Bioinspired Molecular Magnetic Field-Responsive Catalyst for On-Demand Switching of 2e^- and 4e^- Oxygen Reduction

Achieving precise and on-demand steering of the oxygen reduction reaction (ORR) pathway between the efficient 4e^- route to H₂O and the valuable 2e^- route to H₂O₂ remains a pivotal challenge in electrocatalysis. Herein, we address this challenge by designing a bioinspired molecular magnetic field-responsive catalyst (MMFR-C) via magnetic single-atom-anchored Salen-based covalent organic frameworks (MSA-Salen COFs) onto magnetic nanoparticles (single/multi-domain Fe₃O₄). Mimicking cytochrome c oxidase, the MMFR-C employs MSA-Salen COFs as an ordered proton-transfer channel and well-defined N₂-M-O₂ moieties as enzymatic O₂ activation sites, with Fe₃O₄ providing a built-in magnetic field for remote regulation of the active-site electronic structure. The bioinspired MMFR-C exhibits switchable ORR pathways. Relative to the pristine Co-Salen COF (26% H₂O₂ selectivity, n = 3.48), the MMFR-C integrated with a single-domain Fe₃O₄ exhibits a remarkably enhanced H₂O₂ selectivity of 63.9% (n = 2.72), while that with a multi-domain Fe₃O₄ diverts the ORR pathway toward the 4e^- route (n = 3.67). (i) We elucidate that the uniform magnetic field from the single-domain Fe₃O₄ in MMFR-C favors orbital hybridization between its active N₂-M-O₂ moieties and the *OOH intermediate, with moderate *OOH adsorption suppressing O-O scission and thus steering ORR selectivity toward H₂O₂. (ii) In contrast, the enhanced specific magnetism from its multi-domain Fe₃O₄ core optimizes the d-band center of MMFR-C's active sites, stabilizes triplet O₂ adsorption, and reduces spin-forbidden transition barriers, thereby facilitating O-O cleavage and diverting its ORR pathway to the 4e^- route.