## 5 Conclusions  
The goal of the present study was to quantify microstructure degradation phenomena on anodes that can be attributed to chemical and/or electrochemical sintering, to possibly find out the underlying mechanisms, and to propose countermeasures. 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. This was obvious during the dry electrochemical test, where the results were better than expected due to the water that was produced during the reaction.  
The samples have been thermally aged for times from 0 to 1000 h, and additional samples of the same type are subjected to electrochemical loading over the same period in dry and humid conditions.  
Tortuosity and porosity change with time depending on the humidity of the fuel, and the EIS results confirm through the increasing specific resistance the influence of the changes on the microstructure on the electrochemical properties of the cells.  
It has been found that there is a difference in the microstructure and degradation of the cells if we perform a typical chemical (thermal) treatment or if we draw current in a typical electrochemical experiment. This is the case, for example, during the formation of the cell or the stack. This must be performed in a time as short as possible.  
Humidity plays an important role in this degradation process as it helps keep the specific resistance very low compared to the higher specific resistance in dry conditions.  
Electrochemical experiments lead to a different picture of the microstructure and specific resistances. We performed EIS after 25, 50, 100, 250, 500, 750 and 1000 h). These data show that during the long-term electrochemical experiments, the microstructure and, thus, the cell performance (specific resistance) change. A more detailed EIS study using the distribution of relaxation time (DRT) method [32] will follow in order to show the development of different physicochemical processes involved in the measured overall specific resistance.  
Additional modeling and experimental work with several samples under changing conditions needs to be performed in order to be able to elucidate the exact mechanisms of porosity and tortuosity changes and correlate them with electrochemical analysis data for achieving a more general degradation model of SOFC anodes.  
Conceptualization, C.A. and G.S.; methodology, C.A.; validation, C.A. and G.S.; formal analysis, G.S.; investigation, G.S.; resources, G.S.; writing--original draft preparation, C.A.; writing--review and editing, C.A. and G.S.; visualization, C.A.; project administration, G.S.; funding acquisition, C.A. All authors have read and agreed to the published version of the manuscript.  
This study has received funds from the German Federal Ministry of Education and Research (BMBF) for the project \"SOFC-Degradation\" under contract number 03 SF0494D. We acknowledge that the samples were provided by the industrial partner of the TU Clausthal for the project \"SOFC-Degradation\".  
Not applicable.  
Data Availability Statement:Data are available by the authors upon request. Data are not publicly available due to privacy based on contractual regulations.  
**Conflicts of Interest**: The authors declare no conflict of interest. The funders had no role in the design of the study, in the collection, analyses, or interpretation of the data, in the writing of the manuscript, or in the decision to publish the results.