Anisotropic 18O tracer diffusion in epitaxial films of GdBaCo2O5+δ cathode material with different orientations

Abstract

The layered structure of the orthorhombic GdBaCo₂O_5+δ (GBCO) double perovskite compound, currently considered as a promising cathode material in Solid Oxide Fuel Cells (SOFCs), is believed to induce a high degree of anisotropy in the oxygen diffusion coefficient, being maximum along the a–b plane in comparison to the diffusion along the c-axis direction. In this study we have deposited films with different orientation: pure c-axis and a-axis orientation on SrTiO₃(001) and NdGaO₃(110) single crystals, respectively. The oxygen diffusion was analysed by isotopic ¹⁸O exchange depth profiling (IEDP) and Time-of-flight Secondary Ion Mass Spectrometry (ToF-SIMS) in the films along the longitudinal and transverse directions at different exchange temperatures and exposure times. The magnitude of longitudinal D* at low temperatures shows a clear anisotropy. The oxygen diffusion along the a-axis shows comparable values to the bulk polycrystalline GBCO, while it is about one order of magnitude lower along the c-axis of the structure. The corresponding oxygen surface exchange rates k* do not show any anisotropy having comparable values for c-axis and a-axis orientation. These k* values are slightly larger than those reported for bulk material showing that thin film textured cathodes may have enhanced activity for oxygen reduction at low temperatures.