EP0319517B1 - Material based on conductive fibres, its manufacture and its use, in particular in the production of cathode elements - Google Patents

Abstract

Materials consisting of sheets of fibres and more precisely sheets of electrically conductive fibres bonded by means of a fluorine- containing polymer. The invention also relates to a process for the manufacture of these sheets from suspensions containing the fibres, the fluorine-containing polymer and, with a view to certain applications, additives such as electrocatalytic agents. These sheets of fibres and especially those containing electrocatalyst agents can be employed for the production of cathode components for electrolysis cells.

Description

The subject of the present invention is a material which can be used in particular for the production of the cathode element of an electrolysis cell, and in particular of an electrolysis cell of aqueous solutions of alkali halides. It also relates to the cathode element comprising said material. The invention also applies to the process for manufacturing said materials and said cathode elements.

Application EP-A-2787, which relates to electrodes and their use in an energy storage battery, cites general information on materials consisting of carbon fibers and a resin, the carbon fibers being woven and which can be associated with asbestos fibers.

Such electrodes are produced from woven carbon fibers and then bonded as soon as they are prepared to a metal plate.

The material to which the invention relates in the first place consists of a sheet comprising fibers and a binder, this sheet being characterized in that at least part of the fibers consists of electrically conductive fibers, and in that that the binder is chosen from fluorinated polymers.

Within the meaning of the invention, the term “ply” denotes a three-dimensional assembly whose thickness is substantially smaller than the smallest of the other dimensions, said assembly possibly or not having two parallel surfaces. These plies generally have substantially flat and rectilinear surfaces but can also have the most diverse shapes, said shape being able in particular to be determined by the shape of the material with which the ply may be associated, as will be specified below.

For information purposes only, and in the event of the use of the layers according to the invention for the production of the cathode element of a sodium chloride electrolysis cell, the thickness of this layer can be between 0.1 and 5 mm, one of the large dimensions, which can substantially correspond to the height of the cathode element, which can reach 1 m, or even more, the other large dimension, which can correspond substantially to the perimeter of said element up to several tens of meters. It should be recalled that these values are indicated at present for the sole purpose of giving an order of magnitude of sheets in accordance with the invention, but it is obvious that such indications cannot in any way limit the field concerned by the present invention to sheets of precise dimensions.

As has been specified, one of the constituents of the sheets according to the invention consists of fibers, at least part of which are electrically conductive fibers. The choice of conductive fibers and their possible association with non-conductive fibers proceed from various criteria and in particular from compliance with the value chosen for the electrical resistance of the final sheet, taking into account the presence of the polyfluoroolefin binder.

The term “electrically conductive fibers” will denote any material in the form of a filament whose diameter is generally less than 1 mm and preferably between 10 compris and 0.1 mm and whose length is greater than 0.5 mm and preferably between 1 and 20 mm, said material having a resistivity equal to or less than 0.4 Ω.cm.

Such fibers may consist entirely of a material that is intrinsically electrically conductive; as examples of such materials, mention may be made of metallic fibers, and in particular iron fibers, ferrous alloys or nickel or carbon fibers. It is also possible to use fibers originating from material which is not electrically conductive but which are made conductive by a treatment: one can for example cite asbestos fibers, made conductive by chemical or electrochemical deposition of a metal such as nickel , or zirconia fibers (Zr O₂), made conductive by nickel plating. In the case of fibers made conductive by treatment, this will be carried out under conditions such that the resulting fiber has the resistivity mentioned above.

It should be noted that this treatment of fibers, and in particular the nickel-plating mentioned above not only makes it possible to increase the conductivity of fibers and of the resulting sheet but plays a certain electrocatalytic role: more general information on the electrocatalytic agents will be given later.

It goes without saying that the two types of fibers can be combined in the plies according to the invention, namely the intrinsically conductive fibers and the fibers rendered conductive, as explained above. It should also be understood that the invention includes the use of intrinsically conductive fibers, that is to say fibers having the maximum resistivity value mentioned above, these fibers having themselves undergone a treatment such as for example nickel plating to increase their conductivity.

Subject to respecting the maximum resistivity values mentioned above, the conductive fibers are associated with non-electrically conductive fibers, this expression designating, by hypothesis, any filament whose resistivity is greater than 0.4 Ω.cm. In general, these fibers have a diameter less than 1 mm and preferably between 10⁻⁵ and 0.1 mm and a length greater than 0.5 mm and more generally between 1 and 20 mm.

The use of non-conductive fibers meets various requirements and in particular can be justified by the mechanical properties desired for the sheet of fibers. By way of illustration of non-conductive fibers within the meaning of the invention, mention will be made of asbestos fibers.

It has been found that it was advantageous, when the sheet of fibers is intended for application as a cathode element of a sodium chloride electrolysis cell, to effectively combine non-conductive fibers and in particular asbestos fibers to conductive fibers, which advantageously consist of carbon fibers. In such an association, the asbestos fibers and more generally the non-conductive fibers represent 20 to 70% of the weight of the whole conductive fibers / non-conductive fibers.

The binder of the plies according to the invention consists of a fluoropolymer. The term fluoropolymer currently designates a homopolymer or a copolymer derived at least in part from olefinic monomers fully substituted with fluorine atoms, or totally substituted with a combination of fluorine atoms and at least one of chlorine, bromine or iodine atoms per monomer.

Examples of fluorinated homo- or copolymers can be constituted by polymers and copolymers derived from tetrafluoroethylene, hexafluoropropylene, chlorotrifluoroethylene, bromotrifluoroethylene.

Such fluorinated polymers can also contain up to 75 mole percent of units derived from other ethylenically unsaturated monomers containing at least as many fluorine atoms as carbon atoms, such as for example vinylidene (di) fluoride, vinyl and perfluoroalkyl esters, such as perfluoroalkoxyethylene.

It is naturally possible to use in the invention several homo or fluorinated copolymers as defined above. It goes without saying that it would not go beyond the scope of the invention to combine with these fluorinated polymers a small amount, for example up to 10 or 15% by weight of polymers whose molecule does not contain fluorine atoms , such as polypropylene.

The fluoropolymer is currently used as a binder for fibers defined above. The different modes of implementation of said binder will be explained below. It will simply be indicated here that, in the sheets according to the invention, the fluoropolymer represents between 5 and 50% of the total weight of the sheet, that is to say fibers (conductive fibers, possibly associated with non-conductive fibers) + binder.

The layers according to the invention have been defined above by their essential constituents, namely the fibers and the binder. Depending on the different applications for which these layers will be intended, they may, at one or the other moment of their existence, contain other materials or additives. These materials or additives are listed below, it being specified that the additives may be present simultaneously or, on the contrary, succeed one another within the sheet, in the case of treatments carried out on said sheet.

For purely illustrative purposes, mention will first be made of products which are not in the form of fibers and which are capable either of improving the electrical conductivity of the sheet, or of increasing its mechanical properties: such materials may in particular be constituted by powders, whether they are conductive powders such as graphite, nickel, iron or magnetite powders or non-conductive powders, the concept of powder designating a product whose particle size is less than 50 μm and the conductivity being appreciated as in the case of fibers. These powders and in particular non-conductive powders, which may consist for example of asbestos powders or hydrated oxide can contribute with the binder to obtain the cohesion of the sheet of fibers. For purely indicative purposes, the quantity of powdered additive can reach 30% of the weight of the conductive fibers + fluoropolymer assembly.

The sheets may also contain one or more electrocatalytic agents. The use of such catalysts, which can be in the form of powder, the particle size of which can vary, for example between 1 and 100 μm, makes it possible to combine the advantages linked to the use of an elementary cathode directly comprising a deposit of electrocatalytic agent. (voltage gain of the order of 150 mV in the case of sodium chloride electrolysis) and the advantages linked to the use of fiber layers in terms of current distribution, diaphragm support, etc.

By way of illustration of such electrocatalytic agents, mention will be made in particular of the platinum group metals, and in particular platinum itself and palladium, nickel-zinc, nickel-aluminum, titanium-nickel, molybdenum- and alloys and couples. nickel, sulfur-nickel, nickel-phosphorus, cobalt-molybdenum, lanthanum-nickel.

For purely indicative purposes, the quantity of electrocatalytic agent, in whatever form it may appear, can represent up to 50% of the weight of the bonded sheet and more generally from 1 to 30% of this weight, depending on the nature of the catalyst.

The sheets can also contain hydrophilic agents. The use of such agents is particularly recommended when the web is used in an aqueous medium, for example in a process for the electrolysis of aqueous solutions of sodium chloride. The hydrophilic agent contributes to improving the wettability of the sheet of fibers by somewhat counterbalancing the highly hydrophobic nature of the fluoropolymers.

Hydrophilic agents can be chosen from various families of products. It can generally be liquid or pulverulent products, of organic or inorganic nature. As illustrative examples of such agents, mention may be made of surfactants or surfactants, such as sodium dioctylsulfosuccinate, etc. or inorganic compounds such as asbestos powder or short fibers, zirconia, carbon dioxide cerium, potassium titanate, hydrated oxides and in particular alumina.

The amount of hydrophilic agent which may be present in the sheets according to the invention obviously depends on the use intended for this sheet, on the amount of hydrophobic product (essentially the fluorinated binder but also certain fibers contained in these sheets) and on the nature of the hydrophilic agent. As an order of magnitude, it can be indicated that the quantity of hydrophilic agent can reach 10% of the weight of the fluorinated binder and more specifically from 0.1 to 5% of the weight of said binder.

The layers may also contain pore-forming agents, the role of which is to regulate the porosity of the layer, porosity which, in the event of an application in electrolysis, influences the flow of liquids and the evacuation of gases. It should be understood that, when such pore-forming agents are used, the final layer, the porosity of which, under the effect of decomposition or elimination of these agents, has been adjusted or modified, will in principle no longer contain such agents. By way of illustration of the pore-forming agents, mention will be made of the mineral salts which can then be removed by leaching and the salts which can be removed by chemical or thermal decomposition to which preference is given.

These various products can in particular be chosen from alkali or alkaline-earth salts, such as halides, sulfates, sulfites, bisulfites, phosphates, carbonates, bicarbonates. Mention may also be made of amphoteric alumina or silica which can be eliminated in an alkaline medium.

It goes without saying that the quantity and the particle size of the blowing agents - when such agents are used - is closely linked to the application for which the sheets are intended. Simply as an order of magnitude, it will be specified that the particle size of the blowing agents most often varies between 5 and 50 μm, and that the quantity is chosen according to the desired porosity, this porosity being able to reach up to 90% or even more (according to standard ASTM D 276-72).

It should be understood that each of the plies defined above by its essential constituents and by its additives constitutes in itself a new product, directly targeted by the present invention. This applies in particular to sheets comprising the fibers, the binder and the electrocatalytic agent, with or without blowing agent, sheets comprising the fibers, the binder, the hydrophilic agent, with or without electrocatalytic agent and to each of the abovementioned sheets also containing blowing agents and / or electrically conductive or non-conductive powders.

The present invention also relates to a method of manufacturing the plies defined above. It should be understood that the process which will be described below constitutes an embodiment of the sheets, an embodiment by the wet method as will appear on reading the following, but that this description does not in any way constitute a limitation of the field of the invention and that any process making it possible to obtain the plies claimed, whether it is a wet method or a dry process, is part of the field of said invention.

• préparation d'une suspension comprenant les fibres et le liant
• élimination du milieu liquide et séchage de la nappe.

This process essentially comprises the following stages:

• preparation of a suspension comprising the fibers and the binder
• elimination of the liquid medium and drying of the sheet.

The suspension comprises, as indicated above, on the one hand the electrically conductive fibers and on the other hand the binder consisting of a fluoropolymer, these constituents being dispersed in a liquid medium. Although this medium can be of very diverse natures, an aqueous medium or an electrolytic medium is generally used.

In this second hypothesis, the medium may contain, in addition to water, caustic soda, for example at a rate of 5 to 20% and sodium chloride, for example at a rate of 5 to 20%. It goes without saying that this indication is valid for an electrolytic medium corresponding to the electrolysis of sodium chloride but that it is possible, mutatis mutandis, to use any other electrolytic medium.

In general, it is advantageous to incorporate into the aqueous or electrolytic medium a small amount - for example 0.1 to 5% relative to the weight of the solid materials to be dispersed - of dispersing agents or surfactants such as, for example, sodium dioctylsulfosuccinate and, more generally, anionic sulfonic surfactants, such as sulfonates, sulfosuccinates, C sulf to C₂₄ alkyl sulfosuccinamates.

It should be understood that, in the event that the final sheet must contain other additives and in particular those listed above such as non-conductive fibers, conductive or non-conductive powders, hydrophilic agents, pore-forming agents, catalytic agents, these can generally be incorporated as soon as the initial suspension is prepared. It can however be specified that, apart from the case of additional fibers which, in principle, must be dispersed among the conductive fibers, the other additives can also be introduced into the sheet, for example by filtration through said sheet of a suspension containing such agents.

The fluoropolymer is generally in the form of dry powder or of fibers or of aqueous dispersion (latex) generally containing 30 to 70% of dry polymer. In general, the largest dimension of the particles or fibers of fluoropolymer is less than 50 μm, the particle size usually being between 0.1 and 10 μm in the case of powdered polymer.

The suspension defined above, by its essential constituents and by its optional additives is generally greatly diluted in the sense that the ratio: suspension medium / dry matter (fibers, polymer, additives) is of the order of 30 to 100 : 1. These indications correspond to a suspension that can be used industrially, but one could obviously use a much higher ratio.

In order to obtain an easily controllable filtration speed, a thickening agent chosen, for example, from natural or synthetic polysaccharides, can be added to the suspension if necessary.

The various constituents can be directly introduced into the medium, in particular the aqueous medium which may be electrolytic.

According to a variant, and in particular when the fluoropolymer is itself in the form of a dispersion, a dispersion of the fibrous materials (conductive fibers and, optionally non-conductive fibers) is first carried out, with the addition of a dispersing agent, in a fraction, for example 1/5 to 1/2 of the final amount of dispersion medium, then the fluoropolymer is incorporated into this dispersion, the suspension then being diluted and homogenized.

The next phase of the process according to the invention consists in forming the sheet comprising the fibers, the fluorinated binder and possibly the other additives. This sheet can be formed by filtration of the suspension through a highly porous material, such as a metal grid, for example of iron or bronze, the mesh void of which can be between 20 μm and 5 mm. In general, this filtration is advantageously carried out under vacuum, generally following a program ranging, continuously or in stages, from atmospheric pressure to final depression (1.5 × 10 3 to 4.10 Pa).

The sheet resulting from this filtration can be dried, for example at a temperature between 70 and 120 ° C for a period which can range from 1 to 24 hours. The final formation of the web, possibly after the drying mentioned above, comprises heating to a temperature above the melting or softening point of the fluoropolymer, for example from 5 to 50 ° C. above this point, and for a period of time. duration which can range, depending on the polymer and the temperature chosen, from 2 min to 60 min and, more precisely, from 5 to 40 min.

The sheet thus formed, and consisting of a set of conductive fibers linked by a fluoropolymer constitutes, as has been specified, the primary object of the present invention.

The invention also relates, and very particularly the aforementioned plies activated by an electrocatalytic agent. Various electrocatalytic agents have been mentioned previously which can be incorporated and dispersed in said sheet. According to an embodiment of electrocatalytic agents, and as far as the nature of the latter allows, these agents can be deposited on the sheet formed by electrochemical deposition. This technique is particularly interesting when one wishes to use as an agent electrocatalytic nickel, which is deposited in the form of nickel-zinc alloy then leached in an alkaline medium in order to remove the zinc and obtain large surface nickel.

According to this technique, the fiber sheet is deposited on a cathode, the anode is made of nickel and the electrocatalytic bath comprises halides of nickel and zinc. The nickel / zinc couple is deposited on the electrically conductive fibers, the zinc being eliminated as has just been specified.

According to other embodiments and as already indicated, an electrocatalytic agent can be incorporated directly into the suspension in the form of a powder or filtered through the fiber sheet, before or after melting of the binder, a suspension of electrocatalytic agent in any liquid vehicle, most often water, possibly supplemented with surfactant in order to maintain the dispersion of the powders, for example in the case of the reduction of precious metal salts by borohydride sodium.

Another object of the invention consists of a composite material comprising the sheet itself comprising the fibers and the fluoropolymer defined above and an elementary cathode. The expression elementary cathode currently designates the metal part, generally made of iron or nickel, essentially constituted by a mesh or a piece of perforated metal and playing the role of cathode in an electrolysis cell. This elementary cathode may consist of one or an assembly of flat surfaces or, in the case of electrolysis cells of the "thimble" type, may be in the form of a cylinder whose director is a more or less complex surface. , generally substantially rectangular with rounded corners.

There are various methods of assembling the sheet of fibers linked by the fluoropolymer with the elementary cathode. According to a first way of proceeding, the suspension is directly filtered through the elementary cathode and then the elementary cathode / fiber sheet assembly is brought to a temperature allowing the fusion of the fluoropolymer binder, as indicated above. According to another variant, the suspension is filtered separately and a sheet of fibers is formed and the binder is melted, this only operation being carried out after application of the sheet on the elementary cathode. The choice between the different techniques can be linked to the nature of the elementary cathode (grid, perforated metal, expanded metal) and to the desired degree of penetration of the sheet of fibers into the mesh voids or perforations of the elementary cathode.

The composite material comprising the elementary cathode and the sheet of fibers, as described above constitutes in fact the cathode itself of an electrolysis cell, this application to the production of cathode element of electrolysis cell constituting the privileged but not exclusive field of materials according to the invention. In the hypothesis of such an application, it is possible, according to a practice now common, to use in the cell a membrane or a diaphragm between the anode and cathode compartments. In the case of a membrane, which can be chosen from the many electrolysis membranes described in the literature, the composite element according to the invention constitutes an excellent mechanical support and ensures a remarkable distribution of the current. This distribution of the current is naturally linked to the particular structure of the composite elements in accordance with the invention. In addition, the multiplicity of current conductors (conductive fibers) ensures a maximum voltage gain due to the large active area, which gain can be increased when electrocatalytic elements have been, in one or the other form previously disclosed. , dispersed within the fiber sheet.

The composite material can also be associated with a diaphragm. This diaphragm, which can also be chosen from the many diaphragms for electrolysis now known, can be manufactured separately. It can also, and this constitutes an advantageous method, be manufactured directly on the fiber sheet or on the composite fiber sheet / elementary cathode. This direct manufacture is particularly easy when the diaphragm is produced by filtration of a suspension. These techniques for manufacturing porous and microporous membranes or diaphragms are described for example in French patents No. 2,229,739, 2,280,435 or 2,280,609 and French patent application No. 81.9688, the content of these patents and patent application. being incorporated herein by reference.

Composite materials, consisting of an assembly comprising, from one face to the other, the elementary cathode, the sheet of fibers linked by the fluoropolymer and the porous or microporous membrane or diaphragm also constitute an object of the invention. Such composite materials constitute coherent assemblies, benefiting from all the advantages specific to the fiber sheet and to the fiber sheet / elementary cathode composite, to which is added the considerable advantage represented by the elimination of the traditional diaphragm / cathode interface. and its harmful effects, namely a parasitic ohmic drop in the gas-liquid emulsion close to the cathode substrate.

In the examples which follow, practical details of implementation of the invention will be given, these examples having only an illustrative role and can in no case be considered as limiting the invention in any way.

Examples 1 to 3

These examples illustrate the production of sheets of fibers linked by a fluoropolymer.

a) Preparation of carbon fibers

Carbon fibers are prepared as follows:
Dry route: we pass for 4 minutes in a grinder-mixer carbon fiber and the same amount of NaCl (50 or 62.5 g of each ingredient). Fibers are removed whose average length is 1 to 3 mm, the average diameter is 5 to 10 µm. The resistivity is less than 5.10⁻³ Ω.cm.

Wet process: the same carbon fluff is ground in 1 liter of water. The characteristics of the fibers are identical.

b) Preparation of the suspension

We use two methods:

Type I : aqueous route.

• 100 g de fibres constituées par :
  • . 37 ou 50 g des fibres de carbone décrites sous (a)
  • . 63 ou 50 g de fibres d'amiante - type A : variété chrysotile - longueur moyenne comprise entre 1 et 5 mm, diamètre moyen environ 200 Å
  ou
• type B : variété chrysotile - longueur comprise entre 5 et 20 mm, diamètre moyen environ 200 Å
  • . 1 g de dioctylsulfosuccinate de sodium, sous forme de solution aqueuse à 65 %
  • . 7 000 g d'eau adoucie.

A suspension is prepared from:

• 100 g of fibers made up of:
  • . 37 or 50 g of the carbon fibers described under (a)
  • . 63 or 50 g of asbestos fibers - type A: chrysotile variety - average length between 1 and 5 mm, average diameter around 200 Å
  or
• type B: chrysotile variety - length between 5 and 20 mm, average diameter around 200 Å
  • . 1 g sodium dioctylsulfosuccinate, as a 65% aqueous solution
  • . 7,000 g of softened water.

• sous forme de latex dans l'eau à 60 % d'extrait sec
• sous forme de poudre (granulométrie inférieure à 50 µm)

   On agite à nouveau pendant 30 mn.After rotary stirring for 30 min, 40 to 80 g of polytetrafluoroethylene (PTFE) are introduced into this suspension:

• in the form of latex in water with 60% dry extract
• in powder form (particle size less than 50 µm)

The mixture is again stirred for 30 min.

Type II : alkaline route

The procedure is as for the aqueous route but replaces the softened water with the same amount of electrolytic soda (150 g / l of NaCl and 150 g / l of NaOH). Here we used either polytetrafluoroethylene powder or in the form of latex or 30 g of polychlorotrifluoroethylene (PCTFE) in the form of powder with an average particle size of 50 μm.

The suspension is stirred by air for 30 min (air circulation at a flow rate of 10 m³ / h).

c) Manufacture of the sheet of fibers

The suspensions I or II are filtered through a bronze grid opening 40 µm while respecting the following vacuum program: 1 min of decantation then successive stages for 1 min at increasing voids (from 100 to 100 Pa).

The sheet obtained after filtration is detached from the grid and brought to an oven at 350 ° C for 10 min when the polymer is PTFE or at 260 ° C for 30 min when the polymer is PCTFE.

The characteristics of the procedure and the final layers are as follows:

Examples 4 to 9

• . une grille de fer tressé et laminé (diamètre des fils 2 mm, ouverture 2 mm)
• . une plaque de fer perforé (épaisseur 1,5 mm, diamètre des trous 3 mm, entre-axe 5 mm , disposition en quinconce)
• . une plaque de nickel perforé (épaisseur 1,5 mm, diamètre des trous 3 mm, entre-axe 5 mm , disposition en quinconce)

   Le matériau composite résultant de cette filtration et de la fusion du polymère fluoré (12 heures à 100° puis 10 mn à 350°) est utilisé en tant que cathode dans une cellule d'électrolyse du chlorure de sodium (fonctionnement sous 25 A/dm² à 85°C - sortie de la soude : 120 à 140 g/l).The suspensions described under (b) of examples 1 to 3 are used but the filtration of these suspensions is carried out through an elementary cathode constituted by

• . a braided and laminated iron grid (wire diameter 2 mm, opening 2 mm)
• . a perforated iron plate (thickness 1.5 mm, hole diameter 3 mm, center distance 5 mm, staggered arrangement)
• . a perforated nickel plate (thickness 1.5 mm, hole diameter 3 mm, center distance 5 mm, staggered arrangement)

The composite material resulting from this filtration and from the fusion of the fluoropolymer (12 hours at 100 ° then 10 min at 350 °) is used as a cathode in a sodium chloride electrolysis cell (operation at 25 A / dm² at 85 ° C - soda outlet: 120 to 140 g / l).

To make the measurements, place the diaphragm 10 mm from the surface of the composite material and the potential of this composite material (cathode element) is obtained with a Luggin probe applied to its surface (9 measurements distributed over 1/2 dm² and calculation average potential). The electrolyser's active surface is 1/2 dm².

In this new cathode, the excess thickness of the sheet of fibers linked by the fluoropolymer relative to the surface of the elementary cathode varies from 0.1 to 1 mm, depending on the amount of suspension filtered.

The operating characteristics and measurements are collated in the following table:

In this table ΔUmv / ECS designates the potential measured at the surface of the composite material (fiber sheet side) or of the cathode surface with respect to the Saturated Calomel electrode (expressed in mv).

It appears from this table that the composite materials, constituted solely by the fibers and the binder give, in very thin thickness, a potential substantially equal to the potential measured on the elementary cathode.

The increase in the thickness of the sheet of fibers also increases the potential, but this increase remains very acceptable.

Examples 10 to 28 (Examples 13 and 15 are comparative examples)

• a) Le dépôt électrochimique est effectué comme suit : on utilise l'élément cathodique de l'exemple 4 comme cathode d'un électrolyseur dont l'anode est constituée par du nickel. Le bain électrolytique comprend :
  NiCl₂, 6 H₂O = 1 mol/l
  NH₄Cl = 1 mol/l
  ZnCl₂ = 15 g/l
     L'électrolyse est effectuée en milieu agité, à 20° sous une densité de courant de 10 A/dm². L'opération dure 30 mn. Après cette opération, au cours de laquelle se dépose sur les fibres conductrices de l'élément cathodique un alliage nickel-zinc, on immerge cet élément pendant 2 heures dans de la soude électrolytique (concentration 150 g/l) à 80°C. Au terme de cette opération, le zinc a été éliminé et la quantité de nickel déposée représente environ 30 % du poids de la nappe de fibres.
  Les résultats sont les suivants :
  
• b) Dans la deuxième technique d'activation, on a renouvelé l'exemple 4 en utilisant soit des fibres de carbone nickelées (63) et des fibres d'amiante (37), soit uniquement des fibres d'amiante nickelées.
  On observe les résultats suivants :
  
• c) La troisième technique d'activation comporte l'addition d'élément électrocatalytique en poudre.

In this series of tests, the cathode elements were activated by electrochemical deposition (examples 10 and 11), by nickel plating of fibers (examples 12 and 13), and by addition of electrocatalytic element in powder form (examples 14 to 28), the general technique for manufacturing the composite (elementary cathode + sheet of fibers) being that of Examples 4 to 9.

• a) The electrochemical deposition is carried out as follows: the cathode element of Example 4 is used as the cathode of an electrolyser whose anode consists of nickel. The electrolytic bath includes:
  NiCl₂, 6 H₂O = 1 mol / l
  NH₄Cl = 1 mol / l
  ZnCl₂ = 15 g / l
  The electrolysis is carried out in an agitated medium, at 20 ° under a current density of 10 A / dm². The operation lasts 30 min. After this operation, during which a nickel-zinc alloy is deposited on the conductive fibers of the cathode element, this element is immersed for 2 hours in electrolytic soda (concentration 150 g / l) at 80 ° C. At the end of this operation, the zinc was removed and the amount of nickel deposited represents approximately 30% of the weight of the sheet of fibers.
  The results are as follows:
  
• b) In the second activation technique, Example 4 was repeated using either nickel-plated carbon fibers (63) and asbestos fibers (37), or only nickel-plated asbestos fibers.
  The following results are observed:
  
• c) The third activation technique involves the addition of powdered electrocatalytic element.

We operate as follows:
1st method: (examples 14 to 16)
A type I suspension containing 60 g of powdered PTFE, the carbon fiber ratio, is deposited on an elementary cathode of perforated soft iron (thickness 1.5 mm, diameter of the holes 3 mm; center distance 5 mm; staggered arrangement) / asbestos fibers being either 63/37 or 100/0.

On the cathode element obtained (following the general technique of examples 4 to 9), a suspension of platinum or a suspension of palladium is filtered under the following conditions:

Platinum suspension: (for 1 l of suspension)

• . Dissolution of 2.4 g of H₂PtCl₆ in 800 cm³ of water containing 1% oo of poly (oxyethanediyl) α [(tetramethyl 1,1,3,3 butyl) -4 phenyl] ω hydroxy
• . Dissolving 0.9 g of sodium borohydride in 200 cm³ of water
• . Slowly mixing the two solutions

Palladium suspension (for 1 l of suspension)

• . Dissolution of 5.5 g of PdCl₂ in 5 cm³ of 3N HCl and dilution up to 800 cm³ with H₂O containing 1% oo of poly (oxyethanediyl) α [(tetramethyl 1,1,3,3 butyl) -4 phenyl] ω hydroxy
• . Dissolving 0.9 g of sodium borohydride in 200 cm³ of water
• . Mixing with stirring of the two solutions

After filtration, the cathode elements are wrung, dried (100 °, 12 h) and brought to 350 ° for 10 min.

The following results are noted Example Report Activation Potential of the cathode element (ΔUmv / ECS) carbon fiber asbestos fibers Nature g / dm² 14 63/37 Platinum 0.2 - 1250 (- 1380) 15 100/0 Platinum 0.2 - 1280 16 63/37 Palladium 0.2 - 1260

In this table, the amount of activator is expressed by weight of platinum or palladium (metal) deposited per dm² of surface of the cathode element.

The value of the potential given in brackets is that of the elementary cathode alone.

2nd method: (examples 17 to 28)
Powder activators, the particle size of which is equal to or less than 50 μm, are incorporated directly into the suspension.

In the following table the terms or abbreviations have the following meanings:
Type designates the type of suspension (aqueous or alkaline, as in Examples 1 to 3)
C / A denotes the carbon fiber / asbestos fiber weight ratio
P / C + A denotes the weight ratio fluoropolymer / carbon fibers + asbestos fibers
Po / A denotes the porogenic / asbestos fibers weight ratio.

Examples 29 to 40

In the following tests, a cathode element / diaphragm association was made.

(a) Procedure

The cathode element used is made from an elementary braided and laminated iron cathode and a type I suspension, containing a PTFE latex, asbestos fibers (A) and a carbon fiber / fiber ratio. asbestos 63/37. This item may be activated.

• . H₂O 3300 g
• . Sulfosuccinate Na 1 g
• . Fibres d'amiante A 100 g

dans laquelle ont été incorporés, après 1/2 heure d'agitation

• . latex PTFE 133 g (latex à 60 % d'extrait sec)
• . porogène (Al₂O₃ à 25 % Al) 40 g

l'ensemble ayant ensuite été agité pendant 1/2 heure, laissé au repos 24 h, à nouveau dispersé et homogénéisé pendant 1/4 d'heure avant utilisation.The diaphragm is deposited on this element by suction under programmed vacuum of a suspension comprising:

• . H₂O 3300 g
• . Sulfosuccinate Na 1 g
• . Asbestos fibers A 100 g

in which have been incorporated, after 1/2 hour of stirring

• . PTFE latex 133 g (60% dry extract latex)
• . porogenic (Al₂O₃ at 25% Al) 40 g

the whole having then been stirred for 1/2 hour, left to stand for 24 hours, again dispersed and homogenized for 1/4 hour before use.

The programmed vacuum deposition is carried out as follows:
1 min of decantation
1 min under pressure reduced to 9.102 Pa
1 min under pressure reduced to 7.5.10² Pa
1 min under reduced pressure at 6.10² Pa
1 min under pressure reduced to 5.10² Pa
After deposition of the diaphragm, the cathode element / diaphragm assembly is wrung and placed at 100 ° for 12 hours then at 350 ° for 10 minutes.

The porogen is eliminated by alkaline attack before mounting in the electrolyser.

(b) Use in electrolysis.

The electrolysis conditions are those of the previous examples; however the inter-electrode distance is reduced to 6 mm.

RF

: rendement Faraday

ΔU (volts)

: tension aux bornes de l'électrolyseur

NaOH g/l

: concentration en sortie de l'électrolyseur

et, par tracé de Δ U = f(I) ou courbe Intensité/Potentiel la valeur de ΔU_I→0
   Les résultats sont les suivants pour un chlorure anodique constant de 4,8 mole/l :

We measure:

RF

: Faraday yield

ΔU (volts)

: voltage across the electrolyzer

NaOH g / l

: concentration at the electrolyser outlet

and, by plotting Δ U = f (I) or Intensity / Potential curve the value of ΔU _{I → 0}
The results are as follows for a constant anodic chloride of 4.8 mol / l:

• A 180 g/l les rendements Faraday sont équivalents pour tous les essais soit environ 93 %
• La tension d'extrapolation à I₀ baisse par activation par nickelage des fibres et surtout en présence de catalyseur.

Nature de l'activation Témoin Fibres nickelées Platine Ni-Al ΔU_I→0 moyen 2,34 2,31 2,25 2,27

• La tension aux bornes met en évidence les mêmes gains de tension amplifiés.

These results call for the following remarks:

• At 180 g / l the Faraday yields are equivalent for all the tests, ie around 93%.
• The extrapolation voltage at I₀ decreases by activation by nickel plating of the fibers and especially in the presence of catalyst.

Nature of activation Witness Nickel-plated fibers Platinum Ni-Al ΔU _{I → 0} medium 2.34 2.31 2.25 2.27

• The voltage across the terminals highlights the same amplified voltage gains.

Claims (15)

1. Material in the form of a sheet, consisting of fibres and a binder, for use especially in the production of the cathode element of an electrolysis cell, said material being characterized in that

   a) a part of the fibres consists of electrically conductive carbon fibres,

   b) the other part of the fibres consists of electrically nonconductive asbestos fibres,

   c) the weight of the nonconductive fibres represents 20 to 70 % by weight of the total of conductive fibres and nonconductive fibres, and

   d) the binder is selected from fluorinated polymers and represents from 5 to 50 % of the total weight of the sheet, that is to say fibres + binder.
2. Material according to Claim 1, characterized in that the fluorinated polymer is selected from the polymers and copolymers derived from tetrafluoroethylene, hexafluoropropylene, chlorotrifluoroethylene and bromotrifluoroethylene.
3. Material according to Claim 1 or 2, characterized in that the fluorinated polymer contains up to 75 mol% of units derived from other ethylenically unsaturated monomers containing at least as many atoms of fluorine as atoms of carbon.
4. Material according to Claim 1, characterized in that it includes one or more electrocatalytic agents.
5. Material according to Claim 4, characterized in that the electrocatalytic agent is in the form of powder of a particle size of between 1 and 100 µm.
6. Material according to Claim 5, characterized in that the electrocatalytic agent is selected from the metals of the platinum group, and in particular platinum itself and palladium, alloys and couples of nickel-zinc, nickel-aluminium, titanium-nickel, molybdenum-nickel, sulphur-nickel, nickel-phosphorus, cobalt-molybdenum and lanthanum-nickel.
7. Material according to Claim 4, characterized in that the quantity of electrocatalytic agent represents up to 50 % of the weight of the bound sheet and preferably 1 to 30 % of said weight.
8. Process for producing materials according to any one of the Claims 1 to 7, characterized in that it comprises preparing a suspension containing the fibres and the binder and then eliminating the liquid medium and drying the sheet obtained.
9. Process according to Claim 8, characterized in that the suspension additionally contains at least one of the additives selected from nonconductive fibres, conductive or nonconductive powders, hydrophilic agents, porogenic agents and catalytic agents.
10. Process according to Claim 8, characterized in that the suspension is obtained by incorporating a dispersion of fluorinated polymer in a dispersion of fibres, the dispersion of fibres amounting to 1/5 to 1/2 of the final quantity of the dispersion medium.
11. Process according to any one of the Claims 8 to 10, characterized in that the sheet is formed by filtration of the suspension across a highly porous material under a programmed vacuum.
12. Process according to any one of the Claims 8 to 11, characterized in that the sheet is dried for 1 to 24 hours at a temperature between 70 and 120°C and then bound by heating to a temperature 5 to 50°C above the melting point or softening point of the fluorinated polymer for a period which may be from 2 to 60 minutes.
13. Electrode formed by associating material according to any one of Claims 1 to 7 with an elemental cathode consisting of a grid or perforated metal.
14. Electrode according to Claim 13, in which the association is effected by direct filtration of the suspension containing the fibres and the fluorinated polymer across the said elemental cathode, followed by melting of the binder.
15. Use of materials according to any one of the Claims 7, 13 or 14 for electrolysis.