Components for solid oxide electrochemical cell units

Abstract

A component for a solid oxide electrochemical cell is disclosed. The component comprises a stainless-steel substrate comprising a first surface and a second surface, a first coating layer on the first surface, the first coating layer comprising a nickel chromium alloy. Also disclosed is a method for making a component for a solid oxide electrochemical cell by depositing a first coating layer on a surface of a stainless-steel substrate, the first coating layer comprising a nickel chromium alloy.

Description

[0001] COMPONENTS FOR SOLID OXIDE ELECTROCHEMICAL CELL UNITS

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to components for solid oxide electrochemical cell units, to solid state electrochemical cell units comprising such components, to stacks of such solid oxide electrochemical cell units and to processes for making components for solid oxide electrochemical cell units.

[0004] BACKGROUND OF THE INVENTION

[0005] Electrochemical cells may be formed of oxide layers (often known as solid oxide cells: SOC) that may include rare earth oxide layers. SOCs may be used, inter alia, as fuel cells or electrolyser cells.

[0006] SOC fuel cell units produce electricity using an electrochemical conversion process that oxidises fuel. SOC cell units may also, or instead, operate as regenerative fuel cells (or reverse fuel cells) units, often known as solid oxide electrolyser cell units, for example to separate hydrogen and oxygen from water, or carbon monoxide and oxygen from carbon dioxide.

[0007] A solid oxide fuel cell (SOFC) generates electrical energy through the electrochemical oxidation of a fuel gas (usually hydrogen-based) and the device is generally ceramic-based, using an oxygen-ion conducting metal-oxide containing ceramic as its electrolyte. Many ceramic oxygen ion conductors (for instance, doped zirconium oxide or doped cerium oxide) have useful ion conductivities at temperatures in excess of 450 °C or 500 °C (for ceriumoxide based electrolytes) or 650 °C (for zirconium oxide-based ceramics), so SOFCs tend to operate at elevated temperatures.

[0008] In operation, the electrolyte of the SOFC conducts oxygen ions from a cathode to an anode located on opposite sides of the electrolyte. A fuel contacts the anode (usually known as the “fuel electrode”) and an oxidant, such as air or an oxygen rich fluid, contacts the cathode (usually known as the “oxygen electrode”). A solid oxide electrolyser cell (SOEC) may have the same structure as an SOFC, but is essentially the SOFC operating in reverse, or in a regenerative mode, to achieve the electrolysis of water and / or carbon dioxide. The fuel electrode, electrolyte and air (oxygen) electrode of an SOC may each be formed of one or more layers to optimise operation.

[0009] Conventional ceramic-supported (e.g. anode-supported) SOCs have low mechanical strength and are vulnerable to fracture. Hence, metal-supported SOCs have been developed which have the active cell component layers supported on a metal substrate. In these cells, the ceramic layers can be very thin since they only perform an electrochemical function: that is to say, the ceramic layers are not self-supporting but rather are thin coatings / films laid down on and supported by the metal substrate. Such metal supported SOC stacks are more robust, lower cost, have better thermal properties than ceramic-supported SOCs and can be sealed using conventional metal welding techniques.

[0010] Applicant’s WO-A-2015 / 136295 discloses metal-supported SOFCs in which the metal support plate has a porous region surrounded by a non-porous region with the active layers being deposited upon the porous region so that gases may pass through the pores from one side of the metal support plate to the opposite side to access the active layers coated thereon. The porous region comprises small apertures (holes drilled through the metal foil substrate) extending through the support plate.

[0011] WO-A-2016 / 128721 discloses an interconnect for a low temperature solid oxide fuel cell, in particular an interconnect comprising a chromium oxide layer (chromium (III) oxide / chromia). Other coatings on metallic substrates are known. For example, US-A- 2021 / 249666 discloses a metal porous body including a flat plate shape and having continuous pores, a framework of the metal porous body including an alloy layer containing nickel and at least one of chromium and tin, a cobalt layer being formed on a surface of the alloy layer. WO-A-2020 / 217668 discloses a metal porous body with a porous metal skeleton and a metal oxide thin film formed on at least a portion of the metal skeleton surface. WO-A- 2006 / 059943 discloses an interconnect for solid oxide fuel cells, consisting of a metallic substrate, such as stainless steel, and a coating, which in turn comprises at least one metallic layer and one reactive layer. WO-A-2005 / 095672 discloses a base material that is formed by a steel and a coating based on nickel having a chromium portion of at least 7 % by weight. US-A-2011 / 269051 discloses a coated product for use in an electrochemical device including a metal sheet substrate provided with a coating system.

[0012] Components including metal supports, interconnects and other components in SOCs may be formed of SOC-specific materials including steels. There is a desire to use materials containing chromium, that may be lower cost or may have other beneficial properties. Components (e.g. interconnects) in many SOCs have to withstand both the high temperatures during operation and the oxidative environment.

[0013] It has, however, been observed that materials (for example metal alloys) may exhibit chromium volatility. Volatile chromium compounds may cause problems during manufacture and may poison SOC electrodes during operation. There have been attempts to coat components (e.g. with alumina, CoCe or rare earth materials) to reduce the chromium problem. For example, WO-A-2023 / 175353 discloses a method for producing a praseodymium and / or terbium coated chromium-containing component.

[0014] Iron-base alloys have a tendency to corrode on the air side at higher temperatures when exposed to hydrogen on the fuel side, even when alloyed with chromium, which would normally be protective. There are known coatings that are protective against dual atmosphere corrosion where it remains intact, and also block chromium evaporation from the surface of the steel in air at high temperature. Unfortunately, such coatings may be less effective against either issue if the coating is damaged. This can often happen if the steel is coated as a flat strip prior to forming e.g. interconnect components by stamping or hydroforming, as the coating can become damaged where it is deformed during the forming process. The regions where the coating is damaged are then vulnerable to both local corrosion and chromium loss

[0015] Thus, although known methods are generally successful, there is a need to provide alternate methods of reducing or preventing problems associated with chromium and / or dual atmosphere corrosion in SOCs.

[0016] It is an aim of the present invention to address this need.

[0017] SUMMARY OF THE INVENTION

[0018] The present invention accordingly provides, in a first aspect, a component for a solid oxide electrochemical cell, the component comprising: a stainless steel substrate comprising a first surface and a second surface, a first coating layer on the first surface, the first coating layer comprising a nickel chromium alloy.

[0019] Such a component is greatly advantageous because the inventors have surprisingly discovered that the first coating layer suppresses corrosion of the component by tending to form (during heating) a protective chromia scale at the surface of the nickel chromium alloy coating layer. This protective scale may form in-situ during operation (e.g. during heating at 450°C to 600°C), and does not necessarily require an initial high temperature thermal oxidation step.

[0020] The first coating layer comprising a nickel chromium alloy thus may comprise the nickel chromium alloy in metallic form or in partially or fully oxidised form, and may comprise (e.g. after heat treatment and / or diffusion of other elements from the substrate), other elements.

[0021] The component may further comprise a second coating layer on the first coating layer, the second coating layer comprising a cobalt-containing material.

[0022] An advantage of such a second coating layer is that, surprisingly, the second coating layer reduces or prevents chromium evaporation from the first coating layer of nickel chromium alloy. It has been discovered that chromia scale may form in-situ at the interface between the first coating layer of nickel chromium alloy and the second coating layer of the cobalt- containing material (which may oxidise in-situ to cobalt oxide).

[0023] The component may comprise any stainless steel component for use in the SOC apparatus, and may comprise, for example, an interconnect, a spacer, a metal plate, or a substrate, pipe fitting, fixing, pipe, heat exchanger, or a valve component. Usually, the component may comprise an interconnect, a spacer, or a support plate. The support plate may be a support plate upon which electrochemically active layers of the electrochemical cell are deposited. More usually, the component may comprise an interconnect or a support plate. Even more usually, the component may comprise an interconnect.

[0024] The stainless steel substrate may comprise a ferritic stainless steel substrate. The stainless steel of the stainless steel substrate may comprise 11% wt Cr or greater; optionally 15%wt Cr or greater; optionally 17%wt Cr or greater; optionally 19%wt Cr or greater.

[0025] Generally, the stainless steel of the stainless steel substrate may comprise 11 to 25 wt% chromium.

[0026] Examples of stainless steel may be a ferritic stainless steel, optionally comprising SS441, SS444, SS430, Sandvik Sanergy HT, VDM Crofer 22APU, VDM Crofer 22H, and / or Hitachi ZMG232. The first coating layer may have a thickness of 0.1 gm or greater, optionally 0.2 pm or greater, optionally 0.4 pm or greater, optionally 0.5 pm or greater, optionally 0.6 pm or greater, optionally 0.8 pm or greater, optionally 0.9 pm or greater.

[0027] The first coating layer may have a thickness of 10 pm or lower, optionally 6 pm or lower, optionally 4 pm or lower, optionally 2 pm or lower, optionally 1.5 pm or lower, optionally 1.2 pm or lower.

[0028] Thus, the first coating layer may be of thickness in the range 0.1 pm to 10 pm, optionally 0.2 pm to 8 pm, optionally 0.4 pm to 6 pm, optionally 0.5 pm to 4 pm, optionally 0.6 pm to 2 pm, optionally 0.8 pm to 1.5 pm, optionally 0.8 pm to 1.2 pm.

[0029] The second coating layer may comprise cobalt and / or a cobalt alloy.

[0030] In some embodiments, the second coating layer may further comprise a cerium-containing sublayer on the first coating layer, wherein the second coating layer is on the sublayer.

[0031] This is advantageous because the cerium-containing sublayer may reduce the thickness of the chromia subscale when formed and the resulting contact resistance of the component.

[0032] The second coating layer may be of thickness in the range 0.1 pm to 20 pm, optionally 0.1 pm to 7 pm.

[0033] Optionally, the second coating layer may have a thickness of greater than 0.2 pm, optionally greater than 0.3 pm, optionally greater than 0.4 pm, optionally greater than 0.7 pm optionally greater than 0.8 pm, optionally greater than 0.9 pm, optionally greater than 1.5 pm, optionally greater than 4 pm, optionally greater than 7 pm, optionally greater than 12 pm, optionally greater than 15 pm.

[0034] Optionally the second coating layer may have thickness of 15 pm or lower, optionally 12 pm or lower, optionally 10 pm or lower, optionally 5 pm or lower, optionally 3 pm or lower, optionally 1.5 pm or lower, optionally 1.3 pm or lower, optionally 1.1 pm or lower, optionally 1 pm or lower, optionally 0.9 pm or lower, optionally 0.8 pm or lower.

[0035] The sublayer, where present, may have a thickness in the range 10 nm to 50 nm, optionally 15 nm to 40 nm, optionally 20 nm to 35 nm, optionally 25 nm to 35 nm.

[0036] Optionally, the sublayer may have a thickness of 10 nm or greater, optionally 15 nm or greater, optionally 20 nm or greater, optionally 25 nm or greater. Optionally, the sublayer may have a thickness of 50 nm or lower, optionally 40 nm or lower, optionally 35 nm or lower.

[0037] The first coating layer may comprise a nickel chromium alloy comprising 10%wt or higher chromium, optionally 12%wt or higher chromium, optionally 15%wt or higher chromium, optionally 17%wt or higher chromium, optionally 19%wt or higher chromium.

[0038] The first coating layer may comprise a nickel chromium alloy comprising 30%wt or lower chromium, optionally 28%wt or lower chromium, optionally 25%wt or lower chromium, optionally 23%wt or lower chromium, optionally 21%wt or lower chromium.

[0039] Thus, the first coating layer may comprise a nickel chromium alloy comprising 10%wt to 30wt% chromium, optionally 12%wt to 28wt% chromium, optionally 15%wt to 25wt% chromium, optionally 17% to 23wt% chromium, optionally 19%wt to 21wt% chromium.

[0040] Optionally the first coating layer may comprise a nickel chromium alloy comprising about 80wt% Ni, and about 20wt% Cr.

[0041] The present disclosure is directed to components, in particular, for use in solid oxide electrochemical cell units.

[0042] The present invention may, therefore, provide a solid oxide electrochemical cell unit comprising a component according to the first aspect, optionally an interconnect according to the first aspect.

[0043] Thus, in a second aspect, the present invention accordingly provides a solid oxide electrochemical cell unit comprising: an oxygen electrode, an electrolyte, and a fuel electrode, and an interconnect in electrical connection with the oxygen electrode, the interconnect comprising: a stainless steel substrate comprising a first surface and a second surface, a first coating layer on the first surface, the first coating layer comprising a nickel chromium alloy.

[0044] The solid oxide electrochemical cell unit may further comprise a second coating layer on the first coating layer, the second coating layer comprising a cobalt-containing material.

[0045] The first surface is preferably the air-side of the interconnect.

[0046] Preferably, the interconnect comprises raised features (e.g. dimples) contacting the oxygen electrode and defining a gas volume on the oxygen electrode. The invention is particularly advantageous because the first coating layer (and optional second coating layer) may be applied either before or after features such as dimples are formed in the component (e.g. by pressing). Components may still provide the advantages of the invention whether the deposition of the first (and optionally second) coating layer either before or after features such as dimples are formed in the component.

[0047] The electrochemical cell may be an electrolytic cell, an oxygen separator, a sensor or a fuel cell.

[0048] In a third aspect there is provided a stack of solid oxide electrochemical cell units, wherein each solid oxide electrochemical cell unit is according to the second aspect.

[0049] In a fourth aspect, there is provided a process for making a component for a solid oxide electrochemical cell, the process comprising: depositing a first coating layer on the first surface of a stainless steel substrate comprising a first surface and a second surface, the first coating layer comprising a nickel chromium alloy.

[0050] The process may further comprise depositing a second coating layer on the first coating layer, the second coating layer comprising a cobalt-containing material.

[0051] The process may further comprise depositing a sublayer comprising a cerium containing material on the first coating layer, and subsequently depositing the second coating layer on the sublayer.

[0052] In the process, the stainless steel substrate may have undergone forming to form the component (e.g. an interconnect; optionally forming dimples) before deposition of the first coating layer and, optionally, the second coating layer.

[0053] Alternatively, the stainless steel substrate may undergo forming to form the component (e.g. an interconnect; optionally forming dimples) after deposition of the first coating layer and, optionally, the second coating layer.

[0054] Depositing the first coating layer may be by vapour deposition, optionally physical vapour deposition (PVD). Preferably, depositing the first coating layer by physical vapour deposition comprises sputtering using a nickel chromium alloy target.

[0055] Depositing the second coating layer may be by vapour deposition, optionally physical vapour deposition, optionally wherein depositing the second coating layer by physical vapour deposition comprises sputtering using a cobalt-containing target. Depositing the sublayer (where present) may be by vapour deposition, optionally physical vapour deposition. Depositing the sublayer by physical vapour deposition may comprise sputtering using a cerium-containing target.

[0056] The process may further comprise a heating step after deposition of the first coating layer and optional second coating layer. The heating step may preferably be in situ (i.e. wherein the component is raised to the temperature in use) or a pre-heating step.

[0057] Generally the heating step may be to a temperature in the range 450°C to 900°C, optionally 450°C to 900°C. It has been discovered by the inventors that CnCh may form at this temperature (e.g. at 600°C) and this (with the Ni / Cr layer) may provide improved corrosion protection without significantly affecting the underlying substrate.

[0058] It may be beneficial to perform a relatively high temperature oxidation step (e.g. at 700°C to 850°C) as part of the manufacturing process instead of, or in addition to, a heating step that may oxidise the coating in-situ, particularly where there may be coating damage owing to forming.

[0059] The invention in its various aspects is advantageous because it may provide protection to components and devices (including electrochemical cells) from contamination by chromium that may evaporate from components (including stainless steel components such as interconnects) at higher temperature and which may otherwise react to form a stable chromate phase over the active surface of the components (e.g. over electrodes in electrochemical cells) thereby poisoning such electrochemical cells.

[0060] Advantageously, the disclosure allows improved resistance to dual atmosphere (i.e. wherein there is hydrogen containing atmosphere on one side of the electrochemical cell and an oxygen containing atmosphere on the other) corrosion at <650°C as the coating is effective at blocking chromium evaporation.

[0061] Definitions

[0062] In this specification, the terms “rare earth metal” or “rare earth element” refer to metals selected from Y, Sc, and lanthanoid.

[0063] “Lanthanoid”, “lanthanide” and “Ln” are used interchangeably and mean the metallic chemical elements with atomic numbers 57-71. The term “source of’ an element, compound or other material refers to a material comprising the element, compound or other material whether or not chemically bonded in the source. The source of the element, compound or other material may be an elemental source (e.g. Ln, Ni or O2) or may be in the form of a compound or mixture comprising the element, compound or other material including one or more of those elements, compounds or materials.

[0064] “Oxidant electrode,” “oxygen electrode” or “air electrode” and “fuel electrode” are used herein and may be used interchangeably to refer to cathodes and anodes, respectively, of solid oxide fuel cells. Hence, in the context of fuel cells “cathode” may be used inter changeably with “oxygen electrode”, and “anode” may be used interchangeably with “fuel electrode”.

[0065] It will be understood that “attached” and “on” refer to direct or indirect attachment and positioning, respectively.

[0066] It will be understood that each layer referred to in this specification may be comprised of multiple sub-layers (and those sub-layers may have varying compositions).

[0067] In this specification references to electrochemical cell, SOC, SOFC and SOEC may refer to tubular or planar cells. Electrochemical cell units may be tubular or planar in configuration. Planar fuel cell units may be arranged overlying one another in a stack arrangement, for example 100-500 fuel cell units in a stack, with the individual fuel cell units arranged electrically in series.

[0068] Electrochemical cells may be fuel cells, reversible fuel cells or electrolyser cells. Generally, these cells may have the same structure and reference to electrochemical cells may refer (unless the context suggests otherwise) to any of these types of cell. The cell may be based upon a solid oxide electrolyte, optionally a metal-supported solid oxide cell. In fuel cell mode, a fuel contacts the anode (fuel electrode) and an oxidant, such as air or an oxygen-rich fluid, contacts the cathode (air electrode), so in fuel cell mode operation, the air electrode will be the cathode. A solid oxide electrolyser cell (SOEC) may have the same structure as an SOFC, but is essentially the SOFC operating in reverse, or in a regenerative mode, to achieve the electrolysis of water and / or carbon dioxide by using the oxygen electrode, fuel electrode and solid oxide electrolyte to produce hydrogen gas and / or carbon monoxide and oxygen.

[0069] The term “flow path” may be used to define fluid flow paths between various components, and thus it is also to be understood that those components are in fluid flow communication with one another. The various features of aspects of the disclosure as described herein may be used in combination with any other feature in the same or other aspect of the disclosure, if needed with appropriate modification, as would be understood by the person skilled in the art.

[0070] Furthermore, although all aspects of the invention or disclosure preferably “comprise” the features described in relation to that aspect, it is specifically envisaged that they may “consist” or “consist essentially” of those features outlined in the claims.

[0071] The invention will now be described with reference to the accompanying figures and examples.

[0072] BRIEF DESCRIPTION OF THE FIGURES

[0073] Figure 1 shows a schematic cross section of a solid electrochemical cell unit in accordance with the disclosure.

[0074] Figure 2(a) shows a top down, low magnification SEM image of a dimple of a stainless steel substrate with a 500 nm coating of 80wt% Ni, 20wt% Cr after forming and oxidation; and 2(b) shows a high magnification image of the oxide scale on the dimple shoulders.

[0075] Figure 3 shows a SEM cross section of a stainless steel substrate with a 500 nm coating of 80wt% Ni, 20wt% Cr / 10 nm Ce / 600 nm Co.

[0076] Figure 4 shows in (a) a substrate with a protective first coating layer; (b) a substrate with first and second protective coating layers; and (c) a substrate with first and second protective coating layers and a sublayer between the first and second layers.

[0077] Figure 5 shows in (a) a SEM of a cross section through a stainless steel substrate with a 2000 nm coating of 80wt% Ni, 20wt% Cr after oxidation for 2000 hours; (b) shows the EDX results for Ni, Fe and Cr as a function of depth through the surface.

[0078] Figure 6 shows the EDX results for a cross section through a stainless steel substrate with a 500 nm coating of 80wt% Ni, 20wt% Cr and 600 nm Co coating after oxidation for 1000 hours.

[0079] Figure 7 shows the EDX results for a cross section through a stainless steel substrate with a 500 nm layer of 80wt% Ni, 20wt% Cr, 10 nm Ce sublayer, and 600 nm Co layer after oxidation for 1000 hours. Figure 8 shows in (a) a SEM of a cross section through a stainless steel substrate 202 with a first protective layer 204 (2000 nm thick) of 80wt% Ni, 20wt% Cr, a sublayer 208 (10 nm thick) Ce, and a second protective layer 206 (600 nm thick) Co, the coated substrate is shown after oxidation for 1000 hours; (b) shows the EDX results for Ni, Fe and Cr as a function of depth through the surface.

[0080] Figure 9 shows the results of area specific resistance (ASR) for stainless steel substrates with NiCr 500nm / Co 600 nm, NiCr 2000nm / CelO nm / Co 600 nm and NiCr 500nm / CelO nm / Co 600 nm protective coatings.

[0081] Figure 10 shows in (a) an image of part of an interconnect for an electrochemical cell coated with a 80Ni20Cr coating after 1500 hours of operation and in (b) an image of part of an interconnect with no 80Ni20Cr coating after 1500 hours of operation.

[0082] DETAILED DESCRIPTION OF THE INVENTION

[0083] Figure 1 shows, schematically and not to scale (for reasons of clarity), a cross section of a solid electrochemical cell unit 2. A substrate, formed by a ferritic stainless steel metal support 4, is plate-like with a peripheral, non-porous region 6 and a central, porous region 8 where holes have been drilled (e.g. laser-drilled) through the metal support 4. A barrier layer (not shown) to reduce corrosion may be located on the surface of the metal support 4 (on one or both sides thereof). A layer of a fuel electrode layer 10 formed of e.g. Ni:CGO (Ni: cerium gadolinium oxide) is located on the porous region 8 of the metal support 4. An electrolyte layer 12 of rare earth doped ceria (RE=Y, Sc or any Ln) of thickness 4 pm or greater (optionally 6 pm to 12 pm) is located on the fuel electrode layer 10. The electrolyte layer 12 may surround the fuel electrode layer 10 to reduce or prevent gas leaking from the fuel side (or volume) 20 to the air side (or volume) 18 or vice versa. The electrolyte layer 12 may further overlap at least part of the non-porous region 6 of the substrate / support 4. An interlayer 14 of rare earth (RE) stabilised zirconia (RE = Y, Sc or any Ln, e.g. Yb) of thickness 0.5 pm or greater (e.g. 1 pm to 4 pm) is located on the electrolyte layer 12. The fuel cell unit 2 has an oxygen electrode 16 located on the interlayer 14. The oxygen electrode layer 16 may be formed of one or more layers of an electrically conductive ceramic material, for example an oxygen electrode active layer contacting the interlayer 14, an oxygen electrode bulk layer contacting the interconnect 24 and an interfacial layer between the oxygen electrode bulk layer and the oxygen electrode active layer. Cell unit 2 as illustrated in Figure 1 further includes an interconnect 24 (that may also be connected to another cell unit (not shown) in the stack. The interconnect 24 may be a metal sheet pressed or formed to provide the contact features 28 and flanged perimeter 26 which allows the interconnect 24 to contact the support of a second cell unit (above, not shown) at the periphery of both components and they may be sealingly attached to one another around that periphery, for example by welding. The interconnect 24 is provided with contact features 28 (e.g. dimples) which extend and contact the surface of the oxygen electrode 16, to provide electrical contact (there may be contact paste, not shown, between the interconnect dimples 28 and the oxygen electrode 316) and also forming an oxygen fluid volume 18 bounded by the oxygen electrode 16 and the interconnect 24. The interconnect 24 has a protective coating 30 comprising at least a first coating layer comprising a nickel chromium alloy deposited by sputtering (or other form of PVD) on the surface facing the oxygen electrode 16 (i.e. on the air side surface of the interconnect 24). The protective coating 30 may also have at least a second coating layer on the first coating layer, the second coating layer comprising a cobalt-containing material.

[0084] Figure 2(a) shows a top down, low magnification SEM image of a dimple of a stainless steel substrate with a 500 nm coating of 80wt% Ni, 20wt% Cr after forming and oxidation; and 2(b) shows a high magnification image of the oxide scale on the dimple shoulders.

[0085] Figure 4 shows in (a) a stainless steel (e.g stainless steel 441) substrate 102 that may be of an interconnect or other component, the substrate 102 having a protective coating of a first coating layer comprising a nickel chromium alloy 104 (e.g. of about 80wt% Ni, and about 20wt% Cr) that may be around 0.5 pm thick.

[0086] Figure 4(b) shows a stainless steel (e.g stainless steel 441) substrate 102 that may be of an interconnect or other component, the substrate 102 having a protective coating of a first coating layer comprising a nickel chromium alloy 104 (e.g. of about 80wt% Ni, and about 20wt% Cr) that may be around 0.5 pm thick, and a second layer comprising a cobalt containing material (e.g. Co) 106 that may be about 0.6 pm thick on the first coating layer 104.

[0087] Figure 4(c) shows the stainless steel (e.g stainless steel 441) substrate 102 that may be of an interconnect or other component, the substrate 102 having a protective coating of a first coating layer comprising a nickel chromium alloy 104 (e.g. of about 80wt% Ni, and about 20wt% Cr) that may be around 0.5 pm thick. As in Figure 3(b), the substrate also has a second coating layer on the first layer 104, the second coating layer comprising a cobalt containing material (e.g. Co) 106 that may be about 0.6 pm thick, and, in this embodiment, a thin (e.g. about 30 nm thick) sublayer of a cerium containing material 108 located between the first layer 104 and second layer 106.

[0088] Examples

[0089] The present disclosure relates to a coating to reduce or prevent component (e.g. interconnect) corrosion by coating a nickel - chromium alloy (preferably Ni80wt% / Cr20wt%) of approximately 1 micron thickness on to e.g. the air-side of a stainless steel interconnect.

[0090] The coating appears effective at suppressing dual-atmosphere corrosion of the interconnect by forming a protective chromia scale at the surface of the Ni20Cr alloy rather than at the interface between the coating and the steel. This protective oxide scale may form in-situ during operation at e.g. 600°C and does not require an initial high temperature thermal oxidation step.

[0091] Preferably, a second coating layer of either cobalt or cobalt with a thin (e.g. 30 nm) sub-layer of cerium can be deposited on top of the Ni20Cr coating. This may have the effect of preventing chromium evaporation from the Ni20Cr alloy (and reducing the thickness of the chromia subscale and resulting contact resistance if cerium is used). In this instance the chromia scale forms in-situ at the interface between the Ni20Cr alloy and the cobalt (which oxidises in-situ to cobalt oxide), not the interface between the steel and the Ni20Cr alloy. Again the protective oxide scale can be formed during operation with no need for a separate high temperature oxidation step.

[0092] Deposition of each coating can be by Physical Vapour Deposition (PVD), e.g. sputtering.

[0093] In the examples, coating stainless steel 441 substrates were coated and then some samples were formed to produce dimples.

[0094] The samples were examined under an optical microscope after forming.

[0095] Some samples were oxidised at 870°C for 2 hours in flowing humid air to investigate corrosion to the formed regions of the dimples. Other samples were unformed coupons of stainless steel which were not pre-oxidised in air prior to test, but were exposed to flowing humid air on the coated side, and 5% hydrogen / 3% water vapour in nitrogen on the uncoated side for 500-2000h at 600°C, simulating stack conditions. A component exposed to these conditions without an initial oxidation step at higher temperature in air would be subject to severe corrosion after >500h operation.

[0096] The samples were examined using a scanning electron microscope (SEM) and Energy dispersive X-Ray (EDX) analysis to examine elemental composition. The formed dimples were cross sectioned for more detailed SEM analysis, including elemental mapping around the dimple shoulders where cracking of the coating is usually most severe, and more significant corrosion is usually seen (often owing to exposure of the steel).

[0097] The results indicate that NiCr protective coatings are effective, and addition of a Co layer (and Ce sublayer) enhances effectiveness.

[0098] Some cracks may form in the NiCr layer when subjected to tensile stress and elongation during the forming process, however during a subsequent thermal oxidation step a protective chromia scale appears to form both on the exposed edges of the cracked NiCr layer and the exposed base steel below, which should be effective at preventing subsequent breakaway corrosion of the exposed base steel during operation.

[0099] The SEM cross section of Figure 3 shows a stainless steel substrate with a 500 nm coating of 80wt% Ni, 20wt% Cr / 10 nm Ce / 600 nm Co. In this region cracks 222 in the NiCr coating 204 have clearly been filled with chromia, with a layer 207 of cobalt oxide above. There is less evidence of chromium depletion in the base steel and the chromia scale 212 is much thinner; chromia growth has probably been inhibited by the cerium layer 208. Most regions have encouraging microstructure where coating damage from forming appears to have healed through the formation of a protective chromia scale filling the void (as seen in cracks 222). The chromia scale 212 formed is thicker than would be expected from the base steel at the same condition.

[0100] Figure 5 shows in (a) a SEM of a cross section through a stainless steel substrate with a 2000 nm coating of 80wt% Ni, 20wt% Cr after oxidation for 2000 hours; (b) shows the EDX results for Ni, Fe and Cr as a function of depth through the surface. After 500 hours the protective chromia scale Cr2O3 that develops on the surface of the NiCr layer is 0.2 pm thick. After 2000 hours the protective chromia scale CnCh that develops on the surface of the NiCr layer is 0.41 pm thick. 40wt% iron in the coating after 500 hours and as shown in Figure 5(b) around 60% Fe in the coating after 2000 hours. Figure 6 shows the EDX results for a cross section through a stainless steel substrate with a 500 nm coating of 80wt% Ni, 20wt% Cr and 600 nm Co coating after oxidation for 1000 hours. After oxidation, the chromia scale on the surface is 0.25 gm thick.

[0101] Figure 7 shows the EDX results for a cross section through a stainless steel substrate with a 500 nm layer of 80wt% Ni, 20wt% Cr, 10 nm Ce sublayer, and 600 nm Co layer after oxidation for 1000 hours. After oxidation, the chromia scale on the surface is 0.14 gm thick.

[0102] Figure 8 shows in (a) a SEM of a cross section through a stainless steel substrate 202 with a first protective layer 204 of 2000 nm thick 80wt% Ni, 20wt% Cr, a sublayer 208 of 10 nm Ce, and a second protective layer 206 of 600 nm Co, the coated substrate is shown after oxidation for 1000 hours. A chromia scale 212 develops on the surface. The EDX results in Figure 8(b) show Ni, Fe and Cr as a function of depth through the surface. After 500 hours the protective chromia scale CnCh that develops on the surface of the NiCr layer is 0.2 pm thick. After 1000 hours the protective chromia scale CnCh that develops on the surface of the NiCr layer is 0.10 pm thick.

[0103] Figure 9 shows the results of area specific resistance (ASR) for stainless steel substrates with NiCr 500nm / Co 600 nm, NiCr 2000nm / CelO nm / Co 600 nm and NiCr 500nm / CelO nm / Co 600 nm protective coatings. The ASR of the samples with a Ce sublayer are lower. ASR was measured on samples 1 cm2in area sputtered with Pt, contact with Pt paste and subsequent sintering. Area-specific resistance of the oxide scale on the coated steel was measured in air at 600°C, using a 4-wire DC measurement. The oxide scale is electrically conductive and useful for SOC interconnect applications

[0104] Figure 10 shows in (a) an image of part of an interconnect for an electrochemical cell coated with a 80Ni20Cr coating after 1500 hours of operation and in (b) an image of part of an interconnect with no 80Ni20Cr coating after 1500 hours of operation. Figure 10 (b) shows signs of corrosion nucleation 240, which are not present in Fig 10 (a) where the 80Ni20Cr coating is present.

[0105] Reference numerals:

[0106] 2 cell unit

[0107] 4 support 6 non-porous region of support

[0108] 8 porous region of support

[0109] 10 fuel electrode

[0110] 12 electrolyte

[0111] 14 interlayer

[0112] 16 oxygen electrode

[0113] 18 oxygen fluid volume

[0114] 20 fuel side fuel fluid volume

[0115] 24 interconnect

[0116] 26 flanged perimeter (of interconnect)

[0117] 28 contact features (dimples)

[0118] 30 protective coating

[0119] 102 stainless steel substrate

[0120] 104 first coating layer comprising nickel and chromium

[0121] 106 second coating layer comprising a cobalt-containing material

[0122] 108 cerium-containing sublayer

[0123] 202 stainless steel substrate

[0124] 204 first coating layer comprising nickel and chromium

[0125] 206 second coating layer comprising a cobalt-containing material

[0126] 208 cerium-containing sublayer

[0127] 212 Chromia scale

[0128] 222 Chromia filled cracks in the NiCr coating

[0129] 240 Corrosion nucleation sites Other References

[0130] Higuera, V., Belzunce, F.J., Carriles, A. et al. Influence of the thermal-spray procedure on the properties of a nickel-chromium coating. Journal of Materials Science 37, 649-654 (2002).

[0131] Chandrakar, Rituraj, Kumar, Rajesh, Hot corrosion behaviour of nickel chromium coating at different temperatures (800 °C and 900 °C) on SA213 T91 boiler steel weldments, Materials Physics and Mechanics, 14, 2012.

[0132] B. Subramanian, M. Jayachandran & S. Jayakrishnan (2006) Surface characterisation studies of nickel-chromium PVD: EB evaporated low carbon steel samples, Surface Engineering, 22:6, 447-451.

[0133] Anders Harthoj, Tobias Holt, Per Moller, Oxidation behaviour and electrical properties of cobalt / cerium oxide composite coatings for solid oxide fuel cell interconnects, Journal of Power Sources, Volume 281, 2015, Pages 227-237.

[0134] All publications mentioned in the above specification are herein incorporated by reference. Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be performed therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

Claims

Claims1. A component for a solid oxide electrochemical cell, the component comprising: a stainless steel substrate comprising a first surface and a second surface, a first coating layer on the first surface, the first coating layer comprising a nickel chromium alloy.

2. A component as claimed in claim 1, further comprising a second coating layer on the first coating layer, the second coating layer comprising a cobalt-containing material.

3. A component as claimed in either claim 1 or claim 2, wherein the component comprises an interconnect.

4. A component as claimed in any one of the preceding claims, wherein the stainless steel substrate comprises a ferritic stainless steel substrate.

5. A component as claimed in claim 5, wherein the stainless steel of the stainless steel substrate comprises 11% wt Cr or greater; optionally 15%wt Cr or greater; optionally 17%wt Cr or greater; optionally 19%wt Cr or greater.

6. A component as claimed in either claim 4 or claim 5, wherein the stainless steel of the stainless steel substrate comprises 11 to 25 wt% chromium.

7. A component as claimed in any one of the preceding claims, wherein the first coating layer has a thickness of 0.1 pm or greater, optionally 0.2 pm or greater, optionally 0.4 pm or greater, optionally 0.5 pm or greater, optionally 0.6 pm or greater, optionally 0.8 pm or greater, optionally 0.9 pm or greater.

8. A component as claimed in any one of the preceding claims, wherein the first coating layer has a thickness of 10 pm or lower, optionally 6 pm or lower, optionally 4 pm or lower, optionally 2 pm or lower, optionally 1.5 pm or lower, optionally 1.2 pm or lower.

9. A component as claimed in any one of the preceding claims 2 to 8, wherein the second coating layer comprises cobalt and / or a cobalt alloy.

10. A component as claimed in any one of the preceding claims 2 to 9, wherein the second coating layer further comprises a cerium-containing sublayer on the first coating layer, and the second coating layer is on the sublayer.

11. A component as claimed in any one of the preceding claims 2 to 10, wherein the second coating layer is of thickness in the range 0.1 pm to 20 pm.

12. A component as claimed in either claim 10 or claim 11, wherein the sublayer has a thickness in the range 10 nm to 50 nm, optionally 15 nm to 40 nm, optionally 20 nm to 35 nm, optionally 25 nm to 35 nm.

13. A component as claimed in any one of the preceding claims, wherein the first coating layer comprises a nickel chromium alloy comprising 10%wt or higher chromium, optionally 12%wt or higher chromium, optionally 15%wt or higher chromium, optionally 17%wt or higher chromium, optionally 19%wt or higher chromium.

14. A component as claimed in any one of the preceding claims, wherein the first coating layer comprises a nickel chromium alloy comprising 30%wt or lower chromium, optionally 28%wt or lower chromium, optionally 25%wt or lower chromium, optionally 23%wt or lower chromium, optionally 21%wt or lower chromium.

15. A solid oxide electrochemical cell unit comprising a component as claimed in any one of the preceding claims.

16. A solid oxide electrochemical cell unit comprising: an oxygen electrode, an electrolyte, and a fuel electrode, and an interconnect in electrical connection with the oxygen electrode, the interconnect comprising: a stainless steel substrate comprising a first surface and a second surface, a first coating layer on the first surface, the first coating layer comprising a nickel chromium alloy.

17. A solid oxide electrochemical cell unit as claimed in either claim 15 or claim 16, further comprising a second coating layer on the first coating layer, the second coating layer comprising a cobalt-containing material.

18. A solid oxide electrochemical cell unit as claimed in any one of claims 15 to 17, wherein the first surface is the air-side of the interconnect.

19. A solid oxide electrochemical cell unit as claimed in any one of claims 15 to 18, wherein the interconnect comprises raised features (e.g. dimples) contacting the oxygen electrode and defining a gas volume on the oxygen electrode.

20. A solid oxide electrochemical cell unit as claimed in any one of claims 15 to 19, wherein the electrochemical cell comprises an electrolytic cell, an oxygen separator, a sensor or a fuel cell.

21. A stack of solid oxide electrochemical cell units, wherein each solid oxide electrochemical cell unit is as claimed in claims 15 to 20.

22. A process for making a component for a solid oxide electrochemical cell, the process comprising: depositing a first coating layer on the first surface of a stainless steel substrate comprising a first surface and a second surface, the first coating layer comprising a nickel chromium alloy.

23. A process as claimed in claim 22, further comprising depositing a second coating layer on the first coating layer, the second coating layer comprising a cobalt-containing material.

24. A process as claimed in either claim 22 or claim 23, wherein depositing the first coating layer is by vapour deposition, optionally physical vapour deposition (PVD), preferably wherein depositing the first coating layer by physical vapour deposition comprises sputtering using a nickel chromium alloy target.

25. A process as claimed in any one of claims 22 to 24, wherein depositing the second coating layer is by vapour deposition, optionally physical vapour deposition, optionally wherein depositing the second coating layer by physical vapour deposition comprises sputtering using a cobalt-containing target.

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