An Investigation into Polybenzimidazoles as Anion Exchange Membranes

A study was done into three sets of polybenzimidazoles (PBIs). The first set is a quaternized PBI known as poly(1,3-dimethyl benzimidazolium) (P(DMBI)) that was shown to have anion exchange properties. The second set is a novel quarternized PBI that had additional methylation on the phenyl ring known as mesitylene-poly(1,3-dimethyl benzimidazolium) (Mes-P(DMBI)) which also had anion exchange properties. The last set is a series of blend membranes of PBI and Mes-P(DMBI)-OH-, which showed that it was possible to synthesize a stable, hydroxide-conducting polymer. The P(DMBI) membranes were synthesized with various counter-anions (I-, Cl-, Br-, NO3-, HCO3- and OH-) and they were mostly found to have low water uptake at high ion exchange capacities (IECs) and good conductivity values. They were also found to be thermally stable up to approximately 150°C. The hydroxide membrane was unstable due to membrane degradation, which provided the impetus to synthesize the second and third sets of PBIs mentioned above. Mes-P(DMBI) is a novel polymer that was synthesized in order to attempt to create a stable hydroxide-conducting polymer. It was also synthesized with various counter-anions (I-, Cl-, Br-, OH-), and they were found to have much higher water uptake than their P(DMBI) counterparts. The hydroxide membrane was stable, but water-soluble, rendering it unusable as anion exchange membranes (AEMs). The stability of the aforementioned Mes-P(DMBI)-OH- prompted the synthesis of a series of blend membranes of PBI and Mes-P(DMBI)-OH- with different IECs. This utilized the cross-linking and mechanical stability properties of PBI, combined with the hydroxide-conducting property of Mes-P(DMBI)-OH- to make a stable, hydroxide-conducting membrane. The blend membranes were found to conduct hydroxide and to be stable in concentrated (2 M) KOH(aq) at 60°C over the period of a week, showing great promise for use in AEM fuel cells.