Long-term hydrogen absorption/desorption properties and structural changes of LaNi4Co alloy with double desorption plateaus

The long-term hydrogen storage performance of a CaCu5-type LaNi4Co alloy is studied by detailed analysis of its P-C-T curves and structural changes within 1000 cycles. The alloy shows one absorption plateau (α→β phase transition) and two desorption plateaus (γ→β and β→α phase transitions). With increasing cycle number, the plateau pressures for both α→β and β→α phase transitions decrease due to the increase in cell volumes. The increased crystal cells also provide more space to accommodate the simultaneous occupations of 12° and 6 m sites, and thus β→α and β→γ desorption plateaus become less distinguishable. During cycling, partial phase decomposition, crystal grain destruction and surface oxidation occur, resulting in the loss of hydrogen storage capacity. In addition, due to the increase in the atomic disorder and internal microstrains and defects, the average slope factors (SF/H%) of the hydrogen absorption/desorption plateaus are increased. Particularly, the SF/H% value of the γ→β plateau is always higher than those of the α→β and β→α plateaus, and it also increases faster with cycling, reaching about 4 times of the other two plateaus at the 1000th cycle. It is considered that the improvement of the γ→β phase transition process is significant to promote the cycling behavior and practical applications of this alloy system.