Oxygen vacancy localization and anisotropic oxygen anion transport in Sr1−xYxCoO3−δ (x = 0.1, 0.2) under solid oxide fuel cell cathode conditions

Abstract

In-situ neutron diffraction was utilized to study the influence of Y-doping on the crystal structure, oxygen non-stoichiometry, and oxygen anion transport pathways in Sr1−xYxCoO3−δ at temperatures and oxygen partial pressures typical of SOFC cathode conditions, from 500 °C to 800 °C, and oxygen partial pressure, pO2, between 10−3 atm and 1 atm. The resulting diffraction patterns of Sr0.9Y0.1CoO3−δ and Sr0.8Y0.2CoO3−δ were fit to tetragonal space groups P4/mmm and I4/mmm, respectively. While prior reports have suggested a degree of superstructuring in these materials at room temperature, our elevated temperature data demonstrates that this is not present under SOFC conditions. Both materials show similar oxygen stoichiometry with preferential oxygen vacancy formation at the equatorial sites of the Co-centered octahedra. While these equatorial sites are equivalent in Sr0.9Y0.1CoO3−δ, further vacancy localization within Sr0.8Y0.2CoO3−δ leads to a structure consisting of vacancy rich and vacancy poor Co-centered octahedra alternating in the c-axis direction. Both structures suggest a sinusoidal oxygen anion transport pathway via the high vacancy sites, further localized to the alternating vacancy rich layers in Sr0.8Y0.2CoO3−δ. A secondary vacancy ordered Brownmillerite phase was observed at 600 °C and below at the lowest pO2 for Sr0.9Y0.1CoO3−δ.