Identification of <i>In Situ</i> Generated High-Spin Fe(II)-N ₄ Active Sites for Acidic Oxygen Reduction Reaction via <i>Operando</i> ⁵⁷ Fe Mössbauer Spectroscopy

Iron-nitrogen-carbon (FeNC) catalysts are considered among the most promising alternative to Pt catalysts in acidic oxygen reduction reaction (ORR), yet the geometric and electronic structures of the true active site under reaction conditions have not been clearly elucidated. Herein, we synthesized a representative FeNC catalyst by pyrolyzing Fe3+ absorbed on ZIF-8-derived N-doped carbon at a mild temperature under the H2/Ar atmosphere, where a formation mechanism of FeN4 sites through a Zn-mediated Fe nanoparticle atomization process was proposed. The resulting FeNC-750 catalyst shows high acidic ORR activity with a half-wave potential of 0.838 V and a peak power density of 1 W cm-2 in proton-exchange membrane fuel cell (PEMFC). By using operando 57Fe Mössbauer spectroscopy on FeNC-750, it was revealed that the pyrrolic N-coordinated high-spin Fe2+N4 sites, which are in situ generated from high-spin Fe3+N4 during ORR, are identified as catalytic active states. Density functional theory calculations further verified that, compared to the pyridinic N-coordinated low-spin Fe2+N4, the pyrrolic N-coordinated high-spin Fe2+ exhibits optimized adsorption energy for reaction intermediates, thereby lowering the energy barrier of the rate-determining step (RDS) and facilitating OH* desorption. This work provides both experimental and theoretical evidence of the true active site in the FeNC during acidic ORR, offering significant insights for the rational design of high-performance materials for acidic fuel cells.