Degradation kinetics of discharge capacity for amorphous Mg–Ni electrode

Abstract

Amorphous MgNi alloy was prepared by ball-milling of Mg 2Ni and Ni powders. Current density dependence of the discharge capacity was studied in the range from 5 to 250 mA/g for the MgNi electrode by charge–discharge cycle tests in 6 M KOH electrolyte using a conventional two-electrode system. The amount of hydrogen used for the discharge decreased with cycles. The extent of capacity degradation at a given cycle increased with increasing discharge current density. The capacity degradation curves could be expressed by ¢( t)= ¢ ∞+( ¢ 0− ¢ ∞) exp[− kt], where ¢( t) is the discharge capacity at time t, that is the period of time in which the electrode was immersed in the electrolyte solution, ¢ 0 the ideal discharge capacity expected for the virgin electrode, ¢ ∞ the final steady discharge capacity after a number of cycles, and k the apparent rate constant for the degradation. The rate constant increased linearly with the current density. According to diffusion analyses of discharge curves under different current densities, the hydrogen diffusion in the bulk is not the rate-determining step for the measured discharge rate, but surface processes play dominant roles in affecting the MgNi electrode properties.