WO2002037589A2

WO2002037589A2 - Solid oxide fuel cell stack

Info

Publication number: WO2002037589A2
Application number: PCT/US2001/048417
Authority: WO
Inventors: Thomas J. George; G. B. Kirby Meacham
Original assignee: Michael A. Cobb & Company
Priority date: 2000-10-30
Filing date: 2001-10-30
Publication date: 2002-05-10
Also published as: KR20030051764A; NO20031960D0; AU2002230865A1; CA2427501A1; WO2002037589A3; EP1342279A2

Abstract

A solid state electrochemical device incorporating planar sheets (11) of cathode flow passages, in varying configurations and geometries, with thin coatings of electrolyte, anode and interconnect materials, which when assembled and bonded together form a monolithic honeycomb structure defining tubular passages for the passages inside the cell plates, while fuel will flow through passages formed between adjacent cells. Electrically insulating manifolds (15, 17), designed to keep the fuel and air separate, are bonded at each end of the honeycomb. The fuel cell stack and manifolds are encased in a metal housing (9, 10) or cover (19, 20) to provide the outer walls of the manifold, complete the package, and define a discrete fuel cell module that can be used singly or in groups in fuel cell power generation systems.

Description

IMPROVED SOLID OXIDE FUEL CELLS

Contractual Origin of the Invention

The United States Government has rights in this invention pursuant to an employer- employee relationship between the U.S. Department of Energy and the inventors.

Technical Field

The present invention is related to solid oxide fuel cells and particularly to improvements thereto.

Background of the Invention

Fuel cells are electrochemical systems that generate electrical current by chemically reacting a fuel gas and an oxidant gas on the surface of electrodes. Conventionally, the components of a single fuel cell include the anode, the cathode, the electrolyte, and the interconnect material. In a solid state fuel cell, such as solid oxide fuel cells (SOFCs), the electrolyte is in a solid form and insulates the cathode and anode one from the other with respect to electron flow, while permitting oxygen ions to flow from the cathode to the anode, and the interconnect material electronically connects the anode of one cell with the cathode of an adjacent cell, in series, to generate a useful voltage from an assembled fuel cell stack. The SOFC process gases, which include natural or synthetic fuel gas (i.e., those containing hydrogen, carbon monoxide or methane) and an oxidant (i.e., oxygen or air), react on the active electrode surfaces of the cell to produce electrical energy, water vapor and heat.

Several configurations for solid state fuel cells have been developed, including the tubular, flat plate, and monolithic designs. In a tubular design, each single fuel cell includes electrode and electrolyte layers applied to the periphery of a porous support tube. While the inner cathode layer completely surrounds the interior of the support tube, the solid electrolyte and outer anode layers are discontinuous to provide a space for the electrical interconnection of the single fuel cell to the exterior surface of adjacent, parallel cells. Fuel gas is directed over the exterior of the tubular cells, and oxidant gas is directed through the interior of the tubular cells. The flat plate design incorporates the use of electrolyte sheets, which are coated on opposite sides with layers of anode and cathode material. Ribbed distributors may also be provided on the opposite sides of the coated electrolyte sheet to form flow channels for the reactant gases. A conventional cross flow pattern is constructed when the flow channels on the as opposed to co-flow patterns where the flow channels for the fuel gas and oxidant gas are parallel, allow for simpler, more conventional manifolds to be incorporated into the fuel cell structure. A manifold system delivers the reactant gases to the assembled fuel cell. The coated electrolyte sheets and distributors of the flat plate design are tightly stacked between current conducting bipolar plates. In an alternate flat plate design, uncoated electrolyte sheets are stacked between porous plates of anode, cathode, and interconnecting material, with gas delivery tubes extending through the structure.

The monolithic solid oxide fuel cell (MSOFC) design is characterized by a honeycomb construction that is fused together into a continuous structure. The MSOFC is constructed by tape casting or calendar rolling the sheet components of the cell, which include thin composites of anode-electro lyte-cathode (A E/C) material and anode-interconnect-cathode (A/I/C) material. The sheet components are corrugated to form co-flow channels, wherein the fluid gas flows through channels formed by the anode layers, and the oxidant gas flows through parallel channels formed by the cathode layers. The monolithic structure, comprising many single cell layers, is assembled in a green or unfired state and co-sintered to fuse the materials into a rigid, dimensionally stable SOFC core.

These conventional designs have been improved upon in the prior art to achieve higher power densities. Power density is increased by incorporating smaller single unit cell heights and shorter cell-to-cell electronic conduction paths. SOFC designs have thus incorporated thin components which are fused together to form a continuous, bonded structure. However, the large number of small components, layers, and interconnections, in addition to complex fabrication steps, decreases the reliability of operational fuel cells, addition, any given fuel cell design must be commercially viable as an alternative power generating device, and, therefore, factors affecting the economics of power generation by electrochemical activity, such as overall capital and operational costs to the user, must be comparable to those of conventional power generating systems.

Packaging for the fuel cells is known but it usually is a simple box around the stack of solid oxide fuel cells. For example, US Patent Nos. 4,824,724; 4,827,606; and 4,943,494 disclose a box arrangement for a tubular fuel cell design. US Patent Nos. 5,238,754; 5,268,241; and 5,527,634 disclose packages which use end plates, tie bars and spring loaded fasteners to hold the fuel cell plates together. The present invention is to improved solid oxide fuel cells structures including a housing or cover for the solid oxide fuel cell structures, which can be a stack of planar sheets that when joined will resemble a monolithic honeycomb structure. The present invention also includes further improvements are beneficial in optimizing the overall system. In addition, various improvements in cell designs for planar fuel cells are disclosed, as well as processes for making them.

The present invention is a solid state electrochemical device that incorporates planar sheets of cathode flow passages, in various configurations and geometries, with thin coatings of electrolyte, anode and interconnect materials, which when the sheets are assembled and bonded together form a monolithic honeycomb structure defining tubular passages for the air and gas to pass through. Air will flow through cathode flow passages inside the cell plates, while fuel will flow through passages formed by spaces between adjacent cells. Electrically insulating manifolds, that are designed to keep the fuel and air separate, are bonded at each end of the solid oxide fuel cell structure to feed air and fuel to the appropriate passages in the structure. Finally, the fuel cell stack and manifolds are encased in a metal housing or cover to provide the outer walls of the manifold, complete the package, and define a discrete fuel cell module that can be used singly or in groups in fuel cell power generation systems.

Therefore, an object of the present invention is to provide a solid state fuel cell design incorporating an array of parallel cathode material tubes that improves fuel distribution and substantially eliminates the formation of hot spots within the fuel cell assembly.

Another object of the present invention is to provide a solid state fuel cell design incorporating a unique array of parallel cathode material tubes that increase the active surface area per unit fuel cell, such that the overall power density of the assembled fuel cell stack is critically improved.

Yet another object of the present invention is to form the manifolds as an integral part of the cell plates and eliminate secondary operations to form the fuel laterals.

The primary advantage of the present invention is to provide all the advantages of the hollow extruded plate concept, while eliminating secondary operations and integrating components.

Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentation and combinations particularly pointed out in the appended claims. Brief Description of the Drawings

The appended claims set forth those novel features which characterize the invention. However, the invention itself, as well as further objects and advantages thereof, will best be understood by reference to the following detailed description of a preferred embodiment taken in conjunction with the accompanying drawings, where like reference characters identify like elements throughout the various figures, in which:

Fig. 1 is an illustration of an assembled fuel cell stack constructed by stacking planar sheets of integrally connected tubular fuel cells, attaching manifold, and encasing the assembly in a housing;

Fig. 2 is an exploded view of the assembled fuel cell in Fig. 1; Fig. 3 is a cross-sectional view of the planar sheet illustrated in Fig. 1; Fig. 4 is an illustration of a single planar sheet of integrally comiected tubes;

Fig. 5 is a cross-sectional view of the tube sheet illustrated in Fig. 4; Fig. 6 illustrates a stack of tube sheets shown in Figures 4 and 5 that are coated and sintered together to form the monolith;

Figs. 7 and 8 are an exploded and an assembled external manifold with passages and holes to provide fuel and air to each monolith passage;

Fig. 9 is a cross-sectional view of the assembled and attached manifold illustrated in Fij 8;

Fig. 10 shows a different tube configuration combined with an internal manifold configuration; Fig. 11 illustrates an asymmetric tube sheet;

Fig. 12 shows a series of the tube sheets of Fig. 11 assembled to form a monolith stack assembly;

Fig. 13 illustrates a tube shape incorporating ridges or sine-like ripples to increase activ surface area; Fig. 14 illustrates a tube shape incorporating ridges or sine-like ripples in a monolith assembly;

Fig. 15 illustrates, in an exploded view, another tube shape as an extruded sheet which incorporates internal and external manifolds in a monolith assembly;

Figs. 16 and 17 illustrate a tube sheet made by a co-extrusion process having a molded manifold attached;.

^■ A - Fig. 19 is an exploded view and Fig. 20 is an assembled view of a fuel cell incorporating the tube sheet illustrated in Figs. 16 and 17;

Fig. 21 is an illustration of the mold assembly for making tube sheets using a fugitive molding process;

Fig. 22 is an illustration of a stack of tube sheets made using the fugitive molding process;

Fig. 23 is an enlarged illustration of the stack illustrated in Fig. 22; and Fig. 24 is an exploded view and Fig. 25 is an assembled view of a fuel cell incorporating the tube sheet illustrated in Figs. 16 and 17 or the tube sheet illustrated in Figs. 22 and 23.

Detailed Description of the Invention

The present invention relates to a solid state electrochemical device that provides increased active surface area and improves even distribution of a process gas. The present invention is described with respect to a detailed description of its application in the operation of a solid state fuel cell having a solid oxide electrolyte: a solid oxide fuel cell (SOFC). However, it will be obvious to those skilled in the art from the following detailed description that the invention is likewise applicable to any electrochemical system, including electrolysis cells, heat exchangers, chemical exchange apparatuses, and oxygen generators, among other applications. The present invention is directed to improving the process gas distribution and available active surface area in a solid state fuel cell having a unique solid oxide fuel cell structure design. In the fuel cell structure concept fuel and air enter the stack at one surface and flow through closely spaced parallel passages to a second surface where they exhaust together. The stack is made of cathode material cell plates with thin coatings of electrolyte, anode and interconnect materials. The cell plates will be bonded together to form a structure that resembles, but is not limited to, a monolithic honeycomb structure. The air flows through passages inside the cell plates. The fuel flows through passages formed by spaces between adjacent cell plates. Nail current collector members can be positioned within each cell plate to significantly increase the active surface area of the fuel cell stack. This arrangement sequesters the air within the hollow cell plates, and surrounds the cell plates with fuel. By providing electrically insulating manifolds, that are bonded to the ends of the honeycomb monolith, the appropriate flow of the air and fuel is facilitated, without the need for cross-flow of the reactants. The manifolds are designed to keep the fuel and air separate, and feed them to the appropriate holes in the honeycomb-like structure. Further, the designs allow for internal manifolds in the form of fuel or air laterals. These are opening through the plates for the circulation of the air and fuel. These design improvements can reduce hot spots in the fuel cell, but can also provide for electrical continuity since the surfaces can be continuous. An asymmetric tube monolith embodiment allows for air flow through an array of parallel cathode material tubes, and fuel flows in parallel in the passages between the tubes. The tubes are coated with electrolyte and anode material layers except in the interconnect area. The interconnect area is coated with a conductive ceramic film. The cathode tubes are the main structural elements, and the anode layer bonds them together to form unified cell plates that can be easily handled and stacked. The cell plates bond together during burn-in to form a monolithic honeycomb-like structure.

Current flows from the anode through the electrolyte layer to the cathode, and then through the interconnect to the anode of the next cell layer. The fact that the anode layer surrounds the cathode except in the interconnect area provides the electrical continuity to make both the top and bottom surfaces of the plates active. Manifolds are added to the extrusions to form complete cell plates. The inlet manifold forms a vertical air inlet passage with a seal around its perimeter when the cell plates are stacked. Air feed holes lead from the air inlet passage to each air tube in the extrusion. The stack is in a plenum and is surrounded by fuel that flows in between the plates from each side through the fuel inlet passages. Fuel flows into each fuel passage through fuel metering notches. The mixed exhaust manifold forms a vertical exhaust passage with a seal around its outside perimeter when the cell plates are stacked. Air exhaust flows into the mixed exhaust manifold through air exhaust holes and fuel exhaust flows into the mixed exhaust manifold through fuel exhaust notches.

This arrangement sequesters the air within the inlet manifold, cathode air tubes, and mixed exhaust manifold. These components are ceramics and not subject to oxidation. The stack is surrounded by fuel, creating a reducing environment in which metal components such as power takeoffs, pressure plates and tie bars may be used.

The cell plates include cathode, anode and electrolyte layers, and are made using an extrusion process. The manifolds are molded of cathode material and bonded to each end of the extrusions in the green state. The green cell plate assembly is then fired to remove the binder and fugitive materials, and consolidate the cathode and electrolyte layers. The manifolds become a unified part of the plate. Ceramic interconnect material is then applied to the specified areas using a plasma spray process, which will form a fully dense layer without subsequent firing. The plates are then stacked, initial burn-in reduces the anode nickel oxide to metal and sinters together the plates to form a monolith. The complete assembly includes non-repeat features can be appreciated by considering the illustrations in the drawings.

Fig. 1 shows an assembled fuel cell 1, including a housing or cover 2 for enclosing and supporting the elements of the fuel cell. The fuel cell package will have a fuel inlet 3, a fuel outlet 4, an air outlet 6, and terminals 8 at the top and bottom of the fuel cell.

Fig. 2 is an exploded view of the fuel cell package, which further illustrates the assemblage. The cover 2 comprises atop section 9 and a bottom section 10, which are brought together to enclose the fuel cell stack 11. When assembled, the fuel cell stack 11 is placed between a layer of nickel felt and a current collector plate, which is best seen in a cross-section of the fuel cell assembly shown in Fig. 3. The felt is indicated at 13. Finally, a ceramic insulator layer 14 is placed over the current collector plate. When the two halves 9 and 10 are assembled, they can be held in place by, for example, welding the two pieces of the metal shell so as to hold the assembly together in tension. This is facilitated by the use of a spacer 14 in the bottom half of the shell cover. The air and fuel passages in the fuel cell stack are fed and exhausted by manifold assemblies attached at each end of the fuel stack. As seen in Fig. 2, the inlet manifold 15 is attached to the fuel cell stack and defines inlet gallery 16. Manifold 17 defines a fuel exit gallery 22 and has an air outlet 6. Both manifolds 15 and 17 will have packing grooves 18 around the periphery of the manifold for purposes of sealing the manifold in place when the inlet cover 19 and exit cover 20 are assembled with the cover halves. As shown in Fig. 3, air enters the fuel cell via inlet 21 while fuel enters via inlet 3. The fuel is distributed via the fuel inlet gallery 16 and flows through the fuel cell stack 11, exiting the stack via the fuel exit gallery 22 to fuel exit plenum 23, and finally exit through the outlet 4. The air flows through the air tubes in the fuel cell stack and exits via fuel flow passages in the stack.

The fuel cell stack comprises an assembly of extruded tube sheets such as tube sheet 30 illustrated in Figs. 4 and 5. Fig. 4 shows an individual planar tube sheet 30 of integrally connected tubes 31. The sheet 30 maybe constructed from any appropriate fuel cell component material and will include cathode, anode, electrolyte and interconnect material, or any combination thereof. As shown in Fig. 5, the cathode material 32 defines air passages 33 and have an electrolyte and anode coating 34 over the or co-extruded with the extruded cathode material. In addition, the bottom layer defining the air passages 33 will be an interconnect and anode coating 35.

As can be seen in Fig. 6, a plurality of planar sheets 30 are stacked so as to define fuel passages 36. Preferably, the sheets 30 are made from cathode materials in a single extrusion step, or by co-extruding the cathode materials as well as the electrolyte and anode compositions. Although, they can be made by extruding the cathode material and subsequently coating the planar and composed of parallel rows of longitudinally aligned tubes that extend the length of the sheet. Although the tubes are illustrated as triangular in shape, the tubes may have any cross-section and may be symmetrical or asymmetrical. For example, the cross-sections may be triangular, rectangular, trapezoidal, circular, or polygonal in shape, among other geometries.

As shown in Figs. 7 and 8, the assembled stack, which describes a honeycomb stack, are assembled with external manifolds or plate-like structures that bond to the faces of the honeycomb monolith. For example, as shown in Fig. 7, air passages 33 and fuel passages 36 are bonded to manifold structures 37 and 38. Manifold structure 37 is a distributor for air via passages 39 and fuel via passages 40 which connect to air passages 33 and fuel passages 36, respectively, in the fuel cell monolith. Header 38 is joined to distributor 37 and possesses air holes 41 aligned with air passages 39 and 33 so that air passes through openings 41 and 39 to the air passages in the fuel cell stack. Header 38 and distributor 37 together define a fuel gallery 47 with air passages 39 being sealed by the header 38. When the covers or housings 2 and 3 are in place, a seal is formed with header 38 by placing appropriate seal material in seal groove 18. The seal may be refractory fiber rope. The seal provides enough compliance to avoid stresses caused by the differential thermal expansion created between the ceramic stack and the metallic stack containment structure.

The external manifold arrangement shown in Fig. 9 has connected air passages 41, 39 and 36 by joining the distributor 37 to the stack at 44 and the header 38 to the distributor at 45. The fuel gallery 43 distributes fuel through openings 40 into passages 36. The air passages pass straight through to the stack from the flat outside surfaces of the header plate. Fuel surrounds the sides of the stack and feeds into the stack fuel passages through the fuel gallery. As noted earlier, the shape of the air passages and the sheets is not critical. As illustrated in Fig. 10, a tube sheet 50 may be made from octagonal-shaped passageways in which the octagonal shape is cathodic material defining an air passageway 51 and the spaces between the tubes combined with the stack of tube sheet define a fuel passageway 52 with the cathode material being appropriately coated or co-extruded with electrolyte, anode, and/or interconnect material. An additional feature that can be incorporated is the use of internal manifolds where the connecting material between the tubes defining the air passages has been removed to create fuel laterals 53. Fuel laterals facilitate the equalization of the fuel between the levels of the fuel cell and prevent the occurrence of hot spots in the operation of the fuel cell.

Fig. 11 illustrates a tube sheet using an asymmetric tube structure. As shown, the tube sheet 54 can be a single extrusion of a cathode material in an asymmetric shape with the be made by extruding individual tubes and subsequently connecting the tubes prior to firing to form a monolith.

As shown in Fig. 12, the cathode material defines the air passages 55, but when the tube sheets are stacked, the assemblage defines the fuel passages 56, with layer 57 being anode material, layer 58 being electrolyte material, and layer 59 being interconnect material so that there is a continuous path for the electrons in the tube stack.

Another feature shown in Fig. 13, is the use of irregular shaped tube configurations where the extruded cathode material 60 defines the air passage 61 and has coated thereon electrolyte and anode material 62, and intercomiect material 63. When the tubes are assembled, as shown in Fig. 14, the tubes 60 also define fuel passages 64. As shown in Figs. 13 and 14, the small sine-like ripples allow an increase of up to 1.5 times the active area for the anode at, essentially, no increase in material costs.

Fig. 15 illustrates yet another tube shape, which can be made by extrusion and in which the air passages are at 68, and the fuel passages are at 69. This design will increase the surface area of the cathode. The tube stack can have fuel laterals 70 to facilitate passage of fuel between the fuel cell stacks. In addition, header 71 will be joined to the fuel stack to facilitate the distribution of air through air passages 68 and fuel through fuel passage grooves 72.

Another approach to assembling the fuel cells is to co-extrude the stack as shown in Figs. 16, 17 and 18, in which the air passages are extruded tubes 75 and the fuel passages are notches or grooves 76. The sheets 77, when assembled, as shown in Figs. 19 and 20, will result in the grooves 76 and the bottom surface of the next layer of the tube sheet defining the fuel cell passages. The headers can be molded separately and attached by "welding" or the like to the tube sheets. The "welding" is only suggestive in that with ceramic materials, a bonding composition is employed to hold or bond the parts together until they can be fired to produce a more permanent bond. For example, an inlet header 77, which defines an air inlet space 78, and a fuel inlet passage 79, can be attached on the inlet end of the plate 77, and an outlet manifold 80 attached to the outlet end of the plate 77. The outlet has matching passage holes 75 and grooves for fuel passages 76. The exhausted air and fuel will exit together via opening 81 in exhaust manifold 80. When the cell plates 77 are stacked, for example 56 cell plates with manifolds could be stacked to provide a 42 vDC at 0.75 v/cell. A talc or powdered mica could be used on the manifold sealing areas to reduce leakage. The stack is then assembled as shown in Fig. 20 between end plates 84 and 85, having terminals 86 and 87, and ceramic electrical insulating plates 88 and 89 to separate the power take-off plates from the end plate and the base 92 and end plate 93.

Clamping load is provided by an endplate and a baseplate loaded by a pair of tiebars and springs. Ceramic electrical insulating plates separate the power takeoff plates from the endplate and baseplate. The plates can be, for example, Hastelloy X castings or any alumina-coating alloy which can provide long term endurance. Tiebars and springs can be, e.g., conel 716. The tiebars can be extended so that the springs can be positioned outside the hot zone, hiconel springs have a practical limit of about 600°C. Reactant gas inlet and exhaust flows are through ports in the endplate. The endplate is electrically grounded, and com ections may be made by bolting or welding to the power takeoff plate lugs.

An alternative to extruding tube sheets and assembling molded headers would be to mold the tube sheets using a fugitive core material. As illustrated in Figs. 21, 22 and 23, a tube sheet having an integral header could be manufactured by a mold assembly (as illustrated in Fig. 21), which included a lower cathode preform 100, an upper cathode preform 101, and a fugitive core material 102 which is placed between the lower cathode preform 100 and the upper cathode preform 101 to result in air passages when the cathode material is fused to produce a cathode monolith. When the fugitive core is burned off, the space that remains will define air passages 103, the manifold is defined by the molded material 104, and the fuel inlet passage by space 105, the fuel passages by space 106, and the fuel laterals by space 107. Spaces will also result for air laterals at 108.

The upper and lower cathode preforms can be made by compression molding or a similar suitable process. The fugitive cores are made by compression or injection molding from polymer wax, a carbon powder composition, or other material that evaporates, vaporizes, decomposes, or is removed by heat with no residue left behind. A green upper and green lower cathode preforms are assembled with the fugitive core and the assembly is pressed together with the fugitive core in a die to join the preforms into a single green cathode cell plate surrounding the core. The assembly is then dried and fired to consolidate the cathode into a porous ceramic. The core supports the cathode material during the initial part of the firing cycle, and is then removed by vaporization or thermal decomposition. The result is a complete hollow cathode, including inlet and exhaust manifolds, and is ready for subsequent processing to add electrolyte, interconnect and anode layers. The sheets can then be assembled in a similar manner to those illustrated in Figs. 19 and 20 to provide a completed fuel cell.

Figs. 24 and 25 illustrate an alternative packaging design to that shown in Figs. 19 and 20. Fig. 24 shows an exploded view and Fig. 25 shows an assembled view of a packaging similar to that shown in Figs. 1-3. As shown in Figs. 24 and 25, the upper cover 111 and lower plenums. The pieces can be brought together and clamped, welded, or fixed by any appropriate means to maintain the fuel stack 115 in tension and in assembled relationship. This can be facilitated by the use of a mesh spring pack 116 placed in the lower cover or fuel plenum 112. When the assembly is completed, including ceramic insulators 117 and 118, current collector plates 119 and 120, which would produce electrical flow through terminals 121. Air is fed into the housing at 122 and a mixed exhaust is taken out at 123. Fuel is fed into the plenum via fuel inlet 124.

It may be desirable to take the fuel cell stack such as illustrated in Fig. 3 and combine them end-to-end using a coupling header so as to provide an assembly having a lower section for low temperature and a second section for high temperature processing. Also, several monoliths can be combined into a single package module by connecting the modules in series with metal felt pads and/or electrical connections to create a larger package.

The fuel cells will operate by flowing the air and gas through the parallel spaced passages. Fuel flows in from each side through fuel inlet passages between the cell plates. It then flows through fuel metering slots, and on between the cell plates to the exhaust end of the cell stack. The fuel reacts with the air flowing through the internal air passages to generate electric power. The fuel laterals provide extended reactive surface area, as well as electrical continuity between upper and lower sides of the cell plates. The air laterals improve the access of air to the reactive layers compared to simple straight air passages. They also facilitate the handling of the fugitive core. The fuel and air passages empty into the exhaust manifold (i.e., an internal manifold similar to the air inlet manifold) and mix.

The foregoing embodiments of the present invention have been presented for the purposes of illustration and description. These descriptions and embodiments are not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above disclosure. The embodiments were chosen and described in order to best explain the principle of the invention and its practical applications to thereby enable others skilled in the art to best utilize the invention in its various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the invention be defined by the following claims.

Claims

1. A solid oxide fuel cell system comprising

A. a stack of parallel cell passages which passages define air and fuel passages and comprise a cathode layer, an electrolyte layer, and an anode layer, ;

B. manifold means attached at the input end of said fuel cell stack for distributing air and fuel to said air and fuel passages;

C. current collecting means for electrons generated in said fuel cell stack; D. terminal means for withdrawing said electrons as a current from said current collector means;

E. inlet means for feeding fuel to said manifold means;

F. inlet means for feeding air to said manifold means;

G. exhaust manifold means for collecting and emptying exhaust from said stack;

H. exhaust means for removing exhaust from said fuel cell system; and I. housing means for enclosing said fuel cell, defining the outer walls of said input manifold means, and maintaining said stack under tension.

2. The fuel cell system of claim 1, wherein said stack of parallel cell passages is a sintered honeycomb-like structure.

3. The fuel cell system of claim 1, wherein the said stack of parallel cell passages is a honeycomb monolith, and wherein said monolith has internal means for distributing said air and said gas throughout the monolith.

4. The fuel cell system of claim 1, wherein said housing means is a metal shell having at least two pieces which are joined together by welding.

5. The fuel cell system of claim 1 , wherein the cell stack is supported by a resilient means whereby said cell stack can expand within the confines of said housing.

6. The fuel cell system of claim 1, wherein said parallel cell passages are made by extruding a cathode material to produce air passages through said cathode material and said sheets further defines air and fuel passages for operating said fuel cell.

7. The fuel cell system of claim 1, wherein said stack of parallel cell passages is made by a fugitive core molding process, wherein the fugitive core vaporizes or decomposes to leave air passages, and wherein when the stack is assembled, the assemblage will define fiiel passages, and said assembly is dried and fired to consolidate the cathode into a porous ceramic.

8. The fuel cell system of claim 7, wherein the stack of parallel cell passages is made by a fugitive core molding process, and wherein said molding process results in an integral inlet manifold and outlet manifold structure.

9. The fuel cell system of claim 7, wherein said stack of parallel cell passages is made by molding each half of the stack and subsequently joining the halves to produce a full cell stack having integral inlet and outlet manifolds.

10. The fuel cell system of claim 1, wherein said stack of parallel cell passages is made by extruding a ceramic composition to form a sheet of parallel cell passages, and subsequently welding inlet and outlet manifolds to said extruded cell stack.

11. The fuel cell system of claim 1, wherein a layer of nickel felt is placed in the top and bottom of the monolith stack to provide a resilient conductive layer.

12. The fuel cell system of claim 1, wherein the parallel cell passages have a geometry wherein the anode coating increases in thickness in the direction of current flow, while the cathode decreases in thickness.

13. The fuel cell system of claim 1, wherein said parallel cell passages further include a current path means traversing the cathode, electrolyte and anode layers, wherein electrons travel through the cell to facilitate the flow of electrons in the direction of the current path.

14. The fuel cell system of claim 1, wherein said parallel cell passages have defined tubes having a cross-section with a geometry selected from the group consisting of circular, octagonal, hexagonal, rectangular, and triangular.

15. The fuel cell system of claim 1, wherein said cell passages define tubes having a cross-section wherein the geometry is an asymmetric polygon.

16. The fuel cell system of claim 1, wherein the stack of parallel cell passages further includes interconnecting means between the layers of the stack, whereby current can flow between the layers of the stack.