GB2352083A

GB2352083A - Solid oxide fuel cell stacks and manufacturing methods

Info

Publication number: GB2352083A
Application number: GB0012998A
Authority: GB
Inventors: Ashok C Khandkar; Singaravelu Elangovan; Joseph Jay Hartvigsen
Original assignee: Sofco LP
Current assignee: Sofco LP
Priority date: 1999-07-14
Filing date: 2000-05-26
Publication date: 2001-01-17
Also published as: GB0012998D0; DE10033944A1; CA2298967A1; AU4272000A; JP2001035506A

Abstract

A solid oxide fuel cell stack 10 comprises a first solid oxide fuel cell 12, a second solid oxide fuel cell 14, an interconnect 16, and a joining member 18. The first solid oxide fuel cell 12 includes a cathode 32. The second solid oxide fuel cell 14 includes an anode 36. The interconnect 16 is positioned between the cathode 32 of the first solid oxide fuel cell 12 and the anode 36 of the second solid oxide fuel cell 14. The joining member 18 joins the interconnect 16 to the cathode 32 of the first solid oxide fuel cell 12 and/or the anode 36 of the second solid oxide fuel cell 14, wherein the joining member 18 comprises a porous substrate 42, 43. The porous substrate may be formed by impregnating reticulated foam with ceramic slurry and firing. It may have a conductive coating of doped lanthanum cobaltite, lanthanum manganite, praseodymium cobaltite or manganite or other doped conductive oxides or metals.

Description

2352083 SOLID OXIDE FUEL CELL STACKS AND MANUFACTURING METHODS The

invention relates to solid oxide fuel cells (SOFC"s), and more partiCLilartv, to solid oxide fuel cell stacks, methods of manufacturinor solid oxide fuel cell stacks, and methods of manufacturing porous substrates for Joining solid oxide fuel cells to interconnects.

Solid oxide fuel cells have the potential for high fuel efficiency, lwv emissions and distributed generation options. However, due to large system capital costs and market economics, the advantages of solid oxide fuel cells have been difficult to achieve.

One aspect of solid oxide fuel cells which has received much focus has been the lowering of operating temperatures, while increasing stack performance. This, in turn, cuts both stack and balance of plant costs. One manner in which to reduce operating temperatures while obtaining high performance is to join the cells and stacks in a manner which produces low resistance contacts between adjacent cells, and low resistance flow paths for reactant gases - generally termed a solid oxide fuel cell stack.

Stacks are generally joined by using a "flow field" which serves multiple purposes.

The "flow field" provides passageways for the reactant gases, low resistance electrical conduction pathways and effective gas impermeable sealing at the stack perimeter so as to contain the gases. Generally, such a "flow field" is integral with the interconnect, wherein the cells, the interconnect and the flow fields are fabricated and joined together using ceramic bonding layers. With such a stack, it has been difficult to achieve conformity between layers.

Other solutions have involved the co-firing of unfired interconnects, cells and flow fields after joining. For example, Argonne National Labs discloses the use of dense corrugated electrolyte members of zircoma serving as both a flow field and electrolyte. In one embodiment, such members comprise doped lanthanum chromite which is co-fired with the cell. While these methods have demonstrated some success, difficulties have been incurred when such a solution is applied to an entire stack.

The difficulties with producing stacks in this manner are due to the 3 fundamentally conflicting set of microstructural and processing requirements for each ZD of the three layers. More specifically, attempts to co-fire these materials result in 2 either inadequate microstructural development or extensive migration of chemical species amongst adjacent layers, rendering the performance of the stack to be poor.

Even attempts to utilize liquid phase dopants further exacerbate the interdiffusion problems and cause poor performance of the stack.

The invention provides a solid oxide fuel cell stack comprising:

a first solid oxide fuel cell having a cathode; a second solid oxide fuel cell having an anode, an interconnect positioned between the cathode of the first solid oxide fuel cell and the anode of the second solid oxide fuel cell; and means for joining the interconnect to at least one of the cathode of the first solid oxide fuel cell and the anode of the second solid oxide fuel cell, wherein the joining means comprises a porous substrate.

In a preferred embodiment, the orous substrate of the 'onin(y means has a p J 1 0 porosity of 20-80%. In another preferred embodiment, the porous substrate of the joining means has a pore size substantially between 100 and 1000 Irn.

In another preferred embodiment, the solid oxide fuel cell stack further includes means for sealing at least one edge of the porous substrate between the interconnect and one of the cathode of the first solid oxide fuel cell and the anode of the second solid oxide fuel cell. In one such embodiment. the sealinc, means comprises a gas impermeable yttria stabilized zirconia.

In yet another preferred embodiment, the joining means further includes a conductive coating. In one such embodiment, the conductive coating has a thickness of at least 10 4m and preferably in the range of approximately 5 4m to approximately 4m. Preferably, the conductive coating comprises any one of doped lanthanum cobaltite, lanthanurn manganite, praseodymium cobaltite or manganite and/or other doped conductive oxides or metals.

In another preferred embodiment, the joining means includes at least one groove, and, in certain embodiments, the joining means may include two or more grooves, each having at least a portion distally spaced apart. In any such embodiment, the grooves have a depth of at least 500 Lrn. and preferably in the range of approximately 250 tm to approximately 1000 4m.

The invention further provides a method of manufacturing a porous substrate forjoining a solid oxide fuel cell to an interconnect, the method comprising the steps of:

- providing a flow field form; - impregnating the flow field form with an impregnate; - firing the impregnated flow field form, and - volatilizing the flow field form, to in turn form a porous substrate.

In a preferred embodiment, the step of impregnating the flow field comprises the step of introducing the impregnate to the flow field form. Once introduced. the excess impregnate is expelled from the flow field form, The foregoing steps are repeated until the flow field form is impregnated as desired.

In another preferred embodiment, the method further includes the step of impregnating at least one lower surface and an upper surface of the now field form with a conductive coating.

In yet another preferred embodiment, the method further includes the step of introducing grooves into at least one of a lower surface and an upper surface of the flow field form.

In a preferred embodiment, the flow field form comprises an open celled reticulated foam. Preferably, the open celled reticulated foam is selected from one of the group consisting of polyurethanes, polyesters, polyvinyl chlorides, acetates and other copolymers.

In another preferred embodiment, the step of volatilizing the flow field form comprises the step of substantially precluding the formation of carbonaceous residue.

In yet another preferred embodiment, the impregnate may comprise a thixotropic slurry having a ceramic component. Preferably, the impregnate comprises a viscosity of at least 1000 centipoise, and more preferably in the range of 1500-3)000 centipoise. In addition, in any such embodiment, the impregnate may include at least one rheological agent, and, the rheological agent is selected from one of the group consisting of carboxylmethyl cellulose and hydroxymethyl cellulose. In such an 3' 0 embodiment, the rheological agent comprises approximately 0.01% to approximately 10% of the weight of the impregnate.

4 In another preferred embodiment, the impregnate includes at least one binder. Preferably. the binder is selected from one of the group consisting of polyvinyl butyrol and polyvinyl acetate. In an embodiment which includes a binder. the binder comprises approximately 0.01% to approximately 10% of the weight of the joining material.

The invention further provides a method of manufacturing a solid oxide fuel cell stack, the method comprising the steps ofproviding at least two fired solid oxide fuel cells, the solid oxide fuel cells each havina an anode, a cathode and an electrolyte; 0 associating an interconnect with the cathode of one of the at least two fired solid oxide fuel cells and with the anode of the other of the at least two fired solid oxide fuel cells; providing a flow field form having an impregnate, associating the flow field form between and into contact with the interconnect and the cathode or the anode of the at least two fired solid oxide fuel cells; firing the assembled stack of at least two fired solid oxide fuel cells, the interconnect and the flow field form; and volatilizing the flow field form, to, in turn, render a fired solid oxide fuel cell stack.

In a preferred embodiment, the step of associating the flow field form comprises the step of applying the impregnate to one of the interconnect and the respective anode or cathode. Once applied, the flow field form is positioned into contact with each of the interconnect and the respective anode or cathode.

In a preferred embodiment, the step of providing a flow field form comprises the step of impregnating the flow field form with an impregnate. In another preferred embodiment, the step of providing a flow field form comprises the step of impregnating at least one of a lower surface and an upper surface with a conductive coating. In yet another preferred embodiment, the step of providing a flow field form comprises the step of introducing at least one groove in at least one of a lower surface and an upper surface of a conductive coating.

In another preferred embodiment, the step of volatilizing the flow field form occurs at a temperature lower than the temperature required for the firing of the stack.

The invention still further provides a method of manufacturing a solid oxide fuel cell stack, the method comprising the steps of providing at least two solid oxide fuel cells, the solid oxide fuel cells each having an anode, a cathode and an electrolyte, at least one of the at least two solid oxide fuel cells beina unfired, associating an interconnect with the cathode of one of the at least two solid oxide fuel cells and with the anode of the other of the at least two solid oxide fuel cells; providing a flow field form having an impregnate associating the flow field form between and into contact with the interconnect and the cathode or the anode of the at least two solid oxide fuel cells; co-firing the assembled stack of at least two solid oxide fuel cells, the interconnect and the flow field form; and volatilizing the flow field form, to, in turn, render a fired solid oxide fuel cell stack.

In a preferred embodiment, the step of providing at least two solid oxide fuel cells comprises the steps of tape casting an electrolyte and screen printing an anode and a cathode, to, in turn, form an unfired solid oxide fuel cell.

In another preferred embodiment, the step of providing an interconnect comprises the step of tape casting an interconnect, to, in turn, form an unfired interconnect.

In yet another preferred embodiment, the step of providing at least two solid oxide fuel cells comprises the step of providing at least two unfired solid oxide fuel cells. In another preferred embodiment, the step of providing an interconnect comprises the step of providing, a via filled interconnect.

Preferred embodiments of the invention provide improved methods and apparatus forjoining cells and interconnects.

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which:

6 Fila. I is a side elevational view of a solid oxide fuel cell stack embodying the present invention; Fla. 2 is a perspective view of the flow field form and the impregnate is an embodiment of the present invention; and Fig. 3 is a perspective view of another embodiment of the flow field form.

While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail. one specific embodiment with the understanding that the present disclosure can be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiment illustrated.

A solid oxide fuel cell stack 10 is shown in Fig. I as comprising a first solid oxide fuel cell 12, a second solid oxide fuel cell 14, an interconnect 16, means 18 for joining the interconnect 16 to the cells and means 20 for sealing between the interconnect 16 and the first and second solid oxide fuel cells 12, 14.

The first solid oxide fuel cell 12 includes an anode 30, a cathode 32 and an electrolyte 34. Similarly the second solid oxide fuel cell 14 includes an anode 36, a cathode 38 and an electrolyte 40. The solid oxide fuel cells 12 and 14 may comprise conventional solid oxide fuel cells of various desians, havincy various voltage outputs and operational temperatures.

The interconnect 16 comprises an interconnect first surface 48, a second surface 50 and a thickness 52. The first and second surfaces 48, 50, respectively are substantially smooth and planar, although other surface configurations are likewise contemplated. Thus, the thickness 52 generally comprises a substantially uniform thickness which is generally 250-1000 microns. In addition, the interconnect 16 generally comprises conductive ceramic or metal foils, while other materials are likewise contemplated for use.

The means 18 for joining the interconnect 16 to the cells 12, 14 comprises a first porous substrate 42 positioned between the interconnect 16 and the cathode 32 of the first solid oxide fuel cell 12, and a second porous substrate 43 positioned between the interconnect 16 and the anode 36 of the second adjacent solid oxide fuel cell 14.

Each of the first and second porous substrates 42, 43 includes openings extending therethrough which facilitate the passage of air and fuel through the stack.

7 More specifically, the first porous substrate 42 generally comprises a material having a porosity between 20-80%, and preferably between 40-60%. Additionally, the pore size ranges from 100-1000 Lrn in size, and, more preferably 250-1000 Lrn. Of course other pore sizes and porosities are contemplated, as Iona as the porosity allows for suitable passage of gas flow with a low pressure drop.

Similarly, the second porous substrate 43 generally comprises a material having a porosity between 20-80% and more preferably 40-60%. Additionally, the pore size ranges from 100-1000 4m in size and preferably between 250 to 1000 Lrn.

Again, other pore sizes and porosities are likewise contemplated as long as a suitable passage of air can be passed through the substrate. As will be explained in more detail with respect to the method, the material comprises a ceramic material which is capable of conducting charge between the cells.

With either one or both of the porous substrates 42, 43, grooves 71 may be present. These grooves 71 serve to increase the porosity of the porous substrate. In addition, either one or both of the porous substrates 42, 43 may additionally include a conductive coating 47 applied to one or both of the lower and upper surfaces of the porous substrates 42, 43). As will be explained in more detail below, the conductive coating 47 is useful where a low pressure drop and high electron conductivity is desired.

The means 20 for sealing between the interconnect and the cell comprises edge seals 46, which include an yttria stabilized zirconia which is substantially gas impermeable. Of course, other edge seal materials are likewise contemplated for use, such as glasses and glass ceramics, metal foils or ceramic fibre constructs. The sealing means serves to contain and direct the flow of air on one side of the interconnect and gas on the opposite side of the interconnect, so that each is maintained alona, the desired path. It further serves to prevent the undesired comingling, of air and fuel.

In operation, as the air and fuel are directed through the Joining means on either side of the respective interconnect 16, they each proceed through the openings and pores of the joining means. Thus, while providing good joining characteristics for joining the interconnect 16 to the respective cell 12, 14, the joining means nevertheless provide passageways for the required fuel and air for reaction within the SOFC.

8 To manufacture the SOFC stack 10, the first cell 12 and the second cell 14 are provided. In this embodiment, the provided cells have already been fired and are provided in a completed (fired) condition.

Next, an interconnect 16 and the sealing, means 20 are provided- The interconnect 16 is preferably of a smooth surface configuration and of a uniform thickness. However, while such an interconnect 16 is generally cost effective. it is likewise contemplated that other interconnects, such as interconnects havincy various surface configurations and surface variations are likewise contemplated for use. In this embodiment, as with the cells, the interconnect 16 and the sealing means 20 are likewise in a fired condition.

To manufacture the joining means 18, as shown in Fla. a flow field form 60 having a desired dimension is provided along with an impregnate 62. Essentially, the flow field form 60 comprises an open cell foam member which has been cut and trimmed to the desired dimensions. For example, the foam member may be uniform with substantially planar surfaces and a substantially uniform thickness. Likewise. as shown in Flig. 3, the flow field form 60 may include grooves 71 which increase the porosity of the material. The grooves 71 may be of a multitude of different shapes and orientations. Such flow field forms are particularly useful when the porosity of the flow field form is too low, and an increase in porosity is desired.

The flow field form 60 may comprise a multitude of open celled reticulated foams made from polyurethanes, polyesters or the family of polyvinyl chlorides, acetates as well as other different copolymers. Of course, other materials, such as cellulosic materials, for example, may likewise be utilized. While not necessarily limited to such a construction, it is desirable that the flow field form 60 comprises a material that volatilizes or burns out at a temperature at or below that of the temperature at which the impregnate is fired. Further, it is desirable that the material volatilize in the form and leaves behind no carbonaceous residue. Of course, materials which leave certain levels of carbonaceous residue, as well as materials which volatilize or burn out at temperatures which are more elevated may likewise be utilized.

The impregnate 62 comprises a slurry which includes binders, rheological agents and ceramic joining material. The binders aid in the binding function and may 9 comprise any number of different materials, such as, for example, polyvinyl butyrol or polyvinyl acetate. The binder amounts vary between 0 and 10% of the weight of the Slurry; however, amounts outside of this range, i.e. areater than 10%. are likewise contemplated for use.

The rheological agents are used to make the slurry thixotropic. A thixotropic slurry is one which has a high resistance to flow under low shear rates and a low resistance to flow under high shear rates. Thus, a thixotropic slurry will have a viscositv such that it rapidly enters a void or empty space of the flow field form and coats the wall of the fon-n. Yet, once coated.. the slurry does not drain out from the form after impregnation is complete. While various materials are contemplated, the rehological agents may comprise carboxymethyl cellulose or hydroxymethyl cellulose.

While not limited hereto, the rheological agents may comprise between 0 and 10% of the weight of the slurry.

The ceramic Joining material, as will be understood, comprises the porous structure after firing. Thus the ceramic joining material that is utilized may comprise any one of a number of ceramic materials which are suitable for use in association with structures which are associated with interconnects between SOFC's in a stack of SOFC cells.

Once the impregnate 62 has been prepared, the form 60 is impregnated with the impregnate 62. Once initially impregnated, excess impregnate 62 is expelled from the flow field form 60. Subsequently, this procedure is repeated one or more times to ensure a uniform and complete impregnation/coating of the flow field form 60 with the impregnate 62.

In certain embodiments, it may be desirable to impregnate a conductive coating to the upper and lower surfaces of the flow field form 60 which will improve conductivity. An increase of impregnating, or "loading,", of conductive impregnate yields a graded microstructure optimized for a low pressure drop and low electrical resistance.

Once prepared, the stack 10 is assembled by associating the flow field form 60 with the cells 12, 14, the sealing means 20 and the interconnect 16. Specifically, and as shown in Fig. 1, the sequence of the assembly is as follows: the first cell 12 (anode, electrolyte, cathode), the impregnated flow field form 60 with associated sealing

M. the interconnect 16, the impregnated flow field form 60 with associated means t sealing means 20, and the second cell 14 (anode, electrolyte, cathode). Of course, it is contemplated that additional cells and interconnects may be Joined either before the first cell 12 or after the second cell 14 in a similar arrangement. In addition, it is likewise contemplated that the impregnated flow field form 60 may be utilized only between the interconnect 16 and one of the two cells, whereas the interface of the interconnect 16 and the other cell may comprise a conventional or otherwise different interface.

To specifically attach the cathode 32 of the first cell 12 to the impregnated flow field form 60, the surface of the cathode 32 is first coated with a similar cornposition as the impregnate 62, then the two are Joined. This promotes a uniform and effective contact to develop at the surface. However, in other embodiments, the impregnate 62 that is in the flow field form 60 may itself develop a uniform and effective contact with the surface of the cathode 32 without an additional coating beino applied to the surface of the cathode 32. In a similar manner, the surface of the interconnect 16 is coated with an impregnate (or in other embodiments not coated), then the interconnect 16 and the flow field form 60 are joined.

Either prior to attachment of the flow field form 60 to the interconnect 16 or at about the same time, the sealincy means 20 are positioned as desired so as to effectively seal the area between the interconnect 16 and the cathode 32, which in turn, contains the gas and fuel in the operating SOFC.

In a similar manner as with the flow field form 60 between the cathode 32 of the first cell 12 and the interconnect 16, the flow field form 60 that is positioned between the anode 36 of the second cell 14 and the interconnect 16 is assembled in a substantially identical manner. The sealing means 20 are likewise applied in a similar manner as the sealing means between the interconnect 16 and the cathode 32 of the first cell 12.

Additional cells and interconnects can be joined onto the free sides of the first and second cells 12, 14 so as to assemble a stack of any number of SOFC cells. As will be understood, any number of SOFC cells can be assembled so as to achieve a desired power output and a desired performance level.

I Once the stack assembly is complete, the entire assembly is placed in a furnace for heating. Upon heating, the flow field form 60 volatilizes and burns Out, and the ceramic joining material is fired. The result is a rigid, fired cerarnic material that takes ID the shape of the flow field form 60.

In another embodiment, the method of manufacture may comprise the use Of unfired (green) cells and an unfired interconnect. In such an embodiment, the cells comprise a tape cast electrolyte having screen printed anode and cathode on opposite s'des of the electrolyte. Of course, other assemblies Of Unfired (oreen) cells may be used, such as cells which do not comprise tape cast electrol-ytes having screen printed anode s/c athodes. Additionally, the interconnect of such an embodiment may comprise a tape cast interconnect, while other interconnect configurations are likewise contemplated for use. For example, the interconnect may comprise an Linfired (green) interconnect havin- a construction which includes via filled re,-Ilons. Such an interconnect is described in co-pending US patent application Serial No. 09/153,959 entitled "VIA FILLED INTERCONNECT FOR SOLID OXIDE FUEL CELLS", filed on September 16, 1998, and in corresponding EP-A-0 993) 059, to which reference is directed.

The sealing means of such an embodiment likewise comprises an unfired (green) sealing means. Of course, various materials are contemplated for use in the sealing means. As explained. the sealing means provides for gas and fuel separation and containment for the cell.

Once the cells and the interconnect are prepared, the flow field form is impregnated and the stack is assembled in a manner similar to that which is described above with respect to the first embodiment. Once fully assembled into a stack, the entire structure is co-fired. The co-firing of the stack, in turn, fires the cells, and voltalizes the flow field form so as to render a completed and usable stack of SOFC cells. In such an embodiment, due to adhesion properties of the flow field form, it is possible to assemble a stack of unfired cells, interconnects, and impregnated flow field

0 forms and then co-fire all three structures at once to yield a completed SOFC stack.

wings merely explain and illustrate the The foregoing description and dra i invention and the invention is not limited thereto, as those skilled in the art who have 12 the disclosure before them will be able to make modifications and variations therein without departing from the scope of the invention.

Claims

I A solid oxide fuel cell stack comprising:

a first solid oxide fuel cell having a cathode, a second solid oxide fuel cell having an anode; an interconnect positioned between the cathode of the first solid oxide fuel cell and the anode of the second solid oxide fuel cell-, and - means for joining the interconnect to at least one of the cathode of the first solid oxide fuel cell and the anode of the second solid oxide Cuel cell, 'Oierein the 'oininor means comprises a porous substrate.

J 4-1 A solid oxide fuel cell stack according to claim 1, wherein the porous substrate of the joining means has a porosity of 20-80%.

1). A solid oxide fuel cell stack according to claim I or claim 2, wherein the porous substrate of the joining means has a pore size substantially between 100 and 1000 4m.

4. A solid oxide fuel cell stack according to claim 1. claim 2 or claim '), including means for sealing at least one edge of the porous substrate between the interconnect Z:) and one of the cathode of the first solid oxide fuel cell and the anode of the second solid oxide fuel cell.

5. A solid oxide fuel cell stack according to claim 4, wherein the sealing means comprises a gas impermeable yttria stabilized zirconia.

6. A solid oxide fuel cell stack according to any one of the preceding claims, wherein the joining means further includes a conductive coating Z__ 7. A solid oxide fuel cell stack according to claim 6, wherein the conductive coating comprises doped lanthanum. cobaltite, lanthanum manganite, praseodymium cobaltite or manganite and/or other doped conductive oxides or metals.

14 8. A solid oxide fuel cell stack according to claim 6 or claim 7. wherein the conductive coatinc, has a thickness of at least 10 4m and preferably in the range of approximately 5 lim. to approximately 25 tm. 5 9. A solid oxide fuel cell stack according to any one of the preceding claims, wherein the Joining means includes at least one groove.

10. A solid oxide fuel cell stack according to claim 9. havino, two or more orooves 10 each having at least a portion distally spaced apart.

I 11. A solid oxide fuel cell stack according to claim 9 or claim 10, wherein the at least one groove has a depth of at least 500 lirn and preferably in the range of approximately 250 Lm to approximately 1000 Lrn. 15 12. A method of manufacturing a porous substrate for joining a solid oxide fuel cell to an interconnect, the method comprising the steps of:

- providing a flow field form; - impregnating the flow field form with an impregnate; - firing the impregnated flow field form; and - volatilizing the flow field form, to in turn form a porous substrate.

I I A method according to claim 12, wherein the flow field form comprises an open celled reticulated foam. 25 14. A method according to claim I'), wherein the open celled reticulated foam is selected from one of the group consisting of. polyurethanes, polyesters, polyvinyl chlorides, acetates and other copolymers.

15. A method according to claim 12, claim 13 or claim 4, wherein the step of volatilizing the flow field form comprises the step of substantially precluding the ZD formation of carbonaceous residue.

16. A method according to any one of claims 12 to 15, wherein the impregnate comprises a thixotropic slurry having a ceramic component.

17. A method according to claim 16, wherein the impregnate includes at least one rheological agent.

18. A method according to claim 17, wherein the rheological azent is selected from one of the group consisting of carboxylmethyl cellulose and hydroxymethyl cellulose.

19. A method according to claim 17 or claim 18, wherein the rheological agent comprises approximately 0.01% to approximately 10% of the weight of the 0 impregnate.

20. A method according to claim 17, claim 18 or claim 19, wherein the impreanate includes at least one binder.

21. A method according to claim 20, wherein the binder is selected from one of the group consisting of polyvinyl butyrol, and polyvinyl acetate.

22. A method according to claim 20 or claim 21, wherein the binder comprises approximately 0.01% to approximately 10% of the weight of the joining material.

Z 23. A method according to any one of claims 12 to 22, wherein the impregnate has C C a viscosity of at least 1000 centipoise, and preferably in the range of 1500-3000 centipoise.

24. A method according to any one of the claims 12 to 2':), wherein the step of impregnating the flow field form comprises the steps of (a) introducing the impregnate to the flow field form; (b) expelling the excess impregnate from the flow field form; and 16 (c) repeating steps (a) and (b) until the flow field form is impregnated as desired.

25. A method according to any one of claims 12 to 24, comprising the step of 5 impregnating at least one of a lower surface and an Lipper surface of the flow field form with a conductive coating. 26. A method according to any one of claims 12 to 25, comprising the step of t- - introducin2 -rooves into at least one of a lower surface and an tipper surface of the flow field form.

27. A method of manufacturing a solid oxide fuel cell stack, the method comprising the steps of.

providing at least two fired solid oxide fuel cells, the solid oxide fuel cells each havina an anode, a cathode and an electrolyte associating an interconnect with the cathode of one of the at least two fired solid oxide fuel cells and with the anode of the other of the at least two fired solid oxide fuel cells; providing a flow field form having an impregnate; associating the flow field form between and into contact with the interconnect and the cathode or the anode of the at least two fired solid oxide fuel cells; firing the assembled stack of at least two fired solid oxide fuel cells, the interconnect and the flow field form; and volatilizing the flow field form, to, in turn., render a fired solid oxide fuel cell stack.

28. A method according to claim 27, wherein the step of associating the flow field form comprises the steps of.

33 applying the impregnate to one of the interconnect and the respective 0 anode or cathode; and 17 positioning the flow field form into contact with each of the interconnect and the respective anode or cathode.

29. A method according to claim 27 or claim 28, wherein the step of providing a flow field form comprises the step of impregnating the flow field form with an impregnate.

i ing a 30. A method accordina to claim 27 or claim 28, wherein the step of providi, flow field form comprises the step of impregnating at least one of a lower surface and 10 an upper surface with a conductive coating 1. A method according to claim 27 or claim 28, wherein the step of providing a ID flow field form comprises the step of introducing at least one groove in at least one of a lower surface and an upper surface of a conductive coating.

32. A method according to any one of claims 27 to ')I, wherein the step of volatilizing the flow field form occurs at a temperature lower than the temperature required for the firing of the stack.

3. A method of manufacturing a solid oxide fuel cell stack, the method comprising the steps of providing at least two solid oxide fuel cells, the solid oxide fuel cells each having an anode, a cathode and an electrolyte, at least one of the at least two solid oxide fuel cells being unfired; associating an interconnect with the cathode of one of the at least two solid oxide fuel cells' and with the anode of the other of the at least two solid oxide fuel cells; providing a flow field form having an impregnate; associating the flow field form between and into contact with the interconnect and the cathode or the anode of the at least two solid oxide fuel cells; co-firing the assembled stack of at least two solid oxide fuel cells, the interconnect and the flow field form; and 18 volatilizing the flow field form, to, in turn, render a fired solid oxide fuel cell stack.

34. A method according to claim 33, wherein the step of providing at least two solid oxide fuel cells comprises the steps of tape casting an electrolyte; and screen printing an anode and a cathode, to, in turn, form an unfired solid oxide fuel cell.

') 5. A method according to claim 33) or claim 34, wherein the step of providing an interconnect comprises the step of tape casting an interconnect, to, in turn, form an unfired interconnect.

36. A method according to claim 33, claim 34 or claim 35, wherein the step of providing at least two solid oxide fuel cells comprises the step of providing at least two unfired solid oxide fuel cells.

37. A method according to any one of claims 33) to 36, wherein the step of providing an interconnect comprises the step of providing a via filled interconnect.

38. A solid oxide fuel cell stack substantially as herein described with reference to and as illustrated in the accompanying drawings.

39. A method of manufacturing a porous substrate for joining a solid oxide fuel cell to an interconnect, the method being substantially as herein described with reference to and as illustrated in the accompanying drawings.

40. A method of manufacturing a solid oxide fuel cell stack, the method being substantially as herein described with reference to and as illustrated in the 0 accompanying drawings. '