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

In solid oxide fuel cell systems, the stack is the primary component whose performance and lifetime should be maximized while decreasing the cost. In this thesis, leakages, electrical contact resistance, and corrosion resistance in SOFC stacks were studied and developed. Typically, SOFC stacks are assembled at room temperature, then heated up and conditioned, and then operated at temperatures in the range of 600...900 °C. Therefore, the mechanical properties of seals should be understood from room temperature to operating temperature. Of special interest are the mechanical properties of materials during the first heat up, in which the stack is sealed, reduced, and tested. Mechanical properties of glass and compressible sealing materials were studied with different heat-up procedures. It was noticed that with compressible Thermiculite 866 or CL87 materials, the compressibility is diminished after the first heat up, and therefore it is beneficial to apply compression before heating, to obtain maximum deformation capability of the seal. The progress in manufacturing SOFC cells is leading to an increase in cell area. From the perspective of compressible seals, the increase in cell area presents a challenge: the higher the cell area, the higher the required compressive force for the stack. For this purpose, a hybrid sealing material capable of maintaining leak rates below 1% of the inlet fuel flow below 1 MPa of compressive stress was developed. The material consists of a compressible core of Thermiculite 866, a commercial material consisting of vermiculite and steatite, and a conformable glass-based interlayer. The interlayers seal the mating surfaces, thus diminishing the leakages through the interfaces. Using the coating technique, leak rates were diminished by 60...90% compared to the uncoated seals. Post-mortem analyses of a stack also showed no signs of corrosion caused by the glass-coating. A high operating temperature and exposure to both reducing and oxidizing atmospheres is prone to cause corrosion of materials. One example of these corrosion-related deactivation 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. These phenomena can be mitigated with chromium barrier coatings on interconnect steels. A MnCo1.8Fe0.2O4 coating deposited by a high-velocity oxygen flame (HVOF) method was prepared and tested both with ex-situ and stack tests. The prepared coating showed good stability and low areaspecific resistivity, and was found to hinder chromium transport to the electrochemical cell.