## 3 Results
### The Interaction of Electrolyte Preparation and Discharging Performance
A flow chart of the electrolyte preparation and discharging performance is shown in Figure 3. For experimental activities, the positive pole was connected as a working electrode, whereas the negative pole was linked with reference and counter electrodes. It should be emphasized that there is a narrowing of the operating temperature range (10 \({}^{\circ}\)C-40 \({}^{\circ}\)C). It undergoes thermal precipitation at temperatures above 40 \({}^{\circ}\)C and dissipation when below 10 \({}^{\circ}\)C [37]. Low temperatures are also related to the precipitation of species V\({}^{2+}\), V\({}^{3+}\), and VO\({}^{2+}\)[38]. The electrical potential applied to the VRFB cell generated an oxidation reaction on the positive side and reduction on the negative side. When oxidation occurred, VO\({}^{2+}\) was transformed into VO\({}^{2+}\). This transformation can be observedby monitoring the electrolyte's color change from blue to yellow, as presented in Figure 4. The vanadium metal is classified as a transition metal if the oxidation number changes, causing the color change (Equation (3)). In terms of the reaction, the reaction mechanism involved two steps (Equation (4) and (5)) [6]. VO\({}^{2+}\) turned to be V\({}^{3+}\) in the first step, and then a blue solution became green. In the second step, V\({}^{3+}\) accepted the electron, and then V\({}^{2+}\) was produced. The color of the electrolyte changed from green to purple, as indicated in Figure 3. Electrolytes were stored in an external reservoir [39] and were injected into the VRFB with the same flow rate. The redox reaction occurred on the electrode surfaces, and once it was finished, the products there were moved to the electrolyte reservoir for storage. According to this procedure, the migration and concentration change of positive and negative electrolytes were unaffected by the difference in electrolyte volumes. \[\text{Cathode VO}^{2+}+\text{H}_{2}\text{O}\rightarrow\text{VO}_{2} ^{+}+2\text{H}^{+}+\text{e}^{-}\text{E}^{0}=1.00\text{ V} \tag{3}\]  
\[\text{Anode VO}^{2+}+2\text{H}^{+}+\text{e}^{-}\rightarrow\text{V}^{3+}+\text{ H}_{2}\text{O}\text{E}^{0}=0.34\text{ V} \tag{4}\]  
\[\text{V}^{3+}+\text{e}^{-}\rightarrow\text{V}^{2+}\text{E}^{0}=-0.25\text{ V} \tag{5}\]  
Figure 4: The change in electrolyte colors on the positive and negative sides. Figure 3: Flow chart of electrolyte preparation and discharging performance. The actual operating voltage is a critical factor when charging VRFB or preparing electrolytes because the proper operating voltage (applied voltage) can drive an efficient electrolytic reaction with no side reactions. Because the amount of overpotential is required in addition to the thermodynamic voltage, the charging voltage must be greater than 1.26 V. The charging voltage affects the overall kinetics of the electrochemical reaction caused by charge transfer. If the VRFB is overcharged, hydrogen and oxygen gas may be produced at the negative and positive electrodes. Graphite plates can also be corroded, resulting in the production of carbon dioxide gas [6]. As a result, in this experimental section, the optimal charging voltage was investigated. Figure 5 illustrates the current profiles of the VRFB that were measured as a function of time during the electrolyte preparation process. The VRFB was electrically supplied via chronoamperometry by imposing a certain electrical potential (1.6-2.0 V). The system current measured during the preparation step, the reaction duration, the moles of electrons involved, and the electric charge involved in the redox reaction are the values that were observed to determine the appropriate voltage that was applied to initiate the reaction. When the electrolyte is introduced into the VRFB cell, a concentration gradient will cause the diffusion process to occur from a high concentration to a low concentration. Following that, a redox reaction takes place on the electrode surface, resulting in an electron transfer. The two main factors affecting electron mobility are surface morphology and electrode structure.