Tamio Ikeshoji

Non-equilibrium molecular dynamics calculation of heat conduction in liquid and through liquid-gas interface

This paper presents a new algorithm for non-equilibrium molecular dynamics, where a temperature gradient is established in a system with periodic boundary conditions. At each time step in the simulation, a fixed amount of energy is supplied to a hot region by scaling the velocity of each particle in it, subject to conservation of total momentum. An equal amount of energy is likewise withdrawn from a cold region at each time step. Between the hot and cold regions is a region through which an energy flux is established. Two configurations of hot and cold regions are proposed. Using a stacked layer structure, the instantaneous local energy flux for a 128-particle Lennard-Jones system in liquid was found to be in good agreement with the macroscopic theory of heat conduction at stationary state, except in and near the hot and cold regions. Thermal conductivity calculated for the 128-particle system was about 10% smaller than the literature value obtained by molecular dynamics calculations. One run with a 1024-...

The Role of Bridge‐Bonded Adsorbed Formate in the Electrocatalytic Oxidation of Formic Acid on Platinum

The oxidation of formic acid (HCOOH) on platinum electrodes has been extensively investigated as a model electrocatalytic reaction. It is generally accepted that HCOOH is oxidized to CO2 through a dual-pathway mechanism: one pathway (the main pathway) involves a fast reaction via a reactive intermediate and the second pathway includes a step in which a poisoning species is formed. This species, which is oxidized to CO2 at high potentials, has been identified as adsorbed CO, which is formed by dehydration of HCOOH. Adsorbed hydroxycarbonyl (COOHads) has long been assumed to be the reactive intermediate in the main pathway, but the spectroscopic detection of this species has not been reported to date. By using surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATRSEIRAS), Miki et al. observed that formate is adsorbed in a bridge-bonded configuration on Pt electrodes during HCOOH oxidation. On the basis of systematic time-resolved ATR-SEIRAS analysis of the oxidation dynamics, Samjesk et al. suggested that adsorbed formate (HCOOads) is a reactive intermediate in the main pathway and its decomposition to CO2 is the rate-determining step (rds). The adsorbed formate is in equilibrium with HCOOH in the bulk solution and the reaction pathway (formate pathway) can be represented by Equation (1)

On the molecular mechanism of thermal diffusion in liquids

A recently developed non-equilibrium molecular dynamics algorithm for heat conduction is used to compute the thermal conductivity, thermal diffusion factor, and heat of transfer in binary Lennard-Jones mixtures. An internal energy flux is established with local source and sink terms for kinetic energy. Simulations of isotope mixtures covering a range of densities and mass ratios show that the lighter component prefers the hot side of the system at stationary state. This implies a positive thermal diffusion factor in the definition we have adopted here. The molecular basis for the Soret effect is studied by analysing the energy flux through the system. In all cases we found that there is a difference in the relative contributions when we compare the hot and cold sides of the system. The contribution from the lighter component is predominantly flux of kinetic energy, and this contribution increases from the cold to the hot side. The contribution from the heavier component is predominantly energy transfer th...

Water at supercritical conditions: A first principles study

We analyze, via first principles molecular dynamics, the structural and electronic properties of water close to and above the critical point. Contrary to the ordinary liquid state, at supercritical conditions the hydrogen bond network is destabilized to various extents and the continuous breaking and reformation of hydrogen bonded structures allow large density and dipole fluctuations that, in turn, can significantly affect the dielectric properties of the solvent. Close to the critical point, where the density is very low, small clusters, mainly dimers and trimers, are the dominant features, but many molecules exhibit no H-bond. On the other hand, at higher densities, more extended structures appear, but still a continuous network cannot form. In both cases, H-bond configurations that are anomalous with respect to the normal liquid phase appear. These features strongly affect the solvent properties of supercritical water with respect to those of ambient water. They most likely vary continuously as a func...

Electrode Dynamics from First Principles

The investigation of electrode dynamics has been a major topic in the field of electrochemistry for a century. Electrode dynamics consist of electron transfer reactions that give rise to, or are caused by, a bias voltage, and are influenced by surface catalysis, electrolyte solution, transport of electrons and ions. The first-principles molecular dynamics simulation of the electrochemical system has been hampered by the difficulty to describe the bias voltage and the complex solution-electrode interface structure. Here we utilize a new algorithm called the effective screening medium to characterize the biased interface between platinum and liquid water, revealing the microscopic details of the first, Volmer, step of the platinum-catalyzed hydrogen evolution reaction. By clarifying the important roles played by both the water and the bias, we show why this reaction occurs so efficiently at the interface. Our simulations make a significant step towards a deeper understanding of electrochemical reactions.

Direct evidence of magnetically induced phase separation in the fcc phase and thermodynamic calculations of phase equilibria of the Co-Cr system

Abstract Two-phase equilibria between the ferromagnetic fcc and the paramagnetic fcc phase from 800 to 900 °C in the Co-rich region was determined using the diffusion couple technique. It was confirmed that a magnetically-induced miscibility gap of the fcc phase is formed along the Curie temperature. Thermodynamic calculation of the phase equilibria of the Co–Cr system was performed by optimizing the present results and the thermodynamic data in the literature. A set of thermodynamic values for describing the Gibbs energy of liquid, fcc hcp, bcc and sigma phases yielded good agreement between the calculated phase diagram and the experimental data. Moreover, the magnetically-induced miscibility gap between the ferromagnetic and paramagnetic hcp phases was also predicted. This kind of thermodynamic calculation of Co–Cr base alloys is quite useful for the alloy design of the magnetic recording media.

Development of bulk-type all-solid-state lithium-sulfur battery using LiBH4 electrolyte

Stable battery operation of a bulk-type all-solid-state lithium-sulfur battery was demonstrated by using a LiBH4 electrolyte. The electrochemical activity of insulating elemental sulfur as the positive electrode was enhanced by the mutual dispersion of elemental sulfur and carbon in the composite powders. Subsequently, a tight interface between the sulfur-carbon composite and the LiBH4 powders was manifested only by cold-pressing owing to the highly deformable nature of the LiBH4 electrolyte. The high reducing ability of LiBH4 allows using the use of a Li negative electrode that enhances the energy density. The results demonstrate the interface modification of insulating sulfur and the architecture of an all-solid-state Li-S battery configuration with high energy density.

Thermodynamic calculations of Fe-Zr and Fe-Zr-C systems

Thermodynamic calculations of Fe-Zr and Fe-Zr-C systems have been performed using the Thermo-Calc software based on an extensive amount of experimental data, including the thermodynamic measurements and available phase diagram information. The calculated thermodynamic properties and phase diagram in the Fe-Zr system account for the experimental data reasonably, while the calculated solubility of ZrC in γFe in the Fe-Zr-C system is a little smaller than the only experimental data by Narrita; to clear up this discrepancy, further investigations on the Fe-Zr-C as well as the Zr-C systems are needed.

First-principles molecular dynamics study on aqueous sulfuric acid solutions

The properties of aqueous sulfuric acid have been studied employing density functional theory-based molecular dynamics simulations in conjunction with norm-conserving pseudopotentials. The simulations were carried out for two different concentrations whose molar concentrations were fixed at 0.84 and 10.2 mol/l. The structural features of aqueous sulfuric acid solutions show a strong dependency on the concentration. The Grötthuss-type proton transfer mechanism is not effectively operative at the higher concentration because of the broken hydrogen bond network of water induced by ions generated by the dissociation of sulfuric acid. In addition, to evaluate electrical properties, we carried out a simulation that takes an electric field into account. Results are compared with those of the simulation undertaken with no external electric field.

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.