Spin Engineering of Fe─N─C by Axial Ligand Modulation for Enhanced Bifunctional Oxygen Catalysis

Abstract Iron‐based single‐atom catalysts (Fe─N─C) exhibit excellent oxygen reduction activity but struggle with bifunctional performance due to their poor oxygen evolution activity. Although the Fe spin state is found to be closely associated with enhanced bifunctional activity, controllably regulating the Fe spin state remains a challenge. Here, the controllable regulation of Fe spin state is directly achieved through competitive coordination between chlorine and pyridine nitrogen in the axial direction of Fe─N 4 . The spin state of Fe is regulated from high spin to intermediate spin by the modulation of axial ligands from weak‐field ligand chlorine to strong‐field ligand pyridinic nitrogen, which leads to the enhanced bifunctional activity of N─FeN 4 with a small potential gap (Δ E = 0.68 V). Theoretical calculations indicate that the spin state turning is accompanied by an enhanced binding strength between Fe sites and *OH leading to a significant decrease in the OER barrier. Moreover, N─FeN 4 exhibits sufficient durability for oxygen reduction reaction (ORR) (over 50 h), oxygen evolution reaction (OER) (over 200 h), and the assembled zinc–air battery (over 1000 h). Here a novel approach is proposed for designing efficient catalysts based on spin state and profound insights into Fe─N─C spin state for bifunctional oxygen catalysis.