Carbon nanofibers (CNFs) with controllable pore sizes are fabricated by a simple electrospinning method with the help of boron trioxide (B2O3), and their electrochemical properties as electrodes are investigated in aqueous electrolyte. The optimized result is obtained with a B2O3 concentration of 10 wt% in a spinning solution, offering a large specific surface area of 1065 m(2)/g with a mesopore volume fraction up to 35%, pore size range of 2-3 and 20-50nm, and many heteroatoms. The CNF composite with 10 wt% B2O3 concentration (PMB-10) shows a high gravimetric capacitance of 126.31 F/g with a capacitance retention of 84%, and a high energy density of 12-18 W h/kg in the power density range of 400-10,000 W/kg. Thus, the porous structures and heteroatom contents of the PMB composites can be controlled by varying the B2O3 content as the pore expanding agent, as well as heteroatom generator, to optimize the electrochemical performance in an aqueous electrolyte. (C) 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.