High temperature electrolyzers represent a very promising technology to store renewable and nuclear energy over long periods of time in the form of fuels. Solid Oxide Electrolysis Cells (SOEC) operate at high temperatures and efficiently convert steam and CO₂ to hydrogen or syngas. In this presentation, we demonstrate operation of multiple SOECs of different sizes in a wide range of experimental conditions to understand possible ways to reduce cell losses and improve cell efficiency. Cell durability and stability was assessed as a function of temperature, operating voltage, and steam content, often under conditions aimed to accelerate degradations. The data obtained using both small and large size cells was compared. Electrochemical impedance spectroscopy and the distribution of relaxation times methods were employed to identify the cell losses. Extensive post-test characterization was performed using scanning electron and transmission electron microscopies in a combination with energy dispersive spectroscopy. SEM and TEM image analyses were accelerated using machine learning algorithms. The observed changes in the electrodes and at the active electrode/electrolyte interfaces, as well as electrode interactions with gas impurities were used to identify several degradation mechanisms. The electrochemistry models were developed to fit the existing experimental data and predict long-term performance of SOEC cells and stacks.