Porous activated carbon nanofibers have been prepared by electrospinning a H3PO4-containing polyacrylonitrile precursor. A small amount of H3PO4 (<1 wt%) serves as the activation agent during carbonization of the nanofibers. The activated carbon nanofibers have a large surface area (similar to 709 m(2) g(-1)) and high porosity (0.356 cm(3) g(-1)). A high specific capacitance of 156 F g(-1) (at 0.5 A g(-1)) is obtained at a 1:10 mass ratio of H3PO4 to polyacrylonitrile. The energy density of the supercapacitor with the activated carbon nanofibers as the electrodes reaches 10.98 Wh kg(-1) at a power density of 10 kW kg(-1). This is similar to 9 fold larger than that of carbon nanofibers without H3PO4 because the H3PO4-based activation process significantly increases both the micropore volume and the volume ratio of mesopores to micropores.