Nitrogen-doped hollow mesoporous carbon spheres as a highly active and stable metal-free electrocatalyst for oxygen reduction

Nitrogen-doped hollow mesoporous carbon spheres (NHCSs) were successfully prepared via a simple, scalable hydrothermal method, followed by thermal treatment at 650 °C in ammonia atmosphere and subsequent high temperature annealing at 800–1000 °C in nitrogen, respectively. The resulting NHCSs with a particle diameter of ∼150 nm and a shell thickness of ∼20–25 nm have high specific surface areas (738–820 m² g⁻¹), large pore volume (0.50–0.56 cm³ g⁻¹), bimodal pores system (3.9 and 51.6 nm) and adjustable N-doping levels (3.6–7.8 at.%) depending on the pyrolysis temperature. The unique structure of hollow spheres for NHCSs with uniform mesopores throughout the shells can both promote fast mass transfer and provide inner and outer surfaces with high density of N-related active sites, thus improving the reaction kinetics. The NHCS-1000 (annealed at 1000 °C) exhibited not only comparable ORR activity with a direct four-electron reaction pathway in terms of onset and half-wave potentials, and limiting current densities, but superior long-term durability and methanol-tolerance to commercial Pt/C catalyst, because it has the highest relative content of graphitic-N and pyridinic-N groups, indicating their crucial roles in ORR. The NHCS-1000 with excellent ORR performance is of potential to replace Pt/C catalyst for ORR in practical applications.