Single-Site-Level Deciphering of the Complexity of Electrochemical Oxygen Reduction on Fe–N–C Catalysts

Fe–N–C catalysts are emerging as potential alternatives to platinum in the oxygen reduction reaction (ORR) for fuel cell cathodes. The challenge in optimizing these catalysts lies in their structural complexity and the multiplicity of reaction pathways. Here, we employ a series of model catalysts with varying amounts of Fe–N_x and Fe nanoparticles (NPs) and estimate their turnover frequency (TOF) for apparent H₂O and H₂O₂ production at different catalyst loadings. This approach highlights the importance of the surface site density (SD) of Fe–N_x moieties in determining the overall ORR activity, selectivity, and even stability. We uncover that increasing the SD of Fe–N_x moieties fosters the indirect 4e^– ORR pathway and consequently promotes their TOF toward preferential H₂O production. In contrast, Fe NPs, often formed at high Fe contents, behave as anticatalysts (or spectators) in this context. Indeed, an online inductively coupled plasma-mass spectrometry (ICP-MS) study reveals that a higher SD can lead to the faster leaching of Fe–N_x moieties during operation, resulting in accelerated activity decline. Taken together, the comprehensive understanding of the intricate dependence of catalytic activity and stability on the nature and amount of Fe species provides a basis for design principles of next-generation Fe–N–C catalysts.