Insight into the Mechanism of Axial Ligands Regulating the Catalytic Activity of Fe–N ₄ Sites for Oxygen Reduction Reaction

Identifying the actual structure and tuning the catalytic activity of Fe-N-4-based moieties, well-recognized high-activity sites in the oxygen reduction reaction (ORR) are challenging problems. Herein, by using poly(iron phthalocyanine) (PFePc) as an Fe-N-4-based model electrocatalyst, a mechanistic insight into the effect of axial ligands on the ORR catalytic activity of Fe-N-4 is provided and it is revealed that the ORR activity of Fe-N-4 sites with OH desorption as a rate-determining step is related to the energy level gap between the OH p(x)p(y) and Fe 3dz2, which can be tuned by regulating the field strength of the axial ligands. Thus, PFePc coordinated with a weak-field ligand I- (PFePc-I) with a low energy level of Fe 3dz2 exhibits high activity evidenced by an ORR half-wave potential as high as 0.948 V versus RHE. This work develops a novel strategy for tuning the ORR activity of Fe-N-4 and reveals the correlation between the electronic/geometric structure and catalytic activity of Fe-N-4.