Optimized Enhancement Effect of Sulfur in Fe–N–S Codoped Carbon Nanosheets for Efficient Oxygen Reduction Reaction

The study on the design and preparation of oxygen reduction reaction (ORR) electrocatalysts with high efficiency is currently attracting great concern. Among different types of catalysts, heteroatom-doped carbon-based catalysts have exhibited promising potential, and the exploration of optimized matching of the doping elements is crucial to the design and fabrication of this category of catalysts. Herein, by annealing commercially available and cost-effective precursors, Fe-N-S codoped graphene-like carbon nanosheet catalysts were prepared. The atomically dispersed Fe atoms coordinated with the N atoms to form FeN₄ sites as proved by X-ray absorption spectroscopy. By facile modulation of the relative amount of the precursors, the contents of thiophene-S (Th-S) and Fe-N₄ sites could be tuned and a series of catalysts with different Th-S/Fe ratios were prepared. The doped sulfur exhibited an enhancement effect on ORR performance, and strikingly, the enhancement efficiency could be optimized by fine modulation of the Th-S/Fe ratio in the catalysts. Furthermore, it was found that when the Th-S/Fe ratio reached an optimal value of 1.8, the ORR performance was significantly boosted, especially in acidic media. The experimental data were supported by density functional theory calculation results, which indicated that the ORR overpotential of the S₂(FeN₄) configuration model (corresponding to the Th-S/Fe ratio of 2) was lower than that of S₃(FeN₄) and S₁(FeN₄). The optimized catalyst (denoted as FeN/SNC-900-3) displayed highly efficient ORR activity in both alkaline and acidic media. In alkaline media, the half-wave potential was 49 mV more positive than that of the commercial Pt/C catalyst, and in acidic media, the half-wave potential was close to that of Pt/C. Moreover, the stability of FeN/SNC-900-3 was outstanding, and the relative current density showed only a slight decay in both alkaline and acidic media after 40,000 s. A primary Zn-air battery with FeN/SNC-900-3 as the cathode catalyst exhibited a high peak power density of up to 153 mW cm^-2 and superior cycling stability over 200 cycles.