Repetitive hydriding and dehydriding cycles of $MmNi_{4.15}Fe_{0.85}$ in hydrogen containing small amounts of oxygen as a minor impurity was investigated to study the extrinsic degradation of the compound.
The initial stage of cycles exhibited a pronounced degradation of the hydrogen absorption capacity and kinetics which was attributed to the preferential adsorption of oxygen on the active sites for the dissociative chemisorption of hydrogen.
However, the hydrogen absorption capacity was gradually recovered. This may be due to the separation of $MmNi_{4.15}$ $Fe_{0.85}$ to the Mm-oxide and elemental nickel or iron by either pressure cycling or heat treatment.
Especially, the hydrogenation properties of $MmNi_{4.15}$ $Fe_{0.85}$ metal hydride were greatly improved by the heat treatment.
It was suggested that the improvement of hydriding properties of $MmNi_{4.15}$ $Fe_{0.85}$ by the heat treatment was mainly due to the catalytic effect of metallic Ni or Fe segregated at the surface. The fact that surface segregation was enhanced by heat treatment was confirmed by the measurement of magnetization.
