NO169465B - POWER COLLECTOR CONNECTED TO A SOLID POLYMER MEMBRANE - Google Patents

Description

The invention relates to an improved method for producing a current collector/catalyst electrode/membrane assembly which has increased electrical conductivity in the area between the catalyst electrode and the current collector. Such assemblies are useful in a variety of applications, including e.g. fuel cells, water electrolysis cells, chloralkali cells and the like. The assembly formed according to the present invention is essentially structurally stable, which makes it possible for the membrane part to be significantly thinner than those currently available, thereby reducing the ionic resistance in the membrane.

It is highly desirable, on the basis of the strict conditions of many of the applications for the membrane, that the membrane part in the assembly should have essentially structural integrity. Thinner membranes have been shown to be fragile, and yet thinner membranes are desirable because of their reduced ionic resistance. This requires a balance between providing sufficient structural support for the assembly and yet reducing the thickness of the membrane to reduce the ionic resistance in the membrane without sacrificing structural integrity.

References associated with this invention include US Patent No. 4,272,353 which discloses a surface scrubbing technique for scratching a solid polymer electrolyte (SPE) base portion in preparation for subsequent processing. US Patent No. 4,272,560 describes a membrane having a cathode formed by multiple coatings with a textile backing. A dissolved copolymer is used in the production of this electrode. US Patent No. 4,182,670 discloses a combination of cathode and diaphragm using a spray coating of a metal substrate with powdered metal. A polymer-impregnated diaphragm is also described. An electrode body impregnated with powdered metal (typically precious metals) is described in US Patent No. 3,276,911, and a permeable ionic electrolytic material is also mentioned. US Patent No. 4,364,813 discloses catalytic particles deposited on an ion exchange material with an SPE membrane. This patent document also mentions a feature with

a sulfone group as an ion exchanger. US Patent No. 4,366,041

describes a cathode and diaphragm assembly with a sacrificial film formed from wax.

The present invention describes in particular a structurally stable electrode assembly which has lower ion resistance in the membrane part and which has higher electrical conductivity in the catalyst electrode and current collection parts. Membrane thinness is achieved without sacrificing structural integrity and yet resistance to ion movement through the membrane is reduced.

While the foregoing refers in general terms to the present assembly, its structure and method of manufacture are exemplified in the detailed description of the preferred embodiments which follows.

The invention particularly relates to a method for forming an assembly of an ion-permeable membrane, electrode and current collector, and it comprises the steps of: (a) forming a foundation layer of a porous, electrically conductive material, (b) at least partially coating a fluoropolymer binder on at least one surface of the base layer, (c) applying a particulate catalyst material over the fluoropolymer binder on the base layer, (d) dispersing a polymer material as a solution or dispersion over the catalyst material in such a way as to achieve penetration of the polymer material into the porous foundation layer to form a substantially continuous coating on the catalyst material and the at least partially coated foundation layer, and (e) applying heat and/or pressure to the assembly to increase the flow of polymeric material into the foundation layer and around the catalyst material for to achieve adhesion of the catalyst material to the foundation layer and sintering of the polymer material into an essentially non-porous layer around the catalyst material.

The foundation layer is an electrically conductive, hydraulically permeable matrix that acts as a current collector to transfer electrical energy to or from the electrode. It can be composed of a variety of substances, including carbon cloth, carbon paper, carbon felt, metal sieve, metal felt and porous metal sheets. However, the foundation layer is preferably a carbon paper, which is readily available, behaves well, is easy to handle and is relatively cheap.

The paper most preferably used in this invention is one which also has low electrical resistance, has sufficient strength for manufacture and has suitable surface properties, such as roughness, to provide good bonding between the fluoropolymer binder and the foundation layer. It is also advantageous for providing good electrical contact between the carbon paper and the catalytically active particles on the electrode.

As an initial step, the foundation layer is at least partially coated with a suitable polymer binder. This polymer binder may be a fluorocarbon polymer, such as polytetrafluoroethylene, sold under the trademark "Teflon". Other suitable polymers may include thermoplastic nonionic forms of sulfonic acid copolymers, thermoplastic nonionic forms of carboxylic acid copolymers, and the like.

Particularly preferred as the fluoropolymer binder are thermoplastic, nonionic forms of perfluorinated polymers described in the following US Patent Nos. 3,282,875,

3,909,378, 4,025,405, 4,065,366, 4,116,888, 4,123,336, 4,126,588, 4,151,052, 4,176,215, 4,178,218, 4,192,725, 4,209,635, 4,134,721,21 333. 4,478,695, and published European patent application

0.027.009. Such polymers typically have equivalent weights of from 500 to 2000.

Particularly preferred for use as a fluoropolymer binder are copolymers of monomer I with monomer II (as defined below). A third type of monomer can optionally be copolymerized with I and II.

The first monomer type is represented by the general formula:

where Z and Z' are independently selected from -H, -Cl, -F and

- CF3 .

The second monomer consists of one or more monomers selected from compounds represented by the general formula

where:

Y is selected from -SO2Z, -CN, -COZ and -C (R3 f) (R4 f) OH,

Z is selected from -I, -Br, -Cl, -F, OR and -NRiR2,

R is selected from a branched or linear alkyl radical having from 1 to 10 carbon atoms or an aryl radical,

R 3 f and R 4 f are independently selected from perfluoroalkyl radicals having from 1 to 10 carbon atoms,

R 1 and R 2 are independently selected from -H, a branched or linear alkyl radical having from 1 to 10 carbon atoms or an aryl radical,

a is 0-6,

b is 0-6,

c is 0 or 1,

provided that a+b+c is not equal to 0,

X is selected from -Cl, -Br, -F or mixtures thereof when n>l,

n is 0 or 6, and

Rf and Rf, are independently selected from -F, -Cl, perfluoroalkyl radicals having from 1 to 10 carbon atoms, and fluorochloroalkyl radicals having from 1 to 10 carbon atoms.

It is particularly preferred that Y is -SO2F or -COOCH<3>, n is 0 or 1, Rf and Rf , is -F, X is -Cl or -F, and a+b+c is 2 or 3.

The third and optional monomer is one or more monomers selected from compounds represented by the general formula:

where:

Y' is selected from -F, -Cl or -Br,

a<*> and b' are independent of each other 0-3,

c' is 0 or 1,

provided that a 1+b'+c' is not equal to 0,

n' is 0-6,

Rf and R'f are independently selected from -Br, -Cl,

-F, perfluoroalkyl radicals having from 1 to 10 carbon atoms,

and chloroperfluoroalkyl radicals having from 1 to 10 carbon atoms, and

X' is selected from -F, -Cl, -Br or mixtures thereof when n' >1.

The binder is typically applied in solution or dispersion to at least partially coat the foundation layer. The solution or dispersion can be applied to the foundation layer using methods that are well known in the art.

When the electrode is to be used in a fuel cell, the binder is preferably a hydrophobic material such as polytetrafluoroethylene. However, when the electrode is to be used in an electrolytic cell, such as a chloralkali cell, the binder is preferably a hydrophilic material such as a copolymer formed from the monomers I, II and possibly III (described above).

The preferred load, i.e. amount of applied binder is from 0.50 to 50 mg/cm<2> foundation area, with a preferred range of from 2.5 to 30 mg/cm<2> foundation area.

When the binder is applied as a solution or a dispersion, the solvent/dispersant can be a variety of materials, such as includes water, methanol, ethanol and compounds represented by the general formula:

where:

X is selected from F, Cl, Br and I,

X' is selected from Cl, Br and I,

Y and Z are independently selected from H, F, Cl, Br, I and

R' is selected from perfluoroalkyl radicals and chloroperfluoroalkyl radicals having from 1 to 6 carbon atoms.

The most preferred solvents or dispersants are 1,2-dibromotetrafluoroethane (commonly known as Freon 114 B 2): and 1,2,3-trichlorotrifluoroethane (commonly known as Freon 113): Of these two materials, 1,2-dibromotetrafluoroethane is the most preferred solvent or dispersant.

The solution or dispersion used to apply the binder to the foundation layer can have a concentration of from 2 to 30% by weight of polymer in the solvent/dispersant. The concentration is preferably from 8 to 20% by weight of polymer in the solvent/dispersant.

After the solution or dispersion has been applied to the foundation layer, the solvent can then be driven off using heat, a vacuum or a combination of heat and vacuum. The solvent/dispersant can optionally be given the opportunity to evaporate under ambient conditions. The solvent is preferably removed at an elevated temperature. In addition to removing the solvent/dispersant, the heat will sinter. the binder and cause it to more completely penetrate and surround the foundation layer. An example of this is when polytetrafluoroethylene is used as a binder, exposure at a temperature of from 300 to 340°C for approx. 20 min. be sufficient to remove the solvent/dispersant and to sinter the polytetrafluoroethylene.

The next step in the method according to the present invention is the application of the catalytically active and electrically conductive particles to the coated foundation layer. The composite structure will eventually form what is usually referred to as a solid polymer electrolyte or SPE, when the composite is used in a electrochemical cell. The electrode can ultimately be used either as a cathode or as an anode.

Materials suitable for use as electrocatalytic active anode materials include e.g. metals or metal oxides of metals from the platinum group, such as ruthenium, iridium, rhodium, platinum, palladium, either alone or in combination with an oxide of a film-forming metal such as Ti or Ta. Other suitable activating oxides include cobalt oxide, either alone or in combination with other metal oxides, e.g. such as are described in US Patent Nos. 3,632,498, 4,142,005, 4,061,549 and 4,214,971.

Materials suitable for use as electrocatalytic active cathode materials include e.g. metals or metal oxides from the platinum group, such as ruthenium or ruthenium oxide. US Patent No. 4,465,580 describes such cathodes.

The catalytic particles used in the present invention are preferably finely divided and have a preferred size range from 270 to 400 mesh (US standard) (53 to less than 37 pm). The metal powder is applied to the binder-coated foundation layer by methods known to those skilled in the art, and includes e.g. spraying, forming a sheet of catalytic particles and pressing the sheet onto the foundation layer, or forming and applying the particles in the form of a liquid dispersion, e.g. an aqueous dispersion. A suitable loading of catalyst particles has been found to be from 0.2 to 20 mg/cm<2> foundation area with a preferred range of from 1.5 to 5.0 mg/cm<2> foundation area.

A copolymer is formed separately. Such a suitable polymer is a polymer which is formed from the monomers I, II and possibly III, as defined above. The polymer may be a thermoplastic nonionic precursor of a sulfonic acid copolymer or a thermoplastic nonionic precursor of a carboxylic acid copolymer, or a variety of other polymers as defined for use as the binder. The copolymer is preferably formed in a solution or dispersion with a solvent for application to the catalytically active particles. After mixing with a suitable solvent or dispersant, the polymer is applied to the base layer coated with catalyst particles. By applying a vacuum to one side of the foundation layer, the polymer in the solvent or dispersant is drawn down onto the catalyst and into the foundation layer. Although this can in a way be described as coating on one side, nevertheless the coating penetrates sufficiently into the porous sheet.

In the step of binding a fluoropolymer to the surface of the foundation layer coated with catalytic particles, the most convenient method is to use conventional organic solvents. Typical solvents used are 1,2-dibromotetrafluoroethane, methanol, ethanol and the like. The polymer material that is applied forms an essentially non-porous ion exchange layer.

The next step is the application of heat and/or pressure

to remove the solvent/dispersant and to sinter the polymer, thereby forming the polymer into a substantially continuous sheet. Also, the heat and/or pressure improves the coating of the polymer around the catalyst particles and the foundation layer. Exposure to a temperature in the range from 260 to 320°C is e.g. usually suitable for binding the polymer to the particles and the base layer. The temperature range is limited primarily by incipient thermal degradation of the polymer caused by excessive heat. The pressure is preferably sufficiently high and sustained in an interval that provides binding. As an example, the pressure can be applied with up to approx.

5 kg/cm<2> for approx. 1 minute at elevated temperature.

The next step in the preparation of the improved electrode structure is to hydrolyze the structure from the non-ionic to the ionic form. The hydrolysis can be carried out by treating the polymer with a basic reading if the polymer is a thermoplastic nonionic precursor of a sulfonic acid polymer or a thermoplastic nonionic precursor of a carboxylic acid polymer. Also, if the polymer is a thermoplastic, nonionic precursor of a carboxylic acid polymer, an acidic solution may be used to hydrolyze the polymer. For a thermoplastic, non-ionic precursor of a sulphonic acid polymer, e.g. the completed structure is hydrolyzed in 25% by weight sodium hydroxide for 16 hours at an elevated temperature of 80°C.

The completed item is then ready for use. As an example of a typical size, it is not uncommon to encounter a membrane that has a thickness range from 0.125 to 0.25 mm due to the need for structural integrity.

The final product can provide a membrane with a thickness in the range from 0.025 to 0.05 mm, or even less. The resistance to ion movement through the membrane is thus reduced to a significant extent.

In an alternative application, two similar sheets of the same size are placed in contact with each other in such a way that the foundation layers face the outside of the combination, and the polymer layer of each sheet comes into contact with the polymer layer of the other sheet. The composite sheets are then placed in a press and by applying suitable pressure and/or heat they are joined together.

Claims (10)

1. Method for forming an assembly of an ion-permeable membrane, electrode and current collector, characterized in that it comprises the steps of: (a) forming a foundation layer of a porous, electrically conductive material, (b) at least partially coating a fluoropolymer binder on at least one surface of the base layer, (c) applying a particulate catalyst material over the fluoropolymer binder on the base layer, (d) dispersing a polymer material as a solution or dispersion over the catalyst material in such a way as to achieve penetration of the polymer material into the porous foundation layer to form a substantially continuous coating on the catalyst material and the at least partially coated foundation layer, and (e) applying heat and/or pressure to the assembly to increase the flow of polymeric material into the foundation layer and around the catalyst material for to achieve adhesion of the catalyst material to the foundation layer and sintering of the polymer material the material into an essentially non-porous layer around the catalyst material. 2. Method according to claim 1, characterized in that the polymer material contains one or more solvents or dispersants selected from ethanol, methanol, water and a compound represented by the general formula where: X is selected from F, Cl, Br and I, X<*> is selected from Cl, Br and I, Y and Z are independently selected from H, F, Cl, Br, I and R', and R' is selected from perfluoroalkyl radicals and chloroperfluoroalkyl radicals having from 1 to 6 carbon atoms. 3. Method according to claim 2, characterized in that the solvent or dispersant is selected from 1,2-dibromotetrafluoroethane and 1,2,3-trichlorotrifluoroethane. 4. Method according to claim 1, 2 or 3, characterized in that the catalyst particles are selected from ruthenium, iridium, rhodium, platinum, palladium or oxides thereof either alone or in combination with an oxide of a film-forming metal, and cobalt oxide either alone or in combination with other metals or metal oxides from the platinum group. 5. A method according to any one of the preceding claims, characterized in that it includes the step of forming two assemblies of similar size, placing the two assemblies together so that the non-porous polymer surfaces come into intimate contact with each other, and applying heat and/or pressure to form a single planar assembly containing two current collectors and having a non-porous ionically conductive polymer layer between them. 6. Method according to any of the preceding claims, characterized in that said fluoropolymer binder for the foundation layer is selected from a thermoplastic, non-ionic precursor of a sulfonic acid or carboxylic acid copolymer having an equivalent weight in the range from 500 to 2000. 7. Method according to claim 1, characterized in that (a) said conductive material is porous conductive graphite paper, (b) said binder is polytetrafluoroethylene, and (c) said polymer material is a sulfonic acid copolymer in the form of thermoplastic powder in a liquid solvent, and whereby a vacuum is applied to achieve penetration of the thermoplastic powder and solvent into the porous graphite paper. 8. Method according to claim 1, characterized in that it includes the step of exposing the polymer to a base or to an acid at a temperature and a time sufficient to hydrolyze substantially all of the polymer. 9. Method according to any one of the preceding claims, characterized in that the binder is a copolymer formed by polymerization of one or more monomers selected from a first group of monomers represented by the general formula: where: Z and Z' are independently selected from -H, -Cl, -F or -CF3, with one or more monomers selected from another group of monomers represented by the general formula: where: Y is selected from -SO2Z, -CN, -COZ and -C (R3 f) (R4 f) OH, Z is selected from -I, -Br, -Cl, -F, OR and -NRi R2 , R is selected from a branched or linear alkyl radical having from 1 to 10 carbon atoms or an aryl radical, R3 f and R4 f are independently selected from perfluoroalkyl radicals having from 1 to 10 carbon atoms, R1 and R2 are independently selected from -H, a branched or linear alkyl radical having from 1 to 10 carbon atoms or an aryl radical, a is 0-6, b is 0-6, c is 0 or 1, provided that a+b+c is not equal to 0, X is selected from -Cl, -Br, -F or mixtures thereof when n>l, n is 0 or 6, and Rf and Rf- are independently selected from -F, -Cl, perfluoroalkyl radicals having from 1 to 10 carbon atoms , and fluorochloroalkyl radicals having from 1 to 10 carbon atoms, and optionally one or more monomers selected from a third monomer represented by the general formula: where: Y<1> is selected from -F, -Cl or -Br, a' and b' are independently 0-3, c' is 0 or 1, provided that a'+b'+c' is not equal to 0, n' is 0-6, Rf and R1f are independently selected from -Br, -Cl, -F, perfluoroalkyl radicals having from 1 to 10 carbon atoms, and chloroperfluoroalkyl radicals having from 1 to 10 carbon atoms, and X<1> is selected from -F, -Cl, -Br or mixtures thereof when n" >1. 10. Method according to claim 9, characterized by Yer -SO2F or -COOCH<3>, n is 0 or 1, Rf and Rf - , is -F; X is -Cl or -F, and a+b+c is 2 or 3.