## 3 Results
### Observation of Applying Carbon Paper as Electrodes in VRFB  
Electrodes are one of the main components of VRFBs, and they serve the function of providing an electrochemically active surface on which the redox reactions take place. When choosing an electrode, certain criteria must be met, such as being active for the chemical reaction, having a high electrical conductivity and a large surface area, being chemically stable, and being inexpensive [48]. On the surface, C-OH functional groups act as active sites for the oxidation of vanadium ions. Paper electrodes, carbon felt, and graphite felt are the electrode types used for VRFBs. In terms of electrode dimensions, there are two basic categories: 3D structure and 2D structure. While 2D electrodes such as carbon paper electrodes have historically been used for flow-by design, flow-through design frequently uses 3D electrodes such as graphite felt and carbon felt electrodes [49]. This part of the study focused on whether using carbon paper with the flow-through concept was practical. We anticipated that our flow-through architecture would work with both 3D and 2D electrode assemblies. This effort to investigate the use of carbon paper for VRFB with the flow-through design was motivated by its ease of production and economical cost. Carbon felt and graphite felts are typically made using sophisticated processes such as electrospinning followed by needle punching, whereas carbon papers are made using an electrospinning process. Carbon felt and graphite felts are three times more expensive than carbon papers. In addition, carbon paper has high electrical conductivity and high fiber density, resulting in a high specific surface area [46]. To test the impact of the number of carbon paper layers on VRFB performance, a 400 \(\upmu\)m thick stacked carbon paper electrode was created. For this experiment, 3 and 10 layers of carbon paper were utilized as sample layers. As is well known, using multiple electrode layers will cause an increase in contact resistance between the layers [50]. In order to lower the contact resistance, the stacked electrode was compressed, which decreased the electrode's porosity and permeability. Longer charging times and a decreased reaction kinetic rate were the results of this [51], and it caused side flow surrounding the electrode, which resulted in an electrolyte shortage in the electrode's center. The concentration loss of the flow battery is closely proportional to this circumstance [46]. To achieve the desired electrical conductivity and permeability, the electrode is typically compressed in the range of 20-30% of electrode thickness with bipolar plates [51], but at high current density, more than 60 mA/cm\({}^{2}\)[52], the compression should be higher 35-50% of electrode thickness to improve electrical conductivity [53]. There are two case studies in this work; compressing three layers of carbon papers was observed in the case of 0-30% of thickness reduction, and compressing ten layers of carbon papers represented the scenario of 50% of thickness reduction. It can be assumed that the higher compressive force applied to ten-layer carbon papers meaningfully impacted electrolyte diffusion, resulting in a decrease in the reaction efficiency. Figure 9: VRFB performance operated with various flow rates. According to the electrochemical polarization results shown in Figure 9, the VRFB containing three-layer carbon papers had lower ohmic loss than the VRFB containing ten-layer electrodes. As previously stated, when the electrode layer increased, a significant voltage drop was observed at the medium current density (8.00-38.00 mA/cm\({}^{2}\)). When the electrode layers increased from 3 to 10 layers, the slope of the curves in this region decreased 1.25 times. Furthermore, the power of both VRFBs supported the voltage decline phenomenon results. The maximum power of the VRFB using a three-layer electrode was 24.98 mW, while the maximum power of the VRFB using a ten-layer electrode was 21.37 mW.