Compositions – Electrically conductive or emissive compositions – Elemental carbon containing
Reexamination Certificate
2001-11-20
2004-12-14
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Elemental carbon containing
C252S502000, C252S518100, C428S407000, C428S402000, C429S210000, C429S212000, C429S316000, C427S115000, C427S222000
Reexamination Certificate
active
06830710
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a microcomposite powder based on an electrical conductor (for example carbon) and a fluoropolymer, as well as to objects manufactured with this powder. More specifically, the microcomposite powder consists of particles of the order of 0.1 &mgr;m to 0.5 &mgr;m in size, made of fluoropolymer, which is advantageously PVDF, fixed to an electrically conductive element of the order of 1 &mgr;m to 300 &mgr;m in size which is advantageously graphite, a carbon black aggregate, a carbon fibre, an active charcoal or carbon nanotubes. This powder may be obtained by co-atomization.
It is particularly useful for producing bipolar plates used in fuel cells. Fuel cells, which consist of these bipolar plates, are subject to severe working conditions in terms of pressure, temperature and chemical and electrochemical attack. These bipolar plates must thus be able to withstand these various conditions and also be able to be handled easily during assembly of the fuel cell. Several types of functioning of fuel cells exist, which generate different performance specifications as regards the characteristics the bipolar plates need to have in terms of conductivity, mechanical strength and permeability.
BACKGROUND OF THE INVENTION
Patent DE 3538732 discloses an electrode made from a spreadable paste consisting of 70% to 80% by mass of carbon powder with granules 30 to 300 &mgr;m in size and from 10% to 20% by mass of a PVDF solution containing 4% to 8% PVDF in DMF (dimethylformamide) and at least 5% by mass of PTFE powder with granules 10 to 100 &mgr;m in size. The paste is spread on an aluminium substrate and then dried with an infrared lamp for ½ h to 4 h. This electrode based on PVDF and carbon is permeable to gases and liquids.
Patent application JP 08031231 A discloses a formulation based on spherical graphites, thermosetting or thermoplastic agent and carbon black such as conductive ketjenblack. The material shows good mechanical strength and can be used for moulding and calendering. This material can be used in the fuel cell field.
Patent application JP 04013287 A discloses a carbon plate which is porous in the three dimensions with a level of 60% to 80% porosity.
Patent application JP 52122276 A discloses an electrode prepared by the deposition onto a porous textile of pyrolysed anisotropic carbon, which itself is coated with an aqueous dispersion of Teflon® (PTFE) and the whole assembly is dried to form a hydrophobic porous layer.
Patent application WO 2000/25372 discloses a bipolar plate for use in the fuel cell field, which is obtained from the moulding 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% graphite and from 0% to 5% carbon black, along with cotton fibres. The use of fluoro products to improve the removal from the mould and the hydrophobicity are also disclosed therein.
Patent U.S. Pat. No. 5,268,239 discloses the preparation of a separating plate. This graphite-based plate is a mixture containing from 25% to 75% by mass of graphite and from 25% to 75% by mass of phenolic resin. This plate is then pyrolyzed at between 800° C. and 1000° C. and then treated with graphite at between 2300° C. and 3000° C. The said patent also discloses the application of a fluoropolymer film to avoid the migration of the electrolyte.
Patent application WO 2000/24075 discloses the preparation of a substrate which can be used to prepare a membrane, this substrate comprising a porous fibre matrix, characterized in that the fibres are bonded with silica and a fluoropolymer. The process is also disclosed, with, in a first stage, the dispersion of the fibres in water and then, in a second stage, the deposition of this dispersion to form a network. The fibre network is then dried and compacted. An aqueous dispersion of fluoropolymer may be introduced before or after this drying and compacting step.
Patent FR-A-2 355 381 discloses a process for preparing an electrode for fuel cells, characterized by the following steps: (i) first, the formation of an aqueous suspension of catalyst particles, with addition of a cationic surfactant, then (ii) formation of a second colloidal aqueous suspension of hydrophobic polymer, (iii) mixing of the two suspensions to form a uniform aqueous suspension of catalyst particles and of hydrophobic polymer particles. This suspension is then deposited on a conductive support and heated so as to sinter the layer of catalyst and polymer.
Patent FR-A-2 430 100 discloses a process for preparing a finely divided dry powder, characterized in that it consists of grains having a maximum size of about 5 &mgr;m. This powder comprises precatalysed carbon and a hydrophobic fluorocarbon-based polymer, for example PTFE. This powder is obtained by flocculating a co-suspension of the precatalysed carbon grains and the polymer grains.
Patent EP-A-0 948 071 discloses a method for producing an electrode for fuel cells, and also a catalytic powder prepared by mixing a fine carbon powder supporting a catalytic metal with a colloidal dispersion of a polymer. The suspension thus obtained is dried.
Patent EP-A-0 557 259 discloses the preparation of a gas diffusion electrode for an electrochemical cell. 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 is then mixed with a carbon black of acetylenic type supporting a metal catalyst and PTFE to form aggregates. These aggregates are then pressed at 20 kg/cm
2
and sintered at 340° C. for 20 minutes.
Patent EP-A-0 928 036 discloses a method for preparing a gas-permeable electrode by preparing a dispersion of carbon black particles or of carbon black supporting a catalyst, using high-shear equipment to homogenize it, such as microfluidizers, and then adding a binder to the dispersion obtained, followed by a stabilizer. This mixture is then deposited on an electrically conductive fabric, after which it is dried and sintered at 300-400° C.
Patent application WO 2000/30202 discloses a mouldable composition for preparing current-collector plates by compression moulding or injection moulding. This composition comprises a non-fluoro polymer binder; among the polymers which may be used are polyphenylene sulphides, modified polyphenylene ethers, liquid crystal polymers, polyamides, polyimides, polyesters, phenolic resins, epoxy resins and vinyl esters. Among the conductive particles are, more particularly, carbon-based particles. These carbon-based particles are present in a proportion of at least 45% by mass.
Makoto Ushida in J. Electrochem. Soc., vol.142, No. 12, December 1995 has studied the preparation of an MEA (membrane and electrode assembly), based on the formation of a colloid for optimizing the formation of a network in the layer of catalyst and for simplifying the manufacture of the MEA. The preparation by producing, for example, a mixture of perfluorosulfonate ionomer (PFSI) dissolved in ethanol receives an addition of butyl acetate (a poor solvent) to form a colloidal solution. Next, a carbon supporting platinum is mixed with a carbon coated with PTFE. This PTFE-coated carbon is prepared by mixing a carbon suspension and a PTFE suspension along with a surfactant, and the surfactant is then removed during a treatment in air at 290° C. The mixture of the two powders Pt/C and C/PTFE is added to the PFSI colloidal solution, which gives rise to crosslinking of the PFSI chains adsorbed by the carbon, this crosslinking being promoted by a treatment with ultrasound. This colloidal suspension is then spread onto a carbon paper which is pressed at 130° C. and 7.5 MPA for 1 minute.
In Journal of Applied Electrochimstry 28 (1998), pp. 277-282, Fischer studied the preparation of MEA by spraying a mixture of a slurry (suspension) of a metal catalyst, a solution o
Bonnet Anthony
Burchill Michael
Bussi Philippe
Foure Michael
Triballier Karine
Atofina
Kopec Mark
Millen White Zelano & Branigan P.C.
Vijayakumar Kallambella
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