Engineering the first coordination sphere of FeN 4 sites with less electronegative heteroatoms for oxygen reduction catalysis

Rational modulation of Fe-N₄ coordination symmetry through heteroatom doping with distinct electronegativities has emerged as a promising strategy to optimize the performance of the oxygen reduction reaction (ORR). Here, we systematically investigate the less electronegative sulfur- and phosphorus-doped Fe single-atom catalysts and demonstrate that the S-doped catalyst (Fe-SNC) achieves superior ORR activity (E_1/2 = 0.904 V vs. RHE), surpassing both the P-doped Fe-PNC (0.872 V) and undoped Fe-NC control samples while maintaining exceptional durability. Synchrotron radiation X-ray absorption spectroscopy verified the precise engineering of the asymmetric Fe-S₁N₃ and Fe-P₁N₃ configurations within the ZIF-8-derived carbon matrices, confirming successful manipulation of the first coordination sphere. In situ synchrotron radiation infrared spectroscopy further elucidates accelerated *OOH dissociation kinetics in Fe-SNC, which benefits from the optimization of the electronic structure of Fe 3d by S doping. These findings conclusively establish geometric symmetry breaking via electronegativity-driven electronic modulation as an effective strategy for advancing metal-N₄ catalyst design.