*2.2. Results*

Figure 2 demonstrates the internal resistance (*R*int) and capacitance (C) as a function of voltage at a constant current of 50 A. Both the parameters showed approximately ±10% changes for charge/discharge cycles. Similar outcomes were also observed for an applied current of 20 A and 75 A [13]. It was also noticed that both parameters slightly upsurged as the capacitor voltage enhanced from minimum to maximum. Importantly, the tendency of UC capacitance during charge/discharge cycles was identical to the internal resistance.

**Figure 2.** Internal resistance and capacitance of symmetric double-layer UC measured at a constant current of 50 A for charge-discharge cycles.

Figure 3a demonstrates the impedance (|Z|) of the double layer UC at different temperatures for constant voltage of ~2.5 V and frequency range of 1 Hz–100 kHz. Figure 3b shows the impedance (|Z|) for different voltages (0.3376–2.736 V) and frequencies of 1 Hz–100 kHz at constant temperature (15 ◦C). The phase impedance of the double-layer UC is shown in Figure 4. The impedance and phase values remained almost constant for different working temperatures (−30 ◦C to +30 ◦C). These parameters also remain unchanged for different voltage magnitudes of 0.3376–2.736 V.

**Figure 3.** Module impedance (|Z|) as a function of frequency (**a**) at different temperatures for constant voltage (~2.5 V), and (**b**) at different voltages for a constant temperature of 15 ◦C.

**Figure 4.** Phase (θ) of Z as a function of frequency (**a**) at different temperatures for constant voltage (~2.5 V) and (**b**) at different voltages for a constant temperature of 15 ◦C.
