Preparation and characterization of sulfonated poly(aryl ether ketone)s containing 3,5-diphenyl phthalazinone moieties for proton exchange membrane

A series of sulfonated poly(phthalazinone ether ketone)s containing 3,5-diphenyl phthalazinone moieties (SPPEK-dPs) were prepared by the sulfonation of poly(aryl ether ketone)s containing 3,5-diphenyl phthalazinone moieties (PPEK-dPs) which were synthesized via direct nucleophilic polycondensation from 4-(4-hydroxyphenyl)-2,3-phthalazin-1-ketone (DHPZ), 4-(3,5-diphenyl-4-hydroxyphenyl)-2,3-phthalazin-1-ketone (DHPZ-dP) and 4,4-difluorobenzophenone (DFB). The molecular structures were assessed by FTIR and ¹H-NMR spectroscopy. The ion exchange capacity (IEC) of these sulfonated polymers were in the range of 0.99–1.81 mmol g⁻¹. SPPEK-dP proton exchange membranes demonstrated good mechanical properties as well as dimensional, thermal, and oxidative stability. The proton conductivities of SPPEK-dP membranes increased with DHPZ-dP content and temperature. The proton conductivity of SPPEK-dP-55 was 13.18 × 10⁻² S cm⁻¹ at 95 °C. Furthermore, the methanol diffusion coefficients of SPPEK-dP membranes were 0.12 × 10⁻⁷ cm² s⁻¹ to 1.09 × 10⁻⁷ cm² s⁻¹ depending on the molar ratio of DHPZ-dP. Remarkably, the selectivity of SPPEK-dP membranes was 5–7 times higher than that of Nafion 117 membranes under the same conditions. All of the above properties indicate that SPPEK-dPs have potential applications in proton exchange membranes for direct methanol fuel cells.