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Modeling alkali alanates for hydrogen storage by density-functional band-structure calculations

Published online by Cambridge University Press:  01 December 2005

Ole Martin Løvvik ,
Ole Swang  and
Susanne M. Opalka
Ole Martin Løvvik*
Affiliation:
University of Oslo, Centre for Materials Science and Nanotechnology, 0318 Oslo, Norway
Ole Swang
Affiliation:
SINTEF Materials and Chemistry, N-0314 Oslo, Norway
Susanne M. Opalka
Affiliation:
United Technologies Research Center, East Hartford, Connecticut 06018
*
a)Address all correspondence to this author. e-mail: o.m.lovvik@fys.uio.no
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Abstract

The alanates (complex aluminohydrides) have relatively high gravimetric hydrogen density and are among the most promising solid-state hydrogen-storage materials. In this work, the crystal structure and electronic structure of pure and mixed-alkali alanates were calculated by ground-state density-functional band-structure calculations. The results are in excellent correspondence with available experimental data. The properties of the pure alanates were compared, and the relatively high stability of the Li3AlH6 phase was pointed out as an important difference that may explain the difficulty of hydrogenating lithium alanate. The alkali alanates are nonmetallic with calculated band gaps around 5 eV and 2.5–3 eV for the tetra- and hexahydrides. The bonding was identified as ionic between the alkali cations and the aluminohydride complexes, while it is polar covalent within the complex. A broad range of hypothetical mixed-alkali alanate compounds was simulated, and four were found to be stable compared to the pure alanates and each other: LiNa2AlH6, K2LiAlH6, K2NaAlH6, and K2.5Na0.5AlH6. No mixed-alkali tetrahydrides were found to be stable, and this was explained by the local coordination within the different compounds. The only alkali alanate that seemed to be close to fulfilling the international hydrogen density targets was NaAlH4.

Keywords

Ab initio calculationElectronic structureEnergy-storage

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Type
Articles—Energy and The Environment Special Section
Information
Journal of Materials Research , Volume 20 , Issue 12 , December 2005 , pp. 3199 - 3213
Copyright
Copyright © Materials Research Society 2005

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