Browsing by Author "Murakami, T"

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    Dimer-mediated cooperative mechanism of ultrafast-ion conduction in hexagonal perovskite-related oxides
    (American Chemical Society, 2023-11-14) Sakuda, Y; Murakami, T; Avdeev, M; Fujii, K; Yasui, Y; Hester, JR; Hagihala, M; Ikeda, Y; Nambu, Y; Yashima, M
    Oxide-ion and proton conductors have found diverse applications such as in electrolytes of solid-oxide, proton-conducting, and hybrid-ion fuel cells. Research of fuel cells with higher energy efficiency at lower operating temperature has stimulated the search for ion conductors and improved the understanding of the ion-diffusion mechanism. Ion conduction in hexagonal perovskite-related materials is rare, and the mechanism of ion diffusion is unclear. Herein, we report high oxide-ion and proton conductivity (bulk conductivities in wet air: 11 and 2.7 mS cm-1 at 537 and 326 °C, respectively), high chemical, and electrical stability in a new hexagonal perovskite-related oxide Ba7Nb3.8Mo1.2O20.1. Total direct current conductivity at 400 °C in wet air of Ba7Nb3.8Mo1.2O20.1 was 13 times higher than that of Ba7Nb4MoO20. We also report a unique dimer-mediated cooperative mechanism of the high oxide-ion conduction of Ba7Nb3.8Mo1.2O20.1 (bulk conductivities in dry air: 10 mS cm-1 at 593 °C and 1.1 mS cm-1 at 306 °C). Ab initio molecular dynamics (AIMD) simulations, neutron-diffraction experiments at 800 °C, and neutron scattering length density analyses of Ba7Nb3.8Mo1.2O20.1 indicated that the excess oxygen atoms are incorporated by the formation of both 5-fold coordinated (Nb/Mo)O5 monomer and its (Nb/Mo)2O9 dimer with a corner-sharing oxygen atom and that the breaking and reforming of the dimers lead to the high oxide-ion conduction in the oxygen-deficient BaO2.1 c′ layer. The long distance between Nb/Mo and Ba cations sandwiching the c′ layer of Ba7Nb3.8Mo1.2O20.1 was found to be responsible for its low activation energy for oxide-ion conduction, leading to high conductivity at low temperatures. AIMD simulations showed that high proton conduction can be attributed to proton migration in the hexagonal close-packed BaO3 layers of Ba7Nb3.8Mo1.2O20.1. The present findings hold a great promise for the development and design of ion conductors. Copyright © 2023 The Authors. Published by American Chemical Society.
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    High proton conduction in Ba2LuAlO5 with highly oxygen-deficient layers
    (Springer Nature, 2023-06-06) Morikawa, R; Murakami, T; Fujii, K; Avdeev, M; Ikeda, Y; Nambu, Y; Yashima, M
    Proton conductors have found diverse applications, such as electrolytes in proton ceramic fuel cells, which require high ionic conductivity at low temperatures and high chemical stability. Here, we report the oxide, Ba2LuAlO5, which exhibits proton conductivities of 10−2 S cm−1 at 487 °C and 1.5 × 10−3 S cm−1 at 232 °C, high diffusivity and high chemical stability without chemical doping. Ba2LuAlO5 is a hexagonal perovskite-related oxide with highly oxygen-deficient hexagonal close-packed h′ layers, which enables a large amount of water uptake x = 0.50 in Ba2LuAlO5·x H2O. Ab initio molecular dynamics simulations and neutron diffraction show the hydration in the h′ layer and proton migration mainly around cubic close-packed c layers existing at the interface of octahedral LuO6 layers. These results demonstrate that the high proton conduction allowed by the highly oxygen-deficient and cubic close-packed layers is a promising strategy for the development of high-performance proton conductors. © 2023 The Authors - Open Access CC BY
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    High proton conductivity in Ba5Er2Al2ZrO13, a hexagonal perovskite-related oxide with intrinsically oxygen-deficient layers
    (American Chemical Society, 2020-05-15) Murakami, T; Hester, JR; Yashima, M
    For the development of proton-based electrolytes, high proton conductivity at intermediate temperatures (300–600 °C) is crucial, but the available materials have been confined to a limited number of the structure families, such as cubic perovskites. Herein, we report Ba5Er2Al2ZrO13, a hexagonal perovskite-related oxide, as a new class of proton conductors exhibiting higher conductivities than 10–3 S cm–1 between 300 and 1200 °C. The protons as charge carriers are found to exist in the inherently oxygen-deficient h′ layer of Ba5Er2Al2ZrO13, which are supported by Rietveld analysis of neutron-diffraction data, bond-valence-based energy calculations, and thermogravimetric analysis. Our discovery of a new structure family of proton conductors with the inherently oxygen-deficient h′ layer offers a strategy in designing superior proton conductors based on hexagonal perovskite-related oxides. © 2020 American Chemical Society
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    High proton conductivity in β-Ba2ScAlO5 enabled by octahedral and intrinsically oxygen-deficient layers
    (John Wiley & Sons Inc., 2022-12-19) Murakami, T; Avdeev, M; Morikawa, R; Hester, JR; Yashima, M
    Proton conductors are promising materials for clean energy, but most available materials exhibit sufficient conductivity only when chemically substituted to create oxygen vacancies, which often leads to difficulty in sample preparation and chemical instability. Recently, proton conductors based on hexagonal perovskite-related oxides have been attracting attention as they exhibit high proton conductivity even without the chemical substitutions. However, their conduction mechanism has been elusive so far. Herein, taking three types of oxides with different stacking patterns of oxygen-deficient layers (β-Ba2ScAlO5, α-Ba2Sc0.83Al1.17O5, and BaAl2O4) as examples, the roles of close-packed double-octahedral layers and oxygen-deficient layers in proton conduction are shown. It is found that “undoped” β-Ba2ScAlO5, which adopts a structure having alternating double-octahedral layer and double-tetrahedral layer with intrinsically oxygen-deficient hexagonal BaO (h') layer, shows high proton conductivity (≈10−3 S cm−1 above 300 °C), comparable to representative proton conductors. In contrast, the structurally related oxides α-Ba2Sc0.83Al1.17O5 and BaAl2O4 exhibit lower conductivity. Ab initio molecular dynamics simulations revealed that protons in β-Ba2ScAlO5 migrate through the double-octahedral layer, while the h′ layer plays the role of a “proton reservoir” that supplies proton carriers to the proton-conducting double-octahedral layers. The distinct roles of the two layers in proton conduction provide a strategy for developing high-performance proton conductors. © 2022 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH.