WO2011039523A1

WO2011039523A1 - Cell stack

Info

Publication number: WO2011039523A1
Application number: PCT/GB2010/051204
Authority: WO
Inventors: James Alexander Austin; Andreas Karl Backstrom; Alex Sean Blake; Gene Stacey Lewis; Roger Anthony Pitts; Hugh Liam Sutherland
Original assignee: Afc Energy Plc
Priority date: 2009-09-30
Filing date: 2010-07-21
Publication date: 2011-04-07
Also published as: GB0917143D0; TW201125204A

Abstract

A cell stack (20) including a plurality of cells arranged electrically in series, wherein each cell comprises electrodes of opposite polarity constituting an anode (18) and a cathode (19), wherein pairs of electrodes of opposite polarity are integral with each other, being defined by spaced-apart portions of a sheet- like conductive element (10). The conductive element (10) may be folded around a non-conducting separator plate (24) that defines gas flow chambers on opposite faces, for different gases. The cell stack (20) may be used for a liquid electrolyte fuel cell.

Description

Cell Stack

[001] The present invention relates to cell stacks, particularly but not exclusively of fuel cells such as alkaline fuel cells, and to electrodes suitable for such fuel cells. It relates also to cell stacks in

electrolysers and in flow batteries.

Background to the invention

[002] 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 aqueous electrolytes are also of interest. Such fuel cells typically comprise an electrolyte chamber separated from a fuel gas chamber (containing a fuel gas, typically hydrogen) and a further gas chamber (containing an oxidant gas, usually air) . The electrolyte chamber is separated from the gas chambers using electrodes. Typical electrodes for alkaline fuel cells comprise a conductive metal mesh, typically nickel, that provides mechanical strength to the electrode. Onto the metal mesh is deposited a catalyst as a slurry or dispersion of particulate poly tetra-fluoroethylene (PTFE), activated carbon and a catalyst metal, typically platinum. Such electrodes are expensive, electrically inefficient, and suffer from irregular distribution of catalyst. Furthermore, the nickel mesh is prone to breakage and causes local irregularities and unwanted variations in electric field due to resistance at the contact points between the wires of the mesh.

[003] A further problem with such electrodes is that it is necessary to provide a seal around a periphery of the electrode to prevent leakage of gas from the adjacent gas chamber, and this is inherently difficult with a mesh structure .

[004] A single fuel cell typically provides a voltage in the range 0.5 V up to 1.2 V, and to increase the power output it is customary to provide a stack of fuel cells arranged electrically in series, and hence to produce a higher output voltage. Between one cell and the next it is customary to provide a separator plate which may be referred to as a bipolar plate; one face of the separator plate is in contact with an anode, while the other face is in contact with a cathode. The anode and the cathode must be connected electrically together, and this may be achieved by using a bipolar plate that is of a conducting material. However there is inevitably some contact resistance between each bipolar plate and the electrodes on either side of it.

Discussion of the invention

[005] The cell stack of the present invention addresses or mitigates one or more problems of the prior art.

[006] Accordingly the present invention provides a cell stack including a plurality of cells arranged electrically in series, wherein each cell comprises electrodes of opposite polarity constituting an anode and a cathode, wherein pairs of electrodes of opposite polarity are integral with each other, being defined by spaced-apart portions of a conductive element. [007] In one embodiment, the cell stack comprises a separator plate between successive cells in the stack, wherein the separator plate is not electrically

conducting, and defines gas flow chambers on opposite faces thereof; wherein there are electrodes of opposite polarity adjacent to opposite faces of the separator plate, and wherein the electrodes that are adjacent to opposite faces of the separator plate are a pair of electrodes that are integral with each other.

[008] In an alternative embodiment of the cell stack there are no separator plates, and one electrode of each pair of electrodes is interleaved between another pair of electrodes in the stack. In each case the conductive element that defines the two electrodes is a sheet-like element .

[009] Each electrode must also comprise a catalyst to enable the chemical reaction with the gas phase and the electrolyte to occur. In some cases the surface of the electrically-conducting material may be sufficiently catalytic for this purpose, but more usually the

electrode also incorporates a coating of catalytic material. The electrodes must be permeable to enable intimate contact between electrolyte, the catalytic material and the gas phase, so that in use there is a gas/electrolyte interface in contact with the catalytic material. The catalytic material may be provided on an electrically conductive support material, which may be a particulate material such as carbon. [010] The conductive element may be a wire mesh, but it preferably comprises a sheet of electrically

conducting material through which are defined a

multiplicity of through-pores in those portions thereof which are to define the catalytic area of the electrodes; remaining parts of the electrically conducting sheet preferably have no such through-pores, that is to say a peripheral margin around each catalytic portion is not perforated and is non-porous. In a modification the spaced-apart portions of the conductive element are electrically and mechanically bonded to each other by conductive linking members to form the integral

structure; for example two nickel sheets that define through pores may be welded to nickel strips to form an integral structure . [Oil] The layer of particulate catalyst material on a surface of the conductive sheet may be covered by a layer of permeable hydrophilic polymeric material. In this case the conductive sheet is preferably at the side of the electrode further from the electrolyte.

[012] The electrically-conducting material is preferably a metal, although an electrically-conducting polymer material may also be suitable. [013] Preferably the through-pores are discrete through-holes, defined by etched or drilled holes.

Alternatively it may be possible to form the conductive element by electro-forming, or even by sintering, although the latter process is difficult to use when making thin sheets . The preferred structure is formed by laser drilling. The thickness of the electrically conducting sheet may be between 0.1 mm and 3 mm, more preferably between 0.1 mm and 0.4 mm, for example 0.3 mm (300 μπι) or 0.25 mm (250 μηι) ; and the holes may be of width or diameter between 5 μηι and 500 μηι, preferably less than 50 μηι, for example about 20 μηι, and spaced between 50 μηι and 10 mm apart. Such holes may be created by laser drilling. 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 be, for example, circular, oval or elliptical. The holes may also be formed by an etching proces s .

[014] The provision of a non-porous edge region around the perimeter of each catalyst portion simplifies sealing to adjacent components of the fuel cell. The sheet of electrically conducting material through which are defined through-pores by etched or drilled holes provides better electrical conduction than a wire mesh, as no wire-to-wire contacts are involved; it also provides a more uniform distribution of current; and the structure is stiffer for equal values of porosity, as there are no crossing-over wires that can move relative to each other. The size, shape and surface of the pores may assist in controlling the position of the

electrolyte/gas phase interface, using capillary forces. The size and spacing of the holes is also selected to ensure satisfactory diffusion of the reactant species (gas or liquid) to and from that interface.

[015] The electrode preferably has a bubble point between 2 kPa (20 mbar ) and 10 kPa (100 mbar), for example about 4 kPa (40 mbar) . [016] The conductive sheet may be of a metal, for example 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 an electrode of other metals, or may be formed using known deposition techniques such as electrodeposition . 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. In the case of an alkaline fuel cell stack, the preferred material is nickel, as this is resistant to corrosion in contact with an alkaline electrolyte for example of potassium hydroxide solution.

[017] 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 element;

Figure 2 shows a plan view of the electrode element of figure 1;

Figure 3 shows a cross-sectional view of a fuel cell stack incorporating electrode elements as shown in figures 1 and 2;

Figure 4 shows a cross-sectional view through an

alternative electrode element; and

Figure 5 shows a cross-sectional view of an alternative fuel cell stack incorporating electrode elements as shown in figures 1 and 2, or figure 3.

[018] Referring to figures 1 and 2, an electrode element 10 comprises a sheet of ferritic stainless-steel. The sheet is of thickness 0.3 mm, and is rectangular in plan. Most of the sheet - the central region 12 apart from a strip 13 across the middle of the rectangle and a peripheral margin 15 - is perforated by laser drilling to produce a very large number of through holes 14, the holes each being of mean diameter about 30 μηι, and being separated by between 100 μηι and 150 μηι; as a result of the laser drilling process, each hole 14 is in practice slightly tapered along its length. The margin 15 around the periphery of the sheet 10, of width 5 mm, is not perforated, nor is the middle strip 13. Hence there are two perforated regions 12, each of which are generally rectangular, separated by the imperforated middle strip 13. (The hole dimensions and separations are given here by way of example, and in an alternative the holes might be of mean diameter 100 μηι and separated by between 50 and 100 μπ\. )

[019] After forming the through holes 14, the sheet may optionally be subjected to a heat treatment in which it is held at a temperature between 650 and 850°C in air (an oxygen-containing gas) for between 30 minutes and 2 hours, so as to form a protective surface coating of conductive chromium manganese oxide spinel. On one face of the sheet, the perforated regions 12 are then covered in coatings 16a, 16b of catalyst mixtures. The catalyst mixtures 16a or 16b determine whether that portion of the electrode element 10 is an anode 18 or a cathode 19; the only difference would be in the composition of the catalyst mixture, and indeed some catalyst compositions may be suitable in both anodes and cathodes.

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

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

A. Activated carbon, with 10% binder.

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

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

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

D. Leached nickel-aluminium alloy powder with

activated carbon, with 10% binder.

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

[021] Referring now to figure 3, there is shown a cross-sectional view through the structural components of a cell stack 20 with the components separated for clarity. The stack 20 consists of a stack of moulded plastic plates 22 and 24 arranged alternately. Each plate 22 and 24 is hence an electrical insulator. The plates 22 define a generally rectangular through-aperture 26 surrounded by a frame 27; the apertures 26 provide electrolyte chambers. The plates 24 are separator plates (or bipolar plates); they define rectangular blind recesses 28 and 29 on opposite faces, each recess being about 3 mm deep. The blind recesses 28 and 29 provide gas chambers. At one end of the stack 20 is a polar plate 24a which defines only one rectangular blind recess 29; next to that is a rigid end plate 30. Embedded in the polar plate 24a are metal conductors 25 that extend through to the end plate 30, which may be of conducting material. At the opposite end of the stack 20 is another polar plate (not shown), which in this case defines only a rectangular blind recess 28. Each of the plates 22 is covered by a moulded-over gasket 31 of resilient

material, which covers both surfaces of the frame 27. [022] In assembling the cell stack 20 an electrode element 10 is folded around one edge of each separator plate 24, so that the anode regions and cathode regions are on opposite sides and that the imperforated regions 15 and 13 are adjacent to the gasket 31 in the assembled stack 20. (The electrode element 10 is actually in contact with the separator plate 24, although shown separated in the figure for clarity.) They are arranged so that anodes 18 and cathodes 19 are arranged

alternately in the stack 20. It will thus be

appreciated that between one separator plate 24 and the next in the stack 20 there is an electrolyte chamber aperture 26, with an anode 18 on one side and a cathode 19 on the opposite side; and there are gas chambers 28 and 29 at the opposite faces of the anode 18 and the cathode 19 respectively. Hence there is a single fuel cell between one separator plate 24 and the next.

[023] At the ends of the stack 20 are a separate anode 18 (not shown) and a separate cathode 19.

[024] The anodes and cathodes 18 and 19 are arranged with the face that carries the catalyst coating 16 facing the respective blind recess 28 or 29 respectively; in an alternative, they may be arranged with the face carrying the catalyst coating 16 facing the electrolyte chamber 26. The gasket 31 ensures that electrolyte in the chambers 26 cannot leak out, and that gases cannot leak in, around the edges of the anodes and cathodes 18 and 19, and also ensures that gases cannot leak out between adjacent plates 22 and 24. The perforated portions 12 of each electrode element 10 corresponds to the area of the electrolyte chamber 26 and of the gas chamber 28 or 29. The gasket 31 seals onto the non-perforated margin 15 or onto the non-perforated middle strip 13. [025] The flow of electrolyte to and from the electrolyte chambers (apertures 26), and the flows of the gases to and from the gas chambers (recesses 28 and 29), follow respective fluid flow ducts defined by aligned apertures through the plates 22 and 24; only one such set of apertures 33 and 34 are shown. This set of apertures 33 and 34 provides electrolyte to the electrolyte chambers 26 via narrow transverse ducts 35. The gasket 31 has holes so as not to block the apertures 34 or the ducts 35. At one end of the stack 20 is the polar plate 24a which defines a blind recess 29 on one face but is blank on the outer face; this also defines an aperture 33. Outside this is the end plate 30, which also is moulded of polymeric material, and which defines

apertures 36 which align with the apertures 33 and 34 in the plates 22 and 24; at the outside face the end plate 30 also defines ports 38 communicating with the apertures 36 and so with the fluid flow ducts through which the gases and electrolyte flow to or from the stack 20, each port 38 comprising a cylindrical recess on the outer face. At the other end of the stack 20 is another polar plate (not shown) which defines a blind recess 28. 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 the oxidant gas, fuel gas and/or electrolyte.

[026] After assembly of the stack 20 the components may be secured together for example using a strap 40 (shown partly broken away) around the entire stack 20.

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

[027] It will be appreciated that the cell stack 20 is given by way of example, and it may be modified. For example instead of the moulded-over gasket 31 on the frame 27 there might instead be gasket material on the non-perforated parts 13 and 15 of the electrode element 10. As another modification the polar plate 24a might be of nickel (or of another metal) to enable electrical connection to the end of the stack.

[028] As another example a modified electrode, as shown in figure 4, might instead be used in the fuel stack 20. Referring to figure 4, the electrode element 50 comprises a sheet of ferritic stainless-steel. The sheet is of thickness 0.2 mm. As in figure 2, two central regions 52 of the sheet are perforated by laser drilling to produce a very large number of through-holes 54, each hole being of diameter 25 μηι, and the average separation being 150 μηι. A margin 15 around the

periphery of the sheet, of width 5 mm, is not perforated; and a middle strip 13 (not shown in figure 3) between the two perforated regions 52 is not perforated. One surface of each perforated region 52 is covered with a layer 16 of particulate catalyst material with a binder, one of the perforated regions 52 having a layer of anodic catalyst and the other having a layer of cathodic catalyst . [029] The layer of catalyst 16 is covered with a microporous sheet 55 of polypropylene plastics material (SciMAT 700/70, TM) , which is hydrophilic and has an approximate thickness of between 25 and 400 μηι, such as 125 μηι, and a bubble point of between 8.0 to 15.0 kPa gauge (80 to 150 mbar) . This material has a wicking rate of 90 mm per 600 seconds. A range of different nonwoven polymeric materials are suitable for this purpose; for example various polyolefin plastics materials (e.g. Tyvek TM, from DuPont) may be rendered hydrophilic by treatment with a concentrated acid, such as sulfuric or acrylic acid. The microporous sheet 55 is preferably placed over the catalyst layer 16 immediately after depositing the catalyst layer 16, while the binder is still wet, so that the catalyst layer 16 becomes sandwiched between the perforated portion 52 of the metal sheet and the

hydrophilic microporous sheet 55, all of which are bonded together. The electrode element 50 may be used in a similar way to that described in relation to the element 10, being folded around one edge of a separator plate 24, but it may be installed in the opposite orientation to that described in relation to figure 3, so the polymer microporous sheet 55 is that closest to the electrolyte, while the perforated portion 52 is adjacent to the gas chamber. This can provide improved management of the electrolyte flow towards, and flow of water away from, the three-phase interface between electrolyte, gas and catalyst. This may enable the thickness of the

electrolyte chamber defining plate 22 to be decreased. The polymer sheet 55 may also enhance the gas management at the electrode.

[030] Referring now to figure 5 there is shown a sectional view of an alternative fuel cell stack 60, with the components separated for clarity. The stack 60 consists of a stack of moulded frames 62, 63 and 64, each being of an insulating plastics material, and each defining a rectangular through-aperture. Alternate frames 62 provide electrolyte chambers (marked K) , and between successive electrolyte chambers are gas chambers, which are alternately oxygen chambers (marked 0) and fuel chambers (marked H) . All the chambers are separated from neighbouring chambers by electrode elements 70, which are substantially the same as the electrode elements 10 described above: each electrode element 70 defines an anode 18 at one end and a cathode 19 at the other end, and it is folded around the edges of two adjacent frames 62 and 63 or 64, so that the anode 18 of the electrode element 70 is adjacent to a fuel chamber H and that the cathode 19 of the electrode element 70 is adjacent to an oxygen chamber 0. It will thus be appreciated that each electrolyte chamber K is between an oxygen chamber 0 and a fuel chamber H, and is separated from them by a cathode 19 and an anode 18 respectively, these constituting a single fuel cell. Successive fuel cells in the stack are in opposite orientations, but the arrangement of the electrode elements 70 is such that the cells are

electrically in series. Taking the EMF of a single fuel cell as 1 V, the voltages of the folded portions of the electrode elements 70 increase steadily along the stack 60 as marked, so that cell stack 60 of seven cells produces 7 V output.

[031] At the ends of the stack 60 are polar plates

65, 66 that define blind recesses (like the polar plate 24a described above), and the end electrodes, an anode 18 at one end and a cathode 19 at the other end, correspond to half of an electrode element 70. Gaskets (not shown) ensure that the frames 62, 63 and 64 are sealed to the electrode elements 70. The flow of electrolyte to and from the electrolyte chambers K and of gases to the oxygen chambers 0 and the fuel chambers H takes place through respective fluid flow ducts defined by aligned apertures (not shown) through the frames 62, 63 and 64, in a similar way to that described in relation to figure 3. As with the cell stack 20 described previously, the components of the cell stack 60 are secured together after assembly.

[032] The anodes 18 and the cathodes 19 are arranged with the face that carries the catalyst coating 16 facing the respective gas chamber H or 0, with the rectangular perforated portions 12 (see figures 1 and 2) of each electrode element 70 corresponding to the rectangular chambers K, 0 or H defined by the adjacent frames 62, 63 or 64. It will therefore be appreciated that,

considering an unfolded electrode element 70, the catalyst coatings 16 at the two ends are on opposite surfaces .

[033] As mentioned above, the anodes 18 and cathodes

19 may have the structure as shown in figure 1, with the catalyst coating 16 at an exposed surface. The catalyst coating 16 preferably faces the adjacent gas chamber H or 0, although they may be in the opposite orientation.

Alternatively they may have the structure as shown in figure 4 in which the catalyst layer 16 is covered with a microporous sheet 55. And as mentioned in relation to figure 4, in that case the electrode element 70 is desirably installed in the opposite orientation, so that the polymer microporous sheet 55 (and so the catalyst coating 16) is on the surface closest to the electrolyte chamber K. The cell stack 60 may also be modified in various ways. For example alternate electrode elements 70 are shown as being introduced from opposite faces of the stack. In one modification the electrodes 18 and 19, with their perforated margins, are linked together by being welded to linking strips, rather than being formed from a single sheet; if this welding is performed after assembly of the stack 60, then the linking strips may all be provided along the same face of the stack 60 (although staggered in position) . [034] The fuel cell stack 60, as with the fuel cell stack 20, may be used with a liquid electrolyte, such as potassium hydroxide solution in the electrolyte chambers K. The gases would typically be air, supplied to the oxygen chambers 0, and hydrogen supplied to the fuel chambers H. Nevertheless it will be appreciated that the cell stack 60 may be used with different electrolytes and with different gases. Furthermore this structure may be used for other applications, such as a multi-cell electrolyser stack, or a flow battery. [035] In each type of cell stack it will be

appreciated that external electric current takeoff may be achieved by making connections to projecting tabs that are attached to or integral with the end electrodes in the stack. It will also be appreciated that a cell stack may consist of groups of cells arranged as described above, the cells within such a group being electrically in series, while the different groups of cells that make up the stack may be electrically in parallel. This would provide a cell stack that provides a lower voltage but higher current output.

[036] The electrodes described above each comprise a sheet of ferritic stainless-steel, with holes formed by laser drilling. In a modification the stainless steel is coated with a thin layer of nickel, either before or after laser drilling holes through the stainless-steel sheet. The nickel is a good electrical conductor, and also protects the stainless steel against corrosion from the electrolyte.

[037] In use of an electrode of the invention, electrolyte is present at one face and gas is present at the other face, such that there is a gas/liquid 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.

Claims

1. A cell stack including a plurality of cells arranged electrically in series, wherein each cell comprises electrodes of opposite polarity constituting an anode and a cathode, wherein pairs of electrodes of opposite polarity are integral with each other, being defined by spaced-apart portions of a conductive element.

2. A cell stack as claimed in claim 1 wherein the cell stack comprises a separator plate between successive cells in the stack, wherein the separator plate is not electrically conducting, and defines gas flow chambers on opposite faces thereof; wherein there are electrodes of opposite polarity adjacent to opposite faces of the separator plate, and wherein the electrodes that are adjacent to opposite faces of the separator plate are a pair of electrodes that are integral with each other.

3. A cell stack as claimed in claim 1 wherein there are no separator plates between successive cells, and one electrode of each pair of electrodes is interleaved between another pair of electrodes in the stack.

4. A cell stack as claimed in any one of the preceding claims wherein the conductive element comprises a wire mesh .

5. A cell stack as claimed in any one of claims 1 to 3 wherein the conductive sheet comprises a sheet of electrically conducting material through which are defined a multiplicity of through-pores in those portions thereof which define the electrodes.

6. A cell stack as claimed in claim 5 wherein the through-pores are defined by etched or drilled holes.

7. A cell stack as claimed in claim 6 wherein the holes are produced by laser drilling.

8. A cell stack as claimed in any one of claims 5 to 7 wherein those parts of the conductive sheet that surround those parts that define the electrodes define no such through-pores .

9. A cell stack as claimed in claim 8 wherein sealing material is coated on to those parts of the conductive sheet that surround the parts that define the electrodes.

10. A cell stack as claimed in any one of the preceding claims wherein the electrodes incorporate a coating of catalytic material .

11. A cell stack as claimed in claim 10 wherein the catalytic material is provided on a particulate

conducting support material.

12. A cell stack as claimed in claim 10 or claim 11 wherein the coating of catalytic material is covered by a layer of permeable hydrophilic polymeric material.