Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/34—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis
  • C25B1/46—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis in diaphragm cells
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
  • C25B9/17—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof
  • C25B9/19—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof with diaphragms
  • C25B9/23—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof with diaphragms comprising ion-exchange membranes in or on which electrode material is embedded

Definitions

• the present invention relates to a method of bonding an electrode to a cation exchange membrane.
• 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.
• diaphragm methods 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.
• 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.
• an alkali metal hydroxide having a high concentration can be produced at a low cell voltage and with high current efficiency.
• a melt-bonding method has been proposed wherein the surface of the membrane is partially melted to bond it to the electrode.
• 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)- m X 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.
• 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 C 1 -C 20 alkyl group, to the electrode and then converting the groups of formula -COOL to groups of formula ( ⁇ COO) ⁇ m X as defined above.
• 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.
• alkali metal atom as that of the alkali metal chloride to be used as the electrolyte.
• 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.
• 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 -CF 3 and X' represents X or CF 3 (CF 2 ) ⁇ m wherein m represents an integer of 1 to 5.
• 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 C 1 -C 10 perfluoroalkyl group;and A represents a functional group which is convertible to ( ⁇ COO) ⁇ m X in electrolysis.
• 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.
• 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 10 5 to 2 x 10 6 , especially 1.5 x 10 5 to 1 x 10 6 .
• 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.
• a third monomer so as to improve the membrane.
• 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.
• the thickness of the membrane is preferably 20 to 600 microns, especially 50 to 400 microns.
• 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.
• a polyolefin such as polyethylene, polypropylene or more preferably a fluorinated polymer such as polytetrafluoroethylene, with a copolymer of ethylene and tetrafluoroethylene.
• 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.
• the membrane can be bonded to the electrode without any modification.
• 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.
• the membrane in order to convert the ion exchange groups into -COOH groups, 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.%.
• 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.
• 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.
• 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.
• 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 - C 20 alcohol such as methanol, ethanol, propanol, butanol, dodecyl alcohol and sebacyl alcohol.
• 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.
• 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.
• the desired melt-viscosity is usually in a range of 10 2 to 10 10 poise, preferably 10 3 to 10 9 poise.
• the membrane is melted in the appropriate conditions of temperature and pressure so as to give the desired melt-viscosity.
• the pressure is high, the temperature can be lower.
• the pressure is low, the temperature should be high.
• 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 N 2 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.
• 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 k g/cm , preferably 1 to 300 kg/cm 2 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.
• the membrane is in t:ie form of -COOH, it is possible to use a high frequency heating device.
• the bonding condition is depending upon the bonding method, the kind of the fluorinated polymer of the membrane and a thickness of the membrane.
• the bonding operation is carried out at 130 to 350°C under a pressure of 0. 1 to 300 kg/cm 2 for 30 seconds to 1 hour.
• the anode and the cathode is bonded to the cation exchange membrane.
• the electrode bonded to the membrane should have permeability for the gas generated by the electrolysis and the electrolyte.
• the electrode should be a porous substrate, preferably a layer having a thickness of 0. 1 to 100 ⁇ especially 1 to 50 ⁇ . 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 100 ⁇ and a porosity of 30 to 99%.
• the gas such as hydrogen and chlorine generated by the electrolysis are not easily removed from the electrode to cause high electric resistance.
• the electric resistance is disadvantageously large.
• 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.
• the substances for forming the porous electrodes can be as follows.
• 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.
• 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.
• the cell voltage can be advantageously decreased in the electrolysis of an alkali metal chloride.
• 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.
• the overvoltage for forming hydrogen can be advantageously decreased in the electrolysis of water or an aqueous solution of an alkali metal chloride.
• the porous electrodes can be prepared from the substances for the anode and cathode by the following processes.
• the powdery substances having an average particle diameter of from 0. 01 to 100 ⁇ 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 ⁇ 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.
• 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.
• 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 electrolytic cell having the electrode layers and the cation exchange membrane can be a unipolar or bipolar type 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.
• the current collectors 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.
• 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.
• 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.
• 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/dm 2 .
• 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.
• 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/cm 2 ) for an anode. A thin layer made of a stabilized Raney nickel (7 mg/cm 2 ) for a cathode was obtained by the same process.
• Teflon 30 J manufactured by the Du Pont Company
• 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.
• 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.
• the current efficiency for producing sodium hydroxide at a current density of 20 A/dm 2 was 94%.
• the cell voltage was 2.85 V and was not changed from the initiation.
• the thin layers as the cathode and the anode were prepared by the process of Example 1.
• 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/cm 2 to obtain the cation exchange membrane having the electrodes on both surfaces.
• the current efficiency for producing sodium hydroxide at a current density of 20 A/dm 2 was 91%.
• a paste A was prepared by blending 5 wt. parts of platinum black powder having a particle diameter of less than 44 ⁇ , 0.8 wt. part of 60 wt. % of aqueous dispersion of polytetrafluoroethylene (PTFE) having a particle diameter of less than 1 ⁇ 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/cm 2 .
• 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/cm 2 .
• the printed layers were bonded to the cation exchange membrane at 165°C under a pressure of 60 kg/cm 2 and then, dipped into 25 wt.% of aqueous solution of sodium hydroxide at 90°C for 16 hours.
• 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.
• 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.
• the current efficiency for producing sodium hydroxide at a current density of 20 A/dm 2 was 93%.
• 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/cm 2 . 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/cm 2 was 92%.

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• 1979
  • 1979-07-30 JP JP9609779A patent/JPS5620178A/ja active Pending
• 1980
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