Interdiffusion in Multi-Layer Electrolytes for Solid Oxide Cells: Co-Firing and Rapid Densification Techniques

Doped ceria (DC) is a ubiquitous material for high-temperature solid oxide fuel and electrolysis cells (SOFC/SOEC). When used as a fuel-electrode material, DC exhibits high electrochemical activity and good stability in Ni-DC cermets. As an electrolyte material, DC offers high ionic conductivity and low-resistance interfaces to Ni-DC cermet electrodes, but the mixed conductivity necessitates the inclusion of an electron blocking layer. This presentation will cover the processing and co-firing of multi-layer electrolytes of Gadolinia-doped ceria (GDC) and Yttria-stabilized zirconia (YSZ), primarily by suspension-based techniques. A statistical analysis of the parameter space of the paste constituents on the paste rheology reveals the damping factor tan  and the yield point of the paste are the best predictors for the fidelity of the print. It will be shown that the common particle size descriptor of the d50 is not well correlated to tan  and the yield point, and that a bimodality coefficient calculated from skewness and kurtosis of the particle size distribution shows strong correlation with the damping factor, while the yield point of the paste is heavily influenced by the binder content. The importance of these factors for producing desirable printed layers will be shown. Moreover, understanding the correlation of the powder properties to the relevant rheological parameters is essential for the application of machine-learning to design printable pastes.While precise control of the screen-printing parameters enables the fabrication of dense and gas-tight electrolytes, the amount of interdiffusion between the GDC and YSZ phase during co-firing remains an issue. Among the investigated mitigation strategies are several sintering techniques that enable rapid densification in short timeframes, namely ultra-high temperature sintering (UHS), field-assisted sintering (FAST) and black-light sintering (BLS). For these novel techniques, initial investigations show that the rapid densification of GDC-YSZ powder mixtures is accompanied by severe interdiffusion and the formation of mixed phases irrespective of the very short interaction times. In addition, the sample environments present during FAST and UHS lead to reduction of the specimen, negatively impacting density and mechanical stability. While these techniques show promising results for chemically homogeneous materials, the problem of interdiffusion in the YSZ-GDC system is not mitigated.