Shunsuke Yamakawa

Nature of water transport and electro-osmosis in nafion: insights from first-principles molecular dynamics simulations under an electric field.

The effects of water content on water transport and electro-osmosis in a representative polymer electrolyte membrane, Nafion, are investigated in detail by means of first-principles molecular dynamics (MD) simulations in the presence of a homogeneous electric field. We have directly evaluated electro-osmotic drag coefficients (the number of water molecules cotransported with proton conduction) from the trajectories of the first-principles MD simulations and also explicitly evaluated factors that contribute to the electro-osmotic drag coefficients. In agreement with previously reported experiments, our calculations show virtually constant values ( approximately 1) of the electro-osmotic drag coefficients for both low and high water content states. Detailed comparisons of each factor contributing to the drag coefficient reveal that an increase in water content increases the occurrence of the Grotthuss-like effective proton transport process, whose contribution results in a decrease in the electro-osmotic drag coefficient. At the same time, an environment that is favorable for the Grotthuss-like effective proton transport process is also favorable for the transport of water arising from water transport occurring beyond the hydration shell around the protons, whose contribution results in an increase in the electro-osmotic drag coefficient. Conversely, an environment that is not favorable for proton conduction is also not favorable for water transport. As a result, the electro-osmotic drag coefficient shows virtually identical values with respect to change in the water content.

Electronic state calculation of hydrogen in metal clusters based on Gaussian–FEM mixed basis function

Abstract The electronic state calculation combining Gaussian basis functions with finite element method (FEM) basis functions is presented. The electronic state calculation explicitly accounting for the effect of grain boundaries reproduces well the characteristics of hydrogen in metal materials. FEM is one possible technique because of the high applicability to arbitrary boundary conditions. However, since the wave function of an electron significantly changes near the nucleus, a large number of nodal points of FEM must be taken. This fact means that FEM is expected to require high computational costs. We expect that Gaussian–FEM mixed basis function method simultaneously satisfies the applicability to arbitrary boundary conditions and describes the steeply changing electron distribution. Results are presented for the electronic state calculation of H 2 , AlH and Al 2 diatomic molecules and Al 4 H cluster without external fields.

Simulation of growth process of Pt-particles - first-principles calculations

First-principles calculations have been applied to investigate the interactions between Ptn (n = 1 ~ 4) clusters and a graphene sheet as models of Pt/C fuel-cell catalytic electrodes. A Pt atom is stably adsorbed on the bridge site between two carbon atoms with the adsorption energy of about 2eV. For the case of Pt2, both of the Pt atoms are adsorbed on the bridge site. For the case of Pt3, the triangular cluster is more stable than the linear cluster. For the case of Pt4, the tetrahedral cluster is more stable than the planar cluster. The adsorption energies on the surface without defects are 0.55 eV/adatom for Pt2 and 0.05 eV/adatom for Pt3. The interaction energy between the Pt cluster and the graphite surface per Pt atom becomes weaker as the number of Pt atoms increases. The adsorption energy for a Pt atom on the vacant site is 8.00 eV/adatom, which is stronger than the formation energy of a Pt-Pt bond (about 2 eV/bond) for small clusters.

Effects of the microstructure of solid-electrolyte-coated LiCoO2 on its discharge properties in all-solid-state lithium batteries

Li-ion conduction in electrolyte materials and its intercalation properties in active materials are key factors that determine the electrochemical performances of batteries. Sulfide electrolyte coating onto active materials was done to form a favourable electrode–electrolyte interface and to increase the electrochemically active surface area. In this study, to obtain higher packing density pellets, a mixture of LiCoO2 particles with different grain sizes was used as a host material during the electrolyte deposition. The cell performance enhancement was achieved by the formation of a denser environment within composite electrodes. However, the influence of electrode microstructures on ion-conductive pathways has not been elucidated clearly and completely because of difficulties in conducting the experiments. In this context, a simulation model would provide insightful information about the potential of appropriate electrode structural designs. We proposed the implementation of the phase-field method, where the temporal evolution at electrode microstructures is calculated based on a free-energy function to construct realistic 3D images of the electrode structure. This study theoretically evaluated the constant current discharge properties of positive electrodes consisting of LiCoO2 with sulfide electrolyte coatings. The simulation results were able to reproduce an experimentally obtained correlation between the electrode potential and discharge capacity. The present simulation provided an estimation of the material properties required under specific discharge conditions. The preliminary examination demonstrated that the mutually connected fine network of ion-conductive pathways benefits from sulfide electrolyte coatings, and that it affects the discharge properties at higher rates than a conventional microstructure of mixed powder electrodes.

Enhanced Thermal Diffusion of Li in Graphite by Alternating Vertical Electric Field: A Hybrid Quantum-Classical Simulation Study

Enhancing the diffusivity of the Li ion in a Li-graphite intercalation compound that has been used as a negative electrode in the Li-ion rechargeable battery, is important in improving both the recharging speed and power of the battery. In the compound, the Li ion creates a long-range stress field around itself by expanding the interlayer spacing of graphite. We advance the hybrid quantum-classical simulation code to include the external electric field in addition to the long-range stress field by first-principles simulation. In the hybrid code, the quantum region selected adaptively around the Li ion is treated using the real-space density-functional theory for electrons. The rest of the system is described with an empirical interatomic potential that includes the term relating to the dispersion force between the C atoms in different layers. Hybrid simulation runs for Li dynamics in graphite are performed at 423 K under various settings of the amplitude and frequency of alternating electric fields perpen...

Computational Modeling Aspects of PEFC Durability

The aims of this chapter are to provide a concise review of recent (up to early 2009) studies on PEFC degradation using computer modeling approaches and to present our computational modeling studies on catalyst degradation. The review section includes macroscopic and microscopic modeling of catalyst layer degradation, Pt catalyst loss, carbon substrate corrosion, electrolyte membranes decomposition and other phenomena. In the second part, the Pt surface area loss is modeled considering Pt dissolution, diffusion and deposition in a certain thickness of a catalyst layer using a TEM observation-determined particle size distribution. The results show that the Ostwald ripening is the major cause of the Pt surface area loss in the most parts of the catalyst layer although Pt ion effluence plays a certain role near the electrolyte membrane.

Phase-field model for deposition process of platinum nanoparticles on carbon substrate

Platinum supported on a carbon carrier is widely used as a catalyst for polymer electrolyte membrane fuel cells. The catalytic activity is significantly affected by the size distribution and morphologies of the platinum particles. The objective of this study is to extend the phase-field approach to describe the formation process of platinum particles onto the substrate. The microstructural evolution of a nanoparticle was represented by the temporal evolution of the field variables related to the platinum concentration, long-range crystallographic ordering and phase transition. First-principles calculations were performed in order to estimate the interaction energies between several different types of platinum clusters and a graphene sheet. The platinum density profile concentrated over the substrate surface led to the formation of three-dimensional islands in accordance with the Volmer-Weber mode of growth. The size distributions of the platinum particles were sensitive to the heterogeneity of the substrate surface and to the competitive nucleation and growth processes.

Simulation of Initial Growth Process of Pt Clusters on Carbon Materials – First-Principles Calculations

First-principles calculations have been applied to investigate the interactions between Ptn (n=1˜13) clusters and a graphene sheet to model the Pt/C fuel-cell catalytic electrode. For the small clustesr (n 7), the 3D clusters are more stable than the V-2D clusters. In order to investigate the effects of defects and dopants in a graphene sheet, the interactions between the Pt13 cluster and the graphene sheet with an atomic vacancy and an dopant atom, such as boron and nitrogen atoms have been examined. For the atomic vacancy, the Pt atom is directly adsorbed on the C atom vacant site. For the Pt13 cluster adsorbed on the graphene sheet doped with the B or N atoms, the Pt atom is not directly adsorbed on the impurity atoms. The adsorption of the Pt13 cluster above the vacancy of the graphene sheet is more stable that that on the doped graphene sheet in the present calculations.