Papers - OKUYAMA Yuji
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OKUYAMA Yuji
Denki Kagaku 93 ( 1 ) 43 - 43 2025.3
Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Electrochemical Society of Japan
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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 2025.3
Language：English Publishing type：Research paper (scientific journal) Publisher：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 2024.12
Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：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 2024.9
Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：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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Development of defect structure-type proton conductors and its application to sensors and fuel cells Invited Reviewed
OKUYAMA Yuji
Oyo Buturi 93 ( 3 ) 149 - 155 2024.3
Authorship：Lead author, Corresponding author Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Japan Society of Applied Physics
Defect structure-type proton conductors are ceramics in which hydrogen ions (protons) dissolve in oxides and diffuse in the crystal. The material has attracted much 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 explained, and the application to hydrogen sensors and fuel cells will be introduced.
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Yuichi Mikami, Yuki Sekitani, Kosuke Yamauchi, Tomohiro Kuroha, Yuji Okuyama
ACS Applied Energy Materials 7 ( 3 ) 1136 - 1148 2024.2
Authorship：Corresponding author Publishing type：Research paper (scientific journal)
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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Hiroyuki Shimada, Yuichi Mikami, Kosuke Yamauchi, Tomohiro Kuroha, Takayasu Uchi, Kazuo Nakamura, Shun Kobayashi, Ryuma Malik Matsuda, Yuji Okuyama, Yasunobu Mizutani, Masashi Mori
Ceramics International 50 ( 2 ) 3895 - 3901 2024.1
Publishing type：Research paper (scientific journal) Publisher：Elsevier BV
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Development of defect-structure-type proton conductors and their application to sensors and fuel cells Reviewed
Okuyama Yuji
JSAP Review 2024 ( 0 ) n/a 2024
Authorship：Lead author, Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：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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Yuji Okuyama, Keita Kasuga, Masaki Shimomura, Yuichi Mikami, Kosuke Yamauchi, Tomohiro Kuroha, Hirofumi Sumi
Journal of Power Sources 586 2023.12
Authorship：Lead author, Corresponding author Publishing type：Research paper (scientific journal)
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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Kunpeng Li, Hiroyuki Shimada, Yasunobu Mizutani, Yuji Okuyama, Takuto Araki
Energy Conversion and Management 296 2023.11
Publishing type：Research paper (scientific journal)
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 Reviewed
Hirofumi Sumi, Hiroyuki Shimada, Konosuke Watanabe, Yuki Yamaguchi, Katsuhiro Nomura, Yasunobu Mizutani, Ryuma Malik Matsuda, Masashi Mori, Keiji Yashiro, Takuto Araki, Yuji Okuyama
Journal of Power Sources 582 233528 - 233528 2023.10
Authorship：Last author Publishing type：Research paper (scientific journal) Publisher：Elsevier BV
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Mechanisms of photoluminescence in cerium oxide pellets with different types of defects Reviewed
Masaya Nagai, Yuto Furutani, Hikaru Takehara, Masaaki Ashida, Yuji Okuyama, Yukimune Kani
Journal of Luminescence 260 119859 - 119859 2023.8
Publishing type：Research paper (scientific journal) Publisher：Elsevier BV
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Phonons Damped by Proton Doping in Barium Zirconate Reviewed
Hikaru Takehara, Masaya Nagai, Masaaki Ashida, Yuji Okuyama, Yukimune Kani
The Journal of Physical Chemistry C 127 ( 23 ) 10913 - 10921 2023.6
Publishing type：Research paper (scientific journal) Publisher：American Chemical Society (ACS)
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Hirofumi Sumi, Hiroyuki Shimada, Konosuke Watanabe, Yuki Yamaguchi, Katsuhiro Nomura, Yasunobu Mizutani, Yuji Okuyama
ACS Applied Energy Materials 6 ( 3 ) 1853 - 1861 2023.2
Authorship：Last author Language：Japanese Publishing type：Research paper (scientific journal) Publisher：American Chemical Society (ACS)
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Nomura, Katsuhiro, Shimada, Hiroyuki, Yamaguchi, Yuki, Sumi, Hirofumi, Mizutani, Yasunobu, Okuyama, Yuji, Shin, Woosuck
Ceramics International 49 ( 3 ) 5058 - 5065 2023.2
Language：Japanese Publishing type：Research paper (scientific journal)
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Yuji Okuyama, Yoshiaki Harada, Yuichi Mikami, Kosuke Yamauchi, Tomohiro Kuroha, Hiroyuki Shimada, Yuki Yamaguchi, Yasunobu Mizutani
Journal of the Electrochemical Society 170 ( 8 ) 2023
Authorship：Lead author, Corresponding author Publishing type：Research paper (scientific journal)
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 Reviewed
Masaya Nagai, Hikaru Takehara, Masaaki Ashida, Yuji Okuyama, Yukimune Kani
International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz 2023
Publishing type：Research paper (scientific journal)
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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Kwati Leonard, Yuji Okuyama, Mariya E. Ivanova, Wilhelm A. Meulenberg, Hiroshige Matsumoto
ChemElectroChem 9 ( 4 ) 2022.2
Language：English Publishing type：Research paper (scientific journal)
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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Katsuhiro Nomura, Hiroyuki Shimada, Yuki Yamaguchi, Woosuck Shin, Yuji Okuyama, Yasunobu Mizutani
Journal of the Electrochemical Society 169 ( 2 ) 2022.2
Language：Japanese Publishing type：Research paper (scientific journal)
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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Hikaru Takehara, Masaya Nagai, Masaaki Ashida, Yuji Okuyama, Yukimune Kani
Journal of Physical Chemistry C 127 ( 2 ) 968 - 976 2022
Language：Japanese Publishing type：Research paper (scientific journal)
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.