Quaternary ammonium-biphosphate ion-pair based copolymers with continuous H⁺ transport channels for high-temperature proton exchange membrane

Phosphoric acid (PA)-doped polymer electrolyte membranes are the most promising materials for high-temperature proton exchange membranes (HT-PEMs). However, their development is subject to the compromise between proton conductivity and phosphoric acid (PA) doping level. Here, an ether-free QPAF-4 membrane (a copolymer containing perfluoroalkylene and fluorenyl groups with pendant ammonium groups) is directly doped with phosphoric acid and studied as HT-PEMs. Due to the intrinsic micro-phase separation structure of QPAF-4 matrix and strong interaction of quaternary ammonium (QA)-PA, the PA-doped membranes possess high proton conductivity and eminent PA retention at comparatively low PA doping level, respectively. The QPAF-4 membrane with 150% PA uptake (QPAF-4-150%PA) offers a proton conductivity of 52 mS cm⁻¹ at 160 °C. As expected, the single cell with QPAF-4-150%PA membrane shows the maximum peak power density of 683 mW cm⁻² at 160 °C under anhydrous condition. Meanwhile, the single cell exhibits excellent durability over a period of 60 h with only a slight reduction in voltage of 3.1%. These results indicate that the as-prepared PA-doped quaternized polymer with micro-phase separation structure seems to offer a promising way to develop performing and long-life HT-PEMs with low PA doping levels.