Abstract:
A graphite article useful in producing a membrane electrode assembly comprising a pair of electrodes and an ion exchange membrane positioned between the electrodes is presented. 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 opposed surfaces abutting the ion exchange membrane when used in a membrane electrode assembly. At least some of the fluid channels are interconnected to enable flow of fluid therebetween.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an article useful in an electrode assembly for an electrochemical fuel cell. The inventive assembly includes an article formed of flexible graphite sheet that is fluid permeable and has enhanced isotropy with respect to thermal and electrical conductivity.  
         BACKGROUND OF THE INVENTION  
         [0002]    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, especially 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. In 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 may be 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 natural graphites suitable for manufacturing flexible graphite possess a very high degree of orientation.  
           [0003]    As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. 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.  
           [0004]    Natural graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is at least 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 at least 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.  
           [0005]    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.  
           [0006]    Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, such as 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 at least about 80 times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles, which 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 5 pounds per cubic foot to about 125 pounds per cubic foot. 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.  
           [0007]    This very considerable difference in properties, known as 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).  
           [0008]    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.  
           [0009]    The foregoing situations are accommodated by the present invention.  
         SUMMARY OF THE INVENTION  
         [0010]    In accordance with the present invention, a membrane electrode assembly for an electro-chemical fuel cell is provided, comprising a pair of electrodes and an ion exchange membrane 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 through the sheet between first and second opposed surfaces of the sheet, one of the opposed surfaces abutting the ion exchange membrane. Advantageously, the transverse fluid channels are formed by mechanically impacting an opposed surface of the sheet to displace graphite within the sheet at predetermined locations. The transverse fluid channels are adjacently positioned and separated by walls of compressed expanded graphite at least some of which permit interconnection between adjacent channels (such as by having grooves therein) to enable fluid flow therebetween. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a plan view of a transversely permeable sheet of flexible graphite having interconnected transverse channels in accordance with the present invention;  
         [0012]    [0012]FIG. 1(A) shows a flat-ended protrusion element used in making the channels in the perforated sheet of FIG. 1;  
         [0013]    [0013]FIG. 2 is a side elevation view in section of the sheet of FIG. 1;  
         [0014]    FIGS.  2 (A), (B), (C) show various suitable flat-ended configurations for transverse interconnected channels in accordance with the present invention;  
         [0015]    FIGS.  3 ,  3 (A),  3 (B) show a mechanism for making the article of FIG. 1;  
         [0016]    FIGS.  3 (C),  3 (D) show enlarged perspective views of portions of transversely permeable flexible graphite sheet in accordance with the present invention;  
         [0017]    [0017]FIG. 3(E) is a photograph of a portion of transversely permeable flexible graphite sheet corresponding to FIG. 3(C);  
         [0018]    [0018]FIG. 4 shows an enlarged sketch of an elevation view of the oriented expanded graphite particles of flexible graphite sheet material;  
         [0019]    [0019]FIG. 5 is a sketch of an enlarged elevation view of an article formed of flexible graphite sheet in accordance with the present invention;  
         [0020]    [0020]FIG. 5, 6,  7  and  7 (A) show a fluid permeable electrode assembly which includes a transversely permeable article in accordance with the present invention; and  
         [0021]    [0021]FIG. 8 is a photograph at 100× (original magnification) corresponding to a portion of the side elevation view sketch of FIG. 5.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    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, for instance, 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 particles of intercalated graphite 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.  
         [0023]    A common method for manufacturing graphite sheet, e.g., foil from flexible graphite is described by Shane et al. in U.S. Pat. No. 3,404,061, the disclosure of which is incorporated herein by reference. In the typical practice of the Shane et al. method, natural graphite flakes are intercalated by dispersing the flakes in a solution containing an oxidizing agent of, for example, a mixture of nitric and sulfuric acid. 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, such as trifluoroacetic acid, and a strong oxidizing agent soluble in the organic acid.  
         [0024]    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.  
         [0025]    After the flakes are intercalated, any excess solution is drained from the flakes and the flakes are water-washed. The quantity of intercalation solution retained on the flakes after draining may range from 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. Alternatively, the quantity of the intercalation solution may be limited to between 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. up to 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 (or 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.  
         [0026]    Flexible graphite sheet and foil are coherent, with good handling strength, and are suitably compressed, such as by roll-pressing, to a thickness of 0.003 to 0.15 inch and a density of 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. Pat. No. 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 0.15 to 1.5 millimeters. The width of the particles is suitably from 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 2000° F., preferably 2500° F.. 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.  
         [0027]    With reference to FIG. 1 and FIG. 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 FIGS. 5 and 8, and which pass between the parallel, opposed surfaces  30 ,  40  of flexible graphite sheet  10 , and are separated by walls  3  of compressed expandable graphite. The walls  3  are advantageously provided with grooves  5 , having a depth of {fraction (1/10)} to ⅓ the depth of the channels in accordance with the present invention. 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 FIGS.  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 FIGS.  1 (A) and  2 (A),  2 (B),  2 (C),  2 (D), suitably formed of metal, e.g. steel, and integral with and extending from the pressing roller  70  of the impacting device shown in FIG. 3. The smooth flat-ends of the channel-forming protrusion elements  75 ,  175 ,  275 ,  375 , shown at  77 ,  177 ,  277 ,  377 , and the smooth flat ends of the groove-forming protrusion elements  675 ,  775 ,  875 ,  975  shown at  677 ,  777 ,  877 ,  977 , 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 displacement of graphite within the flexible graphite sheet, preferably such that there are no rough or ragged edges or debris resulting from the channel-forming impact. The groove-forming protrusion elements  675 ,  775 ,  875 ,  975  also result in deformation and displacement of graphite within the flexible graphite sheet. Preferred channel-forming protrusion elements  77  have decreasing cross-section in the direction away from the pressing roller  70  to provide larger channel openings on the side of the sheet which 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 .  
         [0028]    In a preferred embodiment, openings at 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 transverse channels  20  are formed in the flexible graphite sheet  10  at a plurality of pre-determined locations by mechanical impact at the predetermined locations in sheet  10  using a mechanism such as shown in FIG. 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 the present invention, the channel-forming protrusions  75  are bridged by groove-forming protrusions  675  which form interconnecting grooves  5  between channels  20  in a row of aligned channels concurrently with formation of channels  20  which is illustrated in the sketch of FIG. 3(C) and the photograph of FIG. 3(E). Additionally, groove-forming protrusion elements  675 ′ can be included as shown in FIGS.  3 (A),  3 (B) to form interconnecting grooves  5 ′ in a parallel row of transverse channels  20  as shown in FIG. 3(D). 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 . FIG. 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 . 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 is 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 FIG. 3, graphite is displaced within flexible graphite sheet  10  by flat-ended (at  77 ) channel-forming 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 FIG. 5. Groove forming protrusions  675  concurrently deform the parallel orientation of expanded graphite particles. This region of  800 , adjacent channels  20  and grooves  5 , shows disruption of the parallel orientation into an oblique, non-parallel orientation is optically observable at magnifications of 100× 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 FIG. 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 . The perforated gas permeable flexible graphite sheet  10  of FIG. 1 can be used as an electrode in an electrochemical fuel cell  500  shown schematically in FIGS. 6, 7 and  7 (A).  
         [0029]    [0029]FIG. 6, FIG. 7 and FIG. 7(A) show, schematically, the basic elements of an electrochemical Fuel Cell, more complete details of which are disclosed in U.S. Pat. Nos. 4,988,583 and 5,300,370 and PCT WO 95/16287 (Jun. 15, 1995) and each of which is incorporated herein by reference.  
         [0030]    With reference to FIG. 6, FIG. 7 and FIG. 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  catalyst coated at surfaces  601 ,  603 , e.g. coated with platinum  600  as shown in FIG. 7(A); 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. Pat. No. 5,300,370.  
         [0031]    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 , as shown in FIG. 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 . In the event of a blockage in a channel  20 , such as indicated at  7  in FIGS. 6 and 7, fluid from adjacent channels can flow through grooves  5  so that gas-catalyst contact adjacent the blocked channel is maintained. 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  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.  
         [0032]    [0032]FIG. 8 is a photograph (original magnification 100×) of a body of flexible graphite corresponding to a portion of the sketch of FIG. 5.  
         [0033]    The articles of FIGS. 1 and 5 and the material shown in the photograph (100×) of FIG. 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 the material of FIG. 4 in which particles of expanded natural graphite unaligned with the opposed planar surfaces are not optically detectable.  
         [0034]    A sample of a sheet of flexible graphite 0.01 inch thick having a density of 0.3 grams/cc, representative of FIG. 4, was mechanically impacted by a device similar to that of FIG. 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.  
         [0035]    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.  
         [0036]    Samples of channeled flexible graphite sheet in accordance with the present invention were placed at the bottom opening (⅜ in. diam.) of a vertical cylinder (3 inch diameter cross-section). The cylinder was filled with 300 cc of air and a weighted piston (5 oz.) 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       (0.01 inch thick; density = 0.3 gms/cc)                    1600 channels per   250 channels per               square inch—0.020   square inch—0.020               inch wide at top;   inch wide at top;           No   0.005 inch wide at   0.007 inch wide at           Channels   bottom   bottom                        Transverse   80   8   0.3       Electrical       Resistance (micro       ohms)       Diffusion Rate-   —   8 seconds   30 seconds       Seconds                  
 
         [0037]    In the present invention, for a flexible graphite sheet having a thickness of 0.003 inch to 0.015 inch adjacent the channels and a density of 0.5 to 1.5 grams per cubic centimeter, the preferred channel density is from 1000 to 3000 channels per square inch 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.  
         [0038]    In the practice of the present invention, the flexible graphite sheet can, 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 20 to 30% by weight, suitably up 60% by weight.  
         [0039]    The article of the present invention can be used as electrical and thermal coupling elements for integrated circuits in computer applications, as conformal electrical contact pads and as electrically energized grids in de-icing equipment.  
         [0040]    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 which 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 which is defined by the following claims. The claims are intended to cover the indicated elements and steps in any arrangement or sequence which is effective to meet the objectives intended for the invention, unless the context specifically indicates the contrary.