Ionic Polybenzimidazoles Crosslinked through Thiol-Ene Reactions for Anion Exchange Membranes

Abstract

This research introduces a study outlining the synthesis and characteristics of innovative anion exchange membranes that utilize m-polybenzimidazole (m-PBI) as the polymer backbone structure. These membranes incorporate quaternary ammonium groups and alkene groups as side chains. These ionic m-PBI membranes were further crosslinked through thermal thiol-ene reactions. By adeptly manipulating the quantities of attached quaternary ammonium groups and alkene groups, the ion exchange capacity (IEC) values and crosslinking degrees were independently controlled. A comprehensive range of m-PBI based anion exchange membranes was developed, showcasing IEC values ranging from 1.51 to 2.50 mmol/g. This series of membranes demonstrated hydroxide conductivity levels ranging between 14 and 58 mS/cm at an operational temperature of 80 °C. Particularly noteworthy is the comparison between membranes with similar IEC values: the crosslinked membranes exhibited reduced water absorption while effectively maintaining a level of hydroxide conductivity comparable to that of uncrosslinked membranes. Moreover, the crosslinked membranes exhibited enhanced dimensional stability. An intriguing finding emerged from a crosslinked membrane with an IEC value of 2.46 mmol/g, which retained a remarkable 84% of its original hydroxide conductivity after being immersed in a 1.0 M sodium hydroxide solution at 60 °C for a duration of 720 hours. These membranes demonstrated mechanical strength even in their hydrated state, exhibiting tensile strengths ranging from 10.8 to 14.6 MPa. Given their heightened hydroxide conductivity, improved resistance to alkaline conditions, and favorable mechanical attributes, the crosslinked ionic m-PBIs emerge as promising candidates for anion exchange membranes in fuel cell applications. Keywords ; anion exchange membranes, crosslinking reaction, polybenzimidazole, back titration, mechanical strength.