Browsing by Author "Ikeda, Y"

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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