Solid Polymer Electrolyte Membranes Carrying Gas-Release Particulates

This invention relates to electrochemical cells, and more particularly to copolymeric perfluorocarbon structures utilized in such cells. More specifically, this invention relates to solid polymeric electrolytes and solid polymer electrolyte electrodes and cell structures and to methods for fabricating solid polymer electrolytes and solid polymer electrolyte electrodes and for attaching these electrodes to copolymeric perfluorocarbon membranes for use in electrochemical cells. Even more specifically, the present invention is concerned with promoting better gas release therefrom by bonding thereto a layer of finely-divided, non-conductive, inorganic particles (such as metal oxides), which bonding employs as the primary bonding agent a dissolved fluorocarbon polymer resin corresponding substantially to the hydrophilic copolymer resin in the membrane itself.

The use of a separator between an anode and cathode in electrochemical cells is known. In the past, these separators have been generally porous separators, such as asbestos diaphragms, used to separate reacting chemicals within the cell. Particularly, for example, in diaphragm chlorine generating cells, such a separator functions to restrain back migration of OH.sup.- radicals from a cell compartment containing the cathode to a cell compartment containing the anode. A restriction upon OH.sup.- back migration has been found to decrease significantly overall electric current utilization inefficiencies in operation of the cells associated with a reaction of the OH.sup.- radical at the anode releasing oxygen.

More recently separators based upon an ion exchange copolymer have found increasing application in electrochemical cells. One copolymeric ion exchange material finding particular acceptance in electrochemical cells such as chlorine generation cells has been fluorocarbon vinyl ether copolymers known generally as perfluorocarbons and marketed by E. I. duPont under the name Nafion.TM..

These so-called perfluorocarbons are generally copolymers of two monomers with one monomer being selected from a group including vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro(alkylvinyl ether), tetrafluoroethylene and mixtures thereof.

The second monomer is selected from a group of monomers usually containing an SO.sub.2 F or sulfonyl fluoride group. Examples of such second monomers can be generically represented by the formula CF.sub.2 .dbd.CFR.sub.1 SO.sub.2 F. R.sub.1 in the generic formula is a bifunctional perfluorinated radical comprising 1 to 8 carbon atoms but occasionally as many as 25 carbon atoms. One restraint upon the generic formula is a general requirement for the presence of at least one fluorine atom on the carbon atom adjacent the --SO.sub.2 F, particularly where the functional group exists as the --(SO.sub.2 NH).sub.m Q form. In this form, Q can be hydrogen or an alkali or alkaline earth metal cation and m is the valence of Q. The R.sub.1 generic formula portion can be of any suitable or conventional configuration, but it has been found preferably that the vinyl radical comonomer join the R.sub.1 group through an ether linkage.

Typical sulfonyl fluoride containing monomers are set forth in U.S. Pat. Nos. 3,282,875; 3,041,317; 3,560,568; 3,718,627 and methods of preparation of intermediate perfluorocarbon copolymers are set forth in U.S. Pat. Nos. 3,041,317; 2,393,967; 2,559,752 and 2,593,583. These perfluorocarbons generally have pendant SO.sub.2 F based functional groups.

Chlorine cells equipped with separators fabricated from perfluorocarbon copolymers have been utilized to produce a somewhat concentrated caustic product containing quite low residual salt levels. Perfluorocarbon copolymers containing perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) comonomer have found particular acceptance in Cl.sub.2 cells.

In chlorine cells using a sodium chloride brine feedstock, one drawback of using perfluorocarbon separators having pendant sulfonyl flouride based functional groups has been a relatively low resistance in desirably thin separators to back migration of caustic including OH.sup.- radicals from the cathode to the anode compartment. This back migration contributes to a lower current utilization efficiency in operating the cell since the OH.sup.- radicals react at the anode to produce oxygen. Recently, it has been found that if pendant sulfonyl fluoride based cationic exchange groups adjacent one separator surface were converted to pendant carboxylate groups, the back migration of OH.sup.- radicals in such Cl.sub.2 cells would be significantly reduced. Conversion of sulfonyl fluoride groups to carboxylate groups is discussed in U.S. Pat. No. 4,151,053.

Presently, perfluorocarbon separators are generally fabricated by forming a thin membrane-like sheet under heat and pressure from one of the intermediate copolymers previously described. The ionic exchange capability of the copolymeric membrane is then activated by saponification with a suitable or conventional compound such as a strong caustic. Generally, such membranes are between 0.5 mil and 150 mil in thickness. Reinforced perfluorocarbon membranes have been fabricated, for example, as shown in U.S. Pat. No. 3,925,135.

Notwithstanding the use of such membrane separators, a remaining electrical power inefficiency in many electrochemical cells has been associated with a voltage drop between the cell anode and cathode attributable to passage of the electrical current through one or more electrolytes separating these electrodes remotely positioned on opposite sides of the cell separator.

Recent proposals have physically sandwiched a perfluorocarbon membrane between an anode-cathode pair. The membrane in such sandwich cell construction functions as an electrolyte between the anode-cathode pair, and the term solid polymer electrolyte (SPE) cell has come to be associated with such cells, the membrane being a solid polymer electrolyte. In some of these SPE proposals, one or more of the electrodes has been a composite of a fluoro resin polymer such as Teflon.TM., E. I. duPont polytetrafluoroethylene (PTFE), with a finely divided electrocatalytic anode material or a finely divided cathode material. In others, the SPE is sandwiched between two such polymeric electrodes. Typical sandwich SPE cells are described in U.S. Pat. Nos. 4,144,301; 4,057,479; 4,056,452 and 4,039,409. SPE composite electrode cells are described in U.S. Pat. Nos. 3,297,484; 4,212,174 and 4,214,958 and in Great Britain Patent Application Nos. 2,009,788A; 2,009,792A and 2,009,795A.

Use of the composite electrodes can significantly enhance cell electrical power efficiency. However, drawbacks associated with present composite electrode configurations have complicated realization of full efficiency benefits. Composite electrodes generally are formed from blends of particulate PTFE TEFLON and a metal particulate or particulate electrocatalytic compound. The PTFE blend is generally sintered into a decal-like patch that is then applied to a perfluorocarbon membrane. Heat and pressure are applied to the decal and membrane to obtain coadherence between them. A heating process generating heat sufficient to soften the PTFE for adherence to the sheet can present a risk of heat damage to cationic exchange properties of the membrane.

These PTFE TEFLON based composites demonstrate significant hydrophobic properties that can inhibit the rate of transfer of cell chemistry through the composite to and from the electrically active component of the composite. Therefore, TEFLON content of such electrodes must be limited. Formation of a porous composite has been proposed to ameliorate the generally hydrophobic nature of the PTFE composite electrodes, but simple porosity has not been sufficient to provide results potentially available when using a hydrophilic polymer in constructing the composite electrode.

To date efforts to utilize a hydrophilic polymer such as NAFION have been largely discouraged by difficulty in forming a commercially acceptable composite electrode utilizing NAFION. While presently composites are formed by sintering particles of PTFE TEFLON until the particles coadhere, it has been found that similar sintering of NAFION can significantly dilute the desirable cationic exchange performance characteristics of NAFION polymer in resulting composite electrodes.

An analogous difficulty has surfaced in the preparation of SPE sandwiches employing more conventional electrode structures. Generally these sandwich SPE electrode assemblies have been prepared by pressing a generally rectilinear electrode into one surface of a NAFION membrane. In some instances, a second similar electrode is simultaneously or subsequently pressed into the obverse membrane surface. To avoid heat damage to the NAFION membrane, considerable pressure, often as high as 6000 psi is required to embed the electrode firmly in the membrane. Depending upon the configuration of the embedded electrode material, such pressure is often required to be applied simultaneously over the entire electrode area, requiring a press of considerable proportions when preparing a commercial scale SPE electrode.

Often where a foraminous electrode such as a mesh of titanium coated with a chlorine release electrocatalyst or a nickel mesh contacts a membrane in a cell, gases released at the electrode adhere to portions of the membrane causing a blinding effect thereby restricting cation passage therethrough. This restriction elevates the electrical voltage required for cell operation, and thereby effectively increases operational power costs.

The use of alcohols to solvate particularly low equivalent weight perfluorocarbon copolymers is known. However, as yet, proposals for formation of perfluorocarbon composite electrodes and for solvent welding the composites to perfluorocarbon membranes where the perfluorocarbons are of relatively elevated equivalent weights desirable in, for example, chlorine cells, have not proven satisfactory. Dissatisfaction has been at least partly due to a lack of suitable techniques for dispersing or solvating in part these higher equivalent weight perfluorocarbons.

A number of patents assigned to Asahi Glass describe the concept of using particulate materials for imparting better gas release characteristics to the membrane. It is believed that the closest Asahi disclosure seen in prepublished Japanese Application No. 163,287/1981 which proposes that particles of "denatured" PTFE be included along with the metal oxide particulates rather than the ordinary PTFE previously proposed, (where "denatured" means that a minor amount of fluorocarbon monomer containing acidic functional groups has been combined with PTFE in forming said particles). However, this document still does not suggest that such "denatured" PTFE should or could be in solution when used to adhere the porous layer of solid oxides, etc. to an ion-exchange membrane, and, instead, makes use of auxiliary polymeric binders (such as carboxy methyl cellulose) in solution in water and/or alcohols. This invention distinguishes in that it comprises a method(s) to improve the adhesion of such gas release particles to the membrane as well as method(s) to apply a uniform coating to a fabric-reinforced membrane.

Most preferably, this invention contemplates the particulate material to be non-electrocatalytic and to aid in the release of gas bubbles from the membrane surface, thereby reducing the cell voltage. The membrane and the polymeric portion of the solid polymer electrolyte or electrode composite are comprised principally of copolymeric perfluorocarbon such as NAFION. The membranes carrying gas-release particles of the instant invention find particular use in chlorine generation cells.

A separator made in accordance with the instant invention includes a perfluorocarbon copolymer based ion exchange membrane and one or more solid polymer electrolytes (SPE) or solid polymer electrolyte electrodes coadhered to the membrane. Coadhered SPE's can include a particulate that is non electrocatalytic forming a composite SPE. Coadhered SPE electrodes include a relatively finely divided material having desired electrode and/or electrocatalytic properties. The SPE electrode is a composite including a quantity of hydrophilic perfluorocarbon copolymeric material at least partially coating the electrode material.

An SPE having included particulates can provide enhanced gas release properties to a membrane chlor-alkali cell. The SPE electrode is a composite of a relatively finely divided conductive electrode material or substance and the copolymeric perfluorocarbon. Generally, if functioning as an anode, such a composite electrode will comprise the copolymeric perfluorocarbon and an electrocatalytic metal oxide such as an oxide of either a platinum group metal, antimony, tin, titanium, vanadium or mixtures thereof. Where functioning as a cathode, such an electrode can be comprised of a relatively finely divided material such as carbon, a group 8 metal, a group IB metal, a group IV metal, stainless steel and mixtures thereof.

In composite electrodes including finely divided metallics providing electrochemical reaction sites, it is advantageous that pores be included generally throughout the composite to provide movement of cell electrochemical reactants to and from the reaction sites. It is desirable that finely divided metallics in such porous composite be only partially coated by the copolymeric perfluorocarbon.

SPE and SPE electrode assemblies of the instant invention are prepared by providing a perfluorocarbon copolymeric membrane and coadhering at least one composite SPE or SPE electrodes to the membrane. Where more than one membrane surface is to have a coadhered SPE or SPE electrode, a composite anode of a conductive anode material and copolymeric perfluorocarbon may be attached to one membrane surface, for example, and a composite cathode of a conductive cathode material and copolymeric perfluorocarbon may be attached to the obverse membrane surface.

SPE or SPE electrode composites can be prepared and coadhered to a selected membrane by any of several interrelated methods. For composites including relatively finely divided material, copolymeric perfluorocarbon is dispersed in a solvating dispersion media, and the finely divided material is blended with the dispersion and deposited in the form of a composite. Dispersion media is removed, and the composite is coadhered to one surface of the membrane. Alternately the dispersion and at least partially dispersion coated finely divided material are applied directly upon one surface of the membrane in the form of a composite, and the dispersion media is removed. Dispersion media removal and coadherence of the composite to the membrane can be enhanced by the timely application of heat and pressure or by a leaching procedure involving a second substance in which the dispersion media is substantially miscible.

Where relatively finely divided metallic electrode material is employed in an electrode composite, it is much preferred that the composite be rendered porous. Composite porosity can be attained by including a pore precursor in preparing the copolymeric perfluorocarbon dispersion and then removing the pore precursor, such as by chemical leaching, after the dispersion media has been removed from the composite electrode. Alternately the porosity can be accomplished by depositing dispersion containing crystallized dispersion media droplets, subsequently removed.

It is preferable, where employing relatively finely divided metallic electrode material, to coat at least partially the material by dispersing it while dispersing the copolymeric perfluorocarbon and any pore precursor. Most preferably, the coating of the membrane surface comprises particulate metal oxides bonded to the membrane by a perfluorosulfonate resin. The result is better adhesion of the metal oxides which in turn lowers cell voltage.

The above and other features and advantages of the invention will become apparent from the following detailed description of the invention made with reference to the accompanying drawing which together form a part of the specification.

The generally sheet-like separator is comprised principally of copolymeric perfluorocarbon such as NAFION. The perfluorocarbon desirably should be available as an intermediate copolymer precursor which can be readily converted to a copolymer containing ion exchange sites. However, the perfluorocarbon is more generally available in sheets already converted to provide active ion exchange sites. These sites on the final copolymer provide the ion exchange functional utility of the perfluorocarbon copolymer in the separator.

The intermediate polymer is prepared from at least two monomers that include fluorine substituted sites. At least one of the monomers comes from a group that comprises vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro(alkyl vinyl ether), tetrafluoroethylene and mixtures thereof.

At least one of the monomers comes from a grouping having members with functional groups capable of imparting cationic exchange characteristics to the final copolymer. Monomers containing pendant sulfonyl, carbonyl or, in some cases phosphoric acid based functional groups are typical examples. Condensation esters, amides or salts based upon the same functional groups can also be utilized. Additionally, these second group monomers can include a functional group into which an ion exchange group can be readily introduced and would thereby include oxyacids, salts, or condensation esters of carbon, nitrogen, silicon, phophorus, sulfur, chlorine, arsenic, selenium, or tellurium.

Among the preferred families of monomers in the second grouping are sulfonyl containing monomers containing the precursor functional group SO.sub.2 F or SO.sub.3 alkyl. Examples of members of such a family can be represented by the generic formulae of CF.sub.2 =CFSO.sub.2 F and CF.sub.2 =CFR.sub.1 SO.sub.2 F where R.sub.1 is a bifunctional perfluorinated radical comprising 2 to 25, preferably 2 to 8 carbon atoms.

The particular chemical content or structure of the perfluorinated radical linking the sulfonyl group to the copolymer chain is not critical and may have F, Cl or H atoms attached to the carbon atom to which the sulfonyl group is attached, although the carbon atom to which the sulfonyl group is attached must also have at least one F attached. Preferably the monomers are perfluorinated. If the sulfonyl group is attached directly to the chain, the carbon in the chain to which it is attached must have an F atom attached to it. The R.sub.1 radical of the formula above can be either unbranched (straight chained) or branched and can have one or more ether linkages. It is preferred that the vinyl radical in this group of sulfonyl fluoride containing comonomers be joined to the R.sub.1 group through an ether linkage i.e., that the comonomer be of the formula CF.sub.2 --CFOR.sub.1 SO.sub.2 F. Illustrative of such sulfonyl fluoride containing comonomers are: ##STR1##

The corresponding esters of the aforementioned sulfonyl fluorides are equally preferred.

While the preferred intermediate copolymers are perfluorocarbon, that is perfluorinated, others can be utilized where there is a fluorine atom attached to the carbon atom to which the sulfonyl group is attached. A highly preferred copolymer is one of tetrafluoroethylene and perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) comprising between 10 and 60 weight percent, and preferably between 25 and 40 weight percent, of the latter monomers.

These perfluorinated copolymers may be prepared in any of a number of well-known manners such as is shown and described in U.S. Pat. Nos. 3,041,317; 2,393,967; 2,559,752 and 2,593,583.

An intermediate copolymer is readily transformed into copolymer containing ion exchange sites by conversion of the sulfonyl groups (--SO.sub.2 F or --SO.sub.3 alkyl) to the form --SO.sub.3 Z by saponification or the like wherein Z is hydrogen, an alkali metal, a quaternary ammonium ion, or an alkaline earth metal. The converted copolymer contains sulfonyl group based ion exchange sites contained in side chains of the copolymer and attached to carbon atoms having at least one attached fluorine atom. Not all sulfonyl groups within the intermediate copolymer need be converted. The conversion may be accomplished in any suitable or customary manner such as is shown in U.S. Pat. Nos. 3,770,547 and 3,784,399.

A separator made from copolymeric perfluorocarbon having sulfonyl based cation exchange functional groups possesses a relatively low resistance to back migration of sodium hydroxide from the cathode to the anode, although such a membrane successfully resists back migration of other caustic compounds such as KOH. A pattern of fluid circulation in the cell zone adjacent the cathode contributes to a dilution in concentration of sodium hydroxide within and adjacent to the cathode and adjacent the membrane, thus reducing a concentration gradient driving force tending to contribute to sodium hydroxide back migration.

In a mode for carrying out the invention, the separator includes a zone having copolymeric perfluorocarbon containing pendant sulfonyl based ion exchange functional groups and a second zone having copolymeric perfluorocarbon containing pendant carbonyl based functional ion exchange groups. The pendant carbonyl based groups provide the copolymeric perfluorocarbon with significantly greater resistance to the back migration of sodium hydroxide, but can also substantially reduce the rate of migration of sodium ions from the anode to the cathode. In order to present a relatively small additional resistance to the desired migration of sodium ions, the carbonyl based zone, usually is provided to be only of sufficient dimension to produce a significant effect upon the back migration of sodium hydroxide.

Alternately said second zone can contain perfluorocarbon containing sulfonamide functionality of the form --R.sub.1 SO.sub.2 NHR.sub.2 where R.sub.2 can be hydrogen, alkyl, substituted alkyl, aromatic or cyclic hydrocarbon. Methods for providing sulfonamide based ion exchange membranes are shown in U.S. Pat. Nos. 3,969,285 and 4,113,585.

Copolymeric perfluorocarbon having pendant carboxylate cationic exchange functional groups can be prepared in any suitable or conventional manner such as in accordance with U.S. Pat. No. 4,151,053 or Japanese Patent Application No. 52(1977)38486 or polymerized from a carbonyl functional group containing monomer derived from a sulfonyl group containing monomer by a method such as is shown in U.S. Pat. No. 4,151,053. Preferred carbonyl containing monomers include

Preferred copolymeric perfluorocarbons utilized in the instant invention therefore include carbonyl and/or sulfonyl based groups represented by the formula

wherein X is sulfonyl fluoride (SO.sub.2 F) carbonyl fluoride (COF) sulfonate methyl ester (SO.sub.2 OCH.sub.3) carboxylate methyl ester (COOCH.sub.3) ionic carboxylate (COO.sup.- Z.sup.+) or ionic sulfonate (SO.sub.3.sup.- Z.sup.+), Y is sulfonyl (--SO.sub.2 --) or carbonyl (--CO--), and Z is hydrogen, an alkali metal such as lithium, cesium, rubidium, potassium and sodium, an alkaline earth metal such as beryllium, magnesium, calcium, strontium, barium and radium, or a quaternary ammonium ion.

Generally, sulfonyl, carbonyl, sulfonate and carboxylate esters and sulfonyl and carbonyl based amide forms of the perfluorocarbon copolymer are readily converted to a salt form by treatment with a strong alkali such as NaOH.

The carbonyl zone where used in cell having foraminous electrodes can contain a particulate such as an oxide of a valve metal. Particularly the oxides of titanium and zirconium have been found to aide in release from the surface of the zone of gases being evolved from the foraminous electrode, particularly where that foraminous electrode is situated in close proximity to the membrane or contacts the membrane directly. Gas release functions to "unblind" membrane surface, thus reducing restriction to the flow of cations through the membrane. The zone thereby functions as an SPE between the electrode and the remaining membrane material, this SPE containing a non-electrolytic particulate.

An SPE or SPE electrode assembly is made in accordance with the instant invention by first providing a copolymeric perfluorocarbon membrane. The membrane can include members of one or more of the ion exchange functional groups discussed previously, depending upon the nature of chemical reactants in the electrochemical cell. Blending of polymers containing different ion exchange functional groups is an available alternate. When chlorine is to be generated from sodium chloride brine, it has been found advantageous to employ copolymer containing pendant sulfonyl based groups throughout most of the membrane and a similar copolymer, but containing pendant carbonyl based groups adjacent what is to be the cathode facing membrane surface which can be attached as an SPE in accordance herewith.

The membrane can be formed by any suitable or conventional means such as by extrusion, calendering, solution coating or the like. It may be advantageous to employ a reinforcing framework within the copolymeric material. This framework can be of any suitable or conventional nature such as TEFLON mesh or the like. Layers of copolymer containing differing pendant functional groups can be laminated under heat and pressure in well-known processes to produce a membrane having desired functional group properties at each membrane surface. Alternately a bifunctional group membrane can be provided in accordance with SPE forming techniques of the invention. For chlorine cells, such membranes have a thickness generally of between 1 mil and 150 mils with a preferable range of from 4 mils to 10 mils.

The equivalent weight range of the copolymer intermediate used in preparing the membrane as well as any SPE or SPE electrode is important. Where lower equivalent weight intermediate copolymers are utilized, the membrane can be subject to destructive attack such as by dissolution by cell chemistry. When an excessively elevated equivalent weight copolymer intermediate is utilized, the membrane may not pass cations sufficiently readily, resulting in an unacceptably high electrical resistance in operating the cell. It has been found that copolymer intermediate equivalent weights should preferably range between about 1000 and 1500 for the sulfonyl based membrane materials and between about 900 and 1500 for the carbonyl based membrane materials.

For an SPE electrode, an electrode substance is selected for compositing with perfluorocarbon copolymers. When the resulting composite electrode is to be an anode, this substance will generally include elements or compounds having electrocatalytic properties. Particularly useful are oxides of either platinum group metals, antimony, tin, titanium, vanadium, cobalt or mixtures thereof. Also useful are platinum group metals, silver and gold. The platinum group includes platinum, palladium, rhodium, iridium, osmium, and ruthenium.

The electrocatalytic anode substance is relatively finely divided, and where relatively finely divided, it may be combined with conductive extenders such as carbon or with relatively finely divided well-known valve metals such as titanium or their oxides. Oxides of the valve metals, titanium, aluminum, zirconium, bismuth, tungsten, tantalum, niobium and mixtures and alloys thereof can also be used.

When the composited electrode is to be a cathode, the active or conductive electrode substance is selected from a group comprising group IB metals, group IVA metals, a group 8 metal, carbon, any suitable or conventional stainless steel, the valve metals, platinum group metal oxides or mixtures thereof. Group IB metals are copper, silver and gold. Group IVA metals are tin and lead. Group 8 metals are iron, cobalt, nickel, and the platinum group metals. As with the anode, these active electrode substances are relatively finely divided.

Where the composite is to be an SPE having an entrained gas release particulate, the particulate is generally a valve metal oxide such as titanium or zirconium oxide or a suitable or conventional gas release particulate such as oxides, hydroxides, nitrides, or carbides of Ti, Zr, Nb, Ta, V, Mn, Mo, Sn, Sb, W, Bi, In, Co, Ni, Be, Al, Cr, Fe, Ga, Ge, Se, Y, Ag, Hf, Pb, or Th.

By relatively finely divided what is meant is the gas release particles are of a size of about 3.0 millimeters by 3.0 millimeters by 3.0 millimeters or smaller in at least one dimension. Particularly particles having at least one dimension considerably larger than the other have been found effective such as particles having dimensions of 1.0 millimeter by 1.4 millimeters by 0.025 millimeters. More preferred are particles having an overall size range of 0.1 to 50 microns in diameter, and most preferred is an average equivalent particle size diameter of not substantially more than about 1 micron. Also preferred are fibers having a diameter of between about 0.025 millimeter and about 1.0 millimeter and between about 1.0 millimeter and 50 millimeter in length.

Perfluorocarbon copolymer is dispersed in any suitable or conventional manner. Preferably relatively finely divided particles of the copolymer are used to form the dispersion. The particles are dispersed in a dispersion medium that preferably has significant capability for solvating the perfluorocarbon copolymer particles. A variety of solvents have been discovered for use as a dispersion medium for the perfluorocarbon copolymer; these suitable solvents are tabulated in Table I and coordinated with the copolymer pendant functional groups with which thay have been found to be an effective dispersion medium. Since these dispersing solvents function effectively alone or in mixtures of more than one, the term dispersion media is used to indicate a suitable or conventional solvating dispersing agent including at least one solvating medium.

Certain of the solvating dispersion media function more effectively with perfluorocarbon having particular metal ions associated with the functional group. For example, N-butylacetamide functions well with the groups COOLi and SO.sub.3 Ca. Sulfolane and N,N-dipropylacetamide function well with SO.sub.3 Na functionality.

It is believed that other suitable or conventional perhalogenated compounds can be used for at least partially solvating SO.sub.2 F or carboxylate ester forms of perfluorocarbon copolymer. It is believed that other suitable or conventional strongly polar compounds can be used for solvating the ionic sulfonate and carboxylate forms of perfluorocarbon polymer.

A composite electrode is formed by blending the conductive electrode materials with the dispersion. The blended dispersion is deposited, and the dispersion media is removed. Relatively finely divided electrode material remains at least partially coated sufficient to assure coadherence between the particles. Preferably this coating of finely divided electrode material is accomplished simultaneously with dispersion of the copolymeric perfluorocarbon.

In at least partially solvating the perfluorocarbon polymers, it is frequently found necessary to heat a blend of the dispersion media and the relatively finely divided perfluorocarbon to a temperature between about 50.degree. C. and 250.degree. C., but not in excess of the boiling point for the resulting dispersion. Depending upon the solvent, a solution of between about 5 and 25 weight percent results. It is not necessary that the perfluorocarbon be dissolved completely in order to form a suitable electrode composite. It is important that undissolved perfluorocarbon be in relatively small particles to avoid isolating relatively large amounts of the conductive electrode material within groupings of larger perfluorocarbon particles. One preferred technique comprises heating the dispersion to at least approach complete solvation and then cooling the dispersion to form a gelatinous dispersion having particles of approximately a desired size. The cooled temperature will vary with the solvent selected. The particle size is controllable using either of mechanical or ultrasonic disruption of the gelatinous dispersion.

Referring to Table I, it may be seen that various solvents have a particularly favorable effect upon only perfluorocarbon copolymers having certain functional groups. Where a composite electrode containing perfluorocarbon having functional groups of a first type is to be at least partially solvent welded to a perfluorocarbon membrane having functional groups of a second type, conversion of one or both types of functional groups may be necessary to achieve solvent compatibility. Particularly, hydrolysis and substitution of metal ions ionically bonded to the functional group can provide a relatively simple tool for coordinating functional groups and solvents. However, other methods such as the use of SF.sub.4 to reform sulfonyl fluoride functional groups from derivatives of sulfonyl fluoride are also available.

The composite of the dispersion and the conductive electrode material are deposited as a sheet-like SPE electrode. This SPE electrode sheet generally has a length and breadth of considerably greater dimension than its thickness. Upon removal of the dispersion media, the SPE electrodes comprise composite SPE electrodes of the perflourocarbon copolymer and the conductive electrode material applied to the separator. Dispersion media removal can be accompanied by heating, vacuum, or both, with temperatures of between 80.degree. C. and 250.degree. C. being preferred. Alternately dispersion media can be extracted using a leaching agent substantially miscible in the dispersion media.

The dispersion, including the coated electrode material, can be deposited separately from the membrane, and subsequently the resulting composite SPE electrode attached or coadhered to the membrane. Alternately the dispersion can be deposited directly upon the membrane. In either alternative, after forming into an SPE electrode sheet, removal of most or all of the dispersion media is effected.

Where the SPE electrode sheet has been deposited separately from the membrane, upon removal of at least most of the dispersion media, the resulting composite SPE electrode can be heated gently and pressed into the membrane until firmly coadhering thereto. Generally a temperature of between 50.degree. C. and 250.degree. C. accompanied by application of between about 500 and 4000 pounds per square inch pressure will suffice to coadhere the composite SPE electrode and the membrane. Where relatively finely divided metallic electrode material has been utilized in preparing the SPE electrode, the pressure need not be applied simultaneous over the entire SPE electrode to effectuate coadherence, but bubbles should be avoided.

From time to time a partially solvating dispersion media compatible with the perfluorocarbon copolymer used in preparation of the composite SPE electrode is also compatible with the perfluorocarbon copolymer present at the surface of te separator to which the composite SPE electrode is to be coadhered or to surfaces where functional groups can be readily modified to be compatible. Composite SPE electrodes prepared using this dually compatible dispersion media can be deposited directly upon the separator surface and the dispersion media removed by suitable or conventional methods. Prior to removal, the solvating dispersion media promotes coadherence between the perfluorocarbon copolymeric composite SPE electrode and the perfluorocarbon copolymeric separator. Exposure to heat within 50.degree. C. and 250.degree. C. and/or pressure between 500 to 4000 pounds enhances this coadherence when the heat and/or pressure are applied either simultaneous to or subsequent to removal of the dispersion media. Where solvent compatibility does not exist, direct deposition upon the membrane is possible, but heat and pressure will be required for coadherence.

When using a relatively finely divided metallic electrode material in preparing a composite SPE electrode, it is preferable to include a plurality of pores in the final composite SPE electrode to facilitate movement of cell chemicals such as brine, caustic, and gaseous chlorine or hydrogen to and from the conductive electrode material. Such pores can be crearted by the inclusion of a pore presursor in the dispersion of copolymeric perfluorocarbon prior to deposition of the dispersion. Subsequent to removal of the dispersion media, the pore precursor is removed from the SPE electrode in any suitable or conventional manner such as by immersing a completed SPE electrode in a solution capable of solvating the pore precursor without damaging the perfluorocarbon copolymer or the metallic electrode material of the composite.

In one alternate of the above embodiment for producing chlorine from sodium chloride brine, the metallic electrode material for the SPE anode is relatively finely divided ruthenium oxide and the metallic electrode material for the SPE cathode is comprised of relatively finely divided platinum and carbon. In such composite SPE electrodes, the pore precursor included in the dispersion can be zinc oxide. Advantageously, the zinc oxide pore precursor can be removed from completed SPE electrodes either before or after coadherence to the membrane. Removal of the pore precursor is effected with a strongly alkaline substance such as caustic, KOH or the like. The strongly alkaline solution also performs to hydrolyze sulfonyl fluoride and methyl carboxylate pendant functional groups in intermediate copolymeric perfluorocarbon to active ion exchange sites. Hydrolysis readies the perfluorocarbon for use in the electrochemical cell.

In an equally preferred alternate, certain solvents can be used to provide pores within the SPE electrode. Particularly, perfluorooctanoic and perfluorodecanoic acids are available to form pores. After dissolution or partial dissolution of perfluorocarbon in these solvents at elevated temperatures, the solution is cooled until a gel begins to form. As the gel forms, syneresis of excess dispersion media occurs from the gel. As cooling continues, these synerizing solvents form droplets within the gel which crystallize. After deposition of the SPE electrode, the deposited SPE electrode is hydrolyzed by saponification with strond caustic or the like. Crystallized droplets are then extracted using a compatible solvent such as FREON 113 or the like to produce the pores. Using a leaching agent like FREON 113 both crystallized and noncrystallized dispersion media can equally be extracted cocurrently. Advantageously, these crystallized droplets tend to migrate to the surface leaving tracks enhancing porosity. Alternatively the crystallized solvent can be sublimed at a temperature below its melting point.

A membrane having an entrained gas release particulate is fabricated in a like manner except using the appropriate gas release particulate in formulating the dispersion. SPE's containing this entrained gas release particulate exhibit far less chalking and sloughing of the particulate than do SPE's formed by pressing of the particulate into the perfluorocarbon membrane.

Particularly for membranes having a fabric reinforcing mesh, the surface of the membrane often resembles a dimpled or checkerboard surface of ridges and valleys. Formation of a separate SPE sheet and subsequent pressing onto the membrane of the separate SPE sheet can avoid pooling of dispersion in the checkerboard surface of the membrane that would produce substantial variation in thickness of the SPE layer. Pressing preferably is accomplished here using a resilient, relatively readily compressible backing between press and SPE to assist in conforming the SPE to contours of the membrane surface. A fibrous board functions well for this surface and materials subject to cold flowing are preferably avoided as a backing material for this service.

The SPE particulate dispersion can also be sprayed upon the membrane using added diluents having a relatively low boiling point so that they may be at least partially removed to thicken the dispersion upon the membrane to forestall drips, sags, and the like.

Experiments have been carried out to coat both surfaces of a commercially-available fabric reinforced membrane (Nafion.TM. 910 or DSXM-9) with particulate TiO.sub.2 or ZrO.sub.2 using sulfolane dispersions of Nafion.TM. lithium perfluorosulfonate resin (NafOLi, 1100 equivalent weight) as the binder or adhesive. Several examples of formulations and coating methods are listed in examples I-VIII. A significant feature of this invention which is illustrated by these examples is the excellent adhesion of the particulate layer to the perfluorocarbon membrane. In the absence of perfluorocarbon binder, the coating is chalky to the touch even after the coating/membrane composite is compressed under elevated temperature and pressure. When the coating contains as little as 10% Nafion.RTM..TM. binder by weight, the coating exhibits excellent mechanical integrity, particularly when pressed into the membrane at elevated temperature and pressure. In comparison, experimental gas-release coated membranes as supplied by DuPont (designated DSXM-32) have characteristically "chalked" easily and lost a significant proportion of coating material during operation at 3.1 KA/M.sup.2 in laboratory chlor-alkali cells. No evidence of coating loss was observed for a TiO.sub.2 /NafOLi coated DSXM-9 membrane of this invention which was operated for one week under similar conditions.

Most preferably, the TiO.sub.2 or ZrO.sub.2 coating is intended to render the membrane surface more hydrophilic, lowering the tendency of gas bubbles to reside on the surface. An experiment was devised to measure the surface properties of coated versus non-coated membranes. Using Nafion 117 films as coating substrates, the contact angle between a fluorinated liquid (3M Co's Fluorinert.TM. FC-70) and the sample film submerged in water was measured using a Rame-Hart NRL C.A. Goiniometer (model 100). This experiment measures the hydrophobicity of the sample surface, where larger contact angles indicate a higher degree of hydrophobicity (and a lower degree of hydrophilicity). The results of these experiments are listed below:

The coated membranes are shown to be significantly more hudrophilic than the non-coated control. Samples 5 and 6 demonstrate that no significant change in contact angle is observed after the sample surface is sanded (to remove the possibility of perfluorocarbon encapsulation of the TiO.sub.2 particles). Therefore, the coated membranes of this invention demonstrate the desired properties for improved gas release.

Three Nafion.TM. 901 membranes were operated in a 20 cm.sup.2 laboratory chlor-alkali cell with 3 mm membrane-cathode gap, a zero membrane-anode gap, a DSA.TM. anode, and a nickel cathode at 3.1 KA/M.sup.2 and 90.degree. C., while producing 400.+-.10 g/1 NaOH. The average cell voltage was 3.39. A ZrO.sub.2 /NafOLi surface modified Nafion 901 membrane (12016-119-2, see example VIII) operated at a cell voltage of 3.34 volts under the same conditions resulting in a 50 mV savings. Cathode current efficiency was not affected by the surface coating, but the voltage/current density slope was reduced from 310 mV/KAM.sup.-2 for the controls to 200 mV/KAM.sup.2 for sample 12016-119-2.

The deposition of a uniform coating over the surface of a fabric-reinforced membrane such as Nafion 901 is a difficult problem. Fabric reinforced membranes are preferred for use in commercial chlor-alkali installations by virtue of superior tear strength and resistance to puncture and rough handling. The strands of the fabric are encapsulated by fluoropolymer and produce ridges which leave a raised pattern on the catholyte surface of the membrane. Direct coating of any paste-like formulation on this irregular surface results in a coating which is thicker in the depressed areas and thinner along the raised areas. A spray-on application can be used to partially overcome these difficulties; but shadowing effects, spotty coverage, and pooling may occur depending upon the viscosity, surface tension and drying characteristics of the coating formulation. An improved method for applying a uniform coating on reinforced membranes is described herein and forms an integral part of this invention. The coating is first uniformly applied to the surface of a flat sacrificial substrate such as aluminum foil or cellulose acetate by standard techniques which are well-known in the art such as direct transfer coating, indirect transfer coating, knife coating, or screen printing. After the coating is substantially dry, it is placed in contact with the membrane and pressed under conditions of high temperature and pressure with a deformable material placed on either side of the membrane/coating laminate.

The deformable material serves to distribute pressure evenly across the irregular membrane surface by compressing preferentially at regions of high stress corresponding to ridges while compressing to a less degree in regions corresponding to low points on the membrane surface. An ideal deformable material is highly compressible in the thickness direction but does not expand in directions parallel to the plane of the sheet during compression. A suitable deformable materials has been found to be fiber board which is inexpensive and readily available. After the pressing operation, it has been observed that (1) the pattern of the reinforcing fabric has been embossed on the fiber board, (2) the coating has evenly draped over the irregularities of the membrane surface and (3) the membrane has not been physically distorted by this process. Adhesion of the coating to the membrane under these conditions has been found to be excellent.

The following examples are offered to illustrate various aspects of the invention.

5.0 grams of duPont NAFION 511 having an equivalent weight of 1100 and having a pendant functionality comprising RSO.sub.3 Li was dispersed in SULFOLANE to form a 10% by weight dispersion. 4.5 grams of titanium dioxide (duPont R-101, 03 micron, dried for 16 hours at 50.degree. C.) was added to the dispersion which was then agitated at high speed for 5-10 minutes. The resulting dispersion was cast on a 1 mil thickness of aluminum foil using a Gardner knife.

The SULFOLANE was then partially removed using radiant heat and the resulting sheet SPE was dried in forced air at 130.degree. C. for 24 hours. A 1 mil thick perfluorocarbon casting having entrained titanium dioxide resulted. The SPE was press laminated to a sheet of NAFION 117 film having pendant functional groups of the form RSO.sub.3 Li using a PASEDENA at 2,000 pounds per square inch.

The aluminum foil was then dissolved from the SPE in 150 gram per liter NaOH to leave a membrane having an attached solid polymer electrolyte (SPE) of a thickness of between 0.5 and 0.75 mils.

A dispersion was prepared in accordance with Example I. The dispersion was sprayed using an air sprayer onto four substrates: a 1 mil thickness of aluminum foil; a 1 mil thickness of anodized aluminum foil; a sheet of cellophane; and a mesh reinforced perfluorocarbon copolymer membrane (duPont Nafion 901), the membrane perfluorocarbon having pendant RSO.sub.3 Li and RCO.sub.2 Li pendant functionality, and being approximately 10 mils in thickness. The applied dispersions were force air dried at 130.degree. C. for 16-24 hours to yield solid polymer electrolytes. The SPE's applied to aluminum foil were transferred to membranes in accordance with Example I, producing substantially similar results. Likewise, the SPE applied to cellophane was transferred to a membrane in accordance with Example I excepting the cellophane being peeled away from the SPE subsequent to the transfer operation. When pressing these SPE's to their rrespective membranes at 2,000 pounds per square inch, a section of cardboard was introduced between each press platen and the SPE's. These SPE's applied to the reinforced perfluorocarbon copolymeric membrane were found to be tightly adhered.

DuPont R-101 titanium dioxide powder was sprinkled on to a perfluorocarbon copolymeric film (NAFION 115) and then pressed into the perfluorocarbon copolymeric film using a hydraulic flat press. Pressing was conducted at 350.degree. F. at 4,000 pounds per square inch for 30 minutes; and upon completion of pressing substantial sloughing of TiO.sub.2 powder from the surface of the membrane was observed, leaving a chalky membrane surface. From observation it was readily apparent that titanium dioxide powder applied in accordance with Examples VI and VII was substantially better adhered to a membrane than when applied in accordance with this example.

Nine parts of titanium dioxide 3 micron powder, 10 parts of a 10 weight percent dispersion of the perfluorocarbon of Example I in SULFOLANE, and 21 parts of isopropanol were blended at high speed. The resulting dispersion was poured into a glass dish and swirled to cover the bottom evenly. A foam rubber roller was rolled in the dish to achieve uniform coverage on the roller and then passed several times across a sheet of aluminum foil to produce a uniform thin coating. The coating on the foil sheet was then dried in a forced air oven at 150.degree. C. for 118 hours. Two 5 inch.times.5 inch squares were cut from the solid polymer electrolyte that resulted. These squares were laminated to 4 inch.times.4 inch pieces of mesh reinforced perfluorocarbon copolymeric 10 mil film (duPont Nafion 901) by hydraulic pressing at 350.degree. F. at 3,000 pounds per square inch for 30 minutes using a sheet of aluminum foil covered cardboard between the press plate and the SPE being pressed into the film. A membrane having a tightly adhered SPE resulted.

The method of Example IV was repeated using zirconium oxide (available from Fisher Scientific) with substantially identical results.

The method of Example IV was repeated except that the ratio of the dispersion components was changed to include 9 parts zirconium oxide, 10 parts of the 10 weight percent dispersion of the perfluorocarbon coploymer of Example I in SULFOLANE and 81 parts isopropanol. The resulting SPE had a substantially similar appearance to that of Example V excepting that the resulting SPE was slightly thicker.

The dispersion of titanium dioxide, perfluorocarbon copolymer in SULFOLANE, and isopropanol of Example VI was rolled directly onto a mesh reinforced perfluorocarbon copolymeric film (duPont Nafion 901) of approximately 10 mils in thickness. Coating was accomplished by resting a 4 inch by 4 inch piece of the reinforced membrane on a vacuum assisted table with the surface having pendant sulfonate functionality facing up. The roller was passed three times over the surface of the film giving a thin uniform coating which dried quite quickly. The film was then flipped over and the ridged side wherein the pattern of the reinforcing mesh could be clearly distinguished was similarly coated. The film was then dried in a forced air oven at 150.degree. C. for 18 hours and then pressed at 350.degree. F. and 3,000 pounds per square inch for 30 minutes with a piece of aluminum foil covered cardboard being interposed between press plates and the coated reinforced perfluorocarbon copolymeric film. A smooth, uniform and thin SPE resulted tightly bonded to the copolymeric membrane.

The method of Example VII was repeated except using zirconium oxide in lieu of titanium dioxide. After pressing the resulting coadhered SPE was substantially the same as that of Example VII.

While a preferred embodiment of the invention has been described in detail, it will be apparent that various modifications or alterations may be made therein without departing from the spirit and scope of the invention as set forth in the appended claims.