 at 1.4 V. b, STM for Co(OH)2/Pt(111) in the HER region. Polarization curves for the HER for this surface are
also shown (50 mV s −1 ). The characteristic height of the clusters shown is ∼5.8 Å with a diameter for 7–8 nm. c, STM for CoOOH/Pt(111) is shown along
with the polarization curves for the OER. The characteristic height of the clusters shown is ∼5.6 Å with a diameter for 15–22 nm. Pure Pt(111) polarization
curves are shown for comparison in both Fig. 1b and c. d, Comparison of OHad charge as a function of oxophilicity of the metal oxide cation (M) for same
coverages of the M 2+δ O δ (OH)2−δ on Pt(111). Inset shows the comparison of voltammograms for bare Pt(111) surfaces with Co 2+δ O δ (OH)2−δ/Pt(111)
surface. Enhanced adsorption of OHad is observed as the larger area under the anodic peak at 0.6 V as well as the early onset of the OHad butterfly region.
Table 1 | XAS analysis for M 2+δ O δ (OH)2−δ systems.
Element HER potential region (E < 0.0 V) OER potential region (E ∼ 1.4 V)
b
d
M n+ N M–O (Å) M n+ N M–O (Å)
Mn 2.0 ± 0.1 1.6 ± 0.3 2.10 ± 0.04 3.5 ± 0.1 3.3 ± 0.4 1.89 ± 0.03
Fe 2.7 ± 0.1 2.5 ± 0.5 1.93 ± 0.03 3.0 ± 0.1 3.4 ± 0.3 1.95 ± 0.02
Co 2.2 ± 0.1 1.8 ± 0.5 2.02 ± 0.05 2.9 ± 0.2 3.9 ± 0.5 1.89 ± 0.02
Ni 2.1 ± 0.1 2.5 ± 0.4 2.06 ± 0.03 2.3 ± 0.1 3.2 ± 0.6 2.02 ± 0.03
M n+ —The valence state of the 3d element; N—first shell co-ordination number; M–O (Å)—characteristic bond distance between the 3d metal centre and the oxygen anion. We show that, depending on
the nature of the 3d metal, different step changes in oxidation states are observed in different potential regions. The rate of change of oxidation states with potential are found to be dependent on the
nature of the elements. In the HER, with the exception of Fe, all elements are in the +2 state. On the other hand, in the OER region, with the exception of Ni, all elements are in an oxidation state > +3. We
note, Fe, with its complex redox chemistry at these potentials, is known to exist in multiple forms 27 , such as Fe(II) and Fe(III) oxides and hydroxides, and therefore exhibits a valence state between 2.5 and
3.0. Nickel, exhibiting lower oxidation state at this potential, is not surprising, because for the Ni-modified surfaces we found that, at 1.4 V, the phase of Ni(OH)2 present on the Pt(111) surface resembles
that of β-Ni(OH)2 and this phase of nickel hydroxide is known to remain stable up to high potentials (

ARTICLES
latter dimension corresponds to approximately two layers of
electrically conductive Co(OH)2. The distribution of the clusters
over the entire surface indicates that the clusters grow in a threedimensional
(3D) (Volmer–Weber 28 ) fashion, seldom achieving
a complete monolayer coverage. Importantly, after recording 50
cyclic voltammograms (CVs) between −0.3V±0.4 V, the STM
images of all 3d-M hydr(oxy)oxide/Pt(111) systems remain the
same, indicating that in this potential range the morphology of
the surface is stable. In contrast, for an electrode held at 1.55 V
the STM image in Fig. 1c indicates significant sintering of the
CoOOH nanoparticles; a distribution of sizes ranging from ∼15
to 25 nm, with approximately constant heights of two layers. This
morphology was found to be stable in the OER region (potentialand
time-independent). Given that similar morphological changes
were exhibited by the other M 2+δ O δ (OH)2−δ/Pt(111) systems, we
conclude that these well-defined, at atomic/molecular level, surface
structures form the basis for any predictive ability in tailoring
catalysts to have desirable reactivity for the HER and the OER in
alkaline environments.
We begin our electrochemical characterization by comparing
the CVs of the Pt(111) and Co(OH)2/Pt(111) surfaces (inset in
Fig. 1b). As was the case for the XAS and STM analyses, the CV
behaviour of Co(OH)2 is a fair representation of the other 3d-M
hydr(oxy)oxide systems (shown in inset of Fig. 1d). Consistent
with earlier reports for Pt(111) (refs 29,30), cyclic voltammetry
of Pt(111) (Fig. 1b) shows that the adsorption of underpotentially
deposited hydrogen (defined as the state of hydrogen adsorbed
at a potential that is positive of the Nernst potential for the
hydrogen reaction, Hupd) between 0.05 and 0.35 V is followed
first by a wide double-layer potential region and then by the
formation of an OHad adlayer (usually termed as the ‘butterfly’
region) between 0.6 and 0.95 V. Figure 2 (bottom inset) shows
that the surface coverage of the Hupd (�Hupd) is reduced by ∼45%
on a Co(OH)2/Pt(111) electrode. We also found that other 3d-M
hydr(oxy)oxide covered surfaces behave in a similar manner (inset
Fig. 1d), suggesting they act as a third body, selectively blocking
the adsorption of Hupd without affecting the Pt–Hupd energetics.
Thus, determination of the �Hupd from CVs of modified Pt(111)
surfaces enables accurate determination of surface coverage by
the 3d-M hydr(oxy)oxide clusters. In the following, Pt(111) is
always modified with nearly identical (∼45%) amounts of these
clusters (Fig. 1d), thereby enabling the use of a well-characterized
M 2+δ O δ (OH)2−δ/Pt(111) electrode (0 ≤ δ ≤ 1.5).
In contrast to the Pt–Hupd bonding, Fig. 1b shows that the
effects of a Co 2+δ O δ (OH)2−δ/Pt(111)-modified Pt(111) surface on
adsorption of OH − are significant. Three distinct features are
noteworthy: (1) the increase in the charge under the OHad (�OHad)
region between 0.6 V < E < 0.95 V, (2) negative shifts for both
the onset of the OH − adsorption and its desorption, and (3)
OHad adsorption/desorption features are strongly irreversible. The
magnitude of the changes in the OHad charge is greater than what
can occur on an unmodified Pt(111) surface; so the oxide clusters’
interaction with the OH must provide an important contribution
to the observed value of �OHad. Similar behaviour is exhibited
by the other 3d elements considered here (Fig. 1d); however,
the charge corresponding to the formation of OHad is strongly
dependent on the nature of the 3d element and they follow the
order Ni < Co < Fe < Mn. This result clearly demonstrates that
the oxophilicity of the 3d elements is strongly dependent on their
‘nobility’ (position in the periodic table). This, in turn, follows
the oxophilicity of the 3d-M hydr(oxy)oxides, a conclusion that
is supported by our density functional theory (DFT) calculations,
specifically for Co and Mn oxide clusters. The DFT analyses show
stronger adsorption of OH on model two Co(OH)2/Pt(111) films
(treated as a film with two layers of Co(OH)2 on the Pt(111) surface)
than for clean Pt(111). Furthermore, the variation in binding
NATURE MATERIALS DOI: 10.1038/NMAT3313
energy moving from Co-based to Mn-based systems is consistent
with the observed oxophilicity derived from the �OHad values (see
Supplementary Information for details).
We thus have a series of well-characterized electrodes showing a
linear variation in the �OHad with the nature of the 3d-M elements
(Fig. 1d). The trend in the values of �OHad as a function of the
3d-M cation indicates that this quantity correlates well with the
OHad–M 2+δ bonding. As such, this result is an excellent basis for
finding variations in the kinetics of electrochemical reactions, if
the rates of these reactions are controlled by the OHad–M 2+δ bond
strength. It should be pointed out, however, that the physical
processes that are associated with the formation of OHad on metal
and metal oxide surfaces have been the subject of controversy even
on well-defined Pt single crystals 30–33 . Nevertheless, there is only
some consensus that in alkaline solution the pseudocapacitance
observed in the CVs of Pt(111) in the range 0.6 < E < 0.95 is
just reversible OH − adsorption on the (111) terrace sites. Given
that the thermodynamic analyses for the OHad adsorption seen
in CVs is not straightforward 34 , relatively little is known about
the true nature of OHad or how the adsorption energies of OHad
may vary between different adsorption sites (that is, terraces versus
defects). As discussed previously 35,36 , OHad species that are present
at potentials below 0.6 V, are found exclusively on the defect sites.
The relatively small number of such defects on Pt(111) makes
them invisible in the CV owing to the larger pseudocapacitive
contributions from the Hupd. To overcome this limitation of the CV
method, it is customary to ‘probe’ the OHad with electrochemical
reactions for which the OHad is a reactant. For our purposes here, we
use the CO oxidation reaction 35,36 to verify/validate the oxophilicity
trends observed in the butterfly region at potentials below 0.6 V.
The reaction mechanism for CO oxidation reaction has
been well established as following the Langmuir–Hinshelwood
(L–H) pathway for Pt bimetallic systems such as PtSn and
PtMo (refs 30,37). These catalysts are bi-functional in nature,
where the CO adsorbs exclusively on the Pt sites and the
OHad groups are present exclusively on the more oxophilic
Sn/Mo sites that facilitate the oxidative removal of CO. In
line with these systems, for the case of Pt(111) modified by
M 2+δ O δ (OH)2−δ clusters, it is reasonable to propose that, whereas
CO is adsorbed exclusively on the Pt sites (CObulk ↔ Pt–COad),
the OHad species adsorb preferentially on 3d-M hydr(oxy)oxides
(OH − + M 2+δ O δ (OH)2−δ ↔ OHad–M 2+δ O δ (OH)2−δ + e − ). The
presence of OHad can then be tested simply by monitoring
the rate of CO oxidation at the constant electrode potential
through a L–H type reaction (Pt–COad +OHad −M 2+δ O δ (OH)2−δ +
2OH − ↔ HCO −
3 + e − + H2O). To see the L–H reaction in
action, we present the polarization curves for CO oxidation on
Co 2+δ O δ (OH)2−δ/Pt(111) and bare Pt(111) as typical examples; the
corresponding activities for other 3d-element-modified electrodes
are summarized in Fig. 2. Clearly, the CO oxidation current on
Co 2+δ O δ (OH)2−δ/Pt(111) is shifted negatively by about 0.3 V with
respect to the bare Pt(111) surface (see top inset of Fig. 2 and
Supplementary Fig. S6), indicating that this surface behaves as a
bi-functional catalyst. As depicted schematically in Fig. 2 (bottom
inset), the reaction proceeds along the perimeters of COad islands
and neighbouring M 2+δ sites. The fact that the onset potential for
the CO oxidation reaction on Co-modified Pt(111) is observed at
0.1 V (see Supplementary Fig. S6), strongly suggests that oxygenated
species must be present on the Co 2+δ O δ (OH)2−δ defect sites at these
potentials. Although the same conclusion commonly holds true for
all other M 2+δ O δ (OH)2−δ/Pt(111) systems, the reactivity of these
surfaces for the CO oxidation reaction is found to be dependent on
the nature of the 3d element.
In general, the kinetics of the CO oxidation reaction on
these surfaces is expected to be a function of both the Pt–COad
and OHad–M 2+δ energetics 30 . However, if we assume that the
552 NATURE MATERIALS | VOL 11 | JUNE 2012 | www.nature.com/naturematerials
© 2012 Macmillan Publishers Limited. All rights reserved

NATURE MATERIALS DOI: 10.1038/NMAT3313 ARTICLES
(mV)
@ 1 mA cm ¬2
η
600
750
900
ΔG Pt–COad = constant
Pt (111)
Pt(111)–Ni2+ δO δ(OH)
2-δ
HCO 3 –
δδ δ
OH –
COb COb OH
OHad OHad –
M2+ O (OH) 2-δ
M
CO CO
ad ad
2+ δ δ O (OH) 2-
Pt
Pt(111)–Co2+ δO δ(OH)
2-δ
HCO
–
3
i (mA cm –2 )
2
1
0
Co2+ Pt(111)
δO δ(OH)
2- δ /Pt (111)
0 0.5 1.0
E (V)
0 50 100 150 200
OHad ¬ Hupd (μC cm–2 θ θ
)
δ
Pt(111)–Fe2+ δO δ(OH)
2-δ
Pt(111)–Mn2+ δO δ(OH)
2-δ
Figure 2 | Trend in overpotential for CO oxidation is shown as a function of the 3d transition elements. The elements are arranged in the order of their
oxophilicity from Mn to Ni. Pt is shown in the figure as a reference. Top inset: a comparison of the polarization curves for Pt(111) and Pt(111) with 40% Co
hydr(oxy)oxides for the CO oxidation reaction. As can clearly be seen, the onset potentials for CO oxidation are shifted ∼300 mV negative from those of
the bare Pt(111) surface. Bottom inset: a schematic showing the L–H mechanism for the CO oxidation reaction. CO from bulk is found to adsorb on the free
Pt site near the oxide clusters. OHad is formed by either adsorption of OH − from the electrolyte and/or a change in oxidation state of the cluster cation
M 2+δ . In the presence of COad and OHad in each others vicinity, reaction between COad and OHad species the occurs forming an intermediate which is
eventually converted to (bi)-carbonates. The free energy for Pt–COad is fixed, which enables the treatment of these bi-functional metal-oxide/metal
catalysts as a ‘pseudo’ mono-functional catalyst with a singular descriptor OHad–M 2+δ .
Pt–COad interaction is independent of the nature of the 3d-M
hydr(oxy)oxide (which is a reasonable assumption, considering that
even Pt–Hupd energies are not affected by the presence of these),
then the rate of the CO oxidation reaction should depend only on
the OHad–M 2+δ energetics. Thus, by fixing the Pt–COad energetics,
we can treat the reaction as a ‘pseudo’ mono-functional reaction
that is controlled by the descriptor related to OHad–M 2+δ bond
strength. Indeed, Fig. 2 reveals that the rate of the CO oxidation
reaction is inversely proportional to the OHad–M 2+δ bond strength,
that is, the activity increases in the order: Mn < Fe < Co < Ni. We
propose that, for a strong interaction, such as that for OHad–Mn 2+δ ,
the CO oxidation reaction is inhibited because of the relatively low
reactivity of OHad. In fact, the OHad–Mn 2+δ bond is so strong that
even pure Pt is more active than the Mn 2+δ O δ (OH)2−δ/Pt(111).
In contrast, for Ni hydr(oxy)oxides that bind OHad neither too
strongly nor too weakly, we observe the maximum activity for
the CO bulk oxidation. The similarity in the trends observed for
the activity for the CO oxidation reaction and the OHad–M 2+δ
interaction strength in the butterfly region, confirms that the same
guiding principle, namely the oxophilicity of the 3d-M cation, is
valid in the Hupd region as well. This indicates that the oxophilicity
trends derived in Figs 1d and 2 are valid between 0.05 and 0.95 V.
By studying the OER (E > 1.5 V) and the HER (E < 0.0V), where
OHad is involved in the reaction pathway, we will further probe if
this trend in OHad–M 2+δ holds true in the potential regions where
these two reactions occur.
The OER is much more complex than the CO oxidation
reaction, involving the formation of many reaction intermediates
before OH − is completely transformed to O2/H2O. So far, many
descriptors have been used to express the overall catalytic activities
of the OER as functions of some physicochemical properties of
the catalyst, including the oxygen adsorption energy in gas phase
environments, the catalyst–OH bond strength, the number of d
electrons, and the lattice spacing in metal electrodes 15,17,18,23,38,39 .
Although these studies identify key surface properties that govern
reactivity for various catalysts, the fundamental understanding
of the elementary processes involved in the OER is still lacking.
This is because the previous experimental results were acquired
on high-surface-area catalysts, which are complex owing to the
presence of ill-defined catalyst centres. In contrast, our well-defined
3d-M hydr(oxy)oxide clusters on Pt(111) provide an excellent
opportunity to establish the guiding principles for designing the
active sites for the OER. A rigorous kinetic analysis of the OER lies
outside the scope of this discussion and will not be presented here.
Rather, we will focus on understanding the reactivity trends for the
OER on M 2+δ O δ (OH)2−δ/Pt(111) surfaces and how these trends
may help in the future design of effective OER catalysts.
As a starting point, we compare the kinetic rates of the OER
on the Pt(111)-oxide electrode (dubbed hereafter as PtO) and the
CoOOH/PtO electrode (shown as inset in Fig. 1c); as before, the
Co system is representative of the other M 2+δ O δ (OH)2−δ systems.
Figure 1c shows that the onset of the OER on the CoOOH/PtO
electrode is shifted by ∼0.25 V, to more negative potentials,
compared with PtO. These differences may reflect variations in the
energetics (activation energies and/or the enthalpies of adsorption)
for the formation of active intermediates at these two surfaces, the
NATURE MATERIALS | VOL 11 | JUNE 2012 | www.nature.com/naturematerials 553
© 2012 Macmillan Publishers Limited. All rights reserved
250

ARTICLES
(mV)
@ 5 mA cm –2
η
450
600
750
Pt (111)
PtO–Ni2+ δO
δ(OH)
2-δ
OH –
O 2
OH ad
PtO/AuO
Intermediate
M2+ δδ δ O (OH) 2¬ δ
PtO–CoO(OH)
OH ad
H 2 O
OH –
NATURE MATERIALS DOI: 10.1038/NMAT3313
I (mA cm –2 )
Au(111)
CoOOH + Au(111)
Pt(111)
CoOOH + Pt(111)
PtO–FeO(OH)
PtO–Mn2+ 1.0 1.5
E (V)
2.0
δO
δ (OH) 2-δ
Ni Co Fe Mn
Figure 3 | Trend in overpotential for the oxygen evolution reaction (OER) is shown as a function of the 3d transition elements. The elements are arranged
in the order of their oxophilicity from Mn to Ni. Pt is shown in the figure as a reference. Top inset: a comparison of polarization curves for Pt(111) and
Au(111) with 40% CoOOH for the OER. As can clearly be seen, the two potential curves are identical, suggesting a limited or no role played by the noble
metal substrate for this reaction. As a result, this reaction is classified as a mono-functional reaction, and the main descriptor (as can be clearly seen from
the trend) is still the OHad–M 3+ interaction. Bottom inset: a schematic showing the OER. OH − from the bulk is found to adsorb on the free catalyst site on
the oxide clusters. The adsorbed OH groups OHad react with other such groups to form a reaction intermediate (re-combination), which is then further
oxidized to O2 and H2O.
exact values of which are unknown. Nevertheless, the low activity of
PtO, consistent with earlier reports 10,14 , is indicative of the weaker
OHad–PtO interaction, and thus it seems that the rate-determining
step for such noble metal catalysts could be the formation of
OHad–PtO intermediates (OH − +PtO ↔ OHad −PtO+e − ). Along
the same lines, the significant activity enhancement of the OER
on the CoOOH/PtO electrode could be due to the enhanced
interaction of OH − with CoOOH. To verify this, we have also
compared the OER on the CoOOH/AuO electrode. The fact that
the rate of reaction on both these surfaces is the same (top inset
of Fig. 3) is confirmation that the OER reaction rate is controlled
only by interaction of reactants and reaction intermediates with
CoOOH and not the metal substrate. Given that the same is
also true for the other M 2+δ O δ (OH)2−δ/Pt(111) electrodes, we
conclude that these catalysts are purely mono-functional, a fact
which is further confirmed by the monotonic variation in OER
activities as a function of ‘loading’ of the CoOOH clusters (see
Supplementary Section S4, Fig. S5). As summarized in Fig. 3, the
OER on M 2+δ O δ (OH)2−δ/PtO exhibits activities increasing from
Mn to Ni hydr(oxy)oxides. This suggests that the overall reaction
rates are driven by the strength of the interaction between the two
oxidic species, the recombination step being the rate determining
(2OHad–M 2+δ O δ (OH)2−δ → products), rather than by the initial
adsorption step (OH − +M 2+δ O δ (OH)2−δOHad–M 2+δ O δ (OH)2−δ +
e − ), which, as shown in Fig. 1d, exhibits the opposite trend
compared with that in Fig. 3. Certainly, we are aware that more than
one recombined species might be formed during OER chemistry
on oxides 10,11 , but those mechanistic details do not affect our
10
5
0
interpretations of the OER trends. In particular, too strong an
interaction between 3d-M hydr(oxy)oxides and OHad can lead to
an adverse effect, wherein the reaction intermediates are stabilized,
leading to a lower turnover frequency (defined as the number
of complete reaction events per site per second). This leads to
‘poisoning’ of the surface and a concomitant decrease in OER
activities, as shown schematically in Fig. 3 (bottom inset). Thus, for
the 3d elements considered here, Ni, with its optimal interaction
strength with OHad, satisfies the Sabatier principle for catalyst
design. Considering that the reactivity trends observed for the OER
(Ni > Co > Fe > Mn) match that observed for the CO reactivity,
we can conclude that the OHad–M 2+δ interaction (oxophilicity)
trends can be extended up to the OER potential regions. In what
follows, we demonstrate that the trends in the energetics between
OHad–M 2+δ for OH − , produced as the water dissociation product,
which are relevant in the hydrogen evolution potential region, are
similar to that for the OHad formed from the supporting electrolyte
between 0.05 and 2.0 V.
Surprisingly, so far, the trends in activity for the HER in
alkaline solutions have never been established independently from
those found for the HER in acidic media 40,41 . Given that, in both
environments, the measured kinetic rate of the HER exhibits a
volcano type behaviour 13,42 when plotted as a function of the
hydrogen adsorption energy, it has been customary to use the
same descriptor for the development of cathode catalysts for the
alkaline HER catalysts. Very recently, however, it was established
that there are both qualitative and quantitative differences between
alkaline and acid HER catalysis 43 . Whereas in acid solutions the
554 NATURE MATERIALS | VOL 11 | JUNE 2012 | www.nature.com/naturematerials
© 2012 Macmillan Publishers Limited. All rights reserved

NATURE MATERIALS DOI: 10.1038/NMAT3313 ARTICLES
(mV)
@ 5 mA cm ¬2
η
200
400
600
ΔG pt-Had = constant
Pt(111)
Pt(111)–Ni(OH) 2
H2 OH ¬ OH ¬
Pt(111)–Co(OH) 2
H 2 O H 2 O
M
Had Had 2+ δδ O δ(OH) 2¬
M δ
2+ δ O δ(OH)
2¬ δ
Pt
Ni
I (mA cm –2 )
0
–6
Au(111) Co(OH)2
+ Au(111)
Pt(111)
Co(OH) 2
+ Pt(111)
Pt(111)–Fe2+ –10
–0.60 –0.26
E (V)
0
δO δ (OH) 2- δ
Pt(111)–Mn (OH) 2
Co Fe Mn
Figure 4 | Trend in overpotential for the hydrogen evolution reaction (HER) is shown as a function of the 3d transition elements. The elements are
arranged in order of their oxophilicity from Mn to Ni. Pt is shown in the figure as a reference. Top inset: a comparison of polarization curves for Pt(111) and
Au(111) with 40% Co(OH)2 for the HER. As can clearly be seen, the Au(111)/oxide surface is significantly less active than the Pt(111) counterpart. This
essentially establishes the role played by the Pt–Hupd descriptor. On fixing this interaction, by using Pt(111) as the main substrate, we have focused on the
‘pseudo’ mono-functional reaction with the OH ∗ ad –M2+δ interaction as the main descriptor. Bottom inset: a schematic showing the HER. Water from the
bulk of the electrolyte dissociatively adsorbs on the oxide cluster, forming OH ∗ ad intermediate on the oxide cluster, along with forming Had intermediate
formed on the Pt substrate. The Had groups re-combine to form H2. Depending on the OHad–M 2+δ strength, the OH ∗ ad is either stabilized (for Mn2+ , Fe 2+δ )
or destabilized (Ni 2+ , Co 2+ ) on the oxide clusters, which is found to dictate the turnover frequencies for these catalysts.
reaction is controlled mainly by the hydrogen recombination (the
Tafel step), in alkaline solutions the kinetics are determined by
a delicate balance between the water dissociation (the Volmer
step) and concomitant interaction of water dissociation products
with a surface. This was recognized by studying the HER on
the Ni(OH)2/Pt systems 43 , where, in a bi-functional effect, the
edges of Ni(OH)2 clusters promote the dissociation of water
(H2O ↔ H + OH − + e − ). The dissociation step is then followed
by H adsorption on the nearby Pt surfaces (H ↔ Pt–Had) and
by adsorption of OH − on Ni(OH)2 (see bottom inset in Fig. 4).
The kinetics of the HER will depend both on the rate of Had
recombination, which is optimized on the Pt substrate 43 , and on the
rate of desorption of OHad to accommodate the adsorption of H2O
on Ni(OH)2 clusters. Although information regarding kinetics of
the Tafel step on metal surfaces has been studied in great detail 40,41 ,
the importance of OH − ↔ OHad–M 2+δ O δ (OH)2−δ energetic, for
the OH − produced from the water dissociation step, for the
alkaline HER is largely unexplored. The presence of a bi-functional
mode of catalysis for the HER is also confirmed by the observed
enhancement for the HER activities on Co(OH)2/Pt(111) systems
(Supplementary Fig. S5). Further confirmation of the bi-functional
mechanism was achieved by observing a distinct maximum in
the activity versus coverage of Co(OH)2 as well as by comparing
the Co(OH)2/Au(111) systems with their Pt(111) counterparts
(Supplementary Section S4 and Fig. S5). Thus, the overall rate
of the HER may, in principle, be controlled by optimizing the
density and the nature of the sites required for dissociation of
water on M 2+δ O δ (OH)2−δ, as well as the OH − –M 2+δ O δ (OH)2−δ and
metal–Had energetics.
Here, using M 2+δ O δ (OH)2−δ/Pt(111) surfaces, we fix the
descriptor related to the adsorption energetics of Pt–Had, which
in turn lets us treat the HER as a ‘pseudo’ mono-functional
reaction: controlled by the descriptor related to OHad–M 2+δ bond
strength. As summarized in Fig. 4, a monotonic relationship
exists between the HER activity and the OHad–M 2+δ energetics,
with the most active catalysts being Ni(OH)2/Pt(111) and the
least active Mn(OH)2/Pt(111). On the basis of the observed
catalytic trends, it is clear that a balance must be found
between the transition state energies for water dissociation
and the final state energies of adsorbed OHad–M 2+δ O δ (OH)2−δ
(refs 44,45). According to the standard Brønsted–Evans–Polanyitype
principles 46 , this results in lower activation barriers for
water dissociation, while also resulting in poisoning of the sites
required for re-adsorption of water molecules. The net result
is that the turnover frequency substantially decreases (below
bare Pt activity) for the Fe and Mn hydr(oxy)oxides on which
OHad is more strongly adsorbed. In essence, this suggests that
the Fe and Mn behave purely as spectators, blocking the Pt
active sites for transforming H2O to H2. The best combination
among the 3d elements considered here, is found for the
Ni(OH)2/Pt(111), which has the most favourable balance between
facilitating water dissociation and preventing ‘poisoning’ with
OHad (water dissociation product), together with the optimal
Pt–Had energetics. The activity trends derived for the HER using
a series of 3d-M cations with different interaction strength
with OHad clearly establishes the presence of OHad in the HER
region (E < 0 V). Most probably, the active sites for OHad
are the defects, which are known to be very active for water
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ARTICLES
dissociation 45,47 . We emphasize that the exact nature of OHad
(electrosorption valency and free energy of adsorption) on the
M 2+δ O δ (OH)2−δ cluster is not known unambiguously. However,
the fact that the reactivity trends for the HER (Ni > Co >
Fe > Mn), on surfaces with constant Pt–Had interaction, are
identical to the trends in oxophilicity, established from the CO
oxidation reaction, OH − adsorption in the butterfly region and
the OER above 1.6 V, strongly validates the use of OHad–M 2+δ
interaction strength as the descriptor controlling the HER on these
M 2+δ O δ (OH)2−δ/Pt(Au) systems.
In summary, we have used well-characterized M 2+δ O δ (OH)2−δ
/Pt(111) catalyst surfaces (M = Ni, Co, Fe, Mn) to establish
clear trends in activity for the HER and the OER of a complex
oxide system, where the conventional methods of comparing highsurface-area
materials present many uncertainties. We determined,
using the OHad–M 2+δ interaction as the primary descriptor, that
the activity for the HER (bi-functional) and the OER (monofunctional)
for these 3d-M hydr(oxy)oxide systems follows the
order Ni > Co > Fe > Mn. The increasing OER activities for
the 3d-M systems (always greater than Pt), as a function of the
decreasing strength of the OHad–M n+ interaction, provides us
with the necessary toolkit to tune transition metal oxide catalysts
for the OER. We anticipate that by further, descriptor-guided
tuning of these oxides, even greater improvements in alkaline
electrocatalyst performance will be possible for the design of the
elusive ‘active centres’ in catalysis.
Methods
Electrode preparation: extended surface electrode preparation. Pt(111) and
Au(111) electrodes were prepared by inductive heating for 15 min at ∼1,100 K
(1,000 K for Au) in an argon hydrogen flow (3% hydrogen) 35 . The electrode
behaviours are known to be significantly influenced by the preparation procedure;
preparation by inductive heating leads to formation of defects such as ad-islands
on the surface. The number of such defects is low enough to be electrochemically
invisible. The annealed specimens were cooled slowly to room temperature
under an inert atmosphere and immediately covered with a droplet of deionized
(DI) water. Electrodes were then assembled into a rotating disk electrode (RDE)
ensemble. Voltammograms were recorded in argon-saturated electrolytes. The
Ag/AgCl reference electrode was used, but all potentials in the paper are shown
versus the reversible hydrogen electrode (RHE).
Metal hydroxide deposition. All substrates considered in this work (freshly
prepared extended surfaces, ad-island covered surfaces, as well as high-surface-area
catalyst electrodes) were washed thoroughly and introduced into an electrolyte
containing various concentrations of transition metal perchlorates/chlorides.
Concentration ranges tested include 150–1,000 ppm. Following the introduction
of the electrode, the oxide layers were then deposited either potentiostatically
(held at potentials above 0.6 V) or by cycling between Hupd and the OHad regions.
Typical coverages of 30–40% were obtained within 10 min of treatment. For higher
coverages, higher concentrations as well as longer times were used. After deposition,
the electrodes were rinsed and introduced into the clean electrochemical cell. The
Hupd of the modified surface is compared against that of the bare surface to estimate
the effective surface coverage of the oxide species.
Chemicals. All alkali metal hydroxides and perchlorate salts used in our
experiments were obtained in the highest purity from Sigma Aldrich. Electrolytes
used for our experiments, 0.1 M KOH/LiOH, were prepared with Millipore DI
water. All gases (argon, oxygen, hydrogen) were of 5N5 quality purchased from
Airgas. A typical three-electrode fluoro ethylene propylene (FEP) polymer based
cell was used to avoid contamination from glass components. Experiments were
controlled using an Autolab PGSTAT 302N potentiostat. Gold or platinum
wires were used as counter electrodes for studying hydrogen evolution reaction.
Precautions were taken to prevent significant accumulation of dissolved counter
electrode ions near the working electrode.
Electrochemical measurements. After extensive rinsing, the electrode was
embedded into the RDE and transferred into a standard three-compartment
electrochemical cell containing 0.1 M KOH/LiOH (Sigma-Aldrich). In each
experiment, the electrode was immersed at 0.05 V in a solution saturated with
argon. After obtaining a stable cycle between 0.05 and 0.7 V the electrolyte
was saturated with H2, following which polarization curves for the HER were
recorded on the disc electrodes between 0.05 V and −0.4 V. The lower potential
limits were chosen so as to avoid significant bubble formation, as well as to
NATURE MATERIALS DOI: 10.1038/NMAT3313
minimize the extended dissolution of the counter electrode. The concentration
of Pt after 1 h of HER measurements was found to be less than 1 ppm in the
working electrode compartment. All polarization curves were corrected for
the infrared contribution within the cell. OER measurements were carried out
by cycling the electrode up to 1.7 V versus RHE. Potential hold experiments
were also carried out for the oxide/Pt(111) systems to study the stability of the
oxide clusters at these potentials, as well as for preparation of samples for the
STM measurements.
STM method. For the as-prepared and the modified surfaces, the STM images
were acquired with a Digital Instruments Multi-Mode Dimension STM controlled
by a Nanoscope III control station. During the measurement, the microscope with
the sample was enclosed in a pressurized cylinder with a CO atmosphere. For
further details, see ref. 35.
XAS/XANES measurements. The X-ray absorption spectroscopy (XAS) data
were acquired at bending magnet beamline 12-BM-B at the Advanced Photon
Source (APS), Argonne National Laboratory. The synchrotron radiation was
filtered by a double crystal Si(111) monochromator with a double mirror system
for focusing and harmonic rejection. A custom-made in situ transmission
electrochemical X-ray cell with a 6 mm diameter Pt(111) single crystal and Ag/AgCl
reference electrode was used in a grazing incidence geometry. A 13-element Ge
detector (CANBERRA) was used to measure the fluorescence yield. Z-1 filters
and grazing incidence geometry was used to minimize the elastic scattering
intensity. The monochrometer calibration was monitored by simultaneously
measuring the same element reference foil in front of a Si diode and looking at
the air-scattered beam.
Received 14 November 2011; accepted 23 March 2012;
published online 6 May 2012
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Acknowledgements
Supported by the Office of Science, Office of Basic Energy Sciences, Division of
Materials Sciences, US Department of Energy, under contract DE-AC02-06CH11357.
R.S. would like to acknowledge the Argonne National Laboratory post-doctoral
fellowship for his funding.
Author contributions
R.S. and N.M.M. developed the idea and designed the experiments. R.S., D.T., K.C.C. and
A.P.P. performed the experiments and data analyses. R.S., N.M.M., D.S., K.C.C., J.G. and
V.S. discussed the results. R.S. and N.M.M. co-wrote the paper.
Additional information
The authors declare no competing financial interests. Supplementary information
accompanies this paper on www.nature.com/naturematerials. Reprints and permissions
information is available online at www.nature.com/reprints. Correspondence and
requests for materials should be addressed to N.M.M.
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ARTICLES
PUBLISHED ONLINE: 11 NOVEMBER 2012 | DOI: 10.1038/NMAT3457
Mesostructured thin films as electrocatalysts with
tunable composition and surface morphology
Dennis F. van der Vliet 1† , Chao Wang 1† , Dusan Tripkovic 1 , Dusan Strmcnik 1 , Xiao Feng Zhang 2 ,
Mark K. Debe 3 , Radoslav T. Atanasoski 3 , Nenad M. Markovic 1 and Vojislav R. Stamenkovic 1 *
Among the most challenging issues in technologies for electrochemical energy conversion are the insufficient activity of the
catalysts for the oxygen reduction reaction, catalyst degradation and carbon-support corrosion. In an effort to address these
barriers, we aimed towards carbon-free multi/bimetallic materials in the form of mesostructured thin films with tailored
physical properties. We present here a new class of metallic materials with tunable near-surface composition, morphology
and structure that have led to greatly improved affinity for the electrochemical reduction of oxygen. The level of activity for
the oxygen reduction reaction established on mesostructured thin-film catalysts exceeds the highest value reported for bulk
polycrystalline Pt bimetallic alloys, and is 20-fold more active than the present state-of-the-art Pt/C nanoscale catalyst.
Over the past decades, extensive research has been devoted to
the development of technologies that can effectively convert
energy and become economically viable for use by the
general public. Great expectations are held for technologies such
as fuel cells and lithium–air batteries that rely on electrochemical
processes. In both cases, satisfactory energy density can be attained;
however, a major challenge lies in the insufficient activity and
durability of the materials that are employed at present as cathode
catalysts for electrochemical reduction of oxygen. These limitations
inevitably lead to a lower operating efficiency of the devices,
which highlights the need for the development of more active
and durable oxygen reduction reaction (ORR) catalysts 1–12 . In the
case of fuel cells, most of the research centres on platinum, the
best monometallic catalyst for the ORR. At the present state of
development, an approximately fivefold reduction in Pt content
is necessary to meet cost requirements for large-scale automotive
applications 5 . Pt-based alloys have already made an impact in
fuel-cell catalyst design by decreasing the amount of platinum while
improving activity and durability 12–18 , which places these materials
at the focus of intensive fundamental and applied research on both
extended (bulk) 17–25 and nanoscale systems 7,14–16,26–34 . The main
challenge in that effort is linked to the possibility of achieving
the unique structural and compositional profile of the Pt3Ni(111)
alloy, which was established from single-crystal studies 12,17 . Such
a profile was obtained on extended surfaces by thermal annealing
that facilitates thermodynamically driven segregation of Pt to form a
pure ordered surface layer, denoted as Pt(111)-skin. The electronic
structure of Pt(111)-skin is altered by the subsurface layer of PtNi
(in 1:1 ratio) and is responsible for the extreme ORR activity, which
is nearly two orders of magnitude higher than the state-of-the-art
Pt/C catalyst. Consequently, the ability to mimic the compositional
profile and structure of Pt-skin in high-surface-area catalysts would
bring unprecedented benefits to technologies that rely on the
ORR. However, despite numerous attempts, this goal has not been
achieved yet for practical catalysts.
Here we present a new class of materials based on mesostructured
multimetallic thin films with adjustable structure and
composition, which have been tailored to emulate the distinctive
properties of the Pt(111)-skin, to be employed in electrochemical
devices. The design of these materials relied on our previous work
related to well-defined extended and nanoscale surfaces in the form
of PtM alloys (M = Ni, Co, Fe, V, Ti; refs 17–20). We aimed
towards catalysts that can bridge the world of extended surfaces
with superior activity and nanoscale systems with high specific
surface area, to harvest maximal utilization of precious metals. Such
synergy is foreseen to be present at the mesoscale, which implies not
only a specific length scale, but rather a principle of operating in
between different physical regimes that exhibit distinct functional
behaviour 35 . Considering that mesoscale materials chemistry is still
in its infancy, it is expected that this field will open pathways in
materials design that arise from the rational control of physicochemical
properties and functionality of mesostructured systems.
In particular, for electrocatalytic materials, most previous work
has emphasized either achievement of high surface area through
small particle size, or the attainment of a better understanding of
fundamental properties through the use of extended surfaces. From
such studies, it is well known that there are substantial differences
in catalytic properties between nanoscale and bulk materials. The
benefits of targeting mesoscale architectures between these extremes
have scarcely been explored, especially in the sense of transferring
superior characteristics from extended surfaces to practical
materials. In view of that, instead of using discrete nanoparticles
(3–5 nm) supported on high-surface-area carbon 26–28,32–34,36 , we deployed
continuous Pt and Pt-alloy nanostructured thin films 29,37–41
(NSTF) over an oriented array of molecular solid whiskers by
physical vapour deposition. Specifically, planar magnetron sputter
deposition was used to deposit thin metal films with a wide range
in composition. Such NSTF catalysts provide good surface area
utilization and eliminate issues related to carbon-support corrosion
and contact resistance at the carbon/metal interface that would lead
to poor utilization and degradation of the catalyst 14 . The capability
to control the deposition rate, as well as the combination and
order of constituents, signifies sputter deposition as an effective
tool to form thin films with desirable thickness, composition profile
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA, 2 Hitachi High Technologies America, Pleasanton, California
94588, USA, 3 Fuel Cell Components Program, 3M, St Paul, Minnesota 55144, USA. † These authors contributed equally to this work.
*e-mail: vrstamenkovic@anl.gov.
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ARTICLES
c
a b
I (μA)
I (mA)
20
0
¬20
0.02
0.00
¬0.02
0.2
Pt as deposited
Pt (111)
0.4 0.6 0.8
E (V)
Pt 3 Ni (111)
As-deposited PtNi/GC
Annealed PtNi/GC
5 nm
0.0 0.2 0.4 0.6 0.8 1.0
E (V versus RHE)
I (μA)
Specific activity (mA cm ¬2 pt)
8
6
4
2
0
NATURE MATERIALS DOI: 10.1038/NMAT3457
Pt thin film annealed
Pt (111)
0.2 0.4 0.6
E (V)
0.8
Pt 3 Ni(111)-skin
Pt poly
As-deposited
Annealed
ΔT
Polycrystalline
ordering
Pt thin film
on GC
(111)
Polycrystalline
(111)
ordering
5 nm
ΔT
PtNi thin film
on GC
Figure 1 | CV and STM images of Pt and Pt alloy 20-nm thin films deposited on a glassy carbon substrate. a, As-deposited Pt thin film (solid line) and
Pt(111) (dashed line). b, Annealed Pt thin film (solid line) and Pt(111) (dashed line). c, CV profile of as-deposited (blue line), annealed PtNi thin film (red
line) and Pt3Ni(111)-skin (dashed line). d, Specific activities measured by RDE in 0.1 M HClO4 with 1,600 rpm, 20 mV s −1 at 0.95 V with corresponding
improvements factors versus polycrystalline Pt.
and surface roughness. However, the surface structure of a thin
film, an important catalytic parameter 33,42–44 , cannot be altered
by this method to match those established on single-crystalline
systems. For that reason, we attempted a thorough examination of
thin-film properties on extended, flat, non-crystalline and chemically
inert substrates such as a mirror-polished glassy carbon
surface. This approach brings an extra level of control in terms of
defined geometric surface area and surface roughness factor that is
unattainable in the case of nanoscale substrates. Consequently, our
efforts have been directed towards exploring structural transitions
in polycrystalline thin films.
The first step comprised the deposition of a pure Pt thin film
onto an ultrahigh-vacuum-cleaned glassy carbon substrate, which
was followed by thermal annealing in a reductive atmosphere
(see Methods). The morphology of the Pt film was validated by
scanning tunnelling microscopy (STM) as shown in Fig. 1a,b. The
difference between as-deposited versus annealed Pt films indicates a
substantial change in the thin-film surface morphology due to rearrangement
of the Pt topmost atoms towards the (111) structure with
minimum surface energy. The as-deposited Pt film has a corrugated
nanostructured three-dimensional surface morphology with an average
grain size of ∼5 nm, whereas the morphology of the annealed
thin film has been transformed into a smooth two-dimensional surface
with large 20 × 100 nm hexagonal (111) facets. In accordance
with the STM results, the characteristic surface features are also
confirmed by electrochemical cyclic voltammetry (CV). Figure 1a
reveals that the CV profile of the as-deposited thin-film surface
d
20
0
¬20
matches the one established for bulk polycrystalline Pt. On the other
hand, Fig. 1b shows that the CV profile of the annealed Pt thin film
underwent extensive transformation from typical polycrystalline
into Pt(111)-like with characteristic fingerprint features between
0.5 and 0.9 V; the so-called butterfly region that corresponds to
adsorption/desorption processes of OHad on Pt(111) facets (see
Supplementary Information). Therefore, it is evident from both
STM and CV that the annealed extended thin film consists of
predominantly (111) facets encompassing the entire surface. In fact,
the degree of resemblance in electrochemical signature between the
annealed thin-film surface and single-crystal Pt confirms that the
(111) facets are both large and interconnected. The synergy between
the surface structure, domain size and functionality defines that the
thin-film surface has a distinct mesostructured morphology. These
findings clearly demonstrate the feasibility of controlling surface
ordering of extended Pt thin films deposited over a non-crystalline
substrate, that is, without the use of templates for epitaxial growth.
Instead of building the crystal lattice from a seed or underlying
crystalline substrate, individual randomly oriented nanoscale grains
coalesce and form large well-ordered (111) facets. All of that greatly
expands the potential for utilization of thin-film materials and
introduces thermal annealing in a controlled atmosphere as a
compelling tool in the fine tuning of a thin film’s structure and
hence electrocatalytic properties.
In the following steps we proceed towards a bimetallic PtNi thin
film with the same thickness to mimic the composition profile of the
Pt3Ni(111) system and to replicate its unique catalytic properties.
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16
12
8
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Improvement factor versus Pt-poly

NATURE MATERIALS DOI: 10.1038/NMAT3457 ARTICLES
a b
z
y
x
50 nm
Pt M N
Substrate growth Physical vapour deposition Nanostructured thin films
c d
e
15 nm 10 nm 10 nm
Figure 2 | HRSEM and TEM micrographs of the NSTF whiskers. Centre: schematic illustration of the vacuum protocol that has been used for the growth of
aligned perylene red substrate, which is then coated by a metallic thin film with an adjustable thickness and composition profile (Pt; M and/or
N = Ni,Co,Fe,Ti,V). a, HRSEM snapshot of a group of whiskers that indicates their length, shape and alignment after thin-film deposition. b, HRSEM
close-up of an intentionally broken single whisker that demonstrates the thickness of the metallic film over perylene red substrate. c, HRTEM close-up of a
single whisker side, which reveals growth of whiskerettes along the whisker. d, HRSEM insight into the whisker’s surface showing a close-packed formation
of whiskerette tips of 5 nm in diameter that facilitates a highly corrugated morphology. e, TEM micrograph of a whisker side that confirms the grained
texture of the sputtered thin film and the average diameter of the whiskerettes.
The results from the electrochemical measurements in Fig. 1c,d
confirm that as for monometallic Pt, the polycrystalline nature
of the as-deposited alloy thin film is predominantly transformed
into a Pt(111)-skin-like surface. This is obvious from both the
CV profile of the annealed alloy thin film that resembles the
one obtained on Pt3Ni(111) and its superior catalytic activity
for the ORR, which was up to now obtained exclusively on the
Pt3Ni(111)-skin surface 17,45 . The combination of the Pt(111)-skinlike
voltammetry and the marked increase in the ORR activity
proves that surface ordering from randomly oriented towards (111)
is indeed feasible for bimetallic thin films, and demonstrates that
the catalytic improvement obeys the same mechanism as previously
reported for Pt-bimetallic single-crystal surfaces; that is, electronic
modification of the topmost Pt layer leads to extreme catalytic
enhancement solely for the (111) orientation 19 . Therefore, the
ORR-specific activity, which equals 70% of the value established
for the most active catalyst, Pt3Ni(111)-skin, serves as a descriptor
that (111)-skin facets are dominating on the annealed thin-film
surface. Together this demonstrates the twofold power of annealing
in facilitating the formation of the mesostructured alloy thinfilm
morphology, characterized by both an energetically more
favourable surface state rich in (111) facets, and the desired
compositional profile.
It is these results that provide the driving force for a shift towards
corresponding thin-film-based high-surface-area materials.
A Pt-alloy NSTF catalyst is deposited by magnetron sputtering
20 nm
over an array of molecular solid whiskers, composed of an organic
pigment N , N -di(3,5-xylyl)perylene-3,4:9,10 bis(dicarboximide);
hereafter denoted as perylene red 29,37–39,46 . Figure 2 illustrates the
step-by-step deposition process of the thin metal films onto the
perylene red support, as well as high-resolution scanning electron
microscopy (HRSEM) and transmission electron microscopy
(TEM) micrographs of the NSTF whiskers. These images reveal
a detailed insight into critical parameters of the NSTF such as
metallic film thickness, length, shape and surface morphology. A
single whisker measures on average about 800 nm in length, and
the film thickness is 5–20 nm. In Fig. 2, it is clearly visible that
on the sides of a single whisker, smaller metal alloy whiskerettes
are formed with a diameter of ∼5 nm (Fig. 2c), and a close-up
of a broken whisker (Fig. 2b) illustrates the metallic film/shell
that surrounds the perylene red substrate. Surface-specific HRSEM
in Fig. 2b,d unveils that the side walls along the whisker have a
very rough surface morphology, consisting mainly of whiskerette
tips bonded closely to each other to produce densely packed
corncob-like features, providing the validity of terming this material
a NSTF. It is important to emphasize that the highly grained texture
of the NSTF side walls made of closely packed whiskerettes is
also confirmed in the TEM micrograph in Fig. 2e. Such structural
parameters greatly affect the functional properties of the NSTF,
and therefore the ability to control and tune them along with the
near-surface compositional profile can lead towards a substantial
gain in catalytic performance.
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ARTICLES
a
b
c
Nanostructured
thin films
Surface
modification
and
substrate
evaporation
Mesostructured
thin films
30 nm
20 nm
15 nm
20 nm
20 nm
20 nm
NATURE MATERIALS DOI: 10.1038/NMAT3457
Figure 3 | In situ transformation from nanostructured into mesostructured thin film during annealing. a–c, Middle: HRTEM images of progressive
annealing between room temperature and 400 ◦ C on a single whisker that capture the ordering from randomly oriented to a homogeneous structure with
visible crystalline domains. Right: HRTEM close-up of the transformation for the same near-surface region during annealing, from highly corrugated into a
flat and smooth surface morphology. Left: schematic illustration of the mesoscale ordering during annealing and formation of the mesostructured thin film
catalyst, associated with the corresponding HRSEM insets.
In what follows, we apply an experimental approach combined
with the knowledge related to highly active well-defined singlecrystalline
and extended thin-film surfaces to develop mesostructured
thin-film electrocatalysts with advanced properties. In situ
HRSEM and TEM are simultaneously employed during NSTF
annealing in a controlled atmosphere. This allows us to visualize
real-time structural changes at the atomic level and to follow
rearrangements of the surface and sub-surface morphology of
thin-film materials. This insight is invaluable in the fine-tuning of
the materials’ properties. Figure 3 depicts in situ results obtained
during thermal annealing of a single PtNi-NSTF whisker. The NSTF
catalyst is mounted onto the HRTEM heating stage and is introduced
to a reductive atmosphere of argon and hydrogen gases. As
the specimen is gradually heated, no change in surface morphology
is observed as depicted in Fig. 3a, which reveals the initial stage and
a close-up of the grained highly corrugated whisker side wall and
its surface. Once the temperature reaches 300 ◦ C, we start to follow
real-time restructuring of the thin film’s morphology. Figure 3b
captures the onset of the surface transformation, which appears as a
smoothening of the near-surface regions. The steady-state structure
is achieved after 30 min and is shown in Fig. 3c. These images
illustrate that the densely packed organization with the initial
three-dimensional surface morphology is being transformed into
a more homogeneous, flat and ordered two-dimensional thin-film
material with clearly observable crystalline features in its walls.
This thermodynamically driven transition releases stress and strain
of the as-deposited thin film and leads towards the state with
minimum surface energy without compromising the overall shape
and dimension of the whisker. As for Pt thin films on glassy carbon,
the initial nanostructured surface morphology that originated from
× 10
× 10
× 10
2 nm
2 nm
2 nm
the closely bonded whiskerettes’ tips is transformed into a smooth
continuous film with large crystalline domains (20–40 nm). Specifically,
randomly oriented nanoscale grains coalesce and give rise to a
mesostructured thin film with unique physicochemical properties;
therefore, the materials after this treatment will be referred to as
mesostructured thin films (Meso-TF). Close inspection of HRTEM
micrographs after applied thermal treatment (see Supplementary
Information) confirms that emerged facets with (111) structure
prevail on the surface whereas undercoordinated sites are diminished,
which also has important implications towards improved
stability. As a side effect, the perylene red substrate is removed
during this procedure (details can be found in the Supplementary
Information). In addition to HRTEM/SEM studies, X-ray diffraction
measurements, which show enhanced alloying and an increase
in the number of (111)-oriented domains on the Meso-TF, are
presented in the Supplementary Information.
The final step in the characterization is to obtain the electrochemical
signature and compare adsorption and catalytic properties
between different classes of thin-film materials and the state-ofthe-art
Pt/C catalyst by rotating-disc electrode (RDE), see Methods
section. As expected, from the CV profile depicted in Fig. 4 we find
that the smooth morphology of the Meso-TF slightly lowers the
electrochemically active surface area (ECSA), from ∼11 m 2 g −1 for
the NSTF to ∼9 m 2 g −1
Pt for the Meso-TF. This implies that most of
the inner portion of the whiskers, which has been vacated by the
perylene red, is not electrochemically active, presumably owing to
lack of penetration of the electrolyte into the hollow of the whisker
(see closed whisker end in Fig. 3). As shown in Fig. 4a, the CV profile
of PtNi NSTF whiskers exhibits similar behaviour to monometallic
Pt NSTF with clearly visible polycrystalline Pt features due to
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NATURE MATERIALS DOI: 10.1038/NMAT3457 ARTICLES
a c
I (mA cm ¬2 )
I (mA)
0.4
0.0
¬0.4
¬0.8
b
PtNi Meso-TF
d
0.0
Pt NSTF
¬0.5
¬1.0
¬1.5
PtNi Meso-TF
PtNi NSTF
Pt NSTF
0.0 0.2 0.4 0.6 0.8 1.0 1.2
E (V versus RHE)
0.0 0.2 0.4 0.6 0.8 1.0 1.2
E (V versus RHE)
E (V versus RHE)
i kin (mA cm pt ¬2 )
1.00
0.95
PtNi Meso-TF
PtNi NSTF
Pt NSTF
Pt/C
0.90
0.01 0.1
ikin (mA cm ¬2 )
2.4
1.8
1.2
0.6
Pt/C Ptpoly
Pt-
NSTF
ordering
Nano Meso
PtNi-
NSTF
1 2 3
ΔT
PtNi-
Meso-TF
Figure 4 | CV on the Pt-based thin-film catalysts. a, Cyclic voltammograms of Pt-NSTF, PtNi-NSTF and PtNi-Meso-TF. b, The ORR polarization curves.
c, Corresponding Tafel plots (Tafel slopes are determined at potentials higher than the half-wave potential (E1/2; potential at which I = 1/2 Idiff), to avoid
diffusion- and solution-resistance-induced errors). d, Specific activities measured at 0.95 V and improvement factor versus Pt-poly (and Pt-NSTF).
the adsorption–desorption processes of underpotentially deposited
hydrogen (Hupd). However, the Hupd region of PtNi Meso-TF is
significantly different with a characteristic flat plateau (Fig. 4a),
which confirms that the surface has a relatively large contribution
of (111) facets compared with the highly corrugated sputtered
thin film that is rich in low-coordinated Pt sites. This is also
in good agreement with HRTEM and X-ray diffraction results.
Moreover, the onset of surface oxide formation is shifted positively
in the following order: Pt-NSTF < PtNi-NSTF < PtNi Meso-TF.
Accordingly, the ORR polarization curves, shown in Fig. 4b, follow
the same trend in activity. Figure 4c and Fig. 4d summarize the
kinetic current densities (specific activities per ECSA of Pt) as
Tafel plots and a bar graph, respectively. As specific activity is a
fundamental property of a material that reflects its intrinsic catalytic
performance, as opposed to mass activity, which emphasizes the
optimized dispersion of a material, our focus has been placed
on boosting specific activity. This approach leads to a higher
turnover frequency (the measure of activity per active site), which
may result in better utilization of Pt, culminating in higher mass
activity. Considering the large increase in specific activity, we report
values measured at 0.95 V to avoid diffusion-induced errors in
kinetic current densities. The order of specific activity becomes
apparent, with Pt/C being the least active, followed by Pt-NSTF
and polycrystalline Pt. One can observe a significant increase in
activity for PtNi Meso-TF, accompanied by a decrease in Tafel
slope from ∼70 mV dec −1 for monometallic Pt to ∼40 mV dec −1 .
This value is considerably lower than those commonly reported
for Pt-based catalysts in the literature 19,47 , but it is in line with the
value obtained on Pt3Ni(111)-skin 47 . The activity of PtNi Meso-TF
exhibits an improvement factor of over 8 versus Pt-poly and
Pt-NSTF. Furthermore, when compared with the state-of-the-art
conventional Pt/C catalyst, the specific activity of the PtNi Meso-TF
achieves an unprecedented 20-fold enhancement. Even though
optimal film thickness, alloy composition and total Pt loading will
be extensively studied in the future, the measured improvement
expressed in A/mgPt corresponds to a mass activity that is already
three times higher than the US Department of Energy technical
target 12 . Together, the flat voltammetric curves, the trend in specific
activity, the low Tafel slope and the structural characterizations
strongly suggest that the annealed PtNi Meso-TF has a Pt-skin-type
near-surface structure.
To review the findings on thin-film-based mesostructured
catalysts and to merge them into the same chart with nanoscale
systems and bulk materials, we present in Fig. 5 the ORR activity
map for different classes of Pt alloys, that is, from nanoparticles
dispersed on high-surface-area carbon, to polycrystalline bulk
materials and to single-crystalline alloys of Pt3Ni(hkl) surfaces 19 .
This map shows a huge span in intrinsic specific activities among
materials of the same bulk elemental composition that differ in
form and surface structure. It also demonstrates the importance
of controlling fundamental properties that determine catalytic
performance; specifically that the ability to alter physical parameters
such as particle size, near-surface composition profile, morphology
and surface structure can lead to great improvements in functional
properties of real catalysts. Notably, we have explored a number
of NSTF catalysts with different compositions as summarized in
Fig. 5; however, for the sake of brevity in this report we have
presented only the results for the PtNi in detail. The activity values
are given for Pt alloys with different transition metals associated
with the atomic number (Z). The main features in Fig. 5 are
designated activity regions for different classes of materials. Metallic
nanoparticles of Pt and Pt alloys dispersed on a high-surface-area
carbon support exhibit profoundly lower activities compared with
their polycrystalline bulk counterparts. The assigned region that
reflects the activity range of metallic nanoparticles is based on
the literature data reported for Pt-alloys obtained by conventional
impregnation methods. The next level in activity is reserved for
extended bulk polycrystalline systems, where the specific activity
of Pt3M-alloys can be improved by a factor of three versus
Pt-poly. As mentioned above, the capability to control the surface
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8
6
4
2
Improvement factor versus Pt-poly

ARTICLES
Activity improvement factor versus Pt-poly
NATURE MATERIALS DOI: 10.1038/NMAT3457
Pt3Ni(111) 20.0 70
8.0
6.0
4.0
2.0
1.0
Single-crystal alloys
Polycrystalline alloys
Pt 3 Ti
Pt-skin
Pt-skeleton
NSTF
Pt 3 V
Metallic nanoparticles dispersed in carbon
Pt 3 Fe
Pt 3 Co
NSTF
Pt 3 Co
Pt 3 CoNi
NSTF
Mesoscale
ordering
22 24 26 28
Atomic number (z)
PtNi
Meso-TF
Figure 5 | Activity map for the ORR obtained for different classes of Pt-based materials. Improvement factors are given on the basis of activities
compared with the values for polycrystalline Pt and the state-of-the-art Pt/C catalyst established by RDE measurements in 0.1 M HClO4 at 0.95 V.
structure leads to an extra boost in activity, and hence the highest
ORR activity ever measured was obtained for the Pt3Ni(111)skin
surface. On the basis of the values depicted in Fig. 5, the
NSTF catalysts can successfully mimic the catalytic behaviour of
polycrystalline bulk materials, while Pt-alloy mesostructured thin
films exceed the range designated for polycrystalline systems. This
is the first practical catalyst to approach the levels of activity
previously reserved only for bulk single-crystalline surfaces, owing
to the formation of a surface and near-surface structure similar
to that of the ideal Pt3Ni(111)-skin. These bimetallic Meso-TF
materials preserve sufficiently high specific surface area, which
enables better utilization of precious metals. Moreover, Pt-based
catalysts with mesoscale features also avoid the activity losses that
are caused by the higher fraction of low-coordinated surface atoms
that are present in nanoscale catalysts 14 . Consequently, thin-film
electrocatalysts are hampered neither by the stability issues that
accompany the use of high-surface-area carbon support, nor by
the loss of active surface area due to particle agglomeration. The
mesostructured thin films, therefore, unite the beneficial properties
of both the nanoscale and the extended bulk systems, and lead
to new design rules for producing highly active and durable
electrocatalysts. These findings provide a proof of concept that the
ability to tailor the composition, morphology and structure of the
thin-film-based materials at the mesoscale allows the harvesting
of maximal performance from the employed constituents. This is
a seeding study on mesoscale ordering in electrocatalysts where
further advancement towards an increased presence of (111) facets
PtNi
NSTF
Pt 3 Ni
Pt 3 Ni
NSTF
30
Pt/C
and optimization of the thin-film thickness may lead to additional
improvements in specific and mass activities.
We report on a new class of mesostructured catalysts based
on thin films with an adjustable composition profile and surface
morphology. These materials are in the form of metallic thin films
with properties that have been tailored to improve the activity
for the ORR. The obtained ORR activity is the highest ever
measured on non-bulk catalysts owing to the beneficial near-surface
compositional profile and its highly crystalline surface morphology.
The exceptional properties of this Meso-TF are comparable to
extended single-crystalline surfaces and improvement factors in
kinetic activity of 8 versus polycrystalline Pt and 20 versus Pt/C
are observed. The substantial advances in catalytic performance
are obtained through structural mesoscale ordering of the thin
film induced by thermal annealing in a reductive atmosphere. The
approach as developed can be applied to generate a wide range
of (electro)catalysts with tailored structure/composition, ultralow
precious metal content and superior functional properties such as
activity and durability.
Methods
Thin-film deposition. Thin metal films were deposited by planar magnetron
sputter deposition on the ultrahigh-vacuum-cleaned surface of a mirror-polished
glassy carbon substrate of 6 mm in diameter (base vacuum 1 × 10 −10 torr). The
deposition rate was set to 0.3 Å s −1 by a quartz-crystal microbalance and an
exposure of 7 s was calibrated for the nominal thickness of 2.2 ∼ 2.3 Å for
a monolayer of Pt. The film thickness was derived from the exposure time of
computer-controlled shutters during deposition. The thickness of all thin films in
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Pt
78
20
15
10
5
Activity improvement factor versus Pt/C

NATURE MATERIALS DOI: 10.1038/NMAT3457 ARTICLES
this study was 20 nm. In the case of NSTF catalysts, consecutive layers of platinum
and the metal of choice were deposited onto the NSTF layer of oriented organic
pigment (perylene red) whiskers also by planar magnetron sputter deposition
in vacuum 48 . The deposition process covered each of the perylene red whiskers
with a thin metallic film. Both the monometallic Pt and the Pt-alloy catalyst were
obtained by this method. The Meso-TF were obtained by thermal annealing of
NSTF at 400 ◦ C in a hydrogen-rich atmosphere. The temperature was increased in
increments of 20 ◦ C per 5 min and the whole process lasted 2 h.
Electrochemical measurements. An Autolab PGSTAT 30 with FI20, ECD,
ADC and SCAN GEN modules was used for the electrochemical measurements.
Perchloric acid diluted with MilliQ water to 0.1 M was the electrolyte in all cases.
The gases used were research grade (5N5+) argon and oxygen. In all experiments,
a silver–silver chloride was the reference electrode. However, all potentials
referred to in this paper are converted to the pH-independent reversible hydrogen
electrode scale. We repeated all experiments 8 times to confirm reproducibility,
and to improve the accuracy in the determination of kinetic activities. Kinetic
current densities were obtained from the measured ORR polarization curves
in accordance with the Koutecky–Levich equation: I −1
ORR = I−1
kinetic + I−1
diffusion . The
ECSA of the nanocatalysts was determined by integrating both the Hupd part of
the CV profile, and the polarization curve obtained by oxidation of a monolayer
of adsorbed carbon monoxide to avoid underestimation of the surface area due
to altered hydrogen adsorption properties. All catalysts were deposited on a
RDE made of glassy carbon and the loading of the nanoscale thin-film catalysts
was adjusted to be 60 µgPt cm −2
disc, whereas the loading for Pt/C obtained from
TKK was 12 µgPt cm −2
disc. Kinetic current densities as reported are normalized
by ECSA in all cases.
Microscopy. A Hitachi H-9500 environmental transmission electron microscope
operated at 300 kV was used to perform the microstructural characterization and
in situ heating TEM study. Powder samples were attached to the heating zone of a
Hitachi gas-injection-heating holder. Images of nanoparticles were first recorded
at room temperature, followed by heating of the specimen inside the microscope
chamber with a vacuum level of about 10 −4 Pa. A CCD (charged-coupled device)
camera was used to monitor the microstructural evolution and record images
and videos. Each heating temperature was held for at least 10 min for detailed
structural characterization, including morphology and atomic structure. A Hitachi
SU70 high-resolution field-emission SEM was used for routine nanoparticle
sample inspection. For the detailed surface morphology study at the nanometre
scale, a Hitachi S-5500 ultrahigh-resolution cold field-emission SEM delivered
a much higher resolution power (0.4 nm secondary electron image resolution
at 30 kV) than normal SEM because of the specially designed objective lens.
On both SU70 and S-5500, secondary electron images were taken at 15 kV or
30 kV to reveal the surface morphology of both the as-deposited, as well as the
annealed nanoparticles.
Received 8 February 2012; accepted 11 September 2012;
published online 11 November 2012
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Pt-skeleton surfaces. J. Am. Chem. Soc. 128, 8813–8819 (2006).
21. Koh, S. & Strasser, P. Electrocatalysis on bimetallic surfaces: Modifying
catalytic reactivity for oxygen reduction by voltammetric surface dealloying.
J. Am. Chem. Soc. 129, 12624–12625 (2007).
22. Zhang, J. L., Vukmirovic, M. B., Xu, Y., Mavrikakis, M. & Adzic, R. R.
Controlling the catalytic activity of platinum-monolayer electrocatalysts
for oxygen reduction with different substrates. Angew. Chem. Int. Ed. 44,
2132–2135 (2005).
23. Mukerjee, S. & Srinivasan, S. Enhanced electrocatalysis of oxygen reduction on
platinum alloys in proton-exchange membrane fuel-cells. J. Electroanal. Chem.
357, 201–224 (1993).
24. Toda, T., Igarashi, H., Uchida, H. & Watanabe, M. Enhancement of the
electroreduction of oxygen on Pt alloys with Fe, Ni, and Co. J. Electrochem. Soc.
146, 3750–3756 (1999).
25. Mavrikakis, M. Computational methods: A search engine for catalysts.
Nature Mater. 5, 847–848 (2006).
26. Mani, P., Srivastava, R. & Strasser, P. Dealloyed Pt–Cu core–shell nanoparticle
electrocatalysts for use in PEM fuel cell cathodes. J. Phys. Chem. C 112,
2770–2778 (2008).
27. Wang, C. et al. Monodisperse Pt3Co nanoparticles as a catalyst for the
oxygen reduction reaction: Size-dependent activity. J. Phys. Chem. C 113,
19365–19368 (2009).
28. Wang, C. et al. Monodisperse Pt3Co nanoparticles as electrocatalyst: The
effects of particle size and pretreatment on electrocatalytic reduction of oxygen.
Phys. Chem. Chem. Phys. 12, 6933–6939 (2010).
29. Van der Vliet, D. et al. Platinum-alloy nanostructured thin film catalysts for
the oxygen reduction reaction. Electrochim. Acta 56, 8695–8699 (2011).
30. Zeis, R., Mathur, A., Fritz, G., Lee, J. & Erlebacher, J. Platinum-plated
nanoporous gold: An efficient, low Pt loading electrocatalyst for PEM fuel cells.
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31. Habas, S. E., Lee, H., Radmilovic, V., Somorjai, G. A. & Yang, P. Shaping
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32. Tao, F. et al. Reaction-driven restructuring of Rh–Pd and Pt–Pd core–shell
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33. Inaba, M. et al. Controlled growth and shape formation of platinum
nanoparticles and their electrochemical properties. Electrochim. Acta 52,
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34. Paulus, U. A. et al. Oxygen reduction on carbon-supported Pt–Ni and Pt–Co
alloy catalysts. J. Phys. Chem. B 106, 4181–4191 (2002).
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voltage stability of nanostructured thin film catalysts for PEM fuel cells.
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Crystallographic characteristics of nanostructured thin-film fuel cell
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Acknowledgements
The research was conducted at Argonne National Laboratory, which is a US Department
of Energy Office of Science Laboratory operated by UChicago Argonne, LLC under
NATURE MATERIALS DOI: 10.1038/NMAT3457
contract no. DE-AC02-06CH11357. The portion of work related to extended
single-crystalline and thin-film surfaces was supported by the US Department of Energy,
Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering
Division. The portion of work exploring practical thin-film-based electrocatalysts
was supported by the Office of Energy Efficiency and Renewable Energy, Fuel Cell
Technologies Program. The authors thank Hitachi High Technologies America for the
access to high-resolution electron microscopy facilities and J. Pearson and A. P. Paulikas
for supporting thin film deposition experiments. V.R.S. is grateful to S. D. Bader and
G.W. Crabtree for productive discussions.
Author contributions
D.F.V., C.W. and V.R.S. designed the experiments. C.W., D.F.V., D.T., D.S., X.F.Z.,
R.T.A., M.K.D. and V.R.S. carried out the experimental work. D.F.V., C.W., M.K.D.,
N.M.M. and V.R.S. discussed the results and V.R.S. wrote the manuscript.
Additional information
Supplementary information is available in the online version of the paper. Reprints and
permissions information is available online at www.nature.com/reprints. Correspondence
and requests for materials should be addressed to V.R.S.
Competing financial interests
The authors declare no competing financial interests.
8 NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials
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PUBLISHED ONLINE: XX MONTH XXXX | DOI: 10.1038/NMAT2883
Selective catalysts for the hydrogen oxidation and
oxygen reduction reactions by patterning of
platinum with calix[4]arene molecules
Bostjan Genorio 1,2 , Dusan Strmcnik 3 , Ram Subbaraman 4 , Dusan Tripkovic 3 , Goran Karapetrov 3 ,
Vojislav R. Stamenkovic 3 , Stane Pejovnik 1 and Nenad M. Marković 3 *
The design of new catalysts for polymer electrolyte membrane
fuel cells must be guided by two equally important fundamental
principles: optimization of their catalytic behaviour as well as
the long-term stability of the metal catalysts and supports
in hostile electrochemical environments1,2 . The methods used
to improve catalytic activity are diverse3–8 , ranging from the
alloying3,4 and de-alloying5 of platinum to the synthesis of
platinum core–shell catalysts6 . However, methods to improve
the stability of the carbon supports and catalyst nanoparticles
are limited9,10 , especially during shutdown (when hydrogen
is purged from the anode by air) and startup (when air is
purged from the anode by hydrogen) conditions when the
cathode potential can be pushed up to 1.5 V (ref. 11). Under the
latter conditions, stability of the cathode materials is strongly
affected (carbon oxidation reaction) by the undesired oxygen
reduction reaction (ORR) on the anode side. This emphasizes
the importance of designing selective anode catalysts that
can efficiently suppress the ORR while fully preserving the
Pt-like activity for the hydrogen oxidation reaction. Here, we
demonstrate that chemically modified platinum with a selfassembled
monolayer of calix[4]arene molecules meets this
challenging requirement.
The problem of cathode degradation during the shutdown/startup
conditions in polymer electrolyte membrane fuel cells
(PEMFCs) was realized two decades ago; however, there are still
few methods to improve the selectivity of anode catalysts. This is
because the required ensemble of active sites to achieve the maximum
reaction rate of the ORR and the hydrogen oxidation reaction
(HOR) is extremely small12 , making the design of selective anode
catalysts extremely challenging. To meet this challenge, significant
improvements in the design of anode materials must be realized.
We present a chemically modified electrode (CME) approach
to the design and improvement of a selective anode catalyst
that can overcome the shutdown and startup limitations that
prevent a full implementation of PEMFC technology. The use
of CME approaches to modify electrocatalytic properties has
been studied previously13,14 . The application of enzyme-mediated
catalysis, specifically using H2-selective hydrogenase enzymes to
carry out selective HOR even under aerobic conditions, has been
reported recently15,16 . So far, the CME approach with a selfassembled
monolayer (SAM) of calix[4]arene or similar organic
molecules has been used as a controllable junction between sizedependant
selectivity for gas separation17,18 and for chloride anion
transport across lipid bilayers 19 . Here, we use such an approach to 44
demonstrate that it is indeed possible to make an ‘oxygen-tolerant’ 45
selective catalyst for the HOR. Although we focus primarily on 46
the selectivity of Pt(111)–calixad systems, we also demonstrate that 47
calix[4]arene-modified Pt(100) and polycrystalline Pt electrodes are 48
highly selective for the ORR and HOR. 49
We start with the analysis of the scanning tunnelling microscopy 50
(STM) images of bare Pt(111) and Pt(111) modified with a 51
calix[4]arene adlayer. In agreement with ref. 19, we found 52
that Pt(111) is not an ideal surface, showing the presence of 53
small adislands of Pt atoms that are separated by well-resolved 54
monoatomic terrace-edge-step sites running roughly parallel to 55
the (111) substrate direction (Fig. 1a). The STM image for a 56
surface modified by the highest coverage of calix[4]arene (Fig. 1b) 57
is characterized by a close-packed, long-range ordered monolayer 58
of parallel arrays of molecules that almost completely cover the 59
(111) terrace sites. Figure 1c is a schematic representation of 60
the Pt(111)/calix interface, in which the wide rim of the cone 61
(representing the anchoring groups) may serve as an electrode 62
surface protector, the narrow rim of the cone as a molecular sieve 63
and the lateral surface of the ‘truncated cone’ as a ‘blocking wall’. 64
The driving force for ordering such large molecules is presum- 65
ably governed by a synergy between the strong chemical interaction 66
of Pt with calix[4]arene molecules and the surface homogeneity of 67
Pt(111) that results in collective interaction of adsorbed molecules 68
on terrace sites. It should be recognized, however, that on the basis 69
of STM data alone it was impossible to deduce either the number 70
of calix[4]arene-free Pt atoms or the nature (coordination) of the 71
remaining bare Pt atoms. Although the structural details of the 72
STM images lie beyond the scope of the present discussion, it is 73
reasonable to suggest that as a result of steric effects most of the 74
step-edges observed in Fig. 1a are not decorated with these large 75
molecules. In contrast, it appears that smaller Pt adislands formed 76
on the terraces of annealed Pt(111) may be ‘buried’ under the 77
large calix[4]arene molecules. The determination of the number of 78
available Pt sites is, however, less challenging given that a reasonable 79
assessment of bare Pt atoms on Pt(111)–calixad can be obtained by 80
monitoring how the fractional coverages (�) of underpotentially 81
deposited hydrogen (Hupd) and hydroxyl species (OHad) are affected 82
by �calix, as we demonstrate further below. 83
Not surprisingly, Fig. 2a shows that pseudo-capacitive features 84
corresponding to Hupd formation, double-layer charging and OHad 85
formation on Pt(111) are almost completely suppressed on the 86
1 Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva cesta 5, 1000 Ljubljana, Slovenia, 2 National Institute of Chemistry,
Hajdrihova 19, SI-1001 Ljubljana, Slovenia, 3 Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA, 4 Nuclear Engineering
Division, Argonne National Laboratory, Argonne, Illinois 60439, USA. *e-mail: markovic@anl.gov.
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LETTERS
a b
c
Lateral surface
Wide rim
40 nm
Narrow rim
20 nm
Figure 1 | Imaging of Pt(111) and Pt(111)–calix electrodes by STM and
corresponding model for adsorbed calix[4]arene molecules on Pt(111).
a, 200×200 nm 2 image of an as-prepared Pt(111) showing large terraces,
divided by mono-atomic steps and covered with a small number of Pt
adislands (the average size of the clusters is 2 nm). b, 100×100 nm 2 image
of the calix[4]arene adlayer (surface coverage of ∼0.98 ML) showing
long-range order of the self-assembled molecules. c, Schematic
representation of calix[4]arene molecules attached to the surface via -SH
groups located on the molecule’s wide rim.
surface covered by organic molecules, confirming a high coverage
of calix[4]arene molecules. Figure 2b also reveals that a systematic
increase in �calix leads to a marked decrease in availability of Pt sites,
that is, for Hupd and OHad from 16% on the least covered surface to
2% on a surface with the most closely packed calix[4]arene adlayer.
When the free Pt sites on the Pt(111)–calix surfaces are transformed
into density of active sites (N), then a noticeable effect of �calix
on the availability of Pt becomes even more obvious, as evidenced
in Table 1. This suggests that the ions/water from the supporting
electrolyte are unable to penetrate the narrow rim of calix[4]arene
molecules. On the basis of these observations, along with the
structural insight derived from the STM images, we propose that
the only sites available for adsorption of electrolyte components are
the relatively small number of Pt unmodified step-edges and/or the
small ensembles of terrace sites between the anchoring groups.
Having illustrated the effect of �calix on the formation of Hupd
and OHad adlayers, a key question arises concerning the extent
to which the �Hupd/OHad versus E curves (Fig. 2b) can be used for
a
b
Current density (µA cm ¬2 )
Charge density (µC cm ¬2 )
NATURE MATERIALS DOI: 10.1038/NMAT2883
0
100
0.2
E (V versus RHE)
0.4 0.6 0.8
50
0
¬50
¬100
30
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I II
III
Pt(111)
Pt(111)-A
Pt(111)-B
Pt(111)-C
Pt(111)-D
¬5
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E (V versus RHE)
Figure 2 | Relationships between surface coverages by calix[4]arene
molecules and Hupd/OHad on Pt(111) in 0.1 M HClO4. a, The effect of �calix
(curve A with 84% coverage, curve B with 95% coverage, curve C with
96% coverage and curve D with 98% coverage) on cyclic voltammetry of
Pt(111) (black dashed line), including the Hupd in region I, double layer
(region II) and OHad adsorption in region III. b, Corresponding charge
density versus E curves for Hupd and OHad, which are assessed on the basis
of the assumption of one Hupd per Pt on the Pt(111)–(1×1) surface and
∼160 µC cm −2 corresponds to the Hupd charge on Pt(111) in potential
range I (ref. 20). A grey potential region in this and all other figures
represents the potential window of importance to the anode selectivity.
Sweep rates 50 mV s −1 .
analysing the polarization curves for the HOR and the ORR in 19
Fig. 3. As in ref. 21, in this analysis we use the general rate expression 20
in which activity (current i) of the ORR and the HOR is simply 21
Table 1 | Calix[4]arene coverages and TOFs for the ORR and HOR for five samples as described in the Methods section.
Sample Hupd charge
(µC cm −2 )
Calix
coverage (%)*
Available
surface (%)*
Number of
available
sites (N cm −2 ) †
Min. TOF for
ORR @0.8 V ‡
Pt(111) 161 0 100 10 15 9 8
Pt(111)-A 26 84 16 1.6×10 14 9 49
Pt(111)-B 9 94 6 5.5×10 13 9 129
Pt(111)-C 7 96 4 4.2×10 13 6 194
Pt(111)-D 4 98 2 2.4×10 13 6 388
*Calix coverages and available surface were calculated by comparing Hupd charge on the bare Pt(111) and modified Pt(111).
† Number of available sites was calculated assuming one H per Pt adsorption and number of total sites on Pt(111) surface 1.5×10 15 .
Min. TOF for
HOR @0.1 V ‡
‡ Values are calculated by taking the measured current density @0.8 and @0.1, using the equation TOF = iE1/nFN. As current is under diffusion control for the HOR and in some cases for the ORR, the
values are presented as minimum TOFs for the reactions.
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NATURE MATERIALS DOI: 10.1038/NMAT2883 LETTERS
a
b
c
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Current (mA)
Current (mA)
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0.6
0.4
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Disc
0.1 M HClO 4
E (V versus RHE)
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Pt (1111)
Pt (1111)-A
Pt (1111)-B
Pt (1111)-C
Pt (1111)-D
¬2.0
0 0.2 0.4 0.6 0.8 1.0
E (V versus RHE)
Figure 3 | Design of ‘O2-tolerant’ selective anode catalysts for the HOR
by controlling �calix on Pt(111). a, Polarization curves for the HOR in 0.1 M
HClO4 on Pt(111) are the same as for all Pt(111)–calix modified surfaces.
The curves for Pt(111)-B (grey) and -C (blue) are the same but they are
omitted for clarity. Small variations may be observed in the hydrogen
evolution region. These arise from variability in the IR correction for the
electrode. For electrodes with high �calix values, the variability in IR values
for the HOR can alter the overall hydrogen evolution reaction currents
because of the large values of these currents. b, Peroxide oxidation currents
recorded at 1.1 V versus RHE on the ring electrode during the ORR on the
Pt(111)–calix disc electrodes. c, Corresponding polarization curves for the
ORR. The rotation rate for all measurements was 1,600 r.p.m.; a sweep rate
of 50 mV s −1 was used in all experiments.
dependent on the number of available Pt sites:
iE1 = nFK1creac.(1−�ad) (1)
where n is the number of electrons, K1 is a constant, creac. is the
concentration of H2 or O2 in the solution and �ad = �Hupd +
�OHad + �calix is the fraction of the surface masked by the siteblocking
species, that is, reaction intermediates are adsorbed at low
coverages. Closely following our analysis in ref. 21, equation (1) is
developed on the basis of a simple assumption that the Pt–O2 and 8
Pt–H2 energetics as well as their reaction intermediates on bare Pt 9
atoms is not affected by the surrounding calix[4]arene molecules. 10
We also assume that if ions/water from the supporting electrolytes 11
are unable to penetrate through the narrow end (see Fig. 1c), then 12
the same should be valid for H2 and O2; that is, the adsorption of 13
H2 and O2 occurs on a small number of calix[4]arene-free Pt sites. 14
To demonstrate that the number of active sites required for the 15
maximum rates of the HOR and ORR is extremely low, the reactivity 16
of Pt(111)–calix surface was converted into a turnover frequency, 17
TOF=iE1/nFN , and is summarized in Table 1. We notice in passing 18
that any rigorous kinetic analyses of the ORR and HOR lie beyond 19
the scope of the present discussion, because this analysis is not 20
necessary to demonstrate the main aspect of our Letter, which is the 21
high selectivity of the Pt(111)–calix surfaces for HOR. 22
We start first with the analysis of polarization curves for the HOR 23
on Pt(111) and Pt(111) modified with calix[4]arene (Fig. 3a). As 24
found previously22 , the HOR on Pt(111) is an extremely fast process 25
that, below 0.1 V, is determined predominantly by the surface 26
coverage of spectator species Hupd (ref. 23). Above this potential, the 27
reaction rate is always under pure diffusion control. An important 28
observation from Fig. 3 is that for various surface coverages of 29
calix[4]arene molecules the HOR is essentially the same. This 30
suggests that the required number of Pt sites for the maximum rates 31
of the HOR is rather small, that is, calculated on the basis of 2% 32
surface site availability, the minimum TOF for HOR on Pt sites 33
could be as high as 388 molecules/(site*s). Notice that this result 34
has fulfilled the first requirement for designing highly active anode 35
catalysts under operating PEMFC conditions—a Pt-like activity for 36
the HOR. The second requirement, to make an O2-tolerant selective 37
catalyst for the anode, is much more challenging and, to the best of 38
our knowledge, a method suitable for the design of such catalysts 39
has not been developed thus far. 40
The ORR is a more complex multi-electron reaction in which 41
O2 is reduced either to water without peroxide formation (4e− 42
reduction) and/or to water plus peroxide formation (a mixed 4e− + 43
2e− reduction) and/or completely to peroxide via a 2e− reduction 44
process24 . As in our previous experiments25 , to analyse possible 45
reaction pathways (neglecting any rigorous kinetic analyses) of the 46
ORR on Pt(111) and Pt(111)–calix surfaces we use the rotating ring- 47
disc electrode (RRDE) method. This method provides information 48
on both the total currents for the ORR on the disc electrode (Fig. 3c) 49
as well as the concomitant production of peroxide on the ring 50
electrode (Fig. 3b). For Pt(111), starting at ∼0.95 V and sweeping 51
the Pt(111) disc potential in the negative direction to 0.45 V, the 52
ring currents were essentially zero, implying that in this potential 53
region the ORR proceeds entirely through the direct 4e− pathway. 54
Figure 3b shows that the appearance of peroxide oxidation currents 55
on the ring electrode begins at potentials below 0.45 V and the 56
limiting current corresponding to an exactly two-electron reduction 57
of O2 is reached at the negative potential limit. 58
There are two general observations concerning the ORR on 59
Pt(111)–calix systems. First, the disc currents show that the 60
ORR is inhibited on the calix[4]arene-modified surfaces and 61
the deactivation increases with an increase of �calix (Fig. 3c and 62
Table 1). The inhibition of O2 adsorption is so strong that the 63
theoretical diffusion-limited currents corresponding to the disc 64
geometric area are never reached. On the basis of our previous 65
discussion in ref. 26, we propose that the currents observed on 66
Pt(111)–calixad surfaces are related to the ORR on uncovered or 67
partially covered active Pt patches. Given that the relative number of 68
these patches is too small to allow a full overlap between the adjacent 69
diffusion zones (spherical diffusion regions), it is reasonable to 70
suggest that the currents below 0.6 V are diffusion-limited currents; 71
but for conditions of highly blocked surfaces. Second, peroxide 72
formation is hardly observed in the potential region of significance 73
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Current density (mA cm ¬2 )
Current density (µA cm ¬2 )
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NATURE MATERIALS DOI: 10.1038/NMAT2883
Pt(100)
0
Pt(Poly)
Pt(100)-D
Pt(Poly)-D
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0 0.2 0.4 0.6 0.8 1.0
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¬60
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b
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f
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E (V versus RHE)
¬100
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E (V versus RHE)
Figure 4 | High selectivity of the ORR and HOR is also observed on the calix[4]arene-covered Pt(100) and polycrystalline Pt electrodes, suggesting that
the Pt–calix systems are of broad fundamental and technological importance. a,b, Polarization curves for the HOR on bare and covered Pt(100) (a) and
Pt(Poly) (b). c,d ORR on bare and calix[4]arene-covered Pt(100) (c) and Pt(Poly) (d) electrodes. e,f, Cyclic voltammograms for these surfaces in 0.1 M
HClO4. Slight kinetic inhibition in HOR between 0.0 and 0.15 V observed for Pt(100) and Pt(Poly) surfaces is most likely the result of the high coverage of
surface species 26 (Hupd) as well as calix for these electrodes. Both Pt(100) and Pt(Poly) electrodes exhibit the same diffusion-limiting currents for HOR as
the bare surfaces while showing significant deactivation for the oxygen reduction reaction at potentials >0.6 V.
for startup/shutdown conditions (E > 0.6 V). This is an important
result as H2O2 may affect degradation of the Nafion membrane.
As revealed in Fig. 3b, below 0.6 V a monotonic increase in the
peroxide production is observed on Pt(111)–calixad. Furthermore,
on this surface the onset potential for peroxide formation is shifted
∼300 mV positive relative to Pt(111), indicating a change in the
reaction pathway on these two surfaces. In short, below 0.6 V the
2e− reduction process is predominant on calix[4]arene-modified
Pt(111) surfaces. This is consistent with our previous proposition
that larger ensembles of Pt sites are required for efficient cleavage of
the O–O bond than for the adsorption of O2 and the concomitant
formation of H2O2 (ref. 27).
Importantly, Fig. 3 shows that on the 98% covered electrode the
ORR is completely inhibited between 0.6 and 0.85 V; however, on
the same surface and within the same potential window the HOR is
under pure diffusion control. This unique selectivity of such CMEs
may be attributed, by inference, to very strong ensemble effects in 17
which the critical number of bare Pt atoms required for adsorption 18
of O2 (that is, the ORR) is much higher than that required for 19
the adsorption of H2 molecules and a subsequent HOR. It is also 20
important to point out that the established selectivity was possible 21
only because the required number of active sites for maximal rates 22
of the HOR is, in fact, extremely small and just 2% of the available 23
active surface sites are sufficient to reach the diffusion limiting 24
currents. Finally, it is important to emphasize that the observed 25
O2 selectivity is not unique to the Pt(111)–calix system and, as 26
summarized in Fig. 4 and detailed in the caption, an exceptional 27
anode selectivity for the ORR and HOR is also observed on the 28
Pt(100) and polycrystalline Pt electrodes. This suggests that Pt–calix 29
systems are of broad fundamental and technological importance. 30
The results presented here indicate some important new 31
directions in the quest to design selective anode catalysts for the 32
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NATURE MATERIALS DOI: 10.1038/NMAT2883 LETTERS
Figure 5 |
HOR that could improve the stability of cathode catalysts in
PEMFC. We have demonstrated that a chemically modified Pt
electrode with a SAM of calix[4]arene molecules can selectively
block the ORR, but in such a way that the HOR proceeds with
Pt-like activity. The optimum selectivity has been achieved by fine
tuning the surface coverage of calix[4]arene molecules, leading to
the formation of a critical ensemble of O2-tolerant Pt sites that
are very active for the adsorption of H2 and consequent H–H
bond breaking. The CME approach we have outlined here is not
restricted to the Pt–calix systems, and may have many applications
in analytical, synthetic and materials chemistry as well as in chemical
energy conversion, selective fuel production and energy storage.
Methods
Synthesis of calix[4]arene. Several functional groups are known to bind to
metal surfaces28 . Thiols are the most widely used compounds in chemisorption
studies because of their well-known chemistry and the favourable self-assembly
characteristics. Our previous studies showed that quadropod thio derivates of
calix[4]arene ensure a strong bond with Pt substrates, resulting in relatively
stable SAMs (ref. 29). For the present study we used protected thio derivatives of
calix[4]arene. Quadropod anchoring compound was synthesized using a three-step
reaction from the corresponding calix[4]arene. The alkyl-protected calix[4]arene
1 was brominated to yield the bromo derivate 2. Lithiation with t-butyllithium,
followed by introduction of sulphur and protection of the thiol group in the form
of thiolacetate, gave a reasonable yield of the final compound 3 (Fig. 5). For the
detailed procedure and data on the compounds see ref. 29.
Preparation of Pt(111), Pt(100) and Pt(Poly) and self-assembly procedure. Pt
electrodes were prepared by inductive heating for 10 min at ≈1,100 K in an argon
hydrogen flow (3% hydrogen). The annealed specimen was cooled slowly to room
temperature in this flow stream and immediately covered by a droplet of water.
The electrode was then immersed in a tetrahydrofuran solution of calix[4]arene
for 24 h, allowing the formation of a calix[4]arene SAM. The concentrations of
calix[4]arene in tetrahydrofuran were 90, 130, 250 and 400 µM to obtain samples
A, B, C and D, respectively, each with different calix coverages for the Pt(111)
electrode (see Table 1). For Pt(100) and Pt(Poly) electrodes, 600 µM solutions were
used to demonstrate the validity of the CME approach even at very high coverages.
Calix coverages were calculated by Hupd comparison between the clean Pt(111)
surface and chemically modified surfaces.
STM method. For the as-prepared surface, the STM images were acquired
with a Digital Instruments Multi-Mode Dimension STM controlled by a
Nanoscope III control station. During the measurement, the microscope with
the sample was enclosed in a pressurized cylinder with a CO atmosphere. For
further details, see ref. 12.
For modified surfaces, STM measurements were carried out on a home-built
low-temperature STM similar to ref. 30 equipped with an RHK SPM1000
controller. The samples were prepared according to the method described above
and transferred to a helium glove box. Any water drops remaining on the sample
were removed by blowing the surface with helium gas. The sample was then
mounted on the STM stage and the STM head was sealed and transferred to the
cryostat. The STM was cooled down to 4.2 K and the surface was scanned at a
bias voltage of 500 mV and tunnelling current of 20 pA. Measurements at 4.2 K
provided minimum drift during scanning (less than a few ångströms per hour).
A high tunnelling resistance was necessary to ensure that the tip did not touch
calix[4]arene molecules on the surface.
RRDE method electrolytes and electrochemical set-up. After extensive rinsing,
the electrode was embedded into the RRDE and transferred into a standard
three-compartment electrochemical cell containing 0.1 M HClO4 (Sigma-Aldrich).
In each experiment, the electrode was immersed at 0.07 V in a solution saturated
with Ar. After obtaining a stable cycle between 0.07 and 0.7 V the polarization
curve for the ORR was recorded on the disc whereas the peroxide oxidation signal
was measured at the ring, which was held at 1.1 V versus the reversible hydrogen
electrode (RHE). Peroxide currents presented are already corrected for the
collection efficiency of 0.24. Subsequently, oxygen was purged from the solution,
1 2 3
replaced with hydrogen and HOR polarization curves were measured. Finally, the 62
voltammetric response was again recorded in an argon-purged solution to confirm 63
that the calix coverage had not changed significantly. 64
All gases were 5N5 quality purchased from Airgas. The sweep rate for all 65
measurements was 50 mV s −1 ; for the ORR measurements the electrode was rotated 66
at 1,600 r.p.m. Electrode potentials are given versus the RHE. 67
Received 25 June 2010; accepted 17 September 2010; 68
published online XX Month XXXX 69
References 70
1. Vielstich, W., Lamm, A. & Gasteiger, H. (eds) Handbook of Fuel Cells 71
(Fundamentals and Survey of Systems, Vol. 1, Wiley, 2003). 72
2. Gasteiger, H. A. & Markovic, N. M. Just a dream—or future reality. Science 73
324, 47–48 (2009). 74
3. Stamenkovic, V. R. et al. Improved oxygen reduction activity on Pt3Ni(111) 75
via increased surface site availability. Science 315, 493–497 (2007). 76
4. Greeley, J. et al. Alloys of platinum and early transition metals as oxygen 77
reduction electrocatalysts. Nature Chem. 1, 552–556 (2009). 78
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cell electrocatalysis on voltammetrically dealloyed Pt–Cu–Co nanoparticles. 80
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voltage stability of nanostructured thin film catalysts for PEM fuel cells. 85
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8. Zhang, J., Sasaki, K., Sutter, E. & Adzic, R. R. Stabilization of platinum 87
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11. Borup, R. et al. Scientific aspects of polymer electrolyte fuel cell durability and 95
degradation. Chem. Rev. 107, 3904–3951 (2007). 96
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13. Bard, A. J. & Stratmann, M. (eds) Modified Electrodes (Encyclopedia of 99
Electrochemistry, Vol. 10, Wiley, 2007). 100
14. Strmcnik, D. S. et al. Nature Chem.; Published online Aug 15 2010. 101
15. Cracknell, J. A. et al. Enzymatic oxidation of H2 in atmospheric O2: 102
The electrochemistry of energy generation from trace H2 by aerobic 103
microorganisms. J. Am. Chem. Soc. 130, 424–425 (2008). 104
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Dalton Trans. 3397–3403 (2005). 107
17. Markowitz, M. A., Janout, V., Castner, D. G. & Regen, S. L. Perforated 108
monolayers—design and synthesis of porous and cohesive monolayers from 109
mercurated calix[n]arenes. J. Am. Chem. Soc. 111, 8192–8200 (1989). 110
18. Zhang, L., Hendel, R. A., Cozzi, P. G. & Regen, S. L. A single Langmuir–Blodgett 111
monolayer for gas separations. J. Am. Chem. Soc. 121, 1621–1622 (1999). 112
19. Sidorov, V., Kotch, F. W., Kuebler, J. L., Lam, Y-F. & Davis, J. T. Chloride 113
transport across lipid bilayers and transmembrane potential induction by an 114
oligophenoxyacetamide. J. Am. Chem. Soc. 125, 2840–2841 (2003). 115
20. Clavilier, J. et al. Study of the charge displacement at constant potential during 116
CO adsorption on Pt(110) and Pt(111) electrodes in contact with a perchloric 117
acid solution. J. Electroanal. Chem. 330, 489–497 (1992). 118
21. Strmcnik, D. et al. The role of non-covalent interactions in electrocatalytic 119
fuel-cell reactions on platinum. Nature Chem. 1, 466–472 (2009). 120
22. Marković, N. M., Grgur, B. N. & Ross, P. N. Temperature-dependent hydrogen 121
electrochemistry on platinum low-index single-crystal surfaces in acid 122
solutions. J. Phys. Chem. B 101, 5405–5413 (1997). 123
23. Strmcnik, D. et al. Adsorption of hydrogen on Pt(1 1 1) and Pt(1 0 0) surfaces 124
and its role in the HOR. Electrochem. Commun. 10, 1602–1605 (2008). 125
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new mechanistic criterion. J. Electroanal. Chem. 69, 195–201 (1976). 127
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reduction reaction on Pt(111): Effects of bromide. J. Electroanal. Chem. 467,
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26. Strmcnik, D. S. et al. Relationship between the surface coverage of spectator
species and the rate of electrocatalytic reactions. J. Phys. Chem. C 111,
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27. Markovic, N. M., Gasteiger, H. A. & Ross, P. N. H2 and CO electrooxidation
on well-characterized Pt, Ru, and Pt–Ru. 1. Rotating disk electrode
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28. Love, J. C. et al. Self-assembled monolayers of thiolates on metals as a form of
nanotechnology. Chem. Rev. 105, 1103–1169 (2005).
29. Genorio, B. et al. Synthesis and self-assembly of thio derivatives of calix[4]arene
on noble metal surfaces. Langmuir 24, 11523–11532 (2008).
NATURE MATERIALS DOI: 10.1038/NMAT2883
30. Renner, Ch. et al. A versatile low-temperature scanning tunneling microscope. 15
J. Vac. Sci. Technol. A 8, 330–332 (1990). 16
Acknowledgements 17
This work was supported by the Director, Office of Science, Office of Basic Energy 18
Sciences, Division of Materials Sciences, US Department of Energy under Contract No. 19
DE-AC03-76SF00098. B.G. acknowledges support from the Center of Excellence Low 20
Carbon Technologies (CO NOT) and the Ministry of Higher Education, Science and 21
Technology of Slovenia (ARRS-3311-04-831034). 22
Additional information 23
The authors declare no competing financial interests. Reprints and permissions 24
information is available online at http://npg.nature.com/reprintsandpermissions. 25
Correspondence and requests for materials should be addressed to N.M.M. 26
6 NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials

Page 1
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the first paragraph. OK?
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Please check ‘transformed into’ here. Can it be
changed to, for example, ‘expressed as’?
Query 4: Line no. 19
Can the text in figure 2b’s caption be changed to
‘which are assessed on the basis of the assumptions
of one Hupd per Pt on the Pt(111)–(1×1) surface
and that the Hupd charge on Pt(111) in potential
range I is ∼160 µC cm −2 (ref. 20).?’
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instances), and can it be defined?
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‘hydrogen evolution reaction’. OK?
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per site per second’?
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Can the sentence ‘The ORR is ... reduction
process 24 .’ be changed to, for example, ‘The ORR is
a more complex multi-electron reaction in which
O2 is reduced to water and/or peroxide. The former
process can occur directly (4e − reduction) and/or
indirectly (via peroxide formation, which is a 2e −
reduction process) 24 .’
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(e) and Pt(Poly) (f)’?
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hydrogen electrode (RHE)’ here. OK?
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an ‘Author contributions’ statement to specify
the contributions of each co-author, such as
experimental work, project planning, data analysis
etc. This statement should be short, and refer to
authors by their initials.
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somewhere relevant in the text.

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2011, Vol. 85, No. 13, pp. 2299–2304. © Pleiades Publishing, Ltd., 2011.
Enhanced Activity in Ethanol Oxidation
of Pt 3Sn Electrocatalysts Synthesized by Microwave Irradiation 1
S. Stevanovi a , D. Tripkovi b , J. Roganc , D. Mini d , A. Gavrilovi e ,
A. Tripkovi a , and V. M. Jovanovi a
c � c � c � c �
c �
c �
a ICTM Department of Electrochemistry, University of Belgrade, Njegoševa 12, Belgrade, Serbia
b Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
c Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, Belgrade, Serbia
d Faculty of Physical Chemistry, University of Belgrade, Studentski trg 12, Belgrade, Serbia
e CEST Centre of Electrochemical Surface Technology, Viktor–Kaplan–Strasse 2, A–2700 Wiener Neustadt, Austria
e-mail: vlad@tmf.bg.ac.rs
Received November 2, 2010
Abstract—High surface area carbon supported Pt and Pt 3Sn catalysts were synthesized by microwave irradiation
and investigated in the ethanol electro-oxidation reaction. The catalysts were obtained using a modified
polyol method in an ethylene glycol solution and were characterized in terms of structure, morphology and
composition by employing XRD, STM and EDX techniques. The diffraction peaks of Pt 3Sn/C catalyst in
XRD patterns are shifted to lower 2θ values with respect to the corresponding peaks at Pt/C catalyst as a consequence
of alloy formation between Pt and Sn. Particle size analysis from STM and XRD shows that Pt and
Pt 3Sn clusters are of a small diameter (~2 nm) with a narrow size distribution. Pt 3Sn/C catalyst is highly
active in ethanol oxidation with the onset potential shifted for ~150 mV to more negative values and with
~2 times higher currents in comparison to Pt/C.
Keywords: enhanced activity, Pt 3Sn electocatalyst, microwave irradiation, ethanol oxidation.
DOI: 10.1134/S0036024411130309
INTRODUCTION
In various applications, fuel cells are widely recognized
as very attractive devices to obtain electric
energy directly from the combustion chemical product.
Alcohols, mainly methanol, are widely proposed
as possible fuels for mobile applications such as electric
vehicles. Methanol is the simplest alcohol (only
one carbon) and the electrocatalysis of its oxidation
should be also simplest, but methanol is a toxic compound.
However, ethanol offers an attractive alternative
as a fuel in low temperature fuel cells because it
can be produced in large quantities from agricultural
products and it is the major renewable biofuel from the
fermentation of biomass. Ethanol has also lower toxicity
and higher mass energy density than methanol (6.1
and 8.0 kWh/kg for methanol and ethanol, respectively).
However, in the case of ethanol as a fuel, the
problem is more complicated because ethanol contains
two carbon atoms and for the complete oxidation
of ethanol to CO 2 a good electrocatalyst must
activate the C–C bond breaking (while avoiding the
poisoning of the catalytic surface by CO species).
The oxidation mechanism of ethanol in acid solution
1 The article is published in the original.
CHEMICAL KINETICS
AND CATALYSIS
2299
may be summarized in the following schema of parallel
reactions:
CH3CH2OH [ CH3CH2OH] ad
C1ad, C2ad CO2( total oxidation),
(1)
CH3CH2OH [ CH3CH2OH] ad
(2)
CH3CHO CH3COOH( partial oxidation).
Carbon supported platinum is commonly used as
anode catalyst in low temperature fuel cells. Pure Pt,
however, is not the most efficient anodic catalyst for
the direct ethanol fuel cell. On the other side platinum
it self is known to be rapidly poisoned by strongly
adsorbed species originating from the dissociative
adsorption of ethanol [1]. Efforts to minimize the poisoning
of Pt have been focused on the addition, of cocatalysts,
such as Ru, Mo, Rh, Bi, Pd or Sn, to platinum
to promote CO oxidation [1–4]. In this sense
platinum is mostly alloyed with Sn to form binary catalysts.
The activity of binary electrocatalyst was attributed
to a bi-functional effect (Pt adsorbs alcohol and
oxidizes H, while Sn oxides supply oxygen providing
oxidation of the blocking intermediate CO) as well as
the electronic interaction between Pt and alloyed metals
[5, 6].

2300
Electrocatalytic performance of the Pt-based catalysts
is highly dependent not only on the nature of the
metals added, but also on a variety of surface conditions
of the synthesized materials (i.e. surface composition,
morphology, impurity, contamination). Hence
an ideal synthesis method of Pt-based electrocatalysts
should be facile, cost effective, controllable and reproducible.
It is also well known that the metal catalytic
activity is strongly dependent on the particle shape,
size and particle size distribution [7]. Conventional
preparation techniques based on wet impregnation or
chemical reduction of metal precursors does not provide
satisfying control of particle size and shape [7].
There has been continuing effort to develop an alternative
synthesis method such as microwave irradiation
[8, 9]. Microwave synthesis method has many advantages
compared with conventional heating synthesis
such as prompt start up and very short heating time
thus enables homogeneous nucleation and shorter
crystallization time leading to formation of small uniform
metal particles.
In this work, Pt and Pt 3Sn nanoparticles, synthesized
by microwave assisted polyol method, were physically
and electrochemically characterized for the ethanol
electrooxidation reaction.
EXPERIMENTAL
Preparation of Pt/C and Pt 3Sn/C Electocatalysts
In this procedure, in a 100 ml beaker mixture of
0.5 ml of 0.05 M H 2PtCl 6 and 0.5 ml of 0.017 M SnCl 2
solution was mixed with 25 ml of ethylene glycol under
magnetic stirring. Then 0.8 M NaOH was added drop
wise to adjust pH ~12. The beaker was placed in the
center of an ordinary microwave oven and heated 60 s
for the Pt and 90 s for Pt 3Sn catalyst at 700 W. After
microwave heating, mixture was uniformly mixed with
20 ml water suspension of 20 mg for Pt and 31.4 mg for
Pt 3Sn of Vulcan XC-72 carbon and 150 ml 2 M H 2SO 4
solution for 3 h with magnetic stirring. The resulting
suspension was filtered and the residue was washed
with high purity water. The solid product was dried at
160°C for 3 h in N 2 atmosphere. The metal loading for
both catalysts should be ~20 wt %. Thermogravimetric
analysis (TGA) confirmed 20 wt % for Pt/C, while for
Pt 3Sn/C a slightly lower portion of metal (~18 wt %)
was found.
Characterization of Pt/C and Pt 3Sn/C Electrocatalysts
The thermogravimetric (TGA) and differential thermal
(DTA) analyses were performed simultaneously
(30–800°C range) on a SDT Q600 TGA/DSC
instrument (TA Instruments). The heating rates were
20 K min –1 and the sample mass was less than 10 mg.
The furnace atmosphere consisted of air at a flow rate
of 100 cm 3 min –1 .
STEVANOVI C� et al.
Unsupported Pt and Pt 3Sn nanoparticles were
characterized by scanning tunnelling microscopy
(STM). STM characterizations were performed using
a NanoScope III A (Veeco, USA) microscope. The
images were obtained in the height mode using a
Pt–Ir tip (set-point current, i t, from 1 to 2 nA, bias
voltage, V b = –300 mV). The mean particle size and
distribution were acquired from a few randomly
chosen areas in the STM images containing about
100 particles.
The X-ray diffraction (XRD) patterns of the powder
catalysts were recorded with an Ital Structure
APD2000 X–ray diffractometer in a Bragg–Brentano
geometry using CuK α radiation (λ = 1.5418 Å) and
step-scan mode (range: 2θ = 15°–85° step-time: 2.50 s,
step-width: 0.02°). The program Powder Cell [10],
was used for phase analysis and calculation of unit cell
parameters.
Microstructural examination was performed by
scanning electron microscopy (SEM). An XL 30
ESEM-FEG (environmental scanning microscope
with field emission gun, manufactured by FEI, Netherlands)
device equipped with an energy dispersive
X-ray spectrometer from EDAX was used. The samples
were inspected using 5, 10 and 20 kV acceleration
voltages at magnifications of 20000× and 10000×
respectively.
Electrocatalytic activity of the catalysts was investigated
by potentiodynamic and chronoamperometric
tests using a PINE-RDE4 model potentiostat/galvanostat
and a three-electrode compartment cell at
room temperature. The working electrode was a thin
layer of Nafion-impregnated Pt/C or Pt 3Sn/C catalysts
applied on a glassy carbon disk electrode with the
loading of 20 μg/cm 2 of the catalyst. The thin layer was
obtained from a suspension of 2 mg of the respective
catalyst in a mixture of 1 ml water and 50 μ1 of 5%
aqueous Nafion solution, prepared in an ultrasonic
bath, placed onto the substrate and dried at room temperature.
Pt wire and a saturated calomel electrode
(SCE) were used as the counter and reference electrode,
respectively. The electrocatalytic activity of as
prepared Pt/C and Pt 3Sn/C was studied in 0.1 M
HClO 4 + 0.5 M C 2H 5OH solution. The electrolyte was
prepared with high purity water and deaerated by N 2.
Ethanol was added to the supporting electrolyte solution
while holding the electrode potential at –0.2 V.
The potential was then cycled up to 0.3 V i.e. the
potential range of technical interest (E < 0.6 V (RHE))
at sweep rate of 20 mV/s. Current–time transient
curves were recorded after immersion of the freshly
prepared electrode in the solution at –0.2 V for 2 s followed
by stepping the potential to 0.2 V and holding
the electrode at that potential for 30 min. A commercially
available Pt/C catalyst from E-TEK with a nominal
Pt loading of 20 wt % and particle size of ~2 nm was
used as a benchmark.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 85 No. 13 2011

RESULTS AND DISCUSSION
Catalysts Characterizations
The particle size and surface morphology of unsupported
Pt and Pt 3Sn catalysts were characterized by
STM. As observed from the top view of STM images
(Fig. 1), both catalysts have rather uniform particles of
a small diameter. Most of particles are in spherical
shape. Cross section analysis (Fig. 1) confirmed particle
size of

2302
Intensity, a.u.
Pt/C
Pt 3Sn/C
STEVANOVI C� et al.
20 40 60 80
2θ, deg
Fig. 2. XRD patterns of Pt/C and Pt 3 Sn/C catalysts.
j, mA/mg Pt 1
120
80
40
0
−0.2 0 0.2 0.4
E, V(vs SCE)
Fig. 3. Potentiodynamic curves for the oxidation of 0.5 M
C 2H 5OH at (1) Pt/C, (2) Pt 3Sn/C and (3) Pt/C–Tanaka
in 0.1 M HClO 4, v = 20 mV/s.
2
3
alyst (table). Because Sn has a bigger atomic radius
than Pt (R Pt = 1.39 Å, R Sn = 1.61 Å) the addition of
some amount of Sn to Pt could induce the extension of
Pt unit cell parameter. Mean particle sizes were calculated
by Scherrer formula [11] and for each catalyst are
larger than that obtained by STM (table) possibly
because unsupported catalysts were used for STM
analysis. Still, the agreement can be described as very
good because the values calculated by Scherrer formula
account for a very probable lattice stress too.
The small particle sizes and homogeneous size distributions
of both catalysts with metal loading of
20 wt % should be attributed to the advantages of
microwave assisted modified polyol process in ethylene
glycol solution. It is generally agreed that the size
of metal nanoparticles is determined by the rate of
reduction of the metal precursor. The dielectric constant
(41.4 at 298 K) and the dielectric loss of ethylene
glycol are high and hence rapid heating occurs easily
under microwave irradiation [12]. In ethylene glycol
mediated reactions (the “polyol” process), ethylene
glycol also acts as a reducing agent to reduce the metal
ions to metal powders [13]. The fast and uniform
microwave heating accelerated the reduction of the
metal ions and the formation of metallic nuclei, thus
greatly facilitated small and uniform particle formation.
Ethylene glycol has also big viscosity and it
could prevent the agglomerations of the obtained
nanoparticles.
EDX analysis of Pt 3Sn/C catalyst revealed that the
elemental composition agrees rather well with the
nominal composition in the initial mixture (table).
Electrochemical Performances
The electrocatalytic activities of as prepared Pt/C,
Pt/C-Tanaka and Pt 3Sn/C catalysts were studied in
0.1 M HClO 4 + 0.5 M C 2H 5OH solution and positive
scan of voltammetric curves are presented in Fig. 3.
Pt 3Sn/C catalyst is highly active in ethanol oxidation
with the onset potential at approximately –0.15 V
(shifted for >0.1 V towards more negative potentials
compared to Pt/C) and rapid kinetics, which is in
accordance with the relevant results in the literature in
the potential range of technical interest, i.e. at E < 0.6 V
(RHE) [14]. Hydrogen adsorption/desorption peaks
are clearly suppressed because the ethanol adsorption
replaces the adsorbed hydrogen from the interface.
The current densities in the whole potential region are
two times higher for Pt 3Sn/C catalyst in comparison
with Pt/C catalyst. Pt/C catalyst exhibits a lower
activity than Pt 3Sn/C catalyst but higher than commercially
Pt/C catalyst—Tanaka with the same metal
loading probably due to its better dispersion and
smaller particles as the results of microwave assisted
synthesis.
The stability of catalyst was studied in the chronoamperometric
experiments and presented in Fig. 4.
The higher initial current density at 0.2 V on Pt 3Sn/C
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 85 No. 13 2011

catalyst in comparison to Pt/C catalyst is in accordance
with potentiodynamic measurments. The current
decreases rapidly at Pt/C catalyst reaching study
state values in a few minutes. On the other hand, the
initial current decreases slightly at Pt 3Sn/C and stabilizes
in the experimental period of time at the value
which is about two times higher than for Pt/C catalyst.
The Pt 3Sn/C catalyst is evidently less poisoned than
Pt/C catalyst.
Although Pt/C synthesized by microwave assisted
polyol method is more active for ethanol oxidation
reaction than commercial Pt/C-Tanaka catalyst, the
activity of Pt 3Sn/C synthesized by polyol method is
lower in comparison with activity of commercial
Pt 3Sn/C–Tanaka catalyst [15]. However, in comparison
with other Pt 3Sn/C catalysts prepared from colloid
solutions but by usual heating, not microwave
irradiation, our Pt 3Sn/C catalyst exhibits larger shift of
onset potential to negative values [1, 6] as well as lower
poisoning [16].
The lower activity of our Pt 3Sn/C catalyst in comparison
with commercial one according to Colmenares
et al. [16] can be tentatively explained by
lower alloying degree in polyol synthetized catalysts.
Beneficial role of Pt–Sn alloy phase has been pointed
out by other authors too [17, 18]. On the other hand,
there are reports in the literature [1, 6] on remarkable
promotion of ethanol oxidation by PtSn/C catalysts
with main part of Sn in non-alloyed oxidized state
what is probably the reason for better performance of
our Pt 3Sn/C in comparison with similar other catalysts
described in the literature.
Thus, the presence of Sn in the catalyst can promote
ethanol oxidation by an electronic effect in the
Pt-based electrode material affecting on adsorption
properties of the surface making this system less
prompt to poisoning by organic species than pure Pt.
Sn or its oxides can supply surface oxygen-containing
species at lower potentials by activation of the interfacial
water molecule necessary to complete the oxidation
of adsorbed reaction intermediates leading to carbon
dioxide, in the situation that the C–C bond was
broken, or the formation of acetic acid [19, 20]. This
oxidative removal of CO-like species strongly
adsorbed on adjacent Pt active sites proceeds through
so-called bi-functional mechanism.
As displayed by the XRD results, the addition of Sn
in the case of Pt 3Sn/C catalyst synthesized by microwave
assisted polyol method induced slight extension
of Pt-Pt distances resulting in rather low alloying
degree. TG and EDX analysis revealed negligible loss
of catalysts components what leads to reasonable
assumption that significant quantity of non-alloyed Sn
is present in the catalyst and on its surface. Thus, the
increased activity of Pt 3Sn/C in comparison to Pt/C
catalyst is most probably promoted by bi-functional
mechanism although the electronic effect of alloyed
Sn and extended Pt–Pt distance could play some role
as well.
ENHANCED ACTIVITY IN ETHANOL OXIDATION 2303
j, mA/mgPt 16
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 85 No. 13 2011
12
8
4
0
500
1000
1500
Fig. 4. Chronoamperometric curves for the oxidation of
0.5 M C 2 H 5 OH at 0.2 V on (1) Pt/C and (2) Pt 3 Sn/C catalysts
in 0.1 M HClO 4 .
2000
t, s
CONCLUSIONS
A microwave assisted rapid heating polyol method
was used to prepare carbon supported Pt and Pt3Sn nanoparticles with high electrocatalytic activities for
the ethanol electrooxidation reaction. Structural
characterization (XRD) and surface characterization
(STM) of the catalysts revealed that by this method the
catalysts with the small particles and with rather uniform
size distribution were synthesized.
Electrochemical measurements revealed the higher
activity for the ethanol oxidation of the Pt3Sn/C. This
catalyst has two times higher oxidation currents and
significantly lower reaction onset potential than Pt/C
catalyst. Chronoamperometric measurements revealed
that Pt3Sn/C catalyst is notably less poisoned than
Pt/C catalyst. Also, in comparison to other similar
catalysts prepared by usual heating, not microwave
irradiation, our catalyst exhibits larger shift of onset
potential to negative values as well as lower poisoning.
The increased activity of Pt3Sn/C in comparison to
Pt/C catalyst is predominantly due to bi-functional
mechanism although the electronic effect of alloyed
Sn and extended Pt–Pt distances should also contribute
the increase in activity.
ACKNOWLEDGMENTS
This work was financially supported by the Ministry
of Science and Technological Development,
Republic of Serbia, contract no. H–142056.
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RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 85 No. 13 2011

Abstract #1510, 221st ECS Meeting, © 2012 The Electrochemical Society
Electrocatalysts for the Oxygen Evolution Reaction
N. Danilovic 1 , R. Subbaraman 2,3 , D. Strmcnik 2 , D.
Tripkovic 2 , A.P. Paulikas 2 , K.-C. Chang 2 , V.R.
Stamenkovic 2 , D. Myers 1 , N.M. Markovic 2
1 Chemical Sciences and Engineering Division,
2 Material Science Division
3 Nuclear Engineering Division
Argonne National Laboratory, Lemont, IL 60439
Electrocatalysis of the oxygen evolution reaction (OER)
is critical to the operation of electrolyzers. Currently,
large-scale electrochemical production of hydrogen from
water splitting is constrained by two limitations: 1) high
overpotentials for the OER and 2) lack of stability of the
electrode materials. While a large number of materials
have been tested for the OER in alkaline and acid
environments, the efforts were guided by a trial-and-error
and/or a combinatorial approach with a resulting lack of
studies focusing on systematic understanding of
fundamental catalytic properties of the OER on wellcharacterized
materials.
In this presentation, using well-defined extended surfaces
we demonstrate that, for proton exchange membrane
(PEM) electrolyzers, activity and stability depend on the
nature of the metal-oxide interface. Extended surfaces of
Ir and Ru were used to investigate the OER on
electrochemically or thermally formed oxides, while the
traditional approach uses high surface area catalysts. By
tuning specific electrochemical or thermal treatments on
the Ir and Ru we selectively grow either amorphous
electrochemical oxides or crystalline oxides with welldefined
surfaces.
Figure 1, illustrates the cyclic voltammograms (CVs) of
polycrystalline Ir, IrO2 electrochemical oxide and IrO2
thermal oxide in 0.1M HClO4.
Figure 1. CVs of different Ir surfaces
The electrochemical and thermal treatments have a drastic
effect on the surface of Ir. Ir metal, prepared by radio
frequency (RF) heat treatment in a reducing atmosphere,
has a well-defined hydrogen underpotenial deposition
(Hupd) and electrochemical double layer regions, with
mixed oxide formation and OER region starting above 1.2
V. Both thermally and electrochemically grown oxides
block the Ir metal surface and prevent Hupd from
forming. The effect of these different surface structures
on the OER is shown in Figure 2 where the OER activity
is shown.
Figure 2. OER on different Ir surfaces
A dramatic increase in activity is seen when the OER is
performed on the electrochemically-grown oxide surface,
while it is suppressed when the reaction is on a thermal
oxide. The OER on the thin oxide on the metal surface,
formed prior to the OER scan has an intermediate
activity. This data suggests that the amorphous
electrochemical oxide, allows promotes breakage of the
O-H bond required for oxygen recombination and
evolution.
Similar data is observed for Ru and its electrochemically-
and thermally-grown extended oxide surfaces.
In conclusion, there is a strong structure-property
relationship between the nature of the surface oxide and
the rate oxygen evolution reaction.

Abstract #1532, 221st ECS Meeting, © 2012 The Electrochemical Society
Fundamental investigations of precious metal
stability in energy conversion systems
D. Strmcnik, D. Tripkovic, R. Subbaraman, N. Danilovic, D. van der
Vliet, A.P. Paulikas, V. R. Stamenkovic and N. M. Markovic
Materials Science Division, Argonne National Laboratory, Argonne,
Illinois 60439 USA
Fundamental understanding of metal electrode-electrolyte
interfaces has long been recognized as one of the
cornerstones of successful commercialization of energy
conversion systems such as fuel cells and electrolyzers.
One of the major challenges faced by these systems today
is the catalyst durability.
State-of-the-art precious metal catalysts and their alloys
are exposed to various harsh conditions under normal
operating cycles, causing degradation of the catalyst and
in their parent devices. In broad terms the degradation of
the catalyst can be understood as any change in catalyst’s
properties, including its size, shape, morphology, activity
etc. While it is well known that precious metal catalysts
degrade in the course of fuel cell/electrolyzer operation,
to date, mostly due to the complexity of real catalyst
systems, there is very little systematic research done into
the fundamental aspects of catalyst degradation.
In this lecture, a systematic scanning tunneling
microscope (STM) and electrochemical study of the
electrochemical interfaces of well-defined single crystal
surfaces during conditions relevant for energy conversion
systems is presented. The results give new insights into
the degradation of electrocatalysts.
Figure 1: An STM image of a) as prepared Pt(111)
surface and b) Pt(111) surface cycled to 1.3 V.
Acknowledgement.
This work was supported by the contract (DE-AC02-
06CH11357) between the University of Chicago and Argonne,
LLC, and the US Department of Energy.

Abstract #1560, 221st ECS Meeting, © 2012 The Electrochemical Society
Controlling Reactivity of Electrochemical Interfaces by Tuning Noncovalent
Interactions
D. Strmcnik, R. Subbaraman, D. Tripkovic, K. Chang, N. Danilovic, D. van
der Vliet, P. Lopes, A. Paulikas, V. Stamenkovic and N. M. Markovic
Argonne National Laboratory, Materials Science Division
Argonne Il 60439
In aqueous electrolytes, depending on the nature of the reacting species, the supporting
electrolyte, and the electrode materials, two types of interactions have traditionally been considered: (i) direct –
covalent - bond formation between adsorbates and electrodes, involving chemisorption, electron transfer, and
release of the ion hydration shell; and (ii) relatively weak non-covalent forces that may affect the concentration
of ions in the vicinity of the electrode but do not involve direct surface-adsorbate bonding. The range of
physical phenomena associated with these two classes of bonds is unusually broad, and are of paramount
importance to understand activity of classical two phase interfaces and Nafion-based three phase interfaces.
In this lecture, we address the importance of both covalent and non-covalent interactions in
controlling catalytic activity of the oxygen reduction reaction (ORR), hydrogen evolution reaction (HER)
and oxidation of organic molecules in alkaline environments. Using surface x-ray scattering measurements
we demonstrate that hydrated cations are located 3.4 Å away from the surface, suggesting that they are
partially hydrated, though not adsorbed at the surface. The effect of these quasi-specifically adsorbed cations
on electrochemical reactions ranges from highly inhibiting (ORR) to highly promoting (HER) to a modest
effects during the oxidation of organic molecules. Although the field is still in its infancy, a great deal has
already been learned and trends are beginning to emerge that give new insight into the relationship
between the nature of bonding interactions and catalytic activity/stability of electrochemical interfaces.

Abstract #1570, 221st ECS Meeting, © 2012 The Electrochemical Society
In-situ Infrared Spectroscopy at Solid-Liquid Interfaces as a
Tool for Evaluation of Nanoscale Surface Morphology
Dusan Tripkovic, Dusan Strmcnik, Dennis F. van der Vliet, Chao Wang, Nenad
M. Markovic and Vojislav R. Stamenkovic
Argonne National Lab, Argonne, IL, USA
Single crustal surfaces of platinum have been characterized electrochemically,
spectroelectrochemically and by scanning tunneling microscopy (STM) in order to reveal
structure-function relationships between surface morphology and electrochemical activity for CO
oxidation. Our findings strongly suggest that the surface morphology along with surface defects
do have dominant role in determining intrinsic catalytic activity for CO electrooxidation. The
results acquired on different single crystal surfaces in rapid-scan mode [1] suggest that the νCO - E
curve, (where E is applied electrode potential), can be used as a spectral fingerprint for recognition
of surface morphologies, including defects and low coordinated sites. FTIR combined with STM
have brought out invaluable details about the true active sites and in combination with
electrochemical methods enabled us to draw a true correlation between stability and morphology of the
surfaces. Moreover, an interesting yet largely unexplored issue in characterizing the morphology of
nanoparticles on the atomic scale is the lack of a surface specific method capable of detecting the
presence of defects - low coordinated sites on nanoscale surfaces. The presence of such undercoordinated
sites is challenging to detect in-situ and to monitor their transformation under reaction
conditions. We demonstrated that a powerful method for characterizing nanoscale surfaces is
utilization of in-situ FTIR in rapid-scan mode due to its sensitivity to the vibrational properties of
adsorbed CO (νCO) which are dependent on surface morphology. Preliminary data, demonstrate that
the adsorbate-induced changes provide a unique possibility for characterizing in-situ structural
changes of nanoscale surfaces that are essential for understanding of structure-function relationships.
1. Stamenkovic et al. Vibrational-properties of CO at the Pt(111) solution interface:the anomalous starktuning
slope. J. Phys. Chem. B 2005, 109, 678-680.

Abstract #1650, Honolulu PRiME 2012, © 2012 The Electrochemical Society
Advanced Electrocatalysts for PEM Fuel Cells
C. Wang, D. van der Vliet, D. Tripkovic, D. Strmcnik, D.
Li, N. M. Markovic and V. R. Stamenkovic
Materials Science Division
Argonne National Laboratory
Argonne, IL 60439, USA
The major barriers to commercial fuel cells are the cost,
performance and durability of the current state-of-the-art
Pt-based electrodes. Advancement in electrocatalysts for
PEM fuel cells relies on the capability to alter their
fundamental properties, employed materials and available
technologies for rational synthesis. In this report we will
summarize the most recent efforts from our group that
have led to substantial improvements in durability and
activity of electrocatalysts. Nanoscale materials in the
form of multimetallic particles dispersed in high surface
area carbon as well as nanostructured thin film catalysts
will be presented. In all cases we relied on the knowledge
obtained from well-defined systems in order to develop
advanced catalysts with tailored properties. The reported
synergy between well-defined systems and corresponding
and nanoparticles emphasizes has been proven as an
efficient approach in design and synthesis of advanced
catalyst for PEM fuel cells.
Such research effort involves the following
steps: 1) characterization of well-defined solid materials by
varying their surface structure, composition profile and
electronic properties [1]; 2) atomic/molecular level
characterization of electrochemical interfaces [2]; 3)
theoretical modeling of structure/function relationship; 4)
identification of the most active and stable sites at atomic
scale during reaction conditions; 5) modification of
electrified solid-liquid interfaces; 6) altering of the surface
and subsurface composition by other constituents aimed to
improving functionality [3]; 7) design and synthesis of
nanomaterials with tailored structures.
�
[1] Stamenkovic et al. Science 315 (2007) 493.
[2] Stamenkovic et al. Nature Materials 6 (2007) 241.
[3] Genorio et al. Nature Materials 9 (2010) 998.
[4] Wang et al. Nano Letters 11 (2011)919.

217th ECS Meeting, Abstract #1755, © The Electrochemical Society
Design catalytic properties of electrochemical interfaces
D. Strmcnik, K. Kodama, D. Tripkovic, D. van der Vliet, C. Wang,, V. Stamenkovic, and N. M.
Marković
Material Sciences Division Argonne National Laboratory, USA
The successful deployment of advanced energy conversion and storage systems depends
critically on our understanding of the fundamental bonding interactions at electrochemical
interfaces []. In aqueous electrolytes, depending on the nature of the reacting species, the
supporting electrolyte, and the electrode material, two types of interactions have traditionally
been considered: (i) direct – covalent - bond formation and (ii) relatively weak non-covalent
metal-ion forces that may affect the concentration of ions in the vicinity of the electrode but do
not involve direct electrode - reactant/intermediate bonding. The range of physical phenomena
associated with these two classes of bonds is unusually broad, ranging from charge transfer,
release of the ion hydration shell, chemisorption, and catalysis.
In this presentation, we address the importance of both covalent and non-covalent interactions
in controlling catalytic activity of electrochemical interfaces. In order to give a brief
overview of the field, this presentation will provide a carefully balanced selection of results
for the oxygen reduction reaction, the hydrogen oxidation reaction and electrooxidation of
small organic molecules first on metal single crystal surfaces and then on corresponding real
nanoparticles. By focusing on the mechanism of action, we demonstrate that the ability to
make a controlled arrangement of surface atoms presages a new era of advances in our
knowledge of the electrochemical reactions.
1

Abstract #1924, 219th ECS Meeting, © 2011 The Electrochemical Society
Electrocatalysis on Well-Defined Solid-Liquid Interfaces
Chao Wang, Dusan Strmcnik, Dusan Tripkovic, Dennis van der Vliet,
Nenad M. Markovic and Vojislav R. Stamenkovic
Argonne National Laboratory
Materials Science Division
Argonne, IL
Novel nanomaterials with uniquely reactive surfaces can solve challenging problems in the areas
as diverse as clean energy production, energy conversion, energy storage, corrosion,
bioengineering, sensors, electronic devices etc. The ultimate goal in the heterogeneous/electrocatalysis
would be to tune the electronic and structural properties of nanoparticles in order to
achieve superior catalytic and stability enhancements [1].
The material-by-design-approach, would be used to demonstrate how the knowledge obtained
from the well-defined extended surfaces can be employed to create tailor-made nanocluster
surfaces with advanced catalytic properties. The surfaces of polycrystalline bimetallic PtM alloys
(M=Ni,Co,Fe,V,Ti,Re) as well as Pt3Ni(hkl) and Pt(hkl) single crystals were characterizaed in
ultra-high vacuum chamber by AES, LEIS and UPS before transfer into electrochemical
environment.
Enhanced catalytic properties are induced by the second metal, and the mechanism of
enhancement may occur through several effects: (1) Electronic effect, due to changes in the
metallic d-band center position vs. Fermi level; and (2) Structural effect, which refers correlation
between surface atom arrangements, and/or corrosion-induced dissolution – surface roughening.
It has been found that each alloy, depending on prehistory, could form surfaces with two different
compositions. The annealed alloys form the outermost Pt-skin surface layer, which consists only
platinum atoms, while the sputtered surfaces have the bulk ratio of alloying components [2,3].
Post-electrochemical UHV surface characterizations revealed that Pt-skin surfaces are stable after
immersion to an electrolyte, while sputtered surfaces formed Pt-skeleton outermost layers as a
result of dissolution of transition metal atoms. Modification in Pt electronic properties induced
by alloying elements alters adsorption/catalytic properties of corresponding bimetallic alloy.
The very same levels of catalytic enhancement have been established between extended surfaces
and corresponding materials in the form of nanoparticles. In addition to the d-band center
position, it has been found how catalytic activity could be affected by the arrangement of surface
atoms, presence of defects and substrate. In order to engineer catalysts with the desirable
physicochemical properties one should consider all relevant factors that could influence catalytic
activity and stability of tailored surfaces.
[1] V. Stamenkovic, B. Fowler, B.S. Mun, G. Wang, P.N. Ross, C.A. Lucas, N.M. Markovic, Science 315
(2007) 493-497.
[2] V. Stamenkovic, B.S. Mun, K.J.J. Mayrhofer, P.N. Ross, N.M. Markovic, J. Rossmeisl, J. Greeley, J.K.
Nørskov, Angewandte Chemie International Edition 45 (2006) 2897.
[3] V. Stamenkovic, B.S. Mun, M. Arenz, K.J.J. Mayrhofer, C. A. Lucas, G. Wang, P.N. Ross, N.M.
Markovic, Nature Materials 6 (2007) 241.

Abstract #549, 221st ECS Meeting, © 2012 The Electrochemical Society
Electrochemical Interfaces for Energy Conversion and Storage
Ram Subbaraman, Dusan Tripkovic, Dusan Strmcnik, Gustav Wiberg, Jakub
Jirkovsky, Chao Wang, Vojislav Stamenkovic and Nenad M. Markovic
Argonne National Laboratory, Materials Science Division
Argonne Il 60439
Design and synthesis of energy efficient electrochemical interfaces (materials and double
layer components) with tailored properties for accelerating and directing chemical transformations is one
key to developing new alternative energy systems – fuel cells, electrolyzers and batteries. In the past,
researchers working in the field of fuel cells (converting hydrogen and oxygen into water) and
electrolyzers (splitting water back to H2 and O2) seldom focused on understanding the electrochemical
compliments of these reactions in battery systems, e.g., the lithium-air system.
In this lecture, we bridge the gap between the “water battery” (fuel cell ↔ electrolyzer) and
the Li-air battery systems. We demonstrate that this would require fundamentally new knowledge in
several critical areas. In water-based environments, we need to identify a common descriptor (the bond
strength) that can be used to establish the guiding principles for designing reactivity of electrochemical
interfaces. We demonstrate that the bonding energy of hydroxyl species to the varieties of wellcharacterized
materials (OHad-M) spanning from metals, metal/metal-oxides and oxides can be used as a
single descriptor to tune the activity of catalysts and double layer components. In turn, successful
identification of such descriptor, together with associated electrocatalytic trends, provides the foundation
for rational design of practical electrocatalysts. In organic-based environments, we need to establish the
missing structure-function relationships for discharging processes in which chemical energy of Li and
oxygen is transformed to LiO 2- /Li2O2/LiO2 and charging processes in which deposited
intermediates/products are transformed back to Li ions and O2. We also need to understand at atomic
and molecular levels a possible interaction of products/intermediates with organic solvents. We conclude
that understanding the complexity (simplicity) of electrochemical interfaces would open new avenues for
design and deployment of alternative energy systems.

Abstract #947, 220th ECS Meeting, © 2011 The Electrochemical Society
Advanced Electrocatalysts: - From Extended to Nanoscale Surfaces
Chao Wang, Dongguo Li, Dennis van der Vliet, Dusan Strmcnik, Dusan Tripkovic, Nenad M. Markovic,
Vojislav R. Stamenkovic
Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
Highly efficient catalysts for energy conversion have become the primary target for the practical
application of renewable energy technologies. The development of novel electrocatalysts relies
on fundamental principles to improving high catalytic activity, durability, and achieving
competitive cost effectiveness. Conventional platinum (Pt) catalysts that have been widely
employed in fuel cells are not only expensive, but also have limited resources considering largescale
applications. Moreover, Pt that is generally considered to be chemically inert becomes
unstable when exposed to hostile electrochemical environments. All of that has led us to develop
a systematic approach toward the rational design and synthesis of advanced Pt-bimetallic and -
multimetallic electrocatalysts for fuel cells. Via synergistic studies on extended surfaces and
high-surface-area catalysts, we managed to control and tune the critical parameters of the
catalysts such as particle size, shape, composition and elemental distribution. These studies were
supported also with theoretical simulation and they provided comprehensive understanding of the
structure-function correlation (Fig. 1). As a result, we developed highly active and durable
nanoscale materials, which can be employed in fuel cells.
Figure 1. Schematic illustration of the stabilization mechanisms in highly durable Au/FePt3 catalysts for
the oxygen reduction.

216th ECS Meeting, Abstract #868, © The Electrochemical Society
Active sites for PEM fuel cell reactions
Dusan Strmcnik 1 , Dusan Tripkovic 1 , Dennis van der Vliet 1 , Jeff Greely 2 ,
Alexander Brownrigg 3 , Chris Lucas 3 , Goran Karapetrov 1 , Vojislav
Stamenkovic 1 and Nenad M. Markovic 1
1 Materials Science Division, 2 Center for Nanoscale Materials,
Argonne National Laboratory, Argonne, Illinois 60439 USA
3 Oliver Lodge Laboratory, Department of Physics, University of
Liverpool, Liverpool, L69 7ZE, UK
.
The development of electrocatalytic materials of
enhanced activity and efficiency through careful
manipulation, at the atomic scale, of the catalyst surface
structure, has long been a goal of electrochemists. To
accomplish this ambitious objective, it would be
necessary to obtain a thorough understanding of the
relationship between the atomic-level surface structure
and the catalytic properties.
In the past, the most common approach for establishing
structure function relationships was to correlate
electrochemical behavior of different single crystal
surfaces, assuming perfectly ordered surfaces.
In our work, however, we focus on the sub-structures
which exist on single crystal surfaces. Using sample
preparation and characterization (STM, FT-IRAS and
SXS) methods of metal single crystal surfaces, we were
able to monitor and control adsorbate-/ potential-induced
changes in the coordination of surface atoms. This, in
turn, allowed us to find correlations between the nature as
well as the number of active sites and the rate of PEM-FC
reactions.
Special focus was awarded to oxygen reduction reaction
and CO oxidation reaction on Pt (100), Pt (111) and Pt
(1099) surfaces to understand, how to combine different
morphological features in one catalyst exhibiting high
activities for both reactions.
The knowledge about the active sites for a particular
reaction is a prerequisite and the first step to
understanding how to tune a catalyst performance.
In our recent work, however, we have already made the
second step, producing a high surface area Pt catalyst that
shows unprecedented activities for CO oxidation as well
as oxygen reduction reaction.
Acknowledgement.
This work was supported by the contract (DE-AC02-
06CH11357) between the University of Chicago and Argonne,
LLC, and the US Department of Energy. Use of the Center for
Nanoscale Materials was supported by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences,
under contract No. DE-AC02-06CH11357. We acknowledge
computer time at the Laboratory Computing Resource Center
(LCRC) at Argonne National Laboratory, The National Energy
Research Scientific Computing Center (NERSC), and the
Molecular Science Computing Facility (MCSF) at Pacific
Northwest National Laboratory.
Wave number [cm -1 ]
Current density [mA/cm 2 ]
4
3
2
1
0
2090
2080
2070
2060
2050
EC
0.0 0.2 0.4 0.6 0.8 1.0
E [V vs. RHE]
STM
DFT
FTIR
0.0 0.2 0.4 0.6 0.8 1.0
E [V vs. RHE]
Figure 1. (top) Cyclic voltammograms (50 mv/s) for
Pt(111) in 0.1 M HClO4 in the presence and absence of
CO. (as prepared surface) (middle) STM image of the as
prepared surface. Inset – DFT model of CO and OH
coverage at the as prepared surface. (bottom) in-situ FT-
IRAS experiment : dependence of νCO and ICO2 on
potential.
CO 2 band intensity [arb. u.]

N M. Markovic
H. Gasteiger and N. M. Markovic

Fuel Cells and Electrocatalysis
�More than just devise for energy conversion - the development of fuel
cell catalysts in the last three decades has transformed electrocatalysis
from art to science
Cattech 4 (2000) 110
H 2 /CO
ORR/OER: a key for the
development of alternative
energies
CO: a key “test molecule”
HOR/HER : the mother of all
electrochemical reactions

Surface Science Approach
Single
Crystals
UHV
Experiment
MATERIALS-BY-DESIGN
DESIGN
Model
Nano
–– SYNTHESIZE
CHARACTIZATION METHODS
SXS
FTIR
STM/AFM RAMAN
(111)
METALS
M -OXIDES
OXIDES
(100)
–– CHARACTERIZE
HRTEM
ACTIVITY, SELECTIVITY AND STABILITY MAPPING
CsOH
LiOH
Au
Pt
Ru
Real
Nano
KOH
Theory/Modeling
ANIONS
WATER
CATIONS
a)
CHEMICAL
Metal-Air Batteries
SYNTHESIS METHODS
Pt(111)
0.85V
�OH = 0.7
Reactants: O 2 , CH 3 OH, H 2
DOUBLE-LAYER-BY-DESIGN
M + M +
M +
–– UNDERSTAND
REAL SYSTEM
Pt/C
–– APPLY
PHYSICAL
Fuel Cell � Electrolyzer
O 2
H 2
3

ORR- The reaction pathway
O 2
(O 2) ad
2e-
2H +
4e-
(H 2O 2) ad
(H 2O 2) solution
2e-
2H +
H 2O

Tailoring Electronic Properties
dN(E)/dE (arb. units)
dN(E)/dE (arb. units)
Intensity [arb. units]
Intensity [arb. units]
Pt Pt 237 237
Pt Pt 158 158
Pt Pt 251 251
AES (a) LEED
200 200 400 400 600 600 800 800
Auger Electron Energy [eV]
LEIS
Pt Pt 390 390
Pt 3 Ni(hkl)
Ni Ni 716 716Ni
Ni 783 783
Ni Ni 848 848
Pt 3Ni(hkl)
Pt 0.71
Surface composition = 100% Pt
0.2 0.4 0.6 0.8
E/E0 UPS
Pt3Ni (111)
(100)
(110)
-8 -6 -4 -2 0
Binding Energy [eV]
Science, 315(2007)493
(b)
(c)
AES 3 keV
[01-1]
[001]
[011]
[1-10]
(d)
p(1x1)
(e) Pt 3 Ni(100)
(f)
Pt 3 Ni(111)
c(5x1)
Pt 3 Ni(110)
(110)-(1x1)
i [�A/cm 2 ]
i [�A/cm 2 ]
i [�A/cm 2 ]
20
0
-20
20
0
-20
20
0
-20
CV
Metal Alloys(Pt 3M)
Pt 3 Ni(111)
Pt(111)
E [V] vs. RHE
Relative Expansion [%]
i [�A/cm 2 ]
i [�A/cm 2 �Hupd [%] ]
i [�A/cm 2 ]
�Hupd H O [%]
2 2
� Hupd [%]
Hi [mA/cm O [%]
2 2 2 ]
i [mA/cm 2 ]
(g) 0.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Pt 3 Ni(100)
Pt(100)
0.1M HClO 4
0.0 0.2 0.4 0.6 0.8 1.0 1.2
E [V] vs. RHE
Pt 3 Ni(110)
Pt(110)
-0.5
60
-1.0
30
-1.5
0
60
-30
60
(h)
-60
30
30
0
0
-30 60
-30
-60 30
-60
0
60
50 60
(i)
30
0
30
0.0 0.2 0.4 0.6 0.8 1.0 1.2
E [V] vs. RHE
-2
0
-4
50
-60
0
-2
-4
-6
Pt[111]-Skin
Pt[111]
i = n F K(1-� s) exp(-γ�G i / RT)
E [V] vs. RHE
0.0 0.2 0.4 0.6 0.8 1.0
E [V] vs. RHE
0.0 0.2 0.4 0.6 0.8 1.0
Pt [at. %]
100
80
60
40
20
0
a’)
1 2 3 4 5 6 7
Atomic layer
20 E [V] vs. RHE
0.0 0.2 0.4 0.6 0.8 1.0
60
80
100
Pt
Pt3Ni(111) 3Ni(111) 0
40
Pt(111)
Pt(111)
0.0 0.2 0.4 0.6 0.8 0.1 1.0 HClO4 E [V] vs. RHE
Pt poly
60 o C
1600 rpm
III
III
0.0 0.2 0.4 0.6 0.8 1.0
E [V] vs. RHE
II
Ni [at. %]
Pt 3 Ni(111)
Pt(111)
II
SXS
0.1 HClO4 60
Pt poly
o 0.0 0.2 0.4 0.6 0.8 1.0
E [V] vs. RHE
C
1600 rpm
Potential/V
I
I
a)
b)
c)
60
30
0
60
30
0
� Oxd [%]
� Oxd [%]
� Oxd [%]

Tailoring Activity by Electronic Properties
Specific Activity: ik @ 0.9V [mA/cm 2
real ]
Activity for ORR
19
18
17
Activity mapping
5
4
3
2
1
0
3.4
Pt 3 Ti
d-band center [eV]
Pt 3 V
3.0
Pt 3 Fe
Pt-skin surfaces
Target Activity
Pt 3 Ni(111)
Pt 3 Co
Pt-skeleton surfaces
2.6
Pt 3 Ni
- 3.4 - 3.0 - 2.6
d-band d-band center center shift [eV] (eV)
Guiding Principles:
(a)
Pt-poly
Pt/C
3
2
1
Activity improvement factor vs. Pt-poly
Pt 3 Co
Pt 3Fe
Pt 3Ti
1 st Layer
2nd (b)
Pt3Ni Layer
3 rd Layer
4th Layer
Pt
3.0 2.8 2.6
-3.0 -2.8 -2.6
Calculated d-band center [eV]
Calculated d-band center [eV]
6
4
2
0
Nanosegregated Profile
6
4
2
0
Pt[111]-Skin surface
Calculated activity [ 10 -2 eV]
Calculated activity [10 -2 eV]
Pt=100 at.%
Pt=48 at.%
Ni=52 at.%
Pt=87 at.%
Ni=13 at.%
Pt=75 at.%
Ni=25 at.%
Nature Materials, 6(2007)241
�Maximize activity by minimizing surface coverage of spectators
�Without compromising activity protect 3d elements by additional Pt layer

Tailoring Electronic Properties
Colloidal solvo - thermal approach for
monodispersed PtM NPs with controlled
size and composition
Pt Lα
Pt Co
Co Kα
Efficient surfactant removal
1 o Particle size effect applies to Pt-bimetallic NPs
Specific Activity increases with
particle size: 3 < 4.5 < 6 < 9nm
Mass Activity decreases with particle size
Optimal size particle size ~5nm
2o Temperature induced segregation in Pt-bimetallic NPs
Agglomeration prevented
(b)
Optimized annealing temperature 400-500 o C
3 o Surface chemistry of homogeneous Pt-bimetallic NPs
Pt xM (1-x) NPs
Dissolution of non Pt surface atoms leads to Pt-skeleton formation
4o Composition effect in Pt-bimetallic NPs
Pt3M 1 nm
PtM PtM 2 PtM 3
0.22 nm
1 nm
Optimal composition of Pt-bimetallic NPs is PtM
1 nm
9

a
b
From Model to Real Catalysts
c
b
c
d
ed
e
Intensity (a. u.)
counts
Intensity (a. u.)
counts
1 nm
0
Intensity Counts
Calculated Profile
0
0
as-prepared acid leached acid leached/annealed
Intensity Counts
Calculated Profile
1 nm 0 1 2 3 4 5
Position (nm)
Pt
Ni
Pt
Ni
1 2 3 4 5
1 Position 2 3 (nm) 4
Position (nm)
5
1
2 3 4 5
Position (nm)
Intensity (a. u.)
counts
Intensity (a. u.)
counts
1 nm
1 nm 0 1 2 3 4 5
Position (nm)
0
0
0
1 2 3 4 5
1 Position 2 3(nm) 4
Position (nm)
5
1 2 3 4 5
Position (nm)
Au core
PtFe shell
counts
counts
0
0
1 2 3 4 5
Position (nm)
1 2 3 4 5
Position (nm)
Nano Letters, 11(2011)919-928
5o 10
0.2
Pt-bimetallic catalysts with mutilayered Pt-skin surfaces
b
400
RDE after 4K cycles @60o c
before
C (0.6-1.05V vs. RHE):
Mass Activity (A/g)
Specific Surface A
Specific Activity (mA/cm 2 )
30
20
0
1.0
0.8
0.6
0.4
0.2
0.0
8-fold 300 after
6
specific and 10-fold mass activity improvements over Pt/C
100
0
before
after
Improvement Factor
(vs. Pt/C)
6
4
2
0
before
Improvement Factor
(vs. Pt/C)
200Advanced
Functional 4 Materials, 21(2011)147
2
0
10
8
6
4
2
0
after
Pt/C acid leached acid leached/annealed
PtNi/C PtNi/C
Pt/C Au/PtFe
c
Mass Activity (A/g)
Specific Activity (
0.6
0.4
0.0
400
300
200
100
0
before
after
Pt/C acid leached
PtNi/C
Improvement
(vs. Pt/C
4
2
0
Improvement Factor
(vs. Pt/C)
6
4
2
0
before
14
10
0
after
acid leached/annealed
PtNi/C
6 o Multimetallic NPs can further improve activity and durability
before
After 60K cycles
before
After 60K cycles
8
4
8

Pt (111)
c)
Beyond electronic effects
- CN- - CN- - CN- - CN- - O 2
Inorganic
2- 3- 3- 3-
- SO 4 / PO 4
Keep electronic properties constant
Maximize activity and selectivity by inert self assembled monolayer
b)
Inert Pt-CN ad adlayer
Nature Chemistry 2 (2010) 880
Molecular patterning
Organic
Inert Pt-Calix[4]arene adlayer
Nature Materials, 9, 2010,881
9

Selectivity controlled activity
•ORR is strongly inhibited by adsorbed phosphoric acid anions
E E [V [V vs. vs. RHE]
RHE]
0.0 0.2 0.4 0.6 0.8 1.0
c)
a)
I II III
Pt (111) I II III
Pt (111)
0.05 M H 3 PO 4
Pt (111) - O – CN 2
d)
2- 3-
- SO 4 / PO 4
Pt (111)
- CN- - CN- - CN- - CN- - M + - M + - M + - M +
0.0 0.2 0.4 0.6 0.8 1.0
E E [V [V vs. vs. RHE]
RHE] - O 2
- M + - M (H2O) x-OHad + - M (H2O) x-OHad + - M (H2O) x-OHad + (H2O) x-OHad 100
50
0
-50
-100
0
-1
-2
-3
-4
-5
-6
Current density [mA/cm2 ] Current density [mA/cm2 Current density [mA/cm ]
2 ] Current density [mA/cm2 ]
Current density [�A/cm2 Current density [�A/cm ] 2 ]
Current density [�A/cm2 Current density [�A/cm ] 2 ]
Pt (111)
c)
“Big vs. Small”
- CN- - CN- - CN- - CN- - O 2
2- 3-
- SO 4 / PO 4
•CN adlayer selectively blocks adsorption of anions
b)
Nature Chemistry 2 (2010) 880
i = n F K (1-� s) exp(-γ�G i / RT)
Current density [mA/cm2 ] Current density [mA/cm2 Current density [mA/cm ]
2 ] Current density [mA/cm2 ]
Current density [�A/cm2 Current density [�A/cm ] 2 ]
Current density [�A/cm2 Current density [�A/cm ] 2 ]
0
-1
-2
-3
-4
-5
E [V vs. RHE]
E [V vs. RHE]
0.0 0.2 0.4 0.6 0.8 1.0
100
50
0
-50
-100
a)
b)
I II III
I II III
Pt (111)
Pt (111) – CN
0.1 M HClO 4
-6
0.0 0.2 0.4 0.6 0.8 1.0
E E [V [V vs. vs. RHE] RHE]
0.0
0.0
18 10

Selectivity controlled stability
a) b)
c)
40 nm
2% of active sites
Nature Materials, 9, 2010,881
Improving stability of cathode catalysts
20 nm
Current density [�A/cm2 Current density [�A/cm ] 2 ]
Chargedensity [�C/cm2 Chargedensity [�C/cm ] 2 ]
30
25
20
15
10
5
0
E [V vs. RHE]
0.0 0.2 0.4 0.6 0.8
100
50
0
-50
-100
a)
b)
I II III
Pt (111)
Pt (111)-A
Pt (111)-B
Pt (111)-C
Pt (111)-D
-5
0.0 0.2 0.4 0.6 0.8
E [V vs. RHE]
11

Tailoring Activity/Stability by Molecular Patterning
�Maintain electronic properties and optimize stability by molecular patterning
Extended
O 2
H 2
Activity
� Calix adlayer prevents adsorption of O 2 but not H 2
Pt(111)-Calix[4]arene
H 2 = 2H + + 2e -
O 2 + 4H + +4e _ = H 2O
Reversible potential (E)
Nano
�Selective anode catalysts for the ORR and HOR are of grate importance for stability of
cathode materials in Fuel Cells
14
17

Three Phase (Nafion) Interface
•What type of interactions are possible at three phase interfaces ?
18

Interplay of Covalent, Electrostatic and Non-covalent Interactions
Sulfonate anions
are “specifically” adsorbed
on Pt (covalent interactions)
Pt-beckbone
interaction is very
week (electrostatic
forces)
Non-covalent
cation-sulfonate interaction
J. Phys. Chem., C. (2010) .
26 19

The Spring Model
J. Phys. Chem., C. (2010) .
i = n F k (1-� anions.) exp(-�G i/RT)
27 20

Perspectives
� Understanding complexity (simplicity) of electrochemical interfaces all
the way down to molecular and atomic levels will pave the way to:
� Greatly improve our predictions for
accelerating and directing chemical
transformations
� Dramatic new energy production
and storage technologies
� Enable new mitigation strategies
for environmental damage
CARBON
CYCLE
WATER
CYCLE
Energy
Conversion
&
Storage Group
ENERGY
Real
16
LITHIUM
CYCLE
16

Dusan Strmcnik,
Chao Wang
Dennis van der Vliet
Ken Kodama
Masa Uchimura
Ram Subbaraman
Nemanja Danilovic
Donguo Li
Pietro Lopez
Voya Stamenkovic
K-C Chang
Gustav Wiberg
Dusan Tripkovic
Chris Lucas
Jeff Greeley

Activity and Stability of platinum and platinum bi metal
alloys for the oxygen reduction reaction
Dusan Tripkovic 1 , Dusan Strmcnik 2 , Vojislav Stamenkovic 2 and
Nenad M. Markovic 2
1 Institute of Chemistry, Technology and Metallurgy – Department of Electrochemistry,
University ofBelgrade, Belgrade,Serbia
2 Materials Science Division, Argonne National Laboratory, Lemont, IL -60559
Tripkovic_dusan@yahoo.com
Much of the research activities in the last two decades have focused on the
development of the polymer electrolyte membrane fuel cells (PEMFCs), a device that
directly converts the chemical energy of hydrogen and oxygen into electrical energy.
Many problems still need to be resolved in order for the PEMFCs to fully reach its
commercial implementation – one of the major issues is the development of cathode
materials that can simultaneously reduce the overpotential for the oxygen reduction
reaction (ORR) and provide long-term stability in hostile electrochemical
environments.
A step toward solution of this foremost hurdle has recently been made by
demonstrating a ninety-fold increase in specific activity (activity per surface Pt atom)
on Pt3Ni(111) vs. the state-of-the-art Pt/C. It was proposed that this exceptional
activity is governed by unique electronic (d-band position) and geometric (111-like
symmetry) properties of nanosegregated (3 atomic layers) structures. Although high
energy conversion efficiencies were achieved on the Pt (111)-skin surfaces, a
fundamental knowledge of how to minimize the degradation of such nanosegregated
structures is still lacking.
Given that the stability of catalysts is the integral part of the materials design
process, establishing the potential window of stability of the these nanosegregated
structures is a major challenge that requires a deeper understanding of the oxideinduced
morphological and activity changes
Here, we present data for the ORR on Pt(111), Pt3Ni(111),
Pt3Ni(1099)=20(111)x(100),Pt3Ni(1077)=8(111)x(100), and Pt3Ni(110) surfaces in
0.1 M HClO4 to address the importance of surface morphology on activity and stability
of Pt-skin atoms using electrochemical (cyclic voltammetry, analytical(ICPMS) and
microscopic(STM) techniques.
Learning from the differences and similarities between these systems could be
a step forward in understanding whether the high activity of Pt3Ni(111) is primarily
influenced by the subsurface concentration of Ni or by the geometry of Pt surface
atoms.

High Activity and Stability of Pt2Bi Catalyst in Formic
Acid Oxidation
J.D. Lović a , M.D. Obradović a , D.V. Tripković a , K.Dj. Popović a ,
V.M. Jovanović a , S.Lj. Gojković b and A.V. Tripković a,*
a ICTM-Institute of Electrochemistry, University of Belgrade,
Njegoševa 12, P.O.Box 473, 11000 Belgrade, Serbia
b Faculty of Technology and Metallurgy, University of Belgrade,
Karnegijeva 4, P.O.Box 3503, 11000 Belgrade, Serbia
*E-mail: amalija@tmf.bg.ac.rs
Formic acid oxidation was studied on Pt2Bi catalyst characterized by
XRD spectroscopy (phase composition), STM (surface morphology) and COads
stripping voltammetry (surface composition). Bulk composition of Pt2Bi
characterized by XRD revealed two phases: 55% PtBi alloy and 45% Pt.
Estimated contribution of pure Pt on the Pt2Bi surface (43.5 %), determined by
COads stripping voltammetry, corresponds closely to bulk composition and
indicates clearly that adsorbed CO prevents leaching of Bi.
-2
j / mA cm R
7
6
5
4
3
2
1
0
Pt 2 Bi
Pt poly
-300 -200 -100 0 100 200 300 400 500 600 700 800 900
E / mV (SCE)
Fig. 1. Cyclic voltammograms for the
oxidation of 0.125 M HCOOH in 0.1 M
H2SO4 solution on Pt2Bi and Pt
catalysts. Scan rate of 50 mV s –1 .
-2
j / mA cm R
6
5
4
3
2
1
Pt poly
Pt 2 Bi
0
0 400 800 1200 1600 2000
Fig. 2. Chronoamperometric curves for
the oxidation of 0.125 M HCOOH at 0.2
V in 0.1 M H2SO4 solution on Pt2Bi and
Pt catalysts.
Pt2Bi exhibits high activity and stability in formic acid oxidation. High
activity is caused by the fact that formic acid oxidation proceeds completely
through dehydrogenation path induced by an ensemble effect. Stability of Pt2Bi
surface in formic acid oxidation is based on preventing of Bi
leaching/dissolution during formic acid oxidation as it was recognized by
insignificant change of surface morphology and roughness demonstrated by
STM images before and after electrochemical treatment in formic acid
containing solution, as well as by the absence of surface poisoning by COads
species.
Pt2Bi is powerful catalyst for formic acid oxidation exhibiting up two
order of magnitude larger current densities at 0.0 V and onset potential shifted
for ~0.2 V to more negative value relative to Pt.
t / s

Insight of Sn influence on formic acid oxidation at
Pt based catalysts
S. Stevanović 1 , D. Tripković 1 , V. Tripković 2 , D. Minić 3 , A.Gavrilović 4 , K. Popović 1 ,
A. Tripković 1 and V.M. Jovanović 1
1 ICTM, Department of Electrochemistry,University of Belgrade, Njegoševa 12, Belgrade,
2 Center for Atomic-scale Materials Design (CAMD), Department of Physics, Nano-DTU,
Technical University of Denmark, Building 311, Kgs. Lyngby 2800, Denmark
3 Faculty of Physical Chemistry, University of Belgrade, Studentski trg 12, Belgrade,Serbia
4 CEST Centre of Electrochemical Surface Technology, Viktor-Kaplan-Str. 2,A-2700 Wiener
Neustadt, Austria
One of the extensively studied bimetallic catalysts is PtSn. It appears that
among other catalysts PtSn is the most active for ethanol oxidation. The increased
activity of Pt based catalyst with added Sn in comparison to pure Pt, is explained by an
electronic effect in the Pt-based electrode material affecting on adsorption properties of
the surface, making this system less prompt to poisoning by organic species than pure
Pt. More importantly, Sn or its oxides can supply surface oxygen-containing species at
lower potentials by activation of the interfacial water molecule necessary to complete
the oxidation of adsorbed reaction intermediates through bi-functional mechanism.
Thus, the addition of Sn is mainly related to its influence on CO. CO is the main poison
of polycrystalline Pt in formic acid oxidation, yet this reaction has been rarely studied
on Pt-Sn catalyst.
The aim of our work is to overview the influence of Sn on the oxidation of
formic acid by studding the reaction when Sn is present only on the surface of Pt (Sn
UPD), when Sn is added to Pt in PtSn catalyst and when Sn is leached out from
surface layers of this catalyst – skeleton structure of PtSn. Both Pt and PtSn catalysts
were prepared by microwave assisted pollyol method and characterized by STM, XRD
and EDX techniques. Oxidation of formic acid was studied in 0.1 M HClO4 + 0.5 M
HCOOH solution by cyclic voltammetry (sweep rate 50 mV/s) and chromoamperometry
at 0.2 V vs SCE.
STM analysis of unsupported catalysts showed small spherical particles of
almost equal size below 2 nm. XRD confirmed small particle sizes for both catalysts
and indicated low alloying degree of Pt and Sn, while EDX revealed almost nominal
Pt:Sn ratio.
If Sn is present on the surface either as ad-atom on Pt or as less noble metal in
PtSn catalyst, the onset potential of formic acid oxidation is shifted to the lower values
and currents are significantly higher in comparison to Pt, thus the reaction is shifted
towards direct path. However, on skeleton surface of PtSn catalyst, when Sn should be
only in subsurface layers, the reaction proceeds as on pure Pt upto 0.3 V, but then
contrary to pure Pt the current continues to increase to the maximum at potential which
is more negative than the second peak at pure Pt. The results are discussed and
explained by Sn influence through bi-functional mechanism, morphology changes and
electronic effect.

Ehanol oxidation on carbon supported platinum based
bimetallic catalysts synthesized by microwave assisted
polyol procedure
S. Stevanović 1 , D. Tripković 1 , J. Rogan 2 , J. Lović 1 , K. Popović 1 , A. Tripković 1 and
V.M. Jovanović 1
1 ICTM- Department of Electrochemistry, University of Belgrade,
Njegoševa 12, Belgrade,
2 Faculty of Technology and Metallurgy, University of Belgrade,
Karnegijeva 4, Belgrade,
Ethanol oxidation was studied in perchloric acid at high surface area
carbon supported Pt/C, PtRh/C and PtSn/C catalysts prepared by microwave
assisted polyol procedure. The catalysts were characterized by XRD, STM and
EDX techniques. The diffraction peaks of the bimetallic catalysts in X-ray
diffraction patterns are slightly shifted to lower (PtSn/C) or higher (PtRh/C) 2θ
values with respect to the corresponding peaks at Pt/C catalyst as a
consequence of alloy formation. Thus, low alloyed PtSn and PtRh catalysts
were obtained. STM analysis of unsupported catalysts shows that the particles
are small (diameter is ~ 2 nm) with a narrow size distribution. This should be
attributed to the advantages of microwave assisted polyol process in ethylene
glycol solution.
j (mA/mg Pt )
210
180
150
120
90
60
30
0
Pt/C
PtRh/C
PtSn/C
-30
-0.2 -0.1 0.0 0.1 0.2 0.3
E (V vs SCE)
Fig. 1: Potentiodynamic curves for the
oxidation of 0.5 M C2H5OH at asprepared
Pt/C, PtRh/C and PtSn/C
catalysts in 0.1 M HClO4, � = 20 mV/s.
It is found that the activity of Pt-based
bimetallic catalyst for ethanol oxidation
greatly depends on secondary metal
and the electrode potential. While
addition of Sn to Pt leads to the
enhancement of ethanol oxidation and
lower poisoning of electrode surface,
addition of Rh does not change overall
electrochemical reaction. The onset
potential for ethanol oxidation at our
PtSn/C catalyst is shifted for ~ 150 mV
to more negative values and the
increase of activity for ~ 3 times in
comparison to Pt/C catalyst is
obtained. The onset potential for this
reaction is remarkably shifted to lower
values in comparison to other PtSn
catalysts with similar composition.
Chronoamperometric measurements
revealed that PtSn/C is notably less poisoned than Pt/C catalyst. The increased
activity of PtSn/C catalyst is mainly due to bifunctional mechanism enabled
most probably by non-alloyed Sn (SnO2) although electronic effect of low
alloyed PtSn could play some role as well.

Advanced Multimetallic Electrocatalysts
Vojislav R. Stamenkovic, Dennis van der Vliet, Dusan Strmcnik, Dusan Tripkovic,
Chao Wang, Nenad M. Markovic
Materials Science Division, Argonne National Laboratory
9700 S. Cass Ave. Argonne, IL 60439, USA
vrstamenkovic@anl.gov
Recently a lot of effort has been placed on the synthesis of alloy nanoparticles
(NPs) for applications in heterogeneous catalysis. 1-3 While it is usually challenging to
reach the optimal catalytic performance for a given chemical reaction by singlecomponent
materials, alloys offer an additional dimension of tailoring electronic and
surface structures of catalysts via ensemble, strain, and/or electronic (ligand) effects.
Fine tuning of these parameters provides the opportunity for catalyst design to reach a
fine balance among activity, selectivity and durability. Bimetallic Pt3M (M = Fe, Co,
Ni, etc.) alloys have been extensively studied and shown to be better catalysts than pure
Pt for the ORR. 2 4 It was first established on well-defined extended surfaces that these
alloys have lower d-band center versus Pt, resulting in reduced binding strength and
adsorption of oxygenated spectator species (e.g., OH − ) on the surface and thus more
active sites accessible for molecular oxygen. A volcano-type dependence of the
electrocatalytic activity on the electronic structures has been revealed, which was
further validated in monodisperse and homogeneous high-surface-area nanocatalysts
prepared by organic solution synthesis. Our successful experience in such
combinational studies of extended surfaces and nanocatalysts have urged us to explore
novel and more complicated systems in the quest for better electrocatalysts.
In this work we have carried out a comprehensive investigation of Pt-ternary
(Pt3(MN)1 with M, N = Fe, Ni or Co) alloy electrocatalysts for the ORR. An organic
solvothermal approach was developed for the synthesis of homogeneous Pt-ternary
alloy NPs. Element mapping based on high-resolution scanning transmission electron
microscopy (STEM) was used to analyze the alloy homogeneity in the NPs. The
obtained NPs were supported on carbon black and then subject to electrocatalytic
studies. We also did fundamental studies of extended surfaces for these ternary alloys, 5
in order to confirm the dependence of ORR activity on the alloying elements in the
NPs. The obtained composition-function correlation was further compared to the
results by theoretical simulations, and the previously established relationship for Ptbimetallic
catalysts.
[1]. Greeley, J.; Mavrikakis, M. Nat. Mater. 2004, 3, 810-815.
[2]. Strasser, P.; Koh, S.; Anniyev, T.; Greeley, J.; More, K.; Yu, C. F.; Liu, Z. C.;
Kaya, S.; Nordlund, D.; Ogasawara, H.; Toney, M. F.; Nilsson, A. Nature Chem. 2010,
2, 454-460
[3]. Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.;
Wang, G. F.; Ross, P. N.; Markovic, N. M. Nat. Mater. 2007, 6, 241-247.
[4] Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C.
A.; Markovic, N. M. Science 2007, 315, 493-497

Electrocatalysts with Multifunctional Properties
Dennis van der Vliet, Dusan Strmcnik, Dusan Tripkovic, Chao Wang, Nenad M.
Markovic, Vojislav R. Stamenkovic,
Materials Science Division, Argonne National Laboratory
9700 S. Cass Ave. Argonne, IL 60439, USA
vrstamenkovic@anl.gov
The ability to alter the electronic and structural properties of electrocatalysts can
lead towards superior catalytic activity. This can be achieved by controlling the size
and shape of nanoparticles, the introduction of another constituent into the catalyst
and/or tailoring the surface morphology. The later has been in focus of this report and
can be controlled either by preparation protocol or electrochemical conditions. The
concept of structure function relationships has been used to investigate the influence of
surface atomic arrangement on the electrocatalytic properties of the surface 1 . By
comparing the rates of an electrocatalytic reaction one is able to draw conclusions
about the reaction mechanism as well as the properties of the material needed to boost
the reaction rate 2-5 . However, in this contribution, we focus on the different atomic
arrangements within the same surface, i.e. on the local surface structure of the catalyst.
We utilized ex- and in-situ tools to characterize and tune the morphology of the catalyst
surfaces. That produced substantial differences in catalytic properties. Moreover, we
demonstrate that surface adatoms can control the overall electrochemical behavior of a
catalyst. For that reason, the knowledge about the active sites for a particular reaction
is a prerequisite and the first step to understanding how to tune a catalyst performance.
In order to elucidate the real active sites we used Pt single crystal surafces to establish
the reaction rates for CO bulk electrooxidation and oxygen reduction reaction as a
function of the applied positive potential limits and surface morphology. We managed
to control the presence of different active sites for the same catalysts and thereofre, we
induced multifunctional behavior. The same analogy was used for nanoscale Pt
catalysts and we proved that multifunctionality of electrocatalyst could be tailored.
References:
[1]. N.M.Markovic, P.N.Ross, Surf. Sci. reports, 45, 117 (2002).
[2]. M. Arenz, K.J.J. Mayrhofer, V. Stamenkovic, B.B. Blizanac, T. Tomoyuki, P.N.
Ross, N.M. Markovic, J.Am.Chem.Soc. 127 6819 (2005)
[3]. N.P. Lebedeva, M. Koper, E. Herrero, J.M. Feliu, R.A. van Santen,
J.Electroanal.Chem., 487, 37-44 (2000).
[4]. N.P. Lebedeva, A. Rodes, J.M. Feliu, M.T.M. Koper, R.A van Santen,
J.Phys.Chem.B, 106, 9863-9872 (2002)
[5] Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C.
A.; Markovic, N. M. Science 2007, 315, 493-497

_n~::;::;D::;n"""':::'>-;J"'"
p:J~""~,p:J>«1)~r::-,-:=
nO:;:j.~
Ox-'::;
'JJ"O-;:Jnn;:J(1)OIOU~
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o:f7
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~
Physical ('hem istry 20 I 0
u-
be mentioned also that Pt/C catalyst synthesized by microwave assisted r
method is ITlOre active then commercial Pt/C-Tanaka catalyst probably due
belter dispersion, However, the activity of PtjSn/C catalyst synthesized by
method is lower in comparison with activity of commercial PtjSn/C-T
catalyst [5], The higher activity of Pt]Sn/C-Tanaka catalyst most likely
consecvcnceof higher alloying degree of this catalyst.
140 -------.---..----
i
-N-
- ---
Pt/C-Tan
Pt/C-MW.aka
I'
120 i -- PI Sn/C-MW
, " 1
~ 100
ci
~ 80
~ "
Fig.I.."
th~ c1ectrooxidation of m
C2H5OH in 0,\ M HCI04,
60 :'
, ,
" ,.
40 : /
, ,
: ,
, ,-
W ,~'
"
,,-
: 0 , / ~,.~-_.":>< /'
-20
"
;:
-0-2 E pJi»s SCE) 0,2 04
Conclusion
The presented results point out the enhanced activity of bimetalic catalyst for
electrooxidation of ethanol which may be mainly ascribed to changes in el
structure due to the al1oy formation.
References
[1] J. Luo, N. Kariuki, L. Han, L. Wang, C. J. Zhong, T. He Electrochil1l.
2006,51,4821. -
L. Colmenares,H. Wang, Z.Jusys, L. Jiang, S. Van, G. Q. Sun, R. .I.
Elcctrochil1l. Acta, 2006, 52, 221.
F. Colmati, E. Antolini, E. R. Gonzalez, J. Alloy and Compounds, 2008,
7(14
Z. Liu, B. Guo, L. Hong, T. H. Lim, Electrochem. Communicatior
83.
A. V. Tripkovic, K. OJ. Popovic, .I. O. Lovic, V. M. .Iovan!
Stevanovic, O. V. Tripkovic, A. Kowal, Electrochem. Comm,.
lmo
270
MANGANESE DIOXII
OL Y ANILINE NANOS
APPLICA TION
B. SUuki~, l. Stojkovic. i'
iversity of' Be/grade, F'ocu/t,/ of
B
stract
nganese dioxide modified carb
,pared via a novel hydrothen
aracterised and examined 1'01' pc
owing a high e\ectrocawlytic a
ueous media,
troduction
recent times, special attention
ide-based catalysts as an altern,
n02) in particular is interesti
rnbination of beneficial ph)
!though having excellent activi
cry low surface areas. Loading
direct synthesis or post graftil
irect synthesis of the oxide onte
ctive phase and the support.
if anoPAN!) were used as a Ii
", .'
,(communICatiOn.
f
~i(J!:xperimental
:~,Carb-nanoPANI was prepared b
!\f)11odification of Carb-nanoPAN
',procedure. First, a reaetion mix!
';in 25 m! of aqueous solution
manganese sulphate (0.06 M) '1
. lined reactor for 15 h at 135"C
". separated from the solution
hom!';