GB2400723A

GB2400723A - Solid oxide fuel cell for a novel substrate and a method for fabricating the same

Info

Publication number: GB2400723A
Application number: GB0308724A
Authority: GB
Inventors: Ahmet Selcuk; Brian Charles Hilton Steele; Derek Corcoran; Gene Lewis; Tom Mccolm; Robert Trezona
Original assignee: Ceres Power Ltd
Current assignee: Ceres Power Ltd
Priority date: 2003-04-15
Filing date: 2003-04-15
Publication date: 2004-10-20
Anticipated expiration: 2023-04-15
Also published as: GB0602247D0; GB2422479A; GB0308724D0; GB2422479B; GB2400723B

Abstract

The present invention provides an intermediate-temperature solid oxide fuel cell, comprising a ferritic stainless steel substrate including a coarsely porous support 3 and a non-porous frame housing 4 the coarsely porous support, a first electrode layer 5 located in the non-porous frame and supported internally by the coarsely porous support of the substrate, an electrolyte layer 6 located over the first electrode layer, and a second electrode layer 7 located over the electrolyte layer. The coarsely porous support is manufactured from expanded foil or woven mesh of ferritic stainless steel and, being bonded to the non-porous stainless steel frame, provides structural support for the fuel cells, collect electricity generated in the cells and allow the gas flow into the porous electrodes. The invention also provides a method of integrating the first electrode and the coarsely porous support into a composite support structure and a fabrication route which allows the sintering of the electrolyte into a dense impermeable layer over such supports at 1000{C or below.

Description

1 2400723

SOLID OXIDE FUEL CELL WITH A NOVEL SUBSTRATE AND

A METHOD FOR FABRICATING THE SAME

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an intermediate temperature solid oxide fuel cell (IT-SOFCs), details of which are known from U.S. Patent Application Publ. No: l0 US20020048699A1, and more specifically to the design and manufacture of a novel substrate for such fuel cells. The invention further relates to the fabrication of electro chemically active membranes of the fuel cell on such substrates. The substrate of the present invention provides structural support for the cells, collects electricity generated in the cells and allows the gas flow into the porous electrodes.

2. Description of the Prior Art

The solid oxide fuel cell (SOFC), known from the US Patent Application Publication No: US20020048699A1, includes a ferritic stainless steel substrate including a porous region and a non-porous region with the nonporous region bounding the porous region, a first electrode layer located over the porous region of the substrate, an electrolyte layer located over the first electrode layer, and a second electrode layer located over the electrolyte layer. The known SOFC further includes a ferritic stainless steel bi-polar plate located under one surface of the porous region of the substrate being sealingly attached to the non-porous region of the substrate about the porous region. The known invention describes a gadolinia doped ceria (COO) based electrolyte which is capable of sistering into an impermeable dense film at temperatures below 1000 C and avoids the need to use brittle seals because the electrolyte layer is hermetically bonded along its periphery to the non porous region of the substrate, preventing mixing of the gaseous oxidant and fuel. Further, the fuel cell compositions provided in the known SOFC permits operations below 550 C, which in turn provides the benefit of having much reduced corrosion rate of the stainless steel, allowing the fuel cell stack to be operated for much extended periods of time, typically in excess of 40,000 hours. Also, the fuel cell operation at such temperatures enables the use of commercial compliant gasket materials, which greatly simplifies design, l0 assembly and operation of the fuel cell stacks.

In the known SOFC, the substrate material is ferritic stainless steel. The incorporation of stainless steel components in solid oxide fuel cells (SOFCs) is recognised as having many advantages over other materials since it offers a considerable reduction in the SOFC fabrication costs, a close thermal expansion matching to the cell materials, excellent mechanical behaviour and enhanced thermal and electrical conductivity. Also, there are commercial grades of stainless steel newly-developed for the SOFC applications, offering much improved hot corrosion resistance and tolerable levels of electrical contact resistance. Moreover, by using the compositions tailored for the SOFC applications, excellent electrical contacts can be maintained between the cell components for extended periods (see, for example, K. Honegger, A. Plas, R. Diethelm and W. Glatz, Electrochemical Society Proceedings, SOFC VII, Volume 2001-16, (2001) 803-810).

In the known SOFC, the flow of the gaseous oxidant or fuel into the first porous electrode layer deposited over the substrate is achieved by the provision of a plurality of through apertures fluidly interconnecting the one and other surface of the substrate. Such apertures have a lateral dimension of from 5 um to 250 am and preferably 30 Am, cover at least 30 area % of the porous region of the substrate and can be formed by processes that include, but are not limited to, hoto-chemical machining and/or laser machining. However, the introduction of such apertures to the substrate presents a number of problems in terms of high manufacturing costs and adoptability of conventional machining methods for high volume substrate manufacturing processes. In addition, in certain circumstances, there is a need to increase the area % of such apertures above 50% in order to ensure sufficient transport of gaseous fuel or oxidant, required for an efficient fuel cell operation. It is therefore necessary to develop alternative substrate structures, which satisfy the above needs and meet the contingent requirements of the SOFC fabrication.

From prior art there are known a number of highly porous structures, such as knitted or woven metal meshes and expanded metal foils, which, in the present invention, are disclosed as structures suitable for the substrate applications.

Also, it has been proposed in patent application WO02101859-A2 to provide a continuous porous support structure for fuel cells, comprising a knitted material or a woven material produced from metal wires. In this patent application, it is further proposed a method of fabricating an anodeelectrolyte-cathode unit on continuous mesh structures using thermal coating processes, in particular plasma spraying and flame-assisted spraying. However, the cell fabrication method disclosed in the said patent application suffers from the disadvantage that the use of thermal spraying to prepare the electro-chemically active layers of the fuel cell is relatively expensive, in particular being wasteful of the ceramic powder and not as convenient as more conventional ceramic processing routes, such as, for instance, tape-casting. The method is further disadvantaged by the requirement of additional fabrication steps in the thermal spraying of the anode in particular. The additional steps include the incorporation of a pore former in the mesh to ensure that the anode is porous enough for the SOFC operation, the construction of a "spray r Harrier" to confine the thermal spraying of the electro- chemically active layers in the mesh volume, and the removal of such temporary structures at later stages of cell fabrication by chemical, electro-chemical or thermal means. Also, more fabrication steps are required to incorporate additional components in the substrate to provide sealing around the periphery of the porous mesh structure in order to prevent the mixing of the gaseous oxidant and fuel.

It is thus an object of the present invention to provide a JO novel stainless steel substrate structure for the solid oxide fuel cells, which substantially avoids the above problems of the fuel cell fabrication over metallic substrates. It is another object of the present invention to provide a cost- effective method of fabricating a planar solid oxide fuel cell Is which utilises such novel substrates, enables the fabrication of electro- chemically active membranes of the fuel cell by sistering at 1000 C or below and avoids the need to use the seals of brittle nature or other sealing attachments.

SUMMARY OF THE INVENTION

Accordingly, a first aspect of the present invention provides a solid oxide fuel cell, comprising a stainless steel substrate including a coarsely porous support and a non-porous frame housing the coarsely porous support or a number of such coarsely porous supports, a first electrode layer located in and in contact with the non-porous frame and reinforced internally by the coarsely porous support of the substrate, an electrolyte layer located over the first electrode layer extending at least over part of the non-porous frame and sealing with the same, and a second electrode layer located over the electrolyte layer.

The non-porous frame is a shaped stainless steel frame.

For purposes of illustration below, the description will be as a squareshaped stainless steel frame with a thickness in the range from 50 um to 2 mm, and more specifically 100,um to 300 Am; although the frame shape can be any form desired - examples of which include, but are not limited to, a pentagonal shape, or a round, or an ovoid shape. Additionally, the frame can be made up to contain more than one frame and these inner frames are not limited in their shape and are not limited in reflecting the shape of the frame - for example, a rectangular frame might contain several octagonal inner frames.

Additionally the inner frames do not have to be limited to being the same shape within the frame - for example a mixture of pentagon shaped inner frames and square shaped inner frames could be placed within a rectangular frame. Preferably, the non-porous frame includes a recess on the inner side in which the coarsely porous support is located.

The coarsely porous support is shaped so that it fits with the corresponding non-porous frame and can be physically bonded along its periphery to the non-porous stainless steel frame.

In one embodiment, the coarsely porous support is an expanded foil having a thickness in the range from one half to two third of the thickness of the stainless steel frame. The expanded foil includes a plurality of through apertures fluidly interconnecting the one and other surfaces of the foil.

Preferably, the number of apertures remains within the range from about 1000 to 12000 per square inch (1.55 x 106 to 1.86 x 107 per m2), more preferably 3000 to 9000 per square inch (4.65 x 106 to 1.40 x 107 per m2). More preferably, the number of apertures is 9000 per square inch (1.40 x 107 per m2).

Preferably, the open area provided by the apertures is from about 30% to 70% of the total area of the porous support. More preferably, the open area provided by the apertures is about 50% of the total area of the porous support. !

In another embodiment, the stainless steel substrate comprises an expanded foil bound by solid regions at two opposite ends and joined to side strips at the other two opposite sides. Additionally, in such a form, several expanded S regions with intervening solid strips could be bound by side strips at opposite sides. Additionally, a process could continuously produce expanded regions with intervening solid strips which could be bound by side strips at opposite sides.

In this embodiment, the use of a non-porous frame is avoided and the expanded region of the substrate includes a recess in which the first electrode layer is at least partially located.

Preferably, the thickness of the support structure outside the recess ranges from 50 Am to 2mm, or more preferably from lOO um to 300um. More preferably, the support structure outside the recess has a thickness of lOO um. Preferably, the recess has a depth of from about 2 um to 70,um, more preferably from 5 Am to 30,um from the planar surface of the substrate. More preferably, the depth of recess is 20 um.

In yet another embodiment, the coarsely porous metallic support is produced from a suitably woven mesh of a stainless steel wire with an average wire diameter in the range from 5 um to 200 um, more preferably 20 um to lOO,um. Preferably the wire diameter of the mesh is 50 um. In a preferred embodiment, the porous support contains two or more layers of woven meshes physically bonded together. Preferably, the porous support comprises two mesh layers, namely a first mesh layer located on the fuel cell side of the substrate and a second mesh layer located on the other side of the substrate. Preferably the second mesh layer is the layer reinforcing the first mesh layer and has a coarser mesh structure than the first mesh layer.

Preferably, the wire diameter of the second mesh layer is at least twice that of the first layer. Preferably, the first mesh layer has a wire diameter of from 5 Am to lOO am, more preferably from 20 um to 40um. More preferably, the first mesh sayer has a wire diameter of 30 um. Preferably the first and second mesh layers has a total thickness of from one half to two third of the thickness of the surrounding section of the non- porous frame.

S Preferably, the stainless steel used for the substrates is a ferritic stainless steel. More preferably, the stainless steel used for the substrates is a titanium/niobium stabilised ferritic stainless steel. More preferably, the stainless steel used for the substrates is a titanium/niobium stabilized ferritic stainless steel containing from 17.5 to 22.0 wt. % Cr.

from 0.3 to 1.0 wt. % An and less than 0.2 wt. % Al.

Preferably, the first and the second electrode layers are sistered materials containing a porosity fraction in the range from 10 to 50 vol. %, and more preferably 30 vol. % of the sistered material. Preferably, the first electrode layer has a thickness equivalent to that of the surrounding non-porous frame section of the substrate and the second electrode layer has a thickness in the range from about 5 um to 50 um, more preferably from 5 am to 15 Am. In one embodiment, the first electrode layer is a single layer in which the coarsely porous support is embedded to provide structural support. In another embodiment, the first electrode layer comprises two sub-layers of the same composition as described above. The first sub-layer is located under the electrolyte layer and the second sub-layer is located under the first sub-layer. The second sub-layer is the layer in which the coarsely porous support is embedded to provide structural support. Preferably, the first sub-layer is about 80 to 60 vol. % dense, more preferably 70 vol.% dense (30 vol. % porous). Preferably, the second sub-layer is about 50 to 70 vol. % dense, more preferably 60 vol. % dense (40 vol. % porous). Preferably, the first sub-layer has a thickness of from 5 Em to 30 um, more preferably from 10 Em to 20 Em and the thickness of the second sub- layer takes a value to make the Octal thickness of the first and the second sub-layers equivalent to that of the surrounding non-porous frame section.

In one embodiment, the first electrode layer is provided as the anode layer and the second electrode is provided as the S cathode layer of the fuel cell. In another embodiment the first electrode layer is provided as the cathode layer and the second electrode layer is provided as the anode layer of the fuel cell.

Preferably, the anode layer comprises from 40 to 60 vol. %, and more preferably about 50 vol. % of nickel oxide and correspondingly from 60 to 40 vol. %, and more preferably about vol. % of rare earth-doped ceria. Preferably, the cathode layer comprises a sistered powder mixture of rare earth-doped ceria and a perovskite oxide mixed conductor. Preferably the perovskite oxide mixed conductor comprises Lal-xSrxcoyFel-yo3- where 0.52x20.2 and 12y20.2. Preferably, the rare earth-doped ceria comprises Ce1-xGdxo3-/2 where 0.32x20.1. More preferably, the rare earth- doped ceria comprises CeO.gGdOlol9s.

Preferably, the electrolyte layer has a thickness in the range from about 5 am to 30,um, and more specifically from about 10 am to 20 um. In one embodiment, the electrolyte layer comprises a dense impermeable layer of a rare earth-doped ceria. Preferably, the rare earth-doped ceria comprises Ce1 xGdxO3-x/2 where 0.32x20.1. More preferably, the rare earth- doped ceria comprises CeO.gGdOlol95. Preferably, the electrolyte layer is sistered at 1000 C or below and at least 97 vol. % dense.

A second aspect of the present invention provides a method of fabricating a solid oxide fuel cell comprising (i) providing a stainless steel substrate including a coarsely porous support and a non-porous frame housing the coarsely porous support or a number of such coarsely porous supports; (ii) providing a first electrode layer in and in contact with the non- porous frame such that the first electrode layer is reinforced internally by the coarsely porous support) (iii) providing an electrolyte layer over the first electrode layer, the electrolyte layer extending over at least part of the non-porous frame and sealing with the samei and (iv) providing a second electrode layer over the electrolyte layer.

It is an advantage of the present invention to provide a stainless steel substrate with a porous region and a non-porous region that can be formed relatively cheaply and allows low cost fabrication of solid oxide fuel cells. It is another advantage of an embodiment of the invention to avoid the need to use brittle seals by providing an electrolyte layer hermetically bonded to the substrate around its periphery over the nonporous region of the stainless steel substrate. It is a further advantage of the invention to reduce the volume of the stainless steel in the porous region of the substrate which brings forth a significant reduction in the contamination of the electro-chemically active layers of the fuel cell with the volatile elements sourced from the stainless steel support.

A preferred embodiment of the present invention will now be described hereinbelow by way of example only with reference 2s to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic showing a vertical cross-sectional view of the solid oxide fuel cell in accordance with specific embodiments of the present invention.

Figure 2 is a schematic showing a plan view of the substrate structure in accordance with one embodiment of the present invention where an expanded foil is used to produce the coarsely porous region of the substrate.

Figure 3 illustrates the AA vertical cross-sectional view of the schematic shown in Figure 2 with no etched recess.

Figure 4 illustrates the AA vertical cross-sectional view of the schematic shown in Figure 2 with an etched recess.

Figure 5 is a schematic showing a plan view of an expanded stainless steel foil with the "solid intersperse design".

Figure 6 illustrates, in accordance with one embodiment, a stainless steel substrate configured to have a porous expanded region and a non- porous region bounding the porous expanded to region and such configuration achieved by joining non-porous side plates to the expanded foil section described by the schematic in Figure 5.

Figure 7 illustrates the AA vertical cross-sectional view of the schematic shown in Figure 5; Figure 8 illustrates fabrication of a stainless steel substrate with expanded regions with intervening solid strips bound by side strips at opposite sides in a continuous process where an expanded region and solid region could be formed and repeated from a sheet of stainless steel foil.

Figure 9 is a schematic showing a plan view of a substrate structure in accordance with one embodiment of the invention where a woven mesh is used to produce the coarsely porous region of the substrate.

Figure 10 illustrates the AA cross-section of the schematic shown in Figure 9.

Figure 11 illustrates the incorporation of an additional mesh layer to the substrate structure whose schematic is provided in Figure 9.

Figure 12 illustrates a vertical cross-sectional view of the solid oxide fuel cell in accordance with one embodiment of the present invention.

Figure 13 illustrates a vertical cross-sectional view of the solid oxide fuel cell in accordance with one embodiment of the present invention.

Figure 14 is the optical image of the anode layer deposited in the mesh region of the substrate in accordance with one embodiment of the present invention as shown in Figures 12 and 13.

Figure 15 is an SEM image showing the anode-mesh microstructure at a vertical cross section of the specimen described in Example 1.

Figure 16 is an SEM image showing a region of the image in Figure 15 at a higher magnification.

DETAILED DESCRIPTION OF THE INVENTION

The solid oxide fuel cell (SOFC) of the example illustrated in Figure 1 is an intermediate-temperature solid oxide fuel cell (IT-SOFC) which is typically used in stacks to generate a power output of from 1 to 100 kW in operations at 700 C or below. They find applications as local power generators, for example, in remote locations or for residential or small commercial combined heat and power (CHP) generation, and in vehicles as an auxiliary power unit (APU), or to drive other equipment, such as air conditioning equipment, or in the application to an uninterruptible power supply (UPS) where, say, they might provide the critical base load.

The example shown in Figure 1 provides a solid oxide fuel cell 1 comprising a stainless steel substrate 2 that includes a coarsely porous region 3 and a non-porous region 4 bounding the coarsely porous region 3, a first electrode layer 5 located over the coarsely porous region 3 of the substrate 2, an electrolyte film 6 located over the first electrode layer 5, and a second electrode layer 7 located over the electrolyte film 6. The electrolyte film 6 acts as a seal to prevent gases leaking from one side of the fuel cell to the other by sealing with the non-porous steel 4 surrounding the coarsely porous region 3. Reference will now be made in detail to specific embodiments.

In the embodiment shown in Figures 2 and 3, the solid oxide fuel cell 1 includes a stainless steel substrate 8 comprising two components bonded together, namely an expanded foil 9 with a coarsely porous structure and a non-porous frame 10 housing the expanded foil 9. The non-porous frame 10 is a shaped frame, shown here as a square-shaped frame although it is not limited to such a shape, and may have a thickness in the range from 50 um to 2 mm, and more specifically 100 um to 300 um. A frame thickness of 200 Em is suitable for the delivery of the robustness required from the substrate without much increase in thermal mass. The non-porous frame can be manufactured by stamping the frame out of a suitable stainless steel foil. The shape and number of the inner frames is not limited, thus, for example, pentagonal and square inner frames could be formed within the boundaries of a rectangular frame.

Alternative machining methods, such as photochemical machining and laser machining may be employed for more precise control over the component shape.

The expanded foil 9 of this embodiment may have a thickness in the range from one half to two third of the thickness of the non-porous frame 10. The use of expanded foil in the form of a porous substrate is highly desirable because the metal foil expansion is known as one of the most economical ways of introducing through apertures to metal foils. The expanded foils are commercially available with a variety of aperture geometries, usually based on a diamond shaped opening and can be configured flexibly to have a range of geometrical variables, for example, an aperture density of up to 9000 per square inch (1.40 x 107 per m2), an average diagonal size ranging from 250 um to several millimetres and an open area fraction up to 90% of the total foil area. For this embodiment, the expanded foil region of the substrate may have an aperture density of from 1000 to 9000 per square inch (1.55 x 106 to 1.40 x 107 per m2), or more preferably of 3000 to 9000 per square inch (4. 65 x 106 to 1.40 x 107 per m2) and an open area fraction of from 30% to 70% of the total area. In the cases where the space and volume limitation is a primary concern for the fuel cell operations, the aperture density can be increased S up to 9000 per square inch (1.40 x 10-7 per m2) and the foil thickness down to 25,um for a significant reduction in the thermal mass and volume of the fuel cells. Furthermore, the expanded foil 9 and the non-porous frame 10 can be integrated into one part by joining all the edges of the expanded foil to the non-porous frame by using relatively cheap welding methods.

In a preferred embodiment shown in Figure 4, the expanded foil is located in a recess 11 on the inner side of the non- porous frame 10 and is shaped to fit the shape of the frame.

The recess is provided to increase the contact area between the expanded foil and the non-porous frame so as to facilitate the welding and the joining of the two components together.

In one embodiment as shown in Figures 5 to 7, the substrate 2 of the solid oxide fuel cell 1 includes an expanded stainless steel foil 12 which has a "solid intersperse design" comprising an expanded region 13 bound by two solid regions 14 at opposite ends, providing a solid contact surface without the necessity of welding on a separate solid section. In order to form a continuous non-porous region bounding the expanded region of the foil, side strips 15 are welded or brazed to the open ends 16 of the expanded region 13. The non-porous region of the substrate in this embodiment has a thickness of about 50 Am to 2mm, or more specifically 100 um to 300 um in accordance with the thickness limitations stated above for the non-porous frame 10. The expanded region 13 includes a square-shaped recess 17 on one surface (although the shape may be other than square), where the first electrode layer 5 is located. The recess has a depth of 2 um to 70 Am, and more preferably from 5 urn to 30um, and more preferably about 20 um, and can be formed by processes such as, but not limited to, cold pressing or photochemical machining relatively cheaply. In another form as shown in Figure 8, such an expanded region and solid region could be formed and repeated in a continuous operation from, say, a sheet of foil, where the size and length of expanded area and interconnected solid regions are controlled as required. In such a continuous operation, a sheet of foil 35 could be continuously fed to apply the following processes in sequence: (i) Form expanded regions 36 by simultaneous slitting and l0 stretching of the foil 35 by shaped tools 37 which determine the form and number of openings in the expanded regions.

(ii) Flatten the expanded regions by rolling between rollers 38 which also provide stretching of the foil.

IS (iii)Join the side strips 39 and the expanded foil 36 using a welding tool 40 suitable for continuous operation using one of the conventional welding techniques.

(iv) Optionally introduce a recess 41 in the expanded region 42 of the foil by die pressing or other conventional shaping methods.

(v) Cut substrate sections 43 using a cutting tool 44 as required.

In another embodiment shown in Figures 9 and 10, the stainless steel substrate 2 includes a single layer of a woven mesh 18, located in a recess 20 formed along the inner edges of the non-porous frame 19. The woven mesh is made from stainless steel wire and bonded physically to the non-porous frame 19 in the recess 20 again by using a relatively cheap method of metal joining. The wire diameter and the weaving pattern of the mesh can be altered to increase the rigidity of the support while maintaining an appreciable amount of open pore space in the mesh for an efficient gas transport into the first electrode layer. Preferably, the mesh support 18 is made from a stainless steel wire with a diameter of from 5 Am to 200,um, and more specifically from 25 um to 100 Am, and more preferably about 50 Am corresponding to a mesh thickness of about 50 Am and 200 Am, S respectively. Because the mesh is bonded to the non-porous frame 19 in the recess 20 regions, the depth of the recess is the limiting value for the mesh thickness. The work of inventors have shown that a mesh thickness of from one half to two third of the frame thickness is sufficient to deliver the l0 mechanical integrity required from the substrate. Accordingly, a mesh wire diameter of from one forth to one third of the frame thicknesses may be maintained for different substrate thicknesses.

Figure 11 shows a mesh support 18 comprising two physically bonded layers of the woven mesh, namely the first mesh layer 22 located on the electrolyte side of the substrate and the second mesh layer 21 is located below the first mesh layer. In this example the wire diameter of the second mesh layer 21 is at least twice that of the first mesh layer 22, provided that the total thickness of the two mesh layers remains in the range from one half to two third of the thickness of the non-porous frame 19. The second mesh layer 21 is incorporated in the mesh support 18 to enhance the robustness of the substrate. The first and second mesh layers can be bonded together by diffusion bonding or welding.

Depending on the substrate thickness, a third or more mesh layers may be included in the mesh support structure for further enhancement of the robustness of the substrate.

A suitable stainless steel material for the substrate components describedabove is a ferritic stainless steel. More preferably, the stainless steel used for the substrates is a titanium/niabium stabilised ferritic stainless steel. More preferably, the stainless steel used for the substrates is a titanium/niobium stabilized ferritic stainless steel containing rom 17.5 to 22.0 wt. % Cr. from 0.3 to 1.0 wt. % Mn and less than 0.2 wt. % Al (e.g. Such steels include, but are not limited to, European designation 1.4509 for about 18 wt.% Cr content or Crofer 22 APU for higher Or contents).

Figure 12 shows a solid oxide fuel cell 1, 23 including a first electrode layer 24 as an anode layer confined within the non-porous region of the stainless steel substrate, an electrolyte film 28 having a thickness of from about 5 am to 30 um, and more specifically from about 10,um to 20,um, located over the first electrode layer, and a second electrode layer 29 as a cathode layer having a thickness of from about 5 Sum to 50 Am, and more specifically from about 5 um to 15um, located over the electrolyte layer.

The anode layer 24 in this embodiment has about 30 to 40 vol. % porosity and a thickness equivalent to that of the non- porous region 26 of the stainless steel substrate 27. The coarsely porous region 25 which in this example is made from either an expanded foil 9, 13, 30 or one or more layers of woven mesh 18, 31 is embedded in the anode layer 24 to provide reinforcement.

In another embodiment shown in Figure 13, the anode layer 24 comprises a first sub-layer 32 having a thickness of about am and a second sub-layer 33 having a thickness with a value to make the total thickness of the first and the second sub layers equivalent to that of the non-porous region of the substrate. The first sub-layer 32 is located under the electrolyte layer 28 and the second sub-layer 33 is located on the other side of the first sub-layer 32. The second sub-layer is the layer in which the coarsely porous region 25 of the substrate (e.g. an expanded foil 30 or one or more layers of a woven mesh 18, 31) is embedded to provide structural support.

In this embodiment, the first sub-layer is about 60 to 80 vol. % dense, and more specifically is about 70 vol.% dense (30 vol. % porous) and the second sub-layer is about 50 to 70 vol. % dense, and more specifically about 60% dense (40 vol.% porous).

The second sub-layer being more porous than the first sub-layer facilitates the flow of gaseous fuel or oxidant into the first sub-layer so that, during the fuel cell operations, the electrode reactions take place at desired rates without being affected by the electrode thickness.

The anode layers 24, 32, 33 described in the above embodiments can be deposited in the coarsely porous region 25 by any conventional deposition technique, in particular, but not limited to, tape casting, screen printing, vacuum casting, electrophoretic deposition and dip coating. The anode layer including its sub-layers may be fabricated from a composition comprising about 40 to 60 vol. %, and more specifically 50 vol. % of NiO and about 40 to 60 vol. %, and more specifically 50 l5 vol. % of Ce09Gd0lo:.9s (CGO). Following its deposition, the anode layer is sistered at about 800 to 1010 C, and more specifically 900 C in a neutral atmosphere to provide a porous composite structure of Ni-CGO with three interpenetrating percolation networks.

Different levels of porosities proposed for the anode in the above embodiments can be introduced by adjusting the particle size distribution of the anode powders to yield the desired pore structure in the deposited anode layer.

Alternatively, the pore volume and size distribution may be optimised by introducing a suitable pore former into the anode structure.

In the above embodiments, a sistered dense electrolyte film 28 is produced comprising Ce09Gd0.1ol.9s (COO) having a thickness of about 10 um to 20 um, more specifically about 20 am. The CGO electrolyte film 28 may be deposited over the anode layer 24, 32 so as to extend beyond the periphery of the anode layer, again using conventional ceramic processing techniques.

Once deposited over the anode layer, the CGO electrolyte layer may be sintered into a dense impermeable film at a temperature below 1000 C, and more specifically below 950 C. Because the periphery of the CGO electrolyte film is located over the non- porous region 26 of the substrate 27, the sistering of the CGO electrolyte leads to the formation of a hermetically bonded region 34, which inhibits the mixing of the gaseous fuel and oxidant. This avoids the need to use glassy seals to achieve the same purpose and greatly simplifies the design of the fuel cells and fuel cell stack construction.

In the above embodiments, a porous cathode layer 29 is provided as the second electrode 7 having a thickness of about 10,um deposited over the electrolyte film 28 again using the conventional ceramic processing routes, in particular screen printing and tape casting. In these embodiments, the cathode layer 29 is fabricated from a composition comprising a powder l5 mixture of CGO and a perovskite oxide mixed conductor being Lal-xsrxcoyFel-yo3-o (LSCF) where 0.52x20.2 and 12y20.2, which is sistered to form a porous composite structure.

The present invention is not limited to the use of the compositions specified above for the anode, electrolyte and cathode layers described in the above specific embodiments.

Alternative electrode and electrolyte materials can be utilised to achieve the goals of the invention. Among many suitable electrolyte and cathode and anode compositions that may be adopted for use in the present invention are those disclosed in the U.S. Pat. Appl. Publ. No: US20020048699, UK Patent Appl.

No: GB0205291.8, and the patent application in preparation by D. Young &Co. with Patent Ref. No: PO16350GB. In general, the inventions disclosed in these references relate to the fabrication of an intermediatetemperature solid oxide fuel cell, based on a CGO electrolyte, by sistering at temperatures below 1000 C. In the latter two references, specific electrolyte compositions and novel cell fabrication procedures have been proposed to avoid the problems associated with the densification of the ceria based electrolytes, particularly when sistered on stainless steel substrates at relatively low temperatures (at or below 1000 C).

The above embodiments provide stainless steel substrates with structural designs which are simple, inexpensive and suitable for mass production, and facilitate the fabrication of the fuel cells. Furthermore, as the stainless steel substrate can easily be joined by brazing or welding onto a metallic bipolar plate, an array of individual fuel cells can be fabricated onto a single bi-polar plate at relatively low costs using conventional metal joining techniques. Another advantageous aspect of the present invention is that the use of brittle seals to prevent the mixing of gaseous fuel and oxidant is avoided by providing an electrolyte layer 28 which is hermetically bonded with its periphery 34 to the non-porous region of the stainless steel substrate 27. A further advantageous aspect of the present invention is the use of a highly porous support structure for the fuel cells which brings forth a significant reduction in the contamination of the electro-chemically active layers with the volatile elements sourced from the stainless steel support. Also, the flow of gaseous fuel or oxidant into the first electrode layer is facilitated by the provision of large aperture volumes (e.g. up to 90 vol. for expanded foils and up to 60 vol.% for woven meshes) in the coarsely porous region of the substrate, which is important for efficient operation of the fuel cell.

The embodiments described above can be used with compositions and fabrication methods for solid oxide fuel cells (e.g. the methods described in the patent applications mentioned in the previous paragraph) in which the electrolyte material can be sistered into a dense impermeable film at temperatures below 1000 C which was not possible before.

Finally, it will be understood that the present invention has been described in its preferred embodiments and can be modified in many different ways without departing from the scope of the invention as defined by the appended claims.

In one modification, the cathode and anode layers could be reversed such that the cathode layer is located on the stainless steel substrate and the anode layer is located on the electrolyte layer. Such modification may be needed to facilitate easier fabrication of the fuel cells.

In another modification, the size of the stainless steel substrate 8, 27 may be increased to house an array of the coarsely porous region where an array of individual fuel cells can be fabricated without the need of joining the individual fuel cells on a bipolar plate.

In a further modification, the solid oxide fuel cell 1 could be of other shape than square.

Although the embodiments described above show a planar substrate, the substrate could be non-planar such as to provide a curved or tubular surface or any design for a desired application.

The present invention also provides a substrate as described above for a fuel cell, oxygen converter or a half cell with an electrode layer located in and in contact with the non-porous frame and reinforced internally by the coarsely porous support of the substrate. Such a substrate may also be provided as a half cell when an electrolyte layer is located over the electrode and extending over at least part of the non- porous frame and sealing with the same.

EXAMPLES

Example 1: Fabrication of an anode layer over a woven mesh region of a stainless steel substrate.

Preparation of a Stainless Steel Substrate: A stainless steel frame with a square-shaped structure was prepared by photo-chemical machining of the component out of a ferritic stainless steel foil (European designation: 1. 4509).

The frame had an internal dimension of 10 mm x 10 mm and an external dimension of 30 mm x 30 mm. A woven mesh of a 430 stainless steel wire was cut to fit into the stainless steel frame and the periphery of the mesh was bonded to the frame by spot welding.

Fabrication of the Anode Layer: The anode layer, comprising 50 vol% NiO powder and 50 vol% COO powder, was deposited over the mesh region of the steel substrate by electrophoretic deposition (EPD). Figure 14 shows an optical image taken from one side of the anode layer after the EPD deposition, revealing that the mesh wires are bridged by the anode particles and embedded inside the anode layer. The substrate coated with the anode layer was then dried and then heated in an inert atmosphere at 950 C in order to obtain a porously sistered anode layer. The scanning electron microscopy images (SEM) shown in Figures 15 and 16 reveal the microstructure of the anode-substrate vertical cross-sections.

It is clear from the SEM images in Figures 15 and 16 that the anode layer is continuous, linking the individual mesh wires and porous enough for the transport of the gaseous fuel needed during the SOFC operation.

Claims

A solid oxide fuel cell, comprising: (i) a stainless steel substrate including a coarsely porous support and a non-porous frame housing the coarsely porous support or a number of such coarsely porous supports; (ii) a first electrode layer located in and in contact with the non- porous frame and reinforced internally by the coarsely porous support of the substrate; (iii)an electrolyte layer located over the first electrode layer extending at least over part of the non- ; porous frame and sealing with the same; and (v) a second electrode layer located over the electrolyte layer.

2. The solid oxide fuel cell of claim l, wherein the coarsely porous support is embedded within the first electrode layer.

3. The solid oxide fuel cell of claim l or claim 2, wherein the electrolyte layer is bonded along its periphery to the non- porous frame.

4. The solid oxide fuel cell of any one of the preceding claims, wherein the internal shape of the non-porous frame is rectangular.

5. The solid oxide fuel cell of claim 4, wherein the internal shape of the non-porous frame is square.

6. The solid oxide fuel cell of any one of claims l to 3, wherein the internal shape of the non-porous frame is non rectangular. l

The solid oxide fuel cell of claim 6, wherein the internal shape of the non-porous frame is pentagonal.

8. The solid oxide fuel cell of claim 6, wherein the internal shape of the non-porous frame is round.

9. The solid oxide fuel cell of claim 6, wherein the internal shape of the non-porous frame is ovoid.

10. The solid oxide fuel cell of any one of the preceding Claims, wherein the non-porous frame contains a plurality of coarsely porous supports.

11. The solid oxide fuel cell of any one of the preceding claims, wherein the non-porous frame has a thickness in the range from 50 Am to 2 mm.

12. The solid oxide fuel cell of claim 11, wherein the non- porous frame has a thickness in the range from 100 um to 300 20,um.

13. The solid oxide fuel cell of claim 1, wherein the coarsely porous support is shaped to fit within the corresponding non- porous frame.

14. The solid oxide fuel cell of claim 1, wherein the non porous frame is provided with a recess in which the coarsely porous support is located.

15. The solid oxide fuel cell of claim 1, wherein the coarsely porous support is physically bonded along its periphery to the non-porous stainless steel frame.

16. The solid oxide fuel cell of claim 1, wherein the coarsely porous support is an expanded foil of stainless steel.

17. The solid oxide fuel cell of claim 16, wherein the coarsely porous support has a thickness in the range from one S half to two third of the thickness of the stainless steel frame.

18. The solid oxide fuel cell of claim 16, wherein the expanded foil includes a plurality of through apertures fluidly interconnecting the one and other surfaces of the expanded foil.

19. The solid oxide fuel cell of claim 18, wherein the number of through apertures ranges from about 1000 to 12000 per square l5 inch (1.55 x 106 to 1.86 x 107 per m2).

20. The solid oxide fuel cell of claim 19, wherein the number of through apertures ranges from 3000 to 9000 per square inch (4.65 x 106 to 1.40 x 107 per m2).

21. The solid oxide fuel cell of claim 20, wherein the number of through apertures is 7000 per square inch (1.09 x 107 per m2), 1 22. The solid oxide fuel cell of claim 18, wherein the open area provided by the apertures is from about 30% to 90% of the total area of the porous support.

23. The solid oxide fuel cell of claim 22, wherein the open area provided by the apertures is between 40% and 70% of the total area of the porous support.

24. The solid oxide fuel cell of claim 23, wherein the open area provided by the apertures is about 70% of the total area of the porous support.

25. The solid oxide fuel cell of any one of claims 16 to 24, wherein the coarsely porous support is an expanded foil bound by solid regions at two opposite ends and joined to side strips at the other two opposite sides.

26. The solid oxide fuel cell of claim 25, wherein the expanded region includes a recess in which the first electrode layer is at least partially located.

27. The solid oxide fuel cell of claim 26, wherein the expanded foil outside the recess has a thickness ranging from 50 um to 2mm.

28. The solid oxide fuel cell of claim 27, wherein the expanded foil outside the recess has a thickness ranging from um to 300 am.

29. The solid oxide fuel cell of claim 28, wherein the expanded foil outside the recess has a thickness of 100 um.

30. The solid oxide fuel cell of any one of claims 26 to 29, wherein the recess has a depth of from about 5 am to 30,um.

31. The solid oxide fuel cell of claim 30, wherein the recess has a depth of 20 um.

32. The solid oxide fuel cell of claims 25 to 31, wherein the expanded foil includes an array of expanded regions with intervening solid strips bound by side strips at opposite sides.

33. The solid oxide fuel cell of claims 25 to 32, wherein the expanded foil is produced in a continuous process to have a single or an array of expanded regions with intervening solid regions bound by side strips at opposite sides. s

34. The solid oxide fuel cell of claim 1, wherein the coarsely porous support is produced from a suitably woven mesh of a stainless steel wire.

JO 35. The solid oxide fuel cell of claim 34, wherein the cross- sectional dimensions of the stainless steel wire has a "wire diameter" which is the average of the diagonal lengths measurable through the centre of symmetry of the wire cross- section.

36. The solid oxide fuel cell of claim 35, wherein the stainless steel wire has a wire diameter in the range from 5,um to 200 Am. 37. The solid oxide fuel cell of claim 36, wherein the stainless steel wire has a wire diameter in the range from 20 um to 100 am.

38. The solid oxide fuel cell of claim 37, wherein the stainless steel wire has a diameter of 50 am.

39. The solid oxide fuel cell of claim 1, wherein the coarsely porous support contains two or more layers of woven meshes where a first mesh layer is located on the fuel cell side of the substrate and a second or more mesh layer(s) is the layer reinforcing the preceding mesh layer(s) and located on the other side of the substrate.

40. The solid oxide fuel cell of claim 39, wherein the first mesh layer is the woven mesh of any one of claims 34 to 38.

41. The solid oxide fuel cell of claim 39 or claim 40, wherein the second or more layers of the woven mesh has a wire diameter s of at least twice that of the first mesh layer.

42. The solid oxide fuel cell of claim 39, wherein the first and second mesh layers have a total thickness of from one half to two third of the thickness of the surrounding section of the l0 non-porous frame.

43. The solid oxide fuel cell of claim 1, wherein the stainless steel used for the substrates is a ferritic stainless steel.

44. The solid oxide fuel cell of claim 43, wherein the stainless steel used for the substrates is a titanium/niobLum stabilized ferritic stainless steel containing from 17.5 to 22.0 wt. % Cr. from 0.3 to 1.0 wt. % Mn and less than 0.2 wt. % Al.

45. The solid oxide fuel cell of claim 1, wherein the first and the second electrode layers are sistered materials containing a porosity fraction in the range from 20 to 50 vol. %.

46. The solid oxide fuel cell of claim 45, wherein the first and the second electrode layers contain a porosity fraction in the range from 30 to 40 vol. %.

47. The solid oxide fuel cell of claim 45, wherein the first electrode layer has a thickness equivalent to that of the surrounding non-porous frame section of the substrate.

18. The solid oxide fuel cell of claim 45, wherein the second electrode layer has a thickness in the range from about 5,um to am.

S 49. The solid oxide fuel cell of claim 48, wherein the second electrode layer has a thickness in the range from 5,um to 15 am.

50. The solid oxide fuel cell of claim 1, wherein the first electrode layer is a single layer in which the coarsely porous support is embedded to provide structural support.

51. The solid oxide fuel cell of claim 1, wherein the first electrode layer comprises two sub-layers, consisting of a first sub-layer located under the electrolyte layer and a second sub- layer located under the first sub-layer.

52. The solid oxide fuel cell of claim 51, wherein the second sub-layer is the layer in which the coarsely porous support is embedded to provide structural support.

53. The solid oxide fuel cell of claim 51, wherein the first sub-layer is about 60 to 80 vol. % dense and the second sub- layer is about 50 to 70 vol. % dense.

54. The solid oxide fuel cell of claim 53, wherein the first sub-layer is about 70 vol. % dense and the second sub-layer is about 60 vol. % dense.

55. The solid oxide fuel cell of claim 51, wherein the first sub-layer has a thickness of from 5,um to 30 am.

56. The solid oxide fuel cell of claim 52, wherein the first sub-layer has a thickness of from 5 um to 15,um. l

57. The solid oxide fuel cell of any one of claims 51 to 56, wherein the thickness of the second sub-layer takes a value to make the total thickness of the first and the second sub-layers equivalent to that of the surrounding non-porous frame section.

58. The solid oxide fuel cell of claim 1, wherein the first electrode layer is provided as the anode layer and the second electrode is provided as the cathode layer of the solid oxide fuel cell.

59. The solid oxide fuel cell of claim 1, wherein the first electrode layer is provided as the cathode layer and the second electrode layer is provided as the anode layer of the fuel cell.

60. The solid oxide fuel cell of claim 58 or claim 59, wherein the anode layer comprises from 40 to 60 % vol. % of nickel oxide and correspondingly from 60 to 40 % vol. % of rare earth- doped ceria.

61. The solid oxide fuel cell of claim 60, wherein the anode layer comprises 50 vol. % of nickel oxide and 50 vol. % of rare earth-doped ceria.

62. The solid oxide fuel cell of claim 58 or claim 59, wherein the cathode layer comprises a sistered powder mixture of rare earth-doped ceria and a perovskite oxide mixed conductor.

63. The solid oxide fuel cell of claim 62, wherein the perovskite oxide mixed conductor comprises Lal-sr where 0.52x20.2 and 12y20.2 and the rare earth-doped ceria comprises Cent Gd,O3./2 where 0.32x20.1.

o4. The solid oxide fuel cell of claim 63, wherein the rare earth-doped ceria comprises CeOgGdO.1ol9s.

65. The solid oxide fuel cell of claim 1, wherein the electrolyte layer comprises a dense impermeable layer of a gadolinia-doped ceria, comprising Cel-xGdxo3-x/2 where 0.32x20.1.

66. The solid oxide fuel cell of claim 65, wherein the gadolinia-doped ceria comprises Ceo.sGdolol.ss.

67. The solid oxide fuel cell of claim 1, wherein the electrolyte layer has a thickness in the range from about 5,um to 30,um.

68. The solid oxide fuel cell of claim 67, wherein the electrolyte layer has a thickness in the range from about 10 um to 20,um.

69. The solid oxide fuel cell of claim 1, wherein the electrolyte layer is sintered at or below 1000 C and at least 97 vol. dense.

70. The solid oxide fuel cell of claim 1, wherein an array of electrocatalytically active membranes each comprising an anode layer, an electrolyte layer and a cathode layer are provided upon each type of the stainless steel substrates according to any of claims 1 to 44.

71. The solid oxide fuel cell of claim 1, wherein the substrate is planar.

72. The solid oxide fuel cell of claim 1, wherein the substrate is curved or tubular. l

3. A substrate for a fuel cell, oxygen converter or a half cell, the substrate comprising a coarsely porous support and a non-porous frame housing the coarsely porous support or a number of such coarsely porous supports and an electrode layer located in and in contact with the nonporous frame and reinforced internally by the coarsely porous support of the substrate.

74. A half cell comprising the substrate of claim 73, and an JO electrolyte layer located over the electrode layer extending over at least part of the non-porous frame and sealing with the same.

75. A method of fabricating a solid oxide fuel cell comprising IS (i) providing a stainless steel substrate including a I coarsely porous support and a non-porous frame housing the coarsely porous support or a number of such coarsely porous supports; (ii) providing a first electrode layer in and in contact with the non- porous frame such that the first electrode layer is reinforced internally by the coarsely porous support; (iii) providing an electrolyte layer over the first electrode layer, the electrolyte layer extending over at least part of the non-porous frame and sealing with the same; and (iv) providing a second electrode layer over the electrolyte layer.

76. A method according to claim 75, wherein the first electrode layer is provided such that the coarsely porous support is embedded within the first electrode layer.

77. A method according to claim 75 or claim 76, wherein the l l Electrolyte layer is bonded along its periphery to the non- porous frame.

78. A method according to any one of claims 75 to 77, wherein s the substrate is produced in a continuous process in which one or more coarsely porous supports are provided with intervening solid regions and solid strips are provided on each side of the coarsely porous supports.