## 4 Conclusions  
Experimental results confirmed that electrochemical polarization curves can be used for observing changes in charging and discharging performances. The discussions about critical operating parameters and materials selection provide a better understanding of cell design and operating condition for further cell performance improvement. The following conclusions were extracted from this research:  
1. The highest charging performance was obtained when 1.7 V of charging voltage was imposed. Because the optimal charging voltage must overcome the standard cell potential and activation loss, this electrical potential has a significant impact on the electrochemical reactions generated during the electrolytic process. Furthermore, overpotential can cause side reactions and impurities, which can reduce charging efficiency. 2. The VRFB, operated using electrolytes prepared by 1.7 V of charging voltages, offered the lowest ohmic loss and concentration losses. Moreover, the completion of the electrochemical reaction during initial charging directly impacts the concentration of species in the catholvte and anolyte, and it brings about the enhancement of maximum power and voltage efficiency. 3. Even though the VRFB operated using 30 mL/min of electrolyte flow rate had the best performance, with a maximum power and voltage efficiency, 10 mL/min of the flow rate was preferred for application to the VRFB operation. Choosing the appropriate flow rate to achieve the desired performance corresponds to striking a balance between VRFB performance and pump energy efficiency. 4. The feasibility of using a carbon paper electrode for a flow-through design was investigated in terms of the number of electrode layers as well as electrolyte flow and diffusion behavior in flow channels and electrode layers. A ten-layer electrode is anticipated to offer a highly active area, but results indicate that using many layers of an electrode may reduce the porosity of the electrode, resulting in a reduction in the active area. This phenomenon causes an increase in the redox reaction rate of the reaction generated in the VRFB assembled with the ten-layer electrode. 5. Feeding electrolytes in different directions, horizontal and vertical, resulted in a better understanding of the effects of flow behavior on retention time and reaction capability. The horizontal flow pattern provided a greater retention time, resulting in improved electrochemical reaction performance. 6. The thickness of the membrane and the number of sulfonate groups are vital factors to consider when selecting a sulfonated membrane for VRFB operation because they have a direct influence on the proton conduction mechanism. The performance of the cell including Nafion 212 was inferior to that of the cell consisting of Nafion 117. The  
Figure 15: Polarization curves and power vs. current density plots of the VRFB containing various membranes. concluding result can be interpreted as follows: the thickness of the membrane has a greater effect on proton transfer than the number of sulfonate groups. Future research will concentrate on connecting the causes to the charging kinetics in order to contribute to the overall reliability of charging VRFB. A systematic reliability analysis will be developed to gain a better understanding of how to operate VRFB with the desired performance. Data curation, K.O. ; investigation, K.O. ; methodology, K.O., R.Y. and J.G. ; writing--original draft, R.Y. and S.C.; writing--review and editing, R.Y. ; writing--final draft, R.Y. ; visualization, R.Y. ; validation, R.Y. and S.C.; funding acquisition, R.Y. and J.G. ; consultant, J.G. ; resources, J.G. ; formal analysis, S.C. All authors have read and agreed to the published version of the manuscript. This research was funded by Thailand Science Research and Innovation (TSRI), Research and Researchers for Industries (RRi) in cooperation with Thai Marine Protection Co., Ltd., grant number PHD62I0003. Not applicable.