Predicting impurity gases and phases during hydrogen evolution from complex metal hydrides using free energy minimization enabled by first-principles calculations

First-principles calculations represent a potent tool for screening metal hydride mixtures that can reversibly store hydrogen. A number of promising new hydride systems with high hydrogen capacity and favorable thermodynamics have been predicted this way. An important limitation of these studies, however, is the assumption that H2 is the only gas-phase product of the reaction, which is not always the case. This paper summarizes new theoretical and numerical approaches that can be used to predict thermodynamic equilibria in complex metal hydride systems with competing reaction pathways. We report thermochemical equilibrium calculations using data obtained from density functional theory (DFT) computations to describe the possible occurrence of gas-phase products other than H2 in three complex hydrides, LiNH2, LiBH4, and Mg(BH4)2, and mixtures of these with the destabilizing compounds LiH, MgH2, and C. The systems under investigation contain N, C, and/or B and thus have the potential to evolve N2, NH3, hydrocarbons, and/or boranes as well as H2. Equilibria as a function of both temperature and total pressure are predicted. The results indicate that significant amounts of these species can form under some conditions. In particular, the thermodynamic model predicts formation of N2 and NH3 as products of LiNH2 decomposition. Comparison with published experimental data indicates that N2 formation must be kinetically limited. Our examination of C-containing systems indicates that methane is the stable gas-phase species at low temperatures, not H2. On the other hand, very low amounts of boranes (primarily BH3) are predicted to form in B-containing systems.