Heteroatom sulfur-doping in single-atom FeNC 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 FeNC catalyst. XAFS demonstrated that the Fe active sites in the FeNSC 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 FeNSC. The energy barrier of the rate-determining step decreased from 0.535 eV (for FeNC) to 0.474 eV for the FeNSC structure, indicating the possible good catalytic activity of the FeNSC catalyst. The following experiments demonstrated that the FeNSC catalyst showed an excellent ORR performance in both acidic medium with a half wave potential (E_1/2) of 0.81 V vs. RHE and basic medium with an E_1/2 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 FeNSC as a cathode, showing a high power density of 240 mW cm^-2. The synthesized single-atom FeNSC 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 of the S atom into the Fe-N₄ matrix. Our findings offer a pathway towards sustainable energy solutions, driving innovation in the field of green energy technologies.