Impact of GDC Interlayer Microstructure on Strontium Zirconate Interphase Formation and Cell Performance

Abstract

In solid oxide cells (SOCs) exhibiting mixed ionic-electronic conductor air electrode (MIEC AE) (e.g. La0.6Sr0.4Co0.2Fe0.8O3–δ LSC(F)), the formation of insulating zirconate interphases at the air electrode/zirconia electrolyte—interface is commonly prevented by an interlayer of doped cerium oxide. This complex interaction has a tremendous influence on cell performance, as the morphology of the interlayer determines the amount and continuity of the zirconate interphases. The performance of fuel electrode supported cells (FESCs) exhibiting gadolinium-doped ceria (GDC) interlayers fabricated from different commercially available powders are compared. All of these layers were fabricated according to the same procedure. The cell performance is analyzed by means of current-voltage (CV) characteristics, electrochemical impedance spectroscopy (EIS) and subsequent impedance analysis by the distribution of relaxation times (DRT). Next to the cell testing at application-oriented conditions, impedance spectra are measured over a wide temperature ranging down to 250 °C to resolve polarization phenomena related to bulk, grain boundary and interfacial effects within and in-between the gadolinium-doped ceria (GDC) and the 8 mol% yttria-stabilized zirconia (8YSZ) electrolyte. Furthermore, symmetrical air electrode (SymAE) cells are analyzed to eliminate impedance contributions from the anode layer (AL) and the substrate. The electrochemical results are correlated to the microstructural features of the GDC/YSZ interface obtained from post-test focused ion beam (FIB)/scanning electron microscope (SEM) analysis. This comparison revealed significant differences in the cell performance, which could be attributed to the amount and continuity of the residual strontium zirconate (SZO) interphase.