Cycle durability of Ca–Mg–Ni alloys and factors which cause degradation of hydrogen storage capacity

The cycle durability of Ca–Mg–Ni hydrogen storage alloys, which are investigated in the Japanese national project (International Clean Energy Network), WE-NET, was examined using pure H2 (99.99999%). The tested alloys were Ca0.25Mg0.66Y0.09Ni1.86 (cubic MgCu2-type structure), Ca0.55Mg0.45Ni3 (rhombohedral PuNi3-type) and Ca0.5Mg0.5Ni2 (MgCu2-type). Although initially the ‘dynamic’ hydrogen storage capacities were H/M ∼0.7, they decreased steeply within a few cycles, and then they declined gradually. At around 400th cycle, they decreased to nearly half of the maximum capacities regardless of crystal structures and compositions of the tested alloys. However, their ‘equilibrium’ capacities did not decrease as the ‘dynamic’ ones with progress of hydrogen sorption. As for the bulk, not serious disproportionation but pulverization and lattice expansion were observed by X-ray diffractometry (XRD) and microscopic analyses. The expansion indicated that some amounts of hydrogen remained in the lattice cells. It was considered that pulverization of the alloy particles and amorphous shell formed on the surface prevented the alloys from smooth hydrogen sorption, and which led to the decrease of the ‘dynamic’ hydrogen storage capacities without that of the ‘equilibrium’ ones in the cycle tests.