Direct Identification of O─O Bond Formation Through Three‐Step Oxidation During Water Splitting by Operando Soft X‐ray Absorption Spectroscopy

Abstract Anionic redox allows the direct formation of O─O bonds from lattice oxygens and provides higher catalytic in the oxygen evolution reaction (OER) than does the conventional metal ion mechanism. While previous theories have predicted and experiments have suggested the possible O─O bond, it has not yet been directly observed in the OER process. In this study, operando soft X‐ray absorption spectroscopy (sXAS) at the O K‐edge and the operando Raman spectra is performed on layered double CoFe hydroxides (LDHs) after intercalation with [Cr(C₂O₄)₃]³⁻, and revealed a three‐step oxidation process, staring from Co²⁺ to Co³⁺, further to Co⁴⁺ (3d6L), and ultimately leading to the formation of O─O bonds and O₂ evolution above a threshold voltage (1.4 V). In contrast, a gradual oxidation of Fe is observed in CoFe LDHs. The OER activity exhibits a significant enhancement, with the overpotential decreasing from 300 to 248 mV at 10 mA cm⁻², following the intercalation of [Cr(C₂O₄)₃]³⁻ into CoFe LDHs, underscoring a crucial role of anionic redox in facilitating water splitting.