Asymmetric electronic regulation of iron sites induced by axial sulfur coordination in the first sphere for enhanced oxygen reduction reaction in Mg-air and Zn-air batteries

In-situ assembly and pyrolysis axially coordinate S to first sphere of Fe in Fe/S-NC. • Axial S induces the charge asymmetry of Fe–N 4 in Fe/S-NC via electronic modulation. • Lowered d-band center via symmetry breaking optimizes ORR intermediate adsorption. • Mg-air and Zn-air batteries with the Fe/S-NC catalyst exhibit superior performance. Developing efficient electrocatalysts for the oxygen reduction reaction (ORR) is essential to boosting the practical performance of Mg-air and Zn-air batteries. However, conventional non-precious Fe-N-C catalysts, with their planar Fe–N 4 sites, exhibit overly strong adsorption toward oxygenated intermediates and thus severely limit their catalytic kinetics. In this study, we report an Fe/S-NC catalyst based on an iron phthalocyanine (FePc) framework, in which a single sulfur atom is axially coordinated directly to the iron site in the first sphere. The resulting asymmetric Fe–N 4 S 1 motif breaks geometric and electronic symmetry of the iron center and thereby accelerates ORR kinetics. Theoretical calculations reveal that electronic reconstruction induced by the axial sulfur effectively optimizes the d-band center of the iron site and weakens the binding strength toward oxygenated intermediates, thus significantly reducing the energy barrier and enhancing the adsorption-desorption thermodynamics. Benefiting from this well-defined structure, the Fe/S-NC catalyst delivers half-wave potentials of 0.88 and 0.63 V (vs. RHE) in 0.1 M KOH and 3.5 wt% NaCl, respectively, and outperforms commercial Pt/C when applied in Mg-air and Zn-air batteries. This work elucidates how electronic asymmetry boosts catalytic performance and provides theoretical guidance for designing high-performance ORR catalysts. Axially coordinated sulfur atoms induce an asymmetric charge distribution at the Fe–N 4 active sites via electronic modulation. This symmetry breaking downshifts the d-band center and refines the adsorption equilibrium of oxygenated intermediates, thereby significantly accelerating the oxygen reduction reaction (ORR) kinetics and enhancing catalyst stability in both alkaline and neutral environments. Download: Download high-res image (77KB) Download: Download full-size image