Resolving Active-Site Heterogeneity in Fe–N–C Catalysts by Integrated Multimodal Spectroscopy and Simulation

Fe–N–C single-atom catalysts are among the most promising electrocatalysts for the oxygen reduction reaction (ORR), yet their atomic-scale structure remains difficult to resolve because high-temperature synthesis generates a diverse population of Fe sites. In this work, we combine Fe K-edge X-ray absorption spectroscopy (XAS), 57Fe Mössbauer spectroscopy, density functional theory (DFT), and spectroscopy simulations to elucidate Fe-site structures before and after air exposure. The as-pyrolyzed catalyst is best described by a heterogeneous distribution of FeN4 sites, dominated by a defect-associated pyrrolic motif with third-shell nitrogen coordination and a smaller pyridinic contribution. We show that coordination beyond the first shell strongly influences Fe–N bond distances, local symmetry, and XANES features. Air exposure converts square-planar Fe(II)-like sites into oxidized, axially coordinated Fe(III)-like species. Overall, this multimodal structure–spectroscopy framework provides a robust strategy for resolving active-site heterogeneity in complex single-atom catalysts.