SURFACE-ANALYSIS OF AB5-TYPE ELECTRODES

By means of photoelectron spectroscopy we have analysed the composition and thickness of surface layers formed on LaNi5 and LaNi4.7Al0.3 powder electrodes after various pretreatments. The exposure at different potentials and for different durations in 6 M KOH electrolyte or electrochemical cycling reveals significant effects on the composition and thickness of the surface layers formed. Apart from the storage in 6 M KOH at 0.4 V above the rest potential, which results in a nickel-rich surface, for all the other treatments a strong lanthanum segregation to the surface occurs. Contrary to earlier suggestions no passivating aluminium surface oxide film could be found on cycled LaNi4.7Al0.3 electrodes. Nevertheless, after prolonged exposures or at higher potentials in KOH electrolyte a slight aluminium enrichment takes place.