Abstract:
The present invention relates to fuel cell modules. The fuel cell module includes a housing that contains a plurality of fuel cell elements called unicells. Each unicell comprises a plurality of microcells. The housing is divided into a plurality of housing sections. A compressible bulkhead disposed between two adjacent housing sections and has a plurality of holes formed therein to allow respective unicells to pass through the bulkhead. A clamp element compresses the bulkhead to form a gas-tight seal between said bulkhead and the unicells.

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application 60/988,906 filed Nov. 19, 2007, which is incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention relates generally to electrochemical cell and fuel cell technology and, more particularly, to fuel cell modules including microfiber fuel cells or microtubular fuel cells, also known as microcells. 
     A fuel cell is a type of electrochemical energy device similar to a battery. A fuel cell includes a cathode and an anode separated by a membrane. A fuel, such as hydrogen, is supplied to the anode side of the fuel cell, while an oxidant, such as oxygen, is supplied to the cathode side of the fuel cell. The hydrogen splits into positive hydrogen ions and negatively-charged electrons. The membrane separator allows the positively-charged ions to pass through to the cathode side of the fuel cell. The negatively-charged electrons, however, must travel through an electric circuit to the cathode, thus creating an electrical current. At the cathode side of the fuel cell, the electrons and positively-charged hydrogen ions combine with oxygen to form water. 
     Microfiber fuel cells or microtubular fuel cells, also called microcells, represent one promising fuel cell technology. A microcell is a fiber-like fuel cell wherein the inner and outer current collectors, membrane separator and catalyst layers are extruded as a single fiber. The fibers may range in size from a few hundred to several thousand microns. One advantage of the fuel cell topology is that is achieves the highest possible Membrane Electrode Assembly (MEA) surface area to volume ratio, resulting in compact fuel cells. Another advantage is its scalability. The microcells can be assembled together in bundles to form units called unicells. The unicells can be further bundled to form larger units called modules. 
     SUMMARY 
     The present invention relates to fuel cell modules and to methods of assembling fuel cell modules from microcells. A plurality of microcells are bundled together to form units called unicells. A plurality of the unicells are, in turn, bundled together to form the fuel cell module. Novel ways of assembling unicells and fuel cell modules are described. 
     In one exemplary embodiment, a fuel cell module comprises a housing including a plurality of housing sections. The housing contains a plurality of elongated cylindrical fuel cell elements. A compressible bulkhead is disposed between two adjacent housing sections and has a plurality of holes formed therein to allow respective fuel cell elements to pass through the bulkhead. A clamp element radially compresses the bulkhead to form a gas-tight seal between the bulkhead and said fuel cell elements. 
     In another exemplary embodiment a fuel cell module comprises a housing including a plurality of housing sections. The housing contains a plurality of elongated cylindrical fuel cell elements. A plurality of bulkheads disposed between the housing sections form seals between adjacent housing sections. A pair of endplates disposed at opposing ends of said housing apply an axially compressive force to the housing to create a gas-tight seal between the housing sections and the bulkheads. 
     In another exemplary embodiment a fuel cell module comprises a housing including a plurality of housing sections. A plurality of bulkheads disposed between said housing sections divide the housing into a plurality of chambers including a reaction chamber for a first gas reactant, an inlet chamber for a second gas reactant, and an outlet chamber for the second gas reactant. The housing contains a plurality of elongated cylindrical fuel cell elements, which pass through respective openings in the bulkheads so that first end portions of said fuel cell elements are contained in said first inlet chamber, intermediate portions of said fuel cell elements are contained in the reaction chamber, and second end portions of said fuel cell elements are contained in the first outlet chamber. Clamps radially compress the bulkheads to form a gas-tight seal between said bulkheads and the fuel cell elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a fuel cell module  10   
         FIG. 2  is a perspective view of an exemplary microcell. 
         FIG. 3  is a perspective view of an exemplary unicell. 
         FIG. 4  is a longitudinal section view at a first end of an exemplary unicell. 
         FIG. 5  is a cross section of a unicell taken through line  4 - 4  of  FIG. 3 . 
         FIG. 6  illustrates an exemplary method for electrically connecting inner current collectors of a unicell. 
         FIG. 7  is a longitudinal section view of another exemplary unicell with flexible connectors. 
         FIG. 8  is an exploded perspective view of a fuel cell module. 
         FIG. 9  is a side elevation view of a fuel cell module. 
         FIG. 10  is a side elevation view of another exemplary fuel cell module. 
         FIG. 11  is a schematic diagram illustrating the electrical connections between unicells at a first end of a fuel cell module. 
         FIG. 12  is a schematic diagram illustrating the electrical connections between unicells at a second end of a fuel cell module. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is directed to a hydrogen fuel cell module  10 . The fuel cell module  10  comprises a plurality of individual microcells  100  ( FIG. 2 ). A plurality of the microcells  100  are bundled to form units referred to herein as unicells  200  ( FIGS. 3-5 ). A plurality of the unicells  200  are then bundled together to form the fuel cell module  10  (FIGS.  1  &amp;  7 - 9 ). 
       FIG. 2  illustrates an exemplary microcell  100 . The basic structure and assembly of the microcell  100  is described in U.S. Pat. Nos. 5,916,514; 5,928,808; 5,989,300; 6,004,691; 6,338,913; 6,399,232; 6,403,248; 6,403,517; 6,444,339; 6,495,281; 6,884,539; and 7,229,712; and U.S. Patent Publ. Nos. 2007/0243439 and 2005/0181269 which are incorporated herein by reference. For convenience, a brief description of the microcell  100  is provided herein. 
     The microcell  100  comprises an inner current collector  102 , a first catalyst layer  104 , a proton exchange membrane (PEM)  106 , a second catalyst layer  108 , and a outer current collector  110 . The microcell  100  could also optionally include a carbon fiber layer between the second catalyst layer  108  and outer current collector  110 . The inner current collector  102  comprises an electrically conductive wire that extends longitudinally through the microcell  100 . In the exemplary embodiments, the inner current collector  102  comprises a layered structure of copper, titanium, and niobium, but such is not required. The inner current collector  102  may be exposed at the ends of the microcell  100 , if desired. The inner current collector  102  may have a plurality of microchannels  112  formed therein to permit the flow of air or oxygen through the microcell  100 . The first catalyst layer  104 , PEM  106 , and second catalyst layer  108  surround the inner current collector  102 . The first and second catalyst layers  104 ,  108  may, for example, comprise layers of a platinum catalyst. The outer current collector  110  winds around the second catalyst layer  108 . In one embodiment, the outer current collector  110  is a titanium wire wound at a uniform pitch P, which may be substantially the diameter of outer current collector  110  or may be larger. A microcell  100  is typically in the range of 200 microns to 3 millimeters in diameter. 
       FIGS. 3-5  illustrate an exemplary unicell  200 . A unicell  200  is a fuel cell assembly comprising a plurality of microcells  100  bundled together. The unicell  200  is elongated and cylindrical in form with a first end  201  and a second end  202 . In general, one of the ends,  201 , 203  functions as an anodic end, while the other end functions as a cathodic end. For simplicity of description, it will be assumed that end  201  functions as the cathodic end and end  203  functions as anodic end; however, it should be understood that the unicell  200  could easily have an opposite anodic/cathodic configuration. 
     In the illustrated embodiment, there are ten microcells  100  in a unicell  200 , although those skilled in the art will appreciate that any number of microcells  100  could be used. The microcells  100  are circumferentially spaced around a central member  203 . The central member  203  may comprise a solid rod or a hollow tube, with either made of an electrically-conductive material or an electrically non-conductive material. The tube may be 1/16 inches to ½ inches in diameter and 1-30 cm in length. In one embodiment, the central member  203  is hollow and functions as a heat exchange tube through which a coolant fluid flows. The central member  203  may be fabricated from the same material as inner current collector  102 . 
     A pair of spaced-apart seals  204 ,  206  is disposed at each end of the unicell  200 . The seals  204 ,  206  are preferably formed by a molded electrically nonconductive epoxy. A gap  208  is formed between the inner seals  206  and outer seals  204  at each end  201 ,  202  of the unicell  200 . The inner current collectors  102  of the microcells  100  extend through the inner seals  206 , across gaps  208 , and at least partially through the outer seals  204 . The inner current collectors  102  of the microcells  100  are exposed within the gap  208  so that one of the gaseous reactants (e.g., air) can enter into the microchannels  112  at one end of the microcells  100  and exit at the opposite end. The first catalyst layer  104 , PEM  106 , and second catalyst layer  108  may terminate at the inner seal  206 . 
     A conductive wrap  212  winds in helical fashion around the microcells  100  and holds the microcells  100  to electrically connect the outer current collectors  110  of the microcells  100 . The conductive wrap  212  also holds the microcells  100  in close contact with the central member  203  so as to provide good electrical contact between the outer current collectors  110  of the microcells  100  and the central member  203 . The conductive wrap  212  preferably includes a substantially flat surface in contact with the outer current collectors  110  of the microcells  100 . The width W of this flat surface is preferably greater than the distance between corresponding points on two consecutive windings (i.e., the pitch) of the outer current collector  110 . Conductive wrap  212  may be wrapped around the microcell bundles so that consecutive windings of wrap  212  do not touch. This arrangement provides interstitial spaces between windings that allow gases to flow around the exterior of microcells  100 . In one embodiment, the wrap  212  is fabricated from the same material as inner current collector  102 . 
     Longitudinally spaced insulators  214  may be disposed around the central member  203  at end  201 ,  203  of the unicell  200  to space and electrically isolate the inner current collectors  102  from the central member  203 . More particularly, insulators  214  may be located at each seal  204 ,  206  so that the seals  204 ,  206  encapsulate and hold the insulators  214 . In addition, an outer retention member  216  may be disposed around the microcells  100  in general alignment with insulators  214 . The insulator  214  and retention member  216  may comprise a non-conductive heat shrink tubing, or other appropriate material. Preferably, inner insulator  214  and retention member  216  are significantly shorter in longitudinal length than the respective seals  204 ,  206  and disposed toward the end of the respective seal  204 ,  206  closest to gap  208 . The inner insulator  214  and retention member  216  may be fully encapsulated by the material forming seals  204 ,  206 , but may extend therefrom in some embodiments. 
     In the exemplary embodiment shown in  FIGS. 3-5 , the central member  203  terminates within the outer seal  204  at a first end  201  of the unicell  200  and protrudes at the second end  202  of the unicell  200 . In this embodiment, a separate stub member  210  protrudes from the outer seal  204  at the first end  201  of the unicell  200 . The outer seal  204  mechanically joins the stub member  210  with the central member  203  while at the same time electrically isolating the stub member  210  from the central member  203 . The stub member  210  functions as a first electrical connector and the inner current collectors  102  of the microcells  100  are electrically connected to the stub member  210 . The protruding end of the central member  203  at second end  202  of the unicell  200  functions as a second electrical connector. The protruding ends of the central member  203  and/or stub member  210  may be externally threaded if desired. In embodiments where the central member  203  comprises a heat exchange tube, the stub member  210  may have an axial opening  211  in fluid communication with the hollow longitudinal passage  203   p  of central member  203 . A fluid passage  205  may be formed in the outer seal  204  at the first end  201  of the unicell  200  to allow coolant fluid to flow from the stub member  210  into the central member passage  203   p.    
       FIG. 6  illustrates one method of electrically connecting the inner current collectors  102  of the microcells  100  to the stub member  210 . The inner current collectors  102  are connected to a conductive strip  220  by any suitable means, such as by adhesive, heat welding, ultrasonic bonding, soldering, crimping, or other suitable techniques. In one exemplary embodiment, five inner current collectors are connected to each of two conductive strips  220 , each made of copper. The conductive strips  220  are then formed around and electrically connected to stub member  210 , such as by soldering. 
     In another exemplary embodiment of the unicell  200  shown in  FIGS. 7A-7B , flexible connectors  222 ,  224  are utilized for making the electrical connections at the ends of unicell  200 . For example, inner current collectors  102  are electrically connected to one or more first flexible connectors  222  at the first end  201  of the unicell  200 . This connection may be by any suitable method, such as the method described above with respect to conductive strip  220 . Outer current collectors  110  and/or the central member  203  electrically connect to one or more second flexible connectors  224  at the second end  202  of the unicell  200 . The flexible connectors  222 , 224  may comprise braided wires, although any other form of flexible connection known in the electrical arts may be used. In one embodiment, two or more flexible connectors  222 ,  224  are disposed at each end  201 ,  202 , respectively. With such connections, central member  203  may protrude at both ends  201 ,  202  of the unicell  200 , with protruding ends of central member  203  externally threaded or otherwise configured to assist in mechanically securing the unicells  200 . Suitable measures should be taken to electrically insulate the central member  203  from the inner current collectors  102  and associated flexible connector  222 , such as by providing an electrically insulating material layer  226  there between at the first end  201  of the unicell  200 . 
     The unicells  200  are bundled together to form fuel cell modules  10 . A fuel cell module typically comprises many unicells  200 . The number of unicells  200  in a fuel cell module could vary from 1 to approximately 1000, but more typically is in the range of twenty-five to a hundred. The fuel cell module  10  will typically have a diameter in the range of 0.25 inches to 12.0 inches. Those skilled in the art will appreciate that the number of unicells  200  in a module  10  is not material and in general is dictated by factors such as size, weight, and desired power output. 
       FIGS. 8-9  illustrate the assembly of an exemplary fuel cell module  10 . The exemplary fuel cell module  10  includes a housing  12 . The exemplary housing  12  is generally cylindrical and comprises five housing sections  14 ,  16 ,  18 ,  20 ,  22 ; four bulkheads  24 ,  26 ,  28 ,  30 ; and two end plates  32 ,  34 . The housing sections  14 ,  16 ,  18 ,  20 ,  22  can be made from a wide range of materials including metals, fiberglass, and carbon-reinforced epoxy composites. For ease of reference, housing sections  14 ,  22  are referred to as the end sections, housing sections  16  and  20  are referred to as the intermediate sections, and housing section  18  is referred to as the center section. The bulkheads  24 ,  26 ,  28 ,  30  are disposed between respective housing sections and may be similar to or slightly larger in diameter than housing sections  14 ,  16 ,  18 ,  20 ,  22 . Bulkheads  24 ,  26 ,  28 ,  30  are made from a compressible material that is also electrically nonconductive, such as silicone or fluoro-silicone. Compression members, such as clamps  36 , are disposed around bulkheads  24 ,  26 ,  28 ,  30  to radially compress bulkheads  24 ,  26 ,  28 ,  30 . End plates  32 , 34  are disposed at opposing ends of the fuel cell module  10  and may be made of a conductive material, such as a metal. A gasket (not shown) may be disposed between the endplates  32 ,  34  and respective housing sections  14 ,  22  to provide a fluid tight seal. The entire assembly is longitudinally held together by tension rods  38  that pass through end plates  32 ,  34 . Tension rods  38  may be threaded at each end and secured by threaded connectors. When the threaded connectors are tightened, the end caps  32 ,  34  apply an axially compressive force that forces housing sections  14 ,  16 , 18 ,  20 ,  22  and bulkheads  24 ,  26 ,  28 ,  30  together so as to form seals therebetween. 
     As shown in  FIG. 8 , bulkheads  24 ,  26 ,  28 ,  30  have a plurality of openings  40  formed therein. The unicells  200  extend through the openings  40  in the bulkheads  24 ,  26 ,  28 ,  30  so that the ends of the unicells  200  terminate within the end sections  14 ,  22  of housing  12 . Seals  204 ,  206  of the unicells  200  align with the bulkheads  24 ,  26 ,  28 ,  30 . When the bulkheads are radially compressed, a gas-tight seal is formed between the bulkheads  24 ,  26 ,  28 ,  30  and the seals  204 ,  206 . The seal formed is sufficient to prevent the undesired flow of the gaseous reactants through the sealed sections at the normal operating pressures of 15-20 psi. The openings  40  are preferably slightly undersized with respect to the size of the corresponding seals  204 ,  206  to provide an interference fit. If desired, one or more openings  40  may of a different size, such as a larger central opening for acceptance of an input/output line, as discussed further below. The openings  40  are advantageously arranged in a close-packed configuration, and the bulkheads  24 ,  26 ,  28 ,  30  are advantageously identical, although neither condition is required for all embodiments. 
     The opposing ends of the housing sections  16 ,  18 ,  20  seat against the opposing faces of respective bulkheads  24 ,  26 ,  28 ,  30 . A first end of housing sections  14 ,  22  seats against a respective end plate  32 ,  34  and a second end of housing sections  14 ,  22  seats against a respective bulkhead  24 ,  30 . When the tension rods  38  are tightened, the entire assembly is axially compressed to form gas-tight seals between the ends of the housing sections  14 ,  16 ,  18 ,  20 ,  22 ; the bulkheads  24 ,  26 ,  28 ,  30 ; and end plates  32 ,  34 . 
     The bulkheads  24 ,  26 ,  28 ,  30  divide the interior of the fuel cell module  10  into five chambers. The three center chambers  16   c ,  18   c ,  20   c  serve as gas chambers for gas reactants. The outer chambers  14   c ,  22   c  may serve as fluid chambers for a coolant. Of course, additional bulkheads and housing sections may be included, so as to form additional chamber(s) if desired. Also, those skilled in the art will appreciate that the housing may include fewer than five chambers in some embodiments. 
     A gas inlet  42  for a first gas reactant is disposed in housing section  16  and a corresponding gas outlet  44  is disposed in housing section  20 . A gas inlet  46  and gas outlet  48  for the second gas reactant may be disposed along the longitudinal axis of the fuel cell module  10  and extend into housing section  18 . In this embodiment, the endplates  32 ,  34 , as well as the bulkheads  24 ,  26 ,  28 ,  30  include a central opening for the gas inlet  46  and gas outlet  48  respectively. The gas inlet  46  may comprise a tube that extends along the axis of the fuel cell module  10  through endplate  32  and bulkheads  24 ,  26  and terminates in the central gas chamber  18   c . Similarly, the gas outlet  48  may comprise a tube that extends along the axis of the fuel cell module  10  through the endplate  34  and bulkheads  30 ,  28 , and terminates in the central gas chamber  18   c . Alternatively, or in addition thereto, gas inlet  46  and gas outlet  48  may connect to the central gas chamber  18   c  via side entry through the periphery of housing section  18 , advantageously toward opposite ends of the central housing section  18 . Coolant, such as de-ionized water or air, is fed into chamber  14   c , flows along the longitudinal passages  203   p  of the unicells  200 , and into chamber  22   c , from which it is exhausted, typically for cooling and recirculation. 
     During operation, a first gas reactant, such as oxygen or air, enters the gas inlet  42  at one end of the fuel cell module  10  into gas chamber  16   c , enters the microchannels  112  of the microcells  100  at gap  208 , flows through the microchannels  112  into a gas chamber  20   c  at the opposite end of the fuel cell module  10 , and exits the fuel cell module  10  through the gas outlet  44 . A second gas reactant, such as hydrogen, enters gas inlet  46 , fills a central gas chamber  18   c  surrounding the central portion of unicells  200 , and exits the gas outlet  48 . In a preferred embodiment, tubular sleeves  250  (shown in  FIG. 3 ) can be placed around each unicell  200  to facilitate an axial flow of gas along each unicell  200 . The tubular sleeves  250  can be made, for example, from a heat shrink material. The hydrogen gas in the central gas chamber  18   c  enters the tubular sleeve  250  at an end adjacent to the gas inlet  46  and exits the tubular sleeve  250  at an end adjacent the gas outlet  48 . For such an arrangement, an additional bulkhead  27  and housing section  17  may be added on the inlet side of the chamber  18   c  as shown in  FIG. 10  so as to force the gas to flow into to the tubular sleeves  250 . In this embodiment, the tubular sleeves  250  may protrude slightly into housing section  17  so that the hydrogen can enter the ends of the tubular selves  250 . 
     The operation of the fuel cell module  10  may result in the generation of water or other liquid in reaction chamber  18   c , and drain  56  is provided for removing such. In addition, the operation of the fuel cell module  10  may result in the generation of significant heat. As previously noted, the central member  203  may function as a heat exchange tube to aid in removing this heat. Alternatively or in addition thereto, dedicated cooling tubes (not shown) may be interposed in the array of unicells  200 . A suitable coolant may be introduced into the central member  203  and/or cooling tubes to absorb and carry off some of the heat produced by the microcells  100  during operation. Accordingly, the central member  203  and/or cooling tubes are advantageously in fluid communication with chambers  14   c ,  22   c  at each end of fuel cell module  10 . A fluid inlet  52  may be disposed in the end section  14  of the housing  12  and a corresponding fluid outlet  54  may be disposed in end section  22 . A liquid coolant, such as de-ionized water, enters fluid chamber  14   c  through fluid inlet  52 , flows through the central member  203  and/or cooling tubes into the fluid chamber  22   c , and exits the fuel cell module  10  through fluid outlet  54 . 
     The unicells  200  are electrically interconnected in chamber  14   c ,  22   c . The interconnections may be arranged so that the unicells  200  are connected in series, are connected in parallel, or in any suitable combination thereof. The unicell interconnections may be made using the flexible connectors  222 ,  224 , or by using conductive plates mechanically and electrically connected to the central members  203  and stubs  210 , or by a combination thereof, or by any other suitable electrical interconnection means known in the electrical arts. When compressible bulkheads are employed, care should be taken to allow for possible relative movement between the unicells  200  as the bulkheads are peripherally compressed. 
       FIGS. 11 and 12  illustrate an exemplary method of interconnecting unicells  200  in a fuel cell module  10 . As shown in  FIGS. 11 and 12 , the individual unicells  200  are interconnected by conductive plates  260  that connect the central members  203  and/or stubs  210  of the unicells  200 . The conductive plates  260  may be held in place by nuts (not shown) that thread onto the ends of the central members  203  and/or stubs  210  of the unicells  200 . The orientation of the unicells  200  is illustrated by solid and dashed lines. The unicells  200  are organized into groups of three unicells  200 . The three unicells  200  in each group are connected in parallel. All of the groups are then connected in series. Of course, the number of unicells  200  in each group can be varied depending on the desired current and/or voltage. 
     The present invention may be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. Further, the various aspects of the disclosed device and method may be used alone or in any combination, as is desired. The disclosed embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.