The effect of transition metals on hydrogen migration and catalysis in cast Mg-Ni alloys

摘要

The inexpensive fabrication technique of casting is applied to develop new Mg-Ni based hydrogen storage alloys with improved hydrogen sorption properties. A nanostructured eutectic Mg-Mg_2Ni is formed upon solidification which introduces a large area of interfaces along which hydrogen diffusion can occur with high diffusivity. After a few cycles of hydro-genation and dehydrogenation, an ultrafine porous structure formed in the eutectic Mg-Mg_2Ni and some cracks developed along the interface between the eutectic and the α-Mg matrix. This indicates that hydrogen atoms introduced into the alloys preferentially migrate along the interfaces in the nanostructured eutectic which enables effective short-range diffusion of hydrogen. Furthermore, transition metals (TMs) such as Nb, Ti and V in the range 240-560 ppm are added directly to molten Mg-10 wt% Ni alloys and are found to form inter-metallic compounds with Ni during solidification. The alloys can store 5.6-6.3 wt% hydrogen at 350 ℃ and 2 Mpa. TM-rich intermetallics distributed homogeneously in the cast alloys appear to play a key role in accelerating the nucleation of Mg from MgH_2 upon dehydrogenation. This leads to a significant improvement in the hydrogen desorption kinetics.