•The size and the density of holes on graphene are controlled by a simple method.•The formation mechanism of holey graphene is proposed.•Holey graphene shows good activity and stability for oxygen reduction reaction (ORR).•The defects, conductivity and ORR performance are quantitatively correlated.Defects or pinholes in graphene are important for tuning its properties to be applicable in various fields, such as supercapacitor and oxygen reduction reaction (ORR). Here, we report a simple carbothermal method to controllably form defects or pinholes in graphene by reacting graphene with CoOx in argon atmosphere. The morphology of defects or pores and the reaction mechanism were investigated and discussed detailedly. The defects, the conductivity and the ORR performance in alkaline electrolyte of the holey graphene were quantitatively analyzed and correlated. The results show that the optimized annealing temperature is 700 °C based on the ORR activity. The content of defects or edge carbon atoms of holey graphene, that is the defect density (nD), determining factor for the ORR activity, can be controlled by controlling the pore size and the density in the basal plane of graphene that was realized by adjusting the cobalt content in the precursor. 10Co/G-700-HCl, possessing the highest edge or defective carbon content, reveals the optimal ORR performance with mixed 4e− and 2e− ORR process, excellent stability and good methanol tolerance.Download high-res image (206KB)Download full-size image

A nanocomposite of manganese dioxide coated on the carbon nanotubes (MnO2/CNTs) was synthesized by a facile direct redox reaction between potassium permanganate and carbon nanotubes without any other oxidant or reductant addition. The morphology, microstructure and crystalline form of this MnO2/CNT nanocomposite were characterized by scanning electron microscopy (SEM), transition electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The electrochemical properties are characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge (GCD). The results show that the facile prepared MnO2/CNTs nanocomposite shows specific capacitance of 162.2 F g−1 at the current density of 0.2 A g−1 and excellent charge/discharge property with 90% of its specific capacitance kept after 2000 cycles at the current density of 5 A g−1.Highlights► MnO2/CNTs are prepared by direct redox reaction between KMnO4 and carbon nanotubes. ► This preparation method is a simple and green without any other additives. ► MnO2/CNTs show specific capacitance of 162.2 F g−1 at the current density of 0.2 A g−1. ► MnO2/CNTs exhibit excellent charge–discharge property.