Effect of Cr2O3 on the O-18 Tracer Incorporation in SOFC Materials

To gain insight into the Cr poisoning mechanism at the cathode side of solid oxide fuel cells (SOFCs) with stainless steel interconnects, we conducted an investigation of the impact of Cr2O3 overlayers on oxygen diffusion in various SOFC electrolyte and cathode materials. High density Gd0.10Ce0.90O2 (GDC), Y0.15Zr0.85O2 (YSZ) and La0.6Sr0.4Co0.2Fe0.8O2 (LSCF) sintered pellets were covered with 3 to 30 nm Cr - overlayers that were subsequently oxidized, forming Cr2O3. Standard 18O tracer diffusion experiments at 800°C were performed and TOFSIMS profiling revealed a Cr2O3 thickness-dependent oxygen uptake process. The oxygen ion diffusion coefficients were found to be unaffected by the Cr2O3 overlayers, which is predictable since they are a bulk property. The extracted surface exchange coefficients however varied with Cr2O3 overlayer thickness. Solid state reaction measurements of Cr2O3 with the three materials of interest, and electronic structure considerations concerning the surface exchange, led to the conclusion that the observed oxygen uptake hindrance for LSCF and the slight increase of the surface exchange coefficient for YSZ can be attributed to the electronic properties of Cr2O3. The rate limitation of the oxygen incorporation into the materials is therefore strongly dependent on the surface electronic properties. A critical thickness of Cr2O3 was determined where the transition from decreased cathode-performance to a Cr2O3-property-governed regime occurs.