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.

Zwitterionic Ionenes toward Processable, Low Water-Containing Anion Exchange Membranes: Synthesis, Characterization, and Application in Electrochemical CO2 Reduction

“…The stability of membranes was reassessed here and compared to the two analogous zwitterionic membranes of 88% dm-12% zw and 75% dm-25% zw. The zwitterionic membranes were found to be stable in 1 M at 80 degrees over several days, but degraded more rapidly with increasing [OH – ], similar to their pristine analogues, as illustrated in Figures c, Figure S3, and Figure S4. , …”

“…As previously reported in the literature, accessing the IEC value in the acidic form of zwitterionic membranes remains a challenge. 46 Despite numerous trials and efforts, we were not successful in determining the IEC (H+) of these materials, which may be a further indication that instead of being charge-balanced by respective counterions; the zwitterion is strongly ion-paired, with the negatively charged sulfonate groups electrostatically bound to the additional positive charge of the polybenzimidazolium. As depicted in Figure 2b, a degree of ionic cross-linking within the membrane might thus be anticipated, which is proved to be the case below.…”

“…The zwitterionic membranes were found to be stable in 1 M at 80 degrees over several days, but degraded more rapidly with increasing [OH − ], similar to their pristine analogues, as illustrated in Figures 1c, Figure S3, and Figure S4. 46,64 Ionic, Salient, and Physical Properties. The experimental IEC of all membranes was evaluated by titration.…”

“…Zwitterionic functionalization or zwitterionation of polymeric systems may offer such a new path. The field of zwitterionically functionalized polymers has so far gained attention due to their antifouling properties − and ability to increase ion selectivity in vanadium redox flow batteries − and, owing to their unique set of properties, as interfacial layers in diverse applications ranging from water desalination to optoelectronic devices. − Recently Shirole et al described a copolymer bearing 1,4 sulfobetaine on poly­(terphenylene piperidinium) exhibiting good conductivities, restricted water-uptake, and chemical stability in 1 M KOH, which makes these types of zwitterionically functionalized AEMs attractive for use in, e.g., AEM-based electrolyzers. As our group has extensively researched poly­(benz)­imidazolium ionenes, we sought to explore whether zwitterionic functionality could be applied to polybenzimidazolium AEMs, serve as a method to circumvent aforementioned trade-offs with cross-linked polymers, and impart advancements to electrochemical devices.…”

Anion Exchange Membrane Based on Cross-Linked Poly(2,6-dimethyl-1,4-phenylene oxide)/Poly(dimethylaminophenyl bisdiphenol) for Fuel Cell Applications

“…Cross-linking involves covalently bonding two polymers, where one is hydrophilic and the other is hydrophobic. This combination of polymers with different properties serves to enhance the hydroxide conductivity of the membrane, making it more robust and effective in facilitating the essential transport of ions and water for efficient fuel cell operation …”