Abstract:
An improved system for more uniformly distributing gaseous fuel over the anode surface of a fuel cell, comprising an interconnect subassembly for electrically connecting anodes and cathodes of adjacent fuel cells in a fuel cell stack. The subassembly includes a perforated plate disposed adjacent the anode surface. The plate may be parallel to or inclined to the anode surface and forms a first wall of a fuel plenum for uniformly distributing fuel via the perforations over the entire surface of the anode. The second wall of the plenum is a plate separating the fuel flow from air flowing across the cathode. Electrical continuity across the interconnect subassembly may be provided, for example, by non-planar upsets such as bumps and dimples in the two plenum plate components, or by metallic foam or filaments disposed between the plates and the electrodes.

Description:
TECHNICAL FIELD 
   The present invention relates to fuel cells; more particularly, to fuel cells wherein a gaseous fuel such as hydrogen or reformed gasoline is flowed across the surface of an anode layer; and most particularly, to such a fuel cell wherein means is included for distributing fresh fuel to all portions of the anode layer surface. 
   BACKGROUND OF THE INVENTION 
   Fuel cells are well known as devices for converting chemically-stored energy directly into electricity. One such type of fuel cell employs a solid-oxide electrolyte having a cathodic layer deposited on a first surface and an anodic layer deposited on a second and opposite surface. Oxygen atoms are reduced to O −2  by the cathodic layer, migrate through the electrolyte, and unite with protons produced from hydrogen by the anodic layer to form water, and, in the case of reformed gasoline, with CO to form CO 2 . Electrons flow from the anode via an external path to the cathode through the cell interconnect. 
   A plurality of such fuel cells may be assembled in series to form a fuel cell stack. The individual fuel cells are electrically connected to each other by interconnect elements between the electrodes to maintain electrical continuity. Each interconnect is mechanically and electrically connected on one side through a fuel flow space to an adjacent anode and on the other side through an air flow space to an adjacent cathode. Such connection is known to be provided by incorporation of conductive filaments or metallic sponge in the respective gas flow spaces between the electrodes and the interconnects. 
   Oxygen is provided to the cathode surface, typically in the form of air, in abundance as a coolant as well as an oxidant for the fuel cell. Fresh air is introduced via a first inlet manifold means to the air flow space at an entry edge of the cathode surface, flows across the surface, and is removed via a first exit manifold at an exit edge of the cathode surface. Hydrogen-containing gas is introduced via a second inlet manifold means to the fuel flow space at an entry edge of the anode surface, flows across the surface, and is removed via a second exit manifold at an exit edge of the anode surface. Typically, but not necessarily, such a fuel cell is rectangular in plan view, and the oxygen and fuel flow through the fuel cell orthogonally to each other. 
   A serious problem is known in the art which adversely affects both fuel utilization efficiency and electrical output of the cell or stack. The anode surface near the entry edge is exposed to fresh fuel with no combustion byproducts in it, such as H 2 O and CO 2 . Thus, the reaction rate and electricity production is relatively high in this region of the anode. However, as the fuel sweeps across the anode toward the exit edge, it picks up, and becomes diluted by, such byproducts while simultaneously becoming relatively depleted of H 2  and CO. Thus, the reaction rate and electricity production become progressively reduced in anode regions farther from the entry edge. Because of this phenomenon, these regions of the anode are sub-optimized, or under-utilized, in production of electricity. 
   Further, a relatively large and potentially damaging temperature difference may result between high-reaction and low-reaction areas of the anode. 
   Therefore, there is a strong need for an improved means for distributing fuel more uniformly over all portions of the anode surface. 
   It is a principal object of the invention to improve temperature uniformity within a fuel cell. 
   SUMMARY OF THE INVENTION 
   Briefly described, the present invention is directed to an improved interconnect system for more uniformly distributing gaseous fuel over the anode surface of a fuel cell. The system comprises an interconnect subassembly for electrically connecting anodes and cathodes of adjacent fuel cells in a fuel cell stack. The subassembly includes a perforated distributor plate disposed adjacent the anode surface. The distributor plate may be parallel to or inclined to the anode surface and forms a first wall of a fuel plenum for uniformly distributing fuel via the perforations over the entire surface of the anode. The second wall of the plenum is a second, imperforate plate separating the fuel flow plenum from air flowing across the adjacent cathode. Electrical continuity across the interconnect subassembly may be provided by non-planar upsets in the two plenum plate components, such as bumps and dimples, or by metallic foam or filaments disposed between the plates and the electrodes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features, and advantages of the invention, as well as presently preferred embodiments thereof, will become more apparent from a reading of the following description in connection with the accompanying drawings in which: 
       FIG. 1  is a schematic elevational cross-sectional view of a first prior art interconnect element disposed between adjacent fuel cells in a fuel cell stack, showing upsets in the interconnect plate, in the form of bumps and dimples, for making electrical contact with the anode and cathode of the adjacent interconnected fuel cells; 
       FIG. 2  is a schematic elevational cross-sectional view of a second prior art interconnect element disposed between adjacent fuel cells in a fuel cell stack, showing metallic sponge and conductive filaments for making electrical contact with the anode and cathode of the adjacent interconnected fuel cells; 
       FIG. 3  is a schematic elevational cross-sectional view of a first embodiment of a combined interconnect and fuel distribution system in accordance with the invention; 
       FIG. 4  is an exploded isometric view from above of the embodiment shown in  FIG. 3 ; 
       FIG. 5  is a detailed plan view of a portion of a perforated distribution plate for forming a first wall of a fuel plenum in accordance with the invention, showing a currently preferred arrangement of perforations, bumps, and dimples; 
       FIG. 6  is a detailed plan view of a portion of a plate forming a second wall of the fuel plenum, showing a currently preferred arrangement of bumps; and 
       FIG. 7  is a schematic elevational cross-sectional view of a second embodiment of an interconnect fuel distribution system in accordance with the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , a first embodiment of a prior art interconnect  10  is disposed in a portion of a fuel cell stack  12 . Stack  12  includes a first fuel cell  14   a  and a second fuel cell  14   b , interconnect  10  providing electrical conductivity therebetween. First fuel cell  14   a  includes a solid-oxide electrolyte  16   a  having a planar anode layer  18   a  attached to one surface thereof and a planar cathode layer  20   a  attached to an opposite surface thereof to define a fuel cell or PEN (positive-electrolyte-negative assembly). Second fuel cell  14   b  is identically constructed of analogously numbered components and includes a solid-oxide electrolyte  16   b  having a planar anode layer  18   b  attached to one surface thereof and a planar cathode layer  20   b  attached to an opposite surface thereof to define a fuel cell or PEN. Interconnect  10  includes an electrically-conductive plate  22  having a plurality of non-planar upsets extending away from both planar surfaces of plate  22  in the form of “bumps,” defined herein as upsets extending toward a cathode, and “dimples,” defined herein as upsets extending toward an anode. Bumps  24  are formed in plate  22  in mechanical and electrical contact with the surface of cathode  20   a  in the first fuel cell PEN  14   a , serving to offspace plate  22  from cathode  20   a  and thereby defining an air flow space  26  therebetween for supply of air  28  to the cathode surface. Dimples  30  are in mechanical and electrical contact with the surface of anode  18   b  in the second fuel cell PEN  14   b , serving to offspace plate  22  from anode  18   b  and thereby defining a fuel flow space  32  therebetween for supply of fuel  34  to the anode surface. Bumps  24  and dimples  30  typically are arranged in predetermined patterns, which may or may not be regular, and the air and fuel flow through their respective spaces  26 , 32  around the bumps and dimples. 
   Referring to  FIG. 2 , a second embodiment of a prior art interconnect  10 ′ is disposed in a portion of a fuel cell stack  12 ′ including a first fuel cell PEN  14   a  and second fuel cell PEN  14   b , interconnect  10 ′ providing electrical conductivity therebetween. Interconnect  10 ′ includes an electrically-conductive plate  22 ′ disposed between PENs  14   a ,  14   b  to form flow spaces  26 , 32 , as in the first embodiment. Instead of bumps and dimples to provide conductivity, interconnect  10 ′ includes either a porous metallic foam  36 , for example, foamed nickel, or a plurality of conductive filaments  38  extending from plate  22 ′ to cathode  20   a  and anode  18   b . 
   As described above, the prior art embodiments as shown in  FIGS. 1 and 2  are unable to prevent fuel from undergoing a continuous change in composition between the entry edge  40  and the exit edge  42  of anode  18   b , by continuous reaction and removal of combustibles and continuous addition of combustion products. 
   Referring to  FIGS. 3 and 4 , a first embodiment  110  of an improved interconnect and fuel distribution system in accordance with the invention, included in an improved fuel cell stack  112 , includes a first interconnect plate  122  similar to prior art plate  22 , having bumps  124  and dimples  130  extending from opposite sides of plate  122 , the dimples  130  forming electrical contact with anode  18   b  as in the prior art to create a fuel flow space  132  for flow of fuel  34  adjacent anode  18   b . Disposed between first plate  122  and cathode  20   a  is a second interconnect plate  144  having bumps  124 ′ extending into electrical contact with cathode  20   a  and thereby forming an air flow space  126  therebetween for flow of air  28  along cathode  20   a . Second plate  144  is off-spaced from first plate  122  by the height of bumps  124 , which bumps alternatively may be provided as dimples in plate  144  to equal effect, to form a plenum  146  therebetween for receiving fuel  34 , which in operation fills plenum  146 . First plate  122  is provided with a plurality of holes  148  extending between plenum  146  and fuel flow space  132  for allowing the fuel to flow from the plenum into the flow space. While the average mass flow from entry edge  40  to exit edge  42  is the same as in the prior art fuel cell stacks, the composition of the gas experienced by the anode surface is very different. The number of holes  148 , their spacing, and the pattern of holes are such that all portions of the anode surface continually receive fresh fuel through holes  148  from plenum  146 . Although a contaminant gradient must still exist in the fuel between entry edge  40  and exit edge  42 , because combustion is still occurring over the entire surface, the gradient is much diminished over that in prior art stack  12  by admixture of fresh fuel to spent fuel over the whole surface. 
   Referring to  FIG. 4 , a fuel cell stack  112  may include other mechanical components not shown schematically in FIG.  3 . As noted previously, air and fuel flow through a fuel cell stack preferably in orthogonal directions. Thus all four peripheries of the elements are provided with flow passages for supplying and exhausting air and fuel. As in the prior art, air  28  is introduced at the lower left of the stack, as shown isometrically in  FIG. 4 , and flows upwards through inlet air ports  50  in the various elements until it reaches distribution spacer  52  wherein the inlet ports  54  are open to air flow space  126 , spacer  52  being substantially the same thickness as the height of bumps  124 ′. Spacer  52  is sealed to cathode  20   a  by a first perimeter seal  56 . Air  28  flows across the surface of cathode  20   a  and exits the flow space via matching exhaust ports similar to inlet ports  54 , 50  (not visible in FIG.  4 ). 
   A similar distribution system is provided for fuel in the orthogonal direction. Fuel  34  enters the stack from the lower back side, flows upwards through inlet fuel ports  58  in PEN  14   b  and first interconnect plate  122  until it reaches fuel entry distribution spacer  60  wherein the fuel inlet ports in spacer  60  (not visible in FIG.  4 ), similar in shape to fuel inlet ports  66  in spacer  64 , are open to plenum  146 , spacer  60  being substantially the same thickness as the height of bumps  124 . Note that the fuel exhaust ports  63  in the opposite edge of first spacer  60  are not open to plenum  146 . Consequently, fuel flows through holes  148  in plate  122  into fuel flow space  132 . A fuel distribution exit spacer  64  is provided between anode  18   b  and plate  122  having open fuel exhaust ports  66  connecting to fuel exhaust ports  68 . Spacer  64  is sealed to anode  18   b  by a second perimeter seal  56 ′. 
   Referring to  FIGS. 5 and 6 , a currently-preferred pattern of holes  148 , bumps  124 , and dimples  130  is shown for a representative portion of plate  122  (FIG.  5 ), the repeating module  55  being a hole bracketed by two bumps in a first direction and by two dimples in a second direction, and a currently-preferred pattern of bumps  124 ′ is shown for a representative portion of plate  144  (FIG.  6 ). In the currently-preferred assembly relationship, bumps  124 ′ are positioned directly over dimples  130  (as shown in FIG.  3 ). Bumps  124 ′ are actually dimples on the underside of plate  144 , from the perspective of plate  122 . The preferred assembly relationship thus provides planar regions  67  between bumps  124 ′ for receiving bumps  124 . 
   Referring to  FIG. 7 , a second embodiment of an improved interconnect  110 ′ and fuel distribution system in accordance with the invention is shown in an improved fuel cell stack  112 ′. System  110 ′ is similar in many respects to improved system  110 , having a first interconnect distribution plate  122 ′ and a second interconnect plate  144 ′ forming a plenum  146 ′ therebetween. Plate  122 ′ is provided with a plurality of holes  148 ′ for distribution of fuel  34  through plate  122 ′ over all portions of the surface of anode  18   b . Second plate  144 ′ may be substantially identical to plate  144  in embodiment  110 , having bumps  124 ′ for electrically contacting cathode  20   a  and forming air flow space  126 . 
   Embodiment  110 ′ differs from embodiment  110  in that electrical contact with plate  144 ′ and anode  18   b  is provided by incorporation of metallic foam  36  or filaments  38  (not shown in  FIG. 7 ) as in the prior art (FIG.  2 ), alternative to the bumps and dimples shown in first plate  122  in the first embodiment. Preferably, plate  144 ′ is canted as shown in  FIG. 7  to progressively diminish the cross-sectional area of plenum  146 ′ in proportion to the reduction in mass flow through the plenum as a function of distance from the plenum entrance. 
   While the invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the scope of the invention. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention include all embodiments falling within the scope and spirit of the appended claims.