###### Abstract  
The goal of the present study was to quantify degradation phenomena on anodes that can be attributed to chemical (thermal) and/or electrochemical sintering, to find out the underlying mechanisms, and to propose countermeasures. The samples were thermally aged for times from 0 to 1000 h, and additional samples of the same type were subjected to electrochemical loading over the same period. The cells were then examined for microstructural changes using FE-SEM/EDS and micro-computed tomography (\(\mu\)-CT), and the results are correlated with electrochemical impedance spectroscopy (EIS) parameters of long-term electrochemical experiments under dry and humid conditions. It has been shown that it is possible to distinguish between the thermal (chemical) and the electrochemical part of the microstructure degradation. Humidity is an important factor that affects the microstructure in the long term. Tortuosity, porosity, and specific resistance change with time, depending on the humidity of the fuel. Tortuosity changes by one order of magnitude in the direction perpendicular to the electrode surface, while in the other two directions in the plane, the changes are only moderate. Porosity increases in all electrochemically treated samples by 1-5% depending on dry ore humidity conditions and time. As all other experimental parameters are the same in all experiments, the EIS results confirm through the increasing specific resistance, mainly the influence of the changes on the microstructure on the electrochemical properties of the cells. SOFCs; anodes; degradation; thermal sintering; electrochemical sintering; tomography +
Footnote †: journal: Medical Engineering, National Technical University of Athens, 15773 Zografou, Greece  
Footnote †: journal: Medical Engineering, National Technical University of Athens, 1573 Zografou, Greece  
Footnote †: journal: Medical Engineering, National Technical University of Athens, 15773 Zografou, Greece
The main degradation effects in SOFC stacks have already been qualitatively described and categorized. This knowledge is available in the literature [10]. However, the exact mechanisms are unknown and controversial. For example, there are quite different theories in the literature as to how chromium poisoning of cathodes occurs, what the driving forces are, and how the mechanism can be countered [11; 12]. The same applies, for example, to sulfur poisoning of anodes, etc. [13; 14], as well as the weighting of individual effects in the overall context of a stack. In addition, recent long-term tests [9] show that unforeseen or underestimated phenomena can lead to sudden failure after longer stack running times (19,000 h). On the other hand, the quantitative description of the phenomena is still largely missing. In addition to many qualitative descriptions of the degradation effects [12; 13], there are only semi-empirical descriptions [14] of the mechanisms and some model descriptions of individual effects [13; 14]. These, in turn, hardly allow any conclusions to be drawn about essential driving parameters, nor do they provide insights into the mechanisms of action that would allow predictions and extrapolations, including lifespan prognosis. Nevertheless, the exact mechanisms are still controversial or have not yet been investigated and understood in sufficient depth. It involves a variety of corrosive and thermodynamic processes, as well as microstructural changes in the materials, which are clearly influenced by the high operating temperatures between 600 and 850 \({}^{\circ}\)C. It is not clear which part of the degradation is caused by pure thermal effects and what is caused or intensified by the current drawing. Investigating them is difficult, lengthy, and often not accessible to direct inspection or observation during ongoing operations because they occur within the closed stack.