GB2316802A - Gas diffusion electrodes based on polyethersulfone Carbon blends - Google Patents

Abstract

All electrocatalytic gas diffusion electrode for fuel cells and a process for its preparation is disclosed. 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 carbon particles and polyethersulfone are distributed so that the matrix is homogeneously porous in a direction lateral to gas flow and asymmetrically porous to gases in the direction of the gas flow. The porosity of the gas diffusion layer decreases in the direction of gas flow. The catalytic layer is made of a coagulated ink suspension containing catalytic carbon particles and a thermoplastic polymer selected from polyethersulfone, poly(vinylidene fluoride) and sulfonated polysulfone and covers the small pore surface of the gas diffusion layer. The gas diffusion layer has a thickness between 50 Ám and 300 Ám. The catalytic layer has thickness between 7 Ám and 50 Ám and a metal catalyst loading between 0.2 mg/cm<SP>2</SP> and 0.5 mg/cm<SP>2</SP>.

Description

2316802 GAS DIFFUSION ELECTRODES BASED ON POLYETHERSULPONE CARBON BLENDS

Field of Invention This invention relates to the preparation of gas diffusion electrodes for use in solid polymer electrolyte fuel cells, the gas diffusion electrodes comprised of polyethersulfone (11PESP11) blended with carbon and a platinum metal electrocatalyst.

Background of the invention

Fuel cells are electrochemical devices in which part of the energy of a chemical reaction is converted directly into direct current electrical energy. The direct conversion of energy into direct current electrical energy eliminates the necessity of converting energy into heat thereby avoiding the is Carnot-cycle efficiency limitation of conventional methods of generating electricity. Thus, without the limitation of the Carnot-cycle, fuel cell technology offers the potential for fuel efficiencies two to three times higher than those of traditional power generator devices, e.g. , internal combustion engines.

other advantages of fuel cells are quietness, cleanliness (lack of air pollution) and the reduction or the complete elimination of moving parts.

i 1 ' .. i i Typically, fuel cells contain two porous electrical terminals called electrodes with an electrolyte disposed therebetween. In the operation of a typical fuel cell, a reductant gas permeates a gas diffusion electrode to a catalyst layer where it reacts to form two protons and two electrons. The protons are transported through the electrolayer to the cathode. The electrons are conducted from the anode to the cathode through outside resistance producing electrical power. oxidant permeates the anode electrode to combine with the electrons at a cathode catalyst layer. Fuel cell reactants are classified as oxidants and reductants on the basis of their electron acceptor or electron donor characteristics. Oxidants include pure oxygent oxygen-containing gases (e. g., air) and halogens (e.g.,, chlorine). Reductants include hydrogen, carbon monoxide, natural 15--- gas, methane, ethane, formaldehyde and methanol.

The electrolyte of the fuel cell serves as the electrochemical connection between the electrodes providing a path for ionic current in the circuit while the electrodes, made of carbon or metal, provide an electrical pathway. Further, the electrolyte prevents transfer of the reactants away from the respective electrodes where the formation of explosive mixtures can occi,n. The electrolyte utilized must not react directly to any appreciable extent with the reactants or reaction products formed during the operation of the fuel cell. Further, the electrolyte must permit the migration of ions formed during operation ofthe fuel 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 sea water, fused salt electrolytes and ion-exchange polymer membranes.

One type of fuel cell is a polymer electrolyte (PEM) fuel cell which is based on a proton exchange polymer membrane.

The PEM fuel cell contains a solid polymer membrane which is an lion-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-.

is The gas diffusion electrodes are exposed to the respective reactant gases, the reductant gas and the oxidant gas. An electrochemical reaction occurs at each of the two junctions (three phase boundaries) where one of the electrodes, electrolyte polymer membrane and reactant gas interface.

For example, when oxygen is the oxidant gas and hydrogen is the reductant gas, the anode is supplied with hydrogen and the cathode with oxygen. The overall chemical reaction in this process is: 2H2 + 02 -> 2H20. The 1 ' i 1 1 1 electrochemical reactions that occur at the metal catalyst sites of the electrodes are as follows:

anode reaction: 2H2 -> 4H + 4e cathode reaction: 0. + 4H + 4e- -> 2H20 During fuel cell operation, hydrogen permeates through the anode and interacts with the metal catalyst, producing electrons and protons. The electrons are conducted via an electronic route through the electrically conductive material and the external circuit to the cathode, while the protons are simultaneously transferred via an ionic route through the polymer electrolyte membrane to the cathode. Concurrently, oxygen permeates to the catalyst sites of the cathode, where the oxygen gains electrons and reacts with the protons to yield water. Consequently, the products of the PEM fuel cell reactions are water and is electricity. In the PEM fuel cell, current is conducted simultaneously through ionic and electronic routes. Efficiency of the PEM fuel cell is largely dependent on the ability to minimize both ionic and electronic resistivity to current.

Gas diffusion electrodes play an important role in fuel cells. During fuel cell operation, fuel gases interact with fuel cell electrodes and heterogeneous reactions occur at catalyst sites of the electrodes. To process these reactions, the electrode catalyst simultaneously must interface with the conductive carbon, electrolyte and fuel gas. Therefore, the electrode must satisfy the following criteria: 1) low diffusion resistance of the gas to reaction sites; 2) high electronic conductivity; 3) mechanical strength for long-term operation; 4) proper hydrophilicity/hydrophobicity balance; and 5) stability.

Gas diffusion electrodes for fuel cells conventionally are made of platinum metal supported on a carbon black and 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 of the electrode while platinum serves as the catalyst for the electrochemical reaction.

A majority of gas diffusion electrodes for fuel cells use polytetrafluoroethylene (IIPTFEII) as the binder. This polymer has high thermal stability and high resistance to chemical degradation. However, PTFE does not dissolve in any known solvent and hence, must be used as a suspension. This complicates the process of fabricating an electrode. More specifically, when PTFE is used as the polymer binder for carbon, it is difficult to control electrode structure, electrode porosity and pore size.

Teflon type gas diffusion electrodes for fuel cells are commonly prepared by mixing PTFE with carbon or graphite powder and pressing it into a sheet in which the PTFE serves as a binder. This sheet is heat treated to a sintering temperature, for example, 300C to 3500C, in which the binder partially degrades creating a porous matrix in which gas can pass and interact with the carbon. U.S. Patent No. 4,847,173 describes a method of preparing a carbon and polymer matrix by either mixing the PTFE in combination with other polymers or with binding agents of other polymers. U.S. Patent No. 3,899,354 describes another method of making a matrix of carbon and PTFE 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 sintering temperature, as described above.

Cabasso and Manassen, in Proceedings. Int. Power Source Symposium, 1990, describe another method for preparing fuel cell electrodes. Instead of pressing or spraying the polymer binder and carbon to form a matrix and then sintering the matrix to form a gas diffusion layer, carbon containing platinum catalyst is mixed with a polyvinylidene fluoride (PW2) solution, cast and then immersed in dimethyl formamide, a non-solvent that precipitates the PW.. Cabasso et al. also state that many other soluble polymers exist that are resistant under the fuel cell conditions employed, i.e., low operating currents up to 200 mA/ca2, relatively low operating temperature (250C to 35C) and a pressure only slightly above atmospheric pressure. In fact, most of the polymer degrades because of the high acidic nature of the membranes, high temperature operation up to 950C and because of the electrical currents of up to several A/cm2 that passes through the matrix.

Cabasso et al. reported two methods of preparing an electrode matrix containing a platinum catalyst therein. In one method, the electrode matrix is prepared by homogeneously casting a solution containing a mixture of platinum catalyst, carbon, PW2 and a solvent onto a glass plate. By doing so, 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 onto a glass plate, a graphite cloth is then put carefully on top of the film mixture and, on top of that, 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 glued to the carbon on one side and on the other side, a carbon polymer layer.

Most research in the past several decades has used PTFE as the binder for carbon substrates in gas diffusion electrodes (Teflon type electrode) and focused on the maximum amount of catalyst used in the electrodes. The function of platinum-oncarbon/electrocatalyst- PTFE blended with carbon mixture as a component of the gas diffusion electrode in the H,/0, fuel cell is well known. Platinum-on-carbon PTFE mixtures commonly have been prepared by mixing platinum black, or platinum-on-carbon (finely mixed) with an aqueous colloidal, negatively charged#.

hydrophobic dispersion of PTFE particles and depositing this mixture onto a carbon cloth substrate (Report No. AFML-TR-77-68).

Wet-proofed, thin, porous carbon paper also has been used as a substrate instead of carbon cloth to make gas diffusion electrodes, as disclosed in U.S. Patent No. 3,912,538. This electrode has overcome the "flooding" problem during fuel cell operation.

Several techniques have been developed to increase platinum catalyst utilization. Methods resulting in tenfold catalyst reduction by using improved electrode structure were developed by Los Alamos National Laboratory, (Gothesfield et al., J. Applied Electrochemistry, 12 (1992), p. 1) Los Alamos, New

15--- Mexico and Texas A&M University, College Station, Texas based on electrodes from Prototech (U.S. Patent No. 4,826,742). In their methods, the electrodes produced by Prototech with a 0.4 mg/CM2 Pt loading then were sputter deposited with Pt to produce a thin layer of Pt (0. 05 mg/cm2) on the front surface of the electrodes.

Fuel cells assembled with these electrodes and a Nafion 112 membrane exhibited 1A/cm2 at 0. 5 v using H2-02 as the reactant gases and had no significant loss of performance, even after 50 days of operation. Gothesfield et al. describe a method by which the Pt loading 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, a fuel cell electrode must have proper morphology and catalyst distribution. The fuel cell electrode requires a porous structure that provides a free transport pathway for the gas permeate and distributes the gas permeate over the entire surface area of the electrode catalyst. How efficiently the fuel gas is distributed to the electrode catalyst highly depends upon the porosity in the electrode, an 10 essential parameter in determining electrode efficiency.

It is, therefore, an aim of this invention to produce a low cost, easy to prepare gas diffusion electrode with favorable chemical and electrical properties for fuel cells and other electrochemical applications.

Another aim of the invention is to provide a gas diffusion electrode with a controlled electrode structure, porosity and pore size.

An aiff of this invention is to provide a process for preparing gas diffusion electrodes with a controlled porosity and pore size by using a blend of activated carbon and polyethersulfone dissolved in an organic solvent, which is then coagulated in a non-solvent for the blend at low temperatures as a porous membrane in a phase inversion mode.

-g- p A further aim of this invention is to provide a process for making gas diffusion electrodes in which a gas diffusion layer and a catalyst layer are fabricated separately, making it possible to formulate each structure with properties that are most suitable for its function.

Yet another aim of this invention is to provide a simple process for making a gas diffusion electrode using a onestep phase inversion technique.

SUIMRY OF THE INVENTION The aims and criteria for the gas diffusion electrode mentioned above and its preparation can be achieved by the practice of this invention. in one aspect, this invention concerns an electrocatalytic gas diffusion electrode for fuel cells comprised of:

an anisotropic gas diffusion layer that is made of a porous carbon matrix through which carbon particles and polyethersulfone are distributed such that the matrix is homogeneously porous in a direction lateral to gas flow and asymmetrically porous to gases in the direction of gas flow, the porosity of the gas diffusion layer decreasing in the direction of gas flow, the gas diffusion layer having a thickness between about 50 Am and about 300 Am, and a catalytic layer that 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 MTa and about 50 MTa and a metal catalyst loading between about 0.1 mg/cm2 and about 0.5 mg/cm2.

In another aspect, this invention concerns a process for preparing a gas diffusion electrode suitable for use in fuel cells, the process comprising:

a. preparing an anisotropic gas diffusion layer that is made of a porous carbon matrix through which carbon particles and polyethersulfone are distributed such that the matrix is homogeneously porous in a direction lateral to gas flow and asymmetrically porous to gases in the direction of gas flow, the porosity of said gas diffusion layer decreasing in the direction of gas flow, the gas diffusion layer having a thickness between about 50 pm and about 300 pm, the gas diffusion layer prepared by 1) casting with a Doctor knife onto a carbon substrate a blend of polyethersulfone and carbon particles dissolved in a solvent for the polyethersulfone and carbon particles to form a layer of film on the carbon substrate, the blend penetrating into at least part of the carbon substrate.; 2) coagulating the film in a coagulation liquid that is a nonsolvent for the polyethersulfone and carbon particles; and 3) removing the coagulation solvent; and b. painting onto the small pore surface of the gas diffusion layer a catalytic layer that is made of an aqueous ink suspension containing catalytic carbon particles and a thermoplastic polymer, said suspension comprising 0.5 to.2% thermoplastic polymer, said 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, said catalytic layer having a thickness between about 7 pm and about 50 pm and a metal catalyst loading between about 0.1 rag/cmz and about 0.5 mg/cm2.

BRIEF DESCRIPTION OF THE DRAWINGS is FIG. 1 is a scanning electron microscope (SEM) photograph depicting a cross-section of a PESF-carbon gas diffusion electrode according to the present invention (a) magnified 200X and (b) magnified 500X. FIG. 2 is a scanning electron microscope photograph depicting a cross-section of a PESF-carbon gas diffusion electrode according to the present invention magnified 400X (a) secondary image (b) pt x-ray mapping.

FIG. 3 is a graph of cell potential (cell voltage (V)) vs. current density (A/cm 2) for a fuel cell assembly according to the invention containing a gas diffusion electrode made as described in Example 1 with Nafion 112 membranes at 800C, 30 psig.

Fig. 4 is a graph of cell potential (cell voltage (V)) vs. current density (A/cm2) f or a fuel cell assembly according to the invention containing a gas diffusion electrode made as described in Example 1 with Nafion 112 membrane tested at various hydrogen-oxygen gas pressures: (0) 10 psig; (f) 20 psig; and psig.

DETAILED DESCRIPTION OF THE INVENTION

Polymeric material serves a number of functions simultaneously in the gas diffusion electrode of the fuel cell.

It acts as a binder to hold the carbon catalyst together to provide the integrity of the electrode, and imparts hydrophobicity. The platinum metal (Pt) catalyst in an electrode functions best if it simultaneously interfaces with the carbon, the electrolyte, and the reactant gases. For high Pt utilization, low ohmic loss and be flood-free, the electrode matrix must be constructed to accommodate these conditions. The structure should be prepared so that the ionic and electronic pathways are short, with minimum tortuosity, while the catalyst should have maximum exposure and utilization of reactant gas without drowning and weeping. Since platinum is an expensive catalyst, it should be used in a minimum amount with maximum efficiency. Therefore, localization of Pt near the surface of the electrode, adjacent to the reactant gas, was found to be most advantageous for electrode performance. Baconts dual layer model structure of the electrode (See British Patent No. 667,298) has been widely accepted. It has an asymmetric anisotropic structure with one layer of side open pores facing the gas side and the other relatively small, fine pores facing'the electrolyte side. The former can facilitate gas transport, the latter will be filled with electrolyte, thus, preventing further gas diffusion that causes gas crossover problems.

Applicants have discovered that an inexpensive 15- thermoplastic polymer, polyethersulfone, can be used to form a blend of polyethersulfone and carbon particles that is suitable as an electron matrix material. Polyethersulfone is a hydrophobic, amorphous polymer with high glass transition temperature. It is resistant to oxidative and reducing environments. Further, it has good durability and a working capability at low pH. Polyethersulfone has proven to be 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. Suitable polyethersulfones according to the present invention have an average molecular weight between about 25,000 and about 100,000 and include those having the following recurring units:

J J io and __0 According to the present invention, when polyethersulfone is blended with carbon particles in a polymer to-carbon ratio by weight between about 20:80 and about 45:65, polyethersulfone functions as an excellent binder, by itself, for the carbon particles in the blend. As a result, polyethersulfone successfully can be used to replace the more expensive PTFE polymers as a binder and a matrix-builder for gas diffusLon electrodes. The polyethersulfone polymer in the blend provides i the electrode structure with properties that are essential for producing high quality fuel cell electrodes.

Gas diffusion electrodes according to applicants' invention are prepared by a two-step method.

The first step uses a phase inversion method to prepare an anisotropic gas diffusion layer according to applicantso invention having a thickness above about 50 Am, preferably above about 75 Am, and below about 300 Am and preferably below about Am. The phase inversion method includes the following sequence: 1) casting onto a carbon substrate or glass plate with a Doctor knife a blend of polyethersulfone and carbon particles dissolved in a solvent for the polyethersulfone forming a layer of film on the carbon substrate; 2) coagulating the film in a coagulation liquid that is a non-solvent for the polyethersulfone; and 3) drying the film to remove the coagulation liquid.

The second step is to prepare a catalyst layer that uses an airbrush to paint a layer of catalyst-carbon-polymer "ink" onto a carbon-polymer gas diffusion layer, the catalytic "ink" having a thickness above about 7 Am, and below about 50 Am, preferably below about 10 Am. The ratio of metal catalyst on carbon to the 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, lower catalyst loading, and higher catalyst utilization. The fuel cell assembled with this electrode has.high performance.

The conductive carbon substrate is a fibrous or porous sheet having a thickness above about 7 pm, preferably above about 10 gm, and below about 35 pm, preferably below 25 pm thick.

Suitable conductive carbon substrates include carbon paper, high conducting carbon cloth, high conducting carbon felt, carbon tape and the like.

The particulate carbon is, for example, a carbon black having a surface area, by the B.E.T. method, of about 50 to about 2000 m2/g. Suitable particulate carbons include active carbon or carbon black, i.e., carbon powder which is in a very finely divided state. When measured by the B.E.T. method, commercially available carbon black powders useful in this invention have a surface area between about 50 m2/g and about 2000 M2/g. Such powders include furnace blacks, lamp blacks, acetylene blacks, channel blacks, and thermal blacks. Furnace blacks having a B.E.T. surface area between about 200 m2/g and about 600 m2/g are preferred. The particle sizes of these active carbon materials can range from about 5 to as much as about 1000 nanometers, but are preferably smaller than about 300 nanometers in average size.

The B.F.T. method refers to the Brunaver-Emmett-Teller method of determining surface area.

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

Patent No. 4,440,167 to Solomon.

Commercially available carbon blacks having a B.E.T.

surface area between about 50 and about 300 m2/g can be steam activated, if desired, to enhance their surface area and thereby increase their B.E.T. value up to about 600 M2/g.

The surface characteristics of carbon blacks can vary.

Some of these carbon blacks have surface functionality, e.g., surface carboxyl groups (and other types of oxygen content) or fluorine-containing groups. Physico-chemical characteristics and ash content also can vary. Moreover, carbon blacks can be graphitized (whereby the carbon black powders acquire some of the structural characteristic of graphite) or graphitized and then treated to restore or enhance surface functionality.

is Preferred commercially available carbon blacks include BLACK PEARLS (trade designation) e.g., BLACK PEARL 2000, VULCAN (trade designation e.g., Vulcan W-72), KEWEN BLACK EC 300J (trade designation of Akzo Chemie Americo of Burt, New York), activated charcoal, Acetylene Black C-100, or mixtures thereof.

The available KEWEN BLACK materials are oil furnace blacks having a B.E.T. surface area ranging from about 900 to about 1000 m2/g, and EC 300J in particular appears to have a surface area of 950 m2/g. KEWEN BLACK EC 300J contains a large fraction of mesophase carbon and hence has regions of long range order.

These regions may make the carbon more corrosion resistant which is important in cathode applications.

According to U.S. Patent No. 4,461,814 to Klinedienst, KEWEN BLACK oil furnace blacks have both high surface area (greater than 900 m2/g) and high dibutyl phthalate (IIDBPII) absorption numbers. Klinedienst discloses that when the DBP absorption is determined by ASTM test D-2414-70, the absorption number should preferably be above 125 cm2 per 100 g of carbon black (e.g., greater than 230 cm3/100 g) and the surface area should be greater than 250 m2/g, to provide a carbon black cathode collector with optimum characteristics. The DBP absorption number for KEWEN BLACK is reported by Klinedienst to be 340 cm3/100 g. Acetylene blacks tend to have high DBP absorption numbers but low B.E.T. surface area. Conversely, Lurgi carbon blacks (from Lurgi Umivett and Chemotechnik GmbH) can have very high B.E.T. surface are (greater than 1200 m2/g) and a low DBP absorption number (less than 100). IICSX" carbon blacks (available from Cabot Corporation of Billerica, MA) also are reported to have high B.E.T. surface areas and high DBP absorption numbers.

Suitable solvents for the polyethersulfone and carbon blend include those selected from the group consisting of N,N dimethyl formamide (I1DMVI), N,N-dimethyl acetamide, N-methyl pyrrolidone and dimethyl sulfoxide. The amount of solvent required to dissolve polyethersulfone will vary depending on the solvent. For example, 10-20% by weight of polyethersulfone will dissolve in DMF.

Suitable coagulation liquids that are non-solvents for the blend of polyethersulfone and carbon particles are those selected from the group consisting of water, isopropanol and hexane, and mixtures of water with isopropanol.

Porous carbon materials such as Vulcan XC-72, Acetylene Black C-100 and Black Pearl 2000 may be used to prepare gas diffusion electrodes according to applicants' method without resulting in water flooding problems that commonly occur when such carbons are used in fuel cell electrodes. Such carbon materials absorb such immense quantities of liquids that flooding of the gas electrodes would be expected to result if the carbon 15--- was cast in a blend. If carbons with low surface area like Vulcan XC-72, etc. are used, they do not absorb much liquid and too much liquid is needed to produce a composition that can be cast as a film. Consequently even a film of a few hundred micrometers thickness so made does not contain enough active carbon material for the electrode. Moreover, carbon materials, such as charcoal, have high electrical resistance and because of their size, they form a highly porous matrix that cannot tolerate the elevated pressures that standard fuel cell assemblies are exposed to, i.e., pressures between 20 psi and 100 psi.

Therefore, charcoal has not been used in the production of fuel cell electrodes.

Applicants unexpectedly discovered that the flooding problem could be overcome and that such carbon materials could be cast in a solvent by using sonication at high frequencies. Thus, to overcome the problem of immense absorption of liquids by carbon materials like Vulcan XC-72, which are used routinely in the production of fuel cell electrodes, an organic solvent (DMF) and polyethersulfone, together with the carbon material are treated to yield a suspension that is well mixed by a sonicator. Sonication at high frequencies results in a slurry that can be cast at the desired thickness on a carbon cloth substrate. Applicants believe that the sonication does not let the carbon absorb sufficient liquid to hinder the formation of an electrode is by a casting step. Applicants have found that when a mixture of the polyethersulfone, platinum catalyst and carbon material are sonicated, a slurry is obtained that can be cast at much thinner thicknesses with much less solvent interfering in the casting process. Therefore, applicants' invention permits the casting of carbons that are popular for fuel cell electrodes.

The formation of good gas diffusion electrodes requires spreading the reactant gases homogeneously within the matrix of the gas diffusion electrode. Gases are fluids and behave like fluids that stream along the least resistant pathway. In the fuel cell, the reactant gases stream toward the catalyst layer where they are consumed. One problem in fuel cell devices, and especially electrodes, is homogeneity of the pathways. If the electrode matrix is denser in one area and less dense in another area, then most of the gas stream will be directed to the less denser area. As a result, the catalyst will not be fully utilized. Applicants'gas diffusion electrode had an electrode matrix that is homogeneous laterally, and asymmetric in the direction of the gas flow. This means that when entering the electrode, the gases penetrate the gas diffusion electrode through the surface which is "open," less resistant, and as the gases% diffuse," toward the surfa--e the electrode matrix is progressively denser and its pores are smaller. Therefore, the electrode matrix of the present invention has an anisotropic porous structure with two asymmetric surface layers, as seen in FIG. 1 herein.

Applicants also have found that when a mixture such as PVF,, carbon material and platinum catalyst is not sonicated and is cast as a solution on a glass substrate, as described in the Cabasso et al. 1990 paper, double density surfaces form because of the way that glass interacts with the polymer carbon mixture. Surprisingly, applicants found that the sonicated slurry must be cast on a carbon cloth or carbon paper to ensure the anisotropic structure of the electrode that facilitates entry of the permeating gases. The sonicated slurry, when cast on a carbon cloth that subsequently is submerged in water ensures the anisotropic structure.

Gas diffusion and distribution in the matrix is important for performance of the electrodes. Calculation of the gas layer on the carbon cloths has been extensively studied.

Applicants also have discovered that when the cast slurry is coagulated in a coagulation liquid that is a non-solvent for the slurry at lower temperatures, a much higher quality of gas diffusion electrode and anisotropically porous structure that is homogeneous laterally results.

Suitable coagulation bath temperatures may range from ambient to -30C. When the coagulation liquid comprises the mixture of water and an alcohol, temperatures below OC and above -20C preferably are employed. When the coagulation liquid is water, temperatures from 25'C to 4C preferably are employed.

Suitable coagulation liquids that are non-solvents for the cast slurry are aqueous solutions made of a mixture of water and alcohol and/or water with inorganic salt in ratios by volume between 99:1 and 1:99. Preferably water is used as the coagulation liquid. When the coagulation liquid is a mixture, preferred is a mixture of water and alcohol or water and salt in ratios by volume between 90:10 and 10:90. Suitable alcohols ,nclude ethanol, isopropanol and methanol. Suitable salts include LiCl, LiN03 and NaNO3 As indicated above, a suitable thermoplastic polymer in the catalytic layer is one selected from the group consisting of polyethersulfone, poly(vinylidene fluoride) and sulfonated polysulfone. Also believed to be suitable thermoplastic polymer for the catalytic layer are sulfonated polyethersulfone and sulfonated poly(phenonel oxide). Preferably, the catalytic layer contains from about 5 to about 25% weight percent of said thermoplastic polymer, the balance being said carbon particles.

The following examples illustrate applicants' invention, but should not be construed as limiting the invention: EXAMPLE 1 Gas diffusion electrodes were prepared using a high surface (DP-5,200) area (200 m2) carbon black (commercially available as Vulcan VX-72R from Cabot, Inc.) and a polyethersulfone by the wet phase inversion technique. The carbon black was dispersed in a 12- 15 wt% solution of polyethersulfone and N,N,-dimethylformamide to form a suspension.

The suspension was mixed well for 10 minutes using a sonicator forming a slurry. Using a Doctor blade knife, the resulting slurry was cast onto a 0.01511 thick, hydrophobic, carbon cloth substrate (commercially available as Panex PWB-3 from Zoltek) until a layer of film having a thickness of.50 gm was formed on the substrate. Care was taken during casting to make sure the slurry at least partially penetrated the cloth. This film then was submerged in a deionized water bath to coagulate the film. The coagulated film was washed extensively with deionized water and placed in a dry box to dry for at least 24 h. The dried film forms the gas diffusion layer of a gas diffusion electrode and had small pores on the surface (see TEM picture). Then, this gas diffusion layer of the electrode was heated at 250C for 1 h.

A catalyst layer aqueous "ink" suspension was prepared as follows: 0.06 g polyvinylidene fluoride (PFV2) was suspended into 4 g 2-propanol and 6 g water by a sonicator. Then, 0.05 g of a non-ionic surfactant (Triton X-100) and 0.3 g of 20 wt of Pt on Vulcan VX-72 carbon black was added to the PFV2 colloidal solution. The mixture was mixed again by the ultrasonicator to form a final "ink" suspension. Then, an art air-brush was used to evenly paint this "ink" onto the surface of the gas diffusion electrode. The painting procedure consisted of applying 7.0 g "ink" suspension to 88 cm2 of the gas diffusion layer. The resulting electrode has a platinum loading of 0.35 mg/cm2 with a 20 pm thick catalytic layer, as illustrated in FIG. 2. The size of the platinum particles was in the range of 20 A to 40 A range.

The electrode then was heated to 300C for at least 2 h.

The gas diffusion electrode made in this way was evaluated in a H2/02 fuel cell. The catalyst side of the electrode was brushed with 0.5 wt% of protonated 117 Nafion solution, and hot pressed to a Nafion 112 or Nafion 117 membrane.

1 1 4 An open cell voltage of 1.0 V was measured. FIG. 3 shows polarization. curves of a fuel cell using a gas diffusion electrode made according to Example 1 and a Nafion 112 membrane.' FIG. 4 shows polarization curves of the fuel cell electrode using an Example 1 electrode with Nafion 112 membrane at different reactant pressures. At 0. 5V, 1A/cm2 current was able to be drawn demonstrating the good performance of this electrode.

EXAMPLE 2

The process of Example 1 was repeated, except that the gas diffusion layer was made by dry phase inversion.

Polyethersulfone and Acetylene Black C-100 carbon were dissolved in DMF to form a paste. The paste was cast onto a carbon cloth substrate and then dried in air allowing the solvent to completely evaporate and forming a cast layer of film. The film is then was pressed through two rollers at room temperature to produce the gas diffusion layer of the electrode. A fuel cell made with this electrode and a Nafion 112 membrane had an open cell voltage of 1.OV and at 0.5V, the current density was 800 mA/cm2.

EXAMPLE 3

The process of Example 1 was repeated, except that a inixture of water and DNF was used as the coagulating solvent to prepare the gas diffusion layer. Sulfonated polysulfone was used instead of PW2 as the polymer in the catalyst layer "ink" suspension. A fuel cell prepared using this electrode and a Nafion 112 membrane had a higher platinum catalyst utilization of approximately 35%. At 0.5V, a current density of 2A/cm2 was measured for the fuel cell.

EXAMPLE 4

0.5 g platinum on Activated Charcoal (10 wtt Pt, Fluka Chemical, Inc.) was suspended in 1.6 g DMF and mixed with 1.6 g_ 15 wt% PESF in DMF solution by a sonicator. This suspension was then cast onto a carbon cloth substrate using a Doctor knife to form a layer of film. The film was submerged in a deionized water bath for 30 seconds to coagulate it. The coagulated film then was removed from the water bath, washed extensively and placed in a dry box to dry for 24 h. The platinum loading of the catalytic layer was 0.5 mg/cm2. The thickness of the gas diffusion electrode formed was approximately 150 pm. A fuel cell made using this gas diffusion electrode and a Nafion 117 membrane d had an over resistance of 0. 69n/cm2 and a Tafel slope of 11o 2 mV/decade. The maximum current density was 800 mA/cm EXAMPLE 5 The process of Example 4 was repeated, except for the addition of a highly hydrophobic, low surface area of 60 m,/g carbon black (5-20% range), Acetylene Black C-100 (Chevron Chemical Co.). The surface concentration of Platinum in the 2 catalyst layer in this example was 0. 1 mg/cm When the amount of the second carbon increased from 0 to 10%, the maximum current density increased from 800 mA/cm2 to 1A/CM2.

EXAMPLE 6

The process of Example 4 was repeated, except that poly(vinylpyrrolidone) PVP 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.

Overall cell voltage of a fuel cell made with this gas diffusion electrode and a Nafion 117 membrane increased to approximately 200mV.

Claims (21)

1. ' An electrocatalytic gas diffusion electrode for fuel cells comprised of:

an anisotropic gas diffusion layer that is made of a porous carbon matrix through which carbon particles and polyethersulfone are distributed such that the matrix is. homogeneously porous in a direction lateral to gas flow and asymmetrically porous to gases in the direction of gas flow, the porosity of said gas diffusion layer decreasing in the direction of gas flow, said gas diffusion layer havi ng a thickness between about 50 pm and about 300 pm. and a catalytic layer that is made of a coagulated "ink" suspension 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 between about 7. gm and about 50 pm and a metal catalyst loading between about 0.1 mg/cm2 and about 0.5 ing/cm2.

2., The electrode of Claim 1, wherein said catalytic layer contains from about 5 to about 25% weight percent of said thermoplastic polymer, the balance being said carbon particles.

3. The electrode of Claim 1 or 2, wherein said gas diffusion layer has a polymer-to-carbon ratio between about 20:80 and about 45:65.

4. The electrode according to any preceding claim, wherein said carbon particles are selected from the group consisting of an activ ated carbon, carbon black and acetylene black, the carbon particles having a B.E.T. surface area between about 50 m2/g and about 2000 M,/g -

5. The electrode according to any preceding claim, wherein said gas diffusion layer further includen poly(vinylpyrrolidone).

6. The electrode according to any preceding claim, wherein the thermoplastic polymer in the catalytic layer is selected from the group consisting of PW., sulfonated polys ulfone, sulfonated polyethersulfone, sulfonated poly(phenonel oxide) and polyethersulfone.

7. The electrode according to any preceding claim, wherein the catalytic carbon particles comprise catalytic metal particles adhered to high surface area carbon carrier particles having a B.E.T. surface between about 200'm2/g and about 2000 m,/g.

8. The electrode of Claim 7, wherein said catalytic metal particles comprise noble metal particles uniformly deposited on said carbon carrier particles, said noble metal being selected from the group consisting of platinum, palladium. rhodium and iridium, and being present in an amount between io20% by weight of said carbon carrier particles.

9. The electrode of any preceding claim, wherein said gas diffusion layer has a thickness between about 75 gm and about 150 Am.

1

10. The electrode of any preceding claim, wherein said catalytic layer has a thickness between 7 gm and 10 g=. and the loadin g of platinum catalyst is between 0.15 ng/c=2 and 0.5 =g/cm2.

11. The electrode of any preceding claim, wherein said catalytic layer consists of platinum alloys mixed with 5-30% PVF, and 7095% carbon particles.

12. A process for preparing a gas diffusion electrode suitable for use in fuel cells, the process comprising:

a. preparing an anisotropic gas diffusion layer that is made of a porous carbon matrix through which carbon particles and polyethersulfone are distributed such that the matrix is homogeneously porous in a direction lateral to gas flow and asymmetrically porous to gases in the direction of gas flow, the porosity of said gas diffusion layer decreasing in the direction of gas flow, said gas diffusion layer having a thickness between about 50 gm and about 300 gm, said gas diffusion layer prepared by 1) casting with a Doctor knife onto a carbon substrate a blend of polyethersulfone and carbon particles dissolved in a solvent for the polyethersulfone to form a layer of film on the carbon substrate, the blend penetrating into at least part of the carbon substrate; 2) coagulating the film in a coagulation liquid that is a non-solvent for the polyethersulfone; and 3) removing the coagulation solvent; and 1 b. painting onto the small pore surface of said gas diffusion layer a catalytic layer that is made of an ink suspension containing catalytic carbon particles and a thermoplastic polymer, said suspension comprising 0.5 to 2% thermoplastic polymer, said thermoplastic polymer being selected from the group consisting of polyethersulfone, poly(vinylidene fluoride) and sulfonated polysulfone, said catalytic layer covering the small pore surface of said gas diffusion layer, said catalytic layer having a thickness between about 7 gm,and about gm and a metal catalyst loading between about 0.1 mg/cm2 and 2 about 0.5 mg/cm

13. The process according to Claim 12, wherein in step(a)(1), said gas diffusion layer is made with a solution that,comprises 5-25 wt% of PESF in N, NI-dimethyl formamide.

14. The process according to Claim 12, wherein in step (a)(1), said carbon particles are selected from the group consisting of activated carbon, a carbon black, an acetylene black and mixture thereof, the carbon particles having a B.E.T. surface area between about 50 m2/g and 2000 mIg.

15. The process according to any of Claims 12 to 14, wherein in step (a)(1), the blend of polyethersulfone and carbon particles dissolved in a solvent for the polyetherscilfone is sonicated for a time period sufficient to homogeneously mix the polyethersulfone with the carbon particles.

16. The process according to any of Claims 12 to 15, wherein in step (aXI), the solvent for the polyethersulfone is selected from the group consisting of N,N-dimethyl formamide, N,N-dimethyl acetamide, Nmethyl pyrrolidone and dimethyl sulfoxide.

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, hexane, ethanol, water/N, NO-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 ambient and -30C.

19. The process according to any of Claims 12 to 18, wherein in step (b), said catalytic layer includes a non-ionic surfactant.

20. The process according to any of Claims 12 to 18, further include the step (c) of sintering the final electrode between 200T and 30TC for a time period between 15 min. and 2 hours.

21. The process according to any of Claims 12 to 18, wherein in step (b), said painting is carried out by the airbrush painting technique.