AbstractIn this work, we report a 4,400 h test of a state‐of‐the‐art Ni‐YSZ (yttria stabilized zirconia) electrode supported solid oxide electrolysis cell. The electrolysis test was carried out at 800 °C, –1 A cm−2 with 10% H2 + 90% H2O supplied to Ni‐YSZ electrode compartment. Except for the first 250 h of fast initial degradation, the cell showed rather stable performance with a moderate degradation rate of around 25 mV per 1,000 h. The electrochemical impedance spectra (EIS) acquired during the test show that both serial resistance and electrode polarization resistance increased during the durability test. Further impedance analyzes show that both the LSCF (strontium and cobalt co‐doped lanthanum ferrite)‐CGO (gadolinium doped ceria) electrode and Ni‐YSZ electrode degraded and the degradation was dominated by the Ni‐YSZ electrode. Post mortem analysis on the Ni‐YSZ electrode revealed loss of contact between Ni‐Ni grains, Ni‐YSZ grains and increased porosity inside the active layer. The microstructural changes were most severe at steam gas inlet and became less severe along the gas flow path. The present test results show that this type of cell can be used for early demonstration of solid oxide cell operation at electrolysis current densities around 1 A cm−2.