Nitrogen-doped carbon nanosheets and nanoflowers with holey mesopores for efficient oxygen reduction catalysis
Efficient structure optimization is one of the key factors for improving the oxygen reduction reaction (ORR) catalytic performance of carbon materials. This paper describes a dual-template method for fabrication of new carbon materials as ORR catalysts, including N-doped carbon nanosheets and nanoflowers with holey mesopores, by employing a polystyrene-b-poly(ethylene oxide) block copolymer as the pore-forming agent, layered double hydroxide (LDH) nanosheets or nanoflowers as the sacrificial morphology-directing template, and m-phenylenediamine as the carbon precursor. The resultant carbon materials possess a honeycomb-like mesoporous structure with similar nitrogen contents of ∼4 wt%, an average pore size of ∼14 nm and a specific surface area of ∼260 m2 g−1. Due to the presence of the holey mesopores that facilitate mass transfer and may shorten the diffusion distance of O2 molecules to the active sites, the nanosheets and the nanoflowers exhibit excellent electrocatalytic performance when serving as metal-free ORR catalysts in basic media with high half-wave-potentials (+0.80 V) and limiting current densities (5.5 mA cm−2) which surpass those of many reported carbon-based materials with much higher surface areas but without holey pores. Moreover, the porous nanoflowers show better electrocatalytic activity than that of the nanosheets, profiting from their 3D structure that can prevent the blockage of partial holey pores caused by the preferential layer-by-layer stacking of the nanosheets.