Regulating Fe Intermediate Spin States via FeN₄‐Cl‐Ti Structure for Enhanced Oxygen Reduction

Abstract

Abstract Modulating the spin states of FeN 4 moieties is critical for enhancing the electrocatalytic oxygen reduction reaction (ORR). In this study, Ti 4 N 3 Cl x and Ti 4 N 3 O x MXenes are synthesized and functionalized with iron phthalocyanine (FePc) to form model catalysts with well‐defined FeN 4 ‐Cl‐Ti and FeN 4 ‐O‐Ti structures, respectively. The FeN 4 ‐Cl‐Ti structure, formed within the Ti 4 N 3 Cl x /FePc composite, enables precise modulation of FeN 4 spin states from low to intermediate spin, significantly enhancing ORR performance. In contrast, the FeN 4 ‐O‐Ti structure in Ti 4 N 3 O x /FePc shows less effective spin state modulation, leading to comparatively lower ORR activity. Compared to FePc and Ti 4 N 3 O x /FePc, Ti 4 N 3 Cl x /FePc demonstrates superior electrochemical performance, with an ORR half‐wave potential of +0.91 V versus RHE and doubled power densities in Zn–air batteries (214.5 mW cm −2 ). Theoretical studies confirm that the intermediate spin states induced by the weak‐field ligand‐modified FeN 4 ‐Cl‐Ti structure in Ti 4 N 3 Cl x /FePc facilitate electron filling in the antibonding orbital composed of Fe 3dz 2 and O 2 π* orbitals, greatly enhancing O₂ activation and ORR activity. These findings underscore the superior catalytic properties of FeN 4 ‐Cl‐Ti compared to FeN 4 ‐O‐Ti, advancing the understanding of spin state‐related catalytic mechanisms and guiding the design of high‐performance ORR catalysts.