Patent Document

TECHNICAL FIELD 
     The present invention is directed to an electrochemical apparatus such as solid-oxide electrolyte fuel cells and fuel cell assemblies for directly converting chemical energy into electricity. More particularly, it is directed to a fuel cell assembly incorporating a thermally integrated vaporizer-reformer. 
     BACKGROUND OF THE INVENTION 
     Fuel cells offer many advantages over conventional power generation systems. It is generally known that such devices are capable of delivering high quality electric power with greater efficiency and lower emissions when compared to comparably sized gas or diesel fueled generators. Further, such systems are generally modular and can fulfill a wide range of energy needs including remote site power generation, light utility, and transportation applications as well as commercial cogeneration and residential applications. 
     Solid oxide fuel cells are well-known devices that are capable of producing electric power at higher efficiency. However, there are a number of major hurdles including issues of operation, scale, and cost. 
     Known solid oxide fuel cells operate by the introduction of air into the cathode and the ionization of oxygen at the cathode/electrolyte surface. The oxygen ions move across the gas non-permeable electrolyte to the anode interface, where they react with the fuel gas flowing into the anode releasing heat and giving up their electrons to the anode. The electrons pass through the anode and separator into the next adjacent cathode. 
     A wide variety of fuels can be used in the electrochemical apparatus of the present invention, including hydrogen, carbon monoxide, alcohols, ammonia, methane, and gaseous- or liquid-hydrocarbon mixtures. The oxidizer gas to, be supplied to the cathode can be oxygen or an oxygen containing gas such as air. 
     Reformation, such as steam reformation, of carbon-containing fuels is necessary to prevent solid carbon deposition (“coking”) at the high solid-oxide stack operating temperatures and also to minimize local stack temperature variations. The output fuel gas (“syngas”) from a reformer intended for a solid-oxide fuel cell will preferably comprise chiefly hydrogen, steam, carbon monoxide, carbon dioxide, (optional) nitrogen, and contain less than about 5 percent of methane. In most cases, the reformer must be preceded by a vaporizer to vaporize water and liquid fuel (if used) followed by a mixer to mix the steam with the fuel gas and/or vapor. 
     Adding the vaporizer and reformation steps to the energy conversion process adds to the cost and size of the unit. The required heat input for vaporization and reforming may be obtained from the surplus heat from the fuel cell stack operation and/or using burners or partial oxidation of the fuel stream. 
     Many different types of vaporizers and reformers exist in the prior art, but, known designs contain their own disadvantages. These disadvantages include bulky subassembly designs, short operating times between required cleaning, and high equipment cost. Many systems rely on partial-oxidation reforming or burners as a heat source, thereby reducing overall system fuel efficiency. Finally, many of the known reformers can produce syngas having high methane content, leading to large temperature variations in the fuel cell stack (cold spots occur when high-methane syngas contacts cells) which tend to reduce both stack power and its operating life. 
     Thus, electrochemical apparatus incorporating a thermally integrated vaporizer-reformer capable of high fuel to electric power efficiency is desirable. It is further desirable to create an electrochemical apparatus incorporating a thermally integrated vaporizer reformer, which is compact, low in cost, requires infrequent cleaning or maintenance, and produces a fuel gas mixture containing low methane levels. 
     SUMMARY OF THE INVENTION 
     The present invention is an electrochemical apparatus incorporating a thermally integrated vaporizer and fuel steam reformer. 
     It is an object of the present invention to produce an electrochemical apparatus incorporating a thermally integrated vaporizer and reformer that has a long operating life, and is of moderate size, weight, and cost. 
     It is another object of the present invention to provide an electrochemical apparatus with a high fuel to electric power efficiency. 
     It is another object of the present invention to provide an electrochemical apparatus that is compact, light weight, and can be mass produced. 
     The present invention provides an electrochemical apparatus including a steam reformer connected to a gaseous or vaporized fuel supply and a vaporized water supply by at least one feed tube; the steam reformer having a mixing orifice and a fuel orifice, wherein the mixing orifice is adapted to receive and mix vaporized water and fuel; a fuel cell stack positioned adjacent the steam reformer and having an air passage, and a fuel passage communicating with the fuel orifice; an annular cap positioned above the fuel cell stack and spaced apart from the stack by a sealing ring, wherein the annular cap and the sealing ring define an air inlet communicating with the air passage and adapted to channel air into the air passage; a can extending downwardly from the annular cap spaced apart from the fuel cell stack and steam reformer defining an exhaust passage therebetween; an enclosure surrounding and spaced apart from the can defining an open space therebetween which is at least partially filled with insulation; and at least one coolant inlet formed within the enclosure for receiving coolant from a source. The coolant can be the oxidizing gas. 
     The present invention further provides an electrochemical apparatus including an enclosure having a base, a top, and an outer wall extending therebetween; a vaporizer positioned generally centrally of the base, the vaporizer including a mixing orifice; at least one feed tube extending through the base into the vaporizer; a steam reformer positioned on top of the vaporizer and communicating with the mixing orifice, the steam reformer having a conductive output end plate defining a cell stack fuel orifice; a compact cell stack located above the steam reformer, the cell stack having at least one air passage and a fuel passage, wherein the fuel passage communicates with the fuel orifice; a stack end plate closing the cell stack, the end plate having at least one air orifice in communication with the air passage; an electrically insulated sealing ring extending upwardly from the stack end plate outside the air orifice; an annular cap mounted on top of the sealing ring and having a hot air inlet, wherein the cap extends radially outward beyond the sealing ring; a can extending downwardly from the annular cap to the base defining an exhaust passage between the cell stack and the can, which communicates with an exhaust vent (preferably annular) within the base; a porous thermal insulation surrounding and spaced apart from the can defining a hot air plenum therebetween; a pin hole sheet surrounding the can defining an open space and spaced inwardly from the enclosure to define a cold air plenum therebetween, wherein the cold air plenum receives coolant air from a coolant supply via a coolant inlet, and the pin hole sheet is provided with a pattern of perforations adapted to distribute the coolant uniformly around the circumference and along the height as desired. 
     The present invention further provides an electrochemical apparatus including a steam reformer having a mixing orifice, the mixing orifice being adapted to receive and mix vaporized water and fuel; a first end plate adjacent to the steam reformer, the first end plate having a fuel inlet in communication with the steam reformer, and at least one air inlet conduit in communication with an air supply; a solid oxide fuel cell stack positioned adjacent to the first end plate, and having an air passage in communication with the air conduit, and a fuel passage communicating with the fuel orifice; an electrically conductive second end plate mounted adjacent to the stack opposite the first end plate; a conducting element extending from the second end plate; a first cylindrical wall extending upwardly from the first end plate spaced from the cell stack defining an annular chamber therebetween; a second cylindrical wall extending from the first end plate opposite the first cylindrical wall and spaced apart from the steam reformer, defining an exhaust passage therebetween, at least one exhaust port formed within the first end plate, wherein the exhaust port communicates with the annular chamber and the exhaust passage, an enclosure surrounding and spaced apart from the first and second cylindrical walls defining an open space therebetween, wherein the open space is at least partially filled with insulation and wherein the enclosure defines an annular exhaust exit communicating with the exhaust passage; and a chamber extending outwardly from the enclosure defining a cold air plenum; wherein, the cold air plenum communicates with the open space and a coolant supply. The coolant can be the oxidizing gas. 
     The present invention further provides and electrochemical apparatus including a steam reformer having a mixing orifice and a fuel orifice, the mixing orifice being adapted to receive and mix vaporized water and fuel; a first end plate adjacent to the steam reformer, the first end plate having a fuel inlet communicating with the fuel orifice and at least one exhaust conduit; a cylindrical wall extending from the first end plate surrounding the steam reformer and spaced therefrom defining an exhaust passage therebetween, wherein the exhaust passage is in communication with the exhaust conduit; a solid oxide fuel cell stack sandwiched between the first end plate and a second end plate; said solid oxide fuel cell stack defining at least one exhaust passage in communication with the exhaust conduit and a fuel passage communicating with the fuel orifice; porous insulation surrounding the solid oxide fuel cell stack and spaced therefrom defining a hot air plenum therebetween; an enclosure surrounding said porous insulation wherein said enclosure defines an exhaust exit registerable with the exhaust passage, and wherein the enclosure is in communication with an oxidant gas supply. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic cross-section of an embodiment of the apparatus in which air is fed to the end of the fuel cell stack opposite the end from which the fuel gas is fed. 
     FIG. 2 is a schematic cross-section of a second embodiment of the apparatus in which air is fed to the same end of the stack from which the fuel is fed. 
     FIG. 3 is a schematic cross-section of a third embodiment of the apparatus in which air is fed to the entire outer surface of the stack. 
     FIG. 4 is a schematic of a fuel cell and a stack of two such cells. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is an electrochemical apparatus incorporating a thermally integrated vaporizer and fuel steam reformer with a solid oxide fuel cell stack. The electrochemical apparatus will be referred to generally by the numeral  10  in the accompanying figures. 
     Referring to FIG. 1, the fuel stack  12  is located within an enclosure  14 . As shown, the enclosure  14  is a cylindrical body having a base  16 , a top  18 , and a cylindrical outside wall  20 . Within the enclosure  14 , a vaporizer  22  is located at about the center of the enclosure&#39;s base  16 . As shown vaporizer  22  may be provided with a pad  23  of insulation located centrally within the vaporizer  22 . A steam reformer  24  rests on top of the vaporizer  22 , and the fuel cell stack  12  is mounted above the reformer  24 . Fuel and water enter the vaporizer through tubes  26 ,  28 . 
     Initially the vaporizer  22  is heated by electric heaters  30  that at least partially surround the vaporizer  22 , steam reformer  24 , and cell stack  12 , and once the reaction starts exhaust gases passing through the annular passage  32  (discussed below) heat the vaporizer  22  and reformer  24 . A mixing orifice  34  is formed within the top  36  of the vaporizer  22  and connects the vaporizer  22  to the adjacent steam reformer  24 . Gaseous or vaporized fuel and vaporized water mix within the mixing orifice  34  and enter the steam reformer  24 . The steam reformer  24  defines a cylindrical chamber  38 . This chamber  38  is filled with beads or pellets  40  of a suitable steam reforming catalyst. The fuel and water vapor mixture reacts with the aid of the catalyst beads  40  to produce syngas fuel. This syngas fuel exits the steam reformer  24  and passes into the cell stack  12  via a fuel orifice  42  within the top of the steam reformer  24 . The steam reformer  24  may contain baffles (not shown) to create a more uniform flow of the gas through the reformer  24 . The fuel orifice  42  is registerable with the inlet  44  of a fuel passage  46  formed within the fuel cell stack  12 . This passage  46  carries the syngas fuel through the stack  12  and to each cell anode. 
     The cell stack  12  is bound at each end by electrically-conductive end plates  48  and  50 . End plate  48  is an annular plate containing the fuel orifice  42 . One power connection to the stack  12  is made via end plate  48 , which is connected, such as by welding, to part of the enclosure  14 . End plate  50  defines a pair of air orifices  52  that communicate with the two internal air passageways  54  formed within the fuel cell stack  12 . Welded to end plate  50  is a heat-resistant power rod (not shown), which conducts electric power from the stack  12  through an insulation layer  56 . An annular electrically insulating seal ring  58  extends from the end plate  50 , outside of the air orifice  52 , to a cap  60  on can  62 . Can  62  has a central hot air inlet or throughway  61  for channeling oxidant gas to the passages  54 . Can  62  is essentially a cylindrical wall extending downwardly from cap  60 . It should be understood that any wall-like structure capable of enclosing the cell stack and other components could act as a can  62 . The can  62  surrounds and defines annular exhaust passage  32 , around the stack  12 , and the reformer  24 , and vaporizer  22 . Exhaust passage  32  terminates in an annular exhaust vent  66  formed within the base. Hot gases  130  (see FIG. 4) expelled from the cell stack  12  are received within the exhaust passage  32  and channeled downwardly through the exhaust passage  32  and out the exhaust vent  66 . 
     If necessary, electrically insulating spacers, such as mullite tubes, can be installed between the can  62  and the stack  12  to maintain the open space and prevent possible shorts from contact between them. Surrounding the can  62  for part of its length is a hot air plenum  68 , bounded by the can  62  and the inside surface  70  of the permeable thermal insulation  56 , that allows air warmed by heat from the hot can  62  to flow to the air inlet  61 . The warmed air is then channeled through the stack air inlets  52  in the top plate  50  and into the air passages  54  of the fuel cell stack  12 . 
     One or more electrical start up heaters  30  surround the can  62  outside the exhaust passage  32 . A cylindrical perforated or pinhole sheet  72  surrounds the can  62  and heaters  30 . The pinhole sheet  72  is provided with a plurality of holes (not shown) that channel air from a cold air plenum  74  (described below) into and through the porous insulation  56 . This airflow circulates and receives heat energy both from passing through the insulation  56  and by convection from the can surface  76 . To minimize local temperature variations along the length of the stack  12 , the hole pattern in the pinhole sheet  72  is chosen to channel greater amounts of coolant air to the hotter regions of the cell stack  12 . 
     The entire apparatus is encased in the airtight enclosure  14 . As shown in FIG. 1, the enclosure  14  is spaced from the cylindrical pinhole sheet  72  to define a cold air plenum  74 , which is fed cold air via an inlet tube (not shown). The inner assembly  78 , consisting of all items from the vaporizer  22  to the top plate may be held tightly against the seal ring  58  by a spring or similar biasing apparatus (not shown). 
     During operation, the assembly  78  is initially preheated by the electrical start up heater(s)  30 . These heaters heat the incoming air within and also the fuel and water entering the vaporizer  22  through tubes  26  and  28 . The heated air in the hot air plenum  68  then enters the air passages  54  formed within the cell stack  12 . At the same time, the heaters  30  continue to act upon the water and fuel as they vaporize and then mix with each other in the mixing orifice  42  and react with the aid of the catalyst pellets  40  in the steam reformer  24  to form syngas fuel. The fuel gas then enters the fuel cell stack  12  through fuel inlet  42 . Once inside the stack  12 , the air and fuel react creating an electrical current. The reactant-depleted fuel and air eventually exit at the stack surface  80  where the remaining fuel is combusted by the oxygen from the exiting partially-depleted air. The exiting exhaust gas is expelled through the exhaust plenum  32 . 
     To thermally contain the reactions within the stack  12 , cold air is blown into the cold air plenum  74 . This cooling air passes from the cold air plenum  74  through the pinhole sheet  72  into the permeable thermal insulation  56 . As it passes through the thermal insulation  56 , the cold air receives heat radiated from the fuel cell stack  12  via the can  62 . The cooling air absorbs excess heat energy, maintaining the stack  12  at the desired substantially constant temperature. The absorbed heat heats the air close to the stack operating temperature. This heated air is received and collected in the hot air plenum  68  and forced through the throughway  61 , in the can  62 , into the air inlets  52  and finally into the fuel cell stack air passages  54 . At the same time, water and fuel entering into the feed tubes  26 ,  28  are vaporized, and then mix with one another in the mixing orifice  42 . 
     Hot exhaust gas flows through the narrow exhaust annular passage  32  adjacent to the steam reformer  24  and vaporizer  22 . Heat from the exhaust gases heats these chambers to their operating temperatures. A wide operating turndown capability is achieved through the use of this proportional-flow heat exchange. At very low stack power levels (when heat losses to the surroundings become a larger fraction of total heat generated), the startup heaters  30  may be powered at low levels if necessary to maintain the desired stack temperature distribution. 
     The air flowrate required to maintain the stack at the desired constant temperature will depend chiefly upon stack temperature, stack electrochemical efficiency, heat losses to the environment, and to a lesser extent upon factors such as fuel composition, air and fuel feed temperatures, air humidity, etc. Typical air flows needed are in the range of 2 to 2.5 times the minimum needed to fully oxidize the fuel (that is, 2 to 2.5 times stoichiometric). 
     The embodiment described herein refers to a single stack fuel cell, however, multiple stacks of fuel cells can be used in any of several different configurations. In one case, the present embodiment is simply repeated in a multiple stack design. For example, a single stack within a nest of stacks is started, and once operation begins, produces heat. This heat in turn heats the adjacent stacks to the appropriate start up temperature, thus, eliminating energy necessary to heat all of the stacks to start up temperature individually with individual heating units. These stacks may be directly adjacent to each other, or disposed with spacing between them to allow the cooling air to flow along surfaces between the individual stacks. 
     FIG. 2 is a schematic view of a second embodiment of the invention. In this embodiment, the hot air enters the fuel cell stack  12  through conduit  82 , formed within the first end plate  48 , from the same end as the hot fuel gas. The second end plate  50  is pressed against one end of the stack  12  by a spring  84  or other known biasing means, which transmits force through a freely-moving layer of thermal insulation  86 . As shown, the spring  84  has a plate  88  adjacent to its free end. This plate  88  contacts the insulative pad  86 . A conducting power rod  90  is welded to the end plate and extends through pad  86  and a hole  92  within the plate  88 . The conducting rod  90  is connected to an electrical conductor  93  such as copper wire at its outer end. This wire conducts current produced by the cell stack  12  to the device to be powered or simply a load. 
     In the second embodiment, a first cylindrical wall  94  extends from first end plate  48  to the top  18  enclosure  14  to which it is sealed. An annular passage  96  is defined between first cylindrical wall  94  and stack  12 . Annular passage is bound at one end by insulative pad  86  and communicates with exhaust passage  32  as will be described below. A second cylindrical wall  98  extends from first end plate  48  in the opposite direction of the first wall  94  and seals to the base  16  of enclosure  14 . This wall  98  surrounds and is spaced from steam reformer  24  and vaporizer  22  defining narrow exhaust passage  32 . The exhaust passage  32  vents outside the outer enclosure  14  though an annular exhaust exit  66 . The hot exhaust gas from annular chamber  96  flows into exhaust passage  32  through at least one exhaust port  100  formed within end plate  48 . A sleeve  101  defines the cold air plenum  74 . Sleeve  101  is mounted to or formed by the side  20  of enclosure  14 , and is located at a height equal to the stack  12 . Pinhole sheet  72  is located within the open surface of sleeve  101  and sealed to the enclosure  14 . 
     FIG. 3 shows a schematic view of a third embodiment of the invention. This embodiment incorporates much of the structure shown in embodiment two with the following differences. The incoming air flows through pinhole sheet  72  and insulation  56  where it is heated to high temperature. In this embodiment first wall  94  is omitted and the edge  70  of insulation  56  defines annular chamber  96 . The heated air exits insulation  56 , and then enters the substantially annular chamber  96  surrounding the stack  12 . Near and at the stack surface  80 , the air mixes with and reacts with the depleted fuel exiting the stack, thus forming a hot oxygen-rich exhaust gas mixture. This exhaust gas then flows radially inwards through each cell cathode where it gives up some of its oxygen. After flowing through the cell, the exhaust mixture flows along the internal passages  54  within the stack  12 , then through exhaust ports or conduits  104  in the first end plate  48  to the exhaust passage  32 . The exhaust gas exits the enclosure through exhaust exit  66 . 
     FIG. 4 shows a schematic exploded view of one preferred embodiment of a solid-oxide fuel cell  105  and a stack  12  of two such cells  105 , which are contained within the apparatus of the present invention. The cell consists of four stacked layers: a separator  106 , a cathode layer  108 , an electrolyte  110 , and an anode layer  112 . Cathode layer  108  and anode layer  112  may be referred to in the general sense as electrodes. A tubular gasket  114  in the cathode layer  108  forms a seal between the separator  106  and electrolyte  110 . A pair of tubular gaskets  116  in the anode layer  112  form seals between the electrolyte  110  and separator  106 . Gaskets  114  and  116  must remain impervious to fuel and air at the relatively high operating temperature of the cell  105  and be capable of maintaining a good seal under operating conditions. Suitable gaskets  114  and  116  may be made from oxidation resistant metal alloys, such as nickel-base alloys, from ceramics, or from glasses or glass-ceramics having suitable softening temperatures. 
     The separator  106  contains an internal hole  118  which is aligned with corresponding holes in the other cell layers to form an internal fuel manifold  46  (FIGS.  1 - 3 ). It also contains a pair of internal holes  120 , which are aligned with corresponding holes in the other cell layers to form a pair of internal air manifolds  54  (FIGS.  1 - 3 ). It is within the scope of the invention to include multiple fuel passages and/or oxygen passages in various locations within the cell, preferably close to the centerline of the cell. 
     A suitable hot fuel gas mixture  122 , represented by an arrow, is fed to the internal fuel manifold  46  and hot air  124 , represented by arrows, is fed to both internal air manifolds  54 . The stack  12  of fuel cells  105  will typically operate at about 850 to 1000° C., but may operate as low as 600° C. with suitable low-temperature solid electrolytes. The separators  106  must be impervious to gases, be good conductors of electrons, and have good 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 from Rolled Alloys, Plansee, Inco Alloys International, and Haynes, respectively. 
     The porous cathode layer  108  is made of a mixed oxide such as strontium-doped lanthanum manganite (LSM). It should have an airflow geometry which produces uniform air distribution with moderate pressure drop at the required flowrate. It may be prepared by conventional ceramic processing procedures, including power pressing, tape casting, extrusion, and the like. 
     The electrolyte  110  is impervious to gases and is a good oxygen ion conductor while having little or no electronic conductivity. Yttria-doped zirconia having about 3 to 10 mole percent Y 2 O 3  is preferred. The electrolyte  110  is preferably coated with a thin fired layer of cathode material on the cathode side and anode material on the anode side. The porous anode layer  112  is preferably made of nickel felt, nickel-zirconia cermet, or other nickel-containing cermet or alloy. The gaskets  114 ,  116  are made of either glass-ceramics or from high-temperature metallic alloys. Cell  105  and stack  12  diameters are typically 50-60 mm and total cell thickness (in use) is typically 1.0-1.5 mm. 
     When the cells  105  are stacked, a series electrical connection is established among all the cells  105  in the stack  12 , such that the stack voltage is the sum of all the cell voltages. 
     The stack  12  is operated by preheating close to operating temperature, supplying air and fuel gas, and connecting an external electric load. Oxygen from the air is ionized at and near the cathode-electrolyte interface. The oxygen ions flow through the electrolyte  110  under the influence of the chemical potential difference. At and 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, and the reaction cycle repeats. Typical power densities are on the order of 150 mW/cm 2  of electrode area at typical cell operating voltages near 0.6 volts. Typical stack volumetric power densities are close to 1.0 kilowatt/liter. 
     In one fuel cell embodiment, shown in FIG. 4, the cathode layer  108  is preferably a porous body having a thickness in the range of about 0.2-0.6 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  108  may comprise mixed ionic/electronic conductors such as an appropriately doped perovskite oxide. The cathode  108  can be prepared, as described above, or 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. 
     Electrolyte  110  is a thin wafer, 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 3 to 10 mole percent of yttria (Y 2 O 3 ), doped cerium oxide, doped bismuth oxide, and oxide ion conducting perovskites. Electrolyte  110  is substantially impervious to gases, however, ionized oxygen can migrate through the electrolyte under the influence of an applied oxygen potential. 
     The quality of the electrical contact between the cathode  108  and the electrolyte  110  may be improved by initially applying a thin layer of substantially the material that comprises the cathode  108  (or is at least electrochemically compatible with the cathode) to the surface of the electrolyte  110  adjacent the cathode  108  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. 
     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  110 , and the more tendency there is for the coating to peel off into the cavities of the electrode pores. Unless specifically stated to the contrary, the electrolyte  110  as mentioned in this Specification shall mean the electrolyte  110  with or without either or both cathode and anode material surface coatings. 
     Anode  112  is a porous body, as described above, and may comprise conventional solid oxide fuel cell anode material. Preferably, the anode  112  comprises either nickel felt or else a finely divided, compressed metallic powder such as nickel blended with a stable oxide powder such as zirconia or cation-doped ceria. As described above regarding the cathode  108 , the anode  112  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 in thickness. Like cathode  108 , anode  112  may be sintered during cell operation or before initial operation in an overheating sintering step. 
     Other embodiments of the solid oxide fuel cells 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. 
     It should 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 variation, modifications, and equivalent embodiments defined by the following claims.

Technology Category: 5