(347a) Investigation of Phase Stability of Magnesium Alanate for Hydrogen Storage From First Principles

Complex hydrides including alanates ([AlH₄]^–) have recently gained attention as alternative hydrogen storage materials. Many of these materials have been known to release hydrogen upon contact with water; however, the hydrolysis reactions are highly irreversible, a process known as “one-pass” hydrogen storage. Nanostructuring and nanocatalysis have been accepted as promising methods to overcome the irreversible hydrogenation process. Thus, predicting which phases may be more stable as a function of nanoparticle size may contribute to nanostructuring complex hydrides for hydrogen storage applications. We have employed density functional theory (DFT) using the projector-augmented wave (PAW) method within the generalized gradient approximation (GGA) to calculate relatively smaller nanoparticles of magnesium alanate (Mg(AlH₄)₂) ranging from 1 to 2 nm. Based upon these results, cluster expansion and Monte Carlo simulation methods were developed to predict the phase stabilities of 2-10 nm Mg(AlH₄)₂ nanoparticles. Our calculations provide phase stability diagrams of Mg(AlH₄)₂ nanoparticles as a function of particle size and temperature. This study may help identify how the relative stability of different compounds (Mg(AlH₄)₂, MgH₂, Al, and H₂) evolves as a function of nanoparticle size and temperature, which will facilitate experimental studies to determine the thermodynamically favored reaction pathways for the hydrogenation/dehydrogenation processes of Mg(AlH₄)₂.