Hollow structured N, S doped carbon sphere with abundant Fe-Nx active sites as an efficient electrocatalyst for oxygen reduction reaction

초록

we proposed preparation method of hollow-structured N, S doped carbon sphere with abundant Fe-Nx active sites (FeNSC). First, Fe-incorporated polypyrrole (PPy) shell was homogeneously coated onto the surface of SiO2 nanosphere via hydrothermal polymerization process and impregnation method. After that, the resultants were carbonized at 900 °C for 2 h and chemically etched out by HF solution to prepare hollow structured FeNC. Finally, as-prepared FeNC nanosphere was annealed with sulfur powder at 400 °C for 2 h to synthesize hollow structured FeNSC. The electrocatalytic activity of FeNSC nanospheres toward ORR was evaluated by various electrochemical measurement in O2 saturated 0.1 M KOH electrolyte using a rotating disk electode (RDE). The hollow-structured FeNSC nanospheres manifested excellent ORR activity with onset-potential and half-wave potential of 0.971 and 0.877 V vs. RHE, respectively, which is much superior than hollow-structured FeNC, NSC, and even state-of-the-art Pt/C electrocatalyst. Furthermore, the long-term durability of the FeNSC was evaluated through chronoamperometry (CA) measurements. After continuous operation over 100 h, the current density of FeNSC was decreased only by 4.3 %, demonstrating extraordinary durability compared to Pt/C (28.3 %). The outstanding ORR performances of FeNSC could be ascribed to the efficient exposure of abundant Fe-Nx active sites induced by well-defined hollow structure. The unique hollow structure of FeNSC also provide reduced ion-diffusion length and large specific surface area, thereby enhancing electrocatalytic ORR performance. Additionally, the electrical conductivity and adsorption of reactant during ORR process can be improved by additional sulfur doping into carbon matrix, which is mainly attributed to high electron spin density of S dopants and introduction of additional active sites for ORR such as thiophen-like C-S-C structure.