Mechanism of SrZrO₃ formation at GDC/YSZ interface of SOFC cathode

SrZrO₃ formation at the interface of gadolinia-doped ceria (GDC) interlayer and yttria-stabilized zirconia (YSZ) electrolyte is analyzed using high-resolution electron microscopy. SrZrO₃ is dispersed in the inter-diffusion layer on the GDC side from the Ce/Zr border. Zr, which diffuses into the GDC grain, contributes to the formation of SrZrO₃. The crystallographic relationship among the SrZrO₃ grains and its neighboring GDC grains reveals that SrZrO₃ is formed at the surface, at the grain boundary, and inside the grain, while maintaining a highly matched boundary with the adjacent GDC grain. The matching of the interface boundary is confirmed by the O-lattice theory, according to which the threshold Zr/Ce ratio is 13/34. If Zr/Ce ratio in the GDC grain is higher than the threshold, SrZrO₃ may significantly grow into the grain. The conduction path for the oxygen ion is retained because the GDC grain containing Zr is split into the SrZrO₃ grain and the less-Zr-containing GDC grain. If Zr/Ce ratio is lower than the threshold, SrZrO₃ may be formed but will be limited by the amount of Zr diffusing from the adjacent region. Thus, the morphology of SrZrO₃ is strongly affected by the state of GDC grains in the inter-diffusion layer.