High Spin State Promotes Water Oxidation Catalysis at Neutral pH in Spinel Cobalt Oxide

In this work, we present Co3O4 quantum dots (QDs) as a highly efficient and stable oxygen evolution reaction (OER) catalyst at neutral pH. The Co3O4 QDs with a mean size of 5 nm were synthesized by reacting cobalt acetate with benzyl alcohol in the presence of ammonia under reflux conditions. The as-synthesized Co3O4 QDs show extraordinary water oxidation activity with onset overpotential as low as 398 mV and mass activity as high as 567 A/g (at 1.75 V vs RHE) in a 0.2 M phosphate buffer electrolyte (pH ∼7), which are among the most efficient Earth-abundant OER catalysts at neutral pH reported in the literature, reaching a stable current density of 10 mA/cm2 at an overpotential of ∼490 mV with a Tafel slope of 80 mV/decade. Through in-depth investigations by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, the high spin Co2+ and Co3+ cations on the surface of Co3O4 QDs were found to be important to promote the OER kinetics at neutral pH.