Under the operation conditions of heat pump, the changes of hydrogenation properties of $MmNi_{4.5}Al_{0.5}$ and $MmNi_{4.5}Fe_{0.85}$ were investigated.
Due to thermal cycling $MmNi_{4.5}Al_{0.5}$, from 303 to 423K, of 3000 times, about 7.6% of the initial reversible hydrogen storage capacity is decreased.
The damage of $MmNi_{4.5}Al_{0.5}$ induced by thermal cyclings was not fully recovered under the annealing condition, 1 atm. hydrogen pressure, 673K and 3 hours.
The degradation rate was not affected after annealing treatment but the recovery of capacity was decreased gradually as the number of annealing treatment was increased.
On cycling $MmNi_{4.15}Fe_{0.85}$ from 298 to 333K, only 6% of the capacity is decreased after 3000 cycles.
Damage of $MmNi_{4.15}Fe_{0.85}$ was not fully recovered either by annealing treatment with 1×$10^{-2}$ torr hydrogen pressure and for 3 hours at 773K.
To certify the phase separation at the surfaces of the particles, $MmNi_{4.15}Fe_{0.85}$ sample which had been thermally cycled 4000 times and recovered to its initial hydrogen capacity was annealed under such a conditions that in the aspects of overall composition $MmNi_{4.15}Fe_{0.85}$ could be fully recovered. The annealing treatment decreased the reversible hydrogen storage capacity by about 13% and hysteresis gap was increased by about two times.
To identify the particle size effect on the degradation rate, $MmNi_{4.15}Fe_{0.85}$ was mechanically pulverized in air into two groups of particle size, -100+200 mesh and -325 mesh. And these two samples were thermally cycled under the same condition. The sample with -325 mesh initial size exhibited more severe capacity loss by about 30% than the -100+200 mesh sample after 1000 cycles.
With the results observed, a new intrinsic degradation model ( Coring Model ) is proposed.
