Spinel Protective Coatings for Solid Oxide Cell (SOC) Interconnects (ICs)

AbstractThe degradation of metal interconnects (ICs) in Solid Oxide Cells (SOCs) primarily results from chromium (Cr) oxide scale growth on stainless-steel substrates, causing ohmic loss and air-side electrode poisoning by Cr. This thesis addresses these challenges by studying the degradation of AB2O4 spinel-coated ICs to enhance protection against Cr diffusion. The research employed four strategies: coating method development, material substitution in the spinel structure, Accelerated Stress Tests (AST) as short-term corrosion aging tests, and an oxidation kinetic study of long-term in-situ aged ICs, up to 40,000 hours of aging. As method development, ink-jet printing (IJP) was used to deposit protective coatings. As a wet-based method, the primary challenge was in formulating highly loaded particulate colloids to achieve sufficiently dense coatings. This induced complexity due to changes in interparticle interactions and an increased risk of particle agglomerations, potentially leading to printing nozzle blocking. Therefore, the focus was on developing a well-dispersed, stable, and agglomerate-free colloid, capable of achieving dense, uniform, and full-surface coverage protective layers. This colloidal stability was achieved using polyacrylic acid as a dispersant, through colloidal characterization and interparticle force modeling. Additionally, magnetron sputtering was used for comparison and for its capability to produce highly dense and uniform coatings, with the idea to evaluate grain boundary diffusion pathways of Cr diffusion. In terms of materials, the research explored substituting cobalt (Co) with copper (Cu) in Mn-Co-based spinel, driven by Cu's favorable thermophysical properties. This was conducted through thermodynamic evaluation and oxidation kinetic studies, including both short-term and long-term corrosion aging tests. Despite promising results in the literature regarding the short-term effectiveness of Cu-based spinels, the current findings revealed that this substitution does not meet the required durability standards for protective coatings. This is primarily due to Cu's high diffusivity and mobility, especially in its interaction with Cr.The study also involved degradation analysis of long-term operated ICs (40,000 hours), particularly with Fe-doped (MnCo)3O4 spinel coating. Special focus was given to studying the steel (scale)-coating interface, with the aim to understand the interactions of Cr scale and the spinel coating as well as identifying the dominant mechanisms of Cr diffusion. In the AST tests, the study focused on assessing how temperature and humidity could potentially accelerate degradation. The study evaluated the reliability of ASTs, particularly regarding changes in diffusion mechanisms under these conditions, and attempted to correlate AST findings with long-term real stack test behaviors. Based on the conducted studies, the findings provide valuable insights into the mechanism of Cr diffusion through spinel coatings. A key outcome is that ionic pathways play a predominant role, diminishing the significance of grain boundary diffusion. This understanding suggests a shift in focus towards enhancing the ionic resistance of spinel coatings, rather than concentrating on developing dense coatings, as a strategy for improving their effectiveness. This outcome opens new paths for the advancement of more robust and efficient protective coatings, thereby enhancing the durability of SOC interconnects.