Abstract:
Compact proton exchange membrane (PEM) electrochemical cell stack. In a preferred embodiment, the cell stack comprises first and second sub-stacks of series-connected, proton exchange membrane (PEM) electrochemical cells. The first sub-stack is stacked between a top end plate and an intermediate plate, and the second sub-stack is stacked between the intermediate plate and a bottom end plate, the top end plate, the intermediate plate and the bottom end plate all extending beyond the peripheries of the first and second sub-stacks. A first set of tie rods is coupled to the top end plate and extends downwardly therefrom through the intermediate plate at points peripheral to the first and second sub-stacks, the first tie rods terminating prior to the bottom end plate. A Belleville washer spring stack is mounted on each of the first tie rods below the intermediate plate and above the bottom end plate for biasing the intermediate plate towards the top end plate. A second set of tie rods is coupled to the bottom end plate and extends upwardly therefrom through the intermediate plate at points peripheral to the first and second sub-stacks, the second tie rods terminating prior to the top end plate. A Belleville washer spring stack is mounted on each of the second tie rods above the intermediate plate and below the top end plate for biasing the intermediate plate towards the bottom end plate. The first and second sub-stacks may be electrically interconnected in series or in parallel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application Ser. No. 60/283,237, filed Apr. 11, 2001, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to proton exchange membrane (PEM) electrochemical cell stacks and relates more particularly to a novel PEM electrochemical cell stack. 
     In certain controlled environments, such as those found in airplanes, submarines and spacecrafts, it is often necessary for oxygen to be furnished in order to provide a habitable environment. An electrolysis cell, which uses electricity to convert water to hydrogen and oxygen, represents one type of device capable of producing quantities of oxygen. One common type of electrolysis cell comprises a proton exchange membrane, an anode positioned along one face of the proton exchange membrane, and a cathode positioned along the other face of the proton exchange membrane. To enhance electrolysis, a catalyst, such as platinum, is typically present both at the interface between the anode and the proton exchange membrane and at the interface between the cathode and the proton exchange membrane. The above-described combination of a proton exchange membrane, an anode, a cathode and associated catalysts is commonly referred to in the art as a membrane electrode assembly. 
     In use, water is delivered to the anode and an electric potential is applied across the two electrodes, thereby causing the electrolyzed water molecules to be converted into protons, electrons and oxygen atoms. The protons migrate through the proton exchange membrane and are reduced at the cathode to form molecular hydrogen. The oxygen atoms do not traverse the proton exchange membrane and, instead, form molecular oxygen at the anode. (An electrolysis cell, when operated in reverse to generate water and electricity using molecular hydrogen and molecular oxygen as starting materials, is referred to in the art as a fuel cell. Electrolysis cells and fuel cells both constitute electrochemical cells, and all discussion herein pertaining to electrolysis cells is correspondingly applicable to fuel cells.) 
     Often, a number of electrolysis cells are assembled together in order to meet hydrogen or oxygen production requirements. One common type of assembly is a stack comprising a plurality of stacked electrolysis cells that are electrically connected in series in a bipolar configuration. In one common type of stack, each cell includes, in addition to a membrane electrode assembly of the type described above, a pair of multi-layer metal screens, one of said screens being in contact with the outer face of the anode and the other of said screens being in contact with the outer face of the cathode. The screens are used to form the fluid cavities within a cell for the water, hydrogen and oxygen. Each cell additionally includes a pair of polysulfone cell frames, each cell frame peripherally surrounding a screen. The frames are used to peripherally contain the fluids and to conduct the fluids into and out of the screen cavities. Each cell further includes a pair of metal foil separators, one of said separators being positioned against the outer face of the anode screen and the other of said separators being positioned against the outer face of the cathode screen. The separators serve to axially contain the fluids on the active areas of the cell assembly. In addition, the separators and screens together serve to conduct electricity from the anode of one cell to the cathode of its adjacent cell. Plastic gaskets seal the outer faces of the cell frames to the metal separators, the inner faces of the cell frames being sealed to the proton exchange membrane. 
     Additional information relating to electrolysis cell stacks includes the following patents and publications, all of which are incorporated herein by reference: U.S. Pat. No. 6,057,053, inventor Gibb, issued May 2, 2000; U.S. Pat. No. 5,466,354, inventors Leonida et al., issued Nov. 14, 1995; U.S. Pat. No. 5,366,823, inventors Leonida et al., issued Nov. 22, 1994; U.S. Pat. No. 5,350,496, inventors Smith et al., issued Sep. 27, 1994; U.S. Pat. No. 5,324,565, inventors Leonida et al., issued Jun. 28, 1994; U.S. Pat. No. 5,316,644, inventors Titterington et al., issued May 31, 1994; U.S. Pat. No. 5,009,968, inventors Guthrie et al., issued Apr. 23, 1991; and Coker et al., “Industrial and Government Applications of SPE Fuel Cell and Electrolyzers,” presented at The Case Western Symposium on “Membranes and Ionic and Electronic Conducting Polymer,” May 17-19, 1982 (Cleveland, Ohio). 
     In order to ensure optimal conversion of water to hydrogen and oxygen by each electrolysis cell in a stack, there must be uniform current distribution across the active areas of the electrodes of each cell, and there must be a proper sealing of cells to prevent the escape of fluids therefrom. Such uniform current distribution and proper sealing require that uniform contact pressure be applied to the cells while, at the same time, permitting movement of the cell components to compensate for thermal expansion and component creeps. These objectives are typically met by providing an electrically-conductive compression pad between adjacent cells in a stack and by compressing the cells of the stack between a top end plate and a bottom end plate. Compression between the two end plates is typically achieved by mounting both end plates on one or more “tie rods”(i.e., posts), with one or both of said end plates being adapted for sliding movement on said “tie rods,” and by mounting springs, typically in the form of Belleville spring washers or the like, on the tie rods external to the end plates in such a way as to bias the end plates towards one another. 
     Because the amount of spring loading that is required for compression can be quite large in many instances (for example, where operating pressures are in the range of 200 to 6000 psi or where a large number of cells are in a stack), it is common to use stacks of Belleville spring washers on each tie rod, with multiple washers being stacked in parallel to increase load and being stacked in series to increase movement. However, as can readily be appreciated, the stacking of such springs external to the end plates adds to the overall height of the cell stack, a result that may, in some instances, be objectionable. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a novel PEM electrochemical cell stack. 
     It is another object of the present invention to provide a novel PEM electrochemical cell stack that overcomes at least some of the shortcomings described above in connection with existing PEM electrochemical cell stacks. 
     It is still another object of the present invention to provide a PEM electrochemical cell stack that has a compact design. 
     Therefore, in accordance with the foregoing objects and/or other objects to be described in or to become apparent from the description which follows, there is provided, according to one aspect of the invention, a proton exchange membrane (PEM) electrochemical cell stack comprising (a) a first sub-stack, said first sub-stack comprising a plurality of proton exchange membrane (PEM) electrochemical cells arranged in series in a bipolar configuration; (b) a second sub-stack, said second sub-stack comprising a plurality of proton exchange membrane (PEM) electrochemical cells arranged in series in a bipolar configuration; (c) a top end plate; (d) a bottom end plate; (e) a first intermediate support, said first sub-stack being stacked between said top end plate and said first intermediate support, said second sub-stack being stacked between said first intermediate support and said bottom end plate; (f) wherein said top end plate, said first intermediate support and said bottom end plate all extend beyond the peripheries of said first and second sub-stacks; (g) a first tie rod, said first tie rod being coupled to said top end plate and extending downwardly from said top end plate through said first intermediate support at a point peripheral to both of said first and second sub-stacks, said first tie rod terminating prior to said bottom end plate; and (h) first biasing means, mounted on said first tie rod below said first intermediate support and above said bottom end plate, for biasing said first intermediate support towards said top end plate. 
     The aforementioned intermediate support may be either a plate or an annular support. Where the intermediate support is a plate, electrical insulation is preferably additionally provided to electrically isolate the plate from the tie rods and/or biasing means. Such insulation may not be necessary for an annular support where the annular support does not extend radially inward to where the support is in contact with the electrically-conductive components of the sub-stack. 
     In a preferred embodiment, the electrochemical cell stack of the present invention comprises first and second sub-stacks of series-connected, proton exchange membrane (PEM) electrochemical cells. The first sub-stack is stacked between a top end plate and an intermediate plate, and the second sub-stack is stacked between the intermediate plate and a bottom end plate, the top end plate, the intermediate plate and the bottom end plate all extending beyond the peripheries of the first and second sub-stacks. A first set of tie rods is coupled to the top end plate and extends downwardly therefrom through the intermediate plate at points peripheral to the first and second sub-stacks, the first tie rods terminating prior to the bottom end plate. A Belleville washer spring stack is mounted on each of the first tie rods below the intermediate plate and above the bottom end plate for biasing the intermediate plate towards the top end plate. A second set of tie rods is coupled to the bottom end plate and extends upwardly therefrom through the intermediate plate at points peripheral to the first and second sub-stacks, the second tie rods terminating prior to the top end plate. A Belleville washer spring stack is mounted on each of the second tie rods above the intermediate plate and below the top end plate for biasing the intermediate plate towards the bottom end plate. The first and second sets of tie rods are preferably interlaced in an alternating pattern around the entire periphery of the sub-stacks. 
     As can readily be appreciated, because, in the electrochemical cell stack of the present invention, the Belleville spring washer stacks are positioned between the top and bottom end plates, the overall size of the cell stack is kept to a minimum. This is a considerable advantage over comparable existing cell stacks. 
     In addition, another particularly advantageous feature of the PEM electrochemical cell stack of the present invention is that the sub-stacks thereof can be electrically interconnected either in series or in parallel. By connecting the sub-stacks in parallel, the current capacity of the stack can be substantially increased while using a single set of end plates and the same compression hardware. Moreover, the current capacity can be further increased by introducing additional intermediate supports into the cell stack. 
    
    
     Additional objects, features, aspects and advantages of the present invention will be set forth, in part, in the description which follows and, in part, will be obvious from the description or may be learned by practice of the invention. In the description, reference is made to the accompanying drawings which form a part thereof and in which is shown by way of illustration specific embodiments for practicing the invention. These embodiments will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are hereby incorporated into and constitute a part of this specification, illustrate preferred embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings wherein like reference numerals represent like parts: 
     FIG. 1 is a simplified schematic front view of a first type of conventional PEM electrochemical cell stack; 
     FIG. 2 is a simplified schematic front view of a second type of conventional PEM electrochemical cell stack; 
     FIG. 3 is a simplified schematic front view of a first embodiment of a PEM electrochemical cell stack constructed according to the teachings of the present invention; 
     FIG. 4 is a front view, broken away in part, of a second embodiment of a PEM electrochemical cell stack constructed according to the teachings of the present invention; 
     FIG. 5 is a simplified schematic front view of a third embodiment of a PEM electrochemical cell stack constructed according to the teachings of the present invention; 
     FIG. 6 is a fragmentary simplified schematic front view of a fourth embodiment of a PEM electrochemical cell stack constructed according to the teachings of the present invention; and 
     FIG. 7 is a graph depicting the operating performance for several runs over one month of the PEM electrochemical cell stack shown in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, there is shown a simplified schematic front view of a first type of conventional PEM electrochemical cell stack, said first type of conventional PEM electrochemical cell stack being represented generally by reference numeral  11 . (For simplicity and clarity, certain components of stack  11 , such as electrical cables external electrical connections and ports for admitting water and for releasing hydrogen gas and oxygen gas, are not shown or further described herein.) 
     Stack  11  comprises a plurality of identical PEM electrochemical cells  13 - 1  through  13 - 20 , cells  13 - 1  through  13 - 20  being arranged in series in a bipolar configuration. For simplicity and clarity, the details of cells  13 - 1  through  13 - 20 , which are well-known and are described above, are not shown in FIG.  1  and are not further described herein. In addition, conventional electrically-conductive compression pads, one of which is positioned in the conventional manner between each adjacent pair of PEM electrochemical cells  13 , are not shown in FIG.  1  and are not further described herein. 
     Stack  11  also comprises a top end plate  15 - 1  and a bottom end plate  15 - 2 , cells  13 - 1  through  13 - 20  being sandwiched between top end plate  15 - 1  and bottom end plate  15 - 2 . For reasons to become apparent below, end plates  15 - 1  and  15 - 2  are sized to extend beyond the peripheries of cells  13 - 1  through  13 - 20 . 
     Stack  11  additionally comprises a plurality of tie rods  17 - 1  through  17 - 3 , tie rods  17 - 1  through  17 - 3  extending perpendicularly relative to end plates  15 - 1  and  15 - 2  at points located around the peripheries of cells  13 - 1  through  13 - 20 . The bottom ends  18 - 1  through  18 - 3  of tie rods  17 - 1  through  17 - 3 , respectively, are fixed to bottom end plate  15 - 2 , and the top ends  19 - 1  through  19 - 3  of tie rods  17 - 1  through  17 - 3 , respectively, extend through transverse openings (not shown) provided in top end plate  15 - 1 . For reasons to be discussed below, top ends  19 - 1  through  19 - 3  of tie rods  17 - 1  through  17 - 3 , respectively, are externally threaded. 
     Stack  11  further comprises a plurality of Belleville washer spring stacks  21 - 1  through  21 - 3 , stacks  21 - 1  through  21 - 3  being inserted over tie rods  17 - 1  through  17 - 3 , respectively, and positioned against the top of end plate  15 - 1 . Each Belleville washer spring stack  21  comprises a plurality of Belleville washer springs  23  arranged in alternating upwardly-facing and downwardly-facing groups. 
     Stack  11  additionally comprises a plurality of annular caps  25 - 1  through  25 - 3 , caps  25 - 1  through  25 - 3  being internally threaded and appropriately dimensioned to permit their being screwed over the top ends  19 - 1  through  19 - 3  of rods  17 - 1  through  17 - 3 , respectively. In this manner, by controllably tightening caps  25 - 1  through  25 - 3  against stacks  21 - 1  through  21 - 3 , respectively, the compressive force applied to end plate  15 - 1  in the direction of end plate  15 - 2  can be adjusted. 
     Unfortunately, however, as can be seen in FIG. 1, there is considerable height added to stack  11  by Belleville washer spring stacks  21 - 1  through  21 - 3 , caps  25 - 1  through  25 - 3  and the top portions of rods  17 - 1  through  17 - 3 . 
     Referring now to FIG. 2, there is shown a simplified schematic front view of a second type of conventional PEM electrochemical cell stack, said second type of conventional PEM electrochemical cell stack being represented generally by reference numeral  51 . (For simplicity and clarity, certain components of stack  51 , such as electrical cables for external electrical connections and ports for admitting water and for releasing hydrogen gas and oxygen gas, are not shown or further described herein.) 
     Stack  51  is similar in many respects to stack  11 . One of the principal differences between the two stacks is that, in stack  11 , rods  17 - 1  through  17 - 3  are fixed to bottom plate  15 - 2  whereas, in stack  51 , there are provided rods  53 - 1  through  53 - 3  having externally threaded bottom ends  55 - 1  through  55 - 3 , respectively, that extend downwardly through transverse openings (not shown) in a bottom end plate  57 . Another principal difference between the two stacks is that stack  51  includes, in addition to Belleville washer spring stacks  60 - 1  through  60 - 3  (stacks  60 - 1  through  60 - 3  being half as big as stacks  21 - 1  through  21 - 3 ) and caps  25 - 1  through  25 - 3  mounted over the externally threaded top ends  59 - 1  through  59 - 3  of rods  53 - 1  through  53 - 3 , respectively, Belleville washer spring stacks  61 - 1  through  61 - 3  and caps  63 - 1  through  63 - 3  mounted over bottom ends  55 - 1  through  55 - 3 , respectively. In this manner, by controllably tightening caps  25 - 1  through  25 - 3  against stacks  60 - 1  through  60 - 3 , respectively, the compressive force applied to end plate  15 - 1  in the direction of end plate  57  can be adjusted, and by controllably tightening caps  63 - 1  through  63 - 3  against stacks  61 - 1  through  61 - 3 , respectively, the compressive force applied to end plate  57  in the direction of end plate  15 - 1  can be adjusted. 
     However, as can readily be appreciated, the problem of space consumption described above in connection with stack  11  is no better in the case of stack  51 . 
     Referring now to FIG. 3, there is shown a simplified schematic front view of a first embodiment of a PEM electrochemical cell stack constructed according to the teachings of the present invention, said PEM electrochemical cell stack being represented generally by reference numeral  101 . (For simplicity and clarity, certain components of stack  101 , such as electrical cables for external electrical connections and ports for admitting water and for releasing hydrogen gas and oxygen gas, are not shown or further described herein.) 
     Stack  101  comprises a first sub-stack  103 - 1  of identical PEM electrochemical cells  13 - 1  through  13 - 20  and a second sub-stack  103 - 2  of identical PEM electrochemical cells  13 - 1  through  13 - 20 , cells  13 - 1  through  13 - 20  being arranged in each of sub-stacks  103 - 1  and  103 - 2  in series in a bipolar configuration. For simplicity and clarity, the details of cells  13 - 1  through  13 - 20 , which are well-known and are described above, are not shown in FIG.  3  and are not further described herein (it being understood, however, that cells  13 - 1  through  13 - 20  are not limited to that which is disclosed herein and may encompass any type of PEM electrochemical cell). In addition, conventional electrically-conductive compression pads, one of which is positioned in the conventional manner between each adjacent pair of PEM electrochemical cells  13  within a sub-stack  103 , are not shown in FIG.  3  and are not further described herein (it being understood, however, that stack  101  may include, instead of conventional electrically-conductive compression pads, electrically-conductive compression pads of the type described in commonly-assigned U.S. patent application Ser. Nos. 09/747,423, now U.S. Pat. No. 6,969,846, and 09/827,368, now U.S. Pat. No. 6,500,319, both of which are incorporated herein by reference). 
     Stack  101  also comprises a top end plate  105 - 1 , an intermediate plate  105 - 2  and a bottom end plate  105 - 3 , first sub-stack  103 - 1  being sandwiched between top end plate  105 - 1  and intermediate plate  105 - 2 , second sub-stack  103 - 2  being sandwiched between intermediate plate  105 - 2  and bottom end plate  105 - 3 . For reasons to become apparent below, plates  105 - 1  through  105 - 3  are sized to extend beyond the peripheries of sub-stacks  103 - 1  through  103 - 2 . For reasons also to become apparent below, intermediate plate  105 - 2  is preferably made of a relatively stiff material to resist bending. 
     Stack  101  additionally comprises a plurality of tie rods  107 - 1  through  107 - 3 , tie rods  107 - 1  through  107 - 3  extending perpendicularly relative to plates  105 - 1  through  105 - 3  at points located around the peripheries of sub-stacks  103 - 1  and  103 - 2 . The top ends  108 - 1  and, 108 - 3  of tie rods  107 - 1  and  107 - 3 , respectively, are fixed to top end plate  105 - 1 , and the bottom ends  109 - 1  and  109 - 3  of tie rods  107 - 1  and  107 - 3 , respectively, extend through transverse openings (not shown) provided in intermediate plate  105 - 2 , terminating a short distance above bottom plate  105 - 3 . The bottom end  109 - 2  of tie rod  107 - 2  is fixed to bottom end plate  105 - 3 , and the top end  108 - 2  of tie rod  107 - 2  extends through a transverse opening (not shown) provided in intermediate plate  105 - 2 , terminating a short distance below top plate  105 - 1 . For reasons to be discussed below, the bottom ends  109 - 1  and  109 - 3  of tie rods  107 - 1  and  107 - 3 , respectively, and the top end  108 - 2  of tie rod  107 - 2  are externally threaded. 
     Stack  101  further comprises a plurality of Belleville washer spring stacks  111 - 1  through  111 - 3 , stacks  111 - 1  through  111 - 3  being identical to Belleville washer spring stacks  60 - 1  through  60 - 3 , respectively. Stacks  111 - 1  and  111 - 3  are inserted over bottom ends  109 - 1  and  109 - 3  of tie rods  107 - 1  and  107 - 3 , respectively, and are positioned against the bottom of intermediate plate  105 - 2 . Stack  111 - 2  is inserted over top end  108  of tie rod  107 - 2  and is positioned against the top of intermediate plate  105 - 2 . 
     Although not shown, stack  101  preferably also includes insulating material to electrically isolate intermediate plate  105 - 2  from tie rods  107 - 1  through  107 - 3  and stacks  111 - 1  through  111 - 3 . 
     Stack  101  additionally comprises a plurality of annular caps  113 - 1  through  113 - 3 , caps  113 - 1  through  113 - 3  being internally threaded and appropriately dimensioned to permit their being screwed over bottom end  108 - 1 , top end  109 - 2  and bottom end  108 - 3  of rods  107 - 1  through  107 - 3 , respectively. Consequently, as can be seen, by controllably tightening caps  113 - 1  and  113 - 3  against stacks  111 - 1  and  111 - 3 , respectively, the compressive force applied to intermediate plate  105 - 2  in the direction of top end plate  105 - 1  can be adjusted, and by controllably tightening cap  113 - 2  against stack  111 - 2 , the compressive force applied to intermediate plate  105 - 2  in the direction of bottom plate  105 - 3  can be adjusted. 
     As can be seen, because stack  101  does not have tie rods extending beyond top end plate  105 - 1  and/or bottom end plate  105 - 3 , stack  101  has a more compact design than either stack  11  or stack  51  (with stack  101  undergoing only a slight increase in end plate diameter to accommodate the positioning of the spring stacks around the peripheries of the cells). 
     It should be understood that, primarily for the sake of clarity, stack  101  has been described as having only three tie rods  107 - 1  through  107 - 3 ; however, notwithstanding the above, stack  101  could readily be modified to have a different number of tie rods. For example, stack  101  could readily be modified so as to include a considerably larger number of tie rods, with the tie rods arranged so as to alternately emanate from top end plate  105 - 1  and bottom end plate  105 - 3 . In fact, one could size the tie rods and spring stacks to provide twice as many tie rods with one-half the number of springs on each than is the case with stack  11 . Such an arrangement would be a volume efficient method of sealing a stack for high-pressure operation (200 to 6000 psi). 
     It should also be noted that one of the advantages of the design of stack  101 , as compared to stack  11  or stack  51 , is that stack  101  can be used to connect sub-stacks  103 - 1  and  103 - 2  either in series or in parallel since end plates  105 - 1  through  105 - 3  also serve as terminal plates. One significant benefit to connecting sub-stacks  103 - 1  and  103 - 2  in parallel is that the current capacity of stack  101  can be substantially increased (i.e., doubled). This is useful in those instances in which system requirements dictate the usage of very large currents but increasing cell size is impractical. Current capacity can be even further increased (tripled or more) by providing additional intermediate plates in stack  101 . 
     Referring now to FIG. 4, there is shown a front view, broken away in part, of a second embodiment of a PEM electrochemical cell stack constructed according to the teachings of the present invention, said PEM electrochemical stack being represented generally by reference numeral  201 . (For simplicity and clarity, certain components of stack  101 , such as electrical cables for external electrical connections, are not shown or further described herein.) 
     Stack  201  is similar in many respects to stack  101 , the principal differences between the two units being that (1) stack  201  includes a significantly greater number of tie rods  203 , Belleville washer spring stacks  205  and caps  207  than the three tie rods  107 , three stacks  111  and three caps  113  of stack  101 ; (2) intermediate plate  105 - 2  of stack  101  is replaced with an annular support  209  in stack  201 , annular support  209  being dimensioned so as not to extend radially inwardly into contact with the electrically-sensitive components of sub-stacks  103 - 1  and  103 - 2 ; and (3) stack  201  additionally includes a pair of reinforcing cylinders  211 - 1  and  211 - 2 , cylinder  211 - 1  peripherally surrounding sub-stack  103 - 1 , cylinder  211 - 2  peripherally surrounding sub-stack  103 - 2 . (Reinforcing cylinder  211  may be of the type disclosed in commonly-assigned U.S. patent application Ser. No. 10/023,428, the disclosure of which is incorporated herein by reference.) 
     Like stack  101 , stack  201  may be operated with its sub-stacks  103 - 1  and  103 - 2  electrically connected in series or in parallel. 
     Referring now to FIG. 5, there is shown a simplified schematic front view of a third embodiment of a PEM electrochemical cell stack constructed according to the teachings of the present invention, said PEM electrochemical cell stack being represented generally by reference numeral  301 . (For simplicity and clarity, certain components of stack  301 , such as electrical cables for external electrical connections and ports for admitting water and for releasing hydrogen gas and oxygen gas, are not shown or further described herein.) 
     Stack  301  is similar in many respects to stack  101 , the principal differences between the two stacks being that (1) whereas stack  101  includes two sub-stacks  103 - 1  and  103 - 2 , stack  301  includes three sub-stacks  303 - 1  through  303 - 3 , each sub-stack  303  comprising a plurality of cells  13 - 1  through  13 - 20 ; (2) whereas stack  101  includes a pair of end plates  105 - 1  and  105 - 3  and an intermediate plate  105 - 2 , stack  301  includes a top end plate  305 , a first intermediate plate  307 , sub-stack  303 - 1  being sandwiched between top end plate  305  and first intermediate plate  307 , a second intermediate plate  309 , sub-stack  303 - 2  being sandwiched between first intermediate plate  307  and second intermediate plate  309 , a bottom end plate assembly  310 , sub-stack  303 - 3  being sandwiched between second intermediate layer  309  and bottom end plate assembly  310 , assembly  310  comprising a plate  311 , a plate  313  and a layer of insulation  315  sandwiched between plate  311  and plate  313 ; and (3) whereas each of tie rods  107 - 1  through  107 - 3  of stack  101  passes through one intermediate plate  105 - 2 , each of tie rods  321 - 1  through  321 - 3  passes through a pair of intermediate plates in stack  301 . 
     It should be noted that, by operating stack  301  so that each of top end plate  305  and intermediate plate  309  serves as a negative terminal and so that each of intermediate plate  307  and plate  311  serves as a positive terminal, the current in plates  307  and  309  will be twice that in plates  305  and  311 . 
     Referring now to FIG. 6, there is shown a fragmentary simplified schematic front view of a fourth embodiment of a PEM electrochemical cell stack constructed according to the teachings of the present invention, said PEM electrochemical cell stack being represented generally by reference numeral  401 . 
     Stack  401  is similar in certain respects to stack  51 , the principal differences between the two stacks being that stack  401  includes two sub-stacks of cells  13 - 1  through  13 - 20 , said two sub-stacks being separated by an intermediate plate  403  having transverse openings (not shown) through which tie rods  53 - 1  through  53 - 3  extend. 
     The two sub-stacks of stack  401  may be electrically connected either in series or in parallel. (Parallel connection affording stack  401  increased current capacity.) Accordingly, where the size of a stack is not critical, stack  401  may be used as an alternative to stack  101 . 
     It should also be noted that, in all of the stacks of the present invention, the electrolysis fluids both input and output may flow through internal porting in either series or parallel flow, independently of the external parallel electrical connections. 
     The following example is provided for illustrative purposes only and is in no way intended to limit the scope of the present invention: 
     EXAMPLE 1 
     An electrolysis unit having the same general construction of stack  201  was operated with its two 20-cell, series-connected, bipolar stacks connected electrically in parallel (i.e., central plate positive, top and bottom plates negative). The average ionic resistance of the 0.3-ft 2  active area cells was 1 milliohm per cell. There was no cell shorting and the stack sealed internally and externally to pressures of 900 psi. Also, the cells were leak-tight at a differential pressure of 900 psi across the membrane and electrode assemblies. The electrolysis unit was subsequently operated at a temperature of 120 to 130° F. for over 200 hours at current densities of 0 to 2000 A/ft 2 . The average cell voltage was 2.1 V/cell at 1000 A/ft 2 . The operating performance of the unit is shown in FIG.  7 . Even though the unit has a maximum current capacity of 600 amps per cell, when the unit was operated with its two stacks in parallel, the unit was able to handle 1200 amps, as 1200 amps were fed to the center plate and 600 amps were fed out of each of the two negative plates. 
     The embodiments of the present invention recited herein are intended to be merely exemplary and those skilled in the art will be able to make numerous variations and modifications to it without departing from the spirit of the present invention. All such variations and modifications are intended to be within the scope of the present invention as defined by the claims appended hereto.