Modelling-Assisted Investigation of Degradation Phenomena in LSM-based SOFC Composite Cathodes

This thesis focuses on the degradation pathways occurring in composite solid oxide fuel cell (SOFC) cathodes based on lanthanum-strontium manganite (LSM), combining modeling at the electrode level with experimental data. LSM composite electrodes are one of the most promising candidates for SOFCs cathodes, yet life-time is currently one of the limiting factors for the deployment of this technology. The combination of modelling tools and analysis of experimental data permitted to achieve a better comprehension of the mechanisms controlling the performance loss during operation and a quantification of the degradation. By acquisition and formalization of knowledge of degradation processes, this thesis offers new tools to predict the life-time of a cell, hypothesize technological solutions for limiting the performance losses, and evaluate the impact of such solutions. A new experimental strategy for SOFC button cell testing has been developed and validated during this work, namely the simultaneous operation of several cathode segments on the same cell support. This allowed, on the one hand, the production of reproducible and reliable data for the evaluation and investigation of degradation processes and, on the other hand, the possibility of rapid identification of experimental problems affecting one segment. The approach has been validated both experimentally and theoretically – through finite element calculations. The vast majority of the experimental results contained in this work has been obtained using this testing configuration. Furthermore, investigation techniques with unprecedented resolution in the SOFC research field have been developed in collaboration between EPFL and external partners, namely a fast Cr quantification in operated cells, and 3D non-destructive reconstruction of cathode microstructures by X-ray computed tomography, giving new instruments for the study of SOFC performance and degradation. Finally, a set of data has been gathered concerning the reactivity of LSM-based systems, aging composite pellets for different times and temperatures and analyzing them with Rietveld-XRD, in order to map and assess the loss of performance caused by the formation of insulating phases in the electrode. A steady-state electrode model present in literature has been selected and improved for simulating the performance of composite cathodes in presence of degradation phenomena, converting it into a time-dependent model. The first phenomenon that has been integrated is the variation of the microstructure of the composite electrode, allowing the prediction of variation in performance for a number of case studies: coarsening of only one phase, coarsening of both phases, coarsening of both phases with variation of porosity. Experimental results obtained by microstructural analysis and electrochemical characterization of cells aged for different times were compared to the simulations of the correspondent case study, showing that the morphological vari