Hydroxide Ion Conducting Membranes Dually Functionalized with Cationic and Zwitterionic Groups

Abstract

A series of ether-free poly(p-terphenyl piperidinium)s carrying both cationic and zwitterionic groups were prepared and evaluated as anion-exchange membranes (AEMs). First, a zwitterionic monomer (zwPip) was synthesized by sulfoalkylation of N-methyl-4-piperidone (Pip) using 1,4-butane sultone. Pip and zwPip were then employed in superacid-mediated polyhydroxyalkylations with p-terphenyl to prepare precursor copolymers containing 20, 30, and 40 mol % of zwPip units. Next, the Pip units were fully quaternized in a Menshutkin reaction with iodomethane. AEMs cast from these copolymers with ion-exchange capacities between 1.8 and 2.2 m equiv g-1 displayed excellent hydroxide ion conductivities, reaching up to 171 mS cm-1 at 80 °C. At lower temperatures, 20 and 40 °C, the water uptake decreased with increasing zwitterionic content, most probably because of the increased ionic cross-linking through piperidinium-sulfonate complexes. AEMs containing 30 and 40% zwPip units showed only a minor ionic loss (<10%) when stored in 1 M aq NaOH solution at 80 °C over a period of 720 h. The overall results show that the co-introduction of zwitterionic and cationic groups is an efficient strategy to initiate ionic cross-linking and improve the performance of hydroxide-conducting AEMs.