The effect of CO impurity on the hydrogenation properties of LaNi₅, LaNi_4.7Al_0.3 and MmNi_4.5Al_0.5 during hydriding-dehydriding cycling

In an investigation of the extrinsic degradation behaviour of LaNi₅, LaNi_4.7Al_0.3 and MmNi_4.5Al_0.5 alloys, the changes of the amount of the absorbed hydrogen on each cycle were measured during the pressure-induced hydriding-dehydriding cycling in hydrogen containing CO as an impurity. For all alloys, the amount of the absorbed hydrogen decreased continuously as the number of cycles was increased. LaNi₅ and LaNi_4.7A1_0.3 are degraded completely within 20 cycles, MmNi_4.5Al_0.5 within 80 cycles. The loss of hydrogen storage capacities is caused by the deactivation of the active sites for the dissociative chemisorption of hydrogen molecules by the preferential adsorption of the CO impurity. This is confirmed by the thermal desorption experiments for the fully degraded samples and the analysis of the composition of the gases evolved from the degraded specimens during the thermal desorption by the gas chromatography. Partial substitution of nickel by aluminium improves the resistance of the alloys to the CO impurity, which is due to the changes of the surface electronic structure induced by the partial substitution of aluminium for nickel. But the resistance of the LaNi_4.7A1_0.3 alloy to the CO impurity is much poorer than that of MmNi_4.5A1_0.5 alloy, which is caused by the successive accumulation of retained hydrogen in LaNi_4.7A1_0.3 which is not desorbed completely during the dehydriding period of each cycle.