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-Fe_3, and FeN₂C₃ configurations induces a square-pyramidal crystal field, which diminishes the spin polarization in the d_z ², d_xz, and d_yz 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.