2,5-Substituted Thiophene Functionalized Semifluorinated Polytriazoles and Evaluation of Low-Temperature Proton Exchange Membrane Application with Elevated Oxidative Stability

Proton conducting membranes with sufficient oxidative stability are vital for a high-performing proton exchange membrane (PEM). Herein, a 2,5-thiophene functionalized 2,5-bis(4-(4-azidophenoxy)-3-(trifluoromethyl)phenyl)thiopheneazide moiety is synthesized, which was subsequently integrated into polymer chains to prepare the semifluorinated sulfonic acid-containing polytriazole-based membrane series, THFTSH-XX. All THFTSH-XX PEMs possessed ion exchange capacities ranging from 1.88 to 2.56 mequiv g–1. The structures of the random copolymers were validated with 1H NMR and Fourier transform infrared (FTIR) spectroscopies. The proton conductivity of the THFTSH-XX membranes was enhanced in comparison to a previously reported PYFTSH-XX series due to the much weaker basicity of the thiophene moiety. The distinct oxidative stability enhancement capability of the thiophene moiety imparts extended durability to the THFTSH-XX membranes compared to other alternative polytriazole-based PEMs. Functionalization with trifluoromethyl and fluoro groups also enhanced the physicochemical properties of the THFTSH-XX PEMs. Specifically, the THFTSH-80 membrane exhibited a maximum proton conductivity of 146 mS·cm–1 at 90 °C. In addition to this, the THFTSH-XX membranes exhibited superior thermomechanical properties with the highest storage modulus value of 2.74 GPa. All membranes exhibited high thermal stability, with Td,10% results in the range of 253–274 °C. As observed in both transmission electron microscopy (TEM) and atomic force microscopy (AFM) micrographs, ionic clusters were highly nanosegregated, prompting lesser dimensional swelling and enhanced mechanical strength. We utilized scanning electron microscopy (SEM) cross-sectional analysis to study how exposure to radical-induced decay causes changes to THFTSH-XX membranes in the microscopic scale. All of the physicochemical results demonstrated in the THFTSH-XX membranes were promising, making thiophene a suitable moiety to mitigate trade-off between proton conductivity and oxidative stability, making THFTSH-XX PEMs prospective for actual application in PEMFC.