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Materials 2021, 14, 1617 8 of 20 for a particular property but rather a cache of properties with additional considerations such as effective IEC and MEA manufacturability. Table 2. Comparison of proton exchange membrane (PEM) properties between the cross-linked diblock ionomer membranes and Nafion 117 membrane. Membrane A50G4S-10 A100G4S-10 A150G4S-10 Nafion 117 (%) (mmol/g) Thickness (μm) 96 105 86 200 a Proton Conductivity b (mS cm−1) 66 ± 2.7 62 ± 3.0 53 ± 2.5 62 ± 3.0 Methanol Permeability c ×10−7 cm2/s 12.7 8.2 5.3 11.6 Characteristic Factor d ×104 (S cm/s) 5.0 7.6 10.0 5.3 Water Uptake IEC 97.9 1.15 32.8 0.87 56.2 0.70 30.0 0.90 a. After acidification treatment (hydrated); b. Test conditions: 23 ◦C and 70% RH; c. [MeOH] = 1 M; d. Characteristic factor = (Proton conductivity)/(Methanol permeability). Figure 4 shows the Arrhenius plot of proton conductivities of the P(AN-co-GMA)-b- SPM membranes and a Nafion 117 membrane as a function of temperature. The proton conductivities of the cross-linked diblock membranes were comparable to that of the Nafion 117 membrane over the temperature range indicated and increased with temperature. In some cases, a proton conductivity higher than Nafion was also possible. Figure 4 also highlights the relationship between proton conductivity and the composition of the diblock ionomers. It is apparent that proton conductivity decreases with the increase in the hydrophobic/hydrophilic ratio of the diblock ionomer. Hence, the ionomer with the lowest hydrophobic/hydrophilic ratio, i.e., the A50G4S-10, forms a membrane with the highest proton conductivity. The dependence of membrane proton conductivity on IEC and water uptake is a well-documented observation. A membrane with high IEC would be highly hydrophilic, and consequently absorbs the most water. This is to be expected since the affinity for water is increased by a larger number of ionizable sulfonic acid groups in the ionomer. The A50G4S-10 membrane, which shows the highest amount of water uptake in Table 2, followed exactly this trend as it also has the highest IEC. This can be attributed to a greater extent of phase separation in membranes made from ionomers with low hydrophobic/hydrophilic ratios. With an increasing extent of phase separation, a more connected and continuous network of hydrophilic channels could be formed to support the transport of protons. On the other end of the spectrum, for the membrane prepared from the largest hydrophobic/hydrophilic ratio, A150G4S-10, the connectivity of primary hydrophilic channels network is probably impeded by the long hydrophobic chains, which decreased the IEC and hence the associated hydrophilicity; thereby increasing the resistance to proton conduction. In the meantime, A150G4S-10 presents the minimum methanol diffusivity of the three specimens studied. The activation energies calculated from the Arrhenius plots of proton conductivity (Figure 4) are summarized in Table 3. The values for the cross-linked diblock ionomer membranes are all in the range of 0.076–0.095 eV; similar to the activation energy of Nafion 117 (0.085 eV measured). Activation energy is commonly used to deduce the dominant mechanism for proton transport. For Nafion membranes the Grotthus mechanism is dominant if the activation energy falls within the range of 0.09–0.012 eV) [19,20]. Hence, the activation energy similar to that of proton transport in Nafion may be used to infer the predominance of the Grotthus mechanism in the diblock ionomer membranes. This could in turn be related to the structure and distribution of the proton transport channels, such as the domain-boundary structure as shown below by the electron micrographs.PDF Image | Hydrophilic Cross-Linked Aliphatic Hydrocarbon Diblock Copolymer
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