Papers - SAKAI Go
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Matsunaga Naoki, Waseda Kousei, Kuroki Masato, Nomura Kouki, Sakai Go
54 1 - 5 2025.10
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：Faculty of Engineering, University of Miyazaki
DOI： 10.34481/0002001934
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Matsunaga N., Kikuchi K., Tokunaga K., Kohno D., Sakai G.
Bulletin of Materials Science 46 ( 3 ) 2023.9
Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：Bulletin of Materials Science
The present study was conducted to prepare precursors by rapid hydrolysis method for realizing lower temperature formation of NiFe2O4 under hydrothermal conditions. The precursor, obtained from a lower concentration of NiCl2–FeCl2 mixed solution, was almost amorphous and could be easily converted to NiFe2O4 crystal phase at around 130°C by hydrothermal treatment. On the other hand, when a higher concentration of NiCl2–FeCl2 solution or trivalent iron salt (FeCl3) was used as starting reagents, individual crystal phases such as α-Ni(OH)2, γ-Fe2O3 and α-FeOOH were recognized in precursors owing to the difference in hydrolysis rates between Ni2+ and Fe2+ (or Fe3+). These individual crystal phases involved precursors that could not be converted easily to NiFe2O4 crystal phase, but needed to treat at higher temperatures for forming NiFe2O4 at least 200°C. Thus, the co-existence of individual crystal phases in precursors might prevent the amorphous precursor from forming NiFe2O4 crystal phase. The formation of NiFe2O4 at lower temperatures is considered to be taken place preferentially from amorphous precursors than individual certain crystal phases.
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Optimum dopant of barium zirconate electrolyte for manufacturing of protonic ceramic fuel cells Reviewed
Kuroha T., Niina Y., Shudo M., Sakai G., Matsunaga N., Goto T., Yamauchi K., Mikami Y., Okuyama Y.
Journal of Power Sources 506 2021.9
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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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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 2020.1
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：International Journal of Hydrogen Energy
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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Experimental and theoretical approaches for the investigation of proton conductive characteristics of La1-xBaxYbO3-d Reviewed
Yuki Obukuro, Yuji Okuyama, Go Sakai, Shigenori Matsushima
Journal of Alloys and Compounds 770 294 - 300 2019
Language：English Publishing type：Research paper (scientific journal)
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Okuyama Yuji, Ymaguchi Takuya, Matsunaga Naoki, Sakai Go
MATERIALS TRANSACTIONS 59 ( 1 ) 14 - 18 2018
Language：English Publishing type：Research paper (scientific journal) Publisher：公益社団法人 日本金属学会
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<sub>1−</sub><i><sub>x</sub></i>Ba<i><sub>x</sub></i>Yb<sub>0.5</sub>In<sub>0.5</sub>O<sub>3−δ</sub> (x = 0.1. 0.3. 0.5, 0.7). The phase transition from the orthorhombic system to the cubic system was over <i>x</i> = 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 influence on the conduction and incorporation of protons into the La<sub>1−</sub><i><sub>x</sub></i>Ba<i><sub>x</sub></i>Yb<sub>0.5</sub>In<sub>0.5</sub>O<sub>3−δ</sub>.
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A direct methane fuel cell with double-layered electrolyte using proton conducting oxide Reviewed
Okuyama Y., Kawano S., Sakai G., Matsunaga N., Mizutani Y.
ECS Transactions 78 ( 1 ) 1953 - 1961 2017.5
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：ECS Transactions
© The Electrochemical Society. BaCe0.8Y0.2O3-δ(BCY) has high proton conductivity, but is less stable against carbon dioxide. La0.9Sr0.1Yb0.8In0.2O3-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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Obukuro Y., Ninomiya K., Arai M., Okuyama Y., Sakai G., Matsushima S.
Computational Materials Science 126 7 - 11 2017.1
Language：English Publishing type：Research paper (scientific journal) Publisher：Computational Materials Science
© 2016 Elsevier B.V. The electronic structure of LaYbO 3 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 LaYbO 3 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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Proton-conducting oxide with redox protonation and its application to a hydrogen sensor with a self standard electrode Reviewed
Yuji Okuyama, Shinya Nagamine, Akira Nakajima, Go Sakai, Naoki Matsunaga, Fusako Takahashi, Koji Kimata, Tomoko Oshima, Koji Tsuneyoshi
RSC Advances 6 34019 - 34026 2016.3
Language：English Publishing type：Research paper (scientific journal)
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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 2016.2
Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Alloys and Compounds
© 2015 Elsevier B.V. Single-phase Sr 2 Bi 2 O 5 was obtained by calcination of the heterobimetallic complex precursor Sr[Bi(DTPA)]·9H 2 O (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 Sr 2 Bi 2 O 5 samples prepared from Sr[Bi(DTPA)]·9H 2 O 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 Sr 6 Bi 2 O 9 and SrCO 3 were observed in Sr 2 Bi 2 O 5 samples with larger La contents of 8 and 10 mol%. The Brunauer-Emmett-Teller surface area of Sr 2 Bi 2 O 5 increased slightly with the content of La. High-resolution transmission electron microscopy revealed clear crystalline planes for both undoped and La-doped Sr 2 Bi 2 O 5 samples without amorphous phases. Meanwhile, X-ray photoelectron spectroscopy indicated that the valence states of constituent metals were Sr 2+ , Bi 3+ , and La 3+ . The solubility limit of La in the Sr 2 Bi 2 O 5 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 Sr 2 Bi 2 O 5 could be modified by La doping, resulting in a red shift of the absorption edge with increasing La content. The band-gap narrowing of Sr 2 Bi 2 O 5 with La-doping was reproduced in energy-band calculations. The photocatalytic activity of Sr 2 Bi 2 O 5 under visible-light irradiation for the oxidation of isopropanol was enhanced by doping with La.
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Electronic structure of photoresponsive Ag<sub>6</sub>M<sub>2</sub>O<sub>7</sub> (M = Si, Ge) Reviewed
OBUKURO Yuki, NINOMIYA Kakeru, MATSUSHIMA Shigenori, NAKAMURA Hiroyuki, OBATA Kenji, SAKAI Go, ARAI Masao, KOBAYASHI Kenkichiro
Journal of the Ceramic Society of Japan 124 ( 1 ) 116 - 121 2016.1
Language：English Publishing type：Research paper (scientific journal) Publisher：The Ceramic Society of Japan
The electronic structures of Ag<sub>6</sub>M<sub>2</sub>O<sub>7</sub> (M = Si, Ge) are investigated by the scalar-relativistic full potential linearized augmented plane wave plus local orbital (FLAPW+lo) method using the modified Becke-Johnson (MBJ) potential combined with the local density approximation (LDA) correlation. For Ag<sub>6</sub>M<sub>2</sub>O<sub>7</sub> (M = Si, Ge), the valence band maximums (VBM) are approximately located at the X (Si) or A (Ge) and the conduction band minimums (CBM) at the Γ both, indicating that Ag<sub>6</sub>M<sub>2</sub>O<sub>7</sub> are an indirect energy gap material. The fundamental band gaps of Ag<sub>6</sub>Si<sub>2</sub>O<sub>7</sub> and Ag<sub>6</sub>Ge<sub>2</sub>O<sub>7</sub> are calculated to be 1.69 and 1.42 eV, respectively, in the MBJ-LDA calculation. The results are a remarkably contrast to the underestimation based on the generalized gradient approximation (GGA) calculation. On the other hand, there is no distinct difference in the effective masses of photogenerated holes and electrons near the VBM and CBM between MBJ-LDA and GGA approaches. The optical properties of Ag<sub>6</sub>M<sub>2</sub>O<sub>7</sub> (M = Si, Ge) are contemplated from spectral dependence of the complex dielectric function, ε (ω) = ε<sub>1</sub> (ω) + <i>i</i>ε<sub>2</sub> (ω).
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Sakai G., Tanaka A., Sueda T., Ogata T., Okuyama Y., Matsunaga N.
Chemistry Letters 45 ( 3 ) 318 - 320 2016.1
Language：English Publishing type：Research paper (scientific journal) Publisher：Chemistry Letters
© 2016 The Chemical Society of Japan. Crystal growth under hydrothermal treatment at 150 °C was observed for the pre-calcined TiO 2 -SnO 2 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
Language：English Publishing type：Research paper (scientific journal) Publisher：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 CaZrO 3 were evaluated by impedance analysis, EMF measurement, IR absorption analysis and ESR measurement. Mn-doped CaZrO 3 acquires a proton from hydrogen by reduction of the manganese ion. 5 mol% Mn doped CaZrO 3 showed proton conduction in a reducing atmosphere and hole conduction in an oxidizing atmosphere. A gas concentration cell using CaZr 0.95 Mn 0.05 O 3-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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CO<inf>2</inf> sensing properties of Zr-added porous CaFe<inf>2</inf>O<inf>4</inf> powder Reviewed
Obata K., Mizuta K., Obukuro Y., Sakai G., Hagiwara H., Ishihara T., Matsushima S.
Sensors and Materials 28 ( 11 ) 1157 - 1164 2016
Language：English Publishing type：Research paper (scientific journal) Publisher：Sensors and Materials
© MYU K.K. The gas sensing properties of Zr-added and pure CaFe 2 O 4 powders for CO 2 in air were examined in the temperature range of 250-450 °C. The semiconductor-type gas sensor made from pure CaFe 2 O 4 powder showed a fairly good response to CO 2 . Furthermore, the addition of a small amount of Zr into CaFe 2 O 4 powder was found to be effective for enhancing the CO 2 response of the present gas sensor. It was also found that the gas response, defined by the ratio of the resistance in air and that of the target gas reached maximum at the operating temperature of 300 °C. The gas response of the Zr-added CaFe 2 O 4 -based sensor at 300 °C was estimated to be 2.5 times higher than that of the sensor made from pure CaFe 2 O 4 powder. However, the 90% response time of the Zr-added CaFe 2 O 4 -based sensor was much quicker at 350 °C than that at 300 °C. Thus, the optimal gas sensing performance of the Zr-added CaFe 2 O 4 -based sensor is expected to be obtained at the operating temperature of 350 °C, considering the still higher response to CO 2 gas at this temperature. It is noted that the present CaFe 2 O 4 -based sensor responded reversibly as well as continuously to CO 2 gas. Infrared analysis revealed that the sensing mechanism of the present CaFe 2 O 4 -based sensor is the change in the electric resistance of CaFe 2 O 4 caused by reactive CO 2 adsorption with negatively charged oxide ions (O - ) resulting in the increase in the hole concentration in the base material of CaFe 2 O 4 .
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Formation of CaFe2O4 porous structure by addition of Zr in malic acid complex Reviewed
Yuki Obukuro, Kenji Obata, Ryosuke Maeda, Shigenori Matsushima, Yuji Okuyama, Naoki Matsunaga, Go Sakai*
Journal of the Ceramic Society of Japan 123 ( 10 ) 995 - 998 2015.10
Language：English Publishing type：Research paper (scientific journal)
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Preparation and Characterization of Heterobimetallic Complex, Sr[Bi(DTPA)]・9H2O, Derived La-doped Sr2Bi2O5 Reviewed
Yuki Obukuro, Go Sakai*, Yuji Okuyama, Naoki Matsunaga, Shigenori Matsushima, Kenji Obata, Eiji Asato
Chemistry Letters 44 ( 7 ) 890 - 892 2015.7
Language：English Publishing type：Research paper (scientific journal)
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酒井 剛, 林 勝義
Electrochemistry 83 ( 4 ) 268 - 268 2015
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Electrochemical Society of Japan
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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
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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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Formation of CaFe<sub>2</sub>O<sub>4</sub> porous structure by addition of Zr in malic acid complex
OBUKURO Yuki, OBATA Kenji, MAEDA Ryosuke, MATSUSHIMA Shigenori, OKUYAMA Yuji, MATSUNAGA Naoki, SAKAI Go
Journal of the Ceramic Society of Japan 123 ( 1442 ) 995 - 998 2015
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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 CaFe<sub>2</sub>O<sub>4</sub>. The CaFe<sub>2</sub>O<sub>4</sub> 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 CaFe<sub>2</sub>O<sub>4</sub> 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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MATSUNAGA Naoki, SUGAHARA Mariko, SAKAI Go
Journal of the Ceramic Society of Japan 122 ( 1428 ) 613 - 617 2014.8
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Ceramic Society of Japan
Synthesis of <i>c</i>-axis grown β-Ni(OH)<sub>2</sub> was attempted by rapid hydrolysis and/or direct hydrothermal treatment of the mixed solution of NiCl<sub>2</sub>, HCl and NH<sub>3</sub>. The UV–vis spectra revealed the addition of NH<sub>3</sub> into NiCl<sub>2</sub> solution shifted the peak position to shorter wavelength, indicating the formation of nickel ammine complex. Hydrolysis of the mixed solution containing nickel ammine complex gave β-Ni(OH)<sub>2</sub> particles. The amount of NH<sub>3</sub> in the mixed solution did not affect the crystallinity or morphology of the β-Ni(OH)<sub>2</sub> particle by the conventional rapid hydrolysis. Hydrothermal treatment of the hydrolyzed β-Ni(OH)<sub>2</sub> particles was changed the morphology from agglomerated particles to nanosheets-linked structure or hexagonal plates. However, any crystal growth or particular-axis growth was not recognized. Then, the direct hydrothermal treatment of the mixed solution was attempted. The direct treatment at certain amount of coexisting NH<sub>3</sub> gave small amount of the precipitates, and the precipitates showed distinctive XRD pattern of <i>c</i>-axis grown β-Ni(OH)<sub>2</sub>. The electrochemical property of the <i>c</i>-axis grown β-Ni(OH)<sub>2</sub> structures indicated that the diffusion control was the rate-determining step for the charge/discharge process.