Patent Application
20060127738
The present invention relates to a membrane electrode assembly for a fuel cell, and to a method and process for preparing a membrane electrode assembly.
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.
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.
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.
The present invention has been developed in view of the above desirability, and provides a fuel cell including an assembly having an ionically conductive member, an electrode, and an electrically conductive member. The assembly also includes an adhesive disposed at a peripheral edge of the assembly that adheres the electrically conductive member, the electrode, and the ionically conductive member, as well as provides mechanical support and inhibits the permeation of reactant gas through the ionically conductive member.
In order to manufacture the above fuel cell, a method has also been developed that includes the steps of applying the adhesive over an edge of the electrode and a peripheral surface of the ionically conductive member such that an electrically conductive member disposed at the electrode may be bonded to the electrode and the peripheral surface of the ionically conductive member. The method also includes, prior to applying the adhesive, pre-treating surfaces of the electrode, the ionically conductive member, and the electrically conductive member.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.
The ionically conductive member 4 is preferably a solid polymer membrane electrolyte, and preferably a PEM. Member 4 is also referred to herein as a membrane 4. Preferably, the ionically conductive member 4 has a thickness in the range of about 10 μm-100 micrometers, and most preferably 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. Preferably, 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.
In the fuel cell of the present invention, 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 H2=2H++2e− (anode) and ½ O2+2H++2e−=H2O (cathode).
The composition of the anode electrode 6 and cathode electrode 8 preferably 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 preferably comprises catalyst-coated carbon or graphite particles. The anode electrode 6 and cathode electrode 8 will preferably 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 invention for the anode 6 and cathode 8 to be of different size (i.e., the cathode larger than the anode or vice versa). A preferred thickness of the anode 6 and cathode 8 is in the range of about 2-30 μm, and most preferably about 10 μm.
The gas diffusion media 10 and 12 and gaskets 14 and 16 may be any gas diffusion media or gasket known in the art. Preferably, 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 preferred 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.
In accordance with a first embodiment of the invention shown in
More particularly, referring to
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.
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 H2 and O2 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.
Now referring to
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., H2 and O2) from diffusing across the membrane 4 at its edge 20, the chemical degradation of the membrane electrolyte 4 is prevented.
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 H2O2 as follows:
O2+2H++2e−=H2O2
This production of H2O2 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
Specifically, when contaminates or impurities are present in the fuel cell environment such as metal cations that have multiple oxidation states, the H2O2 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.
Now referring to
Now referring to
A unique aspect of the second embodiment depicted in
It should be understood that the MEA 2 according to the second embodiment of the present invention 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 H2O2 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.
A method of preparing the MEA 2 shown in
The casting solution is applied to a sheet suitable for use in a decal method, preferably 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).
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.
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. Preferably, the bonding temperature of the adhesive is 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.
In a unique aspect of the invention, 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. Preferably, 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.
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.
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.
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.
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.
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.
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.
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.
The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.
