Papers by Author: Harumi Yokokawa

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Authors: Hiroshi Funakubo, M. Yonetsu, Kazuo Shinozaki, Nobuyasu Mizutani, M. Ishikawa, Naoya Sakai, Harumi Yokokawa
Authors: Teruhisa Horita, Haruo Kishimoto, Katsuhiko Yamaji, Natsuko Sakai, Yue Ping Xiong, Manuel E. Brito, Harumi Yokokawa
Abstract: Oxygen diffusion was measured at oxygen/cathode/electrolyte interfaces and at oxide scales/alloy interconnects in Solid Oxide Fuel Cells (SOFCs). A stable isotope oxygen exchange technique (16O/18O exchange) was adopted to label the diffusion profiles in the oxides, and secondary ion mass spectrometry (SIMS) depth profiles were examined to determine the diffusion coefficients in the oxide ceramic materials. Diffusivity of oxygen in LaMnO3 cathode was measured under polarized condition (that is, considering the electrochemical process and diffusion). Also, oxygen diffusion in the oxide scales formed on the alloy was measured to clarify the formation mechanism of oxide scale.
Authors: Teruhisa Horita, Haruo Kishimoto, Katsuhiko Yamaji, Natsuko Sakai, Yue Ping Xiong, Manuel E. Brito, Harumi Yokokawa
Abstract: The electrochemical reaction of solid oxide fuel cells (SOFCs) was reviewed in terms of mass and charge transports of reaction species around the electrode/electrolyte interfaces. Oxygen reduction and fuel oxidation were analyzed by isotope labeling and secondary ion mass spectrometry as well as conventional electrochemical method. The SIMS images after 18O2 (stable isotope oxygen) labeling suggested that the O2/cathode/electrolyte interfaces were the most active parts for oxygen reduction and incorporation. The widths of active parts of oxygen reduction were about several 100 to some 1000 nm different depending on the cathode materials and reaction mechanism. The isotope labeling-SIMS technique was also applied to visualize the active parts for CH4 decomposition and carbon deposition around the anode metal/electrolyte oxide interfaces. The active parts for carbon deposition were only on the Ni surface on YSZ electrolyte. The effect of substrate oxide on the carbon deposition was also examined at the mesh-shaped metal/oxide interfaces.
Authors: Haruo Kishimoto, Katsuhiko Yamaji, Manuel E. Brito, Teruhisa Horita, Harumi Yokokawa
Abstract: Degradation and disintegration of the nickel base cermet anode is a serious problem in direct utilization of hydrocarbon fuels, especially for liquid fuels such as kerosene. Cell operation test was carried out for the SOFC with nickel – scandia stabilized zirconia (Ni-ScSZ) cermet anode by direct feeding of several liquid hydrocarbons at 1073 K at S/C = 2.0. For n-dodecane (C12H26) fuel, hydrogen produced by reforming reaction on the anode was mainly used for electrochemical oxidation reaction. In contrast, for desulfurized kerosene test, cell operation was only possible by lower current density. Furthermore, noticeable nickel particles growth occurred. This nickel sintering should be enhanced by remaining minor impurities, including sulfur, in the desulfurized kerosene. From the thermodynamic consideration, Ni-S eutectics can be stably formed under some SOFC operation conditions with 100 ppm of sulfur. Carbon deposition was observed on the nickel surface for both Ni-yttria stabilized zirconia (YSZ) and Ni-ScSZ systems, but was noticeably suppressed for Ni-gadolinia doped ceria (GDC) system.
Authors: Yoshitaka Baba, Harukuni Kameda, Yoshio Matsuzaki, Satoshi Yamashita, Nobutaka Yasuda, Toshihiro Uehara, Teruhisa Horita, Katsuhiko Yamaji, Harumi Yokokawa
Abstract: Manganese-cobaltite spinel coatings were produced on Fe-Cr alloys for the improvement of the chemical and mechanical stability of solid oxide fuel cell (SOFC) interconnects. It was found by thermal investigation, i.e., the microscopy of various samples that were heat-treated in air at 800 °C, that the screen-printed coating more effectively inhibited oxide scale growth than the sputtered coating. The reason why the manufacturing method of the spinel coating affected the oxide scale growth rate was investigated. It was demonstrated that the oxide scale in both the samples after annealing in air at 800 °C for 5000 h comprised MnCr2O4 and Cr2O3 with no difference in composition. However, the interface between the alloy and the oxide scale was deeper and rougher and had a larger grain size because of the high oxygen diffusivity in the sputtered coating. In contrast, in the screen-printed sample, the dense spinel layer above the oxide scale blocked oxygen diffusion into the alloy, so the oxide scale growth rate was lower and the interface between the alloy and oxide scale remained flat even after thermal treatment. Introducing a reduction treatment in the manufacturing process made the Mn-Co spinel layer denser and further inhibited the oxide scale growth. Moreover, the addition of Li as a sintering aid into the Mn-Co spinel was found to even more effectively inhibit the oxide scale growth.
Authors: Tatsuya Kawada, Harumi Yokokawa
Authors: Teruhisa Horita, Katsuhiko Yamaji, Natsuko Sakai, Tohru Kato, Tatsuya Kawada, Harumi Yokokawa
Authors: Harumi Yokokawa, Katsuhiko Yamaji, Teruhisa Horita, Natsuko Sakai, Manuel E. Brito
Abstract: Utilization of chemical potential diagrams is powerful in predicting the chemical stability of dissimilar materials and analyzing the diffusion path when reactions are proceeded. Since practical materials consist of many components so that construction of chemical potential diagrams for the multi-component systems becomes crucial. There is one powerful way of treating phase equilibria for such complicated systems in chemical potential diagrams; that is, a construction of three dimensional diagrams, although all phase relations are not visible in such a diagram. To make phase relations visible, there are several ways; one is to make selected phase transparent. This makes it possible to examine the detailed relations between selected materials. Second one is to make dissections at selected values for the chemical potential of selected chemical species. By swinging the dissected value, the change in phase relations can be examined as a function of given chemical potential. Some examples will be given for the formation of oxide scale on ferritic alloys.
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