The present invention relates to the electrochemical industry in general, and more particularly to a membrane-electrode assembly (xe2x80x9cMEAxe2x80x9d) based on fluoro-containing ion-exchange membranes and to a method for its production. Such MEAs are widely used in fuel cells, in water electrolyzers, and in other electrochemical processes.
MEA consisting of the fluoro-containing ion exchange membrane M174-SK(trademark) (Russian trademark [RTM]) and of layers of an electrode material (electrode composition) situated on both sides of the membrane are already known. The electrode composition consists of a mixture of an electrocatalyst and an ion-exchange polymer [USSR patent 1,258,095 IPC S25V 11/10, 1990]. As an ion-exchange polymer in the electrode composition, an inorganic proton-conducting electrolyte (polyantimonic acid, acidic zirconium phosphate) is used. The electrocatalyst is platinum, palladium, or rhodium black.
MF-4 SK membrane is a 300 micron thick cation-exchange membrane (xe2x80x9cCEMxe2x80x9d) made from a hydrolyzed copolymer of tetrafluoroethylene and a perfluorosulfur group-containing vinyl ether, having the following structural formula: 
Our experiments showed that this copolymer has an equivalent weight (EW) of 1200 and a degree of crystallinity of 12%, as shown in control Example 1.
The MEA is produced by applying an electrode composition on both sides of the CEM. The sedimentation method is used. The electrode composition consists of a mixture of an electrocatalyst and an ion-exchange polymer (polyantimonic acid) powder. The composition is fixed by electric current treatment in water at 90xc2x0 C., where the current density is 0.5-1 A/cm2.
A MEA is produced that consists of, for example, CEM MF-4SK and layers of the electrode composition on both sides with an electrocatalyst (platinum black, size of particles 0.01 micron) on both the cathode and the anode sides. The MEA deionized water electrolysis has the following characteristics: the voltage is 2.2V when the current density is 1 A/cm2, and the temperature is 100xc2x0 C. The voltage does not change over a period of time of 1000 hours.
Advantageously, the MEA is stable over a period of time of 1000 hours.
The disadvantage of the MEA (USSR patent 1,258,095) is the impossibility to achieve high adhesion between layers of the electrode composition and CEM because the ion-exchange polymer (polyantimonic acid) is gradually dissolved after a long period of time (more than 1000 hours) of electrolysis of water. Consequently, over a long period of working with MEA, one can observe a tendency for exfoliation of the electrode composition. The method of the MEA production does not permit precise regulation of the composition and the amount of the electrode material applied to CEM. The method is complicated by the fact that layers of electrode composition (electrocatalytic layers) are applied by a method of sedimentation, which requires the following electric current characteristic to fix layers on CEM: a rather high electrocatalyst loading of 1-2 mg/cm2 on the cathode and 4-6 mg/cm2 on the anode.
A MEA with a porous cathode is also known. Such a MEA consists of a polymeric ion exchange membrane of Nafion(copyright) (trademark of CEM by Du Pont) type and a porous layer of an electrode material, a mixture of electrocatalyst particles and a binder (Russian Federation [RF] patent 2,015,207, IPC S25V 11/20, 1994), settled on the cathode side of CEM. The porous cathode layer of the electrode composition is made of a mixture of electrocatalytic particles and the binder, polytetrafluorethylene. The membrane (Nafion(copyright)) is produced from a hydrolyzed copolymer of tetrafluoroethylene and perfluorinated vinyl ethers containing ion-exchange groups. For water electrolysis of CEM (Du Pont) Nafion(copyright) 120 with xe2x80x94SO3H ion exchanging groups, see Russian Federation patent H; see RE patent describing tetrafluoroethylene and perfluorinated vinyl ethers containing ion-exchange groups for water electrolysis of CEM (Du Pont) Nafion(copyright) 120 with SO3 tetrafluoroethylene ion exchanging groups. The membrane (Nafion(copyright)) is produced from hydrol248, 5 American Chemical Society, Washington D.C.
The MEA mentioned above is produced by applying a mixture of electrocatalytic particles and inactive conducting material with the binder (polytetrafluorethylene) and aluminum powder on an aluminum sheet by technique A. After drying at 105xc2x0 C., for example, sintering at 325xc2x0 C. is carried out for 10 minutes. Then, the aluminum sheet with the layer of electrode material is placed on the cathode surface of CEM and pressed at 175xc2x0 C. and a pressure of 50-60 kg/cm2. After pressing, the MEA is dipped in a caustic soda solution to dissolve the aluminum sheet and aluminum powder (the latter is used as a promoter of porosity). Then, the layer of electrode material become porous. The advantage of the MEA of Russian Federation (RF) patent No. 2,015,207 is that MEA""s lifetime increases because the binder (polytetrafluorethylene) does not dissolve during the electrolysis. When such MEA is used for water electrolysis, the cell voltage is 1.8-1.9 V.
The disadvantage of the MEA described above is that the adhesion between the applied porous layer of electrode material and the CEM surface is not as strong as required. During MEA tests of long duration, exfoliation of the porous layer of the electrode material occurs and the evolving gases are deposited at the surface between the CEM and the porous layer. This results in an increase of the MEA voltage. Moreover, as is shown in the Russian Federation patent (example 4), the disadvantage of the described MEA is a comparatively high electrocatalyst loading because of its particular capsulation by polytetrafluorethylene during production (pressing at 325xc2x0 C. and a pressure of 50-60 kg/cm2).
Moreover, the volume porosity of the electrocatalytic layer of the electrode material is uncontrollable, so the transport of gases and liquids in the reaction zone is impeded and the electrochemical properties of the MEA worsen.
The manner of producing the described MEA is rather complicated, because high temperature ( greater than 300xc2x0 C.) sintering and aluminium lixiviation to form the porous layer of an electrode material are required. Moreover, production of the MEA by pressing at 175xc2x0 C. leads to the particular destruction of cation-exchanging groups that worsens CEM electrochemical characteristics and may destroy the whole MEA.
The art having an essential set of attributes closest to the claimed MEA and its method of production is the MEA, and the method of its production, described in U.S. Pat. No. 5,399,184 HOIM 8/10, 1995. U.S. Pat. No. 5,399,184 discloses an MEA that consists of a fluoro-containing cation-exchange membrane made of a tetrafluoroethylene hydrolyzed copolymer and a fluorosulfur group-containing vinyl ether, with an exchange capacity of 0.83-1.43xcx9cmeq/g (as described in the specification) or 1.12-1.43 meq/g (as described in the patent""s Examples and Claims), which corresponds to an EW of 900-1300, and discloses porous layers of an electrode material situated on both of its surfaces. These layers are made of a mixture of an electrocatalyst with an inactive electroconducting material and a fluoropolymer binder. The fluoropolymer binder is a cation-exchange fluoropolymer with a composition identical to the membrane polymer, or may be polytetrafluorethylene. The CEM is made of a hydrolyzed copolymer of tetrafluoroethylene with a perfluorsulfur-containing vinyl ether. Its structural formula is: 
For example, it may be CEM produced by Du Pontxe2x80x94Nafion(copyright) 117. This membrane is made of a copolymer with a degree of crystallinity of 12%. [ACS Symposium Perfluorinated lonomer Membranes, Lake Buena Vista, Fla. Feb. 23-26, 1982, Series 180, pp 217-248, American Chemical Society, Washington D.C.]
The MEA specified by prototype (U.S. Pat. No. 5,399,184) is produced by application of the paste of electrode material on the both surfaces of the CEM. The latter consists of the hydrolyzed copolymer of tetrafluoroethylene with the perfluoro-containing vinyl ether (EW=900-1300). The paste is made of a mixture of inactive electroconducting material (carbon) and an electrocatalyst (platinum) with a fluoro-containing copolymer binder (with 5% solution of cation-exchange fluorocopolymer which has a composition close to the fluorocopolymer which CEM is made of from a 50% dispersion of polytetrafluorethylene in aliphatic alcohol). The paste is applied onto one of the CEM surfaces and then onto the other surface (with a subsequent thermal treatment). When a 5% solution of the cation-exchange fluoropolymer (Nafion(copyright) Solution) is used as the binder with xe2x80x94SO3H ion-exchange groups (see U.S. Pat. No. 5,399,184, Example 1), the paste is treated with a 5% solution of potassium hydroxide in water before application to the CEM to transform the ion-exchange groups in xe2x80x94SO3K groups. The paste is spread on one of the CEM (with xe2x80x94SO3K groups) surfaces in such a way that the layer of an electrode material after drying is not more than 10 microns thick. Then the paste is dried at room temperature for 10 minutes and then under vacuum for 30 minutes to remove the solvent. Then the CEM with the paste is placed between polytetrafluorethylene sheets and pressed at 190xc2x0 C. at a pressure of 50 kg/cm2. Then an analogous layer of an electrode material is placed on the other surface of the membrane by the same procedure. Then MEA is dipped in a 5% sulfuric acid solution at room temperature for 16 hours to convert ion-exchange xe2x80x94SO3K groups to xe2x80x94SO3H groups.
When a polytetrafluorethylene 50% dispersion as fluorpolymeric binder is used in an electrode material (see example 5 of U.S. Pat. No. 5,399,184), the paste of electrode material is applied to a sheet of carbon material, then the sheet is heated at a temperature of 325xc2x0 C. for 10 minutes under pressure. The second sheet is produced in the same way. The electrode material""s layers, which were applied to sheets of carbon material, are then covered with a 5% solution of xe2x80x9cNafion(copyright) Solutionxe2x80x9d containing groups xe2x80x94SO3H and then dried. Then carbon sheets are placed on the CEM""s surface (with sides which are covered with an electrode material facing the CEM) and pressed at a temperature of 135xc2x0 C. and a pressure of 140 kg/cm2 for 60 minutes. The MEA produced by the described method has porous layers of electrode material on the surfaces of the CEM.
MEA produced by the prototype (U.S. Pat. No. 5,399,184) is pressed with a carbon cloth of paper saturated with polytetrafluorethylene on both sides and put into a fuel cell. The voltage of the fuel cell in example 1 is 0.75-0.77 V and the current density is 0.5 A/cm2; in example 5 the voltage is 0.75-0.8 V and the current density is 0.5 A/cm2. The MEA produced can be used not only in fuel cells but also in electrolysis of water.
The disadvantages of the prototype MEA are:
1. Not enough high electrochemical characteristics especially at low electrocatalyst loading. The reason is the low and uncontrolled porosity of the electrode material layers and the high degree of crystallinity. Uncontrolled and low porosity of the electrode material are due to the fact that the porous electrode material layer is formed at a rather high temperature of 190-340xc2x0 C. and a pressure of 50-120 kg/cm2. Our experiments showed that such electrode material layer""s porosity is about 35% (see our control example 2).
Such conditions of MEA production result in deformation and collapse of pores, both those which are in CEM and those which were formed in the electrode material. A local collapse of the membrane can occur. Low and uncontrollable porosity of the electrode material layers and high degree of crystallinity make difficult the reactant supply and the removal of reaction products, and also make difficult achievement of the necessary water balance in MEA. This is the reason for the relatively high MEA resistance and overvoltage on electrodes, finally resulting in fuel cell low voltage to high electric power consumption if MEA is used in water electrolysis. Additionally, partial capsulation of the electrocatalyst with the fluorobinder occurs, which decreases the efficiency of the electrocatalyst and increases its consumption.
2. A not high adhesion between the catalytic layer and CEM decreases the MEA""s lifetime. One reason for the decreased lifetime is the long processing of the MEA with a solution of sulfuric acid which results in significant swelling of catalytic material layers. Another cause for the decreased lifetime is the different degree of swelling of the porous catalytic layer and of the CEM. All this leads to the exfoliation of catalytic material layers when MEA operates during a long time period (Journal of Applied Electrochemistry 22 (1992) pp. 1-7). Such processing with sulfuric acid is necessary if the same cation-exchange copolymer (with xe2x80x94SO3K groups) as CEM""s copolymer is used as the fluoropolymeric binder.
When polytetraethylene is used as a polymeric binder it is almost impossible to reach a uniform distribution of the electrocatalyst because discrete solid lumps of polytetraethylene are usually obtained. These lumps block the surface of CEM and of the electrocatalyst, thus areas on the surface of CEM that are fully covered by polytetraethylene are created, such that liquid and gaseous reactants of the reaction cannot penetrate these areas. The existence of such areas in MEA contributes to the exfoliation of the electrode material layer from the CEM""s surface and the deterioration of electrochemical characteristics.
The disadvantages of MEA production by the prototype are that it is a multistage (6-7 steps) process of long duration. The MEA processing with the solution of sulfuric acid takes about 16 hours, in itself.
The technical result which is achieved by the claimed MEA includes the improvement of electrochemical characteristics of MEA (especially at low catalyst loading), an increase in efficiency of the electrocatalyst usage, and an increase in the lifetime of the MEA.
The claimed method of MEA production permits one to more simply make the process, reduce its duration, and ensure the production of MEA with high electrochemical characteristics.
The mentioned technical result is achieved by using a fluoro-containing cation exchange membrane (xe2x80x9cCEMxe2x80x9d) made of a hydrolyzed copolymer of tetrafluoroethylene with a perfluorosulfur-containing vinyl ester and optimally with a third modifying comonomer, which has a degree of crystallinity of 2-8%, and porous layers of electrode material are produced with porosity 40-70%, decreasing in the direction to the surface of CEM with a gradient of porosity S-1 between 5 and 15% per 1 micron. The MEA consists of a fluoro-containing CEM made of a hydrolyzed copolymer of tetrafluoroethylene with a perfluorosulfur-containing vinyl ether having an EW of 900 to 1300 and porous layers of electrode material made of a mixture containing an electroconducting inactive material and a fluoropolymeric binder located on both surfaces of CEM.
CEM can be produced of hydrolyzed copolymer tetrafluoroethylene with perfluorsulfur-containing vinyl ester and the third modifying comonomer which can be chosen from ethylene, perfluor-2-methylen-4-methyl-1,3-dioxalan and perfluoralkyled vinyl ether with C1-C3 alkyl-group.
The process simplification and reduction of its duration is achieved by using a CEM produced of hydrolyzed copolymer of tetrafluoroethylene with perfluorsulfur-containing with a degree of crystallinity of 2-8% for MEA production. The method includes application of electrocatalyst, inactive electroconducting material with fluor-containing binder mixture onto both surfaces of fluor-containing CEM, which is produced of hydrolyzed copolymer of tetrafluoroethylene with perfluorsulfur-containing vinyl ester with EW=900-1300 with the subsequent thermal treatment. Mixture of electrocatalyst, inactive electroconducting material and 1-5% solution of cation-exchange fluorcopolymer identical to fluorcopolymer of which CEM is made is applied in an organic solvents mixture to the both surfaces of CEM. Heattreatment is carried out with a multistage increase of temperature: from 20-35xc2x0 C. to 80-100xc2x0 C. Another CEM can be used; for example, CEM produced of hydrolyzed copolymer of tetrafluoroethylene with perfluorsulfur-containing vinyl ether and the third modifying comonomer which can be chosen from ethylene, perfluor-2-methylen-4-methyl-1,3-dioxalan and perfluoralkyled vinyl ester with C1-C3 alkyl-group. A mixture of electrocatalyst, inactive electroconducting material and a 1-5% solution of cation-exchange fluorcopolymer identical to the fluorcopolymer of which CEM is made can be applied, in an organic solvents mixture, to both surfaces of CEM.
The inventors of the present invention discovered that a degree of crystallinity of the hydrolyzed cation exchange fluorcopolymer (from which the cation exchange membrane is made of) has a great influence on the electrochemical characteristics of MEA. When the degree of crystallinity of fluorcopolymer is equal to 2-8% there is such water balance in MEA volume which provides necessary inlet of the reagents in MEA and the outlet of the products of the reaction. The degree of crystallinity results in optimum electrochemical characteristics of MEA.
The degree of crystallinity of fluorcopolymer that is used in MEA may be controlled by: (1) the conditions of its synthesis; (2) addition of the third comonomer; (3) the conditions of the hydrolysis when copolymer is transforming from a nonionic form into cationic exchange form. An increase in the degree of crystallinity above 8%, as well as a decrease in the degree of crystallinity below 2% result in a deterioration of the electrochemical characteristics of MEA.
Formation of the layer with porosity that decreases in the direction of cation-exchange membrane with a porosity gradient of 5-15% per 1 xcexcp improves the electrochemical characteristics of MEA. Such porosity is achieved when the layers of an electrode material are applied to the membrane surface containing fluorcontaining binder dissolved in a mixture of organic solvents with different boiling points (preferably as a 1-5% solution). The binder is a fluorcopolymer which is identical to the fluorcopolymer from which the membrane is made. Such combination of the fluorcopolymer with the mixture of solvents, together with removal of the solvents during a multistage increase of the temperature from 20-35xc2x0 C. to 80-100xc2x0 C., provides the necessary porosity gradient, with general porosity preferably 40-70%, without the need to use any special methods to obtain it.
If the heat treatment is carried out at more than 100xc2x0 C., the necessary porosity could hardly be controlled and the porosity gradient 5-15% could hardly be reached. To carry out the heat treatment at lower than 20xc2x0 C. is unexpedient because formation of the electrode layer slows, and therefore reaching the porosity gradient 5-15% is impeded.
When a cation-exchange fluorcopolymer (that is identical to fluorcopolymer from which CEM is made), together with solvents in which CEM fluorcopolymer swells well, is used as a binder of an electrode material, strong adhesion between the layer of an electrode material and CEM could be reached. When comparatively mild conditions of heat treatment (not more than 100xc2x0 C. without any pressure) are used to produce MEA, not pressing of an electrode material in CEA surface but gluing them together takes place when they are combined. Under the mentioned mild conditions of MEA production, the decomposition of fluorcopolymer cation exchange groups does not occur, such that the CEM is not damaged, and the electrochemical characteristics of the CEM do not worsen.
At the claimed MEA, the cation-exchange membrane can be made of hydrolyzed copolymer of tetrafluoroethylene with perfluorsulfur-containing vinyl ethers with the following structural formula: 
The third modifying comonomer at the mentioned fluorcopolymer could be ethylene, perfluor-2-methylene-4-methyl-1,3-dioxalane, perfluoralkylvinyl ether (with C1-C3 in alkyl). The modifying comonomer is brought at the copolymer during the synthesis at the amount of 1-5%, mnllPthvlenP-3-5 moL perfluor-2-methylene-4-methyl-13-dioxalane-1-4% mol; perfluoralkyl ethers with C1-C3 in alkyl-2-5%).
The anologous hydrolyzed copolymers of tetrafluoroethylene with perfluorsulfur-containing vinyl ethers with the above structural formula are described at the mentioned above analogues, in the prototype, and at RF patent No. 2,077,373 (IPC 6VOID 61/00,1997).
Copolymers (CPL) which were used in the following examples of the realization of the invention were synthesized by the inventors. The structural formulae are: 
The layer of an electrode material which is applied on the anodic surface of the CEM as an electrocatalyst may contain platinum, iridium, IrO2, mixed oxides IrO2+RuO2, IrO2+RuO, +TiO2, IrO2+RuO2+SnO2, PbO2+IrO2 and others.
The layer of an electrode material which is applied on the cathodic surface of the CEM as an electrocatalyst may contain platinum, palladium, or platinum with ruthenium, etc.
The layer of electrode material may contain an inactive electroconductive material such as, carbon, lead, lead dioxide, etc.
It is expedient to use polytetrafluorethylene F-4D (RTM) (Russian national standard (RNS) 1496-77) in the composition of an electrode material.
Fluorpolymer binder in the composition of an electrode material is a fluorcopolymer with the composition identical to the fluorcopolymer from which the CEM is made. The binder is used as a 1-5% solution in a mixture of organic solvents with different boiling points. The composition of the mixture depends on: (1) the composition of the copolymer its equivalent weight, and (2) the type of cation-exchange group (xe2x80x94SO3H; xe2x80x94SO3K; xe2x80x94SO3Naxe2x80x94SO3Li) that is included in it.
Moreover, the mixture must contain such solvents that there would not be any coagulation of the system when the solution of cation-exchange fluorcopolymer is combined with the electrocatalyst and inactive electroconductive material. The mixture of organic solvents must include the solvents with low boiling point 20-60xc2x0 C. (1,1,2-trifluor-1,2,-dichloroethane (freon 123); pentane; 1,1,-difluor-1,2-dichloroethene (freon-132B); 1,1,2-trifluorotrichloroethene (freon 113); 1,1,1-trichlorobromidoethane (freon-123B); acetone etc.), solvents with middle boiling point 60-100xc2x0 C. (1,1-difluoro-1,2,2-trichloroethane (freon 122); ethanol; hexane; methyl ethyl ketone, benzene, isopropanol; N-propanol; heptane etc.) and solvents with high boiling point 100-160xc2x0 C. (isobutanol; N-butanol; toluene; dimethylformimide; ciclohexanone etc.).
If the fluorcopolymer contains cation-exchange groups such as xe2x80x94SO3H group, then the mixture of ethanol with freon-113 and methyl ethyl keton preferably can be used. If there are SO3K groups, then dimethylformamide mixed with ethanol and heptane preferably can be used. For copolymer with xe2x80x94SO3Li, groups the mixture of isopropanol with acetone and freon-123B may be used.
The solution of cation exchange fluorcopolymer is obtained by dissolving the cation exchange fluorcopolymer powder in a mixture of organic solvents then heating and stirring the solution. The dissolving temperature depends on the composition and equivalent weight of copolymer, and also on the boiling points of the solvents used.
The properties of the fluorcopolymer from which the CEM is made and the properties of the MEA were determined in the following way:
1. The composition of the fluorcopolymer was determined by infrared spectroscopy using a Perkin-Elmer, 1750 spectrometer.
2. The exchange capacity was determined by titration [RNS 17552-72 and technical conditions (TU) 6.06-041-969-89].
3. The degree of crystallinity was determined by X-ray method using X-ray spectrometer KRM-1.
4. The membrane thickness and the thickness of the layers of an electrode material were determined using the micrometer MK 25-1 (RNS 6507-78).
5. The general porosity and the porosity gradient were determined by the method of ethalone (standard) porometry.