GB2501711A

GB2501711A - Fuel Cell Stack Assembly

Info

Publication number: GB2501711A
Application number: GB1207603.0A
Authority: GB
Inventors: Peter David Hood
Original assignee: Intelligent Energy Ltd
Current assignee: Intelligent Energy Ltd
Priority date: 2012-05-01
Filing date: 2012-05-01
Publication date: 2013-11-06
Also published as: TW201409819A; GB201207603D0; AR090908A1; WO2013164574A1

Abstract

A fuel cell stack assembly comprises a plurality of fuel cells in a stack, the stack defining two opposing parallel end faces. An end plate assembly is provided at each opposing end face of the stack. The end plate assemblies are coupled together to thereby maintain the fuel cells in the stack under compression. At least one, and preferably both, of the end plate assemblies comprises: a master plate 2 defining a master compression face; a slave plate 3 defining a slave compression face, the slave compression face facing the master compression face and being in compressive relationship therewith; and a plastic or viscoplastic interface disposed between the master compression face and the slave compression face. The plastic or viscoplastic interface may be bounded by a containment structure extending along a peripheral edge of the interface. The interface may be acrylic putty, silicone paste , grease or hydraulic bladder containing water or glycerine

Description

FUEL CELL STACK ASSEMBLY

The present invention relates to methods and apparatus suitable for assembling an electrochemical fuel cell stack.

Fuel cell stacks comprise a series of individual fuel cells built up layer by layer into a stack arrangement. Each cell itself may include various layered components such as a polymer electrolyte membrane, gas diffusion layers, fluid flow plates and various sealing gaskets for maintaining fluid tightness and providing fluid fuel and oxidant distribution to the active surfaces of the membrane. At each end face of the stack, a pair of pressure end plates coupled together by tie rods is conventionally used to hold the stack together and maintain compression on the cells in the stack.

It is most important that pressure applied by the end plates to the ends of the fuel cell stack is sufficiently uniform across the surfaces of the stack that all of the individual components of the stack are maintained in proper compressive relationship with one another. Sealing gaskets in particular must be maintained in proper compression across the entire area of each fuel cell to ensure that fluid flow paths are properly defined so that fuel and / or oxidant are correctly conveyed to the active surfaces of each cell and do not leak.

Conventionally, uniform pressure is maintained by providing substantial and robust end plates capable of maintaining sufficient excess pressure across the entire surfaces of the ends of the stack. This results in large and heavy end plates to ensure that they are sufficiently robust that they will not significantly distort under the requisite pressures and will not apply compression forces unevenly. Use of large and heavy end plates results in heavier and larger fuel cell stacks than is desirable. An alternative approach is to use lighter weight end plates but provide an additional mechanism for mitigating the effects of end plate structure distortion when compressive forces are applied. This could be a shim positioned centrally between an end plate and a first inner stack component.

An approach described in US 6040072 is to use bands or straps surrounding the fuel cell stack to transmit load via the end plates and an array of adjustable elements that control the distribution of the compressive load, to minimize the size and weight of the clamping mechanism.

I

Another approach is described in US 2006/0194094 which uses an end plate having a pressure shield which is curved convexly towards the stack and a bearing plate which acts as a transition element to transmit compressive forces to a planar element of the fuel cell stack.

Both of these documents recognise the importance of maintaining a uniform pressure distribution.

A problem exists that when fuel cells are operational, the temperature of the fuel cell stack rises considerably. As a result, thermal expansion of the fuel cell stack occurs resulting in varying loads on the end plates and thus varying degrees of distortion. This can adversely affect any mechanism compensating for distortion of the end plates such that an evenly distributed force is no longer applied across the stack under all operating conditions of the fuel cell stack.

It is an object of the present invention to provide an improved way of ensuring good pressure distribution across the end faces of a fuel cell stack under varying conditions.

According to one aspect, the present invention provides a fuel cell stack assembly comprising: a plurality of fuel cells in a stack, the stack defining two opposing parallel end faces; an end plate assembly at each opposing end face of the stack, the end plate assemblies being coupled together to thereby maintain the fuel cells in the stack under compression; wherein at least one of the end plate assemblies comprises: a master plate defining a master compression face; a slave plate defining a slave compression face, the slave compression face facing the master compression face and being in compressive relationship therewith; and a hydraulic, a plastic or a viscoplastic interface disposed between the master compression face and the slave compression face.

The interface may be a viscoplastic material. The viscoplastic material may be a Bingham plastic. The interface may comprise a generally planar or curved profile layer bounded by a containment structure extending along a peripheral edge of the interface.

The containment structure may be a peripheral seal coupling to the master compression face and to the slave compression face. The peripheral seal may comprise an 0-ring seal.

The interface may comprise a hydraulic fluid disposed within a containment structure.

The containment structure may be a bladder filled with the hydraulic fluid. The bladder may be coupled to or integrated with the slave plate. The slave plate may be integrated with an end element of the stack. The slave plate may comprise a collector plate of the fuel cell stack.

The master compression face may have a first portion and a second portion at a reflex angle to one another and the slave compression face may have a corresponding first portion and second portion at an obtuse angle to one another, and the plastic or viscoplastic interface may comprises a first interface region between the respective first portions and a second interface region between the respective second portions. The first and second interface regions may be separated and each bounded by a separate containment structure. The reflex angle and the obtuse angle may be selected such that the respective portions of the master compression face and the slave compression face are non-parallel prior to application of a load to the end plate assemblies whereas, under the application of the load to maintain the fuel cells under compression, a bending moment in the master plate causes the master compression face and the slave compression face to come into a generally, or more, parallel relationship with one another by distortion of the master plate. The slave plate may comprise a pair of separate elements each defining one of the first portion and the second portion. The master compression face may define a convex surface, the convex surface being configured such that under the application of the load to maintain the fuel cells under compression, the bending moment in the master plate causes the master compression face and the slave compression face to come into a generally, or more, parallel relationship with one another. The slave compression face may define a convex surface, the convex surface being configured such that under the application of the load to maintain the fuel cells under compression, the bending moment in the master plate causes the master compression face and the slave compression face to come into a generally, or more, parallel relationship with one another. The master plate may be formed from a metal and the slave plate may be formed from a non-metal material. The slave plate may extend laterally from the master plate on at least one side defining a lateral extension portion, the lateral extension portion comprising at least one fluid distribution gallery communicating with a fluid distribution gallery passing through the plurality of fuel cells in the stack. The fuel cell stack assembly may include tie bars coupling the end plate assemblies together to maintain the fuel cells in the stack under compression, the tie bars extending through the master plate and the slave plate and being disposed inward of the lateral extension portion. Both of the end plate assemblies may comprise a master plate and a slave plate and a plastic or viscoplastic interface as described above.

According to another aspect, the present invention provides a method of forming a fuel cell stack assembly comprising: forming a plurality of fuel cells into a stack, the stack defining two opposing parallel end faces: positioning a slave plate of an end plate assembly at one end face of the stack, the slave plate having a slave compression face facing outwardly from the stack; positioning a hydraulic, a plastic or a viscoplastic interface onto the slave compression face; positioning a master plate defining a master compression face onto the plastic or viscoplastic interface, such that the plastic or viscoplastic interface is disposed between the master compression face and the slave compression face positioning a second end plate assembly at the opposing end face of the stack; coupling the end plate assemblies together to bring the fuel cells and end plate assemblies into compressive relationship and to maintain the stack under compression.

Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 shows a perspective exploded view of a fuel cell end plate assembly comprising a master plate, a slave plate and a viscoplastic interface between; Figure la shows a schematic cross-section of the master plate and slave plate indicating the angular relationship of compression surfaces; Figure 2 shows a schematic perspective view of an end of an assembled fuel cell stack incorporating an end plate assembly similar to that of figure 1; Figure 3 shows a schematic perspective, partially cut-away view of an alternative fuel cell end plate assembly comprising a master plate, a slave plate and a viscoplastic interface between; Figure 4 shows a cross-sectional view of the fuel cell end plate assembly of figure 3; Figure 5 shows a schematic perspective view of an end of an assembled fuel cell stack incorporating an alternative end plate assembly to that of figure 1; Figure 6 shows (a) a perspective view, (b) a partial cross-sectional view, (c) a plan view, and (d) an edge view of a hydraulic fluid bladder; and Figure 7 shows (a) a perspective view, (b) a partial cross-sectional view, (c) an edge view, (d) a plan view and (e) a cross-sectional view on line A-A of a hydraulic fluid bladder integrated onto a slave end plate.

A first embodiment of a fuel cell end plate assembly I is shown in exploded form in figure 1. The end plate assembly I comprises a master plate 2 and a slave plate 3. The master plate 2 defines a master compression surface 4 which, in this embodiment comprises a first portion 5 and a second portion 6. The first and second portions are at an oblique angle to one another, and more particularly as shown in figure la there is a reflex angle °M between the first and second portions 5, 6 at an apex 7 between the first and second portions. The slave plate 3 has a corresponding slave compression surface 8 facing away from the viewer in figure 1. The slave compression surface 8 also comprises a first portion 9 and a second portion 10, only the edges of which are visible in figure 1. The first and second portions of the slave compression surface 8 are also at an oblique angle to one another, and more particularly as shown in figure la there is an obtuse angle e5 between the first and second portions 9, 10 at an apex 11 between the first and second portions 9, 10. Although the slave plate 3 is shown as a single entity in figure 1, it could be formed in two parts (e.g two halves) each defining one of the first portion 9 and the second portion 10 of the slave compression surface 8. The two halves could abut at the apex 11.

Between the master compression surface 4 and the slave compression surface 8 a viscoplastic interface 12 is provided. In the arrangement of figure 1, the viscoplastic interface is divided into a first interface region 13 and a second interface region 14. In figure 1, the second interface region is shown detached from the second portion of master compression surface 4 for illustrative purposes only. The viscoplastic interface 12 is bounded by a containment structura In the illustrated embodiment, there are separate containment structures in the form of o-ring seals 15, 16 respectively extending around and along a peripheral edge of the respective interface regions 13, 14. Each o-ring ties partially within a respective recess, rebate, groove, depression or channel 17 in the master plate and also partially within a respective recess, rebate, groove, depression or channel in the slave plate (not visible in figure 1). Each 0-ring provides a peripheral seal against the master compression face and the stave compression face. More generally, an 0-ring and a retaining feature (e.g. a recess, rebate, groove, depression or channel in at least one of the master or slave compression faces) may be regarded as examples of a containment structure.

The slave plate 3 includes a planar surface 18 configured for engagement with an end face of a stack of fuel cells. The planar surface 18 may be uniformly flat or may comprise a series of pressure elements which themselves define a series of planar pressure surfaces distributed across the area of the plate which collectively define the planar surface 18.

The slave plate 3 may also include a number of fluid distribution structures 19 for delivery of fuel and / or oxidant to the cells in the stack in known manner. Both the master plate 2 and the slave plate 3 include a number of apertures 20, 21 for the passage of tie-rods 22 (the ends of which can be seen in figure 2) for assembling the stack and for maintaining the stack in compression. The master plate 2 may be formed as an open cell structure with voids 23 and connecting limbs 24 for a lighter weight construction for any given strength.

Referring to figure 2, an end plate assembly 1 is applied to each end of a fuel cell stack 25, one end of which is shown in figure 2. The stack 25 comprises a plurality of fuel cells 26 (not shown separately, but creating the stack 25 with each cell parallel to the slave plate planar surface 18). The stack of cells therefore defines two opposing parallel end faces each of which engages with a respective slave plate. A set of tie rods 22 passes through each end plate assembly 1 and binds them together thereby compressing the cells 26 in the stack 25.

Referring back to figure 1, the viscoplastic interface 12 is preferably formed from any plastic or viscoplastic material capable of limited or constrained plastic flow between the master compression surface 4 and the slave compression surface 8 under the conditions of compression necessary to maintain the fuel cell stack in correct configuration, i.e. with all layers of the fuel cell stack in proper contact and with all gasket seals operational.

The expression "limited plastic flow" is intended to encompass several situations at least including the following: (i) The inherent plastic flow of the plastic or viscoplastic material layer 12 is constrained by the containment structure such that the material cannot escape the boundaries defined by the master compression surface 4, the slave compression surface and the containment structure (e.g. 0-rings 15, 16) and thus the plastic flow is limited to prevent escape of the material from between the master and slave plates.

(ii) The inherent structural integrity of the plastic or viscoplastic material is sufficient to ensure that the plastic flow of the material under the normal compression forces applied to the fuel cell stack are not sufficient to enable the material to escape from between the master and slave plates 2, 3.

(di) A combination of both (i) and (ii) above.

In each case, the performance of the plastic or viscoplastic interface is such as to enable it to locally redistribute itself (a) under the compressive forces applied to the end plate assemblies during assembly of the stack; (b) under any flexure of the master plate or slave plate under load; and (c) during the normal operation of the fuel cell causing thermal expansion of the stack and consequent distortion or changes in distortion of the end plate structures, including thermal expansion / contraction in a repetitive way from thermal cycling during periods of varying electrical load on the fuel cell.

The interface material 12 preferably has sufficient inherent structural integrity to enable it to be physically positioned during assembly and possibly also to be removed and repositioned during reassembly as a single manipulable self-supporting entity.

A hydraulic interface may alternatively be used in place of plastic or viscoplastic material. The expression "hydraulic interface" is intended to encompass an interface comprising a contained volume of hydraulic fluid capable of redistributing itself within the contained volume under the compressive forces applied to the end plate assemblies, such that pressure is evenly transmitted between the master plate and slave plate in analogous manner to that described in connection with the plastic or viscoplastic interface above. Examples of hydraulic interfaces include those using water, glycerine or other known hydraulic fluids, which are generally of very low compressibility or are non-compressible, within the range of pressures experienced between the master plate and the slave plate. Examples of containment structures suitable for use with hydraulic fluids (and also with plastic or viscoplastic materials) include a flexible bladder, formed from any suitable flexible material such as rubber or metal. One example is shown in figure 6.

Figure 6 illustrates a hydraulic fluid bladder 60 comprising a pair of metal foil sheets 61, 62 that are laser welded together to form seam 63 at peripheral edges 64. The metal foil sheets 61, 62 together then define a hydraulic fluid cavity 65 therebetween. The hydraulic fluid 66 fills this cavity 65. As will be discussed further below, such a bladder arrangement can also be used to contain plastic or viscoplastic material encased within.

In the example shown, the bladder may be fabricated from metal foil sheets 61, 62 of 0.2 mm thickness to form a cavity 65 of 0.6 mm thickness. These thicknesses can be adapted to suit any particular fuel cell assembly to provide sufficient compression strength and thickness for accommodating end plate flexure.

The inclusion of the hydraulic, plastic or viscoplastic interface enables the material to locally redistribute itself to accommodate a localised divergence in the master compression surfaces and the slave compression surfaces. Such localised divergence can arise as the compression forces vary as a result of varying thermal expansion. This local redistribution of material ensures that consequential non-uniform pressure applied to the fuel cell stack by the slave plate surface 18 is reduced, minimised or even eliminated. The interface unifies the distributed pressure between the master plate and the slave plate.

The plastic or viscoplastic material forming the interface 12 can be selected from any suitable material such as acrylic putty, silicone paste, mineral or synthetic grease etc. One preferred class of materials for the interface 12 is Bingham plastics. Preferably, the plastic or viscoplastic material is selected from a material behaving as a rigid, semi-rigid or self-supporting body at low stress for ease of assembly of the end plates but which flows as a viscous fluid at high stress. However, other materials such as powders (e.g. powdered marble, powdered plastics) could be used and, if necessary, contained within a bladder or other containment structure as described above in connection with hydraulic fluids. Materials which do not have rigid, semi-rigid or self-supporting properties could be provided as frozen components for assembly, making automated assembly of the fuel cell easier.

In the preferred embodiments the viscoplastic material 12 flows between the master plate 2 and the slave plate 3 compression surfaces 4, 8 in a hydraulic state and is contained within a defined volume with the 0-rings 15, 16. Hydraulic pressure is uniformly distributed regardless of the attitude of the compression surfaces 4, 8 to one another.

Substantial benefits of this arrangement can include (i) integrity of the seals within and between each cell in the fuel cell stack assembly; (ii) uniform interfacial resistance over the electrode areas within the stack even when varied loads are applied to the stack assembly, such as caused by variations in manufacturing, and variations caused by normal operating parameters of the fuel cell stack; (iii) uniform interfacial resistance over the electrode area when under the effects of thermal cycling.

The end plate assembly can facilitate a light weight end plate design that compensates for end plate structure distortion under varied loads and supplies an evenly distributed force onto a stack assembly when thermal and other mechanical forces are cycling the overall compression load. The end plate assembly may allow stack-specific compressive tuning without compromising the parallel attitude of each cell relative to its adjacent cells.

A substantial number of end plate assembly designs can benefit from the hydraulic, plastic or viscoplastic interface as described above. A non-exhaustive selection of alternative designs are discussed below.

Figure 1 shows a master and slave end plate design in which the plates 2, 3 are facetted to include a first portion 5 and a second portion 6 which are each planar, but disposed at an oblique angle to one another. In an alternative configuration each of the first portion and the second portion 6 need not be planar but could be curved surfaces, e.g. each could present a convex surface in the y-direction, i.e. describing an outward going curve when travelling along the surface from the apex 7 to the peripheral edge of the plates near apertures 21. The curved surfaces could be curved (eg. convex or concave) in one direction or curved in both orthogonal directions, e.g. when travelling along the surface in both x and y directions as shown in figure 1.

The transition between first portion 5 and second portion 6 can be a smooth, rounded transition portion 27 as shown in figure 2-The master compression surface 4 and the slave compression surface 8 can be smoothly curving profiles without planar portions in either the x direction and / or the y-direction.

The master plate 2 and the slave plate 3 may be pre-formed such that the master compression surface 4 and the slave compression surface B may be non-parallel to one another when the plates are uncompressed. For example, the reflex angle eM of the master plate 2 and the obtuse angle % of the slave plate can be non-conjugate angles that add up to more than 360 degrees such that the first and second portions 5, 6 of the master compression surface diverge from the respective first and second portions 9, 10 of the slave compression surface when the plates are uncompressed. The divergence can be calculated such that when a correct degree of compression of the stack is applied by the master plate 2 using tie rods 22, a bending moment in the master plate causes the compression surfaces 4, 8 or respective portions of the compression surfaces 5, 6, 9, 10 to become closer to parallel, generally parallel or exactly parallel by distortion of the master plate.

The master compression surface and the slave compression surface can be flat, i.e. without curved portions and without portions at oblique angles to one another. With reference to figures 3 and 4, an end plate assembly 30 comprises a master plate 32 and a slave plate 33. The master plate 32 is a planar plate and includes a planar compression surface 34. The slave plate 33 is a planar plate and includes a planar compression surface 38. Between the compression surfaces 34, 38 lies an elastic or viscoplastic interface 12 bounded by an 0-ring seal 15 disposed within a recess 3T In this arrangement, the containment structure is illustrated as a rebate 39 in the slave plate defining the effective edge of the slave compression surfaca The rebate 39 restrains movement of the 0-ring 15 and thereby confines the elastic or viscoplastic material 12. It will be seen that the peripheral edges of the master plate 32 and the slave plate 33 outside the boundary of the 0-ring 15 do not form parts of compression surfaces in compressive relationship with one another as they do not meet and are spaced apart sufficiently to facilitate a non-parallel attitude in respective plates when distortion in the end plate assembly occurs. Despite any such non-parallel relationship, the slave plate 33 planar surface 18 that presents to the stack can still provide uniform compression to the stack. The operation of this end plate assembly 30 and its viscoplastic interface 12 is analogous to that described in connection with figure 1 One of the master compression surface and the slave compression surface can be profiled with a curve. With reference to figure 5, an end plate assembly 50 comprises a master plate 52 and a slave plate 53. The master plate 52 is a planar plate when fabricated and includes a planar compression surface 54 when uncompressed. The slave plate 53 is a curved convex plate and includes a curved convex compression surface 58. When the master plate 52 is coupled to the slave plate 58 and to the plates 26 of the stack 25 by tie rods 22, the compressive force on the master plate 52 results in an elastic distortion of the master plate about the convex surface of the slave plate 53.

This brings the master compression face and the slave compression face into generally parallel relationship with one another. This provides a more even distribution of force onto the compression surface of the slave plate. Between the compression surfaces 54, 58 of the plates lies an elastic or viscoplastic interface 12 bounded by an 0-ring seal 15 in similar manner to that described in connection with figures 1 to 4 (not visible in figure 5). In this case, the viscoplastic interface provides a curved interfacial layer that follows the profile of the convex surface of the slave plate. The operation of this end plate assembly 30 and its viscoplastic interface 12 is otherwise analogous to that described in connection with figure 1.

Although figure 5 shows a slave plate 53 having a compression surface 58 that is curved (e.g. convex) in one direction (the y-direction) and planar in the orthogonal (x) direction, the compression surface 58 could be curved in both x-and y-directions. The precise curvature profile can be adjusted for optimal pressure distribution taking into account the fixing centres of any tie rods 22 and the characteristics of the resilient master plate and or the slave plate as well as the characteristics of the elastic or viscoplastic interface.

The elastic or viscoplastic interface as described herein can be particularly effective for large fuel cell stacks. Where a stack is formed from only a few cells, or even a few tens of cells, the cumulative effect of the cells on the pressure applied by the end plates during thermal cycling may be quite small or moderate. However, in larger stacks, e.g. in a 192-cell stack, the total thickness variation from thermal cycling may be as much as 1 mm or more. This variation would conventionally be accommodated by increased compression of gaskets throughout the stack and I or by distortion in the end plates.

The hydraulic, plastic or viscoplastic interfaces can be made thick enough to accommodate a magnitude of expected flexure of the master plate and I or slave plate regardless of the size of fuel cell stack.

It is preferred that both ends of a fuel cell stack have an end plate assembly according to one of the embodiments discussed above, but it will be understood that at least some benefit would be achieved if only one end of the stack assembly used an end plate assembly as described and the other end had a conventional end plate.

In a preferred arrangement, the master plate is formed from a metal material for strength and rigidity while possibly in some embodiments allowing a moderate and / or predictable amount of elastic deformation when placing the stack into compression. The slave plate is preferably formed from a rigid non-elastic, non-metal material which will retain its shape under compression and distribute compression forces evenly from interface l2to planar surface 18.

Where the slave plate includes at least one fluid distribution structure (galery) 19, these are preferably disposed laterally outside the positioning of tie bars 22 as exemplified in figures 1 and 2. As shown, the slave plate 3 extends laterally (as shown in the y-direction) from the compression surfaces 4, 5 and laterally outside the master plate 2 defining a lateral extension portion beyond the tie bars and beyond the master plate.

This is beneficial because the compression applied by the tie bars is kept as close as possible to the area of the cell edges which require the uniform pressure distribution.

The fluid distribution gallery 19 communicates with a fluid distribution gallery extending down the side of the stack and this has less demand for a precisely controlled uniform pressure distribution.

The function of the slave plate 3 could be integrated with the end-most element of the fuel cell stack. In some arrangements of fuel cell stack, the final element in a stack may be a collector plate somewhat thicker than other plates in the stack (e.g. anode plates, cathode plates or bipolar plates). Such a collector plate, or other final element, in the stack could be used to form the functions of the slave plate as defined herein.

Figure 7 shows an arrangement in which a hydraulic fluid bladder is integrated with a slave plate 70, e.g. in which the slave plate forms one wall of the bladder 71. In this arrangement, a metal foil 72 is laser or resistance welded to the surface 74 of the slave plate 70 to form seam 73. A cavity 75 is thereby formed and filled with hydraulic fluid.

The cavity 75 can be filled with hydraulic fluid by using a filling port (not shown) drilled in the slave plate which is sealed after filling. Alternatively, the cavity 75 can be filled before final welding of the seam. For example, the cavity 75 may be filled through a breach in a foundation weld then the weld is completed using the mass of the larger component to sink the heat away. The cavity may also be used to confine plastic or viscoplastic materials as discussed above instead of hydraulic fluid. The slave plate 70 can be a final element in the stack, e.g. a current collector plate as discussed above.

Other flexible materials may be used in place of a metal foil.

Although the embodiments described above relate to a proton exchange membrane (PEM) fuel cell for converting hydrogen and oxidant to electrical energy, the concept described can readily be applied to other fuel cell technologies. In particular, other fuel cell types typically operate at even higher temperatures than PEM fuel cells and thus thermal cycling can be an even larger problem to accommodate.

Other embodiments are intentionally within the scope of the accompanying claims.

Claims

CLAIMS1. A fuel cell stack assembly comprising: a plurality of fuel cells in a stack, the stack defining two opposing parallel end faces; an end plate assembly at each opposing end face of the stack, the end plate assemblies being coupled together to thereby maintain the fuel cells in the stack under compression; wherein at least one of the end plate assemblies comprises: a master plate defining a master compression face; a slave plate defining a slave compression face, the slave compression face facing the master compression face and being in compressive relationship therewith; and a hydraulic, a plastic or a viscoplastic interface disposed between the master compression face and the slave compression face.
2. The fuel cell stack assembly of claim 1 in which the interlace is a viscoplastic material.
3. The fuel cell stack assembly of claim 2 in which the viscoplastic material is a Bingham plastic.
4. The fuel cell stack assembly of claim 1 in which the interface comprises a generally planar or curved profile layer bounded by a containment structure extending along a peripheral edge of the interface.
5. The fuel cell stack assembly of claim 4 in which the containment structure is a peripheral seal coupling to the master compression face and to the slave compression face.
6. The fuel cell stack assembly of claim 5 in which the peripheral seal comprises an 0-ring seal.
7. The fuel cell stack assembly of claim 1 in which the interface comprises a hydraulic fluid disposed within a containment structure.
8. The fuel cell stack assembly of claim 7 in which the containment structure is a bladder filled with the hydraulic fluid.
9. The fuel cell stack assembly of claim 8 in which the bladder is coupled to or integrated with the slave plate
10. The fuel cell stack assembly of claim I in which the slave plate is integrated with an end element of the stack.
11. The fuel cell stack assembly of claim 10 in which the slave plate comprises a collector plate of the fuel cell stack.
12. The fuel cell stack assembly of claim 1 in which the master compression face has a first portion and a second portion at a reflex angle to one another and in which the slave compression face has a corresponding first portion and second portion at an obtuse angle to one another, and in which the plastic or viscoplastic interface comprises a first interface region between the respective first portions and a second interface region between the respective second portions.
13. The fuel cell stack assembly of claim 12 in which the first and second interface regions are separated and each bounded by a separate containment structure.
14. The fuel cell stack assembly of claim 12 in which the reflex angle and the obtuse angle are selected such that the respective portions of the master compression face and the slave compression face are non-parallel prior to application of a load to the end plate assemblies whereas, under the application of the load to maintain the fuel cells under compression, a bending moment in the master plate causes the master compression face and the slave compression face to come into a generally, or more, parallel relationship with one another by distortion of the master plate.
15. The fuel cell stack assembly of claim 13 in which the slave plate comprises a pair of separate elements each defining one of the first portion and the second portion.
16. The fuel cell stack assembly of claim 1 in which the master compression face defines a convex surface, the convex surface being configured such that under the application of the load to maintain the fuel cells under compression, the bending moment in the master plate causes the master compression face and the slave compression face to come into a generally, or more, parallel relationship with one another.
17. The fuel cell stack assembly of claim 1 in which the slave compression face defines a convex surface, the convex surface being configured such that under the application of the load to maintain the fuel cells under compression, the bending moment in the master plate causes the master compression face and the slave compression face to come into a generally, or more, parallel relationship with one another.
18. The fuel cell stack assembly of claim 1 in which the master plate is formed from a metal and the slave plate is formed from a non-metal material.
19. The fuel cell stack assembly of claim 1 in which the slave plate extends laterally from the master plate on at least one side defining a lateral extension portion, the lateral extension portion comprising at least one fluid distribution gallery communicating with a fluid distribution gallery passing through the plurality of fuel cells in the stack.
20. The fuel cell stack assembly of claim 19 further including tie bars coupling the end plate assemblies together to maintain the fuel cells in the stack under compression, the tie bars extending through the master plate and the slave plate and being disposed inward of the lateral extension portion.
21. The fuel cell stack assembly of claim 1 in which both of said end plate assemblies comprise a master plate and a slave plate and a plastic or viscoplastic interface as defined in claim 1.
22. A method of forming a fuel cell stack assembly comprising: forming a plurality of fuel cells into a stack, the stack defining two opposing parallel end faces; positioning a slave plate of an end plate assembly at one end face of the stack, the slave plate having a slave compression face facing outwardly from the stack; positioning a hydraulic, a plastic or a viscoplastic interlace onto the slave compression face; positioning a master plate defining a master compression face onto the hydraulic, plastic or viscoplastic interlace, such that the hydraulic, plastic or viscoplastic interface is disposed between the master compression face and the slave compression face positioning a second end plate assembly at the opposing end face of the stack; coupling the end plate assemblies together to bring the fuel cells and end plate assemblies into compressive relationship and to maintain the stack under compression.