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.