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

In many applications, AB(5) type hydrogen storage alloys show passivat ion behavior, i.e. when fully discharged, metal hydride electrodes sho w (especially at higher temperatures) a decrease in activity and there fore a decrease in capacity at normal discharge currents for ensuing c ycles. Passivation may continue to the point where activity becomes so low that the capacity is no longer accessible. Electrochemical measur ements were taken of two different AB(5)-type alloys, one with mangane se and one without manganese (LaNi3.4,Co1.2Al0.4 and LaNi3.4Co1.2Al0.3 Mn0.1). 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. Ph otoelectron spectroscopic analysis together with sputter depth profili ng was used to investigate the surface composition of samples which ha d undergone different surface pretreatments. Surface analysis of elect rodes in the passivated state shows a lower content of metallic nickel and a thicker nickel surface oxide film. We attribute the low electro chemical kinetics of the alloys after passivation to the loss of metal lic nickel and/or cobalt at the electrode-electrolyte interface.