Effective Approaches for Designing Stable M–N_x/C Oxygen-Reduction Catalysts for Proton-Exchange-Membrane Fuel Cells

The large-scale commercialization of proton-exchange-membrane fuel cells (PEMFCs) is extremely limited by their costly platinum-group metals (PGMs) catalysts, which are used for catalyzing the sluggish oxygen reduction reaction (ORR) kinetics at the cathode. Among the reported PGM-free catalysts so far, metal–nitrogen–carbon (M–N_x/C) catalysts hold a great potential to replace PGMs catalysts for the ORR due to their excellent initial activity and low cost. However, despite tremendous progress in this field in the past decade, their further applications are restricted by fast degradation under practical conditions. Herein, the theoretical fundamentals of the stability of the M–N_x/C catalysts are first introduced in terms of thermodynamics and kinetics. The primary degradation mechanisms of M–N_x/C catalysts and the corresponding mitigating strategies are discussed in detail. Finally, the current challenges and the prospects for designing highly stable M–N_x/C catalysts are outlined.