Abstract:
A monopolar fuel cell stack comprises a plurality of sealed unit cells coupled together. Each unit cell comprises two outer cathodes adjacent to corresponding membrane electrode assemblies and a center anode plate. An inlet and outlet manifold are coupled to the anode plate and communicate with a channel therein. Fuel flows from the inlet manifold through the channel in contact with the anode plate and flows out through the outlet manifold. The inlet and outlet manifolds are arranged to couple to the inlet and outlet manifolds respectively of an adjacent one of the plurality of unit cells to permit fuel flow in common into all of the inlet manifolds of the plurality of the unit cells when coupled together in a stack and out of all of the outlet manifolds of the plurality of unit cells when coupled together in a stack.

Description:
RELATED APPLICATIONS 
     The present application is related to U.S. Provisional Patent Application Ser. No. 60/371,053, filed on Apr. 9, 2002, which is incorporated herein by reference and to which priority is claimed pursuant to 35 USC 119. 
    
    
     GOVERNMENT INTERESTS 
     The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 U.S.C. §202) in which the Contractor has elected to retain title. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to fuel cells, and in particular to monopolar fuel stacks. 
     2. Description of the Prior Art 
     Although there are five primary types of fuel cells, one of the most common types is the polymer electrolyte membrane (PEM) fuel cell. A PEM fuel cell consists of several membrane electrode assemblies (MEAs) within gas diffusion layers and bipolar plates. The purpose of a fuel cell is to produce an electrical current. The chemical reactions that produce this current cause the fuel cell to function. In general terms, hydrogen atoms enter a fuel cell at the anode where a chemical reaction strips them of their electrons. The hydrogen atoms are now “ionized” and carry a positive electrical charge. In some cell types oxygen enters the fuel cell at the cathode and combines with electrons returning from the electrical circuit and hydrogen ions traveling though the electrolyte from the anode. In other cell types the oxygen picks up electrons and then travels through the electrolyte to the anode where it combines with hydrogen ions. Regardless of whether oxygen and hydrogen combine at the anode or cathode, together they form water, which drains from the cell. As long as a fuel cell is supplied with hydrogen and oxygen it will generate electricity. 
     To increase the electrical energy available, a plurality of fuel cells can be arranged in series to form a fuel cell stack. In a fuel cell stack, one side of a flow field plate functions as the anode flow field plate for one fuel cell while the opposite side of the flow field plate functions as the cathode flow field plate in another fuel cell. This arrangement may be referred to as a bipolar plate. The stack may also include monopolar plates such as, for example, an anode coolant flow field plate having one side that serves as an anode flow field plate and another side that serves as a coolant flow field plate. As an example, the open-faced coolant channels of an anode coolant flow field plate and a cathode coolant flow field plate may be mated to form collective coolant channels to cool the adjacent flow field plates forming fuel cells. 
     Currently stacks are fabricated with bipolar stacks where the majority of the mass is associated with a bipolar plates that serve to electrically connect cells and distribute the fuel and oxidant. This type of design a “bipolar plate” serves as a repeating element that serves to interconnect the cells and distribute the reactants. Such a bipolar stack is held together under pressure by “end plates” to ensure good contact and sealing. A typical stack would have two or more bipolar plates. The bipolar plates are usually fabricated from graphite composites while the end plates are made from titanium, stainless steel or aluminum. Several tie rods usually run across the stack to hold the plates together. The bipolar stack has the advantage of providing a very low internal resistance which is crucial for minimizing the losses for large currents that may flow through these stacks, and which is especially necessary for stacks which output more than a few tens of watts. 
     However, when the power output is only a few watts, the very low internal resistance offered by the bipolar stack design is not absolutely necessary. Such bipolar plates and end plates are usually machined or molded with flow field from graphite composite and the typical cost $50-$100/sq. foot, and become a major part of the costs of the stack. Most importantly, the biplates, end plates and tie rods constitute about 80% of the weight of a typical stack thus lowering the power density of the stack. Also, once such as stack is assembled, trouble shooting will require the entire stack to be dismantled if any of the cells in the center of the stack has to be accessed. 
     Therefore, a new design that is substantially less expensive to fabricate, lighter, does not require extensive pressure to ensure sealing, that eliminates biplates and endplates totally, and is easy to manufacture and troubleshoot, is highly desirable for commercialization of fuel cells. 
     BRIEF SUMMARY OF THE INVENTION 
     A novel stack design is disclosed below that overcomes the limitations of the conventional bipolar fuel stacks bipolar fuel stacks is proposed. This new stack design offers a two-to-three fold improvement in power densities, suitable for manufacturing, uses inexpensive plastic materials, and straightforward to troubleshoot and assemble. Such a stack design will substantially improve the commercialization of portable direct methanol fuel cell power sources. 
     More particularly, the invention is a monopolar fuel cell stack comprising a plurality of sealed unit cells coupled together. Each unit cell comprises a first cathode, a first membrane electrode assembly disposed adjacent to the first cathode, and an anode plate disposed adjacent to the first membrane electrode assembly. The anode plate has an anode and an internal channel defined therein for flow of fuel through the anode plate. An inlet manifold is coupled to the anode plate and communicates with the channel within the anode plate. An outlet manifold is coupled to the anode plate and communicates with the channel within the anode plate. Fuel flows from the inlet manifold through the channel in contact with the anode plate and flows out through the outlet manifold. A second membrane electrode assembly is disposed adjacent to the anode plate. A second cathode is disposed adjacent to the second membrane. The inlet and outlet manifolds are arranged and configured to couple to the inlet and outlet manifolds respectively of an adjacent one of the plurality of unit cells to permit fuel flow in common into all of the inlet manifolds of the plurality of the unit cells when coupled together in a stack and out of all of the outlet manifolds of the plurality of unit cells when coupled together in a stack. 
     The first and second cathode are included as part of a corresponding first and second cathode plate. Each inlet and outlet manifold is configured to space each unit cell from each adjacent unit cell to provide an air gap between adjacent unit cells when the plurality of unit cells are coupled together to form the stack. At least the inlet and outlet manifolds are composed of plastic. Each of the inlet and outlet manifolds are arranged and configured to snap fit into adjacent inlet and outlet manifolds respectively to provide a common inlet and outlet manifold for the stack for the plurality of unit cells. The snap fit is a zip-lock coupling. 
     The stack may further comprise an inlet and outlet header coupled to the common inlet and outlet manifold for the stack for the plurality of unit cells and a current collector for electrically connecting the anodes of the anode plate with the cathodes in a predetermined connection topology. The stack may further comprise sealing gaskets between the first and second cathode plates on one hand and the first and second membrane electrode assemblies on the other, and further between the first and second membrane electrode assemblies on one hand and the anode plate on the other. In the illustrated embodiment the fuel flowing through the anode plate is liquid methanol. 
     The invention is also a method of fabricating a monopolar fuel cell stack comprising the steps of separately assembling a plurality of sealed unit cells, and snap fitting the plurality of sealed unit cells together to form the stack. 
     While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of “means” or “steps” limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional side view of a single monopolar unit cell devised according to the invention. 
         FIG. 2  is a cross-sectional side view of a three monopolar unit cells combined into a stack according to the invention. 
         FIG. 3  is a cross-sectional side view of a three monopolar unit cells electrically connected as a stack according to the invention. 
         FIGS. 4   a - 4   d  are different perspective views of the assembled stack according to the invention. 
         FIGS. 5   a - 5   i  are a series of perspective views illustrating the assembly of a single unit cell of the invention. 
         FIG. 6  is a photograph of an assembled unit cell of the invention. 
     
    
    
     The invention and its various embodiments can now be better understood by turning to the following detailed description of the preferred embodiments which are presented as illustrated examples of the invention defined in the claims. It is expressly understood that the invention as defined by the claims may be broader than the illustrated embodiments described below. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is a stack  10  as shown in cross-sectional side view in  FIG. 2  used in a methanol fuel cell, although it could be used with other types of fuels as well. Such a methanol fuel cell is described in detail in Surampudi et.al., “ Direct Methanol Feed Fuel Cell and System ,” U.S. Pat. No. 6,303,244 (2001), U.S. Pat. No. 5,599,638 (1997), and U.S. Pat. No. 6,485,851 (2002), each of which are incorporated herein by reference. The stack design of the invention offers a two-to-three fold improvement in power densities, suitable for manufacturing, uses inexpensive plastic materials, and is straightforward to troubleshoot and assemble. Such a stack design brings portable direct methanol fuel cell power sources one big step closer to commercialization. The design achieves the functions and performance of a conventional stack and has the potential to be just 30% of the weight of the conventional bipolar stack. 
     An “all-plastic” monopolar stack design is disclosed. By departing from the bipolar design, such a design eliminates biplates and end plates entirely and is particularly suitable for power output of less than 20 Watts. The stack  10  is assembled from individual “sealed unit cells”  12  as shown in cross-sectional side view in  FIG. 1  that are comprised of two back-to-back sealed fuel cells sharing a common anode. A single unit cell  12  as shown in  FIG. 1  is comprised of a first cathode plate  14   a , which is a conductive frame used as an electrical cathode terminal and which provides mechanical strength and integrity to the unit cell  12  and is shown in perspective view in  FIG. 5   a . First cathode plate  14   a  includes a first planar cathode  18   a  which is exposed to air or oxygen and forms the exterior skin of unit cell  12 . First cathode plate  14   a  is disposed underneath or adjacent to a first gasket  16   a  as shown in the perspective view of  FIG. 5   b.    
     A first membrane electrode assembly  20   a  is disposed on top of or adjacent to a first gasket  16   a  and first cathode  18   a  as shown in the perspective view of  FIG. 5   c . Membrane electrode assembly  20   a  is preferably of the type described in U.S. Pat. Nos. 6,303,244, 5,599,638, and/or 6,485,851, but may include any type of membrane electrode assembly now known or later devised. A second gasket  22   a  is disposed on top of or adjacent to first membrane electrode assembly  20   a  as shown in the perspective view of  FIG. 5   d.    
     The shared or common anode assembly  24  is then disposed on top of or adjacent to first membrane electrode assembly  20   a  and gasket  22   a  as shown in the perspective view of  FIG. 5   e . Anode assembly  24  is preferably molded or made in the most part from plastic and includes a conductive planar anode  26  on each side of a central channel  28  defined within anode assembly  24 . On the extreme left and right ends of anode assembly  24  as shown in  FIG. 1  is a plastic inlet manifold  30  and a plastic outlet manifold  32 . Inlet and outlet manifolds  30  and  32  may be integrally molded with the body of anode assembly  24  or may be welded or glued to it separately. Methanol or another fuel enters inlet manifold  30 , flows into channel  28  in intimate contact with anode  26  on each side of channel  28  and flows out of outlet manifold  32  with such other byproducts which may be produced during the operation of unit cell  12 . 
     Inlet and outlet manifolds  30  and  32  are designed so that they can snap fit or otherwise be readily coupled to adjacent inlet and outlet manifolds  30  and  32  respectively provided on the adjacent unit cell  12  in stack  10  as will be described in connection with  FIG. 2  below. Any type of coupling now known or later devised may be employed to facilitate the combination of adjacent inlet and outlet manifolds  30  and  32 . In the illustrated embodiment, inlet and outlet manifolds  30  and  32  are provided with orifices and sealing edges thereto (not shown) that seal and fit together using a conventional zip-lock or tongue-in-groove pressure or interference fit. The manner in which one manifold  30  or  32  couples to another to form a common inlet or outlet manifold is not material to the invention. One embodiment may include mating zip-lock orifices at the top and bottom of each manifold  30  or  32  with the topmost and bottom most manifold having an orifice sealed with a zip-lock panel or cover. 
     Continuing with the structure of unit cell  12  as assembled as shown in  FIGS. 5   a - 6   i , a third gasket  22   b  is disposed on top of or adjacent to anode assembly  24  as shown in the perspective view of  FIG. 5   f . A second membrane electrode assembly  20   b  is disposed on top of or adjacent to gasket  22   b  and anode  26  as shown in the perspective view of  FIG. 5   g . A fourth gasket  16   b  is disposed on top of or adjacent to second membrane electrode assembly  20   b  as shown in the perspective view of  FIG. 5   h . Finally, a second cathode plate  14   b  including a second cathode  18   b  is disposed on top of or adjacent to gasket  16   b  and second membrane electrode assembly  20   b  as shown in the perspective view of  FIG. 5   i . The entire unit cell  12  is then bolted together along it periphery by a plurality of nuts and bolts as shown in the photograph of  FIG. 6  to form an integral, sealed unit. 
     Such individual “sealed unit cells”  12  are separately assembled and tested and then stacked, coupled or snapped together as shown in  FIG. 2 .  FIG. 2  is a cross-sectional side view of three unit cells  12  assembled together. The vertical extend of manifolds  30  and  32 , which in the illustrated embodiment are prismatic in shape, is such that an air channel  34  is defined between adjacent unit cell  12  through air or oxygen can flow or be forced.  FIG. 4   a  is a three-quarter perspective view of three unit cells  12  showing connection to the top most manifolds  30  and  32  of a header  38  and pipe fitting  36 . Multiple manifolds  30   a  and  32  then combine to form a common manifold for stack  10  as best shown in the side view of  FIG. 4   b , and the end perspective views of  FIGS. 4   c  and  4   d . The actual form and topology of manifolds  30  and  32  may assume any design now known or later devised. 
     By snapping together these individual sealed plastic units  12  to form a seal similar to a “zip loc” type of arrangement, a stack  10  can be easily assembled, disassembled and reassembled. If for any reason on unit cell  12  needs to be serviced or replaced, this can be performed quickly without special tools. Since no pressure is used to hold the many sealed unit cells  12  that constitute the stack  10 , the stack  10  can be easily disassembled and individual sealed unit cells can be replaced. 
     A current collector  40  is included as part of the anode  26  and cathode plate surfaces  18   a  and  18   b  touching each of the electrodes as shown in the cross-sectional side view of  FIG. 3 . The extension of these current collectors  40  are then appropriately connected to obtain the desired voltage and current. For example, connector  42  couples cathode  18   a  to anode  26  in unit cell  12   a . Cathode  18   b  of unit  12   a  is coupled to anode  26  of unit cell  12   b  by connector  44 . Cathode  18   a  of unit  12   b  is coupled to anode  26  of unit cell  12   b  by connector  46 . Cathode  18   b  of unit  12   b  is coupled to anode  26  of unit cell  12   c  by connector  48 . Cathode  18   a  of unit  12   c  is coupled to anode  26  of unit cell  12   c  by connector  50 . Thus, by means of the serpentine arrangement of current collectors a series connection of the anodes and cathodes in cells  12   a - 12   c  is provided. Other types of electrical connections between cells  12   a - 12   c  can also be provided by modifications according to conventional design principles. 
     Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. For example, notwithstanding the fact that the elements of a claim are set forth below in a certain combination, it must be expressly understood that the invention includes other combinations of fewer, more or different elements, which are disclosed in above even when not initially claimed in such combinations. 
     The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself. 
     The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim. Although elements may be described above as acting in certain combinations and even initially claimed as such, it is to be expressly understood that one or more elements from a claimed combination can in some cases be excised from the combination and that the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the claims. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements. 
     The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptionally equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.