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www.rsc.org/materials Volume 22 | Number 41 | 7 November 2012 | Pages 21781–22310
ISSN 0959-9428
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Jieshan Qiu et al.
Hierarchical activated carbon nanofiber webs with tuned structure fabricated by
electrospinning for capacitive deionization
0959-9428(2012)22:41;1-C

Journal of
Materials Chemistry
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Hierarchical activated carbon nanofiber webs with tuned structure fabricated
by electrospinning for capacitive deionization
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Published on 10 September 2012 on http://pubs.rsc.org | doi:10.1039/C2JM34890J
Gang Wang, a Qiang Dong, a Zheng Ling, a Chao Pan, ab Chang Yu a and Jieshan Qiu* a
Received 24th July 2012, Accepted 28th August 2012
DOI: 10.1039/c2jm34890j
Novel hierarchical activated carbon nanofiber (ACF) webs with
tuned structure have been fabricated by incorporating carbon black
(CB) into an electrospun polymer solution, followed by heat treatment.
The as-made electrospun ACF webs show superior capacities
as electrode materials in capacitive deionization (CDI) for desalination
due to their advantageous hierarchical structures.
The consumption and sustainable supply of fresh water have become
one of the top priority issues faced by the global community. 1 In the
past few years, the capacitive deionization (CDI) technology for
desalination of salt water has drawn much attention because of its
potential as an energy-efficient alternative to membrane desalination
and thermal processes currently available for producing fresh water
from salted water sources. 2–10 For the CDI technology, electrode
materials with tuned pore structure and functions, such as conductivity,
are the key to an efficient desalination process. 11 Up to now, a
number of porous carbons with different forms and textures have
been tested as electrosorptive electrodes. 12–15 Unfortunately, the
desalination performance of the electrodes made of conventional
porous carbons such as microporous carbon particles or powders is
far from satisfactory because some micropores in the electrodes are
not accessible to ions. 16 As such, hierarchical porous carbons with
well-defined pore structures are highly desirable, 17,18 in which
micropores, mesopores and macropores are connected well in a
balanced way. The micropores would help lead to an enhanced
electrical double-layer capacitance, the mesopores would provide
low-resistance pathways and channels for the ions through the
porous fibers, while the macropores would function as ion-buffering
reservoirs to minimize the diffusion distances to the interior surfaces
of the porous carbons. 19,20 With this in mind, several kinds of new
carbon materials such as carbon aerogel, 21 carbon nanotubes, 22 graphene
23 and ordered mesoporous carbon 24 have been tested as the
electrode materials for the CDI process. Of the tested carbon materials,
carbon aerogels with a monolithic structure are found to be one
of the promising electrode materials for CDI systems because they
have controllable mesopore size distribution and better solid phase
a
Carbon Research Laboratory, Liaoning Key Lab for Energy Materials
and Chemical Engineering, State Key Lab of Fine Chemicals, Dalian
University of Technology, Dalian 116024, China. E-mail: jqiu@dlut.edu.
cn; Fax: +86 411 84986080; Tel: +86 411 84986024
b
College of Science, Dalian Ocean University, Dalian 116023, China
conductivity, but some bottle-neck problems such as high production
cost and poor mechanical properties limit the carbon aerogels for
practical use as an ideal electrode material in CDI. 25
Electrospinning is a simple yet effective technique that is capable of
fabricating continuous nanofiber webs. 26 In the literature, there are
several reports on the synthesis of self-standing thin webs consisting
of porous carbon nanofibers for energy storage applications, 27,28 but
to our best knowledge, little has been done about the potential of
activated carbon nanofiber (ACF) webs made by electrospinning for
CDI. The concerned ACF webs featuring high porosity and freestanding
nanofibers are expected to be an excellent alternative as
electrode materials in CDI. This has been partly demonstrated
recently by our work that electrospun ACF webs with relatively
developed microporous structure show a high desalination capacity
as CDI electrodes. 29 Nevertheless, there is still room for further
enhancing their desalination capacity, which can be done by further
optimizing the porous structure. Here we report a new strategy for
fabricating hierarchical ACF webs with tuned structure for CDI by
electrospinning, in which carbon black (CB) was incorporated or
embedded in situ into the electrospun ACF webs that were subsequently
activated in flowing CO 2 to tailor the hierarchical pore size
distribution. This helps greatly improve the conductivity and the saltremoval
capacity of the ACF webs that are free-standing and have a
monolithic hierarchical structure with a well-developed yet balanced
micro-, meso- and macro-porosity. In this novel strategy reported
here, no binders are used in contrast to the traditional methods. This
makes it possible to fabricate electrodes with an improved saltremoval
capacity that is much better than other carbon electrodes for
CDI reported in literature. 30,31
The as-made ACF/CB900 webs have a smooth surface (see
Fig. 1a), and are robust and flexible yet free-standing (see Fig. 1b).
Because of this, they can be folded like non-woven cloth, and can
be rolled up easily into a scroll. The continuous skeletons of the
ACF/CB900 webs help reduce the internal resistance of the electrodes,
11 and result in a large electrosorption capacity. The SEM
image in Fig. 1c shows a composite web with 5 wt% CB loading, in
which some small nanoparticles of agglomerated CB can be seen, as
marked in Fig. 1c in the white circles. It seems that the fibers in the
ACF/CB900 webs (Fig. 1d) have a morphology differing from those
without any CB (Fig. 1e and f), and the agglomerated CB would
cause some changes of the smooth cylindrical shape of the fibers to
some degree. In the electrospinning step, the CB particles are oriented
along the electrospinning needle created ‘streamlines’ in the PAN
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Fig. 1 (a) and (b) optical images of ACF/CB900 webs; (c) and (d) SEM images of ACF/CB900 surface; (e) and (f) SEM images of ACF900 surface; (g)
TEM image of ACF/CB900 webs, where small nanoparticles of CB can be seen, as shown in white circles in (c).
solution due to the elongation effect of the fluid jet, and the
agglomerated CB particles are visually parallel to the long-axis of
individual fibers, as evidenced in Fig. 1d. A typical TEM image
(Fig. 1g) shows clearly that the CB particles are embedded in the
PAN-based nanofiber matrix and some pores are formed both in the
nanofibers and in the junction between the nanofiber and CB that
possibly results from the CO 2 activation.
It has been found that the electrical conductivity of the composite
webs can be significantly improved by the embedded CB. The electrical
resistance for the ACF900 webs without CB is 4.61 U cm on
average, while for ACF/CB900 webs with CB, it is only 1.65 U cm.
This significant reduction in the electrical resistance of ACF/CB900
webs is obviously due to the embedded CB in the ACF. Because of
this, the electrical conductivity of ACF/CB900 webs increases by 3
times in comparison to the pristine ACF900 webs without any CB.
This is beneficial for the formation of an electrical double-layer for
charge storage that finally leads to a good capacitive deionization
performance. 22,32
Fig. 2a shows the N 2 adsorption–desorption isotherms of ACF900
and ACF/CB900 webs. According to the IUPAC classification, the
adsorption isotherms of the ACF900 webs are typical type I, indicative
of the dominance of micropores in the porous structure. For the
ACF900 web, its BET surface area is 656 m 2 g 1 (Fig. 2a inset) and its
average pore diameter is 2.3 nm. While for the ACF/CB900 web with
a hysteresis loop at a relative pressure P/P 0 ¼ 0.4–1.0, its surface area
is 428 m 2 g 1 and its average pore diameter is 3.3 nm. The nitrogen
adsorption curves feature typical combined characteristics of type I/II
behaviours due to the capillary condensation in the mesopores and
macropores. 33 The PSDs of ACF900 and ACF/CB900 webs calculated
by dislocalized density function theory (DLDFT) method are
shown in Fig. 2b. Obviously, in comparison with the pristine ACF
with a mesopore ratio of 8%, the ACF/CB900 has a higher mesopore
ratio of 40%, implying that some micropores have merged and
transformed into mesopores with a pore size smaller than 5 nm. For
this phenomenon, the possible reason is below. 34 Firstly, CB used in
the present work has a higher mesopore and macropore volume.
Secondly, more new mesopores are created between the CB and the
PAN matrix due to phase separation and poor contacts, and thirdly,
the incorporation of CB within the ACF leads to some larger pores
due to the difference in shrinkage between the PAN and CB in the
CO 2 burn-off step. The macropores in the ACF/CB900 sample are
actually a combined contribution both from the pores in CB and
from the interconnected open channels between the nanofibers
interconnected in the 3D architecture. Because of these combined
effects, both the mesopore–macropore ratio and the average pore size
of ACF/CB900 webs are higher than the pristine ACF webs, as can
be seen in Fig. 2b inset. The results have evidenced that the
Fig. 2 (a) Nitrogen adsorption–desorption isotherms, inset is the BET,
mesopore volume and mesopore ratio of the corresponding samples; (b)
the micropore and meso-macro-pore size distributions (inset) of ACF900
and ACF/CB900 webs.
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incorporation of CB in the electrospinning process can result in a
well-developed hierarchical pore structure that leads to lower resistance
and shorter diffusion pathways of species in the CDI process.
The as-made webs were tested as the CDI electrode in a batch-type
electrosorptive setup operated at 1.6 V. The CDI cell was constructed
by directly attaching the webs to a graphite current collector. The
CDI cell was first washed with deionized water until the solution
conductivity reached the pure water level. Fig. 3a shows the
adsorptive capacity of electrodes made of ACF/CB900 web and
activated carbon powder (AC). It can be seen that for the ACF/
CB900 electrode, the desalination capacity is much higher than the
AC electrode, and increases more drastically in the initial stage, and
after 60 min, it gradually increases until equilibrium is reached. The
web-like ACF electrode has more benefits than the electrode made of
traditional powder-like AC that must be mixed and combined with a
polymer binder, and the binder would block some of the pores in the
activated carbon material thus increasing the internal resistance,
which subsequently results in a lower adsorption capacity. 12,35,36
To clarify the effect of CB on the desalination of the as-made ACF
electrodes, the desalination capacities of the ACF and the ACF/CB
web electrodes activated at different temperatures are compared, as
shown in Fig. 3b. The ACF/CB electrode shows a much higher
electrosorption capacity than the ACF electrode without CB. For the
ACFs activated at 700 C, the electrosorption capacity of the ACF/
CB electrode is 1.99 mg g 1 , while in the case of the ACF electrode, it
is only 0.92 mg g 1 . The electrosorption capacity of the electrode with
CB activated at 700 C is 116% higher than the ACF electrode
without CB. As the activation temperature increases from 700 Cto
Fig. 4 Electrosorption and regeneration cycles of the ACF/CB900
electrode (left), and applied voltage curves vs. time (right), for which the
test conditions are V NaCl ¼ 50 mL, C 0 ¼ 90 mg L 1 , and M ¼ 0.224 g.
900 C, the electrosorption capacity for all of the electrodes increases
continuously. An electrosorption capacity of 9.13 mg g 1 is achieved
for the ACF/CB900 electrode, which is 90% higher than the electrosorption
capacity of the ACF900 electrode, which is only 4.8 mg
g 1 . In general, the electrosorption capacity of the ACF/CB electrodes
increases by 84–116% in comparison to the ACF electrode
without CB, depending on the activation temperature. The excellent
CDI performance of the ACF/CB electrodes can be attributed to the
hierarchical pore size distribution resulting from the incorporation
and embedment of CB in the electrospinning process.
One of the parameters of key concern for CDI cells is their lifelong
performance. 37 To evaluate the reversibility of the electrosorption
capacity of the CDI cells, tests for cyclic operation and regeneration
were performed, in which the solution with an initial conductivity of
197 mS cm 1 was used. It should be noted that the reversed voltage
of 1.6 V was only applied for 5 min at the beginning of the
regeneration cycle, and the flow rate was increased from 5 mL min 1
to 60 mL min 1 , and this shortens the duration of the ion-release step
from the electrode. However, according to ref. 9, the duration of the
regeneration step is generally equal to the duration of the electrosorption
step if the reversed applied voltage is kept at a constant value
of 0 V. In our case, the adsorption time was 150 min while the
regeneration time used was 100 min, and this is more advantageous in
practice. Fig. 4 shows the variation of the solution conductivity over 4
cycles of adsorption and regeneration, which has evidenced that the
reversibility of the CDI cells is excellent, i.e. the high performance
electrosorption process can be repeated satisfactorily for the CDI cells
Fig. 3 (a) Electrosorption capacities of ACF/CB900 web and AC electrodes
as a function of time in CDI; and (b) the desalination capacity of
ACF and ACF/CB web electrodes activated at different temperatures, for
which the test conditions are V cell ¼ 1.6 V, V NaCl ¼ 50 mL, and C 0 ¼
90 mg L 1 .
Fig. 5
Schematic of the CDI set-up.
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in our work. This has been repeated over 20 times and no decline in
the electrosorption capacity is observed.
Conclusions
In summary, we have developed a simple yet effective strategy to
make free-standing ACF electrodes with well-developed hierarchical
porous structure that show excellent performance in CDI water
desalination. The micro-meso-macroporous structures of the asmade
ACFs can be tuned by incorporating mesoporous carbon
blacks with good electric conductivity in the electrospinning process
followed by CO 2 activation. The ACF web electrodes with a balanced
hierarchical structure help overcome the mass-transport limitation
that is an intrinsic shortcoming for micropore dominated materials.
The novel materials reported here exhibit a high capacity for salt
removal, and are of potential use in CDI desalination as electrodes.
The desalination performance of the ACF web electrodes can be
further improved by optimizing the properties of CB and the process
parameters such as the weight ratio of CB to ACF in the precursor.
Experimental
Materials
The precursor polyacrylonitrile (PAN, M w ¼ 150 000) and solvent
N,N-dimethyl formamide (DMF) were obtained from Aldrich
Chemical Co (USA). Carbon black (CB, ketjen EC-600JD, Japan)
with a mesopore surface area of 393 m 2 g 1 wasusedasafillerof
electrospun activated carbon nanofiber (ACF). For comparison,
powder-like coconut shell-based activated carbon (AC) was used as
an electrode material, of which the BET surface area was 917 m 2 g 1
and the average pore diameter was 2.2 nm.
Preparation method
The PAN-based fibers were prepared by electrospinning using a
10 wt% PAN solution in DMF, and the CB-embedded PAN-based
fibers were prepared by electrospinning using a composite solution
made of 5 wt% CB in 10 wt% PAN solution in DMF. The solution
was sonicated for 2 h to ensure that the CB was dispersed uniformly
before being mixed with the PAN. The two kinds of polymer solutions
with/without CB were electrospun from a syringe tip onto a
rotating metal drum wrapped in aluminium foil. Below are the
conditions for electrospinning: a feeding rate of 1.0 mL h 1 polymer
solution, a supplied voltage of 22 kV, a tip to drum distance of 15 cm,
and a drum rotation rate of 300 rpm. The electrospun fiber webs were
heated at 1 Cmin 1 and stabilized at 280 Cinairfor2h.The
stabilized fibers were activated in flowing CO 2 for 0.5 h at 700–900
C, resulting in ACF webs denoted as ACFX and ACF/CBX (CB
containing ACF), where X stands for the activation temperature.
Measurements
The morphology and structures of the samples were examined using
scanning electron microscopy (SEM, Hitachi S-4700, Japan) and
transmission electron microscopy (TEM, FEI Tecnai G20, USA).
The specific surface area was measured by nitrogen adsorption
(ASAP2020, Micromeritics, USA). The electrical resistance was
measured 5 times with a four-probe method at room temperature, of
which the values were averaged.
Schematic of the CDI set-up
Activated carbon electrode was fabricated using direct coating
method. 38 A carbon slurry was prepared as a suspension of activated
carbon powder, acetylene black and poly(vinylidene fluoride) in dimethylacetamide
in a weight ratio of 75 : 10 : 15. The slurry mixture
was stirred for 6 h before being cast onto a graphite sheet to make a
300 mm thick coating. The coated electrode was dried at 80 Cunder
vacuum for 4 h to get rid of the organic solvent.
The webs were used as CDI electrodes and directly attached to the
graphite current collectors to make a CDI cell. The adsorption
removal efficiency of ions on the web electrodes with a weight of ca.
0.2 g was measured at 25 C in a flow-through setup, as shown in
Fig. 5. For each run, the solution was continuously pumped into the
cell at a rate of 5 mL min 1 using a peristaltic pump (Loner BT100,
China). During the measurement, the potential difference between
the two electrodes was kept at a constant voltage of V cell ¼ 1.6 V
using a programmable DC power supply (PST-3202, Gwinstek,
Taiwan). All experiments were conducted with a C 0 ¼ 90 mg L 1
(197 mScm 1 ) NaCl solution. The variation of NaCl concentration in
the solution was continuously monitored using an ion conductivity
meter (ET915, eDAQ TECH, Australia). In the regeneration process,
the flow rate was kept at 60 mL min 1 . After applying a reverse
voltage of 1.6Vfor5min,itwasswitchedto0Vuntiltheregeneration
process ended.
The electrosorptive capacity (Q t ) is defined as below:
Q t ¼ ðC 0 C t ÞV NaCl
(1)
M
where C 0 (mg L 1 ) is the initial concentration of NaCl, C t is the
instantaneous concentration of NaCl measured at time t; V NaCl is
the solution volume (mL); and M is the mass of the ACF web
electrodes (g).
Acknowledgements
We thank Prof. Yury Gogotsi at Drexel University, USA, for useful
discussion. This work was partly supported by the Dalian Science
and Technology Bureau of China (no. 2010A17GX095), the
Research Start-up Fund in DUT (no. DUT12RC(3)04), and the
NSFC (nos. 20836002, 51102033).
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