Theory of mass transport in sodium alanate

Abstract

Sodium alanate, NaAlH₄, is a well-known hydrogen storage material that decomposes into Na₃AlH₆ and Al while releasing H₂ as a gas. While the thermodynamics of this reaction are ideal for applications in fuel cell vehicles, the reaction rates are prohibitively slow unless the material is doped with transition metals (such as titanium) or rare earths (such as cerium). It has been widely theorized that the flux of point defects through the bulk phases provides the mechanism for long-range metal transport which accompanies the hydrogen release and absorption reactions. In this paper, a quantitative model is introduced to describe mass transport using point defect energies obtained from first-principles density-functional theory (DFT) calculations. It is found that negatively charged sodium vacancies in Na₃AlH₆ have the largest flux of all metal-site defects in any of the phases, at all temperatures examined. Positively charged hydrogen vacancies are predicted to balance the charge of sodium vacancies and have a higher diffusivity than this metal defect. The activation energy for the formation and diffusion of sodium vacancies in Na₃AlH₆ is found to be equal to 50 kJ mol⁻¹ for rehydrogenation and 70 kJ mol⁻¹ for dehydrogenation, in good agreement with experimental values. It is argued that diffusion of sodium vacancies in Na₃AlH₆ represents the rate-limiting process in the dehydrogenation of Ti-doped NaAlH₄ and that Ti must catalyze some other process (or processes) than bulk mass transport.