Short O–O separation in layered oxide Na0.67CoO2 enables an ultrafast oxygen evolution reaction

Abstract

The layered oxide Na0.67CoO2 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–2p and low-spin CoIII/CoIV π-bonding d states is also important; the OER on other CoIII/CoIV oxides is compared with that on Na0.67CoO2 as well as that on IrO2.