## 3 Results and Discussion
Various characteristics of the commercial IEMs and the prepared PFIEMs are summarized in Table 5. In this study, the PFIEMs were prepared according to the molar ratio of VBC, Sty, and OPFMA, and then the membrane properties were systematically evaluated. As the mole ratio of VBC or Sty is increased, the portion of OPFMA is relatively decreased, and therefore, the content of the fluorine part in the membrane is reduced. The amount of ion-exchange groups introduced into VBC or Sty increased while decreasing the fluorine part, resulting in an increase in the IEC. In addition, it can be seen that the WU and \(\sigma\) of the PFIEMs increased, and the electrical resistance decreased due to the increase in the IEC. The high elongation of the prepared membrane was due to the intrinsic characteristics of the porous PE support used, which means that it could be stretched when a strong external force is continuously applied. However, it was proven that the excessive swelling of the membranes did not occur in actual use based on the SR data shown in Table 5. Meanwhile, the \(\sigma\) of the PFIEMs showed a lower value compared with the commercial membranes because the non-ion conducting area was greatly increased due to the use of an inert PE substrate like in the case of a heterogeneous IEM. However, all the PFAEMs and PFCEMs fabricated in the considered composition range showed significantly lower electrical resistance compared with the commercial IEMs, which was mainly due to the relatively thin film thickness. As a result of measuring the surface contact angle, it can be observed that the hydrophobicity of the PFIEMs containing a fluorine moiety was somewhat higher than that of the commercial membranes, which is thought to be owing to the characteristic of the PE substrate with strong hydrophobicity (i.e., contact angle of the PE substrate = ca. 99.0 degree) [46]. It can be seen that the prepared PFIEMs generally exhibited a low water content and high surface hydrophobicity compared with the commercial membranes, which was considered to be advantageous in reducing the crossover of vanadium ions and increasing the oxidation stability of the membrane. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Membrane**} & \multirow{2}{*}{**Thickness (\(\mu\)m)**} & **Tensile Strength** & **Elongation at** \\  & & **(MPa)** & **Break (\%)** \\ \hline AMX (Aatom Corp.) & 140 & 29.47 & 15.60 \\ CMX (Aatom Corp.) & 160 & 25.96 & 18.86 \\ Porous substrate & 25 & 108.5 & 138.4 \\ PFAEM-1 & 25 & 136.6 & 176.5 \\ PFCEM-1 & 25 & 129.9 & 148.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Tensile strength and elongation at break of commercial IEMs, porous substrate, and PFIEMs. Figure 7: Tensile stress–stain curves of commercial IEMs, porous substrate, and PFIEMs. Figure 8 exhibits the results of measuring the weight change of the IEMs during the Fenton oxidation test. It can be seen that PFAEM-1 and PFCEM-1 containing the most fluorine moieties exhibited the best oxidation stability among the membranes tested. The data demonstrate that the oxidation stability elevated as the content of the fluorine moiety increased. Therefore, it can be confirmed that the oxidation stability of the PFIEMs was improved due to the introduction of the fluorine moiety. However, PFCEM-2 and PFCEM-3 showed lower oxidation stability compared with PFCEM-0 without a fluorine moiety. In the case of poly(styrenesulfonic acid), it is known that chain scission by the HO- radical is promoted at low pH conditions [47; 48]. That is, it is believed that the high content of sulfonic acid groups (i.e., high acidity) promotes the oxidative degradation of a cation-exchange polymer. The percentage reduction of the membrane IEC after the Fenton oxidation experiment was determined, and the results are shown in Figure 9. As a result, it was found that PFAEM-1 and PFCEM-1, which had the highest fluorine content among the membrane samples tested, showed the same tendency with the change in weight and had the lowest IEC reduction rate.