Method for manufacturing carbon fiber bipolar plates

Abstract

A method for manufacturing a bipolar plate includes overlapping a first release film (4), at least one carbon reinforced film (2), at least one thermosetting resin film (3) and a second release film (4) along a stack axis (A) to form a stack (1), and pressurizing the stack (1) in a compression system at a predetermined forming pressure and a predetermined forming temperature for a second predetermined time. The carbon reinforced film (2) is a nonwoven reinforced film (2) having a plurality of reinforcing fibers (21), each of which extends along an orientation axis (F), and at least 10% to 60% of the reinforcing fibers (21) have orientation axes (F) oriented along the stack axis Z.

Description

[Technical field]

[0001] The present invention relates to the field of electrochemical devices, and more particularly to a method for manufacturing a composite bipolar plate adapted for mounting within an electrochemical device. [Background technology]

[0002] An electrochemical device is any device capable of carrying out an electrochemical reaction, such as a fuel cell or a proton exchange membrane electrolyzer, which can generate electrical energy or hydrogen, respectively, through a redox reaction (oxidation-reduction reaction). The term "electrochemical device" also refers to redox flow batteries, which can be used to generate electrical energy from potential energy stored within the battery.

[0003] The known electrochemical device comprises a stack of cells extending along a stack axis and two end plates arranged at the ends of the stack, the end plates being connected by compression members that allow the cells to be compressed to ensure a sealing of the electrochemical device.

[0004] According to FIG. 1, which shows 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 fluid distribution within the cell 110. To generate the electrochemical reactions 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 the membrane and react under a catalyst. Each cell 110 is also supplied with a heat transfer fluid, which is used to regulate the temperature of the electrochemical device. In fact, by firmly fastening together two adjacent bipolar plates 130A, 130B of two adjacent cells 110A, 110B, several internal channels 140 can be formed, by which a heat transfer fluid can be circulated between the cells 110.

[0005] 2 and 3, a bipolar plate 130 according to known methods is provided with a number of openings 131 allowing the entry and exit of oxidizing and reducing fluids, and two inlet and outlet openings 132 for a heat transfer fluid. Each bipolar plate 130 also has an active central area 133 in contact with the membrane-electrode assembly and on which the redox reaction takes place, and a peripheral area 134 allowing two bipolar plates 130 to be fixed together.

[0006] The active portion 133 comprises recesses 135 and protrusions 136 (as shown in FIG. 3, which presents a cross-sectional view of the bipolar plate 130 along the plane A:A in FIG. 2) that allow fluid communication between the various inlet / outlet openings 131, 132. The recesses 135 of the two bipolar plates 130 are fixed together, and the protrusions 136 form internal channels 140 (as shown in FIG. 1) for circulating a heat transfer fluid.

[0007] In the prior art, bipolar plates made of graphite or metal (e.g. stainless steel, Inconel, aluminum or titanium) are known which are covered with a protective coating to limit the effects of corrosion caused by the electrochemical device. However, such bipolar plates have a number of disadvantages. Graphite bipolar plates are heavy and generally have a large overall dimension, being thicker than 2 mm. Metallic bipolar plates are heavy and have a limited service life in the acidic and corrosive environment of the electrochemical device in which they are mounted, regardless of the protective coating used.

[0008] As a result, bipolar plates are increasingly made of composite materials. Bipolar plates with conductive elements dispersed in a resin have proven to be suitable for this purpose. The conductive elements may include carbon black, finely ground carbon fibers, graphite, expanded graphite, carbon nanotubes or graphene. The resin is generally a thermoplastic or thermosetting polymer. To produce such a bipolar plate, a large number of conductive elements are mixed into the viscous resin (usually more than 85% of the conductive elements in the bipolar plate as a whole to ensure high conductivity). The bipolar plate is formed by injecting the resin with the conductive elements into a mold or by pressing it in a hot compression press.

[0009] However, the large addition of conductive elements significantly increases the viscosity of the thermosetting or thermoplastic resin, making the mixture thick. The above-mentioned manufacturing methods do not allow the manufacture of thin bipolar plates, which are often very thick, typically more than 2 mm, increasing the overall size and weight of the electrochemical device. This is unsuccessful in constructing electrochemical devices for incorporation into aircraft and other mobile applications, for example. In addition, the injection molding and hot pressing process leave a surface resin layer, such as that caused by the use of a mold release agent, which increases the electrical contact resistance, which is detrimental in electrochemical conversion device applications, where such values ​​must be minimized. To reduce the electrical contact resistance, the bipolar plates must be post-treated by chemical or mechanical treatment, such as ablation.

[0010] Composite bipolar plates comprising reinforcing fibres (e.g. carbon fibres) impregnated with a thermoplastic or thermosetting polymer resin are known in the prior art, and have the advantage that they can be thinner (less than 1 mm thick) and therefore reduce their overall size and weight.

[0011] However, in such bipolar plates, the thermoplastic resin may form a residue layer on the surface similar to an insulating skin, which may affect the electrical conductivity. It is therefore necessary to proceed to expose the reinforcing fibers on the surface of the bipolar plate. Such a procedure, for example carried out by dissolving with plasma irradiation or by grinding, may damage the reinforcing fibers and affect the mechanical properties of the bipolar plate, which is a considerable disadvantage. Also, as known from US Pat. No. 5,999,136, the use of a sacrificial film allows the reinforcing fibers to be exposed on the surface of the bipolar plate without damage. In that case, it is possible to manufacture the bipolar plate using thermocompression (thermocompression bonding), which allows the manufacture of thin and lightweight bipolar plates. However, the manufacturing method in US Pat. No. 5,999,136 does not allow the simple and rapid production of bipolar plates with carbon fiber reinforcement.

[0012] In addition, when using reinforcing fibers, very high forming pressures (typically above 15 MPa) are required to allow proper wetting of the fibers, reduce porosity, and expose the fibers on the surface for better electrical conductivity. Too much pressure can lead to deformation of the initial weave of the reinforcing material and the formation of pores, which can change the gas impermeability of the bipolar plate. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] International Publication No. 2016 / 182131 Summary of the Invention [Problem to be solved by the invention]

[0014] The present invention now aims to overcome at least some of these drawbacks by proposing a method for manufacturing bipolar plates that is simple, rapid, does not require the use of expensive industrial machinery and allows the formation of thin and lightweight bipolar plates. [Means for solving the problem]

[0015] The present invention is a method of manufacturing a bipolar plate, the bipolar plate being adapted to be mounted in an electrochemical device adapted to carry out an electrochemical reaction, the method comprising the steps of: stacking a first release film, at least one reinforcing film, at least one thermoplastic film, and a second release film along a stack axis Z to form a stack; placing the stack in a compression system; Pressurizing the stack in the compression system, the pressurizing step being carried out at a forming pressure and a forming temperature for a second predetermined time such that the thermoplastic resin constituting the thermoplastic resin membrane melts and the thermoplastic resin infiltrates the reinforcing membrane to form a bipolar plate; This relates to those who have the following.

[0016] Notably, in this manufacturing method, the reinforcing membrane is a nonwoven carbon-reinforced membrane having a plurality of reinforcing fibers, each extending along an orientation axis, and the proportion of reinforcing fibers oriented along the stack axis Z is between 10% and 60%.

[0017] Fortunately, non-woven carbon reinforced membranes allow for lower forming pressures than those used for woven reinforced membranes, allowing for the use of less expensive industrial machinery and a simpler and faster manufacturing process. The lower forming pressures also limit deformation of the initial weave of the reinforcement and the formation of pores that can affect the gas impermeability of the bipolar plate.

[0018] Orienting the reinforcing fibers along the stack axis can increase the electrical conductivity of the material along that axis, thereby ensuring optimal electrical transport between the surfaces of a bipolar plate for an electrochemical device.

[0019] A ratio of 10% to 60% of the reinforcing fibers oriented along the stack axis ensures that the bipolar plate has good electrical conductivity while also having high mechanical resistance. This high ratio of reinforcing fibers oriented along the stack axis Z makes it possible to realize both mechanical and electrical conductivity functions.

[0020] Preferably, the reinforcing fibers are oriented by stitching. Some of the reinforcing fibers can be mechanically oriented along the stack axis Z by stitching, which can improve the electrical conductivity of the bipolar plate and allow better charge exchange between the two adjacent cells when the bipolar plate is implemented in an electrochemical device.

[0021] Alternatively, the orientation of the reinforcing fibers can be achieved by other methods, for example by hydroentanglement.

[0022] In a preferred embodiment, the proportion of reinforcing fibres oriented along the stack axis Z is between 15% and 45%, which allows optimizing the electrical conductivity of the bipolar plate. This proportion also means that the proportion of non-oriented reinforcing fibres remains high enough to ensure optimal mechanical strength of the bipolar plate.

[0023] Preferably, the nonwoven carbon reinforced membrane has open porosity, and has a pre-pressing open porosity ratio of greater than 60%, the term "open porosity" being explained in more detail below.

[0024] In one embodiment, the non-woven carbon reinforced membrane has an open porosity ratio of 60% to 70%. Research has shown that certain materials, such as carbon paper and gas diffusion layers, can be particularly effective in producing thin, strong, lightweight bipolar plates with such an open porosity ratio.

[0025] Preferably, the nonwoven carbon reinforced membrane is a carbon felt. The carbon felt has a high proportion of open porosity in the reinforced membrane, preferably more than 70%, which allows optimal flow of the thermoplastic resin in the reinforced membrane, more specifically between the reinforcing fibers. This allows optimal infiltration of the thermoplastic resin into the reinforcing fibers, resulting in a bipolar plate with a high level of consolidation and few pores. "Open porosity" means cavities or porosity channels that open towards the outside of the reinforced membrane. The open porosity is accessible from the outside and can be filled with a polymer resin to ensure the mechanical strength of the bipolar plate. The carbon felt also allows the use of known materials that were traditionally used as thermal insulators or as a diffusion enabling medium for liquid electrolytes in redox flow batteries.

[0026] Preferably, the nonwoven carbon reinforced membrane has an open porosity ratio before the pressing step of more than 80%, which allows for optimal deformation of the reinforced membrane, high compressibility, and adaptation of the reinforced membrane to complex shapes. More preferably, the nonwoven carbon reinforced membrane has an open porosity ratio of more than 90%. Hereinafter, for simplicity, "open porosity ratio" will also be referred to as "open porosity".

[0027] The combination of a high proportion of open porosity in the reinforcing membrane with an optimal proportion of reinforcing fibers oriented along the stack axis Z has the advantage that it provides optimal performance when the bipolar plate is used in an electrochemical system. In particular, the combination of a high level of open porosity with a high proportion of reinforcing fibers oriented along the stack axis Z not only provides a high electrical conductivity of the bipolar plate so that it can optimally perform its role as a conductive element, but also provides a high porosity of the reinforcing material, which allows large deformation and good infiltration of the reinforcing material at low pressures, allowing the correct formation of the channels of the bipolar plate and a composite with negligible porosity for hydrogen, which allows it to perform its role as a fluid separator.

[0028] Preferably, the open porosity has a diameter between 1 μm and 250 μm. Preferably, at least 50% of the open porosity in the reinforced membrane has a diameter greater than 125 μm. More preferably, at least 50% of the open porosity in the reinforced membrane has a diameter greater than 150 μm. Such porosity can be easily filled with polymer resin without affecting the mechanical strength of the bipolar plate. Furthermore, such a porosity size means that the assembly is highly compressible, making it possible to manufacture thin bipolar plates. The expression "diameter of the open porosity" means the diameter of an equivalent circle, i.e. the diameter of a circle having a surface area equivalent to that of the porosity. In the same manner, more precisely, it is desirable that at least 50% of the surface area of ​​the open porosity of the reinforced membrane is occupied by porosity having an equivalent circle diameter greater than 125 μm.

[0029] Preferably, the open porosity has an equivalent circular diameter between 1 μm and 300 μm. In one embodiment, at least 50% of the surface area of ​​the open porosity of the reinforced membrane is occupied by porosity having an equivalent circular diameter of more than 30 μm. More preferably, at least 50% of the surface area of ​​the open porosity of the reinforced membrane is occupied by porosity having an equivalent circular diameter of more than 80 μm, preferably more than 90 μm.

[0030] The preferred forming temperature is 140-400° C., at which a wide family of thermoplastic polymers is feasible.

[0031] In one embodiment, the formation temperature is set to 240 to 360° C., making it possible to form a polyphenylene sulfide (PPS) or polyphenylsulfone (PPSU) type polymer.

[0032] In one embodiment, the formation temperature is 200-260° C., which allows the formation of a polyvinylidene fluoride (PVDF) or polyamide type polymer.

[0033] In one embodiment, the formation temperature is set to 140 to 200° C., making it possible to form a polyolefin-type polymer.

[0034] In one embodiment, the formation temperature is set to 350 to 400° C., making it possible to form a polyaryletherketone type polymer (PEEK, PEK, PEKK, etc.).

[0035] Preferably, the predetermined forming pressure is 6 to 12 MPa, which is sufficient for optimal infiltration of the reinforcing fibers in the reinforcing membrane due to the non-woven carbon, and thus the bipolar plate can benefit from the high level of consolidation, which makes the bipolar plate significantly less permeable to hydrogen in the electrochemical device.

[0036] Preferably, the predetermined forming pressure is 8-10 MPa. According to the invention, there is no need to increase the forming pressure to excessively high pressures. This eliminates the need for particularly expensive industrial machinery. The manufacturing method is carried out faster and more simply, while the manufacturing speed can be increased. This forming pressure also limits the deformation of the reinforcing fibers in the initial reinforcing membrane, which could cause pore-like cavities in the bipolar plate, thereby limiting the risk of malfunctions and ensuring the effectiveness of the bipolar plate.

[0037] Preferably, the reinforcing membrane is less than 5 mm thick. The reinforcing membrane is thick enough to produce bipolar plates with high mechanical resistance, yet flexible enough to allow the reinforcing membrane to be adapted to complex geometric shapes. Preferably, the initial thickness of the reinforcing membrane (i.e. the thickness before the stack is compressed to form the bipolar plate) is between 0.5 and 3.4 mm, allowing the formation of bipolar plates with a post-stack compression thickness of less than 0.5 mm.

[0038] Preferably, the reinforcing membrane is 500 g / m 2A lightweight bipolar plate can be formed by making the mass per unit area below 50 to 400 g / m. Preferably, the mass per unit area is 50 to 400 g / m. 2 More preferably, the mass per unit area is 50 to 300 g / m 2 This allows for the incorporation of formed bipolar plates into electrochemical devices where mass constraints are a premium, such as those in aircraft and other moving bodies.

[0039] In one embodiment, the method further comprises, after the pressing step, removing heat from the formed bipolar plate for a third predetermined period of time.

[0040] In the first embodiment, the heat removal step is carried out at a heat removal rate of 10-100°C / min. Such a high heat removal rate allows the development of crystalline portions of the semi-crystalline polymer, so that the bipolar plate produced has good impermeability to the fluids used in the energy conversion in the electrochemical device. At this heat removal rate, a crystallization ratio of 43-47% can be achieved, which corresponds to a crystallization ratio close to the maximum achievable with the thermoplastic, typically in the range of 40-55%. Studies have shown that the maximum crystallization ratio achievable with other thermoplastics, such as polyaryletherketones, is 20-40%. Similarly, the maximum crystallization ratio achievable with other thermoplastics, such as polyolefins, is 55-80%.

[0041] Additionally, for so-called "amorphous" polymers, i.e., polymers that have no crystalline portions, the heat removal process can be carried out at a heat removal rate of more than 80°C / min, allowing the bipolar plates to be quickly removed from the heat, thereby enabling higher production rates.

[0042] Preferably, the heat removal rate is 40 to 90° C. / min. By setting the heat removal rate at this rate, the heat removal time of the bipolar plate can be shortened while enabling optimal development of the crystal part of the stack, thereby shortening the manufacturing cycle and enabling a rapid method, thereby enabling a high production rate.

[0043] The invention also relates to a bipolar plate adapted for implementation in an electrochemical device and manufactured by the above-mentioned manufacturing method.

[0044] A bipolar plate according to one embodiment comprises a nonwoven carbon reinforced membrane having a plurality of reinforcing fibers, each of which extends along an orientation axis, with the proportion of reinforcing fibers oriented along the stack axis Z being between 10% and 60%.

[0045] Preferably, the bipolar plate has a thickness of 0.5 mm or less, which advantageously allows both reducing the mass and overall size of the electrochemical device and increasing its power density (in kW / kg), which in turn allows the electrochemical device to be easily implemented in moving objects, such as aircraft.

[0046] More preferably, the bipolar plate is less than 0.4 mm thick.

[0047] Preferably, the bipolar plate has a resistance of 12 mΩ.cm 2 It shall have a surface electrical resistivity of less than

[0048] Preferably, the bipolar plate has a porosity of less than 1% to ensure optimal permeability for the bipolar plate.

[0049] The invention will be better understood from a reading of the following description, given by way of example, in conjunction with the following drawings, given by way of non-limiting example and in which the same reference symbols are given for similar parts, in which: [Brief description of the drawings]

[0050] [Figure 1] FIG. 1 is a schematic diagram of a stack comprising membrane-electrode assemblies and bipolar plates within an electrochemical device. [Diagram 2] FIG. 2 is a schematic diagram of the bipolar plate in FIG. [Diagram 3] FIG. 3 is a schematic diagram of a cross-sectional appearance of the bipolar plate in FIG. 2. [Figure 4] FIG. 2 is a schematic diagram of a stack including a non-woven carbon reinforced membrane, a thermoplastic membrane and two release membranes used in the manufacture of a bipolar plate according to the present invention. [Diagram 5] FIG. 5 is a close-up view of the nonwoven reinforced membrane of FIG. 4. [Figure 6] FIG. 1 is a schematic diagram of a first step of a production method according to one embodiment of the present invention. [Figure 7] FIG. 2 is a schematic diagram of a second step of the production method according to one embodiment of the present invention. [Figure 8] FIG. 2 is a schematic diagram of a third step of the production method according to one embodiment of the present invention. [Figure 9] FIG. 4 is a schematic diagram of a fourth step of the production method according to one embodiment of the present invention. [Figure 10] FIG. 2 is a schematic diagram of a fifth step of a production method according to one embodiment of the present invention. [Figure 11] 2 is a graph showing the changes in temperature and pressure in the manufacturing method according to the present invention. [Figure 12] FIG. 2 is a schematic diagram of a sixth step of a production method according to one embodiment of the present invention. [Figure 13] FIG. 2 is a schematic diagram of a bipolar plate manufactured using the manufacturing method according to the present invention. [Figure 14] FIG. 2 is a schematic representation of an image provided by tomographic interpretation of a reinforced membrane. [Figure 15] FIG. 15 is a schematic diagram of skeletonization of the reinforcing fibers of the reinforced membrane shown in FIG. 14. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] It should be noted that the drawings are intended to explain the invention in detail for the purpose of implementing the invention, and of course, the drawings can be used to better define the invention, if necessary.

[0052] The present invention relates to a method for manufacturing composite bipolar plates for electrochemical devices.

[0053] Hereinafter, as stated above, the term "electrochemical device" is intended to refer equally to fuel cells, proton exchange membrane electrolyzers, redox flow batteries, and any other device capable of carrying out an electrochemical reaction.

[0054] As mentioned above, an electrochemical device comprises a stack of cells, each of which has a membrane-electrode assembly and two bipolar plates, also called separator plates, which sandwich the membrane-electrode assembly and allow for fluid dispersion within the cell. To generate electrochemical reactions within the electrochemical device, each cell is supplied with an oxidizing fluid and a reducing fluid, e.g., dihydrogen and dioxygen, in a known manner, which react in a redox reaction when they come into contact. Each cell is also supplied with a heat transfer fluid, which is used to regulate the temperature of the electrochemical device. To allow the passage of fluids, the bipolar plates have recesses and protrusions that form circulation channels.

[0055] According to FIG. 4, a bipolar plate B according to the invention (as shown in FIG. 13) is formed of at least one nonwoven carbon reinforced membrane 2 and at least one thermoplastic resin membrane 3. In this example, the bipolar plate B is formed of a nonwoven carbon reinforced membrane 2 and a thermoplastic resin membrane 3. Although an example of one nonwoven carbon reinforced membrane 2 and one thermoplastic resin membrane 3 is described in this specification, it goes without saying that a different number of membranes is also possible. In particular, it goes without saying that the bipolar plate B can be formed of several nonwoven carbon reinforced membranes 2 and / or several thermoplastic resin membranes 3. In the following, for the sake of simplicity, the nonwoven carbon reinforced membrane is called the reinforced membrane 2.

[0056] According to figure 5, the reinforcing membrane 2 preferably extends along a plane (X,Y) and has two substantially flat and parallel outer surfaces 2A and 2B, namely a lower surface 2A and an upper surface 2B. The reinforcing membrane 2 also comprises a number of reinforcing fibres 21. The reinforcing fibres 21 are preferably made of carbon fibres. The thickness Ep2 of the reinforcing membrane 2 is defined along a vertical axis Z perpendicular to the plane (X,Y).

[0057] Each reinforcing fiber 21 has a first end 21a and a second end 21b. At least some of the reinforcing fibers 21 of the reinforcing membrane 2 extend substantially from the first end 21a to the second end 21b along an orientation axis F. The orientation axis F extends in this example from the lower surface 2A to the upper surface 2B. In other words, the first end 21a of the reinforcing fiber 21 is substantially on the surface of the lower surface 2A, and the second end 21b is substantially on the surface of the upper surface 2B, and each reinforcing fiber 21 extends through the thickness Ep2 of its reinforcing membrane 2. Also according to FIG. 5, the orientation axis F is defined by an angle theta θ formed with respect to the vertical axis Z and an angle phi φ formed with respect to the axis X in the projection plane (X,Y). Preferably, the orientation axis F extends such that the angle theta θ is less than 45° or more than 135° and the angle phi φ is between 45° and 135°. For simplicity, the reinforcing fibers 21 are considered to extend along the stack axis Z if they meet the above conditions.

[0058] More specifically, the orientation axis F extends such that the angle theta θ is between 0° and 45° or between 135° and 180°, and the angle phi φ is between 45° and 135°.

[0059] Preferably, 10% to 60% of the reinforcing fibers 21 of the reinforcing membrane 2 extend along the stack axis Z between the lower surface 2A and the upper surface 2B of the reinforcing membrane 2, as shown in FIG. 5. Such an orientation of the reinforcing fibers 21 allows to increase the electrical conductivity and thus of the bipolar plate B after manufacture, while still maintaining outstanding mechanical properties. The high electrical conductivity along the thickness of the reinforcing membrane 2, i.e. along the stack axis Z, is advantageous since it ensures good electrical conductivity along the thickness of the bipolar plate B, as will be explained in more detail later. In an embodiment, the reinforcing fibers 21 are oriented along the stack axis Z by a method such as needling, stitching or sawing. Such methods are known to those skilled in the art and will not be described in detail here. Alternatively, the orientation of the reinforcing fibers 21 can be performed by other methods, for example by hydroentanglement.

[0060] For example, the orientation of the reinforcing fibers 21 in the reinforcing membrane 2 can be determined by analyzing images obtained by X-ray tomography. In this way, it is possible to obtain a three-dimensional 3D image of the reinforcing membrane 2 and to analyze the reinforcing fibers 21 independently of one another to determine their orientation in the reinforcing membrane 2.

[0061] More specifically, in this example, the reinforcing fibers 21 are identified by distinguishing them from the porosity by a grey level threshold discrimination. As part of this, the 3D image is filtered in contrast areas where the reinforcing fibers 21 are highlighted. The three-dimensional structure of the reinforcing fibers 21 in the image can be identified by using a "morphological operator" type image processing. Preferably, a "skeletonization" type image processing, called in English "morphological skeleton" or "skeletonization", is selected. Such methods are known to those skilled in the art and will not be described in detail here.

[0062] As an example, to determine the orientation of the reinforcing fibers 21 in the reinforced membrane 2, the reinforced membrane is analyzed with a microtomograph of the RX Solutions® brand, for example the EasyTom230 model. The analysis is carried out with an acceleration voltage of 60 kV, a current of 80 μA and a tungsten target. 1120 X-ray projections are taken with an exposure time of 0.4 s and an averaging time of 10 s. The X-rays are captured with the X-Act software (RX Solutions®) and three-dimensionally reconstructed with the “Avizo for Industrial Inspection” software (Thermo Fisher Scientific®).

[0063] Specifically, the following steps are performed: Importing the images of the reinforcement membrane 2 obtained by X-ray tomography analysis into the Thermo Scientific® software “Avizo Software”, the schematic representation of which is shown in FIG. 14 , highlighting the reinforcing fibers 21 by performing a threshold discrimination over a contrast range, Identifying the three-dimensional filamentary structure of each reinforcing fiber 21 by the “Auto Skeleton” module in the software, an example of which is shown in FIG. 15.

[0064] The properties of each reinforcing fiber 21 of the reinforcing membrane 2 can then be investigated, for example their orientation. As an example, in Figure 15 reinforcing fibers 21 with similar orientations are represented by the same continuous or dashed line.

[0065] As mentioned above, the orientation axis F of each reinforcing fiber 21 is determined by the angle theta θ that it makes with the vertical axis Z and the angle phi φ that it makes with the axis X of a reference coordinate system (X,Y,Z), in which the angles theta θ and phi φ vary between 0° and 180°. More precisely, in the reference coordinate system (X,Y,Z), the space is bounded by three separate cones of 45° angle each, whose apex is at the origin of the reference coordinate system and whose centres are respectively on the axes X, Y and Z. The three cones can therefore be defined by the following theta θ·phi φ pairs: Z-oriented cone: 45°<φ<135° and 0°<θ<45° or 135°<θ<180° Y-oriented cone: 45°<φ<135° and 45°<θ<135° X-oriented cone: 0°<φ<45° or 135°<φ<180° and 0°<θ<180°

[0066] The reinforcing fibers 21 are classified among the three cones according to their orientation angles θ and φ, and the reinforcing fibers 21 that belong to two cones (fibers at the border between the two cones) are left out of the classification. Then, the proportion of the reinforcing fibers 21 oriented along each axis X, Y, Z of the reinforcing membrane 2 can be calculated.

[0067] In a preferred embodiment, the reinforcing membrane 2 is a carbon felt, also called a "carbon mat". The carbon felt allows the use of a material with a high open porosity ratio, which allows an efficient exchange between the liquid electrolyte and the bipolar plate of the electrochemical device. The expression "open porosity" means open cavities, i.e. cavities that are not closed by the arrangement and / or the sizing of the carbon reinforcing fibers 21. Preferably, the reinforcing membrane 2 has an open porosity ratio of more than 70%. Even more preferably, the open porosity ratio is 80%, which allows an optimal flow of the thermoplastic resin in the reinforcing membrane 2 and thus a good wetting of the reinforcing fibers 21. Due to the high open porosity ratio of the reinforcing membrane 2, a bipolar plate B with a high consolidation level (a percentage of residual porosity generally less than 1%) can be obtained after the manufacturing method. Carbon felt has the advantage that it can be easily deformed, which allows the reinforcing membrane 2 to be adapted to complex shapes. Carbon felt also has the advantage that it requires a lower forming pressure (typically 6-10 MPa) than that required for woven reinforcement (usually above 15 MPa).More preferably, the non-woven carbon reinforced membrane 2 has an open porosity ratio of more than 90%, which allows the production of bipolar plates B with optimal mechanical properties.

[0068] Preferably, the reinforcing membrane 2 has an open porosity ratio of more than 60%. In one embodiment, the reinforcing membrane 2 has an open porosity ratio of 60% to 70%. Indeed, with certain materials, such as carbon paper or gas diffusion layers, it is possible to manufacture thin, lightweight and highly resistant bipolar plates.

[0069] In a preferred embodiment, the reinforcing membrane 2 has a porosity with a diameter between 1 μm and 250 μm, which allows for effective infiltration of the reinforcing fibers 21 and ensures a high consolidation level in the bipolar plate B produced. Preferably, at least 50% of the porosity has a diameter greater than 125 μm, more preferably greater than 150 μm. The "porosity diameter" refers to the equivalent circular diameter of the porosity, i.e. the diameter of a circle having a surface area equivalent to the surface area of ​​the porosity. In one embodiment, the reinforcing membrane 2 has a porosity with an equivalent circular diameter between 1 μm and 300 μm.

[0070] In one embodiment, at least 50% of the surface area of ​​the open porosity of the reinforcement membrane 2 is occupied by porosity having an equivalent circular diameter of more than 30 μm, preferably more than 80 μm, more preferably more than 90 μm.

[0071] For example, one way in which the porosity in the reinforcing membrane 2 can be characterized is to use images obtained by X-ray tomography and analyze them in three or two dimensions. For example, two-dimensional cut images can be made along the desired direction. As mentioned above, the reinforcing fibers 21 are identified by distinguishing them from the porosity by grey level threshold discrimination. The image can then be filtered with a contrast range to highlight the reinforcing fibers 21. Then, the two-dimensional structure of the porosity can be identified in the image by using "morphological operator" type image processing (e.g. "morphological gradients").

[0072] In a practical example, two-dimensional images are obtained by taking three-dimensional image slices from an X-ray tomography. In this example, these two-dimensional images are imported into the software ImageJ. A threshold discrimination is then performed on the gray level range between 120 and 255 (in the so-called “Dark Background” mode). The two-dimensional structure of the porosity can then be highlighted using a morphological gradient operator in the software (in this example, “Watershed”).

[0073] Once the porosities are identified, the area of ​​each porosity is measured and an equivalent circular diameter is calculated based on the measured area, and the porosities are then characterized and classified according to their equivalent circular diameter, for example, according to whether their equivalent circular diameter is smaller or larger than a diameter threshold for a set of all porosities.

[0074] Preferably, the reinforcing membrane 2 is highly compressible so that it can adapt to the complex geometry of the fluid circulation channels of the bipolar plate. It is also possible to form the bipolar plate B with a thickness of less than 1 mm. Preferably, the formed bipolar plate B has a thickness of less than 0.5 mm, as will be explained in more detail below.

[0075] 5, the reinforcing membrane 2 has a thickness Ep2 along the axis Z, preferably less than 5 mm. Preferably, the thickness Ep2 is between 0.5 and 2.5 mm. Such a thickness Ep2 advantageously allows to limit the overall dimensions of the bipolar plate B after its manufacture.

[0076] In a preferred embodiment, the reinforcing membrane 2 is 500 g / m 2 The mass per unit area is preferably 50 to 300 g / m. 2This allows the use of a low-mass reinforcing membrane 2, which in turn reduces the mass of the bipolar plate B in which the reinforcing membrane 2 is used. Since the bipolar plate B has a low mass in this way, it can be incorporated into an electrochemical device that is configured to be mounted on, for example, an aircraft or other moving object.

[0077] The thermoplastic membrane 3 (as shown in FIG. 4 ) comprises a polymer, designated thermoplastic 31, which after manufacture forms the thermoplastic matrix of the bipolar plate B. For the sake of simplicity, the thermoplastic membrane 3 will hereinafter be referred to as thermoplastic membrane 3.

[0078] 4, the thermoplastic film 3 preferably has a thickness Ep3 of 50 to 600 μm. This thickness allows the amount of thermoplastic resin 31 to be sufficient to form the thermoplastic matrix of the bipolar plate B, while still reducing the volume and mass of the bipolar plate B.

[0079] In one embodiment, the thermoplastic resin of the thermoplastic membrane 3 is of semi-crystalline type, which allows the bipolar plate B to be highly impermeable to fluids, especially hydrogen. The semi-crystalline thermoplastic resin also provides outstanding mechanical and chemical resistance, especially corrosion resistance. In one embodiment, the thermoplastic resin of the thermoplastic membrane 3 is of amorphous type, which allows low dimensional shrinkage and high ductility during the heat removal process.

[0080] The thermoplastic resin can be adapted to meet the chemical environment of the electrochemical device in which the bipolar plate B will be implemented. In this context, for implementing the bipolar plate B in a low-temperature proton exchange membrane (designated by the acronym "PEM"), the thermoplastic resin is preferably of the polyphenylene sulfide type (designated by the acronym "PPS"), polyphenylsulfone (designated by the acronym "PPSU"), polyvinylidene fluoride (designated by the acronym "PVDF"), ethylene chlorotrifluoroethylene (designated by the acronym "ECTFE") or polyolefin, polyaryletherketone or polyamide type.

[0081] To implement the bipolar plate B in a high-temperature proton exchange membrane, the thermoplastic resin is preferably of the polyphenylene sulfide (PPS), polyphenylsulfone (PPSU), ethylene chlorotrifluoroethylene (ECTFE) or polyaryletherketone type.

[0082] For mounting the bipolar plate B in a redox flow battery with vanadium or hydrogen bromide electrolyte or any liquid electrolyte (e.g. organic electrolyte), the thermoplastic resin is preferably of the polyphenylene sulfide (PPS) and polyphenylsulfone (PPSU) type if the electrolyte is of the base type, of the polyvinylidene fluoride (PVDF) or ethylene chlorotrifluoroethylene (ECTFE) type if the electrolyte is of the acid type, or of the polyaryletherketone type.

[0083] Next, a method for manufacturing the bipolar plate B according to one embodiment of the present invention will be described.

[0084] According to Fig. 4, in this example, a bipolar plate B is manufactured from a stack 1 consisting of a reinforcing membrane 2, a thermoplastic resin membrane 3 and two release membranes 4. The stack 1 extends along a stack axis A, as will be described in more detail later. The use of the release membranes 4 makes it possible to expose the reinforcing fibers 21 on the surface of the formed bipolar plate B and make it conductive. Preferably, the reinforcing membrane 2, the thermoplastic membrane 3 and the release membranes 4 are in roll form, which facilitates their storage and handling.

[0085] As shown in Fig. 6, the method comprises a first step E1 of cutting the reinforcing membrane 2, the thermoplastic membrane 3 and the two release membranes 4 (only the reinforcing membrane 2 is shown in Fig. 6). This cutting is carried out, for example manually on a cutting table, making it possible to cut the various membranes to the required dimensions of the final bipolar plate B. The reinforcing membrane 2 and the thermoplastic membrane 3 are preferably of similar dimensions. More preferably, each release membrane 4 has a dimension larger than the dimensions of the reinforcing membrane 2 and the thermoplastic membrane 3 and protrudes from the stack 1 so that it can be easily removed after the bipolar plate B has been formed. Of course, this cutting could also be carried out in other ways, for example by means of a punch or a robotic arm.

[0086] The method then includes a second step E2 in which the first release film 4, the reinforcing film 2, the thermoplastic film 3 and the second release film 4 are stacked along the stack axis A to form the stack 1, as shown in FIG. 7. This arrangement allows the orientation axis F of the reinforcing fibers 21 of the reinforcing film 2 to be oriented along the stack axis A, and thus allows the reinforcing fibers 21 to be oriented so that their ends 21a, 21b are exposed on both the front and back sides of the manufactured bipolar plate B. That is, the reinforcing fibers 21 optimally conduct electricity into the bipolar plate B and thus into the electrochemical device in which it is mounted. The stack 1 in this example can be formed either by sequentially arranging the first release film 4, the reinforcing film 2, the thermoplastic film 3 and the second release film 4, or by sequentially arranging the first release film 4, the thermoplastic film 3, the reinforcing film 2 and the second release film 4.

[0087] Also, if the stack 1 has two thermoplastic resin films 3, in this process, the operator can easily stack the first release film 4, the first thermoplastic film 3, the reinforcing film 2, the second thermoplastic film 3 and the second release film 4 in sequence. The use of two thermoplastic films 3 on either side of the reinforcing film 2 can reduce the migration of the thermoplastic resin through the reinforcing film 2 and facilitate its infiltration.

[0088] The first release film 4, the reinforcing film 2, the thermoplastic film 3 and the second release film 4 can be laid down manually or by, for example, a robotic arm.

[0089] In an embodiment, the stack 1 is consolidated, for example to limit the risk of shifting one film relative to the other. Such consolidation can be achieved, for example, by applying a molten joint (for example, ultrasonic or localized heating) or by fixing all the films together by sawing. Consolidation in this way allows the stack 1 to be more easily transported, for example by a robot arm equipped with gripping means.

[0090] According to FIG. 8, the method then comprises a step E3 of placing the stack 1 in a compression system, in this example a mould M. The mould M comprises in this example a lower member and an upper member, each of which has an inner surface with indentations G. The indentations G are used to form fluid flow channels in the bipolar plate B. In an embodiment, before the stack 1 is placed in the mould M, the mould is coated with a release agent, for example a liquid that can be sprayed on the inner surfaces of the lower and upper members, respectively, to facilitate subsequent release. The mould M is preferably at an initial temperature Ti. Preferably, the initial temperature Ti is between 20 and 210° C.

[0091] The mold M is closed to capture the stack 1, and its temperature is gradually increased from the initial temperature Ti to a predetermined forming temperature Tm. The forming temperature Tm is preferably 140 to 400°C. For example, for PPS type thermoplastic resin, the forming temperature Tm is preferably 305 to 340°C. For PVDF type thermoplastic resin, the forming temperature Tm is 200 to 260°C, preferably 210 to 240°C. For PPSU type thermoplastic resin, the forming temperature Tm is 240 to 360°C, preferably 290 to 330°C. The temperature increase from the initial temperature Ti to the predetermined forming temperature Tm is carried out over a first predetermined time Δt1 (as shown in the graph in FIG. 11). The first time Δt1 is preferably less than 10 minutes, preferably 3 minutes or less, so that the mold M can be heated quickly to save production time and achieve a high operating rate.

[0092] According to Fig. 9, the method then comprises a step E4 of pressing the stack 1 in the compression mould M. This pressing step E4 is carried out at a forming temperature Tm and at a predetermined forming pressure Pm for a second predetermined time Δt2 (that shown in Fig. 11). During the pressing step E4, the thermoplastic resin in the thermoplastic membrane 3 can melt and infiltrate its reinforcing membrane 2 to form the bipolar plate B.

[0093] Preferably, the forming pressure Pm is 6 to 12 MPa. More preferably, the forming pressure Pm is 8 to 10 MPa, which makes it possible to limit the deformation of the reinforcing fibers 21 in the initial reinforcing membrane 2 and the resulting generation of hole-like cavities in the bipolar plate B, and to easily apply the pressure. Preferably, the second time Δt2 is less than 2 minutes, and more preferably less than 1 minute, which makes it possible to reliably infiltrate the reinforcing fibers 21 with the thermoplastic resin 31, and thus limits the porosity of the bipolar plate B.

[0094] As shown in FIG. 10, the method then includes a step E5 of cooling and thus consolidating the formed bipolar plate B for a third predetermined time Δt3 (as shown in the graph in FIG. 11). The temperature is reduced from the forming temperature Tm to a demolding temperature Tr, at which point the pressing is stopped. The demolding temperature Tr is preferably equal to the initial temperature Ti. The third predetermined time Δt3 is set to obtain a heat removal rate of 10-100°C / min for semi-crystalline polymers, which allows for a sufficiently slow heat removal, allowing the development of crystalline portions of the thermoplastic resin, to shorten the heat removal time and shorten the production cycle, while ensuring that the manufactured bipolar plate exhibits low impermeability to hydrogen and other fluids used in the energy conversion system. This heat removal rate results in a crystallinity of 3-47%. More preferably, the heat removal rate is 40-90°C / min, which ensures optimal crystallization of the thermoplastic resin during the consolidation phase. In the case of amorphous polymers, the heat removal rate is preferably greater than 80° C. / min, allowing the bipolar plate B and mold M to be cooled quickly and increasing production rates.

[0095] This formation cycle (i.e., the heat-up step, the formation step and the heat-removal step) is illustrated in FIG. 11, which shows a graph of the evolution of temperature T and pressure P as a function of time t throughout the formation cycle described above.

[0096] When the demolding temperature Tr is reached, the mold M is opened and the formed composite bipolar plate B is removed.

[0097] 12, the release films 4 on either side of the produced bipolar plate B are then removed in step E6 to expose the reinforcing fibers 21 on the surface of the bipolar plate B. Removal of the release film 4 can be performed manually or automatically, for example by rolling it up while leaving its mould.

[0098] In one implementation, the method includes cutting the bipolar plate B, e.g., to remove peripheral edges of the bipolar plate that have manufacturing defects. This can be done, for example, by water jet cutting, milling, or punching. Preferably, centering is taken into consideration to ensure accurate cutting of the bipolar plate (cutting error of 0.1 mm or less).

[0099] Fig. 13 shows a bipolar plate B formed by the above-mentioned manufacturing method. This bipolar plate B extends along a plane (X,Y). Successfully, this method makes it possible to form a bipolar plate B having a thickness Ep (along an axis Z perpendicular to the plane (X,Y)) of less than 1 mm. Preferably, the thickness Ep of the formed bipolar plate B is less than 0.5 mm.

[0100] Preferably, the ratio between the amounts of the reinforcing fibers 21 and the thermoplastic resin 31 in the final bipolar plate B is such that the electrical, thermal and mechanical properties of the bipolar plate B match the operating conditions of the electrochemical device. Therefore, the final bipolar plate B preferably has a volume ratio of the reinforcing fibers 21 of 20-60% and a volume ratio of the thermoplastic matrix 31 of 40-80%. Preferably, the volume ratio of the reinforcing fibers 21 is 30-50% and that of the thermoplastic matrix 31 is 50-70%. The ultra-thin bipolar plate produced in this way makes it possible to ensure optimal electrical conductivity while compensating for the low conductive element ratio compared to the conventional manufacturing method, in which the conductive element ratio is more than 80%. In addition, the mass of the produced bipolar plate is reduced, which makes it possible to reduce the mass of the electrochemical device in which the bipolar plate is mounted.

[0101] For example, the volume ratio of the reinforcing fibers 21 present in the bipolar plate B after manufacture can be determined after separation of the thermoplastic matrix 31, which can be achieved, for example, by dissolving the thermoplastic matrix 31 in acid or by calcination. The mass of the conductive reinforcing fibers 21 is then measured by weighing and related to the initial mass of the material. The volume ratio of the materials is used to convert to mass per unit volume.

[0102] The method of the present invention provides a suitable type of carbon reinforcement and associated manufacturing cycle to provide a method for manufacturing bipolar plates that is simple, rapid, does not require the use of expensive industrial machinery, and allows for the formation of thin, lightweight bipolar plates.

[0103] Preferably, the bipolar plate has a thickness of 0.4 mm or less, which allows both to reduce the mass and overall size of the electrochemical device and to increase the power density (kW / kg) without the need to post-treat the surface of the material to improve its electrical conductivity, thereby allowing the electrochemical device to be easily implemented in moving objects, such as aircraft.

[0104] According to one preferred embodiment of the present invention, the surface electrical resistivity (ASR, an acronym for surface resistivity) is 12 mΩ.cm 2 It is possible to produce a flexible bipolar plate B with a porosity of less than 1%. These properties successfully ensure an impermeability of less than 5×10 mol / m / s / MPa and a high hydrophobicity of the surface of the bipolar plate B with a contact angle of more than 110°.

[0105] The porosity of the manufactured bipolar plate B can be measured by techniques known to those skilled in the art.

[0106] For example, one can use the Archimedes or pycnometer analytical thrust method to measure the actual density of bipolar plate B and compare the actual density to the theoretical density. These two methods allow one to accurately determine the exact volume of a specimen and, if its mass is known, to deduce its mass per unit volume.

[0107] According to a second example, various image analysis methods can be used to distinguish porosity in the bipolar plate B. In this case, the volumes of porosity are identified on a three-dimensional image (or their surface on a two-dimensional image), following a similar manner as described above for the reinforcing membrane 2.

[0108] According to a third example, wave absorption methods, for example those using X-ray tomography or ultrasound, can also be used to determine the porosity of the final bipolar plate B.

[0109] These methods are usually followed by a measurement of the exact composition of the specimen in question. To do this, the reinforcing fibers 21 are separated and weighed independently of the thermoplastic matrix 31. This separation can be achieved by dissolving the matrix or by its thermal degradation (calcination or combustion) or chemical degradation (acid dissolution). If the mass and mass per unit volume of the initial elements of the bipolar plate B are known, the corresponding volumes can be deduced. These volumes are compared with the exact volume of the specimen and the remaining volume is associated with the volume of porosity.

[0110] The volume ratio of porosity Tvp is calculated by the following formula; in which mt corresponds to the mass of the reinforcing fibers 21 obtained, for example, via acid dissolution of the thermoplastic matrix 31, ρf corresponds to the mass per unit volume of the reinforcing fibers 21, and ρr corresponds to that of the thermoplastic matrix 31. Tvp=Vp / Vt=1-mf / (ρf×Vt)-(mt-mf) / (ρr×Vt)

Claims

1. A manufacturing method for a bipolar plate (B), wherein the bipolar plate (B) is configured to be mounted within an electrochemical device, and the electrochemical device is configured to carry out an electrochemical reaction. The aforementioned manufacturing method Step (E2) involves overlapping a first release film (4), at least one reinforcing film (2), at least one thermoplastic resin film (3), and a second release film (4) along the stack axis Z in order to form a stack (1), Step (E3) of placing the stack (1) in the compression system (M), The process (E4) involves pressurizing the stack (1) within the compression system (M), and is performed over a second predetermined time (Δt2) at a predetermined forming pressure (Pm) and a predetermined forming temperature (Tm) such that the thermoplastic resin (31) constituting the thermoplastic resin film (3) melts and the thermoplastic resin (31) permeates the reinforcing film (2) to form a bipolar plate (B). It has, A manufacturing method characterized in that the reinforcing film (2) is a nonwoven carbon reinforcing film (2) comprising a plurality of reinforcing fibers (21), each reinforcing fiber (21) extending along an orientation axis (F), and the ratio of reinforcing fibers (21) oriented along the stack axis Z is 10% to 60%.

2. A manufacturing method according to claim 1, wherein the ratio of reinforcing fibers oriented along the stack axis Z is 15% to 45%.

3. A manufacturing method according to claim 1, wherein the nonwoven carbon-reinforced film (2) is carbon felt.

4. A manufacturing method according to claim 1, wherein the nonwoven carbon reinforced film (2) has open porosity, and the nonwoven carbon reinforced film (2) has an open porosity ratio of more than 60% before the pressurization step (E4).

5. A manufacturing method according to claim 4, wherein the open porosity has a diameter of 1 μm to 300 μm.

6. A manufacturing method according to claim 1, wherein the forming temperature (Tm) is 140 to 400°C.

7. A manufacturing method according to claim 1, wherein the predetermined forming pressure (Pm) is 6 to 12 MPa.

8. A manufacturing method according to claim 1, comprising a step (E5) of removing heat from the formed bipolar plate (B) for a third predetermined time (Δt3) after the pressurization step (E4).

9. A manufacturing method according to claim 8, wherein the rate of heat removal is 40 to 90°C / min.

10. A bipolar plate (B) configured to be mounted within an electrochemical device, the bipolar plate manufactured by the manufacturing method described in claim 1.

11. A bipolar plate (B) according to claim 10, wherein the bipolar plate has a thickness (Ep) of 0.5 mm or less.

12. A bipolar plate (B) according to claim 10, wherein the resistance is 12 mΩcm ２ A bipolar plate having a surface electrical resistivity of less than 1.

13. A bipolar plate (B) according to claim 10, wherein the bipolar plate has a porosity of less than 1%.

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