Evaluation of Protective Coatings for SOFC Interconnects

Chromium poisoning is a widely recognized degradation mechanism in solid oxide fuel cells (SOFC). Stainless steel interconnects (IC), in a direct contact with the cathode, have been identified as the main chromium source. Chromium evaporation from the bulk steel may also cause break-away oxidation of the IC plate, which is equally deleterious for the stack. Thus, protective coatings are needed to prevent chromium from migrating from the steel. A low electrical resistivity and high chemical and physical stability are required of the coatings. This work compares manganese-cobalt and cobalt-cerium protective coatings. The evaluated coatings are fabricated on thin steel samples by commercial companies and research facilities with differing manufacturing methods. The chosen steels are Sandvik Sanergy HT and K41/441 as they are common in SOFC applications. Area specific resistance (ASR) and overall stability were determined with an innovative measurement setup. The steel samples were stacked in a sandwich structure adjacent to thin palladium foils with a screen-printed lanthanum strontium cobalt (LSC) layer. This setup offers two advantages. First, a realistic electrical contact with the cathode material is obtained since the LSC layer is manufactured with the same methods as real SOFC cathodes. Second, the adoption of palladium spacers instead of steel enables electron microscopy analysis on chromium migration into the LSC layer as well as on oxide scale growth. ASR measurements were conducted in a humid atmosphere at 700 °C for a period of 1000 hours. The paper compares the coating solutions in terms of ASR and stability and discusses their usability in SOFC applications.