The effect of thermal cycling of $MmNi_{4.5}Al_{0.5}$ on the degradation was investigated to increase the understanding of P-C-Isotherm curves and hydrogen absorption rate in the isotherm condition.
The reversible hydrogen absorption capacity was decreased about 25% after 5500 cycles.
In activated $MmNi_{4.5}Al_{0.5}$ , the rate controlling steps of hydriding reaction changed from the surface chemisorption to the hydrogen diffusion process through hydride phase sequentially as reaction proceeded.
After 5500 cycles, the sequential change of rate controlling step was same as activated $MmNi_{4.5}Al_{0.5}$ and the intrinsic hydrogen absorption rate significantly increased.
It is suggested that the intrinsic degradation of $MmNi_{4.5}Al_{0.5}$ can be interpreted with coring model, the formation of nickel cluster at the surface of the sample and the atomic site exchange in the bulk are causes of changes of kinetic behavior.
