Hydrogen-storage characteristics of Cu, Nb2O5, and NbF5-added Mg–Ni alloys

From a Mg–23.5 wt%Ni–5 wt%Cu alloy synthesized by the gravity casting method in a large quantity (7.5 kg), Mg–23.5 wt%Ni–x wt%Cu (x = 2.5, 5 and 7.5) samples for hydrogen storage were prepared by melt spinning and crystallization heat treatment. The samples were ground under H2 in order to obtain a fine powder. These alloys contained crystalline Mg and Mg2Ni phases. The Mg–23.5Ni–2.5Cu alloy had the highest hydriding and dehydriding rates after activation among these alloys. The dehydriding curve under 1.0 bar H2 at 573 K exhibits two stages; the dehydriding rate is high for about 2.5 min (the decomposition of Mg2Ni hydride and Mg hydride in small particles), and then it becomes lower (the decomposition of Mg hydride). The hydriding and dehydriding properties of another sample 88 wt%(87.5Mg–10Ni–2.5Cu)–5 wt%Nb2O5–7 wt%NbF5 were also investigated.