Microenvironment Modulation of Single Iron Site in Oxygen Reduction Reaction Revealed by Molecular Model Catalysts

Precise regulation of the coordination environment in iron-based single-site catalysts (Fe SSCs) is essential for promoting the two-electron oxygen reduction reaction (2e^- ORR) toward efficient hydrogen peroxide production. However, the atomic-scale relationship between coordination structure and reaction pathway remains insufficiently understood. In this work, a series of iron molecular catalysts with tunable axial coordination were rationally designed to systematically investigate how ligand-mediated regulation of the metal site and ligand conjugation direct the 2e^- ORR pathway. Combined electrochemical measurements, in situ spectroscopic analysis, and theoretical calculations demonstrate that electron-rich Fe centers weaken *OOH adsorption, thereby enhancing H₂O₂ selectivity. In contrast, increased ligand conjugation modifies the local catalytic microenvironment, strengthens oxygen intermediate adsorption, and reduces H₂O₂ selectivity. A linear structure-activity relationship between the Fe²⁺/Fe³⁺ redox potential and H₂O₂ selectivity is established, identifying this redox potential as a reliable descriptor for rapid catalyst screening. When the optimal Fe-NHC/Py(-H) catalyst was integrated onto monolayer graphene to form a gas diffusion electrode, it delivered 97.6% Faradaic efficiency and an H₂O₂ yield of 72.9 mol g_cat ^-1 h^-1 at 400 mA cm^-2. These findings provide mechanistic insights for designing highly selective 2e^- ORR electrocatalysts via coordination engineering.