Strategies to Mitigate the Degradation of Stainless-Steel Interconnects Used in Solid Oxide Fuel Cells

Doktorsavhandling, 2020

Nowadays the most widely known green energy conversion systems are batteries. However, also other green energy conversion systems exist, for example fuel cells. In a fuel cell hydrogen and oxygen react to form water, and during this reaction energy is set free. This chemical energy is harnessed as electrical energy. In comparison to batteries, fuel cells have some advantages, for example, they do not require recharging and instead are refueled, and hydrogen, as a fuel, has the highest energy density by mass compared to all other energy storage devices. Still fuel cells have not yet managed to become equally important as batteries in our daily life, amongst others, because of their cost and their limited life-times. Different fuel cell types exist, and the present work focuses on solid oxide fuel cells (SOFC), which are a highly efficient fuel cell type. The cost and the limited life-time of SOFCs can be greatly impacted by improving the interconnect. The interconnect is an important part of the fuel cell, where it electrically connects individual fuel cells to form a fuel cell stack. Nowadays, interconnects are made up of thin ferritic stainless-steel (FSS) plates that are formed into a corrugated form. This form allows the gases to pass through channels inside the fuel cell. Because of the very high temperatures in the fuel cell, between 600 °C and 850 °C, degradation of the interconnect occurs. During the exposure of the FSS interconnect to high temperatures, a Cr2O3 layer is formed on the surface of the FSS, and this layer continues to grow. This Cr2O3 layer protects the steel from further corrosion, but it also leads to some issues. For example, under humid conditions hexavalent Cr species evaporate from this layer and poison the cathode of the fuel cell. Additionally, the continued growth of the Cr2O3 layer can increase the interconnect resistance, which can lead to voltage degradation of the fuel cell. Both these degradation mechanisms can be mitigated by applying coatings to the interconnect. Spinel coatings, such as the (Mn,Co)3O4, have been shown to be highly effective at mitigating Cr evaporation, and reactive element coatings, such as Ce coatings have been shown to decelerate the Cr2O3 scale growth. The present work analyzes these coatings further and shows that the state-of-the-art Ce/Co coating (10 nm Ce/640 nm Co) is highly effective at reducing the degradation of the FSS even up to 4 years of exposure at 800 °C. Additionally it is shown that the coating does not increase the resistance of the interconnect, but instead the Cr2O3 scale is the main contributor of the overall resistance. There are many more factors influencing corrosion of interconnects in the SOFC, and thus, the performance of the fuel cell. The present work will elucidate some of these degradation mechanisms further.