Anion exchange membrane and its method of production.

Abstract

The invention relates to an anion exchange membrane containing a cross-linked ionomer composed of polymer chains bearing ionic groups attached to the polymer chains via a cycloaddition group between a 1,3-dipolar compound and an unsaturated polymer. The 1,3-dipolar compound is an aromatic nitrile oxide substituted with an imidazole functional group, and the unsaturated polymer is a polymer containing diene monomer units. Some of the imidazolium ionic groups are linked to a separate imidazolium ionic group on another polymer chain constituting the ionomer by an alkanediyl chain. The invention also relates to a method for obtaining the membrane. The membrane is intended for use in a fuel cell or an electrolyzer.

Description

Title of the invention: Anion exchange membrane and its method of obtaining.

[0001] The field of the present invention is that of anion exchange membranes containing a cross-linked ionomer and intended for use in an electrolyzer or a fuel cell, as well as a method for obtaining such a membrane.

[0002] The core of a fuel cell and an electrolyzer consists of two electrodes, an anode and a cathode, an electrolytic layer that separates the two electrodes, and a catalyst located at the interfaces of the electrolytic layer with each of the electrodes. Fuel cells and electrolyzers include a membrane that constitutes the electrolytic layer. One of the constituent elements of the membrane is the ionomer, a polymer that carries ionic or ionizable groups.

[0003] A key step in the preparation of the membrane is the shaping of the ionomer into a film. The process generally described for forming a film of the ionomer is a coating process that requires dissolving the ionomer.

[0004] Polymers bearing imidazolium ionic groups are described as good candidates for forming anion-exchange membranes. These ionomers, which are generally prepared from a halogenated polymer and a compound bearing an imidazole functional group, are very poorly soluble, or even insoluble, in many solvents. As described in patent application WO 2019010290, the very low solubility or insolubility of these ionomers, which is attributed to ionic interactions present in the ionomer, makes the coating process problematic. To overcome this problem, the film is prepared by coating not from the ionomer, but from its precursor, the halogenated polymer. The halogenated polymer film is then immersed in a bath, a solution of the compound bearing an imidazole functional group, to lead to the formation of the ionomer film.This series of steps must be followed by further steps of rinsing the film and drying before the membrane can be used.

[0005] Furthermore, there is an objective to provide membranes that exhibit good dimensional stability and good resistance to swelling. According to the article Polymers 2023, 15, 1534, this objective can be achieved by cross-linking the constituent ionomer of the membranes. Cross-linking results from the formation of bonds between the ionic groups of different chains of the ionomer. But here again, the conventional coating processes used in the manufacture of membranes that employ the solvents are not suitable for putting a crosslinked ionomer into a film due to its insolubility in most solvents.

[0006] There is therefore a need to provide a membrane containing a cross-linked ionomer which makes it possible to solve the problems mentioned.

[0007] The inventors have discovered a new membrane which contains a cross-linked ionomer whose ionic groups are imidazoliums and whose chemical structure makes it possible to overcome the difficulties mentioned in the preparation of the membranes of the prior art.

[0008] An object of the invention relates to an anion exchange membrane containing a crosslinked ionomer consisting of polymer chains bearing imidazolium ionic groups which are attached to the polymer chains via a group resulting from the cycloaddition reaction between a 1,3-dipolar compound and an unsaturated polymer, the 1,3-dipolar compound being an aromatic nitrile oxide substituted by an imidazole function, the unsaturated polymer being a polymer containing monomeric units of a diene, a part of the imidazolium ionic groups being linked each to a distinct imidazolium ionic group of another polymer chain constituting the ionomer by an alkanediyl chain.

[0009] Another object of the invention is a method for preparing an anion exchange membrane according to the invention, which method comprises the following steps a), b), c) and d): - a) the preparation of a polymer bearing pendant imidazole functions by a cycloaddition reaction of a 1,3-dipolar compound and an unsaturated polymer, the 1,3-dipolar compound being an aromatic nitrile oxide substituted with an imidazole function, the unsaturated polymer being a polymer containing monomeric units of a diene, - b) the incorporation of a reagent into the polymer bearing pendant imidazole functions by thermomechanical mixing, the reagent being a mixture of a monohalogenated alkane and a dihalogenated alkane, - c) a heat treatment of the mixture obtained in step b) to convert the imidazole functions into imidazolium ionic groups, - d) shaping the crosslinked ionomer, obtained at the end of step c), into a film, steps b), c) and d) being carried out in bulk.

[0010] The invention also relates to a membrane-electrode assembly, AME, for a fuel cell or electrolyzer, which assembly comprises an anion exchange membrane according to the invention, which membrane is capable of being obtained by the process according to the invention.

[0011] The invention also relates to a fuel cell or an electrolyzer which contains a membrane according to the invention, which membrane is capable of being obtained by the process according to the invention. Detailed description

[0012] In this description, any interval of values ​​designated by the expression "between a and b" represents the domain of values ​​greater than "a" and less than "b" (i.e., bounds a and b excluded) while any interval of values ​​designated by the expression "from a to b" means the domain of values ​​from "a" to "b" (i.e., including the strict bounds a and b).

[0013] The polymers mentioned in the description may be of fossil origin or bio-based. In the latter case, they may be partially or totally derived from biomass or obtained from renewable raw materials derived from biomass. Similarly, they may also come from the recycling of materials already used, that is to say, they may be partially or totally derived from a recycling process, or even obtained from raw materials themselves derived from a recycling process.

[0014] The terms "membranes" and "films" are well known to those skilled in the art in this technical field. It should be noted that, as is well known, a membrane is a structure as defined by IUP AC in "IUP AC Recommendations 1996". Also as is well known, and in accordance with the definition given by IUP AC, the term "film" is understood according to the definition given by IUP AC in "IUPAC Recommendations 1996".

[0015] The ionomer useful to the invention has as its essential characteristic that it consists of polymer chains bearing imidazolium ionic groups. Preferably, the counter-ions of the imidazolium ionic groups are halide anions, preferably bromides, or hydroxide anions.

[0016] The imidazolium ionic groups are attached to the polymer chains via a group resulting from the cycloaddition reaction between an aromatic nitrile oxide substituted by an imidazole function and a polymer containing monomeric units of a diene.

[0017] The polymer containing monomer units of a diene, an unsaturated polymer useful for the purposes of the invention, is a polymer that includes, in a well-known manner, carbon-carbon double bonds. The unsaturated polymer is preferably a homopolymer of a diene or a copolymer of a diene.

[0018] The diene is preferably a 1,3-diene. The unsaturated polymer is preferably a homopolymer of a 1,3-diene or a copolymer of a 1,3-diene. The unsaturated polymer is then preferably chosen from among the homopolymers. of a 1,3-diene, copolymers of two or more 1,3-dienes, copolymers of a 1,3-diene and a vinylaromatic monomer, and copolymers of a 1,3-diene and ethylene. When the unsaturated polymer is a copolymer of a 1,3-diene, it can be statistical, random, block, or gradient (tapered).

[0019] As 1,3-dienes, 1,3-dienes with 4 to 20 carbon atoms are preferentially suitable, more preferably 1,3-butadiene and isoprene, even more preferably 1,3-butadiene.

[0020] Styrene, or styrenes substituted at the alpha, ortho, meta, or para positions by an alkyl group, are preferably suitable as vinylaromatic monomers. The styrene may be substituted by one or more alkyl groups, which may be identical or different. The alkyl group may contain one or more carbon atoms, in particular one to four carbon atoms. The vinylaromatic monomer is preferably styrene.

[0021] Copolymers of several 1,3-dienes, that is to say copolymers of at least two 1,3-dienes, are known to be copolymers whose constituent monomer units are the monomer units of said several 1,3-dienes. Examples of copolymers of several 1,3-dienes include copolymers of 1,3-butadiene and isoprene, copolymers of 1,3-butadiene and piperylene, and copolymers of isoprene and piperylene.

[0022] As is well known, homopolymers of a 1,3-diene and copolymers of several 1,3-dienes can be prepared in solution, dispersion, or emulsion. They can be prepared by anionic polymerization, by coordination polymerization, for example in the presence of a Ziegler-Natta type catalyst or in the presence of a metallocene, or by radical polymerization.

[0023] Copolymers of a 1,3-diene and a vinylaromatic monomer are well-known copolymers, as are their methods of preparation. They can be prepared in solution, dispersion, or emulsion, particularly in solution or dispersion by anionic polymerization or in emulsion by radical polymerization. Examples include 1,3-butadiene-styrene copolymers, isoprene-styrene copolymers, and 1,3-butadiene-isoprene-styrene copolymers. The proportion of vinylaromatic monomer units, preferably styrene, in the copolymer can vary considerably and is adjusted by those skilled in the art according to the desired properties of the copolymer. It is typically greater than 0% and less than 95% by mol, the percentage being calculated relative to the number of moles of the copolymer.It is preferably less than 75% by mole, more preferably less than 50% by mole, the percentage being calculated in relation to the number of moles of the copolymer.

[0024] Copolymers of 1,3-diene and ethylene are also known polymers, as are their synthesis processes, for example, described in patent applications EP 1 092 731, WO 200754223, WO 200754224, WO 2017103543, and WO 2017103544. Copolymers of 1,3-diene, preferably 1,3-butadiene, and ethylene preferably contain more than 50 mole percent of ethylene. Advantageously, they contain between 50 and 90 mole percent of ethylene units.

[0025] The 1,3-dipolar compound useful for the purposes of the invention is an aromatic nitrile oxide substituted with an imidazole group. Aromatic nitrile oxides are compounds well known to react by cycloaddition, in this case [3+2], with double bonds, as described in the book "Nitrile Oxides, Nitrones and Nitronates in Organic Synthesis, Novel Strategies in Synthesis", 2nd Edition, Henry Feuer, 2008, Wiley-Interscience Edition. Aromatic nitrile oxides substituted with an imidazole group are compounds known from patent application WO 2015059269. Aromatic nitrile oxides substituted with an imidazole group are also known from patent application WO 2015059269 to react with the carbon-carbon double bonds of an unsaturated polymer by cycloaddition.The cycloaddition reaction allows imidazole groups to be grafted onto the polymer, as illustrated in the following diagram on a monomer unit of 1,3-butadiene, with IMIDAZ denoting the imidazole group. The wavy line in the representation of the 1,3-dipolar compound in the diagram symbolizes the attachment of the imidazole group to the aromatic ring substituted by the nitrile oxide. It is through the group resulting from the cycloaddition reaction between the 1,3-dipolar compound and the unsaturated polymer that the imidazole groups are attached to the modified polymer.

[0026] The imidazole function preferably has the formula 177-imidazol-l-yl or 2-alkyl-177-imidazol-l-yl. The alkyl group substituting for the carbon atom at position 2 of the imidazole ring is preferably an alkyl group having 1 to 6 carbon atoms, more preferably an alkyl group having 1 to 3 carbon atoms, and even more preferably a methyl group. Advantageously, the imidazole function has the formula 177-imidazol-l-yl or the formula 2-alkyl-177-imidazol-1-yl, which is preferably 2-methyl-177-imidazol-1-yl.

[0027] The 1,3-dipolar compound preferentially corresponds to formula (I) in which one of the symbols Ri to R5 represents a group containing the imidazole function, the other symbols represent a hydrogen atom or an alkyl, knowing that one of the Ri and R5 is different from a hydrogen atom.

[0028] Preferably, R4 represents the group containing the imidazole function. R4 is preferably a (1H-imidazol-l-yl)alkyl or (2-alkyl-l / / -imidazol-l-yl)alkyl group, more preferably a (177-imidazol-1-y 1)methyl or (2-alkyl-l / / -imidazol-l-yl)methyl group. R4 is even more preferably a l / / -imidazol-l-yl)methyl or (2-methyl-l / / -imidazol-l-yl)methyl group.

[0029] Preferably, in formula (I), Rb R3 and R5 are each an alkyl group and R2 is a hydrogen atom. The alkyl groups of Rh R3 and R5 are preferably methyl or ethyl groups, more preferably methyl groups.

[0030] The 1,3-dipolar compound is advantageously the compound 2,4,6-trimethyl-3-((2-methyl-l / / -imidazol-l-yl)methyl)benzo-nitrile oxide or the compound 2,4,6-triethyl-3-((2-methyl-l / / -imidazol-l-yl)methyl)benzo-nitrile oxide, respectively of formula (lia) and (Ilb).

[0031] The ionomer also has the essential characteristic of being cross-linked. The presence of cross-linking nodes in the ionomer results in the interconnection of some of the polymer chains constituting the ionomer. This interconnection is ensured by the existence of bonds between imidazolium groups of different chains, which are covalently linked via an alkanediyl chain. In other words, some of the imidazolium ionic groups are linked each to an imidazolium ionic group distinct from another polymer chain constituting the ionomer by an alkanediyl chain.

[0032] The membrane according to the invention can be prepared by a process, another object of the invention, which comprises the following steps a), b), c) and d): - a) the preparation of a polymer bearing pendant imidazole functions by a cycloaddition reaction of a 1,3-dipolar compound and an unsaturated polymer, the 1,3-dipolar compound and the unsaturated polymer being those respectively described according to one or more of the embodiments of the invention relating to the membrane according to the invention, - b) the incorporation of a reagent into the polymer bearing pendant imidazole functions by thermomechanical mixing, the reagent being a mixture of a monohalogenated alkane and a dihalogenated alkane, - c) a heat treatment of the mixture obtained in step b) to convert the imidazole functions into imidazolium ionic groups, - d) shaping the crosslinked ionomer obtained at the end of step c) into a film, steps b), c) and d) being carried out in bulk.

[0033] Step a) consists of preparing a polymer bearing imidazole functional groups by grafting the 1,3-dipolar compound onto the unsaturated polymer. The grafting reaction between a polymer containing diene monomer units and a 1,3-dipolar compound such as a nitrile oxide is a well-known [3+2] cycloaddition reaction of the 1,3-dipolar compound onto the carbon-carbon double bonds of the diene monomer units of the polymer. Since the 1,3-dipolar compound used in the present invention is a compound that, in addition to the dipole, bears a chemical functional group, in this case an imidazole functional group, the reaction allows the grafting of pendant chemical functional groups, imidazole functional groups, onto the polymer. The grafting reaction is typically carried out at a temperature above ambient (23°C), preferably above 60°C.

[0034] According to a first embodiment of the invention, the grafting reaction is carried out in solution. The polymer thus modified can be separated from its solution by any type of means known to those skilled in the art, and in particular by a steam stripping operation.

[0035] According to a second embodiment of the invention, the grafting reaction is carried out in bulk, for example in internal mixers, extruders, ovens, or presses. It is then generally preceded by bulk mixing so that the 1,3-dipolar compound is incorporated into the unsaturated polymer. Step a) can be carried out by incorporating the 1,3-dipolar compound into the unsaturated polymer at a mixer temperature below 60°C, then carrying out the grafting reaction under pressure or in an oven at temperatures ranging from 80°C to 200°C. Alternatively, step a) can be carried out by incorporating the 1,3-dipolar compound into the unsaturated polymer at a mixer temperature above 60°C, with the grafting reaction occurring simultaneously with the incorporation. When the grafting reaction is carried out in bulk, it is preferably performed in the presence of an antioxidant of the unsaturated polymer.

[0036] According to a first preferred embodiment of the second embodiment, step a) is a reactive extrusion step of a mixture of the unsaturated polymer and the 1,3-dipolar compound, at the end of which the 1,3-dipolar compound is grafted onto the polymer. Step a) is then typically a reactive extrusion process as described in patent application WO 2018115703. In this embodiment, the unsaturated polymer and the 1,3-dipolar compound feed a twin-screw extruder which conventionally comprises a barrel, a feeding zone, a mixing zone, a set of two worm screws, and a die. The extrusion temperature, the setpoint temperature applied inside the extruder, particularly to the barrel, is preferably above 100°C.Preferably, the extrusion temperature in the area extending from the mixing zone to the end of the screw assembly closest to the die is between 110 and 140°C. This temperature range provides the best compromise between grafting efficiency and productivity. Productivity is governed by the polymer flow rate in the extruder, which is itself adjusted according to the desired residence time of the polymer in the extruder from the mixing zone to the extruder die. Residence times are typically short, ideally no more than 5 minutes, and preferably less than 5 minutes. Residence times ranging from 30 seconds to 2 minutes can be sufficient to achieve both good grafting efficiency and precise control of the grafting ratio, thus ensuring good process reproducibility.Reactive extrusion can be carried out without the need for specific atmospheric conditions inside the barrel. Typically, extrusion takes place under ambient air. At the extruder outlet, after passing through the die, the polymer is recovered, in which some or all of the diene monomer units have reacted with the 1,3-dipolar compound.

[0037] According to a second variant of the second embodiment, step a) takes place in two phases, a first phase during which the unsaturated polymer and the 1,3-dipolar compound are mixed, typically at a temperature below 60°C, for example in an external mixer such as a roller tool, and a second phase during which the mixture resulting from the first phase is heated to a temperature of 80°C to 200°C, preferably from 100°C to 170°C in a press or in an oven, for the time required to perform the grafting. At the end of the second phase, the polymer is recovered, in which some or all of the diene monomer units have reacted with the 1,3-dipolar compound. Such a process is described, for example, in patent application WO 2015059269.

[0038] According to a third embodiment of the second, step a) is carried out by thermomechanical mixing of the unsaturated polymer and the 1,3-dipolar compound, for example in an internal mixer, until a maximum mixing temperature of 110 to 180°C, preferably between 140 and 170°C, is reached. After thermomechanical mixing, the polymer is recovered, in which some or all of the diene monomer units have reacted with the 1,3-dipolar compound. Following this step, the modified polymer can be extruded to form granules, in order to facilitate its storage before use in step b), as described, for example, in patent application WO 2018115704.

[0039] Step a) is advantageously carried out in bulk. The grafting reaction of step a) is preferably carried out in bulk according to the second embodiment, advantageously according to the first variant, the second variant or the third variant, most advantageously according to the first variant which proves to be more efficient with regard to grafting yield and also more productive.

[0040] The amount of 1,3-dipolar compound useful in step a) can vary considerably. It is indexed to the grafting yield and the number of imidazole groups desired to be grafted onto the unsaturated polymer. The grafting yield, which is the ratio between the amount of 1,3-dipolar compound grafted onto the unsaturated polymer and the amount of 1,3-dipolar compound used in step a), is typically greater than or equal to 50% and can reach values ​​greater than or equal to 80%, particularly when step a) is implemented according to the first variant, the amount of 1,3-dipolar compound grafted onto the unsaturated polymer being able to be determined by NMR analysis. The number of imidazole groups desired to be grafted onto the unsaturated polymer is typically greater than or equal to 0.5 mmol per gram of unsaturated polymer. Preferably, it is greater than 0.5 mmol per gram of unsaturated polymer and less than 3.5 mmol per gram of unsaturated polymer. The amount of the 1,3-dipolar compound used in step a) is preferably greater than 0.5 milliequivalents of imidazole function per gram of unsaturated polymer, more preferably greater than 0.6 milliequivalents of imidazole function per gram of unsaturated polymer. It is also preferably less than 4.2 milliequivalents of imidazole function per gram of unsaturated polymer.

[0041] At the end of step a), a polymer is therefore obtained which carries pendant imidazole functions in a quantity which is preferably greater than or equal to 0.5 mmole per gram of polymer, more preferably greater than 0.5 mmole per gram of polymer and less than 3.5 mmoles per gram of polymer.

[0042] Step b) consists of incorporating a reagent into the polymer resulting from step a), a polymer bearing pendant imidazole groups. The reagent is a mixture of a monohalogenated alkane and a dihalogenated alkane. As is well known, a monohalogenated alkane is an alkane substituted with a single halogen atom, and a dihalogenated alkane is an alkane substituted with two halogen atoms.

[0043] The incorporation is carried out by bulk mixing, for example in an internal or external mixer, the aim of which is to distribute the reagent as uniformly as possible within the polymer bearing pendant imidazole groups along its chain without the use of a solvent. At the end of step b), a mixture is obtained containing the polymer bearing pendant imidazole groups and the reagent, the reagent being in the form of a mixture, a mixture of a monohalogenated alkane and a dihalogenated alkane. The composition of the mixture of a monohalogenated alkane and a dihalogenated alkane can vary widely, the molar content of the monohalogenated alkane being able to vary from more than 0% to less than 100%, the remainder being provided by the dihalogenated alkane.The choice of the molar ratio between the dihalogenated alkane and the monohalogenated alkane is generally guided by the desired compromise between the membrane's dimensional stability and its ionic conductivity. The higher the concentration of dihalogenated alkane in the mixture of a monohalogenated and a dihalogenated alkane, the greater the degree of ionomer cross-linking, the lower the membrane's water uptake, and the greater its dimensional stability. This improvement in dimensional stability is accompanied by a decrease in ionic conductivity. The molar ratio between the dihalogenated and monohalogenated alkane preferentially ranges from 0.02 to 1, and more preferably from 0.02 to 0.5.

[0044] The amount of reagent used in step b) is indexed to the rate of imidazole function grafting of the functional polymer obtained at the end of step a), in other words to the number of imidazole functions carried by the polymer at the end of step a), or to the amount of 1,3-dipolar compound used in step a). Preferably, in step b) the amount of reagent is 1 to 2 molar equivalents of imidazole functions carried by the polymer at the end of step a) or is 1.2 to 2.4 molar equivalents of 1,3-dipolar compound used in step a).

[0045] The halogen atom of the reagent can be an iodine atom, a chlorine atom or a bromine atom. It is preferentially a bromine atom. The monohalogenated alkane is preferentially a monobrominated alkane, and the dihalogenated alkane is preferentially a dibrominated alkane. The hydrocarbon chain constituting the reagent can be linear, branched, or cyclic. The monohalogenated alkane and the dihalogenated alkane The alkanes are chosen by a person skilled in the art based on the desired performance compromise of the process. This compromise may include minimizing or avoiding the emission of volatile compounds during step b) or step c), controlling the amount of reagent introduced in step b), and obtaining the best ionomer yield. A person skilled in the art will be able to select the monohalogenated and dihalogenated alkanes based on their physicochemical and chemical properties, such as vapor pressure, boiling point, molar mass, density, and reactivity with imidazole groups, all of which influence this compromise. Preferably, in the dihalogenated alkane, the two halogen atoms are bonded to separate carbon atoms. Advantageously, the halogen atoms bonded to separate carbons are not vicinary halogens. In both the dihalogenated and monohalogenated alkanes, the halogen atoms are bonded to primary carbons. Preferably, both the monohalogenated and dihalogenated alkanes contain at least 6 carbon atoms. More preferably, they contain fewer than 12 carbon atoms. The carbon chain of the monohalogenated alkane is advantageously a linear chain with the halogen atom at its end. The carbon chain of the dihalogenated alkane is advantageously a linear chain with a halogen atom at each end. A single dihalogenated alkane or several dihalogenated alkanes may be used as the dihalogenated alkane, preferably a single dihalogenated alkane, at least for the sake of process simplification. Similarly, as a monohalogenated alkane, one can use a single monohalogenated alkane or several monohalogenated alkanes, preferably a single monohalogenated alkane at least for reasons of process simplification.

[0046] Step c), which follows step b), consists of forming a crosslinked ionomer by reacting the imidazole groups of the polymer and the reagent, a quaternization reaction. In this application, the quaternization reaction is defined as the reaction that allows the formation of an ionic group, the imidazolium cation, with the counterion being the halide anion. The heat treatment in step c) consists of heating the mixture obtained at the end of step b) to a temperature sufficient to carry out the quaternization reaction. A person skilled in the art adjusts the temperature at which the heat treatment is carried out, taking into account the reactivity of the reagent to the quaternization reaction, the thermal stability of the reactants, and the reaction products. In step c), the heat treatment is carried out at a temperature generally above 10°C, preferably above 30°C. The heat treatment is preferably carried out at a temperature below 100°C. Step c) which follows step b) is also carried out in bulk.

[0047] According to one embodiment of the invention, step b) and step c) are carried out in the same device which allows steps b) and c) to be carried out. The device useful for this embodiment may be an internal mixer or an extruder which provides mechanical work to uniformly incorporate the reagent within the polymer bearing pendant imidazole functions and which also provides heat to the mixture formed from the polymer bearing pendant imidazole functions and the reagent to allow the quaternization reaction.

[0048] According to another embodiment of the invention, step b) and step c) are carried out in different devices, a first device such as a roller tool or an extruder allowing the reagent to be incorporated into the polymer bearing pendant imidazole functions at a temperature lower than the quaternization reaction temperature, a second device which brings the mixture formed of the polymer bearing pendant imidazole functions and the reagent to a temperature allowing the quaternization reaction.

[0049] At the end of step c), a crosslinked ionomer is obtained in which the ionic groups are imidazolium groups, and some of the imidazolium groups are linked, each by an alkanediyl chain, to a distinct imidazolium group on another polymer chain constituting the ionomer. The alkanediyl chain linking two imidazolium groups on distinct polymer chains of the ionomer results from the reaction that takes place in step c) between the dihalogenated alkane and the imidazole groups. The content of the imidazolium ionic groups is preferably greater than or equal to 0.5 mmol per gram of crosslinked ionomer, more preferably greater than 0.5 mmol per gram of crosslinked ionomer and less than 3.5 mmol per gram of crosslinked ionomer.

[0050] Step d) of the process according to the invention consists of forming the crosslinked ionomer obtained at the end of step c) into a film intended to form part or all of an anion exchange membrane. Step d) is also carried out in bulk. Any device known for forming a polymer into a film in the absence of a solvent can be used. Examples include calenders, roller nozzles located at the exit of an extruder, and presses. To facilitate its formation into a film, the crosslinked ionomer can be heated to soften it. The film preferably has a thickness of less than 1 mm, more preferably a thickness greater than 10 µm, and preferably less than 500 µm.

[0051] As is well known to those skilled in the art, a membrane is filled with water before being put into operation in an assembly constituting a fuel cell or an electrolyzer. Watering the membrane is generally part of the membrane break-in or activation process. Typically, during this watering, the counterions of the membrane's cross-linked ionomer, which are halides, can be replaced by hydroxide anions, which are known to have much better ionic mobility than halide anions. See, for example, the article Energy Environ Sci 2014, 7, 3135.

[0052] The replacement of halide counterions by hydroxide anions can be achieved by impregnating the crosslinked ionomer film of the membrane with an aqueous solution containing hydroxide anions, for example, by bringing the ionomer film into contact with an aqueous solution containing hydroxide anions, such as an aqueous solution of a strong base. Suitable strong bases include potassium hydroxide and sodium hydroxide, preferably potassium hydroxide. Preferably, the replacement of halide counterions by hydroxide anions is carried out during the membrane break-in or activation process.

[0053] The anion exchange membrane according to the invention can be used in a fuel cell or an electrolyzer, preferably in an electrolyzer. It typically forms part of the electrolytic layer of a membrane-electrode assembly (MEA) for a fuel cell or electrolyzer. MEAs are well-known basic components of fuel cells and electrolyzers. An MEA generally comprises five layers: an electrolytic layer, two catalytic layers, and two gas diffusion layers. The electrolytic layer, which may be a polymer ion exchange membrane, constitutes the central layer of the MEA; on either side of the electrolytic layer is a catalytic layer; the layer adjacent to the catalytic layer is a gas diffusion layer. The assembly formed by the gas diffusion layers, the catalytic layers, and the electrolytic layer is held together by two bipolar plates.One of the two catalytic layers forms the anode, the other the cathode. The membrane-electrode assembly, Membrane-Electrode Assembly (MEA), another object of the invention, has as its essential characteristic of comprising an anion exchange membrane according to the invention.

[0054] In the preparation of the membrane according to the invention, the step of immersion in a solution of a mixture of a dihalogenated alkane and a monohalogenated alkane to carry out the quaternization reaction is eliminated thanks to the chemical structure of the ionomer and its preparation method, which can be carried out in bulk. Similarly, for the same reasons, the coating step is also eliminated. Eliminating these two steps offers the advantage of not having to use solvents in the manufacture of the membrane.

[0055] In summary, the invention can be implemented according to any one of embodiments 1 to 27:

[0056] Mode 1: Anion exchange membrane containing a crosslinked ionomer consisting of polymer chains bearing imidazolium ionic groups which are attached to the polymer chains via a group resulting from the cycloaddition reaction between a 1,3-dipolar compound and an unsaturated polymer, the 1,3-dipolar compound being an aromatic nitrile oxide substituted with an imidazole function, the unsaturated polymer being a polymer containing monomeric units of a diene, a portion of the imidazolium ionic groups being linked each to a distinct imidazolium ionic group of another polymer chain constituting the ionomer by an alkanediyl chain.

[0057] Mode 2: Membrane according to mode 1 in which the unsaturated polymer is a homopolymer of a 1,3-diene or a copolymer of a 1,3-diene.

[0058] Mode 3: Membrane according to mode 1 or 2 in which the unsaturated polymer is selected from homopolymers of a 1,3-diene, copolymers of two or more 1,3-dienes, copolymers of a 1,3-diene and a vinylaromatic monomer and copolymers of a 1,3-diene and ethylene.

[0059] Mode 4: Membrane according to mode 3 in which the vinylaromatic monomer is styrene.

[0060] Mode 5: Membrane according to any one of modes 2 to 4 in which the 1,3-diene is 1,3-butadiene or isoprene.

[0061] Mode 6: Membrane according to any one of modes 2 to 5 in which the 1,3-diene is the 1,3-butadiene.

[0062] Mode 7: Membrane according to any one of modes 1 to 6 in which the imidazole function is of formula l / / -imidazol-l-yl or 2-alkyl-IH-hnidazol-1-yl which is preferably 2-methyl-177-imidazol-1-yl.

[0063] Mode 8: Membrane according to any one of modes 1 to 7 in which the 1,3-dipolar compound corresponds to formula (I) in which one of the symbols Ri to R5 represents a group containing the imidazole function, the other symbols represent a hydrogen atom or an alkyl, knowing that one of the Ri and R5 is different from a hydrogen atom.

[0064] Mode 9: Membrane according to mode 8 in which R4 represents the group containing the imidazole function.

[0065] Mode 10: Membrane according to mode 8 or 9 in which R4 is a (l / / -imidazol-l-yl)alkyl or (2-alkyl-l / / -imidazol-l-yl)alkyl group, preferably a (l / / -imidazol-l-yl)methyl or (2-alkyl-l / / -imidazol-l-yl)methyl group.

[0066] Mode 11: Membrane according to any one of modes 8 to 10 in which Rb, R3 and R5 are each an alkyl and R2 is a hydrogen atom.

[0067] Mode 12: Membrane according to any one of modes 8 to 11 in which Rb, R3 and R5 are methyl or ethyl.

[0068] Mode 13: Membrane according to any one of modes 8 to 12 in which the 1,3-dipolar compound is the compound 2,4,6-trimethyl-3-((2-methyl-l / / -imidazol-l-yl)methyl)benzo-nitrile oxide or the compound 2,4,6-triethyl-3-((2-methyl-l / / -imidazol-l-yl)methyl)benzo-nitrile oxide.

[0069] Mode 14: A method for preparing an anion exchange membrane comprising the following steps a), b), c) and d): - a) the preparation of a polymer bearing pendant imidazole functions by a cycloaddition reaction of a 1,3-dipolar compound and an unsaturated polymer, the 1,3-dipolar compound being an aromatic nitrile oxide substituted with an imidazole function, the unsaturated polymer being a polymer containing monomeric units of a diene, - b) the incorporation of a reagent into the polymer bearing pendant imidazole functions by thermomechanical mixing, the reagent being a mixture of a monohalogenated alkane and a dihalogenated alkane, - c) a heat treatment of the mixture obtained in step b) to convert the imidazole functions into imidazolium ionic groups, - d) shaping the crosslinked ionomer, obtained at the end of step c), into a film, steps b), c) and d) being carried out in bulk.

[0070] Mode 15: Method according to mode 14 in which step a) is carried out in mass.

[0071] Mode 16: A process according to mode 14 or 15 in which the monohalogenated alkane is a monobrominated alkane and dihalogenated alkane is a dibrominated alkane.

[0072] Mode 17: A process according to any one of modes 14 to 16 in which the monohalogenated alkane contains at least 6 carbon atoms and less than 12 carbon atoms.

[0073] Mode 18: A process according to any one of modes 14 to 17 in which the dihalogenated alkane contains at least 6 carbon atoms and less than 12 carbon atoms.

[0074] Mode 19: A process according to any one of modes 14 to 18 in which the amount of the 1,3-dipolar compound used in step a) is greater than 0.5 milliequivalent of imidazole function per gram of unsaturated polymer.

[0075] Mode 20: A process according to any one of modes 14 to 19 in which the amount of the 1,3-dipolar compound used in step a) is greater than 0.6 milliequivalent of imidazole function per gram of unsaturated polymer.

[0076] Mode 21: A process according to any one of modes 14 to 20 in which the amount of the 1,3-dipolar compound used in step a) is less than 4.2 milliequivalents of imidazole function per gram of unsaturated polymer.

[0077] Mode 22: A process according to any one of modes 14 to 21 in which the amount of reagent added in step b) is 1 to 2 molar equivalents of imidazole functions carried by the polymer at the end of step a).

[0078] Mode 23: Process according to any one of modes 14 to 22 in which the amount of reagent added in step b) is 1.2 to 2.4 molar equivalents of the 1,4-dipolar compound used in step a).

[0079] Mode 24: Process according to any one of modes 14 to 23 in which the molar ratio between the dihalogenated alkane and the monohalogenated alkane varies from 0.02 to 1.

[0080] Mode 25: Process according to any one of modes 14 to 24 in which the molar ratio between the dihalogenated alkane and the monohalogenated alkane varies from 0.02 to 0.5.

[0081] Mode 26: Membrane-electrode assembly, MEA, for fuel cell or electrolyzer, which assembly comprises an anion exchange membrane defined in any one of modes 1 to 13, which membrane is capable of being obtained by the process defined in any one of modes 14 to 25.

[0082] Mode 27: Fuel cell or electrolyzer which contains a membrane defined in any one of modes 1 to 13, which membrane is capable of being obtained by the process defined in any one of modes 14 to 25.

[0083] The aforementioned features of the present invention, as well as others, will be better understood upon reading the following description of several examples of embodiments of the invention, given by way of illustration. Examples

[0084] In the examples, the 1,3-dipolar compound substituted with an imidazole function is 2,4,6-trimethyl-3-((2-methyl-l / / -imidazol-l-yl)methyl)benzonitrile oxide. It is prepared according to the procedure described in patent application WO 2015059269.

[0085] Examples 1 to 3: preparation of membranes:

[0086] The unsaturated polymer is a strong cis polybutadiene containing 97% cis 1,4-butadiene units, commercial product "BR Synteca 63" from the Synthos company. The 1,3-dipolar compound is 2,4,6-trimethyl-3-((2-methyl-l / / -imidazol-l-yl)methyl)benzonitrile oxide. It is prepared according to the procedure described in patent application WO 2015059269. It has a mass purity of 60%. The dihalogenated alkane is 1,8-dibromooctane from TCI (purity 97%). The monohalogenated alkane is TCI's 1-bromooctane (99% purity). The press is a Carver brand. The microextruder is a twin-screw with recirculation, made by Xplore, equipped with a barrel with an internal volume of 15 cm3, the chamber volume being 3 cm3, the screw speed being 150 rpm (revolutions / minute).

[0087] Three membranes are prepared according to the following procedure:

[0088] The 1,3-dipolar compound (24.76 g; 58.3 mmol of compound) is incorporated into polybutadiene (25.24 g; 466.5 mmol of butadiene monomer units) in an external mixer at room temperature by performing 12 wallet passes. The resulting mixture is placed in a press for 10 min at 120°C under 10 tonnes. Functional imidazole polybutadiene, the polybutadiene modified by the grafting reaction with the 1,3-dipolar compound, is recovered.

[0089] The functional imidazole polybutadiene is introduced into a microextruder. The dihalogenated alkane or the monohalogenated alkane, or a mixture thereof, is then added to the microextruder at the screw head to be incorporated into the functional imidazole polybutadiene. The residence time in the barrel is 5 minutes at 90°C. A mixture in the form of a rod is collected at the extruder outlet. The mixture is placed in a press at 90°C under 3 to 4 bar for 17 hours. An ionomer is recovered.

[0090] A test sample is taken to create a membrane. Its thickness is shown in Table 1.

[0091] The contents of the species reacted in steps a) to c) of examples 1 to 3 are shown in Table 1. The mass recovered of ionomer is also shown in Table 1. None of the prepared membranes are soluble in chloroform.

[0092] Table 1: Example Functional polymer imid azole Alkane monohalogenated Alkane dihal ogenated Recovered mass Thickness of 1 membrane 1 2.20 g 0.81g 4.2 mmol 0 3.01g 200-250 pm 2 2.22 g 0.76 g 4.0 mmol 0.04 g 0.1 mmol 2.36 g 350-400 p.m. 3 2.3 g 0.42 g 2.2 mmol 0.296 g 1.1 mmol 2.5 g 800-1000 pm

[0093] Only the membranes of examples 2 and 3 conform to the invention.

[0094] Water absorption of the membranes: It is determined by the mass difference between the membrane immersed for 24 hours in deionized water (resistivity on the order of MΩ.cm1) and the membrane dried for 24 hours at 40°C and 100 mb of pressure. The mass percentage of water absorbed by the membrane ("water uptake", WU) is determined using the following calculation: %AVU (mimmersed ≤ mdried) / mdried ≤ 100.

[0095] Ionic conductivity of membranes: To determine the ionic conductivity of the membrane, its electrochemical impedance is measured across the membrane plane at 30°C and 30% relative humidity, with a measurement parameter of 50 mV amplitude variation and an applied potential of 0 V. Prior to this measurement, the halide counterions are replaced by hydroxide anions according to the following procedure: The ionomer film is immersed for 24 h at room temperature (23°C) in a solution of IM potassium hydroxide in demineralized water to perform ion exchange between bromide and hydroxide ions. It is then soaked for 15 min in a demineralized water solution to remove excess hydroxide ions.

[0096] The results are shown in Table 2.

[0097] Table 2: Example: Water uptake (% by mass) Ionic conductivity (mS / cm) 1 55 8 2 30 4 3 38 6

Claims

Demands

1. Anion exchange membrane containing a crosslinked ionomer consisting of polymer chains bearing imidazolium ionic groups which are attached to the polymer chains via a group resulting from the cycloaddition reaction between a 1,3-dipolar compound and an unsaturated polymer, the 1,3-dipolar compound being an aromatic nitrile oxide substituted with an imidazole function, the unsaturated polymer being a polymer containing monomeric units of a diene, a portion of the imidazolium ionic groups being linked each to a distinct imidazolium ionic group of another polymer chain constituting the ionomer by an alkanediyl chain.

2. Membrane according to claim 1 in which the unsaturated polymer is a homopolymer of a 1,3-diene or a copolymer of a 1,3-diene.

3. Membrane according to claim 1 or 2 wherein the unsaturated polymer is selected from homopolymers of a 1,3-diene, copolymers of two or more 1,3-dienes, copolymers of a 1,3-diene and a vinylaromatic monomer and copolymers of a 1,3-diene and ethylene.

4. Membrane according to any one of claims 1 to 3 wherein the imidazole function is of formula l / / -hnidazol-l-yl or 2-alkyl-1 / 7-imidazol-1-ylc which is preferably 2-methyl-1 / 7-imidazol-l-yl.

5. Membrane according to any one of claims 1 to 4, wherein the 1,3-dipolar compound corresponds to formula (I), in which one of the symbols Ri to R5 represents a group containing the imidazole function, the other symbols represent a hydrogen atom or an alkyl group, wherein one of the Ri and R5 is not a hydrogen atom.

6. Membrane according to claim 5 wherein R4 represents the group containing the imidazole function.

7. Membrane according to claim 5 or 6 wherein R4 is a (l / / -imidazol-l-yl)alkyl or (2-alkyl-l / / -imidazol-l-yl)alkyl group, preferably a (l / / -imidazol-l-yl)methyl or (2-alkyl-l / / -imidazol-l-yl)methyl group.

8. Membrane according to any one of claims 5 to 7 wherein Rh, R3 and R5 are each an alkyl and R2 is a hydrogen atom.

9. Membrane according to any one of claims 5 to 8 wherein Rb, R3 and R5 are methyls or ethyls.

10. Membrane according to any one of claims 1 to 9 wherein the 1,3-dipolar compound is the compound 2,4,6-trimethyl-3-((2-methyl-l / / -imidazol-l-yl)methyl)benzo-nitrile oxide or the compound 2,4,6-triethyl-3-((2-methyl-l / / -imidazol-l-yl)methyl)benzo-nitrile oxide.

11. A method for preparing an anion exchange membrane as defined in any one of claims 1 to 10, comprising the following steps (a), (b), (c), and (d): - (a) preparing a polymer bearing pendant imidazole functions by a cycloaddition reaction of a 1,3-dipolar compound and an unsaturated polymer, the 1,3-dipolar compound being an aromatic nitrile oxide substituted with an imidazole function, the unsaturated polymer being a polymer containing monomeric units of a diene, - (b) incorporating a reagent into the polymer bearing pendant imidazole functions by thermomechanical mixing, the reagent being a mixture of a monohalogenated alkane and a dihalogenated alkane, - (c) heat treating the mixture obtained in step (b) to convert the imidazole functions into imidazolium ionic groups, - (d) shaping the crosslinked ionomer, obtained at the end of step c), in the form of a film, steps b),c) and d) being mass-produced.

12. A process according to claim 11 wherein the monohalogenated alkane is a monobrominated alkane and the dihalogenated alkane is a dibrominated alkane.

13. A process according to any one of claims 11 to 12 wherein the amount of the 1,3-dipolar compound used in step a) is greater than 0.5 milliequivalents of imidazole function per gram of unsaturated polymer, preferably greater than 0.6 milliequivalents of imidazole function per gram of unsaturated polymer, and less than 4.2 milliequivalents of imidazole function per gram of unsaturated polymer.

14. A process according to any one of claims 11 to 13 wherein the amount of reagent added in step b) is 1 to 2 molar equivalents of imidazole functions carried by the polymer at the end of step a) or is 1.2 to 2.4 molar equivalents of 1,4-dipolar compound used in step a).

15. Membrane-electrode assembly, MEA, for fuel cell or electrolyzer, wherein the assembly comprises an anion exchange membrane defined in any one of claims 1 to 10, wherein the membrane is capable of being obtained by the process defined in any one of claims 11 to 14.

16. Fuel cell or electrolyzer which contains a membrane as defined in any one of claims 1 to 10, which membrane is capable of being obtained by the process defined in any one of claims 11 to 14.