Engineering the Lewis Acidity of Fe Single-Atom Sites via Atomic-Level Tuning of Spatial Coordination Configuration for Enhanced Oxygen Reduction

Nitrogen-doped carbon-supported Fe catalysts (Fe-N-C) with Fe-N4 active sites hold great promise for the oxygen reduction reaction (ORR). However, fine-tuning the structure of Fe-N4 active sites to enhance their performance remains a grand challenge. Herein, we report an innovative design strategy to promote the ORR activity and kinetics of Fe-N4 sites by engineering their Lewis acidity, which is achieved by tuning the spatial Fe coordination geometry. Theoretical calculations indicated that Fe1-N4SO2 sites (with an axial –SO2 group bonded to Fe) offered favorable Lewis acidity for the ORR, leading to optimized adsorption energies for the key ORR intermediates. To implement this strategy, we developed a molecular-cage-encapsulated coordination strategy to synthesize a Fe single-atom site catalyst (SAC) with Fe1-N4SO2 sites. In agreement with theory, the Fe1-N4SO2/NC catalyst demonstrated outstanding ORR performance in both alkaline (E1/2 = 0.910 V in 0.1 M KOH) and acidic media (E1/2 = 0.772 V in 0.1 M HClO4), surpassing commercial Pt/C and traditional Fe SACs with Fe1-N4 sites or planar S-coordinated Fe1-N4-S sites. Moreover, this newly developed catalyst showed great application potential in quasi-solid-state Zn–air batteries, delivering superior performance across a wide temperature range.