Metallic materials for solid oxide fuel cells and electrolysers - Mitigating high temperature corrosion

Sammanfattning: Solid oxide cells (SOCs) are high-temperature energy conversion devices that have great potential due to their high efficiency, low operating costs, and flexibility. SOCs can produce electricity from a variety of fuels as solid oxide fuel cells (SOFCs), and they can convert electricity to fuels as solid oxide electrolyser cells (SOECs). However, the wide-spread commercialisation of this technology is hindered by high system cost, lack of durability, and poor performance stability during long-term operation. Owing to the high-temperature operation and aggressive environment of SOCs, metallic materials used for interconnects and balance of plant (BOP) components are subject to corrosion. Interconnects are typically made of ferritic stainless steel (FSS), which forms a protective chromia scale at high temperatures. The degradation mechanisms, such as Cr (VI) evaporation and chromia scale growth, lead to electrode poisoning and increased electrical resistance, which degrade cell performance. The primary objective of this thesis is to develop alternative materials and understand the degradative mechanisms so as to effectively reduce the costs and improve the performances of metallic materials in SOC systems. The Cr evaporation, oxide scale growth, the microstructural evolution of the oxide scale, and the area-specific resistances are investigated for the selected materials. The majority of the thesis is focused on Ce/Co coatings. Ce/Co-coated, low-cost, commercial FSS (AISI 441, AISI 430, and AISI 444) are compared to tailor-made Crofer 22 APU in air-side atmospheres. Ce/Co-coated steels are further investigated under dual-atmosphere conditions. The Ce/Co coating is compared to various coatings from research laboratories and universities world-wide. Furthermore, the underlying causes for the improvement in the oxidation resistances of FSS that occur in the presence of the reactive element Ce (in the Ce/Co coating) are investigated, and a new mechanism is proposed. Finally, a model to predict the lifetimes of the coated steels is proposed. Moreover, a new coating system, the Ce/FeNi coating, is proposed as an alternative to the Ce/Co coating. The Ce/FeNi coating is found to be more effective than Ce/Co coating in reducing chromia scale growth. While research on metallic materials for SOC has centred on the interconnects, the metallic materials used in BOP components, which can be a significant source of volatile chromium species, have been largely neglected. Five metallic materials (AISI 441, AISI 444, A197/Kanthal® EF101, alloy 800H, and alloy 600) are examined for potential usage in BOP components. The oxidation and Cr evaporation behaviours of these materials are discussed and correlated to the observed microstructures.