High-coordination Fe-N₄SP single-atom catalysts via the multi-shell synergistic effect for the enhanced oxygen reduction reaction of rechargeable Zn-air battery cathodes

Atomically dispersed iron-nitrogen-carbon (Fe-N-C) catalysts are immensely promising alternatives to noble metal catalysts for the oxygen reduction reaction (ORR), while their activity is bottlenecked by an unsatisfactory electronic structure. Accurately tuning the d-band centers and spin state of Fe sites is significant in enhancing intrinsic ORR activity but remains challenging. Herein, an innovative theoretical model of Fe-N4SP with a highly coordinated structure was rationally designed by decorating Fe-N-4 sites with axial sulfur atoms in the first coordination shell and adjacent phosphorus atoms in the second coordination shell. The theoretical calculations and experimental validations confirmed that the proposed structure can break the symmetric structure of Fe-N-4 sites to rearrange electrons and intensify spin polarization. The increased coordination number conferred enhanced d-orbital interactions in Fe atoms to depress their d-band centers, weakening the adsorption of oxygen intermediates. Fe-N4SP sites were successfully constructed in N, P, and S ternary co-doped hollow carbon nanocages to validate the proposed theoretical model. The obtained Fe-N4SP/NPS-HC catalysts exhibited impressive ORR activity with a decent half-wave potential of 0.912 V in alkaline media and 0.814 V in acidic media, coupled with ultra-long durability of up to 320 h in zinc-air battery systems. Overall, novel insights into the effect of the coordination environment on ORR catalytic activity were provided, useful for modulating the electronic structure of catalysts for efficient storage and energy production systems.