A Solid Oxide Electrolysis (SOE) short stack consisting of anode-supported cells (ASCs in fuel cell mode) was assembled in JÜLICH's F10-design. ASCs are based on Ni/8YSZ (8 mol-% yttria-stabilized zirconia) with an LSCF air electrode (La_0.58Sr_0.4Co_0.2Fe_0.8O_3-δ) and 8YSZ electrolyte. A gadolinium-doped ceria (GDC) (Ce_0.8Gd_0.2O_1.9) barrier layer was deposited between 8YSZ and LSCF by means of physical vapor deposition (PVD). The stack was mainly characterized in a furnace environment in both fuel cell and electrolysis modes, with 50% humidified H₂ at 700 and 800°C. An endothermic long-term electrolysis operation was carried out at 800°C with a current density of −0.5 Acm⁻² and steam conversion rate of 50%. After 2300 h of operation, the stack showed an average voltage degradation rate of 0.7%/kh. Electrochemical impedance spectroscopy (EIS) and analysis of the distribution function of relaxation times (DRT) showed that the degradation was primarily due to the increase in ohmic resistance.

A long-term test with a two-layer solid oxide cell stack was carried out for more than 20,000 hours. The stack was mainly characterized in a furnace environment in electrolysis mode, with 50% humidification of H₂ at 800°C. The endothermic operation was carried out with a current density of −0.5 Acm⁻² and steam conversion rate of 50%. Electrolysis at lower temperatures (i.e., 700°C and 750°C) and fuel cell operation (with 0.5 Acm⁻² and fuel utilization of 50%) at 800°C were also carried out (<2000 h each) for comparison. The voltage and area specific resistance degradation rates were ∼0.6%/kh and 8.2%/kh after ∼18,460 hours of operation. In total, the stack was operated above 700°C for more than 20,000 hours. Impedance measurement and analysis showed that the increase of ohmic resistance was the main degradation phenomenon, while electrode polarizations were kept nearly constant before a severe burning took place in one layer. Ni-depletion in fuel electrodes was confirmed during post-mortem analysis, which was assumed to be the major degradation mechanism observed. The stack performance and degradation analysis under different working conditions, as well as the results of preliminary post-mortem analysis will be presented.