(542d) Achieving High Ion Selectivity with Poly(ionic liquid) Anion-Exchange Membranes

Ion-exchange membranes (IEMs) are crucial for energy and environmental processes that require fast and selective ionic transport. Conventional IEMs are synthesized with hydrophilic charged monomers and hydrophobic cross-linkers or backbones. The hydrophobic contents are introduced to enhance the dimensional stability/mechanical properties of the membrane, resulting in compromised conductivity and selectivity due to insufficient charge content. Therefore, advanced structures composing high charge contents without compromising mechanical strength are needed to overcome this challenge.

Polymerizable surfactants (i.e., Polyionic liquids (PILs)) could overcome the limitations of conventional IEMs due to their well-defined structure, high charge content, and mechanical robustness. Typically, PILs possess densely packed charge groups attached to the backbone along with hydrophobic tails, facilitating hydrophilic/hydrophobic phase separation. The unique structure of PILs reduces the distance between charged units and improves mechanical strength owing to physical interlocking of the hydrophobic segments. Furthermore, the phase-separated morphology of PILs may reduce the ion-conducting pathway across the membrane. These PILs have been extensively investigated in early battery applications due to their excellent conductivity and mechanochemical properties under dry or partially solvated conditions. Despite their potential as ion-conducting materials, their electrochemical transport properties under aqueous conditions as IEMs remain underexplored.

In this study, we synthesized imidazolium-based PIL membranes with finely tuned water and charge content and investigated how the morphological arrangement between the charged backbone and hydrophobic side chain affects ion conductivity and selectivity. The developed membranes exhibited unprecedentedly high selectivity in hypersaline solutions. The membranes also demonstrated exceptionally high ionic conductivity considering their extremely low water content (i.e., water uptake <10%; λ = 1.7) under fully hydrated conditions. Through structural characterization and modeling, we elucidated the mechanism of ion transport governed by the phase separated morphology.

Our findings pave the way for the design of novel structures capable of further overcoming the conductivity-selectivity trade-off in IEMs. This advancement holds promise not only for energy applications (e.g., fuel cells, water electrolysis, and CO₂/NO₃ reduction) but also for environmental applications (e.g., electrodialysis) requiring high conductivity and/or selectivity.