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

Point Defect Dynamics in Sodium Aluminum Hydrides - a Combined Quasielastic Neutron Scattering and Density Functional Theory Study. Q.Shi, J.Voss, H.S.Jacobsen, K.Lefmann, M.Zamponi, T.Vegge: Journal of Alloys and Compounds, 2007, 446-447, 469-73