## 3 Results and Discussion  
Figure 2 shows the FT-IR spectra of the porous PTFE substrate and prepared membranes. In the spectra of both the base membrane (PS + Poly(DMAEMA-DVB)) and PFAEM (PS + Poly (DMAEMA-DVB-XDC)), the absorption bands at 1726 cm\({}^{-1}\) and 1456 cm\({}^{-1}\), which are assigned to the stretch vibration of carbonyl group (C=O), and the C=C in plane stretch vibration of the benzene ring, respectively, indicating the successful in situ synthesis of poly(DMAMEA-DVB) inside the pores of the PTFE substrate [42]. In addition, the absorption band at around 3400 cm\({}^{-1}\), which is assigned to the stretching vibration of the N-H\({}^{+}\) group clearly elucidates the introduction of quaternary ammonium sites into the membrane [42]. The FE-SEM analyses were carried out to investigate the morphology and pore-filling of the membranes and the images are shown in Figure 3. The cross-sectional FE-SEM images of the PTFE substrate films show a highly porous structure, and the pores in the substrates were revealed to be completely filled with ionomer after the membrane fabrication. In addition, nano-sized metal oxide particles (e.g., Al\({}_{2}\)O\({}_{3}\)) were observed in the images of hydrophilic PTFE-based samples (Figure 3c,d), which might enhance the hydrophilicity of the substrate and membrane [43]. The pictures of the porous substrates and prepared membranes are shown in Figure S1 (in the Supplementary Materials). The opaque porous substrates were shown to be changed into a transparent state after the pore-filling by in situ polymerization. In this work, the cross-linking of the PFAEMs was finely controlled by varying the cross-linker (DVB) contents. Some important membrane parameters (i.e. IEC, ER, contact angle, and WU) were correlated with the cross-linker content and the results are depicted in Figure 4a-d. As the DVB content increased, the IEC and WU values were shown to decrease, while the EARs and surface contact angles increased, demonstrating the reduction of free volume and number of hydrophilic fixed charges in the membranes. The images of the surface contact angle measurements are also displayed in Figure S2 (in the Supplementary Materials). The IECs of the PFAEMs fabricated with different porous PTFE substrate films (i.e., hydrophobic and hydrophilic grades) were almost the same at the identical membrane composition, as shown in Figure 4a. This result could prove that the pore size and porosity of the substrate films used, are comparable with each other. However, the PFAEMs prepared using a hydrophilic PTFE substrate (i.e., hydrophilic-PFAEMs) showed much lower EARs compared with those of the hydrophobic PTFE-based PFAEMs (i.e., hydrophobic-PFAEMs), meaning more facilitated ion transport through the more hydrophilic medium. The EAR values of the hydrophilic-PFAEMs  
Figure 2: FT-IR spectra of porous PTFE substrate and pore-filled membranes. started to sharply increase when adding the cross-linker of above 10 wt%, as shown in Figure 4b. Therefore, the optimal cross-linker content was determined as 10 wt%. Figure 4: Changes in (**a**) IEC values, (**b**) electrical resistances (measured in a 0.5 M NaCl), (**c**) surface contact angles, and (**d**) water uptake of the PFAEMs by varying the content of cross-linking agent, divinylbenzene (DVB). Figure 3: Cross-sectional FE-SEM images of porous substrates: ((**a**) hydrophobic PTFE; (**c**) hydrophilic PTFE)) and pore-filled anion-exchange membranes ((**b**) hydrophobic-PFAEM; (**d**) hydrophilic-PFAEM). The various properties of the PFAEMs, which were fabricated with different porous substrate films and identical monomer composition (10 wt% DVB), are compared with those of the commercial membrane (AMX) in Table 2. Note that the same PFAEMs have also been utilized for comparative studies in ADLFC and VRFB systems. The surface contact angles of the PTFE based PFAEMs are shown to be much higher than that of the commercial membrane. Meanwhile, the contact angle of the hydrophilic-PFAEM is much smaller than that of the hydrophobic-PFAEM.