GB2510962A

GB2510962A - Fuel cells and fuel cell electrodes

Info

Publication number: GB2510962A
Application number: GB1322236.9A
Authority: GB
Inventors: Hugh Liam Sutherland; Gene Stacey Lewis; Richard Dawson; Christopher Reynolds; Jody Wheeler
Original assignee: AFC Energy PLC
Current assignee: AFC Energy PLC
Priority date: 2012-12-17
Filing date: 2013-12-16
Publication date: 2014-08-20
Also published as: GB201222728D0; GB201322236D0

Abstract

Electrodes for liquid electrolyte fuel cells comprise a sheet 11 of metal through which are defined a multiplicity of through-holes 14, and a hydrophobic gas-permeable layer 16 of fibrous and/or particulate electrically-conductive material which is bonded to and in electrical contact with the sheet of metal 11. To the outer surface of the gas-permeable layer 16 is bonded a hydrophobic catalyst-containing layer 18, preferably of similar thickness and less hydrophobic than the first layer 16, and to the outer surface of the catalyst-containing layer 18 is bonded a hydrophilic layer 20, such as carbon paper. The components can be assembled by hot pressing, starting with the layers 16 and 18 in a wet state containing the binder polymer in suspension; the hot pressing evaporates the liquid components and sinters the polymer binder, so bonding the layers 16, 18, 20 together and to the sheet 11. The electrode is provided as part of a liquid electrolyte fuel cell (200, fig 2), preferably an alkaline fuel cell, comprising means to define an electrolyte chamber (208, fig 2), and two electrodes 10, one on either side of the electrolyte chamber. The electrode 10 may be arranged in the fuel cell such that the hydrophilic layer 20 faces the electrolyte chamber (208, fig 2), and the metal sheet 11 is adjacent to a gas chamber (207 or 209, fig 2).

Description

Fuel Cells and Fuel Cell Electrodes The present invention relates to liquid electrolyte fuel cells, preferably but not exclusively alkaline fuel cells, and to electrodes suitable for such fuel cells, and to a way of making such electrodes.

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-tetrafluoroethylene (PTFE), activated carbon and a catalyst metal, typically platinum. Such electrodes are expensive, electrically inefficient, and may suffer from irregular distribution of catalyst. W02012/104599 (AFC Energy plc) describes an electrode which includes a perforated metal sheet, and a gas-permeable layer of fibrous or particulate electrically conductive material bonded onto the metal sheet, and provided with a catalytic material at its outer surface.

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 liquid electrolyte fuel cell with means to define an electrolyte chamber, and comprising two electrodes, one electrode on either side of the electrolyte chamber, each electrode comprising: -a sheet of metal through which are defined a multiplicity of through-holes; -a first fluid-permeable layer comprising fibrous and/or particulate electrically-conductive material which is bonded to the sheet of metal, in electrical contact with the sheet of metal, and is hydrophobic; -a second fluid-permeable layer comprising fibrous and/or particulate electrically-conductive material and catalytic material, which is bonded to the outer surface of the first fluid-permeable layer, and which is hydrophobic; and -a third fluid-permeable layer which is hydrophilic, and which is bonded to the outer surface of the second fluid-permeable layer.

In a second aspect, the present invention provides an electrode for use in suchafuelcell.

In a third aspect, the present invention provides a method of making an electrode suitable for use in such a fuel cell, the method comprising the steps of assembling: -a sheet of metal through which are defined a multiplicity of through-holes; -a first fluid-permeable layer comprising fibrous and/or particulate electrically-conductive material and a polymer binder in an un-sintered state, placed on to a surface of the sheet of metal, the first fluid-permeable layer being hydrophobic when the electrode is completed; -a second fluid-permeable layer comprising fibrous and/or particulate electrically-conductive material and catalytic material and a polymer binder in an un-sintered state, placed on to a surface of the first fluid-permeable layer opposite that in contact with the sheet of metal, the second fluid-permeable layer being hydrophobic when the electrode is completed; and -a third fluid-permeable layer which is hydrophilic, placed onto the outer surface of the second fluid-permeable layer; and -hot pressing the assembled items together, so that at least the sheet of metal, the first fluid-permeable layer, and the second fluid-permeable layer are bonded together, and the polymer binder in the first fluid-permeable layer and the second fluid-permeable layer becomes sintered, so that the first fluid-permeable layer and the second fluid-permeable layer are hydrophobic.

By way of example the first fluid-permeable layer and the second fluid-permeable layer may be initially assembled and rolled together. They can then be assembled with the sheet of metal and the third fluid-permeable layer on either side, and hot pressed together.

The hot pressing step may also bond the third fluid-permeable layer onto the electrode. It may use a temperature between 200°C and 300°C, or between 250°C and 300°C. The temperature is preferably lower than the melting point of the polymer binder by between 10°C and 50°C, to ensure the polymer is sintered rather than melting and flowing.

Typically the electrode is arranged such that the fluid-permeable layers face the electrolyte chamber. A gas diffuses through the holes in the sheet of metal and through the first fluid-permeable layer to reach the catalyst in the second fluid- permeable layer. The first fluid-permeable layer may be referred to as the gas-permeable layer. The cell should be operated so that the gas/electrolyte interface is within the thickness of the second fluid-permeable layer. Since this second fluid-permeable layer contains catalyst, it may be referred to as the catalyst-containing layer. In use the third fluid-permeable layer will therefore be within the electrolyte; but since the third fluid-permeable layer is hydrophilic, it is not detrimental to the operation of the cell.

The third fluid-permeable layer may be of carbon paper or thin carbon felt, which is a porous hydrophilic material, which is typically of porosity at least 75%. An alternative is a porous hydrophilic polymer membrane, for example a non-woven polymer membrane. There is no requirement for the third fluid-permeable layer to be electrically conducting. The provision of the third fluid-permeable layer has benefits during production, as it prevents material from the catalyst-containing layer adhering to pressing equipment; and it has benefits during use as it stabilises the outer surface of the catalyst-containing layer, improving the lifetime of the electrode.

The cathode may have the same structure as the anode, although the catalytic material may be different. It will be appreciated that the term "anode" refers to the electrode at which electrochemical oxidation takes place, and is the negative electrode of the fuel cell; the term "cathode" refers to the electrode at which electrochemical reduction takes place, and is the positive electrode of the fuel cell.

The second fluid-permeable layer (the catalyst-containing layer) may be of thickness of at least 50 pm, while the first fluid-permeable layer (the gas permeable layer) may also be of thickness of at least 50 pm. The second fluid-permeable layer may be of thickness between 100 pm and 500 pm, while the first fluid permeable layer may be of thickness between 100 pm and 500 pm.

The catalyst-containing layer is preferably less hydrophobic than the gas-permeable layer. The catalyst-containing layer may be rendered less hydrophobic by incorporating a smaller proportion of a hydrophobic polymer than that within the first fluid-permeable layer. For example the first fluid-permeable layer might comprise between 40% and 80%, for example 55% or 60%, of a hydrophobic polymer such as PTFE, PVdF, or a polyolefin as a percentage by weight of the layer, whereas the second fluid-permeable layer might contain a smaller proportion, for example between 5% and 70% of the percentage within the first fluid-permeable layer.

The catalytic material in the electrode enables the electro-chemical reactions to occur between the gas and liquid phases. In some cases the electrically-conducting material of the second fluid-permeable layer may be sufficiently catalytic for this purpose, but more usually the second fluid-permeable layer also incorporates a separate catalytic material. 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 the gas/liquid interface in contact with the catalytic material.

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 3 mm, more preferably between 0.15mm 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 150 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. 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, might also be used.

As compared to a metal mesh it will be appreciated that the metal sheet of the present invention 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 is also 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 150 pm. 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 first and second fluid-permeable layers may comprise carbon nanotubes, carbon black, and a hydrophobic binder such as PTFE. Other suitable forms of carbon are graphite, graphene and activated charcoal; and potentially buckyballs and nanohorns. These types of carbon provide good electrical conductivity, while the hydrophobic binder inhibits aqueous electrolyte from passing right through the fluid- permeable layers. The fluid-permeable layer may comprise other electrically-conductive particulate materials such as nickel whiskers.

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. The 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.

The metal of 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.

The fluid-permeable layer may be bonded to the sheet of metal by a polymer, or by a ceramic, such as an amorphous ceramic.

A fuel cell system would typically include multiple such fuel cells arranged as a fuel cell stack, in order to provide a larger output voltage.

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 an electrode; and Figure 2 shows a cross-sectional view of a fuel cell stack incorporating electrodes as shown in figure 1.

ELECTRODE STRUCTURE

Referring to figure 1, an electrode 10 comprises a sheet 11 of a metal such as nickel or ferritic stainless-steel. The sheet 11 is of thickness 0.3 mm. Most of the sheet -the central region 12 -is perforated for example by laser drilling to produce a very large number of through holes 14, the holes each being of mean diameter 50 pm and being separated by between 150 pm and 200 pm; as a result of the laser drilling process, each hole 14 is in practice slightly tapered along its length, typically from 70 pm at the front surface, on which the laser is incident, to 30 pm at the opposite surface. A margin 15 around the periphery of the sheet 11, of width 5mm, is not perforated. The dimensions and separations are given here by way of example.

As an alternative the holes 14 might be of mean diameter 300 pm and separated by between 200 pm and 800 pm; such larger holes 14 might be made by chemical etching.

After forming the through holes 14, one surface of the perforated central region 12 is then covered in a gas-permeable layer 16; the exposed surface of the gas-permeable layer 16 is then covered with a layer 18 which contains catalytically active material. The formation of these layers 16 and 18 will be described in the following paragraphs. In this example the gas-permeable layer 16 is of thickness 350 pm, while the catalyst-containing layer 18 is of thickness 250 pm. The outer surface of the catalyst-containing layer 18 is covered with a sheet 20 of carbon paper, which is hydrophilic, of porosity more than 75%, and is less than 50 pm thick.

The gas-permeable layer 16 is made by first mixing carbon with alcohol. A suitable mixture includes electrically-conductive carbon black, multiwall carbon nanotubes to enhance porosity and conductivity, and activated charcoal to enhance porosity, preferably the proportion of carbon black being greater than the proportion of nanotubes. For example it may comprise 60-90 wt% carbon black with lesser proportions of nanotubes and/or activated charcoal. An alternative mixture might comprise only carbon nanotubes, but of a range of different lengths and sizes. The carbon mixture is combined with a large excess of alcohol, typically between three and eight times the mass of carbon, for example five times the mass of carbon. This may for example be ethanol, isopropyl alcohol, or a longer-chain, branched or multifunctional alcohol such as butyl alcohol, pentyl alcohol, or ethylene glycol. The mixture may also contain a small proportion of an alcohol-retaining species, such as calcium acetate or poly(acrylic acid), for example at less than a fifth of the mass of carbon, more typically a tenth. This is then combined with an aqueous suspension of sub-micron sized PTFE particles, typically between two and three times the mass of carbon, for example 2.5 times the mass of carbon. This aqueous suspension of sub-micron sized PTFE particles may be 60 wt% PTFE, containing a branched secondary alcohol ethoxylate surfactant such as the Tergitol TMN series of surfactants (Tergitol is a trade mark). For example this suspension may be DuPont Zonyl (trade mark) FTFE TE-3887, in which the particles are of size between 0.05 and 0.5 pm and of average size 0.2 pm. Mien the PTFE suspension contacts the alcohol, it gels, so the mixture has a dough-like consistency.

In a modification, a solution of surfactant in water may be added to the carbon mixture, to form damp carbon; this is then mixed with an aqueous suspension of FTFE particles also containing surfactant; alcohol such as isopropyl alcohol is then mixed in, and the mixture gels to form a dough-like consistency. This may produce a more uniform coating of PTFE on the carbon particles.

The catalyst-containing layer 18 is made in an equivalent way, combining carbon with an appropriate active catalytic material in particulate form, and with PTFE. By way of example the electrode for the anode may incorporate 10% palladium or 10% palladium/platinum on activated carbon, optionally combined with carbon to provide electrical conductivity, the catalyst-loaded activated carbon constituting between 50% and 100% of the total mass of carbon; and the electrode for the cathode may incorporate carbon for electrical conductivity combined with a spinel MnCoO4, which is a catalytic material, with the spinel forming up to 80% of the total mass of the layer. The catalyst-containing layer 18 incorporates a smaller proportion of PTFE binder than does the gas-permeable layer 16. The catalyst-containing layer is therefore less hydrophobic.

For example the proportion of PTFE may be between 5% and 50% of the proportion in the gas-permeable layer 16, although it may be higher. Typically the catalyst-containing layer 18 would be made with the same 60 wt% aqueous suspension of PTFE particles, but using a much smaller quantity: the mass of the aqueous PIFE suspension is only between 10% and 20%, for example 15%, of the mass of carbon when making the anode catalyst layer; typically between 7% and 12%, for example 10% of the mass of carbon when making the cathode catalyst layer.

The two layers 16 and 18 may then be calendered together, for example through rollers, before finally being rolled out to a thickness of for example 0.3 or 0.7 mm. Whether or not the layers 16 and 18 are first calendered together, they are then placed on to the sheet 11, and covered with the carbon paper 20. At this stage both the layers 16 and 18 may still be wet with the alcohol and the water, the PTFE still being in suspension along with the surfactant, and therefore can readily bond together. Alternatively the layers 16 and 18 may be dried separately, but with the surfactant remaining, and can then be bonded together.

This assembly is then hot pressed, for example between platens at 250°C and with compression. The platens may be covered with a temperatule-resistant release film such as a 15 pm thick polyimide film (e.g. Kapton, trade mark). At this elevated temperature the alcohol and water evaporate, the suifactant breaks down into vapour or evaporates, and the PTFE particles sinter together (although the temperature is below the melting point of PTFE, which would be about 325-328°C).

The gas-permeable layer 16 bonds to the surface of the sheet 11, while the catalyst-containing layer 18 bonds to the gas-permeable layer 16. Both the gas-permeable layer 16 and the catalyst-containing layer 18 become denser, while remaining fluid- permeable. The carbon paper 20 bonds to the outer surface of the catalyst-containing layer 18. The carbon paper 20 prevents any risk of the catalyst-containing layer 18 bonding to the platen or to the release film.

In a modification, the hot pressing is applied between platens at different temperatures, so there is a temperature gradient. For example a platen next to the sheet 11 may be at 250°C, while the platen in contact with the carbon paper 20 may be at a lower temperature such as 150°C or 100°C.

In a further modification, the carbon paper 20 may be replaced by a slightly thicker layer, of carbon felt.

In either case the heating process leads to evaporation of any remaining alcohol and water, and evaporation or breakdown of the surfactant; and at least partial sintering of the PTFE particles, so that the gas-permeable layer 16 and the catalyst containing layer 18 together form a coherent and yet permeable hydrophobic structure which is bonded to the sheet 11, with the carbon paper 20 bonded to the outer surface.

In a modification to the composition described above, the mixture might comprise a different hydrophobic polymer, such as polyvinylidene fluoride, instead of or in addition to the PTFE.

It will be appreciated that the carbon paper 20 is a hydrophilic material, and it is readily permeable, with a high porosity which may be more than 80%. Its thickness is typically less than 500 pm, and may be less than 100 pm, although the thickness is preferably at least 200 pm.

In a modification, a different porous hydrophilic material may be used in place of the carbon paper 20. For example one suitable material is a non-woven chemically-treated polyolefin-based porous membrane such as that referred to as Scimat (trade mark), which is primarily used as a battery separator, and which is hydrophilic and typically of thickness about 150 pm, or less than 100 pm.

It will thus be appreciated that electrodes 10 which are suitable for use in a fuel cell using an aqueous-based electrolyte such as potassium hydroxide can readily be made. The resulting electrode 10 may be used as either an anode ba ora cathode lOb; these may differ in the composition of the catalyst provided within the catalyst-containing layer 18, although some catalyst materials may be suitable in both anodes and cathodes. Two such catalyst materials are mentioned above: 10% palladium or 10% palladium/platinum on activated carbon, for an anode iDa, and the spinel MnCoO4, for a cathode lOb. A variety of other catalyst materials may be used instead.

CELL STACK STRUCTURE

Referring now to figure 2, 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 5mm wide portion 205 of the frame which projects 0.5 mm above the surface of the remaining pad 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 1.0mm 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 ba and lOb locate in the shallow recesses 211 on opposite sides of each bipolar plate 206, with the catalyst-carrying face of the electrode ba or lOb facing the adjacent electrolyte chamber 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 surface, 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 bOa 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 iDa 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 15 is 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 10 closest to the adjacent electrolyte chamber 208.

As shown in figure 1 the gas-permeable layer 16 and the catalyst-containing layer 18 extend partly onto the non-perforated margin 15; and as shown in figure 2 this margin 15 is sealed into the shallow recess 211 by the sealant 215. Both the margin 15 and the recess 211 are 5 mm wide, in this example, so the edges of the gas-permeable layer 16 and the catalyst-containing layer 18 are enclosed by the sealant 215 and are therefore not directly exposed to the electrolyte, and are clamped by the sealant 215 onto the non-perforated margin 15.

It will be appreciated that this cell stack 200 is shown by way of example only, as an illustration of how the electrodes 10 of the invention may be used, and is a schematic view. For example the sealant 215 may be arranged in a somewhat different fashion from that shown. Vvliatever 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 ba and lOb 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.

The flows of fluids to the fuel cells follow respective fluid flow ducts, at least some of which are defined by aligned apertures through the plates 202 and 206. Only one such set of apertures 216 and 218 is shown, which would be suitable for carrying electrolyte to or from the electrolyte chambers 208 via narrow transverse ducts 220.

The flows of the 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.

In a modification, the cell stack is arranged so the aligned apertures 216 and 218 are at the bottom of the cell stack, for supplying electrolyte; and electrolyte leaves the electrolyte chambers 208 through ducts (not shown), similar to the ducts 220, but leading to the outer surface of the cell stack. Although the transverse ducts 220 are shown as being within the plates 202 they may instead be defined by grooves at the surface of the plates 202.

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 also is moulded of polymeric material, and 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 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. There is then another end plate (not shown) which may be blank on the outer face and not define through apertures; alternatively it may define through 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. For example bolts (not shown) may extend through aligned apertures through all the sheets 202 and 206.

In operation of the fuel cell stack 200 gases such as hydrogen and air are provided to the gas chambers 207 and 209 respectively, and an electiolyte solution such as aqueous potassium hydroxide is passed through the electrolyte chambers 208. Each cell produces an electromotive force of about 1 volt, so the electrical output of the stack 200 depends on the number of fuel cells in the stack. Within each cell the electrolyte permeates through the carbon paper 20 (which is hydrophilic) and so contacts the catalytic material in the catalyst-containing layer 18 of each electrode 10. The carbon paper 20 stabilises the outer surface of the catalyst-containing layer 18, and protects it from erosion by the flowing electrolyte, so enhancing the lifetime of the electrode 10.

Claims

Claims 1. An electrode suitable for use in a liquid electrolyte fuel cell, the electrode comprising: -a sheet of metal through which are defined a multiplicity of through-holes; -a first fluid-permeable layer comprising fibrous and/or particulate electrically-conductive material which is bonded to the sheet of metal, in electrical contact with the sheet of metal, and is hydrophobic; -a second fluid-permeable layer comprising fibrous and/or particulate electrically-conductive material and catalytic material, which is bonded to the outer surface of the first fluid-permeable layer, and which is hydrophobic; and -a third fluid-permeable layer which is hydrophilic, and which is bonded to the outer surface of the second fluid-permeable layer.
2. A liquid electrolyte fuel cell comprising means to define an electrolyte chamber, and comprising two electrodes, one electrode on either side of the electrolyte chamber, each electrode comprising: -a sheet of metal through which are defined a multiplicity of through-holes; -a first fluid-permeable layer comprising fibrous and/or particulate electrically-conductive material which is bonded to the sheet of metal, in electrical contact with the sheet of metal, and is hydrophobic; -a second fluid-permeable layer comprising fibrous and/or particulate electrically-conductive material and catalytic material, which is bonded to the outer surface of the first fluid-permeable layer, and which is hydrophobic; and -a third fluid-permeable layer which is hydrophilic, and which is bonded to the outer surface of the second fluid-permeable layer.
3. An electrode or a fuel cell as claimed in claim 1 or claim 2 wherein the second fluid-permeable layer is of thickness of at least 50 pm.
4. An electrode or a fuel cell as claimed in any one of the preceding claims wherein the first fluid-permeable layer is of thickness of at least 50 pm.
5. An electrode or a fuel cell as claimed in any one of the preceding claims wherein the second fluid-permeable layer is of thickness between 100 pm and 500 pm, and the first fluid permeable layer is of thickness between 100 pm and 500 pm.
6. An electrode or a fuel cell as claimed in any one of the preceding claims wherein the second fluid-permeable layer is less hydrophobic than the first fluid-permeable layer.
7. An electrode or a fuel cell as claimed in claim 6 wherein the first fluid-permeable layer incorporate a hydrophobic polymer, and the second fluid-permeable layer incorporates a smaller proportion of hydrophobic polymer than that within the first fluid-permeable layer.
8. An electrode or a fuel cell as claimed in claim 7 wherein the first fluid-permeable layer comprises between 40% and 80% of a hydrophobic polymer, as a percentage by weight of the layer, whereas the second fluid-permeable layer contains between 5% and 70% of the percentage within the first fluid-permeable layer.
9. An electrode or a fuel cell as claimed in any one of the preceding claims wherein the first fluid-permeable layer and the second fluid-permeable layer each incorporates a hydrophobic polymer, the polymer being PTFE, PVdF, or a polyolefin.
10. A method of making an electrode suitable for use in a fuel cell, the method comprising the steps of assembling: -a sheet of metal through which are defined a multiplicity of through-holes; -a first fluid-permeable layer comprising fibrous and/or particulate electrically-conductive material and a polymer binder in an un-sintered state, placed on to a surface of the sheet of metal, the first fluid-permeable layer being hydrophobic when the electrode is completed; -a second fluid-permeable layer comprising fibrous and/or particulate electrically-conductive material and catalytic material and a polymer binder in an un-sintered state, placed on to a surface of the first fluid-permeable layer opposite that in contact with the sheet of metal, the second fluid-permeable layer being hydrophobic when the electrode is completed; and -a third fluid-permeable layer which is hydrophilic, placed onto the outer surface of the second fluid-permeable layer; and -hot pressing the assembled items together, so that at least the sheet of metal, the first fluid-permeable layer, and the second fluid-permeable layer are bonded together, and the polymer binder in the first fluid-permeable layer and the second fluid-permeable layer becomes sintered, so that the first fluid-permeable layer and the second fluid-permeable layer are hydrophobic.
11. A method as claimed in claim 10 wherein the first fluid-permeable layer and the second fluid-permeable layer are initially assembled and rolled together, and are then assembled with the sheet of metal and the third fluid-permeable layer on either side, and hot pressed together.
12. A method as claimed in claim 10 or claim 11 wherein the hot pressing subjects at least part of the assembled items to a temperature below the melting point of the polymer binder by between 100 and 50°C.
13. A method as claimed in any one of claims 10 to 12 wherein the hot pressing step also bonds the third fluid-permeable layer onto the outer surface of the second fluid-permeable layer.
14. An electrode suitable for use in a liquid electrolyte fuel cell substantially as hereinbefore described, with reference to and as shown in the accompanying drawings.
15. A fuel cell substantially as hereinbefore described, with reference to and as shown in the accompanying drawings.
16. A method of making an electrode suitable for use in a fuel cell, the method being substantially as hereinbefore described, with reference to and as shown in the accompanying drawings.