Integration of doped ceria as fuel electrode into a fuel electrode-supported solid oxide cell (FESC) utilizing a YSZ electrolyte is challenging due to interdiffusion during the sintering process. While replacing the Ni-YSZ fuel electrode with Ni-GDC in our state–of-the-art FESC, we observed a drastic performance loss. We therefore investigated the interaction between electrode and electrolyte during processing on the fuel side, and its influence on cell performance.
In general, the difference in diffusion coefficients of Ce and Gd compared to Zr and Y should lead to the formation of Kirkendall voids at the YSZ/GDC interface, in addition to a mixed phase with low conductivity. The observed performance loss of this cell is best explained by the emerging porosity in the most electrochemically active area, namely the YSZ/GDC electrolyte/electrode interface.
In an attempt to supress interdiffusion and pore formation, the dopant in ceria was changed with Y3+ and Sm3+ possessing a smaller and larger ionic radius, respectively, compared to Gd3+. Interestingly, we do not find porosity at YSZ/XDC (X=Y, Gd, Sm) interfaces after sintering at high temperature. There is a slight change in interdiffusion lengths of Ce and the respective X into YSZ, depending on the dopant.
By adding NiO into the XDC, the formation of porosity at the interface was observed. This indicates an enhancement of the interdiffusion coefficients of Ce and X, which is also supported by X-ray diffraction results on powder mixtures.
Differently doped symmetrical ceria cells were measured with impedance spectroscopy.
In order to examine the effect of the porosity at the interface, the polarisation resistance of cells with an additional GDC barrier between the Ni-GDC electrode and the YSZ electrolyte were compared to cells without a barrier. Cells with a barrier show significantly lower polarisation resistance compared to cells without barrier. Interestingly, no difference was observed between cells with sequentially fired and co-fired barrier / electrode multilayers, which is in stark contrast to barrier layers on the air side. Overall, we conclude that prevention of porosity at electrochemical interfaces is more important than the reduced conductivity of an interdiffusion zone, or its minimisation by choosing a sufficient dopant.