PNNL

Abstract

Exploring high-performance and low-cost platinum group metal (PGM)-free catalysts for the oxygen reduction reaction (ORR) is a promising strategic pathway to decrease the cost of current proton exchange membrane fuel cells (PEMFCs). In recent years, transition metals (Fe, Co, etc.) and nitrogen co-doped carbon materials (M-N-C) have demonstrated promising activity and stability. Although significant progress has been achieved for Fe-N-C catalysts, there is a potential concern of Fenton reactions between Fe2+ and H2O2 with the formation of free radicals, which cause the degradation of catalysts, ionomers and membrane used in PEMFCs. Co-N-C catalysts have been explored as an alternative with significantly mitigated Fenton reactions relative to Fe. In this review, we focus on the Co-N-C catalysts in acidic medium relevant to PEMFC applications. Catalyst synthesis, structure/morphology, activity and stability improvement, and reaction mechanism are discussed. Through reviewing experimental and theoretical results, we aim to elucidate the correlations between structure/morphology and activity and provide guidance for rational design of advanced Co catalysts with special emphasis on atomically dispersed Co catalysts. To reduce the peroxide generation during the ORR on Co catalysts, potential strategies are outlined to minimize the detrimental effect of peroxide for the enhancement of fuel cell durability.