Patent Application
20240266571
The present invention relates to a proton exchange membrane, the process for preparing said membrane, and the application of said membrane in fields requiring ion exchange, such as electrochemistry or in energy fields. In particular, this membrane is used in the design of fuel cell membranes, such as proton-conducting membranes for H2/air or H2/O2 fuel cells (these cells being known by the abbreviation PEMFC for “Proton Exchange Membrane Fuel Cell”) or methanol/air fuel cells (these cells being known by the abbreviation DMFC for “Direct Methanol Fuel Cell”).
A fuel cell is an electrochemical generator which converts the chemical energy of a fuel oxidation reaction in the presence of an oxidant, to electrical energy, heat and water. In general, a fuel cell comprises a plurality of electrochemical cells mounted in series, each cell comprising two electrodes of opposite polarity separated by a proton exchange membrane acting as a solid electrolyte. The membrane allows the protons formed during the oxidation of the fuel at the anode to pass to the cathode.
The membranes structure the core of the fuel cell and they must consequently have good proton conduction performance and also low permeability to the reactive gases (H2/air or H2/O2 for PEMFC fuel cells and methanol/air for DMFC fuel cells). The properties of the materials constituting the membranes are essentially thermal stability, resistance to hydrolysis and oxidation, and a degree of mechanical flexibility.
Membranes commonly used and meeting these requirements are membranes obtained from polymers belonging, for example, to the family of polysulfones, polyetherketones, polyphenylenes and polybenzimidazoles. However, it has been found that these nonfluorinated polymers degrade relatively rapidly in a fuel cell environment, and their service life remains, for the time being, insufficient for the PEMFC application.
Most proton exchange membranes are based on the chemistry of perfluorinated polymers possessing long or short branches bearing a sulfonate function. In addition to their high cost, these various polymers have a low resistance to hydroxyl radicals, which limits their durability in a fuel cell environment, and a low mechanical strength. These membranes furthermore have an ionic conductivity/hydrogen permeability ratio that does not make it possible to obtain thin membranes that combine high impermeability and high conductivity. Moreover, perfluorinated membranes have a temperature use limit that does not allow them to operate at temperatures above 80° C. for long periods of time.
In order to obtain long-term efficiency in terms of proton conduction at temperatures above 80° C., some authors have proposed more complex materials comprising, in addition to a polymer matrix, proton-conducting particles, the conductivity thus no longer being attributed solely to the polymer(s) constituting the membranes. This is the case for application WO 2014/173885, which describes composite materials comprising a polymer matrix and a filler consisting of inorganic ion-exchange particles, said particles being synthesized in situ within the fluorinated polymer matrix. These membranes exhibit a more homogeneous distribution of the inorganic particles within the polymer matrix. However, this type of membrane has lower mechanical properties compared to a membrane made of polymer matrix alone, a risk of cavitation at the particle-matrix interface due to variations in dimensions during the operation of the fuel cell, and is difficult to manufacture on an industrial scale.
Ion-conducting membranes produced by radiation-induced grafting are another option for improving their chemical stability. The radiation-induced grafting reaction is controlled by the diffusion of the monomers in the film and the monomer polymerization reactions. The reaction starts at the surface of the irradiated film and gradually moves through the bulk of the film. Films based on ethylene-tetrafluoroethylene (ETFE), fluorinated ethylene-propylene (FEP) and ethylene-chlorotrifluoroethylene (ECTFE) have been described, in particular for amphoteric ion exchange membranes.
There is a real need for proton exchange membranes that have improved properties, in particular improved thermal resistance and a higher conductivity/gas permeability ratio.
To overcome the abovementioned drawbacks, the inventors have developed a membrane that has a very particular morphology obtained starting from a vinylidene fluoride-based polymer (PVDF) in powder form.
According to a first aspect, the invention relates to a material consisting of an irradiated PVDF, in powder form, onto which are grafted a styrene monomer and a nitrile monomer, said irradiated and grafted PVDF bearing proton-exchange sulfonate groups.
Said PVDF is chosen from poly(vinylidene fluoride) homopolymers and copolymers of vinylidene difluoride with at least one comonomer chosen from the list: vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, 3,3,3-trifluoropropene, 2,3,3,3-tetrafluoropropene, 1,3,3,3-tetrafluoropropene, hexafluoroisobutylene, perfluorobutylethylene, 1,1,3,3,3-pentafluoropropene, 1,2,3,3,3-pentafluoropropene, perfluoro(propyl vinyl ether), perfluoro(methyl vinyl ether), bromotrifluoroethylene, chlorofluoroethylene, chlorotrifluoroethylene, chlorotrifluoropropene, ethylene and mixtures thereof.
According to a second aspect, the invention relates to a process for preparing said material, said process comprising the grafting of an irradiated PVDF powder with a mixture of styrene and nitrile monomers, followed by a post-treatment of the PVDF powder thus irradiated and grafted, by sulfonation.
According to a third aspect, the invention relates to a proton exchange polymer electrolyte membrane, said membrane consisting of a film obtained from said PVDF material.
According to a fourth aspect, the invention relates to a process for manufacturing the proton exchange polymer electrolyte membrane from said irradiated, grafted and functionalized PVDF material in powder form, said process comprising converting the PVDF powder into film form.
According to another aspect, the invention relates to a proton exchange polymer composite membrane, said membrane consisting of a porous polymer support impregnated with said PVDF material by the solvent and/or aqueous route.
According to another aspect, the invention relates to a proton exchange polymer composite membrane, said membrane consisting at least partly of fibers of said PVDF material, the remainder being a polymer, and being manufactured by electrospinning. This membrane is then impregnated with said PVDF material by the solvent or aqueous route.
According to another aspect, the invention relates to the applications of the proton exchange polymer electrolyte membrane, in the following fields:
The present invention makes it possible to overcome the disadvantages of the prior art.
More particularly, it provides a technology that makes it possible to:
The invention is now described in greater detail and in a nonlimiting manner in the description that follows.
According to a first aspect, the invention relates to a material consisting of an irradiated PVDF, in powder form, onto which are grafted a styrene monomer and a nitrile monomer, said irradiated and grafted PVDF bearing proton-exchange sulfonate groups.
According to another aspect, the invention relates to a proton exchange polymer electrolyte membrane, said membrane being obtained from said PVDF material.
According to various embodiments, said material and said membrane comprise the following features, combined where appropriate. The contents indicated are expressed by weight, unless otherwise indicated.
The fluoropolymer used in the invention and generically denoted by the abbreviation PVDF is a polymer based on vinylidene difluoride.
According to one embodiment, the PVDF is a poly(vinylidene fluoride) homopolymer or a mixture of vinylidene fluoride homopolymers.
According to one embodiment, the PVDF is a poly(vinylidene fluoride) homopolymer or a copolymer of vinylidene difluoride with at least one comonomer compatible with vinylidene difluoride.
The comonomers compatible with vinylidene difluoride can be halogenated (fluorinated, chlorinated or brominated) or non-halogenated.
Examples of appropriate fluorinated comonomers are: vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, trifluoropropenes and in particular 3,3,3-trifluoropropene, tetrafluoropropenes and in particular 2,3,3,3-tetrafluoropropene or 1,3,3,3-tetrafluoropropene, hexafluoroisobutylene, perfluorobutylethylene, pentafluoropropenes and in particular 1,1,3,3,3-pentafluoropropene or 1,2,3,3,3-pentafluoropropene, perfluoroalkyl vinyl ethers and in particular those of general formula Rf—O—CF═CF2, Rf being an alkyl group, preferably a C1 to C4 alkyl group (preferred examples being perfluoropropyl vinyl ether and perfluoromethyl vinyl ether).
The fluorinated comonomer can comprise a chlorine or bromine atom. It can in particular be chosen from bromotrifluoroethylene, chlorofluoroethylene, chlorotrifluoroethylene and chlorotrifluoropropene. Chlorofluoroethylene can denote either 1-chloro-1-fluoroethylene or 1-chloro-2-fluoroethylene. The 1-chloro-1-fluoroethylene isomer is preferred. The chlorotrifluoropropene is preferably 1-chloro-3,3,3-trifluoropropene or 2-chloro-3,3,3-trifluoropropene.
The VDF copolymer can also comprise non-halogenated monomers, such as ethylene, and/or acrylic or methacrylic comonomers.
The fluoropolymer preferably contains at least 50 mol % of vinylidene difluoride.
According to one embodiment, the PVDF is a copolymer of vinylidene fluoride (VDF) and of hexafluoropropylene (HFP) (P(VDF-HFP)), having a weight percentage of hexafluoropropylene monomer units of from 1% to 35%, preferably from 2% to 23%, preferably from 4% to 20%, by weight, relative to the weight of the copolymer.
According to one embodiment, the PVDF is a mixture of a poly(vinylidene fluoride) homopolymer and of a VDF-HFP copolymer.
According to one embodiment, the vinylidene fluoride copolymer of the invention is a melt-convertible heterogeneous thermoplastic copolymer, and comprises two or more co-continuous phases, said co-continuous phases comprising:
Said heterogeneous copolymer contains two or more phases that produce a co-continuous structure in the solid state. The co-continuous phases are distinct from each other and can be observed with a scanning electron microscope (SEM). The heterogeneous copolymers according to the invention differ from homogeneous copolymers, which comprise a single phase.
According to one embodiment, the PVDF is a mixture of two or more VDF-HFP copolymers.
According to one embodiment, the PVDF comprises monomer units bearing at least one of the following functions: carboxylic acid, carboxylic acid anhydride, carboxylic acid esters, epoxy groups (such as glycidyl), amide, hydroxyl, carbonyl, mercapto, sulfide, oxazoline, phenolic, ester, ether, siloxane, sulfonic, sulfuric, phosphoric or phosphonic. The function is introduced by a chemical reaction which can be grafting or a copolymerization of the fluorinated monomer with a monomer bearing at least one of said functional groups and a vinyl function capable of copolymerizing with the fluorinated monomer, according to techniques well known to a person skilled in the art.
According to one embodiment, the functional group bears a carboxylic acid function which is a group of (meth)acrylic acid type chosen from acrylic acid, methacrylic acid, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate and hydroxyethylhexyl (meth)acrylate.
According to one embodiment, the units bearing the carboxylic acid function additionally comprise a heteroatom chosen from oxygen, sulfur, nitrogen and phosphorus.
According to one embodiment, the functionality is introduced via the transfer agent used during the synthesis process. The transfer agent is a polymer with a molar mass of less than or equal to 20 000 g/mol and which bears functional groups chosen from the following groups: carboxylic acid, carboxylic acid anhydride, carboxylic acid ester, epoxy groups (such as glycidyl), amide, hydroxyl, carbonyl, mercapto, sulfide, oxazoline, phenolic, ester, ether, siloxane, sulfonic, sulfuric, phosphoric or phosphonic. One example of a transfer agent of this type is oligomers of acrylic acid.
The content of functional groups in the PVDF is at least 0.01 mol %, preferably at least 0.1 mol %, and at most 15 mol %, preferably at most 10 mol %.
The PVDF preferably has a high molecular weight. The term “high molecular weight”, as used here, is understood to mean a PVDF having a melt viscosity of greater than 100 Pa·s, preferably of greater than 500 Pa·s, more preferably of greater than 1000 Pa·s, advantageously of greater than 2000 Pa·s. The viscosity is measured at 232° C., at a shear gradient of 100 s−1, using a capillary rheometer or a parallel-plate rheometer, according to the standard ASTM D3825. The two methods give similar results.
The PVDF homopolymers and the VDF copolymers used in the invention can be obtained by known polymerization methods, such as emulsion polymerization.
According to one embodiment, they are prepared by an emulsion polymerization process in the absence of a fluorinated surfactant.
The polymerization of the PVDF results in a latex generally having a solids content of from 10% to 60% by weight, preferably from 10% to 50%, and having a weight-average particle size of less than 1 micrometer, preferably of less than 1000 nm, preferably of less than 800 nm and more preferably of less than 600 nm. The weight-average size of the particles is generally at least 10 nm, preferably at least 50 nm, and advantageously the average size is within the range from 100 to 400 nm. The polymer particles can form agglomerates, referred to as secondary particles, the weight-average size of which is less than 5000 μm, preferably less than 1000 μm, advantageously of between 1 and 80 micrometers and preferably from 2 to 50 micrometers. The agglomerates can break up into discrete particles during the formulation and the application to a substrate.
According to some embodiments, the PVDF homopolymer and the VDF copolymers are composed of biobased VDF. The term “biobased” means “derived from biomass”. This makes it possible to improve the ecological footprint of the membrane. Biobased VDF can be characterized by a content of renewable carbon, that is to say of carbon of natural origin originating from a biomaterial or from biomass, of at least 1 at %, as determined by the content of 14C according to the standard NF EN 16640. The term “renewable carbon” indicates that the carbon is of natural origin and originates from a biomaterial (or from biomass), as indicated below. According to some embodiments, the biocarbon content of the VDF can be greater than 5%, preferably greater than 10%, preferably greater than 25%, preferably greater than or equal to 33%, preferably greater than 50%, preferably greater than or equal to 66%, preferably greater than 75%, preferably greater than 90%, preferably greater than 95%, preferably greater than 98%, preferably greater than 99%, advantageously equal to 100%.
Emulsion polymerization enables the production of a latex of particles of around 200 nm, which after drying, for example spray drying, results in particles having a volume-mean diameter (Dv50) ranging from 10 to 50 m being obtained.
The material according to the invention consists of an irradiated PVDF in powder form onto which are grafted a styrene monomer and a nitrile monomer, said irradiated and grafted PVDF bearing proton-exchange sulfonate groups.
This material is prepared according to a process which comprises the grafting of an irradiated PVDF with a mixture of styrene and nitrile monomers, followed by a post-treatment of the PVDF powder thus irradiated and grafted, by sulfonation.
Each step of this process is detailed below.
In one embodiment, the PVDF powder is first exposed to ionizing radiation to introduce active sites into the PVDF polymer chain. The powder is irradiated by an electron beam, gamma ray or X-ray source at a dose of between 25 and 150 kgray and preferably between 30 and 125 kgray. The irradiation is carried out under vacuum, under air or under nitrogen. An irradiated PVDF powder is thus obtained. The irradiated PVDF powder then undergoes a grafting step using a mixture of monomers comprising a styrene monomer and a nitrile monomer.
According to one embodiment, said styrene monomer is of alpha-alkylstyrene type, with the alkyl group chosen from: methyl, ethyl, propyl, butyl, pentyl, and hexyl.
According to one embodiment, said styrene monomer is chosen from the group: α-methylstyrene, α-fluorostyrene, α-bromostyrene, α-methoxystyrene and α,β,β-trifluorostyrene.
According to one embodiment, said styrene monomer is α-methylstyrene (AMS).
According to one embodiment, said nitrile monomer is chosen from the group: acrylonitrile, 2-methyl-2-butenenitrile, 2-methylene glutaronitrile and methylacrylonitrile.
According to one embodiment, the grafted PVDF powder is passed into a bath at 60° C. containing between 30% and 50% of alpha-methylstyrene, between 30% and 50% of methylene glutaronitrile and between 0 and 40% of isopropanol before being rinsed with isopropanol.
According to one embodiment, the styrene monomer/nitrile monomer molar ratio ranges from 0.7 to 1.3.
According to one embodiment, said nitrile monomer is 2-methylene glutaronitrile (MGN).
According to one embodiment, the PVDF powder is irradiated in the presence of a mixture of monomers comprising said styrene monomer and said nitrile monomer. The powder is irradiated by an electron beam, gamma ray or X-ray source at a dose of between 25 and 150 kgray and preferably between 30 and 125 kgray. The irradiation is carried out under vacuum, under air or under nitrogen.
The irradiated and grafted PVDF powder is then subjected to a post-functionalization reaction with chlorosulfonic acid, followed by hydrolysis in water or an alkaline solution. This enables the cation exchange —SO3H function to be introduced onto the PVDF.
According to one embodiment, the sulfonation of the grafted powder is carried out in a dichloromethane solution containing chlorosulfonic acid at room temperature.
The grafted PVDF powder bearing the covalently bonded —SO3H functions is then rinsed with distilled water until the rinsing water has a neutral pH, before being hydrolysed at 80° C., and then dried in air. The weight of the powder thus converted increases by 25% to 60%, preferably by 35% to 55%.
Measurements by transmission infrared (IR) spectroscopy, via a calibration curve based on the ratio between the area of a specific peak of the aromatic group and/or of a specific peak of the nitrile group relative to a PVDF reference peak, show a degree of grafting of the powdered PVDF, by weight, of between 25% and 60%, preferably between 35% and 55%.
According to another aspect, the invention relates to a process for producing the proton exchange polymer electrolyte membrane from said irradiated, grafted and functionalized PVDF material in powder form, said process comprising converting the PVDF powder into the form of a film which constitutes the membrane. This step of converting the PVDF powder into film form is carried out by any technique known to those skilled in the art: extrusion-blow molding, flat-film extrusion but also, for example, film production by solvent casting.
According to another aspect, the invention relates to a proton exchange polymer electrolyte membrane, said membrane consisting of a film obtained from said PVDF material.
According to another aspect, the invention relates to a process for producing the proton exchange polymer electrolyte membrane from a mixture of said irradiated, grafted and functionalized PVDF material in powder form and another polymer chosen from: polymethyl methacrylate and copolymers thereof, fluoropolymers, polyurethanes and polyesters. Said mixture comprises from 100% to 50% by weight of said irradiated, grafted and functionalized PVDF in powder form. The process comprises converting the mixture into film form. This step of converting the mixture into film form is carried out by any technique known to those skilled in the art: extrusion-blow molding, flat-film extrusion but also, for example, film production by solvent casting.
According to another aspect, the invention relates to a proton exchange polymer composite membrane, said membrane consisting of a porous support impregnated with said PVDF material by the solvent and/or aqueous route, said porous support being a polymer chosen from: polyethylene, polypropylene, polytetrafluoroethylene (PTFE), poly(vinylidene fluoride) (PVDF), polysulfone (PSU), polyethersulfone (PESU), polyimide (PI), the family of polyaryletherketones (PAEK) such as PEEK or PEKK. This porous support can be produced according to techniques known to those skilled in the art such as phase inversion, extrusion followed by sequential stretching, melt-blown or spunbond extrusion, or electrospinning.
According to another aspect, the invention relates to a proton exchange polymer composite membrane, said membrane consisting at least partly of fibers of said PVDF material, the remainder being one of the polymers chosen from: polymethyl methacrylate and copolymers thereof, fluoropolymers, polyurethanes, polyesters, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), poly(vinylidene fluoride) (PVDF), polysulfone (PSU), polyethersulfone (PESU), polyimide (PI), and the family of polyaryletherketones (PAEK) such as PEEK or PEKK. This composite membrane is produced by electrospinning. This membrane is then impregnated with said PVDF material by the solvent or aqueous route.
Advantageously, the ion exchange capacity (IEC) of the electrolyte membrane is greater than 0.6 mmol/g. The IEC is measured as follows: a 1 cm×1 cm sample is immersed in a 0.5M solution of KCl overnight with stirring. The hydrogen ions present in the solution, after exchange with K+ on the sulfonate groups, are then titrated to pH=7 with a 0.05 M KOH solution. The ion exchange capacity is then calculated according to the equation below:
where n(H+) is the number of moles of protons, Wary is the weight of the dry membrane in its H+ form, c(KOH) is the concentration of KOH, V(KOH) is the volume of the KOH solution added for the titration, WK is the weight of the dried membrane in its K+ form, and M(K+) and M(H+) are the masses.
Advantageously, according to one embodiment, the hydrogen permeability of the electrolyte membrane according to the invention is less than 2 mA/cm2. For this measurement, the membrane is placed in a cell of a fuel cell and then a stream of hydrogen is applied at the cathode while a stream of nitrogen is applied at the anode. A potential is then applied on both sides and the current obtained by the transport of hydrogen through the membrane is measured.
Advantageously, according to one embodiment, the hydrogen permeability of the electrolyte membrane according to the invention is less than 2×10−2 mL/min·cm2. For this measurement, the membrane is placed in a cell of a permeameter coupled to a gas chromatograph. The permeameter cell is purged with helium and then a stream of hydrogen is applied to the upper face of the membrane at a pressure of 0.1 MPa. The stream of hydrogen which diffuses through the membrane into the lower part is then measured by gas chromatography.
Dynamic mechanical analysis (DMA) between −40° C. and 140° C. shows that the membrane does not melt. Its elongation at break, measured at 23° C. under 50% relative humidity at a speed of 20 mm/minute, for a film thickness of 20 μm, is greater than 100%.
In membrane-electrode assemblies (MEAs), this powder can be used as a binder between the catalyst, the other electron-conducting additives and the membrane.
According to another aspect, the invention relates to the applications of the proton exchange polymer electrolyte membrane, in the following fields:
According to one embodiment, the polymer electrolyte membrane is intended to be inserted into a fuel cell device within an electrode-membrane-electrode assembly.
These membranes are advantageously in the form of thin films, having, for example, a thickness of 10 to 200 micrometers.
To prepare such an assembly, the membrane may be placed between two electrodes. The assembly formed by the membrane arranged between the two electrodes is then pressed at an appropriate temperature in order to obtain a good electrode-membrane adhesion.
The electrode-membrane-electrode assembly is then placed between two plates ensuring electrical conduction and the supply of reactants to the electrodes. These plates are commonly referred to as bipolar plates.
| Number | Date | Country | Kind |
|---|---|---|---|
| FR21.05700 | May 2021 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2022/051032 | 5/31/2022 | WO |
