<i>Operando</i> Identification of the Dynamic Behavior of Oxygen Vacancy-Rich Co₃O₄ for Oxygen Evolution Reaction

TLDR

The role of defect structures in transition metal compounds for the oxygen evolution reaction remains unclear, yet Co₃O₄ enriched with oxygen vacancies has been reported to catalyze OER efficiently, making operando studies of such defective electrocatalysts essential to understand their intrinsic mechanisms. The study aims to construct pure spinel and oxygen‑vacancy‑rich Co₃O₄ catalyst models to investigate defect mechanisms and the dynamic behavior of defect sites during electrocatalytic OER via operando characterizations. The authors employed operando electrochemical impedance spectroscopy, cyclic voltammetry, quasi‑operando XPS, and operando XAFS to probe the defect sites and surface reconstruction during OER. The operando studies revealed that oxygen vacancies promote low‑valence Co pre‑oxidation at lower potentials, initiate surface reconstruction before OER, and are first filled by OH•, thereby facilitating Co‑OOH• formation and revealing the real active sites, offering dynamic insight into Co₃O₄ defect mechanisms and encouraging further focus on defect electrocatalyst behavior.

Abstract

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 (VO) has been reported to efficiently catalyze OER. Herein, we constructed pure spinel Co3O4 and VO-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 VO could facilitate the pre-oxidation of the low-valence Co (Co2+, part of which was induced by the VO to balance the charge) at a relatively lower applied potential. This observation confirmed that the VO could initialize the surface reconstruction of VO-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 VO-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.