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

pin control of the FeN4 moieties is critical for enhancing the electrocatalytic oxygen reduction reaction (ORR). The relationship between the spin state transitions of the FeN4 moieties and ORR activity is complex and remains debatable due to the discrepancies between the theoretical models and experi-mental catalyst structures, along with potential misinterpreting the 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; this enabled the pre-cise modulation of the FeN4 spin states, established a clear correlation between the intermediate spin states, and improved the ORR performance. Specifically, compared FePc with a 17.1% intermediate spin state, Ti3C2Brx/FePc with an 88.1% intermediate spin state exhibited superior electrochemical per-formance, showing 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). Theoretical studies confirmed that the intermediate spin state led to electron filling in the antibonding orbital composed of the Fe 3dz2 and O2 π* orbitals, significantly improving the O2 activation and ORR activity. This research advances our understanding of the spin-related origins of catalytic activity and facilitates the design and optimiza-tion of advanced ORR catalysts.