Particulate material used in electrodes

PARTICULAR TO BE USED AS ELECTRODE MATERIAL

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Also a subject of the invention is the method for producing this electrode material, which is to mix the catalyst and binder, to bond to the substrate and to form particles of the diameter indicated above.

It is a further object of the present invention an electrolytic cell which uses as electrode material for said particle as well as a method for electrolyzing an aqueous electrolyte in said electrolytic cell.

The invention is based on an improved electrode material and a method for producing such material.

Alkali metal chlorine and hydroxide, for example sodium hydroxide and potassium hydroxide, are commercially prepared by electrolysis of the corresponding alkali metal chloride brines in a

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Electrolytic cell. In a cell type, where the anode is separated from the cathode by an ion permeable barrier, chlorine is released at the anode according to the reaction:

Cl —01₂ + 2e while at the cathode hydroxide ions are produced according to

2H₂0 + 2e -4 H₂ + 20H “which is now a multi-phase reaction in which a species of hydrogen is absorbed onto the cathode surface and the hydrogen molecule is absorbed from it.

The total hydrogen reaction, as a series of postulated absorption and desorbation phases, consumes about 0.8 volts in an alkaline solution, so that if the cathode in a chlorine cell is depolarized with oxygen instead of allowing hydrogen to come off , a 1.2 volts saving can be achieved as the oxygen reduction reaction can theoretically generate 0.4 V. Cathodes previously developed for the use of oxygen as a depolarizer were characterized by a thin sandwich structure of a microporous plastic separator combined with a catalyzed, moisture-proof layer with, for example, polytetrafluoroethylene, and compressed over a current collector of wire mesh. In the prior art depolarized cathodes, oxygen is introduced into the catalyst zone through the microporous liner. These cathodes work. However, they suffer from several deficiencies, including separation or delamination of the various layers and flooding of the micro-layer.

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The invention is based on an electrode material comprising a substrate at least partially coated with a mixture of a binder and an electrochemically active and electrically conductive catalyst, characterized in that the electrode material is a coated particle with an average diameter greater than 0 µm. 3 mm and less than 2.5 cm.

The invention is also based on a method for producing an electrode material comprising forming a fluid mixture of an electrochemically active and electrically conductive catalyst and a binder, linking the catalyst to the substrate by solidifying the fluid mixture and forming it. the particulate electrode material having an average diameter greater than 0.3 mm and up to 2.5 cm.

The invention also resides in a method of electrolyzing an aqueous electrolyte into an electrolytic cell having an anolyte compartment with an anode therein and a catholyte compartment having a cathode therein, which method comprises inducing said aqueous electrolyte into the cell, passing a current between anode and cathode and the recovery of electrolysis products, at least one of the electrodes comprising an electrode material comprising a substrate which is at least partially coated with an electrically conductive mixture of a hydrophobic material and at least one catalyst and wherein the electrode material comprises a series of compacted particles having each having an average diameter greater than 0.3 mm and up to 2.5 cm.

The invention is also based on an electrolytic cell having an anolyte compartment with an anode therein and a catholyte compartment with a cathode therein, a system for introducing an aqueous electrolyte into the cell, a system for passing an electric current between the anode and the cathode. , and a system for recovering the electrolysis products from the cell, at least the anode or | Κ4 «$. liI liSlÈOi η Γ

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-4ί | a cathode formed of an electrode material comprising a substrate which is at least partially coated with an electrically conductive mixture of a hydrophobic material and at least one catalyst, characterized in that the electrode material ii comprises a series of compacted particles each having one of them having an average diameter greater than 0,3 mm and ii less than 2,5 cm. already_f The cell may optionally contain an ion-permeable barrier located between the anode and the cathode. The cell may optionally include a system for feeding the electrode with a gas.

| It has been found that a bed of these partici ... could be useful as a gaseous electrode. The electrode j,_THE | can be used as cathode or as anode.

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The substrate on which a coating is applied must be a material capable of maintaining its shape.

! In order to provide a convenient method of preparing the particles described herein, the substrate is preferably a material that scarcely deforms when heated to a temperature of 350 to 375 ° C for a period of 1 hour.

The substrate may be a sintered material, a solid material, or a bonded agglomeration of small | particles. Suitable building materials include steel, iron, graphite, nickel, platinum, copper and silver, but are not limited to these materials. Particularly preferred is graphite due to its availability and low price. The substrate may be of the same or a different composition than the coating.

The substrate may be electrically conductive or electrically non-conductive. Electrically conductive substrates are preferred because they offer lower resistance to the flow of electrical energy through the particles. Non-conductive substrates are functional because they are at least

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They are partially coated with an electrically conductive coating, thus providing a path for the passage of electronic current. Non-conductive substrates, however, offer greater resistance because the electrical charge must pass around the particle more through its coating than through the particle.

average particle diameter is larger than 0.3 mm and may reach 2.5 cm. For use as an electrode material it is preferable to use particles that are generally smaller than 2.5 cm. This minimizes surface area and gives a high porosity to the particle bed. Particles with a size of 0.7 mm to 4 mm are particularly preferred when the particles are used as an electrode material. Particles smaller than 0.3 mm tend to compact and offer high resistance to the passage of fluid through the particle bed. Substrates larger than 2.5 cm are preferred because there is a minimum amount of surface area for electrical chemical reactions to take place.

The particles may be any shape. The irregularly shaped particles may be conveniently used. However, spherical particles are preferred because they form a bed of optimal porosity and surface area. Irregularly shaped particles tend to compact and minimize bed porosity.

The substrate need not be chemically inert to the electrolyte or electrolysis products of the process in which the particle is used. Preferably, however, the substrate is chemically inert so that the coating does not need to completely cover the substrate. If the substrate is not chemically inert, the coating applied thereon must be a complete coating to prevent reaction between the substrate and the electrolyte or electrolysis products.

coating on the substrate is a

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mixture of a binder and an electrochemically active and electrically conductive catalyst. The binder should be a material that can be fluidly formed by melting, dispersing or dissolving. The binder must be chemically stable for any electrolyte or products it has counted when used in an electrolyte cell. The binder must be firmly stable at the operating temperature of the electrochemical cell in which it is to be used. 0 The binder need not be electrically conductive since the catalyst that is mixed with it is electrically conductive. The coating may be a porous coating or a non-porous coating depending on the substrate building materials. If the substrate is chemically inert to the electrolyte, the coating may be a porous coating. However, if the substrate is not chemically inert to the electrolyte, the coating should be non-porous to prevent reactions occurring between the substrate and the electrolyte or electrolyte products.

Preferably the binder is a hydrophobic material. When used as an electrode material the hydrophobic binder will cause bubbles to form on the surface of the particles and provide maximum contact between gas and liquid. If the binder is not hydrophobic the particle surfaces should be moistened and no bubbles will form.

Various types of hydrophobic material may be used as a binder. The hydrophobic material may be a polyethyl fluorocarbon, for example polytetrafluoroethylene, polychlorotrifluoroethylene, polytrifluoroethylene, polyvinyl fluoride, polyvinylidene fluoride and copolymers, including interpolymers and terpolymers having tetrafluoroethylene, trifluoroethylene trifluoride fluoride fluoride fluoride, Particularly preferred is polytetrafluoroethylene.

The particles also contain in the coating an electrochemically active and electrically conductive catalyst. The choice of catalyst will depend on the type of ι? 4ΐα $ ο.Εβί

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4.V .. · process to which the electrode material will be subjected. Examples of such processes are oxygen reduction and hydrogen oxidation. For oxygen reduction preferred catalysts include things such as carbon black, platinum, silver and activated carbon. Carbon black is particularly preferred because of its good physical characteristics and availability. Preferred are carbon black catalysts with surface areas of 100 to 1,000 square meters per gram of catalyst. Particularly preferred are those having a surface area of ​​150 to 500 square meters per gram of catalyst.

The substrate may be partially or fully coated with the mixture. Preferably the coating covers substantially the entire surface of the particle. The coating may be of any convenient thickness. Thicknesses of about 1 thousandth of an inch are particularly preferred. This provides a coating suitable to cover substantially the entire surface of the particle, and which is still thin enough to retain the catalyst-binder mixture necessary to cover the particle. Thicknesses beyond 5 to 6 milliseconds are unnecessary as a substantial portion of the catalyst is not available for reaction as it is covered by additional catalyst and binder.

The coating may optionally include a peroxide decomposition catalyst. As contemplated herein, the peroxide decomposition catalyst may be either on the outer surface of the particle or on the inner surface of the particle.

Preferred is a particle wherein the peroxide decomposition catalyst is on the outer surface of the substrate because no catalyst is wasted since the unexposed catalyst has no efficacy.

Pero Decomposition Catalysts

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These are known in the art and are typically made of a transition metal with hydrogen adsorption properties. Peroxide decomposition catalysts include copper, silver, platinum, gold and their mixtures or compounds. Silver and platinum and res. Specific mixtures or compounds mixed with carbon black are especially preferred.

Another particularly desirable class of peroxide decomposition catalysts is that of (1) alkaline, rare earth alkaline, and Group IIIB metals with (2) transition metals, which compounds are furthermore characterized by their electrocatalytic properties. or surface catalytic. Especially preferred are the perovskites.

The invention includes within its scope a method for producing coated particles suitable for use as an electrode material. The coated particles are prepared by forming a fluid mixture of particulate substrate, an electrochemically active and electrically conductive catalyst and a binder. 0 The catalyst is attached to the substrate by solidifying the fluid mixture. This forms a particle having a substrate at least partially coated with a binder and a catalyst.

The fluid mixture is initially formed by mixing a particulate substrate with a catalyst and a binder. The mixture may be made fluid in one of several ways. The binder itself may be heated to a temperature at which it is softened or melted.

It may be softened or melted before being mixed with the catalyst and substrate or in situ with the catalyst and substrate. The softening point or melting point of the various materials suitable for use as binders are well known to those skilled in the art. They can be found in many reference books or chemistry manuals.

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An alternative method of forming the fluid mixture is to disperse the binder, catalyst and substrate in a liquid medium. Various liquid media which do not react with the three components of the mixture may be used. Particularly preferred are water or other non-solvent liquid materials for any of the components of the mixture. Dispersion can be done after the components have been mixed or after either component can be dispersed in a liquid before being mixed with the other components. It is important to have the components properly dispersed in the liquid medium.

A third way of forming the liquid mixture is to dissolve the binder in a solvent. The dissolved binder can then be mixed with the catalyst and substrate. The solvent should not have the ability to dissolve neither the substrate nor the catalyst. The binder may be dissolved before or after the components have been mixed.

Once the fluid mixture has been formed it is solidified to bind the catalyst to the substrate.

The mixture may be solidified in one of several different ways. The solidification method is somewhat dependent on the method initially used to form the fluid mixture. If the fluid mixture was formed by melting or softening the binder, simply cooling the mixture will cause the binder to solidify. If the fluid mixture has been formed by dispersing one or more of the components in a liquid medium, solidification may be caused by removing the liquid medium from the mixture. 0 Liquid medium may be removed by heating the mixture to evaporate the liquid medium or subjecting the mixture to a vacuum to vaporize the liquid medium.

If the method used to form the liquid mixture is to dissolve the binder in a solvent, solidification may be performed by removal of the solvent. The solvent may be removed by heating to vaporize the solvent, by vacuum to remove the solvent, or by reacting the solvent with another component.

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Optionally, the mixture during there; The solidification may be treated in such a way as to prevent the coated particles from sticking together to form a single mass. Agitation is a convenient means of preventing particles from adhering to each other during the solidification process. The amount and intensity of agitation used is minimal and can be accomplished by stirring or rolling the material during solidification. Agitation during solidification also helps to | ! increase the uniformity of the coating on the substrate.

Optionally, upon solidification, the coated particles may be heated to a temperature near or above the binder softening point in order to increase the bonding of the material to the substrate. Heating to this temperature causes the binder to soften and become more intermixed with the catalyst and better bonded to the substrate.

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Optionally the particle coating phases may be repeated a number of times. In this way the amount and thickness of the coating can be controlled as a function of the number of times the process is repeated. The thickness of the coating is not critical to the invention.

Optionally a surfactant may be added to the fluid mixture to increase substrate and catalyst wetting by the binder. This ensures better contact and more even distribution of the mixture. It is preferable that a type of surfactant be used which can be removed from the particle after drying. have checked the connection. This is preferred because, when in use as an electrode material, the coated particle is preferably not wetted by the electrolyte. As previously described, the formation of gas bubbles on the particle surface is desirable. The presence of a surfactant at this point will minimize the formation of gas bubbles as the wetting of the particles by an electro^%*^s® «£ S« S33Cln2a »_£w?_u-i

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The rate will be increased. As a result, preferred surfactants are nonionic surfactants which can be thermally decomposed leaving only a charcoal residue. These surfactants are well known in the art and need no further elaboration.

The particles may optionally be washed after being bound to remove any catalyst that has not been bound to the substrate. Water and other non-solvent liquids are suitable for any of the coating components. Water is particularly preferred because of its convenience.

The weight or volumetric ratio of catalyst and binder to substrate depends on the desired degree of coating on the substrate. If a small amount of coating is desired, obviously only a small amount of catalyst and binder should be mixed with the substrate to form a fluid mixture. Conversely if a thick coating is desired, the amount of catalyst and binder should be increased.

The ratio of catalyst amount to binder amount can be varied over a very wide range. For example if carbon black is used as a catalyst and as a binder polytetrafluoroethylene is used, weight ratios of 4: 1 to 1: 4 may be used.

Preferred weight ratios of carbon black to PTEE are 1: 1.5 to 1.5: 1 · Other catalysts and binders may also be used within this same general ratio.

If a metal powder such as silver, platinum or other metals is used as a catalyst, the volume ratio rather than weight ratio should be expressed. The metal powder for the binder may be in the volumetric ratio from 4: 1 to 1: 4. Preferably the volume ratio should be from 1: 1.5 to 1.5: 1.

They can be added to the

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fluid mixing, various types of additional catalysts, one of the preferred embodiments of the invention described herein includes the addition of a peroxide decomposition catalyst in conjunction with a carbon black catalyst. When both of these catalyst types are used 1 to 35 percent by weight of the total catalyst used should be a peroxide decomposition catalyst and 65 to 99 percent of the total amount of catalyst used should be carbon black catalyst. Preferably 3 to 10 percent of the catalyst should be the peroxide decomposition catalyst and 90 to 97 of the catalyst should be the carbon black catalyst. This catalyst mixture should be used in the previously discussed ratios regarding the catalyst to binder ratio.

Usually the peroxide decomposition catalyst used to form the fluid mixture is a precursor catalyst. In other words, the material used in the fluid mixture is a catalyst compound, which must be thermally or chemically decomposed in order to form the catalyst itself. This thermal or chemical decomposition may be performed at any stage of the process of forming the coated particles. Preferably the material is thermally or chemically decomposed before being mixed with the binder and the substrate. This allows for better control of the thermal or chemical decomposition of the catalyst precursor. Additionally it provides maximum contact between carbon black and the peroxide decomposition catalyst. If both are mixed and decomposed before being mixed with the substrate and binder, maximum contact is obtained between carbon black and the peroxide decomposition catalyst.

The coated particles described herein are suitable for use as an electrode material as they are electrically conductive and catalytically active. Conveniently they may be used as a compacted bed electrode. As such they are formed in a bed and supported in some convenient way within

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-13rior of the cell. They are electrically connected with a powder supply associated with the cell. Optionally a current collector may be used. The current collector may be a wire mesh pocket, a wire mesh container, or the like, surrounding the catalyst particles and containing them therein.

If the coated particles include a peroxide decomposition catalyst, the particles may be used in a cell to produce a hydroxide. If the coated particles do not contain a peroxide decomposition catalyst the particles may be used in a cell to produce a peroxide.

According to a preferred embodiment of the method of using the particles described herein, an aqueous alkali metal halide brine is introduced into an electrolytic cell having an anolyte compartment with an anode therein, and a catholyte compartment with a cathode system. and optionally an ion permeable barrier therebetween. Typically, the anode is a metal valve, for example, titanium, tantalum, tungsten, calabium, or the like, with a suitable electrocatalytic surface thereon. Suitable anodic electrocatalytic surfaces are well known in the art and include transition metals, transition metal oxides, transition metal compounds, especially platinum group metals, platinum group metal oxides and metal groups of the platinum group. platinum. Especially preferred are platinum group metal compounds with valve metal oxides, i.e. titanium, tantalum, tungsten, colombium and the like metals.

The ion permeable barrier may be an electrolyte permeable diaphragm, for example, a deposited asbestos diaphragm, a preformed asbestos diaphragm or a microporous synthetic diaphragm. Alternatively, the ion-permeable barrier may be ion-permeable but electrolyte-impermeable as a selected permionic membrane.

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cjiva rive. Typically the cation-selective permionic membranes are fluorocarbon polymers having pendant acid groups on them. Typical pendant acid groups include sulfonic acid groups, carboxylic acid groups, include phosphonic acid groups. , phosphonic acid groups, their precursors, and their reaction products;

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|) anolyte liquor is typically a brine;

An aqueous salt containing from 120 to 250 grams per liter of sodium chloride or 180 to 370 grams per liter of potassium chloride is typically at a pH of 1.5 to 5.5. supplied is typically a saturated or substantially saturated brine containing from 300 to 325 grams per liter of sodium or 450 to 500 grams per liter of potassium chloride. 0 The catholyte liquor recovered from the electrolytic cell may be a catholyte liquor containing approximately 10 to 12 percent by weight of sodium hydroxide and 15 to 25 percent of sodium, or approximately 15 to 20 percent of hydroxide. potassium chloride and approximately 20 to 30 percent potassium chloride when a permeable electrolyte barrier is used. Alternatively, the catholyte product may contain from 10 to 45 percent sodium hydroxide, or from 15 to 65 percent potassium hydroxide, when the ion-permeable barrier is a cation-selective permionic membrane interposed between the anode and the cathode.

An oxidant, for example oxygen, air or oxygen enriched air, is preferably introduced into the catholyte compartment as an electric current is introduced from the cathode compartment to the anode compartment in order to provide a chlorine anode product and an alkali metal hydroxide cathode product, characterized by the substantial absence of hydrogen gas product. 0 The invention is particularly suitable for a preferred compacted bed cathode including coated particles as described herein.

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According to another embodiment of the invention, there is provided an electrolytic cell having an anolyte compartment made of a chlorinated alkali chloride brine resistant material, an anode in the anolyte compartment, a catholyte compartment which is made of a material resistant to a concentrated solution of alkali metal hydroxide, a system ₍ ! cathode chamber in said cathode compartment, and an ion permeable barrier interposed between the anode and the system!

Cathode The electrolytic cell contemplated herein is characterized in that the catholyte compartment has a system for introducing an oxidant into the electrolyte within the cathode compartment and a cathode system comprising single particles.

In another preferred embodiment of the method of using the particles described herein to produce peroxide an aqueous hydroxide solution is introduced into the electrolytic cell already described except that no ion exchange membrane is used. Anolyte feed is typically an aqueous solution containing 15 to 100 grams per liter of sodium hydroxide. 0 The catholyte liquor recovered from the electrolytic cell may be a catholyte liquor containing a weight percent of approximately 0.5 to 3 percent hydrogen peroxide and 15 to 100 grams per liter of sodium hydroxide.

As contemplated herein, an oxidant, for example oxygen, air or oxygen enriched air, is introduced into the catholyte compartment at the same time as an electric current is introduced from the cathode compartment to the anode compartment to provide a product. . oxygen anode duct, and water and a cathode product of an alkali metal hydroxide and a peroxide, characterized by the substantial absence of hydrogen gas product. 0 The invention is particularly directed to the cathode system for carrying out the reaction, whose cathode system comprises coated particles as described herein.

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According to another embodiment of the invention, there is provided an electrolytic cell having an anolyte compartment made of a material resistant to concentrated alkali metal hydroxide solutions, an anode in the anolyte compartment, a compartment catholyte which is made of a material resistant to concentrated alkali metal hydroxide solutions, a system of:

! cathode in said cathode compartment, and an ion permeable barrier interposed between the anode and the ion system; cathode. The electrolytic cell contemplated herein is characteristic! The catholyte compartment has a system for introducing an oxidant into the electrolyte within the cathode compartment and a cathode system comprising individual fibrous particles.

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Example 1

To prepare a catalytically active coating, 0.7 grams of carbon black was mixed with 20 milliliters of an aqueous silver acetate solution having a concentration of 10 grams of silver acetate per liter of solution. A drop of surfactant was added. -Active (Triton® x -100, a product manufactured by Rohm and Haas Co.) to increase carbon black wetting. The mixture was then oven dried at about 100 ° C. Then the mixture was heated for 1 hour at 350 ° C in a nitrogen atmosphere to thermally decompose silver acetate. This material was then mixed with 3.5 grams of a 1:10 aqueous solution (1 part Teflon®30 B, a product manufactured by EI duPont de Nemours &amp; Co., fluoropolymer for 10 parts water). Between them a syrup formed.

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To the syrup was added 10 grams of US-10 + 20 screen graphite particles. After mixing the material was dried at about 100 ° C. Then the material was heated for 1 hour at 35θ ° Ο in a nitrogen atmosphere. The particles produced were graphite particles having a carhono-fluorocarhono-silver coating on their surfaces.

Example 2

An electrolytic cell was assembled as described herein. The cell had an anode and a cathode separated by a porous asbestos diaphragm. The cathode consisted of a compacted bed of the particles produced in Example 1. The ruthenium oxide coated titanium anode.

Sodium chloride brine solution having a concentration of about 300 grams per liter of NaCl was drained into the anode-containing compartment. Gaseous oxygen was allowed to flow into the openings between the cathode constituent particles. Between the anode and cathode an electric current was passed with a voltage of about 2 volts and a density of^- current about 1 amp. per square inch to cause electrolysis of the brine solution. In the anode chlorine gas was produced and in the cathode sodium hydroxide was produced.