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.