Metallic Thin-Film Co- and Ce/Co-Coated Steels as Interconnect Material in IT-SOFC

Abstract

In recent years SOFC manufacturers have been able to decrease the operating temperature significantly and today several anode- and metal-supported designs are able to operate at temperatures as low as 600-700 ˚C. One of several advantages of lower operating temperatures is the possibility to use less expensive materials, for example for the interconnect. Volatilization of Cr(VI) species and the increased electrical resistance, caused by the growing oxide scale, are probably the two most detrimental degradation mechanisms associated with the use of Cr2O3-forming ferritic stainless steels as interconnect material in SOFCs. Within the last decade extensive research has been carried out investigating these two degradation mechanisms. Development of custom-made alloys as well as reactive element surface treatments have been shown to improve corrosion resistance. Additionally a wide range of spinel based coating systems have been suggested to effectively mitigate Cr vaporization. However, the vast majority of studies, which investigate the mitigation of Cr vaporization in combination with oxide scale growth, and the electrical scale resistance were carried out at significantly higher temperatures (800-900 ˚C), often to accelerate the aforementioned degradation mechanisms. Cr vaporization, oxide scale growth, and electrical resistance are, however, all differently affected by a change in temperature, especially with regard to the influence of temperature on the microstructure of the oxide scale. In this work the effect of temperature in the technologically relevant temperature range (650-850 °C) is studied with focus on Cr vaporization, oxide scale growth, microstructural and chemical evolution, as well as the effect these factors have on the electrical resistance of the oxide scale (ASR). Commercially available interconnect materials such as Sanergy HT, Crofer 22 APU and AISI 441, coated with thin-film Co- and Ce/Co-coatings are exposed in air within a typical IT-SOFC temperature regime. Chromium vaporization is measured using the denuder technique, ASR is recorded ex-situ (after the furnace exposures) using platinum as electrode material and the oxide scales are characterized using SEM/EDX. The results from this work show that thin-film Co- and Ce/Co-coatings exhibit excellent properties as interconnect materials for IT-SOFC with respect to Cr vaporization, oxide scale growth, and electrical resistance. The results also show that temperature has a significant effect on the chemical composition of the oxidized Co-coating. However, due to the high electrical conductivity of the relevant Co-spinels, in comparison to the substantially lower electrical conductivity of chromia, this change does not affect the electrical resistance of the total oxide scale.