Patent Publication Number: US-2010129725-A1

Title: Fuel cell bipolar plate with integrated sealing and fuel cell comprising such plates

Description:
The invention mainly concerns a bipolar plate for a fuel cell. 
     The invention also concerns a cell of a fuel cell stack comprising such a bipolar plate. 
     A fuel cell is an electrochemical device that makes it possible to convert chemical energy to electrical energy from a fuel (generally hydrogen) and an oxidant (oxygen or an oxygen-containing gas such as air); the only product of the reaction is water, accompanied by a release of heat and generation of electricity. 
     Inside the fuel cell, the overall chemical reaction produced by the reactions occurring at the electrodes is the following: 
       H 2 +½O 2 →H 2 O 
     A fuel cell can be used to supply electrical energy to any device, such as a computer or a cellular phone, for example, but it can also be used to power a motor vehicle and/or the electrical devices contained in a vehicle. 
     A fuel cell stack can consist of one or more cells. 
     Referring to  FIG. 1 , which represents a cell of a prior art fuel cell stack, such a cell  1  has a proton-conducting electrolyte  2 , sandwiched between a cathode porous electrode  3  and an anode porous electrode  4 , that ensures the electron transfer between these two electrodes  3 ,  4 . 
     To this end, the electrolyte  2  can be a proton exchanging polymer membrane 20 to 200 μm thick, the resulting stack being a PEMFC-type stack (Proton Exchange Membrane Fuel Cell). 
     The assembly consisting of the electrolyte  2  and the two electrodes  3 ,  4  forms a membrane electrode assembly (MEA) plate  5  that is itself sandwiched between first  6  and second  7  bipolar plates that collect the current, distribute the oxidant and the fuel to the electrodes and circulate the heat transfer fluid. 
     The bipolar plates  6 ,  7  commonly used are made of materials that have good corrosion resistance and electrical conductivity properties, such as carbon materials like graphite, polymer-impregnated graphite, or flexible graphite sheets fabricated by machining or molding them. 
     The bipolar plates  6 ,  7  can also be made using metal materials such as titanium-, aluminum- and iron-based alloys, including stainless steels. In this case, the bipolar plate can be fabricated by drawing or stamping thin sheets. 
     In order to distribute the oxidant, the fuel, and the heat transfer fluid to all of the constituent cells of the stack, the second bipolar plate  7  has six drilled holes  7   a ,  7   b ,  7   c ,  7   d ,  7   e ,  7   f , three of which  7   a ,  7   b ,  7   c  are evenly spaced on the top edge  8  of this plate  7 , with the three other holes  7   d ,  7   e ,  7   f  evenly spaced as well, in a symmetrical manner on the bottom edge  9  of this plate  7 . 
     The first bipolar plate  6  has the same holes located in the same places as those on the bipolar plate  7 , with  FIG. 1  showing only the three top holes  6   a ,  6   b ,  6   c  and one bottom hole  6   d.    
     The holes  6   a ,  6   b ,  6   c ,  6   d  in the first bipolar plate  6  and the holes  7   a ,  7   b ,  7   c ,  7   d ,  7   e ,  7   f  in the second bipolar plate  7  must be aligned so that the fluids can circulate through all the constituent cells of the stack when this stack is assembled. 
     In the area of each of these holes  7   a ,  7   b ,  7   c ,  7   d ,  7   e ,  7   f ,  6   a ,  6   b ,  6   c ,  6   d , a conduit that is not shown makes it possible to supply or recover the heat transfer fluid, the fuel or the oxidant circulating on the surface of the plate  6 ,  7  or inside the plate  6 ,  7  in fluid circulation circuits or channels provided for this purpose, which will be described below. 
     Referring to  FIG. 2 , which is a section along the line II-II in  FIG. 1 , the cathode  3  and anode  4  electrodes each have a respective active layer  10 ,  11 , which are the cathode and anode reaction sites, respectively, and a respective diffusion layer  12 ,  13  sandwiched between the active layer  10 ,  11  and the corresponding bipolar plate  7 ,  6 ; this diffusion layer  12 ,  13  can be a paper substrate or a carbon cloth. 
     The diffusion layer  12 ,  13  homogeneously diffuses reagents such as hydrogen and oxygen, which circulate in their respective channels  14 ,  15 , formed by grooves in the respective bipolar plates  7 ,  6 . 
     In this way, the active layer  11  of the anode electrode  4  is supplied with hydrogen via the diffusion layer  13 , and the reaction that occurs in this active layer  11  is the following: 
       H 2 →2 e   − +2H +   (1) 
     In the same way, the active layer  10  of the cathode electrode  3  is supplied with oxygen via the diffusion layer  12 , and the reaction that occurs in this active layer  10  is the following: 
       ½O 2 +2H + +2 e   − →H 2 O  (2) 
     These reactions are made possible by the presence of the conductive membrane  2 , through which protons are transferred from the active layer  11  of the anode  4  toward the active layer  10  of the cathode  3 . 
     Due to the nature of the fluids used and the electrochemical reactions involved, sealing is an important consideration in the design of a fuel cell stack. 
     Referring to  FIG. 3 , which represents a prior art fuel cell, this seal can be formed by the presence of a gasket  16 ,  17  interposed between the substantially rectangular respective bipolar plates  6 ,  7  and the membrane electrode assembly plate  5 , made up of an active area  19  where the electrochemical reactions take place and a frame  18  surrounding this active area  19 . 
     Referring to the anode part of the cell  1  shown in this figure, when the stack is assembled, the gasket  17  is fitted into a substantially rectangular conjugate peripheral groove  20  in the bipolar plate  6  that surrounds the reagent distribution channels  15 . 
     During this same assembly process, the frame  18  of the assembly plate  5  is made to bear on the whole periphery of the bipolar plate  6  and compresses the corresponding gasket  16 , which thereby allows the seal to form between the anode part and the exterior of the stack. 
     Naturally, in a symmetrical fashion, the bipolar plate  7  in the cathode part of the cell  1  also has a peripheral groove surrounding the oxidant distribution channels of this plate  7  into which the gasket  17  fits; they are neither shown nor referenced due to the angle from which this figure is seen. Thus it is understood that the groove  21  and the distribution channels  14 ′ of the bipolar plate  7  that are referenced and depicted belong to the anode part of the cell next to the cell  1 . 
     It is also possible to design the groove  20  and its corresponding groove in the bipolar plate of the cathode part so that they are circular in shape, and in this case, the gasket  16  used is an O-ring. 
     According to prior art, the gasket  16  can also be a flat or serigraphed seal, and in this case, the parts of the cell, particularly the bipolar plates  6 ,  7  have a shape modified to fit. 
     It is also possible to have the gasket positioned on the membrane electrode assembly plate  5  rather than being positioned on the bipolar plate before assembly; in this case as well, the parts that make up the cell are appropriately modified. 
     In the prior art device shown in  FIG. 3 , the bipolar plates  6 ,  7  must therefore be fabricated, but the gasket must also meet strict criteria for resistance particularly, in order to seal off the stack. 
     In this context, the invention particularly concerns a bipolar plate for a fuel cell stack that makes it possible to overcome the difficulties cited above. 
     To this end, the bipolar plate  22  of the invention is essentially characterized in that it has at least one raised border  47 ,  23   a ,  33   a ,  40   a ,  27   a ,  35   a ,  41   a ,  23   a  on at least one of its faces  51 , so as to seal off at least one fluid circuit of said stack from among the oxidant, fuel and heat transfer fluid supply circuits and the oxidant, fuel and heat transfer fluid exhaust circuits; said circuits are formed when the constituent cells  1  of the fuel cell stack are assembled by stacking the openings provided in said plate  22  that respectively form oxidant and fuel inlet and outlet means  33 ,  40 ,  35 ,  41  and openings that form heat transfer fluid inlet and outlet means  23 ,  27 . 
     Advantageously, the bipolar plate of the invention has at least one raised peripheral border  47  enclosing the openings that form reagent inlet and outlet means  33 ,  40 ,  35 ,  41  and the openings that form heat transfer fluid inlet and outlet means  23 ,  27 . 
     By preference, at least one raised border  23   a ,  27   a ,  33   a ,  40   a ,  41   a ,  35   a  encloses at least one opening from among the openings that form reagent inlet and outlet means  33 ,  40 ,  35 ,  41  and the openings that form heat transfer fluid inlet and outlet means  23 ,  27 , so as to seal off the corresponding fluid circuit when the constituent cells of the stack are assembled. 
     In this case, a raised border  23   a ,  27   a ,  33   a ,  40   a ,  41   a ,  35   a  can enclose each opening that forms a reagent inlet or outlet means  33 ,  40 ,  35 ,  41  and each opening that forms a heat transfer fluid inlet or outlet means  23 ,  27 , so as to seal off all of the fluid circuits when the constituent cells of the stack are assembled. 
     In addition, the raised peripheral border  47  can overlap with at least one opening that forms a reagent inlet or outlet means  33 ,  40 ,  35 ,  41  or one opening that forms a heat transfer fluid inlet or outlet means  23 ,  27  on the outermost part of said raised border  23   a ,  24   a ,  27   a ,  28   a.    
     According to a preferred embodiment, at least one raised border  47 ,  33   a ,  35   a ,  40   a ,  41   a ,  23   a ,  24   a ,  27   a ,  28   a  wholly or partly supports a seal  54 . 
     The raised peripheral border  47  of the plate  22  and the raised borders  33   a ,  35   a ,  40   a ,  41   a ,  23   a ,  24   a ,  27   a ,  28   a  of the openings that form inlet and outlet means for reagent  33 ,  35 ,  40 ,  41  and heat transfer fluid  23 ,  24 ,  27 ,  28  are preferably covered by a seal  54 . 
     Moreover, the seal can be a strip and can be serigraphed. 
     The bipolar plate is advantageously made of a metallic material, but can also be made of expanded graphite or loaded composite. 
     The raised borders  47 ,  33   a ,  35   a ,  40   a ,  41   a ,  23   a ,  24   a ,  27   a ,  28   a  are preferably formed by drawing or stamping them. 
     The invention also concerns a cell of a fuel cell stack comprising a membrane electrode assembly plate  50  that has an active area  52  in particular—the anode and cathode reaction sites—and which is sandwiched between two previously described bipolar plates. 
     The membrane electrode assembly plate  50  has a peripheral frame  53  that preferably bears on at least one raised border  47 ,  33   a ,  35   a ,  40   a ,  41   a ,  23   a ,  24   a ,  27   a ,  28   a  of the bipolar plate  22  when said cell is assembled. 
     More preferably, the membrane electrode assembly plate  50  has a peripheral frame  53  that bears on all of the raised borders  47 ,  33   a ,  35   a ,  40   a ,  41   a ,  23   a ,  24   a ,  27   a ,  28   a  of the bipolar plate  22  when said cell is assembled. 
     Advantageously, the membrane electrode assembly plate  50  is mechanically compatible with the bipolar plate  22 . 
     Lastly, the invention also concerns a fuel cell stack comprising at least one above-described cell. 
    
    
     
       The invention will be more easily understood, and other purposes, advantages, and characteristics thereof will become clearer in the following description, written with reference to the attached drawings, which represent non-limiting examples embodying the device of the invention, and in which: 
         FIG. 1  is a perspective exploded view of a prior art fuel cell; 
         FIG. 2  is a sectional view along the line II-II in  FIG. 1 ; 
         FIG. 3  is a perspective exploded view of a prior art fuel cell; 
         FIG. 4  is a front view of the bipolar plate of the invention; 
         FIG. 5  is an enlarged perspective view of the part circled in  FIG. 4 , labeled V; 
         FIG. 6  is a sectional view along the line VI-VI in  FIG. 5  of the upper part of the bipolar plate when it is assembled with the membrane electrode assembly plate; and 
         FIG. 7  is a sectional view along the line VII-VII in  FIG. 5  of the upper part of the bipolar plate when it is assembled with the membrane electrode assembly plate. 
     
    
    
     Referring to  FIG. 4 , the bipolar plate  22  of the invention is rectangular in shape. 
     The plate  22  has an inlet window for heat transfer fluid  23  that runs lengthwise at the periphery of the plate  22  along a first longitudinal edge  31 , and from which two heat transfer fluid inlet channels  25 ,  26  formed in the plate  22  extend from the inlet window  23  to the periphery of a rectangular central surface  46 , where they enter the plate  22 . 
     These channels  25 ,  26  introduce the heat transfer fluid into the plate  22  from the inlet window  23 ; the heat transfer fluid thus introduced circulates within the thickness of the plate in the area of the central surface  46  in distribution channels that are shown schematically and referenced  26   a  and  25   a.    
     The bipolar plate  22  also has a heat transfer fluid outlet window  27  that runs lengthwise at the periphery of the plate  22  along the second, opposite longitudinal edge  32 , from which window two heat transfer fluid outlet channels  29 ,  30  formed in the plate  22  extend from the rectangular central surface  46  to the window  27 , thereby allowing the heat transfer fluid to be collected after circulating through the heat transfer fluid distribution channels  25   a ,  26   a.    
     When the stack is assembled, the heat transfer fluid inlet  23  and outlet  27  windows in all of the constituent cells of the stack are superimposed, forming a heat transfer fluid circuit consisting of a supply circuit and an exhaust circuit for heat transfer fluid. 
     The plate  22  also has an oxidant inlet window  33  located at the periphery of the plate  22 , running transversely along a first half of a first transverse edge  34  of the plate  22 , and an oxidant outlet window  35  located at the periphery of the plate  22 , running transversely along one half of the second, opposite transverse edge  36 , substantially on the diagonal from the oxidant inlet window  33 . 
     An oxidant inlet channel  37  is formed in the plate  22  and runs from the oxidant inlet window  33  toward the rectangular central surface  46  so that the oxidant diffuses from this inlet channel  37  toward and up to an oxidant distribution channel  37   a  formed in the bipolar plate  22  on the rectangular central surface  46 , which channel is open on top in order to diffuse into the cathode electrode of a membrane electrode assembly plate not shown in this figure, which is intended to bear on the bipolar plate, in the central area  46  more particularly, as will be described below. 
     An oxidant outlet channel  39  is formed in the plate  22  and extends from the oxidant outlet window  35  toward the central surface  46  so that the oxidant diffuses from the distribution channel  37   a  through the outlet channel  39  toward the outlet window  35 . 
     When the stack is assembled, the stacking of the windows  33  and  35  of all the constituent cells of the stack forms a fluid circuit that transports the oxidant, composed of an oxidant supply circuit and exhaust circuit. 
     In symmetrical fashion, the bipolar plate  22  also has a fuel inlet window  40  running transversely along the second half of the first transverse edge  34 , and a fuel outlet window  41  running transversely along one half of the second transverse edge  36 , placed substantially on the diagonal from the inlet window  40 . 
     The bipolar plate  22  also has a fuel inlet channel  42  and a fuel outlet channel  43  running from the respective fuel inlet  40  and outlet  41  windows toward the central surface  46 . 
     The fuel thus circulates from the inlet window  40  toward the outlet window  41  through a fuel distribution channel  42   a  formed in the bipolar plate  22 , this distribution channel  42   a  being open on the bottom in order to diffuse into the cathode electrode of a membrane electrode assembly plate not shown in this figure, which is intended to bear on the underside of the bipolar plate. 
     When the stack is assembled, the stacking of the windows  40  and  41  of all the constituent cells of the stack forms a fluid circuit that transports the fuel, composed of a fuel supply circuit and exhaust circuit. 
     Referring to  FIGS. 4 and 5 , the bipolar plate  22  has a peripheral raised border  47  disposed around the entire periphery of the plate  22 , enclosing the heat transfer fluid inlet window  23 , the oxidant inlet window  33 , the fuel inlet window  35 , the heat transfer fluid outlet window  27 , the oxidant outlet window  35 , the fuel outlet window  41 , and the rectangular central surface  46  of the bipolar plate  22 . 
     This raised border makes it possible to seal off the interior of the assembled stack from the exterior of this stack. 
     In addition, the heat transfer fluid inlet window  23 , the oxidant inlet window  33 , the fuel inlet window  40 , the heat transfer fluid outlet window  27 , the oxidant outlet window  35 , and the fuel outlet window  41  each have a respective raised border  23   a ,  33   a ,  35   a ,  27   a ,  40   a ,  41   a  that seals off each of these windows  23 ,  33 ,  40 ,  27 ,  35 ,  41 , respectively, when the stack is assembled, as will be described below. 
     At the heat transfer fluid inlet  23  and outlet  27  windows, the outermost part of the respective raised border  23   a ,  27   a  overlaps with the peripheral raised border  47  of the plate  22 , whereas at the inlet  33 ,  40  and outlet  35 ,  41  windows for oxidant and fuel, respectively, the peripheral border  47  of the bipolar plate  22  encloses each window  33 ,  40 ,  41 ,  35  along with its corresponding raised border  33   a ,  40   a ,  41   a ,  35   a.    
     The peripheral border  47  of the bipolar plate  22 , as well as the respective borders  23   a ,  33   a ,  40   a ,  27   a ,  35   a ,  41   a  of the heat transfer fluid inlet window  23 , the oxidant inlet window  33 , the fuel inlet window  40 , the heat transfer fluid outlet window  27 , the oxidant outlet window  35 , and the fuel outlet window  41  can be formed by drawing or stamping them, and they have a flat front face  48 , shown in  FIG. 5 , parallel to the plane of the bipolar plate  5 , being connected thereto by right-angle or oblique edges  22 . 
     Referring to  FIG. 6 , when the fuel cell stack is assembled, a membrane electrode assembly plate  50  is made to bear on the bipolar plate  22 ; only the upper face  51  of the bipolar plate  22  that has the oxidant distribution channel  37   a  is shown in this figure. 
     The active area  52  of the membrane electrode assembly plate  50  comprises mainly the electrodes and the proton conducting electrolyte, and it bears on the central surface  46  of the bipolar plate  22  in such a way that the reagents circulating in the distribution channel  37   a  diffuse into the electrode in contact with it. 
     According to the invention, the membrane electrode assembly plate  50  has a frame  53  that bears on the peripheral border  47  of the bipolar plate  22  without excessive deformation, this border  47  being covered by a serigraphed seal  54 . 
     Assembling the membrane electrode assembly plate  50  and the bipolar plate  22  in this way makes it possible to form the seal between the active area within which the electrochemical reactions occur and the exterior of the cell, and more generally, it also forms the seal between the interior and the exterior of the assembled stack. 
     It is understood that each bipolar half-plate of the constituent cells in the stack preferably has this peripheral border  47  in order to form the above-mentioned seal. 
     Referring to  FIG. 7 , when the frame  53  of the membrane electrode assembly plate  50  is superimposed onto the bipolar plate  22  where the first heat transfer fluid inlet window  23  is located, this frame  53  has a window  56  that aligns with this heat transfer fluid inlet window  23 . 
     In this way, the frame  53  bears on the whole of the raised border of the window  23 , which makes it possible to form the seal between the heat transfer fluid inlet window  23  and the exterior of the cell. 
     It is understood that the frame  53  of the membrane electrode assembly plate  50  is also made to bear at each respective inlet and outlet window for oxidant  33 ,  35 , fuel  40 ,  41  and heat transfer fluid  23 ,  27 , and that at the respective inlet windows for oxidant  33  and fuel  40  and at the respective outlet windows for oxidant  35  and fuel  41 , the frame  53  bears on the peripheral border  33   a ,  40   a ,  35   a ,  41   a  of each of these windows  33 ,  40 ,  35 ,  41 , as well as onto the peripheral border  47 , which for these four windows  33 ,  40 ,  35 ,  41  encloses these peripheral borders  33   a ,  40   a ,  35   a ,  41   a.    
     Thus, when the stack is assembled, the whole periphery of the bipolar plate and thus all of the raised borders defined above  47 ,  23   a ,  24   a ,  27   a ,  28   a ,  33   a ,  35   a ,  40   a ,  41   a  come into contact with the frame  53  of the membrane electrode assembly plate  50 , and in response to the tightening load, they can be deformed elastically or even plastically so as to conform to the stacking and provide a sufficient linear load on the frame  53 . 
     In this way, the seal between the interior and exterior of the stack is formed by the presence of the peripheral raised border  47  of the bipolar plate, and the specific seal for each of the inlet and outlet windows for reagents  33 ,  40 ,  41 ,  35  or heat transfer fluid  23 ,  27  is formed by the presence of each of the corresponding raised borders  33   a ,  40   a ,  41   a ,  35   a ,  23   a ,  27   a.    
     The frame  53  of the membrane electrode assembly plate  50  is preferably designed to be mechanically compatible with the bipolar plate  22 .