Improving Cyclability of Metal Borohydrides for Hydrogen Storage via Non-Metallic Borohydride Additives.

The theoretical ~15 wt% hydrogen content of magnesium borohydride is inaccessible for cyclical desorption/hydrogenation cycles at reasonable temperatures (<300°C) and pressures (<350 bar). However, a variety of additive compounds lower the Mg(BH₄)₂ hydrogen evolution temperature, changing the end state when hydrogen is released, and lowering the melting point of the system. These additives have included other metal borohydrides, metal hydrides, and organics (ether/glyme). Despite many of these additives lowering the liquidus temperature, desirable due to better mixing and higher theoretical kinetics in the melt, these systems often resulted in undesirable matrices upon re-cooling and cease to thermally cycle as expected. This work investigated an alternative family of additives, non-metallic borohydrides; a broad class of low melting point borohydrides. The temperature and hydrogen-evolution dependent phase diagram of these magnesium borohydride and non-metallic borohydride systems was investigated via calorimetry and in-situ diffraction analysis. Results of heat flow and temperature programmed desorption measurements demonstrate the improved thermal cyclability of these systems. The effect of non-metallic cation size on the behavior of these systems will be discussed in detail.