Structural Frame for an Electrochemical Cell

This invention relates to an electrochemical cell and in particular to a structural frame for use in an electrochemical cell.

It is well established that various chemicals can be produced in an electrochemical cell containing an anode and a cathode. For example, alkali metal chlorates, such as sodium chlorate, have been formed electrolytically from a sodium chloride brine in cells without a separator positioned between the anode and the cathode.

When a separator, such as a liquid permeable asbestos or polytetrafluoroethylene diaphragm or a substantially liquid impervious ion exchange membrane, is used in a cell to electrolyze a sodium chloride brine, the electrolytic products will normally be gaseous chlorine, hydrogen gas, and an aqueous solution containing sodium hydroxide.

For a number of years gaseous chlorine was produced in electrolytic cells wherein an asbestos diaphragm was interposed between finger-like, anodes and cathodes which were interleaved together. During the past several years it has become apparent that the use of a substantially liquid impermeable cation exchange membrane may be preferable to the more well established diaphragm in instances where a higher purity, for example a lower sodium chloride content, higher sodium hydroxide product is desired. It was found to be more convenient to fabricate ion exchange type electrochemical cells from relatively flat or planar sheets of ion exchange membrane rather than to interleave the membrane between the anode and cathode within the older finger-like cells used with asbestos diaphragms.

The newer, so-called flat plate electrochemical cells using a planar piece of ion exchange membrane to separate the anolyte from catholyte compartments also have a plurality of solid, liquid impervious frames adapted to support the anode on one side and the cathode on the opposite side. These frames have previously been constructed of materials such as metal and plastic, but neither of these materials has been found to be entirely satisfactory. In any electrochemical cell, including both monopolar and bipolar cells, there is a possibility that electrolyte may leak from within the cell to the exterior. In instances where such leakage has occurred in cells with iron or other ferrous type frames, it was found that the iron frame corroded or was itself electrolytically attacked. Plastic frames are not generally subject to the electrolytic attack, but are normally not resistant to the anolyte and/or catholyte within the cell under operating conditions for extended periods of time, for example, several years.

It is desired to provide a structural frame for use in electrochemical cells which would minimize the corrosion problems and would increase the relatively short useful life attendant with those frames used by the prior art.

The present invention is a structural frame particularly suitable for use in an electrochemical cell. The frame comprises an organic plastic member with a plurality of horizontally and vertically spaced-apart shoulders protruding outwardly from opposing generally coplanar anolyte and catholyte surfaces of the plastic member. Each of the shoulders annularly encircles and supports an electrically conductive insert extending from an exterior face of a shoulder on the catholyte surface of the plastic member, through the plastic member, to an exterior face of a shoulder on the anolyte surface of the plastic member.

An electrically conductive substantially completely hydraulically impermeable anolyte cover is matingly affixed to the anolyte surface of the plastic member and adapted to minimize contact between the anolyte and the plastic member. The anolyte cover is resistant to the corrosive effects of the anolyte. An electrically conductive catholyte substantially completely hydraulically impermeable cover is matingly affixed to the catholyte surface of the plastic member and adapted to minimize contact between the catholyte and the plastic member. The catholyte cover is resistant to the corrosive effects of the catholyte. Both the anolyte cover and the catholyte cover may be made from a metal, or, optionally made from another material and have metallic inserts molded in at the points where they contact the metallic inserts which pass through the plastic member.

The invention further includes an electrochemical cell utilizing a plurality of the above described structural frames removably and sealably positioned in a generally coplanar relationship with each other and with each of the plastic members being spaced apart at least by an anode on one side of the plastic member and a cathode on an opposing side of the plastic member.

The accompanying drawing further illustrates the invention;

In FIG. 1 is depicted a cross-sectional view of one embodiment of the invention,

In FIG. 2 is an exploded, isometric view of another embodiment of the structural frame in combination with an anode, cathode, and ion exchange membrane,

In FIG. 3 is depicted a cross-sectional side view of another embodiment of the electrochemical cell of the present invention.

Identical numbers, distinguished by a letter suffix within the several figures represent parts having a similar function within the different embodiments.

In FIG. 1 are shown structural frames 10 and 10a, which achieve the above objects. It is illustrated for use in an electrochemical cell for producing gaseous chlorine in aqueous alkali metal hydroxide solution. Although the present invention can be beneficially employed to produce chlorine and various alkali metal hydroxide solutions, it is preferred to use sodium chloride as the primary salt in the starting brine since this particular salt is readily available commercially and there are many well established uses for sodium hydroxide produced electrolytically.

The cell structure 10 includes a generally planar organic plastic member 12 which can be produced by commercial and known procedures into a shape with a plurality of horizontally and vertically spaced apart shoulders 14 and 14a protruding outwardly from cathode and anode sides 16 and 18, respectively. The peripheral surface 20 of the plastic member 12 defines the outer surface of the electrochemical cell when a plurality of the plastic members are positioned together as shown in the drawing. The peripheral configuration of the plastic members 12 is optional and can be varied to suit the particular configuration of the electrochemical cell shape desired.

The number, size, and shape of these shoulders may be an important consideration in both the design and operation of the present invention. They may be square, rectangular, conical, cylindrical, or any other convenient shape when viewed in sections taken either parallel or perpendicular to the central portion. The shoulders may have an elongated shape to form a series of spaced ribs distributed over the surface of the plastic member.

A number of plastic materials are suitable for use in the present invention for the construction of plastic member. Without intending to be limited by the specific organic materials hereinafter delineated, examples of such suitable materials include polyethylene; polypropylene; polyvinylchloride; chlorinated polyvinyl chloride; acrylonitrile, polystyrene, polysulfone, styrene acrylonitrile, butadiene and styrene copolymers; epoxy; vinyl esters; polyesters; and fluoroplastics and copolymers thereof. It is preferred that a material such as polypropylene be used for the structural member since it produces a shape with adequate structural integrity at elevated temperatures, is readily available, and is relatively inexpensive with respect to other suitable materials.

It is surprising that the plastic member 12 can be produced by any of a number of processes known well to those skilled in the art of plastic molding. Such molding processes include, for example, injection molding, compression molding, transfer molding, and casting. Of these processes, injection molding has been found to satisfactorily produce a structure with adequate strength for use in an electrochemical cell. Preferably, the plastic is injected into a mold containing the desired number of inserts (discussed later). In this manner, the plastic member is a one-piece member which fits tightly around the inserts, holds them in place, and provides a high degree of support to them. Such a configuration minimizes the likelihood that the inserts will separate from the plastic member and become loose. The ease of molding relatively complex shapes and the strength of the finished injection molded article contribute to making this process preferred for making the herein described structural member. This a considerable advantage over the prior art where the plastic frame was molded first and then the electrical conductors were subsequently installed.

When the plastic member 12 is employed in an electrochemical cell for producing chlorine, the temperature of the cell and the plastic member will frequently reach, or be maintained at, temperatures of from about 60.degree. to about 90.degree. Celsius. At these temperatures plastics, as do most materials, expand a measurable amount. Any expansion and later contraction on cooling of the plastic frame could result in electrolyte seeping from within the plurality of cells when joined together or, more importantly, could result in distortion of the anode and cathode which are made of metallic expanded mesh or perforated sheets. Furthermore, the differential expansion between the plastic frame 22 and the catholyte cover 22 and anolyte cover 24 would create stress on the welds which affix these covers to the inserts which are themselves molded in the plastic frame.

To reduce, and preferably minimize, the difference in expansion between the covers 22 and 24 and the plastic member 12, it is preferred to incorporate an additive to reduce thermally induced expansion of the plastic member. More preferably, the additive will also increase the structural strength of the finished plastic article. Such additive can be, for example, fiberglass, graphite fibers, carbon fibers, talc, glass beads, pulverized mica, asbestos, and the like, and combinations thereof. It is preferred that the plastic contain from about 5 to about 75 weight percent and more preferably from about 10 to about 40 weight percent of the additive. Glass fibers can be readily mixed with polypropylene to produce an injectable material suitable for use in the present invention which results in a solid, physically strong body with a coefficient of expansion less than polypropylene not containing glass fibers. Of greater importance is the need to minimize the difference in expansion between the plastic member, the electrodes, and the current collector, since these elements are welded together and it is critical that they remain substantially flat and parallel.

It has been determined that the use of commercially available polypropylene which has been specially formulated to afford bonding with the glass works particularly well. This results in a composite having a lower coefficient expansion than a mixture of polypropylene and glass fibers. Such chemicallycombined glass fiber reinforced polypropylene is available from, for example, Hercules, Inc., Wilmington, Del., as Pro-fax PC072 polypropylene.

At least one electric conducting element, such as insert 26, is positioned and preferably molded into the plastic member 12. The insert 26 extends through the plastic member from the catholyte surface 16 to the anolyte surface 18. The inserts 26 and 26a are preferably retained within the plastic member 12 by means of friction between the plastic and the insert. It is more preferable to increase the friction between these two bodies by having an additional means to restrain the insert within the plastic. Such additional means include, for example grooves (one or more) around the circumference of the insert(s), keys welded to the insert, hole(s) extending into and/or through the insert, slots, rings, collars, studs, or bosses.

The insert 26 can be any material which will permit flow of an electric current between the catholyte cover 22 and the anolyte cover 24. Since the covers 22 and 24 are preferably metallic, it is convenient to fabricate the insert from a metal, such as aluminum, copper, iron, steel, nickel, titanium, and the like, or alloys or physical combinations including such metals.

The shoulders and inserts should be spaced so they provide a somewhat uniform and low electrical potential gradient across the face of the electrode to which they are attached. They should be spaced so that they allow free fluid circulation from any unoccupied point within their respective electrolyte compartment to any other unoccupied point within that compartment. Thus the shoulders will be somewhat uniformly spaced apart from one another in their respective compartments.

To improve the flow of DC electric current between the covers 22 and 24, the insert 26 is preferably made of a material weldably compatible with the particular cover it contacts. For example, the insert 26 may be a welded assembly of a steel rod 261 with a vanadium disk 262 interposed between and welded to both the rod 261 and a titanium cup-like member 263 on the anode facing portion of the structure 10. A similar nickel cup-like member 264 may be welded directly to the rod 261 on the cathode facing portion of the insert. The titanium and nickel members 263 and 264, respectively, are then readily weldable to titanium anolyte cover 24 and nickel catholyte cover 22 and 22a preferred for use in an electrochemical cell producing chlorine and an aqueous sodium hydroxide solution.

To prevent catholyte from contacting the plastic member within the electrochemical cell and cause deterioration of the plastic and/or leakage of electrolyte between the plastic and the insert 26 from cathode compartment 30 to anode compartment 32 the cover 22 is matingly contacted with the catholyte surface 16 and the anolyte cover 24 is matingly contacted with the anolyte surface 18. As is shown in FIG. 1, both the anolyte and the catholyte covers are so shaped to correspond closely to the exterior surface of the plastic member 12. The degree of correspondence may be more or less than illustrated in FIG. 1. In some instances, the electrode compartment covers 22 or 24 may abut the frame 10 in one or more locations. It is important that the portions of both of the covers 22 and 24 which are exposed to the anolyte or catholyte and span the plastic member contain no openings through which electrolyte or electrolytic products can pass during operation of the electrochemical cell. The freedom from openings through the covers minimizes the likelihood that electrolyte will leak or seep through holes or spaces around gaskets of other seals and come into contact with the plastic member.

The anolyte cover 24 is made of a material which is resistant to the anolyte during operation of the cell. Normally, this material is not electrolytically active, but the invention is still operable if the material does become or is active electrolytically. Suitable materials are, for example, titanium, tantalum, zirconium, tungsten, and other valve metals not materially affected by the anolyte. Titanium is preferred as the anolyte cover.

The catholyte cover 22 is resistant to attack by the catholyte under the conditions present in the electrochemical cell. Suitable materials for the catholyte cover include, for example, iron, steel, stainless steel, nickel, lead, molybdenum, and cobalt and alloys, including major portions of these metals. Nickel, including nickel base alloys, is preferably used for the catholyte cover, since nickel and nickel alloys are generally resistant to the corrosive effects of the catholyte, especially an aqueous catholyte solution containing up to at least about 35 weight percent sodium hydroxide. Steel has also been found to be suitable, and relatively inexpensive, for use in a cell as a catholyte cover in the presence of a dilute (i.e., up to about 22 weight percent) aqueous solution of sodium hydroxide.

To assist in assembling a plurality of the structural frames 10 into an electrochemical cell it is desirable, although not essential, to have flanges 34 and 34a extending outwardly from the main structural portion of the plastic member 12 along the periphery of such member. In a preferred embodiment the flanges extend outwardly from the plastic member about the same distance as the insert 26. Alternatively, but not preferred, separate spacer elements (not shown) could be utilized to build up the plastic member 12 sufficiently to permit a number of the plastic members to be combined into a cell series without having electrolyte, either anolyte or catholyte, leak from within the catholyte and anolyte compartments 30 and 32, respectively, to an exterior portion of the cell.

FIG. 1 further shows an anode 36, which is positively charged during operation of the cell from an external power source (not shown), electrically connected to the anolyte cover 24. Such electrical connection is readily achieved by welding the anode 36 to the anode cover where the anode cover comes into physical contact with the insert 26. For improved electrical contact, the anolyte cover 24 is welded to the insert 26 and the anode 36 is welded to the anolyte cover 24 adjacent to the insert 26. Various means of welding can be utilized in the present invention, but it has been found highly satisfactory to use resistance or capacitance discharge welding techniques. The anode 36 can additionally be welded to the cover 24 at anode end portion 42 by, for example, resistance or capacitance discharge welding. Other suitable welding techniques include tungsten inert gas (TIG) and metal inert gas (MIG) welding. This welding serves a primary purpose of retaining the anode in position and not for electrical flow, although electric current will naturally pass through the welded areas.

The anode 36 is a metal, such as one of the common film-forming metals, which is resistant to the corrosive effects of the anolyte during the operation of the cell. Suitable metals are well known to include tantalum, tungsten, columbium, zirconium, molybdenum, and preferably, titanium and alloys containing major amounts of these metals, coated with an activating substance, for example, an oxide of a platinum group metals, such as ruthenium, iridium, rhodium, platinum, palladium, either alone or in combination with an oxide of a film-forming metal. Other suitable activating oxides include cobalt oxide either along or in combination with other metal oxides. Examples of such activating oxides are found in U.S. Pat. Nos. 3,632,498; 4,142,005; 4,061,549; and 4,214,971.

The cathode 38 and 38a, which has a negative electric potential during operation of the cell, is electrically connected to the catholyte cover 22 and 22a, respectively, in substantially the same manner as above described for the anode 36. The cathode 38 and 38a should be constructed of a material which is resistant to the corrosive effects of the catholyte during operation of the cell. Materials suitable for contact with the catholyte will depend upon the concentration of the alkali metal hydroxide in the aqueous solution and may be readily determined by one skilled in the art. Generally, however, materials such as iron, nickel, lead, molybdenum, cobalt, and alloys including major amounts of these metals, such as low carbon stainless steel, are suitable for use as the cathode. The cathodic electrodes may optionally be coated with an activating substance to improve performance of the cell. For example, a nickel substrate could be coated with oxides of nickel and a platinum group metal, such as, ruthenium, or nickel and a platinum group metal, or oxide thereof such as ruthenium oxide, to reduce hydrogen overvoltage. U.S. Pat. No. 4,465,580 describes the use of such cathodes.

As is apparent from the drawing, both the anode and the cathode are permeable to the respective electrolyte. The electrodes can be made permeable by several means including, for example, using a punched sheet or plate, an expanded mesh, or woven wire. The anode should be sufficiently porous to permit anolyte and chlorine to pass therethrough and the cathode should be sufficiently porous to permit catholyte to pass therethrough and hydrogen to pass therethrough.

The electrochemical cell of FIG. 1 also shows the anode 36 and the cathode 38 spaced apart by an ion exchange membrane 44 which is in contact with the anode 36. If desired, however, although not preferred, the membrane 44 could be in contact with the cathode 38 or be suspended between the two electrodes. It is important, that the ion exchange membrane 44 separate the anode compartment 32 from the cathode compartment 30a.

Cation exchange membranes are well known to contain fixed anionic groups that permit intrusion and exchange of cations, and exclude ions, from an external source. Generally the membrane has a matrix of a cross-linked polymer, to which are attached charged radicals such as --SO.sub.3 (-1), --COO(-1), --PO.sub.3 (-2), HPO.sub.2 (-1), --AsO.sub.3 (-2), and SeO.sub.3 (-1). Vinyl addition polymers and condensation polymers may be employed. The polymer can be, for example, styrene, divinyl benzene, polyethylene and fluorocarbons. Condensation polymers are, for example, phenol sulfuric acid, and formaldehyde resins. Representative of the types of permselective membranes envisioned for use with this invention are those disclosed in the following U.S. Pat. Nos., 3,909,378; 4,025,405; 4,065,366; 4,116,888; 4,123,336; 4,126,588; 4,151,052; 4,176,215; 4,178,218; 4,192,725; 4,209,635; 4,212,713; 4,251,333; 4,270,996; 4,329,435; 4,330,654; 4,337,137; 4,337,211; 4,340,680; 4,357,218; 4,358,412; and 4,358,545. These patents are hereby incorporated by reference for the purpose of the membranes they disclose.

To minimize leakage of electrolyte from the cell after assembling a number of the structural frames 10 together, at least one gasket 46 is positioned between the frames 10 and 10a. During assembly of the frames a compressive force is applied to the extremes of the frames to compress the gasket material 46 so that it both seals the ion exchange membrane 44 in positioned and minimizes leakage of electrolyte from within the final cell series to the exterior of the cells. Preferably, the membrane 44 is positioned to substantially entirely prevent leakage of electrolyte from within the final cell series to the exterior of the cells. Various gaskets materials can be used including, for example, fluorocarbon, chlorinated polyethylene rubber, and ethylene propylene diene terpolymer rubber.

FIG. 2 is an exploded, partially cross-sectioned isometric view of the structural frame 10b, including a plastic member 12a with a plurality of frustoconical shoulders 14b, with inserts 26a encased therein, extending outwardly from the generally planar anolyte surface 18a. Identical shoulders 14c extend outwardly from the catholyte surface of the plastic member 12a in a mirror image relationship with the shoulders 14b on the anolyte surface. A conduit or recess 48 is provided in the plastic member 12a to permit exit of product produced in the electrochemical cell during operation. Preferably, a pipe, tube, or shaped metal conduit is positioned within the recess 48 and affixed to the cover 24b to facilitate substantially leak free removal of the product from the cell. A similar conduit or recess (not shown) is provided in, for example, a wall portion of the plastic member at a location generally diagonally opposed to the conduit 48 to permit an aqueous sodium chloride solution to be fed through a suitable conduit into the anode compartment. Similar conduits or recesses are provided on the cathode side of the plastic member to permit feeding, for example, water into the cathode compartment and removal of products, such as a solution containing sodium hydroxide, and optionally hydrogen, therefrom. The anolyte cover 24b and catholyte cover 22a are adapted to closely fit over the respective surface of the plastic member 12a and prevent entrance of electrolyte from the respective electrode compartments into the space, if any, between the cover and the plastic member. The covers 22a and 24b also have conduits therein for exit of the brine solution and product produced in the respective electrolyte compartment and feeding of starting solutions to the respective compartments. For example, an opening, such as shaped pipe 50, in the cover 24b corresponds to the recess 48 in the plastic member to afford ready exit of the product chlorine and spent anolyte from the anode compartment. An expanded mesh anode 36b and an expanded mesh cathode 38a are adapted to fit within the respective anolyte and catholyte covers substantially the same as shown in FIG. 1. An ion exchange membrane is shown as sheet 44a. A leak minimizing gasketing material 46a is placed between structural frame members prior to the assembly of an electrochemical cell series.

In FIG. 3 is shown a partially assembled cell series containing three sets of structural frame members with anodes and cathodes spaced apart by ion exchange membranes as shown in the previous figures. The following elements are shown as plastic members 12b, 12c and 12d; gasket 46b; cover 22c; and ion exchange membranes 44b, 44c, and 44d. In this embodiment the inserts 26b, 26c, 26d, and 26e are of a somewhat different configuration than those shown in FIGS. 1 and 2. In particular, the insert 26d is a tubular member with a roughened exterior surface and an electric conducting end portion 52 physically and electrically connected to and covering the entire cross-section of the tubular insert 26d. Such electrical and physically connection can be obtained readily by welding or other known bonding techniques as known to those skilled in the particular art. The peripheral portions of the cover 24b may optionally contain expansion grooves (not shown) to minimize any effects of thermal expansion of the covers upon the operation of the cell.

In operating the cell series as an electrolysis cell series for NaCl brine, certain operating conditions are preferred. In the anode compartment a pH of from about 0.5 to about 5.0 is desired to be maintained. The feed brine preferably contains only minor amounts of multivalent cations (less than about 80 parts per billion when expressed as calcium). More multivalent cation concentration is tolerated with the same beneficial results if the feed brine contains carbon dioxide in concentrations lower than about 70 ppm when the pH of the feed brine is lower than 3.5.

Operating temperatures can range from 0.degree. to 110.degree. C., but preferably from about 60.degree. C. to about 80.degree. C. Brine purified from multivalent cations by ion-exchange resins after conventional brine treatment has occurred is particularly useful in prolonging the life of the solid polymer electrolyte membrane. A low iron content in the feed brine is desired to prolong the life of the solid polymer electrolyte membrane. Preferably the pH of the brine feed is maintained at a pH below 4.0 by the addition of hydrochloric acid.

Preferably the operating pressure is maintained at less than 7 atmospheres.

Usually the cell is operated at a current density of from about 1.0 to about 4.0 amperes per square inch, but in some cases operating above 4.0 amps/in..sup.2 is quite acceptable.