Transition Metal and Nitrogen Co-Doped Carbon-based Electrocatalysts for the Oxygen Reduction Reaction: From Active Site Insights to the Rational Design of Precursors and Structures.

Considering the urgent requirement for clean and sustainable energy, fuel cells and metal-air batteries have emerged as promising energy storage and conversion devices to alleviate the worldwide energy challenges. The key step in accelerating the sluggish oxygen reduction reaction (ORR) kinetics at the cathode is to develop cost-effective and high-efficiency non-precious metal catalysts, which can be used to replace expensive Pt-based catalysts. Recently, the transition metal and nitrogen co-doped carbon (M-Nx /C) materials with tailored morphology, tunable composition, and confined structure show great potential in both acidic and alkaline media. Herein, the mechanism of ORR is provided, followed by recent efforts to clarify the actual structures of active sites. Furthermore, the progress of optimizing the catalytic performance of M-Nx /C catalysts by modulating nitrogen-rich precursors and porous structure engineering is highlighted. The remaining challenges and development prospects of M-Nx /C catalysts are also outlined and evaluated.