Formation Reactions and the Thermodynamics and Kinetics of Dehydrogenation Reaction of Mixed Alanate Na2LiAlH6

Na2LiAlH6 was synthesized by ball-milling a mixture of NaH and LiAlH4 at a molar ratio of 2:1. NaH and LiAlH4 were readily converted in the initial ball-milling process to LiH and NaAlH4, which subsequently reacted with the remaining, excessive NaH to form Na2LiAlH6. The thermodynamic and kinetic mechanisms of dehydrogenation of Na2LiAlH6 were systematically elucidated. Approximately 6.7 wt % of hydrogen was found stored reversibly in Na2LiAlH6 through a few sequential reactions. An enthalpy change of 63.8 kJ/mol-H-2 and an apparent activation energy of about 173 kJ/mol were determined for the first-step hydrogen storage reaction, indicating the reaction was thermodynamically relatively stable with a high kinetic barrier for the decomposition of Na2LiAlH6. In depth kinetic investigations showed that the first-step dehydrogenation reaction of Na2LiAlH6 could be well interpreted with a nucleation and growth model, and its reaction rate was controlled by the diffusion of substance. The dehydrogenation mechanism developed in this work can be helpful for further efforts on the improvement of the hydrogenation/dehydrogenation performances of Na2LiAlH6.