Understanding the Catalytic Sites of Metal–Nitrogen–Carbon Oxygen Reduction Electrocatalysts

Abstract The development of low‐cost catalysts containing earth‐abundant elements as alternatives to Pt‐based catalysts for the oxygen reduction reaction (ORR) is crucial for the large‐scale commercial application of proton exchange membrane fuel cells (PEMFCs). Nonprecious metal–nitrogen–carbon (M‐N‐C) materials represent the most promising candidates to replace Pt‐based catalysts for PEMFCs applications. However, the high‐temperature pyrolysis process for the preparation of M‐N‐C catalysts frequently leads to high structural heterogeneity, that is, the coexistence of various metal‐containing sites and N‐doped carbon structures. Unfortunately, this impedes the identification of the predominant catalytic active structure, and thus, the further development of highly efficient M‐N‐C catalysts for the ORR. This Minireview, after a brief introduction to the development of M‐N‐C ORR catalysts, focuses on the commonly accepted views of predominant catalytic active structures in M‐N‐C catalysts, including atomically dispersed metal–Nx sites, metal nanoparticles encapsulated with nitrogen‐doped carbon structures, synergistic action between metal–Nx sites and encapsulated metal nanoparticles, and metal‐free nitrogen‐doped carbon structures.