Synthesis of Nitrogen-Doped Porous Carbon Spheres with Improved Porosity toward the Electrocatalytic Oxygen Reduction

In this study, a series of activated N-doped porous carbon spheres (ANCSs) have been prepared from biomass as the carbon source to be used as highly active and stable electrocatalysts toward the electrocatalytic oxygen reduction reaction (ORR). Hydrothermal carbonization of biomass glucose, which obtains uniform carbon nanopsheres, is followed by doping N atoms by treatment in ammonia and subsequent activation treatment to form ANCSs. The resultant ANCSs possess a large specific surface area of up to 2813 m²/g and pore volume of up to 1.384 cm³/g, and adjustable N contents (2.38-4.53 atom %) with increasing activation temperature. The graphitic and pyridinic-N groups dominate in various N functional groups in the ANCSs. Remarkably, the 1000 °C-activated sample demonstrates competitive activity and outstanding stability and methanol crossover toward the ORR with a four-electron transfer pathway in alkaline media compared to commercial Pt/C catalyst. This excellent performance should be mainly due to effective N-doping and high porosity which can boost the mass transfer and charge transfer and provide a larger number of active sites for the ORR. The unique spherical morphologies with improved porosity as well as excellent stability and recyclability make these ANCSs among the most promising ORR electrocatalysts in practical applications.