## 1 Introduction
However, despite that good performance, two consecutive solvent steps, in addition to the sulfonation step, were required for the membrane synthesis, complicating the fabrication process. Furthermore, various toxic chemicals, such as borohydride and 1,3-propane sulfone, were used during the synthesis. Furthermore, the authors did not report the methanol permeability, which is a crucial parameter for applications such as DMFCs and artificial photosynthesis. [PERSON] et al. [22] synthesized sPEEK membranes crosslinked via UV light using benzophenone (BP) and triethylamine (TEA) as the photomitiator. Similarly, due to the use of diallyl bisphenol, they were not able to avoid toxic chemicals as membrane polymer building blocks. [PERSON] et al. [23] used a polymer that only showed a weakly phase-separated structure, leading to inadequate proton conductivity. [PERSON] et al. [18] sulfonated PEEK using concentrated sulfuric acid and crosslinked the polymer using benzoxazine. The obtained membranes showed good methanol permeability and proton conductivity, but a trade-off was necessary for those two parameters. With an increasing crosslinking degree, the methanol permeability ranged between 5.79 \(\times\) 10\({}^{-5}\) and 5.46 \(\times\) 10\({}^{-7}\) cm\({}^{2}\)/s, and the proton conductivity ranged between 7.76 \(\times\) 10\({}^{-2}\) and 1.61 \(\times\) 10\({}^{-4}\) S/cm, whilst the commercial Nafion 117 membrane has a methanol permeability and proton conductivity equal to 1.88 \(\times\) 10\({}^{-6}\) cm\({}^{2}\)/s and 0.0891 S/cm, respectively. Again, the use of toxic compounds, such as aniline, paraformaldehyde, and bisphenol A, was required to synthesize the membranes. In another study by [PERSON] et al. [19], more inexpensive and safer chemicals were used for the crosslinking, i.e., styrene, divinylbenzene and AlBN. The resulting membranes showed competitive results compared to Nafion, but they were again not able to obtain a membrane that simultaneously showed better methanol retention and proton conductivity compared to the commercial benchmark. This study aims at tackling the main limitations of the current PEMs, i.e., overcoming the trade-off between methanol rejection and proton conductivity, whilst only using non-hazardous, low-cost chemicals for the PEM synthesis. Methanol crossover and proton conductivity are two key performance parameters of a PEM. More specifically, methanol crossover is seen as one of the most notable technical barriers causing performance losses, whilst the proton conductivity is in direct relation to the energy requirements of a system [28]. This study employed \(\alpha\),\(\alpha^{\prime}\)-dichloro-p-xylene (DCX) as a low-cost, non-hazardous crosslinker. The chloromethyl groups on the benzene ring of DCX enable a Friedel-Crafts reaction between DCX and the aromatic rings of the sPEEK backbone. Additionally, the hydrophobic character of DCX can lower the methanol crossover. To the best of our knowledge, DCX has not been used in the fabrication of PEMs or in the crosslinking of sPEEK. It has only been used in a limited number of studies for the crosslinking of nanofiltration membranes [29; 30]. Furthermore, thin film PEMs having a highly sulfonated PEEK backbone will be fabricated in order to limit the proton resistance.