Patent Publication Number: US-2015072263-A1

Title: Electrochemical cells having current-carrying structures underlying electrochemical reaction layers

Description:
REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 14/152,043, filed Jan. 10, 2014; which application is a continuation of U.S. patent application Ser. No. 13/535,880, filed Jun. 28, 2012, now issued as U.S. Pat. No. 8,628,890 on Jan. 14, 2014; which application is a continuation of U.S. patent application Ser. No. 12/637,422, filed Dec. 14, 2009, now issued as U.S. Pat. No. 8,232,025 on Jul. 31, 2012; which application is a continuation of of U.S. patent application Ser. No. 11/047,560 filed on Feb. 2, 2005, now issued as U.S. Pat. No. 7,632,587 on Dec. 15, 2009; which application claims priority to U.S. provisional patent application Ser. No. 60/567,648 filed May 4, 2004 and U.S. provisional patent application Ser. No. 60/608,879, filed on Sep. 13, 2004, which applications are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     A conventional electrochemical cell  10  is shown in  FIG. 1 . Cell  10  may, for example, comprise a PEM (proton exchange membrane) fuel cell. Cell  10  has a manifold  12  into which is introduced a fuel, such as hydrogen gas. The fuel can pass through a porous current-carrying layer  13 A into an anode catalyst layer  14 A, where the fuel undergoes a chemical reaction to produce free electrons and positively charged ions (typically protons). The free electrons are collected by current-carrying layer  13 A, and the ions pass through an electrically-insulating ion exchange membrane  15 . Ion exchange membrane  15  lies between anode catalyst layer  14 A and a cathode catalyst layer  14 B. Cell  10  has a manifold  16  carrying an oxidant (e.g. air or oxygen). The oxidant can pass through a porous current-carrying layer  13 B to access cathode catalyst layer  14 B. 
     As shown in  FIG. 1A , electrons travel from the sites of chemical reactions in anode catalyst layer  14 A to current-carrying layer  13 A. Protons (or other positively charged ions) travel into and through ion exchange membrane  15  in a direction opposite to the direction of electron flow. Electrons collected in current-carrying layer  13 A travel through an external circuit  18  to the porous current-carrying layer  13 B on the cathode side of cell  10 . In such cells, electron flow and ion flow occur in generally opposite directions and are both substantially perpendicular to the plane of ion exchange membrane  15 . 
     Catalyst layers  14 A and  14 B must be “dual species conductive” (i.e. they must provide conductive paths for the flow of both electrons and ions). Ion exchange membrane  15  must be single species conductive (i.e. it must permit ions to flow while providing electrical insulation to avoid internal short-circuiting of cell  10 ). 
     Many electrochemical devices include some form of porous conductive reactant diffusion media to carry current away from a catalyst layer. This compromises the ability to transport reactants to the catalyst sites, and introduces a difficult material challenge. Further, there are manufacturing and cost issues associated with the inclusion of reactant diffusion layers. A major problem in designing high performance electrochemical cells is to provide current-carrying layers which permit current to be passed into or withdrawn from the cell while permitting reactants to enter the cell and products of the reactions to be removed from the cell. 
     Despite the vast amount of fuel cell research and development that has been done over the past decades there remains a need for more efficient electrochemical cells that can be produced cost effectively and which provide improved access for reactants to the electrochemical reaction sites. 
     SUMMARY OF THE INVENTION 
     The invention relates to electrochemical cells such as fuel cells or electrolyzers. Some embodiments of the invention have application in electrochemical cells of other types such as those used for chlor-alkali processing. Some embodiments of the invention provide electrochemical cell layers comprising arrays of individual or “unit” cells. 
     One aspect of the invention provides a thin layer cell structure comprising an ion exchange membrane having an electrochemical reaction layer on each side thereof. The ion exchange membrane may comprise a layer of unitary construction, or may comprise a composite layer made up of more than one material. The ion exchange membrane may comprise, for example, a proton exchange membrane. An electrical current-carrying structure at least in part underlies one of the electrochemical reaction layers. 
     Another aspect of the invention provides core assemblies for electrochemical cells. A core assembly comprises an ion exchange membrane; an electrically conducting electrochemical reaction layer on at least a first side of the ion exchange membrane; and, an electrically-conductive current-carrying structure in electrical contact with the electrochemical reaction layer. An outer surface of the electrochemical reaction layer overlies at least a portion of the current-carrying structure. 
     A further aspect of the invention provides methods for operating an electrochemical cell. Such methods comprise providing an electrochemical cell having: a catalyst-containing electrochemical reaction layer having an outer face and an inner face; an electrical current-carrying structure underlying the electrochemical reaction layer at least in part; and an ion-conducting layer in contact with the inner face of the electrochemical reaction layer; allowing a reactant to diffuse into the electrochemical reaction layer; allowing the reactant to undergo a catalyzed electrochemical reaction to produce an ion at a location in the electrochemical reaction layer between a surface of the electrochemical layer and the current-carrying layer; and, allowing the ion to travel to the ion-conducting layer along a path that avoids the current-carrying structure. 
     Further aspects of the invention and features of specific embodiments of the invention are described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In drawings which illustrate non-limiting embodiments of the invention: 
         FIG. 1  is a cross-sectional schematic diagram of a prior art electrochemical cell; 
         FIG. 1A  is an enlarged schematic view of a portion of the cell of  FIG. 1 ; 
         FIGS. 2A-D  are schematic views of unit cell structures according to embodiments of the invention; 
         FIG. 3  is a schematic diagram of an electrode according to an embodiment of the invention; 
         FIG. 4  is a schematic diagram showing electron and proton conduction paths according to an embodiment of the invention; 
         FIG. 5  is a schematic view of a unit cell structure according to another embodiment of the invention; 
         FIG. 6  is a cross section through a membrane electrode assembly of an alternative embodiment of the invention wherein unit cells are connected in series; 
         FIG. 6A  is a schematic illustration showing current flow and proton flow in the membrane electrode assembly of  FIG. 6 ; 
         FIG. 6B  is a cross section through a membrane electrode assembly in which unit cells are interconnected by current conductors embedded in a substrate; 
         FIG. 7  is a partial plan view of an electrochemical cell layer having an array of hexagonal unit cells; 
         FIGS. 8A ,  8 B and  8 C are respectively schematic views showing electrochemical cell layers having a plurality of unit cells connected in parallel, in series and in series-parallel; 
         FIG. 9  is a side view of a pleated structure on which unit cells according to the invention may be disposed; 
         FIG. 10  is an exploded view of a fuel cell device according to an embodiment of the invention; 
         FIG. 10A  shows the fuel cell device of  FIG. 10  in assembled form; 
         FIG. 11  shows a fuel cell device according to another embodiment of the invention; 
         FIG. 12  shows a stack of fuel cell layers according to another embodiment of the invention; and 
         FIG. 13  is a section through a fuel cell having a filter layer overlying a catalyst layer. 
     
    
    
     DESCRIPTION 
     Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practised without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
     The invention relates to electrochemical cells such as fuel cells or electrolyzers, and may also have application in other types of electrochemical cells, such as those used for chlor-alkali processing. Some embodiments of the invention provide electrochemical cell layers comprising arrays of individual or “unit” cells. 
     Electrochemical cells according to some embodiments of the invention have a thin layer cell structure wherein an electrical current-carrying structure at least in part underlies an electrochemical reaction layer (referred to herein as a “catalyst layer”). Each cell comprises an ion exchange membrane having a catalyst layer on each side thereof. The ion exchange membrane may comprise, for example, a proton exchange membrane. Certain embodiments of the invention permit construction of an electrochemical cell layer comprising a plurality of individual unit cells formed on a sheet of ion exchange membrane material. 
     The ion exchange membrane may comprise a layer of unitary construction, or may comprise a composite layer made up of more than one material. Some examples of composite structures are described in the commonly-owned U.S. Pat. No. 7,378,176, issued on May 27, 2008, and entitled “MEMBRANES AND ELECTROCHEMICAL CELLS INCORPORATING SUCH MEMBRANES” which is hereby incorporated by reference herein. 
     The configuration of the current-carrying structures in preferred embodiments of the invention provides reactants with improved access to the catalyst layer, and permits the construction of electrochemical cells which are thinner than similar prior art electrochemical cells of the type having current-carrying layers positioned on outer surfaces of the catalyst layers. Throughout this description, the terms “inner” and “outer” are respectively used to refer to directions closer to and farther from the center of the ion exchange membrane. 
       FIGS. 2A and 2B  show unit cell structures  20 A and  20 B according to alternative embodiments of the invention. Structures  20 A and  20 B are similar to one another, and each comprise current-carrying structures  23 A and  23 B positioned on opposite sides of an ion exchange membrane  25 . Electrochemical reaction layers  24 A and  24 B are positioned on the outside of current-carrying structures  23 A and  23 B and ion exchange membrane  25 . The difference between structures  20 A and  20 B is that in structure  20 A current-carrying structures  23 A and  23 B are positioned on the outer surfaces of ion exchange membrane  25 , while in structure  20 B current-carrying structures  23 A and  23 B are embedded in the outer surfaces of ion exchange membrane  25 . 
       FIGS. 2C and 2D  show unit cell structures  20 C and  20 D according to further alternative embodiments of the invention. In structure  20 C, current-carrying structures  23 A and  23 B are formed on a substrate  30 . Substrate  30  is constructed from a non-conducting material. 
     Substrate  30  is penetrated by an opening  32 . Opening  32  is filled with an ion-conducting material. The ion-conducting material may comprise an ionomer or electrolyte suitable to the application. The ion-conducting material may extend outward to the outer edges of current-carrying structures  23 A and  23 B to form ion exchange membrane  25  of unit cell structure  20 C. In the illustrated embodiment, opening  32  is round, but this is not necessary. Opening  32  may be of any suitable shape. In some embodiments, opening  32  is long and narrow. In some embodiments, each unit cell has a plurality of openings  32 . 
     In some embodiments, openings  32  comprise a pattern of openings, which may be microstructured openings, as described, for example in the commonly-assigned U.S. Pat. No. 7,378,176, issued on May 27, 2008 entitled “MEMBRANES AND ELECTROCHEMICAL CELLS INCORPORATING SUCH MEMBRANES” which is referred to above. 
     Examples of materials that may be suitable for substrate  30  in specific applications include: printed circuit board (PCB) material, polyamide films; polyimide films such as Kapton™, polyethylene films, Teflon™ films, other polymer films, reinforced composite materials such as fiberglass, suitable non-polymer materials such as silicon or glass. 
     In some applications it is advantageous that substrate  30  be flexible. In such applications it is desirable that substrate  30  be made of a flexible material. 
     In structure  20 D, current-carrying structures  23 A and  23 B are formed on proton conducting membrane  25  and there is no substrate  30 . Structure  20 D differs from structure  20 A in that current-carrying structures  23 A and  23 B project respectively through the outer surfaces of catalyst layers  24 A and  24 B. A structure like structure  20 D may have its catalyst layers  24 A and  24 B divided into isolated areas by current-carrying structures  23 A and  23 B. Structure  20 D has the disadvantage that the exposed surface area of catalyst areas  24 A and  24 B is somewhat reduced in comparison to structures  20 A,  20 B, and  20 C. 
     In each of unit cell structures  20 A-D, current-carrying structures  23 A and  23 B underlie portions of catalyst layers  24 A and  24 B respectively. In the embodiments of  FIGS. 2A-C , ions liberated at reaction sites which are over current-carrying structures  23 A (or, in  FIG. 2C , over substrate  30 ) are blocked from flowing directly into and through ion exchange membrane  25  by the shortest straight-line path. Ions liberated at such sites must take longer paths to reach catalyst layer  24 B. However, by appropriately positioning current-carrying structures  23 A and  23 B, the thicknesses of the various layers and other dimensions (such as the width D of opening  32  in  FIG. 2C ) one can achieve a situation in which the lengths of paths taken by ions and electrons are not very much longer than corresponding path lengths in comparable prior art electrolytic cells. 
     The embodiment of  FIG. 2C  trades off increased path length for proton conduction against the increased mechanical ruggedness resulting from the presence of substrate  30 . 
     A feature of structures  20 A through  20 C is that the current-carrying structures  23 A and  23 B are not required to be porous because it is not necessary for reactants to pass through these structures. 
     Adjacent unit fuel cells may be connected in parallel by either providing current-carrying structures  23 A and  23 B that are common to the adjacent unit cells, or by electrically interconnecting current-carrying structures  23 A of adjacent cells and current-carrying structures  23 B of adjacent cells. Adjacent unit cells may also be electrically isolated from one another, in which case they may be connected in series, as discussed below with reference to  FIGS. 6 and 6B . Electrical isolation of unit cell structures may be provided by rendering portions of a catalyst layer non-conducting electrically, by making a catalyst layer discontinuous in its portions between unit cells and/or by providing electrically insulating barriers between the unit cell structures. 
     Optimizing catalyst layer  24 A to promote reactions does not always result in the highest electrical conductivity in catalyst layer  24 A. The materials used in the catalyst layer may not be extremely good electrical conductors. However, the losses resulting from the electrical resistivity of catalyst layer  24 A can be minimized by laying out each unit cell so that the distance between any point in catalyst layer  24 A and the closest part of current-carrying member  23 A is small. 
     For example, in some embodiments of the invention the longest path length from any point within either catalyst layer  24 A,  24 B to the corresponding current-carrying member  23 A,  23 B is 5 mm. In other embodiments, the longest path length from any point within either catalyst layer  24 A,  24 B to the corresponding current-carrying member  23 A,  23 B is 0.5 mm. Even smaller diameters are also possible. In general, reducing the diameter decreases the ohmic losses associated with electrical current conduction in the catalyst layer. However, as the structure becomes smaller, the volume taken up current carrying members  23 A,  23 B increases in proportion to the volume of the overall structure, and the space-efficiency of the structure can suffer. 
       FIG. 3  illustrates a geometry that may be used for approximating the potential drop of an electrode  34  (which may be either an anode or a cathode). Electrode  34  comprises a current-carrying structure  23 A having a skin of ion exchange material  25 A therein and catalyst layer  24 A disposed outside thereof. Only the portion of catalyst layer  24 A which is above current-carrying structure  23 A is depicted in  FIG. 3 . Electrode  34  is positioned opposite a corresponding electrode (not shown in  FIG. 3 ) on an outer surface of an ion exchange membrane (not shown in  FIG. 3 ) which may or may not be a composite membrane having substrate  30  embedded therein. In the  FIG. 3  embodiment, current-carrying structure  23 A comprises an annular trace, wherein D T  is the outer diameter of the circular trace, T CL  and T T  are the thicknesses of catalyst layer  24 A and the circular trace, respectively, and W T  is the width of the circular trace. In some embodiments, the ratio of trace diameter to trace width (D T /W T ) is at least 10. 
     Current-carrying structures  23 A and  23 B are constructed from electrically conductive materials. The following table lists some suitable materials for current-carrying structures  23 A and  23 B and their electrical conductivities: 
     
       
         
           
               
               
             
               
                   
               
               
                 Material 
                 Electrical Conductivity 10 7  (S/m) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 Pure Copper 
                 5.88 
               
               
                 Pure Gold 
                 4.55 
               
               
                 Pure Nickel 
                 1.43 
               
               
                 Pure Platinum 
                 0.96 
               
               
                 Tin Oxide (SnO 2 ; applied with 
                 0.003125 
               
               
                 a CO 2  laser) 
               
               
                   
               
            
           
         
       
     
     Any electrically conductive materials may be used to construct current-carrying structures  23 A and  23 B. In some embodiments, current-carrying structures  23 A and  23 B are constructed from metals that are either noble to begin with or are coated with a suitable material (Such as PEMCoat™ from INEOS Chlor™ Americas Inc., Wilmington, Del.) so that they resist corrosion. Corrosion can be a problem when metallic conductors are used in electrochemical cells, and fuel cells in particular. The cross sectional dimensions of current-carrying structures  23 A and  23 B can be chosen based on the total current desired to be carried and the electrical losses which are deemed acceptable in the design. 
     Current-carrying structures  23 A and  23 B may have thicknesses, for example, in the range of 5-75 μm. In some embodiments, the thickness of current-carrying structures  23 A and  23 B is in the range of 25-50 μm. Current-carrying structures  23 A and  23 B need not have the same thickness. Where current-carrying structures  23 A and  23 B comprise annular traces, the traces may have a width of 5-200 μm. In some embodiments, the traces may have a thickness on the order to 5 μm and a width on the order of 25 μm. Current-carrying structures  23 A and  23 B can be formed using any suitable techniques. For example, various printed circuit board fabrication techniques may be used to form structures  23 A and  23 B. Laminating, PVD, sputtering and plating are examples of techniques that may be used alone or in combination to make the traces. 
     Catalyst layers  24 A and  24 B may be constructed from materials which conduct both electrons and the ions formed in the reactions which occur in the cell in which they are employed. (The ions are protons in hydrogen-fuelled PEM fuel cells). Catalyst layers  24 A and  24 B may comprise any type of electrocatalyst suitable for the application at hand. Catalyst layers  24 A and  24 B may comprise electrically-conductive porous sintered powder materials, for example. For fuel cells the catalyst layers may comprise platinum on carbon, for example. In some embodiments, catalyst layers  24 A and/or  24 B comprise mixtures of carbon black and one or more of PTFE powder, PVDF powder, such as Kynar™ powder, and silicon oxide powder. The carbon black may comprise any suitable finely divided carbon material such as one or more of acetylene black carbon, carbon fibers, carbon needles, carbon nanotubes, carbon nanoparticles. 
     In some embodiments, catalyst layers  24 A and  24 B are formed of materials having electrical conductivities in the range of 50-200 S/m. Each catalyst layer  24 A,  24 B may be made up of several layers of different compositions. 
     In some embodiments, catalyst layers  24 A and  24 B have thicknesses of 250 μm or less. In some embodiments, the thickness of catalyst layers  24 A and  24 B is about 10-25 μm. The thickness of catalyst layers  24 A and  24 B may be about 20 μm, for example. Catalyst layers  24 A and  24 B need not have the same thickness. 
     Where ion exchange membrane  25  has a composite structure such as a structure including a substrate  30 , substrate  30  provides mechanical strength to membrane  25 . The presence of substrate  30  permits membrane  25  to be made thinner than ordinary proton conducting membranes. This decreased thickness can compensate to at least some degree for the more tortuous paths taken by protons which are liberated at locations which are not immediately adjacent to apertures in substrate  30 . In some embodiments, the thickness of membrane  25  is in the range of about 5 μm to about 250 μm. The thickness of membrane  25  may be about 25 μm, for example. 
       FIG. 4  shows a portion of a unit cell structure  20 E according to another embodiment of the invention. Unit cell structure  20 E constitutes a PEM fuel cell with substrate  30  having a plurality of openings  32 . A proton exchange material fills openings  32  and surrounds substrate  30  to form ion exchange membrane  25 .  FIG. 4  shows paths taken by protons (H + ) from three example reaction sites  33 A,  33 B and  33 C in catalyst layer  24 A of structure  20 E, through ion exchange membrane  25  and into catalyst layer  24 B to three other example reaction sites  33 D,  33 E and  33 F.  FIG. 4  also shows the paths taken by electrons (e − ) from reaction sites  33 A,  33 B and  33 C to current-carrying structure  23 A, and from current-carrying structure  23 B to reaction sites  33 D,  33 E and  33 F. 
     It can be seen that from reaction site  33 A and  33 B the electron and proton paths through catalyst layer  24 A are roughly equal in length. From reaction site  33 A, which is over current-carrying structure  23 A, the path taken by electrons through catalyst layer  24 A is shorter than that taken by protons which must detour around current-carrying structure  23 A. From reaction site  33 C the path taken through catalyst layer  24 A by protons is significantly shorter than that taken by electrons. In the illustrated examples, the paths taken by electrons and protons in catalyst layer  24 B to reach reaction sites  33 D,  33 E and  33 F have lengths similar to the lengths of the paths taken in catalyst layer  24 A. 
     The paths taken by protons through ion exchange membrane  25  is not equal, due to the presence of substrate  30 . The protons must detour through openings  32 . In the examples illustrated, the path taken by the proton travelling from reaction site  33 B to reaction site  33 E has the shortest distance through ion exchange membrane  25 , while the path taken by the proton travelling from reaction site  33 C to reaction site  33 F has the longest distance through ion exchange membrane  25 . 
     It can be seen in  FIG. 4  that the conductive species generated in catalyst layer  24 A (protons and electrons) both flow in generally the same direction (e.g. downward in  FIG. 4 ) to get from the reaction site where they are liberated to the conductor that will carry them. Likewise, the conductive species used in the reactions in catalyst layer  24 B both flow in generally the same direction (e.g. downward in  FIG. 4 ) to get from the conductor to the reaction site. 
       FIG. 5  shows an electrochemical cell layer  36  comprising two unit cell structures  20 F. In the  FIG. 5  embodiment, cell layer  36  is formed from a nonconducting sheet  26  which has been treated to form two ion-conducting regions  27 . Sheet  26  may, for example, be constructed of a copolymer of tetrafluoroethylene and perfluoro-3,6-dioxa-4-methyl-7-octenesulfonyl fluoride (which is a resin precursor to Nafion™), and may be selectively treated by a hydrolyzation process to form ion-conducting regions  27 , as described, for example in the commonly-assigned application U.S. Pat. No. 7,378,176, issued on May 27, 2008 entitled “MEMBRANES AND ELECTROCHEMICAL CELLS INCORPORATING SUCH MEMBRANES” which is referred to above. 
     Current-carrying structures  23 A and  23 B are placed on opposite sides of sheet  26  around the periphery of each ion-conducting region  27 . Current-carrying structures  23 A and  23 B may be ring-shaped, or may have different shapes. Ion-conducting skins  25 A and  25 B may optionally be placed on the outer surfaces of each ion-conducting region  27  within current-carrying structures  23 A and  23 B, respectively. Ion-conducting skins  25 A and  25 B and ion-conducting region  27  together form ion-conducting membrane  25  for each structure  20 F. Catalyst layers  24 A and  24 B are formed on the outer surfaces of current-carrying structures  23 A and  23 B and ion-conducting skins  25 A and  25 B for each of cell structures  20 F. In the illustrated embodiment, catalyst layers  24 A and  24 B for each cell structure  20 F are formed separately. However, a single catalyst layer  24 A could cover one side of both structures  20 F, and another single catalyst layer  24 B could cover the other side of both structures  20 F, if cell structures  20 F are to be connected in parallel. 
     Neighboring unit cells may be electrically isolated from one another. In this case it is possible to electrically interconnect the unit cells in arrangements other than parallel arrangements. Vias may be used to interconnect adjacent unit cells in series. In embodiments in which unit cells are connected in series, catalyst layers  24 A of the series connected cells are electrically isolated from one another.  FIG. 6  shows a cross section through a part of an electrochemical cell layer  40  in which a number of unit cells  42  are connected in series.  FIG. 6A  illustrates schematically the paths taken by protons and electrons in the assembly of  FIG. 6 . 
     In the embodiment of  FIG. 6 , regions  44  are electrically insulating. Regions  44  may comprise a dielectric material, an air gap, or the like. Regions  44  electrically isolate adjoining electrochemical unit cells from one another. 
     Current-carrying structure  23 A of each unit cell  42  is connected to the current-carrying structure  23 B of the adjacent unit cell  42  by an electrically conductive pathway  23 C which passes through a via in substrate  30 . 
       FIG. 6B  shows an electrochemical cell layer  40 A wherein unit cells are interconnected with one another by way of electrically conducting paths  46  embedded in substrate  30 . Conducting paths  46  may be connected to current-carrying structures  23 A and/or  23 B by way of electrically conducting vias  47  formed in substrate  30 . The conducting paths may be used to interconnect unit cells in series and/or in parallel with one another. A number of independent sets of conducting paths  46  may be provided in or on substrate  30 . 
     Electrochemical cell layer  40 A of  FIG. 6B  may be constructed using a multi-layer circuit board such as a flex circuit. This provides increased current-carrying capacity for the overall current collection system without reducing the surface area available for the cell reactions in the catalyst layers  24 A and  24 B. 
     Unit cells according to embodiments of the invention may have any suitable shapes and may be arrayed in any suitable manner.  FIG. 7  shows one example of an electrochemical cell layer comprising a plurality of unit cell structures  20 D wherein the unit cells have a hexagonal configuration. The entire surface of structures  20 D could be covered with a catalyst layer  24 A if desired. 
     It can be appreciated that various embodiments of the invention described above (e.g., structures  20 D and  40  or  40 A) can be combined to provide assemblies of unit cells which are electrically interconnected in a series-parallel arrangement of any desired complexity. Generally available electrical conductors (such as suitable metals) have much less resistance to the flow of electrons than do generally available proton conductors to the flow of protons. Therefore, the conductors which carry electrons can have significantly smaller cross sectional areas than do the pathways which carry protons. Substrate  30  may comprise a multi-layer structure (as, for example, a multi-layer circuit board) in which case, conductors for carrying electrical currents may be embedded inside substrate  30 . 
       FIGS. 8A ,  8 B and  8 C show various possible ways in which the unit cells in a small array (in this example, a very small array having only  16  unit cells) may be interconnected. In  FIG. 8A , unit cells  42  are connected in parallel. The output voltage is 1 (where 1 is the output voltage of a single unit cell) and the output current is N (in this case 16 times the maximum current of one unit cell). An open circuit failure of any one or more unit cells  42  will not prevent the array from operating (at a reduced output current) at the rated voltage (1 unit). However, a short-circuit failure of any one unit cell can prevent the entire array from functioning. 
     In  FIG. 8B , unit cells  42  are arranged in a series configuration. The voltage output is N (in this case 16 times the voltage of a single unit cell). The maximum current output is 1. An open circuit failure of any one or more unit cells will prevent the array from operating. A short-circuit failure of any one or more unit cells will not prevent the array from providing current at a (reduced) maximum output voltage. 
       FIG. 8C  shows a number of unit cells  42  arranged in a series-parallel configuration. In this case, the array is interconnected so that there are four groups of unit cells connected in series. Each group of unit cells comprises four unit cells connected in parallel. Note that each unit cell is connected to a neighbor which is diagonally adjacent. Note that one of the groups of parallel connected unit cells is split into two parts which are located in spatially separated areas of the array. In some embodiments of the invention, unit cells of a group of unit cells are spatially distributed. This makes it less likely that a failure caused by trauma to an area of the array will cause all of the unit cells of a group to fail. 
     In the embodiment of  FIG. 8C  the output voltage is 4 units at a current of four times the current capacity of one unit cell. The failure of any unit cell in either a short-circuit mode or an open circuit mode will not prevent the array from providing current although the maximum available output voltage or current may be reduced. 
     Large arrays of unit cells can be constructed to provide large power-generating electrochemical cell layers in which the entire electrochemical structure is contained within the layer. This means additional components such as plates for collecting currents etc. can be eliminated, or replaced with structures serving different functions. Structures like those described herein are well adapted to be manufactured by continuous processes. Such structures can be designed in a way which does not require the mechanical assembly of individual parts. Unlike ‘edge collected’ cells, the conductive path lengths within this structure may be kept extremely short so that ohmic losses in the catalyst layer are minimized. 
     An electrochemical cell layer comprising a plurality of unit cells may be constructed by providing a substrate comprising a plurality of ion conducting regions. Such a substrate could be provided, for example by selectively treating a sheet of non- or partially-conducting material to form the ion conducting regions, or by selectively treating a sheet of ion conducting material to form non-conducting regions, as described, for example in the commonly-assigned U.S. Pat. No. 7,378,176, issued on May 27, 2008 entitled “MEMBRANES AND ELECTROCHEMICAL CELLS INCORPORATING SUCH MEMBRANES” which is referred to above. Current-carrying structures may be formed on each side of the substrate around the periphery of each ion conducting region by means of laminating, PVD, sputtering, plating, or other suitable techniques. An electrochemical reaction layer, which may comprise a catalyst, may be deposited on each side of the ion conducting regions, in at least partial contact with the current-carrying structures. 
     Individual unit cells may be very small. Other factors being equal, smaller unit cells can operate at improved efficiencies because the conduction paths for protons and electrons can be shorter in small unit cells than in larger unit cells. The unit cells can be very small, for example, 1 mm in diameter or smaller, or even 500 μm in diameter or smaller. In some embodiments of the invention, unit cells have active areas of about e.g. 0.01 cm 2 . A typical air breathing fuel cell comprising a 1 mm diameter unit cell may produce between about 1 and 3 mW of power. A fuel cell layer comprising 300-1000 such cells could produce 1 W of power. 
     An electrochemical cell according to this invention may have as few as 1 unit cell or may have a very large number, thousands or even millions, of unit cells formed on one substrate. Electrochemical cell structures made according to some prototype embodiments of this invention have in excess of 500 unit cells, for example. 
     So far, substrate  30  and membrane electrode assemblies generally have been described as being planar. This is not necessary. Unit cells according to the invention may be used in an electrochemical cell layer that is pleated or undulating as shown, for example, in  FIG. 9 . Such layers are very compact. Substantially the entire undulating area can be made active. Further, no porous layer is required beyond the catalyst layer and no unsupported face seals are required. Thus the undulating area can be tightly pleated since there is no porous medium between the pleats to interfere with the diffusion of fuel and oxidant to the exposed catalyst layers of the unit cells. Unit cells according to the invention may be incorporated in a pleated layer structure as described, for example, in the commonly-assigned U.S. Pat. No. 7,201,986, issued on Apr. 10, 2007, entitled “ELECTROCHEMICAL CELLS FORMED ON PLEATED SUBSTRATES”, which is hereby incorporated herein by reference. 
       FIGS. 10 and 10A  show a fuel cell device  50  according to one embodiment of the invention. Fuel cell device  50  comprises a fuel cell layer  52  comprising a plurality of unit cells  54 . Fuel cell layer  52  comprises a positive terminal  53  and a negative terminal  55 , which may be connected to an external circuit (not shown). Unit cells  54  may be connected between positive terminal  53  and negative terminal  55  in any suitable manner. Fuel cell layer  52  is sealed to a spacer  56 , which is in turn sealed to a base  58 . Fuel cell layer  52 , spacer  56  and base  58  define a plenum  60  for holding fuel, which may be introduced through fuel inlet  62 . An optional fuel outlet  64  may be provided if fuel flow is required, or if recirculation of fuel is required. Base  58  could optionally be replaced with another fuel cell layer, oriented oppositely to layer  52 . Also, spacer  56  could be built into layer  52 , such that two such layers could be bonded back to back to form a fuel cell device having two fuel cell layers. 
       FIG. 11  shows a non-planar fuel cell device  66  according to another embodiment of the invention. Device  66  is the same as device  50 , except that fuel cell layer  68 , spacer  70  and base  72  are curved. In the example illustrated in  FIG. 11 , layer  68 , spacer  70  and base  72  are shaped to conform to the wall of a cylinder, but it is to be understood that other non-planar configurations are equally possible. 
       FIG. 12  shows a stack of fuel cell layers  52  and spacers  56  according to another embodiment of the invention. Plenums defined by spacers  56  may be filled with fuel and oxidant in alternating fashion to provide reactants to layers  52 . 
     Some embodiments of the invention provide unit cells wherein an exposed area of a catalyst layer is greater than a cross sectional area of an ion-conducting layer through which ions liberated by reactions in the catalyst layer can pass through the cell. This can be seen, for example, in  FIG. 2D  wherein a surface  124  of catalyst layer  24 A has a surface area larger than a cross sectional area of the portion  125  of ion-conducting layer  25  through which ions (e.g. protons) generated in catalyst layer  24 A pass to the opposing catalyst layer  24 B. 
     The invention also provides methods for operating electrochemical cells. One such method comprises: providing an electrochemical cell having: a catalyst-containing electrochemical reaction layer having an outer face and an inner face; an electrical current-carrying structure underlying the electrochemical reaction layer at least in part; and an ion-conducting layer in contact with the inner face of the electrochemical reaction layer; allowing a reactant to diffuse into the electrochemical reaction layer; allowing the reactant to undergo a catalysed electrochemical reaction to produce an ion at a location in the electrochemical reaction layer between a surface of the electrochemical layer and the current-carrying layer; and, allowing the ion to travel to the ion-conducting layer along a path that avoids the current-carrying structure. 
     The path taken by the ion is not substantially anti-parallel to a path taken by the electrical current between the location and the current-carrying structure. 
     Where a component (e.g. a membrane, layer, device, circuit, etc.) is referred to above, unless otherwise indicated, reference to that component (including a reference to a “means”) should be interpreted as including as equivalents of that component any component which performs the function of the described component (i.e., that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure which performs the function in the illustrated exemplary embodiments of the invention. 
     In some embodiments of the invention, a filter layer may be provided on the outer surface of one or both of catalyst layers  24 A,  24 B. The filter layer may be used to remove undesired materials from reactants before they reach catalyst layer  24 A or  24 B. For example, a filter layer placed over the cathode catalyst layer may be impermeable to water but permeable to air, to allow air to reach the cathode of the unit cell, while preventing water from reaching the unit cell.  FIG. 13  illustrates an example of structure  20 A wherein a filter layer  200  is provided on the outer surface of catalyst layer  24 B. 
     It is noteworthy that in a number of the embodiments described above, electrical current from electrochemical reactions occurring in a catalyst layer is collected in the plane of the catalyst layer. 
     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example: 
     This invention has application to fuel cells as well as electrochemical cells of other types such as chlor-alkali reaction cells and electrolysis cells. 
     The invention is not limited to gaseous fuels. Liquid fuels may also be used with appropriate material selections. 
     The anodes and cathodes of the unit cells do not need to be the same size. The anodes may, for example, be somewhat smaller than the cathodes. Any exposed traces could be located on the anode side of the membrane electrode assemblies. 
     The catalyst layers are layers where electrochemical reactions occur. In some embodiments these layers may not comprise catalysts in the strict sense of the term. 
     In some embodiments, the current-carrying structures are depicted as being in direct contact with the ion exchange membrane, but this is not necessary. It is to be understood that the current-carrying structures may be separated from the ion exchange membrane by another material, such as a portion of the catalyst layer. 
     Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.