HK1097110B - Solid oxide fuel cells with novel internal geometry - Google Patents

Description

Solid oxide fuel cell with internal geometry

Technical Field

The present invention relates generally to fuel cells, and more particularly, to tubular Solid Oxide Fuel Cells (SOFC) having an improved anode that enhances the physical and electrochemical properties of the cell through a unique internal geometry system, thereby improving mechanical support, durability, and cell performance characteristics, and a method of manufacture.

Background

Several different solid oxide fuel cell structural devices have been developed, including tubular, planar and monolithic devices, all of which are documented in the technical literature. (see, e.g., Q.M. Nguyen et al, "Science and Technology of Ceramic Fuel Cells", Elsevier Science, Jan.1995). The tubular SOFC device stems from the sealing problems associated with planar Fuel Cell stacks (see g. hoogers, "Fuel Cell technology handbook," CRC Press, aug. 2002). A number of published patents belong to Siemens Westinghouse Power Corp, Orlando, FL, disclosing the so-called air electrode Loading (AES) process (see, e.g., U.S. Pat. No. 5,916,700 to Ruka et al; U.S. Pat. No. 5,993,985 to Borglum and U.S. Pat. No. 6,379,485 also to Borglum).

Despite the significant technical achievements achieved in the field of tubular SOFCs, air electrode-supported tubular fuel cells still suffer from several drawbacks. As one of them, air electrode materials such as lanthanum, strontium, manganite, etc. are expensive, often making the process economics unattractive. In addition, air electrodes are comprised of ceramic materials, which are often mechanically weaker and less durable than fuel electrodes made of cermets (i.e., ceramic and metal composites).

Fuel Electrode Supported (FES) tubular SOFCs have attracted new attention in the art due to some economic improvements (see Song et al, US 6,436,565).

While AES and FES tubular devices have been structurally modified to have an open end and to be closed at one end, there has been little obvious suggestion to modify the internal structural features of tubular SOFCs relative to conventional cylindrical configurations resulting in improvements in the base tubular configuration as a means of enhancing the structural integrity and operating characteristics of such cells.

Accordingly, there is a need for improved tubular SOFCs anodes to enhance structural support, durability, and increase surface area for optimizing the electronic conductivity of the cell.

Disclosure of Invention

It is therefore a primary object of the present invention to provide SOFCs with novel support structures. The geometry of the supporting anode gives the cell enhanced physical, thermal and electrical performance, and is more economically attractive.

It is also a primary object of the present invention to provide a method of making the improved tubular supported anode and SOFCs comprising the same with minimal process steps.

The novel anode layer geometry comprises at least one, and more preferably a plurality of longitudinal, protrusions in the form of internal bumps or protrusions (and grooves therebetween) projecting inwardly from the anode layer inner surface or i.d. to the central opening or bore of the tubular body. In general, the protuberances or protrusions may be parallel or coaxial with the longitudinal axis of the tubular body, or alternatively, may be wound in a conventional helical pattern through a central aperture, for example, without contacting other protuberances or protrusions of the tubular anode surface, or in combination with other sections of the tubular anode surface. The "bosses" or "supporting anodes" impart improved mechanical reliability, durability, and increased active cell area, while enhancing electrochemical performance characteristics by minimizing electrical resistance. The present invention may also enhance the fuel flow characteristics of the cell because the internal protrusions may increase mixing, for example, by causing turbulent or turbulent mixing. It is therefore a primary object of the present invention to provide SOFCs having load cell anodes that include a tubular body forming a central bore, wherein the tubular body includes a load device that protrudes into the central bore for structurally reinforcing the overall fuel cell. Preferably, said load means protruding into said central aperture is integral with the tubular anode body.

For the purposes of the present invention, such expressions as "tubular" or "tubular body" or variations thereof as appearing in the specification and claims are intended to include predominantly round or circular walled fuel cells, e.g. cylindrical shapes, however, the present invention is intended to also include tubular bodies having a polygonal geometry of at least three sides, e.g. triangular tubes, rectangular/square tubes, hexagonal tubes and variations thereof, such as three-sided triangular-like tubes, wherein the apex is e.g. circular, etc. Thus, while the SOFCs of the present invention are described primarily in terms of cylindrical tubular bodies, it is to be understood that this is merely for convenience and is not intended to limit or exclude other geometric configurations like those mentioned hereinabove.

Tubular SOFCs are considered to suffer from potential loss due to their geometry. It is noteworthy that the supporting anode of the novel lobe structural feature provides a preferential "low resistance" electron transfer path to minimize these losses.

It is yet another object of the present invention to provide anode-supporting tubular SOFCs with novel geometric features that not only provide enhanced structural integrity, thermal and electrical performance, but also facilitate the assembly of SOFC stacked multi-function systems due to their unique geometry, whereby the anode bosses act as guides in the placement of fuel injectors, ensuring that the gas passages remain open on both sides of the injectors.

Accordingly, the present invention is directed to SOFCs having novel tubular anode supports comprising fuel electrodes, and more particularly, fuel electrodes having an inner surface anode structure with suitable ridges extending inwardly from the inner surface or radius of the tubular structure into a central bore with intermediate grooves or recesses therebetween for enhancing structural reinforcement of the SOFCs. Like the tubular SOFCs for which the support anodes of the present invention are used, the support anodes of the present invention may be open at both ends or closed at one end. The anode support structure is a relatively thick wall due to the protruding boss structure being generally a non-circular tubular bore. Thus, the unique geometry of the anode imparts rigidity and strength to the overall fuel cell.

Compositionally, the supported fuel electrode of the present invention is comprised of a transition metal (e.g., Ni) and a ceramic material (e.g., stabilized zirconia, doped ceria, or other suitable electrolyte material), i.e., a cermet.

As previously discussed, the primary breakthrough in the present invention relates to a novel anode assembly with modified electrode geometry so that it becomes more physically supporting the entire fuel cell structure. The bore of the fuel electrode comprises an annular anode configured as a tubular but non-circular inner wall structure having at least one, and more preferably a plurality of continuous longitudinal ridges or protrusions, preferably symmetrically spaced from and coaxial with the longitudinal axis of the tubular body. The projections are preferably arranged along the length of the tubular body, but may also be arranged along only a portion of the length of the tubular electrode body. Thus, by introducing longitudinal ridges or protrusions (forming grooves between them) along the inner wall of the tubular structure, there are several advantages:

-increasing the thickness or surface area through the fuel electrode, thereby converting the anode into a highly supported structure of the fuel cell, resulting in greater mechanical strength. Preferably, the internal bosses are symmetrically arranged so that they are spaced equidistant from each other around the inner ring, further structurally reinforcing the entire tubular SOFC;

-creating a larger conductive surface area within the cell;

-achieving a greater electrochemical output by enhancing the electronic conductivity of the anode support;

improved fuel cell assembly efficiency, i.e. ease of handling and assembly into a fuel cell stack, wherein the grooves between the ridges serve as guides for the placement and fixation of the fuel injectors. This reduces or eliminates the damage and leakage problems encountered with conventional round tubes.

Those skilled in the art will recognize that the geometry of the internally protruding bosses/ridges supporting the anode of the present invention is virtually unlimited. Representative protuberances include shapes such as conical, rectangular, square, circular, or semi-circular, to name a few. Generally, they are in a number and size suitable for fuel injector devices to be subsequently introduced during assembly of the SOFC stack.

The invention also relates to an improved method for manufacturing said supported anode.

The method of fabrication of the supported anode is directly related to the fuel electrode mixture composition. Useful extrusion techniques are those typically associated with plastic extrusion. They provide improved economic advantages in the manufacture of supported anodes having internal bosses, particularly extending along the entire length of the tubular body. Casting and pressing techniques are preferred for making more complex internal shapes whereby the internal anode bosses are discontinuous or shorter in length relative to the entire length of the anode support tube.

Further improvements in fuel cell electrochemical properties can be achieved by incorporating artificial pore formers in the fuel electrode mixture to optimize catalytic activity and limit mass transport issues.

It will be apparent to those skilled in the art from this disclosure and the following more detailed description that the present invention provides a significantly improved tubular fuel cell process, and more particularly, a tubular SOFC process. Of particular significance in this regard is the potential that the present invention provides for the production of more economical, high current density fuel cells at lower cost, while having improved mechanical reliability. Additional features and advantages may be better understood in view of the more detailed description that follows.

Brief Description of Drawings

The nature and mode of operation of the present invention will be more fully described in the following detailed description of the invention taken in conjunction with the accompanying drawings in which:

fig. 1 is a perspective view of a tubular SOFC of the present invention comprising a supported anode with portions removed to illustrate the rings/layers of the cell, including the electrolyte and cathode regions of the cell.

Fig. 2 is an enlarged perspective view of the tubular SOFC of fig. 1, showing in more detail the structural features of the invention, including the reinforcement consisting of four continuous longitudinal circular grooves spaced between conical protrusions or ridges as projections on the (full length) internal annular anode of the cell;

fig. 3 is a perspective view of a further SOFC embodiment of the invention, comprising a supporting anode as illustrated in fig. 2, but modified in that the internal conical protrusions extend only along part of the length of the tubular cell on the internal annular anode (short length).

Fig. 4 is a perspective view of another SOFC embodiment of the present invention including a plurality of symmetrically spaced longitudinally continuous rectangular support ridges on the electrode inner ring and forming spaced grooves therebetween a support ring anode;

fig. 5 is a perspective view of an additional SOFC embodiment of the invention comprising a supporting annular anode featuring a plurality of support members in the form of uniformly spaced, circular ridges protruding into the tubular body bore and extending along the length of the cylindrical cell with continuous circular grooves on the inner ring between the ridges (throughout their length);

FIG. 6 is a perspective view of yet another embodiment of the novel solid oxide fuel cell of the present invention comprising a supporting anode characterized by eight conical supporting protrusions extending lengthwise of the tubular cell as ridges directed inwardly away from the inner ring and forming continuous grooves therebetween (the full length);

fig. 7 is a perspective view of another embodiment of the SOFC of the present invention, including a supporting anode, featuring internal elevations in a spiral or helical configuration;

fig. 8 is a perspective view of yet another alternative embodiment of a supported tubular SOFC of the present invention, wherein instead of being cylindrical, the outer cathode includes three surfaces having rounded vertices and the inner supporting annular anode has spaced ridges in a symmetrical configuration protruding into the interior of the cell bore;

fig. 9 is a perspective view of a polygonal SOFC, and more particularly, a fuel cell, wherein the outer annular cathode is hexagonal and reinforced by an inner annular anode characterized by a plurality of rectangular spaced ridges projecting away from the inner surface of the anode into the cell bore.

Fig. 10 is a cross-sectional view illustrating a tubular SOFC of the present invention, fabricated with a support anode mounted on a fuel injector;

FIG. 11 is a top plan view of a tubular SOFC mounted on a fuel injector according to FIG. 10, an

Fig. 12 is a partial end view of an extrusion die for forming a tubular support anode of the present invention having an anode boss projecting from an inner ring.

Description of the preferred embodiments

Turning first to fig. 1, an overall view of the SOFC10 of the present invention is provided as a modified cylindrical tubular body to best illustrate the inner annular anode 12, intermediate electrolyte 14 and outer annular cathode 16. The anode 12 defines an internal bore 18 having a plurality of ridges 20 projecting from the inner wall of the anode into the bore.

Fig. 2 is an enlarged view of the fuel cell of fig. 1, best illustrating the anode support 12 of the present invention, wherein four symmetrically disposed conical bosses 20 extend coaxially along the entire length of the tubular SOFC10, with an oval slot 22 disposed intermediate the bosses 20. The bosses 20 provide increased surface area and strength to the fuel cell, shown as being integral with the annular anode 12.

Fig. 3 illustrates some of the same structural features as in fig. 2, providing an alternative embodiment of a general cylindrical SOFC24 of the present invention, including a supporting anode 26, an intermediate electrolyte 28, and an outer cathode 30. The anode 26 forms an internal bore 32 having elliptical slots 34 disposed between conical bumps 36. The fuel cell 24 is also characterized by shortened ridges 38, the ridges 38 not extending along the entire length of the cell, but rather being shorter than the entire length of the tubular body.

Figure 4 illustrates a cylindrical SOFC40, which is yet another alternative embodiment of the present invention, including a supported inner annular anode 42, an intermediate annular solid electrolyte 44, and an outer cathode 46. The inner annular anode 42 includes a plurality of evenly spaced longitudinal ridges 48 disposed between arcs or grooves 50 as an integral part of an annular anode-like structure. The ridge 48 is of a rectangular or generally square configuration disposed toward the fuel cell bore 52.

Fig. 5 illustrates another embodiment of the anode-reinforced SOFCs of the present invention, with tubular fuel cells 54 also having a cylindrical configuration. The reinforced SOFC54 includes an inner supporting annular anode 56, an intermediate solid electrolyte 58, and an outer annular cathode 60, wherein the anode internal ridges include rounded, uniformly spaced bosses 62 as reinforcing protrusions extending longitudinally through the cell bore 64 along part or all of the length of the cell. Preferably, the protuberances 62 are symmetrically spaced between inner circular regions 66 that support the inner wall of the anode.

Fig. 6 illustrates another embodiment of a cylindrical SOFC68 of the present invention having a supporting anode 70, an intermediate annular solid electrolyte 72, and an outer cathode 74. The support anode 70 features an internal geometry comprising eight generally conical protuberances 76 of full length projecting into a central bore 80 with a slightly circular groove 78 in the middle of the protuberances 76.

Fig. 7 illustrates an embodiment 82 of an additional cylindrical tubular SOFC having a novel internal geometry supporting anode 84 of the present invention, comprising an electrolyte layer 86 and an annular cathode 88, wherein the ridges 90 supporting the anode 84 are continuous and extend in a helical path along the length of the support tube between circular regions 92.

Fig. 8 and 9 illustrate an alternative tubular embodiment of the invention in which the tubular structure is not circular, such as a cylindrical tube, but may be polygonal, for example. Fig. 8 illustrates an embodiment of such an SOFC92 of the present invention including three major outer cathode surfaces 94 that are together triangularly shaped, except that rounded vertices 96 join the surfaces 94. The present invention of course contemplates polygonal structures having three or more surfaces, such as triangles, squares, pentagons, hexagons, and the like. Tubular fuel cell embodiments like those of fig. 8 include a supporting anode as described herein. The fuel cell of fig. 8 also includes an inner supporting anode 98, an intermediate electrolyte 99 and an outer three-sided cathode structure 94. The support anode 98 also includes ridges 100 between the circular regions 102 as protrusions into the cell bore, such as previously described.

Fig. 9 is yet another SOFC104 comprising multiple sides of the present invention, wherein the cell comprises an internally supported annular anode 106, an intermediate tubular electrolyte ring 107, and a cathode outside the polygon having six surfaces 108. This embodiment features a support ring anode 106 having a plurality of raised portions 110 as spaced ridges disposed between circular depressions or arcs 112.

The particular embodiments of fig. 1-9 are intended to be illustrative only and are not intended to be limiting to the wide variety of alternative embodiments that may be apparent to those of ordinary skill in the art, but all such alternatives and variations are intended to be included.

As previously mentioned, the primary aspect of the present invention is an anode having enhanced support characteristics, novel internal geometry, particularly for use in tubular SOFCs, which provides structural reinforcement to the overall fuel cell relative to conventional tubular anodes.

The use of a modified anode as a support structure is most beneficial from a performance characteristics standpoint (see US 6,436,565 to Song et al). In addition, as previously discussed, for fuel cells equipped with the anode of the present invention, where the inner ring thickness is in the range of 0.2-2.0 mm or so, high current density can be achieved by increasing electron conductivity and reducing activation overpotential (see voltage loss due to electrochemical charge transfer reactions).

Compositionally, the electrochemically active material, i.e., the metal, used in the anode, cermet support is preferably present in an amount of from about 30.0 to about 80.0 volume percent, based on the volume of the solid. For metal contents below 30% by volume, the cermet anode composite has a reduced electrical conductivity. When the metal content of the supporting anode cermet is about 30 vol% or more, good interfacial bonding is induced among the metal particles, resulting in increased electron conductivity. Metal contents of up to 80 vol% are sufficient to ensure very high electron conductivity while maintaining sufficient porosity to minimize concentration polarization. The higher content of metal in the anode cermet results in a large coefficient of thermal expansion that is mismatched with the subsequently coated electrolyte, resulting in the formation of cracks during processing or cell operation.

It is also desirable to increase the porosity of the anode in order to enhance cell performance characteristics, thus keeping concentration polarization (see voltage loss associated with gas flow through porous electrodes) at a minimum level. One way to achieve this result is to reduce the metal oxide powder to elemental metal under reducing atmospheric conditions by a substantially in situ treatment, thereby providing greater porosity to the anode substrate. Therefore, higher metal oxide contents are generally preferred in the anode composition.

Additional porosity of the anode may also be created by the introduction of pore formers. Representative examples of useful pore formers include carbon dust, starch, polymer beads, and the like. When the supported anode is fabricated into a complete tubular SOFC structure, the pore former is subsequently removed during sintering. The pore former is preferably used in an amount of up to 50 volume percent based on the cermet powder. The significantly higher content of pore former results in a loss of mechanical strength.

Representative examples of useful ceramic materials for the cermet fuel anode supports of the present invention include stabilized zirconia for high temperature SOFCs (700 ℃ to 1000 ℃). This preferably comprises 8 mol% yttria-stabilized zirconia (YSZ), (ZrO) and₂)_0.92(Y₂O₃)_0.08. Another useful material is doped ceria for moderate temperaturesSOFC (500 ℃ -700 ℃). This preferably comprises gadolinium doped Ceria (CGO), (Ce)_0.09Gd_0.10)O_1.95. Other materials suitable for SOFC electrolyte applications are also suitable for use in the present invention.

In general, the metal phase and cermet electrolyte used in the fuel electrode support of the present invention belong to the transition metals of the periodic table of the elements, and alloys or physical mixtures comprising them. Elemental nickel (Ni) is a preferred metal because of its high electrochemical activity under reducing atmosphere conditions, high electronic conductivity, and its cost effectiveness. The metal may be introduced into the anode support and the cermet electrolyte by different precursors including metal powders, metal oxide powders, metal salts (aqueous or non-aqueous), and the like. Metal oxide powders such as NiO are generally preferred because of their cost effectiveness and their adaptability to ceramic manufacturing processes. A limited amount of very pure metal particles may be introduced by metal salts, such as Ni (NO) dissolved in aqueous and non-aqueous solvents including aqueous or alcoholic solutions₃)₂. This is particularly relevant for anode supports where intimate contact between the metal particles is desired for enhanced electron conductivity.

The protruding longitudinal projections of the anode support may allow the overall thickness of the anode to be reduced as they increase the strength of the anode and the surface area within the anode that is in contact with the gas stream. Thus, the improved supported anode has enhanced structural characteristics as compared to conventional tubes without such structural features. Preferably, the internal ridges are symmetrically arranged so that they and their intermediate grooves are equally spaced from each other. This also gives the tubular support a uniform weight distribution. It is also desirable that this structural characteristic be used to minimize differential shrinkage during drying and sintering of the battery fabrication. Unevenly distributed grooves across the empty tubular support lead to detrimental defects such as warping and/or cracking. The strength of the support tube increases with the number of protruding bosses.

The presence of the protruding bosses also enhances the electrochemical performance characteristics of the supporting anode. Higher current densities are achieved by increasing electron conductivity and reducing the activation overpotential across the thick regions of the cermet anode.

As previously mentioned, the longitudinally projecting bosses also provide excellent ability to assemble the cells into a fuel cell stack assembly. Fig. 10 illustrates a partial cross-sectional view of a tubular SOFC114, including an anode support 116 of the present invention mounted on a fuel injector 118, whereby the anode ridges or bumps 120 are used to position and fix the injector 118 in a predetermined orientation within the cell tubular bore. The ridges 120 between the circular voids 122 physically maintain the center position of the fuel injector 118, optimizing the flow characteristics inside the anode support, thus resulting in better distribution of the fuel into the anode reaction sites.

The processing route for making the raised anode support depends on preparing the fuel electrode mixture comprising metal and ceramic compounds as discussed above. Aqueous or non-aqueous media may be used to suspend the particulates. However, aqueous media are generally preferred because of their cost effectiveness and few environmental concerns associated with flammability and toxicity of organic solvents. Conventional Processing additives (dispersants, binders, plasticizers) may also be used to ensure adequate dispersion of a uniform and stable mixture (see r.j. high et al, "Surface and Colloid Chemistry in advanced Processing", Marcel Dekker, Oct. 1993). The properties of these mixtures, such as viscosity, can be varied by varying the properties or amounts of the different materials. They are thus adapted to suit the particular molding process.

In particular, it is preferred to extrude water-containing, plastic masses when producing profiles of uniform cross section. This is particularly relevant when a continuous projection is desired along the entire length of the support tube. Fig. 12 illustrates a partial view of an extrusion die 124 in which a groove 126 has been machined into an inner mold 128 that exits a die slot 130. Thus, the extrudate exhibits ridges that protrude along the inner wall of the tubular support anode, consistent with those in fig. 1, and so on.

On the other hand, more complex profiles, such as profiles in which the projections are arranged in a spiral (helical) path, can be prepared by casting techniques (liquid phase machining) or pressing techniques (dry methods). Casting techniques include slip casting, centrifugal casting, gel casting, and the like. Pressing techniques include dry pressing and isostatic pressing. All such Processing routes are known and well documented in the literature (see, e.g., J.S. Reed, "Principles of Ceramic Processing, 2 d edition," J.Wiley & Sons, Nov.1994).

As noted above, other additives may be introduced into the construction, such as pore formers, to adjust the porosity of the fuel electrode support. These optional additives are incorporated into the cermet mixture prior to the forming operation.

The novel supported anode of the present invention can be used in anode-supported solid oxide fuel cells that typically employ an intermediate solid electrolyte and an external air electrode (cathode). The fuel electrode supported type (i.e., anode supported) in which the electrolyte layer underlying the air electrode is coated as a thin film on a cermet anode support is well known in the art. The choice of electrolyte material and air electrode (cathode) material depends on the temperature at which the fuel cell is intended to operate and can encompass a wide range.

For example, if the anode-supported SOFC operates at high temperatures of 700 ℃ to 1000 ℃, then the electrolyte is selected from the group consisting of stabilized zirconia, such as (ZrO)₂)_0.92(Y₂O₃)_0.08(YSZ), whereas if operated at moderate temperatures between 500 ℃ and 700 ℃, the electrolyte may be doped ceria, such as (Ce)_0.90Gd_0.10)O_1.95。

A method of manufacturing an anode-supported SOFC using conventional powders includes the steps of:

an electrolyte material (YSZ) and an electrochemically active material such as nickel are blended to form a fuel electrode. The volume% of the electrochemically active material is from about 30 to about 80%, preferably from about 40 to about 60%.

The following examples are offered for purposes of illustration and description. This should not be considered limiting in any way.

Examples

A tubular SOFC with an internal groove supporting an anode is manufactured by:

the green oxide NiO powder was mixed with YSZ powder so that the Ni content introduced (after reduction of the NiO) in the mixture was 30-80 vol%. The paste composition additionally comprises distilled water (solvent), methylcellulose or hydroxypropylmethylcellulose (binder) and glycerol or polyethylene glycol (plasticizer). Suitable paste compositions include solids loadings (NiO + YSZ) of 70 to about 90 wt%; 5-25 wt% of water; 1-15 wt% of a binder; and 0.1 to 5 wt% of a plasticizer. The composition is then mixed using a high shear mixer, such as a sigma blade mixer, under high shear conditions, thereby forming a uniform, plastic mass.

As previously described, optional additives include pore formers (e.g., carbon dust, starch, polymer beads).

The paste is then forced through a die 124 (fig. 12) at high pressure (e.g., 1-30 kilonewtons) to extrude the anode support tube. The shape of the die determines the cross-sectional geometry of the extruded tube. Fig. 12 illustrates machining a suitable die design having die slots 130 and ridges 126 to produce the desired support internal geometry, such as ridges and the like.

The extruded tube may be dried in ambient air for several hours. Short drying times are achieved by using a humidity laboratory where the temperature/humidity can be controlled. The humidity was gradually reduced from a high initial set value (90-100% RH) until the tube was completely dry.

An electrolyte ink or slurry with appropriate solids loading (about 20-60 wt%) and particle size (D50<1 micron) was then used to form the electrolyte layer on the dry support structure. The electrolyte (YSZ) is applied to the drying tube by dip coating, where the dip time and ink viscosity determine the final electrolyte thickness, or by spraying, where the rotation speed, X/Y position, spray distance and other parameters are also used to control the electrolyte layer thickness. Desirably, an electrolyte coating thickness of 5-50 microns can achieve a dense electrolyte layer without cracking after the sintering process. The support and electrolyte are then sintered at a high temperature of 1300 ℃ to 1450 ℃, depending on the particle size and solid loading of the initial electrolyte slurry.

And then co-sintering the anode support and the electrolyte structure for application to the cathode. The cathode is composed of 2-4 layers, the first layer being in direct contact with an electrolyte comprising a higher volume% YSZ than the outer layer, thus forming a gradient cathode structure. The cathode ink was prepared with appropriate solids loading (20 to about 60%) and particle size (D50 ═ 2 microns), and appropriate volume percentages of YSZ or perovskite material (typically LSM of various doping levels) to achieve the desired composition of the different layers. The cathode is applied by a variety of coating techniques including dip coating, spraying and screen printing, spraying being preferred. The entire structure is then sintered at 1000 c-1250 c to form the appropriate interface characteristics and electrode structure for the cathode.

Although the present invention has been described in detail for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.

Claims (36)

1. A solid oxide fuel cell comprising a single tubular body having an inner wall as a supporting anode, said supporting anode defining a hollow central bore of said fuel cell, including at least one solid support means projecting from said inner wall into said hollow central bore of said single tubular body for structurally reinforcing said fuel cell.

2. The solid oxide fuel cell according to claim 1, characterized wherein said support means is integral with said anode and comprises a plurality of ridges or protrusions.

3. The solid oxide fuel cell according to claim 1, characterized wherein said tubular body further comprises an electrolyte layer and a cathode layer in combination with said supporting anode.

4. The solid oxide fuel cell according to claim 3, characterized wherein said tubular body is cylindrical or polygonal.

5. The solid oxide fuel cell according to claim 4, characterized wherein said tubular body is polygonal and comprises at least three sides.

6. The solid oxide fuel cell according to claim 5, characterized wherein said tubular body is hexagonal.

7. The solid oxide fuel cell according to claim 5, characterized wherein said tubular body comprises three sides joined at rounded vertices.

8. The solid oxide fuel cell according to claim 2, characterized wherein said ridges or protrusions extend substantially along the entire length of said central bore of said tubular body.

9. The solid oxide fuel cell according to claim 2, characterized wherein the length of said ridge or protrusion in the longitudinal direction of the tubular body is shorter than the length of the central bore of said tubular body.

10. The solid oxide fuel cell according to claim 2, characterized wherein said ridges or protrusions comprise a generally spiral pattern extending along the length of said central bore.

11. The solid oxide fuel cell according to claim 2, characterized wherein the protruding elevations or bosses are symmetrically arranged with respect to each other within the central bore.

12. The solid oxide fuel cell according to claim 2, characterized wherein said ridges or protrusions are generally conical.

13. The solid oxide fuel cell according to claim 2, characterized wherein said ridges or protrusions are generally square or rectangular.

14. The solid oxide fuel cell according to claim 2, characterized wherein said ridges or protrusions are generally circular.

15. The solid oxide fuel cell according to claim 1, characterized wherein said tubular body is open at both ends or closed at one end.

16. The solid oxide fuel cell according to claim 1, characterized wherein said support means protruding into the central bore of said tubular body comprises the same structural material as the inner wall.

17. The solid oxide fuel cell according to claim 16, characterized wherein the structural material of said anode and said support means comprises a cermet.

18. The solid oxide fuel cell according to claim 17, characterized wherein said cermet comprises stabilized zirconia or doped ceria.

19. The solid oxide fuel cell according to claim 18, characterized wherein the stabilized zirconia is comprised of (ZrO)₂)_0.92(Y₂O₃)_0.08The material of (1).

20. The solid oxide fuel cell according to claim 18, characterized wherein said doped ceria is a ceria-based solution comprising (Ce)_0.90Gd_0.10)O_1.95The material of (1).

21. The solid oxide fuel cell according to claim 17, characterized wherein the metal phase of the cermet is selected from the transition metal group of the periodic table of the elements in an elemental state selected from the group consisting of elemental metals, alloys and mixtures thereof.

22. The solid oxide fuel cell according to claim 21, characterized wherein said transition metal is nickel.

23. The solid oxide fuel cell according to claim 21, characterized wherein the content of the metal phase of the cermet is in the range of 30 vol% to 80 vol%.

24. The solid oxide fuel cell according to claim 3, characterized wherein the cathode thickness in the sintered state is 0.2 mm to 2.0 mm.

25. The solid oxide fuel cell according to claim 1, characterized wherein the thickness of said supporting means protruding into said central bore in a sintered state is 0.1 to 2.0 mm.

26. A method for manufacturing a solid oxide fuel cell comprising at least a supporting anode, characterized by comprising the steps of:

(i) blending a ceramic electrolyte material and an electrochemically active transition metal or transition metal oxide to form a fuel electrode mixture;

(ii) forming said fuel electrode mixture into a single tubular fuel electrode having a hollow central bore, said electrode having at least one solid protuberance or bump projecting inwardly into the longitudinal direction of said hollow central bore, and

(iii) drying the tubular fuel electrode.

27. The method according to claim 26, characterized wherein the ceramic electrolyte material is a ceramic powder selected from the group consisting of stabilized zirconia and doped ceria.

28. The method of claim 26, characterized wherein said electrochemically active transition metal introduced into said fuel electrode mixture is a metal oxide powder.

29. The method of claim 26, characterized wherein said transition metal is at least partially incorporated into said fuel electrode mixture by pre-dissolving a metal compound in an aqueous or non-aqueous solvent.

30. The method of claim 26 including the step characterized by introducing a pore former into said fuel electrode mixture.

31. The method of claim 26 wherein said fuel electrode mixture is a plastic billet suitable for extrusion.

32. The method of claim 26, characterized wherein said fuel electrode mixture is an aqueous or non-aqueous slurry suitable for molding by casting.

33. The method of claim 26 characterized wherein said fuel electrode mixture is a dry blend molded by a compression process.

34. The method according to claim 26, comprising the further step, characterized by:

(iv) applying an electrolyte layer to the dried tubular fuel electrode;

(v) (iii) sintering the fuel electrode-electrolyte structure of step (iv);

(vi) (vi) applying at least one cathode layer to the step (v) sintered fuel electrode-electrolyte structure, and

(vii) sintering the fuel electrode-electrolyte-cathode structure to form a tubular solid oxide fuel cell.

35. A supporting anode for a solid oxide fuel cell, characterized by comprising a single tubular body having an inner wall defining a hollow central bore, said inner wall comprising at least one support means projecting into said hollow central bore for structurally reinforcing the anode.

36. The supported anode of claim 35, wherein said support means is integral with said anode and comprises a plurality of bumps or protrusions.