Method for manufacturing a carbon fiber bipolar plate and associated bipolar plate

Abstract

A method for manufacturing a bipolar plate intended for mounting in an electrochemical device, the method comprising a step of stacking (E2), along a stacking axis (A), a first release film (4), a fibrous element (Q) comprising at least one non-woven carbon reinforcing film (2) and thermoplastic polymer (31), and a second release film (4), to form a stack (1); a step of pressurizing the stack (1) in the compression system (S); and, prior to the stacking step (E2), a step of adding electrically conductive particles (91) to the non-woven carbon reinforcing film (2) and / or the thermoplastic polymer (31) of the fibrous element (Q). Abstract figure: Figure 13

Description

Title of the invention: Method for manufacturing a carbon fiber bipolar plate and associated bipolar plate Technical field

[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 for carrying out an electrochemical reaction, such as a fuel cell or a proton exchange membrane electrolyser, for generating electrical energy or hydrogen respectively from an oxidation-reduction reaction. The term "electrochemical device" also refers to an oxidation-reduction flow battery for 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 [Fig.l], 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 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 half-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 [Fig.2], a bipolar plate 130 comprises, in a known manner, an active central portion 131, in contact with the membrane-electrode assembly s and on which the redox reaction occurs, and a peripheral portion 132 allowing the joining of two bipolar plates 130. The bipolar plate 130 further comprises a plurality of openings 133 for the circulation of the oxidizing and reducing fluids and the heat transfer fluid.

[0006] To enable optimal operation of the electrochemical device, the bipolar plate 130 must have significant electrical conductivity at the active central portion 131. In practice, to determine the electrical conductivity of a bipolar plate 130, it is known to measure an electrical resistance value, known by the acronym ASR meaning “Area-Specific-Resistance” in English. The lower the electrical resistance value, the more electrically conductive the bipolar plate is and the better the electrical exchanges in the electrochemical device will be.

[0007] In the prior art, bipolar plates made of graphite or metal are known, which have a very low electrical resistance. However, graphite plates are heavy and have a significant thickness. This is a disadvantage for an electrochemical device intended to be integrated, for example, in an aircraft or in any other mobility application. Furthermore, 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. In addition, the need for such a coating generates a significant additional cost on the price of 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, a bipolar plate comprising reinforcing fibers (for example carbon fibers) impregnated with a thermoplastic or thermosetting polymer matrix is ​​known in the prior art. Such a bipolar plate has the advantage of being thinner, which makes it possible to limit its size and mass.

[0010] However, in such a bipolar plate, the thermoplastic polymer 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 to reduce the electrical resistance. Such a 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. The use of a sacrificial film is also known for exposing the reinforcing fibers on the surface of the bipolar plate. However, despite the exposure of the reinforcing fibers, the bipolar plate made of reinforcing fibers impregnated in a polymer matrix has a significant electrical resistance which does not allow the expected levels to be reached for optimal operation of the electrochemical device..

[0011] In addition, the reuse of reinforcing fibers involves a very high shaping pressure (generally greater than 15 MPa), to allow the fibers to be properly impregnated, to reduce porosities and to expose the fibers on the surface 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.

[0012] To limit these drawbacks, a bipolar plate manufactured from a stack of a non-woven carbon reinforcement film and a thermoplastic polymer film is also known in the prior art, which is both thin and strong and whose open porosity rate is limited, which makes it possible to maintain significant impermeability to gases. The non-woven carbon reinforcement film comprises reinforcing fibers previously oriented along a stacking axis, so as to increase the electrical conductivity of the bipolar plate.

[0013] However, in such a bipolar plate, the electrical conductivity is not optimal. In other words, such a manufacturing method does not make it possible to achieve the conductivity levels of a bipolar plate made of graphite or metal, for example.

[0014] To date, there is no bipolar plate which is thin, light, mechanically strong and impermeable to gases while being sufficiently conductive to allow optimal operation of the electrochemical device.

[0015] Incidentally, document US2011129737A1 discloses a bipolar plate manufactured from a layer of pre-impregnated composite comprising continuous carbon fibers impregnated in a thermosetting matrix, to which a layer of expanded graphite is added to increase the electrical conductivity of the bipolar plate at its interface. In a second embodiment, a graphite or carbon black powder added to a solvent is sprayed onto the pre-impregnated composite to form the conductive layer on the surface. However, such a conductive layer added to the surface does not improve the overall electrical conductivity of the bipolar plate.

[0016] Also known from document CN103633336A is a bipolar plate manufactured from a woven pre-impregnated composite layer comprising a carbon powder mixed in a polymer resin. To improve the electrical conductivity, conductive particles, such as graphite or carbon black, are then added to the surface of the pre-impregnated composite. Hot pressing makes it possible to impregnate the conductive particles and thus increase the electrical conductivity both on the surface and in the thickness of the bipolar plate. However, as described previously, the use of a pre-impregnated material involves a very high shaping pressure to allow the fibers to be correctly impregnated. (usually between 5 and 50 MPa), which is not desirable because it may affect the mechanical performance of the bipolar plate.

[0017] To improve the electrical conductivity of a bipolar plate manufactured from a stack of a non-woven carbon reinforcement film and a thermoplastic matrix film, one solution would be to add carbon fibers to the final bipolar plate in which the thermoplastic matrix is ​​impregnated into the reinforcement film.

[0018] Such a solution is complex to implement and does not allow satisfactory results to be obtained. Indeed, given the use of a non-woven reinforcing film, adding conductive particles to such a bipolar plate does not allow the overall electrical conductivity of the bipolar plate to be optimally increased.

[0019] The invention thus aims to eliminate at least some of these drawbacks by proposing a bipolar plate that is both thin and strong and whose electrical resistance is sufficiently low to allow significant electrical exchanges in the electrochemical device. The invention aims in particular to manufacture such a bipolar plate simply and quickly by limiting the number of operations and avoiding the use of expensive industrial machines. PRESENTATION OF THE INVENTION

[0020] The invention relates to a method for manufacturing a bipolar plate intended to be mounted in an electrochemical device, the electrochemical device being configured to implement an electrochemical reaction, the method comprising: • a superposition step, along a stacking axis: • a first release film, • a fibrous element comprising at least one non-woven carbon reinforcing film and a thermoplastic polymer, and • a second release film, in order to form a stack, • a step of positioning the stack in a compression system, • a step of thermocompression of the stack in the compression system, so as to form a plurality of circulation channels to form a bipolar plate.

[0021] The manufacturing method is remarkable in that it comprises, prior to the superposition step, a step of adding electrically conductive particles to the non-woven carbon reinforcement film and / or to the thermoplastic polymer of the fibrous element.

[0022] The method according to the invention advantageously makes it possible to increase the electrical conductivity of the bipolar plate manufactured from a non-woven carbon reinforcement film. Thanks to the electrically conductive particles added to the stack, the manufactured bipolar plate has in particular a larger electrical contact surface, which makes it possible to increase the contact between the outer surface of the bipolar plate and the other components of the electrochemical device in which the bipolar plate is mounted. This advantageously makes it possible to improve the thermal and electrical exchanges in the electrochemical device. The electrically conductive particles added to the non-woven carbon reinforcement film also make it possible to form new electrical paths in the material, between the carbon fibers of the carbon reinforcement film, which makes it possible to reduce the electrical resistance of the material.In summary, the method according to the invention makes it possible to fill part of the open porosities of the non-woven carbon reinforcement film and thus increase the thermal and electrical conductivity of the material.

[0023] In addition, the pressurization step, which makes it possible to expose the conductive material on the surface, being carried out after the step of impregnating conductive particles, a greater number of conductive particles is exposed on the surface of the bipolar plate, which makes it possible to increase the conductive contact surface. A larger contact surface on which a greater number of conductive elements are exposed makes it possible to greatly limit the electrical resistance of the bipolar plate and therefore of the electrochemical device.

[0024] In addition, the impregnation of conductive particles directly onto the non-woven carbon reinforcement film or onto the thermoplastic matrix layer of the fibrous element makes it possible to lower the electrical contact resistance of the bipolar plate by a process that is simple and quick to carry out. A limited number of steps are also necessary to implement it.

[0025] In other words, the method according to the invention, simple to implement, advantageously makes it possible to add conductive particles inside and on the surface of the non-woven carbon reinforcement film, between each thickness of the stack, etc. to allow optimal electrical conductivity of the bipolar plate, in particular, in its thickness.

[0026] Preferably, the non-woven carbon reinforcing film comprises a plurality of reinforcing fibers, each reinforcing fiber extending along an orientation axis, the rate of reinforcing fibers oriented along the stacking axis is between 10% and 60%. Due to the reinforcing film comprising reinforcing fibers oriented along the stacking axis, the outer reinforcement surfaces have concavities which are, thanks to the method according to the invention, advantageously filled with the conductive particles. Thus, the manufactured bipolar plate has an electrical conductivity both increased due to the reinforcing fibers oriented along the stacking axis and which allow the electrical charges to be conducted in the thickness of the bipolar plate, and increased due to the conductive particles which are impregnated on the surface and in depth and which increase the electrical contact conductivity of the bipolar plate.

[0027] In one embodiment, the electrically conductive particles are carbon powder particles, which have the advantage of having significant electrical conductivity.

[0028] Preferably, the carbon powder is chosen from at least one of the following elements: graphite (natural, synthetic, expanded), carbon black, ground carbon fibers, carbon nanotubes, graphene. In particular, the carbon powder is chosen from any type of carbon particles or any mixture of carbon particles. All of these materials have the advantage of being able to be inserted into the open porosities of the carbon reinforcement film in an efficient manner and thus reliably increase the electrical conductivity of the bipolar plate.

[0029] According to a preferred aspect, the electrically conductive particles comprise at least 95% pure carbon. In other words, each electrically conductive particle is preferably a carbonaceous element whose chemical composition comprises at least 95% carbon. Such a chemical composition advantageously makes it possible to increase the electrical conductivity in the bipolar plate optimally without affecting the performance of the electrochemical device.

[0030] In one embodiment, each electrically conductive particle comprises at least 95% carbon and the following residual components: moisture, ash, calcium, silicon, iron, sulfur and aluminum.

[0031] More preferably, each electrically conductive particle has a Scott density greater than 0.07 g / cm3. The expression "Scott density" means the apparent density of each particle. Such a density makes it possible to homogenize the distribution of the electrically conductive particles while avoiding any risk of the particles agglomerating together. Thus, the electrical resistance is lowered across the entire bipolar plate in a homogeneous manner.

[0032] Preferably, the electrically conductive particles are particles with a high degree of purity, and which have high quality, mechanical strength and performance. Preferably, the particles are chosen from the following commercial products: TIMREX® KS5-75, TIMREX® KS6, TIMREX® KS15, ENSACO 250G, SuperiorGraphite FormulaBT®, Cabot ATHLOS® CNS, Cabot VULCAN®, AsburyCarbons ASB-4827, AsburyCarbons ASB-TC307, Mingheda SG-06, Mingheda SG-23 and ZEONANO® which have electrical conductivity optimal and are presented in the form of particles whose dimensions are adapted to implement the manufacturing process.

[0033] In a preferred embodiment, each electrically conductive particle is in the form of a chip whose largest dimension corresponding to a characteristic length is less than 250x106 m. Preferably, at least 90% of electrically conductive particles have a characteristic length of less than 110x106 m. More preferably, at least 50% of electrically conductive particles have a characteristic length of less than 50x106 m. Such a dimension advantageously allows the electrically conductive particles to be sufficiently fine to be able to be inserted into the porosities of the carbon reinforcement film and thus increase the electrical conductivity of the bipolar plate.

[0034] Preferably, the fibrous element having an overall mass, the mass rate of electrically conductive particles used in the addition step relative to the overall mass of the fibrous element is between 5% and 50%, preferably between 10 and 30%. Such a mass rate of conductive particles advantageously makes it possible to ensure an electrical resistance value preferably less than 12 mΩ.cm2, more preferably less than 10 mΩ.cm2. Thus the electrical resistance of the bipolar plate manufactured using the manufacturing method is sufficiently low to allow optimal operation of the electrochemical device.

[0035] In a first embodiment, the fibrous element is a stack of at least one non-woven carbon reinforcing film and at least one thermoplastic matrix layer comprising a thermoplastic polymer. This makes it possible to directly use known materials which are simple to superimpose in the raw state. In other words, it is not necessary to carry out a preliminary step, for example impregnation of the thermoplastic polymer in the non-woven carbon reinforcing film, which represents a significant time saving. The electrically conductive particles can be added directly to the thermoplastic polymer layer and / or to the non-woven carbon reinforcing film, which makes it possible to reduce production times.

[0036] In one embodiment, the non-woven carbon reinforcement film comprising two outer reinforcement surfaces, the addition step corresponds to the addition of conductive particles on at least one of the outer reinforcement surfaces of the non-woven carbon reinforcement film. The reinforcement film is thus charged directly by the electrically conductive particles, which makes it possible to ensure optimal distribution in the thickness of the bipolar plate.

[0037] In an alternative embodiment, the thermoplastic matrix layer being a thermoplastic polymer powder, the addition step corresponds to a mixture of the thermoplastic polymer powder and the electrically conductive particles, so as to form a pre-charged thermoplastic matrix layer. The electrically conductive particles are thus mixed with the thermoplastic polymer in a simple manner, ensuring a homogeneous distribution of the electrically conductive particles throughout the thermoplastic matrix layer. When manufacturing a bipolar plate, the electrically conductive particles are automatically added when placing the thermoplastic matrix layer on the non-woven carbon reinforcement film, which allows a simple and rapid addition of the electrically conductive particles to the stack.

[0038] Alternatively, the thermoplastic matrix layer being a thermoplastic matrix film, the adding step corresponds to the addition of electrically conductive particles onto the thermoplastic matrix film, so as to form a pre-charged thermoplastic matrix film. A thermoplastic film in which the electrically conductive particles have been previously impregnated makes it possible to simply store the charged thermoplastic matrix film and to directly use the non-woven carbon reinforcing film and the charged thermoplastic matrix film during the manufacture of a bipolar plate, which facilitates the process and allows significant time savings.

[0039] In a second embodiment, the fibrous element is a non-woven carbon reinforcement film impregnated with thermoplastic polymer, referred to as a "prepreg". In other words, such an embodiment makes it possible to use a prepreg-type material preloaded with electrically conductive particles for the manufacture of the bipolar plate. Such a fibrous element can be easily stored, for example in rolls. It is thus easy to add the prepreg fibrous element loaded with electrically conductive particles during the superposition step without it being necessary to handle a large number of elements. Storage and logistics are also greatly improved.

[0040] In one embodiment, the step of adding electrically conductive particles is carried out on the non-woven carbon reinforcement film before the impregnation of thermoplastic polymer to form the prepreg. The reinforcement film is thus charged directly by the electrically conductive particles, which makes it possible to ensure optimal distribution in the thickness of the fibrous element and therefore of the bipolar plate.

[0041] In an alternative embodiment, the thermoplastic polymer being in the form of a thermoplastic matrix film, the step of adding particles electrically conductive particles is carried out on the thermoplastic matrix film before impregnating the non-woven carbon reinforcement film to form the prepreg. The prepreg-type fibrous element can thus be easily manufactured from a non-woven carbon reinforcement film and a thermoplastic matrix film preloaded with electrically conductive particles.

[0042] Alternatively, since the thermoplastic polymer is in the form of a thermoplastic polymer powder, the step of adding electrically conductive particles corresponds to a mixture of the thermoplastic polymer powder and the electrically conductive particles before pre-impregnating the non-woven carbon reinforcement film. This makes it possible to form a pre-loaded thermoplastic matrix layer which will be used to form the pre-impregnated fibrous element. The electrically conductive particles are thus mixed with the thermoplastic polymer powder in a simple manner, ensuring a homogeneous distribution throughout the thermoplastic matrix layer.In other words, in this embodiment, the mixture of electrically conductive particles and thermoplastic polymer powder, following its deposition on the non-woven carbon reinforcement film, can be melted in order to fix both the thermoplastic polymer and the electrically conductive particles in the non-woven carbon reinforcement film, to form an electrically pre-charged prepreg which can be used directly during the manufacture of the bipolar plate. Its subsequent handling is thus simple and practical.

[0043] In a preferred embodiment, the addition step corresponds to a spraying of a mixture of the electrically conductive particles and a solvent, allowing a simple and rapid deposition to be carried out, while using an optimal quantity of conductive particles and limiting losses. Spraying also makes it possible to ensure a homogeneous deposition of the conductive particles over the entire surface to be impregnated. A mixture with a solvent also makes it possible to impregnate the electrically conductive particles more effectively in the material.

[0044] In an alternative embodiment, the addition step corresponds to immersing the non-woven carbon reinforcement film in a mixture of electrically conductive particles and a solvent. Such an embodiment makes it possible to simultaneously impregnate the entire carbon reinforcement film to be impregnated, which allows a significant saving of time.

[0045] Preferably, the solvent is ethanol, allowing better dispersion of the electrically conductive particles.

[0046] Alternatively, the impregnation step is carried out dry, for example with a sieve, which makes it possible to avoid a solvent evaporation step.

[0047] Preferably, the step of adding electrically conductive particles is carried out on the two outer reinforcement surfaces, which makes it possible to lower the electrical contact resistance on each face of the bipolar plate and in its thickness quickly and efficiently.

[0048] In an alternative embodiment, the step of adding electrically conductive particles is carried out on at least one of the external reinforcement surfaces and by means of a thermoplastic matrix layer in the form of a thermoplastic film loaded with electrically conductive particles, in the recommended proportions, making it possible to increase the number of electrically conductive particles and to homogenize their presence in the non-woven carbon reinforcement film to increase the electrical conductivity at the surface and in the thickness of the bipolar plate.

[0049] The invention also relates to a bipolar plate intended to be mounted in an electrochemical device, the bipolar plate comprising at least one fibrous element comprising at least one non-woven carbon reinforcing film and thermoplastic polymer, the thermoplastic polymer having impregnated the non-woven carbon reinforcing film, electrically conductive particles having been previously added to the non-woven carbon reinforcing film and / or to the thermoplastic polymer of the fibrous element. The manufactured bipolar plate thus advantageously comprises electrically conductive particles distributed homogeneously on the surface and in its thickness, which makes it possible to significantly increase its electrical conductivity.

[0050] According to one aspect, the bipolar plate has an electrical resistance of less than 12 mQ.cm2, preferably less than 10 mQ.cm2, which allows optimal electrical conductivity. The electrical exchanges are thus advantageously optimal in the electrochemical device in which the bipolar plate is mounted. In other words, this allows optimal operation of the electrochemical device with a thin and light bipolar plate manufactured from a non-woven carbon reinforcement film. PRESENTATION OF FIGURES

[0051] The invention will be better understood on reading the description which follows, 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.

[0052] [Fig.l] is a schematic representation of a stack of membrane-electrode assemblies and bipolar plates of an electrochemical device.

[0053] [Fig.2] is a schematic representation of a bipolar plate of [Fig.l].

[0054] [Fig. 3] is a schematic representation of a superposition of a fibrous element, comprising a non-woven carbon reinforcement film and a thermoplastic matrix layer, and two release films for the manufacture of a bipolar plate according to an embodiment of the invention.

[0055] [Fig.4] is a close-up view of the non-woven carbon reinforcement film of [Fig.3].

[0056] [Fig.5] is a schematic representation of a view along a sectional plane of the non-woven carbon reinforcement film of [Fig.4].

[0057] [Fig.6] is a close-up view of the cross-sectional view of the non-woven carbon reinforcement film of [Fig.5].

[0058] [Fig.7] is a schematic representation of an impregnated non-woven reinforcing film of conductive particles.

[0059] [Fig.8] is a close-up view of the non-woven carbon reinforcement film impregnated with conductive particles of [Fig.7].

[0060] [Fig.9] is a schematic representation of a fibrous element comprising a non-woven carbon reinforcing film and thermoplastic polymer according to a first embodiment of the invention.

[0061] [Fig. 10] is a schematic representation of a fibrous element comprising a non-woven carbon reinforcing film and thermoplastic polymer according to a second embodiment of the invention.

[0062] [Fig. 11] is a schematic representation of an addition step of the manufacturing method according to a first embodiment of the invention.

[0063] [Fig. 12] is a schematic representation of an addition step of the manufacturing method according to a second embodiment of the invention.

[0064] [Fig. 13] is a schematic representation of a superposition step of the manufacturing method according to an embodiment of the invention in which the fibrous element is according to the first embodiment shown in [Fig.9].

[0065] [Fig. 14] is a schematic representation of an addition step and a superposition step of the manufacturing process according to an alternative embodiment of the invention.

[0066] [Fig. 15] is a schematic representation of a superposition step of the manufacturing method according to an alternative embodiment of the invention in which the fibrous element is according to the second embodiment shown in [Fig. 10],

[0067] [Fig. 16] is a schematic representation of a positioning step of the manufacturing method according to an embodiment of the invention.

[0068] [Fig. 17] is a schematic representation of a pressurization step of the manufacturing process according to an embodiment of the invention.

[0069] [Fig. 18] is a schematic representation of a cooling step of the manufacturing process according to an embodiment of the invention.

[0070] [Fig. 19] is a graph representing the evolution of temperature and pressure during the manufacturing process according to the invention.

[0071] [Fig.20] is a schematic representation of a removal step of the manufacturing process according to an embodiment of the invention.

[0072] [Fig.21] is a schematic representation of a bipolar plate manufactured using the manufacturing method according to the invention.

[0073] [Fig.22] is a table showing the electrical resistance values ​​of the bipolar plate of [Fig.21] manufactured using the manufacturing method according to the invention as a function of the thermoplastic polymer used and the mass content of electrically conductive particles added.

[0074] [Fig.23] is a table showing the residual components of the chemical composition of the electrically conductive particles according to one embodiment of the invention.

[0075] 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

[0076] The invention relates to a method for manufacturing a bipolar plate made of composite material for an electrochemical device.

[0077] Subsequently, as described previously, the term “electrochemical device” equally designates a fuel cell, a proton exchange membrane electrolyzer, an oxidation-reduction flow battery or any other device making it possible to implement an electrochemical reaction.

[0078] 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.

[0079] In this document, the bipolar plate B (shown in [Fig. 21]) is described in an orthogonal (X, Y, Z) coordinate system (also shown in [Fig. 4]) in which the (X, Y) plane defines a horizontal plane and the Z axis defines a vertical Z axis orthogonal to the horizontal (X, Y) plane. The terms "horizontal" and "vertical" are understood to mean a vertical Z axis that extends from the bottom to the top, as shown in [Fig. 18].

[0080] Fibrous element Q.

[0081] According to one aspect of the invention, the bipolar plate B is formed from a fibrous element Q which comprises at least one non-woven carbon reinforcing film 2 and the thermoplastic polymer 31. In this example, the fibrous element Q comprises a single non-woven carbon reinforcing film 2. It goes without saying that the fibrous element Q could just as well comprise several non-woven carbon reinforcing films 2. Subsequently, for the sake of brevity, the non-woven carbon reinforcing film will be referred to as reinforcing film 2.

[0082] Reinforcing film 2.

[0083] With reference to [Fig. 4], the reinforcing film 2 extends in this example in the plane (X, Y) and has two substantially parallel outer reinforcing surfaces 2A and 2B. In particular, the reinforcing film 2 has a lower reinforcing surface 2A and an upper reinforcing surface 2B. In this example, the terms “lower” and “upper” are described with respect to a vertical axis Z which extends from the bottom to the top, as shown in [Fig. 4]. The thickness Ep2 of the reinforcing film 2 is defined along the vertical axis Z.

[0084] The reinforcing film 2 comprises a plurality of reinforcing fibers 21. Preferably, the reinforcing fibers 21 are formed from carbon fibers.

[0085] 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, which extends from the lower reinforcing surface 2A to the upper reinforcing surface 2B. In other words, at least a portion of the reinforcing fibers 21 extends substantially in the thickness Ep2 of the reinforcing film 2, that is to say along the vertical axis Z. In particular, still with reference to [Fig. 4], the orientation axis F is defined by forming an angle Theta 0 with the vertical axis Z and an angle Phi q> with the axis X, in a projection plane (X, Y). Preferably, the orientation axis F extends so that the angle Theta 0 is less than 45° or greater than 135° and the angle Phi q> 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 vertical axis Z. Such an orientation of the reinforcing fibers 21 makes it possible to increase the electrical conductivity and therefore a . once manufactured, in the bipolar plate B while retaining important mechanical characteristics.

[0086] Preferably, between 10% and 60% of the reinforcing fibers 21 of the reinforcing film 2 extend substantially along the vertical axis Z between the lower reinforcing surface 2A and the upper reinforcing surface 2B. More preferably, between 15% and 45% of the reinforcing fibers 21 of the reinforcing film 2 extend substantially along the vertical axis Z. In one embodiment, the reinforcing fibers 21 are oriented along the vertical axis Z by a needling, stitching or sewing process for example. Such processes are known to those skilled in the art, and will not be described in more detail in this document.

[0087] 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 effective impregnation of the carbon felt by the thermoplastic matrix. The expression “open porosity” means an open cavity, i.e. not closed by the arrangement of the reinforcing fibers 21 and / or by its sizing. The carbon felt also has the advantage of being able to be easily deformed, allowing adaptation of the reinforcing film 2 to complex shapes. In addition, the carbon felt has the advantage of requiring a lower shaping pressure (generally between 6 and 10 MPa) than the shaping pressure necessary for a woven reinforcement (commonly greater than 15 MPa).

[0088] Still with reference to [Fig.4], the reinforcing film 2 has a thickness Ep2, along the vertical axis Z, preferably less than or equal to 5mm. Preferably, the thickness Ep2 is between 0.5 and 2.5mm. Such a thickness Ep2 advantageously makes it possible to limit the size of the bipolar plate B once manufactured.

[0089] 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 B. 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, which makes it possible to limit its size in the electrochemical device in which it will be mounted.

[0090] In a preferred embodiment, the reinforcing film 2 has a surface mass less than or equal to 500g / m2. Preferably, the surface mass is between 50 and 300g / 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 limited mass of the bipolar plate B itself thus allows it to be integrated into an electrochemical device intended to be mounted, for example, in an aircraft or any other vehicle.

[0091] The reinforcing film 2 is configured to be impregnated with thermoplastic polymer 31 to form a thermoplastic matrix of the bipolar plate B after manufacture. For this, the fibrous element Q is presented for example in two different embodiments.

[0092] Fibrous element Q corresponding to a stack.

[0093] In a first embodiment shown in Figures 3 and 9, the fibrous element Q is a stack of a reinforcing film 2 and a thermoplastic matrix layer 3. Such a stack is also known to those skilled in the art under the designation “stacking”. The term “layer” means a quantity of thermoplastic polymer 31 distributed substantially uniformly over the dimensions of the bipolar plate B to be manufactured. For the sake of brevity, the thermoplastic matrix layer 3 will hereinafter be referred to as the “thermoplastic layer” 3.

[0094] In a first embodiment, with reference to [Fig. 3], the thermoplastic layer 3 is in the form of a thermoplastic film which also extends in a plane and has two substantially parallel thermoplastic outer surfaces 3A and 3B.

[0095] Preferably, still with reference to [Fig. 3], the thermoplastic film has a thickness Ep3 of between 50 and 600 μm. Such a thickness makes it possible to have a sufficient quantity of thermoplastic polymer 31 to form the thermoplastic matrix of the bipolar plate B, while limiting its volume and its mass.

[0096] In a second embodiment, the thermoplastic layer 3 is in the form of thermoplastic polymer powder 31. In this embodiment, the thermoplastic layer 3 is formed from a quantity of thermoplastic powder configured to be dry-sprinkled onto the reinforcing film 2, as will be described in more detail later. The powdering is configured to be carried out, for example, by electrostatic spraying, mechanical spraying, dipping, etc.

[0097] The thermoplastic matrix layer 3 is then configured to impregnate the reinforcing film 2 during the manufacture of the bipolar plate B, as will be described in more detail later.

[0098] Fibrous element Q of the “pre-impregnated” type.

[0099] In an alternative embodiment, the fibrous element Q is a “pre-impregnated” type element in which the reinforcing film 2 is pre-impregnated with thermoplastic polymer 31. In a manner similar to a stack, the pre-impregnated type fibrous element Q can be manufactured according to different embodiments.

[0100] In a first embodiment, the pre-impregnated fibrous element Q corresponds to a thermoplastic matrix film 3 pre-impregnated in the reinforcing film 2. In this example, the thermoplastic matrix film 3 has characteristics similar to those stated previously.

[0101] In a second embodiment, the pre-impregnated fibrous element Q corresponds to a thermoplastic polymer powder 31 pre-impregnated in the reinforcing film 2. In this embodiment, a quantity of thermoplastic polymer powder 31 is configured to be dry-sprinkled onto the reinforcing film 2, as will be described in more detail later. The powdering is configured to be carried out for example by electrostatic spraying, mechanical spraying, by dipping, etc.

[0102] The pre-impregnation of the reinforcing film 2 with the thermoplastic polymer 31 is preferably carried out by any known method for producing a prepreg, such as for example thermocompression, draping, hot coating, etc. Such methods for producing a prepreg are known to those skilled in the art and will not be described in more detail in this document.

[0103] Thermoplastic polymer 31.

[0104] In one embodiment, the thermoplastic polymer 31 is of the semi-crystalline type, making it possible to give the bipolar plate B significant impermeability to fluids, in particular to hydrogen, while allowing significant mechanical and chemical resistance. Alternatively, the thermoplastic polymer 31 is of the amorphous type, allowing greater ductility as well as a low rate of dimensional shrinkage during the cooling step.

[0105] The thermoplastic polymer 31 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.

[0106] For mounting the bipolar plate B in a high-temperature proton exchange membrane, the thermoplastic polymer 31 is preferably of the polyphenylene sulfide (PPS), polyphenylsulfone (PPSU) type, of the ethylene chlorotrifluoroethylene (ECTFE) type, of the poly aryl ether ketone type.

[0107] For the assembly of the bipolar plate B in a Redox flow battery with vanadium, hydrogen bromide, or any liquid electrolyte (organic for example) type, the thermoplastic polymer 31 is preferably of polyphenylene sulfide (PPS) and Polyphenylsulfone (PPSU) type if the electrolyte is basic type, and Polyvinylidene fluoride (PVDF) or ethylene chlorotrifluoroethylene (ECTFE) type if the electrolyte is acidic type, or poly aryl ether ketone type.

[0108] Electrically conductive particles 91.

[0109] The bipolar plate B according to the invention is also manufactured from electrically conductive particles 91 configured to be impregnated in the reinforcing film 2 and / or in the thermoplastic polymer 31 of the fibrous element Q, so as to increase the electrical conductivity of the final bipolar plate B. Subsequently, for the sake of brevity, the electrically conductive particles 91 will be designated “conductive particles 91”.

[0110] In a preferred embodiment, the conductive particles 91 are carbon powder particles which have a high electrical conductivity. In this example, the conductive particles 91 are in powder form, however it goes without saying that the conductive particles 91 could alternatively be in a different form, for example in the form of fibers.

[0111] Preferably, the conductive particles 91 in the form of carbon powder are chosen from one or more of the following elements: graphite (natural, synthetic, expanded), carbon black, ground carbon fibers, carbon nanotubes, graphene or any type of carbon particles. In particular, the conductive particles 91 may be in the form of one of the carbon materials mentioned above or a mixture of several of the carbon materials mentioned above. All of these materials have the advantage of being able to be inserted into the open porosities of the reinforcing film 2 in an efficient manner and thus reliably increase the electrical conductivity of the bipolar plate.

[0112] Preferably, the conductive particles 91 are particles with a high degree of purity and have high quality, performance in terms of electrical conductivity and mechanical resistance. The conductive particles 91 are in this example chosen from the following commercial products: TIMREX® KS5-75, TIMREX® KS6, TIMREX® KS15, ENSACO 250G, SuperiorGraphite FormulaBT®, Cabot ATHLOS® CNS, Cabot VULCAN®, AsburyCarbons ASB-4827, AsburyCarbons ASB-TC307, Mingheda SG-06, Mingheda SG-23, Nanocyl®, Entegris®, and ZEONANO®. Each of these products has optimal electrical conductivity and is in the form of particles whose dimensions are adapted to be able to implement the manufacturing method according to the invention which will be described in more detail later.

[0113] In a preferred embodiment, each conductive particle 91 is in the form of a chip whose largest dimension, designated “characteristic length”, is less than 250.106 m. Preferably, at least 90% of the conductive particles 91 have a characteristic length of less than 110.106 m. More preferably, at least 50% of the conductive particles 91 have a length of less than 50.106 m. Such a dimension advantageously allows the conductive particles 91 to be sufficiently fine to be able to be inserted into the porosities of the reinforcing film 2 and thus increase the electrical conductivity of the bipolar plate.

[0114] Preferably, each conductive particle 91 is a carbon compound whose chemical composition comprises at least 95% carbon and whose residual components CR listed in the table shown in [Fig.23] are: humidity Hum, ash CD, Calcium Ca, Silicon Si, Iron Fe, Sulfur S and Aluminum Al. [Fig.21] also shows the maximum quantity Qmax of each component CRes.

[0115] Furthermore, each conductive particle 91 preferably has a Scott density, also referred to as “apparent density”, greater than or equal to 70 kg / m3, which makes it possible to limit the risks of agglomeration of the conductive particles 91 between themselves during the implementation of the method.

[0116] Preferably, the mass ratio of conductive particles 91 relative to the mass of the fibrous element Q, i.e. relative to the mass of the assembly formed by the reinforcing film 2 and the thermoplastic polymer 31, is between 5% and 50%. More preferably, the mass ratio of conductive particles 91 is between 10% and 30%, which makes it possible to lower the electrical resistance characterized by the ASR (meaning "Area-Specific-Resistance" in English) of the bipolar plate B manufactured below 12.103 Q.cm2, preferably below 10.103 Q.cm2, as will be described in more detail later.

[0117] The conductive particles 91 are configured to be added into the reinforcing film 2 and / or into the thermoplastic polymer 31 of the fibrous element Q.

[0118] Addition of electrically conductive particles to the fibrous element Q.

[0119] More specifically, in a first embodiment, the conductive particles 91 are configured to be added to at least one of the outer reinforcing surfaces 2A, 2B of the reinforcing film 2. Preferably, the conductive particles 91 are configured to be added to each of the two outer reinforcing surfaces 2A, 2B so as to impregnate the interior of the reinforcing film 2 more effectively.

[0120] In this respect, as shown in [Fig. 5], due to the reinforcing fibers 21 oriented along the vertical axis Z, the ends 21a, 21b of the reinforcing fibers 21 form peaks or points forming outer reinforcing surfaces 2A, 2B with a relief. More precisely, each outer reinforcing surface 2A, 2B comprises a plurality of concave portions 22, each concave portion 22 being formed between several ends 21a, 21b of reinforcing fibers 21 oriented along the vertical axis Z, as shown in [Fig.6], representing a close-up view of [Fig.5].

[0121] In this example, as shown in Figures 7 and 8, each concave portion 22 is filled with the conductive particles 91, so as to form two substantially planar outer reinforcing surfaces 2A, 2B and increase the electrical conductivity at the surface, as will be described in more detail later. Similarly, the conductive particles 91 are configured to be introduced into the thickness of the reinforcing film 2 so as to increase the electrical conductivity deep in the bipolar plate.

[0122] The addition of conductive particles 91 may be carried out by spraying a mixture 9 of the conductive particles 91 and a solvent 92 onto the outer reinforcing surfaces 2A, 2B or by immersion in such a mixture 9. Preferably, the solvent 92 is ethanol, so as to effectively disperse the mixture 9 on each surface. It goes without saying that the solvent 92 could be different, for example isopropanol, acetone or any solvent evaporating at a temperature below 150°C. Alternatively, the conductive particles 91 are configured to be dry-sprinkled onto the outer surface(s) 2A, 2B of the reinforcing film 2, as will be described in more detail later.

[0123] In other words, in this example, the conductive particles 91 are configured to be added to the reinforcing film 2 before the latter is introduced into the stack to form the fibrous element Q. In the embodiment in which the fibrous element Q corresponds to a prepreg, the assembly formed by the reinforcing film 2 and the conductive particles 91 is configured to be impregnated with the thermoplastic polymer 31 in the form of thermoplastic polymer powder 31 or thermoplastic matrix film 3. An addition of the conductive particles 91 directly into the reinforcing film 2 before impregnation with thermoplastic polymer 31 makes it possible to ensure a homogeneous distribution in the thickness of the reinforcing film 2 and therefore in the thickness of the bipolar plate B which will be manufactured. The electrical conductivity of the bipolar plate B is therefore advantageously increased at the surface and in the thickness of the bipolar plate B.

[0124] In a second embodiment in which the thermoplastic polymer 31 is in the form of a thermoplastic film, the conductive particles 91 are configured to be added to at least one of the thermoplastic outer surfaces 3A, 3B of the thermoplastic film. In this embodiment, preferably, each of the thermoplastic outer surfaces 3A, 3B are impregnated with conductive particles 91.

[0125] The addition of conductive particles 91 can be carried out by spraying a mixture 9 of the conductive particles 91 and a solvent 92 onto the outer surfaces thermoplastics 3A, 3B, in a manner analogous to the reinforcing film 2. Alternatively, the conductive particles 91 are configured to be dry-sprinkled onto the outer surface(s) 3A, 3B of the thermoplastic layer 3 in the form of a thermoplastic film.

[0126] In other words, in this example, the conductive particles 91 are configured to be added to the thermoplastic film before the latter is introduced into the stack to form the fibrous element Q. In the embodiment in which the fibrous element Q corresponds to a prepreg, the thermoplastic film and the conductive particles 91 together impregnate the reinforcing film 2. An addition of the conductive particles 91 directly into the thermoplastic film makes it possible to form an electrically precharged thermoplastic film ready for use for impregnating the reinforcing film 2 during the formation of the fibrous element Q in the form of a prepreg or directly the manufacture of the bipolar plate B, as will be described in more detail later.

[0127] In an alternative embodiment, both the two outer reinforcing surfaces 2A, 2B and the two outer thermoplastic surfaces 3A, 3B are configured to be impregnated with conductive particles 91. It goes without saying that both only one of the outer reinforcing surfaces 2A, 2B and the two outer thermoplastic surfaces 3A, 3B or only one of the outer thermoplastic surfaces 3A, 3B and the two outer reinforcing surfaces 2A, 2B could be impregnated with conductive particles 91.

[0128] The example of a single reinforcing film 2 is presented to form the fibrous element Q, however, it goes without saying that when the fibrous element Q is manufactured from a different number of reinforcing films 2, each outer reinforcing surface 2A, 2B, of each reinforcing film 2 can be impregnated with conductive particles 91. The same is true for a fibrous element Q manufactured from polymer in the form of a thermoplastic film and comprising several thermoplastic films, each thermoplastic outer surface 3A, 3B could be impregnated with conductive particles 91.

[0129] Alternatively, the addition of conductive particles 91 is carried out by mixing them with the thermoplastic polymer 31 which is in the form of thermoplastic powder. In practice, in this example, the conductive particles 91 are added, in the proportions described above, by producing granules from a mixture and kneading of thermoplastic polymer powder 31 and conductive particles 91. The granules can then be extruded by extrusion-inflation, extrusion-drawing, extrusion-cladding, extrusion-calendering, extrusion in a flat die, or even coextrusion. This makes it possible to directly obtain a film highly conductive filled thermoplastic that will be used directly in the stack, as will be described in more detail later.

[0130] In the example in which the fibrous element Q is in the form of a stack of a reinforcing film 2 and a thermoplastic matrix layer 3, the conductive particles 91 are directly added to the stack during the formation of the thermoplastic matrix layer 3 with the thermoplastic polymer 31 and the conductive particles 91 mixed. Similarly, in the example in which the fibrous element Q is in the form of a prepreg, this embodiment makes it possible to mix the conductive particles 91 directly with the thermoplastic polymer powder 31 before the assembly pre-impregnates the reinforcing film 2, as will be described in more detail later.

[0131] Manufacturing process.

[0132] A method of manufacturing a bipolar plate B will now be described according to several embodiments of the invention, with reference to FIGS. 11 to 20.

[0133] With reference to [Fig. 3], the bipolar plate B is manufactured, in this example, from a superposition of a fibrous element Q, comprising a non-woven carbon reinforcing film 2 and thermoplastic polymer 31, and two release films 4 to form a stack 1. The stack 1 extends along a stacking axis A. In this example, the stacking axis A corresponds to the vertical axis Z of the (X, Y, Z) defining reference frame of the reinforcing film 2. In other words, the reinforcing fibers 21 of the reinforcing film 2 oriented along the vertical axis Z described previously, are oriented during the superposition along the stacking axis A. The use of release films 4 makes it possible to expose the reinforcing fibers 21 and the conductive particles 91 on the surface of the formed bipolar plate B and to increase its electrical conductivity.Preferably, the reinforcing film 2 and each release film 4 are in the form of a roll, allowing simple storage and handling. Each film 2, 4 is previously cut to the desired dimensions.

[0134] Fibrous element Q corresponds to a stack.

[0135] In a first embodiment, the fibrous element Q is in the form of a stack of a reinforcing film 2 and a layer of thermoplastic matrix 3, as shown in FIGS. 13 and 14.

[0136] Furthermore, in a first example, the thermoplastic polymer 31 is in the form of a thermoplastic film. In this example, the thermoplastic film is in the form of a roll, allowing simple handling.

[0137] The reinforcing film 2 and the thermoplastic film preferably have similar dimensions. Each release film 4 has dimensions larger than the dimensions of the reinforcing film 2 and the thermoplastic film, so as to protrude from stack 1 so that it can be more easily removed after the formation of bipolar plate B.

[0138] Addition of particles to the reinforcing film.

[0139] Subsequently, the method will be described firstly for an example in which the conductive particles 91 are added to the reinforcing film 2, as shown in [Fig. 13].

[0140] In this example, the manufacturing method comprises a preliminary step of preparing a mixture 9 of conductive particles 91 and a solvent 92. In this example, the conductive particles 91 are graphite powder particles whose characteristic length is less than 50.106 m and which comprises a proportion of carbon greater than or equal to 95%. In particular, in this example, the conductive particles 91 comprise a proportion of carbon greater than or equal to 99.4%, the remaining 0.6% being composed of the CR components listed in the table of [Fig. 21]. In this example, the solvent 92 is ethanol. The mass of conductive particles 91 introduced into the mixture 9 has preferably been previously calculated so that the mass ratio of conductive particles 91 relative to the mass of the entire fibrous element Q is for example 10%. A predetermined quantity of mixture 9 is thus obtained.

[0141] The method then comprises a first step E1 of adding electrically conductive particles 91 to the fibrous element Q. In this example, in the addition step E1, the mixture 9 comprising the conductive particles 91 is added to each outer reinforcing surface 2A, 2B of the reinforcing film 2. Alternatively, the mixture 9 could be applied to only one of the outer reinforcing surfaces 2A, 2B.

[0142] In this addition step E1, in one embodiment, with reference to [Fig.l 1], an operator sprays the mixture 9, for example using a sprayer, onto the lower reinforcement surface 2A. In this example, half of the quantity of mixture 9 prepared is sprayed onto the first lower reinforcement surface 2A. The reinforcement film 2 is then turned over, taking care to keep the conductive particles 91 deposited on the lower reinforcement surface 2A in place. The operator then sprays the second half of mixture 9 onto the upper reinforcement surface 2B. Spraying using a sprayer allows for homogeneous, simple and rapid deposition.

[0143] In a second embodiment, with reference to [Fig. 12], the reinforcing film 2 is immersed in a container containing the mixture 9. This allows an efficient and simultaneous distribution of the conductive particles 91 over all of the external reinforcing surfaces 2A, 2B, the interior of the reinforcing film 2 is also advantageously impregnated.

[0144] The reinforcing film 2 to which the conductive particles 91 have been added is then placed, in this example, in an oven, so as to evaporate the solvent. In this example, the reinforcing film 2 is placed in an oven at 130°C for 10 min.

[0145] In a third embodiment (not shown), in the addition step E1, an operator dry sprinkles a predetermined quantity of conductive particles 91. In a manner analogous to the mixing with a solvent 92, in this example, the quantity of conductive particles 91 has been previously calculated so that the mass ratio of conductive particles 91 relative to the mass of the fibrous element Q is 10%. The sprinkling of the conductive particles 91 is carried out, for example by means of a sieve, on one of the outer reinforcement surfaces 2A, 2B, then on the other of the outer reinforcement surfaces 2A, 2B. This saves time because the drying / evaporation step of the solvent is not necessary.

[0146] Once the addition step E1 is complete, the conductive particles 91 are impregnated in the thickness of the reinforcing film 2 and make it possible to increase the electrical conductivity on the surface and in depth in the reinforcing film 2, as will be described in more detail later.

[0147] When the reinforcing film 2 is impregnated with conductive particles 91, the method comprises, in this example, a superposition step E2, along the stacking axis A, of the first demolding film 4, the reinforcing film 2 impregnated with conductive particles 91, the thermoplastic film and the second demolding film 4, in order to form a stack 1, as shown in [Fig. 13]. 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 the two opposite faces of the manufactured bipolar plate B. The conductive particles 91 on the outer surfaces 2A, 2B make it possible to increase the contact surface of the faces of the bipolar plate B.The reinforcing fibers 21 and the conductive particles 91 thus optimally conduct electricity into the bipolar plate B and therefore into the electrochemical device in which the latter 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 impregnated with conductive particles 91, the thermoplastic film and the second release film 4, or by the successive positioning of the first release film 4, the thermoplastic film, the reinforcing film 2 impregnated with conductive particles 91 and the second release film 4.

[0148] An example is described in which the thermoplastic matrix layer 3 is in the form of a thermoplastic film, the conductive particles being added to the reinforcing film 2, however the thermoplastic matrix layer 3 could alternatively be in the form of polymer powder. thermoplastic 31. In this embodiment, the operator superimposes the first demolding film 4 and the reinforcing film 2 impregnated with conductive particles 91 and sprinkles the thermoplastic polymer powder 31 evenly over the entire reinforcing film 2 before superimposing the second demolding film 4 to form the stack 1.

[0149] Alternatively, in the case of a stack 1 comprising two layers of thermoplastic matrix 3, in this step, an operator could just as easily successively superimpose a first demolding film 4, a first layer of thermoplastic matrix 3, the reinforcing film 2 impregnated with conductive particles 91, a second layer of thermoplastic matrix 3 and a second demolding film 4. The use of two layers of thermoplastic matrix 3 positioned on either side of the reinforcing film 2 makes it possible to minimize the migration of the thermoplastic polymer 31 through the reinforcing film 2 and to facilitate its impregnation.

[0150] Addition of particles to the thermoplastic matrix layer.

[0151] In an alternative embodiment, in the addition step E1, the conductive particles 91 are previously mixed with the thermoplastic polymer 31 of the thermoplastic layer 3. This makes it possible to form, for example, a conductive thermoplastic film used directly in the fibrous element Q. In an exemplary implementation, the conductive particles 91 are mixed with the thermoplastic polymer 31 in viscous or liquid form. The assembly is placed in a mold and cooled to form a thermoplastic film loaded with conductive particles 91. It goes without saying that the formation of a conductive thermoplastic film could be carried out differently by different methods known to those skilled in the art.

[0152] In one example, two conductive thermoplastic films have preferably been previously formed. In the superposition step E2, the operator then superimposes, in this example, successively a first demolding film 4, a first conductive thermoplastic film loaded with conductive particles 91, the reinforcing film 2, a second conductive thermoplastic film loaded with conductive particles 91 and a second demolding film 4. It goes without saying that the stack 1 could just as well comprise both one or more reinforcing film(s) 2 loaded with conductive particles 91 and one or more conductive thermoplastic film(s), which would make it possible to increase even more significantly the electrical conductivity of the manufactured bipolar plate B.

[0153] The superposition of the films can be carried out manually or by means of a robotic arm, for example.

[0154] In an alternative embodiment shown in [Fig.14], in the addition step E1, the conductive particles 91 are mixed with the thermoplastic polymer 31 in powder form, so as to form a powder comprising the thermoplastic polymer powder 31 and the conductive particles 91. In the superposition step E2, with reference to [Fig. 14], the operator superimposes, along the stacking axis A, the first release film 4 and the reinforcing film 2. The operator then sprinkles the mixture of thermoplastic polymer powder 31 and conductive particles 91 onto the reinforcing film 2. The dusting is carried out homogeneously to optimally impregnate the reinforcing film 2 and to ensure that the electrical conductivity throughout the bipolar plate B is increased. In this example, the dusting is carried out, for example, by electrostatic, mechanical spraying or by quenching. The operator then superimposes the second release film 4 onto the mixture of thermoplastic polymer powder 31 and conductive particles 91.

[0155] Fibrous element Q is of the “prepreg” type.

[0156] In a second embodiment, the fibrous element Q is in the form of a reinforcing film 2 impregnated with thermoplastic polymer 31, referred to as a “pre-impregnated”. In other words, in this embodiment, the fibrous element Q is in the form of a pre-impregnated film which can be easily stored, for example, in a roll so that it can be used directly for the manufacturing process of the bipolar plate B.

[0157] In a manner similar to the various embodiments of the addition step E1 described previously for a stack of a reinforcing film 2 and a thermoplastic matrix layer 3, the electrically conductive particles 91 may be added in this example either to the reinforcing film 2 or to the thermoplastic polymer 31 before the formation of the prepreg. In other words, such an embodiment makes it possible to form a fibrous element Q of the prepreg type loaded with conductive particles 91.

[0158] In a preferred embodiment, in the addition step E1, the conductive particles 91 are mixed with the thermoplastic polymer 31 in powder form, so as to form a powder comprising the thermoplastic polymer powder 31 and the conductive particles 91.

[0159] With reference to [Fig. 15], the mixture of thermoplastic polymer powder 31 and conductive particles 91 is dry-sprinkled onto the reinforcing film 2. The assembly is then passed through an oven in order to melt the thermoplastic polymer 31 and fix the conductive particles 91 in the reinforcing film 2. After cooling, the pre-impregnated type fibrous element Q can be wound up and used subsequently. Its handling is thus convenient.

[0160] A method of dry dusting the conductive particles 91 and thermoplastic polymer powder 31 is described, however, the conductive particles 91 could be added in a different manner by means of one of the previously described modes of implementation of the addition step E1 (addition to the reinforcing film 2, additions to a thermoplastic film) for a fibrous element Q in the form of a stack. The only difference in this example being that the conductive particles 91 are added to the upstream fibrous element to form a pre-loaded pre-impregnated fibrous element Q. The fibrous element Q is then preferably in the form of a pre-impregnated film.

[0161] In this example, in the superposition step E2, with reference to [Fig.15], the operator thus directly superimposes successively a first demolding film 4, the fibrous element Q pre-impregnated and pre-loaded with conductive particles 91 and a second demolding film 4.

[0162] 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 (of the ultrasound or localized heating type, for example) or by making seams to secure all the films. Thanks to such consolidation, it is then simpler to transport the entire stack 1, for example by means of a robotic arm comprising gripping means.

[0163] With reference to [Fig. 16], the method then preferably comprises a step E3 of positioning the stack 1 of the fibrous element Q and the demolding films 4 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 SI comprising an EMP imprint to form the fluid circulation channels of the bipolar plate B. In one embodiment, before positioning the stack 1 in the mold M, the latter is coated with a demolding agent, for example a liquid which can be sprayed onto the inner surface SI of each lower and upper member, in order to facilitate demolding thereafter. Preferably, the mold M is then at an initial temperature Ti. Preferably, the initial temperature Ti is between 20 and 210 °C.In this step, the mold M could alternatively comprise two flat surfaces free of impressions to form the circulation channels of the bipolar plate B later.

[0164] 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. By way of example, for PPS type thermoplastics, the shaping temperature Tm is more preferably between 305 and 340°C. For PVDF type thermoplastics, the shaping temperature shape 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 Atl (shown in the graph of [Fig. 19]). Preferably, the first duration Atl 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.

[0165] The method then comprises, in this example, a step E4 of pressurizing the stack 1 in the compression mold M, shown in [Fig. 17]. The pressurizing step E4 is carried out, at the forming temperature Tm, at a predetermined forming pressure Pm, for a second predetermined duration At2 (shown in the graph of [Fig. 19]). The pressurizing step E4 makes it possible to melt the thermoplastic polymer 31 of the thermoplastic layer 3 to impregnate the reinforcing film 2 and form the channels for the circulation of an oxidizing fluid, a reducing fluid and a heat transfer fluid to form a bipolar plate B.Preferably, the shaping pressure Pm is between 6 and 12 MPa, more preferably 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 At2 is less than 2 min, preferably less than 1 min, so as to ensure the impregnation of the reinforcing fibers 21 by the thermoplastic polymer 31 and thus limit the porosity of the bipolar plate B.In this step, in the case of a fibrous element Q in the form of a stack 1 and the addition of the conductive particles 91 to the thermoplastic matrix layer 3, the thermocompression makes it possible to impregnate the conductive particles 91 in the thickness of the bipolar plate B to make it possible to increase the electrical conductivity both on the surface and in depth in the bipolar plate B.

[0166] The method then comprises a cooling step E5, shown in [Fig. 18], of the formed bipolar plate B, for a third predetermined duration At3 (shown in the graph of [Fig. 19]), 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 At3 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 matrix to develop, thereby ensuring that the manufactured bipolar plate B 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 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.

[0167] The shaping cycle (i.e., temperature rise, shaping step, and cooling step) is shown in [Fig. 19], which presents a graph of the evolution of temperature T and pressure P as a function of time t during a complete shaping cycle, as previously described.

[0168] When the release temperature Tr is reached, the mold M is opened and the formed composite bipolar plate B is removed.

[0169] As shown in [Fig.20], the demolding films 4 are then removed, in a removal step E6, on either side of the manufactured bipolar plate B, so as to expose the reinforcing fibers 21 and the conductive particles 91 on the surface of the bipolar plate B. The removal of the demolding films 4 can be carried out manually or automatically by winding the demolding film 4 at the outlet of the mold, for example.

[0170] The method then comprises, in an exemplary implementation, a step of cutting the bipolar plate B, for example to remove a peripheral part which has manufacturing defects. Such cutting can be carried out, for example, by water jet cutting, by milling, or by means of a die. 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).

[0171] [Fig.21] shows 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)) less than 103 μm. Preferably, the formed bipolar plate B has a thickness Ep less than 0.5.103 μm.

[0172] Different modes of implementation of the manufacturing process are presented; it goes without saying that the manufacturing of the bipolar plate B could also be carried out by a known hot coating, pultrusion or draping method for manufacturing a prepreg material in which reinforcing fibers are impregnated in a thermoplastic matrix. Such methods are known to those skilled in the art and will not be described in further detail in this document.

[0173] Thanks to the method according to the invention, the conductive particles 91 impregnated in the reinforcing film 2 and / or in the thermoplastic polymer 31 of the fibrous element Q make it possible to increase the electrical conductivity of the final bipolar plate B by nearly 30% compared to the electrical conductivity of a bipolar plate B manufactured from a stack of a non-woven carbon reinforcing film and a thermoplastic matrix film of the prior art. In certain embodiments, the electrical conductivity of the final bipolar plate B is advantageously increased by more than 50% compared to the electrical conductivity of a bipolar plate B of the prior art, for example in the case of the use of thermoplastic polymer 31 in the form of Polyvinylidene fluoride PVDF.In particular, thanks to the conductive particles 91, the electrical resistance characterized by the ASR (meaning “Area-Specific-Resistance” in English) of the bipolar plate B is advantageously lowered below 12.103 Q.cm2. Depending on the mass rate p of conductive particles 91 added in the stack 1, the electrical resistance ASR can be lowered below 10.103 Q.cm.

[0174] For example, in the case of a bipolar plate having a thickness of 400x106 m, the table shown in [Fig.22] presents the electrical resistance value ASR of the final bipolar plate B as a function of the thermoplastic polymer 31 used (for example Polyphenylene sulfide PPS or Polyvinylidene fluoride PVDF) and the mass rate p of conductive particles 91 in the stack 1, that is to say as a function of the mass rate of conductive particles 91 added to the reinforcing film 2 and / or to the thermoplastic polymer 31. The table makes it possible, for example, to ensure an electrical resistance ASR of the order of 10 mQ.cm2 when the thermoplastic polymer 3 used is Polyphenylene sulfide PPS and a volume rate of conductive particles 91 of 8.1% relative to the mass of the (or) reinforcing film(s) 2 and thermoplastic polymer 31 is used to manufacture the bipolar plate B.In the case of a bipolar plate B manufactured from thermoplastic polymer 31 of the Polyvinylidene fluoride PVDF type, the electrical resistance ASR is also less than 8 mQ.cm2 when a mass rate of 12.5% ​​of conductive particles 31 is added.

Claims

Claims

1. Method for manufacturing a bipolar plate (B) intended to be mounted in an electrochemical device, the electrochemical device being configured to implement an electrochemical reaction, the method comprising: • a superposition step (E2), along a stacking axis (A): • a first demolding film (4), • a fibrous element (Q) comprising at least one non-woven carbon reinforcement film (2) and a thermoplastic polymer (31), and • a second demolding film (4), in order to form a stack (1), • a positioning step (E3) of the stack (1) in a compression system (S), • a thermocompression step (E4) of the stack (1) in the compression system (S), so as to form a plurality of circulation channels to form a bipolar plate (B), • manufacturing method characterized in that it comprises, prior to the superposition step (E2),a step (El) of adding electrically conductive particles (91) to the non-woven carbon reinforcement film (2) and / or to the thermoplastic polymer (31) of the fibrous element (Q).,

2. The manufacturing method according to claim 1, wherein the electrically conductive particles (91) are carbon powder particles.

3. Manufacturing method according to claim 2, wherein the carbon powder is chosen from at least one of the following: graphite (natural, synthetic, expanded), carbon black, ground carbon fibers, carbon nanotubes, graphene.

4. Manufacturing method according to one of claims 2 to 3, in which the electrically conductive particles (91) comprise at least 95% pure carbon.

5. A manufacturing method according to one of claims 1 to 4, wherein each electrically conductive particle (91) is present in the form of a chip whose largest dimension corresponding to a characteristic length is less than 250.106 m

6. ili. Manufacturing method according to one of claims 1 to 5, in which, the fibrous element (Q) having an overall mass, the mass rate of electrically conductive particles (91) used in the addition step (El) relative to the mass of the fibrous element (Q) is between 5% and 50%, preferably between 10 and 30%.

7. Manufacturing method according to one of claims 1 to 6, in which the fibrous element (Q) is a stack of at least one non-woven carbon reinforcement film (2) and at least one layer of thermoplastic matrix (3) comprising a thermoplastic polymer (31).

8. A manufacturing method according to claim 7, wherein the non-woven carbon reinforcement film (2) comprises two outer reinforcement surfaces (2A, 2B), the adding step (El) corresponds to the addition of conductive particles (91) on at least one of the outer reinforcement surfaces (2A, 2B) of the non-woven carbon reinforcement film (2).

9. Manufacturing method according to claim 7, wherein the thermoplastic matrix layer (3) being a thermoplastic polymer powder (31), the adding step (El) corresponds to a mixture of the thermoplastic polymer powder (31) and the electrically conductive particles (91), so as to form a pre-charged thermoplastic matrix layer (3).

10. A manufacturing method according to claim 7, wherein the thermoplastic matrix layer (3) being a thermoplastic matrix film, the adding step (El) corresponds to adding electrically conductive particles (91) onto the thermoplastic matrix film (3), so as to form a preloaded thermoplastic matrix film.

11. Manufacturing method according to one of claims 1 to 6, in which the fibrous element (Q) is a non-woven carbon reinforcement film (2) impregnated with thermoplastic polymer (31), referred to as a “prepreg”.

12. Manufacturing method according to claim 11, wherein the step of adding (El) electrically conductive particles (91) is carried out on the non-woven carbon reinforcement film (2) before the impregnation of thermoplastic polymer (31).

13. Manufacturing method according to claim 11, wherein, the thermoplastic polymer (31) being in the form of a thermoplastic matrix film (3), the step of adding (El) electrically conductive particles (91) is carried out on the thermoplastic matrix film (3) before impregnating the non-woven carbon reinforcement film (2).

14. Manufacturing method according to claim 11, wherein, the thermoplastic polymer (31) being in the form of a thermoplastic polymer powder (31), the step of adding (El) electrically conductive particles (91) corresponds to a mixture of the thermoplastic polymer powder (31) and the electrically conductive particles (91) before impregnating the non-woven carbon reinforcement film (2).

15. Manufacturing method according to one of claims 1 to 14, in which the addition step (El) corresponds to a spraying of a mixture (9) of the electrically conductive particles (91) and a solvent (92).

16. Manufacturing method according to one of claims 8 and 12, in which the addition step (El) corresponds to an immersion of the non-woven carbon reinforcement film (2) in a mixture (9) of electrically conductive particles (91) and a solvent (92).

17. Bipolar plate (B) intended to be mounted in an electrochemical device, the bipolar plate comprising at least one fibrous element (Q) comprising at least one non-woven carbon reinforcing film (2) and thermoplastic polymer (31), the thermoplastic polymer (31) having impregnated the non-woven carbon reinforcing film (2), electrically conductive particles (91) having been previously added to the non-woven carbon reinforcing film (2) and / or to the thermoplastic polymer (31) of the fibrous element (Q).

18. Bipolar plate (B) according to claim 17 having an electrical resistance of less than 12 mQ.cm2, preferably less than 10 mQ.cm2.

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