Heteroatom sulfur-doping in single-atom Fe-N-C catalysts for durable oxygen reduction performance in zinc-air batteries

Heteroatom doping into the transition metal-based catalysts is an effective strategy to improve the oxygen reduction reaction (ORR) kinetics. Herein, we proposed a one-step, soft template assisted, and green method for the synthesis of Sulfur (S) doped single atom Fe-N-C catalyst. XAFS demonstrated that the Fe active sites in the Fe-N-S-C were more likely to possess the Fe-N4 configuration. Density functional theory (DFT) calculations revealed the effect of S-doping into the single atom Fe-N4 symmetric structure, resulting in the delocalization of 3d electrons and asymmetric structure for the single atom Fe-N-S-C. The energy barrier of the ratedetermining step decreased from 0.535 eV (for Fe-N-C) to 0.474 eV for the Fe-N-S-C structure, indicating the possible good catalytic activity of the Fe-N-S-C catalyst. The following experiments demonstrated that the Fe-N-S-C catalyst showed an excellent ORR performance in both acidic medium with a half wave potential (E1/2) of 0.81 V vs. RHE and basic medium with an E1/2 value of 0.93 V vs. RHE. The high ORR performance is validated by assembling a homemade Zinc-air battery (ZAB) using the single atom Fe-N-S-C as a cathode, showing a high power density of 240 mW cm- 2. The synthesized single-atom Fe-N-S-C catalysts outperformed the state-of-the-art 20 % Pt/C catalyst. The combination of physical characterization, experimental results, and DFT calculations unveiled exceptional improvements in the ORR activity through the incorporation