Transition Metal Doping of Manganese Cobalt Spinel Oxides for Coating SOFC Interconnects

Manganese cobalt spinel oxide coatings have been shown to be effective for preventing chromium volatilization from interconnect alloys and associated cathode poisoning in solid oxide fuel cells. A reaction layer consisting of chromium-containing spinel formed due to interaction of the coating with the chromia scale on the alloy surface and increases the overall area specific resistance (ASR) of the system. Reduction in ASR can be accomplished by reducing the thickness and/or increasing the electrical conductivity of the reaction layer, which includes both the high-chromium spinel phase and the chromia scale. This paper reports the effects of transition metal dopants in manganese-cobalt spinel on the reaction products and chromia scale formed on a ferritic stainless steel alloy. Dense single-phase doped spinel ceramics were prepared through process optimization. High temperature diffusion couple experiments with sintered spinel and ferritic stainless steel SS 441 were carried out to understand the spinel-alloy interactions. Doping of the spinel oxide led to a reduction in the thickness of the chromia scale formed on the alloy. The difference in the morphologies and compositions of interaction layers at 800°C and 900°C provides insight on the formation and growth mechanism of a high-chromium spinel.