Sterically Shielded Cations Decouple Ion-Exchange Capacity from Water Uptake in Anion Exchange Membranes
Year: 2026
Type: preprint
Abstract: Anion exchange membranes (AEMs) for electrochemical devices must combine high ion-exchange capacity (IEC) with controlled hydration and dimensional stability. Water uptake is commonly rationalized in terms of IEC and intrinsic cation hydrophilicity, but this picture fails when the cationic site is locally shielded by the polymer architecture. Using the poly(bis-imidazolium) ionene Aemion+® as a model backbone-embedded AEM, we find that it absorbs only ~40% of the water taken up by pendant-cation analogues at matched IEC, reversing the trend expected from solution thermodynamics, which identifies its embedded dimethylimidazolium cation as the more hydrophilic one. All-atom molecular dynamics simulations reveal that adjacent aromatic rings create a three-dimensional steric envelope around the embedded cation, reducing first-shell water coordination by 69% and chloride coordination by 62% relative to the free cation in solution. Restricted access to the cation persists for the membrane in hydroxide form. To separate cation chemistry from architecture, we synthesize poly(phenylene oxide) (PPO) membranes bearing four pendant cations—tetramethylammonium (TMA), dimethylimidazolium (DMIm), piperidinium (Pip), and pyrrolidinium (Mpy)—at matched IEC and find that PPO–TMA and PPO–DMIm have indistinguishable hydration despite the substantially higher intrinsic hydrophilicity of DMIm⁺ in solution, with all four cations falling within a narrow hydration window. These results establish local steric accessibility — the ability of water and counterions to occupy the cation's first solvation shell — as a controlling descriptor for AEM hydration. Engineering cation accessibility therefore offers a molecular design strategy for high-IEC AEMs with controlled hydration and swelling.
Source: ChemRxiv
Institutions: University of Utah, Lawrence Livermore National Laboratory
Language: English
Subfield: Electrical and Electronic Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7223090802