Shunsuke Yagi

Potential positive electrodes for high-voltage magnesium-ion batteries

Magnesium-ion batteries (MIBs) with a Mg-metal negative electrode are expected to combine high energy density and high electromotive force, owing to the divalent ion careers and its low redox potential. However, it has been reported to date that the cell voltage of MIBs is not high enough (∼1.5 V), being far below that of lithium-ion batteries (LIBs) (4–5 V). In this work, we have investigated the potentiality of Mg–Co–O and Mg–Ni–O complex oxides as the positive electrode for MIBs, which are composed of these positive electrodes and a Mg negative electrode in acetonitrile with Mg(ClO4)2 salt as an electrolyte. These MIBs can exhibit a relatively high open circuit voltage, OCV, and can light a blue diode after charge. However, as the combination of the acetonitrile electrolyte and metal Mg can yield the passivation on the surface of the Mg electrode, we have also checked these materials for a well-established Li ion battery system, and confirmed that the charged battery can show high OCV. In order to attain such a high cell voltage, it would be significant to exploit the unstabilized ion after charge in the host complex oxide.

Suppression of Rac1 activity at the apical membrane of MDCK cells is essential for cyst structure maintenance

Using MDCK cells that constitutively express a Förster resonance energy transfer biosensor, we found that Rac1 activity is homogenous at the entire plasma membrane in early stages of cystogenesis, whereas in later stages Rac1 activity is higher at the lateral membrane than at the apical plasma membrane. If Rac1 is activated at the apical membrane in later stages, however, the monolayer cells move into the luminal space. In these cells, tight junctions are disrupted, accompanied by mislocalization of polarization markers and disorientation of cell division. These observations indicate that Rac1 suppression at the apical membrane is essential for the maintenance of cyst structure.

Mechanical-energy influences to electrochemical phenomena in lithium-ion batteries

In lithium-ion batteries, Li ions usually infiltrate into the anode active material, which usually leads to the formation of Li compounds with expanding volumes. It is well known that the volume strain associated with dilatation/contraction at the intercalation/deintercalation cycles gradually deteriorates the electrode. The intention of this work devoting a simple Li/Sn battery system is to clearly show that such a mechanical strain accompanied by the formation of the Li–Sn compounds causes the following more fundamental phenomena: (i) the electrode potential tends to be lower than the value predicted from the chemical thermodynamics consideration, (ii) the kinetics rate of lithiation or delithiation is significantly retarded (i.e., much slower than expected from the diffusion of Li), and (iii) the electromotive force can be controlled by utilizing the elastic strain actively. Through this paper, we demonstrate the mechanical effects of such mechanical strain or energy on the electrochemical reaction with various experimental supports.

Formation of Cu Nanoparticles by Electroless Deposition Using Aqueous CuO Suspension

Cu nanoparticles approximately 30 nm in diameter were electrochemically formed by liquid-phase reduction (i.e., electroless deposition) using hydrazine as a reducing agent and dispersing barely soluble CuO particles in aqueous solution at 353 K. The addition of gelatin into the reaction suspension prevented the coagulation of Cu nanoparticles and also sufficiently suppressed particle growth. Direct reduction of CuO particles did not occur due to the adhesion of gelatin on the surface of CuO particles, and Cu nanoparticles were deposited by the reduction of Cu 2+ ions dissolved in solution from CuO particles. The solubility of CuO particles was extremely small (of the order of 10 -18 mol dm -3 at 353 K) in aqueous solution of pH 12, which regulated the size of Cu particles. Cu nanoparticles approximately 300 nm in diameter were obtained at 323 K, while Cu nanoparticles of two different sizes (less than 50 nm and about 300 nm) were obtained at 333-353 K. The proportion of the smaller particles increased with increasing reaction temperature, and the smaller particles were easily separated by centrifugal classification without aggregation. The formation mechanism of nanoparticles was discussed from the viewpoint of thermodynamics with in situ monitoring of Cu deposition and immersion potential.

Three-Dimensional Nanoelectrode by Metal Nanowire Nonwoven Clothes

Metal nanowire nonwoven cloth (MNNC) is a metal sheet that has resulted from intertwined metal nanowires 100 nm in diameter with several dozen micrometers of length. Thus, it is a new metallic material having both a flexibility of the metal sheet and a large specific surface area of the nanowires. As an application that utilizes these properties, we propose a high-cyclability electrode for Li storage batteries, in which an active material is deposited or coated on MNNC. The proposed electrode can work without any binders, conductive additives, and current collectors, which might largely improve a practical gravimetric energy density. Huge electrode surfaces provide efficient ion/electron transports, and sufficient interspaces between the respective nanowires accommodate large volume expansions of the active material. To demonstrate these advantages, we have fabricated a NiO-covered nickel nanowire nonwoven cloth (NNNC) by electroless deposition under a magnetic field and annealing in air. The adequately annealed NNNC was shown to be an excellent conversion-type electrode that exhibits a quite high cyclability, 500 mAh/g at 1 C after 300 cycles, compared to that of a composite electrode consisting of NiO nanoparticles. Thus, the present design concept will contribute to a game-changing technology in future lithium ion battery (LIB) electrodes.

Oxidation-State Control of Nanoparticles Synthesized via Chemical Reduction Using Potential Diagrams

A general concept for oxidation-state control of nanoparticles synthesized via chemical reduction has been developed. By comparing kinetically determined mixed potential measured in reaction solution and thermodynamically drawn potential diagrams, e.g., potential-pH diagram, it is possible to know what chemical species is stable in the reaction solution?. It is predicted from potential diagrams that nanoparticles in different oxidation states can be selectively synthesized by controlling mixed potential. This concept is verified by selectively synthesizing Cu and Cu 2 O nanoparticles from CuO aqueous suspension via chemical reduction using the concept as an example. The dependency of mixed potential on pH and temperature is discussed in detail for the selective synthesis of nanoparticles.

Room-Temperature Synthesis of Cobalt Nanoparticles by Electroless Deposition in Aqueous Solution

Metallic Co nanoparticles of 24–110 nm diameters are prepared by electroless deposition (chemical reduction) in an aqueous solution at room temperature. The reduction process is monitored by an in situ measurement of a mixed potential. The mixed potential, which is above the redox potential of a Co(II)/Co redox pair, drops by the addition of the nucleating agent and also decreases with an increase in the concentration. Smaller Co nanoparticles are formed. In the smaller particle size, the fraction of the face-centered cubic Co phase increases, and the hexagonal close-packed Co phase decreases.

Electroless Deposition of Ferromagnetic Cobalt Nanoparticles in Propylene Glycol

Ferromagnetic Co nanoparticles with diameters of about 40-400 nm are synthesized by electroless deposition in boiling propylene glycol. The Co particle size is decreased to a certain degree by varying the concentration of starting materials and by adding nucleating agents. The electrochemical behavior of propylene glycol is investigated by in situ measurements of mixed potential to understand the formation of Co nanoparticles in polyol systems. The mixed potential decreases with an increase in temperature and in the presence of NaOH, which suggests the faster decomposition of propylene glycol. It also shifts abruptly to a more negative value when nucleating agents are added. This indicates that nucleating agents catalyze both the oxidation reaction of propylene glycol and the reduction reaction of Co(II) species, as well as aid in the formation of Co nanoparticles as heterogeneous nucleation sites.

Formation of Nickel Nanoparticles by Electroless Deposition Using NiO and Ni ( OH ) 2 Suspensions

Nickel particles ∼300 nm in diameter were fabricated by electroless deposition using hydrazine as a reducing agent in nickel hydroxide/ethylene glycol suspension at 353 K without any dispersing agent. The formation mechanism of nickel nanoparticles is discussed from the viewpoint of thermodynamics with in situ monitoring of nickel deposition and mixed potential. Specifically, in situ monitoring of mixed potential in combination with thermodynamic calculation is useful in discriminating whether or not nickel will be deposited in a reaction. The mixed potential drastically changed at the end point of the nickel deposition reaction, indicating that the cathodic reaction, which determined the mixed potential, switched from the nickel deposition reaction to hydrogen generation reaction.

Alternating Pulsed Electrolysis for Iron-Chromium Alloy Coatings with Continuous Composition Gradient

To develop a surface-finishing technique, an Fe-Cr surface alloying of an iron substrate was investigated using alternating pulsed electrolysis in an aqueous solution containing only Cr(III) ions as a single metal component. In this electrochemical process, the iron substrate was slightly dissolved during the anodic pulse, providing iron ions into the solution in the vicinity of the substrate, while Fe and Cr were both electrodeposited on the substrate surface during the subsequent cathodic pulse. Crack-free or fracture-free Fe-Cr alloy layers with continuous composition gradients were formed on the iron surface under appropriate pulse conditions. Both the thickness and Cr content of the Fe-Cr alloy layer increased with increasing anodic pulse time, suggesting that the Fe(II) ions which dissolved and stayed near the substrate induced the electrodeposition of chromium atoms. The time-dependent concentration distribution of Fe(II) ions in the vicinity of the substrate was digitally simulated through a simple diffusion model using a set of experimental data on current density and current efficiency. The effect of the Fe(II) ion concentration near the substrate surface was discussed in terms of the layer properties, such as chromium content and its anticorrosion behaviors.