Crystallographic and electrochemical characteristics of La0.7Mg0.3Ni5.5? x (Al0.5Mo0.5) x ( x =0 to 0.8) hydrogen storage alloys

The structure, hydrogen storage property, and electrochemical characteristics of the La_0.7Mg_0.3Ni_5.5−x (Al_0.5Mo_0.5) _x (x=0, 0.2, 0.4, 0.6, 0.8) hydrogen storage alloys have been investigated systematically. It has been found by X-ray powder diffraction and Rietveld analysis that the alloys are multiphase and consist of impurity Ni phase and two main crystallographic phases, namely, the La(La, Mg)₂Ni₉ phase and the LaNi₅ phase, and the lattice parameters and the cell volumes of both the La(La, Mg)₂Ni₉ phase and the LaNi₅ phase increase with increasing Al and Mo content in the alloys. The P-C isotherm curves indicated that the hydrogen storage capacity of the alloy first increases and then decreases with increasing x, and the equilibrium pressure decreases with increasing x. The electrochemical measurements show that the maximum discharge capacity first increases from 298.5 (x=0) to 328.3 mAh/g (x=0.6) and then decreases to 304.7 mAh/g (x=0.8). The high rate dischargeability (HRD) of the alloy electrodes increases lineally from 65.4 pct (x=0) to 86.6 pct (x=0.8) at the discharge current density of 1200 mA/g. Moreover, the exchange current density of the alloy electrodes also increases monotonously with increasing x by the linear polarization curves. The hydrogen diffusion coefficient in the alloy bulk, D, increases with increasing Al and Mo content and thus enhances the low-temperature dischargeability (LTD) of the alloy electrodes.