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

Abstract

Modulating the spin states of FeN4 moieties is critical for enhancing the electrocatalytic oxygen reduction reaction (ORR). In this study, Ti4N3Clx and Ti4N3Ox MXenes are synthesized and functionalized with iron phthalocyanine (FePc) to form model catalysts with well-defined FeN4-Cl-Ti and FeN4-O-Ti structures, respectively. The FeN4-Cl-Ti structure, formed within the Ti4N3Clx/FePc composite, enables precise modulation of FeN4 spin states from low to intermediate spin, significantly enhancing ORR performance. In contrast, the FeN4-O-Ti structure in Ti4N3Ox/FePc shows less effective spin state modulation, leading to comparatively lower ORR activity. Compared to FePc and Ti4N3Ox/FePc, Ti4N3Clx/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 FeN4-Cl-Ti structure in Ti4N3Clx/FePc facilitate electron filling in the antibonding orbital composed of Fe 3dz2 and O2 π* orbitals, greatly enhancing O₂ activation and ORR activity. These findings underscore the superior catalytic properties of FeN4-Cl-Ti compared to FeN4-O-Ti, advancing the understanding of spin state-related catalytic mechanisms and guiding the design of high-performance ORR catalysts.