## 4 Discussion
For example, creep processes in the percolated Ni particles, or localized redox processes that cause an increase in the porosity due to the difference in the molar volume of Ni and NiO could be possible explanations for the relaxation of the microstructure. As reported by [PERSON] et al. [41] for Ni-YSZ anode electrodes, it is also possible that the anode degradation could be due to surface oxidation (R1 and R2) of nickel owing to the higher local partial pressure of steam close to electrochemically active sites [42; 43]. \[2\text{Ni}+\text{O}_{2}\to 2\text{NiO}\] (R1)  
\[\text{NiO}+\text{H}_{2}\text{O}\left(\text{g}\right)\to\text{Ni}(\text{OH}) _{2}\] (R2)  
The question that now needs to be answered is why the tortuosity increases in direction 1 and why the nickel prefers to collect on the surface. Capillary forces are unlikely to play a role because the temperatures are not sufficient to give the nickel the necessary flowability. However, the temperatures are sufficient to influence the Ni creep behavior [44; 45], which is strongly determined by diffusion processes in the nickel polycrystals. According to [PERSON] et al. [46], the nickel self-diffusion coefficient at 830 \({}^{\circ}\)C is 3.74 \(\times\) 10\({}^{-13}\) cm\({}^{2}\)/s, which corresponds to an average diffusion depth of approx. 2.76 \(\upmu\)m/h. At approx. 890 \({}^{\circ}\)C, the average diffusion depth is 3.5 \(\upmu\)m/h. These values are more than sufficient to create the image observed after several 100 h in the anodes of this study. The nickel coagulates in large, flat \"grains\" on the surface or around larger GDC particles. The above-mentioned diffusion coefficients, which are relatively large under the prevailing thermodynamic conditions, favor coarsening and/or Ostwald ripening, whereby larger grains grow more strongly to the disadvantage of smaller ones due to their energetic stability and thus lead to abnormal grain growth under formation of pores. This can also explain the observed microporosity in nickel grains, e.g., [41]. This seems to be the only logical explanation for Ni coagulation toward the surface under these conditions. It influences the tortuosity in direction 1 up to 1000 h, but the GDC framework holds (at least up to the upper time limit of 1000 h in this study), and the tortuosity in directions 2 and 3 either remains unchanged or decreases slightly as the pores had been covered with nickel during the experiment. The fact that the coagulation is already very strong after 100 h and does not increase linearly up to 1000 h is probably due to the so-called strong chemical interaction between Ni and GDC (Strong Metal-Support Interaction--SMSI) [32], which may result in a total withdrawal of nickel from the anode cermet. Electrochemically, the situation is such that the specific resistance under wet conditions is the same as in the thermal test. From an \"electrochemical\" perspective, given the electrochemical boundary conditions chosen in this study, there is no big difference between dry and wet electrochemical aging. On the other hand, under dry conditions, a strong increase in specific resistance was expected, analogous to the dry thermal aging test. However, 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 that is produced in the anode department. Given the selected test conditions, this is sufficient to lead to a high proportion of moisture in the anode department due to the high fuel utilization of approx. 60% (determined by mass spectroscopy).