Vacancy-mediated dehydrogenation of sodium alanate

*Department of Chemistry and Biochemistry and
^†California NanoSystems Institute, University of California, Los Angeles, CA 90095; and
^‡Department of Materials Science and Engineering, University of California, Los Angeles, CA 90095-1595

Edited by Peter Edwards, University of Oxford, Oxford, United Kingdom, and accepted by the Editorial Board December 18, 2007 (received for review September 27, 2007)

Abstract

Clarification of the mechanisms of hydrogen release and uptake in transition-metal-doped sodium alanate, NaAlH₄, a prototypical high-density complex hydride, has fundamental importance for the development of improved hydrogen-storage materials. In this and most other modern hydrogen-storage materials, H₂ release and uptake are accompanied by long-range diffusion of metal species. Using first-principles density-functional theory calculations, we have determined that the activation energy for Al mass transport via AlH₃ vacancies is Q = 85 kJ/mol·H₂, which is in excellent agreement with experimentally measured activation energies in Ti-catalyzed NaAlH₄. The activation energy for an alternate decomposition mechanism via NaH vacancies is found to be significantly higher: Q = 112 kJ/mol·H₂. Our results suggest that bulk diffusion of Al species is the rate-limiting step in the dehydrogenation of Ti-doped samples of NaAlH₄ and that the much higher activation energies measured for uncatalyzed samples are controlled by other processes, such as breaking up of AlH₄ ⁻ complexes, formation/dissociation of H₂ molecules, and/or nucleation of the product phases.

Footnotes

^§To whom correspondence should be addressed. E-mail: vidvuds{at}ucla.edu
Author contributions: V.O. designed research; and H.G., K.N.H., and V.O. performed research.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. P.E. is a guest editor invited by the Editorial Board.
¶ Eq. 5.1, formation of AlH₃ vacancies; Eq. 5.2, formation of hexahydride from AlH₃ vacant structure; Eq. 5.3, formation of NaH vacancies; Eq. 5.4, formation of hexahydride from NaH vacant structure.