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.