Effects of mechanical grinding on initial activation and rate capability of Zr–Ti based Laves phase alloy electrode

Abstract

Surface modification of Zr0.9Ti0.1Ni1.1Co0.1Mn0.5V0.2Cr0.1 alloy was accomplished by mechanical grinding (MG). The decrepitation of alloy particles gave rise to a new surface. The effect of MG was systematically studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), pressure-composition (PC) isotherms and electrochemical impedance spectroscopy (EIS). Initial activation and rate capability of a negative electrode made from this alloy were significantly improved by MG treatment at 300 rpm for 0.5–1 h, but prolonged MG treatment (10 h) reduced the nominal discharge capacity from 350 mAh g−1 to 180 mAh g−1. Under these conditions the alloy particles disintegrate and become nanocrystalline, which reduces the discharge capacity owing to the change in the stereology of tetrahedral interstices available for hydrogen storage. The data based on PC isotherms and hydriding kinetics indicates that the equilibrium hydrogen pressure increases and the hydriding rate and storage capacity are significantly reduced by prolonged MG treatment. EIS data reveal that the improved rate capability can be ascribed to an enhanced charge-transfer reaction which is the rate-determining step in the hydriding and dehydriding reactions.