論文 - 奥山 勇治
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九州支部
奥山 勇治
電気化学 93 ( 1 ) 43 - 43 2025年3月
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Li K., Murakami T., Nagata Y., Mikami Y., Yamauchi K., Kuroha T., Okuyama Y., Mizutani Y., Mori M., Araki T.
Applied Energy 381 2025年3月
記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Applied Energy
Protonic ceramic fuel cells (PCFCs) have garnered significant interest due to their theoretically high fuel utilization and excellent energy efficiency at intermediate temperatures (400–600 °C). While the performance of PCFCs has improved dramatically in recent years, the system electrical efficiency is often lower than the corresponding cell energy efficiency due to energy loss in a PCFC system. This study focuses on achieving >70% system electrical efficiency (lower heating value-LHV) in a H2-powered PCFC system. The required property values such as the conductivities and diffusion coefficients of proton and hole in the electrolyte, as well as the exchange current densities and reaction resistances at electrodes are revealed through a validated numerical model compared to the experimental results of a high-performance PCFC. It offers significant empirical insights for advancing high-performance PCFCs capable of achieving >70% system electrical efficiency.
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Nakamura K., Mori M., Okuyama Y.
Solid State Ionics 417 2024年12月
担当区分：最終著者 記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Solid State Ionics
In protonic ceramic fuel cells using Ba(Zr,M)O3-δ (M: trivalent dopant elements) as the electrolyte, the precipitation of BaM2NiO5 due to Ni diffusion from the co-sintered NiO-based electrode causes degradation of protonic ceramic fuel cells. However, BaM2NiO5 itself has been little studied, and even possible stable crystal structures and compositions have not been fully characterized. In this study, we investigated the dynamic and energetic stability of BaM2NiO5 for various trivalent M elements by using first-principles calculations. First, dynamically stable crystal structures were determined for all compositions from phonon dispersion analysis. The formation energies showed negative values in the case of M = lanthanide elements, B, Ga, Tl and Y. The contribution of vibrational entropy to the formation energy of BaM2NiO5 was insignificant, and the internal energy was dominant. The chemical bonding analysis revealed that in BaM2NiO5, the covalent nature of the M-O bond and the ionic nature of the Ba[sbnd]O bond are dominant in the stability of the crystal structure. Precipitation of BaM2NiO5 in Ba(Zr,M)O3-δ was suggested to be dominated by a specific threshold value of formation energy. The validity of that assumption was discussed in terms of the relationship between the factors involved in precipitation and the ionic radius of M element. The formation energy of BaM2NiO5 in M = lanthanide elements and Y showed a downward convex tendency with M = Pm as the minimum value. The reason for this was discussed in terms of the characteristics of the crystal structure of BaM2NiO5, suggesting that the tensile strain in the M-O bonds and the compressive strain in the Ni[sbnd]O and Ba[sbnd]O bonds relax with the ionic radius of the M element.
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Mikami Y., Goto T., Asano H., Kasuga K., Yamauchi K., Kuroha T., Okuyama Y.
Journal of Power Sources 613 2024年9月
担当区分：責任著者 記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Journal of Power Sources
To investigate the effect of Co in cathodes on the durability of protonic ceramic fuel cells (PCFC) using a BaZr0.8Yb0.2O3-δ electrolyte, new cathode materials, specifically Ba(Zr0.875-xYb0.125Cox)O3-δ (x = 0.125, 0.25, 0.375, 0.625, and 0.75), were employed. We examined several material properties of the cathodes and evaluated cell performance, showing that Ba(Zr0.375Yb0.125Co0.5)O3-δ exhibits high performance and a matching thermal expansion coefficient with the electrolyte. During the 1000 h power generation test, the segregation of Co and anode-derived Ni in the cathode progressed. Segregation of Co and Ni was also observed at the grain boundaries in the electrolyte, indicating a decrease in the mechanical strength of the electrolyte caused by the diffusion of Co from the cathode and Ni from the anode. From these results, it became clear that the suppression of Co diffusion into the electrolyte is an important issue for the practical application of PCFCs with a BaZr0.8Yb0.2O3-δ electrolyte and Co-containing cathodes.
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欠陥構造型プロトン伝導体の材料開発とセンサ，燃料電池への応用展開 招待あり 査読あり
奥山 勇治
応用物理 93 ( 3 ) 149 - 155 2024年3月
担当区分：筆頭著者, 責任著者 記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：公益社団法人 応用物理学会
欠陥構造型プロトン伝導体は酸化物などに溶け込んだ水素イオン（プロトン）が結晶内を移動するセラミックスであり，水素センサや燃料電池などの応用が期待されている．本稿では欠陥構造型プロトン伝導体の材料特性や分極特性について解説するとともに水素センサや燃料電池への応用について紹介する．
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Mikami Y., Sekitani Y., Yamauchi K., Kuroha T., Okuyama Y.
ACS Applied Energy Materials 7 ( 3 ) 1136 - 1148 2024年2月
担当区分：責任著者 掲載種別：研究論文（学術雑誌） 出版者・発行元：ACS Applied Energy Materials
Protonic ceramic fuel cells (PCFCs) use a proton-conducting oxide as the electrolyte, leading to a high power density and energy conversion efficiency at intermediate temperatures. Ytterbium-doped barium zirconate has a high proton conductivity and chemical stability against carbon dioxide, making it an attractive electrolyte material for PCFCs. The durability of fuel cells is a critical issue for realizing practical applications of PCFCs, and dissolution of the transition metals from the electrodes into the electrolyte is expected to decrease the durability. In this study, we investigated the effect of dissolution of transition metals (Ni, Co, Fe, and Mn) into BaZr0.8Yb0.2O3−δ on the proton transport properties and on the cell properties. The proton conductivity and proton transport number decreased with an increasing concentration of transition-metal elements, and the Co was found to have a particularly large effect on the material properties of BaZr0.8Yb0.2O3−δ. As for cell properties, anode-supported PCFCs with different cathode firing temperatures were evaluated. The OCV and the maximum power density were low in the cell with high concentrations of Co and Fe in the BaZr0.8Yb0.2O3−δ electrolyte, suggesting the decrease of proton transport number due to dissolution of Co and Fe. Furthermore, based on the measured proton transport properties, the effects of transition-metal dissolution on the power density and energy efficiency of PCFCs were estimated, and the trends were consistent with the results of the cell test. These results indicate that the dissolution of transition metals, especially Co, has a significant effect on the performance and durability of PCFCs using the BaZr0.8Yb0.2O3−δ electrolyte.
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Shimada H., Mikami Y., Yamauchi K., Kuroha T., Uchi T., Nakamura K., Kobayashi S., Matsuda R.M., Okuyama Y., Mizutani Y., Mori M.
Ceramics International 50 ( 2 ) 3895 - 3901 2024年1月
掲載種別：研究論文（学術雑誌） 出版者・発行元：Ceramics International
Long-term durability of anode-supported protonic ceramic fuel cells (PCFCs) with BaZr0.8Yb0.2O3–δ (BZYb20) electrolyte is evaluated at 600 °C over 1000 h operation. To prevent a decrease in cell performance caused by Co diffusion from a La0.6Ba0.4CoO3−δ (LBC)-based cathode into the BZYb20 electrolyte, a porous buffer interlayer of BaZr0.1Ce0.7Y0.1Yb0.1O3–δ (BZCYYb) is placed at the interface between the cathode and electrolyte. By introducing the BZCYYb buffer interlayer, although the power density of the PCFC deteriorates slightly due to adding the ohmic resistance of the interlayer, the long-term durability for 1000 h operation remarkably improves, e.g., degradation rate decreases from 4.8 % kh−1 to 0.9 % kh−1. Moreover, the PCFC with the BZCYYb buffer interlayer achieves a stable operation with a degradation rate of 0.8 % kh−1 for 3000 h. Electron probe microanalysis confirms that the BZCYYb buffer interlayer prevents diffusion of Co from the LBC-based cathode into the BZYb20 electrolyte even after 3000 h operation.
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Development of defect-structure-type proton conductors and their application to sensors and fuel cells 査読あり
Okuyama Yuji
JSAP Review 2024 ( 0 ) n/a 2024年
担当区分：筆頭著者, 責任著者 記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：公益社団法人 応用物理学会
Defect-structure-type proton conductors are ceramics in which hydrogen ions (protons) dissolve in oxides and diffuse in the crystal. The material has attracted considerable attention as an electrolyte for hydrogen sensors and fuel cells. In this review, the material properties and polarization characteristics of defect-structure-type proton conductors will be described, and the application to hydrogen sensors and fuel cells will be introduced.
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Okuyama Y., Kasuga K., Shimomura M., Mikami Y., Yamauchi K., Kuroha T., Sumi H.
Journal of Power Sources 586 2023年12月
担当区分：筆頭著者, 責任著者 掲載種別：研究論文（学術雑誌） 出版者・発行元：Journal of Power Sources
This study elucidates the elementary reactions occurring within the electrode impedance of proton-conducting ceramic fuel cells, using the distribution of relaxation times and the relaxation process involved in proton/deuteron substitution. We identified impedance components attributable to the steam production reaction of protons crossing the electrolyte on the triple phase boundary by observing the change in these components when the gas on the anode was switched from light hydrogen to heavy hydrogen during the power generation and shutdown phases of the fuel cell. It was noted that the steam formation reaction presents the highest resistance in proton-conducting ceramic fuel cells, implying the need for a catalyst to enhance this reaction within the electrode.
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Li K., Shimada H., Mizutani Y., Okuyama Y., Araki T.
Energy Conversion and Management 296 2023年11月
掲載種別：研究論文（学術雑誌） 出版者・発行元：Energy Conversion and Management
The fabrication of high-performance protonic ceramic fuel cells (PCFCs) and the accurate evaluation of their energy efficiency remain challenging because of electron–hole leakage. By combining an established numerical method with experiments, we developed high-performance chemically stable Ce-free PCFCs with a BaZr0.8Yb0.2O3−δ electrolyte that perform comparably to first-tier PCFCs, which typically employ Ce-containing electrolytes. To the best of our knowledge, this study is the first to numerically reproduce experimental performance over a wide range of temperatures, electrolyte thicknesses, and current densities. Importantly, our study can accurately predict the ultra-elevated performance of cells under different conditions without the need for conducting additional experiments (e.g., > 3.0 W·cm−2 power density or > 70% lower heating value energy efficiency)
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Investigation of degradation mechanisms by overpotential evaluation for protonic ceramic fuel cells 査読あり
Sumi H., Shimada H., Watanabe K., Yamaguchi Y., Nomura K., Mizutani Y., Matsuda R.M., Mori M., Yashiro K., Araki T., Okuyama Y.
Journal of Power Sources 582 233528 - 233528 2023年10月
担当区分：最終著者 掲載種別：研究論文（学術雑誌） 出版者・発行元：Journal of Power Sources
Overpotential is frequently estimated from electrochemical impedance spectroscopy to evaluate the electrode performance of fuel cells. However, the overpotential of protonic ceramic fuel cells (PCFCs) is possibly underestimated because of hole conductivity in the electrolyte. In the present work, the overpotentials of PCFCs with BaZr0.8Yb0.2O3-δ (BZYb) and BaCe0.4Zr0.4Y0.1Yb0.1O3-δ (BCZYYb) electrolytes are accurately estimated using distribution of relaxation times analysis and an equivalent circuit model with considering a polarization resistance during durability tests. The activation overpotential in an open circuit state is not zero because leakage current flows in the PCFC electrolytes. The activation overpotential of the cell with the BCZYYb electrolyte is higher than that with the BZYb electrolyte because of the interdiffusion of nickel from the Ni-based anode to the cathode and cobalt from the La0.6Sr0.4Co0.2Fe0.8O3-δ cathode to the anode. The compressive internal stress in the electrolyte, determined by the cos α method using X-ray, decreases after discharging. Intergranular cracks formed at the grain boundary through nickel deposition, causing an increase in the ohmic overpotential with time for the both cells with BZYb and BCZYYb electrolytes. The interdiffusion of transition metals from the anode/cathode to the electrolyte in the manufacturing process should be prevented to improve the PCFC durability.
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Mechanisms of photoluminescence in cerium oxide pellets with different types of defects 査読あり
Nagai M., Furutani Y., Takehara H., Ashida M., Okuyama Y., Kani Y.
Journal of Luminescence 260 119859 - 119859 2023年8月
掲載種別：研究論文（学術雑誌） 出版者・発行元：Journal of Luminescence
We systematically investigated the photoluminescence (PL) spectra of sintered pellets of cerium oxide, CeO2. The crystal defects were controlled by commonly used techniques in the field of solid-state ionics. We observed the transitions between the O-2p and Ce-4f energy levels in intrinsic CeO2. By doping with trivalent lanthanide ions, which results in oxygen vacancies by charge compensation, a new transition band appears. It involves electronic levels that are similar to those of Ce2O3 clusters, rather than to those of oxygen vacancies. In reduced cerium oxide CeO2-δ, where the number of Ce3+ ions is larger, electrons partially fill the energy levels of defects similar to Ce2O3 clusters, and thus an optical response derived from the Ce-4f–Ce-5d transition appears.
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Phonons Damped by Proton Doping in Barium Zirconate 査読あり
Takehara H., Nagai M., Ashida M., Okuyama Y., Kani Y.
Journal of Physical Chemistry C 127 ( 23 ) 10913 - 10921 2023年6月
掲載種別：研究論文（学術雑誌） 出版者・発行元：Journal of Physical Chemistry C
We show that protons strongly damp some phonons in hydrated barium zirconate. To show this, we performed terahertz spectroscopy and infrared reflection spectroscopy measurements on anhydrous and hydrous acceptor-doped barium zirconate sintered pellets. Last and Slater phonon modes are strongly damped with proton doping, whereas phonons are not damped with trivalent dopants. This is due to picosecond motion of protons, such as hydroxyl rotation and attempt motion of proton hopping, coupled with the phonon. This coupling strongly depends on the dopant, suggesting that the proton motion is strongly influenced by the dopant. The results of Raman spectroscopy support this explanation. We consider that such a dynamical long-range proton-lattice correlation will enable new approaches to improve proton conduction in solid-state ionics.
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Sumi H., Shimada H., Watanabe K., Yamaguchi Y., Nomura K., Mizutani Y., Okuyama Y.
ACS Applied Energy Materials 6 ( 3 ) 1853 - 1861 2023年2月
担当区分：最終著者 記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：ACS Applied Energy Materials
Gd-doped ceria (GDC) and Yb-doped barium zirconate (BZYb) are used as electrolytes for intermediate-temperature solid oxide cells (SOCs) and protonic ceramic cells (PCCs), respectively. However, the electron conductivity of GDC under low oxygen partial pressure and the hole conductivity of BZYb under high oxygen partial pressure result in current leakage, which decreases the open circuit voltage and conversion efficiency. In the present work, the effect of polarization resistances on current leakage is investigated through the distribution of relaxation times (DRT) analysis of electrochemical impedance spectra (EIS) for a reversible SOC with GDC electrolyte and PCC with BZYb electrolyte. The polarization resistances increase and decrease with increasing current density in the electrolysis and fuel cell modes, respectively, for the conventional SOC with scandia-stabilized zirconia (ScSZ). In contrast, the polarization resistances exhibit the opposite tendency for the SOC with the GDC electrolyte owing to current leakage. For the PCC with the BZYb electrolyte, the polarization resistances according to air electrode reaction processes indicate maximum values in the fuel cell mode. The effect of hole conductivity on the polarization resistance suggests elementary rate-determining processes of the PCC electrodes.
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Nomura K., Shimada H., Yamaguchi Y., Sumi H., Mizutani Y., Okuyama Y., Shin W.
Ceramics International 49 ( 3 ) 5058 - 5065 2023年2月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Ceramics International
Space groups of Ba(Ce0.8-xZrx)Y0.2O3 (0 ≤ x ≤ 0.8) perovskite-type protonic conductors of wide temperature range from 25 to 900 °C in dry and wet atmospheres are predicted using various machine learning (ML) models. For the prediction, the data including the chemical formulas of the crystal materials as well as space group obtained via high-temperature X-ray diffraction (HT-XRD) and high-temperature neutron diffraction (HT-ND) between 25 and 1000 °C were inputted. 4 different ML models have been constructed using a Random Forest (RF), a Multi-Layer Perceptron (MLP) and a XGBoost, with different data set using experimental data (907 sets of 41 compositions), Magpie descriptor, and additional Inorganic Crystal Structure Database (ICSD) literature data (110 sets of 18 compositions). In the classification problem of space group prediction and phase diagram mapping, accuracy = 0.940 and F1 score = 0.785 is achieved by RF model with coordination number (C.N.) = 9 of oxygen of A-site cation.
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Okuyama Y., Harada Y., Mikami Y., Yamauchi K., Kuroha T., Shimada H., Yamaguchi Y., Mizutani Y.
Journal of the Electrochemical Society 170 ( 8 ) 2023年
担当区分：筆頭著者, 責任著者 掲載種別：研究論文（学術雑誌） 出版者・発行元：Journal of the Electrochemical Society
To measure the proton current in a PCFC (protonic ceramic fuel cell), the proton current detector was developed using a proton conducting oxide. The amount of proton current that flowed in the PCFC can be measured by an apparatus developed based on the electromotive force of an electrochemical cell using 10 mol% In-doped CaZrO3 (hydrogen sensor) and Y-stabilized zirconia (oxygen sensor). The water vapor pressure on the cathode changed when hydrogen ions pass through the PCFC. The hydrogen ion current can be determined by monitoring the water vapor pressure. The electronic leakage was then estimated from the hydrogen ion current and external current. We measured the current of hydrogen flowing through protonic ceramic fuel cell using ytterbium-doped barium zirconate as the electrolyte. Hydrogen ions and holes were found to leak under the OCV at 700 °C. The electronic leakage was found to be suppressed with the increasing electrolyte film thickness. On the other hand, no leakage of hydrogen and holes was observed at 500 °C.
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Strong Proton-Phonon Coupling in Perovskite-type Electrolyte of Proton-Conducting Fuel Cell 査読あり
Nagai M., Takehara H., Ashida M., Okuyama Y., Kani Y.
International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz 2023年
掲載種別：研究論文（学術雑誌） 出版者・発行元：International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz
We performed THz spectroscopy of solid electrolytes with controlled water vapor partial pressure and temperature to reveal picosecond proton motions. We also performed the IR reflection spectroscopy, and showed that the protonic motions in protonated BaZrO3 are strongly influenced by lattice vibrations. Such a dynamical long-range proton-lattice correlation will enable new approaches to improve proton conduction in solid-state ionics.
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Leonard K., Okuyama Y., Ivanova M.E., Meulenberg W.A., Matsumoto H.
ChemElectroChem 9 ( 4 ) 2022年2月
記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：ChemElectroChem
Acceptor-doped barium zirconate cerate electrolytes constitute prospective materials for highly efficient and environmentally friendly electrochemical devices. This manuscript employs a systematic approach to further optimize ionic conductivity in Ba(ZrxCe10−x)0.08Y0.2O3−δ, (1≤x≤9) oxides for moderate temperature electrolysis. We found two new composition variants by fixing a cerium/zirconium ratio of 5/4 at the perovskite B-site with incremental zirconium, an observation that contrasts many reports suggesting a linear decrease in conductivity with increasing zirconium. As a result, the composition BaZr0.44Ce0.36Y0.2O3−δ demonstrates a superior ionic conductivity (10.1 mS cm−1 at 500 °C) to stability trade-off whereas, BaZr0.16Ce0.64Y0.2O3−δ exhibits the highest conductivity (11.5 mS cm−1 at 500 °C) among the studied pellets. The high protonic conductivity is associated with a high degree of hydration, as confirmed by thermo-gravimetric analysis. In addition, both compositions as electrolytes allow successful hydrogen production in a steam electrolyzer prototype. Electrolysis voltage as low as 1.3 V is attainable at current densities of 600 and 500 mA/cm2 respectively at 600 °C, achieving 82 % current efficiencies with the later electrolyte.
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Nomura K., Shimada H., Yamaguchi Y., Shin W., Okuyama Y., Mizutani Y.
Journal of the Electrochemical Society 169 ( 2 ) 2022年2月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Journal of the Electrochemical Society
The phase transitions, thermal expansions, chemical expansions, and CO2 resistances of Ba(Ce0.7Zr0.1Y0.1Yb0.1)O3-δ (BCZYYb7111) and Ba(Ce0.4Zr0.4Y0.1Yb0.1)O3-δ (BCZYYb4411) have been investigated via high-temperature X-ray diffraction in controlled atmospheres (dry N2, wet N2, and dry 33.3 vol% CO2. 66.6 vol% N2) on cooling from 1173 to 298 K. Two phase transitions (cubic (Pm3P m) to trigonal (R3P c) and trigonal (R3P c) to monoclinic (I2/m)) were observed for BCZYYb7111, whereas one phase transition (cubic (Pm3P m) to trigonal (R3P c)) was observed for BCZYYb4411 when the samples were prepared at 1873 K in air Dehydrated BCZYYb7111 and BCZYYb4411 (by firing at 1273 K for 10 h in dry N2) showed trigonal (R3P c) and cubic (Pm3P m) symmetries, respectively, at 298 K in dry N2. BCZYYb7111 and BCZYb4411 exhibited linear thermal expansion coefficients (LTECs) of 10.7 × 10-6 K-1 and 10-1 × 10-6 K.1, respectively, from 973 to 1173 K in dry N2 and maximum chemical expansion rates (CERs) of 0.33% and 0.29%, respectively, at 773 K. BCZYYb7111, which has a higher Ce/Zr ratio, exhibited larger LTEC and CER values than BCZYYb4411, whereas BCZYYb4411, which has a lower Ce/Zr ratio, exhibited higher CO2 resistance than BCZYYb7111.
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Takehara H., Nagai M., Ashida M., Okuyama Y., Kani Y.
Journal of Physical Chemistry C 127 ( 2 ) 968 - 976 2022年
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Journal of Physical Chemistry C
Yttrium-doped barium zirconate is a solid electrolyte with several properties that are advantageous for fuel cells. We measure photoluminescence and photoluminescence excitation spectra of anhydrous yttrium-doped barium zirconate to characterize the atomic arrangement near the oxygen vacancy because the local structure is known to strongly influence the protonic conduction. The photoluminescence spectra of our anhydrous sintered pellets are composed of three components: recombination at the grain boundary region, recombination at a vacancy defect with an unpaired electron, and the transition of the electronic state of the oxygen vacancy. From the peak position of the latter photoluminescence component and the corresponding photoluminescence excitation spectrum, we can obtain information on the atomic arrangement that Zr is adjacent to the oxygen vacancies. This information is important for the design of materials with high protonic conductivity.
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コヒーレントテラヘルツパルスでみる燃料電池で用いられる 固体電解質内の高速イオン伝導
永井 正也, 奥山 勇治, 可児 幸宗
レーザー研究 50 ( 4 ) 198 2022年
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：一般社団法人 レーザー学会
We demonstrate that terahertz spectroscopy, based on ultrashort optical pulse technology, is a powerful
method for evaluating the solid electrolytes of fuel cells and all-solid-state batteries. The results of terahertz
time-domain spectroscopy include information on fast ion motion in solids, which cannot be accessed
with conventional experimental techniques.DOI： 10.2184/lsj.50.4_198
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Evaluation of the Oxygen Migration Energy in Doped Ceria by Terahertz Spectroscopy 査読あり
Nagai M., Furutani Y., Takehara H., Morimoto T., Ashida M., Okuyama Y., Kani Y.
Physical Review Applied 16 ( 6 ) 2021年12月
記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Physical Review Applied
We measure the terahertz conductivity of doped ceria by terahertz time-domain spectroscopy at different temperatures to characterize the shape of the potential associated with microscopic ion hopping. The experimental results for Gd- and La-doped ceria show that the activation energy in doped ceria is larger than that in stabilized zirconia. This result can be explained by a simple rigid-body model with the assumption of doping-induced lattice expansion, i.e., by considering the gap between cations. Our results for conventional fluorite-type solid-oxide electrolytes show that terahertz spectroscopy can be used to investigate fast microscopic ionic conduction in various electrolytes, which cannot be accessed through conventional impedance measurements.
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Hyodo J., Tsujikawa K., Shiga M., Okuyama Y., Yamazaki Y.
ACS Energy Letters 6 ( 8 ) 2985 - 2992 2021年8月
記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：ACS Energy Letters
Proton-conducting perovskite oxides are attractive as electrolytes for environmentally friendly electrochemical devices, giving rise to a demand for a variety of oxides. However, complex phenomena occurring during hydration present challenges for expanding the materials library. Herein, we demonstrate the accelerated discovery of a proton-conducting oxide using data-driven structure-property maps for hydration of 8613 hypothetical perovskite oxides in descriptor spaces characterized as important by gradient boosting regressors. We constructed trustworthy hydration training data sets for 65 compounds, including literature data, by performing thermogravimetry measurements on 22 perovskites. Knowledge-based target variable engineering was necessary to capture the physicochemical fundamentals of hydration and attain high accuracy for predicting proton concentration against temperature in unknown compositions extrapolated from training data sets. The model nominates the SrSnO3 host, which was not previously recognized for proton incorporation or proton conduction, and SrSn0.8Sc0.2O3-δ demonstrated proton incorporation and conduction. The results are promising for accelerating development and applications of proton-conducting oxides.
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Optimum dopant of barium zirconate electrolyte for manufacturing of protonic ceramic fuel cells 査読あり
Kuroha T., Niina Y., Shudo M., Sakai G., Matsunaga N., Goto T., Yamauchi K., Mikami Y., Okuyama Y.
Journal of Power Sources 506 2021年6月
担当区分：最終著者, 責任著者 記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Journal of Power Sources
We examine ceramic tape-casting and anode electrolyte co-firing for large-scale manufacturing of protonic ceramic fuel cells. We confirm the reactivity of Ni, a commonly used anode, with BaZr0.8M0.2O3−δ (BZM20: M = Sc, In, Lu, Yb, Y or Gd). Addition of 0.4 mol% NiO to BZM20 and co-firing at 1778 K, produces BaM2NiO5 for M = Y and Gd. No reaction occurs for M = Sc, In, Lu and Yb. The proton conductivity of all BZM20s decreases by approximately 30% because of NiO doping and dehydration on dissolution NiO. Energy efficiency and power density calculations of the fuel cells based on measured proton and hole conductivities indicate respective decreases of approximately 10% and 75% ecause of the formation of a solid solution of NiO. We fabricate fuel cells by the tape-casting and anode electrolyte co-firing with BZM20 (M = Yb or Y). For M = Y, the cell cannot be fabricated because formation of BaY2NiO5 causes cell cracking. Conversely, when M = Yb, a cell with a maximum output of 0.5 Wcm−2 at 873 K is fabricated. Hence, BZYb20 is a suitable material for tape-casting and anode electrolyte co-firing.
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Sumi H., Shimada H., Yamaguchi Y., Mizutani Y., Okuyama Y., Amezawa K.
Scientific Reports 11 ( 1 ) 10622 2021年4月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Scientific Reports
Protonic ceramic fuel cells (PCFCs) are expected to achieve high power generation efficiency at intermediate temperature around 400–600 °C. In the present work, the distribution of relaxation times (DRT) analysis was investigated in order to deconvolute the anode and cathode polarization resistances for PCFCs supported on yttria-doped barium cerate (BCY) electrolyte in comparison with solid oxide fuel cells (SOFCs) supported on scandia-stabilized zirconia (ScSZ) electrolyte. Four DRT peaks were detected from the impedance spectra measured at 700 °C excluding the gas diffusion process for ScSZ and BCY. The DRT peaks at 5 × 102–1 × 104 Hz and 1 × 100–2 × 102 Hz were related to the hydrogen oxidation reaction at the anode and the oxygen reduction reaction at the cathode, respectively, for both cells. The DRT peak at 2 × 101–1 × 103 Hz depended on the hydrogen concentration at the anode for ScSZ, while it was dependent on the oxygen concentration at the cathode for BCY. Compared to ScSZ, steam was produced at the opposite electrode in the case of BCY, which enhanced the cathode polarization resistance for PCFCs.
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Tanimoto N., Nagai M., Ashida M., Okuyama Y., Kani Y.
Japanese Journal of Applied Physics 60 ( 3 ) 032004 - 032004 2021年3月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Japanese Journal of Applied Physics
© 2021 The Japan Society of Applied Physics. We demonstrate that photoluminescence in solid electrolytes is sensitive to the atomic structure of the defect complex that is decisive to the ionic conduction. We systematically measure the photoluminescence spectra of a typical solid oxide electrolyte, stabilized zirconia sintered pellets. Based on the comparison with the photoluminescence spectrum of a single crystal, we assign the broad long-lived photoluminescence band in the visible region near 2.4 eV to that related to the defect complex. Because the electronic state of the oxygen vacancy is sensitive to the surrounding ions, which has been indicated in previous investigations of the local structure around the dopants and vacancies, we are able to assign each sample's photoluminescence characteristics to a certain atomic arrangement that is considered plausible based on previous investigations. Photoluminescence spectroscopy is applicable to various solid electrolytes and can become a powerful tool for their characterization.
DOI： 10.35848/1347-4065/abdf7a
その他リンク： https://iopscience.iop.org/article/10.35848/1347-4065/abdf7a/pdf
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Li K., Suito F., Takatera R., Ota A., Araki T., Okuyama Y., Mori M.
ECS Transactions 103 ( 1 ) 853 - 860 2021年
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：ECS Transactions
Intermediate-temperature protonic ceramic fuel cells (PCFCs) are expected to be highly efficient energy conversion devices because of their high proton conductivity in the electrolyte. However, besides the main proton conductivity, some other charge carriers, such as electron holes, are also conductive in PCFCs, leading to a decrease in current efficiency. The conductivities are not only affected by the operating temperature but are also strongly influenced by the gas concentrations, which causes complexities in PCFC performance. To reveal the performance and provide feedback to the operating condition selections considering the reaction overpotential, a quasi-two-dimensional Nernst-Planck- Poisson (NPP) numerical model was successfully used to investigate the current efficiency distribution in a planar-type PCFC electrolyte with BaZr0.8Yb0.2O3-δ (BZYb20). The influence of the gas flow direction was also revealed in the present study. The results showed almost the same average current efficiency with co- and counterflows but flat proton distributions with counter-flows.
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Nomura K., Shimada H., Yamaguchi Y., Shin W., Okuyama Y., Mizutani Y.
ECS Transactions 103 ( 1 ) 1753 - 1761 2021年
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：ECS Transactions
The crystal structures, thermal expansions, chemical expansions, and CO2resistances of Ba(Ce0.7Zr0.1Y0.1Yb0.1)O3-δ(BCZYYb7111) and Ba(Ce0.4Zr0.4Y0.1Yb0.1)O3-δ(BCZYYb4411) have been investigated via high-temperature X-ray diffraction (HT-XRD) in controlled atmospheres (dry N2, wet N2, and dry 33.3 vol% CO2- 66.6 vol% N2) on cooling from 1173 to 298 K. Two phase transitions, cubic to trigonal and trigonal to monoclinic, were observed for BCZYYb7111, whereas one phase transition, cubic to trigonal for BCZYYb4411. BCZYYb7111 and BCZYb4411 exhibited the linear thermal expansion coefficients of 10.6 10-6K-1and 9.8 10-6K-1between 773 and 1173 K in dry N2, respectively. The maximum chemical expansion rates of 0.33 % and 0.30 % were observed at 773 K for BCZYYb7111 and BCZYYb4411, respectively. BCZYYb4411 having lower Ce/Zr ratio and higher symmetry showed higher CO2resistance than BCZYYb7111.
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Hyodo J., Kato S., Ida S., Ishihara T., Okuyama Y., Sakai T.
Journal of Physical Chemistry C 125 ( 3 ) 1703 - 1713 2021年
担当区分：責任著者 記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Journal of Physical Chemistry C
© 2021 American Chemical Society. Proton-conducting oxides have garnered much attention as electrolyte materials of electrochemical devices operating at intermediate temperatures ranging 400-700 °C. For the implementation, quantifying the transport number of oxygen ions is crucial to predict the fuel utilization efficiency of solid oxide fuel cells (SOFCs). The acceptor-doped LaYbO3 proton conductors are one of the candidates for the electrolyte material of electrochemical devices such as SOFCs because of their high pure proton conduction. However, the oxide ion conductivity in this material is not quantified yet. Here, we evaluate the tracer diffusion coefficient (D*) of 5 mol % Ba-doped LaYbO3 by the oxygen isotope exchange and depth profile technique using secondary ion mass spectrometry. It is found that the oxygen ion conductivities in this material are lower than 10-4 S cm-1 below 800 °C, less than 2% to the total conductivity. We also visualize the domain of dominant conduction in this material. It reveals that proton conduction is dominant in fairly wide areas of the atmospheric condition, in particular below 600 °C, suggesting that the Ba-doped LaYbO3 is suitable for the SOFC electrolyte in terms of transport number.
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Li K., Araki T., Kawamura T., Ota A., Okuyama Y.
International Journal of Hydrogen Energy 45 ( 58 ) 34139 - 34149 2020年11月
担当区分：最終著者 記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：International Journal of Hydrogen Energy
© 2020 Hydrogen Energy Publications LLC Protonic ceramic fuel cells (PCFCs) operating at intermediate temperatures (400 °C~600 °C) are expected to demonstrate a high performance due to proton (OHO·) conductivity. However, the current efficiencies of PCFCs are bound to decrease due to electron holes (h·) conductivity through the electrolyte and are strongly dependent on influence parameters such as electrolyte thickness and operating temperatures of PCFCs. Consequently, the relationship between current efficiencies and the above-mentioned parameters has been quantitatively evaluated, with the aim of enhancing the former. The defect conductivities in the PCFC have been simulated using two-dimensional Nernst-Planck-Poisson (NPP) system with different operating temperatures and electrolyte thicknesses. Subsequently, the current efficiency distributions are revealed. The influence of the operating temperature on the current efficiency distributions is also clarified. The results further indicated that optimized membrane thickness should be carefully selected according to power density and power efficiency.
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Yamazaki Y., Kuwabara A., Hyodo J., Okuyama Y., Fisher C.A.J., Haile S.M.
Chemistry of Materials 32 ( 17 ) 7292 - 7300 2020年9月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Chemistry of Materials
Copyright © 2020 American Chemical Society. Proton-conducting oxides, specifically doped barium zirconates, have garnered much attention as electrolytes for solid-state electrochemical devices operable at intermediate temperatures (400-600 °C). In chemical terms, hydration energy, Ehyd, and proton-dopant association energy, Eas, are two key parameters that determine whether an oxide exhibits fast proton conduction, but to date ab initio studies have for the most part studied each parameter separately, with no clear correlation with proton conductivity identified in either case. Here, we demonstrate that the oxygen affinity, EO.dopant, defined as the energy released when an oxide ion enters an oxygen vacancy close to a dopant atom, is the missing link between these two parameters and correlates well with experimental proton conductivities in doped barium zirconates. Ab initio calculations of point defects and their complexes in Sc-, In-, Lu-, Er-, Y-, Gd-, and Eu-doped barium zirconates are used to determine Ehyd, Eas, EO.dopant, and the hydrogen affinity, EH.host, of each system. These four energy terms are related by Ehyd = EO.dopant + 2EH.host + 2Eas. Complementary impedance spectroscopy measurements reveal that the stronger the calculated oxygen affinity of a system, the higher the proton conductivity at 350 °C. Although the proton trapping site is also an important factor, the results show that oxygen affinity is an excellent predictor of proton conductivity in these materials.
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Kuroha T., Yamauchi K., Mikami Y., Tsuji Y., Niina Y., Shudo M., Sakai G., Matsunaga N., Okuyama Y.
International Journal of Hydrogen Energy 45 ( 4 ) 3123 - 3131 2020年1月
担当区分：最終著者, 責任著者 記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：International Journal of Hydrogen Energy
© 2019 Hydrogen Energy Publications LLC We investigated the influence of Ni on protonic ceramic fuel cells based on indium-doped barium zirconate. A tubular fuel cell was fabricated and evaluated with BaZr0.8In0.2O3−δ as an electrolyte. The maximum power density was 0.143 W cm−2 and the ohmic resistance of the electrolyte was 0.91 Ω cm2 at 873 K. We used secondary ion mass spectrometry to measure the dissolution of Ni in the electrolyte N to be 0.015. To clarify the effect of Ni on proton transport properties of BaZr0.8In0.2O3−δ, electrical conductivity and proton concentration were measured by AC impedance analysis and thermogravimetric analysis. Electrical conductivity decreased as the NiO content increased. Conversely, proton concentration was independent of the NiO content and proton diffusivity decreased. The sample density also depended on the NiO content. The density decreased as NiO content increased. These results were consistent with the density calculated based on a model describing formation of oxygen vacancies.
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Hyodo J., Kitabayashi K., Hoshino K., Okuyama Y., Yamazaki Y.
Advanced Energy Materials 10 ( 25 ) 2020年
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Advanced Energy Materials
© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim The environmental benefits of fuel cells and electrolyzers have become increasingly recognized in recent years. Fuel cells and electrolyzers that can operate at intermediate temperatures (300–450 °C) require, in principle, neither the precious metal catalysts that are typically used in polymer-electrolyte-membrane systems nor the costly heat-resistant alloys used in balance-of-plant components of high-temperature solid oxide electrochemical cells. These devices require an electrolyte with high ionic conductivity, typically more than 0.01 S cm−1, and high chemical stability. To date, however, high ionic conductivities have been found in chemically unstable materials such as CsH2PO4, In-doped SnP2O7, BaH2, and LaH3−2xOx. Here, fast and stable proton conduction in 60-at% Sc-doped barium zirconate polycrystal, with a total conductivity of 0.01 S cm−1 at 396 °C for 200 h is demonstrated. Heavy doping of Sc in barium zirconate simultaneously enhances the proton concentration, bulk proton diffusivity, specific grain boundary conductivity, and grain growth. An accelerated stability test under a highly concentrated and humidified CO2 stream using in situ X-ray diffraction shows that the perovskite phase is stable over 240 h at 400 °C under 0.98 atm of CO2. These results show great promises as an electrolyte in solid-state electrochemical devices operated at intermediate temperatures.
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岩井 翔, 常吉 孝治, 奥山 勇治
日本エネルギー学会大会講演要旨集 29 ( 0 ) 132 - 133 2020年
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：一般社団法人 日本エネルギー学会
Hydrogen sensor using solid electrolyte for a reduced pressure condition was evaluated. In addition, a response speed of the sensor was evaluated. In the case of introducing 30% of calibrated hydrogen gas into a gas holder, the sensor indicated 30% concentration when the pressure fluctuated from 300 to 1000Pa. The measured value was close to introduced hydrogen gas at 1000Pa even if low concentration of calibrated hydrogen gas. The response speed of the sensor was rapid. It was confirmed that the hydrogen sensor can measure the hydrogen concentration accurately in the reduced pressure condition.
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Microscopic ion migration in solid electrolytes revealed by terahertz time-domain spectroscopy. 査読あり
Morimoto T, Nagai M, Minowa Y, Ashida M, Yokotani Y, Okuyama Y, Kani Y
Nature communications 10 ( 1 ) 2662 2019年6月
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Oishi M., Nakamura T., Watanabe I., Yamamoto T., Doi T., Ina T., Nitta K., Amezawa K., Okuyama Y.
Journal of Physical Chemistry C 123 ( 26 ) 16034 - 16045 2019年6月
記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Journal of Physical Chemistry C
© 2019 American Chemical Society. The protonation mechanism in Mn-doped CaZrO3 (CZM), which involves a direct hydrogen dissolution from the surrounding H2 gas, was investigated by thermogravimetry (TG) and X-ray absorption spectroscopy (XAS). The TG results implied the formation of oxygen vacancies in a H2 atmosphere. The Mn K-edge XAS spectra indicated a reduction of the Mn ions and local structure variations around the Mn ion, but the Zr K-edge spectra were independent of the surrounding atmosphere. The amount of oxygen vacancies was smaller with respect to the reduction of the Mn ions, suggesting direct dissolution of hydrogen. Unlike many typical perovskite-type proton conductors, protonation by direct dissolution of hydrogen and not hydration was the predominant reaction in Mn-doped CaZrO3. Our experimental results demonstrated that the hydration reaction was suppressed because the oxygen vacancy was stable in the distorted ZrO6 symmetry in the CaZrO3 crystal host, whereas protonation proceeded by the direct dissolution of hydrogen being stabilized near the Mn ions in the interstitial sites at the distorted MnO6 octahedral symmetry. The experimental results showed that the structural configurations around dopants play important roles in the stabilization of protons in perovskite-type CZM materials. We demonstrated a new group of proton conductors that can overcome issues with conventional proton conductors by utilizing the direct hydrogen dissolution reaction.
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Obukuro Y., Okuyama Y., Sakai G., Matsushima S.
Journal of Alloys and Compounds 770 294 - 300 2019年1月
記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Journal of Alloys and Compounds
© 2018 Elsevier B.V. The proton conductive characteristics of La1-xBaxYbO3-δ(x = 0.02, 0.06, 0.10) were examined by an impedance analysis in the temperature range of 873–1473 K. The maximum proton concentrations in La1-xBaxYbO3-δdetermined by the weight changes were almost 1/5 of the oxygen vacancies formed by the substitution of Ba. This result indicated that at least two types of oxygen vacancies, a hydratable oxygen vacancy and hydration independent one, exist in the crystal. The bond distances and electronic structure of the Ba-substituted LaYbO3supercell with or without an oxygen vacancy were investigated by the first-principles molecular dynamics calculations. The substitution of Ba for the La-site of LaYbO3was found to induce the local structure change of oxygen atoms surrounding the La atom.
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Okuyama Y., Nagatomo S., Niisaka A., Matsunaga N., Sakai G., Sasamata Y., Ogura Y., Mizutani Y.
ECS Transactions 91 ( 1 ) 2653 - 2659 2019年
記述言語：英語 掲載種別：研究論文（国際会議プロシーディングス） 出版者・発行元：ECS Transactions
© The Electrochemical Society. Steam electrolysis using a proton conducting oxide might offer low-cost hydrogen production using a combination of electrical energy and thermal energy that comes from solar energy. On the other hand, the hydrogen production efficiency is low due to the electrical leakage for the electrolysis of H2O-O2. In this study, the additive effect of methane on the hydrogen production efficiency for stem electrolysis using a proton conducting oxide was examined. The steam electrolysis was performed under a H2O-O2, H2O-CH4 atmosphere at 1073 K. The hydrogen evolution rate of the H2O-CH4 electrolysis with the Ru catalyst was in agreement with the value according to Faraday's law. The hydrogen evolution rate and the electrolysis voltage were found to increase in the order of the H2O-CH4 electrolysis with the catalyst < the H2O-CH4 electrolysis without the catalyst < the H2O-O2 electrolysis.
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Picoseconds ion motions in materials for solid oxide fuel cell 査読あり
Morimoto T., Nagai M., Ashida M., Yokotani Y., Okuyama Y., Kani Y.
International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz 2018-September 2018年10月
記述言語：英語 掲載種別：研究論文（国際会議プロシーディングス） 出版者・発行元：International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz
© 2018 IEEE. We measured the THz conductivity in typical electrolytes of solid oxide fuel cell with THz time-domain spectroscopy up to 1273 K. The measured conductivity reflected the microscopic ion motions just before hopping to the adjacent site. We evaluated the activation energy of the microscopic ion migration, which is difficult to be accessed with other conventional measurements. Our results will open a new application of THz spectroscopy for clarifying the ionic conduction mechanism and exploring novel fuel cell materials.
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Nakamura T., Mizunuma S., Kimura Y., Mikami Y., Yamauchi K., Kuroha T., Taniguchi N., Tsuji Y., Okuyama Y., Amezawa K.
Journal of Materials Chemistry A 6 ( 32 ) 15771 - 15780 2018年1月
記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Journal of Materials Chemistry A
© 2018 The Royal Society of Chemistry. In this study, the efficiency of ionic transport through proton-conducting ceramic electrolytes for energy conversion applications was discussed while considering the energy losses caused by the mixed ionic and electronic conduction and by the ionic transport resistance. It was shown that high energy efficiency cannot be expected with an extremely thin electrolyte because of the significant energy loss due to the internal current leakage. In the case of a fuel cell with a BaZr0.8Y0.2O3-δ electrolyte operating at 873 K, the maximum energy efficiency was estimated to be only 82% when the product of the external current density and the electrolyte thickness was 1.7 × 10-3 A cm-1. The energy efficiency for the ionic transport in BaZr0.8Y0.2O3-δ was compared with those for typical oxide ion conductors, i.e. yttria stabilized zirconia and Ce0.9Gd0.1O1.95-δ, under various fuel cell operating conditions. BaZr0.8Y0.2O3-δ can provide higher energy efficiency than the oxide ion conductors at lower temperatures and under higher current conditions. For the efficient operation of fuel cells using a proton-conducting ceramic as an electrolyte, lower operating temperatures below 873 K is preferable from the viewpoint of energy loss in the electrolyte. The influence of the partial conductivity variation on the energy efficiency was also discussed. It was shown that not only the increase of ionic conductivity but also the decrease of electronic conductivity could improve the energy efficiency if the electrolyte thickness can be optimized. This means that decreasing electronic conductivity as well as increasing proton conductivity can be an effective strategy to develop highly efficient proton conductors for energy conversion applications.
DOI： 10.1039/c8ta05373a
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Leonard K., Okuyama Y., Takamura Y., Lee Y., Miyazaki K., Ivanova M., Meulenberg W., Matsumoto H.
Journal of Materials Chemistry A 6 ( 39 ) 19113 - 19124 2018年1月
記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Journal of Materials Chemistry A
© 2018 The Royal Society of Chemistry. Ceramic proton conductors have the potential to become important components in future clean and efficient energy technologies. In this manuscript, barium cerium yttrium zirconate (Ba(Zr0.5Ce0.4)8/9Y0.2O2.9) and strontium cerium yttrium zirconate (SrZr0.5Ce0.4Y0.1O2.95), proton conducting perovskites were employed as solid oxide electrolysis cell (SOEC) electrolytes for hydrogen production via intermediate temperature steam electrolysis at 550 and 600 °C. Cathode-supported button cells examined for a 12 μm Ba(Zr0.5Ce0.4)8/9Y0.2O2.9electrolyte, with Ni-SrZr0.5Ce0.4Y0.1O2.95as the H2-electrode, and porous Ba0.5La0.5CoO3as the anode reached current densities of 0.2 and 0.5 A cm-2with applied voltage of 1.45 V, at 550 and 600 °C, respectively. Moreover, a hydrogen evolution rate of 127 μmol cm-2per minute was achieved at 0.5 A cm-2, translating to a current efficiency of 82%. In addition, excellent cell performance was obtained using SrZr0.5Ce0.4Y0.1O2.95as an electrolyte. Current densities of 0.2 and 0.5 A cm-2were obtained at 600 °C with applied voltages of 1.28 and 1.63 V, achieving faradaic current efficiencies of 88 and 85%. The NiO-SrZr0.5Ce0.4Y0.1O3-δcomposite cathode was more favorable for the densification of the supported Ba(Zr0.5Ce0.4)8/9Y0.2O2.9electrolyte during sintering and could be promising for use as a cathode substrate in proton-conducting SOECs.
DOI： 10.1039/c8ta04019b
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Okuyama Y., Ymaguchi T., Matsunaga N., Sakai G.
Materials Transactions 59 ( 1 ) 14 - 18 2018年
記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Materials Transactions
© 2017 The Japan Institute of Metals and Materials. In order to clarify the effect of the dopant concentration and phase transition on the proton conduction and proton concentration, the electrical conductivity and proton concentration were determined for La 1-x Ba x Yb 0.5 In 0.5 O 3-d (x = 0.1. 0.3. 0.5, 0.7). The phase transition from the orthorhombic system to the cubic system was over x = 0.3. The proton/deuteron isotope effect on the conductivity was observed for all samples at 673 K. The proton concentration was independent of the barium content above 673 K. The proton concentration increased with the barium content below 673 K, but the concentration ratio of the proton to dopant decreased following an increase in the barium concentration. It was determined that the dopant concentration and phase transition do not have an in uence on the conduction and incorporation of protons into the La 1-x B ax Yb 0.5 In 0.5 O 3-d .
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A direct methane fuel cell with double-layered electrolyte using proton conducting oxide 査読あり
Okuyama Y., Kawano S., Sakai G., Matsunaga N., Mizutani Y.
ECS Transactions 78 ( 1 ) 1953 - 1961 2017年5月
記述言語：英語 掲載種別：研究論文（国際会議プロシーディングス） 出版者・発行元：ECS Transactions
© The Electrochemical Society. BaCe 0.8 Y 0.2 O 3-δ (BCY) has high proton conductivity, but is less stable against carbon dioxide. La 0.9 Sr 0.1 Yb 0.8 In 0.2 O 3-δ (LSYbIn) is chemically stable, but not as conductive as BCY. In this study, the performance of a methane fuel cell using BCY and LSYbIn as electrolytes was evaluated. The fuel cell using BCY showed shortterm degradation. On the other hand, the fuel cell using LSYbIn showed high durability using methane. The protonic ceramic fuel cell with a double-layered electrolyte was developed to improve chemical stability toward methane and the fuel cell performance. LSYbIn was used as the fuel side intermediate layer of the fuel cell using BCY for protection from methane. The fuel cell with a double-layered electrolyte suppressed the degradation by methane.
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Leonard K., Lee Y., Okuyama Y., Miyazaki K., Matsumoto H., Matsumoto H.
International Journal of Hydrogen Energy 42 ( 7 ) 3926 - 3937 2017年2月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：International Journal of Hydrogen Energy
© 2016 Hydrogen Energy Publications LLCDense yttrium doped solid solutions of alkaline earth cerate and zirconate with the formula A(Zr5/9Ce4/9)1−xYxO3−δ, (A = Sr and Ba, x = 0.1, 0.2) have been prepared and studied as potential proton conducting electrolytes. Thermo-gravimetric analysis was used to determine the proton concentration in the specimens. BZCY(54)8/92 showed a maximum proton concentration limit of 17.1 mol % compared to only 6.03 mol % for SZCY(54)8/92 and retained the highest proton conductivity of 1.44 × 10−2 S cm−1 in wet 1% H2 at 600 °C. Sufficient chemical stability under 80% steam for 200 h was confirm for the said composition. Approximately similar proton mobility levels were observed at 600 °C for the compositions. The major difference in total conductivity originates mostly from the differences in proton concentration levels. The bulk and grain boundary contributions on the conductivity were strongly dependent on the A-site host, with the grain boundary response being the main source of resistance in the BZCY specimens relative to the SZCY.
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Obukuro Y., Ninomiya K., Arai M., Okuyama Y., Sakai G., Matsushima S.
Computational Materials Science 126 7 - 11 2017年1月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Computational Materials Science
© 2016 Elsevier B.V.The electronic structure of LaYbO3 was investigated by the full potential linearized augmented plane wave plus local orbital (FLAPW + lo) method with the modified Becke–Johnson potential combined with the local density approximation correlation plus onsite Coulomb interaction (MBJ–LDA + U) for the sake of localized f electrons. This approach was suitable for evaluating electronic structure of LaYbO3 system from view point of calculation cost and time. The band gap, the difference in energy between the valence band (VB) and conduction band (CB), was estimated to be 6.0 eV by the present method. The evaluated value was very close to the reported experimental value. In the VB, Yb 4f orbitals and O 2p orbitals were well hybridized each other to state predominantly. The lower potential region in CB was mainly composed of La 4f orbitals, while the upper region in CB was mainly consisted of La 5d orbitals.
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固体電解質センサによるディーゼル排気ガス中の水素濃度測定 査読あり
常吉孝治、高橋総子、岩井翔、奥山勇治
自動車技術会論文 48 ( 6 ) 1171 - 1175 2017年
記述言語：日本語 掲載種別：研究論文（研究会，シンポジウム資料等）
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ヒーター組み込み型プロトン導電性セラミックスによる気相中水素濃度の測定
常吉 孝治, 高橋 総子, 岩井 翔, 奥山 勇治
東海支部総会講演会講演論文集 2017 ( 0 ) 2017年
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：一般社団法人 日本機械学会
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Obukuro Y., Matsushima S., Obata K., Suzuki T., Arai M., Asato E., Okuyama Y., Matsunaga N., Sakai G.
Journal of Alloys and Compounds 658 139 - 146 2016年2月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Journal of Alloys and Compounds
© 2015 Elsevier B.V.Single-phase Sr2Bi2O5 was obtained by calcination of the heterobimetallic complex precursor Sr[Bi(DTPA)]·9H2O (DTPA is diethylenetriaminepentaacetic acid) at 700 °C for 6 h. The effect of La doping on the crystal structure, surface area, morphology, surface chemical state, and absorption properties of Sr2Bi2O5 samples prepared from Sr[Bi(DTPA)]·9H2O were investigated. No traces of an impurity phase were detected in samples with a lower La content (3 and 5 mol%), while impurity phases such as Sr6Bi2O9 and SrCO3 were observed in Sr2Bi2O5 samples with larger La contents of 8 and 10 mol%. The Brunauer-Emmett-Teller surface area of Sr2Bi2O5 increased slightly with the content of La. High-resolution transmission electron microscopy revealed clear crystalline planes for both undoped and La-doped Sr2Bi2O5 samples without amorphous phases. Meanwhile, X-ray photoelectron spectroscopy indicated that the valence states of constituent metals were Sr2+, Bi3+, and La3+. The solubility limit of La in the Sr2Bi2O5 crystal phase was determined from Raman scattering measurements. La atoms substituted Bi sites when the doping content was low, while both Bi and Sr sites were substituted with La when the content of La was high. The electronic structure of Sr2Bi2O5 could be modified by La doping, resulting in a red shift of the absorption edge with increasing La content. The band-gap narrowing of Sr2Bi2O5 with La-doping was reproduced in energy-band calculations. The photocatalytic activity of Sr2Bi2O5 under visible-light irradiation for the oxidation of isopropanol was enhanced by doping with La.
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Sakai G., Tanaka A., Sueda T., Ogata T., Okuyama Y., Matsunaga N.
Chemistry Letters 45 ( 3 ) 318 - 320 2016年
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Chemistry Letters
© 2016 The Chemical Society of Japan.Crystal growth under hydrothermal treatment at 150 °C was observed for the pre-calcined TiO2-SnO2 precursor obtained by the calcination at 250 °C. This phenomenon was observed for the trivalent titanium ion (Ti3+) used as the starting material for hydrolysis. The peculiar crystal growth phenomenon was thought to be originated from the oxidation of titanium from trivalent to tetravalent, because such peculiar crystal growth under hydrothermal conditions was not observed when tetravalent titanium was used.
DOI： 10.1246/cl.151106
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Okuyama Y., Nagamine S., Nakajima A., Sakai G., Matsunaga N., Takahashi F., Kimata K., Oshima T., Tsuneyoshi K.
RSC Advances 6 ( 40 ) 34019 - 34026 2016年
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：RSC Advances
© The Royal Society of Chemistry 2016.In order to simplify the structure of the EMF-type hydrogen sensor using a proton conductor as the electrolyte, an electrolyte serving as a self-standard electrode in air was developed. The electrochemical properties and proton dissolution mechanism of Mn-doped CaZrO3 were evaluated by impedance analysis, EMF measurement, IR absorption analysis and ESR measurement. Mn-doped CaZrO3 acquires a proton from hydrogen by reduction of the manganese ion. 5 mol% Mn doped CaZrO3 showed proton conduction in a reducing atmosphere and hole conduction in an oxidizing atmosphere. A gas concentration cell using CaZr0.95Mn0.05O3-δ as the electrolyte was constructed for use as a hydrogen sensor. It was found to be dependent only on the hydrogen potential of the working electrode when air was used as the reference gas.
DOI： 10.1039/c5ra23560j
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Okuyama Y., Isa K., Lee Y., Sakai T., Matsumoto H., Matsumoto H.
Solid State Ionics 275 35 - 38 2015年7月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Solid State Ionics
© 2015 Elsevier B.V. All rights reserved.In order to clarify the mixed effect of a cation on proton conduction, the electrical conductivity and the proton concentration of SrCe0.9 - xZrxY0.1O3 - δ (x = 0.0, 0.3, 0.5, 0.7, 0.9) were measured by the AC impedance method and thermogravimetric analysis at 573-1173 K. The electrical conductivity reached a maximum value at x = 0.5. The proton concentration at x = 0.5 is the highest at this experimental temperature. The condition of the oxygen vacancy was examined by a Raman spectroscopic analysis. The Raman band around 350 cm- 1 attributed to the CeO6 stretching mode was observed and the half band width of the peak is broad when the cation on the B site is a mixture of Ce4 + and Zr4 +. It was found that the symmetry of the octahedron around B-site might be reduced due to the cation mix on the B-site.
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Improvement of protonic ceramic fuel cells with thin film BCZY electrolyte
Okuyama Y., Ebihara N., Okuyama K., Mizutani Y.
ECS Transactions 68 ( 1 ) 2545 - 2553 2015年
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：ECS Transactions
© The Electrochemical Society.The performance of protonic ceramic fuel cells (PCFCs) using thin film BaCe<inf>0.6</inf>Zr<inf>0.2</inf>Y<inf>0.2</inf>O<inf>3-δ</inf> (BCZY622) electrolyte has been studied. The reference cell with the La<inf>0.8</inf>Sr<inf>0.2</inf>MnO<inf>3-δ</inf> cathode and Ni anode showed poor performance and short-term degradation. The overpotential of the anode and ohmic loss were the major factors for degradation, and a solid solution of Ni in the BCZY electrolyte has been observed. The electrical conductivity and the proton concentration of BCZY and BCZY622 doped with 5wt% NiO were measured by an impedance analysis and thermogravimetric analysis. The electrical conductivity and the proton concentration of BCZY622 decreased by introducing NiO. To avoid short-term degradation, the cell construction, manufacturing process, and cathode materials were improved. As a result, a carefully designed PCFC with thin film BCZY622, Ba<inf>0.5</inf>Sr<inf>0.5</inf>Co<inf>0.8</inf>Fe<inf>0.2</inf>O<inf>3-δ</inf> cathode and Ni/BCZY622 cermet anode showed good power density of 200-300mW/cm<sup>2</sup> in the wide temperature range of 873-1073K, and had stable power generation.
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Formation of CaFe<inf>2</inf>O<inf>4</inf> porous structure by addition of Zr in malic acid complex
Obukuro Y., Obata K., Maeda R., Matsushima S., Okuyama Y., Matsunaga N., Sakai G.
Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi/Journal of the Ceramic Society of Japan 123 ( 1442 ) 995 - 998 2015年
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi/Journal of the Ceramic Society of Japan
© 2015 The Ceramic Society of Japan.The effect of Zr addition into malic acid complex was examined for the purpose of the improvement of microstructure and specific surface area of CaFe2O4. The CaFe2O4 phase could be obtained for both unadded and Zr-added (5 and 10 mol% with respect to Fe) system by calcination of precursor derived from malic acid complex at 700°C which is almost 200°C lower than that of conventional solid-state reaction method. Among the examined products, only the 5 mol% Zr-added product revealed characteristic smaller grains connected three dimensional porous structures resulting in higher specific surface area compared with unadded and 10 mol% Zr-added products. XPS measurements revealed that there are no notable changes in valence state for all constituent elements. The obtained final product of 5 mol% Zr-added CaFe2O4 is worth further investigate for various application from a view point of improved higher specific surface area and characteristic microstructure as well as the functionality of the material itself.
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Obukuro Yuki, Sakai Go, Okuyama Yuji, Matsunaga Naoki, Matsushima Shigenori, Obata Kenji, Asato Eiji
Chemistry Letters 44 ( 7 ) 890 - 892 2015年
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：The Chemical Society of Japan
Single-phase Sr<sub>2</sub>Bi<sub>2</sub>O<sub>5</sub> was formed by calcination of the heterobimetallic complex, Sr[Bi(DTPA)]·9H<sub>2</sub>O, at 700 °C for 6 h in air. La-doping by 5 and 10 mol % into Sr<sub>2</sub>Bi<sub>2</sub>O<sub>5</sub> was also examined. There were no traces of impurity phase in the 5 mol % La-doped Sr<sub>2</sub>Bi<sub>2</sub>O<sub>5</sub> powder, on the other hand, impurity phases such as Sr<sub>3</sub>Bi<sub>4</sub>O<sub>9</sub> and SrCO<sub>3</sub> were identified for the 10 mol % La-doped sample. UV–vis spectroscopy revealed that the absorption edge of La-doped Sr<sub>2</sub>Bi<sub>2</sub>O<sub>5</sub> exhibits red shift as compared to the pure Sr<sub>2</sub>Bi<sub>2</sub>O<sub>5</sub> and the shift increases with the increase of La<sup>3+</sup> doping. It was confirmed that the photocatalytic activity of Sr<sub>2</sub>Bi<sub>2</sub>O<sub>5</sub> for the oxidation of isopropanol is improved with La doping under visible-light irradiation (λ ≥ 420 nm).
DOI： 10.1246/cl.150178
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Okuyama Y., Okuyama Y., Okuyama K., Mizutani Y., Sakai T., Lee Y., Matsumoto H., Matsumoto H., Matsumoto H.
International Journal of Hydrogen Energy 39 ( 35 ) 20829 - 20836 2014年12月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：International Journal of Hydrogen Energy
© 2014 Hydrogen Energy Publications, LLC. All rights reserved.The electrochemical properties of La0.9Sr0.1Yb0.8In0.2O3-δ (LSYbIn) were investigated by impedance measurements and the fuel cell performance test. The dominant charge carrier is the proton at low temperature. The hole is the dominant charge carrier in an oxidizing atmosphere at high temperature. Based on the conductivity data, the hydrogen potential profile and steady state current in the fuel cell at the polarized state were evaluated by calculation. The performance of the PCFC (protonic ceramic fuel cell) using LSYbIn as the electrolyte was examined in the temperature range of 873-1073 K. The maximum power density was 8.2 mW cm-2 at 1073 K.
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Okuyama Y., Ikeda S., Sakai T., Matsumoto H.
Solid State Ionics 262 865 - 869 2014年9月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Solid State Ionics
In order to clarify the mixing effect of cation on the electrical conductivity and the hydration, the electrical conductivity of La 0.9 Sr 0.1 (Yb 1 - x M x )O 3 - δ (M = In, Y) has been determined by the four-probe AC technique in the temperature range of 673-1173 K. The proton concentration was estimated from the change in weight due to hydration by a thermogravimetric analysis. The conductivity and proton concentration increased with the increase in the In 3 + content on the Yb-site. On the other hand, the electrical conductivity and protonic concentration of La 0.9 Sr 0.1 (Yb 1 - x Y x )O 3 - δ showed minimum values at x = 0.5. © 2013 Elsevier B.V. All rights reserved.
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Preparation of nano-structured cathode for protonic ceramic fuel cell by bead-milling method
Oda H., Yoneda T., Sakai T., Okuyama Y., Matsumoto H.
Solid State Ionics 262 388 - 391 2014年9月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Solid State Ionics
We developed a nano-structured Sm 0.5 Sr 0.5 CoO 3 (SSC55) cathode from slurry dispersed nanoparticles using a planetary bead-milling method. In order to evaluate the cathode performance, the cathode resistances and the protonic ceramic fuel cell properties were examined using electrochemical cells with a BaCe 0.6 Zr 0.2 Y 0.2 O 3 - δ electrolyte disk at intermediate temperatures. The cells with a milled SSC55 cathode showed lower cathode resistance and higher power density compared to those with the SSC55 cathode prepared from the conventional method. The improvement is attributed to an increase in the cathode reaction sites caused by the use of the smaller particles and a good adhesion to the electrolyte surface. © 2013 Published by Elsevier B.V.
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溶融銅用起電力型水素センサの実用化
栗田典明、奥山勇治、祖父江孝之、大島智子、木股幸司、寺西裕記、武津典彦
銅と銅合金 53 ( 1 ) 171 - 176 2014年8月
記述言語：日本語 掲載種別：研究論文（学術雑誌）
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Incorporation and conduction of proton in Sr-doped LaMO<inf>3</inf>(M=Al, Sc, In, Yb, Y)
Okuyama Y., Kozai T., Ikeda S., Matsuka M., Sakai T., Matsumoto H.
Electrochimica Acta 125 443 - 449 2014年4月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Electrochimica Acta
In order to clarify the effect of the B site species in ABO 3 perovskite oxides on the proton transport properties, the proton incorporation into a series of La 0.9 Sr 0.1 MO 3-δ , (M = Al, Sc, In, Yb, Y) was studied by measuring the electrical conductivity and electromotive forces of the gas concentration cells, and by a thermogravimetric analysis. The proton concentration and electrical conductivity increased in the order of the B site species, Al < Y=Yb < Sc < In and Al < Y=Yb < In < Sc, respectively. La 0.9 Sr 0.1 AlO 3-δ showed an oxide ion conductivity, while La 0.9 Sr 0.1 YbO 3-δ and La 0.9 Sr 0.1 YO 3-δ exhibited a protonic conductivity in the temperature range of 573-1173 K. La 0.9 Sr 0.1 ScO 3-δ and La 0.9 Sr 0.1 InO 3-δ showed a protonic conductivity under 873 K, and a mixed proton and oxide ion conductivity at 1073 K. © 2014 Elsevier Ltd.
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Oda H., Okuyama Y., Sakai T., Matsumoto H.
Materials Transactions 55 ( 4 ) 722 - 727 2014年
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Materials Transactions
Nano-structured La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-∞ (LSCF6428) cathode for protonic ceramic fuel cell (PCFC) prepared from planetary bead-milled nanoparticles was investigated. The cathode made by the bead-milled particles showed a porous structure consisting of homogeneous size particles and pores in the range 100 nm. The PCFC comprised of this cathode, Ni anode and BaCe 0.6 Zr 0.2 Y 0.2 O 3-∞ (BCZY622) electrolyte increased the cell performances about 6 times that of the cell using the cathode without milling. The cathode preparation using bead-milling is effective in improving the cell performance with an expanded contact area of the cathode/electrolyte interface due to decreasing particle size and good adhesion of the particles to the electrolyte. © 2014 The Japan Institute of Metals and Materials.
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Proton trapping in yttrium-doped barium zirconate
Yamazaki Y., Blanc F., Okuyama Y., Buannic L., Lucio-Vega J., Grey C., Haile S.
Nature Materials 12 ( 7 ) 647 - 651 2013年7月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Nature Materials
The environmental benefits of fuel cells have been increasingly appreciated in recent years. Among candidate electrolytes for solid-oxide fuel cells, yttrium-doped barium zirconate has garnered attention because of its high proton conductivity, particularly in the intermediate-temperature region targeted for cost-effective solid-oxide fuel cell operation, and its excellent chemical stability. However, fundamental questions surrounding the defect chemistry and macroscopic proton transport mechanism of this material remain, especially in regard to the possible role of proton trapping. Here we show, through a combined thermogravimetric and a.c. impedance study, that macroscopic proton transport in yttrium-doped barium zirconate is limited by proton-dopant association (proton trapping). Protons must overcome the association energy, 29 kJ mol -1, as well as the general activation energy, 16 kJ mol -1, to achieve long-range transport. Proton nuclear magnetic resonance studies show the presence of two types of proton environment above room temperature, reflecting differences in proton-dopant configurations. This insight motivates efforts to identify suitable alternative dopants with reduced association energies as a route to higher conductivities. © 2013 Macmillan Publishers Limited. All rights reserved.
DOI： 10.1038/nmat3638
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Electrochemical hydrogen pumps using Ba doped LaYbO3 type proton conducting electrolyte
Sakai T., Isa K., Matsuka M., Kozai T., Okuyama Y., Ishihara T., Matsumoto H.
International Journal of Hydrogen Energy 38 ( 16 ) 6842 - 6847 2013年5月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：International Journal of Hydrogen Energy
The electrochemical hydrogen pumping using Ba doped LaYbO 3 oxide (La 0.9 Ba 0.1 YbO 3-α , LBYb-91) was investigated in this work. It was found that LBYb-91 can conduct a large amount of protons as much as 100 mA cm -2 and the theoretical hydrogen evolution is actually occurred by the proton conduction. It was also found that LBYb-91 has high chemical stability against CO 2 and H 2 O, and showed a comparable cell performance to SrZr 0.9 Y 0.1 O 3-α (SZY-91) with porous palladium electrode. The present study demonstrated that LBYb-91 is a potential candidate electrolyte material for the electrochemical cell to separate hydrogen from the reformed natural gas. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Proton transport properties of La<inf>0.9</inf>M<inf>0.1</inf>YbO<inf>3-δ</inf>(M = Ba, Sr, Ca, Mg)
Okuyama Y., Kozai T., Sakai T., Matsuka M., Matsumoto H.
Electrochimica Acta 95 54 - 59 2013年4月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Electrochimica Acta
The proton transport properties of La 0.9 M 0.1 YbO 3-δ (M = Ba, Sr, Ca, Mg) were studied by the electrical conductivity measurement technique in the temperature range of 673-1173 K. The proton concentration was estimated by the weight changes due to the hydration reaction. The electrical conductivity and proton concentration of La 0.9 Ba 0.1 YbO 3-δ were the highest in La 0.9 M 0.1 YbO 3-δ . The electromotive force of a hydrogen gas concentration cell using La 0.9 Ba 0.1 YbO 3-δ as the electrolyte was observed at 673-1073 K, and was in a good agreement with the theoretical values assuming that the proton transport number was unity. © 2013 Elsevier Ltd. All rights reserved.
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Proton-conducting oxide and applications to hydrogen energy devices
Matsumoto H., Sakai T., Okuyama Y.
Pure and Applied Chemistry 85 ( 2 ) 427 - 435 2013年2月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Pure and Applied Chemistry
The change of the primary energy source from fossil fuels to the so-called renewable energies, such as solar and wind energy, is a major and necessary trend. The use of these energies needs efficient methods of energy storing and restoring, and hydrogen can serve for an effective conversion between the electricity and the chemical energy. This paper describes the ion-conducting materials applicable to the so-called "hydrogen energy system". Since hydrogen works as an energy medium in this system, hydrogen-ion-transporting materials are beneficial. Proton-conducting oxides will work for this purpose and will be introduced here, viewing from material designing and working mechanism. Application of the proton-conducting oxide materials to water electrolysis is another topic, and how the materials work in principle for conversion between electricity and chemical energy and how their performances could/can be improved will be described. © 2012 IUPAC.
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Long-term stability of sulfated hydrous titania-based electrolyte for water electrolysis
Kim S., Sakai T., Oda H., Okuyama Y., Mizutani Y., Hamagami J., Matsuka M., Ishihara T., Matsumoto H.
Journal of Solid State Electrochemistry 16 ( 11 ) 3587 - 3592 2012年11月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Journal of Solid State Electrochemistry
The long-term stability in water was investigated for an inorganic proton conductor based on sulfated hydrous titania electrolyte in water electrolysis. Heat treatment temperature in the range of 400-700 °C during synthesis of the material was chosen as the critical parameter for the conductivity and stability in water. Electric resistance of the membrane electrode assembly using the sulfated hydrous titania in water and pH were investigated by the effect of heat treatment temperature on the stability of conductivity and acid detachment. TG, Fourier transform infrared spectroscopy, and energy-dispersive X-ray analysis measurements were performed to understand the amount and state of the sulfate groups on the surface of sulfated hydrous titania. Heat treatment at 650 °C was found to obtain good conduction behavior for the sulfated hydrous titania and was confirmed to work for a stable water electrolysis. © 2012 Springer-Verlag Berlin Heidelberg.
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Properties of electrical conductivity in Y-Doped CaZrO<inf>3</inf>
Bao J., Okuyama Y., Shi Z., Fukatsu N., Kurita N.
Materials Transactions 53 ( 5 ) 973 - 979 2012年10月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Materials Transactions
To clarify the high temperature electrochemical properties of Y-doped CaZrO3, the electrical conductivity of the poly-crystalline specimen was measured for various kinds of hydrogen and oxygen-containing atmospheres by employing the two-terminal AC method. The H/D isotope effect of the electrical conductivity was examined in detail in order to specify the conditions in which the proton conduction was dominant. In the temperature range from 973 to 1273 K, it was found that protons are the dominant charge carrier both in hydrogen-rich and oxygen-rich atmospheres. At higher temperatures of 1373 to 1673 K, the dominant charge carrier is estimated to be the positive hole in oxygen-rich atmospheres, whereas to be the oxide ion vacancy in hydrogen-rich atmospheres by the analysis of the atmospheric dependence of the electrical conductivity. These results were almost the same as those of Al-doped CaZrO3 which were reported previously by the present authors. © 2012 The Japan Institute of Metals.
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Sakai T., Matsushita S., Hyodo J., Okuyama Y., Matsuka M., Ishihara T., Matsumoto H.
Electrochimica Acta 75 179 - 184 2012年7月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Electrochimica Acta
Introduction of doped ceria interlayer to cathode/electrolyte interface of the electrochemical cell with proton conducting electrolyte was investigated using thin Ce 0.8 Yb 0.2 O 2-δ (YbDC) interlayer of about 500 nm thickness. YbDC interlayer conducted a large amount of protons as much as 170 mA cm -2 . It was also found that cathode overpotential of the YbDC interlayer cells consistently showed a plateau at about 400 mV, at which that of the non-interlayer cells did not show, suggesting a possibility that cathode reaction is changed by introducing the doped ceria interlayer. This result also indicates that the interlayer showed high activity for cathode reaction when enough cathodic bias was applied. Especially, the interlayer showed high activity for the improvement of poor cathode reaction between SrZr 0.9 Y 0.1 O 3-α (SZY-91) electrolyte and platinum cathode. © 2012 Elsevier Ltd. All rights reserved.
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Determination of the maximum solubility of hydrogen in α-alumina by DC polarization method
Okuyama Y., Kurita N., Sato D., Douhara H., Fukatsu N.
Solid State Ionics 213 92 - 97 2012年4月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Solid State Ionics
In order to evaluate the solubility of hydrogen in polycrystalline α-alumina used as a proton conductor, the amount of absorbed or desorbed hydrogen based on the change in the polarization conditions of the specimen was measured by a coulometric method based on an electrochemical cell using CaZrO 3 (+In 2 O 3 ). The hydrogen chemical potential profile in the specimen was calculated for various states of imposed voltage. The solubility of hydrogen was evaluated from the measured amounts of the absorbed or desorbed hydrogen on the changes in the imposed voltage and the corresponding change in the potential profile. The obtained value of the maximum solubility in the hydrogen atmosphere was (2.0 ± 1.6) × 10 -7 mol cm -3 and found to be independent of temperature in the experimental range of 1273-1573 K. © 2011 Elsevier B.V. All rights reserved.
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Bao J., Ohno H., Okuyama Y., Fukatsu N., Kurita N.
Materials Transactions 53 ( 4 ) 752 - 759 2012年4月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Materials Transactions
In order to clarify the electrochemical properties of the Al-doped CaZrO 3 system, a gas concentration cell was assembled adopting 0.4 mol%Al-doped CaZrO 3 polycrystalline sintered material as the electrolyte and its electromotive force (emf) was measured for various oxygen and hydrogen chemical potential gradients. The measurements were performed in a hydrogen-rich atmosphere for the temperature range from 973 to 1473 K. For almost all the conditions in the experiment, the measured emf's were well explained by regarding that the substantial predominant charge carrier is the proton. Under the conditions that the transport number of proton is less than unity, the agreement was examined between the measured emf and the theoretical one estimated based on the conduction parameters determined by the conductivity measurement reported in the previous work. It was confirmed that they well coincide with each other in all experimental conditions. This fact shows that the conduction parameters of CaZr 0.996 Al 0.004 O 3-α and also the model of defect structure reported in the previous work were reasonable. The proton conduction domain of Al-doped CaZrO 3 in the oxygen-hydrogen chemical potentials plane was examined based on these conduction parameters and it was found to be a little wider than that of In-doped CaZrO 3 system. © 2012 The Japan Institute of Metals.
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Seok-Jun K., Sakai T., Oda H., Hamagami J., Okuyama Y., Matsuka M., Ohta S., Shimizu Y., Ishihara T., Matsumoto H.
Electrochemistry 80 ( 4 ) 246 - 248 2012年4月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Electrochemistry
A sulfonated nanotitania proton conductor based on hydrous titania was applied to water electrolysis as a hydrogen production method from electricity. Hydrous titania in the form of nanoparticles was used as a water absorbing porous electrolyte to allow water transport to the anode via electrolyte part of the water electrolysis cell. Overpotential of the porous electrolyte cell in water was lower than that of the Nafion 117 memb rane. Evolution of hydrogen and oxygen by electrolysis were confirmed although the rates of gas evolution were smaller than those calculated from Faraday's law. © The Electrochemical Society of Japan, All rights reserved.
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The effect of annealing on the proton conductivity of Mg-doped α-Al<inf>2</inf>O<inf>3</inf>
Okuyama Y., Kurita N., Katayama K., Fukatsu N.
Ionics 18 ( 1-2 ) 85 - 90 2012年1月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Ionics
In order to evaluate the effect of annealing treatment on the proton conductivity of α-alumina, the electrical conductivity of Mg-doped polycrystalline α-alumina kept at 1,873 K under various conditions of the surrounding atmosphere and then cooled in the furnace was measured in the temperature range 1,173-1,473 K. The H + /D + isotope effect on the electrical conductivity was also examined under a hydrogen atmosphere at 1,273 K. The protonic conductivity measured at 1,273 K increased with the increase in the activity of oxygen and water vapor in the annealing atmosphere at 1,873 K. It is considered that the solubility limit of magnesium ions in α-alumina in equilibrium with the small amount of the spinel phase increased with the increase in the activity of oxygen and water vapor at 1,873 K. This enhanced amount of magnesium ions is frozen in a non-equilibrium state to 1,273 K and works as the enriched acceptor dopant for the incorporation of protons. © 2011 Springer-Verlag.
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Dissolution of hydrogen into LiTaO<inf>3</inf>
Ishibashi K., Okuyama Y., Kurita N., Fukatsu N.
Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals 75 ( 4 ) 229 - 234 2011年4月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals
It has been known that hydrogen can dissolve into a crystal of LiTaO 3 . However, there is not much literature about the details of the hydrogen in LiTaO 3 . In the present work, the transport properties and the location site of hydrogen in the non-stoichiometric LiTaO 3 single crystal was studied by the electrical conductivity measurement and the IR absorption analysis. H + /D + isotope effect on the conductivity was examined in order to verify whether proton in LiTaO 3 was dominant charge carrier. The result that no isotope effect was detected showed that proton was not the main charge carrier in LiTaO 3 in the measured temperature range 673-873 K. On the displacement reaction of proton and deuteron, the annealing time dependence of IR absorbance was measured in order to estimate the self-diffusion coefficient of proton. The value was determined as D H = 5.5 × 10 -8 cm 2 /s at 973 K. For single crystal of LiTaO3, the crystal orientation dependence of IR absorbance was measured using polarized light. The analysis based on the one-dimensional model of the hydrogen bond showed that proton was locating between the O-O pairs on the basal plane. The amount of dissolution hydrogen in LiTaO 3 equilibrated with gas mixture 1%O 2 -3%H 2 O-Ar at 773-973 K was estimated to be (1.3±0.4) × 10 -8 mol/cm 3 . © 2011 The Japan Institute of Metals.
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Proton conduction in Al-doped CaZrO<inf>3</inf>
Bao J., Ohno H., Kurita N., Okuyama Y., Fukatsu N.
Electrochimica Acta 56 ( 3 ) 1062 - 1068 2011年1月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Electrochimica Acta
To clarify the electrochemical properties of Al-doped CaZrO 3 , the electrical conductivity of the poly-crystalline specimen was measured by employing the two-terminal AC method. By examining the H/D isotope effect of the electrical conductivity, it was found that protons are the dominant charge carrier at temperatures from 873 to 1373 K, both in hydrogen-rich and oxygen-rich atmospheres. At higher temperatures of 1373-1673 K, the atmospheric dependence of the electrical conductivity shows that the dominant charge carrier is the positive hole in oxygen-rich atmospheres, whereas it is the oxide ion vacancy in hydrogen-rich atmospheres. © 2010 Elsevier Ltd. All rights reserved.
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αアルミナを用いた電池型センサーによる溶融Cu-Ni合金中の水素濃度測定 査読あり
奥山勇治, 栗田典明, 武津典彦
銅と銅合金 49 ( 1 ) 292 - 296 2010年8月
記述言語：日本語 掲載種別：研究論文（学術雑誌）
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Electrical conductivity of calcium-doped α-alumina
Okuyama Y., Kurita N., Fukatsu N.
Solid State Ionics 181 ( 3-4 ) 142 - 147 2010年2月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Solid State Ionics
The phenomenon of the incorporation of hydrogen into a single crystal of α-alumina nominally doped with 0.2 mol% CaO was studied by an IR absorption analysis and electrical conductivity measurement in the temperature range of 1073-1673 K. An IR wide band absorption was observed. This band was attributed to the stretching vibration of the OH bond. The H + /D + -isotope effect on the conductivity was observed. It was found that the proton was the dominant charge carrier in the calcium-doped α-alumina under a reducing atmosphere containing hydrogen. The chemical diffusion coefficient of the H + /D + diffusion pair was determined by the relaxation time of the change in the ohmic resistance upon exchanging the atmosphere from H 2 to D 2 . The amount of dissolved hydrogen was evaluated to be (3.8 ± 2.7) × 10 - 6 mol(H)/cm 3 using the determined chemical diffusion coefficient and the value of the total conductivity. © 2009 Elsevier B.V. All rights reserved.
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A new type of hydrogen sensor for molten metals usable up to 1600 K
Okuyama Y., Kurita N., Yamada A., Takami H., Ohshima T., Katahira K., Fukatsu N.
Electrochimica Acta 55 ( 2 ) 470 - 474 2009年12月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Electrochimica Acta
In order to simplify the structure of the galvanic cell-type hydrogen sensor using α-alumina as a solid electrolyte, the availability of employing air as the reference was checked by measuring the electromotive force (emf) of a gas concentration cell in the temperature range from 1073 to 1573 K. The emf was found to depend only on the hydrogen activity of the working electrode when air of normal humidity was used as the reference gas. The hydrogen activities in molten copper melted in a closed chamber were measured by immersing this structure-type hydrogen sensor at 1423 K. The measured values agreed with the hydrogen activities of the molten copper produced by supplying a H 2 /Ar gas mixture over the surface of the melt. The performance of the hydrogen sensor was also tested for molten copper melted in the open air. Reasonable emf responses were obtained by the melt treatments such as the enrichment of hydrogen by covering the surface with charcoal or the reduction of hydrogen by exposing the surface to air. These results suggest that this type of hydrogen sensor is currently the best one for the real time detecting of the amount of hydrogen in molten metal up to 1600 K. © 2009 Elsevier Ltd. All rights reserved.
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Incorporation of hydrogen in barium-doped α-alumina
Okuyama Y., Kurita N., Fukatsu N.
Solid State Ionics 180 ( 2-3 ) 175 - 182 2009年3月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Solid State Ionics
The phenomenon of the incorporation of hydrogen into a single crystal of 0.1 mol% barium-doped α-alumina grown by the Verneuil method was studied using infrared absorption and conductivity measurements. A wide band of infrared absorption was observed in the range of 2500-3500 cm - 1 for the sample annealed in H 2 /H 2 O atmosphere and in the range of 2150-2300 cm - 1 for that annealed in D 2 /D 2 O atmosphere. These absorption bands were found to be attributed to the stretching vibration of the OH and OD bonds, respectively. The solubility of hydrogen was dependent only on hydrogen partial pressure under an oxidizing atmosphere. The IR absorbance measurement using polarized light was carried out to examine the crystal orientation dependence of the integral absorbance. The maximum intensity of integral absorbance was observed when the direction of the electric field was parallel to the direction of the c-axis. The H + /D + -isotope effect on the electrical conductivity was observed under a hydrogen atmosphere, and the chemical diffusion coefficient of the H + /D + diffusion pair was determined by the relaxation time of the ohmic resistance change upon exchanging the atmosphere from H 2 to D 2 . The temperature dependence of the self-diffusion coefficient of a proton was expressed as D Hi• = (1.4 - 1.0 + 3.0 ) × 10 - 3 exp (- frac((0.82 ± 0.12) eV, k T)) cm 2 s - 1 in the temperature range 1073-1373 K. The amount of dissolved hydrogen was estimated to be (1.0 ± 0.3) × 10 - 6 mol cm - 3 using the self-diffusion coefficient and the value of conductivity. © 2008 Elsevier B.V. All rights reserved.
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Diffusion of proton in alumina-rich nonstoichiometric magnesium aluminate spinel
Okuyama Y., Kurita N., Fukatsu N.
Ionics 15 ( 1 ) 43 - 48 2009年2月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Ionics
The phenomenon of the diffusion of proton and deuteron in a single crystal of magnesium aluminate spinel was studied by infrared absorption. The chemical diffusion coefficient of proton was determined by the relaxation time of the absorption intensity upon the substitution of deuteron with proton. The temperature dependence of the chemical diffusion coefficient of proton for Mg 1-x Al 2 O 4 δ x = 0.34 was expressed as ̃ D = 0.16\exp - 210 × 10 3 J mol 1 K 1 vphantom 210 × 10 3 mol 1 K 1 RT. kern-nulldelimiterspaceRT cm 2 /s. The chemical diffusion coefficient of proton was found to be independent of the composition of spinel and of the atmosphere. © 2008 Springer-Verlag.
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Okuyama Y., Kurita N., Fukatsu N.
Materials Transactions 49 ( 1 ) 187 - 192 2008年1月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Materials Transactions
The following gas concentration cell, (-,work.), Pt(P′ O2 , P′ H2 )Iota;Mg 0.8 Al 2 O 4-δ ΙPt(P″ O2 , P″H 2 ), (ref.,+) was constructed and the electromotive force was measured in the temperature range from 1373 to 1673 K. When the oxygen partial pressure on both electrodes was kept at a constant value, hydrogen partial pressure difference on the electrodes did not give any significant electromotive force at the experimental temperatures. When the hydrogen partial pressures were kept at a constant value on both electrodes, the electromotive force dependent on the difference of the oxygen partial pressure on the electrodes was obtained. The electromotive force was in agreement with the theoretical value calculated on assumption that the activity of magnesia was kept constant throughout in the electrolyte and that the transport number of cation was unity. © 2008 The Japan Institute of Metals.
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Defect structure of alumina-rich nonstoichiometric magnesium aluminate spinel
Okuyama Y., Kurita N., Fukatsu N.
Solid State Ionics 177 ( 1-2 ) 59 - 64 2006年1月
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Solid State Ionics
In order to clarify the relation between the dissolution mechanism of hydrogen and the defect structure in alumina-rich nonstoichiometric magnesium aluminate spinels, several compositions of single crystals were grown by the flame fusion method and their compositional dependence of the solubility of hydrogen and that of the mass density were studied by the IR absorption and Archimedes methods, respectively. The solubility of hydrogen increased in proportion to the square root of the water partial pressure and with the increase in the excess amount of alumina. The densities of the single crystals were slightly less than the value calculated based on the reported defect structure. The compositional dependence of the solubility of hydrogen and that of the density can be explained by the model in which the concentration of the oxide ion vacancy increases with the alumina content due to the decrease in the capability of the Mg site to accept the excess Al ion. © 2005 Elsevier B.V. All rights reserved.
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Electrical conductivity of barium-doped a-alumina
Okuyama Y., Kurita N., Fukatsu N.
Advanced Materials Research 11-12 145 - 148 2006年
記述言語：日本語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Advanced Materials Research
The phenomenon of the incorporation of hydrogen into a single crystal of 0.1 mol% Ba-doped α-alumina was studied using IR absorption analysis. A large wide band of IR absorption was observed. This band was attributed to the stretching vibration of the OH bond. The electrical conductivity of the specimens has been measured by the two-probe ac technique in the temperature range 1073-1673K. The H/D-isotope effect on the conductivity was observed. It was found that the dominant charge carrier of barium doped α-alumina is proton under the reducing atmosphere containing hydrogen.