Evaluation of SOFC Interconnects Made of Ferritic Steels with Nano-Structured Oxi-Ceramic Protective Coatings Deposited by the LAFAD Process

Abstract

When Solid Oxide Fuel Cells (SOFCs) are operated in the temperature range from 750 to 800°C, it becomes possible to use inexpensive ferritic steels as interconnects (ICs). Due to the demanding SOFC-IC operating environment, protective coatings are gaining attention as a way to increase long-term stability. In this study, the large area filtered arc deposition (LAFAD) process was used to deposit nano-structured coatings from the (Co,Mn)TiCrAlY(O,N) system. Both nano-laminated as well as nano-composite coating architectures were studied and compared. Coatings were deposited on ferritic steel with the aid of an ultra-thin, adhesion-promoting bond-coat, and were subsequently annealed in air for various time intervals. Surface oxidation was investigated using RBS, SEM, and EDS analyses. Cr-volatilization was evaluated using a modified transpiration apparatus and Area Specific Resistance (ASR) was studied as a function of time using a four-point technique. Significant improvement in oxidation resistance, Cr volatility, and ASR were observed in the coated samples. In addition, a set of sample IC plates were subjected to LAFAD coatings and tested in a SOFC stack. Transport mechanisms for various oxidizing species and coating diffusion barrier properties are discussed, as are the trades-offs between ionic diffusion and electronic conductivity.