Effect of Al and Mo substitution on the structural and hydrogen storage properties of CaNi5

A simple mechanical milling and annealing process has been used to synthesize CaNi5-based hydrogen storage alloys. Heat treatment at 800 °C under vacuum results in the formation of a crystalline CaNi5 phase. Secondary phases, including Ca2Ni7 and Mo–Ni, are formed when substituting Mo for Ni. Replacement of Ni by Al or Mo leads to an increase in the unit cell volume of the CaNi5 phase. The hydrogen storage capacity of all substituted alloys is reduced and the plateau pressures are lower than those of pure CaNi5. Fairly flat plateau regions are retained for all compositions except the CaNi4.8Mo0.2 composition where a Ca2Ni7 phase is dominant. The incorporation of Mo also causes slow sorption kinetics for the CaNi4.9Mo0.1 alloy. CaNi4.9Al0.1 maintains its initial hydrogen absorption capacity for 20 cycles performed at 85 °C but the other substituted alloys lose their capacity rapidly, especially the CaNi4.8Mo0.2 composition.

► The CaNi5 system has been investigated when mechanically alloyed with Mo and Al.
► Addition of Mo into CaNi5 leads to slower hydrogen sorption kinetics.
► Replacement of Al at the Ni site can improve both hydrogen sorption kinetics and cyclic stability of the CaNi5-based alloy.