Efficient oxygen reduction reaction (ORR) is essential for energy conversion and environmental technologies, but directing the reaction toward a specific product remains a major challenge. The four-electron (4e⁻) pathway produces water, whereas the two-electron (2e⁻) pathway generates hydrogen peroxide (H₂O₂), a valuable oxidant for wastewater treatment and chemical processing. Conventional strategies for controlling selectivity often require changes to catalyst composition or structure, limiting the ability to regulate reaction pathways dynamically. Researchers from Hebei University of Science and Technology have developed a bioinspired molecular magnetic field-responsive catalyst that uses built-in magnetic fields to control active-site electronic structures, enabling selective switching between these two oxygen-reduction pathways. 

Why This Magnetic Catalyst Matters

The selectivity of ORR depends strongly on whether the oxygen–oxygen (O–O) bond remains intact or undergoes cleavage during the reaction. Preserving this bond favors H₂O₂ production, while breaking it promotes the four-electron route to water. Achieving precise control over this branching point is difficult because it involves complex interactions among electron transfer, proton transfer, and intermediate adsorption. A catalyst that can regulate these processes without requiring changes to its chemical structure could offer a versatile approach to tailoring electrocatalytic reactions for different applications. 

Innovative Design and Mechanism

Inspired by the biological enzyme cytochrome c oxidase, the research team constructed molecular magnetic field-responsive catalysts (MMFR-C) by integrating magnetic single-atom-anchored Salen-based covalent organic frameworks (MSA-Salen COFs) with single-domain or multi-domain Fe₃O₄ nanoparticles. The COFs provide ordered proton-transfer channels and well-defined N₂–M–O₂ active sites, while Fe₃O₄ generates a built-in magnetic field that regulates the electronic structure and spin polarization of the metal centers. Single-domain Fe₃O₄ produces a relatively weak, uniform magnetic field that optimizes the adsorption of the *OOH intermediate and suppresses O–O bond cleavage, favoring H₂O₂ formation. In contrast, multi-domain Fe₃O₄ generates stronger magnetic interactions that facilitate O–O bond cleavage and direct the reaction toward the four-electron pathway. This design establishes magnetic-domain engineering as a means of controlling catalytic selectivity.  

Outstanding Performance

The catalyst demonstrates substantial improvements in both selectivity and reaction control. For the Co-Salen COF system, incorporating single-domain Fe₃O₄ increases H₂O₂ selectivity from 26% to 63.9%, with an electron-transfer number of 2.72. By comparison, the multi-domain Fe₃O₄ counterpart redirects the reaction toward the four-electron pathway, achieving an electron-transfer number of 3.67. The single-domain Fe₃O₄–Fe-Salen COF system reaches an H₂O₂ production rate of 132 mmol g^-1 h^-1 at 0.3 V versus the reversible hydrogen electrode. Furthermore, the single-domain Fe₃O₄–Co-Salen COF catalyst maintains stable disk and ring currents during 10 hours of continuous operation. These results demonstrate that magnetic-domain structure can regulate ORR selectivity while supporting efficient and stable catalysis.  

Applications and Future Outlook

The team also demonstrated the environmental potential of the system by using electrogenerated H₂O₂ to degrade Rhodamine B, a model organic pollutant. The dye was completely degraded within one hour, highlighting the promise of selective electrochemical H₂O₂ production for water-treatment applications. More broadly, this research introduces a strategy for regulating catalytic pathways through internal magnetic fields rather than relying solely on conventional structural modifications. Further development of magnetic field-responsive catalysts could open new opportunities for controllable electrosynthesis, environmental remediation, and energy-related electrochemical processes.