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MUROYAMA Hiroki

    Department of Applied Chemistry　Associate Professor
Last Updated :2024/05/15

Researcher Information

J-Global ID

Research Interests

  • 触媒化学、電気化学   

Research Areas

  • Manufacturing technology (mechanical, electrical/electronic, chemical engineering) / Catalytic processes and resource chemistry
  • Nanotechnology/Materials / Inorganic compounds/materials chemistry

Education

  • 2006/04 - 2009/01  Kyoto University  Graduate School of Engineering  Department of Energy and Hydrocarbon Chemistry
  • 2004/04 - 2006/03  Kyoto University  Graduate School of Engineering  Department of Energy and Hydrocarbon Chemistry
  • 2000/04 - 2004/03  Kyoto University  Faculty of Engineering  School of Industrial Chemistry

Association Memberships

  • The Electrochemical Society   The SOFC Society of Japan   The Japan Petroleum Institute   The Electrochemical Society of Japan   Catalysis Society of Japan   

Published Papers

  • Toshiaki Matsui; Hiroaki Tsujimoto; Hiroki Muroyama; Koichi Eguchi
    ECS Transactions 2023/05
  • Wenwen Zhang; Hiroki Muroyama; Yuichi Mikami; Qingshi Liu; Xiaojuan Liu; Toshiaki Matsui; Koichi Eguchi
    Chemical Engineering Journal 461 1385-8947 2023/04 
    Here, a novel triple-conducting composite cathode for protonic ceramic fuel cells, with superior oxygen reduction reaction (ORR) activity and stability, is developed by replacing cerium with zirconium in BaCo0.7(Ce0.8Y0.2)0.3O3−δ (BCCY) to construct strongly interacting interfaces. The one-step synthesized BaCo0.7(Zr0.8Y0.2)0.3O3−δ (BCZY) is composed of multiple phases with different functionalities, reaching a polarization resistance of 0.25 Ω cm2 at 600 °C while maintaining excellent performance stably for 206 h. The excellent ORR activity is attributed to the high concentration of active sites and the strong hydration capability at strongly interacting interfaces of Co-rich and Zr-rich phases which is introduced by the interdiffusion of Co and Zr. The low thermal expansion coefficient is also achieved through this interface, contributing to the excellent stability of BCZY. This study highlights the role of strongly interacting interfaces in enhancing the ORR activity and stability of materials and provides a new strategy for developing triple-conducting composite cathodes.
  • Younghwan Im; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    International Journal of Hydrogen Energy 47 (76) 32543 - 32551 0360-3199 2022/09 
    The basic property of the catalyst can influence the catalytic performance for the NH3 decomposition by alleviating associative desorption of N atom, known as a rate-determining step (RDS) in the Ru-based catalysts, but there were few experimental data to prove the alleviation of this step. Therefore, the Ru catalyst supported on rare-earth oxides and zirconium oxide were synthesized to compare the catalytic performance for NH3 decomposition and its dependence on the basic property of the Ru catalysts, including the other characterizations. The catalytic activities at 500 °C decreased in order of Ru/La2O3 > Ru/PrxOy > Ru/Sm2O3 ≅ Ru/Gd2O3 > Ru/Y2O3 > Ru/Yb2O3 > Ru/ZrO2 catalysts. This catalytic activity was related to the basic strength of the catalysts, which can influence the electron state of the impregnated Ru particles. The enhanced basic strength could modify the electronic state of the Ru particles on the support owing to electron donation from the oxide support to the Ru particles, and NH3-temperature programmed surface reaction (NH3-TPSR) confirmed that this enhanced electron donation could promote the desorption step of N atoms.
  • Yu Katayama; Ryoma Kubota; Reshma R. Rao; Jonathan Hwang; Livia Giordano; Asuka Morinaga; Takeou Okanishi; Hiroki Muroyama; Toshiaki Matsui; Yang Shao-Horn; Koichi Eguchi
    JOURNAL OF PHYSICAL CHEMISTRY C 125 (48) 26321 - 26331 1932-7447 2021/12 [Refereed]
     
    Direct methanol fuel cells (DMFCs) using alkaline electrolytes are of interest due to the applicability of nonprecious metal-based materials for electrocatalysts. However, the lack of understanding of the methanol oxidation reaction (MOR) mechanism in alkaline media hinders the development of active catalysts for the MOR. In this work, ambient-pressure XPS and in situ surface-enhanced infrared spectroscopy were performed on the Pt surface in order to gain experimental insights into the reaction pathway for the MOR. We present a comprehensive reaction mechanism for the MOR in alkaline media and show that the MOR proceeds via two different pathways depending on the electrode potential. We confirmed the formation of partially hydrogenated CO adsorbates [HxCOad···(OH) (1 < x < 3)] via water and/or hydroxide ion-mediated dissociation of methanol. The HxCOad···(OH) species were further dehydrogenated to COad in the potential range of 0.40-0.60 VRHE and subsequently oxidized to CO2 by reactive OHad on the Pt surface at 0.65 VRHE (pathway I). Furthermore, H3C-Oad intermediates were observed at potentials higher than 0.9 VRHE, at which the MOR proceeds mainly via H3C-Oad instead of COad intermediates (pathway II). The oxidation current related to this conversion from H3C-Oad to CO2 (pathway II) dominates the overall MOR current, suggesting that the H3C-Oad pathway could be one of the keys to enhancing the MOR activity in an alkaline environment. Our findings pave the way toward a design strategy for MOR electrocatalysts with improved activity based on the experimental reaction mechanisms that have been identified.
  • Toshiaki Matsui; Tomoki Fujinaga; Ritsuki Shimizu; Takashi Ozeki; Hiroki Muroyama; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY 168 (10) ARTN 104509  0013-4651 2021/10 [Refereed]
     
    High energy conversion efficiency in solid oxide fuel cell (SOFC) systems can be achieved by improving the fuel utilization. However, an operation at high fuel utilizations negatively affects Ni-based cermet anodes; the downstream part of the anode is exposed to high P(O2) atmospheres due to the depletion of fuel and the high steam concentration. At this stage, no reports dealt with the degradation phenomena of the anode under such severe conditions. In this study, we aimed to clarify degradation phenomena at the downstream part of practical cells operated at high fuel utilizations by using bottom cells. Fuel gases simulated severe conditions were supplied to single cells. The change in electrochemical properties was monitored during discharge operation, and the microstructural evolution of the anode was quantitatively analyzed by the focused ion beam-scanning electron microscope (FIB-SEM) technique.
  • Ammonia Decomposition over Ni Catalysts Supported on Perovskite-type Oxide
    室山 広樹; 松井 敏明; 江口 浩一
    触媒 = Catalysts & catalysis 触媒学会 63 (2) 101 - 107 0559-8958 2021/04 
    Ammonia decomposition over various Ni catalysts supported on perovskite-type oxide was investigated. The effective species in the perovskite-type oxides for the reaction were examined by systematically changing the A-site and B-site elements. The AEZrO3 (AE = Sr, Ba) compounds were effective as supporting materials for Ni catalysts because of their basic properties. The Ni–BaCe0.4Zr0.4Y0.2O3−δ (BCZY) catalyst was fabricated for a fuel electrode in ammonia-fueled solid oxide fuel cells (SOFCs). The ammonia-fueled SOFC employing this fuel electrode showed comparable performance to the hydrogen-fueled one at 550–700°C due to the high catalytic activity of this electrode for ammonia decomposition.
  • Masitah Hasan; Toshiki Asakoshi; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    PHYSICAL CHEMISTRY CHEMICAL PHYSICS 23 (9) 5551 - 5558 1463-9076 2021/03 [Refereed]
     
    Supported Ni catalysts are active in CO2methanation. It is important to understand the reaction mechanism for the development of highly-active catalysts. In this study, we investigated the reaction pathways of CO2methanation over Ni/Y2O3and Ni/Al2O3based on the adsorbates observed by diffuse reflectance infrared Fourier transform spectroscopy. For Ni/Al2O3, linear and bridged CO adsorbates were converted to nickel carbonyl hydride and/or formyl species, which would be further hydrogenated to methane. In contrast, the formation of formate adsorbates was specifically confirmed over Ni/Y2O3under the CO2methanation condition. The hydrogen molecule was activated by dissociatively-adsorbing on Ni particles. Then, the hydrogenation of formate adsorbates by the activated hydrogen species proceeded sequentially to form methane. The observed bridged CO species would not be a major intermediate for Ni/Y2O3
  • S. Yamaguchi; H. Muroyama; T. Matsui; K. Eguchi
    JOURNAL OF POWER SOURCES ELSEVIER 488 0378-7753 2021/03 
    For an extremely efficient power generation using biomass, a renewable energy, we investigated characteristics of an autothermal downdraft gasifier and SOFCs. Cypress chips were converted into gasified gases with the efficiency of 90%. The gasification rate saturated as the air flow rate increased, because of the heat losses from char surfaces via radiation and convection. For the discharge of SOFCs, electrochemical H-2 oxidation was more dominant than CO oxidation in a model gasified gas, due to water gas shift reaction and selective electrochemical H-2 oxidation via H-spillover. Electrochemical H2O reduction was inhibited by strongly adsorbed CO on Ni in the model gasified gas with the reverse bias operation. The maximum polarization resistances of an anode-supported cell were more than two times larger than the charge transfer resistances in the model gasified gas. The power density in the model gasified gas attained 62% of that in humidified H-2 at 0.8 V.
  • T. Matsui; T. Fujinaga; T. Ozeki; R. Shimizu; H. Muroyama; K. Eguchi
    ECS Transactions 103 (1) 1607 - 1614 1938-6737 2021 
    High energy conversion efficiency in solid oxide fuel cell (SOFC) systems can be achieved by improving the fuel utilization. However, when the SOFCs are operated under high fuel utilization, the downstream part of the anode is exposed to high steam concentration and hence high P(O2) atmosphere. This has a negative impact on the stability of Ni-based cermet anodes. At this stage, no reports dealt with the degradation phenomena of the anode under such severe conditions. In this study, we aimed to clarify degradation phenomena at the downstream part of practical cells operated at high fuel utilizations by using bottom cells. The change in electrochemical properties of single cells was monitored during discharge operation with supplying fuel gases that simulated severe conditions.
  • Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF THE JAPAN PETROLEUM INSTITUTE 64 (3) 123 - 131 1346-8804 2021 [Refereed]
     
    Utilization of hydrogen carriers is an important method for the storage and transportation of hydrogen energy. This review introduced recent research on using supported nickel catalysts for methane production from hydrogen and carbon dioxide, and hydrogen production from ammonia. In the methane production, methane yield was drastically enhanced at 225-250 °C and reached maximum at 300-350 °C for most catalysts tested. CO2 desorption behavior over the catalysts indicated that moderate basic sites should positively affect the catalytic activity for the reaction. IR spectroscopic analyses showed that CO2 methanation over Ni/Al2O3 proceeded via the formation of CO intermediate, whereas the formate adsorbate would be the main intermediate over Ni/Y2O3. For the hydrogen production from ammonia, rare-earth oxide-supported catalysts exhibited relatively high performance despite the low surface area of the support materials. Ammonia conversion over Ni/Y2O3 reached 87% at 550 °C. Rare-earth components were also effective as additives for the Ni/Al2O3 catalyst. These results were derived from the alleviation of hydrogen inhibition by rare-earth materials.
  • Toshiaki Matsui; Naoki Kunimoto; Kohei Manriki; Kazunari Miyazaki; Naoto Kamiuchi; Hiroki Muroyama; Koichi Eguchi
    Journal of Materials Chemistry A Royal Society of Chemistry (RSC) 9 (27) 15199 - 15206 2050-7488 2021 

    The oxygen reduction reaction mechanism on the Ba5SrGd2Co4O15–BaCe0.5Pr0.3Y0.2O3−δ (BSGC–BCPY) composite cathode for proton-conducting fuel cells.

  • Kazunari Miyazaki; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    SUSTAINABLE ENERGY & FUELS 4 (10) 5238 - 5246 2398-4902 2020/10 [Refereed]
     
    Ammonia has been considered a promising alternative energy carrier due to its high hydrogen content and lack of carbon. Since ammonia readily decomposes into hydrogen and nitrogen at high temperatures, it can be used as a fuel for solid oxide fuel cells (SOFCs). In this work, the effect of the ammonia decomposition reaction over an anode on the performance of direct ammonia-fueled SOFCs was evaluated by electrochemical measurements and catalytic activity tests. A Ni-BaCe0.4Zr0.4Y0.2O3−δcermet with excellent catalytic activity for ammonia decomposition was employed as the anode material. Impedance analyses revealed that the total cell resistance strongly depended on the ammonia fuel supply conditions. Particularly, the polarization resistance, which was attributed to a mass transfer process in the anode, increased at high ammonia concentrations and flow rates. On the other hand, in the catalytic activity tests, the conversion of ammonia over the cermet decreased under such fuel supply conditions. Hence, it was concluded that the performance of direct ammonia-fueled SOFCs significantly depended on the ammonia conversion over the anode. Additionally, a considerable reduction in the anode temperature as a result of the endothermic ammonia decomposition reaction was confirmed.
  • Younghwan Im; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 45 (51) 26979 - 26988 0360-3199 2020/10 [Refereed]
     
    The Ni catalysts supported on alkaline earth metal aluminate compounds, Ni/AM-Al-O (AM = Mg, Ca, Sr, Ba) were synthesized to investigate the influence of their basic property on NH3 decomposition activity. The basic strength of the catalysts was confirmed to correspond to that of added alkaline earth metal in the support materials (Ni/Mg–Al–O < Ni/Ca–Al–O < Ni/Sr–Al–O < Ni/Ba–Al–O) from CO2-TPD measurement. This basic strength of the catalysts could influence the catalytic activity for NH3 decomposition, which increased in order of the Ni/Mg–Al–O < Ni/Ca–Al–O < Ni/Sr–Al–O < Ni/Ba–Al–O catalysts. NH3-TPSR showed that the strong basic property weakened H2 adsorption but slightly strengthened N2 adsorption for the catalysts except for the Ni/Mg–Al–O catalyst. From the kinetic analysis, the absolute value of the H2 reaction order decreased with increasing basic strength of the catalysts, indicating that the strong basic property of the catalysts could alleviate the H2 inhibition in ammonia decomposition.
  • 受賞講演 2019年度奨励賞 水素キャリアの製造・利用に向けた金属酸化物担持ニッケル触媒の開発
    室山 広樹
    ペトロテック = Petrotech : 石油学会情報誌 / 石油学会 編 石油学会 ; 1978- 43 (9) 598 - 602 0386-2763 2020/09
  • Yuanhui Wang; Zehui Yang; Jun Yang; Jingxiang Xu; Yaoping Xie; Jieyu Chen; Minghui Yang; Jianxin Wang; Wanbing Guan; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 45 (41) 21816 - 21824 0360-3199 2020/08 [Refereed]
     
    One of the key challenges for ammonia-fed anion exchange membrane fuel cells is to the ammonia electro-oxidation reaction (AEOR) at anode, which has sluggish kinetics and generates atomic nitrogen (Nads) poisoning the Pt catalyst. In this study, a comparative study on Pt/Ta3N5, Pt/Ta2O5, Pt/carbon black, and Pt plate are conducted in order to clarify the promoting effect of the support materials for Pt catalysts. X-ray photoelectron spectroscopy analysis and density functional theory calculations reveal that the support materials significantly affect the electron condition of the Pt, resulting in the tuned adsorption energy of Nads on Pt surface. The electrochemical analysis demonstrates that the weakened adsorption of Nads lowers the coverage of Nads on Pt surface, resulting in the enhanced performance and stability of Pt catalysts for AEOR. In particular, Pt/Ta3N5 catalyst exhibits a current density of 5.92 mA cm−2 of AEOR at −0.34 V vs. SCE, which is higher than that of Pt/Ta2O5 (2.56 mA cm−2, −0.35 V vs. SCE) and Pt/C (4.45 mA cm−2, −0.26 V vs. SCE). The achievements in this study demonstrate the importance of controlling the type of supports for the development of an active electrocatalyst for continuous AEOR.
  • Yuanhui Wang; Yuchen Gu; Hua Zhang; Jun Yang; Jianxin Wang; Wanbing Guan; Jieyu Chen; Bo Chi; Lichao Jia; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi; Zheng Zhong
    APPLIED ENERGY 270 UNSP 115185  0306-2619 2020/07 [Refereed]
     
    Ammonia as an alternative fuel of hydrogen for solid oxide fuel cells is attractive owing to its low cost, ease for transportation and storage, and non-carbon emission. In this work, NH3 power generation was carried out using a flat-tube SOFC with symmetric double-sided cathodes (DSC). The cell performance and durability as well as the catalytic activity of NH3 decomposition within the anode were evaluated simultaneously. The power density of NH3-fueled DSC reached 195 mW cm−2 at 750 °C, which was close to that of H2-fueled one (198 mW cm−2). No apparent degradation of the NH3-fueled DSC cell was observed after stability test for 120 h at 750 °C. In addition, the DSC cell constantly fed with NH3 exhibited stable open circuit voltages throughout a thermal cycling test between 550 and 750 °C for 15 cycles, indicating that no microstructural damage was caused by such severe operation condition. Furthermore, it was found that adding extra catalyst into the inner channels of the anode support promoted the NH3 conversion rate in DSC cell from 83% to 95% at 750 °C, which agreed with the theoretical calculation results. These results demonstrated the promising prospect of the DSC for efficient and durable ammonia power generation.
  • Toshiaki Matsui; Norifumi Yoshida; Hiroki Muroyama; Koichi Eguchi
    JOURNAL OF MATERIALS CHEMISTRY A 8 (23) 11867 - 11873 2050-7488 2020/06 [Refereed]
     
    Recently, segment-in-series tubular solid oxide fuel cells (SOFCs) fabricated by Mitsubishi Hitachi Power System (MHPS) exhibited excellent performance for more than 30 000 h with a degradation rate of -0.10% kh-1; cells consisted of Ni-yttria-stabilized zirconia (YSZ) cermet anode, YSZ electrolyte, Sm-doped ceria (SDC) cathode interlayer, and (La,Sr,Ca)MnO3 (LSCM) cathode. This degradation rate was smaller than those for conventional cells with a LSCF cathode/Gd-doped ceria interlayer/YSZ electrolyte configuration. The reason for such high durability has not been elucidated sufficiently, though the microstructure at the cathode side changed upon discharge operation as in the case of conventional cells. In this study, then, the constituent materials as well as the cell configuration, which were presumed in the vicinity of the cathode/interlayer interface after prolonged operation of real cell stacks, were modeled. Concretely, the composites of (Sm,Ca)MnO3-MnOx were prepared as model cathodes and their electrocatalytic activity for the oxygen reduction reaction was studied with varying a manganese content. Furthermore, the electrochemical activity, the phase stability, and the microstructural stability of (Sm,Ca)MnO3-MnOx were analyzed in detail under the coexistence of SDC ((Sm,Ca)MnO3-MnOx-SDC composite system). This journal is
  • Yuanhui Wang; Jun Yang; Jianxin Wang; Wanbing Guan; Bo Chi; Lichao Jia; Jieyu Chen; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY 167 (6) ARTN 064501  0013-4651 2020/03 [Refereed]
     
    Ammonia is regarded as a promising alternative fuel for solid oxide fuel cells (SOFCs) compared with hydrogen due to its low cost and ease for transportation and storage. The conventional anode Ni/yttria-stabilized zirconia (Ni/YSZ) of SOFCs can serve as an efficient catalyst for ammonia decomposition at high temperatures above 700 °C but its catalytic activity decreases rapidly at lower temperatures, leading to an undesirable degradation of cell performance. Here we report a low-temperature ammonia decomposition catalyst Ba-modified Ni/yttria-stabilized zirconia (Ba-Ni/YSZ) prepared by a one-pot solid-liquid (SL) method. Enhanced performance can be obtained for Ba-Ni/YSZ compared with that of Ni/YSZ catalysts, indicating that the addition of Ba promotes the catalytic activity. X-ray photoelectron spectroscopy (XPS) and Temperature-programmed reduction with H2 (H2-TPR) characterizations of Ba-Ni/YSZ catalysts indicate that Ba changes the electronic structure of Ni and facilitates the interaction between Ni and support. The kinetics analysis demonstrates that the modification by a small amount of Ba species alleviated the negative effect of the hydrogen poisoning on the active sites of Ni/YSZ catalysts. Besides, the Ba-Ni/YSZ catalyst is introduced into the flat-tube SOFC with symmetric double-sided cathodes (DSC), and significantly improves the electrochemical performance of directly ammonia-fed DSC cells at low temperatures.
  • M. Kishimoto; H. Muroyama; S. Suzuki; M. Saito; T. Koide; Y. Takahashi; T. Horiuchi; H. Yamasaki; S. Matsumoto; H. Kubo; N. Takahashi; A. Okabe; S. Ueguchi; M. Jun; A. Tateno; T. Matsuo; T. Matsui; H. Iwai; H. Yoshida; K. Eguchi
    FUEL CELLS 20 (1) 80 - 88 1615-6846 2020/02 [Refereed]
     
    Power generation performance and long-term durability of ammonia-fueled solid oxide fuel cell (SOFC) systems are investigated with SOFC stacks consisting of 30 planar anode-supported cells. SOFC systems with three different operation modes are employed: direct ammonia, external decomposition and autothermal decomposition. A novel BaO/Ni/Sm2O3/MgO catalyst is newly developed for the external ammonia cracker, whereas a Co-Ce-Zr composite oxide catalyst is used for the autothermal ammonia cracker. Initial performance measurement and 1,000 h long-term durability test of the stacks are conducted. The stack fueled with direct ammonia achieves 1 kW power output with 52% direct current (DC) electrical efficiency; a slight decrease in its performance compared with the stack with the mixture fuel of hydrogen and nitrogen is attributed to the decrease in the stack temperature caused by the endothermic ammonia decomposition reaction. The external ammonia cracker helps to maintain the stack temperature, improving the initial performance of the stack. The stack performance with the autothermal ammonia cracker is also comparable to those with the other operation modes. It is also demonstrated that the stacks fueled with ammonia have excellent stability during the long-term tests and 57% energy conversion efficiency at ca. 700 W electrical output is achieved with the external ammonia cracker.
  • Masashi Kishimoto; Hiroki Muroyama; Shinsuke Suzuki; Masaki Saito; Takeshi Koide; Yosuke Takahashi; Toshitaka Horiuchi; Hayahide Yamasaki; Shohei Matsumoto; Hidehito Kubo; Naoya Takahashi; Akinori Okabe; Satoshi Ueguchi; Munsuk Jun; Tomoya Muramoto; Hiroaki Ohara; Takahiro Matsuo; Toshiaki Matsui; Hiroshi Iwai; Hideo Yoshida; Koichi Eguchi
    ICOPE 2019 - 7th International Conference on Power Engineering, Proceedings 839 - 842 2020 
    Technological feasibility of ammonia-fueled solid oxide fuel cell (SOFC) systems are investigated with SOFC stacks consisting of 30 planar anode-supported cells. SOFC systems are investigated with and without external ammonia decomposition. BaO/Ni/Sm2O3/MgO catalyst is newly developed for the external ammonia cracker. Initial performance measurement and 1,000 h long-term durability test of the stacks are conducted. The stack with direct ammonia achieves 1 kW power output with 52% DC electrical efficiency; a slight decrease in performance compared with the stack with mixture fuel of hydrogen and nitrogen is because of the temperature decrease in the stack owing to the endothermic ammonia decomposition reaction. The external ammonia cracker helps to maintain stack temperature, enhancing the initial performance of the stack. It is demonstrated that the stacks fueled with ammonia have excellent stability during the long-term test and that with the external ammonia cracker exhibits 57% energy conversion efficiency at ca. 700 W DC electrical output.
  • Kazunari Miyazaki; Yidan Ding; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    ELECTROCHEMISTRY 88 (1) 28 - 33 1344-3542 2020 [Refereed]
     
    In this study, the applicability of Ba(Ce,Co,Y)O3–δ (BCCY) for a cathode of proton-conducting ceramic fuel cells was investigated. The electrical conductivity and transference number of BCCY were significantly affected by a cobalt content in the oxide. It was found that this material showed a mixed conduction of proton, oxide ion, and electron. The addition of cobalt into Ba(Ce,Y)O3–δ mainly increased the electronic conductivity of materials. Composite electrodes with an optimum composition of La0.6Sr0.4Co0.2Fe0.8O3–δ (LSCF)–BaCe0.7Co0.2Y0.1O3–δ (50:50 wt.%) exhibited lower polarization for the symmetrical cell test with a BaCe0.8Y0.2O3–δ electrolyte in 6.5% humidified oxygen atmosphere, as compared with La0.6Sr0.4Co0.2Fe0.8O3–δ (LSCF) itself and LSCF–BaCe0.9Y0.1O3–δ (50:50 wt.%) composite electrodes. The power generation test was performed at 600°C–700°C using a BaCe0.8Y0.2O3–δ electrolyte-supported single cell employing a LSCF–BaCe0.7Co0.2Y0.1O3–δ (50:50 wt.%) composite cathode, upon feeding 3% humidified hydrogen and pure oxygen to the anode and cathode, respectively. The cell with a LSCF–BaCe0.7Co0.2Y0.1O3–δ (50:50 wt.%) composite cathode exhibited much higher performance than that with a LSCF electrode. Consequently, the introduction of cobalt into Ba(Ce,Y)O3–δ was an effective strategy for an improvement in an oxygen reduction reaction activity of a cathode material.
  • Minkyu Kim; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY 166 (16) F1269 - F1274 0013-4651 2019/11 [Refereed]
     
    In this study, single cells employing a La0.6Sr0.4Co0.2Fe0.8O3.δ (LSCF) cathode were operated with a supply of humidified oxygen to the cathode at 1000°C for 100 h to investigate the influence of water vapor on the performance and microstructure of LSCF cathode. When a gaseous mixture of 20% H2O.80% O2 was supplied to the cathode, the performance of LSCF cathode continuously lowered during a discharge at 300 mA cm-2 for 100 h. Then, the microstructures of surface and cross-section of LSCF cathode were observed by scanning electron microscopy. The surface morphology was drastically changed by the discharge operation. A SrO layer was formed at the outermost surface of cathode, indicating that the strontium segregation was accelerated by water vapor. In response to this phenomenon, the formation amount of cobalt- and/or iron-based oxides enlarged inside the electrode. These microstructural and phase changes would be responsible for the performance deterioration of LSCF cathode.
  • Shimpei Yamaguchi; Tomoatsu Ozaki; Takeshi Suyama; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    INTERNATIONAL JOURNAL OF HYDROGEN ENERGY PERGAMON-ELSEVIER SCIENCE LTD 44 (18) 9338 - 9347 0360-3199 2019/04 [Refereed]
     
    Catalysts that can maintain the activity for a long time are indispensable for the distributed power generation consisting of biomass gasifier and Solid Oxide Fuel Cell (SOFC). This study aims to reveal and develop the regenerative function of composite catalysts containing NiAl2O4 that can be regenerated by reduction and oxidation (Red-Ox) for converting tar from biomass. Regenerative function of composite oxide catalyst containing NiAl2O4 spinel (NAO) and Ru added to NAO (RNAO) by Red-Ox cycles and their activity of autothermal reforming of toluene were examined. Regenerating function of NAO and RNAO were investigated by temperature programmed reduction (TPR) and oxidation at 850 degrees C repeatedly. After 7th times Red-Ox cycles at 850 degrees C, though the regenerated amount of Ni in NAO decreased by 70%, that in RNAO was maintained. By Red-Ox of NAO and RNAO, Ni was oxidized to NiO at 350-650 degrees C and dissolved into gamma-Al2O3 to form the NiAl2O4 spinel at above 750 degrees C. In RNAO, RuO2 particles were more dispersed, decreasing its particle size than the initial, with the Red-Ox cycles, since Ru was dissolved into the Ni particles exsoluted from NiAl2O4. By evaluating the activity of autothermal reforming of toluene, NAO and RNAO showed 1.5-3.0 times higher activity than Pt/gamma-Al2O3, due to the highly dispersive active sites unoccupied by CO. Since the active metals can be redispersed by oxidation and reduction at the range from 750 degrees C to 850 degrees C, NAO and RNAO would contribute the maintenance free system for reforming tar from biomass gasification. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
  • Toshiaki Matsui; Siqi Li; Yuki Inoue; Norifumi Yoshida; Hiroki Muroyama; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 166 (4) F295 - F300 0013-4651 2019/03 [Refereed]
     
    Solid oxide fuel cells with a configuration of (La,Sr)(Co,Fe)O3-δ cathode/doped ceria interlayer/zirconia-based electrolyte have been extensively studied to elucidate the degradation mechanism. Various degradation factors were suggested, such as the formation of highly-resistive SrZrO3 phase, and the reduction in active reaction sites because of the agglomeration of constituent materials. Among them, however, the influence of the ceria-zirconia solid solution formation at the doped ceria interlayer/zirconia-based electrolyte on the cell performance has not been elucidated sufficiently. In this study to achieve a comprehensive understanding of degradation phenomena at the cathode side, the chemical information of constituent materials, as well as the microstructural parameters, were analyzed for single cells before and after discharge operation. Especially, the ionic conductivity of solid solutions formed in the (La,Sr)(Co,Fe)O3-δ/Gd2O3-CeO2/Y2O3-ZrO2 systemwas investigated in detail to clarify the ionic conductivity profile at the Gd2O3-CeO2/Y2O3-ZrO2 interface.
  • 室山 広樹; 奥田 翔大; 松井 敏明; 橋上 聖; 川野 光伸; 稲垣 亨; 江口 浩一
    石油学会 年会・秋季大会講演要旨集 公益社団法人 石油学会 2019 30  2019/01
  • H. Muroyama; M. Kim; T. Matsui; K. Eguchi
    ECS Transactions 91 (1) 1491 - 1501 1938-6737 2019 
    In this study, single cells employing a La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) cathode were operated with a supply of humidified oxygen to the cathode at 1000 ºC for 100 h to investigate the influence of water vapor on the performance and microstructure of LSCF cathode. When the gaseous mixture of 20% H2O−80% O2 was supplied to the cathode, the performance of LSCF cathode continuously lowered during discharge at 300 mA cm−2 for 100 h. Then, the microstructures of surface and cross-section of LSCF cathode were observed by scanning electron microscopy. The surface morphology was drastically changed by discharge operation. The SrO layer was formed at the outermost surface of cathode, indicating that the strontium segregation was accelerated by water vapor. In response to this phenomenon, the formation amount of cobalt- and/or iron-based oxides enlarged inside the electrode. These microstructural and phase changes would be responsible for the performance deterioration of LSCF cathode.
  • S. Yamaguchi; T. Ozaki; T. Suyama; H. Muroyama; T. Matsui; K. Eguchi
    ECS Transactions 91 (1) 391 - 402 1938-6737 2019 
    Gasification of biomass using autothermal downdraft gasifier and the characteristics of gasification were analyzed. The prepared gasified gas was supplied to the electrolyte-supported solid oxide fuel cells (SOFCs) and their electrochemical characteristics were analyzed. In the gasifier, the biomass was converted to the mixed gas containing H2 and CO at the efficiency of ca. 85% and its composition was mainly affected by the water gas shift (WGS) reaction in the reduction area. The exchange current densities of anode electrode in hydrogen and model gasified gas were evaluated. In anode electrode by gasified gases, oxidation of H2 rather than CO mainly occurred due to WGS reaction. The maximum power density attained by gasified gas was 68% of that by H2.
  • Toshiaki Matsui; Siqi Li; Yuki Inoue; Norifumi Yoshida; Hiroki Muroyama; Koichi Eguchi
    ECS Transactions 91 (1) 1247 - 1256 1938-6737 2019 
    Solid oxide fuel cells with a configuration of (La,Sr)(Co,Fe)O3‒δ cathode/doped ceria interlayer/zirconia-based electrolyte have been studied extensively to elucidate the degradation mechanism. Various degradation factors were suggested, such as the formation of highly-resistive SrZrO3 phase, and the reduction in active reaction sites because of the agglomeration of constituent materials. Among them, however, the influence of the ceria-zirconia solid solution formation at the doped ceria interlayer/zirconia-based electrolyte on the cell performance has not been elucidated sufficiently. In this study to achieve a comprehensive understanding of degradation phenomena at the cathode side, the chemical information of constituent materials, as well as the microstructural parameters, were analyzed for single cells before and after discharge operation. Especially, the ionic conductivity of solid solutions formed in the (La,Sr)(Co,Fe)O3‒δ/Gd2O3-CeO2/Y2O3-ZrO2 system was investigated in detail to clarify the ionic conductivity profile at the Gd2O3-CeO2/Y2O3-ZrO2 interface.
  • Toshiaki Matsui; Kohei Manriki; Kazunari Miyazaki; Hiroki Muroyama; Koichi Eguchi
    JOURNAL OF MATERIALS CHEMISTRY A ROYAL SOC CHEMISTRY 6 (29) 14188 - 14194 2050-7488 2018/08 [Refereed]
     
    This is the first report of the application of the barium lanthanide cobaltate of Ba4Sr2Sm2Co4O15 (BSSC) as an oxygen reduction electrocatalyst of proton-conducting ceramic fuel cells. In this study, composite cathodes of Ba4Sr2Sm2Co4O15-BaCe0.5Pr0.3Y0.2O3-δ (BSSC-BCPY) were developed by varying the mixing ratio. The BSSC-BCPY composite with the optimal composition exhibited a polarization resistance of 0.19 Ω cm2, 0.31 Ω cm2, and 0.59 Ω cm2 at 600 °C, 550 °C, and 500 °C, respectively, for the symmetrical cell test in 3% humidified synthetic air. Its activation energy was evaluated to be 68.5 kJ mol-1. In this composite system, BSSC appeared to serve as an active phase for the dissociative adsorption of oxygen. The operation of the anode-supported cell employing the optimized cathode was successfully demonstrated at 600 °C, and a maximum power density of 197 mW cm-2 was achieved upon feeding synthetic air and humidified hydrogen to the cathode and anode, respectively. Finally, the chemical stability of the BSSC-BCPY composite in a CO2-containing atmosphere was evaluated.
  • Yu Katayama; Livia Giordano; Reshma R. Rao; Jonathan Hwang; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi; Yang Shao-Horn
    Journal of Physical Chemistry C American Chemical Society 122 (23) 12341 - 12349 1932-7455 2018/06 [Refereed]
     
    Understanding the surface (electro)chemistry of CO2 and CO on Pt is needed to design active, selective catalysts for CO-tolerant fuel cell reactions and CO2 reduction. In this work, the surface reactivity of Pt in a CO2-saturated alkaline electrolyte was revealed by combining in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) with density functional theory (DFT) calculations. We show that during potential cycling in 1 M KHCO3 electrolyte, CO adsorbates (COad), more specifically, COad surrounded by OH adsorbates (OHad), with linear or bridged configuration, were produced through the reductive adsorption of HCO3- catalyzed by H adsorbates on Pt. The COad coadsorbed with OHad was oxidized to COOHad at potentials as low as ∼0.3 VRHE, which was further oxidized to CO2 at 0.9 VRHE and higher. Further analysis suggests that the proximity between COad and OHad is key to trigger the conversion reaction from COad to CO2 through forming COOHad intermediate at room temperature. The details about how Pt surface adsorbates change as a function of voltage in CO2-saturated alkaline electrolytes can provide strategies to design CO-tolerant catalysts for fuel cell applications and active and selective catalysts for the CO2 reduction reaction.
  • 室山 広樹; 松井 敏明; 江口 浩一
    ケミカルエンジニヤリング = Chemical engineering 化学工業社 63 (9) 632 - 636 0387-1037 2018/01
  • 室山 広樹
    電気化学 公益社団法人 電気化学会 86 (Winter) 340 - 341 2433-3255 2018/01
  • 室山 広樹; 浅越 俊紀; 松井 敏明; 江口 浩一
    石油学会 年会・秋季大会講演要旨集 公益社団法人 石油学会 2018 116  2018/01
  • Hiroki Muroyama; Shota Okuda; Toshiaki Matsui; Satoshi Hashigami; Mitsunobu Kawano; Toru Inagaki; Koichi Eguchi
    Journal of the Japan Petroleum Institute Japan Petroleum Institute 61 (2) 72 - 79 1349-273X 2018 [Refereed]
     
    In situ analyses of dry reforming and partial oxidation of CH4 over 1 wt.% Ni/Al2O3 were performed using an yttria-stabilized zirconia oxygen sensor with five sensing points, which was inserted in the catalyst bed along the gas flow direction. The concentrations of species in the gas flow channel were estimated from the oxygen partial pressure detected by the sensor. The progress of reactions was monitored at the respective points in both reforming reactions under the considered conditions. Additionally, the reaction temperature was measured at three points using thermocouples during CH4 partial oxidation. The plausible reaction mechanism for the partial oxidation of methane over Ni/Al2O3 was proposed based on the distributions of gas species and temperature in the catalyst layer. Monitoring in the stability test showed that the catalyst deactivation tendency was different depending on the position in the catalyst bed.
  • Kaname Okura; Kazunari Miyazaki; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    RSC ADVANCES ROYAL SOC CHEMISTRY 8 (56) 32102 - 32110 2046-2069 2018 [Refereed]
     
    Ammonia decomposition has attracted increasing attention as a promising process for the on-site generation of hydrogen. In this study, Ni catalysts supported on perovskite-type oxides (ABO3) were prepared and the activity for ammonia decomposition was examined. The Ni/ANbO3 (A = Na and K) and Ni/AEMnO3 (AE = Ca, Sr, and Ba) catalysts were less effective for this reaction. Meanwhile, the Ni/REAlO3 (RE = La, Sm, and Gd) catalysts exhibited relatively high activity. For Ni/AETiO3 and Ni/AEZrO3, the performance strongly depended on the A-site element of the perovskite-type oxides, and the Sr and Ba elements were more effective than the Ca one in the respective series. The catalytic activity for Ni/AEZrO3 was higher than Ni/AETiO3 in the case of the same alkaline earth element, and Ni/BaZrO3 was the most active among the samples investigated in this work. For these series, the order in the performance corresponded well with that in the basic property. The nitrogen desorption profiles revealed that the evolution of nitrogen atoms, which is one of the kinetically slow steps, effectively proceeded for Ni/SrZrO3 and Ni/BaZrO3 compared with the conventional Ni catalysts. This promotion effect would be ascribed to the strong basic properties of the SrZrO3 and BaZrO3 supports, resulting in the high activity of Ni/SrZrO3 and Ni/BaZrO3 for ammonia decomposition.
  • Atthapon Srifa; Kaname Okura; Takeou Okanishi; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    APPLIED CATALYSIS B-ENVIRONMENTAL ELSEVIER SCIENCE BV 218 1 - 8 0926-3373 2017/12 [Refereed]
     
    The Cs-modified Co3Mo3N catalysts were successfully prepared by a facile single-step decomposition of a mixture containing hexamethylenetetramine (HMTA) and corresponding metal salts under the flow of nitrogen at 700 degrees C and their catalytic activity for ammonia decomposition was investigated. The Co3Mo3N phase was observed in the XRD pattern of as-prepared sample and acted as the active component for the reaction. The XPS analysis clarified the existence of Cs species, indicating that these species should be dispersed over Co3Mo3N. The addition of a small amount of Cs species significantly improved the catalytic performance of the Co3Mo3N catalyst. The further addition of Cs species had a negative effect on the catalytic activity for the reaction.(NH3)-N-15 isotopic studies revealed that the nitrogen species in Co3Mo3N were exchangeable with nitrogen atoms in gas phase, elucidating the mechanism of the reaction over the Co3Mo3N catalysts. The kinetics analysis indicated that the modification by a small amount of Cs species alleviated the negative effect of the hydrogen poisoning on the active sites of Co3Mo3N catalysts. The desorption behavior of hydrogen and nitrogen suggested that the Cs modification facilitated the recombinative desorption of hydrogen and nitrogen atoms from the active components, resulting in the improvement in the activity of Co3Mo3N. This improvement was due to the electronic modification of Co3Mo3N by the electron donation of Cs promoter. (C) 2017 Elsevier B.V. All rights reserved.
  • Kazunari Miyazaki; Takeou Okanishi; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF POWER SOURCES ELSEVIER SCIENCE BV 365 148 - 154 0378-7753 2017/10 [Refereed]
     
    In this study, the availability of Ni-Ba(Zr,Y)O3-delta (BZY) cermet for the anode of direct ammonia -fueled solid oxide fuel cells (SOFCs) is evaluated. In this device, the anodes need to be active for the catalytic ammonia decomposition as well as the electrochemical hydrogen oxidation. In the catalytic activity test, ammonia decomposes completely over Ni-BZY at ca. 600 degrees C, while higher temperature is required to accomplish the complete decomposition over the conventional SOFC anode of Ni yttria-stabilized zirconia cermet. The high activity of Ni-BZY is attributed to the high basicity of BZY and the high resistance to hydrogen poisoning effect. The electrochemical property of Ni-BZY anode is also evaluated with the anode-supported cell of Ni-BZYIBZYIPt at 600-700 degrees C with feeding ammonia or hydrogen as a fuel. Since the residence time of ammonia fuel in the thick Ni-BZY anode is long, the difference in the cell performance between two fuels is relatively small. Furthermore, it is proved that the steam concentration in the fuel strongly affects the cell performance. We find that this factor is important to satisfy the above mentioned requirements for the anode of direct ammonia-fueled SOFCs. Throughout this study, it is concluded that Ni-BZY cermet will be a promising anode. (C) 2017 Elsevier B.V. All rights reserved.
  • Yu Katayama; Kosuke Yamauchi; Kohei Hayashi; Takeou Okanishi; Hiroki Muroyama; Toshiaki Matsui; Yuuki Kikkawa; Takayuki Negishi; Shin Watanabe; Takenori Isomura; Koichi Eguchi
    ACS APPLIED MATERIALS & INTERFACES AMER CHEMICAL SOC 9 (34) 28650 - 28658 1944-8244 2017/08 [Refereed]
     
    Over the last few decades, because of the significant development of anion exchange membranes, increasing efforts have been devoted the realization of anion exchange membrane fuel cells (AEMFCs) that operate with the supply of hydrogen generated on-site. In this paper, ammonia was selected as a hydrogen source, following which the effect of conceivable impurities, unreacted NH3 and atmospheric CO2, on the performance of AEMFCs was established. As expected, we show that these impurities worsen the performance of AEMFCs significantly. Furthermore, with the help of in situ attenuated total reflection infrared (ATR-IR) spectroscopy, it was revealed that the degradation of the cell performance was primarily due to the inhibition of the hydrogen oxidation reaction (HOR). This is attributed to the active site occupation by CO-related adspecies derived from (bi)carbonate adspecies. Interestingly, this degradation in the HOR activity is suppressed in the presence of both NH3 and HCO3- because of the bicarbonate ion consumption reaction induced by the existence of NH3. Further analysis using in situ ATR-IR and electrochemical methods revealed that the poisonous CO-related adspecies were completely removed under NH3-HCO3- conditions, accompanied by the improvement in HOR activity. Finally, a fuel cell test was conducted by using the practical AEMFC with the supply of NH3-contained H-2 gas to the anode and ambient air to the cathode. The result confirmed the validity of this positive effect of NH3 HCO3 coexistence on CO2-tolerence of AEMFCs. The cell performance achieved nearly 95% of that without any impurity in the fuels. These results clearly show the impact of the chemically induced bicarbonate ion consumption reaction on the realization of highly CO2-tolerent AEMFCs.
  • T. Okanishi; K. Okura; A. Srifa; H. Muroyama; T. Matsui; M. Kishimoto; M. Saito; H. Iwai; H. Yoshida; M. Saito; T. Koide; H. Iwai; S. Suzuki; Y. Takahashi; T. Horiuchi; H. Yamasaki; S. Matsumoto; S. Yumoto; H. Kubo; J. Kawahara; A. Okabe; Y. Kikkawa; T. Isomura; K. Eguchi
    FUEL CELLS WILEY-V C H VERLAG GMBH 17 (3) 383 - 390 1615-6846 2017/06 [Refereed]
     
    Several solid oxide fuel cell (SOFC) systems can be considered for the NH3 utilization. In this study, the catalytic activity and long-term stability of the NH3 decomposition reactor with the Ni/Y2O3-based catalysts and the autothermal NH3 cracker with the Co-Ce-Zr composite oxide catalyst were investigated. Moreover, the NH3 decomposition reactor and the autothermal NH3 cracker were combined with the SOFC stack and their performances of the stack were compared to those fueled with NH3 and H-2/N-2. The power output of each SOFC stack was over 200 W. Moreover, it was demonstrated that the direct NH3-fueled SOFC stack was stable for 1,000 h at 770 degrees C without significant degradation.
  • Takeou Okanishi; Kaname Okura; Atthapon Srifa; Hiroki Muroyama; Toshiaki Matsui; Masashi Kishimoto; Motohiro Saito; Hiroshi Iwai; M Saito; T Koide; H Iwai; S Suzuki; Y Takahashi; T Horiuchi; H Yamasaki; S Matsumoto; S Yumoto; H Kubo; J Kawahara; A Okabe; Y Kikkawa; T Isomura; Koichi Eguchi
    FUEL CELLS WILEY-BLACKWELL 17 (3) 383–390  1615-6854 2017/06 [Refereed]
  • Toshiaki Matsui; Kohei Eguchi; Keisuke Shirai; Takeou Okanishi; Hiroki Muroyama; Koichi Eguchi
    ECS Transactions Electrochemical Society Inc. 78 (1) 1115 - 1123 1938-6737 2017/05 [Refereed]
     
    Recently, the strong chemical interaction between Ni and GDC (Gd2O3.CeO2) was reported for the Ni.GDC cermet anode in solid oxide fuel cells (SOFCs) CeO2 species migrated on the Ni surface after exposure to the reducing atmosphere. This surface modification of nickel affected the catalytic property for carbon deposition and CO adsorption. Generally, such a chemical interaction is promoted by the oxidation-reduction (redox) treatment. In this study, the influence of redox treatment on the chemical interaction between Ni and doped ceria was studied to design the cermet anode with high tolerance to carbon deposition. The Ni.GDC cermet subjected to the redox treatment showed high tolerance to carbon deposition. Furthermore, the redox treatment did not affect the mechanism of electrochemical hydrogen oxidation in humidified hydrogen atmospheres.
  • V. Singh; H. Muroyama; T. Matsui; K. Eguchi
    ECS Transactions Electrochemical Society Inc. 78 (1) 2527 - 2536 1938-6737 2017/05 [Refereed]
     
    In this study, the performance of anode-supported and electrolyte-supported cells was compared under similar operating conditions for direct ammonia-fueled solid oxide fuel cells (SOFCs) the cell configuration of both types of cells was as follows, Ni-YSZ|YSZ|GDC|LSCF. The anode-supported cell (ASC) showed better performance as compared with that of the electrolyte-supported cell (ESC). The influence of the humidity of inlet gas on the performance of ASC was investigated at 700°C. The cell performance did not change substantially under different humidified conditions. In addition, the long-term discharge operation of direct ammonia-fueled ASC was conducted at 600°C for ca. 235 h. The durability of ASC under polarization was evaluated on the basis of the measured current-voltage and power density curves. The cell showed slight degradation in current density and power density after discharge operation. Furthermore, the formation temperature of Ni3N under ammonia-fueled SOFC conditions was investigated in detail.
  • Toshiaki Matsui; Siqi Li; Hiroki Muroyama; Kyosuke Kishida; Haruyuki Inui; Koichi Eguchi
    SOLID STATE IONICS ELSEVIER SCIENCE BV 300 135 - 139 0167-2738 2017/02 [Refereed]
     
    The application of solid oxide fuel cells consisting of (La,Sr)(Co,Fe)O3 - delta (LSCF) cathode/doped ceria interlayer/ zirconia-based electrolyte has been widely examined for the residential cogeneration system. It is well recognized that the microstructural change of cell components proceeds with time, leading to the performance deterioration. Various factors were proposed for this degradation behavior, e.g. the formation of highly-resistive SrZrO3 phase and the agglomeration of LSCF cathode. However, the quantitative influence of the ceria-zirconia solid solutions on the performance has not been studied, with applying the results of dissection analysis of the cells operated; the ceria-zirconia solid solutions are formed at the doped ceria interlayer/zirconia-based electrolyte interface. In this study, the ionic conductivity of these solid solutions formed in the LSCF/Sm2O3-CeO2/Y2O3-ZrO2 system was investigated in detail. The dissolution of Sm3+ into the Y2O3-ZrO2 phase resulted in the significant reduction in ionic conductivity. Furthermore, the influence of dopant species in the doped ceria interlayer, Sm3+ and Gd3+, on the ionic conductivity of solid solutions was studied. (C) 2016 Elsevier B.V. All rights reserved.
  • Toshiaki Matsui; Kohei Eguchi; Keisuke Shirai; Takashi Ozeki; Takeou Okanishi; Hiroki Muroyama; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 164 (13) F1368 - F1374 0013-4651 2017 [Refereed]
     
    Recently, the strong chemical interaction between Ni and GDC (Gd2O3-CeO2) was reported for the cermet anode of Ni-GDC in solid oxide fuel cells (SOFCs);(1) CeO2 species migrated on the Ni surface after exposure to the reducing atmosphere. This surface modification of nickel affected the catalytic properties for carbon deposition and CO adsorption. Generally, such a chemical interaction is promoted by the oxidation-reduction (redox) treatment. In this study, then, the influence of redox treatment on catalytic and electrochemical properties of Ni-GDC cermets was investigated in detail to establish a new design concept of anodes. The Ni-GDC cermet subjected to the redox treatment showed high tolerance to carbon deposition. Furthermore, the redox treatment did not affect the mechanism of electrochemical hydrogen oxidation in humidified hydrogen atmospheres. We also demonstrated the stable power generation of the single cell employing the optimized Ni-GDC anode at 1000 degrees C in a wet methane atmosphere with a steam to carbon ratio of 0.1. (c) The Author(s) 2017. Published by ECS. All rights reserved.
  • Yu Katayama; Takeou Okanishi; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF CATALYSIS ACADEMIC PRESS INC ELSEVIER SCIENCE 344 496 - 506 0021-9517 2016/12 [Refereed]
     
    In recent years, increasing attention has been focused on the utilization of ammonia as a fuel for anion exchange membrane fuel cells (AEMFCs) due to the significant development of anion exchange membranes (AEMs). Although the improvement in catalytic activity for ammonia oxidation reaction was achieved for some Pt-based electrocatalysts, no effective methods were established to enhance the tolerance toward the catalyst poisoning, which is known as the major issue for the utilization of ammonia as a fuel. In this paper, first, the additive effect of Y2O3 was intensely investigated by in situ ATR-IR spectroscopy. For the Y2O3-modified Pt surface, the IR band area of N2H4,ad species, which is the intermediate species of ammonia oxidation reaction, was distinctively increased, suggesting the promotion of NH2,(ad) dimerization process. This effect of Y2O3 was then accurately clarified by applying the Y2O3 modification for Pd surface. Despite that Pd was inactive for the NH2,ad dimerization reaction, the IR band of N2H4,ad species was clearly detected only in the case of Y2O3-modified Pd surface. This is the strong evidence that the Y2O3 additive itself promotes the NH2,ad dimerization reaction. Furthermore, the electrocatalytic performance of Y2O3-modified Pt/C electrocatalyst was evaluated using the commercial AEMFC system. A twofold increase in both maximum power density and tolerance toward the catalyst poisoning was confirmed as compared to the cell with the conventional Pt/C electrocatalyst. These results clearly show the applicability of our catalyst design strategy to improve the performance of the real-world electrocatalyst for the ammonia oxidation reaction. (C) 2016 Elsevier Inc. All rights reserved.
  • Hiroki Muroyama; Yuji Tsuda; Toshiki Asakoshi; Hasan Masitah; Takeou Okanishi; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF CATALYSIS ACADEMIC PRESS INC ELSEVIER SCIENCE 343 178 - 184 0021-9517 2016/11 [Refereed]
     
    Carbon dioxide methanation is well known to offer some advantages and be catalyzed by Ru, Rh, Pd, and Ni. In this study, Ni catalysts supported on various metal oxides were fabricated and their catalytic activity for CO2 methanation was evaluated. The CO2 conversion for most of catalysts drastically increased at 225-250 degrees C and reached a maximal value at 300-350 degrees C. The order of CH4 yield at 250 degrees C was as follows: Ni/Y2O3 > Ni/Sm2O3 > Ni/ZrO2 > Ni/CeO2 > Ni/Al2O3 > Ni/La2O3. The catalytic activity could be partly explained by the basic property of the catalysts. Moreover, the chemical species formed on the catalyst surface during CO2 methanation were examined by in situ infrared spectroscopy. From the obtained results, the difference in the activity depending on the support material of Ni catalysts was discussed. (C) 2016 Elsevier Inc. All rights reserved.
  • Kaname Okura; Takeou Okanishi; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    CHEMCATCHEM WILEY-V C H VERLAG GMBH 8 (18) 2988 - 2995 1867-3880 2016/09 [Refereed]
     
    Ammonia decomposition has attracted the interest of many researchers as a promising process for the on-site generation of H-2. In this study, Ni catalysts supported on various rare-earth oxides were prepared by the impregnation method and employed for ammonia decomposition. The Ni/Y2O3 catalyst exhibited the best performance among the samples investigated. The reaction kinetics study indicated that most of rare-earth oxide supports were effective for the alleviation of the hydrogen inhibition phenomenon in the decomposition reaction. The desorption behavior of hydrogen has revealed that the amount of hydrogen atoms adsorbed strongly on the Ni surface up to high temperatures was relatively small in the case of Ni/Y2O3. Furthermore, for Ni/Y2O3 the optimal Ni loading was 40 wt% in terms of the catalytic activity because of the appropriate Ni dispersion.
  • Toshiaki Matsui; Eri Morikawa; Shintaro Nakada; Takeou Okanishi; Hiroki Muroyama; Yoshifumi Hirao; Tsuyoshi Takahashi; Koichi Eguchi
    ACS APPLIED MATERIALS & INTERFACES AMER CHEMICAL SOC 8 (28) 18119 - 18125 1944-8244 2016/07 [Refereed]
     
    In this study, the heteropolyacids of H3+xPVxMO12-xOx (x = 0, 2, and 3) were applied as redox mediators for the oxygen reduction reaction in polymer electrolyte fuel cells, of which the cathode is free from the usage of noble metals such as Pt/C. In this system, the electrochemical reduction of heteropolyacid over the carbon cathode and the subsequent reoxidation of the partially reduced heteropolyacid by exposure to the dissolved oxygen in the regenerator are important processes for continuous power generation. Thus, the redox properties of catholytes containing these heteropolyacids were investigated in detail. The substitution quantity of V in the heteropolyacid affected the onset reduction potential as well as the reduction current density, resulting in a difference in cell performance. The chemical composition of heteropolyacid also had a significant impact on the reoxidation property. Among the three compounds, H6PV3Mo9O40 was the most suitable redox mediator. Furthermore, the pH of the catholyte was found to be the crucial factor in determining the reoxidation rate of partially reduced heteropolyacid as well as cell performance.
  • Toshiaki Matsui; Masahiro Komoto; Hiroki Muroyama; Kyosuke Kishida; Haruyuki Inui; Koichi Eguchi
    JOURNAL OF POWER SOURCES ELSEVIER SCIENCE BV 312 80 - 85 0378-7753 2016/04 [Refereed]
     
    In this study, the miscrostructural change of the LSCF/SDC/YSZ system upon discharge was analyzed quantitatively by the focused ion beam-scanning electron microscopy as well as the transmission electron microscopy. It is widely recognized that the formation of highly-resistive SrZrO3 phase degrades the cell performance in this system. In fact, the ohmic loss and cathodic overpotential increased in response to the six-fold increase in the volume of SrZrO3 phase after 400 h of discharge at 1000 degrees C. However, the microstructural change proceeded in the whole part of this system, indicating that various degradation factors need to be considered; e.g., 1) the agglomeration of SDC and LSCF phases, 2) the reduction in triple phase boundary length, and 3) the formation of ceria-zirconia solid solution. (C) 2016 Elsevier B.V. All rights reserved.
  • Takeou Okanishi; Yu Katayama; Ryota Ito; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    PHYSICAL CHEMISTRY CHEMICAL PHYSICS ROYAL SOC CHEMISTRY 18 (15) 10109 - 10115 1463-9076 2016/04 [Refereed]
     
    The electrochemical oxidation of 2-propanol over Pt and Pd electrodes was evaluated in alkaline media. Linear sweep voltammograms (LSVs), chronoamperograms (CAs), and simultaneous time-resolved attenuated total reflection infrared (ATR-IR) spectra of both electrodes were obtained in a 0.25 M KOH solution containing 1 M 2-propanol. The onset potential of 2-propanol oxidation for Pt was lower than that for Pd in LSVs while the degree of performance degradation observed for Pd was significantly smaller than that observed for Pt in CAs. The main product of 2-propanol oxidation was acetone over both electrodes and, over Pt only, acetone produced was catalytically oxidized to the enolate ion, which was accumulated on the Pt surface, leading to significant performance degradation. Carbon dioxide and carbonate species (CO32-, HCO3-) were not observed during 2-propanol oxidation over both electrodes, indicating that the complete oxidation of 2-propanol to CO2 will be a minor reaction.
  • Yu Katayama; Takeou Okanishi; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    ACS CATALYSIS AMER CHEMICAL SOC 6 (3) 2026 - 2034 2155-5435 2016/03 [Refereed]
     
    The recent development of anion exchange membranes (AEMs) has increased the potential of anion exchange membrane fuel cells (AEMFCs). Although highly active electro-catalysts for specific reactions have been successfully developed by placing the most importance on the fuel species, only a few studies have focused on OHad (hydroxyl adsorbed species), which is known to be a common reactive species in alkaline environments. In this study, highly oxophilic CeO2 was selected as a surface modifier for a Pt electrode. We first applied in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy to ionomercoated Pt and CeO2-modified Pt surfaces for clarifying the adsorption behavior of OHad. As a result, a distinct change in adsorption behavior of OHad was confirmed in blank KOH solution. These peculiar characteristics were applicable to various electrochemical oxidation reactions. During the ammonia oxidation reaction, the acceleration of the formation of NOad species was observed in CeO2-modified Pt, suggesting the enhancement of OH adsorption. Furthermore, the degree of activity enhancement by CeO2 addition was investigated for the CO oxidation reaction, methanol oxidation reaction, and ethanol oxidation reaction. Under basic conditions, each of these reactions exhibited distinct activity enhancement. In contrast, under acidic conditions, the promoting effect on these reactions was not observed. These results strongly indicate the potential of our catalyst design strategy and the importance of OHad species as reactive species in alkaline environments.
  • Ahmed Fathi Salem Molouk; Jun Yang; Takeou Okanishi; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF POWER SOURCES ELSEVIER SCIENCE BV 305 72 - 79 0378-7753 2016/02 [Refereed]
     
    In the current work, we investigate the performance of solid oxide fuel cells (SOFCs) with Ni-yttria-stabilized zirconia (Ni-YSZ) and Ni gadolinia-dope ceria (Ni-GDC) cermet anodes fueled with H-2 or NH3 in terms of the catalytic activity of ammonia decomposition. The cermet of Ni-GDC shows higher catalytic activity for ammonia decomposition than Ni-YSZ. In response to this, the performance of direct NH3-fueled SOFC improved by using Ni-GDC anode. Moreover, we observe further enhancement in the cell performance and the catalytic activity for ammonia decomposition with applying Ni-GDC anode synthesised by the glycine nitrate combustion process. These results reveal that the high performance of Ni-GDC anode for the direct NH3-fueled SOFC results from its mixed ionic-electronic conductivity as well as high catalytic activity for ammonia decomposition. (C) 2015 Elsevier B.V. All rights reserved.
  • Shohei Suzuki; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 163 (5) F336 - F340 0013-4651 2016 [Refereed]
     
    The effect of carbonate ion species on the direct ammonia fuel cell (DAFC) employing anion exchange membrane (AEM) was evaluated. The OH-form and HCO3-form AEMs were applied as electrolytes for DAFCs. The obtained current density of DAFC with HCO3-form AEM was 1/7 of that with OH-form AEM. This is because carbonate ion species in the AEM suppress the ammonia oxidation reaction on Pt/C anode. Under the presence of carbonate ion species in the electrolyte, the ammonium ion and carbamate ion were formed accompanied with a reduction in the ammonia concentration in the vicinity of anode. This phenomenon affected the open circuit voltage (OCV) of DAFC with HCO3-form AEM. (C) The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. All rights reserved.
  • Muroyama Hiroki; Asajima Hiroki; Okanishi Takeou; Matsui Toshiaki; Eguchi Koichi
    Proceeding of Annual/Fall Meetings of the Japan Petroleum Institute The Japan Petroleum Institute 2016 7 - 7 2016/01 
    ディーゼルエンジン排ガスに含まれる粒子状物質(PM)はディーゼルパティキュレートフィルタ上に担持された触媒を使用して燃焼される。現在、PM燃焼温度を低温化することが検討されており、当研究室では高い酸素吸蔵能を有するセリア系触媒が注目してきた。本研究では、コバルトを添加したセリア系酸化物におけるカーボン燃焼反応を検討した。同位体酸素交換反応試験の結果から、触媒の構成成分のカーボン燃焼反応への寄与を考察した。
  • Vandana Singh; Satoshi Hashigami; Hiroki Muroyama; Toshiaki Matsui; Toru Inagaki; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 163 (11) F3084 - F3090 0013-4651 2016 [Refereed]
     
    The present study focuses on the comparison of Ni-GDC cathodes fabricated by the mechanical mixing (MM) and the spray pyrolysis (SP) methods toward the reduction of CO2 on solid oxide electrolysis cells (SOECs). The synthesized powders are characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The short-term performance and stability of each cathode are measured during the galvanostatic experiment at -1.20 A cm(-2) and 1000 degrees C. A relatively better performance and improved stability have been achieved for the cathode fabricated by SP method, which was supported by the pre- and post-test microstructural analyses. At the same time, effects of preparation methods on the triple phase boundary (TPB) length, two-phase boundary (contact area fraction of active GDC/pore), and isolated phase ratio are also evaluated from the focused ion beam-scanning electron microscope (FIB-SEM) tomography. The computed tomography results reveal that the cathode prepared by SP has larger TPB length and two-phase-boundary as compared with the one prepared by MM, which will be the one of the reasons for the better performance of the cell with Ni-GDC cathode prepared by SP. (C) 2016 The Electrochemical Society. All rights reserved.
  • Toshiaki Matsui; Kohei Eguchi; Takeshi Furukawa; Takeou Okanishi; Hiroki Muroyama; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 163 (10) F1146 - F1150 0013-4651 2016 [Refereed]
     
    The cermet of Ni-oxide ion conductor is widely used as an anode in solid oxide fuel cells (SOFCs). However, the usage of nickel induces various degradation phenomena during discharge operation; e.g., agglomeration and/or oxidation of nickel catalyst, and carbon deposition. In most cases, these degradation phenomena are triggered by the change in the oxygen chemical potential inside the anode. In this study, then, in operando Raman spectroscopy was conducted for the anode of Ni-Ce0.8Sm0.2O2-delta (Ni-SDC) cermet at 700 degrees C with applying SDC as a probe for the detection of oxygen chemical potential. This is because the band related to the oxygen vacancy in the Raman spectrum of SDC varies depending on the partial pressure of oxygen in the ambient atmosphere. The change in oxygen chemical potential at the top surface of anode was successfully quantified under polarization. The effective reaction zone of anode was also discussed by comparing with the data derived from impedance spectra measured simultaneously. (C) The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons. org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. All rights reserved.
  • Kaname Okura; Takeou Okanishi; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    RSC ADVANCES ROYAL SOC CHEMISTRY 6 (88) 85142 - 85148 2046-2069 2016 [Refereed]
     
    ( Recently, ammonia has attracted much attention as a promising hydrogen carrier due to various advantages for on-site hydrogen generation. In this study, Ni/Y2O3 catalysts modified by alkaline earth metals were prepared and their catalytic activity for ammonia decomposition was examined. The addition of a small amount of Sr or Ba species remarkably improved the performance of Ni/Y2O3, while the Mg- and Ca- modification were not effective. The XRD analysis elucidated that the composite oxides consisting of alkaline earth metals and nickel were formed in the as-calcined Sr- and Ba-modified Ni/Y2O3 catalysts, and were decomposed by the reduction treatment. This suggested that the Sr and Ba components were located in the vicinity of Ni particles, resulting in the strong interaction with the Ni species. The NH3-temperature programmed surface reaction measurement revealed that the desorption of nitrogen atoms strongly-adsorbed on the Ni surface has terminated at lower temperatures over the Sr-and Ba-modified catalysts than over the others. This characteristic desorption behavior would be mainly attributed to the enhancement of the electron density in the Ni metal by the strong basic property of Sr and Ba components and the strong Ni-Sr and Ni-Ba interaction. Considering the nitrogen desorption step was kinetically significant, this change in the electronic state of the Ni surface should be responsible for the promotion effect of the Sr-and Ba-modification.
  • Atthapon Srifa; Kaname Okura; Takeou Okanishi; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    CATALYSIS SCIENCE & TECHNOLOGY ROYAL SOC CHEMISTRY 6 (20) 7495 - 7504 2044-4753 2016 [Refereed]
     
    COx-free hydrogen generation via ammonia decomposition has received much attention as an important process for fuel cell applications. In the present study, non-precious Mo nitride-based catalysts with Co, Ni, and Fe additives were synthesized by temperature-programmed reaction of the corresponding oxide precursors with NH3. N-2 adsorption, X-ray diffraction (XRD), NH3-temperature programmed surface reaction (NH3-TPSR), NH3 pulse reaction, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) were carried out to gain better insight into the chemical and textural properties of the catalysts. The XRD analysis confirmed the formation of the Mo2N, Co3Mo3N, Ni3Mo3N, and Fe3Mo3N phases, which act as active species for ammonia decomposition reaction. The Co3Mo3N, Ni3Mo3N, and Fe3Mo3N catalysts were more active for ammonia decomposition than the Mo2N catalysts, indicating that the Co, Ni, and Fe species promoted the reaction. The increase in particle size and the decrease in surface area by the Co, Ni, and Fe addition did not negatively affect the ammonia decomposition activity. Interestingly, the Co, Ni, and Fe addition facilitated the recombinative desorption of N atoms from the active components, resulting in the enhancement in the activity. In addition, kinetic analysis was also conducted in detail to investigate the effects of the NH3 and H-2 partial pressures. The Co, Ni, and Fe addition alleviated the negative effect of the H-2 poisoning on the active sites.
  • Seki Hiroaki; Muroyama Hiroki; Matsui Toshiaki; Eguchi Koichi
    Proceeding of Annual/Fall Meetings of the Japan Petroleum Institute The Japan Petroleum Institute 2016 3 - 3 2016 
    CH4水蒸気改質触媒としてNi–セリア系酸化物サーメットを調製し、レドックス処理の有無によるCH4水蒸気改質反応活性と炭素析出耐性の変化を評価した。レドックス処理により、触媒活性は低下したが、炭素析出耐性は著しく向上した。金属–担体間の強い相互作用により、触媒の微構造が変化したことが観察された。また、表面Ni種の電子状態の違いが触媒性能に影響を与えている可能性が示唆された。
  • Jun Yang; Ahmed Fathi Salem Molouk; Takeou Okanishi; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    ACS APPLIED MATERIALS & INTERFACES AMER CHEMICAL SOC 7 (51) 28701 - 28707 1944-8244 2015/12 [Refereed]
     
    In recent years, solid oxide fuel cells fueled with ammonia have been attracting intensive attention. In this work, ammonia fuel was supplied to the Ni/yttria-stabilized zirconia (YSZ) cermet anode at 600 and 700 degrees C, and the change of electrochemical performance and microstructure under the open-circuit state was studied in detail. The influence of ammonia exposure on the microstructure of Ni was also investigated by using Ni/YSZ powder and Ni film deposited on a YSZ disk. The obtained results demonstrated that Ni in the cermet anode was partially nitrided under an ammonia atmosphere, which considerably roughened the Ni surface. Moreover, the destruction of the anode support layer was confirmed for the anode-supported cell upon the temperature cycling test between 600 and 700 degrees C because of the nitriding phenomenon of Ni, resulting in severe performance degradation.
  • Hiroki Muroyama; Nobutada Yamaguchi; Naoto Kamiuchi; Toshiaki Matsui; Koichi Eguchi
    CATALYSIS TODAY ELSEVIER SCIENCE BV 258 196 - 198 0920-5861 2015/12 [Refereed]
     
    The Pt-SnO2 system has attracted much attention as catalysts and gas sensors. Reduction-oxidation treatment has been reported to significantly affect the microstructure and activity of SnO2-supported platinum (Pt/SnO2) catalyst. In this study, the electrochemical property of Pt-SnOx device was investigated. This device repeatedly exhibited ohmic and rectifying characteristics in response to reduction and oxidation treatments, respectively. The ohmic behavior was also confirmed under reducing conditions, while the rectifying effect of this device appeared in a gaseous mixture of 5% CO-15% O-2-80% N-2. These electrochemical phenomena could be explained by the adsorption of gaseous species and the diffusion of atoms and ions to constituent materials. (C) 2015 Elsevier B.V. All rights reserved.
  • Vandana Singh; Hiroki Muroyama; Toshiaki Matsui; Satoshi Hashigami; Toru Inagaki; Koichi Eguchi
    JOURNAL OF POWER SOURCES ELSEVIER SCIENCE BV 293 642 - 648 0378-7753 2015/10 [Refereed]
     
    The electrochemical performance of Ni-gadolinia-doped ceria (GDC) cathode was studied for CO2 reduction on solid oxide electrolysis cell (SOEC) at 1000 degrees C and compared with that of Ni-yttria stabilized zirconia (Ni-YSZ) cathode. Ni-GDC cathode demonstrated higher performance for CO2 reduction. Furthermore, lanthanum strontium cobalt ferrite (LSCF) anode exhibited lower overpotential than lanthanum strontium manganite-yttria stabilized zirconia (LSM-YSZ) anode. Ni-GDC cathode and LSCF anode were found to be stable under a constant current density of -0.90 A cm(-2) at 900 degrees C. Moreover, no substantial performance degradation was observed for the cell having Ni-GDC cathode and LSCF anode even after 9 h of electrolysis operation under a constant current density of -1.2 A cm(-2) at 1000 degrees C. (C) 2015 Elsevier B.V. All rights reserved.
  • Toshiaki Matsui; Shohei Suzuki; Yu Katayama; Kosuke Yamauchi; Takeou Okanishi; Hiroki Muroyama; Koichi Eguchi
    LANGMUIR AMER CHEMICAL SOC 31 (42) 11717 - 11723 0743-7463 2015/10 [Refereed]
     
    The electrochemical oxidation of ammonia over Pt electrode in alkaline aqueous solutions was studied by in situ attenuated total reflection infrared (ATR-IR) spectroscopy. In 0.1 M NH3-1 M KOH, the band ascribable to the HNH bending mode of adsorbed NH3 was confirmed at 1662-1674 cm(-1) in the potential range of 0.1-1.1 V. The intensity of this band decreased continuously with a rise in potential, indicating the oxidative consumption of adsorbed ammonia. In response to this behavior, the band at 1269 cm(-1) appeared alternatively above 0.2 V, and its intensity reached the local maximal value at ca. 0.4 V. Note that this potential of ca. 0.4 V agreed well with the onset potential of ammonia oxidation, ca. 0.45 V, in the linear sweep voltammogram. This 1269 cm(-1) band was assigned to the NH2 wagging mode of N2H4, which was one of the active intermediates, N2Hx+y,ad (x = 1 or 2, y = 1 or 2), according to the mechanism proposed by Gerischer and Mauere. To the best of our knowledge, this is the first report for the detection of N2H4 as a reaction intermediate over Pt electrode. Furthermore, the formation of bridged NO was also observed above the onset potential of ammonia oxidation, ca. 0.5 V. Such adsorbed NO species probably inhibit the electrochemical reaction due to the occupation of reaction sites at higher potential.
  • Kaname Okura; Takeou Okanishi; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    APPLIED CATALYSIS A-GENERAL ELSEVIER SCIENCE BV 505 77 - 85 0926-860X 2015/09 [Refereed]
     
    Ammonia decomposition has attracted much attention as an efficient method for the on-site generation of hydrogen. In this study, alumina-supported nickel catalysts (Ni/Al2O3) modified by rare-earth elements were prepared by the impregnation method and their catalytic activity for the ammonia decomposition was investigated. The addition of rare-earth elements promoted the decomposition reaction over catalysts and the La-modified catalyst achieved the highest ammonia conversion in this work. For the modified catalysts, the adsorbed hydrogen, which is known to be an inhibitive species for the ammonia decomposition, desorbed at lower temperature compared to the unmodified one. Therefore, the effective alleviation of hydrogen inhibition would be responsible for the activity enhancement for the modified catalysts. The reaction kinetics study also supported this proposed mechanism. For the La-modified catalyst, the optimal pretreatment condition was investigated to enhance the catalytic activity. The catalyst calcined at 400 degrees C followed by reduction at 600 degrees C exhibited the highest ammonia conversion of 94% at 550 degrees C. (C) 2015 Elsevier B.V. All rights reserved.
  • Takeou Okanishi; Yu Katayama; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    ELECTROCHIMICA ACTA PERGAMON-ELSEVIER SCIENCE LTD 173 364 - 369 0013-4686 2015/08 [Refereed]
     
    The electrochemical oxidation of ammonia over two types of SnO2-modified Pt (C-Pt/SnO2, SnO2-Pt/C) and Pt/C electrocatalysts was evaluated in alkaline aqueous solutions. Linear sweep voltammograms (LSVs) and chronoamperograms (CAs) were obtained in a 1 M KOH solution with 0.1 M NH3. The ammonia oxidation current achieved during the LSVs was in the order C-Pt/SnO2 > SnO2-Pt/C > Pt/C. In addition, the apparent activation energies for ammonia oxidation calculated from the CAs for C-Pt/SnO2, SnO2-Pt/C, and Pt/C at various temperatures were 52, 58, and 67 kJ mol (1), respectively. These results indicated that SnO2 activated the dehydrogenation of ammonia over Pt. Moreover, the I-V characteristics of an ammonia-fueled anion exchange membrane fuel cell with the SnO2-Pt/C anode clearly achieved a higher performance than with the Pt/C anode. (C)2015 Elsevier Ltd. All rights reserved.
  • J. Yang; T. Akagi; T. Okanishi; H. Muroyama; T. Matsui; K. Eguchi
    Fuel Cells WILEY-V C H VERLAG GMBH 15 (2) 390 - 397 1615-6846 2015/04 [Refereed]
     
    Recently, the promising prospect of ammonia as a hydrogen carrier for solid oxide fuel cells (SOFCs) has attracted significant interests. In this work, the effects of temperature, fuel content, and total flow rate of anode gas on the performance of Ni/yttria-stabilized zirconia (Ni/YSZ) anode for ammonia-fueled SOFCs were investigated. Based on obtained results, the utilization route of ammonia on Ni/YSZ anode was discussed; the results of electrochemical experiments were related with the catalytic decomposition bahavior of ammonia over Ni/YSZ. Moreover, the catalytic activity for ammonia decomposition and anode performance of Ni/samarium-doped ceria (Ni/SDC) and Ni/yttrium-doped barium cerate (Ni/BCY) were also investigated. Among these anode materials, Ni/BCY exhibited the highest ammonia decomposition activity and anode performance for ammonia-fueled SOFCs at intermediate temperatures.
  • Jun Yang; Ahmed Fathi Salem Molouk; Takeou Okanishi; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    ACS APPLIED MATERIALS & INTERFACES AMER CHEMICAL SOC 7 (13) 7406 - 7412 1944-8244 2015/04 [Refereed]
     
    In this study, Ni/BaCe0.75Y0.25O3-delta (Ni/BCY25) was investigated as an anode for direct ammonia-fueled solid oxide fuel cells, The catalytic activity of Ni/BCY25 for ammonia decomposition was found to be remarkably higher than Ni/8 mol % Y2O3-ZrO2 and Ni/Ce0.90Gd0.10O1.95. The poisoning effect of water and hydrogen on ammonia decomposition reaction over Ni/BCY25 was evaluated. In addition, an electrolyte supported SOFC employing BaCe0.90Y1.10O3-delta (BCY10) electrolyte and Ni/BCY25 anode was fabricated, and its electrochemical performance was investigated at 550-650 degrees C with supply of ammonia and hydrogen fuel gases. The effect of Water content in anode gas on the cell performance was also studied. Based on these results, it was concluded that Ni/BCY25 was a promising anode for direct ammonia-fueled SOFCs An anode supported single cell denoted as Ni/BCY25 vertical bar BCY10 vertical bar Sm0.5Sr0.5CoO3-delta, Was also fabricated, and Maximum powder density Of 216 and 165 mW cm(-2) was achieved at 650 and 600 degrees C, for ammonia fuel respectively.
  • Yu Katayama; Takeou Okanishi; Hiroki Muroyama; Toshiald Matsui; Koichi Eguchi
    JOURNAL OF PHYSICAL CHEMISTRY C AMER CHEMICAL SOC 119 (17) 9134 - 9141 1932-7447 2015/04 [Refereed]
     
    The recent development of anion exchange membranes (AEMs) has increased the potential and the importance of ammonia as a fuel for anion exchange membrane fuel cells (AEMFCs). Although electrocatalysts for ammonia oxidation using Pt-based alloy catalysts have been developed recently, no obvious activity enhancement was reported. In this paper, the electrocatalytic activity of rare earth oxide (RO: CeO2, Y2O3, La2O3, and Sm2O3) modified Pt catalysts for ammonia oxidation reaction was investigated in alkaline aqueous solutions. The ammonia oxidation activity was enhanced in accordance with the amount of OHad species on the electrode surface. From various electrochemical measurements, it was revealed that the RO additive improved the supply capacity of OHad to the reactive Pt sites. Among the catalysts studied, the CeO2-modified Pt electrocatalyst exhibited the highest activity; the peak current density for ammonia oxidation was 3.5 times higher than that of the Pt catalyst. Furthermore, this activity enhancement was also observed at 60 degrees C. These results indicate that the addition of RO is one of the promising ways to design the high performance anode for direct ammonia AEMFCs.
  • Hiroki Muroyama; Hiroki Asajima; Shigeki Hano; Toshiaki Matsui; Koichi Eguchi
    APPLIED CATALYSIS A-GENERAL ELSEVIER SCIENCE BV 489 235 - 240 0926-860X 2015/01 [Refereed]
     
    Since the emission of diesel soot damages both the environment and human health, a method for the catalytic combustion of soot at low temperatures is needed. In this study, CeO2-based oxides were investigated with respect to their catalytic activities in soot combustion to clarify the roles of ceria and an additive. The oxidation behavior of a carbon black layer over CeO2 and Al2O3 substrates in air was examined by in situ microscopy. While the carbon layer over Al2O3 gradually became thinner with an increase in temperature, it disappeared from the circumference over the CeO2 substrate. The produced gas species was also analyzed during carbon oxidation over CeO2-based catalysts containing a transition metal or rare earth oxide. Complete oxidation to CO2 was facilitated by the addition of transition metal elements to the catalysts, which could be explained by the activities of the catalysts for CO oxidation. Moreover, the effect of the support material on the carbon oxidation activity was studied for supported CuO catalysts. The reducing property of CuO over the support was related to its catalytic activity. (C) 2014 Elsevier B.V. All rights reserved.
  • 松井 敏明; 室山 広樹; 江口 浩一
    燃料電池 燃料電池開発情報センター 15 (1) 28 - 34 1346-6623 2015/01 [Refereed]
  • Masaru Kubota; Takeou Okanishi; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 162 (4) F380 - F386 0013-4651 2015 [Refereed]
     
    The performance of nickel-based anode materials for solid oxide fuel cells (SOFCs) could be deteriorated by redox treatments accompanied with the microstructural change. In this study, Ni-yttria-stabilized zirconia (Ni-YSZ) anode was subjected to thermal cycles with redox treatments (thermal-redox cycles: repetition of oxidation in cooling and reduction in heating processes). The microstructural change of anodes was quantified by focused ion beam-scanning electron microscopy. The decrement in the active triple phase boundary (TPB) length caused by sintering and agglomeration of Ni particles was correlated with the performance deterioration. Furthermore, it was found that the onset temperature of reduction treatment in the heating process was one of the critical factors for the microstructural evolution of anode; the agglomeration of Ni particles was promoted significantly under the reduction started from 500 degrees C, while no notable change was confirmed for the case of 900 degrees C. (C) The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. All rights reserved.
  • Ahmed Fathi Salem Molouk; Takeou Okanishi; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 162 (10) F1268 - F1274 0013-4651 2015 [Refereed]
     
    In this study the electrochemical performances of Ni-YSZ anode in humidified H-2, NH3 and CH4 fuels were compared under almost the same partial pressure of oxygen at 500-800 degrees C. The cell performance was significantly affected by fuel species and operating temperature. The single cell exhibited higher performance in wet NH3 as compared with that in CH4, which was caused by the difference in the catalytic activity of anode for the hydrogen production in wet NH3 and CH4. Thus, it was clarified that NH3 is a preferable fuel rather than CH4 for the direct use in SOFCs. Furthermore, the kinetics study on ammonia decomposition over Ni-YSZ anode was conducted at 600 degrees C. Although the ammonia decomposition increased with increasing ammonia concentration, the decomposition reduced in the presence of hydrogen in the reactant gas. Such a behavior was not confirmed at 850 degrees C. These results indicated the inhibition effect of hydrogen for ammonia decomposition, which will be the key factor for the design of anode and the operating condition setting. (C) The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecorrunons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. All rights reserved.
  • Ahmed Fathi Salem Molouk; Jun Yang; Takeou Okanishi; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 162 (14) X27 - X27 0013-4651 2015 [Refereed]
  • Asajima Hiroki; Okanishi Takeou; Muroyama Hiroki; Matsui Toshiaki; Eguchi Koichi
    Proceeding of Annual/Fall Meetings of the Japan Petroleum Institute The Japan Petroleum Institute 2015 23 - 23 2015 
    高い熱効率を持つディーゼルエンジンは有効な動力源として期待されているが、ススなどの粒子状物質の排出量削減が求められている。本研究では、ススの燃焼触媒として、セリア系複合酸化物触媒を調製し、燃焼反応に与える影響を調べた。燃焼試験の結果、遷移金属や希土類成分の添加により活性が向上した。そこで、調製法の違いによる活性への影響について評価を行った。また、各触媒の表面積や物性評価についても検討を行った。
  • Muroyama Hiroki; Ito Ryota; Katayama Yu; Okanishi Takeou; Matsui Toshiaki; Eguchi Koichi
    Proceeding of Annual/Fall Meetings of the Japan Petroleum Institute The Japan Petroleum Institute 2015 173 - 173 2015 
    アルカリ水溶液中における2–プロパノールの電気化学的酸化挙動について検討した。Pd/Cに比べてPt/Cの方が低電位より酸化開始したが定電位保持時における性能低下はPt/Cの方が顕著であった。in situ ATR–IR分光法による電気化学反応解析において、両電極上でアセトン吸着種が生成し、Pt電極上ではさらに反応が進行したエノラート吸着種が経時的に蓄積し性能低下の原因となっていることが示唆された。
  • Katayama Y; Okanishi T; Muroyama H; Matsui T; Eguchi K
    Journal of Physical Chemistry C 119 (22) 12780  1932-7447 2015 [Refereed]
  • T. Matsui; M. Komoto; H. Muroyama; K. Eguchi
    FUEL CELLS WILEY-V C H VERLAG GMBH 14 (6) 1022 - 1027 1615-6846 2014/12 [Refereed]
     
    The stability of interfacial structure between air electrode and ceria-based electrolyte (Sm2O3-CeO2, SDC) was evaluated under various polarization conditions at 1,000 degrees C. This is because the structural change at the interface is strongly related to the diffusion of constituent elements and has the potential to affect the long term stability. In this study, (La,Sr)MnO3 (LSM) and (La,Sr)(Co,Fe)O-3 (LSCF) were applied as air electrodes. The cell with LSM/SDC interface showed the performance enhancement after cathodic polarization for 5 h, accompanied with the change in interfacial structure. The magnitude of the microstructural change as well as activation was dependent on the current density and the oxygen partial pressure. However, the active triple phase boundary (TPB)-length decreased at every condition studied. On the other hand, no appreciable change in interfacial structure was confirmed for the LSCF/SDC system regardless of polarization conditions. In response to this phenomenon, the total TPB-length was almost identical to the active one. The difference in morphological evolution at the interface depending on the electrode material was discussed.
  • Vandana Singh; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    ELECTROCHEMISTRY ELECTROCHEMICAL SOC JAPAN 82 (10) 839 - 844 1344-3542 2014/10 [Refereed]
     
    In this study, effects of operating conditions on the electrochemical reduction of CO2 on solid oxide electrolysis cell (SOEC) were systematically investigated. Experimental results revealed that the reducing gas can influence the behavior of Ni-YSZ cermet cathode. Supply of hydrogen as a reducing gas was found to be the most promising approach to obtain better cell performance for CO2 reduction. Even a small amount of H-2 can facilitate the CO2 reduction due to the kinetically fast reverse water gas shift reaction. High temperature operation is favorable from thermodynamic viewpoint because the Gibbs free energy significantly decreases at elevated temperature. The partial pressure of oxygen at the cathode side is another critical factor that found to be beneficial. A considerable increase in cell voltage was observed when Ni-YSZ vertical bar YSZ vertical bar LSM-YSZ cell operated under constant current densities of -0.75 and -0.90 A cm(-2) at 900 degrees C, whereas the stable cell performance could be possible at -0.15 A cm(-2) for 7 h. The increase in cell voltage was attributed to the degradation of anode. (C) The Electrochemical Society of Japan, All rights reserved.
  • Yi-Hsuan Lee; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF POWER SOURCES ELSEVIER SCIENCE BV 262 451 - 456 0378-7753 2014/09 [Refereed]
     
    The performance degradation of Ni-yttria-stabilized zirconia (Ni-YSZ) cermet anode was measured by impedance spectroscopy at 1000-1200 degrees C and humidity atmospheres under the open circuit condition in SOFCs. More significant crack formation can be observed at 1200 degrees C under 40% H2O-60% H-2 atmosphere. This crack formation gave rise to interruption of the ionic and electronic conduction path in the in-plane direction of anode layer, resulting in performance deterioration of anode. Focused ion beam-scanning electron microscopy (FIB-SEM) analyses were conducted for the anode layers, and then the 3D microstructures of Ni-YSZ anode were reconstructed. According to analysis of these data, the particle size of Ni was grown to the larger under higher temperature and humidity condition, accompanying with increase of isolated Ni-phase and the reduction of triple phase boundary (TPB) length. (C) 2014 Elsevier B.V. All rights reserved.
  • Jun Yang; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF POWER SOURCES ELSEVIER SCIENCE BV 245 277 - 282 0378-7753 2014/01 [Refereed]
     
    A fundamental study on direct ammonia fuel cells with a molten hydroxide electrolyte was conducted. The electrochemical oxidation of ammonia on Pt electrode in a molten NaOH-KOH electrolyte was investigated by cyclic voltammetry and mass spectrometry. Ammonia was proved to be oxidized to N-2 on the Pt electrode during anodic polarization in the molten hydroxide electrolyte. Furthermore, the direct ammonia fuel cell, i.e., Pt gas diffusion electrodelmolten NaOH-KOH electrolyte vertical bar Pt gas diffusion electrode, was assembled and operated at 200-220 degrees C. The cell achieved the maximum power density of ca. 16 mW cm(-2) at 220 degrees C with the supply of NH3 and humidified O-2 to the anode and cathode, respectively. The mechanism of ammonia oxidation over Pt electrodes in a molten hydroxide electrolyte was discussed based on the results obtained. (C) 2013 Elsevier B.V. All rights reserved.
  • V. Singh; H. Muroyama; T. Matsui; K. Eguchi
    SOLID STATE IONIC DEVICES 10 ELECTROCHEMICAL SOC INC 64 (2) 53 - 64 1938-5862 2014 [Refereed]
     
    In this article, two Ni-based cermet electrodes have been fabricated and their electrochemical performances have been evaluated for CO2 electrolysis on solid oxide electrolysis cell (SOEC). It was found that Ni-SDC cermet electrode showed lower cathodic overpotential than Ni-YSZ electrode. The improved cell performance indicated that Ni-SDC cermet was a better and more effective electrode material for CO2 reduction. The results revealed that the conductivity of Ni-SDC cermet cathode for CO2 reduction strongly depended on the operating parameters, and the effect of these operating parameters on the electrochemical activity of Ni-SDC cermet cathode was closely interlinked. The most critical operating parameter for CO2 reduction was appeared to be the gaseous atmosphere, which can be controlled by changing the oxygen partial pressure at Ni-SDC electrode.
  • Toshiaki Matsui; Yuichi Mikami; Hiroki Muroyama; Koichi Eguchi
    JOURNAL OF POWER SOURCES ELSEVIER SCIENCE BV 242 790 - 796 0378-7753 2013/11 [Refereed]
     
    Time-dependent events during operation of SOFCs, i.e., performance enhancement and/or deterioration, can be readily observed for the cell composed of strontium-doped lanthanum manganite (LSM) cathode and yttria-stabilized zirconia (YSZ) electrolyte, concomitant with the change in interfacial structure of LSM/YSZ. The influence of LSM composition on the electrochemical properties and microstructure of LSM/YSZ interface during prolonged operation was investigated. Four different LSM cathodes were used and the change in microstructure, especially TPB-length, was evaluated quantitatively by a focused ion beam scanning electron microscope (FIB-SEM). For LSM cathodes with A-site deficient compositions, the change in TPB-length had a minor contribution to the performance enhancement after 20 h of galvanostatic operation. On the other hand, for 100 h duration an increase in cathode overpotential was confirmed, accompanied with the formation of thin layer of LSM over YSZ electrolyte. A series of phenomena were triggered by the change in oxygen nonstoichiometry of LSM under polarized states. The mechanism for microstructural change was proposed and the long-term stability of LSM/YSZ interface was discussed. (C) 2013 Elsevier B.V. All rights reserved.
  • Jun Yang; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF POWER SOURCES ELSEVIER SCIENCE BV 236 192 - 199 0378-7753 2013/08 [Refereed]
     
    In this work, the effect of porous interlayer on the microstructural change of LSM cathodes and yttria-stabilized zirconia (YSZ) electrolyte was investigated under a current-accelerated condition at 1000 degrees C. The polarization resistance of LSM cathode on bare yttria-stabilized zirconia (YSZ) electrolyte decreased significantly after discharge at 1.2 A cm(-2) for 5 h, and then started to increase gradually. However, when a porous YSZ interlayer was employed between LSM cathode and YSZ electrolyte, the polarization resistance did not increased even after discharge at the same condition for 40 h. The performance changes of the two kinds of cathodes were related with the microstructural change of the interface between LSM cathode and YSZ interlayer. Quantitative analysis for the microstructural change of LSM cathode on YSZ interlayer revealed that even after discharge at 1.2 A cm(-2) for 40 h, 90% of the triple phase boundary of LSM, YSZ and gas phase remained active for oxygen reduction reaction. On the other hand, the LSM cathode on YSZ electrolyte with a porous Sm-doped ceria interlayer showed remarkable long-term stability even after discharge at 1.2 A cm(-2) for 160 h. (C) 2013 Elsevier B.V. All rights reserved.
  • Shohei Suzuki; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    ELECTROCHIMICA ACTA PERGAMON-ELSEVIER SCIENCE LTD 88 552 - 558 0013-4686 2013/01 [Refereed]
     
    Influence of CO2 dissolution into anion exchange membranes (AEMs) on fuel cell performance was investigated. The conductivity of AEM decreased with an increase in carbonate ion species. For the power generation test with a supply of humidified H-2/O-2, the IR-free cell potential difference for the membrane electrode assembly (MEA) with the HCO3- form membrane was lower than that with the OH-form membrane. This low cell potential difference became noticeable by the supply of CO2 to either anode or cathode. The increment of ohmic resistance was confirmed by the supply of CO2 to cathode, but no obvious change in ohmic resistance was observed by the supply of CO2 to anode. Then, a CO2 permeation flux was determined by a gas chromatograph analysis of exhaust gas, and the correlation between the AEM fuel cell performance and the composition of ion species in the membrane was discussed. (C) 2012 Elsevier Ltd. All rights reserved.
  • Toshiaki Matsui; Yuichi Mikami; Hiroki Muroyama; Koichi Eguchi
    ECS Transactions Electrochemical Society Inc. 57 (1) 1683 - 1690 1938-6737 2013 [Refereed]
     
    The cells composed of strontium-doped lanthanum manganite (LSM) cathode and yttria-stabilized zirconia (YSZ) electrolyte show time-dependent events, such as the performance enhancement and/or deterioration, under polarized states. In this study, the influence of LSM composition on the microstructure of LSM/YSZ interface as well as electrochemical properties during prolonged operation was investigated. LSM cathodes with three different compositions were used and the microstructural change was evaluated quantitatively by a focused ion beam-scanning electron microscope (FIB-SEM). It was clarified that for LSMs with A-site deficient compositions, the change in TPB-length had a minor contribution to the performance enhancement upon the initial cathodic current passage. Furthermore, after 100 h of discharge the thin layer of LSM was formed over YSZ electrolyte, accompanied with an increase in cathode overpotential. These phenomena are strongly correlated with the change in oxygen nonstoichiometry of LSM under polarized states. © The Electrochemical Society.
  • Yi-Hsuan Lee; Hirofumi Sumi; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 160 (6) F579 - F584 0013-4651 2013 [Refereed]
     
    Influence of the materials and cell configuration, especially anode thickness, on the performance stability was examined during long-term operation of solid oxide fuel cells (SOFCs) with humidified methane (S/C = 0.1) at 1000 degrees C: Two kinds of anode materials were evaluated; i.e., Ni-yttria-stabilized zirconia (Ni-YSZ) and Ni-samaria-doped ceria (Ni-SDC). The thickness of anode was controlled to be ca. 10, 20, and 30 mu m in this study. Deterioration in performance was observed during a constant current operation with 10 mu m-thick Ni-YSZ anode. On the other hand, the cell performance with 20 and 30 mu m-thick Ni-YSZ was stable for 20 h. These differences in performance stability were significantly related to the anode thickness. In addition, the same experiments were conducted for the cells employing Ni SDC anodes. The cell performance was improved obviously during discharge for 20 h regardless of anode thickness. The improvement upon discharge is ascribable to an increase in catalytic activity of Ni SDC and electronic conductivity of SDC, which is induced by the partial reduction of Ce4+ to Ce3+ under reducing atmospheres. Microstructural observation and Raman spectroscopy were also conducted for the anodes. The electrochemical behavior observed in each anode had a strong correlation with crystallinity of carbon deposited and deposition area. (C) 2013 The Electrochemical Society. All rights reserved.
  • Muroyama Hiroki; Yamaji Hidenori; Matsui Toshiaki; Eguchi Koichi
    Proceeding of Annual/Fall Meetings of the Japan Petroleum Institute The Japan Petroleum Institute 2013 47 - 47 2013 
    ディーゼルエンジン排ガスに含まれる粒子状物質(PM)はディーゼルパティキュレートフィルタ上に担持された触媒を使用して燃焼される。現在、PM燃焼温度を低温化することが検討されており、当研究室では高い酸素吸蔵能を有するCeO2系触媒が注目してきた。本研究では、セリアへの添加元素およびその添加量のPM燃焼活性への影響について調べた。また、同位体酸素交換反応試験、同位体酸素を用いた活性試験によってPM燃焼反応に寄与する触媒酸素量の定量的評価を試みた。
  • Muroyama Hiroki; Kitagawa Kyohei; Kamiuchi Naoto; Matsui Toshiaki; Eguchi Koichi
    Proceeding of Annual/Fall Meetings of the Japan Petroleum Institute The Japan Petroleum Institute 2013 5 - 5 2013 
    Pt–Li2O/TiO2–Al2O3触媒を含浸法とゾル－ゲル法で調製し、TiO2–Al2O3混合担体がNO吸蔵還元特性に与える影響を検討した。Pt–Li2O/TiO2–Al2O3触媒はPt–Li2O/Al2O3触媒よりも低い表面積およびNO吸蔵量を示した。SO2共存下におけるNO吸蔵還元反応では、すべての触媒についてNO吸蔵量が低下したが、Pt–Li2O/TiO2–Al2O3触媒を用いた場合、比較的高いNO吸蔵能が長時間にわたり維持された。この結果よりTiO2–Al2O3担体はSO2共存下における継続的なNO吸蔵に効果的であることが明らかとなった。
  • Hiroki Muroyama; Chikara Saburi; Toshiaki Matsui; Koichi Eguchi
    APPLIED CATALYSIS A-GENERAL ELSEVIER SCIENCE BV 443 119 - 124 0926-860X 2012/11 [Refereed]
     
    Ammonia decomposition is a promising process for on-site generation of hydrogen. In this study, various nickel catalysts supported on metal oxides were prepared and their catalytic activity for ammonia decomposition was investigated. Ni/Al2O3 catalyst achieved the highest ammonia conversion among catalysts investigated due to the high surface area of Al2O3 support (200 m(2) g(-1)). Despite the low surface area of support material (4.7 m(2) g(-1)), the catalytic activity of Ni/La2O3 was comparable to that of Ni/Al2O3. The basicity of support materials was not specifically related with the catalytic activity. For Ni/La2O3 catalyst, the small nickel particles were deposited over La2O3 from LaNiO3 during the reduction treatment. When the Ni loading amount was changed in the range of 10-70 wt% for Ni/La2O3 catalyst, the sample with 40 wt% Ni exhibited the highest conversion of 78.9% at 550 degrees C. The catalytic performance of Ni/La2O3 was also affected by the preparation method and calcination temperature. The LaNiO3 formation was responsible for the high activity of Ni/La2O3 catalyst for the ammonia decomposition. (C) 2012 Elsevier B.V. All rights reserved.
  • Toshiaki Matsui; Jin-Young Kim; Hiroki Muroyama; Megumi Shimazu; Toshiya Abe; Motoyasu Miyao; Koichi Eguchi
    SOLID STATE IONICS ELSEVIER SCIENCE BV 225 50 - 54 0167-2738 2012/10 [Refereed]
     
    Cathode-supported tubular-type cells fabricated by TOTO Ltd. degraded gradually during long-term operation, and a dissection analysis of cells revealed a microstructural change in the anode. In this study, then, the direct observation of the degraded anode was conducted to clarify the influence of this structural change on the performance by using a dual-beam focused ion beam-scanning electron microscopy (FIB-SEM). The microstructural parameters of Ni-(Sc2O3)(0.10)(CeO2)(0.01)(ZrO2)(0.89) cermet anode were quantitatively evaluated. The significant reduction in triple phase boundary (TPB) length was confirmed, accompanied with the migration and sintering of nickel particles especially in the vicinity of anode/electrolyte interface. (C) 2012 Elsevier B.V. All rights reserved.
  • Pramote Puengjinda; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF POWER SOURCES ELSEVIER SCIENCE BV 216 409 - 416 0378-7753 2012/10 [Refereed]
     
    Ceramic material based on composite of yttria-doped SrTiO3 (YST) and samaria-doped ceria (SDC) incorporated with Ni catalyst has been evaluated as an anode of solid oxide fuel cell to substitute the use of conventional Ni-based cermets. The results indicated the advantages of suitable Ni addition which could remarkably enhance the electrochemical performance of anode due to the superior in catalytic activity of SDC and Ni towards fuel oxidation. Temperature-programmed reduction (TPR) profiles indicated the existence of bulk-free and interacted NiO in the composites. The cells with Ni/YST-SDC (20 wt% NiO) anodes exhibited stable performance in the prolong operation with tremendously severe conditions in methane fuel with low steam to carbon ratio (S/C = 0.1) and highly humidified hydrogen fuel (40% H2O-H-2) compared with the conventional Ni-based cermets. Small amount of deposited carbon was observed and played insignificant effects on the electrochemical performance after the operation for 100 h. The stable performance was ascribable to the excellence in YST-SDC ceramic framework that effectively suppressed the carbon formation. Furthermore, the Ni/YST-SDC anode showed high tolerance in encountering with the redox cycles. From these results, Ni/YST-SDC composite is considered to be a promising candidate for SOFC anode material operating in severe conditions. (C) 2012 Elsevier B.V. All rights reserved.
  • Hiroki Muroyama; Tomoharu Akagi; Toshiaki Matsui; Koichi Eguchi
    SOLID STATE IONICS ELSEVIER SCIENCE BV 225 663 - 666 0167-2738 2012/10 [Refereed]
     
    Various alkali metal pentahydrogen diphosphates, MH5(PO4)(2) (M=K, Rb, Cs), were mixed with several molar ratios, and the mixed alkali effect on thermal and electrochemical properties was investigated at intermediate temperatures. The XRD patterns of mixed MH5(PO4)(2) indicated the formation of substitutional solid solution. The mixing of three kinds of alkali metal phosphates with equimolar ratio, KH5(PO4)(2)-RbH5(PO4)(2)-CsH5(PO4)(2), was the most effective for the depression of melting point. The mixed MH5(PO4)(2) and their composites with SiP2O7 matrix attained higher conductivity in the low temperature range (80-150 degrees C) due to the lower melting point of alkali metal phosphates as compared to the corresponding CsH5(PO4)(2) samples. The ionic transference numbers of mixed MH5(PO4)(2)/SiP2O7 composites achieved 0.88-0.92, indicating that these materials were proton conductors. (C) 2012 Elsevier B.V. All rights reserved.
  • K. Eguchi; N. Kamiuchi; J-Y Kim; H. Muroyama; T. Matsui; M. Kishimoto; M. Saito; H. Iwai; H. Yoshida; N. Shikazono; N. Kasagi; J. Akikusa; H. Eto; D. Ueno; M. Kawano; T. Inagaki
    FUEL CELLS WILEY-V C H VERLAG GMBH 12 (4) 537 - 542 1615-6846 2012/08 [Refereed]
     
    The daily start-up and shut-down operations were carried out for the 1?kW-class solid oxide fuel cell stack composed of 46 electrolyte-supported disk-type planar cells made by Mitsubishi Materials Corporation and Kansai Electric Power Company. The representative Nigadolinia doped ceria cermet anodes in the deteriorated cells were analyzed by the dual-beam focused ion beam-scanning electron microscope. The anode microstructures were reconstructed and the microstructural parameters such as triple phase boundary (TPB) length were quantified. The surface area of nickel phase decreased with an increase in the deterioration rate, leading to a reduction in the TPB length. Furthermore, the TPB length had a strong correlation with the voltage deterioration rate.
  • Shohei Suzuki; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF POWER SOURCES ELSEVIER SCIENCE BV 208 257 - 262 0378-7753 2012/06 [Refereed]
     
    The fundamental characteristics of direct ammonia fuel cell employing anion exchange membrane (AEM) as an electrolyte was evaluated. The fuel cells with three different anodes of Pt/C, Pt-Ru/C, and Ru/C were applied for the performance tests. The open circuit voltage (OCV) was notably lower than the ideal electromotive force of 1.17 V and significantly dependent on the electrocatalysts. The OCV achieved was in the following sequence: Pt-Ru/C> Pt/C > Ru/C. Furthermore, the ammonia permeated through the AEM by the solubility-diffusion mechanism and the formation of oxidants such as N-2 and NO was detected in the cathode exhaust gas. This result indicates the oxidation of ammonia in the cathode, which will be another factor for the reduction in OCV. Note that OCV and performance continuously decreased during the consecutive I-V characteristic measurements because of the N-ad poisoning over the surface of anode catalysts. (C) 2012 Elsevier B.V. All rights reserved.
  • Pramote Puengjinda; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF POWER SOURCES ELSEVIER SCIENCE BV 204 67 - 73 0378-7753 2012/04 [Refereed]
     
    Y-doped SrTiO3 (YST) is currently used as an effective anode component to solve the shortcomings of conventional Ni-based cermet anodes. In this study, YST-basecl composite with a different ceramic oxide including samaria-doped ceria (SDC) or yttria-stabilized zirconia (YSZ) was developed as an anode material to evaluate the electrocatalytic performance in hydrogen and methane fuels. The composites showed a good potential in electrical conductivity and compatibility with YSZ electrolyte in the anodic condition. The cell with YST-SDC anode attained the better performance than that with YST-YSZ anode, which was ascribable to the high electrical conductivity and electrocatalytic activity of SDC towards the oxidation of fuels. Addition of nickel remarkably enhanced the electrochemical performance and the stability of the anodes. With 10 wt% of nickel oxide loading, the performance was significantly increased under the operation in humidified hydrogen and methane. Note that in methane fuel, the performance deterioration has not been observed over the short-term operation for 20 h. (C) 2011 Elsevier B.V. All rights reserved.
  • Jun Yang; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF POWER SOURCES ELSEVIER SCIENCE BV 204 25 - 33 0378-7753 2012/04 [Refereed]
     
    In this work, discharge operation at a large current density was conducted for strontium-doped lanthanum manganite (LSM) cathodes, and the change in performance and microstructure of the interface between LSM cathode and YSZ electrolyte was investigated. For (La0.8Sr0.2)(0.97)MnO3 (LSM97) with A-site deficiency, the performance was improved in the initial stage of discharge and then started to deteriorate continuously. The microstructural change in the interface between LSM97 and YSZ was quantitatively analyzed by using a dual-beam focused ion beam-scanning electron microscope. It was found that after discharge at 1.2 A cm(-2) for 60 h, the surface of YSZ electrolyte was covered by a dense layer of LSM, accompanied with the formation of closed nanopores in the internal interface between LSM97 and YSZ. For (La0.8Sr0.2)(1.03)MnO3 (LSM103) with a slight A-site excess, neither apparent formation of dense LSM103 layer along the LSM103/YSZ interface nor performance deterioration was observed during discharge. The mechanism for the structural change of LSM97 cathode was discussed based on the obtained results. (C) 2011 Elsevier B.V. All rights reserved.
  • Pramote Puengjinda; Hiroki Muroyama; Toshiaki Matsui; Mitsunobu Kawano; Toru Inagaki; Koichi Eguchi
    JOURNAL OF THE EUROPEAN CERAMIC SOCIETY ELSEVIER SCI LTD 32 (1) 107 - 113 0955-2219 2012/01 [Refereed]
     
    The microstructural change in composite of nickel oxide and scandia-stabilized zirconia (NiO-ScSZ) under a reducing atmosphere was observed by scanning electron microscopy (SEM). The morphological transformation was noticeable after high-temperature treatment with the formation of two peculiar microstructures; i.e., fibrous zirconia and metallic nickel with wrinkled surface. It was suggested that partial reduction of the nickel species dissolved in ScSZ lattice triggered the formation of these characteristic morphologies. The growth of fibrous zirconia appeared to be promoted via interfacial reaction between the metallic Ni particles and the zirconia phase. The agglomeration of metallic nickel proceeded by the reduction at high temperatures and then the surface was transformed to the wrinkled morphology. The unique wrinkled pattern was often observed for the large agglomerated nickel particles. This drastic change in Ni-ScSZ microstructure upon the high-temperature reduction degraded the electrochemical performance of solid oxide fuel cells. (C) 2011 Elsevier Ltd. All rights reserved.
  • Bin Liu; Hiroki Muroyama; Toshiaki Matsui; Kazuo Tomida; Tatsuo Kabata; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 159 (3) B324 - B330 0013-4651 2012 [Refereed]
     
    Dynamic behavior of the segmented-in-series tubular solid oxide fuel cell upon discharge was investigated. The cell performance decreased initially at 900 degrees C, accompanied by the increase of ohmic resistance. Lower partial pressure of oxygen in the cathode resulted in a more severe decrease, whereas the gas displacement in the anode compartment did not induce such behavior. As the current was cut off, the ohmic resistance rapidly recovered toward the starting value before current load. The possible origin for this increase-recovery behavior of the ohmic resistance has been discussed based on the evaluation of oxygen partial pressure at the cathode/electrolyte interface and element interdiffusion between cathode and electrolyte. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.099203jes] All rights reserved.
  • Toshiaki Matsui; Ryo Kishida; Hiroki Muroyama; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 159 (8) F456 - F460 0013-4651 2012 [Refereed]
     
    The influence of the oxide component in Ni-oxide based cermet anodes on the performance stability was examined at 1000 degrees C with a supply of highly humidified hydrogen, 40% H2O-60% H-2. Three types of anodes were applied; Ni-samaria-doped ceria (Ni-SDC), Ni-yttria-stabilized zirconia (Ni-YSZ), and Ni-scandia-stabilized zirconia (Ni-ScSZ). Abrupt deterioration in performance was confirmed during constant current operation for the cells with Ni-YSZ and Ni-ScSZ anodes, accompanied with a drastic increase in ohmic resistance between the anode and reference electrode. In contrast, the performance of cell with Ni-SDC anode was pretty stable for 48 h. These differences in performance stability were significantly related to the local partial pressure of steam in the vicinity of the electrochemically active sites because the surface oxidation of nickel particles resulted in the deterioration. The microstructural changes in Ni-YSZ and Ni-SDC anodes upon discharge were also quantified by the dual-beam focused ion beam-scanning electron microscopy (FIB-SEM). The electrochemical behavior observed in each anode had a strong correlation with the anode microstructure such as triple phase boundary length and nickel particle size. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.053208jes]
  • Shimoda Naohiro; Muroyama Hiroki; Matsui Toshiaki; Eguchi Koichi
    Proceeding of Annual/Fall Meetings of the Japan Petroleum Institute The Japan Petroleum Institute 2012 29 - 29 2012 
    炭化水素系燃料からの燃料電池への水素供給プロセスにおいてCO低減プロセスである水性ガスシフト反応は非常に重要な反応である。また水素供給方法として、水素含有燃料であるメタノールやジメチルエーテルの水蒸気改質反応の利用も検討されている。これらの反応には金属酸化物担持Cu系触媒が広く用いられているが、本研究では種々の酸化物と混合したCu-Fe系スピネル型酸化物に着目し、新規触媒開発を行った。
  • Okura Kaname; Muroyama Hiroki; Matsui Toshiaki; Eguchi Koichi
    Proceeding of Annual/Fall Meetings of the Japan Petroleum Institute The Japan Petroleum Institute 2012 15 - 15 2012 
    アンモニアは水素含有率が高く、貯蔵や輸送が容易であり、さらにその分解反応においてCO, CO2を排出しないため、水素キャリアとして近年注目されている。本研究では水素製造のためのアンモニア分解触媒として、希土類酸化物を含むNi触媒を含浸法により調製した。活性試験と物性評価を行うことで希土類酸化物の役割を検討するとともに、より高い活性を示す触媒の開発を試みた。
  • Shimoda Naohiro; Muroyama hiroki; Matsui Toshiaki; Eguchi Koichi
    Proceeding of Annual/Fall Meetings of the Japan Petroleum Institute The Japan Petroleum Institute 2012 5 - 5 2012 
    高効率なエネルギー変換効率を有する燃料電池システムにおけるCO低減プロセスである水性ガスシフト(WGS)反応や、燃料水素の供給方法としてのメタノールやジメチルエーテルの水蒸気改質反応における高性能触媒としてCu-Feスピネル酸化物触媒に着目し、触媒開発を行った。本発表では、スピネル触媒のレドックス処理による構造変化に伴う活性ならびに耐久性について詳細に検討した。
  • Yamazaki Hiroki; Muroyama Hiroki; Matsui Toshiaki; Eguchi Koichi
    Proceeding of Annual/Fall Meetings of the Japan Petroleum Institute The Japan Petroleum Institute 2012 40 - 40 2012 
    ディーゼルエンジンは燃費や耐久性が高く大型車に用いられるが空燃比が高いため三元触媒ではNOxを十分に除去することができない。そこで希薄燃焼におけるNOx除去触媒として、NOx吸蔵還元触媒が注目されている。本研究ではNOx吸蔵還元触媒として Pt-Li2O/TiO2-Al2O3 (TiO2/Al2O3 = 1/1)を用いて、Li2Oの担持量、SOx被毒再生処理温度などを変えることによりNOx吸収量、SOx被毒耐性について検討した。
  • N. Shimoda; H. Muroyama; T. Matsui; K. Faungnawakij; R. Kikuchi; K. Eguchi
    APPLIED CATALYSIS A-GENERAL ELSEVIER SCIENCE BV 409 91 - 98 0926-860X 2011/12 [Refereed]
     
    The durability of the CuFe2O4 spinel and gamma-Al2O3 composite catalysts for steam reforming of dimethyl ether (DME) was evaluated at 375 degrees C for 120 h under the daily start-up and shut-down (DSS) operation. No degradation could be observed for the catalyst subjected to the cooling-heating process in O-2 atmosphere. On the other hand, the catalyst performance was significantly degraded by the cooling-heating process in steam atmosphere due to the deactivation of the acid sites on gamma-Al2O3 for DME hydrolysis reaction, leading to the loss of DME steam reforming activity. The higher amount of coke formation was also observed over the catalysts exposed to steam during the cooling-heating process, as compared with the catalysts exposed to O-2 or without DSS operation. The degraded catalyst could be recovered by the heat treatment in air at 375 degrees C, since the active sites of gamma-Al2O3 were regenerated and the coke deposits were removed. Interestingly, the catalytic activity of the regenerated catalysts for methanol steam reforming was higher than the fresh one because of an increase in the metal surface area of Cu species after the regeneration treatment. (C) 2011 Elsevier B.V. All rights reserved.
  • Toshiaki Matsui; Toshimitsu Noto; Hiroki Muroyama; Masahiko Iijima; Koichi Eguchi
    Journal of Power Sources 196 (22) 9445 - 9450 0378-7753 2011/11 [Refereed]
     
    An effective resistance of solid acid/phosphate composites was reduced by fabricating their thin-film electrolyte membranes for fuel cells operating at 100-300 °C. Solid acid and phosphate serve as an ionic conductor and supporting matrix, respectively, in these composites. Three-types of porous matrices were synthesized on a Pd film substrate by the electrostatic spray deposition technique, and then the solid acid was soaked under reduced pressure. The thin-film composite electrolytes showed almost the same conductivity in a wide temperature range of 100-200 °C, regardless of the difference in matrix microstructure. Above 200 °C, however, the microstructure of matrix significantly affected the thermal stability of the thin-film composite. The composite consisting of the matrix with the reticular structure, characterized by a three-dimensional interconnected porous network, achieved high thermal stability as well as low area specific resistance. Fuel cells employing thin-film membrane electrode assemblies were successfully operated at 200 °C, and the electrochemical measurements clarified the improvements. © 2011 Elsevier B.V.
  • Hiroki Muroyama; Koji Katsukawa; Toshiaki Matsui; Koichi Eguchi
    Journal of The Electrochemical Society 158 (9) B1072 - B1075 0013-4651 2011/09 [Refereed]
     
    Tungsten carbides were prepared from ammonium paratungstate via temperature-programmed carburization under flowing a gaseous mixture of CH 4H2 to employ as anode catalysts in fuel cells consisting of CsH2PO4SiP2O7-based composite electrolyte operative at 200C. The resulting materials were characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The heat-treatment at high temperatures promoted the reduction and carburization of tungsten component. The single phase of WC was observed for the samples subjected to the carburization at and above 800°C. The single cell employing the catalyst prepared at 850C attained the best performance. The anode material containing the metallic W exhibited low stability under the power generation condition. With nickel or cobalt additives, the carburization of tungsten species was initiated at low temperatures. The samples with the additives heat-treated at high temperatures were composed of several tungsten carbides including WC. When these samples were applied as anode catalysts, the additive species lowered the cell performance. These results indicated that the WC phase was the most effective electrocatalyst for the hydrogen oxidation. © 2011 The Electrochemical Society.
  • Hirofumi Sumi; Pramote Puengjinda; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    Journal of Power Sources 196 (15) 6048 - 6054 0378-7753 2011/08 [Refereed]
     
    The effect of crystal structure of yttria- (YSZ) and scandia-stabilized zirconia (ScSZ) in nickel-based SOFC anodes was investigated in relation with carbon deposition and oxidation behavior in methane fuel. The lattice parameter of the zirconia decreased by the dissolution of 1-2 mol%Ni to YSZ and ScSZ. For Ni-doped ScSZ, the lattice parameter of the zirconia increased by the Ni dissolution, and the crystal structure of the zirconia was modified after reduction treatment. New finer Ni particles were formed around original Ni grains accompanied by the decrease in Ni solubility to ScSZ after reduction treatment. Carbon deposition was initiated near the boundary between Ni particles and YSZ (or ScSZ) substrate in dry methane atmosphere. Furthermore, the rod-shaped carbon was observed to grow from the new finer Ni particles on the ScSZ substrate. On the other hand, a large amount of amorphous carbon was promoted to be deposited on Ni-YSZ cermet at a high temperature of 1273 K. The amorphous carbon, however, was oxidized at lower temperatures than graphite. The carbon deposition and oxidation behavior was strongly affected by the morphology and crystallinity of deposited carbon. © 2011 Elsevier B.V. All rights reserved.
  • Hirofumi Sumi; Yi-Hsuan Lee; Hiroki Muroyama; Toshiaki Matsui; Motohisa Kamijo; Shin Mimuro; Mitsugu Yamanaka; Yasushi Nakajima; Koichi Eguchi
    Journal of Power Sources 196 (10) 4451 - 4457 0378-7753 2011/05 [Refereed]
     
    The deterioration by carbon deposition was evaluated for electrolyte- and anode-supported solid oxide fuel cells (SOFCs) in comparison with carbon monoxide disproportionation and methane cracking. The polarization resistance of the nickel-yttria stabilized zirconia (Ni-YSZ) anode increased with a rise in CO concentration in H2-CO-CO2 mixture for the electrolyte-supported cells at 923 K. The resistance, however, did not change against CO concentration for the anode-supported cells. In a methane fuel with a steam/carbon (S/C) ratio of 0.1, the cell performance decreased for both of the cells at 1073 K. A large amount of agglomerated amorphous carbon was deposited from the anode surface to the interface between the anode and the electrolyte after power generation at S/C = 0.1 in methane fuel. On the other hand, the crystalline graphite was deposited only at the anode surface for the anode-supported cell after power generation in CO-CO2 mixture. These results suggest that the reaction rate of CO disproportionation is faster than that of methane cracking. The deposited carbon near the anode/electrolyte interface caused the increase in the polarization resistance. © 2011 Elsevier B.V. All rights reserved.
  • Toshiaki Matsui; Chikara Saburi; Shota Okuda; Hiroki Muroyama; Koichi Eguchi
    International Journal of Hydrogen Energy 36 (4) 2945 - 2949 0360-3199 2011/04 [Refereed]
     
    In this study, a new and convenient technique for the in-situ analyisis of methane steam reforming in the chamber was proposed. The YSZ oxygen sensor was used as the sensing device, which provided the partial pressure of oxygen in the reactor. The oxygen sensors were set in the catalytic bed of 1 wt.% Ni/Al 2O3 along the gas flow direction and the progress of catalytic reforming was monitored at each position. The methane conversion derived from the oxygen sensor agreed well with that from the gas chromatograph set at the outlet part of the catalyst layer. Along the gas flow direction in the reactor, the change in the gas composition was clearly observed; the methane conversion changed significantly depending on the reaction temperature and space velocity of reactant gas. Furthermore, the deterioration behavior of catalytic activity was successfully monitiored when a highly humidified methane with a steam to carbon ratio of 4.0 was supplied with a high space velocity of 6250 l kg-1 h-1. © 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
  • Bin Liu; Hiroki Muroyama; Toshiaki Matsui; Kazuo Tomida; Tatsuo Kabata; Koichi Eguchi
    Journal of The Electrochemical Society 158 (2) B215 - B224 0013-4651 2011/02 [Refereed]
     
    Gas phase transport is a very important electrode process in practical solid oxide fuel cells. In this study, we have identified gas conversion impedance and gas diffusion impedance in the Mitsubishi segmented-in-series tubular solid oxide fuel cell. Gas conversion impedance is caused by the weak convection transport in the gas flow channel. It is observed that both the insufficient anode and cathode gas flow rates can result in the gas conversion impedance. Gas conversion impedance appears at less than 0.1 Hz, and its magnitude strongly depends on the gas flow rates. It disappears when the gas flow rates of both the anode and cathode are improved sufficiently. Anode gas diffusion through the porous substrate appears at ∼0.5 Hz and dominates the overall diffusion impedance. Cathode gas diffusion through the porous current collecting layer appears at ∼3 Hz, which significantly contributes to the overall gas diffusion impedance under low cathode oxygen partial pressures. © 2010 The Electrochemical Society.
  • 江口 浩一; 室山 広樹; 松井 敏明
    表面 広信社 49 (9) 291 - 303 0367-648X 2011/01
  • 鈴木 渉平; 室山 広樹; 松井 敏明
    Journal of the Hydrogen Energy Systems Society of Japan 水素エネルギ-協会 36 (2) 44 - 49 1341-6995 2011/01
  • Hiroki Muroyama; Kyohei Kitagawa; Naoto Kamiuchi; Toshiaki Matsui; Koichi Eguchi
    Journal of the Japan Petroleum Institute 公益社団法人石油学会 54 (6) 366 - 372 1346-8804 2011 [Refereed]
     
    The NO storage-reduction properties of 1 wt% Pt -10 wt% Li 2O/TiO 2 -Al 2O 3 catalysts prepared by the impregnation and sol-gel methods were investigated in the presence and absence of SO 2. The surface area and NO storage amount of catalysts were smaller for the Pt -Li 2O/TiO 2 -Al 2O 3 catalyst than for the Pt -Li 2O/Al 2O 3 catalyst. However, the Pt -Li 2O/TiO 2 -Al 2O 3 sample with mixed oxide support achieved high NO removal efficiency for a long period, although the reduction of NO sorption amount by sulfur poisoning was comparable for all catalysts. These results revealed that the mixed oxide support in the catalyst is a key component for continuous NO removal. The preparation method affected the NO storage amount and the crystalline phases of catalysts. The strong diffraction peak of Li 2TiO 3 phase was observed in the XRD pattern of sample prepared by the impregnation method. The formation of Li 2TiO 3 weakened the basicity of the catalyst, resulting in decreased NO storage capacity under a SO 2-free atmosphere and the enhancement of tolerance to sulfur poisoning.
  • Okuda Shota; Muroyama Hiroki; Matsui Toshiaki; Kawano Mitsunobu; Inagaki Toru; Eguchi Koichi
    Proceeding of Annual/Fall Meetings of the Japan Petroleum Institute The Japan Petroleum Institute 2011 72 - 72 2011 
    固体酸化物形燃料電池(SOFC)の一般的なアノードであるNi-YSZ (YSZ: Y2O3-ZrO2)をYSZ管の軸方向に間隔を空けて数点塗布し、電極触媒上でのCH4水蒸気改質反応を検討した。電極近傍における酸素分圧を測定し、系内に存在する各種気体成分の濃度を見積もり、触媒反応をin-situで解析した。各電極上においてCH4転化率の経時変化をモニタリングし、電極における触媒の劣化を観察した。
  • Yamaguchi Nobutada; Kamiuchi Naoto; Muroyama Hiroki; Matsui Toshiaki; Eguchi Koichi
    Catalysis Today 164 (1) 169 - 175 0920-5861 2011 [Refereed]
     
    Carbon monoxide oxidation over the supported platinum catalysts was investigated. The catalyst of Pt/SnO2 exhibited superior catalytic activity despite the low surface area compared to Pt/γ-Al 2O3. It was confirmed by the temperature-programmed reduction analysis that the reduction of platinum species over Pt/SnO 2 was initiated below room temperature. The platinum species reducible at lower temperature should be responsible for the high activity for the complete oxidation of carbon monoxide. Moreover, the influence of the reduction treatments on these supported platinum catalysts was studied. In the case of Pt/γ-Al2O3, the catalytic activity was improved as the reduction temperature was elevated. This is because the adsorption sites of CO on platinum particles increased by the reduction at higher temperature, leading to the high catalytic activity. In contrast, the activity of Pt/SnO2 was enhanced even by the reduction at 25 °C and the catalyst reduced at 90 °C attained the highest activity. Correlations between the amount of adsorbed CO and the catalytic activity were observed in the Pt/γ-Al2O3 system, which were not found in the reduced Pt/SnO2. These results suggest that the oxygen species of the SnO2 support activated by the reduction treatments contribute to carbon monoxide oxidation. The activity of the sample was significantly degraded by the reduction at 400 °C. This behavior was attributed to a decrease in reaction sites due to the formation of intermetallic compounds. © 2010 Elsevier B.V. All rights reserved.
  • Naoto Kamiuchi; Tomohiro Mitsui; Nobutada Yamaguchi; Hiroki Muroyama; Toshiaki Matsui; Ryuji Kikuchi; Koichi Eguchi
    CATALYSIS TODAY ELSEVIER SCIENCE BV 157 (1-4) 415 - 419 0920-5861 2010/11 [Refereed]
     
    The catalytic combustion of toluene over tin oxide-supported metal catalysts (Pt/SnO2 Pd/SnO2 Ru/SnO2 and Rh/SnO2) was studied As a result the Pt/SnO2 catalyst exhibited the highest activity The activity of Pt/SnO2 catalyst is expected to originate from the strong chemical interaction between platinum and tin oxide The correlation between the nano-structure and catalytic activity in the oxidation of ethyl acetate was investigated for the Pt/SnO2 catalysts treated under a reducing or an oxidative atmosphere For the transmission electron microscopy image of the as-calcined Pt/SnO2 catalyst the fine particles composed of platinum species was highly dispersed on the surface of SnO2 support After the reduction treatment at 400 C the peculiar core-shell structure was formed It is noted that the particles with this structure disappeared by the reoxidation treatment in air and the redispersion of nano-particles was confirmed The particles with the core-shell structure were observed again after the second reduction treatment for the reoxidized catalyst The catalytic activities of ethyl acetate combustion over Pt/SnO2 catalysts were deteriorated and activated corresponding to the nano-structural changes induced by the reduction-oxidation treatments (C) 2010 Elsevier B V All rights reserved
  • Jun Yang; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    INTERNATIONAL JOURNAL OF HYDROGEN ENERGY PERGAMON-ELSEVIER SCIENCE LTD 35 (19) 10505 - 10512 0360-3199 2010/10 [Refereed]
     
    The cathodic polarization behavior of strontium-doped lanthanum manganite (LSM) fired on stabilized zirconia electrolyte was investigated. The performance of LSM was enhanced significantly during discharge operation. The subsequent relaxation behavior under open-circuit state proceeded with two steps. A comparative study was also performed on strontium-doped lanthanum cobaltite (LSC) electrodes. The cathode potential of LSC with stabilized zirconia electrolyte decreased significantly at first and then increased gradually under current-static cathodic polarization. The subsequent relaxation in the open-circuit state also proceeded by two steps. When the interlayer of Sm-doped ceria (SDC) was introduced between electrolyte and cathode, the performance enhancement under cathodic polarization became much smaller regardless of the cathode materials, LSM and LSC. The mechanisms for the polarization behavior of LSM and LSC were discussed based on the obtained results. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
  • Hiroki Muroyama; Shigeki Hano; Toshiaki Matsui; Koichi Eguchi
    CATALYSIS TODAY ELSEVIER SCIENCE BV 153 (3-4) 133 - 135 0920-5861 2010/08 [Refereed]
     
    The catalytic soot combustion, surface area, and reducibility for the CeO2-based oxides were investigated The carbon black (CB), which was used as a model of soot particle, was combusted over 700 degrees C, while the CeO2-based catalysts reduced the CB oxidation temperature by more than 150 degrees C Although the addition of are-earth metal oxides to CeO2 increased the surface area, no remarkable improvement of catalytic activity was confirmed The transition metal oxide added to CeO2 significantly promoted the oxidation reaction, and the CuO-CeO2 catalyst exhibited the highest activity despite the lowest surface area of all composite oxides investigated The reduction behavior of catalysts was closely related with the activity of CB oxidation. It was indicated that the oxygen species available for the oxidation reaction was increased by the addition of transition metal components When the Cu content was varied for the CuO-CeO2 catalyst, the optimal amount existed for the CB oxidation. (C) 2010 Elsevier B V. All rights reserved.
  • Naoto Kamiuchi; Tomohiro Mitsui; Hiroki Muroyama; Toshiaki Matsui; Ryuji Kikuchi; Koichi Eguchi
    APPLIED CATALYSIS B-ENVIRONMENTAL ELSEVIER SCIENCE BV 97 (1-2) 120 - 126 0926-3373 2010/06 [Refereed]
     
    Correlation between the catalytic activity and the nano-structure of ruthenium catalyst supported on tin oxide was investigated. As-calcined Ru/SnO(2) catalyst exhibited high catalytic activity for ethyl acetate combustion despite its low surface area of ca. 5.0 m(2) g(-1). The catalytic activity was degraded by the reduction treatment at 400 degrees C, whereas it was partially restored by the subsequent reoxidation treatment at 400 degrees C To elucidate the variation in the catalytic activity, the ruthenium catalysts heat-treated under reductive or oxidative condition were characterized by X-ray diffraction (XRD). X-ray photoelectron spectroscopy (XI'S), and transmission electron microscopy (TEM) In the as-calcined catalyst, It was revealed that the fine particles with the small contact angles were well-dispersed on the surface of tin oxide support In the deactivated catalyst treated at 400 degrees C under a reductive atmosphere, the large particles of intermetallic compounds with the core-shell structure were clearly observed. On the other hand, upon the reoxidation treatment at 400 degrees C the particles with the core-shell structure disappeared accompanied with the appearance of the nano-sized particles. Accordingly, it was clarified that the catalytic activity was strongly influenced by the structural changes of active sites such as sintering and redispersion (C) 2010 Elsevier B.V. All rights reserved
  • Naoto Kamiuchi; Hiroki Muroyama; Toshiaki Matsui; Ryuji Kikuchi; Koichi Eguchi
    APPLIED CATALYSIS A-GENERAL ELSEVIER SCIENCE BV 379 (1-2) 148 - 154 0926-860X 2010/05 [Refereed]
     
    The structural changes in Pd/SnO2 catalyst upon redox treatments were studied by X-ray diffraction (XRD). X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). In the ascalcined catalyst, the fine particles of palladium species with the amorphous and the well-cyrstallized states were highly dispersed on the surface of SnO2 support due to the strong chemical interaction. The heat-treatment at 200 degrees C in hydrogen induced the reduction of palladium species and the particle growth, resulting in the formation of the well-crystallized Pd particles. Some peculiar structures, called the core-shell structure and the particle intrusion structure, were formed by the reduction at 400 degrees C. After the reoxidation treatment, the core-shell structure disappeared while the particle intrusion structure remained. Furthermore, the decrease in the mean particle diameter was confirmed in the reoxidized catalyst. The proposed formation mechanism of the unique structures from TEM observation was supported by the combination of XRD and XPS measurements. (C) 2010 Elsevier B.V. All rights reserved.
  • Iwai, Hiroshi; Shikazono, Naoki; Matsui, Toshiaki; Teshima, Hisanori; Kishimoto, Masashi; Kishida, Ryo; Hayashi, Daisuke; Matsuzaki, Katsuhisa; Kanno, Daisuke; Saito, Motohiro; Muroyama, Hiroki; Eguchi, Koichi; Kasagi, Nobuhide; Yoshida, Hideo
    JOURNAL OF POWER SOURCES ELSEVIER SCIENCE BV 195 (4) 955 - 961 0378-7753 2010/02 [Refereed]
     
    The three-dimensional microstructure of an SOFC anode is quantified using a dual beam focused ion beam scanning electron microscopy (FIB-SEM) system equipped with an energy dispersive X-ray spectroscopy (EDX) unit. The microstructure of the Ni-YSZ anode is virtually reconstructed in a computational field using a series of acquired two-dimensional SEM images. The three-phase boundary (TPB) density and tortuosity factors are carefully evaluated by applying two different evaluation methods to each parameter. The TPB density is estimated by a volume expansion method and a centroid method, while the tortuosity factors are evaluated by a random walk calculation and a lattice Boltzmann method (LBM). Estimates of each parameter obtained by the two methods are in good agreement with each other, thereby validating the reliability of the analysis methods proposed in this study. (C) 2009 Elsevier B.V. All rights reserved.
  • Hiroki Muroyama; Ryo Nakase; Toshiaki Matsui; Koichi Eguchi
    INTERNATIONAL JOURNAL OF HYDROGEN ENERGY PERGAMON-ELSEVIER SCIENCE LTD 35 (4) 1575 - 1581 0360-3199 2010/02 [Refereed]
     
    The Ni-based spinel-type oxides, NiB(2)O(4) (B = Al, Fe, Mn), were investigated for their catalysis of the ethanol steam reforming reaction. Ethanol conversion over spinel-type oxides without reduction treatment was comparable to that over gamma-alumina-supported Ni catalyst with reduction. The spinel oxide of NiAl(2)O(4) showed extremely stable performance for 48 h, while the activity of NiFe(2)O(4) and NiMn(2)O(4) catalysts was reduced by carbon deposition. Catalyst stability for reforming reaction was closely related to the stability of the nickel metal dispersed on the catalyst surface and the spinel structure. Differences in crystallinity and surface area among the catalysts were not crucial factors for determining ethanol conversion for NiAl(2)O(4) calcined between 800 degrees C and 1100 degrees C. The catalyst calcined at 900 degrees C exhibited the highest activity for the reforming reaction. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
  • Toshiaki Matsui; Hiroki Muroyama; Ryuji Kikuchi; Koichi Eguchi
    JOURNAL OF THE JAPAN PETROLEUM INSTITUTE JAPAN PETROLEUM INST 53 (1) 1 - 11 1346-8804 2010/01 [Refereed]
     
    Fuel cells are attractive energy conversion devices with high efficiencies and low emissions, and many studies have been conducted so far. Among them, fuel cells operating at 200-600 degrees C are promising technologies which combine the many advantages of high- and low-temperature fuel cells. However, they have not been developed due to the lack of good ionic-conductors with high thermal stability at intermediate temperatures. Recently, we have developed new proton-conductive electrolytes consisting of solid acid and pyrophosphate, and evaluated their electrochemical, structural and thermal properties at intermediate temperatures. For the composite based on CsH2PO4/SiP2O7, the interfacial chemical reaction between CsH2PO4 and SiP2O7 during heat-treatment gave rise to the formation of a new phase of CsH5(PO4)(2). The temperature dependence of conductivity for this composite was different from that for pure CsH2PO4, and the maximum conductivity achieved was 44 mS.cm(-1) at 266 degrees C. Using potassium and rubidium salts, MH2PO4 (M = K, Rb), as the solid acids for the composite electrolytes, analogous phenomena were confirmed despite the alkaline metal. Operation of a fuel cell employing CsH2PO4/SiP2O7-based composite electrolyte (thickness: ca. 1.3 mm) was demonstrated at 200 degrees C and generated electricity up to 220 mA.cm(-1) at 0.2 V. CsH5(PO4)(2) Composites with SiP2O7 and SiO2 were fabricated, and the composite effects were investigated at intermediate temperatures based on conductivity measurement, thermal analysis, and wettability evaluation. The melting and dehydration processes of CsH5(PO4)(2) in composites were different depending on the matrix species. The composite with SiP2O7 matrix showed the highest conductivity of all composites. The conductivity of the composites appears to correlate with the wettability between the components as examined by contact angle measurement. These findings should be attributed to the differences in the interfacial interactions between CsH5(PO4)(2) and the matrix.
  • Bin Liu; Hiroki Muroyama; Toshiaki Matsui; Kazuo Tomida; Tatsuo Kabata; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 157 (12) B1858 - B1864 0013-4651 2010 [Refereed]
     
    The individual electrode processes from anode and cathode have been clearly identified from practical impedance spectra with high-frequency inductive impedances for the Mitsubishi segmented-in-series tubular solid oxide fuel cell by combining the distribution of relaxation time analysis and the complex nonlinear least square fitting. Anodic gas diffusion process and charge transfer reaction near anodic triple phase boundary appear at similar to 0.3 and 1000 Hz, respectively. Cathodic oxygen reduction processes appear at similar to 10 Hz at 700 degrees C, whereas the impedance arc over 10 000 Hz is dominated by the cathodic oxygen ion transfer through the (La,Sr)MnO(3) (LSM)/Y(2)O(3)-stabilized ZrO(2) (YSZ) interface and YSZ of the composite. The results also exhibit an excellent agreement with those from the analysis of difference in impedance spectra. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3494214] All rights reserved.
  • Hirofumi Sumi; Yi-Hsuan Lee; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 157 (8) B1118 - B1125 0013-4651 2010 [Refereed]
     
    The performance and durability for Ni-YSZ and Ni-ScSZ anodes of solid oxide fuel cells were compared between internal steam and CO2 reforming of methane in the carbon deposition conditions. With a supply of H-2-CO-CO2 mixtures to the anode, the polarization resistance increased with a rise in CO concentrations at 1023 K because of the difficulty of electrochemical oxidation of carbon monoxide. Carbon was produced by the disproportionation of carbon monoxide in CO-CO2 mixture (CO:CO2 = 95:5) at 1023 K, which led to low performance and much degradation of the cells. In comparison between steam and CO2 reforming of methane, the performance and durability in a gaseous mixture at H2O/CH4 = 0.5 were better than that at CO2/CH4 = 0.5. The durability of the Ni-ScSZ anode was superior to that of the Ni-YSZ anode at 1273 K. Amorphous carbon covered the Ni-YSZ anode surface after power generation, which deactivated the nickel catalyst and inhibited gas diffusion. However, crystalline graphite with a rod morphology was grown on the Ni-ScSZ anode, which affected the catalytic activity less than amorphous carbon. The crystallinity and morphology of deposited carbon are important in determining the performance and durability of the cells at low H2O/CH4 and CO2/CH4 ratios. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3435320] All rights reserved.
  • Toshiaki Matsui; Yuichi Mikami; Hiroki Muroyama; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 157 (12) B1790 - B1794 0013-4651 2010 [Refereed]
     
    The microstructural change at the interface between (La,Sr)MnO3+delta (LSM) cathode and yttria-stabilized zirconia (YSZ) electrolyte upon discharge was quantitatively evaluated by focused ion beam-scanning electron microscopy technique so as to elucidate the activation process of cathode. The performance of the single cell was enhanced soon after the current passage up to similar to 5 h at 1000 degrees C due to the reduction in impedance at the cathode. In response to this electrochemical behavior, the interfacial structure between LSM and YSZ changed depending on the time duration of discharge; the roughness of YSZ surface both in and out of contact with LSM particles increased with time. Furthermore, the closed pores were formed at the well-adhered LSM/YSZ interface, which were unexposed to the cathode gas. The triple phase boundary-length (TPB-length) of the as-prepared sample was evaluated to be 1.26 mu m mu m(-2), whereas after the discharge at 200 mA cm(-2) for 300 min the total TPB-length was increased to 1.72 mu m mu m(-2). In the latter case, however, the active TPB-length was 1.35 mu m mu m(-2), which was only 7% larger than that of as-prepared one. Then the contribution rate of TPB-length to the performance enhancement of cathode was discussed. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3499357] All rights reserved.
  • Kota Murakami; Toshiaki Matsui; Ryuji Kikuchi; Hiroki Muroyama; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 157 (6) B880 - B884 0013-4651 2010 [Refereed]
     
    Solid oxide fuel cells with the conventional configuration of Ni-ytttria-stabilized zirconia (Ni-YSZ)vertical bar YSZ vertical bar(La, Sr)MnO(3 +/-delta) (LSM) have been reported to exhibit performance improvement in the initial discharge operation, which is attributed to the activation of LSM. In this study, the time course of potential for LSM was monitored under cathodic polarization to precisely analyze this enhancement. A characteristic phenomenon of potential oscillation was found during the constant current loading. Regardless of electrolyte materials and LSM composition, the potential was oscillated with a frequency of ca. 0.67 Hz soon after current passing. The potential oscillation appeared under the following conditions: (i) at low partial pressure of oxygen, (ii) at high cathodic current loading, and (iii) for an LSM electrode with low porosity. The amplitude of oscillation reduced with the elapsed time of cathodic polarization and then disappeared. The microscopic observation revealed that the structural change in the LSM electrode after cathodic polarization is responsible for the disappearance of potential oscillation. These findings suggest that the potential oscillation originates in the mass-transfer limitation, oxygen diffusion, and densified electrode. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3374407] All rights reserved.
  • Tsugio Ikebe; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 157 (6) B970 - B974 0013-4651 2010 [Refereed]
     
    Composite anodes consisting of Ni, Y-doped SrTiO3 (YST), and Y2O3-stabilized ZrO2 (YSZ) were fabricated; the Ni species was impregnated to YST-YSZ powder and then the resulting powder was fired onto the electrolyte at 1200 degrees C for 5 h in air. The conductive oxide of YST sintered in air showed a conductivity of 7.2 S cm(-1) after reduction in humidified hydrogen (1.2% H2O-H-2) for 100 h and behaved as a metallic conductor with respect to the temperature dependence of conductivity. The influence of the nickel content in the anode of Ni/YST-YSZ on the electrochemical property was studied under 0.6% H2O-H-2 atmosphere. The cermet of 10 wt % Ni/YST-YSZ showed the minimal polarization resistance among the catalysts with the nickel content in the range of 5-50 wt %. An electrolyte-supported cell employing this anode exhibited a power density of 0.34 W cm(-2) at 0.5 V at 1000 degrees C despite the extremely low content of Ni. Stable power generation was attained at a constant terminal voltage of 0.7 V in humidified hydrogen (0.6% H2O-H-2) for 21 h. Even after five consecutive redox cycles, the cell performance remained unchanged mainly due to the stable porous framework of YST-YSZ with a low Ni content. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3421792] All rights reserved.
  • Jun Yang; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 157 (3) B449 - B454 0013-4651 2010 [Refereed]
     
    A numerical model was proposed to simulate the electrochemical behavior of strontium-doped lanthanum manganite (LSM) under cathodic polarization by assuming the transient variation of oxygen concentration and the defect chemistry of LSM. From the simulation, it was revealed that the dynamic response of cathodic potential proceeded in two steps with different time domains. A drastic change in cathodic potential occurred soon after the current loading. In addition, the oxygen nonstoichiometry in LSM changed simultaneously, resulting in an enhancement in the electrochemical performance of the LSM cathode. The simulated phenomenon was validated by experimental results.
  • Hirofumi Sumi; Ryo Kishida; Jin-Young Kim; Hiroki Muroyama; Toshiaki Matsui; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 157 (12) B1747 - B1752 0013-4651 2010 [Refereed]
     
    Reoxidation of a nickel catalyst in anode generally leads to deterioration of cell performance for solid oxide fuel cells (SOFCs). The aim of this study is to clarify the correlation between microstructural and electrochemical characteristics during reduction-oxidation (redox) cycles of a Ni-yttria-stabilized zirconia (YSZ) anode. Cell performance was deteriorated after the first redox cycle because of the increase in the polarization resistance of the anode. Ohmic loss of the anode also increased after the thermal cycle in pure oxygen. For the Ni-YSZ anode after the redox cycles, the increase in surface area of the nickel phase was observed by focused ion beam-scanning electron microscopy, which indicated that the nickel particles became finer and more complicated in shape. The lengths of triple phase boundary (TPB) derived from three-dimensional-reconstructed images of the Ni-YSZ anodes were 2.39 and 2.11 mu m/mu m(3) after the first and fourth redox cycles, respectively, which were smaller than that before the redox treatment with a TPB length of 2.49 mu m/mu m(3). Large cracks were produced on the Ni-YSZ anode after the thermal cycle, which led to the increase in the ohmic loss of the anode. The increase in the polarization resistance was caused by the decrease in the length of TPB during the early stage of the redox cycles for the Ni-YSZ anode. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3491345] All rights reserved.
  • Pramote Puengjinda; Hiroki Muroyama; Toshiaki Matsui; Mitsunobu Kawano; Toru Inagaki; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 157 (11) B1673 - B1678 0013-4651 2010 [Refereed]
     
    Nickel oxide and scandia-stabilized zirconia (NiO-ScSZ) composite powders were prepared through two different processes: mechanical powder mixing (PM) and coprecipitation (CP). The effect of calcination temperatures on the crystalline structure of as-prepared powders and the carbon deposition behavior over the cermets were investigated. Due to the finer precipitates obtained from the CP method, the solid-state reaction between NiO and ScSZ in the as-prepared powder was promoted even at low calcination temperatures, resulting in the stabilization of the cubic phase of ScSZ. The temperature-programmed reduction indicated the strong interaction between NiO and ScSZ in the composite subjected to high temperature treatment regardless of the preparation methods, CP and PM. Phase identification was also conducted for the composites after reduction treatment at high temperatures of 1000 and 1400 degrees C. After reducing at 1400 degrees C, a part of ScSZ in the composite from the CP method transformed from the cubic to the rhombohedral phase, whereas the cubic phase was stable for the composite from the PM method. In addition, the carbon deposition over the sample from the CP method was promoted due to the large surface area of Ni. However, the electrochemical performance of single cells was independent of the cermet anodes from different methods with a supply of both H(2) and CH(4) fuels. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3484094] All rights reserved.
  • Toshiaki Matsui; Ryo Kishida; Jin-Young Kim; Hiroki Muroyama; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 157 (5) B776 - B781 0013-4651 2010 [Refereed]
     
    Nickel-yttria-stabilized zirconia (Ni-YSZ) cermet is a conventional anode for use in solid oxide fuel cells, and its composition and microstructure are carefully controlled to achieve high performance and long-term stability. In this study, the performance stability of the electrolyte-supported cell (Ni-YSZ vertical bar YSZ vertical bar LSM) was examined at 1000 degrees C by feeding humidified fuel, x% H(2)O-(100-x)% H(2). The influence of the cermet composition on degradation was also studied. The degradation behavior was significantly dependent on the fuel humidity and cermet composition. In the case of Ni-YSZ with a volume ratio of 50 to 50, peculiar phenomena were observed. When the fuel of 30% H(2)O-70% H(2) was supplied to the anode at the terminal voltage of 0.7 V, the current density decreased gradually soon after the discharge of up to 69 h, followed by a sudden drop in the current density. After the subsequent open-circuit holding, the performance was partially recovered in the discharge operation. This behavior of the degradation-recovery was reversibly repeated upon the discharge-open-circuit holding operation. Under the 40% H(2)O-60% H(2) atmosphere, an irreversible performance deterioration was observed accompanied with a drastic decrease in the volume-specific triple-phase boundary (TPB) length. The TPB length of the degraded anode was evaluated to be 1.66-1.68 mu m mu m(-3) by a focused ion beam scanning electron microscopy technique, whereas that of the as-prepared anode was 2.49 mu m mu m(-3).
  • Saburi Chikara; Muroyama Hiroki; Matsui Toshiaki; Eguchi Koichi
    Proceeding of Annual/Fall Meetings of the Japan Petroleum Institute The Japan Petroleum Institute 2010 156 - 156 2010 
    アンモニア分解反応による水素製造では、固体高分子型燃料電池の触媒毒となる一酸化炭素が生成されないため、新たな燃料としてアンモニアが近年注目されている。含浸法により調製した担持Ni触媒のアンモニア分解活性を調査したところ、比表面積と塩基性の高い金属酸化物を担体に用いた際に、高いアンモニア分解活性を示した。また、Laを添加することで活性の向上が観察されたことからも、担体の塩基性は触媒活性に影響を与えることが示唆された。
  • Shimoda Naohiro; Muroyama Hiroki; Matsui Toshiaki; Kikuchi Ryuji; Eguchi Koichi
    Proceeding of Annual/Fall Meetings of the Japan Petroleum Institute The Japan Petroleum Institute 2010 155 - 155 2010 
    炭化水素系燃料からの固体高分子形燃料電池への水素供給プロセスにおいてCO低減プロセスである水性ガスシフト反応は非常に重要な反応である。これまで我々は、Cu-Fe、Cu-Mnスピネル触媒が高いシフト活性を示すこと、アルミナと混合したCu-Feスピネル触媒を高温で空気中焼成することで触媒活性が向上することを報告している。本研究では、Cu-Feスピネル酸化物への第三成分添加効果ならびに熱処理効果について検討し、活性ならびにDSS条件下での耐久性の優れたシフト触媒の開発を試みた。
  • Muroyama Hiroki; Tsumura Yoshihiro; Matsui Toshiaki; Eguchi Koichi
    Proceeding of Annual/Fall Meetings of the Japan Petroleum Institute The Japan Petroleum Institute 2010 147 - 147 2010 
    これまでにエタノール水蒸気改質用触媒としてNi系スピネル触媒の検討を行い、BサイトにAlを用いたNiAl2O4触媒が高い耐久性を有する事を見出している。本発表ではNiAl2O4スピネル触媒にMg, Ca, Cu, La, Ceなどの第三成分を添加し、エタノール水蒸気改質における触媒性能を比較検討した。耐久試験においてCe成分の添加が、最も炭素析出の抑制に最も有効であった。添加したCe成分の大部分がスピネル構造内へ固溶せず、表面に分散することによってNiと相互作用したためだと考えられる。
  • Hiroki Muroyama; Toshiaki Matsui; Ryuji Kikuchi; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 156 (12) B1389 - B1393 0013-4651 2009 [Refereed]
     
    Electrochemical hydrogen pumping was investigated with a cell consisting of a CsH2PO4/SiP2O7 composite electrolyte at 200 degrees C. When humidified hydrogen was fed to the anode, the evolution rate of hydrogen at the cathode obeyed Faraday's law up to a current density of 1000 mA cm(-2). In this system, the water-gas shift reaction (WGSR) was promoted electrochemically at the anode during polarization with a feed of humidified carbon monoxide. The overpotential was much higher in humidified carbon monoxide than in humidified hydrogen. With the reactor for the catalytic WGSR located upstream of the anode gas, the overpotential significantly depended on the temperature and space velocity of the reactor. The bilayered electrode composed of a Cu spinel oxide and a Pt catalyst exhibited a low overpotential at low current densities due to the formation of hydrogen over the Cu spinel catalyst. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3232302] All rights reserved.
  • Hano Shigeki; Muroyama Hiroki; Matsui Toshiaki; Eguchi Koichi
    Proceeding of Annual/Fall Meetings of the Japan Petroleum Institute The Japan Petroleum Institute 2009 120 - 120 2009 
    ディーゼル車は熱効率が高く、地球温暖化抑制に有効である。しかし、排ガス中の窒素酸化物(NOx)やパティキュレートマター(PM)は環境問題や健康被害を引き起こす。本研究はPMを低温から燃焼させる触媒の開発及びキャラクタリゼーションを目的とした。カーボン燃焼の良好な触媒であるCeO2へのCuの添加が有効であることを明らかにした。また、反応前後の触媒のキャラクタリゼーションにより、高い触媒活性の要因について調べた。
  • Hiroki Muroyama; Toshiaki Matsui; Ryuji Kikuchi; Koichi Eguchi
    JOURNAL OF PHYSICAL CHEMISTRY C AMER CHEMICAL SOC 112 (39) 15532 - 15536 1932-7447 2008/10 [Refereed]
     
    Proton-conductive CsH5(PO4)(2) composites with various pyrophosphate and silica matrices (e.g., SiP2O7, TiP2O7, and SiO2, etc.) were synthesized, and the composite effects were investigated at intermediate temperatures based on the electrochemical measurement, thermal analysis, and contact angle measurement. The melting and dehydration processes of CsH5(PO4)(2) in composites depended on the matrix species. The composite with SiP2O7 matrix exhibited the highest conductivity among all composites in the temperature range investigated. This result indicates that the conductivity of composites was affected by the pyrophosphate unit and the metal species in the crystalline structure of the pyrophosphate. The conductivity of composites appears to correlate with the compatibility between the components examined by contact angle measurement. These differences in electrochemical and physical properties should be attributed to the interfacial interaction between CsH5(PO4)(2) and the matrix.
  • Hiroki Muroyama; Toshiaki Matsui; Ryuji Kikuchi; Koichi Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 155 (9) B958 - B962 0013-4651 2008 [Refereed]
     
    The influence of thermal cycle on the conduction behavior of CsH(5)(PO(4))(2) was investigated. The temperature dependence of the conductivity was examined in two consecutive heating processes with different sequential operations under a dry atmosphere. In the second heating, the conduction behavior was significantly affected by the thermal history. This phenomenon could be explained consistently by the results from X-ray diffraction (XRD) and thermal analyses of CsH(5)(PO(4))(2). When the sample was heated at the melting point of CsH(5)(PO(4))(2) and then rapidly cooled, the diffraction lines of CsH(5)(PO(4))(2) and the condensed phosphates were observed in the XRD pattern. The differential scanning calorimetry and thermogravimetry-differential thermal analysis were conducted in the heating-cooling process under various experimental conditions. It was noted that the crystallization temperature of CsH(5)(PO(4))(2) was strongly dependent on the extent of dehydration. (C) 2008 The Electrochemical Society.
  • Hiroki Muroyama; Kenji Kudo; Toshiaki Matsui; Ryuji Kikuchi; Koichi Eguchi
    SOLID STATE IONICS ELSEVIER SCIENCE BV 178 (27-28) 1512 - 1516 0167-2738 2007/11 
    The proton-conductive MH2PO4/SiP2O7-based composites (M = K, Rb, Cs) were synthesized, and their structure and proton conductivity were investigated at intermediate temperatures. For all MH2PO4/SiP2O7-based composites, the chemical reaction between MH2PO4 and SiP2O7 has proceeded during the heat treatment at 220 degrees C for 1 h, resulting in the formation of MH5(PO4)(2). The heat-treated composites exhibited higher conductivity than MH2PO4 due to the new conduction phase of MH5(PO4)(2), regardless of the alkaline metal species. The electrolytes of MH5(PO4)(2)/SiP2O7 composites (M=K, Rb, Cs) were also fabricated so as to compare the conductivity with almost the same charge carrier concentration. It is noted that their conductivity is independent of the alkaline metal species. (C) 2007 Elsevier B.V. All rights reserved.
  • Hiroki Muroyama; Kenji Kudo; Toshiaki Matsui; Ryuji Kikuchi; Koichi Eguchi
    SOLID STATE IONICS ELSEVIER SCIENCE BV 178 (27-28) 1512 - 1516 0167-2738 2007/11 [Refereed]
     
    The proton-conductive MH2PO4/SiP2O7-based composites (M = K, Rb, Cs) were synthesized, and their structure and proton conductivity were investigated at intermediate temperatures. For all MH2PO4/SiP2O7-based composites, the chemical reaction between MH2PO4 and SiP2O7 has proceeded during the heat treatment at 220 degrees C for 1 h, resulting in the formation of MH5(PO4)(2). The heat-treated composites exhibited higher conductivity than MH2PO4 due to the new conduction phase of MH5(PO4)(2), regardless of the alkaline metal species. The electrolytes of MH5(PO4)(2)/SiP2O7 composites (M=K, Rb, Cs) were also fabricated so as to compare the conductivity with almost the same charge carrier concentration. It is noted that their conductivity is independent of the alkaline metal species. (C) 2007 Elsevier B.V. All rights reserved.
  • 菊地 隆司; 室山 広樹; 松井 敏明; 江口 浩一
    化学工学会 研究発表講演要旨集 公益社団法人 化学工学会 2007 489 - 489 2007
  • H Muroyama; T Matsui; R Kikuchi; K Eguchi
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY ELECTROCHEMICAL SOC INC 153 (6) A1077 - A1080 0013-4651 2006 
    Proton-conductive electrolytes of CsHSO4 composite were synthesized, and the influence of the matrix on their structure and proton conductivity was investigated at intermediate temperatures. As the supporting matrix, heat-treated TiOSO4 hydrate was used. When the matrix heat-treated at 300 degrees C was employed, formation of new phases was observed in the composite during the heat-treatment at 200 degrees C due to the interfacial chemical reaction between CsHSO4 and the matrix. The resulting composite with a suitable composition exhibited higher conductivity than pure CsHSO4 with linear temperature dependence. This anomalous behavior is quite different from that of the pure CsHSO4, which exhibits the "conductivity jump" at ca. 140 degrees C because of the phase transition. (C) 2006 The Electrochemical Society.
  • H Muroyama; T Matsui; R Kikuchi; K Eguchi
    SOLID STATE IONICS ELSEVIER SCIENCE BV 176 (31-34) 2467 - 2470 0167-2738 2005/10 
    A proton-conductive electrolyte of (NH4)(2)SO4/TiO2 composite was synthesized, and its proton conductivity and chemical stability at intermediate temperatures were investigated. In the composite, partial decomposition of (NH4)(2)SO4 Occurred at around 250 degrees C, and the resulting materials showed high proton conductivity. The highest conductivity achieved was 15 and 35 mS cm(-1) under dry and wet atmospheres at 250 degrees C, respectively. The temperature dependences of the conductivity were almost reversible in any conditions investigated, and the composite was relatively stable. Under a hydrogen atmosphere, however, the reduction of (NH4)(2)SO4 phase occurred and the formation of H2S was observed. (c) 2005 Elsevier B.V All rights reserved.

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