## 2 Modification of Anion-Exchange Membrane
### AEM Prepared via Solution Casting  
Solution casting is the conventional method to fabricate AEM, which generally applies to soluble polymers, their blends, or copolymers [18]. In technical terms, this method comes with a four-step procedure, namely, the dissolution of polymer, introduction of the cation functional group via chloromethylation, polymer solution casting and quaternization. For instance, an alkalized poly(ether imide) (A-PEI) was synthesized by [PERSON] et al. [34] in accordance to the steps given in Figure 1. The chemical structures of the A-PEI were confirmed by FTIR and NMR, which called the successful grafting of the ammonium group on the backbone of polymer. Based on the results obtained, the A-PEI membrane with a ratio between tin chloride and chloromethyl methyl ether (CMME) of 1:80 has shown the highest performance in IEC and hydroxide ionic conductivity at 1.23 meg g\({}^{-1}\) and 44.2 mS cm\({}^{-1}\), respectively. [PERSON] et al. [35] demonstrated a novel AEM, synthesized by Friedel-Crafts acylation of 6-bromo-1-hexanoyl chloride on poly(phenylene oxide) (C6-PPO) followed by alkalization with trimethylamine (TMA). Interestingly, their results present neither a sign of improvement over the performance (IEC reported at 1.8 meg g\({}^{-1}\) compared to membrane without quaternization of 1.7 meg g\({}^{-1}\)) nor its chemical stability in alkaline conditions, which is against their hypothesis regarding the potential of hexyl spacers of C6-PPO in stabilizing the AEM. Such an outcome is unusual because having spacer provides an advantage in side-chain flexibility, and subsequently enhances the anion conduction. This can be seen from the work of [PERSON] et al. [36], whereby an AEM was synthesized by incorporating rotatable ethylene oxide spacers into imidazolium-containing cationic side-chains. The PPO-based imidazolium AEM demonstrates a unique character whereby the facile rotation of C-O-C ether bonds contributes to the reduction in glass transition temperature and promotes the ion transportation. The hydroxide conductivity and IEC were reported at 45 mS cm\({}^{-1}\) and 2.1 meg g\({}^{-1}\), respectively, while the single cell AEMFC performance yielded a value of 437 mW cm\({}^{-2}\) at 65 \({}^{\circ}\)C.  
Figure 1: Synthesis and chemical structures of PEI and chloromethylated-PEI (CMPEI). Reprinted with permission from [34]. Copyright Elsevier, 2019. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Modification** & **Polymer** & **Reagent(s) for Cationic Head** & **Water Uptake** & **Swelling** & **Tensile** & **IEC** & **Ionic** & **Cell** & **Performance** & **Ref. ** \\
**Method** & **Group** & **(\%)** & **Ratio (\%)** & **Strength (MPa)** & **(meq g\({}^{-1}\))** & **(meq g\({}^{-1}\))** & **(meq g\({}^{-1}\))** & **(meq g\({}^{-1}\))** & **(meq g\({}^{-1}\))** & **Ref. ** \\ \hline \begin{tabular}{c} Simple solution \\ casting without \\ modification \\ \end{tabular}  & PEI & Trimethylamine (TMA) & 40.3 & 19.2 & 21.5 & 1.23 & 44.2 (at 90 \({}^{\circ}\)C) & - & [34] \\ \hline Co-PPO & TMA & - & - & - & 1.80 & 1.2 (at 25 \({}^{\circ}\)C) & - & [35] \\ \hline \multirow{3}{*}{\ \begin{tabular}{c} Popy/fluorenyl \\ ketone sulfone (PFRES) \\ \end{tabular}  } & 1,2-bis(2-(2-methylimidazole) & 150.0 (at 60 \({}^{\circ}\)C) & 18.0 & - & 2.10 & 45.0 (at 60 \({}^{\circ}\)C) & 437.0 (at 65 \({}^{\circ}\)C) & [36] \\ \cline{2-9}  & Poly(fluorenyl ether ketone sulfone) & (0) TMA & (0) 99.0 & - & - & (0) 1.80 & (0) 22.3 & - & [37] \\ \cline{2-9}  & (91 -methylimidazole (IM) & (0) 48.0 & - & - & (0) 1.60 & (0) 17.1 & - & [37] \\ \cline{2-9}  & 1-benzyl3-methyl+butyl- & \multirow{2}{*}{128.0} & \multirow{2}{*}{35.0} & \multirow{2}{*}{-} & \multirow{2}{*}{1.21} & \multirow{2}{*}{61.6} & \multirow{2}{*}{-} & \multirow{2}{*}{[38]} \\  & 1,2-3-triazolium & & & & & & & \\  & iodide & & & & & & & \\ \hline \multirow{3}{*}{ \begin{tabular}{c} PTFE \\ (DABC) \\ \end{tabular}