Creation of densely exposed and cavity-edged single Fe active sites for enhanced oxygen electroreduction

The performance of oxygen reduction reaction (ORR) on Fe-N-C single-atom catalysts (SACs) is still less satisfactory due to the rather low atom-utilization of active sites. Here, a novel acetate thermolysis-assisted route is proposed to create hierarchical carbon nanocages of Fe-N-C SACs (Fe_SA-N/Cs-OAc) by selective cleavage of carbon layers, which features abundant edge-sited Fe-N₄ moieties in well-defined mesoporous channels. Benefiting from the ultra-high site density and utilization of Fe-N₄ moieties, the optimized Fe_SA-N/Cs-OAc catalyst demonstrates excellent ORR activities marked by extraordinarily high half-wave potentials (E_1/2) of 0.94 V and 0.82 V in alkaline and acidic electrolytes. Zn-air battery using Fe_SA-N/Cs-OAc as cathode delivers a power density of 165 mW cm⁻², and the maximum output power in H₂-O₂ fuel cell reaches 640 mW cm⁻². The abundant mesoporosity makes most Fe-N₄ sites accessible and simultaneously produces in-plane pore defects that reduced adsorption energy of *OH (−0.72 eV), finally presenting remarkably enhanced ORR performance.