EP2210306A1

EP2210306A1 - Method for making proton conducting membranes for fuel cells by radiografting

Info

Publication number: EP2210306A1
Application number: EP08804642A
Authority: EP
Inventors: Thomas Berthelot; Marie-Claude Clochard
Original assignee: Commissariat a lEnergie Atomique CEA; Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2007-09-26
Filing date: 2008-09-24
Publication date: 2010-07-28
Also published as: CN101809799A; US20100311860A1; FR2921518B1; JP2011501857A; WO2009040365A1; FR2921518A1

Abstract

The invention relates to a method for making a proton-conducting membrane for a fuel cell, that successively comprises: the step of irradiating a polymer matrix; the step of grafting the polymer matrix thus irradiated by a radical reaction with a first compound, that comprises contacting the irradiated polymer matrix with said first compound, wherein said compound includes at least one group capable of forming a covalent bond by radical reaction with said matrix, and includes at least one reactive group capable of reacting with a group of a second compound containing at least one proton-conducting acid group for forming a covalent bond; and the step of contacting the matrix thus grafted with the second compound, whereby a reaction occurs between the reactive groups from the first compound and the appropriate groups of the second compound.

Description

PROCESS FOR PRODUCING FUEL CELL PROTONS CONDUCTIVE MEMBRANES BY RADIOGRAPHY

DESCRIPTION TECHNICAL FIELD

The present invention relates to processes for producing fuel cell proton conductive membranes by a radiografting technique, which technique consists in creating on a polymer matrix free radicals which will be able to react with appropriate compounds by radical reaction.

The field of application of the invention is therefore that of fuel cells, and more particularly of fuel cells, comprising as electrolyte, a proton-conducting membrane, such as PEMFC fuel cells.

("Proton Exchange Membrane Fuel CeIl" for Proton Exchange Membrane Fuel Cell).

STATE OF THE PRIOR ART

A fuel cell generally comprises a stack of elementary cells in which an electrochemical reaction takes place between two reactants which are introduced continuously. The fuel, such as hydrogen, for the cells operating with hydrogen / oxygen mixtures, is brought into contact with the anode, while the oxidant, usually oxygen, is brought into contact with the cathode. The anode and the cathode are separated by an electrolyte of the ionic conductive membrane type. The electrochemical reaction, whose energy is converted in electrical energy, splits into two half-reactions:

an oxidation of the fuel, taking place at the anode / electrolyte interface producing, in the case of hydrogen cells, H + protons, which will cross the electrolyte in the direction of the cathode, and electrons, which join the external circuit, to contribute to the production of electrical energy; a reduction of the oxidant, taking place at the electrolyte / cathode interface, with production of water, in the case of hydrogen cells.

The electrochemical reaction takes place, strictly speaking, at an electrode-membrane-electrode assembly.

The electrode-membrane-electrode assembly is a very thin assembly with a thickness of the order of a millimeter and each electrode is fed with the appropriate gases, for example using a fluted plate.

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

The thickness of this membrane is generally between 50 and 150 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 allows, in general, a battery operation over periods greater than 1000 hours.

The polymer constituting the membrane must therefore fulfill a number of conditions relating to its mechanical, physicochemical and electrical properties which are, inter alia, those defined below.

The polymer must first be able to give thin films, generally 50 to 150 micrometers, dense, without defects. The mechanical properties, elastic modulus, tensile strength, ductility, must make it compatible with assembly operations including, for example, clamping between metal frames. The properties must be preserved by changing from dry to wet.

The polymer must have good thermal stability to hydrolysis and have good resistance to reduction and oxidation. This thermomechanical stability is appreciated in terms of variation of ionic resistance, and in terms of variation of the mechanical properties.

The polymer must finally have a high ionic conductivity, this conductivity being provided by acidic groups, such as carboxylic acid, phosphoric acid or sulfonic acid groups connected to the polymer chain.

For several decades, it has been proposed various types of proton conductive polymers used to form fuel cell membranes. Sulfonated phenolic resins prepared by sulfonation of polycondensed products, such as phenol-formaldehyde polymers, were first used. The membranes prepared with these products are inexpensive but do not have sufficient hydrogen stability at 50-60 ^° C. for long-term applications.

The sulphonated polystyrene derivatives, which have a higher stability than the sulphonated phenolic resins, can then be used, but can not be used at more than 50-60 ^° C.

Currently, acceptable performance is obtained from polymers consisting of a perfluorinated linear main chain and a side chain carrying a sulfonic acid group.

Among these known polymers, which are commercially available, mention may be made of the polymers deposited under the NAFION® brands of the Dupont de Nemours Company.

This polymer has a minimum proton conductivity of 0.10 S / cm and a total acid capacity ranging from 0.95 to 1.01 meq / g. However, this polymer has a high cost in the constitution of a battery (20 to 30% of the total cost of the battery), a limitation in working temperature (of the order of 80 ⁰ C) and a high rate of hydration.

There is therefore a real need to develop proton conducting membranes that are less expensive, allowing to work with less expensive base materials and whose conductivity can be controlled.

STATEMENT OF THE INVENTION

Thus, the invention relates to a method for producing a fuel cell proton conducting membrane comprising successively:

a step of irradiating a polymeric matrix;

a grafting step of said polymeric matrix thus irradiated by radical reaction with a first compound, consisting in bringing said first compound into contact with said irradiated polymeric matrix, said first compound comprising at least one group capable of forming a covalent bond by reaction radical with said matrix and comprising at least one reactive group capable of reacting with a group of a second compound comprising at least one proton-conducting acid group, optionally in the form of salts, to form a covalent bond; - A step of contacting the second compound of the matrix thus grafted, whereby there is reaction with the reactive groups from the first compound and the appropriate groups of the second compound. The aforementioned method is based on the principle of radiografting, that is to say on the principle of grafting by radical reaction with a previously irradiated polymeric matrix.

The introduction of proton conducting acid groups takes place in two stages: firstly, grafting the irradiated matrix by radical reaction of a first compound with said matrix, said first compound comprising a group capable of reacting with a group of a second compound to form a covalent bond;

second, reacting said second compound comprising at least one proton-conducting acid group with the reactive groups derived from the first compound, to form a covalent bond. By means of the method of the invention, by varying the irradiation rate of the matrix, it is possible to modify the quantity of compounds comprising a proton-conducting acid group and thus it is possible to modulate the proton conductivity of the material. resulting. It is also possible to introduce different types of proton conductive groups depending on the nature of the compounds reacted with the irradiated matrix.

The method of the invention comprises a step of irradiating a polymer matrix, this irradiation step having the function of creating free radicals in the material constituting the matrix, this creation of free radicals being a consequence of the transfer of energy of irradiation to said material.

The step of irradiating a polymeric matrix may consist in subjecting said matrix to an electron beam (also called electron irradiation). More particularly, this step may consist in scanning the polymer matrix with an accelerated electron beam, this beam being able to be emitted by an electron accelerator (for example, a Van de Graaf accelerator, 2.5 MeV). In the case of electron beam irradiation, the energy deposition is homogeneous, which means that the free radicals created by this irradiation will be evenly distributed in the volume of the matrix.

The step of irradiating a polymeric matrix may also include subjecting said matrix to heavy ion bombardment. It is specified that heavy ions are ions, whose mass is greater than that of carbon. Generally, these are ions selected from krypton, lead, xenon.

More particularly, this step may consist in bombarding the polymer matrix with a heavy ion beam, such as a 4.5 MeV / mau Pb ion beam or a 10 MeV Kr ion beam. / fc.

From a mechanistic point of view, when the heavy energy vector ion passes through the matrix, its speed decreases. The ion gives up its energy, creating damaged areas, whose shape is approximately cylindrical. These areas are called latent traces and include two regions: the heart and the halo of the trace. The core of the trace is a totally degraded zone, namely an area where there is rupture of the constituent bonds of the material generating free radicals. This core is also the region where the heavy ion transmits a considerable amount of energy to the electrons of the material. Then, from this heart, there is emission of secondary electrons, which will cause defects away from the heart, thus generating a halo.

In the case of heavy ion irradiation, the energy deposition is distributed according to the irradiation angle and is inhomogeneous. It is possible to create traces arranged in a predetermined pattern, and thereby to induce grafting of compounds only in the above-mentioned traces. Thus, it is possible to induce different grafting patterns, by modulating the irradiation angle relative to the normal of the faces of the matrix. This angle is advantageously between 15 ° and 60 °, for example, of the order of 30 °. It is possible to create, for example, a matrix comprising latent traces traversing the oriented matrix in two symmetrical directions. It is possible to use two separate ion sources or to successively proceed to irradiation in two directions to create grafting patterns, where the latent traces are crossed.

According to a particular embodiment, the irradiation step may proceed as follows: irradiation of the polymer matrix with heavy ions; chemical revelation, generally by hydrolysis, of latent traces created by the passage of heavy ions, at the end of which open channels are obtained; electronic irradiation of said open channels, from which the radiografting can proceed.

The chemical revelation consists of bringing the matrix into contact with a reagent able to hydrolyze the latent traces so as to form hollow channels instead of these.

According to this particular embodiment, following the irradiation of the polymer matrix with heavy ions, the latent traces generated have short chains of polymers formed by splitting existing chains during the passage of the ion in the material during the irradiation. In these latent traces, the rate of hydrolysis during the revelation is greater than that of the non-irradiated parts. Thus, it is possible to carry out a selective revelation. The reagents capable of revealing the latent traces are a function of the material constituting the matrix. Thus, the latent traces can in particular be treated with a strongly basic and oxidizing solution, such as a KOH ION solution in the presence of KMnO ₄ at 0.25% by weight at a temperature of 65 ° C., when the polymeric matrix is, for example, constituted polyvinylidene fluoride (PVDF), poly (VDF-co-HFP) (vinylidene fluoride-co-hexafluoropropene), poly (VDF-co-TrFE) (vinylidene fluoride-co-trifluoroethylene), poly (VDF-co- co-TrFE-co-ChloroTrFE) (vinylidene fluoride-co-trifluoroethylene-co-monochlorotrifluoroethylene) and other perfluorinated polymers. A treatment with a Basic solution, optionally coupled with trace sensitization by UV, may be sufficient for example for polymers such as polyethylene terephthalate (PET) and polycarbonate (PC). The treatment leads to the formation of hollow cylindrical pores whose diameter is adjustable as a function of the attack time with the basic and oxidizing solution. Generally heavy ion irradiation will be carried out so that the membrane has a number of traces per cm ² between 10 ⁶ and 10 ¹¹ . Typically it will be from 5.10 ^{7 to} 5.10 ¹⁰ , more especially to 10 ¹⁰ . In any case it should be verified that the mechanical properties of the membrane are not significantly reduced by the amount of traces.

Further information regarding the reagents and operating conditions that can be used for chemical revelation as a function of the material constituting the matrix can be found in Rev.Mod.Phys. , Flight. 55, No. 4, Oct. 1986, p.925.

After this step of revelation, the electron irradiation is carried out to induce the formation of free radicals on the wall of the channels, the implementation being in this case similar to that which has been exposed for the electronic irradiation in general and allows the formation of a polymeric coating to fill the pores. Generally the beam is oriented in a direction normal to the surface of the membrane and the surface thereof is scanned homogeneously. The irradiation dose generally ranges from 10 to 200 kGy for radiografting later, it will typically be close to 100 kGy for PVDF. The dose is generally such that it is greater than the gel dose, which corresponds to the dose from which the recombinations between radicals are favored resulting in the creation of interchain bonds leading to the formation of a three-dimensional network (or crosslinking) that is to say the formation of a gel, in order to induce at the same time crosslinking thus making it possible to improve the mechanical properties of the final polymer. Thus, for PVDF, it is recommended that the dose be at least 30 kGy.

The base polymer matrix may be a matrix made of a polymer chosen from polyurethanes, polyolefins, polycarbonates and polyethylene terephthalates, these polymers being advantageously fluorinated or even perfluorinated.

Preferably, the polymeric matrix may be chosen from matrices made of fluorinated polymers such as polyvinylidene fluoride, copolymers of tetrafluoroethylene and tetrafluoropropylene

(known under the abbreviation FEP), copolymers of ethylene and tetrafluoroethylene (known by the abbreviation ETFE), copolymers of hexafluoropropene and of vinylidene fluoride

(known under the abbreviation HFP-co-VDF), vinylidene fluoride and trifluoroethylene (known under the abbreviation VDF-co-TrFE), vinylidene fluoride, trifluoroethylene and monochlorotrifluoroethylene (known under the abbreviation VDF) -co-TrFE-co-chloroTrFE). Polymeric matrices based on fluoropolymers are advantageous in that they are resistant to corrosion, have good mechanical properties and low gas permeation. They are therefore particularly suitable for constituting fuel cell membranes.

A particularly advantageous matrix of this type is a polyvinylidene fluoride matrix. Polyvinylidene fluoride is chemically inert (especially resistant to corrosion), has good mechanical properties, has a glass transition temperature, which varies from -42 ^° C. to -38 ° C., a melting temperature of 170 ^° C. and a density of 1.75 g / cm ³ . It also has low gas permeation, which makes it particularly useful as a basis for building fuel cell membranes operating with hydrogen as fuel. This polymer is easily extruded and can be in particular in two crystalline forms, depending on the orientation of the crystallites: the α phase and the β phase, the β phase being characterized in particular by piezoelectric properties.

As mentioned above, the irradiation step of the polymeric matrix will make it possible to create free radicals in the matrix material. From a mechanistic point of view, the creation of these free radicals is allowed by the energy generated by the irradiation, which energy is transferred to the material, being concretized by chain breaks and consequently by the creation of these radicals. For example, in the case of polyvinylidene fluoride, the free radicals created are alkyl groups carrying a free electron.

The radicals present in such an irradiated matrix can be trapped in crystallites in order to prolong the life of the matrix in irradiated form. It is therefore recommended to use matrices containing crystallites and preferably between 30% and 50%, generally 40%. For example, PVDF is of a semi-crystalline nature

(It generally has 40% crystallinity and 60% amorphous form) and can be in several crystalline phases, CC, β, γ and δ formed by the association, planar or helical, chains. The α and β phases are the most common. PVDF, which is a thermoplastic polymer which can be melted and then molded, mainly of α phase is generally obtained by cooling from the molten state, for example after simple extrusion. PVDF mainly based on β phase is generally obtained by cold bi-stretching, at less than 50 ⁰ C, PVDF predominantly in α phase. It is recommended to use PVDF mainly comprising the β phase, since the crystallinity is greater in this case.

The first compound intended to be brought into contact with the irradiated matrix is advantageously a compound comprising, as a group capable of reacting by radical reaction to form a covalent bond, an ethylenic group, and as a group a group selected from -CO2H, -NH2, while the second compound will advantageously comprise, as a group reactive with the reactive group of the first compound to form a covalent bond, an -NH2 group when the reactive group of the first compound is a CO ₂ H group or a -CO ₂ H group, when the reactive group of the first compound is a - NH _{2 group} . In both cases, the reaction between the reactive group of the first compound and the group of the second compound is an amidation reaction. It may be necessary to activate the carboxyl function so as to facilitate the reaction with an -NH ₂ function of the second compound. The activation may proceed through the reaction of the -CO ₂ H-function with a succinimide compound, so as to create a -CO-N-succinimide group which is more reactive with the -NH ₂ functions.

As the first compound comprising, as a reactive group, a group -CO ₂ H, acrylic acid may be mentioned. The first compound comprising, as a reactive group -NH ₂ group, vinyl amines.

The grafting step of the first compound, when the group capable of grafting is an ethylenic group, is divided into two phases:

a reaction phase of the first compound with the irradiated matrix, this phase being materialized by an opening of the double bond by reaction with a radical center of the matrix, the radical center thus "moving" from the matrix to a carbon atom from said first compound; a polymerization phase of this first compound from the radical center created on the first grafted compound.

In other words, the free radicals of the material constituting the matrix cause the propagation of the polymerization reaction of the first compound placed in contact with the matrix. The radical reaction is thus, in this case, a radical polymerization reaction of the first compound contacted, from the irradiated matrix.

At the end of the polymerization phase, the membranes obtained will thus comprise a polymer matrix grafted with polymers comprising repeating units resulting from the polymerization of the first compound placed in contact with the irradiated matrix.

If the first compound is represented by the formula = -R (R representing a reactive group capable of reacting with a group of the second compound), the reaction scheme can be as follows:

Matrix Matrix

Matrix

When the first compound is acrylic acid, the membranes at the end of the grafting step comprise a polymeric matrix grafted with grafts of the poly (acrylic acid) type. Such grafts carry -CO2H groups capable of reacting with groups of a second compound (for example, -NH ₂ groups) to form a covalent bond.

The membranes prepared with such a first compound will thus have grafts of the poly (acrylic acid) type, thus comprising a sequence of the following type:

X can represent -CO ₂ H.

The theoretical distances between two acidic protons can be evaluated between 2.3 and 7 Å, which suggests that proton conduction may take place even at very low levels of hydration.

As a second compound comprising an -NH ₂ group, it is advantageous to mention amino acids, that is to say compounds comprising both an acidic group, such as a -CO ₂ H, -SO _{3 group.} H, -PO ₃ H ₂ , and an amino group -NH ₂ . As examples of amino amines that may be suitable, mention may be made of those corresponding to one of the following formulas:

As a second compound comprising a -COOH group, there may be mentioned compounds corresponding to one of the following formulas:

A particular example of a process according to the invention is a process comprising:

a step of irradiating a polyvinylidene fluoride matrix;

a step of grafting said polymeric matrix thus irradiated consisting in bringing acrylic acid into contact with said irradiated polymeric matrix; a step of contacting the matrix thus grafted with taurine.

When the grafts resulting from the reaction of the first compound and optionally of the second compound comprise -CO ₂ H groups, it may be possible to subject the resulting membranes to a sulfonation step allowing the conversion of -CO ₂ H groups into -SO groups. ₃ H by action, for example, chlorosulfonic acid.

The methods of the invention are simple and inexpensive implementation methods. They allow a control of the introduced amount of proton conductive groups in the membrane. By playing on the nature of the grafted compounds, it is possible to access membranes having a wide variety of stoichiometries of proton donor species.

It is conceivable to obtain total acid capacities that may be greater than 0.95 to 1.1 meq / g (the meq / g corresponding to the number of moles of proton exchange molecules or equivalents (here acidic) per gram of membrane) . The capacities Total acids are directly dependent on the degree of grafting used, the number of proton exchange functions introduced during the functionalization and thus the nature of the graft.

Thus, the invention also relates to fuel cell proton conducting membranes obtainable by the method of the invention. In particular, the membranes of the invention may correspond to membranes comprising a polymer matrix grafted with grafts obtained by:

radical polymerization of a first compound comprising an ethylenic group and as a reactive group a group capable of reacting with a -CO2H group, or a -NH2 group, this first compound possibly being acrylic acid _;

reaction of the grafts resulting from the radical polymerization with a second compound comprising, as a group reacting with the group of the first compound to form a covalent bond, an -NH 2 group when the reactive group of the first compound is a CO ₂ H group or a -CO ₂ H group, when the reactive group of the first compound is a group -NH ₂ , said second compound may be taurine, when the first compound is acrylic acid.

More specifically, a particular membrane of the invention is a membrane comprising a polymeric matrix of polyvinylidene fluoride grafted with grafts obtained by: radical polymerization of acrylic acid, generating poly (acrylic acid) chains;

reaction of poly (acrylic acid) chains with taurine. The membranes of the invention can be nanostructured. In particular, they may consist of: a fluorinated polymeric matrix having a nanostructuration induced by irradiation with heavy ions;

nano-domains covalently linked to said matrix, consisting of grafts bearing proton-conducting functions, and / or nano-domains containing chains of said matrix that are covalently bound and interpenetrated with the various polymers (modified or otherwise). ) mentioned above.

The orientation of these nano-domains between them is a function of the conditions of irradiation with heavy ions of said matrix. The route of the heavy ion being rectilinear, the nanodomains are continuous and form conduction channels. There may be mentioned by way of non-limiting illustration: an orientation of the nano-domains perpendicular to the surfaces of said matrix and parallel to each other, an orientation of the nano-domains in cross or mesh.

These nano-domains are covalently bound to said matrix and are impermeable to gases. They constitute privileged conduction pathways for protons. These membranes are intended to be incorporated in fuel cell devices.

Thus, the invention also relates to a fuel cell device comprising at least one membrane as defined above.

This device comprises one or more electrode-membrane-electrode assemblies.

To prepare such an assembly, the membrane may be placed between two electrodes, for example carbon fabric impregnated with a catalyst.

The assembly is then pressed to a suitable temperature in order to obtain good electrode-membrane adhesion.

The electrode-membrane-electrode assembly obtained is then placed between two plates, providing electrical conduction and supply of reagents to the electrodes. These plaques are commonly referred to as bipolar plates.

The invention will now be described with reference to the following examples given for illustrative and not limiting.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a photograph obtained by field effect scanning electron microscopy (SEM) comprising two parts: a part (a) representing a PVDF matrix comprising revealed latent traces and a part (b) representing said radiografted membrane in FIGS. latent traces, obtained according to Example 1.3, before coupling with taurine. Figure 2 is a diagram showing a device for measuring the relative proton conductivity of a membrane.

FIG. 3 is a graph showing the resistivity R (in Ω) (solid curve) and the proton conductivity C (in mS / cm) (dashed curve) as a function of the fluence F (ions / cm ² ) for a membrane obtained according to Example 1.1, before coupling with taurine.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

Example 1

This example illustrates the preparation of a membrane according to the invention according to three variants:

a first variant involving only irradiation with heavy ions Pb ²⁺ ; a second variant involving an electron irradiation;

a third variant successively involving irradiation with heavy Pb ²⁺ ions, chemical revelation followed by electron irradiation. 1.1 - First variant

A matrix with acrylic acid was used. The number of moles of acid introduced was estimated using spectroscopic analyzes.

This matrix was obtained as follows:

Initially, a matrix (6 × 30 cm, 9 μm thick) of polyvinylidene fluoride was subjected to heavy Pb ²⁺ ion bombardment. The fluence varied from 5.10 ⁷ to 5.10 ¹⁰ ions per cm ² . This corresponds to a dose ranging from Gy to 1000 kGy. The electron energy loss (dE / dx) ranged from 2.2 to 72.6 MeV cm ² mg- ¹ (0.39 to 12.8 keV nm ^-1 ) .The irradiation angle was set at 90 ° This stage allowed the creation of latent traces including radical species.

The matrices prepared according to this modality were used immediately or stored under an inert atmosphere, such as nitrogen, and generally cold (-18 ^° C.), for several months before their use.

In a second step, the irradiated matrix was contacted with acrylic acid by immersion in an aqueous solution, sparged with nitrogen for 15 minutes, containing 25% by weight of acid and 0.1 % by mass Mohr salt at 60 ⁰ C for 1h with stirring. Mohr salt has been used to limit the homopolymerization of acrylic acid. The same protocol was carried out with ethyl acetate as a solvent. The membrane obtained was then extracted from the solution and then washed with water and extracted with boiling water using a Sohxlet apparatus for

24. It was then dried for 12 hours under high vacuum.

The degree of grafting, defined with respect to the caking of the membrane before and after radiografting, is between 10 and 20% by weight.

The matrix obtained was immersed in a solution of acetonitrile or a mixture of water / acetonitrile (1/3), N-hydroxysuccinimide (1.2 equivalents relative to the number of moles of acrylic acid introduced into the matrix, this value varies from 3 to 10 mmol / l and is generally located around 8 μmol / l) and carbodiimide (1 equivalent relative to the number of moles of acrylic acid introduced into the matrix) and placed under stirring for 12 hours at room temperature (25 ° C). The matrix was then immersed during

12 h with stirring and at room temperature in a solution of taurine (3 equivalents relative to the number of moles of acrylic acid introduced into the matrix) in a water / acetonitrile mixture (30/70) to which 6 equivalents have been previously added ( compared to taurine) diisopropylethylamine (DIEA).

The resulting membrane was then washed with water and acetonitrile and dried under vacuum. With an acrylic acid grafting rate ranging from 10 to 20% by weight (yield defined by compared to the mass setting of the membrane before and after radiografting), and a functionalization efficiency of 40 to 50 mol% (yield established as a function of the number of modifiable functions introduced by radiografting) the membranes obtained have a total acid capacity of at least 0.58 meq / g. This capacity corresponds to the number of moles of proton exchange molecules or equivalents (here acidic) per gram of membrane.

1.2 - Second variant

A matrix grafted with acrylic acid was used. This matrix was obtained as follows:

Initially, a matrix (6 × 30 cm), 9 μm thick polyvinylidene fluoride was subjected to electron irradiation. The dose varied from 50 to 150 kGy. The irradiation angle was set at 90 °. This step allowed the creation of radicals trapped within the crystallites of PVDF.

In a second step, the irradiated matrix was brought into contact with acrylic acid. For this purpose, the matrix was immersed in a solution, previously degassed, at 25% by weight of acid in water or ethyl acetate and 0.1% by weight of Mohr salt at 60 ^° C. during Ih with stirring. Mohr salt has been used to limit the homopolymerization of acrylic acid. The membrane obtained was then extracted from the solution and then cleaned with water and extracted with boiling water using a Sohxlet machine for 24 hours. It was then dried for 12 hours under high vacuum.

The degree of grafting, defined with respect to the caking of the membrane before and after radiografting, is between 10 and 40% by weight.

The matrix obtained was immersed in a solution of acetonitrile or a mixture of water / acetonitrile (1/3) and N-hydroxysuccinimide (1.2 equivalents relative to the number of moles of acrylic acid introduced into the matrix). and carbodiimide (1 equivalent based on the number of moles of acrylic acid introduced into the matrix) and stirred for 12h at room temperature (25 ° C).

The matrix was then immersed for 12 hours under stirring and at room temperature in a solution of taurine (3 equivalents relative to the number of moles of acrylic acid introduced into the matrix) in a water / acetonitrile mixture (30/70) to which 6 equivalents (relative to taurine) of DIEA 6 equivalents were previously added.

The resulting membrane was then washed with water and acetonitrile and dried under vacuum.

With an acrylic acid grafting rate ranging from 10 to 40% by weight (defined yield relative to the setting of the membrane mass before and after radiografting), and a functionalization efficiency of 70 to 80 mol% (yield established according to the number of modifiable functions introduced by radiografting) the membranes obtained have a total acid capacity of at least 1.3 meq / g.

1.3-Third variant

A matrix grafted with acrylic acid was used.

This matrix was obtained as follows:

In a first step, a matrix was irradiated as explained in section 1.1.

In a second step, the irradiated matrix was put in contact with a solution of KOH ION in the presence of KMnO ₄ at 0.25% by weight at a temperature of 65 ° C. for a variable time of 15 min at 1 h. The treatment led to the formation of hollow cylindrical pores whose diameter varies linearly with the etching time of 25 nm to 100 nm.

In a third step, the previously obtained membrane is subjected to the electron irradiation treatment and brought into contact with the acrylic acid as described in section 1.2. The grafting rate defined with respect to the caking of the membrane before and after radiografting is between 5 and 30% by weight.

FIG. 1 shows an image obtained by Field Scanning Electron Microscopy (SEM) of an acid-grafted membrane. acrylic. Part (a) corresponds to a zone for which traces have been revealed, part (b) corresponds to a part for which radiografting has been carried out in the revealed traces irradiated with electrons after irradiation.

Then, the matrix obtained was immersed in a solution of acetonitrile or a mixture of water / acetonitrile (1/3), N-hydroxysuccinimide (1.2 equivalents relative to the number of moles of acrylic acid introduced into the matrix) and carbodiimide (1 equivalent relative to the number of moles of acrylic acid introduced into the matrix) and placed under stirring for 12 hours at room temperature

(25 ° C). The matrix was then immersed during

12 h with stirring and at room temperature in a solution of taurine (3 equivalents relative to the number of moles of acrylic acid introduced into the matrix) in a water / acetonitrile mixture (30/70) to which 6 equivalents have been previously added ( compared to taurine) from DIEA.

With a degree of acrylic acid grafting ranging from 5 to 30% by weight (defined performance with respect to the setting of the mass of the membrane before and after radiografting), and a functionalization yield of 80 to 90 mol% (yield established depending on the number of modifiable functions introduced by radiografting) the membranes obtained have a total acid capacity of at least 1.5 meq / g.

Example 2

In order to study the influence of the fluence on the proton conductivity, radiografted membranes following the protocol of Example 1-1, with acrylic acid, before coupling with taurine, were tested dry on a device, represented in FIG. 2, measuring a relative proton conductivity, this device comprising: a Plexiglas tank 1 filled with deionized water 3; a pair of platinum electrodes 5, 7; the membrane 9 disposed between the pair of platinum electrodes 5, 7.

As shown by the curves of FIG. 3, the maximum conductivity was obtained for a radiografted PVDF membrane at a fluence of ¹⁰ traces per square centimeter, ie ¹⁰ channels per square centimeter.

Claims

A method of producing a fuel cell proton conducting membrane comprising successively:

a step of irradiating a polymeric matrix;

a step of grafting said polymeric matrix thus irradiated by radical reaction with a first compound, consisting in bringing said irradiated polymeric matrix into contact with said first compound, which comprises at least one group capable of forming a covalent bond by radical reaction; with said matrix and comprises at least one reactive group capable of reacting with a group of a second compound comprising at least one proton-conducting acid group, to form a covalent bond;

- A step of contacting the second compound of the matrix thus grafted, whereby there is reaction between the reactive groups from the first compound and the appropriate groups of the second compound.

The method of claim 1, wherein the irradiating step comprises subjecting said array to an electron beam.

The method of claim 1, wherein the irradiating step comprises subjecting said matrix to heavy ion bombardment.

4. The method of claim 3, wherein the heavy ions are selected from lead, krypton, xenon.

The method of claim 1, wherein the irradiating step consists of the following sequence of steps:

irradiation of the polymer matrix with heavy ions; - chemical revelation of latent traces created by the passage of heavy ions, after which we obtain open channels; electron irradiation of said open channels.

6. Process according to any one of the preceding claims, in which the polymeric matrix is chosen from polyurethane, polyolefin, polycarbonate or polyethylene terephthalate matrices.

7. The method of any of the preceding claims, wherein the polymeric matrix is a fluoropolymer matrix.

8. The method of claim 7, wherein the matrix is polyvinylidene fluoride, copolymer of tetrafluoroethylene and tetrafluoropropylene, copolymer of ethylene and tetrafluoroethylene, copolymer of hexafluoropropene and vinylidene fluoride, fluoride copolymer of vinylidene and trifluoroethylene, a copolymer of vinylidene fluoride, trifluoroethylene and monochlorotrifluoroethylene.

9. A process according to any one of the preceding claims, wherein the matrix is polyvinylidene fluoride.

The method according to any one of the preceding claims, wherein the first compound to be contacted with the irradiated matrix is a compound comprising, as a group capable of reacting by radical reaction to form a covalent bond, a group ethylene, and as a reactive group a -CO2H or -NH _{2 group} .

The process according to claim 10, wherein the first compound comprising as a reactive group a -CO ₂ H group is acrylic acid.

12. The method of claim 10, wherein the first compound comprising as a reactive group -NH ₂ group is selected from vinyl amines.

A process according to any one of the preceding claims, wherein the second compound comprises, as a group reacting with the group of the first compound to form a covalent bond, an -NH ₂ group when the reactive group of the first compound is a -CO ₂ H group or a -CO ₂ H group, when the reactive group of the first compound is -NH ₂ .

The method of claim 13, wherein the second compound is an amino acid.

15. The method of claim 14, wherein the second compound is selected from amino acids of the following formulas:

16. Process for producing according to claim 13, in which the second compound is chosen from compounds of the following formulas:

The method of claim 1, wherein the polymeric matrix is a polyvinylidene fluoride matrix, the first compound is acrylic acid and the second compound is taurine.

18. A fuel cell proton conducting membrane obtainable by a process as defined in any one of claims 1 to 17.

19. Membrane according to claim 18, comprising a polymeric polyvinylidene fluoride matrix grafted with grafts obtained by: - radical polymerization of acrylic acid, generating poly (acrylic acid) chains;

reaction of poly (acrylic acid) chains with taurine.

20. Membrane according to any one of claims 18 or 19, which is nanostructured.

Fuel cell device comprising at least one membrane as defined in any one of claims 18 to 20.