EP0745150B1 - Electroactivated material, its preparation and utilization for producing cathode elements - Google Patents

Description

The present invention relates to an electroactivated material comprising fibers and a binder, and also having an electrocatalytic agent in the form of particles comprising a precious metal oxide or in the form of particles comprising a support and a coating based on such an oxide.

The electroactivated material can be used in particular as a cathode element an electrolysis cell, and in particular a solution electrolysis cell aqueous sodium chloride.

It also relates to a composite material comprising said material as well as processes for the preparation of each of the two materials.

For several years, the use of cathodes made up of a surface metal on which a fibrous sheet is deposited then possibly a diaphragm, developed in the field of electrolysis of aqueous solutions of sodium chloride. Such cathodes have a low overvoltage with respect to the hydrogen release reaction at the cathode and thereby reduced the energy consumption.

It is also known to add electrocatalytic agents, such as nickel for example, to the aforementioned ply, in order to further increase its performance. These agents can be used in the form of a powder dispersed within the sheet aforementioned fibrous, or in the form of a deposit on said sheet, obtained by a electrochemical route for example.

For example, in European patent application EP 319517, the agent electrocatalytic is chosen from nickel, platinum group metals or nickel-titanium type alloys. The electrocatalytic agent is always used in the form of a metal.

However, cathodes of this type do not have resistance to poisoning very important, due to the nature of the electrocatalytic agents, and are therefore deactivated relatively quickly.

In the French patent application FR 2 446 870, the use of oxide has been described. of iridium and ruthenium oxide in fuel cells, that is to say for the in the electrolysis of water. These fuel cells consist of a canvas metal on which is compressed a powder comprising carbon, and a binder. The canvas thus covered is then impregnated with a mixture of the aforementioned salts and then undergoes an oxidizing treatment.

The object of the present invention is to provide an electro-activated material and a composite material comprising the latter, which can be used as a cathode in an electrolysis cell of aqueous sodium chloride solutions, the resistance to poisoning and therefore lifespan are increased.

The present invention therefore has for first object an electroactivated material comprising fibers at least part of which is electrically conductive, a binder, and further comprising an electrocatalytic agent consisting of particles formed ruthenium oxide, platinum, palladium, iridium, or a mixture thereof, distributed in less in part, on an electrically conductive support.

A second object of the invention consists of a composite material comprising, (a) a material with high porosity, (b) the aforementioned electro-activated material.

A third object of the invention consists of a composite material comprising from one face to the other: (a) a metallic surface of high porosity, (b) the aforementioned electro-activated material, (c) a separator.

(a) on prépare une suspension aqueuse comprenant les fibres, le liant, l'agent électrocatalytique, des adjuvants,

(b) on dépose une nappe par filtration sous vide programmé de ladite suspension à travers un matériau à porosité élevée,

(c) on élimine le liquide et l'on sèche éventuellement la nappe ainsi formée,

(d) on fritte éventuellement la nappe ainsi obtenue.

The invention also relates to a method for preparing an electro-activated material which consists in carrying out the following steps:

(a) an aqueous suspension is prepared comprising the fibers, the binder, the electrocatalytic agent, adjuvants,

(b) a web is deposited by programmed vacuum filtration of said suspension through a material with high porosity,

(c) removing the liquid and optionally drying the sheet thus formed,

(d) optionally sintering the sheet thus obtained.

(a) on prépare une suspension aqueuse comprenant les fibres, le liant, l'agent électrocatalytique, des adjuvants,

(b) on dépose une nappe par filtration sous vide programmé de ladite suspension à travers un matériau à porosité élevée,

(c) on élimine le liquide et l'on sèche éventuellement la nappe ainsi formée,

(d) on fritte éventuellement la nappe ainsi obtenue,

(e) on dépose sur ladite nappe, par filtration sous vide programmé, une dispersion dans l'eau ou dans une solution aqueuse d'hydroxyde de sodium, comprenant des fibres, un liant, des adjuvants,

(f) on élimine le liquide et l'on sèche éventuellement le diaphragme ainsi formé,

(g) on fritte l'ensemble.

Finally, the present invention relates to a method of preparing a composite material. This consists of performing the following steps:

(a) an aqueous suspension is prepared comprising the fibers, the binder, the electrocatalytic agent, adjuvants,

(b) a web is deposited by programmed vacuum filtration of said suspension through a material with high porosity,

(c) removing the liquid and optionally drying the sheet thus formed,

(d) the sheet thus obtained is optionally sintered,

(e) a dispersion in water or in an aqueous solution of sodium hydroxide, comprising fibers, a binder, adjuvants, is deposited on said sheet, by filtration under programmed vacuum.

(f) the liquid is eliminated and the diaphragm thus formed is optionally dried,

(g) the whole is sintered.

But other advantages and characteristics of the invention will appear more clearly on reading the description and the examples which follow.

As previously indicated, the electrocatalytic agent, entering the composition of the electroactivated material according to the invention can be presented under the form of particles based on ruthenium oxide, platinum, iridium, palladium, these oxides being alone or in mixture. Said agent may also be in the form of particles made up of an electrically conductive support, comprising for at least part of said particles, a coating in the form of ruthenium oxide, platinum, iridium, palladium; these oxides being alone or in a mixture. In the following, the above-mentioned list will be designated by the term precious metal (metals). It is understood that, thereafter, the term precious metal will indifferently represent one or more of the aforementioned metals.

The combination of these two variants is of course conceivable.

By mixing is first meant particles comprising several oxides or particles containing at least one oxide, mixed with other particles comprising at least one different oxide, or finally these two possibilities simultaneously. Such a definition is valid, that the oxides are located throughout the thickness of the particle or simply in the form of a coating.

The electrocatalytic agent according to the invention is in the form of a coating of a support.

The support is made of an electrically conductive material and stable in the conditions for the subsequent application of the material (pH and temperature in particular).

More particularly, this is chosen from iron, cobalt, nickel, iron of Raney, Raney cobalt, Raney nickel, the elements of columns IVA and VA of the periodic table, carbon, graphite. Here and for the whole description, references to the periodic table of the elements refer to that published in the supplement to the bulletin of the Société Chimique de France (n ° 1 - January 1966).

Preferably, the support is in the form of a powder.

More particularly, the particle size of the support is between 1 and 100 μm.

The specific surface of said support is at most 1000 m ² / g. More specifically, the specific surface varies between 5 and 500 m ² / g.

The proportion by weight, between the coating and the support, varies between 0.5 and 50. It it should be noted that outside proportions, and more particularly higher than those indicated above are possible. This does not, however, particular advantages for the performance of the electro-activated material, while unnecessarily increasing its cost.

The electrocatalytic agent according to the invention can also comprise additives chosen from iron, cobalt, nickel and / or their oxides.

The proportion of said additives, relative to the precious metal oxide (by weight) ranges from 0 to 50%.

The electrocatalytic agent used in the composition of the electroactivated material according to the invention can be distributed either uniformly within said material or else be gathered in a particular area of it, on the periphery for example.

However, according to a preferred embodiment of the invention, the agent electrocatalytic is uniformly distributed in the mass of the electroactivated material.

The amount of electrocatalytic agent in the material according to the invention represents 10 to 70% by weight, based on all the fibers, binder and electrocatalytic agent.

In addition to this electrocatalytic agent, the material according to the invention comprises fibers of which at least part is electrically conductive.

These fibers are in the form of filaments whose diameter is generally less than 1 mm, and preferably between 10 ^-5 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 ohm.cm.

Said fibers may consist entirely of a material intrinsically conductive of electricity. As examples of such materials we can mention metallic fibers, and in particular fibers of iron, ferrous alloys or nickel, carbon fibers or graphite.

It is also possible to use fibers from non-conductive material of electricity but made conductive by a treatment: we can for example cite asbestos fibers, zirconia fibers made conductive by chemical deposition or electrochemical of a metal such as nickel. In the case of fibers made conductive by treatment, this will be carried out under conditions such as the resulting fiber exhibits the resistivity mentioned above.

According to a preferred embodiment of the invention, the electroactivated material includes intrinsically conductive fibers, and more particularly fibers carbon or graphite.

Furthermore, fibers with a monodispersed length, that is to say, are used. that the length of at least 80% and more particularly 90% of the fibers, corresponds to the average fiber length at ± 20% and advantageously at ± 10% near.

The conductive fibers can also be combined with fibers that are not electrically conductive insofar as the resistivity of the material is at most 0.4 ohm.cm. These fibers are generally in the form of filaments whose geometric characteristics are similar to those given for fibers conductive but whose resistivity will, by convention, be greater than 0.4 ohm.cm.

By way of illustration of non-conductive fibers, mention may in particular be made of fibers minerals such as asbestos fibers, glass fibers, quartz fibers, zirconia, titanate fibers, or organic fibers, such as polypropylene or polyethylene, optionally halogenated and in particular fluorinated, the polyhalovinylidene fibers and in particular polyvinylidene fluoride, or fluoropolymer fibers, which will be discussed later on the binder of tablecloths according to the invention.

According to a first variant, asbestos fibers are used in particular in association with carbon or graphite fibers.

According to a second variant, polytetrafluoroethylene fibers are taken, below called PTFE fibers, in particular in combination with fibers previously mentioned minerals.

Preferably, the PTFE fibers have a diameter (D) is generally between 10 and 500 µm and their length (L) is such that the L / D ratio is understood between 5 and 500. Preferably, use is made of PTFE fibers whose dimensions sizes are between 1 and 10 mm for the length and between 50 and 200 µm for the diameter. Their preparation is described in US Patent No. 4,444,640 and this type of PTFE fibers is known to those skilled in the art.

In a combination of conductive and non-conductive fibers, the proportion of non-conductive fibers can represent up to 90% by weight and preferably be between 20 and 70%.

The electroactivated material according to the invention also comprises a chosen binder among fluoropolymers. By fluoropolymer is meant a homopolymer or a copolymer, derivative (s), at least in part, of fully olefinic monomers substituted with fluorine atoms, or fully substituted with a combination fluorine atoms and at least one of chlorine, bromine or iodine atoms by monomer.

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

Such fluoropolymers 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 (di) fluoride vinylidene, vinyl esters and perfluoroalkyl, such as perfluoroalkoxyethylene.

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

The binder can be either in the form of a dry powder or a latex, that is to say of an aqueous suspension of which the dry extract is between 30 and 70%.

The amount of fibers in the electro-activated material of the present invention represents 10 to 65% by weight, based on all the fibers, binder and agent electrocatalytic.

The amount of binder is between 5 and 20% by weight, based on the whole fibers, binder and electrocatalytic agent. However to ensure good consolidation in the electro-activated material, the binder preferably represents 20 to 50% by weight reported to the fiber and binder subset.

The materials according to the present invention can also contain adjuvants such as in particular surfactants.

As nonionic surfactant, it is possible in particular to use ethoxylated alcohols. or fluorocarbon compounds with functionalized groups, alone or as a mixture; these alcohols or these fluorocarbon compounds generally have carbon chains from C ₆ to C ₂₀ . Preferably, ethoxylated alcohols are used which are ethoxylated alkylphenols, such as in particular octoxynols.

The amount of surfactant that may be present in the sheets according to the invention can reach 10% by weight, based on all fibers, binder and agent electrocatalytic and more specifically 0.1 to 5% by weight, based on the whole fibers, binder and electrocatalytic agent.

You can also use a thickener. By "thickener" is meant according to the present invention a compound which increases the viscosity of the solution and which has water retaining properties. We usually use natural polysaccharides or synthetic. Mention may in particular be made of the biopolymers obtained by fermentation a carbohydrate under the action of microorganisms. We use advantageously xanthan gum. Xanthan gum is synthesized using bacteria belonging to the genus Xanthomonas and more particularly to the species described in Bergey's manual of determination bacteriology (Sème edition - 1974 - Williams N. Wilkins C ° Baltimore) such as Xanthomonas begoniae, Xanthomonas campestris, Xanthomonas carotae, Xanthomonas hederae, Xanthomonas incanae, Xanthomonas malvacearum, Xanthomonas papavericola, Xanthomonas phaseoli, Xanthomonas pisi, Xanthomonas vasculorum, Xanthomonas vesicatoria, Xanthomonas vitians, Xanthomonas pelargonii. Xanthomonas campestris is suitable particularly good for the synthesis of xanthan gum.

Among the other microorganisms capable of producing polysaccharides similar properties, mention may be made of bacteria belonging to the genus Arthrobacter, genus Erwinia, genus Azobacter, genus Agrobacter or fungi belonging to the genus Sclerotium.

Xanthan gum can be obtained by any means known per se. Conventionally, the polysaccharide is isolated from the fermentation must by evaporation, drying and grinding or by precipitation using a lower alcohol, separation of the liquid, drying and grinding so as to obtain a powder. Powders commercially available have a particle size generally between 50 and 250 µm and an apparent density greater than about 0.7.

The amount of thickener generally varies between 0.1 to 5% by weight, based on all fibers, binder and electrocatalytic agent.

The materials may still contain blowing agents.

It is understood that when blowing agents are used, the final material whose porosity under the effect of decomposition or elimination of these agents, is regulated or modified, in principle no longer contains such agents. By way of illustration of the agents pore-forming agents, mineral salts can be mentioned, which can then be eliminated by leaching, as well as salts which can be removed by chemical or thermal decomposition.

These various products can in particular be chosen from alkaline salts or alkaline earth, such as halides, sulfates, sulfonates, bisulfites, phosphates, carbonates, bicarbonates. We can also cite alumina amphoteric.

According to a particular embodiment of the invention, use is made of silica or derivatives, as a pore-forming agent, which can be removed subsequently with a alkaline treatment.

All types of silica are suitable for this use and more particularly precipitated silicas or combustion silicas.

The specific surface of said silica is more particularly between 100 and 300 m ² / g.

The quantity and particle size of the blowing agents are closely linked to the application for which the materials are intended. Simply by order of size, the particle size of the blowing agents most often varies between 1 and 50 μm and preferably between 1 and 15 µm. The quantity is chosen according to the porosity desired, it can reach 90% or more (according to ASTM D 276-72).

A second object of the present invention consists of a composite material comprising a high porosity compound and the electroactivated material described previously.

Generally, the compound with high porosity is chosen from surfaces metallic or even among fabrics, like asbestos fabrics, whose vacuum mesh size can be between 20 µm and 5 mm.

According to a preferred embodiment, the compound with high porosity is a metallic surface, called elementary cathode, which is more particularly made of iron, nickel, or else Stainless steel.

It is usually found in the form of a wire mesh or a piece of metal perforated and playing more particularly the role of a cathode in a cell electrolysis. Said cathode may consist of a surface or an assembly of flat surfaces or, in the case of "thimble" type electrolysis cells, present in the form of cylinders, the director of which is a surface more or less complex, generally substantially rectangular with rounded angles.

The composite material can also be combined with a separator which can be a diaphragm or membrane.

Generally, the diaphragms comprise fibers and a binder chosen from fluoropolymers, as well as conventional adjuvants.

All that has been said before concerning fibers, binder and adjuvants usable, remains valid and will therefore not be included in this section.

In order not to enter into too long developments, it is specified that the techniques for making porous and microporous diaphragms and membranes are described in the following French patents: FR 2,229,739, FR 2,280,435, FR 2,280,609 and French patent applications FR 81 9688 and 85 4327, FR 89 10938, FR 89 10937 whose content is incorporated here.

Before describing a method of preparing the activated material and the material composite according to the invention, a process for preparing the electrocatalytic agent will to be present.

The electrocatalytic agent used in the present invention can be obtained by any means known to those skilled in the art, allowing in particular, in the case of supported particles, to access a particle structure comprising a support having a coating.

One of the suitable methods is to prepare a suspension or solution of a compound of ruthenium, platinum, palladium, iridium, or their mixing, optionally in the presence of the aforementioned support and / or additives.

The compounds of said precious metals used for the preparation of electrocatalytic agents according to the invention are chosen from oxides or compounds liable to transform into oxide by heat treatment appropriate (precursor).

As precursors, there may be mentioned without intending to be limited, the acid salts organic or mineral, such as nitrates, halides, carbonates, sulfates, acetates, acetylacetonates, oxalates, tartrates, malonates, succinates.

So, ruthenium chloride, hexachloroplatinic acid, acid hexachloroiridique, palladium nitrate, palladium chloride, iridium chloride, trinitronitrosyl ruthenium.

These salts are therefore suspended or dissolved in a solvent. Usually the solvent is chosen from water, C ₁ -C ₆ alcohols, such as methanol, ethanol, isopropanol.

The precious metal salt content of the solution or suspension is generally between 0.1 and 5 M.

In the case where additives enter into the composition of the agents electrocatalytic, these can be in the form of oxide, oxide precursors, or in metallic form. If precursors are employees, what has been said previously about precursors of precious metals remains valid.

If a support is used and depending on its nature, it may be preferable to use it after undergoing a special surface treatment.

So, if we use carbon as a support, we do everything first an oxidation to increase the concentration of the surface in groupings oxidized. Treatment can be carried out in the presence of inorganic acids such as acid nitric, sulfuric acid in particular, or by heat treatment in an atmosphere oxidizing.

Preferably, the oxidation is carried out in the liquid phase by immersing the carbon. in a boiling nitric acid solution for about an hour. After this treatment, it is filtered and then the product obtained is rinsed with water.

If the support used is pyrophoric, as is notably the case for nickel of Raney, gentle oxidation is carried out in the presence of hydroperoxide hydrogen for example.

According to a first variant of preparation of an electrocatalytic agent according to the invention, all the constituent elements of said agent are mixed in the form of a solution or suspension.

This variant is particularly suitable in the case where the agent comprises a support and in the case where a precursor salt of the precious metal oxides is used, optionally including additives.

The mixing is generally carried out with stirring, at a temperature close to the ambient temperature.

The duration of the contact is from a few minutes to 24 hours.

According to a second variant, a solution or a suspension of the constituent elements of the electrocatalytic agent and a step of precipitation. In such a case, the precious metal is used in the form of a solution precursors.

To the solution or suspension, an agent is added which precipitates at least said metal. he It should be noted that precipitation of the additives, if they are present, is also possible.

All compounds are likely to be used as long as they are combine at least with the precious metal in the form of an insoluble compound. AT by way of example, mention may be made of the hydroxide, carbonate, bicarbonate of alkali metal or alkaline earth, such as sodium, calcium, potassium. We can also quote ammonia.

Conventionally, the precipitating agent is introduced into the mixture support / precious metal, but simultaneous introduction can be carried out.

This operation takes place with stirring and at a temperature close to the ambient temperature.

In such a case, provision may be made after the introduction of the precipitating agent for a ripening period, possibly with stirring, for 1 to 10 hours.

Following filtration, the resulting solid is usually rinsed with a solvent which may or may not be identical to that used for contacting.

Whatever the variant implemented, the next step is a step of drying.

According to a first embodiment, this operation can take place under vacuum or in air at a temperature between room temperature and temperature solvent removal.

The duration of this operation is from a few minutes to 12 hours.

A second embodiment consists in drying the solution by atomization.

This can be achieved using any sprayer known per se, type BUCHI especially.

However, according to a particular mode, an apparatus such as described in French patent applications FR 2 257 326. FR 2 419 754 and FR 2 431 321. In this case, the treating gases are driven in a helical movement and flow in a whirlpool well. The solution or suspension to be dried is injected according to a trajectory combined with the axis of symmetry of the helical trajectories of the gases, which allows to perfectly transfer the momentum of the gases to said solution or suspension. The gases thus perform a dual function, spraying the solution or suspension (transformation into fine droplets) and drying of said droplets. In addition, the residence time is less than 0.1 seconds, which eliminates any risk overheating due to too long contact with gases.

The treatment temperature is such that it allows the solvent to evaporate, or even, in the case where a precious metal salt has been used, to start the transformation of said salt into oxide.

Usually, and depending on the respective flow rates of the gases and the solution or suspension to dry, the gas inlet temperature is between 600 and 900 ° C, preferably between 700 and 900 ° C and the gas outlet temperature between 100 ° C and 300 ° C, preferably between 150 ° C and 250 ° C.

Before drying, a step of separation of the solid can be carried out if the variant implemented comprises passing through a suspension.

The separation is generally carried out by filtration or centrifugation.

Filtration is carried out by any means known to those skilled in the art, at pressure atmospheric or vacuum.

The dried product obtained is then subjected to a heat treatment in order to transform the precious metal salt into oxide.

This operation is carried out under a stream of air or oxygen at a temperature between 200 and 800 ° C, depending on the nature of the precursor.

The duration depends on the nature of the support if it is present. So the duration of the heat treatment may be longer if the support resists high temperatures without degrading or turning into a non-conductive compound of electricity.

As an indication, the duration varies from a few seconds to 1 hour.

At the end of the heat treatment, it is possible to deagglomerate the particles by any means known to those skilled in the art, such as grinding.

Finally, additional rinsing steps followed by filtration or centrifugation and drying can be performed.

Everything described above regarding these steps remains valid and does not will not be repeated here.

Mechanical synthesis represents another suitable method for the preparation of the electrocatalytic agent.

This method is particularly suitable when using a support, a composed of the precious metal and possibly additives, in solid form. In this case, we use a support having a higher hardness than that of the two aforementioned compounds.

In this way, after grinding all the compounds, we obtain particles formed from the support whose periphery is enriched with metal compound valuable and possibly additives.

After grinding, a heat treatment to transform the compound of precious metal in the corresponding oxide may be required.

This operation is then carried out under an oxidizing atmosphere at temperatures varying, depending on the nature of the compound to be transformed, between 200 and 800 ° C.

If the compound is used as an oxide, heat treatment is not necessary.

A method of preparing the electroactivated material will now be described.

(a) on prépare une suspension aqueuse comprenant les fibres, le liant, l'agent électrocatalytique, éventuellement des adjuvants.

(b) on dépose une nappe par filtration sous vide programmé de ladite suspension à travers un matériau à porosité élevée,

(c) on élimine le liquide et l'on sèche éventuellement la nappe ainsi formée,

(d) on fritte éventuellement la nappe ainsi obtenue.

Thus, the material according to the invention can be obtained by implementing the following steps:

(a) an aqueous suspension is prepared comprising the fibers, the binder, the electrocatalytic agent, optionally adjuvants.

(b) a web is deposited by programmed vacuum filtration of said suspension through a material with high porosity,

(c) removing the liquid and optionally drying the sheet thus formed,

(d) optionally sintering the sheet thus obtained.

The amount in each of the various constituents of the aqueous suspension prepared in step (a) is such that it makes it possible to obtain a material having the composition features in particular, set out above.

In a known manner, and mainly for reasons of ease of handling on an industrial scale, the dry matter content (i.e. fibers, binder, the electrocatalytic agent and the adjuvants) of the above dispersion is low. She is usually of the order of 1 to 5% by weight of the whole.

Furthermore, it may be advantageous to incorporate an agent into said suspension. thickener such as natural or synthetic polysaccharides.

The dispersion can be obtained by mixing each of the constituents in the required proportions with water, possibly with additives of the type surfactant, thickener.

A sheet is then formed from the resulting dispersion by programmed vacuum filtration through a material of high porosity. The vacuum program consists of going from atmospheric pressure to a final depression (1.5.10 ^-3 to 4 10 ^-4 Pa) and can be carried out continuously or in stages.

It is indicated here that by tablecloth is meant a material whose thickness is usually between 0.1 and 5 mm and the surface of which can reach several square meters.

The tablecloth, from which the liquid has been removed and possibly dried, can be sintered.

Conventionally, sintering is carried out in air at a temperature higher than softening point of the binder.

In the case where only preparation of the electroactivated material according to the invention is referred, further treatment to remove the blowing agent, if present, is to enforce. Thus, in the case where the blowing agent is silica, a treatment with soda.

If the objective is to prepare a composite material according to the invention, on a sheet obtained by implementing the preceding steps, a dispersion is deposited by programmed vacuum filtration.

This dispersion includes the constituent elements of a diaphragm and is obtained in a manner completely analogous to the process used to prepare the first dispersion.

It should however be noted that the constituent elements of this dispersion can be dispersed in water or in an aqueous solution of sodium hydroxide.

In the second case, care will be taken to choose the constituent elements dispersible in such a medium.

In a manner analogous to the preparation of the first layer, the liquid is eliminated and possibly drying the diaphragm thus formed.

A sintering operation of the assembly is then carried out in conditions identical to those mentioned in step (d).

It should be noted that several variants are possible with regard to sintering.

According to a first variant, each mentioned sintering step is carried out.

It should be noted that this variant is particularly suitable when the binder used in each of the dispersions is different.

Such a variant can likewise be used when the first dispersion includes a blowing agent which can be removed by an alkaline treatment, and that a solution aqueous sodium hydroxide is the second dispersing medium. In such a case, it is specified that a step of elimination of the blowing agent, if it enters the composition of the first dispersion, is not necessary after the sintering operation of step (d) since the subsequent deposition of the diaphragm corresponds to such treatment.

According to a second variant and in the particular case where the two dispersions are in an aqueous medium and where the binder used in the composition of the two dispersions above is the same, advantageously only one sintering step is carried out, the one corresponding to step (f).

Concrete but nonlimiting examples of the invention will now be presented.

EXAMPLES EXAMPLES 1 to 4

Examples 1 to 4 relate to the study of the performance of the stacked electrocatalytic agent obtained according to the following method:
The electrocatalytic agent is mixed with a suspension of PTFE with 60% of dry extract, and the whole is pressed on a nickel mesh with a pressure of 1000 kg / cm ² .
A step of consolidation of the structure is then carried out by bringing the pellet to 350 ° C.
The compact pellets obtained are tested as the cathode of an electrolysis cell of an aqueous solution of sodium hydroxide.
Comparative Example 1: the electrocatalytic agent is a nickel powder (spherical particles 5 μm).
Example 2 : the electrocatalytic agent is ruthenium oxide.
Example 3 : the electrocatalytic agent is a mixture obtained by mechanical synthesis, comprising nickel powder of example 1 and ruthenium oxide powder obtained in example 2, in a weight ratio 70/30 .
This mixture is placed in a steel container under an inert nitrogen atmosphere and in the presence of steel balls (H-440 steel). The whole is stirred for 2 hours on a SPEX-8000 shaker. The grinding process leads to a mixture of Ni and RuO ₂ in the form of particles whose periphery is enriched with ruthenium oxide relative to the core. The average mass composition is Ni ₇₀ Ru ₃₀ .
Example 4: the electrocatalytic agent is based on graphite and ruthenium oxide.
A graphite powder (LONZA) with a specific surface 300 m ² / g is placed for 1 hour in a solution of boiling nitric acid. The powder is then filtered, rinsed and dried.
The powder is introduced into an aqueous solution of RuCl ₃ (10 ^-1 M) and the mixture is stirred for 1 hour.
The mixture is filtered, and the resulting powder is rinsed 4 times with distilled water, dried for 12 hours at 110 ° C.
The final roasting is carried out by bringing the powder to 450 ° C. under a stream of air for 30 min.
A step of disaggregating the aggregates is carried out by grinding.
The electrocatalytic agent thus prepared consists of the starting graphite covered with a layer of RuO ₂ . The mass percentage of RuO ₂ is 9.95 ± 0.05%.

• intensité 300 mA/cm²
• solution NaOH 6N
• température 20°C

Les surtensions représentées dans le tableau sont déterminées à partir des tensions mesurées par rapport à une électrode de référence Hg/HgO reliée à la surface de l'électrode par un capillaire de luggin.
La chute ohmique due à la résistance électrique de l'électrolyte est corrigée par impédancemétrie. Exemples Agent électrocatalytique surtension 1er jour surtension 2ème jour surtension 3ème jour surtension 4ème jour Exemple 1 nickel 284 325 336 335 Exemple 2 RuO₂ 81 77 79 83 Exemple 3 Ni/RuO₂ 84 83 81 84 Exemple 4 RuO₂/graphite 117 112 114 119 On constate un abaissement de la surtension et une stabilisation de celle-ci en utilisant un agent électrocatalytique selon l'invention par rapport au nickel.The electrocatalytic agents are stacked according to the procedure described above, and the pellets obtained are tested for the evolution of hydrogen under the following experimental conditions:

• intensity 300 mA / cm ²
• 6N NaOH solution
• temperature 20 ° C

The overvoltages shown in the table are determined from the voltages measured relative to a Hg / HgO reference electrode connected to the surface of the electrode by a luggin capillary.
The ohmic drop due to the electrical resistance of the electrolyte is corrected by impedance measurement. Examples Electrocatalytic agent 1st day overvoltage 2nd day overvoltage 3rd day overvoltage 4th day overvoltage Example 1 nickel 284 325 336 335 Example 2 RuO ₂ 81 77 79 83 Example 3 Ni / RuO ₂ 84 83 81 84 Example 4 RuO ₂ / graphite 117 112 114 119 There is a lowering of the overvoltage and a stabilization thereof using an electrocatalytic agent according to the invention compared to nickel.

EXAMPLES 5 to 8

The purpose of these examples is to study the performance of electroactivated materials, that is to say of composite materials consisting of an elementary cathode and a sheet comprising an electrocatalytic agent as described in examples 1 to 4.
The preparation of the composite electroactivated material + material with high porosity, which follows is common to Examples 5 to 8:

a) preparation of the suspension

30 g of carbon fibers and 70 g of chrysotile asbestos fibers are placed in 7000 ml of softened water containing 3.3 g of surfactant (Triton® X 100 from the company Rohm and Haas).
After rotary stirring for 30 min, 35 g of PTFE are introduced in the form of a 60% dry extract latex.
After homogenization, 100 g of Tixosil® 33J silica (Rhône-Poulenc) are added and stirring is continued for 30 min.

b) introduction of the electrocatalytic agent

The electrocatalytic agent is incorporated into the suspension described under (a): comparative example 5 120 g of nickel according to example 1 example 6 115 g of ruthenium oxide according to Example 2 example 7 60 g of electrocatalytic agent example 3 example 8 170 g of electrocatalytic agent according to Example 4.

c) preparation of the composite material

The suspension prepared in (b) is filtered through a basic braided and laminated iron cathode (wire diameter 2 mm, opening 2 mm) by applying a vacuum ramp from atmospheric pressure to a vacuum of 300 mbar.
The elementary cathode assembly and the electroactivated sheet is brought to an oven at 350 ° C. for 30 minutes.

d) use in electrolysis

The materials prepared are used as a cathode element in an electrolysis cell of an aqueous solution of 6N sodium hydroxide at 80 ° C. percolating through the cathode assembly.
The voltage is measured relative to a Hg / HgO reference electrode connected to the surface of the sheet by a Luggin capillary.
The ohmic drop due to the electrical resistance of the electrolyte is corrected by impedance measurement.
The electrolyte current density is 300 mA / cm ² . Examples electro-activated sheet overvoltage Weight (kg / cm ² ) Activation m. electrocatalytic agent. (mg / cm ² ) m. Corresponding RuO ₂ (mg / cm ² ) η (mV) Comparative example 5 0.5 Or 16.9 283 Example 6 0.5 RuO ₂ 3 3 146 Example 7 0.5 Ni / RuO ₂ 10.2 3.1 104 Example 8 0.65 RuO ₂ / graphite 27.3 2.74 65

The analysis of this table shows that the incorporation of ruthenium oxide strongly reduces the cathode overvoltage. This decrease is all the more important that the oxide is dispersed on a substrate.

EXAMPLES 9 to 11

These examples relate to the study of the performance of electroactivated materials, consisting of an elementary cathode, an electroactivated sheet and a diaphragm. Comparative example 9 the elementary cathode and electro-activated sheet assembly is obtained according to the method described for examples 5 to 8, except that only graphite powder is used Example 10 the elementary cathode and electro-activated sheet assembly corresponds to that obtained in Example 6. Example 11 the elementary cathode and electro-activated sheet assembly corresponds to that obtained in Example 8.

In all three cases, the electroactivated material / cathode assembly is deposited elementary, a diaphragm according to the following operating mode:

• 3,3 g de tensioactif;
• 100 g de fibres d'amiante chrysotile de longueurs inférieures à 1 mm;
• 20 g de PTFE sous forme de latex à environ 60 % en poids d'extrait sec;
• 30 g de Tixosil® 33 J (Rhône-Poulenc).
• eau permutée, dont la quantité est calculée pour obtenir environ 4 litres de suspension et un extrait d'environ 4,5%;

A suspension is prepared, with stirring, comprising:

• 3.3 g of surfactant;
• 100 g of chrysotile asbestos fibers of lengths less than 1 mm;
• 20 g of PTFE in the form of latex at around 60% by weight of dry extract;
• 30 g of Tixosil® 33 J (Rhône-Poulenc).
• permuted water, the amount of which is calculated to obtain approximately 4 liters of suspension and an extract of approximately 4.5%;

The suspension is allowed to stand for at least 24 hours. The suspension is stirred for 30 minutes before use.

The volume of solution required is withdrawn so that it contains the quantity of dry extract which it is intended to deposit to form the diaphragm (of the order of 1 to 2 kg / m ² ).

Filtration is carried out under programmed vacuum. Depression is established and believes from 50 mbar per minute to reach around 800 mbar. Depression is maintained 15 minutes at 800 mbar.

The assembly is then sintered, after optional drying at approximately 100 ° C., while wearing the cathode and diaphragm assembly at 350 ° C, with a bearing at a temperature about 315 ° C, all for about 1 hour and a half.

The silica is then eliminated by an alkaline attack with electrolytic soda during the first moments of electrolysis (elimination "in situ").

The three composite elementary cathode / diaphragm electro-activated material are tested as the cathode element of an electrolysis cell of an aqueous solution of sodium chloride. The chloride supply is kept constant with a concentration of 280 g / l and a temperature of 80 ° C. Examples Electrocatalytic agent ΔU _{I → O (v)} Comparative example 9 graphite 2.36 Example 10 RuO ₂ 2.21 Example 11 graphite + RuO ₂ 9.95 wt% 2.19 ΔU _{I → O} is calculated from the plot of the cell voltage (ΔU) as a function of the electrolysis current.

This table makes it possible to evaluate the activation of the cathode by the electrocatalytic agent. it is clear that the use of ruthenium oxide significantly reduces the extrapolation voltage and that this effect is increased when the ruthenium oxide is dispersed on a support (graphite for example).

Claims (15)

1. Electroactivated material comprising fibres, of which at least a part is electrically conducting, and a binder, characterized in that it additionally comprises an electrocatalytic agent consisting of particles formed of ruthenium, platinum, palladium or iridium oxide, or their mixture, distributed at least partly over an electrically-conducting support.
2. Material according to the preceding claim, characterized in that the electrocatalytic agent is uniformly distributed throughout the body of the material.
3. Material according to the preceding claim, characterized in that the proportion by weight of coating with respect to the support varies from 0.5 to 50 for each particle.
4. Material according to one of the preceding claims, characterized in that the support is chosen from iron, cobalt, nickel, Raney iron, Raney cobalt, Raney nickel, the elements from columns IVA and VA of the periodic classification, carbon or graphite.
5. Material according to one of the preceding claims, characterized in that the particle size of the support is between 1 and 100 µm.
6. Material according to one of the preceding claims, characterized in that the electrocatalytic agent represents 10-70% by weight, with respect to the combined fibres, binder and electrocatalytic agent.
7. Material according to any one of the preceding claims, characterized in that the said electrocatalytic agent is obtained by mixing all the constituent components of the said agent in the form of a solution or of a suspension, then by optionally carrying out a precipitation stage, then drying, or by mechanical synthesis.
8. Material according to one of the preceding claims, characterized in that the electrocatalytic agent additionally comprises an additive chosen from iron, cobalt, nickel and/or their oxides.
9. Material according to one of the preceding claims, characterized in that the fibres represent 10-65% and the binder 5-20%, with respect to the weight of the combined fibres, binder and electrocatalytic agent, the weight of binder corresponding to 20-50% by weight, with respect to the fibres and binder subsystem.
10. Composite material comprising (a) a material of high porosity and (b) the material according to one of Claims 1 to 8.
11. Material according to the preceding claim, characterized in that it comprises, from one face towards the other: (a) a metal surface of high porosity, (b) the electroactivated material and (c) a separator.
12. Process for the preparation of the material according to one of Claims 1 to 10, characterized in that the following stages are carried out:

    (a) an aqueous suspension comprising the fibres, the binder, the electrocatalytic agent and adjuvants is prepared,

    (b) a sheet is deposited by filtering the said suspension, under programmed vacuum, through a material of high porosity,

    (c) the liquid is removed and the sheet thus formed is optionally dried,

    (d) the sheet thus obtained is optionally sintered.
13. Process for the preparation of a composite material according to Claim 11, characterized in that the following stages are carried out:

    (a) an aqueous suspension comprising the fibres, the binder, the electrocatalytic agent and adjuvants is prepared,

    (b) a sheet is deposited by filtering the said suspension, under programmed vacuum, through a material of high porosity,

    (c) the liquid is removed and the sheet thus formed is optionally dried,

    (d) the sheet thus obtained is optionally sintered,

    (e) a dispersion, in water or in an aqueous sodium hydroxide solution, comprising fibres, a binder and adjuvants is deposited on the said sheet by filtering under programmed vacuum,

    (f) the liquid is removed and the diaphragm thus formed is optionally dried,

    (g) the whole unit is sintered.
14. Process according to the preceding claim, characterized in that, after Stage (g), a treatment is carried out with an aqueous alkali metal hydroxide solution, if the dispersion of Stage (e) is in water and if it comprises silica as porogenic agent.
15. Process according to Claim 13, characterized in that the sintering Stage (d) is carried out if the binder of the dispersions of Stages (a) and (e) are different.