Metallic Interconnects for Solid Oxide Fuel Cells: High Temperature Corrosion and Protective Spinel Coatings
MetadataShow full item record
Metallic Interconnects for Solid Oxide Fuel Cells: Increasing the share of renewable energy is important in the effort to limit global warming. The challenge with increased dependence on renewable energy sources such as sun and wind power is handling the fluctuations in energy production with changes in the weather. A promising solution to this challenge is using hydrogen as an energy carrier. Solid oxide fuel cells (SOFC) offer an environmentally friendly way of efficiently converting hydrogen to electrical energy. These cells can also be operated in the reverse mode to produce hydrogen by electrolysis of water. However, high costs and a limited lifetime inhibit the commercialization of SOFC. These issues are in part related to degradation of the ferritic stainless steel interconnect material. There are two main challenges with the use of ferritic stainless steel as the interconnect material: i) the increase in electrical resistance due to growth of modestly conductive oxide scales; and ii) the vaporization of Cr(VI) species leading to poisoning of the SOFC cathode. This thesis investigates the use of protective spinel oxide coatings as a way of mitigating these issues. The principal aim of the thesis is to contribute to a better understanding of the performance of spinel oxide coatings and their interaction with ferritic stainless steel during operation. It was shown that coatings based on MnCo2O4 are effective for reducing the oxidation rate of ferritic stainless steel and maintaining a low electrical resistance. In the first part of this thesis, iron and copper substitutions in MnCo2O4 were explored as a way of improving coating performance. Iron substitutions proved beneficial for lowering the thermal expansion coefficient, resulting in a better match with the interconnect and other SOFC materials. Copper substituted materials suffered from poor stability, but provided acceptable protection when applied as a coating. In the second part of this thesis the possibility to reduce coating costs by simplifying the heat treatment procedure was investigated. To ensure high density while avoiding excessive damage of the interconnect alloy, spinel coatings have typically been sintered in a two-step reduction and re-oxidation procedure. It was shown that coatings sintered in a single step (in air) can be nearly as effective in reducing chromium evaporation, despite being significantly more porous.