FR2602915A1 - COMPOSITE SUBSTRATE FOR FUEL CELLS AND MANUFACTURING METHOD THEREOF - Google Patents

Abstract

THE COMPOSITE SUBSTRATE COMPRISES A SEPARATOR 1, TWO ELECTRODE SUBSTRATES 2, 2 POROUS AND CARBONES LINKED TO THE SEPARATOR AND PROVIDED WITH GORGES FORMING FLOW CHANNELS 5, 6 FOR REACTIVE GAS ON ONE OF THEIR FACES, AND ONE SURFACE PLANE ON THEIR OTHER FACES, AND SEALING ELEMENTS 3, 3 PERIPHERALS LINKED TO THE SEPARATOR BY A RESIN LAYER 4, 4 FLUOROCARBONED, THESE ELECTRODE SUBSTRATES BEING BOUND TO THE OPPOSITE SURFACES OF THE SEPARATOR, SO AS THE CHANNELS OF FLOW IN ONE OF THE ELECTRODE SUBSTRATES ARE PERPENDICULAR TO THOSE WHICH ARE IN THE OTHER ELECTRODE SUBSTRATE, THE REPORT OF THE TOTAL CROSS SECTION OF THE FUEL ELECTRODE FLOW CHANNELS, A THE TOTAL CROSS SECTION OF THE FLOW CHANNELS ON THE AIR ELECTRODE SIDE, BEING 1: 3 TO 2: 3.

Description

Composite substrate for Fuel Diles and its

manufacturing process.

  The present invention relates to a composite substrate for acid type fuel cells.

  phosphoric acid and its manufacturing process.

  In recent years there has been a great deal of interest in the development and use of a fuel cell and the systems associated therewith as apparatus for producing clean or clean energy. an electricity generating apparatus which is easy to start and to interrupt, which can contribute to the saving of resources by a slowing down of the demand for electricity of thermal or hydroelectric origin, or

  improvement or energy efficiency.

  Hitherto, as a fuel cell, the bipolar type fuel cell has been used in which the bipolar separator obtained by the formation of ribs on a thin gas-impermeable plate made of graphite, after combination with a plate porous carbon plate, which has been made known to the public. However, in addition to this fuel cell, a monopolar type fuel cell, formed by stacking a porous electrode substrate having ribs on one of its faces and a planar surface on its other face, a layer of catalyst, a matrix impregnated with an electrolyte and a separator, has been developed. In this type of monopolar fuel cell, the reactive gas (oxygen or hydrogen) diffuses from the gas flow channel formed by the ribs arranged on the electrode substrate, towards the surface

plane of the electrode.

  In this fuel cell, the two types of reactive gas flow channels formed on the opposite surfaces of the separator, while sandwiching the separator, i.e. the reactive gas flow channels of the side of the electrode

  of fuel, and the reactant gas flow channels on the air electrode side generally had the same total cross-sectional area, with respect to the cross-section perpendicular to the flow direction of the reactive gas.

  As the reaction for the electrodes of the phosphoric acid type fuel cell is given by H2 + 1/2 02 H20, the ideal stoichiometric ratio by volume of hydrogen to oxygen (i.e. fuel ) is 2: 1, and when the two gases mentioned above are used under the same pressure in order to obtain an identical diffusion of the gases, in the fuel cell mentioned above in which the area of the cross section of the oxygen flow channel is identical to that of hydrogen, it is reasonable in theory to use pure gaseous oxygen by volume

  corresponding to 50% of the volume of pure hydrogen gas.

  However, taking into account that in the case of a real operation of a fuel cell, the oxygen supply takes place via air, and that the hydrogen supply takes place by means of a gas which has been obtained by reforming LNG (liquefied natural gas), LPG (liquefied petroleum gas), etc. and at an H2 content of 65 to 80% by volume and in addition, taking into account the rate of use of the combustible gas and of the air, the area of the cross section of the flow channels of the reactive gas on the side of the fuel electrode was too high in the aforementioned fuel cell gas, in which the total cross-sections of the reactive gas flow channels on the side of the fuel electrode, and the total cross-sections flow channels 5 of reactive gas on the side of the air electrode are

  identical for a composite substrate of the bipolar type.

  Although the excessive cross-section of the flow channels of the reactive gas on the side of the fuel electrode, can be compensated by the increase in the air flow correspondingly, taking into account the transfer of the gas to the electrode opposite, as has been mentioned, it is clearly desirable

  to use the two gases under identical pressures.

  As other problems posed by the aforementioned fuel cells, it may be mentioned that there is a risk of exfoliation of the elements and of leakage of the reactive gas by the joined parts, since the connection of each of the elements has been carried out. using a carbon cement which is easily attacked by phosphoric acid, and that there is a problem of mechanical resistance leading to the appearance of cracking during handling, in the case where the surface of the substrate is important because the electrode substrate is made in the form

of a thin plate.

  More recently, a bipolar device for fuel cells, which comprises an impermeable layer formed by impregnating a sealing material selected from the group consisting of a fluorinated copolymer of ethylene and propylene, poLysulfone, polyethersulfone, polyphenylsulfone, a perfluorinated tetrafluoroethylene alkoxy and mixtures thereof, at the interface between the two porous carbon plates

  has been proposed (refer to US Patent No. 4,505,992).

  However, in this type of device also, the problem of imbalance between the supply of reactive gas between the fuel electrode and the air electrode and the problem of leakage of reactive gas towards the side of the porous electrode and have never been resolved. As a result of studies by the present inventors to remedy the above-mentioned drawbacks of conventional electrode substrates, it has been discovered that (1) in a composite substrate 10 comprising (a) a separator, (b) two electrode substrates forming a fuel electrode (hydrogen) and an air electrode between which a separator is interposed and (c) peripheral sealing elements, the production yield by volume can be increased by selecting the ratio of the total cross section of the flow channels of the reactive gas of one of the two electrodes to that of the other electrode, within a fixed range, so as to supply the two gases under a practically identical pressure, and by decreasing the height of the ribs of the electrode substrate on the side of the fuel electrode and (2) that gas leaks to the side of the electrode could be avoided by connecting the peripheral sealing elements to the separator by through a layer of fluorocarbon resin, and based on these discoveries, the present

  inventors have resulted in the present invention.

  More specifically The object of the present invention is to provide a composite substrate for fuel cells, which comprises a separator, two porous and carbonaceous electrode substrates and peripheral sealing elements, which have a cross-sectional ratio of the reactant gas flow channel on the fuel electrode side, at the section on the air electrode side, satisfactory

  under the conditions of the fuel actually used.

  Furthermore, the second object of the present invention is to provide a composite substrate for fuel cells, in which the edge of the porous and carbonaceous electrode substrate 5 has been sealed by connecting the peripheral sealing elements to peripheral extensions of the separator, via a layer of fluorocarbon resin, which obviates the need for a peripheral sealing treatment to prevent leaks

  of reactive gas to the side of the stack.

  The third object of the present invention is to provide a composite substrate for phosphoric acid type fuel cells, which has

  excellent resistance to phosphoric acid.

  According to a first aspect of the present invention, there is provided a composite substrate for fuel cells, comprising: (1) a separator, (2) two porous and carbonaceous electrode substrates which have been respectively provided with a plurality of grooves forming flow channels for the reactant gas on one of their faces, and a flat surface on their other faces, and which have been connected to the opposite surfaces of the separator so that the flow channels of the reactive gas in one of the electrode substrates are perpendicular to those in the other electrode substrate, and the separator extends beyond the two edges of the electrode substrate which are parallel to the channels of flow of reactive gas through the electrode substrate and (3) peripheral sealing elements connected to the peripheral extensions of the separator, which extend beyond the two edges of the electrode substrate, via a fluorocarbo resin layer born, the ratio of the total cross-section of the reactive gas flow channels, formed by the separator and the grooves of the porous carbon electrode substrate, on the side of the fuel electrode, to the total cross-section of the channels of the reactive gas formed by the separator and the grooves of the porous carbon electrode substrate, on the side of

  the air electrode, being from 1: 3 to 2: 3.

  According to a second aspect of the present invention, there is provided a method for producing a composite substrate for fuel cells, which method comprises the steps of: (1) adhering a sheet of flexible Carbon to a surface of each of two porous and carbonaceous electrode substrates, in the form of a flat plate, without grooves and of predetermined dimensions, using an adhesive, (2) subjecting each of the adhesive surfaces 20 of said electrode substrates to a process cutting to make the grooves forming flow channels for the reactive gas thereon, so that the ratio of the total cross section of the flow channels of the reactive gas, on the side of the fuel electrode 25 , to the total cross section of the reactive gas flow channels on the side of the air electrode, ie from 1: 3 to 2: 3, (3) adhere the surfaces of the flexible carbon sheet remaining on the thus cut surfaces of the electric substrates ode, at opposite surfaces of the separator, so that the grooves formed in one of the electrode substrates are perpendicular to those which have been formed in the other electrode substrate, (4) calcining the materials thus fixed at a temperature of at least 800 C in an inert atmosphere and / or under reduced pressure, and (5) connecting the peripheral sealing elements which are constituted by a carbonaceous material 5 compact gas impermeable to extensions

  peripherals of the separator, which extend beyond the two bodies of the electrode substrate which are parallel to the flow channels of the reactive gas therein, using a sheet of fluorocarbon resin.

  According to a third aspect of the present invention, there is provided a method of producing a composite substrate for fuel cells, which comprises the steps of: (1) applying a dispersion of a tetrafluoroethylene resin to the opposite surfaces of 'a separator, (2) bond by adhesion in the molten state each of the surfaces provided with grooves of the two substrates 20 of porous and carbonaceous electrode, at the position determined by the opposite surfaces of the separator on which the dispersion is applied, in order that the grooves in one of the electrode substrates are perpendicular to those in the other electrode substrate, the electrode substrates being respectively provided with a plurality of grooves forming flow channels for the reactive gas on one of their faces, and of a flat surface on their other faces, the size of the grooves being such that the ratio 30 of the total cross section of the flow channels of the reacted gas if, on the fuel electrode side, to the total cross section of the reactive gas flow channels, on the air electrode side, from 1: 3 to 2: 3, and (3) connect Peripheral sealing elements constituted by a compact carbonaceous material impermeable to gases, at peripheral extensions of the separator which extend beyond the two edges of the electrode substrate, which are parallel to the gas flow channels reactive in it, by means

  a sheet of fluorocarbon resin.

  Among the accompanying drawings, Figures 1 to 3 are oblique representations of the substrate

  composite of this intention.

  The present invention relates to a bipolar type composite substrate for fuel cells, which substrate comprises a separator, two porous and carbonaceous electrode substrates forming fuel (hydrogen) electrodes and an air electrode, sandwiching the separator and peripheral sealing elements, and a method for producing

  the composite substrate for fuel cells.

  As previously described, the stoichiometric volume ratio of fuel (hydrogen) to oxygen in a fuel cell of the phosphoric acid type is 2: 1. In the case of the actual operation of a fuel cell, as l the oxygen is supplied by the air, the oxygen content of the gas thus supplied is approximately 20%, and the gas supplying the hydrogen is, as has already been described, a gas of the LNG, LPG, etc. type. . reformed containing C02, steam, etc. and has a hydrogen content

  from about 65 to about 80% by volume.

  Furthermore, with regard to the utilization rate, which is represented by the ratio of the quantity of reactive gas consumed to that of the reactive gas supplied, since the voltage at the terminals of the battery begins to decrease in the case o the utilization rate mentioned above exceeds a given value, this utilization rate is limited. In practice, it is necessary that the rate of use of hydrogen does not exceed 75% and that the rate

  oxygen use does not exceed 50%.

  In the case where the two reactive gases are used at the same pressure, the calculation shows that the ratio of the total cross section of the flow channels of the reactive gas, on the side of the fuel electrode, to the total cross section reactant gas flow channels, on the side of the air electrode, tend towards about 0.325: 1 to 0.41: 1, and taking into account that the hydrogen content

  may be slightly lower depending on the state of the gas from which the hydrogen originates, the ratio mentioned above, from about 1: 3 to about 2: 3, may correspond to the actual state of the feed gases.

  In addition, in the case where the electrode substrate is connected to the separator by making the two materials adhere to one another using a carbonizable adhesive and by calcination of the materials thus bonded in a body, or when the two materials are bonded to each other using a dispersion of tetrafluoroethylene resin, a product can be obtained with satisfactory electrical characteristics and resistance to phosphoric acid, and in the case where the peripheral sealing elements of the compact carbon material are connected to the separator by means of a sheet of fluorocarbon resin, it is possible to obtain a product in which the gas leaks are sufficiently limited, and in which resistance to phosphoric acid and the

  overall mechanical strength are satisfactory.

  The present invention provides a composite substrate for fuel cells, comprising (1) a separator, (2) two porous and carbonaceous electrode substrates which have been respectively provided with a plurality of grooves forming flow channels for the gas reagent on one of their faces, and which have been bonded to opposite surfaces of the separator so that the flow channels of the reagent gas in one of the electrode substrates are perpendicular to those which are located in the other electrode substrate, and that the separator extends beyond the two edges of the electrode substrate which are parallel to the flow channels of the reactive gas in the electrode substrate, 10 and (3) sealing elements connected to the peripheral extensions of the separator, which extend beyond the two edges of the electrode substrate, via a layer of fluorocarbon resin, the ratio of the total cross section of the channels 15 reactive gas flow f formed by the separator and the grooves of the porous carbonaceous electrode substrate, on the side of the fuel electrode, at the total cross section of the reactive gas flow channels formed by the separator and the grooves of the substrate porous carbonaceous electrode, on the side of the air electrode, being from 1: 3 to 2: 3. In addition, the cross section of the reactive gas flow channels mentioned in the present invention designates the cross section of the channel of the reactive gas 25 perpendicular to the direction of flow

reactive gas.

  In the composite fuel cell substrate of the present invention, the porous and carbon electrode substrates and the separator can be bonded to each other by calcination into a single body after bonding them to each other using of a carbonizable adhesive, or can be bonded to each other by a dispersion of tetrafluoroethylene resin. In the case of calcination bonding in a single body, it is desirable to interpose a flexible carbon sheet between the materials, as a buffer sheet absorbing thermal expansion and shrinkage of the materials

during calcination.

  Therefore, the present invention also provides a method of manufacturing the above-mentioned composite substrate for fuel cells, which method comprises the steps of (1) adhering a flexible carbon sheet to a surface 10 of each of two carrier substrates. porous and carbon electrode. in the form of a flat plate, without grooves and of predetermined dimensions, using an adhesive, (2) subjecting each of the adhesive surfaces of said electrode substrates to a cutting treatment 15 in order to produce grooves forming reactive gas flow channels thereon, so that the ratio of the total cross-section of the reactive gas flow channels, on the fuel electrode side, to the total cross-section of the gas channels flow 20 of the reactive gas on the side of the air electrode, ie from 1: 3 to 2: 3, (3) adhering the surfaces of the flexible carbon sheet remaining on the surfaces thus cut from the electrode substrates , to the opposite surfaces of the separator, so that the grooves 25 in one of the electrode substrates are perpendicular to those in the other electrode substrate, (4) calcining the materials thus bonded at a temperature of at least about 800 C in an inert atmosphere and / or under reduced pressure, and (5) connecting peripheral sealing elements comprising a compact carbonaceous material impermeable to gases, to peripheral extensions of the separator, which extend beyond the two edges of the electrode substrate which are parallel to the flow channels 35 of the reactive gas therein, by

  through a sheet of fluorocarbon resin.

  Furthermore, the present invention provides a method of manufacturing the aforementioned composite substrate for fuel cells, which method comprises the steps of (1) applying a dispersion of tetrafluorethyLene resin to the opposite surfaces of a separator, (2) melt bonding each of the surfaces provided with grooves of two porous carbonaceous electrode substrates, at the position determined by the opposite surfaces of the separator on which the dispersion has been applied, so that the grooves located in one of the electrode substrates, are perpendicular to those which are in the other electrode substrate 15, the electrode substrates being respectively provided with a plurality of grooves forming flow channels for the reactive gas, on one of their faces, and of a flat surface, on their other face, the size of the grooves being such that the ratio 20 of the total cross section of the flow channels nt of reactive gas, on the side of the fuel electrode, to the total cross section of the flow channels of the reactive gas, on the side of the air electrode, ie from I: 3 to 2: 3, and ( 3) connecting peripheral sealing elements 25 made of a compact carbon material impermeable to gases, to peripheral extensions of the separator which extend beyond the two edges of the electrode substrate which are parallel to the gas flow channels reagent which is contained therein, via a sheet of fluorocarbon resin. The composite substrate of the invention is described

  in more detail with reference to the accompanying drawings.

  Among the accompanying drawings, Figure I is an oblique representation of the composite substrate of the present invention, in which two porous carbonaceous electrode substrates and a separator were bonded to each other using a dispersion

  tetrafluoroethylene resin.

  The composite substrate of FIG. 1, in accordance with the present invention, has a structure consisting of a separator, of two electrode substrates, namely the electrode substrate for the fuel electrode 2 and the substrate electrode 10 for the air electrode 2 ′ which respectively have grooves forming flow channels 5 and 6 for the reactive gas, in association with the separator mentioned above, and which are located on opposite faces of the separator, as well as peripheral sealing elements 3 and 3 'which seal the two edges of the electrode substrates 2 and 2', which edges are parallel to the direction

  flow channels 5 and 6 of the reactive gas.

  The area of the separator 1 is greater than that of the electrode substrates 2 and 2 ′, and, as shown in FIG. 1, the separator 1 extends beyond the two edges of the electrode substrate, which are parallel to the flow channels 5 and 6 of the reactive gas in the electrode substrate, and the peripheral sealing elements 3 and 3 'are connected to the

  peripheral extensions of the separator 1 mentioned previously (the outer edge of the peripheral extensions of the separator mentioned above, coincides with the outer edge of the peripheral sealing elements).

  The peripheral extensions of the separator, which extend beyond the two edges of the electrode substrate and of the sealing elements 3 and 3 ′, have

  have been linked to each other using fluorocarbon resin 4 and 4 '.

  In the composite substrate shown in FIG. 1, the separator 1 and the electrode substrates 2 and 2 ′ were linked to each other by a dispersion of tetrafluoroethylene resin, and the 5 flow channels 5 and 6 of the reactive gas are determined by the grooves of the electrode substrates 2 and 2 ',

and the separator.

  Furthermore, FIG. 2 represents the composite substrate formed by bonding the separator 1 and the electrode substrates 2 and 2 'to each other, carbon sheets 7 and 7' being interposed between the separator and each of the substrates

electrode 2 and 2 '.

  In the electrode substrate of the present invention, which is shown in Figures 1 and 2, the ratio of the total cross-section of the flow channels 5 of the reactive gas, on the side of the fuel electrode, to the total cross section of the reactant gas flow channels 6 on the side of the air electrode is 1: 3 to 2: 3. The shape of the cross section of the reactant gas flow channels which gives the report of - cross sections mentioned above can be considered optional. However, from the point of view of the possibility of reducing the thickness of the electrode substrate itself, and improving the performance and the mechanical resistance of the cell, it is desirable that the flow channels of the reactive gas have a generally rectangular shape, and that the cross sections of the reactive gas flow channels on either side of the electrode are different, giving the reactive gas flow channels the same width on the side of the fuel electrode and on the side of the air electrode, and by giving them different heights on the side of the fuel electrode and on the side of the air electrode (i.e. the the groove of the electrode substrate is different on the side of the fuel electrode and on the side of the air electrode). The cross-sectional shape of the reactant gas flow channels, which is shown in FIGS. 1 and 2, is rectangular and the reactive gas flow channels extend parallel to the sealingly closed edges, and way

  straight from one end open to the other.

  However, in the case where the reactive gas flow channel can supply a sufficient amount of reactive gas which diffuses into the porous and carbon electrode substrates, the cross section of the reactive gas flow channels can have a any shape. For example, in the case where the cross section of the ribs which form the grooves of the electrode substrate, has the shape of a trapezoid or has a non-rectilinear shape, it is possible to seek a distribution of the stresses on the electrode substrates, this being particularly favorable during the manufacture of the composite substrate. Furthermore, it is also possible to connect the flow channels 25 of the reactive gas to each other in the electrode substrate, and the protruding parts having upper surfaces which transform into contact surfaces with the separator, and which have an optional shape such as for example a circular, ellipsoidal, rectangular shape, etc. may be arranged on the contact surface of the electrode substrate with the separator in successive entangled positions, or in any optional position. It is clear that all 35 combinations of the forms and arrangements mentioned

above are possible.

  The electrode substrate of the present invention is porous and carbonaceous, and it is desirable that the electrode substrate have the properties, an average bulk density of 0.3 to 0.9 g / ml, a

  gas permeability of at least 200 ml / cm2.h.mmAq, and an electrical resistivity not exceeding 200 mS.cm after calcination at a temperature of at least 800 C in an inert atmosphere and / or under reduced pressure.

  It is desirable that the separator used in accordance with the present invention has the properties of an average bulk density of at least 1.4 g / ml, a gas permeability of not more than 10-6 15 ml / cm2.h.mmAq, a electrical resistivity not exceeding

  pitch 10 mS.cm and a thickness not exceeding 2 mm.

  It is desirable that the peripheral sealing elements used in accordance with the present invention have as properties, an average bulk density of at least 1.4 g / ml and a permeability

  gas not exceeding 10-4ml / cm2.h.mmAq.

  As described above, in the composite substrate for fuel cells according to the present invention, although the two edges of the electrode substrate which are parallel to the reactive gas flow channels therein, are sealed by bonding of the peripheral sealing elements constituted by the compact carbonaceous material, to the separator via the sheet of carbon resin, the quantity of gas leaking outwards through the peripheral sealing elements, including through the connecting part, is mainly subject to diffusion, and is not strongly affected by pressure, but in accordance with the present invention, it is desirable that the gas leak does not exceed 10-2 mL / cm.h.mmAq in the case where the quantity of gas leakage per unit of time, per unit of length of the peripheral edge of the connecting part, under a differential pressure of 500 mmAq, is determined by (gas leakage / length of the side. ession

differential).

  The fluorocarbon resin used to connect the peripheral sealing elements to the peripheral extensions of the separator, in accordance with the present invention, is generally a fluorocarbon resin having a melting point at least equal to 200 C, and although this resin is not particularly limited, one can for example cite a tetrafluoroethylene resin (designated under the name of PTFE, having a melting point of 327 C and a temperature of thermal deformation of 121 C under 4.6 kgf * / cm2 with the manometer), a resin copolymer of tetrafluoroethylene and hexafluoropropylene (designated under the name of FEP, and having a melting point of 20 2500C at 280oC and a temperature of thermal deformation of 72 C under 4.6 kgf * / cm2 with the manometer), a fluorinated alkoxyethylene resin (designated as PFA, and having a melting point of 300 to 310 C, and a thermal strain temperature of 751C under 4.6 kgf * / cm2 on a pressure gauge) and a resin

  fluorinated copolymer of ethylene and propylene (designated by TFP and having a melting point of 290 to 300 C).

  The fluorocarbon resins mentioned above have been marketed. Among the fluorocarbon resins mentioned above, the tetrafluoroethylene resin is preferred, the latter being

  marketed under the TEFLON brand name.

  According to the present invention, it is desirable to use the above-mentioned fluorocarbon resin 35, for example in the form of a

  sheet with a thickness of about 50 lum.

  With regard to the electrode substrate of the composite substrate of the present invention, the following materials are used: (1) a material produced by molding a mixture of short carbon fibers, a binder and a substance organic granulate and heating under pressure (refer for example to the US patent

n * 4,522,895).

  A particularly favorable material 10 is that which is prepared by molding a mixture consisting of 20 to 60% by weight of short carbon fibers with a diameter of 5 to 30 μm, with a length of 0.5 to 2 mm. and a linear carbonization shrinkage during their calcination at 2000 ° C., from 0.1 to 3.0%, from 20 to 50% by weight of a carbonaceous binder having a carbonization yield of 30 to 75% by weight , such as a phenolic resin, an epoxy resin and a pitch of petroleum and / or coal, and from 20 to 50% by weight of organic granular substances (a micropore regulator and a release agent (having a yield carbonization of at most 30% by weight, a diameter of 30 to 300 μm and not evaporating at 100 C such as polyvinyl alcohol, polyethylene, polypropylene, polyvinyl chloride, a copolymer resin d ethylene and vinyl acetate and a sugar or mixtures thereof, with conditions of a molding temperature of 100 to 180 C, a molding pressure e from 2 to 100 kgf * / cm2 on the pressure gauge and a pressure maintenance time 30 from 1 to 60 min, and (2) a material prepared by calcination of the material mentioned above in (1) at a temperature of at least 800 C in an inert atmosphere and / or

under reduced pressure.

  However, in the case where the separator and the electrode substrate are linked to each other by a dispersion of tetrafluorethyLene resin, the material having been calcined, i.e. the material (2) ci above is used. As a separator 5 used in accordance with the present invention, a compact carbon plate having a calcination shrinkage of not more than 0.2% when calcined at 2000 C in an inert atmosphere and / or

  under reduced pressure is favorable.

  The materials used to make the composite substrate of the present invention, described by bonding a separator and electrode substrates to each other, are described below by means of a flexible carbon sheet 15 interposed between the joining surfaces of the electrode substrate and the separator, with the use of an adhesive, and calcination of the materials thus bonded into a single body, as well as the manufacturing process

  of the composite substrate of the present invention.

  As a flexible carbon sheet used in accordance with the present invention, there may be mentioned a flexible graphite sheet prepared by compression of expanded graphite granules obtained from graphite granules with a particle diameter of not more than 25 mm when treated with acids and by heating

  then the granules thus treated with acids.

  It is desirable that the flexible graphite sheet thus produced has a thickness of not more than 1 mm and exhibits as properties an apparent density of 1.0 to 1.5 g / ml, a rate of compression deformation (i.e. i.e. a rate of deformation under a compressive load of I kgf * / cm2) not exceeding 0.35 x 10-2 cm2 / kgf * and a flexibility such that it does not break when folded with a radius 35 of curvature of 20 mm. Among the flexible graphite sheets sold, mention may be made, for example, of the brand

  GRAFOIL (manufactured by the company U.C.C.).

  In addition, as flexible carbon sheet also used in accordance with the invention, the following material can be used: A composite material prepared by mixing carbon fibers with a diameter of 4 to 25 Fm and an average length d '' at least 1 mm, which have been obtained by heat treatment of polyacrylonitrile, rayon, a phenolic resin, an isotropic or anisotropic pitch, at a temperature of at least 1000 C, and a binder having a carbonization yield of at least 10% by weight such as a substance selected from the group consisting of phenolic resins, epoxy resins, furan resins, pitches of petroleum or carboniferous layers, polyvinyl alcohols, chlorides polyvinyl, polyacrylonitrile, rayon and siloxane polymers, or a composite material prepared by introducing the binder into. a matrix of carbon fibers, is molded by heating under pressure and the composite material thus molded is calcined at a temperature of at least 850 C in an inert atmosphere and / or under reduced pressure, in order to obtain the flexible carbon sheet . In the flexible carbon sheet thus produced, the lumps of carbon from the binder are dispersed in the matrix of carbon fibers and retain a plurality of carbon fibers, and the carbon fibers are slidably held in contact with each other. by the lumps of carbon. This flexible carbon sheet has as properties a thickness not exceeding I mm, an apparent density of 0.2 to 1.3 g / ml and a rate of compression deformation not exceeding 35 2.0 x 10-1 cm2 / kgf *. The flexible carbon sheet mentioned above has a flexibility such that it does not break when folded to a radius

10 mm curvature.

  As the adhesive used on each of the adherent surfaces in the case of the joining of the electrode substrates and the separator by means of a flexible carbon sheet, any adhesive generally used for adhering can be used. carbonaceous materials, but in particular a thermosetting resin selected from the group consisting of phenolic resins, epoxy resins,

and furan resins.

  Although the thickness of the above-mentioned adhesive layer 15 is not particularly limited, it is generally desirable to apply the adhesive evenly over a thickness not exceeding

pitch 0.5 mm.

  In addition, bonding using the above-mentioned adhesive 20 can be carried out at a temperature of 100 to 180 ° C. under a pressure of 1 to 50 kgf * / cm 2 using a pressure gauge for a pressing time ranging from 1 at 120 min. The composite material prepared by bonding the flexible carbon sheet to the two electrode substrates, although they use the adhesive mentioned above and the bonding conditions mentioned above, is treated in the following manner. The surface of the electrode substrate to which the carbon sheet has been bonded is subjected to a cutting treatment to form grooves of the desired size, which constitute the flow channels of the reactive gas so that the ratio 35 from the total cross-section of the reactive gas flow channels, on the side of the fuel electrode, to the total cross-section of the reactive gas flow channels, on the side of the air electrode, ie I : 3 to 2: 3. The above-mentioned cutting treatment 5 can be carried out by any suitable means, such as for example

using a diamond blade.

  The surfaces of the two flexible carbonaceous sheets remaining on the electrode substrate thus cut, are glued to the opposite surfaces of the separators, as in the case of adhesion of the electrode substrates mentioned above to the carbonaceous sheet, and the material thus combined is calcined at a temperature of at least about 800 C in an inert atmosphere and / or under reduced pressure. The carbonization of the materials thus combined can be completed by carrying out an identical calcination after bonding of the electrode substrates to the carbon sheet, in the same manner as the calcination carried out before the treatment of. cutting. More

  precisely, calcination is carried out twice.

  The peripheral sealing elements constituted by a compact carbonaceous material impermeable to gases are then fixed by adhesion in the molten state of a fluorocarbon resin sheet (more precisely by means of the fluorocarbon resin sheet) to the peripheral extensions. of the separator, which extend beyond the two edges of the electrode substrate which are parallel to the flow channels 30 of the reactive gas therein, and which are adjacent to the two edges of the electrode substrate - mentioned above, at a temperature at least equal to a temperature 50 C lower than the melting point of the fluorocarbon resin, under a pressure of at least 1 kgf * / cm2 on the manometer, the sheet of fluorocarbon resin having been previously interposed between each of the peripheral sealing elements

  and each of the peripheral extensions.

  In order to obtain the structure of the elictrode substrate 5 mentioned above in accordance with the present invention, it is possible, for example, to use several modified methods. By way of example, the flexible carbon sheet is joined to the upper surface of the protuberance produced by the grooves 10 on the electrode substrate by cutting. However, it is more practical to carry out the cutting treatment after having fixed the flexible carbon sheet to the electrode substrate which has not yet been subjected.

to the cutting treatment.

  In addition, in certain production situations, the flexible carbonaceous sheets 7 and 7 ′ can be the same size as the electrode substrate, and can be joined to the entire surface of the separator. This embodiment is in the part of the present invention. However, from the point of view of the thickness of the composite substrate, as the first structure is capable of reducing the thickness of the composite substrate compared to the latter structure, by a value equal to 25 the thickness of the flexible carbon layer , while retaining the same cross-sectional area of the reactive gas flow channels, it will be preferred

the first structure.

  Next, the materials to be used for preparing the composite substrate will be described, in which the separator and the electrode substrates are joined to each other by a dispersion of tetrafluoroethylene resin and the process for producing

this composite substrate.

  As the dispersion of tetrafluoroethylene resins, an aqueous dispersion containing 10 to 70% by weight, for example about 60% by weight, of the aforementioned tetrafluorethylene resin is used. A small amount of surfactant can be added to the dispersion

mentioned above.

  After applying the above-mentioned dispersion of tetrafluorethylene resin on the opposite surfaces of the separator, to a thickness of 0.01 to 0.5 mm, the surfaces provided with grooves of the two electrode substrates, on which the grooves have been formed beforehand in order to form the reactive gas flow channels, so that the ratio of the total cross section of the reactive gas flow channels, on the side of the fuel electrode, to the cross section total transverse of the reactive gas flow channels, on the side of the air electrode, ie from 1: 3 to 2: 3, are brought into contact with the opposite surfaces 20 of the separator and they are bonded by adhesion to L ' molten state at a temperature of at least about 270 C below

  a pressure of at least 1 kgf * / cm2 on the pressure gauge.

  In this regard, although tetrafluorethylene resin is a non-electroconductive substance, the electrical conductivity between

  the porous carbonaceous electrode substrate and the separator is sufficient for the following reasons.

  As at the time of the pressure bonding mentioned above, the two materials are joined together so that the resin

  tetrafluorethyLene applied to the separator, impregnates the porous and carbonaceous electrode substrate, the two materials are joined to each other with sufficient force and, simultaneously, the contact of the two materials is sufficient.

  Subsequently the peripheral sealing elements constituted by a compact carbonaceous material impermeable to gases, is fixed by adhesion in the molten state of a fluorocarbon resin sheet (i.e., via the fluorocarbon resin sheet ) to the peripheral extensions of the separator which extend beyond the two edges of the electrode substrate which are parallel to the flow channels of the reactive gas therein and which are adjacent to the two edges of the above-mentioned electrode substrate , as described above, the fluorocarbon resin sheet having been previously interposed between each of the peripheral sealing elements and each of the peripheral extensions. The previously mentioned junction of the porous and carbon electrode substrates and of the separator, as well as the junction of the separator and of the peripheral sealing elements via the aforementioned sheet of fluorocarbon resin, can be carried out simultaneously, by an appropriate choice of joining conditions, and this operation is particularly advantageous given the reduction in the production stages. The molding of the porous carbonaceous electrode substrate provided with the grooves forming reactive gas flow channels can be carried out by any method. By way of example, after having filled with the raw material mixture a metal mold having the desired shape, the mixture thus introduced is compression molded, or else, after molding the mixture of raw material in the form of a flat plate (after additional calcination of the molded mixture), the grooves are formed by cutting. However, from the point of view of the productivity and uniformity of the products, it is desirable to extrude the mixture of raw materials after kneading it, then to compression mold the mixture thus extruded by rolling.

or mastering.

  In addition, the composite substrate of the present invention may have, as shown in Figure 3, gas distributors 8 and 8 'which are made of the same compact carbonaceous material as the peripheral sealing elements, and which are arranged so as to be near the two

  edges of the electrode substrate which are perpendicular to the flow channels of the reactive gas therein.

  The previously mentioned gas distributors 8 and 8 ′ have grooves which form the flow passages allowing the distribution of the reactive gas in association with the separator, and via the previously mentioned flow passages 9 and 9 ′ for the distribution of the reactive gas, the reactive gas is supplied from the outside to the reactive gas flow channels 5 and 6 (not shown in Figure 3), but it is not necessary that the shape of the cross section flow passages 9 and 9 ′ coincide with the shape of the cross section of the flow channels 5 and 6 of the reactive gas and moreover, it is also not necessary that all of the open ends of the channels of reactive gas flow 5 and 6 opens on the flow passages 9 and 9 ', for the distribution of the reactive gas, and the cross-sectional shape of the flow passages 9 and 9' intended for distribute the reactive gas, may be such that the flow required Reactive gas is guaranteed when the device is used as a composite substrate

for fuel cells.

  With regard to the cross section of the flow passages 9 and 9 'for the distribution of the reactive gas, it is not necessary to provide conditions other than those which have been mentioned above, and therefore it suffices that The ratio of the total cross-section of

  reactant gas flow channels, on the side of the fuel electrode, to the total cross section of the reactive gas flow channels, on the side of the air electrode, from 1: 3 to 2: 3.

  Furthermore, in the composite substrate shown in FIG. 3, like the peripheral sealing element and the gas distributor, which were both made from the same material, are arranged opposite one of the other on either side of the separator, and since the coefficient of thermal expansion of the upper layer coincides with that of the lower layer, the thermal stress existing between the separator and the peripheral sealing element, and the thermal stress existing between the separator and the gas distributor, become identical, and the warping as well as the deformation occurring during calcination can be limited. This situation is particularly favorable in the case where the separator and the electrode substrates are calcined in a single body, and as, in the peripheral region of the composite substrate being in the form of a thin plate, the element d peripheral sealing and the gas distributor 30 have been arranged and fixed face to face, on the

  two surfaces of the separator while holding the separator, this structure has a reinforcing effect, so that the composite substrate of the present invention is very easy to handle during the manufacture of the fuel cell.

  The composite substrate of the present invention, which was obtained in the manner described above, has a total cross section of the reactant gas flow channels, on the side of the two electrodes, in accordance with the conditions imposed by the reactive gas actually available. Therefore, compared to the electrode substrate having the same total cross-section of the reactive gas flow channels on the side of the fuel electrode 10 and on the side of the air electrode, the total cross-section of the channels Reactive gas flow from the fuel electrode side can be reduced while maintaining the same performance when operating the apparatus in the form of a fuel cell. More specifically, the height of the reactant gas flow channels can be reduced. Therefore, it is also possible to decrease.

  the thickness of the composite substrate proper.

  For example, in the ordinary composite substrate, which has a thickness of 3.8 to 4.0 mm, as the flow channels of the reactive gas are formed with a height of 1.0 to 1.4 mm the thickness of the electrode substrate can be reduced by about 0.6 to 0.9 mm at most, and as far as the entire substrate is concerned, the thickness can be reduced by about 24%. This structure not only contributes to making the fuel cell more compact, and also makes it possible to reduce its electrical resistance and its thermal resistance in the same proportions (i.e. around 15 to 24%) thanks to the reduction in the thickness of the composite substrate. Therefore, better fuel efficiency can be expected from the composite substrate of the present invention. In addition, in the composite substrate for fuel cells of the present invention, as the peripheral sealing elements are joined to the peripheral extensions of the separator in the form of a single body via the sheet of fluorocarbon resin, while remaining adjacent to the two edges of the electrode substrate, the composite substrate of the present invention makes it possible to prevent gas leakage very satisfactorily, and it is not necessary to carry out the peripheral sealing treatment required by the ordinary fuel cell in order to avoid leakage of reactive gas from the sides of the fuel cell. More preferably, the electrode substrates 15 and the separator are bonded to each other in a single body via the flexible carbon sheet, or are joined to each other by dispersing a tetrafluorethylene resin, and the sealing elements as well as the separator are joined to each other in the form of a single body by the fluorocarbon resin. Therefore, the composite substrate of the present invention has excellent resistance to phosphoric acid and is particularly useful as a composite substrate

  for fuel cells of the phosphoric acid type.

  Furthermore, in the composite substrate of the present invention, as the peripheral sealing elements have been uniformly disposed on the peripheral region of the plate-shaped composite substrate and bonded thereto, while maintaining the

  separator alternately on both sides, this structure provides a reinforcing effect, which makes the above-mentioned composite substrate suitable for handling during the manufacture of the fuel cell.

  The nonlimiting example is given by way of

  Illustrative of the present invention.

Example.

  The composite substrate for fuel cells of the present invention was produced using the following materials: (1) Electrode substrates: 1-1: a porous and carbonaceous flat plate (manufactured by the company KUREHA KAGAKU KOGYO Co., Ltd. 10 under the trade name KES- 400) having been previously calcined at a temperature of at least 800 C in a nitrogen atmosphere and having for dimensions a width and a length of 690 mm and

a thickness of 1.47 mm.

  1-2: a porous and carbonaceous flat plate identical to that used in 11, except that it has a thickness of 0.97 mm and not a thickness

1.47 mm.

(2) Separator.

  The separator was obtained by cutting a compact carbon plate (produced by the company SHOWA DENKO Co., Ltd., under the trade mark SG-2, with a thickness of 0.6 mm) into a piece of

690 mm in length and width.

  (3) Sheet of fluorocarbon resin.

  Four pieces of a resin sheet

  fluorocarbons were obtained by cutting a TEFLON sheet (produced by the company NICHIAS Co, Ltd., having a thickness of 0.10 mm) as a function of the size of the peripheral sealing element.

(4) Soft carbon sheet.

  Two pieces of a flexible carbon sheet were obtained by cutting a sheet of GRAFOIL (produced by the company UCC, having an apparent density of 1.10 g / ml and a thickness of 0.13 mm) as a function of the size of the junction surface

  (690 mm in length and 650 mm in width).

  (5) Peripheral sealing elements: -I: two pieces of a compact carbon plate 5 (produced by the company TOKAI Carbon Co, Ltd, having an apparent density of 1.85 g / ml) were cut with size 690 mm in length, 20

  mm wide and 1.5 mm thick.

  -2: two pieces of a compact carbon 10 plate (identical to the plate above) were cut to size 690 mm in length, 20 mm

wide and 1.0 mm thick.

  After applying an adhesive from a series of phenolic resins to one face of each of the two electrode substrates and to one face of each of the two sheets of GRAFOIL, the adhesive thus applied

was allowed to dry.

  Subsequently, the junction surface of the GRAFOIL was joined to the junction surface of the electrode substrate, with the exception of the surface which must lie opposite the peripheries of the separator to which the material peripheral sealing is fixed, at a temperature of 140 ° C., under a pressure of 10 kgf * / cm2 on the manometer and with a holding time of the pressure of 20 min. The surface adhering to the GRAFOIL of the electrode substrate, which consisted of the 1.47 mm thick porous carbonaceous flat plate was then cut in order to produce a plurality of parallel grooves 30 having a rectangular cross section of a height 1.0 mm and 2 mm wide at

  intervals of 4 mm, using a diamond blade.

  Likewise, the surface of the electrode substrate adhering to the GRAFOIL, which was constituted by the porous and carbonaceous flat plate with a thickness of 0.97 mm, was cut in order to form a plurality of parallel grooves having a rectangular cross section with a height of 0.5 mm and a width

  2 mm, at 4 mm intervals.

  - Subsequently on the remaining GRAFOIL surfaces of the materials thus treated, the adhesive mentioned above was applied and dried. Similarly, on opposite surfaces of the separator,

  the above adhesive was applied and dried.

  Each of the remaining GRAFOIL surfaces of the two electrode substrates was then bonded to the opposite surface of the separator so that the plurality of parallel grooves in one of the two electrode substrates were perpendicular to those which found in a

  other electrode substrate, at a temperature of 140 ° C., at a pressure of 10 kgf * / cm 2 on the manometer and with a pressure holding time of 20 min, and the materials thus bonded were calcined at 2000 ° C. in a nitrogen atmosphere.

  After calcination, the parts of the electrode substrate facing the peripheries of the separator to which the peripheral sealing elements are joined, were removed in order to expose the peripheral extensions of the separator, and a sheet of TEFLON was interposed between each of the peripheral sealing elements and the separator, then the two materials were bonded to each other by adhesion in the molten state of the sheet 30 of TEFLON at 350 C, under a pressure of 20 kgf * / cm2 on the pressure gauge for a pressure maintenance period of 20 min. Thanks to the procedures mentioned

  above, a composite substrate for fuel cells having a thickness of 3.3 mm was obtained.

  Comparative example: In the same way as in the previous example, except that two pieces of the porous and carbonaceous flat plates with a thickness of 1.47 mm were used as electrode substrates, and that one cut the two surfaces adhering to the GRAFOIL of the two electrode substrates in order to form a plurality of parallel grooves having a rectangular cross section with a height of 1.0 mm and a width of 2 mm at intervals of 4 mm, a composite substrate for fuel cells was produced. Thickness

  of the composite substrate thus produced was 3.8 mm.

  The results of measurement of the electrical resistance and of the thermal resistance of the composite substrates thus produced in the EXAMPLE and in the COMPARATIVE EXAMPLE, are provided in the following table.

BOARD

:. Em * Example

: Example ::

  comparative :::: 25: Thickness (mm): 3.3: 3.8:: Electrical resistance: 15 18: (m.cm2):: :::: Thermal resistance: 34 40 (m2.h.OC / kcal)::

:::

:::

  to == = = = = = = = = = = = == = = = = = = = = = = = = =

  As clearly shown in the table, the electrical resistance and thermal resistance of the composite substrate for fuel cells of the present invention could be reduced by about 15 to 16% compared to that of ordinary composite substrates, thanks to the decrease of

the thickness of the ccmposite substrate.

Claims (21)

  1. A composite substrate for fuel cells comprising: (1) a separator (1), (2) two porous and carbonaceous electrode substrates (2, 2 ') which have been respectively provided with a plurality of grooves forming channels of flow (5, 6) for the reactive gas on one of their faces, and of a flat surface on their other face, and having been joined to the opposite surfaces of said separator (1), so that said reactive gas flow channels in one of said electrode substrates (2, 2 ') are perpendicular to those in another electrode substrate, and that said separator (1) extends beyond the two edges of said electrode substrate which are parallel to said reactive gas flow channels in said electrode substrate, and (3) sealing elements (3, 3 ') connected to the peripheral extensions of said separator (1), which extend beyond the two edges of said electrode substrate (2, 2 ') via a layer of res ine fluorocarbon (4, 4 '), the ratio of the total cross section of said 25 flow channels (5) of reactive gases formed by said separator (1) and the grooves of said porous and carbonaceous electrode substrate, on the side of the fuel electrode (2) at the full cross section of said reactive gas flow channels (6) formed by said separator (1) and the grooves of said porous and carbonaceous electrode substrate, on the electrode side of air (2 ') being from 1: 3 to 2: 3.   2. Composite substrate for fuel cells according to claim 1, characterized in that a flexible carbide sheet (7, 7 ') has been interposed between the joining surfaces of said porous and carbonaceous electrode substrate (2,2' ) and said separator (1). 3. Composite substrate for combustible combus5 cells according to claim 1, characterized in that said electrode substrates (2, 2 ') porous and carbonaceous and Said separator (1) have been linked using   of a dispersion of tetrafluoroethylene resin.   4. compo.site substrate for fuel cells according to claim 1, characterized in that said porous carbonaceous electrode substrate (2, 2 ′) has an apparent density of 0.3 to 0.9 g / ml, a permeability of at least 200 ml / cm2.h.mmAq and an electrical resistivity not exceeding 200 mlcm, after calci15 nation at a temperature of at least 1000 C in a   inert atmosphere and / or under reduced pressure.   5. A composite substrate for fuel cells according to claim 1, characterized in that said separator (1) is a compact carbonaceous material having an apparent density of at least 1.4 g / ml, a gas permeability not exceeding 106 ml / cm2.h.mmAq, an electrical resistivity not exceeding 10 m - .. cm   and a thickness not exceeding 2 mm.   6. Composite substrate for fuel cells in the fuel according to claim 1, characterized in that said peripheral sealing element (3, 3 ′) is a compact carbon material having an apparent density of at least 1.4 g / ml and permeability   gas not exceeding 10-4 ml / cm2.h.mmAq.   7. A composite substrate for fuel cells according to claim 2, characterized in that said flexible carbon sheet (7, 7 ') was obtained by carbonization of a composite material comprising carbon fibers having an average length of at least 35 minus I mm and a binder, and in that it has a thickness not exceeding 1 mm, an apparent density of 0.2 to 1.3 g / ml, a rate of compression deformation not exceeding 2.0 x 10-1 cm2.kgf * and a flexibility such that it does not break when folded up to a radius of curvature of 10 mm, in that in said flexible carbon sheet (7, 7 '), the carbon clusters derived from said binder, dispersed in the matrix of said carbon fibers and retaining a plurality of said carbon fibers, said carbon fibers are held in sliding manner against each other by the carbon clusters. 8. A composite substrate for fuel cells according to claim 2, characterized in that said flexible carbon sheet (7, 7 ') was produced by compression of expanded graphite particles obtained by exposure of graphite particles having a particle diameter not exceeding 5 mm in an acid treatment, then in heating the particles thus treated with the acids, and in that it has a thickness not exceeding 20 I mm, an apparent density of 1.0 to 1 , 5 g / ml, a compression deformation rate not exceeding 0.35 x 10-2 cm2 / kgf * and a flexibility such that it does not break when folded to a radius 20 mm curvature.   9. composite substrate for fuel cells according to claim 1, characterized in that said fluorocarbon resin (4, 4 ') has a point at least 200 C.   10. A method for producing a composite substrate for fuel cells according to claim 2, which method comprises the steps of: (1) bonding a flexible carbon sheet (7, 7 ') to a surface of each of two porous and carbonaceous electrode substrate (2,2 ′) in the form of a flat plate, without groove and of determined dimensions, using an adhesive, (2) subjecting each of the adherent surfaces of said 5 - substrates of an electrode to a cutting treatment for producing grooves forming flow channels for the reactive gas therein, so that the ratio of the total cross section of said flow channels (5) of reactive gas 10 on the side of the fuel electrode (2), at the total cross-section of said flow channels (6) of reactive gases, on the side of the air electrode (2 '), ie from 1: 3 to 2 : 3, (3) Glue the surfaces of said flexible carbon-containing sheet 15 remaining on the thus-cut surfaces of said electro substrates from to the opposite surfaces of said separator (1), so that the grooves in one of said electrode substrates are perpendicular to those in the other electrode substrate, (4) Calcining the materials thus bonded at a temperature of at least 800 ° C. in an inert atmosphere and / or under reduced pressure, and (5) bonding peripheral sealing elements (3.3 ′) made of a compact carbon material impermeable to gases to peripheral extensions of said separator, which extend beyond the two edges of said electrode substrate which are parallel to said flow channels 30 of the reactive gas therein using a sheet fluorocarbon resin.   11. Method according to claim 10, characterized in that said porous and carbonaceous electrode substrate (2, 2 ') is selected from the group consisting of (1), a molded material obtained by molding   of a mixture of short carbon fibers, of a binder and of an organic granulated substance, in a single body, by heating under pressure, and (2) a calcined material obtained by calcination of said molded material 5 defined above (1) in an inert atmosphere and / or under reduced pressure.   12. Method according to claim 10, characterized in that said flexible carbon sheet (7, 7 ') was obtained by carbonization of a composite material comprising carbon fibers having an average length of at least I mm and a binder, and in that it has a thickness of not more than I mm, an apparent density of 0.2 to 1.3 g / ml, a rate of compression deformation not exceeding 2.0 x 15 10-1 cm2 / kgf * and a flexibility such that it does not break when it is folded up to a radius of curvature of 10 mm, in that in said sheet: carbonaceous (7, 7 ′) flexible, the carbon masses derived from said binder are dispersed in the matrix of said carbon fibers and retain a plurality of said carbon fibers, said carbon fibers being slidably held against each other the others by carbon pools.   13. The method of claim 12, characterized in that the flexible carbon sheet (7, 7 ') is obtained by molding a composite material comprising carbon fibers having an average length of at least I mm and a binder having a carbonization yield of at least 10%, by heating under pressure and calcination of the composite material thus molded at a temperature of at least 850 C in a   inert atmosphere and / or under reduced pressure.   14. Method according to claim 10, characterized in that said flexible carbon sheet 35 (7, 7 ') is produced by compression of particles   of expanded graphite obtained by exposing graphite particles having a particle diameter of not more than 5 mm, to an acid treatment, then by heating the graphite particles thus treated with acids, and in that it has a thickness not exceeding 1 mm, an apparent density of 1.0 to 1.5 g / ml, a rate of compression deformation not exceeding -35.3 x 102 cm2 / kgf * and a flexibility such that it does not does not break when bent to a radius of curvature of 20 mm.   15. The method of claim 10,   characterized in that the adhesive is a thermosetting resin selected from the group consisting of phenolic resins, epoxy resins, and furan resins.   16. The method of claim 10, characterized in that the conditions prevailing during the joining of said electrode substrate to said separator are a joining temperature of 100 20 to 180 C, a compression pressure of 1 to 50 kgf * / cm2 pressure gauge and pressure hold time from I to 120 minutes.   17. A method of manufacturing a composite substrate for fuel cells according to claim 3, which method comprises the steps of: (1) applying a dispersion of tretrafluoroethylene resin to the opposite surfaces of a separator, (2) bond by adhesion in the molten state each of the surfaces provided with grooves of two electrode substrates (2, 2 ′) porous and carbonaceous at the defined position of the opposite surfaces of said separator on which the dispersion has been applied, so as to that the grooves located in one of said electrode substrates are perpendicular to those which are in another electrode substrate, said electrode substrates being respectively provided with a plurality of grooves forming flow channels ( 5, 6) for the reactive gas 5 on one of their faces, and of a flat surface on their other face, the size of the grooves being such that the ratio of the total cross section to the flow channels (5) reactive gas from side of the fuel electrode 10 (2) at the total cross section of said flow channels (6) of reactive gas, on the side of the air electrode (2 ') is from 1: 3 to 2 3, and (3) bonding peripheral sealing elements (3, 3 ') made of a compact carbon material impermeable to gases, to peripheral extensions of said separator (1) which extend beyond the two edges of said substrate. electrode   which are parallel to said flow channels of the reactive gas therein, by means of a sheet 20 of fluorocarbon resin.   18. The method of claim 17, characterized in that said porous and carbonaceous electrode substrate (2, 2 ') is produced by calcination of the molded material obtained by molding a mixture of short carbon fibers, a binder and an organic granular substance, in a single body by pressure heating.   19. The method of claim 17, characterized in that the joining by adhesion in the molten state of said separator (1) to said porous and carbonaceous electrode substrate (2, 2 ') is carried out at a temperature of at least about 270 C, under a   pressure of at least I kgf * / cm2 at the namometer.   20. The method of claim 10 or   17, characterized in that said separator (1) is a compact carbon plate having a calcination shrinkage not exceeding 0.2% when it is calcined at 2000 ° C. in an inert atmosphere and / or under reduced pressure .   21. The method of claim 10 or   17, characterized in that said sheet of fluorocarbon resin (4, 4 ') has a melting point of at least 10,200 C.