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
  • 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 an improved membrane-electrode structure for use in an ion exchange membrane electrolytic cell. More particularly, the invention is concerned with the use of an intermediate layer for the membrane-electrode structure of chlor- alkali electrolyzers to reduce the amount of hydrogen in chlorine. It is known to attain an electrolysis by a so called solid polymer electrolyte (SPE) type electrolysis of an alkali metal' chloride wherein a cation exchange membrane of a fluorinated polymer is bonded with a gas-liquid permeable catalytic anode on one surface and/or a gas-liquid permeable catalytic cathode on the other surface of the membrane.
• SPE solid polymer electrolyte
• This prior art electrolytic method is remarkably advantageous as an electrolysis at a lower cell voltage because the electric resistance caused by the electrolyte and the electric resistance caused by bubbles of hydrogen gas and chlorine gas generated in the electrolysis can effectively be decreased. This has been considered to be difficult to attain in the electrolysis with cells of other configurations.
• the anode and/or the cathode in the prior art electrolytic cell are bonded on the surface of the ion exchange membrane so as to be partially embedded.
• the gas and the electrolyte solution are readily permeated so as to remove, from the electrode, the gas formed by the electrolysis at the electrode Javer contacting the membrane. That is, there are few gas bubbles adhering to the membrane after they are formed.
• Such a porous electrode is usually made of a thin porous layer which is formed by uniformly mixing particles which act as an anode or a cathode with a binder.
• Perfluoro membranes which are used as membranes for electrolysis reactions usually have fairly low water contents. As compared with conventional ion exchangers with same amount of water contents, the conductivity of the perfluoro membranes are abnormally high. This is because of phase separation existing in the perfluoro ionic membranes. The phase separation greatly reduces the tortuosity for sodium ion diffusion.
• the hydrogen diffusion path is the aqueous ionic region and the amorphous fluorocarbon region. Therefore, the tortuorsity experienced by the hydrogen molecules is also low for the phase- segregated fluorocarbon membranes as compared with conventional hydrocarbon ionic membranes.
• phase-segregation characteristics of the fluorocarbon membranes provides the high migration rates for sodium ions, thus relatively lower ionic resistivity is also the cause for the high hydrogen diffusion rates and the resulting high percentage of hydrogen in chlorine.
• the high permeation rate of hydrogen is even more enhanced by the high solubility of hydrogen in the fluorocarbon membranes because of the hydrophobic interaction between hydrogen molecules and the fluorocarbon chains. Therefore, reducing hydrogen permeation rates by increasing the thickness of the membranes or modifying the structure of the membranes would not be very effective because the sodium migration rate would be reduced as one tries to reduce the hydrogen diffusion rate; and the tortuosity effect is difficult to introduce because of the phase separation.
• a retardation layer is defined as a layer between the electrode layer and the membrane to retard hydrogen permeation. Any kind of layer can have a certain effect to retard hydrogen permeation as long as it is (1) inactive for electrolytic hydrogen generation, and (2) flooded. The latter requirement is also important for low resistance (that is, lower voltage and good performance) . With these considerations a layer of a blend of inert solid particles (usually inorganic) and binders (usually organic) would serve the purpose best.
• the binder can (1) bind the components in the barrier layer together and also (2) provide the necessary adhesion between the retardation layer and the electrode layer and that between the retardation layer and the membrane.
• the function of the solid particles is also two fold: (1) providing the physical strength to the barrier layer so that there is very limited interpenetration between different layers during fabrication, and (2) forming an agglomerate with the binder.
• the retardation layer is better than the membrane itself in retarding hydrogen permeation is because (1) it allows hydroxide and sodium ions to migrate at a faster rate so relatively small voltage penalty has to be paid.
• sodium ion diffusion is slowned down by the coulombic interaction exerted by the sulfonate or carboxylate groups. The situation is even worse when the membrane is immersed in strong caustic solution as in the clor-alkali membrane. This is particularly severe for the carboxylic membranes. Ion pairing between sodium ions and carboxylate groups and hydroxide ions is believed to be the cause for the very slow diffusion rate when membrane dehydration occurs under this condition.
• the solubility of hydrogen is much lower in caustic solution than in the membrane, so the permeation rate (the product of diffusion coefficient and solubility) of hydrogen can be reduced by a larger factor compared with that of the sodium and hydroxyl ions.
• the present invention provides an improved membrane-electrode structure for use in electrochemical cells which comprises a retardation layer between an ion exchange membrane and the cathode.
• the retardation layer comprises a blend of 5 to 80 percent by weight of inorganic solid particles with 95 to 20 percent by weight of a thermoplastic polymers binder having a melting point of 230°F to 540°F (110 to 232°C) .
• the function of hte retardation layer is to provide porosity and tortuosity so as to impede hydrogen diffusion. That is, when a retardation layer with a porosity in the range of 5 percent to 90 percent is prepared, it is preferable to have a porosity in the range of 20 percent to 60 percent, preferably, in the range of 30 percent to 50 percent.
• the tortuosity/porosity ratio is in the range of 2-500, preferably in the range of 5-100, and more preferably in the range of 10-50.
• the inorganic solid particles comprise one or more of the borides, carbides and nitrides of metals of Groups IIIB, IVA, IV B, VB and VI B of the Periodic Table.
• suitable materials include SiC, YC, VC, Tic, BC, TiB, HfB, BV 2 , NbB 2 MOB , W 2 B, VN, Si 3 N 4 , Zr ⁇ 2 , NbN, BN and TiB.
• silicon carbide is used.
• the binder which is used in the invention comprises novel ion exchange polymers which can be used alone or blended with nonionic thermoplastic binders. These polymers are copolymers of the following monomer I with monomer II.
• Y is -S0 2 Z Z is -I, -Br, -Cl, -F, -OR, or -NR- ⁇ ;
• R is a branched or linear alkyl radical having from 1 to 10 carbon atoms or an aryl radical
• Rl and R 2 are independently selected from the group consisting of -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 a+b+c is not equal to O;
• X is -Cl, -Br, -F, or mixtures thereof when n>l; n is O to 6; and .
• Rf and Rf are independently selected from the group consisting of -F, -Cl, perfluoroalkyl radicals having from 1 to 10 carbon atpms and fluorochloroalkyl radicals having from 1 to 10 carbon atoms.
• the ionic binders can be mixed with a nonionic binder such as a fluorinated hydrocarbon, that is Teflon.
• At least one of the electrodes is bonded to an ion exchange membrane through a retardation layer for use in an electrolytic cell, particularly a chlor-alkali cell, so as to retard the diffusion of hydrogen.
• the composition for preparing the retardation layer is preferably in the form of a suspension of agglomerate of particles having a diameter of 0.1 to 10 microns, preferably 1 to 4 microns.
• the suspension can be formed with an organic solvent which can be easily removed by evaporation such as halogenated hydrocarbons, alkanols, ether, and the like. Preferable is Freon.
• the inorganic particles are admixed with the particles of the binder so as to comprise 5 to 80 percent by weight of total particles. A higher percentage of inorganic particles results in poor adhesion of the electrode layer to the retardation layer.
• the mixture of the inorganic particles and the binder are then suspended in an organic solvent and applied on an ion exchange membrane. The suspension can be applied by spraying, brushing, screen-printing, and the likeso as to distribute the inorganic particles substantially throughout the retardation.
• the composition is preferably heat pressed by a roller or press at 80 to 220°C under a pressure of 0.01 to 150 kg/cm 2 to bond the layer to the membrane.
• An electrode layer can then be heat pressed on the barrier layer under the same conditions.
• the retardation layer is 0.3 to 3.0 mils (0.0076 to 0.0762 mm) in thickness, preferably 0.4 to 1.0 mils (0.0102 to 0.0254 mm).
• the cation exchange membrane on which the porous non-electrode layer is formed can be made of a polymer having cation exchange groups such as carboxylic acid groups, sulfonic acid groups, phosphoric acid groups and phenolic hydroxy groups.
• Suitable polymers include copolymers of a vinyl monomer such as tetrafluoroethylene and chlorotrifluoroethylene, and a perfluorovinyl monomer having an ion- exchange group, such as a sulfonic acid group, carboxylic acid group and phosphoric acid group or a reactive group which can be converted into the ion-exchange group.
• a membrane of a polymer of trifluoroethylene in which ion exchange groups, such as sulfonic acid groups, are introduced or a polymer of styrene-divinyl benzene in which sulfonic acid groups are introduced.
• the cation exchange membrane is preferably made of a polymer having the following units:
• Y-A wherein X represents fluorine, chlorine or hydrogen atom or -CF 3 ; X' represents X or CF 3 (CH2) m ; m represents an integer of 1 to 5.
• the typical examples of Y have the structures bonding A to fluorocarbon group such as
• Z Rf x, y and z respectively represent an integer of 1 to 10; Z and Rf represent -F or a C ⁇ -C ⁇ Q perfluoroalkyl group; and A represents -COOM or S0 M, or a functional group which is convertible into -COOM or -
• a cation exchange membrane having an ion exchange group content of 0.5 to 4.0 miliequivalence/gram dry polymer, especially 0.8 to 2.0 miliequivalence/gram dry polymer, which is made of said copolymer.
• the ratio of the units (N) is preferably in a range of 1 to 40 mol percent preferably 3 to 25 mol percent.
• the cation exchange membrane used in this invention is not limited to one made of only one kind of the polymer. It is possible to use a laminated membrane made of two kinds of the polymers having lower ion exchange capacity in the cathode side, for example, having a weak acidic ion exchange group such as carboxylic acid group in the cathode side and a strong acidic ion exchange group, such as sulfonic acid group, in the anode side.
• the cation exchange membrane used in the present invention can be fabricated by blending a polyolefin, such as polyethylene, polypropylene, preferably a fluorinated polymer, such as polytetrafluoroethylene, and a copolymer of ethylene and tetrafluoroethylene.
• a polyolefin such as polyethylene, polypropylene
• fluorinated polymer such as polytetrafluoroethylene
• copolymer of ethylene and tetrafluoroethylene ethylene and tetrafluoroethylene
• the electrode used in the present invention has a lower over- voltage than that of the material of the porous non-electrode layer bonded to the ion exchange membrane.
• the anode has a lower .
• chlorine over-voltage than that of the porous layer at the anode side
• the cathode has a lower hydrogen over-voltage than that of the barrier layer at the cathode side in the case of the electrolysis of alkali metal chloride.
• the material of the electrode used depends on the material of the retardation layer bonded to the membrane.
• the anode is usually made of a platinum group metal or alloy, a conductive platinum group metal oxide or a conductive reduced oxide thereof.
• the cathode is usually a platinum group metal or alloy, a conductive platinum group metal oxide or an iron group metal or alloy or silver.
• the platinum group metal are Pt, Rh, Ru, Pd, Ir.
• the cathode is iron, cobalt, nickel, Raney nickel, stabilized Raney nickel, stainless steel, a stainless steel treated by etching with a base.
• the preferred cathodic materials for use with the retardation layer of the present invention are Ag and Ru ⁇ .
• the polymers comprising the binders of the present invention desirably have an equivalent weight within a certain desired range namely, 550 to 1200. It is possible to tailor the polymer preparation steps in a way to produce a polymer having an equivalent weight within the desired range.
• Equivalent weight is a function of the relative concentration of the reactants in the polymerization reaction.
• the polymers comprising the binders of the present invention desirably have a melt viscosity within a certain desired range. It is possible to tailor the polymer preparation steps in a way to produce a polymer having a melt viscosity within the desired range.
• the melt viscosity is based upon the concentration of the initiator and by the temperature of the reaction.
• the polymer obtained by one of the above process is then hydrolyzed in an appropriate basic solution to convert the nonionic thermoplastic form of the polymer to the ionic functional form which will have ion transport properties.
• the hydrolysis step is particularly important in the process because during the hydrolysis step the nonfunctional polymer film is heated and reacted as shown below during which process, the polymer is softened and swollen with moisture in a controlled manner. Incomplete hydrolysis leaves covalentently bonded functional groups whose lack of mobile ions lead to insulating regions within the binder.
• the density of the hydrolysis solution is preferably between 1.26 and 1.28 grams per ml at ambient temperature.
• the hydrolysis process requires two moles of NaOH for each mole of the functional group in the polymer, as shown in the following equation:
• Z is -I, -Br, -Cl, -F, -OR, or -NR ⁇ ;
• R is a branched or linear alkyl radical having from 1 to 10 carbon atoms or an aryl radical;
• R-L and Rj are independently selected from the group consisting of —H, a branched or linear alkyl radical having from 1 to 10 carbon atoms or an aryl radical, preferably phenyl or a lower alkyl substituted phenyl.
• the copolymers are placed in the hydrolysis bath at room temperature, with inert, mesh materials holding the copolymers in the liquid, making sure that there are no trapped bubbles around the films. The bath is then heated to 60°C to 90°C and then held at that temperature for a minimum of four hours to insure complete hydrolysis and expansion to the correct level.
• the bath is allowed to cool to room temperature and the polymers are then removed from the bath and rinsed with high purity deionized water, then placed in a deionized water bath to leach out residual ionic substances.
• a pore former which can be leached out after fabrication may be used in forming the retardation layer. It is advantageous that the pores formed by the pore former are interconnected and extended from the membrane-electrode interface to the electrode-catholyte interface.
• This example shows the preparation of a sulfonic fluoropolymer having an equivalent weight of 794 and a low shear melt viscosity of 50,000 poise (dyne sec-cm-2) at
• a 132 liter glass-lined reactor equipped; with an anchor agitator, H-baffle, a platinum resistance temperature device, and a temperature control jacket was charged with 527 grams of ammonium perfluorooctanoate, 398.4 grams of Na 2 HP0 4 7H 2 0, 328.8 grams NaH 2 P0 4 H 2 0 and 210.8 grams of (NH ) 2 S 2 0g.
• the reactor was then evacuated down to 0.0 atmosphere, as measured on the electronic pressure readout, and then an inert gas (nitrogen) was added to pressure up the reactor to a pressure of 448 kPa. This was done a total of 4 times, then the reactor was evacuated one more time.
• TFE tetrafluoroethylene
• the feed was stopped and then nitrogen was blown through the gas phase portion of the system and ambient temperature water was added to the reactor jacket.
• the materials react to form a latex.
• the latex was transferred to a larger vessel for separation and stripping of residual monomer. After the contents were allowed to settle, a bottom dump valve was opened to allow separate phase monomer to be drained away. The vessel was then heated and a vacuum was applied to remove any further monomer components. After this, a brine system circulates 20°C brine through cooling coils in the vessel to freeze the latex, causing coagulation into large polymer agglomerates.
• the latex was allowed to thaw with slight warming (room temperature water) and the latex was transferred into a centrifuge where it was filtered and washed repeatedly with deionized water. The latex polymer cake was then dried overnight in a rotary cone dryer under vacuum (969 Pa) at 110°C. The water content of the polymer was tested by Karl Fischer reagent and found to be 140 ppm. The isolated polymer was weighed and found to be 23,18 kg. The equivalent weight of the above polymer was determined to be 794.
• the binder can be prepared in either thermoplastic form or ionic form.
• the dried polymer was dispersed in a suitable solvent and attrited to a fine dispersion.
• To prepare it in ionic form the polymer was then hydrolyzed in an approximately 25 weight percent NaOH solution.
• the density of the hydrolysis solution was between 1.26 and 1.28 grams per ml at ambient temperature. The hydrolysis process consumed two moles of NaOH for each mole of the functional group in the polymer, as shown in the following equation:
• the polymers were placed in the hydrolysis bath at room temperature, with inert, mesh materials holding the polymers the liquid-making sure that there were no trapped bubbles.
• the bath was then heated to 60°C to 90°C and then held at that temperature for a minimum of four hours to insure complete hydrolysis and expansion to the correct level.
• Example 2 After the hydrolysis heating step, the bath was allowed to cool to room temperature and the polymers were then removed from the bath and rinsed with high purity deionized water, then placed in a deionized water bath to leach out residual ionic substances.
• Example 2 After the hydrolysis heating step, the bath was allowed to cool to room temperature and the polymers were then removed from the bath and rinsed with high purity deionized water, then placed in a deionized water bath to leach out residual ionic substances.
• Example 2 Example 2
• a suspension of particles SiC and the copolymer of Example 1 was formed in Freon at a ratio of 70:30.
• the resultant layer had a tortuosity/porosity ratio in the range of 5-50.
• the composition was sprayed onto a high performance sulfonic carboxylic bilayer ion exchange membrane of Dow. After the solvent was evaporated the composition was hot pressed at 475°F (246°C) and 0.5-100 PSI to form a layer 0.4 mil (0.0102 mm) in thickness.
• An electrode layer of Ag/Ru0 2 / binder (76 percent:10 percent:8 percent) of 1 mil (0.0254 mm) in thickness was then sprayed on top of the barrier layer. The entire unit was then heated at 400°F (204°C) and pressed together at 0.5-100 Psi .
• the electrode/retardation layer/membrane was then treated to get the final form for electrolysis. This could involve electrolysis in an appropriate solution to hydrolyze the membrane and/or the binder if either of them is in the thermoplastic form.
• the resulting structure can be used as the membrane for a chlor- alkali electrolyzer.

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