Influence of the substitution Ce for La on structural and electrochemical characteristics of La0.75-xCexMg0.25Ni3Co0.5 (x=0, 0.05, 0.1, 0.15, 0.2 at. %) hydrogen storage alloys

Structural and electrochemical performances of La0.75-xCexMg0.25Ni3Co0.5 (x = 0, 0.05, 0.1, 0.15, 0.2 at.%) alloys prepared by an induction melting under helium atmosphere followed by annealing treatment at 1173 K in a vacuum furnace for 8 h are studied. Rietveld refinement results from the XRD data suggest that the alloys are mainly composed of (La, Mg)Ni3, (La, Mg)2Ni7 and LaNi5 phases. Increasing the content of Ce, the amounts of (La, Mg)Ni3 phase and (La, Mg)2Ni7 phase decrease, while the amount of LaNi5 phase increases. Electrochemical performance measurement results show that the La0.75-xCexMg0.25Ni3Co0.5 alloys are completely activated within 2 cycles, and the cyclic stability after 100 charge/discharge cycles of the alloys increases from 62.39% to 84.94% with the rising of Ce content, and the discharge capacity of the 100th cycle reaches the maximum value of 259 mAh/g when Ce content is 0.1 at.%. Meanwhile, the high rate discharge ability of the alloy electrodes increases to the maximum value when Ce content is 0.1 at.%, and then decreases. Thus, the La0.65Ce0.1Mg0.25Ni3Co0.5 alloy exhibits optimum comprehensive electrochemical properties.