Vacancy-mediated dehydrogenation of sodium alanate

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.