CA1185126A

CA1185126A - Electrolytic cell membrane/spe formation by solution coating

Info

Publication number: CA1185126A
Application number: CA000403654A
Authority: CA
Inventors: Michael J. Covitch; Mark F. Smith; Leo L. Benezra
Original assignee: Eltech Systems Corp
Current assignee: Eltech Systems Corp
Priority date: 1981-06-26
Filing date: 1982-05-25
Publication date: 1985-04-09
Also published as: JPS586985A; GB2101160A; DE3223830A1; US4386987A

Abstract

ELECTROLYTIC CELL MEMBRANE/SPE
FORMATION BY SOLUTION COATING

ABSTRACT

A method for forming a membrane upon an electrode from a dispersed, perfluorocarbon copolymer. Perfluorocarbon is dispersed in a solvating medium, a substantial portion, but not necessarily all of the perfluoro-carbon being solvated. The dispersion is applied to an electrode and the dispersion medium is removed.

Description

5~

ELECTROLYTIC CELL MEMBRANE/SPE
FORMATION BY SOLVTION COATING

FIELD OF THE INVENTION

This invention relates to batteries, fuel cells and electrochemical 5 cells, and particularly to separators utilized in such cells. More specifically, this inven~ion relates to solid polymeric electrolyte cell separators, polymeric cellmembranes and methods for fabricating and attaching electrodes to such solid polymeric electrolytes and polymeric membranes for use in electrochemical cells.

BACKGROUND OF THE INVENTION

The use of a separator between an anode and cathode in batteries, fuel cells, and electrochemical cells is knownO In the past, these separators have been generally porous separators, such as asbestos diaphragms, used to separate reacting chemistry within the cell. Particularly, for example9 in diaphragm 15 chlorine generating cells, such a separator functions to restrain back migration of OH radicals from a cell compartment containing the cathode to a cell compartment containing the anode. A restriction upon OH back migration has been found to significantly decrease current inefficiencies associated with a reaction of the OH radical at the anode releasing oxyOen.
More recently separators based upon an ion exchange copolymer have found increasing application in batteries, fuel cells, and elec~rochemical cells.
One copolymeric ion exchange material finding particular acceptance in electro-chemical cells such as chlorine generation cells has been fluorocarbon vinyl ether copolymers known generally as perfluorocarbons and marketed by E. 1. duPont - 25 under the name NAFION ~.

,, `~

- 2 -These so-called perfluorocarbons are generally copolymers of two monomers with one monomer being selected frorn a group including vinyl fluoride! hexafluoropropylene, vinylidene fluoride, trifluoroethylene, chlorotri-fluoroethylene, perfluoro~alkylvinyl ether), tetrafluoroethylene and mixtures 5 thereof.
The second monomer is selected from a group of monomers containing an SO2F or sulfonyl fluoride group. Examples of such second monomers can be generically represented by the formula CF2=CFRlSO2F Rl in the generic formula is a bifunctional perfluorinated radical comprising one to 10 eight 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 -S021~, particularly where the -S02F group exists as the -(-SO2NH)mQ form. In this form, Q can be hydrogen or an alkali or alkaline earth metal cation and m is the valence of Q. The Rl generic for~ula portion 15 can be of any suitable or conventional configuration, but it has been founcl preferably that the vinyl radical comonomer join the Rl group through an ether linkage.
Typical sulfonyl fluoride containing monomers are set for~h in U.S.
Patent Nos. 3,282,875; 3,041,317; 3,560,568; 3,718,627 and methods of 20 preparation of intermediate perfluorocarbon copolymers are set forth in U.S~
Patent Nos. 3,041,317; 2,393,967; 2,559,752 and 2,593,583. These perfluoro-carbons generally have pendant SO2F based functional groups.
Chlorine cells equipped with separators fabricated from perfluoro-carbon copolymers have been utilized to produce a somewhat concentrated 25 caustic product containing quite low residual salt levels. Perfluorocarbon copolymers made from perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) comonomer have found particular acceptance in C12 cells.
Many chlorine cells use a sodium chloride brine feedstock. One draw-back to the use in such cells of perfluorocarbon separators having pendant 30 sulfonyl fluoride oased functional groups has been a relatively low resistance in desirably thin separators to back migration of caustic formed in these cells, including OH 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 radicals react at the anode to produce oxygen. Recently, it 35 has been found that if pendant sulfonyl fluoride based cationic exchange groups adjacent one separator surface were converted to pendant carbonyl groups, -the r`

back migration of OH radicals in such ~12 cells would be significantly reduced.
Conversion of sulfonyl fluoride ~roups to carboxylate groups is discussed in U.S.
Patent No. 4,151,053.
Presently, perfluorocarbon separators are generally fabricated by 5 forming a thin membrane-like sheet under heat and pressure from one of the interrned;ate 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. Patent No.

3,925,135.
Notwithstanding the use of such membrane separators, a remaining electrical power inefficiency in many batteries, fuel cells and electrochemical cells has been associated with a voltage drop between the cell anode and cathodeattributable to passage of the electrical current through one or more electro-lytes 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 theterm solid polymer electrolyte ~SPE~ cell has come to be associated with such cells, the membrane being a solid polymer electrolyte. Typical sandwich SPE
cells are described in U.S. Patent Nos. ~,144,301; 4,G57,479; 4,056,4S2 and

4,039,4û9.
At least one difficul~y has surfaced in the preparation of SPE
sandwiches employing reticulate electrode structures. Generally these sandwich SPE electrode assemblies have been prepared by pressing a generally rectilinear electrode into one surface of a perfluorocarbon copolymeric membrane. In some instances, a second similar electrode is simultaneously or subsequen~ly pressed 30 into the obverse membrane surface. To avoid heat damage to the copolymeric membrane, considerable pressure, often as high as 6000 psi is required to embed the electrode firmly in the rnembrane. For reasons related t~ re~iculate electrode structural configuration, such pressure is generally required to be applied simultaneously over the entire electrode area, requiring a press of 35 considerable proportions when preparing a commercial scale SPE electrode. As yet, the solution coatin~ of such electrodes with perfluorocarbon copolymer has not been feasible principally due to difficulties in developing a suitable solvent for perfluorocarbon copolyrner.

5~6 The use of alcohols to solvate particularly low e-luiva1ent weight perfluorocarbon copolymers is known. ~owever~ as yet, proposals for Eormation of at least partially solvated perfluorocarbon dispersions and for solution coating electrodes With the copolymer perfluorocarbon where the perfluorocarbon is of a relatively elevated equivalent weight desira~le in, for ex~nple, chlorine cells, have not proven satisfactory. Dissatisfaction nas been at least partly due to a lack of suitable techni~ues for dispersing and/or solvating these nigher ~uivalent ~Jeiynt perfluorocarbons~

DISCLOSU~E O~ T~E II~VEI~TIO~

The present invention provides a method for forminy an integral electrolytic cell mem~rane and solid pol~ner electrolyte (SPE~ while cocurrently attaching an electrodeO A cell melnbrane that is integral with a solid polymer electrolyte and carried b~ a cell electrode results from the method.
Therefore, the invention provides for a method for forming an electrolyte electrode assernbly Eor use in an electrochemical cell comprising the steps of:
(1~ dispersing a quantity of a copolymeric perfluorocarbon having an equivalent weight greater than 900 but less than 1500 and having one of sulfonyl, carbonyl and phosphorous ~ased pendant functional groups in a solvating dispersion m,edia;
(2) providing an electrode structure that includes interstices;
(3) at least once applyiny the dispersion to the electrode structure, whereby tne dispersion at least partiall~ coats the electrode structure ~ridgir.y the interstices; and (4) removing the dispersion rr,edia.
~hè invention also provides for a method for ma~iny an electrode assernbly for use in an electrochemical cell com~rising the steps of:
(1) dispersiny a quantity of a copol~neric perfluorocarbon ~,, ,"

having an equivalent weiyht grateY than 90U DUt less than 1500 and having on of sulfollyl, carbonyl and phosphorous ~ased pendant functional grcups in a solvating dispersion media;
(2) providing a reticulate electrode structure;
(3) at least once coating the reticulate electrode witn the dispersion and removiny the dispersion m~dia until a coatilly of a desired thickness has been attained on the reticulate electrode structure, and interstices between elements of the reticulate electrode structure have been bridged;
~4) removing a portion of the coating to expose a portion of the reticulate electrode structure.
The invention also provides for a method for ma~iny a perfluorocarbon copol~neric membrane and solid pol~ner electrolyte electrode assembly for use in an electrochemical cell com~rising the steps of:
(1) dispersing a quantity of a copol~meric perfluorocar~on having an equivalent weight oE greater then 9U0 but less then 1500 and haviny on oE sulfon~l, carbonyl and phosphorous based pendant functional yroups in a solvating dispersion mRdia;
(2) providing a reticulate electrode structure includiny a surface portion comprising at least one electrocatalytic comoound;
(3) masking the electrocatalytic surface portion;
(4) at least once coating the reticulate electrode structure with the dispersion media whereby the dispersion bridges interstices between the elements of the reticulate electrode structure;
(5) removing the dispersion media;

(6) repeatiny steps 4 and 5 until a coating of desired thickness is achieved upon the reticulate electrode structure completely bridging the interstices;

(7) removing the J~sking.
The invention provides Eor a metnod Eor making a perfluorocarbon copol~neric melnbrane having an equivalent weiyht o.
greater than 900 but less than 1500 and solid pol~mer electrol~te h5~

electrode assel~bl~ Eor use in an electrochemical cell comprisin~ the steps of:
(1) dispersiny a quantity of the copol~neric perFluorocarbon in solvating dispersion media;
(2) providing a reticulate electrode structure including a surface portion comprising at least one electrocatalytic compound;
(3) placiny a sheet of resinous Inaterial ~eneath a sheet of aluminum foil;
(4) placing the reticulate electrode structure upon tne al~inum foil with the electrocatalytic surface portion opposing the alurninwm foil;
(5) pressing the reticulate electrode structure into tne alurninum foil, the resinous material undergoing cold flow whereby the aluminum foil generally conforms to contours vf the reticulate electrode structure;
(6) at least once coating the reticulate electrode structure with the dispersion and removing the dispersion media until a coating oE a desired thickness has been built upon the electrode bridging between the elements of the reticulate electrode structure;
(7) removing the aluminwn foil and resinous Ina~erial~
The invention also provides for a method for forming a perfluorocarbon copolyrner coated structure com~rising the steps of:
(1~ dispersing a quantity of a copolymericperfluorocarbon having an equivalent weight greater than 900 but less than about 1500 and having one of sulfonyl, carbon~l and phosphorous Dased pendant functional groups in a solvating dispersion media;
(2) providing a substrate structure that includes interstices;
(3) at least once applying the dispersion to the substrate structure, whereby the disperslon at least partially coats the substrate structure bridging the interstices; and (4) removing the dispersion media.
A device Inade in accordance with the instant inventlon includes an electrode structure suitable for use in a fuel cell, .,,3, - ~c -battery, electrochemicdl cell or the like. This electrode structure includes interstices. A portion of the electrode s~ructure is coated with a copol~,leric perfluorocarbon, the perfluorocarbon coating bridging the interstices of the electrode structure. ~he thickness and continuity oE the copol~ eric perEluorocarboll b-cidyiny the interstices should be contiyuous and sufEiciently thick to preclude free ~ovement o liquids within the cell from one side of the coated electrode structure to the other. ~ilore than one coatiny of one or more perfluorocarbon copolymers may be a~lied where~y the integral membrane and SPE possess more than one desirable ~endant functional ~roup attribute of the perfluorocarbon copol~ne{s.
A solid polylr,er electrolyte-electrode of the instant invention is prepared by a process begun when a selected perfluorocarbon copolymer is dispersed in an at least partially solvating dispersion media. A desired electrode structure is then at least partially coated with the dispersion~ the dispersion bridging the interstices. The dispersion media is removed following coating. Repeated cycles of coating and subse~uent removal of the dispersion media may be desira`ole in achieviny an inteyral mel~rane and SPE having desired polymeric functional group properties an~/or to achieve a desired thickness.
In certain preferred embodiments, the electrode structure can include surface portions comprising one or more electrocatalytic compounds. In forming solid polymer electrolyte-electrodes usiny such electrode structures, it , , .S~
`\

is desirable that these electrocatalytically active surfaces not be coated accomplished by a method such as masking the electroca-talytic surface portions prior to coating.
Where the electrode structure is coated with the dispersion to an 5 extent providing a coating over a greater area of the surface of the electrodestructure than is desired, in certain preferred embodiments, the coating covering the undesirable electrode structure surface areas can be removed.
The above and other features and adYanta~es of the invention will become apparent from the following detailed descriptien of the inven-tion made 10 with reference to the accompany~ng drawings which form a part of the specif ication.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is an elevational view of the solid polymer electrolyte-electrode of this invention viewed from the coated side.
Figure 2 is a partial side elevational cross sectional view of the solid polymer electrolyte-electrode of the instant invention.

~EST EFl,lBODlMENT OF THE INVENTION

Referring to Figures 1 and 2, an integral membrane and solid polymer electrolyte-electrode is shown generally at 10. The solid polymer electrolyte 20 ~SPE) electrode 10 is comprised of an electrode structure 15 and a polymer coating 20.
The electrode structure 15 is generally o~ reticulate form but equally may be of sintered metal or other suitable or conventional configuration. The electrode structure 15 includes interstices 25.
The polymer coating 20 coats generally one surface of the electrode structure 15 and bridges or blinds the interstices 25. All interstices to be immersed in electrolyte contained in the electrochemical cell must be entirely blinded. The thickness of the coating, particularly that coating bridging the interstices, can be varied, but generally ranges between 0.5 and Ij0 mils and 30 preferably ranges between 4 and 10 mils.
Where the SPE-electrode 10 is to be used as an anode, the surface 30 remaining uncoated can include an electrocatalytic surface por tion 35. This electrocatalytic portion 35 includes at least one compound selected from the l ~h r~,r~ 3 .

group consisting of gold, silver and oxides of: iron, nickel, chromiurn, antimony, tin, cobalt, copper, lead, manganese, titanium, and a platinum group metal; the platinurn group comprising platinum, palladium, osmium, iridium, rhodium and ruthenium.
S The electrode structure 15 is made principally from a suitable or conventional substrate sus~h as: Periodic Table Group IVA metals tin and lead;
Periodic Table Group IB metals copper, silver and goldj Periodic Table Group 8 metals cobalt, nickel, iron including stainless steels, ruthenium, rhodium, palladium, osmium, iridium and platinum; as well as manganese, chromium, 10 vanadium, titanium, niobium, zirconium, bismuth, tantalum, aluminum and carbon. Where the SPE-electrode 10 is to function as an anode, the electro-catalytic compound is applied to the anode in any well-known manner.
The SPE electrode 10 can be employed in an electrolytic cell such as a sodium chloride brine based chlorine generation cell. Where the electrode 15 structure 15 is to function as an anode, it advantageously includes the electro-catalytic surface portion 35. Sodium chloride brine present in the cell generally at 37 reacts at the anode to release C12 and Na+ cations. The Na~~ cations negotiate the membrane-SPE 20 carrying current between cell anode and cathode and are thereafter available for reaction at a cell cathode of suitable or 20 conventional configuration. Alternately, the reticulate electrode structure can perform as a cathode whereby sodium ions negotiating the coating 20 react to form caustic NaOH with hydroxyl ions liberated by the cathodic dissociation of water.
The SPE electrode 10 of the instant invention is prepared by at !east 25 partially coating the reticulate electrode structure within a dispersion of perfluorocarbon copolymer having pendant functional groups capable of being converted to ion exchange functional groups such as groups based upon or derivedfrom sulfonyl, carbonyl, or in some cases phosphoric functional groups. The coating can be accomplished in any suitable or conventional manner such as by 30 dipping, spraying, brushing or with a roller. Following coating, the dispersing media for the perfluorocarbon copolymer is removed, usually by the application of gentle heat and, if desired, vacuum, or by leaching with a suitable or conventional light solvent such as acetoneJ 2-propanol or a ha!ogenated hydro-carbon such as FREON~) 113s a product of duYont. One or more coa-tings may be 35 required to provide a coating of desired thickness and one that effectively b!inds all the interstices of tne electrode structure 15.

5~

The copolymeric perfluorocarbon dispersed for use in coating the electrode structure is generally an intermediate copolymer having functional groups providing latent ion exchange capability later activated or an ion exchange activated copolymer. The intermediate polymer is prepared from at 5 least two monomers that include fluorine substituted si-tes. At least one of the monomers comes from a group that comprises vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, chlorotrifluoro-ethylene, perfluoro(alkyl vinyl ether), tetrafluoroethylene and mixtures thereof.
At least one of the monomers comes from a grouping having members 10 with functional groups capable of imparting cationic exchange characteristics to the final copolymer. Monomers containing pendant sulfonic acid, carboxylic acid or, in some cases phosphoric acid 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 15 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, phosphorus, sulfur, chlorine, arsenic, selenium, or tellurium.
Among the preferred ~amilies of monomers in the second grouping are sulfonyl containing monomers containing the precursor functional group 20 SO2F or SO3 alkyl. Examples of members of such a family can be represented by the generic formulae of CF2=CFSO2F and CF2=CFRlSO2F where Rl is a bifunctional perfluorinated radical comprising 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 25 have fluorine, chlorine or hydrogen atorns 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 fluorine atom attached. If the sulfonyl group is attached directly to the chain, the carbon in the chain to which it is attached must have a fluorine atom attached to it. The Rl radical of the 30 formula above can be either branched or unoranched, i.e., straight chained, 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 Rl group through an ether linkage, i.e., that the comonomer by of the formula CF2=CFO~lSO2F. Illustrative of such sulfonyl fluoride containing comonomers 35 are:

5~
S --C~=CFCF2CF2S2F' CF2=CFOCF2C FCF2CF252F' CF2=cFocF2cFocF2cFocF2cF2so2F~ CF2=CFCF2CF2502F, O

The corresponding esters of the aforementioned sulfonyl fluorides are 5 equally preferred.
While the preferred intermediate copolymers are pe-rfluorocarbon3 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-4-10methyl-7-octenesulfonyl fluoride) comprising between 10 and 60 weight percer.t, and preferably between 25 and 40 weight percent, of the latter monomers.
These perfluorinated copolymers may be prepared in any o~ a number of well-known manners such as is shown and described in U.S. Patent Nos.
3,041,317; 2,393,967; 2,559,752 and 2,593,583.
15An intermediate-copolymer is readily transformed intG a copolymer containin~ ion exchange sites by conversion of the sulfonyl groups (-SO2F or --SO3 alkyl~ to the form --SO3X by saponification or the like wherein X is hydrogen, an alkali metal, or an alkaline earth metal. The converted copolymer contains sulfonyl fluoride based ion exch~nge sites contained in side chains of the 20 copolymer and attached to carbon atorns having at least one attached fluorine atom. Not all sulfonyl groups within the intermediate copolymer need be converted. The conYersion may be accomplished in any suitable or customary manner such as is shown in U.S. Patent Nos. 3,770,547 and 3,784,399.
A coating 20 made from copolymeric perfluorocarbon having sulfonyl 25 based cation exchange functional groups possesses a relatively low resistance to back migra1:ion of sodium hydroxide frorn cathodic areas of the celi 39 to -the anodic cell areas 37, although such a membrane successfully resis-ts back migration of other caustic compounds such as KOH~ Where the sulfonyl fluoride yroup is at least partially converted to a sulfonalnide ~ ,reatiny with propylal-nine or the like, usefulness in a chlorine cell based upon NaCl electrolysis may be improved.
In some preferred modes for carrying out the invention, the coating includes pendant carbonyl based functional ~roups. The pendant carbonyl base~ yroups provide the copolymeric perfluorocarbon with siynificantly yreater resistance to the miyration of sodium hydroxide, but can also subs~antially re~uce the rate of migration of soc~ium ions from the anode to the cathode.
Copol~neric perfluorocarbon having perldant carbonyl based cationic exchange functional groups can be prepared in an~ suitable or conventional manner such as in accordance with U.S. Patent No.
4,151,053 or polymerized from a carbonyl functional group containiny monomer derived from a ~ulfonyl group containin~ nomer by a method such as is shown in U.S. Patent 4,151,053 Preferred carbon~l containiny monomers include CF2=CF-O-CF~CF(CF3)O f CF2)2COOCH3 and CF2=CF-O-CF2CF(CF3)0CF2COOCH3-Preferred copolyrneric perfluorocarbons utilized in the instant invention therefore include carbonyl and/or sulfonyl basedgroups represented by the formula - OCF2CF2X and/or - OCF2CF2Y-B-YCF2CF2O - wherein X is sulfonyl fluoride (SO2F) carbonyl fluoride (CO2F) sulfonate methyl ester (SOOCH~) carbox~late methyl ester (CCCX~13) ionic carboxylate (COO Z ) or ionic sulfonate (SO3Z-~), Y is sulfonyl or carbonyl ~-SO - - CO -), and ~ is a cross-linking structure such as -O-, -O-C-, -S S-, and di and poly amines of the form N~i(CRlR2)XNH2 where Rl, R2 are selected froln short chain alkanes, alkenes, hydroyen, and amine yroups and Z is hydroyen/ an alkali metal such as lithi~n, cesiurn, rubidi~lrn, potassium and sodium or an alkaline earth su-h as bariuun, beryllium, rraynesi~,l, calciurn, strontium and radiurn or a ~uaternary arr~oniurn ion B forms oE other than -O- display relativel~ low cation exchanye functionality, nowever.
Generally, sulEon~l, carbonyl, sulronate and car~oxylate ~' r~

esters and sulfollyl and carbonyl based amide fOrrllS of the perfluorocarbon copol~ner are readily converted to a salt forln by treatment with a strong alkali such as NaO~.
The equivalent weight ranye of the co~ol~ner inter,nediate used in preparing the membralle 15 is important. Where lower equivalent weight copol~ners are utilized, the mel,~ra.ne can be subject ~o destructive attack such as dissolution in cell chemistry. When an excessively elevated eluivalent weiyht co~olymer is utilized, the membrane may not pass cations sufficiently readily ,J, ,, , 5 :~L 2 resulting in an unaccep-tably low electrical efficiency 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.
The electrocatalytic anode substance can be applied as a component of one or more coatings to an electrode structure. ~Vhen applied to an elec-trode structure, the electrocatalytic compound can be applied directly over an electrode substrate, generally a valve metal such as ti~anium or the like well known in the art9 or it may be applied over a primary coating first applied to the 10 substrate of types also well known in the art. The electrocatalytic coating is generally applied to electrode structure portions not intended to be coated by the copolymeric perfluorocarbon. Coverage of the electrode with the electro-catalytic substance is usually constrained to surfaces not coated with the copolymer to avoid a separation of the coating from the electrode structure 15 15 that would accompany generation of chlorine gas at copolymer coated electrodestructure surfaces. For the same reasons, it is necessary to season or render inactive those portions of the electrode substrate structure 15 to be coated by the copolymer. Seasoning avoids generation of chlorine gas beneath the coating adjacent the electrode structure 15 that would cause a separation of the coating.
20 Desired portions of the electrode structure 15 can be rendered inactive by the brief actual generation of chlorine using the electrode structure before copolymer coating.
Perfluorocarbon copolymer is dispersed in any suitable or conventional manner. Preferably relatively finely divided particles of the 25 copolymer are used to form the dispersion. The particles are dispersed in a dlspersion media that preferably has significant capability for solvating the perfluorocarbon copolymer particles. A variety of solvating dispersers have beendiscovered for use with the perfluorocarbon copolymers; these suitable solvatingdispersers are tabulated in Table I and coordinated with the copolymer pendant 30 functional groups with which they have been found to be an effective dispersion medium. Since one or more of the dispersers may be used together in preparing a perfluorocarbon dispersion, as well as one or more of the dispersers suitably diluted, the term dispersion media is used to refer to a suitable or conventional solvating dispersion agent having at least one solvating dispersion medium.

~I. L~ f~

+~
o~ X.XXXXXXXXXX

V X X X
o~ o o o .'~ o V

E ~7 o x X x X X X,: X E ~

Z ~ T, _ :.~ E
o ~ ~ 3 ~ ~ o r ~ Y ~ ~ ~ o ~I v c v :, V t~ ~ 'V-- Z- Z Z Z Z Z E
T ~ 2, Z Z Z Z Z Z --~ I _ ~

-- l2 --Certain o~ the solvating dispersion media function more effectively with perfluorocarbon copolymer having particular metal ions associated with the functional group. For example, N-butylacetamide functions well with the groups COOLi and SO3Ca. Sulfolane and N9N-dipropylace tamide function well with 5 SO3Na functionality.
It is believed that other suitable or conventional perhalogenated compounds lil~e perfluorotrialkyl amines can be used for at least partially solvating the SO2F or carboxylate ester forms of perfluorocarbon copolymer. I-t is believed that other suitable or conventional strongly polar compounds can be 10 used for solvating the ionic sulfonate and carboxylate forms of the perfluorocarbon copolymer.
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 50C
15 and 250C but not in excess of the boiling point for the resulting dispersion.
Depending upon the solvating dispersion medium, 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 coating. It is important that perfluorocarbon particles remaining unsolvated be relatively 20 small to produce a smooth void free coating particularly in bridging the interstices. In one alternate technique, the dispersion is heated to at least approach complete solvation and then cooled to from a gel having particles of approximately the size desired to form the coating. The particle size is controllable using either of mechanical or ultrasonic disruption of the gelatinous 25 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. An SPE coated electrode 10 containing perfluorocarbon having functional groups of a first type can be at least partially 30 solvent welded to a perfluorocarbon coated electrode having functional groups of a second type; however, conversion of one or both types of functional groups maybe necessary to achieve solvent compatability. 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,35 other methods such as the use of SF4 to reform sulfonyl fluoride functional groups from derivatives of sulfonyl fluoride are also available.

One simple method for constraining dispersion from coating electro-catalytic portions 35 of ~he electrode structure 15 is to mask those electrocatalytic portions 35 while coating the electrode structure 15 with the dispersion. A reticulate electrode can be effectively masked by pressing the electrode structure into a sheet of aluminum foil covering a sheet of a resinousmaterial that relatively readily undergoes cold flow. Cold flow is the relatively slow flowing of a ma~erial away from an object being pressed into the material.
Particularly, an E. 1. duPont product, TEFLON(~, in -the form o-f fluoroethylene polymer (~EP) or polytetrafluoroethylene (PTFE) has been found 10 to be particularly useful for use as the resinous sheet. As the electrode structure is pressed into the aluminum foil, the TEFLON supporting the foil coldflows from beneath the electrode structure towards the inters-tices of the electrode structure. The foil is urged by the cold flowing TEFLON to conform closely to contours of the electrode structure including portions of the electrode 15 structure surrounding the interstices. ~here the surface of the electrode structure pressed into the foil includes electrocatalytic portions, the electro-catalytic portion can thereby be effectively masked.
Where an entire electrode structure has been immersed in dispersed copolymer and thereby coated, it is desirable to expose some portion of the 20 electrode structure. Selective removal of the coating can be accomplished by any suitable or conventional method such as grinding9 scarifying, cutting or thelike.
Where desired, ion exchange functional groups adjacent one coating surface can be converted from, for example, sulfonyl based groups to 25 carboxylate based groups. Conversion, such as by methods shown in U.S. Patent4,151,053 can provide a carboxylate based layer 40 in the coating that assists in resisting sodium hydroxide backmigration from the cell cathode to the cell anodewhile retaining a desirable sulfonyl based layer 45 more freely permeable -to sodium ions seeking to migrate to the cell cathode.
ln a preferred alternate, one or more coatings of a copolymer containing a particular functional group is applied to the electrode 15 followedby one or more coatings of copolymer containing a second functional group.
Where the copolymers are mutually soluble in dispersing media used for dispersing the second copolymer, a solvent bond between the coating applications35 is established by which ~hey become coadhered.
In one typical example, perfluorocarbon containing pendant sulfonyl fluoride groups is applied to unmeshed portions of an electrode to be used as an 5~

anode. The sulfonyl fluoride group containing copolymer is dispersed in Halocarbon Oil, perfluorodecanoic acid or perfluorooctanoic acid.
After establishing a contiguous coating of desired thickness, a further coating of a second copolymer containing pendant methyl carboxylate ester groups is applied over the original coating again using ~lalocarbon Oil, perfluoro-octanoic acid or perfluorodecanoic acid as the dispersion media.
Functional groups in both copolymers are then saponified using KOH
to yleld an integral SPE and membrane having sulfonyl based cationic exchange groups opposing the anode, and carbonyl based functional groups opposing a cathode utilized in conjunction with the anode in a cell.
Further, a cathode coated on one surface with a functional copolymeric fluorocarbon containing pendant first functional groups can be solvent adhered to an anode having a perfluorocarbon coating containing pendant second functional groups, or each can be solvent adhered to an intervening perfluorocarbon copolymeric film. Heat and/or pressure may be necessary to assure acceptable coadherence using solvents, but under extremes, of temperature and pressure, such as 2000-6000 psig and temperatures in excess of 100C+ a solvent may be unnecessary for coadherence.
The following examples are offered to illustrate further the inventiOn.

EXAMPLE I

Perfluorocarbon copolymer having pendant SO2F functional groups and polymerized from polytetrafluoroethylene and perfluoro(3,6-dioxa-4-methyl-7-octenesulfonylfluoride) and having an equivalent weight of about 1100 was dissolved in hot (240C) Halocarbon Oil to yield a 12 percent (weight) solution-dispersion. A titanium expanded mesh, 10 Ti 14-3/0 (read as titanium mesh having a wire thickness of 10 mils, a wire width of 14 mils, a mesh opening having a long dimension of about 1/8 inch and a short dimension of about 5Q mils) is coated on one side with an electrocatalytic coating such as is described and shown in U.S. Patent 3,751,296. A sheet of aluminum foil was sandwiched between the electrocatalytic surface and a sheet of TEFLON and the electrode pressed into the foil and TEFT ON.
The rnesh was then mounted upon a frame and irnmersed in the dispersion, withdrawn and the Halocarbon Oil removed by extraction using 35 FREON 113. TmMersion and ex~trac~ion were repeated. The mesh was demounted from the frame and hydrolyzecl in weak KOH for 96 hours at room temperature which served also to leach the aluminum foil from the mesh. A
mil contiguous cationic exchange coating resulted on the mesh.

EXAMPLE II

A procedure identical to that of Example I was performed using a sheet of porous titanium, made by sintering titaniurn particles coated with an electrocatalytic coating as in Example 1. A contiguous 4 mil coating resulted upon the sheet.

EXAMPLE III

lQ A titanium mesh 5 Ti 7-3/0 electr-ocatalytically coated as in ExarnpleI and a nickel mesh 5 Ni 7-3/0 were each masked on one side using aluminum foii and TEFLON under pressure in accordance with Example I. The meshes were installed in a frame and coated in accordance with Example I. After removal of the dispersion media, the coated surfaces were then aligned with a 15 perfluorocarbon film between them and pressed a-t 180~ and 2000 pslg until each coadhered to the film. The resulting composite film was a 23 mil thickness including both electrodes.
The laminated electrode structure was saponified in weak KOH.

EXAMPLE IV

Sulfonyl fluoride functional groups in the coatings of Examples I, II
and III are converted in part by n-propyl amine to sulfonamide functionality before saponification. The resulting coating provides superior chlorine cell performance to coatings including only saponified sulfonyl fluoride functional groups.

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 scope of the invention as se-t forth in the appended claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for forming an electrolyte electrode assembly for use in an electrochemical cell comprising the steps of:
(1) dispersing a quantity of a copolymeric perfluorocarbon having an equivalent weight greater than 900 and less than 1500 and having one of sulfonyl, carbonyl and phosphorous based pendant functional groups in a solvating dispersion media;
(2) providing an electrode structure that includes interstices;
(3) at least once applying the dispersion to the electrode structure, whereby the dispersion at least partially coats the electrode structure bridging the interstices; and (4) removing the dispersion media.

2. The method of Claim 1 including the additional step of applying at least one additional coating of a further copolymeric perfluorocarbon compound in an equivalent weight range of from 900 to 1500 and having a second pendant functional group.

3. The method of Claim 1 including the step of masking portions of the electrode structure prior to immersion in the dispersion.

4. The method of Claim 1 including the step of removing coating from a portion of the electrode structure after completion of coating and dispersion media removal.

5. A method for making an electrode assembly for use in an electrochemical cell comprising the steps of:
(1) dispersing a quantity of a copolymeric perfluorocarbon having an equivalent weight greater than 900 but less than 1500 and having one of sulfonyl, carbonyl and phosphorous based pendant functional groups in a solvating dispersion media;
(2) providing a reticulate electrode structure;
(3) at least once coating the reticulate electrode with the dispersion and removing the dispersion media until a coating of a desired thickness has been attained on the reticulate electrode structure, and interstices between elements of the reticulate electrode structure have been bridged;
(4) removing a portion of the coating to expose a portion of the reticulate electrode structure.

6. The method of Claim 5 wherein masking comprises the steps of:
(1) placing a sheets of a relatively thin masking material over a sheet of a resinous material capable of relatively readily undergoing cold flow;
(2) placing the reticulate electrode structure upon the masking material sheet with the eletrocatalytic surface portions opposing the sheet;
(3) pressing the reticulate electrode structure into the masking material sheet until the resinous material undergoes cold flow, thereby supporting the masking material sheet in conforming to contours of the reticulate electrode electrocatalytic surface portion.

7. The method of Claim 5 including the additional step of applying at least one coating of a further perfluorocarbon compound having an equivalent weight greater than 900 but less than 1500 and having a second pendant functional group.

8. A method for making a perfluorocarbon copolymeric membrane and solid polymer electrolyte electrode assembly for use in an electrochemical cell comprising the steps of:
(1) dispersing a quantity of a copolymeric perfluorocarbon having an equivalent weight of greater than 900 but less than 1500 and having one of sulfonyl, carbonyl and phosphorous based pendant functional groups in a solvating dispersion media;
(2) providing a reticulate electrode structure including a surface portion comprising at least one electrocatalytic compound;

(3) masking the electrocatalytic surface portion;
(4) at least once coating the reticulate electrode structure with the dispersion media whereby the dispersion bridges interstices between the elements of the reticulate electrode structure;
(5) removing the dispersion media;
(6) repeating steps 4 and 5 until a coating of desired thickness is achieved upon the reticulate electrode structure completely bridging the interstices;
(7) removing the masking.

9, The method of Claim 8 wherein the reticulate electrode structure comprises nickel.

10. The method of Claim 8 including the additional step of applying at least one coating of a further copolymeric perfluorocarbon compound having an equivalent weight of greater than 900 but less than 1500 and having a second pendant functional group.

11. A method for making a perfluorocarbon copolymeric membrane having an equivalent weight of greater than 900 but less than 1500 and solid polymer electrolyte electrode assembly for use in an electrochemical cell comprising the steps of:
(1) dispersing a quantity of the copolymeric perfluorocarbon in solvating dispersion media;
(2) providing a reticulate electrode structure including a surface portion comprising at least one electrocatalytic compound;
(3) placing a sheet of resinous material beneath a sheet of aluminum foil;
(4) placing the reticulate electrode structure upon the aluminum foil with the electrocatalytic surface portion opposing the aluminum foil;
(5) pressing the reticulate electrode structure into the aluminum foil, the resinous material undergoing cold flow whereby the aluminum foil generally comforms to contours of the reticulate electrode structure;
(6) at least once coating the reticulate electrode structure with the dispersion and removing the dispersion media until a coating of a desired thickness has been built upon the electrode bridging between the elements of the reticulate electrode structure;
(7) removing the aluninum foil and resinous material.

12. The method of either of Claims 5 or 11 wherein the electrocatalytic compound comprises at least one compound selected from a group consisting of oxides of manganese, tin, antimony, titanium, vanadium and a platinum group metal.

13. The method of any of Claims 1, 2, or 3, wherein the perfluorocarbon copolymer is polymerized from at least two monomers, one such monomer consisting essentially of at least one fluorinated vinyl compound and said other monomer consisting essentially of at least one monomer having the structure wherein R1 is a bifunctional perfluorinated radical containing from 2 to 8 carbon atoms which carbon atoms can be at least once interrupted by one or more oxygen atoms and X is selected from a group consisting of sulfonyl fluoride, carboxyl fluoride, sulfonate ester, carboxylate ester, and saponification products of sulfonyl fluoride and carboxyl fluoride.

14. The method of any of Claims 4, 5, or 6, wherein the perfluorocarbon copolymer is polymerized from at least two monomers, one such monomer consisting essentially of at least one fluorinated vinyl compound and said other monomer consisting essentially of at least one monomer having the structure wherein R1 is a bifunctional perfluorinated radical containing from 2 to 8 carbon atoms which carbon atoms can be at least once interrupted by one or more oxygen atoms and X is selected from a group consisting of sulfonyl fluoride, carboxyl fluoride, sulfonate ester, carboxylate ester, and saponification products of sulfonyl fluoride and carboxyl fluoride.

15. The method of any of Claims 7, 8, or 9, wherein the perfluorocarbon copolymer is polymerized from at least two monomers, one such monomer consisting essentially of at least one fluorinated vinyl compound and said other monomer consisting essentially of at least one monomer having the structure wherein R1 is a bifunctional perfluorinated radical containing from 2 to 8 carbon atoms which carbon atoms can be at least once interrupted by one or more oxygen atoms and X is selected from a group consisting of sulfonyl fluoride, carboxyl fluoride, sulfonate ester, carboxylate ester, and saponification products of sulfonyl fluoride and carboxyl fluoride.

16. The method of either of Claims 10 or 11, wherein the perfluorocarbon copolymer is polymerized from at least two monomers, on such monomer consisting essentially of at least one fluorinated vinyl compound and said other monomer consisting of at least one monomer having the structure wherein R1 is a bifunctional perfluorinated radical containing from 2 to 8 carbon atoms which carbon atoms can be at least once interrupted by one or more oxygen atoms and X is selected from a group consisting of sulfonyl fluoride, carboxyl fluoride, sulfonate ester, carboxylate ester, and saponification products of sulfonyl fluoride and carboxyl fluoride.

17. The method of any of Claims 1, 2, or 3, wherein the dispersion media is selected from a group of chlorotrifluoroethylene;
perfluorooctanoic acid; perfluorodecanoic acid;
perfluorotributylamine; perfluoro-1-methyldecalin;
decafluorobiphenol; penafluorophenol; pentafluorobenzoic acid;
N-butylacetamide; tetrahydrothiophene-1, 1-dioxide (tetramethylene sulfone); N-N-diethylacetamide; N-N-dimethylpropionamide;
N,N-dibutylformamide; N,N-dimethylacetetamide;

perfluorotrialkylamine; and dipropylamide.

18. The method of any of Claims 4, 5, or 6, wherein the dispersion media is selected from a group of chlorotrifluoroethylene;
perfluorooctanoic acid; perfluorodecanoic acid;
perfluorotributylamine; perfluoro-1-methyldecalin;
decafluorobiphenol; pentafluorophenol; pentafluorobenzoic acid;
N-butylacetainide; tetrahydrothiophene-1, 1-dioxide (tetrarnethylene sulfone); N-N-diethylacetamide; N-N-dimethylpropionamide;
N,N-dibutylformamide; N,N-dimethylacetetamide;
perfluorotrialkylamine, and dipropylamide.

19. The method of any of Claims 7, 8, or 9, wherein the dispersion media is selected from a group of chlorotrifluoroethylene;
perfluorooctanoic acid; perfluorodecanoic acid;
perfluorotributylamine; perfluoro-1-methyldecalin;
decafluorobiphenol; pentafluorophenol; pentafluorobenzoic acid;
N-butylacetamide; tetrahydrothiophene-1, 1-dioxide (tetramethylene sulfone); N-N-diethylacetamide; N-N-dimethylpropionamide;
N,N-dibutylformamide; N,N-dimethylacetetamide;
perfluorotrialkylamine; and dipropylamide.

20. The method of either of Claims 10, or 11, wherein the dispersion media is selected from a group of chlorotrifluoroethylene;
perfluorooctanoic acid; perfluorodecanoic acid;
perfluorotributylamine; perfluoro-1-methyldecalin;
decafluorobiphenol; pentafluorophenol; pentafluorooenzoic acid;
N-butylacetamide; tetrahydrothiophene-1, 1-dioxide (tetramethylene sulfone); N-N-diethylacetamide; N-N-dimethylpropionamide;
N,N-dibutylformamide; N,N-dimethylacetetamide;
perfluorotrialkylarnine; and dipropylamide.

21. The method of either of Claims 1 or 5, including the additional step of adhering coated portions of a second electrode assembly to the electrode assembly using at least one of heat, pressure and solvent.

22. The method of either of Claims 8 or 11, including the additional step of adhering coated portions of a second electrode assembly to the electrode assembly using at least one of neat, pressure and solvent.

23. A method for forming a perfluorocarbon copolymer coated structure comprising the steps of:
(1) dispersing a quantity of a copolymeric perfluorocarbon having an equivalent weight greater than 900 but less than 1500 and having one of sulfonyl, carbonyl and phosphorous based pendant functional groups in a solvating dispersion media;
(2) providing a substrate structure that includes interstices;
(3) at least once applying the dispersion to the substrate structure, whereby the dispersion at least partially coats the substrate structure bridging the interstices; and (4) removing the dispersion media