Regulating single-Fe-atom spin-state via implanting second-shell symmetrical sulfur coordination

Noble-metal-free iron-nitrogen-carbon (Fe–N–C) catalysts with well-defined FeN 4 active sites are promising alternatives to replace platinum (Pt) catalysts for oxygen reduction reaction (ORR). However, the precise design of the peripheral heteroatom-doped FeN 4 centers and the clarification of the underlying electrocatalytic ORR mechanism remain a significant challenge. Herein, we employ a coordination engineering strategy to implant two sulfur (S) atoms into the second coordination shell of the FeN 4 center (FeN 4 S 2 ), which effectively modulates the Fe spin state via d z 2 - p , d xz - p , and d yz - p interactions. The symmetrical S-doping in FeN 4 S 2 can weaken the spin polarization of FeN 4 that induces a transition of single-Fe-atom from a high-spin to a medium-spin state, thereby optimizing the oxygen intermediates adsorption and transforming the ORR potential-determining step from *O → *OH over FeN 4 to *O 2 → *OOH over FeN 4 S 2 with a lower free energy barrier, which greatly boosts the ORR performance in both alkaline and acidic media. The zinc-air battery with a FeN 4 S 2 air electrode delivers a peak power density as high as 248.2 mW cm –2 with exceptional charge-discharge stability, significantly outperforming the state-of-the-art Pt/C air electrode.