Coated Ferritic Stainless Steels as Interconnects in Solid Oxide Fuel Cells - Material Development and Electrical Properties

Doctoral thesis, 2015

Solid oxide fuel cells (SOFCs) are attracting increasing interest as devices with potential uses in decentralized and clean electricity and heat production. Several challenges with respect to materials have to be overcome to achieve efficiencies and life-spans that are sufficient for long-term applications. An important element of an SOFC stack is the interconnect component, which connects two adjacent fuel cell elements. Interconnects, which are commonly composed of ferritic stainless steels, have to be corrosion-resistant, mechanically stable and costoptimized. This work aimed to investigate economic solutions for interconnect materials and to understand the underlying mechanisms of degradation and electrical conduction of these materials. Mainly two substrates, a commercially available steel (AISI 441) and a ferritic stainless steel that was optimized for an SOFC application (Sandvik Sanergy HT) were combined with different barrier coatings and exposed to a cathode-side atmosphere. A method was developed that allows for the electrical characterization of promising material systems and model alloys, thereby facilitating a fundamental understanding of the dominant electrical conduction processes linked to the oxide scales that grow on interconnects. The AISI 441 steel coated with reactive elements and cobalt showed good corrosion and chromium evaporation profiles, while AISI 441 coated with cerium and cobalt also had promising electrical properties. The Sanergy HT steel was examined with coatings of copper and iron and copper and manganese, respectively. The corrosion and chromium evaporation profiles of Sanergy HT were improved by coating with copper and iron. The copper and iron-coated Sanergy HT showed lower area specific resistance values than cobalt-coated Sanergy HT. Chromia, which is the main constituent of oxide scales, was synthesized using different methods. The electrical properties of chromia were found to be sensitive to not only impurities, but also heat treatment. Finally the electrical properties of cobalt- and cobalt cerium-coated Sanergy HT steels were investigated. It was revealed that the addition of cerium improved the conductivity of the interconnect by both slowing down chromia growth and preventing the outward diffusion of iron into the spinel.