Tuning the Electronic Structure and Spin State of Fe─N─C Catalysts Using an Axial Oxygen Ligand and Fe Clusters for High‐Efficiency Rechargeable Zinc–Air Batteries

ABSTRACT Fe─N─C catalysts with square‐planar FeN 4 sites show excellent activity for the oxygen reduction reaction (ORR), but poor activity for the oxygen evolution reaction (OER), hampering their use as air–electrode catalysts in rechargeable zinc–air batteries (ZABs). Herein, an acetate (Ac) ligand‐regulation strategy was developed to prepare FeN 4 ─O/Clu@NC‐ x Ac catalysts with square‐pyramidal FeN 4 ─O sites and adjacent Fe clusters. By optimizing the mass of sodium acetate ( x = 0.05–0.3 g) used during the catalyst syntheses, the electronic properties of FeN 4 ─O sites could be tuned to allow efficient bifunctional ORR/OER activity. A FeN 4 ─O/Clu@NC‐0.1Ac catalyst delivered a half‐wave potential of 0.89 V for ORR and a low overpotential of 330 mV at 10 mA cm −2 for OER in 0.1 m KOH. Furthermore, FeN 4 ─O/Clu@NC‐0.1Ac demonstrates exceptional ORR activity in acidic ( E 1/2 = 0.72 V) and neutral ( E 1/2 = 0.70 V) media. An aqueous zinc–air battery assembled using FeN 4 ─O/Clu@NC‐0.1Ac afforded a remarkable power density (284.5 mW cm −2 ) and ultralong cycling stability. Magnetic data combined with DFT calculations reveal that the axial O ligand at FeN 4 sites altered the spin state of Fe 2+ from low‐spin to medium‐spin, which along with electron transfer from adjacent Fe clusters, fine‐tunes the adsorption energy of oxygen intermediates to allow efficient ORR/OER activity.