Comparisons of metallic clusters imbedded in the surface oxide of AB2, AB5, and A2B7 alloys

In order to clarify the relationship between the saturation magnetization of the activated metal hydride material and electrode performance, the specific power measured at both low temperature (−30 °C) and operating temperature (35 °C) of a nickel metal hydride battery were compared with magnetization measurements. It is found specific powers measured at both temperatures correlate well with the saturated magnetic susceptibility, which is proportional to the total amount of metallic nickel clusters distributed within the surface oxide layer after activation. Both the gas phase storage and electrochemical properties of the activated AB2, AB5, and A2B7 alloys were investigated. While the AB2 alloy has the highest storage capacity, it also has the lowest high-rate dischargeability among the three alloys. This suggests a competition between storage capacity and rate capability in these materials. In an alkaline etching experiment, it is established the saturation magnetization increases with etching time in these AB2, AB5, and A2B7 alloys. The saturation magnetization after a 4 h etches track the changes in the high-rate dischargeability. The increase is attributed to the growth in size of the metallic inclusions for the AB2 and Nd–A2B7 samples, and from an increase in the number of metallic inclusion in the case of AB5 and La–A2B7. Transmission electron microscope studies calibrate the size inferred from magnetic susceptibility studies and also the Ni-dominated FCC structure of the metallic clusters.

Research highlightsDifferent growth mechanisms have been found on the metallic inclusions imbedded in the surface oxides of AB2, AN5, and A2B7. The high power performance has been correlated to the size and distribution of these inclusions.