Carbon fiber bipolar plate and associated manufacturing process

Abstract

A bipolar plate (B) configured for mounting in an electrochemical device, the electrochemical device being configured to carry out an electrochemical reaction, the bipolar plate (B) comprising a plurality of reinforcing fibers (21) impregnated in a thermoplastic polymer matrix (3), each reinforcing fiber (21) comprising an outer surface, at least 50% of the reinforcing fibers (21) having an average surface roughness (Ra) greater than 0.4 x 10⁻⁶ m, preferably greater than 0.75 x 10⁻⁶ m. Abstract figure: Figure 4

Description

Title of the invention: Bipolar carbon fiber plate and associated manufacturing process. Technical field

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

[0002] An electrochemical device is defined as any device that enables the implementation of an electrochemical reaction, such as a fuel cell or a proton exchange membrane electrolyzer, enabling the generation of electrical energy or hydrogen, respectively, from a redox reaction. The term "electrochemical device" also refers to a redox flow battery that generates 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 stacking axis and two end plates, placed at the ends of the stack. The end plates are connected by compression elements that allow the cells to be compressed and ensure the sealing of the electrochemical device.

[0004] With reference to [Fig. 1], representing an electrochemical device 100, each cell 110 comprises, in a 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 within the cell 110. To carry out 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 flow across a membrane and react with the aid of a catalyst. Each cell 110 is also supplied with a heat transfer fluid, used for the thermal regulation of the electrochemical device.In practice, two adjacent bipolar half-plates 130A, 130B of two adjacent cells 110A, 110B are joined together to form several internal channels 140 which allow the passage of the heat transfer fluid between the cells 110. Hereafter, for the sake of brevity and simplicity, the term "bipolar plate" refers to a bipolar half-plate.

[0005] With reference to [Fig. 2], a bipolar plate 130 comprises, in a known manner, a central active portion 131, in contact with the membrane-electrode assembly and on which the redox reaction takes place, and a peripheral portion 132 allowing the two bipolar plates 130A, 130B to be joined together. The bipolar plate 130 further includes a plurality of openings 133 for the circulation of the oxidizing and reducing fluids and the heat transfer fluid.

[0006] To allow optimal operation of the electrochemical device, the bipolar plate 130 must have the lowest possible hydrogen permeability to allow exchanges with the membrane-electrode assembly 120 and avoid any risk of mixing with the heat transfer fluid.

[0007] In the prior art, bipolar plates made of graphite, expanded graphite, or metal are known, which exhibit very low permeability and thus allow optimal heat exchange within the cell. However, graphite plates are heavy, fragile, and have a significant thickness. This is a drawback for an electrochemical device intended for integration, for example, in an aircraft or any other mobility application. Furthermore, expanded graphite plates use thermosetting, non-recyclable materials and are not suitable for high-temperature proton exchange membrane technologies. Metal bipolar plates have a limited lifespan, given the acidic and corrosive environment of the electrochemical devices in which they are mounted, despite the protective coating used.Furthermore, the need for such a coating results in a significant additional cost to the final bipolar plates, and reduces their competitive advantage.

[0008] Bipolar plates made of composite material are thus very well positioned for heavy mobility applications.

[0009] In this respect, prior art discloses a bipolar plate comprising woven reinforcing fibers (for example, carbon fibers) impregnated with a thermoplastic or thermosetting polymer matrix. Such a bipolar plate has the advantage of being thinner, which reduces its size and mass.

[0010] However, the use of reinforcing fibers requires very high forming pressure (generally exceeding 15 MPa) to properly impregnate the fibers, reduce porosity, and expose the surface fibers for good electrical conductivity. High pressure can deform the initial weave of the reinforcement and generate porous cavities that can impair the gas impermeability of the bipolar plates.

[0011] A bipolar plate made from a stack of a non-woven carbon reinforcement film and a layer of thermoplastic polymer (in the form of a film, thermoplastic powder or other), which is both thin (generally less than 0.5mm), resistant and has a limited open porosity rate, which allows for lower levels of gas permeability.

[0012] However, such a thin bipolar plate does not achieve the permeability levels of a bipolar plate made of graphite or metal, for example. In particular, thermoplastic materials exhibit weak interaction with their environment, which limits the formation of bonds with reinforcing fibers in composite applications. Consequently, a low affinity between the reinforcing fiber and the thermoplastic matrix can create pathways for molecules, altering the gas permeability properties.

[0013] To date, there is no bipolar plate that is simultaneously thin, light, mechanically resistant and sufficiently impermeable to gases while being sufficiently conductive to allow optimal operation of the electrochemical device.

[0014] The invention thus aims to eliminate at least some of these drawbacks by proposing a thin bipolar plate made of composite material whose level of permeability and electrical conductivity are optimal. PRESENTATION OF THE INVENTION

[0015] The invention relates to a bipolar plate intended to be mounted in an electrochemical device, the electrochemical device being configured to carry out an electrochemical reaction, the bipolar plate comprising a plurality of reinforcing fibers impregnated in a thermoplastic polymer matrix. The bipolar plate is notable in that, for each reinforcing fiber comprising an outer surface, at least 50% of the reinforcing fibers have an average surface roughness greater than 0.4 x 0.6 m, preferably greater than 0.75 x 0.6 m.

[0016] By "surface roughness" is meant the measurement of the average of the peaks and troughs of the outer surface, that is to say, the average deviation from a mean line, the mean line corresponding to the line on either side of which all the peaks and troughs are equally distributed. In other words, the reinforcing fibers do not have a smooth outer surface, but have a surface grain that gives them a raised texture.

[0017] Such surface roughness increases the interactions between the thermoplastic polymer matrix and the outer surface of the reinforcing fibers. Thanks to a greater affinity of the thermoplastic polymer for the reinforcing fibers, the gas flow paths in the bipolar plate are more tortuous, which advantageously reduces the gas permeability in the fabricated bipolar plate. This improves the exchanges in each cell of the electrochemical device and thus enhances the operation of the electrochemical device.

[0018] According to a preferred aspect, at least 60% of the reinforcing fibers have an average surface roughness greater than 0.4 x 0.6 m, preferably greater than 0.75 x 0.6 m, Preferably, at least 68% of the reinforcing fibers have an average surface roughness greater than 0.4 x 0.6 m, and preferably greater than 0.75 x 0.6 m. This optimizes the interactions between the reinforcing fibers and the thermoplastic polymer matrix, reducing the permeability of the bipolar plate to an optimal value. Optimal permeability allows for optimal exchanges within each cell of the electrochemical device, thus enabling optimal operation of the electrochemical device.

[0019] Preferably, for each reinforcing fiber to have a length, at least 50% of the reinforcing fibers have a length greater than or equal to 1 x 10² m, allowing for optimal electrical conductivity in the bipolar plate. Long fibers can also advantageously be inserted (for example, by needle punching) into the thickness of the bipolar plate to further increase electrical conductivity. The "length of the reinforcing fibers" in the bipolar plate refers to the length of the reinforcing fibers used in the manufacture of the bipolar plate, i.e., before the plate is formed.

[0020] Preferably, each reinforcing fiber is a carbon fiber, enabling the formation of a lightweight bipolar plate with high electrical conductivity. A lightweight bipolar plate is particularly advantageous for integration into mobile systems.

[0021] In a preferred embodiment, the thermoplastic polymer matrix comprises a semi-crystalline thermoplastic polymer, which allows for optimal impregnation of the thermoplastic polymer onto the surface of each reinforcing fiber, thus significantly limiting porosity in the bipolar plate. This reduces the permeability of the bipolar plate.

[0022] Preferably, the thermoplastic polymer is selected from: 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 is of the polyolefin, polyaryl ether ketone, or polyamide type. Even more preferably, the thermoplastic polymer is of the polyphenylene sulfide type, known by the acronym "PPS," allowing the use of a thermoplastic polymer with optimal adhesion to the surface of the reinforcing fibers during the polymerization of the material.

[0023] Preferably, the bipolar plate comprises a plurality of non-woven, thermally activated carbon reinforcing fibers impregnated in a thermoplastic polymer matrix of the polyphenylene sulfide (PPS) type. Such a reinforcing fiber / thermoplastic polymer pair makes it possible to form a bipolar plate of which The degree of permeability is optimal for optimal functioning of the electrochemical device.

[0024] Preferably, the thermoplastic polymer has a crystallization rate of between 45 and 56%. Thanks to the surface roughness of the reinforcing fibers, the interactions between the reinforcing fibers and the thermoplastic polymer matrix are improved and the crystallization of the thermoplastic polymer is advantageously optimized around the reinforcing fibers in the case of a semi-crystalline thermoplastic polymer.

[0025] In a preferred embodiment, the bipolar plate has a thickness less than or equal to 0.5 x 10³ m, preferably less than or equal to 0.3 x 10³ m, which makes it possible both to limit the mass and size of 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.

[0026] The invention also relates to a method for manufacturing a bipolar plate as described above, the bipolar plate being intended to be mounted in an electrochemical device, the electrochemical device being configured to carry out an electrochemical reaction, the method comprising: • a layering step, along a stacking axis of at least: a reinforcing layer comprising a plurality of reinforcing fibers and a layer of a thermoplastic polymer, so as to form a stack, at least 50% of the reinforcing fibers of the reinforcing layer comprising an outer surface having a surface roughness greater than 0.4 x 0.6 m, preferably greater than 0.75 x 0.6 m, and • a thermocompression step of the stack in a compression system, so as to form a plurality of circulation channels to form the bipolar plate.

[0027] In a preferred embodiment, the reinforcing layer is a non-woven carbon reinforcing film, allowing for a simpler and faster process. A non-woven carbon reinforcing film advantageously allows the application of lower forming pressure than that used for a woven reinforcing film, thus enabling the use of less expensive industrial machinery. Lower forming pressure also limits deformation of the initial weave of the reinforcement and the formation of porous cavities that could impair the gas impermeability of the bipolar plates.

[0028] Preferably, the non-woven carbon reinforcement film is a carbon felt with a high open porosity, which allows optimal flow of Thermoplastic polymer between the reinforcing fibers. The reinforcing fibers are thus optimally impregnated with thermoplastic resin, resulting in a bipolar plate with a high level of consolidation and few pores.

[0029] In one embodiment, with each reinforcing fiber extending along an orientation axis within the non-woven carbon reinforcing film, the proportion of reinforcing fibers oriented along the stacking axis is between 10% and 60%. This ensures good conductivity in the bipolar plate while also providing high mechanical strength. Such a proportion of reinforcing fibers oriented along the stacking axis thus fulfills a dual function: mechanical strength and electrical conductivity.

[0030] In one embodiment, the process includes, prior to the layering step, a step of adding electrically conductive particles to the reinforcing fibers and / or the thermoplastic polymer, thereby increasing the electrical conductivity of the fabricated bipolar plate by increasing the electrical contact surface. Thermal and electrical exchanges are thus advantageously improved in the electrochemical device. PRESENTATION OF THE FIGURES

[0031] 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.

[0032] Fig. 1 is a schematic representation of a stack of membrane-electrode assemblies and bipolar plates of an electrochemical device.

[0033] Fig. 2 is a schematic representation of a bipolar plate of Fig. 1.

[0034] The [Fig.3] is a schematic representation in a transverse plane of a bipolar plate according to one embodiment of the invention.

[0035] Fig. 4 is a close-up perspective view of the bipolar plate of Fig. 3.

[0036] Figure 5 is a close-up view of a reinforcing fiber impregnated in a matrix polymer in which the interface between the reinforcing fiber and the polymer matrix has a low affinity.

[0037] Fig. 6 is a close-up view of a reinforcing fiber impregnated in a polymer matrix of the bipolar plate of Fig. 3 in which the interface between the reinforcing fiber and the polymer matrix has a high affinity.

[0038] Fig. 7 is a close-up view of the outer surface of the reinforcing fiber of Fig. 6.

[0039] Fig. 8 is a schematic representation of a first step of a manufacturing process for the bipolar plate of Fig. 3 according to an embodiment of the invention.

[0040] Fig. 9 is a schematic representation of a second step in the manufacturing process of the bipolar plate of Fig. 3.

[0041] The [Fig. 10] is a schematic representation of a third step in the manufacturing process of the bipolar plate of the [Fig.3].

[0042] It should be noted that the figures set out the invention in detail to implement the invention, said figures being of course able to serve to better define the invention where appropriate. DETAILED DESCRIPTION OF THE INVENTION

[0043] The invention relates to a bipolar plate made of composite material intended to be mounted in an electrochemical device.

[0044] Subsequently, as described above, the term "electrochemical device" refers equally to a fuel cell, a proton exchange membrane electrolyzer, a redox flow battery or any other device enabling the implementation of an electrochemical reaction.

[0045] As described previously, 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 within the cell. To carry out the electrochemical reaction in the electrochemical device, each cell is supplied, in a known manner, with an oxidizing fluid and a reducing fluid, for example, dihydrogen and dioxygen, which react when brought into contact in a redox 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 and convex portions to form circulation channels.

[0046] In this document, with reference to [Fig. 3] and 4, the bipolar plate B is described in an orthogonal coordinate system (X, Y, Z) in which the (X, Y) plane defines a horizontal plane and the Z axis defines a vertical axis Z orthogonal to the horizontal (X, Y) plane. The terms "horizontal" and "vertical" refer to a vertical axis Z extending from bottom to top.

[0047] The bipolar plate B according to the invention comprises reinforcing fibers 21 impregnated in a thermoplastic polymer matrix 3, as shown in the [Fig.4]. In practice, in this example, the bipolar plate B is formed from a non-woven carbon reinforcement film 2 and a layer of thermoplastic polymer 3.

[0048] The thermoplastic polymer 3 is configured to impregnate the reinforcing fibers 21 during the manufacture of the bipolar plate B, as will be described in more detail later.

[0049] In one embodiment, the thermoplastic polymer 3 is semi-crystalline, which gives the bipolar plate B significant impermeability to fluids, particularly hydrogen, while also providing high mechanical and chemical resistance. Figures 5 and 6 show a plurality of crystals 31 formed during the crystallization of the thermoplastic polymer 3. Alternatively, the thermoplastic polymer 3 is amorphous, allowing for greater ductility and a low rate of dimensional shrinkage during a cooling step required in the fabrication of the bipolar plate B.

[0050] Preferably, the thermoplastic polymer 3 is selected from 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 is of the polyolefin, polyaryl ether ketone, or polyamide type. Preferably, the thermoplastic polymer 3 is of the polyphenylene sulfide type (known by the acronym "PPS"), which can achieve a high degree of crystallization upon polymer cooling. The degree of crystallization defines the volume of crystals formed relative to the total volume of thermoplastic polymer 3. A high degree of crystallization reduces the permeability of the fabricated bipolar plate B, as will be described in more detail later. In this example, thermoplastic polymer 3 has a crystallization rate between 45 and 56%.

[0051] Alternatively, the thermoplastic polymer 3 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 type (known by the acronym "PPS"), polyphenylsulfone type (known by the acronym "PPSU"), polyvinylidene fluoride type (known by the acronym "PVDF"), ethylene chlorotrifluoroethylene type (known by the acronym "ECTFE") or of the polyolefin, polyaryl ether ketone, or polyamide type.

[0052] As described previously, the thermoplastic polymer 3 is configured to impregnate a plurality of reinforcing fibers 21 during the manufacture of the bipolar plate B. Figures 5 and 6 show a close-up view of a reinforcing fiber 21 impregnated in the thermoplastic polymer 3.

[0053] Preferably, the majority of the reinforcing fibers 21 are carbon fibers with high electrical conductivity properties. Preferably, at least 90% of the reinforcing fibers 21 are carbon fibers. Even more preferably, each reinforcing fiber 21 is a carbon fiber, which makes it possible to form a lightweight bipolar plate B while possessing high electrical conductivity.

[0054] Preferably, at least 50% of the reinforcing fibers 21 are so-called "long" fibers having a specific length greater than or equal to 1 x 10² m. By "specific length" is meant the length of the reinforcing fiber 21 when it extends longitudinally in a straight line along an axis (for example, an axis A shown in [Fig. 7]). Such a length increases the electrical conductivity of the bipolar plate B by forming a significant entanglement of reinforcing fibers 21 in the thermoplastic polymer matrix 3. In this example, each reinforcing fiber 21 has a diameter between 7 and 10 x 10⁶ m. In this example, the length of the reinforcing fibers 21 corresponds to the length of the reinforcing fibers 21 used to manufacture the non-woven carbon reinforcing film 2.In other words, the latter can be measured before the formation of a reinforcing film 2, that is to say before an entanglement of reinforcing fibers 21 is achieved.

[0055] Each reinforcing fiber 21 has an outer surface S21 defined along the entire length of the fiber. According to one aspect of the invention, the outer surface S21 of the reinforcing fibers 21 comprises a plurality of asperities (or peaks) and hollows to form a rough outer surface S21, as shown in [Fig. 6]. In particular, [Fig. 5] shows a reinforcing fiber 21 whose outer surface S21 is substantially smooth, and [Fig. 6] shows a reinforcing fiber 21 whose outer surface S21 is rough. [Fig. 7] shows a close-up view of the rough outer surface S21 of the reinforcing fiber 21 of [Fig. 6]. A rough outer surface S21 allows the crystals 31 of the thermoplastic polymer 3 to adhere to the reinforcing fiber 21 more effectively (as shown in [Fig. 6] and in comparison with [Fig. 7].5] on which few crystals adhere to the surface of the reinforcing fiber 21), which allows the permeability of the bipolar plate B to be lowered to fluids and in particular to hydrogen. .

[0056] In practice, at least 50% of the reinforcing fibers 21 preferably have an average surface roughness Ra greater than 0.4 x 10⁶ m. Even more preferably, the surface roughness Ra is greater than 0.75 x 10⁶ m. By definition, the surface roughness Ra corresponds to the distance between the arithmetic mean of the deviations Ec between the peaks and troughs (shown in [Fig. 7]) and the mean line LM of the reinforcing fiber 21. The mean line LM, also shown in [Fig. 7], and also known as the "center line," corresponds to the line of on either side of which all the peaks and troughs are equally distributed. Such a surface roughness Ra allows optimal adhesion of the thermoplastic polymer crystals 31 to the outer surface S21 of the reinforcing fiber 21. In this example, such a surface roughness Ra can be obtained by oxidative treatment of the reinforcing fiber 21, electrochemical treatment, plasma treatment, or functionalization of the surface by sizing or grafting.

[0057] Preferably, 60% of the reinforcing fibers 21 have a surface roughness Ra greater than 0.4 x 10⁶ m, preferably greater than 0.75 x 10⁶ m. Even more preferably, 68% of the reinforcing fibers 21 have a surface roughness Ra greater than 0.4 x 10⁶ m, preferably greater than 0.75 x 10⁶ m. In one embodiment, each reinforcing fiber 21 has a surface roughness Ra greater than 0.4 x 10⁶ m, preferably greater than 0.75 x 10⁶ m, allowing optimal adhesion of the thermoplastic polymer crystals 31 to the outer surface S21 of all the reinforcing fibers 21. In this example, more than 99% of the reinforcing fibers have a surface roughness Ra greater than 0.4 x 10⁶ m, preferably greater than 0.75 x 10⁶ m.

[0058] The surface roughness Ra of the reinforcing fibers 21 as described above can be measured using an Olympus® LEXT OLS5100 3D scanning laser microscope with a laser wavelength of 405 nm. The measurements can be performed with an MPLAPON50XLEXT objective and an image resolution of 1024 x 1024 pixels. The reinforcing fibers 21 are observed, for example, on surfaces of 250 pm x 250 pm. The roughness of the reinforcing fibers 21 can then be determined using a surface roughness measurement module of the Gwyddion® software (v2.62), by taking at least twenty reinforcing fibers 21 from each reinforcing film 2, with an analysis width of 2.5 pm. A threshold parameter can then be applied to obtain a surface roughness value Ra, the threshold preferably being 0.08, corresponding to a recommended value for the roughness range of 0.1 to 2 pm.The surface roughness Ra is thus calculated from the following equation | y. | and corresponds to the arithmetic mean of the absolute values ​​of the deviations of the evaluation profile (Yi) of the reinforcing fiber 21 from the mean line. .

[0059] It is known to those skilled in the art that the surface roughness Ra is related to the measurement of the Brunauer-Emmett-Teller (BET) specific surface area, and that an increase in the surface roughness Ra implies an increase in the specific surface area of ​​the reinforcing fiber 21. "Specific surface area" is understood to mean the ratio between the actual surface area of ​​the outer surface (i.e., including the increase in surface area due to asperities) of the reinforcing fiber 21 and the amount of material in that reinforcing fiber. 21. Such a specific surface area makes it possible to increase the number of attachment points on the surface of the reinforcing fiber 21, which makes it possible to increase tenfold the number of nucleation points of the thermoplastic polymer 3 around the reinforcing fiber 21. In other words, the crystallization around the reinforcing fibers 21 is greatly improved.

[0060] In one embodiment, as shown in [Fig.8], the bipolar plate B is formed from a carbon reinforcing film 2 and a layer of thermoplastic polymer 3.

[0061] More specifically, in this example, the thermoplastic polymer layer 3 is a thermoplastic polymer film 3. It goes without saying that the thermoplastic polymer matrix 3 could be formed, for example, from thermoplastic polymer powder 3.

[0062] Similarly, in this example, the carbon reinforcement film 2 is a non-woven carbon reinforcement film 2. Preferably, the carbon reinforcement film 2 is a carbon felt.

[0063] In this example, with reference to Figures 3 and 4, the carbon reinforcing film 2 extends in a plane (X, Y) and comprises a plurality of non-woven reinforcing fibers 21, so as to form an entanglement of reinforcing fibers 21. The thickness of the carbon reinforcing film 2 is defined along the vertical axis Z, orthogonal to the plane (X, Y). Preferably, 10% to 60% of the reinforcing fibers 21 extend substantially through the thickness of the carbon reinforcing film 2, i.e., along the vertical axis Z, so as to increase the electrical conductivity of the bipolar plate B.

[0064] It goes without saying that the reinforcement could be achieved in a different way, for example by means of free reinforcing fibers 21.

[0065] In one embodiment, the bipolar plate B is also formed by adding electrically conductive particles to its surface and throughout its thickness, so as to increase its electrical conductivity. In this example, the electrically conductive particles are carbon, graphite, or graphene particles.

[0066] A method for manufacturing a bipolar plate B as described previously, with reference to Figures 8 to 10, according to a preferred implementation method, will now be described.

[0067] In this example, the bipolar plate B is formed from a non-woven carbon reinforcing film 2, in which at least 50% of the reinforcing fibers 21 have a surface roughness Ra greater than 0.4 x 10⁶ m, and a thermoplastic polymer layer 3 in the form of a thermoplastic polymer film 3. Preferably, the surface roughness Ra of the reinforcing fibers 21 is greater than 0.75 x 10⁶ m. It is understood that the manufacturing process is analogous. for a bipolar plate formed from a thermoplastic polymer matrix 3, used, for example, in the form of thermoplastic polymer powder 3. Similarly, it is understood that the process also applies to the stacking of several carbon reinforcement films 2 and / or several layers of thermoplastic polymer 3. The process also applies to the production of a prepreg-type semi-finished product from one or more carbon reinforcement films 2 and thermoplastic polymer powder 3, and / or from one or more thermoplastic polymer films 3. In this example, the carbon reinforcement film 2 is a carbon felt and the thermoplastic polymer 3 is polyphenylene sulfide (PPS). The carbon reinforcement film 2 and the thermoplastic polymer film 3 have been previously cut to the desired dimensions.

[0068] With reference to [Fig. 8], the process comprises a first stacking step El, along a stacking axis (corresponding in this example to the vertical axis Z), of a first release film 4, the carbon reinforcement film 2, the thermoplastic polymer film 3 and a second release film 4, in order to form a stack 1. In one embodiment, the stack 1 is doped with electrically conductive particles to increase the electrical conductivity of the bipolar plate B. The electrically conductive particles can be previously added to the carbon reinforcement film 2 and / or to the thermoplastic polymer layer 3. In one embodiment, the stack 1 is consolidated, for example by the application of tack welds or by making stitches, so as to limit the risk of one of the films shifting, for example.

[0069] The stack 1 is then positioned in a mold M which preferably includes an EMP cavity to form the fluid circulation channels in the bipolar plate B. It is understood that the mold M could be without an EMP cavity to form the channels subsequently. In a second step E2 shown in [Fig. 9], the mold M is subjected to high-temperature pressure to form the bipolar plate B by thermocompression. Preferably, the temperature is raised to a forming temperature Tm, which in this example, where the thermoplastic polymer 3 is a PPS, is between 305 and 340°C. Similarly, the pressure is raised in this example to a forming pressure Pm of between 6 and 12 MPa, so as to apply sufficient pressure while limiting the deformation of the reinforcing fibers 21 to prevent the formation of porous cavities in the bipolar plate B.

[0070] This thermocompression leads to the crystallization of the thermoplastic polymer 3, in which a plurality of crystals 31 are formed. Thanks to the PPS-type thermoplastic polymer 3, the ratio of crystals 31 formed to the total volume of thermoplastic polymer 3 is between 45 and 56%. Furthermore, thanks to the reinforcing fibers 21 whose outer surface S21 has a surface roughness greater, in this example, than 0.75x106 m, the thermoplastic polymer 3 adheres more effectively to the outer surface S21 of the reinforcing fibers 21, which optimizes the impermeability of the bipolar plate B.

[0071] The assembly is then cooled and demolded in a third step E3. The release films 4 are removed, so as to expose the reinforcing fibers 21 on the surface of the manufactured bipolar plate B. In one embodiment, the bipolar plate B is cut to remove the peripheral portion of the bipolar plate B that has manufacturing defects.

[0072] Such a process advantageously allows the formation of a bipolar plate B having a thickness Ep, along the vertical axis Z, of less than 0.5 mm. Preferably, the process allows the formation of a bipolar plate B having a thickness Ep, along the vertical axis Z, of less than 0.3 mm. Such a thin bipolar plate B also advantageously has a hydrogen permeability of less than 2.5 x 10⁻¹⁰ mol / m / s / MPa, which allows optimal operation of the electrochemical device in which the bipolar plate B is mounted.

Claims

Demands

1. Bipolar plate (B) configured to be mounted in an electrochemical device, the electrochemical device being configured to carry out an electrochemical reaction, the bipolar plate (B) comprising a plurality of reinforcing fibers (21) impregnated in a thermoplastic polymer matrix (3), the bipolar plate (B) being characterized in that each reinforcing fiber (21) comprising an outer surface (S21), at least 50% of the reinforcing fibers (21) have an average surface roughness (Ra) greater than 0.4 x 10⁶ m, preferably greater than 0.75 x 10⁶ m.

2. Bipolar plate (B) according to claim 1, wherein at least 60% of the reinforcing fibers (21) have an average surface roughness (Ra) greater than 0.4 x 0.6 m, preferably greater than 0.75 x 0.6 m, preferably at least 68% of the reinforcing fibers (21) have an average surface roughness (Ra) greater than 0.4 x 0.6 m, preferably greater than 0.75 x 0.6 m.

3. Bipolar plate (B) according to any one of claims 1 to 2, wherein each reinforcing fiber (21) having a length, at least 50% of the reinforcing fibers (21) have a length greater than or equal to 1x102 m.

4. Bipolar plate (B) according to any one of claims 1 to 3, wherein each reinforcing fiber (21) is a carbon fiber.

5. Bipolar plate (B) according to claim 4, wherein the thermoplastic polymer (3) is selected from: polyphenylene sulfide (known as "PPS"), polyphenylsulfone (known as "PPSU"), polyvinylidene fluoride (known as "PVDF"), ethylene chlorotrifluoroethylene (known as "ECTFE") or of the polyolefin, polyaryl ether ketone, or polyamide type.

6. Bipolar plate (B) according to any one of claims 1 to 5, the bipolar plate (B) having a thickness less than or equal to 0.5x103 m, preferably less than or equal to 0.3x10 3m.

7. A method for manufacturing a bipolar plate (B) according to any one of claims 1 to 6, 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:

8.

9.

10.

11. • a stacking step, along a stacking axis (A) of at least: a reinforcing layer (2) comprising a plurality of reinforcing fibers (21) and a layer of a thermoplastic polymer (3), so as to form a stack (1), at least 50% of the reinforcing fibers (21) of the reinforcing layer (2) comprising an outer surface (S21) having a surface roughness (Ra) greater than 0.4x106 m, preferably greater than 0.75x106 m, and • a thermocompression step of the stack (1) in a compression system (S), so as to form a plurality of circulation channels to form the bipolar plate (B). Manufacturing method according to claim 7, wherein the reinforcement layer (2) is a non-woven carbon reinforcement film. Manufacturing method according to claim 8, wherein the non-woven carbon reinforcement film is a carbon felt. A manufacturing method according to any one of claims 8 to 9, wherein each reinforcing fiber (21) extends in the non-woven carbon reinforcing film along an orientation axis (F), the ratio of reinforcing fibers (21) oriented along the stacking axis (A) is between 10% and 60%. Manufacturing method according to any one of claims 7 to 10, comprising previously in the layering step, an addition step (El) of electrically conductive particles (91) to the reinforcing fibers (21) and / or the thermoplastic polymer (3).

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