Ion-conducting membrane and its process for preparing a polyethersulfone by electrospinning.

Abstract

The invention relates to a method for preparing a membrane comprising the successive steps of preparing a layer of polyethersulfone nanofibers by electrospinning a polyethersulfone solution, heating the nanofibers to soften them, cooling the nanofibers, and impregnating the nanofibers with an ionomer to form the membrane. Such a membrane is composed of polyethersulfone nanofibers that have junction points with adjacent nanofibers. It has ionic conductivity properties and can be used in a fuel cell or electrolyzer.

Description

Title of the invention: Ion-conducting membrane and its method of preparation by electrospinning of a polyethersulfone.

[0001] The field of the present invention is that of methods for manufacturing ion-conducting membranes intended for use in electrochemical devices such as fuel cells and electrolyzers.

[0002] The core of a fuel cell and an electrolyzer consists of two electrodes, an anode and a cathode, an electrolytic layer separating the two electrodes, and a catalyst located at the interfaces of the electrolytic layer with each electrode. Fuel cells and electrolyzers may include a membrane. The membranes used in these electrochemical devices are generally made of a porous support impregnated with an ionomer and form the electrolytic layer. They have a physical barrier function to prevent the diffusion of gases between the two electrodes of the electrochemical device and an ion-conducting function to participate in the exchange of ions from one electrode to the other.During the operation of the electrochemical device, the membranes undergo dimensional changes that can be caused by stresses resulting from the differential pressure between the two electrodes, or, for example, by swelling caused by variations in the humidity level within the device. To fulfill their two functions—physical barrier and ion conductivity—the membranes must possess both mechanical strength and ion conductivity properties. Among the membranes that meet these two requirements are those with an expanded polytetrafluoroethylene (ePTFE) porous support and Nafion ionomer, a perfluorinated polymer containing sulfonate groups.However, there is a concern to reduce the use of perfluorinated substances in many areas of application, including in electrochemical devices, for environmental reasons.

[0003] The inventors have discovered a process which makes it possible to produce a membrane which offers a good compromise between the properties of mechanical resistance, the properties of ionic conductivity and the content of perfluorinated substances.

[0004] Thus, a first object of the invention is a method for preparing a membrane which comprises the following successive steps: (a) prepare a layer of polyethersulfone nanofibres by electrospinning a polyethersulfone solution, (b) heat the nanofibres to soften them, (c) cool the nanofibers, (d) impregnate the nanofibers with an ionomer to form the membrane.

[0005] Another object of the invention is a membrane, which can be obtained by the process according to the invention, which membrane comprises non-woven polyethersulfone nanofibers impregnated with an ionomer, which nanofibers have junction points with those adjacent to them.

[0006] The invention also relates to an electrochemical device which includes a membrane according to the invention. Detailed description of the invention

[0007] 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.

[0008] The polyethersulfone useful for the purposes of the invention is, as is well known, poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene), the constituent units of which have the formula (Ci2H8O3S). Polyethersulfone is typically a commercially available product, generally in the form of granules, for example from BASF, Solvay, and Sumitomo under the respective trade names Ultrason® E, Varadel®, and Sumikaexcel®. It is used without prior chemical modification in the membrane preparation process. Polyethersulfone preferentially has a weight average molar mass (Mw) greater than 10,000 g / mol and less than 250,000 g / mol, more preferably greater than 20,000 g / mol and less than 200,000 g / mol, and even more preferably greater than 30,000 g / mol and less than 150,000 g / mol.Mw values ​​are measured by SEC RI (size-exclusion chromatography coupled to a differential refractometer with poly(2-vinylpyridine) calibration).

[0009] The polyethersulfone solution is typically prepared by dissolving the polyethersulfone in a solvent at room temperature (23 °C) or at a temperature higher than room temperature, generally at a temperature less than or equal to the boiling point of the solvent. Any solvent known to solubilize polyethersulfones is suitable, such as dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), or mixtures thereof. Preferably, the polyethersulfone solution is a solution in dimethyl sulfoxide. Dimethyl sulfoxide is preferred to other solvents. Solvents are used to ensure the good solubility of polyethersulfone in dimethyl sulfoxide, the non-toxicity of dimethyl sulfoxide, and its solubility in water, which makes the residual presence of dimethyl sulfoxide in the membrane compatible with its use in a fuel cell or electrolyzer, which typically operates in the presence of water. The polyethersulfone solution is prepared at a concentration adjusted by a person skilled in the art, particularly based on the weight-average molar mass of the polyethersulfone and the nature of the solvent, to be suitable for use in an electrospinning device to form nanofibers. The polyethersulfone solution preferably has a concentration greater than 5% and less than 30%, with these percentages being mass percentages calculated relative to the total mass of the polyethersulfone solution.

[0010] In step a), the polyethersulfone nanofibers are formed by electrospinning of the polyethersulfone solution. Electrospinning is a process well known to those skilled in the art for forming nanofibers. The electrospinning process of a polymer solution consists of applying an electrical potential difference between a polymer solution and a collecting surface to obtain nonwoven articles or fabrics made of nanofibers with cross-sections ranging from the order of nanometers to several micrometers. An electrode connected to a high positive (or negative) voltage source is introduced into the polymer solution contained in a vessel fitted with a capillary tube. The solution is maintained, by its surface tension, in the form of a droplet at the end of the capillary tube.Under the influence of the electric field generated by the potential difference between the polymer solution and the collecting surface, the hemispherical surface of the droplet stretches to form a cone, called a Taylor cone. When the forces created by the electric field overcome the surface tension, a jet of solution is ejected from the cone. The solvent evaporates as the jet travels from the tip of the cone to the collecting surface. Solidified nanofibers are deposited on the collecting surface. The collecting surface can be the surface of a commutator on a rotating drum coated with an electrically conductive metal. The commutator is connected to ground or to a high-voltage negative source. Alternatively, the collecting surface can be the surface of a film or liner placed as close as possible to the commutator surface, between the commutator and the capillary.The deposited nanofibers together form a porous layer composed of non-woven nanofibers. This process allows for the creation of nanofiber layers of controlled, variable dimensions, particularly nanofibers with diameters on the order of tens or hundreds of nanometers, or even microns. Those skilled in the art know how to adjust the parameters of the electrospinning process to control the diameter and orientation of the nanofibers, notably by manipulating the... The concentration of the polymer solution, the distance between the capillary tip and the collecting surface, the electrical potential difference applied to the polymer solution, and the drum rotation speed are all factors. For example, it is known that a high drum rotation speed favors the orientation of nanofibers in preferred directions, such as in the production direction and substantially in the production direction, and leads to the formation of a layer with strong anisotropy in the plane formed by the directions defined by the production direction and the transverse direction, whereas a low drum rotation speed leads to the formation of nanofibers deposited in random directions.A person skilled in the art defines the production direction (in English "Machine Direction") as the direction in which the layer moves during its manufacturing process in step a), the transverse direction (in English "Transverse Direction" or "Cross Direction") being perpendicular to the production direction.

[0011] According to a first embodiment of the invention, the nanofibers constituting the layer prepared in step a) extend in random directions. The layer according to this first embodiment typically has a ratio of the layer's transverse tensile modulus to its production-direction tensile modulus that is greater than 0.5, preferably greater than 0.8.

[0012] According to a second embodiment of the invention, the layer prepared in step a) has nanofibers oriented in preferred directions, preferably in the production direction or substantially in the production direction. The layer according to the second embodiment of the process according to the invention therefore exhibits anisotropy in the plane formed by the directions defined by the production direction and the transverse direction. As is well known, an anisotropic layer has different tensile moduli depending on the directions considered. The layer according to this second embodiment typically has a ratio of the layer's transverse tensile modulus to its production tensile modulus that is less than 0.5, preferably less than 0.4, more preferably less than 0.3, and even more preferably less than 0.1.

[0013] To characterize the anisotropy of the layer prepared in step a), the secant tensile moduli respectively in the transverse direction and in the production direction are determined under the same conditions.

[0014] The polyethersulfone nanofiber layer prepared in step a) preferably has a thickness greater than or equal to 5 pm and less than or equal to 100 pm. The nanofibers of the layer prepared in step a) preferably have a diameter greater than or equal to 100 nm and less than or equal to 3000 nm, more preferably a diameter greater than or equal to 500 nm and less than or equal to 2500 nm. The diameter of a nanofiber is understood to be the diameter of the cross-section of the nanofiber. In the case where the cross-section is not circular, the diameter is the largest length of the cross-section.

[0015] According to a first embodiment of the invention, a single layer of polyethersulfone nanofibers is prepared in step a).

[0016] According to a second embodiment of the invention, several layers of polyethersulfone nanofibers are prepared in step a) and are superimposed one on top of the other to form an assembly before step b). According to this embodiment, step b) is performed on the layers once they have been assembled. The process for preparing a membrane according to the second embodiment comprises the following successive steps: (a) prepare several layers of polyethersulfone nanofibres by electrospinning a polyethersulfone solution, and stack the layers one on top of the other to form an assembly, (b) heat the nanofibres to soften them, (c) cool the nanofibers, (d) impregnate the nanofibers with an ionomer to form the membrane.

[0017] According to a third embodiment of the invention, several layers of polyethersulfone nanofibers are prepared in step a) and are layered one on top of the other between steps c) and d) to form an assembly. According to this embodiment, the process for preparing a membrane comprises the following successive steps: (a) preparing several layers of polyethersulfone nanofibers by electrospinning a polyethersulfone solution, (b) heating the nanofibers to soften them, (c) cool the nanofibers, layer them one on top of the other to form an assembly, (d) impregnate the nanofibers with an ionomer to form the membrane. The third variant differs from the second variant in that step b) and step c) are carried out before the layers are superimposed on each other.

[0018] According to a preferred embodiment of the second variant and the third variant, the layers prepared in step a) are superimposed one on top of the other in such a way that their respective production directions are aligned in the assembly.

[0019] Preferably, the layers prepared in step a) have nanofibers oriented in preferred directions, preferably in the direction of production or substantially in the direction of production.

[0020] According to a particularly preferred embodiment of the second and third variants, the layers prepared in step a) have nanofibers oriented in preferred directions, preferably in the production direction or substantially in the production direction, and are superimposed one on top of the other such that their respective production directions are aligned in the assembly. In other words, the assembly is carried out in such a way as to maintain anisotropy in the assembly in the plane formed by the directions defined by the production direction and the cross direction.

[0021] By multiple layers, we mean at least two layers, preferably two layers. The at least two layers can be prepared successively by electrospinning a first time, then by repeating step a) until the desired number of layers is obtained. Alternatively, the at least two layers can be prepared by electrospinning to obtain a first layer of nanofibers which is cut at the end of step a) to the dimensions required to obtain the desired number of layers.

[0022] Step b) is a step for softening the nanofibers. The softening of the nanofibers leads to the creation of junction points between a fiber and those adjacent to it. In other words, under the effect of their softening, the nanofibers form junction points with those adjacent to them; at these junction points, the nanofibers have fused and are bonded to one another. The creation of junction points between a nanofiber and those adjacent to it has the effect of increasing the mechanical strength of a layer of nanofibers. When step b) is carried out on an assembly made according to the second embodiment of the invention, the creation of junction points between a nanofiber and those adjacent to it has the effect of increasing the mechanical strength of the constituent layers of the assembly and of the assembly itself. The softening of the nanofibers is achieved by heating them, typically to a temperature that allows the polyethersulfone chains to move freely and that is known to those skilled in the art to be close to the glass transition temperature (Tg) of polyethersulfone, the Tg of polyethersulfone being approximately 225°C. According to any one embodiment of the invention, step b) is preferably carried out at a temperature above 210°C and below 240°C. Step b) can be carried out in air or under an inert atmosphere, for example, nitrogen.

[0023] Step b) is followed by step c) to fix the configuration of the layer or assembly obtained at the end of step b), that is, to maintain the state of the layer or assembly with the junction points between adjacent nanofibers and to obtain the mechanical strength properties. Typically, the nanofibers are cooled to a temperature that no longer allows mobility of the polyethersulfone chains and that is below the glass transition temperature of the polyethersulfone. Preferably, the nanofibers are cooled to a temperature close to room temperature, typically to a temperature ranging from 20 to 25 °C, to facilitate step d).

[0024] Step d) consists of impregnating the nanofibers with an ionomer, the nanofibers being in the form of a layer or an assembly. Impregnation can be carried out by immersing the nanofibers in a solution or a dispersion of the ionomer. The immersion time and temperature are adjusted by those skilled in the art according to the thickness of the layer or assembly, the desired ionomer content in the layer or assembly, or the concentration of the ionomer solution or dispersion. Alternatively, impregnation can be carried out by coating the nanofibers, that is, by applying a solution or a dispersion of the ionomer to the surfaces of the layer or assembly. The temperature at which the coating is carried out is chosen by a person skilled in the art, in particular according to the viscosity of the solution or the dispersion of the ionomer, the boiling point of the solvent of the solution or the dispersion of the ionomer.The coating is preferably carried out at a temperature close to room temperature, typically 20°C to 25°C, or at a temperature above room temperature and below the boiling point of the solvent of the solution or dispersion of the ionomer. The concentration of the ionomer solution or dispersion is adjusted by a person skilled in the art, taking into account the solubility of the ionomer in the solvent and the viscosity of the solution or dispersion. The solvent is selected by a person skilled in the art, taking into account the ionomer's solubility in that solvent and its boiling point. The solvent preferably has a relatively low boiling point, typically 100°C or lower, to allow for easy removal from the layer or assembly, particularly by evaporation under vacuum, air currents, or an inert gas such as nitrogen or argon.According to any one of the embodiments of the invention, the impregnation step is preferably followed by a solvent evaporation step from the solution or ionomer dispersion.

[0025] The ionomer useful for the purposes of the invention can be any polymer known to be an ionomer. Its chemical structure is not limited as long as it possesses the ion-exchange properties characteristic of ionomers. It typically carries ionic, cationic functions such as quaternary amine functions, or anionic functions such as sulfonate, sulfate, phosphonate, phosphate, or carboxylate functions, ionic functions that participate in the ion exchange between the two electrodes of an electrochemical device such as a fuel cell or an electrolyzer. The ionomer is typically a polymer that comprises a main hydrocarbon chain bearing pendant groups, which are the ionic functions. which can also be substituted by fluorinated groups or by non-ionic groups bearing heteroatoms other than fluorine, or be interrupted by heteroatoms other than fluorine. In this respect, we can mention, for example, styrene polymers or styrene substituted by an alkyl, tetrafluoroethylene polymers, which polymers have pendant groups comprising an ionic function, preferably a quaternary amine function, a sulfonate function, a sulfate function, a phosphonate function, a phosphate function or a carboxylate function.Preferably, the ionomer is a polymer comprising monomeric units of styrene or styrene substituted with an alkyl group, or alternatively a polymer comprising monomeric units of tetrafluoroethylene, which polymer has pendant groups comprising an ionic function, preferably a quaternary amine function, a sulfonate function, a sulfate function, a phosphonate function, a phosphate function, or a carboxylate function.

[0026] The process according to the invention, described in any one of the embodiments of the invention and any one of its variants, allows the preparation of a membrane that has the properties of transporting ions. Such a membrane can be described as an ion-conducting membrane.

[0027] The membrane, another object of the invention, comprises non-woven polyethersulfone nanofibers impregnated with an ionomer, which nanofibers have junction points with those adjacent to them. The nanofibers preferably have a diameter greater than or equal to 100 nm and less than or equal to 3000 nm, more preferably a diameter greater than or equal to 500 nm and less than or equal to 2500 nm. Preferably, the membrane has a thickness greater than or equal to 10 pm and less than or equal to 200 pm.

[0028] The membrane according to the invention is typically a membrane consisting of a porous support impregnated with an ionomer, the porous support being made up of polyethersulfone nanofibers. The ionomer and the polyethersulfone constituting the membrane are respectively those described as being useful in the process for preparing a membrane according to the invention. Preferably, the ionomer and the polyethersulfone are the only constituent elements of the membrane. According to any one of the embodiments of the invention, the ionomer is preferably a polymer comprising monomeric units of styrene or styrene substituted with an alkyl group, or a polymer comprising monomeric units of tetrafluoroethylene, which polymer has pendant groups comprising an ionic function, preferably a quaternary amine function, a sulfonate function, a sulfate function, a phosphonate function, a phosphate function, or a carboxylate function..

[0029] The membrane can be prepared by the process according to the invention in any one of the described embodiments relating to the process according to the invention, including variants thereof, namely the first, second, and third variants described therein. The membrane can consist of a single layer of polyethersulfone nanofibers or of several layers of polyethersulfone, preferably two layers, which are superimposed one on top of the other. Preferably, a layer of polyethersulfone nanofibers has a thickness greater than or equal to 5 pm and less than or equal to 100 pm.

[0030] According to one embodiment of the invention, the polyethersulfone nanofibers extend in random directions within the membrane, and the membrane is preferably composed of a single layer of polyethersulfone nanofibers. A membrane according to this embodiment can be prepared according to the first embodiment of the process according to the invention combined with the first variant of the process.

[0031] According to a preferred embodiment of the invention, the membrane is made up of several layers of polyethersulfone nanofibers, the layers being superimposed one on top of the other to form an assembly, their respective production directions being aligned in the assembly. The membrane according to this preferred embodiment can be prepared according to the particularly preferred embodiment of the second or third variant of the process according to the invention.

[0032] According to a particularly preferred embodiment of the invention, the membrane is made up of several layers of polyethersulfone nanofibers, preferably two layers of polyethersulfone nanofibers, the layers having nanofibers oriented in preferred directions, preferably in the production direction or substantially in the production direction, the layers being superimposed one on the other to form an assembly, their respective production directions being aligned in the assembly. The membrane according to the invention and made up of several layers having nanofibers oriented in preferred directions can be prepared according to the particularly preferred embodiment of the second or third variant of the process according to the invention.

[0033] The membrane according to the invention is typically an ion-conducting membrane and is intended for use in an electrochemical device typically comprising two electrodes, respectively an anode and a cathode. The electrochemical device comprising the membrane according to the invention is preferably a fuel cell or an electrolyzer.

[0034] 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 Size exclusion chromatography (SEC):

[0035] Size exclusion chromatography (SEC) separates macromolecules in solution according to their size using columns filled with a porous gel. The macromolecules are separated according to their hydrodynamic volume, with the largest eluting first. While not an absolute method, SEC allows for the determination of the molar mass distribution of a polymer. Using commercial standards, the various weight-average molar masses (Mw) can be determined via a Moore calibration. Commercial standards are poly(2-vinylpyridine), and the range of analyzable masses is between 890,000 g / mol and 458 g / mol. No special treatment is required for the polymer sample prior to analysis. It is dissolved at a concentration of approximately 1 g / L in dimethylformamide with 0.1 M LiBr, then stirred for 2 hours at 50°C before injection for analysis according to the conditions described in the table below. Elisant DMF + 0.1M LiB injection volume Temperature Detector Mobile phase flow rate Columns 100 jiL 50°C Refractometer (RI) 1 mL / min 2 Polargel-L

[0036] Membrane preparation:

[0037] Two membranes M1 and M2 according to the invention are prepared according to the following procedure:

[0038] Polyethersulfone nanofiber layers are prepared from a 25% by mass solution of poly(oxy-1,4-phenylensulfonyl-1,4-phenylene (CAS 25667-42-9, purchased from Goodfellow under trade name SU30-GL-000111, Mw of 59230 g / mol, calibration poly(2-vinylpyridine)) polyethersulfone in DMSO. Solubilization is carried out at 90°C. The solution is fed through a needle, and the nanofibers formed by electrospinning are deposited onto a collector roller of a rotating drum. The electrospinning parameters are as follows: 1 Solution flow rate: 1.2 mL / h 2 Needle / collector distance: 15 cm 3 Needle sweep speed: 30 mm / s 4 Sweeping across the entire roller dimension (0-200 mm) 5 High voltage: -5 kV (needle) / +8.7 kV (roller) 6 Roller rotation (200 or 2000 rpm)

[0039] One layer is prepared with a rotation speed of 200 revolutions / min (rpm), two layers are prepared with a rotation speed of 2000 rpm.

[0040] The C1 layer prepared at a rotation speed of 200 rpm has a thickness between 30 and 40 µm, while the two layers C2 and C3, prepared at a rotation speed of 2000 rpm, each have a thickness between 15 and 20 µm. The thickness of the polyethersulfone nanofiber layers is measured using a mechanical probe comparator (Nikon Digimicro MFC-101A model). The dimensions of the nanofibers are measured by scanning electron microscopy (SEM, FEG GeminiSEM 360).

[0041] The polyethersulfone nanofibers of layer C1 extend in random directions, whereas the polyethersulfone nanofibers of layers C2 and C3 are oriented in preferred directions, notably in the production direction and substantially in the production direction. These arrangements of the nanofibers in the layers, random for C1 and oriented for C2 and C3, can be observed by scanning electron microscopy.

[0042] The two layers C2 and C3 are superimposed on each other to form a C2 / C3 assembly such that their respective production directions are aligned in the assembly.

[0043] The nanofiber layers, layer Cl and C2 / C3 assembly, are held flat and placed in a ventilated oven at 225°C for 3 hours (step b) under air, then removed from the oven and left at room temperature (23°C) to be cooled to room temperature (step c). Junction points between the nanofibers and those adjacent to them are highlighted by scanning electron microscopy, as shown in [Fig. 1] and [Fig. 2], scanning electron microscopy images of the nanofibers obtained at the end of step c).

[0044] The measurement of the tensile secant moduli of the layers is carried out at the end of step c) to define the anisotropic character of the layers prepared in step a). The tensile secant moduli of the layers are measured on test specimens of dimension of 10 mm x 50 mm with a “Discovery TA HR” rheometer at 23°C and 0% relative humidity (RH), the deformation rate being 0.17 s1. A test specimen is cut from the layer, with the length of the specimen running in the production direction, the production direction being the direction in which the layer flows during its production. A first nominal secant modulus is measured at 4% deformation, called the production direction modulus. Another specimen is cut from the layer, with its length perpendicular to the production direction (crosswise direction). A second nominal secant modulus is measured at 4% strain, called the crosswise modulus. In the case of layers C2 and C3, the ratio of the cross direction modulus to the production direction modulus is less than 0.5 (in this case equal to 0.03 for C2 and C3). In the case of the Cl layer, the ratio of the cross-direction modulus to the production-direction modulus is equal to 1.46.

[0045] The Cl nanofiber layer and the C2 / C3 assembly are then impregnated with an ionomer, Nafion®, by coating them with a dispersion of Nafion® D2021CS (dispersion at 20% mass concentration) according to the coating procedure described below to manufacture the M1 and M2 membranes according to the invention respectively.

[0046] For the fabrication of the membrane M1, a first 18 µm layer of the dispersion is deposited onto a fluorinated ethylene propylene film (FEP substrate) using a bar-coater, and then the Cl layer is deposited on this first layer. After drying at 100°C for 1.5 hours, a second 500 µm layer of the dispersion is deposited at room temperature using a slot-die coating system. The resulting membrane M1 is then dried at 100°C.

[0047] The M2 membrane is prepared according to the same procedure, except that the C2 / C3 assembly is used instead of the CL layer

[0048] A third membrane, M3, is produced using the same coating method as membranes M1 and M2, except that no layer of polyethersulfone nanofibers is deposited. Membrane M3 is a membrane not conforming to the invention and consists solely of a perfluorinated substance, "Nafion".

[0049] A fourth membrane, M4, is produced using the same coating method as membrane M1, except that an expanded polytetrafluoroethylene (ePTFE) film is used instead of the nanofiber layer. Membrane M4 is a membrane not conforming to the invention, and all its constituent elements are perfluorinated substances, "Nafion" and PTFE.

[0050] Characterization of membranes M1 to M4: To determine the ionic conductivity of the prepared membrane, its electrochemical impedance is measured across the plane of the membrane at 30°C and 30% relative humidity, the measurement parameters being an amplitude variation of 50 mV, an applied potential of 0V.

[0051] The secant tensile moduli are measured on specimens of dimensions of 10 mm x 50 mm with a “Discovery TA HR” rheometer at 23°C and 0% relative humidity (RH), the strain rate being 0.17 s1. A test specimen is cut from the membrane, its length running along the production direction. The production direction is the direction in which the layer is advanced during its production. A first nominal secant modulus is measured at 4% deformation, referred to as the production direction modulus. Another test specimen is cut from the membrane, with its length perpendicular to the production direction (transverse direction). A second nominal secant modulus is measured at 4% deformation, called the transverse modulus. The result used is the average of the two modules, namely the sum of the production direction module and the transverse direction module divided by 2, known as the average of the modules at 4%. The thickness of a membrane is measured using a mechanical probe comparator.

[0052] The characteristics of the membranes are shown in Table 1.

[0053] [Tables] Membrane Thickness (pm) Average modulus at 4% (MPa) Ionic conductivity θ (mS.cm1) M1 90 174 3 M2 90 145 6 M3 95 150 8 M4 75 256 2

[0054] The M1 and M2 membranes according to the invention are the membranes which present the best compromise between the properties of mechanical resistance, the properties of ionic conductivity and the proportion of perfluorinated substances, compared to the M4 membrane which is made up of a support and an ionomer both perfluorinated.

Claims

Demands

1. A method for preparing a membrane comprising the following successive steps: (a) preparing a layer of polyethersulfone nanofibers by electrospinning a polyethersulfone solution, (b) heating the nanofibers to soften them, (c) cooling the nanofibers, (d) impregnating the nanofibers with an ionomer to form the membrane.

2. A process according to claim 1 wherein the polyethersulfone solution is a solution in dimethyl sulfoxide.

3. A method according to claim 1 or 2 wherein a single layer of polyethersulfone nanofibers is prepared in step a).

4. A method according to claim 1 or 2 wherein several layers of polyethersulfone nanofibers are prepared in step a) and are superimposed one on top of the other to form an assembly before step b).

5. A method according to claim 1 or 2 wherein several layers of polyethersulfone nanofibers are prepared in step a) and are superimposed one on top of the other between step c) and step d) to form an assembly.

6. A method according to claim 4 or 5 wherein the layers prepared in step a) are superimposed one on top of the other such that their respective production directions are aligned in the assembly.

7. A method according to any one of claims 4 to 6 wherein the layers prepared in step a) have nanofibers oriented in preferred directions.

8. A method according to any one of claims 1 to 6 wherein the nanofibers constituting the layer prepared in step a) extend in random directions.

9. A method according to any one of claims 1 to 8, wherein the ionomer is a polymer comprising monomeric units of styrene or styrene substituted with an alkyl group, or a polymer comprising monomeric units of tetrafluoroethylene, said polymer having pendant groups comprising an ionic function, preferably a quaternary amine function, a sulfonate function, sulfate function, phosphonate function, phosphate function or carboxylate function.

10. Membrane comprising non-woven polyethersulfone nanofibers impregnated with an ionomer, which nanofibers have junction points with those adjacent to them, the nanofibers being formed by electrospinning.

11. Membrane according to claim 10 wherein the polyethersulfone nanofibers and the ionomer are the only constituent elements of the membrane.

12. Membrane according to claim 10 or 11, wherein the membrane is made up of several layers of polyethersulfone nanofibers, the layers being superimposed one on top of the other to form an assembly, their respective production directions being aligned in the assembly.

13. Membrane according to any one of claims 10 to 12, wherein the polyethersulfone nanofibers extend in random directions.

14. Membrane according to any one of claims 10 to 12, wherein the membrane is made up of several layers of polyethersulfone nanofibers, the layers having nanofibers oriented in preferred directions.

15. Electrochemical device comprising a membrane according to any one of claims 10 to 14, which device is a fuel cell or an electrolyzer.

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