3.2.2. Electrochemical impedance spectroscopy (EIS)
In the EIS spectrum, the bulk resistance (Rb) is related to the interruption on the real axis at high frequency. At the same time, the semicircle at the mid-frequency zone corresponds to the charge transfer resistance (Rct) at the electrolyte/electrode interface, which is strongly related to the capability of ion transfer into the pores of an electrode (Table 1). As seen in Fig. 5, the device based on NirGO3 has the lowest bulk resistance (2 Ω) and the charge transfer resistance (1 Ohm) compared to other devices. This is because the π-π agglomeration in bare rGO is less in NirGO-based electrodes. During the formation of the foam structure, this agglomeration was prevented, and thus lower resistance was obtained. Although CB prevents this agglomeration, there is an increase in bulk resistance due to the usage of a dielectric binder. Among the NirGOs, the highest resistance was observed in NirGO6. As seen in the SEM images, the reason is that as the Ni ratio increases, the regular agglomeration and the formation of porous structure between the rGO layers deteriorates (Fig. S1). Therefore, the bulk resistance (11 Ω) at the electrode and the charge transfer resistance (4 Ω) between the electrolyte and the electrode also increases.
3.2.3. Galvanostatic charge-discharge (CDC) characteristics
The capacitance of each electrode materials was also calculated by using the galvanostatic CDC curves (Fig. 6 (a)). Calculated capacitance of rGO, rGO:CB, NirGO1, NirGO3, and NirGO6, were 606, 1000, 714, 833, and 270 Fg-1, respectively (Table 1). When the IR (current-resistance) drops in each electrode are examined, it is seen that they are very close to each other but slightly higher than that of NirGO6 (0.3 Ohm). This shows that the total resistances in the devices are close to each other. Then, CDC curves were obtained for all electrode materials at different currents between 1-5 mA. In this way, charge transfer and interaction between electrode and electrolyte were investigated. All devices retained their capacitive behaviour even though their capacitance values ​​decreased with the current increase.