Co-based bifunctional OER/ORR Electrocatalysts: From nanostructure engineering to catalytic mechanism innovation for cutting-edge research in empowering energy conversion systems

The focal point of new energy research lies in the advancement of novel energy conversion and storage apparatus. The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) serve as two fundamental processes in energy conversion systems. In the process of catalytic reaction, the reactants need to diffuse to the surface of the catalyst to carry out the reaction, including mass transfer, adsorption, desorption and other processes between the two-phase interface. This is also the main reason for the slow kinetics of ORR and OER. Cobalt (Co)-based materials stand out as a prime alternative to traditional precious metal catalysts, owing to their earth-abundance, excellent electronic effects and controllability of structure and morphology. This review focuses on the structural configuration, synthesis methodologies, and emerging applications of Co-based bifunctional electrocatalysts, and investigates their catalytic performances in ORR/OER reactions within alkaline electrolytes. Firstly, by analyzing the catalytic traits and potential reaction routes of ORR/OER, the main reaction mechanisms of both are elaborated respectively. Secondly, the synthesis strategies, activity enhancement mechanisms, and electrocatalytic performances of Co-based electrocatalysts containing Co-based oxides, Co-based phosphides, Co-based sulfides, Co-N-C materials, and Co-based single atoms are emphasized. Moreover, the applications of these catalysts in ORR/OER bifunctional electrocatalysis are investigated. Finally, we focus on the strategies and application prospects of Co-based bifunctional electrocatalysts in the rapidly growing field, as well as facing the challenges and opportunities in the future. This review is expected to provide a reference for peer researchers in designing and developing the next-generation bifunctional ORR/OER electrocatalysts with better activity and long-term stability. This review analyzes the research status of Co-based bifunctional electrocatalysts in the ORR and OER fields, focuses on the synthesis strategies, activity enhancement mechanisms, and bifunctional electrocatalytic performances of Co-based oxides, sulfides, nitrides, phosphides, Co-N-C, and Co-based single-atom electrocatalysts. Moreover, looking ahead to future directions, providing a reference for the development of the next generation of such catalysts. • Covers research progress of Co-based bifunctional electrocatalysts in ORR/OER. • Elaborating on reaction mechanisms of ORR/OER for better understanding. • Focusing on synthesis, activity enhancement mechanisms of various Co-based catalysts. • Putting forward the challenges/prospects of Co-based electrocatalysts for ORR/OER.