Improving sealing, electrical contacts, and corrosion resistance in solid oxide fuel cell stacks

Solid oxide fuel cells are high temperature electrochemical devices for direct conversion of fuel and oxidant to electricity and heat. The high temperature and dual atmosphere creates challenges for the materials in solid oxide fuel cell stacks. In this thesis SOFC stack materials were studied and developed. SOFC stacks are assembled at room temperature, then heated up, conditioned and operated at 600...900 °C. Therefore, the mechanical properties of seals should be understood from room temperature to operating temperature. Mechanical properties of SOFC sealing materials were studied with different heat-up procedures. Increasing cell footprint presents a challenge for the sealing: the higher the cell area, the higher the required compressive force for the stack. To diminish the required compressive force, a hybrid sealing material consisting of a compressible core and a conformable glass-based interlayer was developed. This hybrid sealing material decreased leak rates by 60...90% compared to the conventional compressible seals. High operating temperature and exposure to both reducing and oxidizing atmospheres is prone to cause corrosion of materials. One example of these corrosion-mechanisms is chromium evaporation from interconnect steel materials. The evaporated chromium is transported in the gas phase to the electrochemically active cell, where it can solidify to chromium oxide, causing loss of performance. A chromium barrier coating based on MnCo1.8Fe0.2O4 was developed and deposited by a high-velocity oxygen flame method on the interconnect steel. The coating showed good stability and low area-specific resistivity, and effectively hindered chromium transport to the electrochemical cell.