WO2015198042A1

WO2015198042A1 - Solid oxide fuel cell stack

Info

Publication number: WO2015198042A1
Application number: PCT/GB2015/051837
Authority: WO
Inventors: Antony MEADOWCROFT; Kevin Kendall
Original assignee: Adelan Limited
Priority date: 2014-06-24
Filing date: 2015-06-24
Publication date: 2015-12-30
Also published as: EP3161891A1; GB201411205D0

Abstract

The invention provides a solid oxide fuel cell stack comprising two or more solid oxide fuel cells, wherein each solid oxide fuel cell is a tubular fuel cell, comprising an inner electrode tube,an electrolyte layer coaxially surrounding the inner electrode, and an outer electrode layer coaxially surrounding the electrolyte layer, wherein one of the inner electrode tube and the outer electrode layer is an anode and the other is a cathode. Each solid oxide fuel cell is electrically connected to an adjacent solid oxide fuel cell, said connected fuel cells forming a pair. In each pair of solid oxide fuel cells at least one of the fuel cells includes an access portion, said portion comprising a part of the tubular fuel cell where the inner electrode tube is not covered by the electrolyte layer and the outer electrode layer, such that an electrical contact can be made with the inner electrode tube at this portion. In each pair of solid oxide fuel cells the electrical connection is from the access portion of one tubular fuel cell to the outer electrode layer of the other tubular fuel cell. The electrical connection is made via an interconnect wire, the wire comprising a first section that electrically contacts the inner electrode tube of one tubular fuel cell at the access portion thereof, said first section extending around the circumference of the inner electrode tube and being secured thereto, and the wire comprising a second section that electrically contacts the outer electrode layer of the other tubular fuel cell, said second section extending along a portion of the outer electrode layer and being secured thereto.

Description

SOLID OXIDE FUEL CELL STACK

Field of the Invention The present invention relates to solid oxide fuel cell stacks, in which a plurality of fuel cells is interconnected.

Background to the Invention Solid oxide fuel cells (SOFCs) are well known in the art. These types of fuel cell generate a direct electric current (DC) by reacting a fuel gas with an oxidant. SOFCs have an anode, an electrolyte and a cathode. The fuel cells can be made from a variety of solid oxide materials (e.g. they may be based on ceramics such as zirconium oxide, ceria, gadolinia, or other oxide materials, and they may optionally be doped with a dopant such as yttria, scandia, magnesia or calcia). In addition, SOFCs may be provided in a variety of geometries, e.g. they may be provided as tubular SOFCs where the fuel cell is in the shape of a tube.

The electrolyte is normally a dense layer of ceramic that conducts oxygen ions. Suitable materials include yttria stabilized zirconia (YSZ) (e.g. the 8% form, Y8SZ), scandia stabilized zirconia (ScSZ) (e.g. the 9 mol% form of Sc₂0₃, 9ScSZ) and gadolinium doped ceria (GDC).

The ceramic anode layer may, for example, be a cermet (ceramic-metal composite) made up of nickel mixed with ceramic, such as the ceramic material that is used for the electrolyte in that particular cell, e.g. YSZ.

The cathode layer must be, at minimum, electronically conductive . Lanthanum strontium manganite (LSM) is often used. Composite cathodes consisting of LSM and YSZ have also been used. Mixed ionic/electronic conducting (MIEC) ceramics, such as the perovskite LSCF, can also be used.

Two or more SOFCs can be connected to form what is known as a solid oxide fuel cell stack. In the assembling of solid oxide fuel cell stacks, the interconnect component connecting the cells is very important. The purpose of the interconnect is to collect the electric current generated by each fuel cell and to connect the cells in series, so that the electricity that each cell generates can be combined. For example, in stacks of tubular SOFCs, such interconnects electrically and physically connect the anode of one fuel cell tube to the cathode of an adj acent fuel cell tube.

Because these interconnects are exposed to both the oxidizing and reducing side of the cell, at high temperatures (about 500-950°C), they must be extremely stable. In addition, good electrical conductance is important for increasing recovery of the electrical energy generated by the fuel cell.

Typically interconnects are metallic or ceramic layers that are located between the individual fuel cells. For example, ceramic-based interconnects, such as lanthanum chromite interconnects, exhibit good high temperature conductivity and thermal expansion behaviour. However this ceramic is very expensive, has low toughness and is difficult to manufacture as a suitable interconnector. Metallic alloys such as a 95Cr-5Fe alloy may also be used.

US 6, 106,967 discloses a solid oxide fuel cell stack which utilizes metallic foils as interconnects which are disposed between a porous anode and a porous cathode of adjacent integral component fuel cell units.

US 2010/0021791 A l discloses an interconnect for a solid oxide fuel cell, comprising a wire mesh or a metal foil positioned between adjacent components of a fuel cell power plant, for example, between a fuel cell and a bipolar plate, for the purpose of passing and collecting current from one cell to the next. The interconnect in this location contacts a surface of a separator or bipolar plate and an external surface of a fuel cell electrode.

The use of interconnects in the form of wires is also known. For example US 201 1/0189587 Al discloses a plurality of fuel cell tubes, each fuel cell including an active area, an anode outer surface and an interconnect member. The wire interconnect member is disposed circumferentially around the fuel cell tube electrically contacting the anode outer surface. US 2007/0148523 Al also discloses a system for electrically interconnecting a plurality of fuel cells, each one having a plurality of discrete electrical connection points along an outer surface. The electrical connections are made directly between the discrete electrical connection points of adjacent fuel cells and can include a wire.

US2007/0141447 A l discloses a system in which there is a cathode current collector that electrically connects the cathode of one fuel cell tube to the anode of another fuel cell tube. The cathode current collector may comprise ribbons of metal or wires. Linear segments of wire extend parallel to the longitudinal axis of the tube and a spiral wire is wrapped around the segments.

However, silver wires wrapped around the cathode of a tubular SOFC in a spiral fashion can move with respect to the tube. This can lead to unreliable electric contacts. This is the case even if they are tightly attached when initially assembled, due to the high operating temperatures of fuel cells which can relax the elastically tight fits in use.

In addition, designs using wires wrapped around SOFC tubes use a large amount of silver wire, making the system expensive and heavy.

In view of the above, there is still a need for improving interconnects for SOFCs. For example, it would be desirable to provide more efficient connections between fuel cells. This in turn can lead to the SOFC stack being more powerful. It would also be desirable to provide SOFC stacks which use significantly less amount of interconnect material. This would have benefits of making the SOFC stack less expensive and lighter.

Summary of the Invention The invention provides, in a first aspect, a solid oxide fuel cell stack comprising two or more solid oxide fuel cells, wherein each solid oxide fuel cell is a tubular fuel cell comprising an inner electrode tube, an electrolyte layer coaxially surrounding the inner electrode, and an outer electrode layer coaxially surrounding the electrolyte layer, wherein one out of the inner electrode tube and the outer electrode layer is an anode and the other is a cathode . Each solid oxide fuel cell is electrically connected to an adjacent solid oxide fuel cell, said connected fuel cells forming a pair. In each pair of solid oxide fuel cells at least one of those fuel cells includes an access portion, said portion comprising a part of the tubular fuel cell where the inner electrode tube is not covered by the electrolyte layer and the outer electrode layer, such that an electrical contact can be made with the inner electrode tube at this portion. In each pair of solid oxide fuel cells the electrical connection is from the access portion of one tubular fuel cell to the outer electrode layer of the other tubular fuel cell. Said electrical connection is made via an interconnect wire. This wire comprises a first section that electrically contacts the inner electrode tube of one tubular fuel cell at the access portion thereof, said first section extending around the circumference of the inner electrode tube and being secured thereto. The wire also comprises a second section that electrically contacts the outer electrode layer of the other tubular fuel cell, said second section extending along a portion of the outer electrode layer and being secured thereto.

The fuel cell stack of the invention can utilise a smaller amount of interconnect material than prior art designs, thereby giving rise to a product that is lighter in weight as well as being cheaper to manufacture. It is preferred that the inner electrode tube is an anode and the outer electrode layer is a cathode .

It is preferred that the access portion is located substantially centrally along the length of the tubular fuel cell. For example, it may be located at a position that is a distance from the distal end of the tubular fuel cell that is from 40 to 60% of the total length of the tubular fuel cell, e.g. from 45 to 55% of the total length of the tubular fuel cell. Thus it may be located centrally, i.e. where the distance from the distal end to the access portion is 50% of the total length of the tubular fuel cell, or it may be close to but not at the centre of the length of the tubular fuel cell. The skilled person will appreciate that the access portion will extend over a distance that is a portion of the length of the tubular fuel cell. When reference is made to the location of the access portion the intention is that this is with reference to the centre point of that distance, i.e. the mid-point lengthways of the access portion. It may be that the access portion serves to divide the outer electrode layer into two portions, one of which extends from the access portion towards the distal end of the tubular fuel cell and one of which extends from the access portion towards the proximal end of the tubular fuel cell. In one embodiment the two portions have lengths that are substantially similar, for example the lengths may be the same + 25%, or + 20%, or ± 15%, or ± 10%.

It may be that the portion of the outer electrode layer that extends from the access portion towards the distal end of the tubular fuel cell extends over from 10 to 75% of the total length of the tubular fuel cell, such as from 15 to 70% or from 20 to 65% or from 25 to 60% of the total length of the tubular fuel cell. In one embodiment it may be from 30 to 55% of the total length of the tubular fuel cell.

It may be that the portion of the outer electrode layer that extends from the access portion towards the proximal end of the tubular fuel cell extends over from 10 to 75% of the total length of the tubular fuel cell, such as from 15 to 70% or from 20 to 65 % or from 25 to 60% of the total length of the tubular fuel cell. In one embodiment it may be from 30 to 55% of the total length of the tubular fuel cell. It may be that the access portion serves to divide the electrolyte layer into two portions, one of which extends from the access portion towards the distal end of the tubular fuel cell and one of which extends from the access portion towards the proximal end of the tubular fuel cell. In one embodiment the two portions have lengths that are substantially similar, for example the lengths may be the same + 25%, or + 20%, or ± 15%, or ± 10%.

It may be that the portion of the electrolyte layer that extends from the access portion towards the distal end of the tubular fuel cell extends over from 10 to 75% of the total length of the tubular fuel cell, such as from 15 to 70% or from 20 to 65% or from 25 to 60% of the total length of the tubular fuel cell. In one embodiment it may be from 30 to 55% of the total length of the tubular fuel cell.

It may be that the portion of the electrolyte layer that extends from the access portion towards the proximal end of the tubular fuel cell extends over from 10 to 75% of the total length of the tubular fuel cell, such as from 15 to 70% or from 20 to 65% or from 25 to 60% of the total length of the tubular fuel cell. In one embodiment it may be from 30 to 55% of the total length of the tubular fuel cell.

The use of an arrangement that is relatively symmetrical, e.g. with the access portion located substantially centrally along the length of the tubular fuel cell and/or with the two portions of the outer electrode layer having lengths that are substantially similar, and/or or with the two portions of the electrolyte layer having lengths that are substantially similar is beneficial. This results in a product where there are less likely to be strains on the connection, making the interconnect wire less likely to move with respect to the tube, and resulting in more reliable electrical contacts.

It is also easier to then line up multiple connected fuel cell tubes.

Preferably, the second section of the interconnect wire comprises two branches, each of which electrically contacts the outer electrode layer of the tubular fuel cell, with each branch extending along a portion of the outer electrode layer and being secured thereto. This provides an improved attachment and electrical connection. Again, there is a relatively symmetrical arrangement which therefore has less strain and more reliable electrical contacts.

In one embodiment, the branches extend in opposite directions. For example, one branch may contact the outer electrode layer of the other tubular fuel cell and extend towards the proximal end of the tubular fuel cell and the other branch may contact the outer electrode layer of the other tubular fuel cell and extend towards the distal end of the tubular fuel cell.

In one embodiment the branches have direct physical contact with the outer electrode layer. In general, in the solid oxide fuel cell stack, it can be preferred that the second section of the interconnect wire has direct physical contact with the outer electrode layer of the other tubular fuel cell, said second section extending along a portion of the outer electrode layer and being secured thereto. This requires less material, as an intervening layer of electrically conductive material is then not required. An improved and more secure connection can be achieved via direct physical contact. Likewise, in the solid oxide fuel cell stack it can be preferred that the first section of the interconnect wire has direct physical contact with the inner electrode tube of one tubular fuel cell at the access portion thereof. This requires less material, as an intervening layer of electrically conductive material is then not required. An improved and more secure connection can be achieved via direct physical contact.

In one embodiment of the invention there are no wires wrapped around the length of the outer electrode layer of the solid oxide fuel cell in a spiral fashion - i.e. there are no wires wrapped with multiple (three or more) turns around the circumference of the outer electrode layer of the solid oxide fuel cell.

Wires wrapped around the outer electrode layer (e.g. cathode) of a tubular SOFC in a spiral fashion can move with respect to the tube . This can lead to unreliable electric contacts. This is the case even if they are tightly attached when initially assembled, due to the high operating temperatures of fuel cells which can relax the elastically tight fits in use. In addition, designs using wires wrapped around SOFC tubes with multiple turns use a large amount of wire, making the system expensive and heavy. It is therefore desirable that wires are only wrapped around the access portion, i.e. where the first section extends around the circumference of the inner electrode tube and being secured thereto. It is preferred that there are not multiple turns of wire around the circumference of the outer electrode layer. In one embodiment the second section extends longitudinally along the length of the outer electrode layer. In one such embodiment the second section is secured to the outer electrode layer without the use of a spiral wire with multiple (three or more) turns extending along some, most or all of the length of the outer electrode layer. It may be that the second section is secured to the outer electrode layer only using tie wires or sealant or combinations thereof.

In one particular embodiment, the interconnect wire may be a wire having two or more strands. In the first section of the interconnect wire the strands may be temporarily or permanently secured together, e.g. by being twisted together or adhered together. In the second section of the interconnect wire one or more strands form the first branch and one or more strands form the second branch.

It may be that the second section of the wire extends along a portion of the outer electrode layer in a direction that is substantially linear. Each tubular fuel cell will have a longitudinal axis. It may be that the second section of the wire extends along a portion of the outer electrode layer in a direction substantially parallel to said longitudinal axis. Where the second section comprises two branches, it may be that each branch extends along a portion of the outer electrode layer in a direction substantially parallel to said longitudinal axis.

Detailed description of the invention

The solid oxide fuel cell stack of the invention comprises two or more solid oxide fuel cells. It may comprise only two solid oxide fuel cells, or it may comprise more than two solid oxide fuel cells. There is no upper limit on the number of cells that can be joined in a stack and clearly this will depend of the intended application for the fuel cell stack. For example, there could be from two to 1000 fuel cells or more, or from two to 200 fuel cells, or from 2 to 50 fuel cells, e.g. from 3 to 20 fuel cells.

Each solid oxide fuel cell is electrically connected to an adjacent solid oxide fuel cell, said connected fuel cells forming a pair. However, although each solid oxide fuel cell is connected to an adj acent solid oxide fuel cell, forming a "pair", this does not limit the invention to even numbers of fuel cells in the stack. It will be appreciated by the skilled person that any given fuel cell can be in a "pair" with two different fuel cells. Thus, for example, in a stack of three fuel cells the first and second fuel cells form a "pair", and the second and third fuel cells form a "pair". It will in fact be the case that all but the first and last fuel cells in the stack will be in a "pair" with two fuel cells, one on each side . It is only the first fuel cell and the last fuel cell that are only part of one "pair".

Each solid oxide fuel cell is a tubular fuel cell comprising an inner electrode tube, an electrolyte layer coaxially surrounding the inner electrode, and an outer electrode layer coaxially surrounding the electrolyte layer, wherein one out of the inner electrode tube and the outer electrode layer is an anode and the other is a cathode. It is preferred that the inner electrode tube is an anode and the outer electrode layer is a cathode .

In each pair of solid oxide fuel cells at least one of those fuel cells includes an access portion, said portion comprising a part of the tubular fuel cell where the inner electrode tube is not covered by the electrolyte layer and the outer electrode layer, such that an electrical contact can be made with the inner electrode tube at this portion. It will be appreciated that in a stack of fuel cells according to the invention it is possible for at least the first or at least the last fuel cell to not include an access portion. It may be that all but one of the fuel cells in the stack include an access portion. However, in another embodiment all of the fuel cells in the stack include an access portion.

The access portion will involve a gap in the electrolyte layer and a gap in the outer electrode layer. These gaps may be correspondingly sized, or they may be different sizes. As long as the gaps are aligned sufficiently that an electrical contact can be made with the inner electrode tube any configuration can be utilized.

Preferably electrical contact can be made with the inner electrode tube around most or all of its circumference.

In one embodiment, the gap in the electrolyte layer extends for a distance A along the length of the tubular fuel cell and the gap in the outer electrode layer extends for a distance B along the length of the tubular fuel cell. It may be that A is less than or equal to B .

In one embodiment, the gap in the electrolyte layer extends for a distance A along the length of the tubular fuel cell, wherein A is from 0. 1 to 15% of the length of the fuel cell, preferably from 0.5 to 10%, more preferably from 1 to 5%, e.g. from 1 to 3%.

In one embodiment, the gap in the outer electrode layer extends for a distance B along the length of the tubular fuel cell, wherein B is from 0. 1 to 15% of the length of the fuel cell, preferably from 0.5 to 10%, more preferably from 1 to 5%, e.g. from 1 to 3%. In one embodiment the access portion is located substantially centrally along the length of the tubular fuel cell. For example, it may be at a position that is a distance from the distal end of the tubular fuel cell that is from 40 to 60% of the total length of the tubular fuel cell, e.g. from 42 to 58% of the total length of the tubular fuel cell or from 43 to 57% of the total length of the tubular fuel cell or from 45 to 55% of the total length of the tubular fuel cell.

In each pair of solid oxide fuel cells the electrical connection is from the access portion of one tubular fuel cell to the outer electrode layer of the other tubular fuel cell. Said electrical connection is made via an interconnect wire.

Preferably each tubular fuel cell is in electrical contact with one or two such interconnect wires. Specifically, it is preferred that each tubular fuel cell either (a) is in electrical contact with only one such interconnect wire, with said interconnect wire contacting the access portion of said tubular fuel cell; or (b) is in electrical contact with only one such interconnect wire, with said interconnect wire contacting the outer electrode layer of said tubular fuel cell (optionally with said interconnect wire having multiple branches, each of which may contact the outer electrode layer of said tubular fuel cell); or (c) is in electrical contact with two such interconnect wires, with one of said interconnect wires contacting the access portion of said tubular fuel cell and with the other of said interconnect wires contacting the outer electrode layer of said tubular fuel cell (optionally with said interconnect wire having multiple branches, each of which may contact the outer electrode layer of said tubular fuel cell) . The interconnect wire comprises a first section that electrically contacts the inner electrode tube of one tubular fuel cell at the access portion thereof, said first section extending around the circumference of the inner electrode tube and being secured thereto. In one embodiment, the first section makes electrical contact with the inner electrode tube around most or all of its circumference (e.g. 60% or more, or 70% or more, or 80% or more, or 90% or more, or 95% or more, or 99% or more, of the circumference) . Preferably there is electrical contact with the inner electrode tube around all of its circumference . However, it will be appreciated that if there are small sections of the circumference where electrical contact is not made this will not unduly affect the effectiveness of the solid oxide fuel cell stack.

The first section is suitably wound round the circumference of the inner electrode tube to achieve the electrical contact. In one preferred embodiment, the first section is, however, not solely attached by winding.

For example, the first section may be secured to the inner electrode tube, at least in part, by a conductive sealant. In particular, the first section may be secured to the inner electrode tube by winding in combination with the use of a conductive sealant.

In one embodiment the sealant is a used is a metallic ink or paint, such as a silver ink or paint. An example of a suitable ink is the Silver Paste DAD 87 available from Shanghai Research Institute of Synthetic Resins.

In general, conductive sealants known in the art are noble metals or noble metal alloys. These may be in the form of inks or paints or pastes. These may include metals selected from the group consisting of: ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and gold.

A benefit of using such a sealant is that it can serve to reduce or prevent fuel leakage from inside the fuel cell. In one embodiment the sealant covers most or all of the access portion. Another benefit of using such a sealant is that it improves the electrical contact between the first section and the inner electrode tube .

In one embodiment the conductive sealant is applied to the access portion before the first section is wound round the circumference of the inner electrode tube. For example, the conductive sealant may be applied to the inner electrode tube on some, most or all of the area of its surface that is exposed by the access portion. The conductive sealant may be applied by being painted on, or sprayed on, or by any other suitable means, e.g. screen printing. It may be that the conductive sealant is heated after it is applied. Suitably it may be heated to sinter to full density.

Metallic ink materials that can be soldered are known in the art. These include copper-containing solders, titanium-containing solders, as well as silver pastes and the like . When a silver-based solder is used this can be used with or without added elemental copper, e.g. copper oxide. When a silver-based solder is used this can be used with or without added titanium, to improve wetting. Therefore the skilled person would be able to obtain a suitable metallic ink for use as conductive sealant, and apply it and heat it as required, using his common general knowledge .

One or more further sealant layers may optionally be applied on top of the conducting sealant. Thus there may be one layer of sealant, or there may be two or more layers of sealant. Any such further sealant layers may cover some, most or all of the conducting sealant. Each further sealant layer may be the same or different. Any further sealant layers may be conductive or they may be non-conductive. The wire also comprises a second section that electrically contacts the outer electrode layer of the other tubular fuel cell, said second section extending along a portion of the outer electrode layer and being secured thereto.

It may be that the second section of the wire extends along a portion of the outer electrode layer in a direction that is substantially linear. Thus in one embodiment the second section of the wire is not wrapped around the circumference of the tubular fuel cell.

The second section may be secured to the outer electrode layer by any suitable means. It may be that the second section is secured to the outer electrode layer by one or more tie wires.

The tie wires may be formed from any suitable material. Preferably the tie wires are metallic. Preferably the material is a high temperature resistant metal, e.g. a metal that can withstand being heated to 700 degrees C. In one embodiment, the tie wires are formed from a metal or an alloy. It may, for example, be an iron-based alloy, such as steel, e.g. stainless steel, carbon steel or mild steel. Stainless steel is generally preferred but other high temperature resistant alloys could certainly be contemplated.

As described above, prior art silver windings are both heavy and uneconomical. Also silver tie wires may expand more than the SOFC and may soften considerably at the operating temperature.

Therefore a benefit of using tie wires formed from high temperature resistant alloys (e.g. stainless steel) is that this provides an efficient and cheaper way of inhibiting any movement of the second section of the interconnect wire extending along the outer electrode layer.

In one embodiment, therefore, the tie wires are not formed from silver.

The second section may be secured to the outer electrode layer solely by the use of ties to achieve the electrical contact. In one preferred embodiment, the second section is, however, not solely attached by the use of ties. For example, the second section may be further secured to the outer electrode layer by a conductive sealant. The conductive sealant may be as discussed above . In one embodiment the sealant is a used is a metallic ink, such as a silver ink. Preferably, the second section of the interconnect wire comprises two branches, each of which electrically contacts the outer electrode layer of the tubular fuel cell, with each branch extending along a portion of the outer electrode layer and being secured thereto. This provides an improved attachment and electrical connection. In one embodiment, the branches extend in opposite directions. For example, one branch may contact the outer electrode layer of the other tubular fuel cell and extend towards the proximal end of the tubular fuel cell and the other branch may contact the outer electrode layer of the other tubular fuel cell and extend towards the distal end of the tubular fuel cell. Each tubular fuel cell will have a longitudinal axis. It may be that the second section of the wire extends along a portion of the outer electrode layer in a direction substantially parallel to said longitudinal axis. Where the second section comprises two branches, it may be that each branch extends along a portion of the outer electrode layer in a direction substantially parallel to said longitudinal axis.

In one preferred embodiment, the interconnect wire may be a wire having two or more strands. For example, it may have from two to 50 strands, or more; such as from two to 40 strands or from two to 30 strands. It may have from four to 20 strands, e .g. from six to 18 strands. In one preferred embodiment it has from 8 to 16 strands, e.g. 10, 12, 14 or 16 strands. . For example, in certain embodiments, the wire has 8, 10, 12 or 14 strands. Preferably the wire has 8 strands.

Whilst it is preferred that the wire has an even number of strands, this is not essential.

In general, the use of a multi strand wire is advantageous as compared to a single strand wire of equivalent thickness. It is easier to handle and more flexible .

In the first section of the interconnect wire the strands may be temporarily or permanently secured together, e.g. by being twisted together or adhered together.

Twisting the strands together can have the benefit of improving contact. The multi strand twisted wire is also easier to handle. It is also more reliable . In one embodiment, the interconnect wire may be a wire having two or more strands twisted together, e.g. from two to 50 strands twisted together; such as from two to 40 strands or from two to 30 strands twisted together. It may have from four to 20 strands, e.g. from six to 18 strands twisted together. In one preferred embodiment it has from 8 to 16 strands, e.g. 10, 12, 14 or 16 strands, twisted together. For example, in certain embodiments, the wire has 8, 10, 12 or 14 strands twisted together.

In the second section of the interconnect wire one or more strands form the first branch and one or more strands form the second branch. It may be that the number of strands in the first branch is the same as the number of strands in the second branch, but this is not essential. This must of course be the case if the wire has an odd number of strands. However, even when the wire has an even number of strands these do not need to be split evenly between the two branches. It is, however, preferred that the two branches have the same number of strands or the number of strands in the first branch is only one more or less than the number of strands in the second branch.

In one embodiment the interconnect wire has an overall diameter of 5mm or less, such as from 0.5 to 5mm.

In one embodiment the interconnect wire has a plurality of strands, wherein each strand has diameter of 0.6 mm or less, e.g. from 0.05 to 0.6mm. For example, in certain embodiments, the diameter of each strand is less than 0.5 mm, such as less than 0.4 mm. It may be that strand diameter is between 0. 1 mm and 0.6 mm, or between 0.2 mm and 0.5mm, e.g. from 0.3 to 0.4mm. The interconnect wire may be formed from any suitable material. Clearly it is preferred that the material has sufficient long-term oxidation resistance at the SOFC operating temperatures. Equally, it is preferred that the material has good electron conductance . Preferably the wire is metallic. In one embodiment, the wire is formed from a material selected from the group consisting of Ag, Au, Pt, Ni, Co, W, Ti, Cu, Pd, Mn, Mo, Rh, Ir and Si, or an alloy based on combinations of two or more of these metals.

In one embodiment the wire is made from an alloy. The alloy may, for example, be an alloy containing Fe, Ni or Cr or the like. The skilled person would be able to identify alloys based on such metals that are suitable . In particular alloys based on such metals and that include minor additions of other elements to promote conductive oxidised surface layers could be contemplated. In one preferred embodiment the wire is made from a silver-based alloy or from an iron-based alloy, e .g. stainless steel. Stainless steel 440 and stainless steel 441 may be mentioned as examples of suitable alloys. In one preferred embodiment the wire is made of a material selected from silver (Ag), gold (Au), platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir) and alloys based on combinations of two or more of these metals. In a preferred embodiment the wire is made from silver or a silver alloy.

In a preferred embodiment, the interconnect wire is a silver wire that has from four to twenty strands, e.g. from six to eighteen strands, with each strand having a diameter of from 0. 1 to 0.5 mm.

The electrolyte of the solid oxide fuel cell may be made from any suitable material for a solid oxide fuel cell electrolyte . It may, for example, be a ceramic selected from yttria stabilized zirconia (YSZ) (e.g. the 8% form, Y8SZ), scandia stabilized zirconia (ScSZ) (e .g . the 9 mol% form of Sc₂0₃, 9ScSZ) and gadolinium doped ceria (GDC). In one embodiment it is a yttria stabilized zirconia.

The anode of the solid oxide fuel cell may be made from any suitable material for a solid oxide fuel cell anode. It may, for example, be nickel or a cermet (ceramic-metal composite) made up of nickel mixed with ceramic, such as the ceramic material that is used for the electrolyte in that particular cell, e.g. YSZ. In one embodiment the anode material is porous nickel.

The cathode of the solid oxide fuel cell may be made from any suitable material for a solid oxide fuel cell cathode. It may, for example be a lanthanum strontium manganite (LSM), a composite of LSM and YSZ, or the perovskite strontium-cobalt-lanthanum- ferrite (LSCF). In one embodiment the cathode material is LSCF.

Each tubular fuel cell may have any suitable dimensions. In one embodiment, each tube has a length of from 0. 1 mm to 500 mm, such as from 1 mm to 250mm or from 2 mm to 100m. It may be that each tube has a diameter of from 0. 1 mm to 100 mm, such as from 1mm to 50mm or from 3 mm to 10mm.

Description of the drawings The invention will now be further described, by means of example only, with reference to the drawings in which:

Figure 1 is a cross-sectional view of a tubular SOFC provided with an interconnect wire.

Figure 2 shows an SOFC stack according to the invention, comprising two tubular SOFCs side-by-side connected by an interconnect wire.

Figure 3 shows an SOFC stack according to the invention, comprising three tubular SOFCs side-by-side, with each SOFC being connected to the adjacent SOFC by an interconnect wire.

Figure 1 shows a solid oxide fuel cell that has a hollow inner electrode tube ( 1), an electrolyte layer (2) on the outside of the electrode tube, and an outer electrode layer (3) coaxially surrounding the outer surface of the electrolyte layer (2).

The fuel cell has a central access portion (4) involving a gap in the electrolyte layer (2) extending for a distance A along the length of the tubular cell and exposing the underlying inner electrode tube ( 1 ). The outer electrode layer (3) has also a gap extending for a distance B along the length of the tubular fuel cell and located such that the underlying inner electrode tube ( 1) can be contacted. This therefore provides an access portion (4) where the inner electrode tube ( 1 ) is not covered by the electrolyte layer and the outer cathode layer, such that an electrical contact can be made with the inner electrode tube at this portion. The wire has a first section (5) that is wrapped around the central access portion (4) of the fuel cell to make electrical contact with the conducting outer surface of the inner electrode tube ( 1). The wire has a second section (6) in which the wire splits into two branches (6a, 6b). In Figure 2 two such SOFC tubes are electrically connected by the interconnect wire to form a stack. The first section (5) of the wire is wrapped around the inner electrode tube ( 1) to make electrical contact with the conducting outer surface of the inner electrode tube of the first SOFC tube (I). The second section (6) of the wire electrically contacts the outer electrode layer of the second SOFC tube (II) . It can be seen that the two branches (6a, 6b) of the second section extend along the surface of the tube in opposite directions and are secured thereto with tie wires (7).

As can be seen in Figure 3, it is also possible to connect more than two SOFC tubes in series. In Figure 3 three fuel cells are connected in series.

The three fuel cells can be seen as forming two "pairs".

A first pair is made up of the first SOFC and the second SOFC. The first section (5) of the wire is wrapped around the inner electrode tube ( 1 ) of the first SOFC (I) to make electrical contact with the conducting outer surface of the inner electrode tube of the first SOFC tube (I). The second section (6) of the wire electrically contacts the outer electrode layer (3) of the second SOFC tube (II). It can be seen that the two branches (6a, 6b) of the second section extend along the surface of the tube in opposite directions and are secured thereto with tie wires (7).

A second pair is made up of the second SOFC (II) and the third SOFC (III). The first section (5) of the wire is wrapped around the anode tube of the second SOFC (II) to make electrical contact with the conducting outer surface of the inner electrode tube ( 1) of the second SOFC tube (II) . The second section (6) of the wire electrically contacts the outer electrode layer (3) of the third SOFC tube (III). It can be seen that the two branches (6a, 6b) of the second section extend along the surface of the tube in opposite directions and are secured thereto with tie wires (7). The invention will now be further described, in a non-limiting manner, by reference to the following examples.

Examples Example 1 :

Cell tubes from Ultra Electronics AMI were obtained. The 6.8mm diameter tubes have a nickel cermet tubular support coated with a dense yttria stabilized zirconia (YSZ) electrolyte layer Ι Ομιη thick. 100mm of this tube was covered with a 40μιη layer of strontium-cobalt-lanthanum- ferrite (LSCF) cathode.

In the central part of the tube, a 5mm length of the LSCF layer was scraped away, forming a central ring around the tube. This left a gap in the centre of the cathode revealing the YSZ layer. Then in the middle of this gap, a further 1 mm length of the YSZ was also ground down, to reveal the conducting anode layer underneath the YSZ layer. A silver wire made of 8 strands, each strand having a diameter of 0.35 mm, was wrapped around the gap to make good electrical contact between the wire and the anode layer. The surface was sealed with a silver ink (Silver Paste DAD 87, obtained from Shanghai Research Institute of Synthetic Resins). The silver wire stretching away from the tube was split into two branches having 4 strands each. These branches were laid on the cathode surfaces of the neighbouring tube. The wire was secured to the tube using three stainless steel tie wires on each side. The silver and tie wires were coated with porous silver ink (SPI Supplies SPI 5002- AB silver ink, which is a highly concentrated suspension (43 % ±3 % silver solids) of silver powder combined with an organic suspending and binder system, obtained from Aztek Trading) to make good conducting contact with the cathode layers. Testing this arrangement in a 4-point test, an I-V curve was plotted at 700 degrees C with 150ml/min of hydrogen passing inside the tube and air on the outside. This standard test is well-known and is described in, for example, 'High-Temperature Solid Oxide Fuel Cells - Fundamentals, Design and Applications' by K Kendall et al, published by Elsevier Science, Oxford, 2003.

The power output per 100mm long cell at 0.7V was found to be 9W with high fuel utilisation (70%).

Thus the SOFC stack according to the invention has an efficient connection between fuel cells, leading to the SOFC stack being more powerful. The SOFC stack according to the invention also utilises a relatively low amount of interconnect material, making the SOFC stack less expensive and lighter.

Claims

1. A solid oxide fuel cell stack comprising two or more solid oxide fuel cells, wherein each solid oxide fuel cell is a tubular fuel cell, comprising an inner electrode tube, an electrolyte layer coaxially surrounding the inner electrode, and an outer electrode layer coaxially surrounding the electrolyte layer, wherein one of the inner electrode tube and the outer electrode layer is an anode and the other is a cathode,

wherein each solid oxide fuel cell is electrically connected to an adjacent solid oxide fuel cell, said connected fuel cells forming a pair,

wherein in each pair of solid oxide fuel cells at least one of the fuel cells includes an access portion, said portion comprising a part of the tubular fuel cell where the inner electrode tube is not covered by the electrolyte layer and the outer electrode layer, such that an electrical contact can be made with the inner electrode tube at this portion,

and wherein in each pair of solid oxide fuel cells the electrical connection is from the access portion of one tubular fuel cell to the outer electrode layer of the other tubular fuel cell,

and wherein said electrical connection is made via an interconnect wire, the wire comprising a first section that electrically contacts the inner electrode tube of one tubular fuel cell at the access portion thereof, said first section extending around the circumference of the inner electrode tube and being secured thereto, and the wire comprising a second section that electrically contacts the outer electrode layer of the other tubular fuel cell, said second section extending along a portion of the outer electrode layer and being secured thereto.

2. The solid oxide fuel cell stack of claim 1 , wherein the access portion is located substantially centrally along the length of the tubular fuel cell.

3. The solid oxide fuel cell stack of claim 2, wherein each tubular fuel cell has a length that extends from a distal end to a proximal end, and wherein the access portion is located at a longitudinal position along the length of the tube that is a distance from the distal end of the tubular fuel cell that is from 40 to 60% of the total length of the tubular fuel cell.

4. The solid oxide fuel cell stack of any one of the preceding claims, wherein the second section of the interconnect wire comprises two branches, each of which electrically contacts the outer electrode layer of the tubular fuel cell, with each branch extending along a portion of the outer electrode layer and being secured thereto.

5. The solid oxide fuel cell stack of claim 4, wherein the branches extend in opposite directions.

6. The solid oxide fuel cell stack of claim 5, wherein each tubular fuel cell has a length that extends from a distal end to a proximal end, and wherein one branch extends towards the distal end of the tubular fuel cell and one branch extends towards the proximal end of the tubular fuel cell.

7. The solid oxide fuel cell stack of any one of the preceding claims, wherein each solid oxide fuel cell has a length that extends from a distal end to a proximal end, and wherein the access portion serves to divide the outer electrode layer into two portions, one of which extends from the access portion towards the distal end and one of which extends from the access portion towards the proximal end.

8. The solid oxide fuel cell stack of any one of the preceding claims, wherein each solid oxide fuel cell has a length that extends from a distal end to a proximal end, and wherein the access portion serves to divide the electrolyte layer into two portions, one of which extends from the access portion towards the distal end and one of which extends from the access portion towards the proximal end.

9. The solid oxide fuel cell stack of any one of the preceding claims, wherein the interconnect wire comprises a first section that directly electrically contacts the inner electrode tube of one tubular fuel cell at the access portion thereof, said first section extending around the circumference of the inner electrode tube and being secured thereto.

10. The solid oxide fuel cell stack of any one of the preceding claims, wherein the interconnect wire comprises a second section that directly electrically contacts the outer electrode layer of the other tubular fuel cell, said second section extending along a portion of the outer electrode layer and being secured thereto.

1 1. The solid oxide fuel cell stack of any one of the preceding claims, wherein there are no wires wrapped around the length of the outer electrode layer of the solid oxide fuel cell in a spiral fashion.

12. The solid oxide fuel cell stack of any one of the preceding claims, wherein the inner electrode tube is an anode and the outer electrode layer is a cathode .

13. The solid oxide fuel cell stack of any one of the preceding claims, wherein the interconnect wire is a wire having two or more strands.

14. The solid oxide fuel cell stack of claim 13, wherein in the first section of the interconnect wire the strands are temporarily or permanently secured together.

15. The solid oxide fuel cell stack of claim 13 or claim 14 wherein in the second section of the interconnect wire one or more strands form the first branch and one or more strands form the second branch.

16. The solid oxide fuel cell stack of any one of claims 13 to 15, wherein the interconnect wire is a wire having four or more strands.

17. The solid oxide fuel cell stack of claim 16, wherein the interconnect wire is a wire having from six to thirty strands.

18. The solid oxide fuel cell stack of any one of the preceding claims, wherein the second section of the wire extends along a portion of the outer electrode layer in a direction that is substantially linear.

19. The solid oxide fuel cell stack of any one of the preceding claims, wherein the first section is secured to the inner electrode tube by a conductive sealant.

20. The solid oxide fuel cell stack of any one of the preceding claims, wherein the second section is secured to the outer electrode layer by one or more tie wires.

21. The solid oxide fuel cell stack of claim 20, wherein the second section is further secured to the outer electrode layer by a conductive sealant.

22. The solid oxide fuel cell stack of any one of the preceding claims, wherein the interconnect wire is metallic.

23. The solid oxide fuel cell stack of claim 22, wherein the interconnect wire is formed from Ag, Au, Pt, Pd, Rh, Ir and alloys based on combinations of two or more of these metals; or is formed from an alloy based on one or more of Fe, Ni and Cr.