Employing a mechanically robust metal support as the structural element in SOFC has been the objective of various development efforts. The EU-sponsored project "METSOFC", completed at the end of 2011, resulted in a number of advancements towards implementing this strategy. These include robust metal supported cells (MSCs) having low ASR at low temperature, incorporation into small stacks of powers approaching ½kW, and stack tolerance to various operation cycles. DTU Energy Conversion's (formerly Risø DTU) research into planar MSCs has produced an advanced cell design with high performance. The novel approach has yielded roboust, defect-free cells fabricated by a unique and well-tailored co-sintering process. At low operation temperatures (650°C), these cells have shown remarkable ASRs: 0.35 Ωcm 2 in cell tests (16 cm 2 active area) and under 0.3 Ωcm 2 in button cells (0.5 cm 2 active area). Further success was attained with even larger cell areas of 12 cm squares, which facilitated integration into stacks at Topsoe Fuel Cell. Development of MSC stacks showed that the MSCs could achieve similar or better performance, compared to SoA anode supported ceramic cells. The best stacked MSCs had power densities approaching 275 mW/cm 2 (at 680°C and 0.8V). Furthermore, extended testing at AVL determined extra stack performance and reliability characteristics, including behavior towards sulfur and simulated diesel reformate, and tolerance to thermal cycles and load cycles. These and other key outcomes of the METSOFC consortium are covered, along with associated work supported by the Danish National Advanced Technology Foundation.
The potential of MS-SOFCs was demonstrated through the previous EU METSOFC project, which concluded that the development of oxidation resistant novel metal-supported solid oxide fule cell (MS-SOFC) design and stack is the requirement to advance this technology to the next level. The following EU METSAPP project has been executed with an overall aim of developing advanced metal-supported cells and stacks based on a robust, reliable and up-scalable technology. During the project, oxidation resistant nanostructured anodes based on modified SrTiO 3 were developed and integrated into MS-SOFCs to enhance their robustness. In addition, the manufacturing of metal-supported cells with different geometries, scalability of the manufacturing process was demonstrated and more than 200 cells with an area of~1 50 cm 2 were produced. The electrochemical performance of different cell generations was evaluated and best performance and stability combination was observed with doped SrTiO 3 based anode designs. Furthermore, numerical models to understand the corrosion behavior of the MS-SOFCs were developed and validated. Finally, the cost effective concept of coated metal interconnects was developed, which resulted in 90% reduction in Cr evaporation, three times lower Cr 2 O 3 scale thickness and increased lifetime. The possibility of assembling these cells into two radically different stack designs was demonstrated.
High temperature electrolysis (HTE) of steam, CO2, and steam and CO2 for highly efficient generation of hydrogen, carbon monoxide as well as syngas was investigated for four solid oxide cell stacks, all supplied by different stack manufacturers. The SOCs employed within the stacks were planar, and electrolyte or electrode supported with an industrial size between 80 and 128 cm². A comprehensive electrochemical characterization of both stacks and individual cells within the stacks was conducted by means of electrochemical impedance spectroscopy and polarization curve measurement. Detailed performance analyses showed the highest efficiency when operating the stack under H2O electrolysis, followed by co-electrolysis and eventually CO2 electrolysis. Subsequently, the stacks were operated under reversible system-relevant steady-state conditions, thus varying the working temperatures, the current density and the gas inlet flow. For that purpose both the conversion rate and fuel utilization were set to be between 70% and 80%. All stacks were operated for long-term periods of >1,000 h, during which degradation monitoring was applied. The results obtained within the present study allow a better understanding of the electrochemical processes that occur during reversible operation and especially HTE, and provide a guideline for optimized operation of a fully autonomous rSOC system.
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