Highly alkali-stable zwitterionic poly(arylene quinuclidinium) anion exchange membranes

Alkaline electrochemical energy conversion devices generally require anion exchange membranes (AEMs) which combine limited water uptake and swelling with high hydroxide conductivity and high resistance against attack by the ubiquitous hydroxide ions. In this context, zwitterionic AEMs may offer distinct possibilities to tune AEM properties in relation to purely cationic AEMs. Here, we report on the preparation and characterization of a series of poly(p-terphenyl quinuclidinium) AEMs functionalized with both zwitterionic and cationic groups. In order to control the ionic contents, a zwitterionic N-sulfobutylquinuclidinium-3-one monomer was synthesized and employed together with 3-quinuclidone in superacid-mediated polyhydroxyalkylations with p-terphenyl, followed by complete quaternization of the resulting copolymers. With hydroxide exchange capacities between 2.08 and 2.45 mequiv g^-1, the AEMs reached high hydroxide conductivities, 100-139 mS cm^-1, with 94 to 143% water uptakes at 80 ºC. In addition, the AEMs showed outstanding alkaline stability with no signs of degradation by NMR analysis after 720 h storage in 5 M aq. NaOH at 90 ºC. After storage in 10 M aq. NaOH at 90 °C, the zwitterionic AEMs showed a mere 4% ionic loss through the substitution of the sulfoalkyl chain. In comparison, a corresponding zwitterionic AEM based on poly(p-terphenyl piperidinium) suffered a severe ionic loss, i.e., 63% by Hofmann β-elimination and 11% through methyl substitution, under the same conditions. The study showed that polymers functionalized with both zwitterionic and anionic groups based on quinuclidine are attractive AEM materials, combining restricted water uptake and swelling with high hydroxide conductivity. Moreover, these AEMs possess an outstanding alkaline stability because the cage-like structure of the quinuclidinium cation efficiently hinders degradation by Hofmann β-elimination.