Electrically conductive spinel and perovskite phases for solid oxide fuel cell interconnect applications

As the operating temperature of solid oxide fuel cell (SOFC) reduces to below 800°C, metallic alloys can be used as the interconnect material. Mn1+xCr2-xO4 spinel was found to form on the surface of the interconnect alloy Crofer, as well as near the interface of LaMnO3-based cathode and Cr2O3-forming interconnect alloys such as Ebrite during long-term operation. However, the effect of the spinel formation on the performance of the fuel cell stack is yet unclear.; The fundamental properties of the Mn1+xCr2-xO 4 spinel were studied in this dissertation. Spinel phase based on Mn 1+xCr2-xO4 (0 ≤ x ≤ 1) was synthesized and characterized. The lattice parameter of the spinel was found to increase with increasing Mn content up to x = 0.7. At x > 0.7, the cubic spinel changed to tetragonal because of Jahn-Teller distortion. The electrical conductivity of the spinel also increased with increasing Mn content. Electrical conduction in this phase was likely through a small polaron hopping mechanism. The spinel phase showed similar electrical conductivity in both oxidizing and reducing environments expected in solid oxide fuel cell operation.; In addition, the chemical compatibility between the spinel phase Mn 1.5Cr1.5O4 and the SOFC cathode materials La 0.8Sr0.2MO3 (M = Mn, Co, Fe) was studied and its impact on SOFC stack performance was discussed. While no new phases were formed after annealing, interdiffusion of the cations in the perovskite and spinel phases was detected. After high temperature annealing, the bulk and interfacial resistivities changed due to interdiffusion of the cations in the two phases, which was revealed from AC impedance spectra.; Finally, electrically conductive perovskite coating on Cr2O 3-forming alloy interconnect was proposed to slow down the Cr2 O3 scale growth on metallic alloy and to mitigate the evaporation problem. An economical dip coating process was used to synthesize LaCrO 3 coating on the ferritic alloy Ebrite substrate. XRD results indicated that the LaCrO3 perovskite phase was formed after annealing for both the LaCrO3 and La2O3 precursors. The LaCrO3 coatings were found to cause a pronounced reduction in oxidation rate of the alloy, especially with low La content precursors. The area specific resistance of the oxide scales formed on the bare and coated alloy substrates was also evaluated and discussed.