Method of bonding electrode to cation exchange membrane

Abstract

 A method of bonding an electrode to a cation exchange membrane of a fluorinated polymer, which in use has ion exchange groups of formula

wherein X represents an alkali metal atom, an alkaline earth metal atom or -NRR' in which R and R' respectively represent a hydrogen atom or a lower alkyl group; and m is the valence of the group X. The membrane, in a form having ion exchange groups of formula

wherein L represents a hydrogen atom or a C₁-C₂₀ alkyl group, is nelt-bonded to said electrode and then the ion exchange groups are converted into groups of formula (̵COO)_mX.

Description

[0001] The present invention relates to a method of bonding an electrode to a cation exchange membrane. In particular, embodiments it relates to a method of bonding a porous, gas permeable, catalytic electrode to a cation exchange membrane of a fluorinated polymer having carboxylic acid groups as cation exchange groups which is used in an electrolytic cell for producing an alkali metal hydroxide by electrolysis of an aqueous solution of an alkali metal chloride at low voltage and with high current efficiency.

[0002] As processes for producing an alkali metal hydroxide by electrolysis of an aqueous solution of an alkali metal chloride, diaphragm methods have generally taken over from mercury methods since they present less of a pollution hazard.

[0003] More recently, ion exchange membranes have been used in place of asbestos diaphragms, and processes using such membranes are found to give alkali metal hydroxides of high purity in high concentration.

[0004] On the other hand, there is a continuing need to save energy, and for this reason it is desirable to minimize cell voltages, in such processes.

[0005] Various methods have been proposed for decreasing cell voltages. Various ways of improving materials, components, shapes and configurations of anodes and cathodes have been tried, as well as new formulations and types of an ion exchange membrane. These improvements have had some success. Most of them, however have the disadvantages that the maximum concentration of alkali metal hydroxide obtained is not high and a remarkable increase in cell voltage or decrease in current efficiency occurs if the concentration of alkali metal hydroxide is to high. Also, the durability of low cell voltage is not satisfactory.

[0006] It has been also proposed to use a cation exchange membrane of a fluorinated polymer which is bonded to gas-liquid permeable catalytic anode on one surface and a gas-liquid permeable catalytic cathode on the other (British Patent Specification 2,009,795). This method is remarkably advantageous for electrolysis at low cell voltage because the electrical resistance, caused by the electrolyte and by bubbles of hydrogen gas and chlorine gas generated in the electrolysis, can be remarkably decreased, a result which has hitherto been difficult to achieve.

[0007] When a cation exchange membrane of a fluorinated polymer having carboxylic acid groups or sulfonic acid groups, especially carboxylic acid groups is used as the ion exchange membrane, an alkali metal hydroxide having a high concentration can be produced at a low cell voltage and with high current efficiency.

[0008] One of the important factors in achieving good results in this process is uniform and firm bonding of the catalytic electrodes to the cation exchange membrane. When these are not satisfactorily bonded or bonding strength is low, the cell voltage is liable to increase and the electrodes may be peeled off from the membrane by gas generation at the interface between the electrode and the membrane. It has been proposed to use an adhesive for effective bonding. However, most adhesives have low conductivity and thus increase the electrical resistance. No adhesive effective for bonding an electrode to a cation exchange membrane of a fluorinated polymer has so far been found.

[0009] A melt-bonding method has been proposed wherein the surface of the membrane is partially melted to bond it to the electrode.

[0010] When a cation exchange membrane of a fluorinated polymer having carboxylic acid groups is used, it has been found that the electrode is not satisfactorily bonded to the membrane or if the bonding is satisfactory the electrolytic characteristics of the resulting composite are unsatisfactory, particularly, where the carboxylic acid groups of the membrane are of the formula 4 COO)-_mX wherein X represents an alkali metal or alkaline earth metal atom or -NRR' wherein R and R' respectively represent a hydrogen atom or a lower alkyl group; and m is the valence of X.

[0011] The present invention provides a process for producing a fluorinated polymer membrane, having ion exchange groups of this latter formula and bonded to an electrode, which process comprises melt-bonding a cation exchange membrane having ion exchange groups of formula -COOL, wherein L represents a hydrogen atom or a C₁-C₂₀ alkyl group, to the electrode and then converting the groups of formula -COOL to groups of formula (̵COO)̵_mX as defined above.

[0012] The cation exchange membrane is preferably bonded to a porous gas-liquid permeable electrode, and is particularly suitable for the electrolysis of an aqueous solution of an alkali metal chloride.

[0013] In the above formula X is preferably the same. alkali metal atom as that of the alkali metal chloride to be used as the electrolyte.

[0014] The ion exchange capacity of carboxylic acid groups is important since it affects the characteristics of the membrane in the electrolysis. It is dependent upon the type of fluorinated polymer used for the membrane, and is preferably in a range of 0.5 to 2.5 meg/g. dry polymer, especially 1.0 to 2.0 meg/g. dry polymer. This latter range gives good electrochemical and mechanical characteristics.

[0015] The cation exchange membrane is preferably made of a fluorinated polymer having the following units



wherein X represents a fluorine, chlorine, or hydrogen atom or -CF₃ and X' represents X or CF₃(CF₂)̵_m wherein m represents an integer of 1 to 5.

[0016] Y is preferably a group having a structure in which A is bonded to a fluorocarbon group, such as



and

wherein x, y and z respectively represent an integer from 1 to 10; Z and Rf represent -F or a C₁-C₁₀ perfluoroalkyl group;and A represents a functional group which is convertible to (̵COO)̵_mX in electrolysis.

[0017] The N mole % of the units of

is preferably in a range of 1 to 40 mole %. especially 3 to 25 mole % to impart the desired ion exchange capacity to the membrane.

[0018] The molecular weight of the fluorinated polymer used is important since it affects the electrochemical characteristics of the resulting membrane. The molecular weight of the fluorinated polymer is preferably in a range of 1 x 10⁵ to 2 x 10⁶, especially 1.5 x 10⁵ to 1 x 10⁶.

[0019] In the production of the perfluoro polymer, various processes can be employed.

[0020] In the preparation of the perfluoro polymer, one or more monomers for forming the units (M) and (N) can be used, if necessary with a third monomer so as to improve the membrane. For example, flexibility can be imparted to the membrane by incorporating CF₂ = CFORf(Rf is a C₁-C₁₀ perfluoroalkyl group), and its mechanical strength can be improved by crosslinking the copolymer with a divinyl monomer such as

[0021] The copolymerization of the fluorinated olefin .. monomer with the monomer having carboxylic acid groups or functional groups convertible into carboxylic acid groups, and the third monomer where used can be carried out by any suitable conventional process. The polymerization can be carried out, if necessary using a solvent such as a halohydrocarbon by catalytic polymerization, thermal polymerization or radiation-induced polymerization. The method used for fabrication of the ion exchange membrane from the resulting copolymer is not critical. For example known methods such as press-molding, roll-molding, extrusion- molding, solution spreading, dispersion molding and powder molding can be used.

[0022] The thickness of the membrane is preferably 20 to 600 microns, especially 50 to 400 microns.

[0023] The cation exchange membrane used in the present invention can be fabricated by blending a polyolefin such as polyethylene, polypropylene or more preferably a fluorinated polymer such as polytetrafluoroethylene, with a copolymer of ethylene and tetrafluoroethylene.

[0024] The membrane can be reinforced by supporting said copolymer on a fabric such as a woven fabric or a net, a non-woven fabric or a porous film made of said polymer to be blended. The weight of the polymers for the blend or the support is not considered in the measurement of the ion exchange capacity.

[0025] When the functional groups of the cation exchange membrane for bonding to the electrode are in the form of -COOL (L is defined above), the membrane can be bonded to the electrode without any modification.

[0026] When the functional groups of the membrane are in the form of (̵COO)̵_mX ( X and m are defined above), the groups are converted into the groups in the form of -COOL (L is defined above).

[0027] The conversion of the ion exchange groups into the form of -COOL need not be carried out throughout the membrane. Only the surface layer to be bonded to the electrode need be converted, usually to a depth of less than 50p and preferably less than 30p. The method of conversion of the ion exchange groups can be selected according to the kind of groups X and L. For example, in order to convert the ion exchange groups into -COOH groups, the membrane can be brought into contact with an aqueous solution of an inorganic acid or an organic acid, preferably in the presence of a polar organic compound. The inorganic acid can be hydrochloric acid, sulfonic acid, nitric acid or phosphoric acid. The organic acid can be acetic acid, propionic acid, perfluoroacetic acid, or p-toluenesulfonic acid. The acid is usually used as an aqueous solution having a concentration of 0.5 to 90 wt.%.

[0028] The polar organic compound which may optionally be added, can be methanol, ethanol, propanol, ethyleneglycol, dimethylsulfoxide acetic acid and phenol. The polar organic acid is preferably added to the aqueous solution of the acid at a concentration of 5 to 90 wt. %. The contacting treatment of the membrane with the aqueous solution of the acid is preferably carried out at 10 to 120°C for 30 minutes to 20 hours.

[0029] When the ion exchange groups are converted into -COOL groups wherein L is a C₁ - C₂₀ alkyl group, the groups are converted into the acid form and then further converted into the ester form by reacting with the corresponding alcohol. The acid form can be also converted into the acid halide form by reacting with phosphorous trichloride or phosphorus oxychloride, and then converted into the ester form by reacting with an alcohol. The groups in the acid form can be also converted into the acid anhydride form by reacting with acetice anhydride or perfluoroacetic anhydride and then converted into the groups in'the ester form by reacting with an alcohol. If necessary, the membrane ((̵COO

X type) is treated with a chloride such as thionyl chloride, phosphorus trichloride, phosphorus oxychloride at 0 to 120°C for 1 to 25 hours so as to convert the groups (̵COO

X into the groups in the form of acid anhydride and then, is treated with an alcohol to convert the groups in the ester form. The membrane ((̵COO

X type) can be treated in an alcohol in the presence of the organic acid or the inorganic acid to convert the groups of (̵COO

X into the groups of -COOL. The alcohol used for the esterification of the acid, the acid halide or the acid anhydride is preferably a C_l - C₂₀ alcohol such as methanol, ethanol, propanol, butanol, dodecyl alcohol and sebacyl alcohol. In the esterification, the membrane can be dipped into an aqueous solution of an inorganic acid or organic acid which is the same or different from the acid used for the conversion of the groups of 4COO)mX. The dipping treatment is preferably carried out at 30 to 120°C for 30 minutes to 40 hours.

[0030] When the cation exchange membrane of a fluorinated polymer having the groups of -COOL is bonded to the electrode, it is preferable to have a specific melt-viscosity in the molten state rather than simply melting the fluorinated polymer for the membrane.

[0031] The inventors have found that the desired melt-viscosity is usually in a range of 10² to 10¹⁰ poise, preferably 10³ to 10⁹ poise. The membrane is melted in the appropriate conditions of temperature and pressure so as to give the desired melt-viscosity. When the pressure is high, the temperature can be lower. On the other hand, when the pressure is low, the temperature should be high.

[0032] When the ion exchange groups of the membrane are in the form of -COOL, the decomposition temperature of the fluorinated polymer (the temperature at which a 5% weight loss of the polymer occurs in raising a temperature at a rate of 10 C/min. in a N₂ atmosphere) is high, in a range of 350 to 370°C. Therefore, decomposition of the fluorinated polymer for the membrane does not occur in bonding the porous electrode to the membrane. Although part of the membrane intrudes into the pores on the surface of the electrode during bonding, the porous electrode is not damaged and maintains stable bonding properties for a long time, and thus also maintains a stable low cell voltage for a long time.

[0033] In the bonding process, the surface of the cation exchange membrane of a fluorinated polymer is usually heated to about 100 to 330°C, preferably about 120 to 300°C. It is enough to apply a pressure of from 0.01 to 1000 _kg/cm , preferably 1 to 300 kg/cm² to the part of the membrane to be bonded. The heating means used in the bonding step can be a press-heating device, an ultrasonic wave heating device, an impulse heating device and a friction heating device. When the membrane is in t:ie form of -COOH, it is possible to use a high frequency heating device.

[0034] In order to improve the bonding strength, it is possible to pretreat the surface of the membrane, for example by a sand-blast treatment of the bonding surface or a coating of a swelling agent or a solvent for a fluorinated polymer (-COOL type) on the bonding surface. The bonding condition is depending upon the bonding method, the kind of the fluorinated polymer of the membrane and a thickness of the membrane. For example, in the case of the impulse heating device, the bonding operation is carried out at 130 to 350°C under a pressure of 0. 1 to 300 kg/cm² for 30 seconds to 1 hour.

[0035] In the present invention, at least one of the anode and the cathode is bonded to the cation exchange membrane. As is clear, it is enough to convert the groups on the bonding surface into the groups in the form of -COOL in the case of bonding the electrode on only one surface of the membrane. The electrode bonded to the membrane should have permeability for the gas generated by the electrolysis and the electrolyte. In order to give such property, the electrode should be a porous substrate, preferably a layer having a thickness of 0. 1 to 100µ especially 1 to 50µ. In such porous electrode, the pore diameter, the porosity and the air permeability should be in the desired ranges. The electrodes as the anode and the cathode preferably have an average porosity of 0. 01 to 100µ and a porosity of 30 to 99%.

[0036] When the average pore diameter and the porosity are less than said ranges, the gas such as hydrogen and chlorine generated by the electrolysis are not easily removed from the electrode to cause high electric resistance. On the other hand, when they are more than said ranges, the electric resistance is disadvantageously large.

[0037] When the average pore diameter is in a range of 0. 1 to 50µ, and the porosity is in a range of 35 to 95%, the gas is easily removed from the electrode and the electric resistance can be small. The stable operation can be continued for a long time.

[0038] The substances for forming the porous electrodes can be as follows.

[0039] The substances suitable for the anode include platinum group metals such as Pt, Ir, Pd and Ru, alloys thereof and oxides of the platinum group metal or alloy, a heat-stabilized reducible oxide and graphite. When the platinum group metal, the alloy or the oxide of the metal or alloy is used for the anode, the cell voltage can be advantageously decreased in the electrolysis of an alkali metal chloride.

[0040] The substances suitable for the cathode include platinum group metals, alloys thereof, graphite, nickel, Raney nickel, developed Raney nickel and stainless steel and iron group metals.

[0041] When the platinum group metal or the alloy or the Raney nickel is used for the cathode, the overvoltage for forming hydrogen can be advantageously decreased in the electrolysis of water or an aqueous solution of an alkali metal chloride.

[0042] The porous electrodes can be prepared from the substances for the anode and cathode by the following processes.

[0043] The powdery substances having an average particle diameter of from 0. 01 to 100µ preferably 0. 1 to 50_fl is adhered, if necessary with a suitable binder. The binder is preferably a fluorinated polymer especially polytetrafluoroethylene. An aqueous dispersion of polytetrafluoroethylene having an average diameter of less than 1 µ is preferably used. The ratio of the binder to the powdery substrate for the electrode is preferably in a range of 0. 05 to 5 wt. parts especially 0. 1 to 3 wt. parts per 10 wt. parts of the powdery substance for the electrode. When the ratio of binder is too high, the potential of the electrode is disadvantageously high whereas when it is too low, the powdery substance for the electrode is disadvantageously separated. In the preparation of the electrodes, it is possible to incorporate a desired solvent or surfactant so as to uniformly blend the powdery substance for the electrode and the binder. It is also possible to incorporate an electric conductive filler such as graphite or a water soluble additive such as carboxymethyl cellulose and polyvinyl alcohol. The components are thoroughly mixed and deposited as a cake on a filter by a filtering method. The cake is brought into contact with the cation exchange membrane under a pressure. The mixture of the components for the electrode can be prepared in a form of a paste and the paste is coated on the cation exchange membrane. The paste can be also coated on an aluminum foil and the paste layer is brought into contact with the cation exchange membrane to form the electrode layer on the membrane. The method of forming the electrode layer on the cation exchange membrane disclosed in U. S. Patent 3, 134, 697 can be employed.

[0044] The porous electrode layer on the cation exchange membrane can be bonded on the membrane by the press-bonding machine etc. according to this invention. A part of the porous electrode layer is preferably embedded into the surface layer of the membrane. The cation exchange membrane bonded to the electrode is in the form of -COOL. The ion exchange groups in the form of -COOL is converted into the groups in the form of +COO 7=X by a suitable treatment such as hydrolysis or neutralization.

[0045] The electrolytic cell having the electrode layers and the cation exchange membrane can be a unipolar or bipolar type electrolytic cell.

[0046] As a material for the electrolytic cell, a material with is resistant to an aqueous solution of an alkali metal chloride and chlorine such as titanium is used for the anode compartment and a material which is resistant to an alkali metal hydroxide having high concentration and hydrogen such as iron, stainless steel or nickel is used for the cathode compartment in an electrolysis of an alkali metal chloride.

[0047] When the porous electrodes are used in the present invention, .a current collector for feeding the current is placed at the outside of each electrode. The current collectors usually have the same or higher overvoltage for chlorine or hydrogen in comparison with that of the electrodes. For example, the current collector at the anode side is made of a precious metal or a valve metal coated with a precious metal or oxide thereof and the current collector at the cathode side is made of nickel, stainless steel or expanded metal in a form of a mesh or a net. The current collectors are brought into contact with the porous electrodes under pressure.

[0048] In the present invention, the process condition for the electrolysis of an aqueous solution of an alkali metal chloride can be the known condition in the prior arts as British Patent 2, 009, 795.

[0049] For example, an aqueous solution of an alkali metal chloride (2. 5 to 5.0 Normal) is fed into the anode compartment and water or a dilute solution of an alkali metal hydroxide is fed into the cathode compartment and the electrolysis is preferably carried out at 80 to 120°C and a current density of 10 to 100 A/dm².

[0050] In the electrolysis, calcium ions, magnesium ions or other heavy metal ions in the aqueous solution of the alkali metal chloride cause a deterioration in the cation exchange membrane and accordingly, the content of such ions should be reduced as far as possible. In order to prevent the generation of oxygen in the anode compartment, it is advantageous to incorporate an acid such as hydrochloric acid in the aqueous solution of the alkali metal chloride.

[0051] The process for producing the alkali metal hydroxide and chlorine by electrolysis of the aqueous solution of the alkali metal chloride has been illustrated. The present invention is not limited to the embodiment described and can also be applied to the preparation of cells for electrolysis of water, or of another alkali metal salt such as sodium sulfate, and to the construction of fuel cells.

[0052] The present invention will be further illustrated by the following examples and references which are provided for purposes of illustration only.

EXAMPLE 1:

[0053] Platinum black powder was suspended in water and a dispersion of polytetrafluoroethylene (Teflon 30 J manufactured by the Du Pont Company) was added at a ratio of polytetrafluoroethylene to platinum black of 1/10 and a non-ionic surfactant (Triton X-100 manufactured by Rhom & Haas Co.) was added dropwise and the mixture was blended with an ultrasonification under cooling with ice. The mixture was sucked on a porous polytetrafluoroethylene membrane to obtain a thin layer made of platinum black (5 mg/cm²) for an anode. A thin layer made of a stabilized Raney nickel (7 mg/cm²) for a cathode was obtained by the same process.

[0054] A cation exchange membrane made of a copolymer of CF₂ ⁼ CF₂ and CF_{2 =} CFO(CF₂)₃COOCH₃ having an ion exchange capacity of 1. 45 meg/g. polymer and a thickness of 250 µ was used. Both of the electrode layers were brought into contact with each of the surfaces of the cation exchange membrane so as to be each of the porous polytetrafluoroethylene membrane at the outer surface. They were heated and pressed at 150°C under a pressure of 25 kg/cm² to bond the electrode layers to the cation exchange membrane and then, the porous polytetrafluoroethylene membranes were peeled off to obtain the cation exchange membrane bonding the electrodes.

[0055] The cation exchange membrane bonding the electrodes dipped in 25 wt. % of an aqueous solution of sodium hydroxide at 90°C for 16 hours to hydrolyze the cation exchange membrane. A nickel mesh (40 mesh) and a platinum mesh (40 mesh) as the current collectors were respectively brought into contact with the anode and the cathode under a pressure.

[0056] An electrolysis was carried out under maintaining 4 Normal of a concentration of sodium chloride in the anode compartment and maintaining 35 wt. % of a concentration of sodium hydroxide as the catholyte by feeding water into the cathode compartment.

[0057] The results are as follows.

[0058] The current efficiency for producing sodium hydroxide at a current density of 20 A/dm² was 94%.
When the electrolysis at 20 A/dm² was continued for 100 days, the cell voltage was 2.85 V and was not changed from the initiation.

REFERENCE 1:

[0059] The thin layers as the cathode and the anode were prepared by the process of Example 1. A cation exchange membrane made of a copolymer of CF_{2 =} CF₂ and CF₂ = CFO(CF₂)₃COOH₃ having an ion exchange capacity of 1.45 meg/g. polymer and a thickness of 250µ was used. Both of the electrode layers were heat-bonded on each of the surfaces of the cation exchange membrane at 200°C under a pressure of 100 kg/cm² to obtain the cation exchange membrane having the electrodes on both surfaces.

[0060] In accordance with the process of Example 1 except using the electrodes and the membrane, the electrolysis was carried out. The results are as follows.

[0061] The current efficiency for producing sodium hydroxide at a current density of 20 A/dm² was 91%.

[0062] When the electrolysis at 20 A/dm² was continued for 10 days, the electrodes were partially peeled off from the cation exchange membrane thereby being impossible to continue the electrolysis.

EXAMPLE 2:

[0063] A cation exchange membrane made of a copolymer of CF_{2 =} CF₂ and CF₂ ⁼ CFO(CF₂)₃COOCH₃ having an ion exchange capacity of 1.43 meg/g. polymer and a thickness of 240 µ was dipped into 25 wt. % of aqueous solution of sodium hydroxide at 90°C for 16 hours and then, it was dipped into 1N-HC1. at the ambient temperature for 24 hours and dried in air.

[0064] A paste A was prepared by blending 5 wt. parts of platinum black powder having a particle diameter of less than 44 µ, 0.8 wt. part of 60 wt. % of aqueous dispersion of polytetrafluoroethylene (PTFE) having a particle diameter of less than 1µ and 10 wt. parts of 1. 5 wt. % of aqueous solution of carboxymethyl cellulose. The paste A was screen-printed on one surface of the treated cation exchange membrane and the printed layer was dried in air to solidify the paste thereby forming an anode layer containing platinum black at a ratio of 2 mg/cm².

[0065] A paste B was prepared by blending 5 wt. parts of stabilized Raney nickel obtained by dissolving aluminum component from Raney nickel obtained by dissolving aluminum component from Raney nickel alloy with a base and partially oxidizing it, 10 wt. parts of an aqueous solution of 1. 5 wt.% of carboxymethyl cellulose and 0.8 wt. part of 60 wt. % of aqueous dispersion of polytetrafluoroethylene. The paste B was screen-printed on the other surface of the treated cation exchange membrane thereby forming a cathode layer containing stabilized Raney nickel at a ratio of 5 mg/cm². The printed layers were bonded to the cation exchange membrane at 165°C under a pressure of 60 kg/cm² and then, dipped into 25 wt.% of aqueous solution of sodium hydroxide at 90°C for 16 hours.

[0066] A platinum gauze (40 mesh) was brought into contact with the platinum black layer and a nickel gauze (20 mesh) was brought into contact with the stabilized Raney nickel layer under a pressure.

[0067] An electrolysis was carried out under maintaining 4 Normal of a concentration of sodium chloride in the anode compartment and maintaining 35 wt. % of a concentration of sodium hydroxide as the catholyte by feeding water into the cathode compartment.

[0068] The results are as follows.

[0069] The current efficiency for producing sodium hydroxide at a current density of 20 A/dm² was 93%.

EXAMPLE 3:

[0070] The cathode and anode thin layers were prepared by the same process as in Example 1 except that polytetrafluoroethylene was not added in the electrode layer. Both of the electrode layers which do not contain polytetrafluoroethylene as a binder were heat-bonded on each surface of the cation exchange membrane at 160°C under a pressure of 60 kg/cm². The cation exchange membrane with electrode layers on both surface was obtained. In accordance with the process and condition of Example 1, the electrolysis was carried out. The results are as follows.

The current efficiency for producing sodium hydroxide at a current density of 20 A/cm² was 92%.

Claims

1. A method of bonding an electrode to a cation exchange membrane of a fluorinated polymer which in use has ion exchange groups of formula:

wherein X represents an alkali metal atom, an alkaline earth metal atom or -NRR' in which R and R' respectively represent a hydrogen atom or a lower alkyl group; and m is the valence of the group X, characterized in that said membrane, in a form having ion exchange groups of formula:

wherein L represents a hydrogen atom or a Cl-C20 alkyl group, is melt-bonded to said electrode and the ion exchange groups of formula -COOL are then converted into the ion exchange groups of formula (̵COO)̵_mX.

2. A method according to claim 1 characterised in that said electrode is a gas and liquid permeable porous substrate having an average pore diameter of from 0.01 to 100µ, a porosity of 30 to 99% and a thickness of 0.1 to loop.

3. A method according to claim 1 or claim 2 characterised in that said electrode is a porous anode substrate obtained by binding a powder of a platinum group metal, an electrically conductive oxide thereof or a heat stabilized reduced oxide, with a binder.

4. A method according to claim 1 or 2 characterised in that said electrode is a porous cathode substrate obtained by bonding a powder of a platinum group metal, an electrically conductive oxide thereof, an iron group metal or Raney nickel, with a binder.

5. A method according to claim 3 or claim 4 characterised in that said binder is a fluorinated polymer.

6. A method according to any preceding claim characterised in that the ion exchange capacity of carboxylic acid groups of said cation exchange membrane is in a range of 0.5 to 2.5 meg/g. dry polymer.

7. A method according to any preceding claim characterised in that said cation exchange membrane is made of a fluorinated polymer having the units



wherein X represents a fluorine, chlorine or hydrogen atom or -CF₃; X' represents X or CF₃(CF₂)̵_m; m represents an integer from 1 to 5; Y represents a unit having one of the formulae:



and

x, y and z each represent an integer from 1 to 10; Z and Rf each represent -F or a C₁-C₁₀ perfluoroalkyl group; and A represents a functional group which is convertible during electrolysis into a group having the formula (̵COO)̵_mX defined in claim 1.

8. A method according to any preceding claim characterised in that said cation exchange membrane is made of a perfluoro- polymer.

9. A method according to any preceding claim characterised in that said cation exchange membrane is melt-bonded to said electrode with the surface layer having ion exchange groups in the form of -COOL in a thickness of not more than 50µ.

10. A method according to any preceding claim characterised in that the part of said cation exchange membrane at which said bonding takes place is melted at a melt-viscosity of from 10² to 10⁹ poise.

11. A method according to any preceding claim characterised in that the part of said cation exchange membrane at which said bonding takes place is heated at 100 to 330°C and is bonded under a pressure of 0.01 to 1000 kg/cm².

12. A method according to any preceding claim characterised in that said bonding is carried out by heating with a press-heating device, an ultrasonic heating device, an impulse heating device, a friction heating device or a high frequency heating device.

13. A method according to any preceding claim characterised in that said electrode is an electrode for electrolysis of an aqueous solution of an alkali metal chloride for producing an alkali metal hydroxide and chlorine.

14. A method according to any preceding claim characterised in that said electrode is formed from an electrically conductive powder and a binder and said binder is bonded to said cation exchange membrane in a form having ion exchange groups of formula -COOL as defined in claim 1.

15. A method according to claim 14 characterised in that said binder is a fluorinated polymer.