GB2472450A

GB2472450A - Cell Stack Plates

Info

Publication number: GB2472450A
Application number: GB0913833A
Authority: GB
Inventors: Roger Anthony Pitts
Original assignee: AFC Energy PLC
Current assignee: AFC Energy PLC
Priority date: 2009-08-07
Filing date: 2009-08-07
Publication date: 2011-02-09
Also published as: GB0913833D0

Abstract

A plate for use in a cell stack (160), the plate (100,120) being formed of a polymeric plastic material e.g. acrylonitrilie butadiene styrene , with electrically conducting elements (112, 116, 128) embedded in the plastic material. The electrically conducting elements may be of metal e.g. nickel or stainless steel, and they provide at least part of an electrical conduction path for electric current within or from the cell stack (160). The plate may be a bipolar plate (100) or a polar plate (120). This provides a cheaper and lighter structure than using metal plates.

Description

Cell Stack Plates The present invention relates to plates that may be used in electric cell stacks such as fuel cell stacks, flow batteries or electrolysis cell stacks, preferably but not exclusively stacks of alkaline fuel cells, for separating successive cells or for forming an end to a stack, and also to cell stack assemblies including the plates.

Background to the invention

Fuel cells have been identified as a relatively clean and efficient source of electrical power. Alkaline fuel cells are of particular interest because they operate at relatively low temperatures, are efficient and suitable for operation in an industrial environment.

Acid fuel cells and fuel cells employing other aqueous electrolytes are also of interest. Such fuel cells typically comprise an electrolyte chamber separated from a fuel gas chamber (containing a fuel gas, typically hydrogen) and a further gas chamber (containing an oxidant gas, usually air) . The electrolyte chamber is separated from the gas chambers using electrodes.

Typical electrodes for alkaline fuel cells comprise a conductive metal mesh, typically nickel, that provides mechanical strength to the electrode. Onto the metal mesh is deposited a catalyst as a slurry or dispersion of particulate poly tetra-fluoroethylene (PTFE), activated carbon and a catalyst metal, typically platinum. A single cell provides only a low voltage, and cells are customarily arranged in stacks to provide increase the available power and voltage.

Discussion of the invention There is provided in accordance with the present invention a plate for use in a cell stack, the plate being formed of a polymeric plastics material, with electrically conducting elements embedded in the plastics material, wherein the electrically conducting elements provide at least part of an electrical conduction path for electric current from or within the cell stack.

In one embodiment the electrically conducting elements extend from one face of the plate to the opposite face of the plate. In another embodiment one or more embedded electrically conducting elements are exposed at one face of the plate.

Where the plate is used as a bipolar plate to separate successive fuel cells, then the conducting elements extend from one face of the plate to the opposite face, and so provide an electrical conduction path between electrodes on opposite sides of the bipolar plate. Where the plate is at an end of a cell stack, and is therefore a polar plate, then conducting elements may extend from one face to the opposite face, in the same way as with the bipolar plate, to provide an electrical conduction path from the last electrode in the stack to an outlet conductor of the cell stack; but alternatively the end plate may incorporate a conducting element exposed at its outer surface, to which an outlet conductor would make contact, and the periphery of the plate on both faces may be coated with an electrically conductive material.

It will be appreciated that this provides a significantly cheaper and lighter structure than the use of metal plates as bipolar plates and as polar or end plates.

The present invention also provide a cell stack assembly that includes at least one such plate.

The electrically conducting elements are preferably metal elements. The metal of the embedded electrically- conducting elements may be nickel, or may be stainless-steel, or other metals that do not readily react with the fluids within the cell stack. If the metal is a steel that contains both cobalt and manganese, heat treatment of the steel may generate a manganese cobalt oxide spinel coating on the surface, which is itself electrically conductive and protective to the metal. Similar protective coatings may be formed on an electrode of other metals, or deposited using known deposition techniques such as electrophoresis. The provision of a protective coating on the surface would enhance the durability of the embedded metal elements; where no such protective layer is present, the durability of the embedded metal elements, and so of the plate, would be decreased. A preferred material is nickel, as this is not susceptible to corrosion in contact with an alkaline electro]yte for examp]e of potassium hydroxide so]ution.

In a second aspect the present invention provides an electrolyte chamber plate for use in a cell stack, the electrolyte chamber plate being of a plastics polymeric material and being over-moulded with a gasket material, so the gasket material is on both faces of the plate and also around the edge of the electrolyte chamber.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a cross-sectional view through a bipolar plate of the invention; Figure 2 shows a perspective view of a metal insert that may be used in a bipolar plate similar to that of figure 1; Figure 3 shows a cross-sectional view through a polar plate of the invention; Figure 4 shows a cross-sectional view through an alternative polar plate of the invention; Figure 5 shows a cross-sectional view of a fuel cell stack incorporating plates of the invention; Figure 6 shows a perspective view of an alternative bipolar plate; and Figure 7 shows a perspective view of an electrolyte plate for use with the bipolar plate of figure 6.

The invention is generally applicable to low temperature fuel cells, and will be described in relation to a fuel cell and fuel cell stack using potassium hydroxide aqueous solution as the electrolyte. An individual fuel cell consist of spaced apart permeable electrodes between which is an electrolyte chamber. The electrodes are respectively an anode and a cathode, and they separate the e]ectro]yte chamber from respective gas chambers: the anode separates the electrolyte chamber from a chamber containing a fuel gas such as hydrogen, whereas the cathode separates the electrolyte chamber from a chamber containing an oxidising gas such as oxygen. Typically both the anode and the cathode incorporate not only an electrical conductor but also a catalyst for the corresponding electrode reaction.

By way of example, catalyst mixtures for both cathode and anode electrodes may use a combination of catalyst and binder which is spray-coated onto the surface of the sheet 10. The binder may for example be a polyolefin (such as polyethylene) which been made tacky by heat treatment with a liquid such as a hydrocarbon (typically between C6 and C12), the liquid then acting as a dispersing agent for the catalyst particles and for the binder, and evaporating after the coating step.

Percentage weights refer to the total mass of the dry materials. Some example compositions are as follows: The cathode catalyst mixtures A to C below include an oxygen reduction catalyst.

A. Activated carbon, with 10% binder.

B. 10% Pd/Pt on activated carbon, with 10% binder.

C. Silver on activated carbon, with 10% binder.

The anode catalyst mixtures D and B below include a hydrogen oxidation catalyst.

D. Nickel-aluminum alloy powder with activated carbon, with 10% binder.

F. 10% Pd/Pt on activated carbon, with 10% binder.

In a fuel cell stack there are plates to separate successive cells in the stack. In particular these may be bipolar plates, separating an anode (and a hydrogen chamber) on one side from a cathode (and an oxygen chamber) on the opposite side. Referring now to figure 1 there is shown a bipolar plate 100 of the invention. The plate 100 is rectangular in plan, and of thickness 10 mm, and it defines rectangular blind recesses 102 and 104 on opposite faces, each recess being about 3 mm deep, surrounded by a frame 106 in which there is a 5 mm wide shallow recess 108 of depth 1.0 mm surrounding each blind recess 102 and 104. The blind recesses 102 and 104 provide the gas chambers. Within the frame 106 are defined a number of fluid flow passages 109 (only one of which is shown in figure 1) . When assembled into a fuel cell stack, electrodes 10 (an anode lOa and a cathode lOb -shown in figure 5) completely cover each blind recess 102 or 104 and locate into those shallow recesses 108.

The plate 100 can be made by injection moulding from a plastics material, such as acrylonitrile butadiene styrene (ABS) . It incorporates several nickel pins or studs 112 that extend between opposite surfaces of the plate 100, being embedded in the plastics material so the pins or studs 112 are a tight fit in the plastics material; they are tight enough to ensure no gas leakage.

These pins or studs 112 may be incorporated and embedded during the production of the plate 100, which may for example be by an injection moulding process. They are of diameter 3 mm, and are arranged spaced apart for example at 10 mm spacings all around the portion of the plate 100 where there is the shallow recess 108.

In a modification, instead of pins or studs 112 there might instead be one or more strips of metal. For example, referring to figure 2, instead of the pins or studs 112 there might instead be a continuous metal strip 116 embedded in the plastics material around the shallow recess 108, the strip 116 being castellated along its length so that the raised portions are flush with one face of the bipolar plate 100 and the lowered portions are flush with the opposite faces of the bipolar plate 100. Typically the strip 116 itself would be of rectangular cross-section, of thickness between 0.4 and 1.5 mm and of width between 2 and 6 mm, for example being 0.5 mm by 2.5 mm; and the castellations would be of length between 10 mm and 20 mm, for example 15 mm.

Preferably the castellations are symmetrical, with the length of the raised portions being substantially equal to the length of the lowered portions. The strip 116 shown in figure 2 would be embedded in the plastics material of the plate 100. Like the pins or studs 112, the strip 116 may be of nickel. Alternatively the pins or studs 112, or the strips 116, might instead be of stainless steel.

Instead of such a continuous metal strip 116 it will be appreciated that there might be a number of separate castellated shaped strips, for example a straight castellated strip along each of the four sides that make up the rectangular periphery. It will also be appreciated that the exact shape of the castellations is not of concern, as long as the castellated strip is exposed at both faces of the plate 100 in which it is embedded.

At the end of the fuel cell stack must be an similar plate, which defines a gas chamber on only one side, and so may be referred to as a polar plate. Referring to figure 3 there is shown one such polar plate 120. This is similar to the bipolar plate 100, in being moulded of plastics material such as ABS. and being of the same rectangular shape in plan, and of thickness 10 mm, and in that it defines a rectangular blind recess 102 on one face which is surrounded by a shallow recess 108. Around the blind recess 102 the plate 120 defines a frame 122 through which are defined flow channels 129 (only one of which is shown) Embedded at the rear face of the polar plate 120 is a rectangular nickel sheet 128 whose surface is flush with the rear surface of the polar plate 120. The frame 122 including the shallow recess 108 is plated with nickel, for example by electroless deposition, the plating extending over the edge of the nickel sheet 128.

In use an electrode 10 (see figure 5) of the last cell in the stack locates in the shallow recess 108. The plated nickel coating provides electrical conduction from the electrode 10 in the recess 108 around the outside of the frame 122 to the embedded nickel sheet 128. An external connection can therefore be made to the embedded nickel sheet 128, which is in good electrical connection with the electrode 10.

Referring to figure 4 there is shown a polar plate with many features identical to those of the polar plate 120. In the polar plate 130, however, there is no plating onto the plastic frame 122; instead the embedded nickel plate 128 incorporates projecting studs 132 that are also embedded within the plastics material of the end plate 130, so that the ends of the studs 132 are flush with the surface of the shallow recess 108. Such studs 132 may be of diameter 4 mm, and may be spaced apart at 10 mm intervals around the shallow recess 108. The studs 132 provide good electrical conduction from the electrode to the embedded nickel sheet 128. An external connection can therefore be made to the embedded nickel sheet 128.

Referring now to figure 5, there is shown a cross-sectional view through the structural components of a cell stack 160 with the components separated for clarity.

The stack 160 consists of a stack of moulded plastic plates 100 and 200 arranged alternately. The plates 100 are bipolar plates as described in relation to figure 1.

The blind recesses 102 and 104 provide gas chambers. The plates 200 define a generally rectangular through-aperture 202; the apertures 202 provide electrolyte chambers; immediately surrounding the aperture 202 is a 5 mm wide portion 205 which projects 0.5 mm above the surface of the remaining part of the plate 200. At each end of the stack 160 there is a polar plate 120, outside which is an end plate 170.

It will thus be appreciated that between one bipolar plate 100 and the next in the stack 160 (or between the last bipolar plate 100 and the polar plate 120), there is an electrolyte chamber 202, with an anode lOa on one side and a cathode lOb on the opposite side; and there are gas chambers 102 and 104 at the opposite faces of the anode lOa and the cathode lOb respectively. These components Electrodes lOa and lOb locate in the shallow recesses 108 on opposite sides of each bipolar plate 100, with the catalyst-carrying face of the electrode lOa or lOb facing the respective blind recess 102 or 104 respectively. Before assembly of the stack components, the opposed surfaces of each plate 200 (including that of the raised portion 205) is covered with gasket sealant 215; this adheres to the plate 200 and dries to give a non-tacky outer surface, while remaining resilient. The components are then assembled as described, so that the raised portions 205 locate in the shallow recesses 108, securing the electrodes lOa and lOb in place. The sealant 215 ensures that electrolyte in the chambers 202 cannot leak out, and that gases cannot leak in, around the edges of the electrodes lOa and lOb, and also ensures that gases cannot]eak out between adjacent p]ates 100 and 200. There is an appropriate catalyst coating on the face of each electrode plate 10 closest to the adjacent gas chamber 102 or 104.

The flow of electrolyte to and from the electrolyte chambers (apertures 202), and the flows of the gases to and from the gas chambers (recesses 102 and 104), follow respective fluid flow ducts defined by aligned apertures through the plates 100 and 200; only one such set of apertures 109 and 209 are shown in figure 5. This set of apertures 109 and 209 provides electrolyte to the electrolyte chambers 202 via narrow transverse ducts 212.

-10 -The sealant 215 is placed so as not to block the apertures 209. At one end of the stack 200 is the polar plate 120 and the end plate 170. The end plate 170 is also of a plastics material, and defines through holes 179 to align with the aligned apertures 109, 209 and 129; and at the outer face of the end plate 170 each hole terminates in a larger-diameter socket 175. At the other end of the stack 200 is an end plate (not shown) which does not define through apertures.

After assembly of the stack 160 the components may be secured together, for example using a strap 220 (shown partly broken away) around the entire stack 160.

Electrical contact to the cell stack 160 may be made by a lead (not shown) through the end plate 170 to contact the embedded nickel sheet 128.

It will he appreciated that the cell stack 160 is given by way of example, and it may be modified while remaining within the scope of the present invention, which is that defined by the claims. In particular it is not restricted to plates of particular shapes and sizes.

Where the conduction path is provided by pins or studs 112 or 132, these may typically be of diameter in the range 0.5 mm up to 10 mm, more preferably between 2 mm and 6 mm. They are preferably arranged so as to make contact with the adjacent electrode at spaced positions around its entire perimeter, to minimise the variations in the current density through the electrode. Typically the spacing between one pin or stud and the next should be no more than 30 mm, more preferably no more than 20 mm, and more preferably no more than three times the diameter of the pin or stud. And as mentioned above the contact between successive electrodes may be provided by alternative-shaped embedded conducting elements.

-11 -Referring now to figure 6 there is shown a perspective view of an alternative bipolar plate 300 which is made of a polymeric plastics material such as ABS. The plate 300 defines rectangular blind recesses on each face, only the blind recess 302 for air being shown in figure 6; this blind recess 302 communicates through three channels 305 at each end that communicate with air inflow and air outflow holes 309. The blind recess (not shown) on the opposite face communicates similarly with two hydrogen inflow and hydrogen outflow holes 310 at each end. Between the portions of the plate 300 that define the inflow and outflow holes 309 and 310 is a thin web 320 defining holes for bolts (not shown) . Around each blind recess is a shallow peripheral recess 308 to locate an electrode, the depth of this recess 308 being equal to the thickness of the electrode, and near each corner of this peripheral recess 308 is a blind hole 311 for a locating pin 421 (see figure 7) . Along each side of the plate 300 there are four projecting thin flanges 314 each defining a through hole 315.

There are several nickel pins or studs 312 embedded in the plastics material of which the bipolar plate 300 is made, extending between the surfaces of the peripheral recesses 308 on each face of the bipolar plate 300.

Referring now to figure 7 is shown a perspective view of an electrolyte plate 400 for use with the bipolar plate 300 of figure 6. The plate 400 is made of a plastics material such as ABS. and is in the form of a generally rectangular frame enclosing an electrolyte chamber 402, and is substantially the same on both faces.

The plate 400 defines a peripheral rim 403 and raised ribs 404 at each end that correspond to the shape of the ends of the plate 300, with the thin web 320 extending over the raised ribs 404. The plate 400 defines air -12 -holes 409 and hydrogen holes 410 that align with the holes 309 and 310 in the bipolar plate 300, and defines through-holes 415 that align with the holes 315 on the bipolar plate 300. The through-holes 415 communicate through narrow ducts 416 with the electrolyte chamber 402. The ends and the sides of the plate 400 also define several holes 418 for bolts.

There is a flat area surrounding the electrolyte chamber 402, this being of a width greater than that of the peripheral recess 308 (so as to extend over the surrounding thicker portion of the bipolar plate 300); near each corner there is a projecting locating pin 421.

This flat area, along with the portions of the plate 400 defining the holes 409 and 410, which are all of the same thickness, are covered by a resilient gasket 430, this gasket also extending around the edge of the electrolyte chamber 402 and onto the opposite face of the electrolyte plate 400. In addition the gasket 430 includes thicker portions 432 around each of the through-holes 415. This gasket 430 may be produced by moulding it over the plate 400. There are gaps 434 in the gasket 430 at a number of locations along the flat area, and there are gaps aligned with the e]ectro]yte ducts 416. The gaps 434 ensure that the electrolyte plate 400 remains flat during the over-moulding process, so the thickness of the gasket 430 is uniform on both faces of the electrolyte plate 400.

A cell stack can hence be made by assembling bipolar plates 300, electrodes 10 and electrolyte plates 400 in a similar way to that described in relation to figure 5.

The gasket 430 holds the adjacent electrodes 10 firmly in contact with the adjacent bipolar plates 300, and seals the bipolar plate 300 to the electrolyte plate 400 around the outside of each electrode 10, preventing the electrolyte or gases from leaking. The gasket 430 also -13 -ensures that the aligned holes 309 and 409, and the aligned holes 310 and 410, define flow channels for air and for hydrogen that are leak tight, while the thicker portions 432 ensure that the aligned holes 315 and 415 define flow channels for electrolyte that are leak tight.

The locating pins 421 ensure the correct orientation and alignment of the electrodes (through which there are corresponding holes) . The cell stack, which may also include an end plate analogous to that shown in figure 5, is then secured using bolts through the holes 418.

In the examples given above all the plates are made of plastics materials. It will be appreciated that a cell stack might for example comprise bipolar plates as described above, but with polar plates that are entirely of metal; equally the bipolar plates of a cell stack might be of metal, while the polar plates might be as described above. Furthermore at least some of the polar or bipolar plates might be entirely of a conductive plastic material.

In another alternative the bipolar plates may not provide a shallow recess (such as 108 or 308) to locate the electrode, particularly where the electrode is also located by locating pins (such as 421) . In that case, as with the electrolyte plate 400, there would be no need in the plate 200 for the raised portion 205 around the edge of the electrolyte chamber 202. In another modification, an electrolyte plate 200 might be provided with an over-moulded gasket, like the gasket 430, in place of the separate coatings of gasket sealant 215.

It should also be understood that the plates with the embedded electrically conducted inserts, for use as bipolar plates and polar plates, and the plates that define electrolyte chambers and are provided with over- -14 -moulded gaskets, would be equally suitable for use in other types of cell stacks, for example in electrolysis cell stacks.

Claims

-15 -Claims 1. A plate for use in a cell stack, the plate being formed of a polymeric plastics material, with one or more electrically conducting elements embedded in the plastics material, wherein the electrically conducting elements provide at least part of an electrical conduction path for electric current from or within the cell stack.
2. A plate as claimed in claim 1 wherein the polymeric plastics material is electrically conducting.
3. A plate as claimed in claim 1 wherein the electrically conducting elements extend from one face of the plate to the opposite face of the plate.
4. A plate as claimed in claim 1 wherein the electrically conducting elements are exposed at one face of the plate.
5. A plate as claimed in claim 3 wherein the plate is for use as a bipolar plate and defines gas chambers on opposite faces thereof.
6. A plate as claimed in claim 3 wherein the plate is for use as a polar plate and defines a gas chamber on one face thereof.
7. A plate as claimed in claim 4 wherein the plate is for use as an end plate and defines a gas chamber on the face opposite to that at which the embedded metal elements are exposed.
8. A plate as claimed in any one of the preceding claims wherein the electrically conducting elements are of metal.

-16 -
9. A plate as claimed in claim 8 wherein the metal elements are of nickel or of stainless-steel.
10. A plate for use in a cell stack, the plate defining an electrolyte chamber and being of a plastics polymeric material, wherein the plate is over-moulded with a gasket material, so the gasket material is on both faces of the plate and also around the edge of the electrolyte chamber.
11. A cell stack assembly including at least one plate as claimed in any one of the preceding claims.
12. A plate for use in a cell stack substantially as hereinbefore described with reference to, and as shown in, any one of the accompanying drawings.
13. A cell stack assembly substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.