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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 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 (<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. One of the critical challenges to the application and commercialization of solid oxide cell (SOC) technology is the long-term stability of complete systems, stacks and single components. Despite the fact that accelerating methods have long been under discussion, the time-consuming and costly endurance tests of stacks and cells under relevant conditions are still necessary for reliable degradation analyses and lifetime prediction. Compared to the tests with single cell or other stack and system component, the results of long-term degradation tests with stacks are quite limited. [1][2][3][4][5] Previous results have shown stable performance of stacks working in the temperature range of 700-800• C in SOFC mode. [6][7][8][9] Recently, a short stack achieved a 10 years lifetime, and remains in operation. 10 With proper protective coating, a voltage degradation rate of less than 0.3%/kh and lifetime of more than 40,000 h at 700• C is possible with current stack design and components. In contrast to the low degradation rate in SOFC mode, higher degradation rates of ∼0.6-1.5%/kh were observed with similar types of cells in the same stack design in SOEC mode, as described by Nguyen et al. 11 Therefore, the functionality and optimization potential of the cells and stacks, as well as their long-term degradation behavior and mechanisms in SOEC mode, needs to be further investigated. Amongst the stacks operated in SOEC mode, a two-layer stack was operated under different stationary conditions for more than 20,000 h. The stack performance was regularly monitored by open circuit voltage, voltage-current curves and impedance measurements. After cooling down, one third of the stack was embedded in resin for cross-section preparation, and the rest was disassembled for visual inspection. The prelim...

A Solid Oxide Electrolysis (SOE) short stack consisting of anode-supported cells (ASCs in fuel cell mode) was assembled in J ÜLICH's F10-design. ASCs are based on Ni/8YSZ (8 mol-% yttria-stabilized zirconia) with an LSCF air electrode (La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ ) and 8YSZ electrolyte. A gadolinium-doped ceria (GDC) (Ce 0.8 Gd 0.2 O 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 H 2 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.

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A long-term test with a two-layer solid oxide electrolyzer stack was carried out for more than 20 000 hours. The stack was mainly operated in a furnace environment in electrolysis mode, with 50% humidification of H2 at 800 °C, a current density of -0.5 Acm -2 and steam conversion rate of 50%. After ~18 000 hours of operation in electrolysis mode, the voltage and area specific resistance degradation rates were ~0.6%/kh and 8.2%/kh, respectively. A detailed post mortem analysis of cells including ICP-OES and microstructural analysis was conducted. Two main degradation phenomena were observed in the cells: In the fuel electrode, the depletion and agglomeration of nickel were visible. At the air electrode, demixing of the air electrode and diffusion of strontium took place. This was observed in the formation of strontium zirconate at the interface between the electrolyte and the GDC barrier layer as well as in the formation of strontium oxide and strontium chromate on top of the cells. Strontium oxide was even found in pores on top of the electrolyte.

Long-term stable and robust stack technology is a pre-requisite for a successful implementation of SOFC in the market and so it is one of the main issues in SOFC development. Also bigger stacks are desirable for systems with increased power output. Progress in stack and systems development is illustrated. Enhanced thermo-mechanical robustness of stacks in the kW range was demonstrated by successfully cycling a five layer stack incorporating cells of 20 × 20 cm 2 100 times between 200 and 700 • C. Aiming at the utilization of anode-supported cells from other manufacturers, the design was modified to a window frame layout, integrating in this case four 10 × 10 cm 2 cells in one layer. A 20 kW system was operated for about 7000 hours in total, thereby demonstrating the suitability of the module and system concept. As special highlights, long-term tests with short stacks have now reached operation times of 66,500 h and 34,500 h, respectively. As a relatively new subject, activities entailed by high temperature electrolysis are briefly outlined.

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