## 4 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 \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Membranes** & **CE (\%)** & **VE (\%)** & **EE (\%)** & \(K_{A}\) (v\(\times\)10\({}^{6}\), m s\({}^{-1}\))** \\ \hline AMX (Aatom Corp.) & 93.6 & 87.7 & 82.1 & 2.16 \\ Hydrophobic-PFAEM & 97.2 & 89.4 & 86.9 & 2.63 \\ Hydrophilic-PFAEM & 90.9 & 91.3 & 83.0 & 7.69 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Various characteristics of commercial and prepared membranes. Figure 8: Time course change of vanadium ion transported through AMX, Hydrophobic-PTFE based PFAEM, and Hydrophilic-PTFE based PFAEM, respectively. Figure 9: Time course changes in V\({}^{4+}\) ion concentration during the oxidative stability tests of AEMs. 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 ADLPCs, 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. The following are available online at [[http://www.mdpi.com/1996-1073/13/18/4761/s1](http://www.mdpi.com/1996-1073/13/18/4761/s1)]([http://www.mdpi.com/1996-1073/13/18/4761/s1](http://www.mdpi.com/1996-1073/13/18/4761/s1)). Figure S1. Pictures of porous substrates (a) hydrophobic polytetrafluoroethylene (PTFE); (b) hydrophilic PTFE and pore-filled anion-exchange membranes (c) hydrophobic-pore-filled anion-exchange membranes (PFAEM); (d) hydrophilic-PFAEM. Figure S2. Surface contact angles of pore-filled anion-exchange membranes. Conceptualization, M.-S.K. ; methodology, M.-S.K. ; experimentation, D.-H.K. and M.-S.K. ; validation, M.-S.K. ; investigation, D.-H.K. and M.-S.K. ; resources, M.-S.K. ; writing-original draft preparation, D.-H.K. and M.-S.K. ; writing--review and editing, M.-S.K. ; supervision, M.-S.K. ; project administration, M.-S.K. ; funding acquisition, M.-S.K.