Highly mesoporous Fe and N Co-doped graphitic catalysts prepared from short-time synthesis of precursor towards highly efficient oxygen reduction

Abstract

Fe and N co-doped graphitic catalysts (Fe–N_x-Cs) were prepared with a high mesopore volume and graphitic structure towards application in high performance oxygen reduction reaction (ORR). To prepare the catalysts, a core–shell precursor (Fe₃C@NDC) was prepared with ferrous cores and N-doped graphitic shells by floating catalytic pyrolysis (synthesis time was only a few seconds). By acid treatment, the N-doped structure and some carbon structures were oxidized and the ferrous cores were dissolved, leading to a sharply increased concentration of ferrous ions. Thus, the N-doped structure with negative charge was easily attached with Fe²⁺/Fe³⁺ to form Fe–N_x active sites for ORR. To greatly increase the specific surface area of the catalysts without significantly destroying the graphitic structure of their shells, lots of the oxidized structure, which was not coordinated with Fe ions, was removed to create nanopores in the shells by further annealing at 300 °C (890 vs. 550 m² g⁻¹ and mesopore volume: 1.6 vs. 0.8 cm³ g⁻¹, after annealing). The Fe–N coordination made N-doped structure of Fe–N stable even with 300 °C annealing to protect the Fe–N_x active sites. Thus, the Fe–N_x–C900 catalyst performed well towards ORR (onset potential: 0.97 V of Fe–N_x–C900 (0.98 V of commercial Pt/C); number of electrons transferred per oxygen molecule of Fe–N_x–C900 was close to 4).