Investigation of the Electrode-Electrolyte Interfaces in Solid Oxide Cells

Abstract

The interfaces between electrodes and electrolyte are critical locations in a solid oxide cell (SOC). These interfaces originate from the chemical interaction of two different materials during processing, and are therefore very sensitive to the chemical nature of the materials, as well as the thermal history of the cell. On the air side, perovskite air electrodes tend to form insulating zirconates when sintered on stabilized zirconia, the most common electrolyte material. On the fuel side, using an ionic conductor with a different chemical composition as zirconia can lead to pronounced interdiffusion and the formation of new phases.Interlayers of doped ceria are frequently used in order to suppress these undesired chemical reactions between electrodes and stabilized zirconia electrolytes. Prior investigations have focused extensively on the chemical composition of the interface and its consequences for cell performance. The focus of this contribution is the microstructure of the interface, as well as the microstructural development during processing.On the fuel side, the interdiffusion of ceria and zirconia is known to lead to an intermixed phase with decreased conductivity. However, the reduced cell performance of anode-supported cells with Ni-GDC electrodes cannot be explained by an increase in the electrolyte resistance alone. We show that the formation of porosity due to a difference in the diffusion coefficients of ceria and zirconia leads to an increase in the fuel electrode polarization, and investigate possible countermeasures. It is shown that specifically the presence of NiO leads to the formation of porosity at the interface.On the air side, we investigate the role of a dense interdiffusion layer between ceria and zirconia on the air electrode polarization. We confirm that only a dense interdiffusion layer is necessary by using Pr-doped ceria as a barrier layer, which delaminates after sintering and leaves behind a submicron barrier layer. Finally, we investigate the hypothesis that the densification of the barrier layer during air electrode sintering is essential for electrode adhesion and performance.