Sulfur-induced electronic modification of Co-N-C frameworks for enhanced ORR activity and rechargeable Zn-air batteries

Rechargeable Zn-air batteries require bifunctional air electrodes capable of simultaneously enhancing oxygen reduction reaction (ORR) selectivity, oxygen evolution reaction (OER) kinetics, and cycling durability. To address this requirement, Co/CoO-decorated N,S-dual-doped carbon sheets (Co-NSC) were prepared through a template-free self-assembled pyrolysis strategy using guanine, dibenzyl disulfide, and cobalt nitrate. Sulfur incorporation modifies the Co-containing N-doped carbon framework, producing altered Co valence distribution, thiophene-like sulfur species, and pyridinic nitrogen environments. These structural and electronic modifications promoted a greater contribution from the four-electron ORR pathway while supporting the evolution of Co-centered oxygen intermediate during OER. As a result, the bifunctional oxygen electrocatalysis was greatly enhanced. As a bifunctional air cathode, Co-NSC reduced the ORR/OER voltage gap and improved rechargeable Zn-air battery performance compared with the Co-NC and Pt/C + RuO2 benchmarks. The assembled cell delivered high power output and stable cycling over 68 h (408 cycles), retaining 73.8% of its initial round-trip efficiency. These findings demonstrate that sulfur-assisted modification of Co-N-C framework is an effective strategy for simultaneously enhancing ORR performance, OER kinetics, and the cycling durability of rechargeable metal-air batteries.