Influence of oxygen impurity on the hydrogenation properties of LaNi₅, LaNi_4.7Al_0.3 and MmNi_4.5Al_0.5 during long-term pressure-induced hydriding-dehydriding cycling

The extrinsic degradation behaviours of LaNi₅, LaNi_4.7Al_0.3 and MmNi_4.5Al_0.5 alloys are investigated by measuring changes in the amount of absorbed hydrogen in each cycle during pressure-induced hydriding-dehydriding cycles extending up to 2450 cycles in hydrogen containing oxygen. For all alloys, the absorbed hydrogen content is reduced drastically during the first cycle and recovered during subsequent cycles. This may be due to the catalytic effects of the hydroxides of the rare earth metals and/or to metallic nickel clusters on the surface of the specimens. However, as cycling is extended over a few thousand cycles, the storage capacity gradually decreases again. The gradual decrease may be caused by the decomposition of the alloy into the hydroxides of the rare earth metals and metallic nickel clusters. The decomposition is verified by the observation of the characteristic peaks of La(OH)₃ and nickel clusters in the X-ray diffraction pattern of fully degraded LaNi₅ samples after 2272 cycles. The decomposition is confirmed by magnetization measurements which detect the metallic nickel clusters in the degraded samples. LaNi_4.7A1_0.3 and MmNi_4.5-A1_0.5 alloys can absorb hydrogen after 2450 cycles, although LaNi₅ is fully degraded. The substituted aluminium atoms seem to reduce the amount of the absorbed oxygen by the formation of A1₂O₃ and retard the decomposition by restricting the exchange between lanthanum and nickel atoms in the neighbouring sites due to the fact that the atomic size of aluminium is much larger than that of nickel. The resistance of aluminium substituted alloys to oxygen is therefore improved.