Takuya Mabuchi

Effect of bound state of water on hydronium ion mobility in hydrated Nafion using molecular dynamics simulations

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 modified two-state empirical valence bond model for proton transport in aqueous solutions

A detailed analysis of the proton solvation structure and transport properties in aqueous solutions is performed using classical molecular dynamics simulations. A refined two-state empirical valence bond (aTS-EVB) method, which is based on the EVB model of Walbran and Kornyshev and the anharmonic water force field, is developed in order to describe efficiently excess proton transport via the Grotthuss mechanism. The new aTS-EVB model clearly satisfies the requirement for simpler and faster calculation, because of the simplicity of the two-state EVB algorithm, while providing a better description of diffusive dynamics of the excess proton and water in comparison with the previous two-state EVB models, which significantly improves agreement with the available experimental data. The results of activation energies for the excess proton and water calculated between 300 and 340 K (the temperature range used in this study) are also found to be in good agreement with the corresponding experimental data.

Molecular Dynamics Study of Proton and Water Transport in Nafion Membrane

Polymer electrolyte fuel cells (PEFCs) are highly expected as a next-generation power supply system due to the purity of its exhaust gas, its high power density and high efficiency. The polymer electrolyte membrane is a critical component for the performance of the PEFCs and it is important to understand the nanostructure in the membrane to enhance proton transport. We have performed an atomistic analysis of the vehicular transport of hydronium ions and water molecules in the nanostructure of hydrated Nafion membrane by systematically changing the hydration level which provides insights into a connection between the nanoscopic and mesoscopic structure of ion clusters and the dynamics of hydronium ions and water molecules in the hydrated Nafion membrane. In this study, classical molecular dynamics simulations are implemented using a model of Nafion membrane which is based on DREIDING force field and newly modified and validated by comparing the density, water diffusivity, and Nafion morphology with experimental data. The simulated final density after the annealing procedure agrees with experiment within 1.3 % for various water contents and the trends that density decreases with increasing hydration level are reproduced. In addition to determination of diffusion coefficients of solvent molecules as a function of hydration level (from λ = 1 up to λ = 18), we have also calculated radial distribution functions and static structure factors not only to clarify the structure of water molecules and hydronium ions around the first solvation shell of sulfonate groups but also to validate the mesoscopic periodic structure among water clusters. The diffusion coefficient of water molecules increases with increasing hydration level and is found to be in good agreement with experimental data. The diffusion coefficient of hydronium ions has showed that general trends in the experimental data are reproduced by the simulations although the classical models have the limitation of probing hydronium dynamics. The static structure factors of liquid molecules at low wave length provide insights into the periodic structure of the inter-water clusters. These results are consistent with the Gebel’s model based on small-angle X-ray scattering that considers the dry membrane to be made of isolated spherical ionic clusters of radius ∼7.5 A that swell with increasing hydration.Copyright

Relationship between Proton Transport and Morphology of Perfluorosulfonic Acid Membranes: A Reactive Molecular Dynamics Approach

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.

Dynamics of oxygen scattering on ionomer surface in catalyst layer of PEFC

Molecular dynamics simulations have been performed to clarify the scattering phenomena of oxygen molecules on ionomer thin films, which affect the transport resistance of oxygen in catalyst layers in polymer electrolyte fuel cells. We have evaluated the probability density functions of the translational energy and scattering angle of scattered molecules, and the residence time of oxygen molecules on the ionomer surface. It was found that the energy distributions of scattered oxygen molecules depend on the incident energy and differ from that of thermally equilibrated molecules. On the other hand, the angular distributions are independent of the incident energy, and well reproduced by the diffusive scattering model. These results indicate that oxygen molecules do not accommodate completely with ionomer surface during the collision. We also evaluated the trapping dynamics of oxygen molecules on the ionomer surface in the trajectory calculations. Increasing the normal component of the incident energy results in the longer residence time on the ionomer surface.

Analysis of the oxygen scattering behaviour on ionomer surface in Catalyst Layer of PEFC

Mass transport significantly affects the reaction efficiency of polymer electrolyte fuel cells. In particular, the oxygen transport in catalyst layers is important for the improvement of its efficiency. However, the mechanism of oxygen scattering on ionomer surface, which is one of the dominant factors of transport phenomena, has not been clarified. Therefore, we analyzed the oxygen scattering behaviour on ionomer surface using molecular dynamics simulation. Oxygen molecules are impinged to ionomer surface with different incident energies and angles. According to the total energy of oxygen molecule, the trajectories of oxygen molecules are classified into trapping or scattering. The trapping probability of oxygen molecule on ionomer surface decreases as the normal component of the incident energy increases. Oxygen molecules with low normal incident energy get energy during the gas–surface interaction on the surface and desorb from the surface. The number of collisions with the surface does not affect the energy transfer between oxygen molecule and ionomer surface.

Scattering dynamics of oxygen molecules on Nafion membrane

The scattering behaviors of oxygen molecules on a Nafion membrane, which is a typical polymer electrolyte membrane used in polymer electrolyte fuel cells, have been investigated using molecular dynamics simulations. We have evaluated the probability density functions of the translational energy and scattering angle of the scattered oxygen molecules for a wide range of incident conditions and water contents. It was found that the translational energy of oxygen molecules does not accommodate with the Nafion membrane during the collision, and oxygen molecules are reflected diffusely on the surface. Two types of collision behaviors, i.e., single and multiple collisions, were observed in the simulations. Increasing the normal component of the incident energy and the water content results in the longer residence time on the ionomer surface.

Atomistic study of proton hopping mechanism in hydrated Nafion membrane

We have investigated the transport phenomena of hydronium ions and water molecules in the nanostructure of hydrated Nafion membrane by systematically changing the hydration level using classical molecular dynamics simulations. The new empirical valence bond (EVB) model is developed in order to improve the description of proton mobility in both aqueous and Nafion environments. The new EVB model predicts a significantly enhanced transport in comparison with previous hopping models as well as the classical hydronium diffusion, which largely improves the agreement with the available experimental data. We have determined diffusion coefficients of hydronium ions and water molecules in hydrated Nafion membrane as a function of hydration level to investigate the impact of the Grotthuss mechanism on the proton transport property. Proton hopping mechanism was found to become more significant at higher hydration levels.