Point defect dynamics in sodium aluminum hydrides—A combined quasielastic neutron scattering and density functional theory study

Understanding the catalytic role of titanium-based additives on the reversible hydrogenation of complex metal hydrides is an essential step towards developing hydrogen storage materials for the transport sector. Improved bulk diffusion of hydrogen is one of the proposed catalytic effects, and here we study hydrogen dynamics in undoped and TiCl₃-doped samples of NaAlH₄ and Na₃AlH₆ using a combination of density functional theory calculations and quasielastic neutron scattering. Hydrogen dynamics is found to be limited and mediated by hydrogen vacancies in both alanate phases, requiring an activation in excess of 0.3 eV. Even at 390 K, less than 1% of the hydrogen in NaAlH₄ performs long range diffusion, and only localized hydrogen dynamics is observed in Na₃AlH₆. The effect of the TiCl₃ dopant on hydrogen bulk diffusion is found to be negligible.

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