Abstract: Water electrolysis for hydrogen production holds great promise as an energy conversion technology. The electrolysis process contains two necessary electrocatalytic reactions, one is the hydrogen evolution reaction (HER) at the cathode, and the other is the oxygen evolution reaction (OER) at the anode. In general, the kinetics of OER is much slower than that of HER, dominating the overall of performance electrolysis. As identified, the slow kinetics of catalytic OER is mainly resulted from multiple electron transfer steps, and the catalysts often undergo compositional, structural, and electronic changes during operation, leading to complicated dynamic reaction mechanisms which have not been fully understood. Obviously, this challenge presents formidable obstacles to the development of highly efficient OER electrocatalysts. To address the issue, it is crucial to unravel the origins of intrinsic OER activity and stability and elucidate the catalytic mechanisms across diverse catalyst materials. In this context, in-situ/operando characterization techniques would play a pivotal role in understanding the catalytic reaction mechanisms by enabling real-time monitoring of catalyst structures under operational conditions. These techniques can facilitate the identification of active sites for OER and provide essential insights into the types and quantities of key reaction intermediates. This comprehensive review explores various catalyst design and synthesis strategies aimed at enhancing the intrinsic OER activity and stability of catalysts and examines the application of advanced in-situ/operando techniques for probing catalyst mechanisms during the OER process. Furthermore, the imperative need for developing innovative in-situ/operando techniques, theoretical artificial intelligence and machine learning and conducting theoretical research to better understand catalyst structural evolution under conditions closely resembling practical OER working states is also deeply discussed. Those efforts should be able to lay the foundation for the improved fabrication of practical OER catalysts.

Key words: Oxygen evolution reaction, Electrocatalysts, Reaction mechanism, In-situ techniques