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Scientific Report 2007-2009<br />
Condensed matter physics and biophysics<br />
C39. Nanomaterials for alternative energies. Solid-state hydrogen<br />
storage<br />
Hydrogen is attracting renewed interest as an energy<br />
carrier, due to the necessity of finding ecological energy<br />
media which may decrease the environmental pollution<br />
from fossil fuels. Hydrogen storage represents a nodal<br />
point for the development of a hydrogen economy. Of<br />
the three possible ways to store hydrogen, i.e. as high<br />
pressure gas, as a liquid (≃20 K at atmospheric pressure),<br />
or as hydrides in solids, the latter one appears<br />
as the most promising, due to the high mass and volume<br />
density and safety. The development of hydrogen<br />
storage media is currently considered as the most technologically<br />
challenging way for achieving a hydrogen-based<br />
economy. There are many compounds with promising<br />
hydrogen absorption capacities which, however, display<br />
serious drawbacks, like lack of reversibility, slow kinetics<br />
or deterioration with proceeding cycling. There is currently<br />
general agreement that adopting only traditional<br />
materials and methods will not lead to the achievement<br />
of the strict requirements necessary for the solid state<br />
hydrogen storage. At present, a considerable part of the<br />
international research efforts is devoted to the study of<br />
solid state hydrogen storage materials finely dispersed on<br />
artificial nanoporous supports. This approach is considered<br />
promising, since the use of thin layers of hydrogen<br />
absorbing compounds is expected to avoid sample compacting<br />
and to increase the surface to volume ratio to<br />
very high values. However the real interest in the dispersion<br />
of nanoparticles into nanoscaffolds resides in the<br />
possibility of obtaining a compound with better storage<br />
properties than the starting bulk material.<br />
of the most promising compounds, ammonia borane<br />
(NH 3 BH 3 ), finely dispersed in the channels of mesoporous<br />
silica does not undergo the structural phase transition<br />
present in the bulk, thus providing a clear indication<br />
that the basic physical properties of this material<br />
are strongly modified by such assembling on a nanoscale.<br />
The occurrence of different electronic and lattice interactions<br />
in nanostructured scaffolded hydrogen storage systems<br />
could provide an alternative approach to modify<br />
the thermodynamic features of bulk materials to obtain<br />
enhanced dehydrogenation properties and to accomplish<br />
reversibility. Finally anelastic spectroscopy has been<br />
proven to be a powerful tool, complementary to NMR<br />
and neutron scattering, in the study of the hydrogen dynamics.<br />
This information is highly demanded, because<br />
the hydrogenation/dehydrogenation of storage materials<br />
resides on the possibility for hydrogen atoms to perform<br />
short or long range diffusion processes. Anelastic spectroscopy<br />
investigations conducted in alanates allowed us<br />
to propose a model for the dehydrogenation process.<br />
The extension of the studies in ammonia borane lead<br />
us to identify the rotational and torsional dynamics of H<br />
atoms and to derive their activation energies [2]. Moreover,<br />
we used the measurements of the dynamic elastic<br />
modulus, which is very sensitive to the occurrence of<br />
phase transformations, to characterize the nature and<br />
the kinetics of the phase transition in NH 3 BH 3 [3].<br />
E/E<br />
0 . 3<br />
0 . 2<br />
0 . 1<br />
0 . 0<br />
3<br />
1<br />
1 M C M - 4 1<br />
2 N H 3 B H 3 b u l k p o w d e r<br />
3 N H 3 B H 3 : M C M - 4 1 ( 1 : 2 )<br />
heat exchange (W/g)<br />
- 0 . 1<br />
- 0 . 2<br />
1 . 2<br />
0 . 8<br />
0 . 4<br />
0 . 0<br />
- 0 . 4<br />
- 0 . 8<br />
1<br />
2<br />
- 1 . 2<br />
1 8 0 2 0 0 2 2 0 2 4 0 2 6 0 2 8 0<br />
T ( K )<br />
2<br />
3<br />
Figure 1: Effect on the Young modulus of the confinement<br />
of ammonia borane into mesoporous silica, MCM-41 [2].<br />
An important task of the research conducted in our<br />
Laboratory is the understanding of the basic mechanisms<br />
of the hydrogenation/dehydrogenation process and the<br />
changes induced by the nanoconfinement. A combined<br />
study performed by anelastic spectroscopy and differential<br />
scanning calorimetry allowed us to show that one<br />
Figure 2: Schematic representation of the rotational and<br />
torsional dynamics of the NH 3 BH 3 molecule with the corresponding<br />
energy profile [3].<br />
References<br />
1. O. Palumbo et al., Int. J. Hydr. En. 33, 3107 (2008).<br />
2. A. Paolone et al., J. Phys. Chem C 113, 5872 (2009).<br />
3. A. Paolone et al., J. Phys. Chem C 113, 10319 (2009).<br />
Authors<br />
R. Cantelli, A. Paolone 3 , O. Palumbo 3 , P. Rispoli<br />
<strong>Sapienza</strong> Università di Roma 92 Dipartimento di Fisica