Synergistic Effect of LiBH4 and LiAlH4 Additives on Improved Hydrogen Storage Properties of Unexpected High Capacity Magnesium Hydride

A significant improvement in reversible de/rehydrogenation properties of MgH2 doped with 2.5 wt % LiAlH4 and 2.5 wt % LiBH4 additives is achieved by reactive ball milling. Careful comparison studies show that a small amount (total mass ratio of ≤5 wt %) of LiBH4 and/or LiAlH4 can improve the hydrogen desorption performance of MgH2 in terms of reducing hydrogen desorption temperature, enhancing hydrogen desorption kinetics, and increasing hydrogen capacity. In particular, the MgH2 co-doped with LiAlH4–LiBH4 exhibits the best dehydrogenation properties, which starts to release hydrogen at about 280 °C and releases a high hydrogen capacity of 7.62 wt % with superior reaction kinetics. The activation energy of hydrogen desorption is decreased from 187.8 kJ/mol (ball-milled MgH2) to 155.8 kJ/mol. It can also uptake 7.7 wt % H2 within 15 min at 300 °C with a stable hydrogen absorption kinetics in the first 10 cycles. This synergistic effect of LiBH4 and LiAlH4 additives on improved hydrogen absorption/desorption properties is attributed to the in situ formation of Li3Mg7, Mg17Al12, and MgAlB4 new phases acting as the “catalytic active sites” to facilitate the diffusion of hydrogen through the reaction barriers in the de/hydrogenation cycles of MgH2. Moreover, these new phases can be rehydrogenated into MgH2, LiBH4, and Li3AlH6 hydrides, which further enhance the hydrogen capacity of co-doped sample more than the theoretical capacity of primitive MgH2.