WO2008019503A1

WO2008019503A1 - Method for operating a fuel cell and a fuel cell stack

Info

Publication number: WO2008019503A1
Application number: PCT/CA2007/001443
Authority: WO
Inventors: Hao Tang; Dingrong Bai; David ELKAÏM; Jean-Guy Chouinard
Original assignee: Hyteon Inc.
Priority date: 2006-08-18
Filing date: 2007-08-20
Publication date: 2008-02-21

Abstract

There is described a method of operating a fuel cell comprising: inputting an anode reactant into an anode inlet and a cathode reactant into a cathode inlet of the fuel cell; flowing the anode reactant and the cathode reactant on respective flow field plates over a network of flow channels bounded by a series of passages; decreasing at least one of anode reactant water partial pressure and cathode reactant water partial pressure to adjust a water transfer rate from a cathode to an anode across a membrane; chemically reacting the anode reactant and the cathode reactant on catalysts in order to generate an electrical current; and outputting un-reacted anode reactant and un-reacted cathode reactant through an anode outlet and a cathode outlet, respectively.

Description

METHOD FOR OPERATING A FUEL CELL AND A FUEL CEIiL STACK

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application, claims priority of US Provisional Patent Application filed on August 18, 2006 and bearing serial number 60/838,437.

TECHNICAL FIELD

[0002] The present invention relates to the field of fuel cells, and more particularly to a method of operating a fuel cell in a robust way to optimize its lifetime by adjusting water transfer between the anode and the cathode .

BACKGROUND OF THE INVENTION

[0003] Normally, Proton Electrolyte Membrane (PEM) fuel cell stacks are operated at -100% relative humidity

(RH) to enhance Membrane Electrode Assembly (MEA) and stack lifetime. This is especially important for stationary applications, where five to ten years stack lifetime is required. However, to improve fuel cell system efficiency, it is desirable to have an anode reactant stoichiometry in the range of 1.1 to 1.5.

[0004] One of the common problems for PEM fuel cell stacks operating at low anode reactant stoichiometry is anode flooding, which causes stack cell voltage variation, performance loss, and more seriously, system shutdown. Anode flooding is caused mainly by (1) low anode reactant flow rate at the outlet portions due to high hydrogen utilization and (2) 0 to 40% of production water can be transferred from cathode to anode during fuel cell system operation.

[0005] Therefore, there is a need to provide improved methods for operating fuel cells and stacks that will optimize water transfer between anode and cathode in a fuel cell stack.

SUMMARY

[0006] To limit the water transfer from cathode to anode, new MEA/stack design and operation strategies are disclosed herein.

[0007] In accordance with a broad aspect of the present invention, there is provided a method of operating a fuel cell comprising: inputting an anode reactant into an anode inlet and a cathode reactant into a cathode inlet of the fuel cell; flowing the anode reactant and the cathode reactant on respective flow field plates over a network of flow channels bounded by a series of passages; varying at least one of anode reactant water partial pressure and cathode reactant water partial pressure to adjust a water transfer rate from a cathode to an anode across a membrane electrode assembly; chemically reacting the anode reactant and the cathode reactant on catalysts in order to generate an electrical current; and outputting unreacted anode reactant and unreacted cathode reactant through an anode outlet and a cathode outlet, respectively.

[0008] In accordance with another broad aspect of the present invention, there is provided a method of operating a fuel cell stack comprising: inputting an anode reactant into an anode inlet and a cathode reactant into a cathode inlet of the fuel cell stack; flowing the anode reactant and the cathode reactant into a plurality of fuel cells in the fuel cell stack on respective flow field plates over a network of flow channels bounded by a series of passages,- varying at least one of anode reactant water partial pressure and cathode reactant water partial pressure to adjust a water transfer rate from a cathode to an anode across a membrane electrode assembly; chemically reacting the anode reactant and the cathode reactant using catalysts in order to generate an electrical current; and outputting unreacted anode reactant and unreacted cathode reactant through an anode outlet and a cathode outlet, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:

[0010] Fig. 1 is a graph showing reactant water partial pressure versus location along the fuel cell flow field;

[0011] Fig. 2 is a flow chart of a method of operating a fuel cell;

[0012] Fig. 3 is a schematic of an anode plate with water-rich zones identified; [0013] Fig. 4 is a schematic of a cathode plate with water-rich zones identified;

[0014] Figs. 5a, 5b, and 5c show cross-sectional views of grooves in a flow field plate having different sizes;

[0015] Figs. 6a, 6b and 6C illustrate the connection of two flow field channels having different cross- sectional size.

[0016] Fig. 7 illustrates the connection of two series of flow field channels having different cross-sectional size;

[0017] Fig. 8 is a flow chart illustrating the method of operating a fuel cell stack; and

[0018] Pig. 9 illustrates a fuel cell stack with two end cells and a middle cell .

[0019] It will be noted that throughout the appended drawings, like features are identified by like reference numerals.

DETAILED DESCRIPTION

[0020] Typically, homogenous proton exchange membranes and uniform-cross sections of flow fields are used for common stack designs. In the case of co-flow flow field stack designs, the water content is higher (production water + reactant saturation water) at the cathode outlet portion and consequently some of the cathode water is transferred from cathode to anode. In addition to the significant increase in water partial pressure due to consumption of hydrogen in the anode side, the water transferred from the cathode will further increase anode reactant water saturation temperature (dew point) so that water flooding may occur in the anode. The water transfer rate from cathode to anode can be mathematically expressed as :

[0021] Where Q is water transfer rate, D is water diffusion coefficient in the membrane, S is water solubility in the membrane, L is membrane thickness, and Pi and P₂ are H₃O partial pressures; from the cathode and anode sides, respectively. One way to reduce the water transfer from the cathode side to the anode side is to reduce the partial pressure difference (Pi-P₂) • This can be achieved by at least one of reducing the water partial pressure P_x at the cathode side and increasing the water partial pressure P₂ at the anode side. By varying the local anode/cathode partial pressure difference (P1-P2) , it is possible to adjust the water transfer rate from cathode to anode at certain locations of the flow field or for some individual cells of the stack and thus avoid/reduce anode flooding- at water rich, regions and/or humidify the anode reactant at water-lean regions .

[0022] In one embodiment, the local water partial pressure difference is altered by varying the flow channel cross sectional area. At the moment when the reactant fluid flows from one passage of a first cross. - sectional area to a next passage of a second cross- sectional area, there will be a decrease in local pressure, in addition to a variation in the reactant flow rate. As such the local pressure difference between anode and cathode can be varied.

[0023] Fig. 1 presents the water partial pressure of reactants as a function of the location along a fuel cell . Line 11 represents the water partial pressure of the anode reactant along flow field channels having a constant cross-section. Line 10 shows the water partial pressure in which a drop in anode pressure occurs locally along the flow field plate at location 14. This drop in pressure can be caused by the transition from a first flow area to a second flow area, the two areas being differently sized. Line 13 represents the water partial pressure of the cathode reactant along flow field channels having a constant cross-section. Line 12 shows the water partial pressure in which a drop in cathode pressure occurs locally along the flow' field plate at location 15. This drop in pressure can be caused by the transition from a first flow area to a second flow area, the two areas being differently sized.

[0024] For the purpose of this example, region 14 represents an anode water-lean region and the pressure fluctuation for the anode water partial pressure occurs in this region. Region 15 represents a cathode water-rich region and the pressure fluctuation occurs in this region. Since the drop is caused by the transition from one flow area to another, the pressure recovers to some extent once the turbulent period caused by the change of partial pressures is passed. However, it should be noted that the recovery is generally not complete, i.e. the partial water pressure does not: return to the same level as it would have been if the drop in pressure had not occurred. This is illustrated in the graph of figure 1. In order to obtain a post-recovery pressure higher than 13 or 11, certain operating parameters of the fuel cell may be modified.

[0025] In one embodiment of the method described herein, the reduction of the water transfer from cathode to anode in a fuel cell is performed by increasing the local pressure of the anode reactant at preferable location (s) of the anode flow field and/or decreasing the local pressure of the cathode reactant at the corresponding location (s) of the cathode flow field. As illustrated in Fig. 2, the anode and cathode reactants enter a fuel cell via an anode inlet and cathode inlet, respectively. The fuel cell comprises at least an anode flow field plate, a cathode flow field plate and an MEA in between. The MEA comprises, in one embodiment, an anode gas diffusion layer, a cathode gas diffusion layer, an anode catalyst, a cathode catalyst and a membrane in between. In the flow field plates, the corresponding reactant flows over a network of flow channels bounded by a series of passages having grooves which can be parallel . The flow partial pressures of the anode and cathode reactants are changed as the reactants flow from one passage to another passage of the corresponding flow field plate. The local pressure of the anode reactant is increased as the anode reactant flows along the anode flow field plate and the local pressure of the cathode reactant can be decreased as the cathode reactant flows along the cathode flow field plate. So the P1-P₂ is altered with the variations of anode and cathode partxal pressures. The reactants diffuse in their corresponding gas diffusion layer until they reach the catalyst layers and they chemically react to generate an electrical current and the water (i.e. the production water). The production water adds to the humidifying water present in the reactants. Furthermore, the unused anode reactant and cathode reactant exit the fuel cell by an anode outlet and a cathode outlet, respectively. The decrease in water partial pressure difference of (P1-P2) between anode side and cathode side reduces the water back diffusion from the cathode side to the anode side.

[0026] In one embodiment of the method, only the local pressure of the anode reactant is varied.

[0027] In another embodiment of the present method, only the local pressure of the cathode reactant is varied.

[0028] In still another embodiment of the present method, both pressures of anode and cathode are varied, but respectively to different extents.

[0029] It should be understood that changing the local flow area of anode flow channels and/or cathode flow channels during fuel cell operation as discussed above is only an example of optimizing stack water management. In another embodiment of the method, under some special fuel cell system operation conditions, other appropriate means can also be applied to alter the water partial pressures. such as decreasing the anode water partial pressure and/or increasing the cathode water partial pressure, in order to optimize stack water management. For example, this may be the case when it is desired to stimulate water transfer from cathode to anode for anode humidifiσation purpose.

[0030] In an embodiment of the method, the partial pressure of at least one of the reactants is varied in the water-rich regions of the fuel cell. For example, in the anode flow field plate 20, the water-rich regions may comprise the anode outlet region 28 (i.e. the region substantially adjacent to the anode outlet 26) and the corner regions such as 32, as illustrated in Fig. 3. In the cathode flow field plate 50, the water-rich regions may comprise the cathode outlet region 58 (i.e. the region substantially adjacent to the cathode outlet 56) and the corners regions βuch as 62, as illustrated in Fig. 4. The configurations of the anode flow field plate 20 and the cathode flow field plate 50 of Figs. 3 and 4, respectively, are purely for exemplary purposes and can vary without departing from the scope of the present method.

[0031] In an embodiment of the present method, the increase of the pressure of the anode reactant along the flow field is achieved by flowing the anode reactant from a first passage of flow channels to a second passage of flow channels having a different cross-section area than the flow channels of the first passage. [0032] In an embodiment of the present method, the decrease of the pressure oZ the cathode reactanu dloi-cj the flow field is achieved by flowing the cathode reactant from a first passage of flow channels to a second passage of flow channels having a different cross- section than the flow channels of the first passage.

[0033] Figs. 5a, 5b, and 5c illustrate comparative cross-sections for grooves of a flow field plate. Fig. 5a illustrates a flow field plate 102 having grooves 104 of a given depth 106. Fig. 5b illustrates a flow field plate 108 having grooves 110 of a depth 112 larger than the depth 106 of the grooves 104. Flowing a cathode reactant from a passage of channels as illustrated in Fig 5a to a passage of flow channels as illustrated in Fig 5b may decrease the cathode reactant local partial pressure.

[0034] Fig 5c illustrates a flow field plate 114 having grooves 116 of a depth 118 smaller than the depth 106 of the grooves 104. Flowing an anode reactant from a passage of channels as illustrated in Fig 5a to a passage of flow channels as illustrated in Fig 5c may increase the anode reactant partial pressure.

[0035] According to the present method, an anode flow field plate can comprise flow channels of a smaller cross-sectional size in at least one of the water-rich regions of the fuel cell and/σr a cathode flow field plate can comprise flow channels of a larger cross- sectional size in at least one of the water-rich regions of the fuel cell. [0036] It should be understood that the width of the grooves Try also vary. Λ

oZ different

and depths is also possible.

[0037] It should also be understood that channels of any form having a varying cross-sectional size can be utilized in the present method.

[0038] In one embodiment of the present method, the fluid reactant flows from a first passage of flow channels to a second passage of flow channels, each channel of the second passage being fluidly connected to a corresponding channel of the first passage.

[0039] Fig. 6a illustrates the connection of a flow channel 200 of a first passage 210 to a flow channel 201 of a second passage 212. The flow channel 200 of width 202 directly emerges into the flow channel 201 of width 204 being larger than the width 202 of the flow channel 201.

[0040] If the flow channels illustrated in Fig 6a are located on the cathode flow field plate of a fuel cell and if the first passage 210 is located in a region of normal water content while the second passage 212 is located in a water-rich region, then the cathode reactant flows in the direction of arrow 206. As the cathode reactant flows from the flow channel 200 to the flow channel 201, its partial pressure may be decreased.

[0041] If the flow channels illustrated in Fig 6a are located on the anode flow field plate of a fuel cell and if the second passage 212 is located in a region of normal water content where the first passage 210 is locatαd in α water -rich region, cϊiari the anode reactaiiL flows in the direction of arrow 208. As the anode reactant flows from the flow channel 201 to the flow channel 200, its partial pressure may be increased.

[0042] Fig. 6b illustrates the connection of a flow channel 220 of a first passage 236 to a flow channel 222 of a second passage 238. The flow channel 220 of width 230 and the flow channel 222 of width 232 are connected by a connection channel 224 of width 234 being inferior to the width 232 of the flow channel 222 but superior to the width 230 of the flow channel 220. It should be understood that the connection region 240 may comprise several interconnected flow channels of which the width decreases from the flow channel 222 to the flow channel 220.

[0043] If the flow channels illustrated in Fig 6b are located in the cathode side of a fuel cell and if the first passage 236 is located in a region of normal water content where the passage 238 is located in a water-rich region, then the cathode reactant flows in the direction of arrow 226. As the cathode reactant flows from the flow channel 220 to the flow channel 222, its partial pressure may be decreased.

[0044] If the flow channels illustrated in Fig 6b are located on the anode flow field plate of a fuel cell and the passage 238 is located in a region of normal water content where the passage 236 is located in a water-rich region, the anode reactant flows in the direction of arrow 228. As the anode reactant flows from the flow channel 222 to the flow channel 220, its partial pressure may be increased.

[0045] Alternatively, as illustrated in Fig. 6c, the connection region 240 may comprise a single flow channel 242 connected to the flow channels 220 and 222. The flow channel 242 has a width 244 that gradually decreases from the width 232 of the flow channel 222 to the width 230 of the flow channel 220.

[0046] It should be understood that the depths of the grooveβ may also vary, which varies the depth of the flow channels. A combination of different widths and depths of flow channels is also possible.

[0047] In another embodiment of the method, the fluid reactant flows from a first passage of flow channels to a second passage of flow channels and at least two flow channels of the second passage are fluidly connected to at least two flow channels of the first passage as illustrated in Fig. 7. The two series of flow channels 300 and 302 are separated by a ridge 316 which comprises a chamber 304. The chamber 304 fluidly connects a series of flow channels 300 of width 306 to a series of flow channels 302 of width 308 being larger than the width 306 of the flow channels 300.

[0048] It should be understood that a chamber 304 of any form and size can be used without departing from the present method. [0049] If the system of flow channels illustrated in Fig 7 is located on. the cathode plate of a fuel cell and if the passage 318 represents a region of normal water content where the passage 314 represents a water-rich region, the cathode reactant flows in the direction of arrow 310. As the cathode reactant flows from the flow channels 300 to the flow channels 302, its partial pressure may be decreased.

[0050] If the system of flow channels illustrated in Fig 7 is located on the anode plate of a fuel cell and if the passage 314 represents a region of normal water content where the passage 318 represents a water-rich region, the anode reactant flows in the direction of arrow 312. As the anode reactant flows from the flow channels 302 to the flow channels 300, its water partial pressure may be increased.

[0051] It should be understood that the number of larger cross-section flow channels 302 may be equal to the number of smaller cross-section channels 300. Alternatively, the number of larger cross-section flow channels 302 may be superior to the number of smaller cross-section flow channels 300.

[0052] It should be understood that the present method can be applied to fuel cells having non-grooved flow field plates. These plates do not have flow channels in the one or more passages present, each passage being simply a recess. In this case, the flow rate of the reactant can vary by varying the depth and/or the area of the passages/recess of the corresponding flow field plate. The different passages of a non-grooved flow field plate may be filled ^τvith electric conductive material Lo enhance the conductance of electrons and the mechanical support of the membrane.

[0053] In an embodiment of the method, the reduction of the water transfer in a fuel cell stack is performed by decreasing the water partial pressure of the anode reactant and increasing the water partial pressure of the cathode reactant along the flow field in the end fuel cells. As illustrated in Fig. 8, the anode and cathode reactants are injected into the fuel cell stack through their corresponding inlets. The reactants flow in their respective flow field plate of each fuel cell in the fuel cell stack. The pressure of the anode reactant and/or the pressure of the cathode reactant are varied in at least one water-rich region of the top fuel cell and the bottom fuel cell. The reactants chemically react to generate an electrical current and to the water (i.e. the production water) . The production water adds to the humidifying water present into the reactants. Furthermore, the un- reacted anode reactant and cathode reactant exit the fuel cell stack at their corresponding outlet. The increase of the water partial pressure of the anode reactant and the decreasing of the water partial pressure of the cathode reactant in the water-rich regions of the end fuel cells will reduce the water transfer from the cathode side to the anode side in the end fuel cells in comparison to the fuel cell substantially at the center of the fuel cell stack. [0054] Aa illustrated in Fig. 9, a fuel cell stack comprise", a top fuel cell 350, z. bottom fuel cell 352 α.ul a plurality of fuel cells 354 between the top fuel cell 350 and the bottom fuel cell 352. Each fuel cell 350, 352, 354 comprises a cathode flow field plate 360, 370, 380, an anode flow field plate 362, 372, 382, a cathode 364, 374, 384, an anode 366, 376, 386, and a membrane 368, 378, 388 therebetween.

[0055] The cathode flow field plates 360 and 370 of the end fuel cells 350, 352 are provided with flow channels of larger cross-sectional size in at least one water-rich region of the flow field plates 360, 370 in comparison to the flow channels in the corresponding region of the cathode flow field plate 380 in the middle fuel cell 354. The anode flow field plates 362 and 372 of the end fuel cells 350, 352 are provided with flow channels of smaller cross-sectional size in at least one water-rich region of the flow field plates 362, 372 in comparison to the flow channels in the corresponding region of the anode flow field plate 382 in the middle fuel cell 354.

[0056] As a result, the local partial pressure of the cathode reactant is decreased and/or the local partial pressure of the anode reactant is decreased in the water- rich regions of the end fuel cells 350, 352 in comparison to the flow rate of the anode and cathode reactants in the middle fuel cell 354. Hence, the water transfer form cathode side to anode side is reduced in the end fuel cells 350, 352. [0057] In an embodiment of the method, the local part: -^~* procures of the an_^Je αnJ cathode reacL_^ats along the flow fields are different from the corresponding local partial pressures in at least one water-rich region of the fuel cells located at the top of the fuel cell stack and/or at least one of the fuel cells located at the bottom of the fuel cell stack.

[0058] In one embodiment of the present method, the local partial pressure of the cathode reactant gradually increases in at least one water-rich region from the middle cell to at least one end cell and the local partial pressure of the anode reactant gradually decreases in at least one water-rich region from the middle fuel cell to at least one end cell,

[0059] The present method can be applied to a fuel cell stack comprising non-grooved flow field plates. The non-grooved flow field plates do not have flow channels in the one or more passages, each passage being simply a recess. At least one passage of a flow field plate can have a varying depth and/or area throughout the fuel cell stack. The varying depth and/or area flow field plate can be a cathode flow field plate and/or an anode flow field plate. The non-grooved flow field plate may only comprise a single recess corresponding to a single passage between the inlet and the outlet of the flow field plate. The localized partial pressure of a reactant can vary- throughout the fuel cell stack by varying the depth and/or the area of the recess from one flow field plate to another . [0060] The passages of a non-grooved flow field plate may be filled with, electric conductive material to enhance the conductance of electrons and the mechanical support of the MEA. The method also applies to fuel cell stacks comprising a combination of grooved flow field plates and non-grooved flow field plates.

[0061] In one embodiment, only the local partial pressure of the cathode reactant may vary according to water-rich regions through the fuel cell stack. Alternatively, only the local partial pressure of the anode reactant may be vary according to water-rich regions through the fuel cell stack. Also alternatively, the local partial pressures of both the anode and cathode reactants may vary according to water-rich regions through the fuel cell stack.

[0062] It should be understood that any technique permitting the decrease or the increase of the flow rate of a fluid reactant can be used in the present method. The reactant local partial pressure may vary without taking into account the water-rich region but simply in accordance with the position of the fuel cell in the stack.

[0063] It can be appreciated that any technique enabling the variation of the water partial pressure of a reactant known to a person skilled m the art can be used to adjust the water transfer rate across the membrane/MEA of a fuel cell, thereby optimizing water management in an individual fuel cell or in a fuel cell stack. [0064] The embodiments of the invention described above are intended to be ε::α.πplary only. The scope o∑ the invention is therefore intended to be limited solely by the scope of the appended claims .

Claims

I /WE CLAIM :

1. A method of operating a fuel cell comprising: inputting an anode reactant into an anode inlet and a cathode reactant into a cathode inlet of said fuel cell; flowing said anode reactant and said cathode reactant on respective flow field plates over a network of flow channels bounded by a series of passages; varying at least one of anode reactant water partial pressure and cathode reactant water partial pressure to adjust a water transfer rate from a cathode to an anode across a membrane electrode assembly; chemically reacting said anode reactant and said cathode reactant on catalysts in order to generate an electrical current ; and outputting unreacted anode reactant and unreacted cathode reactant through an anode outlet and a cathode outlet, respectively.

2. A method as claimed in claim 1, wherein said varying at least one of anode reactant water partial pressure and cathode reactant water partial pressure comprises at least one of decreasing said partial pressure of said anode reactant and increasing said partial pressure of said cathode reactant .

3. A method as claimed in claim 1, wherein said varying at least one of anode reactant , water partial pressure and cathode reactant water partial pressure comprises at least one of increasing said partial pressure of said anode reactant and decreasing said partial pressure of said cathode reactant .

4. A, method as claimed in claim 3, wherein said flow field plates have grooves in said passages, and said at least one of increasing said partial pressure of said anode reactant and decreasing said partial pressure of said cathode reactant comprises flowing at least one of said anode reactant and said cathode reactant from a first passage to a second passage having grooves of a different cross-section than grooves of said first passage.

5. A method as claimed in claim 4, wherein said increasing a partial pressure of said anode reactant comprises flowing said anode reactant from a first passage to a second passage having grooves of a smaller cross-section than grooves of said first passage.

6. A method as claimed in claim 4, wherein said decreasing a partial pressure of said cathode reactant comprises flowing said cathode reactant from a first passage to a second passage having grooves of a larger cross-section than grooves of said first passage.

7. A method as claimed in claim 4, wherein said at least one of increasing a partial pressure of said anode reactant and decreasing a partial pressure of said, cathode reactant comprises flowing said anode reactant from a first passage of an anode plate to a second passage of said anode plate having grooves of a smaller cross-section than grooves of said first passage of an anode plate and flowing said cathode reactant from a first passage of a cathode plate to a second passage of said cathode plate having grooves of a larger cross- section than grooves ^' of said first passage of a cathode plate .

8. A method as claimed in claim 5 or 7, wherein said flowing said anode reactant from a first passage to a second passage comprises flowing said anode reactant from, said first passage to said second passage having grooves of a smaller width than grooves of said first passage.

9. A method as claimed in any one of claims 5, 7 and 8, wherein said flowing said anode reactant from a first passage to a second passage comprises flowing said anode reactant from said first passage to said second passage having grooves of a smaller height than groov.es of said first passage.

10. A method as claimed in claim 6 or 7 , wherein said flowing said cathode reactant from a first passage to a second passage comprises flowing said cathode reactant from said first passage to said second passage having grooves of a larger width than grooves of said first passage.

11. A method as claimed in any one of claims 6 and 7, wherein said flowing said cathode reactant from a first passage to a second passage comprises flowing said cathode reactant from said first passage to said second passage having grooves of a larger height than grooves of said first passage.

12. A method as claimed in claim 4, wherein said increasing a partial pressure of said anode reactant comprises flowing said anode reactant across said at least one of said series of passages having a gradually decreasing cross-section.

13. A method as claimed in claim 4, wherein said decreasing a partial pressure of said cathode reactant comprises flowing said cathode reactant across said at least one of said series of passages having a gradually increasing cross-section.

14. A method of operating a fuel cell stack comprising: inputting an anode reactant into an anode inlet and a cathode reactant into a cathode inlet of said fuel cell stack; flowing said anode reactant and said .cathode reactant into a plurality of fuel cells in said fuel cell stack on respective flow field plates over a network of flow channels bounded by a series of passages; varying at least one of anode reactant water partial pressure and cathode reactant water partial pressure to adjust a water transfer rate from a cathode to an anode across a membrane electrode assembly; chemically reacting said anode reactant and said cathode reactant using catalysts in order to generate an electrical current ; and outputting un-reacted anode reactant and un-reacted cathode reactant through an anode outlet and a cathode outlet, respectively.

15. A method as claimed in claim 14, wherein said varying at least one of anode reactant water partial pressure and cathode reactant water partial pressure comprises at least one of decreasing said water partial pressure of said anode reactant and increasing said water partial pressure of said cathode reactant across flow field plates of at least one end cell of said fuel cell stack.

16. A method as claimed in claim 15, wherein said varying at least one of anode reactant water partial pressure and cathode reactant water partial pressure comprises at least one of increasing a water partial pressure of said anode reactant and decreasing a water partial pressure of said cathode reactant .

17. A method as claimed in claim 16, wherein said flow field plates have grooves in said passages, and said at least one of increasing a water partial pressure of said anode reactant and decreasing a water partial pressure of said cathode reactant comprises flowing at least one of said anode reactant and said cathode reactant from a first passage to a second passage having grooves of a different cross-section than grooves of said first passage.

18. A method as claimed in claim 17, wherein said increasing a water partial pressure of said anode reactant comprises gradually increasing said water partial pressure of said anode reactant from a middle fuel cell to at least one end cell.

19. A method as claimed in claim 17, wherein said decreasing a water partial pressure of said cathode reactant comprises gradually decreasing said water partial pressure of said cathode reactant from a middle fuel cell to at least one end cell.

20. A method as claimed in claim 17, wherein said increasing a flow resistance of said anode reactant comprises flowing said anode reactant from a first passage to a second passage having grooves of a smaller cross-section than grooves of said first passage.

21. A method as claimed in claim 17, wherein said decreasing a water partial pressure of said cathode reactant comprises flowing said cathode reactant from a first passage to a second passage having grooves of a larger cross-section than grooves of said first passage.

22. A method as claimed in claim 17, wherein said at least one of increasing a water partial pressure of said anode reactant and decreasing a water partial pressure of said cathode reactant comprises flowing said anode reactant from a first passage of an anode plate to a second passage of said anode plate having grooves of a smal.ler cross-section than grooves of said first passage of an anode plate and flowing said cathode reactant from a first passage of a cathode plate to a second passage of said cathode plate having grooves of a larger cross-section than grooves of said first passage of a cathode plate .

23. A method as claimed in claim 18 or 19, wherein said flowing said anode reactant from a first passage to a second passage comprises flowing said anode reactant from said first passage to said second passage having grooves of a smaller width than grooves of said first passage.

24. A method as claimed in any one of claims 18, 20 and 21, wherein said flowing said anode reactant from a first passage to a second passage comprises flowing said anode reactant from said first passage to said second passage having grooves of a smaller height than grooves of said first passage.

25.' A method as claimed in claim 19 or 20, wherein said flowing said cathode reactant from a first passage to a second passage comprises flowing said cathode reactant from said first passage to said second passage having grooves of a larger width than grooves of said first passage.

26. A method as claimed in any one of claims 19 .or 20, wherein said flowing said cathode reactant from a first passage to a second passage comprises flowing said cathode reactant from said first passage to said second passage having grooves of a larger height than grooves of said first passage.