Nanoporous carbon nanofibers (CNFs) are produced by incorporating tetraethyl orthosilicate (TEOS) into polyacrylonitrile (PAN) via electrospinning, and their electrochemical properties are investigated as an electrode in supercapacitors. TEOS is used as a pore generator in the PAN-based precursor for the CNFs with stabilized Si-related functional structures. The microstructures (e.g., nanometer-size diameters, high specific surface areas, narrow pore size distributions, and tunable porosities) and some of the surface functionalities of the CNFs are affected by the TEOS concentration. The electrode with these characteristics demonstrates better supercapacitor performance in terms of capacitance, energy, and power efficiency, which is attributed to the synergistic effect between the double-layer capacitance and the pseudo-capacitive effect.