Microenvironment Design and Regulation of M-N-C Catalysts for Oxygen Reduction Reactions: Mechanisms, Synergies, and Future Pathways.

Fuel cells represent the core components of next-generation high-efficiency electrochemical energy conversion technologies. The performance of the cathode oxygen reduction reaction (ORR) catalyst determines battery efficiency and lifespan. Thus, developing efficient, cost-effective ORR single-atom catalysts is essential for fuel cell commercialization. However, the symmetrical metal with a four-nitrogen (MN4) coordination and its electronic structure hinder the adsorption and conversion of oxygen intermediates, resulting in insufficient intrinsic activity and poor structural stability, which limits the efficiency of the ORR. To address this issue, several strategies have been proposed to improve catalytic performance and stability. This review introduces the mechanism of ORR and analyzes the effects of coordination engineering and substrate modification on the electronic structure, coordination environment, and geometric configuration of active sites in transition metal-nitrogen-carbon (M-N-C) catalysts, based on representative experimental results and theoretical calculations. Particular emphasis is placed on the relationship between the microenvironment and intrinsic activity. Subsequently, this review analyzes the origins of discrepancies between theoretical calculations and electrochemical experiments and summarizes optimization strategies to bridge this gap. Furthermore, the value of in situ characterization techniques is highlighted in investigating the microscale coordination and interfacial microenvironment of active sites, thereby promoting an understanding of catalytic mechanisms.