GB2502079A

GB2502079A - Fuel Cells

Info

Publication number: GB2502079A
Application number: GB1208482.8A
Authority: GB
Inventors: Hugh Liam Sutherland; Christopher Reynolds
Original assignee: AFC Energy PLC
Current assignee: AFC Energy PLC
Priority date: 2012-05-15
Filing date: 2012-05-15
Publication date: 2013-11-20
Also published as: GB201208482D0

Abstract

A fuel cell comprises an electrolyte chamber (208), and comprising two electrodes (10a, 10b), one on either side of the electrolyte chamber (208). Each electrode comprises a fluid-permeable layer of electrically-conductive material, and also a catalytic material. The fluid-permeable layer comprises a permeable polymer matrix comprising polymer particles (120) bonded together at points of contact (125), and particulate electrically-conductive material such as carbon nanotubes (122) for electrical conductivity.

Description

Fuel Cells The present invention relates to fuel cells, preferably but not exclusively liquid electrolyte fuel cells, and to electrodes suitable for such fuel cells.

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 mechanically and electrochemically durable. Acid fuel cells and fuel cells employing other liquid 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. Such electrodes are expensive, electrically inefficient, and may suffer from irregular distribution of catalyst. Furthermore, the nickel mesh may cause local irregularities in electric properties due to resistance at the contact points between the wires of the mesh. If the mesh extends to the edge of the electrode, this can lead to sealing problems, because a mesh allows fluid flow in the plane of the mesh.

Some of these problems can be solved using instead of the mesh, a sheet of metal through which holes are defined for example by laser drilling, as described in WO 2011/01584 to (AFC Energy). However it may be difficult to achieve adequate and uniform gas diffusion to reach the catalyst carried by the electrode.

Discussion of the Invention The electrode of the present invention addresses or mitigates one or more

problems of the prior art.

Accordingly the present invention, in a first aspect, provides a fuel cell with a polymer electrolyte or means to define an electrolyte chamber for a liquid electrolyte, and comprising two electrodes, one electrode on either side of the electrolyte or electrolyte chamber, each electrode comprising a fluid-permeable layer of electrically-conductive material which comprises catalytic material; wherein the fluid-permeable layer comprises a porous and fluid-permeable polymer matrix comprising polymer particles bonded together at points of contact, and electrically-conductive particulate material for electrical conductivity.

The conductive particulate material may comprise carbon, in the form of carbon black or carbon nanotubes, or metallic whiskers. Preferably at least some of the particulate material is in the form of particles that have a length greater than their width, such as carbon nanotubes or metallic whiskers, as this provides good electrical conductivity along the length of each particle.

The electrically-conductive particulate material may be incorporated within the polymer particles, or may be at the surfaces of the polymer particles.

Each electrode may also comprise a sheet of metal through which are defined a multiplicity of through-holes, the fluid-permeable layer of electrically-conductive material being in electrical contact with the sheet of metal and being bonded to it.

The electrode may be arranged such that the fluid-permeable layer faces the electrolyte or electrolyte chamber, or the electrode might instead be arranged with the fluid-permeable layer facing a gas chamber.

The electrode must comprise a catalyst to enable the chemical reaction with the gas phase to occur. In some cases the material of the fluid-permeable layer may be sufficiently catalytic for this purpose, but more usually the electrode also incorporates a separate catalytic material, which may be a coating. The electrode is at least partially permeable to gas, so as to enable intimate contact between the liquid electrolyte, the catalytic material and the gas phase, with a gas/liquid interface in contact with the catalytic material.

Where there is a sheet of metal in the electrode, the through-holes may be defined by etched or drilled holes, so there are discrete holes. One suitable structure is formed by laser drilling. The holes may also be formed by a chemical etching process. The thickness of the metal sheet may be between 0.1 mm and 3mm, more preferably between 0.15 mm and 0.5 mm, for example 0.3 mm (300 pm) or 0.2 mm (200 pm); and the holes may be of width or diameter between 5 pm and 2 mm, for example typically about 20 pm or 50 pm if formed by laser drilling, or about 200 pm or 300 pm if formed by chemical etching, and spaced between 50 pm and 10 mm apart. As an alternative, a much thinner layer of metal, for example a film of thickness less than 20 pm or less than 5 pm, which may be supported on a polymer substrate, may be perforated either by laser ablation or by chemical etching; and metal then deposited by electroplating onto the perforated metal film so as to achieve the desired thickness of metal. In some cases the diameter of the hole gradually decreases through the thickness of the sheet, so the holes are slightly tapered, while in other cases the holes taper from both surfaces with longitudinally curved walls, so the minimum diameter is near the centre-plane of the metal sheet, while in yet other cases the holes are of substantially uniform diameter. The holes may be formed using a combination of two or more techniques, for example using chemical etching in conjunction with laser ablation. In cross-section, the holes may for example be circular, oval or elliptical. Somewhat larger holes, for example up to 2 mm or 3 mm across, and which might be circular, oval or slit-shaped, or of polygonal or irregular shape, might also be used.

As compared to a metal mesh it will be appreciated that such a metal sheet provides better electrical conduction, as no wire-to-wire contacts are involved; it also provides a more uniform distribution of current; and the structure is stiffer, as there are no crossing-over wires that can move relative to each other. The size and spacing of the holes would also be selected to ensure satisfactory diffusion of the reactant species (gas) to and from the gas-permeable layer and so the interface.

Preferably the holes are of average diameter between 30 pm and 300 pm, for example 50 pm or 200 pm, and are at a centre-to-centre separation of at least 0.15 mm. In any event the holes may occupy less than 50% of the area of the metal sheet, preferably less than 25% and optionally less than 10%; indeed the proportion may be less than 1%.

The electrode must be sealed to adjacent components of the fuel cell, and where the electrode incorporates a sheet of metal, the sheet may therefore define a peripheral margin without any through-holes. The sealing between the electrode and adjacent components would therefore be to this peripheral margin, simplifying the sealing. Where no such metal sheet is provided, the fluid-permeable layer would preferably be integral with a peripheral margin of the same material, against which adjacent components may be sealed, the peripheral margin preferably being impermeable to fluid to simplify sealing.

An electrode which does not include a metal sheet avoids any risk of delamination; however the provision of a metal sheet as part of the electrode may provide lower electrical resistance.

The use of a hydrophobic polymer for the polymer matrix may be advantageous. A matrix of hydrophobic polymer inhibits aqueous electrolyte from passing right through the fluid-permeable layer, which may therefore be referred to as a gas-permeable layer. Some suitable polymers would include polyethylene (PE), polypropylene (PP). polytetrafluoroethylene (PTFE). polyether-ether-ketone (PEEK), or a copolymer such as poly (hexafluoropropylene-co-tetrafluoroethylene). The hydrophobic polymer must be inert to other components of the fuel cell that it comes into contact with, such as any liquid electrolyte.

The fluid-permeable layer may include other types of carbon such as carbon fibres, graphite, graphene and activated charcoal; and potentially buckyballs and nanohorns. These types of carbon also provide good electrical conductivity. Where metal whiskers are used, they may for example be of nickel or silver. Where there is a sheet of metal in the electrode, the thickness of the fluid-permeable layer may be greater than or equal to the separation between the holes through the metal sheet.

In any event, the fluid-permeable layer is preferably at least 0.15 mm thick, but preferably less than 4 mm thick, for example between 0.2 mm and 1.5 mm thick. The fluid-permeable layer is formed of polymer particles, and the pores within it should therefore be small and close together, forming a fluid-permeable network of pores throughout the layer. For example the pores would typically be of width less than 20 pm, and may be spaced apart by less than 50 pm. Such a fluid-permeable layer may be formed by laser processing to sinter polymer particles together.

In the fuel cell, the electrode may be sealed by gaskets to adjacent structural components, for example to a frame to define the electrolyte chamber. If the electrode includes a sheet of metal and the fluid-permeable layer does not extend to the edges of the metal sheet, an edge of the fluid-permeable layer may also be covered by the gasket. The gasket may be stepped to enclose an edge region of the fluid-permeable layer. In any event the edge region of the fluid-permeable layer is desirably held onto the metal sheet, for example by a seal or gasket, in addition to the fluid-permeable layer being bonded onto the metal sheet.

A suitable metal for the metal sheet may be nickel, or may be stainless-steel; other metals that are not significantly affected by the electrolyte may also be used. In some cases it may be preferable to use a metal such as silver, gold or titanium, either to form the sheet or to provide a coating on the sheet. If the metal is a steel that contains both chromium and manganese, heat treatment of the steel may generate a chromium manganese oxide spinel coating on the surface, which is itself electrically conductive and protective to the underlying metal. Similar protective coatings may be formed on an electrode of other metals, or may be formed using known deposition techniques such as electro-deposition. The provision of a protective coating on the surface may enhance the chemical durability of the metal sheet; where no such protective layer is present, the durability of the metal sheet would be decreased. The preferred material is nickel, as this is resistant to corrosion in contact with an alkaline electrolyte for example of potassium hydroxide solution.

In another aspect, the present invention provides an electrode suitable for use in such a fuel cell. Such an electrode may therefore comprise a fluid-permeable layer of electrically-conductive material which comprises catalytic material; wherein the fluid-permeable layer comprises a permeable polymer matrix comprising polymer particles bonded together at points of contact, and carbon nanotubes for electrical conductivity. The electrode may also include a sheet of metal defining through holes.

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 la shows a cross-sectional view through an electrode; Figure lb shows a cross-sectional view through a modification of the electrode of figure la; Figure lc shows a cross-sectional view through an alternative electrode; Figure 2 shows a gas-permeable portion of the electrode of figure 1 a to a larger scale; Figure 3 shows an alternative gas-permeable portion of the electrode of figure 1 a, to the same scale as figure 2; and Figure 4 shows a cross-sectional view of a fuel cell stack incorporating electrodes as shown in figure 1; Electrode Structure Referring to figure 1 a, an electrode 10 comprises a sheet 11 of a metal such as nickel. The sheet 11 is of thickness 0.3 mm. Most of the sheet -the central region 12 -is perforated by chemical etching to produce a very large number of through holes 14, the holes each being of mean diameter 300 pm and being separated by between 300 pm and 1000 pm; as a result of the etching process, each hole 14 has a cusped shape along its length. A margin 15 around the periphery of the sheet 11, of width 5 mm, is not perforated. The hole dimensions and separations are given here by way of example; as an alternative the holes might be of mean diameter only -100 pm and separated by between 100 pm and 300 pm; such smaller holes might be made by laser drilling. As shown in Figure lb, if the holes 14 are made by a different process, such as laser drilling, they may have a different shape along their length, for example being tapered.

After forming the through holes 14, one surface of the perforated central region 12 is then covered in an electrically-conductive gas-permeable layer 16 of polymer-based material; the exposed surface of the gas-permeable layer 16 is provided with a coating 18 of catalytically active materiaL Referring to figure lc, an alternative electrode 110 comprises an electrically-conductive sheet 111 layer of polymer-based material. The sheet 111 is of thickness 1.2 mm. Most of the sheet -the central region 112 -is porous and gas-permeable. A margin 115 around the periphery of the sheet 111, of width 5 mm, is not gas-permeable. The exposed surface of the gas-permeable region 112 is provided with a coating 18 of catalytically active material.

The gas-permeable layer 16 of figures la and lb may be of the same material as the sheet 111 of figure lc.

Referring now to figure 2 the gas-permeable layer 16 consists of multiple polymer particles 120, which in this case are of PTFE, each particle 120 being generally spherical and containing carbon nanotubes 122 within each polymer particle 120. The particles 120 may be of diameter in the range 10-100 pm, for example between 20 and 40 pm or between 30 and 80 pm, so they may be described as a powder. Alternatively they may be somewhat larger particles, for example between 50 and 200 pm.

The gas-permeable layer 16 is made by selective laser sintering. This process involves fusing together of thin layers of these particles 120 using a focused laser beam in such a way that the pores 124 between the particles 120 remain open.

In one example the powder is deposited in layers of thickness between 20 and 60 pm, and each layer is then scanned with a high intensity laser beam so that the particles 120 within the layer fuse together at their points of contact 125, as do the points ot contact with the layer below. Powder deposition and laser scanning are repeated many times to create the entire gas-permeable layer 16. This is typically done using a piston or support plate which is covered with the powder, and lowering the piston or support plate every time a thin layer of the powder is deposited. After scanning the selected parts of each layer with the laser, the powder bed is lowered by one layer thickness, and the process repeated. Preferably the entire bed of powder is warmed up to slightly below the temperature required for sintering, so that the power required by the laser is reduced.

This procedure can provide a gas-permeable layer with a void fraction of more than 10%, preferably more than 15%, such as 20%, 25% or 30%. The void fraction depends on the uniformity of size of the particles 120, and on the degree of sintering.

After performing this process the gas-permeable layer 16 can be removed, and any un-sintered powder is then emptied out of the space above the piston or support plate.

The sheet 111 of figure 1 c may be made in substantially the same way. The central region 112 is made exactly as described above, whereas the impermeable margin 115 is made by sintering the particles 120 at a greater laser intensity or longer duration, so that the pores 124 between the particles 120 are blocked. The material in the margin 115 is therefore densified, whereas the material in the central region 112 remains porous, with a high void fraction as discussed above.

Referring to figure 3, the gas-permeable layer 16 may instead consist of multiple polymer particles 130, which in this case are of PTFE, each particle 130 being generally spherical, where the surfaces of each particle 130 are coated with carbon nanotubes 122. The particles 130 may be of diameter in the range 10-1 00 pm, for example between 20 and 40 pm or between 30 and 80 pm, so they may be described as a powder. Alternatively they may be somewhat larger particles, for example between 50 and 200 pm. The gas-permeable layer 16 and the sheet 111 may be made from these coated particles by selective laser sintering in substantially the same way as described above, but using these particles 130 that are coated with carbon nanotubes 122.

In either case the resulting gas-permeable layer 16 or the sheet 111 is electrically conductive because of the presence of the carbon nanotubes, which form a conductive network throughout the polymer matrix.

Bonding Processes As mentioned above, the gas-permeable layer 16 of Figure 1 a or lb is bonded to the sheet 11. This may be achieved by compressing the gas-permeable layer 16 on to the sheet 11 at an elevated temperature (below the temperature at which the polymer matrix would deform), this heat treatment bonding the gas-permeable layer 16 onto the sheet 11. Adhesion between them may be enhanced by the provision of a thin polymer coating on the metal before the gas-permeable layer 16 is put into position, leaving part of the metal exposed to ensure electrical contact; the polymer bonds them together Adhesion can also be enhanced by roughening the surface of the sheet 11, and this may be achieved using a laser, or by an etching process for example using a grain-boundary-specific etchant. Adhesion can also be enhanced by controlling the shape of the holes 14 such that the holes 14 taper along part of their length, as the gas-permeable layer 16 may then be pressed firmly onto the sheet 11 such that portions of the gas-permeable layer 16 are extruded through the tapered part of the holes l4so as to protrude slightly beyond the narrowest part, providing a rivet-like mechanical bond onto the sheet 11. In the case of chemically etched holes, the holes may taper from both surfaces to a narrowest part or cusp near the middle of the sheet (as shown in Figure 1 a).

An alternative way of bonding the gas-permeable layer 16 onto the metal sheet 11 is by forming an interfacial bond comprising an amorphous ceramic. This may be achieved by providing the surfaces of the sheet 11 and of the gas-permeable layer 16 with coatings no more than 1 pm thick of an amorphous-ceramic precursor.

These coatings are preferably no more than 0.5 pm thick, and more preferably no more than 0.1 pm thick. The sheet 11 and the gas-permeable layer 16 are then assembled and pressed together, and treated so as to form the amorphous ceramic from the precursor material. In some cases this may only require drying, while in other cases heating is needed. Clearly the heating must not exceed temperatures at which the polymer in the gas-permeable layer 16 would deteriorate, so the heating is to a much lower temperature than would be used when forming a crystalline ceramic.

For example it may be appropriate to heat at between 200°C and 300°C, or between 230°C and 270°C, for example 250°C, and to hold at that temperature for between mm and 1.5 hours. This leads to formation of the amorphous ceramic, bonding the materials together.

By way of example, a solution is made of about 0.085 M zirconium acetylacetonate in 50:50 methoxypropanol-denatured ethanol as solvent, with the addition of 50 drops of surfactant (e.g. Tergitol TMN6 (trade mark)) per 100 ml of solution. This is sprayed with an ultrasonically atomised spray system onto those surfaces of the metal sheet 11 and of the gas-permeable layer 16 that are to be bonded together. The solvents are allowed to evaporate, leaving an oily film of the zirconium salt and surtactani. Spraying and allowing the solvents to evaporate may be repeated two or three more times.

The sheet 11 and the layer 16 are then further dried at an elevated temperature, for example between 25° and 45°C, for at least 2.5 minutes, and then pressed together, dried at the elevated temperature for between 10 and 30 mm, and then baked at 250°C for 30 mm. The pressing step may be carried out at 1.4 kN/cm2 = 14 MPa for 10 seconds.

It is surmised that the zirconium salt forms a waxy film with the surfactant.

The salt melts at about 190°C, running over the surface to some extent, and then decomposes to form amorphous zirconia in surface cracks and at the interface.

It will be appreciated that the ceramic may comprise other metal oxides, such as those of cerium, indium, tin, manganese, or cobalt, or mixtures of oxides. The precursor may be an acetylacetenoate, or an alkoxide such as a formate or acetate, or other metal-organic compounds in which organic groups are bound to a metal atom via an oxygen atom, and which can be broken down thermally. An alternative is to use a salt (such as zirconium chloride, indium chloride or tin chloride) dissolved in alcohol, as this behaves analogously to the corresponding alkoxide when heated.

Another option is to use an oxide in a colloidal form, for example using flame hydrolysed zirconia or tin oxide dispersed in water, optionally with an organic binder such as polyvinyl alcohol; in this case it may be sufficient to dry the colloid to form the amorphous oxide. In each case it may be advantageous to incorporate a surfactant into the initial solution, to enhance the contact with the polymer in the gas permeable layer 16.

Catalyst Coating The electrode 10 or 110 may be used in either a cathode or an anode; the principal difference would be in the composition of the catalyst mixture that forms the coating 18, and indeed some catalyst compositions may be suitable in both anodes and cathodes.

By way of example, catalyst mixtures for both cathode and anode electrodes may use a combination of catalyst, binder and solvent which is spray-coated onto the surface of the gas-permeable layer 16 or of the porous central region 112 to form the coating 18. 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 E below include a hydrogen oxidation catalyst.

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

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

As an alternative, the catalyst might comprise silver particles, deposited by spraying a suspension of silver particles in a liquid, and then baking the electrode 10 or 110 so that the silver particles partly sinter together. Whatever type of catalyst is deposited as the coating 18, it is important that the exposed surface of the electrode or 110 remains permeable, as liquid electrolyte must permeate the coating 18, to meet the gas that permeates through the gas-permeable layer 16 or the portion 112, so there is a gas/electrolyte interface within the coating 18, where the catalyst is present. Furthermore it follows that the coating 18 should be at least partly hydrophilic. In a modification, before depositing the coating 18, the exposed surface of the gas-permeable layer 16 is given a surface texture, for example by rolling with a textured roller, before spraying on the catalyst-containing coating 18. The surface texture may for example provide variations in thickness of up to 50 pm.

Another alternative way of introducing the catalyst would be to form a thin catalyst layer by an extrusion process and pressing such an extruded layer onto the gas-permeable layer 16 or 112. Screen printing would be another technique.

Cell Structure Referring now to figure 4, there is shown a cross-sectional view through the structural components of a cell stack 200 with the components separated for clarity.

The stack 200 consists of a stack of moulded plastic plates 202 and 206 arranged alternately. Each plate 202 defines a generally rectangular through-aperture 208 surrounded by a frame 204; the apertures 208 provide electrolyte chambers, and immediately surrounding the aperture 208 is a 5 mm wide portion 205 of the frame which projects 0.5 mm above the surface of the remaining part of the frame 204. The plates 206 are bipolar plates; each defines rectangular blind recesses 207 and 209 on opposite faces, each recess being about 3 mm deep, surrounded by a frame 210 generally similar to the frame 204, but in which there is a 5 mm wide shallow recess 211 of depth 1.0 mm surrounding each recess. The blind recesses 207 and 209 provide gas chambers.

It will thus be appreciated that between one bipolar plate 206 and the next in the stack 200 (or between the last bipolar plate 206 and an end plate 230), there is an electrolyte chamber 208, with an anode ba on one side and a cathode lOb on the opposite side; and there are gas chambers 207 and 209 at the opposite faces of the anode ba and the cathode lOb respectively. These components constitute a single fuel cell.

Electrodes bOa and lOb locate in the shallow recesses 211 on opposite sides of each bipolar plate 206, with the catalyst-carrying face of the electrode 1 Oa or 1 Ob facing the adjacent electrolyte chambei 208. Before assembly of the stack components, the opposed surfaces of each frame 204 (including that of the raised portion 205) may be covered with gasket sealant 215; this adheres to the frame 204 and dries to give a non-tacky outer surtace, while remaining resilient. The components are then assembled as described, so that the raised portions 205 locate in the shallow recesses 211, securing the electrodes ba and lOb in place. The sealant 215 ensures that electrolyte in the chambers 208 cannot leak out, and that gases cannot leak in, around the edges of the electrodes ba and lOb, and also ensures that gases cannot leak out between adjacent frames 204 and 210. The perforated central section 12 of each electrode 10 corresponds to the area of the electrolyte chamber 208 and of the gas chamber 207 or 209; the non-perforated peripheral margin iSis sealed into the peripheral shallow recess 211; and the gas-permeable layer 16 with the catalytic coating 18 is on the face of the electrode 1 Oa or lOb closest to the adjacent electrolyte chamber 208.

The electrodes 1 Oa and 1 Ob may have the structure of the electrode 10 of figure 1 a, or they may have the structure of the electrode 110 of figure lb. It will be appreciated that this cell stack 200 is shown by way of example only, as an illustration of how the electrodes 10 or 110 of the invention may be used.

Whatever the detailed arrangements of the cell stack 200 may be, in each case a single fuel cell consists of an electrolyte chamber 208 with electrodes 1 Oa and 1 Ob on either side which separate it from adjacent gas chambers 207 and 209. Within the stack 200 several fuel cells are arranged so as to be electrically in series, to provide a greater voltage than is available from a single cell. In an alternative cell stack, for example, the electrodes may extend to the outside surface of the cell stack, rather than locating in recesses; this may require a wider impermeable margin 15, 115 than described above.

The flows of fluids to the fuel cells follow respective fluid flow ducts, at least some of which may be defined by aligned apertures through the plates 202 and 206.

One such set of apertures 216 and 218 is shown, which (in this example) would be suitable for carrying electrolyte to or from the electrolyte chambers 208 via narrow transverse ducts 220. The flows of one or both gases to and from the gas chambers (recesses 207 and 209) may similarly be along ducts defined by aligned apertures through the plates 202 and 206.

At one end of the stack 200 is a polar plate 230 which defines a blind recess 209 on one face but is blank on the outer face. Outside this is an end plate 240, which defines apertures 242 which align with the apertures 216 and 218 in the plates 202 and 206; at the outside face the end plate 240 also defines ports 244 communicating with the apertures 242 and so with the fluid flow ducts through which the gases and electrolyte flow to or from the stack 200, each port 244 comprising a cylindrical recess on the outer face. At the other end of the stack 200 is another polar plate (not shown) which defines a blind recess 207, and there may be another end plate (not shown) which may be blank on the outer face and not define through apertures; alternatively it may define apertures for one or more of oxidant gas, fuel gas and electrolyte.

After assembly of the stack 200 the components may be secured together for example using a strap 235 (shown partly broken away) around the entire stack 200.

Other means may also be used for securing the components, such as bolts.

The electrodes 10 described above each comprise a sheet 11 of nickel.

Another material may be used instead. For example the sheet 11 may be of steel, which may be coated with a thin layer of nickel. The nickel is a good electrical conductor, and also protects the stainless steel against corrosion from the electrolyte.

In use of the electrode 10 in the cell of figure 4, aqueous electrolyte such as potassium hydroxide solution (KOH) is present at the face carrying the catalytic coating 18, while gas is present at the other face. The gas permeates the gas-permeable layer 16; while the electrolyte at least partly permeates the catalytic coating 18, but not the gas-permeable layer 16 because of the hydrophobic nature of the polymer matrix of PTFE. Consequently there is a gas/liquid-electrolyte interface in the vicinity of the catalyst. The gas does not bubble through the electrode into the electrolyte, as the interface is at a substantially constant position. The gas undergoes a chemical reaction in the vicinity of the coating 18. Fresh gas diffuses into the gas-permeable layer 16 from the holes 14, and any reaction products either diffuse back out through the holes 14 or are taken away by the liquid electrolyte. Although the holes 14 are spaced apart, the gas-permeable layer 16 is sufficiently thick that the gas flow reaching the coating 18 is substantially uniform over its entire area. The electrode 110 would operate in substantially the same way.

Claims

Claims 1. A fuel cell with a polymer electrolyte or means to define an electrolyte chamber for a liquid electrolyte, and comprising two electrodes, one electrode on either side of the electrolyte or electrolyte chamber, each electrode comprising a fluid-permeable layer of electrically-conductive material which comprises catalytic material; wherein the fluid-permeable layer comprises a porous and fluid-permeable polymer matrix comprising polymer particles bonded together at points of contact, and electrically-conductive particulate material for electrical conductivity.
2. A fuel cell as claimed in claim 1 wherein the conductive particulate material comprises carbon, in the form of carbon nanotubes.
3. A fuel cell as claimed in claim 1 or claim 2 wherein the conductive particulate material comprises metallic whiskers.
4. A fuel cell as claimed in any one of the preceding claims wherein the conductive particulate material is incorporated within the polymer particles.
5. A fuel cell as claimed in any one of claims 1 to 3 wherein the conductive particulate material is at the surfaces of the polymer particles.
6. A fuel cell as claimed in any one of the preceding claims wherein the electrode also incorporates catalytic material in a coating on the fluid-permeable layer.
7. A fuel cell as claimed in any one of the preceding claims, also comprising a sheet of metal through which are defined a multiplicity of through-holes, the fluid-permeable layer of electrically-conductive material being in electrical contact with the sheet of metal and being bonded to it.
8. A fuel cell as claimed in claim 7 wherein the thickness of the metal sheet is between 0.1 mm and 3 mm.
9. A fuel cell as claimed in claim 7 or claim 8 wherein the holes are of width or diameter between 5 pm and 4 mm.
10. Afuel cell as claimed in claim 7, claim B or claim 9 wherein the holes occupy less than 25% of the area of the metal sheet.
11. A fuel cell as claimed in any one of the preceding claims wherein the electrode defines a peripheral margin without any through-holes.
12. A fuel cell as claimed in any one of the preceding claims wherein the polymer of the polymer matrix is selected from polyethylene (PE), polypropylene (PP), polytetrafluoroethylene (PTFE). polyether-ether-ketone (PEEK), and poly (hexafluoropropylene-co-tetrafluoroethylene).
13. Afuel cell as claimed in any one of the preceding claims wherein the fluid-permeable layer also includes at least one other type of carbon such as graphite, graphene, activated charcoal, buckyballs and nanohorns.
14. An electrode suitable for use in a fuel cell as claimed in a one of the preceding claims.
15. A fuel cell substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.
16. An electrode substantially as hereinbefore described with reference to, and as shown in, figures la, ib, 3 or4 of the accompanying drawings.