Structure and Electrical Conductivity of Mn-based Spinels Used as Solid Oxide Fuel Cell Interconnect Coatings

Abstract

<p>At solid oxide fuel cell (SOFC) operating temperatures (650^oC--800^oC), the chromia scale growth on the interconnect surface and chromium poisoning of cathode can lead to performance degradation of the whole cell. A spinel coating can be effective for blocking chromium outward diffusion to overcome this issue. In this thesis, two spinel-forming systems, Zn-Mn-O and Co-Cu-Mn-O were studied to identify a suitable coating.</p> <p>In-situ high temperature XRD was used to identify the phases in the Zn-Mn-O system between 600^oC and 1300^oC. The results showed that cubic spinel phase was stable only at high temperatures (above 1200^oC) and when the temperature decreases, the cubic phase tends to deform to tetragonal structure. In addition, the conductivity results showed low conductivities (below 3 S/cm) at SOFC operating temperature. Thus, the Zn-Mn-O system is not suitable for SOFC interconnect coatings.</p> <p>Another potential coating material analyzed was the Co-Cu-Mn-O system. This system exhibited promising conductivity values. Electrodeposition was used to apply Co-Cu-Mn-O coatings on both ferritic stainless steel and chromium-based alloy (Cr-5Fe) followed by oxidation in air at 800^oC. The spinel coating formed nicely on the stainless steel substrate. However, on the chromium plate, nitride formation, blistering and metal isolation were the common problems that occurred during the oxidation process. In order to improve the quality of coating on the chromium alloy, different heat treatments were explored, such as annealing in reducing atmosphere, oxidation in pure oxygen / mixed gas and decreasing the oxidation temperature. The objective of modifying the heat treatment was to produce adherent, dense coatings.</p>