Interactions between SOFC Interconnect Spinel Coating Materials and Chromia

Abstract

The SOFC technology has been developed toward lower operation temperature in the range of 500C - 800C, which brings wider materials choice for the interconnect. Ferritic stainless steels have been widely used as SOFC interconnect candidates, but their high temperature oxidation and chromium volatilization can lead to cathode poisoning and cell degradation. Applying ceramic conductive coating is an effective technique to protect the metallic interconnects, and manganese cobalt spinel oxides Mn1.5Co1.5O4 have demonstrated promising performances as coating material. However, the reasons for their excellent properties have not been completely understood. This work aims to provide a fundamental understanding of (Mn,Co)3O4 coating in high-temperature operation, and to improve the performance of protective coatings. The interactions between (Mn,Co)3O4 spinel coatings and chromia at high temperature were investigated, and Chapter 2 discusses the mass transport behavior. The interactions lead to the change in chemical composition and microstructure of the original coating. To evaluate the long-term stability, the properties of the reaction layer need further characterization. Chapter 3 focuses on the effect of chromium doping on the electrical conductivity, cation distributions and thermal expansion of Mn1.5Co1.5O4 at SOFC operation condition. The relationship of cation distributions with transport properties in the reaction layer was also discussed. In addition, the effects of different transition metal dopants in the spinel coating material on the mass transport behavior were studied to identify the improved coatings. The effects of iron doping in spinel oxides are investigated on transport behavior in the interaction, and compared with that of titanium doping in Chapter 4. The interaction of nickel or copper substituted (Mn,Co)3O4 with chromia at high temperature are evaluated in Chapter 5. The differences in the mass transport behavior of transition metal dopants studied in this work provide helpful information on selection in novel spinel coatings.