Abstract:
A method of forming a fuel cell may include treating a surface of a membrane electrode assembly (MEA) of the fuel cell, positioning a preformed adhesive insert on the treated surface, and bonding an electrically conductive member to the treated surface with the adhesive. Treating the surface may include a pre-treatment to increase adhesive properties thereof. Positioning the adhesive insert may include locating the adhesive insert on a surface of the membrane electrolyte adjacent to an edge of the electrode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional application and claims the benefit of U.S. patent application Ser. No. 11/010,770, filed on Dec. 13, 2004. The disclosure of the above application is incorporated herein by reference. 
     
    
     FIELD  
       [0002]     The present disclosure relates to a membrane electrode assembly for a fuel cell, and to a method and process for preparing a membrane electrode assembly.  
       BACKGROUND  
       [0003]     Fuel cells are being developed as a power source for electric vehicles and other applications. One such fuel cell is the PEM (i.e. Proton Exchange Membrane) fuel cell that includes a so-called “membrane-electrode-assembly” (MEA) comprising a thin, solid polymer membrane-electrolyte having a pair of electrodes (i.e., an anode and a cathode) on opposite faces of the membrane-electrolyte. The MEA is sandwiched between planar gas distribution elements.  
         [0004]     In these PEM fuel cells, the electrodes are typically of a smaller surface area as compared to the membrane electrolyte such that edges of the membrane electrolyte protrude outward from the electrodes. On these edges of the membrane electrolyte, gaskets or seals are disposed to peripherally frame the electrodes. Due to the limitations of manufacturing tolerances, however, the seals, MEA, and gas distribution elements are not adequately closely aligned. Due to the misalignment of these elements, failures at the edges of the membrane electrolyte can develop and shorten the life span of the fuel cell and decrease the performance of the fuel cell.  
         [0005]     Moreover, tensile stresses on the membrane electrolyte that are caused by membrane shrinkage when the membrane electrolyte is cycled from wet to dry conditions, and chemical degradation of the membrane electrolyte due to chemical attack of the electrolyte in the membrane and the electrodes by free radicals produced by reaction of cross-over gases (hydrogen from the anode to the cathode, and oxygen from the cathode to the anode) also affect the life span and performance of a fuel cell. As such, it is desirable to develop a PEM fuel cell that eliminates the above drawbacks.  
       SUMMARY  
       [0006]     Accordingly, a method of forming a fuel cell may include treating a surface of a membrane electrode assembly (MEA) of the fuel cell, positioning a preformed adhesive insert on the treated surface, and bonding an electrically conductive member to the treated surface with the adhesive. Treating the surface may include a pre-treatment to increase adhesive properties thereof. Positioning the adhesive insert may include locating the adhesive insert on a surface of the membrane electrolyte adjacent to an edge of the electrode.  
         [0007]     Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0009]      FIGS. 1A and 1B  are exploded, cross-sectional views of a membrane electrode assembly (MEA) according to a principle and first embodiment of the present disclosure;  
         [0010]      FIG. 2  is a cross-sectional view of a prior art membrane electrode assembly;  
         [0011]      FIG. 3  is a cross-sectional view of the MEA shown in  FIGS. 1A and 1B  in an assembled form;  
         [0012]      FIG. 4  is a cross-sectional view of the MEA shown in  FIG. 3  depicting the prevention of a condensed flux of gases from crossing a membrane electrolyte; and  
         [0013]      FIG. 5  is a cross-sectional view of MEA according to a principle and second embodiment of the present disclosure.  
     
    
     DETAILED DESCRIPTION  
       [0014]     The following description is merely exemplary in nature and is in no way intended to limit the disclosure, its application, or uses.  
         [0015]      FIGS. 1A and 1B  are exploded, cross-sectional views of a membrane electrode assembly (MEA) according to a principle of the present disclosure. As shown in  FIGS. 1A and 1B , the MEA  2  includes an ionically conductive member  4  disposed between an anode electrode  6  and a cathode electrode  8 . The MEA  2  is further disposed between a pair of electrically conductive members  10  and  12 , or gas diffusion media  10  and  12 . The gas diffusion media  10  and  12  are peripherally surrounded by frame-shaped gaskets  14  and  16 . The gaskets  14  and  16  and diffusion media  10  and  12  may or may not be laminated to the ionically conductive member  4  and/or the electrodes  6  and  8 .  
         [0016]     The ionically conductive member  4  is a solid polymer membrane electrolyte, and more specifically a PEM. Member  4  is also referred to herein as a membrane  4 . The ionically conductive member  4  has a thickness in the range of about 10 μm-100 micrometers, and more specifically a thickness of about 25 micrometers. Polymers suitable for such membrane electrolytes are well known in the art and are described in U.S. Pat. Nos. 5,272,017 and 3,134,697 and elsewhere in the patent and non-patent literature. It should be noted, however, that the composition of the ionically conductive member  4  may comprise any of the proton conductive polymers conventionally used in the art. For example, perfluorinated sulfonic acid polymers such as NAFION® are used. Furthermore, the polymer may be the sole constituent of the membrane, contain mechanically supporting fibrils of another material, or be interspersed with particles (e.g., with silica, zeolites, or other similar particles). Alternatively, the polymer or ionomer may be carried in the pores of another material.  
         [0017]     In the fuel cell of the present disclosure, the ionically conductive member  4  is a cation permeable, proton conductive membrane, having H +  ions as the mobile ion; the fuel gas is hydrogen (or reformate) and the oxidant is oxygen or air. The overall cell reaction is the oxidation of hydrogen to water and the respective reactions at the anode and cathode are H 2 =2H + +2e −  (anode) and  ½ O   2 +2H + +2e − =H 2 O (cathode).  
         [0018]     The composition of the anode electrode  6  and cathode electrode  8  comprises electrochemically active material dispersed in a polymer binder which, like the ionically conductive member  4 , is a proton conductive material such as NAFION®. The electrochemically active material comprises catalyst-coated carbon or graphite particles. The anode electrode  6  and cathode electrode  8  may include platinum-ruthenium, platinum, or other Pt/transition-metal-alloys as the catalyst. Although the anode  6  and cathode  8  in the figures are shown to be equal in size, it should be noted that it is not out of the scope of the disclosure for the anode  6  and cathode  8  to be of different size (i.e., the cathode larger than the anode or vice versa). A thickness of the anode  6  and cathode  8  is in the range of about 2-30 μm, and more specifically about 10 μm.  
         [0019]     The gas diffusion media  10  and  12  and gaskets  14  and  16  may be any gas diffusion media or gasket known in the art. For example, the gas diffusion media  10  and  12  are carbon papers, carbon cloths, or carbon foams with a thickness of in the range of about 50-300 μm. Further, the gas diffusion media  10  and  12  may be impregnated with various levels of Teflon® or other fluorocarbons to achieve more or less hydrophobicity. The gaskets  14  and  16  are typically elastomeric in nature but may also comprise materials such as polyester and PTFE. However, the gaskets  14  and  16  may be any material sufficient for sealing the membrane electrode assembly  2 . A thickness of the gaskets  14  and  16  is approximately ½ the thickness of the gas diffusion media  10  and  12  to about 1½ times the thickness of the gas diffusion media  10  and  12 .  
         [0020]     In accordance with a first embodiment of the disclosure shown in  FIGS. 1A and 1B , an adhesive  18  that is used to bond the diffusion media  10  and  12  to the MEA  2  is disposed at an edge  20  or peripheral surface  20  of the membrane electrolyte  4  to overlap the electrodes  6  and  8  and membrane electrolyte  4 . The adhesive  18  is a hot-melt adhesive such as ethyl vinyl acetate (EVA), polyamide, polyolefin, or polyester. By disposing an adhesive  18  between the diffusion media  10  and  12  and membrane  4  ( FIG. 1A ), or between the electrodes  6  and  8  and membrane  4  ( FIG. 1B ), the durability of the membrane edge  20  is improved. It should be understood that the application of a hot melt adhesive  18  is merely exemplary and the present disclosure should not be limited thereto. More particularly, other adhesives  18  such as silicone, polyurethane, and fluoroelastomers may be used as the adhesive  18 . Further, elastomer systems such as thermoplastic elastomers, epoxides, phenoxys, acrylics, and pressure sensitive adhesive systems may also be used as the adhesive  18 . The application of the adhesive  18  at the peripheral surface  20  of the membrane electrolyte  4  reduces and homogenizes the tensile stresses located at the edge  20  of the membrane electrolyte  4  that is not supported by the electrodes  6  and  8 , and prevents a chemical degradation of the membrane electrolyte  4 .  
         [0021]     More particularly, referring to  FIG. 2 , a prior art MEA  22  is depicted. The prior art MEA  22  includes electrodes  24  and  26  with a much smaller surface area in comparison to the membrane electrolyte  28  such that edges  30  of the membrane electrolyte  28  protrude outward from the electrodes  24  and  26 . On these edges  30  of the membrane electrolyte  28 , rest sub-gaskets  32  and  34 , that are disposed to surround the electrodes  24  and  26 . Gas diffusion media  36  and  38  sit upon the sub-gaskets  32  and  34 . Gaskets  40  and  42  surround the gas diffusion media  36  and  38 .  
         [0022]     Due to difficulty in manufacturing to tight tolerances, there is a gap  44  between the electrode  24  and  26  and sub-gaskets  32  and  34 . Such a gap  44  acts as a living hinge, permitting the membrane  28  to flex. Such a hinge action leads to stress and tears, rips, or holes in the edges  30  of the membrane electrolyte  28 . This also leads to stress as the compressive force acting on membrane electrolyte  28  differs due to such difference in height. For example, if the sub-gaskets  32  or  34  are higher than the electrode  24  or  26 , the compressive forces on the sub-gaskets  32  and  34  will be too high, if the sub-gasket  32  or  34  is shorter than the electrode  24  or  26 , the compressive forces on the electrode  24  or  26  will be too high. Thus, the arrangement typical in the prior art causes the small gap  44  formed between the sub-gaskets  32  and  34  and the electrodes  24  and  26 . This small gap  44  leaves a small portion of the membrane electrolyte  28  unsupported.  
         [0023]     Furthermore, if the sub-gaskets  32  and  34  are thicker than the electrodes  24  and  26 , they form a “step” upon which gas diffusion media  36  and  38  rest. Gas diffusion media  36  and  38  assist in dispersing reactant gases H 2  and O 2  over the electrodes  24  and  26  and conduct current from the electrodes  24  and  26  to lands of the electrically conductive bipolar plates (not shown). As such, in order to facilitate electrical conductivity between the gas diffusion media  36  and  38  and electrodes  24  and  26 , the membrane electrode assembly  22  needs to be compressed at a high pressure. This puts a great deal of stress on the unsupported portion of the membrane electrolyte  28  which may cause it to develop small pinholes or tears. The pinholes are also caused by the carbon or graphite fibers of the diffusion media  36  and  38  puncturing the membrane electrolyte  28 . These fiber punctures cause the fuel cell to short and produce a lower cell potential.  
         [0024]     Now referring to  FIG. 3 , a cross-sectional view of the membrane electrode assembly  2  according to a principle of the present disclosure, in its assembled form, is depicted. In  FIG. 3 , it can be seen that each of the elements of the membrane electrode assembly  2  have been bonded together by the adhesive  18 . Since the gas diffusion media  10  and  12  are a porous material, the adhesive  18  enters the pores of the gas diffusion media  10  and  12  when the elements of the fuel cell are compressed together. Upon solidification of the adhesive  18 , the adhesive  18  acts as a seal around the peripheral surface  20  of the membrane electrolyte  4  that bonds the peripheral surface  20  of the membrane electrolyte  4 , the electrodes  6  and  8 , and the gas diffusion media  10  and  12  together. Since the membrane electrolyte  4 , electrodes  6  and  8 , and gas diffusion media  10  and  12  are bonded together, a unitary structure is formed. As such, no gaps are present between each of the elements of the fuel cell, and the membrane electrolyte  4  can be subjected to uniform pressures throughout its surface. The uniform pressures prevent the exertion of any tensile stresses on the membrane electrolyte  4 , which prevents the occurrence of pinholes and degradation of the membrane electrolyte  4 . A long-lasting and robust fuel cell with high performance is thus achieved.  
         [0025]     Moreover, the adhesive  18  prevents the diffusion of hydrogen and oxygen across the membrane electrolyte  4  at the membrane electrolyte edge  20  because the adhesive  18  has a sealing property. Since the adhesive  18  has a sealing property that prevents the constituent reactants (i.e., H 2  and O 2 ) from diffusing across the membrane  4  at its edge  20 , the chemical degradation of the membrane electrolyte  4  is prevented.  
         [0026]     That is, during the normal operation of a fuel cell, hydrogen and oxygen gas may permeate across the membrane electrolyte  4  to both the cathode  8  and anode  6 , respectively, such that oxygen is in the presence of the hydrogen. When these reactant gases comes into contact with the electrochemically active material of the electrodes  6  and  8 , the oxygen is reduced and reacts with H +  ions produced from the oxidation of the hydrogen fuel gas. This ensuing side reaction between the reduced oxygen and H +  ions produces H 2 O 2  as follows: 
 
O 2 +2H + +2e − =H 2 O 2  
 
         [0027]     This production of H 2 O 2  has been known to cause a degradation of the membrane electrolyte  4  and, thus, a diminished fuel cell life and performance. Furthermore, it is generally understood that other possible mechanisms of chemical degradation of the electrolyte in the membrane and the electrodes can be mitigated in the absence of gas cross-over through the membrane  4 . Again referring to the prior art membrane electrode assembly shown in  FIG. 2 , these gases are more prone to permeate the membrane  28  at the edges of the membrane  28  at the so-called gaps  44  between the elements of the fuel cell caused by manufacturing tolerances of the elements. As such, a condensed flux  46  of the reactant gases may collect at a region located where edges of the electrodes  24  and  26  meet the unsupported and unsealed membrane electrolyte  28  which can form H 2 O 2  and chemically degrade the membrane electrolyte  28 . That is, when the condensed flux  46  that collects in this gap  44  contacts the electrochemically active material of the electrodes  24  and  26 , the production of H 2 O 2  occurs.  
         [0028]     Specifically, when contaminates or impurities are present in the fuel cell environment such as metal cations that have multiple oxidation states, the H 2 O 2  in the presence of these metal cations may break down into a peroxide radical that may attack the ionomer of the membrane  28  and electrodes  24  and  26 . Since a condensed flux  46  tends to form at the edges of the membrane  28 , the edges of the membrane  28  are particularly susceptible to degradation.  
         [0029]     Now referring to  FIG. 4 , where the peripheral surface of the membrane electrolyte  20  is supported and sealed by the adhesive  18 , the condensed flux of gases  46  that may collect at the peripheral surface  20  of the membrane is prevented from diffusing across the membrane electrolyte  4  by the adhesive  18 . As such, the condensed flux of gases  46  are prevented from contacting the electrochemically active area of the electrodes  6  and  8 , which prevents the production of H 2 O 2 . The degradation of the membrane electrolyte  4  at the edge  20  of the membrane electrolyte  4 , therefore, is prevented.  
         [0030]     Now referring to  FIG. 5 , a second embodiment of the present disclosure will be described. As shown in  FIG. 5 , the adhesive  18  is applied to the edge of MEA  2  such that no gaskets are needed. That is, the adhesive  18  may be applied by way of injection molding or applied as a plug or insert that is heated and compression molded to seal the entire outer portion of the MEA  2 . When the adhesive  18  is applied as a plug that is compression molded, the adhesive  18  takes the form as shown by the lines in phantom. In this manner, the elements of the MEA  2  are bonded together to form a unitary structure that provides uniform mechanical support throughout the entire structure of the MEA  2  when the MEA  2  is compressed in fuel cell.  
         [0031]     A unique aspect of the second embodiment depicted in  FIG. 5  are the projecting portions  19  formed on the edges of the adhesive  18 . These bulbous portions  19  may serve as gaskets for the MEA  2  such that when the MEA  2  is compressed along with a plurality of the MEA&#39;s  2  in a fuel cell stack, further mechanical support is provided at the edges of the MEA  2  in the stack. This is because the adhesive  18 , even after it solidifies after molding onto the MEA  2 , will remain a bendable and pliable material.  
         [0032]     It should be understood that the MEA  2  according to the second embodiment of the present disclosure also provides, in addition to the above-described mechanical support characteristics, the same sealing properties that prevent cross-over of the reactant gases across the membrane as described with reference to the first embodiment. That is, the adhesive  18  reduces or prevents the cross-over of hydrogen and oxygen across the membrane  4  such that the production of H 2 O 2  can be prevented. Moreover, the adhesive  18  that is applied by injection molding or as a plug that is compression molded also may imbibe into the gas diffusion media  10  and  12 .  
         [0033]     A method of preparing the MEA  2  shown in  FIGS. 1A and 1B  according to the present disclosure will now be described. In order to prepare the anode  6  and cathode  8  of the MEA  2 , catalyzed carbon particles are prepared and then combined with the ionomer binder in solution with a casting solvent. For example, the anode  6  and cathode  8  comprise ⅓ carbon or graphite, ⅓ ionomer, and ⅓ catalyst. Casting solvents may be aqueous or alcoholic in nature, but solvents such as dimethylacetic acid (DMAc) or trifluoroacetic acid (TFA) also may be used.  
         [0034]     The casting solution is applied to a sheet suitable for use in a decal method, more specifically the sheet is a Teflonated sheet. The sheet is subsequently hot-pressed to the ionically conductive member  4  (membrane electrolyte), such as a PEM, to form a catalyst coated membrane (CCM). The sheet is then peeled from the ionically conductive member  4  and the catalyst coated carbon or graphite remains embedded as a continuous electrode  6  or  8  to form the MEA  2 . Alternatively, the casting solution may be applied directly to the gas diffusion medium  10  or  12  to form a catalyst coated diffusion medium (CCDM).  
         [0035]     It should also be understood that it may be desirable to have a microporous layer  11  and  13  formed on the gas diffusion media  10  or  12 . The microporous layer  11  and  13 , which is a water management layer that wicks water away from the membrane  4 , may be formed in the same manner as the electrodes  6  and  8 , described above, but the casting solution is comprised of carbon particles and a Teflon® solution.  
         [0036]     To apply the adhesive  18 , a variety of methods may be employed. That is, the adhesive  18  may be applied as a film, as a slug, or sprayed onto the edge  20  of the membrane electrolyte  4 , the electrodes  6  and  8 , and gas diffusion media  10  and  12 . Further, as described above with reference to the second embodiment, the adhesive may be injection molded onto the edge of the MEA  2 . After the adhesive  18  has been applied, the elements of the MEA  2  are bonded to form a unitary structure by heating the adhesive to a melting point dependent on the type of material being used as the adhesive and applying pressure in the range of 10-20 psi. The bonding temperature of the adhesive may be in the range of 270 F-380 F. Utilizing temperatures in this range prevents subjecting the delicate materials of the MEA  2  such as the membrane electrolyte  4  and electrodes  6  and  8  to temperatures that may cause a degradation of these materials.  
         [0037]     In a unique aspect of the disclosure, before applying the adhesive  18 , the membrane electrolyte  4 , electrodes  6  and  8 , and gas diffusion media  10  and  12  are subjected to a pre-treatment. That is, the membrane electrolyte  4 , electrodes  6  and  8 , and gas diffusion media  10  and  12  are pre-treated with a surface treatment that activates the surfaces of these materials. For example, a radio-frequency glow discharge treatment is used. Additional pre-treatments that also activate the surfaces of these materials are a sodium napthalate etching treatment, a corona discharge treatment, a flame treatment, a plasma treatment, a UV treatment, a wet chemical treatment, a surface diffusion treatment, a sputter etching treatment, an ion beam etching treatment, an RF sputter etching treatment, and the use of a primer.  
         [0038]     With respect to plasma treatments, a variety of plasma-based techniques can be used such as plasma-based flame treatment, a plasma-based UV or UV/ozone treatment, an atmospheric pressure discharge plasma treatment, and a low pressure plasma treatment. These plasma treatments clean, chemically activate, and coat the elements of the MEA  2 . Other plasma treatments that may be used are a dielectric barrier discharge plasma treatment, a sputter deposition plasma treatment (DC and RF magnetically enhanced plasma), an etching plasma treatment (RF and microwave plasmas, and RF and microwave magnetically enhanced plasmas), a sputter etching plasma treatment, an RF sputter etching plasma treatment, an ion beam etching plasma treatment, a glow discharge plasma treatment, and a capacitive coupled plasma treatment.  
         [0039]     The use of a pre-treatment increases the adhesive force between the elements of the MEA  2  by exciting or activating the polymeric groups of the membrane electrolyte  4 , the electrodes  6  and  8 , and the gas diffusion media  10  and  12 . This is advantageous because polymers and plastics are low surface energy materials and most high strength adhesives do not spontaneously wet their surfaces. This is also advantageous because a surface pre-treatment provides a reproducible surface so that the adhesive effects of the adhesive  18  can be consistent from product to product. As such, by activating the surfaces of the membrane electrolyte  4 , electrodes  6  and  8 , and gas diffusion media  10  and  12 , the adhesive force of the adhesive  18  is increased which results in an increased sealing effect of the MEA  2 . Further, the increased adhesive force between the elements of the MEA  2  provides a more robust MEA  2  that increases resistance to mechanical and chemical stresses.  
         [0040]     That is, by using a pre-treatment, the surface energy of the elements will rise such that radicals will form at the ends of the polymeric groups that form the membrane electrolyte  4 , the electrodes  6  and  8 , and the diffusion media  10  and  12 . These radicals attract the molecules of the adhesive  18  when the adhesive  18  is applied to thereby “bond” the elements of the MEA  2  with the adhesive  18 . Further, it should be understood that the above surface treatments increases the surface energy of the elements of the MEA  2  by inducing chemical changes and physical changes in the polymeric elements of the MEA  2 .  
         [0041]     More specifically, the elements of the MEA  2  may be chemically altered by the above pre-treatments by the incorporation of a new chemical species, the loss of a chemical species, radical formation, and interaction of the treated surfaces of the elements of the MEA  2  with the atmosphere in which the pre-treatment is conducted. Physical changes that can occur in the elements of the MEA  2  include chain scission, the creation of low molecular weight fragments, surface cross-linking, the reorientation of surface groups, and the etching and removal of surface species. It should be noted, however, that the physical changes usually change the surface chemistry of the elements of the MEA  2  in addition to providing the physical changes.  
         [0042]     Moreover, if the pretreatment of the elements of the MEA  2  is performed in an atmosphere consisting of air with a reactive gas containing a suitable chemical species such as argon, nitrogen, silane, or any other gas that can produce radicals that is bled in, the adhesion characteristics between the elements can be further augmented. That is, when the radicals form at the ends of the polymeric groups that form the membrane  4 , the electrodes  6  and  8 , and the diffusion media  10  and  12 , the chemical species bled into the atmosphere also form radicals that can bond to the radicals formed at the ends of the polymeric groups. When the elements of the MEA  2  are then compressed together to facilitate contact between the elements of the MEA  2 , the chemical species may then bond together to tightly connect the elements of the MEA  2 . For example, if a nitrogen containing reactive gas is bled into the atmosphere during the pretreatment, nitrogen radicals will form at the ends of the polymeric groups of the elements of the MEA  2 . When the elements are compressed together, the nitrogen radicals of one element will bond with the nitrogen radicals of another element to form nitrogen bonds, which are very strong.  
         [0043]     In the case of a corona treatment, it is desirable that the treatment be conducted in an atmosphere containing air with a nitrogen or argon gas bled in. With respect to a radio frequency glow discharge treatment, it is desirable that the treatment be conducted in a vacuum with a reactive gas such as argon or nitrogen bled in. Alternatively, a carbonaceous or salacious gas may be bled in, or other gases such as oxygen or He—O blends may be used.  
         [0044]     It should also be understood that, after performing a pretreatment and before compressing the elements of the MEA  2  together, a primer or coupling agent may be applied to the elements of the MEA  2 . In this regard, the primer or coupling agent may be any primer or coupling agent known in the art, but should be selected specifically to the application used as the pretreatment.  
         [0045]     The description of the disclosure is merely exemplary in nature and, thus, variations that do not depart from the gist of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure.