A Molecular Dynamics Study of a Nafion Polyelectrolyte Membrane and the Aqueous Phase Structure for Proton Transport

Abstract

A molecular dynamics simulation study of hydrated Nafion at water contents ranging from 5 to 20 wt % was performed to examine the structure and dynamics of the hydrated polyelectrolyte system. The simulations show that the system forms segregated hydrophobic regions consisting primarily of the polymer backbone and hydrophilic regions with an inhomogeneous water distribution. We find that the water clustering strongly depends on the water content. At low water content, only isolated small water clusters are formed. As the water content increases, it becomes increasingly possible that a predominant majority of water molecules form a single cluster, suggesting that the hydrophilic regions become connected. We characterize the atomic structures formed within the system by various atomic pair correlation functions. The water structure factor shows a peak at q values corresponding to an intercluster distance about 2.5 nm and greater. With increasing water content, the distance moves to larger values, consistent with findings from scattering experiments. We find that the degree of solvation of hydronium ions by water molecules is a strong function of water content. At 5 wt %, a majority of the hydronium ions are hydrated by no more than two water molecules, prohibiting structural diffusion. As water content increases, the hydronium ions continue to become increasingly hydrated, resulting in structures capable of forming eigen ions, a necessary step in structural diffusion. Addressing the experimentally observed fact that conductivity in these membranes abruptly drops near 5 wt %, we find that both the local structure of the poorly hydrated hydronium ions and the disconnected nature of the global morphology of the water nanonetwork at low water content should contribute to poor conductivity.

Role of water states on water uptake and proton transport in Nafion using molecular simulations and bimodal network

“…These results are in good agreement with experiments [18] and available MD results [53,54], and our multiscale approach better predicts compared to the full atomistic simulations [53,54], where the limited simulation size may not represent the domain-size-dependent water transport. With a consideration of hydration-dependent Nafion morphol-ogy (swelling), i.e., reduced domain sizes at low water activity, the water uptake is expected to decrease especially at the low water content, and this consideration results in better agreement with the experimental data.…”

Molecular Simulations of Hydrated Proton Exchange Membranes: the Structure

“…4). This is at variance with the work of Cui et al [68,69] who observed a less dispersed water distribution in Nafion as compared to Hyflon.…”

Simulation and Experimental Studies on Proton Diffusion in Polyelectrolytes Based on Sulfonated Naphthalenic Copolyimides

“…25 Results of molecular modeling have been reported that give information relating the structure of the membrane with the mechanisms of proton conduction. 18,[33][34][35] Using MD simulation, Cui et al 36 studied the structure and dynamics of hydrated Nafion with water uptake ranging from 5 to 20 wt %. These authors found that at 5% the majority of the hydronium ions are hydrated by no more than two molecules of water, prohibiting structural diffusion.…”