WO2013130515A1 - Coatings for sofc metallic interconnects - Google Patents

Abstract

Various methods of treating a chromium iron interconnect for a solid oxide fuel cell stack and coating the interconnect with a ceramic layer are provided.

Description

Atty. Dkt. No. 7917-451 WO

Coatings For SOFC Metallic Interconnects

RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Application Serial No. 13/409,629, filed March 1, 2012, U.S. Provisional Application Serial No. 61/605,309, filed March 1, 2012 and U.S. Provisional Application Serial No. 61/702,397, filed September 18, 2012, all of which are hereby incorporated by reference in their entirety.

FIELD

[0002] The present invention is directed to fuel cell stack components, specifically to interconnects and methods of making interconnects for fuel cell stacks.

BACKGROUND

[0003] A typical solid oxide fuel cell stack includes multiple fuel cells separated by metallic interconnects (IC) which provide both electrical connection between adjacent cells in the stack and channels for delivery and removal of fuel and oxidant. The metallic interconnects are commonly composed of a Cr based alloy such as an alloy known as CrF which has a composition of 95wt Cr-5wt Fe or Cr-Fe-Y having a 94wt Cr-5wt Fe- lwt Y composition. The CrF and CrFeY alloys retain their strength and are dimensionally stable at typical solid oxide fuel cell (SOFC) operating conditions, e.g. 700-900C in both air and wet fuel atmospheres. However, during operation of the SOFCs, chromium in the CrF or CrFeY alloys react with oxygen and form chromia, resulting in degradation of the SOFC stack.

[0004] Two of the major degradation mechanisms affecting SOFC stacks are directly linked to chromia formation of the metallic interconnect component: i) higher stack ohmic resistance due to the formation of native chromium oxide (chromia, Ο¾θ3) on the Atty. Dkt. No. 7917-451 WO interconnect, and ii) chromium poisoning of the SOFC cathode.

[0005] Although ¾(¾ is an electronic conductor, the conductivity of this material at SOFC operating temperatures (700-900C) is very low, with values on the order of O.OlS/cm at 850C (versus 7.9 x 10⁴ Scm^"1 for Cr metal). The chromium oxide layer grows in thickness on the surfaces of the interconnect with time and thus the ohmic resistance of the interconnect and therefore of the SOFC stack due to this oxide layer increases with time.

[0006] The second degradation mechanism related to the chromia forming metallic interconnects is known as chromium poisoning of the cathode. At SOFC operating temperatures, chromium vapor diffuses through cracks or pores in the coating and chromium ions can diffuse through the lattice of the interconnect coating material into the SOFC cathode via solid state diffusion. Additionally, during fuel cell operation, ambient air (humid air) flows over the air (cathode) side of the interconnect and wet fuel flows over the fuel (anode) side of the interconnect. At SOFC operating temperatures and in the presence of humid air (cathode side), chromium on the surface of the &₂θ₃ layer on the interconnect reacts with water and evaporates in the form of the gaseous species chromium oxide hydroxide, Cr0₂(OH)₂. The chromium oxide hydroxide species transports in vapor form from the interconnect surface to the cathode electrode of the fuel cell where it may deposit in the solid form, Ο¾θ₃. The C¾0₃ deposits on and in (e.g., via grain boundary diffusion) the SOFC cathodes and/or reacts with the cathode (e.g. to form a Cr-Mn spinel), resulting in significant performance degradation of the cathode electrode. Typical SOFC cathode materials, such as perovskite materials, (e.g., LSM, LSC, LSCF, and LSF) are particularly vulnerable to chromium oxide degradation.

SUMMARY

[0007] An embodiment relates to a coated interconnect for a solid oxide fuel cell Atty. Dkt. No. 7917-451 WO including an interconnect substrate comprising iron and chromium and a manganese cobalt oxide spinel coating formed over an air side of the interconnect substrate and a method of making and treating thereof. The manganese cobalt oxide spinel coating comprises a composition in the (Mn,Co)₃0₄ family between the end compositions of Ο¾0₄ and Mn₃0₄.

[0008] An embodiment relates to a method of coating an interconnect for a solid oxide fuel cell, comprising providing an interconnect substrate comprising Cr and Fe, and coating an air side of the interconnect substrate with a manganese cobalt oxide spinel coating using a plasma spray process.

[0009] Another embodiment relates to a coated interconnect for a solid oxide fuel cell, comprising an interconnect substrate comprising at least 70 weight percent chromium, and a manganese cobalt oxide spinel coating formed over an air side of the interconnect substrate, wherein the spinel comprises a Co:Mn atomic ratio of at least 1:3.

[0010] Another embodiment relates to a coated interconnect for a solid oxide fuel cell, comprising an interconnect substrate comprising iron and chromium, a manganese cobalt oxide spinel coating formed over an air side of the interconnect substrate, a manganese- cobalt-chromium intermediate spinel layer located between the spinel coating and the air side of the interconnect substrate, and a perovskite layer located over the spinel coating.

[0011] Another embodiment relates to a coated interconnect for a solid oxide fuel cell. The interconnect includes an interconnect substrate including iron and chromium and a composite spinel and perovskite coating formed over an air side of the interconnect substrate. In an aspect, the spinel phase comprises manganese cobalt oxide spinel and the perovskite phase comprises lanthanum strontium manganate.

[0012] Another embodiment relates to a method of making an interconnect for a solid oxide fuel cell including forming a manganese cobalt oxide spinel coating on an air side of Atty. Dkt. No. 7917-451 WO the interconnect except in at least one seal region on the air side of the interconnect.

[0013] Another embodiment relates to a method of making an interconnect for a solid oxide fuel cell including coating an air side of a Cr-Fe interconnect with a ceramic layer to form a native chromium oxide layer on a fuel side of the interconnect, depositing a nickel containing material on the native chromium oxide layer and diffusing the nickel into the native chromium oxide layer to form a nickel metal-chromium oxide composite layer.

[0014] Another embodiment relates to a method of making an interconnect for a solid oxide fuel cell including coating an air side of the interconnect with a ceramic layer to form a native chromium oxide on a fuel side of the interconnect and annealing the interconnect at a temperature of at least 900 C in an atmosphere having a partial pressure of oxygen between 10^"16 and 10^"24 atm to reduce or eliminate the native chromium oxide layer.

[0015] Another embodiment relates to a method of making an interconnect for a solid oxide fuel cell including providing a interconnect comprising a pressed green mixture of Cr and Fe powder, coating an air side of an interconnect with a layer of material comprising ceramic powder and sintering the green mixture of Cr and Fe powder and the layer of ceramic powder in the same sintering step.

[0016] Another embodiment relates to a method of making an interconnect for a solid oxide fuel cell including providing an interconnect comprising Cr and Fe, coating the interconnect with manganese cobalt oxide precursors comprising one or more of MnO, CoO, Mn metal, Co metal and combinations thereof and sintering the manganese cobalt oxide precursors to form a manganese cobalt oxide spinel coating.

[0017] Another embodiment relates to a coated interconnect for a solid oxide fuel cell including an interconnect substrate comprising iron and chromium and a manganese cobalt oxide spinel coating formed over an air side of the interconnect substrate. The manganese Atty. Dkt. No. 7917-451 WO cobalt oxide spinel coating comprises a composition in a (Mn,Co)₃0₄ family between the end compositions of C0₃O₄ and Mn₃0₄, wherein an atomic ratio of Mn:Co > 5: 1.

[0018] Another embodiment relates to a coated interconnect for a solid oxide fuel cell including an interconnect substrate comprising iron and chromium and a manganese cobalt oxide spinel coating formed over an air side of the interconnect substrate. The manganese cobalt oxide spinel coating further comprises at least one of iron, titanium, vanadium, chromium, aluminum, manganese, calcium, silicon and/or cerium.

[0019] Another embodiment relates to a coated interconnect for a solid oxide fuel cell including an interconnect substrate comprising iron and chromium, a manganese cobalt oxide spinel coating formed over an air side of the interconnect substrate and a barrier layer between the interconnect substrate and the manganese cobalt oxide spinel coating.

[0020] Another embodiment relates to a method of making a coated interconnect for a solid oxide fuel cell including forming a reactive layer and a manganese cobalt oxide spinel layer over the interconnect and diffusing the dopant from the reactive layer into the manganese cobalt oxide spinel layer.

[0021] Another embodiment relates to a method of making a coated interconnect for a solid oxide fuel cell including forming an interconnect comprising Fe, Cr and a dopant, coating the interconnect with a manganese cobalt oxide spinel layer and diffusing dopant from the interconnect into the manganese cobalt oxide spinel layer.

[0022] Another embodiment relates to a method of making a coated interconnect for a solid oxide fuel cell including providing an interconnect comprising a chromium iron alloy, removing a first native chromium oxide from an air side but not a fuel side of the interconnect, coating the air side of the interconnect with a manganese cobalt oxide spinel layer and forming a second native chromium oxide on the fuel side of the interconnect, and Atty. Dkt. No. 7917-451 WO removing the second native oxide from the fuel side of the interconnect.

[0023] Another embodiment relates to a method of making an interconnect for a solid oxide fuel cell including providing an interconnect comprising Cr and Fe and coating the interconnect with a ceramic layer with an air plasma spray process. The ceramic layer includes a sintering aid.

[0024] Another embodiment relates to a method of making an interconnect for a solid oxide fuel cell stack including providing first metallic powder particles comprising Cr and Fe in a mold cavity, providing second powder particles comprising one or more of Sr, La, Mn and Co oxides in the mold cavity and compacting the first and second powder particles to form the interconnect.

[0025] Another embodiment relates to a coated interconnect for a solid oxide fuel cell including an interconnect substrate comprising iron and chromium, a manganese cobalt oxide spinel coating formed over an air side of the interconnect substrate, wherein the manganese cobalt oxide spinel coating further comprises at least one or nickel and copper, a manganese and chromium containing oxide intermediate spinel layer located between the manganese cobalt oxide spinel coating and the air side of the interconnect substrate and a perovskite layer located over the manganese cobalt oxide spinel coating.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1A is a micrograph showing a Mn-Cr spinel phase inside the pores of an LSM based cathode.

[0027] FIG. IB is a micrograph showing a Cr containing phase in the cracks of an LSM interconnect coating that was deposited by air plasma spray. The SOFC stack was operated for 2000 rs at 850C.

[0028] FIG. 2A is a side schematic illustration of an embodiment of an interconnect with Atty. Dkt. No. 7917-451 WO a spinel MCO coating and an underlying interfacial oxide. FIG. 2B illustrates a micrograph of an as-deposited APS Mn1.5C01.5O4 coating according to an embodiment.

[0029] FIG. 2C illustrates a plot of voltage versus time of SOFC stacks tested in dry air comparing degradation rates of repeat elements with LSM to Mn1.5C01.5O4 spinel interconnect coatings.

[0030] FIG. 2D illustrates a plot of voltage versus time of SOFC stacks tested in wet air comparing degradation rates of repeat elements with LSM, Mn1.5C01.5O4 spinel, and double layer LSM-Mni 5C01 5Ο4 interconnect coatings.

[0031] FIGS. 2E and 2F illustrate micrographs of Mn1.5C01.5O4 spinel coating and LSM perovskite coating, respectively after lOOOhrs of operation in SOFC stack.

[0032] FIGs. 3A-3C are a schematic illustration of steps in a method of making an interconnect according an embodiment.

[0033] FIG. 4 is another schematic illustration of a method of making an interconnect according an embodiment.

[0034] FIG. 5 is a plot illustrating the increase in area specific resistance (ASR) with time of doping coated and undoped coated interconnects.

[0035] FIG. 6A is a side schematic illustration of an embodiment of an interconnect with a bilayer composite coating. FIG. 6B is a side schematic illustration of an embodiment of a solid oxide fuel cell stack which includes an interconnect with a bilayer coating on its ribs.

[0036] FIGs. 7A-7C are schematic illustrations illustrating: (A) the air side of an interconnect according to an embodiment, (B) a close up view of the seal portion of the air side of the interconnect and (C) the fuel side of the interconnect.

[0037] FIG. 8 is a micrograph illustrating chromium oxide on a fuel side (uncoated side) Atty. Dkt. No. 7917-451 WO of an interconnect after a reduction sintering step.

[0038] FIG. 9 is a micrograph of a portion of a SOFC stack illustrating reduction of an MCO coating (in a strip seal area) at the coating/IC interface due to fuel diffusing through porous IC.

[0039] FIG. 10 is a phase diagram illustrating the Μη3θ₄-Ο¾θ4 system.

[0040] FIG. 11 is a schematic illustration of a fuel inlet riser in a conventional fuel cell stack.

[0041] FIG. 12 is a schematic illustration of a SOFC illustrating a theory of electrolyte corrosion.

[0042] FIG. 13 is a micrograph illustrating an embodiment of an interconnect with a composite LSM-MCO coating.

DETAILED DESCRIPTION

[0043] To limit the diffusion of chromium ions (e.g., Cr³⁺) through the interconnect coating material to the SOFC cathode, materials may be selected that have few cation vacancies and thus low chromium diffusivity. A series of materials that have low cation diffusivity are in the perovskite family, such as lanthanum strontium oxide, e.g.

Lai__xSr_xMn0₃ (LSM), where 0.1<x<0.3, such as 0.1<x<0.2. These materials have been used as interconnect coating materials. In the case of LSM, the material has high electronic conductivity yet low anion and cation diffusion.

[0044] A second role of the interconnect coating is to suppress the formation of the native oxide on the interconnect surface. The native oxide is formed when oxygen reacts with chromium in the interconnect alloy to form a relatively high resistance layer of Cr₂0₃. If the interconnect coating can suppress the transport of oxygen and water vapor from the air to the Atty. Dkt. No. 7917-451 WO surface of the interconnect, then the kinetics of oxide growth can be reduced.

[0045] Similar to chromium, oxygen (e.g., O^2" ions) can transport through the coating via solid state diffusion or by gas transport through pores and cracks in the coating. This mechanism is also available for airborne water vapor, an accelerant for Cr evaporation and possibly oxide growth. As discussed above, in a humid air environment, chromium evaporates from the surface of Q^C in the form of the gas molecule Cr0₂(OH)₂ that can subsequently diffuse through defects, such as pore and cracks, in the coating(s). In the case of oxygen and water vapor, the molecules diffuse through the defects by either bulk diffusion or by a Knudsen diffusion process, depending on the size of the defect or pore.

[0046] If a Cr0₂(OH)₂ molecule touches the coating surface, it may react to form a crystal and then re-evaporates to continue diffusing in the gas stream (in the crack or pore). Experiments have shown that Cr0₂(OH)₂ reacts with the LSM interconnect coating 104 to form a spinel phase 105, e.g. manganese chromium oxide (Mn, Ο")₃0₄ as shown in Figure 1A. Although Cr0₂(OH)₂ reacts with LSM to form the spinel phase, the chromium species is not prohibited from re-evaporating and diffusing farther down the crack or defect.

Chromium has been observed transporting along the lengths of cracks in LSM IC coatings that have operated in fuel cells for extended periods of time. Figure IB shows chromium crystals 105 in cracks 103 in an LSM IC coating 104 that was operated in an SOFC stack for 2000 hrs under normal conditions of 800-850C with ambient air on the cathode side. The chromium-containing crystal formations are characteristic of those formed from a vapor-to- solid phase transformation. SEM and EDS analysis of the bulk LSM coating away from the cracks do not show the presence of chromium. Therefore, it may be concluded that the majority of chromium transport from the CrF interconnect is through the LSM IC coating is via gas phase transport through and along micro- and macro-cracks, inter-particle spaces, and Atty. Dkt. No. 7917-451 WO porosity in the LSM coating.

[0047] In the case of solid state transport, materials are chosen that have few oxide ion vacancies and thus low oxide ion conductivity. For example, the perovskite LSM is unique in that it exhibits both low cation and anion conductivity yet possesses high electronic conductivity, making it a very good coating material. Other perovskites such as Lai_ _xSr_xFe03-d, Lai__xSr_xCo03_d, and Lai__xSr_xCoi__yFe_y03_d all exhibit high electronic conduction and low cation conduction (low chromium diffusion rates). However, these particular materials also exhibit high oxide ion conductivities and thus are less effective at protecting the interconnect from oxidation (oxide growth).

[0048] A second material family that can be used for interconnect coating are the manganese cobalt oxide (MCO) spinel materials. Figure 2 A illustrates the MCO spinel layer 102 over an air side of a chromium containing interconnect 100. The MCO spinel layer 102 may have the following formula (Ml, Μ2)₃θ4₊ο.ι where Ml comprises at least 70 atomic percent, such as 70-100 at manganese, and M2 comprises at least 70 atomic percent, such as 70-100 at cobalt. Ml and/or M2 may contain additional elements, as will be described with respect to the subsequent embodiments below. The MCO spinel encompasses the compositional range from MI2M21O4+0.1 to M22MI 1O4+0.1.

[0049] In an embodiment, Ml consists of Mn (and unavoidable impurities, if any) and M2 consists of Co (and unavoidable impurities, if any) and the spinel is stoichiometric (i.e., the metal to oxygen atomic ratio is 3:4).

[0050] In an embodiment, the MCO spinel encompasses the compositional range from Mn2Co04 to Co2Mn04. That is, any spinel having the composition Mn2-_xCoi_+x04 (0 < x < 1) or written as z(M¾04) + (I-ZXC0₃O4), where (1/3 < z < 2/3) or written as (Mn, Co)₃04 may be used, such as Mn1.5C01.5O4, MnCo204 or Mn2Co04. Many of the spinels that contain Atty. Dkt. No. 7917-451 WO transition metals exhibit good electronic conductivities and reasonably low anion and cation diffusivities and are therefore suitable coating materials.

[0051] Preferably the spinel composition contains at least 25 atomic percent of cobalt oxide, such as 25 to 60 atomic percent cobalt oxide. Another way to phrase this is that the atomic ratio of Co to Mn in the spinel is preferably at least 1 :3, such as 1 :3 to 6:4, preferably 1 : 1. Thus, the preferred but non-limiting spinel composition is Mn1.5C01.5O4 which comprises 50 atomic percent manganese oxide and fifty atomic percent cobalt oxide. The MCO coating 102 may have any suitable thickness, such as 20 to 100 microns, preferably greater than 20 microns, such as 25 to 40 microns.

[0052] Any suitable chromium containing interconnect substrate 100 may be used.

Preferably, the substrate 100 is a chromium based alloy, such as an alloy containing at least 70 weight percent chromium, for example 92 to 97 weight percent chromium, 3 to 7 weight percent iron, and optionally 0 to 1 weight percent of yttrium, yttria, other alloying elements and/or unavoidable impurities. Preferably, the substrate 100 comprises the so-called CrF alloy (e.g., 95 weight percent Cr and 5 weight percent Fe). The alloy may be oxidized on its surface and/or throughout its volume, such that the substrate contains a chromium and/or iron oxide layer on its surface or oxide regions in its volume. However, other suitable substrate 100 materials may be used instead, such as nicrofer, Inconel 600 or X750, Crofer 22 APU or other chromium containing stainless steels.

[0053] As shown in Figure 2A, the interconnect may contain an intermediate (i.e., interfacial) oxide layer 101 between the substrate 100 and the MCO spinel coating 102. The intermediate oxide layer 101 may comprise a native chromia layer on the CrF interconnect substrate 100 and/or the intermediate oxide layer 101 may comprise an intermediate spinel layer containing Cr, Mn, O and optionally Co. For example, the intermediate oxide layer 101 Atty. Dkt. No. 7917-451 WO may comprise a (Mn, Cr, Co)₃04 spinel layer formed by reacting the MCO coating 102 with the chromium containing substrate 100 during the SOFC stack high temperature operation or stack annealing to melt the seals or reduce a nickel oxide in the cermet SOFC anode electrodes to nickel, as will be described below. The intermediate layer 101, if present, may have a thickness of 5 microns or less, such as 1 to 5 microns.

[0054] The MCO coating 102 is deposited on the interconnect substrate 100 using any suitable deposition method. Preferably, the coating 102 is deposited by a plasma spray process, such as an air plasma spray (APS) process. In a plasma spray process, a feedstock powder is introduced into a plasma jet or spray, emanating from a plasma source, such as a plasma torch. The feedstock powder is melted in the plasma jet (where the temperature is over 8,000K) and propelled towards the interconnect substrate 100. There, the molten droplets flatten, rapidly solidify and form the MCO spinel coating 102. Preferably, the feedstock powder comprises MCO powder having the same composition as the coating 102. However, metal (e.g., Mn, Co or Mn-Co alloy) powder may be used instead and subsequently oxidized to form the MCO spinel coating 102.

[0055] The plasma may be generated by either direct current (e.g., electric arc DC plasma) or by induction (e.g., by providing the plasma jet through a center of an induction coil while a RF alternating current passes through the coil). The plasma may comprise a gas stabilized plasma (e.g., argon, helium, etc,). Preferably, the plasma spraying is air plasma spraying (APS) which is performed in ambient air. Alternatively, a controlled atmosphere plasma spraying (CAPS) method may be used which is performed in a closed chamber, which is either filled with an inert gas or evacuated.

[0056] Preferably, the native oxide layer is removed from the interconnect substrate 100 prior to the deposition of the coating. For example, the native chromia layer may be removed Atty. Dkt. No. 7917-451 WO from the CrF substrate 100 by grinding, polishing, grit blasting, etching or other suitable methods before deposition of the MCO coating 102, such that the native chromia does not substantially reform prior to MCO coating deposition. Figure 2B is a micrograph illustrating an as-deposited Mn1.5C01.5O4 coating 102 on an interconnect 100 after the native chromia layer is removed. The Mn1.5C01.5O4 coating 102 exhibits good density with some porosity and microcracking.

[0057] Planar SOFC stacks containing some interconnects coated with the Mn1.5C01.5O4 coating and some interconnects with an LSM coating were tested to give a head-to-head comparison of the coatings. The results of these tests are illustrated in Figures 2C and 2D. Figures 2C and 2D are plots of the voltage verses time for two different SOFC stacks that both contain Mn1.5C01.5O4 and LSM coated interconnects 100. The results shown in Figure 2C are for a stack tested with dry air while the results shown in Figure 2D are for a stack tested with humid air. In both cases, the degradation rates of the repeat layers with the Mn1.5C01.5O4 coatings are significantly lower than those with the LSM coatings.

[0058] In the case of dry air testing (Figure 2C), the Mn1.5C01.5O4 coating exhibited a lower ASR degradation relative to the LSM coating. The lower resistivity of the Mn and Co doped Cr spinel layer 101 results in lower degradation of the interconnects and the adjacent SOFC cathode electrodes for Mn1.5C01.5O4 coated interconnects relative to LSM coated interconnects (and their adjacent SOFC cathode electrodes). The resistance of this intermediate chromium containing oxide layer is dependent on both the thickness of the intermediate oxide layer and electrical conductivity of the oxide layer. The coating layer affects the resistance of the oxide layer in two ways, by: i) reducing the growth rate and thus thickness of oxide layer at a given time, and ii) reacting with the oxide layer to produce a secondary oxide phase that has a different composition and conductivity. Atty. Dkt. No. 7917-451 WO

[0059] The MCO coating 102 acts as a barrier layer suppressing the diffusion of oxygen from the air stream to the intermediate oxide layer 101 on the interconnect substrate 100. This in turn reduces the growth rate of the native chromia layer and/or the intermediate spinel layer 101. Coatings that are effective in reducing oxygen transport from the air stream to the native oxide include materials that exhibit low oxygen diffusivity (solid state diffusion of oxide ions), such as spinel phases. The physical characteristics of a good protective coating include having high density, low connected porosity, no microcracking, and complete coverage of the interconnect.

[0060] The coating 102 also affects the resistance of the native oxide by interdiffusion and the formation of secondary phases. The oxide layer that forms on an uncoated CrF interconnect is the native oxide, Q^C^. This oxide exhibits conductivity on the order of O.OlS/cm at 850C. However, with a coating on the interconnect, a reaction occurs between the native Ο¾θ₃ oxide and the coating material. This reaction results in the formation of a reaction zone oxide layer which has a conductivity different from either the &2<¾ native oxide or the coating material.

[0061] In the case of a CrF interconnect coated with LSM, the reaction zone oxide layer that forms is in the spinel family (Mn, Cr^C . The conductivity of the (Mn, Ο")₃0₄ spinel is dependent on the composition, with examples given as follows: MnC^C : 0.003 S/cm, Mn_1.2Cr_1.80₄ 0.02 S/cm and Mn_1.5Cr_1.50₄ 0.07 S/cm at 800C.

[0062] Generally, the conductivity of the (Mn, Cr)₃0₄ spinels are slightly better than the native Ο¾0₃. However, the thickness of the reaction zone oxide can be thicker than the native oxide. Thus the total ohmic resistance can be larger. With a Mn1.5C01.5O t spinel coating on CrF materials, the reaction zone intermediate oxide layer 101 includes a (Mn, Cr, Co)₃0₄ spinel phase. Layer 101 may comprise 60 to 100 volume percent of the (Mn, Cr, Atty. Dkt. No. 7917-451 WO

Co)₃0₄ spinel phase, with the balance (if any) being chromia or other phases. The conductivity of the cobalt containing (Mn, Cr, Co^C family of spinels is considerably higher than that of the (Mn, Cr^C spinels as given by the examples: MnCo₂0₄: 36 S/cm, CoC^C : 7 S/cm, and CoM^C : 6 S/cm. The higher conductivity of the reaction zone oxide created with the Mn1.5C01.5O4 spinel coating (which preferably has an electrical conductivity of at least 20 S/cm, such as at least 38 S/cm) on CrF results in lower ohmic resistance losses from this interface and thus lower SOFC performance degradation with time.

[0063] In wet air atmospheres and at SOFC operating temperatures, the evaporation rates of chromium from the surface of the interconnect are relatively high. Therefore containment by a coating is preferable. The results of the SOFC stack tested in humid air are illustrated in Figure 2D. These results also show that the repeat layers with the Mn1.5C01.5O4 spinel coating exhibit lower degradation as compared to the repeat layers with LSM coating.

[0064] The lower degradation with the Mn1.5C01.5O4 spinel coating may be attributed to both i) the lower ohmic resistance of the reaction zone oxide layer, and ii) the reduction in the rate of chromium evaporation. Figure 2E shows a micrograph of the Mn1.5C01.5O4 coating 102 on a CrF substrate 100 after operating in a SOFC stack (as described below) at elevated operating temperature for 1000 hrs. Figure 2F shows a micrograph of an LSM coating 104 on the CrF substrate 100 in a SOFC stack operated at elevated operating temperature for 1000 hrs. As evident in the micrographs, the Mn1.5C01.5O4 coating 102 is dense, has no open porosity, and does not have microcracks 103. This is in contrast to the LSM coating 104 which has microcracking after SOFC operation.

[0065] The LSM coating 104 tends to sinter during SOFC operation, leading to the formation of microcracks 103 which can allow chromium vapor transport through the coating. Comparison of Figure 2B with Figure 2E indicates that even though the as- Atty. Dkt. No. 7917-451 WO deposited APS coating of M¾ .5C01 5Ο4 shown in Figure 2B may contain some microcracking and fissures, after a period of time at SOFC operating temperatures (e.g., 700 to 900 C), the MCO coating appears to densify in such a way to heal and eliminate connected microcracks, as shown in Figure 2F. Likewise, the intermediate chromium containing spinel layer 101 is formed by reaction between the MCO coating 102 and the CrF interconnect substrate 100 during stack fabrication annealing and/or during stack operation at elevated temperature.

[0066] In an embodiment, the spinel, e.g. (Mn, Co^C , powder is doped with Cu to reduce the melting temperature of the spinel. The lowered melting temperature improves (increases) the coating density upon deposition with a coating method, such as air plasma spray (APS) and increases the conductivity of reaction zone oxide. The improvement in the density of the coating due to the lower melting temperature can occur during APS deposition and during operation at SOFC temperature for extended periods of time.

[0067] The addition of Cu to the spinel layer has an additional advantage. The Cu doping of the spinel, such as (Mn, Co)₃0₄, may result in higher electrical conductivity of the base spinel phase as well as any reaction zone oxides that form between the spinel and the native C¾0₃ oxide. Examples of electrical conductivities of oxides from the (Mn, Co, Cu, Cr)s0₄ family include: CuCr₂0₄: 0.4 S/cm at 800C, Cui.3Mni.7O4: 225 S/cm at 750C, and CuMn₂0₄: 40 S/cm at 800C.

[0068] The spinel family of materials has the general formula AB2O4. These materials may form an octahedral or cubic crystal structure depending on the elements occupying the A and B sites. Further, depending on the doping conditions, the copper atoms may occupy either the A site, the B site or a combination of the A and B sites. Generally, Cu prefers to go into B site. When the A element is Mn, the B element is Co, and the spinel is doped with Cu, the spinel family may be described with the general formula (Mn, Co, Cu)₃0₄. More Atty. Dkt. No. 7917-451 WO specifically, the spinel family may be described with the following formulas depending on location of the Cu alloying element:

(1) Mn₂__x__yCo_1+xCu_y0₄ (0 < x < 1), (0 < y < 0.3) if Cu goes in A site

(2) Mn₂__xCo_1+x__yCu_y0₄ (0 < x < 1), (0 < y < 0.3) if Cu goes in B site

(3) Mn₂-_x-_y/2Co_1+x__y/2Cu_y0₄ (0 < x < 1), (0 < y < 0.3) if Cu goes equally in both A and B site.

[0069] Specific (Mn, Co, Cu)₃0₄ compositions include, but are not limited to, Mn_1.5C0_1.2Cuo.3O t , Mni_.5Coi_.4Cuo_.i0₄; Mn₂Coo_.8Cuo.2O t and C0₂Mno_.8Cuo.2Ozt. Additional compositions include Mn₂Coi__yCu_y0₄, where (0 < y < 0.3), if Cu goes in B site. These composition may also be written, (M¾0₃) + (1-Z)(COO)+Z(CUO), where (0 < z < 0.3). Other compositions include Co₂Mni__yCu_y0₄ where (0 < y < 0.3) if Cu goes in B site. These composition may also be written, (C0₂O₃) + (l-z)(MnO) + z(CuO) where (0 < z < 0.3). In one preferred Mn, Co spinel composition, the Mn/Co ratio is 1.5/1.5, e.g. Mn_1.5C0₁.5Ozt. When B site doped with Cu, preferred compositions include Mni_.5Coi.5__yCu_y0₄, where (0 < y < 0.3).

[0070] In another embodiment, (Mn, Co)₃0₄ or (Mn, Co, Cu)₃0₄ spinel families are doped with one or more single valence species. That is, one or more species that only have one valence state. Doping with single valence species reduces cation transport at high temperature and thus reduces the thickness of the intermediate oxide layer 101. The primary ionic transport mechanism in spinels is through cation diffusion via cation vacancies in the lattice structure. In spinels with multivalent species M^{2+ 3+}, such as Mn^3+/4+ and Co^{2+ 3+}, cation vacancies are generated when M species are oxidized from lower to higher valance states to maintain local charge neutrality. The introduction of a single valence species typically decreases the amount of cation vacancies and decreases the amount of interdiffusion Atty. Dkt. No. 7917-451 WO between the spinel coating 102 and the native 0"₂(¾ oxide or the CrF substrate 100. In this manner, the amount of the intermediate oxide layer 101 that forms is decreased. Examples of single valence species that may be introduced into the spinel coating include Y³⁺, Al³⁺, Mg ²⁺ and/or Zn²⁺ metals. In an aspect, the spinel coating has a composition of (Mn, Co, M^C , where M = Y, Al, Mg, or Zn. For example, if M=A1 doped in the A position, then the spinel compositions may include Mn₂-_yAl_yCo0₄ (0 < y < 0.3) or (l-z)(Mn₂0₃) +z(Al₂0₃)+CoO, where (0 < z < 0.15).

[0071] In an embodiment, the interconnect coating is deposited on the Cr based alloy interconnect, such as an IC containing 93-97 wt Cr and 3-7 wt Fe, such as the above described Cr-Fe-Y or CrF interconnects with an air plasma spray (APS) process. The air plasma spray process is a thermal spray process in which powdered coating materials are fed into the coating apparatus. The coating particles are introduced into a plasma jet in which they are melted and then accelerated toward the substrate. On reaching the substrate, the molten droplets flatten and cool, forming the coating. The plasma may be generated by either direct current (DC plasma) or by induction (RF plasma). Further, unlike controlled atmosphere plasma spraying (CAPS) which requires an inert gas or vacuum, air plasma spraying is performed in ambient air.

[0072] Cracks in the coatings can arise at two distinct times, a) during deposition, and b) during operation in SOFC conditions. Cracks formed during deposition are influenced by both the spray gun parameters and the material's properties of the coating material. The cracks that form during operation are largely a function of the material's properties and more specially the density and sinterability of the material. Without being bound by a particular theory, it is believed that the cracking that occurs during operation is the result of continuing sintering of the coating and therefore increased densification of the coating with time. As the Atty. Dkt. No. 7917-451 WO coatings densify, they shrink laterally. However, the coatings are constrained by the substrate and thus cracks form to relieve stress. A coating that is applied with a lower density is more likely to densify further during operation, leading to crack formation. In contrast, a coating that is applied with a higher density, is less likely to form cracks.

[0073] In an embodiment, a sintering aid is added to the IC coating to reduce crack formation and thus decrease chromium evaporation. The sintering aid is a material which increases the as-deposited coating density and/or decreases the densification after coating deposition. Since the sintering aid increases the as-deposited density of the coating materials, it thereby reduces crack formation that occurs after the coating formation due to subsequent densification and/or operating stress on a relatively porous material. Suitable sintering aids include materials that either a) lower the melting temperature of the bulk phase of the coating materials, b) melt at a lower temperature than the bulk phase resulting in liquid phase sintering, or c) form secondary phases with lower melting temperatures. For the perovskite family, including LSM, sintering aids include Fe, Co, Ni, and Cu. These transition metals are soluble in LSM and readily dope the B-site in the ABO₃ perovskite phase. The melting temperature of oxides in the 3d transition metals tend to decrease in the order Fe > Co > Ni > Cu. The addition of these elements to the B-site of LSM will lower the melting temperature and improve the as-sprayed density. In an embodiment, one or more of Fe, Co, Ni and Cu are added to the coating such that the coating comprises 0.5 wt to 5 wt , such as 1% to 4%, such as 2% to 3% of these metals. In an alternative embodiment, the coating composition is expressed in atomic percent and comprises Lai__xSr_xMni__yM_y0₃__d where (M = Fe, Co, Ni, and/or Cu), 0.1 < x < 0.3, 0.005 < y < 0.05 and 0 < d < 0.3. It should be noted that the atomic percent ranges of the Fe, Co, Ni and Cu do not necessarily have to match the weigh percent ranges of these elements from the prior embodiment. Atty. Dkt. No. 7917-451 WO

[0074] Other elements can also be added in combination with the above transition metals to maximize conductivity, stability, and sinterability. These elements include, but are not limited to, Ba, Bi, B, Cu or any combination thereof (e.g. Cu + Ba combination), such as in a range of 5 wt or less, such as 0.5-5 wt . Additionally, sintering aids that specifically dope the A-site of LSM, such as Y, may be added for similar effect. An example according to this embodiment is La_yY_xSri__x__yMn0₃, where x=0.05-0.5, y=0.2-0.5, such as LacuYo_.iSro_.sMnC^. For coating materials other than LSM, copper may be used as the sintering aid in the above described MCO spinel material.

[0075] In another embodiment, rather than introducing a transition metal powder into the air plasma spray during deposition, a metal oxide powder that is easily reduced in the APS atmosphere to its metal state is added to the plasma. Preferably, the metal of the metal oxide exhibits a melting temperature lower than that of the coating phase (perovskite or spinel phase). For example, the binary oxides cobalt oxide (e.g., CoO, C0₃O4, or C02O₃), NiO, Ι¾θ3, SnO, B2O₃, copper oxide (e.g., CuO or Q12O), BaO, B12O₃, ZnO or any combination thereof (e.g., (Cu,Ba)0) may be added as a second phase to the coating powder (i.e. LSM powder or La + Sr + Mn powders or their oxides). This addition, results in a two-phase powder mixture that is fed to the gun. The amount of second phase could be less than or equal to 5 wt , such as in the range from 0.1 wt to 5wt of the total powder weight.

[0076] In the APS gun, the metal oxide is reduced to its metal phase, melts, and promotes sintering of the melted LSM particles as the LSM particles solidify on the surface of the IC. The lower melting temperature of the metals and binary oxides promotes densification during deposition and solidification.

[0077] In another embodiment, a material that reacts with the coating material (such as LSM) and forms a secondary phase with a lower melting temperature is added to the coating Atty. Dkt. No. 7917-451 WO feed during the APS process. The lower melting temperature secondary phase promotes densification. For example, silicate and/or calcium aluminate powders may react with the coating material powder(s) in the hot plasma portion of the APS gun to form glassy phases. In an embodiment, La from the LSM material reacts with a Si-Ca-Al oxide (which may also include K or Na) to form a glassy phase such as La-Ca-Si-Al oxide that forms between LSM particles. The coating may include less than or equal to 5 wt , such as 0.5-5% of silicate, Ca-Al oxide or Si-Ca Al oxide.

[0078] In an embodiment, the coating is post-treated in such a manner as to cause stress- free densification. This post-treatment may be performed in combination with or without the addition of the sintering aids of the prior embodiment. In an example post-treatment according to this embodiment, "redox" cycling in N2 and O2 atmospheres is performed. In this cycling, the coating is alternatively exposed to neutral and oxidizing atmospheres. For example, the coating may be treated in a neutral atmosphere comprising nitrogen or a noble gas (e.g., argon) and then treated in an oxidizing atmosphere comprising oxygen, water vapor, air, etc. One or more cycles may be performed, such 2, 3, 4, or more as desired. If desired, a reducing (e.g., hydrogen) atmosphere may be used instead of or in addition to the neutral atmosphere. Redox cycling in N2 and O2 atmospheres may cause cation vacancy concentration gradients that increase the diffusion of cation vacancies and thereby effectively increase sintering rates. This effect can be further increased by using a lower Sr content LSM coating of Lai__xSr_xMn0₃__d where x<=0.1, e.g., 0.01<x<0.1, d<0.3, such that the oxygen non- stoichiometry is maximized. Use of this sintering procedure may enhance any or all of the sintering aid techniques described above.

[0079] In another embodiment, the surface area for electrical interaction between the coating and the underlying Cr-Fe IC surface is enlarged. The chromia layer that forms Atty. Dkt. No. 7917-451 WO between the coating and the IC causes millivolt drops over time as the chromia layer grows in thickness. The total voltage drop is dependent on the area and thickness over which the voltage drop occurs. Increasing the area of the oxide growth between the IC and the coating lowers the impact on voltage losses, thereby increasing the life of the stack. By adding what would be depth penetrations of the coating, this embodiment effectively increases the surface area of contact and thereby reduces the impact of the growing chromia layer.

[0080] A method according to this embodiment includes embedding small quantities of coating materials into the IC. There are two alternatives aspects of this embodiment. One aspect includes fully and uniformly distributing the coating material, such as LSM or MCO, within the IC powder (e.g., Cr-Fe powder) before compacting to form the IC. The coating powder (e.g., LSM and/or MCO powder) could be included when mixing the lubricant and Fe, Cr (or Cr-Fe alloy) powders together before compaction. Preferably, the powder mixture is able to withstand sintering temperatures and a reducing environment. The second aspect includes incorporating (e.g., embedding) a predetermined amount of coating powder only in the top surface of the Cr alloy IC. The oxide regions embedded in the surface of the CrF or CrFeY IC increase the surface roughness of the IC after the IC sintering step. The full coating is deposited on the Cr alloy interconnect after the pressing and sintering steps.

[0081] A method for embedding the coating material in the top surface of the interconnect is illustrated in Figures 3A-3C. The lubricant and Cr/Fe powder 202 which is used to form the bulk of the IC are added to the mold cavity 200 with a first shoe (not shown) or by another suitable method, as shown in Figure 3A. The coating material powder 204 (e.g., LSM or MCO) or a mix of the coating material power 204 and lubricant/Cr/Fe powder 202 is provided into the mold cavity using a second shoe 206 over the powder 202 located in the mold cavity before the compaction step, as shown in Figure 3B. The powders 204, 202 Atty. Dkt. No. 7917-451 WO are then compacted using a punch 208, as shown in Figure 3C, to form the interconnect having the coating material embedded in its surface on the air side (i.e., if the air side of the IC is formed facing up in the mold).

[0082] Alternatively, the coating material powder 204 (e.g., LSM or MCO) (or a mix of the coating material power 204 and lubricant/Cr/Fe powder 202) is provided into the mold cavity 200 first. The lubricant/Cr/Fe powder 202 is then provided into the mold cavity 200 over powder 204 before the compaction step if the air side of the IC is formed in the mold facing down. In this manner, the coating material is incorporated into the IC primarily at the top of the air side surface of the IC.

[0083] Alternatively, as shown in Figure 4, the coating powder 204 may be

electrostatically attracted to the upper punch 208 of the press. Then, the upper punch 208 presses the coating powder 204 and the lubricant / interconnect powder materials 202 in the mold cavity 200 to form an IC with the coating material 204 embedded in the top of the air side.

[0084] Using the above methods, the coating powder may be uniformly incorporated in the surface of the air side of the IC after the compaction step. The compaction step is then followed by sintering and coating steps, such as an MCO and/or LSM coating step by APS or another method described herein.

[0085] The ratio of the coating powder and Fe in the Cr-Fe alloy is preferably selected so that the top coating material has a similar coefficient of thermal expansion (CTE) to that of the sintered and oxidized interconnect. The coefficient of thermal expansion of the Cr-Fe alloy is a function of the composition of the alloy and can be chosen by selecting a Cr to Fe ratio. The sintering process may be adjusted to keep the powder oxidized and stable. For example, sintering may be performed using wet hydrogen, or in an inert atmosphere, such as Atty. Dkt. No. 7917-451 WO nitrogen, argon or another noble gas. The wet hydrogen or inert gas atmosphere is oxidizing or neutral, respectively, and thereby prevents the oxide powder from reducing.

[0086] In another embodiment, the coating is a multi-layer composite. Figure 6A illustrates an example of this embodiment of an IC with a composite coating. The composite coating is composed of a spinel layer 102 and a perovskite layer 104. The spinel layer 102 is deposited first on the Cr alloy (e.g., CrF) interconnect 100. The perovskite layer 104 layer described above is then deposited on top of the spinel layer 102. The native chromium containing interfacial spinel layer 101 may form between the interconnect 100 and layer 102 during layer 102 deposition and/or during high temperature operation of the fuel cell stack containing the interconnect.

[0087] The perovskite layer 104 may comprise any suitable perovskite layer described above, such as LSM. LSM may have the following formula: Lai__xSr_xMn0₃ (LSM), where 0.1<x<0.3, such as 0.1<x<0.2. The spinel coating 102 is deposited first and is in direct contact with the interconnect substrate 100 (if the native layer 101 is removed from the substrate 100) or is in direct contact with the native layer 101 on the substrate 100.

[0088] Preferably, the lower spinel layer 102 comprises the above described MCO spinel containing Cu and/or Ni. Layer 102 acts as a doping layer that increases the conductivity of the underlying manganese chromium oxide (Mn, Cr^C or manganese cobalt chromium oxide (Mn, Co, Cr^C interfacial spinel layer 101. In other words, the Cu and/or Ni from the spinel layer 102 diffuses into the interfacial spinel layer 101 during and/or after formation of layer 101. This results in a Cu and/or Ni doped layer 101 (e.g., (Mn and Cr)3__x__y Co_x(Cu and/or Ni)_y0₄ where (0 < x < 1), (0 < y < 0.3)) which lowers layer 101 resistivity.

[0089] Layer 102 may comprise the above described Cu containing MCO layer and/or a Ni containing MCO layer and/or a Ni and Cu containing MCO layer. In the MCO layer, Atty. Dkt. No. 7917-451 WO when the A element is Mn, the B element is Co, and the spinel is doped with Cu and/or Ni, the spinel family may be described with the general formula (Mn, Co)₃__y(Cu, Ni)_y0₄ , where (0 < y < 0.3) More specifically, the spinel family may be described with the following formulas depending on location of the Cu and/or Ni alloying elements:

(1) Mn₂-x-_yCo_1+x (Cu, Ni)_y0₄ (0 < x < 1), (0 < y < 0.3) if Cu and/or Ni goes in A site

(2) Mn₂-_xCo_1+x__y(Cu, Ni)_y0₄ (0 < x < 1), (0 < y < 0.3) if Cu and/or Ni goes in B site

(3) Mn₂__x__y/2Co_1+x__y/2(Cu, Ni)_y0₄ (0 < x < 1), (0 < y < 0.3) if Cu and/or Ni goes equally in both A and B site.

[0090] Figure 5 illustrates the reduction of area specific resistance (ASR) degradation as a function of time of CrF interconnects coated with a Cu and Ni containing manganese oxide doping layer (e.g., (Cu,Ni,Mn)₃0₄ spinel) compared to an interconnect coated with an LSM layer. The interconnects with a doping layer initially start with a higher ASR than the interconnects with the LSM layer coating. However, the rate of increase of ASR for the LSM layer coated ICs is much higher than that of the doping layer coated ICs. Within 1000-2000 hours, the ASR of the LSM layer coated ICs is higher than those with the doping layer coating. Further, after approximately 2500 hours of operation, the ASR of the interconnects with the doping layer stays constant, while the ASR of the LSM layer coated interconnects keeps rising with time.

[0091] While the Cu and/or Ni containing spinel doping layer 102 decreases the ASR of the interconnects, it is permeable to both oxygen and chromium. Thus, in the present embodiment, a second perovskite barrier layer 104 is formed over the doping layer 102. Preferably, layer 104 is a dense LSM layer that reduces or prevents Cr and oxygen diffusion. Layer 104 may be formed with the sintering aid described above to increase its density. The dense layer 104 reduces or prevents the growth of the interfacial spinel layer 101 by blocking Atty. Dkt. No. 7917-451 WO diffusion of air and oxygen from the fuel cell cathode side to the CrF IC surface during stack operation. Layer 104 also reduces or prevents chromium poisoning of the fuel cell cathodes in the stack by reducing or preventing chromium diffusion from the ICs to the cathodes.

[0092] Thus, the composite coating 102/104 reduces or eliminates the area specific resistance (ASR) degradation contribution from interconnects to the stacks and lowers the overall degradation of the fuel cell stack by reducing or eliminating Cr poisoning of the fuel cell cathodes. First, the spinel layer 102 dopes the chromium containing interfacial spinel layer 101 with elements (e.g. Co and/or Mn and if present, one of optional Ni and/or Cu) that decrease the resistance of the spinel layer 101. Second, the spinel layer 102 prevents direct interaction between the perovskite 104 layer and the Cr containing interfacial spinel layer 101 which can lead to the formation of unwanted and resistive secondary phases. Third, the spinel (e.g. Mn containing spinel having Co, Cu and/or Ni) layer 102 is less prone to cracking than the LSM layer 104, which enhances the integrity of the coating. Fourth, the top perovskite layer 104 is a second barrier layer that decreases the transport of oxygen to the interfacial oxide 101 on the interconnect surface. The top perovskite layer 104 thus reduces the growth rate of the native oxide layer 101, and decreases transport of chromium from layer 101 to the fuel cell cathodes through the spinel layer 102.

[0093] An example of a solid oxide fuel cell (SOFC) stack is illustrated in Figure 6B. Each SOFC 1 comprises a cathode electrode 7, a solid oxide electrolyte 5, and an anode electrode 3. Fuel cell stacks are frequently built from a multiplicity of SOFC's 1 in the form of planar elements or other geometries. Fuel and air is provided to the electrochemically active surfaces of the anode 3 and cathode 7 electrodes, respectively. The interconnect 9 containing gas flow passages or channels 8 between ribs 10, separates the individual Atty. Dkt. No. 7917-451 WO cells in the stack. The interconnect 9 electrically connects the anode or fuel electrode 3 of one cell 1 to the cathode or air electrode 7 of the adjacent cell 1. The interconnect

9 separates fuel, such as a hydrocarbon fuel, flowing in the fuel channels 8 between ribs 10 on the fuel side of the interconnect to the fuel electrode (i.e. anode 3) of one cell 1 in the stack, from oxidant, such as air, flowing in the air channels 8 between ribs

10 on the air side of the interconnect to the air electrode (i.e. cathode 7) of an adjacent cell 1 in the stack. At either end of the stack, there may be an air end plate or fuel end plate (not shown) for providing air or fuel, respectively, to the end electrode. Figure 6B shows that the lower SOFC 1 is located between two interconnects 9.

[0094] As shown in Figure 6B, the MCO spinel coating 102 is located over the ribs 10 and the air channels 8 on the air side of the interconnects 9 facing the adjacent SOFC 1 cathode 7 in the stack. The perovskite layer 104 (e.g., LSM) is located over the MCO coating 102 only over the rib 10 areas, but not in the air channel 8 areas. This allows the perovskite (e.g., LSM) layer 104 to contact the same or similar perovskite (e.g., LSM) cathode 7, without coating the entire air side of the

interconnect 9 with the perovskite layer 104. The intermediate oxide layer (not shown in Figure 6B) above may be formed between the CrF interconnect 9 substrate 100 and the MCO spinel coating 102 after stack annealing and/or operation.

[0095] In another embodiment, a second phase is added to a (Mn, Co^C spinel to act as a getter for impurities, such as sulfur and silicon. In this manner, the adhesion of the coating to the CrF interconnect substrate 100 may be improved. For example, metal oxide phases, such as non-spinel metal oxides, for example, AI2O₃, Y2O₃, or T1O2 may be added to the Atty. Dkt. No. 7917-451 WO spinel phase of the coating 102 as a second phase. In one aspect, when the metal oxide phase is alumina, the coating composition may be (l-x)(Mn, Ο))₃0₄ and x(Al₂(¾), where (0 < x < 0.02). In this case, the AI2O₃ primarily exists as a second phase and not as a doping agent in the spinel structure. During deposition and at SOFC operation temperatures, however, some interdiffusion may occur. In this case, aluminum, yttrium or titanium doping of the spinel phase will occur.

[0096] Figure 7A shows the air side of an exemplary interconnect 100. The interconnect may be used in a stack which is internally manifolded for fuel and externally manifolded for air. The interconnect contains air flow passages or channels 8 between ribs 10 to allow air to flow from one side 13 to the opposite side 14 of the interconnect. Ring (e.g. toroidal) seals 15 are located around fuel inlet and outlet openings 16A, 16B (i.e., through holes 16A, 16B in interconnect 100). Strip seals 19 are located on lateral sides of the interconnect 100.

[0097] Figure 7B shows a close up view of an exemplary seal 15, passages 8 and ribs 10. The seals 15 may comprise any suitable seal glass or glass ceramic material, such as borosilicate glass. Alternatively, the seals 15 may comprise a glass ceramic material described in US Application Serial Number 12/292,078 filed on November 12, 2008, incorporated herein by reference.

[0098] The interconnect 100 may contain an upraised or boss region below the seal 15 if desired. Additionally, as illustrated in Figure 7B, the seal 15 is preferably located in a flat region 17 of the interconnect 100. That is, the seal 15 is located in a portion of the interconnect that does not include ribs 10. If desired, the interconnect 100 may be configured for a stack which is internally manifolded for both air and fuel. In this case, the interconnect 100 and the corresponding fuel cell electrolyte would also contain additional air inlet and outlet openings (not shown). Atty. Dkt. No. 7917-451 WO

[0099] Figure 7C illustrates the fuel side of the interconnect 100. A window seal 18 is located on the periphery of the interconnect 100. Also shown are fuel distribution plenums 17 and fuel flow passages 8 between ribs 10. It is important to note that the interconnect 100 shown in Figure 7C has two types of fuel flow passages; however, this is not a limitation of the present invention. The fuel side of an interconnect 100 may have fuel flow passages that are all the same depth and length, or a combination of short and long, and/or deep and shallow passages.

[00100] In an embodiment, the interconnect 100 is coated with the Mr^ sCoj 5Ο4 (MCO) spinel at room temperature using an aerosol spray coating method and further processed with one or more heat treatments. Generally, the MCO coating is omitted in the seal regions (toroid 15, strip 19) by masking or removing MCO deposited in these regions.

[00101] The MCO coating may be reduced by the fuel in the riser hole and then reacts with the glass sealing materials at the toroid-shaped seal 15. Thus, in an embodiment, for the interconnect 100 shown in Figure 7B, the MCO coating is removed from the flat region 17 (e.g., by grit blasting) on the air side of the interconnect before stack assembly and testing. Alternatively, the flat region 17 may be masked during aerosol deposition to prevent coating of the flat region 17. Thus, the MCO coating is omitted in the region 17 under the toroidal seal 15 adjacent to the fuel inlet and/r outlet openings 16A, 16B.

[00102] In another embodiment, the interconnect 100 is manufactured by a powder metallurgy process. The powder metallurgy process may result in parts that have connected porosity within the bulk of the interconnect 100 that allows fuel to diffuse from the fuel side to the air side. This fuel transported via the pores may to react with the MCO coating on the air side at the coating/interconnect interface. This reaction may lead to seal failure and stack separation. In an embodiment, this failure may be mitigated by omitting the MCO coating Atty. Dkt. No. 7917-451 WO under the strip seal 19 by masking the seal 19 locations on the edges of the interconnect during MCO deposition, thereby eliminating coating in these seal areas and allowing the glass seals 19 to bond directly to the metallic interconnect.

[00103] In another embodiment, interconnects 100 form a thin, green colored &₂θ₃ oxide layer 25 on the fuel side of the interconnect 100. A cross-sectional micrograph of this fuel side oxide is illustrated in Figure 8. The Ο¾θ₃ oxide thickness was found to be between 0.5 to 2 microns. Three methods described below may be used to convert or remove this undesirable chromium oxide layer.

[00104] In one embodiment of the method, this oxide layer is removed by any suitable method, such as grit blasting. This method is effective. However, this method is time consuming and adds processing costs.

[00105] Alternatively, the &₂θ₃ oxide layer 25 may be left in place and converted to a composite layer. In this embodiment, a nickel mesh anode contact is deposited on the Ο¾θ₃ oxide layer 25 and allowed to diffuse into the chromium oxide layer. The nickel reacts with the &₂θ₃ oxide layer 25 and forms a Ni-metal/Cr₂0₃ composite layer that reduces ohmic resistance of layer 25. If desired, the mesh may be heated after contacting layer 25 to expedite the composite formation.

[00106] In another embodiment, oxide layer 25 is reduced or completely eliminated by firing the MCO coated interconnect in an ambient having a low oxygen partial pressure. For instance, based on thermodynamics, &₂θ₃ can be reduced to Cr metal at a p0₂ (partial pressure) of 10^"24atm at 900° C, while CoO reduces to Co-metal at a p0₂ of 10^"16 atm at 900° C. By lowering the partial pressure of oxygen (i.e., lowering the dew point) of the firing

-24

atmosphere to less than 10^" atm at 900° C, the formation of the Ο¾θ₃ oxide on the fuel side (uncoated side) may be prevented, while allowing the reduction of the MCO coating on the Atty. Dkt. No. 7917-451 WO air side of the interconnect to MnO (or Mn metal if 02 < 10^~27atm) and Co-metal for sintering benefits. At 02 < 10^~27atm, MCO would be reduced to both Mn-metal and Co- metal which may lead to better sintering and denser coatings as compared with MnO/Co- metal. In general, the MCO coated interconnect may be annealed at T>850 °C, such as 900 °C to 1200 °C, at p0₂ of 10^~24 atm, e.g. 10^"25 atm to 10^~30 atm, including 10^"27 atm to 10^~30 atm for 30 minutes to 40 hours, such as 2-10 hours.

[00107] In another embodiment, to reduce costs of the MCO coating process, the MCO coating may be annealed (e.g. fired or sintered) during the sintering step for the powder metallurgy (PM) formed interconnect. The sintering of the powder metallurgy interconnect 100 and of the MCO coating on the interconnect may be conducted in the same step in a reducing ambient, such as a hydrogen reduction furnace with a dew point between -20 and - 30° C, at temperatures between 1300 and 1400 °C, and for a duration between 0.5 and 6 hrs. At these temperatures and partial pressures of oxygen, the MCO coating will reduce completely to Co-metal and Mn-metal. However, the melting temperature of Mn is around 1245 °C, the melting temperature of Co is around 1495 °C, and the Co-Mn system has a depressed liquidus line. Thus, sintering at temperatures between 1300 and 1400° C may result in the formation of an undesirable liquid phase.

[00108] Possible solutions to avoid the formation of liquid include lowering the sintering temperature below 1300 °C, such as below 1245 °C, for example from 1100 °C to 1245 °C, increasing the partial pressure of oxygen to reduce the Mn (but not oxidize the Cr) in MCO to MnO (melting temp 1650° C) as opposed to Mn-metal, decreasing the Mn:Co ratio in MCO to increase the melting temperature of the Mn-Co metal system, adding dopants to MCO, such as Cr, to increase melting temperature of Co-Mn-Cr metal system, and/or adding dopants, such as Fe, V and or Ti to the MCO coating to stabilize binary and ternary oxides (to Atty. Dkt. No. 7917-451 WO prevent reduction to metal phase). For example, at a sintering temperature of 1400° C, MnO reduces to Mn-metal at a p(¾ of 10^~17atm while Cr₂(¾ reduces to Cr-metal at a p(¾ of 10^{" 15}atm, which gives a small window (a p(¾ between 10^"17 and 10^"15atm) where Cr is reduced to metal yet the MnO stays as an oxide which has a high melting point. Thus, the interconnect and the MCO coating may be sintered at 1300-1400 °C at p0₂= 10^"15-10^"17 atm.

[00109] In another embodiment, the IC sintering step could be conducted first after which the MCO coating is applied to the sintered IC. The IC and coating are then put through a reduction step described in the previous embodiment that is more suitable for the MCO coating.

[00110] In another embodiment, interconnect fabrication costs may be reduced by depositing the MCO layer as a mixture of already reduced components such as MnO, CoO, Mn metal, Co metal, or any combination of these constituents. The mixture is then to be sintered, preferably under low p0₂ conditions. However, such sintering may be easier or the starting material may be denser, thereby reducing the time for sintering. Additionally, these precursor particles may be much less expensive than MCO precursor, which requires expensive synthesis methods to produce.

[00111] Additionally, a grit blast step may be performed before coating the interconnect with the MCO layer to remove the native chromium oxide layer from both the air and fuel sides of the interconnect. To reduce costs, the native oxide may be removed only from the air side of the interconnect before forming the MCO coating on the air side of the interconnect. The MCO coating is then deposited on the air side and the interconnect is anneals as described above. Removal of oxide from the fuel side, such as by grit blasting, may then take place after the anneal is complete. In this manner, the number of grit blast steps is reduced because no additional grit-blast steps are required to remove the oxide growth that occurs on Atty. Dkt. No. 7917-451 WO the fuel side of the interconnect during the anneal of the MCO coating.

[00112] In other embodiments, the composition of MCO coating is modified to increase stability at SOFC operational temperatures, such as 800-1000 °C. The MCO composition of some of the prior embodiments is Mn1.5C01.5O4. This material has a high electric conductivity. However, the MCO material is reducible to the binary oxides, MnO and CoO, or to the binary oxide MnO and Co-metal.

[00113] In some fuel cell geometries, the MCO coating is only directly exposed to the fuel stream at the riser opening(s) 16 A, 16B. This fuel/coating interface can be eliminated by not coating the flat region 17 around the opening (Figure 7B). However, interconnects which are fabricated by a powder metallurgy method results in a part with some connected (open) porosity that can allow fuel to diffuse through the part to the air side. The fuel that diffuses through the pores may react with and reduce the MCO at the MCO/interconnect interface (shown in Figure 9) resulting in a porous layer consisting of MnO and Co-metal. The coating/IC interface may be compromised, leading to adhesive failure and separation of the cell from the interconnect during routine handling, as shown in Figure 9.

[00114] It is desirable to have a coating material that is more stable and less likely to be reduced when exposed to a fuel environment. The embodiments described below optimize the composition and/or dope the MCO with other elements in order to stabilize the material in a reducing atmosphere.

[00115] Figures 11 and 12 illustrate a theory of electrolyte corrosion. In the prior art SOFC stack shown in Figures 11 and 12, LSM coating 11 on an interconnect is located in contact with the ring seal 15. The seal 15 contacts the cell electrolyte 5. Without wishing to be bound by a particular theory, it is believed that manganese and/or cobalt from the manganese and/or cobalt containing metal oxide (e.g., LSM of LSCo) layer 11 leaches into Atty. Dkt. No. 7917-451 WO and/or reacts with the glass seal 15 and is then transported from the glass to the electrolyte. The manganese and/or cobalt may be transported from the glass to the electrolyte as manganese and/or cobalt atoms or ions or as a manganese and/or cobalt containing compound, such as a manganese and/or cobalt rich silicate compound. For example, it is believed that manganese and cobalt react with the glass to form a (Si, Ba)(Mn,Co)06+5 mobile phase which is transported from the glass seal to the electrolyte. The manganese and/or cobalt (e.g., as part of the mobile phase) at or in the electrolyte 5 tends to collect at the grain boundaries of the zirconia based electrolyte. This results in intergranular corrosion and pits which weaken the electrolyte grain boundaries, ultimately leading to cracks (e.g., opening 16A to opening 16B cracks) in the electrolyte 5. Without being bound by a particular theory, it is also possible that the fuel (e.g., natural gas, hydrogen and/or carbon monoxide) passing through the fuel inlet riser 36 may also react with the metal oxide layer 11 and/or the glass seal 15 to create the mobile phase and to enhance manganese and/or cobalt leaching from layer 11 into the seal 15, as shown in Figure 11.

[00116] As discussed above, in other embodiments, the composition of MCO coating is modified to increase stability at SOFC operational temperatures, such as 800-1000 °C. Thus, the MCO composition may be optimized based on stability and electrical conductivity.

Example compositions include, but are not limited to, Mn2Co0₄, Mn1.75Coo.25O !,

Co_{1 7}5Mno.₂50₄, Co₂Mn0₄, and Co₂.5Mn₀.5O₄.

[00117] Based on the phase diagram (Figure 10) and from a stability point of view, it may be beneficial to have a multi-phased composition rich in Mn such as Mn2.5Coo.5O t and Mn2.75Coo_.250₄ (e.g. Mn.Co atomic ration of 5: 1 or greater, such as 5: 1 10 11 : 1. A higher Mn content may also result in a more stable composition because the composition is in a higher oxidation state than the two phase spinel + binary oxide found at high Co content. However, Atty. Dkt. No. 7917-451 WO any composition in the (Mn,Co)₃0₄ family between the end compositions of C0₃O₄ and M¾0₄ may be suitable.

[00118] In another embodiment, MCO is stabilized by adding an additional dopant that is less prone to reduction. For example, it is known that MCO reacts with Cr in the IC alloys to form (Cr, Co, Mn^C spinel. If Cr is added intentionally to the MCO coating in low levels, such as 0.1 atomic % to 10%, this would result in a spinel (Cr, Co, Μη)₃θ₄ which is more stable than MCO because Cr³⁺ is very stable. Other transition metal elements that are soluble in the spinel structure which may increase stability include Fe, V, and Ti. Example coating materials include the spinel (Fe, Co, Mn)₃0₄ with 1% to 50 at% Fe, (Ti, Co, Mn)₃0₄ with 1% to 50% Ti, or a combination of (Fe, Ti, Co, Μη)₃θ₄.

[00119] The addition of Ti may lead to more stable secondary phases including C0₂T1O₄, ΜηΤΐ₂θ₄, or FeTi₂0₄. These phases benefit overall coating stability. Spinels with any combination of the above mentioned dopants are possible including (Fe, Cr, Co, Μη)₃θ₄, (Cr, Ti, Co, Mn)₃0₄, etc.

[00120] It is known that spinels based on Mg, Ca, and Al are very stable and resist reduction. However, these spinels have low electrical conductivity and thus are not preferred for application as an interconnect coating. In contrast, low levels of doping of Ca, Mg, and/or Al into a conductive spinel, such as MCO, increases the stability of the material while only marginally lowering the electrical conductivity. Example spinels include (Ca, Co, Μη)₃θ₄ with 1% to 10 at% Ca, (Mg, Co, Mn)₃0₄ with 1% to 10 at% Mg, (Al, Co, Mn)₃0₄ with 1% to 10 at% Al, or combinations such as (Ca, Al, Mn, Co)₃0₄, where Ca, Al and/or Mg are added at 1-10 at%. Si and Ce are other elements that may be use as dopants (1-10 at%) for the MCO spinel.

[00121] In addition to the methods described above that fall in the general category of Atty. Dkt. No. 7917-451 WO material- specific stabilization efforts, alternative embodiments are drawn to design changes that can be made that improve the stability of the coating, either in combination with or in the alternative to the above embodiments. In a first alternative embodiment, a stable barrier layer can be added to the interconnect before the addition of the MCO coating. This barrier layer would preferably be made of a more stable oxide than MCO and would be conductive and thin enough to not detrimentally affect the conductivity of the interconnect component.

Further, this barrier layer is preferably dense and hermetic. Example barrier layers include, but are not limited to, a doped Ti-oxide (e.g. T1O₂) layer or lanthanum strontium manganate (LSM).

[00122] A second alternative embodiment includes the addition of a reactive barrier layer between the interconnect and the MCO coating which includes any of the elements discussed above (e.g. Cr, V, Fe, Ti, Al, Mg, Si, Ce and/or Ca) as possible dopants. This layer diffuses these element(s) into the MCO coating upon heating the interconnect to standard operating temperatures (800-1000°C), creating a graded doping profile with higher concentration of dopant at the interconnect interface where reduction occurs. In this manner, a majority of the coating contains relatively little dopant and hence the conductivity may be less affected than by a uniform doping of the coating material. A reactive layer is a metal layer (e.g. Ti or metal containing compound that allows outdiffusion of the metal at 800 °C or higher.

[00123] Another embodiment includes designing the interconnect material to contain a reactive doping element (e.g. Si, Ce, Mg, Ca, Ti and/or Al for a Cr-4-6 Fe interconnect) that diffuses into the MCO coating in the same manner just described. Thus, the interconnect would contain > 90wt Cr, 4-6 Fe and 0.1-2% Mg, Ti, Ca and/or Al.

[00124] Additionally, any method of deposition or treatment of the IC to reduce or close the porosity of the part, beyond the standard oxidation methods, would help limit the Atty. Dkt. No. 7917-451 WO reduction of the MCO coating. For example, a Cr layer may be electroplated onto the porous part before the MCO annealing step to further reduce the porosity. Or, as described above, the addition of a reactive barrier layer, if dense and hermetic, would also reduce or block hydrogen diffusion from surface pores.

[00125] The embodiment of the invention provides a composite perovskite and spinel coating rather than the bilayer spinel and perovskite coating. Figure 13 is a micrograph illustrating the interconnect with a composite LSM-MCO coating. The composite LSM/MCO coating 110 according to this embodiment is designed to utilize the best features of each of these individual coatings discussed above. The composite coating 110 illustrated in Figure 13 comprises 40 wt MCO and 60 wt LSM and is conditioned (two day cycle at 850°C) in a SOFC stack. The light phase 112 is LSM while the dark phase 114 is MCO. Thus, the LSM and MCO phases are present as distinct regions in the composite coating. Without wishing to be bound by a particular theory, it is believed that the MCO phase may form plate-like or pancake-like (e.g., longer than thicker) structures 114 in the LSM phase matrix 112. However, the structure may be different for different compositions and/or deposition methods of the composite coating 110.

[00126] The presence of crack-healing pancake-like MCO structures within the composite coating 110 suppresses Cr evaporation through cracks generated in the LSM. The presence of LSM stabilizes the composite LSM/MCO coating in reducing atmospheres such that spallation does not occur and coating integrity is maintained. Preferably, the MCO content of the composite coating 110 is sufficiently high to form the Mn-Cr-Co oxide (e.g., spinel 101) scale on the IC, which provides lower ohmic resistance compared to a single-phase LSM coating, which may only form a MnCr oxide spinel on the interconnect surface. As can be seen in Figure 13, the composite coating 110 does not exhibit any cracking or spalling after Atty. Dkt. No. 7917-451 WO two days of conditioning at 850°C in the SOFC stack.

[00127] The composition of the composite coating 110 may be any ratio of LSM:MCO as long as there is a mix of the two materials (not a bi-layer coating). For example, the perovskite to spinel (e.g., LSM:MCO) weight ratio may range between 20:80 and 90:10, such as 50:50 to 80:20. The composition of the individual LSM and MCO materials in the composite coating 110 may vary as described above and may have any level or ratio of non- oxygen constituents, and may include other phases besides the pervoskite and spinel phase and/or other elements besides Mn, Co, La, Sr and O. For example, the spinel phase 114 of the coating 110 may comprise Mn₂-_xCoi_+x0₄, where 0<x<l and the perovskite phase 112 may comprise La!__xSr_xMn0₃ (LSM), where 0.1<x<0.3, such as 0.1<x<0.

[00128] The composite coating 110 may be deposited on the interconnect using any deposition method, such as, but not limited to APS. Preferably, the perovskite and spinel are deposited together in one step. For example, APS feedstock powder provided into the plasma in the APS process may comprise a mixture of LSM and MCO powder having the same weight ratio as that desired for the coating 110.

[00129] The microstructure, thickness, or any other physical property of the coating may vary and can be of any form. However, a dense coating is preferred. The composite coating 110 may be deposited on any location on the interconnect. That is, the composite coating is not limited to any specific portion of the interconnect, but is preferred to be deposited on the cathode side of the interconnect.

[00130] Although the foregoing refers to particular preferred embodiments, it will be understood that the invention is not so limited. It will occur to those of ordinary skill in the art that various modifications may be made to the disclosed embodiments and that such modifications are intended to be within the scope of the invention. All of the publications, Atty. Dkt. No. 7917-451WO patent applications and patents cited herein are incorporated herein by reference in their entirety.

Claims

Atty. Dkt. No. 7917-451 WO WHAT IS CLAIMED IS:

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

providing an interconnect substrate comprising Cr and Fe; and

coating an air side of the interconnect substrate with a manganese cobalt oxide spinel coating using a plasma spray process.

2. The method of claim 1, wherein the spinel comprises Mn2-_xCoi_+x0₄, wherein

0<x<l.

3. The method of claim 2, wherein the spinel comprises Mn1.5C01.5O4.

4. The method of claim 1 , wherein the spinel further comprises Cu.

5. The method of claim 4, wherein the spinel has a formula:

Mn₂__x__yCoi_+xCu_y0₄ (0 < x < 1), (0 < y < 0.3), if Cu is located on an A site of the spinel;

Mn2-_xCoi_+x__yCu_y0₄ (0 < x < 1), (0 < y < 0.3), if Cu is located on a B site of the spinel; or

Mn₂-x-_y 2Coi_+x__{y 2}Cu_y0₄ (0 < x < 1), (0 < y < 0.3), if Cu is located on both the A site and the B site of the spinel.

6. The method of claim 1 , wherein the spinel further comprises one or more single valence elements.

7. The method of claim 6, wherein the one or more single valence elements is selected from the group consisting of Y³⁺, Al³⁺, Mg²⁺ and Zn²⁺ and the spinel comprises (Mn, Co, M)₃0₄, where M = Y, Al, Mg, or Zn.

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

9. The method of claim 8, wherein the getter comprises at least one of AI2O₃, Y2O₃, or

Ti0₂. Atty. Dkt. No. 7917-451 WO

10. The method of claim 1, further comprising depositing a perovskite layer over the spinel coating.

11. The method of claim 10, wherein the spinel coating is located over both ribs and air channels over the air side of the interconnect substrate and the perovskite layer is located over the spinel coating over the ribs but not over the air channels of the interconnect substrate.

12. The method of claim 2, wherein:

the 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.

13. The method of claim 12, further comprising:

placing the coated interconnect substrate into a solid oxide fuel cell stack;

removing a chromia layer from the air side of the interconnect substrate prior to the step of coating and after the step of placing such that the spinel coating is formed directly on the chromium-iron alloy surface of the air side of the interconnect substrate that is not coated with chromia; and

forming a manganese-cobalt-chromium intermediate spinel layer between the spinel coating and the chromium-iron alloy surface of the air side of the interconnect substrate by reacting the spinel coating with the chromium-iron alloy surface of the air side of the interconnect substrate at an elevated temperature after the step of placing.

14. The method of claim 1, wherein the manganese cobalt oxide spinel coating further comprises a perovskite component to form a composite coating.

15. The method of claim 14, wherein the composite coating comprises plate shaped spinel phase regions in a perovskite phase matrix;

the perovskite phase comprises lanthanum strontium manganate; and

a manganese-cobalt-chromium intermediate spinel layer is located between the composite coating and the air side of the interconnect substrate. Atty. Dkt. No. 7917-451 WO

16. A coated interconnect for a solid oxide fuel cell, comprising:

an interconnect substrate comprising iron and chromium; and

a composite spinel and perovskite coating formed over an air side of the interconnect substrate.

17. The coated interconnect of claim 16, wherein the composite coating comprises plate shaped spinel phase regions in a perovskite phase matrix.

18. The coated interconnect of claim 17, wherein:

the spinel phase comprises manganese cobalt oxide spinel;

the perovskite phase comprises lanthanum strontium manganate;

a manganese-cobalt-chromium intermediate spinel layer is located between the composite coating and the air side of the interconnect substrate; and

the composite coating is formed using a air plasma spray process.

19. A coated interconnect for a solid oxide fuel cell, comprising:

an interconnect substrate comprising iron and chromium;

a manganese cobalt oxide spinel coating formed over an air side of the interconnect substrate;

a manganese-cobalt-chromium intermediate spinel layer located between the spinel coating and the air side of the interconnect substrate; and

a perovskite layer located over the spinel coating.

20. The coated interconnect of claim 19, wherein the spinel of the spinel coating comprises Mn2-_xCoi_+x0₄, wherein 0<x<l and wherein the spinel comprises a Co:Mn atomic ratio of at least 1:3.

21. The coated interconnect of claim 20, wherein the spinel of the spinel coating comprises Mn1.5C01.5O4 and the perovskite layer comprises lanthanum strontium manganate.

22. The coated interconnect of claim 19, wherein the spinel coating is located over both ribs and air channels over the air side of the interconnect substrate and the perovskite Atty. Dkt. No. 7917-451 WO layer is located over the spinel coating over the ribs but not over the air channels of the interconnect substrate.

23. The coated interconnect of claim 20, wherein:

the spinel coating comprises the Co:Mn atomic ratio of 1:3 to 6:4; 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.

24. The coated interconnect of claim 23, wherein the interconnect is located in a solid oxide fuel cell stack.

25. A coated interconnect for a solid oxide fuel cell, comprising:

an interconnect substrate comprising at least 70 weight percent chromium; and a manganese cobalt oxide spinel coating formed over an air side of the interconnect substrate, wherein the spinel comprises a Co:Mn atomic ratio of at least 1:3.

26. The coated interconnect of claim 25, wherein the spinel comprises Mn2-_xCoi_+x04, wherein 0<x<l.

27. The coated interconnect of claim 26, wherein the spinel comprises Mn1.5C01.5O4.

28. The coated interconnect of claim 25, wherein the spinel further comprises Cu.

29. The coated interconnect of claim 28, wherein the spinel has a formula:

Mn₂-x-_yCoi_+xCu_y0₄ (0 < x < 1), (0 < y < 0.3), if Cu is located on an A site of the spinel;

Mn2-xCoi_+x__yCu_y0₄ (0 < x < 1), (0 < y < 0.3), if Cu is located on a B site of the spinel; or

Mn₂-x-_y 2Coi_+x__{y 2}Cu_y0₄ (0 < x < 1), (0 < y < 0.3), if Cu is located on both the A site and the B site of the spinel.

30. The coated interconnect of claim 25, wherein the spinel further comprises one or more single valence elements. Atty. Dkt. No. 7917-451 WO

31. The coated interconnect of claim 30, wherein the one or more single valence elements is selected from the group consisting of Y³⁺, Al³⁺, Mg²⁺ and Zn²⁺ and the spinel comprises (Mn, Co, M)₃0₄, where M = Y, Al, Mg, or Zn.

32. The coated interconnect of claim 25, wherein the coating further comprises a getter.

33. The coated interconnect of claim 32, wherein the getter comprises at least one of A1₂0₃, Y₂0₃, or Ti0₂.

34. The coated interconnect of claim 25, further comprising a perovskite layer located over the spinel coating.

35. The coated interconnect of claim 34, wherein the spinel coating is located over both ribs and air channels over the air side of the interconnect substrate and the perovskite layer is located over the spinel coating over the ribs but not over the air channels of the interconnect substrate.

36. The coated interconnect of claim 26, wherein:

the spinel comprises the Co:Mn atomic ratio of 1:3 to 6:4; 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.

37. The coated interconnect of claim 36, further comprising a manganese-cobalt- chromium intermediate spinel layer located between the spinel coating and the chromium- iron alloy surface of the air side of the interconnect substrate.

38. The coated interconnect of claim 25, wherein the interconnect is located in a solid oxide fuel cell stack.

39. A method of making an interconnect for a solid oxide fuel cell comprising:

providing an interconnect comprising Cr and Fe; and

coating the interconnect with a ceramic layer with an air plasma spray process, Atty. Dkt. No. 7917-451 WO wherein the ceramic layer comprises a sintering aid.

40. The method of claim 39, wherein the sintering aid performs at least one of:

a) lowering of a melting temperature of a bulk phase of the ceramic;

b) melting at a lower temperature than the bulk phase; or

c) forming secondary phases with lower melting temperatures than the bulk phase ceramic.

41. The method of claim 39, wherein the ceramic layer comprises LSM and the sintering aid comprises B-site dopant selected from Fe, Co, Ni, Cu and combinations thereof.

42. The method of claim 39, wherein the ceramic layer comprises LSM and the sintering aid comprises an A-site dopant comprising Y.

43. The method of claim 39, wherein the air plasma spray process comprises spraying a mixture of a powder of the ceramic and a metal oxide that is reduced in the APS atmosphere to its metal state, the reduced metal having a melting temperature lower than that of the ceramic layer.

44. The method of claim 43, wherein:

the metal oxide comprises cobalt oxide, nickel oxide, indium oxide, tin oxide, boron oxide, bismuth oxide, barium oxide, copper oxide, or zinc oxide or any combination thereof; the metal oxide comprises from 0.1 wt to 5wt of the total ceramic and metal oxide powder weight; and

the ceramic and the reduced metal melt in a gun of an air plasma system and the molten metal promotes sintering of the melted ceramic powder as they solidify on a surface of the interconnect.

45. The method of claim 40, wherein the sintering aid comprises a compound selected from a silicate, a calcium aluminate, or a calcium aluminum silicate that reacts with the ceramic and forms one or more glassy phases.

46. A coated interconnect for a solid oxide fuel cell, comprising:

an interconnect substrate comprising iron and chromium; Atty. Dkt. No. 7917-451 WO a manganese cobalt oxide spinel coating formed over an air side of the interconnect substrate, wherein the manganese cobalt oxide spinel coating further comprises at least one or nickel and copper;

a manganese and chromium containing oxide intermediate spinel layer located between the manganese cobalt oxide spinel coating and the air side of the interconnect substrate; and

a perovskite layer located over the manganese cobalt oxide spinel coating.

47. A coated interconnect for a solid oxide fuel cell, comprising:

an interconnect substrate comprising iron and chromium; and

a manganese cobalt oxide spinel coating formed over an air side of the interconnect substrate,

wherein the manganese cobalt oxide spinel coating comprises a composition in a (Mn,Co)₃04 family between the end compositions of C0₃O₄ and M¾04, wherein an atomic ratio of Mn:Co > 5:l.

48. The coated interconnect of claim 47, wherein the manganese cobalt oxide spinel coating comprises, Mn1.75Coo.25O4 or Mn2.5Coo.5O4.

49. A coated interconnect for a solid oxide fuel cell, comprising:

an interconnect substrate comprising iron and chromium;

a manganese cobalt oxide spinel coating formed over an air side of the interconnect substrate; and

a barrier layer between the interconnect substrate and the manganese cobalt oxide spinel coating.

50. The coated interconnect of claim 49, wherein the barrier layer comprises titanium oxide or a perovskite comprising lanthanum strontium manganate.

51. The coated interconnect of claim 49, wherein the barrier is a reactive barrier comprising a dopant and the coating comprises a gradient doped manganese cobalt oxide spinel layer. Atty. Dkt. No. 7917-451 WO

52. The coated interconnect of claim 51, wherein the barrier layer comprises a dopant selected from one or more of Cr, Fe, Ti, V, Mg, Ca, Al, Si or Ce.

53. A method of making a coated interconnect for a solid oxide fuel cell, comprising: forming a reactive layer and a manganese cobalt oxide spinel layer over the interconnect; and

diffusing the dopant from the reactive layer into the manganese cobalt oxide spinel layer.

54. The method of claim 53, wherein the manganese cobalt oxide spinel layer has a gradient doping profile.

55. The method of claim 53, wherein the dopant comprises Cr, Fe, Ti, V, Mg, Ca, Al, Si, Ce or combinations thereof.

56. A method of making a coated interconnect for a solid oxide fuel cell, comprising: forming an interconnect comprising Fe, Cr and a dopant;

coating the interconnect with a manganese cobalt oxide spinel layer; and

diffusing dopant from the interconnect into the manganese cobalt oxide spinel layer.

57. The method of claim 56, wherein the manganese cobalt oxide spinel layer has a gradient doping profile.

58. The method of claim 56, wherein the dopant comprises Ti, V, Mg, Ca, Al, Si, Ce or combinations thereof.

59. A coated interconnect for a solid oxide fuel cell, comprising:

an interconnect substrate comprising iron and chromium; and

a manganese cobalt oxide spinel coating formed over an air side of the interconnect substrate, wherein the manganese cobalt oxide spinel coating further comprises at least one of iron, titanium, vanadium, chromium, aluminum, manganese, calcium, silicon and/or cerium.

60. The coated interconnect of claim 59, wherein the manganese cobalt oxide spinel comprises at least one of 1-50 at% Fe, 1-50 at% Ti, 1-50 at% V, 0.1-10 at% Cr, 1-10 at% Mg, Ca, Al, Si or Ce. Atty. Dkt. No. 7917-451 WO

61. A method of making an interconnect for a solid oxide fuel cell comprising:

forming a manganese cobalt oxide spinel coating on an air side of the interconnect except in at least one seal region on the air side of the interconnect.

62. The method of claim 61, further comprising masking the at least one seal region before forming the manganese cobalt oxide spinel coating.

63. The method of claim 61, further comprising removing manganese cobalt oxide spinel coating formed on the at least one the seal region.

64. The method of claim 61, further comprising forming at least one seal in the at least one seal region.

65. A method of making an interconnect for a solid oxide fuel cell comprising:

providing a interconnect comprising a pressed green mixture of Cr and Fe powder; coating an air side of an interconnect with a layer of material comprising ceramic powder; and

sintering the green mixture of Cr and Fe powder and the layer of ceramic powder in the same sintering step.

66. The method of claim 65, wherein the sintering is performed in a hydrogen reduction furnace with a dew point between -20 and -30° C, at temperatures between 1300 and 1400° C, and for durations between 0.5 and 6 hrs.

67. The method of claim 66, wherein sintering is performed with an oxygen partial pressure between 10^"17 and 10^"15 atm.

68. A method of making an interconnect for a solid oxide fuel cell comprising:

providing an interconnect comprising Cr and Fe;

coating the interconnect with manganese cobalt oxide precursors comprising one or more of MnO, CoO, Mn metal, Co metal and combinations thereof; and

sintering the manganese cobalt oxide precursors to form a manganese cobalt oxide spinel coating. Atty. Dkt. No. 7917-451 WO

69. The method of claim 68, further comprising removing a native oxide from at least one of an air side or a fuel side of the interconnect.

70. The method of claim 69, comprising removing a native oxide from the air side and the fuel side of the interconnect.

71. The method of claim 70, wherein removing comprises grit blasting.

72. A method of making an interconnect for a solid oxide fuel cell stack, comprising: providing first metallic powder particles comprising Cr and Fe in a mold cavity; providing second powder particles comprising one or more of Sr, La, Mn and Co oxides in the mold cavity; and

compacting the first and second powder particles to form the interconnect.

73. The method of claim 72, wherein the first and second powder particles are mixed together to form a powder mixture prior to being provided to the mold cavity as the powder mixture.

74. The method of claim 73, further comprising adding a lubricant to the powder mixture prior to the step of compacting.

75. The method of claim 74, wherein the interconnect has at least one of Sr, La, Mn and Co oxide regions throughout its thickness.

76. The method of claim 72, wherein the first powder particles are provided in the mold cavity first and the second powder particles provided on top of the first powder particles in the mold cavity and wherein the compacting results in lanthanum strontium manganate (LSM) or manganese cobalt oxide (MCO) regions on a top surface of a Cr-Fe alloy interconnect.

77. The method of claim 72, wherein the second powder particles are provided in the mold cavity first and the first powder particles provided on top of the second powder particles in the mold cavity and wherein the compacting results in lanthanum strontium manganate (LSM) or manganese cobalt oxide (MCO) regions on a bottom surface of a Cr-Fe alloy interconnect. Atty. Dkt. No. 7917-451 WO

78. The method of claim 72, wherein:

the first powder particles are provided to the mold first and the second powder particles are electrostatically attracted to a bottom surface of a punch used to compact the powder particles; and

the punch presses the second powder onto the first powder to compact the first and the second power particles to form a Cr-Fe alloy interconnect with lanthanum strontium manganate (LSM) or manganese cobalt oxide (MCO) regions on a top surface of the interconnect.

79. The method of claim 72, wherein a surface area of the interconnect is increased relative to an interconnect made only by compacting the first powder particles in the same mold cavity.

80. The method of claim 72, wherein the first powder particles comprise a mixture of elemental Cr and elemental Fe particles or Cr-Fe alloy powder particles.

81. The method of claim 72, wherein the second powder particles comprise lanthanum strontium manganate (LSM) or manganese cobalt oxide (MCO) powder particles.