Local structural analysis on hydration behavior in doped AZrO3 (A = Ba, Ca) protonic conductors

Conference Dates

March 10-14, 2019


Perovskite-type protonic conductors are candidate electrolytes for intermediate temperature solid oxide fuel cells. For practical application, further improvement in proton conductivity is needed. Proton migration in these materials has been reported to be limited by trapping effects of protons by acceptor dopants [1]. In addition, it is suggested that the formation of percolation path with the trapped protons can lower the activation barrier for long-range proton migration and result in enhancement of proton conductivity. The formation of this percolation path is dominated by the location of protons and their concentrations, and higher proton concentration than the percolation limit is necessary for long range transport of protons. To increase or control the proton concentration in the material, understanding of hydration behavior is important because protons are incorporated into the material via hydration reaction of water vapor and oxygen vacancies. Therefore, this study focuses on investigation of hydration behavior from the local structural viewpoint with respect to protons and oxygen vacancies to explore a strategy to improve proton conductivity. For this purpose, solid-state nuclear magnetic resonance combined with density functional theory (DFT) calculations is used to elucidate the local structure of protons and oxygen vacancies since this technique is sensitive to the difference in chemical environment of probing nuclei. In the previous study base on 45Sc NMR analysis, difference in the local structure around oxygen vacancies in Sc-doped AZrO3 (A = Ba, Ca) is suggested to be related to the hydration behavior [2]. To clarify the influence of the local structure around oxygen vacancies to the hydration behavior, the electric-field gradient (EFG) at Sc sites, an indicator of the local structure, in Sc-doped AZrO3 is calculated by DFT calculation and compared with 45Sc NMR results. EFG at Sc sites is derived from the charge of oxide ions surrounding the nucleus. When the symmetry of the surrounding oxide ions is lowered due to the formation of the oxygen vacancy, large gradient in the electric-field at the Sc site is generated. This EFG interacts with the electric quadrupole moment of Sc nucleus, and this interaction can be observed by 45Sc NMR.

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