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 FeN 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 FeN configuration, the axial-N ligand in the FeN, FeN-Fe and FeNC configurations induces a square-pyramidal crystal field, which diminishes the spin polarization in the d , d, and d orbitals, and alters the electronic delocalization of Fe atom. In a 0.10 M HClO electrolyte, the ORR activity increases with enhanced electronic delocalization, following the trend: FeN>FeN+Fe>FeNC>FeN. Operando technique further reveals that the dissociation of Fe─N bond in the FeN configuration occurs alongside the insertion of oxygen, leading to the formation of FeNO and FeNO structures that could accelerate the ORR kinetics. Consequently, the FeN 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 at 3.2 A cm in PEMFC.