Synergistic Effect of LiBH4and LiAlH4Additives on Improved Hydrogen Storage Properties of Unexpected High Capacity Magnesium Hydride

A significant improvement in reversible de/rehydrogenation properties of MgH 2 doped with 2.5 wt % LiAlH 4 and 2.5 wt % LiBH 4 additives is achieved by reactive ball milling. Careful comparison studies show that a small amount (total mass ratio of ≤5 wt %) of LiBH 4 and/or LiAlH 4 can improve the hydrogen desorption performance of MgH 2 in terms of reducing hydrogen desorption temperature, enhancing hydrogen desorption kinetics, and increasing hydrogen capacity. In particular, the MgH 2 co-doped with LiAlH 4 –LiBH 4 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 MgH 2 ) to 155.8 kJ/mol. It can also uptake 7.7 wt % H 2 within 15 min at 300 °C with a stable hydrogen absorption kinetics in the first 10 cycles. This synergistic effect of LiBH 4 and LiAlH 4 additives on improved hydrogen absorption/desorption properties is attributed to the in situ formation of Li 3 Mg 7, Mg 17 Al 12, and MgAlB 4 new phases acting as the “catalytic active sites” to facilitate the diffusion of hydrogen through the reaction barriers in the de/hydrogenation cycles of MgH 2 . Moreover, these new phases can be rehydrogenated into MgH 2, LiBH 4, and Li 3 AlH 6 hydrides, which further enhance the hydrogen capacity of co-doped sample more than the theoretical capacity of primitive MgH 2 .