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 (E_1/2 = 0.910 V in 0.1 M KOH) and acidic media (E_1/2 = 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.