SURFACE-TREATMENT OF A LANI5-TYPE METAL-HYDRIDE ELECTRODE WITH AN ALKALINE-SOLUTION DISSOLVING COBALT(II) HYDROXIDE

The effects of KOH aqueous solution saturated with Co(OH)(2) on LaNi4. 7Al0.3 electrodes were studied using electrochemical impedance spectro scopy and X-ray photoelectron spectroscopy. The electrochemical activi ty of an LaNi4.7Al0.3 electrode was considerably improved by a treatme nt with Co(OH)(2)-KOH solution below the potential of -0.8 V (vs. Hg/H gO) after several cycles in pure KOH electrolyte. These electrodes exh ibited higher metallic-nickel and lower lanthanum concentrations in th e subsurface region of the alloy than those without any treatment. Thi s metallic-nickel enrichment was concluded to be caused by oxidation o f metallic lanthanum with water catalyzed by Co(OH)(4)(2-) ions follow ed by a dissolution of lanthanum hydroxide. In contrast, the electrode treated with the Co(OH)(2)-KOH solution without regulation of potenti al before cycling in KOH solution exhibited lower electrochemical acti vity and metallic-nickel concentration than that without any treatment . In this case, Co(OH)(4)(2-) ions were considered to act as an oxidan t to the alloy.