context: 3.2.1 Cyclic voltammetry. CV studies have been performed first to optimize the operating potential difference range of the supercapacitor cells. For this purpose, the CV profiles of a typical cell (Cell#4) have been recorded for varying potential difference ranges as shown in Fig. 4a. A close inspection indicates that CV responses show almost rectangular-box like patterns without much deviation, indicating stable capacitive behavior, for the range from 0 to 2.5 V. Thus, all the cells have been electrochemically characterized up to 2.5 V in two electrode configuration, in the present study. This has been confirmed from GCD studies also, as discussed later. It may be noted that the stability window of the dual redox-active electrolyte film is high (∼6.2 V), as discussed above, and the charging voltage of the device is restricted to ∼2.5 V only. The most possible reason for the limited charging voltage of the device is related to the possible reaction(s) of the electrolyte ions with the surface functional groups (carbonyl, carboxyl, hydroxyl, etc.) attached to the activated carbon electrode surface. The electrochemical performance of all the supercapacitor cells (Cell#1 to Cell#4) has been compared, comparing their CV patterns, recorded at a scan rate of 10 mV s−1, as shown in Fig. 4b. Cell#1 (with the GPE without redox additives) shows almost a rectangular pattern, similar to the characteristics of capacitors with carbon electrodes, indicating the dominance of double-layer-type capacitive nature. The CV profiles of Cell#2 and Cell#3 (containing GPEs with single redox additives DPA and KI, respectively) illustrate distinct reversible redox peaks (Fig. 4b). The reversible redox peaks in Cell#2 are associated with the reversible conversion between diphenyl benzidine, DPB (colorless) and DPB (violet), which is initiated after irreversible redox transformation of DPA into DPB (colorless), as shown in the following Scheme (1).