2, Northwestern University, Evanston, Illinois, United States
Lanthanum strontium cobalt ferrite (LSCF) cathodes in solid oxide fuel cells can experience rapid degradation under high operating temperatures. Coarsening of the microstructure is one of the mechanisms that lead to such degradation because of the loss of reactive surface area. Thus, insight into the dynamics of coarsening in morphologically complex LSCF microstructures is required to improve overall performance of SOFC. Serial sectioning with FIB-SEM of both as-fired and experimentally annealed microstructures was carried out to reconstruct the three-dimensional microstructures. Using the reconstructed as-fired LSCF microstructure as the initial condition, coarsening by surface diffusion is simulated with a phase-field model based on the conserved Cahn-Hilliard equation with a concentration dependent mobility term. We find that the characteristic length scale, Sv-1, obeys the temporal power law, ~t1/4. This is expected for microstructural systems coarsening via surface diffusion, thereby verifying the phase-field simulation code. The morphological evolution of the as-fired LSCF microstructure is examined statistically with interfacial shape distribution (ISD) and locally with isosurfaces colored with local curvatures. We find that interfaces of the microstructure are mostly saddle-shaped, which is consistent with morphology of bicontinuous structures. As coarsening proceeds, high-curvature interfaces disappear and the overall curvature distribution becomes more symmetric with respect to the average mean curvature of the microstructure. Lastly, we have calculated tortuosities of both the LSCF and pore phases. The tortuosity values are nearly constant during the coarsening process; therefore, the electrode performance is not significantly affected by the tortuosity change. The simulation indicates that, while the morphology of the structure undergoes significant change during coarsening, the connectivity of the bicontinuous microstructure remains relatively unchanged. Therefore, we hypothesize that the performance degradation is due to the reactive surface area change and reactivity reduction due to strontium segregation.