Superprotonic Membranes from Phosphonic Acid-Anchored Pyridinyl Sulfonated Polytriazoles: Toward Intrinsic PA Doping

Designing high-performing and cost-effective hydrocarbon-based proton exchange membranes (PEMs) is an effective strategy for replacing state-of-the-art perfluorosulfonated membranes. In this study, we developed a series of phosphonic ester-containing sulfonated polytriazoles, designated as PYPDSNa-XX, which intrinsically generate phosphonic acid-based sulfonated polytriazoles (PTs; PYPDSH-XX) with a pyridinyl moiety. We also synthesized a series of purely sulfonated PTs without any phosphonic esters (PYMASH-XX) to investigate the impact of the phosphonic acid [PA; –PO(OH) 2 ] groups on the properties of the PEMs. To analyze the polymer structures and estimate the conversion of phosphonic ester to –PO(OH) 2, we employed various one-dimensional ( 1 H, 13 C, 19 F, and 31 P) and two-dimensional ( 1 H- 1 H COSY and 1 H- 13 C HSQC) NMR techniques. X-ray photoelectron spectroscopy (XPS) was also used to confirm the formation of PA groups in the PYPDSH-XX copolymers. The copolymers exhibited high thermal, mechanical, and dimensional stability. Cross-sectional high-resolution transmission electron microscopy (HRTEM) investigations revealed a more distinct, interconnected, and well-segregated hydrophobic–hydrophilic phase morphology in the PYPDSH-XX membranes compared to the PYMASH-XX membranes. Notably, the PYPDSH-90 membrane demonstrated proton conductivity values as high as 388 mS/cm in water at 90 °C, outperforming the solely sulfonic acid-containing reference by a factor of 2, thus proving successful PA doping in the membranes. Additionally, under varying relative humidity conditions, the phospho-sulfonated PT (PYPDSH-XX) membranes displayed easier and less water-dependent proton conduction than the purely sulfonated PTs (PYMASH-XX). Furthermore, the copolymers demonstrated high oxidative stability, with a lifetime (τ) of at least 11 h and residual weight exceeding 92%. This work presents an effective method for synthesizing sulfonated proton exchange membranes (PEMs) that are intrinsically doped with PA, unlike traditional methods that involve adding phosphoric acid (H 3 PO 4 ) externally.