Method for manufacturing a bipolar plate made of carbon fibers

Abstract

A method for manufacturing a bipolar plate comprising a step of superposing, along a stack axis (A), a first release film (4), at least one carbon reinforcing film (2), at least one thermosetting resin film (3) and a second release film (4), in order to form a stack (1), and a step of pressurizing the stack (1) in a compression system, the pressurizing step being carried out at a predetermined forming pressure and a predetermined forming temperature for a second predetermined time. The carbon reinforcing film (2) is a nonwoven reinforcing film (2) comprising a plurality of reinforcing fibers (21), each reinforcing fiber (21) extending along an orientation axis (F), the orientation axis (F) of at least 10% to 60% of the reinforcing fibers (21) is oriented along the stack axis (A).

Description

Manufacturing process of a carbon fiber bipolar plate

[0001] The present invention relates to the field of electrochemical devices and in particular aims at a method of manufacturing bipolar plates made of composite material, intended to be mounted in an electrochemical device.

[0002] An electrochemical device is any device capable of carrying out an electrochemical reaction, such as a fuel cell or a proton exchange membrane electrolyser, capable of generating electrical energy or hydrogen respectively from a redox reaction. The term "electrochemical device" also refers to a redox flow battery capable of generating electrical energy from potential energy stored in the battery.

[0003] In a known manner, an electrochemical device comprises a stack of a plurality of cells extending along a stack axis and two end plates, placed at the ends of the stack. The end plates are connected by compression members which make it possible to compress the cells and ensure the sealing of the electrochemical device.

[0004] With reference to the, representing an electrochemical device 100, each cell 110 comprises, in known manner, a membrane-electrode assembly 120 and two bipolar plates 130, also called separator plates, which sandwich the membrane-electrode assembly 120 and allow the distribution of fluids in the cell 110. To form the electrochemical reaction in the electrochemical device, each cell 110 is supplied with an oxidizing fluid and a reducing fluid, for example dihydrogen and dioxygen, which circulate on either side of a membrane and react by means of a catalyst. Each cell 110 is also supplied with heat transfer fluid, used for the thermal regulation of the electrochemical device.In practice, two adjacent bipolar plates 130A, 130B of two adjacent cells 110A, 110B are secured together so as to form several internal channels 140 which allow the passage of the heat transfer fluid between the cells 110.

[0005] With reference to Figures 2 and 3, a bipolar plate 130 comprises, in a known manner, a plurality of openings 131, which allow the entry and exit of the oxidizing and reducing fluids, and two inlet and outlet openings 132 for the heat transfer fluid. Each bipolar plate 130 further comprises an active central portion 133, in contact with the membrane-electrode assembly and on which the oxidation-reduction reaction occurs, and a peripheral portion 134 allowing the joining of two bipolar plates 130.

[0006] The active portion 133 comprises concave portions 135 and convex portions 136 (shown in the showing a sectional view of the bipolar plate 130 along a plane A:A shown in the) which allow the circulation of fluids between the different inlet / outlet openings 131, 132. The concave portions 135 of two bipolar plates 130 are secured together so that the convex portions 136 form the internal channels 140 (shown in the) for the circulation of the heat transfer fluid.

[0007] In the prior art, bipolar plates are known made of graphite or metal (for example stainless steel, inconel, aluminum or titanium) covered with a protective coating in order to limit the impact of corrosion due to the electrochemical device. However, such bipolar plates have many drawbacks. Graphite bipolar plates are heavy and have a large footprint due to their thickness, which is generally greater than 2 mm. Metal bipolar plates have a significant mass and a limited service life, given the acidic and corrosive environment of the electrochemical devices in which they are mounted, despite the protective coating used.

[0008] Bipolar plates are increasingly made of composite material. A bipolar plate is known for this purpose, comprising conductive elements dispersed in a resin. The conductive elements are of the carbon black, crushed carbon fibers, graphite, expanded graphite, carbon nanotubes, or graphene type, for example. The resin is generally a thermoplastic or thermosetting polymer. When forming such a bipolar plate, the viscous resin is mixed with a very large number of conductive elements (generally more than 85% of conductive elements for the entire bipolar plate, to ensure high conductivity). The bipolar plate is formed by injecting the resin loaded with conductive elements into a mold or molded by pressing in a thermocompression press.

[0009] However, the massive addition of conductive elements significantly increases the viscosity of the thermosetting or thermoplastic resin, making the mixture thick. Also, the manufacturing processes described above do not allow the manufacture of thin bipolar plates and the bipolar plate often has a significant thickness, generally greater than 2 mm, which increases the size and mass of the electrochemical device. This is a disadvantage for an electrochemical device intended to be integrated, for example, in an aircraft or in any other mobility application. In addition, injection or thermocompression molding leaves a surface layer of resin, which may be due to the use of mold release agents for example, and which increases the electrical contact resistance, which is detrimental for an electrochemical conversion device application where this value must be minimized.In order to reduce the electrical contact resistance, it is then necessary to implement a post-treatment of the bipolar plate, by chemical treatment, or mechanical treatment, such as abrasion for example.

[0010] Thus, in the prior art, a bipolar plate is known in composite material comprising reinforcing fibers (for example carbon fibers) impregnated with thermoplastic or thermosetting polymer resin. Such a bipolar plate has the advantage of being thinner (less than 1 mm thick), which makes it possible to limit its size and mass.

[0011] However, in such a bipolar plate, the thermoplastic resin forms a residual layer on the surface similar to an insulating skin, which can affect the electrical conductivity. It is then necessary to expose the reinforcing fibers on the surface of the bipolar plate. Such treatment, carried out for example by sanding or dissolution by plasma irradiation, can damage the reinforcing fibers and affect the mechanical properties of the bipolar plate, which presents a significant drawback. Also known from document WO2016182131A1 is the use of a sacrificial film allowing the reinforcing fibers to be exposed on the surface of the bipolar plate, without damaging them. The manufacture of bipolar plates is then carried out by thermo-compression, allowing the manufacture of thin and light bipolar plates.However, in document WO2016182131A1, the manufacturing method does not allow a bipolar plate to be produced simply and quickly using carbon fiber reinforcement.

[0012] In addition, the use of reinforcing fibers requires a very high forming pressure (generally greater than 15 MPa), to allow the fibers to be properly impregnated, to reduce porosities and to expose the surface fibers to allow good electrical conductivity. High pressure can deform the initial weave of the reinforcement and generate the formation of porous cavities which can alter the impermeability of the bipolar plates to gases.

[0013] The invention thus aims to eliminate at least some of these drawbacks by proposing a method for manufacturing a bipolar plate that is simple, rapid and does not require the use of expensive industrial machines, allowing the formation of a thin and light bipolar plate. PRESENTATION OF THE INVENTION

[0014] The invention relates to a method for manufacturing a bipolar plate, the bipolar plate being intended to be mounted in an electrochemical device, the electrochemical device being configured to implement an electrochemical reaction, the method comprising:a step of superimposing, along a stacking axis Z, a first demolding film, at least one reinforcing film, at least one thermoplastic resin film and a second demolding film, in order to form a stack,a step of positioning the stack in a compression system,a step of pressurizing the stack in the compression system, the pressurizing step being carried out at a predetermined forming pressure and at a predetermined forming temperature for a second predetermined duration,so as to melt the thermoplastic resin of the thermoplastic resin film and impregnate the reinforcing film with the thermoplastic resin to form a bipolar plate.,

[0015] The manufacturing method is remarkable in that the reinforcing film is a non-woven carbon reinforcing film comprising a plurality of reinforcing fibers, each reinforcing fiber extending along an orientation axis, the rate of reinforcing fibers oriented along the stacking axis Z is between 10% and 60%.

[0016] A non-woven carbon reinforcement film advantageously allows the application of a lower forming pressure than the pressure used for a woven reinforcement film, which allows the use of less expensive industrial machines, allowing a simpler and faster manufacturing process. A lower forming pressure also makes it possible to limit the deformation of the initial weave of the reinforcement and the formation of porous cavities which could alter the impermeability of the bipolar plates with respect to gases.

[0017] An orientation of the reinforcing fibers along the stacking axis makes it possible to increase the electrical conductivity of the material along this axis, in order to ensure optimal electrical transport between the faces of the bipolar plate for use in electrochemical devices.

[0018] A rate of reinforcing fibers oriented along the stacking axis of between 10% and 60% allows good conductivity in the bipolar plate while ensuring that the bipolar plate has significant mechanical strength. Such a rate of reinforcing fibers oriented along the stacking axis Z thus allows it to fulfill a dual mechanical and electrical conductivity function.

[0019] Preferably, the orientation of the reinforcing fibers is achieved by stitching. Stitching makes it possible to mechanically orient a portion of the reinforcing fibers along the stacking axis Z, allowing better electrical conductivity in the bipolar plate, allowing better electrical exchanges in the cells adjacent to the bipolar plate when the latter is mounted in an electrochemical device.

[0020] Alternatively, the orientation of the reinforcing fibers can be achieved by a different method, for example by hydroentangling.

[0021] In a preferred embodiment, the rate of reinforcing fibers oriented along the stacking axis Z is between 15 and 45%, allowing optimal conductivity in the bipolar plate. Such a rate also makes it possible to maintain a rate of unoriented reinforcing fibers sufficiently high to guarantee optimal mechanical strength of the bipolar plate.

[0022] Preferably, the nonwoven carbon reinforcing film comprising open porosities, the nonwoven carbon reinforcing film has an open porosity rate greater than 60% before the pressurizing step. The term "open porosity" will be described in more detail later.

[0023] In one embodiment, the non-woven carbon reinforcement film has an open porosity rate of between 60% and 70%. Following studies, it has been discovered that certain materials such as carbon papers or gas diffusion layers, particularly effective for the manufacture of a light and strong bipolar plate of thin thickness, can have such an open porosity rate.

[0024] Preferably, the non-woven carbon reinforcing film is a carbon felt. A carbon felt has a high open porosity rate in the reinforcing film, preferably greater than 70%, allowing optimal flow of the thermoplastic resin in the reinforcing film and more precisely between the reinforcing fibers. The reinforcing fibers are thus optimally impregnated with thermoplastic resin, which makes it possible to obtain a bipolar plate having a high level of consolidation, in which few pores are present. By "open porosity" is meant cavities or channels of porosity open to the outside of the reinforcing film. The open porosities are accessible from the outside and can be filled with the polymer resin to allow good mechanical strength of the bipolar plate.A carbon felt also allows the use of a known material, traditionally used as a thermal insulator or as a medium allowing the diffusion of liquid electrolyte in the case of a Redox flow battery.

[0025] Preferably, the non-woven carbon reinforcing film comprising open porosities, the non-woven carbon reinforcing film has an open porosity rate greater than 80% before the pressurizing step. Such an open porosity rate also allows optimal deformation of the reinforcing film allowing high compressibility and adaptation of the reinforcing film to complex shapes. More preferably, the non-woven carbon reinforcing film has an open porosity rate greater than 90%. Subsequently, for the sake of brevity, the “open porosity rate” may be designated by the expression “open porosity”.

[0026] A high open porosity in the reinforcing film combined with an optimal rate of reinforcing fibers oriented along the stacking axis Z has the advantage of obtaining optimal performance for the use of the bipolar plate in an electrochemical system. In particular, a high open porosity, combined with a high rate of reinforcing fibers oriented along the stacking axis Z allows both a high conductivity of the bipolar plate allowing it to optimally fulfill its role as a conductive element and a high porosity of the reinforcement allowing good impregnation and significant deformation of the reinforcement at low pressure to correctly shape the channels of the bipolar plate, and obtain a composite with negligible porosities to hydrogen in order to fulfill its role as a fluidic separator.

[0027] Preferably, said open porosities have a diameter of between 1 µm and 250 µm. Preferably, at least 50% of the open porosities of the reinforcing film have a diameter greater than 125 µm. More preferably, at least 50% of the open porosities of the reinforcing film have a diameter greater than 150 µm. Such porosities can easily be filled with the polymer resin without impacting the mechanical strength of the bipolar plate. Such a size of the porosity also allows the assembly to be highly compressible, which advantageously allows the manufacture of thin bipolar plates. The expression "diameter of the open porosities" means the equivalent circle diameter, i.e. the diameter of a circle having a surface area equivalent to the surface area of ​​the porosity.Similarly, it is described here more precisely that, preferably, at least 50% of the surface of the open porosities of the reinforcing film is occupied by porosities whose equivalent circle diameter is greater than 125 µm.

[0028] Preferably, said open porosities have an equivalent circle diameter of between 1 µm and 300 µm. In one embodiment, at least 50% of the surface area of ​​the open porosities of the reinforcing film is occupied by porosities whose equivalent circle diameter is greater than 30 µm. More preferably, at least 50% of the surface area of ​​the open porosities of the reinforcing film is occupied by porosities whose equivalent circle diameter is greater than 80 µm, preferably greater than 90 µm.

[0029] Preferably, the forming temperature is between 140 and 400°C, allowing the use of a wide range of thermoplastic polymer families.

[0030] In one embodiment, the shaping temperature is between 240 and 360°C, allowing the shaping of polymers of the polyphenylene sulfide PPS or polyphenylsulfone PPSU type.

[0031] In one embodiment, the shaping temperature is between 200 and 260°C, allowing the shaping of polyvinylidene fluoride (PVDF) or polyamide type polymers.

[0032] In one embodiment, the shaping temperature is between 140 and 200°C, allowing the shaping of polyolefin-type polymers.

[0033] In one implementation mode, the shaping temperature is between 350 and 400°C, allowing the shaping of polyaryl ether ketone type polymers (PEEK, PEK, PEKK, etc.).

[0034] Preferably, the predetermined shaping pressure is between 6 and 12 MPa. Thanks to the non-woven carbon, such a shaping pressure is sufficient to optimally impregnate the reinforcing fibers in the reinforcing film and benefit from a bipolar plate having a high level of consolidation and therefore significant impermeability of the bipolar plates to hydrogen in an electrochemical device.

[0035] Preferably, the predetermined shaping pressure is between 8 and 10 MPa. Thanks to the invention, it is not necessary to raise the shaping pressure to an excessively high pressure. It is therefore not necessary to use a particularly expensive industrial machine. The manufacturing method is faster and simpler to implement, while allowing high manufacturing rates. Such a shaping pressure also makes it possible to limit the deformation of the reinforcing fibers in the initial reinforcing film which could generate porous cavities in the bipolar plate, thus ensuring the effectiveness of the bipolar plate by limiting any risk of malfunction.

[0036] Preferably, the reinforcing film has a thickness of less than 5 mm. The reinforcing film is thick enough to produce a bipolar plate having significant mechanical strength while having sufficient flexibility for the reinforcing film to adapt to complex geometries. Preferably, the initial thickness of the reinforcing film (i.e. the thickness before compression of the stack to form the bipolar plate) is between 0.5 and 3.4 mm, to allow the formation of bipolar plates with a thickness of less than 0.5 mm after compression of the stack.

[0037] Preferably, the reinforcing film has a surface mass less than or equal to 500 g / m², making it possible to form a lightweight bipolar plate. Preferably, the surface mass is between 50 and 400 g / m². More preferably, the surface mass is between 50 and 300 g / m², making it possible to integrate the formed bipolar plate into an electrochemical device, for example in an aircraft or other vehicle, in which mass constraints are significant.

[0038] In one embodiment, the manufacturing method comprises, after the pressurizing step, a step of cooling the formed bipolar plate, for a third predetermined duration.

[0039] In a first embodiment, the cooling step is carried out at a cooling rate of between 10 and 100 °C / min. Such a cooling rate allows the development of the crystalline part of the semi-crystalline polymers to allow the manufactured bipolar plate to have good impermeability to the fluid used for energy conversion in the electrochemical device. Such a cooling rate thus makes it possible to obtain a crystallization rate of between 43 and 47%, corresponding to a crystallization rate close to the maximum crystallization rate that can be obtained on the thermoplastic resins used, generally of the order of 40 to 55%. After studies, the maximum crystallization rate that can be obtained on other types of thermoplastic resins, for example for polyaryl ether ketone is between 20 and 40%.Similarly, the maximum crystallization rate that can be obtained on different types of thermoplastic resins, such as polyolefins, is between 55 and 80%.

[0040] Alternatively, for so-called "amorphous" polymers, i.e. polymers which do not include crystalline parts, the cooling step is carried out at a cooling rate greater than 80°C / min, so as to rapidly cool the bipolar plate, allowing faster production rates.

[0041] Preferably, the cooling rate is between 40 and 90 °C / min. Such a cooling rate allows the optimal development of the crystalline part of the stack, while minimizing the cooling time of the bipolar plate to reduce manufacturing cycles and allow a rapid process allowing high manufacturing rates.

[0042] The invention also relates to a bipolar plate, intended to be mounted in an electrochemical device, the bipolar plate being manufactured using the manufacturing method as described above.

[0043] According to one aspect, the bipolar plate comprises a non-woven carbon reinforcing film comprising a plurality of reinforcing fibers, each reinforcing fiber extending along an orientation axis, the rate of reinforcing fibers oriented along the stacking axis Z being between 10% and 60%.

[0044] Preferably, the bipolar plate has a thickness less than or equal to 0.5 mm, which advantageously makes it possible both to limit the mass and the size in the electrochemical device and to increase its power density (number of kW / kg). The electrochemical device can thus be easily mounted in a vehicle such as an aircraft for example.

[0045] More preferably, the bipolar plate has a thickness of less than 0.4 mm.

[0046] Preferably, the bipolar plate has a specific surface electrical resistance of less than 12 mΩ.cm².

[0047] Preferably, the bipolar plate has a porosity rate of less than 1%, ensuring optimal permeability for the bipolar plate. PRESENTATION OF FIGURES

[0048] The invention will be better understood upon reading the following description, given by way of example, and referring to the following figures, given by way of non-limiting examples, in which identical references are given to similar objects.

[0049] This is a schematic representation of a stack of membrane-electrode assemblies and bipolar plates of an electrochemical device.

[0050] This is a schematic representation of a bipolar plate of the.

[0051] This is a schematic representation of a view along a sectional plane of the bipolar plate of the.

[0052] This is a schematic representation of a stack of a non-woven carbon reinforcement film, a thermoplastic resin film and two release films for the manufacture of a bipolar plate according to the invention.

[0053] This is a close-up view of the non-woven reinforcement film of the.

[0054] This is a schematic representation of a first step of the manufacturing process according to an embodiment of the invention.

[0055] This is a schematic representation of a second step of the manufacturing process according to an embodiment of the invention.

[0056] A schematic representation of a third step of the manufacturing process according to an embodiment of the invention.

[0057] This is a schematic representation of a fourth step of the manufacturing process according to an embodiment of the invention.

[0058] This is a schematic representation of a fifth step of the manufacturing process according to an embodiment of the invention.

[0059] This is a graph representing the evolution of temperature and pressure during a manufacturing process according to the invention.

[0060] This is a schematic representation of a sixth step of the manufacturing process according to an embodiment of the invention.

[0061] This is a schematic representation of a bipolar plate manufactured using the manufacturing method according to the invention.

[0062] This is a schematic representation of an image from a tomographic characterization of a reinforcing film.

[0063] This is a schematic representation of a skeletonization of the reinforcing fibers of the reinforcing film shown in the.

[0064] It should be noted that the figures set out the invention in detail to implement the invention, said figures can of course be used to better define the invention where appropriate. DETAILED DESCRIPTION OF THE INVENTION

[0065] The invention relates to a method for manufacturing a bipolar plate made of composite material for an electrochemical device.

[0066] Subsequently, as described above, the term “electrochemical device” also designates a fuel cell, a proton exchange membrane electrolyzer, an oxidation-reduction flow battery or any other device making it possible to carry out an electrochemical reaction.

[0067] As described above, an electrochemical device comprises a stack of a plurality of cells, each comprising a membrane-electrode assembly and two bipolar plates, also called separator plates, which sandwich the membrane-electrode assembly and allow the distribution of fluids in the cell. To form the electrochemical reaction in the electrochemical device, each cell is, in a known manner, supplied with an oxidizing fluid and a reducing fluid, for example dihydrogen and dioxygen, which react when brought into contact in an oxidation-reduction reaction. Each cell is also supplied with a heat transfer fluid, used for the thermal regulation of the electrochemical device. To allow the passage of fluids, the bipolar plate comprises concave portions and convex portions to form circulation channels.

[0068] With reference to the, the bipolar plate B (shown in the) according to the invention is formed from at least one non-woven carbon reinforcing film 2 and at least one thermoplastic resin film 3. In this example, the bipolar plate B is formed from a non-woven carbon reinforcing film 2 and a thermoplastic resin film 3. This document describes the example of a single non-woven carbon reinforcing film 2 and a single thermoplastic resin film 3, however it goes without saying that the number of films could be different. In particular, it goes without saying that the bipolar plate B could be formed from several non-woven carbon reinforcing films 2 and / or several thermoplastic resin films 3. Hereinafter, for the sake of brevity, the non-woven carbon reinforcing film will be referred to as reinforcing film 2.

[0069] With reference to the, the reinforcing film 2 preferably extends in a plane (X, Y) and has two substantially planar and parallel outer surfaces 2A and 2B, a lower surface 2A and an upper surface 2B. The reinforcing film 2 further comprises a plurality of reinforcing fibers 21. Preferably, the reinforcing fibers 21 are formed of carbon fibers. The thickness Ep2 of the reinforcing film 2 is defined along a vertical axis Z, orthogonal to the plane (X, Y).

[0070] Each reinforcing fiber 21 comprises a first end 21a and a second end 21b. At least a portion of the reinforcing fibers 21 of the reinforcing film 2 extends substantially, from the first end 21a to the second end 21b, along an orientation axis F. The orientation axis F extends, in this example, from the lower surface 2A to the upper surface 2B. In other words, the first end 21a of the reinforcing fibers 21 is substantially on the surface of the lower surface 2A and the second end 21b, substantially on the surface of the upper surface 2B and each reinforcing fiber 21 extends in the thickness Ep2 of the reinforcing film 2. Still with reference to 1, the orientation axis F is defined by forming an angle Theta θ with the vertical axis Z and an angle Phi φ with the axis X, in a projection plane (X, Y).Preferably, the orientation axis F extends so that the angle Theta θ is less than 45° or greater than 135° and the angle Phi φ is between 45° and 135°. For the sake of clarity, when the reinforcing fibers 21 meet the preceding conditions, it is considered that the reinforcing fibers 21 extend along the stacking axis Z.

[0071] More precisely, the orientation axis F extends so that the angle Theta θ is between 0° and 45° or between 135° and 180° and the angle Phi φ is between 45° and 135°.

[0072] Preferably, between 10% and 60% of the reinforcing fibers 21 of the reinforcing film 2 extend along the stacking axis Z between the lower surface 2A and the upper surface 2B of the reinforcing film 2, as shown in the. Such an orientation of the reinforcing fibers 21 makes it possible to increase the electrical conductivity and therefore, once manufactured, in the bipolar plate B while retaining significant mechanical characteristics. Significant conductivity in the thickness of the reinforcing film 2, i.e. along the stacking axis Z, is advantageous because it makes it possible to ensure good conductivity in the thickness of the bipolar plate B, as will be described in more detail later. In one embodiment, the reinforcing fibers 21 are oriented along the stacking axis Z by a needling, stitching or sewing process, for example. Such methods are known to those skilled in the art, and will not be described in further detail in this document.The orientation of the reinforcing fibers 21 can alternatively be achieved by a different method, for example by hydroentangling.

[0073] For example, the orientation of the reinforcing fibers 21 in the reinforcing film 2 can be determined by analyzing images obtained by X-ray tomography. Such a method makes it possible to obtain a three-dimensional 3D image of the reinforcing film 2 and to analyze the reinforcing fibers 21 independently of one another, in order to define their orientations in the reinforcing film 2.

[0074] More specifically, in this example, the reinforcing fibers 21 are defined by gray level thresholding in order to distinguish them from the porosities. This involves filtering the 3D image in a contrast range making it possible to highlight the reinforcing fibers 21. A “morphological operator” type image processing is used to identify the three-dimensional structure of the reinforcing fibers 21 in the image. Preferably, a “skeletonization” type image processing, known as “morphological skeleton” or “Skeletonization” in English, is chosen. Such a method is known to those skilled in the art and will not be described in more detail in this document.

[0075] In one example, to determine the orientation of the reinforcing fibers 21 in the reinforcing film 2, the latter is analyzed by a micro-tomograph of the RX Solutions™ brand, for example the EasyTom 230 model. The analysis is carried out with an accelerating voltage of 60 kV, an intensity of 80 µA, and a tungsten target. The exposure time is 0.4 s, averaging over 10 s, 1120 radiographic projections. The acquisition of the X-rays is carried out using the X-Act software (RX Solutions™) and the three-dimensional reconstruction, using the “Avizo for Industrial Inspection” software (Thermo Fisher Scientific™).

[0076] In particular, the following steps are carried out: the images of the reinforcing film 2 resulting from an X-ray tomography characterization are imported into the “Avizo Software” software from Thermo Scientific™, a schematic representation of which is presented on the, the thresholding is carried out on a range of contrasts making it possible to highlight the reinforcing fibers 21, the three-dimensional filamentary structure of each reinforcing fiber 21 is identified by means of a skeletonization module (“Auto Skeleton”) of the software, an example of a representation of which is presented on the.

[0077] The properties of each reinforcing fiber 21 of the reinforcing film 2, such as their orientation, can then be studied. For example, in the, the reinforcing fibers 21 which have a similar orientation are represented by identical continuous or dotted lines.

[0078] As described above, the orientation axis F of each reinforcing fiber 21 is defined by the angle Theta θ that it forms with the vertical axis Z and the angle Phi φ that it forms with the axis X of the reference frame (X, Y, Z). In this reference frame, the angles Theta θ and Phi φ vary between 0° and 180°. More precisely, in the (X, Y, Z) frame, the space is delimited into three distinct cones of angle 45° having as vertex the origin of the frame and centered respectively on the X, Y and Z axes. Thus the three cones can be defined by the following pairs Theta θ and Phi φ:Cone oriented on Z: 45° < φ < 135° and 0° < θ < 45° or 135° < θ < 180°Cone oriented on Y: 45° < φ < 135° and 45° < θ < 135°Cone oriented on X: 0° < φ < 45° or 135° < φ < 180° and 0° < θ < 180°

[0079] The reinforcing fibers 21 are classified according to their orientations θ and φ between these three cones, the reinforcing fibers 21 belonging to two cones not being classified (fibers at the border between two cones). The rate of reinforcing fibers 21 oriented along each axis X, Y and Z of the reinforcing film 2 can then be calculated.

[0080] In a preferred embodiment, the reinforcing film 2 is a carbon felt, also known as a “carbon mat”. A carbon felt allows the use of a material having a high level of open porosity allowing efficient exchange between the liquid electrolyte and the bipolar plate of the electrochemical device. The expression “open porosity” means an open cavity, i.e. not closed by the arrangement of the carbon reinforcing fibers 21 and / or by its sizing. Preferably, the level of open porosity in the reinforcing film 2 is greater than 70%. More preferably, the level of open porosity is greater than 80%, which allows optimal flow of the thermoplastic resin in the reinforcing film 2, allowing good impregnation of the reinforcing fibers 21.Thanks to the high open porosity rate of the reinforcing film 2, it is possible, after the manufacturing process, to obtain a bipolar plate B having a high level of consolidation (with a percentage of remaining porosities generally less than 1%). The carbon felt also has the advantage of being able to be easily deformed, allowing the reinforcing film 2 to be adapted to complex shapes. The carbon felt also has the advantage of requiring a lower forming pressure (generally between 6 and 10 MPa) than the forming pressure necessary for a woven reinforcement (commonly greater than 15 MPa). More preferably, the non-woven carbon reinforcing film 2 has an open porosity rate greater than 90%, allowing the manufacture of a bipolar plate B whose mechanical characteristics are optimal.

[0081] Preferably, the reinforcing film 2 has an open porosity rate greater than 60%. In one embodiment, the reinforcing film 2 has an open porosity rate of between 60% and 70%. Indeed, certain materials such as carbon papers or gas diffusion layers, having such an open porosity rate, allow the manufacture of a light and strong bipolar plate of thin thickness.

[0082] In a preferred embodiment, the reinforcing film 2 has porosities having a diameter of between 1 µm and 250 µm, making it possible to effectively impregnate the reinforcing fibers 21 and to guarantee a high level of consolidation in the manufactured bipolar plate B. Preferably, at least 50% of the porosities have a diameter greater than 125 µm, more preferably greater than 150 µm. By "diameter of the porosities" is meant the equivalent circle diameter of the porosities, that is to say the diameter of a circle having a surface area equivalent to the surface area of ​​a porosity. In one embodiment, the reinforcing film 2 has porosities having an equivalent circle diameter of between 1 µm and 300 µm.

[0083] In one embodiment, at least 50% of the surface area of ​​the open porosities of the reinforcing film 2 is occupied by porosities whose equivalent circle diameter is greater than 30 µm, preferably greater than 80 µm, more preferably greater than 90 µm.

[0084] For example, to characterize the porosities in the reinforcing film 2, one method is to use images obtained by X-ray tomography and analyze them in three dimensions or in two dimensions. For example, a two-dimensional section can be made in a desired direction. As described above, the reinforcing fibers 21 are defined by gray level thresholding in order to distinguish them from the porosities. The image is then filtered according to a range of contrasts making it possible to highlight the reinforcing fibers 21. An image processing of the “morphological operator” type (for example “morphological gradient”) is then used to identify the two-dimensional structure of the porosities in the image.

[0085] In practice, in one example, three-dimensional image sections from X-ray tomography are made to obtain two-dimensional images. These two-dimensional images are imported in this example into the ImageJ™ software. Thresholding is then carried out on a range of gray levels between 120 and 255 (in a so-called “Dark Background” mode). A morphological gradient operator in the software (in this example “Watershed”) then makes it possible to highlight the two-dimensional structure of the porosities.

[0086] Once the porosities have been identified, the area of ​​each porosity is measured and the equivalent circle diameter is calculated from the measured area. The porosities are then characterized and classified according to their equivalent circle diameter. For example, porosities are classified according to whether their equivalent circle diameter is less than or greater than a diameter threshold of a proportion of all porosities.

[0087] Preferably, the reinforcing film 2 is highly compressible, in order to adapt to the complex geometries of the fluid circulation channels of the bipolar plate. It is also possible to form a bipolar plate B with a thickness of less than 1 mm. Preferably, the formed bipolar plate B has a thickness of less than 0.5 mm, as will be described in more detail below.

[0088] Still with reference to the, the reinforcing film 2 has a thickness Ep2, along the Z axis, preferably less than or equal to 5 mm. Preferably the thickness Ep2 is between 0.5 and 2.5 mm. Such a thickness Ep2 advantageously makes it possible to limit the size of the bipolar plate B once manufactured.

[0089] In a preferred embodiment, the reinforcing film 2 has a surface mass less than or equal to 500 g / m². Preferably, the surface mass is between 50 and 300 g / m², allowing the use of a reinforcing film 2 having a limited mass, which makes it possible to limit the mass of the bipolar plate B in which the reinforcing film 2 is used. The bipolar plate B then has a limited mass, allowing it to be integrated into an electrochemical device intended to be mounted for example in an aircraft or in any other vehicle.

[0090] The thermoplastic resin film 3 (shown in the) comprises a polymer, designated thermoplastic resin 31, for forming the thermoplastic matrix of the bipolar plate B after manufacture. For the sake of brevity, the thermoplastic resin film 3 will hereinafter be referred to as thermoplastic film 3.

[0091] Preferably, with reference to the, the thermoplastic film 3 has a thickness Ep3 of between 50 and 600 µm. Such a thickness allows a sufficient quantity of thermoplastic resin 31 to form the thermoplastic matrix of the bipolar plate B, while limiting the volume and mass of the bipolar plate B.

[0092] In one embodiment, the thermoplastic resin of the thermoplastic film 3 is of the semi-crystalline type, making it possible to give the bipolar plate B significant impermeability to fluids, in particular to hydrogen. A semi-crystalline thermoplastic resin also provides significant mechanical and chemical resistance, in particular resistance to corrosion. In one embodiment, the thermoplastic resin of the thermoplastic film 3 is of the amorphous type, allowing greater ductility as well as a low rate of dimensional shrinkage during the cooling step.

[0093] The thermoplastic resin can be adapted according to the chemical environment of the electrochemical device in which the bipolar plate B will be mounted. As such, for mounting the bipolar plate B in a low-temperature proton exchange membrane (known by the acronym "PEM"), the thermoplastic resin is preferably of the polyphenylene sulfide (known by the acronym "PPS"), polyphenylsulfone (known by the acronym "PPSU"), polyvinylidene fluoride (known by the acronym "PVDF"), ethylene chlorotrifluoroethylene (known by the acronym "ECTFE") or of the polyolefin, poly aryl ether ketone, or polyamide type.

[0094] For mounting the bipolar plate B in a high temperature proton exchange membrane, the thermoplastic resin is preferably polyphenylene sulfide (PPS), polyphenylsulfone (PPSU), ethylene chlorotrifluoroethylene (ECTFE), poly aryl ether ketone.

[0095] For the assembly of the bipolar plate B in a Redox flow battery with vanadium, hydrogen bromide, or any liquid electrolyte (organic for example), the thermoplastic resin is preferably of the polyphenylene sulfide (PPS) and Polyphenylsulfone (PPSU) type if the electrolyte is of the basic type, and of the Polyvinylidene fluoride (PVDF) or ethylene chlorotrifluoroethylene (ECTFE) type if the electrolyte is of the acid type, or of the poly aryl ether ketone type.

[0096] A method of manufacturing a bipolar plate B according to an embodiment of the invention will now be described.

[0097] With reference to the, the bipolar plate B is manufactured, in this example, from a stack 1 of a reinforcing film 2, a thermoplastic resin film 3 and two release films 4. The stack 1 extends along a stacking axis A, as will be described in more detail later. The use of release films 4 makes it possible to expose the reinforcing fibers 21 on the surface of the formed bipolar plate B and to make it electrically conductive. Preferably, the reinforcing film 2, the thermoplastic film 3 and the release film 4 are in the form of a roll, allowing simple storage and handling.

[0098] As shown in the, the method comprises a first step E1 of cutting the reinforcing film 2, the thermoplastic resin film 3 and the two release films 4 (only the reinforcing film 2 is shown in the). The cutting is carried out for example manually on a cutting table and makes it possible to cut the different films to the dimensions of the desired final bipolar plate B. The reinforcing film 2 and the thermoplastic film 3 preferably have similar dimensions. More preferably, each release film 4 has dimensions larger than the dimensions of the reinforcing film 2 and the thermoplastic film 3, so as to protrude from the stack 1 so that it can be more easily removed after the formation of the bipolar plate B. It goes without saying that the cutting could also be carried out in a different way, for example using a die cutter or using a robotic arm.

[0099] The method then comprises, with reference to the, a second step E2 of superposition, along the stacking axis A, of the first demolding film 4, the reinforcing film 2, the thermoplastic film 3 and the second demolding film 4, in order to form the stack 1. Such positioning makes it possible to orient the orientation axis F of the reinforcing fibers 21 of the reinforcing film 2 along the stacking axis A, which makes it possible to orient the reinforcing fibers 21 so that their ends 21a, 21b are exposed on two opposite faces of the manufactured bipolar plate B. The reinforcing fibers 21 thus optimally conduct electricity in the bipolar plate B and therefore in the electrochemical device in which it will be mounted.In this example, the stack 1 can be formed either by the successive positioning of the first release film 4, the reinforcing film 2, the thermoplastic film 3 and the second release film 4, or by the successive positioning of the first release film 4, the thermoplastic film 3, the reinforcing film 2 and the second release film 4.

[0100] Alternatively, in the case of a stack 1 comprising two thermoplastic resin films 3, in this step, an operator could just as easily successively superimpose a first demolding film 4, a first thermoplastic film 3, a reinforcing film 2, a second thermoplastic film 3 and a second demolding film 4. The use of two thermoplastic films 3 positioned on either side of the reinforcing film 2 makes it possible to minimize the migration of the thermoplastic resin through the reinforcing film 2 and to facilitate its impregnation.

[0101] The superposition of the first release film 4, the reinforcing film 2, the thermoplastic film 3 and the second release film 4 can be carried out manually or by means of a robotic arm, for example.

[0102] In one embodiment, the stack 1 is consolidated, so as to limit the risks of one of the films shifting relative to the others, for example. Such consolidation can be achieved, for example, by applying welding points (such as ultrasound or localized heating, for example) or by making seams to secure all the films. Thanks to such consolidation, it is then easier to transport the entire stack 1, for example by means of a robotic arm comprising gripping means.

[0103] With reference to the, the method then comprises a step E3 of positioning the stack 1 in a compression system, in this example in a mold M. The mold M comprises, in this example, a lower member and an upper member, each comprising an inner surface having an imprint G. The imprint G makes it possible to form the fluid circulation channels in the bipolar plate B. In one embodiment, before positioning the stack 1 in the mold M, the latter is coated with a mold release agent, for example a liquid which can be sprayed onto the inner surface of each lower and upper member, in order to facilitate subsequent demolding. Preferably, the mold M is then at an initial temperature Ti. Preferably, the initial temperature Ti is between 20 and 210 °C.

[0104] When the mold M is closed, trapping the stack 1, the temperature is gradually increased from the initial temperature Ti to a predetermined shaping temperature Tm. Preferably, the shaping temperature Tm is between 140 and 400°C. As examples, for PPS type thermoplastics, the shaping temperature Tm is more preferably between 305 and 340°C. For PVDF type thermoplastics, the shaping temperature Tm is between 200 and 260°C, more preferably between 210 and 240°C. For PPSU type thermoplastics, the shaping temperature Tm is between 240 and 360°C, more preferably between 290 and 330°C. The temperature rise from the initial temperature Ti to a predetermined shaping temperature Tm is carried out during a first predetermined duration Δt1 (represented on the graph of the).Preferably, the first duration Δt1 is less than 10 min, preferably less than or equal to 3 min, so as to quickly heat the mold M to allow time savings in production and high production rates.

[0105] With reference to the, the method then comprises a step E4 of pressurizing the stack 1 in the compression mold M. The pressurizing step E4 is carried out, at the forming temperature Tm, at a predetermined forming pressure Pm, for a second predetermined duration Δt2 (represented on the graph of the). The pressurizing step E4 makes it possible to melt the thermoplastic resin of the thermoplastic film 3 to impregnate the reinforcing film 2 and form a bipolar plate B.

[0106] Preferably, the shaping pressure Pm is between 6 and 12 MPa. More preferably, the shaping pressure Pm is between 8 and 10 MPa, making it possible both to limit the deformation of the reinforcing fibers 21 in the initial reinforcing film 2, which could generate porous cavities in the bipolar plate B, and to apply the pressure in a simple manner. Preferably, the second duration Δt2 is less than 2 min, preferably less than 1 min, so as to ensure the impregnation of the reinforcing fibers 21 by the thermoplastic resin 31 and thus limit the porosity of the bipolar plate B.

[0107] The method then comprises a cooling step E5, shown in the, of the formed bipolar plate B, for a third predetermined duration Δt3 (shown in the graph of the), so as to consolidate it. The temperature drops from the forming temperature Tm to a release temperature Tr, at which the pressurization is stopped. Preferably, the release temperature Tr is equal to the initial temperature Ti.The third predetermined duration Δt3 is defined, in the case of semi-crystalline polymers, to obtain a cooling rate of between 10 and 100 °C / min, allowing cooling slow enough to allow the crystalline part of the thermoplastic resin to develop, thereby ensuring that the manufactured bipolar plate will have low permeability to hydrogen or any other fluid used in an energy conversion system, while minimizing cooling times in order to reduce manufacturing cycles. Such a cooling rate makes it possible to obtain a crystallinity rate advantageously of between 43 and 47%. More preferably, the cooling rate is between 40 and 90 °C / min, which ensures optimal crystallization of the thermoplastic resin during the consolidation phase.In the case of amorphous polymers, the cooling rate is preferably greater than 80°C / min, so as to quickly cool the bipolar plate B and the mold M and accelerate production rates.

[0108] The forming cycle (i.e., temperature rise, forming step, and cooling step) is shown in, which presents a graph of the evolution of temperature T and pressure P as a function of time t during a complete forming cycle, as previously described.

[0109] When the release temperature Tr is reached, the mold M is opened and the formed composite bipolar plate B is removed.

[0110] As shown in the, the release films 4 are then removed, in a step E6, on either side of the manufactured bipolar plate B, so as to expose the reinforcing fibers 21 on the surface of the bipolar plate B. The removal of the release films 4 can be carried out manually or automatically by winding the release film 4 at the exit of the mold for example.

[0111] The method then comprises, in an exemplary implementation, a step of cutting the bipolar plate B, for example to remove the peripheral part of the bipolar plate which has manufacturing defects. Such cutting can be carried out, for example, by water jet cutting, by milling, or by means of a die cutter. Preferably, centering is taken into account in order to ensure precise cutting of the bipolar plate (cutting error less than or equal to 0.1 mm).

[0112] The present invention is a bipolar plate B formed by the manufacturing method described above. The bipolar plate B extends in a plane (X, Y). Such a method advantageously makes it possible to form a bipolar plate B having a thickness Ep (along the Z axis, orthogonal to the plane (X, Y)) of less than 1 mm. Preferably, the formed bipolar plate B has a thickness Ep of less than 0.5 mm.

[0113] Preferably, the proportions between the quantity of reinforcing fibers 21 and thermoplastic resin 31 of the final bipolar plate B are ensured so that the electrical, thermal and mechanical properties of the bipolar plate B satisfy the requirements of the applications of the electrochemical devices. The final bipolar plate B thus preferably has a volume ratio of reinforcing fibers 21 of between 20 and 60%, and a volume ratio of thermoplastic matrix 31 of between 40 and 80%. Preferably, the volume ratio of reinforcing fibers 21 is between 30 and 50%, and that of the thermoplastic matrix 31 between 50 and 70%. The very thin thickness of the bipolar plate thus produced makes it possible to compensate for a lower rate of conductive elements than in the manufacturing methods of the prior art, in which the rate of conductive elements is greater than 80%, while ensuring optimal conductivity.The mass of the fabricated bipolar plate is also reduced, which makes it possible to limit the mass of the electrochemical device in which the bipolar plate will be mounted.

[0114] For example, the volume ratio of reinforcing fibers 21 present in the bipolar plate B after manufacture can be determined after separation from the thermoplastic matrix 31. This separation can be carried out by acid dissolution of the thermoplastic matrix 31 or by calcination for example. The mass of the conductive reinforcing fibers 21 is then measured by weighing and related to the initial mass of the material. The change in volume ratio is carried out using the density of the materials.

[0115] The method according to the invention describes a suitable type of carbon reinforcement and the associated manufacturing cycles, in order to propose a method for manufacturing a bipolar plate that is simple, rapid and does not require the use of expensive industrial machines, while allowing the formation of a thin and light bipolar plate.

[0116] Preferably, the bipolar plate has a thickness less than or equal to 0.4 mm, which advantageously makes it possible both to limit the mass and the size in the electrochemical device and to increase its power density (kW / kg), without having to resort to post-treatment of the surface of the material in order to increase its electrical conductivity. The electrochemical device can thus be easily mounted in a vehicle such as an aircraft for example.

[0117] In a preferred manufacturing example, the present invention makes it possible to manufacture a flexible bipolar plate B, having a specific electrical surface resistance (known by the acronym ASR meaning “Areal Specific Resistance” in English) of less than 12 mΩ.cm², a porosity rate of less than 1%. Such characteristics advantageously make it possible to guarantee a permeability of less than 5x10^-8 mol / m / s / MPa and a high hydrophobicity of the surface of the bipolar plate B, with contact angles greater than 110°.

[0118] The porosity rate of the manufactured bipolar plate B can be measured in different ways known to those skilled in the art.

[0119] For example, the Archimedes' thrust method or Pycnometer analysis can be used to measure the actual density of the bipolar plate B and compare this actual density with a theoretical density. These two methods allow the exact volume of a sample to be precisely determined, and knowing its mass, its density to be deduced.

[0120] In a second example, different image analysis methods make it possible to distinguish the porosities in the bipolar plate B. In this case, the aim is to identify the volume of the porosities on a three-dimensional image (or their surface on a two-dimensional image), in a manner analogous to the method described previously for the reinforcement film 2.

[0121] In a third example, wave absorption methods such as X-ray tomography or the use of ultrasound also make it possible to determine the porosity rate of the final bipolar B plate.

[0122] These methods are conventionally followed by a measurement of the exact composition of the sample of interest. For this, the reinforcing fibers 21 are separated and weighed independently of the thermoplastic matrix 31. This separation can be achieved by dissolution of the matrix or by its thermal degradation (calcination or combustion) or chemical degradation (acid dissolution). Knowing the masses and densities of the initial components of the bipolar plate B, it is possible to deduce the corresponding volumes. These volumes are compared to the exact volume of the sample and the remaining volume corresponds to the volume of the porosities.

[0123] The volumetric rate of porosity is calculated according to the following formula, in which corresponds to the mass of reinforcing fiber 21 obtained for example after acid dissolution of the thermoplastic matrix 31, corresponds to the density of the reinforcing fiber 21 and to that of the thermoplastic matrix 31: .

Claims

A method for manufacturing a bipolar plate (B), the bipolar plate (B) being intended to be mounted in an electrochemical device, the electrochemical device being configured to carry out an electrochemical reaction, the method comprising: a stacking step (E2), along a stacking axis Z, of a first release film (4), at least one reinforcing film (2), at least one thermoplastic resin film (3) and a second release film (4), in order to form a stack (1), a positioning step (E3) of the stack (1) in a compression system (M), a pressurization step (E4) of the stack (1) in the compression system (M), the pressurization step (E4) being carried out at a predetermined forming pressure (Pm) and at a predetermined forming temperature (Tm) for a second predetermined duration (Δt2),in order to melt the thermoplastic resin (31) from the thermoplastic resin film (3) and impregnate the reinforcing film (2) with the thermoplastic resin (31) to form a bipolar plate (B), manufacturing process characterized in that the reinforcing film (2) is a non-woven carbon reinforcing film (2) comprising a plurality of reinforcing fibers (21), each reinforcing fiber (21) extending along an orientation axis (F), the proportion of reinforcing fibers (21) oriented along the stacking axis Z is between 10% and 60%. Manufacturing method according to claim 1, wherein the proportion of reinforcing fibers oriented along the Z stacking axis is between 15% and 45%. A manufacturing method according to any one of claims 1 and 2, wherein the non-woven carbon reinforcement film (2) is a carbon felt. Manufacturing method according to any one of claims 1 to 3, wherein the non-woven carbon reinforcement film (2) comprising open porosities, the non-woven carbon reinforcement film (2) has an open porosity rate greater than 60% before the pressurization step (E4). Manufacturing method according to claim 4, wherein said open porosities have a diameter between 1 µm and 300 µm. Manufacturing process according to any one of claims 1 to 5, wherein the shaping temperature (Tm) is between 140 and 400°C. Manufacturing method according to any one of claims 1 to 6, wherein the predetermined forming pressure (Pm) is between 6 and 12 MPa. Manufacturing process according to any one of claims 1 to 7, comprising subsequent to the pressurization step (E4), a cooling step (E5) of the formed bipolar plate (B), for a third predetermined duration (Δt3). Manufacturing method according to claim 8, wherein the cooling rate is between 40 and 90°C / min. Bipolar plate (B), intended to be mounted in an electrochemical device, the bipolar plate (B) being manufactured using the manufacturing process according to any one of claims 1 to 9. Bipolar plate (B) according to claim 10 having a thickness (Ep) less than or equal to 0.5mm. Bipolar plate (B) according to any one of claims 10 to 11, having a specific surface electrical resistance of less than 12 mΩ.cm². Bipolar plate (B) according to any one of claims 10 to 12, having a porosity rate of less than 1%.

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