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.