## 4 Conclusions  
Crosslinked sPEEK PEMs were fabricated using a facile fabrication process with low-cost and non-hazardous chemicals, with the aim of overcoming the traditional trade-off between methanol rejection and proton conductivity. The highly sulfonated PEEK backbones were crosslinked via a Friedel-Crafts reaction with the hydrophobic monomer p-xylylene dichloride (DCX). The crosslinked PEMs did not dissolve in DMAc, confirming a successful crosslinking. The membranes were optimized by varying the sulfonation  
Figure 12: Proton resistance versus methanol permeation of Nafion 117 and the synthesized membranes. and crosslinking intensities. A sulfonation temperature of 60 \({}^{\circ}\)C resulted in a sulfonation degree of 56% and IECs higher than Nafion 117. A higher sulfonation temperature of 90 \({}^{\circ}\)C (sPEEK/DCX-s90\({}^{\circ}\)C) even led to an IEC of more than 3.5 mmol/g; however, at that degree of sulfonation, sPEEK became soluble in water, which is a practical issue. Furthermore, the sPEEK/DCX-s90\({}^{\circ}\)C had fewer available crosslinking spots due to the higher sulfonation degree, resulting in a lower performance. Therefore, a sulfonation temperature of 60 \({}^{\circ}\)C was considered to be optimal. In addition, all the PEMs showed lower proton resistance values than the commercial benchmark, which can be explained by the low thickness and the high IECs. The amount of crosslinking was controlled by varying the crosslinking time and temperature, and the crosslinking degree was measured using XPS. The crosslinking degree had a significant effect on the water and methanol uptake, cation/anion permeation, and methanol permeability. An increase in crosslinking degree makes the membrane more dense and, hence, less permeable for all solutes. Furthermore, since DCX is hydrophobic, it increases the hydrophobic character of the membrane, which rejects specific compounds, such as water and methanol. Due to those effects, the highly crosslinked membranes, i.e., sPEEK/DCX-CX24h and sPEEK/DCX-CX190\({}^{\circ}\)C, showed high performances, i.e., low water- and methanol uptake values, high cation/anion permeation and a low methanol permeability, as opposed to the poorly crosslinked PEMs, i.e., sPEEK and sPEEK/DCX-s90\({}^{\circ}\)C.  
In conclusion, it could be stated that both sPEEK/DCX-CX24h and sPEEK/DCX-CX190\({}^{\circ}\)C overcame the traditional trade-off between proton conductivity and methanol rejection. Both membrane types showed lower resistance and lower methanol permeability. Therefore, this work showed the possibility of using a facile fabrication procedure using non-hazardous compounds for the synthesis of PEMs, which are able to overcome the trade-off between proton conductivity and methanol rejection. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app14073089/s1](https://www.mdpi.com/article/10.3390/app14073089/s1)]([https://www.mdpi.com/article/10.3390/app14073089/s1](https://www.mdpi.com/article/10.3390/app14073089/s1)), Figure S1: Pictures of the sPEEK/DCX membranes; Figure S2: High-resolution O 1s XPS spectra of the self-synthesized membranes; (a) sPEEK, (b) sPEEK/DCX, (c) sPEEK/DCX-s90\({}^{\circ}\)C, (d) sPEEK/DCX-CX24h, (e) sPEEK/DCX-CX190\({}^{\circ}\)C; Figure S3: The impedance spectroscopy data of the proton resistance experiment. The resistance value is derived from the intersect at high frequencies on the complex impedance plane with the Re(Z) axis. Conceptualization, S.D. and B.V.d.B. ; methodology, S.D., L.T., G.V.E., S.K. and B.V.d.B. ; software, G.V.E., S.K. and G.H. ; validation, S.D. and L.T. ; investigation, S.D., L.T., G.V.E., S.K. and G.H. ; resources, B.V.d.B. and G.H. ; data curation, S.D., L.T. and S.K. ; writing--original draft preparation, S.D., G.V.E. and B.V.d.B. ; writing--review and editing, S.D. and B.V.d.B. ; supervision, B.V.d.B. and G.H. ; project administration, B.V.d.B. and G.H. ; funding acquisition, B.V.d.B. and G.H. All authors have read and agreed to the published version of the manuscript.