Significantly enhanced oxygen reduction reaction performance of N-doped carbon by heterogeneous sulfur incorporation: synergistic effect between the two dopants in metal-free catalysts

Developing highly active non-noble metal oxygen reduction reaction (ORR) catalysts is crucial for a variety of renewable energy applications including fuel cells and metal-air batteries. Heteroatom doped carbon materials, known as metal-free catalysts, show potential applications in the ORR, and may be promising replacement candidates for expensive, scarce platinum catalysts. Despite the inspiring progress made, the performance of the current metal-free carbon catalysts is still far from satisfactory for large-scale applications. Herein, we introduce an effective and robust ORR catalyst based on N, S co-doped carbon materials with abundant surface active sites. Electrochemical results indicate that the incorporation of sulfur into nitrogen-doped carbon (S-NCx) can dramatically improve the stability of the catalyst by improving the selectivity of O-2 electro-reduction to H2O. Density functional theory calculations reveal that sulfur doping lowers the energy barrier of O-2(ads) hydrogenation to form OOH(ads), thus leading to enhanced intrinsic activity. In particular, the correlation between ORR activity and nitrogen and sulfur species in these materials is studied in-depth, and it is found the ORR performance of S-NCx catalysts is significantly affected by pyridinic N and C-S-C contents.