Kyoto University · Engineering
Professor Yuko Yokoyama's research lab specializes in electrochemical science and materials engineering, with a focus on fundamental electrochemistry in aqueous systems, particularly concentrated electrolytes and local pH dynamics near electrodes. The lab investigates electrochemical energy conversion devices such as aqueous batteries, exploring ways to expand the electrochemical window and improve performance through novel electrolyte design. Additional research directions include magnetic nanostructures, such as NiFe wires for spintronic applications, and advanced microfabrication techniques for functional materials like molten-solder droplet generation and superconducting thin films.
Figures are computed from collected data and may differ slightly.
Aqueous solutions have attracted considerable attention for use in aqueous-based energy-conversion devices such as aqueous lithium-ion batteries. While aqueous solutions have some desirable properties as electrolyte solutions, one drawback is the narrow potential window of water due to the electrolysis reactions of water (oxygen/hydrogen evolution reaction, OER/HER). Recently, it has been reported that the potential windows were expanded in concentrated electrolyte solutions. In this study, we i
In electrochemical reactions involving protons or hydroxide ions, local pH in the vicinity of a working electrode dynamically changes. In this study, we introduce the measurements of the local pH i...
Abstract When electrochemical reactions that generate/consume protons or hydroxide ions take place, the local pH in the vicinity of an electrode becomes considerably different from the bulk pH. However, measuring the local pH is very difficult, and the idea of using a rotating ring‐disk electrode (RRDE) is not applicable to solutions with neutral pH. Herein, we extend the theory of local pH measurement using a RRDE and make it applicable to solutions with a wide pH range. This extended theory is
The magnetization process in microfabricated NiFe wires was observed using a Kerr microscope. Magnetic wires were made from a 20-nm-thick NiFe film by using lift-off techniques. Their width W and length L were designed as W=0.5, 1.0 and 2.0 μm and L=50 μm, respectively. One end of the wire was connected to a square shaped head with a side of 2W, which is designed to act as a domain wall source. In each wire, necks with different width of 0.2W, 0.6W, and 0.8W were introduced as artificial pinning
<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We have developed a molten-solder ejection method by which molten-solder droplets from a 13- (1 pL) to 450- <formula formulatype="inline"><tex Notation="TeX">$\mu\hbox{m}$</tex></formula> (48 nL) diameter could be realized. This method has been developed as a technique to supply small solder balls. The authors have developed a newly designed solder ejection head and confirmed that solder balls with a
Fluoride ions are used in battery electrolytes in fluoride shuttle batteries. Since organic solvents are used in battery electrolytes, there is a growing demand to develop appropriate methods for quantifying fluoride ion concentration in organic solvents. In this study, a fluoride ion-selective electrode (ISE) for organic solutions is proposed as an electrode of the second kind. A Ag|AgF electrode was made via the anodization of a silver wire in propylene carbonate (PC) containing dissolved fluo
The flux penetration and trapping behavior in superconducting YBa 2 Cu 3 O X films have been observed quantitatively by means of the magnetooptical Faraday effect in iron garnet films with perpendicular magnetization. The mazelike domain pattern of the iron garnet film placed on the superconducting film is affected by the flux in the superconducting film and is observed using a polarizing microscope. Bi- and Ga-substituted YIG films grown by the liquid-phase epitaxy (LPE) method provide suitable
The flux penetration in superconducting thin films has been observed by means of the magnetooptical Faraday effect in iron garnet films. Bi- and Ga-substituted YIG films were used for the observation of the flux density distribution for the disk-shaped YBa 2 Cu 3 O x films with a radius of 0.5 mm and a thickness of 1800 Å. Critical current density J c was estimated from the whole profile of the flux density distribution without using Bean's model. The estimated J c is about 6×10 6 A/cm 2 at 10 K
Ca-doped Bi substituted iron garnet films (BiGdYb) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3-x</inf> Ca <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</inf> ( <tex xmlns:mml="http://w
The change of the lattice constant, the absorption loss and Faraday rotation due to reduction of Ca:YIG films are investigated. It was found that the absorption loss of the reduced Ca:YIG films at 1.15 µm is 2∼3 dB/cm irrespective of the content of Ca,Pb,Pt ions and the vacancies, and the Faraday rotation increased in the near infrared region and decreased in the visible region. It turns out that the reduction of Pb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.o
Annealing effects in H2-atmosphere on the optical absorption loss and lattice constant are studied for Ca- and Si-doped YIG and Ca-doped (BiGd)3(FeAlGa)5O12 films grown by LPE technique. H2-annealing was done at 150∼500°C for 22 hrs. The absorption loss and lattice constant in YIG films with poor impurities or doped Si did not change by H2-annealing at 150 ∼450°C, however, those in Ca-doped YIG and Bi-substituted garnet films changed notably. The changes of the absorption loss are explained by t
Reducing treatment effects on magneto-optical properties were studied for Ca-doped Bisubstituted iron garnet (BiGdCa)3(FeAlGa)5O12 films grown by the LPE technique. The films were reduced by using a hydrazine solution at 100∼110°C. Faraday rotation and absorption loss of as-grown and reduced films were measured at wavelength λ between 0.5 and 2.0 μm. By reducing treatment, the Faraday rotation remained unchanged, however, the absorption loss decreased for both Ca-free and Ca-doped films. The red
Optical absorption losses a and Faraday rotation θ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> in the wavelength region λ=0.5 to 2.5 μm were investigated for YIG films doped with Pb, Ca and other ions. The contributions of Pb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> , Pb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> ,
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