Porous carbon nanofiber composites (NFCs) were prepared by electrospinning blended solutions of polyacrylonitrile (PAN) and polymethylhydrosiloxane (PMHS) in N,N-dimethylformamide (DMF). A PMHS concentration of 5 wt% was regarded as the optimum concentration to obtain fibers of a uniform size with a homogeneous dispersion of silica, the maximum specific surface area and the highest conductivity (4.91 S cm−1) after heat treatment at 800 °C. The supercapacitor electrode prepared with 5 wt% PMHS had the highest specific capacitance, 126.86 F/g, and the highest energy density, 17.0–10.0 Wh/kg, in the range of 400–20,000 W/kg in a 6 M KOH aqueous solution.