Online ISSN : 2186-2451
Print ISSN : 1344-3542
ISSN-L : 1344-3542
Current issue
Displaying 1-7 of 7 articles from this issue
The 70th special feature “Research Frontiers of Electrochemical Capacitors”
  • Soshi SHIRAISHI, Koki URITA, Hirotomo NISHIHARA, Takeshi KONDO
    2024 Volume 92 Issue 7 Pages 074001
    Published: July 01, 2024
    Released on J-STAGE: July 01, 2024
    Advance online publication: May 14, 2024

    To further improve the energy density and reliability of carbon based electrochemical capacitors, such as electric double-layer capacitors and lithium ion capacitors, it is necessary to increase both the electric double layer capacitance and stability of the porous carbon electrodes used in the capacitors. Not only the specific surface area and pore size, but also the pore shape and pore size distribution, and the analytical methods to properly evaluate the pore structure should be considered when improving the capacitance. In the case of the high stability to high voltage charging, it is important to design and control the three-dimensional structure of the electrode as well as the crystal structure and crystallinity of the carbon. In this review, some advanced examples of the recent research on these topics will be discussed.

  • Masashi ISHIKAWA, Kaoru DOKKO, Hsisheng TENG, Simon LINDBERG, Jon AJUR ...
    2024 Volume 92 Issue 7 Pages 074003
    Published: July 03, 2024
    Released on J-STAGE: July 03, 2024
    Advance online publication: June 08, 2024

    Aqueous electrolytes containing sulfuric acid and organic electrolytes made by dissolving quaternary ammonium salts in propylene carbonate have long been used as electrolytes in supercapacitors. In the organic electrolyte system, acetonitrile was later also used as a solvent. This paper will describe more recent electrolytes for supercapacitors, including those in the research and development stage. The main electrolytes discussed here are novel-solvent-based electrolytes, highly concentrated electrolytes, deep eutectic electrolytes, polymer gel electrolytes, and solid-state electrolytes. The advantages as well as problems of these electrolytes are discussed, and the prospects of electrolytes for supercapacitors are presented.

  • Yoshikiyo HATAKEYAMA, Ryusuke FUTAMURA
    2024 Volume 92 Issue 7 Pages 074004
    Published: July 01, 2024
    Released on J-STAGE: July 01, 2024
    Advance online publication: May 22, 2024

    A detailed understanding of the energy storage mechanism is essential to enhance the energy density of electrochemical capacitors. This necessity has led to the widespread use of in situ and operando measurements of electrochemical devices with laboratory X-ray scattering equipment and synchrotron X-rays. For electrochemical capacitors, it is crucial to obtain comprehensive information on the behavior of the electrolytes within the nanopores, the formation of the electric double-layer, and the structural changes in both the electrode and the porous carbon used as the electrode material. This review will present recent studies on electric double-layer capacitors, highlighting the insights gained from X-ray scattering measurements.

  • Yu-Ting WENG, Tsung-Yi CHEN, Jeng-Lung CHEN, Nae-Lih WU
    2024 Volume 92 Issue 7 Pages 074005
    Published: July 01, 2024
    Released on J-STAGE: July 01, 2024
    Advance online publication: May 16, 2024

    Possessing high electronic conductivity and the nature of chemical inertness, the Magnéli phase titanium oxide Ti4O7 is a promising material for various electrochemical applications. Herein, the Ti4O7 electrode in aqueous Li2SO4 electrolyte is characterized for its supercapacitor applications. The oxide electrode exhibits pseudocapacitive behavior over a wide potential range of ±1.0 V (vs. Ag/AgCl), showing a specific capacitance of 105 F g−1, equivalent to 85 µF cm−2–oxide, along with outstanding high-rate performance and cycle stability (96 % capacitance retention after 5000 cycles). In situ X-ray absorption near-edge spectroscopy analysis on the Ti K-edge absorption reveals that the pseudocapacitance does not involve the redox reaction of the oxide electrode material. A pseudocapacitance mechanism attributed to the reversible redox reactions of the hydrogen and oxygen atoms adsorbed on the oxide surface is proposed.

Regular Papers
  • Kazuyuki IWASE, Takeyuki KAMIMURA, Itaru HONMA
    2024 Volume 92 Issue 7 Pages 077001
    Published: July 04, 2024
    Released on J-STAGE: July 04, 2024
    Advance online publication: June 11, 2024
    J-STAGE Data

    Ag-Sn bimetallic alloys were synthesized via mechanical alloying using a ball-milling process as electrocatalysts for the carbon dioxide (CO2) reduction reaction. Single-phase intermetallic compounds or solid solutions of bimetallic Ag-Sn alloy were successfully synthesized. The main reaction product for the CO2 reduction reaction was formate over the synthesized Ag-Sn alloy catalysts, and the catalyst with an intermetallic phase exhibited the highest activity toward formate generation, especially at high current density. This study demonstrated that mechanical alloying is a potential approach for the development of CO2 reduction reaction electrocatalysts.

  • Kana ONOUE, Tomoyuki WATANABE, Christopher C. JOHN, Akira NASU, Hiroak ...
    2024 Volume 92 Issue 7 Pages 077002
    Published: July 10, 2024
    Released on J-STAGE: July 10, 2024
    Advance online publication: June 14, 2024
    J-STAGE Data

    We apply the hydroflux method as a potential cathode-coating process for solid-state batteries. The garnet-type Li-ion conductor Li6.4La3Zr1.4Ta0.6O12 reacts with the precursor material of LiCoO2. Water molecules in the molten alkaline hydroxide initiate Li+/H+ ion exchange and dissolution of the Zr4+ species. The NASICON-type lithium-ion conductor LATP also reacts with molten hydroxide owing to the formation of soluble species of Al(OH)4 under high pH conditions. Perovskite-type Li0.33La0.55TiO3 is stable under the hydroflux condition because titanium and lanthanum do not form soluble species in alkaline solution. The chemical compatibility of the solid-state electrolyte is mostly estimated using Pourbaix diagram of each element in the system. The solid-state electrolyte containing only insoluble species in an alkaline solution is preferable for the hydroflux cathode-coating process.

  • Shinji NARUSE, Yuki OSHIRO, Shintaro NAKADA, Hiroki MUROYAMA, Toshiaki ...
    2024 Volume 92 Issue 7 Pages 077003
    Published: July 13, 2024
    Released on J-STAGE: July 13, 2024
    Advance online publication: June 22, 2024
    J-STAGE Data

    A H15P3V6Mo18O84 aqueous solution containing phosphovanadomolybdate anions such as [PV3Mo9O40]6− and [PV2Mo10O40]5− were applied to a catholyte of redox flow polymer electrolyte fuel cells (PEFCs). This system enables continuous power generation by the electrochemical reduction of the phosphovanadomolybdate anions over the carbon cathode and subsequent reoxidation of the reduced phosphovanadomolybdate anions in the cathode tank. Currently, investigations on H15P3V6Mo18O84 aqueous solution have been limited to compositional analysis only, and there are no reports of its application to the catholyte. In this study, the performance of H15P3V6Mo18O84 aqueous solution using as the catholyte in redox flow PEFCs was investigated. In addition, the pH of catholytes was increased, and effects on the performance were compared. Current–voltage measurements showed that increasing the pH (0.3 → 2.5) decreased the power density (42 mW cm−2 → 17 mW cm−2). On the other hand, the reduction–reoxidation operation showed that increasing the pH increased the reoxidation rate of the phosphovanadomolybdate anions (0.5 mmol min−1 → 2.2 mmol min−1). This study is the first step toward the use of H15P3V6Mo18O84 aqueous solution as a catholyte in redox flow PEFCs.