Effects of transition metal oxide and Ni addition on the hydrogen-storage properties of Mg

Mg-based hydrogen-storage materials with the compositions of Mg–10 wt%oxide (oxide = Cr₂O₃, Fe₂O₃, MnO, and SiO₂) and Mg–xFe₂O₃–yNi were prepared by reactive mechanical grinding (RMG). Taking into consideration the hydriding and dehydriding rates and the cost of materials, Fe₂O₃ prepared by spray conversion is an appropriate oxide additive to Mg. Mg–5 wt%Fe₂O₃–15 wt%Ni exhibited the best hydrogen-storage performance among the Mg–xFe₂O₃–yNi hydrogen materials. It stored 5.47 wt%H under 1.2 MPa H₂ for 60 min and released 5.42 wt%H under 0.1 MPa H₂ for 15 min at 593 K. The addition of Fe₂O₃ and Ni to Mg by the RMG shortens the diffusion distances through the reduction of the particle size of Mg. These additives are also considered to facilitate nucleation by creating many defects on the surface and in the interior of Mg. The added Fe₂O₃ and Ni themselves may also act as active sites for the nucleation. Ni forms the Mg₂Ni phase by a reaction with Mg, and Fe appears from the reduction of Fe₂O₃ by hydrogen after hydriding–dehydriding cycling.