GB2529149A

GB2529149A - Fuel cell

Info

Publication number: GB2529149A
Application number: GB1413781.4A
Authority: GB
Inventors: Hossein Ostadi; Zachary Elliott
Original assignee: Intelligent Energy Ltd
Current assignee: Intelligent Energy Ltd
Priority date: 2014-08-04
Filing date: 2014-08-04
Publication date: 2016-02-17
Also published as: WO2016020659A1; GB201413781D0

Abstract

A fuel cell array (1, fig 1) comprises a substantially laminar assembly 2 including a plurality of fuel cells 3, 5, 6 arranged in an array over exclusive areas of the assembly, the fuel cells each including an anode 3a, 5a, 6a spaced from the anode of the or each other fuel cell and a cathode 3c, 5c, 6c spaced from the cathode of the or each other fuel cell, the anodes arranged on an opposite side of a proton exchange membrane 11 to the cathodes, wherein the proton exchange membrane is common to the plurality of fuel cells and at least two of the fuel cells are connectable in series. The fuel cells 3, 5, 6 may also include anode gas diffusion layers 21a, 23a, 24a and cathode gas diffusion layers 21c, 23c, 24c. Interconnections 12, 13 may connect respective anodes and cathodes of adjacent fuel cells. In a further aspect, anodes and cathodes are connected to an interconnection controller (30, fig 3) configured to reconfigure interconnections between the fuel cells so that they are connected in series or in parallel or disconnected from the array.

Description

Intellectual Property Office Application No. GB1413781.4 RTI\4 Date:1 April 2015 The following terms are registered trade marks and should be read as such wherever they occur in this document: N aflon Intellectual Property Office is an operating name of the Patent Office www.gov.uk/ipo

FUEL CELL

This invention relates to a fuel cell array. In particular, it relates to a plurality of individual fuel cells arranged side by side on a sheet. The invention also relates to an interconnection controller configured to reconfigure electrical interconnections between a plurality of the fuel cells.

Conventional electrochemical fuel cells convert fuel and oxidant, generally both in the form of gaseous streams, into electrical energy and a reaction product, A common type of electrochemical fuel cell for reacting hydrogen and oxygen comprises a polymeric ion (proton) exchange membrane (PEM), with fuel and air being passed over respective sides of the membrane. Protons (that is, hydrogen ions) are conducted through the REM, balanced by electrons conducted through a circuit connecting the anode and cathode of the fuel cell. To increase the available voltage, a stack may be formed comprising a number of such membranes arranged with separate anode and cathode fluid flow paths. Such a stack is typically in the form of a block comprising numerous individual fuel cell plates held together face to face by end plates at either end of the stack.

In an alternative configuration, the fuel cells may be arranged side by side, rather than face to face, in a substantially laminar sheet.

According to a first aspect of the invention we provide a fuel cell array comprising a substantially laminar assembly including a plurality of fuel cells arranged in an array over exclusive areas of the assembly, the fuel cells each including an anode spaced from the anode of the or each other fuel cell and a cathode spaced from the cathode of the or each other fuel cell, the anodes arranged on an opposite side of a proton exchange membrane to the cathodes, wherein the proton exchange membrane is common to the plurality of fuel cells and at least two of the plurality of fuel cells are connectable in series.

This is advantageous as the two-dimensional fuel cell array may be formed of a plurality of individual fuel cells but the PEM may be common to all of fuel cells. This may be advantageous for manufacture. Further, while the fuel cells may be arranged side by side in the array, through-assembly interconnections may be provided for connecting adjacent fuel cells in series, This alternative array arrangement to the conventional fuel cell stack may be advantageous for space-limited applications, such as in electronic devices.

Optionally, at least two of the fuel cells of the plurality of fuel cells include an interconnection configured to connect the anode of one fuel cell to the cathode of another of the fuel cells.

Optionally, each fuel cell includes a first gas diffusion layer electrically isolated from the gas diffusion layer of the other fuel cells in the array and a second gas diffusion layer electrically isolated from the gas diffusion layer of the other fuel cells in the array, the first gas diffusion layer arranged between the common proton exchange membrane and the anode of a particular fuel cell and the second gas diffusion layer arranged between the common proton exchange membrane and the cathode of a particular fuel cell. The gas diffusion layer may be conductive and abut the PEM to form part of an electrode assembly. Thus, the gas diffusion layer of one cell may be electrically isolated from the gas diffusion layer of an adjacent cell so that the adjacent cells are not connected in parallel.

Optionally, the gas diffusion layer of each fuel cell is physically separated from the gas diffusion layer of the or each other fuel cell. The physical separation may provide the electrical isolation.

Optionally, an insulator region extends between the gas diffusion layer of one of the fuel cells and the gas diffusion layer of another of the fuel cells on a common side of the proton exchange membrane. The insulator region may comprise an insulator part of the gas diffusion layer that is less conductive than a part of the gas diffusion layer that forms the fuel cells. The insulator part may be gas diffusion layer material that has been treated to reduce its conductivity, such as by chemical or thermal treatment. The insulator region may comprise an insulating foam. Preferably the insulator region has substantially the same compressibility as the parts of the gas diffusion layer that form the fuel cells. This is advantageous to achieve a uniform compression across the assembly.

Optionally, the fuel cell array includes a first substrate and a second substrate wherein the plurality of fuel cells are arranged therebetween over complimentary areas of the first and second substrates that are exclusive to each fuel cell. In other embodiments only one of the first or second substrates may be provided. The substrates may be rigid or flexible. For flexible substrates a support structure may be provided. The first and/or second substrates may be common to the plurality of fuel cells or at least common to two or more of the plurality of fuel cells.

The fuel cell array may include a first flow field arrangement configured to provide a first reactant (e.g. fuel) to the anode of each of the fuel cells in the array and a second flow field configured to provide a second reactant (e.g. oxidant) to the cathode of each of the fuel cells in the array.

Optionally, the first substrate includes a first flow field comprising a plurality of channels for delivering a first reactant to a first side of the proton exchange membrane and the second substrate includes a second flow field comprising a plurality of channels for delivering a second reactant to an opposed second side of the proton exchange membrane. It will be appreciated that only one of the first or second substrates may be provided with the flow field. Alternatively, the flow fields may be provided by channels in the gas diffusion layer(s).

Optionally, the first substrate includes the anodes of the fuel cells and/or the second substrate includes cathodes of the fuel cells. Optionally, the first substrate and/or second substrate comprises a printed circuit board. Thus, the anodes and/or cathodes may be formed by metallization of the printed circuit board, which connects to the REM via a conductive gas diffusional layer.

Optionally, the plurality of fuel cells extend over different sized areas and or over areas having different shapes. This provides flexibility to ensure the array can fit within desired bounds. Further, different shaped and/or sized fuel cells may have different power outputs and therefore the total output of the fuel cell array may be controlled based on the interconnections between fuel cells in the array.

Optionally, the anodes and cathodes of at least a subset of the fuel cells are connected to an interconnection controller, the interconnection controller configured to provide for the reconfiguration of interconnections between the fuel cells such that said fuel cells are connected in series or connected in parallel or disconnected from the array. The interconnection controller may provide for reconfiguration of the interconnections between fuel cells of the array. Optionally, the interconnection controller is configured to actively reconfigure the interconnections between cells during operation of the fuel cell array.

Optionally, the interconnection controller is configured to receive a measure of the electrical output of a plurality of cells in the array and reconfigure interconnections between the cells in response to said measure. Optionally, the measure of the electrical output comprises the total output of the fuel cell array.

Optionally, the interconnection controller is configured to reconfigured interconnections between the fuel cells to maintain a predetermined output voltage for the fuel cell array.

According to a further aspect of the invention we provide a fuel cell array comprising a substantially laminar assembly including a plurality of fuel cells arranged in an array over exclusive areas of the assembly, the fuel cells each including an anode spaced from the anode of the or each other fuel cell and a cathode spaced from the cathode of the or each other fuel cell, the anodes arranged on an opposite side of a proton exchange membrane to the cathodes, wherein the anodes and cathodes of at least a subset of the fuel cells are connected to an interconnection controller, the interconnection controller configured to provide for the reconfiguration of interconnections between the subset of fuel cells such that said fuel cells are connected in series or connected in parallel or disconnected from the array.

Thus, the interconnection controller may be configured to form interconnections either in series or disconnect one or more of the fuel cells from the output of the array. The interconnection controller may be configured to form interconnections either in series or in parallel.

Optionally, the interconnection controller is configured to actively reconfigure the interconnections between cells during operation of the fuel cell array. This is advantageous as the reconfiguring of interconnections may be performed by transistors mounted on the PCB substrates. The active reconfiguration of interconnections may also allow the array to output a plurality of different voltages or powers. This may obviate the need for a DC-DC convertor.

Optionally, the interconnection controller is configured to receive a measure of the electrical output of a plurality of cells in the array and reconfigure interconnections between the cells in response to said measure. Thus, the interconnection controller may be configured to measure the output voltage of some or all of the fuel cells and reconfigured the interconnections accordingly. Optionally, the measure of the electrical output comprises the total output of the fuel cell array or the electrical output of a subset of the fuel cells in the array.

Optionally, the interconnection controller is configured to reconfigure interconnections between the fuel cells to maintain a predetermined output voltage for the fuel cell array.

There now follows, by way of example only, a detailed description of embodiments of the invention with reference to the following figures, in which: Figure 1 shows a plane view of an example fuel cell array; Figure 2 shows a section through an example fuel cell array; Figure 3 shows a section through a second example fuel cell array; and Figure 4 shows a flow chart illustrating a method of manufacturing the fuel cell array.

Figures 1 and 2 show an example a fuel cell array 1 comprising a substantially laminar assembly 2 including a plurality of fuel cells 3, 4, 5, 6 arranged in an array over exclusive areas of the assembly 2. Thus, the assembly is formed of a two dimensional array of fuel cells. The fuel cells 3, 4, 5, 6 each including an anode 3a, 5a, Ba (the anode for the fuel cell 4 is not visible) spaced from the anode of the or each other fuel cell by gaps 7, 8, The fuel cells 3, 4, 5, 6 also comprise a cathode 3c, 5c, 6c (the cathode for the fuel cell 4 is not visible) spaced from the cathode of the or each other fuel cell 3, 4, 5, 6 by gaps 9, 10. The anodes 3a, 5a, 6a are arranged on an opposite side of a proton exchange membrane 11 to the cathodes 3c, Sc, 6c. The proton exchange membrane 11 is common to the plurality of fuel cells 3, 4, 5, 6. Thus, while the fuel cells 3, 4, 5, 6 are arranged over distinct areas of the assembly, a unitary proton exchange membrane (PEM) 11 extends through each of them. Each fuel cell thus includes a distinct region of the PEM 11. In the example of Figure 1, the first fuel cell 4 is connected in series with the third fuel cell 5 and the third fuel cell 5 is connected in series with the fourth fuel cell 6. A first interconnection 12 extends between the anode 3a of the first fuel cell 3 and the cathode Sc of the third fuel cell 5. A second interconnection 13 extends between the anode Sa of the third fuel cell 5 and the cathode 6c of the fourth fuel cell 6.

Figure 1 shows the fuel cells 3, 4, 5, 6 arranged over exclusive areas of the assembly 2 with gaps 7, 8 and 14 electrically separating the electrodes (anode and cathode) of the fuel cells. The fuel cells can then be advantageously interconnected with the interconnections 12, 13 in series or parallel configurations as required.

Figure 2 shows a cross section along line A-A of Figure 1. The laminar assembly 2 is formed of a plurality of layers, which make up the array of fuel cells. The assembly 2 comprises a first substrate 15 and a second substrate 16. The first and second substrates 15, 16, in this example, comprise printed circuit boards (PCBs), which may be of silicon metalized with tin. The substrates 15, 16 could be of other materials, whether rigid or flexible. A flexible substrate may allow the laminar assembly 2 to conform to a three dimensional shape for example.

The first substrate 15 includes an anode flow field 17 formed in the surface thereof. The anode flow field 17 comprises a plurality of channels 18 for delivering a reactant, comprising a hydrogen fuel in this example, to the anode side of each of the fuel cells 3, 4, 5, 6. The channels 18 are formed by a plurality of ridges that define the channels therebetween. The channels 18 are formed by milling or etching of the PCB, i.e. a subtractive process, although they may be formed by printing or depositing the ridges onto the PCB in an additive process. The ridges do not extend in the gaps 7, 8, 9, 10, although in other embodiments they may.

The second substrate 16 includes a cathode flow field 20 formed in the surface thereof.

The cathode flow field 20 likewise comprises a plurality of channels 21 for delivering a reactant, comprising air in this example, to the cathode side of each of the fuel cells 3, 4, 5, 6. The channels 20 are defined by a plurality of ridges, which may be formed in a similar way to the ridges of the anode flow field 17.

The anodes 3a, 5a, 6a comprise conductive pads that extend over the distinct areas of the anode flow field 17. The cathodes 3c, 5c, 6c also comprise conductive pads that extend over distinct areas of the cathode flow field 20. It will be appreciated that the anodes and cathodes may be arranged in other ways. For example, the PCB may have a substantially laminar electrode deposited thereon and conductive ridges may be applied to (or printed on) the laminar electrode for forming the channels 18 or 21.

Typically copper is used as the conductive material with printed circuit boards. In this example, the PCB is tin coated as copper has been found to affect the performance of the catalysts used in the PEM 11.

The interconnections 7 and 8 may be formed by vias extending through the PCBs 15, 16 and conductive tracks on the outer faces. Wires may extend between the PCBs at an edge of the array. Alternatively, the interconnections 7, 8 may be provided in the gaps 7, 8, 9 10 and extend through the PEM 11. Thus, the PCBs may be perforated to allow for interconnections to extend through them.

Each fuel cell 3, 4, 5, 6 additionally includes a gas diffusion layer between the proton exchange membrane 11 and each electrode. Thus, the first fuel cell 3 has an anode gas diffusion layer 21a which abuts the anode 3a on one side and the PEM 11 on the other side. The first fuel cell 3 also has a cathode gas diffusion later 21c which abuts the cathode 3c on one side and the PEM 11 on the other. The second, third and fourth fuel cells 4, 5, 6 also include anode and cathode gas diffusion layers 22a, 22c; 23a, 23c; and 24a, 24c respectively.

The gas diffusion layers are configured to aid the diffusion of reactant received from the spaced channels 18 (flow fields) such that it is presented in a substantially uniform concentration across the fuel cell's area of PEM 11. The gas diffusion layer typically has an open cell foam structure and may be of a conductive material such as carbon.

The gas diffusion layers 21a, 21c of one fuel cell 3 are electrically separated by the physical separation provided by the gaps 7, 9 from the gas diffusion layers 23a, 23c of an adjacent fuel cell 5. In other embodiments, the anode gas diffusion layers 21a, 22a, 23a, 24a may be continuous with one another and extend across said gaps 7, 8.

However, the portions of the gas diffusion layer that extend across the gaps may be treated or adapted such that it is non-conductive, thereby electrically separating regions of the gas diffusion layer that make up the individual fuel cells, Likewise, the cathode gas diffusion layers 21c, 22c, 23c, 24c may be continuous with one another and extend across said gaps 9, 10. However, the portions of the cathode gas diffusion layer that extend across the gaps 9, 10 may be treated or adapted such that they are non-conductive, thereby electrically separating regions of the gas diffusion layer that make up the individual fuel cells. The gas diffusion layers may be made conductive by the addition of conductive particles. Therefore, the regions that span the gaps may be formed without conductive particles.

In this example, the gaps 7, 8, 9, 10 are unfilled, although, as discussed later, the gaps may be filled with an insulating material. The width of the gaps is dependent on the conductivity of the PEM 11. Thus, the PEM 11 must have sufficient sheet resistance or be of sufficiently low conductivity that adjacent cells do not form short circuits with one another. It has surprisingly been found that providing a separation distance of 0.5 mm or more between fuel cells on the common PEM 11 is sufficient to substantially prevent a short circuit. The in plane conductivity of the PEM is poor and of the order of hundreds of Ohms per sq. rn.

The PEM 11 typically includes a catalyst layer on its surface. It has been found that removing the catalyst layer from the common PEM 11 in the regions between fuel cells further reduces the chance of unwanted connections between adjacent cells. The PEM is typically a polymer material, such as Nafion, with a catalytic coating applied to a surface thereof.

Thus, the first substrate 15, second substrate 16 and PEM 11 may be common to all of the fuel cells in the assembly but the anodes, cathodes and conductive regions of the gas diffusion layers are configured to extend over distinct regions of the assembly to thus form a plurality of individual fuel cells arranged in an array. Thus, the fuel cells are arranged in the same plane but may be connected in series with trans-plane interconnections 7, 8. The flow fields may transport reactants across a plurality of the fuel cells of the array between a reactant inlet into the array and reactant exhaust outlet out of the array.

Figure 3 shows a second embodiment having the same general layout as that shown in Figures 1 and 2. Accordingly the same reference numerals have been used. As mentioned above, the gaps 7, 8, 9, 10 are filled with insulating material. This may act to support the PEM 11 where it extends between the fuel cells. The insulating material may comprise a foam or rubber material, such as silicon rubber. Preferably the insulating material has a similar compressibility as the gas diffusion layer material. Further, an interconnection controller 30 is provided which is connected by interconnections to the anodes and cathodes of the fuel cells 3, 4, 5, 6, although fuel cell 4 is not visible in this cross-sectional view. Thus, interconnections 31 and 32 connect the anode 3a and cathode 3c respectively of the first fuel cell 3 to the controller 30. Interconnections 33 and 34 connect the anode 5a and cathode 5c respectively of the third fuel cell 5 to the controller 30 Interconnections 35 and 36 connect the anode 6a and cathode 6c respectively of the fourth fuel cell 6 to the controller 30. a

The interconnection controller 30 is programmable such that the interconnections between the fuel cells 3, 4, 5, 6 can be reconfigured as required, such as between series and parallel arrangements or between series and disconnected or between parallel and disconnected. Thus, the interconnection controller 30 may connect the fuel cells 4, 5, 6, 7 in series if a higher voltage is required. The interconnection controller 30 may connect the fuel cells 4, 5, 6, 7 in parallel if a lower voltage but higher current is required.

Alternatively, the interconnection controller may disconnect certain fuel cells from contributing to the output of the array, as required, while the remaining fuel cell(s) are connected in series. Alternatively, a combination of series, parallel and disconnected configurations may be used to achieve a desired output voltage.

In a series configuration, for the first and third fuel cells 3 and 5, the interconnection 31 may be connected to interconnection 34. In a parallel configuration the interconnection controller may be configured to connect interconnections 33 and 35 and is interconnections 32 and 34.

In a further embodiment the interconnection controller 30 includes a fuel cell array output sensor 37 configured to measure an electrical output of the fuel cell array 1. The interconnection controller 30 may be configured to receive the measure of electrical output and form the interconnections between the fuel cells accordingly. This is particularly advantageous as the power output by a fuel cell can vary with temperature, fuel concentration, fuel cell age and other factors. The interconnection controller 30 provides for control of the power output of the array by interconnecting the plurality of fuel cells in different series or parallel or disconnected configurations or combinations thereof. The granularity of output voltages achieved may be reduced by including more fuel cells in the array. Thus, the interconnection controller 30 may be provided with a target output voltage and using the measure from the sensor 37, which forms a closed loop feedback arrangement, to modify actively the interconnections between fuel cells while in use to obtain or move towards the target output voltage. The interconnection controller 30 may replace a DC-DC converter commonly used in fuel cell power sources for providing a particular output voltage.

While in this example the interconnection controller 30 is shown as a centralised switch, it will be appreciated it may be distributed over the substrates 15, 16 as a network of switching elements. Thus, the switching elements may comprise transistors that connect or disconnect interconnections from each of the electrodes. Control signals for the switching elements may be provided by a controller.

Figure 4 shows a method of forming the fuel cell array 1. Step 40 comprises receiving the first substrate 15 and milling or etching the channels 18 of the flow field therein.

Step 41 comprises applying a unitary gas diffusion layer onto the first substrate 15 over the channels 18 with conductive adhesive. The adhesive may comprise a conductive epoxy. The adhesive layer may have a thickness of between 40 pm to 1000 pm.

Step 42 comprises milling or forming a pattern in the unitary gas diffusion layer and optionally any metal electrode layers on the substrate 15 to define the individual anode gas diffusion layers 21 a, 22a, 23a, 24a. Accordingly, gaps 7, 8 are formed by the removal of material from the unitary gas diffusion layer. The gaps 7, 8 may optionally be filled. The milling may define separation distance between the gas diffusion layer of substantially 0.5 mm.

Step 43 comprises applying a single PEM 11 over the patterned gas diffusion layer.

Step 44 comprises receiving the second substrate 16 and milling or etching the

channels 21 of the cathode flow field therein.

Step 45 comprises applying a unitary gas diffusion layer onto the second substrate 16 over the channels 21 with conductive adhesive.

Step 46 comprises milling or forming a complimentary pattern to the anode side in the unitary gas diffusion layer and optionally any metal electrode layers on the second substrate 16 to define the individual cathode gas diffusion layers 21c, 22c, 23c, 24c.

Step 47 comprises affixing the second substrate and cathode gas diffusions layers to the cathode side of the PEM 11. The "halves" may be glued together using an epoxy adhesive or pre-preg material.

Steps 41 and 42 and step 45 and 46 may be replaced by a step of applying a pre-partitioned gas diffusion layer onto the first substrate 15 and the second substrate 16 respectively rather than patterning a unitary layer. The separate gas diffusion layers may be formed by punching from a larger sheet of gas diffusion layer material. The individual anode side gas diffusion layers 21a, 22a, 23a, 24a for each of the fuel cells may be arranged on or formed on a removable backing layer in a particular arrangement. The gas diffusion layers may then be affixed to the first substrate 15 and the backing layer removed. Similarly the cathode side gas diffusion layers 21c, 22c, 23c, 24c for each of the fuel cells may be affixed to the second substrate 15 by a similar method. Thus, the backing layer assists in ensuring accurate placement of the gas diffusion layers on the substrate and, consequently, the alignment of the anode side and cathode side gas diffusion layers of each fuel cell 3, 4, 5, 6.

The first and second substrates 15, 16 may be compressed together to seal the structures formed therebetween together. The compression of the array I may be Ia improved by the use of a resilient sponge material between the substrates and as part of one or more of the fuel cells 3, 4, 5, 6. The use of a sponge material, which may be conductive, may assist in ensuring an even compression force is applied across all of the fuel cells.

The area of the fuel cells at different locations in the array may be selected to ensure reliable compression across the array. For example, the area of the fuel cells near the edge of the array may be larger than those at the centre of the array. This has been found to result in a substantially constant compression force applied across all of the fuel cells in the array.

In another example, the area of each fuel cell in the array may be based on the distance of that fuel cell from the reactant source, Thus, the array may have a reactant inlet at a point or edge of the array to introduce reactant in to the one of the flow fields 17 or 20.

The area of the fuel cells may be determined based on the distance from said reactant inlet. This may form an aspect of the invention. It will be appreciated that the concentration of reactant as it flows along the channels from the inlet will decrease as it is consumed along the way by each fuel cell it passes. Accordingly, the fuel cells further from the reactant inlet may have a larger area.

The method may include the step of modifying the PEM 11 in the regions between the cells. For example, the PEM 11 may be treated with chemicals, by heat treatment or by laser ablation to reduce its conductivity in the areas between fuel cells. This may allow the fuel cells to be arranged closer together. This is advantageous as the conductivity of the PEM 11 has been found to be primarily at the surface.

In other embodiments the first and second substrates 15, 16 are flexible. A further support structure, which may be of any shape, may support the flexible substrates across its surface. Thus, the substrate provides the frame for electrical connections between the fuel cells andIor an interconnection controller and the support structure provides rigidity for the assembly. The support structure may comprise a casing or an internal frame of an electronic device. In other embodiments the flow fields 17, 20 may be formed by channels arranged in the gas diffusion layers rather than the substrate to which the gas diffusion layers are affixed. Thus, a gas diffusion layer may be milled to form channels therein before placement on the substrate. The flow fields may comprise a combination of channels formed in the substrates and channels formed in the gas diffusion layers. For example, half the channel depth may be provided by a groove in the substrate and the remaining half of the channels depth may be provided by a groove in the gas diffusion layer.