Interdiffusion between gadolinium-doped ceria and yttria-stabilized zirconia and its impact on cell performance

Advantageous aspects of gadolinium doped ceria (GDC) as an anode layer in anode-supported solid oxide fuel cells are well known regarding its high electrochemical performance and improved tolerance against sulfur poisoning and coking [1]. A high-sintering temperature of 1400 °C is necessary to fabricate a dense electrolyte, and to achieve the desired shrinkage of the anode substrate. An anode-supported cell with a Ni-GDC anode and a yttria-stabilized zirconia (YSZ) electrolyte showed a strongly decreased performance during cell testing compared to cells using Ni-YSZ as the anode. Hence, the interdiffusion of GDC-YSZ has been studied in detail to better understand its physical properties. Powders of 20GDC and 8YSZ have been mixed in equal weight ratio, and sintered at the same conditions as for the anode-supported cells to examine the sintering behavior of the intermixing phase. Ex-situ as well as in-situ high-temperature X-ray diffraction (XRD) was carried out up to 1400 °C to observe the intermixing of GDC-YSZ as a function of temperature. Pawley refinements and Rietveld analysis were performed on the powder XRD data to calculate the lattice parameters and to quantify the intermixing phases, respectively. Since the lattice parameters of the GDC-YSZ solid solution follows Vegard’s rule [2], the calculated lattice parameters were used to determine the stoichiometry of the intermixing phases. Using these constraints, corresponding Rietveld analysis were undertaken to quantify GDC, YSZ and GDC-YSZ intermixing phases as a function of temperature. A complete interdiffusion of GDC-YSZ already took place at 1300 °C, which suggests a formation of this mixed layer during cell sintering process at the interface of anode layer and electrolyte. Besides, we observed that the presence of NiO enhanced the diffusion kinetics of GDC and YSZ. X-ray photoelectron spectroscopy, 4-point conductivity and thermo-gravimetric analysis were conducted to investigate the reducibility, ionic and electronic conductivity of the GDC-YSZ mixed phase in air and under reducing conditions [3]. Impedance spectroscopy and microstructure analysis reveals an increase in the ohmic resistance of the cell due to the GDC-YSZ intermixing, and a strong increase in anode polarization which is probably due to the porosity at the interface caused by the difference in diffusion coefficients of Ce4+ and Zr4+ (Kirkendall effect)