Second Coordination Shell Engineering Tunes Fe 3d Orbital Occupancy for Enhanced Oxygen Reduction Catalysis

Abstract

Second coordination shell (SCS) engineering offers a powerful strategy for fine‐tuning the intrinsic activity of Fe–N‐C single‐atom catalysts, yet achieving targeted SCS regulation remains a formidable synthetic challenge. Herein, we propose a rational precursor‐engineering approach driven by the steric hindrance of pyrazole‐borate ligands to precisely construct Fe─N 4 sites with tunable boron SCS doping levels. The optimized FeN 4 B 3 /NC catalyst exhibits an exceptional half‐wave potential of 0.92 V for the oxygen reduction reaction (ORR) in alkaline media and a specific energy density of 967.03 Wh kg Zn −1 in a zinc‐air battery. Combined experimental and theoretical investigations reveal that second‐shell boron coordination induces ligand‐field asymmetry around the Fe center, leading to an occupancy transition of ‐to‐ in the Fe 3 orbital. This reconfiguration of Fe 3d electron distribution fine‐tunes the adsorption energetics by forcing electron injection into the Fe─O σ∗ anti‐bonding orbital, which generates enhanced Pauli repulsion to weaken the * OH binding affinity and accelerate its desorption. This work demonstrates the pivotal role of the SCS environment in intrinsic activity modulation but also provides an orbital‐level design principle for high‐performance single‐atom catalysts.