We have performed a detailed analysis of the structural properties of the sulfonate groups in terms of isolated and overlapped solvation shells in the nanostructure of hydrated Nafion membrane using classical molecular dynamics simulations. Our simulations have demonstrated the correlation between the two different areas in bound water region, i.e., the first solvation shell, and the vehicular transport of hydronium ions at different water contents. We have employed a model of the Nafion membrane using the improved force field, which is newly modified and validated by comparing the density and water diffusivity with those obtained experimentally. The first solvation shells were classified into the two types, the isolated area and the overlapped area. The mean residence times of solvent molecules explicitly showed the different behaviors in each of those areas in terms of the vehicular transport of protons: the diffusivity of classical hydronium ions in the overlapped area dominates their total diffusion at lower water contents while that in the isolated area dominates for their diffusion at higher water contents. The results provided insights into the importance role of those areas in the solvation shells for the diffusivity of vehicular transport of hydronium ions in hydrated Nafion membrane.
A reactive molecular dynamics simulation has been performed for the characterization of the relationship between proton transport and water clustering in polymer electrolyte membranes. We have demonstrated that the anharmonic two-state empirical valence bond model is capable of describing efficiently excess proton transport through the Grotthuss hopping mechanism within the simplicity of the theoretical framework. To explore the long-time diffusion behavior in perfluorosulfonic acid membranes with statistical certainty, simulations that are longer than 10 ns are needed. The contribution of the Grotthuss mechanism to the proton transport yields a larger fraction compared to the vehicular mechanism, when the estimated percolation threshold of λ = 5.6 is surpassed. The cluster analyses elicit a consistent outlook in regard to the relationship between the connectivity and the confinement of water clusters and proton transport. The cluster growth behavior findings reveal that, below the percolation threshold, the water domains grow along the channel length to form the connected, elongated clusters, thus contributing to an increase in connectivity and a decrease in confinement, whereas above the percolation threshold the channel widths of water domains increase, while the elongated structure of clusters is retained, thereby contributing to further confinement decreases.
Molecular dynamics simulations were performed to analyze the water content dependence of the structural properties of ionomer and the oxygen permeation properties in ionomer on a Pt surface. The ionomer/gas interface, bulk region, and ionomer/Pt interface were defined based on the density distribution of ionomer. Oxygen pathways are clearly observed in the ionomer/Pt interface for quite a long period. It was found that the oxygen permeability decreases with increasing water content, which is the opposite of what takes place in the Nafion membranes. Then the oxygen diffusivity, solubility, and permeability in each region were analyzed. It was found that the oxygen permeability in the ionomer/Pt interface was dominant in the oxygen permeability in the ionomer. Furthermore, a decrease in the oxygen solubility has a larger effect on the water content dependence of oxygen permeability. Considering the structural properties of the ionomer, the oxygen solubility in the ionomer/Pt interface decreased because the voids were blocked at higher water content.
Coarse-grained molecular dynamics simulations using explicit solvent models were performed to understand Nafion ionomer morphology in 1-propanol (NPA)/water solutions under various conditions (ionomer concentration, NPA/water fraction, and salt addition). The self-assembly behavior of ionomers into a cylindrical aggregate with a diameter of ∼2−3 nm was observed. At low ionomer concentration (≤5.0 wt %), the ionomer aggregate becomes smaller in size and thinner with increasing NPA fractions. At high ionomer concentration (7.5 wt %), the size of aggregates in a longitudinal direction increases significantly at high NPA fractions, suggesting that excess addition of NPA does not necessarily contribute to the dispersivity of ionomers, particularly at high ionomer concentration. These nonmonotonic behaviors of ionomer aggregation are controlled by the electrostatic repulsion among the sulfonate groups, which is determined by the balance among the dielectric constants of solvents, the distribution of hydronium ions, and the surface density of sulfonate groups. Upon the addition of salts, the size of aggregates increases significantly, and the formation of a larger disk-shaped aggregate and a secondary aggregate of multiple bundles for low and high NPA contents, respectively, was found.
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