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 FeN 3 O-O-Ti can induce a low-to-medium spin-state transition and optimize O 2 adsorption by FeN 3 O. 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 H 2 /O 2 anion exchange membrane fuel cells in a wide-temperature range.