## 3 Results
### Performance Testing: Proton Resistance/Methanol Diffusivity  
As mentioned before, very often, a trade-off has to be made between methanol rejection and proton conductivity when considering PEMs for applications in DMFCs and artificial photosynthesis. Figures 11 and 12 show those two key performance parameters for Nafion 117 and the PEMs of this study. The methanol permeation data and the proton resistance values in Figure 11 are normalized by thickness and surface area. A thickness of 183 \(\upmu\)m is assumed for Nafion 117, and the measured thickness of 20 \(\upmu\)m is used for the PEMs of this study. Due to the normalization, Figure 11 shows the intrinsic properties of the membrane material without the influence of the membrane thickness. The most important observations are the lower resistance of the Nafion 117 membrane and the superior methanol diffusion coefficient of the PEMs of this study. These results are in line with the expectations. The morphology of Nafion consists of connected ionic clusters that serve as proton conducting channels [8], leading to high conductivity, while sPEEK-based membranes are known to have limited conductivities [49]. The relatively high standard deviations for the proton resistance come from the high sensitivity of the measurement values due to the low membrane resistance (see Figure 12) and low membrane thickness. The shortcoming of the methodology of how the data are represented in Figure 11 is that it gives the impression that the membrane performance can easily be tuned by the thickness. However, a different membrane morphology will be formed during the solvent evaporation step if a thicker membrane is cast. Therefore, the membrane performance does not change linearly with the membrane thickness. A better way to compare the results of the synthesized PEMs of this study with Nafion 117 is via Figure 12. In Figure 12, the parameters are not normalized by the membrane thickness such that the effect of the low membrane thickness becomes visible. The methanol permeability and the proton resistance of Nafion 117 are indicated with a dotted line. Similar results in the literature were obtained for the proton resistance of Nafion 117 (see [PERSON] et al. [50]), indicating the validity of this approach. Their study showed an average proton conductivity of 0.0605 S/cm, tested in 1 M H\({}_{2}\)SO\({}_{4}\), compared to 0.0407 S/cm in this study. The difference between the two values can be explained by a different setup and different pretreatments. The conversion from the measured 0.45 \(\Omega\) cm\({}^{2}\) in this study to 0.0407 S/cm follows the calculations explained  
Figure 11: Proton resistance versus methanol diffusion coefficient of Nafion 117 and the synthesized membranes, all normalized by the thickness. by [PERSON] et al. [50], including 183 \(\upmu\)m as the thickness for Nafion 117. The results from the electrochemical impedance spectroscopy are shown in the Supporting Information in Figure S3. The membrane proton resistance equals the intersect at high frequencies on the complex impedance plane with the Re(Z) axis. Figure 12 shows that although the proton resistance normalized by the membrane thickness of Nafion 117 is lower than the PEMs of this study, once the membrane thickness is taken into account, all sPEEK membranes have superior proton resistance values. The low thickness of the membranes in combination with the high IEC explains this significantly lower membrane resistance. In summary, the membranes with the lowest and highest resistance are, respectively, sPEEK/DCX-s90\({}^{\circ}\)C, which is sulfonated at the highest temperature, and sPEEK/DCX-CX24h, which has one of the highest crosslinking degrees. However, the difference in conductivities is limited. The crosslinking degree has the largest influence on the methanol permeability. The non-crosslinked sPEEK membrane has a methanol permeability more than 50% higher than the commercial benchmark.