Iron-Free Cathode Catalysts for Proton-Exchange-Membrane Fuel Cells: Cobalt Catalysts and the Peroxide Mitigation Approach.

High-performance and inexpensive platinum-group-metal (PGM)-free catalysts for the oxygen reduction reaction (ORR) in challenging acidic media are crucial for proton-exchange-membrane fuel cells (PEMFCs). Catalysts based on Fe and N codoped carbon (Fe-N-C) have demonstrated promising activity and stability. However, a serious concern is the Fenton reactions between Fe and H O generating active free radicals, which likely cause degradation of the catalysts, organic ionomers within electrodes, and polymer membranes used in PEMFCs. Alternatively, Co-N-C catalysts with mitigated Fenton reactions have been explored as a promising replacement for Fe and PGM catalysts. Therefore, herein, the focus is on Co-N-C catalysts for the ORR relevant to PEMFC applications. Catalyst synthesis, structure/morphology, activity and stability improvement, and reaction mechanisms are discussed in detail. Combining experimental and theoretical understanding, the aim is to elucidate the structure-property correlations and provide guidance for rational design of advanced Co catalysts with a special emphasis on atomically dispersed single-metal-site catalysts. In the meantime, to reduce H O generation during the ORR on the Co catalysts, potential strategies are outlined to minimize the detrimental effect on fuel cell durability.