Abstract:
The present invention is directed to mitigating overuse of limited membrane regions in electrochemical conversion assemblies, particularly under cold start conditions. In accordance with one embodiment of the present invention, the anode and/or cathode flowfield plates of an electrochemical conversion assembly are configured such that the fluid header region defines an anode fluid header, a cathode fluid header, and a coolant fluid header configured such that a feed region of the plate defines an array of substantially linear fluid channels extending from an acutely angled header/feed interface defined on the plate to a feed/active interface defined across the entire active area of the plate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. patent application Ser. No. 11/009,378 (GP-305619), for REACTANT FEED FOR NESTED STAMPED PLATES FOR A COMPACT FUEL CELL, filed Dec. 10, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to electrochemical conversion cells, commonly referred to as fuel cells, which produce electrical energy by processing first and second reactants. For example, electrical energy can be generated in a fuel cell through the reduction of an oxygen-containing gas and the oxidation of a hydrogenous gas. By way of illustration and not limitation, a typical cell comprises a membrane electrode assembly positioned between a pair of flow fields accommodating respective ones of the reactants. More specifically, a cathode flowfield plate and an anode flowfield plate can be positioned on opposite sides of the membrane electrode assembly. The voltage provided by a single cell unit is typically too small for useful application so it is common to arrange a plurality of cells in a conductively coupled “stack” to increase the electrical output of the electrochemical conversion assembly.  
         [0003]     By way of background, the conversion assembly generally comprises a membrane electrode assembly, an anode flowfield, and a cathode flowfield. The membrane electrode assembly in turn comprises a proton exchange membrane separating an anode and cathode. The membrane electrode assembly generally comprises, among other things, a catalyst supported by a high surface area support material and is characterized by enhanced proton conductivity under wet conditions. For the purpose of describing the context of the present invention, it is noted that the general configuration and operation of fuel cells and fuel cell stacks is beyond the scope of the present invention. Rather, the present invention is directed to particular flowfield plate configurations and to general concepts regarding their design. Regarding the general configuration and operation of fuel cells and fuel cell stacks, applicants refer to the vast collection of teachings covering the manner in which fuel cell “stacks” and the various components of the stack are configured. For example, a plurality of U.S. patents and published applications relate directly to fuel cell configurations and corresponding methods of operation. More specifically, FIGS. 1 and 2 of U.S. Patent Application Pub. No. 2005/0058864, and the accompanying text, present a detailed illustration of the components of a fuel cell stack—this particular subject matter is expressly incorporated herein by reference.  
       BRIEF SUMMARY OF THE INVENTION  
       [0004]     The present invention is directed to mitigating overuse of limited membrane regions in electrochemical conversion assemblies, particularly under cold start conditions. In accordance with one embodiment of the present invention, an electrochemical conversion assembly is provided comprising at least one electrochemical conversion cell configured to convert first and second reactants to electrical energy. The cell comprises a membrane electrode assembly, an anode flowfield portion and a cathode flowfield portion defined on opposite sides of the membrane electrode assembly. A first reactant supply is configured to provide a first reactant to an anode side of the membrane electrode assembly via the anode flowfield portion and a second reactant supply is configured to provide a second reactant to a cathode side of the membrane electrode assembly via the cathode flowfield portion. At least one of the anode and cathode flowfield portions comprises an enhanced flowfield plate defining an active region, a fluid header region, and a feed region.  
         [0005]     The active region comprises a plurality of fluid flow channels and the feed region is configured to transfer fluid from the header region to the fluid flow channels of the active region. The fluid header region defines an anode fluid header, a cathode fluid header, and a coolant fluid header fluidly decoupled from each other. A portion of the feed region interfaces with the active region along a feed/active interface extending across a substantial entirety of the active region. A portion of the fluid header region corresponding to at least one of the fluid headers interfaces with the feed region along a header/feed interface oriented at an acute angle with respect to the feed/active interface. The feed region defines an array of substantially linear fluid channels extending from the header/feed interface to the feed/active interface.  
         [0006]     In accordance with another embodiment of the present invention, an electrochemical conversion assembly is provided comprising an enhanced anode flowfield plate and an enhanced cathode flowfield plate positioned on opposite sides of a membrane electrode assembly.  
         [0007]     Accordingly, it is an object of the present invention to mitigate overuse of limited membrane regions in electrochemical conversion assemblies. Of course, other objects of the present invention will be apparent in light of the description of the invention embodied herein. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0008]     The following detailed description of specific embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:  
         [0009]      FIG. 1  is a plan view of the reactant side of an anode flowfield plate according to one embodiment of the present invention; and  
         [0010]      FIG. 2  is a mirror image plan view of the reactant side of a cathode flowfield plate configured to complement the embodiment of the present invention illustrated in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION  
       [0011]     An enhanced anode flowfield plate  10  according to one embodiment of the present invention is illustrated in  FIG. 1  and comprises a fluid header region  20 , a feed region  30 , and an active region  40 . The active region  40  comprises a plurality of fluid flow channels  42 . The feed region  30  is configured to transfer fluid from the header region  20  to the fluid flow channels  42  of the active region  40 . The active region  40  extends along two dimensions x,y defined within the X-Y plane of the enhanced flowfield plate  10 . As will be appreciated by those familiar with fuel cell design, the extent of the x,y dimensions of the active region  40  corresponds directly to the size of the associated membrane electrode assembly, which assemblies typically approximate a substantially orthogonal quadrilateral.  
         [0012]     The fluid header region  20  defines an anode fluid header  22 , a cathode fluid header  24 , and a coolant fluid header  26  fluidly decoupled from each other. In the illustrated embodiment, the anode fluid header  22  interfaces with the feed region  30  along a header/feed interface  32  oriented at an acute angle θ with respect to the feed/active interface  34  where the feed region  30  interfaces with the active region  40 . The feed/active interface  34  extends across the substantial entirety of the active region  40 . In addition, the feed region  30  defines an array of substantially linear fluid channels  36  extending from the header/feed interface  32  to the feed/active interface  34 . In this manner, the enhanced flowfield plate  10  illustrated in  FIG. 1  can be utilized to minimize the extent to which the initial heat generated from current distribution during start-up is localized in specific areas of the active region  40  of the flowfield plate  10  and portions of the membrane electrode assembly lying in register with the plate  10 .  
         [0013]     More specifically, the present inventors have recognized that initial current density, and therefore initial heat generation, is concentrated near the anode inlet area of a fuel cell because it is the first to see reactants when a cold start is initiated. As more current load is applied to the fuel cell stack, more current gets generated near the fuel cell stack header anode inlet and it continues to warm in a relatively concentrated area. The heat generated in the relatively concentrated area is gradually dispersed to more of the active region of the cell, melting any ice present and rendering more catalyst sites active until the active region is up to normal operating temperature. The limited, high temperature/high current density region that appears near the anode inlet during cold starting is undesirable because the region would become relatively over utilized over many repeated cold start cycles and would negatively impact the durability of the membrane electrode assembly in the over utilized regions. To exacerbate the problem, pre-freeze purge operations in fuel cells often make the anode inlet area one of driest areas of the cell when a cold start is initiated. The enhanced flowfield plate design of the present invention is proposed herein as a means for increasing the utilization of more of the active region during start-up by distributing more of the anode reactants to more of the active region at start-up. In effect, although other objects of the present invention can be gleaned from the present description, particular embodiments of the present invention seek to distribute the high temperature/high current density region over more of the active region of the cell.  
         [0014]     Referring to  FIG. 2 , an enhanced cathode flowfield plate  10 ′ is illustrated, where like structure is indicated with like reference numerals in  FIGS. 1 and 2 . In the embodiment illustrated in  FIG. 2 , the cathode fluid header  24  interfaces with the feed region  30  along the header/feed interface  32  and the feed region  30  interfaces with the active region  40  along the feed/active interface  34 . As is the case with the anode flowfield plate of  FIG. 1 , the feed/active interface  34  extends across the substantial entirety of the active region  40 . Comparing  FIGS. 1 and 2 , it is noted that the respective header/feed interfaces  32  of the respective enhanced flowfield plates associated with the anode and cathode flowfield plates  10 ,  10 ′ of  FIGS. 1 and 2  face the respective feed/active interfaces  34  from opposite directions.  
         [0015]     Referring to the header regions  20  illustrated in  FIGS. 1 and 2 , it is noted that the anode fluid header  22 , the cathode fluid header  24 , and the coolant fluid header  26  each extend a given distance from a common edge  12  of the respective enhanced flowfield plates  10 ,  10 ′ in the direction of the active region  40 . The spacing between the active region  40  and the fluid header region  20  varies from a minimum value a in an area of the anode fluid header  22  or the cathode fluid header  24  to a maximum value b in an area of the coolant fluid header  26 . In this manner, the feed region  30  can be configured such that the array of substantially linear fluid channels  36  defines a substantially triangular configuration where the longest side of the triangular configuration is defined at the feed/active interface  34  and the shortest side of the triangular configuration is defined at the header/feed interface  32 .  
         [0016]     Regarding the active regions  40  illustrated in  FIGS. 1 and 2 , it is noted that the fluid flow path defined by the fluid flow channels  42  in the active region maintains a continuous progression away from the feed/active interface. More specifically, the continuous progression of fluid flow is characterized by changes in flow direction of substantially less than 90 degrees. In this manner, fluid within the fluid flow channels  42  is less likely to become trapped therein because it can be readily drained from the active region  40 .  
         [0017]     The fluid flow channels  42  in the active region  40  define respective fluid inlets along the feed/active interface  34  and respective fluid outlets along an opposite feed/active interface  34 ′ defined at a distal edge of the fluid flow channels  42 . An opposite feed region  30 ′ interfaces with the fluid outlets and communicates with an opposite header region  20 ′, also defined beyond the distal edge of the fluid flow channels in the active region  40 .  
         [0018]     Further, it is noted that the active region  40  is configured to be rotationally symmetric in a plane defined by a major face of the enhanced flowfield plate  10 ,  10 ′. The inlet and outlet sides of each plate  10 ,  10 ′ can be interchanged by merely rotating the plate  1800  in the plane of the plate. In this manner, the enhanced flowfield plate  10 ,  10 ′ is configured such that the header and feed regions  20 ,  30  are functionally interchangeable with the opposite header and feed regions  20 ′,  30 ′.  
         [0019]     For the purposes of describing and defining the present invention it is noted that an acute angle is an angle of greater than zero and less than 90 degrees. Further, the term “linear” is utilized herein to correspond to a unidirectional projection, as opposed to a curved path or a path defined by a plurality of linear segments extending in different directions.  
         [0020]     It is noted that terms like “preferably,” “commonly,” and “typically” are not utilized herein to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention.  
         [0021]     For the purposes of describing and defining the present invention it is noted that the term “substantially” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. For example, in the context of the “substantially linear fluid channels” identified herein, it is noted that the insignificant introduction of curved or otherwise non-linear portions in the linear channels illustrated herein should not be taken as a departure from the scope of the terms “substantially linear” absent a showing that the non-linear portions result in a change in the basic function of the “substantially linear fluid channels.” 
         [0022]     Having described the invention in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. For example, it is contemplated that a vehicle may be configured to incorporate an electrochemical conversion assembly according to the present invention to permit the electrochemical conversion assembly to serve as a source of motive power for the vehicle.  
         [0023]     Although some aspects of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the present invention is not necessarily limited to these preferred aspects of the invention.