Static energy storage cell

Abstract

A static energy storage cell comprising first and second carbon electrodes 101a, 101b and an electrolyte system having a membrane 103 comprising a cross-linked hydrophilic polymer hydrated in an aqueo

Description

Field of the invention The present invention relates to a static energy storage cell comprising a dual or multi redox electrolyte and an ion selective polymer membrane. Background of the invention Non-flow cells, or static energy storage cells are a type of energy storage cell. These cells do not comprise pumps to pump the liquids through the system. Static metal halide non-flow batteries have not been widely commercialised due to issues with charge rates, cost and cycle life. Low charge rates are a result of high resistance materials and electrodes. Mitigation strategies may be used and are aimed at stopping unfavourable redox active species migrating across the separator and interacting with the opposite electrode redox reaction. These interactions (also known as crossover) lead to a drop in energy out and coulombic efficiency eventually leading to the device's end of life. Furthermore, in some instances the use of commercial ion separation membranes have been used in order to prohibit unfavourable crossover; however, these membranes often have a relatively low ionic conductivity and high cost per m2. Low cycle lifetimes may be attributed to parasitic side reactions such as gas evolution. There therefore exists a need for an improved energy storage cell. In particular, an energy storage cell which can overcome one or more of the disadvantages associated with known static non-flow redox batteries. Summary of the invention In a first aspect, the present invention relates to a static energy storage cell comprising: (i) a first electrode and a second electrode, (ii) an electrolyte system comprising: a. a membrane comprising a cross-linked hydrophilic polymer hydrated in an aqueous solution, wherein the membrane is situated between the first electrode and the second electrode, b. a first electrolyte comprising a first species which can undergo a redox reaction at the first electrode, wherein the first electrolyte is situated between the membrane and the first electrode, c. a second electrolyte comprising a second species which can undergo a redox reaction at the second electrode, wherein the second electrolyte is situated between the membrane and the second electrode, wherein the first electrode and the second electrode are carbon electrodes, wherein the first species is a metallic species, and wherein the second species is a halogen species. According to another aspect of the present invention there is a method of manufacture of a static energy storage cell comprising: a. soaking a hydrophilic cross-linked polymer membrane in an aqueous solution to form a hydrated hydrophilic cross-linked polymer membrane, b. assembling a cell stack comprising two carbon electrodes either side of the hydrated hydrophilic cross-linked polymer membrane, c. adding an electrolyte composition comprising an oxidant species between the membrane and one carbon electrode, d. adding an electrolyte composition comprising a reductant species between the membrane and the other carbon electrode. Advantageously, the hydrated cross-linked hydrophilic polymer membrane of the present invention provides desirable ion selectivity. The ion selectivity of the polymer membrane prohibits unfavourable crossover / ion migration of species in the energy storage cell. As a result, the hydrated cross-linked hydrophilic polymer membrane of the present invention prevents a drop in the energy out and coulombic efficiency of the energy storage cell. Furthermore, the hydrated cross-linked hydrophilic polymer membrane of the present invention advantageously has a high ionic conductivity whilst maintaining good mechanical stability. This combination of ion selectivity, ionic conductivity and mechanical stability facilitates high power and quick charge / discharge times compared to established static redox batteries using similar redox pairs. High ionic conductivity for certain favourable ions passing through the membrane enables a lower resistance system compared to traditional ion selective membranes. Further, the good mechanical stability of the hydrated cross-linked hydrophilic polymer membranes of the present invention allows for electrochemical cells to operate with a high pressure being applied between electrodes, compared to, for example, electrochemical cells with a hydrogel separator. A high pressure between electrodes reduces contact resistance within an electrochemical cell, and thus allows for a more efficient and high-power density cell. Advantageously, the combination of (i) the carbon electrodes (ii) the electrolyte system comprising at least two species: one of which can be reduced on one carbon electrode, and one of which can be reduced on the other carbon electrode and (iii) the membrane comprising a cross-linked hydrophilic polymer results in a system which exhibits exceptional ion selectivity across the membrane, yielding a desirable coulombic and energy efficiency cell at low cost. The energy density of the energy storage cells of the present invention are improved compared to redox flow batteries using similar redox pairs. Furthermore, the cross-linked hydrophilic polymer membrane uses low-cost input raw materials and economical production processes compared to high cost traditional ion selective separators. Advantageously, the addition of non-redox active ions, such as those provided by way of a supporting salt, alongside the redox active ions present in the electrolyte means that the electrochemical cell can implement two storage mechanisms. These are: (i) physical storage of ions in an electric double layer mechanism (EDLC); and (ii) chemical redox storage in redox storage reactions of the ions. In particular, the proportion of EDLC storage is between 2 to 50% of the total storage of the system. The provision of the chemical storage mechanisms by the hydration with the ionic salt can enable a higher energy density relative to EDLC only storage systems. This supercapacitance element enables fast response and charge / discharge times when coupled with the aforementioned attributes to create a 'hybrid' energy storage technology. 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 simplified diagram of a single energy storage cell according to the present invention. Figure 2 is a simplified diagram, of several, single energy storage cells according to the present invention connected in series. Figure 3 is a simplified diagram of a H-cell where two electrolytes are separated by a hydrated cross-linked hydrophilic polymer membrane according to the present invention. Figure 4 is a graph showing the energy out comparison of single and dual electrolyte systems. Figure 5 is a graph showing the coulombic efficiency comparison of single and dual electrolyte systems. Figure 6 is a graph showing the halogen absorbing ability of the woven activated carbon fabric cathodes. Figure 7 is a graph showing the coulombic efficiency comparison of three example polymer membranes according to the present invention. Figure 8 is a graph showing the coulombic efficiency comparison of a symmetric and asymmetric electrolyte composition according to the present invention. Detailed description Figure 1 shows an electrochemical cell in accordance with the present invention. The electrochemical cell of Figure 1 comprises a first carbon electrode 101a, a second carbon electrode 101b, an electrolyte system comprising a cross-linked hydrophilic polymer hydrated in an aqueous solution 103 located between the two electrodes, a first electrolyte 102a, a second electrolyte 102b and carbon current collectors at the device terminals 104. The first electrolyte comprises a metal ionic redox species which can undergo a redox reaction at the first electrode, and the second electrolyte comprises a halogen redox species which can undergo a redox reaction at the second electrode. The electrolyte system of the present invention comprises: • a membrane comprising a cross-linked hydrophilic polymer hydrated in an aqueous solution, • a first electrolyte comprising a first species, wherein the first species is a metallic species; and • a second electrolyte comprising a second species, wherein the second species is a halogen species. Membrane As used herein, 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. As used herein, 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. As used herein, the term "about", when qualifying a number or value, is used to refer to values that lie within ± 5% of the value specified. For the avoidance of doubt, when a term such as "0 to 5" is employed herein, this will be understood by the skilled person to mean 0 and 5, inclusively. Unless otherwise stated, the same reasoning will apply to other such terms used herein. As used herein, the term "membrane" refers to a separator in an electrochemical cell which provides some selectivity to the rate at which some ions pass through. As used herein, the term "hydrated cross-linked hydrophilic polymer membrane" means that the cross-linked hydrophilic polymer membrane is hydrated such that there has been uptake of aqueous solution by the hydrophilic polymer. 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 "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. References herein to the term "polymer" will be understood to mean the co-polymer formed from the co-monomer mixture. 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 "cross-linker" refers to a molecular compound capable of forming chemical bonds between polymer chains. The cross-linker may be hydrophobic or hydrophilic. The cross-linker may be a type of monomer. As used herein, the term "thermoset" refers to a material which has a high degree of crosslinking which forms a three-dimensional network of bonds and does not have a melting transition. Accordingly, the cross-linked hydrophilic polymers of the present invention may be described as thermoset polymers. The inventors believe that the polymers used in the present invention are thermoset polymers. As used herein, the term "self-supporting" refers to a polymer membrane which does not need to be polymerized onto a substrate. Accordingly, static energy storage cells which comprise the self-supporting membrane do not require a further additional solid support, such as a glass sheet. As used herein, the term "water properties" when used in relation to a polymer material, refers to the properties and behaviour of that polymer material in relation to water and other aqueous environments, such as saline solution i.e. its hydrophilicity and stability in an aqueous environment. As used herein, the term "homogenous", when used in relation to a polymer material, refers to a polymer material whose physical properties (e.g. ionic properties and water properties) are substantially uniform throughout its entire structure. As used herein, the term "isotropic", when used in relation to a polymer material, refers to a polymer material whose properties are the same in all orientations. As used herein, the term "homogenous" when used in relation to a co-monomer mixture, refers to a co-monomer solution or dispersion comprising miscible monomers that are uniformly dissolved or mixed. As used herein, the term "hydrophilic polymer" refers to a polymer that dissolves in water when it is not cross-linked and absorbs water and swells to form a stable elastic solid when cross-linked. As used herein, the term "hydrophilic monomer" takes its usual definition in the art, and so refers to a monomer with an affinity for water molecules. The term "hydrophobic monomer" also takes its usual definition in the art, and so refers to a monomer that repels water molecules. As used herein, the term "amino acid" takes its usual definition in the art, and so refers to an organic compound with amino and carboxylic acid functional groups, and a side-chain that is specific to each amino acid. The term encompasses the traditional "natural" amino acids but also any compound with an amino acid backbone (i.e. with any side-chain). As used herein, the term "hydrophilic polymer membrane" refers to a continuous isotropic and homogenous layer of hydrophilic polymer. The static energy storage cell of the present invention comprises a hydrated cross-linked hydrophilic polymer membrane. The hydrated cross-linked hydrophilic polymer membrane is an ion selective separator. The membrane is selective such that it allows favorable ions to pass through, while stopping unfavorable ions from passing through. The membrane is reguired to ensure separation of the redox species that are formed at the anode and cathode during charging. If a non-selective separator is used, these redox products will crossover to the counter electrode and react spontaneously with their counterparts. This means that electrons can move across the cell, but not through the external circuit (as they do in charge and discharge). This in turn lowers the efficiency of the cell, as electrons that were moved during charging are no longer stored / available and so there are less electrons available for discharge. The polymer membrane therefore mitigates against parasitic species crossover and provides dual and multi redox electrolyte cells with a substantially greater energy and coulombic efficiency than non-selective separators. Without wishing to be bound by theory, it is believed that the irregular porous nature of the hydrophilic cross-linked material creates a tortuous diffusion path for different species through the polymer membrane, slowing crossover of unfavourable species. By way of example, when the electrolyte comprises zinc bromide (ZnBn), Bq is formed during charging and is prevented from migrating through the hydrated cross-linked hydrophilic polymer membrane and reacting with the solid Zn metal deposited on the opposite electrode for an extended period of time. Ionic conductivity is necessary for redox reactants and products to move to and from the electrode surfaces within the dual redox cell of the present invention. Advantageously, the hydrated cross-linked hydrophilic polymer membrane of the present invention provides good ionic conductivity between electrodes. For instance, when the electrolyte comprises zinc bromide (ZnBrz), the hydrated cross-linked hydrophilic polymer membrane allows Zn2+ and Br ions to pass with ease, as well as water molecules, H+, H3O+, OH- and any ions of a supporting salt, for example. A high ionic conductivity is advantageous for efficient and high power electrochemical storage devices because the ions can flow and equilibrate between charging and discharging with ease. Hydrogels are reported to provide ion selectivity. These gels possess low mechanical strength, however, and as such can only be incorporated within electrochemical cells with low pressure applied between electrodes so as to maintain a homogeneous gel layer, or by incorporating a commercial separator such as glass fiber to provide the mechanical stability. The low pressure applied to these electrochemical cells leads to low efficiency and low power density as the contact between cell components is poor and contact resistance is high. Additionally, the incorporation of a gel plus a commercial separator increases the mass of components required and thus limits the energy density of such electrochemical cells. Advantageously, the hydrated cross-linked hydrophilic polymer membrane of the current invention provides the important ion selectivity and high ionic conductivity whilst maintaining good mechanical stability. So much so that the hydrated cross-linked hydrophilic polymer membrane requires no supporting separator such as glass fiber. The hydrated cross-linked hydrophilic polymer membrane possesses beneficial mechanical robustness from strong covalent bonds within the polymer backbone, and the hydrophilic / hydrophobic moieties described below. The good mechanical stability of the hydrated cross-linked hydrophilic polymer membranes of the present invention allows for electrochemical cells to operate with a high pressure being applied between electrodes, compared to electrochemical cells with a hydrogel separator, for example. A high pressure between electrodes reduces contact resistance within an electrochemical cell, and thus allows for a more efficient and high power density cell. Although the hydrated cross-linked hydrophilic polymer membranes described in the present invention provide good mechanical robustness and a high pressure between electrodes compared with hydrogels, for example, they may additionally be composited within a solid support. Such solid supports may include glass fibers, papers, woven polymer sheets, or any materials the skilled person could composite with these cross-linked hydrophilic polymer membranes. Advantageously, the membrane of the present invention does not contain any fluorinated components, unlike, for example, typical costly ion exchange membranes which rely on a polytetrafluoroethylene supporting backbone. The manufacturing of these fluorinated polymers requires toxic reagents which may leach into the environment and cause environmental damage, and when consumer goods contain such fluorinated polymers strict safety controls are required in the event of a fire, as extremely toxic fluorinated components may be evolved. In an embodiment, the the membrane is a self-supporting membrane. The polymers of the type that can be used in the invention are described herein. The cross-linked hydrophilic polymer is preferably homogenous and isotropic in its ionic properties, and preferably also in its water properties. It is preferably hydrophilic and ionically conductive, throughout its entire structure. In an embodiment, the cross-linked hydrophilic polymer is formed by a process comprising the steps of: a. adding at least one hydrophilic monomer, and at least one cross-linker to an aqueous solution to form a co-monomer mixture; b. polymerising the co-monomer mixture. In an embodiment, the co-monomer mixture of step a) further comprises an amino acid. In an embodiment, the cross-linked hydrophilic polymer is formed by a process comprising the steps of: a. adding at least one hydrophilic monomer, at least one hydrophobic monomer and at least one cross-linker to an aqueous solution to form a co-monomer mixture; b. polymerising the co-monomer mixture. Preferably, the co-monomer mixture is polymerised via free-radical polymerisation. In an embodiment, the co-monomer mixture of step a) further comprises an amino acid. In an embodiment, the polymer membrane may be a cross-linked hydrophilic polymer produced by a process comprising the steps of: a. providing a co-monomer solution comprising at least one hydrophobic monomer, at least one hydrophilic monomer, at least one amino acid and at least one crosslinker; and b. polymerising the co-monomer solution. The cross-linked hydrophilic polymer membranes of the type that can be used in the invention are described further below. Examples of such polymers may also be described in WO 2017 / 153706, WO 2017 / 115064 and WO 2017 / 153705. In a preferred embodiment, the hydrophilic monomer is selected from the group consisting of l-vinyl-2-pyrrolidone, 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, 2-acrylamido-2-methyl-propanesulfonic acid, acrylic acid, methacrylic acid, ethyl acrylate, sodium 4-styrenesulfonate, 4,4'-diamino-2,2'-stilbenedisulfonic acid, bis[2-(methacryloyloxy)ethyl]phosphate, acrylamide and N-[3- (d imethyla mino)propyl] methacrylate. In a more preferred embodiment, the hydrophilic monomer is selected from the group consisting of l-vinyl-2-pyrrolidone and 2-hydroxyethyl methacrylate. In an embodiment, the hydrophobic monomer is selected from the list consisting of methyl methacrylate, acrylonitrile and allyl methacrylate. In an 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, preferably allyl methacrylate, poly(ethylene glycol) diacrylate and tetra(ethylene glycol) diacrylate. Preferably, the cross-linking agent is allyl methacrylate or ethylene glycol dimethacrylate. The hydrophobic monomer and the cross-linker may be the same or different. For example, both the crosslinker and the hydrophobic monomer may be allyl methacrylate. As used herein, the term "aqueous solution" refers to solutions, including water, wherein the solvent is water. The aqueous solutions referred to solutions where the aqueous component is between 1% and 100% of the solution composition, or preferably 50% to 100% of the solution composition. Preferably, the amount of aqueous solution in the co-monomer mixture must be sufficient to provide a uniformly mixed homogenous solution or dispersion. The amount of aqueous solution in the co-monomer mixture may be 1 % to 60% by weight, preferably 5% to 55% by weight, most preferably 35% to 55% by weight based on the total weight of the comonomer mixture. Preferably, the aqueous solution in the co-monomer mixture is 50% to 100% by weight water, and most preferably 100% by weight water, being distilled or deionised water. Preferably, the at least one amino acid is selected from phenylalanine, tryptophan, histidine, ethylenediaminetetraacetic acid (EDTA) and tyrosine, or a combination thereof. Even more preferably, the at least one amino acid is selected from phenylalanine and tryptophan, or a combination thereof. For instance, the polymer membrane may be a vinyl pyrrolidone formula (e.g., l-vinyl-2-pyrrolidone), cross-linked with AMA (allyl methacrylate) and containing at least one of the amino acids selected from the list consisting of phenylalanine, tryptophan, histidine, ethylenediaminetetraacetic acid (EDTA) and tyrosine, or a combination thereof. The inventors believe that the polymers described herein are thermoset polymers. In a preferred embodiment, the co-monomer solution further comprises a polymerisation initiator. As used herein, the term "polymerisation initiator" takes its usual definition in the art, and so refers to an agent capable of initiating the process of chemical polymerisation, for example free-radical polymerisation. Azobisisobutyronitrile (AIBN) 4,4'azobis(cyanovaleric acid), 2,2'-azobis(2-methylpropionamidine) dihydrochloride and 2-hydroxy-2-methylpriophenone are examples of such initiators. Azobisisobutyronitrile (AIBN) and 2,2'-azobis(2-methylpropionamidine) dihydrochloride have utility when polymerisation is by thermal means, and 2-hydroxy-2-methylpriophenone is suitable for use with UV polymerisation. Preferably, the polymerisation initiator may be azobisisobutyronitrile (AIBN), 2,2'-azobis(2-methylpropionamidine) dihydrochloride or 2-hydroxy-2-methylpriophenone. The polymer formulation is cast into a membrane using established manufacturing methods, such as casting into a mould, drop casting or doctor blading. The polymerisation reactions occur simultaneously (i.e., chain propagation and cross-linking) once initiated. Preferably, the polymerisation step is carried out by thermal, UV or gamma radiation. Without wishing to be bound by theory, the inventors believe that this method of manufacturing the polymer membrane results in a thermoset copolymer. The hydrated cross-linked hydrophilic polymer membrane is hydrated by an aqueous solution. In an embodiment, the cross-linked hydrophilic polymer membrane is hydrated in water. In an embodiment, the cross-linked hydrophilic polymer membrane is hydrated in an aqueous salt solution. Preferably, the aqueous salt solution is an inorganic halide salt solution, preferably a metal halide salt solution. In a particular embodiment, the aqueous salt solution has a concentration of from 0.1 to 10 M, preferably from 2 to 6 M, more preferably from 3 to 6 M. As used herein, the term "inorganic halide salt" refers to a salt comprising halide anions and inorganic cations. The term "inorganic" in "inorganic halide salt" refers to cationic species which lack carbon-hydrogen bonds. As used herein, the term "metal halide salt" refers to a salt comprising halide anions and metal cations. Preferably, the metal halide salt solution comprises a metallic species selected from the list consisting of a transition metal or a group 1, 2, 13, 14, 15 or 16 metal. Preferably, the metal halide salt solution comprises a metallic species selected from the list consisting of a transition metal ora group 13, 14, 15 or 16 metal. In an embodiment, the metallic species is iron, cobalt, nickel, copper or zinc, preferably zinc. Preferably, the metal halide salt solution comprises a halogen species selected from the list consisting of bromine, iodine or chlorine, preferably bromine. In an embodiment, the cross-linked hydrophilic polymer membrane is hydrated in the first electrolyte and / or the second electrolyte. In an embodiment, the aqueous salt solution may be the first electrolyte and / or the second electrolyte. Hydration of the cross-linked hydrophilic polymer membrane is performed by immersing the cross-linked hydrophilic polymer membrane in an aqueous solution such that there is some expansion or contraction of the hydrated cross-linked hydrophilic polymer membrane. In an embodiment, the hydration of the cross-linked hydrophilic polymer membrane is performed by immersing the cross-linked hydrophilic polymer membrane in an aqueous solution for such a time that there is no more expansion or contraction of the hydrated cross-linked hydrophilic polymer membrane. The cross-linked hydrophilic polymer membrane may be hydrated, at least to some degree, prior to it being placed into the energy storage cell. Hydration of the polymer membrane will also occur in the energy storage cell, when the membrane is contacted by the first electrolyte and the second electrolyte. The hydrated cross-linked hydrophilic polymer membrane of the present invention advantageously maintains sufficient hydration of the energy storage cell described herein, such that electrolyte starvation is minimised and the electrochemical cell performs well for an extended period of time or cycle number. The electrolyte system As used herein, the term "electrolyte" takes its usual definition in the art and so refers to a solution of cations and anions dissolved in a solvent, such as an aqueous solution (e.g., water), acetonitrile, propylene carbonate or tetra hydrofuran. The electrolyte may be an aqueous electrolyte. As used herein, the term "aqueous electrolyte" takes its usual definition in the art, and so refers to an aqueous solution containing cations and anions. As used herein, the term "organic electrolyte" refers to an organic solvent containing an ionically conducting species (i.e., anions or cations). As used herein, the term "redox" (reduction-oxidation) takes its usual definition in the art and so refers to a type of chemical reaction in which the oxidation states of a substance changes. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a decrease in the oxidation state. As used herein, the term "oxidant" refers to the species which gains electrons during the charging of the cell. As used herein, the term "reductant" refers to the species which loses electrons during the charging of the cell. The static energy storage cell of the present invention comprises an electrolyte system comprising: • a first electrolyte comprising a first species which can undergo a redox reaction at the first electrode, wherein the first electrolyte is situated between the membrane and the first electrode, • a second electrolyte comprising a second species which can undergo a redox reaction at the second electrode, wherein the second electrolyte is situated between the membrane and the second electrode, wherein the first electrode and the second electrode are carbon electrodes, wherein the first species is a metallic species, and wherein the second species is a halogen species. The first species and the second species are therefore different species. Accordingly, the static energy storage cell of the present invention comprises an electrolyte system comprising: • a first electrolyte comprising a metallic species which can undergo a redox reaction at the first carbon electrode, wherein the first electrolyte is situated between the membrane and the first carbon electrode, • a second electrolyte comprising a halogen species which can undergo a redox reaction at the second carbon electrode, wherein the second electrolyte is situated between the membrane and the second carbon electrode. One species (e.g., the first species) undergoes an oxidation reaction on one electrode during cell discharge. The other species (e.g., the second species) undergoes a reduction reaction on the other electrode during cell discharge. During cell charging, the first species will be reduced at one electrode, and the second species will be oxidised at the other electrode. The first electrolyte may comprise more than one species (e.g., in addition to the first species). These one or more additional species may also be a redox active species. Accordingly, in an embodiment, the first electrolyte comprises a first species and one or more additional redox active species. The second electrolyte may also comprise more than one species (e.g., in addition to the first species). These one or more additional species may also be a redox active species. Accordingly, in an embodiment, the second electrolyte comprises a second species and one or more additional redox active species. In an embodiment, both the first electrolyte and the second electrolyte comprise more than one species. The electrolyte system used in the energy storage cell of the present invention may be described as a dual redox electrolyte system or a multi redox electrolyte system. As used herein, the term "dual redox electrolyte system" refers to an electrolyte system which involves two reactants within the electrolyte system that undergo redox reactions on the surface of opposing electrodes during charging and discharging. For example, when the electrolyte system of the energy storage cell is a dual redox electrolyte system, one species (e.g., the first species) undergoes an oxidation reaction on one electrode during cell discharge. The other species (e.g., the second species) undergoes a reduction reaction on the other electrode during cell discharge. During charging, the species which was oxidised during discharge (e.g., the first species) will be reduced. During charging, the species which was reduced during discharge (e.g., the second species) will be oxidised. As used herein, the term "multi redox electrolyte system" refers to an electrolyte system which involves more than two species which undergo redox reactions on the surface of opposing electrodes during cell charging and discharging. Accordingly, in the present invention, when the first electrolyte comprises more than one redox species, the electrolyte system may be described as a multi redox electrolyte system. When the second electrolyte comprises more than one redox species, the electrolyte system may be described as a multi redox electrolyte system. When both the first and second electrolytes comprise more than one redox species, the electrolyte system may be described as a multi redox electrolyte system. A multi redox electrolyte system may also refer to an electrolyte system wherein one of the first species or second species is a species which has more than one oxidation state change during charging / discharging. An example of a species with more than one oxidation state change during charging / discharging state is vanadium. A multi redox species may also refer to an electrolyte system wherein one of the first species or the second species is a species which has more than one oxidation state during charging / discharging, and one of the first species or the second species comprises more than one redox species. In an embodiment, the first species undergoes a singular redox reaction on the first electrode. In an embodiment, the second species undergoes a singular redox reaction on the second electrode. In an embodiment, the first species undergoes a multi redox reaction on the first electrode. In an embodiment, the second species undergoes a multi redox reaction on the second electrode. In dual or multi redox electrolyte systems, energy is stored within the redox products which enhances the capacity of an energy storage device, in addition to any other storage mechanisms, such as electric double layer capacitance (EDLC). In an embodiment, the first electrolyte and the second electrolyte may be the same electrolyte composition. In this embodiment, the first electrolyte therefore comprises both the first and second species, and the second electrolyte also comprises the first and the second species. The difference between the first electrolyte and the second electrolyte is that, during the operation of the cell, the first species (as present in the first electrolyte) undergoes a redox reaction at the first electrode, whereas in the second electrolyte, the second species undergoes a redox reaction at the second electrode. In an embodiment, the first electrolyte and the second electrolyte may be different. The solvent that the ions are dissolved in may be different. Alternatively, or additionally, one or more of the ion species may be different. In an embodiment, the first electrolyte is an aqueous electrolyte. In an embodiment, the second electrolyte is an aqueous electrolyte. In an embodiment, the first electrolyte is an aqueous electrolyte and the second electrolyte is an aqueous electrolyte. The first species, which is a metallic species, may be depicted as Mn+, wherein n is an integer (e.g., 1, 2, 3, 4 or 5) representing the oxidation state on the species M. The second species, which is a halogen species, may be depicted as Xy\ wherein y is an integer (e.g., 1, 2, 3, 4 or 5) representing the oxidation state on the species X. In an embodiment, the first species (Mn+) and the second species (Xy) come homogeneously from the same species. By way of example, when the first species is Zn2+ and the second species is Br, the zinc ions and bromide ions come homogeneously from zinc bromide (ZnBrz). In an embodiment, the first species (Mn+) and the second species (Xy) come homogeneously from a different species. For example, when the first species is Zn2+ and the second species is Br, the zinc ions may come from zinc chloride and the bromide may can from sodium bromide. During charging of the cell, the following reaction occurs at the negative electrode: Mn+ + e^ M When the first species is zinc, this reaction can be depicted as: Zn2+ + 2e Zn(S) During charging of the cell, the following reaction occurs at the positive electrode: Xy X + e- When the second species is bromide, this reaction can be depicted as: 2Br Br2(aq)+ 2e In an embodiment, the first electrolyte is a metal halide salt solution. In an embodiment, the second electrolyte is a metal halide salt solution. In an embodiment, the first and second electrolytes are metal halide salt solutions. Examples of metallic halide salts that may be present in the electrolyte system of the energy storage cell of the present invention include zinc bromide (ZnBr2), zinc iodide (Znl2), iron bromide (FeBn), iron (II) iodide (Fel2) and vanadium bromide (VBn). The electrolyte system may further comprise vanadium ions. Vanadium ions may be present by adding vanadium pentoxide (V2Os) to the electrolyte solution. In an embodiment, the first and / or second electrolyte is a zinc / bromine / vanadium solution. Preferably, the dual redox system comprises an inorganic halide salt at a molarity of 0.1 to 30 M, preferably 1 to 15 M, more preferably 2 to 6 M. The first species is a metallic species. In an embodiment, the metallic species is a is a transition metal or a group 13, 14, 15 or 16 metal. In an embodiment, the metallic species is iron, cobalt, nickel, copper or zinc, preferably zinc. When the first species is a metallic species, during the charging of the cell, the reduction of the metallic species on the negative electrode may be a metal depositing step. Metals that undergo a metal depositing reaction include, but are not limited to, iron, cobalt, nickel, copper, zinc, manganese, aluminium, tin, lead, bismuth, cadmium and titanium. Preferably, the metal halide salt comprises iron, cobalt, nickel, copper and / or zinc, preferably zinc. Alternatively, the balancing reduction reaction during charging could be of a metal ion (e.g., M3+, M2+) where the metal does not return to a neutral electronic state (i.e., 0), such as Fe3+ -» Fe2+. These reactions may also occur when the metal is in a molecular state (such as MnOT) or potentially complexed with an organic compound. Examples of these systems are: • Titanium, Ti3+Ti2+ • Vanadium, V3+ —> v2+ • Chromium, Cr3+ -► Cr2+ • Manganese, MnOT + 2H2O + 3e- -► MnO? + 4OIT • Iron, Fe3+ -► Fe2+ • Cobalt, Co3+ -+ Co2+ • Copper, Cu2+ -> Cu+ The second species is a halogen species, preferably bromine, iodine or chlorine, preferably bromine. In an embodiment, the first species is zinc and the second species in bromine. In an embodiment, the first and / or second electrolyte comprises non-redox active ions. These non-redox active ions may come from a supporting salt. In an embodiment, the first and / or second electrolyte may further comprise a supporting salt. Preferably, the first and second electrolyte comprises a supporting salt. The addition of a supporting salt into the electrolyte system is thought to improve the ionic conductivity of the electrolyte. In an embodiment, the supporting salt may be the same in the first and second electrolyte. In an embodiment, the supporting salt may be different in the first and second electrolyte. Preferably, the supporting salt is one or more from the list consisting of sodium chloride, sodium bromide, potassium chloride, potassium bromide, ammonium chloride, sodium acetate, potassium acetate, potassium perchlorate and sodium perchlorate. When a support salt is present in the electrolyte, the ratio of inorganic halide salt (e.g., metal halide salt): supporting salt is in a ratio of 10:1 to 1:10, preferably in a ratio of from 2:1 to 1:1. In an embodiment, when a zinc bromide electrolyte system is used (e.g., the inorganic halide salt is ZnBn) and sodium chloride (NaCI) is selected as the supporting salt, the ratio of ZnBrz : NaCI is preferably from 2:1 to 1:1. The addition of non-redox active ions, such as those provided by way of a supporting salt, alongside the redox active ions present in the electrolyte means that the electrochemical cell can implement two storage mechanisms. These are: (i) physical storage of ions in an electric double layer mechanism (EDLC); and (ii) chemical redox storage in redox storage reactions of the ions. Advantageously, the addition of an EDLC energy storage mechanism to the multi-redox energy storage system allows for faster rates of charging and discharging, thus a higher power dense device. The non-redox active ions provide the EDLC storage mechanism and the redox active species (e.g., the dual or multi redox active system) provide the chemical redox storage. As used herein, the term "non-redox active species" refers to a species which does not undergo a reduction or oxidation reaction at the voltages typically experienced by an energy storage cell. For example, the typical voltage range experienced by a single cell is less than 5 V, typically less than 2.5 V. However, as the skilled person will appreciate, when the cells are connected in series or parallel, the stack may be charged to a greater potential than 5 V. However, each individual cell would not typically surpass 5 V under normally operating conditions. The electrochemical cell of the present invention may also include an additive in order to improve metal plating reversibility. These additives are selected from the list consisting of include nicotinamide, polyethylene glycol (PEG 200), tris(trimethylsilyl) borate (TMSB), DMSO, defoaming agents and brightening agents. Electrodes The electrochemical cell of the present invention comprises two electrodes (an anode and a cathode). The electrodes act as charge collectors and to conduct current into and out of the cell during charging or discharging. The electrodes are carbon electrodes. The electrodes present in the cell may be of the same type or may be of a different type of carbon. The preferred carbon electrode is a carbon foil electrode ora graphite electrode. The energy storage cell of the present invention comprises: • a carbon anode, • a carbon cathode, and • carbon current collectors. Preferably, the carbon current collectors are carbon foil current collectors. Electrodes of either type (e.g., metal type or carbon type) may be coated with activated carbon to provide an extended surface area. It is preferable for the cathode, where halide oxidisation occurs, to have a high surface area, to increase the reactivity. It is preferable for the anode to have a high electric conductivity, typically more graphitised, to enable smoother, more efficient metal plating during the metal ion reduction reaction. As used herein, the term "activated carbon layer" refers to a layer of carbon material that includes pores to increase its surface area. Examples of activated carbon include activated carbon powder and activated carbon fibres that may be woven, felted or pressed into a cloth-like fabric, or combinations thereof. Preferably, the activated carbon layer comprises electrically conductive carbon in the form of particulates (such as activated carbon powder), a plurality of individual activated carbon fibres, a plurality of activated carbon fibres woven, felted or pressed into a cloth-like fabric, or combinations thereof. Using a plurality of individual activated carbon fibres, a plurality of activated carbon fibres woven, felted or pressed into a cloth-like fabric, or combinations thereof may advantageously result in a reduced manufacturing time. It will be understood that the term "woven" refers to the interlacement of fibres to form the cloth-like fabric, the term "felted" refers to the packing together of fibres to form the cloth-like fabric, the term "pressed" refers to the packing together of fibres under pressure in order to form a cloth-like fabric. It may be desirable to reduce the void space between the fibres or particles so that when used in a cell the volume (and weight) of additional liquid electrolyte is minimised and the energy density per unit mass is thereby maximised. In some embodiments, where halide ions are oxidised on the cathode surface, that electrode is formed of a woven fabric of activated carbon fibres. Preferably, the woven activated carbon fibres of the electrode, where halide ions are oxidised, is a porous woven activated carbon fabric which is able to absorb resulting species formed on the electrode. For example, when bromide is oxidised to bromine, the porous electrode absorbs the resulting bromine or polybromide species (e.g., Bn", Brs ) Advantageously, when the porous woven activated carbon fabrics absorb resulting halogens, such as bromine or polybromide species, the reactants are available for the reverse reaction as they have not travelled away from the electrode. Additionally, the molecules or ions are not as free to travel across the cell to the membrane or opposite electrode, reducing parasitic reactions and electrolyte starvation. When the electrically activated carbon material is in particulate form, it can be provided as a plurality of particles with dimensions small enough to fall in the nm to pm range. For example, the activated carbon particles material can be provided as a plurality of particles with dimensions of 500nm to 200 pm. Preferably, the material is provided as a plurality of particles with dimensions of at least 1 pm, more preferably at least 10 pm. Preferably, the electrically conductive material is provided as a plurality of particles with dimensions of less than 150 pm, more preferably of less than 100 pm. The skilled person will be familiar with the techniques necessary to measure the relevant dimensions of the particles. Preferably, the activated material is in particulate form, or is provided as a plurality of fibres woven, felted or pressed into a cloth-like fabric, and so preferably, the carbon is in the form of activated carbon powder, or a plurality of activated carbon fibres woven, felted or pressed into a cloth-like fabric, or combinations thereof. An example of an electrically conductive material is provided by a plurality of activated carbon fibres in a woven, felted or cloth-like fabric. In an embodiment, the electrode may be a printed carbon electrode. Printed carbon electrodes involve using an activated carbon powder with a high surface area, a conductive additive (such as a sub-micron acetylene black powder (typical trade name is Super P)) and a polymer binder, typically PVDF are dissolved in a solvent, typically NMP, and coated on the current collector. The coating is then dried in vacuum oven and used as the electrode. The electrodes described herein may be hydrated such as to provide adequate wetting and enough aqueous solution to prevent electrolyte starvation. The hydration may be performed by immersion or addition of an aqueous solution to the electrodes. The electrochemical cell of the current invention includes at least two current collectors. The current collectors of the present invention are carbon current collectors. The carbon of the current collector is such to provide a high conductivity between the cell electrode components and the electrical load device. Preferably, the carbon current collectors are formed of graphitic carbon. Advantageously, the carbon current collectors of the present invention do not corrode in the aqueous electrolyte, increasing cell longevity and reducing side reactions. This is different to non-carbon current collectors such as aluminium or copper which would demonstrate significant and catastrophic corrosion. Advantageously, the carbon current collectors of the present invention demonstrate a low catalytic activity to hydrogen evolution in the electrochemical cell. The reduced catalytic hydrogen evolution reaction is advantageous for reducing electrolyte starvation, reducing cell gassing and reducing side reactions, all increasing the efficiency of the electrochemical cell of the present invention and increasing cell longevity. The carbon electrodes of the present invention additionally allow for efficient surface reactions, such as redox reactions and EDLC. Additionally, all cell components have sufficient chemical stability to withstand hydrolysis or corrosion from the aqueous electrolyte and / or products of redox reactions during charging / discharging, leading to superior longevity. Advantageously, the carbon anodes of the present invention provide a high conductivity surface for redox reactions with metal ionic species. Advantageously, the carbon anodes provide a low energy required for plating of metal ionic species, such as Zn2*, compared with conventional metal electrodes. Applications Figure 1 shows a single electrochemical cell in accordance with the present invention. In an alternative embodiment, several, single electrochemical cells according to the present invention may be connect in series or in parallel, in order to increase the capacity or operating voltage of the final device. Figure 2 shows several, single electrochemical cells according to the present invention connected in series. The device exemplified in Figure 2 comprises multiple electrodes (a first electrode 201a and a second electrode 201b, multiple hydrated cross-linked hydrophilic polymer membranes 203 located between the first electrodes and the second electrodes, a first electrolyte 202a, and a second electrolyte 202b, and multiple carbon current collectors 204 at the device terminals and between individual cells. The first electrolyte comprises a first redox species which can undergo a redox reaction at the first electrode, and the second electrolyte comprises a second redox species which can undergo a redox reaction at the second electrode. The energy storage cell of the present invention comprises a dual or multi redox electrolyte systems within a static, no macroscopic flow, cell. In some embodiments, the electrochemical energy storage cell may be a supercapacitor or a hybrid supercapacitor. In some embodiments, the energy storage cell may be a hybrid asymmetric supercapacitor. As used herein, the term "hybrid supercapacitor" refers to energy storage devices with a proportion of electrostatic double layer capacitance greater than 2% of the total energy stored. In some embodiments, the electrochemical energy storage cell may be a pseudocapacitor. In pseudocapacitors the redox reaction I charge movement occurs on the surface of the electrode(s), whereas in hybrid supercapacitors the redox reaction I charge movement occurs inside the electrode(s) (e.g., intercalation). In both cases EDLC is used to balance I contribute to the energy storage. In some embodiments, the energy storage cells described herein are particularly useful within stationary energy storage applications. Due to an increasingly turbulent energy landscape, there is a growing demand for residential stationary energy storage solutions where the need for backup power supplies during brownouts and a requirement to integrate renewables, such as solar panels. According to another aspect of the present invention there is a method of manufacture of a static energy storage cell comprising: a. soaking a hydrophilic cross-linked polymer membrane in an aqueous electrolyte solution to form a hydrated hydrophilic cross-linked polymer membrane, b. assembling a cell stack comprising two carbon electrodes either side of the hydrated hydrophilic cross-linked polymer membrane, c. adding an electrolyte composition comprising an oxidant species between the membrane and one carbon electrode, d. adding an electrolyte composition comprising a reductant species between the membrane and the other carbon electrode. In an embodiment, the hydrophilic cross-linked polymer membrane is a self-supporting hydrophilic cross-linked polymer membrane. In an embodiment, the first electrolyte is an aqueous electrolyte. In an embodiment, the second electrolyte is an aqueous electrolyte. In an embodiment, the first electrolyte is an aqueous electrolyte and the second electrolyte is an aqueous electrolyte. Examples Example 1: Ionic conductivity of membrane To demonstrate the high ionic conductivity of an example of a hydrated cross-linked hydrophilic polymer membrane described in the present invention, the ionic conductivity was measured and compared with a commercial membrane separator used widely in electrochemical energy storage cells, Nation® N-117. The example cross-linked hydrophilic polymer membrane was prepared by the following method. A solution was formed by mixing 2.5 mL of deionised water, 3 mL of l-vinyl-2-pyrrolidone (VP) and 0.04 g phenylalanine with a magnetic stirrer bar. 0.165 mL of allyl methacrylate (AMA) as the cross-linker and 0.110 mL of 2-hydroxy-2-methylpropiophenone as the initiator were added to the mixture, which was then formed as a thin film between glass slides under UV radiation for 4 minutes on one side then 1 minute on the reverse to form a cross-linked hydrophilic polymer membrane. Alternative initiators such as 2,2'-azobis(2-methylpropionamidine) dihydrochloride can be used for thermal polymerization. All membrane separators were hydrated in 6 M NaBr prior to testing, then the hydrated membrane separators were held between two carbon foil electrodes and analysed by electrochemical impedance spectroscopy. Cell resistance is thus measured, and conductivity calculated in S m L When a hydrated cross-linked hydrophilic polymer membrane, such as an VP / AMA phenylalanine polymer membrane, is hydrated in a NaBr solution, it possesses an ionic conductivity of 2.4 S m x, whereas the commercial membrane separator Nation® N-117 hydrated in the same solution possesses an ionic conductivity of 0.3 S m L Separator Ionic conductivity (S m1) Hydrated cross-linked hydrophilic polymer membrane 2.4 Nation® commercial ion exchange membrane N-117 0.3 Furthermore, the combination of membrane ionic conductivity and ion selectivity (discussed above) allow for high power charge / discharge times. Example 2: Selectivity of membrane Figure 3 shows electrolytes separated by a hydrated cross-linked hydrophilic polymer membrane described in the present invention in a H-cell. The H-cell of Figure 3 comprises two electrolyte chambers 301 separated by a separator under investigation 302. A platinum electrode 303 was immersed into each chamber equidistance from the separator, and a Ag / AgCI reference electrode 304 also immersed into the left-hand chamber. The parasitic crossover species, such as Bn or Brn+2’, is introduced into an electrolyte chamber via galvanostatic charge discharge cycling, in this instance the left chamber 305 contains the cathode and so produces Bn or Brn+2 , for example. Bn or Brn+2 is monitored easily via UV / vis spectrophotometry as it has a strong yellow / orange colour. A ZnBn solution was therefore used as the electrolyte to produce the crossover species Br2 / Brn+2’ on charging the electrochemical H-cell, and NaCI was also added as a support electrolyte. 20 GCD cycles were conducted at 0.2 A, producing Br2 / Brn+2_ on charging which was allowed to disperse throughout the H-cell. Both chambers were gently agitated with a stirrer bar during testing. Almost immediately after the 20 cycles were completed, UV-vis analyses were conducted of both chambers to calculate the concentration of Br2 / Brn-2 present based on a calibration curve of known concentrations. To demonstrate the ion selectivity of hydrated cross-linked hydrophilic polymer membranes of the present invention, one example was compared with a non-selective glass mat. Specifically, a hydrated cross-linked hydrophilic polymer, as described in Example 1, was compared with a fine glass mat, whereby the fine glass mat is the non-selective separator. The table below shows how the hydrated cross-linked hydrophilic polymer membrane in the present invention disrupts Br2 / Brn+2_ migration. The concentration of Br2 / Brn+2" in the anode compartment when a non-selective separator is used is 17x that of the concentration when a hydrated cross-linked hydrophilic polymer separator is used. When a non-selective separator is used to separate the chambers, the Bn / Brn-z migrates between the chambers with ease, and rapidly forms an equilibrium of concentration between the chambers. Separator Anode chamber concentration (M) Hydrated cross-linked hydrophilic polymer separator 0.02 Fine glass mat 0.34 Example 3: Method of manufacturing energy storage cell, Zn / Br polymer cell construction Graphite foil (25pm) was used as the current collector for both the anode and cathode. The electrolyte was made by dissolving IM NaCI and ZnBr2 in deionized water using a magnetic stirrer. The anode and cathode were produced by placing a high surface area (>1200m2 / g) woven activated carbon material in an excess of electrolyte and vacuum degassing the mixture to remove air cavities within the fabric and ensure homogeneity of the electrolyte. The polymer formulation used was as described in Example 1 and was hydrated in the cell electrolyte of NaCI I ZnBr2 mixture. The cell was then constructed with current collectors on the outside, carbon fabric with membrane between the anode and cathode. The cell stack was then placed in polypropylene or aluminized pouch cell material and sealed under vacuum to a pressure of <100mbar. Example 4: Method of manufacturing energy storage cell, NaBr polymer cell construction The electrolyte was 6M NaBr in deionized water. The polymer formulation used was as described in Example 1 and was hydrated in the electrolyte solution overnight. The anode and cathode were prepared as above in Example 3. Example 5: Zn / Br glass fiber cell construction This cell was prepared as in Example 3 above; however, the polymer separator was replaced with 7 layers of 25pm glass fiber weave. Example 6: Energy out and columbic efficiency comparison data The three cells described were tested using a constant current charge methodology of and a constant current discharge methodology at the same current density of llmA / cm2 from 100% to 0% state of charge (SoC). The NaBr polymer cell demonstrates a single redox system, the Zn / Br systems demonstrate a dual redox system with and without the presence of the hydrated crosslinked hydrophilic polymer. Glass fiber should be treated as a non-selective separator. Figures 4 and 5 are graphs showing these results. The graphs are limited to 20 cycles, as the glass fiber cell also demonstrated poor stability and showed degradation above this range. The primary benefit of the polymer in this system is the improvement of in columbic efficiency between the Zn / Br system with polymer over the same system with glass fiber. This leads to a significant improvement in the energy out of the system. When comparing single, NaBr, and dual, ZnBr2 systems, which utilize the same polymer membrane, a substantial increase in energy out is observed. This is due to the addition of the zinc plating redox couple to allow for an additional energy storage mechanism in the cell. Additionally, the presence of the redox couple also provides charge balancing of the bromide / bromide redox couple, that is present in both systems, further enhancing the cells performance. Example 7: Halogen absorption by the cathode To demonstrate the ability of the woven activated carbon fiber cathodes to absorb halogen species, UV-vis spectrophotometry was used to show a decreasing halogen concentration. By way of example, a bromine solution was used which possessed a characteristic yellow colour. A piece of woven activated carbon fiber cathode was immersed in the yellow bromine solution, and the UV-vis absorption for the peak corresponding to bromine analysed over time. Figure 6 shows the absorption of bromine over twelve hours decreases, demonstrating the halogen absorbing ability of the woven activated carbon fabric cathodes. Example 8: Other membrane material examples A range of different self-supporting cross-linked hydrophilic polymer membranes were formed as per Example 1. Example 1 provided a VP-AMA based example, but 2-hydroxyethyl methacrylate (HEMA) may also be used as a hydrophilic co-monomer. To demonstrate the versatility of these hydrophilic copolymers, two other cross-linking monomers are demonstrated, poly(ethylene glycol) diacrylate (PEGDA) and tetra(ethylene glycol) diacrylate (TEGDA), in addition to allyl methacrylate (AMA) as described in Example 1. Figure 7 shows when these hydrated hydrophilic copolymer membranes are used in electrochemical cells of the present invention, good Coulombic efficiencies are also obtained of ~95%. Example 9: Symmetric and asymmetric electrolyte composition comparisons To demonstrate the different electrolyte compositions described in this invention, two electrochemical cells were constructed: one possessed an electrolyte system where the catholyte and anolyte compositions were the same composition (symmetric), and the other possessed an electrolyte composition where the catholyte and anolyte compositions were different compositions (asymmetric). Both cells were constructed as per Figure 1, with a hydrated cross-linked hydrophilic polymer membrane described in Example 1. For both cells, a woven activated carbon fabric was used as the cathode, and a carbon felt was used as the anode. Figure 8 shows that in an electrochemical cell of the present invention, the cells possess high Coulombic efficiencies with a symmetric or asymmetric electrolyte composition.

Claims

1. A non-flow energy storage cell comprising:(i) a first electrode and a second electrode,(ii) an electrolyte system comprising:a. a membrane comprising a cross-linked hydrophilic co-polymer hydrated in an aqueous solution, wherein the membrane is situated between the first electrode and the second electrode,b. a first electrolyte comprising a first species which can undergo a redox reaction at the first electrode, wherein the first electrolyte is situated between the membrane and the first electrode,c. a second electrolyte comprising a second species which can undergo a redox reaction at the second electrode, wherein the second electrolyte is situated between the membrane and the second electrode,wherein the first electrode and the second electrode are carbon electrodes, wherein the first species is a metallic species, wherein the metallic species is a transition metal or a group 13, 14, 15 or 16 metal, wherein the second species is a halogen species, andwherein the membrane does not contain any fluorinated components.

2. A non-flow energy storage cell according to claim 1, wherein the first electrolyte and the second electrolyte are the same electrolyte composition.

3. A non-flow energy storage cell according to claim 1, wherein the first electrolyte and the second electrolyte are a different electrolyte composition.

4. A non-flow energy storage cell according to any one of the preceding claims, wherein the first electrolyte is an aqueous electrolyte and the second electrolyte is an aqueous electrolyte.

5. A non-flow energy storage cell according to any one of the preceding claims, wherein the membrane is a self-supporting membrane.

6. A non-flow energy storage cell according to claim 1, wherein the cross-linked hydrophilic co-polymer is formed by the process comprising the steps of:a. adding at least one hydrophilic monomer, and at least one cross-linker to an aqueous solution to form a co-monomer mixture,b. polymerising the co-monomer mixture.29 01 257. A non-flow energy storage cell according to claim 6, wherein the co-monomer mixture of step a) further comprises a hydrophobic monomer.

8. A non-flow energy storage cell according to claim 7, wherein the hydrophobic monomer and the cross-linker are the same species.

9. A non-flow energy storage cell according to any one of claims 6 to 8, wherein the hydrophilic monomer is selected from the group consisting of l-vinyl-2-pyrrolidone, 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, 2-acrylamido-2-methyl-propanesulfonic acid, acrylic acid, methacrylic acid, ethyl acrylate, sodium 4-styrenesulfonate, 4,4'-diamino-2,2'-stilbenedisulfonic acid, bis[2-(methacryloyloxy)ethyl]phosphate, acrylamide and N-[3-(dimethylamino)propyl]methacrylate, preferably l-vinyl-2-pyrrolidone and 2-hydroxyethyl methacrylate.

10. A non-flow energy storage cell according to any one of claims 7 to 9, wherein the hydrophobic monomer is selected from the list consisting of methyl methacrylate, acrylonitrile and allyl methacrylate.

11. A non-flow energy storage cell according to any one of claims 6 to 10 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, poly(ethylene glycol) diacrylate and tetra (ethylene glycol) diacrylate.

12. A non-flow energy storage cell according to any one of the preceding claims wherein the cross-linked hydrophilic co-polymer is hydrated in water or an aqueous salt solution.13.A non-flow energy storage cell according to claim 1 wherein the cross-linked hydrophilic co-polymer is hydrated in the first electrolyte and / or the second electrolyte.14.A non-flow energy storage cell according to any one of the preceding claims, wherein the first electrode and / or the second electrode comprise activated carbon.29 01 2515.A non-flow energy storage cell according to claim 14, wherein the first and / or second electrode comprise a plurality of activated carbon fibres woven into a clothlike fabric.16.A non-flow energy storage cell according to any one of the preceding claims, comprising carbon current collectors.17.A non-flow energy storage cell according to any one of the preceding claims, wherein the first species undergoes a singular redox reaction on the first electrode.18.A non-flow energy storage cell according to any one of the preceding claims, wherein the first species undergoes a multi redox reaction on the first electrode.19.A non-flow energy storage cell according to any one of the preceding claims, wherein the second species undergoes a singular redox reaction on the second electrode.20.A non-flow energy storage cell according to any one of the preceding claims, wherein the second species undergoes a multi redox reaction on the second electrode.

21. A non-flow energy storage cell according to any one of the preceding claims, wherein the first electrolyte comprises an additional species which can undergo a redox reaction at the first electrode.22.A non-flow energy storage cell according to any one of the preceding claims, wherein the second electrolyte comprises an additional species which can undergo a redox reaction at the second electrode.

23. A non-flow energy storage cell according to any one of the preceding claims, wherein the metallic species is iron, cobalt, nickel, copper or zinc.

24. A non-flow energy storage cell according to any one of the preceding claims, wherein the halogen species is bromine, iodine or chlorine.

25. A non-flow energy storage cell according to any one of the preceding claims wherein the first species is zinc and the second species is bromine.

26. A non-flow energy storage cell according to any one of the preceding claims wherein the electrolyte system further comprises a supporting salt.

27. A non-flow energy storage cell according to claim 26 wherein the supporting salt is one or more from the list consisting of sodium chloride, sodium bromide, potassium chloride, potassium bromide, ammonium chloride, sodium acetate, potassium acetate, potassium perchlorate and sodium perchlorate.

28. A non-flow energy storage cell according to any one of the preceding claims which is a hybrid asymmetric supercapacitor.

29. A non-flow energy storage cell according to any one of the preceding claims which is a pseudocapacitor.

30. A non-flow energy storage cell according to any one of the preceding claims for use in stationary energy storage.29 01 25

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