Different concentrations of potassium iodide (KI) as redox additive had been added to 1 M sulfuric acid (H2SO4) electrolyte with an aim of enhancing the capacitance and energy density of ultracapacitors via redox reactions at the interfaces of electrode–electrolyte. Ultracapacitors were fabricated using chemically treated activated carbon as electrode with H2SO4 and H2SO4–KI as an electrolyte. The electrochemical performances of fabricated supercapacitors were investigated by impedance spectroscopy, cyclic voltammetry and charge–discharge techniques. The maximum capacitance ‘C’ was observed with redox additives-based electrolyte system comprising 1 M H2SO4–0.3 M KI (1072 F g− 1), which is very much higher than conventional 1 M H2SO4 (61.3 F g− 1) aqueous electrolyte-based ultracapacitors. It corresponds to an energy density of 20.49 Wh kg− 1 at 2.1 A g− 1 for redox additive-based electrolyte, which is six times higher as compared to that of pristine electrolyte (1 M H2SO4) having energy density of only 3.36 Wh kg− 1. The temperature dependence behavior of fabricated cell was also analyzed, which shows increasing pattern in its capacitance values in a temperature range of 5–70 °C. Under cyclic stability test, redox electrolyte-based system shows almost 100% capacitance retention up to 5000 cycles and even more. For comparison, ultracapacitors based on polymer gel electrolyte polyvinyl alcohol (PVA) (10 wt%)—{H2SO4 (1 M)–KI (0.3 M)} (90 wt%) have been fabricated and characterized with the same electrode materials.