FR2827998A1 - Hydrophilizing a plate comprising a fluoropolymer and an electrically conductive material, e.g. for use in fuel cells or batteries, comprises contacting the plate with a solution of a base - Google Patents

Abstract

Method for hydrophilizing a plate comprising a fluoropolymer and an electrically conductive material comprises contacting the plate with a solution of a base and then removing the plate, rinsing it with water or a solvent and optionally drying it. An Independent claim is also included for plates produced by the above process.

Description

at least two steam generators (1).

  HYDROPHILIC PLATE BASED ON FLUORINATED POLYMER

[Field of the invention]

  The invention relates to a hydrophilic plate based on fluoropolymer.

  Energy generators such as fuel cells as well as energy storage devices such as batteries contain plates based on fluorinated polymer. These plates generally consist of a fluoropolymer such as PVDF (polyvinylidene fluoride) and a conductive product such as for example graphite or carbon black. These plates can be obtained, for example, by injection into a mold or by hot pressing of a mixture consisting of powdered PVDF and carbon black or graphite. The size of these plates can be from 0.1 m x 0.1 m to 1 m x 1 m and their thickness can vary from 1 to 10 mm. According to their arrangement and their function these plates are

  sometimes called bipolar plates.

  There are several types of fuel cell operation which generate different specifications with regard to the characteristics which the bipolar plates must have in terms of conductivity, mechanical resistance and water permeability and therefore

hydrophilic behavior.

  These PVDF and carbon black or graphite based plates are

  hydrophobic, the invention describes a process for making them hydrophilic.

  [Prior art and the technical problem] The prior art below has described numerous bipolar plates and has sometimes mentioned that they were hydrophobic: Patent DE 3538732 describes an electrode made of a paste which can be spread and made up of 70 to 80% by mass of carbon powder of granule size 30 to 300 μm and from 10 to 20% by mass of a solution of PVDF with 4 to 8% of PVDF in DMF (dimethylformamide) and at least minus 5% by mass of PTFE powder with a granule size of 10 to 100 μm. The paste is spread on an aluminum substrate and then dried by an infrared lamp for '/ z h to 4 h. This PVDF and carbon based electrode is gas permeable and

liquids.

  Patent application JP 2000077079-A describes a separator consisting of 85 to 97% graphite and 3 to 15% of thermosetting resin. This separator is produced by cold forming at 100 to 150 kg / m2 and then formed by molding at 150 to 1500 kg / m2. These separators are

electrically conductive and uniform.

  In patent application JP 08031231 A there is described a formulation based on spherical graphite, thermosetting or thermoplastic and carbon black of the conductive ketjenblack type. The material has good mechanical strength and can be used for molding and

  calendering. This material can be used in the field of fuel cells.

  In patent application JP 04013287 A, a plate is described.

  porous carbon in the three dimensions up to 60 to 80% porosity.

  In patent application JP 52122276 A there is described an electrode prepared by deposition on a porous textile of pyrolyzed anisotropic carbon, itself covered with an aqueous dispersion of TEFLON (PTFE) and the whole is

  dried to form a hydrophobic porous layer.

  In patent application WO 200025372 there is described a bipolar plate for use in the field of fuel cells and originating from the molding of a vinyl ester resin and a graphite powder making it possible to obtain a conductivity of at least 10 S / cm. These plates can contain from 20 to 95% of graphite and from 0 to 5% of carbon black as well as cotton fibers. It also describes the use of fluorinated products to improve the

demoulding as well as hydrophobia.

  In US Patent 5,268,239 describes the preparation of a separator plate. This graphite-based plate is a mixture containing 25 to

  % by mass of graphite and 25 to 75% by mass of phenolic resin.

  This plate is then pyrolyzed between 800 and 1000 C, then graphitized between 2300 C and 3000 C. This patent also describes the application of a film of

  fluoropolymer to prevent electrolyl migration.

  In patent FR 2430100 there is described a process for the preparation of a finely divided dry powder characterized in that it consists of grains having a maximum dimension of approximately 5 μm. This powder comprises precatalysed carbon and a hydrophobic fluorocarbon polymer, for example PTFE. This powder is obtained by flocculation of a cosuspension of

  grains of precatalysed carbon and grains of polymers.

  In patent EP 0948071 there is a method for producing an electrode for fuel cells, as well as a catalytic powder prepared by mixing a fine carbon powder supporting a catalytic metal with a collodal dispersion of a polymer. The suspension thus obtained is dried. In patent EP 0557259 the preparation of a gas diffusion electrode for an electrochemical cell is described. This electrode is prepared using a carbon black powder dispersed in an organic solvent in the presence of soluble polyethylene. The dispersion is then dried which allows the polyethylene to cover the surface of the black. This polyethylene is then fluorinated. This hydrophobic carbon black powder powder is then mixed with an acetylene type carbon black supporting a catalyst metal as well as PTFE to form aggregates. These aggregates are

  then pressed at 20 kg / cm2 and sintered at 340 C for 20 minutes.

  In patent application WO 200030202 there is described a moldable composition allowing the preparation of current collector plates by compression molding or injection molding. This composition comprises a non-fluorinated polymeric binder; among the polymers which can be used are polyphenylene sulfides, modified polyphenylene ether, liquid crystal polymers, polyamides, polyimides, polyesters, phenolics, epoxy resins and vinyl esters. Among the conductive particles are more particularly carbonaceous particles. These

  carbonaceous particles are present at least at 45% by mass.

  The prior art has also described hydrophilic plates, thus patent application WO 9853514 describes a method for preparing a proton exchange membrane as well as a method allowing the production of hydrophilic bipolar plates by adding a hydrophilic agent which can be by example of silica dispersed uniformly throughout the material. This addition of silica leads to a larger water intake but also to an increase in the electrical resistivity of the plate. Most of the prior art concerning the production of bipolar plates or electrodes shows a large number of methods allowing the production of hydrophobic non-porous graphite plates or involving agents allowing the increase in hydrophilicity but to the detriment of other properties, such as conductivity, for example. Hydrophilic plates have now been found which do not have the aforementioned drawbacks. They are essentially made up in general of a fluorinated polymer such as PVDF (polyvinylidene fluoride) and of a conductive product such as for example graphite or carbon black. These plates can be obtained very easily. We start from a plate essentially consisting of a fluoropolymer such as PVDF and a conductive product such as for example graphite or carbon black, this plate is naturally hydrophobic, then it is brought into contact with a compound basic

  in solution and rinsed, for example with water.

[Brief description of the invention]

  The present invention relates to a method for rendering hydrophilic a plate essentially comprising a fluoropolymer and an electroconductive product in which: A the plate is brought into contact with a basic compound in solution, A the plate and the basic compound in solution are separated, A the plate is rinsed with water or a solvent and then optionally dried.

  The invention also relates to the hydrophilic plate as an object.

  [Detailed description of the invention]

  As regards the electroconductive product, these are all the conductors of electricity. By way of example, mention may be made of metals, metal oxides and products based on carbon. Examples of carbon-based products that may be mentioned include graphite, carbon black aggregates, carbon fibers, carbon nanotubes and activated carbon. It would not go beyond the scope of the invention to use several electrically conductive elements for example; (i) graphite and carbon black aggregates; (ii) graphite, aggregates of carbon black and carbon fibers; (iii) carbon black aggregates and carbon fibers; (iv) from

graphite and carbon fibers.

  By carbon-based products is meant carbon black, carbon fibers, carbon nanotubes and preferably graphite. The carbon elements that can be used are described in Handbook of fillers 2nd Edition published by Chem Tec Publishing 1999 page 62 2.1. 22, page 92 2.1.33 and page 184 2.2.2. Preferably, it is possible to use either TIMCAL3 KS15 graphite or ASBURY 4012 graphite. Among the carbon blacks which can be used, there may be mentioned Ketjen EC 600 JD blacks with a specific surface 1250 m2 1gr or KetjenQ3 EC 300 J. with a specific surface 800 m2 / g. Among the carbon fibers which can be used, mention will be made of fibers

  ASBURY AGM99. Advantageously the electro carbon elements -

  conductors may be replaced by electrically conductive elements without leaving the framework of the invention. We would not go outside the framework of

  the invention using two or more electrically conductive products.

  With regard to the fluoropolymer (fluoropolymer), any polymer having in its chain at least one monomer chosen from compounds containing a vinyl group capable of opening to polymerize and which contains, directly attached to this vinyl group, is thus designated. minus one atom of

  fluorine, fluoroalkyl group or fluoroalkoxy group.

  By way of example of a monomer, mention may be made of vinyl fluoride; vinylidene fluoride (VF2); trifluoroethylene (VF3); chlorotrifluoroethylene (CTFE); 1,2-difluoroethylene; tetrafluoroethylene (TFE); Hexafluoropropylene (HFP); perfluoro (alkyl vinyl) ethers such as perfluoro (methyl vinyl) ether (PMVE), perfluoro (ethyl vinyl) ether (PEVE) and perfluoro (propyl vinyl) ether (PPVE); perfluoro (1,3-dioxole); perfluoro (2,2 dimethyl-1,3 -dioxole) (PDD); the product of formula CF2 = CFOCF2CF (CF3) 0CF2CF2X in which X is SO2F, C02H, CH20H, CH20CN or CH20P03H; the product of formula CF2 = CFOCF2CF2S02F; the product of formula F (CF2) nCH20CF = CF2 in which n is 1, 2, 3, 4 or 5; the product of formula R1 CH20CF = CF2 in which R1 is hydrogen or F (CF2) z and z is 1, 2, 3 or 4; the product of formula R30CF = CH2 in which R3 is F (CF2) z- and z is 1, 2, 3 or 4; perfluorobutyl ethylene (PFBE); 3.3.3

  trifluoropropene and 2-trifluoromethyl-3,3,3 -trifluoro- 1 -propene.

  The fluoropolymer can be a homopolymer or a copolymer, it can

  also include non-fluorinated monomers such as ethylene.

  Advantageously, the fluoropolymer is PVDF homopolymer or copolymer containing at least 60% by weight of VF2, the optional comonomer is chosen from the fluorinated monomers mentioned above and is advantageously HFP. The fluoropolymer may contain plasticizers or

  additives, such as, for example, a well known plasticizer, dibutyl sebacate.

  It would not be departing from the scope of the invention to use two or more

fluoropolymers.

  the production of the plates involves the mixture of a fluoropolymer and an electroconductive product which constitutes a powder. This powder can be obtained either by using an intimate mixture of fluorinated materials and electrically conductive elements, either by using the co-atomization technique, or by using a simple powder mixture, or by using the

  flocculation or coagulation technique.

  The microcomposite powder can be produced by (co) atomization of one or more emulsions or aqueous dispersions containing its various constituents. For example, the microcomposite powder of the invention can be produced by customization between a fluoropolymer latex (an emulsion or a suspension) and an aqueous dispersion of electroconductive product. The latex and the dispersion are introduced into the coatomization head according to the usual techniques. It is also possible before co-atomization to make a mixture by direct introduction of the electroconductive product into the fluoropolymer latex, or else to mix a dispersion of the product.

  electroconductive and fluoropolymer latex. Liatomization (or co-

  atomization) consists in spraying an aqueous dispersion (or a suspension) or even a solution in a current of hot air to remove the water and

  recover dissolved, dispersed or suspended products.

  As regards the manufacture of the microcomposite powder by flocculation or coagulation, the technologies for finishing an aqueous dispersion are known in themselves. Coagulation is described in the chapter "drying" ("Drying" by P.Y. Mc Cormick, in "Encyclopedia of Polymer Science and Engineoring>" Volume 5, pp. 187-203, Wiley Intersciences, 1990). It describes the coagulation of an aqueous dispersion containing the latex or polymer latex mixture as well as the fillers, in which we speak for example of rotary dryers. Flocculation is also discussed in the chapter "Flocculation" by GR Rose, in "Encyclopedia of Polymer Science and Engineering", Volume 7, pp. 211-233, Wiley Intersciences, 1990). aqueous dispersions based on polymers and fillers by abrupt cooling of the dispersion, or by reducing the dry extract of the dispersion by partial drying, or by adding thereto a surfactant or a destabilizing salt. flocculated dispersion, the aqueous phase is removed to recover the composite powder, for example by drying or filtration This powder can be transformed by the usual techniques of processing thermoplastic materials such as injection, extrusion, compression , Compression transfer injection, compression transfer extrusion. The plates obtained are composed of 40 to 99.9 g% (by weight) of

  electroconductive product and from 0.01 to 60% fluoropolymer.

  Advantageously, the plate consists of 10 to 40% of fluoropolymer and 90 to 60% of electroconductive product. Preferably, the plate is made up of 15 to 30% of fluoropolymer and 85 to 70% of electroconductive product. The plates obtained using the preceding formulation have a density ranging from 500 to 2170 kg / m3, a resistivity between 0.01 and 1 Q.cm in volume resistivity and from 0.01 to 0.5 Q in surface resistivity. The flexural modulus of the plates obtained using the formulation according to the invention is between 1000 and 12000 MPa and the

  bending stress at break is between 1 and 50 MPa.

  As regards contacting with the basic compound, the porous plates thus obtained are then treated by contact with an organic or mineral base (also called basic compound), which is totally or partially dissolved in water, an organic solvent. or a mixture between water and an organic solvent. A catalyst for activating the base can also be advantageously introduced into the treatment medium. The treatment process can be carried out by any technique allowing contact between the porous plate and the base, such as

  sprinkling with a basic solution or immersion in a basic solution.

  The treatment conditions are adjusted in duration, temperature and pressure so as to provide the desired hydrophilic characteristics, while ensuring maximum yield for the process, that is to say by privileging the

shortest possible duration.

  As regards the basic reagents which can be used for the treatment, a distinction can be made, for example, with hydroxide derivatives such as potassium hydroxide, ammonium hydroxide, sodium hydroxide, lithium hydroxide or tetraalkylammonium hydroxides; carbonates such as potassium carbonate or sodium carbonate; primary, secondary or tertiary hydrocarbon amines; metal alcoholates

  alkaline, such as methanolates or ethanolates of potassium or sodium.

  The catalysts, optionally added in combination with these different bases, are, for example, tetraalkylammonium halides, such as tetrabutylammonium bromide; as well as the halides of

  tetraalkylphosphonium or alkylarylphosphonium.

  The organic solvents which can be used to carry out the treatment, either in pure form or in the form of a mixture with one another or with water, are those which allow the total or partial dissolution of the basic reagents previously described. Mention will be made, for example, of alcohols, such as methanol, ethanol or isopropanol, ketones, such as acetone or methyl ethyl ketone, esters, such as ethyl acetate or methyl proplonate, ethers, such as dioxane

  or diglyme, amides, such as dimethylformamide or N-

  methylpyrrolidone. Carbonates, such as diethylcarbonate or

  dibutylcarbonate, and halogenated derivatives, such as chloroform or 1,1-

  dichloro-2,2,2-trifluoroethane. In the case of mixtures with water, preferably use the miscible or partially miscible solvents in the phase

  aqueous, such as alcohols or certain ketones.

  Advantageously, the treatment of the bipolar plate will be carried out by immersion in a stirred and temperature-controlled bath containing the base dissolved in a mixture of water and alcohol. The composition of the mixture will advantageously be richer in water to obtain a more economical process, preferably a fraction by weight of alcohol of between 5 and% will be used. The base can be introduced into the medium, either beforehand or continuously; in solid, liquid or gaseous form. The concentration of the base in the medium will depend on its nature, preferably, in the particular case of sodium hydroxide the concentration of the base will be between 5 to 25% by weight in the mixture of water and alcohol . Preferably, the treatment temperature will be chosen to be lower than the boiling temperature of the azeotrope or the boiling temperature of the most volatile constituent. As indicated above, the treatment time will be as short as possible to obtain maximum yield from the process. The contact time can be between 5 min and 2 hours. Typically contact times of less than 60 minutes will be preferred. This treatment duration will obviously lead to obtaining satisfactory hydrophilic characteristics, which can be determined later by water recovery measures applied to the dry treated plate. At the end of this step, the plate is removed from the bath containing the base to be, advantageously, washed from the treatment residues, for example by spraying with an organic solvent, a mixture of organic solvent and water, or pure water , or else, by immersion in an organic solvent bath, a mixture of organic solvent and water, or pure water. An easy control of the washing efficiency can then be carried out by pH-metry or

again conductimetry.

  After washing, the bipolar plate can be dried or used directly in wet form to be integrated into a battery-type device.

1 5 fuel.

  The hydrophilicity of the plates can be measured by the ratio between the quantity of water absorbed and the quantity that can be incorporated in the plate for a

  of the immersion level of the plate at 23 C in a water bath at 23 C.

  Typically values are greater than 80% for immersion times

between 20 s and 60 s.

[Examples]

  Example 1: (comparative) non-hydrophilic permeable plate From a PVDF KynarO 1000 powder and an ASBURY graphite (0 Graphite 4012, 80% graphite and 20% Kynar formulation is produced in a powder mixer The Kyna 1000 is a PVDF homopolymer of MFI 2 g / 10 min (at 230 C under 5 kg) 34 g of this powder are then placed in a mold and compressed at 230 C for 10 s then cooled for 10 s and finally demolded. The plate thus produced is 10 cm x cm x 0.2 cm This plate has a density d = 1.7, its volume conductivity is 16. 6 S / cm and its nitrogen permeability is 100.1046m2 and its permeability to the water is 50.1046m2 (value of the law's diffusion coefficient

  DARCY in very porous media).

  A drop of water is deposited on this plate and a very slow absorption of water can be observed for 30 minutes. The water intake after 30 s is then 0.3% of the mass of water that can be incorporated. Example 2: (according to the invention) permeable hydrophilic plate A plate is made as in ex 1 this plate is then soaked in a solution formed by a mixture of 1500g of water and 180g of ethanol in which has been previously dissolved 240g of sodium hydroxide. The solution is brought to 70 ° C. for 60 minutes with stirring. The plate is then removed, then dried at 75 ° C. in a ventilated oven to constant weight. The dry plate is then immersed in 1800 g of pure water at 23 C with stirring. After one hour, all of the wash water is changed to a new charge of pure water. This operation is repeated a total of 4 times to reach a pH of the aqueous washing solution close to the pH of

  pure water. The plate is finally dried at 75 ° C. in a ventilated oven.

  This plate is again evaluated in terms of permeability and conductivity, its density is 1.7, its conductivity is 16.6 s / cm and its permeability to nitrogen and to water remains unchanged. A drop of water is placed on this plate and is instantly absorbed by this plate, thus exhibiting a much more hydrophilic behavior than in Example 1. The uptake after 30 s is then 87% of the mass in

incorporable water.

  In Figure 1 we present the evolution of water uptake over time for the plate from the plate of Example 1 and for the plate

from example 2.

Fig 1 water recovery test j =

520 --- _- __ -

  -100 0 100 200 300 400 500 600 700 800 900 10001100 1200 1300 1400

  time (min) Example 3: (according to the invention) Hydrophilic non-permeable plate We start with a latex of KYNAR 710 from the company ATOFINA having a dry extract of 25% and a dispersion of graphite prepared according to the following procedure : The following products are used: KYNAR (3710 is a PVDF homopolymer of MVFR (Melt Volume

  Flow Rate) 10 cm3 / 10 min at 230 C under 5 kg.

  A Kynar 710 latex (dry extract = 20%), a carbon element, Graphite a

  antifoam (Byk 019), a surfactant (Coadis 1 23K).

  A graphite dispersion is prepared by mixing the water, the surfactant, the antifoam and then after dissolution, the filler is added with vigorous stirring, aiming for a high dry extract (50%). The latex is added with moderate stirring and then it is made up with water to obtain an easily pumpable liquid dispersion (ES at 20%). The composition of a graphite dispersion is given below: 973 g of water, 0.5 g of antifoam, 129.8 g of surfactant (moderate stirring) then add 1040g of Graphite. Shake vigorously until a liquid solution does not settle. Add with stirring

  moderate 780 g of PVDF KYNAR 710 Latex and 3475.1 g of water.

  The PVDF latex graphite I dispersion mixture thus prepared is then pumped with moderate stirring and then coatomized using the following operating conditions: Coatomizer inlet temperature: 170 C Coatomizer outlet temperature: 60 C Total flow rate: 17 kg / h The co-atomisation on MINOR PRODUCTION of the company NIRO (I) of PVDF latex particles and graphite particles allows the preparation of 1000 gr of microcomposite powder with a composition of 87% by mass of graphite and 13% by mass of Kynar (3,710. This powder is pressed at 1T / cm2

  at 185 C to prepare a plate with a density of 2050 kg / cm3.

  This plate is then soaked in a solution formed by a mixture of 1500g of water and 180g of ethanol in which 240g of sodium hydroxide has previously been dissolved. The solution is brought to 70 ° C. for 60 minutes with stirring. The plate is then removed, then dried at 75 ° C. in a ventilated oven to constant weight. The dry plate is then immersed in 1800 g of pure water at 23 C with stirring. After an hour, the

  all of the wash water is changed with a new charge of pure water.

  This operation is repeated a total of 4 times to reach a pH of the aqueous washing solution close to the pH of pure water. The plaque is finally

  dried at 75 C in a ventilated oven.

Claims (8)

  1 Method for rendering a plate hydrophilic essentially comprising a fluoropolymer and an electroconductive product in which: A the plate is brought into contact with a basic compound in solution, A the plate and the basic compound in solution are separated, A the plate is rinsed with water or a solvent then optionally dried. 2 The method of claim 1 wherein the electroconductive product is chosen from carbon black, carbon fibers,   carbon nanotubes and graphite.   3 Method according to any one of the preceding claims   in which the fluoropolymer is the homopolymer or copolymer PVDF   containing at least 60% by weight of VF2.   4 Method according to any one of the preceding claims   in which the proportion of fluoropolymer is from 10 to 40% by weight for   respectively 90 to 60% of electroconductive product.   Process according to Claim 4, in which the proportion of fluoropolymer is from 15 to 30% by weight for respectively 85 to 70% of electroconductive product.   6 Method according to any one of the preceding claims   in which the plate which is brought into contact with the basic compound has a   density ranging from 500 to 2170 kg / m3.   7 Method according to any one of the preceding claims   in which the bending module of the plate which is brought into contact with the   basic compound has a flexural modulus between 1000 and 12000 MPa.   8 A method according to any of the preceding claims   in which the basic compound is in solution in a miscible mixture water and alcohol.   9 The method of claim 8 wherein the solution contains between 5 and 20% by weight of alcohol.   Method according to any one of the preceding claims   in which the contact time of the plate with the basic compound is between 5 min and 2 hours.   11 Plates obtained by the process of any one of