Coatings for metal interconnects to reduce SOFC degradation

1. A method of coating an interconnect for a solid oxide fuel cell, comprising:

providing an interconnect substrate comprising a chromium-iron alloy comprising at least 70 weight percent chromium;

removing a native chromia layer from the interconnect substrate to expose a chromium-iron alloy surface of the interconnect substrate;

providing a mixture of manganese cobalt oxide spinel feedstock powder and strontium-doped lanthanum manganate perovskite feedstock powder into a plasma to coat an air side of the interconnect substrate with a single layer composite coating containing strontium-doped lanthanum manganate and manganese cobalt oxide directly on the exposed chromium-iron alloy surface of the interconnect substrate using a plasma spray process;

placing the coated interconnect substrate into a solid oxide fuel cell stack, such that the single layer composite coating directly contacts a solid oxide fuel cell of the stack and directly contacts the air side of the interconnect substrate, the air side of the interconnect substrate comprising the chromium-iron alloy comprising at least 70 weight percent chromium; and

conditioning the interconnect substrate, having on its air side only the single layer composite coating, in the solid oxide fuel cell stack, to form a manganese-cobalt-chromium intermediate spinel layer between the composite coating and the chromium-iron alloy surface of the air side of the interconnect substrate, the intermediate spinel layer comprising a (Mn, Cr, Co) 3 O 4 spinel phase and the composite coating comprising a strontium-doped lanthanum manganate perovskite phase and a manganese cobalt oxide spinel phase;

wherein:

forming the manganese-cobalt-chromium intermediate spinel layer during the step of conditioning comprises forming the intermediate spinel layer by reacting the single layer composite coating with the chromium-iron alloy surface of the air side of the interconnect substrate at an elevated temperature after the step of placing;

the composite coating comprises distinct regions of the manganese cobalt oxide spinel phase in a matrix comprising the strontium-doped lanthanum manganate perovskite phase;

the manganese cobalt oxide spinel phase regions have plate-like or pancake-like structures;

the plate-like or pancake-like structures comprise crack-healing structures within the composite coating which suppress Cr evaporation through cracks generated in the matrix comprising the strontium-doped lanthanum manganate perovskite phase during operation of the solid oxide fuel cell stack; and

the matrix comprising the strontium-doped lanthanum manganate perovskite phase stabilizes the composite coating in reducing atmospheres such that spallation does not occur during operation of the solid oxide fuel cell stack.

2. The method of claim 1 , wherein the manganese cobalt oxide spinel comprises Mn 2−x Co 1+x O 4 , wherein 0≤x≤1.

3. The method of claim 2 , wherein the manganese cobalt oxide spinel comprises Mn 1.5 Co 1.5 O 4 .

4. The method of claim 2 , wherein:

the manganese cobalt oxide spinel comprises a Co:Mn atomic ratio of at least 1:3; and

the interconnect substrate comprises a chromium-iron alloy containing 92 to 97 weight percent chromium, 3 to 7 weight percent iron, and 0 to 1 weight percent of yttrium or yttria.

5. The method of claim 1 , wherein the composite coating further comprises a getter.

6. The method of claim 5 , wherein the getter comprises at least one of Al 2 O 3 , Y 2 O 3 , or TiO 2 .

7. The method of claim 1 , wherein the intermediate spinel layer comprises 60 to 100 volume percent of the (Mn, Cr, Co) 3 O 4 spinel phase.

8. The method of claim 1 , wherein the composite coating comprises a perovskite phase to spinel phase weight ratio between 20:80 and 90:10.

9. The method of claim 1 , wherein the composite coating comprises a perovskite phase to spinel phase weight ratio between 50:50 and 80:20.