Improved Oxidation Resistance and Reduced Cr Vaporization from Thin-Film Coated Solid Oxide Fuel Cell Interconnects
High electrical efficiency, clean emissions, and the possibility to operate on a great variety of fuels, such as hydrogen, alcohols and hydrocarbons are some advantages of Solid Oxide Fuel Cell (SOFC) technology. Too short lifetimes and high production costs have, however, limited the commercialization of this technology. Volatilization of Cr(VI) species and the increased electrical resistance caused by a growing oxide scale are two major degradation mechanisms associated with the use of Cr2O3-forming ferritic stainless steels as interconnect material in a SOFC. In this thesis the possibility to mitigate Cr vaporization and improve oxidation resistance in a cost-effective way, by the application of thin-film metallic Co- and Ce/Co-coatings, was investigated. Uncoated and coated ferritic stainless steels were exposed for up to 3300 h at 650-850 °C. Cr vaporization, oxide scale growth, microstructural and chemical evolution of the oxide scales, as well as the effect these factors have on the electrical resistance of the oxide scale were studied. Cr vaporization was measured using the denuder technique. Oxide scale growth kinetics were studied mainly gravimetrically and the electrical scale resistance was measured ex-situ in an ASR setup using platinum as the electrode material. For chemical, structural and microstructural analysis, SEM/EDX, XRD, BIB/FIB, TEM/EELS and SIMS techniques were utilized. In order to study the oxide scale growth mechanism a two-stage, 18/16O-tracer exposure setup was used. Within the studied temperature interval it was shown that thin-film Co- and Ce/Co-coated ferritic stainless steels exhibit excellent properties as interconnect material in SOFCs. Cr vaporization can be mitigated with a Co-coating. The addition of a Ce-coating can improve the oxidation resistance, and thus decrease the electrical resistance of the oxide scale. The possibility to coat high volumes of steel using Physical Vapour Deposition (PVD) technology, and subsequently press the pre-coated steel into interconnects to allow for gas distribution, was also investigated. Cracks are formed within the coating as the coated steel is pressed into interconnects. The results in this study, however, show that these cracks are able to heal upon exposure at high temperatures.