Abstract:
A solid oxide fuel cell employs an array of tubular flat plates that are sealed only at their extremities using compliant seals. The seals may be formed of talc and surround gas inlet and outlet openings in the tube extremities. Locating the seals remote from the higher temperature central areas of the tubes increases the service life of the seals and their tolerance to thermal cycling. The seals may incorporate an annular conductor for electrically interconnecting adjacent tubes into a common circuit.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     This invention generally relates to electrochemical devices such as fuel cells, and relates more particularly to improvements in solid oxide fuel cells (SOFCs) of the flat plate design, as well as methods for manufacturing the cells. 
     2. Background of the Invention 
     Fuel cells are electrochemical energy conversion devices that generate electricity and heat by converting the chemical energy of fuels. Solid-oxide fuel cells (SOFCs) are made from solid-state materials, such as ceramic oxide. SOFCs consist of three components: a cathode, anode, and an electrolyte sandwiched between the cathode and anode. Oxygen from the air is reduced at the cathode and is converted into negatively charged oxygen ions. These ions travel through the electrolyte to the anode, where they react with fuel, such as hydrogen. The fuel is oxidized by the oxygen ions and releases electrons to an external circuit, thereby producing electricity. The electrons then travel to the cathode, where they release oxygen from air, thus continuing the electricity-generating cycle. Individual cells can be stacked together in series to generate larger quantities of electricity. 
     Generally, SOFCs that employ planar cell units present design challenges because of a need to separate the air from the fuel by a seal substantially around the entire edge of the ceramic fuel cell plate. Also, the interconnection of stacks in a fuel cell assembly is made difficult because of the relatively high operating temperatures of SOFC which have ceramic, rather than metallic interconnects. Metallic interconnections are subject to oxidation, which leads to a loss of conductivity. Finally, the high operating temperature of SOFCs presents a further design challenge from the standpoint of the mechanical integrity of the fuel cell stack. When brought up to operating temperature and then back to room temperature, the fuel cell stack experiences dramatic thermal and mechanical stresses, which can lead to mechanical fatigue and failure, particularly where the fuel cells must be thermally cycle many times. The design problems discussed above are exacerbated when SOFCs are used in automotive applications such as auxiliary power units (APU) for vehicles. The automotive environment is particularly challenging and demanding, compared to the use of SOFCs in stationary applications, because of the need for higher power densities dictated size constraints, impact on fuel economy and emissions, crash worthiness, and deep thermal cycling over many cycles of use. 
     Several configurations for SOFCs have been developed, including monolithic, planar and tubular. The monolithic SOFC design is characterized by a honeycomb construction that is fused together into a continuous structure. Planar stacks, which have good energy densities, suffer from the fact that they require large perimeter seals around the entire edges of the ceramic fuel cell plates. Neither of these design elements lend themselves to rapid or uneven thermal cycling. Planar stacks need long, slow heat up cycles which is inconsistent with automotive applications where SOFCs are called upon to operate “on demand”. Tubular SOFCs require sealing only at the ends of the tubes over a relatively small area. The tubular seals are therefore distant from the hottest area of the fuel cell stack, consequently tubular SOFCs can be thermally cycled faster, and for more cycles. Unfortunately, tubular SOFCs exhibit markedly lower power per unit volume, compared to flat plate SOFCs, because their physical geometries do not allow high density, close stacking of the individual tubular units. 
     Accordingly, there is a clear need in the art for improved SOFCs that exhibit exceptionally high power per unit volume, which can not only withstand stresses stemming from deep thermal cycling over many cycles of use. 
     SUMMARY OF INVENTION 
     A primary object of the invention is to provide an SOFC that exhibits high power density, but is not subject to performance deterioration due to deep thermal cycling over many cycles of use. In accordance with a preferred embodiment of the invention, a fuel cell assembly includes a plurality of flat tubes arranged in parallel, spaced relationship, each having first and second anode-electrolyte-cathode surfaces. The tubes are sealed only at their outer extremities, thereby reducing the total sealing area and placing the seal distance from the higher temperature, central areas of the tubes. The seals, which isolate the fuel gas from the air, are preferably in the form of annular, compliant members formed of talc that surround through holes in the ends of the tubes which allow fuel gas to enter and exit the tubes. An annular, compliant conductor may be incorporated into the talc seal member in order to electrically connect adjacent tubes. Tolerance of deep thermal cycling can be improved by employing the compliant seal members only at the gas outlet ends of the tubes, and using noncompliant seals at the inlet of the tubes using long flat tubes with seals outside of the hot zones. 
     In accordance with another important object of the invention, in a preferred embodiment, a method is provided for making a flat fuel cell tube assembly, comprising the steps of forming an open ended tube by sintering two strips of uncured ceramic forming the sides of the tube, and sealing only the ends of the tube using compliant seals. One or more gas passageways within the tubes may be formed using a fugitive material as a filler which is burned out during the sintering process, or by laminating the ceramic strips in a shallow cavity bowl. 
     An advantage of the present tubular flat plate fuel cell is that the area of tube requiring sealing is substantially reduced, thereby making the fuel cell suitable for use in applications requiring higher operating temperatures. A related advantage is that the seals are located distant from central portions of the tube which experience higher operating temperatures that are otherwise deleterious to the seals. 
     An added advantage of the invention is that the seals are preferably made of compliant material allowing thermal and mechanical expansion of the fuel cell components during thermal cycling. A still further advantage of the invention is that the tubes can be manufactured using commonly available materials and well proven production techniques. 
     These and other features and advantages of the present invention may be better understood by considering the following details of a description of a preferred embodiment of the invention. In the course of this description, reference will frequently be made to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a combined schematic and diagrammatic view showing the basic operation of a solid oxide fuel cell; 
         FIG. 2  is diagrammatic, side view of a fuel cell stack forming the preferred embodiment of the present invention, and the depicting individual layers of each flat tube and gas passageways for carrying fuel gas through the tubes; 
         FIG. 3  is a side view of the fuel cell stack shown in  FIG. 2 ; 
         FIG. 4  is a plan view of one of the tubes forming part of the fuel cell stack of  FIGS. 2 and 3 ; 
         FIG. 5  five is a side view of the tubes shown in  FIG. 4 ; 
         FIG. 6  is enlarged, cross sectional view taken through line  6 — 6  in  FIG. 5 ; 
         FIG. 7  is an enlarged, fragmentary, cross sectional view taken through one side of the fuel cell stack of  FIGS. 2 and 3 , depicting more clearly the compliant seals; 
         FIG. 8  a perspective view of the one of the compliant seals of  FIG. 7 ; 
         FIG. 9  is a cross sectional view of the compliant seal shown in  FIG. 8 ; 
         FIG. 10  is a plan view of one strip of ceramic tape forming part of the flat tube shown in  FIG. 5 ; 
         FIG. 11  is a plan view of another strip of ceramic tape before being bonded to the strip shown in  FIG. 10 ; 
         FIG. 12  is a graph showing the electrical performance of a fuel cell according to the preferred embodiment; 
         FIG. 13  a plan view of a fuel cell module employing a flat fuel cell tube according to an alternate embodiment of the invention, the cover of the module having been removed for clarity; 
         FIG. 14  is a side elevational view of the fuel cell shown in  FIG. 13 ; 
         FIG. 15  is an enlarged, cross sectional view of one of the flat tubes used in the fuel stack shown in  FIG. 13 ; 
         FIG. 16  is a view similar to  FIG. 15  but showing a plurality of the tubes in stacked, interconnected relationship; and, 
         FIG. 17  is a side elevational view of the fuel cell tube shown in  FIG. 15 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIG. 1 , the present invention relates to improvements in solid oxide fuel cells (SOFCs) whose fundamental operation is well known in the art. An SOFC, generally indicated by numeral  20  comprises an electrolyte  22  sandwiched between an anode  24  and a cathode  26 . A first process gas, such as the oxygen contained in air is exposed to and reduced at the cathode  26 , while a second process gas, such as hydrogen, is exposed to the anode  24 . The oxygen reduced at the cathode  26  is converted into negatively charged oxygen ions which travel through the electrolyte  22  to the anode  24  where they react with the hydrogen fuel gas. The fuel gas is oxidized by the oxygen ions and releases electrons to an external circuit where they produce an electromotive force or voltage indicated by the voltmeter  28 . The electrons then travel to the cathode  26  where they reduce oxygen from the air, thus continuing the electricity-generating cycle. A plurality of the fuel cells  20  may be stacked together and connected in series to supply larger quantities of electricity. 
     Reference now made to  FIGS. 2–6  which show the details of flat tube fuel cells according to the preferred embodiment of the invention, arranged in a fuel cell stack generally indicated by the numeral  30 . The fuel cell stack  30  includes a plurality of stacked, flat tube fuel cells  32  which are held in parallel, compressed relationship by any suitable clamping or mounting means (not shown), and are typically surrounded by a protective housing (not shown). The flat tubes  32  are held in spaced relationship to each other by spacers  34 , to create a series of gas flow channels  36  between the opposed, facing sides of the tubes  32 . The fuel cell stack  30  is of a so-called cross flow or orthogonal flow type, in which the first and second process gases flow in orthogonal directions. In the present example, a first process gas, such as hydrogen fuel, flows longitudinally through later discussed gas passageways in the tubes  32 , while a second process gas such as air flows through the gas flow channels  36 , in a direction transverse to the flow of gas through the tubes  32 . 
     Each of the flat tubes  32  includes one or more internal, longitudinal gas flow passageways  44  that extend substantially throughout its entire length. The passageways  44  are formed within the body of an anode  42  having its opposite flat sides covered by a layer of solid oxide electrolyte  40 . Both sides of the electrolyte  40  are covered with a cathode  38 , so that each of the flat tubes  32  possesses first and second anode-electrolyte-cathode surfaces, wherein the anode  42  is contacted by the first process gas, and the cathode  38  is contacted by the second processed gas. 
     Each of the flat tubes  32  includes a first transversely extending through hole  46  forming a gas inlet in one end of the tube, and a second transversely extending through hole  48  forming a gas outlet in the opposite end of the tube  32 . Similar through holes are formed in the spacers  34 , coaxial with the through holes  46 ,  48 . As best seen  FIG. 2 , the inlet and outlets  46 ,  48  respectively communicate with the opposite ends of the gas passageway  44  in the tube  32 . The aligned openings  46 ,  48  in the tubes  32  and those formed in the spacers  34  create a delivery channel  46  which delivers the first process gas from a source indicated by the arrow  43 , to the inlet sides of the passageways  44 . Similarly the aligned through holes  48  create an exhaust channel  48  which carries first process gas from the outlet ends of the passageways  44  to a stack exhaust indicated by the arrow  45 . 
     As shown in  FIG. 6 , depending upon the method of manufacture and the size of the tubes  32 , the anode  42  may be formed with one or more internal support walls  42 B, which together with sidewalls  42 A, divide the gas passageway  44  into multiple flow channels. The support walls  42 B function to prevent the collapse of the tubes during their manufacture. 
     In accordance with the present invention, the gas inlet and outlet openings  46 ,  48  are sealed by means of annularly shaped, compliant seals  52  ( FIG. 7 ) which may be formed from a compliant material such as talc or mica. The talc used in the seals  52  may have an average grain size of about 0.5 to about 10.0 micrometers and will have a sufficient thickness so that when the tubes  32  are compressed together in a stack, the talc will seal the voids in the opposing, clamped surfaces of the tubes  32 , creating a gas tight seal therebetween. The seals  52  may be preformed under low compression in a suitable mold. Although the seals  52  may be interposed directly between the facing surfaces of a spacer  34  and a cathode surface of the joining tube plate  32 , it is preferred that they be placed within cylindrically shaped recesses  50  formed in the spacers  34 , coaxial with the corresponding delivery channel  46 ,  48 . By placing the seals  52  within the recesses  50 , the sides of the talc seal are restrained which tends to prevent lateral deformation of the seal, thereby improving the sealing quality. 
     An alternate embodiment of the talc seal  52  is shown in  FIGS. 8 and 9 . As an additional feature, a compliant, cylindrically shaped metal conductor  90  formed of copper, for example, is concentrically disposed within an outer talc body  88 . The conductor  90  forms an electrical interconnect that extends the entire length of the seal  52  and has a central opening  92  therein to allow the first process gas to flow therethrough. One end of the interconnect conductor  90  contacts an anode  42  and the opposite end contacts a cathode  38  of an adjoining tube  32 , thereby electrically connecting the tubes  32  in series relationship to each other. 
     In those cases where higher operating temperatures and deep thermal cycling are encountered in the fuel cell, it may be desirable to use the talc seals  52  only at the outlet openings  48 , where the temperature of the first process gas is lower than at the inlet openings  46 . In this case, noncompliant seals such as those made from glass beads, may be used to seal the inlet openings. The noncompliant seals are better suited to create an effective seal at the inlet openings  46  where the gas pressure is higher than at the outlet openings  48 . Even though the noncompliant seals do not expand and contract with temperature, the use of compliant seals at the outlet opening allows the entire assembly to expand and contract with temperature. 
     Reference is now made to  FIGS. 13–17  which depict a flat fuel cell tube and fuel cell stack forming an alternate embodiment of the present invention. The fuel cell tube  54  has a flat, tubular anode  64  which, like the previously described embodiment, possesses an internal, longitudinally extending passageway  62  through which a first process gas may flow. A layer of solid oxide electrolyte  66  is formed over both flat sides of the anode  64 . A cathode  56  formed over the electrolyte  66 , and as best seen in  FIGS. 15 and 17 , extends downwardly, but not completely over the sides of the tube  54 , so as to leave a strip  58  of the electrolyte  66  exposed. The exposed strip of electrolyte  58  forms a reaction area since it is exposed to the second process gas flowing over the exterior surfaces of the tube  54 . An electrically conductive strip  60  is formed on the bottom of the tube  54 . As shown in  FIG. 16 , the conductive strip  60  is used to interconnect two stacks of  69  of the fuel cells. Specifically, the conductive strips  60  connect the anodes  64  of the upper stack with the cathode  56  of the stacks therebeneath. 
     In contrast to the previously described preferred embodiment in which adjacent tubes are interconnected by transverse through holes in the tubes, the flat tubes  54  do not possess such through holes, but rather have their ends open, forming gas inlets and outlets. 
     As seen in  FIG. 13 , a fuel cell module is formed by interconnecting a plurality of the flat tubes  54  within a gas tight housing  68 . The housing  68  includes a gas inlet  78  coupled with a suitable source of fuel gas, such as hydrogen, and a gas outlet  80 . Gas received through the inlet  78  passes through a first plenum  70  in the direction of the arrow  72 . The plenum  70  distributes the gas to the inlet openings in the ends of the tubes  56 . The gas exiting the tubes  54  at the outlet ends are collected in a second plenum  74  which directs the gas in the direction of arrow  76  to the gas outlet  80 . The flat tubes are held in parallel, spaced apart relationship by means of talc seals  84  and insulators  82 , each of which extends the entire height of the tube  56 . The talc seals  84  create a seal between the gas flow channels  85  between adjacent tubes, and the gas plenums  70 ,  74 . Insulators  82  function to thermally isolate the seals  84  from the higher temperatures generated toward the middle of the tubes  54 . The insulators  82  may be formed of alumina zirconia fiberboard, and also act as spacers to maintain the spacing between adjacent tubes  54 . A waffle like current collector  86  formed, for example of gold plated stainless steel, is disposed between and electrically interconnects the cathodes of adjoining tubes  54 . The current collectors  86  electrically connect the tubes in each stack in parallel relationship to each other, and also function to help distribute heat more uniformly over the stack. 
     The component parts of the previously described flat tubes are manufactured using commonly available materials and proven manufacturing techniques involving tapecasting, screen printing and lamination. These manufacturing techniques allow close control over the thickness of anodes, the electrolyte and the cathodes. The flat tubes are formed of green ceramic tape which may be composed of 40–60 wt % NiO, 60–40 wt % fully stabilized ZrO 2  (stabilized by 13 wt % Y 2 O 3 ) in the blend), pore formers of rice starch or others, a binder such as polyvinyl butyual, and a plastizer such as Di (propyleneglycol)dibenzoate. The method for manufacturing green ceramic tape is well known in the art and therefore need not be described in detail herein. Briefly, however, the green tape is manufactured by mixing milled powders of a desired particle size with solvents, binders and plastizers to form a slurry which is then spread on a moving Mylar carrier using a “doctor blade” to achieve a film thickness of between 50 and 400 microns. The moving carrier continues slowly down an enclosed casting table for several hundred feet, passing through a number of controlled rate drying stages. Volatilization of the solvents stabilizes the solidifying dispersion. The tape emerges from the table dry and ready for blanking or cutting into sheets. 
     The next step involves screen printing a thin paste layer of ZrO 2  with 8 mole % Y 2 O 3  unto the surface of the tape. This printing is performed using a stainless steel screen, the thickness of which controls the thickness of the wet paste. The film applied with this process may have a thickness of between 3 and 40 microns. Multiple coatings of the same or different materials may be deposited. The tape and the coating must be dried after each printing. The flexible green tape having the desired patterns printed thereon is then cut to size with a hot knife or shear. 
     There are several methods for forming the tape into the shape of a desired flat tube. One method of forming the tube consists of stencil printing a fugitive layer of material on one side of one strip of dried tape, consisting of a mixture carbon black and organic binders; the fugitive layer is printed on a side of the tape opposite of the side containing the layer of ZrO 2 . Next, a second strip of tape having a layer of ZrO 2 —Y 2 O 3  layer on one side there of is laminated to the first ceramic strip, with the stencil printed fugitive layer sandwiched between the two strips, in a heated press, where lamination is performed at 90° C. and a pressure of 3,000 psi. Following lamination, the formed tubes are placed in an oven at a temperature that is sufficient to incinerate the fugitive material so as to leave an internal passageway within the tube. 
     A second method for forming the tubes consists of cutting a piece of the green tape into thin strips and then screen printing thereon a layer of the Zirconia composite ink in the shape that the strips will form, and then laying them onto the wet ink. A second printing is performed with the same pattern on top of the strips and another of tape is added to form the tube. The composite piece is then dried. 
     A third method of forming a tube consists of pre-forming the tape in a shallow cavity mold heated to a temperature of 80 to 100° C., and then printing a binder on the raised edges of the tape. Then, a second strip of green tape is laminated on top of the first piece to form the tube. Through holes may be punched and trimming may be performed before the tubes are fired. 
       FIG. 10  shows a strip of green tape to which there has been applied a patterned layer of material that forms a perimeter side wall  42 A and a central support wall  42 B of the tube shown in  FIG. 6 .  FIG. 11  shows a second strip of tape that is applied over the patterned strip of  FIG. 10  to form the fully assembled tube. 
     Regardless of the manufacturing technique used above, the formed tubes are then placed into an oven and cured at a temperature of between 1400 and 1500° C. The tubes are then coated with a suitable cathode material of the lanthanum perovskite family (ABO 3 ) and then refired to a lower temperature of between 900 and 1000° C. In the case of the alternate embodiment of the flat tube shown in  FIGS. 13–17 , one longitudinal edge of the tubes are left uncovered by the ZrO 2  electrolyte. This edge is then covered by a layer of nonporous conductive material which as previously described, is exposed to the first and second process gases on its opposite sides. The nonporous conductor may be of the lanthanum chromate family or a layer of a metal such as platinum, gold or titanium nitrite. This interconnecting conductor is not a structural element and therefore may be applied as a composite material and co-fired with either the tube during the initial sintering step or when the cathode is fired on or sputtered or painted. 
     The tubes having been formed as described above, are then assembled with the talc seals positioned therebetween surrounding the gas inlet and outlet openings in the ends of the tube. The tubes and seals are clamped together by any suitable means and installed in an appropriate enclosure such as a stainless steel house. 
     From the forgoing, it may appreciated that the tubular flat plate fuel cells and method of making the same described above not only provides for the reliable accomplishment of the objects of the invention but do so in a particularly effective and economical manner. It is recognized, of course, that those skilled in the art may make various modifications or additions chosen to illustrate the invention without departing from the spirit and scope of the present contribution to the art. Accordingly, it is to be understood that the protection sought and to be afforded hereby should be deemed to extend to the subject matter claimed and all equivalents thereof fairly within the scope of the invention.