Ionically conductive membranes and methods of making ionically conductive membranes by electrospinning polyethersulfone

Abstract

The present invention relates to a process for the preparation of a membrane, said process comprising the following successive stages: preparation of a layer of polyethersulfone nanofibres by electrospinning of a solution of polyethersulfone; heating of the nanofibres to soften them; cooling of the nanofibres; and impregnation of the nanofibres with a ionomer so as to form a membrane. This membrane is composed of polyethersulfone nanofibres, these nanofibres having points of connection between them and their neighbouring nanofibres. The membrane has ionic conductivity properties and can be used in a fuel cell or an electrolyser.

Description

Technical Field

[0001] The present invention pertains to the field of manufacturing processes for ion-conducting membranes intended for use in electrochemical devices such as fuel cells and electrolyzers. Background Technology

[0002] The core of a fuel cell and electrolyzer consists of two electrodes (anode and cathode), an electrolytic layer separating the two electrodes, and a catalyst located at the interface between the electrolytic layer and each electrode. Fuel cells and electrolyzers may also include membranes. Membranes used in these electrochemical devices typically consist of a porous support impregnated with an ionomer, forming the electrolytic layer. They function as a physical barrier, preventing gas diffusion between the two electrodes of the electrochemical device; they also function as ion conductors, participating in ion exchange from one electrode to the other. During operation of the electrochemical device, the membrane undergoes dimensional changes, which may be caused by stress from the pressure difference between the two electrodes, or by swelling, for example, due to changes in humidity levels within the device. To achieve both physical barrier and ion conduction functions, the membrane must possess both mechanical strength and ion conduction properties. Among membranes that meet these requirements, those using expanded polytetrafluoroethylene (known as "ePTFE") as the porous support and those using Nafion (a perfluoropolymer with sulfonic acid functional groups) as the ionomer can be mentioned. In fact, for environmental reasons, the focus is on reducing the use of perfluorinated materials in many application areas, including electrochemical devices. Summary of the Invention

[0003] The inventors have discovered a method that allows for the preparation of membranes that achieve a good trade-off between mechanical strength, ionic conductivity, and perfluorinated content.

[0004] Therefore, the first subject of the present invention is a method for preparing a membrane, the method comprising the following sequential stages: (a) A layer of polyethersulfone nanofibers was prepared by electrospinning a polyethersulfone solution; (b) Heating the nanofibers to soften them; (c) Cooling the nanofibers; (d) Impregnate the nanofibers with an ionomer to form a membrane.

[0005] Another subject of the invention is a membrane that can be obtained by the method according to the invention, the membrane comprising nonwoven polyethersulfone nanofibers impregnated with ionomers, the nanofibers having connection points with their adjacent nanofibers, the membrane being obtainable by the method according to the invention.

[0006] The present invention also relates to an electrochemical device comprising a membrane according to the invention. Detailed Implementation

[0007] The polymers mentioned in the specification can be fossil-derived or bio-based. In the latter case, they can be produced partly or entirely from biomass, or obtained from renewable raw materials derived from biomass. In the same way, they can also originate from the recycling of pre-used materials, that is, they can be produced partly or entirely from the recycling process, or obtained from starting materials that are themselves produced by the recycling process.

[0008] The polyethersulfone used to meet the requirements of this invention is the well-known poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene), and the constituent unit of the polyethersulfone is of formula (C 12 (H8O3S). Polyethersulfone is generally a commercially available product, typically in granular form, and is obtained from BASF, Solvay, or Sumitomo under their respective trade names, such as Ultrason® E, Varadel®, or Sumikaexcel®. It can be used in membrane preparation methods without chemical modification. The weight-average molar mass (Mw) of polyethersulfone is preferably 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. The Mw value is determined using SEC RI (size exclusion chromatography coupled with differential refractometer, calibrated with poly(2-vinylpyridine)).

[0009] Polyethersulfone solutions are typically prepared by dissolving polyethersulfone in a solvent at ambient temperature (23°C) or at a temperature above ambient temperature, typically at a temperature below or equal to the boiling point of the solvent. Any solvent known to dissolve polyethersulfone can be used, 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 over other solvents because polyethersulfone has good solubility in dimethyl sulfoxide, which is non-toxic and soluble in water, making the residual dimethyl sulfoxide in the membrane compatible with its use in fuel cells or electrolyzers (which typically operate in the presence of water). The concentration of the polyethersulfone solution is specifically adjusted by those skilled in the art based on the weight-average molecular weight of the polyethersulfone and the properties of the solvent to enable its use in electrospinning devices to form nanofibers. The concentration of the polyethersulfone solution is preferably greater than 5% and less than 30%, which is a weight percentage calculated relative to the total weight of the polyethersulfone solution.

[0010] In stage a), polyethersulfone nanofibers are formed by electrospinning a polyethersulfone solution. Electrospinning is a method well known to those skilled in the art for forming nanofibers. The method of electrospinning a polymer solution involves applying a potential difference between the polymer solution and a collection surface to obtain an article or nonwoven fabric composed of nanofibers, the cross-section of which can range from nanometers to several micrometers. Electrodes connected to a positive (or negative) high-voltage power supply are introduced into a container containing the polymer solution and a capillary tube. The solution remains in droplet form at the end of the capillary tube due to its surface tension. Under the action of an electric field generated by the potential difference between the polymer solution and the collection surface, the hemispherical surface of the droplet is stretched to form a cone (called a Taylor cone). When the force generated by the electric field overcomes the surface tension, a jet of solution is ejected from the cone. As the jet flows from the end of the cone to the collection surface, the solvent is evaporated. The solidified nanofibers are deposited on the collection surface. The collection surface can be the surface of a collector consisting of a rotating drum covered with a conductive metal. The collector is grounded or connected to a negative high-voltage power supply. Alternatively, the collecting surface can be a thin film or pad placed as close as possible to the surface of the collector, located between the collector and the capillary. Thus, the deposited nanofibers combine to form a porous layer composed of nonwoven nanofibers. This method allows for the fabrication of nanofiber layers with controlled variable sizes, particularly nanofibers with diameters on the order of one-tenth or one-hundredth of a nanometer or micrometer. Those skilled in the art know how to adjust the parameters of the electrospinning process, particularly by varying the concentration of the polymer solution, the distance between the capillary tip and the collecting surface, the potential difference applied to the polymer solution, and the rotational speed of the roller, to control the diameter and orientation of the nanofibers. For example, it is known that high roller rotation speeds promote nanofiber orientation along a preferred direction (e.g., along the processing direction and substantially along the processing direction) and result in the formation of a strongly anisotropic layer in a plane defined by the processing direction and the transverse (or intersecting) direction, while low roller rotation speeds lead to the formation of nanofibers deposited in random directions. Those skilled in the art define the processing direction as the direction in which the layer travels forward during the fabrication process of stage a), with the transverse (or intersecting) direction perpendicular to the processing direction.

[0011] According to a first embodiment of the invention, the constituent nanofibers of the layer prepared in stage a) extend in a random direction. The ratio of the secant tensile modulus of the layer according to this first embodiment in the transverse or intersecting direction to its secant tensile modulus in the processing direction is typically greater than 0.5, preferably greater than 0.8.

[0012] According to a second embodiment of the invention, the layer prepared in stage a) has nanofibers oriented along a preferred direction, preferably along the processing direction or substantially along the processing direction. Therefore, the layer of the second embodiment of the method according to the invention exhibits anisotropy in a plane defined by the processing direction and the transverse or intersecting direction. It is well known that the tensile modulus of anisotropic layers varies depending on the direction considered. The ratio of the secant tensile modulus of the layer according to this second embodiment in the transverse or intersecting direction to its secant tensile modulus in the processing direction is typically 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 layers prepared in stage a), the secant tensile modulus in the transverse (or cross) direction and the processing direction was measured under the same conditions.

[0014] The thickness of the polyethersulfone nanofiber layer prepared in stage a) is preferably greater than or equal to 5 micrometers and less than or equal to 100 micrometers. The diameter of the nanofibers in the layer prepared in stage a) is preferably greater than or equal to 100 nanometers and less than or equal to 3000 nanometers, more preferably greater than or equal to 500 nanometers and less than or equal to 2500 nanometers. The term "diameter of nanofiber" should be understood as referring to the diameter of the cross-section of the nanofiber. In the case where the cross-section is not circular, the diameter is the maximum length of the cross-section.

[0015] According to a first variant of the invention, a monolayer of polyethersulfone nanofibers is prepared in stage a).

[0016] According to a second variant of the invention, multilayered polyethersulfone nanofibers are prepared in stage a), and then stacked on top of each other to form a module prior to stage b). According to this variant, stage b) is performed on the assembled nanofiber layers. The method for preparing the membrane according to the second variant comprises the following sequential stages: (a) Multilayer polyethersulfone nanofibers were prepared by electrospinning a polyethersulfone solution and the layers were stacked together to form a component; (b) Heating the nanofibers to soften them; (c) Cooling the nanofibers; (d) Impregnate the nanofibers with an ionomer to form a membrane.

[0017] According to a third variant of the invention, multilayered polyethersulfone nanofibers are prepared in stage a), and then stacked together between stages c) and d) to form a module. According to this variant, the method for preparing the membrane comprises the following sequential stages: (a) Multilayer polyethersulfone nanofibers were prepared by electrospinning a polyethersulfone solution; (b) Heating the nanofibers to soften them; (c) Cool the nanofibers and stack the layers together to form a component; (d) Impregnate the nanofibers with an ionomer to form a membrane.

[0018] The difference between the third variant and the second variant is that stages b) and c) are performed before the layers are superimposed on each other.

[0019] According to preferred embodiments of the second and third variants, the layers prepared in stage a) are superimposed on each other such that their respective processing directions are aligned in the component.

[0020] Preferably, the layer prepared in stage a) has nanofibers oriented along a preferred direction, preferably along the processing direction or substantially along the processing direction.

[0021] According to particularly preferred embodiments of the second and third variants, the layers prepared in stage a) have nanofibers oriented along a preferred direction (preferably along or substantially along the processing direction), and are stacked on top of each other such that their respective processing directions are aligned in the assembly. In other words, the assembly is prepared such that it remains anisotropic in a plane defined by the processing direction and the transverse (or intersecting) direction.

[0022] The term "multilayer" should be understood as referring to at least two layers, preferably two layers. The at least two layers can be prepared sequentially by performing a first electrospinning, followed by repeating stage a) until the desired number of layers is obtained. Alternatively, the at least two layers can be prepared by obtaining nanofibers for the first layer through electrospinning, and then cutting them to the desired size at the end of stage a) to obtain the desired number of layers.

[0023] Stage b) is the stage of softening the nanofibers. Softening the nanofibers leads to the formation of connection points between the fibers and their adjacent fibers. In other words, under the softening effect, connection points are formed between the nanofibers and their adjacent nanofibers; at these connection points, the nanofibers have fused and bonded together. The formation of connection points between the nanofibers and their adjacent nanofibers can improve the mechanical strength of the nanofiber layer. When stage b) is performed on the component prepared according to the second variant of the invention, the formation of connection points between the nanofibers and their adjacent nanofibers can improve the mechanical strength of the constituent layers of the component and the component itself.

[0024] The softening of the nanofibers is achieved by heating them, typically to a temperature that allows the polyethersulfone chains to move, and this temperature (in a manner known to those skilled in the art) is close to the glass transition temperature (Tg) of polyethersulfone, which is approximately 225°C. According to any embodiment of the invention, stage b) is preferably carried out at a temperature above 210°C and below 240°C. Stage b) can be carried out in air or under an inert atmosphere, such as nitrogen.

[0025] Stage b) is followed by stage c), which permanently establishes the structure of the layer or component obtained at the end of stage b), that is, maintains the state of the layer or component, provides connection points between adjacent nanofibers, and achieves mechanical strength properties. Typically, the nanofibers are cooled to a temperature at which the polyethersulfone chains no longer have mobility, and this temperature is below the glass transition temperature of polyethersulfone. Preferably, the nanofibers are cooled to a temperature close to ambient temperature, typically in the range of 20°C to 25°C, to facilitate further progress to stage d).

[0026] Stage d) involves impregnating nanofibers with an ionomer, the nanofibers existing in the form of layers or components. Impregnation can be performed by immersing the nanofibers in a solution or dispersion of the ionomer. The impregnation time and temperature are adjusted by those skilled in the art based on the thickness of the layer or component, the desired ionomer content in the layer or component, or the concentration of the ionomer solution or dispersion. Alternatively, impregnation can be achieved by coating the nanofibers, i.e., applying a solution or dispersion of the ionomer to the surface of the layer or component. The coating temperature is specifically selected by those skilled in the art based on the viscosity of the ionomer solution or dispersion and the boiling point of the solvent used in the ionomer solution or dispersion. Coating is preferably performed at a temperature close to ambient temperature, typically 20°C to 25°C, or at a temperature above ambient temperature but below the boiling point of the solvent used in the ionomer solution or dispersion.

[0027] The concentration of the ionomer solution or dispersion is adjusted by those skilled in the art specifically based on the solubility of the ionomer in the solvent used for the ionomer solution or dispersion, and the viscosity of the ionomer solution or dispersion. The solvent used for the ionomer solution or dispersion is selected by those skilled in the art specifically based on the solubility of the ionomer in the solvent and its boiling point. The solvent used for the ionomer solution or dispersion preferably has a relatively low boiling point, typically less than or equal to 100°C, so that it can be easily removed from the layer or assembly, particularly by evaporation under vacuum or under a flow of air or an inert gas (e.g., nitrogen or argon). According to any embodiment of the invention, a stage of solvent evaporation from the ionomer solution or dispersion is preferably performed after the impregnation stage.

[0028] The ionomer used to satisfy the requirements of this invention can be any polymer known as an ionomer. Its chemical structure is not limited as long as it possesses the ion exchange properties characteristic of ionomers. It typically contains cationic functional groups (e.g., quaternary ammonium functional groups) or anionic functional groups (e.g., sulfonate, sulfate, phosphonate, phosphate, or carboxylate functional groups) that participate in ion exchange between the two electrodes of an electrochemical device (e.g., a fuel cell or electrolyzer). The ionomer is typically a polymer comprising a hydrocarbon backbone with side groups that are ionic functional groups. This hydrocarbon chain may also be substituted with fluorinated groups or nonionic groups containing heteroatoms other than fluorine, or interrupted by heteroatoms other than fluorine. Thus, for example, styrene polymers or alkyl-substituted styrene polymers, tetrafluoroethylene polymers, etc., can be mentioned, these polymers having side groups containing ionic functional groups, preferably quaternary ammonium, sulfonate, sulfate, phosphonate, phosphate, or carboxylate functional groups. Preferably, the ionomer is a polymer containing styrene monomer units or alkyl-substituted styrene monomer units, or a polymer containing tetrafluoroethylene monomer units, the polymer having side groups containing ionic functional groups, preferably quaternary ammonium functional groups, sulfonate functional groups, sulfate functional groups, phosphonate functional groups, phosphate functional groups or carboxylate functional groups.

[0029] The method according to the invention, as described in any embodiment or variation thereof, enables the preparation of membranes with ion transport properties. Such membranes may be referred to as ion-conducting membranes.

[0030] Another subject of the invention is a membrane comprising polyethersulfone nanofibers, said polyethersulfone nanofibers being nonwoven and impregnated with an ionomer, wherein the nanofibers have connection points with their adjacent nanofibers. The nanofibers are typically obtained by electrospinning. The diameter of the nanofibers is preferably greater than or equal to 100 nanometers and less than or equal to 3000 nanometers, more preferably greater than or equal to 500 nanometers and less than or equal to 2500 nanometers. Preferably, the thickness of the membrane is greater than or equal to 10 micrometers and less than or equal to 200 micrometers.

[0031] The membrane according to the invention is typically composed of a porous support impregnated with an ionomer, said porous support being composed of polyethersulfone nanofibers. The constitutive ionomer and constitutive polyethersulfone of the membrane are those described in the method for preparing the membrane according to the invention. Preferably, the ionomer and polyethersulfone are the only constituent elements of the membrane. According to any embodiment of the invention, the ionomer is preferably a polymer containing styrene monomer units or alkyl-substituted styrene monomer units, or a polymer containing tetrafluoroethylene monomer units, said polymer having side groups containing ionic functional groups, preferably quaternary ammonium functional groups, sulfonate functional groups, sulfate functional groups, phosphonate functional groups, phosphate functional groups, or carboxylate functional groups.

[0032] The membrane can be prepared by the method according to the invention, i.e., any embodiment of the method according to the invention, including its variants (i.e., the first, second, and third variants of the method according to the invention). The membrane can consist of a single layer of polyethersulfone nanofibers or multiple layers of polyethersulfone nanofibers, preferably two layers stacked on top of each other. Preferably, the thickness of the polyethersulfone nanofiber layer is greater than or equal to 5 micrometers and less than or equal to 100 micrometers.

[0033] According to one embodiment of the invention, polyethersulfone nanofibers extend in a random direction within the membrane, and the membrane is preferably composed of a monolayer of polyethersulfone nanofibers. The membrane according to this embodiment can be prepared according to a first embodiment of the method of the invention in combination with a first variant of that method.

[0034] According to a preferred embodiment of the invention, the membrane is composed of multiple layers of polyethersulfone nanofibers, which are stacked on top of each other to form an assembly, with their respective processing directions aligned within the assembly. The membrane according to this preferred embodiment can be prepared according to a particularly preferred embodiment of a second or third variant of the method of the invention.

[0035] According to a particularly preferred embodiment of the invention, the membrane is composed of multiple layers of polyethersulfone nanofibers, preferably two layers, wherein the nanofibers in these layers are oriented along a preferred direction, preferably along or substantially along the processing direction, and these layers are stacked together to form an assembly in which their respective processing directions are aligned. The membrane of the invention, composed of multiple layers of nanofibers oriented along the preferred direction, can be prepared according to a particularly preferred embodiment of a second or third variant of the method according to the invention.

[0036] The membrane according to the invention is typically an ion-conducting membrane and is intended for use in electrochemical devices, which typically include two electrodes, an anode and a cathode. Electrochemical devices incorporating the membrane according to the invention are preferably fuel cells or electrolyzers.

[0037] The above and other features of the invention will be better understood by reading the following description of several illustrative embodiments of the invention.

[0038] Example Size exclusion chromatography (SEC): Size exclusion chromatography (SEC) allows the separation of macromolecules in solution based on their size using columns packed with porous gels. Macromolecules are separated according to their hydrodynamic volume, with the largest molecules eluting first. While not an absolute method, SEC allows for the determination of the molar mass distribution of polymers. Various weight-average molar masses (Mw) can be determined using commercial standards via the Moore calibration method. The commercial standards are for poly(2-vinylpyridine) products, with analyzable mass ranges between 890,000 g / mol and 458 g / mol.

[0039] No special treatment was performed on the polymer sample prior to analysis. The polymer sample was dissolved in dimethylformamide containing 0.1 M LiBr at a concentration of approximately 1 g / L, and then stirred at 50 °C for 2 hours. Injection analysis was then performed according to the conditions described in the table below. Membrane preparation: The two membranes M1 and M2 according to the present invention are prepared according to the following procedure: Polyethersulfone poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene) (CAS 25667-42-9, purchased from Goodfellow, trade number SU30-GL-000111, Mw 59 230 g / mol, poly(2-vinylpyridine) calibrated) was dissolved in DMSO to prepare a 25 wt% solution, thereby preparing a layer of polyethersulfone nanofibers. Dissolution was carried out at 90 °C. The solution was fed through a needle, and the nanofibers formed by electrospinning were deposited on the collecting roller of a rotating drum. The electrospinning parameters are as follows: One layer was prepared at a rotation speed of 200 rpm (rev / min), and two layers were prepared at a rotation speed of 2000 rpm.

[0040] Layer C1, prepared at 200 rpm, had a thickness between 30 and 40 μm, while layers C2 and C3, prepared at 2000 rpm, had thicknesses between 15 and 20 μm respectively. The thickness of the polyethersulfone nanofiber layers was measured using a mechanical comparator (Nikon Digimicro MFC-101A). The nanofiber dimensions were measured using a scanning electron microscope (SEM FEG GeminiSEM 360).

[0041] The polyethersulfone nanofibers in layer C1 extend in a random direction, while the polyethersulfone nanofibers in layers C2 and C3 are oriented in a preferred direction, particularly along the processing direction and substantially along the processing direction. The arrangement of these nanofibers in each layer (random arrangement in C1, oriented arrangement in C2 and C3) can be observed using a scanning electron microscope.

[0042] Two layers, C2 and C3, are stacked on top of each other to form component C2 / C3, so that their respective processing directions are aligned in the component.

[0043] The nanofiber layers, namely layer C1 and components C2 / C3, were laid flat and placed in a ventilated oven at 225°C for 3 hours (stage b). They were then removed from the oven and cooled to ambient temperature (23°C) (stage c). The connection points between the nanofibers and their adjacent nanofibers were confirmed by scanning electron microscopy. Figure 1 and Figure 2 As shown, these two images are scanning electron microscope images of nanofibers obtained at the end of stage c).

[0044] At the end of stage c), the secant tensile modulus of each layer was measured to determine the anisotropic properties of each layer prepared in stage a). A Discovery TA HR rheometer was used at 23°C and 0% relative humidity (RH) with a modulus of 0.17 s⁻¹. -1 The strain rate was measured, and the secant tensile modulus of each layer was measured for a 10 mm x 50 mm specimen.

[0045] A specimen is cut from the layer along the processing direction, which refers to the direction in which the layer travels forward during its preparation. The first nominal secant modulus (referred to as the processing direction modulus) is measured at 4% strain.

[0046] Another specimen is cut from the layer, its length along a direction perpendicular to the processing direction (transverse or cross direction). The second nominal secant modulus (referred to as the transverse or cross direction modulus) is measured at 4% strain.

[0047] For layers C2 and C3, the ratio of the lateral or cross modulus to the machining modulus is less than 0.5 (in this example, the ratio for C2 and C3 is 0.03).

[0048] For layer C1, the ratio of the modulus in the lateral or cross direction to the modulus in the machining direction is equal to 1.46.

[0049] Subsequently, layers C1 and components C2 / C3 of nanofibers were impregnated with the ionomer Nafion® by coating them with a dispersion of Nafion® D2021CS (dispersion concentration of 20% by weight) according to the following coating procedure, thereby preparing membranes M1 and M2 according to the invention, respectively.

[0050] To prepare membrane M1, an 18-micron dispersion as the first layer was first deposited on a fluorinated ethylene propylene film (FEP substrate) using a membrane applicator (bar coating equipment), followed by the deposition of a C1 layer on top of this first layer. After drying at 100°C for 1 hour and 30 minutes, a 500-micron dispersion as the second layer was deposited at ambient temperature using a system employing "slit-die coating" technology. The resulting membrane M1 was then dried at 100°C.

[0051] Membrane M2 was prepared using the same procedure, except that layer C1 was replaced with component C2 / C3.

[0052] The third membrane M3 was prepared using the same coating procedure as membranes M1 and M2, except that a layer of polyethersulfone nanofibers was not deposited. Membrane M3 is not conforming to the present invention, as it consists solely of the perfluorinated substance "Nafion".

[0053] A fourth membrane, M4, was prepared using the same coating procedure as membrane M1, except that the nanofiber layer was replaced with a membrane made of expanded polytetrafluoroethylene (ePTFE). Membrane M4 is not in accordance with the present invention, as all its components are perfluorinated materials (“Nafion” and PTFE).

[0054] Characterization of membranes M1 to M4: To determine the ionic conductivity of the prepared membrane, the electrochemical impedance on the membrane plane was measured at 30 °C and 30% relative humidity, with the measured parameters being an amplitude change of 50 mV and an applied potential of 0 V.

[0055] Using a Discovery TA HR rheometer, at 23°C and 0% relative humidity (RH), the rheometer was used to measure the rheometry at a rate of 0.17 s⁻¹. -1 The strain rate was measured, and the secant tensile modulus was measured on a 10 mm x 50 mm specimen.

[0056] A sample is cut from the membrane along the processing direction, which is the direction in which the layer travels forward during its fabrication. The length of the sample is along the direction in which the nanofiber layer travels during its fabrication (processing direction). The first nominal secant modulus at 4% strain is measured and is called the processing direction modulus.

[0057] Another sample is cut from the membrane, its length along a direction perpendicular to the processing direction (transverse or cross direction). The second nominal secant modulus at 4% strain is measured, referred to as the transverse or cross direction modulus.

[0058] The result used is the average of two moduli, namely the sum of the processing direction modulus and the transverse or cross direction modulus divided by 2, which is called the average of the 4% modulus. The film thickness is measured using a mechanical comparator.

[0059] The properties of the membrane are shown in Table 1.

[0060] Table 1 Compared to membrane M4 (which consists of a carrier and an ionomer, both of which are perfluorinated), membranes M1 and M2 according to the present invention achieve the best trade-off between mechanical strength properties, ionic conductivity properties and the proportion of perfluorinated material.

Claims

1. A method for preparing a membrane, the method comprising the following sequential stages: (a) A layer of polyethersulfone nanofibers was prepared by electrospinning a polyethersulfone solution; (b) Heating the nanofibers to soften them; (c) Cooling the nanofibers; (d) Impregnate the nanofibers with an ionomer to form a membrane.

2. The method according to claim 1, wherein, The polyethersulfone solution is a solution in dimethyl sulfoxide.

3. The method according to claim 1 or 2, wherein, In stage a), a monolayer of polyethersulfone nanofibers is prepared.

4. The method according to claim 1 or 2, wherein, Multilayer polyethersulfone nanofibers are prepared in stage a) and stacked on top of each other to form a component before stage (b).

5. The method according to claim 1 or 2, wherein, Multilayer polyethersulfone nanofibers are prepared in stage a), and then stacked on top of each other between stages c) and d) to form a component.

6. The method according to claim 4 or 5, wherein, The layers prepared in stage a) are stacked on top of each other such that their respective processing directions are aligned in the assembly.

7. The method according to any one of claims 4 to 6, wherein, Each layer prepared in stage a) has nanofibers oriented along a preferred direction.

8. The method according to any one of claims 1 to 6, wherein, The constituent nanofibers of the layer prepared in stage a) extend in a random direction.

9. The method according to any one of claims 1 to 8, wherein, The ionomer is a polymer containing styrene monomer units or alkyl-substituted styrene monomer units, or a polymer containing tetrafluoroethylene monomer units. The polymer has side groups containing ionic functional groups, preferably quaternary ammonium functional groups, sulfonate functional groups, sulfate functional groups, phosphonate functional groups, phosphate functional groups, or carboxylate functional groups.

10. A membrane comprising nonwoven polyethersulfone nanofibers impregnated with an ionomer, wherein the nanofibers have connection points with their adjacent nanofibers, the membrane being obtainable by the method according to any one of claims 1 to 9.

11. The membrane according to claim 10, wherein, Polyethersulfone nanofibers and ionomers are the only constituent elements of the membrane.

12. The membrane according to claim 10 or 11, wherein the membrane is composed of multiple layers of polyethersulfone nanofibers, the layers being stacked on top of each other to form an assembly, the respective processing directions of which are aligned in the assembly.

13. The membrane according to any one of claims 10 to 12, wherein, Polyethersulfone nanofibers extend in random directions.

14. The membrane according to any one of claims 10 to 12, wherein the membrane is composed of multiple layers of polyethersulfone nanofibers, each layer having nanofibers oriented along a preferred direction.

15. An electrochemical device comprising the membrane as described in any one of claims 10 to 14, wherein the device is a fuel cell or an electrolyzer.

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