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

ABSTRACT 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 H 2 O 2 selectivity. In contrast, increased ligand conjugation modifies the local catalytic microenvironment, strengthens oxygen intermediate adsorption, and reduces H 2 O 2 selectivity. A linear structure‐activity relationship between the Fe 2+ /Fe 3+ redox potential and H 2 O 2 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 H 2 O 2 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.