Comparison of different manganese-cobalt-iron spinel protective coatings for SOFC interconnects

Chromium poisoning is a well-known degradation mechanism in solid oxide fuel cell (SOFC) stacks. Stainless steel interconnects (IC) have been identified as a major source of chromium. Additionally, depletion of chromium in very thin IC plates can lead to destructive break-away oxidation. This calls for protective coatings to inhibit the evaporation of chromium from the IC plates and to improve the SOFC stack durability. Such coatings should have a low electric resistivity and high physical and chemical stability in high temperatures. Much literature has been published on the performance of coatings. However, comparison between them is difficult due to the wide range of testing conditions. This work contributes to the field by comparing coating solutions from different companies and research centres, manufactured by different methods. The evaluated coatings include manganese-cobalt-iron and cerium-cobalt protective layers. The developed coatings build on previous work within the SCoReD2.0 project. Thin steel samples of AISI441, Sandvik Sanergy HT and Crofer 22 H were used as substrates. These steels were chosen since they are commercially available and widely used in SOFC applications. Area specific resistance (ASR) and overall stability were investigated with a measurement setup that mimics the conditions found in SOFC stacks. The steel samples were placed on top of thin palladium foils with a screen-printed lanthanum-strontium-cobalt (LSC) layer. The measurement setup replicates the interactions at an SOFC cathode since the LSC layer is manufactured the same way as real cathodes. In addition, the use of palladium spacers instead of steel enables electron microscopy analysis of chromium migration into the LSC layer as well as of oxide scale growth. ASR measurements were carried out in a humid air atmosphere at 700 °C for 1000 hours. The paper compares the protective coatings in terms of ASR, chromium retention and overall stability and discusses their usability in SOFC stacks. This work has been conducted within the SCoReD2.0 project, which has received funding from the European Union's Fuel Cells and Hydrogen Joint Technology Initiative under contract no. 325331. Additionally, the NELLHI (grant agreement no. 621227) and INNO-Sofc (grant agreement no. 671403) projects are acknowledged.