The Dynamic Behavior of Electrocatalysts Oxygen Vacancy-Rich Co₃O₄ for Oxygen Evolution Reaction: An in situ/Operando X-ray Spectroscopic Study

Yu-Cheng Huang^1*, Zhaohui Xiao², Zhiwei Hu³, Shuangyin Wang², Ying-Rui Lu⁴, Jeng-Lung Chen⁴, Chi-Liang Chen⁴, K. Thanigai Arul⁵, Wu-Ching Chou¹, Chung-Li Dong⁵

¹Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan
²State Key Laboratory of Chem/Bio-Sensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Hunan 410082, China
³Max Planck Institute for Chemical Physics of Solids, Dresden 01187, Germany
⁴National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
⁵Research Center for X-ray Science & Department of Physics, Tamkang University, New Taipei City 25137, Taiwan

* Presenter:Yu-Cheng Huang, email:ychuang0129.04g@g2.nctu.edu.tw

The exact effect of defect structure in transition metal-based electrocatalysts on the oxygen evolution reaction (OER), which is a highly dynamic process, remains unclear. During this electrochemical reaction, determination of electronic structure of the catalyst and the structure-activity relationship of the defective electrocatalyst under operando conditions is very important for unraveling the reaction mechanism. Co₃O₄ with rich oxygen vacancy (Vo) has been reported to efficiently catalyze OER. However, the stability of Vo, especially under the highly oxidizing conditions of the OER, is a concern. Stabilizing Vo in Co₃O₄ while retaining excellent electrocatalytic activity is a real challenge. In this study, pure spinel Co₃O₄ and Vo-Co₃O₄ are investigated by in situ x-ray absorption spectroscopy. The defect formation mechanism is revealed through various operational conditions, and the dynamic behavior of defect sites in the electrocatalytic OER process is also examined. Soft X-ray spectroscopy was used to track the valence and spin states of Co ions in these Co₃O₄ in operational OER condition. It is suggested that Vo can initialize the surface reconstruction of Vo-Co₃O₄ before the OER process. The oxygen vacancies are firstly filled with OH-, promoting the formation of Vo-Co₃O₄ and the pre-oxidation of Co with low charge state, and causing the formation of intermediate Co-OOH-. A large proportion of Co ions undergo a voltage-dependent transition from Co3+ 3d⁶ to Co4 + 3d⁶L (L stands for the hole in the ligand O 2p states). Highly oxidized Co4+ sites, instead of Co3+ sites or oxygen vacancies, are mostly responsible for high OER activity. The dynamic evolution of the surface structure of the defective electrocatalyst was observed and the active sites in the electrocatalysis process were identified. Accordingly, the effect of defective and active sites on Co₃O₄ for OER was thus revealed in this study.

Keywords: Oxygen Vacancy (Vo), In situ/Operando X-ray Spectroscopy, X-ray Absorption Spectroscopy, Crystal/Ligand Field Theory