Papers - OKUYAMA Yuji
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Fast Ionic Conduction in Solid Electrolyte of Fuel Cells Revealed Using Coherent THz Pulses
NAGAI Masaya, OKUYAMA Yuji, KANI Yukimune
The Review of Laser Engineering 50 ( 4 ) 198 2022
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Laser Society of Japan
DOI： 10.2184/lsj.50.4_198
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Evaluation of the Oxygen Migration Energy in Doped Ceria by Terahertz Spectroscopy Reviewed
Masaya Nagai, Yuto Furutani, Hikaru Takehara, Tomohide Morimoto, Masaaki Ashida, Yuji Okuyama, Yukimune Kani
Physical Review Applied 16 ( 6 ) 2021.12
Language：English Publishing type：Research paper (scientific journal)
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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Junji Hyodo, Kota Tsujikawa, Motoki Shiga, Yuji Okuyama, Yoshihiro Yamazaki
ACS ENERGY LETTERS 6 ( 8 ) 2985 - 2992 2021.8
Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC
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-delta demonstrated proton incorporation and conduction. The results are promising for syrthesis 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 Reviewed
Kuroha T., Niina Y., Shudo M., Sakai G., Matsunaga N., Goto T., Yamauchi K., Mikami Y., Okuyama Y.
Journal of Power Sources 506 2021.6
Authorship：Last author, Corresponding author Language：English 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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Hirofumi Sumi, Hiroyuki Shimada, Yuki Yamaguchi, Yasunobu Mizutani, Yuji Okuyama, Koji Amezawa
SCIENTIFIC REPORTS 11 ( 1 ) 10622 2021.4
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：NATURE RESEARCH
Protonic ceramic fuel cells (PCFCs) are expected to achieve high power generation efficiency at intermediate temperature around 400-600 degrees 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 degrees C excluding the gas diffusion process for ScSZ and BCY. The DRT peaks at 5x10(2)-1x10(4) Hz and 1x10(0)-2x10(2) 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 2x10(1)-1x10(3) 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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Naoki Tanimoto, Masaya Nagai, Masaaki Ashida, Yuji Okuyama, Yukimune Kani
Japanese Journal of Applied Physics 60 ( 3 ) 032004 - 032004 2021.3
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：IOP Publishing
DOI： 10.35848/1347-4065/abdf7a
Other Link： 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
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：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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Junji Hyodo, Shogo Kato, Shintaro Ida, Tatsumi Ishihara, Yuji Okuyama, Takaaki Sakai
JOURNAL OF PHYSICAL CHEMISTRY C 125 ( 3 ) 1703 - 1713 2021
Authorship：Corresponding author Language：Japanese Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC
Proton-conducting oxides have garnered much attention as electrolyte materials of electrochemical devices operating at intermediate temperatures ranging 400-700 degrees 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 degrees 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 degrees C, suggesting that the Ba-doped LaYbO3 is suitable for the SOFC electrolyte in terms of transport number.
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Kunpeng Li, Takuto Araki, Toshiki Kawamura, Atsuhito Ota, Yuji Okuyama
INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 45 ( 58 ) 34139 - 34149 2020.11
Authorship：Last author Language：Japanese Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD
Protonic ceramic fuel cells (PCFCs) operating at intermediate temperatures (400 degrees C similar to 600 degrees C) are expected to demonstrate a high performance due to proton (OHO center dot) conductivity. However, the current efficiencies of PCFCs are bound to decrease due to electron holes (h(center dot)) 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. (c) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Yoshihiro Yamazaki, Akihide Kuwabara, Junji Hyodo, Yuji Okuyama, Craig A. J. Fisher, Sossina M. Haile
CHEMISTRY OF MATERIALS 32 ( 17 ) 7292 - 7300 2020.9
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC
Proton-conducting oxides, specifically doped barium zirconates, have garnered much attention as electrolytes for solid-state electrochemical devices operable at intermediate temperatures (400-600 degrees C). In chemical terms, hydration energy, E-hyd, and proton-dopant association energy, E-hyd, 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, E-O.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 E-hyd, E-as, E-O.dopant, and the hydrogen affinity, E-H.host, of each system. These four energy terms are related by E-hyd = E-O.dopant 2E(H.host )2E(as). Complementary impedance spectroscopy measurements reveal that the stronger the calculated oxygen affinity of a system, the higher the proton conductivity at 350 degrees 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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Tomohiro Kuroha, Kosuke Yamauchi, Yuichi Mikami, Yoichiro Tsuji, Yoshiki Niina, Mizuki Shudo, Go Sakai, Naoki Matsunaga, Yuji Okuyama
INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 45 ( 4 ) 3123 - 3131 2020.1
Authorship：Last author, Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD
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-delta as an electrolyte. The maximum power density was 0.143 W cm(-2) and the ohmic resistance of the electrolyte was 0.91 Omega cm(2) 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-delta, 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. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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4-12 Hydrogen Sensor Using Solid Electrolyte for a Vacuum Carburizing Furnace
IWAI Tsubasa, TSUNEYOSHI Koji, OKUYAMA Yuji
Proceedings of the Annual Conference of The Japan Institute of Energy 29 ( 0 ) 132 - 133 2020
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Japan Institute of Energy
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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Junji Hyodo, Koki Kitabayashi, Kenta Hoshino, Yuji Okuyama, Yoshihiro Yamazaki
ADVANCED ENERGY MATERIALS 10 ( 25 ) 2020
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：WILEY-V C H VERLAG GMBH
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 degrees 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 degrees 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 degrees 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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Microscopic ion migration in solid electrolytes revealed by terahertz time-domain spectroscopy. Reviewed
Morimoto T, Nagai M, Minowa Y, Ashida M, Yokotani Y, Okuyama Y, Kani Y
Nature communications 10 ( 1 ) 2662 2019.6
Language：English Publishing type：Research paper (scientific journal)
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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 2019.6
Language：English Publishing type：Research paper (scientific journal) Publisher：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 2019.1
Language：English Publishing type：Research paper (scientific journal) Publisher：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
Language：English Publishing type：Research paper (international conference proceedings) Publisher：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 Reviewed
Morimoto T., Nagai M., Ashida M., Yokotani Y., Okuyama Y., Kani Y.
International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz 2018-September 2018.10
Language：English Publishing type：Research paper (international conference proceedings) Publisher：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 2018.1
Language：English Publishing type：Research paper (scientific journal) Publisher：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