Degradation Mechanisms of the Lanthanum Strontium Cobalt Ferrite Used as Oxygen Electrode in Solid Oxide Cells

This thesis investigates solid oxide cell degradation through a threefold approach, comprising multiscale physics-based modeling, long-term tests, and advanced post-test analyses. The model adapted to the studied cell (Ni-YSZ/YSZ/GDC/LSCF) and validated against experimental data, provides an understanding of cell-level processes under varied operating conditions. The model includes morphological and microscale electrochemical kinetic models for the hydrogen and oxygen electrodes, complemented by a macroscale model simulating the complete cell response. Durability tests are performed to assess the impact of various operating conditions, including 2000-hour long-term tests at temperatures of 750, 800, and 850 °C for 2000 hours, using 10/90 vol.% H2/H2O and dry air at the hydrogen and oxygen electrodes, respectively, and 1000-hour ageing tests at 800 °C to explore the effect of air humidity (0%, 3%, and 8%) under both SOEC and SOFC modes. Advanced post-test analyses, including synchrotron- and laboratory-based techniques down to the atomic scales, are selected to investigate microstructural, chemical, and structural material degradation. The findings demonstrate the model capability to predict both stationary and dynamic electrochemical responses, including local responses along the cell radius. The model interpretation of electrochemical impedance spectra (EIS), for the cells operated in SOEC mode under dry air, identified the hydrogen electrode as the primary source of performance loss. Conversely, the oxygen electrode-related performance remains relatively stable after ageing, indicating limited degradation. Post-test analyses confirmed a substantial microstructural evolution of the hydrogen electrode, including Ni migration and coarsening. In contrast, the oxygen electrode experienced slight Sr segregation. Elemental inter-diffusion across cell layers is identified in all samples, while the commonly observed SrZrO3 phase is not detected. Exploring the impact of operating temperature in SOEC mode revealed an exacerbated cell degradation at higher temperatures, particularly for Ni migration in the hydrogen electrode and, in a minor way, Sr segregation in the oxygen electrode. Ageing under humid air introduced an additional degradation phenomenon at the oxygen electrode in both SOEC and SOFC modes. This consisted of an aggravated LSCF decomposition and the formation of a Sr oxide-based insulating phase on top of the current collector, especially in SOEC mode. The presence of this phase at the cell inlet impacted the current distribution along the cell radius, resulting in a non-uniform hydrogen electrode degradation.