A dual system of redox additive by incorporating Na2MoO4 and KI into H2SO4 solution has been developed to highly elevate the capacitance of supercapacitors primarily owing to the synergistic effect between them at the superposed redox voltage. Furthermore, the synergistic effect therein is attributed to the formation of complex substance of (MoxIyO4x)C-n-(z), which can promote redox reaction of Mon+ and In- at the interface of carbon electrode and electrolyte. On the other hand, many crucial factors mainly including the molar ratio, concentration of redox additive and voltage window strongly determine the final capacitive behaviors. For example, when adding Na2MoO4 and KI into H2SO4 with the same concentration of 0.1 mol L-1, the resultant capacitance has remarkably increased by 17.4 times, compared with the one without any redox additive, at 3 A g(-1) in a two-electrode system. What's more, the homologous energy density can reach up to 653 Wh kg(-1) at the suitable voltage window (0-1 V). Hence, the present synergism of various kinds of redox additives is intriguing and easily extended to other systems, which could highly elevate the capacitive performances of supercapacitors. (C) 2016 Elsevier B.V. All rights reserved.