Patent Publication Number: US-2005142435-A1

Title: Apparatus and method for maintaining compression of the active area in an electrochemical cell

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application is a divisional application, which claims priority to U.S. patent application Ser. No. 09/965,675 filed Sep. 27, 2001, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/235,944 filed Sep. 27, 2000, and U.S. Provisional Patent Application Ser. No. 60/235,975 filed Sep. 28, 2000, the entire contents of both applications being incorporated herein by reference. 
    
    
     BACKGROUND  
      This disclosure relates to electrochemical cells, and, more particularly, to an apparatus for maintaining compression within the active area of an electrochemical cell.  
      Electrochemical cells are energy conversion devices that are usually classified as either electrolysis cells or fuel cells. Proton exchange membrane electrolysis cells can function as hydrogen generators by electrolytically decomposing water to produce hydrogen and oxygen gases. Referring to  FIG. 1 , a section of an anode feed electrolysis cell of the related art is shown at  10  and is hereinafter referred to as “cell  10 .” Reactant water  12  is fed to cell  10  at an oxygen electrode (e.g., an anode)  14  where a chemical reaction occurs to form oxygen gas  16 , electrons, and hydrogen ions (protons). The chemical reaction is facilitated by the positive terminal of a power source  18  connected to anode  14  and a negative terminal of power source  18  connected to a hydrogen electrode (e.g., a cathode)  20 . Oxygen gas  16  and a first portion  22  of the water are discharged from cell  10 , while the protons and a second portion  24  of the water migrate across a proton exchange membrane  26  to cathode  20 . At cathode  20 , hydrogen gas  28  is formed and is removed for use as a fuel or a process gas. Second portion  24  of water, which is entrained with hydrogen gas, is also removed from cathode  20 .  
      Another type of water electrolysis cell that utilizes the same configuration as is shown in  FIG. 1  is a cathode feed cell. In the cathode feed cell, process water is fed on the side of the hydrogen electrode. A portion of the water migrates from the cathode across the membrane to the anode. A power source connected across the anode and the cathode facilitates a chemical reaction that generates hydrogen ions and oxygen gas. Excess process water exits the cell at the cathode side without passing through the membrane.  
      A typical fuel cell also utilizes the same general configuration as is shown in  FIG. 1 . Hydrogen gas is introduced to the hydrogen electrode (the anode in the fuel cell), while oxygen, or an oxygen-containing gas such as air, is introduced to the oxygen electrode (the cathode in the fuel cell). The hydrogen gas for fuel cell operation can originate from a pure hydrogen source, a hydrocarbon, methanol, or any other source that supplies hydrogen at a purity level suitable for fuel cell operation. Hydrogen gas electrochemically reacts at the anode to produce protons and electrons, the electrons flow from the anode through an electrically connected external load, and the protons migrate through the membrane to the cathode. At the cathode, the protons and electrons react with oxygen to form water.  
      Conventional electrochemical cell systems generally include one or more individual cells arranged in a stack, with the working fluids directed through the cells via input and output conduits formed within the stack structure. The cells within the stack are sequentially arranged, each including a membrane electrode assembly (hereinafter “MEA”) defined by a cathode, a proton exchange membrane, and an anode. Each cell typically further comprises a first flow field in fluid communication with the cathode and a second flow field in fluid communication with the anode. The MEA may be supported on either or both sides by flow field support members such as screen packs or bipolar plates disposed within the flow fields, and which may be configured to facilitate membrane hydration and/or fluid movement to and from the MEA.  
      Referring to  FIG. 2 , a conventional electrochemical cell system suitable for operation as an anode feed electrolysis cell, a cathode feed electrolysis cell, or a fuel cell is shown at  30 . Cell system  30  includes the MEA defined by anode  14 , cathode  20 , and proton exchange membrane  26 . Regions proximate to and bounded on at least one side by anode  14  and cathode  20  respectively define flow fields  31 ,  32 . A flow field support member  33  is disposed adjacent to anode  14  and is retained within flow field  31  by a frame  34  and a cell separator plate  35 . A flow field support member  36  is disposed adjacent to cathode  20  and is retained within flow field  32  by a frame  40  and a pressure pad separator plate  37 . A pressure pad  38  is disposed between pressure pad separator plate  37  and a cell separator plate  39 . The cell components, particularly frames  34 ,  40  and cell separator plates  35 ,  39 , are formed with the suitable manifolds or other conduits to facilitate fluid communication through cell system  30 .  
      A pressure differential often exists within the cell system and particularly across the cell. Such a pressure differential may cause variations in the pressure distribution over the surface area of the MEA. In order to compensate for the pressure differential while maintaining intimate contact between the various cell components under a variety of operational conditions and over long time periods, compression is applied to the cell components via pressure pad  38 . However, because pressure pads  38  are generally fabricated from materials incompatible with system fluids and/or the material from which the cell membrane is fabricated, pressure pads  38  are oftentimes separated from the active area by pressure pad separator plate  37  or enclosed within protective casings (not shown).  
      While existing pressure pads are suitable for their intended purposes, there still remains a need for improvements, particularly regarding the compression of the components in the electrolysis cell and support of the MEA, particularly at high pressures. Therefore, a need exists for a pressure pad that is compatible with the cell environment and that provides uniform compression of the cell components and support of the MEA, thereby allowing for the optimum performance of the electrolysis cell.  
     SUMMARY  
      The above-described drawbacks and disadvantages are alleviated by pressure pads and methods for forming a pressure pads. In one embodiment, the method of forming a pressure pad can comprise: disposing a first electrically conductive member at a first compression member to form a first ring assembly; disposing a second electrically conductive member at a second compression member to form a second ring assembly; and arranging the first ring assembly at the second ring assembly in a concentric pattern.  
      In another embodiment, the method of forming a pressure pad can comprise: disposing an electrically conductive member at a compression member, and arranging the electrically conductive member and the compression member in a spiral pattern.  
      In yet another embodiment, the method of forming a pressure pad can comprise: forming a plate of electrically conductive material, and disposing a compression member at the plate.  
      In one embodiment, the pressure pad can comprise an electrically conductive member interwoven with a compression member. In another embodiment, the pressure pad can comprise an electrically conductive member inter-stitched with a compression member.  
      The above discussed and other features and advantages will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Referring now to the drawings, which are meant to be exemplary and not limiting, and wherein like elements are numbered alike in the several FIGURES:  
       FIG. 1  is a schematic representation of a conventional anode feed electrolysis cell;  
       FIG. 2  is a cross sectional schematic representation of a conventional electrochemical cell system showing the spatial relationship of the cell components;  
       FIG. 3  is a cross sectional schematic representation of an electrochemical cell system showing the spatial relationship of the cell components and a pressure pad;  
       FIG. 4  is a plan view of a pressure pad having a plurality of concentrically arranged ring assemblies;  
       FIGS. 5 and 6  are schematic representations of a ring assembly of a pressure pad;  
       FIG. 7  is a plan view of a pressure pad having a spirally wound configuration;  
       FIG. 8  is a sectional view of a pressure pad having an electrically conductive member and a compression member of complementary configuration;  
       FIGS. 9A and 9B  are sectional views of pressure pads having compression members longitudinally disposed within electrically conductive members;  
       FIGS. 10A through 10H  illustrate various cross sectional geometries of electrically conductive members and compression members;  
       FIG. 11  is an exploded perspective view of a pressure pad configured as an electrically conductive plate on which compression members are disposed;  
       FIG. 12  is an edge sectional view of the electrically conductive plate of the pressure pad of  FIG. 11 ;  
       FIG. 13  is an edge sectional view of a pressure pad having a contoured plate;  
       FIG. 14  is a plan view of the pressure pad of  FIG. 11 ;  
       FIG. 15  is an edge sectional view of a pressure pad having a plate configured to capture compression members therein;  
       FIG. 16  is an edge sectional view of a pressure pad having compression members into which trans-radial grooves are disposed;  
       FIG. 17  is a perspective view of a pressure pad wherein electrically conductive and elastomeric members are woven together; and  
       FIG. 18  is a side sectional view of a pressure pad in which an electrically conductive member is stitched into an elastomeric member. 
    
    
     DETAILED DESCRIPTION  
      Disclosed herein is a novel apparatus and methods for maintaining the compression of the active area in an electrochemical cell. The active area generally refers to the electrically associated electrodes and the space between two or more electrically associated electrodes of the cell. A compression device, e.g., a pressure pad as is described below, is disposed at the cell proximate to one of the electrodes. Other compression devices may further be disposed proximate to the other electrodes. The pressure pad, which comprises an electrically conductive material and a resilient elastomeric material selected for its compatibility with the cell environment, is typically disposed at a flow field adjacent to the electrode where it is exposed to the system fluids.  
      Although the disclosure below is described in relation to a proton exchange membrane electrochemical cell employing hydrogen, oxygen, and water, other types of electrochemical cells and/or electrolytes may be used, including, but not limited to, phosphoric acid and the like. Various reactants can also be used, including, but not limited to, hydrogen, bromine, oxygen, air, chlorine, and iodine. Upon the application of different reactants and/or different electrolytes, the flows and reactions change accordingly, as is commonly understood in relation to that particular type of electrochemical cell. Furthermore, while the discussion below is directed to an anode feed electrolysis cell, it should be understood by those of skill in the art that cathode feed electrolysis cells, fuel cells, and regenerative fuel cells are also within the scope of the embodiments disclosed.  
      Referring to  FIG. 3 , an electrochemical cell system incorporating an exemplary embodiment of a pressure pad capable of providing improved compression in the active area of the cell is shown at  50 . Cell system  50  typically includes a plurality of cells employed in a cell stack as part of the system. When cell system  50  is utilized as an electrolysis cell, power inputs are generally between about 1.48 volts and about 3.0 volts, with current densities being between about 50 A/ft 2  (amperes per square foot) and about 4,000 A/ft 2 . When utilized as a fuel cell, power outputs range between about 0.4 volts and about 1 volt, with current densities being between about 0.1 A/ft 2  and about 10,000 A/ft 2 . Current densities exceeding 10,000 A/ft 2  may also be obtained depending upon the fuel cell dimensions and configuration. The number of cells within the stack and the dimensions of the individual cells is scalable to the cell power output and/or gas output requirements.  
      Cell system  50  is substantially similar to cell system  30  as described above and shown with reference to  FIG. 2 . In particular, cell system  50  comprises an MEA defined by a proton exchange membrane  51  having a first electrode (e.g., an anode)  52  and a second electrode (e.g., a cathode)  53  disposed on opposing sides thereof. Regions proximate to and bounded on at least one side by anode  52  and cathode  53  respectively define flow fields  54 ,  55 . A flow field support member  56  may be disposed adjacent to anode  52  and retained within flow field  54  by a frame  57  and a cell separator plate  59 . A gasket  58  is optionally positioned between frame  57  and cell separator plate  59  to effectively seal flow field  54 .  
      A flow field support member  60  may be disposed adjacent to cathode  53 . A pressure pad  64  is typically disposed between flow field support member  60  and a cell separator plate  66 . Flow field support member  60  and pressure pad  64  are retained within flow field  55  by a frame  67  and cell separator plate  66 . Because pressure pad  64  may be fabricated from materials that are compatible with the cell environment, cell system  50  is generally operated without a pressure pad separator plate. A gasket  68  is optionally positioned between frame  67  and cell separator plate  66  to effectively seal flow field  55 . The cell components, particularly frames  57 ,  67 , cell separator plates  59 ,  66 , and gaskets  58 ,  68 , are formed with the suitable manifolds or other conduits to facilitate fluid communication through cell system  50 .  
      Frames  57 ,  67  can be formed of any dielectric material that is compatible with the electrochemical cell environment and is capable of holding flow field support members  56 ,  60  in position within flow fields  54 ,  55 . Materials from which frames  57 ,  67  can be fabricated include, but are not limited to, thermosets, thermoplastics, and rubber-based materials, such as polyetherimide, polysulfone, polyethersulfone, polyarylether ketone (PEEK), and mixtures comprising at least one of the foregoing materials.  
      Membrane  51  comprises electrolytes that are preferably solids or gels under the operating conditions of the electrochemical cell. Useful materials from which membrane  51  can be fabricated include proton conducting ionomers and ion exchange resins. Useful proton conducting ionomers include complexes comprising an alkali metal salt, an alkali earth metal salt, a protonic acid, or a protonic acid salt. Counter-ions useful in the above salts include halogen ion, perchloric ion, thiocyanate ion, trifluoromethane sulfonic ion, borofluoric ion, and the like. Representative examples of such salts include, but are not limited to, lithium fluoride, sodium iodide, lithium iodide, lithium perchlorate, sodium thiocyanate, lithium trifluoromethane sulfonate, lithium borofluoride, lithium hexafluorophosphate, phosphoric acid, sulfuric acid, trifluoromethane sulfonic acid, and the like. The alkali metal salt, alkali earth metal salt, protonic acid, or protonic acid salt is complexed with one or more polar polymers such as a polyether, polyester, or polyimide, or with a network or cross-linked polymer containing the above polar polymer as a segment.  
      Useful polyethers include polyoxyalkylenes, such as polyethylene glycol, polyethylene glycol monoether, and polyethylene glycol diether; copolymers of at least one of these polyethers, such as poly(oxyethylene-co-oxypropylene)glycol, poly(oxyethylene-co-oxypropylene)glycol monoether, and poly(oxyethylene-co-oxypropylene)glycol diether; condensation products of ethylenediamine with the above polyoxyalkylenes; and esters, such as phosphoric acid esters, aliphatic carboxylic acid esters or aromatic carboxylic acid esters of the above polyoxyalkylenes. Copolymers of, e.g., polyethylene glycol with dialkylsiloxanes, maleic anhydride, or polyethylene glycol monoethyl ether with methacrylic acid, are known in the art to exhibit sufficient ionic conductivity to be useful.  
      Ion-exchange resins useful as proton conducting materials include hydrocarbon- and fluorocarbon-type resins. Hydrocarbon-type ion-exchange resins include phenolic resins, condensation resins such as phenol-formaldehyde, polystyrene, styrene-divinyl benzene copolymers, styrene-butadiene copolymers, styrene-divinylbenzene-vinylchloride terpolymers, and the like, that are imbued with cation-exchange ability by sulfonation, or are imbued with anion-exchange ability by chloromethylation followed by conversion to the corresponding quaternary amine.  
      Fluorocarbon-type ion-exchange resins can include hydrates of tetrafluoroethylene-perfluorosulfonyl ethoxyvinyl ether or tetrafluoroethylene-hydroxylated (perfluoro vinyl ether) copolymers. When oxidation and/or acid resistance is desirable, for instance, at the cathode of a fuel cell, fluorocarbon-type resins having sulfonic, carboxylic and/or phosphoric acid functionality are preferred. Fluorocarbon-type resins typically exhibit excellent resistance to oxidation by halogen, strong acids and bases. One family of fluorocarbon-type resins having sulfonic acid group functionality is NAFION™ resins (commercially available from E.I. du Pont de Nemours and Company, Wilmington, Del.).  
      Anode  52  and cathode  53  are fabricated from catalyst materials suitable for performing the needed electrochemical reaction (i.e., electrolyzing water to produce hydrogen and oxygen). Suitable materials for anode  52  and cathode  53  include, but are not limited to, platinum, palladium, rhodium, carbon, gold, tantalum, tungsten, ruthenium, iridium, osmium, alloys thereof, and the like. Anode  52  and cathode  53  may be adhesively disposed on membrane  51 , or they may be positioned adjacent to, but in contact with, membrane  51 .  
      Flow field support members  56 ,  60  allow the passage of system fluids and are preferably electrically conductive. Such support members  56 ,  60  may comprise, for example, screen packs or bipolar plates. Screen packs include one or more layers of perforated sheets or a woven mesh formed from metal strands. Typical metals that may be used to fabricate screen packs include, for example, niobium, zirconium, tantalum, titanium, carbon steel, stainless steel, nickel, cobalt, and alloys thereof. Bipolar plates are commonly carbon or carbon composite structures incorporating a polymeric binder. Bipolar plates may also be fabricated from metal. Typical metals that may be used to fabricate bipolar plates include, but are not limited to, niobium, zirconium, tantalum, titanium, carbon steel, stainless steel, nickel, cobalt, and alloys thereof.  
      Electrical communication is maintained between adjacently positioned cells in the electrochemical system (and across the cell stack) through the cell separator plates. In order to facilitate the electrical communication, continuity of structure is provided between an anode and a cathode and its respective associated cell separator plate through a compression of the cell componentry. Such compression is effected in cell system  50  via pressure pad  64 , which is disposed in direct contact with a flow field and is positioned adjacent to the cell separator plate on either the anode or the cathode side of membrane  51 . To effect an optimum compression (and optimum electrical communication), pressure pads  64  may be disposed on both sides of membrane  51 , and they may be positioned within either or both of the flow fields of cell system  50  in place of either or both of the flow field support members.  
      Pressure pad  64  comprises an electrically conductive material configured to provide for the electrical communication across the cell. Pressure pad  64  further comprises a compression member, which may be fabricated from an elastomeric material, to provide for the substantially uniform distribution of compression within the cell system. Both the electrically conductive material and the elastomeric material are preferably compatible with the system fluids and the material from which membrane  51  is fabricated. Pressure pad  64  is optionally porous to allow passage of water or system gases, is capable of allowing intimate contact to be maintained between cell components at high pressures, and is configured to withstand high pressures while maintaining its operability over extended time periods. In particular, pressure pad  64  is configured to withstand pressures up to or exceeding about 100 pounds per square inch (psi), about 500 psi, about 1000 psi, about 5000 psi, and more preferably about 10,000 psi. Pressure pad  64  may be configured and dimensioned to withstand pressures exceeding 10,000 psi.  
      It should be appreciated by those of skill in the art that electrically conductive components, e.g., rings, members, conductive plates, and other devices as are described herein, are fabricated from an electrically conductive material, and preferably an electrically conductive material that is compatible with the cell system fluids. Metallic materials from which electrically conductive components can be fabricated include, but are not limited to, conductive metals and alloys and superalloys thereof, for example copper, silver, gold, aluminum, zirconium, tantalum, titanium, niobium, iron and ferrous alloys, for examples steels such as stainless steel, nickel and nickel alloys such as HASTELLOY™ (commercially available from Haynes International, Kokomo, Ind.), cobalt and cobalt superalloys such as ELGILOY™ (commercially available from Elgiloy® Limited Partnership, Elgin, Ill.) and MP35N™ (commercially available from Maryland Specialty Wire, Inc., Rye, N.Y.), hafnium, and tungsten, among others, with titanium preferred because of its strength, durability, availability, low cost, ductility, low density, and its compatibility with the electrochemical cell environment. Non-metallic materials from which electrically conductive components can be fabricated include, but are not limited to, refractory materials, electrically conductive carbon, electrically conductive polymers, and electrically conductive graphite. Additionally, an electrically conductive component can comprise a substrate plated with a suitable metallic material. A substrate material can be plated by any suitable means (e.g., electroplating, chemical vapor deposition, etc.) with any of the foregoing metallic materials.  
      Compressible components, e.g., rings, members, and other devices as are described herein are fabricated from a compressible material such as an elastomeric material. Examples of such elastomeric materials include, but are not limited to silicones, such as fluorosilicones, fluoroelastomers, such as KALREZ® (commercially available from E.I. du Pont de Nemours and Company, Wilmington, Del.), VITON® (commercially available from E.I. du Pont de Nemours and Company, Wilmington, Del.), and FLUOREL® (commercially available from Minnesota Mining and Manufacturing Company, St. Paul, Minn.), and combinations and mixtures comprising at least one of the foregoing elastomeric materials. The elastomeric material is preferably inert to the electrochemical cell environment such that the pressure pad may be employed in fluid communication with the cell fluids and the cell membrane. Examples of such inert elastomeric materials include, but are not limited to fluoroelastomers, such as KALREZ®, VITON®, and FLUOREL®.  
      The elastomeric materials may themselves be made conductive, typically by the incorporation of electrically conductive particulate materials as is known in the art. Suitable electrically conductive particulate materials include, but are not limited to, the above-mentioned electrically conductive metals and alloys and superalloys thereof, preferably copper and nickel. Also useful are non-conductive particles coated with conductive materials, for example silver-coated glass spheres, as well as conductive, particulate carbon, for example acetylene blacks, conductive furnace black, super-conductive furnace black, extra-conductive furnace black, vapor grown carbon fibers, carbon nanotubes, and the like. Copper, nickel, conductive carbon, or a combination thereof is presently preferred because of their conductivity, availability, low cost, and compatibility with the electrochemical cell environment. The particular shape of the particles is not critical, and includes spheres, plates, whiskers, tubes, drawn wires, flakes, short fibers, irregularly-shaped particles, and the like. Suitable particle sizes and amounts vary widely, and are readily determined by one of ordinary skill in the art depending on factors including, but not limited to, the particular materials chosen, the desired elastomeric characteristics and conductivity of the pressure pad, the cost of the materials, the size of the pressure pad, the method of manufacture, and other considerations. Regardless of the exact size, shape, and composition of the conductive fillers particles, they should be thoroughly dispersed through the polymeric resin. Such compositions and their method of manufacture have been described, for example, in U.S. Pat. Nos. 4,011,360; 5,082,596; 5,296,570; 5,498,644; 5,585,038; and 5,656,690.  
      Referring now to  FIG. 4 , an exemplary embodiment of pressure pad  64  is shown. Pressure pad  64  comprises a plurality of concentrically arranged ring assemblies  69 . In its simplest form, each ring assembly  69  is defined by an electrically conductive ring  70  and a compression ring  71  positioned adjacent to conductive ring  70 . Rings  70 ,  71  may be continuous, or they may be broken to facilitate assembly of each ring assembly  69 . Other configurations of the ring assembly (not shown) may be defined by at least two conductive rings and/or at least two compression rings. Ring assemblies  69  may be configured such that rings  70 ,  71  interengage, each ring being supported by an adjacently positioned ring.  
      Ring assembly  69  may be mounted or otherwise supported within the cell system structure by a support device (not shown) such as a plate or an arrangement of spacers. The size and geometry of pressure pad  64  is based upon the size and geometry of the cell into which pressure pad  64  is incorporated and the pressure range over which the cell operates. While pressure pad  64  is depicted in  FIG. 4  as being round across a major plane thereof, it should be understood by those of skill in the art that pressure pad  64  may be configured as being elliptical or polygonal as dictated by the geometry of the cell. Fluid communication can be maintained across pressure pad  64  by configuring ring assemblies  69  to include openings, channels, or other fluid flow conduits (not shown).  
      Referring to  FIGS. 5 and 6 , the compression and decompression of ring assembly  69  is shown. In  FIG. 5 , a pressure pad into which ring assembly  69  is incorporated is not subject to a pressure. For pressure loads up to about 4000 psi, compression ring  71  has an uncompressed thickness A of between about 0.05 inches and about 1.5 inches (about 1.27 mm and about 38.1 mm). Upon compression of compression ring  71 , as is illustrated in  FIG. 6 , compression ring  71  has a compressed thickness B that is less than uncompressed thickness A. Compression of compression ring  71  allows the pressure pad to be securely retained within the flow field of the electrochemical cell system. The dimensions of the pressure pad (including, but not limited to, thicknesses A and B) are defined such that a spring rate of the pressure pad is within a predetermined range. Moreover, while the cross sectional geometry of each ring  70 ,  71  is shown to be rectangular, it should be understood that rings  70 ,  71  may be of other cross sectional geometries, as is shown and described below with reference to  FIGS. 10A through 10H .  
      Referring now to  FIG. 7 , an exemplary embodiment of a pressure pad is shown at  164 . Pressure pad  164  comprises an electrically conductive member  170  and a compression member  171  positioned adjacent to conductive member  170 . Members  170 ,  171  are wound in a spiral configuration and can be wound loosely or tightly to provide for varying degrees of fluid communication between opposing sides of pressure pad  164 . Furthermore, pressure pad  164  can be positioned adjacent other similarly or differently configured pressure pads to provide support to the MEA of the electrochemical cell system. Variations in the tension with which the members of adjacently positioned pressure pads are wound can provide a porosity gradient across an assembly of adjacently positioned pressure pads, thereby allowing for the controlled flow of fluid through the cell system. The thickness of compression member  171 , as above, is typically greater than the thickness of conductive member  170  to enable pressure pad  164  to be securely retained in the cell system.  
      With reference to  FIG. 8 , another exemplary embodiment of a pressure pad is shown at  264 . Pressure pad  264  comprises a plurality of electrically conductive rings  270  of a particular cross sectional geometry between which are disposed compression rings  271  of a complementary cross sectional geometry. Compression of pressure pad  264  into which rings  270 ,  271  are incorporated enables contact to be maintained between mating surfaces thereof, thereby providing for a substantially uniform distribution of radial compression within pressure pad  264 . Furthermore, although pressure pad  264  is described as being a plurality of rings, it should be understood by those of skill in the art that pressure pad  264  may comprise adjacently positioned individual members having complementary surfaces wound in a spiral pattern.  
      Another exemplary embodiment of a pressure pad is shown generally at  364  in  FIGS. 9A and 9B . Pressure pad  364  comprises a plurality of rings  369  concentrically arranged, each ring  369  being defined by an electrically conductive member  370  and a compression member  371  integrally disposed with each other. Such an arrangement provides for substantially even compression within an electrochemical cell system, particularly under the high pressures at which cell systems typically operate.  
      Compression member  371  is longitudinally disposed within electrically conductive member  370  in an annular arrangement. Although compression member  371  can be disposed longitudinally anywhere within the boundaries of conductive member  370 , as is shown in  FIG. 9B , it should be appreciated by those of skill in the art that compression member  371  is preferably concentrically disposed within electrically conductive member  370 , as is shown in  FIG. 9A , such that compression member  371  is surrounded by an electrically conductive surface of substantially uniform thickness. Rings  369  are configured to have a geometry across a major plane thereof that corresponds with the cross sectional geometry of the cell stack into which they are incorporated. In particular, rings  369  may be round, elliptical, or polygonal. Moreover, while the cross sectional geometry of each member  370 ,  371  is shown to be round, it should be understood that the cross sectional geometries of the conductive and compression members may be of other shapes, e.g., shapes as depicted below with reference to  FIGS. 10A through 10H . Similar to the rings of pressure pads described above, each ring  369  has an uncompressed thickness of between about 0.05 inches and about 1.5 inches (about 1.27 mm and about 38.1 mm).  
      Pressure pad  364  may also be defined by a continuous resilient cord spirally arranged. The spiral configuration is typically effected by winding the resilient cord around a central axial point. In such a configuration, compression member  371  is longitudinally disposed within electrically conductive member  370  to form the resilient cord, which, in a manner similar to that of the rings of pressure pad  364 , incorporates both electrically conductive member  370  and compression member  371  in an annular arrangement that may or may not be concentric. As above, the cross sectional shapes of both the electrically conductive member and the compression member may be of various geometries. Similar to the rings, the resilient cord has an uncompressed thickness of between about 0.05 inches and about 1.5 inches (about 1.27 mm and about 38.1 mm).  
      The annular arrangement of the electrically conductive member and the compression member can be formed by a number of different operations. In one exemplary forming operation of pressure pads having either a ring or a spiral wound cord configuration, the compression member is wrapped (e.g., wound or braided over) or coated (e.g., through a dipping, spraying, or pultrusion process) with the electrically conductive member. In another exemplary forming operation, the conductive member can be chemically welded or adhesively bonded to the compression member. In yet another exemplary forming operation, the compression member, and particularly the outer surface of the compression member, can be impregnated with electrically conductive powders, fibers, or other elements to form the electrically conductive member.  
      Referring to  FIGS. 10A through 10H , the various cross sectional geometries of the electrically conductive or compression members employable in pressure pads are illustrated. In particular, it should be noted that structures defined by the geometries as depicted in  FIGS. 10A and 10B  can be employed with a structure having a geometry such as that depicted in  FIG. 10G  to provide a pressure pad structure (as is shown in  FIG. 8 ) in which adjacently positioned components are supported in a complementary fashion. Furthermore, structures having geometries such as those shown in  FIGS. 10C and 10D  can provide complementary support, as can structures having geometries shown in  FIGS. 10E and 10F . Rings or a spirally wound member having a geometry as is illustrated in  FIG. 10H  may be employed by itself or in conjunction with any of the others shown in  FIGS. 10A through 10G .  
      Yet another exemplary embodiment of a pressure pad is shown at  464  with reference to  FIGS. 11 through 14 . Pressure pad  464  comprises an electrically conductive plate  470  having raised portions formed or otherwise disposed annularly (and preferably concentrically) over a major surface thereof and compression members disposed between the raised portions. Conductive plate  470  is generally formed in a stamping, casting, molding, or machining operation. The raised portions on major surface  469   a  of conductive plate  470  define alternating “peaks” and “troughs” that alternate and correspond with opposing troughs and peaks on an opposing major surface  469   b  of conductive plate  470 . The alternating peaks and troughs define annularly positioned areas in which the compression members can be received to provide compressibility to pressure pad  464 . The annularly positioned areas define a first receiving area  485   a , a second receiving area  485   b , and a third receiving area  485   c  on one major surface  469   a  of conductive plate  470 , while correspondingly defining a fourth receiving area  485   d  and a fifth receiving area  485   e  on major surface  469   b  of conductive plate  470 . It should be realized by those of skill in the art, however, that although conductive plate  470  is shown and described as having five receiving areas, any number of receiving areas can be disposed thereon. Compression members  471   a ,  471   b ,  471   c ,  471   d ,  471   e  are accordingly disposed within their respective receiving areas on the appropriate sides of conductive plate  470 .  
      As can be seen in  FIGS. 11 and 12 , transition surfaces  493  defining the raised portions and defining the receiving areas may be angled from the general plane of conductive plate  470 . Transition surfaces  493  generally extend between major surfaces  469   a ,  469   b  at angles greater than ninety degrees. Transition surfaces  493  may also be configured such that a plurality of edges  487  are defined thereon, as is shown in the exemplary embodiment illustrated in  FIG. 13 . In either configuration, the compression members disposed therein may include an adhesive material  494  disposed between the surfaces thereof and the surfaces of the respective receiving areas to facilitate the retention of compression members  471  within the receiving areas.  
      Alternately, or additionally, compression members  471  may include an adhesive material integral therewith to provide bondability with the surfaces of conductive plate  470 .  
      Referring now to  FIG. 14 , conductive plate  470  may include a plurality of interruptions  492  extending radially from a center location  491  of conductive plate  470  to define an arrangement of wedges joined at the center location. Interruptions  492  impart a flexibility to conductive plate  470  by allowing the wedges to independently respond to variations in pressure exerted on the face of conductive plate  470 . Such a flexibility substantially reduces the rigidity of conductive plate  470  and enables pressure pad  464  to provide for the even distribution of compression within the cell system while maintaining electrical communication across the opposing faces of conductive plate  470 .  
      In another exemplary embodiment of a conductive plate shown at  570  with reference to  FIG. 15 , the edges defined by transition surfaces  593  are such that the angles between the major surfaces are less than ninety degrees. In such a configuration, compression members  571  disposed within the receiving areas are physically retained or captured therein by edges  595  formed by the major surfaces  569   a ,  569   b  and transition surfaces  593 . Adhesive materials may optionally be employed to assist in the retention of compression members  571  within the receiving areas.  
      An exemplary embodiment of a compression member  671  employable in a pressure pad  664  includes grooves  696  disposed therein, as is shown in  FIG. 16 . Grooves  696  typically extend trans-radially across at least one surface of each compression member  671 . Upon compression of a plate  670  into which a grooved compression member  671  is mounted, the pressure exerted normally on the major surface of plate  670  facilitates the radially outward dispersion of a compressive force F applied to pressure pad  664  through compression member  671 . Dispersion of such pressure further facilitates the compression of compression member  671  against an adjacent surface, e.g., a frame of the cell system.  
      In other exemplary embodiments of the pressure pad, an electrically conductive material and an elastomeric material are integrated with each other by inter-weaving strands of the electrically conductive and elastomeric materials (as is shown in  FIG. 17 ) or by stitching strands of one material into the other (as is shown in  FIG. 18 ). Additionally, the pressure pad can comprise a plurality of woven or stitched layers where the faces of each individual pressure pad can be disposed adjacent to each other. The individual pressure pads can be interconnected to form a unitary pad, or they can be stacked and held in place within the cell by the frames and the cell separator plates.  
      For configurations in which the pressure pad is woven, as is shown at  764  in  FIG. 17 , electrically conductive material  770  is generally provided as a cord or ribbon (i.e., a flattened cord). The thickness of electrically conductive material  770  is typically between about 0.005 inches and about 0.1 inches (about 0.127 mm and about 2.54 mm) and preferably between about 0.005 inches and about 0.01 inches (about 0.127 mm and about 0.254 mm). Elastomeric material  771  may similarly be provided in a cord or ribbon form having a diameter or other cross-sectional dimension that is substantially less than the length. The cross-sectional shape of electrically conductive material  770  or elastomeric material  771  can be circular, oval, square, rectangular, triangular, polygonal, or any other shape suited to weaving. One exemplary suitable elastomeric material  771  has a circular cross-section, for example, with a diameter from about 0.05 inches to about 0.1 inches (about 1.27 mm to about 2.54 mm), and preferably from about 0.075 inches to about 0.1 inches (about 1.9 mm to about 2.54 mm).  
      Referring now to  FIG. 18 , an exemplary embodiment of a stitched pressure pad is shown at  864 . In pressure pad  864 , a first material (e.g., an electrically conductive material  870 ) is stitched into a second material (e.g., an elastomeric material  871 ), wherein the second material is provided in the form of a flat sheet. The flat sheet includes perforations provided therein to facilitate the stitching operation. Either electrically conductive material  870  or elastomeric material  871  may be stitched into the other, for example, flat layers of a perforated conductive metal or a felt of conductive carbon fibers may be stitched into a flat layer of an elastomer such as a fluorosilicone; or flat layers of an elastomer such as fluorosilicone may be stitched into a layer of metal. In one exemplary embodiment of pressure pad  864 , the elastomeric material is a polytetrafluoroethylene, such as VITON®, in the form of a perforated pad having a durometer from about 45 to about 90 and preferably from about 70 to about 75. An electrically conductive ribbon of titanium is stitched through the polytetrafluoroethylene. Alternately, VITON® in the form of a cord may be woven through a sheet of electrically conductive carbon fibers.  
      In any of the foregoing exemplary embodiments, the pressure pads are typically disposed at the side of the cell at which the pressure is greater. It should be understood by those of skill in the art, however, that pressure pads may be disposed at either side or at both sides of the cell. Furthermore, it should be understood that a suitable number of pressure pads can be stacked to replace either or both of the flow field support members in the cell system.  
      The electrochemical cell system as described above incorporates pressure pads preferably formed of metals and elastomeric materials that are compatible with the cell system fluids as well as the cell membrane. The pressure pads are capable of withstanding pressures of up to or in excess of 100 psi, 500 psi, 1000 psi, 5000 psi, and, more preferably, up to or in excess of 10,000 psi, with the upper limit being a function of the cell system capabilities. The electrically conductive material and the elastomeric material is generally selected and the pressure pad configured such that the overall electrical resistance of the cell system is minimal, thereby resulting in an overall stack resistance that is minimal.  
      While the disclosure has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.