Ink composition

Abstract

Ink compositions comprising hydrophilic, cross-linked polymer particles and methods for making the same. The ink compositions can be used in aqueous supercapacitor applications. Electrodes comprising the homogenised ink compositions deposited on the electrode or within the electrode structure and methods for making the same. The electrodes can be used in energy storage devices.

Description

[0001]INK COMPOSITION Field of the invention The present invention relates to an ink composition comprising hydrophilic, cross-linked polymer particles, electrodes comprising said ink compositions and their use in aqueous supercapacitor applications. Background of the invention Dielectric capacitors consist of a dielectric or insulator material pressed between two conductive plates (electrodes). The energy storage effect originates from charge building up on both of the conductive plates due to the application of an external voltage polarising the dielectric material. The charges remain on the conductive plates after removing the external voltage, and discharge occurs when the electrodes are electrically connected. A supercapacitor, or ultra capacitor, is an energy storage technology. Typically, a supercapacitor can be expected to display more energy stored per unit mass and per unit volume than a conventional electrolytic capacitor by a factor of 10-100. A supercapacitor can typically be expected to possess an approximately 100-1000 times greater specific power than Li-ion batteries, and a supercapacitor should also be able to undergo far more recharge cycles than Li-ion batteries. As such, supercapacitors are considered as a technology that bridges the gap between traditional dielectric capacitors and battery technologies. In a supercapacitor, two electrodes are kept apart by an electrolyte separator which is ionically conducting but electronically insulating. The electrolytes employed include aqueous electrolytes, non-aqueous electrolytes, polymer electrolytes, and gel electrolytes. The two electrodes may be identical (symmetric electrode configuration) or distinct from each other in terms of material composition, mass loading, or surface area (asymmetric electrode configuration). In general, supercapacitors can be classified into three main categories based on their charge storage mechanism. One type of energy storage mechanism in supercapacitors originates from electric double layer capacitance (EDLC), whereby charge storage occurs via build-up of charge between the electrolyte(s) and each of the electrodes. Connecting the two electrodes electrically discharges the stored energy and allows work to be done. This is also known as non-Faradaic charge storage. While all electrode materials display EDLC behaviour to some extent, carbon materials are particularly effective. The second energy storage mechanism involves a combination of electrostatic and electrochemical processes. This type uses transition metal oxides or conductive polymers for fast and reversible redox reactions. These are called pseudocapacitors. The third type, called hybrid capacitors, combines the characteristics of both EDLCs and pseudocapacitors. This combination offers high capacitance and high energy density. In all the types of supercapacitors, carbon-based materials are found to be a common choice of electrode materials. Such carbon materials can often form structures which are difficult for the electrolyte to penetrate, which in turn results in poor electrode wettability. Accordingly, the energy densities of EDLCs based on commercially available activated carbon have been limited to about 10 Wh kg-1. Additionally, aqueous supercapacitors with or without redox additives suffer from further energy density reduction due to limited wettability. Compared to organic electrolytes, aqueous electrolytes exhibit inferior wetting behaviour on the carbon structure, resulting in reduced accessible surface area and consequently lower energy storage capacity. Furthermore, the operational voltage window of aqueous-based supercapacitors is inherently restricted by water splitting reactions at higher voltages, further limiting the achievable energy density. It is important to ensure that the electrode materials are adequately wetted by a continuum of electrolyte. Insufficient electrolyte wetting of electrodes can lead to irregular reactions in the electrodes and unstable formation of the solid-electrolyte interface film. This can deteriorate the cell performance and cause poor cycle life. In addition, incomplete wetting can result in dendrite formation of metals, which causes severe safety issues. If the electrode material is unwetted, this may also lead to underutilisation of electrode capacity and increase electrode resistance. The performance of a carbon-based supercapacitor electrode in aqueous electrolyte with or without redox additives can therefore be improved by increasing the electrode’s wettability. This can be achieved by the chemical grafting of hydrophilic polymers, such as polyvinylpyrrolidone, directly onto the surface of the carbon electrode. Typically, this is achieved by placing the carbon electrode into a solution containing the polymer’s constituent monomers, and then electrochemically polymerising the monomers into polymer chains that grow out from the surface of the carbon electrode, yielding what is commonly referred to as a “polymer brush” structure. This modification of the electrode surface is often a multi-step process, requiring the use of oxidising agents and solvents. These methods are therefore time consuming and require the use of undesirable oxidising agents and other chemical materials. There therefore exists a need for alternative methods of increasing an electrode’s wettability. In particular, methods which do not require the use of undesirable materials, while not compromising on the electrode performance. Summary of the invention In a first aspect, the present invention relates to an ink composition comprising hydrophilic, cross-linked polymer particles. According to a second aspect of the present invention there is a method of forming the ink composition as described in the first aspect, wherein the hydrophilic, cross-linked polymer particles are formed by: a. adding monomer units, and at least one cross-linker to an aqueous solution to form a monomer mixture; b. polymerising the monomer mixture to form a cross-linked hydrophilic polymer; c. drying the cross-linked hydrophilic polymer; and d. forming into a polymer particle material. In a third aspect of the present invention there is an electrode comprising an electronically conductive surface, with a homogenised ink composition according to the first aspect deposited on the electrode and / or within electrode structure. In a fourth aspect of the present invention there is a method of forming an electrode, the method comprising the following steps: a. homogenising the ink composition of the first aspect; and b. depositing the homogenised ink composition onto an electronically conducting surface. In a fifth aspect of the present invention there is an energy storage device comprising an electrode as described in the fourth aspect. Advantageously, the ink composition of the present invention improves a carbon electrode’s wettability. Improving the electrode’s wettability in turn improves the electrode / electrolyte interactions. These improved interactions result in improved cell performance. For example, the inventors of the present invention have found that the addition of hydrophilic, cross-linked polymer particles to a carbonaceous material in an ink composition for the fabrication of an electrode can increase both the energy out and the mass energy density of a cell by 20-25% when the potential difference limit is 2 V. It was also observed that as the potential difference limit is increased, the percentage improvement in energy density is also increased. Advantageously, electrodes fabricated via the deposition of inks according to the present invention produce electrodes wherein the structure is uniform and the hydrophilic polymer is distributed evenly throughout the carbonaceous structure. Advantageously, the ink composition of the present invention does not comprise undesirable materials and does not require harsh chemicals and / or multi-step polymerisation processes in order to fabricate an electrode according to the present invention. Advantageously, the ink composition of the present invention was also found to improve both the capacitance and discharge time of cells. Advantageously, the ink composition of the present invention is compatible with a wide range of pH conditions in the resulting electrochemical cell. Brief description of the drawings Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 is a graph which compares the energy out and mass energy density of devices where crosslinked hydrophilic vinylpyrrolidone based polymer has been incorporated into the negative electrode structure. Figure 2 is a graph which expresses the data shown in Figure 1 as % improvement in energy out and mass energy density. Figure 3 is a graph which compares the capacitance against the current for the cells of Examples 4a, 4b and 4c. Figure 4 is a graph which compares the potential against time for the cells of Examples 4a, 4b and 4c. Figure 5a and 5b are graphs which compare the capacitance density and energy density of devices comprising printed carbon electrodes, where the both the negative and positive electrode incorporates vinyl pyrrolidone-based crosslinked hydrophilic polymer into its structure. Detailed description The term “comprising” or variants thereof will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. The term “consisting” or variants thereof is to be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, and the exclusion of any other element, integer or step or group of elements, integers or steps. The term “about”, when qualifying a number or value, is used to refer to values that lie within ± 5%, preferably ±1%, of the value specified. As used herein, the term “polymer” takes its usual definition in the art and so refers to a homopolymer or a co-polymer formed from the polymerisation of one or more monomers. As used herein, the term “homopolymer” takes its usual definition in the art and so refers to a polymer whose polymer chains comprise one type of monomer. As used herein, the term “co-polymer” takes its usual definition in the art and so refers to a polymer whose polymer chains comprise two or more different types of monomers. As used herein the term “monomer” takes its usual definition in the art, and so refers to a molecular compound that may chemically bind to another monomer to form a polymer. As used herein, the term “repeat units” refers to the sections of a polymer whose repetition would produce the complete polymer chain (except for the end-groups) by linking the repeat units together successively along the chain. As used herein, the term “hydrophilic polymer” refers to a polymer that dissolves in water and other suitable polar liquids when it is not cross-linked, but when cross-linked absorbs water and other suitable polar liquids and swells to form a stable elastic solid. Hydrophilic polymers offer certain benefits due to their affinity for water. As used herein, the term “polymer particles” includes powdered polymer materials, or dispersed in liquid dispersion medium. As used herein the term “powdered” refers to a dry bulk solid composed of particles. As used herein, the term “carbonaceous material” refers to a material comprising carbon. Accordingly, the term "carbonaceous material" covers both the terms “activated carbon" and "conductive carbon”. As used herein, the term “activated carbon” takes its usual definition in the art and so refers to carbon particles that have been processed to have small, low-volume pores that increase the surface area available for adsorption or chemical reactions. As used herein, the term “conductive carbon” takes its usual definition in the art and so refers to various forms of carbon with excellent electrical conductivity, such as carbon black, graphite, carbon nanotubes, and graphene powder. As used herein, the term “binder” refers to material which provides cohesion, adhesion and / or dispersion properties. For example, in the present invention, a binder binds the ink composition to the electrode which eventually improves the conductivity pathway of the electrode material. As used herein, the term “active binder” refers to a material which offers both adhesion and wettable properties. For example, in the present invention, a hydrophilic polymer acts as an active binder by adhering the ink composition to the carbon electrode surface while simultaneously enhancing its wettability. As used herein, the term “liquid dispersion medium” refers to a suitable aqueous solvent that can act as a medium to hold the homogenous ink composition. As used herein, the term "co-monomer mixture", takes its usual definition in the art, and so refers to a solution or dispersion of miscible monomers that, when polymerised, forms a co-polymer. As used herein, the term "cross-linker" refers to a molecular compound capable of forming chemical bonds between polymer chains, and includes compounds such as methylenebisacrylamide, N-(1-Hydroxy-2,2-dimethoxyethyl)acrylamide, allyl methacrylate and ethylene glycol dimethacrylate. The cross-linker may be hydrophobic or hydrophilic. The ink composition In a first aspect, the present invention relates to an ink composition comprising hydrophilic, cross-linked polymer particles. In an embodiment, the hydrophilic, cross-linked polymer particles may be described as a powdered, cross-linked hydrophilic polymer. Accordingly, the present invention relates to an ink composition comprising a powdered, cross-linked, hydrophilic polymer. In a particular embodiment, the ink composition further comprises a binder. In a particular embodiment, the ink composition further comprises a liquid dispersion medium. Accordingly, in an embodiment, the ink composition of the present invention comprises: a. hydrophilic, cross-linked polymer particles; b. a binder; and c. a liquid dispersion medium. In a particular embodiment, the hydrophilic, cross-linked polymer particles comprise at least one type of hydrophilic repeat units selected from the list consisting of 1-vinyl-2- pyrrolidone, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-acrylamido-2- methyl-propanesulfonic acid, acrylic acid, methacrylic acid, ethyl acrylate and N-[3- (dimethylamino)propyl]methacrylate. In a particular embodiment, the hydrophilic, cross-linked polymer particles comprise 1- vinyl-2-pyrrolidone repeat units. The skilled person will understand that the term 1-vinyl- 2-pyrrolidone may be used interchangeably with vinylpyrrolidone, In an embodiment, the ink composition of the present invention comprises: a. a powdered, cross-linked, hydrophilic polymer comprising 1-vinyl-2-pyrrolidone repeat units; b. a binder; and c. a liquid dispersion medium. In an embodiment, the hydrophilic, cross-linked polymer particles comprise at least one additional type of repeat units, wherein the additional type of repeat units are not vinylpyrrolidone repeat units. In an embodiment, the hydrophilic, cross-linked polymer particles comprise at least one type of hydrophobic repeat units. In an even more particular embodiment, the hydrophobic repeat units are selected from the list consisting of methyl methacrylate, acrylonitrile and allyl methacrylate. In an even more particular embodiment, the hydrophobic repeat units are selected from the list consisting of methyl methacrylate and allyl methacrylate. Accordingly, the hydrophilic, cross-linked polymer particles comprise vinylpyrrolidone repeat units and methyl methacrylate repeat units. In particular, the hydrophilic, cross-linked polymer particles are comprised of a co-polymer of vinylpyrrolidone and methyl methacrylate. Alternatively, the hydrophilic, cross-linked polymer particles are comprised of vinylpyrrolidone repeat units and allyl methacrylate repeat units. In particular, the hydrophilic, cross-linked polymer particles are comprised of a co-polymer of vinylpyrrolidone and allyl methacrylate. In a particular embodiment, the at least one additional type of repeat units are selected from the list consisting of allyl methacrylate, methyl methacrylate, acrylonitrile, 2- hydroxyethyl acrylate, and 2-hydroxyethyl methacrylate. In a particular embodiment, the ratio of hydrophilic repeat units : hydrophobic repeat units in the polymer particles is from 1:30 to 30:1, preferably from 1:20 to 20:1, preferably from 1:10 to 10:1, preferably from 1:5 to 5:1, preferably from 1:4 to 4:1. In a particular embodiment, the ratio of vinylpyrrolidone repeat units : at least one additional type of repeat units in the polymer particles is from 1:30 to 30:1, preferably from 1:20 to 20:1, preferably from 1:10 to 10:1, preferably from 1:5 to 5:1, preferably from 1:4 to 4:1. In a particular embodiment, the ratio of hydrophilic repeat units : hydrophobic repeat units in the polymer particles is from 1:30 to 30:1, preferably 1:10 to 10:1, more preferably from 1:5 to 5:1. In a particular embodiment, the ratio of vinylpyrrolidone repeat units : at least one additional type of repeat units in the polymer particles is from 1:30 to 30:1, preferably 1:10 to 10:1, more preferably from 1:5 to 5:1. In a more particular embodiment, the ratio of vinylpyrrolidone repeat units : at least one additional type of repeat units in the polymer particles is 4:1. In a particular embodiment, the polymer in the hydrophilic, cross-linked polymer particles is cross-linked using a cross-linker selected from the list consisting of allyl methacrylate, ethylene glycol dimethacrylate, vinyl methacrylate, divinyl benzene, bisphenol A glycerolate dimethacrylate, poly(ethylene glycol) diacrylate, di(ethylene glycol) diacrylate, tetra(ethylene glycol) diacrylate and N,N’-methylenebis(acrylamide). In a more particular embodiment, the polymer in the hydrophilic, cross-linked polymer particles is cross-linked using the cross-linking agent allyl methacrylate. In an even more particular embodiment, both the cross-linker and the at least one additional type of repeat units are allyl methacrylate. In an even more particular embodiment, amount of cross-linker is from 0.5 wt.% to 10 wt.%, preferably 0.5 to 5 wt.%, more preferably 0.5 to 1 wt.% of the respective polymer formulation. In a particular embodiment, the binder is selected from the list consisting of polyvinyl alcohol, polyvinylidene fluoride. In a more particular embodiment, the binder is polyvinyl alcohol. In a particular embodiment, the ratio of hydrophilic, cross-linked polymer particles : binder is from 50 : 50 to 90 : 10, preferably 60 : 40 to 90 : 10. In a particular embodiment, the ratio of hydrophilic, cross-linked polymer particles: binder is in a ratio of 60:40, 70:30, 80:20, or 90:10. In a particular embodiment, the composition further comprises a carbonaceous material. For example, the present invention is an ink composition comprising: a. a powdered, cross-linked, hydrophilic polymer comprising vinylpyrrolidone repeat units; b. a binder; c. a liquid dispersion medium; and d. a carbonaceous material. In a particular embodiment, the carbonaceous material is selected from the list consisting of activated carbon powder, carbon black, graphene powder, carbon nanotubes, or a combination thereof. In a more particular embodiment, the carbonaceous material is activated carbon powder. In a more particular embodiment, the carbonaceous material is conductive carbon powder, preferably carbon black. Advantageously, the hydrophilic polymer used in the present invention plays a dual role as it not only binds the carbon material but also contributes additional capacitance to the electrode. Accordingly, the hydrophilic polymer of the present invention can act as an active binder in the carbon electrode composition. In a particular embodiment, the ratio of hydrophilic, cross-linked polymer particles : carbonaceous material is from 1 : 10 to 10 : 1. In a particular embodiment, the ratio of hydrophilic, cross-linked polymer particles : binder : carbonaceous material is from 60 : 20 : 20 to 90 : 5 : 5. In a more particular embodiment, the ratio of hydrophilic, cross-linked polymer particles : binder : carbonaceous material is 60 : 20 : 20. In a more particular embodiment, the ratio of hydrophilic, cross-linked polymer particles : binder : carbonaceous material is 70 : 20 : 10. In a more particular embodiment, the ratio of hydrophilic, cross-linked polymer particles : binder : carbonaceous material is 90 : 5 : 5. In a particular embodiment, the liquid dispersion medium comprises an aqueous solvent. In a more particular embodiment, the aqueous solvent is water. In an even more particular embodiment, the water is deionised water. In a particular embodiment, the liquid dispersion medium comprises an organic solvent. In an even more particular embodiment, the organic solvent is selected from the list consisting of N-Methyl-2-pyrrolidone, isopropyl alcohol, ethanol, propan-2-ol, and ethylene glycol. In a particular embodiment, the binder is selected from the list consisting of polyvinyl alcohol, polyvinylidene fluoride, polyvinylpyrrolidone, styrene butadiene rubber, and sodium alginate. In a more particular embodiment, the binder is polyvinyl alcohol. In a particular embodiment, the composition further comprises redox-active additives. In a more particular embodiment, the redox-active additive is selected from the list consisting of quinone and anthraquinone derivatives, metal oxides, and redox active polymers such as polyaniline and poly(3,4-ethylenedioxythiophene). In a particular embodiment, additives may also be included in the ink composition. Additives may increase the electrode performance, and the identity of the additive will depend on the intended use of the electrode. For instance, platinum group metals may be incorporated into the ink if the electrode is to be used for water electrolysis, and transition metal oxides can be incorporated into the ink to introduce pseudo capacitance contributions when fabricating supercapacitor electrodes. In a particular embodiment, the ink composition of the present invention comprises: a. a powdered, cross-linked hydrophilic polymer comprising vinylpyrrolidone repeat units and methyl methacrylate repeat units; b. activated carbon; c. polyvinyl alcohol; and d. water. In a particular embodiment, the ink composition of the present invention comprises: a. a powdered, cross-linked hydrophilic polymer comprising vinylpyrrolidone repeat units and methyl methacrylate repeat units; b. conductive carbon; c. polyvinyl alcohol; and d. water. In a more particular embodiment, the cross-linker is allyl methacrylate. In a particular embodiment, the ink composition of the present invention comprises: a. a powdered, cross-linked hydrophilic polymer comprising vinylpyrrolidone repeat units and allyl methacrylate repeat units; b. polyvinyl alcohol; and c. water. In a more particular embodiment, the cross-linker is allyl methacrylate. Method of forming the ink composition According to another aspect of the present invention there is a method of forming an ink composition. The method firstly comprises the steps of forming the hydrophilic, cross- linked polymer particles. The hydrophilic, cross-linked polymer particles are formed by a method comprising: a. adding monomer units, and at least one cross-linker to an aqueous solution to form a monomer mixture; b. polymerising the monomer mixture to form a cross-linked hydrophilic polymer; c. drying the cross-linked hydrophilic polymer; and d. forming into a polymer particle material. The polymer particle material may be a powdered polymer material. In a particular embodiment, the method step a. further comprises adding an additional type of monomer to an aqueous solution to form a co-monomer mixture. For the avoidance of doubt, when an additional type of monomer is added to the aqueous solution, the aqueous solution comprises at least two types of monomers. For example, the first type of monomer may be a vinylpyrrolidone monomer and the additional type of monomer is different to the vinylpyrrolidone monomer. In a particular embodiment, the cross-linker is selected from the list consisting of allyl methacrylate, ethylene glycol dimethacrylate, vinyl methacrylate, divinyl benzene, bisphenol A glycerolate dimethacrylate, poly(ethylene glycol) diacrylate, di(ethylene glycol) diacrylate, tetra(ethylene glycol) diacrylate and N,N’methylenebis acrylamide. In a more particular embodiment, the cross-linker is allyl methacrylate. In a particular embodiment, the aqueous solution in step a. is selected from the list consisting of water, ethanol, methanol, and acetic acid. In a particular embodiment, the co-monomer mixture may be polymerised using UV, gamma or thermal radiation. The UV or gamma radiation may be carried out under ambient temperature and pressure, whilst thermal polymerisation may be carried out at temperatures up to 70 °C. According to step c. of the method of the present invention, the resulting polymer material is dried. According to step d. of the method of the present invention, the dried polymer material is then put into a particle form. For example, a powdered form. This can be achieved, for example, by drying the polymer material in an oven at 80 °C for 12 to 36 hours. The skilled person will appreciate that the length of time that the polymer is in the oven for will depend on the size of the pieces of polymer. When the polymer material is dry (i.e., the polymer no longer reduces in mass due to the evaporation of water) the dried polymer material is ground in a ball mill at ambient temperature. In situations where particles of 1 to 10 μm or submicron size are desired, cryogenic grinding may be used instead of ball mill grinding. In a preferred embodiment, the dried, polymer material is made up of polymer particles with an average particle size when measured across the particles largest dimension of 1 μm to 500 μm. Preferably, the polymer particles are less than 100 μm, preferably between 1 μm and 100 μm. The skilled person will be aware of how to measure particles of micrometre size. Nevertheless, when the polymer particles used in the present invention are required to be less than 100 μm, these particles may be selected as those which pass through a 100 μm sieve. Similarly, when the polymer particles used in the present invention are required to be less than 50 μm, these particles may be selected as those which pass through a 50 μm sieve. In a particular embodiment, the first type of monomer is a hydrophilic monomer. In a more particular embodiment, the first type of monomer is selected from the list consisting of 1-vinyl-2-pyrrolidone, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-acrylamido-2-methyl-propanesulfonic acid, acrylic acid, methacrylic acid, ethyl acrylate and N-[3-(dimethylamino)propyl]methacrylate. In a more particular embodiment, the first type of monomer is 1-vinyl-2-pyrrolidone. Accordingly, the monomer units in step a. are 1-vinyl-2-pyrrolidone repeat units. In a particular embodiment, the additional type of monomer is a hydrophobic monomer. In a more particular embodiment, the hydrophobic monomer is selected from the list consisting of methyl methacrylate, acrylonitrile and allyl methacrylate. In an even more particular embodiment, the hydrophobic monomer is selected from the list consisting of methyl methacrylate and allyl methacrylate. Accordingly, the hydrophilic, cross-linked polymer particles comprise vinylpyrrolidone repeat units and methyl methacrylate repeat units. In particular, the hydrophilic, cross-linked polymer particles are comprised of a co-polymer of vinylpyrrolidone and methyl methacrylate. Alternatively, the hydrophilic, cross-linked polymer particles are comprised of vinylpyrrolidone repeat units and allyl methacrylate repeat units. In particular, the hydrophilic, cross-linked polymer particles are comprised of a co-polymer of vinylpyrrolidone and allyl methacrylate. In a particular embodiment, both the cross-linker and the second type of monomer are allyl methacrylate. In a particular embodiment, the second type of monomer is also a hydrophilic monomer. In a more particular embodiment, the hydrophilic monomer is selected from the list consisting of 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-acrylamido-2- methyl-propanesulfonic acid, acrylic acid, methacrylic acid, ethyl acrylate and N-[3- (dimethylamino)propyl]methacrylate. In a particular embodiment, the second type of monomer is selected from the list consisting of allyl methacrylate, methyl methacrylate, acrylonitrile, 2-hydroxyethyl acrylate, and 2- hydroxyethyl methacrylate. In a particular embodiment, the ratio of hydrophilic monomer : at least one additional type of monomer is from 1:30 to 30:1, preferably from 1:20 to 20:1, preferably from 1:10 to 10:1, preferably from 1:5 to 5:1, preferably from 1:4 to 4:1. In a particular embodiment, the ratio of vinylpyrrolidone monomer : at least one additional type of monomer is from 1:30 to 30:1, preferably from 1:20 to 20:1, preferably from 1:10 to 10:1, preferably from 1:5 to 5:1, preferably from 1:4 to 4:1. In a particular embodiment, the ratio of hydrophilic monomer : a second type of monomer in the co-monomer mixture is from 1:30 to 30:1, preferably 1:10 to 10:1, more preferably from 1:5 to 5:1. In a particular embodiment, the ratio of vinylpyrrolidone monomer : a second type of monomer in the co-monomer mixture is from 1:30 to 30:1, preferably 1:10 to 10:1, more preferably from 1:5 to 5:1. In a more particular embodiment, the ratio of vinylpyrrolidone monomer : a second type of monomer is 4:1. Once the polymer particles have been formed, they can then be added to a mixture comprising a binder and a liquid dispersion medium. Optionally, the mixture may also comprise a carbonaceous material. Accordingly, in a particular embodiment, the method of forming an ink composition according to the present invention further comprises the following step: e. adding the hydrophilic polymer particles into an ink mixture comprising a binder and / or a liquid dispersion medium. In a more particular embodiment, the ink mixture further comprises a carbonaceous material. In a more particular embodiment, the carbonaceous material is selected from the list consisting of activated carbon powder, carbon black, graphene powder, carbon nanotubes, or a combination thereof. In a more particular embodiment, the carbonaceous material is activated carbon powder. In a more particular embodiment, the carbonaceous material is conductive carbon powder, preferably carbon black. In a particular embodiment, the ratio of hydrophilic, cross-linked polymer particles : binder : carbonaceous material is from 60 : 20 : 20 to 90 : 5 : 5. In a more particular embodiment, the ratio of hydrophilic, cross-linked polymer particles : binder : carbonaceous material is 60 : 20 : 20. In a more particular embodiment, the ratio of hydrophilic, cross-linked polymer particles : binder : carbonaceous material is 70 : 20 : 10. In a more particular embodiment, the ratio of hydrophilic, cross-linked polymer particles : binder : carbonaceous material is 90 : 5 : 5. In a particular embodiment, the liquid dispersion medium comprises an aqueous solvent. In a more particular embodiment, the aqueous solvent is water. In an even more particular embodiment, the water is deionised water. In a particular embodiment, the liquid dispersion medium comprises an organic solvent. In an even more particular embodiment, the organic solvent is selected from the list consisting of N-Methyl-2-pyrrolidone, isopropyl alcohol, ethanol, propan-2-ol, and ethylene glycol. In a particular embodiment, the binder is selected from the list consisting of polyvinyl alcohol, polyvinylidene fluoride. In a more particular embodiment, the binder is polyvinyl alcohol. In a particular embodiment, the composition further comprises redox-active additives. In a more particular embodiment, the redox-active additive is selected from the list consisting of quinone and anthraquinone derivatives, metal oxides, and redox active polymers such as polyaniline and poly(3,4-ethylenedioxythiophene). Method of forming an electrode According to another aspect of the present invention, there is a method of forming an electrode, wherein the method comprises the following steps: a. homogenising the ink composition as described in any of the embodiments above, and b. depositing the homogenised ink composition onto an electronically conducting surface. In a particular embodiment, in step b. the method of ink deposition is selected from the list consisting of bar coating, doctor blading, spray coating, ink jet printing, transfer printing, screen printing, brush-coating, vacuum filtration and stamping. In a particular embodiment, the electronically conductive surface is heated during step b. The skilled person will appreciate that electrodes fabricated via the deposition of inks or slurries consisting of carbonaceous powder and hydrophilic polymer powder will produce an electrode with a very different structure to a polymer brush electrode where un- crosslinked hydrophilic polymer chains have been grown out from the surface of the carbon electrode via direct polymerisation onto the carbon electrode. For example, electrodes fabricated via the above method will produce electrodes where the structure is uniform and the hydrophilic polymer is distributed evenly throughout the carbonaceous structure. Electrode According to another aspect of the present invention there is an electrode comprising hydrophilic, cross-linked polymer particles. In a particular embodiment, there is an electrode comprising an ink composition, as described in any of the embodiments above. For example, the electrode may be modified with the ink composition. In a particular embodiment, there is an electrode comprising an electronically conductive surface, with a homogenised ink composition, as described in any of the embodiments above, on the electrode and / or within electrode structure. In a particular embodiment, the electronically conductive surface is selected from the list consisting of carbon foil, carbon paper, carbon fabric, carbon felt, glassy carbon sheet, boron doped diamond, nitrogen doped diamond, aluminium foil, and stainless steel. In a particular embodiment, the electrode is a carbon electrode. In a particular embodiment, the electrode is a negative electrode. In a particular embodiment, the electrode is a positive electrode. Both positive and negative electrodes can have the ability to store charges by non-Faradaic as well as Faradaic process. In the non-Faradaic process, the electrolyte ions adsorb and desorb in the electrode surface upon polarising the electrodes. In the Faradaic process, the positive electrode undergoes oxidation process whereas negative electrode simultaneously undergoes reduction process. Both the electrodes are either supported with carbon materials or a metal-based material as electronically conductive surface. In an embodiment, the electrodes of the present invention comprise a carbon source. The carbon source may be the electrically conductive surface. Alternatively, the carbon source may a carbonaceous material present in the ink composition. Alternatively, the carbon source may be both the electronically conducive surface and the carbonaceous material in the ink composition. Energy storage device According to another aspect of the present invention there is a storage device comprising an electrode as described in any one of the above embodiments. In a particular embodiment, the energy storage device is a supercapacitor. In a more particular embodiment, the energy storage device is a hybrid supercapacitor. Some supercapacitor applications include renewable and off-peak energy storage, portable and wearable electronics, medical devices, power emergency actuators, memory protection, power enhancement for battery systems, and energy sources for automobiles. In a particular embodiment, the energy storage device comprises a positive and a negative electrode with a membrane located therebetween. In a more particular embodiment, the membrane comprises a membrane is a cross-linked hydrophilic polymer. The cross-linked hydrophilic polymer membranes of the type that can be used in the invention are described in WO 2017 / 153706, WO 2017 / 115064 and WO 2017 / 153705. Examples Figures 1 and 2 (as referred to in examples 1-3) depict a representative performance of energy storage devices where the negative electrode incorporates vinyl pyrrolidone-based crosslinked hydrophilic polymer into its structure compared to energy storage devices where the negative electrode does not incorporate vinyl pyrrolidone-based crosslinked hydrophilic polymer into its structure. Example 1: cell construction (VPMA-AMA) The example cells from Figures 1 and 2 have been constructed as follows: (i) A negative electrode constructed in the following manner: a. An ink compromising of 750 mg of conductive carbon (CC, carbon black), 75 mg of polyvinyl alcohol (13,000 – 23,000 g mol 1) in 10 mL ofdeionised water is made. b. Powdered 1:4 methyl methacrylate:vinylpyrrolidone with 1% allyl methacrylate crosslinker (VPMA-AMA) is added when making an electrode that is considered to be a vinyl pyrrolidone-based crosslinked hydrophilic polymer containing electrode. The ratio of in the ink composition is 1 VPMA-AMA : 1 poly-vinyl alcohol : 10 CC (ratio by mass). c. The ink is homogenised. d. Ink is deposited onto one side of a heated (approximately 80 C) 3 cm x 3 cm carbon paper by brush coating until the electrode has a dry mass of 500 mg (430 mg of ink, after the deionised water has been evaporated, onto a 70 mg, 3 cm x 3 cm carbon paper). e. The dry electrode is dipped into 0.5 M H2SO4negative electrolyte. (ii) The side of the negative electrode that has not been brush-coated is pressed against a carbon foil current collector. (iii) The side of the negative electrode that has been brush-coated is pressed against a 6 M NaBr hydrated separator. (iv) A carbon fabric positive electrode that has been fully soaked in 6 M NaBr is pressed against the side of the separator opposite to the side that the negative electrode is pressed against. (v) There is no direct physical contact between the positive and negative electrodes, although the separator allows ionic conduction between both electrodes. (vi) The side of the positive electrode not pressed against the separator is pressed against a carbon foil current collector. (vii) There is no direct physical contact between the current collectors, although they can be electronically connected via an electrical load or testing apparatus. The performance of the example cells from Figures 1 and 2 have been assessed using the following testing regime: (i) The cells are set to charge at 100 mA until a certain potential difference limit is met. (ii) The cells are then immediately discharged at 100 mA until the potential difference between the electrodes is 0 V. (iii) The potential difference limit for the charging step is increased by 0.1 V after every 50 charge / discharge cycles have been completed. (iv) The starting potential difference limit for a charge step is 1.5 V. Example 2: Energy out and mass energy density Figure 1 shows the performance of an electrochemical cell utilising an electrode fabricated by the method described in example 1, and the performance of an electrochemical cell utilising an electrode fabricated by the deposition of 430 mg of 1 poly-vinyl alcohol : 10 CC (ratio by mass) on a 3 cm x 3 cm carbon paper conducting surface. In both cases, a cell is charged and discharged at 100 mA to a defined potential difference limit. In both cases, the potential difference limit starts at 1.5 V. In both cases, the limit increases by 0.1 V every 50 cycles. Figure 1 compares the energy out and mass energy density of devices where crosslinked hydrophilic vinylpyrrolidone based polymer has been incorporated into the negative electrode structure. Figure 1 shows that the presence of the crosslinked hydrophilic vinylpyrrolidone based polymer in the negative electrode increase the energy yields of the Example 3: Improvement in energy out and mass energy density Figure 2 expresses the data shown in Figure 1 as the %improvement for energy out / J and mass energy density / Wh per kg observed when using an electrode fabricated by the method described in example 1 over when using an electrode fabricated by the deposition of 430 mg of 1 poly-vinyl alcohol : 10 CC (ratio by mass) on a 3 cm x 3 cm carbon paper conducting surface for the negative electrode in an energy storage device. Figure 2 expresses the data shown in Figure 1 as %improvement in energy out and mass energy density. Figure 2 shows that the presence of the crosslinked hydrophilic vinylpyrrolidone based polymer in the negative electrode increases a device’s energy yield charge / discharge cycles are completed, indicating increased performance stability. Figure 2 also shows that the %improvement in energy out and mass energy density increases with potential difference limit, indicating complicated behaviour. Example 4: Cell construction Three further examples cells were constructed according to the following examples. A first cell was constructed comprising a carbon electrode and an ink composition comprising a powdered, cross-linked, hydrophilic polymer comprising vinylpyrrolidone repeat units (VP- AMA). A comparative cell was made which did not comprise the ink composition. A third cell was made according to a “polymer brush method”. Example 4a: Cell construction of VP-AMA cell Preparation of VP-AMA using the monomer of 1-vinyl-2-Pyrrolidone monomer (3 ml), cross linked with allyl methacrylate (0.165 ml) with the assistance of UV initiator 2-Hydroxy-2- Methylpropiophenone (0.11ml) in 2.5 ml of deionized water. In order to form a solid form, the cured polymer membrane is subject to physical treatment process such as vacuum drying or freeze drying over a period of 12 hours. The resultant product after the drying process is subject to physical grinding step, preferably ball milling or preferably mortar and pestle. The VP-AMA cell was constructed in the following manner: a. An ink compromising of 180 mg of VP-AMA fine powder, 20 mg of polyvinyl alcohol (13,000 – 23,000 g mol 1) in 2 mL of deionised water in a 15 mlglass vial. b. The ink is homogenised using ultrasonication for 5 mins and followed by magnetic stirring overnight. c. Ink was deposited onto one side of a heated (approximately 80 °C) 3 cm x 3 cm carbon felt by brush coating. After drying the electrode, the target mass of 30 mg was achieved on 3 cm x 3 cm carbon felt. This VP-AMA coated carbon felt acted as the positive electrode. d. The carbon fabric of 3 cm x 3 cm was used as the negative electrode. e. A separator soaked in 6M NaBr used for the cell construction. f. The positive and negative electrodes are either wetted equally with 2 ml of 1 M of H2SO4for two-electrode cell constructions or flooded with 1 M of H2SO4 for three-electrode cell constructions. Example 4b: Cell construction of Pristine Carbon Felt The pristine carbon felt was constructed in the following manner: a. Pristine carbon felt acted as the positive electrode without polymer coating. b. The carbon fabric of 3 cm x 3 cm was used as the negative electrode. c. A separator soaked in 6M NaBr used for the cell construction. d. The positive and negative electrodes are either wetted with equally with 2 ml of 1 M of H2SO4 for two-electrode cell constructions or flooded with 1 M of H2SO4 for three-electrode cell constructions. Example 4c: Polymer brush, VP-PEGDA The VP-PEGDA was constructed in the following manner: a. At first, the carbon felt electrode was hydrated in a 20 ml jar with 50:50 ratio of water and ethanol mixture and allowed to marinate for few seconds until effervescence was excessive. Then, a monomer solution of 2ml was added to the jar and allowed to rest for 12 hours. The jar was placed into a water bath and allowed to cure for 16 h at 60 °C. Then, the thermally-cured carbon felt material was collected and dried on a hot plate. The final product acted as the positive electrode. b. The carbon fabric of 3 cm x 3 cm was used as the negative electrode. c. A separator soaked in 6M NaBr used for the cell construction. The positive and negative electrodes were wetted equally with 2 ml of 1 M of H2SO4for two-electrode cell constructions or flooded with 1 M of H2SO4for three- electrode cell constructions. Example 5: Testing of cells from Examples 4a-4c The performance of the cells from Examples 4a-4c has been assessed using the following testing regime: (i) The cells were charged and discharged at various current rates ranging from 10 mA to 50 mA at every 10 cycles interval. (ii) The potential limit was set to 0 V during discharge conditions for the cells. The results are shown in figures 3 and 4. Figure 3 shows the wide range of current selectivity of the VP-AMA coated polymer in comparison with the pristine and VP-PEGDA-based electrode cell. Figure 4 shows the VP- AMA coated positive electrode cell delivering a longer discharge time than the pristine electrode and the polymer brush electrode cell. Figure 3 shows the VP-AMA-supported electrode exhibited highest specific capacitance irrespective of current conditions in comparison with the pristine carbon felt and the VP- PEGDA cells. For instance, at 10 mA conditions, the VP-AMA cell delivered 52.5 F / g as compared to the VP-PEGDA (30 F / g) and pristine carbon felt (21 F / g). Figure 4 presents the 10thcharge discharge profiles of each cell at a specific condition of 10 mA for comparison purposes. It is very evident that VP-AMA cell exhibited longer discharge time as compared to the VP-PEGDA and pristine cell. Example 6: pH conditions The electric double layer capacitance value of the VP-AMA coated electrode was measured under the influence of various pH mediums (Table 1). The data was acquired using cyclic voltammetry technique under the scan rate condition of 5 mVs-1. Electrolyte medium, and pH Capacitance observed. ranges (F) Acid, 1 MH2SO4 1.02 (pH range: 0 to 3) Neutral, 1M Na2SO4 0.41 (pH of 7) Alkaline, 1 M KOH 0.81 (pH range of 10 to 13) Table 1. Example 7: Printed carbon electrode demonstration In addition to the brush coated electrodes, the performance of the VP-AMA powder, as prepared according to example 1, was investigated using a typical symmetric planar carbon electrode. These electrodes were prepared by making a slurry composed of various ratio of components such as activated carbon material, polyvinylidene fluoride (PVDF) binder, conductive carbon (carbon black), and with or without VP-AMA polymer particle as listed in the Table 2, below. Cell ID Slurry component ratio VP-AMA 9 wt% 73:9:9:9 (activated carbon: conductive carbon: VP-AMA: PVDF) VP-AMA 13 wt.% 69.6: 13: 8.7 :8.7 (activated carbon: VP-AMA: conductive carbon :PVDF) VP-AMA 15 wt.% 67.6: 15: 8.7 :8.7(activated carbon:VP-AMA: conductive carbon: PVDF) Control 80: 10: 10 (activated carbon: conductive carbon: PVDF) Table 2. The slurry was mixed, using N-methyl-2-pyrrolidone (NMP) solvent of a suitable volume, with the assistance of Thinky mixer equipment for a period of 6 minutes. The homogeneously mixed slurry was then coated onto Graphite foil and vacuum-dried overnight at a temperature of 80°C. The dried electrodes, with an area of 3 x 3 cm and a specified thickness, were used to sandwich the VP-AMA membrane. Prior to cell assembly, both the positive and negative electrodes were wetted with 1 M H2SO4electrolyte, with a volume of 0.5 ml applied to each electrode. Further, to demonstrate the electrochemical cell performance, galvanostatic cycling study at the constant current value of 10 mA. Figure 5 (a & b) represents the capacitance and energy density plots of VP-AMA cells in comparison with a control cell. The increment in cell performance was observed which follows the trend of VPA-AMA 15 wt.%> VPA-AMA 13wt.%> VPA-AMA 9 wt.%. This study reveals the potential use of VP-AMA powder as an active binder in a typical symmetric planar carbon electrode.

Claims

Claims 1. An ink composition comprising hydrophilic, cross-linked polymer particles.

2. An ink composition according to claim 1 further comprising a binder.

3. An ink composition according to claim 1 or claim 2 further comprising a liquid dispersion medium.

4. An ink composition according to any one of the preceding claims, wherein the hydrophilic, cross-linked polymer particles comprise at least one type of hydrophilic repeat units selected from the list consisting of 1-vinyl-2-pyrrolidone, 2- hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-acrylamido-2-methyl- propanesulfonic acid, acrylic acid, methacrylic acid, ethyl acrylate and N-[3- (dimethylamino)propyl]methacrylate.

5. An ink composition according to claim 4, wherein the hydrophilic, cross-linked polymer particles comprise 1-vinyl-2-pyrrolidone repeat units.

6. An ink composition according to claim 5, wherein the hydrophilic, cross-linked polymer particles comprise at least one additional type of repeat units, wherein the additional type of repeat units is not vinylpyrrolidone repeat units.

7. An ink composition according to any one of the preceding claims, wherein the hydrophilic, cross-linked polymer particles comprise at least one type of hydrophobic repeat units.

8. An ink composition according to claim 7, wherein the hydrophobic repeat units are selected from the list consisting of methyl methacrylate, acrylonitrile and allyl methacrylate.

9. An ink composition according to claim 8, wherein the hydrophobic repeat units are selected from the list consisting of methyl methacrylate and allyl methacrylate.

10. An ink composition according to claim 6, wherein the at least one additional type of repeat units are selected from the list consisting of allyl methacrylate, methyl methacrylate, acrylonitrile, 2-hydroxyethyl acrylate, and 2-hydroxyethyl methacrylate.

11. An ink composition according to claim 6, wherein the hydrophilic, cross-linked polymer particles comprise vinylpyrrolidone repeat units and methyl methacrylate repeat units.

12. An ink composition according to claim 6, wherein the hydrophilic, cross-linked polymer particles comprise vinylpyrrolidone repeat units and allyl methacrylate repeat units.

13. An ink composition according to any one of claims 6 to 12, wherein the ratio of vinylpyrrolidone repeat units : at least one additional type of repeat units is from 1:30 to 30:1, preferably 1:10 to 10:1, more preferably from 1:5 to 5:

1.

14. An ink composition according to any one of claims 6 to 13, wherein the ratio of vinylpyrrolidone repeat units : at least one additional type of repeat units is 4:

1.

15. An ink composition according to any one of the preceding claims, wherein the hydrophilic, cross-linked polymer particles have a particle size in the range of from 1 μm to 500 μm.

16. An ink composition according to any one of the preceding claims, wherein the hydrophilic, cross-linked polymer particles comprise a cross-linker selected from the list consisting of allyl methacrylate, ethylene glycol dimethacrylate, vinyl methacrylate, divinyl benzene, bisphenol A glycerolate dimethacrylate, poly(ethylene glycol) diacrylate, di(ethylene glycol) diacrylate, tetra(ethylene glycol) diacrylate and N,N’-methylenebis(acrylamide).

17. An ink composition according to claim 16, wherein the crosslinking agent is allyl methacrylate.

18. An ink composition according to any one of the preceding claims, wherein the amount of cross-linker is from 0.5 wt.% to 10 wt.%, preferably 0.5 to 5 wt.%, more preferably 0.5 to 1 wt.% of the respective polymer formulation.

19. An ink composition according to any one of claims 2 to 18, wherein the binder is selected from the list consisting of polyvinyl alcohol and polyvinylidene fluoride, preferably polyvinyl alcohol.

20. An ink composition according to any one of claims 2 to 19, wherein the hydrophilic cross-linked polymer acts as an active binder.

21. An ink composition according to any one of claims 2 to 20, wherein the ratio of hydrophilic, cross-linked polymer particles: binder is from 50 : 50 to 90 : 10, preferably 60 : 40 to 90 :

10.

22. An ink composition according to any one of the preceding claims, wherein the composition further comprises a carbonaceous material.

23. An ink composition according to claim 22, wherein the carbonaceous material is selected from the list consisting of activated carbon powder, carbon black, graphene powder, carbon nanotubes, or a combination thereof.

24. An ink composition according to claim 23, wherein the carbonaceous material is carbon black.

25. An ink composition according to any one of the claims 3 to 24 wherein the liquid dispersion medium comprises an aqueous solvent.

26. An ink composition according to claim 25, wherein the aqueous solvent is water.

27. An ink composition according to any one of claims 3 to 26, wherein the liquid dispersion medium comprises an organic solvent.

28. An ink composition according to claim 27, wherein the organic solvent is selected from the list consisting of N-Methyl-2-pyrrolidone, isopropyl alcohol, ethanol, propan-2-ol, and ethylene glycol.

29. An ink composition according to any one of the preceding claims, wherein the composition further comprises redox-active additives.

30. An ink composition according to claim 29, wherein the redox-active additive is selected from the list consisting of quinone and anthraquinone derivatives, metal oxides, and redox active polymers such as polyaniline and poly(3,4- ethylenedioxythiophene).

31. A method of forming the ink composition of any one of the preceding claims, wherein the hydrophilic, cross-linked polymer particles are formed by:a. adding monomer units, and at least one cross-linker to an aqueous solution to form a monomer mixture; b. polymerising the monomer mixture to form a cross-linked hydrophilic polymer; c. drying the cross-linked hydrophilic polymer; and d. forming into a polymer particle material.

32. A method of forming an ink composition according to claim 31, wherein the method further comprises the following step: e. adding the hydrophilic, cross-linked polymer particles into an ink mixture comprising a binder and / or a liquid dispersion medium.

33. A method according to claim 32, wherein the ink mixture further comprises a carbonaceous material.

34. A method according to any one of claims 31 to 33, wherein the monomer units in step a. are 1-vinyl-2-pyrrolidone repeat units.

35. A method according to claim 34, wherein step a. further comprises adding an additional type of monomer to an aqueous solution to form a co-monomer mixture.

36. A method according to claim 35, wherein the additional type of monomer is a hydrophobic monomer.

37. A method according to claim 36, wherein the hydrophobic monomer is selected from the list consisting of methyl methacrylate, acrylonitrile and allyl methacrylate.

38. A method according to any one of claims 33 to 37 wherein the carbonaceous material is selected from the list consisting of activated carbon powder, carbon black, graphene powder, carbon nanotubes, or a combination thereof, preferably wherein the carbonaceous material is carbon black.

39. A method according to any one of claims 32 to 38, wherein the binder is selected from the list consisting of polyvinyl alcohol and polyvinylidene fluoride, preferably polyvinyl alcohol.

40. A method according to any one of claims 32 to 39, wherein the liquid dispersion medium comprises an aqueous solvent.

41. A method according to any one of claims 31 to 40, wherein the cross-linker is selected from the list consisting of allyl methacrylate, ethylene glycol dimethacrylate, vinyl methacrylate, divinyl benzene, bisphenol A glycerolate dimethacrylate, poly(ethylene glycol) diacrylate, di(ethylene glycol) diacrylate, tetra(ethylene glycol) diacrylate and N,N'-methylenebis(acrylamide), preferably allyl methacrylate.

42. An electrode comprising an electronically conductive surface, with a homogenised ink composition according to any one of claims 1-30 deposited on the electrode and / or within electrode structure.

43. An electrode according to claim 42, wherein the electrode further comprises carbonaceous material.

44. An electrode according to claim 42 or 43, wherein the electronically conductive surface is selected from the list consisting of carbon foil, carbon paper, carbon fabric, carbon felt, glassy carbon sheet, boron doped diamond, nitrogen doped diamond, aluminium foil, and stainless steel.

45. An electrode according to any one of claims 42 to 44, wherein the electrode is a carbon electrode.

46. An electrode according to any one of claims 42 to 45, wherein the electrode is a negative electrode.

47. An electrode according to any one of claims 42 to 45, wherein the electrode is a positive electrode.

48. A method of forming an electrode, the method comprising the following steps: a. homogenising the ink composition of any one of claims 1-30; and b. depositing the homogenised ink composition onto an electronically conducting surface.

49. The method according to claim 48, wherein in step b. the method of ink deposition is selected from the list consisting of bar coating, doctor blading, spray coating, ink jet printing, transfer printing, screen printing, brush-coating, vacuum filtration and stamping.

50. The method according to claim 48 or claim 49, wherein the electronically conductive surface is heated during step b.

51. An energy storage device comprising an electrode as described in any one of claims 42 to 47.

52. An energy storage device according to claim 51, wherein the energy storage device is a supercapacitor.

53. An energy storage device according to claim 52, wherein the energy storage device is a hybrid supercapacitor.

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