Thioether-based sulfonated polytriazoles appended with phosphine oxide moieties with enhanced oxidative stability for PEMFC applications

Abstract

Ether-free heteroaromatic sulfonated polymer electrolytes are gaining attention for use in proton exchange membranes in durable fuel cells, mainly due to the vulnerability of ether linkages to reactive oxygen species. In this study, we synthesized ether-free sulfonated polytriazoles (SPTs) with phosphine oxide (PO) and thioether units, with varying sulfonic acid content designated as POTPSH-XX. The polymer compositions were determined through NMR analysis. The copolymers exhibited high thermal, mechanical, and viscoelastic properties and excellent dimensional stability, attributed to PO and thioether moieties. Morphological studies revealed well-separated and interconnected phase morphology. Among them, the POTPSH-90 membrane achieved the highest proton conductivity of 125 mS cm⁻¹ at 80 °C and 137 mS cm⁻¹ at 90 °C. They also demonstrated strong oxidative stability, with degradation times of 19 to 30 h and a residual weight of at least 96.8% after keeping for an hour in Fenton's reagent at 80 °C. The antioxidant or radical-capturing properties of the thioether groups in the polymer backbone were investigated using FTIR, NMR, XPS, and DSC analysis of the non-sulfonated (POTP) homopolymer and sulfonated (POTPSH-XX) copolymer at various time intervals during the Fenton test. In H₂–O₂ single-fuel cell tests, POTPSH-90 showed an open circuit voltage of 1.01 V and a peak power density of 106.4 mW cm⁻² at 80 °C without back pressure.