Solid oxide fuel cells (SOFCs) can be used for efficient energy conversion with a wide variety of fuels from hydrogen to gasified coal to fuels derived from renewable biomass. The fuel tolerance of SOFCs is due primarily to the high operating temperature, which accelerates reactions in the fuel. However, the high temperature also accelerates unwanted reactions and thus creates materials challenges, so intermediate temperature SOFCs are being developed to balance the benefits of efficiency and fuel tolerance at high temperatures against the detrimental effects of chemical reaction and thermal stresses generated during high-temperature operation. Although the reduced operating temperature mitigates some degradation mechanisms, degradation still occurs, particularly for long operating times, so there are remaining materials challenges for the development of long-life cost-effective SOFCs.
Through funding from the National Science Foundation (NSF) and the Department of Energy (DOE), Auburn University is addressing some of these challenges. One issue is the oxidation behavior of metallic interconnect materials in dual-atmosphere exposure. During SOFC operation, the interconnect is exposed simultaneously to air and fuel. The diffusion of hydrogen from the fuel through the alloy has been shown to affect the oxidation behavior of the alloy on the side exposed to air. In particular, locally high oxidation rates in dual atmosphere exposures can lead to the formation of nodules or thick oxide scales. Understanding this mechanism is the focus of a project funded by NSF through the Division of Materials Research.
Another degradation issue is chromium poisoning of the cathode, which occurs when chromium-containing vapor species from alloy scale are deposited on the cathode. The amount of chromium poisoning depends on the compositions of the various SOFC components, so an understanding of the effects of different elements on the cathode poisoning is needed to select the best materials for long-term operation. Work at Auburn has shown that small amounts of manganese or cobalt dissolved in the zirconia electrolyte increases the amount of chromium poisoning.
One of the approaches for reducing the amount of chromium poisoning is to apply ceramic coatings to the metallic alloys to reduce the amount of chromium volatilization. Auburn University is working with Pacific Northwest National Laboratory (PNNL) the development of ceramic coatings for SOFC interconnects. The DOE has funded fundamental thermodynamic and transport studies at Auburn University to support the coating development work being performed at PNNL. This includes characterization of the reaction layer between chromium oxide, which forms on the stainless steel interconnect material, and the ceramic coating, such as the (Mn,Co)3O4 spinel, as well as analysis of how this interaction will affect SOFC performance after many hours or operation.
