GB2530022A

GB2530022A - Fuel cell compression

Info

Publication number: GB2530022A
Application number: GB1415500.6A
Authority: GB
Inventors: Hossein Ostadi; Zachary Elliott
Original assignee: Intelligent Energy Ltd
Current assignee: Intelligent Energy Ltd
Priority date: 2014-09-02
Filing date: 2014-09-02
Publication date: 2016-03-16
Also published as: GB201415500D0

Abstract

A fuel cell assembly 1 comprises one or more layered fuel cells 2 compressed between coupled end plates 3, 4 located on two opposing faces of the cell stack. At least one of the end plates comprises a fluid chamber 6 in communication, preferably via a pressure regulator, with a reactant fluid flow path 15 that extends across an active area of the fuel cells. The chamber may have an inlet port 8 coupled to a fuel supply conduit 9 and an outlet port 10 coupled to an anode fluid path. The chamber may comprise a flexible membrane sealed to the end plate. The chamber may comprise a fluid tight envelope containing a permeable structure such as foam. Also claimed is a fuel cell including means for varying the pressure in a chamber as a function of pressure in a reactant flow path, such that increased pressure in the cell is resisted by increased pressure from the end plate.

Description

FUEL CELL COMPRESSION

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

Fuel cell stacks comprise a series of individual fuel cells built up layer by layer into a stack arrangement. Each cell may include various layered components such as a polymer electrolyte membrane, gas diffusion layers, anode and cathode 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 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 important that pressure applied by the end plates to the end faces 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 such that fuel and / or oxidant is I are correctly conveyed to the active surfaces of each cell and the cells do not leak.

Uniform pressure may be maintained by providing substantial and robust end plates capable of maintaining sufficient 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.

An approach described in US 6200698 is to use an end plate assembly having a sealed bladder containing a two-phase fluid operable to maintain, over an extended period of time, a generally uniform distributed stack compression pressure. The use of a two phase fluid results in the bladder being pressurized even if gas slowly leaks from the bladder.

Another approach is described in US 6258475 which uses an end plate assembly having a silicone oil filled liquid chamber sandwiched between a back-up plate and an end plate.

The chamber is sealed at pressure.

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 or fuel cell stack under varying conditions.

According to one aspect, the present invention provides a fuel cell assembly comprising: one or more fuel cells assembled in a layered construction, the layered construction defining two opposing end faces, the or each fuel cell having a reactant fluid flow path extending across an active area of the fuel cell; an end plate at each opposing end face of the layered construction, the end plates being coupled together to thereby maintain the layered construction of the one or more fuel cells under compression; wherein at least one of the end plates comprises a fluid chamber extending across the end face and in fluid communication with the reactant fluid flow path.

The reactant fluid flow path may be an anode fluid flow path. The fluid chamber may have an inlet port coupled to a reactant fuel supply conduit of the fuel cell assembly. The fluid chamber may have an inlet port coupled to an anode fuel supply conduit of the fuel cell assembly and an outlet port coupled to the anode fluid flow path of at least one of the one or more fuel cells. The fluid chamber may be an expandable fluid chamber. The fluid chamber may comprise a flexible membrane coupled and sealed to an end plate of the at least one end plates. The fluid chamber may comprise an electrically conductive medium.

The fluid chamber may comprise a flexible fluid tight envelope with a resilient fluid-permeable structure within the envelope. The resilient fluid permeable structure may comprise a foam. The fluid chamber may comprise multiple compartments each in fluid communication with the anode fluid flow path. The fluid chamber may comprise a flexible membrane coupled and sealed to a current collector plate of the at least one end plates.

The layers of the one or more fuel cells and the end plates may be flexible and / or non-planar. The fluid chamber may be in fluid communication with the reactant fluid flow path by way of a pressure regulator.

According to another aspect, the present invention provides a fuel cell assembly comprising: one or more fuel cells assembled in a layered construction, the layered construction defining two opposing end faces, each fuel cell having a reactant fluid flow path extending across an active area of the fuel cell; an end plate at each opposing end face of the layered construction, the end plates being coupled together to thereby maintain the layered construction of the one or more fuel cells under compression; wherein at least one of the end plates comprises a fluid chamber extending across the end face, and further includes means for varying the pressure in the fluid chamber as a function of pressure in the reactant fluid flow path.

According to another aspect, the present invention provides a method of maintaining layers of a layered fuel cell assembly in compression during use, comprising the steps of: providing a reactant fluid supply to the fuel cell assembly; pressurising a reactant fluid flow path in a fuel cell of the fuel cell assembly with the reactant fluid supply and simultaneously pressurising a fluid chamber in an end plate of the fuel cell assembly adjacent to the fuel cell with the anode fluid supply to thereby maintain the layers of the fuel cell 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 schematic side view of a fuel cell stack incorporating a flexible fluid chamber adjacent to a stack end plate; Figure 2 shows a schematic perspective view of a fuel cell incorporating an inflatable fluid chamber adjacent to an anode of the fuel cell; Figure 3 shows a schematic cross-sectional side view of the fuel cell of figure 2.

With reference to figure 1, a fuel cell stack 1 comprises a stack of fuel cells 2 in a layered construction disposed between two end plates 3, 4. Any practical number of fuel cells 2 may be envisaged within a stack. Each of the fuel cells 2 is provided with an anode fluid supply conduit 15 configured to distribute anode fluid such as hydrogen to an anode flow field plate in each cell 2. The anode fluid supply conduit 15 may be a manifold extending through all the cells 2 in the stack 1. Each of the fuel cells 2 is provided with a cathode fluid supply conduit 16 configured to distribute cathode fluid such as air from a cathode fluid supply 17 to a cathode flow field plate in each cell 2. The cathode fluid supply conduit 16 may be a manifold extending through all the cells 2 in the stack 1. A cathode exhaust conduit 18 may be a manifold extending through all the cells 2 proximal to an opposing face of the stack, and leading to a cathode exhaust line 19. The anode flow field plate and the cathode flow field plate each generally provide a reactant fluid flow path extending across an active area of each fuel cell.

The end plate 4 comprises a support plate 5 and a fluid chamber 6 disposed between the support plate 5 and an end face of the stack of cells 2. The fluid chamber 6 has an inlet port 8 which can be coupled to a hydrogen or other fuel source 9. The fluid chamber 6 also has an outlet port 10 which is coupled, by way of a fluid communication path 11 to the anode fluid supply conduit 15. The fluid communication path 11 is shown schematically for clarity, and may be internal to the fuel cell stack 1. The stack of cells may include electrical collector plates 7 disposed between end-most cells 2 and their respective end plates 3, 4, for providing external electrical connections 13, 14 for output power from the fuel cell. The electrical collector plate 7 could form one wall of the fluid chamber 6, the other wall being formed by the inner surface of the support plate 5. Alternatively, the fluid chamber 6 could have walls defined by a separate bladder arrangement as will be discussed later.

The end plate 3 at the opposite end of the stack may also be constructed with a support plate 5 and a fluid chamber 6, and the fluid chamber 6 in the end plate 3 may also be coupled to the fuel source 9.

The fuel cell stack 1 is held together in compression by a suitable clamping arrangement bearing against the end plates 3 and 4, schematically indicated by the arrows 14.

The clamping arrangement 14 and the stiffness of the support plate 5 of each of the end plates 3, 4 may be generally sufficient to hold the fuel cell stack in compression such that a fluid tight relationship is maintained between all of the fuel cells. However, during operation of the fuel cell stack, it may become necessary to increase the anode fluid supply pressure to maintain adequate electrical performance of the stack. The increase in anode pressure may generally cause the internal pressures within the fuel cells to bear outwards on the end plates 3, 4 to such an extent that the stiffness or rigidity of the support plates 5 is no longer sufficient to prevent bowing or distortion of the plates and therefore may not, alone, be capable of providing sufficient compressive force across the entire surfaces of the support plates to maintain the fuel cells 2 in proper parallel relationship and undistorted.

However, under the circumstances of increased anode fluid supply pressure at the inlet port 8, the pressure will not only rise in the fuel cells 2, but also in the fluid chamber 6, since this is in fluid communication with the anode fluid supply conduit 15. The rise in pressure in the fluid chamber 6 will generate an expansion force in the fluid chamber 6 which will tend to counteract the corresponding expansion force in fuel cells 2. Thus, although the support plates 5 may now tend to distort outwards under the increased pressure, the fuel cells 2 in the stack should be maintained in, or closer to, a parallel condition by virtue of the compression profile asserted by the fluid chamber 6. Because of the fluid communication between the fluid chamber 6 and the anode fluid supply conduit and the anode fluid flow path through the cells 2, pressure increases and decreases in the anode fluid flow path can be followed by pressure increases and decreases in the fluid chamber 6.

The fuel cell stack 1 may deploy an end plate assembly such as that exemplified by the support plate 5 and fluid chamber 6 at one or at both ends of the stack 1. The fluid chamber 6 at either end of the stack may be coupled to the anode fluid supply 9 (e.g. hydrogen) or to the cathode fluid supply 17 (e.g. air), according to the most appropriate supply pressure variation. The fluid chamber 6 and the fuel cell reactant supply conduit (e.g. anode fluid supply conduit 15 or cathode fluid supply conduit 16) may be coupled to the anode fluid supply 9 or the cathode fluid supply 17 either in series (as exemplified by the arrangement shown in figure 1) or in parallel. In other words, in the series arrangement, the anode fluid supply 9 may be coupled to the inlet port 8 of the fluid chamber 6 and the anode fluid supply conduit 15 may be coupled to the outlet port 10 of the fluid chamber 6, where the inlet and outlet ports 8, 10 are preferably disposed on opposite sides of the fluid chamber 6. In the parallel arrangement, the fluid chamber 6 may have only a single port, e.g. port 10, and this port 10 and the anode fluid supply conduit 15 may both be coupled by a T-connection to the fuel source 9. The series or parallel arrangement can also be used if a fluid chamber 6 is connected to a cathode fluid supply 17 instead of the anode supply.

Thus, in a general aspect, at least one of the end plates 3, 4 has at least one fluid chamber 6 extending across the end face of the fuel cell stack 1, in which the fluid chamber is in fluid communication with the reactant fluid flow path, e.g. the anode fluid flow path or the cathode fluid flow path. This enables the pressure in the reactant fluid flaw path to be communicated to the fluid chamber 6.

The principles described above in connection with figure 1 can generally be applied in any fuel cell or fuel cell stack arrangement, and may be particularly useful in a single fuel cell configuration, and in a fuel cell or fuel cell stack with a somewhat flexible structure, as will now be described in greater detail.

With reference to figure 2, a single fuel cell assembly 20 has a fuel cell 21 comprising an anode flow field plate 22, a cathode flow field plate 23, a polymer electrolyte membrane 24 between the anode and cathode flow field plates and a gas diffusion layer 25 between the anode plate 22 and the membrane 24 and a gas diffusion layer 26 between the cathode plate 23 and the membrane 24. A suitable clamping arrangement shown schematically at 27 is provided at multiple points around the circumference of the fuel cell 21. A combined sealing and clamping arrangement could be provided by peripheral seals at the edges of the plates and/or C-shape cross-section clip around part or all of the circumference of the fuel cell.

As seen in the arrangement of figure 2, the anode 22 may be effectively integrated into or form part of an end plate and the cathode 23 may be effectively integrated into or form part of the other end plate. In the arrangement shown, the cathode 23 may be a slotted or ridged copper-coated polyimide (e.g. Kapton) film, e.g. where the slots or ridges define the cathode fluid flow channels. The anode 22 may similarly comprise a copper-coated polyimide (e.g Kapton) film, which may also be slotted or ridged to define anode fluid flow channels. For both the anode 22 and the cathode 23, the end plate may therefore generally comprise a flexible dielectric film on which is coated the anode or cathode electrode in the form of an electrically conductive coating. The fuel cell 21 may be held together by bonding the polyimide films together at the peripheral edge of the fuel cell.

Figure 3 shows schematically the fuel cell 21 in cross-section. The anode end plate 34 incorporates a flexible fluid chamber 36 operable on similar principles to that described in relation to figure 1. In this arrangement, however, the support plateS (figure 1) is replaced by a further flexible film 30. The flexible film 30 and the anode film 22 together define an end plate assembly 34 forming a bladder-like arrangement. An inlet port 31 is configured to allow anode fluid (e.g. hydrogen) into the flexible fluid chamber 36. A communication port 32 (e.g. a restrictive orifice) is configured to allow anode fluid to pass from the fluid chamber 36 to fluid flow paths in the anode fuel cell, and from there into the gas diffusion layer 25 and to the membrane 24, An anode fluid outlet 35 (figure 2) may be provided to vent the anode fluid flow paths if required.

In the arrangement shown in figures 2 and 3, the cathode 23 defines the end plate 33 which has no fluid chamber, although it could have one, if required.

An electrical contact 38 may extend from the anode electrode surface of the anode flexible film 22 to a point external to a sealing peripheral edge of the fuel cell 21, for external electrical connection to the anode. An electrical contact 39 may extend from the cathode electrode surface of the cathode flexible film 23 to a point external to the sealing peripheral edge of the fuel cell, for external electrical connection to the cathode.

In use, reactant such as hydrogen fuel is applied to the fuel cell via the inlet port 31, which pressurizes both the fluid chamber 36 and the anode fluid flow paths between the anode film 22 and the membrane 24. This causes a bulge in the fluid chamber 36, shown in greatly exaggerated form in figure 3, and the pressurization of the fluid chamber 36 ensures that the anode and cathode layers 22 and 23 are compressed together, to maintain the layered construction of the fuel cell under compression.

The fuel cell 21 may be mounted on any suitable substrate, or the end plate 33 or 34 could be made of material sufficiently stiff to resist excessive distortion in the fuel cell under pressure. If flexible layers are used for all layers of the fuel cell 21, the fuel cell could be formed to be generally non-planar, e.g. disposed on or around other shaped structures, e.g. around a cylinder. The pressurization of the fluid chamber 36 will ensure a good pressure distribution across an entire active area of the fuel cell to ensure optimum performance and good electrical conductivity between the various layers 22-26 of the fuel cell. The pressurization of the fluid chamber 36 can be maintained at a higher pressure than the anode fluid flow path, e.g. by use of a suitable pressure regulator such as a restrictive orifice as the communication pod 32.

Although the embodiments described above use a single fluid chamber 6, 36 in an end plate, it is possible to provide multiple fluid chambers, or compartments to a single fluid chamber, distributed over the area of an end plate. The fluid chambers can be interconnected to ensure even pressure distribution. The fluid chambers could be separate, with differential pressures from fluid chamber to fluid chamber, to deliberately create a non-uniform pressure distribution across the area of the end plate. This non-uniform pressure distribution could be, for example, increased pressure towards the central part of the end plate. Preferably, the fluid chamber 6, 36, or multiple fluid chambers, extend across the full extent of the active areas of the fuel cell or fuel cells. Alternatively, the fluid chamber or chambers can be positioned towards a central part of the end plate area, or distributed at discrete locations across the end plate area, The fluid chamber or chambers could be filled with a porous foam to provide a degree of shaping to the end plate, while still allowing the fluid chamber or chambers to be filled and pressurized with a reactant fluid, More generally, the or each fluid chamber may be defined by an envelope which could be completely or partially filled with any resilient fluid-permeable structure within the envelope.

The fluid chamber or chambers can be expandable or non-expandable in total volume.

Thus, the chamber 6, 36 may be defined by at least one wall having some elasticity to create an expandable chamber. For example, the flexible film 30 may be elastic.

Alternatively, the chamber 6, 36 may be defined by walls that are inelastic, so that any variation in the shape of the end plate arising from increased pressure does not increase the volume of the chamber.

The fluid chamber may generally be defined by a flexible membrane which is coupled to and sealed to a support plate which is more rigid than the flexible membrane, such as exemplified in figure 1. Alternatively, the fluid chamber may be entirely defined by flexible membranes, such as exemplified in figures 2 and 3.

The fluid chamber may be defined by an envelope constructed at least in part with an electrically conductive medium such that the fluid chamber wall can operate as a collector plate.

The embodiments described above have generally illustrated arrangements in which a fluid chamber, which forms part of an end plate for a fuel cell or a fuel cell stack, is in fluid communication with a reactant fluid flow path through the fuel cell or cells in a stack. The fluid communication conveniently and efficiently ensures that a variation in the pressure of the reactant (e.g. hydrogen) in the fluid flow path of the cell or cells is accompanied by a corresponding variation in pressure applied by the end plate to the fuel cell or cells. The corresponding variation may generally comprise a linear or non-linear monotonic relationship. Thus, any tendency for the layers of the fuel cell or cells to expand under increased reactant pressure can be resisted by a related increase in pressure in the fluid chamber.

The fluid communication path between the fluid chamber and the reactant fluid flow path through the fuel cell or cells provides a solution which is relatively simple and low cost to implement. However, it will be recognised that varying the pressure in a fluid chamber extending across the end face of the fuel cell or stack as a function of the pressure in a reactant fluid flow path in the cell or stack could be achieved by electronic, mechanical or electromechanical control of suitable valves independently supplying pressurized fluid to a fluid chamber and to the reactant fluid flow path. Thus, the fluid supplied to the fluid chamber could be different from the reactant fluid, though this would represent a more technically complex solution.

Use of a fluid chamber, the pressurization of which can be varied as a function of the pressurization of a reactant fluid flow path (such as the anode or cathode fluid flow paths), allows a light weight fuel cell design where distortion in an end plate is acceptable and can be accommodated. The fluid chamber also allows single fuel cells to be formed in which the end plates (i.e. outer layers) of the fuel cell can be formed in fully flexible materials which can be shaped around or over non-planar substrates. For fully flexible end plates, the distortion in the fuel cell arising from the pressurization can be limited by constraining inward displacement of the peripheral edge of the fuel cell.

The fluid chamber arrangements discussed above can be particularly useful for fuel cells where there is a significant pressure difference between the anode fluid flow paths and the cathode fluid flow paths, e.g. where the fuel cell operates with high pressure hydrogen and an open cathode arrangement at or close to atmospheric pressure.

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

Claims

CLAIMS1. A fuel cell assembly comprising: one or more fuel cells assembled in a layered construction, the layered construction defining two opposing end faces, the or each fuel cell having a reactant fluid flow path extending across an active area of the fuel cell; an end plate at each opposing end face of the layered construction, the end plates being coupled together to thereby maintain the layered construction of the one or more fuel cells under compression; wherein at least one of the end plates comprises a fluid chamber extending across the end face and in fluid communication with the reactant fluid flow path.
2. The fuel cell assembly of claim 1 in which the reactant fluid flow path is an anode fluid flow path.
3. The fuel cell assembly of claim 1 in which the fluid chamber has an inlet port coupled to a reactant fuel supply conduit of the fuel cell assembly.
4. The fuel cell assembly of claim 1 in which the fluid chamber has an inlet port coupled to an anode fuel supply conduit of the fuel cell assembly and an outlet port coupled to the anode fluid flow path of at least one of the one or more fuel cells.
5. The fuel cell assembly of claim I in which the fluid chamber is an expandable fluid chamber.
6. The fuel cell assembly of claim I in which the fluid chamber comprises a flexible membrane coupled and sealed to an end plate of the at least one end plates.
7. The fuel cell assembly of claim 1 in which the fluid chamber comprises an electrically conductive medium.
8. The fuel cell assembly of claim 1 in which the fluid chamber comprises a flexible fluid tight envelope with a resilient fluid-permeable structure within the envelope.
9. The fuel cell assembly of claim 8 in which the resilient fluid permeable structure comprises a foam.
10. The fuel cell assembly of claim 1 in which the fluid chamber comprises multiple compartments each in fluid communication with the anode fluid flow path.
11. The fuel cell assembly of claim 1 in which the fluid chamber comprises a flexible membrane coupled and sealed to a current collector plate of the at least one end plates.
12. The fuel cell assembly of claim 1 in which the layers of the one or more fuel cells and the end plates are flexible and for non-planar.
13. The fuel cell assembly of claim I in which the fluid chamber is fluid communication with the reactant fluid flow path by way of a pressure regulator.
14. A fuel cell assembly comprising: one or more fuel cells assembled in a layered construction, the layered construction defining two opposing end faces, each fuel cell having a reactant fluid flow path extending across an active area of the fuel cell; an end plate at each opposing end face of the layered construction, the end plates being coupled together to thereby maintain the layered construction of the one or more fuel cells under compression; wherein at least one of the end plates comprises a fluid chamber extending across the end face, and further includes means for varying the pressure in the fluid chamber as a function of pressure in the reactant fluid flow path.
15. A method of maintaining layers of a layered fuel cell assembly in compression during use, comprising the steps of: providing a reactant fluid supply to the fuel cell assembly; pressurising a reactant fluid flow path in a fuel cell of the fuel cell assembly with the reactant fluid supply and simultaneously pressurising a fluid chamber in an end plate of the fuel cell assembly adjacent to the fuel cell with the anode fluid supply to thereby maintain the layers of the fuel cell under compression.