Durability Testing of a High-Temperature Steam Electrolyzer Stack at 700 °C

A 5‐cell solid oxide electrolyzer cell (SOEC) stack has been operated at ca. 700 °C for 2,000 h under static conditions followed by a short‐term transient operation. The best repeating unit (RU) showed an area specific resistance of 0.4 Ω cm2 and an initial voltage of 1.125 V at 705 °C and –0.6 A cm–2 when fed with 90% H2O/10% H2 at the hydrogen‐electrode. One instability factor has been observed, which led to dramatic voltage fluctuations for most RUs in the stack. Such kind of voltage fluctuation is correlated predominantly to the variation of ohmic resistances of RUs, pointing out most probably unstable electrical contact in the stack under the present testing conditions. Disregarding the worst RU (RU1), the average degradation rate (voltage increase rate) of all RUs in the stack has been determined to be around 2% kh–1 (24 mV kh–1) for the last 500 h of static operation free of voltage fluctuations. The stack showed no noticeable degradation during the short‐term transient operation (totally 756 ON/OFF current cycles), indicating preliminarily that it could be possible to couple intermittent power sources with high‐temperature steam electrolyzers.