Investigating the origin of Co(IV)’s high electrocatalytic activity in the oxygen evolution reaction at a Na_xCoO₂ interface

We present a study based on the layered oxide system Na_xCoO₂ as an oxygen evolution reaction (OER) catalyst, where the formal oxidation state of Co can be continuously increased, above 3+, through Na-ion deintercalation. Accompanying this increase of formal Co oxidation state, the overpotential for the catalytic oxygen evolution reaction (OER) at 10 mA/cm² decreases from 470 mV to 415 mV, while all the Na_xCoO₂ materials (x = 0.75, 0.65, 0.52, 0.36) share a common Tafel slope of 41(±5) mV/decade, implying a common rate-limiting process for the whole series. Our use of X-ray photoelectron spectroscopy (XPS) on the de-intercalation-derived Na_xCoO₂ catalysts, on the other hand, led to the unexpected observation that the percentage of surface Co(III) increases from 23% to 37% even as the Na deintercalation increases the formal Co oxidation state above 3+. These observations suggest that when the formal oxidation state of the central metal is pushed to an unusually high value, i.e. Co(IV) in this case, oxygen vacancies on the materials surface formed from the relaxation of the highly oxidized metal to a more stable oxidation state are the key to improving catalytic OER activity.