FR2755542A1 - GAS DIFFUSION ELECTRODES BASED ON POLYETHERSULFONE AND CARBON MIXTURES - Google Patents

Abstract

A fully electrocatalytic gas diffusion electrode for electrochemical cells and a process for their manufacture are described. The electrode includes an anisotropic gas diffusion layer and a catalytic layer. The gas diffusion layer consists of a porous carbon matrix through which the carbon particles and the polyethersulfone are distributed so that the matrix is uniformly porous in the direction lateral to the gas flow and asymmetrically porous with respect to the gases in the direction of the gas flow. The catalytic layer consists of a coagulated ink suspension containing catalytic carbon particles and a thermoplastic polymer selected from polyethersulfones, poly(vinylidene fluoride) and sulfonated polysulfones and covers the small pore area of the gas diffusion layer . The gaseous diffusion layer has a thickness of between 50 and 300 µm. The catalytic layer has a thickness of between 7 mum and 50 mum and a metal catalyst charge of between 0.2 mg/cm**2 and 0.5 mg/cm**2.

Description

GAS DIFFUSION ELECTRODES BASED ON MIXTURES

POLYETHERSULFONE AND CARBON

  The present invention relates to the manufacture of gas diffusion electrodes for use in electrochemical cells (in particular cells, fuel cells) with solid polymer electrolyte, the gas diffusion electrodes comprising polyethersulfone ("PESF") mixed with carbon and to a

  platinum metal electrocatalyst.

  STATE OF THE ART Electrochemical cells of the fuel cell type are electrochemical devices in which part of the energy released by a chemical reaction is transformed directly into direct current electrical energy. The direct transformation of energy into direct current electrical energy dispenses with the obligation to convert energy into heat, thereby avoiding the efficiency limits of the Carnot cycle of conventional energy production methods. Consequently, without the limitation of the Carnot cycle, the fuel cell type electrochemical cell technology offers a potential combustion efficiency two to three times greater than that of traditional energy production devices, such as combustion engines internal. Other advantages of electrochemical cells constitute the fact that they are silent, clean (absence of air pollution) and the fact of a reduction or a complete elimination of

moving parts.

  Typically, fuel cell type electrochemical cells contain two porous electrical terminals called electrodes and an electrolyte located between them. During the operation of a typical electrochemical cell, a reducing gas passes through a gas diffusion electrode to reach

  a layer of catalyst, where it reacts to lead to two protons and two electrons.

  Protons are transported to the cathode through the electro-

  chemical. The protons are transported from the anode to the cathode via an external resistance producing electrical energy. The oxidant diffuses by permeation through the anode to combine with the electrons near a layer of cathode catalyst. Electrochemical cell reagents are classified as oxidants and reducers according to their characteristics of electron acceptor or electron donor. Oxidizers include pure oxygen, oxygen-containing gases (eg air) and halogens (eg chlorine). Reducers include hydrogen, carbon monoxide, natural gas,

  methane. ethane, formaldehyde and methanol.

  The electrolyte present in the electrochemical cell serves as an electrochemical bond between the electrodes, providing a path for the ion current in the circuit, while the electrodes, made of carbon or metal, constitute an electrical track. In addition, the electrolyte prevents the reagents from moving away from the respective electrodes where the formation of explosive mixtures can take place. The electrolyte used does not react significantly with the reagents or reaction products formed during the operation of the electrochemical cell. In addition, the electrolyte must allow the migration of the ions formed during the operation of the electrochemical cell. Examples of electrolytes that have been used are aqueous solutions of strong bases such as alkali metal hydroxides, aqueous solutions of acids such as sulfuric acid and hydrochloric acid, aqueous saline electrolytes such as water salt, molten salt electrolytes and membranes

of ion exchange polymers.

  One type of fuel cell type electrochemical cell is a polymer electrochemical cell (PEM, in English polymer electrolyte fuel cell) based on a proton exchange polymer membrane. The PEM electrochemical cell contains a solid polymer membrane, an "ion exchange membrane" which acts as an electrolyte. The ion exchange membrane is sandwiched between two "gas diffusion" electrodes, an anode and a cathode, each containing

  generally a metal catalyst on a conductive support material.

  The gas diffusion electrodes are exposed to the respective gas reactants, the reducing gas and the oxidizing gas. An electrochemical reaction takes place at each of the two junctions (three-phase interfaces) o one of the electrodes, the membrane

  polymer electrolyte and the gaseous reagent are in contact.

  For example, when oxygen is the oxidizing gas and hydrogen is the reducing gas, the anode is supplied with hydrogen and the cathode with oxygen. The overall chemical reaction of this reaction is then written: 2 H2 + 2 ----> 2 H20. The electrochemical reactions that take place at the metal catalyst sites are as follows

  anodic reaction: 2H2 ------> 4H + + 4th-

  cathodic reaction: O2 + 4H + + 4th- -----> 2 H20O During the operation of the fuel cell type electrochemical cell, the hydrogen diffuses through the anode and interacts with the metal catalyst producing electrons and protons. Electrons are conducted to the cathode via electronic conduction and through the electrically conductive material and the external circuit while simultaneously, protons are transferred to the cathode via ionic conduction through the membrane electrolyte

  polymer. At the same time, oxygen diffuses to the catalytic sites at the cathode.

  o oxygen collects electrons and reacts with protons to give water.

  Therefore, the products of the reactions of a PEM electrochemical cell are water and electricity. In a PEM electrochemical cell, the current is conducted simultaneously by ionic conduction and by electronic conduction. The efficiency of the electrochemical cell strongly depends on the ability to minimize both the

  ionic resistivity as the electronic resistivity with respect to the current.

  Gas diffusion electrodes play an important role in electrochemical cells. During the operation of the electrochemical cell, the combustible gases interact with the electrodes of the electrochemical cell and give rise to heterogeneous reactions near the catalyst sites of the electrodes. In order to carry out these reactions, the electrode catalyst must be simultaneously in contact with the conductive carbon, the electrolyte and the combustible gas. This is why the electrode must meet the following criteria: 1) low resistance to the diffusion of gas towards the reaction sites; 2) high electronic conductivity; 3) high mechanical strength allowing long-term operation;

  4) appropriate hydrophilic / hydrophobic balance; and 5) stability.

  Conventional gas diffusion electrodes for electrochemical cells consist of platinum metal on a support made of a carbon black and polymer substrate. The polymer serves as a binder for the carbon black particles and ensures physical cohesion, that is to say the mechanical resistance of the electrode. Carbon is used to minimize the electronic resistance of

  the electrode while platinum serves as a catalyst for the electrochemical reaction.

  The majority of gas diffusion electrodes for electrochemical cells use polytetrafluoroethylene ("PTFE") as a binder. This polymer has high thermal stability as well as very good chemical resistance. However, PTFE does not dissolve in any known solvent and must therefore be used in the form of a suspension. This complicates the manufacturing process of the electrode. More specifically, when PTFE is used as a polymeric binder for carbon, it is

  difficult to control the structure, porosity and size of the pores of the electrode.

  Gas diffusion electrodes for Teflon-type electrochemical cells are usually manufactured by mixing PTFE with carbon or graphite powder and pressing it into a sheet in which PTFE serves as a binder. This sheet is subjected to a heat treatment at a sintering temperature, for example 300 ° C. to 350 ° C., the temperature at which the binder partially decomposes, forming a porous matrix allowing the passage of the gas and the interaction with the carbon. U.S. Patent No. 4,847,173 describes a process for manufacturing a carbon and polymer matrix, either by mixing the PTFE in

  combination with other polymers, either with binders of different polymers.

  US Patent No. 3,899,354 describes another method of manufacturing a carbon matrix and PTFE or other polymeric binders by spraying a suspension of a mixture of PTFE and carbon on carbon paper until a thick layer which forms the matrix of the electrode and subsequent heating of the matrix to a sintering temperature as described above. Cabasso and Manassen describe in Proceedings, Int. PowerSource Symposium, 1990, another method of manufacturing electrodes for electrochemical cells. Instead of pressing or spraying the polymer binder and the carbon to obtain the matrix and sintering the matrix to obtain a gas diffusion layer, a platinum catalyst containing carbon is mixed with a solution of polyvinylidene fluoride (PVF2), spread out and then immersed in dimethylformamide, a non-solvent which precipitates PVF2. Cabasso et al. also claim that many other soluble polymers exist that withstand the conditions used in electrochemical cells, for example, low operating currents up to 200 mA / cm2, a relatively low operating temperature (25 C to 35 C) and a pressure only slightly higher than atmospheric pressure. Indeed, most polymers decompose due to the highly acidic nature of the membranes, an operation at high temperature up to 95 C and electric currents

  crossing the matrix which can reach several A / cm2.

  Cabasso et al. report two methods of making electrode arrays that contain a platinum catalyst. According to a method, the electrode matrix is produced by uniformly spreading a solution containing a mixture of

  platinum, carbon, PVF2 and solvent catalyst on a glass plate.

  This has the effect of uniformly distributing the platinum catalyst throughout the matrix of the electrode. According to the other method, a solution of a mixture of carbon, platinum catalyst, polymer and solvent is spread on a glass plate, then a graphite veil is carefully placed over the film of the mixture and . above it is spread a mixture of polymer and carbon without the platinum catalyst. The whole is immersed in water and has the structure of three layers, carbon, catalyst. polymer bonded to carbon on one side

  and on the other side a layer of carbon and polymer.

  During the last decades, PTFE has been generally used in research as a binder for carbon substrates in gas diffusion electrodes (Teflon_ type electrode) and we have focused on the maximum amount of catalyst used in the electrodes. The operation of platinum on carbon / electrocatalyst-PTFE mixed with a mixture of carbon as a constituent of a gas diffusion electrode in the electrochemical cell 1-12 / O2 is well known. Platinum on carbon-PTFE mixtures have generally been produced by mixing platinum black, or platinum on carbon (intimately mixed) with an aqueous colloidal dispersion, negatively charged and hydrophobic of PTFE particles and by spreading this mixture on a substrate of carbon veil (Report no AFML-TR-77-68). Thin, porous, waterproof carbon paper has also been used in the manufacture of a gas diffusion electrode as a substrate instead of carbon veil, as described in U.S. Patent 3,912,538. This electrode overcame the submersion problem

  ("flooding") during the operation of the electrochemical cell.

  Several techniques have been developed in order to increase the use of platinum catalyst. Methods for dividing the amount of catalyst by ten using an improved electrode structure have been developed by the laboratories of Los Alamos, (Gothesfield et al., J. Appl. Electrochemistry, 22 (1992), p. 1) Los Alamos, New Mexico and Texas A&M University, College Station, Texas, based on Prototech electrodes (US Patent 4,826,742). According to their methods, Pt is deposited on electrodes manufactured by Prototech, charged with 0.4 mg / cm2 of Pt, by sputtering in order to obtain a thin layer of Pt (0.05 mg / cm2) on the surface. front of the electrodes. The electrochemical cells assembled with these electrodes and a Nafion 112 membrane have 1 A / cm2 at 0.5 V using H2 / O2 as gaseous reagents and no significant loss of performance is observed even after 50 days of operation. Gothesfield et al. describe a process for reducing the platinum load to 0.15 mg / cm2. This process involves painting the PTFE membrane with an ink composed of solvents

  organic, Pt-C and Nafion solution.

  In order to present good performance, a cell electrode

  electrochemical must have an appropriate morphology and distribution.

  The electrochemical cell electrode requires a porous structure which provides a free transport path for the gas permeate and distributes the gas permeate over the entire area of electrode catalyst. The efficiency of fuel gas distribution over the electrode catalyst is a function of the porosity of

  the electrode, an essential parameter for determining the efficiency of the electrode.

  Therefore. the object of the present invention is the easy manufacture of low-cost gas diffusion electrodes having advantageous chemical and electrical properties for electrochemical cells and for other

electrochemical applications.

  Another object of the present invention is to provide a diffusion electrode

  gas with an electrode structure, porosity, and controlled pore size.

  An object of the present invention is to provide a process for the manufacture of gas diffusion electrodes with controlled porosity and pore size through the use of a mixture of activated carbon and polyethersulfone dissolved in an organic solvent, that the Then allowed to coagulate, by phase inversion, in a non-solvent of the mixture, at low temperature, in the form of a porous membrane. Another object of the present invention is to provide a method of manufacturing a gas diffusion electrode in which the gas diffusion layer and the catalyst layer are manufactured separately, thus allowing a formulation of each structure such that one obtains the most suitable properties for their

function.

  Yet another object of the present invention is to provide a method of manufacturing simple gas diffusion electrodes using the technique

  single stage phase inversion.

  Summary of the Invention The objectives and criteria for the gas diffusion electrodes mentioned above as well as their manufacture can be achieved by the present invention. According to one aspect, the invention relates to an electrocatalytic gas diffusion electrode for electrochemical cells of the fuel cell type, comprising: an anisotropic gas diffusion layer made from a porous carbon matrix in which the particles of carbon and polyethersulfone are distributed in such a way that the matrix is uniformly porous in the direction lateral to the gas flow and asymmetrically porous in the direction of the gas flow, the porosity of the gas diffusion layer decreasing according to the direction of the gas flow, the layer of gas diffusion having a thickness between 50 im and 300 gim, and a catalytic layer made up of a suspension of coagulated "ink" containing catalytic particles of carbon and a thermoplastic polymer. the

  catalytic layer covering the small pore surface of said diffusion layer.

  said catalytic layer having a thickness between 7 gLm and 50 ptm and a

  load of metallic catalyst of between 0.2 mg / cm2 and 0.5 mg / cm2.

  According to one embodiment of the invention, said catalytic layer contains from 5 to 25% by weight of said polyethersulfone polymer. the rest being carbon particles. According to a second embodiment, said diffusion layer has a

  polymer: carbon ratio between 20:80 and 45:65.

  Said gas diffusion layer may further comprise poly (vinylpyrrolidone). According to a preferred embodiment, the carbon particles are chosen from the group composed of: active carbon, carbon black and acetylene black and their mixtures, the carbon particles having a B.E.T. between 50

m2 / g and 2000 m2 / g.

  The thermoplastic polymer in the catalytic layer can be chosen from the group composed of PVF2, sulfonated polysulfones, sulfonated polyethersulfones, and

sulfonated poly (phenylene oxide).

  The catalytic carbon particles can comprise particles of metallic catalyst adhering to support carbon particles, with a high active surface, the support carbon particles having a B.E.T. between

m2 / g and 2000 m2 / g.

  Said catalytic metal particles can comprise noble metal particles uniformly deposited on said support carbon particles, said noble metal being chosen from the group composed of platinum, palladium, rhodium and iridium, and being present in an amount between 10 and

  % by weight relative to the supporting carbon particles.

  The thickness of said gas diffusion layer can be between 75

gm and 150 ium.

  The thickness of said catalytic layer can be between 7 PLm and 10

  ptm, and the load of platinum catalyst can be 0.15 mg / cm2 and 0.5 mg / cm2.

  The catalytic layer can consist of platinum alloys mixed with - 30% PESF and 70 - 95% carbon particles. The invention also relates to an electrochemical cell, in particular a battery with

  combustible, containing such an electrode.

  According to another aspect, this invention relates to a method of manufacturing gas diffusion electrodes suitable for use in electrochemical cells, this method comprising the following steps: a. fabricating an anisotropic gas diffusion layer consisting of a porous carbon matrix in which the particles of carbon and polyethersulfone are distributed in such a way that the matrix is uniformly porous in the direction lateral to the gas flow and asymmetrically porous in the direction of the flow gaseous. the porosity of said gas diffusion layer decreasing in the direction of the gas flow, the gas diffusion layer having a thickness of between 50 [tm and 300 ttm, the gas diffusion layer being produced 1) by spreading on a substrate of carbon a mixture of polyethersulfone and carbon particles dissolved in a solvent for the polyethersulfone and the carbon particles using a doctor blade in order to form a film layer on the carbon substrate, the mixture penetrating at least one part of the carbon substrate; 2) by allowing the film to coagulate in a coagulation liquid which is a non-solvent for the polyethersulfone and the carbon particles; and 3) removing the coagulation liquid; and B. painting on the small pore surface of the gas diffusion layer a catalytic layer consisting of an aqueous suspension ink containing catalytic carbon particles and a thermoplastic polymer, said suspension comprising 0.5 to 2% of thermoplastic polymer, said polymer thermoplastic being selected from a group composed of polyethersulfone, poly (vinylidene fluoride) and sulfonated polysulfones, the catalytic layer covering the small pore surface of said gas diffusion layer, said catalytic layer having a thickness of between 7 gLm and 50 μm and a metal catalyst charge

  between 0.2 mg / cm2 and 0.5 mg / cm2.

  According to one embodiment, in step (a) (l), said gas diffusion layer is made from a solution comprising 5% by weight of PESF

in N, N'-dimethylformamide.

  According to yet another embodiment, in step (a) (1), the mixture of polyethersulfone and carbon particles dissolved in a solvent for the polyethersulfone is subjected to sonication for a sufficient time to mix

  homogeneously the polyethersulfone with the carbon particles.

  In step (a) (l), the solvent for the polyethersulfone can be chosen from

  a group composed of N, N'-dimethylformamide, N, N-dimethylacetamide, N-

  methylpyrrolidone and dimethyl sulfoxide.

  In step (a) (2), said gas diffusion layer can be manufactured at

  starting from a coagulation liquid chosen from a group composed of water, hexane.

  ethanol, eat / NN'-dimethyl-formamide, water / ethanol, water / methanol, water / isopropanol.

  tetrahydro-furan, and mixtures thereof.

  In step (b) said coagulation liquid can have a temperature

  between room temperature and -30 C.

  In step (b), said catalytic layer can comprise a non-surfactant

ionic.

  In step (b). painting can be done with painting technique

by sprayer.

  The manufacturing method according to the invention may also include step (c) of sintering the finished electrode between 200 C and 300 C for a period of between

15 minutes and 2 hours.

  Other goals, characteristics and advantages will become apparent from reading the

  description of the various embodiments of the invention, given without

  limiting and with reference to the appended figures, in which: FIG. I is a photograph by scanning electron microscopy (SEM) of a cross section of a PESF-carbon gas diffusion electrode

  according to the present invention (a) 200X magnification, (b) 500X magnification.

  Figure 2 is a scanning electron micrograph of a cross section of a PESFcarbon gas diffusion electrode according to the present invention (a) 400X magnification, (b) Mapping of Pt by X-rays. Figure 3 is a graph of the cell potential (cell voltage (V)) as a function of the current density (A / cm2) for an electrochemical cell assembled according to the invention, containing a gas diffusion electrode manufactured

  according to Example 1 with Nafion 112 membranes at 80 C, under 30 psig (207 kPa).

  FIG. 4 is a graph of the cell potential (cell voltage (V)) as a function of the current density (A / cm 2) for an electrochemical cell assembled according to the invention, containing a gas diffusion electrode manufactured according to Example I with Nafion 112 membranes tested at different hydrogen-oxygen gas pressures: (o) 10 psig (69kPa) (.) 20 psig (138kPa); and (V) 30

psig (209 kPa).

  The polymer material simultaneously has several functions in the gas diffusion electrode of the electrochemical cell. It acts as a binder to ensure the

  cohesion of the catalyst to carbon and the integrity of the electrode and makes it hydrophobic.

  The platinum metal (Pt) catalyst works best in an electrode if

  it is simultaneously in contact with carbon, electrolyte, and gaseous reactants.

  In order to allow a significant use of platinum, a low ohmic loss and to prevent immersion, the matrix of the electrode is constructed in order to satisfy these conditions. The structure is manufactured in such a way that the ionic and electronic conduction paths are short and have a minimum of sinuosities, while the catalyst and exposed to the maximum and the reactive gases are used without drowning and without condensation. In view of the fact that platinum is an expensive catalyst, it will be used in minimum quantity with maximum efficiency. For this purpose, it has been

  highlighted as the location of the Pt near the surface of the electrode.

  adjacent to the gaseous reagents, leads to the most advantageous electrode performance. The Bacon dual layered structure model (see G.B. Patent No. 667,298) has been widely accepted. It has an asymmetric anisotropic structure with on the gas side a layer of open pores on the sides and on the electrolyte side the other relatively small and fine pores. The former can facilitate the transport of gas, the latter will be filled with electrolyte and thus prevent subsequent diffusion

  gas which would cause crossover problems.

  The applicants have discovered that an inexpensive thermoplastic polymer, polyethersulfone, can be used to form a mixture of polyethersulfone and

  carbon particles which is suitable as an electrode matrix material.

  Polyethersulfone is a hydrophobic amorphous polymer with a high glass transition temperature. It resists an oxidizing and reducing environment. In addition, it has good durability and a workability at low pH. Polyethersulfone has been shown to be an excellent building block for various porous membranes for gas separation (I. Cabasso in "Encyclopedia Polymer Science and Engineering", second edition, John Wiley & Sons, Inc. 9, 509 (1987) ) and ultrafiltration. The polyethersulfones suitable according to the present invention have an average molecular weight of between 25,000 and 100,000 and include those having the following monomers: and according to the present invention, when the polyethersulfone is mixed with carbon particles in a weight ratio of polymer with carbon between: 80 and 45:65, the polyethersulfone acts by itself as an excellent binder for the carbon particles present in the mixture. Therefore, polyethersulfone can be successfully used to replace more expensive PTFE polymers as a binder and matrix adjuvant for gas diffusion electrodes. The polyethersulfone polymer in the blend provides an electrode structure having properties necessary for making electrodes of electrochemical cells

High quality. The gas diffusion electrode according to the invention is prepared according to a

  proceeded in two stages. The first step uses the phase inversion method to manufacture an anisotropic gas diffusion layer according to the invention. having a thickness greater than 50 μm, preferably greater than 75 μm and less than 300 g.m and preferably below 150 μm. The phase inversion method includes the following sequence of steps: 1) spread using a doctor blade on a carbon substrate or a glass plate a mixture of polyethersulfone and carbon particles dissolved in a solvent to polyethersulfone, forming a film layer on the carbon substrate; 2) coagulating the film in a coagulation liquid which is a non-solvent for polyethersulfone; and 3) dry the film so

  to remove the coagulation fluid.

  The second step is to make a layer of catalyst using

  a sprayer to paint a layer of "polymer-carbon" ink

  catalyst on the carbon-polymer gas diffusion layer, the catalytic "ink" having a thickness greater than 7 µm, and less than 50 lim, preferably less than 10 lim. The ratio of the metal to carbon catalyst to the "ink" is between 25:75 and 40:60 by weight. The electrode according to this invention has a higher porosity in the gas diffusion layer, a lower catalyst charge and a higher catalyst use. The electrochemical cell

  assembled with this electrode has high performance.

  The carbon substrate is a fibrous or porous sheet having a thickness greater than 7 gm, preferably greater than 10 µm and less than 35 ltm, preferably less than 25 gm. Suitable carbon substrates include carbon paper, conductive carbon fabric, highly carbon felt

  conductor, carbon tape and the like.

  The particulate carbon can be, for example, carbon black with an active surface of about 50 to 2000 m2 / g, determined with the B.E.T. A suitable particulate carbon includes activated carbon or carbon black, i.e. very finely divided carbon powder. When measured by the B.E.T. method, the commercial carbon powders useful in this invention have an active surface of between 50 m2 / g and 2000 m2 / g. Such powders include furnace blacks, smoke blacks, acetylene blacks, tunnel blacks, inactive soot. Furnace blacks having an active surface of between m2 / g and 600 m2 / g are preferred. The particle size of these active carbon materials can be in the range of 5 to 1000 nanometers, but are of

  preferably of average size less than 300 nanometers.

  The B.E.T. refers to the method of determining the area

active of Brunaver-Emmett-Teller.

  The term "carbon black" is used as defined in U.S. Patent n

4,440,167 to Solomon.

  Commercial carbon blacks having a B.E.T. between 50 and 300 m2 / g of commerce can be activated by steam if desired, with the effect of increasing their active surface and thus increasing their B.E.T. that can

reach up to 600 m2 / g.

  The surface characteristics of carbon blacks can be variable. Some of these carbon blacks have surface functionalities, i.e., carboxyl groups (and other types carrying oxygen) or groups containing fluorine on the surface. The physico-chemical characteristics and the ash content are also variable. In addition, carbon blacks can be graphitized (carbon black powders thus acquire some of the structural characteristics of graphite) or graphitized and then treated to restore or increase the

surface functionality.

  Preferred commercial carbon blacks include BLACKs

  PEARLS, i.e. BLACK PEARL 2000, VULCAN (e.g. Vulcan VX-

  72), KETJEN BLACK EC 300J (Akzo Chemie Americo of Burt, New York), activated charcoal, acetylene black C-100, or mixtures thereof. The available KETJEN BLACK materials are oil stove blacks with a B.E.T. in the range of 900 up to around 1000 m2 / g, and the EC 300J in particular seems to have an active surface of 950 m2 / g. The KETJEN BLACK EC 300J contains a significant proportion of mesophasic carbon and therefore presents regions with long-range order. These regions can make the carbon more resistant to corrosion, which is important for cathodic applications. In accordance with U.S. Patent No. 4,461,814 to Klinedienst. KETJEN BLACK oil furnace blacks have both a high active surface (greater than 900 m2 / g) and high absorption indices of dibutyl diphtalate ("DBP"). Klinedienst describes that when the absorption index is determined by the ASTM D-2414-70 test, the adsorption index will preferably be greater than 125 cm3 per g of carbon black (for example greater than 230 cm3 / 100 g) and the active surface will be greater than 250 m2 / g. to provide carbon black for a cathode collector with optimal characteristics. The DBP adsorption index for KETJEN BLACK is reported by Klinedienst equal to 340 cm3 / 100 g. Acetylene blacks

  often have high adsorption indices but weak active surfaces.

  In contrast, carbon blacks from Lurgi (from Lurgi Umivett and Chemoteclmnik GmbH) may have B.E.T. very high (greater than 1200 m2, g) and a low absorption index (less than 100 cm3 / 10Og). It has also been reported that

  "CSX" carbon blacks (sold by Cabot Corporation of Billerica.

  MA) have B.E.T. high and high DBP absorption indices.

  Solvents suitable for blending polyethersulfone and carbon include those selected from the group consisting of N.N'-dimethylformamide ("DMF"), N, N-dimethyl acetamide. N-methylpyrrolidone and dimethyl sulfoxide. The amount of solvent required to dissolve the polyethersulfone is variable depending on the solvent. For exemple. 10% by weight of polyethersulfone are dissolved in

DMF.

  Suitable coagulation liquids or solvents which are polyethersulfone nonsolvents and carbon particles are those selected from the group.

  composed of water, isopropanol and hexane, and mixtures of water and isopropanol.

  Porous materials such as Vulcan XC-72, Acetylene Black C-100 and Black Pearl 2000 can be used to make gas diffusion electrodes according to the process of depositors without leading to problems of water immersion (flooding ) generally encountered when such carbons are used in electrochemical cells. Such carbons absorb liquids in such immense quantities that one can expect immersion of the gas electrodes if the carbon is spread in a mixture. If low active area carbons such as Vulcan XC-72 or the like are used, they do not absorb much liquid and too much liquid is required to produce a composition which can be spread into a film. Therefore, even a film a few hundred micrometers thick thus produced does not contain enough active carbon material for the electrode. In addition, carbon materials such as coal, wood, have a high electrical resistance and due to their size, they form a very porous matrix which cannot tolerate the high pressures to which the assembled electro-chemical cells are exposed, for example pressures between 20 psi

(138kPa) and 100 psi (690kPa).

  Therefore, charcoal was not used in the manufacture

  electrodes for electrochemical cells.

  Applicants have unexpectedly discovered that the flooding problem can be overcome and that such materials can be spread in a solvent using high frequency sonication. Thus, in order to overcome the problem of the immense absorption of liquids by carbon materials such as the Vulcan CX-72, which are usually used in the manufacture of electrodes for electrochemical cells. an organic solvent (DMF) and the polyethersulfone are treated together with the carbon material to obtain a suspension which is thoroughly mixed using a sonicator. The high frequency sonication leads to a paste which can be spread on a carbon veil substrate in the desired thickness. The applicants assume that the sonication prevents the carbon from absorbing enough liquid to interfere with the formation of an electrode using the spreading step. The applicants have found that when a mixture of polyethersulfone, platinum catalyst and carbon material is subjected to sonication. a paste is obtained which can be spread in much smaller thicknesses, with reduced solvent interference during spreading. Consequently, the invention of the applicants allows the spreading of carbons

  which are common for electrochemical cells.

  The formation of good gas diffusion electrodes requires the homogeneous diffusion of the gas reactants within the matrix of the gas diffusion electrode. Gases are fluids and behave like fluids that flow along the path of least resistance. In the electrochemical cell, the gaseous reactants flow to the catalyst layer where they are consumed. A problem in the devices of electrochemical cells, and in particular of electrodes, is the homogeneity of the channels. If the electrode matrix is denser in one region and less dense in another, the gas flow will be directed to the

  less dense region. As a result, the catalyst will not be used completely.

  The depositing gas diffusion electrode has an electrode matrix which is laterally homogeneous and asymmetrical according to the direction of the gas flow. This means that upon entering the electrode, the gases penetrate the gas diffusion electrode through the surface which is "open", less resistant, and as the gases diffuse, the matrix of the electrode becomes progressively denser and its pores smaller. Therefore, the electrode array of the present invention has an anisotropic porous structure with two surface layers

  asymmetrical, as seen in Figure 1.

  The applicants have also found that when a mixture such as PVF2.

  carbon material and a platinum catalyst is not sonicated and is spread as a solution on a glass substrate as described in the publication by Cabasso et al. from 1990. double density surfaces are

  form due to the nature of the interaction between the glass and the polymer-carbon mixture.

  Contrary to all expectations, the applicants have found that the dough subjected to sonication must be spread on a carbon cloth or carbon paper to ensure the anisotropic structure of the electrode which facilitates the entry of the diffused gases. The dough subjected to sonication, when spread on a carbon cloth which is

  then submerged in water ensures the anisotropic structure.

  The diffusion and distribution of gases in the matrix are important with regard to the performance of the electrodes. The calculation of the gaseous layer on the carbon fabric has been studied intensively. The depositors also discovered

  only when the spread dough is coagulated in a coagulation liquid which is a non

  solvent for dough at lower temperatures. we get an electrode of

  gas diffusion of much higher quality and an anisotropic porosity structure.

homogeneous in lateral direction.

  The appropriate coagulation bath temperatures can be between ambient and -30 C. When the liquid comprises the mixture of water and an alcohol, temperatures below 0 C and above -20 C are preferably used . When the coagulation liquid is water, temperatures

  S between 25 C and 4 C are preferably used.

  Suitable coagulation liquids, which are non-solvents for the spread paste, are aqueous solutions from a mixture of water and an alcohol and / or

  water with an inorganic salt in volume ratios between 99: 1 and 1:99.

  Preferably, water is used as the coagulation liquid. When the coagulation liquid is a mixture, a mixture of water and alcohol or water and salt is preferred in a volume ratio of between 90:10 and 10:90. Suitable alcohols include ethanol, isopropanol and methanol. Suitable salts include

LiCI, LiNO3 and NaNO3.

  As indicated above, a suitable thermoplastic polymer in the catalytic layer is chosen from the group consisting of polyethersulfone, poly (vinylidene fluoride) and sulfonated polysulfone. Thermoplastic polymers also suitable for the catalytic layer are the sulfonated polyethersulfones.

  and sulfonated poly (phenylene oxide).

  Of course, the present invention is not limited to the embodiments described and shown, but it is susceptible of numerous variants accessible to

  a person skilled in the art without departing from the spirit of the invention.

Example 1

  Gas diffusion electrodes were fabricated using high surface active carbon black (DP-5,200) (200 m2) (marketed as Vulcan VC-72R from Cabot, Inc.) and polyethersulfone using of the wet phase inversion technique. The carbon black is dispersed in a 12-15% by weight solution of polyethersulfone and N.N'-dimethylformamide for

form a suspension.

  The suspension is thoroughly mixed for 10 minutes using a sonicator, and a paste is obtained. Using a doctor blade, the paste is spread on a hydrophobic carbon fabric substrate with a thickness of 0.015 inches (0.381 mm) (marketed under the name of Panex PWB-3 from Zoltek) until a layer of film is obtained. '' a thickness of 50 gm on the substrate. Particular attention is paid to the fact that the dough penetrates the fabric at least partially. This film is then immersed in a bath of demineralized water in order to coagulate the film. The coagulated film is rinsed thoroughly with deionized water and placed in a dry box to dry for at least 24 hours. The dried film constitutes the gas diffusion layer of a gas diffusion electrode and has pores of I (6 small size (see SEM photomicrograph) on the surface. This gas diffusion layer of the electrode is then heated to 250 ° C. for 1 h. An "ink" of an aqueous suspension for the catalyst layer is produced as follows: 0.06 g of polyvinylidene fluoride (PFV2) is suspended in 4 g of 2-propanol and 6 g of water using a sonicator. Then 0.05 g of a nonionic surfactant (Triton - X-100) and 0.3 g of 20% by weight of Pt on carbon black Vulcan VX-72 are added to the colloidal solution of PFV2. The mixture is mixed again by an ultrasonic device in order to form the final "ink" suspension. Then, an art sprayer is used to paint the ink "evenly on the surface of the gas diffusion electrode. The painting step involves applying 7.0 g of the ink suspension at 88 c m2 of the gas diffusion layer. the electrode obtained has a platinum charge of 0.35 mg / cm2

  with a catalytic layer with a thickness of 20 glm, as shown in Figure 2.

  The size of the platinum particles is in the range of 20 A to 40 A. The electrode is then heated to 300 C for at least 2 h. The gas diffusion electrode manufactured in this way is evaluated in an H2 / O2 electrochemical cell. The catalyst side of the electrode is brushed with a 0.5% by weight solution of protonated Nafion 117 and hot pressed against a Nafion 112 or Nafion 117 membrane. With an open cell, a voltage of 1 V is measured. The figure 3 shows the curve of the polarization of an electrochemical cell using a gas diffusion electrode manufactured according to example 1 and a Nafion 112 membrane at different pressures of the reagents. At 0.5 V, a current density of I A / cm2 can be obtained, highlighting the good performance of

this electrode.

  Example 2 The process of Example 1 is repeated, except that the gas diffusion layer

  is produced by dry phase inversion.

  The polyethersulfone and the acetylene black carbon C-100 are dissolved in DMF to form a paste. The paste is spread on a carbon fabric substrate and then air dried, allowing the solvent to completely evaporate and forming a spread layer of film. The film is then pressed between two rollers at room temperature to lead to the gas diffusion layer. An electrochemical cell made with this electrode and a Nafion 112 membrane has a cell voltage

  open at 1.0 V and at 0.5 V, the current density is 800 mA / cm2.

  Example 3 The process of Example 1 is repeated, except that for the manufacture of the gas diffusion layer. a mixture of water and DMF is used as a coagulation solvent. Sulfonated polyethersulfone is used in place of PVF2 as the polymer in the catalyst layer suspension ink; an electrochemical cell made using this electrode and a Nafion 112 membrane has a higher platinum catalyst use than 35% At 0.5 V, a density of

  current of 2 A / cm2 is measured for the electrochemical cell.

  Example 4 0.5 g of platinum on activated charcoal (10% by weight of platinum, Fluka Chemical, Inc.) is suspended in 1.6 g of DMF and mixed with 1.6 g of a PESF solution at 15% by weight in DMF using a sonicator. This suspension is then spread on a carbon canvas substrate using a doctor blade to form a film layer. The film is submerged in a demineralized water bath for seconds to coagulate. The coagulated film is removed from the bath, rinsed thoroughly and placed for 24 h in a dry box to dry. The platinum charge of the catalytic layer is 0.5 mg / cm2. The thickness of the gas diffusion electrode formed is 150 μm. The electrochemical cell manufactured using this gas diffusion electrode and a Nafion 117 membrane with surface resistance ("over resistance") of 0.69 f2 / cm2 and a Tafel slope of 110 mV / decade. The density

  maximum current is 800 mA / cm2.

Example 5

  The process of Example 4 is repeated, except for the addition of a black of

  highly hydrophobic carbon with low active surface of 60 m2 / g (range of 5-

  %), acetylene black C-100 (Chevron Chemical Co.). The concentration of

  platinum surface in the catalyst layer in this example is 0.1 mg / cm2.

  When the amount of the second carbon is increased from 0 to 10%, the density of

  maximum current increases from 800 A / cm2 to I A / cm2.

  Example 6 The process of Example 4 is repeated, except that poly (vinypyrrolidone) PVP is used as a pore filter in order to control the porosity of the gas diffusion layer and obtain an open pore structure. The PVP is mixed with the polymer solution before spreading the gas diffusion layer. The PVP is then removed by rinsing the electrode with water for three days. The overall cell voltage of an electrochemical cell manufactured with this gas diffusion electrode and

  a Nafion 117 membrane increases by around 200 mV.

Claims (17)

  1.- A gas diffusion electrode for electrochemical cells.   comprising: an anisotropic gas diffusion layer made from a porous carbon matrix in which the carbon and polyethersulfone particles are distributed in such a way that the matrix is uniformly porous in the direction lateral to the gas flow and asymmetrically porous according to the direction of the gas flow, the porosity of the gas diffusion layer decreasing according to the direction of the gas flow, the gas diffusion layer having a thickness of between 50 gim and 300 μm, and a catalytic layer consisting of a suspension of " coagulated ink containing catalytic carbon particles and a thermoplastic polymer, the catalytic layer covering the small pore surface of said gas diffusion layer, said catalytic layer having a thickness of between 7 μm and 50 μm and a charge of metallic catalyst between 0.2 mg / cm2 and 0.5mg / cm2 2.- The electrode according to claim 1. wherein said catalytic layer contains from 5 to 25% by weight of said polyethersulfone polymer, the remainder being carbon particles.   3. The electrode according to claim I or 2. wherein said layer of   diffusion has a polymer: carbon ratio between 20:80 and 45:65.   4.- The electrode according to any one of claims 1 to 3, in which   said carbon particles are chosen from the group consisting of: active carbon, carbon black and acetylene black and their mixtures, the carbon particles   having a B.E.T. between 50 m2 / g and 2000 m2 / g.   5.- The electrode according to any one of claims I to 4, in which   said gas diffusion layer further comprises poly (vinylpyrrolidone).   6.- The electrode according to any one of claims 1 to 5, in which   said thermoplastic polymer in the catalytic layer is chosen from the group consisting of PVF2, sulfonated polysulfones, sulfonated polyethersulfones. and sulfonated poly (phenylene oxide).   7.- The electrode according to any one of claims I to 6, in which   the catalytic carbon particles comprise particles of metallic catalyst adhering to support carbon particles, with a high active surface, the support carbon particles having a B.E.T. between 200 m2 / g and 2000 m2 / g. 8. The electrode according to claim 7, in which said particles of catalytic metal comprise particles of noble metal uniformly deposited on said particles of support carbon, said noble metal being chosen from the group composed of platinum, palladium, rhodium and iridium, and being present in an amount between 10 and 20% by weight relative to the particles of carbon support.   9. The electrode according to any one of claims 1 to 8, in which   the thickness of said gas diffusion layer is between 75 lm and 150 lm.   10.- The electrode according to any one of claims I to 9, in which   the thickness of said catalytic layer is between 7 Lm and 10 lm, and the   load of platinum catalyst is 0.15 mg / cm2 and 0.5 mg / cm2.   11.- The electrode according to any one of claims I to 10, in which   said catalytic layer consists of platinum alloys mixed with 5 - 30% of   PESF and 70 - 95% carbon particles.   12.- A method of manufacturing a gas diffusion electrode according to one   any one of claims I to 11, the method comprising the following steps:   at. manufacture of the gas diffusion layer l) by spreading on a carbon substrate a mixture of polyethersulfone and carbon particles dissolved in a solvent for the polyethersulfone and the carbon particles using a doctor blade in order to form a layer film on the carbon substrate, the mixture penetrating at least part of the carbon substrate; 2) by coagulating the film in a coagulation liquid which is a non-solvent for the polyethersulfone and the carbon particles; and 3) removing the coagulation liquid; and B. paint on the small pore surface of the gas diffusion layer the catalytic layer consisting of an aqueous suspension ink containing catalytic carbon particles and a thermoplastic polymer. said suspension   comprising 0.5 to 2% of thermoplastic polymer.   13.- The method according to claim 12. wherein in step (a) (1).   said gas diffusion layer is made from a solution comprising -25% by weight of PESF in N, N'-dimethylformamide. 14.- The method according to claim 12 or 13, wherein in step (a) (1), the mixture of polyethersulfone and carbon particles dissolved in a solvent for the polyethersulfone is subjected to sonication for a sufficient time for homogeneously mix the polyethersulfone with the particles of carbon.   15.- The method according to claim 12, 13 or 14, wherein in step (a) (1), the solvent for the polyethersulfone is chosen from a group composed of N, N'-dimethylformamide, N, N- dimethylacetamide, Nmethylpyrrolidone and dimethvl sulfoxide.   16.- The method according to any one of claims 12 to 15, wherein   in step (a) (2), said gas diffusion layer is made from a   coagulation liquid chosen from a group composed of water, hexane, ethanol.   water / N, N'-dimethyl-formamide, water / ethanol, water / methanol, water / isopropanol.   tetrahydro-furan, and mixtures thereof.   17.- The method according to any one of claims 12 to 16 in which.   in step (a) (2), said coagulation liquid has a temperature between room temperature and -30 C.   18.- The method according to any one of claims 12 to 17, wherein   in step (b), said catalytic layer comprises a nonionic surfactant.   19.- The method according to any one of claims 12 to 18, comprising   further step (c) of sintering the finished electrode between 200 C and 300 C for a   duration between 15 minutes and 2 hours.   20.- The method according to any one of claims 12 to 19. wherein   in step (b), said painting is carried out with the spray painting technique. 21.- Electrochemical cell, in particular fuel cell, containing a   electrode according to any one of claims 1 to 1 1.