Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
  • H01B1/20—Conductive material dispersed in non-conductive organic material
  • H01B1/24—Conductive material dispersed in non-conductive organic material the conductive material comprising carbon-silicon compounds, carbon or silicon
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
  • H01M8/0204—Non-porous and characterised by the material
  • H01M8/0221—Organic resins; Organic polymers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
  • H01M8/0204—Non-porous and characterised by the material
  • H01M8/0223—Composites
  • H01M8/0226—Composites in the form of mixtures
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/10—Fuel cells with solid electrolytes
  • H01M2008/1095—Fuel cells with polymeric electrolytes
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/50—Fuel cells
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
  • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

• the present invention relates to a method for preparing a conductive article of electricity.
• This method is particularly suitable for the preparation of current collectors, in particular bipolar plates used in proton exchange membrane fuel cells (known as PEMFC cells, meaning PEMFC).
• PEMFC cells proton exchange membrane fuel cells
• One of the main fields of the invention can therefore be considered as that of fuel cells.
• a fuel cell comprises a stack of elementary cells in which an electrochemical reaction takes place between two reactants which are introduced continuously.
• Fuel such as hydrogen, for batteries operating with H2 / O2 mixtures, or methanol for batteries operating with methanol / oxygen mixtures, is brought into contact with the anode, whereas the oxidant, generally oxygen is brought into contact with the cathode.
• the anode and the cathode are separated by an electrolyte, proton exchange membrane type.
• the electrochemical reaction the energy of which is converted into electrical energy, splits into two half-reactions: fuel oxidation, taking place at the anode / electrolyte interface producing, in the case of H 2 cells, protons H + , which will cross the electrolyte in the direction of the cathode, and electrons, which join the external circuit, in order to contribute to the production of electrical energy;
• the electrochemical reaction takes place, strictly speaking, at an electrode-membrane-electrode assembly.
• the electrode-membrane-electrode assemblies are arranged, most often in the form of a stack, the electrical continuity between the different assemblies being ensured by means of conductive plates, called bipolar plates.
• the bipolar plates In addition to the current collector function, the bipolar plates must also perform the following functions: the distribution of the reagents and the evacuation of the products at the level of the anode and the cathode, the reactants being hydrogen and oxygen and the products the water for the batteries operating with H 2 / O 2 ;
• the constituent materials of the bipolar plates must meet the following criteria: sufficient electrical conductivity in order to efficiently collect the electric current produced by the elementary cells; good thermal conductivity to evacuate the heat produced during the electrochemical reaction to the elementary cells; good mechanical properties so as to be able to withstand the stresses associated with the assembly of elementary cells constituting the cell and also to withstand handling during assembly of the fuel cell; a thermal stability to guarantee the integrity of the assembly in the ranges of temperature of use of the battery; chemical stability against fluids in the core of the cell (eg, water, acid) so that the performance of the material can be maintained and decomposition can be avoided of it and thus a pollution of the anode and the cathode with which it is in contact; a reagent (eg, hydrogen and oxygen) impermeability greater than that of the proton exchange membrane; surface hydrophobicity to facilitate evacuation of water formed during the electrochemical reaction; an ability to be shaped, so as to allow the formation of distribution channels on the surface of the plates and without requiring, preferably, machining phase for the formation of
• the bipolar plates currently used can be subdivided into three categories:
• bipolar plates made from metal sheets shaped by stamping the arrival of the fluids and the evacuation of the formed products are carried out in areas of the bipolar plate locally flat, which requires the use of a frame having a shape adapted and able to ensure the peripheral sealing, as described in WO 2007/03743.
• This technique has the disadvantage of requiring for the same bipolar plate a complementary piece intended to ensure the junction between two associated plates to form the bipolar plate and ensure the supply and evacuation of fluid and product. This results in final bipolar plates less compact than expected.
• the plates made of organic composite materials consist of plates comprising an organic polymer matrix in which electrically conductive particles are dispersed.
• the particles give the bipolar plates the electrical conductivity necessary for the collection of the current and the polymer matrix the mechanical strength necessary for the assembly of the different constituent elements of the cell in which the bipolar plates are arranged.
• the conductive particles may be metallic, which has the advantage of good electrical conductivity. However, they have the disadvantage of having a high density and being sensitive to the chemical environment.
• the conductive particles may also be carbon-based products, in the form of powders, such as powders of carbon black, graphite or carbon fibers.
• the plates are made by incorporating the conductive particles into a liquid resin followed by curing of the resin as described in US 6,248,467.
• a raw material in liquid form in this case a liquid resin, has the following disadvantages:
• the invention relates to a method of manufacturing an electrically conductive article comprising the following steps:
• thermocompression of the mixture of powders obtained in the preceding step in a mold of shape adapted to the article and at a temperature effective to obtain the crosslinking of the resin, at the end of which is obtained; conductive article of electricity.
• the implementation of this process results in the following advantages: an energy-saving process, in particular because the mixing step is carried out by the dry method not involving heating, as is the case with the mixing steps intended to obtain products in the form of pastes; a method that consumes less energy because the resin is a thermosetting resin, which implies that the article at the end of the process does not need to be cooled when it needs to be demolded, which is not the case generally when the article is made from thermoplastic resin;
• the article can be, after the powder mixing step, carried out in a single thermocompression step, without the need for prior transformation and without the need for a subsequent step of machining;
• the method comprises a first step of dry mixing a powder comprising at least one thermosetting resin comprising at least two epoxide groups, a hardener compound powder of said resin and a powder of an electrically conductive filler.
• thermosetting resin by dry means, conventionally, a mixing step not involving liquid compounds, such as solvents, and melted phases, the various constituents of the mixture remaining in the form of solid powders and not requiring heater. It follows a simple and energy-saving step and a mixture of powders that have not undergone a crosslinking step of the thermosetting resin before transformation thereof during the thermocompression step.
• thermocompression is meant, conventionally, a step of shaping the powder mixture into the desired article comprising pressurizing said mixture to a temperature effective to ensure the cohesion of the powder mixture so as to form the article, this step can be carried out, for example, in an injection press or in a simple compression press and does not require a prefusion step of the powder mixture before shaping.
• the powder comprising at least one thermosetting resin comprising at least two epoxide groups may have an average particle size ranging from 10 to 500 ⁇ m, as can the hardening compound powder of said resin and the conductive filler powder of the resin. 'electricity.
• the powder comprising at least one thermosetting resin comprising at least two epoxide groups, the hardener compound powder of said resin and the electrically conductive filler powder has an average particle size ranging from 75 to 150 ⁇ m.
• thermosetting resin is an epoxy resin comprising at least two epoxide groups, namely groups having the following pattern:
• n 2 representing the repetition number of the motif taken in square brackets, and mixtures thereof.
• the hardener compound is conventionally, according to the invention, a compound capable of generating the crosslinking reaction (it could therefore also be described as a crosslinking agent) of the abovementioned resin, for example by opening the epoxide rings, these hardening compounds thus comprising at least one function capable of reacting with the epoxide rings of said resins.
• Suitable hardening compounds may be compounds comprising at least one amine function. These may include aliphatic amine compounds, amidoamines, polyamides, polyetheramines, cycloaliphatic amines, anhydrides, aromatic amines, imidazole compounds.
• crosslinking reaction of an epoxy resin with an amino compound can be schematized as follows:
• the imidazole compounds used as curative compounds are particularly preferred. Indeed, they generate a homopolymerization of an epoxy resin, which means that the patterns resulting from the polymerization of the resin are directly connected to each other, without there being any cross-linking nodes from the hardener compound molecules. . It follows, after polymerization, a denser material, more rigid and having a higher glass transition temperature than those obtained with a hardener including during the polymerization between two patterns from the resin.
• the electrically conductive filler powder may be a powder of any electrically conductive material.
• it may be metal powders, metal oxide powders, powders of carbonaceous materials.
• the electrically conductive filler powder used in the context of the process of the invention is a powder of carbon material which is particularly advantageous because of its chemical inertness and its low density.
• carbonaceous material powder mention may in particular be made of graphite powders, carbon black powders, in particular synthetic graphite powders, which have the advantage of not including impurities harmful to the conductivity of the material, unlike the grades natural resources from mining.
• carbon black powders mention may be made of commercial Ensaco 350G powders with a specific surface area of 770 m 2 / g, Ensaco 250G with a specific surface area of 65 m 2 / g.
• synthetic graphite powders mention may be made of Timrex KS 150 commercial powders with a specific surface area of 3 m 2 / g, Timrex KS 75 with a specific surface area of 6.5 m 2 / g.
• lamellar graphite As other types of electrically conductive fillers, it is possible to envisage, in addition to those mentioned above, lamellar graphite, carbon fibers, carbon nanotubes and, in particular, lamellar graphite.
• the lamellar graphite goes, in addition to conferring an electronic conductivity to the material, also allow, after the thermocompression step, when the graphite lamellae are disposed of. perpendicular to the flow of gas arriving on the formed article, to increase the tortuosity of the gas, and consequently to improve the impermeability of the material to these gases.
• the mixing step may incorporate other additives, such as catalysts, plasticizers, flexibilizers, nucleating agents, hyperdispersing agents, anti-caking agents, reactive diluents.
• additives such as catalysts, plasticizers, flexibilizers, nucleating agents, hyperdispersing agents, anti-caking agents, reactive diluents.
• additives will also be in the form of solid powders having, advantageously, an average particle size ranging from 10 to 500 ⁇ m.
• Suitable catalysts may be compounds capable of readily giving off hydrogen, such as phenolic, alcoholic, acidic, amino compounds. More specifically, they may be aliphatic amines, imidazole compounds or substituted ureas, such as a compound of the following formula:
• the various powders constituting the mixture have an average particle size identical.
• the charge powder electrically conductive may represent from 50 to 95% by weight relative to the total mass of the mixture, while the other powders (resin powder, hardener compound powder and optionally other additive powders) may represent from 5 to 50% by weight relative to the total mass of the mixture.
• the electrically conductive filler powder represents from 80 to 92% by weight relative to the total mass of the mixture.
• the various powders mentioned above are mixed dry, so as to avoid any preliminary melting and a beginning of crosslinking of the resin, the mixture can be produced, for example, at room temperature.
• the mixing can be carried out by moving the particles relative to each other. This displacement can be generated by a stirring system (for example, the shares or ribbons of a convective mixer, the blades of a high-shear mixer), by a flow of air (for example, with a mixer with impaction) or by the rotation of a tank comprising said mixture. In this case, the rotation of the tank must cause mixing until the dynamic angle of the natural slope is exceeded, so that the particles then move in relation to each other.
• a stirring system for example, the shares or ribbons of a convective mixer, the blades of a high-shear mixer
• a flow of air for example, with a mixer with impaction
• the rotation of the tank must cause mixing until the dynamic angle of the natural slope is exceeded, so that the particles then move in relation to each other.
• the mixing step is carried out in a convective or rotating-tank mixer, which does not bring too much energy to the particles, which could lead to an increase in the temperature and possibly a crosslinking of the resin during the mixing. mixing step, which is prohibited in the method of the invention.
• the mixing step can be divided into two sub-steps:
• the mixture of the reagent system comprising the resin powder, the hardening compound powder and optionally the catalyst powder;
• the powder mixtures may be the following:
• ni is greater than 1, 5;
• n 2 representing the number of repetitions of the pattern taken in square brackets
• the powder mixtures resulting from the mixing step of the process of the invention are particularly stable and can be stored at ambient temperature, ready for use, for several months, without degradation of the properties of the article formed by transformation of said powder mixtures.
• the resulting powder mixture is subjected to a step of thermocompression of the mixture of powders obtained in the preceding step in a mold of suitable shape for obtaining the desired article and at a temperature of effective to obtain the crosslinking of the resin, at the end of which is obtained the conductive article of electricity.
• the mixture of powders is converted by densification of the resulting mixture generated by the crosslinking of the resin. It is advantageously exerted a pressure less than or equal to 1 t / cm 2 , ideally between 250 and 750 kg / cm 2 .
• the exercise of this pressure on the mixture of powders makes it possible, in addition to compacting the mixture, to reduce the distance between the electrically conductive filler particles.
• the pressure is preferably maintained for a time longer than the curing time of the epoxy resin.
• the mixture is heated to a temperature necessary for the crosslinking of the resin.
• the temperature and duration of the heating cycle are a function of the epoxy resin / hardener couple and the catalyst, when it is present.
• the heating cycle can vary from 2 to 30 minutes for a temperature ranging from 50 to 250 ° C.
• the step is performed in a mold whose shape is adapted to obtain the desired article, which means, in other words, that the mold must have a design and shape such as after the thermocompression step, the article resulting from this step has the desired final shape, without having to machine said article to give it its final shape.
• thermocompression step can be implemented as follows:
• the pressure can be applied gradually, in order to evacuate the air included in the material in powder form. Under the effect of heat, the compounds of the mixture begin to melt.
• the polymerization reaction is initiated when the compounds have reached the activation temperature of the hardener. The pressure is maintained until the end of the polymerization reaction of the resin; once the polymerization is complete, demolding the resulting article without prior cooling.
• the mold can be covered with a layer of a release agent or a specific surface treatment, in order to optimize the extraction of the molded articles.
• thermosetting resin makes it possible to reduce the energy cost of manufacturing the aforementioned articles. Indeed, if a thermoplastic resin was used, it would be necessary, after the thermocompression step, to cool the mold so that the article becomes solid and can be extracted from the mold.
• the process of the invention is particularly suitable for the preparation of bipolar plates for fuel cells, especially bipolar plates advantageously having the following characteristics: an electrical conductivity of at least 150 S / cm; a thermal conductivity of at least 15 W / mK; a flexural modulus of at least 10 GPa;
• a permeability to hydrogen of less than 2 * 10 -6 cm 3 / s. cm 2 (measured at 80 ° C. under 3 atm).
• FIG. 1 is a graph illustrating planar electrical conductivity ⁇ (in S / cm) as a function of the average size of electrically conductive filler particles t (in ⁇ m) for plates manufactured in accordance with FIG. 1.
• FIG. 2 is a diagram illustrating the directions in which the electrical conductivity and the thermal conductivity, respectively planar (arrow 1) and transverse (arrow 2), are measured, the arrow 3 indicating the charging direction.
• FIG. 3 is a graph illustrating the planar electrical conductivity ⁇ (in S / cm) as a function of the charge ratio% (in mass%) of electrically conductive particles for plates manufactured according to example 2.
• FIG. 4 is a graph illustrating the planar electrical conductivity ⁇ (in S / cm) as a function of the operating pressure P (in t / cm 2 ) for plates manufactured according to example 3.
• FIG. 6 is a graph illustrating the polarization curve, that is to say the T cell voltage (in V) as a function of the current density D (in A / cm 2 ) for an electrode-membrane assembly made according to Example 4.
• Examples 1 to 3 above are intended to highlight the reasoned choice of the characteristics of the raw materials and the operating conditions within the scope of the invention to produce bipolar plates.
• the conduction of electricity by the plate reflects a possible passage of electrons within the composite material constituting the bipolar plates. This electronic transfer is done via electrically conductive filler particles.
• This overall resistance is due to both the constriction resistance (due to the low contact area between the filler particles) and the tunnel resistance (due to the thickness of the insulating film on the surface of these same particles).
• the constriction resistance is a function of the geometry of the electrically conductive particles and the compaction of the composite material (notably the settling and the deformation of the particles on each other).
• the tunnel resistance is, in turn, a function of the proportion of resin in the composite material.
• Electronic conduction is then done by electronic jump between isolated particles, generating a local electric field.
• the electrical conductivity ⁇ (in S / cm) is determined by measuring the electrical resistivity p (in ⁇ .cm) of the material by the Werner method, known as the 4-point method.
• An alternating current I is sent into the material to be characterized from two extreme points.
• the potential difference V is measured between the two internal points.
• the points are equidistant from a distance d.
• the resistivity p (in ⁇ .cm) corresponds to the following formula:
• thermal conductivity is related to two types of behavior: the movement of charge carriers, electrons or holes;
• the thermal conductivity is therefore linked, on the one hand, to the electrical conductivity (movement of the charge carriers) and, on the other hand, to the very structure of the material (vibration of the atoms).
• the thermal conductivity ⁇ (W / mK) is measured directly on the article of the invention (hot compression molded plate).
• the senor is positioned on the main surface of the article (to determine the planar conductivity) or on its edge (to determine the transverse conductivity).
• a resistive sensor is used.
• the electrical current applied to the sensor generates a known amount of heat transmitted to the article.
• the dissipation of this amount of heat is analyzed by the sensor (which manifests as a decrease in electric current).
• Y represents the arrow at break
• - D represents the distance between supports
• b represents the width of the test piece
• h represents the thickness of the test piece
• Wettability-Hydrophobicity This property is evaluated by measuring the contact angle of a drop of water on the surface of the plate. This method consists in depositing a drop of liquid on the surface of the substrate. The wettability of the surface is then characterized by the contact angle ⁇ formed by the solid surface and the tangent to the liquid surface at the connection point.
• This example is intended to illustrate the influence of the average particle size of the electrically conductive charge on the electrical conductivity of the material and, consequently, on the thermal conductivity.
• the reagent system for its part, comprises the following ingredients: a DGEBA resin of formula as defined in the example 4 below of functionality 2, epoxide equivalent 475-550 g / eq (DER 671 from Dow)
• a substituted urea catalyst of formula as defined in Example 4 below (Amicure UR2T from Air Products) (0.5 percent resin parts) having an average particle size of less than 50 ⁇ m.
• thermocompression at 180 ° C. under 1 t / cm 2 for 30 minutes.
• FIG. 1 representing the planar electrical conductivity ⁇ (in S / cm) as a function of the average size of electrically conductive filler particles t (in ⁇ m).
• planar electrical conductivity corresponds to the conductivity measured in a material plane perpendicular to the load direction.
• an electrically conductive composite material comprising graphite particles must have an average particle size.
• an average particle diameter due to the overall shape of the particles advantageously greater than 10 microns and less than 500 microns, preferably ranging from 75 to 150 microns.
• the lamellar filler particles may be foamed graphite sheets.
• the synthetic graphite used (Timrex KS150 TIMCAL Ltd) has an average particle size such that 90% by volume of the particles have an average particle size of less than 150 ⁇ m and the lamellar graphite is in the form of sheets of several hundred 2 ⁇ m surface area and a few ⁇ m thick.
• thermocompression parameters are as follows: 30 minutes at 180 ° C. under lt / cm 2 .
• this lamellar charge is arranged in a plane perpendicular to the direction of movement of the gas molecules (this provision being induced by the implementation of the method), which is also the charging direction.
• the lamellar filler arranged in the form of parallel planes interferes with the progression through the plate of the reaction gas molecules.
• This example is intended to illustrate the influence of the rate of electrically conductive filler particles on the properties of the constituent material of the bipolar plate produced according to this example.
• the aforementioned mixtures are then shaped according to a method of thermocompression at 180 ° C. under 1 t / cm 2 for 30 minutes.
• FIG. 3 represents the planar electrical conductivity ⁇ (in S / cm) as a function of the% charge rate (in% by weight) of the electrically conductive particles.
• This example is intended to illustrate the influence of the pressure implemented during the thermocompression step on the properties of the constituent material of the bipolar plate made according to this example.
• the same mixture (mixture Mi as explained in the table of Example 4) is shaped by thermocompression at different pressures (0.25, 0.5, 0.75, 1; 25 and 1.5 t / cm 2 ) for an unchanged time and temperature (30 minutes at 180 ° C.).
• planar electrical conductivity ⁇ (in S / cm) is then measured.
• FIG. 4 represents a graph illustrating the planar electrical conductivity ⁇ (in S / cm) as a function of the pressure implemented P (in t / cm 2 ). From these results, it is clear for the particular mixture used, the following findings: up to 0.5 t / cm 2 , the compactness and thus the planar electrical conductivity are increased; for a certain range of pressures applied (in this case between 0.5 and 0.75 t / cm 2 ), the planar electrical conductivity has maximum values. This can be explained by the fact that the particles of electricity are approaching each other thus increasing their contact surface; for too high pressures
• planar electrical conductivity begins to decrease. This can be explained by the fact that the organic matrix exudes, thus helping to form an insulating film on the surface of the composite material.
• This phenomenon of exudation is also detectable with the measurement of the contact angle of a drop of water placed on the surface of the plates resulting from the thermocompression.
• FIG. 5 is a graph illustrating the evolution of the contact angle ⁇ (in °) as a function of the pressure implemented P (in t / cm 2 ).
• the contact angle ⁇ decreases sharply from a pressure of 750 kg / cm 2 , which reflects an increase in the hydrophilic nature of the surface of the material due to the exudation of the matrix. organic. Compared to the mixture used, it appears the need to work at pressures that are not too high to overcome the phenomenon of exudation of the organic matrix.
• the pressure applied for this type of mixture is preferably less than 1 t / cm 2 , ideally between 200 and 750 kg / cm 2 , so as to sufficiently compact the composite material to ensure optimal properties.
• This example illustrates the preparation of various powder mixtures (respectively Mi to M 4 ) to be shaped by thermocompression according to the method of the invention.
• the DGEBA resin has the following formula:
• the Novolac modified DGEBA resin is a mixture of DGEBA resin of formula above and a resin of the following formula:
• n 2 representing the repetition number of the pattern taken in parentheses, the mixture comprising at least 80% by weight of DGEBA resin.
• the dicyandiamide hardener has the following formula:
• the "2-phenylimidazole” catalyst has the following formula:
• the solid epoxy resins supplied in the form of crystals, are premixed to the desired particle size so that 90% by volume of the particles have an average particle size of less than or equal to 300 ⁇ m and from the total population of particles, 50% by volume of the particles have an average particle size less than or equal to 150 microns.
• the grinding is carried out using a sieve mill provided with a suitable perforated grid.
• a powder mixer specifically a rotating bowl mixer.
• an oscillating cradle is set in motion by two axes rotating in opposite directions.
• the resulting movement combines rotation, translation and inversion. This random movement of the particles guarantees a homogeneous mixture without segregation between components.
• the mixing step takes place under optimal hygiene conditions, without dust, thus reducing the cleaning operations.
• the mixing step can be split into two sub-steps:
• the mixture of the reagent system comprising the resin, the hardener and the catalyst
• the resulting mixture has the particularity of being very stable and can be preserved in the state for several months without degradation.
• the molding is implemented as follows:
• the mixture of powders is introduced into a hot mold (temperature higher than the polymerization temperature of the resin), the mold having dimensions corresponding to the dimensions of the plate; desired, the mold including fingerprints so as to generate the design of the distribution channels on the surface of the plates; the mold is then closed and put under load.
• the pressure can be applied gradually, in order to evacuate the air included in the material in powder form. Under the effect of heat, the compounds of the mixture begin to melt.
• the polymerization reaction is initiated when the compounds have reached the activation temperature of the hardener. The pressure is maintained until the end of the polymerization reaction of the resin;
• the resulting plate is then demolded, without prior cooling.
• thermocompression forming step of the compositions The precise operating conditions of the thermocompression forming step of the compositions are summarized in the table below for the various mixtures.
• the thermal stability was also evaluated by measuring the glass transition temperature Tg of the reactive system (ie comprising a resin, a hardener and optionally a catalyst).
• the plates obtained with an EPN resin have a greater thermal stability and a better resistance to bending than for the plates obtained with a DGEBA resin; since the EPN resin is more functional (more than two epoxide sites per molecule), the crosslinking density of the material formed is greater than for a difunctional resin (such as DGEBA);
• the plates obtained with an imidazole type hardener have excellent rigidity and therefore increased mechanical strength over an extended temperature range; the presence of a lamellar filler makes it possible to improve the impermeability to gases.
• the plate made with the mixture Mi was subjected to stack tests carried out using a single-cell test bench. These tests consist in testing an electrode-membrane assembly positioned between two composite plates made by the method of the invention allowing the distribution of gases and the collection of electrons. More specifically, the tested assembly consists of a Nafion NRE212 membrane and two Nafion-impregnated commercial "GDE" electrodes (E-tek HT250EW 0.5 mg Pt / cm 2 , 0.7 mg Nafion / cm 2 ). .
• FIG. 6 This figure represents a graph illustrating the polarization curve, that is to say the cell voltage T (in V) as a function of the current density D ( in A / cm 2 ) obtained with an assembly as mentioned above.
• Bipolar plates do not deteriorate the electrochemical performance of the battery core. The new plates are therefore not consos the point of view of the electrochemical performance of the PEMFC cell core, as can be seen from the curve of FIG.

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