Metallic materials in Solid Oxide Fuel Cells: Oxidation and chromium evaporation properties
Sammanfattning: Solid oxide fuel cells (SOFCs) are high-temperature energy conversion devices which have great potential due to their high efficiency, low operating costs and flexibility in using conventional hydrocarbon based fuels. However, system cost, durability and performance stability in long-term operation are barriers to the widespread commercialization of SOFC technology. Due to the high-temperature operation and aggressive environment in SOFCs, metallic materials - used for interconnects and balance of plant (BOP) - are subject to corrosion. Interconnects are typically made of ferritic stainless steel, which forms a protective chromia scale at high temperatures. This results in two main degradation mechanisms: 1) chromium evaporation, which leads to cathode poisoning and 2) chromia scale growth, which leads to increased electrical resistance. To date, research into metallic materials in SOFC has focused mostly on interconnects. However, metallic materials used in BOP components can be a significant source of volatile chromium species and are often overlooked. The aim of this thesis is to find high performance, cost-effective metallic materials for SOFC systems. Five metallic materials AISI 441, AISI 444, A197/Kanthal® EF101, alloy 800H and alloy 600, were studied for potential use in BOP components. Lowcost steels, AISI 441 and AISI 444, and tailor-made Crofer 22 APU in combination with different coatings were evaluated for the interconnect application. Chromium evaporation and oxide-scale growth of the materials are investigated, and the oxide scale is studied further, using XRD, SEM, EDX and ASR. The alumina former, A197, showed the lowest chromium evaporation and oxidation in all exposure conditions. Alloy 800H showed poor oxidation behaviour at lower temperatures but its performance improved significantly after pre-oxidation. Alloy 800H has higher chromium evaporation than A197 but significantly lower than 441 and 444. This low chromium evaporation is due to the formation of an Fe, Ni-rich oxide cap layer. Alloy 600 showed intermediate performance. 441 and 444 showed the highest oxidation and chromium evaporation of the selected alloys, making them a poor choice for BOP components. The uncoated low-cost steels, 441 and 444, showed higher chromium evaporation and/or oxide scale growth than the tailor-made Crofer 22 APU. The oxide scale structure was similar for all the steels, with (Cr, Mn)3O4 spinel on top and Cr2O3 scale underneath after 500 hours. The Ce/Co coated steels showed lower oxide scale growth and a chromium evaporation at least 60 times lower than the uncoated steels. In addition, all the coated steels showed similar chromium evaporation, oxide scale structure ((Co, Mn)3O4 spinel on top and Cr2O3 scale underneath), oxide scale thickness and area specific resistance after 1,000 hours.
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