Thermodynamic destabilization and reaction kinetics in light metal hydride systems

Experimental approaches for altering the thermodynamics and kinetics of light element hydride systems are discussed. Equilibrium hydrogen pressures and reaction enthalpies can be varied with additives that form new alloy or compound phases upon dehydrogenation. The formation of new phases lowers the dehydrogenated state enthalpy and effectively destabilizes the component hydrides. This strategy is illustrated for LiBH4 destabilized by MgH2, MgF2, MgS, and MgSe. The slow rates of hydrogen exchange in light element hydrides can be improved with catalysts and by reducing diffusion distances to the nanometer scale. The catalytic effects of a variety of transition metal sources on hydrogen exchange in the LiBH4/MgH2 system are described. The effects of reduced diffusion distances are illustrated using LiBH4 incorporated into a nanoporous carbon aerogel.