Insight into the Mechanism of Axial Ligands Regulating the Catalytic Activity of Fe–N4 Sites for Oxygen Reduction Reaction

Abstract

Identifying the actual structure and tuning the catalytic activity of Fe–N₄-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–N₄-based model electrocatalyst, a mechanistic insight into the effect of axial ligands on the ORR catalytic activity of Fe–N₄ is provided and it is revealed that the ORR activity of Fe–N₄ sites with OH desorption as a rate-determining step is related to the energy level gap between the OH p_xp_y and Fe 3$d_{z^2}$, 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 3$d_{z^2}$ 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–N₄ and reveals the correlation between the electronic/geometric structure and catalytic activity of Fe–N₄.