Pore-Filled Anion-Exchange Membranes with Double Cross-Linking Structure for Fuel Cells and Redox Flow Batteries

In this work, high-performance pore-filled anion-exchange membranes (PFAEMs) with double cross-linking structures have been successfully developed for application to promising electrochemical energy conversion systems, such as alkaline direct liquid fuel cells (ADLFCs) and vanadium redox flow batteries (VRFBs). Specifically, two kinds of porous polytetrafluoroethylene (PTFE) substrates, with different hydrophilicities, were utilized for the membrane fabrication. The PTFE-based PFAEMs revealed, both excellent electrochemical characteristics, and chemical stability in harsh environments. It was proven that the use of a hydrophilic porous substrate is more desirable for the efficient power generation of ADLFCs, mainly owing to the facilitated transport of hydroxyl ions through the membrane, showing an excellent maximum power density of around 400 mW cm−2 at 60 °C. In the case of VRFB, however, the battery cell employing the hydrophobic PTFE-based PFAEM exhibited the highest energy efficiency (87%, cf. AMX = 82%) among the tested membranes, because the crossover rate of vanadium redox species through the membrane most significantly affects the VRFB efficiency. The results imply that the properties of a porous substrate for preparing the membranes should match the operating environment, for successful applications to electrochemical energy conversion processes.

Among various energy conversion systems, the application to fuel cells has been the most actively researched. As is well known, there are several types of fuel cells, depending on the kinds of electrolytes used and operation conditions, including the proton-exchange membrane fuel cell (PEMFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC), and direct methanol fuel cell (DMFC) [21]. Recently, alkaline direct liquid fuel

Membrane Characterizations
The synthesis of the anion-exchangeable polymer was confirmed by FT-IR (FT/IR-4700, Jasco, Tokyo, Japan) analysis. The morphological characteristics of the porous substrates and prepared PFAEMs were investigated by employing field emission scanning electron microscopy (FE-SEM, TESCAN, Czech). The surface hydrophilicity of the membranes was also evaluated using a contact

Membrane Characterizations
The synthesis of the anion-exchangeable polymer was confirmed by FT-IR (FT/IR-4700, Jasco, Tokyo, Japan) analysis. The morphological characteristics of the porous substrates and prepared PFAEMs were investigated by employing field emission scanning electron microscopy (FE-SEM, TESCAN, Czech). The surface hydrophilicity of the membranes was also evaluated using a contact angle analyzer (Phoenix 150, SEO Co., Suwon-si, Korea). The water uptake (WU) of the membranes was determined using the following equation: where W dry and W wet are the dry and wet membrane weights, respectively. A traditional titration method was employed to determine the ion-exchange capacity (IEC) of the membranes. After the pre-equilibrium in 0.5 M NaCl, chloride ions in the membrane were fully exchanged with sulfate ions in 0.25 M Na 2 SO 4 . The amount of Cl − was then quantitatively analyzed by titration with an AgNO 3 standard solution, and the IEC values were calculated using the following equation: where N Cl − is the normal concentration of Cl − (meq./L), V s is the solution volume (L), and W dry is the dry membrane weights (g). Both the ion conductivity (σ) and electrical area resistance (EAR) of the membranes were evaluated in a 1.0 M KOH solution at room temperature using a lab-made clip cell and an LCZ meter. The σ values were obtained from the following equation: where R memb is the resistance (Ω), l is the thickness (cm), and A is the effective area (cm 2 ) of the tested membrane. The EAR was estimated using the following equation: where |Z| is the magnitude of impedance (Ω), θ is the phase angle, and A is the effective area (cm 2 ) of the tested membrane. The transport number (t − ) for counter ions (Cl − ) was obtained by measuring the cell potential (=electromotive force, emf) using a two-compartment cell (membrane area = 0.785 cm 2 ; each volume = 0.23 dm 3 ) equipped with a pair of Ag/AgCl reference electrodes. As a result, the t − values were determined by the following equation: where E m is the cell potential, F the Faraday constant, T the absolute temperature, R the molar gas constant, and a H and a L the activity in high and low concentration compartments, respectively. The alkaline stability of the membranes was evaluated by conventional soaking tests under a harsh alkaline environment (1 M KOH; 60 • C; 500 h). The oxidative stability of the membranes was also checked The time-course changes in the transport number and membrane weight were recorded to evaluate the alkaline and oxidative stabilities, respectively. The fuel (potassium formate, KHCOO) crossover rates through the membranes were estimated via conventional 2-compartment diffusion cell tests. The time-course change in KHCOO content in the low concentration compartment was quantitatively analyzed using UV/Vis spectroscopy (UV-2600i, Shimadzu Co., Kyoto, Japan). The diffusion coefficient (D) of KHCOO through a membrane was calculated by the following equation [16]: where t is the time, δ is the membrane thickness, V L is the low concentration compartment volume, A is the membrane area, and C H and C L are the molar concentrations, in high and low concentration compartments, respectively. In addition, the overall dialysis coefficient (K A ) of vanadium cations through a membrane was determined using a two-compartment cell (effective area = 4 × 4 cm 2 ), filled with 1 M VOSO 4 /2.0 M H 2 SO 4 (feed) and 1 M MgSO 4 /2.0 M H 2 SO 4 (permeate). During the tests, the time-course change in vanadyl (VO 2+ ) ion concentration in the permeate compartment was recorded by measuring the solution absorbance using UV/Vis spectroscopy. The K A values were determined from the dependence of the component concentration and volume changes upon time, using the following equation [40]: where c I A0 is the initial molar concentration of component A in the feed compartment. c I A and c II A are the molar concentrations of component A in the feed (I) and permeate (II) compartments, respectively. A is the membrane area, τ is time, V I and V II are the solution volume in the feed (I) and permeate (II) compartments, respectively, and k v is the solution volume ratio of both compartments (=V I /V II ). In this work, the vanadium oxidative stability of the membranes was also confirmed. The membrane specimens (2 × 2 cm 2 ) were immersed in 0.1 M V 2 O 5 solution (in 5 M H 2 SO 4 ) and stored at 40 • C for about 200 h to evaluate the oxidative stability of membranes in a vanadium electrolyte solution. The time-course change in the oxidation state of vanadium ions was monitored by measuring the solution absorbance using UV/Vis spectroscopy [41].

MEA Performance Tests (ADLFC)
Commercially available Pd/C (40%, Premetek Co., Cherry Hill, NJ, USA) and Pt/C (46.7%, Tanaka Co., Tokyo, Japan) were chosen as anode and cathode electrocatalysts, respectively. The electrocatalyst solutions were directly sprayed on the surface of the membranes (3 × 3 cm 2 ; OH-form), and the total loading amount of Pd and Pt was revealed to be about 1 mg cm −2 . As a binder in the electrocatalyst inks, commercially available anion-exchange ionomer (AS-4, Tokuyama Co., Tokyo, Japan) was utilized. The catalyst-coated membrane (CCM) was inserted between two sheets of carbon fiber composite paper (TGP-H-060, Toray Inc., Tokyo, Japan) used as a gas diffusion layer (GDL). The CCM and GDLs were then assembled with a clamping pressure of about 5.5 MPa. The current-voltage (I-V) polarization characteristics of the prepared MEAs were investigated using a single cell, having an effective area of 9 cm 2 at 60 • C. Liquid fuel (6 M HCOOK/4 M KOH) and humidified O 2 gas (of 100% relative humidity (RH)) were fed to the anode and cathode at the flow rate of 5 mL min −1 and 500 sccm, respectively. In addition, for the I-V polarization curve measurement, a current was stepped up by 0.01 A and then maintained for 30 s at each step to gain a stable response. systems utilizing various membranes. The galvanostatic charge-discharge experiments were performed utilizing a lab-made RFB cell (membrane area = 12.5 cm 2 ) containing a pair of carbon felt electrodes (GF20-3, Nippon Graphite, Otsu-shi, Japan) with a battery cycler (WBCS3000S, Wonatech, Seoul, Korea) in the potential range of 0.9-1.9 V at 0.25 A. All the tests were carried out at room temperature. 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., Al2O3) 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.  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.

Results and Discussion
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 Energies 2020, 13, 4761 7 of 14 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%.
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., Al2O3) 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 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%. 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. This is one of the  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. This is one of the intrinsic characteristics of pore-filling types of membranes employing an inert porous substrate, that is, the hydrophobic nature of the porous substrate dominates the surface contact angles. The IEC values of the compared membranes were almost the same as each other, while the hydrophilic PFAEM revealed much higher conductivity for hydroxyl ions than those of both the hydrophobic-PFAEM and commercial membrane. As a result, the EAR of the hydrophilic-PFAEM was shown to be reduced by about four times compared with that of both the hydrophobic-PFAEM and the commercial membrane, because of the relatively high conductivity and thin membrane thickness. The prepared membranes exhibited excellent transport numbers for an anion (Cl − ), which were superior to that of the commercial membrane. Moreover, the alkaline stability of the AEMs was also checked via soaking tests under a harsh alkaline condition (i.e., 1 M KOH/60 • C/500 h). The transport numbers of the AEMs were shown to be significantly reduced after the alkaline soaking tests. The decrement in the transport numbers could have mainly originated from the degradation of quaternary ammonium sites under a harsh alkaline environment. The decrease in the transport number of the PTFE-based PFAEMs was much smaller than that of the commercial membrane, demonstrating that both, the use of chemically stable PTFE substrates, and the highly cross-linked ionomer, could largely enhance the alkaline stability of the membranes. The oxidative stability of the commercial and prepared membranes was also evaluated with the soaking experiment, using Fenton's reagent (3% H 2 O 2 containing 2 ppm FeSO 4 ). As shown in Table 2 and Figure 5, the chemical stabilities of the PFAEMs were superior to that of the commercial membrane. The result demonstrates that the combination of a chemically stable PTFE substrate and a highly cross-linked hydrocarbon ionomer significantly enhances the chemical stability of the membranes. In addition, the differences in the chemical stability of the two PFAEMs were not that significant. The MEAs utilizing two different PFAEMs were evaluated by the I-V polarization test, using a liquid fuel of 4 M KOH and 6 M HCOOK at 60 • C and 100% RH. The I-V and current-power (I-P) polarization curves of the MEAs are displayed in Figure 6. As a result, the power generation performance of the MEA was dramatically improved by employing the hydrophilic membrane instead of the hydrophobic one. The maximum power density of the MEA employing the hydrophilic-PFAEM was shown to be about 400 mW cm −2 at 1 A cm −2 . This result is almost comparable with that of the state-of-the-art membranes such as the Tokuyama A901 [28]. Since the crossover of liquid fuel through a membrane largely affects the energy conversion efficiency in such types of fuel cell [44], we also evaluated the diffusion coefficients of KCOOH through the PFAEMs, by means of conventional two-compartment diffusion cell tests. As shown in Table 3, the diffusion coefficient of the fuel molecule through the hydrophilic-PFAEM was revealed to be somewhat higher than that of the Energies 2020, 13, 4761 9 of 14 hydrophobic-PFAEM. It is believed that the more hydrophilic nature of the membrane increases the diffusion rate of the hydrophilic molecules. This means that the energy conversion efficiency of the hydrophilic-PFAEM is expected to be poorer than that of the hydrophobic-PFAEM, in terms of the fuel crossover rate. Therefore, it could be concluded that the dramatic improvement of the power density by employing the hydrophilic-PFAEM mainly resulted from the facilitated hydroxyl ion transport through the membrane.  The MEAs utilizing two different PFAEMs were evaluated by the I-V polarization test, using a liquid fuel of 4 M KOH and 6 M HCOOK at 60 °C and 100% RH. The I-V and current-power (I-P) polarization curves of the MEAs are displayed in Figure 6. As a result, the power generation performance of the MEA was dramatically improved by employing the hydrophilic membrane instead of the hydrophobic one. The maximum power density of the MEA employing the hydrophilic-PFAEM was shown to be about 400 mW cm −2 at 1 A cm −2 . This result is almost comparable with that of the state-of-the-art membranes such as the Tokuyama A901 [28]. Since the crossover of liquid fuel through a membrane largely affects the energy conversion efficiency in such types of fuel cell [44], we also evaluated the diffusion coefficients of KCOOH through the PFAEMs, by means of conventional two-compartment diffusion cell tests. As shown in Table 3, the diffusion coefficient of the fuel molecule through the hydrophilic-PFAEM was revealed to be somewhat higher than that of the hydrophobic-PFAEM. It is believed that the more hydrophilic nature of the membrane increases the diffusion rate of the hydrophilic molecules. This means that the energy conversion efficiency of the hydrophilic-PFAEM is expected to be poorer than that of the hydrophobic-PFAEM, in terms of the fuel crossover rate. Therefore, it could be concluded that the dramatic improvement of the power density by employing the hydrophilic-PFAEM mainly resulted from the facilitated hydroxyl ion transport through the membrane.

Membranes
Diffusion Coefficient (×10 9 , cm 2 s −1 ) Hydrophobic-PFAEM 5.75 Hydrophilic-PFAEM 6.99 VRFB experiments were also performed to investigate the influence of the membranes on the battery characteristics, as shown in Figure 7. The charge-discharge performances were revealed to be largely affected by the membrane properties, and the efficiencies are summarized in Table 4. The hydrophobic-PFAEM showed the highest value of coulombic efficiency (CE) among the membranes tested, indicating that the crossover of redox species through the membrane was effectively  Table 3. Diffusion coefficients of potassium formate (KHCOO) through the PFAEMs prepared by using different porous PTFE substrates.

Membranes
Diffusion Coefficient (×10 9 , cm 2 s −1 ) Hydrophobic-PFAEM 5.75 Hydrophilic-PFAEM 6.99 Energies 2020, 13, 4761 10 of 14 VRFB experiments were also performed to investigate the influence of the membranes on the battery characteristics, as shown in Figure 7. The charge-discharge performances were revealed to be largely affected by the membrane properties, and the efficiencies are summarized in Table 4. The hydrophobic-PFAEM showed the highest value of coulombic efficiency (CE) among the membranes tested, indicating that the crossover of redox species through the membrane was effectively suppressed owing to its high cross-linking degree and hydrophobic nature. The crossover rate of vanadyl ions (VO 2+ ) through the membranes could be estimated from a two-compartment diffusion cell experiment, by recording the time-course change of VO 2+ concentration (Figure 8). The overall dialysis coefficient (K A ) values calculated from Equation (7) are also summarized in Table 4. The hydrophobic-PFAEM showed almost similar K A values, despite its considerably reduced thickness compared to that of the commercial AMX membrane. However, the hydrophilic-PFAEM revealed a K A value increased by about three times, owing to its hydrophilic nature and much-reduced thickness compared to the hydrophobic-PFAEM. As a result, it can be seen that the lowest CE value of the hydrophilic-PFAEM among the membranes compared, is due to the high crossover rate of the vanadium redox species. On the other hand, the hydrophilic-PFAEM exhibited the highest voltage efficiency (VE) among the membranes tested, due to the lowest mass transport resistance (note the data in Table 2). Overall, the VRFB employing the hydrophobic-PFAEM showed the highest energy efficiency (EE), of about 87%. In the case of ADLFC, it was preferable to use a hydrophilic-membrane because ion conductivity was the most critical factor determining the efficiency of the system. However, unlike ADLFC, the crossover of the redox species through the membrane more significantly influenced the system efficiency in VRFB. Therefore, in this case, it was found that the employment of a hydrophobic-PTFE-based PFAEM can result in a more improved energy efficiency. adyl ions (VO 2+ ) through the membranes could be estimated from a two-compartment diffusion experiment, by recording the time-course change of VO 2+ concentration ( Figure 8). The overall lysis coefficient (KA) values calculated from Equation (7) are also summarized in Table 4. The rophobic-PFAEM showed almost similar KA values, despite its considerably reduced thickness pared to that of the commercial AMX membrane. However, the hydrophilic-PFAEM revealed a alue increased by about three times, owing to its hydrophilic nature and much-reduced thickness pared to the hydrophobic-PFAEM. As a result, it can be seen that the lowest CE value of the rophilic-PFAEM among the membranes compared, is due to the high crossover rate of the adium redox species. On the other hand, the hydrophilic-PFAEM exhibited the highest voltage ciency (VE) among the membranes tested, due to the lowest mass transport resistance (note the a in Table 2). Overall, the VRFB employing the hydrophobic-PFAEM showed the highest energy ciency (EE), of about 87%. In the case of ADLFC, it was preferable to use a hydrophilic-membrane ause ion conductivity was the most critical factor determining the efficiency of the system. wever, unlike ADLFC, the crossover of the redox species through the membrane more ificantly influenced the system efficiency in VRFB. Therefore, in this case, it was found that the ployment of a hydrophobic-PTFE-based PFAEM can result in a more improved energy efficiency.        Figure 9. The results show that the V 4+ ion concentration increased continuously owing to the oxidative degradation of the polymer. However, the PTFE-based PFAEMs exhibited a muchreduced increase rate of V 4+ ion concentration, compared to that of the commercial AMX membrane. In addition, it was shown that the difference in hydrophilic properties of the porous substrates did not appear to have a significant effect in this case. These are well coincident with the results of the Fenton test and demonstrate that the PTFE-based PFAEMs have excellent oxidative stabilities in the harsh conditions of both ADLFC and VRFB. The excellent oxidative stability for the PFAEMs could be attributed to the use of chemically stable PTFE substrates and the decreased free volume due to  Figure 9. The results show that the V 4+ ion concentration increased continuously owing to the oxidative degradation of the polymer. However, the PTFE-based PFAEMs exhibited a much-reduced increase rate of V 4+ ion concentration, compared to that of the commercial AMX membrane. In addition, it was shown that the difference in hydrophilic properties of the porous substrates did not appear to have a significant effect in this case. These are well coincident with the results of the Fenton test and demonstrate that the PTFE-based PFAEMs have excellent oxidative stabilities in the harsh conditions of both ADLFC and VRFB. The excellent oxidative stability for the PFAEMs could be attributed to the use of chemically stable PTFE substrates and the decreased free volume due to the high cross-linking density, which reduces the influence of oxygen radicals on the polymer degradation [45].
Energies 2020, 13, x FOR PEER REVIEW 13 of 16 the high cross-linking density, which reduces the influence of oxygen radicals on the polymer degradation [45].

Conclusions
In this work, high-performance PFAEMs were successfully developed by combining a thin porous PTFE substrate and anion-exchangeable polymers with a double cross-linking structure, for application to electrochemical energy conversion systems, such as ADLFC and VRFB. In particular, two different kinds of porous PTFE substrates (i.e., hydrophilic and hydrophobic grades) were utilized for the fabrication of the PFAEMs. The PFAEMs exhibited excellent electrochemical characteristics and chemical stabilities, both in strong alkaline and oxidative conditions. In addition, the optimum membrane composition was investigated by adjusting the cross-linking degree. From the correlation studies, the membrane characteristics were systematically analyzed, and as a result, the optimal cross-linker (DVB) content was determined as 10 wt%. It was also proven that the use of

Conclusions
In this work, high-performance PFAEMs were successfully developed by combining a thin porous PTFE substrate and anion-exchangeable polymers with a double cross-linking structure, for application to electrochemical energy conversion systems, such as ADLFC and VRFB. In particular, two different kinds of porous PTFE substrates (i.e., hydrophilic and hydrophobic grades) were utilized for the fabrication of the PFAEMs. The PFAEMs exhibited excellent electrochemical characteristics and chemical stabilities, both in strong alkaline and oxidative conditions. In addition, the optimum membrane composition was investigated by adjusting the cross-linking degree. From the correlation studies, the membrane characteristics were systematically analyzed, and as a result, the optimal cross-linker (DVB) content was determined as 10 wt%. It was also proven that the use of hydrophilic PTFE porous substrate (rather than hydrophobic grade) can significantly enhance the power generation of ADLFCs, mainly due to the greatly facilitated hydroxyl ion transport through the membrane. As a result, an excellent maximum power density of around 400 mW cm −2 at 1 A cm −2 , which is almost comparable with that of the state-of-the-art membrane, was achieved by employing the hydrophilic PTFE-based PFAEM. The PFAEMs were also applied to VRFB for electrochemical energy storage. The results revealed that the crossover of vanadium redox species through the membrane most significantly affects the system efficiency in VRFB. The VRFB employing the hydrophobic-PFAEM exhibited the highest energy efficiency (EE), of 87%, among the membranes tested (cf. hydrophilic-PFAEM = 83% and AMX = 82%), mainly owing to its low crossover rate for vanadium redox ions and moderate membrane resistance. The PTFE-based PFAEMs also showed excellent oxidative stabilities in a highly acidic vanadium solution, which were superior to that of the commercial AMX membrane. Consequently, through this study, high-performance AEMs capable of long-term use under harsh alkaline and acidic conditions have been developed through the combination of porous PTFE substrates and an ionomer having both a high cross-linking degree and IEC. In particular, it was also revealed that the characteristics (e.g., hydrophilicity) of the porous substrate are critical and should match the operating environment for successful applications to electrochemical energy conversion processes.