Thermochemical Expansion and Protonic and Electronic Hole Conductivity of Grain Interior and Grain Boundaries in 10 Mole% Y-Substituted SrZrO3

Abstract

Proton conducting acceptor-doped SrZrO₃ has a history as long as that of BaZrO₃ , but has attracted less interest. Inspired by its higher transport number of ionic conduction in wet oxygen revealed by our recent work, we here explore further aspects of doped SrZrO₃ as electrolyte in proton ceramic electrochemical cells. In-situ high temperature XRD (HT-XRD) analysis of SrZr_0.9 Y_0.1 O_3-δ (SZY10) indicated an anisotropic chemical expansion of hydration, stronger along the b than the a direction, and negative in the c direction. A systematic electromotive force (EMF) and impedance spectroscopy study as a function of $p_{O_2}$ and $p_{H_{2}O}$ allowed determination of partial conductivities of electron holes and ions (mainly protons) in bulk (grain interior) and grain boundaries. Enthalpies and preexponentials were determined and interpreted for bulk and grain boundary partial conductivities based on defect chemistry and a brick layer model. The hole conductivity in bulk is modest and ensures high ionic transport numbers in oxidizing atmospheres, while grain boundaries exhibit lower ionic transport numbers from a relatively higher hole conductivity attributed primarily to tunnelling past the deepest part of the space charge region. Y-doped SrZrO₃ (SZY) materials exhibit lower proton conductivities but excel over Y-doped BaZrO₃ (BZY) in terms of thermal expansion compatibility with electrode materials and higher ionic transport numbers in oxidizing atmospheres and may hence be candidates for functional layers between BZY-based electrolytes and positrodes in proton ceramic electrochemical cells.

Keywords: grain boundary; proton conductor; space charge; strontium zirconate; transport property.