## 3 Results and Discussion
This result demonstrates that the fluorine moiety introduced into the membranes can also improve the oxidative stability of the ion-exchange groups. Figure 8: Time course changes in residual weight fraction during the Fenton oxidation test of (**a**) AEMs and (**b**) CEMs. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Membrane**} & **Thickness** & \multirow{2}{*}{**WU (\%)**} & **SR** & **IEC** & \(\sigma\) & **MER** & **Transport Number** & **Contact** & \(k_{\text{VQ}^{2+}}\) \\  & **(\(\mu\)m)** & & **(\%)** & **(meq-/g)** & **(mS/cm)** & **(\(\Omega\cdot\text{cm}^{2}\))** & **(-)** & **Angle (\({}^{\circ}\))** & **(\(\times 10^{-7}\), m/s)** \\ \hline AMX & 140 & 22.4 & 16.8 & 1.54 & 5.24 & 2.67 & 0.988 & 54.14 & 3.27 \\ PFAEM-0 & 25 & 23.8 & 20.7 & 2.02 & 3.97 & 0.63 & 0.988 & - & 2.17 \\ PFAEM-1 & 25 & 12.8 & 19.3 & 1.34 & 1.58 & 1.58 & 0.978 & 58.17 & 1.18 \\ PFAEM-2 & 25 & 14.8 & 20.0 & 1.67 & 1.87 & 1.34 & 0.985 & - & 2.57 \\ PFAEM-3 & 25 & 16.8 & 20.9 & 2.00 & 2.69 & 0.93 & 0.991 & - & 2.73 \\ \hline CMX & 160 & 27.1 & 14.8 & 1.89 & 5.93 & 2.70 & 0.977 & 45.72 & 6.54 \\ PFCEM-0 & 25 & 17.1 & 12.7 & 2.26 & 4.31 & 0.58 & 0.991 & - & 3.97 \\ PFCEM-1 & 25 & 13.1 & 10.4 & 1.96 & 1.52 & 1.64 & 0.979 & 52.53 & 3.27 \\ PFCEM-2 & 25 & 15.7 & 15.1 & 2.47 & 1.82 & 1.37 & 0.983 & - & 4.47 \\ PFCEM-3 & 25 & 18.3 & 17.1 & 2.74 & 2.16 & 1.16 & 0.987 & - & 5.39 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Various properties of commercial membranes and PFIEMs. Meanwhile, IEMs applied to a VRFB operating under strongly acidic conditions may cause problems, such as a decrease in the degree of cross-linking and decomposition of functional groups when used for a long period of time [49]. Therefore, the chemical stability of the prepared PFiEMs by measuring the rate of membrane decomposition in a vanadium electrolyte solution was also evaluated. When immersed in a VO2+/H2 SO4 solution, VO2+ is reduced to VO2+ due to the oxidation reaction of the membrane. Thus, there is a proportional relationship between the generation rate of VO2+ ions and the decomposition rate of the IEMs [50]. Figure 10 shows the results of measuring the concentration of VO2+ generated by the oxidation reaction of each IEM. As a result, it was confirmed that the oxidation stability of the PFiEMs in the vanadium electrolyte was significantly higher than that of the commercial membranes. This is considered to be a result of the excellent stability of the porous PE film used as the reinforcing material. In addition, as the ratio of the fluorine moieties increased, a lower VO2+ ion production rate was exhibited. From this, it was also confirmed that the chemical stability of the IEMs for VRFB application could be improved by introducing fluorine moieties into the membrane. Figure 10: Time course changes in VO2+ concentration during the chemical stability test of (**a**) AEMs and (**b**) CEMs in 0.1 M V2 SO5 (in 5 M H2 SO4). Figure 9: Initial IEC values and percentage reduction of IEC after Fenton oxidation at 80 \({}^{\circ}\)C for 8 h of (**a**) AEMs and (**b**) CEMs. Vanadium ion permeability through a membrane is a parameter indicating the crossover characteristic of vanadium ions. The crossover of vanadium ions as the active electrode material can be regarded as a self-discharge process in a VRFB system. Vanadium ions pass through the IEM and chemically react with vanadium ions of different oxidation numbers, resulting in efficiency loss and capacity reduction [51]. Since the crossover behavior is mainly determined by the membrane characteristics, fabricating the IEMs with a reduced crossover rate is one of the important issues in the RFB technology field. An ideal IEM for VRFB should have low vanadium ion permeability to reduce self-discharge and achieve high current efficiency [52].