We report a facile method for obtaining extremely high surface area and uniformly porous carbon nanofibers for supercapacitors. Blends of polyacrylonitrile and sacrificial Nafion at different compositions have been electrospun into non-woven nanofiber mats with diameters in the range of 200–400 nm. Electrospun nanofiber mats are then subjected to carbonization to obtain porous carbon nanofibers (CNFs) as polyacrylonitrile converts to carbon and Nafion decomposes out creating intra-fiber pores. Resultant porous CNFs exhibit specific surface area of up to 1600 m2 g−1 with a large fraction of mesopores (2–4 nm). No additional chemical or physical activation process was used. We demonstrate the tunability of the pore sizes within CNFs by varying the amount of Nafion. The non-woven fiber mats of porous CNFs are studied as free-standing electrode materials for supercapacitors eliminating the need for polymeric binding agents. Electrochemical measurements showed large specific gravimetric and volumetric capacitances of up to 210 F g−1 and 60 F cm−3 in 1 M H2SO4 at a high cyclic voltammetry scan rate of 100 mV s−1 due to the large fraction of mesopores. These materials retain 75% performance at a large current density of 20 A g−1 indicating excellent power handling capability. ► Fabrication of free-standing porous carbon nanofibers (CNFs) using electrospinning. ► We demonstrate interconnected pore structure inside carbon nanofibers using TEM. ► Porous CNFs exhibit 1600 m2 g−1 specific surface with large fraction of mesopores. ► Porous CNFs exhibit 210 F g−1 at 1 A g−1 current and retain 75% capacitance at 20 A g−1. ► Porous CNFs exhibit high power due to hierarchical pore structure.