Operando Identification of the Dynamic Behavior of Oxygen Vacancy-Rich Co₃O₄ for Oxygen Evolution Reaction

The exact role of a defect structure on transition metal compounds for electrocatalytic oxygen evolution reaction (OER), which is a very dynamic process, remains unclear. Studying the structure–activity relationship of defective electrocatalysts under operando conditions is crucial for understanding their intrinsic reaction mechanism and dynamic behavior of defect sites. Co₃O₄ with rich oxygen vacancy (V_O) has been reported to efficiently catalyze OER. Herein, we constructed pure spinel Co₃O₄ and V_O-rich Co₃O₄ as catalyst models to study the defect mechanism and investigate the dynamic behavior of defect sites during the electrocatalytic OER process by various operando characterizations. Operando electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) implied that the V_O could facilitate the pre-oxidation of the low-valence Co (Co²⁺, part of which was induced by the V_O to balance the charge) at a relatively lower applied potential. This observation confirmed that the V_O could initialize the surface reconstruction of V_O–Co₃O₄ prior to the occurrence of the OER process. The quasi-operando X-ray photoelectron spectroscopy (XPS) and operando X-ray absorption fine structure (XAFS) results further demonstrated the oxygen vacancies were filled with OH^• first for V_O–Co₃O₄ and facilitated pre-oxidation of low-valence Co and promoted reconstruction/deprotonation of intermediate Co–OOH^•. This work provides insight into the defect mechanism in Co₃O₄ for OER in a dynamic way by observing the surface dynamic evolution process of defective electrocatalysts and identifying the real active sites during the electrocatalysis process. The current finding would motivate the community to focus more on the dynamic behavior of defect electrocatalysts.