Plasma-treatment carbon nanotubes (CNTs) grafted with maleic anhydride (MA) were embedded in polyacryonitrile nanofibers via electrospinning and subsequently carbonizated at 800 degrees C to fabricate carbon nanofibers (CNFs). The grafted degree of MA on CNTs (CNTs-MA) was determined via Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The morphology, surface composition and conductivity of the CNTs-MA/CNF were characterized using electron microscopy, X-ray photoelectron and electrochemical impedance spectroscopy, respectively. CNTs-MA not only affected the conductivity of the CNFs but also the types of the nitrogen functional groups that could be represented as active sites on the CNFs to enhance the performance of the supercapacitors. When 2.5 wt.% CNTs-MA was embedded in the CNFs, the highest conductivity was obtained at 5.2 s/cm, and the amount of pyridinic and pyrrolic species increased to 70.3%. However, the highest capacitance was not obtained with 2.5 wt.% CNTs-MA added because of current leakage present in the system. The highest capacitance was 382 F/g with 1.0 wt.% CNTs-MA embedded in CNF with proper conductivity of 2.2 s/cm. Furthermore, galvanostatic charge/discharge, cyclic voltammetry and electrochemical impedance spectroscopy measurements also demonstrated that 1.0 wt.% CNTs-MA embedded in CNF resulted in better electrochemical reversibility and impendence properties. (C) 2015 Elsevier Ltd. All rights reserved.