| The extrinsic degradation behaviors of LaNi_5 alloy are investigated by the changes of the amount of absorbed hydrogen in each cycles during the pressure induced hydriding-dehydriding cycling in hydrogen containing CO or O₂as impurity. In the mixed gases of H₂and CO, the amount of absorbed hydrogen decreases continuous1y with the increase of the number of cycling. Thus, LaNi_5, is degraded completely within 15 cycles. The lost of hydrogen storage capacity is due to the deactivation of the active sites for the dissociative chemisorption of hydrogen molecules by the preferential adsorption of the CO impurity. This was verified by the thermal description 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. The contamination characterstic is such that CO poisons strongly some particles which are in touch with it immediately after the mixed gas are introduced newly during the hydriding reaction. In H₂-0.1%O₂, the absorbed hydrogen contents are decreased drastically at first cycle and recovered by the subsequent cycling, which may be caused by the formation of the metatlic Ni clusters or the surface of specimen. However, as the cycling is extended over a few thousand cycles, the storage capacity is reduced gradually again. The gradual decrease may be due to the decomposition of the alloy into the hydroxides of the rare earth metals and the metallic Ni clusters. The decomposition was confirmed through the observation of the characteristic peaks of La(OH)₃ and Ni clusters in the x-ray diffraction pattern of the fully degraded LaNi_5 samples after 2272 cycles. The decomposition was confirmed by detecting the metallic Ni clusters in the degraded samples by the magnetization measurements. |
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