## 3 Results
### Impedance Spectroscopy  
From the impedance spectra of the thermally aged samples under dry conditions (Figure 2a), it is evident that the resistance is higher compared to the sample under wet conditions (Figure 2b).  
In the electrochemical experiments (Figure 2), the influence of the anode gas humidification is also clearly measurable, although this time, it is the opposite of that in the thermal aging experiments. Here, the total resistance is smaller in dry than in wet H\({}_{2}\).  
From an analysis of the measured Nyquist plots in Figure 2c,d and an appropriate combination of the impedance elements, the following equivalent circuit model (ECM) that fits the measured impedance data can be proposed (Figure 2e).  
Figure 3 shows the development of the obtained specific resistance of the cells as a function of the aging time (both thermal and electrochemical). The influence of the moisture  
Figure 2: (**a**) Impedance spectra time development of the thermally stressed Cell 8 in dry H\({}_{2}\) for 1000 h at 850 \({}^{\circ}\)C and (**b**) of the thermally stressed Cell 10 in humid H\({}_{2}\) for 1000 h at 850 \({}^{\circ}\)C. (**c**) Impedance spectra time development of the electrochemically stressed Cell 2 in dry H\({}_{2}\) for 1000 h at 850 \({}^{\circ}\)C (**d**) and of the electrochemically stressed Cell 5 in humid H\({}_{2}\) for 1000 h at 850 \({}^{\circ}\)C. (**e**) Equivalent circuit model used for fitting the impedance data.  
in the anode compartment is very clear in the thermal aging tests. In dry conditions, the specific resistance increases significantly with time in contrast to the wet thermal aging test. The increase of the specific resistance after 250 h under thermal treatment in dry conditions is especially remarkable.  
Electrochemically (besides the current drawing, practically the same experiment), the situation is such that the specific resistance under wet conditions is the same as in the thermal test. In the dry electrochemical test, the specific resistance does not increase as significantly as in the thermal test. This is probably because, in electrochemical experiments, there is always a certain amount of moisture on the anode side due to the water being produced. This effect is stronger under the selected test conditions due to the high fuel utilization of approx. 60% (determined by mass spectroscopy), which leads to a high proportion of moisture in the anode. The observed difference in the development of the specific resistance between thermal and electrochemical experiments can stem only from processes related to the current draw. Besides, the increased humidity under high fuel utilization, possibly local ohmic heating, leads to changes in the local chemistry and microstructure.  
From the above impedance spectra, it is obvious that, with a few exceptions, the greatest influence on the total resistance is caused by low-frequency processes, i.e., the gas transport in the electrodes, and fewer processes, such as the oxidation reaction of the fuel or electronic or ionic transport phenomena [39].  
U-t curves (not presented here) of electrochemically aged cells for 1000 h under dry and wet conditions have shown by linear extrapolation that the mean value of the voltage drop after 1000 h with non-humidified fuel gas is ca. 183 mV/year, whereas humidified fuel gas results in a loss of only 53 mV/year, which is clearly less than 3% and 1% per year respectively. During the long-term tests, a voltage change has been observed. According to that, after approx. 200 h, the cell appears to recover in both cases, and the  
Figure 3: Comparison of the specific resistance development as a function of the treatment time in chemical and electrochemical experiments under humid and dry conditions.  
loss for t > 250 h is 122 mV/year under dry conditions and an average of 96 mV/year when running with wet fuel gas, again being smaller than 2% and 1% per year, respectively. These changes are in accordance with microstructure relaxation, as described in the results and discussion sections.