Optimization of Alloy-Coating Compositions for Use as Solid Oxide Fuel Cell Interconnects
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Optimization of Alloy-Coating Compositions for Use as Solid Oxide Fuel Cell Interconnects Jeffrey W. Fergus1, Yu Zhao1 and Yingjia Liu1 1 Auburn University, Materials Research and Education Center, 275 Wilmore Laboratories, Auburn, AL 36849, U.S.A. ABSTRACT Reduction in the operating temperature of solid oxide fuel cells (SOFCs) allows for the use of ferritic stainless steels as the interconnect material. However, cathode poisoning due to volatilization of chromium from the oxide scale formed on these alloys requires the use of a ceramic coating on the alloy. The reaction layer formed between the coating and the alloy affects the performance of the fuel cell, so the composition and properties of this layer must be considered in selecting the alloy and coating compositions. In this paper, the factors relevant to selection of the alloy/coating materials and the effect of the alloy-coating interaction on these factors are discussed. INTRODUCTION The high operating temperature of solid oxide fuel cells (SOFCs) allows for the use of a variety of fuels, such as natural gas, reformed diesel fuel and biogas, which expands the range of potential SOFC applications. However, the high operating temperature can also lead to materials degradation. One important example of degradation in SOFCs is poisoning of the cathode from chromium deposits as a result of chromium volatilization from the stainless steel interconnect. The volatilization can be reduced through alloy design by the addition of manganese, which leads to the formation of a (Mn,Cr)3O4 spinel phase on the scale surface. However, additional reduction in volatilization is needed for long lifetimes, so ceramic coatings are applied to reduce chromium volatilization. One promising coating material is a manganese cobalt spinel oxide, which has high electrical conductivity and a coefficient of thermal expansion that is similar to those of other SOFC coating materials. After high-temperature exposure, the coating material reacts with the oxidation scale formed on the stainless steel alloy. The thickness of the resulting multilayer scale-coating combination increases with time, but the growth mechanism is different from that of the scale on uncoated alloy. Thus, the optimal composition of a coated alloy may be different than that of an uncoated alloy. Similarly, the optimal coating composition depends not only on the properties of the coating material, but also on those of the reaction layer formed between the coating and the alloy. This paper includes discussion of the factors considered in selection of the alloy and coating materials for SOFCs, the reaction layer that forms at the alloy-coating interface during long-term exposure and how the reaction layer, and how its properties affect the selection of the alloy and coating materials.
SOFC INTERCONNECT ALLOY-COATING DEVELOPMENT Alloy selection The SOFC interconnect is exposed to air at high temperatures, so oxidation resistance is a critical property for interconnect alloys. Oxidation resistance is generally achieved by fo
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