Template-free synthesis of hollow spherical carbons: Curvature-dependent electrocatalytic performance for oxygen reduction reaction

Developing high-efficiency and economical non-PGM catalysts for oxygen reduction reaction (ORR) is of great significance for energy conversion and storage devices. In this work, a template-free strategy has been developed to prepare Fe, N co-doped hollow spherical carbon (FeNHSC) with a certain curvature, through a simple hydrothermal method and then NH3 activation. The obtained FeNHSC exhibits remarkably improved ORR activity, good long-term stability, and outstanding resistance to methanol crossover, which are much higher than the commercial Pt/C in alkaline medium. The enhancement of the above electrocatalytic performances can be attributed to the curved shell of the synthesized hollow spherical carbon, which contributes to exposure active Fe in form of the Fe-N-4 moieties anchored on the carbon. Density functional theory (DFT) calculation results further reveal that the FeNHSC with a curved structure can efficiently boost the ORR rate, compared with the non-curved carbon. For the curved carbon, the positively shifted d-band center (epsilon(d)) and increased positive surface charges of the Fe sites in form of Fe-N-4 structure should be responsible for the curvature-dependent ORR activity. This finding may open a way to fabrication next-generation transition metal-nitrogen-carbon catalysts for clean energies.