GB2540592A

GB2540592A - Fuel cell stack insert

Info

Publication number: GB2540592A
Application number: GB1512945.5A
Authority: GB
Inventors: Michael Tracy Richard
Original assignee: AFC Energy PLC
Current assignee: AFC Energy PLC
Priority date: 2015-07-22
Filing date: 2015-07-22
Publication date: 2017-01-25
Anticipated expiration: 2035-07-22
Also published as: GB201512945D0; GB2540592B

Abstract

A fuel cell stack (5, fig 1) consists of multiple fuel cells (6, fig 1), each comprising a frame 30 to define an electrolyte chamber 20 for a liquid electrolyte. Each of the frames 30 in the stack define apertures 34 that are aligned in the fuel cell stack to define fluid flow headers, there being at least one fluid flow header for the liquid electrolyte, and each cell defines a linking passage 36 between the electrolyte chamber 20 and the fluid flow header for the liquid electrolyte. The cell stack also comprises a flow-modifying insert 50 within the aperture 34 providing the fluid flow header for the liquid electrolyte, the insert 50 in combination with the header defining a flow channel whose cross-sectional area varies along the length of the insert 50 (figs 3 and 4), decreasing in the flow direction along the flow channel. The insert 50 improves flow uniformity within the stack. Preferably, the insert 50 is in the form of an open channel, with a cross-section which tapers linearly along its length.

Description

Fuel Cell Stack Insert

The present invention relates to a system that includes several fuel cells in a stack, enabling fluids to flow through all the cells in the stack; it relates to an insert such that the flows through different cells are more uniform.

Background to the invention

Fuel cells have been identified as a relatively clean and efficient source of electrical power. Alkaline fuel cells are of particular interest because they operate at relatively low temperatures, are efficient and suitable for operation in an industrial environment. Acid fuel cells and fuel cells employing other aqueous electrolytes are also of interest. Such fuel cells typically comprise an electrolyte chamber separated from a fuel gas chamber (containing a fuel gas, typically hydrogen) and a further gas chamber (containing an oxidant gas, usually air). The electrolyte chamber is separated from the gas chambers using electrodes. Typical electrodes for alkaline fuel cells comprise a conductive metal mesh, typically of nickel, that provides mechanical strength to the electrode. Onto the metal mesh is deposited a catalyst which may for example contain activated carbon and a catalyst metal such as platinum. A single fuel cell does not produce a large voltage, and it is usually desirable to assemble a number of fuel cells into a stack to provide a larger electrical power output. Each fuel cell consists of a number of components including a surrounding frame that defines the edges of the cell. In order to feed fluids into the fuel cells of such a stack, typically each frame within the stack defines a number of apertures, so that in the stack these apertures are aligned and define fluid flow headers. In particular, considering liquid electrolyte, each such electrolyte header would be connected by linking channels to the electrolyte chambers of the cells. These linking channels would be narrow and long so as to impose a significant pressure drop and to ensure a significant ionic electrical resistance between the cell and the header. These are both advantageous effects, as the pressure drop would be expected to enhance the uniformity of the flow through different cells in the stack (comparing one cell with another), while the ionic electrical resistance suppresses ionic leakage currents between different cells. Nevertheless it has been found that in a stack with many cells there can be significant non-uniformity in the electrolyte flow through different cells, which may for example lead to a localised overheating.

Discussion of the invention

The system of the present invention addresses or mitigates one or more problems of the prior art.

In accordance with the present invention there is provided a fuel cell stack consisting of multiple fuel cells, each fuel cell comprising a frame to define an electrolyte chamber for a liquid electrolyte, and each of the frames in the stack defining apertures that are aligned in the fuel cell stack to define fluid flow headers, there being at least one fluid flow header for the liquid electrolyte, and each cell defining a linking passage between the electrolyte chamber and the fluid flow header for the liquid electrolyte; wherein the fuel cell stack also comprises a flow-modifying insert within the fluid flow header for the liquid electrolyte, the insert in combination with the header defining a flow channel whose cross-sectional area varies along the length of the insert, decreasing in the flow direction along the flow channel.

Surprisingly the provision of such an insert can have a significant effect on the uniformity of the electrolyte flows through different cells, leading to more consistent electrical output from the cells and more uniform temperature distribution within the stack.

The insert may be in the form of an open channel, for example a U-shaped channel, the open face of the channel being oriented towards the linking passages.

In this case the cross-sectional area of the open channel decreases in the flow direction. By way of example the walls of the open channel may vary in thickness along the length of the insert, becoming thicker in the flow direction so that the open channel becomes narrower in the flow direction. Alternatively or additionally the base of the open channel may also vary in thickness, so for example the depth of the open channel may become less in the flow direction.

In a preferred embodiment the cross-sectional area of the open channel decreases linearly in the flow direction. For example the insert may be in the form of an open channel of uniform depth, whose width tapers linearly in the flow direction. Alternatively the insert may be in the form of an open channel whose width is constant, but whose depth decreases linearly in the flow direction.

In a modification the insert defines a port at an intermediate position along its length, and the insert in combination with the header defines an inlet flow channel leading to the port, and outlet flow channels that communicate between the port and the linking passages. In this case the cross-sectional area of each outlet flow channel decreases in the flow direction along the outlet flow channel. Preferably the positions of the port or ports are no more than 10% of the length of the insert away from the midpoint, more preferably no more than 5% of the length of the insert away from the midpoint. Where there is a single port, its width may be between 2% and 5% of the length of the insert, for example 3%.

It will be appreciated that the provision of such an insert does not necessitate any change or alteration in the components that make up the fuel cell stack. The insert would merely be inserted into the header that would otherwise carry the electrolyte to the cells. Preferably the insert is of substantially the same external shape in cross-section as that of the apertures that define the header, but allowing sufficient clearance for the insert to be inserted. It may be of a substantially rigid engineering plastic; suitable plastics would be ABS (acrylonitrile butadiene styrene) or Noryl ™ (which is an amorphous blend of polyphenylene ether resin with polystyrene).

Where an insert is in the form of an open channel, it will be appreciated that the channel may be open along its entire length, or there may be struts linking the walls at spaced apart positions along the channel to hold the walls at the required separation, the positions of the struts being selected so that they do not obstruct the linking passages.

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 partly schematic sectional view of fuel cells in a stack;

Figure 2 shows a plan view of an electrolyte chamber frame of the fuel cell stack of figure 1, corresponding to the view on the line 2-2 of figure 1, and showing an insert; Figure 3 shows a plan view of an insert in the fuel cell stack of figure 2;

Figure 4 shows a perspective view of the insert in the fuel cell stack of figure 2 Figure 5 shows a longitudinal sectional view of an alternative insert; and Figure 6 shows a longitudinal sectional view of another alternative insert. A fuel cell consists of two electrodes, an anode and a cathode, separated by an electrolyte, and each electrode is in contact with a respective gas stream. Chemical reactions that take place at the electrodes cause ions to migrate through the electrolyte, and generate an electric current in an external circuit. It is customary to arrange fuel cells in stacks, to obtain a larger voltage or power output than is available from a single fuel cell. Each such fuel cell stack must be supplied with appropriate fluids. For example the electrolyte may be an aqueous solution of potassium hydroxide (KOH), and the gas streams may be hydrogen, and air or oxygen. In the system described below an aqueous KOH electrolyte and an air stream are passed through the fuel cell stacks, and similarly a hydrogen stream is passed in parallel through the cell stacks.

Referring to figure 1, a fuel cell stack 5 comprises a stack of several fuel cells 6, only three of which are shown. The cells 6 are typically connected electrically in series. Each cell produces an electromotive force of about 1 volt, so the electrical output of the stack 5 depends on the number of fuel cells in the stack. There may in practice be fifty or a hundred or even more fuel cells 6 in the stack 5. Each fuel cell 6 includes two opposed electrodes 10, between which is defined an electrolyte chamber 20, as described below, one electrode 10 being an anode, marked 10a, and the other being a cathode marked 10c.

Each electrode 10 comprises a generally rectangular sheet 11 of a metal such as nickel or ferritic stainless-steel. The sheet 11 may for example be of thickness 0.3 mm. A central region 12 of the sheet is perforated for example by drilling, etching or cutting to produce a very large number of through holes 14; in this example the central region 12 is of rectangular shape. The holes 14 may for example be of mean diameter 50 pm if produced by laser drilling; or for example of mean diameter 300 pm if produced by chemical etching; or for example of diameter 1 or 2 mm if made by mechanical drilling, punching or cutting. A margin 15 around the periphery of the central region 12 is not provided with such small holes 14.

One surface of the perforated central region 12 is covered by a gas-permeable layer 16 whose outer surface is covered by a layer 18 which contains catalytically active material. The metal sheet 11 and the layers 16 and 18 are bonded together. 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 layers 16 and 18 may each comprise carbon with a hydrophobic polymer binder, the catalyst-containing layer 18 also including an appropriate active catalytic material in particulate form.

The catalyst-containing layers 18 may incorporate a smaller proportion of the binder than the gas-permeable layers 16, so the catalyst-containing layer 18 is less hydrophobic. The polymer binder may be polytetrafluoroethylene (PTFE), or for example polyvinylidene fluoride.

In the fuel cell stack 5 each electrode 10 is arranged so that the gas-permeable layer 16 and the catalyst containing layer 18 face the electrolyte chamber 20; that may be referred to as the front face of the electrode 10. The fuel cell stack 5 also includes gas chambers 21 and 22 that alternate along the stack with the electrolyte chambers 20. Each anode 10a separates the electrolyte chamber 20 from a gas chamber 21 through which a fuel gas such as hydrogen is passed, while each cathode 10c separates the electrolyte chamber 20 from a gas chamber 22 through which an oxidising gas such as air is passed.

Each gas chamber 21 is defined by a rectangular peripheral frame 24 which is sealed to the rear surface of each adjacent anode 10a by a gasket (not shown). Each gas chamber 22 is defined by a rectangular peripheral frame 26 which is sealed to the rear surface of each adjacent cathode 10c by a gasket (not shown).

The electrolyte chamber 20 is defined by a rectangular peripheral frame 30 which is sealed by gaskets 32 onto the non-perforated margin 15 of each electrode 10 (the frames 24, 26 and 30 and the gasket 32 are shown in broken lines in figure 1). The electrolyte chambers 20 and the gas chambers 21 and 22 are of substantially the same size as the perforated central region 12 of each electrode 10, and align with the central region 12.

In operation of the fuel cell stack 5 a fuel gas such as hydrogen is supplied to the gas chambers 21 and air is provided to the gas chambers 22 respectively, and an electrolyte solution such as aqueous potassium hydroxide solution is supplied to or passed through the electrolyte chambers 20. Each fuel cell 6 provides an electromotive force of slightly less than 1 V. Within each cell 6 the electrolyte permeates through the outer portion of the catalyst-containing layer 18 (which may be at least partly hydrophilic) and so contacts the catalytic material in the catalyst-containing layer 18 of each electrode 10.

Each electrode 10 in this example extends to the periphery of the stack 5, and the components that make up the stack 5 are held together for example by tie-rods or bolts (not shown). Within the stack 5 the electrolyte, fuel gas, and air, are supplied to and withdrawn from the respective chambers 20, 21,22. At least some of the requisite fluid flow paths are defined by aligned apertures through the frames 24, 26, 30, gaskets 32, and electrodes 10. By way of example a header to supply electrolyte to all the electrolyte chambers 20 may be defined by aligned apertures 34 near the bottom of each electrode 10, each frame 24, 26 and 30 and each gasket 32. Headers for inflow of fuel gas to all the fuel gas chambers 21 may be similarly defined by aligned apertures, for example apertures 35 near the top of each electrode 10, each frame 24, 26 and 30 and each gasket 32. The header defined by the apertures 34 may communicate with the electrolyte chambers 20 through channels such as grooves 36 indicated by broken lines within the frames 30. The header defined by the apertures 35 may communicate with the fuel gas chambers 21 through channels such as grooves 37 indicated by broken lines within the frames 24. It will be appreciated that the grooves 36 and 37 are shown schematically; the grooves 36 and 37 may follow convoluted paths, and the aligned apertures 34 and the aligned apertures 35 may be in positions other than those shown.

Referring also to figure 2, this shows a plan view of a frame 30 defining an electrolyte chamber 20; part of the bottom of the frame 30 is shown in some detail, whereas less detail is provided in relation to the ends and the top of the frame 30. In use, electrolyte is fed into the header defined by the aligned apertures 34, flows through a path or groove 36 which is narrow and convoluted, and which communicates with multiple outlets 42 across the entire width of the chamber 20.

The electrolyte then flows upwardly through the chamber 20, and may then for example flow through grooves such as the grooves 39 defined in the surface of the frame 30 into headers defined by aligned apertures such as the apertures 40. This is merely an example as to how the electrolyte may leave the electrolyte chamber 20.

As shown in the bottom of the frame 30, each frame defines several holes 44 for the bolts or tie rods (not shown) to hold all the elements of the stack 5 together; and also defines apertures 46 to define headers for withdrawing hydrogen, corresponding generally to the apertures 35 that define headers for supplying hydrogen. A seal 48 surrounds each hole 44 and each aperture 34, 35, 40 and 46, and also the groove 36. In an alternative arrangement, the hydrogen may be supplied through the apertures 46 and may be withdrawn through the apertures 35.

The long and convoluted path of the inlet path 36 ensures high ionic electrical resistance between each electrolyte chamber 20 and the header defined by the apertures 34, so suppressing leakage currents. In addition it ensures a significant pressure drop between the header defined by the apertures 34, and each electrolyte chamber 20.

In accordance with the present invention, an insert 50 is provided in the header defined by the apertures 34 (the insert 50 is not shown in figure 1). The insert 50 extends the entire length of the stack 5, through every cell 6, and is of generally U-shaped or open-channel shape cross-section, consisting of a base portion and two side walls, and being open along the top (as shown). The insert 50 has a cross-sectional shape substantially the same as that of the aperture 34, but is slightly smaller to provide clearance as the insert 50 is inserted into the header defined by all the apertures 34 along the entire length of the stack 5. For example the aperture 34 and therefore the header may be of width 37 mm and of height 10.5 mm; and the insert 50 may be of width 35 mm or 36 mm and of height typically between 7.0 mm and 8.0 mm, for example 7.5 mm. The insert 50 is therefore a loose fit in the header, which ensures that the insert 50 can be freely inserted through the entire length of the header, and the height difference ensures that the insert 50 does not obstruct the open end of the groove 36.

Referring now to figures 3 and 4 there is shown a plan view of the insert 50 in isolation, and a perspective view of the insert 50 in isolation, respectively. It will be seen that the overall cross-sectional shape is rectangular, corresponding to the shape and size of the header defined by the apertures 34 as mentioned above. The length of the insert 50 is the length of the header defined by the apertures 34, on the supposition that the electrolyte is fed into the header at only one end. The side walls of the insert 50 have a linear taper, so that the fluid flow space between the side walls tapers linearly from a maximum width at the end 51 to a minimum width at the end 52. The insert 50 may for example be machined from a rigid engineering plastic such as ABS.

The insert 50 shown in figures 3 and 4 is for use in a header that has an inlet for the electrolyte at one end, and is closed at the other end. The insert 50 is inserted into the header defined by the apertures 34 such that the end 51 with the maximum width of fluid flow space is adjacent to the inlet end, whereas the end 52 with the minimum width of fluid flow space is adjacent to the closed end.

It has been found that the provision of the insert 50 within the header defined by the apertures 34 leads to considerable improvement in the uniformity of the electrolyte flow through all the electrolyte chambers 20. Consequently there is less variation in temperature within the stack 5, and the electrical properties of the cells 6 are more consistent and uniform. This is particularly advantageous in the context of a fuel cell stack 5 where the electrolyte is supplied at a pressure that is less than 10 kPa (100 mbar), corresponding to a pressure head of less than 100 cm water, for example with electrolyte supplied to the inlet of the header defined by the apertures 34 at a pressure of 5 kPa (50 mbar).

In one modification suitable for use in the stack 5 described above, the insert may be of open channel shape as described above, but with one side wall slightly higher than the other side wall, the lower side wall being adjacent to the side of the aperture 34 with which the grooves 36 communicate. For example with the size of aperture 34 described above, one side wall might be of height 9.5 mm and the other side wall of height 7.5 mm. This again ensures that the open ends of the grooves 36 are not obstructed by the insert.

In another modification the insert may obstruct one side of the header, rather than obstructing the bottom part of the header. So for example an insert for use with the size of aperture 34 described above may comprise a linear wedge that varies in width between say 2 mm and 33 mm, and is of height say 8 mm or 9 mm. In the stack 5 shown in figure 2, such a linear wedge would be inserted on the left-hand side so that the ends of the grooves 36 not obstructed. Such a linear wedge may also be combined with one or more side walls.

Referring now to figure 5 there is shown a longitudinal sectional view of an alternative insert 60. The insert 60 includes a projecting end plate 62, and is intended to be inserted into the header defined by the apertures 34 such that the end plate 62 blocks part of the inlet into the header; the broken line 63 indicates the top edge of the header; and the insert 60 is of the same length as the header. The insert 60 defines an inlet channel 64 of constant rectangular cross-section between side walls 65 and a top wall 66, the inlet channel 64 starting at the end of the insert 60 which is at the inlet into the header, and extending slightly over half way along the length of the insert 60. Near the closed end of the inlet channel 64 a number of apertures 68 are defined through the top wall 66. The upper surface of the insert 60 slopes linearly upwards between the section of the top wall 66 with the apertures 68, and both the ends of the insert 60, so defining two tapering outlet channels 69 between the upper surface of the insert 60 and the top edge of the header 63.

Hence in use when electrolyte is fed into the inlet of the header, the end plate 62 ensures that the electrolyte flows along the inlet channel 64. As indicated by the arrows P, the electrolyte then flows through the apertures 68 and flows in both directions along the tapering channels towards both ends of the header, so communicating with the grooves 36 that lead to the cells 6.This has the same benefits as with the insert 50, in providing more uniform flows of electrolyte through the cells 6 in the stack 5. In this case the apertures 68 are at positions that are within 5% of the length of the insert either side of the midpoint.

It will be appreciated that the insert 60 may be modified in various ways, for example the insert may also define side walls for the tapering flow channels 69. The apertures 68 may be circular, or may be rectangular slots, or may be replaced by a single large aperture.

Referring now to figure 6 there is shown a longitudinal sectional view of an insert 70 which has a number of features in common with the insert 60, the features that are the same being referred to by the same reference numerals. The broken line 63 indicates the top of the header. The insert 70 includes a projecting thicker end portion 72 which fits tightly into the orifice of the end manifold so as to block part of the inlet into the header; the remainder of the insert 70 is of the same length as the header. The insert 70 is of sheet material, and includes a top plate 76 which in longitudinal section is in the form of a shallow V; side plates 75 extend downward from this top plate 76 along its entire length. The insert 70 thus defines an inlet channel 74 of tapering rectangular cross-section between the side walls 75 and the top plate 76, the inlet channel 74 starting at the end of the insert 70 which is at the inlet into the header, and extending slightly over half way along the length of the insert 70, where it is blocked by a thin end plate 77. Shortly before the closed end of the inlet channel 74 a number of apertures 68 are defined through the top plate 76. The shallow V shape of the top plate 76 defines two tapering outlet channels 69 between the upper surface of the insert 70 and the top edge of the header 63.

Hence in use when electrolyte is fed into the inlet of the header, the end portion 72 ensures that the electrolyte flows along the inlet channel 74. As indicated by the arrows P, the electrolyte then flows through the apertures 68 and flows in both directions along the tapering channels 69 towards both ends of the header, so communicating with the grooves 36 that lead to the cells 6.This has the same benefits as with the inserts 50 and 60, in providing more uniform flows of electrolyte through the cells 6 in the stack 5. As with the insert 60, the apertures 68 are at positions that are within 5% of the length of the insert 70 either side of the midpoint.

The insert 60 is shown as being of solid material throughout its length, out of which the channels 64 and 69 are moulded or formed, whereas the insert 70 is of sheet structure. It will be appreciated that an insert may combine these structural approaches, for example the insert 70 might be modified by having the right-hand side (as shown) of solid material as in the insert 60. As another alternative the insert 70 might also include an additional plate parallel to the bottom of the header to define an upper boundary to the inlet flow channel 75, so that the flow channel 75 would be of constant cross-sectional area rather than tapering.

The inserts 60 and 70 are described as if the flow direction through the apertures 68 is upward, and such that the broken lines 63 represent the top of the header. It will be appreciated that an alternative insert might be of such a cross-sectional shape as to fit sideways into the header, so that the broken lines 63 would represent the side wall of the header, and the flow direction through the apertures 68 would be horizontal. And in this case also the apertures 68 might be of circular or rectangular shape, or might be replaced by a single large aperture.

Claims

1. A fuel cell stack consisting of multiple fuel cells, each fuel cell comprising a frame to define an electrolyte chamber for a liquid electrolyte, and each of the frames in the stack defining apertures that are aligned in the fuel cell stack to define fluid flow headers, there being at least one fluid flow header for the liquid electrolyte, and each cell defining a linking passage between the electrolyte chamber and the fluid flow header for the liquid electrolyte; wherein the fuel cell stack also comprises a flow-modifying insert within the fluid flow header for the liquid electrolyte, the insert in combination with the header defining a flow channel whose cross-sectional area varies along the length of the insert, decreasing in the flow direction along the flow channel.

2. A fuel cell stack as claimed in claim 1 wherein the insert is in the form of an open channel.

3. A fuel cell stack as claimed in claim 2 wherein the cross-sectional area of the open channel decreases linearly in the flow direction.

4. A fuel cell stack as claimed in claim 3 wherein the open channel is of uniform depth, and of width that tapers linearly in the flow direction.

5. A fuel cell stack as claimed in claim 1 wherein the insert defines a port at an intermediate position along its length, and is such that when the insert is located in the header an inlet flow channel is defined that leads to the port, and outlet flow channels are defined that communicate between the port and the linking passages, the cross-sectional area of each outlet flow channel decreasing in the flow direction along the outlet flow channel.

6. A fuel cell stack as claimed in claim 5 wherein the cross-sectional area of each outlet flow channel decreases linearly in the flow direction.

7. A fuel cell stack as claimed in claim 5 or claim 6 wherein the port is no more than 10% of the length of the insert away from the midpoint of the insert.

8. A fuel cell stack as claimed in any one of the preceding claims wherein the insert is of the same external shape in cross-section as that of the apertures that define the header, but allowing sufficient clearance for the insert to be inserted.

9. A fuel cell stack as claimed in any one of the preceding claims wherein the insert is of a substantially rigid engineering plastic.

10. A fuel cell stack substantially as hereinbefore described with reference to, and as shown in, figures 1 and 2 of the accompanying drawings, including an insert substantially as hereinbefore described with reference to, and as shown in, figures 2 to 4, or figure 5, or figure 6 of the accompanying drawings.

11. An insert for use in a fuel cell stack as defined in any one of the preceding claims.