Electrochemistry
Online ISSN : 2186-2451
Print ISSN : 1344-3542
ISSN-L : 1344-3542
Volume 70 , Issue 8
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  • Yoshinori KIDA, Katsunori YANAGIDA, Atsuhiro FUNAHASHI, Toshiyuki NOHM ...
    2002 Volume 70 Issue 8 Pages 590-594
    Published: August 05, 2002
    Released: September 10, 2019
    JOURNALS OPEN ACCESS

    We analyzed the states of lithium intercalated into carbon and of lithium compounds on the surface of carbon by 7Li nuclear magnetic resonance (NMR) spectroscopy. In the case of graphite-coke hybrid carbon, the ratio of the relative peak area of inactive lithium after 500 cycles to that after 10 cycles was a smaller than that in the case of graphite or coke. The quantity of inactive lithium, which did not include the lithium compounds produced at initial charging, in graphite, coke, and graphite-coke hybrid carbon significantly differed after 500 charge/discharge cycles. In order to clarify the voltage region in which inactive lithium is produced, we carried out charge/discharge cycle tests in the higher and the lower voltage regions, respectively, and analyzed the electrodes by 7Li NMR spectroscopy. In the case of a coke negative electrode, the electrode deterioration was large after the cycle test in the higher voltage region, but was almost non-existent after that in the lower voltage region. Conversely, in the case of a graphite negative electrode, the electrode deterioration was significantly larger after the lower voltage region cycles. A battery using graphite-coke hybrid carbon as the negative electrode performed better with a reduced generation amount of inactive lithium under charge/discharge cycle.

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  • Masayuki ITAGAKI, Ken-ichi SHIMODA, Kunihiro WATANABE
    2002 Volume 70 Issue 8 Pages 595-601
    Published: August 05, 2002
    Released: September 10, 2019
    JOURNALS OPEN ACCESS

    The anodic dissolution mechanisms of iron in acetonitrile solution containing water have been studied by electrochemical impedance spectroscopy (EIS). Firstly, the influence of water was investigated by measuring the i-E curve. There were four anodic regions in the i-E curve, i.e., (i) the inert region, (ii) the first active dissolution region, (iii) the current plateau region and (iv) the second active dissolution region. The potential of first active dissolution region (ii) shifted to negative with the increase of the water concentration. The electrochemical impedance was measured at each anodic region. An inductive loop was described on the Nyquist plot of electrochemical impedance in the anodic regions (ii) and (iv), indicating the presence of reaction intermediate. A negative resistance was observed in the anodic region (iii), meaning the formation of oxide films on the iron electrode. The dissolution mechanisms of iron were proposed on the basis of the above-mentioned results and the spectrophotometric analyses of Fe(II) and Fe(III) dissolved during electrolysis. The numerical simulations were performed to confirm the proposed mechanisms and to obtain the kinetic parameters.

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  • Masashi MORI
    2002 Volume 70 Issue 8 Pages 602-608
    Published: August 05, 2002
    Released: September 10, 2019
    JOURNALS OPEN ACCESS

    The shrinkage mechanism of nondoped lanthanum manganite perovskites, La1−xMnO3+δ (0 ≤ x ≤ 0.1), during a thermal cycling in air has been investigated. For the La1−xMnO3+δ samples annealed at 600°C, they shrank during a thermal cycle measurement in the temperature range from 600 to 1100°C, and a decrease of their linear thermal expansion coefficients at temperatures > around 940°C was observed. It was found during the thermal cycle measurement in the temperature range from 50 to 800°C that the LaMnO3+δ sample quenched from 1000°C exhibited a large shrinkage at around 700°C by phase change from the orthorhombic into the hexagonal perovskite. For the La0.95MnO3+δ and La0.9MnO3+δ perovskite samples without the phase change, no shrinkage behavior was observed.

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  • Marius CHEMLA, Valérie BERTAGNA, François ROUELLE, Sébastien PETTTDI ...
    2002 Volume 70 Issue 8 Pages 609-614
    Published: August 05, 2002
    Released: September 10, 2019
    JOURNALS OPEN ACCESS

    A careful study of the impedance at zero current of a silicon substrate in a semiconductor/oxide/electrolyte (SOE) structure permitted to identify the contribution of the depletion layer under various bias potentials. Modelling the equivalent circuit proved that the imaginary component was a pure capacitor C in parallel with a pure resistance R. Experimental data showed that these two components undergo a steep variation when the system approaches the silicon flat band potential situation. A theoretical development is presented under the assumption that the gradient of potential inside the material is small enough for a simplified treatment based on the linearization of the exponential function. The steep increase in the vicinity of the flat band potential of the space charge capacitance and the conductance was confirmed. It constitutes a useful tool for electrochemical studies to determine the band level curvature as a function of the sample potential measured vs. a reference electrode.

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  • Hideo MAEDA, Hisatoshi FUKUMOTO, Kenro MITSUDA
    2002 Volume 70 Issue 8 Pages 615-621
    Published: August 05, 2002
    Released: September 10, 2019
    JOURNALS OPEN ACCESS

    The possibility of the electrolytic oxidation of CO as a means of reducing the CO content in reformed gas for PEFCs was investigated. A two-cell stack of PEFCs was used as a CO electrolysis module. To study the CO adsorption rate and CO selectivity of the module, three types of electrolyzing methods, i.e., constant current electrolysis, potential stepped electrolysis, and repeating pulse electrolysis, were used. It was found that the CO concentration can be reduced from 1% to below the 50 ppm level when a voltage exceeding more than 0.4 V is applied to each cell of the module. The maximum selectivity of the CO electrolytic oxidation was observed to reach 18% using the repeating pulse electrolysis method in a low current region. However, in a high current region, the selectivity of the CO oxidation did not improve.

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  • Akio TANAKA, Hiroki TAMURA, Shinichi KIKKAWA
    2002 Volume 70 Issue 8 Pages 622-629
    Published: August 05, 2002
    Released: September 10, 2019
    JOURNALS OPEN ACCESS

    Spinel-type manganese dioxide, λ-MnO2, has a three dimensional network of tunnels connecting vacant sites in the oxide lattice. This oxide can preferentially incorporate lithium ions into the lattice vacancies since lithium ions fit the tunnels in size. With the incorporation, the lattice Mn(IV) reduces to Mn(III) maintaining electric neutrality. The rate of incorporation decreased with time, suggesting inhibition of the reaction due to reaction products as pre-formed Mn(III) in the oxide decreased the incorporation rate. The incorporation rate increased with increasing pH, temperature, the mass concentration and specific surface area of the λ-MnO2 samples, and the amount of a radical scavenger added. A kinetic model is proposed by considering the following reaction processes: (1) an Mn(IV)-vacancy pair on the oxide surface oxidizes a hydroxide ion in solution and forms a free vacancy, lattice Mn(III), and a hydroxyl radical. A part of the products reacts backward because the reaction products do not satisfy electric neutrality; (2) the free vacancy remaining takes up the lithium ion from the solution to reestablish electric neutrality; (3) the formed Mn(III)-lithium ion pair reduces the Mn(IV)-vacancy pair in the oxide bulk and the lithium ion transfers to the bulk through the tunnel, regenerating the Mn(IV)-vacancy pair on the surface; and (4) the hydroxyl radical decomposes to oxygen and water. The steps (1) and (2) were assumed to be rate determining, and the rate equation was derived. The model rate equation reproduced the observed results, and the lithium ion incorporation properites of λ-MnO2 could be evaluated with the model parameters.

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