NO160725B - PARTICLES SUITABLE FOR THE PREPARATION OF SUCH PARTICLES, AND THE USE OF THE PARTICLES AS ELECTRO-MATERIALS, BY ELECTROLYSE OF A Aqueous Electrolyte. - Google Patents

Description

This invention relates to particles suitable for use as electrode material, a method for producing such particles, and the use of the particles as electrode material in the electrolysis of an aqueous electrolyte.

Chlorine and alkali metal hydroxide, for example sodium hydroxide and potassium hydroxide, are produced industrially by electrolysis of the corresponding alkali metal chloride solutions in an electrolysis cell. In a cell type, where the anode is separated. the cathode by an ion-permeable barrier, chlorine is evolved at the anode according to the reaction: while hydroxyl ions are formed at the cathode according to

which is actually a multi-step reaction in which hydrogen is absorbed on the surface of the cathode and the hydrogen molecule is desorbed from it.

The overall hydrogen reaction, as a series of postulated adsorption and desorption steps, consumes approx. 0.8 volts in an alkaline solution, so that if the cathode in a chlorine cell is depolarized with oxygen instead of being allowed to evolve hydrogen, it is possible to "save" approx. 1.2 volts, as the oxygen-reduction reaction can theoretically generate 0.4 volts. The previously developed cathodes for the use of oxygen as a depolarizing agent typically had a thin sandwich structure consisting of a microporous separator made of plastic combined with a catalyzed layer that was made water-resistant with, for example, polytetrafluoroethylene and pressed onto a current collector of wire cloth.

At the known depolarized cathodes, the catalyst zone becomes

added oxygen through the microporous material. Such cathodes work. However, they show various disadvantages, including separation or delamination of the different layers and flooding of the microporous layer.

The invention relates to particles suitable for use as electrode material, characterized in that they comprise a spherical substrate selected from a sintered material, a massive material and a connected agglomerate of small particles, the substrate being preferably selected from steel, iron, nickel, platinum, copper, silver and graphite and is at least partially coated with a mixture of a binder and an electrochemically active, electrically conductive catalyst selected from carbon black, activated carbon, platinum and silver and mixtures thereof, and wherein the particles have an average diameter between 0.3 mm and 2.5 cm.

The invention also relates to a method for producing particles suitable for use as an electrode material according to claims 1-4, characterized in that a fluid mixture is formed of an electrochemically active, electrically conductive catalyst selected from carbon black, activated carbon, platinum, silver and mixtures thereof , and a hydrophobic polymeric binder, a spherical substrate selected from steel, iron, nickel, platinum, copper, silver and graphite is coated with said mixture, and this is caused to solidify, so that particles with an average diameter between 0.3 mm and 2.5 cm is formed.

The invention also relates to the use of particles according to claim 1 as electrode material in the electrolysis of an aqueous electrolyte. Examples of electrolysis cells which are suitable for the use of the particles according to the invention as electrode material are described below. The particles can be used as anode material and/or as cathode material.

A layer of particles according to the invention has been found

to be well suited as a gas electrode. The electrode can be used as a cathode or as an anode.

The substrate on which a coating is applied should be a material that will retain its shape. In order to achieve a good result in the production of the particles described here, the substrate is preferably a material which is not significantly deformed when heated to a temperature of 350-375°G for a period of 1 hour.

The substrate can be a sintered material, a massive material or a connected agglomerate of small particles. Preferred materials include steel, iron, graphite, nickel, platinum, copper and silver. Graphite is particularly preferred because its availability and low price. The substrate can be of the same material or of a different material than the coating.

The substrate can be electrically conductive or electrically non-conductive. Electrically conductive substrates are preferred, as they show less resistance to the flow of electrical energy through the particles. Non-conductive substrates can be used, as they are at least partially coated with an electrically conductive coating, whereby a flow path is provided for the electric current. Non-conductive substrates, however, show greater resistance, as the electric charge must flow around the particle through its coating instead of flowing through the particle.

The average diameter of the particles is greater than 0.3 mm and up to as much as 2.5 cm. Particles that are clearly smaller than 2.5 cm are preferably used as electrode material, as decreasing particle size results in increasing surface area and a high porosity in the layer of particles. Particles having a size of from 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 pack and exhibit a high resistance to fluid flow through the bed of particles. Particles larger than 2.5 cm result in a smaller surface area than is desired for electrochemical reactions.

The spherical (ie spherical or ball-like) shape of the particles has been found to be particularly advantageous. Spherical particles form a layer with optimal porosity and surface area. Irregularly shaped particles tend to pack themselves so that the porosity of the layer becomes smaller.

The substrate need not be chemically inert towards the electrolyte or the electrolysis products from the process in which the particle is used. However, the substrate is preferably chemically inert, so that the coating does not need to cover the substrate completely. If the substrate is not chemically inert, the applied coating should be a complete coating, so that it prevents reaction between the substrate and the electrolyte or electrolysis products.

The coating on the substrate is a mixture of a binder

and an electrochemically active, electrically conductive catalyst. The binder should be a material that can be brought into fluid form by melting, dispersing or dissolving. The binder should be chemically stable to any electrolyte or products with which it will come into contact during use in an electrolytic cell. The binder should have good stability at the operating temperature of the electrochemical cell in which it is to be used. The binder itself need not be electrically conductive, as the catalyst mixed with it is electrically conductive.The coating may be a porous coating or a

non-porous coating depending on the materials from which the substrate is formed. If the substrate is chemically inert to the electrolyte, the coating can be a porous coating. If, however, the substrate is not chemically inert to the electrolyte, the coating should be

non-porous to prevent reactions between the substrate and the electrolyte or electrolysis products.

The binder is preferably a hydrophobic material. When used as an electrode material, the hydrophobic binder will cause the formation of bubbles on the surface of the particles and provide maximum contact between the gas and the liquid. If the binder is not hydrophobic, the surface of the particles will be wetted and no bubbles will form.

Different types of hydrophobic materials can be used as binders. The hydrophobic material can be a polyfluorocarbon, for example polytetrafluoroethylene, polychlorotrifluoroethylene, polytrifluoroethylene, polyvinyl fluoride, polyvinylidene fluoride and copolymers, including interpolymers and terpolymers that have tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, vinylidene fluoride and viriyl fluoride. Particularly preferred is polytetrafluoroethylene.

The particles also contain an electrochemically active, electrically conductive catalyst in the coating. The selection of the catalyst will depend on the type of process in which the electrode material is to be used. Examples of such processes include reduction of oxygen and oxidation of hydrogen. For the reduction of oxygen, the preferred catalysts include such materials as carbon black, platinum, silver and activated carbon. Black smoke is particularly preferred because its good physical properties and its availability. Black smoke catalysts with a surface area of 100 to 1,000 m 2 are preferred. grams of catalyst. Particular preference is given to those with a surface area of 150 to 500 m 2pr. grams of catalyst.

The substrate can be completely or partially coated with the binder/catalyst mixture. Preferably, the coating essentially covers the entire surface of the particle. The coating can be of any suitable thickness. Thicknesses of approx. 25 ym is particularly preferred. This provides a sufficient coating to cover essentially the entire particle surface, while the coating is sufficiently thin so that too much catalyst/binder mixture is not included for coating the particles. Thicknesses greater than 125 - 155 ym involve waste insofar as a substantial part of the catalyst is unavailable for the reaction as it is covered by additional catalyst and binder.

Optionally, the coating may include a peroxide-decomposing catalyst. This catalyst can be present both on the outer surface of the particle as well as in the inner surface of the particle. A particle in which the peroxide splitting catalyst is on the outer surface of the substrate is preferred, because catalyst is not wasted since unexposed catalyst is inactive.

The peroxide cleavage catalyst is known in the art and is typically a transition metal with hydrogen adsorption properties. Preferred peroxide cleavage catalysts include copper, silver, platinum, gold and mixtures or compounds thereof. Silver and platinum and mixtures or compounds thereof are mixed with carbon black and are particularly preferred.

Another particularly preferred class of known peroxide decomposition catalysts are compounds of (1) alkali metals, alkaline earth metals and metals from group UIB in the periodic system of the elements with (2) transition metals, which compounds are further characterized by electrocatalytic or surface catalytic properties. Particularly preferred are perovskites.

The invention also includes a method for producing coated particles suitable for use as an electrode material. The coated particles are produced by forming a fluid mixture of a particulate substrate, an electrochemically active, electrically conductive catalyst and a binder. The catalyst is bound to the substrate by bringing the fluid mixture to solidify. This creates a particle with a substrate that is at least partially coated with a binder and a catalyst. The fluid mixture is first formed by mixing a particulate substrate with a catalyst and a binder. The mixture can be made fluid in one of several ways. The binder itself can be heated to a temperature at which it softens or melts. It can be softened or melted before mixing with the catalyst and substrate or in situ with the catalyst and substrate. The softening point or melting point of various materials suitable as binders is well known to one skilled in the art. They can be found in a number of chemical reference books.

An alternative method for forming the fluid mixture is to disperse the binder, catalyst and substrate in a liquid medium. Various liquid media which are non-reactive with the three components of the mixture can be used. Particularly preferred are water or other liquid materials which are not solvents for any of the components in the mixture. The dispersion can be prepared after the ingredients are mixed, or any of the ingredients can be dispersed in a liquid before it is mixed with the other ingredients. It is important that the components are dispersed well in the liquid medium.

A third way to form the liquid mixture is

to dissolve the binder in a solvent. The dissolved binder is then mixed with the catalyst and the substrate. The solvent should not have the ability to dissolve either the substrate or the catalyst. The binder can be dissolved after or before the components have been mixed.

After the fluid mixture is prepared, it is brought to

to solidify, whereby the catalyst is bound to the substrate. The mixture can be made to solidify in one of several different ways. The solidification method is somewhat dependent on the method initially used to produce the fluid mixture. If the fluid mixture was prepared by melting or softening the binder, cooling the mixture will be sufficient to cause the binder to solidify. If the fluid mixture was prepared by dispersing one or more of the components in a liquid medium, solidification can be effected by removing the liquid medium from the mixture. The liquid medium can be removed by heating the mixture so that the liquid medium evaporates, or by placing the mixture under a negative pressure to evaporate the liquid medium. If the method used to produce the liquid mixture was dissolving the binder in a solvent, the solidification can be achieved by removing the solvent. The solvent can be removed by heating and evaporating the solvent, by applying vacuum to remove the solvent or by reacting the solvent with another component.

Optionally, the mixture can be treated during solidification

such a way that the coated particles are prevented from sticking together to form a single mass. Agitation is an appropriate way to prevent the particles from sticking to each other during the solidification process. The degree and intensity of agitation used is minimal and can be achieved by simply stirring or rolling the material during solidification. Agitation during solidification also helps to improve the uniformity of the coating on the substrate.

Optionally, after solidification, the coated particles can be heated to a temperature close to or above the binder's softening point, whereby the material's bond to the substrate is strengthened. Heating to this temperature causes the binder to soften and is better mixed with the catalyst and better bonded to the substrate.

Optionally, the steps relating to the coating of the particle can be repeated several times. In this way, the amount and thickness of the coating can be regulated depending on the number of times the process is repeated. The thickness of the coating is not critical or

essential for the invention.

Optionally, a surface-active substance can be added to the fluid mixture, whereby the substrate and the catalyst are better wetted by the binder. This ensures better contact and a more even distribution of the mixture. It is preferred to use a type of surfactant that can be removed from the particle after binding has taken place. This is preferred because the coated particle during use as an electrode material should preferably not be wetted by the electrolyte. As discussed above, the formation of gas bubbles on the surface of the particle is desired. The presence of a surfactant at this stage would minimize the formation of gas bubbles, as the particles would be more wetted by an electrolyte. The preferred surfactants are for this reason those non-ionic surfactants which can be thermally decomposed and leave only a carbon residue. Such surface-active substances are well known in the field, so that further explanation is considered superfluous.

Optionally, the particles can be washed after binding to remove catalyst that does not have to be bound to the substrate. Water and other liquids that are not solvents for any of the components of the coating are suitable. Water is particularly preferred as a convenient agent.

The amount of catalyst and binder in relation to the substrate, on a weight or volume basis, depends on the degree of coating desired 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, a larger amount of catalyst and binder is used.

The ratio between the amount of catalyst and the amount of binder can be varied over a wide range. If, for example, carbon black is used as a catalyst and polytetrafluoroethylene is used as a binder, a weight ratio of from 4:1 to 1:4 is preferred, especially a ratio between carbon black and PTFE of from 1:1.5 to 1.5:1 on a weight basis. Other catalysts and binders should also be used within this same general condition.

If a metal powder is used as a catalyst, such as silver, platinum or a mixture thereof, it is appropriate to express

the share on a volume basis rather than on a weight basis. The relationship between

lom metal powder and binder can be in the range from 4:1 to 1:4 on a volume basis. Preferably, the ratio should be from 1:1.5 to 1.5:1 on a volume basis.

Optionally, various types of additional catalysts can be added to the fluid mixture. One of the preferred embodiments of the present disclosure includes the addition of a peroxide cracking catalyst in conjunction with a carbon black catalyst. When using both of these catalyst types, from 1 to 35 percent by weight of the total catalyst used should be a peroxide splitting catalyst, and

from 65 to 99% of the total amount of catalyst should be the carbon black catalyst. Preferably, 3 - 10% of the catalyst should

pure be the peroxide cracking catalyst, and from 90 to 97% of the catalyst should be the carbon black catalyst. This catalyst mixture should be used in the conditions discussed above regarding the catalyst/binder ratio.

Usually, the peroxide cleavage catalyst used is

des for the formation of the fluid mixture, a precursor-catalyst-

dare. The material used in the fluid mixture is, in other words, a compound of the catalyst that must be decomposed ter-

misc or chemical to form the catalyst itself. This thermal or chemical cleavage can be carried out at any stage in the process by which the coated particles are formed. Preferably, the material is cleaved thermally or chemically

mix before mixing with the binder and substrate. This enables better control of the thermal or chemical decomposition of the catalyst precursor. Furthermore, it provides maximum contact between the carbon black and the peroxide decomposition catalyst. If these two are mixed and split before mixing with the substrate and binder, maximum contact is achieved between the carbon black and the peroxide splitting catalyst.

The coated particles described herein are eg

net for use as an electrode material, as they are electrically conductive and catalytically active. Appropriately, they can be used as a filler layer electrode. They are then formed into a layer and supported in an appropriate manner in the cell. They are electrically connected to a power source for the cell. Optionally, a current collector can be used. The current collector can be a wire mesh bag,

wire mesh container or similar, which surrounds the catalyzed particles and contains them.

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

According to a preferred embodiment of the method for using the particles described herein, an aqueous solution of alkali metal halide is supplied to an electrolysis cell which has an anolyte chamber with an anode and a catholyte chamber with cathode devices, as well as possibly an ion-permeable barrier between them. Typically, the anode is a valve metal, for example titanium, tantalum, tungsten, niobium or the like, with a suitable electrocatalytic surface. Suitable anodic electrocatalytic surfaces are well known in the art and include transition metals, oxides of transition metals, compounds of transition metals, especially platinum group metals, oxides of platinum group metals and compounds of platinum group metals. Particularly preferred are compounds of oxides of platinum group metals with oxides of valent metals, i.e. titanium, tantalum, tungsten, niobium and the like.

The ion-permeable barrier may be an electrolyte-permeable diaphragm, e.g. 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 such as a cation-selective permionic membrane. Such membranes are typically made of fluorocarbon polymers that have attached acid groups. Typical pendant acid groups include sulfonic acid groups, carboxylic acid groups, phosphonic acid groups, phosphoric acid groups, precursors of these and reaction products thereof.

The anolyte liquid is typically an aqueous salt solution containing 120 - 250 g/l sodium chloride or 180 - 370 g/l potassium chloride and typically has a pH from 1.5 to 5.5. The salt solution that is fed in is typically a saturated or essentially saturated salt solution containing 300 - 325 g/l sodium chloride or 450 - 500 g/l potassium chloride. The catholyte liquid recovered from the electrolysis cell may be a catholyte liquid containing 10-12 weight percent sodium hydroxide and 15-25 weight percent sodium chloride, or 15-20 weight percent potassium hydroxide and 20-30 weight percent potassium chloride, as when an electrolyte-permeable barrier is used. Alternatively, the catholyte product may contain from 10 to 45 weight percent sodium hydroxide or from 15 to 65 weight percent potassium hydroxide, which when the ion-permeable barrier is a cation-selective permionic membrane placed between the anode and the cathode.

An oxidizing agent, e.g. oxygen, air or oxygen-enriched air, is preferably supplied to the catholyte chamber, just as an electric current is passed from the cathode chamber to the anode chamber, thereby obtaining an anode product of chlorine and a cathode product of alkali metal hydroxide, characterized by the fact that gaseous hydrogen product is practically absent. The invention is particularly advantageous when using a filler layer cathode with coated particles as described herein.

According to a further exemplification of the invention, an electrolysis cell is used with an anolyte chamber built up from a material that is resistant to concentrated, chlorine-containing alkali metal chloride solutions, an anode in the anolyte chamber, a catholyte chamber that is built up from a material that is resistant to concentrated alkali metal hydroxides solutions, a cathode device in said cathode chamber, as well as an ion-permeable barrier placed between the anode and the cathode device. The electrolysis cell's equipment includes devices for supplying an oxidizing agent to the electrolyte within the catholyte chamber and a cathode device comprising individual particles.

In another preferred embodiment of the method for using the particles described herein for the production of a peroxide, an aqueous hydroxide solution is fed into the electrolysis cell already described, with the exception that no ion exchange membrane is used. The anolyte that is fed in is typically an aqueous solution containing from 15 to 100 g/l sodium hydroxide. The catholyte liquid which is recovered from the electrolysis cell can be a catholyte liquid containing 0.5-3 weight percent hydrogen peroxide and 15 - 100 g/l sodium hydroxide.

According to a further exemplification of the invention, an electrolysis cell is used which has an anolyte chamber built up of a material which is resistant to concentrated alkali metal hydroxide solutions, an anode in the anolyte chamber, a catholyte chamber which is built up of a material which is resistant to concentrated alkali metal hydroxide solutions solutions, a cathode device in said cathode chamber, and an ion-permeable barrier placed between the anode and the cathode device. The electrolytic cell's equipment includes devices for supplying an oxidizing agent to the electrolyte within the cathode chamber and a cathode device comprising individual porous particles.

EXAMPLE 1

To produce a catholytically active coating, 0.7 g of carbon black was mixed with 20 ml of an aqueous silver acetate solution with a concentration of 10 g of silver acetate per liter of solution. A drop of surfactant ("Triton" X-100, a product supplied by Rohm and Haas Co.) was added to improve the wetting of the carbon black. The mixture was then oven-dried at approx. 100°C. The mixture was then heated for 1 hour at 350°C in a nitrogen atmosphere, whereby the silver acetate was thermally decomposed. This material was then mixed with 3.5 g of a 1:10 aqueous emulsion (1 part "Teflon" 30B, a product supplied by E.I. duPont de Nemours & Co., a fluoropolymer, to 10 parts water). A slurry was formed from this. 10 g of graphite particles of size -10 +20 US Mesh were added to the slurry. After mixing, the material was dried at approx. 100°C. The material was then heated at 350°C for 1 hour in a nitrogen atmosphere. The particles obtained were graphite particles having a

carbon-fluorocarbon-silver coating on its surfaces.

EXAMPLE 2

An electrolysis cell was constructed as described herein. The cell had an anode and a cathode separated by a porous asbestos diaphragm. The cathode was a packed layer of the particles produced in example 1. The anode was ruthenium oxide-coated titanium.

A sodium chloride solution with a concentration of approx. 300 g of NaCl per liters was added to a chamber containing the anode. Oxygen gas was supplied through the openings between the particles that made up the cathode. An electric current was at a voltage of approx. 2 volts and a current density of about. 0.155 amps per cm<2 >conducted between the anode and the cathode, which caused electrolysis of the salt solution. Chlorine gas was produced at the anode, and sodium hydroxide was produced at the cathode.

Claims (14)

1. Particles suitable for use as electrode material, characterized in that they comprise a spherical substrate selected from a sintered material, a massive material and a connected agglomerate of small particles, the substrate being preferably selected from steel, iron, nickel, platinum, copper, silver and graphite and is at least partially coated with a mixture of a binder and an electrochemically active, electrically conductive catalyst selected from carbon black, activated carbon, platinum and silver and mixtures thereof, and wherein the particles have an average diameter between 0.3 mm and 2.5 cm. 2. Particles according to claim 1, characterized in that the binder is a hydrophobic fluorocarbon polymer, and the catalyst has a surface area of 100-1000 m<2>/g. 3. Particles according to claim 1, characterized in that the catalyst is carbon black and has a surface area of 150-500 m<2>/g, the binder is polytetrafluoroethylene, and where the weight ratio between carbon black and binder is from 4:1 to 1:4. 4. Particles according to claim 1, 2 or 3, characterized in that the coated particle has an average diameter of 0.7-4 mm. 5. Process for producing particles suitable for use as an electrode material according to claims 1-4, characterized in that a fluid mixture is formed of an electrochemically active, electrically conductive catalyst selected from carbon black, activated carbon, platinum, silver and mixtures thereof, and a hydrophobic polymeric binder, a spherical substrate selected from steel, iron, nickel, platinum, copper, silver and graphite is coated with said mixture, and this is caused to solidify, so that particles with an average diameter between 0.3 mm and 2, 5 cm is formed. 6. Method according to claim 5, characterized in that the fluid mixture is formed by heating the polymeric binder to a temperature that is at least as high as its softening temperature, and cooling the mixture to a temperature below the softening temperature, whereby the coating is bonded to the substrate. 7. Method according to claim 5, characterized in that the fluid mixture is formed by dispersing the catalyst, the binder and the substrate in a liquid, and heating the mixture, whereby the liquid evaporates and the coating is bonded to the substrate. 8. Method according to one or more of claims 5-7, characterized in that the fluid mixture is formed by dissolving the binder in a solvent and heating the mixture, whereby the solvent evaporates and the coating is bonded to the substrate. 9. Method according to one or more of claims 5-8, characterized in that carbon black is used as catalyst and polytetrafluoroethylene is used as binder, where the weight ratio between carbon black and binder is from 4:1 to 1:4, preferably from 1:1 .5 to 1.5:1. 10. Method according to one or more of claims 5-8, characterized in that if a metal powder is used as catalyst, a ratio between catalyst and binder in the range from 4:1 to 1:4 is used on a volume basis, preferably in the range from 1: 1.5 to 1.5:1 on a volume basis. 11. Method according to one or more of claims 5-10, characterized in that a surfactant is added to the mixture, preferably a non-ionic surfactant. 12. Method according to one or more of claims 5-11, characterized in that a second catalyst is added which is a peroxide decomposition catalyst. 13. Method according to claim 12, characterized in that, based on the total amount of catalyst in the mixture, 1-35% by weight of peroxide decomposition catalyst and 65-99% by weight of carbon black are used, preferably 3-10% by weight and 90-97% by weight. 14. Use of particles according to claim 1 as electrode material in the electrolysis of an aqueous electrolyte.