Hydrogen storage and electrochemical properties of the La 0.7Mg 0.3Ni 3.825− xCo 0.675Mn x hydrogen storage electrode alloys

Abstract

In this paper, the structure, hydrogen storage and electrochemical properties of the La 0.7Mg 0.3Ni 3.825− x Co 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)Ni 3 phase with the rhombohedral PuNi 3-type structure and an LaNi 5 phase with the hexagonal CaCu 5-type structure. Moreover, with increasing x, the abundance of the (La,Mg)Ni 3 phase decreases, but the abundance of the LaNi 5 phase increases progressively, which indicates that the Mn element is beneficial for the formation of LaNi 5 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 I 0, 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.