FUEL CELL ELECTRODE ASSEMBLY WITH SELECTIVE CATALYST LOADING

Abstract: A membrane electrode assembly is provided. More particularly, an assembly is provided which includes a pair of electrodes (10) and an ion exchange membrane (550) having opposed major surfaces positioned between the electrodes, at least one of the electrodes being formed of a sheet of a compressed mass of expanded graphite particles having a plurality of transverse fluid channels (20) passing through the sheet between first and second opposed surfaces (30, 40) of the sheet, one of the opposed surfaces abutting the ion exchange membrane. At least one of the surfaces of the ion exchange membrane has a catalyst metal (600) selectively loaded thereon.

Description: Description FUEL CELL ELECTRODE ASSEMBLYWITH SELECTIVE CATALYST LOADING Technical Field This invention relates to an electrode assembly for an electrochemical fuel cell that includes an article formed of flexible graphite sheet that is fluid permeable and has enhanced isotropy with respect to thermal and electrical conductivity. The electrode assembly also includes an ion exchange membrane having catalyst metal selectively loaded thereon. Background of the Invention An ion exchange membrane fuel cell, more specifically a proton exchange membrane (PEM) fuel cell, produces electricity through the chemical reaction of hydrogen and oxygen in the air. Within the fuel cell, an anode and cathode surround a polymer electrolyte. A catalyst material stimulates hydrogen molecules to split into hydrogen atoms and through an internal load at the membrane the atoms each split into a proton and an electron. The electrons generated are utilized as electrical energy. The protons migrate through the electrolyte and combine with oxygen from the air and electrons to form water. A PEM fuel cell is advantageously formed of a membrane electrode assembly sandwiched between two graphite flow field plates. Conventionally, the membrane electrode assembly consists of electrodes (anode and cathode) with a thin layer of a catalyst material, particularly platinum or a platinum group metal coated on graphite or carbon particles, bonded to either side of a proton exchange membrane disposed between the electrodes. In operation, hydrogen flows through channels in one of the flow field plates to the anode, where the catalyst promotes its separation into hydrogen atoms and thereafter into protons and electrons. Air flows through the channels in the other flow field plate to the cathode, where the oxygen in the air attracts the protons through the proton exchange membrane and the electrons through the circuit, which join to form water. Since electrons cannot pass through the PEM, they travel through the anode, through a circuit in which the electricity is utilized, and back to the cathode. The air stream on the cathode side removes the water formed by combination of the hydrogen and oxygen. Combinations of such fuel cells are used in a fuel cell stack to provide the desired electrical power. One limiting factor to the more widespread use of PEM fuel cells is the cost of the catalyst material. Metals such as platinum and the other platinum group metals are relatively expensive, especially as compared to the other elements of the cell, such as the graphite flow field plates. In conventional fuel cells, the catalyst material is bonded to the PEM or electrodes in a non-selective manner. That is, the catalyst material is distributed relatively uniformly on the opposed surfaces of the PEM, with result that catalyst deployment is not as efficient as possible. Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion. Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces, h considering the graphite structure, two axes or directions are usually noted, to wit, the "c" axis or direction and the "a" axes or directions. For simplicity, the "c" axis or direction maybe considered as the direction perpendicular to the carbon layers. The "a" axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the "c" direction. The graphites suitable for manufacturing flexible graphite possess a very high degree of orientation. As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the "c" direction and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained. Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or "c" direction dimension which is as much as about 80 or more times the original "c" direction dimension can be formed without the use of a binder into cohesive or integrated flexible graphite sheets of expanded graphite, e.g. webs, papers, strips, tapes, or the like. The formation of graphite particles which have been expanded to have a final thickness or "c" dimension which is as much as about 80 times the original "c" direction dimension into integrated flexible sheets by compression, without the use of any binding material is believed to be possible due to the excellent mechanical interlocking, or cohesion which is achieved between the voluminously expanded graphite particles. In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation. Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a "c" direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.08 g/cc to about 2.0 g/cc. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increased density. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the "c" direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the "a" directions and the thermal, electrical and fluid diffusion properties of the sheet are very different, by orders of magnitude, for the "c" and "a" directions. This very considerable difference in properties, i.e. anisotropy, which is directionally dependent, can be disadvantageous in some applications. For example, in gasket applications where flexible graphite sheet is used as the gasket material and in use is held tightly between metal surfaces, the diffusion of fluid, e.g. gases or liquids, occurs more readily parallel to and between the major surfaces of the flexible graphite sheet. It would, in most instances, provide for greater gasket performance, if the resistance to fluid flow parallel to the major surfaces of the graphite sheet ("a" direction) were increased, even at the expense of reduced resistance to fluid diffusion flow transverse to the major faces of the graphite sheet ("c" direction). With respect to electrical properties, the resistivity of anisotropic flexible graphite sheet is high in the direction transverse to the major surfaces ("c" direction) of the flexible graphite sheet, and very substantially less in the direction parallel to and between the major faces of the flexible graphite sheet ("a" direction). In applications such as fluid flow field plates for fuel cells and seals for fuel cells, it would be of advantage if the electrical resistance transverse to the major surfaces of the flexible graphite sheet ("c" direction) were decreased, even at the expense of an increase in electrical resistivity in the direction parallel to the major faces of the flexible graphite sheet ("a" direction). With respect to thermal properties, the thermal conductivity of a flexible graphite sheet in a direction parallel to the upper and lower surfaces of the flexible graphite sheet is relatively high, while it is relatively very low in the "c" direction transverse to the upper and lower surfaces. The foregoing situations are accommodated by the present invention. Summary of the Invention The present invention provides a membrane electrode assembly for an electrochemical fuel cell comprising a pair of electrodes and an ion exchange membrane having opposed major surfaces positioned between the electrodes. At least one of the electrodes is formed of a sheet of a compressed mass of expanded graphite particles having a plurality of transverse fluid channels passing through the sheet between first and second opposed surfaces of the sheet, one of the opposed surfaces abutting the ion exchange membrane. The transverse fluid channels are formed by mechanically impacting an opposed surface of the graphite sheet to displace graphite within the sheet at predetermined locations to provide a channel pattern. The transverse fluid channels are adjacently positioned and separated by walls of compressed expanded graphite. The ion exchange membrane has catalyst metal selectively loaded on it in a manner complementary to the channel pattern of at least one of the electrodes. Brief Description of the Drawings Figure 1 is apian view of a transversely permeable sheet of flexible graphite having transverse channels in accordance with the present invention; Figure 1(A) shows a flat-ended protrusion element used in making the channels in the perforated sheet of Figure 1; Figure 2 is a side elevation view in section of the sheet of Figure 1; Figures 2(A), (B), (C) show various suitable flat-ended configurations for transverse channels in accordance with the present invention; Figures 3, 3(A) shows a mechanism for making the article of Figure 1; Figure 4 shows an enlarged sketch of an elevation view of the oriented expanded graphite particles of prior art flexible graphite sheet material; Figure 5 is a sketch of an enlarged elevation view of an article formed of flexible graphite sheet in accordance with the present invention; Figures 6, 7 and 7(A) show a fluid permeable electrode assembly which includes a transversely permeable article in accordance with the present invention; and Figure 8 is a photograph at 100X (original magnification) corresponding to a portion of the side elevation view sketch of Figure 5. Detailed Description of the Invention Graphite is a crystalline form of carbon comprising atoms covalently bonded in flat layered planes with weaker bonds between the planes. By treating particles of graphite, such as natural graphite flake, with an intercalant of, e.g. a solution of sulfuric and nitric acid, the crystal structure of the graphite reacts to form a compound of graphite and the intercalant. The treated particles of graphite are hereafter referred to as "particles of intercalated graphite". Upon exposure to high temperature, the intercalant within the graphite decomposes and volatilizes, causing the particles of intercalated graphite to expand in dimension as much as about 80 or more times its original volume in an accordion-like fashion in the "c" direction, i.e. in the direction perpendicular to the crystalline planes of the graphite. The exfoliated graphite particles are vermiform in appearance, and are therefore commonly referred to as worms. The worms may be compressed together into flexible sheets that, unlike the original graphite flakes, can be formed and cut into various shapes and provided with small transverse openings by deforming mechanical impact. Graphite starting materials suitable for use in the present invention include highly graphitic carbonaceous materials capable of reversibly intercalating alkali metals. These highly graphitic carbonaceous materials have a degree of graphitization above about 0.80 and most preferably about 1.0. As used in this disclosure, the term "degree of graphitization" refers to the value g according to the formula: g - 3.45 - d(002) 0.095 where d(002) is the spacing between the graphitic layers of the carbons in the crystal structure measured in Angstrom units. The spacing d between graphite layers is measured by standard X-ray diffraction techniques. The positions of diffraction peaks corresponding to the (002), (004) and (006) Miller Indices are measured, and standard least-squares techniques are employed to derive spacing which minimizes the total error for all of these peaks. Examples of highly graphitic carbonaceous anode materials include synthetic graphites and natural graphites from various sources, as well as other carbonaceous materials such as petroleum cokes heat treated at temperatures above 2500°C, carbons prepared by chemical vapor deposition or pyrolysis of hydrocarbons and the like. The graphite starting materials used in the present invention may contain non-carbon components so long as the crystal structure of the starting materials maintains the required degree of graphitization. Generally, any carbon-containing material, the crystal structure of which possesses the required degree of graphitization, is suitable for use with the present invention. Such graphite preferably has an ash content of less than six weight percent. A common method for manufacturing graphite sheet is described by Shane et a in U.S. Patent 3,404,061, the disclosure of which is incorporated herein by reference. In the typical practice of the Shane et a method, natural graphite flakes are intercalated by dispersing the flakes in a solution containing e.g., a mixture of nitric and sulfuric acid, advantageously at a level of about 50 to about 300 parts by weight of intercalant solution per 100 parts by weight of graphite flakes (pph). The intercalation solution contains oxidizing and other intercalating agents known in the art. Examples include those containing oxidizing agents and oxidizing mixtures, such as solutions containing nitric acid, potassium chlorate, chromic acid, potassium permanganate, potassium chromate, potassium dichromate, perchloric acid, and the like, or mixtures, such as for example, concentrated nitric acid and chlorate, chromic acid and phosphoric acid, sulfuric acid and nitric acid, or mixtures of a strong organic acid, e.g. trifluoroacetic acid, and a strong oxidizing agent soluble in the organic acid. Alternatively, an electric potential can be used to bring about oxidation of the graphite. Chemical species that can be introduced into the graphite crystal using electrolytic oxidation include sulfuric acid as well as other acids. In a preferred embodiment, the intercalating agent is a solution of a mixture of sulfuric acid, or sulfuric acid and phosphoric acid, and an oxidizing agent, i.e. nitric acid, perchloric acid, chromic acid, potassium permanganate, hydrogen peroxide, iodic or periodic acids, or the like. Although less preferred, the intercalation solutions may contain metal halides such as ferric chloride, and ferric chloride mixed with sulfuric acid, or a halide, such as bromine as a solution of bromine and sulfuric acid or bromine in an organic solvent. The quantity of intercalation solution may range from about 20 to 150 parts of solution by weight per 100 parts by weight of graphite flakes (pph) and more typically about 50 to 120 pph. After the flakes are intercalated, any excess solution is drained from the flakes and the flakes are water-washed. Alternatively, the quantity of the intercalation solution may be limited to between about 10 to 50 parts of solution per hundred parts of graphite by weight (pph) which permits the washing step to be eliminated as taught and described in U.S. Pat. No. 4,895,713, the disclosure of which is also herein incorporated by reference. The thus treated particles of graphite are sometimes referred to as "particles of intercalated graphite". Upon exposure to high temperature, e.g. about 700°C to 1000°C and higher, the particles of intercalated graphite expand as much as about 80 to 1000 or more times its original volume in an accordion-like fashion in the c-direction, i.e. in the direction perpendicular to the crystalline planes of the constituent graphite particles. The expanded, i.e. exfoliated graphite particles are vermiform in appearance, and are therefore commonly referred to as worms. The worms may be compressed together into flexible sheets that, unlike the original graphite flakes, can be formed and cut into various shapes and provided with small transverse openings by deforming mechanical impact as hereinafter described. Flexible graphite sheet and foil are coherent, with good handling strength, and are suitably compressed, e.g. by roll-pressing, to a thickness of about 0.075 mm to 3.75 mm and a density of about 0.1 to 1.5 grams per cubic centimeter. From about 1.5-30% by weight of ceramic additives, can be blended with the intercalated graphite flakes as described in U.S. Patent 5,902,762 (which is incorporated herein by reference) to provide enhanced resin impregnation in the final flexible graphite product. The additives include ceramic fiber particles having a length of about 0.15 to 1.5 millimeters. The width of the particles is suitably from about 0.04 to 0.004 mm. The ceramic fiber particles are non-reactive and non-adhering to graphite and are stable at temperatures up to about 1100°C, preferably about 1400°C or higher. Suitable ceramic fiber particles are formed of macerated quartz glass fibers, carbon and graphite fibers, zirconia, boron nitride, silicon carbide and magnesia fibers, naturally occurring mineral fibers such as calcium metasilicate fibers, calcium aluminum silicate fibers, aluminum oxide fibers and the like. The flexible graphite sheet can also, at times, be advantageously treated with resin and the absorbed resin, after curing, enhances the moisture resistance and handling strength, i.e. stiffness of the flexible graphite sheet. Suitable resin content is preferably about 20 to 30% by weight, suitably up to about 60% by weight. In the practice of this invention, the flexible graphite sheet is provided with channels, which are preferably smooth-sided, and which pass between the parallel, opposed surfaces of the flexible graphite sheet and are separated by walls of compressed expandable graphite. It is the walls of the flexible graphite sheet that actually abut the ion exchange membrane, when the inventive flexible graphite sheet functions as an electrode in an electrochemical fuel cell. The channels preferably have openings on one of the opposed surfaces that are larger than the openings in the other opposed surface. The channels can have different configurations, which can be formed, for instance, using flat-ended protrusion elements of different shapes. The smooth flat-ends of the protrusion elements preferably ensure deformation and complete displacement of graphite within the flexible graphite sheet, i.e. there are no rough or ragged edges or debris resulting from the channel-forming impact. Preferred protrusion elements have decreasing cross-section in the direction away from the pressing force to provide larger channel openings on the side of the sheet that is initially impacted. The development of smooth, unobstructed surfaces surrounding channel openings enables the free flow of fluid into and through smooth-sided channels. In a preferred embodiment, openings one of the opposed surfaces are larger than the channel openings in the other opposed surface, e.g. from 1 to 200 times greater in area, and result from the use of protrusion elements having converging sides. The channels are formed in the flexible graphite sheet at a plurality of locations by mechanical impact. Thus, a pattern of channels is formed in the flexible graphite sheet. That pattern can be devised in order to control, optimize or maximize fluid flow through the channels, as desired. For instance, the pattern formed in the flexible graphite sheet can be uniform about the sheet (i.e., the channels are relatively evenly distributed on the sheet) or it can comprise selective placement of the channels, as described, or it can comprise variations in channel density or channel shape in order to, for instance, equalize fluid pressure along the surface of the electrode when in use, as well as for other purposes which would be apparent to the skilled artisan. The impact force is preferably delivered using a patterned roller, suitably controlled to provide well-formed perforations in the graphite sheet. In the course of impacting the flexible graphite sheet to form channels, graphite is displaced within the sheet to disrupt and deform the parallel orientation of the expanded graphite particles. In effect the displaced graphite is being "die-molded" by the sides of adjacent protrusions and the smooth surface of the roller. This reduces the anisotropy in the flexible graphite sheet and thus increases the electrical and thermal conductivity of the sheet in the direction transverse to the opposed surfaces. A similar effect is achieved with frusto-conical and parallel-sided peg-shaped flat-ended protrusions. As noted above, the inventive membrane electrode assembly comprises an ion exchange membrane sandwiched between two electrodes, at least one of which is the above- described graphite sheet. A typical substrate for the ion (or proton) exchange membrane is a porous material, such as a glass cloth or a polymeric material such as a porous polyolefin like polyethylene or polypropylene. Preferably, for use in a commercial practical electrochemical fuel cell, the substrate for the PEM should be between about 10 and 200 microns thick, with an average pore diameter of about 0.1 to about 1.0 microns and porosity of about 50 to 98%. Perfluorinated polymers, like polytetrafluoroethylene, are sometimes preferred. The substrate can then be impregnated to control properties such as porosity. Styrene impregnants such as trifluorostyrene and substituted trifluorostyrenes have been suggested as particularly suitable for use in fuel cell proton exchange membranes. One preferred impregnant useful in the practice of the invention is available from Ion Power Inc. under the tradename Liquione-1100; an especially preferred impregnant is a perfluorinated polymer membrane sold under the tradename Nafion® by DuPont Company. Suitable materials for use as the proton exchange membrane are described in U.S. Patents 5,773,480 and 5,834,523, the disclosures of each of which are incorporated herein by reference. In order to facilitate and/or enable the dissociation/association reactions required for fuel cell operation, a catalyst metal is loaded on the two opposed major surfaces of the PEM. Most commonly, the catalyst is a noble metal like platinum or a platinum group metal, often loaded on graphite or carbon particles. The catalyst can be loaded directly to the surface of the PEM, or a catalyst-loaded moiety, such as activated carbon paper can be bonded to either surface of the PEM, as would be familiar to the skilled artisan. Indeed, the catalyst can also be loaded on the inventive membrane electrode. In operation, the fluid (t.e., either hydrogen gas or oxygen gas, depending on the "side" of the membrane electrode assembly in question) contacts the catalyst, i the case of hydrogen, on the anodic side of the assembly, the catalyst catalyzes the dissociation of the hydrogen to its constituent protons and electrons; the protons then migrate through the proton exchange membrane, and the electrons are then utilized as electrical energy. In the case of oxygen, on the cathodic side of the assembly, the catalyst catalyzes the association of the protons and electrons, with the oxygen, to form water. In accordance with the present invention, distribution of the catalyst metal on the opposed surfaces of the PEM is controlled, or selected, by known methods, such as "printing" processes, such that the catalyst metal is arrayed about the surface of the PEM at locations at or near where the electrode contacts the PEM, i.e., where the desired reactions occur. In this way, catalyst is not present at locations where it will not employed in the dissociation/association reactions necessary for fuel cell operation, and where, therefore, the catalyst would be relatively ineffective. It will be understood that the expression "the catalyst metal is arrayed about the surface of the PEM" is also meant to include the situation where the metal is loaded on the electrode where it abuts the PEM. The catalyst is selectively loaded on the proton exchange membrane in a pattern determined by the channel pattern of the electrode that faces the particular PEM side. Most preferably, the catalyst is arrayed on the PEM at locations corresponding to the walls forming channels of the graphite sheet electrode as described above (such as by loading the catalyst metal on the electrode walls themselves), in order to maximize catalyst effectiveness. In this way, less catalyst is needed for similar electrical outputs from the fuel cell, resulting in significant savings and other advantages, such as reduced catalyst recycle costs. More particularly, in operation, the reaction catalyzed by the catalyst metal, that is, the dissociation of hydrogen molecules and atoms into constituent protons and electrons, and the re-association of the protons and electrons, in combination with oxygen, into water, occurs at the point where the surfaces of the electrode meet (or abut), or are in electrical contact with, the ion exchange membrane. It is at this location and, effectively, only at this location, where dissociated electrons can be conducted along the electrode and dissociated protons can migrate across the membrane (and vice versa with respect to the cathodic side of the fuel cell). Thus, it is only at or near where the walls of the channels formed in the inventive graphite sheet abut the ion exchange membrane, where catalyst is selectively loaded in accordance with the present invention. Detailed Description of the Preferred Embodiment With reference to Figure 1 and Figure 2, a compressed mass of expanded graphite particles, in the form of a flexible graphite sheet is shown at 10. The flexible graphite sheet 10 is provided with channels 20, which are preferably smooth-sided as indicated at 67 in Figures 5 and 8, and which pass between the opposed surfaces 30, 40 of flexible graphite sheet 10, and are separated by walls 3 of compressed expandable graphite. The channels 20 preferably have openings 50 on one of the opposed surfaces 30 which are larger than the openings 60 in the other opposed surface 40. The channels 20 can have different configurations as shown at 20' - 20'" in Figures 2(A), 2(B), 2(C) which are formed using flat-ended protrusion elements of different shapes as shown at 75, 175, 275, 375 in Figures 1(A) and 2(A), 2(B), 2(C), suitably formed of metal, e.g. steel and integral with and extending from the pressing roller 70 of the impacting device shown in Figure 3. The smooth flat-ends of the protrusion elements, shown at 77, 177, 277, 377, and the smooth bearing surface 73, of roller 70, and the smooth bearing surface 78 of roller 72 (or alternatively flat metal plate 79), ensure deformation and complete displacement of graphite within the flexible graphite sheet, i.e. there are no rough or ragged edges or debris resulting from the channel-forming impact. Preferred protrusion elements have decreasing cross- section in the direction away from the pressing roller 70 to provide larger channel openings on the side of the sheet that is initially impacted. The development of smooth, unobstructed surfaces 63 surrounding channel openings 60, enables the free flow of fluid into and through smooth-sided (at 67) channels 20. In a preferred embodiment, openings on one of the opposed surfaces are larger than the channel openings in the other opposed surface, e.g. from 1 to 200 times greater in area, and result from the use of protrusion elements having converging sides such as shown at 76, 276, 376. The channels 20 are formed in the flexible graphite sheet 10 at a plurality of predetermined locations by mechanical impact at the predetermined locations in sheet 10 using a mechanism such as shown in Figure 3 comprising a pair of steel rollers 70, 72 with one of the rollers having truncated, i.e. flat-ended, prism-shaped protrusions 75 which impact surface 30 of flexible graphite sheet 10 to displace graphite and penetrate sheet 10 to form open channels 20. In practice, both rollers 70, 72 can be provided with "out-of-register" protrusions, and a flat metal plate indicated at 79, can be used in place of smooth-surfaced roller 72. Indeed, the protrusion elements, such as protrusions 75, 175, 275, 375 which impact surface 30 to form channels 20 can also have varying sizes or configurations, in order to impart differing characteristics to flexible graphite sheet 10 at differing locations, most preferably in a selected pattern. For instance, it may be desirable to have the permeability of sheet 10 greater at certain locations than at others in order to, e.g., facilitate water vapor dispersal from or through flexible graphite sheet 10 where doing so may be needed or to equalize or accommodate fluid or gas pressure along flexible graphite sheet 10. This orientation of the expanded graphite particles 80 results in anisotropic properties in flexible graphite sheets; i.e. the electrical conductivity and thermal conductivity of the sheet being substantially lower in the direction transverse to opposed surfaces 130, 140 ("c " direction) than in the direction ("a" direction) parallel to opposed surfaces 130, 140. In the course of impacting flexible graphite sheet 10 to form channels 20, as illustrated in Figure 3, graphite is displaced within flexible graphite sheet 10 by flat- ended (at 77) protrusions 75 to push aside graphite as it travels to and bears against smooth surface 73 of roller 70 to disrupt and deform the parallel orientation of expanded graphite particles 80 as shown at 800 in Figure 5. This region of 800, adjacent channels 20, shows disruption of the parallel orientation into an oblique, non-parallel orientation is optically observable at magnifications of 100X and higher. In effect the displaced graphite is being "die-molded" by the sides 76 of adjacent protrusions 75 and the smooth surface 73 of roller 70 as illustrated in Figure 5. This reduces the anisotropy in flexible graphite sheet 10 and thus increases the electrical and thermal conductivity of sheet 10 in the direction transverse to the opposed surfaces 30, 40. A similar effect is achieved with frusto-conical and parallel-sided peg-shaped flat-ended protrusions 275 and 175. As comparison, Figure 4 is an enlarged sketch of a sheet of flexible graphite 110 that shows a typical prior art orientation of compressed expanded graphite particles 80 substantially parallel to the opposed surfaces 130, 140. The perforated gas permeable flexible graphite sheet 10 of Figure 1 are used as an electrode in an electrochemical fuel cell 500 shown schematically in Figures 6, 7 and 7(A). Figure 6, Figure 7 and Figure 7(A) show, schematically, the basic elements of an electrochemical Fuel Cell, more complete details of which are disclosed in U.S. Patents 4,988,583 and 5,300,370 and PCT WO 95/16287 (15 June 1995) and each of which is incorporated herein by reference. With reference to Figure 6, Figure 7 and Figure 7(A), the Fuel Cell indicated generally at 500, comprises electrolyte in the form of a plastic e.g. a solid polymer ion exchange membrane 550; perforated flexible graphite sheet electrodes 10 in accordance with the present invention; and flow field plates 1000, 1100 which respectively abut electrodes 10. Pressurized fuel is circulated through grooves 1400 of fuel flow field pate 1100 and pressurized oxidant is circulated through grooves 1200. In operation, the fuel flow field plate 1100 becomes an anode, and the oxidant flow field plate 1000 becomes a cathode with the result that an electric potential, i.e. voltage is developed between the fuel flow field plate 1000 and the oxidant flow field plate 1100. The above described electrochemical fuel cell is combined with others in a fuel cell stack to provide the desired level of electric power as described in the above-noted U.S. Patent 5,300,370. One significant difference lies in the fact that the catalyst 600 is selectively loaded on the surfaces of the solid polymer exchange membrane 550. hi this way, the catalyst metal is only present in a selected pattern on membrane 550 rather than relatively uniformly distributed thereon and, therefore, the amount of catalyst employed is minimized while maximizing the effectiveness of the catalyst. This is because catalyst is now only disposed on the surfaces of the membrane where the walls 3 of channels 20 abut membrane 550. The operation of Fuel Cell 500 requires that the electrodes 10 be porous to the fuel and oxidant fluids, e.g. hydrogen and oxygen, to permit these components to readily pass from the grooves 1400, 1200 through electrodes 10 to contact the catalyst 600 on the surfaces of the membrane 550, as shown in Figure 7(A), and enable protons derived from hydrogen to migrate through ion exchange membrane 550. In the electrode 10 of the present invention, channels 20 are positioned to adjacently cover grooves 1400, 1200 of the flow field plates so that the pressurized gas from the grooves passes through the smaller openings 60 of channels 20 and exits the larger openings 50 of channels 20. The initial velocity of the gas at the smaller openings 60 is higher than the gas flow at the larger openings 50 with the result that the gas is slowed down when it contacts the catalyst 600 on the surface of membrane 550 and the residence time of gas-catalyst contact is increased and the area of gas exposure at the membrane 550 is maximized. This feature, together with the increased electrical conductivity of the flexible graphite electrode of the present invention enables more efficient fuel cell operation. Figure 8 is a photograph (original magnification 100X) of a body of flexible graphite corresponding to a portion of the sketch of Figure 5. The articles of Figures 1 and 5 and the material shown in the photograph (100X) of Figure 8 can be shown to have increased thermal and electrical conductivity in the direction transverse to opposed parallel, planar surfaces 30, 40 as compared to the thermal and electrical conductivity in the direction transverse to surfaces 130, 140 of prior art material of Figure 4 in which particles of expanded natural graphite unaligned with the opposed planar surfaces are not optically detectable. A sample of a sheet of flexible graphite 0.25 mm thick having a density of 0.3 g/cc, representative of Figure 4, was mechanically impacted by a device similar to that of Figure 3 to provide channels of different size in the flexible graphite sheet. The transverse ("c" direction) electrical resistance of the sheet material samples was measured and the results are shown in the table below. Also, the transverse gas permeability of channeled flexible graphite sheet samples, in accordance with the present invention, was measured, using a Gurley Model 4118 for Gas Permeability Measurement. Samples of channeled flexible graphite sheet in accordance with the present invention were placed at the bottom opening (9.5 mm diam.) of a vertical cylinder (76.2 mm diameter cross-section). The cylinder was filled with 300 cc of air and a weighted piston (142 g) was set in place at the top of the cylinder. The rate of gas flow through the channeled samples was measured as a function of the time of descent of the piston and the results are shown in the table below. Flexible Graphite Sheet (0.25 mm thick; density = 0.3 gms/cc) In the present invention, for a flexible graphite sheet having a thickness of 0.075 mm to 0.375 mm adjacent the channels and a density of 0.5 to 1.5 grams per cubic centimeter, the preferred channel density is from 160 to 465 channels per square cm and the preferred channel size is a channel in which the ratio of the area of larger channel opening to the smaller is from 50:1 to 150:1. The above description is intended to enable the person skilled in the art to practice the invention. It is not intended to detail all of the possible variations and modifications that will become apparent to the skilled worker upon reading the description. It is intended, however, that all such modifications and variations be included within the scope of the invention that is defined by the following claims. The claims are intended to cover the indicated elements and steps in any arrangement or sequence that is effective to meet the objectives intended for the invention, unless the context specifically indicates the contrary.

Claims: What is claimed is: 1. A membrane electrode assembly comprising a pair of electrodes and an ion exchange membrane having opposed major surfaces positioned between the electrodes, at least one of the electrodes being formed of a sheet of a compressed mass of expanded graphite particles having a plurality of transverse fluid channels passing througli the sheet between first and second parallel, opposed surfaces of the sheet in a selected pattern, the channels being separated by walls of compressed expandable graphite, the walls formed on one of the opposed surfaces abutting the ion exchange membrane, wherein the ion exchange membrane has catalyst selectively loaded on or at at least one of its opposed major surfaces. 2. The assembly of claim 1 wherein the compressed mass of expanded graphite particles is characterized by expanded graphite particles adjacent the channels extending obliquely with respect to the opposed surfaces. 3. The assembly of claim 1 wherein the channel openings at the first surface are larger than the channel openings at the second surface. 4. The assembly of claim 3 wherein the graphite sheet has a thickness of about 0.075 mm to 0.375 mm adjacent said channels and a density of about 0.5 to 1.5 grams per cubic centimeter. 5. The assembly of claim 1 wherein the catalyst is loaded on or at both major surfaces of the ion exchange membrane. 6. The assembly of claim 5 wherein the catalyst is loaded on or at a surface of the ion exchange membrane in a pattern corresponding to the pattern of the walls of the channels of the sheet of a compressed mass of expanded graphite particles abutting that surface of the ion exchange membrane. 7. The assembly of claim 1 wherein the configuration and/or size of the transverse fluid channels passing through the sheet between first and second parallel, opposed surfaces of the sheet varies in a selected pattern. 8. An electrode for a proton exchange membrane fuel cell, comprising a sheet of a compressed mass of expanded graphite particles having a plurality of transverse fluid channels passing through the sheet between first and second parallel, opposed surfaces of the sheet in a selected pattern. 9. The electrode of claim 8 wherein the pattern of channels can be formed by selective placement of the channels, variations in channel density, variations of channel shape, or combinations thereof. 10. The electrode of claim 9 wherein the compressed mass of expanded graphite particles is characterized by expanded graphite particles adjacent the channels extending obliquely with respect to the opposed surfaces. 11. The electrode of claim 10 wherein the channel openings at the first surface are larger than the channel openings at the second surface. 12. The electrode of claim 11 wherein the graphite sheet has a thickness of about 0.075 mm to 0.375 mm adjacent said channels and a density of about 0.5 to 1.5 grams per cubic centimeter. 13. The electrode of claim 8 wherein the graphite sheet comprises walls lying between the transverse fluid channels thereof, the graphite sheet having catalyst selectively loaded on at least some of the walls. * 14. A process for forming a membrane electrode assembly for a proton exchange membrane fuel cell, comprising forming channels in a sheet of a compressed mass of expanded graphite particles having two opposed major surfaces and combining the graphite sheet with the ion exchange membrane having catalyst selectively loaded on or at at least one of its opposed major surfaces. 15. The process of claim 14 wherein the channels are formed in the graphite sheet by a pressing force using flat-ended protrusion elements. 16. The process of claim 15 wherein the channels have opemngs on one of the opposed surfaces that are larger than the openings in the other opposed surface. 17. The process of claim 16 wherein the channels can have different configurations formed using flat-ended protrusion elements of different shapes 18. The process of claim 17 wherein the protrusion elements have decreasing cross-section in the direction away from the pressing force to provide larger channel openings on the side of the graphite sheet that is initially impacted. 19. The process of claim 14 wherein the channels are formed using at least one patterned roller. 20. The process of claim 14 wherein the graphite sheet comprises walls lying between the transverse fluid channels thereof, the graphite sheet having catalyst selectively loaded on at least some of the walls. 21. The process of claim 14 wherein the catalyst is loaded on or at at least one of the opposed surfaces of the ion exchange membrane by a printing process.

Inventors: MERCURI ROBERT ANGELO, MERCURI, ROBERT, ANGELO
Applicants: GRAFTECH INC, GRAFTECH INC.
Application year: 2001
Main classification: H01M4
IPC codes: H01M4, H01M8