Method of manufacturing a component for an electrochemical cell

Abstract

A method of manufacturing a component for an electrochemical cell. The method comprising, mechanically flattening a substrate comprising a porous region and a non-porous region, applying a first material on at least part of a surface of the porous region of the substrate as a precursor for, and to form, a first electrode on the substrate, and sintering the component to densify the first material and to thermally grow an oxide coating on the substrate.

Description

[0001] METHOD OF MANUFACTURING A COMPONENT FOR AN ELECTROCHEMICAL CELL

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to methods of manufacturing components for electrochemical cells, to methods of manufacturing electrochemical cell units comprising such components, to methods of manufacturing stacks of electrochemical cell units comprising such components and to electrochemical cells, electrochemical cell units and electrochemical cell stacks.

[0004] BACKGROUND OF THE INVENTION

[0005] Electrochemical cells formed of oxide layers (often known as solid oxide cells: SOC) may be used as fuel cells or electrolyser cells.

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

[0007] A solid oxide cell (SOC) 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 cerium-oxide based electrolytes) or 650 °C (for zirconium oxide-based ceramics), so SOCs tend to operate at elevated temperatures. The fuel electrode, electrolyte and air electrode of an SOFC may each be formed of one or more layers to optimise operation. 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 a fuel electrode (anode in SOFC, cathode in SOEC) and an oxidant, such as air or an oxygen rich fluid, contacts an oxygen electrode (sometimes known as the “air electrode”, cathode in SOFC, anode in SOEC). Other types of electrochemical cells exist, such as alkaline electrolyte cells, which operate in a similar way to SOCs.

[0008] 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 electrochemical cell component layers supported on a 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 substrate. Such supported SOFC stacks are more robust, lower cost, have better thermal properties than ceramic-supported SOFCs and where the substrate is a metal can be sealed using conventional metal welding techniques.

[0009] EP-A-3 790 092 discloses metal-supported electrochemical elements with reduced degree of warping. Applicant’s earlier patent applications WO-A-2002 / 35628, WO-A-2009 / 090419 and WO-A-2015 / 136295 disclose metal-supported SOCs in which the electrochemically active layers (or active fuel cell component layers) comprise anode, electrolyte and cathode layers deposited (e.g. as thin coatings / films) on, and supported by, a metal substrate support plate (e.g. foil). The metal substrate 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 contact the active layers coated thereon. The porous region comprises discrete apertures extending through the support plate.

[0010] The pores in the substrate may be formed by drilling, for example, laser drilling. This process may lead to stress being induced in the substrate and distortion of the substrate. Other processes prior to forming a first electrode on a substrate may lead to distortion of said substrate. Thermal annealing processes are typically required (e.g., after drilling) to relax and flatten the substrate and these may use significant amounts of energy and add a further step in the manufacturing process. However, said thermal annealing processes may also be used to form a thermally grown oxide (TGO) layer on the substrate - either in the same or a subsequent thermal process to the thermal annealing to flatten the substrate. Such thermal annealing to produce a TGO layer usually requires a temperature of at least several hundred degrees centigrade for at least an hour (a longer time being required at a lower temperature to produce a given TGO layer thickness), thereby consuming significant amounts of energy. Typical conditions for a TGO process may be a temperature of 600-1000 °C for 1-3 hours. Electrochemical cells manufactured using this method (thermal annealing to flatten the substrate) may be referred to as “standard” cells in this disclosure.

[0011] There is a need, therefore, to reduce or prevent problems associated with thermal annealing of the substrate.

[0012] It is an aim of the present invention to address this need. SUMMARY OF THE INVENTION

[0013] The present invention accordingly provides, in a first aspect, a method of manufacturing a component for an electrochemical cell, the method comprising, mechanically flattening a substrate comprising a porous region and a non-porous region, applying (i.e., subsequently to mechanically flattening) a first material on at least part of a surface of the porous region of the substrate as a precursor for, and / or to form, a first electrode on the substrate, and sintering (i.e., subsequently to the applying) the component to densify the first material and to thermally grow an oxide coating on the substrate (including parts of the substrate covered and not covered by the first material). In other words, the method preferably does not comprise a thermal annealing step to grow a thermally grown oxide before applying the first material. Electrochemical cells manufactured according to this method may be referred to as “levelled” cells in this disclosure.

[0014] This is greatly advantageous because mechanical flattening results in significant reduction in stress in the substrate and in flattening of the substrate.

[0015] The method may further comprise providing a substrate comprising a porous region and a non-porous region.

[0016] The porous region may be provided by drilling (e.g. laser drilling) the substrate. Generally, drilling (e.g. laser drilling) of the substrate leads to deformation of the substrate. In some cases, the deformation may be relatively uniform and repeatable, for example deformation of a substrate into an approximate saddle shape. The deformation makes further flat-bed processing steps difficult. Thermal annealing processes to flatten the substrate remove the deformation shape, but result in a relatively wavy shape (that differs between substrates, albeit having a lower amplitude than the deformation shape) and are highly energy intensive.

[0017] Mechanical flattening greatly improves flatness and the resulting shapes, even if they deviate from flatness, are consistent and relatively reproducible between substrates.

[0018] Furthermore, mechanical flattening allows the reduction or elimination of a thermally intensive step of annealing in the process of flattening the substrate. Advantageously, thermal processing that may be required to densify one or more layers of the cell and / or to thermally grow one or more oxide coatings on the substrate may be nonetheless achieved in a subsequent step. Electrochemical cell units using components manufactured according to the invention, advantageously exhibited similar performance to standard electrochemical cell units using substrates that had been thermally annealed to flatten the substrate and to grow an oxide coating thereon before applying the first material / first electrode. Accordingly, electrochemical cell units using components manufactured according to the invention, are flatter (and have a more consistent flatness profile), and may be manufactured using one fewer energy intensive thermal processing step (due to sintering the component to densify the first material and to thermally grow an oxide coating on the substrate).

[0019] Providing the substrate comprising a porous region and a non-porous region may comprise providing a substrate workpiece and drilling a plurality of pores in the substrate workpiece over at least a portion thereof to form a porous region. Preferably, drilling the substrate may comprise laser drilling the substrate.

[0020] Generally, before the mechanical flattening step, drilling (e.g. laser drilling) the substrate to form the porous region may involve providing a substrate workpiece having a first side / surface and a second side / surface, and drilling from the second side to the first side to form each pore of the porous region. The pores allow fluid communication between the first and second sides. The first material as a precursor for, or to form, the first electrode may be subsequently applied to the first side.

[0021] The substrate may preferably comprise metal, optionally the substrate may comprise a steel substrate, optionally the substrate may comprise a stainless steel substrate.

[0022] The substrate may have a thickness in the range of about 50 pm to 500 pm, often about 100 pm to 400 pm, in some cases about 200 pm to 350 pm.

[0023] Mechanically flattening may comprise mechanical flattening by rolling. Thus, mechanical flattening may comprise roller levelling. Roller levelling may be performed by passing the substrate through a roller leveller.

[0024] Roller levelling may comprise a bending / flattening process in which the substrate is repeatedly deformed in a plurality of alternating bends produced by passing the substrate between upper and lower sets of levelling rollers of a roller leveller. The centres of the levelling rollers may be arranged with an offset (in the direction of thickness of the substrate) from the infeed (into which the substrate enters the roller levelling machine) to the outlet (from which the substrate exits the roller levelling machine). The centres of the lower and upper rollers may be staggered - i.e., spaced from one another along the direction from infeed to outlet. The offset and stagger is typically dependent upon the radius of the rollers and the offset tailored to substrate thickness and material properties, and the offset will typically change (optionally continuously change) from the infeed / inlet to the outlet. The stagger may be equal to the radius of the rollers plus a spacing factor (e.g. in the range of 0.2-0.8 multiplied by the radius of the rollers).

[0025] The levelling rollers provide that the substrate, during the roller levelling process, may be subjected to large to progressively smaller alternating bends. Thus, the substrate may follow a diminishing wave-like path (e.g. a decaying sine wave) through the roller leveller. During the levelling process the first bend may take the substrate past its yield point (which releases stress in the material) the repeated further bending reduces or removes residual unevenness and stress. As the substrate passes through the rollers it may be subject to elastic-plastic alternating bends and the constant reduction of bending intensity may produce the flattened substrate.

[0026] Optionally, the offset (vertical distance) between the centres of the upper and lower sets of rollers of the roller leveller may be adjusted at the infeed / inlet (entry) of the roller leveller and / or at the outlet (exit) of the roller leveller.

[0027] Optionally, the offset (vertical distance) between the centres of the upper set of rollers and the lower set of rollers at the inlet of the roller leveller is less than the sum of radii of the upper and lower roller. Optionally, the offset at the inlet may be equal to rl+r2+xt, where rl and r2 are the radii of the upper and lower rollers respectively, t is the thickness of the substrate, and x is an offset factor. In such cases, x is less than zero at the infeed, which means that the substrate is bent as it passes through the rollers proximal to the infeed. Optionally the offset factor x, at the infeed, is in the range -0.2 to -2.5, optionally x is in the range -0.4 to -2, optionally x is in the range -0.6 to -1.7, optionally x is in the range -0.8 to -1.5.

[0028] Optionally, the offset (vertical distance) between the centres of the upper set of rollers and the lower set of rollers at the outlet of the roller leveller is approximately equal to the sum of the radii of upper roller, lower roller, and thickness of the substrate. The offset at the outlet may be equal to rl+r2+wt, where rl and r2 are the radii of the upper and lower rollers respectively, t is the thickness of the substrate, and w is an offset factor. In such cases, w is approximately one at the outlet. Optionally the offset factor w, at the outlet, is in the range 0.8 to 1.2, optionally w is in the range 0.9 to 1.1, optionally w is in the range 0.95 to 1.05, optionally w is in the range 0.98 to 1.02.

[0029] Optionally, there are a plurality of upper rollers and a plurality of lower rollers, for example at least two upper rollers and at least two lower rollers (the substrate being passed between the upper rollers and lower rollers), optionally at least three upper rollers and at least three lower rollers, optionally at least four upper rollers and at least four lower rollers, optionally at least five upper rollers and at least five lower rollers. In general, increasing the number of rollers leads to a flatter substrate, but increases cost. Optionally, there may be at most twenty upper rollers and at most twenty lower rollers, optionally at most eighteen upper rollers and at most eighteen lower rollers, optionally at most fifteen upper rollers and at most fifteen lower rollers, optionally at most twelve upper rollers and at most twelve lower rollers, optionally at most ten upper rollers and at most ten lower rollers. Often, there will be one more or one less upper roller than lower roller.

[0030] Advantageously, mechanical flattening of the substrate may result in a flatness of 4 mm or lower determined as deviation from a mean (flat) plane of the substrate surface over a predetermined area of the substrate and / or over the area of the substrate, optionally mechanical flattening of the substrate may result in a flatness of 3 mm or lower, optionally mechanical flattening of the substrate may result in a flatness of 2 mm or lower, optionally mechanical flattening of the substrate may result in a flatness of 1.7 mm or lower, optionally mechanical flattening of the substrate may result in a flatness of 1.5 mm or lower. The predetermined area may be that of the area of the substrate, or may be that of the area of the substrate upon which the first material is applied. The predetermined area may be at least 100 mm x 100 mm, optionally at least 150 mm x 150 mm, optionally at least 200 mm x 200 mm, optionally at most 1000 mm x 1000 mm, optionally at most 800 mm x 800 mm, optionally at most 600 mm x 600 mm, optionally at most 500 mm x 500 mm.

[0031] Flatness may be determined, for example, by the methods of BSI BS EN ISO 9445-2 (2010) by which maximum deviation from a flat surface may be determined with the substrate lying on the flat horizontal surface and the maximum distance between the lower surface of the substrate and the flat horizontal surface measured. Preferably, flatness may be determined using a non-contact Coordinate-Measuring Machine (CMM) (e.g. using a NEXIV measuring system). Alternatively a feeler wedge (also known as wedge feeler gauge) may be used to measure distance between the flat horizontal surface and the substrate thereon to determine flatness.

[0032] The first material may be applied directly on at least part of the surface of the porous region of the substrate. Thus, applying the first material on at least part of the surface of the porous region of the substrate as a precursor for and / or to form a first electrode may comprise applying the first material directly on at least part of a surface of the porous region of the substrate.

[0033] Alternatively, the first material may be applied indirectly on at least part of the surface of the porous region of the substrate, for example, the first material may be applied on to one or more intermediate layers disposed between the substrate surface and the first material. In some cases at the time of applying the first material to the substrate, the substrate may be uncoated or may comprise a barrier layer on at least one surface thereof, upon which the first material is applied (and in other words, the method preferably does not comprise a thermal annealing step to grow a thermally grown oxide before applying an optional barrier layer). The barrier layer may comprise CGOIO.

[0034] Optionally, the first material may comprise doped ceria, optionally the first material may comprise cerium gadolinium oxide (CGO)

[0035] If the first electrode is to be a fuel electrode, the first material may comprise a source of nickel, optionally, the source of nickel may comprise nickel oxide. Optionally, the first material may comprise at least one layer comprising a transition metal CGO cermet.

[0036] Optionally, the first electrode may have a thickness of 3 pm or higher, optionally 5 pm or higher, optionally 10 pm or higher, optionally 15 pm or higher.

[0037] Optionally, the first electrode may have a thickness of 50 pm or lower, optionally 45 pm or lower, optionally 40 pm or lower, optionally 35 pm or lower.

[0038] The method may further comprise, before sintering the component, applying a second material on the first electrode as a precursor for and / or to form an electrolyte on the first electrode.

[0039] Optionally, sintering the component further densifies the second material (i.e. co-sintering of the first material and the second material to form the first electrode and the electrolyte). The second material may comprise doped ceria, optionally selected from samarium-doped ceria (SDC), gadolinium-doped ceria (GDC, or CGO), praseodymium doped ceria (PDC), samaria- gadolinia doped ceria (SGDC) and mixtures thereof.

[0040] The method may further comprise applying a third material on the electrolyte as a precursor for and / or to form a second electrode on the electrolyte.

[0041] Optionally, a further sintering step or further sintering steps may be performed on the component after applying the third material.

[0042] The third material may comprise doped ceria, optionally cerium gadolinium oxide (CGO).

[0043] Applying the layers of the first material / first electrode, the second material / electrolyte and / or third material / second electrode on the substrate may be by any suitable method. Each of these layers may be comprised of one or more sub-layers. Each sub-layers may have a varying composition.

[0044] Applying the first material, the second material and / or the third material may be by printing, optionally roller printing, jet printing, screen-printing, gravure printing or slot-die printing, or by spraying (for example atomised spraying), or by vapor deposition such as using chemical vapor deposition (CVD), or using physical vapor deposition (PVD).

[0045] Further materials to form further layers of the component may be applied to the component if desired.

[0046] Sintering the component, preferably sintering the first and / or second material (optionally cosintering the first and second material), may comprise heating the component to a temperature of 800°C or higher, optionally to a temperature of 850°C or higher, optionally to a temperature of 900°C or higher, optionally to a temperature of 1000°C or higher.

[0047] Sintering the component, preferably sintering the first and / or second material (optionally cosintering the first and second material), may comprise heating the component to a temperature of 1200°C or lower, optionally to a temperature of 1100°C or lower.

[0048] Thus, sintering the component, preferably sintering the first and / or second material (optionally co-sintering the first and second material), may comprise heating the component to a temperature in the range 800°C to 1200°C, optionally to a temperature in the range 900°C to 1200°C, optionally to a temperature of 1000°C to 1100°C. Sintering the component, preferably sintering the third material may comprise heating the component to a temperature of 1000°C or lower, optionally to a temperature of 900°C or lower, optionally to a temperature of 850°C or lower. Sintering the component, preferably sintering the third material may comprise heating the component to a temperature of 600°C or higher, optionally to a temperature of 700°C or higher, optionally to a temperature of 750°C or higher. Thus, sintering the component, preferably sintering the third material, may comprise heating the component to a temperature in the range 600°C to 1000°C, optionally to a temperature in the range 700°C to 900°C, optionally to a temperature of 750°C to 850°C.

[0049] Sintering the component may comprise heating the component in an oxidising atmosphere (for example in air or oxygen, preferably air).

[0050] The component manufactured by the method as set out above may form part of an electrochemical cell. Thus, in a second aspect, the present invention accordingly provides a method of manufacturing an electrochemical cell.

[0051] In a third aspect, the component manufactured by the method as set out above may form part of an electrochemical cell unit. Thus, the present invention accordingly provides a method of manufacturing an electrochemical cell unit comprising providing a component manufactured according to the first aspect, providing an interconnect; and attaching the interconnect to the component.

[0052] In a fourth aspect, the present invention accordingly provides a method of manufacturing an electrochemical cell unit stack, the method comprising providing a plurality of electrochemical cell units according to the third aspect and stacking a second electrochemical cell unit upon a first electrochemical cell unit in a spaced, opposed, relationship

[0053] In a fifth aspect, the present invention accordingly provides an electrochemical cell unit obtainable by a method according to the third aspect.

[0054] In a sixth aspect, the present invention accordingly provides an electrochemical cell stack obtainable by a method according to the fourth aspect.

[0055] Definitions

[0056] In this specification, 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. Pr, Tb 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.

[0057] In this specification references to electrochemical cell, SOC, SOFC and SOEC may refer to planar cells. Electrochemical cell units may be generally planar in configuration. Planar fuel cell units may be arranged overlying one another in a stack arrangement, for example 100- 400 fuel cell units in a stack, with the individual fuel cell units often arranged electrically in series.

[0058] Electrochemical cells may be fuel cells, reversible fuel cells or electrolyser cells. Generally, these cells may have the same or similar structures 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). A solid oxide electrolyser cell (SOEC) may have the same structure as an SOFC, but may be the SOFC operating in reverse, or in a regenerative mode, to achieve the electrolysis of water, NO2 and / or carbon dioxide by using the solid oxide electrolyte to produce hydrogen gas and / or carbon monoxide and oxygen. In SOEC mode, a fuel contacts the cathode (fuel electrode), a potential is applied across the cell so that oxygen evolves from the anode (oxygen electrode) and hydrogen is produced at the fuel electrode.

[0059] The terms “flattening” and “levelling” are unused interchangeably unless the context otherwise requires.

[0060] 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.

[0061] 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.

[0062] It will be understood that “attached” and “on” refer to direct or indirect attachment and positioning, respectively. It will be understood that each layer may be comprised of multiple sub-layers (and those sublayers may have varying compositions).

[0063] The invention will now be described with reference to the accompanying figures and examples.

[0064] BRIEF DESCRIPTION OF THE FIGURES

[0065] Figure 1 shows in (a) a photograph of the front of a substrate (e.g. of metal, e.g. of steel, e.g. of stainless steel) for an electrochemical cell after laser drilling, (b) a photograph of the side view of the substrate in (a) after laser drilling, (c) a photograph of the front of the substrate after roller levelling, (d) a photograph of the side view of the substrate after roller levelling, and (e) a schematic cross section through a roller leveller.

[0066] Figure 2 shows a schematic (not to scale) and magnified cross section of the porous region of a substrate illustrating steps of the method, in (a) a drilled and mechanically levelled portion of the substrate, in (b) the drilled and mechanically levelled portion of the substrate having a first electrode layer thereon, and in (c) the drilled and mechanically levelled portion of the substrate having the first electrode and electrolyte thereon and having undergone co-sintering (i.e. sintering together of the first electrode and electrolyte) in air at 1020 °C.

[0067] Figure 3 shows surface plots for an example of a substrate (units of axes in mm) of (a) a surface plot of a portion of the substrate before roller levelling, and (b) a surface plot of a portion of the substrate after roller levelling.

[0068] Figure 4 shows a schematic cross section of an electrochemical cell unit having a mechanically flattened substrate.

[0069] Figure 5 shows a boxplot of the open circuit voltage (OCV) of electrochemical cells in a test stack either having mechanically levelled substrates according to the disclosure (levelled) or thermally annealed substrates (STD).

[0070] Figure 6 shows SEM images for a cell unit whose substrate was levelled (Figure 6a) and a cell unit whose substrate was thermally annealed (Figure 6b).

[0071] DETAILED DESCRIPTION OF THE INVENTION Figure 1 (a) to (d) show photographs of a substrate (e.g. of stainless steel) for an electrochemical cell after laser drilling. The substrate shown in Figure 1(a) (front view) and (b) (side view) is shown to be deformed into an approximate, shallow saddle shape. The substrate was mechanically flattened by roller levelling and is shown after roller levelling in the photographs Figure 1(c) (front view) and (d) (side view). After roller levelling, the substrate is much more even and flat.

[0072] Figure 1(e) shows a schematic cross section of a roller levelling machine 42 that may be used to mechanically flatten substrates according to the disclosure. The roller leveller 42 comprises a set of upper levelling rollers 46 and a set of lower levelling rollers 44. The roller leveller 42 may comprise secondary rollers (not shown) to support the set of upper levelling rollers 46 and the set of lower levelling rollers 44.

[0073] A workpiece 4a (the substrate to be flattened) passes between the upper levelling rollers 46 and lower levelling rollers 44 which results in the substrate being repeatedly deformed in large to progressively smaller alternating bends. Thus, the workpiece 4a may follow a diminishing wave-like path (e.g. a decaying sine wave) through the roller leveller 42. Centres of lower rollers are offset from centres of upper rollers along the direction from inlet to outlet of the roller leveller 42. The offset between the centres of the upper levelling rollers 46 and lower levelling rollers 44 (a vertical distance, in a direction perpendicular to the direction from the inlet to outlet of the roller leveller) is set - typically so that at the inlet the offset is less than the sum of the radii of the upper and lower roller and at the outlet is similar to the sum of the radii of the upper roller, lower roller and thickness of the substrate.

[0074] During the levelling process the first bend may take the substrate past its yield point (which releases stress in the material) the repeated further bending reduces or removes residual unevenness and stress. As the workpiece 4a passes through the rollers it may be subject to elastic-plastic alternating bends and the constant reduction of bending intensity until it exits the roller leveller as a flattened substrate 4b. The roller leveller 42 has six lower levelling rollers and five upper levelling rollers, but it would be understood that more or fewer levelling rollers are possible. However, two upper levelling rollers and one lower levelling roller, or one upper levelling roller and two lower levelling rollers may be considered a minimum to expose the workpiece to alternating bends.

[0075] The substrate 4a, 4b may have a thickness in the range of about 50 pm to 500 pm, often about 100 pm to 400 pm, in some cases about 200 pm to 350 pm. The offset (vertical distance) between the centres of the upper levelling rollers 46 and lower levelling rollers 44 (a vertical distance, in a direction perpendicular to the direction from the inlet to outlet of the roller leveller) may be adjusted at the inlet (entry or infeed) of the roller leveller 42 and / or at the outlet (exit) of the roller leveller 42. The offset may be adjusted depending on the thickness of the substrate. The offset may be adjusted depending on material properties of the substrate.

[0076] For example, at the inlet the offset (offl) between the centres of the upper levelling rollers 46 and lower levelling rollers 44 may be less than the sum of the radii of the upper and lower levelling roller. The offset (vertical distance) between the centres of the upper set of rollers and the lower set of rollers at the inlet of the roller leveller may be equal to offl = rl+r2+xt, where rl and r2 are the radii of the upper and lower rollers respectively, t is the thickness of the substrate, and x is an offset factor. In such cases, x is less than zero at the infeed, which means that the substrate is bent as it passed through the rollers proximal to the infeed. Optionally the offset factor x, at the infeed, is in the range -0.2 to -2.5, optionally x is in the range -0.4 to -2, optionally x is in the range -0.6 to -1.7, optionally x is in the range -0.8 to - 1.5. In the example of Fig. 1 a) to d), x was set at -1.3. The rotational speed of the rollers - and so speed of the substrate through the rollers is often adjustable. In the case of Fig. la) to d) the substrate passed through the rollers at a speed of 7 m / min, but any reasonable speed range may be used (e.g., 0.5 - 40 m / min).

[0077] The offset (vertical distance) between the centres of the upper set of rollers and the lower set of rollers at the outlet of the roller leveller is approximately equal to the sum of the radii of upper roller, lower roller, and thickness of the substrate. The offset at the outlet may be equal to off2 = rl+r2+wt, where rl and r2 are the radii of the upper and lower rollers respectively, t is the thickness of the substrate, and w is an offset factor. In such cases, w is approximately one at the outlet. Optionally the offset factor w, at the outlet, is in the range 0.8 to 1.2, optionally w is in the range 0.9 to 1.1, optionally w is in the range 0.95 to 1.05, optionally w is in the range 0.98 to 1.02. In the example of Fig. 1 a) to d), w was set at 1.0.

[0078] The flattened substrate 4b may have a flatness of 4 mm or lower (optionally 2 mm or lower, optionally 1.7 mm or lower). Flatness may be determined, for example, by the methods of BSI BS EN ISO 9445-2 (2010) by which maximum deviation from a flat horizontal surface may be determined with the substrate lying on the flat surface and the maximum distance between the lower surface of the substrate product and the flat horizontal surface measured. Preferably, flatness may be determined using a non-contact Coordinate-Measuring Machine (CMM) (e.g. using a NEXIV measuring system). Alternatively a feeler wedge (also known as wedge feeler gauge) may be used to measure distance between the flat horizontal surface and the substrate thereon to determine flatness.

[0079] Figure 2 shows a schematic (not to scale) and magnified cross section of the porous region of a substrate illustrating steps of the method, in (a) a drilled and mechanically levelled portion of the substrate, (b) the drilled and mechanically levelled portion of the substrate having first electrode layer thereon, and in (c) the drilled and mechanically levelled portion of the substrate having the first electrode and electrolyte thereon and having undergone co-sintering (i.e. sintering of the first electrode and electrolyte together) in air at 1020 °C.

[0080] Figure 2(a) shows a component comprising the substrate 4 having a porous portion 8 with laser drilled pores 7. The substrate 4 has not undergone a thermal process (e.g. creep annealing) in order to flatten the substrate 4. Instead, the substrate 4 has been mechanically flattened using a roller leveller.

[0081] Figure 2(b) shows the component in the second step of the method comprising the substrate 4 with a first electrode 10 (e.g. a fuel electrode) deposited on the porous region 8 of the substrate 4. The first (fuel) electrode 10 may comprise e.g. Ni:CGO (Ni: cerium gadolinium oxide). The pores 7 in the substrate 4 allow fuel gas(es) in the fuel side below the substrate 4 (in use) to contact the fuel electrode 10. The top surface 3 of the substrate 4 and the interface 5 between the fuel electrode 10 and the substrate 4 does not have a thermally grown oxide (since the substrate has not been subjected to creep annealing).

[0082] Figure 2(c) shows the component with a half-cell (i.e. one electrode and electrolyte) deposited on the surface thereof comprising the substrate 4 with the first electrode 10 on the porous region 8 of the substrate 4 and an electrolyte layer 12 on the first electrode 10. After deposition of the electrolyte 12, the component is subjected to a sintering process in air at 1020 °C, which co-sinters (i.e. sinters in one process) the first electrode 10 and electrolyte 12. Sintering in air results in the formation of a thermally grown oxide on the surface of the substrate 4. The thermally grown oxide forms on the surface where it is not coated, for example at the interface 9 between the first electrode 10 and the substrate 4, but also where it is coated.

[0083] Figure 3 shows surface plots for an example of a substrate (units of axes in mm) of (a) a surface plot of a portion of a substrate after laser drilling but before mechanical flattening, and (b) a surface plot of a portion of the substrate after mechanical flattening by roller levelling (x=-1.5, w=1.0 in the formulas described with reference to Fig. 1). By comparison of Figure 3(a) with Figure 3(b), it can be seen that the substrate is significantly flattened by said mechanical flattening, e.g., to a flatness of 1.7 mm or lower. The x, y values of the surface plots are not limiting, the substrate may be larger (or smaller) than indicated in Figure 3.

[0084] Figure 4 shows, schematically and not to scale (for reasons of clarity), a cross section of an electrochemical cell 2.

[0085] A mechanically flattened substrate 4 (which, for example, may be formed of a ferritic stainless steel metal plate) flattened to a flatness of 4 mm or lower (optionally 3 mm or lower, or 1.7 mm or lower), 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 substrate 4. A barrier layer (not shown) to reduce corrosion may be located on the surface of the substrate 4 (on one or both sides thereof). The substrate may be mechanically flattened by roller levelling.

[0086] A fuel electrode 10 is located on the porous region 8 of the substrate 4. The fuel electrode 10 may be formed of e.g. Ni:CGO (Ni: cerium gadolinium oxide). A barrier layer (not shown) may surround the substrate. It may be formed on the substrate 4 prior to deposition of the fuel electrode 10. The barrier layer may comprise CGO.

[0087] An electrolyte layer 12 of rare earth doped ceria (dopant may be e.g. Y, Sc or any Ln) of thickness 4 pm or greater is located on the fuel electrode 10. Optionally, the electrolyte may have a thickness of 5 pm or greater, 6 pm or greater or 7 pm or greater. The electrolyte may have a thickness of 17 pm or lower. Optionally, the electrolyte may have a thickness of 15 pm or lower, optionally 12 pm or lower. Thus, the electrolyte may have a thickness in the range 4 pm to 17 pm, a thickness in the range 5 pm to 15 pm, or a thickness in the range 6 pm to 12 pm.

[0088] The electrolyte layer 12 may surround the fuel electrode 10 to reduce or prevent gas leaking from the fuel side 22 to the oxygen side 24 or vice versa. The electrolyte layer 12 may further overlap at least part of the non-porous region 6 of the mechanically flattened substrate 4.

[0089] 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 interlayer 14 may act as an electron blocking layer to improve efficiency of the electrochemical cell 2.

[0090] A second interlayer 16 of RE doped ceria (RE = Y, Sc or any Ln) of thickness of up to 1 pm is located on the interlayer 14, and is in direct contact with the interlayer 14.

[0091] The electrochemical cell 2 has an oxygen electrode 18 of, for example, rare earth strontium cobaltite / cerium gadolinium oxide (e.g. RE SC / CGO) located on the second interlayer 16. The oxygen electrode may have a second oxygen electrode layer 20 (that may act as a bulk cathode layer in SOFC mode) of, for example, an electrically conductive perovskite material.

[0092] The materials to form the layers of the electrochemical cell 2 may be applied to the substrate and / or to other layers by any suitable method. Examples of suitable methods include printing, optionally roller printing, jet printing, screen-printing, gravure printing or slot-die printing, or spraying (for example atomised spraying), vapor deposition such as chemical vapor deposition (CVD), or physical vapor deposition (PVD). Reference may, for example, be made to WO 2009 / 090419 A2, which discusses methods for laying down, as well as exemplary compositions of electrochemically active layers for electrochemical cells, together with the application of the materials of such layers upon a metal substrate, especially upon a stainless steel substrate. The layers (including air electrode layers) may show good adhesion.

[0093] The first material, the precursor to form the first electrode (fuel electrode 10 as illustrated in Figure 4) may be applied to the substrate as discussed above, optionally dried and then the second material as a precursor to form the electrolyte 12 may be applied to the first electrode and optionally dried. The first electrode and the electrolyte may be sintered in one step (i.e. co-sintered) at a temperature of 800° C or higher. During said sintering, a thermally grown oxide layer (not shown in Fig. 4) is grown on the substrate both where it is coated and where it is not coated by the first electrode, and where present electrolyte, and barrier layer.

[0094] Further materials to form the other layers may be applied subsequently to the co-sintering step. After application of each material, the material of the layer may be optionally dried (if appropriate) and / or may be sintered at a temperature of 800°C or above.

[0095] The third material, the precursor to form the second electrode (oxygen electrode 18 as illustrated in Figure 4) may be applied as discussed above, optionally dried and then may be sintered at a temperature of 600° C or higher. One undesirable process during sintering of the electrolyte is contamination of the electrolyte layer with volatile transition metal oxide species (particularly chromium oxides) evaporating from the steel. Cells made using a levelled substrate and sintered in the same thermal treatment step as thermally growing oxide on the substrate were tested and determined that transition metal oxide species detrimental to performance were in greater concentration than standard reference parts.

[0096] Electrochemical cells using either standard (thermally annealed substrates) or levelled (substrates flattened using a roller leveller and with a thermally grown oxide on the substrate surface or interface grown in the same process as co-sintering of the first electrode and electrolyte layers as discussed herein, see in particular Figure 2 and discussion above) were made and placed in a test stack (8 levelled cells and 7 standard cells) and tested in SOFC mode.

[0097] The cell units (levelled and standard) were operated under the same conditions and open circuit voltages (OCV) for the standard (“STD”) and levelled cells were measured and are shown as boxplots in Figure 5. The results indicating that there is no significant difference in performance between levelled and standard cells, while the levelled cells enable a significant reduction in cost and manufacturing time by removal of the separate thermal annealing / thermally grown oxide process and sintering process.

[0098] Figure 6 shows SEM images for a cell unit whose substrate was levelled in Figure 6a and a standard cell unit - i.e., a cell unit whose substrate was thermally annealed to level it - in Figure 6b. Each image is a cross section of a cell unit showing the substrate 6 (partially), a thermally grown oxide (TGO) layer 30, a barrier layer 32 (in this case CGO), a first electrode 10 (in this case, fuel electrode), and electrolyte 12 (partially). The image of the levelled cell unit in Figure 6a is subsequent to a step of sintering to densify both the first electrode and electrolyte, and to thermally grow the oxide coating. In contrast, the standard cell of Figure 6b was prepared by levelling using thermal annealing to also grow the thermally grown oxide prior to applying the barrier layer. In Figure 6a, it can be seen that the TGO layer 30 of the levelled cell unit is grown over the entire substrate 6 despite the presence of the other layers. The TGO layer of the levelled cell unit is thinner than that of the standard cell unit of Figure 6b, which leads to a lower resistance for the levelled cell unit, and thereby improved performance. The TGO layer of the levelled cell unit is around 30-60% of the thickness of the TGO layer of the standard cell unit, which may be around 1300 to 1800 nm. Such lower thickness for the levelled cell unit is due to a lower temperature being used for sintering the first electrode 10 and electrolyte 12 than was used for thermal annealing to level the substrate (and grow the TGO layer). Nonetheless, the thinner TGO layer of the levelled cell unit is continuous and sufficient for corrosion resistance. 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 method of manufacturing a component for an electrochemical cell, the method comprising, mechanically flattening a substrate comprising a porous region and a non- porous region, applying a first material on at least part of a surface of the porous region of the substrate as a precursor for, and to form, a first electrode on the substrate, and sintering the component to densify the first material and to thermally grow an oxide coating on the substrate.

2. A method as claimed in claim 1, further comprising providing the substrate comprising a porous region and a non-porous region comprising providing a substrate workpiece and drilling a plurality of pores in the substrate workpiece over at least a portion thereof to form a porous region.

3. A method as claimed in claim 2, wherein drilling the substrate comprises laser drilling the substrate.

4. A method as claimed in any one of the preceding claims, wherein the substrate comprises metal, optionally wherein the substrate comprises a steel substrate, optionally wherein the substrate comprises a stainless steel substrate.

5. A method as claimed in in any one of the preceding claims, wherein mechanically flattening comprises mechanical flattening by rolling, optionally by roller levelling.

6. A method as claimed in any one of the preceding claims, wherein mechanical flattening of the substrate results in a flatness of 4 mm or lower determined as deviation fromthe mean plane over a predetermined area, optionally 3 mm or lower, optionally 2 mm or lower, optionally 1.7 mm or lower.

7. A method as claimed in any one of the preceding claims, wherein applying the first material on at least part of the surface of the porous region of the substrate to form a first electrode comprises applying the first material directly on at least part of a surface of the porous region of the substrate.

8. A method as claimed in any one of the preceding claims, wherein the first material comprises doped ceria, optionally wherein the first material comprises cerium gadolinium oxide (CGO)9. A method as claimed in any one of the preceding claims, further comprising, before sintering the component, applying a second material on the first material and / or on the first electrode as a precursor for and to form an electrolyte on the first electrode.

10. A method as claimed in claim 9, wherein sintering the component further densifies the second material.

11. A method as claimed in either claim 9 or claim 10, wherein the second material comprises doped ceria, optionally selected from samarium-doped ceria (SDC), gadolinium- doped ceria (GDC, or CGO), praseodymium doped ceria (PDC), samaria- gadolinia doped ceria (SGDC) and mixtures thereof.

12. A method as claimed in any one of claims 9 to 11, further comprising, applying a third material on the second material and / or on the electrolyte as a precursor for and to form a second electrode on the electrolyte.

13. A method as claimed in claim 12, further comprising sintering the component after applying the third material.

14. A method as claimed in either claim 12 or claim 13, wherein the third material comprises doped ceria, optionally cerium gadolinium oxide (CGO).

15. A method as claimed in any one of claims 12 to 14, wherein applying the third material is by printing, optionally by screen-printing or inkjet printing.

16. A method as claimed in any one of claims 9 to 15, wherein applying the second material is by printing, optionally by screen-printing or inkjet printing.

17. A method as claimed in any one of the preceding claims, wherein applying the first material is by printing, optionally by screen-printing or inkjet printing.

18. A method as claimed in any one of the preceding claims, wherein sintering the component comprises heating the component to a temperature of 800°C or higher, optionally to a temperature of 1000°C or higher.

19. A method as claimed in any one of the preceding claims, wherein sintering the component comprises heating the component in an oxidising atmosphere.

20. A method of manufacturing an electrochemical cell unit comprising providing a component according to any one of claims 1 to 19, providing an interconnect; and attaching the interconnect to the component.

21. A method of manufacturing an electrochemical cell unit stack, the method comprising providing a plurality of electrochemical cell units according to claim 20 and stacking a second electrochemical cell unit upon a first electrochemical cell unit in a spaced, opposed, relationship22. An electrochemical cell obtainable by a method as claimed in any one of claims 1 to 19.

23. An electrochemical cell unit obtainable by a method as claimed in claim 20.

24. An electrochemical cell stack obtainable by a method as claimed in claim 21.

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