Modeling alkali alanates for hydrogen storage by density-functional band-structure calculations
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Ole Swang SINTEF Materials and Chemistry, N-0314 Oslo, Norway
Susanne M. Opalka United Technologies Research Center, East Hartford, Connecticut 06018 (Received 14 April 2005; accepted 10 June 2005)
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.
I. INTRODUCTION
The search for solid-state hydrogen-storage materials is a crucial part of the path toward implementation of a hydrogen economy.1–7 Alkali alanates [complex hydrides with the formula MnAlH(n+3), where M is an alkali element: Li, Na, K, . . .] are among the most promising materials for this application, with high hydrogen density available at moderate conditions.8–13 The fully hydrogenated alkali alanate phase, the monoalkali aluminium tetrahydride, MAlH4 desorbs hydrogen stepwise through a series of reactions 3MAlH4 → M3AlH6 + 2 Al + 3 H2 , M3AlH6 → 3MH + Al + 3/2 H2 3MH → 3M + 3/2 H2
,
,
(1) (2) (3)
where M is Li, Na, or K. In the first reaction, the MAlH4 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0397 J. Mater. Res., Vol. 20, No. 12, Dec 2005
phase disproportionates to form the trialkali aluminium hexahydride, M3AlH6 phase, which during the second reaction disproportionates into the monoalkali hydride, MH. The last MH decomposition reaction takes place at temperatures too high to allow it to be counted as accessible for in situ mobile applications. The total and accessible hydrogen content of the alkali alanates are listed in Table I. A. Lithium alanate
Since LiAlH4 is the lightest alanate, it is the most attractive candidate from a gravimetric point of view. The reversible hydrogenation of lithium alanate has been pursued by several groups,14–35 and it is now clear that, for lithium alanate, the decomposition of
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