Axial‐N Induced Square‐Pyramidal Crystal Filed of Atomically Iron Sites for Enhancing Acidic Oxygen Reduction

Atomically dispersed Fe-based catalyst represents a promising alternative to platinum for the oxygen reduction reaction (ORR). However, the prevalent FeN4 configuration exhibits limited intrinsic activity in acidic media owing to its inherent instability, thereby restricting its application in proton exchange membrane fuel cell (PEMFC). Herein, we introduce axial-N coordination to enhance the activity and stability of atomically dispersed Fe sites for acidic ORR by establishing a barrier to prevent Fe dissolution. Compared to the FeN4 configuration, the axial-N ligand in the FeN5, FeN5-Fe3, and FeN2C3 configurations induces a square-pyramidal crystal field, which diminishes the spin polarization in the dz 2, dxz, and dyz orbitals, and alters the electronic delocalization of Fe atom. In a 0.10 M HClO4 electrolyte, the ORR activity increases with enhanced electronic delocalization, following the trend: FeN5>FeN5+Fe3>FeN2C3>FeN4. Operando technique further reveals that the dissociation of Fe─N bond in the FeN5 configuration occurs alongside the insertion of oxygen, leading to the formation of FeN3O2 and FeN4O1 structures that could accelerate the ORR kinetics. Consequently, the FeN5 configuration shows a positive shift of 30 mV in half-wave potential compared to Pt/C and achieves a peak power of 1.2 W cm-2 at 3.2 A cm-2 in PEMFC.