## 4 Discussion  
The cells were then examined for microstructural changes using FESEM and \(\upmu\)-CT, and the results are correlated with the ones of long-term electrochemical experiments. In the thermal tests, the porosity increases more strongly with humidified fuel gas with time, regardless of whether the current is drawn or not. This can be attributed to an increased reaction rate of reaction R2 (v.i. ), which runs preferably under these conditions. Further, in dry conditions performed thermal tests, coarsening seems to be the main reason for degradation, especially at the beginning of the treatment coarsening is obvious (Figure 6a). Overall, we observed that, with only thermal loading, the increase in porosity is higher than in the electrochemical tests. This can be ascribed to some nickel losses due to the higher mobility of the volatile nickel hydroxide species, as reported by [PERSON] et al. [3]. In contrast to nickel microstructure, which is strongly changed due to the nickel coagulation and maybe losses in thermal treatment, the GDC microstructure remains largely intact even after 1000 h. The increased porosity under humid conditions does not affect the specific resistance in the electrochemical experiments as it does not so dramatically change the percolation in the Ni and GDC subnets. A sufficient nickel distribution with humidified fuel gas leads to a lower specific resistance of the cell despite an increase in porosity. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **Treatment** & **Time [h]** & \begin{tabular}{c} **Tortuosity** \\ **Direction 1** \\ \end{tabular} & \
\begin{tabular}{c} **Tortuosity** \\ **Direction 2** \\ \end{tabular} &
\begin{tabular}{c} **Tortuosity** \\ **Direction 3** \\ \end{tabular} \\ \hline Cell 6 (Ref.) & No & 4 & 3.158 & 1.948 & 1.759 \\ \hline Cell 7 & thermal & 100 & 36.11 & 1.902 & 1.899 \\ \hline Cell 8 & thermal & 1000 & 5.045 & 1.361 & 1.403 \\ \hline Cell 9 & thermal & 100 & 9.864 & 1.615 & 1.697 \\ \hline Cell 10 & thermal & 1000 & 4.632 & 1.809 & 1.704 \\ \hline Cell 1 & electrochemical & 100 & 11.61 & 1.217 & 1.448 \\ \hline Cell 2 & electrochemical & 1000 & 13.42 & 2.076 & 2.71 \\ \hline Cell 3 & electrochemical & 100 & 12.01 & 1.290 & 1.711 \\ \hline Cell 5 & electrochemical & 1000 & 4.454 & 1.824 & 1.648 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results for tortuosity factors using TauFactor® code. Figure 8: Total porosity for the electrochemically treated cells derived from \(\upmu\)-CT data. The tortuosity in the two directions (2 and 3) is not perpendicular to the electrolyte (direction 1) and either remains unchanged or decreases slightly, consistent with the above statement that the GDC framework largely persists. The tortuosity factor increases significantly perpendicular to the electrolyte, probably due to the strong nickel coagulation on the anode surface, which is obvious in compacted anode layers in SEM micrographs (e.g., Figure 7b). This hinders the access of fuel to the active layer and leads to the increased specific resistance of all cells with time (Figure 3). Since the molar volume of Ni is only 60% compared to that of NiO, reducing the anode initially increases the porosity. However, this also represents a weakening of the entire anode structure and thus promotes later compaction of the anode over time due to high-temperature creep and diffusion processes. The clamping pressure applied to contact the electrodes does not appear to have a significant influence on the anode microstructure since, in this case, the tortuosity in directions 2 and 3 should also increase over time. It is also clear that the tortuosity in direction 1 (Table 2), which increases extremely after 100 h, relaxes over time and almost returns to normal values. An explanation of this phenomenon is not possible with the available data and would be only a matter of speculation.