Discrimination between OH− and H2O oxidation for oxygen evolution reaction

Abstract

OH⁻/H₂O-reactant discrimination for the oxygen evolution reaction (OER) is a critical but not well resolved issue. This has led to unreasonable activity comparisons, misinterpreted OER mechanisms, and ununified models for theoretical calculations regardless of the thermodynamic/kinetic difference between OH⁻ and H₂O oxidation. Here, we discriminate between OH⁻ and H₂O oxidation by tuning the interfacial OH⁻ concentration. Combining OER kinetic analysis with in situ ¹⁶OH⁻/H₂¹⁸O isotopic labeling-based differential electrochemical mass spectrometry, we examine the respective electrochemical oxidation behaviors between OH⁻ and H₂O oxidation. We find that OH⁻ oxidation presents ∼550 mV lower onset potential relative to H₂O oxidation and that Tafel plotting gives slopes of ∼50 mV dec⁻¹ for OH⁻ oxidation, which is substantially lower than those of ∼200 mV dec⁻¹ for H₂O oxidation on a model CoOOH catalyst. This work calls for the discrimination of OH⁻/H₂O oxidation as the prerequisite for future OER activity evaluation and mechanism studies.