3.6.3. EIS Study of PPy and PPY/GA Composites

Impedance spectroscopy is very beneficial for obtaining information about the electrode materials' resistive and capacitance properties. At a constant DC potential of 0.5 V with an AC of 0.01 V, a potentiostatic EIS study was performed from 0.1 Hz to 100 kHz. Figure 7a depicts a Nyquist plot of pure PPy and various PPy/GA composites. However, Figure 7b shows an equivalent circuit for EIS. The Nyquist plot of pure PPy reveals a distorted semicircle at a high-frequency region [44]. This semicircle is followed by a 45◦ slanted or sloped line, which is followed by a straight line in the low-frequency region. The intercepts on the *X*-axis and the real axis are termed solution resistance (Rs), and the diameters of semicircles indicate electrode resistance (Rct) in the high-frequency area because of charge transfer resistance in the active compounds. When compared to pure PPy, the PPy/GA 1 composite had a somewhat narrower semicircle, indicating a low Rct value. In the low-frequency region, the PPy/GA 2, PPy/GA 3, and PPy/GA 5 display a straight line with an angle of 45◦ to 65◦, which resembles an ideal capacitor and fast ion diffusion in electrode materials. In the high-frequency region shown in Figure 7a, as the GA loading in the composite increases from 0.125 to 1 wt%, Rs increases from 12.8 to 2682 ohm, and the diameter of the semicircle (Rct) grows. Despite the composites' 0.5 wt% GA content, the high Rct suggests that as GA content increases, the number of surface electrochemical reaction sites decreases. When the GA loading in the composite is increased to 1 wt%, the number of bulk electrochemical reaction sites increases relative to the number of surface electrochemical reaction sites and the Rs again decrease. Because of the non-homogeneity of samples, porosity, and non-uniform distribution of current, a constant phase element (CPE) is used in the equivalent circuit instead of a capacitor. PPy/GA 4 indicates poor contact between the current collector and active materials, as well as high intrinsic resistance of the active material. The high resistance to ion transport between the electrolyte solution and the electrode interface causes the semi-circle or Rct value to rise. The data is summarized in Table 2.


**Table 2.** Solution resistance of the PPy/GA 1–5 composites.

**Figure 7.** (**a**) EIS spectra of (a) PPy, (b) PPy/GA 1, (c) PPy/GA 2, (d) PPy/GA 3, (e) PPy/GA 4, and (f) PPy/GA 5. (**b**) Equivalent circuit for EIS.

3.6.4. Galvanostatic Charge-Discharge (GCD) Study of PPy and PPY/GA Composites

GCD has also described the electrochemical performance of the produced electrodes [45] as well as the galvanostatic charge-discharge curves of PPy/GA 1 for the supercapacitors device at varied current densities of 1, 1.5, 2, and 2.5 A/g Figure 8a. The GCD curves for the fabricated electrodes PPy and PPy/GA 1 to PPy/GA 5 at various loading concentrations of gum arabic and at a fixed current density of 1 A/g are shown in Figure 8b. The shape of the curves depicts optimal capacitor behavior for supercapacitors. The charge curves are symmetric to discharge curves between potential intervals indicating feasibility of PPy/GA surface for supercapacitor [46].

The following equations were used to determine various parameters such as specific capacitance (Cs), energy density (E), and power density (P) from the GCD curves of modified supercapacitor electrodes [46].

The specific capacitance (Cs) of the modified supercapacitor electrodes was calculated by using Equation (3) [47].

$$\text{Cs} = \frac{I \times \Delta t}{m \times \Delta V} \tag{3}$$

where "*I*" is the charge-discharge current (A), Δ*t* is the discharge time, "*m*" is the mass deposited on the electrode, and Δ*V* is the voltage difference in the discharge segment. The total energy density *E* (Wh kg<sup>−</sup>1) and power density *P* (Wkg<sup>−</sup>1) of the supercapacitor device were calculated using Equations (4) and (5) [48].

$$E = \frac{1}{2 \times 3.6} \times \text{Cs} \times \Delta V^2 \tag{4}$$

$$P = \frac{E}{\Delta t} \times 3600\tag{5}$$

**Figure 8.** (**a**) GCD curves of PPy/GA 1 composite at different current densities. (**b**) GCD curves of neat PPy and PPy/GA composites at 1 A/g current density. (**c**) GCD curves of PPy/GA 1 for 1st and 1000th cycles at 1 A/g current density. (**d**) Plot of specific capacitance and capacitance retention versus cycle number.

In Equations (4) and (5) of energy density (*E*, Wh/kg) and power density (*P*, W/kg), *Cs* is the specific capacitance, Δ*V* is the potential window, and Δ*t* is the discharge time as mentioned previously. The values of *Cs*, *E*, and *P* are tabulated for PPy and PPy/GA composites in Table 3.

**Table 3.** The specific capacitance, energy density, and power density of PPy and PPy/GA 1–5.


Table 4 compares the specific capacitance of PPy/biodegradable polymers-based electrodes to that of a PPy/GA composite developed in this study. Table 4 demonstrates that PPy/GA has a relatively high specific capacitance.


**Table 4.** Comparison of specific capacitance of PPy/biodegradable polymers-based electrodes.

The apparent behavior of the GCD curves is well-adapted to the typical behavior of supercapacitors, which reveals that specific capacities have a declining nature and an increase in current density. The PPy/GA-based electrode proved its characteristic double-layer capacitance behavior as well as good electrochemical reversibility with a highly symmetric triangular-shaped charge/discharge curve [54]. The addition of 0.125% GA to the PPy matrix increases the charge and discharge time, which demonstrates the increase in the specific capacitance. The incorporation of GA in the PPy matrix may result in a mesoporous structure, which increases surface area and ionic conductivity. Figure 8c demonstrates the cyclic stability of the modified electrode, which was evaluated for 1000 charge–discharge cycles at a current density of 1 A/g and still had an 85% specific capacitance [2] Figure 8d. The ohmic drop in the GCD curves can be attributed the solution resistance.
