Effect of La/Ce ratio on the structure and electrochemical characteristics of La 0.7− xCe xMg 0.3Ni 2.8Co 0.5 ( x = 0.1–0.5) hydrogen storage alloys

Abstract

The effect of La/Ce ratio on the structure and electrochemical characteristics of the La 0.7− x Ce x Mg 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 LaNi 2, all these alloys mainly consist of two phases: the La(La, Mg) 2Ni 9 phase with the rhombohedral PuNi 3-type structure and the LaNi 5 phase with the hexagonal CaCu 5-type structure. The abundance of the La(La, Mg) 2Ni 9 phase decreases with increasing cerium content whereas the LaNi 5 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− x Ce x Mg 0.3Ni 2.8Co 0.5 ( x = 0.1–0.5) alloy electrodes.