Mechanism of improving hydrogenation of Mg by in-situ formation of Al* in hydriding combustion synthesis

Abstract

Magnesium hydride (MgH2) is an ideal solid hydrogen storage material and on-line hydrolysis hydrogen source. However, the synthesis of MgH2 usually requires harsh conditions of high temperature, high pressure and long duration, bringing great challenges to production cost and safety. In this paper, MgH2 is prepared by hydriding combustion synthesis (HCS) and the mechanism of improving Mg hydrogenation properties by the in-situ formation of Al* is revealed. Under low hydrogen pressure of 1 MPa, the hydrogenation degree of Mg can be greatly increased from 9.6% of pure Mg to 90.5% with 10 mol% Al added. In the activation stage, Al reacts with Mg to form Mg17Al12, which brings rich grain boundaries and provides rapid diffusion channels for hydrogen. Subsequently, in the hydrogenation stage, Mg17Al12 reacts with H2 to generate Al* in-situ, which are evenly distributed and act as nucleation sites for MgH2. Hydrogen molecules diffuse rapidly along the interface between Mg/MgH2 and Al* and are easily adsorbed and dissociated on the surface of nano-scale Al* . The unique disproportionation hydrogenation reaction of in-situ forming highly active metallic substances as excellent catalysts provides a new idea for the modification of Mg-based hydrogen storage materials.