Michihisa Koyama

The compositional space of high-entropy-alloy nanoparticles (HEA NPs) significantly expands the diversity of the materials library. Every atom in HEA NPs has a different elemental coordination environment, which requires knowledge of the local electronic structure at an atomic level. However, such structure has not been disclosed experimentally or theoretically. We synthesized HEA NPs composed of all eight noble-metal-group elements (NM-HEA) for the first time. Their electronic structure was rev

Overpotential and ac impedance spectra were measured to construct a model to describe porous (SSC) cathodes for solid oxide fuel cells (SOFCs). Analysis of the impedance spectra revealed that there are three processes involved in the overall electrochemical reaction; (i) the adsorption/desorption process on the surface of the electrode, (ii) the ionic conduction in the bulk SSC, and (iii) the diffusion of oxygen in the gas phase. It was found that in air atmosphere, the reaction processes (i) an

To study the atomistic behavior of the phosphoric ester molecule on the nascent Fe surface under boundary lubrication conditions, we adopted a hybrid tight-binding quantum chemical molecular dynamics method. First, we investigated chemical interactions between phosphoric ester and the nascent Fe surface. Phosphoric ester was shown to interact with the nascent Fe surface, forming both covalent and ionic bonds. Formation and dissociation dynamics of covalent bonds during tribochemical reaction was

Chemical degradation of perfluorosulfonic acid (PFSA) membrane is one of the most serious problems for stable and long-term operations of the polymer electrolyte fuel cell (PEFC). The chemical degradation is caused by the chemical reaction between the PFSA membrane and chemical species such as free radicals. Although chemical degradation of the PFSA membrane has been studied by various experimental techniques, the mechanism of chemical degradation relies much on speculations from ex-situ observa

The electronic structure of surface atoms has a great effect on catalytic activity because the binding energy of reactants is closely related to the electronic structure. Therefore, designing and controlling the local density of states (LDOS) of the catalyst surface would be a rational way to develop innovative catalysts. Herein, we first demonstrate a highly active AuIr solid-solution alloy electrocatalyst for the oxygen reduction reaction (ORR) by emulating the Pt LDOS profile. The calculated

Since 1970, people have been making every endeavor to reduce toxic emissions from automobiles. After the development of a three-way catalyst (TWC) that concurrently converts three harmful gases, carbon monoxide (CO), hydrocarbons (HCs), and nitrogen oxides (NO_x ), Rh became an essential element in automobile technology because only Rh works efficiently for catalytic NO_x reduction. However, due to the sharp price spike in 2007, numerous efforts have been made to replace Rh i

The cathodic reaction mechanism of a solid oxide fuel cell (SOFC) was investigated for an electrode‐electrolyte system of , under both the conducting and mixed‐ionic conducting conditions. AC impedance measurements were carried out, and the electrode interfacial conductivities were calculated. The experimental results revealed that the processes dominating the electrode resistance for the conducting and the mixed‐ionic conducting conditions are different. It was also found that the process domin

We studied the binding energies of O species on face-centered-cubic Pt₃M nanoparticles (NPs) with a Pt-skin layer using density functional theory calculations, where M is Co, Ni, or Cu. It is desirable to express the property by structural parameters rather than by calculated electronic structures such as the <i>d</i>-band center. A generalized coordination number (GCN) is an effective descriptor to predict atomic or molecular adsorption energy on Pt-NPs. The GCN was extended to the p

We studied the adsorption energies of the NO species on face-centered-cubic M405-nanoparticles (NPs) (M = Rh, Pd, Ag, Ir, or Pt) using density functional theory calculations. Various adsorption sites including the on-top, bridge, and hollow sites in the ridge, (100) facet, and (111) facet were considered. The molecular adsorption energy on the slab model is affected by the adsorption configurations. By contrast, we observed different NO adsorption energies even for the same configuration on the

New power generation technologies are expected to reduce various environmental impacts of providing electricity to urban regions for some investment cost. Determining which power generation technologies are most suitable for meeting the demand of a particular region requires analysis of tradeoffs between costs and environmental impacts. Models simulating different power generation technologies can help quantify these tradeoffs. An Internet-based modelling infrastructure called DOME (distributed