MXPA98002670A - Field field structures for combusti cells membrane electrode assemblies - Google Patents

Abstract

An electrochemical fuel cell having a membrane electrode assembly (1) and a flow field (4) adjacent thereto wherein the flow field comprises an electrically conductive porous material having a porosity of at least 50 percent and an average pore size of at least 35 microns. This fuel cell is capable of operating at relatively high current densities and relatively high voltages at reduced gas flow rates.

Description

FIELD FIELD STRUCTURES FOR FUEL CELLS MEMBRANE ELECTRODE ASSEMBLIES This invention relates to electrochemical fuel cells and, more particularly, to membrane electrode assemblies of fuel cells and flow field structures adjacent thereto. The electrochemical fuel cells generate electrical current through the oxidation of a fuel. One type of fuel cell employs a membrane electrode assembly ("MEA") that includes a membrane having an anode side and a cathode side, depending on the direction of the current with respect thereto. The membrane itself serves as an electrolyte. A suitable catalyst for the electrochemical reaction is applied to the membrane, or incorporated into the polymer composition from which the membrane is prepared. Alternately, the catalyst is applied to carbon fiber paper, which is then laminated to the membrane to form the membrane electrode assembly. Located on either side of the membrane electrode assembly is a flow field which typically consists of a graphite plate that has been machined to provide a series of channels on its surface, as shows, for example, in the Patents of the United States of North America Numbers: 5,300,370 and 5,230,966. The channels transport fuel to the anode side and oxidant to the side of the cathode, and transport the products of the reaction mainly from the cathode side, and are typically separated from the membrane electrode assembly by a thin layer of a carbon material porous, like carbon fiber paper. Figure 1 illustrates an embodiment of the membrane electrode assembly and the flow field of the fuel cell of the first aspect of the invention. Figure 2 illustrates a configuration of a repeating units that can be used in the preparation of a fuel cell stack containing a plurality of fuel cells arranged in series, which incorporates the membrane electrode assembly and the field flow illustrated in Figure 1. Figures 3, 4 and 5 illustrate the operation of the fuel cells referenced in Examples 1, 2, and 3. Figure 6 illustrates a membrane electrode assembly having two active layers placed on the same side of the membrane. Figure 7 shows the operation of the membrane electrode assemblies prepared as described in Example 4.

Figure 8 illustrates a membrane electrode assembly having a porous layer and a flow field adjacent thereto. Figure 9, Figure 10, Figure 11, and Figure 12 illustrate the operation of fuel cells which incorporate the porous layers prepared as described in the examples. Figure 13 illustrates the operation of the membrane / electrode assemblies described in Example 9. Figure 14 illustrates the operation of the membrane / electrode assemblies described in Example 10. In one aspect, this invention is an electrochemical fuel cell. having a membrane electrode assembly and a flow field adjacent thereto wherein the flow field comprises an electrically conductive porous material having a porosity of at least 50 percent and an average pore size of at least 35 micras . It has been found that the fuel cell of the first aspect of the invention is capable of operating at relatively high current densities and at relatively high voltages at lower gas flow rates. Typically, in a fuel cell, the membrane and polymer layer containing catalytically active metal particles ("active layer") must be hydrated in order to be ionically sufficient conductive During the operation of the fuel cell, water is formed on the cathode side of the active layer, which condenses within the adjacent flow field. Water may also be present due to the wetting of one or both of the reactive gases. However, if too much water condenses or otherwise accumulates adjacent to the active layer or within the active layer, the efficiency of the fuel cell is reduced, since the diffusion of the gas through the liquid water is slow in relationship with its diffusion through water vapor. The porosity characteristics and the pore size of the flow field in the fuel cell of the first aspect of the invention are believed to improve the mass transfer capabilities, which results in higher voltages at high current densities. It is believed, without pretending to be attached to any particular theory, that relatively high porosity and large pores combine to preserve the effective gas transport in the presence of liquid water. Because the flow of gas fed is in the plane of the flow field and is substantially parallel to the active layer, the liquid is swept away from the active layer and out of the flow field by the gas stream, thus maintaining the pores open for the effective transport of the reactive gas to the catalytically active particles. However, when the flow field is relatively thick, (eg, greater than 0.508 millimeters for an air supply stoichiometry equal to 2 to 30 psig) the gas regime becomes inadequate to maintain pores without water. In these cases, increasing the humidity of the flow field is believed to promote the type of flow condition therein which can be referred to as an annular flow, where the liquid is dispersed on the solid surfaces of the porous structure, leaving the center of the large pores open and available for the transport of effective gas. In a second aspect, this invention is a membrane electrode assembly having a solid polymer electrolyte, and at least two active layers placed on the same side of the membrane; wherein the active layers comprise catalytically active particles and an ionomer; the equivalent average weights of the ionomers in the layers differ by at least 50; and the active layer placed closest to the membrane (hereinafter, the "first" active layer) contains the ionomer with the lowest equivalent average weight. The "second" active layer (the layer placed next to the first active layer that is opposite the side facing the membrane) can either be placed adjacent to or in contact with the first active layer, or one or more additional active layers. they can be placed between the first and second active layers.

In a third aspect, this invention is a membrane electrode assembly having a solid polymer electrolyte, and at least one active layer positioned on one side of the membrane; wherein the active layer comprises (a) catalytically active particles, and (b) an ionomer having an equivalent weight in the range of 650 to 950 and which is substantially insoluble in water at temperatures below 100 ° C. It has been found that the membrane electrode assemblies ("MEA") of the second and third aspects of the invention when used in a fuel cell provide a relatively high voltage at a given current density and gas flow rate. . This equivalent weight of the ionomer is believed to affect the water content of the active layer. It is believed, without attempting to adhere to any particular theory, that the lower equivalent weight ionomer maintains a higher water content at low current densities. This higher water content improves the proton conductivity and the accessibility of the catalytically active particles, thereby increasing the voltage. However, this increase in water content can lower the performance (voltage) at higher current densities. It has been found that performance at both high and low current densities can be optimized using an active layer of multiple layers with an ionomer of different equivalent weight in each layer. It is believed, without pretending to adhere to any theory in particular, that the improved performance is a result of the differences in hydrophilicity between the layers. The lower equivalent weight ionomer adjacent to the membrane is believed to provide an area within the membrane electrode assembly that has a high water content, for better performance at low current densities, while the higher equivalent weight ionomer Less hydrophilic helps transport water away from the membrane at higher current densities. In the third aspect of the invention, the use of a relatively low equivalent weight ionomer gives better performance at lower current densities. In a fourth aspect, this invention is an electrochemical fuel cell having a membrane electrode assembly and a layer of an electrically conductive porous material adjacent thereto which has at least two portions with different average pore sizes, wherein one first portion of the layer adjacent to the membrane electrode assembly has a porosity no greater than a second portion of the layer adjacent to the opposite side of the layer; the second portion has a porosity of at least 82 percent; and the second portion has an average pore size that is at least 10 microns and at least ten times larger than the average pore size of the first portion. In a fifth aspect, this invention is an electrochemical fuel cell having a membrane electrode assembly with a porous non-woven layer of an electrically conductive porous material adjacent thereto which has at least two portions with different average pore sizes, wherein a first portion of the layer adjacent to the membrane electrode assembly has a porosity no greater than a second portion of the layer adjacent to the opposite side of the layer; the second portion has a porosity of at least 50 percent; and the second portion has an average pore size which is at least 35 microns and at least ten times larger than the average pore size of the first portion. In a sixth aspect, this invention is a process for preparing an electrochemical fuel cell having a membrane electrode assembly comprising the steps of: (a) applying a layer of a conductive composition to a sheet of a porous conductive material that has a porosity of at least 82 percent under conditions sufficient to form a solid, porous layer of the conductive composition on one side of the sheet of porous conductive material, forming a composite therewith, and (b) placing the composite adjacent to the membrane electrode assembly so that the side of the compound to which it is applied the conductive composition is in front of said assembly. It has been discovered that the fuel cells of the fourth and fifth aspects of the invention as well as the fuel cells prepared by the process of the sixth aspect of the invention are capable of operating at high current density at a relatively high voltage, have a relatively high energy density, and provide a high energy density even when operated under relatively low gas pressures. In a seventh aspect, this invention is a composition comprising (a) catalytically active particles, (b) an organic compound having a pKa of at least 18 and a basicity parameter, β, less than 0.66, and (c) a polymeric binder. In an eighth aspect, this invention is a process for preparing a membrane / electrode assembly, which comprises the sequential steps of (i) applying a layer of the composition of the first aspect of the invention to a solid polymer electrolyte, a paper of carbon fiber, or a release substrate; (ii) heating the composition under conditions sufficient to volatilize at least 95 percent of component (b); and (iii) placing the composition in contact with the solid polymer electrolyte, if the composition was not applied directly to the solid polymer electrolyte, forming the membrane / electrode assembly by this. It has been found that the composition and process of the seventh and eighth aspects of the invention, when used to prepare a membrane electrode assembly (MEA) having a solid polymer electrolyte, provides a membrane electrolyte assembly that provides a relatively high voltage at a current density and a flow rate given in a fuel cell. It is believed, without pretending to be attached to any particular theory, that the improved performance is a result of the ability of the organic compound to easily volatilize when heated, which is believed to result from a low incidence of ionic, hydrogen, or covalent bonds or partial bonds formed between the organic compound and the polymeric binder, particularly when the binder is in ionic form. Although the propensity of an organic compound to bind to the binder is difficult to quantify, the characteristics of the organic compound presented in the above compendium are measurable characteristics that are believed to be indicative of a compound with a minimal or nonexistent propensity to bind with an ionomer or a polar polymer. The pKa and the basicity parameter reflect the acidity and basicity of the compound, respectively. It is believed that the ease with which the organic compound can be removed from the ink affects significantly the pore characteristics of the resulting active layer. The easy removal of the organic compound is believed to promote a "foaming" effect in the layer, which increases the porosity of the pores of the layer. The characteristics of the pore affect the transport of water through the layer, which significantly affects the performance of the membrane electrode assembly in which it is incorporated. Further, if the composition of the seventh aspect of the invention (hereinafter "catalyst ink") is applied directly to the membrane, it will not cause it to swell excessively, since the organic compound will not bind significantly with the ionomer in the membrane . In addition, the composition of the invention allows the use of Na-i- or H + ionomer forms as the binder without significant degradation thereof when the catalyst ink is heated to volatilize the organic compound, and provides an active layer with good stability long-term. These and other advantages of these inventions will be apparent from the description that follows. Now referring to Figure 1 and the Figure 8, the term "membrane electrode assembly" (1) as used herein refers to the combination of the solid polymer electrolyte (also referred to as a "membrane") and catalytically active particles in the fuel cell assembly, regardless of your configuration or method of preparation. The layer of membrane material containing these particles is known as the "active layer", regardless of whether these particles are incorporated in a discrete layer of polymer (2) and applied or laminated to the surface of the membrane (3), or they are incorporated in the same membrane. Referring now to Figure 1, the flow field (4) is a layer of an electrically conductive porous material having a gas stream inlet and outlet connected thereto. The flow field may comprise porous carbon material. The fuel cell of the first aspect of the invention preferably contains no impervious flow field components having engraved, polished or molded flow channels configured across its entire active face. These channels administer gases directly to the active layer through a "backing layer" of porous carbon that supports the active layer, as illustrated in Figures 1 and 4 of U.S. Patent Number: 5,108,849. However, the flow field, the bipolar plates, and / or the end plates used as supports for the fuel cell and for separating the cells in a multiple cell configuration may contain one or more ducts therein to increase the flow of the reactive gases to the flow field in the fuel cell of the first aspect of the invention. An example of these pipelines is illustrated in Figure 2. Examples of suitable porous carbon materials that can be used as the flow field in the first aspect of the invention include carbon paper, graphite paper, carbon felts, or other carbon-based composites comprising at least 20 weight percent of carbon. The flow field may have interdigitated channels cut in it to decrease the pressure drop introduced into the reactive gases. When desirable, the porous carbon material can be treated with a perfluorosilane or fluorine composition to increase its hydrophobicity, or be oxidized, sulfonated, or coated with a hydrophilic material to increase its hydrophilicity. If the flow field has a thickness of at least 0.508 millimeters, it preferably has a high wettability. The wettability of the flow field can be determined experimentally by the following method: a sample of 7.62 centimeters by 7.62 centimeters of the flow field is held vertically in a tray containing water 0.5 centimeters deep. The amount of water absorbed in 10 seconds is measured by weight. Flow fields with a thickness greater than 0.508 millimeters preferably absorb at least 0.5 grams of water per gram of porous material, more preferably at least 1 gram of water per gram of porous material. The conductivity of the flow field layer in the The first aspect of the invention is preferably at least 0.01 Siemens / centimeter (S / cm), more preferably at least 0.1 Siemens / centimeter, and more preferably at least 0.2 Siemens / centimeter. The preferred thickness of the flow field will depend on the optimum pressure drop across the flow field, but is preferably at least 0.0254 millimeters, more preferably at least 0.127 millimeters, and is more preferably at least 0.254 millimeters, but is preferably no greater of 6.35 mm, more preferably not greater than 2.54 mm, and is more preferably not greater than 1.27 mm. The porosity of the flow field is preferably at least 75 percent, more preferably at least 80 percent. The average pore size of the flow field is preferably at least 45 microns, more preferably at least 50 microns; but it is preferably not more than 250 microns. The term "average pore size" as used herein means that half the open volume of the material is contained in pores larger in diameter than the average pore size, and half is contained in pores equal to or less than the pore size. average pore size. The average pore size can be measured by any convenient method, such as by mercury porosimetry. The device used to measure the distribution of the average pore size of the layer can be calibrated using calibration standards of silicon oxide / aluminum oxide (available from Micrometics, Norcross, GA). All membrane electrode assemblies as described herein may be prepared by any convenient technique unless otherwise advised. In one technique, a catalyst "ink" layer is first applied to a solid polymer electrolyte, a carbon fiber paper, or a release substrate. Catalyst inks typically comprise catalytically active particles (such as platinum supported on carbon), binder, solvent or dispersion aid and, optionally, a plasticizing agent. Preferably, the ink comprises catalytically active particles, and at least one compound that functions as an ionomer, such as a polytetrafluoroethylene polymer having sulfonic acid groups and an equivalent weight (based on acid groups) in the range of 650 to 1400 The ink also preferably contains an organic solvent or dispersion aid which allows the application of a thin, uniform layer of the catalyst / ionomer mixture to the solid polymer electrolyte, carbon fiber paper, or release substrate. In order to prepare the membrane electrode assembly of the second aspect of the invention, a layer of catalyst "ink" is first applied to a solid polymer electrolyte, and then a layer of a second and third ink is applied. applies to the portion of the membrane electrode assembly opposite to the first active layer or to a release substrate, or to the top of the first active layer. The term "active layer" as used herein refers to a layer comprising a mixture of ionomer and catalytically active particles. The assembly in the fuel cell of the first aspect of the invention can be prepared by any convenient method, but is preferably prepared by applying the catalyst ink (a suspension or dispersion of the catalytically active particles) directly to a solid polymer electrolyte as described , for example, in U.S. Patent Number 5, 211,984. The ink is applied to the membrane in one or more applications sufficient to give a desired charge of catalytically active particles. Preferably, the layer of catalytically active particles is prepared by applying at least two inks in separate passages to form layers of the different inks, with the binder ink of higher equivalent weight applied in such a way that it will be placed adjacent to the flow field in the fuel cell. In these cases, the membrane electrode assembly comprises a solid polymer membrane having at least two layers of catalyst ink on at least one side thereof, wherein at least two layers of the ink The catalyst comprises polytetrafluoroethylene polymers having pendant sulfonic acid groups, the equivalent weights of which differ by more than 50, and wherein the layer having the highest equivalent weight is placed adjacent to the flow field. As soon as it is prepared, the membrane electrode assembly is placed next to the flow field in the fuel cell assembly. The fuel cells described herein may be incorporated in a multi-cell or "stacked" assembly comprising a number of fuel cells arranged in series. An example of a repeating unit is illustrated in Figure 2, which shows an anode flow field (5), a membrane electrode assembly (6), a cathode flow field (7), and a separator plate. bipolar (8). The bipolar separator plate has ducts (9) and (10) which transport the reactants and the products of the reaction to and from the flow field. In this configuration, the membrane electrode assembly is located between two porous flow fields having an inert material impregnated in the boundary regions (the darker areas in the figure) in order to prevent the reactive gases from escaping to the Exterior. The holes in the boundary regions of all the elements together form a gas manifold when they are stacked together and placed under compression. The material used to prepare the bipolar plate separator can be selected from a variety of rigid or non-rigid materials, and the plate has gas delivery ducts molded or embossed on its surface. These pipelines administer reactive gases to, and remove products from the reaction of, the porous flow fields. In an alternative mode, gases and products can be introduced or removed via ducts or open spaces in the porous flow field connected to the manifolds. The bipolar separating plate may also have an internal structure for circulating a cooling fluid therein. The ink from which the second active layer is prepared is applied to the upper part of the first active layer, the first active layer is preferably dried first sufficiently before the application of the second ink to avoid too much mixing of the inks. However, a lower degree of mixing of the inks at their point of contact with each other may be desirable since it will promote electrical and ionic conductivity between the active layers. After the inks have been applied, they are preferably heated under conditions sufficient to volatilize at least 95 percent of any organic solvent or dispersion aid present in the inks. The term "solid polymer electrolyte" as used herein refers to a porous layer composed of a solid polymer having a conductivity of at least 1 x 10"3 Siemens / centimeter (S / cm) under the conditions of operation of the fuel cell or electrolytic cell, or which can be reacted with acid or base to generate a porous layer having this conductivity. Preferably, the solid polymer electrolyte comprises a film of a sulfonated fluoropolymer, or a layered composite or films of sulfonated fluoropolymers having different equivalent weights. After the application of a catalyst ink to a solid polymer electrolyte, the ink is preferably heated under conditions sufficient to remove sufficient organic solvent or dispersing aid so that the active layer comprises at least 99 weight percent, more preferably when minus 99.9 weight percent of the mixture of catalytically active particles and the ionomer. The ink is applied in an amount sufficient to provide a layer of the mixture which has a thickness of at least 1 μm, more preferably at least 5 μm, and more preferably at least 10 μm; but it is preferably not greater than 30 μm. The porosity of the layer is preferably at least 30 percent, more preferably at least 50 percent; but it is preferably not greater than 90 percent, more preferably not greater than 60 percent. The average pore size of the layer is preferably at least 0.01 μm, more preferably at least 0.03 μm; but it is preferably not greater than 10 μm, more preferably not greater than 0.5 μm, and more preferably 0.1 μm. The thickness, porosity, and pore size characteristics referred to above refer to the measurements taken when the ionomer (ionomers) contained in the layer are in their dry and protonated form. After this, the components of the membrane electrode assemblies of the second and third aspects of the invention are assembled by placing one of the active layers in contact with the solid polymer electrolyte, and then placing the second and third active layer in such a way that is between the first active layer and the porous carbon material, forming the membrane / electrode assembly therethrough. The term "catalytically active particles" as used herein refers to particles of a metal or compound that are catalytic for the electro-reduction of oxygen or electro-oxidation of hydrogen or methanol under the conditions of pressure and temperature in the cell made out of fuel. Examples of these particles that are useful include particles of platinum, ruthenium, gold, palladium, rhodium, iridium, electroconductive and reduced oxides thereof, and alloys of these materials, either in combination with each other or with other transition metals. The particles may be supported in a convenient material, if desired, as carbon black. Preferably, the particles catalytically active are platinum particles supported on carbon, which preferably contains from 10 percent to 30 percent by weight of platinum. The size of the catalytically active particles (on an unsupported base) is preferably 10 A, more preferably at least 20 A; but is preferably not greater than 500 Á, more preferably not greater than 200 Á. Larger size particles can also be used, or they can be formed during the operation of the cell by agglomeration of smaller particles. However, the use of these particles can result in decreased cell performance. The catalytically active particles are preferably used in an amount sufficient to provide an optimum catalytic effect under the operating conditions of the electrochemical device in which they are employed. Preferably, they are used in an amount sufficient to provide a level of charge on the cathode side of the membrane of at least 0.05 milligram / square centimeter, more preferably at least 0.1 milligram / square centimeter, and more preferably at least 0.15 milligram / square centimeter; but is preferably not greater than 0.45 milligrams / square centimeter, more preferably not greater than 0.35 milligrams / square centimeter, and is more preferably not greater than 0.25 milligrams / square centimeter. The level of load on the side The anode of the membrane is preferably at least 0.01 milligrams / square centimeter, but not greater than 0.15 milligrams / square centimeter. Relative to the amount of the ionomer, however, the particles are preferably present in the ink in an amount, based on the weight of the catalytic particles, including their support, if any, sufficient to provide a weight ratio of particles: ionomer at least 2: 1, but preferably not greater than 5: 1. Examples of organic compounds suitable for use in the preparation of the catalyst ink (except for the ink of the seventh aspect of the invention) include polar solvents such as glycerin, alcohols of 1 to 6 carbon atoms, and other compounds such as ethylene carbonate, propylene carbonate, butylene carbonate, ethylene carbamate, propylene carbamate, butylene carbamate, acetone, acetonitrile, difluorobenzene, and sulfolane, but is more preferably propylene carbonate. The organic compound is preferably present in an amount, based on the weight of the composition, of at least 10 percent, more preferably at least 20 percent, and more preferably at least 30 percent; but it is preferably not greater than 90 percent. These solvents in the ink work mainly as solvents or auxiliary dipersers.

Suitable ionomers for use in the preparation of the catalyst inks described herein include any polymer or oligomer having an ionic conductivity of at least 1 x 10 ~ 3 Siemens / centimeter, more preferably at least 10"1 Siemens / centimeter (low the operating conditions of the fuel cell or electrolytic cell), or which can be reacted with acid or base to generate an oligomer or polymer having ionic conductivity Examples of suitable ionomers include fluoropolymers having pendant ion exchange groups, as sulfonic acid groups in proton or salt form Examples of these include sulfonic fluoropolymers having fluoropolymer backbones and pending ion exchange groups of 1 to 5 carbon atoms attached thereto and ending in a sulfonyl group are convenient for use in the present invention Examples of these fluoropo Liters of sulfonic ion exchange groups are illustrated, for example, in U.S. Patent Number: 4,578,512; 4,554,112; 4,515,989; 4,478,695; 4,470,889; 4,462,877; 4,417,969; 4,358,545; 4,358,412; 4,337,211; 4,337,137; and 4,330,654. Preferably, the ionomer has a substantially fluorinated polymer core structure and a recurring pendant group having the formula: -O- (CFR) a- (CFR ') b-S03M (I) wherein: a and b are independently 0 or an integer from 1 to 3; a + b is at least 1; R and R 'are independently selected from halogen, perfluoroalkyl, and fluorochloroalkyl; and M is independently selected from hydrogen or an alkali metal. Other useful ionomers to form both thick and thin composite membrane layers are characterized by a substantially fluorinated polymer core structure and a recurring pendant group having the formula: 0- (CFR) a - (CFR ') b -0- ( CF2) c -S03M (II) wherein: a and b are independently 0 or an integer from 1 to 3; c is an integer from 1 to 3; a + b is at least 1; R and R 'are independently selected from perfluoroalkyl, halogen and fluorochloroalkyl; and M is independently selected from hydrogen or an alkali metal. Ionomers having the above formulas are described in the Patents of the United States of North America Numbers: 4,478,695; 4,417,969; 4,358,545; 4,940,525; 3,282,875; and 4,329,435. The ionomer is preferably present in an amount, based on the weight of the composition, of at least 0.5 percent but preferably not more than 5 percent. The ionomer can be used in any ionic form, such as the proton form or salt form of the corresponding oligomer or polymer.

Examples of salt forms include quaternary ammonium, sodium, lithium, and potassium. In the second aspect of the invention, the ionomers used to prepare the inks preferably have an equivalent weight, based on the number of outstanding ionic groups per molecule, of at least 600, more preferably at least 700, and preferably not greater than 1200, more preferably not greater than 950. However, the ionomer must also be substantially insoluble in water at temperatures below 100 ° C; therefore, the minimum equivalent weight for certain fluoropolymers may be higher. The term "substantially insoluble in water" as used herein means that the pure ionomer in the ionic form is at least 75 percent insoluble in distilled water at any concentration. The difference between the equivalent weight of the ionomers in at least two of the inks used to prepare the membrane electrode assembly is preferably at least 50, more preferably at least 100, and more preferably at least 300; but is preferably not greater than 800, more preferably not greater than 600, and more preferably not greater than 400. In the second and third aspects of the invention, the ionomer used to prepare the ink preferably has an equivalent weight of at least 650 , more preferably at least 700, and is more preferably at least 770; but it is preferably not greater than 950, more preferably not greater than 900, and more preferably not greater than 840. The equivalent weight of the ionomer can be determined by any convenient technique, such as titration with a base, as illustrated in the United States Patent United States of America Number: 4,940,525. Referring now to Figure 6, which illustrates, the membrane electrode assembly of the second aspect of the invention, a membrane (11) is shown, having two active layers placed on each side of the membrane. The active layers closest to the membrane (12, 13) contain ionomers having lower equivalent weights than the active layers placed adjacent thereto (14, 15). Referring now to Figure 8, which illustrates the membrane electrode assembly of the fourth and fifth aspects of the invention, the porous layer (16) is a layer of electrically conductive porous material having at least two portions with different average sizes. of pores and is located between the active layer and the flow field. The flow field (17) may comprise a machined graphite plate, or may be primarily composed of a thicker layer of porous carbon material as described, for example, in U.S. Patent Number: 5,252,410. However, the porous layer (16) does not It contains no catalyst that are typically present in the active layer, such as platinum. The fuel cells of the fourth and fifth aspects of the invention contain a layer of an electrically conductive porous material (hereinafter, "intermediate layer") which is adjacent to the membrane electrode assembly and has at least two portions with different average pore sizes. The portion of the layer adjacent to the membrane electrode assembly (18) (hereinafter "small pore region") has an average pore size that is at least ten times smaller than the portion of the layer adjacent to the opposite side of the layer (19) (hereinafter, "large pore region"). Suitable compositions for use in the preparation of the intermediate layer include any organic or inorganic composition that can be manufactured into a solid layer having the porosity and pore size characteristics referred to above, and which also has sufficient dimensional, hydrolytic and oxidative stability under the operating conditions of the fuel cell. One method of preparing an intermediate layer having an asymmetric pore size characteristics-is to prepare this layer from two or more materials having different average pore sizes. An example of this method is to first obtain or prepare a material that has a size of intermediate pore suitable for the large pore region (hereinafter, "large pore material"), and then infiltrate and / or coat one side of the material with a composition that will reduce the porosity of a portion of the material sufficiently to obtain the desired smaller pore size, and / or forming a discrete layer of the composition on the outside of the material having the desired small pore characteristics. Typically, in a fuel cell, the membrane and the polymer layer containing a metal catalyst ("active layer") must be hydrated in order to be sufficiently ionically conductive. During operation of the fuel cell, water is formed on the cathode side of the active layer, which condenses within the adjacent flow field. Water may also be present due to the wetting of one or both of the reactive gases. However, if too much water is condensed or otherwise accumulated adjacent to the active layer or within the active layer, the efficiency of the fuel cell is reduced, since the diffusion of the gas through the liquid water is slow with relation to its diffusion through water vapor. It is believed, without pretending to stick, that the region of small pores of the layer reduces the accumulation of excess liquid water in or near the active layer because it serves as a semipermeable layer or membrane that allows the water vapor generated inside the active or present layer due to the wetting of the reactive gases to pass between the active layer and the flow field, but reduces or prevents condensation of the water in the active layer and reduces or prevents liquid water present in the flow field or in the large pore region of the intermediate layer from passing back through the small pore region to the active layer. Preferably, the wettability (determined by the pore size and the contact angle of the solid with water) of the region of small pores is such that for a sufficiently large fraction of the pores the displacement pressure required to force the liquid water into these pores is greater than the hydraulic pressure in the components of the flow field under the prevailing condition of pressure and temperature in the fuel cell. Examples of suitable organic compositions that can be used to prepare or infiltrate the large pore material include polymeric or oligomeric thermoplastic or thermoset materials, such as polytetrafluoroethylenes, including those having sulfonic acid groups, (such as Nafion ™, available from DuPont), oxides of (poly) alkylene, polyolefins, polycarbonates, benzocyclobutanes, perfluorocyclobutanes, polyvinyl alcohols, and polystyrene, epoxy resins, copolymers of perfluoroalkyl / acrylic, polyanilines, polypyrroles, as well as mixtures thereof. Preferably, the composition is a polytetrafluoroethylene, perfluoroalkyl / acrylic copolymer, or a perfluorocyclobutane, and is more preferably a perfluorocyclobutane. Examples of suitable inorganic compositions that can be used include clays, silicates, and titanium-based compositions. The composition used to prepare the small pore region of the intermediate layer preferably contains polymer, carbon particles, and a convenient vehicle. The vehicle will typically infiltrate the entire large pore material, although most of the polymer and carbon particles were collected on or near the surface of the side of the material to which it is applied (depending on its porosity and the size of the contained particles). in the composition), thereby forming the region of small pores on the side of the material to which the composition is applied. Accordingly, the regions or portions of the intermediate layer having different average pore sizes are not necessarily discrete layers, as long as at least the first depth of the small pore region and at least the first 50 microns of Depth of the large pore region (measured from the surface of the layer in a direction perpendicular to the layer) has the necessary pore characteristics.

The intermediate layer can also be prepared by applying the composition used in the preparation of the small pore region to the membrane electrode assembly, and then placing or laminating a layer of a large pore material adjacent thereto. Alternatively, a film of the composition used in the preparation of the small pore region can be prepared separately using conventional filmmaking techniques, and then placed or laminated between the membrane electrode assembly and the large pore material. If the composition is applied to the membrane electrode assembly, it can be applied using any convenient coating technique such as painting or screen printing. The small pore region of the intermediate layer is preferably at least as hydrophobic as the active layer. The composition used to prepare the small pore region is preferably a liquid-based composition that will solidify after application. If the composition to be applied is solvent-based, sufficient solvent is removed to form a solid layer of material before assembling the fuel cell. This solvent can be removed either at ambient conditions or at elevated temperatures. If appropriate, the composition is heated to increase its stability and uniformity, such as by crosslinking, advancing molecular weight, or agglomerating latex particles. If the composition used to prepare the small pore region is to be applied directly to the membrane electrode assembly, most of the dissolved solids contained therein (such as the polymer) are preferably hydrophilic in nature, since the membrane and the Active layer is usually prepared from a hydrophilic composition, and application of a solution of mostly hydrophobic solids will normally be expected to adversely affect the properties of the active layer. However, the composition used to prepare the small pore region is still preferably hydrophobic after it has been cured. Hydrophobic fillers, such as carbon fibers and / or powders treated with hydrophobic compositions such as the silane and fluorine based compositions, can be used in the compositions used to prepare the small pore region to give it some hydrophobic character and affect the wettability of its pores, as well as increasing the porosity and the average pore size of the solidified composition. In these cases, the weight ratio of the carbon fibers or powders against the other components in the composition is preferably at least 1: 1, more preferably at least 3: 1, and preferably not greater than 10: 1, more preferably not greater than 5: 1; and is more preferably 3: 1.

If the small pore region is prepared by applying the composition to a large pore material, such as a graphite paper, the relatively fine pore structure of the paper will help keep most of the fillers in the composition near the surface on the side. of the paper to which it is applied. Alternatively, the composition may be one that is primarily hydrophilic as applied, but hydrophobic after cure, such as a polytetrafluoroethylene latex. If the small pore region is prepared by applying a hydrophilic composition to a large pore material, a thin coating of a highly hydrophobic material can be applied as ZonylMR 7040, a perfluoroalkyl-acrylic copolymer available from DuPont, next to the pore region small that faces the membrane electrode assembly to further increase hydrophobicity. Other examples of highly hydrophobic materials include Fluorad ™ FC722 and FC 724 available in 3M. The membrane electrode assembly of the fourth and fifth aspects of the invention is preferably prepared by applying the catalyst ink (a suspension or dispersion of the catalyst) directly to the membrane as described, for example, in the U.S. Patent Number : 5, 211,984. If the catalyst is to be applied to a porous carbon material, the composition used to prepare the small pore region is preferably applied first, followed by the catalyst ink, so that the infiltrated porous carbon material can be used as an intermediate layer, as well as a support layer for the catalyst. However, this method, as well as any method that requires the preparation of a separate film for the intermediate layer is less preferred since these films and catalyst-containing structures typically have to be laminated to the membrane portion of the membrane electrode assembly with In order to assemble the fuel cell. These lamination processes, where heat and / or excessive pressure is applied to the intermediate layer, can alter or damage its pore structure. In addition, the composition of the intermediate layer can be formulated to optimize the maximum voltage at which the fuel cell will operate at a given current density. It is believed that higher voltages at higher current density require that the small pore region be more hydrophobic than at lower current densities. For example, if a higher voltage is desired at a lower current density, compositions having a higher carbon / polymer ratio (such as 5: 1) are preferred for use in the preparation of the small pore region, particularly when applied to a graphite paper having a relatively low porosity. In the same way, if higher voltages are preferred at more current densities high, lower carbon / polymer ratios (such as 3: 1) are preferred, particularly when applied to a graphite paper having a relatively high porosity. The small pore region preferably has a thickness in the range of from 1 micron to 150 microns (measured in a direction perpendicular to the intermediate layer), and has the desired porosity and pore size characteristics. More preferably, the region has a thickness in the range of from 5 to 25 microns. Preferably, the portion of the region adjacent to the membrane electrode assembly is sufficiently porous to allow the transmission of water vapor through the region. The porosity of this portion of the region is preferably at least 10 percent. The average pore size of the small pore region is preferably at least 0.1 micron, more preferably at least 1 micron; but preferably it is not greater than 10 microns. The average pore size can be measured by any convenient method, such as by mercury porosimetry. The device used to measure the average pore size distribution of the layer can be calibrated using silicon oxide / aluminum oxide calibration standards (available from Micrometics, Norcross, GA). The term "average pore size" as used herein means that half the open volume of the material is contained in pores larger in diameter than the average pore size, and half is contained in pores equal to or less than the average pore size. The porosity of the small pore region is preferably at least 10 percent. Conductive fillers and non-conductive inert or fugitive fillers can be incorporated into the composition to achieve the desired pore structure. Intrinsically, conductive polymers such as doped polyaniline or polypyrrole can also be used to prepare the composition in order to increase its conductivity. The pore structure of the small pore region can also be controlled to some extent by selecting the polymer or using an oligomeric composition. The region of large pores has a thickness of at least 0.0508 millimeters, more preferably at least 0.1524 millimeters; but it is preferably not greater than 1.27 millimeters. The porosity of this region is preferably at least 82 percent, more preferably at least 85 percent, and most preferably at least 87.5 percent. The average pore size of the large pore region is preferably at least 30 microns. The porosity and pore size values given above represent the characteristics of the small pore region for at least the first half of its depth from the side of the intermediate layer near the membrane electrode assembly and at least the first 50 microns of its depth from the opposite side of the intermediate layer regardless of its method of preparation. Examples of suitable porous carbon materials that can be used as the large pros material include carbon paper, graphite paper, carbon felts, or other carbon-based composites comprising at least 20 percent of the weight of the carbon. When desired, the porous carbon material can be treated with a perfluorosilane or fluorine composition to increase its hydrophobicity, or it can be oxidized, sulfonated or coated with a hydrophilic material to increase its hydrophilicity. If a porous carbon material is used both as a flow field and as a large pore material, it may have interdigitated channels cut in it to lower the pressure drop introduced into the reaction gases. The conductivity of the intermediate layer is preferably at least 0.01 Siemens / centimeter (S / cm), more preferably at least 0.1 Siemens / centimeter, and more preferably at least 10 Siemens / centimeter. The conductivity of the layer can be increased by the addition of conductive fillers, such as fibers or carbon particles, or by the incorporation of conductive salts or polymers. The term "catalytically active particles" as used herein refers to particles of a metal or compound that is catalytic for electro-reduction of oxygen or electro-oxidation of oxygen or electro-oxidation of hydrogen or methanol under the conditions of pressure and temperature in the fuel cell . Examples of these particles that are useful include particles of platinum, ruthenium, gold, palladium, rhodium, iridium, electroconductive and reduced oxides thereof, and alloys of these materials, either in combination with each other or with other transition metals. The particles may be supported in a convenient material, if desired, as carbon black. Preferably, the catalytically active particles are platinum particles supported on carbon, which preferably contains from 10 percent to 30 percent by weight of platinum. The size of the catalyst particles (on an unsupported base) is preferably 10 A, more preferably at least 20 A; but is preferably not greater than 500 Á, more preferably not greater than 200 Á. The particles are preferably used in an amount sufficient to provide an optimum catalytic effect under the operating conditions of the electrochemical device in which they are employed. In relation to the amount of binder, however, the particles are preferably present in the ink in an amount, based on the weight of the catalytic particle, including its support, if sufficient. to provide a proportion of component (a): component (c) at least 2: 1, but preferably not more than 5: 1. Convenient organic compounds include organic compounds having a pKa (the negative logarithm (to base 10) of the equilibrium constant, K, for the reaction between the compound and water) of at least 18 and a basicity parameter, β, less than 0.66. Preferably, the pKa is at least 25. Preferably, β is less than 0. 48, and is more preferably less than 0.40. The basicity parameter for many organic compounds, as well as reference procedures for their determination, are described in Kamlet et al., "Linear Solvation Energy Relationships." 23. A Comprehensive Collection of the Solvochromatic Parameters, n *, ot, and j, and Some Methods for Simplifying the Generalized Solvatochromic Equation, "J. Pray Chem., Vol. 48, pp. 2877-2887 (1983). Preferably, the compound is volatilized at temperatures in the range of from 100 ° C to 250 ° C without significant degradation which may impair the performance of the active layer. A relatively low volatilization temperature is also preferred, since the organic compounds (component (b)) that are not removed from the layer can add to the electrical resistance of the layer, causing an operation of the meaner membrane electrode assembly. This characteristic is particularly important when the binder is used in its proton form, since the binder will act as a catalyst to further promote the degradation of any residual organic compound. The use of a proton form of the binder has advantages, however, since the quaternary ammonium cations present in an ink composition are difficult to remove and can contribute to a long period of "outage" when a fuel cell or battery of fuel cells is initially started. Preferably the boiling point of the solvent is greater than 100 ° C so that after curing the ink, water or low boiling solvents may be present in the ink (typically introduced into the ink as a commercially available binder). available containing these components) are removed first. Examples of organic compounds suitable for use as component (b) include ethylene carbonate, propylene carbonate, butylene carbonate, ethylene carbamate, propylene carbamate, and butylene carbamate, acetone, acetonitrile, difluorobenzene, and sulfolane, but prefers more propylene carbonate. The organic compound is preferably present in an amount, based on the weight of the composition, of at least 10 percent, more preferably at least 20 percent, and is more preferably at least 30 percent; but preferably it is not greater than 90 percent. Suitable polymeric binders for use in the preparation of the composition of the invention include any polymer or oligomer having ionic conductivity of at least 1 x 10"3 Siemens / centimeter (under the fuel cell operating conditions) or the electrolytic cell, or which can be reacted with acid or base to generate an oligomer or polymer having ionic conductivity.If the binder has pendent ionic groups, it preferably has an equivalent weight of at least 600, more preferably at least 700, and preferably not greater than 1200, more preferably not greater than 950. The equivalent weight of the binder is based on the number of outstanding ionic groups per molecule, as can be determined by any convenient technique, such as titration with a base, as illustrated in U.S. Patent Number: 4,940,525 Examples of suitable binders i They include perfluorinated polymers and polytetrafluoroethylene polymers, and polytetrafluoroethylene polymers having pendant sulfonic acid groups, (such as Nafion®, available from DuPont). The binder is preferably present in an amount, based on the weight of the composition, of at least 0.5 percent, but preferably not more than 5 percent. An advantage of present invention is that the ionomer can be used in any ionic form, such as the proton form or salt form of the oligomer or polymer. Examples of salt forms include quaternary ammonium, sodium, lithium and potassium. The membrane electrode assembly can be prepared by any convenient technique, including the process of the second aspect of the invention. Preferably, the membrane electrode assembly is prepared by applying one or more layers of the catalyst ink (the composition of the invention) directly to the solid polymer electrolyte as described, for example, in the United States of America Patent Number: 5,211,984. The term "solid polymer electrolyte" as used herein refers to a membrane composed of a solid polymer having a conductivity of at least 1 x 10"3 Siemens / centimeter under the operating conditions of the fuel cell or electrolytic cell, or which can be reacted with acid or base to generate a membrane having this conductivity Preferably, the solid polymer electrolyte comprises a film of a sulfonated fluoropolymer Another method comprises applying one or more layers of the catalyst ink to a release material, such as a substrate coated with polytetrafluoroethylene, cure the ink, and then laminate the material cured to the membrane A third method comprises applying one or more layers of the catalyst ink to one side of a sheet of porous carbon material, such as carbon paper or graphite, and then place the side of the material to which the ink is applied adjacent to the membrane. If the ink is cured before being placed near the membrane, it should preferably be laminated to the membrane to ensure good contact between the two. The ink can be cured using any convenient method to remove at least 95 percent of the component (b), as well as any other volatile organic solvent contained in the ink, such as by heating to an elevated temperature optionally under reduced pressure. Preferably, the ink is heated to a temperature at which component (b) is volatile, but below its boiling point. If more than one ink is used to prepare the active layer of the membrane electrode assembly, the inks preferably contain a polytetrafluoroethylene polymer having pendant sulfonic acid groups as the binder, and the ink layer closest to the membrane has a equivalent weight that differs from the equivalent weight of the binder in the ink layer adjacent thereto by at least 50. In addition, the layer having the binder of lower equivalent weight is preferably placed adjacent to the solid polymer electrolyte. Preferably, the ink is heated under conditions sufficient to remove at least 99 by percent, more preferably at least 99.9 percent of component (b). The ink is applied in an amount sufficient to provide a layer of the composition which, when dried and protonated, has a thickness of at least 1 μm, more preferably at least 5 μm, and more preferably at least 10 μm; but it is preferably not greater than 30 μm. The porosity of the layer is preferably at least 30 percent, more preferably at least 50 percent; but it is preferably not greater than 90 percent, more preferably not greater than 60 percent. The average pore size of the layer is preferably at least 0.01 μm, more preferably at least 0.03 μm; but is preferably not greater than 10 μm, more preferably not greater than 0.5 μm, and is more preferably 0.1 μm. The following examples are given to illustrate the invention and should not be construed as limiting in any way. Unless stated otherwise, all parts and percentages are given by weight. Example 1 The membrane and electrode structures were prepared as follows: An ion exchange membrane prepared from perfluorosulfonic acid ionomer having an equivalent weight of 800, a thickness of 60 microns dry and 127 microns fully hydrated (available in The Dow Chemical Company as XUS 13204.20) was obtained and cut in leaves of 11 centimeters by 11 centimeters and was placed in a bath of NaOH to convert it to the Na + form. The electrode ink was prepared by mixing 1.08 grams of a 5.79 weight percent solution of the above ionomer (in a 50:50 volume percent ethanol / water solution), 0.1875 grams of 20 weight percent platinum in carbon (available from E-TEK (Natick, MA)) and 0.114 grams of an IM solution of tetrabutylammonium hydroxide (TBAOH, a plasticizing agent) in methanol, and 0.6 grams of propylene carbonate (dispersing aid). The mixture was stirred with a stir bar overnight or until the mixture was uniformly dispersed. An additional 1.2 grams of propylene carbonate was added to the mixture. The catalyst ink was painted on clean, 9 square centimeter polytetrafluoroethylene coated glass fiber parts (CHR Industries, New Haven, CT) that had been oven dried at 110 ° C and preweighed. The pieces were painted twice with the catalyst ink, which was completely dried before the application of the second layer. The Pt charges were 0.14 milligrams / square centimeter on the anode and 0.25 milligrams / square centimeter on the cathode. The membrane electrode assembly was formed by aligning a coated piece on each side of the ionomer membrane that had been dried on a vacuum board. The pieces and the membrane were placed between two pieces of stainless steel to hold them while they were placed in the press. In assembly it was placed in a press at 195 ° C and pressed at a pressure of 45 kilograms per square centimeter of the piece for 5 minutes. The press pack was allowed to cool to room temperature before opening. The piece of the layer containing catalytically active particles was detached, leaving the film adhered to the surface of the membrane. The cathode flow field was carbon paper with a porosity of 90 percent and a thickness of 0.6 millimeters (available as Spectracarb ™ paper from Spectracorp (Lawrence, MA)). The wettability of the paper was increased by oxidation in a medium having 0.006M of silver sulfate, 0.2M of sodium persulfate, and 0.5M of sulfuric acid, at a temperature of 60 ° C for 1 hour. A sample of 7.62 centimeters by 7.62 centimeters of paper oxidized in this way imbibed 2.7 grams of water per gram of carbon, when held vertically in a tray with water 0.5 centimeters deep for 10 seconds. The anode flow field was carbon paper with a porosity of 79 percent and a thickness of 0.35 millimeters. The membrane electrode assembly and the porous flow fields were tested in a test fuel cell prepared by Fuel Cell Technologies, Inc. (Santa Fe, NM). The membrane electrode assembly and the flow fields were placed between two blocks of solid graphite, each one with a single gas administration channel and a single output channel. The cell was placed on a single cell test pedestal made by Fuel Cell Technologies, Inc. The operation of the flow field is illustrated in Figures 4 and 5, under the flow conditions presented in Table I. Air was used as oxidizing gas unless otherwise indicated in Table I. * oxygen as oxidant Example 2 A membrane / electrode electrode assembly structure was prepared using the procedure described in Example 1, except that the electrode ink was prepared by mixing 1 gram of a 5 percent solution of Nafion ™ (a polytetrafluoroethylene of equivalent weight of 1100 having sulfonic acid groups, available from DuPont), 0.130 grams of platinum on carbon with 20 weight percent of platinum, and 0.76 grams of an IVM solution of tetrabutylammonium hydroxide (TBAOH) in methanol, and 1.2 grams of propylene carbonate (an auxiliary dispersant). A fuel cell was assembled and tested according to the procedure described in Example 1. The behavior of the flow field is illustrated in Figures 3, 4 and 5, under the flow conditions presented in Table I. An examination careful of the curve for Example 2a suggests that the limitation of apparent mass transport in the curve was not caused by the presence of liquid in the flow field but by the consumption of all the oxygen in the air fed to the fuel cell . The limiting current was only below 2 A / cm2 when the stoichiometry was IX at 2A / cm2. The curve shows that the fuel cell can be operated at 2 A / cm2 with air stoichiometry only slightly more than 1. When this fuel cell is incorporated into a multiple cell stack as part of a power generation system, the ability to operate at this low stoichiometry would help minimize the cost of a gas pressurization subsystem. Other increases in the flow regimes (Example 2b, 2c) flatten the curve, suggesting that the behavior was almost completely limited by the cell resistance and also confirmed the absence of mass transfer limitations. A performance curve obtained using pure oxygen as the fed gas is shown in Figure 3 as Example 2d. Example 3 A membrane / electrode electrode assembly structure was prepared using the procedure described in Example 1, except that two electrode inks were prepared separately as described in Examples 1 and 2. The ink of Example 2 was applied on the piece of fiberglass to be used with the cathode side of the membrane, and allowed to dry completely, followed by an application of the ink described in Example 1. The ink described in Example 1 was applied to the piece of fiberglass to be used with the anode side of the membrane. The platinum charges were 0.14 milligrams / square centimeter on the anode side of the membrane and 0.25 milligrams / square centimeter on the cathode side of the membrane. A fuel cell was assembled and tested as described in Example 1. The operation of the fuel cell was illustrated in Figures 4 and 5, under the flow conditions presented in Table I. Example 4 An assembly was prepared. membrane / electrode as follows: an ion exchange membrane prepared from a perfluorosulfonic acid ionomer with a weight equivalent (EW) of 800, a thickness of 0.06 millimeters dry and 0.12 millimeters completely hydrated was obtained and cut into sheets of 11 centimeters by 11 centimeters and placed in a bath of NaOH to convert it to the Na + form. The electrode ink was prepared by mixing 2.03 grams of a 3.7 weight percent solution of a perfluorosulfonic acid ionomer with an equivalent weight (EW) of 770 (in a 50:50 volume percent solution of ethanol / water), 0.1875 grams of 20 weight percent platinum in carbon (available from E-TEK, Natick, MA) and 0.105 grams of tetrabutylammonium hydroxide (TBAOH), and 0.6 grams of glycerin. The mixture was stirred with a stir bar overnight or until the mixture was uniformly dispersed. An additional 1.2 grams of glycerin was added to the mixture. The catalyst ink was applied to clean, 9 square centimeter polytetrafluoroethylene coated fiberglass parts (CHR Industries, New Haven, CT) that had been dried in an oven at 110 ° C and preweighed. The pieces were painted twice more with the catalyst ink, which was completely dried before the application of the second and third layers. The membrane electrode assembly was formed by aligning a coated piece on each side of the ionomer membrane that had been dried on a vacuum board. The pieces and the membrane were placed in a press at 195 ° C and pressed at a pressure of 45 kilograms per square centimeter of the piece for 5 minutes. The press pack was allowed to cool to room temperature before opening. The piece of the catalyst layer was detached, leaving the film adhered to the surface of the membrane. The platinum charges and the thicknesses of the catalyst layer were 0.14 milligram / square centimeter and 5 μm on the anode side of the membrane, 0.25 milligram / square centimeter and 8 μm on the cathode side of the membrane, respectively. Separate intermediate layers (between the membrane electrode assembly and the flow field) of a graphite cloth impregnated with a mixture of carbon and polytetrafluoroethylene particles (available as ELAT from E-TEK, Inc., Natick, MA) were placed close to both active layers in the cell assembly and held in place by a polytetrafluoroethylene film packing and compression of the cell. The resulting assemblies were tested in a test fuel cell prepared by Fuel Cell Technologies, Inc. (Santa Fe, NM). The flow fields were composed of blocks of solid graphite with serrated machined channels. The cell was placed on a single cell test pedestal made by Fuel Cell Technologies, Inc. (Santa Fe, NM). The flows of the anode (H2) and the cathode (air) were fixed and did not vary with the current density. The Flow rates for a given test were defined by specifying a current density. For example, if the flow rate of the H2 anode was 2X stoichiometric at 1.0 Amps / square centimeter (A / cm2), then the flow rate was twice that required to sustain a current density of 1 A / cm2. . Thus, when the cell was operated at 0.5 A / cm2, the same flow was 4 times that which was required to sustain the current density. The anode and cathode pressures were sustained at 20 and 30 psig, respectively. The temperature of the cell was 80 ° C while the external humidifiers were set at 100 ° C for the anode and 85 ° C for the cathode. The cell was preconditioned at 0.5 V load for 12 hours. The operation of the cell is shown in Figure 7. The flow rate of the anode of the H2 was 2X stoichiometric at 1.0 A / cm2, and the flow rate of the cathode of the air was 3X stoichiometric at 1.0 A / cm2. Example 5 The structures of the membrane and the electrode were prepared as follows (MEA 1): An ion exchange membrane prepared from a perfluorosulfonic acid ionomer having an equivalent weight of 800, a thickness of 60 microns dry was obtained and 127 micron fully hydrated (available from The Dow Chemical Company as XUS 13204.20) and cut into 11-cent by 11-centimeter sheets and placed in an NaOH bath to convert to the Na + form. The electrode ink was prepared by mixing 1.08 grams of a 5.79 weight percent solution of the above ionomer (in a 50:50 volume percent ethanol / water solution), 0.1875 grams of 20 weight percent platinum in carbon (available from E-TEK (Natick, MA)) and 0.114 grams of an IM solution of tetrabutylammonium hydroxide (TBAOH) in methanol, and 0.6 grams of propylene carbonate (dispersing aid). The mixture was stirred with a stir bar overnight or until the mixture was uniformly dispersed. An additional 1.2 grams of propylene carbonate was added to the mixture. The catalyst ink was painted on clean, 9 square centimeter polytetrafluoroethylene coated glass fiber parts (CHR Industries, New Haven, CT) that had been oven dried at 110 ° C and preweighed. The pieces were painted twice with the catalyst ink, which was completely dried before the application of the second layer. The Pt charges were 0.14 milligrams / square centimeter on the anode and 0.25 milligrams / square centimeter on the cathode. The membrane electrode assembly was formed by aligning a coated piece on each side of the ionomer membrane that had been dried on a vacuum board. The pieces and the membrane were placed between two pieces of stainless steel to hold them while they were placed in the press. In assembly it was placed in a press at 195 ° C and pressed at a pressure of 45 kilograms per square centimeter of the piece for 5 minutes. The press pack was allowed to cool to room temperature before opening. The piece of the layer containing catalytically active particles was detached, leaving the film adhered to the surface of the membrane. Another sample of membrane electrode assembly (MEA 2) was prepared by applying the catalyst ink directly to the surface of the ionomer membrane. The amount of ink transferred was determined by weighing the bottle and the brush before and after applying the ink. Again the ink was applied in multiple layers but in this case the successive layers were applied without requiring the ink to dry completely between applications. The membrane was held in place on a vacuum board having a porous glass of fine sintered stainless steel on top of a hot vacuum manifold plate. The vacuum table was operated between 45 ° C and 60 ° C as the ink was applied. The second side of the membrane can be coated in the same manner. This structure was pressed as described for MEA 1. The membrane and the ionomer binder of both samples were converted back to the proton form by refluxing in normal 1 sulfuric acid for 0.5 hour. The membrane electrode assembly was again dried on the table under vacuum and stored in a dry environment until used.

An intermediate layer (IL 1) was prepared as follows: An ink was prepared from 3 grams of Vulcan carbon "11 XC-72, 2 grams of a 50 weight percent solution of a perfluorocyclobutane polymer (poly (1)). , 1,2-tris (4-trifluorovinyloxyphenyl) ethane, prepared as described in U.S. Patent Number: 5,037,919 and B-stepped in mesitylene to produce a polymer with an average molecular weight in the range of from 4,000 to 8,000) and 31 grams of mesitylene The ink was applied in two applications to an untreated graphite paper of 0.25 g / cm3-0.2 millimeters thick with a porosity of 87 percent and an average pore size of 50 microns ( Spectracorp, Lawrence, MA) to obtain a charge of 2 mg / cm3 of polymer and carbon The ink was not required to dry completely between applications The solvent was allowed to evaporate and the polymer was completely cured at 200 ° C under vacuum during 1 hour C / polymer layer c It housed near the active layer in the cell assembly and was held in place by a polytetrafluoroethylene-coated packing and cell compression. Another sample of intermediate layer / membrane electrode assembly (IL 2) was prepared by the following method: A coating ink was prepared by mixing vulcan carbon powder XC-72R with 2 weight percent of a dispersing agent (Triton X- 100, DuPont) to make a solution with percent solids which was stirred overnight. Next, polytetrafluoroethylene (PTFE) latex (T-30B, available from DuPont) diluted to 6 weight percent solids in an amount sufficient to provide a carbon / PTFE weight ratio of 3: 1 was added and mixed gently for 1 to 2 minutes. The ink was applied to the untreated graphite paper (0.22 millimeters thick, 0.25 g / cm3 density, 87 porosity percent and 50 microns average pore size, obtained in Spectracorp) using a Meyer Rod # 40 and drying air. The sample was placed in an inert atmosphere oven at 340 ° C for 6 hours to sinter the polytetrafluoroethylene to make it hydrophobic. This procedure also produced a coated weight of 2 milligrams / square centimeter (carbon solids and polytetrafluoroethylene). The membrane electrode assemblies were tested in a test fuel cell prepared by Fuel Cell Technologies, Inc. (Santa Fe, NM). The flow fields were composed of blocks of solid graphite with machined channels snaking. The membrane electrode assembly is placed in the cell with an intermediate layer on each side. The cell is placed on a single cell test stand made by Fuel Cell Technologies, Inc. The anode (H2) and cathode (air or 02) fluxes remained fixed and did not vary with the current density. The flow rates used for a given test they were defined by specifying a current density and a stoichiometric multiple for this current density. For example, the air cathode flow rate can be specified as stoichiometric 2X at 1.0 A / cm3. In this case, the flow rate was double that required to sustain a current density of 1 A / cm2. Thus, when the cell was operating at 0.5 A / cm2, this same flow was 4 times that which is required to sustain the current density. The anode and cathode pressures were maintained at 30 and 40 psig, respectively. The temperature of the cell was 80 ° C while the external humidifiers were set at 100 ° C for the anode and 85 ° C for the cathode. The cell was preconditioned at 0.5 V load for 12 hours. Figure 9 illustrates the operation of a fuel cell containing MEA 1 and IL1 prepared as described above. The Figure shows that the behavior of the cell using air as a fuel approaches that of oxygen. The flow regime of the hydrogen was at the same stoichiometry as the air or oxygen flow, and the gas pressure at the anode and cathode was 30 psig and 40 psig, respectively. Figure 10 shows the operation of the same fuel cell at a hydrogen pressure of 30 psig at a flow rate of 2X at 1.0 Amp / cm2, and an air pressure of 40 psig at a flow rate of 3X to 1.0. A / cm2 over the entire curve. Figure 11 illustrates the operation of a fuel cell containing MEA 1 and IL2 prepared as described above, under the same flow conditions as those used in the example shown in Figure 10. Example 6 A membrane electrode assembly was prepared as described in Example 5 (MEA 1). The ink used to prepare the small pore region was prepared as described in Example 5 (IL 1), except that it was applied to an 84.5 percent porous 225 μm thick graphite paper obtained from Toray (Tokyo, JP) . The large pore graphite paper functions both as part of the intermediate layer and as the flow field on the single cell test stand. The outer edges of the graphite paper were filled with an inert material to prevent escape of the reaction gases. In these single-cell tests, the reactive gases were applied to the surface of the graphite paper so that the flow then occurred in the plane of the graphite paper in the direction parallel to the plane of the active catalyst layer. The gases were administered to the porous graphite paper by pyrolyzed graphite blocks with a single inlet and outlet channel (available from POCO Graphite, Decatur, TX). Figure 12 illustrates the operation of the membrane electrode assembly described above (for Figure 11) in a flow of hydrogen and air at 2 stequios at 2.0 A / cm2 at a pressure of 20 psig of hydrogen and psig of air for the entire curve. Example 7 An assembly was prepared as described in Example 6 (MEA 1). Graphite and carbon powder fibers used in the preparation of the small pore layer were previously treated by soaking in a 1 percent by weight solution of tridecafluoro-1,2,1-tetrahydrooctyl-l-trichlorosilane toluene at 80 ° C for 3 minutes to make them hydrophobic. The fibers and particles were then rinsed in excess toluene and dried at room temperature. The graphite fibers were received in lengths of 6.35 millimeters. The fibers were mixed in a Waring blender in a glycerol mixture for 10 minutes or until the desired length (less than 1 millimeter) was obtained. The fibers were rinsed with plenty of water and dried. The small pore layer was formed by applying a second ink on the catalyst layer. The ink was prepared by combining 0.05 grams of the treated graphite fibers (7 μm diameter in Fortafil), 0.05 grams of carbon powder (Vulcan XC-72), 1.0 grams of a 5 weight percent solution of Nafion "11 , 0.07 grams of TBAOH (1 molar solution in methanol), and 1.2 grams of propylene carbonate The membrane electrode assembly with the small pore layer applied to it was placed in a single test cell with a flow field porous graphite (paper graphite 0.2 g / cm3, 600 μm thick, of Spectracorp with a porosity of about 90 percent) and tested according to the procedure described in Example 6. Figure 12 illustrates the operation of the membrane electrode assembly described above, to a flow of hydrogen and air at 2 stechios at 2.0 A / cm2 at a pressure of 20 psig of hydrogen and 30 psig of air for the entire curve. Example 8 A membrane / electrode assembly was prepared as follows: An ion exchange membrane prepared from an ionomer of perfluorosulfonic acid having an equivalent weight of 800, a thickness of 60 microns dry and 127 microns fully hydrated, was obtained, available from The Dow Chemical Company, and cut into sheets of 11 centimeters by 11 centimeters and placed in an NaOH bath to convert it to the Na-i- form. The electrode ink was prepared by mixing 1.08 grams of a 5.79 weight percent solution of the ionomer (in a 50:50 volume percent solution of ethanol / water), 0.1875 grams of 20 weight percent platinum in carbon (available from E-TEK, Natick, MA) and 0.114 grams of an IM solution of tetrabutylammonium hydroxide (TBAOH), and 0.6 grams of propylene carbonate. The mixture was stirred with a stir bar overnight or until the mixture was uniformly dispersed. An additional 1.2 grams of propylene carbonate was added to the mixture. The catalyst ink was applied to clean, 9 square centimeter polytetrafluoroethylene coated fiberglass parts (CHR Industries, New Haven, CT) that had been dried in an oven at 110 ° C and preweighed. The pieces were painted twice with the catalyst ink, which was completely dried before the application of the second layer. The membrane electrode assembly was formed by aligning a coated piece on each side of the ionomer membrane that had been dried on a vacuum board. The pieces and the membrane were placed in a press at 195 ° C and pressed at a pressure of 45 kilograms per square centimeter of the piece for 5 minutes. The press pack was allowed to cool to room temperature before opening. The piece of the catalyst layer was detached, leaving the film adhered to the surface of the membrane. The platinum charges and the thicknesses of the catalyst layer were 0.14 milligram / square centimeter and 10 μm on the anode side of the membrane, 0.25 milligram / square centimeter and 17 μm on the cathode side of the membrane, respectively. Separate intermediate layers (between the electrode assembly and flow field) of a graphite cloth impregnated with a mixture of carbon particles and polytetrafluoroethylene (available as ELAT from E-TEK, Inc., Natick, MA) were placed near both active layers in the cell assembly and were held in place by a polytetrafluoroethylene film packing and cell compression. The resulting assemblies were tested in a test fuel cell prepared by Fuel Cell Technologies, Inc. (Santa Fe, NM). The flow fields were composed of blocks of solid graphite with serrated machined channels. The cell was placed on a single cell test pedestal made by Fuel Cell Technologies, Inc. (Santa Fe, NM). The flows of the anode (H2) and the cathode (air) were fixed and did not vary with the current density. The flow rates for a given test were defined by specifying a current density. For example, if the flow rate of the H2 anode was 2X stoichiometric at 1.0 Amps / square centimeter (A / cm2), then the flow rate was twice that required to sustain a current density of 1 A / cm2. . Thus, when the cell was operated at 0.5 A / cm2, the same flow was 4 times that which was required to sustain the current density. The anode and cathode pressures were held at 30 and 40 psig, respectively. The temperature of the cell was 80 ° C while the external humidifiers were set at 100 ° C for the anode and 85 ° C for the cathode. The cell was preconditioned at 0.5 V load for 12 hours. The operation of the cell is shown in Figure 13. The flow rate of the H2 anode was 2X stoichiometric at 1.0 A / cm2, and the air cathode flow rate was stoichiometric 3X at 1.0 A / cm2. Example 9 A membrane / electrode assembly was prepared as described in Example 8 except that TBAOH was not added to the catalyst ink. The platinum charges and the thickness of the catalyst layer on the anode side and the cathode of the membrane were 0.15 mg / cm2, 10 μm and 0.25 mg / cm2, 16 μm, respectively. Intermediate layers were prepared as follows: An ink was prepared from 3 grams of Vulcan carbon "11 XC-72, 2 grams of a 50 weight percent solution of a perfluorocyclobutane polymer (poly (1, 2, tris (4-trifluorovinyloxyphenyl) ethane), prepared as described in U.S. Patent Number: 5,037,919 and B-stepped in mesitylene to produce a polymer with an average molecular weight in the range of from 4,000 to 8,000) and 31 grams of mesitylene The ink was applied in two applications to an untreated graphite paper of 0.25 g / cm3-0.2 millimeters thick with a porosity of 87 percent and an average pore size of 50 microns (Spectracorp, Lawrence, MA) to obtain a charge of 2 mg / cm3 of polymer and carbon The ink was not required to dry completely between applications The solvent was allowed to evaporate and the polymer was completely cured at 200 ° C under vacuum for 1 hour. The polymer / C layer was placed near the active layer in the cell assembly and held in place by a polytetrafluoroethylene-coated gasket and cell compression. The membrane electrode assembly and the intermediate layers were assembled together and tested in a fuel cell as described in Example 8. Although the cell was previously conditioned for 12 hours, its peak operation was reached in only 1 hour. The behavior curve is shown in Figure 14. The flow rates were the same as in Example 8.

Claims (79)

1. CLAIMS 1. An electrochemical fuel cell having a membrane electrode assembly and a flow field adjacent thereto wherein the flow field comprises an electrically conductive porous material having a porosity of at least 50 percent and a size of average pore of at least 35 micras.
2. 2. The fuel cell of claim 1 wherein the porous material has a thickness of at least 0.508 millimeters, and a 7.62 centimeter by 7.62 centimeter portion can imbibe at least 0.5 grams of water per gram of material in ten seconds when Hang vertically at a depth of 0.5 centimeters of water.
3. 3. The fuel cell of claim 2 wherein the porous material has a thickness of at least

   0. 508 millimeters and you can imbibe at least 1 gram of water per gram of material.
4. 4. The fuel cell of claim 1 wherein the flow field has a porosity of at least 75 percent.
5. 5. The fuel cell of claim 1 wherein the flow field has a porosity of at least 80 percent.
6. 6. The fuel cell of claim 1 wherein the flow field has an average pore size of at least 45 micras.
7. 7. The fuel cell of claim 1 wherein the flow field has an average pore size of at least 50 microns.
8. 8. The fuel cell of claim 1 wherein the flow field has an average pore size not greater than 250 microns.
9. The fuel cell of claim 1 wherein the membrane electrode assembly comprises a solid polymer membrane having at least two layers of catalytically active ink particles on at least one side thereof, wherein at least two layers of the catalytically active ink particles comprises polytetrafluoroethylene polymer having groups of sulph acids pending, of equivalent weights differing by more than 50, and wherein the layer having the highest equivalent weight is placed adjacent to the flow field.
10. 10. The fuel cell of claim 9 wherein the porous material has a porosity of at least 80 percent and an average pore size of at least 50 microns.
11. 11. The fuel cell of claim 10 wherein the porous material has a thickness of at least 0.508 millimeters, and a 7.62 centimeter by 7.62 centimeter portion can imbibe at least 1 gram of water per gram of material in ten seconds when it hangs vertically in a depth of 0.5 centimeters of water.
12. 12. A stacked assembly comprising at least 3 of the fuel cells of claim 1 arranged in series.
13. The assembly of claim 12 wherein the repeating units of the fuel cells are as shown in Figure 2.
14. 14. A membrane electrode assembly having an ion exchange membrane, and at least two layers active placed on the same side of the membrane; wherein the active layers comprise catalytically active particles and an ionomer; the average equivalent weights of the ionomers in the layers differ by at least 50; and the active layer placed closest to the membrane contains the ionomer with the lowest average equivalent weight.
15. 15. The membrane electrode assembly of claim 14 wherein the active layer comprises at least 99 percent by weight of the mixture of catalytically active particles and the ionomer.
16. 16. The membrane electrode assembly of claim 14 wherein the active layer has a thickness of at least 1 μm.
17. 17. The membrane electrode assembly of claim 14 wherein the active layer has a thickness of at least 5 μm.
18. 18. The membrane electrode assembly of claim 14 wherein the active layer has a thickness of at least 10 μm.
19. 19. The membrane electrode assembly of claim 14 wherein the active layer has a thickness of at least 30 μm.
20. 20. The membrane electrode assembly of claim 14 wherein the active layer has a porosity of at least 30 percent.
21. 21. The membrane electrode assembly of claim 14 wherein the active layer has a porosity of at least 50 percent.
22. 22. The membrane electrode assembly of claim 14 wherein the active layer has an average pore size in the range of 0.01 μm to 10 μm.
23. 23. The membrane electrode assembly of claim 14 wherein the active layer has an average pore size in the range of 0.03 μm to 0.5 μm.
24. 24. The membrane electrode assembly of claim 14 wherein the catalytically active particles are present in an amount sufficient to provide a level of charge on the cathode side of the membrane in the range of 0.05 milligram / square centimeter to 0.45 milligrams / square centimeter, and a level of load on the anode side of the membrane in the range of 0.01 milligram / square centimeter to 0.15 milligram / square centimeter.
25. 25. A membrane electrode assembly having an ion exchange membrane, and at least one active layer positioned on one side of the membrane; wherein the active layers comprise (a) catalytically active particles, and (b) an ionomer having an equivalent weight in the range of from 650 to 950 and which is substantially insoluble in water at a temperature of less than 100 ° C.
26. 26. The membrane electrode assembly of claim 25 wherein the active layer comprises at least 99 percent by weight of the mixture of catalytically active particles and the ionomer.
27. 27. The membrane electrode assembly of claim 25 wherein the active layer has a thickness of at least 1 μm.
28. 28. The membrane electrode assembly of claim 25 wherein the active layer has a thickness of at least 5 μm.
29. 29. The membrane electrode assembly of claim 25 wherein the active layer has a thickness of at least 10 μm.
30. 30. The membrane electrode assembly of claim 25 wherein the active layer has a thickness of at least 30 μm.
31. 31. The membrane electrode assembly of claim 25 wherein the active layer has a porosity of at least 30 percent.
32. 32. The membrane electrode assembly of claim 25 wherein the active layer has a porosity of at least 50 percent.
33. 33. The membrane electrode assembly of claim 25 wherein the active layer has an average pore size in the range of 0.01 μm to 10 μm.
34. 34. The membrane electrode assembly of claim 25 wherein the active layer has an average pore size in the range of 0.03 μm to 0.5 μm.
35. 35. The membrane electrode assembly of claim 25 wherein the catalytically active particles are present in an amount sufficient to provide a charge level on the cathode side of the membrane in the range of 0.05 milligram / square centimeter to 0.45 milligram / square centimeter, and a load level on the anode side of the membrane in the range of 0.01 milligram / square centimeter to 0.15 milligram / square centimeter.
36. 36. A composition comprising (a) catalytically active particles, (b) an organic compound, and (c) an ionomer having an equivalent weight in the range of 650 to 950 and which is substantially insoluble in water at a temperature of less than 100 ° C.
37. 37. An electrochemical fuel cell having a membrane electrode assembly and a layer of electrically conductive porous material adjacent thereto having at least two portions with different average pore sizes, wherein a first portion of the layer adjacent to the assembly of membrane electrode has a porosity no greater than a second portion of the layer adjacent to the opposite side of the layer; the second portion has a porosity of at least 82 percent; and the second portion has an average pore size that is at least 10 microns and at least ten times greater than the average pore size of the first portion.
38. 38. The fuel cell of claim 37 wherein the first portion of the porous layer is hydrophobic.
39. 39. The fuel cell of claim 37 wherein the first portion of the porous layer comprises a polytetrafluoroethylene polymer.
40. 40. The fuel cell of claim 37 wherein the first portion of the porous layer comprises a perfluoroalkyl / acrylic copolymer.
41. 41. The fuel cell of claim 37 wherein the first portion of the porous layer comprises a perfluorocyclobutane polymer.
42. 42. The fuel cell of claim 37 wherein the average pore size of the first portion of the porous layer is at least 0.1 miera.
43. 43. The fuel cell of claim 37 wherein the average pore size of the first portion of the porous layer is at least 1 miera.
44. 44. The fuel cell of claim 37 wherein the porosity of the second portion of the porous layer is at least 82.5 percent.
45. 45. The fuel cell of claim 37 wherein the porosity of the second portion of the porous layer is at least 85 percent.
46. 46. The fuel cell of claim 37 wherein the porosity of the second portion of the porous layer is at least 87.5 percent.
47. 47. The fuel cell of claim 37 wherein the second portion of the porous layer is more hydrophilic than the first portion.
48. 48. The fuel cell of claim 37 wherein the first portion of the porous layer has an average pore size of at least 30 microns.
49. 49. An electrochemical fuel cell having a membrane electrode assembly with a porous non-woven layer of an electrically conductive porous material adjacent thereto that has at least two portions with different average pore sizes, wherein a first portion of the layer adjacent to the membrane electrode assembly has a porosity no greater than a second portion of the layer adjacent to the opposite side of the layer; the second portion has a porosity of at least 50 percent; and the second portion has an average pore size that is at least 35 microns and at least ten times larger than the average pore size of the first portion.
50. 50. The fuel cell of claim 49 wherein the first portion of the porous layer is hydrophobic.
51. 51. The fuel cell of claim 49 wherein the first portion of the porous layer comprises a polytetrafluoroethylene polymer.
52. 52. The fuel cell of claim 49 wherein the first portion of the porous layer comprises a perfluoroalkyl / acrylic copolymer.
53. 53. The fuel cell of claim 49 wherein the first portion of the porous layer comprises a perfluorocyclobutane polymer.
54. 54. A process for preparing an electrochemical fuel cell having a membrane electrode assembly comprising the steps of: (a) applying a layer of a conductive composition to a sheet of a porous conductive material having a porosity of at least 82 percent under sufficient conditions to form a solid, porous layer of the conductive composition on one side of the sheet of porous conductive material, forming a compound therefrom, and (b) placing the compound adjacent to the membrane electrode assembly so that the side of the compound to which the conductive composition was applied facing the assembly.
55. 55. The process of claim 54 wherein the conductive composition comprises a polymer of polytetrafluoroethylene, perfluoroalkyl / acrylic copolymer, or perfluorocyclobutane.
56. 56. The process of claim 55 wherein the conductive composition additionally comprises carbon fibers and / or powders and the weight ratio of the fibers and / or powders against the polymers is at least 1: 1.
57. 57. The process of claim 55 wherein the conductive composition additionally comprises carbon fibers and / or powders and the weight ratio of the fibers and / or powders against the polymers is at least 3: 1.
58. 58. The process of claim 55 wherein the conductive composition additionally comprises carbon fibers and / or powders and the weight ratio of the fibers and / or powders against the polymers is at least 10: 1.
59. 59. The process of claim 56 wherein the conductive composition additionally comprises carbon fibers and / or powders and the weight ratio of the fibers and / or Powders against polymers is not greater than 5: 1.
60. 60. A composition comprising (a) catalytically active particles, (b) an organic compound having a pKa of at least 18 and a basicity parameter, β, less than 0.66, and (c) a polymeric binder.
61. 61. The composition of claim 60 wherein the binder is a proton form of an ionomer.
62. 62. The composition of claim 60 wherein the binder has an equivalent weight in the range of from 600 to 1200.
63. 63. The composition of claim 60 wherein the binder has an equivalent weight in the range of from 700 to 950.
64. 64. A composition comprising (a) catalytically active particles, (b) an organic compound selected from ethylene carbonate, propylene carbonate, butylene carbonate, ethylene carbamate, propylene carbamate, and butylene carbamate, acetone, acetonitrile, difluorobenzene, and sulfolane; and (c) a polymeric binder.
65. 65. A composition comprising (a) catalytically active particles, (b) propylene carbonate, and (c) a polymeric binder.
66. 66. A process for preparing a membrane / electrode assembly, which comprises the sequential steps of (i) applying a layer of the composition of the claim 1 to a solid polymer electrolyte, a carbon fiber paper, or a release substrate; (ii) heating the composition under conditions sufficient to volatilize at least 95 percent of component (b); and (iii) placing the composition in contact with a solid polymer electrolyte, if the composition was not applied directly to the solid polymer electrolyte, thereby forming the membrane / electrode assembly.
67. 67. The process of claim 66 wherein the ink is applied in an amount sufficient to provide a layer of the composition which, when dried and protonated, has a thickness of at least 10 μm.
68. 68. The process of claim 66 wherein the ink is applied in an amount sufficient to provide a layer of the composition which, when dried and protonated, has a thickness of at least 15 μm.
69. 69. The process of claim 67 wherein the porosity of the dry and protonated layer is in the range of 30 percent to 90 percent.
70. 70. The process of claim 67 wherein the porosity of the dry and protonated layer is in the range of 50 percent to 90 percent.
71. 71. The process of claim 67 wherein the average pore size of the dry and protonated layer is in the range of 0.03 μm to 0.5 μm.
72. 72. A process for preparing a membrane / electrode assembly, comprising the sequential steps of (i) applying a layer of the composition of claim 5 to a solid polymer electrolyte, a carbon fiber paper, or a release substrate; (ii) heating the composition under conditions sufficient to volatilize at least 95 percent of component (b); and (iii) placing the composition in contact with a solid polymer electrolyte, if the composition was not applied directly to the solid polymer electrolyte, thereby forming the membrane / electrode assembly.
73. 73. A process for preparing a membrane / electrode assembly, comprising the sequential steps of (i) applying a layer of the composition of claim 6 to a solid polymer electrolyte, a carbon fiber paper, or a substrate of liberation; (ii) heating the composition under conditions sufficient to volatilize at least 95 percent of component (b); and (iii) placing the composition in contact with a solid polymer electrolyte, if the composition was not applied directly to the solid polymer electrolyte, thereby forming the membrane / electrode assembly.
74. 74. The process of claim 73 wherein the ink is applied in an amount sufficient to provide a layer of the composition which, when dried and protonated, It has a thickness of at least 10 μm.
75. 75. The process of claim 73 wherein the ink is applied in an amount sufficient to provide a layer of the composition which, when dried and protonated, has a thickness of at least 15 μm.
76. 76. The process of claim 74 wherein the porosity of the dry and protonated layer is in the range of 30 percent to 90 percent.
77. 77. The process of claim 74 wherein the porosity of the dry and protonated layer is in the range of 50 percent to 90 percent.
78. 78. The process of claim 74 wherein the average pore size of the dry and protonated layer is in the range of 0.03 μm to 0.5 μm.
79. 79. The process of claim 73 wherein at least two compositions of claim 6 are applied sequentially to the electrolyte, paper, or substrate, the polytetrafluoroethylene polymer compositions containing pendant sulfonic acid groups as binders, the average weights of which vary by weight. at least 50 for each composition, and wherein the composition having the lowest average equivalent weight binder is placed adjacent to the solid polymer electrolyte.