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 spectroscopy and X-ray photoelectron spectroscopy. The electrochemical activity of an LaNi4.7Al0.3 electrode was considerably improved by a treatment with Co(OH)2-KOH solution below the potential of −0.8 V (vs. Hg/HgO) after several cycles in pure KOH electrolyte. These electrodes exhibited higher metallic-nickel and lower lanthanum concentrations in the subsurface region of the alloy than those without any treatment. This metallic-nickel enrichment was concluded to be caused by oxidation of metallic lanthanum with water catalyzed by Co(OH)42− ions followed by a dissolution of lanthanum hydroxide. In contrast, the electrode treated with the Co(OH)2-KOH solution without regulation of potential before cycling in KOH solution exhibited lower electrochemical activity and metallic-nickel concentration than that without any treatment. In this case, Co(OH)42− ions were considered to act as an oxidant to the alloy.