PA-doped high-temperature proton exchange membranes containing bis-cation pairs with excellent PA retention capability for fuel cells

The thiol-imidazole functionalized (p-triphenyl-pentafluorobenzaldehyde) polymer (IMPTP) was prepared and quaternized with different side chains to obtain imidazolium-modified Me-IMPTP, He-IMPTP and BIM-IMPTP membranes for application in high-temperature proton exchange membrane fuel cells (HT-PEMFCs). The presence of the thioether group in the polymers enabled radical scavenging for antioxidant properties, while imidazolium cations interacted strongly with H₂PO₄⁻ to prevent phosphoric acid (PA) leaching. The prepared BIM-IMPTP membrane incorporating bisimidazolium cation string with a long alkyl spacer demonstrated the highest mass retention of 82.93% after being immersed in Fenton's reagent for 24 h. Additionally, the PA-doped BIM-IMPTP membranes exhibited excellent PA retention under high-humidity conditions (80 °C/100% RH). The single cell equipped with the BIM-IMPTP/320%PA membrane achieved a maximum power density (PD_max) of 945 mW cm⁻² at 160 °C. Among the four membranes with a similar acid doping content (ADC), the BIM-IMPTP/163%PA membrane with bis-cation pairs in the side chains exhibited a well-developed microphase-separated structure and high proton conductivity (119.0 mS cm⁻¹ at 180 °C). The single cell assembled with BIM-IMPTP/163%PA membrane maintained a PD_max of 613 mW cm⁻² at 160 °C and demonstrated long-term operational stability under both 150 °C/400 mA cm⁻² and 80 °C/200 mA cm⁻² conditions. These results indicate that the introduction of thioether and bis-cation pairs in the structural design of polymers contributes significantly to the long-term stability of HT-PEMs.