Abstract:
The present invention is directed generally to an electrochemical apparatus for oxidation or consumption of a fuel, and the generation of electricity, such as, a solid electrolyte fuel cell. The electrochemical apparatus ( 1 ) comprises at least one cell ( 2 ), wherein the cell ( 2 ) has a solid electrolyte ( 10 ) disposed between a unitary oxygen electrode ( 12 ) and a unitary fuel electrode ( 8 ), and at least one separator ( 6 ) contacting the surface of one of the electrodes ( 13 ) opposite of the electrolyte ( 10 ). At least one electrode ( 13 ) of the cell ( 2 ) defines a micro-channel pattern ( 26 ), wherein the micro-channel cross-section is preferably varied, such that reactant gas flowing through the micro channels achieves tailored local flow, pressure, and velocity distributions.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of copending application U.S. Ser. No. 09/455,149, filed on Dec. 6, 1999 (allowed) which is incorporated herein by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to fuel cells, and more particularly to fuel cells constructed of stacked plate components. More particularly, the present invention relates to fuel cells containing enhanced flow electrodes for fuel and/or air.  
         BACKGROUND OF THE INVENTION  
         [0003]    The invention is directed generally to an electrochemical apparatus for oxidation or consumption of a fuel, and the generation of electricity, such as, a solid electrolyte fuel cell.  
           [0004]    Although particular embodiments are applicable to conventional co-fired solid electrolyte fuel cell apparatus, the present invention is particularly useful when utilizing non-cofired solid oxide electrolyte fuel cells, preferably planar fuel cells, that contain a stack of multiple assemblies. Each assembly comprises a solid electrolyte disposed between a cathode and an anode, being bounded by separators, which contact the surfaces of the electrodes opposite the electrolyte.  
           [0005]    The fuel cell operates by conducting ions through the electrolyte. For solid oxide fuel cells in particular, oxygen or air is introduced at the cathode, and ionization of oxygen occurs at the cathode/electrolyte surface. The oxygen ions move across the gas non-permeable electrolyte to the anode interface, where it reacts with the fuel flowing into the anode at the anode/electrolyte interface, releasing heat and supplying electrons to the anode. Distribution of the air and fuel reactants is typically performed by a manifold assembly within the fuel cell apparatus.  
           [0006]    Conventionally, each reactant is supplied through a flow conduit to the appropriate electrode, and distribution to the electrode/electrolyte interface is accomplished by internal porosity and/or grooved channels.  
           [0007]    Minh, U.S. Pat. No. 5,256,499, discloses a monolithic fuel cell having an integrally formed manifold constructed by corrugations formed within the anode and cathode with aligned ribs and columns arranged to force fuel and oxidant along aligned pathways. Reactants are fed from the sides of the fuel cell and travel along these pathways.  
           [0008]    Hsu, U.S. Pat. No. 5,747,485, discloses a conductor plate for a solid oxide fuel cell with ridges extending therefrom. These ridges form grooves used to channel reacting gases out of the cell.  
           [0009]    Datta, U.K. Patent No. 2,219,125A discloses an electrolyte with a three-dimensional groove arrangement used to control hot spots within the electrolyte block.  
           [0010]    Hsu, Minh and Datta employ external manifolding and rectangular geometries driving the reactants from one side of the cell to the other. Despite the use of channels, reactants entering from a single side of the cell deplete as they travel across the cell. Further, when reactants are fed externally from more than one side, the flows converge creating localized areas of increased reaction. The increased number of reactions generates an undesirable thermal gradient, which can damage the cell.  
           [0011]    Moreover, Hsu, Minh and Datta employ grooves of uniform cross section along the length of these grooves. These grooves are essentially pathways within the cell, and fail to control gas flow rate or pressure distribution. The flow rate is controlled at its source and not tailored or controlled within the cell.  
           [0012]    In fuel cells which have their anode fuel-exit edges exposed to an oxidizing environment, any anode local exit regions having low fuel mixture velocities may allow oxygen back diffusion into the cell stack, causing premature combustion and loss of active anode area. The electrochemical processes inherent in the fuel cell&#39;s operation become less effective and performance suffers.  
           [0013]    Custom flow pattern design is desirable to achieve substantially uniform reactant concentration distribution within the cell and from cell to cell within a stack, which also helps minimize unnecessary and undesirable thermal gradients within the cell.  
           [0014]    It is desirable to provide a compact, centrally fed radial fuel cell utilizing micro-channels to tailor the flow distribution of reacting gases within the fuel cell and amongst all the cells in a stack.  
           [0015]    It is also desirable to provide a compact fuel cell utilizing variable cross-section micro-channels to tailor the flow, pressure, and velocity distribution of reacting gases within the fuel cell and amongst all the cells in a stack.  
           [0016]    It is also desirable to provide an enhanced flow electrode produced by simple scalable production techniques.  
         SUMMARY OF THE INVENTION  
         [0017]    We have found that micro-channels integrated within the electrode structure can be formed in a compact fuel cell. Integrated micro-channels minimize the complexity of stack components. Channels of smaller dimension than those existing in the prior art can be manufactured by a variety of techniques. Using these techniques, flow and pressure distribution can be customized and controlled through the channel design, enhancing reactant distribution to the cell. It has further been found that a fuel cell apparatus employing a network of micro-channels can improve overall cell reactant balance through controlled pressure distribution. It has further been found that employing controlled flow and pressure in a compact integrated device results in an apparatus exhibiting improved volumetric power density and efficiency.  
           [0018]    The present invention therefore provides an electrochemical apparatus comprising at least one cell, wherein the cell has a solid electrolyte disposed between an oxygen electrode and a fuel electrode, with at least one separator between adjacent cells contacting the surface of one of the electrodes opposite the electrolyte; wherein at least one electrode of the cell defines a variable cross-section micro-channel pattern, wherein this pattern serves to distribute the flowing gas uniformly within the electrode, regulates the pressure drop of this gas, and also creates preferred local gas velocities, especially where the gas exits the electrode.  
           [0019]    The present invention further provides an electrochemical apparatus comprising at least one cell, having a solid electrolyte disposed between an oxygen electrode and a fuel electrode; and at least one separator contacting the surface of one of the electrodes opposite the electrolyte. In one embodiment, at least one separator preferably defines a micro-channel pattern; wherein the micro-channel pattern narrows towards the cell rim, such that gas flowing out the rim is accelerated.  
           [0020]    The micro-channel is preferably a small size, on the order of about 0.5 millimeter or less, such that the micro-channel can be defined within at least one electrode or separator by low-cost manufacturing techniques.  
           [0021]    The present invention also provides an electrochemical apparatus comprising an electrode defining a pattern of micro-channels for directing the flow of reactant; wherein the cross sectional area of the micro-channels is varied along the micro-channel length.  
           [0022]    The present invention also provides an electrochemical apparatus comprising a plurality of cells forming a stack; each cell within the stack has a solid electrolyte disposed between an oxygen electrode and a fuel electrode, with at least one separator contacting the surface of one of the electrodes opposite the electrolyte. In substantially each of these cells, at least one electrode defines a variable cross-section micro-channel pattern.  
           [0023]    The present invention also provides an electrochemical apparatus comprising at least one cell having a solid electrolyte disposed between an oxygen electrode and a fuel electrode, and at least one separator contacting the surface of one of the electrodes opposite the electrolyte; wherein at least one electrode or the electrolyte or the separator surface has a plurality of columns extending therefrom; said columns defining variable cross-section micro-channels therebetween.  
           [0024]    The present invention also provides an electrochemical apparatus comprising at least one circular cell having a cell rim; said cell has a solid electrolyte layer disposed between an oxygen electrode layer and a fuel electrode layer; at least one separator layer contacting the surface of one of the electrodes opposite the electrolyte; wherein each of the layers define at least one air hole and at least one fuel hole and wherein the respective holes in each layer are registrable with one another and define generally central internal air and fuel manifolds; wherein at least one layer has a plurality of circular columns extending longitudinally outwardly from the respective air or fuel manifold, defining a micro-channel pattern. Preferably, the columns are arranged in radially expanding rows; and an increasing number of columns extend from said at least one layer in each of said rows, such that said columns define a variable cross-section micro-channel that narrows toward the cell rim.  
           [0025]    The present invention also provides an electrochemical apparatus comprising at least one fuel cell, wherein the cell has a solid electrolyte disposed between an oxygen electrode and a fuel electrode, and at least one separator contacting the surface of one of the electrodes opposite the electrolyte; wherein the cell defines at least one air manifold and at least one fuel manifold located substantially centrally within the cell; and at least one of the electrodes defines a micro-channel pattern.  
           [0026]    The present invention also provides an electrochemical apparatus comprising at least one cell; wherein the cell has a solid electrolyte disposed between an oxygen electrode and a fuel electrode, wherein at least one said electrode is a unitary electrode, at least one separator contacting the surface of one of the electrodes opposite the electrolyte; wherein the at least one electrode defines a variable cross-section micro-channel pattern adapted to control flow distribution and pressure drop of at least one reactant gas.  
           [0027]    The present invention further provides an electrochemical apparatus comprising at least one cell, having a solid electrolyte disposed between oxygen electrode and a fuel electrode; wherein at least one said electrode is a unitary electrode; and at least one separator contacting the surface of one of the electrodes opposite the electrolyte; wherein the at least one separator defines a pattern of micro-channels; wherein the micro-channel cross-sectional area narrows toward the cell rim adapted to control flow distribution and pressure drop of at least one reactant gas.  
           [0028]    The present invention also provides an electrochemical apparatus comprising a unitary electrode defining a pattern of micro-channels, wherein the electrode has a rim; and said micro-channels have a cross-sectional area that decreases towards the electrode rim for the controlled flow distribution and pressure drop of at least one reactant gas.  
           [0029]    The present invention further provides, in a process for the fabrication of a solid oxide fuel cell comprising at least one cell having a cell rim, wherein said cell has a solid electrolyte layer disposed between an oxygen electrode layer and a fuel electrode layer, and at least one separator layer contacting the surface of one of the electrodes opposite said electrolyte; wherein each of the layers define at least one air hole and at least one fuel hole and wherein the respective holes within each layer are registerable with one another and define generally central internal air and fuel manifolds; the improvement including providing reactant micro-channels in at least one layer, said micro-channels having a width of not more than about 0.5 mm.  
           [0030]    The micro-channel patterns may be fabricated by a variety of known fabrication methods. One preferred method is the use of mechanical pressing. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    [0031]FIG. 1 is a partially schematic, partially exploded side view of fuel cells capable of having an enhanced flow micro-channel pattern in one of the layers according to the present invention.  
         [0032]    [0032]FIGS. 2A and 2B are plan views of enhanced flow micro-channel containing electrodes according to the present invention.  
         [0033]    [0033]FIG. 3 is a partially schematic sectional side view of a cell according to the present invention as seen along line  3 - 3  in FIG. 2.  
         [0034]    [0034]FIG. 4 is a partially schematic sectional side view of an enhanced flow electrode containing micro-channels according to another embodiment of the present invention.  
         [0035]    [0035]FIG. 5 is a partially schematic sectional side view of a cross flow layer channel according to the prior art. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]    Although applicable to other types of electrochemical apparatus, for purposes of this description the invention will be described in relation to its incorporation into a solid electrolyte (oxide) fuel cell as described in U.S. Pat. No. 5,445,903, incorporated by reference as if reprinted herein. The electrochemical apparatus  1  of one embodiment of the present invention is represented in FIG. 1, which shows a schematic exploded view of one preferred embodiment of a solid-oxide fuel cell  2  and a stack of two such cells  4 .  
         [0037]    A cell  2  generally comprises four stacked layers: a separator  6 , a cathode layer  8 , an electrolyte  10 , and an anode layer  12 . Cathode layer  8  and anode layer  12  may be referred to in the general sense as electrodes  13  and in a preferred embodiment at least one such electrode is comprised of a unitary (single) component as contrasted to an active electrode structure and an associated porous element. A tubular gasket  14  in a cathode layer forms a seal between the nonporous separator and electrolyte. A pair of tubular gaskets  16  in the anode layer form seals between the electrolyte and separator. Gaskets  14  and  16  must remain impervious to fuel and air respectively at the relatively high operating temperature of the cell and must be capable of maintaining a good seal under cell operating conditions. Suitable gaskets  14  and  16  can be made from oxidation resistant metal alloys such as nickel-base alloys, from ceramics, or from glasses or glass-ceramics having suitable softening temperatures.  
         [0038]    As shown in FIGS. 1 and 3, the separator contains an internal fuel hole  18 , which is aligned with corresponding holes in the other cell layers to form an internal fuel manifold  19 . It also contains a pair of internal air holes  20 , which are aligned with corresponding holes in the other cell layers to form a pair of internal air manifolds  21 . It is within the scope of the invention to include single or multiple fuel passages and/or oxygen passages in various cell locations, preferably close to the centerline of the cell.  
         [0039]    A suitable hot fuel gas mixture  22 , represented by an arrow, is fed to the internal fuel manifold  19  and hot air  24 , represented by arrows, is fed to both internal air manifolds  21 . The stack of fuel cells will typically operate at about 850° to 1000° C., but may operate as low as 600° C. with suitable low-temperature solid electrolytes.  
         [0040]    The separators  6  must be impervious to gases, be good conductors of electrons, and have long-term compatibility with both the adjacent material and with the air and fuel mixtures. They should also be fairly good conductors of heat. Suitable materials include doped lanthanum chromite or high-temperature metallic alloys, such as RA330, Ducralloy, Inconel 601, or Haynes 230 available from Rolled Alloys, Plansee, Inco Alloys International, and Haynes respectively.  
         [0041]    The porous cathode layer or oxygen electrode  8 , is generally made of a mixed oxide preferably such as strontium-doped lanthanum manganite (LSM). The electrolyte  10  is impervious to gases and is a good oxygen ion conductor while having little or no electronic conductivity. Yttria-doped zirconia having about 6 to 10 mole percent Y 2 O 3  is preferred. The electrolyte  10  is preferably coated with a thin, fired layer of LSM on the cathode side and nickel oxide/doped ceria on the anode side.  
         [0042]    The porous anode layer or fuel electrode  12  is preferably made of nickel felt, nickel-zirconia cermet, or other nickel-containing cermet or alloy.  
         [0043]    Cell and stack diameters are typically about 50 to about 80 mm and total cell thickness (in use) is typically about 1 to about 2 mm, but can be of slightly larger diameter.  
         [0044]    When the cells  2  are stacked, a series electrical connection is established among all the cells in the stack, such that the stack voltage is the sum of all the cell voltages. In use, in one embodiment a stack is clamped between a pair of high-temperature electrical contact blocks equipped with mating holes for feeding gaseous fuel and air via feed tubes (not shown). At one end of the stack, the separator  6  is omitted and, thus, the stack is bounded by a cathode layer  8  at one end and an anode layer  12  at the other end. The fuel gas and air may be fed into opposite ends or the same ends of the stack.  
         [0045]    The stack is operated by preheating the apparatus close to operating temperature, supplying air and fuel gas, and connecting an external electric load. Oxygen from the air is ionized at, or near, the cathode-electrolyte interface. The oxygen ions flow through the electrolyte under the influence of the chemical potential difference. At, or near, the electrolyte-anode interface the oxygen ions combine with fuel molecules (chiefly hydrogen and carbon monoxide), releasing electrons which flow into the next cell. Typical power densities are on the order of about 150 mW/cm 2  of electrode area at typical cell operating voltages near about 0.7 volts. Typical stack volumetric power densities are close to about 1.0 kilowatt/liter.  
         [0046]    The cathode layer  8  is preferably a porous body having a thickness in the range of about 0.2 to about 0.6 mm, and composed of conventional cathode material, most preferably an oxide having the perovskite crystalline form such as strontium doped lanthanum manganite (LaMnO 3 ), doped calcium manganite (CaMnO 3 ), lanthanum chromite (LaCrO 3 ), lanthanum cobaltite, (LaCoO 3 ), lanthanum nickelite (LaNiO 3 ), lanthanum ferrite (LaFeO 3 ), or mixtures thereof. The cathode  8  may comprise mixed ionic/electronic conductors such as an appropriately doped perovskite oxide listed above. The cathode  8  can be prepared by conventional ceramic processing procedures for making a flat, planar structure, including pressing a powder, or extruding or tape casting a green body, and sintering either prior to or during the initial operation of the apparatus.  
         [0047]    Electrolyte  10  is a thin wafer, generally less than about 0.4 mm thick, preferably about 0.2 mm or less of conventional solid oxide fuel cell electrolyte material. Representative electrolytes include zirconia (ZrO 2 ) stabilized with 6 to 10 mole percent of yttria (Y 2 O 3 ), doped cerium oxide, doped bismuth oxide, and oxide ion conducting perovskites. Electrolyte  10  is substantially impervious to gases, however, ionized oxygen can migrate through the electrolyte under the influence of applied oxygen potential.  
         [0048]    The quality of the electrical contact between the cathode  8  and the electrolyte  10  may be improved by initially applying a thin layer of substantially the material that comprises the cathode  8  (or is at least electrochemically compatible with the cathode) to the surface of the electrolyte  10  adjacent the cathode  8  in the form of a paint or ink including a volatile vehicle to form an electrical contact zone. Likewise, a paint or ink containing substantially anode material such as nickel or nickel oxide may be applied to the surface of the electrolyte adjacent the anode to form such an electrical contact zone. This electrolyte surface coating may be applied by other conventional techniques also, such as plasma deposition, spin casting, spraying, or screen printing.  
         [0049]    The thickness of the electrolyte surface coatings is generally on the order of about 1 to less than about 100 microns, and preferably less than 50 microns. It has been found that the thicker this surface coating is applied, the less gas is able to contact the electrolyte  10 , and the more tendency there is for the coating to peel off. Unless specifically stated to the contrary, the electrolyte  10  as mentioned in this Specification shall mean the electrolyte  10  with or without either or both cathode and anode material surface coatings.  
         [0050]    Anode  12  is a porous body, and may comprise conventional solid oxide fuel cell anode material. Preferably, the anode comprises either nickel felt or else a finely divided, compressed metallic powder such as nickel blended with a stable oxide powder such as zirconia, cation-doped ceria. As described above regarding the cathode  8 , the anode  12  may comprise a mixed conductor, optionally combined with an electronically conducting material. Other examples include ceria, which can be doped with an oxide of lanthanum, zirconium or thorium, optionally containing an electronically conducting phase such as Co, Ru, or Pt. The thickness of the anode is preferably about 0.1 mm to about 0.5 mm. Like cathode  8 , anode  12  may be sintered during cell operation or before initial operation in an overheating sintering step.  
         [0051]    In the preferred embodiment as shown in FIG. 2A, at least one electrode  13  defines a plurality of micro-channels  26 , as necessary. In the alternative, the separator  6  might define the micro channels  26  on either or both of its surfaces. The narrowing of the micro-channels  26  is not linear based on the distance from the flow source in the cell element but rather narrow non-linearly progressively towards the cell rim  32 , such as a function r n  where r is the distance along the channel toward the cell rim and n is a number greater than 1, that depends upon the system parameters. Since the separators contact the anode and cathode surfaces, micro-channels  26  defined within the separator surfaces would also provide reactant channeling. For sake of simplicity, the description, while referring to electrode micro-channels, encompasses micro-channels formed within the separator  6  as well.  
         [0052]    As shown in FIG. 2A, micro-channels  26  may be formed within an electrode  13 . These micro-channels  26  create a preferential path for reactant flow across the electrode  13 . As shown, in simplified form, a micro-channel  26  may be defined by a quantity of regularly spaced circular columns  34  extending between surfaces of adjacent layers. (Although circular columns are preferred, columns of other geometries may be utilized to provide customized flow characteristics.) The spaces between the columns  34  provide a preferential path for gas flow. Using cathode  8  as an example, air enters the micro-channel  26  from internal air manifold  21  via air holes  20 . Gaskets or seals  14  isolate the air from fuel manifolds  19  and fuel hole  18  formed within cathode  8 . The entering gas spreads outwardly amongst the columns  34  of electrode material, successively passing the columnar rows from inner row  36  to outer row  38  before exiting at the rim  32 . It should be understood that a preferred pattern of columns  34  would utilize many more columns than shown in the simplified figure, with each column having a diameter on the order of about 1 mm or less. The height of each column 34 is generally on the order of about 0.05 mm to about 0.4 mm, preferably about 0.1 mm. It should be appreciated that the depth of micro-channels  26  may comprise substantially the entire thickness of the electrode  13 .  
         [0053]    The preferred pattern may be designed to control flow distribution within a cell  2  by defining pathways that offer reduced resistance in comparison with the surrounding material. The flow distribution may be further controlled by the number, size, or arrangement of the micro-channels  26  within the cell  2 .  
         [0054]    The preferred pattern is designed with consideration to the column spacing and the contact-area percentage. Column spacing may be relatively wide to help minimize the cell pressure drop. Pressure is controlled by the size of the column (diameter) and the number of columns per square centimeter. The column diameter and the contact-area percentage may be selected by a compromise between minimizing electrical resistance, achieving good reacting gas distribution to and from the active electrode sites, achieving the target pressure drop within a minimum pattern thickness, and fabrication limitations, if any.  
         [0055]    The pattern may be designed to achieve a specific overall pressure drop at its design gas flow rate. It is also possible to manufacture a pattern with a desired lack of symmetry, to account for any expected side-to-side temperature difference within the stack, for example. Both the column shape and pattern layout may vary to produce the desired result. While the columns are shown in the Figures to be of a circular cross section, it is within the scope of the invention that the columns be formed with other cross sectional shapes, such as ovals, squares, rectangles, and other regular or irregular polygonal shapes. It should further be understood that in addition to columnar patterns, continuous channels may be formed within electrode  13  including grid channels, spiral channels, and radial line channels. The distribution of flow and achievement of a desired pressure drop may be controlled by using these types of channels as described above.  
         [0056]    At the stack level, the flow distribution along the length of the stack may similarly be controlled by varying the number, size, and distribution of micro-channels  26  in different cells in accordance with the desired stack-wide distribution of reactants.  
         [0057]    [0057]FIG. 2B is a simplified schematic illustration of an example fuel electrode  12  micro-channel pattern with variable cross-section flow channels formed on a separator  6 . The pattern consists of a quantity of circular posts or columns 34 with open spaces between them where the gas flows. The fuel gas is fed into the micro-channel  26  pattern from fuel manifold  19  via a fuel hole  18 . Seals  16  isolate the fuel from air manifolds  21  and air holes  20 . The gas flows outwards amongst the columns, first passing the inner row of columns 36 and finally the outer row of columns 38 before exiting at the rim  32 . The preferred pattern would utilize many more columns than shown in this simplified figure, with each column having a diameter on the order of about 1 mm or less. The preferred height of each column is very short, on the order of about 0.1 mm.  
         [0058]    Using variable cross-section micro-channels, the preferred pattern would be designed using several considerations as follows. The column spacing would be relatively wide near the center of the cell, where the gas flow diameter is small, to help minimize the cell pressure drop. The spacing would be relatively narrow near the rim of the cell in order to achieve a good gas exit velocity, thereby preventing the surrounding gas mixture from diffusing backwards into the cell. The diameter of the columns and their contact-area percentage based on the area of the adjacent layer would be selected as a compromise between minimizing electrical resistance, achieving good reactant gas distribution to and from the active electrode sites, achieving the target pressure drop with a minimum pattern thickness, and fabrication limitations, if any. If the inner row of columns were arranged in a circular pattern as shown, good circumferential symmetry of gas flow could be achieved even when the center cavity is non-circular.  
         [0059]    The pattern may be designed to achieve a specific target overall pressure drop at its designed gas flow rate. It would also be possible to manufacture a pattern with a desired lack of circular symmetry, if so wished due to an expected side-to-side temperature difference of the stack, for example. Both the column shape and the pattern layout could vary in many different ways as still be able to produce the desired results. Additionally, the thickness or height of the pattern might be varied from center to rim as another means for tailoring local flow, pressure, and velocity. It should be understood that micro-channels  26  may comprise substantially the entire thickness of the electrode.  
         [0060]    It should further be understood that in addition to columnar patterns, continuous channels may be formed within electrodes  13 . Some examples include, grid channels, spiral channels, and radial line channels. In a manner similar to the patterns, the flow, pressure, and velocity of reactants may be controlled by varying the cross-section of these channels.  
         [0061]    The micro-channels  26  may be fabricated into the surface of electrode  13 , electrolyte  10  or separator  6  by a variety of conventional subtractive techniques including electrical-discharge machining, stamping, laser ablation, chemical etching, ultrasonic etching, scribing, and grinding. As a benefit of the present invention, the micro-channels  26  may be formed by photolithography, pressing, calendering, micro electro mechanical systems (MEMS) techniques, or additive deposition techniques, air brush painting, stenciling, or screen printing. MEMS techniques include microetching, and micro- or nano-machining. The use of these techniques is possible because of the electrode  13  and micro-channel  26  size. The micro-channels may be formed by additive or subtractive techniques as set forth above, as applied to an electrode, electrolyte or separator. Material can be removed from the surface of one of the layers to provide the micro-channel, or material can be added to the surface of at least one of the layers. For example, electrode material can be deposited on the electrode, or the adjacent separator or electrolyte surface, to form columns which define the micro-channels as the space therebetween.  
         [0062]    In the electrodes, the pillars  34  or micro-channels  26  are preferably made by uniaxially pressing a pattern into an unfired electrode preform. This preform is made of electrode powder or premixed ceramic-metallic powders mixed with an organic binder material. This combination of components is processed into a soft, ductile mixture having a dough-like consistency that can be easily pressed into a variety of shapes. The mixture is sufficiently rigid, however, to retain any impressed pattern including columns 34 and micro-channels  26 .  
         [0063]    [0063]FIG. 4 depicts a porous electrode  13  having micro-channels  26  formed between columns 34 of electrode material. The width of the micro-channels is generally on the order of about 0.1 to about 0.5 mm, and the depth of the micro-channels is generally on the order of about 0.1 to about 0.5 mm, although the micro-channel can be as deep as the thickness of the electrode layer, if the electrode is formed on an adjacent layer such as the electrolyte or separator. As an example, for an electrode  13  having a thickness “a” of 0.5 mm, an effective micro-channel could be on the order of 0.15 mm×0.15 mm height×depth. Comparatively, the prior art, depicted in FIG. 5, provides crossflow channels  51  in metallic separators  52  having a thickness “b” on the order of 3 mm, in which the height and depth “c” of the crossflow channels are on the order of 1 mm×1 mm.  
         [0064]    To begin operation of the electrochemical apparatus, the fuel cells  2  are heated by an outside heat source to near their operating temperature. Once the reaction is initiated, it sustains itself by producing sufficient heat to support the ongoing cell operations. At the same time, an electrical current flows through the stack by virtue of the oxygen ionization and neutralization within each cell. This electrical current, driven by the oxygen potential difference, is the electrical output energy. To produce useful quantities of electric power having a useful voltage, fuel cells  2  of the type shown in FIG. 1 are typically arranged in a series connected stack. Because each of the fuel cells  2  is so thin, up to hundreds of cells can be assembled in a single stack of reasonable physical size.  
         [0065]    Respectively, a gaseous fuel  22  is supplied to fuel manifold  19  and an oxygen-bearing gas  24 , such as air, is supplied to air manifold  21 . The oxygen-bearing gas flows through pores (and micro-channels, if used) in the cathode  8 , driven by the difference in the gas pressures in the manifold and outside the cathode  8 . The oxygen becomes negatively ionized in the cathode  8  at or near the electrolyte  10 . The electrolyte  10  is a good conductor of oxygen ions. Oxygen ions, thus, flow through the electrolyte  10  to reach the anode  12 . At the anode  12 , these ions give up their excess electrons to become oxygen atoms and molecules, fuel  22  flows through the porous anode (and micro-channels if used) and combines with the oxygen to form water (and other products if fuels other than hydrogen are used), releasing thermal energy.  
         [0066]    At the stack level, the micro-channel cross-sectional area within each cell  2  can also be varied from fuel cell to fuel cell to improve the overall reactant balance within the stack. To illustrate, reactants enter the stack at one end. The fuel manifold  19  has some finite pressure drop, so as reactant flows along the manifold, there is a gradation in pressure from one end of the stack to the other. For uniform electrodes, the gradient in pressure in the fuel manifold  19  results in a differential flow across each anode  12 . However, the cross-section of the micro-channels  26  can be tailored such that the pressure drop (or resistance to flow) across each anode  12  compensates for the pressure drop within the fuel manifold  19 , thereby enabling consistent reactant distribution from one end of the stack to the other. Reducing or increasing the number of micro-channels  26  can be used to produce the same effect.  
         [0067]    In a stack with reactants being fed from the top, the pressure of reactants within the internal manifold will decrease progressively towards the bottom of the stack. To compensate for this decrease, the net cross-sectional area of the micro-channels  26  in each cell within the stack can be progressively increased from top to bottom. By increasing the net cross-sectional area from top to bottom, a generally even distribution of reactants across the stack height will result. To achieve a balanced distribution of reactants in other flow arrangements, for instance where fuel is fed from one end and oxygen bearing gas from the opposite end, the cross-sectional area of the micro-channels on the anode  12  and cathode  8  may be varied according to the direction of the flow. In a stack that receives fuel  22  from the bottom of the stack and oxygen bearing gas  24  from the top, the cross-sectional area of the cathode micro-channels in each cell would be increased from top to bottom, and the cross-sectional micro-channel area of the anode would be increased from bottom to top to balance the distribution of reactants across the stack.  
         [0068]    Balanced flow distribution of reactants reduces thermal gradients within the cell  2 . Reactant depleted areas produce less heat than reactant rich areas, thus, uniform reactant supplies across the cell  2  and stack reduce the thermal gradients.  
         [0069]    Cells  2  incorporating the varied micro-channel  26  are preferably symmetrical about a central access. Oval, circular, or other symmetrical shapes offer good performance. Most preferably, the cell&#39;s major surface will have a circular shape with central feed holes. The central feed design facilitates uniform reactant flow distribution and allows high reactant utilization rates.  
         [0070]    As can be appreciated, an almost infinite number of pattern configurations are possible. It should further appreciated that while the above description is made with reference to a planar fuel cell, the present invention will include non-planar configurations including but not limited to tubular fuel cells. Therefore, the above pattern is presented as an example only and does not limit the scope of the claimed invention.  
         [0071]    Other embodiments of the solid oxide fuel cell and its components are disclosed in U.S. Pat. Nos. 5,445,903 and 5,589,285, assigned to the common assignee of the present invention, which patents are hereby incorporated by reference as if fully written out below.  
         [0072]    It should now be apparent that various embodiments of the present invention accomplish the objects of this invention. It should be appreciated that the present invention is not limited to the specific embodiments described above, but includes variations, modifications, and equivalent embodiments defined by the following claims.