Molecular Dynamics Analysis of Hydroxide Ion Transport Mechanism in Anion Exchange Membrane

In order to address global climate change, clean energy sources that reduce CO₂ emissions must be urgently developed. Hydrogen has emerged as a promising energy carrier due to its high energy density and environmentally friendly combustion, producing only water. Among hydrogen production technologies, green hydrogen, which is produced via water electrolysis powered by renewable electricity, is a key solution for decarbonization. In particular, anion exchange membrane water electrolysis is considered a cost-effective alternative because it allows the use of non-precious metal catalysts and offers the potential for operation with pure water. However, the relatively low conductivity of hydroxide ions in anion exchange membrane (AEM) is a major challenge in improving system performance.

To investigate hydroxide ion transport mechanisms at the molecular scale, this study employs classical molecular dynamics (MD) simulations using QPAF-4, an AEM containing pendant trimethylammonium groups. Focusing on the vehicle mechanism, we explore how changes in water contents (λ) affect hydroxide ion diffusion. Our MD simulation reveals that the diffusion coefficient of hydroxide ion increases rapidly with water content at low λ, particularly between λ = 6 and 9, but shows a more gradual rise at higher λ values.  This behavior is considered to be influenced by structural changes in the solvation structure around the trimethylammonium groups.

We defined a “solvation number” to quantify the number of first solvation shells surrounding each hydroxide ion and categorized the local structures accordingly. At low λ, hydroxide ions were predominantly trapped in overlapped areas with strong electrostatic interactions, whereas at high λ, they mainly resided in isolated areas or the second solvation shell, exhibiting higher mobility.

These results suggest that increased water content increases the distance between trimethylammonium groups and decreases the overlap of solvation shells, thus weakening ion trapping and promoting diffusion. Our results provide basic insights into the relationship between solvation structure and ion transport. These insights offer guidelines for the rational design of high-performance AEM materials for next-generation water electrolysis systems.