F. Meli et al., PASSIVATION BEHAVIOR OF AB(5)-TYPE HYDROGEN STORAGE ALLOYS FOR BATTERY ELECTRODE APPLICATION, Journal of alloys and compounds, 221, 1995, pp. 284-290
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.