Abstract:
A fuel cell assembly provides for direct water delivery to water injection points in the active area of fluid flow field plates of the assembly. The fluid flow field plate has a plurality of channels formed in the surface thereof which extend across the surface of the plate in a predetermined pattern, defining the active areas of the plate. A distribution foil has a plurality of channels formed in a surface thereof which channels extend from a first edge of the distribution foil to a second edge of the distribution foil. The channels terminate at the second edge at positions substantially coincident with respective ones of the field plate channels at water injection points. A cover foil extends over the distribution foil to enclosed the distribution foil channels and thereby form conduits for the water between the two foils.

Description:
This patent application is a national stage application of PCT application no. PCT/GB03/02973, which claims priority to GB02157907. This patent application claims priority to both PCT/GB03/02973 and to GB02157907. 
     TECHNICAL FIELD 
     The present invention relates to fuel cells, and in particular to flow field plates suitable for use in solid polymer electrolyte fuel cells, which flow field plates act as fluid delivery conduits to electrode surfaces of the fuel cell. 
     BACKGROUND 
     Conventional electrochemical fuel cells convert fuel and oxidant into electrical energy and a reaction product. A typical layout of a conventional fuel cell  10  is shown in  FIG. 1  which, for clarity, illustrates the various layers in exploded form. A solid polymer ion transfer membrane  11  is sandwiched between an anode  12  and a cathode  13 . Typically, the anode  12  and the cathode  13  are both formed from an electrically conductive, porous material such as porous carbon, to which small particles of platinum and/or other precious metal catalyst are bonded. The anode  12  and cathode  13  are often bonded directly to the respective adjacent surfaces of the membrane  11 . This combination is commonly referred to as the membrane-electrode assembly, or MEA. 
     Sandwiching the polymer membrane and porous electrode layers is an anode fluid flow field plate  14  and a cathode fluid flow field plate  15 . Intermediate backing layers  12   a  and  13   a  may also be employed between the anode fluid flow field plate  14  and the anode  12  and similarly between the cathode fluid flow field plate  15  and the cathode  13 . The backing layers are of a porous nature and fabricated so as to ensure effective diffusion of gas to and from the anode and cathode surfaces as well as assisting in the management of water vapour and liquid water. Throughout the present specification, references to the electrodes (anode and/or cathode) are intended to include electrodes with or without such a backing layer. 
     The fluid flow field plates  14 ,  15  are formed from an electrically conductive, non-porous material by which electrical contact can be made to the respective anode electrode  12  or cathode electrode  13 . At the same time, the fluid flow field plates must facilitate the delivery and/or exhaust of fluid fuel, oxidant and/or reaction product to or from the porous electrodes. This is conventionally effected by forming fluid flow passages in a surface of the fluid flow field plates, such as grooves or channels  16  in the surface presented to the porous electrodes  12 ,  13 . 
     With reference also to  FIG. 2(   a ), one conventional configuration of fluid flow channel provides a serpentine structure  20  in a face of the anode  14  (or cathode  15 ) having an inlet manifold  21  and an outlet manifold  22  as shown in  FIG. 2(   a ). According to conventional design, it will be understood that the serpentine structure  20  comprises a channel  16  in the surface of the plate  14  (or  15 ), while the manifolds  21  and  22  each comprise an aperture through the plate so that fluid for delivery to, or exhaust from, the channel  20  can be communicated throughout the depth of a stack of plates in a direction orthogonal to the plate as particularly indicated by the arrow in the cross-section on A-A shown in the  FIG. 2(   b ). 
     Other manifold apertures  23 ,  25  may be provided for fuel, oxidant, other fluids or exhaust communication to other channels in the plates, not shown. 
     The channels  16  in the fluid flow field plates  14 ,  15  may be open ended at both ends, ie. the channels extending between an inlet manifold  21  and an outlet manifold  22  as shown, allowing a continuous throughput of fluid, typically used for a combined oxidant supply and reactant exhaust. Alternatively, the channels  16  may be closed at one end, ie. each channel has communication with only an input manifold  21  to supply fluid, relying entirely on 100% transfer of gaseous material into and out of the porous electrodes of the MEA. The closed channel may typically be used to deliver hydrogen fuel to the MEA  11 - 13  in a comb type structure. 
     With reference to  FIG. 3 , a cross-sectional view of art of a stack of plates forming a conventional fuel cell assembly  30  is shown. In this arrangement, adjacent anode and cathode fluid flow field plates are combined in conventional manner to form a single bipolar plate  31  having anode channels  32  on one face and cathode channels  33  on the opposite face, each adjacent to a respective membrane-electrode assembly (MEA)  34 . The inlet manifold apertures  21  and outlet manifold apertures  22  are all overlaid to provide the inlet and outlet manifolds to the entire stack. The various elements of the stack are shown slightly separated for clarity, although it will be understood that they will be compressed together using sealing gaskets if required. 
     In order to obtain high and sustained power delivery capability from a fuel cell, it is generally necessary to maintain a high water content within the membrane-electrode assembly, and in particular within the membrane. 
     In the prior art, this is conventionally achieved by humidifying the feed gases, either fuel, air or both, fed via manifolds  21 ,  22  or  23  and channels  16 . A disadvantage with this technique is that in order to maintain sufficient humidification levels, the inlet gas streams often require heating and supplementary apparatus to introduce water vapour into the flowing gas streams. 
     In the prior art, the supplementary apparatus has been implemented in a number of ways. Bubbling the fuel or oxidant gases through heated water columns prior to introduction into the fuel cell has been applied. Alternatively, permeable membranes have been utilised as water transfer media such that water is carried into a gas stream from an adjacent plenum containing liquid water. Wicks have similarly been adopted to act as water transport media, liquid to vapour phase. 
     The additional apparatus may be separate from, or form an integral part of, the fuel cell stack. In either case, there is an associated increase in size and complexity of the assembly as a whole. 
     An alternative method is to deliver water directly to the membrane  11 ,  34 , eg. directly to the electrode surfaces or into the channels  16  of the bipolar plates  31 . This technique has the advantage of not only supplying the water to maintain a high membrane water content but also can act to cool the fuel cell through evaporation and extraction of latent heat of vaporisation. 
     This direct heat removal process that provides for the extraction of energy via the exit gas stream has distinct advantages associated with the elimination of intermediate cooling plates within the fuel cell stack assembly. 
     In the prior art, it is common to adopt a cooling regime which intersperses heat exchange plates between the electrochemically active plates so as to extract the thermal energy resulting from resistive and thermodynamic inefficiency of the fuel cell. These heat exchange, or cooling, plates utilise a recirculating or, less commonly, once-through fluid flow which carries heat away from the fuel cell stack. The cooling plates are in general of a different design to the active plates thereby adding to the complexity, size and cost of the fuel cell assembly. 
     A difficulty that can be encountered in the direct introduction of water is to deliver precise quantities of water to the many fluid flow fuel plate channels  16  within a fuel cell stack  30 . Typically, this requires the delivery of precise quantities of water to many thousands of individual locations. To achieve this, a complex design of fluid flow field plate  14 ,  15  or  31  is required, which is more difficult to achieve and which increases costs of production. 
     If the water delivery process is uneven then the cooling effect can be poorly distributed, resulting in localised hot spots where overheating may result in physical stress and a deterioration of the membrane  11  mechanical properties and ultimately rupture. This effect applies with both poor (uneven) delivery across a plate surface and uneven delivery to each of the individual cells that make up the stack. In other words, temperature variations mail occur within a cell, or from cell to cell. 
     SUMMARY 
     It is an object of the present invention to provide an improved method and apparatus for controlled delivery of water to individual channels in the fluid flow plates. It is a further object of the invention to provide such a method and apparatus which is easy to manufacture and assemble. 
     According to one aspect, the present invention provides a fuel cell assembly comprising:
         a fluid flow field plate having a plurality of channels formed in the surface thereof and extending across the surface of the plate in a predetermined pattern;   a distribution foil having a plurality of channels formed in a surface thereof and extending from a first edge of the distribution foil to a second edge of the distribution foil, the channels terminating at the second edge at positions substantially coincident with respective ones of the field plate channels; and   a cover foil extending over the distribution foil to enclose the distribution foil channels and thereby form conduits for water between the two foils.       

     According to another aspect, the present invention provides a fuel cell assembly comprising:
         a fluid flow field plate hating a plurality of channels formed in the surface thereof and extending across the surface of the plate in a predetermined pattern;   a distribution foil having a plurality of channels formed in a surface thereof, the channels each extending from first positions proximal to or at a first edge of the distribution foil to second positions proximal to or at a second edge of the distribution foil, the channels terminating at the second positions substantially coincident with respective ones of the underlying plate channels; and   a cover foil co-extensive with a substantial part of the distribution foil to enclose the distribution foil channels over at least part of their length between the first and second positions and thereby form conduits for water between the two foils.       

     According to a further aspect, the present invention provides a fuel cell assembly comprising:
         a fluid flow field plate having a plurality of channels formed in the surface thereof and extending across the surface of the field plate in a predetermined pattern;   an adjacent membrane-electrode assembly (MEA) in contact with the fluid flow field plate over an active area of the MEA and;   a distribution membrane interposed between the fluid flow field plate and the MEA, the membrane having a plurality of water conduits extending therethrough between first positions proximal to or at a first edge of the membrane to second positions proximal to or at a second edge of the membrane, the conduits terminating at the second positions substantially coincident with respective ones of the plate channels.       

    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which: 
         FIG. 1  shows a schematic cross-sectional view through a part of a conventional fuel cell; 
         FIGS. 2(   a ) and  2 ( b ) respectively show a simplified plan and sectional view of a fluid flow field plate of the fuel cell of  FIG. 1 ; 
         FIG. 3  shows a cross-sectional view through a conventional fuel cell stack with bipolar plates; 
         FIG. 4(   a ) shows a plan view of a fuel cell fluid flow field plate with a serpentine fluid conduit, showing in outline the overlay position of a water distribution foil and cover foil according to the present invention; 
         FIG. 4(   b ) shows a plan view of a fuel cell fluid flow field plate with interdigitated comb fluid conduit, showing in outline the overlay position of a water distribution foil and cover foil according to the present invention; 
         FIG. 5  shows a plan view of a water distribution foil according to the present invention; 
         FIG. 6  shows a cross-sectional view of the fluid flow field plate, water distribution foil and cover foil of  FIGS. 4 and 5 ; 
         FIG. 7  shows a perspective view of part of the assembly of  FIG. 6 ; 
         FIG. 8  shows a cross-sectional view of a fluid flow field plate, water distribution foil and cover foil in which the relative positions of the water distribution foil and cover foil are reversed; and 
         FIG. 9  shows a schematic plan view of water injection points for an interdigitated comb channel structure. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIGS. 4(   a ) and  4 ( b ), the present invention provides a series of water injection conduits extending between a water inlet manifold  25  and the individual channels  16  of a fluid flow field plate  40   a  or  40   b . Generally speaking, the water injection conduits are provided by way of a membrane or laminated structure which lies on the surface of the fluid flow field plate  40 . The water injection conduits are provided with inlets communicating with the water inlet manifold  25  and outlets which define predetermined water injection points over the channels  16  in the fluid flow field plate. 
     In a preferred arrangement, the laminated structure is provided in the form of two foil layers  41 ,  42  overlying the plate  40 , the position of which foils are shown in dashed outline in  FIGS. 4(   a ) and  4 ( b ). 
       FIG. 4(   a ) illustrates a plan view of a fluid flow field plate  40   a  with serpentine channel  16 , with foils  41   a ,  42   a  having first edges  43   a ,  44   a  coincident with the water inlet manifold  25 , and second edges  45   a ,  46   a  located at or adjacent to predetermined water injection points  49  of the channels  16 . 
       FIG. 4(   b ) illustrates a plan view of a fluid flow field plate  40   b  with two interdigitated comb channels  47 ,  48  each communicating with a respective manifold  21 ,  22 , and foils  41   b ,  42   b  having first edges  43   b ,  44   b  coincident with the water inlet manifold  25 , and second edges  45   b ,  46   b  located at or adjacent to predetermined water injection points of the channel  47 . It will be noted that the foils may be repeated on the opposite edge of the plate  40   b  between a second water inlet manifold  25  and predetermined water injection points on the channel  48 . 
       FIG. 5  shows a detailed plan view of the water distribution foil  41  layout, illustrating the preferred paths of the water injection conduits  50 . The conduits  50  are formed by a first series of channels  51  which extend from the first edge  43  of the foil  41  located at the water inlet manifold  25 , to a pressure distribution gallery or plenum  52  that extends along the length of the water injection foil  41 . The pressure distribution gallery  52  communicates with a second series of channels  53  which extend to the second edge  45  of the foil for communication with the channels  16  in the fluid flow field plate. For this purpose, the second series of channels  53  are grouped to terminate at respective convergence structures  54  at the second edge  45  of the water injection foil  41 . 
     In the preferred embodiment as illustrated, the convergence structures  54  comprise arcuate recesses  55  cut into the second edge  45  of the foil  41  at water injection points  49  adapted to be coincident with predetermined positions over channels  16 , shown in outline on the figure. 
     The pressure distribution gallery  52  preferably comprises an array of intercommunicating channels  56  which baffle the incoming water from the first series of channels  51  and effectively distribute it along the entire length of the foil  41  so that each group of the second series of channels  53  receives water at a substantially similar pressure. 
     Referring back to  FIGS. 4(   a ) and  4 ( b ), the cover foil  42  comprises an unpatterned foil (ie. without channels) of substantially similar peripheral shape to the lower foil. The cover foil  42  extends beyond the edge of the distribution foil  41  at least at the ends of the second series of channels to ensure that water is directed downwards into the desired flow field plate channel  16 . Most conveniently, this overlap is achieved by the recesses  55  being formed in the distribution foil  41 , but not in the cover foil  42 . Thus, as best seen in the cross-sectional diagram of  FIG. 6 , in exaggerated form, the cover foil  42  forms a top closure to the channels  51 ,  52  and  53  to form the water injection conduits  50 , leaving open the ends of the channels  51  and  53 . In the embodiment shown, the cover foil  42  may be formed slightly larger than the distribution foil  41  such that it overlaps the second edge  45  (and possibly the first edge  43 ) to achieve a similar effect. 
     It is noted that the foil layers are very thin compared with the plate  40  thickness, the thickness of the foil layers being easily absorbed by the MEA  34  and any gaskets interposed between the plates. The components in the  FIG. 6  are shown slightly separated for clarity, although they will, of course, be compressed together. 
       FIG. 7  shows a perspective diagram of the water distribution foil  41  in position over the flow field plate  40  showing alignment of the various channels and manifolds. 
     It will be recognised that the water distribution channels  51 ,  52 ,  53  need not be formed in the lower foil  41 . In another embodiment, shown in  FIG. 8 , the water distribution channels  80  are formed in the lower surface of upper foil  82 , while the lower foil  81  serves to form the closure of the channels  80  to form the water injection conduits. In other words, the distribution foil  82  and cover foil  81  are inverted compared with the arrangement of  FIG. 6 . 
     In the  FIG. 8  arrangement, at least the second series of channels (compare channels  53  in  FIG. 5 ) will not extend right to the second edge  83  of the upper foil, but will terminate at positions proximal to the second edge. The lower (cover) foil  81  will extend almost to the end of the channels  80 , but will preferably stop slightly short thereof in order that there is fluid communication from the end of the channel  80  into the plate channel  16  at the water injection points  49 . 
     As indicated above, the lower (cover) foil  81  provides a closure to the channels  80  forming a barrier preventing water from escaping into underlying channels  16  in the fluid flow plate  40  in the wrong places, eg. where the water injection conduits traverse the fuel and/or oxidant channels  16  (eg. at location  85 ). 
     Preferably, the foils as described above are formed from a metal, such as stainless steel. However, any suitable material having appropriate pressurised water containment properties could be used, and the expression “foil” used throughout the present specification is to be construed accordingly. Preferably, the foils are electrically conductive but they need not be so, since they do not impinge on the active area of the MEA. 
     In a preferred embodiment, the fluid flow channels  16  in the anode or cathode plates  40  are tropically between 0.4 mm and 1.2 mm in width and depth. It is found that a channel width and depth of 10 μm, chemically etched into the water distribution foil, selves to provide the necessary degree of water injection. 
     In use, the pressure of water being delivered via manifold  25  is controlled to ensure a significant pressure difference between the water supply and the gas pressure in the fluid flow channels  16 , achieving an equal distribution of water between the thousands of flow paths. In the preferred embodiment, water is delivered to the manifold at a pressure in the range 0.5-3 bar H 2 O. 
     An advantage of this approach is that the water distribution membrane is extremely thin and can easily be located within the available space within bipolar plates or in the gasket area. 
     The volumetric water dispensing accuracy can also be very precisely controlled by suitable design of the water injection conduit pattern and channel dimensions. 
     The water can be dispensed to either the fuel stream (anode) or the oxidant (cathode) side of the bipolar plate  34 , or both. Preferably, the water is injected into the cathode side. 
     As illustrated in  FIG. 9 , water that is dispensed into interdigitated channels  90  in the flow field plate  40  can be introduced at either the entry point  91  to the channel, after the feeder channel  92 , or alternatively into the exit track  93  at an injection point  94  at the same end of the bipolar plate as the feed manifold. 
     An advantage of water injection into the exit tracks is a reduction of pressure drop in reactant gas flows. This is because the water does not pass through the diffusion medium causing masking of void space for the gas passage. Similarly the elimination of water flow through the diffusion medium will also reduce the attrition of the medium and its gradual fragmentation and structural deterioration. 
     The evaporative cooling process is effective in the exit tracks and water content of the membrane is maintained due to saturation of the air with water vapour. 
     Although embodiments of the present invention have been described in the context of water injection into a proton exchange membrane fuel cell, it will be understood that the same structures may be used to inject any fluid material to injection points on a field plate. 
     Other embodiments are within the accompanying claims.