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.

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Heterogeneous catalysis plays an important role in modern industry. Exploring catalysts with high efficiency, low-cost, and high stability is an important issue for the research of heterogeneous catalysis. In recent years, researchers have prepared a variety of defective catalysts and found that the defects in catalysts have an important effect on their catalytic activity. However, the relationship between defects and catalytic activity remains to be clarified. In this Review, three aspects including recognition, understanding, and utilization for defect chemistry in heterogeneous catalysis have been proposed. Based on recent progress, this work mainly introduces the fundamental concept, types, and characterization of defects in catalysts, significant effects of defects on catalytic properties, and controllable construction methods of defects for efficient catalyst synthesis. Finally, the conclusion and outlook for the research of defect chemistry in heterogeneous catalysis are presented. It is hoped that this work can provide guidance for the controllable synthesis and mechanism research of defective heterogeneous catalysts.

2,5‐Furandicarboxylic acid was obtained from the electrooxidation of 5‐hydroxymethylfurfural (HMF) with non‐noble metal‐based catalysts. Moreover, combining the biomass oxidation with the hydrogen evolution reaction (HER) increased the energy conversion efficiency of an electrolyzer and also generated value‐added products at both electrodes. Here, the reaction pathway on the surface of a carbon‐coupled nickel nitride nanosheet (Ni3N@C) electrode was evaluated by surface‐selective vibrational spectroscopy using sum frequency generation (SFG) during the electrochemical oxidation. The Ni3N@C electrode shows catalytic activities for HMF oxidation and the HER. As the first in situ SFG study on transition‐metal nitride for the electrooxidation upgrade of HMF, this work not only demonstrates that the reaction pathway of electrochemical oxidation but also provides an opportunity for nonprecious metal nitrides to simultaneously upgrade biomass and produce H2 under ambient conditions.

The solid-electrolyte interphase (SEI) layer is pivotal for the stable and rechargeable batteries especially under high rate. However, the mechanism of Li+ transport through the SEI has not been clearly...

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