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  Method for preparing membrane electrode assemblies from fluoropolymer dispersionsUSPTO Application #: 20070278094Title: Method for preparing membrane electrode assemblies from fluoropolymer dispersions Abstract: Described are methods for producing membrane electrode assemblies containing crosslinked membranes and electrodes, which membranes and electrodes are prepared from fluoropolymer organic-liquid dispersions containing a homogeneous mixture of reacted and unreacted sulfonyl halide groups. (end of abstract) Agent: E I Du Pont De Nemours And Company Legal Patent Records Center - Wilmington, DE, US Inventor: Robert D. Lousenberg USPTO Applicaton #: 20070278094 - Class: 204280000 (USPTO) Related Patent Categories: Chemistry: Electrical And Wave Energy, Apparatus, Electrolytic, Elements, Electrodes The Patent Description & Claims data below is from USPTO Patent Application 20070278094. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This application claims the benefit of U.S. Provisional Application No. 60/810,096 filed Jun. 1, 2006. FIELD OF INVENTION [0002] The invention is directed to membrane electrode assemblies that include membrane and electrodes each made from fluoropolymer organic-liquid dispersions containing a homogeneous mixture of reacted and unreacted sulfonyl halide groups to facilitate cross linking between the membrane and electrodes. BACKGROUND [0003] Electrochemical cells generally include an anode electrode and a cathode electrode separated by an electrolyte. Fuel cells are electrochemical cells that convert the chemical energy of a fuel, such as a hydrogen gas, and an oxidant, such as air, into electrical energy. Fuel cells are typically formed as stacks or assemblages of membrane electrode assemblies (MEAs), which each include an electrolyte, an anode (a negatively charged electrode) and cathode (a positively charged electrode), and other optional components. A polymeric proton exchange membrane (PEM) is frequently used as the electrolyte. A metal catalyst and electrolyte mixture is generally used to form the anode and cathode electrodes. Fuel cells typically also comprise a porous electrically conductive sheet material that is in electrical contact with each of the electrodes and permits diffusion of the reactants to the electrodes, and is know as a gas diffusion layer, gas diffusion substrate or gas diffusion backing. When the electrocatalyst is coated on or adhered to the PEM, the MEA is said to include a catalyst coated membrane (CCM). In other instances, where the electrocatalyst is coated on the gas diffusion layer, the MEA is said to include gas diffusion electrode(s) (GDE). The functional components of fuel cells are normally aligned in layers as follows: conductive plate/gas diffusion backing/anode electrode/membrane/cathode electrode/gas diffusion backing/conductive plate. In a fuel cell, a reactant or reducing fluid such as hydrogen or methanol is supplied to the anode, and an oxidant such as oxygen or air is supplied to the cathode. The reducing fluid electrochemically reacts at a surface of the anode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode, while hydrogen ions transfer through the electrolyte to the cathode, where they react with the oxidant and electrons to produce water and release thermal energy. [0004] Long term MEA stability is critically important for fuels cells. MEA stability depends on the stability of the PEM, the anode and cathode electrodes, and the engagement between the PEM and the electrodes. One mechanism that has been used to improve membrane stability has been to provide cross-links within the body of the membrane. However, when the PEM is highly crosslinked, it becomes difficult to make CCMs in which the electrodes fully engage the membrane and remain physically and chemically engaged over the life of the MEA. This is especially the case when the electrodes are prepared as decals that are transferred to the surface of the membrane to produce a CCM. [0005] Solvent or dispersion casting is a common and advantageous fuel cell membrane fabrication process. Well-known fluoropolymer electrolyte dispersions that are in widespread commercial use are Nafion.RTM. perfluoroionomers available from E. I. du Pont de Nemours and Company, Wilmington Del. The solutions and dispersions used to form the membranes are also frequently used to make catalyst ink formulations that are used to form the electrodes of the fuel cell MEA. Fluoropolymer electrolyte dispersions suitable for casting membranes are disclosed in U.S. Pat. Nos. 4,433,082 and 4,731,263, which teach aqueous organic and organic-liquid fluoropolymer electrolyte dispersion compositions in sulfonic acid (SO.sub.3H) and sulfonate (SO.sub.3.sup.-) form with no significant sulfonyl fluoride (SO.sub.2F) concentrations. MEA electrodes made from such fluoropolymer electrolyte dispersions and a variety of catalysts are disclosed in WO2005/001978. [0006] U.S. Pat. No. 3,282,875 discloses that the SO.sub.2F group of a precursor fluoropolymer electrolyte might be used to crosslink or "vulcanize" the fluoropolymer by reaction with di- or multifunctional crosslinking agents but did not disclose a method to do this homogeneously or a way to securely attach electrodes to membranes comprised of a crosslinked fluoropolymer. US2005/0124769 discloses functionalized aromatic main chain polymers useful in membranes and electrodes that can be crosslinked. U.S. Pat. No. 6,733,914 discloses a method for heterogeneously converting a significant fraction of the SO.sub.2F groups of Nafion.RTM.-like polymer membranes to SO.sub.3.sup.- and sulfonamide (SO.sub.2NH.sub.2) groups by reaction with aqueous ammonia. The membranes were subsequently crosslinked by a heat-annealing step at high temperature in which some of the SO.sub.2NH.sub.2 groups presumably reacted with residual the SO.sub.2F groups to form sulfonimide (--SO.sub.2NHSO.sub.2--) crosslinks. The heterogeneous nature of the front reaction with aqueous ammonia did not provide a homogeneous crosslink density throughout the film and there is no disclosure of a way to securely attach electrodes to membranes comprised of a crosslinked fluoropolymer. There remains a need for methods to produce durable MEAs that include a crosslinked fluoropolymer membrane and fluoropolymer-based electrodes securely engaged to the membrane. SUMMARY [0007] The invention is directed to a method to prepare a membrane electrode assembly comprising the steps of: [0008] a) providing a first solution comprising a polymer solvent and a first polymer containing pendant SO.sub.2X groups, wherein the first polymer comprises a fluorinated backbone containing pendant groups described by the formula --(O--CF.sub.2CFR.sub.f).sub.a--(O--CF.sub.2).sub.b--(CFR'.sub.f).sub.cSO- .sub.2X, where X is a halogen, R.sub.f and R'.sub.f are independently selected from F, Cl or a perfluorinated alkyl group having 1 to 10 carbon atoms, a=0 to 2, b=0 to 1, and c=0 to 6; [0009] b) combining the first solution of step a) with a nucleophilic compound Y and a first polar liquid, to form a first reaction mixture; [0010] c) removing by distillation substantially all of the polymer solvent from the first reaction mixture of step b) to form a first dispersion wherein about 5% to about 95% of the pendant SO.sub.2X groups of the first polymer have reacted with the nucleophilic compound Y and about 95% to about 5% of the pendant SO.sub.2X groups remain unreacted; [0011] d) forming a layer of the first dispersion of step c), and removing the first polar liquid from the layer of the first dispersion to form a polymer membrane; [0012] e) providing a second solution comprising a polymer solvent and a second polymer containing pendant SO.sub.2X groups, wherein the second polymer comprises a fluorinated backbone containing pendant groups described by the formula --(O--CF.sub.2CFR.sub.f).sub.a--(O--CF.sub.2).sub.b--(CFR'.sub.f).sub.cSO- .sub.2X, where X is a halogen, R.sub.f and R'.sub.f are independently selected from F, Cl or a perfluorinated alkyl group having 1 to 10 carbon atoms, a=0 to 2, b=0 to 1, and c=0 to 6; [0013] f) combining the second solution of step e) with a nucleophilic compound Y and a second polar liquid, to form a second reaction mixture; [0014] g) removing by distillation substantially all of the polymer solvent from the second reaction mixture of step f) to form a second dispersion wherein about 5% to about 95% of the pendant SO.sub.2X groups of the second polymer have reacted with the nucleophilic compound Y and about 95% to about 5% of the pendant SO.sub.2X groups remain unreacted; [0015] h) combining the second dispersion of step g) and an electrocatalyst to form an electrode ink; [0016] i) forming a layer of the electrode ink and removing the second polar liquid to form an electrode, and positioning said electrode against the membrane of step d); and [0017] j) forming crosslinks within said membrane and between said membrane and said electrode. [0018] In one embodiment of the invention, in the first dispersion of step c), about 25% to about 75% of the pendant SO.sub.2X groups in the first polymer of the first dispersion have reacted with the nucleophilic compound Y and about 75% to about 25% of the pendant SO.sub.2X groups in the first polymer of the first dispersion remain unreacted. It another embodiment of the invention, in the second dispersion of step g) about 25% to about 75% of the pendant SO.sub.2X groups in the second polymer of the second dispersion have reacted with the nucleophilic compound Y and about 75% to about 25% of the pendant SO.sub.2X groups in the second polymer of the second dispersion remain unreacted. [0019] The method of the invention may include the step of mixing the first reaction mixture of step b) or the first dispersion of step c) with a crosslinkable compound. In one embodiment of the invention, the crosslinkable compound is of the formula HNR.sup.1, R.sup.2, and about 1% to about 100% of the remaining pendant SO.sub.2X groups in step c) are converted to pendant SO.sub.2NR.sup.1R.sup.2 groups, wherein R.sup.1 and R.sup.2 are independently hydrogen or optionally substituted alkyl groups. The invention may also include the additional step of contacting the membrane of step d) and the electrode of step i) with a crosslinking promoter so that crosslinks are formed between pendant groups in the membrane and pendant groups is the electrode. In one embodiment of the invention, the crosslinks comprise one or more sulfonimide moieties such as SO.sub.2NR.sup.7SO.sub.2R.sup.8SO.sub.2NR.sup.9SO.sub.2, wherein R.sup.7 and R.sup.9 are independently hydrogen or optionally substituted alkyl groups, and R.sup.8 is a substituted or unsubstituted alkyl, a substituted or unsubstituted aryl, a substituted sulfonimide polymer, an ionene polymer, or a substituted or unsubstituted heteroatomic function. [0020] The invention is also directed to membrane electrode assemblies and electrochemical cells comprising the membrane electrode assemblies produced as described above. In one embodiment the electrochemical cell is a fuel cell. DETAILED DESCRIPTION [0021] Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values recited when defining a range. Moreover, all ranges set forth herein are intended to include not only the particular ranges specifically described, but also any combination of values therein, including the minimum and maximum values recited. [0022] Membrane electrode assemblies made by the processes described herein, particularly when converted to ionomeric acid form, can be used in fuel cells. Examples include hydrogen fuel cells, reformed-hydrogen fuel cells, direct methanol fuel cells or other organic/air (e.g. those utilizing organic fuels of ethanol, propanol, dimethyl- or diethyl ethers, formic acid, carboxylic acid systems such as acetic acid, and the like). The membrane electrode assemblies can also be advantageously employed in other electrochemical cells. Other uses for the membrane electrode assemblies described herein include use in batteries and use in cells for the electrolysis of water to form hydrogen and oxygen. [0023] The PEM is typically comprised of an ion exchange polymer, also known as an ionomer. Following the practice of the art, the term "ionomer" is used to refer to a polymeric material having a pendant group with a terminal ionic group. The terminal ionic group may be an acid or a salt thereof as might be encountered in an intermediate stage of fabrication or production of a fuel cell. Proper operation of an electrochemical cell may require that the ionomer be in acid form. Highly fluorinated ionomers are frequently used in PEMs. "Highly fluorinated" means that at least 90% of the total number of univalent atoms in the polymer are fluorine atoms. Most typically, the polymer of the PEM is perfluorinated, which means that 100% of the of univalent atoms in the polymer are fluorine atoms. It is typical for polymers used in fuel cell membranes to have sulfonate ion exchange groups. The term "sulfonate ion exchange groups" as used herein means either sulfonic acid groups or salts of sulfonic acid groups, typically alkali metal or ammonium salts. [0024] MEAs typically include an anode electrode facing one side of the PEM and a cathode electrode facing the opposite side of the PEM. The anode and cathode electrodes are typically applied on or adhered to the PEM so as to form a CCM, which is sometimes referred to as an MEA3. In other MEA arrangements, one or both of the electrodes may be coated on or adhered to the PEM-facing side of gas diffusion layers positioned on opposite sides of the PEM. For the electrodes to function effectively in the fuel cells, effective anode and cathode electrocatalyst sites must be provided in the anode and cathode electrodes. In order for the anode and cathode to be effective: (1) the electrocatalyst sites must be accessible to the reactant, (2) the electrocatalyst sites must be electrically connected to the gas diffusion layer, and (3) the electrocatalyst sites must be ionically connected to the fuel cell electrolyte. The electrocatalyst sites are ionically connected to the electrolyte via an ionomer binder in the electrode. The binder employed in the electrode serves as a binder for the electrocatalyst particles and it help to physically and ionically connect the electrode to the membrane. It is therefore important that the ion-exchange polymers in the binder composition be compatible with the ion-exchange polymer in the membrane. Ionomers suitable for the binder of the anode and cathode electrodes of the MEAs of the invention are the fluorinated ion-exchange polymers, and most typically highly fluorinated polymers. These ionomers typically have end groups in sulfonyl halide form, but may alternatively have end groups in the sulfonic acid form. [0025] The present invention is directed to the preparation of MEAs in which one or more of the electrodes is attached to the PEM and where chemical crosslinks are formed between the polymer of the PEM and the ionomer binder in at least one of the electrodes. In addition, in the MEAs prepared according to the invention, there are also crosslinks formed within the PEM. In the preferred method for producing MEAs, the ionomer of the PEM and the ionomer binder of an adjoining electrode are produced from polymer dispersions containing significant and homogeneously dispersed sulfonyl halide (SO.sub.2X) groups in a non-fluorinated liquid. The method for producing the polymer dispersions comprises the steps of: [0026] a) providing a solution comprising a polymer solvent and a polymer containing pendant SO.sub.2X groups, wherein the polymer comprises a fluorinated backbone containing pendant groups described by the formula --(O--CF.sub.2CFR.sub.f).sub.a--(O--CF.sub.2).sub.b--(CFR'.sub.f).sub.cSO- .sub.2X, where X is a halogen, R.sub.f and R'.sub.f are independently selected from F, Cl or a perfluorinated alkyl group having 1 to 10 carbon atoms, a=0 to 2, b=0 to 1, and c=0 to 6; [0027] b) combining the solution of step a) with a nucleophilic compound Y and a polar liquid, in any order, to form a reaction mixture; and [0028] c) removing by distillation substantially all of the polymer solvent from the reaction mixture of step b) to form a dispersion wherein about 5% to about 95% of the pendant SO.sub.2X groups have reacted with the nucleophilic compound Y and about 95% to about 5% of the pendant SO.sub.2X groups remain unreacted. [0029] The polymer may be a homopolymer or a copolymer of any configuration, such as a block or random copolymer. By "fluorinated backbone" it is meant that at least 80% of the total number of halogen and hydrogen atoms on the backbone of the polymer are fluorine atoms. The polymer backbone may also be perfluorinated, which means that 100% of the total number of halogen and hydrogen atoms on the backbone are fluorine atoms. One type of suitable polymer is a copolymer of a first fluorinated vinyl monomer and a second fluorinated vinyl monomer having one or more SO.sub.2X groups. Possible first monomers include tetrafluoroethylene (TFE), hexafluoropropylene, vinylidine fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoroalkylvinyl ether, and mixtures thereof. Possible second monomers include a variety of fluorinated vinyl ethers with a SO.sub.2X group. X can be any halogen or a combination of more than one halogen, and is typically F. [0030] Suitable homopolymers and copolymers that are known in the art include those described in WO 2000/0024709 and U.S. Pat. No. 6,025,092. A suitable fluoropolymer that is commercially available is Nafion.RTM. fluoropolymer from E. I. du Pont de Nemours and Company, Wilmington Del. One type of Nafion.RTM. fluoropolymer is a copolymer of tetrafluoroethylene (TFE) with perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) (PSEPVE), as disclosed in U.S. Pat. No. 3,282,875. Other suitable fluoropolymers are copolymers of TFE with perfluoro(3-oxa-4-pentenesulfonyl fluoride) (PSEVE), as disclosed in U.S. Pat. No. 4,358,545 and U.S. Pat. No. 4,940,525, and copolymers of TFE with CF2=CFO(CF.sub.2).sub.4SO.sub.2F, as disclosed in U.S. Patent Application 2004/0121210. The polymer may comprise a perfluorocarbon backbone and pendant groups of the formula --O--CF.sub.2CF(CF.sub.3)--O--CF.sub.2CF.sub.2SO.sub.2F. Polymers of this type are disclosed in U.S. Pat. No. 3,282,875. All of these copolymers can be converted later to the ionomeric form by hydrolysis, typically by exposure to an appropriate aqueous base, as disclosed in U.S. Pat. No. 3,282,875. [0031] The polymer is typically first dissolved in a solvent for the polymer at a concentration typically between 1 and 30% (weight % or w/w) and preferably between 10 and 20% (w/w). By "polymer solvent" is meant a solvent that will dissolve and solvate the SO.sub.2X form of the polymer and not otherwise react with or degrade the polymer. Typically the polymer solvent is fluorinated. By "fluorinated" it is meant that at least 10% of the total number of hydrogen and halogen atoms in the solvent are fluorine. Examples of suitable polymer solvents include, but are not limited to, fluorocarbons (a compound containing only carbon and fluorine atoms), fluorocarbon ethers (a fluorocarbon additionally containing an ether linkage), hydrofluorocarbons (a compound containing only carbon, hydrogen and fluorine atoms), hydrofluorocarbon ethers (a hydrofluorocarbon additionally containing an ether linkage), chlorofluorocarbons (a compound containing only carbon, chlorine and fluorine atoms), chlorofluorocarbon ethers (a chlorofluorocarbon additionally containing an ether linkage), 2H-perfluoro(5-methyl-3,6-dioxanonane), and Fluorinert.RTM. electronic liquids (3M, St. Paul, Minn.). Suitable solvents also include fluorochemical solvents from E. I. DuPont de Nemours (Wilmington, Del.) A mixture of one or more different polymer solvents may also be used. [0032] The SO.sub.2X form polymer is dissolved with stirring and may require heating for efficient dissolution. The dissolution temperature may be dependent on the polymer composition or SO.sub.2X concentration as measured by the equivalent weight (EW). For the purposes of this application, EW is defined to be the weight of the polymer in sulfonic acid form required to neutralize one equivalent of NaOH, in units of grams per mole (g mol.sup.-1). High EW polymers (i.e. low SO.sub.2X concentration) may require higher dissolution temperatures. When the maximum dissolution temperature at atmospheric pressure is limited by the boiling point of the solvent, a suitable pressure vessel may be used to increase the dissolution temperature. The polymer EW may be varied as desired for the particular application. Herein, polymers with EW less than or equal to 1500 g mol.sup.-1 are typically employed, more typically less than about 900 g mol.sup.-1. [0033] Next, a reactive mixture is formed by mixing a nucleophilic compound, Y, and a polar liquid, with the polymer solution. The terms "nucleophilic" and "nucleophile" are recognized in the art as pertaining to a chemical moiety having a reactive pair of electrons. More specifically herein, the nucleophilic compound Y is capable of displacing the halogen X of the polymer SO.sub.2X groups through a substitution type reaction, and forming a covalent bond with sulfur. Suitable nucleophilic compounds may include but are not limited to, water, alkali metal hydroxides, alcohols, amines, hydrocarbon and fluorocarbon sulfonamides. The amount of the nucleophilic compound Y added is generally less than stoichiometric and will determine the % of SO.sub.2X groups that will remain unreacted. Continue reading... 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