Solid Oxide Electrolyzer Stack with 20,000 h of Operation

Tuesday, 25 July 2017: 14:20
Atlantic Ballroom 1/2 (The Diplomat Beach Resort)
Q. Fang, L. Blum, N. H. Menzler (Forschungszentrum Jülich GmbH), and D. Stolten (Forschungszentrum Jülich GmbH, 52425, Jülich, Germany, Chair for Fuel Cells, RWTH Aachen, 52072, Aachen, Germany)
One of the critical challenges for application of solid oxide cell (SOC) technology is the long-term stability of the complete system and/or single component. Despite of different debating accelerating methods, the time and cost consuming endurance tests of stacks and cells under relevant conditions are still necessary for reliable degradation analysis and lifetime prediction. In parallel to the stationary solid oxide fuel cell stack tests (>34,000 h), a test with a two-layer solid oxide electrolyser stack in JÜLICH’s F10-design was started on September 2014 (still in operation as of 26.01.2017). The short stack consists of Ni/YSZ-supported cells (ASCs in fuel cell mode) with LSCF air electrode (La0.58Sr0.4Co0.2Fe0.8O3-δ) and 8YSZ (8 mol-% yttria-stabilized zirconia) electrolyte. A gadolinium-doped ceria (GDC) (Ce0.8Gd0.2O1.9) 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 electrolysis mode, with 50% humidified H2 at 800 °C. The endothermic electrolysis 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 shortly (<2000 h each) for comparison. After altogether ~20,000 hours of operation, the voltage and ASR degradation rates during electrolysis at 800 °C (8000~20,000 h) are ~0.4%/1000h and 3%/1000h, respectively. 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. An increasing polarization in fuel electrode in one of the layers was also observed during the last 2000 h of operation, most likely due to the increased leakage rate inside the stack.

The stack is planned to be operated for another 2000 h before post mortem analysis. Stack performance and degradation analysis under different working conditions, together with preliminary post analysis will be presented.