Conductive open-porous material

Abstract

A material having an open-porous structure and a conductive coating comprising carbon fibre fragments formed on the open-porous structure. An electrode for a bioelectrochemical system may be formed using the material.

Description

[0001] P606415PC00

[0002] Conductive Open-Porous Material

[0003] Field of the Invention

[0004] The invention relates to a conductive open-porous material and an electrode for a bioelectrochemical system comprising the conductive open-porous material. The invention also relates to a method of forming the conductive open-porous material and a method of forming an electrode for a bioelectrochemical system comprising the conductive open-porous material.

[0005] Background of the Invention

[0006] Conductive open-porous materials find numerous applications, for example, as electrodes for bioelectrochemical systems, where the open-porous structure may be colonised by electrogenic microorganisms. They also find applications as materials for absorbing radiofrequency (RF) radiation, such as microwaves or radar.

[0007] A conductive open-porous material can be formed by coating a non-conductive open-porous material (such as foam) with a conductive coating (such as carbon black). However, existing conductive open-porous materials tend to have quite poor conductivity.

[0008] It would be desirable to improve the conductivity and other material properties of existing conductive open-porous materials.

[0009] Summary of Invention

[0010] According to a first aspect of the invention, there is provided a material comprising an open-porous structure and a conductive coating formed on the open-porous structure, wherein the conductive coating comprises carbon fibre fragments.

[0011] The carbon-fibre fragments are part of a conductive coating which is applied to the surface of the open-porous structure (such as an open-cell foam). The conductive coating may penetrate within the open-porous structure, covering the surface of substantially all the pores with the conductive coating. The carbon fibre fragments allow a non-conductive open-porous structure (such as open-cell foam) to be made highly conductive. The carbon fibre fragments provide a conductive coating with improved conductivity compared to previous conductive coatings, such as carbon black, allowing an open-cell foam to achieve low electrical resistance (typically around 1 Ohm) usually associated with metals such asP606415PC00

[0012] steel. This is particularly the case when an open-cell foam is compressed during use as the compression helps to reduce electrical resistance.

[0013] Materials with an open-porous structure and high levels of conductivity are advantageous in numerous applications, for example, as electrodes for bioelectrochemical systems which require an open-porous structure which mimics the soil-like conditions electrogenic microbes prefer while providing the high levels of conductivity required to function as an efficient electrode.

[0014] Materials with an open-porous structure and high levels of conductivity are also suited to other applications, including applications which require absorption of radio-frequency (RF) radiation, such as microwaves or radar. The carbon fibre fragments improve the RF absorption properties of conductive foam, and allows for the use of thinner materials for a given level of absorption than with standard conductive foam.

[0015] The conductive coating may further comprises a binder and optionally one or more of: conductive activated carbon, amorphous carbon particles, conductive carbon black, and graphite flakes.

[0016] To help adhere the carbon fibre fragments to the open-porous structure, the conductive coating may include a binder. The conductive coating may include additional components, in addition to the carbon-fibre fragments, to improve adhesion or conductivity, or to otherwise beneficially modify the surface properties of the open-porous structure.

[0017] The conductive coating may comprise a redox mediator (such as magnetite) to facilitate the transfer of electrons between microorganisms inhabiting an electrode and the electrode itself which helps to make the electrode a more favourable environment for electroactive microorganisms to inhabit.

[0018] The conductive coating may further comprise magnetite, optionally wherein at least a portion of the magnetite covers at least a portion of the carbon fibre fragments.

[0019] While the carbon fibre fragments are biocompatible with electrogenic microorganisms, meaning that the material is ideal as an electrode material for bioelectrochemical systems, the addition of magnetite to the conductive coating has been found to further improve the biocompatibility, particularly when the carbon fibre fragments are at least partially coated with magnetite. The presence of magnetite further enhances the conductivity of an electrode for a bioelectrochemical system. Therefore, an electrode combining carbon fibreP606415PC00

[0020] fragments and magnetite may provide improved conductivity and biocompatibility relative to an electrode with carbon fibre fragments or magnetite alone. This could lead to improved hydrogen generation in a microbial electrolysis cell employing an anode made from open cell foam with a conductive coating combining carbon fibre fragments and magnetite.

[0021] The open-porous structure may comprise a non-conductive polymer.

[0022] The open-porous structure may be a foam, for example, comprising one or more of: polyethylene, polypropylene, polyurethane, polystyrene, polyester, polyether, polyetheretherketone and acrylonitrile butadiene styrene.

[0023] Conductive foams make ideal anode materials for bioelectrochemical systems, such as microbial electrolysis cells (MECs). The open-porous structure of foam mimics the open structure of soil and therefore provides favourable conditions for colonization by electrogenic microorganisms. The open-porous structure of foam also enables fluid, such as wastewater, to flow deeply within the open-porous structure even with fairly thick pieces of foam, offering a high surface area and good diffusion properties to enhance fluid throughput and treatment rate particularly in comparison to carbon felt where it is harder for wastewater to penetrate the inner parts of the felt meaning most interactions with electrogenic organisms happen near the surface limiting the effective surface area of the carbon felt. To overcome the intrinsically non-conductive nature of foams in order for them to make a suitable electrode for a bioelectrochemical system, the open-porous structure incorporates a conductive material as a coating throughout the open-porous structure. The inventors have discovered that coating a non-conductive open cell foam with a conductive coating containing carbon fibre fragments allows an open cell foam to achieve low electrical resistance (typically around 1 Ohm) usually associated with metals such as steel (particularly when compression is applied to the open-cell foam which helps to reduce contact resistance). As a result, an open cell foam coated in a conductive coating including carbon fibre fragments provides an ideal electrode material for electrodes for bioelectrochemical systems such as microbial electrolysis cells in terms of biocompatibility, electrical conductivity and fluid flow properties.

[0024] The open-porous structure may have a porosity of between 5 and 100 pores per inch (PPI), preferably less than 20 pores per inch, preferably between 10 and 12 pores per inch.

[0025] Generally, materials with a higher number of pores per inch (i.e., where each pore is smaller) work better as filter materials. For use as an electrode in a bioelectrochemicalP606415PC00

[0026] system, it is preferable to use a material with a porosity of less than 20 pores per inch to ensure that the pores do not tend to get clogged and form a resistance to the flow of wastewater. For a flow-through anode, where it is desirable to encourage water to flow freely through the pores to increase surface area and improve hydrogen generation efficiency, it is preferable for the material to have a porosity of between 10 and 12 pores per inch.

[0027] The carbon fibre fragments may comprise recycled carbon fibre. Suitable carbon fibre fragments may be derived from the recycling of carbon fibre composite parts which have reached the end of their life. The recycling process involves thermal treatment in addition to solvent washing to remove the organic binder components of the composite. The treatment that the old carbon fibre goes through as part of the recycling process has been found to benefit the surface properties (e.g., porosity / roughness) of the carbon fibre fragments for use in a conductive coating for bioelectrochemical systems, for example, by improving the biocompatibility of the carbon fibre fragments.

[0028] Instead of using recycled carbon fibre as the carbon fibre fragments, virgin carbon fibre that is ground up could be used instead. The properties of the carbon fibre fragments or the conductive coating may be improved by applying chemical treatments, some of which are similar to those which carbon fibre composite parts would undergo during recycling.

[0029] For example, the carbon fibre fragments or the conductive coating may be treated with an oxidation process involving high temperature carbon dioxide, mineral acids or thermal treatment in the presence of low concentrations of oxygen. This oxidation process may increase roughness / porosity of the carbon fibre fragments or modify surface group functionality (moieties) of the carbon fibre fragments or the conductive coating to make them more biocompatible.

[0030] The carbon fibre fragments may be heat treated to greater than 1000°C to increase their graphitic character. In other words, to increase the ratio of sp2 to sp3 hybridised carbon-carbon bonds. This process will further increase the electrical conductivity of the carbon fibres.

[0031] The carbon fibre fragments may have an average length that is shorter than the average pore size of the open-porous structure. The carbon fibre fragments may have an average length that is longer than an amorphous carbon particle (such as carbon black or activated carbon), for example, greater than 10 μm. By having an average length for the carbon fibre fragments that is greater than an amorphous carbon particle but less than theP606415PC00

[0032] average pore size of the open-porous structure, a low resistance of less than 1 ohm / cm may be achieved. For example, the carbon fibre fragments may have a length between 10 μm and 1000 μm. To reduce clumping and provide a more uniform coating, the average length of the carbon fibre fragments may be less than 200 μm, for example, around 100 pm.

[0033] According to a second aspect of the invention, there is provided an electrode for a bioelectrochemical system comprising a material according to the first aspect.

[0034] The bioelectrochemical system may be a microbial electrolysis cell or a microbial fuel cell. The electrode may be an anode or a cathode of the microbial electrolysis cell or microbial fuel cell.

[0035] The material may comprise a biocompatible polymer that is chemically stable at a pH of the electrode environment. The pH of the electrode environment may be the pH of the electrolytic solutions surround the electrode (i.e., the anolyte for an anode, or the catholyte for a cathode).

[0036] According to a third aspect of the invention, there is provided a method of forming a material according to the first aspect. The method comprises receiving an open-porous structure and forming a conductive coating comprising carbon fibre fragments over the open-porous structure.

[0037] The open-porous structure may be foam, for example, comprising one or more of: polyethylene, polypropylene, polyurethane, polystyrene, polyester, polyether, polyetheretherketone and acrylonitrile butadiene styrene.

[0038] The conductive coating may comprise a binder. The conductive coating may be spray coated onto the open-porous structure.

[0039] The conductive material may further comprise one or more of: conductive activated carbon, conductive carbon black, graphite flakes, amorphous carbon particles, and magnetite.

[0040] The method may further comprise at least partially melting the open-porous structure after applying the carbon fibre fragments in order to fix the carbon fibre fragments to the open-porous structure.P606415PC00

[0041] The carbon fibre fragments may be heated to greater than 1000°C to increase their graphitic character. This may improve electrical conductivity of the conductive coating.

[0042] The carbon fibre fragments may be treated with a chemical or physical activation process to increase the surface area of the carbon fibre fragments.

[0043] The carbon fibre fragments and / or the conductive coating may be treated with an oxidation process, optionally wherein the oxidation process includes one or more of: high temperature carbon dioxide, mineral acids and thermal treatment in a low oxygen environment. This may enhance porosity and may modify surface group functionality of the carbon fibre fragments.

[0044] According to a fourth aspect of the invention, there is provided a method of forming an electrode for a bioelectrochemical system. The method may comprise receiving an open-porous structure for colonisation by electrogenic microorganisms and forming a conductive coating comprising carbon fibre fragments on the open-porous structure.

[0045] Brief Description of the Drawings

[0046] The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0047] Figure 1 is a diagram of an example of a pouch for a microbial electrolysis cell; Figure 2 is a diagram showing a cross-section through the pouch of Figure 1, illustrating the arrangement of a number of flexible layers making up the microbial electrolysis cell;

[0048] Figure 3 is a diagram showing an expanded view of a cathode chamber shown by Figure 2;

[0049] Figures 4A and 4B are diagrams illustrating how inter-layer distances within the pouch reduce as a result of a compressive force being applied to outer layers of the pouch;

[0050] Figure 5 is a diagram of an alternative arrangement of flexible layers making up the microbial electrolysis cell;

[0051] Figure 6 is a diagram of a tank having a number of chambers for holding one or more microbial electrolysis cells;

[0052] Figure 7 is a diagram showing a single row of chambers of the tank and illustrates how fluid flows between neighbouring chambers;

[0053] Figure 8 is a diagram showing a cross-sectional view of a single chamber of the tank of Figure 6, showing how one or more pouches and anodes are positioned in each chamber of the tank.P606415PC00

[0054] Figure 9A is a diagram of a lid for the tank having a number of portions with openings for making electrical or gas connections to the pouches in each chamber. Figures 9B and 9C are cross-sections through the lid, illustrating how the electrical and gas connections are made through the portions of the lid;

[0055] Figure 10 is a diagram of an alternative tank design which does not require a lid. In Figure 10, the electrical connections for each of the microbial electrolysis cells within the tank are made along a common edge of the tank; and

[0056] Figures 11a and 11b illustrate an open-cell foam and Figures 11c and 11d illustrate the open-cell foam after coating with conductive carbon fibre fragments.

[0057] Detailed Description

[0058] Figures 1 and 2 illustrate a pouch design for a microbial electrolysis cell which dramatically increases the anode and cathode surface area for a given volume wastewater tank by reducing "dead space" (space where water can flow without being in contact with the anode for treatment). This greatly increases the current density and therefore the capacity for hydrogen production and wastewater treatment. The pouch is made from readily available and cost-effective flexible layers of materials that are sealed (for example, by heat sealing or ultrasonically) to form a gas-tight pouch that facilitates hydrogen gas collection, enhancing the ability to mass manufacture microbial electrolysis cells. Overall, the pouch design described below allows larger quantities of wastewater to be processed and hydrogen generated.

[0059] Figure 1 shows an example of a microbial electrolysis cell 100 where a plurality of flexible layers 101 making up the microbial electrolysis cell 100 are sealed together to form a gastight pouch 110.

[0060] Figure 2 shows a cross-section through the line A-A of pouch 110 illustrating the arrangement of flexible layers making up the microbial electrolysis cell 100. Anodes 130 are located on the external faces on either side of the pouch 110 where the anodes 130 can be immersed in wastewater. The anodes 130 are formed from a conductive open-porous structure that can be colonised by microorganisms that generate hydrogen ions and electrons as they feed on the wastewater.

[0061] A suitable conductive open-porous structure can be formed from an intrinsically conductive material that is formed into an open-porous structure, for example, by weaving conductive fibres (such as carbon fibre) into a mat. Alternatively, as will be discussed in more detail later, a conductive open-porous structure for the anodes 130 can be formed by taking aP606415PC00

[0062] non-conductive open-porous material (such as an open-cell foam) and coating it with a conductive material (such as conductive activated carbon, amorphous carbon particles, conductive carbon black, and graphite flakes).

[0063] The anodes 130 each have an anode current collector 133 formed from a stainless-steel mesh. Alternatively, the anode current collector may be formed from titanium, or a highly conductive material such as copper or brass that is plated in a precious or semi-precious material such as platinum.

[0064] Hydrogen ions generated at the anodes 130 pass through the outer layers 132 on either side of the pouch 110 into the cathode chambers 134 on either side of the pouch 110 where the hydrogen ions meet electrons (that transit an external circuit) and are converted into hydrogen gas. The outer layers 132 are made from a material, such as nylon or a polyethersulfone (PES) membrane, which allows the hydrogen ions to pass into the pouch 110 but prevents hydrogen gas from leaving the pouch 110, and which also prevents the microorganisms colonising the anode 130 from passing into the pouch 110.

[0065] Gas diffusion layers 136 (such as a porous PTFE membrane) separate the cathode chambers 134 from a hydrogen collection chamber 138 at the centre of the pouch 110. The hydrogen gas passes through the gas diffusion layers 136 into the hydrogen collection chamber 138 from where it can be extracted. The gas diffusion layers 136 prevent water that is generated as a byproduct in the cathode chamber 134 from passing into the hydrogen collection chamber 138. The hydrogen collection chamber 138 contains a resilient material (for example, a sheet of open cell foam) which helps to maintain the volume of the hydrogen collection chamber 138 (in other words, that prevents the hydrogen collection chamber 138 from being crushed which would inhibit hydrogen collection).

[0066] The outer layers 132 and gas diffusion layers 136 are made from materials which allow them to be sealed together (for example, by heat sealing, ultrasonic sealing, or adhesive) along three or four of their edges to form a cathode chamber 134 between each outer layer 132 and the adjacent gas diffusion layer 136, and to form a hydrogen collection chamber 138 between each of the gas diffusion layers 136. A sealing member 120 is sealed onto all or part of the fourth edge of the outer layers 132 and the gas diffusion layers 134 to complete the gas tight seal of the pouch 110 and the sealing of the cathode chambers 134 from the hydrogen collection chamber 138. The sealing member 120 has a hydrogen gas chamber port 121, connected to the hydrogen collection chamber 138, to which gas connections can be made for tapping off the hydrogen. The sealing member 120 also hasP606415PC00

[0067] cathode chamber ports 122 for making electrical connections to cathode assemblies positioned within the cathode chambers 134.

[0068] The sealing member 120 may be formed as follows. The fourth edge of each of the outer layers 132 and of the gas diffusion layers 136 is positioned into a mould for forming a housing for the sealing member 120. Hydrogen gas piping is inserted through the hydrogen gas chamber port 121 into the hydrogen gas collection chamber 138 to allow hydrogen gas to be extracted from the hydrogen gas collection chamber 138, and cathode wiring is inserted through the cathode chamber ports 122 for making electrical connections within the cathode chambers 134. A resin (or other suitable bonding means) is then poured into the mould such that the outer layers 132 and the gas diffusion layers 136 are bonded together to make the pouch 110 gas tight, and a gas-tight seal is formed between the hydrogen gas piping and the hydrogen gas chamber port 121 and between the cathode wiring and the cathode chamber ports 122. The mould may comprise a further injection port (not shown) for injecting air into the pouch 110 to test the gas-tightness of the pouch 110 before the pouch 100 is used in a microbial electrolysis cell. Once the gas-tightness of the pouch 110 is verified, the further injection port is sealed using further resin.

[0069] This design allows an anode 130 and cathode chamber 134 located on either side of the pouch 110 to feed hydrogen into a central hydrogen collection chamber 138, which helps to simplify the collection of hydrogen and reduce the amount of hydrogen piping required per cell. As will be described in more detail in relation to Figure 8, this design also allows multiple pouches 110 to be stacked together, with each anode 130 shared between neighbouring pouches 110. This design helps to increase the anode and cathode surface area for a given volume wastewater tank and greatly increases the current density and therefore the capacity for hydrogen production and wastewater treatment.

[0070] Figure 3 is an expanded view of one of the cathode chambers 134 shown in Figure 2. A cathode assembly 140 is placed within each of the cathode chambers 134, such that the cathode assembly 140 substantially fills an available volume within the cathode chamber 134. The cathode assembly 140 is formed from a conductive cathode base layer 141 and a cathode current collector 143. The conductive cathode base layer 141 comprises a hydrogen evolution catalyst formed from nickel-plated conductive fibres. The nickel-plated conductive fibres could be a nickel-plated carbon fibre mat, or alternatively a nickel-plated recycled carbon paper which may be easier and quicker to nickel-plate than a carbon fibre mat. Alternatively, the hydrogen evolution catalyst may be formed from nickel or stainless-steel mesh. The cathode current collector 143 comprises a nickel mesh. Alternatively, the cathode current collector 143 may comprise stainless-steel, titanium, or a conductiveP606415PC00

[0071] mesh coated in a precious metal (as explained above with respect to the anode current collector). The conductive cathode base layer 141 and the cathode current collector 143 may be coupled together, for example using nylon fixings, so as to eliminate any galvanostatic corrosion between the conductive cathode base layer 141 and the cathode current collector 143. To collect current from the cathode current collector 143, cathode wiring 142 is coupled to the cathode current collector 143. The cathode wiring 142 extends out of the cathode chamber 134 and through the respective cathode chamber port 122 so that an electrical connection can be made with the cathode assembly 140 once the outer layers 132 and the gas diffusion layers 136 have been sealed together by the sealing member 120, as described above.

[0072] Figure 4 shows an arrangement where a compressive force F is applied to opposite sides of the pouch 110 (when viewed along the line A-A as seen in Figure 2). The compressive force F may be applied as a result of the pouch 110 being placed into a tank (discussed in more detail later in relation to Figures 5-7), either by inner walls of the tank (which can apply the compressive force F to the anodes 130 shown in Figure 2) and / or by other neighbouring pouches 110 (in the event that multiple pouches are provided in the tank in a sandwich arrangement), which in turn transfer the compressive force to opposing sides of the pouch 110. As is shown by Figure 4A, the outer layer 132 and the gas diffusion layer 136 are initially separated by a first inter-layer distance a (defining a width of the cathode chamber 134). Similarly, the cathode chamber 134 and the hydrogen collection chamber 138 are separated by a second inter-layer distance b (defining a width of the gas diffusion layer 136). The gas diffusion layers 136 are separated from one another by a third interlayer distance c (defining a width of the hydrogen collection chamber 138).

[0073] Figure 4B illustrates the compressive force F being applied to opposing sides of the pouch 110 (e.g., by the inner walls of the tank and / or by neighbouring pouches), which results in a reduction to at least some of the inter-layer distances within the pouch 110. For example, the compressive force F reduces the first inter-layer distance a between the outer layer 132 and the gas diffusion layer 136 to a reduced first inter-layer distance a' (i.e., the width of the cathode chamber 134 reduces to a')- The compressive force F also reduces the second inter-layer distance b to a reduced second inter-layer distance b' (i.e., the width of the gas diffusion layer 136 reduces to b')- These reduced inter-layer distances a' and b'aid the transfer of hydrogen ions into the cathode chamber 134, and the transfer of hydrogen gas across the gas diffusion layer 136 and into the hydrogen collection chamber 138 for collection. The reduced inter-layer distance a' is substantially constant along a common axis of the outer layer 132 and the gas diffusion layer 136. The reduced inter-layer distance b' is also substantially constant along a common axis of the cathodeP606415PC00

[0074] chamber 134 and the hydrogen collection chamber 138. In other words, the reduced interlayer distances a' and b' are consistent across a length of the pouch 110 when the compressive force F is applied, such that any local bulges in the cathode chambers 134 and gas diffusion layers 136 are substantially avoided.

[0075] In contrast, and as shown by Figure 4B, the inter-layer distance c between the gas diffusion layers 136 on opposing sides of the hydrogen gas collection layer 138 (i.e., the width of the hydrogen collection chamber 138) is substantially unchanged by the application of the compressive force F to opposing sides of the pouch 110. Any reduction in the inter-layer distance c between the gas diffusion layers 136 is significantly less than that represented by the reduced interlayer distances a' and b'. As introduced above, the hydrogen collection chamber 138 contains a separator formed from a resilient material (for example, a sheet of open cell foam) which substantially maintains the volume of the hydrogen collection chamber 138. The resilient material is configured to at least partially resist compression of the pouch 110 arising from the compressive force F so that a minimum interlayer distance c' is maintained, where c' ~ c. As such, the capacity to collect hydrogen gas from the hydrogen collection chamber 138 is not compromised by the compressive force F (i.e., the available volume of the hydrogen collection chamber 138 is substantially unchanged). The open cell foam used as the resilient material has a suitable pore size such that hydrogen gas may freely pass through the open cell foam to be collected from the hydrogen collection chamber 138 via the hydrogen gas chamber port 121.

[0076] In this example, the compressive force F has been described as being applied to opposing sides of the pouch 110, for example, by squeezing the pouch 110 by inner walls of a tank and / or by neighbouring pouches. However, the compressive force F could be applied in other ways, for example, by rolling the pouch 110 up into a tube.

[0077] Figure 5 illustrates an alternative arrangement of the flexible layers making up the microbial electrolysis cell 100. In Figure 5, gas diffusion layers 136 are omitted, and only a single central cathode chamber 134' is retained which also acts as hydrogen collection chamber 138'. The outer layers 132 are instead located adjacent to opposing sides of the central cathode chamber 134'. The cathode assembly 140 is located within the cathode chamber 134'. The cathode assembly 140 may contain a resilient material (such as the resilient material described above with respect to the hydrogen collection chamber 138) or the cathode itself may be formed from a resilient conductive material (such as a metal mesh or a conductive open-cell foam as described below) to help maintain the volume of the cathode chamber 134 to prevent the cathode chamber 134 from collapsing under theP606415PC00

[0078] compressive force F and thereby preventing hydrogen from being extracted from the cathode chamber 134. The cathode assembly 140 may contain a single cathode, or may contain multiple cathodes arranged so as to have gaps therebetween to allow hydrogen to escape.

[0079] The outer layers 132 are made from materials which allow them to be sealed together (for example, by heat sealing, ultrasonic sealing, or adhesive) along three or four of their edges to form the cathode chamber 134. A sealing member (similar to sealing member 120 described above and shown in Figure 1) is sealed onto all or part of the fourth edge of the outer layers 132 to complete the gas tight seal of the pouch 110 and to provide electrical and gas connections. The sealing member has at least a hydrogen gas chamber port 121 connected to the cathode chamber 134 (to which gas connections can be made for tapping off the hydrogen) and a cathode connection port 122 (for making electrical connections to the cathode assembly 140). The sealing member used with the pouch 110 arrangement of Figure 5 may also have an additional port that acts as an inlet / outlet for the recirculation of catholyte around the cathode assembly 140.

[0080] As with Figure 4, the anodes 130 shown in Figure 5 are located on the external faces on either side of the pouch 110 where the anodes 130 can be immersed in wastewater. The anodes 130 are formed from a conductive open-porous structure that can be colonised by microorganisms that generate hydrogen ions and electrons as they feed on the wastewater. This example shows how a biocompatible material, such as a felt 131 constructed from biocompatible carbon fibres, may be additionally located on opposing sides of the pouch 110, between the pouch 110 and the anodes 130. The felt 131 provides a hospitable environment for microorganisms (including electrogenic microorganisms that donate electrons and contribute hydrogen ions) around the anodes 130, thereby generating higher levels of current in the microbial electrolysis cell 100. The felt 131 may also act as a filter to prevent large particles of non-dissolved solids from reaching the outer layers 132. In the case where the outer layers 132 are formed from a membrane (such as PES membrane), non-dissolved solids can block pores in the membrane and therefore reduce the number of available sites for hydrogen ions to transit through the membrane towards the cathode assembly 140. The felt 131 may also assist in the management of the pH in the microbial electrolysis cell 100, by providing a more gradual pH gradient between analyte (which typically has a pH of 6-8) and catholyte (which can reach pH 12) in the microbial electrolysis cell 100, compared with when the felt 131 is absent. This gradual pH gradient decreases the likelihood of deposits (such as calcium carbonate) from forming on the membrane which might block transit sites for hydrogen ions.P606415PC00

[0081] Each anode 130 has an anode current collector 133, which may be formed from the same materials described above with respect to Figure 2. The anode current collector 133 may be positioned between the conductive open-porous structure of the anode 130 and the felt 131. Alternatively, the conductive open-porous structure of the anode 130 may be positioned between anode current collector 133 and the felt 131. This alternative arrangement reduces the number of anode current collectors 133 required when scaling to a multiple microbial electrolysis cell 100 designs as an anode current collector 133 can be shared between two microbial electrolysis cells 100.

[0082] When the compressive force F is applied to the conductive open-porous structure of the anodes 130, the anode current collector 133 is essentially pushed into / against the anode 130, which causes a decrease in electrical resistance. The inventors have found that a greater than 10-fold reduction in electrical resistance can be achieved when the compressive force F is applied to the anodes 130 (e.g., when the microbial electrolysis cell 100 is placed into the tank / squeezed against other microbial electrolysis cells). Additionally, the inventors have also discovered that compression of the open-porous structure of the anodes 130 under the compressive force F, an increase in conductivity is achieved. For example, if a foam having a thickness of 10 mm is used as the conductive open-porous structure, is it possible to achieve conductivity > 1 Siemens when the foam is under compression. In comparison, if the same foam is uncompressed, a conductivity of only 0.5 Siemens is achieved.

[0083] Figure 6 illustrates a tank 200 (which may be referred to as a waste water tank) for holding a number of pouches 110. The tank 200 has walls 202 defining an outer shell of the tank 200. The tank 200 also has a number of baffles 204 which, together with the walls 202, define a grid of chambers 206 within the tank 200. One or more MECs 100 are held within each chamber 206 between opposing baffles 204.

[0084] The tank 200 is filled with wastewater to be treated by the MECs 100. Figure 7 illustrates a single row of chambers 206 within the tank 200. Each baffle 204 within the tank 200 may have a recessed portion 208 at one end such that the wastewater can flow between neighbouring chambers 206 (as shown by fluid flow arrow 210). As shown by Figure 7, the recessed portions 208 may be formed at alternating ends of neighbouring baffles 204 such that the wastewater can pass between chambers 206 by flowing over-under-over neighbouring baffles 204. Although Figure 7 only shows a single row of chamber 206, the skilled person will appreciate that recessed portions 206 and 208 may be formed between adjacent rows of the chamber 206, for example at the ends of each row recessed portionsP606415PC00

[0085] may be provided to allow wastewater to flow between rows and throughout the entire tank 200.

[0086] Figure 8 shows a cross-sectional view of a single chamber 206 of the tank 200 taken along line B-B of Figure 6. One or more pouches 110 and anodes 130 may be positioned into each chamber 206 of the tank 200. In the example of Figure 8, three pouches 110 and four anodes 130 are positioned within chamber 206, arranged in an alternating fashion, i.e., anode-pouch-anode-pouch-anode-pouch-anode. More or fewer pouches 110 and anodes 130 may be inserted into each chamber 206 between adjacent baffles 204 as required in order to substantially fill the volume of the chamber 206, meet particular wastewater treatment requirements, etc. As alluded to earlier, as a result of being placed between adjacent baffles 204 in each chamber 206 of the tank 200, a compressive force F is applied to opposite sides of each pouch 110. More specifically, by virtue of the fact that the anodes 130 are also located in the chamber 206 with the pouches 110, the compressive force F is applied by adjacent baffles 204 to the anodes 130 at either side of the pouch / anode sandwich shown in Figure 8. The compressive force F is then transferred from the anodes 130 to the pouches 110. The magnitude of the compressive force F can be adjusted by varying the number of anodes 130 and pouches 110 in each anode / pouch sandwich accordingly, i.e., the magnitude of the compressive force F will increase as the number of anodes 130 and pouches 110 in the anode / pouch sandwich increases, as there will be less unoccupied space available within each chamber 206 and so the amount of "squeeze" on the anode / pouch sandwich will increase.

[0087] Figure 9 shows a lid 300 for tank 200. As shown in Figure 9A, the lid 300 is divided up into lid portions 302 where each lid portion 302 is aligned with one of the chambers 206 of tank 200. Each of the lid portions 302 may contain a number of openings 304, 306 and 308 for making electrical or gas connections to the pouches 110 in each chamber 206. The opening 304 allows electrical connections to be made to the cathode assembly 140, the opening 306 allows electrical connections to be made to the anode 130, and the opening 308 is for gas piping to allow removal of the hydrogen from the hydrogen collection chambers 138. There is also an opening 310 to allow access to the region of the chamber 206 around the pouch 110 to allow for removal of debris / contaminants that may have been deposited by the wastewater in the chamber 206.

[0088] Figure 9B is a cross-section view through the line C-C in Figure 9A and Figure 9C is a cross-section view through the line D-D in Figure 9A. Together, Figures 9B and 9C show how sealing members, such as glands, are formed around cables or pipes penetrating through openings 304, 306, 308 and 310 in the lid portion 302 to provide a gas-tight sealP606415PC00

[0089] around them. Figure 9B shows gland 310 around cathode wires 320 and gland 312 around anode wires 322. Figure 9C shows gland 314 around hydrogen pipe 324 and gland 316 around desludge tube 326. Figure 9B illustrates multiple cathode wires 320 and anode wires 322 passing through their respective glands 310 and 312, where each of the cathode wires 320 and anode wires 322 is routed to the cathode or anode of an individual pouch 110. Figure 9C illustrates a single hydrogen pipe 324 passing through its gland 314, with this hydrogen pipe 324 being split off under the lid 300 to connect to each individual pouch 110.

[0090] The cathode wires 320 and anode wires 322 are connected to an electrical power supply that provides a suitable potential difference across each microbial electrolysis cell 100. To simplify wiring, all of the cathode wires 320 and anodes wires 322 may be connected to a respective cathode and anode busbar which is connected to the electrical power supply. The hydrogen pipes 324 are connected to a manifold which is connected to a hydrogen collection vessel.

[0091] Figure 10 shows an alternative design for the tank 200 which does not require a lid, such as the lid 300 shown in Figure 9. In Figure 10, the tank 200 houses a number of microbial electrolysis cells 1010, which may be arranged as described with respect to any of the earlier microbial electrolysis cell examples. The microbial electrolysis cells 1010 are held in position and at regular spacing by baffles located within the tank 200 (not shown in Figure 10). An electrical connection housing 1020 is provided for each of the microbial electrolysis cells 1010 along a common edge of the tank 200. Wiring 1030 extends between the electrical connection housing 1020 and the anodes and cathodes of each microbial electrolysis cell 1010. A magnified view of an individual electrical connection housing is shown in Figure 10, which illustrates that each electrical connection housing has: an anode connection portion 1080, a cathode connection portion 1085, and a control electronics portion 1090. Wiring 1030 connects the anode(s) of each microbial electrolysis cell 1010 to the anode connection portion 1080 of a respective electrical connection housing 1020. Similarly, wiring 1030 also connects the cathode(s) of each microbial electrolysis cell 1010 to the cathode connection portion 1080 of the respective electrical connection housing 1020. The control electronics portion is configured to control the application of electrical power from the electrical power supply to the anode(s) and cathode(s) of the microbial electrolysis cells 1010.

[0092] Each microbial electrolysis cell 1010 within the tank 200 has a hydrogen collection tube 1040 to allow for hydrogen to be tapped off. The hydrogen collection tube 1040 may formP606415PC00

[0093] part of a sealing member, such as sealing member 120 discussed above with respect to Figure 1.

[0094] The tank has an inlet 150 to allow wastewater to be fed into the tank 200 for treatment, and an outlet 1060 to allow for clean water to be removed from the tank following treatment of the wastewater by the microbial electrolysis cells 1010. The tank 200 may also have one or more outlets 1070 disposed along a lower section of the tank that are configured to allow sludge and other detritus / contaminants to be drained out of the tank 200.

[0095] As mentioned previously, a conductive open-porous structure for the anodes 130 can be formed by taking a non-conductive open-porous material (such as an open-cell foam) and coating it with a conductive material. Conductive foams make ideal anode materials because the open-porous structure of foam mimics the open structure of soil and therefore provides favourable conditions for colonization by electrogenic microorganisms. The open- porous structure of foam also enables fluid, such as wastewater, to flow deeply within the open-porous structure even with fairly thick pieces of foam, offering a high surface area and good diffusion properties that enhance fluid throughput and treatment rate particularly in comparison to carbon felt where it can be harder for wastewater to penetrate the inner parts of the felt meaning most interactions with electrogenic organisms happen near the surface which tends to limit the effective surface area of the carbon felt.

[0096] To overcome the intrinsically non-conductive nature of foams for them to make a suitable anode for a microbial electrolysis cell, the open-porous structure incorporates a conductive material as a coating throughout the open-porous structure. The inventors have discovered that a non-conductive open-cell foam coated with a conductive coating containing carbon fibre fragments has greatly reduced electrical resistance due to the graphitic character of the carbon fibres, allow open-cell foam to achieve the kind of electrical resistance (typically around 1 Ohm) usually associated with metals such as steel. This is particularly the case when the foam is compressed as a result of being used as an electrode in the microbial electrolysis cell design described above where the compression of the open-cell foam aids in reducing the electrical resistance. Therefore, open-cell foam coated in a conductive coating containing carbon fibre fragments provides an excellent anode material given its biocompatibility, fluid flow properties and conductivity. Even though the cathode of the microbial electrolysis cell is not colonised by microorganisms, the resilient nature and excellent conductivity, enhanced under compression, makesP606415PC00

[0097] open-cell foam coated in a conductive coating containing carbon fibre fragments an excellent choice for the cathode 140 in the microbial electrolysis cell 100 as well.

[0098] The open-porous structure may be made from a non-conductive biocompatible polymer foam that is chemically stable at a pH of the anode or cathode environment (i.e., at the pH of the anolyte or catholyte). Examples include polyethylene, polypropylene, polyurethane, polystyrene, polyester, polyether, polyetheretherketone and acrylonitrile butadiene styrene.

[0099] For use as an anode 130, the porosity is ideally less than 20 PPI (pores per inch) to ensure that the pores do not tend to get clogged and form a resistance to the flow of wastewater. To encourage wastewater to flow freely through the anode 130, to increase surface area and improve hydrogen generation efficiency, it is preferable to have a porosity of between 10 and 12 PPI (pores per inch).

[0100] After the open-porous structure has been formed, for example using any foaming process known to the skilled person, a conductive coating containing the carbon fibre fragments and a binder is spray coated onto the open-porous structure, covering the surface and penetrating into the porous structure to coat substantially all of the pores throughout the open-porous structure with the conductive coating. The carbon fibre fragments typically have an average length that is longer than an amorphous carbon particle (such as carbon black or activated carbon) and shorter than the average pore size of the open-porous structure, for example, the carbon fibre fragments may have a length of between 10 μm to 1000 μm. To reduce clumping and provide a more uniform coating, it may be preferable for the carbon fibre fragments to have an average length that is less than 200 μm, or around 100 pm. After coating the open-porous structure with the carbon fibre fragments, the open-porous structure may be partially melted in order to fix the carbon fibre fragments to the open-porous structure.

[0101] Figure 11a shows an electron microscope image at 25x magnification of an open-cell foam 1100 prior to coating with carbon fibre fragments. Figure 11b shows a close-up of the same open-cell foam 1100 at 1,000x magnification. Figure 11c shows the open-cell foam 1100 after coating with carbon fibre fragments 1110 at 25x magnification, and at 1000x magnification in Figure 11d, where the carbon fibre fragments 1110 can be seen distributed across the surface of the open-cell foam 1100.

[0102] The conductive coating may include additional components to improve adhesion, biocompatibility, or conductivity. For example, the conductive coating may containP606415PC00

[0103] additional conductive components, such as conductive activated carbon, conductive carbon black, graphite flakes, amorphous carbon particles, and magnetite.

[0104] Magnetite may be added to the conductive coating as it has been found to improve biocompatibility, particularly when a magnetite coating is added after the open-porous structure has been coated with the carbon fibre fragments so that the carbon fibre fragments are at least partially coated with magnetite. The presence of magnetite further enhances the conductivity of the anode. Therefore, an anode combining carbon fibre fragments and magnetite provides improved conductivity and biocompatibility relative to an anode with carbon fibre fragments or magnetite alone (assuming that the amount of binder is not increased, and the amount of carbon fibre is not decreased, when the magnetite is added to the conductive coating), which leads to improved hydrogen generation in a microbial electrolysis cell employing an anode made from open cell foam with a conductive coating combining carbon fibre fragments and magnetite.

[0105] The carbon fibre fragments may be derived from virgin carbon fibre that is ground up or from the recycling of carbon fibre composite parts which have reached the end of their life. The use of recycled carbon fibre has been found to be advantageous over using virgin carbon fibre because the recycling process has been found to benefit the surface properties (e.g., porosity / roughness) of the carbon fibre fragments by improving their biocompatibility. In fact, if virgin carbon fibre is used, it is preferable to treat it with thermal and chemical processes that are similar to those carbon fibre undergoes during recycling. For example, the carbon fibre fragments or the conductive coating may be treated with an oxidation process involving high temperature carbon dioxide, mineral acids or thermal treatment in the presence of low concentrations of oxygen to increase roughness / porosity of the carbon fibre fragments or modify surface group functionality (moieties) of the carbon fibre fragments to make them more biocompatible. The carbon fibre fragments may be heat treated to greater than 1000°C to increase their graphitic character (increase the ratio of sp2to sp3hybridised carbon-carbon bonds) to further increase the electrical conductivity of the carbon fibres.

[0106] Although the material with an open-porous structure and high levels of conductivity owing to its conductive coating containing carbon fibre fragments has been described as an anode or cathode for a microbial electrolysis ceil, it could also be used as an electrode in any other kind of bioelectrochemical system, such as an anode or cathode in a microbial fuel ceil, snorkel, etc.P606415PC00

[0107] Also, materials with an open-porous structure and high levels of conductivity are also suited to other applications, including applications which require absorption of radio¬ frequency (RF) radiation, such as microwaves or radar. The carbon fibre fragments improve the RF absorption properties of conductive foam, and allows for the use of thinner materials for a given level of absorption than with standard conductive foam.

Claims

P606415PC00Claims1. A material comprising an open- porous structure and a conductive coating formed on the open-porous structure, wherein the conductive coating comprises carbon fibre fragments.

2. The material of claim 1, wherein the conductive coating further comprises a binder and optionally one or more of: conductive activated carbon, amorphous carbon particles, conductive carbon black, and graphite flakes.

3. The material of either of claims 1 or 2, wherein the conductive coating further comprises magnetite, optionally wherein at least a portion of the magnetite covers at least a portion of the carbon fibre fragments.

4. The material of any preceding claim, wherein the open-porous structure comprises a non-conductive polymer.

5. The material of any preceding claim, wherein the open-porous structure is a foam, for example, comprising one or more of: polyethylene, polypropylene, polyurethane, polystyrene, polyester, polyether, polyetheretherketone and acrylonitrile butadiene styrene.

6. The material of any preceding claim, wherein the open-porous structure has a porosity of between 5 and 100 pores per inch (PPI), preferably less than 20 pores per inch, preferably between 10 and 12 pores per inch.

7. The material of any preceding claim, wherein the carbon fibre fragments comprise recycled carbon fibre.

8. The material of any preceding claim, wherein the carbon fibre fragments have an average length that is shorter than the average pore size of the open-porous structure.

9. The material of any preceding claim, wherein the carbon fibre fragments have a length of 10 μm to 1000 μm.

10. An electrode for a bioelectrochemical system comprising a material according to any of claims 1 to 9.P606415PC0011. The electrode of claim 10, wherein the bioelectrochemical system is a microbial electrolysis cell or a microbial fuel cell and the electrode is an anode or cathode of the microbial electrolysis cell or microbial fuel cell.

12. The electrode of either of claims 10 or 11, wherein the material comprises a biocompatible polymer that is chemically stable at a pH of the electrode environment.

13. A method of forming a material comprising receiving an open-porous structure and forming a conductive coating comprising carbon fibre fragments over the open-porous structure.

14. The method of claim 13, wherein the open-porous structure is foam, for example, comprising one or more of: polyethylene, polypropylene, polyurethane, polystyrene, polyester, polyether, polyetheretherketone and acrylonitrile butadiene styrene.

15. The method of either of claims 13 or 14, wherein the conductive coating comprises a binder and the conductive coating is spray coated onto the open-porous structure.

16. The method of any of claims 13 to 15, wherein the conductive material further comprises one or more of: conductive activated carbon, conductive carbon black, graphite flakes, amorphous carbon particles, and magnetite.

17. The method of any of claims 13 to 16, further comprising at least partially melting the open-porous structure after applying the carbon fibre fragments in order to fix the carbon fibre fragments to the open-porous structure,18. The method of any preceding claim, wherein the carbon fibre fragments are heated to greater than 1000°C to increase their graphitic character,19. The method of any of claims 13 to 18, wherein the carbon fibre fragments are treated with a chemical or physical activation process to increase the surface area of the carbon fibre fragments.

20. The method of any of claims 13 to 19, where the carbon fibre fragments and / or the conductive coating are treated with an oxidation process, optionally wherein the oxidation process includes one or more of: high temperature carbon dioxide, mineral acids and thermal treatment in a low oxygen environment.P606415PC0021. A method of forming an electrode for a bioelectrochemical system, the method comprising receiving an open-porous structure for colonisation by electrogenic microorganisms and forming a conductive coating comprising carbon fibre fragments on the open-porous structure.

22. A bioelectrochemical system comprising an electrode according to any of claims 10 to 12.

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