Abstract in English:

Because of their tunable porosity and specific surface area, porous materials are of high interest for the purification of wastewater by adsorption as well as in electrocatalysis, where, in particular, developing metal-free carbon-based catalysts for the oxygen reduction reaction (ORR) is researched. The carbon spheres presented here can meet these two requirements simultaneously. Porous polyacrylonitrile (PAN) spheres were firstly prepared by droplet shaping cum nonsolvent induced phase separation. Then, they were converted to nitrogen-doped carbon spheres by two-step carbonization via preoxidation and pyrolysis. During the pyrolysis step, carbon dioxide was used to reopen the pores of the materials that had been blocked after the preoxidation step. By this way, specific surface area values of carbon spheres of more than 1000 m²/g could be obtained. Best performing carbon materials (C#3) showed high adsorption capacity (e.g., 296 mg/g for methyl orange at solute equilibrium concentration of 24 mg/L in water). The XPS analysis revealed that only quaternary (N-Q) and pyridinic (N-6) nitrogen sites were found in these carbon spheres. The best ORR performance was found also for C#3 carbon spheres, with a potential of 0.81 V versus reversible hydrogen electrode (RHE) at −1 mA/cm² and electron transfer number of 3.5 at 0.6–0.8 V versus RHE, as determined by rotating disk or rotating ring disk electrode tests. In conclusion, all results confirm that these PAN-derived carbon spheres are potentially valuable materials for both wastewater treatment by adsorption and electrocatalytic oxygen reduction.