Hydrogen storage and electrochemical properties of the La _0.7Mg_0.3Ni_3.825-xCo_0.675Mn _x hydrogen storage electrode alloys

In this paper, the structure, hydrogen storage and electrochemical properties of the La_0.7Mg_0.3Ni_3.825-xCo _0.675Mn_x (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys have been investigated systematically. It is found that, by X-ray diffraction and Rietveld analysis, all alloys consist mainly of two phases: an (La,Mg)Ni₃ phase with the rhombohedral PuNi ₃-type structure and an LaNi₅ phase with the hexagonal CaCu₅-type structure. Moreover, with increasing x, the abundance of the (La,Mg)Ni₃ phase decreases, but the abundance of the LaNi ₅ phase increases progressively, which indicates that the Mn element is beneficial for the formation of LaNi₅ phase in the alloy. The pressure-composition (P-C) isotherm curves reveal that the equilibrium pressure decreases and the hydrogen storage capacity increases first and then decreases with increasing x. Electrochemical studies show that the maximum discharge capacity of the alloy electrodes increases from 225.2 to 328.8 mAh/g and then decreases to 292.2 mAh/g with increasing x from 0.0 to 0.5. Meanwhile, the high rate dischargeabilities of the alloy electrodes are also improved with an optimum Mn content in the alloy (x=0.4). In addition, the exchange current density I₀, the limiting current density I_L and the hydrogen diffusion coefficient D of the alloy electrodes all increase first and then decrease with increasing x.