Formation of a microporous MPL layer on the surface of an active layer for an electrochemical converter

Abstract

Formation of a microporous layer MPL on the surface of an active layer for an electrochemical converter The invention relates to a method for forming an electroconductive and hydrophobic microporous layer (MPL), on the surface of an active layer intended for an electrochemical converter, comprising at least the following steps: (a) providing a non-aqueous dispersion, called "ink", comprising at least one carbon-based particulate material and at least one organic solvent; (b) forming a deposit of said ink on the surface of said active layer; and (c) evaporating said solvent(s) to form said microporous layer MPL, step (c) being carried out simultaneously and / or subsequently to step (b); wherein said ink comprises at least one poly(vinylidene fluoride-co-hexafluoropropene) copolymer, denoted PVDF-HFP, in solution in said organic solvent.It also relates to the ink for the preparation of such a microporous layer; a multilayer structure comprising at least one active layer supported by a solid electrolyte membrane and being in contact, at its face opposite said solid membrane, with a microporous layer obtained by the method of the invention, a membrane / electrode assembly (MEA), comprising such a multilayer structure, as well as their use in a cell of an electrochemical converter, in particular in a PEMFC.

Description

Title of the invention: Formation of a microporous MPL layer on the surface of an active layer for an electrochemical converter Technical field

[0001] The present invention relates to the preparation of a microporous layer (MPL), electrically conductive and hydrophobic, useful in the field of manufacturing electrochemical converters, in particular fuel cells, in particular proton exchange membrane fuel cells, and polymer membrane electrolysers.

[0002] More particularly, the invention relates to a new method for forming a microporous layer directly on the surface of an active layer in the context of the preparation of a membrane-electrode assembly intended for an electrochemical converter. Prior art

[0003] Proton Exchange Membrane Fuel Cells (PEMFCs) are electrochemical energy conversion devices considered a promising energy source for transportation applications. Although this technology has been at the forefront of commercialization efforts over the past two decades, especially by automobile manufacturers, breakthroughs are still needed to meet all cost and durability specifications.

[0004] The operating principle of a PEMFC cell is based on the conversion of chemical energy into electrical energy by catalytic reaction of hydrogen and oxygen. A cell comprises at least one cell, but more generally a series stack of several cells, to meet the needs of the applications. Each cell comprises a membrane-electrode assembly (better known by the acronym "MEA"), commonly called the cell core, which constitutes the basic element of PEMFCs.

[0005] In the heart of the battery, all the phenomena giving rise to the energy conversion take place. As in any electrochemical system, it is made up of two electrodes, an anode and a cathode, separated by an electrolyte. In the case of the PEMFC, the latter is a polymer membrane with a thickness of between 10 and 20 μm. The electrodes are made up of two main parts: an active layer (or CL for "catalyst layer" in English), the site of the electrochemical reactions catalyzed by platinum (5 to 15 μm thick), and a diffusion layer (GDL for "Gas Diffusion Layer" in English), with a thickness between 150 and 300 pm.

[0006] This diffusion layer has a significant impact on both the performance and durability of the PEMFC, due to its role in all transport phenomena occurring within the core of the cell. Thus, the diffusion layer serves to collect current, supply reactive gas, but also to eliminate water and heat produced within the core of the cell.

[0007] To meet all of these requirements, and in particular to allow good water elimination, it has been proposed to include a carbon-based microporous layer, called "MPL" (for "Microporous layer" in English terminology), between the catalytic layer and the gas diffusion layer.

[0008] Several alternatives for the formation of an MPL at the level of a membrane-electrode assembly are implemented to date. The most common technique, represented in [Fig. l](b) (extract from publication [2]) consists of depositing the layer to form the MPL at the level of a GDL, then the MPL+GDL assembly undergoes a hydrophobic heat treatment at very high temperature, before being assembled at the level of a CCM structure (“Catalyst Coated Membrane” in English terminology, designating the assembly of an electrolyte membrane coated on each of its opposite faces with a catalytic layer), with or without hot pressing.

[0009] Another technique, used in research, consists of preparing a self-supporting MPL. The microporous layer is thus produced separately on an inert substrate, from which it can be separated after heat treatment. The production of a self-supporting MPL, separate from the GDL, advantageously allows only the nature of the GDL to be varied during the tests. However, this method is not suitable for an industrial approach because the self-supporting MPL layers have very low mechanical strength, which makes their handling very delicate.

[0010] In the context of either of these techniques, the presence of interfacial spaces between the active layer and the MPL is inevitable, in particular due to the roughness of the surfaces of the active layer and the MPL. However, these spaces are detrimental to the performance of the cell, in particular because water tends to accumulate there, in particular for high operating current densities.

[0011] To overcome this disadvantage, Daniel et al. [1] propose a method for preparing the MPL directly on the active layer, by spraying a dispersion (more commonly called "ink"), formed from the mixture of a solution of PTFE AF 1600 (poly[4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole-co-tetrafluoroethylene]) in a fluorinated solvent, such as FC-72 or Fluorinert® FC-40, a solution of carbon black, and dilution with isopropanol.

[0012] This technique, however, has several drawbacks, in particular to enable its implementation on an industrial scale. On the one hand, it requires, for the solubilization of the polymer, the use of a fluorinated solvent, which is undesirable due to its toxicity.

[0013] On the other hand, such a method does not allow the ink to be deposited by a technique other than spraying. In fact, the ink is based on isopropanol, a solvent that reacts with the materials of the active layers. Deposition of the ink by spraying allows rapid evaporation of the solvent and a reduced contact time of the surface of the active layer with the solvent. On the other hand, with this method, deposition of the ink by coating is not possible due to the risk of damaging the active layer.

[0014] Finally, this method requires carrying out, after the deposition of the layer, a sintering step, a heat treatment requiring a high temperature, in order to make the layer hydrophobic.

[0015] Thus, there remains a need to be able to have a simplified preparation method for a microporous layer (MPL) directly on the surface of the active layer in the context of the preparation of a membrane-electrode assembly intended for an electrochemical converter, for example a PEMFC, making it possible to overcome the aforementioned constraints.

[0016] In particular, there remains a need to have a method for preparing an MPL directly on the surface of the active layer, without risk of damaging the latter, which can involve the deposition of the ink by spraying or coating, while making it possible to access an MPL having the required physicochemical properties, in particular good hydrophobicity properties and good mechanical strength. Summary of the invention

[0017] The invention aims precisely to meet these expectations.

[0018] More particularly, the invention relates, according to a first of its aspects, to a method for forming a microporous layer (MPL), electrically conductive and hydrophobic, on the surface of an active layer intended for an electrochemical converter, said method comprising at least the following steps: (a) having a non-aqueous dispersion, called “ink”, comprising at least one carbonaceous particulate material and at least one organic solvent; (b) forming a deposit of said ink on the surface of said active layer; and (c) evaporating said solvent(s) to form said microporous MPL layer, step (c) being carried out simultaneously and / or subsequently to step (b); wherein said ink comprises at least one poly(vinyl fluoride) copolymer nylidene-co-hexafluoropropene), noted PVDF-HFP, in solution in said organic solvent.

[0019] To the inventors' knowledge, it has never yet been proposed to form a microporous layer using a PVDF-HFP copolymer.

[0020] The invention also relates, according to another of its aspects, to a non-aqueous dispersion or ink, for the preparation of a microporous layer intended for an electrochemical converter, said ink comprising at least: - at least one carbonaceous particulate material dispersed in at least one organic solvent; and - at least one poly(vinylidene fluoride-co-hexafluoropropene) copolymer, denoted PVDF-HFP, solubilized in said organic solvent.

[0021] It also relates to the use of an ink as defined previously to form a microporous layer directly on the surface of an active layer in the context of the preparation of a membrane-electrode assembly intended for an electrochemical converter, for example a PEMFC.

[0022] By "non-aqueous dispersion" is meant a dispersion of solid particles in one or more organic solvents, and not containing water or failing that containing a very small quantity of water, in particular less than 1% by mass, in particular less than 0.1% by mass.

[0023] Advantageously, the use of a PVDF-HFP copolymer according to the invention allows the use of a wide range of solvents capable of solubilizing said polymer.

[0024] Advantageously, the process for preparing an MPL according to the invention makes it possible to avoid the use of fluorinated solvent.

[0025] Advantageously, the ink for forming the MPL uses one or more organic solvents which are inert with respect to the active layer on the surface of which the MPL layer is formed.

[0026] By “inert” solvent is meant, in the context of the present invention, a solvent which is non-reactive with respect to the material of the active layer, and consequently not capable of damaging or degrading said active layer with which it is brought into contact.

[0027] The use according to the invention of an organic solvent inert with respect to the active layer advantageously allows the deposition of the ink for the preparation of the MPL layer, directly on the surface of the active layer, and by any deposition technique, in particular by spraying, but also by coating, without risk of deterioration of the active layer.

[0028] Thus, unlike the process described by Daniel et al. [1] which, given the solvent of the ink used, isopropanol, reactive with the active layer, can only implement the application of the ink by spraying so as to allow very rapid evaporation of the solvent and to minimize the contact time with the active layer, the ink according to the invention is suitable for application by any deposition technique, in particular by spraying or by coating.

[0029] Preferably, the ink for the formation of the MPL comprises a single organic solvent, allowing both the dispersion of the carbonaceous particulate material(s) and the solubilization of the PVDF-HFP.

[0030] In a preferred embodiment, the ink comprises, as organic solvent, in particular as sole organic solvent, ethyl acetate.

[0031] Furthermore, advantageously, the preparation of a microporous layer according to the invention does not require a sintering step. As indicated previously, in the methods usually proposed for forming the MPL layer, the sintering step is necessary in order to modify the crystalline structure of the polymer of the MPL layer so as to achieve the desired hydrophobicity for the MPL. On the other hand, the preparation of an MPL layer based on a PVDF-HFP copolymer according to the invention makes it possible to obtain a layer having good hydrophobicity without needing to resort to a sintering step.

[0032] Thus, as detailed in the rest of the text, the preparation of an ink, and the formation of an MPL according to the method of the invention, are particularly easy.

[0033] For the purposes of the invention, a layer is said to be “hydrophobic” if the external surface of its constituent material is such that a drop of water deposited thereon does not spread. In particular, the liquid / gas interface of the drop of water forms a contact angle with the surface greater than 90°. This hydrophobic character is of course also reproduced throughout the thickness of the layer, in particular in the porosity of the material. It can in particular be controlled by XPS analysis of the surface of a cross-section of this layer.

[0034] Advantageously, the microporous layer formed according to the invention combines good hydrophobicity properties and good mechanical properties. The method of the invention makes it possible in particular to obtain homogeneous, flexible and crack-resistant microporous layers.

[0035] The invention also relates, according to another of its aspects, to a multi-layer structure, useful for the preparation of a membrane / electrode assembly intended for an electrochemical converter, for example a PEMFC cell, comprising at least one active layer supported by a solid electrolyte membrane; and more particularly belonging to a catalytically coated membrane called CCM (“Catalyst Coated Membrane”); said active layer being in contact, at its face opposite said solid membrane, with a microporous layer (MPL), electroconductive and hydrophobic, obtained by a method of the invention, as defined previously, in particular obtained by deposition on the surface of said active layer of an ink according to the invention.

[0036] As indicated previously, the microporous layer is formed according to the invention in the context of the manufacture of a membrane / electrode assembly, called AME, intended for an electrochemical converter, for example a PEMFC.

[0037] Thus, according to another of its aspects, the invention relates to the use of a multilayer structure as defined previously for the preparation of a membrane / electrode assembly intended for an electrochemical converter, for example a PEMFC.

[0038] The invention also relates to a membrane / electrode assembly, called AME, intended for an electrochemical converter, for example a PEMFC, comprising a multilayer structure according to the invention, said assembly comprising more particularly the following stack: GDL / MPL / CCM / MPL / GDL, GDL designating diffusion layers, typically of the carbon substrate type, for example carbon paper, impregnated with PTFE; MPL designating microporous layers; CCM (“catalyst coated membrane”) designating an electrolyte membrane coated on either side with a cathodic catalytic layer (CCL) and an anodic catalytic layer (ACL for “anode catalyst layer”); at least one of the MPL layers being formed by a method according to the invention, as defined previously, using an ink as defined previously.

[0039] Advantageously, the formation of the MPL directly on the surface of an active layer, in particular at the level of the CCM structure, makes it possible to vary the nature of the GDL, in order to optimize the membrane / electrode assembly.

[0040] The invention also relates, according to another of its aspects, to the use of a multilayer structure according to the invention as defined previously or of a membrane / electrode assembly according to the invention as defined previously, in a cell of an electrochemical converter, in particular in a fuel cell and more particularly in a proton exchange membrane fuel cell (PEMFC).

[0041] Other characteristics, variants and advantages of the formation of an MPL according to the invention, of its implementation at the level of a membrane / electrode assembly for an electrochemical converter, will emerge more clearly on reading the description, examples and figures which follow, given for illustrative and non-limiting purposes of the invention.

[0042] In the remainder of the text, the expressions “between ... and ...”, “ranging from ... to ...” and “varying from ... to ...” are equivalent and are intended to mean that the limits are included, unless otherwise stated. Brief description of the drawings

[0043] [Fig.l] is a prior art diagram (extract from publication [2]) showing different membrane-electrode assembly (MEA) architectures: (a) MPL-free AME, (b) MPL with a commercial MPL, (c) MPL with a modified MPL on CCM and (d) MPL with a double MPL;

[0044] [Fig.2] shows a photograph ([Fig.2](a)) and a schematic representation ([Fig.2](b)) of the device used for coating the ink for preparing the MPL layer according to example 1;

[0045] [Fig.3] shows images obtained by scanning electron microscopy (SEM) of the complete AME assembly formed in example 1 (GDL / MPL / CCM / MPL / GDL, the upper MPL layer being a layer formed according to the invention), for different magnifications;

[0046] [Fig.4] schematically shows the production of the reference AMEs, formed in a conventional manner by assembling MPL / GDL structures at each of the active layers of a CCM structure ([Fig.4](a)); and the production of the AMEs according to the invention, formed by assembling on the cathode side a GDL at the CCM / MPL structure formed according to the invention integrating an MPL formed according to the invention directly on the surface of the cathode active layer of the CCM ([Fig.4](b)).

[0047] [Fig.5] represents the polarization curves (voltage (V) as a function of current density (A / cm2)) of the cells obtained with each of the AMEs in example 2. Detailed description INK FOR PREPARING MPL

[0048] As indicated previously, the invention is based on the formation of the microporous layer, referred to in the remainder of the text as "MPL", directly on the surface of the active layer, from a non-aqueous dispersion, referred to as "ink", comprising at least one carbon-based particulate material and at least one poly(vinylidene fluoride-co-hexafluoropropene) copolymer, referred to in the remainder of the text as PVDF-HFP, solubilized in at least one organic solvent.

[0049] PVDF-HFP copolymer

[0050] A PVDF-HFP copolymer used according to the invention more particularly has the following structure (I):

[0051] [Chem.l] FF CF 3 PVDF-HFP (I) x corresponding to the average number of monomeric units derived from vinylidene fluoride and y the average number of monomeric units derived from hexafluoropropene.

[0052] The sequence of monomeric units derived from vinylidene fluoride and hexafluoropropene in PVDF-HFP may be random, of the monoblock or multiblock type, preferably monoblock or multiblock. The HFP units are preferably grafted onto the chain ends of the PVDF polymer.

[0053] According to a particular embodiment, a PVDF-HFP suitable for the invention advantageously has a number-average molecular mass Mn of between 300 g.mol1 and 600 g.mol *. The number-average molar mass can be measured by size exclusion chromatography (or SEC). It can also be obtained from the 'H NMR analysis of the (co)polymer obtained.

[0054] The PVDF-HFP copolymers can be synthesized by methods known to those skilled in the art, or even be commercially available.

[0055] By way of example, a PVDF-HFP copolymer suitable for the invention may be marketed under the reference PVDF-HFP Solef® 21216 by SOLVAY.

[0056] Said PVDF-HFP copolymer(s) may be used in the ink used for the formation of the MPL layer at a rate of 0.3 to 5% by mass, in particular 0.4% to 2% by mass, relative to the total mass of the ink.

[0057] Organic solvent

[0058] As regards the organic solvent, it is chosen so as to solubilize the PVDF-HFP copolymer and to disperse the said carbonaceous particulate material(s).

[0059] In particular, the PVDF-HFP copolymer can be solubilized in said organic solvent at a rate of at least 2% by mass, in particular at a content of 2% to 5% by mass.

[0060] Advantageously, as mentioned above, the use of a PVDF-HFP copolymer according to the invention, in particular compared to PTFE as used in the process described by Daniel et al. [1], allows the use of a wide range of solvents capable of solubilizing the polymer.

[0061] Also, advantageously, the said organic solvent(s) used according to the invention are distinct from fluorinated solvents, the latter being undesirable for reasons of toxicity.

[0062] The organic solvent of the ink may be chosen in particular from acetone, acetonitrile, ethyl acetate, butanone (MEK), tetrahydrofuran (THF), dimethylacetamide (DMAC), α,α-dimethylformamide (DMF), and mixtures thereof; preferably from acetone, acetonitrile, ethyl acetate, butanone, tetrahydrofuran (THF) and mixtures thereof.

[0063] Advantageously, the method of the invention uses an organic solvent which is inert with respect to the active layer on the surface of which the MPL is intended to be formed.

[0064] As mentioned previously, the use according to the invention of an organic solvent inert with respect to the active layer advantageously allows the deposition of the ink for the preparation of the MPL layer, directly on the surface of the active layer, and by any deposition technique.

[0065] Thus, unlike the method described by Daniel et al. [1] which, given the solvent of the ink used, isopropanol, reactive with the active layer, can only implement the application of the ink by spraying so as to allow very rapid evaporation of the solvent and reduce to a minimum the contact time with the active layer, the ink according to the invention is suitable for application by any deposition technique, in particular by spraying but also by coating.

[0066] According to a particular embodiment of the invention, the organic solvent used, in addition to dissolving the polymer, is inert with respect to the active layer on the surface of which the ink is intended to be deposited.

[0067] Preferably, the ink uses, as organic solvent, in particular as sole organic solvent, ethyl acetate.

[0068] Said organic solvent(s), in particular ethyl acetate, may represent from 70 to 90% by mass, in particular from 80 to 85% by mass, of the total mass of the ink.

[0069] Carbonaceous particulate material

[0070] The ink comprises at least one carbonaceous particulate material dispersed in said organic solvent(s).

[0071] The carbonaceous particulate material is dedicated to giving the MPL layer its electrically conductive properties. It also makes it possible to increase the thermal conductivity allowing the heat produced in the fuel cell to be evacuated.

[0072] Generally speaking, the carbonaceous particulate material has an average particle size of less than one millimeter, in particular less than 5 pm and more particularly less than 100 nm, in particular between 20 nm and 50 nm.

[0073] The average particle size can be assessed by electron microscopy at scanning.

[0074] It is understood that the nature of the carbon material(s) used in the ink, in particular the average particle size of the carbon material(s) used, is adjusted with regard to the means chosen for depositing the ink on the surface of the active layer.

[0075] In particular, when the ink is deposited by spraying, the said carbonaceous particulate material(s) must have a particle size adapted to the spraying device, in particular adapted to the diameter of the nozzle of the spraying device, to avoid obstruction of the nozzle.

[0076] In particular, in the case of an ink intended to be deposited by spray, said carbon-based particulate material(s) advantageously have an average particle size less than or equal to 5 μm and more particularly between 20 and 100 nm.

[0077] Said carbonaceous particulate material(s) may be chosen from carbon black, activated carbon, graphite, carbon nanotubes, carbon nanofibers, ground carbon fibers, and mixtures thereof, preferably from carbon black, carbon nanofibers, in particular produced in the vapor phase, and mixtures thereof.

[0078] In particular, the ink may comprise a single type of particulate carbon material, or a mixture of at least two particulate carbon materials.

[0079] The particulate carbon material may comprise at least carbon black, for example sold under the brand name Vulcan XC72R® with a 99% dry extract marketed by the company TANAKA.

[0080] The particulate carbon material may comprise carbon nanofibers.

[0081] Advantageously, these carbon fibers are produced in the vapor phase. In particular, these fibers may comprise graphitized carbon. They are generally characterized by a length of 1 to 50 μm and preferably 5 to 25 μm. The carbon fibers produced in the vapor phase advantageously make it possible to increase the thermal and electrical conductivities while limiting the number of cracks forming during the drying of the MPL layer.

[0082] For example, it may be carbon nanofibers, marketed under the name VGCF® of 99% dry extract by the company Showa Denko.

[0083] According to a particular embodiment, the ink comprises a mixture of carbon black and carbon nanofibers, in particular produced in the vapor phase.

[0084] The carbonaceous particulate material(s) may be used at a rate of 2 to 7% by mass, in particular 2 to 5% by mass, relative to the total mass of the ink.

[0085] Preparation of the ink for the MPL layer

[0086] The contents of the various components of the ink, in particular of said PVDF-HFP copolymer(s) and of said carbonaceous particulate material(s), in said organic solvent(s), are adjusted in order to obtain an MPL layer combining porosity suitable for gas diffusion and optimal product transport, low electrical resistivity, satisfactory mechanical stability and satisfactory thermal conductivity.

[0087] In particular, the ink for preparing the MPL layer may comprise from 2% to 7%, in particular from 2 to 5% by mass of carbonaceous particulate material(s), from 0.3% to 5%, in particular from 0.4 to 2% by mass of PVDF-HFP and from 70% to 90%, in particular from 80 to 85% by mass of organic solvent(s), in particular as defined above, the solvent preferably being ethyl acetate.

[0088] The PVDF-HFP copolymer content in the ink can vary between 5% and 10% by mass relative to the total mass of the ink, expressed as dry extract.

[0089] In a particular embodiment, said carbonaceous particulate material(s) and PVDF-HFP copolymer may be used in a carbonaceous material(s) / PVDF-HFP mass ratio ranging from 2 to 6.

[0090] The ink according to the invention can be obtained by mixing, in said organic solvent(s), in particular in ethyl acetate, PVDF-HFP and said carbon-based particulate material(s).

[0091] Preferably, the PVDF-HFP is previously solubilized in the organic solvent, in particular in ethyl acetate, before being combined with the other components.

[0092] Preferably, the ink is obtained by mixing, in particular in this order, said particulate carbon material(s), PVDF-HFP, preferably previously solubilized in an organic solvent, in particular in ethyl acetate, and the organic solvent.

[0093] Preferably, the mixture is dispersed.

[0094] Thus, the preparation of the ink is easy and advantageously requires few steps; it can thus be obtained beforehand by (i) solubilizing the PVDF-HFP in the organic solvent, preferably in ethyl acetate, (ii) mixing with the said carbon-based particulate material(s) in the organic solvent, then (iii) dispersing the mixture.

[0095] The components may for example be dispersed with a mechanical disperser, for example of the rotor-stator type. Preferably, the dispersion is prepared using a vacuum disperser. In particular, before the dispersion step, the different carbonaceous particulate materials are first mixed, then the PVDF-HFP is added, and finally the organic solvent, in particular ethyl acetate.

[0096] The dispersion obtained can preferably then be subjected to stirring, for example using a roller tube type stirrer. Examples of stirrers that may be mentioned include: roller tube type agitators. Zirconium beads, for example with a diameter between 2 mm and 3 mm, for example 3 mm in diameter, can be added to the dispersion.

[0097] The ink prepared according to the invention advantageously exhibits good dispersion of the carbonaceous particulate materials in the organic solvent, in particular ethyl acetate, in which the PVDF-HFP copolymer is dissolved. FORMATION OF THE MPL LAYER

[0098] As indicated previously, the preparation of the MPL layer according to the invention involves the formation of a deposit of said ink, in particular as defined previously, directly on the surface of the active layer, and the evaporation of the organic solvent(s), to form the MPL layer.

[0099] The active layer on the surface of which the MPL layer according to the invention is formed may be a cathodic catalytic layer, called "CCL" for "Cathode catalyst layer" in English terminology, or an anode catalytic layer, called "ACL" for "Anode catalyst layer" in English terminology.

[0100] The active layer is more particularly supported by a solid membrane, in particular a solid electrolyte membrane, the deposition of the ink in step (b) being carried out on the face of said active layer opposite the solid membrane.

[0101] The active layer on the surface of which the ink according to the invention is deposited may thus more particularly belong to a catalytic coated membrane, called CCM ("Catalyst Coated Membrane" in English terminology), designating the assembly of a membrane coated on each of its opposite faces with a catalytic layer (active layer).

[0102] The CCM membrane used can be chosen from those commonly used for the preparation of electrochemical converters, in particular fuel cells and polymer membrane electrolysers.

[0103] As mentioned previously, the ink can be deposited advantageously by coating or by spraying.

[0104] Advantageously, the deposition is carried out so as to control the thickness of the deposition on the surface of the active layer.

[0105] The coating can be carried out, for example, with a roller, a scraper or a knife.

[0106] Advantageously, the deposition of the ink is carried out at a temperature between 60 and 80°C, in particular between 70 and 80°C.

[0107] The evaporation of the organic solvent, in particular of ethyl acetate, can be carried out simultaneously with the deposition of the ink, in particular during deposition by spraying, and / or after deposition of the ink, in particular during deposition by coating. Drying after deposition can be carried out for example for a few minutes, for example from 1 to 15 minutes, in particular from 1 to 10 minutes, for example approximately 5 minutes.

[0108] Preferably, when the ink is deposited by coating, the structure comprising the active layer superimposed on a membrane, in particular the CCM membrane, is fixed, for example using an adhesive strip, advantageously on a rigid support, for example on a polytetrafluoroethylene (PTFE) substrate, in order to avoid a phenomenon of retraction of the membrane in the presence of a large quantity of solvent.

[0109] The evaporation of said organic solvent(s), in particular ethyl acetate, may be carried out by heating to a temperature less than or equal to 80°C, in particular between 60 and 80°C, in particular between 70 and 80°C.

[0110] Advantageously, as mentioned above, the formation of the MPL layer according to the invention does not require any sintering step. Sintering is understood to mean a heat treatment at a temperature above the melting temperature of the polymer present in the MPL layer. This sintering step is generally necessary, for example in the case of the use of PTFE, in order to modify the crystalline structure of the polymer and achieve the desired hydrophobicity of the MPL layer.

[0111] In the context of the present invention, PVDF-HFP makes it possible to provide the MPL layer formed according to the invention with the necessary hydrophobicity without resorting to a sintering step.

[0112] The MPL layer formed according to the invention on the surface of an active layer advantageously has a thickness of between 30 pm and 70 pm, in particular between 40 pm and 60 pm, even more particularly between 45 pm and 55 pm.

[0113] As explained in the examples which follow, the method of the invention advantageously makes it possible to access MPL layers of thinner thickness than commercial MPL layers. A reduction in thickness is likely to allow a reduction in the resistance to oxygen transport, and thus to increase the performance of the electrochemical converter. ELECTROCHEMICAL CONVERTER

[0114] As mentioned previously, the method of the invention makes it possible to prepare MPL layers in the context of the manufacture of a membrane / electrode assembly, known as AME, for an electrochemical converter, in particular for proton exchange membrane fuel cells (PEMFC) or polymer membrane electrolysers.

[0115] The invention thus aims, according to another of its aspects, at the use of a multi-layer structure according to the invention comprising an MPL layer formed according to the invention, for the preparation of a membrane / electrode assembly intended for an electrochemical converter, for example a PEMFC.

[0116] A membrane / electrode assembly (MEA) according to the invention comprising an MPL layer formed according to the invention more particularly comprises the following stack: GDL / MPL / CCM / MPL / GDL, GDL designating gas diffusion layers, for example of carbon substrate type (e.g. carbon non-woven, carbon cloth, carbon felt, carbon paper, etc.) impregnated with PTFE; MPL designating microporous layers; CCM designating a membrane coated on either side with a cathodic catalytic layer (CCL) and an anodic catalytic layer (ACL for “anode catalyst layer”); at least one of the MPL layers being a layer formed according to the invention, in particular both MPL layers being layers formed according to the invention.

[0117] The preparation of a membrane / electrode assembly (MEA) according to the invention includes in particular the implementation, at the level of the face of the MPL layer formed according to the invention, opposite the face in contact with the active layer, of a GDL layer.

[0118] The GDL layer can be simply bonded to the MPL layer, without requiring hot pressing.

[0119] The invention will now be described by means of the following examples, given of course for illustrative and non-limiting purposes of the invention. EXAMPLE 1

[0120] Formation of the microporous MPL layer according to the invention directly on an active layer and integration into a complete membrane / electrode assembly (MEA)

[0121] Preparation of ink for the manufacture of MPL

[0122] The following raw materials were used: - VGCF® carbon fibers with 99% dry extract marketed by Showa Denko; - Vulcan XC72® carbon black with 99% dry extract marketed by TANAKA; - polymer: poly(vinylidene fluoride / hexafluoropropene) or PVDF-HFP (marketed under the reference PVDF-HFP Solef® 21216 by SOLVAY), solubilized at 2% by weight in ethyl acetate; - solvent: ethyl acetate.

[0123] The ink for the formation of the MPL was prepared from these raw materials, by mixing in the following order and in the amounts indicated in Table 1 below.

[0124] [Tables 1] Order of introduction Component Mass in g 1 Vukan carbon 0.438 2 VGCF (carbon nanofibers) 0.6 3 2%wt of PVDF-HFP in ethyl acetate HT 4 Ethyl acetate 14.4

[0125] The solution was then dispersed in a DISPERMAT® pot for 30 minutes at 1000 rpm. Then, the pot was then placed on a roller tube type agitator after adding 1 / 3 of the ink volume of 3mm diameter zirconium beads to the mixture. The zirconium beads are added directly to the mixture. Thus, by placing the pot on the roller tubes, they allow shear to be added as for a classic ball mill ("bail milling") but much gentler.

[0126] Formation of MPL by coating of ink

[0127] The coating was carried out the same day on a coating table equipped with a porous suction and heating support.

[0128] The ink is applied by coating at the cathodic side of a CCM (“Catalyst coated membrane”) comprising catalytic layers on either side of a membrane (solid electrolyte), marketed under the reference Gore® A510.1 / M735.18 / C580.4, i.e. at the level of the cathodic active layer.

[0129] To avoid shrinkage of the membranes when in contact with a large amount of solvent, the CCM is fixed on a rigid 250 μm PTFE support during the coating of the ink on the catalytic layer.

[0130] The coating was then carried out with the following parameters: • Knife height: 250 pm • Speed: 1 cm / s • Table temperature: 70°C.

[0131] A thin film of ethylene polynaphthalate (PEN) 50 μm thick is added over the TLC, before coating, in order to delimit the deposition zone, as shown schematically in [Fig.2].

[0132] The deposit is dried after coating at 70°C for approximately 5 minutes. The temperature corresponds more particularly to the set temperature of the heating plate on which the CCM membrane is deposited, the CCM membrane thus being at a slightly lower temperature.

[0133] Integration of the CCM / MPL according to the invention at the level of a complete AME

[0134] A complete membrane / electrode assembly (MEA) is then formed by applying a GDL formed from a carbon fiber substrate impregnated with 5% by mass of PTFE, marketed under the reference Sigracet® GDL 25 BA at the level of the MPL formed at the level of the cathodic active layer; and, on the anodic side, a GDL / MPL assembly, commercially available under the reference Sigracet® GDL 25 BC, formed from a carbon fiber substrate impregnated with 5% by mass of PTFE and covered with a standard MPL (PTFE and carbon black).

[0135] In both cases the GDL, with or without MPL, is attached to the CCM during the assembly of FAME in the cell without hot pressing.

[0136] [Fig.4](b) schematically represents the production of the AME according to the invention integrating the CCM / MPL assembly produced according to the invention.

[0137] Results

[0138] The microporous layer obtained on the surface of the cathodic active layer has good adhesion to the active layer. The layer is flexible and does not crack during handling of the TLC.

[0139] Observation, in section, by scanning electron microcopy (SEM) of the complete AME assembly thus formed ([Fig.3]) indicates that the microporous MPL layer formed according to the invention has a thickness of the order of 50 μm.

[0140] The deposit has a homogeneous thickness, and the active layer is intact and has therefore not undergone any deterioration during the direct formation of the MPL according to the invention.

[0141] The method of the invention thus makes it possible to obtain MPL layers that are thinner than commercial layers, generally of the order of 80 μm in thickness.

[0142] Obtaining a thinner layer is not a disadvantage. On the contrary, this reduction in thickness is likely to advantageously reduce the resistance to oxygen transport. EXAMPLE 2 Differential cell evaluation

[0143] For comparison, a standard AME is also studied, formed from the assembly of diffusion layers marketed under the reference Sigracet® GDL 25 BC, each formed from a carbon fiber substrate impregnated with 5% by mass of PTFE and covered with a standard MPL, on either side of the CCM marketed under the reference Gore® 735.18.

[0144] The two AMEs tested are shown in [Fig.4].

[0145] Each AME is reproduced twice ((B1) and (B2) designating the reference AMEs; and (B3), (B4) the AMEs according to the invention.

[0146] The AMEs produced are tested in a PEMFC type differential cell with a surface area of ​​1.8 cm2.

[0147] The AME is first conditioned for 6 hours by applying a voltage of 0.7 V. The cell is heated to 80°C under H2 on the anode side and air on the cathode side at a relative pressure of 1.5 bars on each side. The gases are at a relative humidity of 80% and a stoichiometry of 20 at the anode and 30 at the cathode. The polarization curves are produced at the end of the conditioning under the same conditions as this one, i.e. 80°C; 1.5 bars; 80% RH, H2 / air - stoichiometry 20-30. They are voltage controlled and the scanning is carried out starting from the OCV up to 0.1 V then back to the OCV at a scanning speed of 10 mV / s.

[0148] [Fig.5] represents the polarization curves (voltage (V) as a function of current density (A / cm2)), of the cells obtained with each of the AMEs (Bl), (B2), (B3) and (B4).

[0149] The performances of the microporous layer (MPL) prepared according to the invention are very close to the reference, confirming the effectiveness of the preparation method according to the present invention. Reference

[0150] [1] Daniel et al, 2021, J. Electrochem. Soc. 168 104513; [2] Daniel et al., New CCLIMPL Architecture Reducing Interfacial Gaps and Enhancing PEM Fuel Cell Performance of FUEL CELLS, Volume 20, 2020, No. 2, 224-228.

Claims

Claims

1. A method for forming an electroconductive and hydrophobic microporous layer (MPL) on the surface of an active layer intended for an electrochemical converter, comprising at least the following steps: (a) providing a non-aqueous dispersion, called "ink", comprising at least one carbon-based particulate material and a single organic solvent; (b) forming a deposit of said ink on the surface of said active layer; and (c) evaporating the solvent to form said microporous layer MPL, step (c) being carried out simultaneously and / or subsequently to step (b); wherein said ink comprises at least one poly(vinylidene fluoride-co-hexafluoropropene) copolymer, denoted PVDF-HFP, in solution in said organic solvent, the single organic solvent being ethyl acetate.

2. Method according to the preceding claim, in which said PVDF-HFP copolymer has a number-average molecular mass Mn of between 300 g.mol1 and 600 g.mol *.

3. Method according to any one of the preceding claims, in which said carbonaceous particulate material(s) have an average particle size of less than one millimeter, in particular less than 5 pm and more particularly less than 100 nm, in particular between 20 nm and 50 nm.

4. Method according to any one of the preceding claims, in which said carbonaceous particulate material(s) are chosen from carbon black, activated carbon, graphite, carbon nanotubes, carbon nanofibers, ground carbon fibers, and mixtures thereof, preferably from carbon black, carbon nanofibers, in particular produced in the vapor phase, and mixtures thereof.

5. A method according to any preceding claim, wherein said ink comprises from 2% to 7% by weight of carbonaceous particulate material(s), from 0.3% to 5% by weight of PVDF-HFP copolymer and from 70% to 90% by weight of organic solvent(s).

6. A method according to any preceding claim, wherein said ink is previously obtained by (i) solubilizing said PVDF-HFP copolymer in said organic solvent (ii) mixing with said carbonaceous particulate material(s) in said organic solvent, then (iii) dispersing said mixture.

7. Method according to any one of the preceding claims, in which said active layer is supported by a solid electrolyte membrane, in particular said active layer belongs to a catalytically coated membrane, called CCM; the deposition of the ink in step (b) being carried out on the face of said active layer opposite the solid membrane.

8. Method according to any one of the preceding claims, in which the deposition in step (b) of the ink is carried out by coating or by spraying, in particular at a temperature between 60 and 80°C, in particular between 70 and 80°C.

9. Method according to any one of the preceding claims, wherein said MPL layer has a thickness of between 30 pm and 70 pm, in particular between 40 pm and 60 pm, even more particularly between 45 pm and 55 pm.

10. Ink for the preparation of a microporous layer (MPL) intended for an electrochemical converter, said ink comprising at least: - at least one carbonaceous particulate material dispersed in a single organic solvent; and - at least one poly(vinylidene fluoride-co-hexafluoropropene) copolymer, denoted PVDF-HFP, solubilized in said organic solvent, the single organic solvent being ethyl acetate.

11. Ink according to the preceding claim, said ink being as defined in any one of claims 2 to 6.

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