Crystallographic and electrochemical characteristics of La 0.7Mg 0.3Ni 3 − x(Al 0.5Mo 0.5) x ( x = 0–0.4) hydrogen storage alloys

Abstract

The effect of partial substitution of Al and Mo for Ni on the structure and electrochemical properties of the La 0.7Mg 0.3Ni 3 − x (Al 0.5Mo 0.5) x ( x = 0–0.4) hydrogen storage alloys have been investigated systematically. The result of X-ray powder diffraction (XRD) and Rietveld analysis show that all the alloys consist of the La(La, Mg) 2Ni 9 phase and the LaNi 5 phase. Meanwhile, the lattice parameter and the cell volume of both the La(La, Mg) 2Ni 9 phase and the LaNi 5 phase increase with increasing Al and Mo contents in the alloys. The pressure composition isotherms curves indicate that the hydrogen storage capacity first increases and then decreases with increasing x. The electrochemical measurements show that the maximum discharge capacity of the alloy electrodes first increases from 343.3 ( x = 0) to 377.6 mAh/g ( x = 0.3) and then decreases to 350.4 mAh/g ( x = 0.4). Moreover, the high rate dischargeability (HRD) and the exchange current density of the alloy electrodes decrease first and then increases with the increase of x in the alloys. The hydrogen diffusion coefficient increases with increasing Al and Mo content and thus increases the low temperature dischargeability (LTD) of the alloy electrodes.