## 3 Results and Discussion  
Figure 4 shows photographs of the prepared PFIEMs. Unlike the opaque porous substrate film, it can be seen that the prepared PFIEMs are shown to be transparent, and from this, it can be indirectly confirmed that the pores of the substrate are completely filled with a polymer. In addition, in the case of PFCEM, the color was changed to dark yellow after the post-treatment, and therefore, it can be expected that a sulfonation reaction occurred in the pore-filled polymer. The content of ionomer filled in the porous substrate is measured and summarized in Table 3. The content of the filled ionomer is similar to the porosity of the porous substrate shown in Table 1.  
In addition, field-emission scanning electron microscopy (FE-SEM, JSM-7500F, JEOL Ltd., Japan) analysis was performed to check the surface morphology of the prepared IEMs. As shown in Figure 5, it was confirmed that the pores of the porous substrate were completely filled with a polymer after the membrane preparation, and there were no open pores [35]. Figure 4: Pictures of (**a**) pore-filled anion-exchange membrane (PFAEM-1) and (**b**) pore-filled cation-exchange membrane (PFCEM-1). Figure 5: FE-SEM images (surface, \(\times\)20,000) of (**a**) porous PE substrate, (**b**) PFAEM-1, and (**c**) PFCEM-1. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Membrane** & **PFAEM-0** & **PFAEM-1** & **PFAEM-2** & **PFAEM-3** \\ \hline Ionomer content (wt\%) & 40.7 & 41.3 & 42.6 & 41.3 \\ \hline
**Membrane** & **PFCEM-0** & **PFCEM-1** & **PFCEM-2** & **PFCEM-3** \\ \hline Ionomer content (wt\%) & 44.1 & 44.0 & 44.1 & 43.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Ionomer contents in prepared PFIEMs. The FTIR spectra of the porous PE substrate and the prepared PFAEMs and PFCEMs are shown in Figure 6. In the FTIR spectra of PFAEM, the successful introduction of quaternary ammonium groups was confirmed from the absorption peaks observed at 1372, 975, 890, and 812 cm\({}^{-1}\)[36; 37; 38; 39]. Additionally, the presence of C=C bonds and aromatic rings was checked from the absorption peaks found at 1640 and 1390 cm\({}^{-1}\), respectively [40; 41]. Meanwhile, CF\({}_{2}\) stretching vibration was observed at 1170 cm\({}^{-1}\) in the spectrum of PFAEM-1 [42]. In addition, C=O and C-O-C bonds were confirmed at 1740 and 1120 cm\({}^{-1}\), respectively, indicating the introduction of a fluorine moiety due to the copolymerization of OFPMA monomer [43; 44]. Meanwhile, in the spectra of PFCEM, the absorption peaks assigned to sulfonic acid groups were found at 1127, 1037, 1008, and 678 cm\({}^{-1}\), elucidating the successful introduction of cation-exchange groups [38]. In the case of the PFCEM, the existence of the fluorine moiety could not be checked from the FTIR spectra due to the overlapping of the absorption bands. Overall, it can be demonstrated that the preparation of the PFAEM and PFCEM was successfully performed through the monomer pore filling, in situ radical polymerization, and post-treatment reaction. The IEMs used in the RFB are required to have excellent mechanical strength to resist pressure drop owing to high flow rates. Tensile strength and elongation at break are important parameters indicating the mechanical strength of IEMs [45]. The results of tensile strength and stress measurement for the commercial membranes, the porous substrate film, and the prepared PFIEMs are summarized in Figure 7 and Table 4. It can be seen that the porous PE film used as the reinforcing material has an excellent tensile strength (125.1 MPa) and elongation at break (46.47%) despite a relatively thin film thickness compared with those of commercial IEMs. In addition, the prepared PFIEMs revealed largely improved tensile strength and toughness compared with the porous substrate film. From the results, it was confirmed that the PFIEMs fabricated in this work had superior mechanical strength despite having a thickness of about 1/6 of the commercial membranes. Figure 6: FTIR spectra of porous substrate, PFAEMs, and PFCEMs.