## 3 Results
### The Influence of Different Nafion Membrane Specifications on VRFB Performance
Research literature stated that Nafion 117 shows superior performance than the other membranes in this group since it offers many advantages such as the greatest protection against vanadium diffusion to prevent self-discharge, the highest coulombic efficiency, the lowest swelling value (63%), and the highest value of ion exchange capacity (0.88 mmol/g) [62]. Nafion 212 and Nafion 117 were used to study the performance of the flow battery. Proton transfer in the membrane is an important factor affecting the performance of the flow battery. The thickness of the membrane and the sulfonated group affect the proton transfer in the membrane. Nafion 212 and Nafion 117 have different thicknesses and amounts of functional groups. The functionality of Nafion 117 and Nafion 212 membranes, representing widely used membranes in the market [63], for VRFB operation was investigated [64]. These Nafion membranes have sulfonate (SO3-) groups at the end of the polymer chain, which allow only protons to transfer [23]. The thickness of the membranes and the amount of SO3- groups in them were the focus of this experiment. Serial numbers were used. Nafion membranes were labelled with the gram of polymer/mole of sulfonate group and thickness as follows: Nafion 117 means the membrane has 1100 g of polymer/mole of sulfonate group and a thickness of 0.007 inches (183.0 \(\upmu\)m), while Nafion 212 has 2100 g of polymer/mole of sulfonate group and a thickness of 0.002 inches (50.8 \(\upmu\)m). Note that Nafion 117 is the commercial membrane that contains the highest amount of sulfonate groups compared to the other commercial Nafion membranes. Figure 15 shows that the VRFB containing Nafion 212 performed better than the cell containing Nafion 117. It produced open-circuit voltages of 1.51 V, a power of 47.33 mW, a maximum current density of 66.95 mA/cm\({}^{2}\), and maximum energy of 0.13 Wh. The VRFB assembled with Nafion 117, on the other hand, provided 1.48 V open-circuit voltage, 21.81 mW power, 46.26 mA/cm\({}^{2}\) maximum current density, and 0.04 Wh maximum energy. In terms of voltage loss at medium current density, the VRFB with Nafion 212 had a significantly lower ohmic loss than the VRFB with Nafion 117 [65]; thus, the material resistance was reduced. The lower resistance had a positive effect on proton transfer through the membrane that enhanced the redox kinetics [66]. As a result, it can be interpreted that the proton transfer through the membrane via vehicle and Groththus mechanisms strongly influences on VRFB performance in comparison with the surface mechanism. As a result, this could be observed from Nafion 212, which contained a smaller number of sulfonate groups than Nafion 117; therefore, the proton transfer ability via the surface mechanism should be lower than that when Nafion 117 is used. The results can confirm that the bulk mechanism had a greater effect than the surface mechanism, and the proton transfer was coupled with the water movement via electro-osmotic drag [23]. In actuality, the performance of the cell containing Nafion 212 as shown in the polarization curves was inferior to that of the cell containing Nafion 117. When it comes to membrane thickness, Nafion 212 is less thick than Nafion 117, and it performs better than Nafion 117. This indicates that a smaller membrane thickness improves proton transfer capabilities through the Groththus and vehicle processes [67; 68]. Furthermore, the transfer resistance will decrease as the membrane thickness decreases, according to [PERSON]'s law. Thinner membranes aid in improving proton conductivity and proton permeability. In terms of mechanical properties, Nafion 212 has a lower swelling ratio than Nafion 117 [67; 68]. With this rational information, the thickness of the membrane has a greater impact than the number of sulfonate groups; thus, Nafion 212 performed better than Nafion 117.