Deciphering Spin-Activity Relationship of FeN4 Moieties Bridged in Halogenated MXenes for Oxy-gen Reduction

Spin control of FeN4 moieties is critical for enhancing the electrocatalytic oxygen reduction reaction (ORR). The relationship between Fe-N4 spin state transitions and ORR activity is complex and remains controversial due to discrepancies between theoretical models and experimental catalyst structures, along with potential misinterpreting characterization data. In this study, we utilized Ti3C2Tx MXenes with various terminations (−I, −Br, −Cl, −F, and −O), integrated with iron phthalocyanine (FePc), to form model catalysts with defined FeN4-Tx-Ti structures, allowing for precise modulation of the FeN4 spin states and establishing a clear correlation between the intermediate spin state and improved ORR performance. Specifically, Ti3C2Brx/FePc, featuring 88.1% intermediate spin state, exhibited superior electrochemical performances, with an ORR half-wave potential of 0.94 V versus RHE, and doubled power densities in Zn-air batteries (252.5 mW cm-2) and H2-O2 fuel cells (350.7 mW cm-2) compared to FePc with 17.1% intermediate spin state. Theoretical studies confirmed that the intermediate spin state leads to electron filling in the anti-bonding orbital composed of Fe 3dz2 and O2 π* orbitals, significantly improving O2 activation and ORR activity. This research advances our understanding of the spin-related origins of catalytic activity and sets the stage for the design and optimization of advanced ORR catalysts.