CA2241566A1

CA2241566A1 - Flow field plate

Info

Publication number: CA2241566A1
Application number: CA002241566A
Authority: CA
Inventors: Winston R. Mackelvie
Original assignee: Bondface Technology Inc
Current assignee: Bondface Technology Inc
Priority date: 1998-06-23
Filing date: 1998-06-23
Publication date: 1999-12-23
Also published as: AU4355399A; WO1999067845A1

Abstract

A flow field plate comprising surface layers of electrically conductive material, and a core layer of electrically conductive material between the surface layers within the thickness of the plate; the plate defining multiple sets of fluid passages comprising first sets of passages, one set formed in the thickness of each surface layer and open to and parallel to the surface of that layer, a second set of passages formed in the thickness of the core layer and extending transversely to the passages of the first sets to provide points of intersection with the latter when viewed in plan, ports placing passages of the second set in communication with passages of one or other of the first sets at points of intersection of the passages, and a third set of passages extending perpendicularly through the layers, without intersecting the first sets of passages, and each communicating with a passage or passages of the second set to provide fluid paths into, out of, or through the first sets of passages via the second set of passages

Description

This invention relates to flow field plates. Flow field plates are typically used in fuel cell stacks in which they perform several functions.
In a typical fuel cell stack of the membrane type as described for example in U.S. Patent No. 3, 134, 696, membranes are sandwiched between porous catalytic electrode layers, and in turn between flow field plates which separate the cells in the stack. The flow field plates perform multiple functions.
They act as current collectors for the electrodes and they provide electrical continuity between adjacent cells. They separately distribute reagent gases (oxygen and hydrogen) across opposite faces of the plate in contact with opposite polarity electrodes of adjacent cells They remove the product of reaction (water) typically from the oxygen side, and should supply adequate moisture to the hydrogen side to prevent dehydration of the membrane. They act to conduct away heat generated at the membrane during operation of the cello These multiple functions result in such plates having a complex structure and being expensive to produce. Flow field plate construction of diverse types are exemplified by U.S.
Patents Nos. 3,814,631 (Warszawski et al); 4,125,676 (Maricle et al); 4,649,091 (McElroy); 5,108,849 (Watkins et al);
5,300,370 (Washington et al); 5,445,904 ( Kaufman); 5,484,666 (Gibb et al); 5,514,487 (Washington et al); 5,683,828 (Spear et al); 5,707,755 (Grot) and 5,709,961 (Cisar et al), It is an object of the invention to provide a flow field plate which is effective to carry out its function, but relatively simple and economical to manufacture.
According to the invention, there is provided a flow field plate comprising surface layers of electrically conductive material, a core layer of electrically conductive material between the surface layers within the thickness of the plate, wherein the plate defines multiple sets of fluid passages comprising first sets of passages, one set formed in the thickness of each surface layer and open to and parallel to the surface of that layer, a second .set of passages formed in the thickness of the core layer and extending transversely to the passages of the first sets to provide points of intersection with the latter when viewed in plan, ports placing passages of the second set in communication with passages of one or other of the first sets at points of intersection of the passages, and a third set of passages extending perpendicularly through the layers, without intersecting passages of the first sets, and each communicating with a passage or passages of the second set to provide fluid paths into, out of, or through the first sets of passages via the second set of passages. The surface or core layers may be formed integrally or as a sandwich construction. The first sets of passages are preferably machined into the surface layer in a concentric circular or helical pattern. The layers may be formed of graphite or formed or metallized with a metal resistant to corrosion under the operating conditions of a fuel cell.
Further features of the invention will be apparent from the appended claims and from the following description of presently preferred embodiments of the invention with reference to the accompanying drawings, in which:
Figure 1 is an exploded view of a fuel cell stack incorporating a first embodiment of flow field plate;
Figure 2 is a plan view of the flow field plate shown in Figure 1;
Figure 3 is a plan view of a modification of the flow field plate shown in Figure 2;
Figure 4 is a fragmentary cross-section on the line 4-4 in Figure 3;

2 Figures 5-7 are plan views from the same side of separately formed layers of a second embodiment of flow field plate;
Figure 8 is an exploded isometric view of the embodiment of Figures 5-7;
Figure 9 is a plan view of a third embodiment of flow field plate;
Figure 10 is a cross-section of a variation of the embodiment of Figure 9; and Figure 11 is a plan view of a fourth embodiment of flow field plate.
Referring first to Figure 1, there is shown in exploded view components of a fuel cell stack incorporating flow field plates 1 in accordance with the invention; end flow field plates la in the stack may, as shown, be single rather than double sided since the face adjacent an end cap will not form part of a cell. The cells in the stack are formed by electrode assemblies, of which only one is shown, inserted between adjacent flow field plates. Each electrode assembly comprises in this example, a semipermeable proton exchange membrane B, on each side of which are located porous graphitic electrode layers C&F. It should be understood that flow field plates in accordance with the invention could also be utilized with other types of electrode assembly presenting planar electrode surfaces to the plates, and in other types of electrochemical cell stacks, for example cells using electrical power to disassociate electrolytes into gases rather than the reverse process that takes place in fuel cells, although fuel cells are presently seen as a primary electrochemical application for the plates. Further possible application are in filter presses or fluid purification units in which the electrode assembly would be replaced by a

3 suitable filter element of osmotic membrane, or ion transfer a l ement The membrane B is clamped adjacent its outer periphery and adjacent a central aperture by 0-rings 31b and 31e located in grooves 31d (see Figure 4) in the adjacent flow field plates when the stack of flow field plates and electrode assemblies is clamped between end plates (of which only one is shown) by an axial tie rod (not shown) passing through a central bore 12 in a core member 20 on which the electrode assemblies and flow field plates are assembled. Elastomeric collars G within the central bores 9 of the flow field plates interact with the apices of core 20 to define three channels 4d, 5d and 6d extending through the bores 12 longitudinally of the stack forming fluid passages for oxygen, hydrogen and water, these passages communicating with ports J in the end cap H.
Washers E may optionally or alternatively be used to seal the passages so formed at the membranes B.
Opposite surface layers of the plates 1 are formed with a series of concentric grooves forming first sets of channels covering an annular area between the O-rings 31b and 31e, this area corresponding to that of the electrodes C and Fo On the sides of the plates seen in Figures 1 and 2, the-set of grooves comprises alternating grooves 2 and 3, while on the opposite side (see Figure 4) there is one set of grooves 10.
Drilled radial bores 4, 5 and 6, forming a second set of channels, extend through core layers of the plates 1 between the surface layers, and communicate respectively with the grooves 2, 3 and 10 through ports or vias 4a, 5a and 6a respectively The bores 4, 5 and 6 communicate with the channels 4d, 5d and 6d, forming a third set of channels though ports of which only port 6c is referenced. The channels 4d, 4 and 2 of each plate conduct oxygen to fields adjacent the electrodes F adjoining electrode assemblies on one side of the plates, and the channels 6d, 6 and 10 of each

4 plate conduct hydrogen to fields adjacent the electrodes of the electrode assemblies adjoining the other sides of the plates. The channels 3, 5a and 5d conduct water, formed by reaction between the oxygen and the hydrogen of the membrane under the influence of the catalyst treated electrodes, away from the reaction zone. The width and shape of lands 3a between the grooves 2 and 3 may be controlled (compare Figure 10) so as to maximize the area of the electrodes exposed to the reagent fields, and having regard to the porosity of the electrode material to allow oxygen and water to migrate the channels 2 towards the channels 3. The width and shape of lands between the channels 10 may be similarly controlled.
Since the channels nearest the centre of the plate are shorter, it may be desirable to make these channels narrower so as to reduce the fluid flow through these channels compared to those of greater radius.
The drillings forming channels 4, 5 and 6 are closed at their outer ends by a further 0-ring 31 retained in a channel 8 around the periphery of each plate 1.
The reaction between the hydrogen and the oxygen at the membrane is exothermic, and it may be desirable to provide additional cooling of the assembly during operation. This is facilitated by the modification of the plate shown in Figure 2 as illustrated in Figure 3. As compared with the plate of Figure 2, the core member 20, instead of being approximately triangular, is in the form of a five pointed star so as to define five rather than three passages within the bores 9.
The additional passages 11 and lla communicate with additional radial bores llc and lld in the plate, while the 0-ring 31 is replaced by a sealing collar 31c so as to enclose the channel 8 around the periphery of the plate. The channels 4, 5 and 6 are closed at their outer ends by plugs 4c, 5c and 6c. Cooling liquid may be fed to the stack through the channel 11 and exit through the channel 11a after

5 passing through the plates via the channels llc, lld and 8.
The plates 1 may be constructed in various ways. In one presently preferred form, a disc of graphite is machined on its opposite faces to form the grooves 2, 3 and 10 and on its periphery to form the channel 8. Such circular grooves are readily machined even in a material such as graphite. The radial bores are drilled. Rather than graphite, the plate may be formed of metal such as a noble metal or corrosion resistant alloy, but noble metals are very costly, and corrosion resistant alloys or metal may have inadequate corrosion resistance, or, in the case of metals such as titanium or tantalum, may be costly and difficult to machine.
Another possible approach is to mould or cast the plate with at least the surface grooves, drill the radial passages, and metallize the completed plate using a noble metal. In this case the plate may be machined, cast or moulded from base metal or synthetic resin, provided that the integrity of the metallization of the various passages can be assured if the substrate material is not itself corrosion resistant. In the drawings the ports or vias 4a, 5a and 6a are shown as separately formed, but it may be practical to displace the radial drillings sufficiently towards the relevant surfaces of the plate that the primary and secondary passages intersect without additional drillings. If the plates are used in a non-electrochemical application, then their conductivity may be immaterial, and they can be moulded or machined from synthetic resin.
In use in a fuel cell, the stack incorporating the plates is preferably operated with the passages 2 and 3 facing upwardly, so that water formed by interaction at the membrane of oxygen and hydrogen accumulates in and is drained from the passages 3.

6 Turning now to Figures 5 to 8, an alternative embodiment of plate is shown, in which the same reference numerals are utilized to indicate similar parts. The opportunity has been taken in the several figures to illustrate variations of this embodiment, but collectively the Figure shows respectively a first surface layer 100, a core layer 101 and a second surface layer 102 which are assembled in the relationship shown in Figure 8 to form a complete plate. Each layer may for example be formed by either as already described above, or by embossing a sheet of a deformable graphitic composition to the various passages. For example, the grooves 2, 3 and 10 may be pressed into the outer layers, and the secondary passages pressed into the appropriate side of the centre layer 10. Such pressed passages, such as the passage 50o in the centre layer will weaken it less than punched or drilled slots such as 6 or 40. The passage may communicate with a central passage 9, divided by a core 20, through ports 40d, 50d, or with off-centre through passages such as 13. As seen in Figure 6, the arrangement may incorporate cooling passages, as described above with reference to Figure 3.
Referring now to Figures 9 and 10, these Figures illustrate an embodiment incorporating certain variations of the embodiments of Figures 1-4. For example, the drilled passages in the core layer of the plate need not be radial, so long as they can intersect the passages 1, 2 and 3.
Moreover, the third passages extending longitudinally of the stack need not be located in the centre portions of the plates. In Figure 8, a non radial passage 90 extends between longitudinal passages 70, 80, sealed to passages of adjacent plates by 0-rings 70b, 80b. The passages 70, 80 are radially outward of the 0-rings 31b. In Figures 9 and 10 a group of longitudinal passages 13 sealed by 0-rings 13a provide for admission of hydrogen, oxygen and cooling water, while an central passage 12 provides for drainage of water produced by reaction, and for the passage of a tie-rod (not shown).

7 Figure 11 shows how multiple stacks of cells may be assembled using a single set of plates 1. The plates in this case are rectangular, the stack being held together by the rods (not shown) through passages 12. The drillings 4, 5 and 6 (closed at the edge of the plate by plugs such as 6c) may be connected to longitudinal passages formed either by the passages 12, or segments of a central (relative to the grooves 2, 3) core 20, as previously described.
Although the passages 1, 2 and 3 have been described as circular and concentric, helical grooves could be employed to form the passages and are easily machined. Separation of the passages 2 and 3 can be achieved in this case by use of a multi-start helix. In cases where the grooves must be machined from a material such as graphite, complex groove layouts should be avoided.

8

Claims

CLAIMS:

1. A flow field plate comprising surface layers of electrically conductive material, and a core layer of electrically conductive material between the surface layers within the thickness of the plate: the plate defining multiple sets of fluid passages comprising first sets of passages, one set formed in the thickness of each surface layer and open to and parallel to the surface of that layer, a second set of passages formed in the thickness of the core layer and extending transversely to the passages of the first sets to provide points of intersection with the latter when viewed in plan, ports placing passages of the second set in communication with passages of one or other of the first sets at points of intersection of the passages, and a third set of passages extending perpendicularly through the layers, without intersecting the first sets of passages, and each communicating with a passage or passages of the second set to provide fluid paths into, out of, or through the first sets of passages via the second set of passages.

2. A flow field plate according to claim 1, wherein the surface and core layers are integral, and the second set of passages is formed by drillings in the plane of the core layer.

3. A flow field plate according to claim 1, wherein the plate is of sandwich construction, with the surface and core layers formed separately, and the first and second sets of passages are formed separately in the layers, the second set of passage being formed by slots in the core layer.

4. A flow field plate according to claim 3, wherein at least the core layer is formed from compressible conductive material, and the passages are pressed into the material.

5. A flow field plate according to claim 1-3, wherein each first set of passages is machined in the material of its respective surface layer.

6. A flow field plate according to any one of claims 1-5, wherein the passages of each first set are concentric and circular or single or multiple start helices.

7. A flow field plate according to any one of claims 1-6, wherein the passages of each first set have different widths or spacings, according to their function, and/or their radial position.

8. A flow field plate according to any one of claims 1-7, wherein a set of first passages on one side of the plate is to carry hydrogen, and adjacent passages on the other side carry oxygen and water respectively.

9. A flow field assembly according to any one of claims 1-8, wherein the first passages are generally circular and concentric, the second passage are radial, and the third passages are parallel to the central axis of the first passages.

10. A flow field plate according to any one of claims 1-7, wherein at least one of the surface and core layers is formed of graphite.

11. A flow field plate according to any one of claims 1-10, wherein at least one of the surface and core layers is formed of a metal resistant to corrosion under the operating conditions prevailing in a fuel cell.

12. A flow field plate according to any one of claims 1-9, wherein the material at least one of the surface and core layers is rendered conductive by metallizing with a metal resistant to corrosion under the operating conditions prevailing in a fuel cell.

13. A flow field plate according to claim 6, wherein the passages of the third set are located in plan radially inside or outside of an area occupied by the passages of the first sets.

14. A flow field plate according to claim 13, wherein the passages of the third set are formed by subdivision of a peripheral portion of the axial passage, with a central portion reserved for passage of an axial tension rod.

15. A flow field plate according to any one of the claims 1-14 wherein the plate defines multiple coplanar flow fields.

16. A fuel cell stack comprising a stack of flow field plates according to any one of claims 1-15, interleaved by electrode assemblies comprising membranes sandwiched between porous electrode layers, the passages of the third set in each plate being in longitudinal communication parallel to a longitudinal axis of the stack, and means to compress the stack longitudinally.

17. A fuel cell stack according to claim 16, wherein the passages of the first sets occupy annular areas on opposite sides of the flow field plates, and 0-rings engaged in annular grooves radially inwardly and outwardly of these areas engage the membranes inwardly and outwardly of the electrode layers.

18. A fuel call stack according to claim 16 or 17, wherein sealing members associated with the third passages provide continuity and sealing of longitudinal passages through the stack formed by cooperation of the third passages.

19. A flow field plate substantially in accordance with any of the embodiments hereinbefore described with reference to the accompanying drawings.

20. A fuel cell stack substantially in accordance with any of the embodiments hereinbefore described with reference to the accompanying drawings.

21. A stack of flow field plates configured substantively as described in accordance with any of the embodiments shown in the accompanying drawings, interleaved with selectively permeable membranes.