Direct air capture system

Abstract

An electrode for use in an electrochemical cell, the electrode comprising a composite material comprising graphite coated with one or more quinone species, the quinone species being capable of bonding with carbon dioxide when the quinone species is in a reduced state and releasing bound carbon dioxide when the quinone species is in an oxidized state. Electrochemical cells and gas separation systems comprising the electrode, and methods of using said gas separation systems to remove CO2 from a gas mixture are also described herein.

Description

[0001] Direct Air Capture System

[0002] Field of the Invention

[0003] The present invention relates electrodes for use in an electrochemical cell, and electrochemical cells and gas separation systems comprising said electrode.

[0004] Background of the Invention

[0005] It is well known that carbon dioxide (CO2) is a greenhouse gas and that increasing concentrations of this gas in the atmosphere of the earth from fossil fuel burning and other sources is the main contributor to global heating. The Intergovernmental Panel on Climate Change (IPCC) has emphasised the importance of not only reducing greenhouse gas emissions but also deploying negative emissions technologies to limit global warming to well below 2°C.

[0006] Direct air capture (DAC) of CO2 has been developed as a negative emission technology. One form of DAC is electrochemical. The significance of electrochemical DAC lies in its capacity to remove CO2 from the air regardless of its source, making it a valuable tool for addressing emissions from both concentrated and dispersed sources.

[0007] The economic balance point for carbon dioxide removal from the atmosphere is a cost of $100 per ton. At or below this cost significant increase in accessibility and deployment of DAC technology can be accessed.

[0008] Summary of the Invention

[0009] The present inventors have identified that increasing the longevity of electrochemical DAC cells and reducing the costs of these cells is required to achieve the economic balance point. To achieve cost reductions, the capability of the cell to adsorb CO2 cannot be significantly reduced.

[0010] Currently, multiwalled carbon nanotubes (MWCNTs) are utilised within electrochemical DAC systems due to their long cycle life. However, MWCNTs are costly, complex to manufacture, and have restricted particle diameter. The present invention provides an electrode for use in an electrochemical cell, wherein the electrode comprises a composite material, the composite material comprising graphite coated with one or more quinone species capable of bonding with carbon dioxide when the electroactive species is in a reduced state and releasing bound carbon dioxide when the electroactive species is in an oxidized state.

[0011] Graphite has been found to possess good cycle durability and CO2 capture performance. Graphite is also a low-cost material. That these characteristics allow the production of a low cost, yet durable electrode for use in electrochemical DAC cells.

[0012] Moreover, unlike MWCNTs the particle diameter of particulate graphite can be readily modified. The particle diameter of the composite material is preferably as small as possible, to increase the surface area available for DAC. However, a small particle diameter (c. 1 / rm) is only suitable for dry climates. In humid climates, degradation time / effective lifetime of the composite material decreases as particle diameter decreases. There is therefore a balance to be found between efficient small particles and durable larger particles (c. >100 / rm). The flexible particle diameter of graphite allows the composite material to be adapted to different climates, including humid climates, providing durable electrodes irrespective of the climate. This contrasts with MWCNTs, the particle diameter of which is small and difficult to modify meaning MWCNT composite materials are unsuitable for humid climates.

[0013] In a second aspect the invention provides an electrochemical cell comprising the electrode of the first aspect.

[0014] In a third aspect, the invention provides a gas separation system comprising one or more electrochemical cells of the second aspect, wherein the one or more electrochemical cells is in fluid communication with one or more gas inlet and one or more gas outlet.

[0015] In a fourth aspect, the invention further provides a method of reducing the concentration of CO2 in a gas mixture. The method comprises:

[0016] (i) providing the gas separation system of the present invention and a gas mixture comprising CO2; (ii) passing the gas mixture comprising CO2 into the gas separation system through the one or more gas inlet such that it is in contact with the one or more electrochemical cell and applying a positive potential across the one or more electrochemical cell to reduce the quinone species and adsorb at least a portion of CO2 from the gas mixture onto the surface of the electrode, thus reducing the concentration of CO2 in the gas mixture;

[0017] (iii) passing the gas mixture in which the concentration of CO2 has been reduced out of the one or more gas outlets.

[0018] Detailed Description of the Invention

[0019] The present invention provides an electrode for use in an electrochemical cell, wherein the electrode comprises a composite material, the composite material comprising: graphite coated with one or more quinone species capable of bonding with carbon dioxide when the quinone species is in a reduced state and releasing bound carbon dioxide when the quinone species is in an oxidized state.

[0020] The graphite may be synthetic graphite, naturally occurring graphite, exfoliated graphite, graphene nanoplatelets or a combination thereof.

[0021] Synthetic graphite may also be referred to as artificial graphite. The synthetic graphite may be primary synthetic graphite, secondary synthetic graphite or a combination thereof.

[0022] The naturally occurring graphite may be amorphous graphite, flake graphite, crystalline vein graphite or combinations thereof.

[0023] Natural graphite is abundant and is relatively cheap in comparison to other forms of graphite.

[0024] In comparison to natural graphite, synthetic graphite demonstrates a higher purity, and improved electrical conductivity and chemical stability.

[0025] Exfoliated graphite has a high surface area relative to natural graphite. As surface area increases, so does the amount of quinone species that can be coated onto the graphite. Graphene nanoplatelets (GNP) have a very high surface area and good electrical conductivity relative to natural graphite.

[0026] The graphite may be particulate graphite. Particulate graphite may have a particle diameter of 1 mm or less, such as 750 pm or less, 600 pm or less, 500 pm or less, 400 pm or less, 300 pm or less, or 200 pm or less. Particulate graphite may have a particle diameter of 1 pm or greater, such as 10 pm or greater, 25 pm or greater, 50 pm or greater, 100 pm or greater. The diameter of graphite particles may also be referred to as the mesh size of the graphite particles. Particle diameter may be determined using laser diffraction, for example by generating a suspension of particles in deionised water and measuring the particle size distribution using a Mastersizer 2000 (Malvern P analytical).

[0027] Where the graphite is particulate graphite, the electrode further comprises an electrode support material on which the composite material is disposed. The electrode support material can be any suitably conductive support material. Examples of electrode support material include a metal structure (e.g. foils, meshes or fabrics), a metal oxide structure (e.g. TiCE, ZnCh, NiO), carbon paper, carbon cloth, woven or non-woven carbon fibre mat, carbon materials (e.g. graphite, graphene nanoplatelets, activated carbon, graphite mesh), a polymer material (e.g. polyvinylidene fluoride, polyethylene oxide, sulfonated tetrafluoroethylene based fluoropolymer-copolymers such as Nafion™ (CAS No: 66796-30-3) or polyacrylonitrile) or combinations thereof. The skilled person would be aware of other suitable electrode support materials.

[0028] Graphite particle diameters below 1 mm, and in particular below 500 pm, improve the conductivity of the electrode, allowing more efficient CO2 capture. Without wishing to be bound by theory, it is thought that particle diameters below 1 mm, and in particular below 500 pm result in a more even coat of the composite material on the electrode support material. Smaller particle diameters also increase the surface area of the electrode, which enables the electrode to act as a more effective charge carrier, whilst allowing large quantities of quinone to adhere to the surface, thereby providing an electrode with improved conductivity. Providing the quinone species coated onto graphite particles on an electrode support material rather than directly coated onto the electrode support material improves adherence of the quinone species to the electrode surface and improves the distribution of charge. The improved distribution of charge allows the quinone species to be charged to the potential required for CO2 capture at improved speeds and improves CO2 capture efficiency, wherein efficiency is the number of mols of CO2 captured in a cycle divided by the number of mols of sites available for CO2 capture.

[0029] The one or more quinone species is one capable of bonding with carbon dioxide when the quinone species is in a reduced state and releasing bound carbon dioxide when the quinone species is in an oxidized state.

[0030] Quinones, such as an ortho-benzoquinone moiety or a para-benzoquinone moiety, are suitable species for CO2 capture. In a reduced state, the anion is capable of interacting with and binding two CO2 molecules as shown below. Oxidation of this anion releases the CO2 molecule.

[0031] The one or more quinone species may comprise or consist of substituted or unsubstituted quinone. For example, the one or more quinone species may be selected from benzoquinone, 1,4-naphthoquinone, 1,2-naphthoquinone, anthraquinone, phenanthrenequinone, benzanthraquinone, dibenzoanthraquinone, or 4,5,9, 10- pyrenetetrone. The one or more quinone species may also be selected from polymers, co-polymers, homo-oligomers or co-oligomers comprising repeating units derived from a substituted or unsubstituted quinone or tetrone. For example, the one or more quinone species may also be polymers, co-polymers, oligomers or co-oligomers comprising repeating units selected from benzoquinone, 1,4-naphthoquinone, 1,2-naphthoquinone, anthraquinone, phenanthrenequinone, benzanthraquinone, dibenzoanthraquinone, or 4,5,9, 10-pyrenetetrone. The one or more quinone species may be polybenzoquinone (PBQ) or polyanthraquinone (PAQ). The quinone species may be coated onto the graphite by any suitable method. For example, the quinone species may be dissolved in a suitable solvent and coated onto the graphite by dip-coating, spray coating or brush-coating. Where the graphite is particulate graphite, the graphite may be combined with to a solution of quinone species to form a suspension. This suspension may optionally be dried, for example by spray drying.

[0032] Where present, the electrode support material may be coated with the composite material by any suitable method, such as dip-coating, spray coating or brush-coating.

[0033] The invention further provides an electrochemical cell comprising one or more first electrode, wherein the first electrode is the electrode of the present invention.

[0034] The electrochemical cell may comprise a second electrode. The second electrode may be any suitable electrode as would be understood by the skilled person. A function of the second electrode is to complete the circuit of the electrochemical cell when the cell is connected to a power source. The second electrode may be the same or different to the electrode of the present invention.

[0035] The second electrode may be selected from those with corrosion resistance to the conditions within the reactor system. For example, the secondary electrode may be a polyvinyl-ferrocene (PvFC) electrode, a nickel electrode or a platinum electrode. A PvFC electrode may be used due to the inert nature of the electrode over thousands of cycles.

[0036] The second electrode may comprise one or more electroactive species that is the same as or different to the electroactive species of the first electrode. For example, the one or more electroactive species of the second electrode may comprise or consist of substituted or unsubstituted quinone. The one or more electroactive species of the second electrode may comprise or consist of an electroactive inorganic complex, for example an alkali metal-transition metal oxide or an alkali metal-transition metal phosphate of the formula AMX, wherein A is Li, Na, or K; M is Ni, Co, Mn, Al, Ti, Mo, Fe, V, Si, or a combination thereof; and X is O2 or PO4. The second electroactive species may be selected from LiFePCL, polyvinyl-ferrocene, or combinations thereof. Combinations of the above electroactive materials of the second electrode are also envisioned. The electroactive species may be coated onto a particulate material, such as multiwalled carbon nanotubes, graphite particles, carbon black, fullerene or combinations thereof, which is then disposed on an electrode support material.

[0037] The second electrode may comprise an electrode support material on which the one or more electroactive species is disposed. The electrode support material of the second electrode can be a conductive material and act as a current collector within the electrochemical cell.

[0038] The electrochemical cell may further comprise one or more separator. The one or more separator may be positioned between the first and second electrodes. Where the cell contains more than one first electrode, more than one second electrode or more than one each of the first and second electrode, a separator may be positioned between each of the electrodes. The separator may comprise any suitable material. For example, the separator may comprise or consist of material selected from polymer film, ceramic, cellulose, polymer / ceramic composites, or combinations thereof. Examples of suitable polymer films include a polyamide, a polyolefin, a polyaramid, a polyester, a polyurethane, an acrylic resin, or combinations thereof. The separator may be a cellulose paper.

[0039] The electrochemical cell may comprise an electrolyte. The electrolyte will have suitable conductivity at room temperature (20°C). The electrodes, the separator or both may be wetted with the electrolyte. The skilled person will be aware of electrolytes suitable for electrochemical cells. The electrolyte may be confined to the separator, or to the separator and one or both of the electrodes. This construction allows the electrochemical cell to be exposed directly to the gas mixture from which CO2 is to be absorbed, improving the rate of diffusion of CO2 into the electrochemical cell over a cell immersed in the electrolyte.

[0040] The electrolyte may comprise or consist of one or more ionic liquid, one or more organic liquid, or a combination of one or more ionic liquid and one or more organic liquid.

[0041] Ionic liquids as electrolytes show improved charge transfer, carbon dioxide migration and operating potentials compared to other electrolytes. The one or more ionic liquid comprises an anion component and a cation component. The anion of the ionic liquid may comprise halide, sulfate, sulfonate, carbonate, bicarbonate, phosphate, nitrate, nitrate, acetate, PF3, BF4, PFe, triflate, nonaflate, bis(trifluoromethylsulfonyl)amide, trifluoroacetate, heptafluorobutanoate, haloaluminate, triazolide, or an amino acid derivative (e.g., proline with the proton on the nitrogen removed) or combinations thereof. The cation of the ionic liquid may comprise imidazolium, pyridinium, pyrrolidinium, phosphonium, ammonium, sulfonium, thiazolium, pyrazolium, piperidinium, triazolium, pyrazolium, oxazolium, guanadinium, an alkali cation, or dialkylmorpholinium or combinations thereof.

[0042] Suitable ionic liquids may be l-ethyl-3-methylimidazolium acetate ([EMIm] [Ac]), 1- butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIm] [Tf2N]), 1- hexyl-3-methylimidazolium hexafluorophosphate ([HMIm] [PFe]), 1-butyl-l- methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMPyr] [Tf2N]), Lithium bis(trifluoromethylsulfonyl)imide (Li[Tf2N]), l-ethyl-3-methylimidazolium hydroxide ([EMIm] [OH]), l-butyl-3-methylimidazolium hydroxide ([BMIm] [0H]), l-hexyl-3- methylimidazolium hydroxide ([HMIm] [OH]), 1 -butyl- 1-methylpyrrolidinium hydroxide ([BMPyr] [OH]), lithium hydroxide (LiOH), or combinations thereof.

[0043] The ionic liquid of the electrolyte may be a basic ionic liquid. The present inventors have identified that acidic ionic liquids are involved in the degradation of graphite and so reduce the durability of the electrode of the present invention. By using basic ionic liquids the durability of the electrode is therefore further increased. Examples of basic ionic liquids from which the electrolyte may be selected include l-ethyl-3- methylimidazolium hydroxide ([EMIm] [0H]), l-butyl-3-methylimidazolium hydroxide ([BMIm] [0H]), l-hexyl-3-methylimidazolium hydroxide ([HMIm] [OH]), 1 -butyl- 1- methylpyrrolidinium hydroxide ([BMPyr] [OH]), lithium hydroxide (LiOH), or combination thereof.

[0044] Methods of assessing the basicity of an ionic liquid are known to those skilled in the art and the basicity of an ionic liquid may be assessed using any appropriate method. For example, the basicity of an ionic liquid may be assessed using the Hammett function, and / or could be measured from cell performance under a strict set of conditions. These conditions would be measured using capture quantity, efficiency, cell degradation percentage and electrical efficiency. Computational chemistry may be used to provide bond strengths, quinone protonation tendencies and carbon dioxide solubility markers.

[0045] The electrolyte may be l-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Bmim] [TfSI]).

[0046] The electrochemical cell may comprise one or more current collector. The first electrode may be in direct physical contact with the first current collector. The second electrode may be in direct physical contact with the second current collector. A current collector may be integrated into one or more of the electrodes. Where a current collector is integrated into an electrode, this electrode may be configured receive electrical power from a power source that does not form part of the electrochemical cell, optionally via the integrated current collector.

[0047] The one or more current collector may be metallic. For example, the current collector may be steel, stainless steel, aluminium, copper, nickel, titanium, alloys thereof or combinations thereof. Suitable structures include meshes including woven and nonwoven meshes, perforated foils or sheets, sintered structures or combinations thereof. The one or more current collector may be a stainless-steel mesh.

[0048] The electrochemical cell may be constructed such that a separator is disposed between the first electrode and the second electrode, the first electrode is disposed between the separator and a first current collector, and the second electrode is disposed between the separator and a second current collector. The current collectors may be configured to receive electrical power from a power source that does not form part of the electrochemical cell. The separator, first electrode and second electrode may be in contact with the electrolyte.

[0049] The electrochemical cell may be constructed such that a separator is disposed between the first electrode and the second electrode where a first current collector is integrated into the first electrode and a second current collector is integrated into the second electrode. The first and second electrode are configured receive electrical power from a power source that does not form part of the electrochemical cell. The separator, first electrode and second electrode may be in contact with the electrolyte. The invention further provides a gas separation system comprising one or more electrochemical cell of the invention, wherein the one or more electrochemical cell is in fluid communication with one or more gas inlet and one or more gas outlet.

[0050] The gas separation system may contain a single electrochemical cell or two or more electrochemical cells, such as from one to one thousand electrochemical cells, one to one hundred electrochemical cells, two to fifty electrochemical cells, four to twenty electrochemical cells or ten to fifteen electrochemical cells.

[0051] Each electrochemical cell is in fluid communication with one or more gas inlet and one or more gas outlet. Gas inlets allow gas mixture to access the system and interact with the one or more electrochemical cell.

[0052] The gas mixture may be any suitable gas mixture, such as atmosphere or a flue gas mixture from an industrial process. When interacting with the one or more electrochemical cell, CO2 in the gas mixture can be adsorbed onto the electrode of the invention, when that electrode is in a reduced state, and so removed from the gas mixture. After interacting with the electrochemical cell, the gas mixture therefore has a reduced CO2 content. Gas outlets allow gas mixture with reduced CO2 content to exit the system.

[0053] The gas separation system may comprise one gas inlet, or multiple gas inlets. Each electrochemical cell in the gas separation system may be in fluid communication with a single gas inlet or multiple gas outlets. Electrochemical cells may share gas inlets with other electrochemical cells in the system. The gas separation system may comprise one gas outlet, or multiple gas outlets. Each electrochemical cell in the gas separation system may be in fluid communication with a single gas outlet or with multiple gas outlets. Electrochemical cells may share gas outlets with other electrochemical cells in the system.

[0054] The electrochemical cells of the system may be housed in one or more chamber forming part of the system. For example, an individual chamber for each electrochemical cell, a chamber containing multiple electrochemical cells or a single chamber containing every electrochemical cell within the system. The one or more chamber of the system may be substantially sealed from gas ingress or outflow save for via the gas inlets and gas outlets. It will be understood that the term “substantially sealed” refers to limited and / or minimal gas exchange occurring between the inside of the chamber and the outside of the chamber. However, it is not necessary for the container to be perfectly sealed.

[0055] The system may be configured to received electrical power from a power source that does not form part of the system, and transfer this to the electrochemical cells of the system.

[0056] The gas separation system may further comprise a fan to promote air flow through the system.

[0057] The invention further provides a method of reducing the concentration of CO2 in a gas mixture. The method comprises:

[0058] (iv) providing the gas separation system of the present invention and a gas mixture comprising CO2;

[0059] (v) passing the gas mixture comprising CO2 into the gas separation system through the one or more gas inlet such that it is in contact with the one or more electrochemical cell and applying a positive potential across the one or more electrochemical cell to reduce the quinone species and adsorb at least a portion of CO2 from the gas mixture onto the surface of the electrode, thus reducing the concentration of CO2 in the gas mixture;

[0060] (vi) passing the gas mixture in which the concentration of CO2 has been reduced out of the one or more gas outlets.

[0061] The step of applying a positive potential across an electrochemical cell of the invention to reduce the quinone species and adsorb CO2 from the gas mixture may be referred to as a charging cycle, as the electrode surface is charged with CO2.

[0062] It will be appreciated that the magnitude of the potential applied across the one or more electrochemical cells will vary according to the specifics of the system and circumstances, including the required speed of operation, the nature of the quinone species, construction of the electrochemical cell, and available power source. The skilled person will be able to determine the potentials required. The positive potential applied may be 0.1 V or greater. For example, it may be 0.5 V or greater, 0.7 V or greater, 0.9 V or greater, 1 V or greater, 1.5 V or greater, 1.7 V or greater, or 2 V or greater. The positive potential applied may be 4 V or less, such as 3.5

[0063] V or less, 3 V or less or 2.7 V or less. The positive potential applied may be from 0.1

[0064] V to 4 V, such as from 0.5 V to 3.5 V, from 1 V to 3 V or from 1.5 V to 2.7 V.

[0065] The method may further be a method of providing an exhaust gas mixture enriched with CO2. Here, the method further comprises after step (iii) the additional step of applying a negative potential across the one or more electrochemical cells to oxidise the quinone species and release adsorbed CO2 from the surface of the electrode. The CO2 may then be extracted from the gas separation system via the one or more gas outlets. The CO2 may then be transported to another location, such as a storage location or further processing location.

[0066] Applying a negative potential across an electrochemical cell of the invention to oxidise the quinone species and release adsorbed CO2 from the surface of the electrode may be referred to as a discharge step, as CO2 is discharged from the electrode surface.

[0067] During or before the discharge step, a carrier gas or carrier gas mixture may be introduced to the system to aid the transportation of the CO2 released from the system through the gas outlet. It will be appreciated that other methods of extracting the released CO2 from the system are available.

[0068] The negative potential applied may be -0.1 V or less, such as -0.3 V or less, -0.5 V or less, -0.7 V or less, or -1 V or less. The negative potential applied may be -4 V or greater, such as -3.5 V or greater, -3 V or greater, -2.5 V or greater, or -2 V or greater. The negative potential applied may be from -4 V to -0.1 V, such as from -3 V to -0.5 V, from -2.5 V to 0.7 V or from -2 V to -1 V.

[0069] Detailed Description of the Drawings

[0070] Figure 1 is a schematic view of an electrochemical cell according to the invention.

[0071] Figure 2 shows the change in current at 0.55V at an electrode comprising a polyanthraquinone and MWCNT composite material as the number of cycles changes, and the extrapolation of this trend to find the point at which current at 0.55V drops to 60% of the current at 0.55 V of the first cycle.

[0072] Figure 3 shows the change in current at 0.55V at an electrode comprising a polyanthraquinone and graphite composite material as the number of cycles changes, and the extrapolation of this trend to find the point at which current at 0.55V drops to 60% of the current at 0.55 V of the first cycle.

[0073] Figure 4 shows the change in current at 0.55V at an electrode comprising a polyanthraquinone and activated carbon (AC) composite material as the number of cycles changes, and the extrapolation of this trend to find the point at which current at 0.55V drops to 60% of the current at 0.55V of the first cycle.

[0074] Figure 5 shows the change in current at 0.55V at an electrode comprising a polyanthraquinone and AC / graphite 1 : 1 mix composite material as the number of cycles changes, and the extrapolation of this trend to find the point at which current at 0.55 V drops to 60% of the current at 0.55V of the first cycle.

[0075] Figure 6 shows the change in current at 0.55V at an electrode comprising a polyanthraquinone and carbon black composite material as the number of cycles changes, and the extrapolation of this trend to find the point at which current at 0.55 V drops to 60% of the current at 0.55V of the first cycle.

[0076] Figure 7 shows SEM images of (A) a polyantraquinone and activated carbon (d < 250pm) composite material on carbon mesh, (B) a polyantraquinone and activated carbon (d < 500pm) composite material on carbon mesh, (C) a polyantraquinone and MWCNT carbon composite material on carbon mesh, (D) a carbon mesh.

[0077] Figure 8 is a schematic view of a gas separation system according to the invention.

[0078] Figure 9 shows release of CO2 from an electrochemical cell according to the invention during a discharge steps.

[0079] Figure 10 shows the current profile of an electrochemical cell according to the invention during charging and discharging steps.

[0080] Figure 11 shows current profile corresponding to CO2 release during a discharge steps.

[0081] Referring to Figure 1 of the accompanying drawings, Figure 1 shows an electrochemical cell 1 comprising an electrolyte separator 3 disposed between the first electrode 5 and the second electrode 7, the first electrode 5 disposed between the electrolyte separator 3 and a first current collector 9, and the second electrode 7 disposed between the electrolyte separator 3 and a second current collector 1 1. The electrolyte separator 3, first electrode 7 and second electrode 7 are in contact with the electrolyte 13, which is disposed between the electrolyte separator 3 and the first electrode 5, and the electrolyte separator 3 and the second electrode 7. An additional electrolyte separator 15 is disposed next to the second current collector 11.

[0082] The additional electrolyte separator 15 is used to prevent external factors from affecting the charging process across the cell. The electrolyte separator 3 prevents contact between the first and second electrode, mitigating the possibility of an internal short circuit.

[0083] Referring to Figure 8 of the accompanying drawings, Figure 8 shows a gas separation system 17 comprising a gas inlet 19, a gas outlet 21 and three electrochemical cells 23 as defined herein and according to Figure 1. The direction of gas flow through the system is indicated.

[0084] Examples

[0085] Example 1 - Preparation of Electrochemical Cells

[0086] Polyanthraquinone Preparation

[0087] A solution of 1,4-dichloroanthranquinone (3.02 g) in DMF (40 ml) was prepared. A second solution was made by adding bis(l,5-cyclooctadiene) nickel (0) (4.00 g) , 2,2 bipyridyl (2.27 g), and cyclooctadiene (1.5 ml) to DMF (100 ml). The second solution was heated to 65 °C and the first solution added to the second solution dropwise until homogenous. The mixture was stirred for 72 hours. After 72 hours the mixture was cooled to room temperature. 1 M hydrochloric acid (200 ml) was added and the mixture stirred for 1 hour. The mixture was then filtered to obtain the solid portion which was washed twice with 1 M hydrochloric acid (2 x 400 ml), twice with deionized water (2 x 200 ml), twice with DMF (2 x 200 ml), and twice with methanol (2 x 200 ml). The residue was then dried under vacuum for 24 hours. This residue was then extracted with DMF via Soxlet extraction for 5 hours to obtain the polyanthraquinone. Preparation of Composite Material

[0088] For each composite, 200 mg of PAQ and 200 mL of NMP were sonicated at 51 °C for 20 minutes, then 600 mg of quinone species support material was added, and that mixture was sonicated for another 20 minutes to form a composite material ink. The quinone species support material studied are shown in Table 1.

[0089] Table 1. Quinone Species Coated Materials

[0090] Preparation of Composite Material Electrodes

[0091] 25 cm x 25 cm sheets of carbon mesh were cut. For each composite material, two carbon mesh sheets were dipped in the composite material ink and then allowed to dry for 2 hours at 120 °C.

[0092] Preparation of Electrochemical Cells

[0093] Electrochemical cells comprising a composite material electrode as the first electrode were constructed as shown in Figure 1.

[0094] The second electrode was constructed of polyvinylferrocene coated MWCNTs disposed on carbon mesh, prepared in the same manner as the composite material electrodes.

[0095] The electrolyte was the ionic liquid l-butyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide. The electrodes were wetted with the electrolyte before being laminated into the cell. The electrolyte separator was cellulose filter paper, used to ensure no contact between the first electrode and second electrode.

[0096] The current collectors were stainless-steel mesh.

[0097] The current collectors were configured to allow attachment of potentiostat connections to the electrochemical cell.

[0098] Example 2 - Durability Testing

[0099] Cyclic voltammetry scanning was used to cycle a charge of 0 - 1.3 V through a 1 cm2portion of the composite material electrodes of Example 1. A scanning rate of 0.05 V / s was used, and a CV curve was recorded at cycle number 1, 10, 50, 100, 200, and every 100 cycles after that for 72 hours or 5400 cycles. A 1 cm2portion of the composite material electrodes were used as the working electrode, a 25 cm long wire of coiled platinum was used as the counter electrode, Ag / Ag-Cl catalyst was used as the reference electrode, and 40 ml of [Bmim][Tf2N] was used as the electrolyte. The electrochemical cells were cycled to testing failure, providing the number of measured cycles of each cell. At this point the potentiostat was unable to measure the potential accurately. The number of cycles achieved is referred to as the number of measured cycles.

[0100] The results were extrapolated to approximate the durability of the electrodes as shown in Figures 2-6. The quinone oxidation peak was found at 0.55 V. Graphs were produced that showed the oxidation current of the working electrode at 0.55 V and compared this to the cycle number. This facilitated an estimate of how the electrode would degrade over time. The durability was given as the number of cycles the electrode could undergo until the oxidation current at 0.55 V reached 60% of it is value on the first cycle. The results of this analysis are shown in Table 2.

[0101] Table 2. Results of Durability Testing

[0102] In the durability testing, the average particle diameter range for each quinone species coated material was from 250 to 300 pm.

[0103] As is shown in Table 2, MWCNTs have a predicted durability of 7490 cycles, similar to literature values. Graphite then has the second highest predicted durability of 3296. The AC / Graphite 1 : 1 mixture has a predicted durability about half that of graphite. This indicates a linear relationship between durability and the amount of graphite in the composite material, as there was half as much graphite on the electrode substrate. AC has the lowest predicted durability.

[0104] The cyclic voltametric testing revealed an upwards trend before a decrease. This is due to impurities being present on the working electrodes surface, reducing the conductivity of the electrode. As these are oxidized and removed from the electrodes surface, the current increase.

[0105] The durability tests show that use of graphite particles as the quinone species support material results in acceptable durability.

[0106] Example 3 - SEM imaging

[0107] As seen in Figure 7, SEM images of activated carbon (d < 250pm) composite material, activated carbon (d < 500pm) composite material, and a MWCNT carbon composite material, each disposed on carbon mesh, were obtained. Further SEM images of a plain carbon mesh were also obtained.

[0108] The SEM image (C) of the MWCNT composite material showed many small white dots on the topographical image. SEM images (A) and (B) of activated carbon (d < 250pm) composite material and activated carbon (d < 500pm) composite material, respectively, show far fewer white dots. These images demonstrate AC was found on the surface of the carbon mesh. Without wishing to be bound by theory, it is thought that this occurs because the particle size of the AC composite materials was larger than that of the MWCNT composite material (which had a particle diameter of around 12 pm) and so the AC composite materials did not coat as evenly as the MWCNT composite material.

[0109] Example 4 - CO2 Extraction and Discharge

[0110] An electrochemical cell was prepared having the general construction shown in Figure 1. The cell comprised an electrolyte separator disposed between a first electrode and a second electrode. The first electrode was disposed between the electrolyte separator and a first current collector. The second electrode was disposed between the electrolyte separator was and a second current collector. The electrolyte separator, first electrode and second electrode were in contact with the electrolyte, which was disposed between the electrolyte separator and the first electrode, and the electrolyte separator and the second electrode. An additional electrolyte separator was disposed next to the second current collector. The composite material of the electrode was polyanthraquinone coated onto graphite powder having a particle diameter of <20 pm. The electrolyte separator was filter paper wetted with an electrolyte, which was l-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. The current collectors were steel mesh.

[0111] The cell was exposed to ambient atmosphere comprising CO2 (c. 500ppm). A 2.2 V potential was applied across the cell in a charging cycle. A flow rate of 5 L / min across the cell and an open system was maintained during a charge cycle in order to provide a substantially constant CO2 concentration around the cell. During the application of the 2.2 V potential, the polyanthraquinone of the electrode is reduced, allowing causing the cell to absorb CO2 from the atmosphere.

[0112] The system was then sealed to gas ingress or egress. A -1.5 V potential was applied across the cell in the sealed system to switch the cell into a discharge cycle. A negative potential of -1.5 V reverses the redox process, causing the cell to release CO2 from the electrode surface. Figure 9 correlates the CO2 concentration in the atmosphere with the potential applied across the cell. CO2 is released from the surface of the electrode, increasing the concentration of CO2 in the atmosphere around the cell following the change in potential applied across the cell from 2.2 V to -1.5 V, moving the cell from a charge cycle into a discharge cycle.

[0113] Figure 10 correlates the current response with the potential applied across the cell. During the initialisation step when the potential is increased from 0 V to 2.2 V, a sharp increase in current is seen as the electric field is stabilized. As CO2 is captured during the charge cycle a small increase in current is seen. This is a result of CO2 capture onto the surface of the electrode. The reverse redox process is displayed clearly in this figure, as the reversing of applied potential polarity shows a large negative spike in current followed by a decaying negative current over time - indicating the slow release of CO2 from the electrode surface.

[0114] Figure 11 correlates the current response with CO2 level in the atmosphere. Response of current vs CO2 concentration is a useful validation tool to display the redox behaviour of the system. As the current reaches negative values, the CO2 concentration increases. This is indicative of a discharge phase during which the reverse redox process occurs, and CO2 is released from the surface of the electrode.

[0115] In each of figures 9 to 11, “Max CO2 Concentration (ppm)” is the theoretical maximum quantity of CO2 able to be captured and subsequently discharged into the sealed system given the relationship between quantity released and volume available to diffuse into.

Claims

CLAIMS1. An electrode for use in an electrochemical cell, wherein the electrode comprises a composite material, the composite material comprising: graphite coated with one or more quinone species capable of bonding with carbon dioxide when the quinone species is in a reduced state and releasing bound carbon dioxide when the quinone species is in an oxidized state.

2. The electrode of claim 1, wherein the graphite has a particle diameter of less than 1 mm, such as 500 pm, less than 400 pm, or less than 300 pm.

3. The electrode of any one of claim 1 or claim 2, wherein the quinone species is selected from benzoquinone, 1 ,4- naphthoquinone, 1,2-naphthoquinone, anthraquinone, phenanthrenequinone, benzanthraquinone, dibenzoanthraquinone, or 4,5,9, 10-pyrenetetrone.

4. The electrode of any one of claims 1 to 3, wherein the quinone species is selected from homopolymers, co-polymers, homooligomers or co-oligomers comprising repeating units selected from benzoquinone, 1,4-naphthoquinone, 1,2- naphthoquinone, anthraquinone, phenanthrenequinone, benzanthraquinone, dibenzoanthraquinone, or 4,5,9, 10-pyrenetetrone.

5. The electrode of any one of claims 1 to 4, wherein the quinone species is selected from polybenzoquinone or polyanthraquinone.

6. The electrode of any one of claims 1 to 5, wherein the electrode further comprises an electrode support material upon which the composite material is disposed.

7. The electrode of claim 6 wherein the electrode support material is selected from a metal structure, a metal oxide structure, carbon paper, carbon cloth, a wovenor non-woven carbon fibre mat, carbon materials, a polymer material, or combinations thereof.

8. An electrochemical cell comprising a one or more first electrode, wherein the first electrode is the electrode of any one of claims 1 to 7.

9. The electrochemical cell of claim 8, wherein the electrochemical cell further comprises one or more second electrode, and a separator disposed between the one or more first and one or more second electrode.

10. The electrochemical cell of claim 8 or claim 9, wherein the electrochemical cell further comprises one or more current collector.1 1. The electrochemical cell of any one of claims 8 to 10 wherein the electrochemical cell further comprises an electrolyte.

12. The electrochemical cell of claim 1 1, wherein the electrolyte is an ionic liquid, optionally a basic ionic liquid.

13. The electrochemical cell of claim 1 1 or claim 12, wherein the electrolyte is selected from the group consisting of l-ethyl-3-methylimidazolium acetate ([EMIm] [Ac]), l-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIm] [Tf2N]), l-hexyl-3-methylimidazolium hexafluorophosphate ([HMIm] [PFe]), 1 -butyl- 1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMPyr] [Tf2N]), lithium bis(trifluoromethylsulfonyl)imide (Li[Tf2N]), l-ethyl-3-methylimidazolium hydroxide ([EMIm] [OH]), l-butyl-3-methylimidazolium hydroxide ([BMIm] [0H]), l-hexyl-3-methylimidazolium hydroxide ([HMIm] [0H]), 1- butyl- 1-methylpyrrolidinium hydroxide ([BMPyr] [OH]), lithium hydroxide (LiOH), or combinations thereof.

14. A gas separation system comprising one or more electrochemical cells of any one of claims 7 to 13, wherein the one or more cells is in fluid communication with one or more gas inlet and one or more gas outlet.

15. A method of reducing the concentration of CO2 in a gas mixture, the method comprising:(i) providing the gas separation system of claim 14 and a gas mixture comprising CO2; (ii) passing the gas mixture comprising CO2 into the gas separation system through the one or more gas inlet such that it is in contact with the one or more electrochemical cell and applying a positive potential across the one or more electrochemical cell to reduce the quinone species and adsorb at least a portion of CO2 from the gas mixture onto the surface of the electrode, thus reducing the concentration of CO2 in the gas mixture;(iii) passing the gas mixture in which the concentration of CO2 has been reduced out of the one or more gas outlets.

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