Abstract:
A fuel cell system made up of a plurality of fuel cells. Each cell includes a fuel inlet, an oxidant inlet, a fuel side product outlet and an oxidant side product outlet. A common fuel supply line is provided for the fuel inlets. A common oxidant supply line is provided for the oxidant inlets. A common product purging mechanism is coupled to the outlets for purging the same of unused fuel, unused oxidant, fuel side product and oxidant side product. The product purging mechanism includes valving structure operable to selectively and independently open the outlets of a given cell. A method for operating such a fuel cell system includes supplying fuel to the fuel inlets from a common source of fuel and supplying an oxidant to the oxidant inlets from a common source of oxidant. The outlets of a given cell are selectively opened to purge fuel product and oxidant product from the given cell while the outlets of other cells are kept closed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a divisional of presently pending application Ser. No. 09/552,419 filed Apr. 19, 2000, and priority is claimed therefrom pursuant to 35 U.S.C. § 120. The entirety of the disclosure of said application Ser. No. 09/552,419 is specifically incorporated herein by this specific reference thereto. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to the field of fuel cells and, more particularly to methodology and apparatus for controlling the supply of reactant fluids to the cells and the purging of reaction products and inert fluids from the individual cells and/or groups of cells of a cell stack. In particular the invention relates to methodology and apparatus for tailoring the supply of fuel to and removal of reaction products from cells so as to meet the individual demands of each respective cell and/or group of cells in a stack of cells.  
           [0004]    2. The State of the Prior Art  
           [0005]    Fuel cells are electromechanical devices that convert chemical energy in the form of fuel and an oxidant directly into electrical energy. Fuel cells are generally classified in accordance with the type of electrolyte (e.g., alkaline, phosphoric acid, solid polymer, molten carbonate, solid oxide) used to provide ionic conduction between an anode and a cathode. Useful fuel cells include PEM fuel cells, acidic fuel cells and alkaline fuel cells. In this regard, PEM is the abbreviation for polymer electrolyte membrane or proton exchange membrane, and such membranes are proton-conducting thus facilitating the use thereof as an electrolyte for transporting protons from the anode to the cathode of the cell. The present invention is generally applicable to each of these types of cells.  
           [0006]    A fuel cell generally includes seven major components, namely, 1) an anode current collector acting as the negative terminal to conduct electrons away from the cell; 2) a fluid plate to distribute and supply fuel to the anode and provide electrical contact between the anode current collector and the anode; 3) an anode comprising a porous diffusion layer and a catalyst layer where fuel is oxidized (e.g. hydrogen fuel is oxidized to form protons and electrons); 4) an electrolyte to provide ionic conduction between the anode and the cathode, which in the case of a PEM fuel cell is a proton conducting membrane; 5) a cathode with similar structure to that of the anode where an oxidant is reduced (e.g., oxygen reacts with protons and electrons to form water); 6) a fluid distributor plate to distribute and supply oxidant to the cathode and provide electrical contact between the cathode and a cathode current collector; and 7) a cathode current collector acting as a positive terminal to conduct electrons back to the cell.  
           [0007]    During operation of a fuel cell, a fuel such as hydrogen is caused to flow into the cell through a fuel inlet, and the fuel is distributed by the fuel distributor plate on the anode side of the fuel cell. Hydrogen diffuses from the fuel distributor plate through the anode diffusion layer to the catalyst layer in the anode, where it is oxidized to form protons and electrons. The electrons are conducted out of the anode, through the fuel distributor plate, to the anode current collector, and out of the cell to power an electrical device. The protons are transported across the PEM to the cathode.  
           [0008]    An oxidant such as oxygen is caused to flow into the cell through an oxidant inlet, and the oxidant is distributed by the oxidant distributor plate on the cathode side of the cell. Oxygen diffuses from the oxidant distributor plate through the cathode diffusion layer to the catalyst layer in the cathode, where it reacts with the protons generated in the anode and electrons to form water. The electrons are conducted from the electrical device back to the cell and to the cathode via the cathode current collector and the oxidant fluid distributor plate. The product water produced at the catalyst layer in the cathode diffuses through the cathode diffusion layer to flow channels in the oxidant fluid distributor plate and is removed from the cell by the flow of excess oxidant and/or inert materials in the incoming oxidant fluid stream.  
           [0009]    Fuel cells typically produce a voltage which varies up to about 1.0V. Accordingly, in order to generate greater voltages, a plurality of fuel cells are stacked in series to form a fuel cell stack. The cells of a given stack generally are arranged with a common inlet for the fuel (e.g., hydrogen or methanol and water), a common outlet for the fuel, a common inlet for the oxidant (e.g., oxygen or air), and common outlet for the oxidant. The fuel and oxidant for each cell of the stack is fed to the respective cells from these common inlets, and the product water, inert materials, excess fuel and/or excess oxidant from each cell of the stack is removed from the respective cells through these common outlets. To prevent buildup of product water, inert materials, excess fuel and/or excess oxidant in the cells, and to maintain appropriately high concentrations of all reactants in all cells, continuous or frequent purging of fuel and/or oxidant has generally been required in the past.  
           [0010]    This leads to low fuel and/or oxidant utilization, whereby large amounts of parasitic power are required to provide the necessary flows of fuel and/or oxidant to the cell stack. Moreover, it is essentially impossible to provide fuel cells such that the same have identical dimensions and morphological properties (e.g., porosity, tortuosity, wetting characteristics). Thus, the flow resistance properties of the cells of a given stack are not uniform during operation.  
           [0011]    This nonuniformity in flow resistance among the cells of a stack results in nonuniform fluid flow into and through the cells leading to nonuniform cell-to-cell performance and non-optimal stack performance. Cells having so much flow resistance that incoming flow of fuel or oxidant is inappropriately restricted and/or that proper purging of product or inert fluids is prevented become starved for fuel and/or fluid oxidant resulting in poor performance or perhaps even total failure of the cell. Adding to the non-optimal performance is the danger of explosion. In a fuel stack, cells receiving insufficient reactants to support the electrical current being drawn from the stack may go into reverse, thus generating a potentially explosive mixture in the cell. For example, when a hydrogen-oxygen cell experiences reversal, hydrogen is generated in the cathode where oxygen is present and oxygen is generated in the anode where hydrogen is present, thus provided an explosive fluid mixture of hydrogen and oxygen in both compartments.  
           [0012]    In an effort to address this problem, fuel cell stacks have been used only in low current density operations and/or the same have been designed in such a way to insure that cells with the highest flow resistance obtain a sufficient supply of new reactants and are adequately purged of product and inert fluids. The latter is often accomplished by using high flow rates of reactants; however, as explained above, the use of high reactant flow rates results in low fuel and oxidant utilization and high parasitic power consumption. To minimize fuel loss in such a case, a recirculation loop is sometimes used. Such a loop is discussed in U.S. Pat. No. 5,316,869. In this scheme, recirculation blowers or pumps are used to provide sufficiently high flow rates of reactant fluids through the cell stack to assure adequate flow of fresh reactants to and purging of product and inert fluids from each cell. To assure that the entire loop is not saturated with product or inert fluids, a purging line is generally incorporated in the loop. Such purging line may be designed to be open continuously to remove the fluid mixture at such a rate that the system is not saturated with product or inert fluids. Alternatively the purging line may be designed for opening only when reactant fluid concentrations drop below a certain set value or when concentrations of product and/or inert fluids exceed certain set values. In the alternative case, a fluid sensor may be used to detect the concentrations which are being monitored. Either purging process results in high fuel loss rate and high parasitic power consumption to power the recirculation system.  
           [0013]    The problem of properly disposing of product and inert gas is further addressed in U.S. Re. 36,148. This patent describes a fuel stack arrangement wherein the cells are divided into groups of cells that, on both the fuel side and the oxidant side, have parallel gas feed within each group, but serial feed from group to group. That is to say, on both the cathode side and the anodes side, reactant fluid is fed to the cells of the first group of cells in parallel. On the anode side, the exhaust from the first group is collected and fed to the cells of a second group of cells in parallel. Flow on the anode side continues in like manner for as many groups as are included in the stack. On the cathode side, liquid water product is separated from the exhaust, and the remainder of the first group exhaust is collected and fed to the cells of the second group of cells in parallel. Flow and water product removal on the cathode side continues in like manner for as many groups as are included in the stack.  
           [0014]    The sequential feeding of the exhaust from one group of cells to the inlet of another group of cells presents a number of disadvantages as follows:  
           [0015]    1) The fluid feed composition varies from group to group as the fluid traverses the stack. For example, when air is used as an oxidant, the oxidant containing feed stream becomes leaner in oxygen and richer in nitrogen as the fluid flows from one group to the next. Consequently, the cells of downstream groups have relatively poorer performance than the cells of upstream groups. The same is true on the fuel side of the stack.  
           [0016]    2) travel path from stack inlet to stack outlet is longer and the amount of reactant fluid flowing through each cell group includes the amount needed for subsequent cell groups. As a result, the cell stack has a high pressure drop and requires a great deal of parasitic power to pump the fluid.  
           [0017]    3)  5  Since the cells of a given group are arranged for parallel fluid flow, nonuniformity among the individual cells results in nonuniform flow through the cells of a group. This problem has been discussed above.  
         SUMMARY OF THE INVENTION  
         [0018]    The above problems and shortcomings of the prior art are minimized, if not eliminated entirely, through the utilization of a device and methodology which embodies the concepts and principles of the present invention. Thus, the invention provides a fuel cell system that may preferably include a plurality of fuel cells, each including a fuel inlet, an oxidant inlet, a fuel product outlet and an oxidant product outlet. The fuel cell system of the invention also includes a common fuel supply line interconnecting the fuel inlets and a common oxidant supply line interconnecting the oxidant inlets.  
           [0019]    In one important aspect of the invention, the system may preferably include a fuel product purging mechanism that is coupled to the fuel product outlets and which incorporates fuel product valving structure operable to selectively and independently open each of the fuel product outlets. In another important aspect of the invention, the system may instead include an oxidant product purging mechanism that is coupled to the oxidant product outlets and which incorporates oxidant product valving structure operable to selectively and independently open each of said oxidant product outlets. In another important aspect of the invention, the system may ideally include a product purging mechanism that is coupled to the product outlets and which incorporates product valving structure operable to selectively and independently open each of said product outlets.  
           [0020]    An important feature of the preferred embodiment of the invention is the inclusion of a separate fuel product valve for each fuel product outlet and/or a separate oxidant product valve for each oxidant product outlet.  
           [0021]    In one highly preferred form of the invention, the fuel product valving structure include a stationary element and a moveable element. Preferably, the stationary element has an external surface and includes a plurality of fuel product ports that extend therethrough. Each of the fuel product ports preferably has one end that is connected to a respective fuel product outlet and a second end that opens through the external surface. The moveable element preferably has a valve surface that is in engagement with the external surface. The moveable element preferably has at least one fuel product collection conduit that extends therethrough and opens at said valve surface. The moveable element preferably is moveable relative to the stationary element so as to register said conduit with the second end of a selected one of said ports.  
           [0022]    In another highly preferred form of the invention, the oxidant product valving structure include a stationary element and a moveable element. Preferably, the stationary element has an external surface and includes a plurality of oxidant product ports that extend therethrough. Each of the oxidant product ports preferably has one end that is connected to a respective oxidant product outlet and a second end that opens through the external surface. The moveable element preferably has a valve surface that is in engagement with the external surface. The moveable element preferably has at least one oxidant product collection conduit that extends therethrough and opens at said valve surface. The moveable element preferably is moveable relative to the stationary element so as to register said conduit with the second end of a selected one of said ports.  
           [0023]    In a particularly preferred form of the invention, the system may include a common product valving structure. In this highly preferred form of the invention, the product valving structure includes a stationary element and a moveable element. Preferably, the stationary element has an external surface and includes a plurality of product ports that extend therethrough. Each of the product ports preferably has one end that is connected to a respective product outlet and a second end that opens through the external surface. The moveable element preferably has a valve surface that is in engagement with the external surface. The moveable element preferably has at least one oxidant product collection conduit and at least one fuel product collection conduit, which conduits extend therethrough and opens at said valve surface. The moveable element preferably is moveable relative to the stationary element so as to register said conduits with the second ends of selected ports. Ideally, in each of the cases outlined above, the elements are each disc-shaped, the surfaces are generally planar, and the moveable element is rotatable about an axis which is generally perpendicular to the plane of said surfaces. In this highly preferred form of the invention, the second ends of ports that are connected to fuel product outlets and said fuel product collection conduit may be arranged in a first circle that is concentric with said axis and has a first diameter, and the second ends of ports that are connected to oxidant product outlets and said oxidant product collection conduit may be arranged in a second circle that is concentric with said axis and has a second diameter that is different than said first diameter.  
           [0024]    In another highly important embodiment, the invention provides a method for operating a fuel cell system made up of a plurality of fuel cells, each including a fuel inlet, an oxidant inlet, an openable and closeable fuel product outlet and an openable and closeable oxidant product outlet. In this embodiment, the method may comprise supplying fuel to the fuel inlets, supplying an oxidant to said oxidant inlets, and selectively opening the outlets of a given cell to purge fuel product and oxidant product from said given cell. An important preferred feature of this embodiment involves selectively keeping the outlets of other cells in a closed condition while one or more outlets of the given cell are open. In accordance with the invention, the step of selectively opening the outlets of a given cell may be conducted in such a way that the outlets of the given cell are open at the same time so as to simultaneously purge fuel product and oxidant product from the given cell. Ideally, the invention provides such a method which comprises closing the outlets of said given cell and selectively opening the outlets of a second cell to purge fuel product and oxidant product from said second cell. In this latter situation, said step of selectively opening the outlets of a second cell may be conducted in such a way that the outlets of the second cell are open at the same time so as to simultaneously purge fuel product and oxidant product from the second cell.  
           [0025]    In a still further embodiment of the invention, a method for operating a fuel cell is provided wherein the method includes selectively opening the outlets of each cell of a given group of cells to purge fuel product and oxidant product from each of the cells of said given group of cells. In this aspect of the invention, the groups of cells may preferably be operated in the same manner as the individual cells discussed previously.  
           [0026]    In accordance with the concepts and principles of the invention, each of the cells has an identical travel path. Moreover, the total gas travel path for each gas portion is such that only a single cell is traversed from stack inlet to outlet. In addition, in accordance with the invention, no cell must support a sufficient flow to provide reactants to a downstream cell. Furthermore, when the invention is employed, both pressure drop across the cell stack and parasitic power consumption are much lower than has been thought possible in the past. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    [0027]FIG. 1 is a schematic diagram of a conventional PEM fuel cell showing the major components and electrode reactions for a hydrogen/oxygen system;  
         [0028]    [0028]FIG. 2 is a schematic diagram of a conventional PEM fuel cell stack with three cells connected electrically in series;  
         [0029]    [0029]FIG. 3 is a schematic diagram of a three-cell fuel cell stack which embodies the principles and concepts of the invention with common fuel and oxidant inlets and individual outlets for each cell in the stack;  
         [0030]    [0030]FIG. 4 is a block diagram of the fuel cell stack of FIG. 3;  
         [0031]    [0031]FIG. 5 is a schematic diagram of a fuel cell stack which embodies the principles and concepts of the invention with common fuel and oxidant inlets and outlets and individual outlets from each cell compartment connected internally to the common outlets through micro-electromechanical (MEM) valves;  
         [0032]    [0032]FIG. 6 is a block diagram of the fuel cell stack of FIG. 5;  
         [0033]    [0033]FIG. 7 is a schematic diagram of a device useful in connection with the fuel cell stack of FIGS. 3 and 4 to allow only one outlet or sets of outlets to open at a time; and  
         [0034]    [0034]FIGS. 8A and 8B are schematic diagrams of a MEM valve useful in connection with the fuel cell stack of FIGS. 5 and 6 to open and close the outlets individually. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    A conventional Proton-Exchange-Membrane (PEM) fuel cell  100  is shown schematically in FIG. 1. Fuel cells of this sort generally include an anode current collector  101  which acts as the negative terminal for the cell, a fluid distributor for the fuel  102 , an anode  103  where a fuel like hydrogen is oxidized to protons and electrons (H 2 →2H + +2e − ), a proton conducting membrane (membrane electrolyte)  104 , a cathode  105  where an oxidant, for example oxygen, reacts with protons and electrons to form water (½O 2 +2H + +2e − →H 2 O), a fluid distributor for the oxidant  106 , and a cathode current collector  107  which acts as the positive terminal for the cell. The electrical power generated by the fuel cell is used to power an electrical load  108 . The three-component unit of the cell which consists of the anode  103 , the proton conducting membrane  104  and the cathode  105  is generally referred to as a membrane-and-electrode assembly (MEA).  
         [0036]    As discussed above, fuel cells are generally arranged in a stack capable of producing a required voltage. The fuel cell stack may include a plurality of fuel cells connected in series. By way of example, a fuel cell stack  200 , which includes three cells  210 ,  220 ,  230  that are electrically connected in series is shown schematically in FIG. 2. It is to be noted here that those skilled in the fuel cell art will readily recognize that the stack may include any number of cells as may be required by an application and that the present invention is in no way limited to a three cell stack.  
         [0037]    The three-cell stack illustrated in FIG. 2 includes a negative current collector  201 , a monopolar fuel distributor plate  211  having fluid channels  214  for delivering fuel to the fuel side of fuel cell  210 , a MEA  212  for fuel cell  210 , a bipolar fluid distributor plate  213  that has fluid channels  215  on one side for delivering oxidant to the oxidant side of fuel cell  210  and fluid channels  216  on the other side for delivering fuel to the fuel side of fuel cell  220 , a MEA  222  for fuel cell  220 , another bipolar fluid distributor plate  223  with channels  225  on one side to distribute the oxidant to fuel cell  220  and channels  226  on the other side to distribute fuel to fuel cell  230 , a MEA  232  for fuel cell  230 , and a monopolar oxidant distributor plate having fuel distributor channels  234  for delivering fuel to the fuel side of fuel cell  230 . The total voltage of the stack is the sum of the individual cell voltages.  
         [0038]    A three-cell fuel cell stack  249 , which includes cells  210 ,  220  and  230 , and which embodies the principles and concepts of the invention, is shown in FIGS. 3 and 4. The cells  210 ,  220  and  230  are connected electrically in series in the same way as shown for stack  200  of FIG. 2. Cell  210  has a fuel inlet  261  (on the back side of plate  211 ), an oxidant inlet  262 , a fuel side product outlet  253  and an oxidant side product outlet  256 . Cell  220  has a fuel inlet  263  (on the back side of plate  213 ), an oxidant inlet  264 , a fuel side product outlet  254  and an oxidant side product outlet  257 . Cell  230  has a fuel inlet  265  (on the back side of plate  223 ), an oxidant inlet  266 , a fuel side product outlet  255  and an oxidant side product outlet  258 . Operationally, outlet  253  is in fluid communication with channels  214  (on the back side of plate  211 ), outlet  254  is in fluid communication with channels  216  (on the back side of plate  213 ), outlet  255  is in fluid communication with channels  226  (on the back side of plate  223 ), outlet  256  is in fluid communication with channels  215 , outlet  257  is in fluid communication with channels  225 , and outlet  258  is in fluid communication with channels  234 .  
         [0039]    The fluid distribution system for the three-cell stack of FIG. 3 includes a common fuel supply line  251  which interconnects fuel inlets  261 ,  263  and  265  and a common oxidant supply line  252  which interconnects oxidant inlets  262 ,  264  and  266 . The distribution system also includes distribution channels  214 ,  215 ,  216 ,  225 ,  226  and  234 , individual fuel side product outlets  253 ,  254  and  255 , and individual oxidant side product outlets  256 ,  257  and  258 . Outlets  253  through  258  may all be connected externally to a device  240  as illustrated in FIG. 8 which may allows only one outlet or one set of outlets to be open at any given time. The device  240 , which is explained in greater detail below, may be referred to hereinafter as an external sequential purging device or as a common product purging mechanism. In either case, the same includes valving structure operable to selectively and independently open the outlets of a given cell or group of cells. As illustrated in FIG. 4, device  240  may be arranged to accommodate purging of number of cells in addition to cells  210 ,  220  and  230 . For example device  240  as shown is arranged to accommodate purging of both fuel and oxidant sides of  8  cells. In practice, as would be readily recognized by the routineer in the fuel cell art, device  240  may be arranged to accommodate as many cells as might be included in a group or stack of cells.  
         [0040]    As illustrated in FIGS. 3 and 4, common fuel supply line  251  distributes fuel to fuel cells  210 ,  220  and  230  of stack  249 . That is to say, common fuel supply line  251  interconnects fuel inlets  261 ,  263  and  265  and thereby distributes fuel to fuel channel  214  on the back face of plate  211  for fuel cell  210  and to the fuel channels  216  and  226  on the back faces of plates  213  and  223  respectively for fuel cells  220  and  230 . Unused fuel and fuel side products from fuel cells  210 ,  220  and  230  are removed through fuel side outlets  253 ,  254  and  255  which are openable and closeable by virtue of being associated with device  240 . From the foregoing it is clear that the fuel sides of the cells  210 ,  220  and  230  are operated in parallel with regard to fuel side flow.  
         [0041]    As is also illustrated in FIGS. 3 and 4, common oxidant supply line  252  distributes oxidant to fuel cells  210 ,  220  and  230  of stack  249 . That is to say, common oxidant supply line  252  interconnects fuel inlets  262 ,  264  and  266  and thereby distributes oxidant to oxidant channel  215  on the front face of plate  213  for fuel cell  210  and to the oxidant channels  225  and  234  on the front faces of plates  223  and  233  respectively for fuel cells  220  and  230 . Unused oxidant and oxidant side products from fuel cells  210 ,  220  and  230  are removed through oxidant side outlets  256 ,  257  and  258  which are openable and closeable by virtue of being associated with device  240 . From the foregoing it is also clear that the oxidant sides of the cells  210 ,  220  and  230  are also operated in parallel with regard to oxidant side flow.  
         [0042]    As shown in FIG. 7, external sequential purging device  240  may preferably have two main components, namely a preferably disc shaped stationary plate  241  and a preferably disc shaped moveable plate  242 . For a three cell group, stationary plate  241  may preferably have three fuel side ports  270 ,  271  and  272  which extend therethrough and which are aligned on a circle  276 . Stationary plate  241  may also preferably have three oxidant side ports  273 ,  274  and  275  which extend therethrough and which are aligned on a circle  277 . Fuel side outlets  253 ,  254  and  255  carrying unused fuel and fuel side product from cells  210 ,  220  and  230  are respectively connected in fluid communication with the upstream ends of of ports  270 ,  271  and  272 , and oxidant side outlets  256 ,  257  and  258  carrying unused oxidant and oxidant side product are respectively connected in fluid communication with the upstream ends of ports  273 ,  274  and  275 .  
         [0043]    Disc  241  preferably has an external generally planar surface  290  which engages a generally planar valve surface  291  on the back of plate  242 . As can be appreciated viewing FIG. 7, ports  270  through  275  have respective downstream ends which open through surface  290 . Moveable plate  242  is preferably mounted for rotation relative to plate  241  about an axis  292  that is generally perpendicular to surfaces  290  and  291 . Circles  276  and  277  are concentric with axis  292 . Rotating plate  242  has two through-holes  280  and  281  which are aligned on circles  282  and  283  that match with circles  276  and  277  on stationary plate  241 . Circles  282  and  283  are also concentric with axis  292 . Hole  280  is connected in fluid communication with a common fuel side collection conduit  295  which serves as a purge outlet for the fuel outlets from the fuel cell stack  249 . Hole  281  is connected in fluid communication with a common oxidant side collection conduit  296  which serves as a purge outlet for the oxidant outlets from fuel cell stack  249 .  
         [0044]    During operation rotating plate  242  rotates so as to register hole  280  with the downstream end of a selected one of the ports  270 ,  271  and  272  and so as to register hole  281  with the downstream end of a selected one of the ports  273 ,  274  and  275 . Thus, each set of outlets opens in sequence to purge a given cell which is connected to such set of outlets of product and inert fluids. This purging also causes the given cell to be supplied with new reactants. The duration and frequency of purging for each cell or group of selected cells may be controlled by the rotation speed of the rotating plate and placement of ports thereon. It should be noted that it is not critical that the fuel side and the oxidant sides of a given cell be purged simultaneously. In this same regard, it may be necessary to purge one side of a cell more often than the other side. Thus, the arrangement of the common purging mechanism may need to be designed to accommodate the necessities of a given system insofar as the timing and sequencing of the various product outlets is concerned. Also it is to be recognized that it might be desirable in a given situation to include separate respective purging mechanisms for the fuel and oxidant sides of a cell or group of cells.  
         [0045]    As discussed above, FIG. 4 is a block diagram of the fluid distribution in the fuel and oxidant sides of each fuel cell for a fuel cell stack configured similarly to that shown in FIG. 3. Unused oxidant and/or fuel in the respective product stream from the sequential purging device  240  can be recovered in a respective condenser  250 A,  250 B and recycled to the fuel cell stack for reuse. For example, when pure oxygen is used as an oxidant, the oxidant side product stream will generally contain unused oxygen and water. The water may be condensed, separated from the unused oxygen and used to humidify the fuel stream or discarded, while the unused oxygen may be recycled back to the fuel cell stack for reuse. When air, which is readily available is used as an oxidant, water may be separated from the oxidant product stream for reuse and the product stream, which includes unused oxygen and nitrogen and other inert materials, may simply be purged back into the atmosphere. Similarly, when pure hydrogen is used as a fuel, the fuel product stream may contain both unused hydrogen and water. Since water is needed to humidify the hydrogen stream there generally is no need to separate water from the fuel product stream. The fuel product stream may be recycled directly back to the fuel cell stack for reuse. However, when a non-pure hydrogen gas, such as, for example, a reformate gas containing hydrogen, nitrogen and carbon dioxide is used as a fuel, it may be desirable to separate water from the fuel side product stream and recycle the recovered water for reuse. Unused hydrogen along with inert gases like nitrogen and carbon dioxide may be sent to a gas burner to generate heat or additional useful work.  
         [0046]    Another three-cell fuel cell stack which embodies the principles and concepts of the invention is shown schematically in FIG. 5 where it is identified by the reference numeral  350 . As shown, stack  350  includes cells  310 ,  320  and  330 . Again, the number of cells in the stack is variable and dependent upon the voltage required for a given application. The cells of stack  350 , which are supplied with reactants via common fuel inlet line  351  and common oxidant inlet line  352 , are essentially the same as the cells of stack  249 , except for the inclusion of an individual MEM valve  340  in each of the fuel side and oxidant side outlets  353 ,  354 ,  355 ,  356 ,  357  and  358 . That is to say, the fuel side outlet and the oxidant side outlet of each cell has its own individual openable and closeable MEM valve  340 . FIG. 8A depicts valve  340  in an open condition while FIG. 8B depicts valve  340  in a closed condition. The MEM valves are connected externally to an electronic controller  361  by electrical wiring  360 . With the exception of the MEM valves  340 , the other components of the cell stack  350  are essentially the same as those described for the fuel cell stack  249  of FIG. 3.  
         [0047]    Sequential purging of each cell or group of selected cells is achieved by sequentially opening the MEM valves of one cell or group of selected cells while keeping those of other cells or groups of cells closed. Purging frequency and duration of the opening and closing of valves  340  may be controlled by an electronic controller using predetermined time intervals and/or time intervals determined empirically for a given cell stack. It should also be noted that the oxidant side of a given cell may need purging more often than the fuel side, or vice versa. The arrangement of stack  350  facilitates such operation. Downstream from the valves  340 , the oxidant side outlets  356 ,  357  and  358  are connected in fluid communication with a common excess oxidant and oxidant side collection conduit  396 . Similarly the fuel side outlets  353 ,  354  and  355  on the back faces of plates  301 ,  311  and  321  are connected in fluid communication with a common excess fuel and fuel side collection conduit  395 .  
         [0048]    [0048]FIG. 6 is a block diagram of the fluid distribution in the fuel and oxidant sides of each fuel cell for a fuel cell stack configured similarly to that shown in FIG. 5. With the exception of the inclusion of MEM valves between the fuel and oxidant outlets of each cell and the common fuel and oxidant outlets, all other features are essentially the same as those shown in the FIG. 4 block diagram for the fuel cell stack with external sequential purging device.