Papers - OKUYAMA Yuji
-
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 2013.7
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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
-
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 2013.5
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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.
-
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 2013.4
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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.
-
Proton-conducting oxide and applications to hydrogen energy devices
Matsumoto H., Sakai T., Okuyama Y.
Pure and Applied Chemistry 85 ( 2 ) 427 - 435 2013.2
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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.
-
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 2012.11
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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.
-
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 2012.10
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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.
-
Sakai T., Matsushita S., Hyodo J., Okuyama Y., Matsuka M., Ishihara T., Matsumoto H.
Electrochimica Acta 75 179 - 184 2012.7
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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.
-
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 2012.4
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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.
-
Bao J., Ohno H., Okuyama Y., Fukatsu N., Kurita N.
Materials Transactions 53 ( 4 ) 752 - 759 2012.4
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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.
-
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 2012.4
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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.
-
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 2012.1
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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.
-
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 2011.4
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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.
-
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 2011.1
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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.
-
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 Reviewed
奥山勇治, 栗田典明, 武津典彦
銅と銅合金 49 ( 1 ) 292 - 296 2010.8
Language：Japanese Publishing type：Research paper (scientific journal)
-
Electrical conductivity of calcium-doped α-alumina
Okuyama Y., Kurita N., Fukatsu N.
Solid State Ionics 181 ( 3-4 ) 142 - 147 2010.2
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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.
-
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 2009.12
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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.
-
Incorporation of hydrogen in barium-doped α-alumina
Okuyama Y., Kurita N., Fukatsu N.
Solid State Ionics 180 ( 2-3 ) 175 - 182 2009.3
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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.
-
Diffusion of proton in alumina-rich nonstoichiometric magnesium aluminate spinel
Okuyama Y., Kurita N., Fukatsu N.
Ionics 15 ( 1 ) 43 - 48 2009.2
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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.
-
Okuyama Y., Kurita N., Fukatsu N.
Materials Transactions 49 ( 1 ) 187 - 192 2008.1
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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.
-
Defect structure of alumina-rich nonstoichiometric magnesium aluminate spinel
Okuyama Y., Kurita N., Fukatsu N.
Solid State Ionics 177 ( 1-2 ) 59 - 64 2006.1
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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.