Hemispherical Cavity Cobalt Oxyhydroxides as Oxygen Evolution Electrocatalysts Electrodeposited in Borate-Buffered Solution Using a Polystyrene Colloidal Template Method

Cobalt oxyhydroxide (CoOOH) electrocatalysts with a hemispherical cavity structure were electrodeposited in borate-buffered solution using a polystyrene colloidal template method. The hemispherical-cavity catalysts exhibited higher activity for the oxygen evolution reaction than the corresponding flat thin-film catalysts. Analysis by operando Co K-edge X-ray absorption spectroscopy revealed that, under the electrode working conditions, the local structure of the Co species was composed of nanosized CoOOH clusters with high-oxidation-number Co species. In addition, a B K-edge X-ray absorption spectroscopic analysis showed that borate anions were adsorbed onto the catalysts, stabilizing the catalyst structure. Thus, the highly oxidized CoOOH clusters with adsorbed borate anions are likely to function as active sites, possibly because the clusters are generated around anisotropic Co sites in the catalysts with hemispherical cavities.