Vacancy-mediated dehydrogenation of sodium alanate.

Clarification of the mechanisms of hydrogen release and uptake in transition-metal-doped sodium alanate, NaAlH(4), 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, H(2) 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 AlH(3) vacancies is Q = 85 kJ/mol.H(2), which is in excellent agreement with experimentally measured activation energies in Ti-catalyzed NaAlH(4). The activation energy for an alternate decomposition mechanism via NaH vacancies is found to be significantly higher: Q = 112 kJ/mol.H(2). Our results suggest that bulk diffusion of Al species is the rate-limiting step in the dehydrogenation of Ti-doped samples of NaAlH(4) and that the much higher activation energies measured for uncatalyzed samples are controlled by other processes, such as breaking up of AlH(4)(-) complexes, formation/dissociation of H(2) molecules, and/or nucleation of the product phases.