Short O–O separation in layered oxide Na _0.67 CoO ₂ enables an ultrafast oxygen evolution reaction

Abstract

The layered oxide Na_0.67CoO₂ with Na⁺ occupying trigonal prismatic sites between CoO₂ layers exhibits a remarkably high room temperature oxygen evolution reaction (OER) activity in alkaline solution. The high activity is attributed to an unusually short O-O separation that favors formation of peroxide ions by O^--O^- interactions followed by O₂ evolution in preference to the conventional route through surface O-OH^- species. The dependence of the onset potential on the pH of the alkaline solution was found to be consistent with the loss of H⁺ ions from the surface oxygen to provide surface O^- that may either be attacked by solution OH^- or react with another O^-; a short O-O separation favors the latter route. The role of a strong hybridization of the O-2<i>p</i> and low-spin Co^III/Co^IV π-bonding <i>d</i> states is also important; the OER on other Co^III/Co^IV oxides is compared with that on Na_0.67CoO₂ as well as that on IrO₂.