Hydrogen Storage For Fuel Cells - A Density Functional Theory
Study of Hydrogen Adsorption on Aluminum Clusters
Researchers from Accelrys have carried out a study of hydrogen
adsorption by Aluminum clusters, a promising candidate for fuel
cell hydrogen storage devices.
The simulations revealed that a lot of H atoms can be adsorbed
easily on the surface of the Al clusters.
These findings are important because light metals such as Aluminum
are inexpensive and can store large amounts of hydrogen - factors
that are crucial in fuel cell design.
Hydrogen storage is a crucial issue in fuel cell design and application
- a safe, compact, and inexpensive system that is capable of storing
a relatively large amount of hydrogen is highly desirable. Many
light metals, e.g. magnesium and Aluminum, show promise for hydrogen
storage devices owing to their potential high storage densities
and safe endothermic hydrogen release.
However, the experimental study of the structure of metal clusters
is problematic due to high reactivity. It is important to understand
cluster size and hydrogen-site specific effects in order to provide
a contribution to a unified description of the bridging between
the properties of clusters, nanowires, and cluster crystals.
In this study, researchers from Accelrys,1 using MS
simulated the interaction of both hydrogen atoms and hydrogen molecules
with the surface of an Al13 cluster. Optimized and and
transition state (TS) structures were obtained for Al13Hn(n = 1-12) clusters.
According to jellium model, the Al13- anion,
having a complete outer shell of electrons, should be more stable
than its neutral counterpart, Al13. The simulations revealed
this to be the case - the anion being 2.75eV lower in energy than
the neutral ground sate. It was also found that the Al132-
anion is also more stable than the neutral one. This finding shows
that the Al13 cluster can favor, from an electronic point
of view, the surface adsorption of more than one hydrogen atom.
The optimized structures of three different isomers of the Al13H
cluster with the hydrogen atom being adsorped in different positions
on the cluster surface were obtained and studied. The transition
states were also obtained.
Structures with two or more hydrogen atoms adsorped on the cluster
surface were also investigated.
It was found that:
Hydrogen can be adsorped on the Al13 cluster surface
without crossing a potential barrier. However, the binding energy
was not high enough to dissociate the hydrogen molecule (the hydrogen
molecule being repelled by the cluster surface)
The Al13H cluster has three different stable minima
with the hydrogen atom being attached to one Al atom, being displaced
between two Al atoms and being displaced between three Al atoms.
The most stable isomer is with hydrogen atom attached to one Al
atom. The most stable isomer leads to significant distortions
of the cluster.
With the increase of the number of hydrogen atoms adsorped on
the Al13 cluster surface the distortion is reduced
and the binding energy with small fluctuations does not change.
Al clusters of different sizes, Al nanowires, and Al cluster crystals
can be considered as potential materials for hydrogen storage devices.2,3
A. Goldberg, M. Mori, and A. Bick, Al Clusters as a Tool for
Hydrogen Storage, Proceedings of the Fourth International Conference
on Intelligent Processing and Manufacturing of Materials, IPMM'03,
ed. John A. Meech, Yoshiyuki Kawazoe, John F. Maguire, Vijay Kumar,
and Haiping Wang, May 18-23, 2003, Sendai Japan.
R. Ahlrichs and S. D. Elliott, Clusters of Aluminum, a Density
Functional Study, Phys. Chem. Chem. Phys., 1999, 1,
J. A. Alonso, M. J. Lopez, L. M. Molina, F. Duque, and A. Mananes,
Conditions for the Self-assembling of Cluster Materials, Nanotechnology,
2002, 13, 253 257; Y. Kawakami, T. Kikura, K. Doi,
K. Nakamura, and A. Tachibana, First-Principles on the Reactions
and Dynamical Electronic Characteristics of Electromigration on
Aluminum Nanowires, to be published in the Proceedings of the
International Conference on Processing and Manufacturing of Advanced
Materials. Processing, Fabrication, Properties, Applications,
July 7-11, 2003. Leganes, Madrid, Spain.