Hydroxide Ion Conducting Membranes Dually Functionalized with Cationic and Zwitterionic Groups

A series of ether-freepoly(p-terphenylpiperidinium)scarrying both cationic and zwitterionic groups were prepared and evaluatedas 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 insuperacid-mediated polyhydroxyalkylations with p-terphenylto prepare precursor copolymers containing 20, 30, and 40 mol % ofzwPip units. Next, the Pip units were fully quaternized in a Menshutkinreaction with iodomethane. AEMs cast from these copolymers with ion-exchangecapacities between 1.8 and 2.2 m equiv g(-1) displayedexcellent hydroxide ion conductivities, reaching up to 171 mS cm(-1) at 80 & DEG;C. At lower temperatures, 20 and 40 & DEG;C,the water uptake decreased with increasing zwitterionic content, mostprobably because of the increased ionic cross-linking through piperidinium-sulfonatecomplexes. AEMs containing 30 and 40% zwPip units showed only a minorionic loss (<10%) when stored in 1 M aq NaOH solution at 80 & DEG;Cover a period of 720 h. The overall results show that the co-introductionof zwitterionic and cationic groups is an efficient strategy to initiateionic cross-linking and improve the performance of hydroxide-conductingAEMs.