Fe nanoparticles encapsulated in doped graphitic shells as high-performance and stable catalysts for oxygen reduction reaction in an acid medium

Abstract

Doped carbon nanostructures with transition metals and nitrogen (N) as doping sources used as promising non-precious metal (NPM) catalysts for oxygen reduction reaction (ORR) have been intensively researched. However, the NPM catalysts, prepared using a mixture of transition metal, nitrogen, and carbon sources, contained both the transition metal-N and N-doped carbon-coated transition metal nanoparticles (NPs) in the surface structure. However, among the transition metal-N and N-doped carbon-coated transition metal NPs, the predominant electrocatalytic active sites for ORR in the NPM catalysts remain uncertain. In this study, we proposed the NPM catalysts for ORR with various active sites such as the doped carbon-coated Fe NPs and/or N- and/or S-doped carbon structures through one- or two-step heating processes. The sample synthesized using a two-step heating process with doped carbon supported Fe NPs and dicyandiamide exhibited significantly improved activity and stability for ORR in O2-saturated H2SO4, due to a synergistic effect by the co-doping of S and N in the carbon structure and the Fe NPs encapsulated in the doped carbon, comparable to a commercial Pt/C catalyst. In particular, the Fe NPs encapsulated in the N- and S-doped carbon layers with a nanometer scale thickness are found to be an electrocatalytic active site for ORR. ? 2018