Thin film perovskite coatings and their application for SOFC ferritic steel interconnects

High electrical contact resistance and Cr evaporation are two well recognised technical issues in reliable long-term use of ferritic stainless steel interconnects in solid oxide fuel cells (SOFCs). They have a crucial negative impact on the cell performance and stability, if not adequately addressed. During the last years, many types of conductive ceramic oxides with either a spinel or perovskite lattice structure have been investigated as protective oxide layers for SOFC interconnect applications. For example perovskites show sufficiently high electronic conductivity, good matching of the thermal expansion coefficient (TEC), chemical stability in SOFC-operating environments and low cation mobility. Nevertheless, their performance has been often reported to be below expectations due to poor adherence and higher difficulty in obtaining densely sintered layers in comparison to spinel coatings. As a new attempt to address such aspects, a novel chemical conversion process has been developed for producing dense thin films (below 1 μm) of LaFeO3-based perovskite coatings on ferritic stainless steel surfaces, under relatively low temperature conditions. Commercially available ferritic 22Cr steels (Crofer 22H and Sanergy HT steels) have been used to evaluate electrical contact resistance, corrosion stability and Cr evaporation of the perovskite-modified stainless steel surfaces in medium-term tests at 700°C. X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) equipped with energy dispersive X-ray analysis (SEM-EDX) have been used to characterise the materials before and after testing. Results show that a stable electrical contact resistance is obtained at 700°C, well below the target value of 0.05 Ωcm2 at this temperature, for both coated steels. Coated Sanergy HT steel show a somewhat better Cr retention, although not to a fully satisfactory degree. Further efforts are still required for obtaining improved Cr barrier performance on 22Cr steels.