Feasibility study of the direct synthesis of Mg(BH 4) 2 complex hydrides by mechanical milling

Abstract

Magnesium borohydride [Mg(BH 4) 2] is a promising complex hydride for hydrogen storage. This study reports a feasibility study of the synthesis of Mg(BH 4) 2 by planetary ball milling of pure Mg–B powder and then a hydrogenation process under a hydrogen atmosphere. The ball milling parameters were firstly optimized. The Mg–B powders with the smallest average particle size of 1.91 ± 1.61 μm and the smallest average crystallite size of 9.4 ± 1.1 nm were used. However, no successful synthesis of Mg(BH 4) 2 was achieved. The obtained nanocrystalline hydride was MgH 2. It was found that the addition of B into magnesium strongly enhanced the kinetic properties of magnesium hydrogenation. The reduction of crystallite size showed a more obvious effect on improving the absorption kinetic properties than that of the particle size. The powder of the smallest crystallite size after hydrogenation comprised a minute amount of B–H compounds. The formation of Mg(BH 4) 2 from pure elements was difficult because of the barriers against breaking the B–B atomic bonds and the migration of the B atoms.