Tuning the Spin State of the Iron Center by Bridge‐Bonded Fe‐O‐Ti Ligands for Enhanced Oxygen Reduction

Exploring functional substrates and precisely regulating the electronic structures of atomic metal active species with moderate spin state are of great importance yet remain challenging. Hereon, we provide an axial Fe‐O‐Ti ligand regulated spin‐state transition strategy to improve the oxygen reduction reaction (ORR) activity of Fe centers. Theoretical calculations indicate that Fe‐O‐Ti ligands in FeN3O‐O‐Ti can induce a low‐to‐medium spin‐state transition and optimize O2 adsorption by FeN3O. As a proof‐of‐concept, the oriented catalyst was prepared from atomic‐Fe‐doped polymer‐like quantum dots and ultrathin o‐terminated MXene. The optimal catalyst exhibits an intrinsic activity that is almost 5 times higher than the control sample (without axial Fe‐O‐Ti ligands). It also delivers a superior performance in Zn‐air batteries and H2/O2 anion exchange membrane fuel cells in a wide‐temperature range. The spin‐state tuning for atomic Fe centers (FeN3O) is attained through introducing an axial Fe‐O‐Ti bridge bond in atomic‐Fe doped polymer‐like quantum dots/ultrathin o‐terminated MXene (o‐MQFe) hybrids. This low‐to‐medium spin‐state transition gives the o‐MQFe catalyst stronger O2 adsorption affinity and thus a higher intrinsic oxygen reduction reaction (ORR) activity.