On the conductivity degradation and phase stability of solid oxide fuel cell (SOFC) zirconia electrolytes analysed via XRD

The degradation of ionic conductivity in 6ScSZ and 3YSZ SOFC electrolytes was analysed via X-ray diffraction (XRD), 4-point conductivity and electrochemical impedance spectroscopy (EIS) and the results present a consistent story: in 6ScSZ subtle but reproducible changes to the phase assembly in the form of a shrinking “cubic shoulder” are obtained with ageing at 900 °C, which closely corresponds to the decrease in ionic conductivity. This appears to be the gradual transformation of the residual high-temperature c or t″ phase to t′. Both the shoulders and the conductivity return after “healing” at temperatures above the ageing temperature but below the sintering temperature, bringing the samples back to the as-sintered state. Evidence of a type of short-range ordering (SRO) acting in parallel with the phase changes was seen in the EIS. Distortions on the XRD peaks such as humps on the left shoulder were almost never seen and appear to be a polishing artefact unrelated to changes in the conductivity.