Proton Diffusion in Hydrated Acceptor-Doped Barium Zirconate
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0972-AA01-04
Proton Diffusion in Hydrated Acceptor-Doped Barium Zirconate Dirk Wilmer1, Tilo Seydel2, and Klaus-Dieter Kreuer3 1 Institute of Physical Chemistry, University of Münster, Corrensstr. 30, Münster, 48149, Germany 2 Institut Laue-Langevin, Grenoble, 38042, France 3 Max-Planck-Institute for Solid State Research, Stuttgart, 70569, Germany
ABSTRACT Quasielastic neutron scattering experiments have been performed on proton-conducting hydrated Y-doped BaZrO3 at temperatures between 473 K and 673 K. Quasielastic broadening is observed at all temperatures, but only at the higher ones the scattering exhibits the characteristics of the proton transport process. Attempts to interpret our data in the framework of the two-state model yield unconvincing results. A simpler approach using the isotropic Chudley-Elliott model yields reasonable values for the proton self-diffusivities but also unusually high values for the jump distance which are close to the mean distance between dopants.
INTRODUCTION Acceptor-doped perovskites incorporate water by dissociative absorption. Owing to the mobility of oxygen vacancies and protonic defects, these materials are ionic conductors. If such a material is fully hydrated, the transference number of the protonic defects is close to unity. For use in electrochemical devices such as solid oxide fuel cells (where reducing the operating temperature is a key objective), high proton conductivity has to be accompanied by reasonable thermodynamics phase stability. In this respect, acceptor-doped BaZrO3 exhibits promising properties [1]. Despite its significantly higher ionic radius when compared to Zr4+, Y3+ turns out to be optimal as an acceptor-dopant for BaZrO3. For temperatures below 700 °C, the observed proton conductivities clearly exceed the oxide ion conductivities of the best oxide ion conductors. Proton migration in perovskites is usually described by the Grotthuss mechanism [2,3] which consists of two steps: reorientation of the proton-carrying hydroxide groups and proton transfer to neighboring oxide ions. Experimentally [4,5] and by quantum MD simulations [6,7] rotational diffusion has been characterized as a fast motion with low activation barriers. In addition, the strong red-shift of the OH-stretching vibration indicates strong hydrogen bond interactions, favoring fast proton transfer steps rather than reorientation processes requiring the breaking of these bonds. We performed incoherent quasielastic neutron scattering measurements on Y-doped and hydrated BaZrO3 in order to study proton diffusion in this material. Key questions to be addressed are i) how does the proton conduction mechanism look like and ii) does the dopant play the role of a trap for the proton [8], or are the protonic defects more or less delocalized.
EXPERIMENT BaZr0.9Y0.1O3-δ (10Y:BZ for short) was prepared by solid state reaction as described elsewhere [9]. A cylindrical ceramic 10Y:BZ sample (32.1636 g after 3 h drying in nitrogen, about 90 % of the theoretical density) was loaded in water vapor at 6
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