In parallel to the stationary solid oxide fuel cell stack tests (>34,000 h), a long-term test with a two-layer solid oxide electrolyzer stack comprising JULICH's F10 design was carried out. The stack was mainly characterized in a furnace environment in electrolysis mode, with 50% humidification of H_2 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 for short durations (<2000 h each) for comparison. After ～21,000 hours of operation in total, the voltage and ASR degradation rates during electrolysis at 800°C (8000-20,000 h) were ～0.4%/kh and 2.7%/kh, respectively. In this work, stack performance and degradation analysis under different working conditions is presented.
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