Utilizing green solvents to synthesize high performance crosslinked sulfonated PEEK cation exchange membranes

One of the major drawbacks in the fabrication of cation exchange membranes (CEMs) is the reliance on non-sustainable, harmful and toxic solvents. Until recently, dimethyl sulfoxide (DMSO) was the only green solvent reported for the fabrication of CEMs. Herein, we explore the viability of seven alternative green solvents for the synthesis of a sulfonated PEEK-based (sPEEK) CEM crosslinked with α,α′-dichloro-p-xylene (DCX), namely: DMSO, Tamisolve NxG, Rhodiasolv® PolarClean, Cyrene™, acetyl triethyl citrate, acetyl tributyl citrate and γ-valerolactone. Based on Hansen solubility analysis, and upon preliminary testing, only Tamisolve NxG and DMSO were deemed suitable for CEM fabrication. Several membranes were therefore synthesized by using either Tamisolve NxG, DMSO, or a blend of the two solvents, and subsequently tested for ion conductivity and methanol rejection. The variation of the solvent had a significant impact on the membrane performance. The membrane fabricated with DMSO as the sole solvent but without DCX, resulted in a non-crosslinked sPEEK structure. In this case, the sPEEK polymer chains tend to crystallize upon solvent evaporation, reducing the methanol crossover. However, using DMSO as the sole solvent in combination with the crosslinker resulted in a CEM with a too high electrical resistance due to the high degree of crosslinking together with the crystalline regions formed after evaporation. The use of Tamisolve NxG, instead, inhibited the crosslinking, and was therefore blended with DMSO to optimize the crosslinking degree. The solvent blend resulted in crosslinked sPEEK/DCX-10/90 % having higher electrical conductivity and methanol rejection than Nafion 117, respectively, 2.06∗10⁻³ vs. 2.8∗10⁻³/min and 0.036 vs. 0.089 Ω.