Effect of Iron-Sulfur Bond on Tailoring the Electron Structure in Dual-Atomic Iron Sites for Enhanced Oxygen Reduction Reaction.

In oxygen reduction reaction (ORR), increasing metal loading in dual-atomic catalyst easily leads to metal aggregation, resulting in the formation of clusters or nanoparticles. Herein, a new approach involving sulfur incorporation is developed to preserve the dual-atomic structure and regulate the electrons of Fe₂-NC dual atomic catalyst, without resorting to simply increasing metal loading. The optimized Fe₂-S/NC-6 catalyst with Fe─S bond demonstrated exceptional ORR activity in pH-universal electrolytes, boosting the most positive E_1/2 values (0.902 V in alkaline, 0.689 V in neutral and 0.781 V in acidic solution). Theoretical study revealed that Fe₂-S/NC catalyst with Fe─S bond and Fe₂-NC/S catalyst with thiophene-like sulfur both can decrease the d-band center of Fe sites compared to Fe₂-NC without sulfur, and weaken the adsorption with OH* intermediate. In the case of Fe─S bond, this decline is more notable. The predicted ORR performance ranked in the sequence of Fe₂-S/NC > Fe₂-NC/S > Fe₂-NC. The Fe₂-S/NC-6-based Zn-Air battery (ZAB) and microbial fuel cell (MFC) exhibited remarkable power density (317.1 mW cm^-2 for ZAB, 2074 ± 66 mW m^-2 for MFC) with prominent stability. This work innovatively highlighted the role of Fe─S bond in regulating the electron structure of dual-atomic Fe₂-NC catalyst aiming to the excellent ORR performance.