Comprehensive Molecular Dynamics Study of Oxygen Diffusion in Carbon Mesopores: Insights for Designing Fuel-Cell Catalyst Supports.

Mesoporous carbon is often used as a support for platinum catalysts in polymer electrolyte fuel-cell catalyst layers. Mesopores in the carbon support improve the performance of fuel cells by inhibiting the adsorption of ionomer onto the catalyst particles. However, the mesopores may impair mass transport. Hence, understanding molecular behaviors in the pores is essential to optimizing the mesopore structures. Specifically, it is crucial to understand the oxygen transport in the high-current region. In this study, the diffusion coefficients of oxygen molecules in carbon mesopores were calculated for various pore lengths, pore diameters, filling rates, and water contents in the ionomer via molecular dynamics simulations. The results show that oxygen diffusion slows by 2 orders of magnitude because of pore occlusion, and it slows down by an additional 1 or 2 orders of magnitude if ionomers are present in the pores. The occlusion can be theoretically predicted by considering the surface free energy. This theory provides some insight into mesoporous carbon designs; for instance, the theory suggests that narrow pores should be shortened to prevent occlusion. Slow diffusion in the presence of ionomers was attributed to the localization of oxygen at the dense ionomer-carbon interface. Thus, to improve oxygen transport properties, carbon surfaces and ionomer structures may be designed in such a manner as to prevent densification at the interface.

Self-Learning Molecular Design for High Lithium-Ion Conductive Ionic Liquids using Maze Game.

A self-learning artificial intelligence system for an autonomous molecular search was recently utilized in place of laborious material development processes by humans. In this approach, because the evaluation of unsuitable or unrealistic candidates considerably decreases the search efficiency, prior knowledge of the chemistry and engineering requirements should be embedded into the molecular-generative algorithm. However, when using naive rule-based restrictions, one must implement the complex rule logic into the code each time, depending on the materials and potential applications. Herein, we propose a molecular-generative method using a maze game to control the allowable constituent fragments of molecules, which improves the flexibility and consistency to implement the rules. We performed an autonomous search for optimized cation structures of high Li-ion conductive ionic liquids evaluated by molecular dynamics simulations, in its practically reasonable scope defined by the maze game. From the search, we discover that acyl ammonium cations are favorable for high Li-ion conductivity because of the high association between the cations and Li ions. These results broaden our existing insight owing to the ability to explore beyond our practical experiences.

Computational study of effect of water finger on ion transport through water-oil interface.

When an ion transports from water to oil through water-oil interface, it accompanies hydrated water molecules and transiently forms a chain of water, called "water finger." We thoroughly investigated the role of the water finger in chloride ion transport through water-dichloromethane interface by using molecular dynamics technique. We developed a proper coordinate w to describe the water finger structure and calculated the free energy landscape and the friction for the ion transport as a function of ion position z and the water finger coordinate w. It is clearly shown that the formation and break of water finger accompanies an activation barrier for the ion transport, which has been overlooked in the conventional free energy curve along the ion position z. The present analysis of the friction does not support the hypothesis of augmented local friction (reduced local diffusion coefficient) at the interface. These results mean that the experimentally observed rate constants of interfacial ion transfer are reduced from the diffusion-limited one because of the activation barrier associated to the water finger, not the anomalous local diffusion. We also found that the nascent ion just after the break of water finger has excessive hydration water than that in the oil phase.

Microscopic Barrier Mechanism of Ion Transport through Liquid-Liquid Interface.

Microscopic mechanism of ion transport through water-oil interface was investigated with molecular dynamics simulation. The formation/breaking of a water finger during the ion passage was explicitly formulated in the free energy surface. The calculated 2D free energy surface clearly revealed a hidden barrier of ion passage accompanied by the water finger. This barrier elucidates the retarded rate of interfacial ion transfer.

Mechanism of Ionomer Film Formation via Solution Drying.

Film formation via the drying of ionomer solutions is a crucial process that has a strong influence on the morphology and transport properties of polymer electrolyte membranes and thin films. However, the microscopic mechanism of this process remains unclear. Here, we elucidate this mechanism using a coarse-grained model based on all-atom molecular dynamics that accurately reproduces small-angle X-ray scattering spectra. In dilute ionomer solutions, ionomers form rod-like bundles with diameters of 1.5-2 nm. As the water solvent evaporates, these bundles gradually aggregate and connect to each other, while maintaining their diameter. Finally, the remaining water forms nanosized clusters surrounded by the surfaces of the bundles with hydrophilic sulfonate groups.

Accurate Multiscale Simulation of Frictional Interfaces by Quantum Mechanics/Green's Function Molecular Dynamics.

Understanding frictional phenomena is a fascinating fundamental problem with huge potential impact on energy saving. Such an understanding requires monitoring what happens at the sliding buried interface, which is almost inaccessible by experiments. Simulations represent powerful tools in this context, yet a methodological step forward is needed to fully capture the multiscale nature of the frictional phenomena. Here, we present a multiscale approach based on linked ab initio and Green's function molecular dynamics, which is above the state-of-the-art techniques used in computational tribology as it allows for a realistic description of both the interfacial chemistry and energy dissipation due to bulk phonons in nonequilibrium conditions. By considering a technologically relevant system composed of two diamond surfaces with different degrees of passivation, we show that the presented method can be used not only for monitoring in real-time tribolochemical phenomena such as the tribologically induced surface graphitization and passivation effects but also for estimating realistic friction coefficients. This opens the way to in silico experiments of tribology to test materials to reduce friction prior to that in real labs.

Interfacial Distribution of Nafion Ionomer Thin Films on Nitrogen-Modified Carbon Surfaces.

Chemically modified carbon supports for the cathode catalyst layers of polymer electrolyte fuel cells (PEFCs) show considerable promise for boosting the oxygen reduction reaction. This study evaluated the ionomer distribution of Nafion ionomer thin films on nitrogen (N)-modified carbon surfaces along their depth direction. Neutron reflectivity (NR) measurements performed using the double-contrast technique with H2O and D2O revealed that the introduction of N functional groups to carbon thin films promoted ionomer adsorption onto the surface under wet conditions (22 °C, 85% relative humidity). Molecular dynamics (MD) simulations conducted to verify the origin of the robust contact between the ionomer and N-modified carbon surface revealed an ionomer adsorption mechanism on the N-modified carbon surfaces, which involved Coulomb interactions between the positively charged carbon surface and the ionomer side chains with negatively charged sulfonic acid groups. The positive surface charge, which was determined using the contents of the N functional groups estimated by X-ray photoelectron spectroscopy, was found to be sufficient as an impetus for ionomer adsorption. This strategy involving NR measurements and MD simulations can provide insights into the solid-ionomer interfacial structures in a cathode catalyst layer and can therefore be extensively employed in studies on PEFCs.

The origins of binding specificity of a lanthanide ion binding peptide.

Lanthanide ions (Ln3+) show similar physicochemical properties in aqueous solutions, wherein they exist as + 3 cations and exhibit ionic radii differences of less than 0.26 Å. A flexible linear peptide lanthanide binding tag (LBT), which recognizes a series of 15 Ln3+, shows an interesting characteristic in binding specificity, i.e., binding affinity biphasically changes with an increase in the atomic number, and shows a greater than 60-fold affinity difference between the highest and lowest values. Herein, by combining experimental and computational investigations, we gain deep insight into the reaction mechanism underlying the specificity of LBT3, an LBT mutant, toward Ln3+. Our results clearly show that LBT3-Ln3+ binding can be divided into three, and the large affinity difference is based on the ability of Ln3+ in a complex to be directly coordinated with a water molecule. When the LBT3 recognizes a Ln3+ with a larger ionic radius (La3+ to Sm3+), a water molecule can interact with Ln3+ directly. This extra water molecule infiltrates the complex and induces dissociation of the Asn5 sidechain (one of the coordinates) from Ln3+, resulting in a destabilizing complex and low affinity. Conversely, with recognition of smaller Ln3+ (Sm3+ to Yb3+), the LBT3 completely surrounds the ions and constructs a stable high affinity complex. Moreover, when the LBT3 recognizes the smallest Ln3+, namely Lu3+, although it completely surrounds Lu3+, an entropically unfavorable phenomenon specifically occurs, resulting in lower affinity than that of Yb3+. Our findings will be useful for the design of molecules that enable the distinction of sub-angstrom size differences.

Hydrated Ion Clusters in Hydrophobic Liquid: Equilibrium Distribution, Kinetics, and Implications.

Hydrophilic ions in oil phase tend to be hydrated in the presence of trace water and form hydrated clusters. The present paper elucidates the equilibrium size distribution of hydrated ion clusters and the microscopic rates of adsorption and desorption of water with the help of molecular dynamics simulations. The size distribution is derived from reversible work of hydration, which is nearly constant over the hydration number except for small clusters. The intrinsic rate constants of adsorption and desorption are evaluated to be in several psec order after correcting the diffusion. The microscopic hydration properties of ions in the oil phase play key roles in chemical reactions involving both hydrophilic and hydrophobic reactants as well as in the transport and reactivity of the ions in oil phase and at the water-oil interface.

[A report of two cases--two patients of the extremely advanced hepatocellular carcinoma have responded completely for a long time after a combination therapy consisting of arterial chemotherapy and injection of interferon-alpha].

The prognosis of advanced hepatocellular carcinoma (HCC) is extremely poor. Patient 1 was a 43-year-old male with major portal tumor thrombi. He received combination therapy consisting of continuous arterial infusion (MTX 30 mg/m2, day 1, CDDP/5-FU 6 mg/m2: 250 mg/m2, day 1-14) and subcutaneous injection of IFN-alpha (500 x 10(4) U, 3 times a week, 4 weeks). Patient 2 was a 66-year-old male with major hepatic venous tumor thrombi. He received combination therapy consisting of continuous arterial infusion (5-FU 6 mg/m2: 250 mg/m2, day 1-14) and subcutaneous injection of IFN-alpha (500 x 10(4) U, 3 times a week, 4 weeks). Decrease in tumor was observed in both patients markers and marked regression of tumor was observed in both patients. They are still in complete response. This combination therapy is an effective strategy for advanced HCC.