## 3 Results and Discussion
Additionally, under both wet and dry conditions, the E3-C10 PFAEM exhibited tensile strengths of 100.2 and 183.9 MPa, respectively, which are higher than the 91.29 and 181.9 MPa recorded for the E3-C11 PFAEM. Based on the stress-strain curves, the higher mechanical strength of the PFAEMs compared to the PP substrate is attributed to the filling of the cross-linked ionomer into the PP substrate. Moreover, differences in mechanical strength between E3-C10 and E3-C11 PFAEMs exhibited based on the type of the cross-linking monomers. As shown in Table 1, C10 contains a triazine structural formula. It is well known that triazine-based compounds, including the C=N bonds in C10, enhance physical and mechanical properties when reacting with other components [43]. In other words, the triazine-containing C10 cross-linking-monomer-based PFAEM exhibited improved mechanical strength compared to the trimethylolpropane-containing C11 cross-linking-monomer-based PFAEM. These structural differences account for the observed variations in mechanical strength. Figure 8: Tensile strength–strain curves of the porous PP substrate and the fabricated PFAEMs: (**a**) wet E3-C10 and E3-C11 PFAEMs; (**b**) dry E3-C10 and E3-C11 PFAEMs. Figure 7: Hydrogen permeability of E3-C10 and E3-C11 PFAEMs. QA groups in the ionomer-filled region of PFAEMs can degrade, reducing their availability, through Hofmann elimination, nucleophilic substitution, and E1 elimination. Nonetheless, QA groups have been widely used in AEMs as OH\({}^{-}\) conduction pathways due to their high potential for OH\({}^{-}\) ion conduction in membranes. To overcome disadvantages such as Hofmann elimination, nucleophilic substitution, and E1 elimination, chemically stable polymer structures have been developed through structural improvements by selecting and utilizing highly stable monomers. To confirm the effect of the structural formula of the cross-linking monomer on the stability of AEMs, specifically PFAEMs, alkaline stability tests were conducted. These tests aimed to investigate the relationship between the structural formulas of the cross-linking monomers and the chemical stability of the fabricated PFAEMs. Herein, alkaline stability measurements of the triazine-structured E3-C10 and the trimethylolpropane-structured E3-C11 PFAEMs were conducted to investigate the structural effect of the cross-linking monomers. The PFAEMs were stored in 4 M KOH at 60 \({}^{\circ}\)C for up to 960 h to evaluate changes in OH\({}^{-}\) ion conductivity over time. As shown in Figure 9a, the E3-C10 PFAEM exhibited higher BOT and EOT OH\({}^{-}\) ion conductivities compared to the E3-C11 PFAEM. Furthermore, as illustrated in Figure 9b, the loss in ion conductivity was 5.35% for the E3-C10 PFAEM and 17.7% for the E3-C11 PFAEM. To analyze the degradation effects of the membranes based on the different cross-linking monomers, the structures of the monomers were compared. As shown in Figure 9b, the structural formula of C10 contains triazine groups without C-O bonds from ether or ester linkages. These features enhance chemical stability by suppressing degradation mechanisms such as Hofmann elimination, nucleophilic substitution, E1 elimination, and ring opening due to the bulky structure and ether-free linkage. In contrast, the C11 monomer contains ester linkages, which are susceptible to nucleophilic attack by water and OH\({}^{-}\) ions. Specifically, the C-O bonds in the ester group are vulnerable to hydroxyl radicals, leading to reduced chemical stability of the AEMs [44]. As shown in Figure 10a, the single cell performance increases in the order of E3-C10 (285.2 mA cm\({}^{-2}\) at 1.9 V) \(<\) E3-C11 (338.7 mA cm\({}^{-2}\) at 1.9 V). Additionally, as depicted in Figure 10b, the E3-C10 PFAEM-based single cell exhibited a Tafel slope of 153.3 mV dec\({}^{-1}\), which is higher than that of the E3-C11 PFAEM-based single cell.