ELECTROCHEMICAL-BEHAVIOR OF SOME MECHANICALLY ALLOYED MG-NI-BASED AMORPHOUS HYDROGEN STORAGE ALLOYS

The electrochemical behaviours of some amorphous Mg50Ni50-x-yMxNy (M, N = Co, Al and Si) hydrogen storage alloys prepared by mechanical alloying (MA) were investigated. It is found that the MA amorphous alloys are easier to activate electrochemically than crystalline alloys. All MA amorphous alloys reach-their respective maximum capacities at the first charge/discharge cycle. At a current density of 20 mA/g the MA Mg50Ni50 has a maximum discharging capacity around 500 mAh/g, which is ten times higher than that of the crystalline alloy. The partial substitution of Co, Si and Al for Ni in Mg50Ni50 alloy decreases its discharging capacity each to a different extent. However, the durability of the amorphous Mg50Ni50-x-yMxNy alloys is rather poor, and their capacity degradation rates amount to 10 - 60 mAh/g per cycle. XRD results reveal that the rapid capacity degradation of the amorphous Mg-Ni-based alloy is due to the oxidation of magnesium, its hydrogen storage component, into magnesium oxides during electrochemical charge/discharge cycling.