Ion-conducting membrane and method for preparing same by electrospinning a polyethersulfone

Abstract

The invention relates to a method for preparing a membrane which comprises the following steps, in succession: preparing a layer of polyethersulfone nanofibres by electrospinning a polyethersulfone solution; heating the nanofibres to soften them; cooling the nanofibres; and impregnating the nanofibres with an ionomer to form the membrane. A membrane of this kind consists of polyethersulfone nanofibres that have points at which they join to the nanofibres adjacent to them. The membrane has ionic conductivity properties and can be used in a fuel cell or an electrolyser.

Description

[0001] Ion-conducting membrane and its preparation process by electrospinning of a polyethersulfone.

[0002] The field of the present invention is that of methods of manufacturing ion-conducting membranes for use in electrochemical devices such as fuel cells and electrolysers.

[0003] The core of a fuel cell and an electrolyzer consists of two electrodes, an anode and a cathode, an electrolyte layer that separates the two electrodes, and a catalyst that is located at the interfaces of the electrolyte layer with each of the electrodes. 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 electrolyte 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 are subject to dimensional changes which can be caused by stresses resulting from the differential pressure between the two electrodes or for example by swelling phenomena caused by variations in the humidity level in the device. To ensure their two functions, physical barrier and ion conduction, the membranes must have mechanical resistance properties and ion conduction properties. Among the membranes which satisfy these two requirements, we can cite those which have as a porous support an expanded polytetrafluoroethylene, known as "ePTFE", and as an ionomer "Nafion" which is also a perfluorinated polymer carrying sulfonate functions.However, there is a concern to reduce the use of perfluorinated substances in many application areas, including electrochemical devices, for environmental reasons.

[0004] The inventors have discovered a process that makes it possible to produce a membrane that offers a good compromise between mechanical strength properties, ionic conductivity properties and the content of perfluorinated substances.

[0005] Thus, a first object of the invention is a process for preparing a membrane which comprises the following successive steps:

[0006] (a) preparing a layer of polyethersulfone nanofibers by electrospinning a polyethersulfone solution,

[0007] (b) heating the nanofibers to soften them,

[0008] (c) cooling the nanofibers,

[0009] (d) impregnate the nanofibers with an ionomer to form the membrane.

[0010] Another subject of the invention is a membrane, obtainable by the method 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, which membrane is obtainable by the method according to the invention. The invention also relates to an electrochemical device which comprises a membrane according to the invention.

[0011] Detailed description of the invention

[0012] The polymers mentioned in the description may be of fossil or bio-sourced origin. 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 in use, that is to say, they may be, partially or totally, derived from a recycling process, or obtained from raw materials themselves derived from a recycling process.

[0013] 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 the polyethersulfone are of formula (CHsChS). The polyethersulfone is typically a commercially available product, generally in the form of granules, for example from BASF, Solvay, Sumitomo under the respective trade names Ultrason® E, Varadel®, Sumikaexcel®. It is used without chemical modification prior to its use in the membrane preparation process. The polyethersulfone preferably has a weight average molar mass (Mw) greater than 10000 g / mol and less than 250000 g / mol, more preferably greater than 20000 g / mol and less than 200000 g / mol, even more preferably greater than 30000 g / mol and less than 150000 g / mol.Mw values ​​are measured by SEC RI (size exclusion chromatograph coupled to a differential refractometer with poly(2-vinyl pyridi ne) calibration).

[0014] 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 lower than or equal to the boiling point of the solvent. Suitable solvents are any solvents known to solubilize polyethersulfones, 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 for 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 the use of the membrane in a fuel cell or an electrolyzer which generally operates in the presence of water. The polyethersulfone solution is prepared at a concentration which is adjusted by a person skilled in the art in particular as a function of the weight-average molar mass of the polyethersulfone and the nature of the solvent in order to be able to be used in an electrospinning device to form nanofibers. The polyethersulfone solution preferably has a concentration greater than 5% and less than 30%, the percentages being mass percentages calculated relative to the total mass of the polyethersulfone solution.

[0015] In step a) the polyethersulfone nanofibers are formed by electrospinning the polyethersulfone solution. Electrospinning is a process well known to those skilled in the art for forming nanofibers. The process of electrospinning a polymer solution consists of applying an electrical potential difference between a polymer solution and a collecting surface to obtain nonwoven articles or fabrics consisting of nanofibers with a cross-section that can range from the order of a nanometer to several micrometers. An electrode connected to a positive (or negative) high voltage source is introduced into the polymer solution contained in a container equipped with a capillary. The solution is maintained, by its surface tension, in the form of a drop at the end of the capillary.Under the effect of the electric field generated by the electric 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 is evaporated during the jet's path between the tip of the cone and the collecting surface. Solidified nanofibers are deposited on the collecting surface. The collecting surface can be the surface of a collector on a rotating drum covered with an electrically conductive metal. The collector is connected to ground or to a negative high voltage source. Alternatively, the collecting surface can be the surface of a film or liner placed as close as possible to the collector surface, between the collector and the capillary.All the deposited nanofibers thus form a porous layer made up of non-woven nanofibers. This process makes it possible to produce layers of nanofibers of controlled variable dimensions, and in particular nanofibers whose diameter is in the order of magnitude 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 of the nanofibers and the orientation of the nanofibers by varying in particular the concentration of the polymer solution, the distance between the end of the capillary and the collecting surface, the difference in electrical potential applied to the polymer solution and the rotation speed of the drum.For example, it is known that a high rotational speed of the drum promotes the orientation of the nanofibers in preferential directions, such as in the production direction and substantially in the production direction, and leads to the formation of a layer having a high anisotropy in the plane formed by the directions defined by the production direction and the cross direction, whereas a low rotational speed of the drum 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 being the direction of travel of the layer during its manufacturing process in step a), the cross direction (in English "Transverse Direction" or "Cross Direction") being perpendicular to the production direction.

[0016] 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 secant tensile modulus of the layer in the cross direction to its secant tensile modulus in the production direction which is greater than 0.5, preferably greater than 0.8.

[0017] According to a second embodiment of the invention, the layer prepared in step a) has nanofibers oriented in preferential directions, preferentially in the production direction or substantially in the production direction. The layer according to the second embodiment of the method according to the invention therefore has anisotropy in the plane formed by the directions defined by the production direction and the cross direction. In a well-known manner, 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 secant tensile modulus of the layer in the cross direction to its secant tensile modulus in the production direction which is less than 0.5, preferentially less than 0.4, more preferentially less than 0.3, even more preferentially less than 0.1.

[0018] To characterize the anisotropy of the layer prepared in step a), the secant tensile moduli in the cross direction and in the production direction respectively are determined under the same conditions.

[0019] The layer of polyethersulfone nanofibers prepared in step a) preferably has a thickness greater than or equal to 5 μm and less than or equal to 100 μm. 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 mean 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.

[0020] According to a first variant of the invention, a single layer of polyethersulfone nanofibers is prepared in step a).

[0021] According to a second variant of the invention, several layers of polyethersulfone nanofibers are prepared in step a) and are superimposed on each other to form an assembly before step b). According to this variant, step b) is carried out on the layers once assembled. The method for preparing a membrane according to the second variant comprises the following successive steps:

[0022] (a) preparing several layers of polyethersulfone nanofibers by electrospinning a polyethersulfone solution, superimposing the layers on top of each other to form an assembly,

[0023] (b) heating the nanofibers to soften them,

[0024] (c) cooling the nanofibers,

[0025] (d) impregnate the nanofibers with an ionomer to form the membrane.

[0026] According to a third variant of the invention, several layers of polyethersulfone nanofibers are prepared in step a) and are superimposed on each other between step c) and step d) to form an assembly. According to this variant, the method for preparing a membrane comprises the following successive steps:

[0027] (a) preparing multiple layers of polyethersulfone nanofibers by electrospinning a polyethersulfone solution,

[0028] (b) heating the nanofibers to soften them,

[0029] (c) cool the nanofibers, superimpose the layers on each other to form an assembly,

[0030] (d) impregnating 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.

[0031] According to a preferred embodiment of the second variant and the third variant, the layers prepared in step a) are superimposed on each other in such a way that their respective production directions are aligned in the assembly.

[0032] Preferably, the layers prepared in step a) have nanofibers oriented in preferential directions, preferentially in the production direction or substantially in the production direction.

[0033] According to a particularly preferred embodiment of the second variant and the third variant, the layers prepared in step a) have nanofibers oriented in preferential directions, preferentially in the production direction or substantially in the production direction, and are superimposed on each other in such a way that their respective production directions are aligned in the assembly. In other words, the assembly is carried out so as to maintain an anisotropy in the assembly in the plane formed by the directions defined by the production direction and the cross direction.

[0034] By several layers is meant at least two layers, preferably two layers. The at least two layers can be prepared successively by electrospinning a first time, then 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 desired dimensions to obtain the desired number of layers.

[0035] 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 each other. 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 produced according to the second variant 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.

[0036] The softening of the nanofibers is carried out by heating the nanofibers, typically at a temperature which allows the mobility of the polyethersulfone chains and which is, in a manner known to those skilled in the art, close to the glass transition temperature (Tg) of the polyethersulfone, the Tg of the polyethersulfone being approximately 225°C. According to any one of the embodiments of the invention, step b) is preferably carried out at a temperature above 210°C and below 240°C. Step b) may be carried out in air or under an inert atmosphere, for example under nitrogen.

[0037] Step b) is followed by step c) to freeze the configuration of the layer or assembly obtained at the end of step b), i.e. 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 which no longer allows mobility of the polyethersulfone chains and which is lower than 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 more easily proceed to step d).

[0038] Step d) involves impregnating the nanofibers with an ionomer, the nanofibers being in the form of a layer or an assembly. The impregnation may be carried out by immersing the nanofibers in a solution or dispersion of the ionomer. The immersion time and temperature are adjusted by a person skilled in the art according to the thickness of the layer or assembly, the desired content of the ionomer in the layer or assembly, or the concentration of the solution or dispersion of the ionomer. Alternatively, the impregnation may be carried out by coating the nanofibers, i.e., by applying a solution or 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 as a function of the viscosity of the solution or dispersion of the ionomer, the boiling point of the solvent of the solution or 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 ionomer solution or dispersion.

[0039] The concentration of the ionomer solution or dispersion is adjusted by a person skilled in the art, in particular as a function of the solubility of the ionomer in the solvent of the ionomer solution or dispersion and as a function of the viscosity of the ionomer solution or dispersion. The solvent of the ionomer solution or dispersion is chosen by a person skilled in the art, in particular as a function of the solubility of the ionomer in said solvent and as a function of its boiling point. The solvent of 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, in particular by evaporation under vacuum, under a stream of air 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 step of evaporation of the solvent from the solution or dispersion of the ionomer.

[0040] The ionomer useful for the purposes of the invention may be any polymer known to be an ionomer. Its chemical structure is not limited as long as it has the ion exchange properties which are specific to ionomers. It typically carries ionic, cationic functions such as quaternary amine functions or anionic functions such as sulfonate, sulfate, phosphonate, phosphate, carboxylate functions, ionic functions which 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 which comprises a main hydrocarbon chain carrying pendant groups which are the ionic functions, the hydrocarbon chain being able to also be substituted by fluorinated groups or by non-ionic groups carrying heteroatoms other than fluorine or else be interrupted by heteroatoms other than fluorine.In this respect, mention may be made, for example, of styrene or alkyl-substituted styrene polymers, 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 monomer units of styrene or alkyl-substituted styrene or a polymer comprising monomer 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.

[0041] The method according to the invention described according to any of the embodiments of the invention and any of its variants, allows the preparation of a membrane which has the properties of transporting ions. Such a membrane can be described as an ion-conducting membrane.

[0042] The membrane, another subject of the invention, comprises non-woven polyethersulfone nanofibers impregnated with an ionomer, which nanofibers have junction points with those adjacent to them. The nanofibers are typically obtained by electrospinning. 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 μm and less than or equal to 200 μm.

[0043] The membrane according to the invention is typically a membrane consisting of a porous support impregnated with an ionomer, the porous support being constituted by 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 monomer units of styrene or styrene substituted by an alkyl or a polymer comprising monomer 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.

[0044] The membrane may be prepared by the process according to the invention according to any of the described embodiments relating to the process according to the invention, including in their variants which are the first variant, the second variant and the third variant described of the process according to the invention. The membrane may consist of a single layer of polyethersulfone nanofibers or of several layers of polyethersulfone, preferably two layers, which are superimposed on each other. Preferably, a layer of polyethersulfone nanofibers has a thickness greater than or equal to 5 μm and less than or equal to 100 μm. According to one embodiment of the invention, the polyethersulfone nanofibers extend in random directions in the membrane and the membrane preferably consists of a single layer of polyethersulfone nanofibers.A membrane according to this embodiment can be prepared according to the first embodiment of the method according to the invention combined with the first variant of the method.

[0045] According to a preferred embodiment of the invention, the membrane is made up of several layers of polyethersulfone nanofibers, the layers being superimposed on each 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 variant or the third variant of the process according to the invention.

[0046] 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 on each 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 variant or the third variant of the process according to the invention.

[0047] The membrane according to the invention is typically an ion-conducting membrane and is intended to be used in an electrochemical device typically comprising two electrodes, respectively an anode and a cathode. The electrochemical device which comprises the membrane according to the invention is preferably a fuel cell or an electrolyser.

[0048] The above-mentioned characteristics of the present invention, as well as others, will be better understood upon reading the following description of several exemplary embodiments of the invention, given for illustrative purposes.

[0049] Examples

[0050] 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 being eluted first. Although not an absolute method, SEC allows us to understand the molar mass distribution of a polymer. Using commercial standard products, the various weight-average molar masses (Mw) can be determined using a so-called MOORE calibration. The commercial standard products are poly(2-vinyl pyridine), and the range of analyzable masses is between 890,000 g. mol 1 and 458 g. mol ~ 1. There is no particular treatment of the polymer sample before analysis. It is dissolved at a concentration of approximately 1 g / L in dimethylformamide with 0.1 M of Li Br, then stirred for 2 hours at 50°C before injection for analysis according to the conditions described in the table below.

[0051] Preparation of membranes:

[0052] Two membranes M1 and M2 in accordance with the invention are prepared according to the following procedure:

[0053] Polyethersulfone nanofiber layers are prepared from a 25% by mass solution of polyethersulfone poly(oxy-1,4-phenylensulfonyl-1,4 phenylene (CAS 25667-42-9, purchased from Goodfellow under the commercial reference SU30-GL-000111, Mw of 59230 g / mol, calibration poly(2-vinylpyridine)) in DMSO. Solubilization is carried out at 90°C. The solution is fed through a needle and the nanofibers formed by electrospinning are deposited on a collector roll of a rotating drum. The electrospinning parameters are as follows:

[0054] One layer is prepared with a rotation speed of 200 revolutions per minute (rpm), two layers are prepared with a rotation speed of 2000 rpm.

[0055] The Cl layer prepared with a rotation speed of 200 rpm has a thickness between 30 and 40 μm, the two C2 and C3 layers prepared with 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 size of the nanofibers is measured by scanning electron microscopy, SEM, (SEM FEG GeminiSEM 360).

[0056] The polyethersulfone nanofibers of the Cl layer extend in random directions, whereas the polyethersulfone nanofibers of the C2 and C3 layers are oriented in preferential directions, notably in the production direction and substantially in the production direction. These arrangements of the nanofibers in the layers, random for Cl and oriented for C2 and C3, can be observed by scanning electron microscopy.

[0057] 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.

[0058] The nanofiber layers, layer C1 and assembly C2 / C3, are kept flat and placed in a ventilated oven at 225°C for 3 hours (step b) in 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 Figure 1 and Figure 2, scanning electron microscopy images of the nanofibers obtained at the end of step c).

[0059] The measurement of the secant tensile 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 secant tensile moduli of the layers are measured on specimens of dimension 10 mm x 50 mm with a rheometer "Discovery TA HR" at 23°C and 0% relative humidity (RH), the strain rate being 0.17 s -1 .

[0060] A specimen is cut from the layer, the length of the specimen being in the production direction, the production direction being the direction of travel of the layer during its production. A first nominal secant modulus is measured at 4% deformation, called the production direction modulus.

[0061] Another specimen is cut from the layer, the length of the specimen being in the direction perpendicular to the production direction (cross direction). A second nominal secant modulus is measured at 4% deformation, called the cross direction modulus.

[0062] 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).

[0063] In the case of layer Cl, the ratio of the cross-direction modulus to the production direction modulus is equal to 1.46.

[0064] The layer of nanofibers C1 and the assembly C2 / C3 are then impregnated with an ionomer, Nafion®, by coating them with a dispersion of Nation® D2021CS (dispersion at 20% mass concentration) according to the coating procedure described below to respectively manufacture the membranes M1 and M2 in accordance with the invention.

[0065] For the manufacture of the Ml membrane, a first 18 μm layer of the dispersion is deposited on a fluorinated ethylene propylene film (FEP substrate) with a film applicator, a “bar-coater” instrument, then the Cl layer is deposited on this first layer. After drying at 100°C for 1h30, a second 500 μm layer of the dispersion is deposited at room temperature with a system using the “slot die coating” technology. The resulting membrane Ml is dried at 100°C.

[0066] The M2 membrane is prepared using the same procedure except that the C2 / C3 assembly is used instead of the Cl layer.

[0067] A third membrane, M3, is produced using the same coating procedure as membranes M1 and M2, except that no layer of polyethersulfone nanofibers is deposited. Membrane M3 is a membrane not in accordance with the invention and is made solely of a perfluorinated substance, “Nafion”.

[0068] A fourth membrane, M4, is produced using the same coating procedure as membrane M1, except that an expanded polytetrafluoroethylene (ePTFE) film is used instead of the nanofiber layer. Membrane M4 is a membrane not in accordance with the invention and all its constituent elements are perfluorinated substances, "Nafion" and PTFE.

[0069] Characterizations of membranes Ml to M4:

[0070] To determine the ionic conductivity of the prepared membrane, its electrochemical impedance across the membrane plane is measured at 30°C and 30% relative humidity, the measurement parameters being an amplitude variation of 50 mV, an applied potential of 0V.

[0071] The tensile secant moduli are measured on specimens of dimensions 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 s -1 .

[0072] A specimen is cut from the membrane, the length of the specimen being in the production direction, the production direction being the direction of travel of the layer during its production, the length of the specimen being in the direction of advancement of the layer of nanofibers during its production (production direction). A first nominal secant modulus is measured at 4% deformation, called the production direction modulus.

[0073] Another specimen is cut from the membrane, the length of the specimen being in the direction perpendicular to the production direction (cross direction). A second nominal secant modulus is measured at 4% deformation, called the cross direction modulus.

[0074] The result used is the average of the two modules, namely the sum of the production direction module and the cross direction module divided by 2, known as the average of the modules at 4%. The thickness of a membrane is measured using a mechanical feeler comparator.

[0075] The characteristics of the membranes are shown in Table 1.

[0076] Table 1 The membranes M1 and M2 in accordance with the invention are the membranes which present the best compromise between the mechanical resistance properties, the ionic conductivity properties and the proportion of perfluorinated substances, compared to the membrane M4 which is constituted by a support and an ionomer both perfluorinated.

Claims

Claims 1. Process for preparing a membrane which comprises 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) impregnate the nanofibers with an ionomer to form the membrane.

2. Method according to claim 1 in which the polyethersulfone solution is a solution in dimethyl sulfoxide.

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

4. Method according to claim 1 or 2 in which several layers of polyethersulfone nanofibers are prepared in step a) and are superimposed on each other to form an assembly before step b).

5. Method according to claim 1 or 2 in which several layers of polyethersulfone nanofibers are prepared in step a) and are superimposed on each other between step c) and step d) to form an assembly.

6. Method according to claim 4 or 5 in which the layers prepared in step a) are superimposed on each other in such a way that their respective production directions are aligned in the assembly.

7. Method according to any one of claims 4 to 6 in which the layers prepared in step a) have nanofibers oriented in preferential directions.

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

9. Method according to any one of claims 1 to 8 in which the ionomer is a polymer comprising monomer units of styrene or styrene substituted by an alkyl or a polymer comprising monomer 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.

10. Membrane which comprises non-woven polyethersulfone nanofibers impregnated with an ionomer, which nanofibers have junction points with those adjacent to them, which membrane is capable of being obtained by the method defined in any one of claims 1 to 9.

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

12. Membrane according to claim 10 or 11, which 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, which membrane consists of several layers of polyethersulfone nanofibers, the layers having nanofibers oriented in preferential directions.

15. An electrochemical device which comprises a membrane according to any one of claims 10 to 14, which device is a fuel cell or an electrolyser.

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