## 2 Materials and Methods  
In this study, electrolyte-supported button cells with a 23 mm diameter electrolyte made of stabilized zirconia with 3% Y\({}_{2}\)O\({}_{3}\) were examined. The anode and cathode had a diameter of 10 mm. The anode material was a nickel/10% gadolinium doped ceria (Ni/GDC) cermet with an active layer directly on the electrolyte support and a contact layer on top. Nickel was used as an additional contact. Strontium-doped lanthanum manganate (LSM) was used as standard cathode material. All materials used in the present study are state-of-the-art SOFC materials, as the goal is to avoid microstructure degradation of standard materials and to extend their service time in SOFCs and not to test more novel or exotic materials. In order to find out the different contributions (chemical and electrical) to the total degradation, we performed thermal and electrochemical experiments with similar cells under the same conditions. The only difference was that during the electrochemical experiments, a current of 230 mA/cm\({}^{2}\) was drawn. The cells have been contacted during the thermal treatment, similarly to the electrochemical experiments. The time was varied between 100 h and 1000 h while the temperature was kept constant at 850 \({}^{\circ}\)C. Further, in all electrochemical experiments, a current density of 230 mA/cm\({}^{2}\) was set over the entire duration of the experiment. The used hydrogen fuel was humidified by passing H\({}_{2}\) through a bubble saturator at room temperature (25 \({}^{\circ}\)C), leading to a humidified gas composed of 3 vol.% H\({}_{2}\)O/H\({}_{2}\) (3.17 kPa) as reported, e.g., in [35]. The addition of steam was achieved by passing He, as a carrier gas, through a saturator at room temperature (25 \({}^{\circ}\)C). The experimental conditions for all cells are summarized in Table 1.  
During the electrochemical experiments, impedance spectra (EIS) were regularly taken. After the chemical (thermal) and electrochemical treatment, all samples were investigated using scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and micro-computed tomography (\(\upmu\)-CT). Electrochemical Impedance Spectroscopy (EIS) measurements were conducted using a potentiostat-galvanostat (BIOLOGIC SP-150 (BioLogic, Seyssinet-Pariset, France)) equipped with a frequency response analyzer (FRA). EIS data were collected over the frequency range of 1 MHz-1 MHz, using a stimulus of 10 mV amplitude. The EIS experiments were performed at open current-voltage (OCV) at certain times from 0 to 1000 h.  
The equivalent electrical circuit applied for the calculation of internal resistances consists of a two--time constant model equivalent circuit considering the overall solution resistance of the cell (RS) in series with two parallel RC components followed by a Warburg element (W). Linear Sweep Voltammetry (LSV) from the OCV to the short-circuit of the cell was carried out for the construction of polarization curves in order to assess the stack performance. The SEM analysis was performed using a FEI Helios NanoLab(tm) 600, equipped with a high-resolution electron column with a Field Emmission Gun (FEG) electron source and a light element X-ray EDS detector. The samples were impregnated using a low viscosity epoxy resin, and after curing, they were ground using diamond abrasives and polished with diamond suspensions with 7, 3, and 1 \(\upmu\)m particle diameter. On the samples, we deposited a 20 nm thin carbon layer in order to avoid charging, thus improving the image stability under the electron beam. X-ray tomography was performed using a BRUKER SkyScan2011(tm) X-ray nano-tomograph with spatial resolution of \(<\)100 nm pixel size (isotropic) using an X-ray source of 20-80 kV, a focal spot size of \(<\)400 nm, and a 12 bit CCD detector. The spatial resolution in volume terms is according to the manufacturer equal to or better than that of synchrotron tomography. The samples used in the \(\upmu\)-CT have had dimensions of about 0.2 \(\times\) 1.5 \(\times\) 3 mm\({}^{3}\) (X, Y, Z).