On the Nature of Durable Active Sites in Fe–N–C Fuel Cell Catalysts

Fe–N–C catalysts have the potential to replace the costly platinum catalysts in fuel cells but face the challenge of instability. It is of vital importance to identify the chemical nature of durable active sites in Fe–N–C. In this perspective, we analyze the geometric and electronic factors that affect the intrinsic durability of the FeN_xC_y moieties and propose that iron–oxygen binding energy is most relevant. A weak Fe–O binding is beneficial to mitigate the oxidation attack to the iron center by reaction intermediates thus enhancing its resistance to demetalation. We then propose the iron oxidation (valence) state as an apparent descriptor of the Fe–O binding strength. A valence state of +2 indicates a high anti-oxidation ability and promises superior stability. Our proposal will deepen the understanding of the activity–stability trade-off for Fe–N–C catalysts and guide future active site optimization.