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EP1680378A2 - Porous structures useful as bipolar plates and methods for preparing same - Google Patents

Porous structures useful as bipolar plates and methods for preparing same

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Publication number
EP1680378A2
EP1680378A2 EP04767923A EP04767923A EP1680378A2 EP 1680378 A2 EP1680378 A2 EP 1680378A2 EP 04767923 A EP04767923 A EP 04767923A EP 04767923 A EP04767923 A EP 04767923A EP 1680378 A2 EP1680378 A2 EP 1680378A2
Authority
EP
European Patent Office
Prior art keywords
carbon
porous
matrix
layer
face
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP04767923A
Other languages
German (de)
French (fr)
Inventor
Renaut Mosdale
Sylvie Escribano
Pierre Olry
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Original Assignee
Commissariat a lEnergie Atomique CEA
SNECMA Propulsion Solide SA
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Commissariat a lEnergie Atomique CEA, SNECMA Propulsion Solide SA filed Critical Commissariat a lEnergie Atomique CEA
Publication of EP1680378A2 publication Critical patent/EP1680378A2/en
Withdrawn legal-status Critical Current

Links

Classifications

    • 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
    • H01M8/023—Porous and characterised by the material
    • H01M8/0234—Carbonaceous material
    • 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
    • H01M8/023—Porous and characterised by the material
    • H01M8/0241—Composites
    • H01M8/0245—Composites in the form of layered or coated products
    • 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 porous structures which can be used in particular as bipolar plates or as a bipolar plate / electrode assembly in fuel cell devices.
  • the invention also relates to a method of manufacturing such porous structures.
  • the general field of the invention can be defined as that of fuel cells - in particular fuel cells of the solid polymer electrolyte type.
  • a fuel cell is an assembly generally comprising a plurality of elementary cells stacked one on the other. In each of the elementary cells of the fuel cell, an electrochemical reaction is created between two reagents which are introduced continuously into the elementary cells.
  • the fuel usually used is hydrogen or methanol, depending on whether one is respectively in the presence of a cell operating with mixtures of the hydrogen / oxygen type (cell of the PEMFC type) and in the presence a cell operating with mixtures of the methanol / oxygen type (cell of the DMFC type).
  • the fuel is brought into contact with the anode while the oxidant - in this case oxygen, is brought into contact with the cathode.
  • the cathode and the anode are separated by an electrolyte of the ion exchange membrane type.
  • an oxidation reaction of the fuel for example hydrogen, represented by the following equation: 2H 2 ⁇ 4H + + 4th "
  • a reaction takes place reduction of the oxidant, in general oxygen, represented by the following equation: 0 2 + 4H + + 4th " ⁇ 2H 2 0
  • Electrons produced at the anode are sent to the cathode by an external circuit in order to contribute to the production of electrical energy.
  • the bipolar plates must also fulfill other functions than that of ensuring the electrical connection. Indeed, one must for example proceed, via these bipolar plates, to the continuous supply of reagents to the anode of a first assembly, and to the cathode of a second adjacent assembly, the bipolar plates filling , at that time, the role of reagent distributor.
  • the bipolar plates are also used to evacuate products at the cathode, by integrating elements for removing excess water.
  • the bipolar plates can also incorporate a heat exchanger used to counter any overheating within the stack of electrode-membrane-electrode assemblies.
  • bipolar plates may reside in the mechanical strength of the electrode-menbran-electrode assemblies, in particular when the latter are stacked on top of each other. Such an assembly ensures an overall volume of the thin battery, which is fully compatible with the intended applications, such as that relating to an electric vehicle.
  • bipolar plates for distributing the reagents.
  • channels are machined on at least one face of the bipolar plates. These channels are provided to ensure the most homogeneous distribution possible of the reagents on a surface of the electrode with which they are in contact. These channels are usually organized so that the reagents injected into these channels wind over a large part of the surface of the electrode.
  • the means used to obtain such a result are horizontal sections spaced by elbows descending at 180 °. Note that these sections are also capable of recovering and discharging the water produced at the cathode.
  • this particular arrangement of means does not make it possible to obtain a sufficiently large exchange surface to result in an acceptable electrochemical conversion yield for industrial application.
  • another configuration has been proposed in the prior art. According to this configuration, it is a question of using a metallic foam with high porosity to be added to the metallic parts in which are machined, this metal foam to ensure good distribution of reagents as well as the evacuation of different products.
  • the structures, used as bipolar plates in the prior art all have one or more of the following drawbacks: - they do not allow efficient distribution of the reagents, due to an insufficient exchange surface between the structure and the element to be supplied with fluid; - They cause, by the fact that they can be made up of several parts possibly made of different materials, contact resistance and corrosion problems.
  • PRESENTATION OF THE INVENTION The object of the present invention is therefore to propose a porous structure which can be used in particular for constituting bipolar plates as well as bipolar plate / electrode assemblies, said structure overcoming the drawbacks of the aforementioned prior art.
  • the object of the invention is also to propose a method for manufacturing such porous structures.
  • the present invention relates to a porous structure comprising a porous carbon fiber matrix, said porous matrix being delimited at at least one of its faces by an impermeable layer in a selected carbon element. among carbon fibers, carbon nanotubes, vitreous carbon or a combination thereof, said waterproof layer being linked to the porous matrix by carbon-carbon bonds.
  • Such a porous structure has the following advantages: - the fact that it is made up entirely of carbon, this structure has electrical continuity, good conductivity and great chemical inertness, which the porous structures of the art do not have anterior; - the fact that the parts of this structure (matrix and waterproof layer) are no longer only linked by mechanical connection but by carbon-carbon bonds, this structure, when will be dedicated to fluid circulation, will not experience any fluid leakage problem; and - for the same reasons as those mentioned above, when it is dedicated to electrical conduction, the porous structure of the invention will not exhibit a drop in potential, insofar as the contact resistance inherent in the structures of the prior art no longer exists, owing to the fact that the various constituent elements of the porous structure of the invention consist of the same material (carbon) and are linked by carbon-carbon bonds; - Finally, the fact of using only carbon elements as explained above to constitute the porous structure makes it possible to limit the size and the mass thereof.
  • the present invention relates to a method of manufacturing a porous structure as defined above, said method comprising a step of producing said waterproof layer (s): 1) by growth of carbon elements chosen from carbon fibers, carbon nanotubes, on one face or two opposite faces of a carbon fiber matrix followed by densification of said carbon elements; and / or 2) by the formation of vitreous carbon on one face or two opposite faces of a carbon fiber matrix, when the carbon element is vitreous carbon.
  • the method of the invention has the following advantages: - it provides a simplification in the design of porous zones, in that, unlike the methods of the prior art, such porous zones are no longer designed by superposition of materials of different natures; - allows control of the porosity of the various constituent parts of the porous area; - it allows, thanks to the materials used all based on carbon, to obtain an area having excellent chemical, electrochemical and thermal stability; - it implements stages which can be carried out in a continuous production line.
  • the present invention relates to a bipolar plate or a bipolar plate / electrode assembly comprising a porous structure according to the invention.
  • FIGS. 1 to 6 are sectional views of various porous structures according to the invention. DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS
  • the invention relates to porous structures which can be used as a bipolar plate and / or as a bipolar plate / electrode assembly.
  • the porous structures consist of a porous carbon fiber matrix, said porous matrix being delimited at at least one of its faces by a waterproof layer in an element chosen from carbon fibers, carbon nanotubes , vitreous carbon, said waterproof layer being linked to the porous matrix by carbon-carbon bonds.
  • the porous structure generally has an open porosity overall.
  • porous matrix of carbon fibers it is specified that, by porous matrix of carbon fibers, one understands, in what precedes and what follows, a flexible part made up of an entanglement of son in carbon fibers, the degree of entanglement being function of the desired porosity .
  • the porous matrix is delimited on at least one of its faces by a waterproof layer, that is to say a layer impermeable to gases and liquids.
  • This waterproof layer has the particularity of being made of a carbon element chosen from carbon fibers, carbon nanotubes, vitreous carbon, and of not being associated with the porous matrix by a mechanical bond but by carbon bonds. -carbon.
  • the porous structure constitutes a part which does not have, as is the case with the porous structures of the prior art, a contact resistance, responsible in particular, when the porous structures are used as a bipolar plate, d 'a drop in potential.
  • a contact resistance responsible in particular, when the porous structures are used as a bipolar plate, d 'a drop in potential.
  • these can be presented in different configurations.
  • the porous structure 1 comprises a porous matrix 3 of carbon fibers delimited at the level of a first face 5 by a waterproof layer 7, having the characteristics mentioned above and at a second face 9 opposite the first face 5 by a porous layer 11 of a carbon element chosen from carbon fibers, carbon nanotubes, said porous layer 11 being linked by carbon-carbon bonds to the porous matrix 3. It is understood that the porous layer will have a predetermined porosity depending on the use dedicated to this layer.
  • the porous structure 13 comprises a porous matrix 15 delimited at the level of a first face 17 by a waterproof layer 19 and at the level of a second face 21 opposite the first face by another waterproof layer 23, said waterproof layers 19, 23 being as defined above.
  • the porous structures of the invention may also comprise a porous layer of a carbon element chosen from carbon fibers, carbon nanotubes, on the above-mentioned waterproof layer or layers and / or on one face of the porous matrix, such as this is in particular shown in FIGS. 3 and 4.
  • FIG. 3 represents a porous structure 25 comprising a porous matrix 27, delimited on one face 30 by a waterproof layer 29 and on the opposite face 32 by a porous layer 31 such that shown in FIG. 1, and in addition another porous layer 33 on said waterproof layer 29.
  • FIG. 3 represents a porous structure 25 comprising a porous matrix 27, delimited on one face 30 by a waterproof layer 29 and on the opposite face 32 by a porous layer 31 such that shown in FIG. 1, and in addition another porous layer 33 on said waterproof layer 29.
  • FIG. 3 represents a porous structure 25 comprising a porous matrix 27, delimited on one face 30 by a waterproof layer 29 and on the opposite face 32 by a porous layer 31 such that shown in FIG. 1,
  • porous structure 35 represents a porous structure 35 comprising a porous matrix 37 delimited on two opposite faces 40, 42 by two waterproof layers 39, 41 on either side of said porous matrix 37, on which two porous layers 43, 45 are fixed by carbon-carbon bond.
  • the porous structures may include an active layer (referenced respectively 12 in Figures 1, 5 and 6) deposited on the aforementioned porous layers.
  • Figure 5 corresponds to a complex porous structure resulting from the association by their waterproof layers 7 of two porous structures 13 as shown in FIG. 1.
  • FIG. 6 corresponds to a complex porous structure resulting from the association by their impermeable layers (7, 19, 23) of a porous structure 13 in accordance with FIG. 2 with two porous structures 1 in accordance with FIG. 1.
  • the porous structures of the invention can be used as a bipolar plate and / or as a bipolar plate / electrode assembly.
  • the porous structures of the invention can also be used in heat exchangers. It will be recalled that a bipolar plate is a part ensuring the physical separation between two electrodes of opposite polarity from two adjacent battery cells, while ensuring electrical continuity.
  • a bipolar plate can ensure, in addition to its role of separation, a role in the distribution of suitable reagents (namely fuel or oxidizer) to the above-mentioned electrodes.
  • a bipolar plate / electrode assembly is an assembly resulting from the association of a bipolar plate as defined above with at least one part of an electrode, that is to say the reagent diffusion zone (which may correspond to the porous layers previously mentioned) and possibly the active area (which may correspond to the above-mentioned active layer).
  • active layer is meant according to the invention a layer comprising at least one catalyst capable of catalyzing the electrochemical reaction suitable for the electrode concerned.
  • the porous structures shown in Figures 1, 4, 5 and 6 can be used as bipolar plates and / or electrode / bipolar plate assembly.
  • the waterproof layer 7 and the porous matrix 3 correspond to a half-plate, insofar as it rests only on one electrode and the porous layer 11 corresponds to the zone of diffusion of the reagents of the electrode and the catalytic layer 12 corresponds to the active area of the electrode.
  • the porous matrix 37 corresponds to the zone of circulation of the cooling liquid; - The porous layers 43, 45 correspond to the reagent distribution zones; - The sealed layers 39, 41 provide a separation between the coolant circulation zone and the reagent distribution zones.
  • the porous matrices 3 correspond to the zone of distribution of the reagents;
  • the porous layers 11 correspond to the diffusion zone of the electrodes belonging to two adjacent cells;
  • the active layers 12 correspond to the active area of the electrodes belonging to two adjacent cells;
  • the waterproof layers 7 ensuring the separation between the two reagent distribution zones.
  • the porous matrix 15 corresponds to the zone of circulation of the coolant and the porous matrices 3 correspond to the two zones of distribution of reagents;
  • the porous layers 11 correspond to the diffusion zones of the electrodes belonging to two adjacent cells;
  • the active layers 12 correspond to the active area of the electrodes belonging to two adjacent cells;
  • the waterproof layers 7, 19, 23 ensuring the separation between the zone of circulation of the liquid and the two reagent distribution zones.
  • the structure represented by FIG. 2 can correspond to a bipolar plate in which: - the porous matrix 15 corresponds to a zone of circulation of a cooling fluid; - the waterproof layers 19 and 23 can ensure the separation between two electrodes of two adjacent cell cells.
  • the structure shown in FIG. 3 can correspond to an electrode / bipolar plate assembly without a cooling circuit in which: - the porous matrix 27 and the porous layer 31 correspond to a reagent distribution zone; - The porous layer 33 corresponds to a reagent distribution zone distinct from the previously mentioned distribution zone; - The waterproof layer 29 ensures the separation between the two aforementioned distribution zones.
  • the porosity, within the same porous layer can be variable, depending on the use of this porous layer.
  • the porosity between two distinct porous layers can also be different depending on whether these porous layers are dedicated to the distribution of gas (such as 0 2 ) and the distribution of liquid (such as methanol).
  • the invention relates to a method for manufacturing such a porous structure as defined above, said method comprising a step of producing said waterproof layer (s) by growing elements of carbon on one or two opposite sides of a carbon fiber matrix followed by densification of said carbon elements (when these carbon elements are carbon fibers or carbon nanotubes), or by the formation of carbon glassy.
  • carbon fiber matrix is understood to mean a part resulting from the entanglement of carbon fibers, the entanglement being more or less dense depending on the desired porosity.
  • the carbon fiber matrices may be commercially available or may be prepared beforehand, for example by needling carbon fibers. It is specified that the needling technique consists of mechanically entangling in the three directions of the space of the fibers of a web, using a needling machine, the entanglement being able to be adjusted according to the desired porosity.
  • the step of producing the waterproof layer (s) consists in producing these waterproof layers, so that they are anchored, in whole or in part in the carbon fiber matrix, more precisely in the pores constituting this matrix in carbon fibers through carbon-carbon bonding.
  • the porous area is thus obtained (formed by the structure of the carbon fiber matrix) delimited on at least one side thereof by an impervious layer, which interpenetrates the pores of said matrix, the resultant piece thus being a part "Monoblock", that is to say a part not resulting from the addition of several parts joined together, for example, by welding and not having the drawbacks inherent in this type of parts, as has been mentioned more high.
  • such a waterproof layer can be obtained by growth of carbon elements on at least one of the faces of a carbon fiber matrix followed by densification of said carbon elements, when the carbon elements are carbon fibers. , carbon nanotubes.
  • Such a waterproof layer can also be obtained by the formation of vitreous carbon on at least one of the faces of a carbon fiber matrix. It is also conceivable to combine both the growth of carbon elements and the formation of vitreous carbon, when the tight layer comprises both carbon elements, such as carbon fibers or carbon nanotubes and with both glassy carbon.
  • the step of growing said carbon elements carbon fibers may consist of pyrolizing precursor fibers of carbon fibers, said fibers possibly being polymer fibers such as polyacrylonitrile fibers (PAN), fibers obtained from pitch, the pyrolysis step being preceded by the following steps : - a step of impregnating the adequate face of the carbon fiber matrix with suitable monomers or petroleum pitch; - in the case where the precursor fibers are polymer fibers, a step of polymerization of said monomers followed by spinning, to obtain the appropriate polymer fibers; - In the case where the precursor fibers are pitch fibers, a spinning step so as to obtain pitch fibers.
  • PAN polyacrylonitrile fibers
  • the spinning will be done so as to obtain a network of fibers sufficiently entangled so that, at the end of the pyrolysis, the resulting layer is a waterproof layer.
  • the carbon nanotubes growth step can be carried out on the carbon fiber matrix according to a process such as that described in FR 2 844 510.
  • This process notably comprises the following steps: - a step of impregnating the face adequate matrix with an aqueous solution comprising one or more metal catalyst salts for growth of carbon nanotubes, such as Co, Ni or Fe salts in the form of nitrates, acetates; - A step of decomposition into oxide (s) of said salt or salts by heat treatment, for example by bringing the impregnated matrix to a temperature between 100 ° C and 250 ° C; - A step of reduction of the oxide (s) formed, for example, by introducing the matrix into an oven under a reducing atmosphere; a step for synthesizing carbon nanotubes by bringing the matrix into contact with a gaseous carbon precursor in an oven heated to a temperature allowing the formation of carbon by decomposition (cracking) of the gaseous precursor.
  • aqueous solution comprising one or more metal catalyst salts for growth of carbon nanotubes, such as Co, Ni or Fe salts in the form of nitrates, acetate
  • the gaseous precursor can be an aromatic or non-aromatic hydrocarbon.
  • acetylene, ethylene, propylene or methane are used.
  • the oven temperature required for cracking can range from 450 ° C to 1200 ° C.
  • the structure obtained (whether the waterproof layer is made of carbon fibers or carbon nanotubes) is then densified by liquid means or by chemical vapor infiltration, as described in document FR 2 844 510.
  • the step of Vitreous carbon can be formed by impregnating the carbon fiber matrix on the appropriate face with a furan or phenolic resin followed by a pyrolysis step.
  • the porous layer (s) can be obtaining by growth of carbon elements, such as carbon fibers and carbon nanotubes, the growth being regulated so as to obtain at the end of this growth a layer having the desired porosity.
  • the porous structure also includes an active layer based on catalyst, the latter can be obtained by techniques conventionally employed in the manufacture of active layers, such as coating or spraying with suspensions comprising the appropriate catalyst. Such suspensions can be a platinum carbon suspension.
  • the porous structures of the invention thanks to the presence of different zones with determined porosity, find their applications in the field of fuel cells of the PEMFC or DMFC type operating at low temperature and cells operating at intermediate temperature (such as phosphoric acid batteries operating at 250 ° C) as bipolar plates but also in the field of heat exchangers.

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  • Chemical & Material Sciences (AREA)
  • Sustainable Energy (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Sustainable Development (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • General Chemical & Material Sciences (AREA)
  • Composite Materials (AREA)
  • Inert Electrodes (AREA)
  • Fuel Cell (AREA)
  • Ceramic Products (AREA)

Abstract

The invention concerns a porous structure characterized in that it comprises a porous carbon fabric matrix (15), said porous matrix being delimited at one of its surfaces (17, 21) by a sealing layer (19, 23) made of an element selected among carbon fibers, carbon nanotubes, glassy carbon, said sealing layer being bound to the porous matrix by carbon-carbon bonds. The invention also concerns a method for making such porous structures. The invention is applicable to fuel cells and heat exchangers.

Description

STRUCTURES POREUSES UTILISABLES EN TANT QUE PLAQUES BIPOLAIRES ET PROCÉDÉS DE PRÉPARATION DE TELLES STRUCTURES POREUSES. DESCRIPTION POROUS STRUCTURES FOR USE AS BIPOLAR PLATES AND PROCESSES FOR PREPARING SUCH POROUS STRUCTURES. DESCRIPTION
DOMAINE TECHNIQUE La présente invention a trait à des structures poreuses- pouvant être utilisées notamment en tant que plaques bipolaires ou ensemble plaque bipolaire/électrode dans des dispositifs de pile à combustible. L'invention concerne également un procédé de fabrication de telles structures poreuses. Le domaine général de l'invention peut être défini comme celui des piles à combustible- en particulier des piles à combustible du type à électrolyte polymère solide.TECHNICAL FIELD The present invention relates to porous structures which can be used in particular as bipolar plates or as a bipolar plate / electrode assembly in fuel cell devices. The invention also relates to a method of manufacturing such porous structures. The general field of the invention can be defined as that of fuel cells - in particular fuel cells of the solid polymer electrolyte type.
ETAT DE LA TECHNIQUE ANTERIEURE Une pile à combustible est un ensemble comportant généralement une pluralité de cellules élémentaires empilées les unes sur les autres. Dans chacune des cellules élémentaires de la pile à combustible, une réaction électrochimique se crée entre deux réactifs qui sont introduits de manière continue dans les cellules élémentaires. Le combustible habituellement utilisé est l'hydrogène ou le méthanol, suivant que l'on se trouve respectivement en présence d'une pile fonctionnant avec des mélanges du type hydrogène/oxygène (pile du type PEMFC) et en présence d'une pile fonctionnant avec des mélanges du type méthanol/oxygène (pile du type DMFC) . Le combustible est amené au contact de l'anode tandis que le comburant- en l'occurrence l'oxygène, est amené au contact de la cathode. La cathode et l'anode sont séparées par un électrolyte du type membrane échangeuse d'ions. Au niveau de l'anode, il se produit une réaction d'oxydation du combustible, par exemple l'hydrogène, représentée par l'équation suivante : 2H2 → 4H+ + 4e" Au niveau de la cathode, il se produit une réaction de réduction de l'oxydant, en général l'oxygène, représentée par l'équation suivante: 02 + 4H+ + 4e" → 2H20 On assiste alors à une réaction électrochimique dont l'énergie créée est convertie en énergie électrique. Des protons H+ circulent de l'anode en direction de la cathode en traversant l' électrolyte . Des électrons produits à l'anode sont acheminés vers la cathode par un circuit extérieur afin de concourir à la production d'énergie électrique. Dans un même temps, au niveau de la cathode, on assiste à une production d'eau qui est évacuée de l'ensemble électrode-membrane-électrode. Dans les piles à combustible de l'art antérieur, plusieurs ensembles électrode-membrane- électrode sont empilés les uns sur les autres, afin d'obtenir une puissance supérieure à celle fournie par un seul de ces ensembles. La jonction et la continuité électrique entre ces ensembles s'effectuent généralement à l'aide de plaques conductrices, ces plaques étant également appelées plaques bipolaires . C'est donc à l'aide de ces plaques bipolaires, que l'on peut joindre la cathode d'un ensemble avec l'anode d'un ensemble adjacent. Ces plaques bipolaires permettent en outre d'assurer les plus grandes conductivités électriques possibles, de manière à éviter les chutes ohmiques préjudiciables au rendement de la pile à combustible. Les plaques bipolaires doivent également remplir d'autres fonctions que celle d'assurer la jonction électrique. En effet, on doit par exemple procéder, par l'intermédiaire de ces plaques bipolaires, à l'alimentation continue en réactifs de l'anode d'un premier ensemble, et de la cathode d'un second ensemble adjacent, les plaques bipolaires remplissant, à ce moment là, le rôle de distributeur de réactifs. De plus, les plaques bipolaires servent aussi à l'évacuation des produits au niveau de la cathode, en intégrant des éléments d'élimination de l'eau en excès. Les plaques bipolaires peuvent en outre incorporer un échangeur thermique servant à contrer toute surchauffe au sein de l'empilement d'ensembles électrode-membrane-électrode . Notons enfin qu'une autre fonction de ces plaques bipolaires peut résider dans la tenue mécanique des ensembles électrode-menbrane-électrode, notamment lorsque ces derniers sont empilés les uns sur les autres. Un tel assemblage assure un volume global de la pile de faible épaisseur, ce qui est tout à fait compatible avec les applications prévues, comme par exemple celle concernant un véhicule électrique. Dans l'art antérieur, il existe différentes configurations de plaques bipolaires, pour réaliser la distribution des réactifs. On note tout d' abord une configuration selon laquelle des canaux sont usinés sur au moins une face des plaques bipolaires. Ces canaux sont prévus pour assurer une distribution la plus homogène possible des réactifs sur une surface de l'électrode avec laquelle ils sont en contact. Ces canaux sont habituellement organisés de sorte que les réactifs injectés dans ces canaux serpentent sur une grande partie de la surface de l'électrode. Les moyens mis en œuvre pour obtenir un tel résultat sont des tronçons horizontaux espacés par des coudes descendant à 180°. Notons que ces tronçons sont également susceptibles de récupérer et d'évacuer l'eau produite au niveau de la cathode. Cependant, il a été constaté que cet agencement particulier de moyens ne permettait pas d'obtenir une surface d'échange suffisamment importante pour aboutir à un rendement de conversion électrochimique acceptable en vue d'une application industrielle . Pour pallier cet inconvénient, une autre configuration a été proposée dans l'art antérieur. Il s'agit selon cette configuration d'utiliser une mousse métallique à forte porosité à adjoindre aux pièces métalliques dans lesquelles sont pratiqués des usinages, cette mousse métallique permettant d'assurer une bonne distribution des réactifs ainsi que l'évacuation des différents produits . Néanmoins, le fait d'adjoindre une mousse métallique au niveau de la plaque bipolaire contribue à créer une résistance importante, ce qui entraîne une diminution de la conduction électrique au sein de 1' ensemble. Même si le problème afférent à la conduction électrique peut être partiellement résolu en compressant la mousse métallique, il s'avère en tout état de cause que des problèmes de corrosion persistent, du fait de la nature chimique très agressive de l'environnement de ce type de pile à combustible, même en utilisant des revêtements inoxydables, et notamment en raison de la présence de nombreux défauts comme des ruptures de brins au sein de la mousse métallique. Ainsi, les structures, utilisées en tant que plaques bipolaires dans l'art antérieur, présentent toutes l'un ou plusieurs inconvénients suivants : - elles ne permettent pas une distribution efficace des réactifs, du fait d'une surface d'échange insuffisante entre la structure et l'élément à alimenter en fluide ; - elles engendrent, par le fait qu'elles peuvent être constituées de plusieurs pièces éventuellement en matériaux différents, une résistance de contact et des problèmes de corrosion. EXPOSE DE L'INVENTION Le but de la présente invention est donc de proposer une structure poreuse utilisable notamment pour constituer des plaques bipolaires ainsi que des ensembles plaque bipolaire/électrode, ladite structure remédiant aux inconvénients de l'art antérieur précité. Le but de l'invention est également de proposer un procédé de fabrication de telles structures poreuses .STATE OF THE PRIOR ART A fuel cell is an assembly generally comprising a plurality of elementary cells stacked one on the other. In each of the elementary cells of the fuel cell, an electrochemical reaction is created between two reagents which are introduced continuously into the elementary cells. The fuel usually used is hydrogen or methanol, depending on whether one is respectively in the presence of a cell operating with mixtures of the hydrogen / oxygen type (cell of the PEMFC type) and in the presence a cell operating with mixtures of the methanol / oxygen type (cell of the DMFC type). The fuel is brought into contact with the anode while the oxidant - in this case oxygen, is brought into contact with the cathode. The cathode and the anode are separated by an electrolyte of the ion exchange membrane type. At the anode, there is an oxidation reaction of the fuel, for example hydrogen, represented by the following equation: 2H 2 → 4H + + 4th " At the cathode, a reaction takes place reduction of the oxidant, in general oxygen, represented by the following equation: 0 2 + 4H + + 4th " → 2H 2 0 We then witness an electrochemical reaction whose created energy is converted into electrical energy. H + protons flow from the anode towards the cathode through the electrolyte. Electrons produced at the anode are sent to the cathode by an external circuit in order to contribute to the production of electrical energy. At the same time, at the cathode, there is a production of water which is discharged from the electrode-membrane-electrode assembly. In fuel cells of the prior art, several electrode-membrane-electrode assemblies are stacked on each other, in order to obtain a power greater than that supplied by a single one of these assemblies. The junction and the electrical continuity between these sets are made generally using conductive plates, these plates also being called bipolar plates. It is therefore with the aid of these bipolar plates that the cathode of an assembly can be joined with the anode of an adjacent assembly. These bipolar plates also make it possible to ensure the highest possible electrical conductivities, so as to avoid ohmic drops detrimental to the performance of the fuel cell. The bipolar plates must also fulfill other functions than that of ensuring the electrical connection. Indeed, one must for example proceed, via these bipolar plates, to the continuous supply of reagents to the anode of a first assembly, and to the cathode of a second adjacent assembly, the bipolar plates filling , at that time, the role of reagent distributor. In addition, the bipolar plates are also used to evacuate products at the cathode, by integrating elements for removing excess water. The bipolar plates can also incorporate a heat exchanger used to counter any overheating within the stack of electrode-membrane-electrode assemblies. Finally, note that another function of these bipolar plates may reside in the mechanical strength of the electrode-menbran-electrode assemblies, in particular when the latter are stacked on top of each other. Such an assembly ensures an overall volume of the thin battery, which is fully compatible with the intended applications, such as that relating to an electric vehicle. In the prior art, there are different configurations of bipolar plates, for distributing the reagents. We first note a configuration according to which channels are machined on at least one face of the bipolar plates. These channels are provided to ensure the most homogeneous distribution possible of the reagents on a surface of the electrode with which they are in contact. These channels are usually organized so that the reagents injected into these channels wind over a large part of the surface of the electrode. The means used to obtain such a result are horizontal sections spaced by elbows descending at 180 °. Note that these sections are also capable of recovering and discharging the water produced at the cathode. However, it has been found that this particular arrangement of means does not make it possible to obtain a sufficiently large exchange surface to result in an acceptable electrochemical conversion yield for industrial application. To overcome this drawback, another configuration has been proposed in the prior art. According to this configuration, it is a question of using a metallic foam with high porosity to be added to the metallic parts in which are machined, this metal foam to ensure good distribution of reagents as well as the evacuation of different products. Nevertheless, the fact of adding a metal foam at the level of the bipolar plate contributes to creating a significant resistance, which brings about a reduction in the electrical conduction within the assembly. Even if the problem relating to electrical conduction can be partially solved by compressing the metal foam, it turns out in any event that corrosion problems persist, due to the very aggressive chemical nature of the environment of this type. of a fuel cell, even using stainless coatings, and in particular due to the presence of numerous defects such as breakage of strands within the metal foam. Thus, the structures, used as bipolar plates in the prior art, all have one or more of the following drawbacks: - they do not allow efficient distribution of the reagents, due to an insufficient exchange surface between the structure and the element to be supplied with fluid; - They cause, by the fact that they can be made up of several parts possibly made of different materials, contact resistance and corrosion problems. PRESENTATION OF THE INVENTION The object of the present invention is therefore to propose a porous structure which can be used in particular for constituting bipolar plates as well as bipolar plate / electrode assemblies, said structure overcoming the drawbacks of the aforementioned prior art. The object of the invention is also to propose a method for manufacturing such porous structures.
Ainsi, selon un premier objet, la présente invention a trait à une structure poreuse comprenant une matrice poreuse en fibres de carbone, ladite matrice poreuse étant délimitée au niveau d'une au moins de ses faces par une couche etanche en un élément de carbone choisi parmi les fibres de carbone, les nanotubes de carbone, le carbone vitreux ou une combinaison de ceux-ci, ladite couche etanche étant liée à la matrice poreuse par des liaisons carbone- carbone. Une telle structure poreuse présente les avantages suivants : - du fait qu'elle soit constituée uniquement de carbone, cette structure présente une continuité électrique, une bonne conductivité ainsi qu'une grande inertie chimique, que ne présentent pas les structures poreuses de l'art antérieur ; - du fait que les parties de cette structure (matrice et couche etanche) ne sont plus uniquement liées par liaison mécanique mais par des liaisons carbone-carbone, cette structure, lorsqu'elle sera dédiée à la circulation de fluide, ne connaîtra pas de problème de fuite de fluide ; et - pour les mêmes raisons que celles mentionnées ci-dessus, lorsqu'elle sera dédiée à la conduction électrique, la structure poreuse de l'invention ne présentera pas de chute de potentiel, dans la mesure où la résistance de contact inhérente aux structures de l'art antérieur n'existe plus, du fait que les différents éléments constitutifs de la structure poreuse de l'invention sont constitués du même matériau (le carbone) et sont liés par des liaisons carbone-carbone ; - enfin, le fait de n'utiliser que des éléments de carbone tels qu'explicités ci-dessus pour constituer la structure poreuse permet de limiter l'encombrement et la masse de celle-ci.Thus, according to a first object, the present invention relates to a porous structure comprising a porous carbon fiber matrix, said porous matrix being delimited at at least one of its faces by an impermeable layer in a selected carbon element. among carbon fibers, carbon nanotubes, vitreous carbon or a combination thereof, said waterproof layer being linked to the porous matrix by carbon-carbon bonds. Such a porous structure has the following advantages: - the fact that it is made up entirely of carbon, this structure has electrical continuity, good conductivity and great chemical inertness, which the porous structures of the art do not have anterior; - the fact that the parts of this structure (matrix and waterproof layer) are no longer only linked by mechanical connection but by carbon-carbon bonds, this structure, when will be dedicated to fluid circulation, will not experience any fluid leakage problem; and - for the same reasons as those mentioned above, when it is dedicated to electrical conduction, the porous structure of the invention will not exhibit a drop in potential, insofar as the contact resistance inherent in the structures of the prior art no longer exists, owing to the fact that the various constituent elements of the porous structure of the invention consist of the same material (carbon) and are linked by carbon-carbon bonds; - Finally, the fact of using only carbon elements as explained above to constitute the porous structure makes it possible to limit the size and the mass thereof.
Selon un second objet, la présente invention a trait à un procédé de fabrication d'une structure poreuse telle que définie précédemment, ledit procédé comprenant une étape de réalisation de ladite ou desdites couche (s) etanche (s) : 1) par croissance d'éléments de carbone choisis parmi les fibres de carbone, les nanotubes de carbone, sur une face ou deux faces opposées d'une matrice en fibres de carbone suivie d'une densification desdits éléments de carbone ; et/ou 2) par formation de carbone vitreux sur une face ou deux faces opposées d'une matrice en fibres de carbone, lorsque l'élément de carbone est du carbone vitreux. Ainsi, le procédé de l'invention présente les avantages suivants : - il apporte une simplification dans la conception des zones poreuses, dans la mesure, où contrairement aux procédés de l'art antérieur, de telles zones poreuses ne sont plus conçues par superposition de matériaux de natures différentes ; - permet un contrôle de la porosité des différentes parties constitutives de la zone poreuse ; - il permet, grâce aux matériaux utilisés tous à base de carbone, d'obtenir une zone présentant une excellente stabilité chimique, électrochimique et thermique ; - il met en œuvre des étapes pouvant être réalisées dans une ligne de production continue.According to a second object, the present invention relates to a method of manufacturing a porous structure as defined above, said method comprising a step of producing said waterproof layer (s): 1) by growth of carbon elements chosen from carbon fibers, carbon nanotubes, on one face or two opposite faces of a carbon fiber matrix followed by densification of said carbon elements; and / or 2) by the formation of vitreous carbon on one face or two opposite faces of a carbon fiber matrix, when the carbon element is vitreous carbon. Thus, the method of the invention has the following advantages: - it provides a simplification in the design of porous zones, in that, unlike the methods of the prior art, such porous zones are no longer designed by superposition of materials of different natures; - allows control of the porosity of the various constituent parts of the porous area; - it allows, thanks to the materials used all based on carbon, to obtain an area having excellent chemical, electrochemical and thermal stability; - it implements stages which can be carried out in a continuous production line.
Enfin, selon un troisième objet, la présente invention a trait à une plaque bipolaire ou un ensemble plaque bipolaire/électrode comprenant une structure poreuse conforme à l'invention.Finally, according to a third object, the present invention relates to a bipolar plate or a bipolar plate / electrode assembly comprising a porous structure according to the invention.
D'autres avantages et caractéristiques de l'invention apparaîtront dans la description non limitative détaillée ci-dessous.Other advantages and characteristics of the invention will appear in the non-limiting description detailed below.
BRÈVE DESCRIPTION DES DESSINS Les figures 1 à 6 sont des vues en coupe de différentes structures poreuses conformes à 1' invention. EXPOSE DETAILLE DE MODES DE REALISATION PARTICULIERSBRIEF DESCRIPTION OF THE DRAWINGS Figures 1 to 6 are sectional views of various porous structures according to the invention. DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS
Comme mentionné précédemment, l'invention a trait à des structures poreuses pouvant être utilisées en tant que plaque bipolaire et/ou en tant qu'ensemble plaque bipolaire/électrode. Ainsi, les structures poreuses sont constituées d'une matrice poreuse en fibres de carbone, ladite matrice poreuse étant délimitée au niveau d'une au moins de ses faces par une couche etanche en un élément choisi parmi les fibres de carbone, les nanotubes de carbone, le carbone vitreux, ladite couche etanche étant liée à la matrice poreuse par des liaisons carbone-carbone. On précise que, selon l'invention, la structure poreuse présente, de manière générale, globalement une porosité ouverte. On précise que, par matrice poreuse en fibres de carbone, on entend, dans ce qui précède et ce qui suit, une pièce souple constituée d'un enchevêtrement de fils en fibres de carbone, le degré d'enchevêtrement étant fonction de la porosité souhaitée. La matrice poreuse est délimitée sur au moins l'une des ses faces par une couche etanche, c'est-à-dire une couche imperméable aux gaz et aux liquides. Cette couche etanche présente la particularité d'être réalisée en un élément de carbone choisi parmi les fibres de carbone, les nanotubes de carbone, le carbone vitreux, et de ne pas être associée à la matrice poreuse par une liaison mécanique mais par des liaisons carbone-carbone. De ce fait, la structure poreuse constitue une pièce ne présentant pas, telle que cela est le cas des structures poreuses de l'art antérieur, une résistance de contact, responsable notamment, lorsque les structures poreuses sont utilisées en tant que plaque bipolaire, d'une chute de potentiel. Selon l'utilisation envisagée des structures poreuses de l'invention, celles-ci peuvent se présenter sous différentes configurations. Ainsi, selon un premier mode de réalisation, représentée notamment par la figure 1, la structure poreuse 1 comporte une matrice poreuse 3 en fibres de carbone délimitée au niveau d'une première face 5 par une couche etanche 7, présentant les caractéristiques précédemment mentionnées et au niveau d' une deuxième face 9 opposée à la première face 5 par une couche poreuse 11 en un élément de carbone choisi parmi les fibres de carbone, les nanotubes de carbone, ladite couche poreuse 11 étant liée par des liaisons carbone-carbone à la matrice poreuse 3. Il est entendu que la couche poreuse présentera une porosité prédéterminée en fonction de l'usage dédié à cette couche . Selon un second mode de réalisation, représentée par la figure 2, la structure poreuse 13 comporte une matrice poreuse 15 délimitée au niveau d'une première face 17 par une couche etanche 19 et au niveau d'une seconde face 21 opposée à la première face par une autre couche etanche 23, lesdites couches étanches 19, 23 étant telles que définies précédemment. Les structures poreuses de l'invention peuvent comprendre également une couche poreuse en un élément de carbone choisi parmi les fibres de carbone, les nanotubes de carbone, sur le ou les couches étanches susmentionnées et/ou sur une face de la matrice poreuse, telles que cela est notamment représenté sur les figures 3 et 4. Ainsi, la figure 3 représente une structure poreuse 25 comprenant une matrice poreuse 27, délimitée sur une face 30 par une couche etanche 29 et sur la face opposée 32 par une couche poreuse 31 telle que représentée sur la figure 1, et en plus une autre couche poreuse 33 sur ladite couche etanche 29. La figure 4, quant à elle, représente une structure poreuse 35 comprenant une matrice poreuse 37 délimitée sur deux faces opposées 40, 42 par deux couches étanches 39, 41 de part et d'autre de ladite matrice poreuse 37, sur lesquelles sont fixées par liaison carbone-carbone deux couches poreuses 43, 45. On note que, selon l'un quelconque des modes de réalisation, les structures poreuses peuvent comprendre une couche active (référencée respectivement 12 sur les figures 1, 5 et 6) déposée sur les couches poreuses susmentionnées.As mentioned above, the invention relates to porous structures which can be used as a bipolar plate and / or as a bipolar plate / electrode assembly. Thus, the porous structures consist of a porous carbon fiber matrix, said porous matrix being delimited at at least one of its faces by a waterproof layer in an element chosen from carbon fibers, carbon nanotubes , vitreous carbon, said waterproof layer being linked to the porous matrix by carbon-carbon bonds. It should be noted that, according to the invention, the porous structure generally has an open porosity overall. It is specified that, by porous matrix of carbon fibers, one understands, in what precedes and what follows, a flexible part made up of an entanglement of son in carbon fibers, the degree of entanglement being function of the desired porosity . The porous matrix is delimited on at least one of its faces by a waterproof layer, that is to say a layer impermeable to gases and liquids. This waterproof layer has the particularity of being made of a carbon element chosen from carbon fibers, carbon nanotubes, vitreous carbon, and of not being associated with the porous matrix by a mechanical bond but by carbon bonds. -carbon. Therefore, the porous structure constitutes a part which does not have, as is the case with the porous structures of the prior art, a contact resistance, responsible in particular, when the porous structures are used as a bipolar plate, d 'a drop in potential. Depending on the intended use of the porous structures of the invention, these can be presented in different configurations. Thus, according to a first embodiment, shown in particular in FIG. 1, the porous structure 1 comprises a porous matrix 3 of carbon fibers delimited at the level of a first face 5 by a waterproof layer 7, having the characteristics mentioned above and at a second face 9 opposite the first face 5 by a porous layer 11 of a carbon element chosen from carbon fibers, carbon nanotubes, said porous layer 11 being linked by carbon-carbon bonds to the porous matrix 3. It is understood that the porous layer will have a predetermined porosity depending on the use dedicated to this layer. According to a second embodiment, represented by FIG. 2, the porous structure 13 comprises a porous matrix 15 delimited at the level of a first face 17 by a waterproof layer 19 and at the level of a second face 21 opposite the first face by another waterproof layer 23, said waterproof layers 19, 23 being as defined above. The porous structures of the invention may also comprise a porous layer of a carbon element chosen from carbon fibers, carbon nanotubes, on the above-mentioned waterproof layer or layers and / or on one face of the porous matrix, such as this is in particular shown in FIGS. 3 and 4. Thus, FIG. 3 represents a porous structure 25 comprising a porous matrix 27, delimited on one face 30 by a waterproof layer 29 and on the opposite face 32 by a porous layer 31 such that shown in FIG. 1, and in addition another porous layer 33 on said waterproof layer 29. FIG. 4, for its part, represents a porous structure 35 comprising a porous matrix 37 delimited on two opposite faces 40, 42 by two waterproof layers 39, 41 on either side of said porous matrix 37, on which two porous layers 43, 45 are fixed by carbon-carbon bond. It is noted that, according to any one of the modes d e embodiment, the porous structures may include an active layer (referenced respectively 12 in Figures 1, 5 and 6) deposited on the aforementioned porous layers.
Il est entendu que les différentes structures simples décrites ci-dessus peuvent être associées pour fournir des structures plus complexes. Ainsi, la figure 5 correspond à une structure poreuse complexe résultant de l'association par leurs couches étanches 7 de deux structures poreuses 13 telles que représentés sur la figure 1.It is understood that the various simple structures described above can be combined to provide more complex structures. Thus, Figure 5 corresponds to a complex porous structure resulting from the association by their waterproof layers 7 of two porous structures 13 as shown in FIG. 1.
La figure 6 correspond à une structure poreuse complexe résultant de l'association par leurs couches étanches (7, 19, 23) d'une structure poreuse 13 conforme à la figure 2 avec deux structures poreuses 1 conformes à la figure 1. Les structures poreuses de l'invention peuvent être utilisées en tant que plaque bipolaire et/ou en tant qu'ensemble plaque bipolaire/électrode. Les structures poreuses de l'invention peuvent être également utilisées dans des échangeurs thermiques. On rappellera qu'une plaque bipolaire est une pièce assurant la séparation physique entre deux électrodes de polarité opposée de deux cellules de pile adjacente, tout en assurant la continuité électrique. Une plaque bipolaire peut assurer, en plus de son rôle de séparation, un rôle dans la distribution des réactifs adéquats (à savoir combustible ou comburant) aux électrodes susmentionnées. Un ensemble plaque bipolaire/électrode est un ensemble résultant de l'association d'une plaque bipolaire telle que définie précédemment avec au moins une partie d'une électrode, c'est-à-dire la zone de diffusion des réactifs (pouvant correspondre aux couches poreuses précédemment mentionnées) et éventuellement la zone active (pouvant correspondre à la couche active susmentionnée) . On précise que, par couche active, on entend selon l'invention une couche comprenant au moins un catalyseur apte à catalyser la réaction électrochimique adéquate à l'électrode concernée. Ainsi, en particulier les structures poreuses représentées sur les figures 1, 4, 5 et 6 peuvent être utilisées en tant que plaques bipolaire et/ou ensemble électrode/plaque bipolaire. Ainsi, la structure représentée sur la figure 1 peut correspondre à un ensemble électrode/plaque bipolaire situé en fin d'empilement, lorsque ledit ensemble est destiné à être incorporé dans une pile constituée d'un empilement de cellules élémentaires. Dans ce cas, la couche etanche 7 et la matrice poreuse 3 correspondent à une demi-plaque, dans la mesure où elle ne repose que sur une électrode et la couche poreuse 11 correspond à la zone de diffusion des réactifs de l'électrode et la couche catalytique 12 correspond à la zone active de l'électrode. La structure représentée sur la figure 4 peut correspondre à une plaque bipolaire comprenant un circuit, de refroidissement, dans laquelle : - la matrice poreuse 37 correspond à la zone de circulation du liquide de refroidissement ; - les couches poreuses 43, 45 correspondent aux zones de distribution des réactifs ; - les couches étanches 39, 41 assurent une séparation entre la zone de circulation du liquide de refroidissement et les zones de distribution des réactifs . La structure poreuse représentée sur la figure 5 peut correspondre à un ensemble électrode/plaque bipolaire ne comprenant pas de circuit de refroidissement, dans lequel : - les matrices poreuses 3 correspondent à la zone de distribution des réactifs ; - les couches poreuses 11 correspondent à la zone de diffusion des électrodes appartenant à deux cellules adjacentes ; - les couches actives 12 correspondent à la zone active des électrodes appartenant à deux cellules adjacentes ; - les couches étanches 7 assurant la séparation entre les deux zones de distribution des réactifs . La structure représentée sur la figure 6 peut correspondre à un ensemble électrode/plaque bipolaire comprenant un circuit de refroidissement, dans lequel : - la matrice poreuse 15 correspond à la zone de circulation du liquide de refroidissement et les matrices poreuses 3 correspondent aux deux zones de distribution des réactifs ; - les couches poreuses 11 correspondent aux zones de diffusion des électrodes appartenant à deux cellules adjacentes ; - les couches actives 12 correspondent à la zone active des électrodes appartenant à deux cellules adjacentes ; - les couches étanches 7, 19, 23 assurant la séparation entre la zone de circulation du liquide de refroidissement et les deux zones de distribution des réactifs.FIG. 6 corresponds to a complex porous structure resulting from the association by their impermeable layers (7, 19, 23) of a porous structure 13 in accordance with FIG. 2 with two porous structures 1 in accordance with FIG. 1. The porous structures of the invention can be used as a bipolar plate and / or as a bipolar plate / electrode assembly. The porous structures of the invention can also be used in heat exchangers. It will be recalled that a bipolar plate is a part ensuring the physical separation between two electrodes of opposite polarity from two adjacent battery cells, while ensuring electrical continuity. A bipolar plate can ensure, in addition to its role of separation, a role in the distribution of suitable reagents (namely fuel or oxidizer) to the above-mentioned electrodes. A bipolar plate / electrode assembly is an assembly resulting from the association of a bipolar plate as defined above with at least one part of an electrode, that is to say the reagent diffusion zone (which may correspond to the porous layers previously mentioned) and possibly the active area (which may correspond to the above-mentioned active layer). It is specified that, by active layer, is meant according to the invention a layer comprising at least one catalyst capable of catalyzing the electrochemical reaction suitable for the electrode concerned. Thus, in particular the porous structures shown in Figures 1, 4, 5 and 6 can be used as bipolar plates and / or electrode / bipolar plate assembly. Thus, the structure shown in FIG. 1 can correspond to an electrode / bipolar plate assembly located at the end of the stack, when said assembly is intended to be incorporated in a stack consisting of a stack of elementary cells. In this case, the waterproof layer 7 and the porous matrix 3 correspond to a half-plate, insofar as it rests only on one electrode and the porous layer 11 corresponds to the zone of diffusion of the reagents of the electrode and the catalytic layer 12 corresponds to the active area of the electrode. The structure shown in FIG. 4 can correspond to a bipolar plate comprising a cooling circuit, in which: the porous matrix 37 corresponds to the zone of circulation of the cooling liquid; - The porous layers 43, 45 correspond to the reagent distribution zones; - The sealed layers 39, 41 provide a separation between the coolant circulation zone and the reagent distribution zones. The porous structure represented in FIG. 5 can correspond to an assembly electrode / bipolar plate not comprising a cooling circuit, in which: - the porous matrices 3 correspond to the zone of distribution of the reagents; - The porous layers 11 correspond to the diffusion zone of the electrodes belonging to two adjacent cells; the active layers 12 correspond to the active area of the electrodes belonging to two adjacent cells; - the waterproof layers 7 ensuring the separation between the two reagent distribution zones. The structure shown in FIG. 6 may correspond to an electrode / bipolar plate assembly comprising a cooling circuit, in which: the porous matrix 15 corresponds to the zone of circulation of the coolant and the porous matrices 3 correspond to the two zones of distribution of reagents; - The porous layers 11 correspond to the diffusion zones of the electrodes belonging to two adjacent cells; the active layers 12 correspond to the active area of the electrodes belonging to two adjacent cells; - the waterproof layers 7, 19, 23 ensuring the separation between the zone of circulation of the liquid and the two reagent distribution zones.
La structure représentée par la figure 2 peut correspondre à une plaque bipolaire dans laquelle : - la matrice poreuse 15 correspond à une zone de circulation d'un fluide de refroidissement ; - les couches étanches 19 et 23 peuvent assurer la séparation entre deux électrodes de deux cellules de pile adjacentes.The structure represented by FIG. 2 can correspond to a bipolar plate in which: - the porous matrix 15 corresponds to a zone of circulation of a cooling fluid; - the waterproof layers 19 and 23 can ensure the separation between two electrodes of two adjacent cell cells.
Enfin, la structure représentée par la figure 3 peut correspondre à un ensemble électrode/plaque bipolaire dépourvu de circuit de refroidissement dans lequel : - la matrice poreuse 27 et la couche poreuse 31 correspondent à une zone de distribution de réactifs ; - la couche poreuse 33 correspond à une zone de distribution de réactifs distincte de la zone de distribution précédemment mentionnée ; - la couche etanche 29 assure la séparation entre les deux zones de distribution susmentionnées .Finally, the structure shown in FIG. 3 can correspond to an electrode / bipolar plate assembly without a cooling circuit in which: - the porous matrix 27 and the porous layer 31 correspond to a reagent distribution zone; - The porous layer 33 corresponds to a reagent distribution zone distinct from the previously mentioned distribution zone; - The waterproof layer 29 ensures the separation between the two aforementioned distribution zones.
Pour les différents cas de figure explicités ci-dessus, il est entendu que la porosité, au sein d'une même couche poreuse peut être variable, selon l'usage de cette couche poreuse. La porosité entre deux couches poreuses distinctes peut être également différente selon que ces couches poreuses sont dédiées à la distribution de gaz (tel que 02) et à la distribution de liquide (tel que le méthanol) .For the different cases explained above, it is understood that the porosity, within the same porous layer can be variable, depending on the use of this porous layer. The porosity between two distinct porous layers can also be different depending on whether these porous layers are dedicated to the distribution of gas (such as 0 2 ) and the distribution of liquid (such as methanol).
Comme mentionné précédemment, l'invention se rapporte à un procédé de fabrication d'une telle structure poreuse telle que définie précédemment, ledit procédé comprenant une étape de réalisation de ladite ou desdites couche (s) etanche (s) par croissance d' éléments de carbone sur une face ou deux f ces opposées d'une matrice en fibres de carbone suivie d'une densification desdits éléments de carbone (lorsque ces éléments de carbone sont des fibres de carbone ou des nanotubes de carbone) , ou par la formation de carbone vitreux.As mentioned above, the invention relates to a method for manufacturing such a porous structure as defined above, said method comprising a step of producing said waterproof layer (s) by growing elements of carbon on one or two opposite sides of a carbon fiber matrix followed by densification of said carbon elements (when these carbon elements are carbon fibers or carbon nanotubes), or by the formation of carbon glassy.
Selon l'invention, on précise que l'on entend par matrice en fibres de carbone une pièce résultant de l'enchevêtrement de fibres de carbone, l'enchevêtrement étant plus ou moins dense selon la porosité souhaitée. Les matrices en fibres de carbone peuvent être disponibles dans le commerce ou peuvent être préparées préalablement par exemple par aiguilletage de fibres de carbone. On précise que la technique d' aiguilletage consiste à enchevêtrer mécaniquement dans les trois directions de l'espace des fibres d'un voile, à l'aide d'une aiguilleteuse, l'enchevêtrement pouvant être réglé en fonction de la porosité recherchée. L'étape de réalisation de la ou les couches étanches consiste à réaliser ces couches étanches, de manière à ce qu'elles soient ancrées, en tout ou partie dans la matrice en fibres de carbone, plus précisément dans les pores constitutifs de cette matrice en fibres de carbone par le biais de liaison carbone-carbone . L'on obtient ainsi une zone poreuse (constituée par la structure de la matrice en fibres de carbone) délimitée sur au moins une de ses faces par une couche etanche, qui interpénètre les pores de ladite matrice, la' pièce résultante étant ainsi une pièce « monobloc », c'est-à- dire une pièce ne résultant pas de l'adjonction de plusieurs pièces réunies, par exemple, par soudage et ne présentant pas les inconvénients inhérents à ce type de pièces, tel que cela a été mentionné plus haut. Ainsi, une telle couche etanche peut être obtenue par croissance d'éléments de carbone sur au moins une des faces d'une matrice en fibres de carbone suivie d'une densification desdits éléments de carbone, lorsque les éléments de carbone sont des fibres de carbone, des nanotubes de carbone. Une telle couche etanche peut être obtenue également par formation de carbone vitreux sur au moins une des faces d'une matrice en fibres de carbone. Il est envisageable également d'associer à la fois la croissance d'éléments de carbone et la formation de carbone vitreux, lorsque la couche etanche comprend à la fois des éléments de carbone, tels que des fibres de carbone ou des nanotubes de carbone et à la fois du carbone vitreux. Lorsque les éléments de carbone sont des fibres de carbone, l'étape de croissance desdites fibres de carbone peut consister à pyroliser des fibres précurseurs des fibres de carbones, lesdites fibres pouvant être des fibres polymères telles que des fibres de polyacrylonitrile (PAN) , des fibres obtenus à partir de brai, l'étape de pyrolyse étant précédée des étapes suivantes : - une étape d'imprégnation de la face adéquate de la matrice en fibres de carbone par des monomères adéquats ou du brai de pétrole ; - dans le cas où les fibres précurseurs sont des fibres polymères, une étape de polymérisation desdits monomères suivi d'un filage, pour obtenir les fibres polymères adéquats ; - dans le cas où les fibres précurseurs sont des fibres de brai, une étape de filage de manière à obtenir des fibres de brai . Il est entendu que le filage se fera de manière à obtenir une réseau de fibres suffisamment enchevêtrées afin, qu'à l'issue de la pyrolyse, la couche résultante soit une couche etanche. L'étape de croissance des nanotubes de carbone peut s'effectuer sur la matrice en fibres de carbone selon un procédé tel que celui décrit dans FR 2 844 510. Ce procédé comprend notamment les étapes suivantes : - une étape d' imprégnation de la face adéquate de la matrice par une solution aqueuse comprenant un ou plusieurs sels de catalyseur métallique de croissance de nanotubes de carbone, tels que des sels de Co, Ni ou Fe sous forme de nitrates, d'acétates ; - une étape de décomposition en oxyde (s) du ou desdits sels par traitement thermique, par exemple en portant la matrice imprégnée à une température comprise entre 100 °C et 250 °C ; - une étape de réduction du ou des oxyde (s) formés, par exemple, en introduisant la matrice dans un four sous atmosphère réductrice ; - une étape de synthèse des nanotubes de carbone en amenant la matrice en contact avec un précurseur gazeux de carbone dans un four chauffé à une température permettant la formation de carbone par décomposition (craquage) du précurseur gazeux. Le précurseur gazeux peut être un hydrocarbure aromatique ou non aromatique. On utilise, par exemple, de l'acétylène, de l'éthylène, du propylène ou du méthane. La température du four nécessaire au craquage peut s'échelonner de 450°C à 1200°C. La structure obtenue (que la couche etanche soit en fibres de carbone ou en nanotubes de carbone) est ensuite densifiée par voie liquide ou par infiltration chimique en phase vapeur, tel que cela est décrit dans le document FR 2 844 510. L' étape de formation du carbone vitreux peut être réalisée par imprégnation de la matrice en fibres de carbone sur la face adéquate par une résine furanique ou phénolique suivi d'une étape de pyrolyse. Lorsque la structure poreuse de l'invention comporte une ou plusieurs couches poreuses délimitant la matrice en tissu ou déposée (s) sur les couches étanches, la ou lesdites couches poreuses peuvent être obtenir par croissance d'éléments de carbone, telles que les fibres de carbone et les nanotubes de carbone, la croissance étant réglée de manière à obtenir à l'issue de cette croissance une couche présentant la porosité souhaitée. Lorsque la structure poreuse comporte également une couche active à base de catalyseur, cette dernière peut être obtenue par des techniques classiquement employées dans la fabrication des couches actives, telles que l'enduction ou la pulvérisation par des suspensions comprenant le catalyseur adéquat . De telles suspensions peuvent être une suspension de carbone platiné. Ainsi, grâce à leurs caractéristiques susmentionnées, les structures poreuses de l'invention grâce à la présence de différentes zones à porosité déterminée, trouvent leurs applications dans le domaine des piles à combustible de type PEMFC ou DMFC fonctionnant à basse température et des piles fonctionnant à température intermédiaire (tels que les piles à acide phosphorique fonctionnant à 250 °C) en tant que plaques bipolaires mais également dans le domaine des échangeurs thermiques . According to the invention, it is specified that the term “carbon fiber matrix” is understood to mean a part resulting from the entanglement of carbon fibers, the entanglement being more or less dense depending on the desired porosity. The carbon fiber matrices may be commercially available or may be prepared beforehand, for example by needling carbon fibers. It is specified that the needling technique consists of mechanically entangling in the three directions of the space of the fibers of a web, using a needling machine, the entanglement being able to be adjusted according to the desired porosity. The step of producing the waterproof layer (s) consists in producing these waterproof layers, so that they are anchored, in whole or in part in the carbon fiber matrix, more precisely in the pores constituting this matrix in carbon fibers through carbon-carbon bonding. The porous area is thus obtained (formed by the structure of the carbon fiber matrix) delimited on at least one side thereof by an impervious layer, which interpenetrates the pores of said matrix, the resultant piece thus being a part "Monoblock", that is to say a part not resulting from the addition of several parts joined together, for example, by welding and not having the drawbacks inherent in this type of parts, as has been mentioned more high. Thus, such a waterproof layer can be obtained by growth of carbon elements on at least one of the faces of a carbon fiber matrix followed by densification of said carbon elements, when the carbon elements are carbon fibers. , carbon nanotubes. Such a waterproof layer can also be obtained by the formation of vitreous carbon on at least one of the faces of a carbon fiber matrix. It is also conceivable to combine both the growth of carbon elements and the formation of vitreous carbon, when the tight layer comprises both carbon elements, such as carbon fibers or carbon nanotubes and with both glassy carbon. When the carbon elements are carbon fibers, the step of growing said carbon elements carbon fibers may consist of pyrolizing precursor fibers of carbon fibers, said fibers possibly being polymer fibers such as polyacrylonitrile fibers (PAN), fibers obtained from pitch, the pyrolysis step being preceded by the following steps : - a step of impregnating the adequate face of the carbon fiber matrix with suitable monomers or petroleum pitch; - in the case where the precursor fibers are polymer fibers, a step of polymerization of said monomers followed by spinning, to obtain the appropriate polymer fibers; - In the case where the precursor fibers are pitch fibers, a spinning step so as to obtain pitch fibers. It is understood that the spinning will be done so as to obtain a network of fibers sufficiently entangled so that, at the end of the pyrolysis, the resulting layer is a waterproof layer. The carbon nanotubes growth step can be carried out on the carbon fiber matrix according to a process such as that described in FR 2 844 510. This process notably comprises the following steps: - a step of impregnating the face adequate matrix with an aqueous solution comprising one or more metal catalyst salts for growth of carbon nanotubes, such as Co, Ni or Fe salts in the form of nitrates, acetates; - A step of decomposition into oxide (s) of said salt or salts by heat treatment, for example by bringing the impregnated matrix to a temperature between 100 ° C and 250 ° C; - A step of reduction of the oxide (s) formed, for example, by introducing the matrix into an oven under a reducing atmosphere; a step for synthesizing carbon nanotubes by bringing the matrix into contact with a gaseous carbon precursor in an oven heated to a temperature allowing the formation of carbon by decomposition (cracking) of the gaseous precursor. The gaseous precursor can be an aromatic or non-aromatic hydrocarbon. For example, acetylene, ethylene, propylene or methane are used. The oven temperature required for cracking can range from 450 ° C to 1200 ° C. The structure obtained (whether the waterproof layer is made of carbon fibers or carbon nanotubes) is then densified by liquid means or by chemical vapor infiltration, as described in document FR 2 844 510. The step of Vitreous carbon can be formed by impregnating the carbon fiber matrix on the appropriate face with a furan or phenolic resin followed by a pyrolysis step. When the porous structure of the invention comprises one or more porous layers delimiting the fabric matrix or deposited on the waterproof layers, the porous layer (s) can be obtaining by growth of carbon elements, such as carbon fibers and carbon nanotubes, the growth being regulated so as to obtain at the end of this growth a layer having the desired porosity. When the porous structure also includes an active layer based on catalyst, the latter can be obtained by techniques conventionally employed in the manufacture of active layers, such as coating or spraying with suspensions comprising the appropriate catalyst. Such suspensions can be a platinum carbon suspension. Thus, thanks to their aforementioned characteristics, the porous structures of the invention thanks to the presence of different zones with determined porosity, find their applications in the field of fuel cells of the PEMFC or DMFC type operating at low temperature and cells operating at intermediate temperature (such as phosphoric acid batteries operating at 250 ° C) as bipolar plates but also in the field of heat exchangers.

Claims

REVENDICATIONS 1. Structure poreuse comprenant une matrice poreuse (3, 15, 27, 37) en fibres de carbone, ladite matrice poreuse étant délimitée au niveau d'une au moins de ses faces (5, 17, 21, 30, 40, 42) par une couche etanche (7, 19, 23, 29, 39, 41) en un élément de carbone choisi parmi les fibres de carbone, les nanotubes de carbone, le carbone vitreux • ou des combinaisons de ceux-ci, ladite couche etanche étant liée à la matrice poreuse par des liaisons carbone- carbone . CLAIMS 1. Porous structure comprising a porous matrix (3, 15, 27, 37) made of carbon fibers, said porous matrix being delimited at at least one of its faces (5, 17, 21, 30, 40, 42 ) by a waterproof layer (7, 19, 23, 29, 39, 41) in a carbon element chosen from carbon fibers, carbon nanotubes, vitreous carbon • or combinations thereof, said waterproof layer being linked to the porous matrix by carbon-carbon bonds.
2. Structure poreuse selon la revendication 1, dans laquelle ladite matrice poreuse (3) est délimitée au niveau d'une première face (5) par une couche etanche (7), telle que définie dans la revendication 1 et au niveau d'une deuxième face (9) opposée à la première face (5) par une couche poreuse (11) en un élément de carbone choisi parmi les fibres de carbone, les nanotubes de carbone, ladite couche poreuse étant liée par des liaisons carbone-carbone à la matrice poreuse.2. Porous structure according to claim 1, wherein said porous matrix (3) is delimited at a first face (5) by a waterproof layer (7), as defined in claim 1 and at a second face (9) opposite the first face (5) by a porous layer (11) made of a carbon element chosen from carbon fibers, carbon nanotubes, said porous layer being linked by carbon-carbon bonds to the porous matrix.
3. Structure poreuse selon la revendication 1, dans laquelle ladite matrice poreuse (15) est délimitée au niveau d'une première face (17) par une couche etanche (19) et au niveau d'une seconde face (21) opposée à la première face (17) par une autre couche etanche (23) , lesdites couches étanches étant telles que définies dans la revendication 1. 3. Porous structure according to claim 1, wherein said porous matrix (15) is delimited at a first face (17) by a waterproof layer (19) and at a second face (21) opposite to the first face (17) by another waterproof layer (23), said waterproof layers being as defined in claim 1.
4. Structure poreuse selon l'une quelconque des revendications 1 à 3, comprenant, en outre, une couche poreuse (31, 33, 43, 45) en un élément de carbone choisi parmi les fibres de carbone, les nanotubes de carbone, sur le ou lesdites couches étanches (29, 39, 41) et/ou sur une face (32) de la matrice poreuse (27) .4. Porous structure according to any one of claims 1 to 3, further comprising a porous layer (31, 33, 43, 45) in a carbon element chosen from carbon fibers, carbon nanotubes, on the said sealed layer (s) (29, 39, 41) and / or on one face (32) of the porous matrix (27).
5. Structure poreuse selon la revendication 2, caractérisée en ce qu'elle comprend, en outre, une couche active (12) sur ladite ou lesdites couches poreuses (11) .5. Porous structure according to claim 2, characterized in that it further comprises an active layer (12) on said at least one porous layer (11).
6. Plaque bipolaire ou ensemble plaque bipolaire-électrode comprenant une structure poreuse telle que définie selon l'une quelconque des revendications 1 à 5.6. Bipolar plate or bipolar plate-electrode assembly comprising a porous structure as defined according to any one of claims 1 to 5.
7. Procédé de fabrication d'une structure poreuse telle que définie selon l'une quelconque des revendications 1 à 6, caractérisé en ce que ledit procédé comprend une étape de réalisation de ladite ou desdites couche (s) etanche (s) : 1) par croissance d'éléments de carbone choisis parmi les fibres de carbone, les nanotubes de carbone, sur une face ou deux faces opposées d'une matrice en fibres de carbone suivie d'une densification desdits éléments de carbone ; et/ou 2) par formation de carbone vitreux sur une face ou deux faces opposées d'une matrice en fibres de carbone, lorsque l'élément de carbone est du carbone vitreux.7. A method of manufacturing a porous structure as defined according to any one of claims 1 to 6, characterized in that said method comprises a step of producing said waterproof layer (s): 1) by growth of carbon elements chosen from carbon fibers, carbon nanotubes, on one face or two opposite faces of a carbon fiber matrix followed by densification of said carbon elements; and / or 2) by the formation of vitreous carbon on one face or two opposite faces of a fiber matrix of carbon, when the carbon element is glassy carbon.
8. Procédé de fabrication selon la revendication 7, comprenant une étape de préparation de ladite matrice en fibres de carbone par aiguilletage de fibres de carbone. 8. The manufacturing method according to claim 7, comprising a step of preparing said carbon fiber matrix by needling carbon fibers.
EP04767923A 2003-07-29 2004-07-28 Porous structures useful as bipolar plates and methods for preparing same Withdrawn EP1680378A2 (en)

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FR0350379A FR2858465A1 (en) 2003-07-29 2003-07-29 POROUS STRUCTURES USED AS BIPOLAR PLATES AND METHODS OF PREPARING SUCH POROUS STRUCTURES
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