Abstract:
A fuel cell assembly provides for the delivery of fluid into a channel of a fluid flow field plate in alternating flow directions through the channel for delivery of the fluid to a membrane-electrode assembly. The fuel cell includes a fluid flow field plate having a channel for delivery of fluid to a membrane-electrode assembly, the channel having a first inlet/outlet port communicating therewith and a second inlet/outlet port communicating therewith; and a fluid delivery system connected to the fluid flow field plate adapted for bi-directional delivery of fluid into the channel of the fluid flow field plate.

Description:
TECHNICAL FIELD 
     The present invention relates to fuel cells, and in particular to methods and apparatus for the delivery of fuel and oxidant to flow field plates 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  which deliver fuel and oxidant respectively to the MEA. 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 . Hydrogen and/or other fluid fuels or fuel mixes are delivered to the anode channels. Oxidant is delivered to the cathode channels, and reactant product water vapour is extracted from the cathode channels. 
     Throughout this specification, the expression “channel” will be used to indicate any suitable conduit for delivery of fluid fuel or oxidant to the MEA and/or for the exhaust of unused fuel or oxidant together with any purge or reactant products from the MEA. 
     With reference also to  FIG. 2(   a ), one conventional configuration of fluid flow channel  16   a  in the cathode fluid flow field plate  15  for delivery of oxidant to, and exhaust of water vapour from, the MEA is an open-ended channel having an inlet  21  and an outlet  22 . This allows a continuous through-purge of gas to provide the requisite exhaust purge. 
     With reference also to  FIG. 2(   b ), one conventional configuration of fluid flow channel  16   b  in the anode fluid flow field plate  14  for delivery of hydrogen fuel to the MEA is a “dead-ended” channel arrangement  16   b , typically in a comb-like structure. Such a dead-ended channel  16   b  has an inlet  24 , but no outlet, the hydrogen fuel being consumed as it enters the MEA from the channels  16   b . As shown, two interdigitated comb structures may be used, with two inlets  24 . 
     For simplicity, the channels  16   b  are shown in this diagram simply as single lines although it will be understood that they have finite width. An outline of an underlying open-ended cathode channel  16   a  is shown in dashed outline. The depiction of the channels  16  in the drawings is highly simplified for clarity; the channel widths and separations may both be of the order of a millimetre or so. 
     The dead-ended channel arrangement for the anode channels  16   b  suffers from at least one significant disadvantage. Although the reactant product, typically water vapour, is primarily produced on the cathode side of the MEA, and can be exhausted from the open-ended channel outlet  22 , some water is typically transported back to the anode side of the MEA by diffusion. Unless managed, this water can accumulate locally and impede the access of hydrogen to the catalytically active sites for electrochemical reaction, effectively deactivating the portions of the electrode from which the hydrogen is blocked. This is sometimes referred to as ‘flooding’ of the anode and results in gradual but persistent performance decline in the fuel cell. A lower power output capability at any given operating voltage is the result. 
     In the prior art, one solution to this problem is to also use an open-ended channel  16   a  as the anode channel, allowing a continuous or intermittent purge of excess hydrogen to exit the fuel cell, carrying water with it to remove the water from the ‘water masked’ surfaces, thereby re-admitting hydrogen to the previously blocked sites. 
     It will be recognised that this is wasteful of hydrogen fuel which is either lost as an exhaust gas, or else it must be dehumidified and/or reconditioned so that it can be recycled to the fuel inlet. This can contribute substantially to overall system inefficiencies or complexity of fuel delivery equipment and therefore large volumes of unused purge hydrogen are undesirable. 
     SUMMARY 
     Therefore, it is an object of the present invention to increase the efficiency with which accumulated water or water vapour can be removed from the water masked surfaces of the MEA. 
     According to one aspect, the present invention provides a fuel cell assembly comprising:
         a fluid flow field plate having a channel for delivery of fluid to a membrane-electrode assembly, the channel having a first inlet/outlet port communicating therewith and a second inlet/outlet port communicating therewith; and   a fluid delivery system connected to the fluid flow field plate adapted for bi-directional delivery of fluid into the channel of the fluid flow field plate.       

     According to another aspect, the present invention provides a method of operating a fuel cell assembly comprising the step of delivering fluid into a channel of a fluid flow field plate in alternating flow directions through the channel for delivery of the fluid to a membrane-electrode assembly. 
    
    
     
       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  is a schematic cross-sectional view through a part of a conventional fuel cell; 
         FIG. 2(   a ) is a simplified plan view of a fluid flow field plate of the fuel cell of  FIG. 1  with an open-ended channel; 
         FIG. 2(   b ) is a simplified plan view of a fluid flow field plate of the fuel cell of  FIG. 1  with closed-ended channels; 
         FIG. 3  is a schematic block diagram of a fuel delivery system allowing bi-directional flow of fuel through the fuel cell; 
         FIG. 4  is a graph illustrating the improved performance of a fuel cell operated with bi-directional fuel feed and an absolute cell voltage purge trigger; 
         FIG. 5  is a graph illustrating the improved performance of a fuel cell operated with bi-directional fuel feed and a proportionate decline cell voltage purge trigger; and 
         FIG. 6  is a schematic block diagram of an oxidant delivery and purge system allowing bi-directional flow of oxidant and exhaust gases through the fuel cell. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention recognises that a major contributor to performance decline in fuel cells is ‘stagnant’ or trapped water which, under constant operating conditions, is not perturbed and therefore tends to realise the water masked surfaces of the MEA. Not all of this accumulated water is eliminated during conventional purge processes and an important factor in improving the efficiency of the cell is in disturbing this water layer. 
     The water layer can be disturbed, to allow greater hydrogen access to the MEA, by use of the gas stream itself with or without venting. 
     The fuel cell may be fed alternately from two or more separate inlet ports (preferably symmetrical and opposite), with or without an outlet port. 
     When hydrogen is fed bi-directionally from one of two (or more) alternate ports, both the hydrogen and water move in alternating directions. It has been established that this significantly reduces the effect of water accumulation and increases the utilisation of the MEA without necessarily venting of hydrogen. 
     Although water does accumulate, its effect in terms of masking catalyst sites in the MEA is much less due to the regular perturbation in the hydrogen flow. 
     This means that, even where purging is still required to remove water build up, the purge requirement is significantly reduced and the time period between purge cycles can be greatly extended. Thus hydrogen utilisation increases markedly, and fuel cell performance is consequently significantly enhanced. 
     With reference to  FIG. 3 , an exemplary hydrogen fuel fluid delivery system provides bi-directional delivery of fluid into the anode fluid flow field plate of a fuel cell  30  by way of a first inlet/outlet port  31  and a second inlet/outlet port  32 . A fluid supply line  33  provides hydrogen to the input port of a multi-way valve  34 , which may particularly be of the three-way type. 
     A first output port  34   a  of the multi-way valve  34  is coupled to the first inlet/outlet port  31  of the fuel cell  30 . A second output port  34   b  of the multi-way valve  34  is coupled to the second inlet/outlet port  32  of the fuel cell  30 . The switching of the multi-way valve  34  is controlled by a controller  35 , the operation of which will be described later. 
     A purge valve  36  has its input port  36   a  coupled to the second inlet/outlet port  32  of the fuel cell  30 , and its output port  36   b  coupled to an exhaust or recycle line  37 . The switching of the purge valve  36  is controlled by a controller  35 , the operation of which will be described later. 
     In a typical fuel cell stack, a plurality of fuel cells  30  will all have their respective anode fluid flow field plates connected via a suitable manifold arrangement well known in the art, and the first and second inlet/outlet ports  31 ,  32  may be common to the plurality of parallel fuel cells (not shown). 
     During operation, the controller  35  switches the multi-way valve  34  between a first configuration in which the fluid supply line  33  is connected to the first inlet/outlet port  31  and a second configuration in which the fluid supply line  33  is connected to the second inlet/outlet port  32 . Thus, by cyclically switching the valve  34  between its first and second configurations, a bi-directional flow of hydrogen (or other fuel or fuel mix) within the fuel cell  30  flow plate channels is achieved. 
     Preferably the switching occurs on a regular periodic basis, with a duty cycle of between 0.1 Hz and 100 Hz. More preferably, the switching occurs with a duty cycle of approximately 2.5 Hz+/−1.5 Hz. 
     The controller  35  also preferably switches the purge valve  36  on and off. We refer to the ‘off’ condition to mean that the exhaust or recycle line  37  is isolated from the second inlet/outlet port  32  and the ‘on’ condition to mean that the exhaust or recycle line  37  is coupled to the second inlet/outlet port  32 . 
     Preferably the controller  35  only switches the purge valve  36  on when at least one predetermined trigger condition is sensed. 
     One possible trigger condition is when the fuel cell voltage has fallen to a predetermined absolute threshold level, for example approximately 0.65 V per cell. This is monitored by the controller  35 , using voltage sense line  39 . 
     Another possible trigger condition is when the fuel cell voltage has fallen by a predetermined relative or proportional amount since a previous purge cycle, for example approximately 0.3% of the cell or fuel cell stack voltage. 
     Another possible trigger condition is upon expiry of a predetermined time period. 
     The controller  35  switches the purge valve  36  on when the multi-way valve  34  is in the first configuration so that the first inlet/outlet port  31  is acting as a fuel inlet and the second inlet/outlet port  32  is acting as an outlet, thereby ensuring an efficient purge mechanism. Thus, this may constitute a further trigger condition that can be used in conjunction with other trigger conditions, eg. on a Boolean ‘and’ basis. 
     In this case, the control means  35  may also be operative to ensure that the purge valve  36  operation is controlled such that when the purge is required, its ‘on’ cycle is coextensive with, or shorter than, the duration that the multi-way valve  34  is in the first configuration. The required duration of a purge cycle could be greater than the duty cycle of the multi-way valve  34 , in which case the purge valve may operate for several successive cycles of the multi-way valve  34 , when it is in the first configuration. 
     Referring now to  FIG. 4 , the graph illustrates the effects of operating a fuel cell such that the multi-way valve  34  is toggled between the first and second configurations with a duty cycle of 1 Hz (ie. an “oscillating, bi-directional feed” at 1 Hz); and the purge valve  36  is switched on to vent to exhaust upon sensing a trigger condition of 0.648 V cell voltage. This is contrasted with the performance of the same cell operated with a “direct feed”, in which the multi-way valve  34  is held permanently in the first configuration, and again with a vent triggered at 0.648 V per cell, under the same load conditions. 
     It can readily be seen that the mean time of 55 seconds between vent cycles for the direct feed is extended to a mean time of 101 seconds between vent cycles for the oscillating, bi-directional feed, also providing a slightly increased mean voltage from 0.6508 V to 0.6521 V. It will be understood that an increased interval between purge cycles necessarily results in a decreased loss of hydrogen to the purge exhaust and an increase in fuel cell efficiency. 
     Referring now to  FIG. 5 , the graph illustrates the effects of operating a fuel cell such that the multi-way valve  34  is toggled between the first and second configurations with a duty cycle of 1 Hz (ie. an “oscillating, bi-directional feed” at 1 Hz); and the purge valve  36  is switched on to vent to exhaust upon sensing a trigger condition indicated by a proportionate fall in stack voltage of about 0.3%. 
     This is contrasted with the performance of the same cell operated with a “direct feed”, in which the multi-way valve  34  is held in the first configuration, and again with a vent triggered at 0.3% voltage drop. 
     It can readily be seen that although the period between vent cycles is substantially the same for the direct feed and for the oscillating feed, the oscillating, bi-directional feed provides a higher mean and absolute voltage level taking the mean cell voltage from 0.6344 V to 0.6369 V and therefore a higher fuel cell efficiency. 
     Although described in the context of hydrogen as a fluid fuel, and oxygen as a fluid oxidant, delivery of other fuel streams, including hydrogen rich gas streams, such as hydrogen with CO 2  or N 2  or fuel streams with or without additional purge gases or humidification may benefit from the oscillating bi-directional fuel feed. Similarly, delivery of oxidant may include air, or oxidant in any proportion with diluent or inert gases or as the sole fluid. 
     Although the preferred embodiments have been described in the context of displacing water from the catalytic sites of the anode side of the MEA, by way of a bi-directional fuel feed, it will be understood that for optimum efficiency, it may be desirable to provide a similar displacement mechanism to the cathode side of the MEA, such that oxidant, with or without additional purge gases, is fed into the cathode fluid flow plate channel in a bi-directional mode. 
     With reference to  FIG. 6 , an alternative fuel cell assembly provides an oscillating, bi-directional fluid feed of, for example oxidant and purge through the fluid flow channels of fuel cell  60 . The fuel cell  60  has a first inlet/outlet port  62  and a second inlet/outlet port  61  which are both coupled to first and second outputs of a two-gang multi-way valve. 
     The two-gang multi-way valve effectively comprises a first valve  64  and a second valve  66  that operate in concert with one another. The first valve  64  has an input connected to a fluid source  63  (eg. oxidant), a first output  64   a  connected to the first inlet/outlet port  62  and a second output connected to the second inlet/outlet port  61 . The second valve  66  has an input connected to a purge line  67 , a first output  66   a  connected to the second inlet/outlet port  61  and a second output  66   b  connected to the first inlet/outlet port  62 . 
     The valves  64  and  66  are configured so that only the first outputs  64   a ,  66   a  or the second outputs  64   b ,  66   b  can be switched to the respective valve inputs at any one time. 
     In this manner, a bi-directional flow of fluid and exhaust can be maintained within the fuel cell  60  at all times. In the preferred arrangement, the fluid supply  63  is oxidant (with or without additional carrier/purge/inert gases) that is delivered to the cathode fluid flow field plate. However, it will be recognised that this arrangement can also be used for delivery of fuel such as hydrogen to the anode fluid flow field plate if a permanent exhaust or recycle connection is required. In this way, a reduced purge flow may be utilised due to the greater efficiency in displacing water from the MEA by the bi-directional flows. 
     While the examples of the invention have illustrated use of a dual port fuel cell  30 ,  60 , it will be understood that more than two ports can be used to effect oscillating, multi-directional fluid flow through the channel or channels of the fluid flow field plates. Alternatively, the ports used for venting need not necessarily be combined with the ports used as inlets, but could be provided separately, for example at one or more positions intermediate a pair of alternating inlet ports. 
     Other embodiments are intentionally within the scope of the appended claims.