MXPA97006504A - Gas diffusion electrodes based on carbon poly (vinylidene) fluoride mixtures - Google Patents

Abstract

Electrocatalytic gas diffusion electrodes for energy cells and a method for their preparation are described. The electrode comprises an anisotropic gas diffusion layer and a catalytic layer. The gas diffusion layer is made of a porous carbon matrix through which particles of carbon and poly (vinylidene) fluoride are distributed, so that the matrix is homogeneously porous in a direction lateral to the gas flow and asymmetrically porous to gases in the direction of gas flow. The porosity of the gas diffusion layer is reduced in the direction of gas flow. The catalytic layer is made of a coagulated ink suspension containing particles of catalytic carbon and a thermoplastic polymer selected from polyester sulfone, poly (vinylidene fluoride) and sulfonated gas polysulfone. The gas diffusion layer has a thickness between 50æm and 300æm. The catalytic layer has a thickness between 7æm and 50æm and a catalyst load between 0.2 mg / cmýy 0.5 mg / c

Description

GAS DIFFUSION ELECTRODES BASED ON FLUORIDE MIXTURES OF POLKVINYLIDENE) WITH CARBON FIELD OF THE INVENTION This invention relates to the preparation of gas diffusion electrode for use in solid polymer electrolyte energy cells, gas diffusion electrodes comprise poly (vinylidene) fluoride ("PVF2") mixed with carbon and a metal electrocatalyst of platinum.

BACKGROUND OF THE INVENTION Energetic cells are electrochemical devices, in which part of the energy of a chemical reaction is converted directly to direct current electrical energy. The direct conversion of energy to direct current electric power eliminates the need to convert energy into heat, thus avoiding the elimination of Carnot cycle efficiency from conventional methods to generate electricity. In this way, without limiting the Carnot cycle, energy cell technology offers the potential for fuel efficiencies two to three times higher than those of traditional power generating devices, for example, internal combustion engines. Other advantages of energetic cells are calmness, cleanliness (lack of air pollution) and the reduction or complete elimination of moving parts. Typically, the energetic cells contain two porous electrical terminals called electrodes with an electrolyte disposed therebetween. During the operation of a typical energetic cell, a reducing agent penetrates an anodic electrode into a catalyst layer, where it reacts to form two protons and two electrons. The protons are transported through an electrocatalyst to the cathode. The electrons are conducted from the anode to the cathode through external resistance producing electrical energy. An oxidant penetrates the anode electrode to combine with the electrons towards a cathode catalyst layer. The reducing agents of energy cell with classified as oxidants and reducing agents based on their characteristics of electron donor or acceptor. Oxidants include pure oxygen, gases that contain oxygen (for example, air) and halogens (for example, chlorine). Reducing agents include hydrogen, carbon monoxide, natural gas, methane, ethane, formaldehyde and methanol. The electrolyte of the energy cell serves as the electrochemical connection between the electrodes providing a path for the ion current in the circuit, while the electrodes, made of carbon or metal, provide an electrical path. In addition, the electrolyte prevents the transfer of reagents away from the respective electrodes, where the formation of explosive mixtures may occur. The electrolyte used must not react directly to any appreciable degree with the reactants or reaction products formed during the operation of the energy cell. In addition, the electrolyte must allow the migration of ions formed during the operation of the energy 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 salt electrolytes, such as seawater, molten salt electrolytes. , and ion exchange polymer membranes. One type of energy cell is a polymer electrolyte energetic cell (PEM), which is made of a proton exchange polymer membrane. The PEM energy cell contains a solid polymer membrane, which is an "ion exchange membrane" that acts as an electrolyte. The ion exchange membrane is sandwiched between two "gas diffusion" electrodes, an anode and a cathode, each commonly containing a metal catalyst supported by an electrically conductive material. The gas diffusion electrodes are exposed to the respective reactive gases, the reducing gas and the oxidizing gas. An electrochemical reaction is presented in each of the junctions (limits of three phases), where one of the electrodes, the polymer membrane and the reducing gas are interconnected. For example, when the oxidizing gas is oxygen and the reducing gas is hydrogen, the anode is supplied with the hydrogen and the cathode with the oxygen. The complete chemical reaction in the procedure is: 2 H2 + O2? 2H2O. The electrochemical reactions that occur at the metal catalyst sites of the electrodes are as follows: anode reaction: 2H2? 4H + + 4e 'cathode reaction: O2 + 4H + + 4e' - 2H2O During the operation of the energy cell, hydrogen penetrates through the cathode and interacts with the metal catalyst, producing electrons and protons. The electrons are conducted via an electronic path through the electrically conductive material and the external circuit to the cathode, while the protons are simultaneously transferred via an ion path through the polymer electrolyte membrane to the cathode. Concurrently, oxygen penetrates the catalyst sites of the cathode, where oxygen gains electrons and reacts with the protons to produce water. Consequently, the products of the reactions of the PEM energy cell with water and electricity. In the PEM energy cell, the current is conducted simultaneously through ionic and electronic routes. The efficiency of the PEM power cell depends greatly on the ability to minimize both ionic and electronic resistivity to the current. The gauge diffusion electrodes play a very important role in the energetic cells. During the operation of the energy cell, the fuel gases interact with the electrodes of the energy cell and heterogeneous reactions occur at the catalyst sites of the electrodes. To process these reactions, the electrode catalyst must simultaneously be interconnected with the conductive carbon, electrolyte and fuel gas. Therefore, the electrode must meet the following criteria: 1) low gas diffusion resistance to the reaction sites; 2) high electronic conductivity; 3) mechanical resistance for long-term operations; 4) appropriate balance of hydrophilic / hydrophobic character; and 5) stability. The gas diffusion electrodes for energy cells are conventionally made of platinum metal supported on carbon black and a polymer substrate. The polymer serves as a binder for the carbon black particles to ensure physical integrity, i.e., the mechanical strength of the electrode. Carbon is used to minimize electronic resistance, while platinum serves as the catalyst for the electrochemical reaction. A greater part of the gas diffusion electrodes for energy cells use polytetrafluoroethylene ("PTFE") as the binder. This polymer has a high thermal stability and a high resistance to chemical degradation. However, PTFE does not dissolve in any known solvent and, therefore, must be used as a suspension. This complicates the manufacturing process of the electrode. More specifically, when PTFE is used as the polymer binder for carbon, it is difficult to control the electrode structure, the porosity of the electrode and the pore size. Teflon®-type gas diffusion electrodes for energetic cells are commonly prepared by mixing PTFE with carbon or graphite powder and compressing them to a sheet, in which the PTFE serves as a binder. This sheet is treated with heat at a concreting temperature, for example, from 300 ° C to 350 ° C, where the binder is partially degraded, creating a porous matrix, in which the gas can pass and interact with the carbon. The patent of E.U.A. No. 4,847,173 discloses a method for preparing a carbon and polymer matrix either by mixing the PTFE in combination with other polymers or by binding agents of other polymers. The patent of E.U.A. No. 3,899,354 describes another method for making a carbon and PTFE matrix or other polymer binder by spraying, carbon paper, with a suspension of a mixture of PTFE and carbon until a thick layer is obtained, forming an electrode matrix, and then heating the matrix to a concreting temperature, as described above. Cabasso and Manassen in Proceedings, Int. Power Source Svmposium, 1990, describe another method for preparing energy cell electrodes. Instead of compressing or spraying the polymer binder and carbon to form a matrix and then concreting the matrix to form a gas diffusion layer, the carbon-containing platinum catalyst is mixed with a PVF2 solution, cast and then submerged in dimethyl formamide, a non-solvent that precipitates PVF2. Cabasso and others also state that there are many other soluble polymers that are resistant under the conditions employed in the energy cell, ie, low operating currents up to 200 mA / cm2, relatively low operating temperature (25 ° C to 40 ° C ) and a pressure only slightly above atmospheric pressure. In fact, most of the polymer degrades, due to the highly acid nature of the membranes, high operating temperature of up to 95 ° C and due to electric currents of up to several A / cm2 passing through the matrix. Cabasso et al. Reported two methods for preparing an electrode matrix containing a platinum catalyst therein. In one method, the electrode matrix is prepared by homogeneously casting a solution containing a catalyst mixture of platinum, carbon, PVF2, and a solvent on a glass plate. By doing this, the platinum catalyst is uniformly spread through the electrode matrix. In the other method, a solution of a mixture of carbon, platinum catalyst, polymer and solvent is cast on a glass plate, then a graphite cloth is carefully placed on top of the film mixture and, on top of this, a mixture of carbon polymer without the platinum catalyst, is cast. This was immersed in water and had the structure of three layers of carbon catalyst polymer adhered to the carbon on one side and on the other side, a layer of carbon polymer. The majority of the search in recent decades has used PTFE as the binder for carbon substrates in gas diffusion electrodes (Teflon®-type electrode) and has focused on the maximum amount of catalyst used in the electrodes. The role of platinum on carbon / electocatalyst-PTFE combined with a mixture of carbon as a component of the gas diffusing electrode in the H2 / O2 energy cell is well known. Mixtures of platinum on carbon PTFE have commonly been prepared by mixing black platinum, or platinum on carbon (finely mixed) with a hydrophobic, aqueous, colloidal, negatively charged dispersion of PTFE particles and depositing this mixture on a cloth substrate. of carbon (Report No. AFML-TR-77-68). Also, porous, thin, moisture-proof carbon paper has been used as a substrate instead of the carbon cloth to make the gas diffusion electrodes, as described in the U.A. No. 3, 912, 538. This electrode has overcome the problem of "flooding" during the operation of the energy cell. Several techniques have been developed to increase the utilization of the platinum catalyst. Methods that result in a ten-fold catalyst reduction using the improved electrode structure were developed by Los Alamos National Laboratory (Gottesfield et al., J. Applied Electrochemistry, 22 (1992), page 1) Los Alamos, New Mexico , and Texas A & M University, College Station, Texas, based on Prototech electrodes (U.S. Patent No. 4,826,742). In their methods, the electrodes produced by Prototech with a charge of 0.4 mg / cm2 of Pt were then deposited by sputtering with Pt to produce a thin layer of Pt (0.05 mg / cm2) on the front face of the electrodes. The energy cells assembled with these electrodes and a Nafion 112 membrane exhibited 1A / cm2 at 0.5 V, using H2-O2 as the reactive gases and had no significant loss of performance, even after 50 days of operation. Gottesfield and others describe a method by which the Pt load was reduced to 0.15 mg / cm2. This method involved painting a PTFE membrane sheet with ink made of organic solvents, Pt-C and Nafion solution. For good performance, an energy cell electrode must have an appropriate morphology and catalytic distribution. The energy cell electrode requires a porous structure that provides a free transport path for the penetration gas and that distributes the penetration gas over the entire surface area of the electrode catalyst. How efficiently the fuel gas is distributed to the electrode catalyst depends highly on the porosity of the electrode, an essential parameter for determining the efficiency of the electrode. Therefore, it is an object of this invention to produce a low-cost, easy-to-prepare gas diffusion electrode with favorable chemical and electrical properties for energy cells and other electrochemical applications. Another object of the invention is to provide a gas diffusion electrode with a controlled structure of electrode, porosity and pore sites. An object of this invention is to provide a method for preparing gas diffusion electrodes with controlled porosity and pore size using a mixture of activated carbon and a poly (vinylidene) fluoride dissolved in an organic solvent, which is then coagulates in a non-solvent for mixing, at low temperatures such as a porous membrane in a phase inversion mode. A further object of this invention is to provide a method for making gas diffusion electrodes, in which a gas diffusion layer and a catalyst layer are manufactured separately, making it possible to formulate each structure with properties that are very suitable for its function. Yet another object of this invention is to provide a simple method for making a gas diffusion electrode using a one-step phase inversion technique.

COMPENDIUM OF THE INVENTION The aforementioned objects and criteria for the gas diffusion electrode, and their preparation, can be achieved through the practice of this invention. In one aspect, this invention relates to an electrocatalytic gas diffusion electrode for energetic cells comprising: an anisotropic gas diffusion layer which is made of a porous carbon matrix, through which particles of carbon and poly (vinylidene) fluoride, so that the matrix is homogeneously porous in a direction lateral to the gas flow and asymmetrically porous to the gases in the direction of gas flow, the porosity of the gas diffusion layer decreasing in the direction of the gas flow, the gas diffusion layer having a thickness between about 50 μm and about 300 μm, and a catalytic layer which is made of a coagulated "ink" suspension containing catalytic carbon particles and a polymer thermoplastic, the catalytic layer covering the small pore surface of the gas diffusion layer, the catalytic layer having a thickness of between at 7 μm and about 50 μm, and a metal catalyst load of between about 0.2 mg / cm2 and about 0.5 mg / cm2.

In another aspect, this invention relates to a method for preparing a gas diffusion electrode suitable for use in energetic cells, the method comprising: a. preparing an anisotropic gas diffusion layer that is made of a porous carbon matrix, through which particles of carbon and poly (vinylidene) fluoride are distributed, so that the matrix is homogeneously porous in a lateral direction to the flow gas and asymmetrically porous to the gases in the direction of gas flow, the porosity of said gas diffusion layer decreasing in the gas flow direction, the gas diffusion layer having a thickness of between approximately 50 μm and approximately 300 μm, the gas diffusion layer is prepared, 1) by casting with a fixed blade on a substrate, a mixture of poly (vinylidene) fluoride and carbon black dissolved in a solvent for the poly (vinylidene) fluoride and the carbon black to form a layer or film on the carbon substrate, the mixture penetrating at least part of the carbon substrate; 2) coagulating the film in a coagulation liquid that is a non-solvent for poly (vinylidene) fluoride and carbon black; and 3) removing the coagulation solvent; and b) painting on the small pore surface of the gas diffusion layer, a catalytic layer which is made of a coagulated aqueous ink suspension containing catalytic carbon particles and a thermoplastic polymer, the thermoplastic polymer being selected from the group consisting of of polyethersulfone, poly (vinylidene fluoride) and sulfonated polysulfone, the catalytic layer covering the small pore surface of said gas diffusion layer, the catalytic layer having a thickness of between about 7 μm and about 50 μm, and a charge of Metal catalyst of between about 0.2 mg / cm2 and about 0.5 mg / cm BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a 100X amplified photograph, of scanning electron microscope, of a PVF2-carbon gas diffusion electrode, showing (a) a cross-section; and (b) a surface. Figure 2 is an amplified photograph 200X, scanning electron microscope, of a cross section of a PVF2-carbon gas diffusion electrode, according to the present invention, showing (a) a secondary image; (b) an X-ray Pt mapping. Figure 3 is a graph of a cell potential (cell voltage (V)) vs. current density (A / cm2) for the energy cell assembly according to the invention, containing a gas diffusion electrode made as described in Example 1 with membranes Nafion 1 12 (•) and 1 17 (0) a 80 ° C, 2,109 kg / cm2m, and 80 ° C, 4,218 kg / cm2m, respectively. Figure 4 is a polarization and energy density curve for an energy cell assembly according to the invention containing a gas diffusion electrode made as described in Example 1 with the Nafion 1 12 membrane tested at 80 ° C and 2.109 kg / cm2m. Figure 5 is a graph of current density (A / cm2) vs. log pressure of the cathode reactive gas for an energy cell assembly according to the invention, containing a gas diffusion electrode made as described in the electrode of Example 1 with Nafion membrane 117 at 80 ° C.

DETAILED DESCRIPTION OF THE INVENTION The polymeric material serves a number of functions simultaneously in the gas diffusion electrode of the energy cell. This acts as a binder to hold the carbon catalyst together to provide the integrity of the electrode, and impart a hydrophobic character. The platinum metal catalyst (Pt) in an electrode works best if it is connected to the carbon, the electrolyte, and the reactive gases simultaneously. For high utilization of Pt, low ohmic loss and be free of flood, the electrode matrix must be built to adapt to these conditions. The structure must be prepared in such a way that the ionic and electronic trajectories are short, with minimum tortuosity, while the catalyst must have a maximum exposure and use of reactive gas without flooding and leakage. Since platinum is a very expensive catalyst, it must be used in a minimum amount with maximum efficiency. Therefore, it was found that the location of Pt near the surface of the electrode, adjacent to the reactor gas, is the most advantageous for the performance of the electrode. The Bacon double layer model structure of the electrode (see British Patent No. 667,298) has been widely used. It has an asymmetric anisotropic structure with a layer of lateral open pores that looks at the side of the gas and the other with relatively fine pores, looking at the side of the electrolyte. The first can facilitate the transport of gas, the last one will be full of electrolyte, avoiding, thus, additional diffusion of gas that causes problems of crossing. Applicants have discovered that a non-expensive thermoplastic polymer, poly (vinylidene) fluoride, can be used to form a mixture of poly (vinylidene) fluoride and carbon particles which is suitable as an electron matrix material. Poly (vinylidene) fluoride is a semi-crystalline, hydrophobic polymer with a high melting temperature (Tm of about 168 ° C) and a low glass transition temperature (Tg of about 35 ° C). It is resistant to oxidation and reduction environments. In addition, it has good duration and a working capacity at a low pH. It has been proven that poly (vinylidene) fluoride is an excellent building block for a variety of porous membranes for gas separation (I. Cabasso in "Encyclopedia Polymer Science and Engineering", 2nd Ed., John Wiley & Sons, Inc., 9, 509 (1987)) and ultrafiltration. According to the present invention, when the poly (vinylidene) fluoride is mixed with carbon particles at a polymer to carbon ratio, by weight, between about 20:80 and about 45:65, the poly (vinylidene) fluoride ) works as an excellent binder, by itself, for the carbon particles in the mix. As a result, poly (vinylidene) fluoride can successfully be used to replace the more expensive PTFE polymers such as a binder and a matrix-binder for gas diffusion electrodes. Poly (vinylidene) fluoride polymer in the mix, provides the electrode structure with properties that are essential to produce high quality energy cell electrode. The gas diffusion electrodes according to the invention of the applicants are prepared by a two-step method.

The first step uses a phase inversion method to prepare an anisotropic gas diffusion layer according to the applicants' invention having a thickness of about 50 μm, preferably above about 75 μm, and below about 300 μm, preferably below about 150 μm. The phase reversal method includes the following sequences: 1) cast with a fixed blade on a conductive carbon substrate, a mixture of poly (vinylidene) fluoride and dissolved carbon particles in a solvent for poly (vinylidene) fluoride ) forming a layer of film on the carbon substrate; 2) coagulate the film in a coagulation liquid that is non-solvent for poly (vinylidene) fluoride; and 3) drying the film to remove the coagulation liquid. The second step is to prepare a catalyst layer that uses an air brush to paint a diffusion layer of carbon-polymer gas with an "ink" layer of catalyst-carbon-polymer, the catalytic "ink" layer having a thickness of about 7 μm and below about 50 μm, preferably below about 10 μm. The ratio of metal to carbon catalyst to polymer in the "ink" is between 25:75 and 40:60, by weight. The electrode in this invention has a higher porosity in the gas diffusion layer, a lower catalyst load, and a higher utilization of the catalyst. The energy cell assembled with that electrode has a high performance. The conductive carbon substrate is a fibrous or porous sheet having a thickness above about 7 μm, preferably above about 10 μm, and below about 35 μm, preferably below about 25 μm. Suitable carbon conductive substrates include carbon paper, high conductive carbon cloth, high carbon carbon felt, carbon tape, and the like. The particulate carbon is, for example, carbon black having a surface area, through the B.E.T method, from about 50 to about 2000 m2 / g. Suitable particulate carbon include active carbon or carbon black, that is, carbon powder, which is in a very finely divided state. When measured through method B E .T. , commercially available black powder powders useful in this invention have a surface area of between about 50 m / g and about 2000 m / g. Such powders include furnace blacks, lamp blacks, acetylene blacks, channel blacks, and thermal blacks. Furnace blacks having a surface area B.E.T. are preferred. between approximately 200 m2 / g and approximately 600 m2 / g. The particle sizes of these active carbon materials can vary from about 5 to as much as about 1000 nanometers, but preferably they are smaller than about 300 nanometers in average size. The B.E.T. refers to the Brunaver-Emmett-Teller method to determine the surface area. The term "carbon black" is used as defined in the U.S. patent. No. 4,440,617 issued to Solomon. Commercially available carbon blacks having a surface area B.E.T. between about 50 and about 300 m2 / g can be activated with steam, if desired, to increase its surface area and thus increase its B.E.T. up to approximately 600 m2 / g. The surface characteristics of the carbon blacks may vary. Some of these carbon blacks have surface functionality, for example, carboxyl groups on the surface (and other types of oxygen content) or groups containing fluorine. The physical-chemical characteristics and the ash content may also vary. In addition, the carbon blacks may contain graphite (so that the carbon black powders acquire some of the structural characteristic of the graphite) or contain graphite and then be treated to restore or improve the functionality of the surface. Preferred commercially available carbon blacks include BLACK PEARLS (trade designation), eg, BLACK PEARL 2000, VULCAN (commercial designation, for example Vulcan VX-72), KETJEN BLACK EC 300J (commercial designation Akzo Chemie Americo de Burt, New York), activated carbon, C-100 acetylene black, or mixtures thereof. The KETJEN BLACK materials available with furnace blacks having a surface area B.E.T. which varies from approximately 900 to approximately 1000 m2 / g, and EC 300J, in particular, appears to have a surface area of 950 m / g. KETJEN BLACK EC 300J contains a large fraction of mesophase carbon and, therefore, has regions of the long scale order. These regions can make carbon more resistant to corrosion, which is important in cathode applications. According to the patent of E.U.A. No. 4,461,814 issued to Klinedienst, oil furnace blacks, KETJEN BLACK, have both a high surface area (greater than 900 m2 / g) and high absorption number of dibutyl phthalate ("DBP"). Klinedienst discloses that when the DBP absorption is determined by the ASTM D-2414-70 test, the absorption number should preferably be above about 125 cm2 per 100 g of carbon black (eg, greater than 230 m2 / 100. g) and the surface area must be greater than 250 m2 / g, to provide a cathode collector of carbon black with optimum characteristics. The DBP absorption number for KETJEN BLACK is reported by Klinedienst and is 340 cm3 / g. Acetylene blacks tend to have high DBP absorption numbers but a surface area B.E.T. low. Conversely, Lurgi carbon blacks (from Lurgi Umivet and Chemotechnik GmbH) may have a surface area B.E.T. very high (greater than 1200 m2 / g) and a low DBP absorption number (less than 100). It is also reported that "CSX" carbon blacks (available from Cabot Corporation of Billerica, MA) have high surface areas B.E.T. and high numbers of DBP absorption. Suitable solvents for the mixture of poly (vinylidene fluoride) and carbon include those selected from the group consisting of cyclohexane, d-butyrolactone, ethylene carbonate, N, N-dimethyl formamide ("DMF"), dimethyl sulfoxide (" DMSO "), N-methyl pyrrolidone, NN-dimethyl acetamide (" DMA "), and a mixture of DMF with tetrahydrofuran (" THF "). The amount of solvent required to dissolve poly (vinylidene) fluoride will vary depending on the solvent. For example, 10-20% by weight of poly (vinylidene) fluoride will be dissolved in DMF. Suitable coagulation liquids which are non-solvent for the mixture of poly (vinylidene) fluoride and carbon particles are those selected from the group consisting of water, aliphatic and cycloaliphatic hydrocarbons, alcohols such as ethanol and isopropanol, ketones such as acetone and methyl isobutyl ketone, hexane and mixtures thereof of water and other coagulation liquids which are miscible with water. Porous carbon materials, such as Vulcan XC-72, Acetylene Black C-100 and Black Pearl 2000, can be used to prepare gas diffusion electrodes according to the applicant's method without resulting in water flooding problems that they commonly occur when said carbons are used in energy cell electrodes. Said carbon materials absorb said immense amount of liquids that the flooding of the gas electrodes could be expected as a result, if the carbon was cast in a mixture. If carbon is used with a low surface area, such as Vulcan XC-72, etc., they do not absorb much liquid and too much liquid is necessary to produce a composition that can be cast as a film. Consequently, even a film with a thickness of a few hundred micrometers, thus made, does not contain enough active carbon material for the electrode. In addition, carbon materials, such as carbon, have a high electrical resistance and, because of their size, form a highly porous matrix that can not tolerate the high pressures to which standard energy cell assemblies are exposed, ie, pressures between 1,406 kg / cm2 and 7.03 kg / cm2. Therefore, coal has not been used in the production of energy cell electrodes. The applicants unexpectedly discovered that the problem of flooding can be overcome and that said carbon materials can be cast in a solvent through the application of sound at high frequencies. In this way, to overcome the problem of immense absorption of liquids through carbon materials such as Vulcan XC-72, which are routinely used in the production of energy cell electrodes, an organic solvent (DMF) and poly (vinylidene) fluoride , along with the carbon material are treated to produce a suspension that is well mixed through a sound applicator. The application of sound at high frequencies results in a sludge that can be cast to the desired thickness on a carbon cloth substrate. Applicants believe that the sound application does not allow the carbon to absorb enough liquid to prevent the formation of an electrode through a casting step. Applicants have found that when sound is applied to a mixture of poly (vinylidene) fluoride, platinum metal catalyst and carbon material, a slurry is obtained which can be cast at much thinner thicknesses with much less solvent by interfering with the casting procedure. Therefore, Applicants' invention allows the casting of carbons that are popular for energy cell electrodes. The formation of good gas diffusion electrodes requires the reactive gases to be spread homogeneously within the matrix of the gas diffusion electrode. The gases are fluid and behave like fluids that run along the last resistant trajectory. In the energetic cell, the reactive gases flow towards the catalyst layer, where they are consumed. A problem in the energy cell devices, and especially in electrodes, is the homogeneity of the trajectories. If the electrode array is denser in one area and less dense in another area, then most of the gas stream will be directed toward the less dense area. As a result, the catalyst will not be completely used. The gas diffusion electrode of the Applicant has an electrode array that is laterally homogeneous, and asymmetric in the direction of gas flow. This means that when the electrode enters, the gases penetrate the gas diffusion electrode through the surface, which is "open", less resistant, and as the gases diffuse towards the surface, the electrode matrix is made progressively denser and its pores smaller. Therefore, the electrode matrix of the present invention has an anisotropic porous structure with two asymmetric surface layers, as can be seen in Figure 1 hereof. Applicants have also found that when the mixture of poly (vinylidene) fluoride, carbon material and platinum metal catalyst is not applied to sound and is cast as a solution on a glass substrate, as described in the document. of Cabasso et al., 1990, double density surfaces are formed due to the way in which the glass interacts with the polymer-carbon mixture. Surprisingly, applicants have found that mud with applied sound must be cast on a conductive carbon cloth or conductive carbon paper to ensure the anisotropic structure of the electrode that facilitates the ingress of penetration gases. The mud with applied sound, when cast on a carbon cloth that subsequently submerges in water, ensures the anisotropic structure. The diffusion and distribution of gas in the matrix is important for the performance of the electrodes. In an extensive way, the calculation of the gas layer on the carbon fabrics has been studied. Applicants have also discovered that when the cast slurry coagulates in a coagulation liquid that is a non-solvent for the sludge at lower temperatures, a much higher quality of the gas diffusion electrode and an anisotropically porous structure that is laterally present homogeneous Suitable coagulation bath temperatures can vary from environment to -30 ° C. When the coagulation liquid comprises the mixture of water and an alcohol or water and an inorganic salt, temperatures are used below 0 ° C and, preferably, above -20 ° C. When the coagulation liquid is water, temperatures of 25 ° C to 4 ° C are preferably employed. Suitable coagulation liquids that are non-solvent for poly (vinylidene) fluoride are aqueous solutions made of water or a mixture of water and alcohol and / or water with inorganic salt in volume ratios of between 20:80 and 80: twenty. 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 in volume ratios of between 10: 90 and 90: 10 is preferred. Suitable alcohols include ethanol and isopropanol. Suitable salts include CaCl 2, LiCl, NaCl, and LiNO 3. Other suitable coagulation liquids which are non-solvent for poly (vinylidene) fluoride are aliphatic and cycloaliphatic hydrocarbons, alcohols, acetone and methyl isobutyl ketone. The following examples illustrate the applicant's invention, but should not be construed as limiting the invention.

EXAMPLE 1 Gas diffusion electrodes were prepared using carbon black (commercially available as Vulcan VX-72R from Cabot, Inc.) with a large surface area (DP-5,200) (200 m2), and a poly (vinylidene) fluoride which has a number average molecular weight of 60,000, through a wet phase inversion technique. The carbon black was dispersed in a 20% by weight solution of poly (vinylidene) fluoride and N, N-dimethylformamide to form a suspension. The suspension was mixed well for 30 minutes using a sound applicator to form a slurry. Using a fixed blade, the resulting slurry was cast onto a hydrophobic carbon cloth substrate, 0.0381 cm thick (commercially available as Panex PWB-3 from Zoltek) until a film layer was formed on the substrate. a thickness of 100 μm. Care was taken during the casting to ensure that the mud at least partially penetrated the fabric. Then, the fabric was immersed in a bath of deionized water to coagulate the film. The coagulated film was extensively washed with deionized water and placed in a drying box to dry for at least 24 hours. The dried film formed the anisotropic gas diffusion layer of a gas diffusion electrode and had pore sizes progressively increasing from the top to the bottom, the smallest pores on the surface (see Figure 1). then, this gas diffusion layer of the electrode was heated at 250 ° C for 1 hour. An "ink" suspension of catalyst layer was prepared as follows: 0.06 g of poly (vinylidene) fluoride (PVF2) was suspended in 4 g of 2-propanol and 6 g of water through a sound applicator. 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, were added to the colloidal solution of PVF2. The mixture was again combined through an ultrasound applicator to form a suspended, final "ink" solution. Then, an art air brush was used to uniformly paint, with the "ink", the surface of the gas diffusion electrode. The painting process consisted of the application of 6.98 g of the "ink" suspension to 162 cm2 of the gas diffusion layer. The resulting electrode has a platinum load of 0.30 mg / cm2 with a catalytic layer with a thickness of 20 μm. The size of the platinum particles was on the 40 A scale. Then, the electrode was heated at 250 ° C for at least 2 hours. The cross section of this electrode is shown in Figure 2. The gas diffusion electrode made in this manner was evaluated in an H2 / O2 energy cell. The catalyst side of the electrode was brushed with 0.5% by weight of a protonated Nafion 117 solution, and compressed under heat to a membrane of Nafion 112 (or Nafion 117). An open cell voltage of 1.02 V was measured. Figure 3 shows polarization curves of an energy cell using a gas diffusion electrode made according to Example 1 and the Nafion membranes 112 and 117. Figure 4 shows curves of polarization of the energy cell electrode using an electrode of Example 1 with the Nafion membrane 112 at 80 ° C, and 2,109 kg / cm2 of the H2 / O2 reagent. It was allowed to expel, at 0.5 V, an energy density of 0.6 W / cm2 at a current density of 1A / cm2, demonstrating the good performance of this electrode. The use of this electrode was 26% and the Ohmica resistance, R, was 0.185 O / cm2, and the Tafel inclination was 0.059 V / decade. This energy cell was also treated with H2 / air as the reagent. Figure 5 shows a graph of the current density of the cell at a constant voltage of 0.5 V vs. oxygen or air / gas pressure. It was clearly observed that, using air as the cathode reagent, this electrode has a better performance than any other electrode.

EXAMPLE 2 The procedure of Example 1 was repeated, except that the gas diffusion layer was made through dry phase inversion. Poly (vinylidene) fluoride and C-100 Acetylene Black carbon were dissolved in DMF to form a paste. The paste was cast on a carbon cloth substrate and then dried in air allowing the solvent to completely evaporate and a cast film layer to be formed. The film was then compressed through two rollers at room temperature to produce the gas diffusion layer of the electrode. An energetic cell made with this electrode and a Nafion 112 membrane had an open cell voltage of 1.0V and at 0.7V under 25 ° C, 1 atm., The current density was 200 mA / cm ' EXAMPLE 3 0.5 g of platinum was suspended on Activated Carbon (10% by weight of Pt, Fluka Chemical, In.) In 1.6 g of DMF and mixed with 1.6 g of 15% by weight of PVF2 in a solution of DMF through a sound applicator. Then, this suspension was cast onto a carbon cloth substrate using a fixed blade to form a film layer. The film was immersed in a bath of deionized water for 30 minutes to coagulate it. The coagulated film was removed after the water bath, washed extensively and placed in a drying box to dry for 24 hours. The platinum load of the catalytic layer was 0.5 mg / cm2. The thickness of the diffusion electrode formed was approximately 50 μm. An energy cell, made using this gas diffusion electrode and a Nafion 1 17 membrane, had a current density of 500 mA / cm2 at 0.45V, below 25 ° C, 1 atm. , test conditions.

EXAMPLE 4 The procedure of Example 1 was repeated, except for the addition of carbon black (scale 5-20%), highly hydrophobic, Acetylene Black C-100 (Chevron Chemical Co.) having a surface area of 60 m2 / g. The platinum surface concentration in the catalyst layer in this example was 0.1 mg / cm2. The energy cell prepared by this electrode, which introduced carbon particles secondary to the gas diffusion layer, had an improvement of 100 mV at a current density of 200 mA / cm2 on an energy cell without secondary carbon particles.

EXAMPLE 5 0.6 g of Vulcan carbon black was mixed with 0.4 g of PVF2 in 6.7 g of DM F through a sound applicator to form a paste. The resulting paste was then cast on a carbon cloth with a fixed blade. The plate was then immersed in a non-solvent bath, in the present one called D.I. water, or tetrahydrofuran ("THF"), or ethanol. The coagulated film was then dried with air. The second layer, which contained 0.5 ge Pt on Vulcan VX-72 carbon black (10% by weight of Pt), was suspended in 1.6 g of DMF and mixed with a 1.6 g solution of polysulfone in DMF (15% by weight), then slipped over the first gas diffusion layer through a fixed blade. The resulting air-dried electrode allows the solvent to evaporate completely from the cast film. The electrode was then heated at 250 ° C for 4 hours under a nitrogen atmosphere. The final electrode had a Pt load of ~ 0.5 mg / cm2.

EXAMPLE 6 Example 3 was repeated, except that PVP of poly (vinipyrrolidone) was used as a pore filter to control the porosity of the gas diffusion layer and obtain the necessary open pore structure. PVP was mixed with the polymer solution before casting the gas diffusion layer. The PVP was subsequently removed by rinsing the electrode with water for three days. The total cell voltage of an energy cell made with this gas diffusion electrode and a Nafion 117 membrane was increased to approximately 200mV.

EXAMPLE 7 Two gas diffusion electrodes were made according to the procedure of Example 1, except that two different coagulation baths were used to make the gas diffusion layer. One of the coagulation baths consisted of 50 parts of water by volume and 50 parts of ethanol by volume. The second coagulation bath consisted of an aqueous solution saturated with CaCl. Two samples of carbon black dispersed in poly (vinylidene) fluoride and dimethylformamide in a 10% by weight solution, sound was applied, and sludge with applied sound was cast on a carbon substrate and subsequently coagulated, respectively, with the first and second coagulation baths at -10 ° C. The coagulation of both sludge was extremely slow, while a controlled gas diffusion layer was formed, essentially free of defects. Energy cells made using these gas diffusion electrodes produced 15% better energy densities than an energy cell built with the gas diffusion electrode in Example 1.

Claims (23)

1. - An electrocatalytic gas diffusion electrode for energetic cells comprising: an anisotropic gas diffusion layer that is made of a porous carbon matrix through which particles of carbon and poly (vinylidene) fluoride are distributed, in a manner that the matrix is homogeneously porous in a direction lateral to the gas flow and asymmetrically porous to the gases in the direction of the gas flow, the porosity of said gas diffusion layer decreasing in the direction of the gas flow, said diffusion layer of gas having a thickness between about 50 μm and about 300 μm, and a catalytic layer which is made of a coagulated "ink" suspension containing catalytic carbon particles and a thermoplastic polymer, the catalytic layer covering the small pore surface of the gas diffusion layer, the catalytic layer having a thickness between about 7 μm and about 50 μm and a charge of a of metal catalyst of between about 0.2 mg / cm2 and about 0.5 mg / cm2.

2. The electrode according to claim 1, wherein said catalytic layer contains from about 5 to about 25% by weight of said poly (vinylidene fluoride) polymer, the rest being said catalytic carbon particles.

3. The electrode according to claim 1, wherein said gas diffusion layer has a polymer to carbon ratio of between about 20: 80 and about 45:65.

4. The electrode according to claim 1, wherein the gas diffusion layer, the carbon particles are selected from the group consisting of an activated carbon, carbon black, acetylene black, and mixtures thereof, the carbon particles having a BET surface area between approximately 50 m2 / g and 2000 m2 / g.

5. The electrode according to claim 1, wherein said gas diffusion layer further includes poly (vinylpyrrolidone).

6. The electrode according to claim 1, wherein the polymer in the catalytic layer is selected from the group consisting of PVF2, sulfonated polysulfone, sulfonated polyethersulfone, and sulfonated poly (phenonele) oxide.

7 - The electrode according to claim 1, wherein the catalytic carbon particles comprise particles of catalytic metal adhered to the carbon carrier particles having a surface area B. E.T. between approximately 200 m2 / g and 2000 m2 / g.

8 - The electrode according to claim 7, wherein the catalytic metal particles comprise noble metal particles deposited on the carbon carrier particles, said noble metals being selected from the group consisting of platinum, palladium, rhodium and iridium, and being present in an amount of between 10-20% by weight of said carrier particles.

9. The electrode according to claim 1, wherein said gas diffusion layer has a thickness between about 75 μm and about 150 μm.

10. The electrode according to claim 1, wherein said catalytic layer has a thickness of between 7 μm and 10 μm and the charge of the platinum catalyst is between 0.15 mg / cm2 and 0.5 mg / cm2.

11. The electrode according to claim 1, wherein the catalytic layer comprises platinum alloys mixed with 5-30% PVF2 and 70-95% carbon particles.

12. A method for preparing a gas diffusion electrode suitable for use in energetic cells, the method comprising: a. preparing an anisotropic gas diffusion layer that is made of a porous carbon matrix, through which carbon and PVF2 particles are distributed, so that the matrix is homogeneously porous in a direction lateral to the gas flow and asymmetrically porous to the gases in the direction of gas flow, the porosity of said gas diffusion layer decreasing in the gas flow direction, the gas diffusion layer having a thickness of between about 50 μm and about 300 μm, the layer of gas diffusion is prepared, 1) by casting with a fixed blade on a substrate, a mixture of PVF2 and carbon particles dissolved in a solvent for the PVF2 to form a layer or film on the carbon substrate, the mixture penetrating into the the least part of the carbon substrate; 2) coagulating the film in a coagulation liquid that is a non-solvent for the PVF2; and 3) removing the coagulation solvent; and b) painting on the small pore surface of the gas diffusion layer, a catalytic layer which is made of a coagulated ink suspension containing catalytic carbon particles and a thermoplastic polymer comprising 0.5 ~ 2% thermoplastic polymer, the thermoplastic polymer being selected from the group consisting of polyethersulfone, poly (vinylidene fluoride) and sulfonated polysulfone, the catalytic layer covering the small pore surface of said gas diffusion layer, the catalytic layer having a thickness of between about 7 μm and about 50 μm, and a metal catalyst load of between about 0.2 mg / cm2 and about 0.5 mg / cm2.

13. The process according to claim 12, wherein in step (a) (1), said gas diffusion layer is made with a solution comprising 5-25% by weight of PVF2 in N, N ' -dimeti1 formamide.

14. The process according to claim 12, wherein in step (a) (1), said carbon particles are selected from the group consisting of an activated carbon, carbon black, acetylene black, and mixtures of the themselves, the carbon particles having a BET surface area between approximately 50 m2 / g and 2000 m2 / g.

15. - The method according to claim 14, wherein in step (a) (1), the mixture of PVF2 and the dissolved carbon particles in a solvent for the PVF2 sound is applied for a period sufficient to homogeneously mix the PVF2 with carbon particles.

16. The process according to claim 14, wherein in step (a) (1), the solvent for PVF2 is selected from the group consisting of cyclohexane, d-butyrolactone, ethylene carbonate, N, N- dimethyl formamide, dimethyl sulfoxide, N-methyl pyrrolidone, N, N-dimethyl acetamide, and a mixture of N, N-dimethyl formamide with tetrahydrofuran.

17. The process according to claim 12, wherein in step (a) (2), said gas diffusion layer is made using a coagulation liquid selected from the group consisting of water, ethanol, water / N , N'-dimethyl formamide, water / ethanol, water / methanol, water / isopropanol, tetrahydrofuran and mixtures thereof.

18. The process according to claim 17, wherein in step (a) (2), said coagulation liquid has a temperature between room temperature and -30 ° C.

19. The process according to claim 12, wherein in step (b), said catalytic layer includes a nonionic surfactant.

The method according to claim 12, wherein it further includes step (c) of concreting the electrode between 200 ° C and 300 ° C for a period of between 0.25 hours and 2 hours.

The method according to claim 12, wherein in step (a) (1), said gas diffusion layer is made with a solution comprising 10-20% by weight of PVF2 in N, N- dimethyl formamide.

22. The process according to claim 12, wherein in step (b), the ratio of Pt over carbon to the thermoplastic polymer is between 25:75 and 40:60.

23. The process according to claim 12, wherein in step (a) (2), the coagulation solvent is selected from the group consisting of water, ethanol, water and mixture of N, N-dimethyl formamide, and tetrahydrofuran.