Abstract:
An electrochemical cell has an anode cavity and a cathode cavity. The anode cavity and the cathode cavity sandwich an electrochemically conductive medium. The anode cavity and/or the cathode cavity have electrically conductive plates assembled using a solid bonding material. Each plate has nesting volumes and protrusions provided in the perimeter seal area. The protrusions of one plate fit into the volumes of an adjacent plate to eliminate the offset introduced by the bonding material thickness.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to an electrochemical cell capable of operating at elevated pressures with respect to ambient pressure.  
         [0002]     As a way of example, electrochemical cells may be used to electrolytically separate water into its components, hydrogen and oxygen or, alternatively, combine hydrogen with oxygen to form water and generate electricity. Each electrochemical cell typically has an anode cavity, a cathode cavity, and an electrolyte layer in contact with the electrodes. The electrolyte may be an ionically conductive material such as an ion-exchange membrane or liquid electrolyte solution contained in a porous matrix. The electrolyte is positioned between the anode cavity and the cathode cavity and serves to facilitate or enable the exchange of electrical charge and to separate the fluids between the two cavities.  
         [0003]     The electrolyte area positioned between the anode and cathode cavities represents the working cell area or active area, where electrochemical processes take place. Several electrochemical cells of appropriate active area are assembled together to meet the capacity requirements for a specific application. There are numerous methods of connecting electrochemical cells together described by the way the fluids are admitted and removed to and from individual cells and the way electrical energy is provided for the cells&#39; operation. A particular case refers to an assemblage of cells electrically connected in series and having the fluids managed in a parallel connection. Such an assembly is commonly known as a bipolar stack. The illustrations in this discussion will be made with respect to this particular mode of assembly without limiting the object of the disclosure.  
         [0004]     Either the anode cavity or the cathode cavity or both may be formed from electrically conductive, essentially round metal plates, which are stacked upon one another as taught in Titterington et al. (U.S. Pat. No. 5,316,644) which is hereby incorporated by reference. Each plate may have a central area comprising a mesh screen that permits the passage of fluid through the plate. A peripheral area surrounds the mesh screen to form a seal that prevents the leakage of pressurized fluid out of the central area as well as between anode and cathode cavities. When all of the central areas of these plates are aligned, the stack of mesh screens forms a fluid cavity.  
         [0005]     The fluid exchange between the fluid cavities and the outside is accomplished through openings located in the seal area of each of the metal plates. These openings are in fluid communication with either the anode or the cathode cavities. Several options are described for providing the fluid connections in Titterington, et al. (U.S. Pat. No. 5,316,644).  
         [0006]     During the manufacturing process, the round metal plates are sealed together by a suitable adhesive applied to the peripheral area of each plate. The adhesive has a thickness that accumulates over the number of adjacent layers, creating a gap between the plates. The electrochemical cell and cell stack operation requires that the plates must be in intimate physical contact with one another to ensure adequate electrical conductivity between the plates. Accordingly, manufacturers typically deform the central areas of these plates together to place them in contact with one another.  
         [0007]      FIGS. 1 and 2  illustrate this manufacturing process in detail. As shown in  FIG. 1 , either the anode cavity or the cathode cavity or both, also known in the trade as a bipolar plate assembly, are formed by aligning first conductive member  34 , an essentially round metal plate, over second conductive member  38 , another round metal plate. Adhesive  42  is then applied by means of a spray between first conductive member  34  and second conductive member  38  to form a seal in peripheral area  30  to prevent the leakage of fluid out of central area  26  of stacked members  34  and  38 . As a consequence of the thickness of the spray adhesive, bond gap  46  is formed between first conductive member  34  and second conductive member  38 . As shown in  FIG. 2 , gap  46  is eliminated by deforming second conductive member  38  towards first conductive member  34  to place them in electrical contact with one another.  
         [0008]     Each cavity may comprise ten or more bonded plates. Deforming the working area  26  can be accomplished before the individual cells are assembled into a stack or after the stack of cells is completed. The force required to deform these plates as an assembly may, however, damage the electrolyte. A pressure pad is required to bridge the gap created by the accumulated adhesive thickness and to press the plates together for adequate electrical contact. The pressure pad is an expensive component, which is needed to close the gaps between plates. The deforming operation using the pressure pad also adds labor expense to the assembly of the electrochemical cell.  
         [0009]     Furthermore, a sprayed adhesive usually requires an organic solvent for its application. The spray adhesive may also require the addition of neat solvent to ensure uniformity of application for a thin layer of adhesive. These solvents create volatile organic compound (VOC) emissions that, for a large-scale production, raise environmental concerns.  
         [0010]     A need therefore exists for an alternative bonding solution that eliminates the gap created by the spray adhesive between the plates, eliminates the need for a pressure pad, and reduces VOC emissions in the manufacturing of high pressure electrochemical cells.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention comprises a structure for an electrochemical cell having an anode cavity, a cathode cavity, and an electrolyte sandwiched between the anode cavity and the cathode cavity and in contact with catalyst electrodes. The anode cavity or the cathode cavity, or both, are formed from stacked conductive members, such as metal plates, suitably formed to provide all the functionality needed for cell operation such as described in Titterington et al. (U.S. Pat. No. 5,316,644). Each conductive member of the present invention has at least one protrusion that nests within a corresponding oversized volume, or groove, of an adjacent conductive member. In the seal area of each conductive member, the adhesive is accommodated in the oversized volume. Accordingly, during the assembly process, excess adhesive that might normally create a gap between the conductive members flows into the oversized volume of the adjacent conductive member to thereby eliminate any bonding gap.  
         [0012]     Preferably, the adhesive is an adhesive tape that has a liquid state and a solid state. The use of an adhesive tape permits greater control over the amount of adhesive used to prevent the creation of a bonding gap or a spillover as well as to encourage uniform application. Moreover, an adhesive tape reduces the emissions of VOCs that might raise environmental issues. While in solid state, the tape may be arranged between a protrusion of one conductive member and a volume of another conductive member in the peripheral area of each conductive member. When adequate heat is applied to the stack of conductive members, the adhesive tape melts and it flows into the volume. The adhesive tape may have a surface area sized the same or smaller than the peripheral area of the conductive members to prevent an overflow of adhesive and further prevent the creation of a bonding gap.  
         [0013]     The invention further encompasses a method of manufacturing the unique electrochemical cell. Adhesive tape is arranged between one conductive member having protrusions and one conductive member having volumes. The adhesive tape is then liquefied under pressure and heat to form a seal between the conductive members. In this way, an anode or a cathode cavity or both may be formed. The cavities may then sandwich an electrolyte to complete the cell stack assembly. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows:  
         [0015]      FIG. 1  illustrates bonding gaps created by prior art techniques.  
         [0016]      FIG. 2  illustrates the deformation of the plates of the prior art to place them in electrical contact with one another.  
         [0017]      FIG. 3  illustrates a cross-sectional view of an electrochemical cell of the present invention, including anode cavity and cathode cavity.  
         [0018]      FIG. 4  illustrates a cross-sectional view of the cavity of an inventive electrochemical cell.  
         [0019]      FIG. 5  illustrates an overhead view of two matching conductive members of the inventive electrochemical cell of  FIG. 4 .  
         [0020]      FIG. 6  shows a close up view of channels and ridges of each conductive member of  FIG. 5 .  
         [0021]      FIG. 7  illustrates an overhead view of two other matching conductive members of an electrochemical cell.  
         [0022]      FIG. 8  shows a close up view of an alternative channel and ridge design of  FIG. 7 .  
         [0023]      FIG. 9  shows a close up view of an alternative channel and ridge design.  
         [0024]      FIG. 10  shows a close up view of an alternative channel and ridge design.  
         [0025]      FIG. 11  shows a close up view of an alternative channel and ridge design. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]      FIG. 3  illustrates a side cross-sectional view of electrochemical cell  10 . Electrochemical cell  10  has anode cavity  14  spaced from cathode cavity  18 . Anode cavity  14  and cathode cavity  18  sandwich electrochemically conductive medium  22 , such as a polymer electrolyte membrane or a liquid contained in a porous matrix. Moreover, as further known, electrochemical cell  10  has central area  26 A of anode cavity  14 , central area  26 C of cathode cavity  18 , which is also a fluid cavity. These fluid cavities are sealed by peripheral areas  30 A and  30 C to contain the fluid interactions between anode cavity  14  and the cathode cavity  18 , respectively. As known, anode cavity  14  and cathode cavity  18  may comprise a stack of bonded cylindrical conductive members, such as metal plates, using the bonding material as the sealing agent.  
         [0027]      FIG. 4  illustrates in detail the inventive structure  100 , a stack of conductive members forming either anode cavity  14  or cathode cavity  18  of electrochemical cell  10 . First conductive member  50 , here a round metal plate screen, has central area  26  and peripheral area  30 . Central area  26  has first set of openings  86  to permit fluid to pass through first conductive member  50  and has peripheral area  30  to contain fluid within central area  26 . Also, second conductive member  54 , another round metal plate screen, has central area  26  and peripheral area  30 . Second conductive member  54  also has second set of openings  98  in central area  26  to permit fluid to pass through second conductive member  54 . First set of openings  86  is in fluid communication with second set of openings  98 . When stacked together with other conductive members, first conductive member  50  and second conductive member  54  form cylinder  106  having axis A.  
         [0028]     One aspect of the present invention eliminates bond gap  46  (shown in  FIG. 1 ) by providing volumes  58  to receive excess adhesive. As shown in  FIG. 4 , first conductive member  50  is spaced from second conductive member  54 . First conductive member  50  has volumes  58  and protrusions  62  in peripheral area  30 . Likewise, second conductive member  54  has volumes  58  and protrusions  62  in peripheral area  30 . Volumes  58  and protrusions  62  may extend radially about axis A. Volumes  58  have width W 1  greater than width W 2  of protrusions  62 . In this way, volumes  58  are oversized and can easily accommodate protrusions  62  so that there is volume leftover to receive adhesive in liquid state.  
         [0029]     As shown in  FIG. 4 , first conductive member  50  is aligned relative to second conductive member  54  so that protrusions  62  of first conductive member  50  are disposed over volumes  58  of second conductive member  54  while protrusions  62  of second conductive member  54  are aligned over volumes  58  of first conductive member  50 . First conductive member  50  may be aligned relative to second conductive member  54  by placing holes at predetermined locations of each member and then stacking the members through these holes on rods as known. As shown in  FIG. 4 , volumes  58  may comprise volumes  111  and  114  that are spaced radially from each other relative to axis A.  
         [0030]     Further, unlike the prior art, adhesive tape  64 , rather than a spray adhesive, is used to bond and seal first conductive member  50  to second conductive member  54 . The use of an adhesive tape promotes uniform application of the adhesive as well as reduces the emission of VOCs. Adhesive tape  64  may be die-cut to fit over peripheral area  30  of first conductive member  50  and second conductive member  54 . Preferably, adhesive tape  64  has a surface area less than area G, the surface area of peripheral area  30 , to reduce excess adhesive. Adhesive tape  64  preferably should also not be tacky at room temperature, should have very high bond strength, and should require only moderately high temperatures to form a bond. Applicant has tested a number of adhesive tapes and has found tapes using an ethylene acrylic acid copolymer, such as product number TBF  406  by 3M™ Company, to be particularly well suited for the present application.  
         [0031]     When in solid state  72 , adhesive tape  64 , a polymer melt adhesive, is placed over peripheral area  30  between protrusions  62  and volumes  58  of first and second conductive member  50 ,  54 . First conductive member  50  and second conductive member  54  thus sandwich adhesive tape  64  along peripheral area  30 . Pressure is applied on first conductive member  50  and second conductive member  54 , for example 1,083 kPa (157 psi), on peripheral area  30  before heating. The temperature of adhesive tape  64  is then gradually increased from ambient temperature to 160° C. to 163° C. following a predetermined schedule of about 3° C. per minute. The compressive load between first conductive member  50  and second conductive member  54  is then increased to about 10,345 kPa (1,500 psi) on peripheral area  30  when the temperature reaches 154° C.  
         [0032]     During this process, adhesive tape  64  liquefies and excess adhesive flows into volumes  58  of first conductive member  50  and second conductive member  54 . Then, protrusions  62  of second conductive member  54  extend into volumes  58  of first conductive member  50  while volumes  58  of second conductive member  54  receive protrusions  62  of first conductive member  50 . To prevent the formation of a bonding gap, it is important that volumes  58  are large enough to accommodate protrusions  62  as well as anticipated excess adhesive tape  64 . First conductive member  50  and second conductive member  54  are then allowed to reach room temperature under the higher pressure. The pressure is released when the assembly reaches ambient temperature. In this way, two or more conductive members may be formed together to form anode cavity  14  or cathode cavity  18  of a cylindrical shape.  
         [0033]     While here, adhesive tape  64  is used as a securing member, other adhesive materials and techniques, such as adhesive pastes and solids, may be used too. Adhesive pastes, such as caulking compounds, uncured silicone rubber, polymer solutions in solvents and other similar materials may be used in some pre-formed state such as a paste rolled into a sheet die-cut to size, and applied to the peripheral area of the conducting members. For such a bonding material, compression would be required from the beginning until the bond is established. For adhesive solids, such as an adhesive film or brazing alloy, the approaches are similar. Clamping pressure is increased at the point when the solid material becomes fluid and pressure is maintained until the bond is formed.  
         [0034]     By using this inventive technique, the offset due to the bond line is completely eliminated. There is also no need to exert additional force on the part during assembly. Further, the invention does not use organic solvents or create hazardous waste or fumes. Also, the inventive technique uses lower compression force during the bonding process and requires shorter time to bond than other techniques. In fact, this technique reduces assembly time significantly.  
         [0035]      FIG. 5  illustrates an overhead view of first conductive member  50  and second conductive member  54 . As shown in  FIGS. 5 and 6 , openings  71  are provided in peripheral area  30  to communicate fluid for the active area as taught by Titterington, et al. In  FIG. 6 , a close-up view is provided of peripheral area  30  of each member. As shown, for each member  50  and  54 , volumes  58  may extend entirely around peripheral area  30  forming channel volumes while protrusions  62  may likewise extend around peripheral area  30  forming ridges. Volumes  59  and ridges  63  may also be provided around openings  71 .  
         [0036]      FIG. 7  illustrates an alternative pattern for first conductive member  104  and second conductive member  108 . As shown in  FIG. 8 , like the patterns shown in  FIG. 5 , first conductive member  104  has volumes  58  and protrusions  62  that extend around peripheral area  30 . Likewise, second conductive member  108  also has volumes  58  and protrusions  62  that extends around peripheral area  30 . The patterns of  FIGS. 6 and 8  may be etched by known techniques.  
         [0037]      FIGS. 9-11  show ridges  102  and  103  in a broken or interrupted pattern. It should be noted that ridges  102 ,  103  and  106  are formed around holes  71 . Ridges  102  and  106  of first conductive member  108  are concentric around hole  71  and are formed in an interrupted pattern. Ridges  103  are also concentric around hole  71  of second conductive members  110 . When first conductive member  108  is overlayed on second conductive member  110 , ridges  103  (see dashed lines) are received in volume  112  of first conductive member  108 . Again, volume  112  is oversized relative to ridges  103  to receive excess adhesive during the cell assembly process. Also, it should be noted that ridges  102  and  106  are staggered angularly relative to ridges  104  so that a tortuous path is formed between peripheral area  30  and hole  71 .  
         [0038]     Interrupted ridges  102 ,  103  and  106  allow venting of trapped gas formed in the bonding process to vent from peripheral area  30  to atmosphere. Trapped gas bubbles may otherwise form between members  108  and  100 , especially when these members are thin and bonding thick. However, to ensure an adequate seal of peripheral area  30  during normal cell operation, continuous and uninterrupted sealing ridges  108  and  112  are provided. Gas is allowed to escape to the atmosphere before sealing ridges  108  and  112  seal peripheral area  30 .  
         [0039]      FIG. 10  illustrates first conductive member  114  and second conductive member  116 . Interrupted ridges  118  are provided on second conductive member  116 .  
         [0040]     Finally,  FIG. 11  shows broken ridges  120  on both first conductive member  124  and second conductive member  126 . Ridges  120  have wider spacing between each other. The spacing of ridges and the pattern will be dictated by the needs of the particular application.  
         [0041]     The aforementioned description is exemplary rather than limiting. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed. However, one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. Hence, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For this reason the following claims should be studied to determine the true scope and content of this invention.