Ion Transport Analysis of Anion-Exchange Membranes in Various Hydration States: Combining Experiments with Molecular Dynamics Simulation

Anion exchange membrane (AEM)-based systems are a viable approach for fuel cells and water electrolyzers, since they operate under alkaline conditions and allow the use of cost-effective, abundant catalysts such as iron and nickel. The current challenges of AEM are the low anion conductivity and insufficient chemical (alkaline) stability. Especially, the anion conductivity drops in low hydration states at reduced humidities. To overcome these issues, it is crucial to understand how the polymer morphology changes with hydration states and why low humidity conditions lead to a decrease in anion conductivity. In the present study, all-atom molecular dynamics (MD) simulations have been carried out to elucidate the hydration structure of trimethylammonium-substituted polyfluorene (TMA-PF), previously reported by the authors [Nara et al., Polym. Adv. Technol. 2022, 33, 2863–2871], with various water contents (number of water molecules per anion exchange group: λ = 0–10). The simulation results indicate that the anion exchange groups were aggregated with water molecules as the proportion of the water contents increased, which led to well-connected hydrophilic phases constructing anion conduction pathways. However, such hydrophilic channels remained connected even at low hydration states. Further analyses on local structures around the anion exchange groups revealed that most anions were trapped by multiple anion exchange groups under low-humidity conditions. The strong electrostatic interactions between the anions and the anion exchange groups suppress the anion mobility and lead to low anion conductivity under low-humidity conditions. The present findings obtained from combined experimental and computational works are useful for promoting practical applications of AEM-based systems.