*3.6. Galvanostatic Charge–Discharge (GCD)*

The specific capacitance of the electrodes obtained from GCD curves is calculated at various current densities by the following equation [37]:

$$\mathbb{C} = \frac{I \cdot t\_d}{m \cdot \Delta V} \tag{2}$$

I is a discharge current (A), *td* is a discharge time (s), m is an active material mass, and Δ*V* is a potential drop during discharge.

Specific capacitance values of G95 and G70L25 for each current density are shown in Table 1. The specific capacitances of both the G95 and G70L25 electrodes were similar to those of the CV

experiments. When the current density increased from 0.5 to 20 A/g, specific ion diffusion rates tended to reduce specific capacities of the G95 and G70L25 electrodes at a high current density [38]. As a result, the variation of the specific capacitance of the G95 and G70L25 electrodes over current density shows the typical behavior of a supercapacitor. The low value of capacitance at a high current density is due to the low ion penetration on the electrode surface [39]. The Ragone plot of G95 and G70L25 is shown in Figure 7d. For the G95 and G70L25 electrodes, the energy density is 21.80–16.24 Wh/kg and 17.10–15.12 Wh/kg at the same range of current density, respectively. The G70L25 electrode shows a 22% decrease in energy density at 0.5 A/g, but only a 7% decrease at 20 A/g compared to G95, which means catalytic effects of the perovskite enhances at a higher current density. In addition, we fabricated composite electrodes of LSMCO and graphene nanoplatelets by mixing and dispersing using magnetic bars and ultrasonic. In this process, we believe that structural stability increases because graphene nanoplatelets wrap around LSMCO [40].


**Table 1.** The specific capacitance of G95 and G70L25 according to current density.

**Figure 7.** Galvanostatic charge-discharge graph of (**a**) G95 electrode and (**b**) G70L25 electrode; (**c**) specific capacitance variation of G95 and G70L25 with current density; (**d**) comparative Ragone plots of the G95 and G70L25 electrodes.
