DOE Hydrogen and Fuel Cells Program FY 2012 Annual Progress Report

Our studies comprise two classes of materials: metal hydrides and complex hydrides. Metal hydrides can store large amounts of hydrogen, but due to the high atomic mass of the host element(s) the weight-percent efficiency is typically low. We are focusing on materials in which the atomic mass of the metal is low, such as MgH2 and AlH3. Comprehensive studies of point defects and migration enable us to identify the dominant diffusion mechanisms. We are also performing Kinetic Monte Carlo simulations of the dehydrogenation process. For complex hydrides, a major result of our studies is that the point defects that are relevant for transport are all charged. Their formation energy (and hence the kinetics of diffusion and decomposition) thus depends on the electron chemical potential, which in turn is affected by the presence of additives. This explains, for instance, the effect of transition metal impurities on the kinetics. Our recent work has focused on LiBH4, LiAlH4, and Li2NH/LiNH2. For the latter, we have been able to explain the particle-size dependence of the activation energy for decomposition.