GB2628145A - Bidirectional (staggered) bipolar plate pattern for fuel cell cooling - Google Patents

Abstract

A fuel cell bipolar plate having anode and cathode plates and a plurality of gaseous media coolant flow channels between them. At least one of the coolant channels varies in size between its inlet 112 and outlet 110 and includes an expansion area. At least one cooling channel has an inlet smaller than its outlet. The channels may have deflection barriers 114 configured to force the gas coolant from its initial path within the channel. The barriers may be staggered between the inlet and outlet and be of varying lengths and positions. The pattern of barriers may provide a gradual increase of heat rejection efficiency between the inlets and outlets. A fuel cell stack may comprise a plurality of the bipolar plates and the stack may be used in a vehicle such as a fuel cell powered aircraft.

Description

BIDIRECTIONAL (STAGGERED) BIPOLAR PLATE PATTERN FOR FUEL CELL

COOLING

FIELD OF THE DISCLOSURE

[0001] The present disclosure relates to fuel cells and methods for making the same. The disclosure has particular utility in the creation of high temperature metal composite bipolar plates (BPPs) for high temperature proton exchange membrane (HT-PEM) fuel cells for use in fuel cell powered vehicles including aircraft, and will he described in connection with such utility, although other utilities are contemplated including, by way of example, formation of batteries and other electronic devices.

BACKGROUND AND SUMMARY

[0002] This section provides background information related to the present disclosure which is not necessarily prior art. This section provides a general summary of the disclosure and is not a comprehensive disclosure of its full scope or all its features.

[0003] A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. A typical hydrogen fuel cell includes a proton exchange membrane (PEM), that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce hydrogen protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations: H2 4 21-r-F2e-at the anode of the cell, and Equation I 02+41-r-F4e-2H20 at the cathode of the cell. Equation 2 [0004] Proton Exchange Membrane (PEM) fuel cells are made from several layers of different materials. The heart of a PEM fuel cell is the membrane electrode assembly (MEA), which includes a PEM membrane, catalyst layers, and gas diffusion layers (GDLs).

[0005] Fig. 1 is a perspective view of a conventional PEM fuel cell 10. Fuel cell 10 includes a Proton Exchange Membrane (PEM) 12, typically formed in a specially treated polymer material that conducts only positive charged ions, and blocks electrodes.

[0006] Catalyst layers are provided on both sides of the PEM 12 -an anode catalyst layer 14 on one side, and a cathode catalyst layer 16 on the other. Gas Diffusion Layers (GDLs) 18, 20 sit to the outside of the catalyst layers 14, 16 and facilitate transport of reactants into the catalyst layers, as well as removal of the product water. The PEM 12, catalyst layers 14, 16 and the GDLs 18, 20 together make up the so-called Membrane Electrode Assembly (MEA) 22. The MEA 22 is the part of the fuel cell where power is produced.

[0007] Each individual MEA 22 produces less than one volt under typical operating condition, but most applications require higher voltages. Therefore, multiple MEAs 22 usually are connected in series by stacking them on top of one other to provide a usable output voltage. Each cell in the stack is sandwiched between two bipolar plates (BPPs) anode plate 24 and cathode plate 26 to separate it from neighboring cells. These plates 24, 26, which may be made of metal, carbon, or composites, provide electrical conduction between cells, as well as providing physical strength to the stack. The surfaces of the plates typically contain flow channels 28, 30 machined or stamped into the plates 24, 26 to allow gases to flow over the MEA 22. Such flow channels are provided to distribute reactants over an active area of the fuel cell thereby maximizing performance and stability. Additional flow channels 32, 34 inside each plate 24, 26 are used to circulate a liquid coolant.

I-00081 Referring also to Fig. 2, each MEA 22 in a fuel cell stack is sandwiched between two bipolar plates 24, 26, and gaskets 36, 38 are added around the edges of the MEA 22 to make a gas-tight seal. These gaskets usually are made of a rubbery polymer. Each fuel cell 10 includes current collectors 40, 42 and end plates 44, 46.

[0009] Referring also to Fig. 3, typically reactant flow channels 28, 30 on the one hand and coolant flow channels 32, 34 on the other hand are formed to run essentially parallel to one another, and reactant flow channels 28, 30 on the one hand and coolant flow channels 32, 34 on the other hand are formed to run traverse to each other, typically essentially perpendicular to each other as illustrated in Fig. 3.

[00010] Summarizing to this point, fuel cell systems, particularly for surface-based applications, typically have bipolar plates (BPPs) to provide both working gases and coolant liquid flowing through separate channels. However, the high volumetric density of coolant liquid is problematic for applications such as aircraft. Air-cooling is a more lightweight method but its low density, high thermal conductivity, and low thermal capacity pose challenges to cooling efficiency. Current state-of-art air-cooled BPPs typically are formed by sheet-metal into simple rectangular or linear flow field patterns with thick and shallow channels which lead to inlet overcooling and outlet undercooling. Alternatively, the anode plate may be formed of metal and the cathode plate formed of a polymeric composite material according to Applicant's internal prior art as disclosed in PCT Application Serial Nos. PCT/US2022/025162 and PCT/US2022/028976, and Applicant's co-pending UK Patent Application No. GB2303807.8, filed 15 March 2023 (Attorney Docket ClZERO 23.05 UK, the contents of which are incorporated herein in its entirety by reference). Since air is not able to evenly remove enough heat from the aluminum BPP, the resulting temperature gradient across a single plate can be up to 35° C. Additionally, for aviation, required power output and hence cooling parameters vary at different stages of flight (e.g., an aircraft in cruise may typically use 30% of takeoff/initial climb power). Thus there is a need to ensure sufficient and even cooling with air throughout different stages of flight and for different aircraft types.

[0oon] In one embodiment of the disclosure there is provided a fuel cell bipolar plate (BPP) having an anode plate and a cathode plate, and a plurality of gaseous media coolant flow channels therebetween, wherein the gaseous media coolant flow channels each have an inlet and an outlet, wherein at least one of the gaseous media coolant flow channels varies in size between its inlet and outlet and includes an expansion area, and wherein at least one of the gaseous media coolant flow channels has an inlet that is smaller in size than its outlet.

[00012] In one embodiment one or more of the gaseous media coolant flow channels increases in size downstream of their respective inlets.

[00013] In another embodiment one or more of the gaseous media coolant flow channels include deflection barriers configured to force the gaseous media coolant from a straight path within said channel (s).

[00014] In a further embodiment two or more of the gaseous media coolant flow channels have staggered deflection barriers between their respective inlets and outlets.

[00015] In yet another embodiment two or more of the gaseous media coolant flow channels have deflection barriers of varying lengths and positions between their respective inlets and outlets.

[000161 In a still further embodiment one or more of the gaseous media coolant flow channels are configured in a pattern that provides a gradual increase of heat rejection efficiency between their respective inlets and outlets.

[00017] In yet another embodiment one or more of the gaseous media coolant flow channels have deflection barriers configured to cause the gaseous media coolant to change flow direction.

[000181 In still yet another embodiment one or more of the gaseous media coolant flow channels have deflection harriers configured to cause the media gaseous coolant to gaseous media coolant flow channel.

[ooms] In another embodiment two or more of the gaseous media coolant flow channels increase in size downstream of their respective inlets.

[000201 In a further embodiment the gaseous media coolant flow channels are narrowest at their respective inlets as compared to their respective outlets.

[00021] In yet another embodiment the BPP has a rectangular shape in plan.

[00022] In a further embodiment the BPP has a trapezoidal shape in plan.

[00023] In another embodiment the BPP has a ring-sector shape in plan.

[000241 In still yet another embodiment the anode plate is formed of a metal, and the cathode plate is formed of a polymeric composite material.

[000251 In a further embodiment the anode plate has a plurality of anode reactant gas flow channels running parallel to one another, the cathode plate has a plurality of cathode reactant gas flow channels running parallel to one another and parallel to the anode reactant gas flow channels, and the gaseous media coolant flow channels run at an angle to the anode reactant gas flow channels and the cathode reactant gas flow channels.

[00026] In another embodiment the coolant media flow channels run perpendicular to the anode reactant gas flow and the cathode reactant gas channels.

[00027] The present disclosure also provides a fuel cell stack comprising a plurality of fuel cell bipolar plates.

[00028] The present disclosure also provides a fuel cell powered vehicle comprising a fuel cell stack as above described, in particular fuel cell powered vehicle in the form of a fuel cell powered aircraft.

[00029] According to aspect A of the present invention there is provided a fuel cell bipolar plate (BPP) having an anode plate and a cathode plate, and a plurality of gaseous media coolant flow channels therebetween, wherein the gaseous media coolant flow channels each have an inlet and an outlet, wherein at least one of the gaseous media coolant flow channels varies in size between its inlet and outlet, and includes an expansion area, and wherein at least one of the gaseous media coolant Clow channels has an inlet that is smaller in size than its outlet.

[00030] Preferably one or more of the gaseous media coolant flow channels increases in size downstream of their respective inlets.

[00031] Preferably one or more of the gaseous media coolant flow channels have deflection barriers configured to force the gaseous media coolant from its initial path within said channel(s).

[00032] Preferably two or more of the gaseous media coolant flow channels have staggered deflection barriers between their respective inlets and outlets.

[00033] Preferably two or more of the gaseous media coolant flow channels have deflector barriers of varying lengths and positions between their respective inlets and outlets.

[00034] Preferably two or more of the gaseous media coolant flow channels are configured in a pattern that provides a gradual increase of heat rejection efficiency between their respective inlets and outlets.

[00035] Preferably two or more of the gaseous media coolant flow channels increase in size downstream of their respective inlets.

[000361 Preferably the gaseous media coolant flow channels are narrowest at their respective inlets as compared to their respective outlets.

[00037] In one alternative the BPP has a rectangular shape in plan.

[000381 In another alternative the BPP has a trapezoidal shape in plan.

[00039] In a further alternative the BPP has a ring-sector shape in plan.

[000401 Preferably the anode plate is formed of a metal, and the cathode plate is formed of a polymeric composite material.

[00041] Preferably the anode plate has a plurality of anode reactant gas flow channels running parallel to one another, the cathode plate has a plurality of cathode reactant gas flow channels running parallel to one another and parallel to the anode reactant gas flow channels, and the gaseous media coolant flow channels run at an angle to the anode reactant gas flow channels and the reactant cathode gas flow channels.

[000421 Preferably the coolant media flow channels run perpendicular to the anode reactant gas flow channels and the cathode reactant gas channels.

[00043] Preferably one or more of the gaseous media coolant flow channels have deflection barriers configured to cause the gaseous media coolant to change flow direction.

[00044] Preferably the deflection barriers are configured to cause the gaseous media coolant to divide into parts, wherein a part moves forward, and a part moves horizontally around a deflection barrier in a direction of an adjacent gaseous media coolant flow channel [00045] According to aspect B of the present invention there is provided a fuel cell stack comprising a plurality of fuel cell bipolar plates according to aspect A of the present invention.

[00046] According to aspect C of the present invention there is provided a fuel cell powered vehicle comprising a fuel cell stack according to aspect B of the present invention.

1-00047-1 Preferably the vehicle comprises a fuel cell powered aircraft.

[09048] Further areas of applicability will become apparent form the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

Brief Description of the Drawings

[00049] Further features and advantages of the disclosure will be seen in the following detailed description, taken in conjunction with the accompanying drawings. The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations and are not intended to limit the scope of the present disclosure.

In the drawings: Figs. 1 is a perspective view, Fig. la is a cross-sectional view and Fig. 2, an exploded perspective view of a fuel cell in accordance with the prior art; Fig. 3 is a cross-sectional view of a fuel cell in accordance with the prior art; Figs. 4 is a perspective view of a fuel cell and Fig. 4a is a top plan view of the cooling channels of an anode plate of a fuel cell in accordance with the present disclosure; Figs. 5-6 are top plan views of an anode or cathode plate, illustrating the gaseous media coolant flow channels in accordance with the present disclosure; and Fig. 7 is a schematic depiction of a hydrogen powered aircraft in accordance with the present disclosure.

Detai led Description

wow Example embodiments will now be described more fully with reference to the accompanying drawings. Example embodiments are provided so that this disclosure will be thorough and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.

[99051] The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms "a," "an," and "the" may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms "comprises," "comprising," "including," and "having, are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.

[09052] When an element or layer is referred to as being "on," -engaged to,-"connected to," or "coupled to" another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being "directly on," "directly engaged to," "directly connected to," or "directly coupled to" another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., "between" versus "directly between," "adjacent" versus "directly adjacent," etc.). As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

[00053] Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as "first," "second," and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.

r000541 Spatially relative terms, such as "inner," "outer," "beneath," "below," "lower," "above," "upper," and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "below" or "beneath" other elements or features would then be oriented "above" the other elements or features. Thus, the example term "below can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

[00055] As used herein in describing the flow channels as running adjacent to one another, the term "parallel" shall mean the flow channels as having essentially the same distance continuously between them, or running substantially parallel where the distance between adjacent flow channels varies depending on the shape of the plate. Thus, in the case of a bipolar plate that is substantially rectangular in shape, the flow channels may run essentially parallel to one another, while in the shape of a BPP formed as a trapezoid or ring-sector shape, the flow channels may deviate from one another by 5, 10, 15, 20, 25 or even 30" depending on the shape of the BPP.

[00056] Also, as used herein, in describing the gaseous media coolant flow channels as running transverse or perpendicular to the gaseous reactant gas flow channels shall mean that the respective channels are "perpendicular i.e., running at right angles (90°) to one another, or running transverse to one another at 90° ± 5, 10, 15, 20, 25 or even 30° from perpendicular.

[00057] Referring to Figs. 4 and 4a, there is illustrated a BPP 100 formed as a trapezoid in accordance with one embodiment of the present disclosure. BPP 100 is similar in overall construction and function as BPP 10 illustrated in Figs. 1, la, 2 and 3, and is patterned with deep and narrow channels 102, 104, 106 and 108 on both sides, where the channels 104, 106 function to deliver reactants and the channels 102, 108 function to deliver cooling media (e.g., air). The individual reactant channels 104, 106 respectively run substantially parallel to one another, as do the coolant channels 102, 108. As a whole, the reactant channel and the coolant channels run substantially perpendicular to each other. The reactant and cooling channels forming the BPP may be comprised of metal, or preferably are formed of a novel metal-composite material as disclosed in our aforesaid UK Application No. GB2303807.8 (Attorney Docket No. Cl-ZERO 23.05 UK) in a novel layout, in which the cooling channels 102, 108 have varying shapes and including an expansion area between the inlet 112 and outlet 110, any may include staggered deflection barriers with increasing spaces between channels 116.

[00058] A coolant deflection path is designed to be increasingly staggered as it proceeds from the inlet-side 112 of the BPP to the outlet-side 100. The inlet side 112 of the BPP starts with straight cooling channels 102 spaced closely to each other. The deflection barriers 114 force the coolant media to deviate from the initially linear path and allows for "bidirectional" motion, where not only vertical but also horizontal flow motion occurs. While part of the coolant continues its vertical path, the rest will take the longer path completely around the deflector.

[000591 Including narrow inlet 112 channel dimensions allows inlet channels to minimize heat rejection by the cold air on the initial part of its way through the BPP. In one embodiment of a trapezoidal or ring-sector BPP, a ratio of 1/3 to 1/10 between the initial channel width at the inlet to the BPP sector was found to prevent fuel cell overcooling from the edge where cold air enters.

[00060] In other embodiments, the coolant channel pattern can be adapted to have more narrow, shallow, and longer channels for transitioning from air cooling to evaporative cooling or for varying power output range, depending on the type of aviation application.

[000611 The BPP can be stamped, etched, or rolled into a trapezoidal or ring-sector shape to encompass a described coolant channel pattern.

[000621 The novel BPP multipath coolant channel pattern has linear rectangular channels with deflectors of varying lengths and positions in order to optimize efficiency of heat rejection. To achieve an evenly-distributed temperature across the cell, the pattern enables a gradual increase of heat rejection efficiency as cold air is heated so that maximum efficiency is provided at the exhaust end, where there is a very small temperature gradient between the fuel cell and the air, while at the intake end, the heat exchanging should be smaller to counter the already high gas velocity and avoid the air being saturated at its maximum temperature at the beginning of the cell.

[00063] Providing this novel Clow channel shape with an expansion area and deflection barriers allows a 3-4 times reduction in coolant flow rate and more efficient cooling, thus enabling air vs liquid cooling. By way of example, in one embodiment as compared to employing a liquid which is heavy, only 0.8 kg/s air flow was required to reject 100 kW of heat rather than 2 9 kg/s. Thus, only 3% rather than 20% of the generated power is consumed for cooling.

[00064] Providing our BPPs with cooling channel patterns according to the present disclosure increases contact area of working gasses, enabling greater thermal flux for more evenly distributed temperature. Thus, we are able to achieve as little as a 100 variation for 80% of the active area. As a result, the cooling channel pattern enables higher altitude aircraft where, at a lower air density, the thermal capacity of air is even lower which would otherwise unacceptably limit air-cooling efficiency. The cooling channel pattern also lends itself to a radial fuel cell stack designs such as trapezoid or ring-se tan plan shapes, which fit 11101- a coaxial system, airplane fuselage or nacelle inner space. The cooling channel patterns in accordance with the present disclosure also allows lightweighting for a lighter/smaller system. By way of example, the overall size of a heat exchanger cooling portion of BPPs made in accordance with the present disclosure may be reduced by up to 4 times resulting in a lighter/smaller system.

[00065] BPPs made with cooling channel patterns in accordance with the present disclosure also provide greater design flexibility, mechanical stability, and fuel cell stack rigidity, especially in a ring--*tor shaped embodiment compared to traditional rectangular shape stacks that are more difficult to align for taller stacks t higher output. This is especially important under vibration as may be encountered by an aircraft during take-off and climb.

[00066] The present disclosure provides several advantages: BPPs have not previously had a multi-path pattern combining linear rectangular channels to run perpendicular to uniform circular channels. With the present disclosure, thinner, narrower channels with intentionally-varied size throughout the cell readily can he formed by the novel manufacturing methods and composite material described in our aforesaid UK Application No. GB2303807.8 (Attorney Docket Cl-ZERO 23.05 UK). This also provides us with flexibility of design depending on desired power output range and aircraft type.

[00067] Fig. 5 shows a BPP having a generally rectangular plan shape, and Fig. 6 shows a BPP having a ring-shaped plan shape in accordance with the present disclosure.

[00068] A plurality of fuel cell bipolar plates 10 made in accordance with the present disclosure may be stacked together to form a fuel cell stack.

[00069] Fig. 7 illustrates an aircraft 140 including two electric motors 142, 14 which are powered by two parallel HT-PEM hydrogen fuel cells 146, 148 incorporating BPPs made in accordance with the present disclosure.

[00070] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure. Various changes and advantages may be made in the above disclosure without departing from the spirit and scope thereof.

Claims (19)

1. What is claimed: I. A fuel cell bipolar plate (BPP) having an anode plate and a cathode plate, and a plurality of gaseous media coolant flow channels therebetween, wherein the gaseous media coolant flow channels each have an inlet and an outlet, wherein at least one of the gaseous media coolant flow channels varies in size between its inlet and outlet, and includes an expansion area, and wherein at least one of the gaseous media coolant flow channels has an inlet that is smaller in size than its outlet.
2. 2. The BPP of claim 1, wherein one or more of the gaseous media coolant flow channels increases in size downstream of their respective inlets.
3. 3. The BPP of claim 1 or claim 2, wherein one or more of the gaseous media coolant flow channels have deflection barriers configured to force the gaseous media coolant from its initial path within said channel(s).
4. 4. The BPP according to any of claims 1-3, wherein two or more of the gaseous media coolant flow channels have staggered deflection barriers between their respective inlets and outlets.
5. 5. The BPP of any of claims 1-4, wherein two or more of the gaseous media coolant flow channels have deflector harriers of varying lengths and positions between their respective inlets and outlets.
6. 6. The BPP of any of claims 1-5, wherein two or more of the gaseous media coolant flow channels are configured in a pattern that provides a gradual increase of heat rejection efficiency between their respective inlets and outlets.
7. 7. The BPP of claim 6, wherein two or more of the gaseous media coolant Clow channels increase in size downstream of their respective inlets.
8. 8. The BPP of any of claims 1-7, wherein the gaseous media coolant flow channels are narrowest at their respective inlets as compared to their respective outlets.
9. 9. The BPP of any of claims 1-8, wherein the BPP has a rectangular shape in plan.
10. 10. The BPP of any of claims 1-8, wherein the BPP has a trapezoidal shape in plan.
11. 11. The BPP of claims 1-8, wherein the BPP has a ring-sector shape in plan.
12. 12. The BPP of any of claims 1-11, wherein the anode plate is formed of a metal, and the cathode plate is formed of a polymeric composite material.
13. 13. The BPP of any of claims 1-12, wherein the anode plate has a plurality of anode reactant gas Clow channels running parallel to one another, the cathode plate has a plurality of cathode reactant gas flow channels running parallel to one another and parallel to the anode reactant gas flow channels, and the gaseous media coolant flow channels run at an angle to the anode reactant gas Clow channels and the reactant cathode gas Clow channels.
14. 14. The BPP of claim 13, wherein the coolant media flow channels run perpendicular to the anode reactant gas flow channels and the cathode reactant gas channels.
15. 15. The BPP of any of claims 1-14, wherein one or more of the gaseous media coolant flow channels have deflection barriers configured to cause the gaseous media coolant to change flow direction.
16. 16. The BPP of claim 15, wherein the deflection barriers are configured to cause the gaseous media coolant to divide into parts, wherein a part moves forward, and a part moves horizontally around a deflection barrier in a direction of an adjacent gaseous media coolant flow channel.
17. 17. A fuel cell stack comprising a plurality of fuel cell bipolar plates as claimed in any of claims 1-16.
18. 18. A fuel cell powered vehicle comprising a fuel cell stack as claimed in claim 17.
19. 19. The fuel cell powered vehicle as claimed in claim 18 wherein the vehicle comprises a fuel cell powered aircraft.