Nitrogen/sulfur co-doped non-noble metal material as an efficient electrocatalyst for the oxygen reduction reaction in alkaline media

Abstract

This work demonstrates the feasibility of nitrogen/sulfur co-doped non-noble metal materials (Fe–N/C–TsOH) as platinum-free catalysts for the oxygen reduction reaction (ORR) in alkaline media. Electrochemical techniques such as cyclic voltammetry (CV), rotating disk electrodes (RDEs) and rotating ring-disk electrodes (RRDEs) are employed with the Koutecky–Levich theory to investigate the ORR kinetic constants and the reaction mechanism. It is found that the catalysts doped with TsOH (p-toluenesulfonic acid) show significantly improved ORR activity relative to a TsOH-free catalyst. The overall electron transfer numbers for the catalyzed ORR are determined to be 3.899 and 3.098, respectively, for the catalysts with and without TsOH-doping. Catalysts heat treated at 600 °C exhibit relatively higher activity. In addition, the catalyst doped with TsOH (Fe–N/C–TsOH-600) not only exhibits exceptional stability in 0.1 mol L⁻¹ KOH solution but also has higher methanol tolerance compared to commercial Pt/C catalyst in 0.1 mol L⁻¹ KOH. To some extent, increasing the Fe–N/C–TsOH-600 loading on the electrode favors a faster reduction of H₂O₂ to intermediate to H₂O. X-ray photoelectron spectroscopy analysis indicates that pyrrolic N groups are the most active sites, and that sulfur species are structurally bound to carbon in the forms of C–S_(n)–C and oxidized –SO_(n)– bonds, an additional beneficial factor for the ORR.