Area specific resistance of oxide scales grown on ferritic alloys for solid oxide fuel cell interconnects

Planar solid oxide fuel cells (SOFC) are considered to be power generators with high efficiency and low emission at small power units (1-200 kW_el). Many prototype systems are already successfully realized. For mass production the costs have to be reduced and the long-term stability has to be enhanced. Power losses <0.5%/1000 h is the target value for stacks in stationary SOFC-based power systems. To reach this goal, the factors influencing degradation have to be found and reduced. In this work the interaction between interconnect and different ceramic materials such as perovskites (La_0.8Sr_0.2(Mn_,Co)O₃, La_0.65Sr_0.3MnO₃, La_0.65Sr_0.3(Mn,Co)O₃) and spinels (Mn(Co,Fe)O₄, (Cu,Ni)Mn₂O₄) was investigated on the cathode (air) side of conventional ferritic interconnect materials (CroFer22APU, ITMLC, ZMG232L). The method to determine the value of the area specific resistance between interconnect and contact layer (R_#ICC) within a tolerance of 10% has been developed to provide reliable data for ASR values and their degradation.The R_#ICC-value increases with annealing time. The degree of this increase depends on used materials and their combination. The spinel contact layers form a thin dense ceramic layer at the beginning of the annealing process. This layer reduces the oxidation rate of the alloy. Because of this protection layer a thinner oxide scale grows and the ASR aging rate is much lower (0.4-0.9 mΩ cm²/1000 h). The comparison of the aging rates of different alloys with La_0.8Sr_0.2(Mn,Co)O₃ contact layer reveals remarkable differences: 3.1 mΩ cm²/1000 h for CroFer22APU, 10.9 mΩ cm²/1000 h for ITMLC and 21.2 mΩ cm²/1000 h for ZMG232L.The degradation in a stack has been determined from the R_#ICC-values and geometric factors. The impact of oxidation at the cathode side of interconnect is about one third of the total stack degradation. The method opens the possibility for comparing area specific resistances of special material combinations with high accuracy. By optimized material combinations the degradation in stacks can be reduced to <0.5%/1000 h.