Balanced Ionic Conductivity and Permselectivity of Cation Exchange Membranes Prepared from Sulfonated Poly(ether sulfone)

Sulfonated poly-(ether sulfone) (S-PES) was synthesized through the postsulfonation of poly-(ether sulfone) (PES) with chlorosulfonic acid as the sulfonating agent. The degree of sulfonation (DS) of the produced S-PES was controlled between 13.2 and 36.2% by adjusting the reaction time. The DS values were determined by elemental analysis, ¹H NMR, and titration against 0.01 M NaOH. The presence of sulfonic acid groups was confirmed by using ¹H NMR and FT-IR analyses. The produced S-PES with different DS values was cast into membranes. The performance of these materials as cation exchange membranes (CEMs) was evaluated in detail, in light of their ion exchange capacity, water uptake, hydration number, charge density, contact angle, thermal and long-term stability, ionic conductivity, and permselectivity. The properties of the obtained membranes were compared to various reported CEMs, including industrial benchmarks such as Nafion-117, Fuji CEM, FKS-20, CMX, and CSE. We demonstrate that our work achieved the best balance between ionic conductivity and permselectivity through finely controlled DS optimization. The best-performing membrane was found to have a DS of 32.4% with a significantly high ionic conductivity of 16.85 mS cm^-1 and a high permselectivity of 98.0% in 0.5 M NaCl. Our work sheds light on the crucial interplay between various membrane properties as the degree of functionalization varies stepwise. The fabricated membranes are promising candidates for advancing electrodialysis-based desalination and salinity-driven renewable energy production technologies.