Charge/Discharge Characteristics of Amorphous MgNi Electrode and Influence of Heat Treatment in Argon

An amorphous MgNi alloy was prepared by mechanical alloying, and its charge/discharge characteristics were examined in a 6 M KOH solution at room temperature. The discharge capacity of the alloy was large than that of conventional AB5 alloys in the initial stage of charge/discharge cycles, but it decreased promptly with an increase in cycle number. The cell voltage in the charge process increased with the cycles. Degradation in the discharge capacity also occurred owing to immersion in the KOH solution under an open circuit potential. Since the formation of a Mg(OH)2 layer was observed by X-ray diffraction analysis, the degradation of the electrode was ascribed to the increase in overvoltage in the charge process by the Mg(OH)2 layer formed on the alloy surface through the chemical reaction between the alloy and the KOH solution. The effect of heat treatment on the charge/discharge characteristics was also examined. The discharge capacity of amorphous MgNi and MgNi0.6Co0.4 alloys decreased in the initial stage of the cycle test by heating in argon owing to recrystallization. The degradation rates of these alloys, however, were also reduced by heating at 300℃, and the heated alloys showed large discharge capacity compared with unheated ones after 10 cycles. These results were attributed to the increase in surface area of the alloys brought about by selective evaporation of magnesium