An investigation of the intrinsic degradation mechanism of LaNi₅ by thermal desorption technique

The intrinsic degradation behavior of LaNi₅ was investigated by the changes of pressure-composition-temperature (P-C-T) curves of LaNi₅H₂ system and the hydrogen desorption technique after the pressure induced hydrogen absorption-desorption cycling. Thermal desorption experiment of LaNi₅ hydrides showed only one peak in the hydrogen evolution rate vs temperature plot for the activated samples but two peaks for the degraded samples. The low temperature peak of the degraded samples and a peak of the activated sample related to the hydrogen evolved from the sites which hydrogen could absorb into and desorb from reversibly. reversibly. The high temperature peak of the degraded samples seems to be associated with the hydrogen from the stable hydrides such as several series of LaNi stoichiometric compound hydrides and LaH₂. Such stoichiometric compound hydrides and LaH₂ would be formed by the disordering of the lattice and the changes of local composition of the degraded LaNi5 hydrides. Ni clusters formed at the surface by the cycling were confirmed by the magnetization measurement, which indicates the formation of a series of stable hydrides. Thus it is concluded that the formation of the various stoichiometric compound hydrides and LaH₂ were responsible for the intrinsic degradation of LaNi₅ through the cycle.