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

Sammanfattning: Interconnects are a vital part of solid oxide fuel cells (SOFC), where they electrically connect individual cells to form a fuel cell stack. They are a main contributor to the overall stack cost and the limited life-time of fuel cells, and, therefore, improvements carried out on the interconnect level could further the commercialization of SOFCs. The limited life-time of the interconnect is related to the material used today, ferritic stainless steels (FSS). FSS interconnects are more cost-effective than previously used ceramics, but they degrade under the conditions prevalent in an SOFC: high temperatures between 600 °C and 850 °C, and a p(O2) gradient. Certain corrosion phenomena that occur, such as Cr evaporation and continuous oxide scale growth, negatively impact cell performance due to cathode poisoning and increased electrical resistance, respectively. These phenomena have been found to be effectively mitigated by coatings, such as the (Co,Mn)3O4 (MCO) coating, or reactive element coatings, such as Ce. The present thesis examines these coatings with regard to three aspects: (i) does the semi-conducting spinel coating affect the electrical resistance of the interconnect negatively, or is its conductivity negligible in comparison to the continuously growing Cr2O3 scale below it; (ii) does the coating self-heal if it is cracked even at intermediate temperatures, i.e. 650 °C and 750 °C, or do the cracks persist and increase Cr evaporation; and (iii) is the long-term stability of the state-of-the-art Ce/Co coating (10 nm Ce/640 nm Co) still effective after 35 000 h, or not. The second aspect is not only important to understand corrosion behavior, but it would also allow for large-scale roll-to-roll PVD coating, which is significantly more cost-effective than batch coating. Another corrosion phenomenon that is elucidated within the scope of this work is the dual atmosphere effect. This effect leads to increased corrosion on the air-facing side of the interconnect if the FSS is exposed to a dual atmosphere, i.e. air on one side and hydrogen on the other side, compared to if the FSS is exposed to an air-only atmosphere. A new theory as to why the dual atmosphere effect occurs is proposed, and it is indirectly verified by means of excluding all other possibilities. Factors that influence the dual atmosphere effect are discussed, and it is shown how the dual atmosphere effect could, in part, be mitigated.