GB2487836A

GB2487836A - Fuel Cell Assembly

Info

Publication number: GB2487836A
Application number: GB1201656.4A
Authority: GB
Inventors: Martin Thomas
Original assignee: AFC Energy PLC
Current assignee: AFC Energy PLC
Priority date: 2011-02-04
Filing date: 2012-01-31
Publication date: 2012-08-08
Also published as: GB201201656D0; GB201101968D0

Abstract

A liquid electrolyte fuel cell assembly (30) comprises two electrodes (10) arranged with one electrode on either side of an electrolyte space (22). Each electrode (10) comprises a support sheet (11) of electrically conducting material through which are defined a multiplicity of laser drilled through-holes (14), and a gas-permeable layer (16) of fibrous and/or particulate electrically-conductive material which is in electrical contact with the support sheet, and which comprises catalytic material (18) at least in the vicinity of a surface in contact with the electrolyte space (22). These components are held together by a multiplicity of electrically-insulating linkages that are distributed over the active area of the fuel cell assembly (30). The linkages may be in the form of plastic rivets (25)

Description

Fuel Cell Assembly The present invention relates to liquid electrolyte fuel cells, preferably but not exclusively alkaline 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.

More generally, a fuel cell consists of a number of functional layers, and in most cases compression is required to minimise electrical contact resistance between certain of those functional layers. In the case of a fuel cell stack, sealing of successive cells is usually ensured by compression of the stack. The compression of both the functional layers as well as the stack seals is typically carried out using rigid plates at each end of the stack, which are linked by tie rods or an equivalent linkage, usually arranged around their periphery. To avoid uneven compression, the end plates need to be very rigid and usually contribute significantly to the weight of the fuel cell stack.

In many cases each fuel cell contains chambers for electrolyte or for gas, and such chambers add to the difficulty of compressing cells or layers together. The compression force may be transmitted through these chambers by locating solid, porous structures in the chambers, but these add to the complexity of the fuel cell and have a negative impact on the flows of gases or electrolyte through the chambers in the fuel cell stack.

Discussion of the invention The cell assembly of the present invention addresses or mitigates one or more

problems of the prior art.

Accordingly the present invention provides a liquid electrolyte fuel cell assembly comprising two electrodes and an electrolyte space, arranged with one electrode on either side of the electrolyte space, each electrode comprising: -a support sheet of electrically conductive material through which are defined a multiplicity of through-holes, and -a gas-permeable layer of fibrous and/or particulate electrically-conductive material which is in electrical contact with the support sheet, and which comprises catalytic material; wherein the electrodes are held together by a multiplicity of electrically-insulating linkages that are distributed over the active area of the fuel cell assembly.

The electrically-insulating linkages may for example be plastic rivets, or plastic bolts, or plastic clips. The effect of these linkages is to ensure that the pressure holding the components of the cell assembly together can be substantially uniform over the active area of the cell assembly. This is achieved without disrupting or disturbing the flow of gas in the gas chambers. Furthermore the electrolyte chamber is held at a substantially constant width, which is desirable in order to obtain uniform electrolyte flow.

In the preferred arrangement the electrodes are spaced apart by a porous liquid-permeable spacer which defines the electrolyte space. In this case there are apertures through the liquid-permeable spacer. Indeed there may also be aligned apertures through the electrodes, for the linkages to extend through. The apertures are of substantially the same diameter as the linkages that pass through them, and might for example be circular, and of diameter less than 5 mm, for example 3 mm. The spacing between the apertures, and so between the linkages, may be in the range 20 mm up to 200 mm, more preferably above 25 mm, and more preferably less than 100 mm, for example 30 mm or 50 mm. There may be sealing elements to seal the end portions of the linkages to the outer surfaces of the support sheet, to prevent any leakage of electrolyte through the aligned apertures. In an alternative, the electrically-insulating linkages extend through discrete spacers, or themselves define spacers, and these spacers hold the electrodes apart.

Although the linkages are electrically insulating, in that they must not conduct electricity between the opposed electrodes of the cell assembly, they may be at least partly of conductive material. They may be integral with the support sheet. If the support sheets are of metal, and the rivets are partly of metal, then the rivets may be welded to the support sheets.

The electrode must have properties that enable the chemical reaction with the gas phase to occur. In some cases the material of the gas-permeable layer may be sufficiently catalytic for this purpose, but more usually the electrode also incorporates a separate catalytic material, which may be in the form of a coating. The electrode is 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.

Preferably the through-holes in the support sheet are defined by etched or drilled holes, so there are discrete holes. The preferred structure is formed by laser drilling. The support sheet may be of metal, or of conductive polymer, or metal-reinforced conductive polymer. The thickness of the support sheet may be between 0.1 mm and 3 mm, more preferably between 0.15 mm and 0.5 mm, for example 0.3 mm (300 pm) or 0.25 mm (250 pm); and the holes may be of width or diameter between 5 pm and 500 pm, for example about 20 pm or 50 pm, and spaced between pm and 10 mm apart. Such holes may be created by laser drilling. 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. In cross-section, the holes may for example be circular, oval or elliptical. The holes may also be formed by an etching process. Preferably there are no through-holes in a zone around each aperture, for example a zone of radial width 2 mm.

As compared to a metal mesh it will be appreciated that the support 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 70 pm, for example pm, and are at a centre-to-centre separation of at least 0.15 mm. In any event the holes preferably occupy less than 25% of the area of the support sheet, more preferably less than 10%; indeed the proportion may be less than 1%.

The electrode of the invention, assuming the apertures are blocked by the electrically-insulating linkages, preferably would have a bubble point between 10 mbar and 100 mbar, for example about 40 mbar. This is determined by the properties of the exposed surface of the gas-permeable layer. For example, if the electrolyte has a surface tension of 0.07 NIm then a bubble point of less than 100 mbar corresponds to a pore diameter of greater than 14 pm. The gas-permeable layer may comprise carbon nanotubes, carbon black, and PTFE as a hydrophobic binder. Alternative suitable forms of carbon are graphite, graphene and activated charcoal. These types of carbon provide good electrical conductivity, while the hydrophobic binder inhibits aqueous electrolyte from entering the gas-permeable layer. Preferably the thickness of the gas-permeable layer is greater than or equal to the separation between the holes through the metal sheet. Hence the gas-permeable layer is preferably at least 0.15 mm thick, but preferably less than 1 mm thick, for example between 0.2 mm and 0.5 mm thick. Since the gas-permeable layer is formed of particulate material, the pores within it are small and close together, forming a gas-permeable network of pores throughout the layer. For example the pores would typically be of width less than 30 pm, and may be spaced apart by less than 50 pm.

The support sheet may be of a metal such as nickel, or 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 a support sheet 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.

A fuel cell stack may be made of several such fuel cell assemblies. If the linkages have projecting heads that are electrically conducting and are in electrical contact with the metal sheet, then in such a fuel cell stack the projecting heads on one fuel cell assembly may make electrical contact with the projecting heads on an adjacent fuel cell assembly. The projecting heads may be of two sorts, one defining a recess or socket and the other defining a projection that mates with the recess or socket. Adjacent fuel cell assemblies can therefore be plugged together, with the projections fitting into the sockets.

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; Figure 2 shows a plan view of the electrode shown in Figure 1, in the direction of arrow A; and Figure 3 shows a cross-sectional view of a fuel cell assembly incorporating electrodes as shown in figures 1 and 2.

Referring to figures 1 and 2, which are schematic views, an electrode 10 comprises a sheet 11 of ferritic stainless-steel. The sheet 11 is of thickness 0.3 mm.

Most of the sheet -the central region 12 -is perforated 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 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 100 pm and separated by between 200 pm and 300 pm. In figure 2 a few of the through holes 14 are shown schematically, and the boundary between the central region 12 and the non-perforated margin 15 is indicated by a broken line.

In addition the sheet 11 is perforated to provide an array of much larger holes 17, of diameter 3 mm, forming a rectangular array with a centre-to-centre spacing of mm in the horizontal direction (as shown) and a centre-to-centre spacing of 35 mm in the vertical direction (as shown) over the central region 12 of the sheet 11. An annular zone 20 of radial width 2 mm around each of the holes 17 does not have any of the through holes 14; each annular zone 20 is indicated by a broken line. After forming the through holes 14 and 17, one surface of the perforated central region 12 is then covered in a coating to provide a gas-permeable layer 16; the exposed surface of the gas-permeable layer 16 is then coated with a coating 18 of catalytically active material. These coating processes will now be described.

Firstly the sheet 11 is treated to ensure that the surfaces within the holes 14 and 17 are hydrophobic. This uses an aqueous suspension of sub-micron sized PTFE particles, 60 wt% PTFE, containing a branched secondary alcohol ethoxylate surf actant such as the Tergitol TMN series of surfactants (Tergitol is a trade mark).

For example this suspension may be DuPont Zonyl (trade mark) PTFE TE-3887, in which the particles are of size between 0.05 and 0.5 pm and of average size 0.2 pm.

This suspension is diluted with water to 4.5 wt% PTFE, and a cloth is soaked with this dilute suspension. The sheet 11 is then placed on to this cloth, so that the surface that is to be covered with the gas-permeable layer 16 is the surface that touches the cloth, so that the suspension is absorbed into the holes 14 by a capillary effect. The surfaces within the holes 17 may also be coated using a paintbrush. The sheet 11 is then placed on to a dry non-shedding cloth, so that any excess suspension in the holes 14 is absorbed into the cloth. The sheet 11 is then allowed to dry.

The gas-permeable layer 16 is made by first mixing carbon with alcohol. The preferred 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 80 wt% carbon black (e.g. Cabot XC72F{ (trade mark)); 10 wt% nanotubes (e.g. Nanocyl 7000); and 10 wt% activated charcoal (e.g. Norit SX Ultra). An alternative mixture might comprise only carbon nanotubes, but of a range of different lengths. 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 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 a wt% aqueous suspension of sub-micron sized PTFE particles containing Tergitol surf actant, as described above, typically between two and three times the mass of carbon, for example 2.5 times the mass of carbon. When the PTFE suspension contacts the alcohol, it gels, so the mixture has a dough-like consistency. The mixture is repeatedly extruded or calendered, for example through rollers, and then folded up, which has been found to create fibrous structures, so the layer becomes stratified, for example being rolled out and folded up six times, before finally being rolled out to a thickness of 0.4 mm. This is then pressed on to a flat substrate, and is then heated to between 250°C and 300°C and held at that temperature for between 30 mm and 1.5 hours. This leads to evaporation of any remaining alcohol, and evaporation or breakdown of the surfactant; and sintering of the PTFE particles, so that the gas-permeable layer 16 is a coherent and yet permeable hydrophobic structure. The same heating process is also applies to the sheet 11, which sinters the PTFE coating within the holes 14 and 17, ensuring that the surfaces within the holes 14 and 17 are also hydrophobic.

In an alternative to the above procedure for forming the gas-permeable layer 16, two different layers may be superimposed, the layers differing in the carbon components. For example a first layer may contain carbon nanotubes of a range of different lengths, and a second layer may contain carbon black as well as carbon nanotubes. The heating process would bond both the layers together, providing a gas-permeable layer 16 which is the required coherent and yet permeable hydrophobic structure but which has somewhat different electrical properties through its thickness.

The flat substrate on to which the mixture is pressed for the heating process may be the sheet 11, in which case the gas-permeable layer 16 will adhere to the sheet 11 by virtue of the sintering of the PTFE coating on the sheet 11 and in the permeable layer 16. Alternatively the flat substrate may be a smooth substrate from which the resultant gas-permeable layer 16 is removed after the heating process. In this case the gas-permeable layer 16 would be placed on to the surface of the sheet 11, but would not adhere to it.

In the electrode 10 of figure 1 the gas-permeable layer 16 is shown as being in contact with the rear surface of the metal sheet 11, that is to say the surface remote from that on which the laser had been incident to form the holes 14. In an alternative arrangement, the gas-permeable layer 16 is in contact with the front surface of the metal sheet 11. The laser drilling of the sheet 11 may create a slightly rough finish around the holes 14, which would enhance adhesion between the metal sheet 11 and the gas-permeable layer 16. In a further alternative the surface of the metal sheet 11 onto which the gas-permeable layer 16 is to be placed is subjected to a roughening pretreatment before the gas-permeable layer 16 is adhered to it. Such a roughening pre-treatment may provide roughness at a scale of less than 20 pm, preferably around pm, and this may be achieved using a laser. This roughening pre-treatment would enhance adhesion between the metal sheet 11 and the gas-permeable layer 16.

The electrode 10 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 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 06 and 012), 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 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 10 remains permeable, as liquid electrolyte must permeate the coating 18, to meet the gas that permeates through the gas-permeable layer 16, 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, as described above for forming the gas-permeable layer, and pressing such an extruded layer onto a gas-permeable layer or co-extruding the catalyst layer onto a gas-permeable layer. Screen printing would be another technique. Furthermore catalyst may be incorporated into the mixture forming the gas-permeable layer.

Referring now to figure 2, there is shown a cross-sectional view through a fuel cell assembly 30. The fuel cell assembly 30 consists of two electrodes 10 each consisting of a metal sheet 11 and a gas-permeable layer 16 with a catalyst-containing coating 18. In one case the coating 18 contains an anode catalyst, while in the other case the coating 18 contains a cathode catalyst. As mentioned above, the gas-permeable layer 16 may in each case be adhered to the metal sheet, or may be only in contact with it. Between the two electrodes 10 is a porous liquid-permeable spacer 22, which may for example be of an open-cell foam material. The spacer 22 is sufficiently liquid-permeable that electrolyte solution, which may for example be aqueous potassium hydroxide, can flow through it. The apertures 17 in the two electrodes 10 are aligned with each other, and are aligned with apertures 17 through the liquid-permeable spacer 22. The metal sheets 11 on opposite sides of the assembly 30 are secured to each other by plastic rivets 24. In this example each rivet 24 has rounded heads 25 at each end which are sealed to the non-perforated zones 20 by resilient washers 26, and each rivet 24 consists of two parts that clip together: a male part 27 defining a conically tapering prong and a female part 28 defining a conically tapering recess. The mating faces of the male part 27 and the female part 28 define inclined barbs (not shown) so that they can be pushed together, but cannot then be pulled apart because the barbs latch into each other. The rivets 24 may for example be of ASS, PTFE or nylon.

Such a rivet 24 is inserted and clipped together within each of the apertures 17.

Consequently the electrodes 10 are held together with substantially uniform pressure over their active area. This ensures that the gas-permeable layer 16 remains in contact with the metal sheet 11 even if they are not adhered together. In use it will be understood that different gases are provided to the zones outside the metal sheets 11, for example hydrogen adjacent to one metal sheet 11 and air adjacent to the other metal sheet, and these gases diffused through the through-holes 14 and the respective gas-permeable layer 16 to reach the catalytic coating 18. The gases undergo chemical reactions at the three-phase boundary where the electrolyte is in contact with the catalytic coating 18, and the electrons produced by or required by those chemical reactions are conducted through the carbon of the gas-permeable layer to the metal sheet 11. Hence, as is well known, a voltage is developed between the opposed metal sheets 11. The gases do not flow through the apertures 17 firstly because the heads of the rivets 24 are sealed to the metal sheets 11 by the washers 26, and secondly because the liquid electrolyte is in contact with the external surface of the rivet 24 where it passes through the liquid-permeable spacer 22 and so prevents any passage of gases between opposite sides of the fuel cell assembly 30.

It will be appreciated that the rivets 24 are shown only by way of example, and that other means may be used to extend through the apertures 17 to link the electrodes 10 together, for example a nut and bolt, or a rivet-like item of a different shape. It will also be appreciated that the laser-drilled through-holes 14 are by way of example. As an alternative the metal sheet might instead define rectangular cutouts, each say 50 mm by 30 mm, leaving a coarse grid of strips, for example of width less than 12 mm, for example of width between 5 mm and 8 mm, along the lines where the apertures 17 are provided; each rectangular cutout may be provided with a rectangular metal mesh, the mesh being welded into the cutout. This may be cheaper to produce, but its electrical and mechanical properties are significantly less uniform than the sheet 11 with the laser-drilled holes 14. In place of a sheet made of metal, the support sheet might be formed of an electrically-conductive plastic material. Whatever material the support sheet is made of, it must be sufficiently rigid to support itself across the space between rivets.

Where a larger voltage is required it is conventional to provide a stack of fuel cells. Referring now to figure 4 there is shown a part of a fuel cell stack 40 comprising a plurality of fuel cell assemblies 42; each fuel cell assembly 42 is similar to the fuel cell assembly 30 described above, and identical components are referred to by the same reference numerals. Each fuel cell assembly 42 consists of two electrodes 10 separated by a liquid-permeable spacer 22, each electrode 10 consisting of a metal plate 11, a gas-permeable layer 16 and a catalyst coating 18. The region between the non-perforated margins 15 of the plates 11 is occupied by a peripheral seal 44. The components of each fuel cell assembly 42 are held together by rivets 46 which differ from the rivets 24 in that the head 48 is flat and that the head and the immediately adjacent portion of the shank is of metal, while the projecting and interlocking male and female portions are plastic.

As an alternative the rivets 46 may be partly of non-conductive plastic, but with the head 48 and the immediately adjacent portion of the shank of electrically-conductive plastic.

It will therefore be appreciated that the outside faces of the electrodes 10 are spaced apart by the heads 48 of the rivets 46, and that the heads 48 of successive fuel cell assemblies 42 therefore make electrical contact with each other. The resulting spaces between the faces of the electrodes 10 consequently define gas supply chambers 50, air and hydrogen being supplied to chambers 50a and 50h respectively which alternate along the stack 40. In a modification some of the rivets 46 may have heads that define recesses, the other rivets having heads that define projections, so that when the fuel cell assemblies 42 are stacked together, the projections mate with the recesses.

In this case there are apertures through the liquid-permeable spacer. Indeed there may also be aligned apertures through the electrodes, for the linkages to extend through. The apertures are of substantially the same diameter as the linkages that pass through them, and might for example be circular, and of diameter less than 5 mm, for example 3 mm. The spacing between the apertures, and so between the linkages, may be in the range 20 mm up to 200 mm, more preferably above 25 mm, and more preferably less than 100 mm, for example 30 mm or 50 mm. There may be sealing elements to seal the end portions of the linkages to the outer surfaces of the support sheet, to prevent any leakage of electrolyte through the aligned apertures. In an alternative, the electrically-insulating linkages extend through discrete spacers, or themselves define spacers, and these spacers hold the electrodes apart.

The invention allows the functional layers of each fuel cell in a stack to be compressed together, without introducing any channel-filling components within the gas flow channels. It therefore allows the size of the active area to be increased, without running up against the problem of spreading the compression load across a large area. Furthermore the present invention may be used both in monopolar cell stack designs, and bipolar cell stack designs.

Claims

Claims 1. A liquid electrolyte fuel cell assembly comprising two electrodes and an electrolyte space, arranged with one electrode on either side of the electrolyte space, each electrode comprising: -a support sheet of electrically conductive material through which are defined a multiplicity of through-holes, and -a gas-permeable layer of fibrous and/or particulate electrically-conductive material which is in electrical contact with the support sheet, and which comprises catalytic material at least in the vicinity of a surface in contact with the porous liquid-permeable spacer; wherein the electrodes are held together by a multiplicity of electrically-insulating linkages that are distributed over the active area of the fuel cell assembly.
2. An assembly as claimed in claim 1 wherein the electrically-insulating linkages comprise plastic rivets, or plastic bolts, or plastic clips.
3. An assembly as claimed in claim 1 or claim 2 wherein the electrically-insulating linkages are partly of electrically-conductive material.
4. An assembly as claimed in any one of the preceding claims wherein the electrodes are spaced apart by a porous liquid-permeable spacer which defines the electrolyte space.
5. An assembly as claimed in claim 4 wherein there are aligned apertures through the electrodes and the liquid-permeable spacer through which the linkages extend.
6. An assembly as claimed in any one of the preceding claims wherein the spacing between the linkages, is in the range 20 mm up to 200 mm.
7. An assembly as claimed in any one of the preceding claims also comprising sealing elements to seal the end portions of the linkages to the outer surfaces of the support sheet.
8. An assembly as claimed in any one of the preceding claims wherein the through-holes in the support sheet are defined by discrete holes of width or diameter between pm and 500 pm.
9. An assembly as claimed in any one of the preceding claims wherein there are apertures through the support sheets through which the linkages extend, and there are no through-holes in a zone around each aperture.
10. An assembly as claimed in any one of the preceding claims wherein the thickness of the gas-permeable layer is greater than or equal to the separation between the through-holes in the metal sheet.
11. An assembly as claimed in any one of the preceding claims wherein the gas-permeable layer is at least 0.15mm thick.
12. An assembly as claimed in any one of the preceding claims wherein the gas-permeable layer comprises carbon and a hydrophobic binder.
13. An assembly as claimed in any one of the preceding claims wherein the surfaces of the support sheet within the through-holes are provided with a hydrophobic coating.
14. An assembly as claimed in any one of the preceding claims wherein the gas-permeable layer comprises carbon nanotubes and carbon black.
15. The liquid electrolyte fuel cell assembly substantially as hereinbef ore described with reference to, and as shown in, the accompanying drawings.
16. A fuel cell stack comprising a plurality of fuel cell assemblies as claimed in any one of the preceding claims.