Abstract:
A gas diffusion layer that is usable with a fuel cell includes a material that is adapted to permit a reactant of the fuel cell to diffuse through the material. The material includes flow channels for communicating the reactant so that at least a portion of the reactant diffuses through the material to a membrane of a fuel cell. The gas diffusion layer may be received by a recessed portion of a plate.

Description:
BACKGROUND 
     The invention generally relates to the overall construction and configuration of a proton exchange membrane fuel cell, and more particularly, the invention relates to a combined fuel cell flow plate and gas diffusion layer. 
     A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM), a membrane that may permit only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is oxidized to produce hydrogen protons that pass through the PEM. The electrons produced by this oxidation travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions may be described by the following equations: 
     
       
         H 2 →2H + +2 e   −  at the anode of the cell, and 
       
     
     
       
         O 2 +4H + +4 e   − →2H 2 O at the cathode of the cell. 
       
     
     Because a single fuel cell typically produces a relatively small voltage (around 1 volt, for example), several serially connected fuel cells may be formed out of an arrangement called a fuel cell stack to produce a higher voltage. The fuel cell stack may include different plates that are stacked one on top of the other in the appropriate order, and each plate may be associated with more than one fuel cell of the stack. The plates may be made from a graphite composite material or metal and may include various channels and orifices to, as examples, route the above-described reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. In addition to the membrane, a catalyst and gas diffusion layer are sandwiched between the anode and cathode plates. The catalyst layer may be placed on the membrane or on the gas diffusion layer. The gas diffusion layer may be made out of an electrically conductive and porous diffusion material, such as a carbon cloth or paper material, for example. 
     Referring to FIG. 1, as an example, a fuel cell stack  10  may be formed out of repeating units called plate modules  12 . In this manner, each plate module  12  includes a set of plates that may form several fuel cells. For example, for the arrangement depicted in FIG. 1, an exemplary plate module  12   a  may be formed from a cathode cooler plate  14 , a bipolar plate  16 , a cathode cooler plate  18 , an anode cooler plate  20 , a bipolar plate  22  and an anode cooler plate  24  that are stacked from bottom to top in the listed order. The cooler plate functions as a heat exchanger by communicating a coolant through coolant flow channels that are formed in either the upper or lower surface of the cooler plate to remove heat from the plate module  12   a . The surface of the cooler plate that is not used to route the coolant includes flow channels to route either hydrogen (for the anode cooler plates  18  and  24 ) or oxygen (for the cathode cooler plates  14  and  20 ) to an associated fuel cell. The bipolar plates  16  and  22  include flow channels on one surface to route hydrogen to an associated fuel cell and flow channels on the opposing surface to route oxygen to another associated fuel cell. Due to this arrangement, each fuel cell may be formed in part from one bipolar plate and one cooler plate, as an example. 
     For example, one fuel cell of the plate module  12   a  may include an anode-membrane-ecathode sandwich, called a membrane-electrode-assembly (MEA), that is located between the anode cooler plate  24  and the bipolar plate  22 . In this manner, the upper surface of the bipolar plate  22  includes flow channels to route oxygen near the cathode of the MEA, and the lower surface of the anode cooler plate  24  includes flow channels to route hydrogen near the anode of the MEA. 
     As another example, another fuel cell of the plate module  12   a  may be formed from another MEA that is located between the bipolar plate  22  and the cathode cooler plate  20 . The lower surface of the bipolar plate  22  includes flow channels to route hydrogen near the anode of the MEA, and the upper surface of the cathode cooler plate  24  includes flow channels to route oxygen near the cathode of the MEA. The other fuel cells of the plate module  12   a  may be formed in a similar manner. 
     In order to manufacture the fuel cell stack in a cost effective manner, the time that is required to assemble the stack needs to be minimized. One way to accomplish this is to minimize the number of components that are handled in the assembly of the stack. 
     SUMMARY 
     The components that are used to assemble a fuel cell stack may be reduced by creating flow channels in gas diffusion layers of the stack. In this manner, in an embodiment of the invention, a gas diffusion layer that is usable with a fuel cell includes a material that is adapted to permit a reactant of the fuel cell to diffuse through the material. The material includes flow channels for communicating the reactant so that at least a portion of the reactant diffuses through the material to a membrane of a fuel cell. In some embodiments, the gas diffusion layer may be received by a plate, and the plate may be stacked with other plates to form the fuel cell stack. Advantages of the present invention may include, for example, decreasing the number and cost of parts and materials required to manufacture a fuel cell stack. Other advantages may include increased structural integrity of a fuel cell stack, and increased fuel cell performance. It will be appreciated that in embodiments where a planer surface of the combined flow plate and gas diffusion layer contacts the membrane electrode assembly (as opposed to such contact being limited to contact along flow plate lands), still other advantages may include providing continuous support of the fuel cell membrane electrode assembly, homogenized diffusion, permeability, and electrical contact between the membrane electrode assembly and the combined flow plate and gas diffusion layer, and reducing the amount of stack compression needed for satisfactory electrical conductivity between the membrane electrode assembly and the combined flow plate and gas diffusion layer. 
    
    
     Advantages and other features of the invention will become apparent from the following description, from the drawing and from the claims. 
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a side view of a fuel cell stack according to the prior art. 
     FIG. 2 is a side view of a fuel cell stack according to an embodiment of the invention. 
     FIG. 3 is an exploded bottom up perspective view of an anode cooler plate assembly of the fuel cell stack of FIG. 2 according to an embodiment of the invention. 
     FIG. 4 is a bottom up perspective view of the anode cooler plate assembly of FIG.  3 . 
     FIG. 5 is a cross-sectional view of the anode cooler plate assembly taken along line  5 — 5  of FIG.  4 . 
     FIG. 6 is a bottom view of a gas diffusion layer of the anode cooler plate assembly according to an embodiment of the invention. 
     FIG. 7 is a more detailed exploded cross-sectional view of a plate module of the fuel cell stack of FIG. 2 according to an embodiment of the invention. 
     FIG. 8 is a bottom up perspective view of an anode cooler plate assembly according to an embodiment of the invention. 
     FIG. 9 is a cross-sectional view of a portion of a gas diffusion layer according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 2, an embodiment  50  of a fuel cell stack in accordance with the invention includes gas diffusion layers that each have built-in flow channels to communicate reactants to membranes (proton exchange membranes (PEMs), for example) of the stack  50 . Because flow channels are formed in the gas diffusion layers, reactant flow channels do not need to be formed in flow plates (graphite composite or metal flow plates, for example) of the fuel cell stack  50 . Thus, due to this technique, the gas diffusion layers and flow plates are combined into plate assemblies (described below) that may reduce assembly time of the stack  50  and thus, may reduce the cost of manufacturing the fuel cell stack  50 . 
     More specifically, the fuel cell stack  50  may be formed from repeating units called plate modules  51 . A side view of an exemplary plate module  51   a  (having a design similar to the other plate modules  51 ) is depicted in FIG.  2 . The plate module  51   a  includes flow plates that serve as electrically conductive frames to hold the gas diffusion layers. In this manner, each gas diffusion layer is electrically conductive, diffuses a fuel cell reactant to an associated membrane and supports the associated fuel cell membrane. As an example, in some embodiments, the plate module  51   a  may include the following flow plates to form four fuel cells: bipolar plates  54  and  60 ; cathode cooler plates  52  and  58 ; and anode cooler plates  56  and  62 . 
     FIG. 3 depicts a bottom up exploded perspective view of an anode cooler plate assembly  63  that is formed from the anode cooler plate  62  and an inset gas diffusion layer  84 . The plate  62  includes openings that align with corresponding openings of other flow plates of the stack  50  to form a manifold for communicating the reactants and a coolant. In this manner, the plate  62  may include an opening  170  that forms part of a vertical inlet manifold passageway of the manifold for the hydrogen, and the plate  62  may include an opening  168  that forms part of a vertical outlet passageway of the manifold for the hydrogen. Similarly, openings  174  and  164  in the plate  62  may form vertical inlet and outlet passageways, respectively, of the manifold for communicating air (to furnish oxygen to the fuel cells); and the plate  62  may include openings  172  and  166 , respectively, to partially form vertical inlet and outlet passageways of the manifold for communicating the coolant. 
     The anode cooler plate  62  may include a generally rectangular recessed portion  82  for receiving the inset gas diffusion layer  84 . In this manner, the gas diffusion layer  84  may be placed in the recessed portion  82  so that a top surface  91  of the gas diffusion layer  84  is generally flush with a rim  69  of the anode cooler plate  62  that circumscribes the recessed portion  82 , as depicted in FIG.  4 . Inlet  92  and outlet  90  extensions of the gas diffusion layer  84  include inlet and outlet flow channel ports, respectively, that include flow channels to establish communication between the flow channels of the gas diffusion layer  84  and the hydrogen manifold passageways. The inlet  92  and outlet  90  extensions may rest in corresponding channels  88  and  86 , respectively, that are formed in the anode cooler plate  62 , as depicted in FIG.  4 . For example, each channel  86 ,  88  may be formed by the absence of a section of the rim  69 . 
     Referring to FIG. 5, the upper surface of the anode cooler plate  62  may include a gasket groove  99  to receive a gasket  98  that seals the various reactant and coolant regions that are formed by the anode cooler plate  62  and the gas diffusion layer  84 . Also shown in FIGS. 3,  4  and  5  is another gasket  98  that contacts the lower surface of the anode plate  62  and resides in a corresponding gasket groove that is formed in the bipolar plate  60  (see FIG.  2 ). 
     Referring to FIG. 6, the upper surface (that contacts the surface  91  of the plate  62 ) of the gas diffusion layer  84  includes flow channels  102  to communicate hydrogen from the opening  170  of the hydrogen manifold passageway to the opening  168  of the hydrogen outlet manifold passageway. Thus, due to the above-described arrangement, the flow channels  102  in combination with the adjacent surface  91  of the plate  62  form closed flow serpentine conduits for communicating a hydrogen flow. Hydrogen from the flow diffuses through the lower surface  91  (see FIG. 4) of the gas diffusion layer  84  to contact a membrane, such as a proton exchange membrane (PEM), that is located below the gas diffusion layer  84 . While serpentine channels are shown in FIG. 6 as an illustration, the present invention is not limited to serpentine flow channels. Embodiments of the present invention may also include, for example, non-serpentine channels, channels that are discontinuous (e.g., branching into additional channels), and any other suitable channel configurations. 
     The fuel cell reaction produces water near the membrane. For purposes of preventing this water from collecting in the gas diffusion layer  84 , the gas diffusion layer  84  may include a wet proofing agent, or other agents that may promote water diffusion to prevent water collection. For example, a cross-sectional portion  290  of the gas diffusion layer  84  is depicted in FIG.  9 . As shown, the gas diffusion layer  84  includes a diffuser body  302  and an upper region  300  in which the flow channels  102  are formed. In some embodiments, a hydrophobic material (Teflon®, for example) may be deposited in the diffuser body  302  to prevent water from collecting in the diffuser body  302 . In some embodiments, fluorinated carbon particles may be deposited on the surface  91  to prevent migration of the water. 
     In some embodiments, the gas diffusion layer  84  may be formed by injection molding or other molding techniques known in the art. For example the gas diffusion layer  84  may be formed of a powdered metal material with a thermally or chemically removable binder material. For example, a wax binding material may be removed and the remaining structure sintered to form a porous metal gas diffusion layer substrate. Other molding techniques may allow formation of the gas diffusion layer  84 , for example, by simple compression of a metal or carbon based powder. Other materials and techniques are possible. 
     The gas diffusion layer may also be used to communicate air flows. For example, the plate module  51   a  (as well as the entire stack  50 ) may include gas diffusion layers that include flow channels for communicating air and gas diffusion layers that include flow channels for communicating oxygen. For example, FIG. 7 depicts a more detailed cross-sectional view of the plate module  51   a  taken along the same cross-sectional slice as the cross-sectional view of the anode cooler plate  62  that is depicted in FIG.  5 . As shown, the bipolar plate  60  may include an upper surface recessed region  140  for receiving an inset gas diffusion layer  109 . The gas diffusion layer  109 , in turn, includes flow channels for communicating an air flow to cause oxygen (from the air flow) to diffuse through the gas diffusion layer  109  to an upper membrane  104 . The bipolar plate  60  may also include a lower surface recessed region  142  for receiving a gas diffusion layer  110 . The gas diffusion layer  110 , in turn, includes flow channels for communicating hydrogen to cause the hydrogen to diffuse through the gas diffusion layer  110  to a lower membrane  106 . It will be appreciated that, while members  109  and  110 , for example, are generally referred to as gas diffusion layers, they might also be referred to as porous flow plates, such that designs under the present invention could be referred to as being without gas diffusion layers. In fact, members  109  and  110  are combined flow plates and gas diffusion layers. The scope of the present invention is not limited by arbitrary designations of such terms as they might be construed with respect to the prior art. Rather, for purposes of the present invention, the relative meaning of traditional terms such as flow plates and gas diffusion layers should be taken in the context of the true spirit of the claims of the present invention. 
     Similarly, the cathode cooler plate  58  includes an inset gas diffusion layer  112 , the anode cooler plate  56  includes an inset gas diffusion layer  114 , the bipolar plate  54  includes an inset gas diffusion layer  118 , and the cathode cooler plate  58  includes an inset gas diffusion layer  120 . Similar to the gas diffusion layers  84 ,  109  and  110 , the gas diffusion layers  112 ,  114 ,  116 ,  118  and  120  include reactant flow channels. 
     Thus, as depicted in FIG. 7, in some embodiments, three basic plates may be used to form the stack  50 . In this manner, each bipolar plate (such as the bipolar plates  54  and  60 ) may be identical. In this manner, each bipolar plate may receive and support an upper air diffusion layer that diffuses oxygen to an upper membrane, and each bipolar plate may receive and support a lower gas diffusion layer that diffuses hydrogen to a lower membrane. Each anode cooler plate (anode cooler plates  56  and  62  ) may also be identical. In this manner, each anode cooler plate receives and supports a hydrogen diffusion layer. A gasket groove may be formed in the upper surface of the anode cooler plate. Similarly, each cathode cooler plate (cathode cooler plates  54  and  58 ) may also be identical. In this manner, cathode cooler plate receives and supports an oxygen diffusion layer. A gasket groove may be formed in the upper surface of the cathode cooler plate. As depicted in FIG. 7, the cathode cooler plate may be similar in design to the anode plate except for the placement of the gasket groove. Other arrangements are possible. 
     Other embodiments are within the scope of the following claims. For example, an alternative anode cooler plate assembly  205  is depicted in FIG.  8 . The assembly  205  includes an anode cooler plate  204  that receives an inset gas diffusion layer  200  that has built-in flow channels. Unlike the anode cooler plate  62 , the plate  205  has built in extensions  209  and outlet  207  for communicating hydrogen between the flow channels of the gas diffusion layer  200  as the hydrogen manifold passageways. 
     As another example, other layers besides the gas diffusion layers may be placed in the recessed portions of the plates. Thus, layers that enhance performance aspects of the fuel cells may be used. For example, a layer may be used to control the effective pore size through which reactant gasses must diffuse. A layer may also be used to enhance the electrical contact between the gas diffusion layer and the plate. A layer may also be used to inhibit water collection in the gas diffusion layer, such as by using a hydrophobizing agent or by other methods known in the art. The gas diffusion layer may itself receive various coatings or treatments to promote performance objectives including those described above. A catalyst layer may be bonded either to the membrane or to the gas diffusion layer. Other layers or combinations of layers are possible. As yet another example of an embodiment of the invention, the membrane may be bonded between two gas diffusion layers. 
     In the preceding description, directional terms, such as “upper,” “lower,” etc. may have been used for reasons of convenience to describe the fuel cell stack and its associated components. However, such orientations are not needed to practice the invention, and thus, other orientations are possible in other embodiments of the invention. For example, the fuel cell stack  50  and its associated components, in some embodiments, may be tilted by 90°. 
     While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.