Identification of active sites in nitrogen and sulfur co-doped carbon-based oxygen reduction catalysts

Different heteroatoms co-doped carbons are a burgeoning class of metal-free catalysts to replace Pt for the oxygen reduction reaction (ORR), but the lack of understanding of active sites delays their further improvement. Here we combined experimental designs and theoretical simulations with attempts to understand the correlation between N and/or S doping configurations and their catalytic activities. The results indicated that there is no obvious synergistic effect between N and S co-doping, in contrast with previous observations. S doping followed by N doping contributes to a large pyridinic N content in the catalyst due to the low formation energy for N to substitute doped S, leading to greatly enhanced ORR activity. Inversely, N doping followed by S doping takes pyridinic N away, resulting in an obvious ORR performance loss. Therefore, the doping sequence of S and N is crucial for the ORR activity of the co-doped catalysts. Furthermore, the pyridinic N is determined as the active functional group in N, S co-doped carbons by first-principle density functional theory calculations. (C) 2019 Elsevier Ltd. All rights reserved.