Effect of La/Ce ratio on the structure and electrochemical characteristics of La0.7−xCexMg0.3Ni2.8Co0.5 (x = 0.1-0.5) hydrogen storage alloys

The effect of La/Ce ratio on the structure and electrochemical characteristics of the La_0.7−xCe_xMg_0.3Ni_2.8Co_0.5 (x = 0.1, 0.2, 0.3, 0.4, 0.5) alloys has been studied systematically. The result of the Rietveld analyses shows that, except for small amount of impurity phases including LaNi and LaNi₂, all these alloys mainly consist of two phases: the La(La, Mg)₂Ni₉ phase with the rhombohedral PuNi₃-type structure and the LaNi₅ phase with the hexagonal CaCu₅-type structure. The abundance of the La(La, Mg)₂Ni₉ phase decreases with increasing cerium content whereas the LaNi₅ phase increases with increasing Ce content, moreover, both the a and cell volumes of the two phases decrease with the increase of Ce content. The maximum discharge capacity decreases from 367.5 mAh g⁻¹ (x = 0.1) to 68.3 mAh g⁻¹ (x = 0.5) but the cycling life gradually improve. As the discharge current density is 1200 mA g⁻¹, the HRD increases from 55.4% (x = 0.1) to 67.5% (x = 0.3) and then decreases to 52.1% (x = 0.5). The cell volume reduction with increasing x is detrimental to hydrogen diffusion D and accordingly decreases the low temperature dischargeability of the La_0.7−xCe_xMg_0.3Ni_2.8Co_0.5 (x = 0.1-0.5) alloy electrodes.