Abstract:
An electrolytic cell employs a plastic molded frame component with raised ridges on one surface to create seal with a proton exchange membrane and on the opposite surface a groove with an interlocking feature for accepting a tabbed elastomer gasket. The gasket and frame design when combined with a proton exchange membrane can be stacked in multiple layers using mechanical hardware. The frame captures the softer elastomeric sealing material preventing elastomeric creep and loss of positive seal caused by the relaxation of mechanical hardware under load and internal pressure fluctuations. The addition of the ridged sealing surface provides positive surface contact with the polymeric membrane to further prevent the loss of seal under mechanical load. The interlocking feature reduces assembly time and improves assembly accuracy.

Description:
PRIORITY 
     This application is a national phase of International Application No. PCT/US2008/075414 filed Sep. 9, 2008 and published in the English language, which claims priority of U.S. Provisional Application No. 61/055,419 filed May 22, 2008, which are hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention herein described relates generally to electrolytic cell stacks and more particularly to electrolytic cell stacks used to generate hydrogen and oxygen by the electrolytic dissociation of water, or as a fuel cell that produces electricity by converting hydrogen and oxygen back into water. 
     BACKGROUND 
     Gases, such as oxygen, hydrogen or chlorine, may be generated by disassociating a chemical compound into its constituent elements. Electrolytic cell stacks heretofore have been used for this purpose. The cell stack may include a catalytic anode, a catalytic cathode and an adjacent solid polymeric ion-exchange membrane as an electrolyte that is in electrical contact with both the anode and the cathode. A DC voltage is applied across the catalytic electrodes to cause the feed compound, such as water, to dissociate into its constituent ionic forms, such as oxygen and hydrogen. The evolved gas or gasses may be collected as desired. 
     U.S. Pat. No. 5,037,518, which is hereby incorporated herein by reference, discloses an electrolytic cell, also herein referred to as a cell stack. The cell has a watertight housing which clamps a solid polymeric ion-exchange membrane electrolyte between a catalytic cathode and a catalytic anode. Gaskets are provided to insure a water-tight construction for the electrolytic cell. The gaskets may be laminates having a catalytic screen disposed between two non-conductive annular gaskets. The screen is in electrical contact with the adjacent electrode and functions as a part of that electrode. To ensure that the screens firmly engage the respective electrodes, as well as the solid-electrolyte membrane, a pressure disc of the same diameter as the screens is disposed between the cathode and the uppermost gasket so that when the two housing sections are drawn together, the pressure disc exerts pressure on the screens. 
     SUMMARY OF THE INVENTION 
     The arrangement shown in FIG. 2 of U.S. Pat. No. 5,037,518 works well for stacks with one or two cell layers. When more than two cell layers are used, pressure leakage problems have been found to occur, such as hydrogen leaking into the oxygen and outboard gas leakage to the atmosphere. In a prior art four layer cell stack using cells similar to the single cell shown in FIG. 2 of U.S. Pat. No. 5,037,518, the oxygen and hydrogen gaskets (gaskets 106 and 109 in U.S. Pat. No. 5,037,518 were replaced with gaskets made from an elastomeric material. This design has performed acceptably, but the inventors found that over time the elastomeric gaskets “relax”. This relaxation can cause the cell stack to leak outboard and/or hydrogen to leak into the oxygen. The relaxation may also cause the screens to be driven into the ion-exchange membranes and this can lead to holes or tears. 
     The inventors also discovered that internal leaks arose from a poor seal just inside the hydrogen and/or oxygen ports in the prior art two-cell stack design that used a rigid gasket frame, which is shown in  FIG. 3 . 
     In order to resolve these problems and drawbacks, the inventors conceived of various design features that may be used individually or in combination with any one or more of the other features to provide a superior cell stack design that provides performance characteristic heretofore not attainable by prior art designs. The novel and inventive features include, among others:
         (a) seating, as by molding, a screen, in particular a fine screen, into a plastic gasket frame in order to provide a flat surface for the ion-exchange membrane to sit on, thereby improving the seal around the ion-exchange membrane while minimizing or eliminating driving of a screen pack (or other flow field member) into the ion-exchange membrane;   (b) welding or otherwise securing a flow field member, in particular a coarse screen package, to the corresponding electrode plate, such as a titanium electrode plate, to reduce part count and aid in alignment;   (c) adding raised ridges, such as 0.03 inch ridges, to the plastic frame to improve sealing along the membrane perimeter and around the O 2  and H 2  ports;   (d) adding a small seal insert, such as a “trapezoidal” seal insert, to the frame for improving transmission of pressure through the stack radially inwardly of an adjacent port to aid in sealing at the ports; and/or   (e) adding slots in the frame and corresponding tabs to the seals, or vice versa, to create a mechanical interlock between the frame and seal to ensure proper seating of the seals.
 
Again, any one of these features may be used with any one or more of the other features to obtain improved performance in a cell stack, particularly to enable cell stacks including one, two, three, four, five, six or more cells in a stacked arrangement, while affording desirable sealing performance without degradation of the cells.
       

     Accordingly, the present invention provides an improved electrolytic cell. The electrolytic cell, in a preferred embodiment, employs a plastic molded frame component with raised ridges on one surface to create seal with a proton exchange membrane and on the opposite surface a groove with an interlocking feature for accepting a tabbed elastomeric gasket. The gasket and frame design when combined with a proton exchange membrane can be stacked in multiple layers using mechanical hardware. The frame captures the softer elastomeric sealing material preventing elastomeric creep and loss of positive seal caused by the relaxation of mechanical hardware under load and internal pressure fluctuations. The addition of the ridged sealing surface provides positive surface contact with the polymeric membrane to further prevent the loss of seal under mechanical load. The interlocking feature reduces assembly time and improves assembly accuracy. 
     Further features of the invention will become apparent from the following detailed description when considered in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the annexed drawings, 
         FIG. 1  is a diagrammatic illustration of a prior art electrolysis cell system in which hydrogen gas can be generated; 
         FIG. 2  is side elevational view of a prior art electrolytic cell stack assembly; 
         FIG. 3  is an exploded perspective view of a prior art electrolytic cell used in the assembly of  FIG. 2 ; 
         FIG. 4  is a top plan view of a rigid hydrogen gasket frame according to the invention; 
         FIG. 5  is an edge view of the rigid hydrogen gasket frame of  FIG. 4 ; 
         FIG. 6  is a bottom plan view of the rigid hydrogen gasket frame of  FIG. 4 ; 
         FIG. 7  is a perspective view of a seal insert according to the present invention; 
         FIG. 8  is a plan view of the seal insert of  FIG. 7 ; 
         FIG. 9  is an end view of the seal insert of  FIG. 7 ; 
         FIG. 10  is an elevational view of the seal insert of  FIG. 7 ; 
         FIG. 11  is a plan view of a hydrogen sealing gasket according to the invention; 
         FIG. 12  is a perspective view of the hydrogen gasket seal of  FIG. 11 ; 
         FIG. 13  is an exploded perspective view illustrating assembly of the hydrogen gasket seal and seal insert to the hydrogen gasket frame; 
         FIG. 14  is a fragmentary perspective view, partly broken away in section, showing the hydrogen gasket seal and seal insert assembled to the hydrogen gasket frame; and 
         FIG. 15  is another fragmentary perspective view, partly broken away in section, showing the hydrogen gasket seal and seal insert assembled to the hydrogen gasket frame. 
     
    
    
     DETAILED DESCRIPTION  
     Referring now in detail to the drawings and initially to  FIG. 1 , an exemplary gas generator system is indicated generally by reference numeral  20 . The illustrated system  20  is suitable for generating hydrogen for use as a fuel or for various other applications. While the improvements described below are described in relation to an electrolytic cell, the improvements are applicable to electrolytic cells, fuel cells, and the like. Furthermore, although the description and figures are directed to the production of hydrogen and oxygen gas by the electrolysis of water and will be described chiefly in this context, the apparatus is applicable to the generation of other gases from other reactant materials. 
     The system  20  includes a water-fed electrolytic cell stack  22  capable of generating hydrogen gas from reactant water. The reactant water utilized by system  20  is supplied from a water source  24  and is fed by gravity or pumped through a pump  26  into the electrolytic cell stack  22 . The supply line may have an electrical conductivity sensor  28  disposed in fluid communication therewith to monitor the electrical potential of the water, thereby determining its purity and ensuring its adequacy for use in system  20 . 
     The cell stack  22  comprises one or more cells that may be encapsulated within sealed structures, such as that shown in  FIG. 2 . The reactant water is received by manifolds or other types of conduits that are in fluid communication with the cell components. An electrical source  38  is disposed in electrical communication with each cell within cell stack  22  to provide a driving force for the dissociation of the water. The electrical source is operatively communicable with a cell control system (not shown) that controls the operation of system  20 . 
     An oxygen-rich water stream exits the cell stack  22  and may fed back to the water supply via conduit  32 . A hydrogen stream, which will normally be entrained with water, exits cell stack and is fed to a hydrogen/water separator  36  where the gas and liquid phases are separated. 
     The hydrogen gas exiting the separator  36  may be further dried in a drying unit  38 , which may be, for example, a diffuser, a pressure swing absorber, desiccant or the like. This wet hydrogen stream can have a pressure of about 1 pound per square inch (psi) up to and exceeding about 20,000 psi. More particularly, the hydrogen stream pressure can be about 1 psi to about 10,000 psi, or from about 100 psi to about 6,000 psi, or from about 1,500 psi to about 2,500 psi, or a pressure of about 100 psi to about 275 psi for various applications. The hydrogen from drying unit may be fed to a storage facility, or directly to an application, e.g., a refueling system, for use as a fuel. 
     Water with trace amounts of hydrogen entrained therein may be returned to the water source  24  from the separator  36  through a low-pressure hydrogen separator  40 . The separator  36  may have associated therewith a release  42 , such as a relief valve, to rapidly purge hydrogen to a hydrogen vent  44  if the pressure or pressure differential exceeds a pre-selected limit. 
     In the illustrated system, a hydrogen output sensor  46  monitors the hydrogen pressure. The hydrogen output sensor is interfaced with a transducer  48 , which is capable of converting the voltage or current value into a pressure reading. A display (not shown) may be disposed in operable communication with transducer to provide a reading of the pressure, for example, at the location of hydrogen output sensor on the hydrogen line. 
     Referring now to  FIG. 2 , a exemplary cell stack  22  can be seen to include a housing  50  including upper and lower housing structures  51  and  52  typically referred to as plates. The plates have stacked and clamped therebetween one or more electrolytic cells  54  according to the present invention. In the illustrated embodiment, water enters the cell housing  50  through a tee connection  55  connected to the housing by a stem, with the other end of the tee leading to a drain port. Water and oxygen are removed from the housing via an outlet  56 , and water and hydrogen are removed via an outlet  57 . 
     An exemplary prior art electrolytic cell  54  is shown in  FIG. 3 , with the normally stacked together components shown separated from one another. Going from one end of the cell to the opposite end, the illustrated cell includes the bottom plate  52 , a heat transfer gasket  58 , a second heat transfer gasket  60 , an oxygen anode terminal plate  62 , a membrane support assembly screen  64 , an anode-side gasket seal (or sealing gasket)  66 , a rigid oxygen gasket frame  68 , an electrode and membrane assembly  70 , a rigid hydrogen gasket frame  72 , a membrane support assembly screen  74 , a cathode-side gasket seal (or sealing gasket)  76 , a hydrogen cathode terminal plate  78 , a pressure pad  80 , and a top gasket  82  that interfaces with the top plate  51  or another cell stack. The references to top, upper, bottom, lower, etc. are being used in relation to the illustrated orientation of the cell  54  or other components, but this should not viewed as limiting since the cell and/or other components can be otherwise oriented as may be desired for a particular application. Thus, the herein referred to bottom plate may be disposed at the top, side or otherwise depending on the orientation of the cell. 
     The illustrated membrane assembly  70  includes a solid polymeric ion-exchange membrane  86  that is bounded by a carrier or gasket portion  88 . The membrane  86  is disposed between the anode and cathode terminal plates  62  and  78  and in electrical contact therewith via the anode and cathode screens  64  and  74 . 
     The gaskets and associated seals are provided to insure a water-tight (or more generally fluid-tight) construction for the electrolytic cell. In addition, the gaskets  58  and  60  are provided between the anode and the bottom plate  52  of the housing to electrically insulate the anode from the base of the housing. These gaskets should also have sufficient thermal conductivity to ensure good heat transfer from the electrodes to the housing, which serves as a heat sink. The housing in turn may be fastened to a metal chassis which then would also become part of the heat sink. 
     The pressure disk  80  is provided to ensure that the screens  64  and  74  firmly engage the respective electrodes as well as the solid-electrolyte membrane  107 . The pressure disk  80  has essentially the same diameter as the screens and may be disposed, as shown, between the cathode terminal plate  78  and the uppermost gasket  82  so that when the two housing sections are drawn together, the disc  80  exerts pressure on the screens. 
     The electrolytic cell  54  as thus far described generally is of a conventional design. Consequently, further details of the construction and operation of the cell  54  need not be described except as needed to facilitate an understanding of the invention, the features of which will not be described in greater detail. 
     The foregoing cell arrangement works well for stacks with one or two cell layers. When more than two cell layers are used, pressure leakage problems have been found to occur, such as hydrogen leaking into the oxygen and outboard gas leakage to the atmosphere. In a prior art four layer cell stack using cells similar to the single cell shown in FIG. 2 of U.S. Pat. No. 5,037,518, the oxygen and hydrogen gaskets (gaskets 106 and 109 in U.S. Pat. No. 5,037,518 were replaced with gaskets made from an elastomeric material. This design has performed acceptably, but over time the elastomeric gaskets “relax”. This relaxation can cause the cell stack to leak outboard and/or hydrogen to leak into the oxygen. The relaxation may also cause the screens to be driven into the ion-exchange membranes and this can lead to holes or tears. 
     The inventors also discovered that internal leaks arose from a poor seal just inside the hydrogen and/or oxygen ports in the prior art two-cell stack design that did use a rigid frame. 
     In order to resolve these problems and drawbacks, the inventors conceived of various design features that may be used individually or in combination with any one or more of the other features to provide a superior cell stack design that provides performance characteristics heretofore not attainable by prior art designs. The novel and inventive features include, among others:
         seating, as by molding, a screen, in particular a fine screen, into a plastic gasket frame in order to provide a flat surface for the ion-exchange membrane to sit on, thereby improving the seal around the ion-exchange membrane while minimizing or eliminating driving of a screen pack (or other flow field member) into the ion-exchange membrane;   welding or otherwise securing a flow field member, in particular a coarse screen package, to the corresponding electrode plate, such as a titanium electrode plate, to reduce part count and aid in alignment;   adding raised ridges, such as 0.03 inch ridges, to the plastic frame to improve sealing along the membrane perimeter and around the O 2  and H 2  ports;   adding a small seal insert, such as a “trapezoidal” seal insert, to the frame for improving transmission of pressure through the stack radially inwardly of an adjacent port to aid in sealing at the ports; and/or   adding slots in the frame and corresponding tabs to the seals, or vice versa, to create a mechanical interlock between the frame and seal to ensure proper seating of the seals.
 
Again, any one of these features may be used with any one or more of the other features to obtain improved performance in a cell stack, particularly to enable cell stacks including one, two, three, four, five, six or more cells in a stacked arrangement, while affording desirable sealing performance without degradation of the cells.
       

     Referring now to  FIGS. 4-6 , an exemplary rigid gasket frame according to the invention, herein also referred to as a planar frame component, is indicated generally by reference numeral  90 . The frame  90  can be configured for use as the rigid oxygen (anode) gasket frame  68  or the rigid hydrogen (cathode) gasket frame  72 . 
     The frame  90  has a frame body  91  that bounds and supports the flat ion-exchange membrane. The frame body  91  surrounds a central through opening  92  that defines a flow field region adjacent a respective side of the ion-exchange membrane  86  and in fluid communication with the respective electrode plate (anode  62  or cathode  78 ). The frame  90  and central opening  92  may be of any suitable configuration. In the illustrated embodiment the frame is generally circular and the opening is circular. Those skilled in the art will appreciate that other shapes may be used, such as rectangular, square or other polygonal shape, or other non-circular shapes. 
     The frame  90  retains therein a screen  94 . In a preferred embodiment, the screen is molded into the frame that preferably is made of plastic. That is, the frame is molded to the peripheral edge portion of the screen to form a unitary structure. As above noted, the screen may be a fine mesh screen that provides a flat surface for the ion-exchange membrane to sit on, thereby improving the seal around the ion-exchange membrane while minimizing or eliminating driving of a screen pack (or other flow field member) into the ion-exchange membrane. 
     The screen  94  may be formed as part of a flow field member or provided in addition to the flow field member. The flow field member may be, for example, a conventional fine or coarse screen package, such as the package  64 / 74  shown in  FIG. 3 . As above indicated, this package or other flow field member may be welded to the adjacent electrode plate  62 / 78 , such as a titanium electrode plate, to reduce part count and aid in alignment. The screen package or other flow field member allows fluid to flow therethrough for carrying out the electrolytic reaction. The flow field member may be a screen pack, bipolar plate, porous plate, gas diffusion member, for example. In the illustrated embodiment, the flow field member is a screen or screen pack, and the reference to a screen pack herein is intended to mean a screen, screen pack or any other type of flow field member unless expressly indicated otherwise to the contrary. With reference to  FIG. 3 , the screen pack may be the screen  64  or screen  74  depending on which side of the ion-exchange membrane the frame  90  is located. The radially inner portion of the frame that borders the central opening  92  may be recessed to form a pocket  95  for closely receiving and locating the peripheral edge of the screen pack, with the fine mesh screen forming a bottom of this recessed area as best illustrated in  FIGS. 13-15 . 
     In addition to radially locating the screen  94 , the frame  90  preferably also functions to radially locate a gasket seal  98  as depicted in  FIGS. 13-15 . To this end, the side face of the frame  90  opposite the ion-exchange membrane has formed therein an annular groove  100  for receiving and locating the gasket seal  98  that preferably has a corresponding shape. The gasket seal  90  may be either the anode-side gasket seal  66  of  FIG. 1  or the cathode-side gasket seal  76 , depending on which side of the ion-exchange member the frame  90  is located (whether it forms the rigid frame  68  or rigid frame  72 ). 
     In a preferred embodiment, the frame  90  and gasket seal  98  have respective integral interlocking features that cooperate with one another for fixedly locating the gasket seal relative to the frame. In the illustrated exemplary embodiment seen in  FIGS. 4 ,  6 ,  11  and  12 , the gasket seal  98  may have one or more axially extending locating tabs  102  forming the interlocking feature thereof that are received in respective locating slots  104  in the frame  90  that form the interlocking feature of the frame. In the illustrated gasket seal, a series of projecting tabs are circumferentially spaced-apart around the inner periphery of the gasket seal and the slots are openings that are correspondingly spaced-apart around the inner periphery of the frame  90 . As best seen in  FIG. 4 , the slots may open radially inwardly to the screen-receiving recess  95  in the frame plate  90 . 
     The gasket seal  90  forms a peripheral seal that surrounds the flow field region to prevent escape of fluid to the atmosphere. The gasket seal further has portions thereof configured to seal around one or more port openings in the frame  90  that are not intended to communicate with the flow field region. With reference to FIGS.  4  and  11 - 15 , the gasket seal has diametrically opposed lobe portions  108  and  110  provided with respective openings  112  and  114  that align with respective port openings  116  and  118  in the frame plate  90 , as well in the cell components between which the frame plate is sandwiched, these being the ion-exchange membrane assembly  70  on one side and the anode/cathode electrode plate  62 / 78  on the opposite side in the case of the cell stack shown in  FIG. 3 . If the gasket seal  76  is the cathode gasket seal in the water-fed electrolytic cell stack  22  ( FIG. 3 ) capable of generating hydrogen gas from reactant water, the port openings  116  and  118  will provide for flow of supply water and oxygen-rich water into and out of the cell. 
     The frame plate  90  also has one or more port openings  120  and  122  that are intended to communicate with the hydrogen flow field region. These port openings are the port openings through which hydrogen gas is discharged from the flow field region. In the regions of these port openings, the gasket seal  98  extends only around the exterior of these openings. Moreover, the frame plate has at each port one or more generally radially extending grooves  126  and  128  for fluidly connecting the flow field region to respective port openings. In the illustrated embodiment, the gasket seal has diametrically opposite lobes  130  and  132  that have interior recesses  134  and  136  extending between the flow field region and the port openings. As shown, the recesses may be generally triangular or trapezoidal in shape, and the frame plate may have two circumferentially spaced apart grooves in the membrane side face thereof that from therebetween an acute angle and which generally form respective sides of a respective trapezoidal region  138 ,  140  that has the other two sides coinciding with the port opening and the central opening in the frame. 
     As above mentioned, the inventors discovered that internal leaks arose from a poor seal just inside the hydrogen and/or oxygen ports in the prior art two-cell stack design that used a rigid frame, which would be compounded as the number of cell layers is increased. The inventors discovered that this problem can be surprisingly resolved by the provision of a sealing insert  148  in the frame. The sealing insert is located between the respective port and the central opening in the region of the flow passage or passages connecting the flow field region to the port. In the illustrated embodiment using converging recesses, the seal insert has a trapezoidal shape as seen in  FIGS. 7-10 . 
     In order to insure proper locating and retention of the seal inserts, each seal insert  148  is provided with a tab portion  150  that fits in a correspondingly shaped slot  152  in the frame. The tab portion projects from a main body portion  154 , the thickness of which is selected preferably to provide a facing surface essentially flush with the surface of the gasket seal for engaging along with the gasket seal a planar surface of the adjacent electrode plate. The sealing insert may be made of any suitable material, such as for example the same elastomeric material as the gasket seal whereby the sealing insert and gasket seal will have similar compression characteristics in the cell stack. In this latter regard, the generally planar body of the seal insert may have a thickness essentially equal the thickness of the gasket seal. 
     Thus, the sealing insert  148  provides backing for the frame  90  in the region of the flow passages  126  and  128 , thereby to insure a tight seal between the frame and the peripheral sealing portion of the ion-exchange membrane assembly. Additionally or alternatively, the side surface of the frame facing the ion-exchange membrane assembly may be provided with one or more annular ridges  160  to improve sealing around the membrane perimeter and around the O 2  and H 2  ports. The ridge or ridges  160  may have an axial thickness of 0.03 inch, for example. The ridges may be radially spaced apart, such as concentrically. Although three ridges are shown, the number may be varied as desired. 
     Although the invention has been illustrated with only one electrolytic cell, i.e., one pair of electrodes and one ion-exchange membrane, it will be understood that two, three, four, five, six or more cells may be stacked on top of each other in a single housing in order to increase the hydrogen-producing capacity of the unit. 
     Although the invention has been shown and described with respect to a certain embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.