By combining introducing functional groups on the surface of carbon nanotubes (CNTs) with adding redox-active molecules-hydroquinone (HQ) into 1 M H2SO4 aqueous electrolyte, CNTs' specific capacitance has been significantly improved. After being treated by alkali and mixed acid, CNTs show higher specific surface area and appropriate pore size distribution, resulting in higher specific capacitance. In H2SO4 electrolyte, a specific capacitance of 85 F g(-1) is achieved for those CNTs treated in alkali and mixed acid (CNTs-T), much higher than that of raw CNTs (only 28 F g(-1)). Furthermore, by introducing redoxactive molecules-HQ into H2SO4 electrolytes, CNTs-T electrode presents a specific capacitance as high as 3199 F g(-1) at 5 mV s(-1) in 0.075 M HQ + H2SO4 electrolyte, about 38 times as large as that obtained in H2SO4 electrolyte. In addition, HQ's concentration affects the electrochemical performance of CNTs-T electrode. High HQ concentration results in the aggregration of free ions and charges, and therefore a slow diffusion of ions and charges and relative poor electrochemical performance. For CNTs-T electrode, the optimum concentration of HQ is 0.075 M. In such electrolyte, CNTs-T displays a stable specific capacitance of about 2250 F g(-1) at 5 mV s(-1) after 350 cycles. (C) 2013 Elsevier Ltd. All rights reserved.