EP1259995A1

EP1259995A1 - Method for preparing electrode-membrane assemblies, resulting assemblies and fuel cells comprising same

Info

Publication number: EP1259995A1
Application number: EP01911830A
Authority: EP
Inventors: Didier Marsacq; Franck Jousse; Michel Pineri; Régis Mercier
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2000-03-03
Filing date: 2001-03-02
Publication date: 2002-11-27
Also published as: FR2805927A1; FR2805927B1; WO2001065623A1; JP2003526184A; CA2400146A1

Abstract

The invention concerns a method for preparing an assembly comprising at least an electrode having an active surface, and a heat-stable polymer membrane, comprising the following steps which consist in: a) pouring on a support a heat-stable polymer so as to obtain a heat-stable polymer solution film; then b) partly drying said heat-stable polymer solution film by evaporating the solvent from said solution; c) depositing an electrode on the surface of said heat-stable polymer solution film, during the drying process, before it is completely dry, the active surface of the electrode facing said surface, so as to obtain an assembly comprising a heat-stable polymer membrane and said electrode; d) completely drying said assembly resulting from step c); then e) separating the assembly comprising said membrane and said electrode from the substrate. The invention also concerns electrode-membrane and electrode-membrane-electrode (EME) assemblies obtained by said method and a fuel cell comprising said assemblies.

Description

PROCESS FOR THE PREPARATION OF ELECTRODE-MEMBRANE ASSEMBLIES, ASSEMBLIES THUS OBTAINED AND FUEL CELLS COMPRISING SUCH ASSEMBLIES

DESCRIPTION

The present invention relates to a process for preparing electrode-membrane and electrode-membrane-electrode assemblies and to the assemblies thus obtained.

These assemblies are more precisely

15 electrode-membrane-electrode assemblies, in which the membranes are ion-exchange polymer membranes, such assemblies find their application more particularly in fuel cells, in particular low-level fuel cells.

Temperatures generally operating from room temperature, up to about 100 ° C, such as proton exchange membrane fuel cells operating with either the gas couple

(H ₂ / air oxygen), known as PEMFC, or

25 with the methanol / oxygen pair of air, known as DMFC ("Direct Methanol Fuel Cell", in English).

Consequently, the invention also relates to a fuel cell device, in

30 particular of the solid electrolyte type, comprising at least at least one of said electrode - membrane - electrode assemblies.

The technical field of the invention can thus be defined as that of fuel cells, in particular fuel cells of the solid electrolyte type.

Fuel cells of the solid polymer electrolyte type find, in particular, their application in electric vehicles which are currently the subject of numerous development programs, in order to provide a solution to the pollution caused by vehicles with thermal engine.

Fuel cells with solid polymer electrolyte could make it possible, by playing the role of an electrochemical energy converter, associated with an on-board energy reservoir, for example of hydrogen or an alcohol, to overcome the problems, in particular of time of charging and autonomy, linked to the use of batteries in electric vehicles.

The schematic assembly of a fuel cell, allowing the production of electrical energy, is shown in part in Figure 1 attached. The essential element of such a cell is an ion exchange type membrane, more specifically a proton exchange membrane, formed of a solid polymer electrolyte, more precisely of a proton conducting polymer (1); this membrane is used to separate the anode compartment (2), where produces the oxidation of fuel, such as hydrogen H ₂ (4), according to the diagram:

2H ₂ → 4H ⁺ + 4th ^" ,

of the cathode compartment (3), where the oxidant, such as oxygen in the air 0 (5) is reduced, according to the diagram:

0 ₂ + 4H ⁺ + 4th ^" → 2H ₂ 0,

with production of water (6), while the anode and the cathode are connected by an external circuit (10). The water thus produced circulates between the two compartments by electro-osmosis and by diffusion (arrows 11, 12).

The volume electrodes (13), conductive, electronic, placed on either side of the membrane, generally comprise an active area (14) and a diffusion area (15). The active area, generally provided on one of the surfaces of the electrode, consists of a porous teflon-coated felt, loaded with carbon black or porous graphite, covered with a finely divided noble metal (16) (for example , in the form of grains), such as platinum, and a thin deposit of ionic conductive polymer, of structure generally similar to that of the membrane. The diffusion zone (15) is made up of a porous material, for example of the same porous teflon-coated felt, loaded with carbon black, or of the same porous graphite, made hydrophobic by the integration of a hydrophobic polymer, such as PTFE. The hydrophobic nature allows the evacuation of liquid water. The noble metal, such as platinum, located in the active zone, makes it possible either to oxidize hydrogen or methanol at the anode, or to reduce oxygen at the cathode.

The protons produced at the anode, by oxidation, for example hydrogen, on the surface of the noble metal grains, such as platinum, are transported (9) through the membrane to the cathode where they recombine with the ions produced by the reduction, for example oxygen from the air to give water (6).

The electrons thus produced (IV) make it possible to supply, for example, an electric motor (18) placed in the external circuit (10), with water as the only by-product of the reaction.

The membrane and electrodes assembly is a very thin assembly with a thickness of the order of a millimeter, called “electrode-membrane-electrode assembly (EME)” and each electrode is fed from the rear, for example using of a grooved plate, by the gases.

The power densities obtained by this recombination and which are generally of the order of 0.5 to 2 W / cm ² , in the case where hydrogen and oxygen are used, require the association of several of these volume electrode-membrane-volume electrode structures to obtain, for example the 50 kW required for a standard electric vehicle. In other words, it is necessary to assemble a large number of these structures, the elementary surfaces of which can be of the order of 20 × 20 cm ² , in order to obtain the desired power, in particular in the case where the fuel cell is put used in an electric vehicle.

For this purpose, each assembly formed by two electrodes and a membrane, defining an elementary cell of the fuel cell, is thus disposed between two sealed plates (7, 8) which, on the one hand, ensure the distribution of the hydrogen on the anode side and, on the other hand, oxygen on the cathode side. These plates are called bipolar plates.

The ion-conducting membrane is generally an organic membrane containing ionic groups which, in the presence of water, allow the conduction of the protons (9) produced at the anode by oxidation of hydrogen.

The thickness of this membrane is from a few tens to a few hundred microns and results from a compromise between the mechanical strength and the ohmic drop. This membrane also allows the separation of gases. The chemical and electrochemical resistance of these membranes generally allows battery operation over periods of more than 1,000 hours.

The polymer constituting the membrane must therefore fulfill a certain number of conditions relating to its mechanical, physico-chemical and electrical properties. The polymer must first of all be able to give thin films, from 50 to 100 micrometers, dense, without defects. The mechanical properties, tensile stress modulus, ductility, must make it compatible with assembly operations including, for example, clamping between metal frames.

The properties must be preserved by passing from the dry state to the wet state. The polymer must have good chemical stability with respect to hydrolysis and have good resistance to reduction and to oxidation up to 100 ° C. This stability is assessed in terms of variation in ionic resistance, and in terms of variation in mechanical properties.

Finally, the polymer must have a high ionic conductivity, this conductivity is provided by strong acid groups, such as phosphonic acid groups, but especially sulfonic groups linked to the polymer chain. Therefore, these polymers will generally be defined by their equivalent mass, that is to say by the weight of polymer in grams per acid equivalent.

For example, the best systems currently developed are capable of providing a specific power of 1 W.cm ^"2 , or a current density of 2 A. cm ^{" 2} for 0.5 Volts.

The most commonly used polymers are sulfonated fluorinated thermoplastic copolymers whose linear main chain is perfluorinated and whose side chain carries a sulfonic acid group.

These thermoplastic copolymers are commercially available under the trade name Nafion ^® of Du Pont, or ACIPLEX-S ^® from Asahi Chemical Company, others are experimental, produced by Dow Company to manufacture the so-called membrane "XUS".

We have seen that the electrochemical reactions, described by the operations mentioned above, involve protons coming from the membrane, electrons, the catalyst located on one of the surfaces of the electrode and finally either the reducing agent, such that hydrogen, or the oxidant, such as oxygen in the air, these reactions occurring essentially at the limit or interface between the membrane and the electrode.

It is therefore clear that the performance of an electrode-membrane assembly and therefore of the fuel cell are closely linked to the quality of the electrode-membrane interface, on which fundamentally depends the probability of the simultaneous presence in this zone of the different species, cited above. The manufacturing process for electrode - membrane - electrode assemblies or assemblies has a decisive influence on the quality of the electrode - membrane interface.

The manufacture of electrode - membrane - electrode (EME) assemblies is not or very little described in the literature. Indeed, it is most often a specific know-how for each laboratory or industrial, involved in the manufacture of fuel cells.

The EME manufacturing process, most often cited, consists in producing the EME assemblies by hot passage of the electrodes facing each other over the proton exchange membrane, said membrane having been previously, separately prepared, generally by casting, and completely dried.

This technique is commonly used in the case of proton exchange membranes, the most commonly used at present and already mentioned above; namely, NAFION ^® type polymer membranes.

The production EME sets, the electrodes are previously impregnated, for example, a solution of Nafion ^®, are then hot-pressed between 120 ° and 150 ° C, on both sides of the membrane. The thermoplastic nature of NAFION and the impregnation of the electrodes, using a polymer identical to that which composes the membrane, make it possible to obtain an excellent quality of the electrode-membrane interface, both in terms of properties. mechanical, represented by excellent adhesion, than that of the proton-electrode exchange surface.

The electrochemical performance of fuel cells incorporating such assemblies is therefore satisfactory.

However, this method of manufacturing EME assemblies has several drawbacks major: first, this method is difficult to industrialize and NAFION ^® polymers are extremely expensive. However, from the perspective of the development of fuel cells usable for automobile traction, another essential problem, now well identified by the experts, is the cost of the membrane, the latter being with that of bipolar plates the predominant factor influencing the cost price of the fuel cell. In 1995, the cost of membranes produced or in development was around 3,000 to 3,500 F / m ² and it is estimated that this cost must be divided by 10, or even 20, to assist in development. industrial fuel cell for the automotive industry. With a view to lowering costs, poly 1, - (2,6-diphenyl-6) -phenyl ether sulfonated on the main chain, polyethersulfones and polyetherketones have been synthesized and tested without really competing with fluorinated membranes as regards instant performance and durability.

In order to provide membranes meeting the relative conditions, in particular their mechanical, physicochemical and electrical properties, while presenting a manufacturing cost clearly lower than that, prohibitive of the perfluorinated membranes, described above, new polymers have been developed. of sulfonated polyimides which are described in document FR-A-2 748 485. However, the process described above is not suitable for other types of membranes, that is to say for membranes that are not made of a thermoplastic polymer, such as NAFION ^® .

In particular, the process used in the case of NAFION membranes is absolutely not suitable for membranes which are composed of a sulfonated polymer with a thermostable skeleton, a polymer among which, mention may be made of polyimides, polyethersulfones, polyetheretherketones, polybenzoxazoles, polybenzimidazoles, polyphenylenes and their derivatives, etc.

Indeed, such membranes have neither the thermoplastic nature or the chemical structure of Nafion ^® and they therefore have no affinity for the electrode impregnated with a solution of Nafion and the quality of the interface electrode - membrane is poor .

In addition, in the case where the membrane is a thermally stable polymer, the method involves the use of several compounds, i.e. a sulfonated polymer thermostable for proton-exchange membrane and one Nafion ^® solution for impregnating

1 electrode.

In addition, the process is then a complex discontinuous process comprising multiple steps, among others: development of the proton exchange membrane, impregnation of the electrodes, pressing, heating.

As a result, the performance and even the advantage of such assemblies, involving a thermostable polymer membrane and an electrode impregnated with NAFION type polymer, are extremely limited from the industrial point of view.

If it is wished to retain all the advantages of thermostable sulfonated polymer membranes in terms of, in particular, cost price, and to overcome the drawbacks mentioned above by improving in particular the quality of the electrode-membrane interface, it has been envisaged in the same way as for the membranes NAFION ^® type polymer, impregnating the electrodes with a polymer solution which comprises the membrane in the case of sulfonated polymers to thermostable skeleton, such as polyimides, polyethersulfones, polyetheretherketones , polybenzoxazoles, polybenzimidazoles, polyphenylenes and their derivatives, etc.

However, the mechanical rigidity of this family of polymers causes the appearance of strong mechanical stresses at the interfaces during the drying phase by evaporation of the solvent.

In addition, the absence of a thermoplastic nature of these polymers does not allow sufficient contact to be established between the membrane and the impregnated electrodes. In this case, the electrochemical performance is not as high and cannot be reproduced from one operation to another. The quality of the interfaces is not sufficient to allow the proton conductor to be intimately in contact with the electronic conductor and the catalyst on the surface of the electrode. This guy assembly is likely to be sensitive to aging and to evolve rapidly over time. Despite pressing at high temperature, the electrode-membrane adhesion remains weak. Finally, the multiple stages, indicated above, which mark the production of these assemblies constitute an obstacle to continuous manufacturing.

Furthermore, from document US-A-5,242,764, an assembly process is known which makes it possible to avoid the use of a proton exchange membrane. This technique is based on the impregnation of the electrodes, using a high quantity of a NAFION solution, followed by hot bonding of the two electrodes thus impregnated. This technique is, again, only suitable for thermoplastic polymers of the NAFION® type and makes it difficult to obtain a layer of homogeneous proton conducting polymer impermeable to gas. There is therefore a need for a process for manufacturing, preparing, assemblies comprising an electrode and a membrane made of a thermostable polymer, also called elementary assemblies, and of electrode - membrane - electrode assemblies, which is simple, reliable, reproducible , and on ; which has only a limited number of steps, which is of limited cost, and which can be implemented continuously, this process having, moreover, all the advantages inherent in the use of membranes made of thermostable polymers . This process must also make it possible to obtain electrode-membrane interfaces of excellent quality, without defects, with very high cohesion of the electrode-membrane bond and intimate contact of the catalyst with the membrane, these properties being stable to over time and not very sensitive to aging.

The electrode-membrane-electrode assemblies obtained must finally have excellent and perfectly reproducible electrochemical properties.

The aim of the present invention is to provide a method for preparing an assembly comprising an electrode and at least one membrane made of a thermostable polymer, more precisely a method for preparing an electrode - membrane - electrode assembly, which responds, between others, to all of the needs indicated above.

The object of the present invention is also to provide a process for preparing an assembly comprising an electrode and a membrane made of a thermostable polymer, more specifically an electrode - membrane - electrode (EME) assembly consisting of a membrane in one thermostable polymer and two electrodes, which does not have the disadvantages, defects, limitations and disadvantages of the methods of the prior art and which solves the problems posed by the methods of the prior art.

This object and others still are achieved, in accordance with the invention by a process for preparing an assembly comprising at least one electrode (preferably one) having an active face, and a membrane made of a thermostable polymer, in which the following steps are carried out: a) a solution of a thermostable polymer is poured onto a support so as to obtain a film of solution of thermostable polymer; then b) partially drying said film of thermostable polymer solution by evaporation of the solvent from said solution; c) an electrode is deposited on the surface of said film of thermostable polymer solution, during drying, before it is completely dry, the active face of the electrode facing said surface, so as to obtain an assembly comprising a thermostable polymer membrane (formed by said partially dried polymer solution film) and said electrode; d) said assembly obtained in step c) is completely dried; then e) the assembly comprising said membrane and said electrode is detached from the substrate.

The method according to the invention makes it possible to meet the needs and remedy the drawbacks mentioned above. The process according to the invention is particularly suitable for membranes made of thermostable polymer, the advantages of which are inherent in this type of polymer, which also have repercussions on the process which implements them. The method according to the invention comprises a limited number of simple steps which are easy to carry out by proven means, it is reliable and reproducible, achievable at low temperature, without significant energy consumption, requires only a relatively limited duration, and involves only a few raw materials, these being limited to the polymer, solvent and to the electrode.

On the contrary, methods of the prior art, it allows continuous manufacturing and at low cost. Basically, according to the invention, the electrode-membrane assembly is carried out during the development of the membrane by casting.

According to the invention, in accordance with step c) of the method, the electrode is deposited simply, directly and without other operations (such as pressing or others, as in the prior art), on the surface of the membrane during drying thereof, that is to say that the membrane then consists of a film of thermostable polymer solution still wet and not completely dry.

It is known that, in a conventional manner, the membrane is prepared by pouring a solution of the polymer onto a substrate or support, so as to obtain a film of polymer solution, in particular of thermostable polymer, then this polymer film in solution. is then dried by total evaporation of the solvent, the dry extract obtained constituting the membrane such as the proton exchange membrane.

In the methods of manufacturing electrode-membrane assemblies and EME assemblies of the prior art, such as the pressing processes at hot, the assembly operation aimed at carrying out the assembly and establishing a connection between the electrode (s) and the membrane is in all cases carried out with a membrane whose development process has been completed and which is completely dry. It is therefore a fully, completely formed and dry membrane which is used in the processes of the prior art.

According to the invention, one goes against this approach, unanimously followed in the prior art, by using, during the preparation of the assembly of a membrane during formation, not yet fully formed, still damp and not completely dry. In essential step c) of the process of the invention, during drying, and when the viscosity of the polymer film in solution has reached an optimal level, an electrode is carefully deposited on the surface of the film.

A well-determined fraction of the polymer solution then permeates the electrode, and more precisely, the active layer situated on the active face thereof which faces the surface of the polymer film in solution. This impregnation is done simply under the action of the weight of the electrode in the still viscous polymer and without any pressure being applied.

Thanks to the process of the invention and in particular because of step c) specific that it comprises the electrode-membrane interface is of excellent quality. It is totally surprising that an interface of such quality is obtained with thermostable polymer membranes; such a result, which was up to now obtained only for thermosplastiques polymer type NAFION ^®, is reached for the first time by implementing the method of the invention. It has been shown that the electrode-membrane interface prepared by the method of the invention is perfectly regular and free from defects.

The cohesion between the electrode and the membrane and the electrode is such that it is no longer possible to separate them, unlike the assemblies produced by the methods of the prior art. This excellent adhesion is, among others, one of the fundamental effects and advantages provided by the method of the invention, compared to the methods of the prior art, such as methods based on hot pressing of the electrode or electrodes on the membrane.

It can be explained that, in the process of the invention, the deposition operation according to step c) being carried out when the membrane is not yet completely formed or dry, this more precisely means that the substance which permeates the electrode is composed of thermostable polymer, more particularly of proton conducting polymer, and of a slight fraction of solvent. This allows the proton conductor to be drawn homogeneously within the active layer of the electrode. The assembly thus obtained is then dried under precise conditions at moderate temperature, generally from 70 ° C. to 150 ° C., preferably at a temperature of 100 ° C to 120 ° C. An example of an adequate temperature is in particular close to 70 ° C. This promotes the presence of proton conducting polymer in the vicinity of the electronic conductor and the catalyst contained in the electrode.

In other words, thanks to the quality of the interfaces obtained by the process of the invention, there is close contact between the membrane and the electrode (s), that is to say that the proton conductor is intimately in contact with the electronic conductor and the catalyst on the surface of the electrode. It follows that the assemblies prepared by the process of the invention have very high electrochemical properties and performances and perfectly reproducible, that is to say, in particular, an evolution of the voltage as a function of the current density at less similar to "all NAFION ^® " assemblies.

In order to further improve the mechanical properties of the assemblies according to the invention, it is possible at the end of step a) to have a reinforcement within the film of thermostable polymer solution, for example, by rolling.

Or, for the same purpose, a reinforcement may be placed on the support or substrate, prior to step a) of the method according to the invention.

The invention relates more specifically to a method of preparing an electrode-membrane-electrode assembly consisting of a thermostable polymer membrane and two electrodes. This method comprises, first of all, the production of a first membrane electrode assembly by the method described above, then at the end of step e), we proceed to the following step f): we pour on the face of the assembly constituted by the membrane a solution of a thermostable polymer, so as to obtain a film of solution of thermostable polymer; then steps b) to e) are substantially repeated. In other words, the electrode-membrane assembly obtained at the end of step c) is then used as a substrate during a second casting operation, partial drying, deposition of a second electrode, and total drying. . The steps of this process will therefore be, in addition to steps a), b), c), d), e) and f), the following steps: g) said film of thermostable polymer solution is partially dried by evaporation of the solvent from said solution; h) a second electrode is deposited on the surface of said film of thermostable polymer solution, during drying, before it is completely dry, the active face of the second electrode facing the surface of said film, so as to obtain an electrode - thermostable polymer membrane - electrode assembly; then i) said electrode - membrane - electrode assembly obtained during step h) is completely dried. The advantages and effects brought by this method are those already described above, in particular the assemblies have mechanical, electrochemical properties (evolution of the voltage as a function of the current density), etc., superior to the assemblies obtained by the method of prior art.

According to a variant of the process of the invention, called the “impregnation” process as opposed to the “coating” process, described in the above, an electrode - membrane - electrode assembly is prepared by the following steps: a) one impregnates a reinforcement with a solution of a thermostable polymer, so as to obtain a film of reinforced and self-supporting thermostable polymer solution; then b) partially drying said film of reinforced and self-supported thermostable polymer solution, by evaporation of the solvent from said solution; c) an electrode is deposited on each of the faces of said film of thermostable polymer solution, during drying, before it is completely dry, the active face of each of the electrodes facing each of the surfaces of said film; d) said assembly obtained in step c) is completely dried.

According to a particularly advantageous characteristic of the process of the invention, whether it is the coating process or the process by impregnation, it can be carried out continuously, which is not the case with the methods of the prior art.

The invention also relates to the assemblies comprising at least one membrane and at least one electrode, as well as the electrode - membrane - electrode assemblies capable of being obtained by the above process. We have seen that these assemblies, as such and because of the excellent and surprising quality of their interface and of the mechanical (adhesion, etc.) and electrochemical properties (evolution of the voltage as a function of the current density) which ensued, inherently possessed properties which differentiated them from the assemblies of the prior art and made them superior to the latter.

The invention further relates to a fuel cell device comprising at least one electrode - membrane - electrode assembly obtained by the method according to the invention. In the same way, the batteries have, as such, excellent and surprising properties, due both to the properties of the thermostable membranes and to the properties of the EME assemblies, the properties resulting directly from the implementation of the process of l 'invention. The invention will now be described in more detail, with reference to the accompanying drawings, in which:

- Figure 1 schematically shows a fuel cell comprising several elementary cells with an electrode - membrane - electrode assembly, as well as bipolar plates; FIG. 2 is an image obtained by scanning electron microscopy of an electrode-membrane interface obtained by the method of the invention with a membrane of sulfonated polyimide. The scale is 10 μm;

- Figure 3 is an image obtained by scanning electron microscopy of an electrode-membrane interface obtained by a method of the prior art with a sulfonated polyimide membrane. The scale is 10 μm.

More specifically, the method according to the invention, in its variant, called "coating", firstly comprises the preparation of a solution, in a solvent, of a thermostable polymer. The thermostable polymer can be any known polymer. The method according to the invention is suitable for all polymers capable of giving membranes by casting. By thermostable, is generally meant a polymer whose glass transition temperature (case of amorphous polymers) or of melting point (case of semi-crystalline polymers) is higher than the degradation temperature of the polymer.

Preferably, the polymer is an ion-exchange polymer, more preferably a proton-conducting polymer, such as a sulfonated polymer, but a polymer carrying phosphate or other functions may also be suitable. Examples of suitable polymers that may be mentioned include sulfonated polyimides, sulfonated polyethersulfones, sulfonated polystyrenes and their sulfonated derivatives, sulfonated poletheretherketones and their derivatives. sulfonated, sulfonated polybenzoxazoles, sulfonated polybenzimidazoles, sulfonated polyparaphenylenes and their sulfonated derivatives.

Particularly preferred polymers are the sulfonated polyimides described in document FR-A-2 748 485 incorporated herein by reference, in particular for the parts of this document describing these polymers.

Other polymers of sulfonated polyimide type are the sulfonated polyimides blocks formed by the blocks or blocks represented by the following formulas (I _x ) and (I _y ):

in which: - x is a real number, preferably greater than or equal to 4, more preferably from 4 to 15; and

- y is a real number, preferably greater than or equal to 5, more preferably from 5 to 10;

- and the groups Ci and C ₂ may be identical or different and each represent a tetravalent group comprising at least one aromatic carbon ring, optionally substituted, having from 6 to 10 carbon atoms and / or a heterocycle of aromatic character, optionally substituted, having from 5 to 10 atoms and comprising one or more heteroatoms chosen from S, N and 0; Ci and C ₂ each forming, with neighboring imide groups, rings with 5 or 6 atoms, - the groups Ar _x and Ar ₂ may be identical or different and each represent a divalent group comprising at least one aromatic carbon ring,

Optionally substituted, having from 6 to 10 carbon atoms and / or a heterocycle of aromatic nature, optionally substituted, having from 5 to 10 atoms and comprising one or more heteroatoms chosen from S, N and 0; at least one of said aromatic rings

15 carbonaceous and / or heterocycle of Ar ₂ being further substituted by at least one sulfonic acid group.

Such sulfonated polyimides can correspond to the following general formula (I):

in which Ci, C ₂ , Ar _x and Ar ₂ , x and y have the meaning already given above, z is a number, preferably from 1 to 10, more preferably from 2 to 6, and where each of the groups Ri and R ₂ represents NH ₂ , or a group of formula:

where C ₃ is a divalent group comprising at least one aromatic carbon ring, optionally substituted, having 6 to 10 carbon atoms and / or a heterocycle of aromatic nature, optionally substituted, having 5 to 10 atoms and comprising one or more heteroatoms chosen from S, N and 0, C ₃ forming with the neighboring imide group a ring with 5 or 6 atoms.

Most such polymers are readily available commercially and at low cost. The thermostable polymer must also be soluble in the solvent of the solution, this solvent can be easily chosen by a person skilled in the art depending on the polymer used.

The solvent is generally an organic solvent which is chosen, for example, from polar aprotic solvents, such as dimethylformamide (DMF), dimethylacetamide (DMAC), N-methylpyrrolidone (NMP), alone or in admixture, with, for example, aromatic solvents, such as xylene or solvents of the glycol ether type.

The solvent can also be a phenolic type solvent, that is to say it is chosen, for example, from phenol, phenols substituted by one or more halogens (Cl, I, Br, F), cresols (o-, m- and p-cresol), cresols substituted by a halogen

(Cl, I, Br, F) and mixtures thereof. The concentration, viscosity and temperature of the polymer solution are adjusted, in order to allow the production of a homogeneous film by the coating system. By way of example, this casting system is preferably chosen from so-called “Hand Coater” systems or manual applicators.

The concentration, viscosity, and temperature of the applied polymer solution depend on the nature of the latter, but suitable ranges will be, for example, from 30 to 100 g / l, for the concentration, from 1 to 10 Pa. s, for viscosity, and from 80 to 130 ° C for the temperature of the solution applied by casting (in the case of a polymer of sulfonated polyimide type).

This polymer solution poured onto a support or substrate which can be both flexible and rigid.

As suitable material for the substrate or support, mention may be made of: glass, aluminum, polyester, etc. The shape of this substrate or support generally corresponds to that of the membrane and the final assembly that we want to prepare. This substrate is generally flat.

It is further preferable that the substrate or support is perfectly clean during casting.

At the end of the casting, a film, generally planar, of a solution of thermostable polymer on the surface of the substrate or support is obtained. It is a "wet" film, that is to say a rich film. as a solvent and comprising substantially all of the solvent present in the solution used for casting. The thickness of the wet film is variable, it is generally calibrated at a thickness of 500 to 5,000 μm, for example 3,000 μm. The polymer solution film is then partially dried by evaporation of the solvent from said solution. For this purpose, the substrate or support is generally maintained at a temperature of 40 to 150 ° C, for example 120 ° C, in order to cause rapid evaporation of the solvent. Such a temperature can be obtained by placing the substrate or support provided with the film of polymer solution in an oven.

Drying is partial drying, that is to say that the polymer solution film still contains solvent, generally the fraction of solvent still present is 5 to 20% of the amount of solvent initially present.

In other words, the drying is stopped after a variable duration, generally from 60 to 120 minutes, when the viscosity of the polymer film has reached a level high enough to withstand the electrode. This viscosity can be easily determined by a person skilled in the art, but it is generally 20 to 30 Pa.s.

The electrode is therefore deposited on the surface of said film of thermostable polymer solution during drying, before it is completely dry, the active face of the electrode facing said surface.

The electrode is a conventional readily available commercially available electrode of the type commonly used in fuel cells and has already been described above. Such an electrode, generally planar, and with a thickness of 100 to 500 μm, generally comprises a face called the active face containing the catalyst, for example platinum carbon, it is this active face which is delicately deposited on the surface of the film. wet with thermostable polymer solution.

This slowly impregnates the active layer, for example the carbon layer platinized on the surface of the electrode and a membrane (still wet) - electrode assembly is thus formed.

In the next step, the drying of the electrode-membrane assembly obtained is continued for a variable period of 30 to 60 minutes at a temperature of 70 to 150 ° C, for example 120 ° C, in order to eliminate all residual solvent still present and form the final assembly. In fact, it is during this stage that the "membrane" is actually formed. Finally, in a last step, the electrode-membrane assembly is detached from the support or substrate.

The thickness of such an assembly is from 100 to 500 μm.

When it is desired to carry out a complete assembly of electrode - membrane - electrode by the coating process according to the invention, one begins, first of all, by preparing a first assembly, in accordance with the description given above. The electrode-membrane assembly thus obtained is then used as a substrate to produce a second elementary electrode-membrane assembly, that is to say that a new wet film of thermostable polymer solution is poured onto the membrane of the first elementary assembly. It is generally the same solution of the same polymer as that poured to make the first assembly. Then, during the drying of this wet film, a second electrode, generally analogous to the first, is carefully deposited under the conditions already mentioned above. Again, it is the active face of the electrode which is deposited on the surface of the wet film. Complete drying is then carried out under the same conditions as those described for the preparation of the first elementary assembly.

At the end of this process, a complete assembly of electrode - membrane - electrode is obtained. It should be noted that this method also allows improve the gas impermeability of the proton exchange membrane.

Advantageously, according to the invention, it is possible to produce assemblies in which the membrane is reinforced by a reinforcement in order in particular to improve its mechanical properties.

In the coating process, it is thus possible to apply, spread, the solution of thermostable polymer on a substrate or support, as described above, a reinforcement having been previously disposed on said support.

Such a reinforcement can consist of a fabric, for example glass, PEEK, PTFE; a mat, for example glass; a porous material, for example PEEK, PTFE.

The preparation of the solution and all the other steps of the process and the conditions thereof are similar to those described above for the coating process, the only difference being that the reinforcement is available before step a ) on the substrate or support.

In other words, the polymer, for example the sulfonated polyimide used, is in solution in a solvent which can be of variable nature, such as phenol, chlorophenol, cresol, NMP, DMF, DMAc, etc. The concentration, the viscosity and temperature of the solution are adjusted to allow the production of a homogeneous film by a coating system such as a "Hand-Coater". The solution of polymer, for example of sulfonated polyimide is then spread on a substrate flexible or rigid, for example made of perfectly clean glass on which the reinforcement is placed. The thickness of the wet film is calibrated to a thickness of 500 to 5,000 μm, for example in the vicinity of 3,000 μm. The temperature of the substrate is maintained, for example, in the vicinity of 120 ° C., in order to cause the rapid evaporation of the solvent.

After a few minutes (60 to 120), the viscosity of the wet film has reached a level high enough to support the electrode. The active face of the electrode containing, for example, the platinum carbon is then placed delicately on the surface of the wet film. This slowly impregnates the active layer, for example, of platinum carbon on the surface of the electrode. The drying of the electrode-membrane assembly is continued for several more minutes in order to remove all of the residual solvent.

The electrode-reinforced membrane assembly is then detached from the initial substrate and is then, in turn, used as a substrate to perform a second step.

A new wet film is poured onto the membrane of the previous assembly. During the drying of this wet film, a second electrode is carefully deposited in accordance with the description already given above.

At the end of these two stages, a complete assembly of electrode - reinforced membrane - electrode is obtained. Or, in the coating process and with the same objective of reinforcing the membrane, the reinforcement can be placed within the wet film of polymer solution prepared by casting during step a). Such reinforcement is analogous to that already described above.

The preparation of the solution and all the other stages of the process, as well as the conditions thereof, are similar to those described above for the coating process, the only difference being that the reinforcement is placed within the same wet film, at the end of step a). In other words: the polymer, for example sulfonated polyimide used, is in solution in a solvent which may be of variable nature, such as phenol, chlorophenol, cresol, NMP, DMF, DMAc, etc. The concentration, the viscosity and the temperature of the solution are adjusted in order to allow a homogeneous film to be produced by a coating system, such as a “Hand-Coater”. The solution of polymer, for example of sulfonated polyimide, is then spread on a substrate, for example of glass, perfectly clean. The thickness of the wet film is calibrated to a thickness of 500 to 5,000 μm, for example in the vicinity of 3,000 μm. The reinforcement is then placed by any suitable technique, for example by rolling within the wet film, on the substrate. The temperature of the substrate is maintained, for example in the vicinity of 120 ° C., in order to cause the rapid evaporation of the solvent. After a few minutes, the viscosity of the wet film has reached a level high enough to support the electrode. The active face of the electrode containing, for example, the platinum carbon is then placed delicately on the surface of the wet film. This perfectly impregnates the active layer, for example carbon platinized on the surface of the electrode. The drying of the electrode-membrane assembly is continued for several more minutes in order to remove all of the residual solvent.

The electrode-membrane assembly is then detached from the substrate and used to make a reinforced electrode-membrane-electrode assembly, in accordance with what has already been described above.

The method according to the invention, for preparing a complete assembly of electrode - membrane - reinforced electrode, can, according to a variant, be carried out by "impregnation".

A polymer solution is prepared in the same manner as above, the polymers and solvents used for the preparation of this solution are the same as those already mentioned above for the coating process.

The concentration, viscosity, and temperature of the polymer solution are adjusted in this case, in order to allow the impregnation of a reinforcement, this reinforcement being of the type already described above, namely, for example, fabric, matt or porous material, for example glass, PEEK or PTFE. As a result, these concentrations, viscosities and temperatures may differ from those indicated in the case of the coating process. The concentration, viscosity and temperature of the polymer solution used for the impregnation depend on the nature of the polymer and possibly that of the reinforcement, but suitable ranges will be, for example from 80 to 120 g / l, for the concentration , from 5 to 15 Pa.s, for the viscosity and from 70 to 120 ° C for the temperature of the solution impregnating the reinforcement (in the case of a polymer of sulfonated polyimide type). The impregnation is generally carried out by simply immersing the reinforcement in the polymer solution.

At the end of this impregnation, a film of self-supported reinforced thermostable polymer solution is obtained, this film is a “wet” film, that is to say rich in solvent and comprising substantially all of the solvent present in the solution used for impregnating the reinforcement. The thickness of the wet film of polymer solution is variable, it is generally calibrated to a thickness of 1,000 to 2,000 μm, for example 1,500 μm. The film of the reinforced and self-supporting polymer solution is then partially dried, by evaporation of the solvent from the said solution. For this purpose, the reinforced and self-supporting wet film is generally maintained at a temperature of 70 to 150 ° C, for example 120 ° C, in order to cause rapid evaporation of the solvent. Such a temperature can be obtained by placing the reinforced self-supporting wet film of polymer solution in an oven. Drying is partial drying, that is to say that the self-supported reinforced film of polymer solution still contains solvent, generally the fraction of solvent still present is 5 to 15% of the amount of solvent initially present.

In other words, the drying is stopped after a variable duration, generally from 60 to 120 minutes, when the viscosity of the self-supporting reinforced polymer film has reached a sufficiently high level to support the electrodes on either side. . This viscosity, which may be different from that, during the analogous step of the coating process, can be easily determined by a person skilled in the art, but it is generally 15 to 20 Pa.s.

The electrodes are therefore deposited on each of the surfaces of said reinforced wet film, self-supported with thermostable polymer solution during drying, before it is completely dry, the active face of each of the electrodes containing, for example platinum carbon, forming facing each of said surfaces.

This operation is carried out by a device called "co-laminating". The electrodes are conventional electrodes, of the type commonly used in fuel cells and they have already been described above. These electrodes generally comprise a face called the active face containing the catalyst, for example platinum carbon, it is the active face of each of the electrodes which is delicately deposited on each of the self-supporting reinforced wet film surfaces of thermostable polymer solution.

This slowly impregnates each of the active layers, for example the carbon layers platinized on the surface of the electrodes, and thus directly forms a complete electrode-membrane assembly.

(still wet) - electrode.

In the next step, the drying of the electrode-membrane assembly obtained is continued for a variable period of 30 to 60 minutes at a temperature of 70 to 150 ° C, for example 120 ° C, in order to eliminate all residual solvent still present and form the complete final EME assembly. In fact, it is during this stage that the "membrane" is actually formed.

This process allows in just two simple steps to obtain a complete assembly.

The EME assemblies, prepared according to the invention, can be used, in particular, in a fuel cell that can operate, for example, with the following systems:

- hydrogen, alcohols, such as methanol, at the anode;

- oxygen, air, at the cathode. The present invention also relates to a fuel cell device comprising at least one EME assembly prepared by the method according to the invention.

Such a cell has all the properties linked to thermostable membranes: for example due to the excellent mechanical properties, the membrane can undergo without deterioration the stresses (tightening, etc.) associated with mounting in such a device.

The properties of thermostable membranes of sulfonated polyimide type are, for example, described in document FR-A-2,748,485, already cited.

The fuel cell can, for example, correspond to the diagram already given in FIG. 1.

Such a fuel cell, in which the EME assembly or assemblies are prepared by the process according to the invention has, therefore, all the advantages due to these assemblies and to the excellent quality of their interface: in particular, excellent solidity assemblies, reliability, excellent mechanical and electrochemical properties (change in voltage as a function of current density at least similar to “all NAFION®” assemblies), impermeability to gases, etc., all of these properties being perfectly reproducible and not subject to no degradation over time.

These properties are clearly superior to those of cells comprising assemblies of the prior art, for example: the temperature of the cell is generally maintained between 50 and 80 ° C. and, under these conditions, it produces for example a current density of 0.5 A / cm ² with a voltage of 0.6 V and this over a very long period of time up to 3000 hours, which demonstrates the excellent thermal and mechanical and other stability properties of the assemblies and its excellent properties electric. The invention will now be described with reference to the following examples, given by way of illustration and not limitation.

Examples

Example 1

Realization of an elementary electrode-membrane assembly by the method according to the invention

This involves the production of an electrode-membrane assembly in a sulfonated polyimide, the molecular structure of which is described below.

the values of x are generally 0 <x, y <20; and for example in the present case x = 8 and y = 10.

The sulfonated polyimide used is in solution in metacresol. The concentration, viscosity and temperature of the solution are adjusted in order to allow the production of a homogeneous film by a “Hand-Coater” system and are as follows:

- concentration: 70 g / 1; - viscosity: 4 Pa.s;

- temperature: 120 ° C.

The solution of the sulfonated polyimide is then spread over a glass substrate of rectangular shape 3 mm thick and perfectly clean. The thickness of the wet film is calibrated in the vicinity of 3000 μm. The temperature of the substrate is maintained in the vicinity of 120 ° C. in order to cause the rapid evaporation of the solvent. After a few minutes, the viscosity of the wet film has reached a level high enough to support the electrode. This electrode is an electrode provided by the SORAPEC ^® Company.

The face of the electrode containing the platinum carbon is then gently placed on the surface of the wet film. This slowly impregnates the layer of platinum carbon on the surface of the electrode. The drying of the electrode-membrane assembly is continued for a further ... minutes in order to remove all of the residual solvent.

The electrode - membrane assembly is then detached from the substrate.

Example 2

Realization of a complete electrode - membrane - electrode assembly by coating

It involves the realization of an electrode - membrane - electrode assembly in sulfonated polyimide in two distinct steps: Step 1

Creation of an elementary electrode-membrane assembly

A first electrode-membrane assembly is carried out in accordance with the description of the example

1. The electrode-membrane assembly thus obtained is used as a substrate to carry out the second step.

2nd step

Creation of a second elementary electrode-membrane assembly

A new wet film is poured onto the membrane of the previous assembly. During the drying of this wet film, a second electrode is carefully deposited, as described in Example 1.

At the end of these two stages, a complete assembly of electrode - membrane - electrode is obtained. This methodology also improves the gas impermeability of the proton exchange membrane. Example 3

Realization of a complete electrode - reinforced membrane - electrode by coating assembly

The sulfonated polyimide used is the same as above. The concentration, viscosity and temperature of the solution are adjusted in order to allow the production of a homogeneous film by a “Hand-Coater” system and are the same as in Example 1. The solution of sulfonated polyimide is then spread on a glass substrate perfectly clean on which is arranged a reinforcement, which is a fabric PEEK from the SEFAR ^® Corporation. The thickness of the wet film is calibrated in the vicinity of 3000 μm. The temperature of the substrate is maintained in the vicinity of 120 ° C. in order to cause the rapid evaporation of the solvent. After a few minutes, the viscosity of the wet film has reached a level high enough to support the electrode, namely 15 Pa.s. The face of the electrode containing the platinum carbon is then gently placed on the surface of the wet film. This slowly impregnates the layer of platinum carbon on the surface of the electrode. The drying of the electrode-membrane assembly is continued for 60 minutes in order to remove all of the residual solvent therefrom.

The electrode-reinforced membrane assembly is then detached from the substrate and used as a substrate to perform a second step. A new wet film is poured onto the membrane of the previous assembly. During the drying of this wet film, a second electrode is carefully deposited, as described in Example 1.

At the end of these two stages, a complete assembly of electrode - reinforced membrane - electrode is obtained.

Example 4

This involves the creation of an electrode - membrane - electrode assembly reinforced with a PEEK fabric from the company SEFAR ^® .

The sulfonated polyimide used is the same as above and it is in solution in a solvent which is metacresol.

The concentration, the viscosity and the temperature of the solution are adjusted in order to allow the production of a homogeneous film by a “Hand-Coater” system, they are identical to those of Example 1. The solution of sulfonated polyimide is then spread on a perfectly clean glass substrate. The thickness of the wet film is calibrated in the vicinity of 3000 μm. The reinforcement is then deposited within the wet film, on the substrate. Under the action of his own weight, the reinforcement penetrates the thickness of the wet film until it is in contact with the substrate. The temperature of the substrate is maintained in the vicinity of 120 ° C. in order to cause the rapid evaporation of the solvent. After a few minutes, the viscosity of the wet film has reached a level high enough to support the electrode. The face of the electrode containing the platinum carbon is then gently placed on the surface of the wet film. This perfectly impregnates the layer of platinum carbon on the surface of the electrode. The drying of the electrode-membrane assembly is continued for another 60 minutes in order to remove all of the residual solvent.

The electrode-membrane assembly is then detached from the substrate and used to make a reinforced electrode-membrane-electrode assembly, as described in Example 2.

Example 5

Realization of a complete electrode - membrane - electrode assembly reinforced by impregnation

It involves the production of an electrode - membrane - reinforced electrode assembly. The sulfonated polyimide used is in solution in a solvent which is metacresol. The concentration, viscosity and temperature of the solution are adjusted to allow the reinforcement to be impregnated and are as follows: - concentration: 80 g / 1;

- viscosity: 2 Pa.s;

- temperature: 120 ° C.

The thickness of the self-supporting reinforced wet film is calibrated in the vicinity of 3000 μm.

The temperature of the self-supporting reinforced wet film is maintained in the vicinity of 120 ° C., in order to cause the rapid evaporation of the solvent. After a few minutes, the viscosity of the self-supporting reinforced wet film has reached a sufficiently high level, namely 15 Pa.s, to support the electrodes on both sides. The face of the electrodes containing the platinum carbon is then placed delicately on the surface of the self-supporting reinforced wet film. This perfectly impregnates the layer of platinum carbon on the surface of the electrodes. The drying of the electrode - membrane - electrode assembly is continued for several more minutes, in order to remove all of the residual solvent. The assemblies obtained in Example 1, with the sulfonated polyimide membrane, the structure of which is described in Example 1, are characterized by the fact that the electrode-membrane interface is of very good quality as shown in the figure. 2. Indeed, in this photograph, obtained by scanning electron microscopy, the electrode-membrane interface is perfectly regular and free from defects and no homogeneity is visible.

By browsing the photograph from bottom to top, there are several levels which correspond respectively to the heart of the electrode (Teflon felt charged with carbon black, part 1), with the layer of platinum carbon (clear gloss level, part 2) with a thickness close to 20 μm and finally with the proton exchange membrane (part 3) with a thickness close to 15 μm.

The cohesion between the electrode and the membrane is such that it is no longer possible to separate them, unlike the assemblies produced by existing methods. This type of analysis by scanning electron microscopy aimed at characterizing the membrane-electrode interface makes it possible to distinguish the assemblies obtained by the process of the invention, from the assemblies obtained by any other process based on the pressing of a formed membrane and an electrode.

Indeed, any other process based on the pressing of a formed ("dry") membrane and an electrode leads to the formation of defects at the membrane - electrode interface, as shown in FIG. 3 in which the reference numbers have the same meaning as in Figure 2.

In this FIG. 3, which represents the membrane - electrode interface of an assembly obtained by pressing, according to the prior art, there are clearly different vacuoles and various defects. These defects are the cause of the poor electrochemical performance of these assemblies.

In fact, in addition to the adhesion problems, various heterogeneities, such as air vacuoles, bending zones, etc., are visible at the electrode-membrane interface of the assemblies obtained by the conventional methods. In addition, a mapping of the sulfur element was carried out using a castaing probe on an assembly obtained by the method of the invention according to the description of Example 1. In this mapping the sulfur element identifies the presence of the proton conductor. It clearly appears that the process of the invention makes it possible to entrain a fraction of the proton conductor in the area rich in platinum.

Claims

1. Method for preparing an assembly comprising at least one electrode having an active face, and a membrane made of a thermostable polymer, in which the following steps are carried out: a) a solution of a thermostable polymer of so as to obtain a film of thermostable polymer solution; then b) partially drying said film of thermostable polymer solution by evaporation of the solvent from said solution; c) an electrode is deposited on the surface of said film of thermostable polymer solution, during drying, before it is completely dry, the active face of the electrode facing said surface, so as to obtain an assembly comprising a thermostable polymer membrane and said electrode; d) said assembly obtained in step c) is completely dried; then e) the assembly comprising said membrane and said electrode is detached from the substrate.

2. Method according to claim 1, wherein, prior to step a), a reinforcement is arranged on said support.

3. Preparation process according to claim 1, wherein, at the end of step a), there is a reinforcement within the film of thermostable polymer solution.

4. The method of claim 3, wherein said reinforcement is arranged by rolling within the film of thermostable polymer solution.

5. Method according to any one of claims a 4, for preparing an electrode-membrane-electrode assembly (EME) consisting of a thermostable polymer membrane and two electrodes further comprising at the end from step e), the following steps: f) a solution of a thermostable polymer is poured over the face of the assembly, so as to obtain a film of solution of thermostable polymer; then g) partially drying said film of thermostable polymer solution by evaporation of the solvent from said solution; h) a second electrode is deposited on the surface of said film of thermostable polymer solution, during drying, before it is completely dry, the active face of the second electrode facing the surface of said film, so as to obtain an electrode - thermostable polymer membrane - electrode assembly; then i) said electrode - membrane - electrode assembly obtained during step h) is completely dried.

6. Method for preparing an electrode-membrane-electrode assembly (EME), in which the following steps are carried out: a) a reinforcement is impregnated with a solution of a thermostable polymer, so as to obtain a film reinforced and self-supporting thermostable polymer solution; then b) partially drying said film of reinforced and self-supported thermostable polymer solution, by evaporation of the solvent from said solution; c) an electrode is deposited on each of the faces of said film of thermostable polymer solution, in the event of drying, before it is completely dry, the active face of each of the electrodes facing each of the surfaces of said film; d) said assembly obtained in step c) is completely dried.

7. Method according to any one of claims 1 to 6, carried out continuously.

8. Method according to any one of claims 1 to 7, wherein said thermostable polymer is an ion exchange polymer, such as a proton conducting polymer.

9. The method of claim 8, wherein said polymer is chosen from sulfonated polyimides, sulfonated polyethersulfones, sulfonated polystyrenes and their sulfonated derivatives, sulfonated polyetheretherketones and their sulfonated derivatives, sulfonated polybenzoxazoles, sulfonated polybenzimidazoles, polyparaphenylenes and their sulfonated derivatives.

10. An assembly comprising at least one electrode and a membrane capable of being obtained by the method according to any one of claims 1 to 4 and 7 to 9.

11. An electrode-membrane-electrode assembly capable of being obtained by the method according to any one of claims 5 to 9.

12. Fuel cell comprising at least one electrode - membrane - electrode assembly according to claim 11.