PASSIVATION BEHAVIOR OF AB(5)-TYPE HYDROGEN STORAGE ALLOYS FOR BATTERY ELECTRODE APPLICATION

In many applications, AB(5) type hydrogen storage alloys show passivation behavior, i.e. when fully discharged, metal hydride electrodes show (especially at higher temperatures) a decrease in activity and therefore a decrease in capacity at normal discharge currents for ensuing cycles. Passivation may continue to the point where activity becomes so low that the capacity is no longer accessible. Electrochemical measurements were taken of two different AB(5)-type alloys, one with manganese and one without manganese (LaNi3.4,Co1.2Al0.4 and LaNi3.4Co1.2Al0.3Mn0.1). Both alloys showed passivation behavior after remaining in the discharged state. The ahoy with manganese showed a stronger tendency to passivation which is in contradiction with earlier observations. Photoelectron spectroscopic analysis together with sputter depth profiling was used to investigate the surface composition of samples which had undergone different surface pretreatments. Surface analysis of electrodes in the passivated state shows a lower content of metallic nickel and a thicker nickel surface oxide film. We attribute the low electrochemical kinetics of the alloys after passivation to the loss of metallic nickel and/or cobalt at the electrode-electrolyte interface.