Water management in the catalyst layers (CLs) of proton-exchange membrane fuel cells is crucial for its commercialization and popularization. However, the high experimental or computational cost in obtaining water distribution and diffusion remains a bottleneck in the existing experimental methods and simulation algorithms, and further mechanistic exploration at the nanoscale is necessary. Herein, we integrate, for the first time, molecular dynamics simulation with our customized analysis framework based on a multiattribute point cloud dataset and an advanced deep learning network. This was achieved through our workflow that generates simulated transport data of water molecules in the CLs as the training and test dataset. Deep learning framework models the multibody solid-liquid system of CLs on a molecular scale and completes the mapping from the Pt/C substrate structure and Nafion aggregates to the density distribution and diffusion coefficient of water molecules. The prediction results are comprehensively analyzed and error evaluated, which reveals the highly anisotropic interaction landscape between 50,000 pairs of interacting nanoparticles and explains the structure and water transport property relationship in the hydrated Nafion film on the molecular scale. Compared to the conventional methods, the proposed deep learning framework shows computational cost efficiency, accuracy, and good visual display. Further, it has a generality potential to model macro- and microscopic mass transport in different components of fuel cells. Our framework is expected to make real-time predictions of the distribution and diffusion of water molecules in CLs as well as establish statistical significance in the structural optimization and design of CLs and other components of fuel cells.

We investigated the fundamental mechanisms determining the limit of the atomic layer deposition (ALD) temperature window by density functional theory (DFT) simulations. Boron nitride (BN) ALD experiments were performed using BCl3 or tris-dimethylaminoborane (TDMAB) and NH3 on Si(1 0 0) substrates to check whether the ALD temperature window exists or not. With increasing the growth temperature, an increase in growth per cycle (GPC) between 700 and 900 °C was relatively small in the BCl3 and NH3 system, while the GPC exponentially increased in the TDMAB and NH3 system. Based on the experimental results, we performed DFT simulations to investigate the reaction mechanisms determining the lower ALD temperature window in the BCl3 and NH3 systems. The two initial reactions and two cyclic reactions were numerically analyzed: (a) BCl3 reactions on OH-terminated Si(1 0 0) surface, (b) NH3 reactions on Cl-terminated Si(1 0 0) surface, (c) BCl3 reactions on H-terminated BN surface, and (d) NH3 reactions on Cl-terminated BN surface. We found activation energies for reaction (c) is 30.0 kcal/mol, determining the lower limit of the ALD temperature window. The activation temperature of reaction (c) is obtained 789 °C which is just in the ALD temperature window by our experiments.

In recent years, the construction of artificial cells using molecular robots have attracted much attention. In order to achieve selective transport of multiple ion species in artificial ion channels, it is essential to understand the mechanism of ion transport in antiporters and symporters of natural membrane proteins. The CLCF F−/H+ Antiporter (CLCF) has been attracting attention for its ability to specifically transport F− as an antiporter. The CLCF exchanges intracellular F− with extracellular H+ and releases F− from the cell when bacteria's intracellular F− concentration reaches the toxic concentration of 10–100 μM. On the other hand, it has been reported that certain drugs inhibit bacterial growth by regulating membrane transport proteins and decreasing ion transport function. Regulation of CLCF is also expected to inhibit bacterial growth, andit is necessary to understand the ion exchange mechanism for the regulation of CLCF. The structure of CLCF is similar to that of CLC-ec1, which exchanges Cl− and H+. The CLC-ec1utilizes Gluex for ion exchange, and CLCF also possesses Gluex and is thought to contribute to ion exchange. However, the role of Gluex in the ion exchange mechanism is still unclear. In this study, we performed MD simulations to investigate the role of Gluex in the ion exchange mechanism of the CLCF protein by analyzing the relationship between Gluex and surrounding Gluex structures in the different states of Gluex.

Abstract Growth morphology of carbon clusters deposited on different substrates were investigated by theoretical and experimental approach. For theoretical approach, molecular dynamics was employed to evaluate an adsorptive stability of different size of carbon clusters placed on different substrates. The adsorptive stability was estimated by the difference of total energy of supercell designed as carbon cluster placed on a certain crystal plane of substrate. Among the simulations of this study, carbon cluster flatly settled down on the surface of SrTiO$$_{3}$$(001). The result was experimentally verified with layer by layer growth of graphene by pulsed laser deposition in carbon dioxide atmosphere. The absorptive stability can be useful reference for screening substrate for any target material other than graphene.

Direct growth of oxide films on silicon is usually difficult because of extensive diffusion or chemical reaction between silicon (Si) and oxide materials. Schlom et. al. comprehensively investigate the thermodynamic stability of binary oxides. However the thermodynamic stability does not include factors for epitaxial growth such as lattice mismatch or crystallographic arrangement of oxide deposited on silicon surface. A magnesium oxide (MgO) was taken as an example of oxide materials and the absorption energy estimated on MgO placed on Si surface predicted the crystal orientation of epitaxial growth to be cubic on cubic [MgO(001) // Si(001) and MgO(100) // Si(100)]. These results agreed with experimental results observed on epitaxial MgO films deposited on Si surface. The evaluation of adsorption energy can be guideline for an epitaxial growth of oxide materials on silicon surface.

Hypothesis: Recent advances in deep learning (DL) have enabled high level of real-time prediction of thermophysical properties of materials. On the other hand, molecular dynamics (MD) have been long used as a numerical microscope to observe detailed interfacial conditions but require separate simulations that are computationally costly. Hence, it should be possible to combine MD and DL to obtain high resolution interfacial details at a low computational cost. Experiment: We proposed a novel DL encoding–decoding convolutional neural network (CNN) coupled with MD to realize the mapping from micro solid–liquid interface geometry to molecular temperature and density distribution of liquid containing surfactant. A multi-nanoscale optimization scheme was further proposed to reduce the uncertainty of DL prediction at the expense of local details to obtain more resilient predictors. Findings: The statistical results showed that the proposed CNN had high prediction accuracy and could reproduce the heat transfer and adsorption phenomena under the influence of various factors including liquid composition, wettability, and solid surface roughness, while the computational efficiency was greatly improved. Our DL method with the support of multi-nanoscale learning strategies can achieve the fast and accurate visualization and prediction of various interfacial properties of liquid and assist for interfacial material design.

Molecular dynamics (MD) simulation can effectively analyze the transport properties of liquid at the solid surface with different nanoscale roughness, while high computational costs are required. Herein, a deep learning encoding-decoding convolutional neural network is proposed to predict the adsorption density distribution of atomic and organic liquids under different molecular scale surface roughness. Compared with the previous deep learning studies focusing on simple macro adsorption parameters, our deep learning method realizes the prediction and visualization of micro scale adsorption behavior with very high accuracy. The data-driven deep learning algorithm replaces the MD extensive sampling and simplifies the operation process, which improves the computational efficiency of a single model 36000-fold. This study proves the good coupling between MD and deep learning method, which is helpful for designing surface geometry to obtain desirable interfacial transport properties of molecular liquid and complementing the nanoscale model system enabling the interactive visualization.

Reactive force-field molecular dynamics simulation of Si thin film deposition in order to prove the availability of the simulation model including both chemical reactions and physical dynamics for a plasma-enhanced chemical vapor deposition process was performed. We simulated surface reactions of SiHx (x = 2–4) formed in the plasma on a Si(1 0 0)-(2 × 1) surface covered with H atoms and evaluated the surface reactions with different substrate temperature and H coverage. The existing potential parameter set was modified to fit the dissociation energies in the gaseous species. The gaseous species collided with the surface one by one, and then the surface reactions were classified into four events: reflection, desorption, chemisorption, and physisorption. Our results show that an increase in substrate temperature affects both the bond dissociation in the gaseous species and the desorption of gaseous species on the surface. The H atoms inhibit the chemisorption by terminating dangling bonds on the surface and promote reflection and desorption by thermal movement. This method should facilitate the simulation of much more complex systems such as SiC and SiGe.

In this paper, a quantum effect of hydrogen molecules (uncertainties of each atomic nuclear position and momentum) on the bubble nucleation rate was investigated. The homogeneous bubble nucleation analyses were performed using a density functional theory (DFT) reflecting equations of state (EOSs) constructed to reproduce the thermophysical properties of hydrogen obtained from a classical molecular dynamics (MD) method and a quantum MD method. The results showed that the quantum nature of liquid hydrogen decreases the bubble nucleation rate when compared in the same reduced temperature and reduced superheat ratio condition. Further, it was indicated that the results might be caused by the increase of the energy barrier arising from the difference of the density profile and its position at the critical bubble (in other words, the differences of the critical bubble size and the liquid–vapor interface thickness). Furthermore, the DFT analysis was validated through the evaluation of the bubble nucleation rate using the classical MD method and the quantum MD method made as numerical experiments, and qualitatively the same result was obtained between the DFT and the MD simulations.

We analyzed the transport properties of cerium ions, which are added to improve the chemical durability of a polymer electrolyte membrane (PEM), using molecular dynamics simulations. For the analysis, the diffusion coefficient of cerium ions and the radial distribution function around sulfonic acid groups were obtained. The results show that at low water content, the diffusion coefficient is small because of the direct coordination of cerium ions to the sulfonic acid groups due to the small number of water molecules around the sulfonic acid groups. At high water content, water molecules gather around the sulfonic acid groups, and cerium ions are coordinated far from the sulfonic acid groups, resulting in a weaker binding than at low water content and an increase in the diffusion coefficient of cerium ions.

We analyzed the influence of the interaction between oxygen molecules and ionomer surfaces on the overall oxygen transport in the catalyst layer of polymer electrolyte fuel cells using a Monte Carlo (MC) method and the molecular dynamics (MD) method. We analyzed how gas-surface interaction parameters of oxygen molecules on the ionomer surface affects the overall transport by the MC method, and found that the conditions of surface diffusion affect much on the overall transport properties. We analyzed how oxygen molecules colliding with the ionomer surface behave using the MD method. When the results were compared, it was found that the ease of movement in the surface diffusion greatly affects the overall transport. We obtained the results of the residence time of the oxygen molecules on the ionomer surface. Moreover, it was suggested that further analysis on the behavior of oxygen molecules on the ionomer surface is necessary in order to more accurately calculate the overall oxygen transport in the catalyst layer.

Coarse-grained molecular dynamics simulations were performed to understand the morphological evolution and adsorption mechanism of Nafion ionomers from the aqueous solutions to the Pt/C substrate surface under various solution compositions and substrate properties. We found that the ionomer coverage did not increase with the increasing ionomer-to-carbon ratio but was related to the size and concentration of the ionomer aggregates, following the Langmuir adsorption model that shows a wettability switching behavior due to their changed morphology from solution to the surface. Ionomer aggregates in the solution tended to unfold and spread on the carbon substrate rather than Pt particles, although the cylindrical ionomer aggregates were easily attracted by Pt particles initially due to their hydrophilic ionic shells. The smaller Pt particles had a greater effect on ionomer adsorption. With the increasing number of Pt particles, ionomer coverage increased first and then decreased, depending on whether there was enough carbon surface to anchor the ionomer backbone. A balanced Pt/C ratio and the appropriate distribution of the Pt particles were required for tuning the ionomer coverage and distribution toward the design of the catalyst ink structure to improve the power performance.

We simulated the growth of SiGe alloys by using reactive force-field (ReaxFF) molecular dynamics simulations in binary systems of SiHx and GeHx and identify the effect of gaseous species on the compositions and crystallinity in alloys. The compositions in alloys could be estimated if compositions of gaseous species can be identified because of the observation of the linear increase in Ge content in alloys. Supplying GeH2 decreased the crystallinity compared to GeH3. The mechanism is described by the mobility and reactivity of the gaseous species and surface structures. These results suggest that the mobility and reactivity of both surface and gaseous species are essential in identifying the effect of gaseous species on composition and crystallinity.

Atomistic analysis of the ion transport in polymer electrolytes for all-solid-state Li-ion batteries was performed using molecular dynamics simulations to investigate the relationship between Li-ion transport and polymer morphology. Polyethylene oxide (PEO) and poly(diethylene oxide-alt-oxymethylene), P(2EO-MO), were used as the electrolyte materials, and the effects of salt concentrations and polymer types on the ion transport properties were explored. The size and number of LiTFSI clusters were found to increase with increasing salt concentrations, leading to a decrease in ion diffusivity at high salt concentrations. The Li-ion transport mechanisms were further analyzed by calculating the inter/intra-hopping rate and distance at various ion concentrations in PEO and P(2EO-MO) polymers. While the balance between the rate and distance of inter-hopping was comparable for both PEO and P(2EO-MO), the intra-hopping rate and distance were found to be higher in PEO than in P(2EO-MO), leading to a higher diffusivity in PEO. The results of this study provide insights into the correlation between the nanoscopic structures of ion solvation and the dynamics of Li-ion transport in polymer electrolytes.

Polymer electrolyte membrane (PEM) composed of polymer or polymer blend is a vital element in PEM fuel cell that allows proton transport and serves as a barrier between fuel and oxygen. Understanding the microscopic phase behavior in polymer blends is very crucial to design alternative cost-effective proton-conducting materials. In this study, the mesoscale morphologies of Nafion/poly(1-vinyl-1,2,4-triazole) (Nafion-PVTri) and Nafion/poly(vinyl phosphonic acid) (Nafion-PVPA) blend membranes were studied by dissipative particle dynamics (DPD) simulation technique. Simulation results indicate that both blend membranes can form a phase-separated microstructure due to the different hydrophobic and hydrophilic character of different polymer chains and different segments in the same polymer chain. There is a strong, attractive interaction between the phosphonic acid and sulfonic acid groups and a very strong repulsive interaction between the fluorinated and phosphonic acid groups in the Nafion-PVPA blend membrane. By increasing the PVPA content in the blend membrane, the PVPA clusters’ size gradually increases and forms a continuous phase. On the other hand, repulsive interaction between fluorinated and triazole units in the Nafion-PVTri blend is not very strong compared to the Nafion-PVPA blend, which results in different phase behavior in Nafion-PVTri blend membrane. This relatively lower repulsive interaction causes Nafion-PVTri blend membrane to have non-continuous phases regardless of the composition.

Coarse-grained molecular dynamics simulations were performed to understand the evolution of ionomer morphologies in solutions during solvent evaporation. To reproduce experimental fabrication conditions, the simulation conditions, such as evaporation and sedimentation rate, were determined based on a dimensionless parameter, the Peclet number, providing a direct link between simulation results and experimental findings. The effects of ionomer loading and substrate wettability on the morphologies of ionomer thin films after drying were investigated extensively, which exhibit similar trends reported in experiments. At low ionomer loading, a discontinuous patchy film with a minimum thickness of approximate to 3 nm was formed on the hydrophobic substrate, whereas a high-coverage continuous film of approximate to 2 nm thickness was found on the hydrophilic substrate. At high ionomer loading, regardless of the substrate wettability, a lamellar-like morphology with multiple water-rich layers was observed, although the stability of the layer structures and the ionomer surface roughness differ between the substrate types because the substrate wettability strongly affects the adsorption behaviors of ionomers and water as the first layer in the interfacial region. Our findings of a decrease in the degree of phase segregation with increasing thickness suggest an eventual collapse of the lamellar structure at a certain thickness within a few tens of nanometers or even thinner.

In order to clarify the proton transport phenomena in Nafion/carbon nanotubes (CNTs) composite membranes, we focused on the interfacial transport between Nafion and graphene sheets (GSs). We investigated the water content dependence of proton transport and structural properties at the interface between Nafion and GSs using molecular dynamics simulations. The density distribution, water cluster analysis at the interface region, and the self-diffusion coefficient of protons at the interface region were obtained at different water contents. The self-diffusion coefficients of protons at the interface were larger than those of the Nafion bulk for all the water contents because of the formation of water layer near the interface. Our findings provide insight into the relationship between the proton transport and the interfacial structure at molecular level, which suggests a possible improvement on proton transport with addition of CNTs with high specific surface area.

We analyzed the transport properties of oxygen molecules using the Monte Carlo method. We obtained the effective diffusion coefficient of oxygen molecules in a catalyst layer of polymer electrolyte fuel cells. In this calculation system, when oxygen molecules hit a wall, it diffuses on the ionomer surface for a period of time and then diffusely reflected. The simulation was performed using various conditions of the surface diffusion. The conditions are defined by the surface diffusion coefficient and the time constant that controls the residence time of oxygen molecules. When the surface diffusion coefficient is small, the increase in the time constant results in the decrease in the effective diffusion coefficient of oxygen. However, when the surface diffusion coefficient is large, as the time constant increases, the effective diffusion coefficient of oxygen has a peak value and then decreases. This phenomenon occurs due to the three-dimensional bending structure of the catalyst layer. When an oxygen molecule moves along the bending three-dimensional structure, the distance along the ionomer surface becomes larger than the straight line segment between the start and end points.

The modified free volume (MFV) theory was applied to evaluation of the self-diffusion coefficient of liquid hydrogen in which the nuclear quantum effect appears. The free volume of the hydrogen was defined via the generic van der Waals (GvdW) equation of state, which was derived based on the centroid path integral notation. The computational results of the self-diffusion coefficient were compared to those of the experimental data, centroid molecular dynamics (CMD) methods, and previous studies. This comparison revealed that the self-diffusion coefficient of liquid hydrogen computed by means of the MFV theory reproduces the experimental data with the same precision as CMD and previous studies. In conclusion, even though the MFV theory is a completely classical mechanism, the self-diffusion coefficient of a quantum liquid can be calculated on the basis of the MFV theory by treating the centroid as a representative of the quantum molecule.

We carried out quantum chemical calculations to analyze the effects of fluorination on the activation energy (Ea) of sulfonic group deprotonation by water molecules. The model molecule was 2,3,4,5,6-pentafluorobenzenesulfonic acid (5FBSA), which was obtained by substituting all aromatic hydrogen atoms of benzenesulfonic acid (BSA) for fluorine atoms. The target hydration level was three. Our analysis indicated that the Ea of deprotonation in 5FBSA was lower than that of BSA, suggesting that the cation of 5FBSA was stabilized. Previous studies have reported that fluorinated molecules have a lower Ea to deprotonation and a stabilized deprotonated state even at a hydration level of three. This effect is attributed to the strong electron withdrawing ability of fluorine. However, compared with non-aromatic molecules, the Ea of deprotonation of aromatic molecules is slightly higher, and the overall energy change (Delta E) is lower, even if the molecule is fluorinated.

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 similar to 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.

The self-assembly of Nafion ionomer in a mixture of 1-propanol (NPA) and water was investigated using coarse-grained molecular dynamics simulations. Ionomer formation into cylindrical bundle-like aggregates is observed when the ionomer chain size is sufficiently large. The size of ionomer bundles decreases (the diameter decreased from 2.5 to 1.9nm) with increasing NPA content, which indicates that the ionomers tend to be more dispersed at higher NPA content. The results of simulations are in good quantitative agreement with the stable size of an ionomer bundle estimated from our free energy calculations as well as the available experimental data. Furthermore, in contrast to the polarizable NPA model, the standard nonpolarizable NPA model shows the opposite trend that the ionomer size increases with increasing NPA content because of a higher localization of hydronium ions at the bundle surface, revealing that the polarization effect plays a significant role in determining the aggregation behaviors. The present study provides insight into the control of ionomer self-assembly toward obtaining targeted structures for specific purposes. (c) 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020, 58, 487-499

© The Electrochemical Society The SiGe thin films, such as hydrogenated amorphous SiGe (a-SiGe:H) and hydrogenated microcrystalline SiGe (μc-SiGe:H), are known as a potential material. In order to form promising SiGe thin films using chemical vapor deposition, relationship between materials/process and structure/composition should be clarified at the atomic level. We analyzed the influence of the substrate temperature and the ratio of SiH3 and GeH3, which are considered to be the dominant gaseous species on the deposition, on the crystallinity and atomic content in SiGe thin films by reactive force-field molecular dynamics simulations. The crystallinity increased as the substrate temperature increased, and the lowest crystallinity was obtained when the ratio of SiH3 and GeH3 is approximately 0.5. The hydrogen content decreased as the substrate temperature increased, while silicon and germanium content tended to increase as substrate temperature increased. These results were in good agreement with relevant studies.

© The Electrochemical Society To spread polymer electrolyte fuel cells, improving the cell performance is required. The cell performance depends on various factors. One of the factors that lowers cell performance is proton transport resistance in catalyst layers. Catalyst layers have multiscale structures which influence on proton transport resistance. In this study, to investigate the relationship between structures of catalyst layers and cell performances, mass transport and chemical reactions are calculated in the 3D catalyst layer models. The information about the ionomer thickness dependence on the diffusion coefficient of protons based on the molecular dynamics simulation is introduced to transport calculation in order to analyze nanoscale structure influence on the cell performance. As a result, we have found that the output voltage increases over the whole range of current density with increasing ionomer/carbon ratio considering ionomer thickness dependence. This result suggests that the nanoscale structure in catalyst layer has a large influence on the cell performance. Furthermore, cell performance analysis with ionomer thickness distribution based on experimental values is conduced in this study. The result suggests that contribution of nanoscale proton transport characteristic increases by improving the ionomer distribution model.

© The Electrochemical Society We analyzed the transport properties of cerium ions, which are added to improve the chemical durability of a polymer electrolyte membrane (PEM), using molecular dynamics simulations. A system of the PEM in which incorporate cerium ions are incorporated and water flow is set was constructed to investigate the convective ion transport properties. At a low water contents, cerium ions are hardly affected by water flow. This is because many cerium ions are trapped by sulfonate group of Nafion polymer and the connectivity of water cluster is low. As the water contents increase, free cerium ions without being trapped by sulfonate groups increase and water clusters are connected and the cluster size increases. Hence, cerium ions are easily affected by water convection at high water contents.

In order to form a SiGe thin film by chemical vapor deposition (CVD) with a suitable quality for advanced devices, the relationships between materials/process and structure/composition are needed to be clarified at the atomic level. We simulated SiGe CVD by using reactive force-field (ReaxFF) molecular dynamics simulations, especially on binary systems of SiHx + GeHx, and derived the influence of the substrate temperature and these ratios of gaseous species on the crystallinity and compositions in the thin films. The crystallinity increases as the substrate temperature increases, and the lowest crystallinity is obtained at the ratios of gaseous species 0.5 and 0.7 for the SiH3 and SiH2, respectively. As the substrate temperature increases, the hydrogen content decreases while Si and Ge content tend to increase. These trends can be seen in relevant studies. Through this simulation we successfully observe that the reactivity of gaseous species greatly affects the crystallinity and compositions in the thin films.

A molecular dynamics simulation is performed to understand the effects of ferrous ion contaminations on the proton transport property and nanostructures of solvent molecules in hydrated Nafion membranes while considering the Grotthuss mechanism. At low hydration conditions, the proton diffusivity has a local maximum at a certain concentration of ferrous ions. In the case of low ferrous ion concentration (approximate to 25% of total cation charge), proton diffusivity is similar to that in the pure membrane. Also, in the case of middle ferrous ion concentration (50%), proton diffusivity is enhanced because the water clusters are more connected compared with those in the pure membrane. In the case of high ferrous ion concentration (larger than 70%), proton diffusivity is inhibited because the water clusters are disintegrated by the ferrous ions. Moreover, the correlation between proton diffusivity and nanostructure of the Nafion membrane is different among low, middle, and high water content. The results imply that the proton diffusivity is affected by ferrous ion contaminations by different mechanisms according to water content.

We consider a Couette flow of a rarefied Ar gas with heat transfer between two wall surfaces and investigate the scattering behavior of gas molecules reflected either at a clean Pt surface or at a surface contaminated with adsorbates. Water molecules abundantly present in the atmosphere were adopted as the adsorbates. The reflection of gas molecules on the lower wall surface was simulated by Molecular Dynamics (MD) method to obtain accommodation coefficients and velocity distribution functions of gas molecules. We applied the modified reflection model of gas molecule and investigated the velocity distribution functions of the model by comparing the MD results to verify the validity. The accommodation coefficients obtained by the MD method depend on the number of adsorbed water molecules on the lower wall surface. Specifically, tangential momentum accommodation coefficient (TMAC) tended to increase and then decrease with the increase in adsorbed water molecules, but normal momentum accommodation coefficient (NMAC) tended to decrease monotonically. The velocity distribution functions of the modified reflection model approximately show the good agreement with the MD calculation but the degree of coincidence depends on the speed difference between the upper and lower wall surfaces, and the number of adsorbed water molecules on the surface.

Molecular dynamics simulations are performed to investigate the nucleation and growth of cavities in a hydrated Nafion membrane under mechanical deformation. The simulation model used in this study accurately reproduces the experimental values of the elastic modulus of the membrane as a function of water content. The results obtained from triaxial tensile tests reveal a ductile to brittle transition as the water content increases. The nucleation and growth of the cavities have been quantitatively analyzed in terms of the number and size of cavities, illustrating the ductile to brittle transition uncovered by the stress/strain curves. Further local analyses have been carried out to identify the nucleation sites. The analysis of local plasticity indicates that as the water content increases, the membrane accumulates more plastic deformation in the hydrophilic domain than in the hydrophobic domain during the rupture stage of the tensile tests. These results suggest that the water network significantly impacts the nucleation and expansion of cavities induced by mechanical deformation. Furthermore, the local mechanical properties of the Nafion membrane are evaluated. The results show that the mechanical properties are heterogeneous at the nanoscale and that the cavities nucleate in soft regions of the membrane. A statistical analysis of the local water density of nucleation sites indicates that the polymer-water interfaces are more likely to nucleate cavities. The expansion and coalescence of cavities is facilitated by the high molecular reorganization of the water network, which explains the brittle behavior of membranes with high water content.

© 2019 IEEE. We calculate a deposition process of hydrogenated amorphous silicon (a-Si:H) films on a silicon (100) substrate by reactive force-field molecular dynamics simulations. The influences of (a) substrate temperatures and (b) coverage of hydrogen atoms on the substrate on the adsorption probability are investigated, and it is found out that (a) the adsorption probability is almost constant for SiH2 and SiH3, but decrease with increase in the substrate temperature for SiH4, (b) it decreases with the increase in hydrogen coverage.

The effects of water nanochannel diameter on proton transport pathways and properties have been studied using reactive molecular dynamics simulations. The proton distributions and diffusivities have been evaluated using the cylinder model of water domains at various diameters that is the most typical proposed morphological model in proton-exchange membranes. The proton distributions are analyzed to clarify proton pathways by classifying the water channel into two regions in parallel: an inner channel (free water) and an outer channel (bound water). For all the water contents, the nonmonotonic trends that show a peak at a certain diameter are found to be observed in the proton diffusivity, which is dominated by the proton diffusivity in the free water region and has a strong correlation with the proton distribution that is controlled by the balance between the volume fraction of free water and the surface density of sulfonate groups. The electroosmotic drag coefficients are found to increase monotonically with increasing channel diameter as a result of the increase in the volume fraction of free water. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 867-878

The scattering dynamics of oxygen molecules on perfluorosulfonic acid (PFSA) ionomer thin films was investigated using molecular dynamic simulations. We constructed PFSA ionomer thin films with different water contents on a carbon surface at 300 K and then shot oxygen molecules into the films with different incident angles and incident energies corresponding to a temperature range from 150 to 600 K. The effective softness of the film for gas-surface collisions increased on increasing the hydration level of the film. The surface regions exposing PFSA polymers and water molecules had different softness because of polymer entanglement and the mobility of water molecules, respectively. PFSA polymers make the surface more rigid than water molecules and lead to poor accommodation of oxygen molecules during the collisions. In contrast, impinging molecules are more likely to be trapped on the water molecules and trapped molecules are better accommodated than molecules reflected directly from the surface. The angular distributions of scattered molecules that were reflected directly or trapped on the surface both showed good agreement with Knudsen's cosine distribution. The diffusive reflection from the surface was caused by the roughness of the ionomer film rather than by the thermal accommodation with the surface.

We demonstrated epitaxial growth of magnesium oxide (MgO) on a silicon (Si) substrate with cubic on cubic structure (MgO(001)//Si(001) and MgO(100)//Si(100)]. Epitaxial films were prepared using the pulsed laser deposition method, and with dependent on deposition conditions of oxygen partial pressure and substrate temperature, the lattice constants of MgO decreased along both the surface normal and in-plane directions. To support the facts of (1) constriction of lattice constants and (2) cubic on cubic growth, we show (1) optimal lattice constants with defects in the crystal structure, (2) domain mismatch between MgO and Si instead of lattice mismatch, and (3) stability of MgO crystals on the surface of the Si substrate. (C) 2018 The Japan Society of Applied Physics

Molecular dynamics simulations were performed to analyze the water content dependence of the oxygen transport resistance through an ionomer thin film on a Pt surface, based on the phenomenological equations which make a correlation between the oxygen flux and the chemical potential. The oxygen transport resistance of the ionomer/Pt interface was found to be much larger than that of other regions, which indicated that the ionomer/Pt interface was dominant for oxygen transport through the ionomer thin film. Moreover, in the conventional method that assumes that the oxygen flux is driven by the concentration gradient, the oxygen transport resistance in the ionomer/Pt interface was found to be underestimated, whereas that in the ionomer/gas interface was found to be overestimated, which was caused by the exclusion of the contribution of the oxygen-film interaction on the oxygen flux. Furthermore, the oxygen transport resistance of the ionomer thin film increased with increasing water content. In particular, in the ionomer/Pt interface the transport resistance increased because the voids in the region were filled with water at a higher water content.

© The Electrochemical Society. To spread polymer electrolyte fuel cells, improving the cell performance is required. The cell performance depends on various factors. One of the factors that lowers cell performance is proton transport resistance in catalyst layers. Catalyst layers have Nano/Microscale structures which influence on proton transport resistance. In this study, to investigate the relationship between structures of catalyst layers and cell performances, mass transport and chemical reactions are calculated in the 3D catalyst layer models. The information about the ionomer thickness dependence on the diffusion coefficient of protons based on the molecular dynamics simulation studies, is introduced to transport calculation in order to analyze nanoscale structure influence on the cell performance. As a result, we have found that the output voltage increases over the whole range of current density with increasing ionomer/carbon ratio considering ionomer thickness dependence. This result suggests that the nanoscale structure in catalyst layer has a large influence on the cell performance.

© The Electrochemical Society. Chemical decomposition of polymer electrolyte membrane (PEM) by hydroxyl radicals has been reported as one of causes of polymer electrolyte fuel cells (PEFCs) degradation, and as a countermeasure, a method of adding cerium ions into the PEM has been studied. Using molecular dynamics simulations, we constructed a system of the PEM that incorporate cerium ions to investigate the effect of cerium ions on the proton transport property. We have obtained proton diffusion coefficient, radial distribution function, coordinate number, number of cluster and cluster size. As a result, we found that at low water content, diffusion coefficient of proton increases by adding cerium ions because cerium ion attract the surrounding water molecules and clusters are connected to make better diffusion path of protons. At high water content. The presence of cerium ions in the cluster hinders proton transport, which results in the decrease in the diffusion coefficient of proton.

The scattering behaviors of oxygen molecules on Nafion ionomer thin film, which is a typical polymer electrolyte membrane used in polymer electrolyte fuel cells, has been investigated using molecular dynamics simulations. The scattering processes and energy accommodation of impinging molecules depend on the surface composition of the film. The local softness of the initial collision partner on the film and the number of collisions with the surface characterize the degree of energy accommodation.

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 lambda = 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 structure of clusters is retained, thereby contributing to further confinement decreases. increase, while the elongated

New equations for oxygen transport, which is driven by the chemical potential gradient through an ionomer thin film on a Pt surface, were developed. The chemical potential was described by two terms which indicated the effect of the oxygen concentration and that of the interaction between oxygen molecules and the ionomer film on the oxygen flux. The oxygen transport resistance of the ionomer/Pt interface was the largest, which is suggested to be attributed to the structural properties; the density of the ionomer/Pt interface is very high compared with the rest of regions, and the oxygen concentration is lower since the oxygen molecules are distributed locally in the ionomer/Pt interface while the oxygen molecules are distributed widely in the rest of regions. The oxygen transport resistance of the ionomer/Pt interface was underestimated while that of the ionomer/gas interface was overestimated if one considers only the concentration gradient as the driving force. Therefore, the oxygen-film interaction was found to have a significant effect on the oxygen transport resistances of the both interfaces.

In order to improve cell performance of polymer electrolyte fuel cells, it is important to understand mass transport phenomena at the nanoscale. In this study, we have investigated the effects of thickness of ionomer thin films on proton diffusivity using molecular dynamics simulations. We have obtained self-diffusion coefficient of protons, maximum cluster lengths corresponding to connectivity of water clusters, and water density distributions at different ionomer thickness. As a result, we found that at high water contents, proton diffusion coefficients have a little relationship with ionomer thickness, whereas at low water contents, proton diffusion coefficients show a peak with the ionomer thickness of around seven nm. The maximum cluster lengths show the similar trends as the proton diffusion coefficient.

© 2018 by The Electrochemical Society. All rights reserved. New equations for oxygen transport, which is driven by the chemical potential gradient through an ionomer thin film on a Pt surface, were developed. The chemical potential was described by two terms which indicated the effect of the oxygen concentration and that of the interaction between oxygen molecules and the ionomer film on the oxygen flux. The oxygen transport resistance of the ionomer/Pt interface was the largest, which is suggested to be attributed to the structural properties; the density of the ionomer/Pt interface is very high compared with the rest of regions, and the oxygen concentration is lower since the oxygen molecules are distributed locally in the ionomer/Pt interface while the oxygen molecules are distributed widely in the rest of regions. The oxygen transport resistance of the ionomer/Pt interface was underestimated while that of the ionomer/gas interface was overestimated if one considers only the concentration gradient as the driving force. Therefore, the oxygen-film interaction was found to have a significant effect on the oxygen transport resistances of the both interfaces.

To obtain a basic insight for developing hydrocarbon based polymer membranes that have high proton conductivity, we designed new molecules by introducing the sulfonyl group (SO2) into various positions of benzene sulfonic acid and analyze their deprotonation reaction. We evaluated the activation and stabilization energies of deprotonation reactions of these model molecules with three water molecules. The energies were highly dependent on the positions of the sulfonyl group. For the case of a sulfonyl group introduced between the carbon and sulfur atom of the sulfonic group (SO3H), the activation energy decreased and the stabilization energy increased. These changes were attributed to the charge distribution around the oxygen atoms averaged owing to the high electronegativity of the sulfonyl group. Our results suggest that introducing highly electronegative functional groups can improve the performance of hydrocarbon-based membranes, without the need for fluorine atoms. (C) 2017 Elsevier B.V. All rights reserved.

In this paper, the nuclear quantum effect of the hydrogen molecule on its diffusivity was analyzed using the molecular dynamics (MD) method. The centroid MD (CMD) method was applied to reproduce the time evolution of the molecules. The diffusion coefficient of hydrogen was calculated using the Green-Kubo method over a wide temperature region, and the temperature dependence of the quantum effect of the hydrogen molecule on its diffusivity was addressed. The calculated results were compared with classical MD results based on the principle of corresponding state (PCS). It was confirmed that the difference in the diffusion coefficient calculated in the CMD and classical MD methods was small, and the PCS appears to be satisfied on the temperature dependence of the diffusion coefficient, even though the quantum effect of the hydrogen molecules was taken into account. It was clarified that this result did not suggest that the quantum effect on the diffusivity of the hydrogen molecule was small but that the two changes in the intermolecular interaction of hydrogen due to the quantum effect offset each other. Moreover, it was found that this tendency was related to the temperature dependence of the ratio of the kinetic energy of the quantum fluctuational motion to the classical kinetic energy. Published by AIP Publishing.

A mathematical model focusing on local gas transport in a single carbon primary particle was developed by combining the classical agglomerate model with local gas transport in the vicinity of a Pt particle. Two local gas transport processes gas transport through an ionomer thin film at Pt particles deposited on the surface of the carbon primary particle and gas transport through water at Pt particles deposited inside the carbon primary particle were newly introduced. The model showed qualitative agreement with the experimentally measured gas transport resistance of catalyst layers with different effective Pt surface areas, which had not been captured by the classical agglomerate approach. The experimental data were fitted by significantly reducing the gas permeability for the local gas transport processes in the vicinity of the Pt surface, which was 1/4-1/20 of the permeability measured in the bulk materials. The corrections of gas permeability were all related to the local gas transport in the vicinity of the Pt surface, which implied the presence of a high energy barrier for reactant gas transport at ionomer/Pt and water/Pt interfaces. The influence of structural parameters of catalyst layers on the gas transport resistance was presented to provide a guideline toward the reduction of the gas transport resistance at low Pt loading. (C) 2017 Elsevier B.V. All rights reserved.

There is a rising pressure on industry to meet the steady climb of energy demand while concurrently reducing harmful emissions exhausted into the atmosphere. For the past decade, oxygen transport membrane reactors (OTMs), have been receiving growing attention due to their ability to provide high volumes of pure oxygen that react with incoming fuel at minimal energy costs. However, one significant concern is the OTM's stability when exposed to high concentrations of CO2, a potentially harmful acidic gas. To preserve the integrity of the OTM in acidic environments, many have adapted dual-phase OTMs, combining the advantages of the perovskite-type material's high oxygen permeation performance with a stable additive material's tolerance of CO2. In this study, dual-phase OTMs comprised of varying weight ratios between SrSc0.1Co0.9O3-delta (SSC) and Sm0.2Ce0.8O1.9 ( SDC) were successfully prepared and studied. Specifically, the dual-phase OTMs' oxygen permeation flux and combustion performance are reported. The results show that the oxygen permeation flux through dual-phase OTMs decreases with the increase in SDC wt.% in the composition using a helium or methane sweeping gas. The highest oxygen permeation flux is found to be a pure SSC OTM at 5.27 ml.min(-1).cm(-2) using methane sweeping gas with a flow rate of 80 ml. min(-1). Additionally, a pure SSC OTM exhibits a CO2 selectivity of 97.7% with a methane sweeping gas flow rate of 5 ml.min(-1). Despite the SSC OTM's higher oxygen permeation flux and CO2 selectivity, a dual-phase OTM exhibited a higher oxygen permeation flux and membrane stability compared to a pure SSC membrane after exposed to a CO2 sweeping gas for 60 hours. This suggests the potential for a highly stable dual-phase OTM design that can maintain an oxygen permeation performance in any environment and be potentially implemented in future carbon capture technologies.

Effects of polymer structure on the electroosmosis in proton exchange membranes (PEMs) have been investigated using a reactive molecular dynamics simulation. An anharmonic two-state empirical valence bond (alpha TS-EVB) model has been used to describe efficiently excess proton transport via the Grotthuss hopping mechanism. The electroosmotic drag coefficients (i.e., the number of water molecules transferred through the membrane per proton) has been evaluated directly in PEMs consisting of various equivalent weights (EWs). The electroosmotic drag coefficients from the MD simulations are in good agreement with the available experimental values at studied levels of hydration. It is shown that for low water contents the connectivity of the water clusters decreases with increasing EW, leading to a decrease in the electroosmotic drag coefficients. The proton and water mobility is also found to decrease with increasing EW because of the structural changes in the water cluster domains. The present simulations provide quantitative information about the direct link between electroosmosis and nanoscopic water domain structure in different polymer structures

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. (C) 2017 The Electrochemical Society. All rights reserved.

© The Electrochemical Society. Institute of Fluid Science, Tohoku University, Sendai, Miyagi 980-8577, Japan Structural properties of ionomer aggregations in a mixture of 1-propanol (NPA) and water have been investigated using coarsegrained molecular dynamics simulations. To perform simulations of larger systems and longer time spans, the reduced spatial resolution of the bead representation was used as a coarse-grained model for the enhancement of the computational efficiency compared to all atomistic simulations. The dependence of NPA content on the ionomer structures was studied by systematically changing the NPA content in the system. The self-assembly behavior of ionomers into cylindrical bundle-like aggregates was observed for all NPA content solutions. The radius (thickness) of an ionomer bundle was also found to be in good agreement with experimental measurements. Formation of ionomer aggregations showed different behavior at each NPA content, although the ionomers aggregated into one cluster at all NPA contents.

© The Electrochemical Society. We investigated the gas-surface interaction between oxygen molecules and ionomer surface using molecular dynamics simulations because the scattering phenomena of gas molecules on the surface play a crucial role for the gas transport in catalyst layers (CLs). The probability density functions (PDFs) of the translational energy of scattered oxygen molecules was analyzed for a wide range of incident condition and water contents for different scattering process. The energy distributions show the dependence on the incident temperature and independent of water content. The characteristic temperature of translational energy was also investigated by fitting to the Maxwell-Boltzmann distribution. This result suggests that there is a different tendency of the thermal accommodation to the ionomer surface depending on the scattering process.

Gas flow in porous media can be seen in various engineering devices such as catalytic converters and fuel cells. It is important to understand transport phenomena in porous media for improvement of the performance of such devices. Porous media with pores as small as the mean free path of gas molecules are used in such devices as proton exchange membrane fuel cells. It is difficult to measure molecular transport through such small pores in the experimental approach. In addition, even when using theoretical or numerical approaches, gas flow through nanoscale pores must be treated by the Boltzmann equation rather than the Navier-Stokes equations because it cannot be considered as a continuum. Thus, conventional analyses based on the continuum hypothesis are inadequate and the transport phenomena in porous media with nanoscale pores are not yet clearly understood. In this study, we represented porous media by randomly arranged solid spherical particles and simulated pressure-driven gas flow through the porous media by using the direct simulation Monte Carlo (DSMC) method based on the Boltzmann equation. DSMC simulations were performed for different porosities and different sizes of solid particles of porous media. It was confirmed that Darcy's law holds even in the case of porous media with micro-/nanoscale pores. Using the obtained results, we constructed expressions to estimate the pressure-driven gas transport in porous media with micro-/nanoscale pores and porosity ranging from 0.3 to 0.5. The flow velocities estimated by using the constructed expressions agreed well with those obtained in the DSMC simulations.

Ionomer adsorption at the surface of a graphite sheet in a solvent-saturated environment was examined using molecular dynamics simulations. The amount of ionomer adsorbed on the graphite surface greatly depended on the equivalent weight (EW) of the ionomer and the alcohol content in the solvent. The maximum ionomer coverage was obtained in water-rich solvent for low EW ionomer, while the addition of alcohol to the solvent was necessary to enhance adsorption of high EW ionomer. The ionomer coverage at the surface with ionized functional groups was lower than that of the bare graphite surface, which indicated a significant effect of the electrostatic repulsive interaction between negatively charged functional groups and sulfonic acid groups on the ionomer adsorption. The effect of the alcohol content and EW of the ionomer on ionomer coverage still remained even in the presence of strong electrostatic interactions between sulfonic acid groups and the functionalized carbon surface. (C) 2016 Elsevier Ltd. All rights reserved.

© 2016, European Association for the Development of Renewable Energy, Environment and Power Quality (EA4EPQ). All rights reserved. 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.

We have performed a numerical analysis of the effect of ferrous ion contamination on proton transport property and nanostructure of solvent molecules in hydrated Nafion using molecular dynamics simulation with considering Grotthuss mechanism. The local maximum point of diffusion coefficient of proton changes according to the amount of ferrous ions. The distribution of ferrous ions depends on the balance between the energy of structural change of side chain of Nafion and the coordination energy of ferrous ion around sulfonate groups. The coordination energy of ferrous ion is dominant in the case of lambda = 3. On the other hand, the energy of structural change of side chain is dominant in the case of lambda = 9 or higher. These two energies are balanced in the case of lambda = 6. Positive correlation between the number of water clusters and the diffusion coefficients in the case of lambda = 12.

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.

Molecular dynamics simulations were carried out to elucidate the effect of the wettability of the carbon support used for Nafion ionomer thin films on proton transport in the ionomer, which is related to the power density of polymer electrolyte fuel cells. The Lennard-Jones wall model was used as the support model for the ionomer to generate two different hydrophobic walls: the high hydrophobic wall (H-wall) and the low hydrophobic wall (L-wall). The proton transport model, including the Grotthuss mechanism, was used to express real proton transport phenomena (In early work, we confirmed that it well reproduces the experimentally measured proton self-diffusion coefficient in a Nafion membrane.) The obtained proton self-diffusion coefficient (DH+) indicated that the DH+ for the H-wall case is larger than for the L-wall case. This is related to the morphology of the films. For the H-wall case, the sulfonic groups that form part of the hydrophilic Nafion side chains were confirmed to be oriented in the direction to the wall in the upper side of the film and opposite from the wall in the lower side of the film, which can lead to the alignment of Nafion molecules and also create lamellar water structures in the film. It was also confirmed that such water structures have better cluster connectivity and larger cluster size, meaning that they serve as better proton transport pathways.

In order to estimate the proton transport property of the degraded Nafion membrane, molecular dynamics simulations have been carried out. The degraded Nafion molecules are made by breaking side chains randomly. Degradation level is controlled by changing the number of broken side chains. In the simulation systems, there were four ten-unit degraded Nafion molecules and equimolar fragments generated by degradation in various conditions, such as different water contents and degradation levels. Density, diffusion coefficient, and number of water cluster have been evaluated. Our findings are, in the degraded Nafion membranes, water molecules and hydronium ions are more dispersed which suppresses Grotthus mechanism, and the system becomes much fluidity owing to the existence of various fragments which facilitates Vehicle mechanism.

The effects of water cluster structures on proton transport properties have been studied using reactive molecular dynamics simulations. The proton diffusivity and proton distributions have been evaluated in the two hydrophilic cluster structures (i.e., the cylinder model and the lamellar model) that are the most typical proposed morphological models in polymer electrolyte membranes. The proton transport with the Grotthuss mechanism has been incorporated using the modified two-state empirical valence bond (aTS-EVB) model. The diffusion coefficients in each dimension are correlated with the cluster size as well as the type of cluster models. It is found that the proton diffusion coefficients strongly depend on the cluster model and its size. The proton distributions are also investigated to elucidate the proton pathway in each cluster model. Our simulation results provide insight into quantitative information about the water cluster structure dependence of the proton transport properties at an atomic level.

We investigated the scattering phenomena of oxygen molecules on an ionomer surface, which plays a crucial role in oxygen transport in catalyst layers, using molecular dynamics simulations. The probability density functions (PDFs) of the translational energy and scattering angle of scattered oxygen molecules were analyzed for a wide range of incident condition and water contents. The translational energy distribution depends on the incident energy, but is insensitive to the incident angle and the water content. The scattering angle distribution is independent of the incident condition and the water content. Two types of collision behaviors, i.e., single and multiple collisions, were observed in the trajectory calculations. The fraction of multiple collisions and the residence time of oxygen molecules on ionomer surface were also investigated. These results provide an insight into the atomic level dynamics of oxygen scattering on ionomer surfaces.

Molecular dynamic simulations were carried out to investigate the effects of wettability of carbon supports and the thickness of thin films on proton transport. The simplified Lennard-Jones (LJ) wall model was used as the support model, which allows us to reproduce different hydrophobic walls by just tuning the LJ parameters. The thickness dependence was investigated in the observed thickness range in a catalyst layer: 4 and 10 nm. The proton transport property was evaluated in terms of selfdiffusion coefficients (DH+) and water cluster structures formed in the thin films. The obtained DH+ indicated that the DH+ increases with increasing hydrophobicity of the wall. It is also indicated that, for the high hydrophobic wall case, the DH+ decreases with increasing the thickness of thin films, while, for the low hydrophobic wall case, the DH+ is almost the same. It was confirmed that these trends were associated with the water structures formed in the thin films, i.e., water-layer-like structures are formed when the DH+ are larger.

We analyzed the dependence of the structure properties and the permeation properties of oxygen molecules in ionomer on water content. We constructed a system in which oxygen molecules permeated ionomer on a Pt surface. The ionomer/gas interface, bulk region, and ionomer/Pt interface were defined based on the density distribution of ionomer at each water content. Furthermore, the oxygen diffusivity, solubility, and permeability in each region of the ionomer were evaluated to analyze the oxygen permeation properties. As a result, the oxygen permeability in the ionomer/Pt interface is the lowest, for all water content. Moreover, comparing the contributions of the oxygen diffusivity and solubility to the oxygen permeability, it was found that the oxygen solubility plays a dominant role in the oxygen permeability, indicating that the oxygen solubility in the ionomer/Pt interface is the dominant factor in the oxygen permeability in the ionomer.

The rapid spread of micro/nanoelectromechanical systems necessitates detailed understanding of fluidics within nanoscale Structures. In this paper, the dynamics of a water droplet in nanochannels are analyzed using molecular dynamics simulations. As the channel size decreases, the shear stress between the droplet and the solid wall becomes much larger than predictions based on conventional slip boundary conditions. Our analysis shows that the Navier friction coefficient is quite sensitive to liquid pressure, which tends to be significantly large in hydrophobic nanochannels because of the Laplace pressure. We propose a modified version of the Young Laplace,equation that can accurately estimate the liquid pressure in nanochannels, By accounting for these nanochannel characteristics, we have successfully derived an expression that describes the channel size dependence of the shear stress between the droplet and the solid wall.

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. (C) 2015 AIP Publishing LLC.

We conducted a quantum chemical analysis of the deprotonation reaction of the sulfonic group (SO3H) in a model of a hydrocarbon (HC) membrane, which has been proposed as a new proton conductor for polymer electrolyte membranes (PEMs) of polymer electrolyte fuel cells (PEFCs). By comparison with perfluorosulfonic acid (PFSA) species, activation energies are higher at all hydration levels. When deprotonation occurs in the PFSA at hydration level three, the activation energy in the model HC is still higher than the thermal energy of the PEFC operation temperature and therefore it is difficult or impossible to overcome it. Moreover, at hydration level three, the deprotonated state is not stable, in contrast to PFSA, and deprotonation of SO3H and protonation of the sulfonate (SO3-) should occur with the same probability. Because the activation energy is high and the deprotonation state is unstable, it is difficult to deprotonate the SO3H of the model HC to SO3- at hydration level three. Moreover, a bond-order analysis shows that SO3- is more strongly connected to H3O+ than it is for PFSA. These appear to be the main causes of the remarkably reduced proton conductivity in HC membranes at low hydration levels. (C) 2015 Elsevier B.V. All rights reserved.

The objective in this study is the investigation of the principle of corresponding state for the density fluctuation around the critical points of non-polar diatomic fluids. In this paper, we conducted Molecular Dynamics (MD) simulation for the extraction of the fluctuation structure around the critical points of 2-Center-Lennard-Jones (2CLJ) fluids, which have anisotropy depending on the molecular elongation. As a result, in the 2CLJ fluids which have comparatively shorter molecular elongations, the principle of corresponding state can be satisfied because almost all density fluctuations in each elongation showed the similar values. On the other hand, some of the results suggested that the 2CLJ fluids which have the longer elongation decrease the density fluctuation although the further detailed investigation is necessary.

We have performed a detailed analysis of proton solvation and transport properties in hydrated Nafion using molecular dynamics simulation. The revised empirical valence bond (EVB) method was developed in order to treat the excess proton transport through the Grotthuss mechanism. 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. Our results suggest that a proton hopping mechanism has a small effect on the proton dissociation from the first solvation shell of sulfonate groups, namely that protons are not enhanced to separate from the sulfonate groups by the hopping mechanisms. From diffusion comparison between the Grotthuss and vehicular mechanism, the Grotthuss mechanism dominates the proton diffusion at the studied hydration levels including a hydration level of 3. It was also found that the vehicular mechanism dominates the electroosmotic transport of water molecules at the studied hydration levels.

In this study, the channel size dependence of the shear stress between water droplets and solid walls in nm-order channel was analyzed. We considered a several different-sized and highly hydrophobic channel whose macroscopic contact angle was about 150 degrees. We have evaluated the shear stress and the normal pressure by molecular dynamics simulation. Analyzing shear stress and normal pressure based on the macroscopic model, we have discussed the difference between the macroscopic model based on hydrodynamics and the microscopic model. As a result, in the high hydrophobic case, it became clear that the shear stress depends on the channel size due to the large Laplace pressure. Furthermore, in the case that the channel size was less than 50 angstrom, the normal pressure by the molecular simulation didn't agree with the expected value from the Young-Laplace equation. From this study it was clear that molecular simulation is needed when the channel size is less than 40 angstrom.

We analyzed the oxygen permeability through the ionomer on the Pt surface using molecular dynamics simulations. We calculated the density distributions of ionomer components and oxygen, the number of permeated oxygen through the ionomer, and the oxygen solubility. As a result, we have found that the oxygen permeability decreases as water content increases. In the ionomer/gas interface and the bulk region, the oxygen solubility decreases. Beside, compared with the structure properties of bulk membranes, the oxygen diffusivity is considered to increase as water content increases due to the swelling of the regions. In the ionomer/Pt interface, the oxygen solubility decreases with increasing water content, besides it is considered that the oxygen diffusivity decreases with increasing water content, because the solvent fills the pores which are pathways of oxygen.

Proton transport properties in Nafion thin films have been investigated using molecular dynamics simulations to characterize the nanoscopic flow phenomena in the electrode of polymer electrolyte fuel cell. Protons transfer in the ionomer by a combination of the Vehicle mechanism and the Grotthuss mechanism. In this study, the Grotthuss mechanism is described using the anharmonic two-state empirical valence bond method to evaluate quantitative proton transport within the framework of classical molecular dynamics simulations. The electrode consists of platinum-supported carbon and ionomers, and it is well known that the wettability of supported carbon depends on materials and operating environment. Our results suggested that the wettability affects morphology of the ionomer and proton transport.

Molecular dynamics simulations have been performed to study the effects of water cluster structure on proton transport (PT) properties. The systems were built to closely approximate the proposed hydrophilic cluster structures (e.g. the cylinder model and the lamellar model) that are the most typical morphological models of Nafion. The PT properties were estimated in terms of diffusion coefficient and proton pathway using a modified twostate empirical valence bond (EVB) model. The diffusion coefficients in each dimension were calculated and correlated with the cluster size as well as the type of cluster models. It was found that proton diffusion is strongly affected by the cluster model and its size, which can be associated with the proton pathway in each cluster model. The influence of the sulfonate groups on the PT properties were also investigated. Our simulation results provide insight into quantitative information about PT properties in atomic level.

We have performed a numerical analysis of the effect of ferrous ion contamination on proton transport property and nanostructure of solvent molecules in hydrated Nafion using molecular dynamics simulation. In the case of hydration level 3, our results suggest that ferrous ion contamination has an influence of either enhancing or inhibiting of proton transport depending on the amount of contaminated ferrous ions. The ferrous ion has two coordinate sulfonate groups. Structural changes of Nafion membrane were observed with increasing contaminated ferrous ions. In the case of hydration level 9, the correlation between the trend of diffusion coefficient of hydronium ions and the results of structural analysis was not observed. In the case of large amount contaminated condition, the number of coordinated ferrous ions around sulfonate groups was saturated and the majority of contaminated ferrous ions does not exist in first solvation shell.

Wurzite aluminum nitride is prepared on a c-plane sapphire substrate by electron cyclotron resonance plasma-enhanced sputtering deposition (ECR sputtering). Atomic force microscopy (AFM) showed flat AlN thin-film surfaces, and X-ray diffraction (XRD) analysis verified the epitaxial growth of AlN films with the full-width at half-maximum (FWHM) of the rocking curve of 0.025 deg on the film with the thickness of 100 nm. XRD analysis also verified the change in the peak position for the AlN film along both out-of-plane and in-plane directions. The effect of lattice constants on the energy gap was theoretically estimated by the first principles method. (C) 2014 The Japan Society of Applied Physics

The microstructure of a TGP-H-120 Toray paper gas diffusion layer (GDL) was investigated using high resolution X-ray computed tomography (CT) technique, with a resolution of 1.8 gm and a field of view (FOV) of similar to 1.8 x 1.8 mm. The images obtained from the tomography scans were further post processed, and image thresholding and binarization methodologies are presented. The validity of Otsu's thresholding method was examined. Detailed information on bulk porosity and porosity distribution of the GDL at various Polytetrafluoroethylene (PTFE) treatments and uniform/non-uniform compression pressures was provided. A sample holder was designed to investigate the effects of non-uniform compression pressure, which enabled regulating compression pressure between 0, and 3 MPa at a gas channel/current collector rib configuration. The results show the heterogeneous and anisotropic microstructure of the GDL, non-uniform distribution of PTFE, and significant microstructural change under uniform/nonuniform compression. These findings provide useful inputs for numerical models to include the effects of microstructural changes in the study of transport phenomena within the GDL and to increase the accuracy and predictability of cell performance. (C) 2014 Elsevier B.V. All rights reserved.

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. (C) 2014 AIP Publishing LLC.

Ease of deprotonation what is called acidity about sulfonic group of model molecules of perfluorosulfonic acid (PFSA) membrane and hydrocarbon (HC) membrane was focused on in this study. The deprotonation reaction of sulfonic group by water molecules were analyzed by density functional theory in low hydration levels. Target hydration levels were one to three. Calculation level was B3LYP/6-311+G(d,p). As a result, sulfonic group of model molecules of PFSA is deprotonated in hydration level three via low activation energy. In addition, deprotonated state (SO&lt inf&gt 3&lt /inf&gt &lt sup&gt -&lt /sup&gt group, H&lt inf&gt 3&lt /inf&gt O&lt sup&gt +&lt /sup&gt and two water molecules) is more stable than the protonated state (sulfonic group and three water molecules). On the other hand, deprotonated state of model molecule of HC in hydration level three is unstable. In addition, activation energy is relatively high. Those properties indicate sulfonic group of HC membrane is relatively hard to deprotonate in comparison with PFSA membrane in the same hydration level.

Transport phenomena within gas diffusion layers (GDLs) of of proton exchange membrane fuel cells (PEMFCs) are crucially important for an optimum PEM fuel cell performance. The microstructure properties of the GDLs, such as porosity, tortuosity and pore size distribution govern the transport of fluid, heat and charge. Additionally, PEM fuel cells are assembled under high compression pressure to minimize the contact resistance between components. The compression significantly alters the microstructure of the GDLs, and affects the cell performance substantially by blocking the pathways of liquid and gas species to and from the catalyst layer reaction sites. This study gives detailed understanding on the effect of compression on a GDL microstructure using high resolution Xray Computed Tomography. Porosity distributions were shown when the GDL is compressed. A significant GDL penetration into the gas channel was observed at high compression pressures.

We have performed a detailed analysis of proton solvation and transport properties in hydrated Nafion using molecular dynamics simulation. The revised EVB model predicts a significantly enhanced transport in comparison with the classical hydronium models, which largely improves the agreement with the available experimental data. The excess proton was found to have weaker interaction with the sulfonate groups in comparison with the classical hydronium ion, reflecting the delocalization nature of the proton charge defects in the EVB model. Our results suggest that the hopping transport has a large contribution to the proton diffusion even at lower hydration levels, as low as 3. It was also found that the Grotthuss component becomes more important and its contribution increases linearly as the hydration level increases.

In this paper, the quantum effect of hydrogen molecule on its diffusivity is analyzed using Molecular Dynamics (MD) method. The path integral centroid MD (CMD) method is applied for the reproduction method of time evolution of the molecules. The diffusion coefficient of liquid hydrogen is calculated using the Green-Kubo method. The simulation is performed at wide temperature region and the temperature dependence of the quantum effect of hydrogen molecule is addressed. The calculation results are compared with those of classical MD results. As a result, it is confirmed that the diffusivity of hydrogen molecule is changed depending on temperature by the quantum effect. It is clarified that this result can be explained that the dominant factor by quantum effect on the diffusivity of hydrogen changes from the swollening the potential to the shallowing the potential well around 30 K. Moreover, it is found that this tendency is related to the temperature dependency of the ratio of the quantum kinetic energy and classical kinetic energy.

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.

We performed a numerical analysis of the influences of contaminated metal ion on the dynamic properties of proton in a polymer electrolyte membrane (PEM). It was revealed that the nanostructure of the PEM was changed greatly with increasing the concentration of ferric ions. The diffusion coefficient of hydronium ions increases larger at higher water contents with increasing the concentration of ferric ions because of the lower concentration of cations. At higher water contents, the hydronium diffusion coefficient increases largely because ferric ions reduce the electrostatic interactions between hydronium ions and sulfonate groups. At lower water contents, by contrast, the structures between ferric ions and sulfonate groups hinder the hydronium motions resulting in a small increase in the hydronium diffusion coefficient.

A reaction analysis for deprotonation of the sulfonic group in a model molecule of perfluorosulfonic acid (PFSA) at low hydration levels was performed. PFSA is usually adopted as a polymer electrolyte membrane in polymer electrolyte fuel cells. In hydration level three, the deprotonation reaction certainly occurs. The deprotonated state produced is more stable than the predeprotonated state by 3.72 kcal/mol. In addition, its activation energy is very low. Although quantitative discussion of this activation energy is difficult considering the computational error, it can be said qualitatively that H+ is abstracted smoothly from the sulfonic group because of a low activation energy. From the results of bond-order analysis, the produced H3O+ is strongly bound by the SO3- group. Thus, diffusivity of H3O+ would be low. In hydration level four or more, we found a possibility that the diffusivity of H3O+ increases because the hydrogen-bond strength between H3O+ and SO3- is lower or SO3- cannot bind H3O+ directly by forming an Eigen cation.

We analyzed oxygen permeability through the two different types of thin ionomer films, composed of Nafion and hydrocarbon (HC), on Pt surface. From the results of the number of permeated oxygen, density distributions of ionomer components and oxygen, and solubility distributions of oxygen, we have found that the number of permeated oxygen molecules decreases in both ionomers as water content increases. In the ionomer-gas interface and the bulk region of both ionomers, the oxygen solubility decreases, and solvent block the diffusion of oxygen with increasing water content. In the ionomer-Pt interface of the Nafion-based ionomer, the oxygen solubility also decreases, and the voids in the ionomer, which can be the pathways for oxygen, are occupied by solvent at higher water contents. On the other hand, in the interface of the HC-based ionomer, solvent molecules are piled up on the surface of high-density polymers, and block the permeation of oxygen.

In this paper, we have analysed an effect of quantum nature of the hydrogen molecule on its thermodynamic and transport properties using molecular dynamics (MD) method based on the path integral method. We performed NVE constant MD simulation and the quantum effect on the molecular mechanism was analysed. The simulation results were compared with experimental data. As a result, we clarified that the quantum nature makes the virial pressure larger than in classical mechanics and taking account the quantum nature makes smaller intermolecular interaction energy and larger repulsive force than classical representation. Besides, we have confirmed that the path-integral-based MD method well reproduces the thermal conductivity and quantum effect on the transport properties is also large.

We derive the equation of motion for non-Markovian dissipative particle dynamics (NMDPD) by introducing the history effects on the time evolution of the system. Our formulation is based on the generalized Langevin equation, which describes the motions of the centers of mass of clusters comprising microscopic particles. The mean, friction, and fluctuating forces in the NMDPD model are directly constructed from an underlying molecular dynamics (MD) system without any scaling procedure. For the validation of our formulation, we construct NMDPD models from high-density Lennard-Jones systems, in which the typical time scales of the coarse-grained particle motions and the fluctuating forces are not fully separable. The NMDPD models reproduce the temperatures, diffusion coefficients, and viscosities of the corresponding MD systems more accurately than the dissipative particle dynamics models based on a Markovian approximation. Our results suggest that the NMDPD method is a promising alternative for simulating mesoscale flows where a Markovian approximation is not valid.

Nafion/poly(vinylphosphonic acid) blends were synthesized and characterized in this work. Poly(vinylphosphonic acid), PVPA, was synthesized by the free-radical polymerization of vinylphosphonic acid. Then Nafion/PVPA blend membranes were prepared by means of film casting from Nafion/PVPA solutions with several molar ratios of PVPA repeat unit to - SO3H. Homogeneous Nafion/PVPA films were produced. Nafion-PVPA interactions were studied by Fourier transform infrared (FT-IR) spectroscopy. Thermal properties were investigated via thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The TGA results illustrated that all of these Nafion/PVPA electrolytes are thermally stable up to 400 degrees C. The membrane properties were further characterized by studying their morphologies using scanning electron microscopy (SEM). The proton conductivity of the Nafion/P(VPA)(3) blend membrane was 1.1x10(-5) S/cm in an anhydrous state at 130 degrees C. The conductivities of the blends increased by at least three orders of magnitude upon hydration, exceeding 10(-2) S/cm with RH=50 % at ambient temperature.

The lubrication phenomenon occurring by shearing a nanoscale liquid bridge was simulated using the molecular dynamics method by varying the width of the liquid bridge, and the momentum transport phenomena of the liquid bridge were analyzed. The Fennell method was used to calculate the coulombic interaction and the Lees-Edwards method was used to maintain the velocity gradient in the liquid bridge. First, to estimate the overall viscosity coefficient of the liquid bridge, the width and interfacial region of the liquid bridge were determined. The overall viscosity coefficient was then modeled by considering two contributions from the bulk and interfacial region and the momentum fluxes or viscosity coefficients in the bulk and interfacial region were obtained. The model approximately expresses the simulation results, and the viscosity of the interfacial region was determined to be between one fourth and one third of that of the bulk. In addition, the partial momentum fluxes were calculated to verify the validity of the proposed model. (C) 2012 Elsevier Ltd. All rights reserved.

In this study, we focused on the chemical feature about sulfonic group of perfluorosulfonic acid (PFSA) by first principle calculation in low hydration levels. Our study could elucidate two facts about the chemical property of PFSA, the one is why deprotonation reaction of sulfonic group of PFSA is not occurred in hydration level one and two and the other is notable feature of deprotonation reaction in hydration level three by B3LYP/6-311+G(d,p) level calculation. In the hydration level three, activation energy of deprotonation reaction of sulfonic group was 0.70 kcal/mol. Although this activation energy includes computational error, it is considered that this deprotonation reaction would proceed in the operation temperature of polymer electrolyte fuel cell smoothly. And total energy of product was lower than the reactant by 3.72 kcal/mol. From these results, deprotonation reaction of sulfonic group in hydration level three would proceed to product efficiently. In addition, reverse reaction is hard to occur by the thermal energy of 350 K which is operation temperature of polymer electrolyte fuel cell. © The Electrochemical Society.

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 hythonium 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 2 = 1 up to A = 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.

We have performed an atomistic analysis of the vehicular transport of hydronium ions and water molecules in the nanostructure of hydrated Nafion membrane using classical molecular dynamics simulations with a model of Nafion membrane based on DREIDING force field, which is newly modified and validated by comparing the density, water diffusivity, and morphology of the membrane with those obtained experimentally. In addition to determination of radial distribution function of solvent molecules vicinity of sulfonate groups as a function of hydration level, we have also calculated mean residence time of solvent molecules in the solvation shells which were classified into three types, overlapped shell, single shell, and second solvation shell. The mean residence time of solvent molecules explicitly showed different behaviors in each region, and they provided insights into the correlation between the nanoscopic structure of ion clusters and the dynamics of hydronium ions and water molecules in the membrane. © 2013 The Japan Society of Mechanical Engineers.

In this paper, we conducted analysis of p-V-T relation of cryogenic hydrogen using classical Molecular Dynamics (MD) and path integral Centroid MD (CMD) method to understand an effect of quantum nature of hydrogen molecules. We performed NVE constant MD simulation across a wide density-temperature region to obtain an Equation Of State (EOS). Simulation results were compared with experimental data. As a result, it was confirmed that classical MD cannot reproduce the experimental data at the high density region. On the other hand, CMD well reproduces the thermodynamic properties of liquid hydrogen. Moreover, it was clarified that taking the quantum effect into account makes repulsion force larger and the potential well smaller. Because of this mechanism, the intermolecular interaction of hydrogen diminishes and the virial pressure increases. ©2013 The Japan Society of Mechanical Engineers.

Polymer electrolyte fuel cell (PEFC) is focused worldwide as the energy conversion device for next generation. However, it has never been popular because of high price and low efficiency. In cathode catalyst layer of polymer electrolyte fuel cell, an ionomer with which the catalyst is covered is very important on the point of transferring protons to the catalytic surface on the cathode side. On the other hand, it is said that ionomer interferes with oxygen permeation to the catalytic surface. The mechanism of oxygen permeation through the ionomer was not analyzed in detail because it is too small to research by experiment. In this research, we constructed the system including polymer electrolyte membrane, water molecule, oxonium ion and platinum surface by using molecular dynamics study, and researched about the effect of the water content of the ionomer on the structure of the ionomer and oxygen permeability.

Liquid Hydrogen plays an important role in hydrogen energy society. Therefore it is important to understand its thermal and transport properties accurately. However cryogenic hydrogen has unusual thermodynamic properties because of its quantum nature. The thermal de Broglie wavelength of cryogenic hydrogen molecule becomes the same order as molecular diameter. Therefore, each molecular position and its momentum cannot be defined classically. Because of this nature, hydrogen molecules show higher diffusivity than classical counterpart. Until now, the effects of quantum nature of hydrogen and its mechanism on the thermodynamic properties have not been clarified in detail. Especially, how the quantum nature would effect on the energy transfer in molecular scale has not been clarified. An accurate understanding of the effect and mechanism of quantum nature is important for hydrogen storage method and energy devices which use hydrogen as a fuel. In this study, therefore, we investigated the effect of this quantum nature and its mechanism on the thermodynamic and transport properties of cryogenic hydrogen using classical Molecular Dynamics (MD) method and quantum molecular dynamics method. We applied path integral Centroid Molecular Dynamics (CMD) method for the analysis. First, we have conducted thermodynamic estimation of cryogenic hydrogen using the MD methods. This simulation was performed across a wide density-temperature range. Using these results, equations of state (EOS) were obtained by Kataoka's method, and these were compared with experimental data according to the principle of corresponding states. As a result, it was confirmed that both quantitative and qualitative effect of the quantum nature on the thermodynamic properties of hydrogen are large. It was also found that taking account the quantum nature makes larger virial pressure and weaker intermolecular interaction energy. Second, we have calculated the diffusion coefficient of liquid hydrogen to clarify the effect of the quantum nature on the transport properties'. We used Green-Kubo form for the calculation using velocity autocorrelation function. The simulation was performed across a wide temperature range. CMD simulation results were compared with classical simulation results and experimental data. We clarified the effect of quantum nature on the transport properties of liquid hydrogen.

We have performed an atomistic analysis of transport phenomena of hydronium ions and water molecules in the hydrated Nafion membrane. The new empirical valence bond (EVB) model is developed based on the previous study of EVB model reported by Walbran et al. in order to improve the description of proton mobility in both aqueous and Nafion environments. We have calculated mean square displacement (MSD) to determine diffusion coefficients of hydronium ions and water molecules as a function of hydration level for the investigation of the impact of Grotthuss mechanism on the transport property of solvent molecules as well as the validation of our simulation by comparing with the experimental data of diffusion coefficients. A large contribution of Grotthuss mechanism for the diffusion of hydronium ions has been found at high water contents, testifying the important impact of Grotthuss mechanism in the membrane as well as in the bulk aqueous solutions.

In this paper, we conducted analysis of p-V-T relation of cryogenic hydrogen using classical Molecular Dynamics (MD) and path integral Centroid MD (CMD) method to understand an effect of quantum nature of hydrogen molecules. We performed NVE constant MD simulation across a wide density-temperature region to obtain an Equation Of State (EOS). Simulation results were compared with experimental data. As a result, it was confirmed that classical MD cannot reproduce the experimental data at the high density region. On the other hand, CMD well reproduces the thermodynamic properties of liquid hydrogen. Moreover, it was clarified that taking the quantum effect into account makes repulsion force larger and the potential well smaller. Because of this mechanism, the intermolecular interaction of hydrogen diminishes and the virial pressure increases.

In this study, the channel size dependence of transport properties of a water droplet was clarified by molecular dynamics simulations. Considering several models of a micro channel, we evaluated forces between the water droplet and the solid wall. From these results, we clarified that the friction forces depend on the width of the channel and the size of the droplet. Moreover, the effect of the liquid-vapor interface was considered.

Computer simulation is a very powerful tool to analyze transport phenomena in the membrane electrode assembly (MEA) of polymer electrolyte fuel cells (PEFCs). In particular, there are many nanoscale structures in this flow field, and therefore the phenomena should be analyzed from the microscopic point of view rather than computational fluid dynamics. In this paper, we report large-scale molecular dynamics (MD) simulations to analyze these flows. In particular, dissociation phenomena of a hydrogen molecule on a Pt catalyst, transport phenomena of proton and water in a polymer electrolyte membrane (PEM), oxygen permeability of ionomers in a catalyst layer (CL), and transport phenomena of a water droplet in a nanopore were simulated, and their characteristics are discussed. In the analysis of the dissociation phenomena of the hydrogen molecule, it was found that the trend of dissociation probability as a function of impinging energy considering the motion of the molecule differs from that without considering the motion of the molecule. In the analysis of proton transfer in a PEM, the diffusion coefficients obtained by this simulation were consistent with the experimental data. In the analysis of oxygen permeability of ionomers, the dependence of water content on the permeability was estimated and the difference between ionomer on catalyst layer and that in bulk state was clarified. In the analysis of transport phenomena of a water droplet in a nanopore, we compared the results of our simulation with the macroscopic governing equation.

The dissociation phenomena of H-2 molecule on Pt(111) surface was simulated by Molecular Dynamics (MD) method and the effect of motion of the gas molecule or surface atoms on dissociation phenomena was analyzed in detail. The Embedded Atom Method (EAM) was used to model the interaction between an H-2 molecule and Pt(111) surface. Using this potential, simulations of an H-2 molecule impinging on a Pt(111) surface were performed and the characteristics of the collision were observed. Using MD data the dynamic dissociation probability were obtained and compared with the static dissociation probability to analyze the effect of atomic motion on dissociation phenomena.

Polymer electrolyte membrane fuel cell (PEFC) is focused worldwide as the energy conversion device of next generation. In the PEFC cathode catalyst layer, an ionomer with which the catalyst is covered is very important on the point of transferring protons to the catalytic surface on the cathode side. On the other hand, it is said that an ionomer interferes with oxygen permeation to the catalytic surface. The mechanism of oxygen permeation through an ionomer was not analyzed in detail because it is too small to research by experiment. Moreover molecular dynamics simulation of the catalyst layer and oxygen permeability has not yet studied. In this research, we constructed the system including nafion, water, oxonium ion, platinum layers by using molecular dynamics study, and studied about the effect of the water content of the ionomer on the structure of the ionomer and permeability of the oxygen molecule. As the results, a lot of oxygen molecules permeated through a dried ionomer and reached to the catalytic surface but there were few oxygen molecules that permeated through a hydrated ionomer and reached there. In addition, it is found that the shape of the ionomer in the case of water content rate gamma = 3, 7, 11 changed.

These days Polymer Electrolyte Fuel Cell (PEFC) is the most developed fuel cell. A polymer electrolyte membrane (PEM) is used in PEFC. Its efficiency is proportional to the proton transferring efficiency, which depends on the nanoscale structure of water. In this study, the property of proton transfer was analyzed by Molecular Dynamics (MD) method including Grotthus mechanism by Empirical Valence Bond (EVB) method. Nafion membrane was adopted as PEM. The potential energy barrier of proton hopping obtained by EVB method was adjusted to reproduce the energy barrier obtained by Density Functional Theory (DFT). In MD simulation, the distribution of water in Nation was firstly analyzed. The results showed that liquid molecules gather around sulfo groups. Next, the property of proton transfer was analyzed by Mean Square Displacement (MSD).

Gas flow in porous media occurs in various engineering devices such as catalytic converters and fuel cells. In order to improve the performance of such devices, it is important to understand transport phenomena in porous media. In porous media with pores as small as a molecular mean free path, molecular motions need to be directly considered instead of treating gas flow as a continuum, and effects of complicated channels need to be taken into account. Therefore, such gas flow was analyzed by using the direct simulation Monte Carlo (DSMC) method, which is the stochastic solution of the Boltzmann equation. Numerical simulations of gas flow driven by pressure gradient without surface reaction were performed to clarify transport phenomena in porous media imitated by arranging nanoscale solid particles randomly. The effects of pressure gradient, diameter of particles and porosity on gas flow rates and permeability of porous media were investigated.

In this paper, we estimated the thermodynamic and transport properties of cryogenic hydrogen using classical molecular simulation to clarify the limit of classical method on the estimation of those properties of cryogenic hydrogen. Three empirical potentials, the Lennard-Jones (LJ) potential, two-center Lennard-Jones (2CLJ) potential, and modified Buckingham (exp-6) potential, and an ab initio potential model derived by the molecular orbital (MO) calculation were applied. Molecular dynamics (MD) simulations were performed across a wide density-temperature range. Using these data, the equation of state (EOS) was obtained by Kataoka's method, and these were compared with NIST (National Institute of Standards and Technology) data according to the principle of corresponding states. Moreover, we investigated transport coefficients (viscosity coefficient, diffusion coefficient and thermal conductivity) using time correlation function. As a result, it was confirmed that the potential model has a large effect on the estimated thermodynamic and transport properties of cryogenic hydrogen. On the other hand, from the viewpoint of the principle of corresponding states, we obtained the same results from the empirical potential models as from the ab initio potential, showing that the potential model has only a small effect on the reduced EOS: the classical MD results could not reproduce the NIST data in the high-density region. This difference is thought to arise from the quantum effect in actual liquid hydrogen.

Polymer electrolyte membrane fuel cell (PEFC) is focused worldwide as the energy conversion device of next generation. In the PEFC cathode catalyst layer, an ionomer with which the catalyst is covered is very important on the point of transferring protons to the catalytic surface on the cathode side. On the other hand, it is said that an ionomer interferes with oxygen permeation to the catalytic surface. The mechanism of oxygen permeation through an ionomer was not analyzed in detail because it is too small to research by experiment. Moreover molecular dynamics simulation of the catalyst layer and oxygen permeability has not yet studied. In this research, we constructed the system including nafion, water, oxonium ion, platinum layers by using molecular dynamics study, and studied about the effect of the water content of the ionomer on the structure of the ionomer and permeability of the oxygen molecule. As the results, a lot of oxygen molecules permeated through a dried ionomer and reached to the catalytic surface but there were few oxygen molecules that permeated through a hydrated ionomer and reached there. In addition, it is found that the shape of the ionomer in the case of water content rate gamma = 3, 7, 11 changed.

This paper describes characteristics of proton transfer in polymer electrolyte membrane by Molecular Dynamics (MD) simulation. Nafion was used as a membrane. Grotthus mechanism as well as Vehicle mechanism was considered in the simulation. To treat Grotthus mechanism, Empirical Valence Bond (EVB) method was used The parameters or functions of the interaction potential of EVB method were determined so that potential energy barrier of proton hopping obtained by EVB method is consistent with that obtained by Density Functional Theory (DFT) and adjusted so that the diffusion coefficient of hydronium ion in water obtained by MD simulation is consistent with that of experimental results. After annealing the system, radial distribution function or mean square displacements of hydronium ion and water, which correspond to diffusivity of each compound, was obtained These results show the nanoscale characteristics of proton transfer in polymer electrolyte membrane.

In this study, we investigated the limits of classical molecular simulation on the estimation of thermodynamic properties of cryogenic hydrogen. Three empirical potentials, the Lennard-Jones (LJ) potential, two-centre LJ (2CLJ) potential, and modified Buckingham (exp-6) potential and an ab initio potential model derived by the molecular orbital calculation were applied. Molecular dynamics (MD) simulations were performed across a wide density-temperature range. Using these data, the equation of state (EOS) was obtained by Kataoka's method, and they were compared with National Institute of Standards and Technology (NIST) data using the principle of corresponding states. As a result, it was confirmed that the potential model has a large effect on the estimated thermodynamic properties of cryogenic hydrogen. On the other hand, from the viewpoint of the principle of corresponding states, we obtained the same results from the empirical potential models as from the ab initio potential showing that the potential model has only a small effect on the reduced EOS: the classical MD results could not reproduce the NIST data in the high-density region. This difference is thought to arise from the quantum effect in actual liquid hydrogen.

In this study, the transport property of a water droplet in the nano hydrophobic pore was clarified by molecular dynamics simulation. The size dependence of the transport property of the water droplet in a nano slit pore was focused. Considering several models of the nano pore, the differences of the structures of water droplets were shown and these differences were discussed from the viewpoint of the interaction between hydrophobic walls. Furthermore, forces by the hydrophobic walls were evaluated. From these results, the dependence of the width of the pore and the size of the droplet were clarified. Moreover, the effect of the liquid-vapor interface was considered. © The Electrochemical Society.

We have proposed a model of Nafion membrane based on DREIDING force field validated by comparing the density, water diffusivity, and Nafion morphology with experimental data. The simulated final density agrees with experiment within 1.3 % for various water contents. In addition to determination of diffusion coefficients of liquid molecules as a function of hydration level for dynamical validation, we have also calculated static structure factors among liquid molecules for mesoscopic structural validation. The diffusion coefficient of water molecules 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 although the classical models have the limitation of hydronium dynamics. The static structure factors of liquid molecules at low wave length provide insights into the periodic structure of the inter-clusters which are consistent with the experimental data based on small-angle X-ray and neutron scattering. © The Electrochemical Society.

To remove liquid water from a gas diffusion layer (GDL) effectively and minimize an increase of transport resistance, hydrophobic treatment with a polytetrafluoroethylene (PTFE) is widely used. Because the GDL micro-structure has strong impact on transport properties, detailed understanding of fiber and porous network and PTFE distribution is strongly requested. To investigate correlation between the GDL micro-structure, PTFE treatment, and transport properties, micro-structure of the GDL was visualized by high-resolution X-ray computed tomography (X-ray CT). Newly designed sample holder allowed to regulate compression pressure. Visualization results with spatial resolution of 1.0 μm made it possible to distinguish carbon fibers, binder material, PTFE, and voids (air), and non-uniform distribution of PTFE was observed. Because non-uniform porosity and contact angle distribution in the direction of thickness change the liquid water morphology within the GDL, understanding and controlling the GDL micro-structure is important and manufacturing process of GDL should be scientifically developed. © The Electrochemical Society.

In this paper, we analyzed the nanoscale transport phenomena in membrane electrode assembly (MEA) of polymer electrolyte fuel cell (PEFC) by large scale molecular dynamics (MD) simulations. Especially, (1) transport phenomena of proton and water in polymer electrolyte membrane (PEM), (2) oxygen permeability of ionomer in catalyst layer (CL), and (3) transport phenomena of water droplet in a nano pore were simulated, and the nanoscale transport characteristics were analyzed in detail. In the analysis of proton transfer in PEM, the simulation results were compared with some experimental results to evaluate the validity of our simulation. With regards to the oxygen permeability of ionomer, the dependence of water content on the permeability was estimated and the difference of characteristics between ionomer and bulk membrane was discussed. In the analysis of transport phenomena of a water droplet in a nano pore, we compared the results of our simulation with macroscopic governing equation.

Energy is commonly dissipated in molecular dynamics simulations by using a thermostat. In non-isothermal shear simulations of confined liquids, the choice of the thermostat is very delicate. We show in this paper that under certain conditions, the use of classical thermostats can lead to an erroneous description of the dynamics in the confined system. This occurs when a critical shear rate is surpassed as the thermo-viscous effects become prominent. In this high-shear-high-dissipation regime, advanced dissipation methods including a novel one are introduced and compared. The MD results show that the physical modeling of both the accommodation of the surface temperature to liquid heating and the heat conduction through the confining solids is essential. The novel method offers several advantages on existing ones including computational efficiency and easiness of application for complex systems. (C) 2011 American Institute of Physics. [doi:10.1063/1.3644938]

The dissociation probabilities of H-2 and D-2 molecules on a Pt(111) surface with thermal motion were analyzed using the molecular dynamics (MD) method. The potential constructed using the embedded atom method was used as the interaction potential between a gas molecule and the surface. The effects of changing the translational energy and incident polar angle of D2 molecules impinging on a Pt(111) surface were analyzed using MD simulations. The effect of initial orientation, incident azimuthal angle, rotational energy of gas molecules, and the impinging points on the surface were averaged by setting the initial values in a random manner. When the molecules approach normal to the surface, the dissociation probability increases with the initial translational energy. At larger incident angles, the probability becomes smaller. The impinging processes were categorized in terms of reaching the chemisorption layer by analyzing the repulsion forces from the surface. The effective translational energies for impingement, both normal and parallel to the surface, play important but different roles in terms of molecules reaching the chemisorption layer and the dissociation probability. The results were compared to those obtained by molecular beam experiments to check the validity of the simulations. The results indicate that the dependence of the dissociation probability on the translational energy and incident angle is in approximate agreement with that from experiments. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3606434]

The effects of the motion of atoms or molecules on the dissociation probability of the H-2-Pt(111) system were analyzed by molecular dynamics. The embedded atom method (EAM) was used to model the interaction between a Pt(111) surface and an H-2 molecule to consider the dependence of electron density. Initially, the EAM potential was constructed to express the characteristics of the system, such as the electron density or dissociation barrier at certain sites and orientations, as obtained by density functional theory (DFT). Using this potential, simulations of an H-2 molecule impinging on a Pt(111) surface were performed, and the characteristics of the collision were observed. These simulations were performed many times, changing the orientation of the H-2 molecule, and a dynamic dissociation probability at each site against impinging energy was obtained. On the other hand, a static dissociation probability was defined from the dissociation barrier of a hydrogen molecule obtained by the EAM potential. These results were compared to one another, and the effects of the motion of atoms or molecules, which were called dynamic effects, on the dissociation probability were analyzed. The dynamic effects on the dissociation phenomena were very large at the top site, but were small at bridge or fcc sites. (C) 2011 American Institute of Physics. [doi:10.1063/1.3554690]

Gas flow with surface reaction in porous media appears in various regions of engineering. In porous media with holes as small as a molecular mean free path, Kn of gas flow in the narrow channel is on the order of unity. Therefore, the direct simulation Monte Carlo (DSMC) method is suitable to solve transport phenomena in such kind of porous media. We perform 2D DSMC simulations of such a flow. The shape of narrow channel in porous media is complicated. To reduce complexity, we propose the simplification for porous structures by cubes and polyhedra. Results for the simplification by polyhedra agree well with the result obtained in the case without simplification. Results for the simplification by cubes also show good agreement with the result without simplification when surface reaction probability is modified.

Molecular Dynamics (MD) was used to simulate dissociative adsorption of a hydrogen molecule on the Pt(111) surface considering the movement of the surface atoms and gas molecules. The Embedded Atom Method (EAM) was applied to represent the interaction potential. The parameters of the EAM potential were determined such that the values of the dissociation barrier at different sites estimated by the EAM potential agreed with that of DFT calculation results. A number of MD simulations of gas molecules impinging on a Pt(111) surface were carried out randomly changing initial orientations, incident azimuth angles, and impinging positions on the surface with fixed initial translational energy, initial rotational energy, and incident polar angle. The number of collisions in which the gas molecule was dissociated were counted to compute the dissociation probability. The dissociation probability was analyzed and expressed by a mathematical function involving the initial conditions of the impinging molecule, namely the translational energy, rotational energy, and incident polar angle. Furthermore, the utility of the model was verified by comparing its results with raw MD simulation results of molecular beam experiments.

The bond dissociation energy of a perfluorosulfonic acid (PFSA) molecule was investigated by density functional theory (DFT) to provide some dissociation trends of the PFSA molecule and the basic information for the design of more durable PFSA membranes. As the preliminary analysis, the chemical bond strengths of the PFSA molecule were analyzed exhaustively, and the vulnerable bond was identified by comparison of the results. The same calculations were performed for an ionized PFSA molecule to determine the influence of the ionization state on the bond strength. The C-S bond was the weakest among the side chain backbone in the neutral molecule; however, the C-S bond became stronger when ionized while it weakened the C-O bond connecting the side chain with the main chain. Analysis of the C-F bonds in the side chain showed that the dissociation energy decreases in the order of primary, secondary, and tertiary bonds, as also reported in the literature. Analysis of the main chain showed that the secondary C-F bonds neighboring the tertiary C-F bond connecting the side and main chains were the weakest. Ionization of the PFSA molecule weakens the average dissociation energy of C-F bonds. As the realistic analysis, the same calculations were performed considering the solvation effects to analyze the effects on the dissociation trend. Moreover, the possibility for improving the durability of PFSA membranes was investigated by partial substitution of H atoms or CH3 groups for F atoms. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3518421] All rights reserved.

Dissociative adsorption processes of hydrogen molecules on a platinum surface were simulated by a molecular dynamics (MD) method. The effect of the incidence angle of the impinging gas molecules on the dissociation probability was analyzed. The embedded atom method (EAM) was used to construct an interaction potential between the gas molecule and the Pt(111) surface. Initially, the effect of the motions of atoms or molecules (dynamic effects) on the dissociation probability was studied. It showed that dynamic effects on the dissociation phenomena are very large for adsorption at the top site. A number of MD simulations of H-2 or D-2 molecules impinging on a Pt(111) surface were also carried out, with varying initial orientations and rotational energy of the molecule, and differing collision locations on the surface. The results were averaged and the effects of the isotopic nature, incident angle, and translational energy of the adsorbate were analyzed. The results were also compared to those obtained from molecular beam experiments to check the validity of the simulations. This comparison showed that the dependences of the dissociation probability on translational energy and incident angle roughly agree with experiments.

Protons transfer in polyelectrolyte membrane by both Vehicle and Grotthus mechanisms. This study describes the property of proton and water transfer in perfluorosulfonic acid (PFSA) membrane using molecular dynamics (MD) simulation in view of both Vehicle and Grotthus mechanisms. To treat Grotthus mechanism, Empirical Valence Bond (EVB) method was introduced to MD simulation. The potential energy barrier of proton hopping in EVB method was adjusted to the computational result of Density Functional Theory (DFT) to replicate the experimental value of diffusion coefficient of oxonium ions in water. The transport property of water and oxonium ions in PFSA membrane was analyzed by such as Radial Distribution Function (RDF) and Mean Square Displacement (MSD) at different water contents. From this analysis, some perception of the structure of water cluster and the diffusivity of water and proton in PFSA membrane was derived.

The aim of this study is refinement of proton(s) hopping rate by applying new model for Molecular Dynamics (MD) calculation with Empirical Valence Bond (EVB) method. The overview about this new model is an acceptor selection method based on energy prediction instead of the one based on distance. The effect of water molecules outside the Eigen cation was mainly considered in this new model. Then, microenvironment interaction to the formation of the cluster and proton hopping were reasonably expressed as an actual condition. This procedure was applied for proton hopping. The total number of cluster formation and cancel were also analyzed.

In polymer electrolyte membrane fuel cell (PEFC) cathode catalyst layer, PFSA ionomer with which the catalyst is covered is very important on the point of transferring protons to the catalytic surface on the cathode side. On the other hand, it is said that an ionomer interferes with oxygen permeation to the catalytic surface. The mechanism of oxygen permeation through an ionomer was not analyzed in detail because it is too small to research by experiment. Moreover molecular dynamics simulation of the catalyst layer and oxygen permeability has not yet studied. In this research, we constructed the system of ionomer on the platinum surface by using molecular dynamics study, and studied about the effect of the water content of the ionomer on the structure of the ionomer and permeability of the oxygen molecule.

In polymer electrolyte membrane fuel cell (PEFC) cathode catalyst layer, PFSA ionomer with which the catalyst is covered is very important on the point of transferring protons to the catalytic surface on the cathode side. On the other hand, it is said that an ionomer interferes with oxygen permeation to the catalytic surface. The mechanism of oxygen permeation through an ionomer was not analyzed in detail because it is too small to research by experiment. Moreover molecular dynamics simulation of the catalyst layer and oxygen permeability has not yet studied. In this research, we constructed the system of ionomer on the platinum surface by using molecular dynamics study, and studied about the effect of the water content of the ionomer on the structure of the ionomer and permeability of the oxygen molecule.

This paper describes characteristics of proton transfer in polymer electrolyte membrane by Molecular Dynamics (MD) simulation. Nafion was used as a membrane. Grotthus mechanism as well as Vehicle mechanism was considered in the simulation. To treat Grotthus mechanism, Empirical Valence Bond (EVB) method was used. The parameters or functions of the interaction potential of EVB method were determined so that potential energy barrier of proton hopping obtained by EVB method is consistent with that obtained by Density Functional Theory (DFT) and adjusted so that the diffusion coefficient of hydronium ion in water obtained by MD simulation is consistent with that of experimental results. After annealing the system, radial distribution function or mean square displacements of hydronium ion and water, which correspond to diffusivity of each compound, was obtained These results show the nanoscale characteristics of proton transfer in polymer electrolyte membrane.

Dissociation phenomena of gas molecule on a metal surface, especially, the effect of motion of a gas molecule on dissociation probability of the molecule on a metal surface was analyzed by Molecular Dynamics (MD) method. Platinum (111) surface and hydrogen were chosen as the metal surface and the gas molecule, respectively. Embedded Atom Method (EAM) was used to express the interaction potential as the functional of the electron density. Parameters or functions of the EAM potential were determined so that the characteristics of the interaction potential obtained by the EAM method were consistent with those obtained by Density Functional Theory (DFT). Dissociation probabilities at specific sites (top, brg and fcc) were obtained by MD method against impinging energy. On the other hand, the dissociation probabilities were determined from dissociation barriers of the sites and orientations of hydrogen molecule. These results were compared with each other and the effect of motion of atoms or molecules on dissociation probability was analyzed. © 2010.

In this research the momentum transport phenomena in a nanoscale liquid bridge were simulated by Molecular Dynamics method and its nanoscale characteristics were analyzed. Water was assumed as the lubricant. Momentum flux was generated in a liquid bridge by controlling the velocity of a part of the liquid bridge. Using the velocity gradient of the liquid bridge and the momentum flux, viscosity coefficients of the liquid bridge were obtained. We analyze the dependence of the width of the liquid bridge on the viscosity coefficient and the differences of the viscosity coefficients between liquid bridges and bulk water were confirmed.

This study describes the property of proton transfer in Nafion membrane analyzed by molecular dynamics (MD) simulation including both Vehicle and Grotthus mechanism. To treat Grotthus mechanism, Empirical Valence Bond(EVE) method was introduced to MD simulation. The potential energy barrier of proton hopping obtained by EVB method was adjusted to the computational result of Density Functional Theory (DFT). After adjusting EVB potential, it is confirmed that protons hop along the hydrogen bond network consecutively. The parameter for the simulation of Nafion membrane was water contents λ, which is defined as the ratio of water molecules and hydronium ions to sulfo groups, SO_<3^->, obtained by λ=N_<H2O,H3O+>/N_<SO3->. The changes of transferring properties and structure of molecules with the changes of λ were analyzed by Mean Square Displacement and Radial Distribution Function, respectively.

Molecular Dynamics (MD) was used to simulate dissociative adsorption of hydrogen molecule on Pt(1l1) surface considering the movement of the surface atoms and gas molecules. Embedded Atom Method (EAM) was applied to represent the interaction potential. A number of MD simulations of gas molecules impinging on Pt(111) surface were carried out changing initial orientations, incident azimuth angles, and impinging positions on the surface using uniform random numbers with fixed initial translational energy, initial rotational energy, and incident polar angle. The number of incidents that dissociated gas molecules was counted to compute dissociation probability. The dissociation probability was analyzed and expressed by a mathematical function of initial conditions of impinging molecule, namely translational energy, rotational energy, and incident polar angle. And the model was verified by comparing with MD simulation results of molecular beam experiments.

Gas flow with surface reaction in porous media appears in various regions of engineering. In porous media with holes as small as a molecular mean free path, i. e., Knudsen number Kn of gas flow in the narrow channel is in the order of unity. Therefore, the direct simulation Monte Carlo (DSMC) method is suitable to solve transport phenomena in porous media. We perform 2D DSMC simulations of such a flow. Porous media are modeled as an aggregation of randomly-placed spherical particles. The shape of narrow channel in porous media is complicated. To reduce complexity, we propose the simplification for porous structures, i. e., we simplify solid bodies in porous media as aggregations of cubes or as aggregations of polyhedra. We perform simulations with and without simplification. We compare results and investigate effects of the simplification.

In this research, we reported the dependence of pore size on the characteristics of transport phenomena of confined water in a nano slit pore by molecular dynamics (MD) simulation. The two surfaces were made of graphite, whose distance was set at 15, 30, and 60 A. The interaction potential between water and carbon was Lennard-Jones potential, and that between water molecules was SPC/E. The water formed a film in each system. The film moved when constant fictitious force parallel to the walls was given to water molecules in the pore. From the results, we examined dependence on width of the velocity of water in the slit pores.

In this paper, we investigated the effect of intermolecular potential model on thermodynamic properties of cryogenic hydrogen. We applied three empirical potential models and one ab-initio potential which was derived by Molecular Orbital (MO) calculation. We performed NVE constant Molecular Dynamics (MD) calculation to obtain Equation Of State (EOS). Calculation results were compared with NIST data using the principle of corresponding date. As a result, it was confirmed that every potential showed the same tendency and cannot reproduce NIST data at the high density region.

The effect of incident angle on dissociation probability is analyzed by Molecular Dynamics (MD) method. Hydrogen is used as the gas molecule and Pt(111) is used as the surface. The embedded atom method (EAM) is used as the interaction potential between the gas molecule and the Pt(111) surface. Initially, the effect of motion of atoms or molecules (dynamic effects) on dissociation probability is discussed. It shows that dynamic effects on the dissociation phenomena are very large at top site. MD simulations of D-2 molecules impinging on a Pt(111) surface are also carried out, changing the incident angle and its effects are analyzed. The results are also compared to those obtained by the molecular beam experiments to check the validity of the simulations. Consequently the results show that the dependence of translational energy and incident angle on dissociation probability roughly agree to those of experiments.

The effect of incident angle on dissociation probability is analyzed by Molecular Dynamics (MD) method. Hydrogen is used as the gas molecule and Pt(111) is used as the surface. The embedded atom method (EAM) is used as the interaction potential between the gas molecule and the Pt(111) surface. Initially, the effect of motion of atoms or molecules (dynamic effects) on dissociation probability is discussed. It shows that dynamic effects on the dissociation phenomena are very large at top site. MD simulations of D 2 molecules impinging on a Pt(111) surface are also carried out, changing the incident angle and its effects are analyzed. The results are also compared to those obtained by the molecular beam experiments to check the validity of the simulations. Consequently the results show that the dependence of translational energy and incident angle on dissociation probability roughly agree to those of experiments. ©The Electrochemical Society.

This study describes the property of proton transfer in Nafion membrane analyzed by molecular dynamics (MD) simulation including both Vehicle and Grotthus mechanism. To treat Grotthus mechanism, Empirical Valence Bond (EVB) method was introduced to MD simulation. The potential energy barrier of proton hopping obtained by EVB method was adjusted to the computational result of Density Functional Theory (DFT). After adjusting EVB potential, it is confirmed that protons hop along the hydrogen bond network consecutively. The parameter for the simulation of Nafion membrane was water contents lambda, which is defined as the ratio of water molecules and hydronium ions to sulfo groups, SO3-, obtained by lambda =N-H2O,(H3O+)/N-SO3-. The changes of transferring properties and structure of molecules with the changes of lambda were analyzed by Mean Square Displacement and Radial Distribution Function, respectively.

Protons transfer in polyelectrolyte membrane by both Vehicle and Grotthus mechanism. Due to the Grotthus mechanism, protons transfer at higher velocity in water. The mechanism, however, hasn't been widely considered in studies about proton transfer in polyelectrolyte membrane by molecular dynamics (MD) because of its complexity, whereas Vehicle mechanism has been done. This paper describes proton transfer behavior in Nafion as a polyelectrolyte membrane by MD simulation including both Vehicle and Grotthus mechanism. To treat Grotthus mechanism, Empirical Valence Bond (EVB) method is used. The potential energy barrier of proton hopping obtained by EVB method is adjusted to the computational result of Density Functional Theory (DFT). The structure of water and Nafion molecules is analyzed by Radial Distribution Function (RDF). Diffusion coefficient of proton is analyzed by Mean Square Displacement (MSD).

Dissociation process of hydrogen molecule on platinum surface was simulated by Molecular Dynamics method. EAM potential was used as an interaction between a hydrogen molecule and a platinum surface. Firstly a dissociation probability against translational energy was obtained using the potential without considering the motion of atoms. Another dissociation probability was obtained by the Molecular Dynamics simulation and they were compared with each other to analyze the effect of the motion of atoms on the dissociation probability. The results showed that the tendency of the dissociation probability is different depending on the dissociation site, for instance, top, bridge and fee site.

In this study the correlation between thermophysical properties of low temperature hydrogen and its intermolecular potential was investigated by classical molecular dynamics method (MD). Lennard-Jones (LJ) potential, 2-Center Lennard-Jones (2CLJ) potential and modified Buckingham (exp-6) potential were applied for this calculation. MD simulations were performed at a wide density-temperature range. Equation of state (EOS) was obtained based on the MD results, and they were compared with NIST (National Institute of Standards and Technology) data. As a result, it was confirmed that the thermal properties obtained by MD results can reproduce the NIST data at gas phase, while it cannot at liquid phase and around its critical point. This distinction are considered to arisee from the many-body effect or quantum effect of real hydrogen liquid.

Durability of perfluorosulfonic acid (PFSA) membrane is an important factor to improve the lifetime of PEFC. In this study the bond dissociation energy of PFSA molecule was investigated by quantum chemistry method to provide basic information for designing more durable PFSA membrane. Nafion (R) was used as a PFSA molecule. All calculations were performed by DMol(3). Chemical bond strengths of PFSA molecule were analyzed exhaustively. The vulnerable bond was identified by comparing the results. The same calculations were performed for ionized PFSA molecule to find out the influence of the ionization state on the bond strength. Moreover, the possibility for improving durability of PFSA membrane was investigated by substituting H atoms or CH3 groups for a part of F atoms.

Dissociation phenomena of gas molecule on metal surface was analyzed by Molecular Dynamics Method. Platinum (111) surface and hydrogen were chosen to be the metal surface and the gas molecule, respectively. Embedded Atom Method was used as the interaction between the surface and the atoms in order to express the dependence of electron density. The parameters were determined so that the results such as the electron density, adsorption energy of an H atom on a Pt(111) surface and the interaction between H atoms of an H_2 molecule obtained by EAM method were consistent with that obtained by Density Functional Theory or empirical function. Collisions of a hydrogen molecule with a Platinum surface were simulated by Molecular Dynamics method and the dissociation probability was obtained. Using the results, effect of motion of the surface atoms or the hydrogen molecule on the dissociation probability was analyzed.

Dissociative adsorption phenomena of hydrogen molecule on platinum surface was simulated by the Molecular Dynamics Method. The Embedded Atom Method (EAM) was used as an interaction potential between an H_2 molecule and a Pt surface in order to reproduce the dissociation barrier which the molecule has to pass over to dissociate on the surface. The functions of the EAM were adjusted so that they can describe the orders of dissociation barriers at some dissociation sites on a Pt(111) surface calculated by the Density functional theory (DFT). Many cases of collision of an H_2 molecule with a Pt(111) surface were simulated and dissociation probability was obtained. Using the results, the dependence of dissociation probability on dissociation barrier at some dissociation sites on a Pt(111) surface was analyzed.

The dissociation phenomena of a gas molecule on a metal surface were analyzed by the molecular dynamics method. A platinum (111) surface and hydrogen were chosen as the metal surface and the gas molecule, respectively. The embedded atom method was used as the interaction between atoms in order to express the dependence of electron density. The parameters were determined so that the results such as the electron density, adsorption energy of an H atom on a Pt(111) surface, and the interaction between H atoms of an H2 molecule obtained by the EAM method were consistent with those obtained by the density functional theory or empirical function. Collisions between a hydrogen molecule and the platinum surface were simulated by the molecular dynamics method, and the dissociation probability was obtained. Using these results, the effect of the motion of the surface atoms or the hydrogen molecule on the dissociation probability was analyzed. © 2008 Wiley Periodicals, Inc.

For experimental investigations of the thermodynamic effect on a cavitating inducer it is nesessary to observe the cavitation. However, visualizations of the cavitation are not so easy in cryogenic flow. For this reason, we estimated the cavity region in liquid nitrogen based on measurements of the pressure fluctuation near the blade tip. In the present study, we focused on the length of the tip cavitation as a cavitation indicator Comparison of the tip cavity length in liquid nitrogen (80 K) with that in cold water (296 K) allowed us to estimate the strength of the thermodynamic effect. The degree of thermodynamic effect was found to increase with an increase of the cavity length. The temperature depression was estimated from the difference of the cavitation number of corresponding cavity condition (i.e., cavity length) between in liquid nitrogen and in cold water. The estimated temperature depression caused by vaporization increased rapidly when the cavity length extended over the throat. In addition, the estimated temperature inside the bubble nearly reached the temperature of the triple point when the pump performance deteriorated.

The dependence of molecular motion on the dissociative adsorption mechanism of hydrogen molecule (H-2) on platinum (Pt) surface was studied by Molecular Dynamics (MD) method. An interaction between atoms was considered by the Embedded Atom Method (EAM). A potential between an H atom and a Pt atom was determined from results of Density Functional Theory (DFT). Dissociation probabilities of three surface conditions, that is, (1) when the surface temperature is 300 K, (2) when the surface temperature is 0 K with allowing motion of the surface atoms and (3) when the surface temperature is 0 K with prohibiting motion of the surface atoms, were obtained. From results of the simulations, the effect of surface motion on dissociation probability was analyzed as a function of initial energy of the dissociating molecule or the surface conditions. First, it was concluded that the increase in the dissociation probability of the case (3) by the increase in the initial translational energy of H2 molecule is gentle compared with those of the other cases. Additionally, the minimum initial translational energy of H2 molecule of case (3) at which the H2 molecule can dissociate is the smallest among all of three cases. It was found that this is because the range of the dissociation barrier distribution for the case (3) is wider than those for the other cases due to the thermal motion of surface atoms. Moreover, the effect of translational and rotational motion of molecule on the dissociation probability was analyzed. It was concluded that the dissociation probability increases with the increase in the translational energy while it decreases with the increase in the rotational energy when the rotational energy is small.

The thermal conductivity of a diatomic liquid in a narrow channel including a nanobubble is analyzed using a nonequilibrium molecular dynamics method. Oxygen is assumed as the liquid and the two-center Lennard-Jones model is used to express the intermolecular potential acting on liquid molecules. Heat conduction of the liquid at various states (single phase, two-phase including a nanobubble and separated two-phase) at T = 0.7T(cr) is simulated and the thermal conductivity is estimated, where T is temperature and T-cr is the critical temperature. These values are shown to be consistent with experimental data within 10% accuracy. The thermal conductivity decreases with the decrease in the pressure in a single phase, whereas they are almost independent of the bubble size in two-phase including a nanobubble. Detailed analysis of the molecular contribution to the thermal conductivity reveals that the contribution of the heat flux caused by translational energy transfer to the thermal conductivity is dominant and the heat flux passing through the liquid phase increases with the decrease in the effective cross-section of liquid. (c) 2006 The Japan Society of Fluid Mechanics and Elsevier B.V. All rights reserved.

The dissociative adsorption of H_2 on Pt surface is studied by molecular dynamics (MD) simulation using Embedded Atom Method (EAM). The parameters or functions of this method are determined based on the results of Density Functional Theory (DFT). The dissociation barrier of each orientation of molecule obtained by EAM is analyzed and compared with that obtained by DFT. Many cases of collision of a H_2 molecule on Pt surface are simulated and dissociation probability is obtained. Consequently, dissociation probability changes greatly by the thermal motion of surface atoms. Moreover, the effect of translational and rotational energy of molecule on the dissociation probability is analyzed.

For experimental investigations of the thermodynamic effect on a cavitating inducer, it is neccesary to observe the cavitation. However, visualizations of the cavitation are not so easy in cryogenic flow. For this reason, we estimated the cavity region in liquid nitrogen based on measurements of the pressure fluctuation near the blade tip. The degree of the thermodynamic effect was found to increase with the increase of the cavity length of the tip cavitation. The estimated temperature depression caused by vaporization increased rapidly when the cavity length extended over the throat. In addition, the estimated temperature inside the bubble nearly reached the temperature of the triple point when the pump performance deteriorated.

The present study investigates bubble nucleation in liquid oxygen with dissolved impurities (nitrogen or helium molecules) using molecular dynamics simulations. When the mole fraction of impurities is 0.05, there is a fundamental difference in the bubble nucleation mechanism between the two dissolved impurities cases vaporization in the homogeneous bulk makes a bubble in the case of a nitrogen-dissolved liquid while phase separation of impurities and liquid molecules makes a nucleus in the case of a helium-dissolved liquid. Fluctuations can cause local voids, which in turn can grow to be bubbles, and this effect is stronger in the case of a helium-dissolved liquid with a lower mole fraction (0.01) than in the case of a nitrogen-dissolved liquid with a higher mole fraction (0.05). From these results, we conclude that helium molecules have a much stronger action to raise the bubble formation pressure compared with nitrogen. In this paper, the kinetically-defined critical nucleus, which is a very important factor in quantitatively evaluating the nucleation mechanism, is also estimated through the calculation of the size change rate of each nucleus. © 2005 Wiley Periodicals, Inc.

The adsorption phenomena of H-2 on Pt(111) surface were simulated by the combination of accelerated Quantum Molecular Dynamics (QMD) method and classical Molecular Dynamics (MD) method. About QMD method, only the valence orbitals were considered and a Hamiltonian matrix and an overlap integral matrix were approximated to the analytical function of the distance between the atoms. It was confirmed that the probability density function of electron obtained by this method was very consistent with the results of Density Functional Theory (DFT). About the binding energy of Pt bulk and the partial density of state (PDOS), the results obtained by this method were qualitatively consistent with each other. On the viewpoint of energy conservation and temperature control, it was confirmed that this method can simulate the reaction phenomena on the catalyst.

For experimental investigations of the thermodynamic effect on a cavitating inducer, it is nesessary to observe the cavitation. However, visualizations of the cavitation are not so easy in cryogenic flow. For this reason, we estimated the cavity region in liquid nitrogen based on measurements of the pressure fluctuation near the blade tip.In the present study, we focused on the length of the tip cavitation as a cavitation parameter. Comparison of the tip cavity length in liquid nitrogen (80 K) with that in cold water (296 K) allowed us to estimate the strength of the thermodynamic effect. The degree of thermodynamic effect was found to increase with an increase of the cavity length. The estimated temperature depression caused by vaporization increased rapidly when the cavity length extended over the throat. In addition, the estimated temperature inside the bubble nearly reached the temperature of the triple point when the pump performance deteriorated.

The dissociation mechanism of H_2 on Pt(111) surface is studied by molecular dynamics (MD) simulation using embedded-atom method (EAM). First, we investigate the precision of the EAM by comparing some results, such as potential energy surface (PES), of the EAM calculation with those of density functional theory (DFT) calculation. The PES by the EAM calculation is in reasonable agreement with DFT calculation. Using this EAM potential, the dissociation process of H_2 on Pt(111) surface is simulated. The dissociation probability in the case that surface atoms are fixed is higher than the probability in the case that surface atoms are moved.

With regard to the development of a liquid-fuel rocket engine, knowledge of unsteady characteristics of turbopumps is essential in attempts to increase rocket reliability. Numerical simulation is very advantageous in determining the unsteady characteristics of turbopumps, and knowledge thus obtained can contribute to decreasing the cost and the number of experiments. In the present study, the effect of compressibility of cryogenic propellants, such as liquid hydrogen (LH2) and liquid oxygen (LOX), and the effect of nonlinearity of flow on the dynamic response of a high-pressure turbopump were considered. The results of calculation were compared with a nonlinear incompressible mathematical model. The effects of dynamic characteristics of a cavitating inducer, such as cavitation compliance and mass flow gain factor, as well as the effect of pipe elasticity and that of an accumulator for the POGO suppressor, were also analyzed.

The temperature depression of liquids due to latent heat of vaporization causes vapor pressure depression and suppresses cavity growth. This phenomenon is called "thermodynamic effect of cavitation." This effect is especially significant in cryogenic fluids such as LOX and LH2. Due to this effect, the performance of hydraulic equipment for cryogenic fluids, such as turbopumps of rocket engines, is not as bad as predicted. In this paper, the size of the cavity in cryogenic fluid is estimated numerically taking the thermodynamic effect of cavitation into consideration. A cavity is assumed to be a sheet cavity. From the results, the effects of the properties of liquid and Reynolds number on the thermodynamic effect of cavitation are investigated.

The thermal conductivity of diatomic liquids was analyzed using a nonequilibrium molecular dynamics (NEMD) method. Five liquids, namely, O-2, CO, CS2, C-12 and Br-2, were assumed. The two-center Lennard-Jones (2CLJ) model was used to express the intermolecular potential acting on liquid molecules. First, the equation of state of each liquid was obtained using MD simulation, and the critical temperature, density and pressure of each liquid were determined. Heat conduction of each liquid at various liquid states [metastable (p = 1.9Pcr), saturated (p = 2.1p(cr),), and stable (p = 2.3p(cr))] at T = 0.7T(cr) was simulated and the thermal conductivity was estimated. These values were compared with experimental results and it was confirmed that the simulated results were consistent with the experimental data within 10%. Obtained thermal conductivities at saturated state were reduced by the critical temperature, density and mass of molecules and these values were compared with each other. It was found that the reduced thermal conductivity increased with the increase in the molecular elongation. Detailed analysis of the molecular contribution to the thermal conductivity revealed that the contribution of the heat flux caused by energy transport and by translational energy transfer to the thermal conductivity is independent of the molecular elongation while the contribution of the heat flux caused by rotational energy transfer to the thermal conductivity increases with the increase in the molecular elongation. Moreover, by comparing the reduced thermal conductivity at various states, it was found that the increase of thermal conductivity with the increase in the density, or pressure, was caused by the increase of the contribution of energy transfer due to molecular interaction. (C) 2004 Elsevier Ltd. All rights reserved.

Effect of compressibility of liquid hydrogen on dynamic response of a high pressure LH<SUB>2</SUB> turbopump system including pipes has been analyzed. Simulation results of a nonlinear compressible mathematical model have been compared with those of a nonlinear incompressible mathematical model. Design parameters of LE-7 engine were used to simulate steady state condition and the boundary condition of tank pressure as well as pump outlet pressure was controlled to simulate unsteady condition. Effects of dynamic characteristics parameters of a cavitating inducer such as cavitation compliance and mass flow gain factor on dynamic response of a compressible flow system were analyzed as well.

The adsorption phenomena of H_2 on Pt (111) surface were simulated by the combination of accelerated Quantum Molecular Dynamics (QMD) method and classical Molecular Dynamics (MD) method. About QMD method, only the valence orbitals were considered and a Hamiltonian matrix and an overlap integral matrix were approximated to the analytical function of the distance between the atoms. It was confirmed that the probability density function of electron obtained by this method is very consistent with the results of Density Functional Theory (DFT). About the binding energy of Pt bulk and the partial density of state (PDOS), the results obtained by this method were qualitatively consistent with each other. On the viewpoint of energy conservation and temperature control, it was confirmed that this method can simulate the reaction phenomena on the catalyst well.

Effect of compressibility of liquid hydrogen on dynamic response of a high pressure LH<SUB>2</SUB> turbopump system including pipes has been analyzed. Simulation results of a nonlinear compressible mathematical model have been compared with those of a nonlinear incompressible mathematical model. Design parameters of LE-7 engine were used to simulate steady state condition and the boundary condition of tank pressure as well as pump outlet pressure was controlled to simulate unsteady condition. Effects of dynamic characteristics parameters of a cavitating inducer such as cavitation compliance and mass flow gain factor on dynamic response of a compressible flow system were analyzed as well.

The effect of molecular elongation on the thermal conductivity of diatomic liquids has been analyzed using a nonequilibrium molecular dynamics (NEMD) method. The two-center Lennard-Jones model was used to express the intermolecular potential acting on liquid molecules. The simulations were performed using the nondimensional form of the potential so that the molecular elongation, d/sigma, was the only parameter varied in the simulation. The simulations were performed for five values of this parameter. First, the equation of state of each liquid was obtained using equilibrium molecular dynamics simulation, and the critical temperature, density, and pressure of each liquid were determined. Then, NEMD simulations of heat conduction in the five liquids were performed using values for temperature and density which were identical among the five liquids when they were reduced by their respective critical temperature and density (T=0.7 T-cr and rho=2.24 rho(cr)). Obtained thermal conductivities were reduced by the critical temperature, density, and molecular mass of each compound, and these values were compared with each other. It was found that the reduced thermal conductivity increased as molecular elongation increased. Detailed analysis of the molecular contribution to the thermal conductivity revealed that (a) the contribution of the heat flux caused by energy transport and by translational energy transfer to the thermal conductivity is independent of the molecular elongation, and (b) the contribution of the heat flux caused by rotational energy transfer to the thermal conductivity increases with the increase in the molecular elongation. (C) 2003 American Institute of Physics.

The Dynamic Molecular Collision (DMC) model can accurately estimate, energy transfer between the translational and rotational degrees of freedom at a collision. In this model, a probability density function (PDF) of energy after collision at each degree of freedom is modeled using an exponential function. Properties of the model are obtained by results of the Molecular Dynamics (MD) method. A total and inelastic collision cross section are also constructed. The defect of the model is that a large number of binary collisions of diatomic molecules have to be simulated in advance in order to construct a table of the properties. In this paper, the dependence of initial energy on the properties of the DMC model is analyzed in detail and some relations between these properties and the initial energy are obtained. Using these results, each property is expressed by fundamental functions of the initial energy. In order to verify the validity of the model function, equilibrium or nonequilibrium flows are simulated by the model and the results are compared with theoretical or experimental results.

In the present study, molecular dynamics simulation is applied to the analysis of the bubble nucleation in liquid oxygen including helium or nitrogen as impurities. All molecular interactions are given by the Lennard-Jones potential and simulations are excuted under the standard boiling point of oxygen (90K). Under a high concentration, helium molecules make a cluster based on the phase separation while nitrogen molecules do not. In addition, it is found that the effect of helium molecules which make a bigger local void under a lower concentration is strong while the nitrogen molecules make a smaller one even under a higher concentration. Based on the simulation results, it is pointed out that the helium molecules have a possibility to raise the nucleation point even under a lower concentration while nitrogen ones do not so much.

In cryogenic fluids such as LH2 or LOX, temperature depression of liquids due to latent heat of vaporization suppresses the growth of cavitation, which is called "thermodynamic effects of cavitation." Thermodynamic effects of cavitation are significant in these fluids, because they are generally operated close to the critical point and are also characterized by a strong dependence of vapor pressure on temperature. Owing to this phenomenon, the performance of the rocket pump, inducer and other hydraulic equipment is sustained. In this paper, the thermodynamic effects of cavitation are investigated numerically. To predict these effects, a cavitation model introduced by Deshpande et al. is improved. Using this model a sheet cavity around a 2-D hydrofoil is simulated and the dependence of the properties of fluids or Reynolds number on the thermodynamic effects of cavitation is analyzed. The numerical results explain the thermodynamic effects well.

Heterogeneous bubble nucleation in liquid oxygen including helium, nitrogen, or argon is simulated by using the molecular dynamics method. Molecular interaction is given as Lennard-Jones potential, and, basically, each potential parameter is determined so that a saturation curve obtained by MD data is consistent with an experimental value. In the case that helium is the impurity, a bubble is caused by density fluctuation at a lower concentration, while clusters of helium molecules become bubble nuclei at a higher concentration, and the point of bubble formation moves closer to the saturation point of pure oxygen when they form clusters. In the case that nitrogen or argon is the impurity, the above-mentioned clustering is not observed at a concentration where helium makes clusters, and these impurities have weaker action to make clusters compared with helium.

In this paper the effect of rotational degree of freedom on the properties of liquid is analyzed by the Molecular Dynamics (MD) Method. Oxygen is assumed as the liquid and molecules are assumed as both monatomic and diatomic one. 2 center Lennard-Jones (2CLJ) potential is assumed as the intermolecular potential for diatomic molecules and Lennard-Jones (LJ) potential is used for monatomic molecule. Simulations are performed at various combinations of density and temperature and an Equation of State (EOS) of each liquid is obtained by using these results. The parameters of each potential are determined so that the critical temperature and density are consistent with each other. The properties of liquid such as, latent heat, specific heat, thermal conductivity and surface tension are compared with each other and the effect of rotational degree of freedom on the properties of liquid is investigated.

Heterogeneous bubble nucleation in liquid oxygen including helium or nitrogen is simulated by usint the molecular dynamics method. Molecular interaction is given as Lennard-Jones potential, and each potential parameter is determined so that it well expresses the thermodynamic properties of factual fluids. In the case that helium is the impurity, clustering of helium molecules is observed through the simulation and these clusters are considered to become bubble nuclei. On the other hand, nitrogen molecules do not make a cluster in liquid oxygen under the same condition, and bubble is formed based on the density fluctuation as same as unary liquid oxygen. Therefore, helium in the liquid oxygen has a fairly stronger action to make clusters as bubble nuclei.

Many rocket launch failures are caused by engine problem, especially by the propellant feed system problem. Analysis of transient or unsteady characteristics of turbopump system is necessary on conducting rocket engine stability analysis such as POGO analysis. Dynamic characteristics of cavitating inducer isn't governed only by cavitation compliance defined as the ratio of cavity volume change rate to inducer inlet pressure change rate, but also by mass flow gain factor defined as the ratio of cavity volume change rate to inducer inlet mass flow change rate. This paper provides a new dynamic model of rocket engine LH_2 feed system and gives some stability analysis results.

Dynamic Molecular Collision (DMC) model is extended to calculate collisions of N-2 and He. An intermolecular potential is obtained as the sum of the two potentials between each atom of a N-2 molecule and He. Lennard-Jones (12-6) potential is used as the interatomic potential and potential parameters are determined by combination rule. N-2-He collisions are simulated in many cases by Molecular Dynamics (MD) method in order to construct the collision model between N-2 and He. A collision cross section is determined based on a diffusion coefficient and a probability density function of energy after collision is determined by the MD method. In the present paper, moreover, this model is applied to simulations of the free jet expansion of N-2-He mixture by Direct Simulation Monte Carlo (DSMC) method and the flow field is analyzed. Especially the number density and energy distributions at the axis of a free jet are analyzed in detail.

Dynamic Molecular Collision (DMC) model is extended to calculate collisions of N-2 and He. An intermolecular potential is obtained as the sum of the two potentials between each atom of a N-2 molecule and He. Lennard-Jones (12-6) potential is used as the interatomic potential and potential parameters are determined by combination rule. N-2-He collisions are simulated in many cases by Molecular Dynamics (MD) method in order to construct the collision model between N-2 and He. A collision cross section is determined based on a diffusion coefficient and a probability density function of energy after collision is determined by the MD method. In the present paper, moreover, this model is applied to simulations of the free jet expansion of N-2-He mixture by Direct Simulation Monte Carlo (DSMC) method and the flow field is analyzed. Especially the number density and energy distributions at the axis of a free jet are analyzed in detail.

In our 3rd report, we introduced a Lennard-Jones (LJ) potential parameter based on the potential obtained by ab initio calculation and collision cross section from the Wang-Chang, Uhlenbeck and Taxman's theory and molecular dynamics (MD) simulation. In the present study, we have improved the dynamic molecular collision (DMC) model to calculate the property of MD simulation better than the previous model. To confirm its validity we calculated the equilibrium state, the transport coefficient (viscosity coefficient and heat conductivity) at various temperatures and the normal shock wave by the direct simulation Monte Carlo (DSMC) method using the DMC model and compared the results with other theoretical and experimental results. Consequently, we found that the diatomic rarefied gas flows could be simulated very well using our model. These results were compared with those obtained by the Larsen Borgnakke model. It was found that this model was more efficient than the previous model.

In our 3rd report, we introduced a Lennard-Jones (LJ) potential parameter based on the potential obtained by ab initio calculation and collision cross section from the Wang-Chang, Uhlenbeck and Taxman's theory and molecular dynamics (MD) simulation. In the present study, we have improved the dynamic molecular collision (DMC) model to calculate the property of MD simulation better than the previous model. To confirm its validity we calculated the equilibrium state, the transport coefficient (viscosity coefficient and heat conductivity) at various temperatures and the normal shock wave by the direct simulation Monte Carlo (DSMC) method using the DMC model and compared the results with other theoretical and experimental results. Consequently, we found that the diatomic rarefied gas flows could be simulated very well using our model. These results were compared with those obtained by the Larsen Borgnakke model. It was found that this model was more efficient than the previous model.

The Dynamic Molecular Collision (DMC) model is constructed for accurate and realistic simulations of rarefied gas flows of nonpolar diatomic molecules by the Direct Simulation Monte Carlo (DSMC) method. This model is applicable for moderate temperatures (up to a few hundred K for nitrogen), where most molecules are in the vibrational ground state and the vibrational degree of freedom can be neglected. In this range, moreover, the rotational energy can be considered as a continuous one. The collisions of diatomic molecules are simulated many times by the Molecular Dynamics (MD) method at various initial conditions. The site to site potential is used as an intermolecular one. The collision cross section is developed from the database obtained by MD simulation and kinetic theory of viscosity coefficient of diatomic molecules. The probability density function of energy after collision is also developed using the database. In order to verify the DMC model, two flow fields are simulated. First, the DMC model is applied to the simulation of the translational and rotational energy distribution at the equilibrium condition and the results are compared with the Maxwell distribution. The results agree very well with each other. Second, the DMC model is applied to the simulation of the rotational relaxation through low and high Mach number normal shock wave. These results also agree very well with the experimental results of Robben and Talbot, although the upstream rotational temperature is a little lower. (C) 1999 American Institute of Physics. [S1070-6631(99)02607-0].

In the 3rd report, we introduced an LJ potential parameter which is based on the potential obtained from ab-initio calculation and collision cross section from the Wang-Chang Uhlenbeck and Taxmas's theory and Molecular Dynamics (MD) calculation. In this 4th paper, we improved a Dynamic Molecular Collision (DMC) model to calculate the property of MD simulation better than the former one. To make sure of its validity we calculated equilibrium state, transport coefficient (viscosity coefficient, heat conductivity) at some temperatures and normal shock wave by the DSMC method using DMC model and compared these results with other theoretical and experimental results. Consequently, we found that the diatomic rarefied gas flows could be simulated very well using our model.

A parallel algorithm for direct simulation Monte Carlo calculation of diatomic molecular rarefied gas flows is presented. For reliable simulation of such flow, an efficient molecular collision model is required. Using the molecular dynamics method, the collision of N-2 molecules is simulated. For this molecular dynamics simulation, the parameter decomposition method is applied for parallel computing. By using these results, the statistical collision model of diatomic molecule is constructed. For validation this model is applied to the direct simulation Monte Carlo method to simulate the energy distribution at equilibrium condition and the structure of normal shock wave. For this DSMC calculation, the domain decomposition is applied. It is shown that the collision process of diatomic molecules can be calculated precisely and the parallel algorithm can be efficiently implemented on the parallel computer. (C) 1997 Elsevier Science B.V.

At present, the DSMC method is the best scheme to analyze rarefied gas flows. However, polyatomic inelastic collisions which have complicated energy transfer between translational and internal degree of freedom cannot be analyzed accurately. In this paper, a model for energy exchange between translational and rotational degrees of freedom is constructed using the results of MD simulation. In this 1st report, we describe how to simulate the collision of diatomic molecules by the MD method and the results of the simulation, We find that the energy transfer is distributed without a characteristic relation to the direction and orientation of the rotational vector, and that both the energy transfer and collisional cross section strongly depend on the initial translational and rotational energy.

In the 1st report, we made some calculations to obtain data in order to construct a model for collision of diatomic molecules. Consequently, we found that the energy transfer is distributed without a characteristic relation to the direction and orientation of the rotational vector, and that both the energy transfer and collisional cross section strongly depend on the initial translational and rotational energy In this 2nd paper, we constructed a collision model for diatomic molecules using these data. To make sure of its validity we calculated equilibrium state, viscosity coefficient, heat conductivity diffusion coefficient and normal shock wave by the DSMC method using our model and compared these results with other theoretical and experimental results. Consequently, we found that the diatomic rarefied gas flows can be simulated very well using our model.

The collison of N2 molecules in which only rotation of molecules is regarded as internal degree of freedom is simulated and the energy transfer between translational and rotational energy is investigated numerically at low temperature. The results are visualized and it is shown that energy transfer changes by the direction and orientation of rotational vector and the impact parameter and it is also distributed in some form according to the initial translational and rotational energy. Using these results, the collision model of diatomic molecule is constructed and is applied to the Direct Simulation Monte Carlo Method and some properties of fluids, for instance, the energy distribution at equiliblium condition and so on, are calculated for making sure of its validity. It is shown that the collision process of diatomic molecules can be calculated well by use of this model.