## 3 Results
### Membrane Parameters  
The ion exchange capacity (IEC) shows the amount of sulfonic acid groups per gram of dry membrane mass. These groups are responsible for the transport of cations through the membrane. The higher the IEC, the higher the proton conductivity. Figure 9a shows the IEC of Nafion 117 compared to the membranes synthesized in this study. The IEC of Nafion 117 (1.04 mmol/g) agrees with the values found in reference [48]. The results show that all the crosslinked sPEEK membranes show a higher IEC, which indicates that the sulfonation with concentrated H2 SO4 for 24 h at 60 \({}^{\circ}\)C led to a sufficient number of functional groups. Increasing the sulfonation temperature to 90 \({}^{\circ}\)C triples the IEC compared to Nafion 117. Furthermore, the IEC decreases with increasing crosslinking density since the crosslink increases the membrane mass without adding sulfonic acid groups. Figure 9b,c show the water and methanol uptake of the synthesized membranes compared to Nafion 117, respectively. The non-crosslinked sPEEK membrane is very hydrophilic and not dense due to the absence of DCX and, therefore, has the highest uptake values. However, the uptake values decrease with increasing crosslinking degree. For example, sPEEK/DCX-s90\({}^{\circ}\)C has relatively high uptake values due to its high sulfonation degree and limited crosslinking degree. Increasing the crosslinking degree further by increasing the crosslinking time (sPEEK/DCX-CX24h) and temperature (sPEEK/DCX-CX190\({}^{\circ}\)C) eventually leads to uptake values even lower than the commercial benchmark. Those membranes are very dense, making them more resistant to swelling, and contain more hydrophobic regions, which reject water and methanol. Cation/anion permeation: Figure 10 shows the cation/anion permeation of Nafion 117 and the membranes of this study, tested for KCl (a) and NaHCO3 (b). The crosslinking degree has the largest influence on this performance parameter. The non-crosslinked sPEEK and the poorly crosslinked sPEEK/DCX-s90\({}^{\circ}\)C almost had no selectivity when KCl was used (16 and 33%, respectively) and a very poor selectivity when NaHCO3 was used (47 and 56%, respectively), despite having a high IEC. These results indicate that the electrostatic repulsion forces coming from the sulfonic acid groups are not sufficient to inhibit the anions from permeating through the thin, non-dense membrane. Since the higher sulfonation temperature led to fewer available crosslinking sites, a sulfonation temperature of 60 \({}^{\circ}\)C was considered optimal for this study. Crosslinking the membranes  
Figure 8: High-resolution S 2p XPS spectra of the synthetized membranes; (**a**) sPEEK, (**b**) sPEEK/DCX, (**c**) sPEEK/DCX-CXs90\({}^{\circ}\)C, (**d**) sPEEK/DCX-CX24h, and (**e**) sPEEK/DCX-CX190\({}^{\circ}\)C.  
led to a higher cation/anion permeation due to the presence of more hydrophobic regions formed by DCX and a denser structure. This could also be linked to the lower water and methanol uptake values. The sPEEK/DCX membrane showed selectivity ratios of 66 and 74% when using KCl and NaHCO\({}_{3}\), respectively. Although these values are higher than sPEEK and sPEEK/DCX-s90\({}^{\circ}\)C, they are still inferior to Nafion 117, which has selectivity values of 91 and 84% for KCl and NaHCO\({}_{3}\), respectively. However, the selectivity values become comparable with the commercial benchmark when the crosslinking density is further increased. sPEEK/DCX-CX24 has selectivity ratios of 85 and 86% for KCl and NaHCO\({}_{3}\), respectively, and sPEEK/DCX-CX190\({}^{\circ}\)C has selectivity ratios of 87 and 81% for KCl and NaHCO\({}_{3}\), respectively. Therefore, a sufficiently crosslinked membrane can reach competitive selectivities compared to the commercial benchmark. Figure 10: Cation/anion permeation of Nafion 117 compared to the synthesized membranes; (**a**) KCl cation/anion permeation, (**b**) NaHCO\({}_{3}\) cation/anion permeation.