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The efficient high temperature electrolysis is of high interest for potential application in a renewable energy system. The possibility to use high temperature heat for the electrolysis reaction makes the SOEC technology an attractive option for the integration in process concepts for the production of valuable chemicals from renewable energies. Moreover, the SOEC allows the electrolysis of water and carbon dioxide in a so-called Co-electrolysis. The produced syngas can be used in a subsequent synthesis step, e.g. a Fischer-Tropsch synthesis. The development of these power-to-product processes as well as of necessary components are in focus of current research activities at IKTS. The main component is the high temperature electrolyzer (SOEC). In collaboration with Plansee SE, the CFY stack technology has been developed. This stack technology can be applied for SOFC, SOEC and reversible operation (rSOC) and establishes an universal platform for different applications. Performance maps of a MK352 CFY stack in steam- (H2O) and Co- (H2O/CO2) electrolysis operation show a difference in electric power demand of less than 5 %. This low additional power demand of Co-electrolysis together with the advantages of direct syngas production makes it promising for highly efficient integration within a process concept for the production of valuable hydrocarbons. The output syngas can be directly adjusted by choosing appropriate operating conditions for the SOEC to typical compositions used for reactions like Fischer-Tropsch synthesis. Based on this approach the operating conditions were set and durability tests with stacks (>1500 h) were conducted. The results raised several scientific questions which were answered by splitting the “black box” of the stack in separate parallel investigated experiments in the fields of cell tests, tests of interconnect materials, contact resistances, protective layer and glasses. The surprising behavior of reversible degradation of the stacks was analyzed by single cell experiments and could be reduced by adjusting of operating conditions and resulted in a fine tuning of the electrode design. Nevertheless still an irreversible degradation of about 1%/1000 h was detected. Several material tests consolidated the common understanding of degradation phenomena and will be shown with comparison to the real stack tests. The results show the excellent suitability of the CFY stack for electrolysis applications and give an outlook on further improvements. Figure: 30-cell MK352 stack for SOFC/SOEC and rSOC operation Figure 1

The 3YSZ substrates with thickness between 90-200 μm are excellent from mechanical and economical points of view, but its main disadvantage is rather low ionic conductivity when compared with scandia-doped zirconia or fully stabilized 8YSZ. With decreasing electrolyte thickness between 40-60 μm, it is possible to significantly improve the electrochemical performance of the 3YSZ based electrolyte-supported cell (ESC) thereby fully utilizing the available mechanical stability. Development and progress in manufacturing of high power density electrolyte supported cell based on a thin (50 µm) 3YSZ substrate is presented. The cells with improved cathode based on LSMM’/ScSZ and the multilayer anode based on Ni/GDC cermet show very good electrochemical performance. The maximum power density increases up to 750 mW/cm2 for developed cell without additional contact layer at 0.7V@860 °C and is greater than one of the commercial cells based on partially scandia stabilized zirconia electrolytes as well as 95 µm 3YSZ. The changes in polarization resistance of tested cells under different operating conditions as well as during redox-cycling and durability tests are discussed on basis of analysis of impedance spectra. The developed cells show a good long-term stability (proved for >1300 h) under high current density (500 mA/cm²@850 °C, N2:H2:H2O=55:40:5, air). The estimated power degradation rate is lower than 0.5%/1300 h. By using Ni/GDC anode the redox cycle ability of cell under real operating conditions is considerably improved. To reduce the contacting losses in stack different contacting layers have been tested and optimized in test bench. The influence of different parameters on electrochemical performance of the cell as well as first results for cells integration in stack are presented and discussed.

Membrane-electrode-assembly (MEA) is a heart component of SOFC and defines the limits for power density, efficiency and durability available for exploitation in stacks and systems. MEA performance is often given as measure for possibilities of fuel cell technology. Most widespread way to communicate the MEA performance is to present the I-U-characteristic at a constant temperature and to estimate the area specific resistance from the slope of this curve. Other methods for characterization of performance like current density, impedance and local resistance measurement under defined operating conditions also have been used for this purpose. The fuel composition worldwide used for MEA tests (from dry hydrogen to H2:H2O=50:50) is connected with open circuit voltage which has great impact on measured performance. Moreover the realized fuel utilization strongly depends on operating conditions and varies in broad range. For these reasons it is difficult to compare results achieved by different researchers. It is beneficial to define unified value which would characterise electrochemical MEA performance. To resolve this challenge the voltage drop during the cell performance have been mathematically separated in two parts: (i) voltage drop due to change of Nernst voltage as a result of fuel utilization (and humidification) by current flow and (ii) voltage drop due to ohmic and polarization losses in the cell. This approach allows to separate the influence of non-linearity of Nernst voltage dependence from current on estimated cell resistance. It was found that at temperatures above 750°C the area specific resistance of the cell corrected to the Nernst voltage drop (fuel utilization) is a constant value, which only weakly depend on fuel composition. Favourable operating conditions as well as influence of operating conditions for estimation of cell resistance will be presented.

The stack concept of electrolyte supported cells (ESC) is applicable for both, solid oxide fuel cells as well as solid oxide electrolysis cells. High power densities and efficiencies comparable to anode supported cells (ASC) can be achieved, however, for the stack design presented in this paper, in unison with high robustness. Incorporating an electrolyte made of fully scandia stabilized zirconia paired with chromium-based CFY (Cr5FeY) interconnects of matched CTE (coefficient of thermal expansion), a feasible stack concept was created with the focus on long-term stability in regard to high temperature corrosion and thermal cycling. Stack data of more than 20.000 hours of a single stack is presented in addition to industrial onward development of the stack design. Using the same material combinations and the same footprint as for the MK351-design, the newly developed MK352-design was simulated, drafted and produced and allows for easier stack setup and system integration. Minor changes in interconnect (IC) design enable lower pressure drops over the stack, which, paired with enhanced tolerance chains lead to enhanced stack performance, especially in regard to thermal cycling. Cross-checked, test results for the MK352-stack are in good agreement with the simulation.

Metal-supported cells (MSC) for SOFC have attracted growing attention over the last decade, especially for portable and mobile applications. To avoid oxidation of the metal support, sintering steps are typically executed in inert or reducing atmosphere. Standard cathode materials, which are normally used for electrolyte-supported (ESC) or anode-supported (ASC) cells, decompose in such atmospheres. Therefore cathodes should be applied to metal-supported cells only after all the non-oxidizing-atmosphere processing steps are finished. Recently, we have developed new in-situ sintered composite cathodes with sintering temperature < 950 °C. Good adhesion as well as low and long-term stable polarization resistance of new electrodes has been obtained.

Chromium-based interconnects (CFY) in combination with electrolyte supported cells are perfectly matching the requirements of different SOFC applications. Furthermore, this type of stacks and the related manufacturing of cells and interconnects is already proven in very high volumes today. Plansee SE and Fraunhofer IKTS, together with partners in the SOFC20 project and lead customers, have developed the advanced MK351 stack-platform, which is now available for interested new customers. A suitable, well-adjusted combination of materials, especially regarding the cathode protection and contacting, has been developed. Sample test results have been verified in long term stack testing for more than 8,000 h. The stack has proven very low power degradation of less than 0.7 %/ 1,000 h.

Fraunhofer IKTS and Plansee SE continuously develop SOFC stacks with Chromium based interconnects (CFY) and electrolyte supported cells based on Scandia doped Zirconia. This technology has a great potential for cost-effective manufacturing and rapid up-scaling due to availability of major stack components and essential improvements in manufacturing made last year. The stacks are already available on the market and are in test at several potential customers. The paper gives an overview of the current status of CFY-stack development focused on performance map and system cycling. The results of operation of CFY-stacks in modules with stacks working on different fuels with and without internal steam reforming in partial and full load are presented.

Development and progress in manufacturing of high power density electrolyte supported cell is presented. The maximum power density increased from 529 mW/cm2 for commercial cell (Fa. Kerafol) up to 973 mW/cm2 for novel IKTS cell at 0.7V@860°C and is comparable with the performance of anode-supported SOFC cells. The changes in polarization resistance of tested cells under different conditions are discussed on basis of analysis of impedance spectra.

SOFC modules with a power of 5 to 50 kW are a serious competitor to commercially available combined heat and power plants for decentralized electricity and heat supply with high electrical efficiency. Electrical efficiency and electrical power of a SOFC stack are essential for its profitable operation. The highest power density and electrical efficiency can only be achieved with planar SOFC stacks. Electrolyte supported cells with 10ScSZ electrolytes allow power densities of more than 500 mW/cm². On account of the thermomechanical properties of electrolyte this potential can only be used in stacks with chromium based interconnects (CFY from Plansee SE). High power, robustness and long-term stability are essential for stationary SOFC systems. In order to fulfill these requirements Fraunhofer IKTS in close collaboration with Plansee SE develops a new stack design with larger active area, simple cathode contacting, external cathode manifold and optimized interconnect layer. This paper will give an overview of the current SOFC stack development focused on power output, long-term stability, temperature cycling and reliability of SOFC stack assembling.

The electrolyte supported cells with low area specific resistance (ASR

Gastight and stable sealings are crucial for reliable service of solid oxide fuel cells (SOFCs). Failures of sealings are conceivable for many reasons resulting in damage of the whole SOFC-stack. Reactions between sealing and interconnector materials (Crofer 22 APU) at high temperatures under oxidizing and reducing conditions affect the bonding strength and microstructure of the glass to metal interface. Electric voltage across the glass sealings lead to additional effects at anodically and cathodically polarized interfaces. Tests of model sealings under electric voltage in combination with dual atmosphere provide information about the stability of sealing glasses under SOFC-relevant operating conditions.

SOFCs can be operated with various fuel types which make them attractive to use for different applications. In many of these applications carbon containing gases are used as fuel that can cause soot formation in the cells especially during heating up - cooling down the SOFC stacks. Experiments have been performed in order to understand the effect of temperature, heating rate and gas composition for carbon containing fuel gas by simulating stack operation parameters. The soot formation phenomenon is studied on the pellets made from anode powders as well as on SOFC one cell stacks. The electrochemical behaviour evaluated by I-V-characteristics and impedance spectroscopy and microscopic analysis of the cells confirm soot formation when SOFC stack is fuelled under unfavourable temperature regions and heating rates. However, it is also found that stack can regenerate itself during operation after a small amount of coke has been deposited on the electrode.