Synthesis of ionic polybenzimidazoles with broad ion exchange capacity range for anion exchange membrane fuel cell application

In this study, new anion exchange membranes (AEM) based on crosslinked polybenzimidazole (m-PBI) with quaternary ammonium groups, crosslinkable allyl groups, and hydrophobic ethyl groups as side chains are synthesized and characterized. The AEMs are crosslinked by thermal thiol-ene reaction using a dithiol crosslinker. The ion exchange capacity (IEC) values and crosslinking density were controlled by the number of quaternary ammonium groups and allyl groups, respectively. The introduction of ethyl groups improved the solubility of ionic PBIs even at very low IEC values by eliminating the hydrogen bonding interaction of imidazole rings. This method allows ionic PBIs with broad IEC values, from 0.75 to 2.55 mmol/g, to be prepared. The broad IEC values were achieved by independently controlling the numbers of quaternary ammonium groups, allyl groups, and hydrophobic ethyl groups during preparation. The crosslinked ionic PBIs revealed hydroxide conductivity from 16 to 86 mS/cm at 80°C. The wet membranes also showed excellent mechanical strength with tensile strength of 12.2 to 20.1 MPa and Young's Modulus of 0.67 to 1.45 GPa. The hydroxide conductivity of a crosslinked membrane (0.40Q0.60Et1.00Pr, IEC = 0.95 mmol/g) decreased only 7.9% after the membranes was immersed in a 1.0 M sodium hydroxide solution at 80°C for 720 h. A single fuel cell based on this membrane showed a maximum peak power density of 136 mW/cm² with a current density of 377 mA /cm² at 60°C.