In the context of growing CO₂ emissions, the need of more efficient low carbon technologies for energy storage and energy efficiency is pressing. Owing to their unique characteristics, ceramic based solid oxide cells present the valuable feature to be able of operating in fuel cell and electrolyzer mode. If the latter allows primarily efficient conversion of steam into hydrogen, it has the huge benefit of allowing co-electrolysis of CO₂ and H₂O into a valuable synthetic gas consisting in a mixture of CO and H₂ that can be processed downstream into chemicals, for instance via the Fischer-Tropsch process. As practical operation requires a stable function of the electrochemical cells, it is of crucial importance to understand their behavior and the underlying degradation mechanisms in order to be able to identify countermeasures for improving durability.

In this context, the electrochemical behavior of solid oxide cells is being investigated at DLR, from cell to stack level, in different configurations ranging from steam electrolysis to co-electrolysis and reversible operation. In this respect fuel electrode supported cells and electrolyte supported cells have been operated in electrolysis and co-electrolysis mode in different short-term and long-term runs of more than 1000 hours of operation with various gas conditions, operating temperatures and current densities. The electrochemical behavior was investigated by electrochemical impedance spectroscopy while the degradation was monitored by chronopotentiometry. Post-test microstructural analysis of functional ceramic layers was systematically performed with scanning electron microscopy in order to correlate possible microstructural changes with degradation phenomena.

Among others, it is found that the standard Nickel / Yttria-Stabilized-Zirconia cermet in fuel electrode supported cells suffers irreversible degradation through Nickel agglomeration and Nickel depletion, a phenomenon which was found to be correlated with temperature and current density. In electrolysis operation, cracking within the electrolyte along the air electrode is often observed. The main degradation features will be presented and discussed on the basis of the key identified drivers. Advanced materials solutions to mitigate the degradation issues will be presented and prospective applications will be introduced.