Development of a Hydrogen-Storage Alloy with a High Capacity of Approximately 6 wt% by Adding a Transition Metal and a Halide

A sample with a composition of 80 wt% Mg + 14 wt% Ni + 6 wt% TaF₅ (designated 80Mg + 14Ni + 6TaF₅) was prepared by reactive mechanical grinding. The hydrogenation and dehydrogenation properties of the sample were then examined and we analyzed a rate-determining step for the dehydrogenation of the activated 80Mg + 14Ni + 6TaF₅ using a spherical moving-boundary model. Two cycles of hydrogenation in 12 bar H₂ and dehydrogenation in a vacuum were required for activation. At the third cycle at 593 K, the sample absorbed 4.86 wt% H for 5 min and 5.77 wt% H for 60 min in 12 bar H₂ and released 5.04 wt% H for 30 min and 5.21 wt% H for 60 min in 1.0 bar H₂. The activated 80Mg + 14Ni + 6TaF₅ had quite high hydrogenation and dehydrogenation rates and a large hydrogen-storage capacity with an effective hydrogen-storage capacity of approximately 6 wt%. The Ni and halide (TaF₅)-added Mg sample had a higher initial hydrogenation rate and a larger hydrogen-storage capacity than an Ni and oxide (Fe₂O₃)-added Mg sample.