*3.5. Cyclic Voltammetry (CV)*

In the CV curve, the specific capacity of the electrode can be obtained through the following equation:

$$C\_s = \frac{1}{s \cdot m \cdot \Delta V} \int\_{V\_1}^{V\_2} i dV \tag{1}$$

Here, CS is a specific capacitance (F/g), s is a scan rate (V/s), m is a mass of active material, ΔV is a voltage window (0 V–0.8 V), I is a current (A), V1 is a lower voltage limit (V), and V2 is an upper voltage limit (V) [36].

In Figures 5 and 6, the CV curve has a nearly rectangular shape and represents the capacitive behavior of the EDLC. Figure 5 shows the CV curves of the G95 electrode and G70L25 electrode at a scan rate of 5 mV/s in 1 M H2SO4 after 5 cycles. The specific capacitance of the G95 electrode is 69.34 F/g, and the standard deviation is 6.961. The specific capacitance of the G70L25 electrode is 52.31 F/g, and the standard deviation is 2.576. The specific capacitance of the G95 electrode is about 10% higher than that of G70L25. The higher specific surface area of G95 can explain the higher initial specific capacitance values of G95 in CV measurements. However, the standard deviations of the results for G95 are quite high compared to those of G70L25. It might mean that producing supercapacitors with the expected design values in the mass production phase can be difficult.

Figure 6 shows a CV curve measured for 4990 cycles by increasing the scan rate to 20 mV/s. Figure 6a,b is the CV curve of the G95 electrode. The specific capacitance is 62.23 F/g at 11 cycles and 61.38 F/g at 4990 cycles. The reduction of 1.38% can be seen in Figure 6a. On the other hand, specific capacitance is 57.07 F/g at 11 cycles and 59.81 F/g at 4990 cycles, which means an increase of the specific capacitance (Figure 6b). However, Figure 6c,d shows a CV curve of the G70L25 electrode. In Figure 6c, the specific capacitance was 47.37 F/g at 11 cycles and 45.94 F/g at 4990 cycles, so it decreased by 3.01%. In Figure 6d, specific capacitance is 50.98 F/g at 11 cycles and 48.41 F/g at 4990 cycles. For the G70L25 electrode, after 4990 cycles, the performance of the G70L25 electrode is consistent for several data sets, and up to 7% reduction can be expected.

**Figure 5.** Cyclic voltammetry at 5 mV/s of G95 electrode and G70L25 electrode in1MH2SO4.

**Figure 6.** Cyclic voltammetry at 20 mV/s (**a**) specific capacitance decreased after 4990 cycles for the electrode of G95 in 1M H2SO4; (**b**) specific capacitance increased after 4990 cycles for the electrode of G95 in 1 M H2SO4; (**c**,**d**) specific capacitance decreased after 4990 cycles for the electrodes of G70L25 in 1MH2SO4; (**e**) the plot of specific capacitance verse cycle at 20 mV/s in H2SO4.

The results of the G95 electrode, however, show that it is difficult to expect a reproducibility of the experiments because specific capacitance tends to both increase or decrease, which means that we cannot guarantee some level of the supercapacitor capacity after long term operation. However, all of the G70L25 electrodes tend to decrease. Therefore, even though the specific capacitance of the electrode G70L25 with the perovskite LSMCO added onto graphene nanoplatelets is lower than that of the G95 electrode, the stability seems to be superior.
