The Active Sites and Corresponding Stability Challenges of the M-N-C Catalysts for Proton Exchange Membrane Fuel Cell

Proton exchange membrane fuel cells (PEMFCs) as promising alternatives to traditional internal combustion engines have attracted massive concerns to promote their wide application in society. However, the biggest challenge to the commercialization of PEMFCs remains the high cost due to the adoption of the platinum group metal (PGM) catalysts in the cathode. Thus, the development of PGM-free catalysts based on earth-abundant elements with a much lower cost is considered as the most favorable solution. Although the reported activity of the state-of-the-art PGM-free catalyst is comparable with that of a typical commercial Pt/C catalyst, the precise structure of active sites on the PGM-free catalysts is debatable. In addition, the stability of the highly active PGM-free catalysts still needs improvement. Herein, we reviewed the recent research about the nature of the active sites, the progress in stability improvement, and the degradation mechanisms of active sites on PGM-free catalysts.