Multi-hydride systems with enhanced hydrogen storage properties derived from Mg(BH4)2 and LiAlH4

Abstract

A two-step ball-milling method has been provided to synthesize Mg(BH4)2 using NaBH4 and MgCl2 as starting materials. The method offers high yield and high purity (96%) of the compound. The as-synthesized Mg(BH4)2 is then combined with LiAlH4 by ball-milling in order to form new multi-hydride systems with high hydrogen storage properties. The structure, the dehydrogenation and the reversibility of the combined systems are studied. Analyses show that a metathesis reaction takes place between Mg(BH4)2 and LiAlH4 during milling, forming Mg(AlH4)2 and LiBH4. Mg(BH4)2 is excessive and remains in the ball-milled product when the molar ratio of Mg(BH4)2 to LiAlH4 is over 0.5. The onset dehydrogenation temperature of the combined systems is lowered to ca. 120 °C, which is much lower than that of either Mg(BH4)2 or LiAlH4. The dehydrogenation capacities of the combined systems below 300 °C are all higher than that of both Mg(BH4)2 and LiAlH4. The combined systems are reversible for hydrogen storage at moderate hydrogenation condition, and rapid hydrogenation occurred within the initial 30 min. Moreover, the remained Mg(BH4)2 in the combined systems is found also partially reversible. The mechanism of the enhancement of the hydrogen storage properties and the dehydrogenation/hydrogenation process of the combined systems were discussed.