TWI836119B - Fuel cell interconnect with iron rich rib regions and method of making thereof - Google Patents

Abstract

An interconnect for a solid oxide fuel cell stack includes a chromium alloy interconnect having ribs separated by channels, and metal or metal oxide caps located directly on the top surfaces of the ribs on both air and fuel sides of the interconnect but not on bottom of the channels, where the metal or metal oxide caps comprise iron, manganese, cobalt, copper, a superalloy or an oxide thereof.

Description

Fuel cell interconnects having iron-rich rib regions and methods of making the same

The present invention relates to a fuel cell stack assembly, and more particularly, to an interconnector and a method for preparing the interconnector for a fuel cell stack.

A typical solid oxide fuel cell stack includes multiple fuel cells separated by metallic interconnects (ICs), which provide electrical connections between adjacent cells in the stack and channels for transporting and removing fuel and oxidant. Metallic interconnects are typically composed of Cr-based alloys, such as those known as CrFe, with a composition of 95 wt% Cr-5 wt% Fe or Cr-Fe- with a composition of 94 wt% Cr-5 wt% Fe-1 wt% Y. Y. CrFe and CrFeY alloys maintain their strength and are dimensionally stable under typical solid oxide fuel cell (SOFC) operating conditions, such as 700-900°C in air and wet fuel atmospheres. However, during operation of the SOFC, chromium in the CrFe or CrFeY alloy reacts with oxygen and forms chromium oxide, causing degradation of the SOFC stack.

Two of the main degradation mechanisms affecting SOFC stacks are directly related to chromium oxide formation on metal interconnect components: i) higher stack ohmic resistance due to native chromium oxide (chromia, Cr ₂ O ₃ ) formation on the interconnects, and ii) chromium poisoning of the SOFC cathode.

Although Cr ₂ O ₃ is an electronic conductor, the material has extremely low conductivity at SOFC operating temperatures (700-900°C), with a value of about 0.01S/cm at 850°C (while the conductivity of Cr metal is 7.9×10 ⁴ S/cm). The thickness of the chromium oxide layer on the surface of the interconnect increases over time, and therefore the ohmic resistance of the interconnect, and therefore the SOFC stack due to this oxide layer, increases over time.

The second degradation mechanism associated with the chromium oxide that forms the metal interconnects is called 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 through the air (cathode) side of the interconnect and wet fuel flows through the fuel (anode) side of the interconnect. At SOFC operating temperatures and in the presence of moist air (cathode side), chromium on the surface of the Cr ₂ O ₃ layer on the interconnect reacts with water and oxidizes chromium hydroxide (CrO ₂ (OH)) as a gaseous species ₂ ) evaporates in the form. The chromium oxide hydroxide species is transported in vapor form from the interconnect surface to the cathode electrode of the fuel cell, where it can be deposited in solid form (Cr ₂ O ₃ ). Cr ₂ O ₃ deposits on and in the SOFC cathode (eg, via grain boundary diffusion) and/or reacts with the cathode (eg, to form Cr-Mn spinel), resulting in significant performance degradation of the cathode electrode. Typical SOFC cathode materials such as perovskite materials (eg, LSM, LSC, LSCF, and LSF) are particularly susceptible to chromium oxide degradation.

One embodiment includes a method of preparing an interconnect for a solid oxide fuel cell stack, comprising providing an iron-rich material into a channel of a mold; providing a powder containing 4-6 wt% Fe, 0-1 wt% Y and the balance Cr over the iron-rich material containing at least 25 wt% iron in the mold; pressing the iron-rich material containing at least 25 wt% iron and the powder containing 4-6 wt% Fe, 0-1 wt% Y and the balance Cr in the mold to form an interconnect; and sintering the interconnect to form a sintered interconnect having an iron-rich region with an iron concentration greater than 10% in a rib of the interconnect.

Another embodiment includes a method of preparing an interconnect for a solid oxide fuel cell stack, comprising providing a chromium alloy interconnect with ribs separated by channels, and directly on the top surface of the ribs Rather than forming a metal or metal oxide coating layer on the bottom of the channels, the metal or metal oxide coating layer includes iron, manganese, cobalt, copper, superalloys and their oxides.

Another embodiment includes an interconnect for a solid oxide fuel cell stack that includes a chromium alloy interconnect having ribs separated by channels; and a metal or metal oxide capping layer, It is located directly on the top surface of the ribs rather than on the bottom of the channels, wherein the metal or metal oxide coating includes iron, manganese, cobalt, copper, superalloys or oxides thereof.

3: Anode electrode

5:Battery electrolyte

8:Air flow channel

10: Ribs

11:LSM coating

13: One side

14: Opposite side

15: Ring seal

16A: Fuel inlet opening/through hole

16B: Fuel outlet opening/through hole

17: Flat area/fuel distribution air chamber

18: Window seals

19: Strip seal

25: Oxide layer

27: Metal or metal oxide contact layer

29: Reaction area

31: Nickel mesh

33: Impurities

35: Gap

36:Fuel inlet riser pipe

37A:Welding point

37B: welding point

37C: Welding point

37D: Welding point

100:Interconnectors

101:Spinel phase/chromium crystal

102: Spinel layer

103:Crack

104: LSM interconnect coating

111: Coating

129: Iron-rich area

200:Mold cavity

201: Mould

202: Lubricant and Cr/Fe powder

204: Coating material powder

205P:Pure Fe powder

205S:Fe foil tape

206: Second shoe-shaped part

208:Punch

305:Fe-Cr powder

308: Channel

310: Ribs

312: Adhesive

470:Metal coating

570:Metal coating

Figure 1 is a micrograph showing the Mn-Cr spinel phase within the pores of an LSM-based cathode.

Figure 2 is a micrograph showing Cr-containing phases in cracks in an LSM interconnect coating deposited by air plasma spraying. The SOFC stack was operated at 850°C for 2000 hours.

3A to 3C are schematic diagrams of steps in a method of preparing an interconnect according to an embodiment.

FIG. 4 is another schematic diagram of a method for preparing an interconnect according to an embodiment.

FIG5 is a schematic side view of an embodiment of an interconnect having a double-layer composite coating.

Figures 6A to 6B and Figure 7 are schematic diagrams illustrating the following: (6A) the air side of an interconnect according to one embodiment, (6B) a close-up view of a sealing portion of the air side of the interconnect, and (7) the fuel side of the interconnect.

Figure 8 is a micrograph illustrating chromium oxide on the fuel side (uncoated side) of the interconnect after the reduction sintering step.

Figure 9 is a micrograph of a portion of a SOFC stack illustrating the reduction of MCO coating at the coating/IC interface (in the strip seal area) due to diffusion of fuel through the porous IC.

FIG. 10 is a phase diagram illustrating the Mn ₃ O ₄ -Co ₃ O ₄ system.

Figure 11 is a schematic diagram of a fuel inlet riser in a known fuel cell stack.

Figure 12 is a schematic diagram of SOFC to illustrate the electrolyte corrosion theory.

Figure 13 is a schematic side cross-sectional view of a portion of an interconnect according to an embodiment.

Figures 14A, 14B and 14C are top views of interconnects according to alternative embodiments.

15A to 15C are schematic diagrams of steps in a method of preparing an interconnect according to another embodiment.

16 is a schematic diagram of a method of preparing an interconnect according to another embodiment.

17A-17B are schematic diagrams of steps in a method of preparing an interconnect according to another embodiment.

Figures 18A to 18D are schematic diagrams of interconnects prepared according to various embodiments.

x

0.3，諸如0.1

x

0.2。此等材料已用作互連件塗層材料。在LSM之情況下，材料具有高電子導電性，但具有低陰離子及陽離子擴散。 To limit the diffusion of chromium ions (e.g., Cr ³⁺ ) through the interconnect coating material to the SOFC cathode, a material with very few cation vacancies and thus low chromium diffusion rate can be selected. A family of materials with low cation diffusion rates belongs to the calcium-titanium family, such as strontium oxides, such as La _1-x Sr _x MnO ₃ (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 but low anion and cation diffusion.

A secondary function of the interconnect coating is to inhibit the formation of native oxide on the interconnect surface. Native oxide forms when oxygen reacts with chromium in the interconnect alloy to form a relatively high resistance layer _{of Cr2O3} . If the interconnect coating can inhibit the transport of oxygen and water vapor from the air to the surface of the interconnect, the kinetics of oxide growth can be reduced.

Similar to chromium, oxygen (eg, ^O2- ions) can be transported through the coating via solid-state diffusion or through gaseous transport through pores and cracks in the coating. This mechanism can also be used for floating water vapor, an accelerator for Cr evaporation and possible oxide growth. As discussed above, in a humid air environment, chromium evaporates _from the surface of _CrO2 as gas molecules _CrO2 (OH) ₂ , which can then diffuse through imperfections in the coating, such as pores and cracks. In the case of oxygen and water vapor, molecules diffuse through the defect by bulk diffusion or the Knudsen diffusion process, depending on the size of the defect or pore.

If CrO ₂ (OH) ₂ molecules contact the coating surface, they can react to form crystals and then re-evaporate to continue diffusion in the gas flow (in cracks or pores). Experiments have shown that CrO ₂ (OH) ₂ reacts with the LSM interconnect coating 104 to form a spinel phase 101, such as manganese chromium oxide (Mn,Cr) ₃ O ₄ as shown in Figure 1 . Although CrO ₂ (OH) ₂ reacts with LSM to form a spinel phase, the chromium species does not inhibit reevaporation and further diffuses into cracks or defects. Chromium transport has been observed along the length of cracks in the coating of LSM ICs operating in fuel cells for extended periods of time. Figure 2 shows chromium crystals 101 in cracks 103 in an LSM IC coating 104 on the cathode side operating in a SOFC stack for 2000 hours under typical conditions of 800-850°C in ambient air. The formation of chromium-containing crystals is characteristic of those crystals formed from gas phase to solid phase transition. SEM and EDS analysis of the overall LSM coating away from the cracks did not show the presence of chromium. Therefore, it can be concluded that the majority of chromium transport from CrFe interconnects through the LSM IC coating occurs via gas phase transport through and along micro- and macro-cracks, interparticle spaces and pores in the LSM coating happen.

In the case of solid-state transport, materials are chosen that have very few oxide ion vacancies and therefore low oxide ion conductivity. For example, calcite LSM is unique in that it exhibits low cation and anion conductivity, but has high electron conductivity, making it an excellent coating material. Other calcites (such as La1 _{- xSrxFeO3 -d} , La1 _- xSrxCoO3 _-d , and La1 _{- xSrxCo1- yFeyO3 - d} ) all exhibit high electron conductivity and low cation conductivity (low chromium diffusion _rate ). However, these particular materials also exhibit high oxide ion conductivity and are therefore less effective in protecting interconnects from oxidation (oxide growth).

x

1)或書寫為z(Mn₃O₄)+(1-z)(Co₃O₄)(其中(

z

))或書寫為(Mn,Co)₃O₄之任何尖晶石，諸如Mn_1.5Co_1.5O₄、MnCo₂O₄或Mn₂CoO₄。含有過渡金屬之許多尖晶石展現良好的電子導電性及相當低的陰離子及陽離子擴散率，且因此為適合之塗層材料。 A second family of materials that can be used for interconnect coatings is manganese cobalt oxide (MCO) spinel materials. In one embodiment, MCO spinel covers a range of combinations from Mn ₂ CoO ₄ to Co ₂ MnO _4. That is, a material having the composition Mn _2-x Co _1+x O ₄ (0

x

1) or written as z(Mn ₃ O ₄ )+(1-z)(Co ₃ O ₄ )(where (

z

)) or any spinel written as (Mn,Co) ₃ O ₄ , such as Mn _1.5 Co _1.5 O ₄ , MnCo ₂ O ₄ or Mn ₂ CoO ₄ . Many spinels containing transition metals exhibit good electronic conductivity and relatively low anion and cation diffusion rates and are therefore suitable coating materials.

In one embodiment, a spinel (e.g., (Mn,Co) ₃ O ₄ ) powder is doped with Cu to lower the melting temperature of the spinel. The lowered melting temperature improves (increases) the coating density when deposited by coating methods such as air plasma spraying (APS) and increases the conductivity of the oxide in the reaction zone. Due to the lower melting temperature, the increase in coating density may occur during APS deposition and during long-term operation at SOFC temperatures.

Adding Cu to the spinel layer has additional advantages. Cu-doped spinels such as (Mn,Co) ₃ O ₄ can result in higher conductivity of the base spinel phase and any reaction zone oxides formed between the spinel and native Cr ₂ O ₃ oxides Rate. Examples of conductivities of oxides from the (Mn, Co, Cu, Cr) ₃ O ₄ family include: CuCr ₂ O ₄ : 0.4 S/cm at 800°C, Cu _1.3 Mn _1.7 O ₄ : 225 S at 750°C /cm and CuMn ₂ O ₄ : 40S/cm at 800°C.

The family of spinel materials has the general formula AB ₂ O ₄ . Depending on the elements occupying the A and B sites, these materials can form octahedral or cubic crystal structures. In addition, depending on the doping conditions, the copper atoms can occupy the A site, the B site, or a combination of the A and B sites. Generally speaking, Cu prefers to enter the B site. When the A element is Mn, the B element is Co and the spinel is doped with Cu, the spinel family can be described by the general formula (Mn, Co, Cu) ₃ O _4. More specifically, depending on the position of the Cu alloying element, the spinel family can be described by the following formula:

x

1)，(0

y

0.3) (1) If Cu enters the A site, then Mn _2-xy Co _1+x Cu _y O ₄ (0

x

1),(0

y

0.3)

x

1)，(0

y

0.3) (2) If Cu enters the B site, then Mn _2-x Co _1+xy Cu _y O ₄ (0

x

1), (0

y

0.3)

x

1)，(0

y

0.3) (3) If Cu enters the A and B sites equally, then Mn _2-xy /2Co _1+xy /2Cu _y O ₄ (0

x

1),(0

y

0.3)

y

0.3)。此等組合物亦可寫為(Mn₂O₃)+(1-z)(CoO)+z(CuO)，其中(0

z

0.3)。若Cu進入B位點，則其他組合物包括Co₂Mn_1-yCu_yO₄，其中(0

y

0.3)。此等組合物亦可寫為(Co₂O₃)+(1-z)(MnO)+z(CuO)，其中(0

z

0.3)。在一種較 佳的Mn,Co尖晶石組合物中，Mn/Co比率為1.5/1.5，例如M_1.5Co_1.5O₄。當B位點摻雜有Cu時，較佳組合物包括Mn_1.5Co_1.5-yCu_yO₄，其中(0

y

0.3)。 Specific (Mn, Co, Cu) ₃ O ₄ compositions include, but are not limited to, Mn _1.5 Co _1.2 Cu _0.3 O ₄ , Mn _1.5 Co _1.4 Cu _0.1 O ₄ ; Mn ₂ Co _0.8 Cu _0.2 O ₄ , and Co ₂ Mn _0.8 Cu _0.2 O ₄ . If Cu is incorporated into the B site, additional compositions include Mn ₂ Co _1-y Cu _y O ₄ , wherein (0

y

0.3). These compositions can also be written as (Mn ₂ O ₃ )+(1-z)(CoO)+z(CuO), where (0

z

0.3). If Cu enters the B site, the other compositions include Co ₂ Mn _1-y Cu _y O ₄ , where (0

y

0.3). These compositions can also be written as (Co ₂ O ₃ )+(1-z)(MnO)+z(CuO), where (0

z

0.3). In a preferred Mn, Co spinel composition, the Mn/Co ratio is 1.5/1.5, such as M _1.5 Co _1.5 O ₄ . When the B site is doped with Cu, the preferred composition includes Mn _1.5 Co _1.5-y Cu _y O ₄ , wherein (0

y

0.3).

y

0.3)或(1-z)(Mn₂O₃)+z(Al₂O₃)+CoO，其中(0

z

0.15)。 In another embodiment, the (Mn,Co) ₃ O ₄ or (Mn,Co,Cu) ₃ O ₄ spinel family is doped with one or more monovalent species. That is, one or more species having only one valence state. Doping with monovalent species reduces cation transport at high temperatures and thus reduces the thickness of the intermediate oxide layer. The main ion transport mechanism in spinel is cation diffusion through 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 the M species oxidizes from a lower valence state to a higher valence state to maintain local charge neutrality. The introduction of monovalent species generally reduces the amount of cation vacancies and reduces the amount of interdiffusion between the spinel coating 102 and the native Cr ₂ O ₃ oxide or CrFe substrate 100. In this way, the amount of intermediate oxide layer formed is reduced. Examples of monovalent species that can be introduced into the spinel coating include Y ³⁺ , Al ³⁺ , Mg ²⁺ and/or Zn ²⁺ metals. In one embodiment, the spinel coating has a composition of (Mn, Co, M) ₃ O ₄ , where M = Y, Al, Mg or Zn. For example, if M = Al is doped in the A position, the spinel composition may include Mn _{2-y AlyCoO 4} (0

y

0.3) or (1-z)(Mn ₂ O ₃ )+z(Al ₂ O ₃ )+CoO, where (0

z

0.15).

In one embodiment, the interconnect coating is deposited on a Cr-based alloy interconnect, such as an IC containing 93-97 wt% Cr and 3-7 wt% Fe, such as the Cr-Fe-Y or CrFe interconnects described above, using an air plasma spray (APS) process. The air plasma spray process is a thermal spray process in which a powdered coating material is fed into a coating apparatus. The coating particles are introduced into a plasma jet, where they are melted and then accelerated toward the substrate. Upon reaching the substrate, the molten droplets are planarized and cooled, thereby forming a coating. Plasma can be generated by direct current (DC plasma) or by induction (RF plasma). In addition, unlike controlled atmosphere plasma spraying (CAPS), which requires an inert gas or vacuum, air plasma spraying is performed in ambient air.

Cracks in the coating can appear at two different times, a) during deposition, and b) during operation under SOFC conditions. Cracks formed during deposition are affected by the spray gun parameters and the material properties of the coating material. Cracks formed during operation vary greatly with the material properties, and more specifically, the density and sinterability of the material. Without being bound by a particular theory, it is believed that cracks that appear during operation are a result of continued sintering of the coating and therefore increasing densification of the coating over time. As the coating densifies, it shrinks laterally. However, the coating is constrained by the substrate and therefore forms cracks to relieve stress. Coatings applied at lower densities are more likely to densify further during operation, leading to crack formation. In contrast, coatings applied at higher densities are less likely to form cracks.

x

0.3，0.005

y

0.05且0

d

0.3。應注意，Fe、Co、Ni及Cu之原子百分比範圍不一定必須與先前實施例之此等元素的重量百分比範圍相匹配。 In a first embodiment, a sintering aid is added to the IC coating to reduce crack formation and therefore chromium evaporation. Sintering aids are materials that increase the density of the as-deposited coating and/or reduce the densification of the coating after deposition. Because sintering aids increase the as-deposited density of the coating material, they thereby reduce crack formation after coating formation due to subsequent densification and/or operating stresses on relatively porous materials. Suitable sintering aids include materials that: a) reduce the melting temperature of the bulk phase of the coating material, b) melt at a temperature lower than the bulk phase, thereby causing liquid phase sintering, or c) form a layer with a lower melting temperature Secondary phase. For the perovskite family including LSM, sintering aids include Fe, Co, Ni and Cu. These transition metals are soluble in LSM and easily doped into the B sites in the ABO ₃ perovskite phase. The melting temperatures of oxides in 3d transition metals tend to decrease in the order Fe>Co>Ni>Cu. Adding these elements to the B site of LSM will lower the melting temperature and increase the density of states as sprayed. In one embodiment, one or more of Fe, Co, Ni, and Cu are added to the coating such that the coating contains 0.5 to 5 wt%, such as 1 to 4%, such as 2 to 3% ) of such metals. In an alternative embodiment, the coating composition is expressed in atomic percent and includes La _1-x Sr _x Mn _{1-y My} O _3-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 Fe, Co, Ni, and Cu do not necessarily have to match the weight percent ranges of these elements of the previous embodiments.

Other elements may also be added in combination with the above transition metals to maximize conductivity, stability and sinterability. Such elements include, but are not limited to, Ba, Bi, B, Cu or any combination thereof (e.g., Cu+Ba combination), such as in the range of 5 wt% or less, such as 0.5-5 wt%. In addition, sintering aids (such as Y), especially doping the A-site of the LSM, may be added to obtain similar effects. An example according to this embodiment is La _y Y _x Sr _1-xy MnO ₃ , wherein x=0.05-0.5, y=0.2-0.5, such as La _0.4 Y _0.1 Sr _0.5 MnO ₃ . For coating materials other than LSM, copper can be used as a sintering aid in the MCO spinel materials described above.

In another embodiment, metal oxide powders that are easily reduced to their metallic state in an APS atmosphere are added to the plasma rather than introducing transition metal powders into the air plasma jet during deposition. Preferably, the metal of the metal oxide exhibits a melting temperature lower than that of the coating phase (perovskite or spinel phase). For example, binary oxides cobalt oxide (for example, CoO, Co ₃ O ₄ or Co ₂ O ₃ ), NiO, In ₂ O ₃ , SnO, B ₂ O ₃ , copper oxide (for example, CuO or Cu ₂ O), BaO, Bi ₂ O ₃ , ZnO, or any combination thereof (e.g., (Cu,Ba)O) can be added as a second phase to the coating powder (i.e., LSM powder or La+Sr+Mn powder or its oxide). This addition creates a two-phase powder mixture that is fed to the spray gun. The amount of the second phase may be less than or equal to 5 wt%, such as in the range of 0.1 wt% to 5 wt% of the total powder weight.

In the APS gun, as the LSM particles solidify on the surface of the IC, the metal oxide is reduced to its metallic phase, melting and promoting sintering of the molten LSM particles. The lower melting temperatures of metals and binary oxides promote densification during deposition and solidification.

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 feed during the APS process. The lower melting temperature secondary phase promotes densification. For example, silicate and/or calcium aluminate powders can react with coating material powder in the hot plasma portion of the APS gun to form a glass phase. In one embodiment, La from the LSM material reacts with Si-Ca-Al oxide (which may also include K or Na) to form a glass phase, such as La-Ca-Si-Al oxide formed between LSM particles things. The coating may include less than or equal to 5 wt%, such as 0.5-5% silicate, Ca-Al oxide or Si-Ca-Al oxide.

x

0.1，d

0.3)之較低Sr含量LSM塗層來進一步增加此效應，使得氧之非化學計量最大化。使用此燒結步驟可增強任何或所有上文所描述之燒結助劑技術。 In a second embodiment, the coating is post-treated in a manner that causes stress-free densification. The post-processing can be performed with or without adding the sintering aid of the first embodiment. In an example post-processing according to the second embodiment, an "oxidation-reduction" cycle is performed in an N ₂ and O ₂ atmosphere. During this cycle, the coating is alternately exposed to neutral and oxidizing atmospheres. For example, the coating may be treated in a neutral atmosphere containing nitrogen or an inert gas (eg, argon) and then in an oxidizing atmosphere containing oxygen, water vapor, air, and the like. If desired, one or more cycles may be performed, such as 2, 3, 4 or more. If necessary, a reducing (eg, hydrogen) atmosphere may be used instead of or in addition to a neutral atmosphere. Redox cycling in N ₂ and O ₂ atmospheres can create a cation vacancy concentration gradient, which increases the diffusion of cation vacancies and thereby effectively increases the sintering rate. By using La _1-x SrxMnO _3-d (where x<=0.1, for example 0.01

x

0.1,d

0.3) The lower Sr content LSM coating further increases this effect, maximizing the non-stoichiometry of oxygen. Use of this sintering step can enhance any or all of the sintering aid technologies described above.

In a third embodiment, the surface area of electrical interaction between the coating and the underlying Cr-Fe IC surface is expanded. The chromium oxide layer that forms between the coating and the IC causes a millivolt drop over time as the thickness of the chromium oxide layer grows. The total voltage drop depends on the area over which the voltage drop occurs and the thickness. Increasing the oxide growth area between the IC and the coating reduces the impact on voltage losses, thereby increasing the useful life of the stack. By adding something that will penetrate deep into the coating, this embodiment effectively increases the surface area of contact and thereby reduces the impact of the growing chromium oxide layer.

A method according to this third embodiment includes embedding a small amount of coating material into the IC. There are two alternative aspects of this embodiment. One aspect includes fully and uniformly distributing a coating material, such as LSM or MCO, within the IC powder (eg, Cr-Fe powder) prior to pressing to form the IC. Coating powders (eg, LSM and/or MCO powders) may be included when the lubricant and Fe, Cr (or Cr-Fe alloy) powders are mixed together prior to pressing. Preferably, the powder mixture is able to withstand sintering temperatures and reducing environments. A second aspect includes incorporating (eg, embedding) a predetermined amount of coating powder into only the top surface of the Cr alloy IC. After the IC sintering step, the oxide regions embedded in the surface of the CrFe or CrFeY IC increase the surface roughness of the IC. After the pressing and sintering steps, the entire coating is deposited on the Cr alloy interconnect.

A method for embedding coating material in the top surface of an interconnect is illustrated in Figures 3A-3C. As shown in Figure 3A, lubricant and Cr/Fe powder 202 used to form the body of the IC are added to the mold cavity 200 using a first shoe (not shown) or by another suitable method. As shown in Figure 3B, prior to the pressing step, the mold cavity is filled with coating material powder 204 (eg, LSM or MCO) or coating material powder 204 and lubricant/Cr/Fe powder 202 using a second shoe 206 The mixture is provided into the mold cavity above the powder 202 in the mold cavity. Next, as shown in Figure 3C, the powders 204, 202 are pressed using a punch 208 to form an interconnect on the air side with a coating material embedded in its surface (i.e., if the air side of the IC is facing in the mold formed above).

Alternatively, coating material powder 204 (eg, LSM or MCO) (or a mixture of coating material powder 204 and lubricant/Cr/Fe powder 202) is first provided to the mold cavity. 200 in. If the air side of the IC is formed facing downwards in the mold, lubricant/Cr/Fe powder 202 is provided to the mold cavity 200 above the powder 204 before the pressing step. In this manner, the coating material is incorporated into the IC primarily at the top of the IC's air side surface.

Alternatively, as shown in FIG. 4 , the coating powder 204 may be electrostatically attracted to the upper punch 208 of the press. The upper punch 208 then presses the coating powder 204 and the lubricant/interconnect powder material 202 in the mold cavity 200 to form an IC having the coating material 204 embedded in the top portion of the air side.

Using the above method, after the pressing step, the coating powder can be uniformly incorporated into the surface of the air side of the IC. The pressing step is then followed by a sintering and coating step, such as an MCO and/or LSM coating step by APS or another method described herein.

The ratio of coating powder to Fe in the Cr-Fe alloy is preferably selected so that the top coating material has a coefficient of thermal expansion similar to the coefficient of thermal expansion (CTE) 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 selected by selecting the ratio of Cr to Fe. The sintering process can be adjusted to keep the powder oxidized and stable. For example, sintering can be performed using humid hydrogen, or in an inert atmosphere such as nitrogen, argon, or another inert gas. Humid hydrogen or inert gas atmospheres are oxidizing or neutral, respectively, and thereby prevent the oxide powder from reducing.

In a fourth embodiment, the coating is a multi-layer composite. FIG. 5 illustrates an example of a fourth embodiment of an IC having a composite coating. The composite coating consists of a spinel layer 102 and a calcite layer 104. The spinel layer 102 is first deposited on a Cr alloy (e.g., CrFe) interconnect 100. A calcite layer 104 (e.g., an LSM layer described above) is then deposited on top of the spinel layer 102. During deposition of layer 102 and/or during high temperature operation of a fuel cell stack containing the interconnect, an interfacial spinel layer 101 containing native chromium may form between the interconnect 100 and layer 102.

x

1)，(0

y

0.3))，其降低層101之電阻率。 Preferably, the lower spinel layer 102 comprises the MCO spinel containing Cu and/or Ni described above. Layer 102 acts as a doping layer that increases the conductivity of the underlying manganese chromium oxide (Mn,Cr) ₃ O ₄ or manganese cobalt chromium oxide (Mn,Co,Cr) ₃ O ₄ interface spinel layer 101. In other words, during and/or after forming layer 101, Cu and/or Ni from the spinel layer 102 diffuse into the interface spinel layer 101. This produces a Cu and/or Ni doped layer 101 (e.g., (Mn and Cr) _3-xy Co _x (Cu and/or Ni) _y O ₄ , where (0

x

1),(0

y

0.3)), which reduces the resistivity of layer 101.

0.3)。更特定言之，視Cu及/或Ni合金元素之位置而定，尖晶石家族可用下式描述： Layer 102 may include the MCO layer containing Cu and/or the MCO layer containing Ni and/or the MCO layer containing Ni and Cu described above. In the MCO layer, when the A element is Mn, the B element is Co and the spinel is doped with Cu and/or Ni, the spinel family can be described by the general formula (Mn, Co) _3-y (Cu, Ni) _y O ₄ , where (0<y

0.3). More specifically, depending on the position of the Cu and/or Ni alloying elements, the spinel family can be described by the following formula:

x

1)，(0

y

0.3) (1) If Cu and/or Ni enter the A site, then Mn _2-xy Co _1+x (Cu,Ni _y O ₄ (0

x

1), (0

y

0.3)

x

1)，(0

y

0.3) (2) If Cu and/or Ni enter the B site, then Mn _2-x Co _1+xy (Cu,Ni) _y O ₄ (0

x

1),(0

y

0.3)

x

1)，(0

y

0.3)。 (3) If Cu and/or Ni enter the A and B sites equally, then Mn _2-xy/2 Co _1+xy/2 (Cu,Ni _y O ₄ (0

x

1), (0

y

0.3).

Although the spinel doped layer 102 containing Cu and/or Ni reduces the ASR of the interconnect, it is permeable to both oxygen and chromium. Therefore, in an embodiment of the present invention, a second calcium titanate barrier layer 104 is formed above the doped layer 102. Preferably, the layer 104 is a dense LSM layer that reduces or prevents the diffusion of Cr and oxygen. The layer 104 can be formed with a sintering aid as described above to increase its density. The dense layer 104 reduces or prevents the growth of the interfacial spinel layer 101 by blocking the diffusion of air and oxygen from the cathode side of the fuel cell to the CrFe IC surface during stack operation. Layer 104 also reduces or prevents chromium poisoning of the cathodes of fuel cells in the stack by reducing or preventing chromium from diffusing from the IC to the cathode.

Thus, the composite coating 102/104 reduces or eliminates the area specific resistance (ASR) degradation contribution of the interconnects and stack and reduces overall degradation of the fuel cell stack by reducing or eliminating Cr poisoning of the fuel cell cathode. First, the spinel doped layer 102 dopes the chromium-containing interfacial spinel layer 101 with elements (e.g., Ni and/or Cu) that reduce the electrical resistance of the spinel layer 101. Second, the spinel layer 102 prevents direct interaction between the calcite layer 104 and the Cr-containing interfacial spinel layer 101, which can result in the formation of an undesirable and resistive secondary phase. Third, the spinel (e.g., Mn-containing spinel with 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 calcium titanate layer 104 is a second barrier layer that reduces the transfer of oxygen to the interfacial oxide 101 on the surface of the interconnect. Thus, the top calcium titanate layer 104 reduces the growth rate of the native oxide layer 101 and reduces the transfer of chromium from the layer 101 to the fuel cell cathode via the doped layer 102.

Figure 6A shows the air side of an exemplary interconnect 100. Interconnects can be used in stacks with manifolds for fuel on the inside and air manifolds on the outside. The interconnect contains passages/channels 8 between the ribs 10 to allow air to flow from one side 13 of the interconnect to the opposite side 14 . An annular (eg annular) seal 15 is located around the fuel inlet opening 16A and the outlet opening 16B (ie, the through holes 16A, 16B in the interconnect 100). Strip seals 19 are located on the sides of the interconnect 100 .

FIG. 6B shows a close-up view of an exemplary seal 15, channel 8, and rib 10. Seal 15 may include any suitable sealing glass or glass ceramic material, such as borosilicate glass. Alternatively, seal 15 may include a glass ceramic material as described in U.S. Application No. 12/292,078, filed November 12, 2008, which is incorporated herein by reference.

If desired, the interconnect 100 may contain a raised or boss area below the seal 15. In addition, as illustrated in FIG. 6B , the seal 15 is preferably located in a flat area 17 of the interconnect 100. That is, the seal 15 is located in a portion of the interconnect that does not include the rib 10. If desired, the interconnect 100 may be configured for use with a stack that has manifolds for air and fuel installed internally. In this case, the interconnect 100 and the corresponding fuel cell electrolyte will also contain additional inlet and outlet openings (not shown).

Figure 7 illustrates the fuel side of interconnect 100. Window seal 18 is located on the edge of interconnect 100 . Also shown are the fuel distribution plenums 17 and fuel flow channels 8 between the ribs 10 . It is worth noting that the interconnect 100 shown in Figure 7 has two types of fuel flow channels; however, this is not a limitation of the present invention. The fuel side of interconnect 100 may have fuel flow channels that are deep and of the same length, or a combination of short and long channels, and/or a combination of deep and shallow channels.

In one embodiment, interconnect 100 is coated with _Mn1.5Co1.5O4 (MCO) _spinel at room temperature using _an aerosol spraying method and further treated with one or more thermal treatments. Generally, MCO coating is omitted in the sealing areas (rings 15, stripes 19) by masking or removing the MCO deposited in these areas.

The MCO coating can be reduced by fuel in the riser bore and then react with the glass seal material at the annular seal 15. Therefore, in one embodiment, for the interconnect 100 shown in Figure 6B, the MCO coating is removed from the flat area 17 (eg, by sandblasting) on the air side of the interconnect prior to stack assembly and testing. remove. Alternatively, flat areas 17 may be masked during aerosol deposition to prevent coating of flat areas 17 . Therefore, the MCO coating is omitted in the area 17 under the annular seal 15 adjacent the fuel inlet opening 16A and/or the outlet opening 16B.

In another embodiment, the interconnect 100 is manufactured by a powder metallurgy process. The powder metallurgy process can produce a part that has connected pores within the body of the interconnect 100, which allow fuel to diffuse from the fuel side to the air side. This fuel transmitted through the pores can react with the MCO coating on the air side at the coating/interconnect interface. This reaction can cause seal failure and stack separation. In one embodiment, this failure can be mitigated by masking the seal 19 location on the edge of the interconnect during MCO deposition, omitting the MCO coating under the strip seal 19, thereby eliminating the coating in these sealed areas and allowing the glass seal 19 to be directly bonded to the metal interconnect.

In another embodiment, the interconnect 100 forms a thin _{green Cr2O3} oxide layer 25 on the fuel side of the interconnect 100. A cross-sectional micrograph of this fuel side oxide is shown in Figure 8. The _{Cr2O3 oxide} thickness was found to be between 0.5 and 2 microns. The three methods described below can be used to convert or remove this undesirable chromium oxide layer.

In a first embodiment of the method, the oxide layer is removed by any suitable method, such as sandblasting. This method is effective. However, this method is time consuming and increases processing costs.

Alternatively, the _{Cr2O3 oxide} layer 25 can be left in place and converted to a composite layer. In this embodiment, a nickel mesh anode contact is deposited on the _Cr2O3 oxide layer 25 and allowed to diffuse into the chromium oxide layer. The nickel reacts with _{the Cr2O3 oxide} layer 25 and forms _a Ni _- metal/ _Cr2O3 composite layer that reduces the ohmic resistance of layer 25. If necessary, the mesh can be heated after contacting the layer 25 to accelerate composite formation.

In another embodiment, the oxide layer ₂₅ is reduced or completely eliminated by firing the MCO coated interconnect in an environment with low oxygen partial pressure. For example, based on thermodynamics, _Cr2O3 can be reduced to Cr metal at 900°C at a _pO2 (partial pressure) of 10-24 atm, while CoO can be reduced to Co-metal at 900°C at a _pO2 of 10-16 atm. By reducing the oxygen partial pressure of the firing atmosphere (i.e., lowering the dew point) to less than _10-24 atm at 900°C, the formation of _Cr2O3 oxide on the fuel side (uncoated side) can be prevented, while allowing the MCO coating on the air side of the interconnect to be reduced to MnO (or Mn metal if _pO2 <10-27 atm) and Co-metal to facilitate sintering. At _pO2 <10-27 atm, MCO will be reduced to Mn-metal and Co-metal, which can result in better sintering and a denser coating compared to MnO/Co-metal. Generally, the MCO coated interconnect can be annealed at T>850°C (eg, 900°C to 1200°C) at a _pO2 of 10-24 atm (eg, 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.

In another embodiment described below, the formation of a Cr ₂ O ₃ oxide layer 25 is reduced or avoided in the location between the nickel mesh anode contact compliant layer and the fuel side of the interconnect.

As described above, in SOFC stacks, a compliant layer in the form of nickel is typically introduced to conform to surface shape changes to improve contact with the battery's anode electrode. At the beginning of the service life, simple contact is sufficient to provide the expected power from the active area. However, when the interconnect is made primarily from chromium, an oxide layer 25 may then form on the fuel side during operation due to the water content in the fuel. In the following embodiments, the growth of this oxide layer 25 may be reduced or eliminated beneath the Ni mesh.

The inventors have observed that interconnects containing oxide layer 25 growth between the IC and the Ni mesh generally have high voltage losses and degradation rates that are closely related to the area specific resistance degradation ("ASRD") rate (via Ohm's law and active area). The growth of low conductivity oxides can generally cause an increase in ASRD. Conversely, interconnects that provide minimal oxide layer 25 growth between the IC and the Ni mesh generally have low ASRD. With the absence of oxide layer 25, the inventors have also observed that the interconnect immediately below the Ni mesh forms a Cr-Fe-Ni alloy. Without wishing to be bound by a particular theory, the inventors believe that the formation of this Cr-Fe-Ni alloy or Cr-Ni alloy can lead to lower ASRD and that this alloy is more resistant to oxide growth than Cr-Fe interconnects. Therefore, it is advantageous to form this alloy at as many contact points as possible between the Ni mesh and the IC, especially in areas of high current density, such as in the middle of the interconnect. Furthermore, during stack operation, the current must be forced through the resistive oxide layer 25 that grows late in the stack life, or consolidate to higher conductivity points, thereby reducing the effective active area.

Furthermore, the inventors also believe that the formation of this alloy is affected by several factors, including the compression pressure between the Ni mesh and the interconnect, the undiffused Fe percentage in the local interconnect below the Ni mesh, surface contamination between the interconnect and the Ni mesh, the connection between the mesh and the interconnect, and/or the addition of nickel to the interconnect alloy. For example, if the undiffused Fe percentage is low and the contaminants are high, high pressure can be placed to overcome these obstacles. In contrast, if the undiffused Fe percentage increases and the contaminant content decreases, less pressure can be placed to avoid an increase in ASRD.

In a first aspect of this embodiment, the compressive pressure between the Ni mesh and the interconnect in the fuel cell stack is increased to reduce the formation of the chromium oxide layer 25. One way to increase the pressure on the mesh in the stack is to make the interconnect thickness non-uniform to create a pressure field or gradient on the mesh. Preferably, the height of the interconnect ribs in the middle of the interconnect is slightly greater than the height of the ribs in the edge of the interconnect (i.e., the thickness of the middle of the interconnect is slightly greater than the thickness of the outer portion of the interconnect). This creates a pressure field in the middle of the interconnect (where most of the current is generated in adjacent fuel cells in the stack) and, after the mesh and interconnect are placed in the fuel cell stack, a higher pressure is applied to the nickel mesh contacting the middle of the interconnect than to the edge of the interconnect. This, in turn, is believed to increase the formation of Cr-Fe-Ni alloy beneath the mesh and/or reduce ASRD.

In a second aspect of this embodiment, contamination between the fuel side of the interconnect and the mesh is reduced. This can be done by reducing the presence of contaminants during the stack manufacturing process and/or by cleaning surface of the interconnect.

In a third aspect of this embodiment, a sufficiently high percentage of undiffused iron remains on at least the fuel side of the interconnect to form a Cr-Fe-Ni alloy. Undiffused iron includes regions of iron that have not yet been alloyed with the chromium matrix of the interconnect (e.g., in an interconnect having 4-6 wt % Fe and the balance being Cr and optionally 0-1 wt % yttrium oxide or yttrium). Achieving a high percentage of undiffused iron can be achieved by any suitable method, such as less sintered pressed powder interconnects and/or starting with larger iron particles. Less sintering involves partially sintering the interconnect after pressing the chromium and iron-containing powders into the interconnect at a lower temperature or for a shorter duration than required to fully alloy the pressed iron and chromium powder particles. Larger iron particles are effective in achieving the desired percentage of undiffused iron for ASRD reduction purposes, but may require longer sintering times and/or higher sintering temperatures. Thus, one method of achieving undiffused iron involves pressing a mixture of chromium powder having a first average particle size and iron powder having a second particle size larger than the first particle size (e.g., 30-200% larger in diameter, such as 50-100% larger) to form the interconnect, followed by sintering the interconnect.

In a fourth aspect of this embodiment, a nickel mesh is physically connected to the fuel side surface of the interconnect to prevent chromium oxide from forming between the mesh and the surface of the interconnect. For example, at least in the middle of the interconnect, and preferably in the middle and edges of the interconnect, the nickel mesh can be thermally welded, welded or brazed to the interconnect surface over the entire surface of the mesh. By soldering the mesh to the interconnect in a plurality of locations, especially in the middle of the interconnect where low voltages are often found, the effective active area is increased and high conductivity is found in this active area. Alternatively, pressed powder Cr-Fe interconnects may be placed in contact with a Ni mesh and then sintered while in contact with a Ni mesh below the melting point of Ni (eg, below 1450°C, such as at 1350-1425°C) . This sintering temperature maintains the CTE of the part as the sintering time increases while thermally fusing the Ni mesh to the interconnect in all contact points.

In a fifth aspect of this embodiment, nickel is added to the Cr-Fe interconnect gold to promote the formation of a Cr-Fe-Ni alloy at least on the fuel surface of the interconnect. Iron powder is added to the base Cr powder to increase the CTE of the interconnect beyond that of chromium and match the CTE of the solid oxide fuel cell. In the case of Ni, which has approximately the same CTE as Fe, it is reasonable for Ni to replace Fe. Ni powder is doped into chromium powder or chromium and iron (or chromium-iron alloy) powder mixture in a powder metallurgy press/die, followed by pressing the powder, resulting in a part containing Cr-Ni or Cr-Fe-Ni The pressed interconnect portion of the alloy. Additionally, Co and Ni are slightly more compressible than Fe, so replacing Fe with these elements will only aid the compaction process. It is known that adding Fe to the Cr matrix reduces the degree of oxidation, so it may still be advantageous to maintain a certain Fe content. Therefore, all or part of the iron in the interconnect may be replaced by nickel (e.g., 1-100%, such as 10-90%, e.g., 30-70% of the iron in a Cr-Fe (4-6 wt%) alloy may be replaced by nickel). Nickel is substituted to form Cr-M (4-6wt%) alloy, where M=1-99% Ni and 99-1Fe%.

Powder composition adjustments are described in the above examples for assisting the function of the coating on the air side of the interconnect by partially replacing Fe in the powder mixture with LSM, MCO, Co and/or Mn in order to facilitate the A Mn-Co-Cr spinel layer is formed on the cathode side (air side) of the interconnect.

In another aspect of this embodiment, alloying elements applicable to the air side (e.g., Co and/or Mn) and the fuel side (e.g., Ni) are incorporated into the Cr-Fe interconnect. Thus, the powder composition placed in the press/die includes Cr, Fe, Ni, and at least one of Co and Mn. However, only Co and/or Mn are required on the air side, and only Ni is required on the fuel side. The inventors have observed that a quantitative separation of the Fe and Cr powders occurs in the press/die, resulting in smaller Cr particles sifting down in the press pressing cavity (i.e., in the die cavity), resulting in the fuel side having a more dilute Fe content, and the air side having a more concentrated Fe content. This phenomenon can be exploited to layer the IC with the desired materials by mixing the powder composition and placing it into a press cavity where the Ni particle size is smaller than the Cr and Fe particles and the Cr and Fe particles are the same size. If particles containing Co and/or Mn are also used (e.g., Co and/or Mn metal particles and/or MCO and/or LSM metal oxide particles), they are larger than the Cr and Fe particles of the interconnect pressed with the air side up. The punch and die can then be vibrated as the press cavity is filled to help the separation process occur. This will cause Ni to settle on the bottom of the cavity that will form the fuel side of the interconnect, Co and/or Mn to settle on the top of the cavity that will form the air side of the interconnect, and Fe and Cr to remain in the middle. For interconnects pressed with the fuel side up, the Ni particle size is larger than the Cr and Fe particles, the Cr and Fe particles are the same size, and the particles containing Co and/or Mn are smaller than the Cr and Fe particles. As used herein, particle size refers to the average particle size, and larger particles can have an average particle size that is 25-200% larger than the average particle size of Fe and Cr, and smaller particles can have an average particle size that is 25-200% smaller than the average particle size of Fe and Cr.

The following is a non-limiting example of the average particle size of this embodiment.

If chromium oxide management does not require Fe, then: 2.5% Co, particle size is about 100μm

95% Cr, particle size about 50μm

2.5% Ni, particle size is about 25μm

If chromium oxide management requires Fe, then: 2% Co, particle size is about 100μm

95% Cr, particle size is about 50μm

1% Fe, particle size is about 50μm

2% Ni, particle size is about 25μm

If a Mn-based spinel is formed, then: 1% Co, particle size is about 100μm

1% Mn, particle size is about 100μm

95% Cr, particle size is about 50μm

1% Fe, particle size is about 50μm

2% Ni, particle size is about 25μm

In another aspect of this embodiment, nickel powder may be added only to the fuel side of the IC using the method described above with respect to Figures 3 and 4. If desired, nickel powder may be added to the fuel side while Co, Mn, cobalt oxide, and/or manganese oxide powders may be added only to the air side of the interconnect.

A method for embedding alloy material in the top surface of the interconnect is illustrated in FIGS. 3A-3C. As shown in FIG. 3A, a lubricant and Cr/Fe powder 202 for forming the body of the IC are added to a mold cavity 200 using a first shoe (not shown) or by another suitable method. As shown in FIG. 3B, prior to the pressing step, a second shoe 206 is used to provide an alloy material powder 204 (e.g., Ni) or a mixture of alloy material powder 204 and lubricant/Cr/Fe powder 202 to the mold cavity above the powder 202 in the mold cavity. The powders 204, 202 are then pressed using a punch 208 as shown in FIG. 3C to form an interconnect having an alloy material (e.g., Ni) embedded in its surface on the fuel side (i.e., if the fuel side of the IC is formed facing upward in the mold). Prior to the alloy (e.g., Ni) material being formed on the top side of the interconnect, an air side coating material powder (e.g., LSM, MCO, Co and/or Mn) may be formed on the opposing bottom side of the interconnect as described above with respect to FIGS. 3A-3C .

Alternatively, alloy material powder 204 (or a mixture of alloy material powder 204 and lubricant/Cr/Fe powder 202) is first provided into mold cavity 200. If the fuel side of the IC is formed in the mold facing downward, lubricant/Cr/Fe powder 202 is then provided above powder 204 in mold cavity 200 prior to the pressing step. In this way, Ni is incorporated into the IC primarily at the top of the fuel side surface of the IC. Air side coating material powder (e.g., LSM, MCO, Co and/or Mn) may then be formed on the opposite top side of the interconnect as described above with respect to FIGS. 3A-3C.

Alternatively, as shown in FIG. 4 , the alloy powder 204 may be electrostatically attracted to the upper punch 208 of the press. The upper punch 208 then presses the alloy powder 204 and lubricant/interconnector powder material 202 in the die cavity 200 to form an interconnector having the alloy material 204 embedded in the top portion of the fuel side.

Using the above method, after the pressing step, the alloy powder can be uniformly incorporated into the surface of the fuel side of the interconnect. Then, the pressing step is followed by the sintering and nickel mesh forming steps.

In a sixth aspect of this embodiment, as shown in FIG13, a metal or metal oxide contact layer 27 is formed on at least the top portion of the rib 10 on the fuel side of the interconnect. The contact layer 27 contacts the nickel mesh 31, which in turn contacts the anode electrode 3 of an adjacent fuel cell in the stack.

Without wishing to be bound by a particular theory, it is believed that the contact layer 27 breaks the continuous chromium oxide scale of the chromium oxide layer 25 and promotes the formation of Cr- in the "reaction zone" 29 in the interconnect rib 10 below the mesh 31 Fe-Ni or Cr-Ni alloy, especially when the contact layer 27 contains nickel or nickel oxide. Specifically, nickel may diffuse from the nickel or nickel oxide contact layer 27 into the chromium oxide layer 25, thereby creating a conductive path through the oxide (eg, a nickel path formed by solid nickel diffusion) and diffusing to the interconnect. in the "reaction zone" 29 of the rib, thereby increasing the formation of Cr-Fe-Ni or Cr-Ni alloy and increasing the size (eg, depth and/or width) of the "reaction zone" 29. Furthermore, the contact layer 27 increases the contact surface area between the nickel or nickel oxide material and the interconnect ribs 10 (compared to the contact surface area between the conductors of the nickel mesh 31 and the ribs 10). Therefore, contact layer 27 may facilitate interconnection Good contact between the ribs 10 and the mesh 31. Furthermore, it is believed that the contact layer 27 prevents or reduces the diffusion of impurities 33 from the mesh 31 into the chromium oxide layer 25 , which could otherwise potentially lead to an undesirable increase in the resistance of the chromium oxide layer 25 .

The contact layer 27 may be made of any suitable metal, such as nickel, platinum or a platinum group metal (such as rhodium, palladium or ruthenium), copper, iron, cobalt, silver, gold, tungsten, any other transition group metal or any alloy of the foregoing metals. The metal may be applied in the metallic (reduced) phase (e.g., nickel metal), or may be applied as an oxide (e.g., nickel oxide). The oxide will be reduced to the metallic, conductive phase during normal operation of the hydrogen-rich fuel stream.

Contact layer 27 in the form of metal or metal oxide can be applied by any suitable method, such as screen printing, sputtering, electron beam deposition, evaporation, atomic layer deposition, electroplating, electroless plating, thermal spraying, painting, Dip coating, aerosol spray coating, electrophoretic deposition, etc. Any of the above fabrication processes may optionally be performed after thermal processes (eg, annealing) to achieve bonding and interdiffusion, such as sintering, reduction, oxidation, diffusion bonding, or brazing.

For example, a contact layer 27 (such as a contact layer metal or metal oxide containing ink) can be screen printed on the ribs 10 , on another layer (such as a Ni mesh 31 ) or on the fuel cell anode electrode 3 . The anode print may be in a pattern of ribs aligned with the ribs 10, perpendicular to the ribs (or at any other angle), or in a continuous layer.

For interconnects with screen printed contact layer 27 and mesh 31 soldered to ribs 10, it is preferred that contact layer 27 does not coat the entire top surface of all ribs to provide that mesh 31 will be soldered to the ribs. Part 10 of the uncoated area. Generally speaking, screen printed layers can have poor electrical conductivity. Therefore, the contact layer 27 pattern preferably has gaps 35 to accommodate mesh bonding points. For example, as shown in Figure 14A, there may be four soldering points 37A, 37B, 37C, and 37D for soldering, one in each corner of the interconnect. The contact layer 27 pattern should have points 37A exposed to The gap 35 of 37D is such that it is not coated by the contact layer 27 .

For example, the contact layer 27 pattern shown in FIG. 14A contains printed lines that do not extend all the way to the ends of the ribs to expose the solder points 37A to 37D in the strip gaps 35 . However, the area covered with anode contact layer 27 will have better electrical contact with the mesh than the area in gap 35 .

More complex patterns are shown in Figure 14B. This contact layer 27 pattern has more ribs coated with anode contact ink, thereby forming better electrical contact. Figure 14C illustrates the intermediate background between the patterns of Figures 14A and 14B. The contact layer 27 pattern in Figure 14C is cross-shaped so that gaps 35 are located only at the corners of the interconnects. Because the contact layer 27 ink has limited thickness and compressibility, sufficient gap 35 space should be provided around the weld points 37A to 37D so that the mesh does not deform excessively when pressed down onto the bare ribs 10 to be welded. The pattern shown in Figure 14C has a large amount of space to accommodate soldering points 37A to 37D and changes in the automated soldering process, while still maintaining good contact with the ends of the ribs in the center of the cell, which is important for electrochemical and recombination systems. important.

The patterns described above are not limited and other contact layer 27 patterns may be used. For example, a hatch ("dashed") pattern may be used. It may be advantageous to print the contact ink in dashed lines (eg, discontinuous). This can increase local contact pressure and therefore electrical contact. In another configuration, each printed rib or every 2nd rib (or every 3rd rib, etc.) is printed with contact layer ink.

The contact layer may have a thickness of 5 microns to 1000 microns (e.g. 25 microns). If a thicker print is required, multiple layers may be screen printed in consecutive steps. The anodic contact ink should be printable, sufficiently stable under ambient conditions and printed with suitable wear resistance. The powder may be a metal, a metal alloy or an oxide such as nickel oxide which is reduced to Ni metal during operation. The ink may contain a solvent such as water, ethanol, ethylene glycol, terpineol, isopropyl alcohol, toluene, hexane or acetone. The ink may also contain a dispersant, a binder and/or a plasticizer. An anti-wear component may also be added to the ink. Depending on the particle size of the powder and the relevant surface area, a solid loading of 50-90% may be used, such as about 80%. After printing, the ink may be dried in a low temperature process to make the printed layer more stable and wear-resistant. It may be dried at a temperature of 80°C-200°C, such as about 120°C.

The contact layer 27 may be selectively formed on the top of the rib, the top and sides of the rib, or the ribs 10 and channels 8 may be coated over at least a portion of the entire surface of the interconnect. Preferably, if the contact layer 27 is printed, it is located only on the top of the rib 10. If ink pours down into the fuel channel 8, the fuel flow may be affected, which in turn may affect the fuel distribution in the hot box. In severe cases, it may be necessary to reduce the hot box fuel utilization, which reduces system efficiency and/or power output. The printing can be carefully aligned and periodically checked. Manual inspection or an automated vision system can be implemented to screen out misprinted interconnects. The ink may be tinted with contrast additives to improve the accuracy of automated vision systems. Screening should be carefully matched to "pitch" (rib spacing). Therefore, interconnect manufacturing variations may require multiple screens with different rib spacings to ensure well-registered printing.

In summary, the formation of the chromium oxide layer is reduced or avoided by at least one of: increasing the press pressure between the nickel mesh and the interconnect, providing undiffused Fe in the interconnect below the nickel mesh, reducing surface contamination between the interconnect and the nickel mesh, bonding the nickel mesh to the interconnect, adding nickel to the interconnect alloy, or applying a metal or metal oxide contact layer over the ribs on the fuel side of the interconnect, including any combination of two, three, four, five, or all six of the above steps.

In another embodiment, to reduce the cost of the MCO coating process, the MCO coating can be annealed (e.g., sintered or sintered) during the sintering step of the powder metallurgy (PM) formed interconnect. The sintering of the powder metallurgy interconnect 100 and the MCO coating on the interconnect can be performed in the same step in a reducing environment, such as a hydrogen reduction furnace with a dew point between -20 and -30°C, a temperature between 1300 and 1400°C, and a duration between 0.5 and 6 hours. At such temperatures and oxygen partial pressures, the MCO coating will be completely reduced to Co-metal and Mn-metal. However, the melting temperature of Mn is about 1245°C, the melting temperature of Co is about 1495°C, and the Co-Mn system has a low-pressure liquidus. Therefore, sintering at temperatures between 1300 and 1400°C may result in the formation of an undesirable liquid phase.

Possible solutions to avoid liquid formation include lowering the sintering temperature below 1300°C, such as below 1245°C, for example 1100°C to 1245°C; increasing the oxygen partial pressure to reduce the Mn (but not oxidize Cr) in the MCO to Mn-metal opposite MnO (melting temperature 1650°C); reduce the Mn:Co ratio in MCO to increase the melting temperature of the Mn-Co metal system; add dopants to MCO (such as Cr) to increase Co-Mn- Melting temperature of the 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 prevent reduction to the metallic phase). For example, at a sintering temperature of 1400° _C , MnO is reduced to Mn-metal at a _pO2 of 10-17 atm, while _Cr2O3 is reduced to Cr-metal at a _pO2 of ^10-15 atm, which provides where Cr is reduced to metal but MnO remains as a small range oxide with a high melting point (pO ₂ between 10 ⁻¹⁷ and 10 ⁻¹⁵ atm). Therefore, interconnects and MCO coatings can be sintered at 1300-1400°C at pO ₂ =10 ^-15 -10 ^-17 atm.

In another embodiment, the IC sintering step can be performed first and then the MCO coating is applied to the sintered IC. The IC and coating were then subjected to the reduction steps described in the previous examples and more suitable for MCO coatings.

In another embodiment, interconnect manufacturing 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 components. The mixture is then sintered, preferably under low _pO2 conditions. However, such sintering may be easier or the starting material may be denser, thereby reducing sintering time. Additionally, such precursor particles may be significantly cheaper than MCO precursors, which require expensive synthetic methods to produce.

Additionally, a sandblasting step may be performed to remove native chromium oxide layers from both the air and fuel sides of the interconnect prior to coating the interconnect with the MCO layer. To reduce costs, native oxide may be removed only from the air side of the interconnect prior to 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 annealed as described above. After the annealing is complete, the oxide may then be removed from the fuel side, such as by sandblasting. In this manner, the number of sandblasting steps is reduced because an additional sandblasting step is not required to remove oxide growth on the fuel side of the interconnect that occurs during annealing of the MCO coating.

In other embodiments, the composition of the MCO coating is modified to increase stability at SOFC operating temperatures, such as 800-1000°C. The MCO composition of some previous examples was Mn _1.5 Co _1.5 O ₄ . This material has high electrical conductivity. However, MCO materials can be reduced to binary oxides, MnO and CoO, or to binary oxides MnO and Co-metal.

In some fuel cell geometries, the MCO coating is only directly exposed to the fuel flow at riser openings 16A, 16B. This fuel/coating interface can be eliminated by not coating the flat area 17 around the opening (Figure 6B). However, interconnects fabricated by powder metallurgy methods allow for portions with some connected (open) pores to allow fuel to diffuse to the air side through the portion. Fuel diffusing through the pores can react with and reduce the MCO at the MCO/interconnect interface (shown in Figure 9), creating a porous layer composed of MnO and Co-metal. As shown in Figure 9, the coating/IC interface can be damaged, leading to adhesion failure and cell separation from the interconnect during routine handling.

It is desirable for the coated material to be more stable and less likely to reduce when exposed to a fuel environment. The embodiments described below optimize the composition and/or dope the MCO with other elements to stabilize the material in a reducing atmosphere.

Figures 11 and 12 illustrate the theory of electrolyte corrosion. In the prior art SOFC stack shown in Figures 11 and 12, the LSM coating 11 on the interconnect is in contact with the annular seal 15. The seal 15 is in contact with the cell electrolyte 5. Without wishing to be bound by a particular theory, it is believed that manganese and/or cobalt from the metal oxide (e.g., LSM of LSCo) layer 11 containing manganese and/or cobalt filters into and/or reacts with the glass seal 15 and is then transported from the glass to the electrolyte. Manganese and/or cobalt can be transferred from the glass to the electrolyte as manganese and/or cobalt atoms or ions or as compounds containing manganese and/or cobalt (such as manganese and/or cobalt-rich silicate compounds). For example, it is believed that manganese and cobalt react with the glass to form a (Si,Ba)(Mn,Co)O _6+δ mobile phase, which is transferred from the glass seal to the electrolyte. Manganese and/or cobalt at or in the electrolyte 5 (e.g., as part of the mobile phase) tend to accumulate at the grain boundaries of the zirconia-based electrolyte. This produces intergranular corrosion and pits that weaken the electrolyte grain boundaries, ultimately leading to cracks in the electrolyte 5 (e.g., opening 16A to opening 16B cracks). 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 produce a mobile phase and enhance the filtration of manganese and/or cobalt from the layer 11 into the seal 15, as shown in Figure 11.

As discussed above, in other embodiments, the composition of the MCO coating is modified to increase stability at SOFC operating temperatures, such as 800-1000°C. Therefore, MCO compositions can be optimized based on stability and conductivity. Example compositions include, but are not limited to, Mn ₂ CoO ₄ , Mn1.75Co _0.25 O ₄ , Co _1.75 Mn _0.25O4 , CoMnO ₄ and Co _2.5 Mn _0.5 O ₄ .

Based on the phase diagram (Fig. 10) and from a stability point of view, it can be beneficial to have Mn-rich heterogeneous compositions, such as Mn _2.5 Co _0.5 O ₄ and Mn _2.75 Co _0.25 O ₄ (e.g. Mn:Co atomic ratio of 5 :1 or greater, such as 5:1 to 11:1). Higher Mn contents also produce more stable compositions because the compositions are in a higher oxidation state than the binary spinel + binary oxide found at high Co contents. However, any composition in _{the (Mn,Co)3O4 family between the} final composition of _Co3O4 and _Mn3O4 may be suitable.

In another embodiment, MCO is stabilized by adding additional dopants that are less susceptible to reduction. For example, MCO is known to react with Cr in IC alloys to form (Cr,Co,Mn) ₃ O ₄ spinel. If Cr is intentionally added to the MCO coating at low levels, such as 0.1 atomic % to 10%, this will produce spinel (Cr,Co,Mn) ₃ O ₄ that is more stable than MCO due to the Cr ^{3 +} is extremely stable. Other transition metal elements that are soluble in the spinel structure and can increase stability include Fe, V and Ti. Example coating materials include spinel (Fe,Co,Mn) ₃ O ₄ with 1% to 50 at% Fe, (Ti,Co,Mn) ₃ O ₄ or (Fe,Ti ,Co,Mn) ₃ O ₄ combination.

Adding Ti can lead to more stable secondary phases including _{Co2TiO4 , Mn2TiO4} or _Fe2TiO4 . These phases are beneficial to the overall coating stability. _{Spinels with any combination of the dopants mentioned above may include (Fe,Cr,Co,Mn)3O4 , ( Cr} ,Ti,Co,Mn) _{3O4 ,} etc.

It is known that spinels based on Mg, Ca and Al are extremely stable and resistant to reduction. However, these spinels have low electrical conductivity and are therefore not well suited for interconnect coatings. In contrast, doping Ca, Mg and/or Al at low levels into conductive spinels such as MCO increases the stability of the material while only slightly reducing the conductivity. Example spinels include (Ca,Co,Mn) ₃ O ₄ with 1 to 10 at% Ca, (Mg, Co, Mn) 3 O 4 with 1 to 10 at% Mg, (Mg, Co, Mn) ₃ O ₄ with 1 to 10 at% Al (Al, Co, Mn) ₃ O ₄ or a combination such as (Ca, Al, Mn, Co) ₃ O ₄ , where Ca, Al and/or Mg are added at 1-10 at%. Si and Ce are other elements that can be used as dopants (1-10at%) of MCO spinels.

In addition to the above-described methods belonging to the general category of material-specific stabilization efforts, alternative embodiments are intended to devise modifications that can be combined with or as an alternative to the above-described embodiments to improve the stability of the coating. In a first alternative embodiment, a stabilizing barrier layer may be added to the interconnect prior to adding the MCO coating. This barrier layer will preferably be made of a more stable oxide than MCO, and will be conductive and thin enough not to adversely affect the conductivity of the interconnect assembly. In addition, the barrier layer is preferably dense and sealed. Example barrier layers include, but are not limited to, a Ti-doped oxide (eg, _TiO2 ) layer or lanthanum strontium manganate (LSM).

A second alternative embodiment includes adding a reactive barrier layer between the interconnect and the MCO coating, the MCO coating including any of the elements discussed above as possible dopants (e.g., Cr, V, Fe, Ti, Al, Mg, Si, Ce, and/or Ca). When the interconnect is heated to standard operating temperatures (800-1000°C), this layer diffuses these elements into the MCO coating, thereby creating a gradient doping profile with a higher concentration of dopants at the interconnect interface where reduction occurs. In this way, the majority of the coating contains relatively few dopants and thus the conductivity may be less affected by the uniform doping of the coating material. The reaction layer is a metal layer (such as Ti or a metal-containing compound that allows metal to diffuse outward at 800°C or higher).

90wt% Cr、4-6wt% Fe及0.1-2wt% Mg、Ti、Ca及/或Al。 A third embodiment includes designing the interconnect material to contain reactive dopants (e.g. Si, Ce, Mg, Ca, Ti and/or Al for Cr-4-6% Fe interconnects) that diffuse into the MCO coating in the same manner as just described. Thus, the interconnect will contain

90wt% Cr, 4-6wt% Fe and 0.1-2wt% Mg, Ti, Ca and/or Al.

Additionally, any method of depositing or treating the IC to reduce or close the porosity of the part, in addition to standard oxidation methods, will help limit MCO coating reduction. For example, a Cr layer can Plating onto the porous parts prior to the MCO annealing step to further reduce porosity. Alternatively, as described above, if dense and sealed, adding a reactive barrier layer will also reduce or block hydrogen diffusion from surface pores.

To ensure a long-term good electrical connection between the Ni mesh 31 and the Cr-Fe alloy rib 10, it is desirable to have Fe at the top of the rib 10 to ensure a long-term good electrical connection between the Ni mesh 31 and the top of the Cr/Fe rib 10 Electrical connection. However, in the case where the interconnect 100 has an iron content of 4 to 6 wt%, the Fe particles are not sufficient to ensure that all or at least most of the Ni is in contact with the Fe-containing Cr. By intentionally placing Fe on top of the ribs 10 , preferably prior to sintering, electrical contact life can be increased without unacceptably changing the coefficient of thermal expansion of the entire interconnect 100 .

The following non-limiting example methods of increasing the iron concentration at the tips of interconnect ribs 10 compared to the base portion of interconnect 100 between channels 8 and ribs on opposite sides of the interconnect include (1 ) placing pure Fe particles, such as 99% pure, such as 99.9% pure, on top of the ribs 10 , (2) using Fe foil strips and/or (3) placing Cr containing a higher concentration of Fe into the ribs 10 on the entire fuel side.

Particles may be provided to the top of the ribs 10 by spraying particles over the fuel side of a pressing tool, such as a mold 201 containing a cavity 200 containing the ribs 10 and channels 8 in an opposite arrangement to the interconnects. Ribs 310 are separated by channels 308. Gravity keeps the particles at the bottom of the passage 308 of the pressing tool. If the mold consists of a hollow cylinder with upper and lower punches, the ribs 310 and channels 308 may be located in the lower punch of the pressing tool, or if the bottom of the mold is fixed and the tool contains only one upper punch, then It is located in the bottom of mold 201.

As shown in FIG. 15A , pure Fe powder 205P (e.g., 99.9 wt% pure Fe particles) is provided (e.g., sprayed) into the channel 308 of the mold cavity 200. As shown in FIG. 15B , a powder 202 comprising 4-6 wt% Fe, 0-1 wt% Y, and the balance Cr is provided onto the top of the pure Fe powder 205P in the channel 308 of the mold cavity 200. Then, as also illustrated in FIG. 15B , the powder 202 is pressed with a punch 208 (e.g., an upper punch). As illustrated in FIG. 15C , the result is an interconnect 100 having an iron-rich region 129 at the top of the rib 10 on the fuel side of the interconnect 100. Region 129 has greater than 10 wt% Fe (e.g., 15-99 wt% Fe, e.g., 25-75 wt% Fe), optionally 0 to 1 wt% Y, and the balance is chromium. The advantage of this method is that it is inexpensive and results in lower high area ratio resistive degradation ("ASRD"). One feature of this embodiment is that a small amount of iron powder grains are placed in place using a spraying method rather than a powder shoe method.

In another embodiment, as illustrated in FIG. 16 , a Fe foil strip 205S is placed in the channel 308 of the cavity 200 of the press tool 201. In one embodiment, the Fe foil strip 205 may be 20-80 microns thick, such as 30-70 microns thick. The foil strip 205S may be about one-half to one millimeter wide, such as 0.25 to 0.75 mm wide. In one embodiment, the foil strip 205S may be held in place at the bottom of the channel 308 with an adhesive 312, such as an acrylic adhesive or another suitable adhesive. An advantage of this method is that the amount of Fe provided to the top of the rib 10 of the interconnect 100 is more controlled than a powder spraying method. Similar to the method illustrated in FIG. 15B , a powder 202 comprising 4-6wt% Fe, 0-1wt% Y and the balance Cr is provided on top of the Fe foil strip 205S in the channel 308 of the cavity 200 of the mold 201. The powder 202 is then pressed with a punch 208. The result is an interconnect 100 having an iron-rich region 129 at the top of the rib 10, similar to the interconnect 100 illustrated in FIG. 15C . Advantageously, when a shoe filled with Cr-Fe 4-6wt% powder is used, displacement of Fe particles associated with the above method is avoided.

In another embodiment illustrated in FIG. 17A , pure iron powder (e.g., 99% pure iron powder) or Fe-Cr powder 305 having a higher Fe concentration (e.g., 25-50wt% Fe, 50-25wt% Cr) than the base portion of the interconnect 100 is provided to the channel 308 of the cavity 200 of the pressing tool 201 before the Fe-Cr powder 202 having a lower Fe concentration (e.g., 4-6wt% Fe) is provided to the cavity 200 of the pressing tool 201. In this embodiment, the first shoe 206 is used to place the powder 305 containing a higher Fe content into the rib channel 308. After the second shoe 206 fills the remaining space in the cavity 200 with powder 202 comprising 4-6wt% Fe, 0-1wt% Y and the balance Cr, as illustrated in FIG. 17B , a punch 208 is used in a pressing step similar to that illustrated in FIG. 15B to reduce the height of the area containing powder 305 by about half. The target value of at least about 25% Fe is sufficient to provide sufficient Fe particles at the top of the interconnect 100 rib 10 without unacceptably affecting the CTE of the interconnect. In this embodiment, the interconnect 100 rib 10 has an intentional gradient in the Fe concentration of the powder.

In another embodiment, the fuel side of the metal interconnect 100 contacts the nickel screen 31 in the anode chamber in the presence of a wet fuel atmosphere. Embodiments include materials and methods that alter the chemistry of the top of the rib 10 and the microstructure of the rib 10 to reduce the resistance of the layer formed between the nickel screen 31 and the top of the rib 10. It is believed that depositing certain materials onto the top of the rib 10 will reduce the ohmic resistance. Such materials include Fe, Mn, Co, Cu, and high temperature superalloys (including but not limited to Inconel 625 and 718, Haynes 230, and various Hastelloy alloys) and their oxides. The chemical formulas of Inconel Nickel 625, 718 and Haynes 230 and various Herschel alloys are provided below.

Inconel 625(wt%)

Inco Nickel 718 (wt%)

Haynes 230

Hersted combination

For example, if an Fe capping layer is deposited and metallurgically bonded to the top of the rib, Cr ₂ O ₃ is not formed between the Fe capping layer and the Cr-Fe top of the rib 10 . However, Cr can still diffuse from the interconnect ribs through the Fe capping layer and deposit on the nickel wire mesh 31 via Cr evaporation and deposition. Therefore, the "new" interface coating will be between the Fe coating and the nickel mesh 31. If an oxide forms at this interface, it will be an oxide of an alloy of Fe-Ni-Cr (Fe and Ni are in contact with Cr added by evaporation/diffusion). This Fe-Ni-Cr alloy may be in an austenitic phase and join the nickel mesh 31 to the Fe capping layer without the presence of oxides. However, if an oxide forms, it will be the Fe,Ni,Cr) _{3O4 spinel} phase _, which is more conductive than _Cr2O3 . In the case where the Mn coating is bonded to the top of the rib 10, the principle is similar but will be different. For example, in addition to Mn-Ni-Cr alloys, highly conductive (Mn,Ni,Cr) ₃ O ₄ oxides can also be formed. However, a lower resistance interface is also formed.

Another embodiment of reducing the ohmic resistance of the oxide on top of the rib 10 includes depositing a capping layer of inconel or other Ni-Cr superalloy on top of the Cr-Fe interconnect material. As is well known, the oxidation rate of Cr-Fe, and therefore the subsequent _Cr2O3 layer thickness, is much greater than that of Ni-Cr superalloy. By depositing a capping layer of dense superalloy and metallurgically bonding it to the top of the rib 10, the thickness of _{the Cr2O3} layer formed under the _nickel wire (on top of the capping layer) should be significantly thinner relative to the Cr-Fe alloy, such as less than half the thickness, such as between 10-50% the thickness. In addition, many superalloys form dual oxide coatings, including Cr ₂ O ₃ and (Mn,Cr) ₃ O ₄ spinel, which is more conductive than Cr ₂ O ₃ and has lower Cr-evaporation.

In the above embodiments, an additional benefit is that the metal capping layer 470 can fill the pores and increase the density of the top of the rib 10. There is some evidence that a porous and poorly formed rib top can lead to excessive oxide formation and thicker oxide layers. In embodiments where the metal or metal oxide capping layer 470 is formed on the top of the rib 10 of the interconnect 100, the Ni contact layer 27 described with respect to FIG. 13 can be omitted. However, the Fe-rich top portion of the rib 10 of the embodiments of FIGS. 15A to 17B can be included below the capping layer 470 or omitted.

To obtain a dense metal coating on top of the sintered interconnect 100 formed by powder metallurgy, a laser can be used to melt the metal powder that has been deposited on the top of the rib 10. In a first aspect, the interconnect 100 is placed in a bed of metal powder of the desired coating material. Additional powder is then spread over the interconnect 100 to fill the channels 8 and to a given depth (e.g., 10-50 μm) on top of the rib 10. The laser is then scanned over the top of the rib 10 to subsequently melt the metal powder and bond it to the top of the rib 10. In a second embodiment, the selected metal powder is mixed with one or more organic compounds and screen printed onto the top of the rib 10 with the desired width and thickness. The top of the rib 10 is then laser scanned to melt and bond the powder. In both cases, an appropriate inert atmosphere (argon, neon, helium, etc.) should be selected so that the metal powder does not oxidize easily. Other parameters (such as laser power, laser type, scan rate, and powder particle size) can be optimized for each material set to achieve the desired bonding, thickness, and density of the metal coating.

In the embodiment shown in FIG. 18A , the metal or metal oxide coating layer 470 is formed only on the top surface of the rib 10 , but not on the sidewalls of the rib 10 or the bottom of the channel 8 . In an alternative embodiment shown in FIG. 18B , the metal or metal oxide coating layer 470 is formed on the top of the sidewalls of the rib 10 and at least a portion thereof.

The capping layer 470 can advantageously be formed at different stages of interconnect 100 fabrication. For example, the capping layer 470 can be added to the interconnect 100 after pressing or after oxidizing the interconnect 100, each of which has different advantages. For example, adding a Fe capping layer 470 to the top of the rib 10 after pressing a powder comprising 4-6wt% Fe, 0-1wt% Y, and the balance Cr will result in a higher density Fe-rich region, but still contain enough Cr to prevent Fe oxidation. This ensures that enough Fe metal remains on the top of the rib 10 to perform the resistance reduction function.

If Fe is added after oxidation of the pressed and sintered interconnect 100 (e.g., as described in U.S. Patent No. 8,652,691, hereby incorporated by reference in its entirety), subsequent exposure to water (e.g., wet fuel) in the SOFC stack during stack operation can result in some oxidation of the Fe capping layer 470. Some oxidation states of iron are electrically conductive as oxides. The advantage is to produce even higher local concentrations of Fe without negatively affecting the thermal expansion distribution coefficient of the interconnect.

In one embodiment of forming the Mn capping layer 470, due to the high vapor pressure of Mn, it is preferable to form Mn on top of the ribs 10 after sintering and before oxidizing the interconnect 100. A subsequent pure oxidation step then forms a higher quality, more conductive manganese oxide than when Mn is oxidized in wet fuel as part of the SOFC operating conditions.

In an alternative embodiment shown in FIG. 18C , a metal capping layer 570 is formed on top of the air side ribs 10 of the interconnect 100 . This metal covering layer 570 can also reduce the ASRD at the air side interface. As described in previous embodiments, the air side of interconnect 100 may be coated by atmospheric plasma spraying (APS) with LSM, MCO, or LSM/MCO composite materials that reduce or prevent Cr-evaporation. However, a resistive Cr ₂ O ₃ layer may still form beneath the coating. In one embodiment, the interconnect is sandblasted, the top of the air side ribs are capped with a metal overlay 570 (such as Inconel), and then the entire air side of the interconnect is coated with APS LSM, MCO Or LSM/MCO layer. Inconel (or other superalloy) preferably reduces the thickness of Cr ₂ O ₃ formed on top of the ribs. Furthermore, the reduced Cr ₂ O ₃ thickness reduces the ASRD on the air side because the tops of the ribs 10 carry a greater proportion of the current. Similarly, the Fe or _{Mn capping layer promotes the formation of (Cr,Fe)3O4 and} (Cr,Fe) _{3O4 spinel} phases that are more conductive than the _{native Cr2O3 oxide} skin. Capping layer 570 may be formed in addition to, or without, metal or metal oxide capping layer 470 on the fuel side of interconnect 100 .

In other alternative embodiments shown in Figure 18D, the first metal coating 470 described above with respect to Figure 18A or 18B is formed on the fuel side of the rib 10, and the second metal coating 470 described above with respect to Figure 18C is formed on the fuel side of the rib 10. A metal capping layer 570 is formed on top of the air side ribs 10 of the interconnect 100 .

Thus, in an alternative embodiment, a metal or metal oxide coating (e.g., Fe, Mn, superalloy, Co, Cu, etc.) 570 is formed on top of the ribs 10 on the air side of the interconnect 100 beneath the LSM, MCO, or LSM/MCO coating 111, which covers the ribs 10 and channels 8 of the interconnect 100 on the air side of the coating.

15A-17B include a method of preparing an interconnect for a solid oxide fuel cell stack comprising at least 7 wt%, such as at least 10 wt%, such as at least 20 wt%, such as at least 25 wt% iron. , such as about 10 wt% to about 99.9 wt%, such as about 10 wt% to about 95 wt% iron, such as about 20 wt% to about 80 wt% iron, including about 25 wt% to about 50 wt% iron, an iron-rich material (205P, 205S, 305) are provided to the channel 308 of the mold 201; the powder 202 containing 4-6wt% Fe, 0-1wt% Y and the rest is Cr is provided to the iron-rich material (205P, 205S, 305) in the mold 201 Above; pressing an iron-rich material and powder in mold 201 (e.g., using punch 208 ) to form interconnect 100 ; and sintering interconnect 100 to form rib 10 having an iron concentration ratio of interconnect 100 The sintered interconnect of the iron-rich region 129 with a 10% greater iron concentration, as shown in Figure 15C.

In one embodiment, the method further includes connecting the Ni mesh 31 (shown in FIG. 13 ) to the iron-rich region 129 in the rib 10 on the fuel side of the interconnect 100 . In one embodiment, iron-rich region 129 contains 10-99 wt% iron, such as 15-99 wt% iron, such as 20-80 wt% iron, such as 25-75 wt% iron. The interconnect 100 may be placed in a solid oxide fuel cell stack containing solid oxide fuel cells.

In one embodiment, the iron-rich region 129 is located only in the top ends of the ribs 10 on the fuel side of the interconnect beneath the nickel mesh 31 (shown in FIG. 13 ) that contacts the top ends of these ribs. In another embodiment, the iron-rich region 129 is located only in the top ends of the ribs 10 on the air side of the interconnect 100. In another embodiment, the iron-rich region 129 is located in the top ends of the ribs 10 on both the fuel side and the air side of the interconnect.

In one embodiment shown in Figure 15A, the step of providing an iron-rich material containing at least 25 wt% iron into the channels of the mold includes spraying iron powder 205P containing at least 99 wt% iron into the channels 308 of the mold 201 middle.

In another embodiment shown in FIG. 16 , the step of providing the iron-rich material into the channels of the mold includes providing iron foil strips 205S into the channels 308 of the mold 201 . Depending on the situation, the adhesive 312 can be disposed between the iron foil strip 205S and the channel 308 of the mold 201.

In another embodiment shown in Figure 17A, the step of providing the iron-rich material into the channels of the mold includes using a first shoe 206 to provide a powder 305 containing pure iron or chromium and at least 25 wt% iron. to the channel 308 of the mold 201. As shown in Figure 17B, the step of providing a powder containing 4-6 wt% Fe, 0-1 wt% Y, and the remainder Cr includes using a second shoe 206 to provide a powder containing 4-6 wt% Fe, 0-1 wt% Y, and Powder 202, the balance being Cr, is provided in mold 201 over powder 305 containing pure iron or chromium and at least 25 wt% iron.

In another embodiment shown in Figures 18A to 18C, a method of preparing an interconnect for a solid oxide fuel cell stack includes providing a chromium alloy interconnect 100 having ribs 10 separated by channels 8, and forming a metal or metal oxide coating (470, 570) directly on the top surface of the ribs 10 on the air side and the fuel side of the interconnect, but not on the bottom of the channel 8. The metal or metal oxide coating (470, 570) includes iron, manganese, cobalt, copper, superalloys or oxides thereof.

In one embodiment shown in FIG. 18A , the covering layer 470 is located only on the top surface of the rib 10 and not on the side surface of the rib 10 . In another embodiment shown in FIG. 18B , the covering layer 470 is located on the top surface of the rib 10 and on the side surface of the rib 10 and not on the bottom of the channel 8 .

In one embodiment, the method further comprises pressing a powder 202 comprising 4-6 wt% Fe, 0-1 wt% Y and the balance Cr in a mold 201 to form an interconnect 100; and sintering the interconnect before forming a metal or metal oxide coating (470, 570); and placing the interconnect into a solid oxide fuel cell stack containing a solid oxide fuel cell after forming the metal or metal oxide coating.

In one embodiment, the iron-rich region 129 described above and illustrated in Figure 15C may be located in the metal or metal oxide coating (470, 570) on the fuel side and/or the air side of the interconnect 100 below.

In one embodiment shown in FIG. 18C , the method further includes forming a lanthanum strontium manganate layer, a manganese cobalt oxide layer, or a composite lanthanum strontium manganate layer on the metal or metal oxide capping layer 570 and on the bottom of the channel 8 /Manganese cobalt oxide layer 111.

Although the foregoing relates to preferred embodiments, it should be understood that the invention is not limited thereto. It will be apparent to those skilled in the art that various changes may be made to the disclosed embodiments and that such changes are intended to be within the scope of the invention. All publications, patent applications, and patents cited herein are incorporated by reference in their entirety.

8: Air flow channel

10: Ribs

100:Interconnectors

129: Iron-rich area

Claims (3)

A method of preparing interconnects for solid oxide fuel cell stacks, comprising: providing an iron-rich material into a channel of a mold; containing 4-6wt% Fe, 0-1wt% Y and the remainder Cr The powder is provided over the iron-rich material in the mold; the iron-rich material and the powder containing 4-6wt% Fe, 0-1wt% Y and the remainder Cr are pressed in the mold to form the an interconnect; and sintering the interconnect to form a sintered interconnect having iron-rich regions with an iron concentration greater than 10% in ribs of the interconnect; wherein the iron-rich material is provided to The step in the channels of the mold includes providing iron foil strips into the channels of the mold. A method for preparing an interconnect for a solid oxide fuel cell stack, comprising: providing an iron-rich material into a channel of a mold; providing a powder comprising 4-6 wt% Fe, 0-1 wt% Y and the balance Cr over the iron-rich material in the mold; pressing the iron-rich material and the powder comprising 4-6 wt% Fe, 0-1 wt% Y and the balance Cr in the mold to form the interconnect; and sintering the interconnect to form a sintered interconnect having an iron-rich region with an iron concentration greater than 10% in a rib of the interconnect; wherein: the step of providing the iron-rich material into the channels of the mold comprises providing a powder comprising chromium and at least 25 wt% iron into the channels of the mold with a first shoe; and the providing the powder comprising 4-6 wt% Fe, 0-1 wt% Y and the balance Cr comprises pressing the iron-rich material and the powder comprising 4-6 wt% Fe, 0-1 wt% Y and the balance Cr in the mold to form the interconnect; and sintering the interconnect to form a sintered interconnect having an iron-rich region with an iron concentration greater than 10% in a rib of the interconnect; wherein: the step of providing the iron-rich material into the channels of the mold comprises providing a powder comprising chromium and at least 25 wt% iron into the channels of the mold with a first shoe; and the providing the powder comprising 4-6 wt% Fe, 0-1 wt% The step of providing the powder containing 4-6wt% Fe, 0-1wt% Y and the rest Cr to the top of the powder containing chromium and at least 25wt% iron in the mold using a second shoe-shaped piece. A method for preparing an interconnect for a solid oxide fuel cell stack, comprising: providing an iron-rich material into a channel of a mold; providing a powder comprising 4-6 wt% Fe, 0-1 wt% Y and the balance Cr to the mold above the iron-rich material; pressing the iron-rich material and the powder comprising 4-6 wt% Fe, 0-1 wt% Y and the balance Cr in the mold to form the interconnect; and sintering the interconnect to form a sintered interconnect having an iron-rich region with an iron concentration greater than 10% in a rib of the interconnect; wherein: the step of providing the iron-rich material into the channels of the mold comprises providing a pure iron powder into the channels of the mold with a first shoe-shaped member; and the providing the powder comprising 4-6 wt% Fe, 0-1 wt% Y and the balance Cr comprises pressing the iron-rich material and the powder comprising 4-6 wt% Fe, 0-1 wt% Y and the balance Cr in the mold to form the interconnect; and The step of preparing the powder containing 4-6wt% Fe, 0-1wt% Y and the rest Cr comprises providing the powder containing 4-6wt% Fe, 0-1wt% Y and the rest Cr to the top of the pure iron powder in the mold using a second shoe-shaped member.