Supercapacitors with high capacitance, high energy density and a small time constant have been developed successfully using functionalized activated carbon nanoparticles (FACNs) and a cross-linkable polymeric binder in neutral aqueous Na2SO4. Due to the functional groups on the FACNs’ surface and the slenderness of the FACNs’ nanocomposite electrode, the FACNs’ nanocomposite electrode exhibits superior specific capacitance of 154F/g, energy density of 18Wh/kg, and a time constant of 0.25s. Furthermore, the cyclic voltammogram is still rectangular in shape even at exceedingly high scan rates of 5V/s. These characteristics show that our proposed method is suitable for the fabrication of high-performance supercapacitors.