Improved Water Electrolysis Over Cobalt Oxyhydroxide by Carbonate Anion Adsorption: Direct Observation of Active Species Using Operando Hard/Soft X-Ray Absorption Spectroscopy.

Cobalt oxide electrocatalysts electrodeposited from carbonate-based electrolyte solutions (Co-C_i) and exhibiting high oxygen evolution reaction (OER) activity at neutral pH are investigated using operando hard/soft X-ray absorption fine structure (XAFS) analyses. Operando Co K-edge XAFS data indicate that the Co-C_i comprises a collection of CoOOH nanoclusters and that the Co in this material is oxidized to higher valence states upon applying an electrode potential. Operando C K-edge XAFS results show that carbonate anions are adsorbed on the CoOOH nanoclusters and that this phenomenon is promoted at the active potential for the OER. The carbonate anions are found to remain adsorbed on the CoOOH and to be catalytically active even in unbuffered solutions without carbonate anions. The carbonate anions evidently stabilize the active sites associated with Co⁴⁺ to enhance the electrocatalytic activity. This study provides new insights into the function of anions adsorbed on CoOOH nanoclusters, and the results of this work should be applicable to a wide range of metal oxide catalysts in neutral solutions.