Method for preparing ion exchange membranes

Abstract

The present invention relates to a method for producing an ion exchange membrane, comprising the steps of: a. applying an ionic liquid containing at least one polymerizable and / or crosslinkable group on a cellulosic substrate, wherein the cation of the ionic liquid is a heterocyclic aromatic ring containing at least one nitrogen atom as a heteroatom; and b. polymerizing and / or crosslinking the at least one polymerizable and / or crosslinkable group to form a polymer or copolymer layer on the cellulosic substrate.

Description

[Technical Field]

[0001] The present invention relates to an ion exchange membrane. [Background technology]

[0002] Ion exchangers are materials that can replace dissolved ions with other ions. They are usually commercially available in the form of columns containing ion exchange resins or membranes through which the solution to be treated flows. The ion exchange process can be divided into two mechanisms depending on the application: (i) the ions to be exchanged are bound to the ion exchange material, which then releases the equivalent charge of the previously bound ions into the solution (hopping mechanism); (ii) the ions are bound to charge carriers that can freely diffuse in the liquid (vehicle mechanism).

[0003] The principle behind ion exchange is that the higher the charge of ions, the stronger they bind to the ion exchange material and, for the same charge, the larger their ionic radius. Ions that bind more strongly to the ion exchange material displace other ions from their respective binding sites. Important influencing factors are the pH value and substance concentration. The transport of ions through an ion exchange membrane is driven by convection, diffusion, and migration.

[0004] Ion exchange membranes are thin membranes that allow the passage of positively or negatively charged ions. Such membranes therefore represent charge-selective filters and can be used to concentrate or separate charge-carrying species in a solution.

[0005] Ion exchange membranes typically contain charged functional groups covalently attached to the membrane material (e.g., organic and / or inorganic polymers). The membrane's negative or positive charge is compensated by mobile ions in the surrounding solution. These mobile counterions can be exchanged for any ions of the same charge, allowing them to pass through the membrane. For ions with the same sign as the fixed ions, the membrane represents a barrier.

[0006] Ion exchange membranes are used, for example, in fuel cells and batteries. In such cells and batteries, these membranes act as separators between two compartments and their electrolytes, as ionic conductors, and as electrical insulators to prevent short circuits in the cell. These membranes are a major cost factor in fuel cell and battery systems. The most commonly used membranes are perfluorinated polymer membranes, such as Nafion™. Perfluorinated polymer membranes are a major cost factor, and undesirable crossover properties remain a limiting factor for these membranes. Alternative ion exchange membranes are primarily based on synthetic polymers and have their own advantages and disadvantages.

[0007] It is therefore an object of the present invention to provide an ion exchange membrane that overcomes the drawbacks of ion exchange membranes known in the art, that has superior properties, can be manufactured at lower cost, and has a lower environmental impact than widely applied perfluorinated polymer membranes. Summary of the Invention

[0008] Accordingly, the present invention provides a method for producing an ion exchange membrane, the method comprising: a. applying an ionic liquid containing at least one polymerizable and / or crosslinkable group in the cation and / or anion onto a cellulosic substrate, wherein the cation of the ionic liquid is a heteroaromatic or derivative thereof containing at least one nitrogen as a heteroatom; b. polymerizing and / or crosslinking at least one polymerizable and / or crosslinkable group to form a polymer or copolymer layer on the cellulosic substrate.

[0009] Surprisingly, it has been shown that the process of the present invention can be used to produce ion exchange membranes having the desirable properties described above.

[0010] The use of a cellulosic substrate and copolymerized ionic liquids results in low diffusivity of the active species and high ionic conductivity, both properties surprisingly superior to commercially available membranes.

[0011] Furthermore, the use of a combination of different materials and new manufacturing methods allows for individual adaptation to different types of environments, which opens up a wide range of opportunities.

[0012] The ion exchange membrane of the present invention is produced by applying an ionic liquid containing at least one polymerizable and / or crosslinkable group in the cation and / or anion onto a cellulosic substrate. The cation of the ionic liquid is a heteroaromatic ring containing one, two, or three nitrogen atoms as heteroatoms. The heteroaromatic ring can be five- or six-membered and can contain substituents.

[0013] Therefore, another aspect of the present invention relates to an ion exchange membrane obtainable by the method according to the invention.

[0014] The ion exchange membranes produced according to the present invention can be used in a variety of applications. Accordingly, a further aspect of the present invention relates to a redox flow cell comprising an ion exchange membrane of the present invention between a cathode cell and an anode cell.

[0015] The ion exchange membranes obtained by the method according to the invention can further be used to separate any cathode compartment from any anode compartment, an application in which they are particularly useful in redox flow cells, fuel cells and galvanic cells. [Brief explanation of the drawings]

[0016] [Figure 1a] The contact angle and volume of the droplet on each film surface are shown. [Figure 1b] The contact angle and volume of the droplet on each film surface are shown. [Figure 2] Figure 2A shows the through-plane cell stack used for redox flow battery membrane measurements, and Figure 2B shows the corresponding equivalent circuit. [Figure 3] 1 shows the results of a battery test using a paper / [EMIM][AC] membrane of the present invention, showing a 30 minute step time between charge and discharge as well as the coulombic efficiency. [Figure 4] 1 shows the voltage / current curves of a flow battery with a paper-based membrane and a commercial vanadium electrolyte. DETAILED DESCRIPTION OF THE INVENTION

[0017] The method of the present invention allows the production of ion exchange membranes using cellulose-based substrates, which can be used as an alternative to ion exchange membranes known in the art. Depending on the ionic liquid used and the resulting polymerization, cation exchange membranes and / or anion exchange membranes can be produced. Cation ion exchange membranes facilitate the transport of positively charged moieties across the membrane. Positive groups in anion exchange membranes repel cations. The method of the present invention can also be used to produce amphoteric membranes. Amphoteric membranes contain both anionic and cationic active groups. Thus, the ion exchange membranes obtained by the method of the present invention can be anion exchange membranes, cation exchange membranes, or amphoteric / bipolar exchange membranes.

[0018] As used herein, "cellulosic substrate" refers to a thin material produced by the papermaking process, i.e., by pressing wet fibers, typically cellulose pulp derived from wood, grass, or weeds, together and drying them into a flexible sheet. Cellulosic substrates can also be produced from recycled materials, such as recycled paper. Cellulosic substrates can also be produced from nano- and microfibrillated cellulose or by purifying pulp fibers. Cellulosic substrates in the context of the present invention also include films produced by the regeneration of cellulose in the viscose process. Cellulosic substrates used in the present invention can be, for example, paper filters or cellophane films. Cellulosic substrates can have an average thickness of 10 to 500 μm, preferably 50 to 250 μm, more preferably 100 to 200 μm, and even more preferably 130 to 150 μm. Preferably, the cellulosic substrate used in the present invention can be bulk cellulose, such as paper. Bulk cellulose is characterized by being composed of much larger fibers with lower aspect ratios and strengths when dry and larger fiber diameters than other cellulosic substrates, such as nanocellulose. In addition, bulk cellulose is inexpensive and, as mentioned above, readily available. Therefore, it is particularly advantageous that bulk cellulose, when used in the method of the present invention, can outperform commercially available membranes. Hereinafter, "ionic liquid" is understood to mean a compound consisting only of ions and having a melting point below 100°C. Typically, ionic liquids are non-volatile, non-flammable, thermally stable, and characterized by high ionic conductivity. Their physicochemical properties can be tailored to the corresponding application. Ionic liquids consist of anions and cations. Due to their low vapor pressure, such solvents are more environmentally friendly than classical solvents.

[0019] The term "polymerizable group" as used herein refers to a chemical group / moiety suitable for polymerization reactions, such as, for example, free radical or ionic chain polymerization, polyaddition, or polycondensation. Monomers containing polymerizable groups can form oligomers or polymers in which the monomers are covalently bonded to each other to form polymer chains. A "crosslinkable group" as used herein is a chemical group / moiety that can form a covalent bond between polymer chains. The polymerizable group and the crosslinkable group may be chemically similar or identical.

[0020] According to a particularly preferred embodiment of the present invention, the polymerizable and / or crosslinkable group is an alkenyl or alkynyl group.

[0021] The use of these groups has been shown to be advantageous because the double or triple bond creates a reactive center that can react with free radicals. The formed compound can then react again with the reactive center of the monomer, thus resulting in the formation of a polymer. Therefore, it is even more preferred that the polymerizable and / or crosslinkable group is selected from the group consisting of vinyl, allyl, and methacrylate groups.

[0022] The cation of the ionic liquid is a heteroaromatic group containing one, two or three nitrogen heteroatoms, and is preferably selected from the group consisting of imidazole salt cations, pyrrole salt cations, pyridine salt cations and derivatives thereof.

[0023] The heteroaromatic rings of the present invention may contain substitutions, preferably on one or more nitrogen atoms of the heterocycle. These substitutions result in derivatives of the heteroaromatic rings, particularly derivatives of imidazole (salt cation), pyrrole (salt cation), and pyridine (salt cation). When the heteroaromatic ring derivative contains multiple substituents, the substituents may be the same or different. The substituents may independently be C1-C5 alkyl, alkenyl, or alkynyl groups. The substituents are preferably selected from the group consisting of methyl, ethyl, and allyl groups. The imidazole derivatives may be substituted, for example, with methyl and ethyl or allyl groups, and combinations thereof.

[0024] Ionic liquids can be divided into alkylammonium, dialkylimidazolium, phosphonium, and N-alkylpyridinium-based ionic liquids. For the method according to the present invention, dialkylimidazolium-based ionic liquids are preferably used. Dialkylimidazolium-based ionic liquids consist of an imidazolium ring as a cation and a suitable anion. This special structure provides high stability under oxidation and reduction conditions, low viscosity, and is relatively easy to synthesize.

[0025] According to a particularly preferred embodiment of the present invention, the cations of the ionic liquid are selected from the group consisting of imidazole salt cations and derivatives thereof, preferably 1-ethyl-3-methylimidazolium, 1-allyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-vinyl-3-butylimidazolium, 1-ethyl-3-methylimidazolium, 1-hexyl-3-ethylimidazolium, 1-methylimidazolium and 1-hexyl-3-methylimidazolium, quaternary ammonium salt cations, preferably 2-hydroxyethyl-trimethylammonium, pyrrole salt cations, preferably 1-butyl-1-methylpyrrolidinium, quaternary phosphine salt cations, preferably triethyl(4-vinylbenzyl)phosphonium tetrafluoroborate and trihexyltetradecylphosphonium, and pyridine salt cations, preferably 1-butyl-1-methylpiperidinium. The use of 1-allyl-3-methylimidazolium and 1-ethyl-3-methylimidazolium as cations in ionic liquids is particularly preferred.

[0026] According to a particularly preferred embodiment of the present invention, the anion of the ionic liquid is selected from the group consisting of acrylate, dicyanamide, acetate, preferably vinyl acetate, phosphonate, preferably vinyl phosphonate, bis((trifluoromethyl)sulfonyl)imide, bis((pentafluoromethyl)sulfonyl)imide, hexafluorophosphate, tetrafluoroborate, methyl sulfate, triflate, thiocyanate, trifluoroacetate, hydrogen sulfate or a halide.

[0027] According to a particularly preferred embodiment of the invention, the ionic liquid is selected from the group consisting of 1-ethyl-3-methylimidazolium acrylate, 1-allyl-3-methylimidazolium dicyanamide, 1-ethyl-3-methylimidazolium vinyl acetate, 1-ethyl-3-methylimidazolium vinylphosphonate, 1-allyl-3-methylimidazolium methanesulfonate, 1-butyl-3-methylimidazolium acrylate or 1-hexyl-3-methylimidazolium acrylate. The use of 1-ethyl-3-methylimidazolium acrylate and 1-ethyl-3-methylimidazolium vinyl acetate is particularly preferred.

[0028] According to a particularly preferred embodiment of the present invention, polymerization is initiated by a radical reaction induced by chemical, photochemical, thermochemical, and / or plasma methods. It has been shown that polymerization can occur with several combinations of ionic liquids and monomers. To initiate the polymerization process, it is advantageous to have a reactive cation or anion of the ionic liquid. This reactive ion can react with both the monomer and the ionic liquid, resulting in free radical polymerization. The formation of the reactive ion, and thus the radical, is preferably initiated by one of the methods mentioned. Chemically induced reactions are preferably charge-change effects induced primarily by functional groups or individual atoms. On the other hand, photochemical induction is a chemical reaction induced by the action of light. Therefore, thermochemical induction is preferably a reaction induced by the action of heat, and plasma induction is preferably a reaction induced by the action of an external induction coil on the plasma.

[0029] According to a particularly preferred embodiment of the present invention, polymerization is initiated by vapor deposition, including oxidative chemical vapor deposition, chemically initiated vapor deposition, and plasma-enhanced chemical vapor deposition, or by the use of UV radiation, thermal activation, and radical formation by transition metal complexes using alkyl halides as initiators. Particularly preferred is chemical vapor deposition using a thermal initiator. The thermal initiator is particularly preferably a peroxide, preferably tert-butyl peroxide, benzophenone, dibenzoyl peroxide, perfluorooxanesulfonyl fluoride, triethylamine, 2,2'-azobis(2-methylpropane), benzophenone, or 2-bromoisobutyryl bromide.

[0030] According to a particularly preferred embodiment of the present invention, the cellulosic substrate is contacted with at least one polymerizable monomer and / or at least one crosslinking agent after step a. This particularly preferred embodiment has the advantage of immobilizing the ionic liquid in the polymer matrix by copolymerization. Copolymerization further improves mechanical stability and minimizes loss of ionic liquid resulting from weak interactions. Furthermore, the introduction of the monomer and / or crosslinking agent can have a particularly positive effect on the growth rate of the polymer layer, since this influence is influenced by the adsorption of the monomer and / or crosslinking agent on the substrate surface.

[0031] According to a particularly preferred embodiment of the present invention, at least one polymerizable monomer and / or at least one crosslinking agent contains at least one ring structure or at least one alkenyl or alkynyl group that can be opened during the reaction. The double or triple bonds of the alkenyl and alkynyl groups and ring structures form reactive centers, which allow bonds to be formed more easily at these sites. The reactive centers of the monomers cause polymer growth, while the reactive centers allow polymer chains to be linked. Even more preferably, the polymerizable monomer and / or crosslinking agent contains an alkenyl group and / or ring structure that can be opened during the reaction.

[0032] According to another particularly preferred embodiment of the present invention, the at least one polymerizable monomer and / or the at least one crosslinker is selected from the group consisting of methacrylic acid and methacrylates, among which ethylene glycol dimethacrylate, 2-hydroxyethyl methacrylate, and C1-C6 methacrylate compounds are particularly preferred, as well as divinylbenzene, hexavinyldisiloxane, 1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane, and ethylene glycol dimethacrylate. Particularly preferred compounds are ethylene glycol dimethacrylate and 2-hydroxyethyl methacrylate.

[0033] According to another particularly preferred embodiment of the present invention, the cellulosic substrate is paper or other substrate derived from a cellulosic raw material that can be used in acidic or alkaline environments without decomposition, can act as a support material for polymers, and allows for covalent or physical bonding of polymers to cellulose or cellulose derivatives. Substrates made from cellulosic raw materials often have a structure. Therefore, it is particularly preferred that a polymer film completely covers the fibers of the substrate and penetrates the pores of the structure. This preferred embodiment provides mechanical and chemical stability against flowing electrolytes. In a more particularly preferred embodiment, the cellulosic substrate is a general-purpose paper filter with or without a wet strength additive. Pulp for producing the filter paper can be from softwood, hardwood, fiber crops, and combinations thereof. Particularly preferred filters are made from dissolving pulp or mercerized pulp. Particularly preferred general-purpose paper filters have an average thickness of 10 to 300 μm, more preferably 50 to 250 μm, preferably 100 to 200 μm, more preferably 100 to 150 μm, and more preferably 120 to 140 μm.

[0034] According to particularly preferred embodiments of the present invention, the application of the ionic liquid onto the cellulosic substrate is carried out by gas phase deposition using solution-based coating techniques such as blade casting, drop casting, spin coating, dip coating, spray coating, and / or vapor deposition in a vacuum.

[0035] In blade casting, the solution is spread onto a flat surface or substrate using a so-called blade, which ensures uniform spreading and uniform thickness.

[0036] Drop casting is a coating technique that involves dropping a solution onto a substrate and then allowing the solution to evaporate, forming a thin layer on the substrate.

[0037] In spin coating, the substrate is fixed onto a turntable by vacuum. The desired amount of solvent is applied onto the substrate. The acceleration, final speed, and time are adjusted to distribute the solvent evenly. Excess material is spun off. The solvent is removed by baking to obtain a solid layer.

[0038] Dip coating is a coating technique in which a substrate is immersed in a solution at a constant rate. After a sufficient time, the substrate is withdrawn. During the withdrawal, a thin coating layer forms on the surface. Excess liquid drips off. Finally, the solvent evaporates from the liquid.

[0039] Spray coating is the application of a solution in the form of accelerated spray particles to the surface of a substrate. When the spray particles hit the surface, a layer is formed. The spray particles are somewhat flattened by the impact.

[0040] Vapor deposition includes a variety of film growth techniques, with physical vapor deposition, chemical vapor deposition, atomic layer deposition, and molecular beam epitaxy being important categories. In these processes, solid components are deposited on a substrate surface by reaction from the gas phase.

[0041] The present invention further relates to an ion exchange membrane obtainable by the method disclosed herein. The ion exchange membrane according to the present invention allows the exchange of specific dissolved ions while exchanging other ions or neutral molecules. The ion exchange membrane is a conductor. It can be used for the exchange of protons or anions in various fields. The ion exchange membrane according to the present invention is preferably used in combination with an aqueous solution or water. Applications include industrial water treatment, electronics industry, energy storage, food industry, beverage industry, purification of a wide range of products, pharmaceutical industry, and medical applications.

[0042] The present invention also relates to a redox flow cell comprising an ion exchange membrane according to the present invention between a cathode cell and an anode cell. The redox flow cell comprises two tanks with a membrane according to the present invention, electrodes, bipolar plates, current collectors, and a connected pump. The active material is stored in the tanks in dissolved form. The ion exchange membrane preferably acts as a separator between the two compartments and their electrolytes, as an ion conductor, and as an electrical insulator. In the redox flow cell, power and energy are separated.

[0043] The redox flow cell according to the invention can be used in particular in the fields of load sharing, energy storage, peak shaving, uninterruptible power supplies, power conversion, electric vehicles and as stand-alone power systems.

[0044] The redox flow cell according to the present invention is preferably charged with surplus power. The surplus power can come from any type of power production. Preferably, the surplus power comes from a sustainable energy source. Particularly preferred are sustainable energy sources whose energy production is influenced by nature. Such energy sources include solar energy, hydropower, wind power, and marine energy. Natural fluctuations in this type of energy production can be compensated for by planned charging and discharging of the redox flow cell. The redox flow cell can be used to store energy from surplus power. The cell can be discharged during times of high energy demand. In particular, the cell can also be used to ensure power supply in the event of a power outage. Power outages include local or regional interruptions. Therefore, the use of the redox flow cell according to the present invention can be particularly useful in facilities where a constant power supply is essential. The redox flow cell can be installed anywhere on the power grid. However, this location may preferably be in close proximity to the energy production or demand site.

[0045] The present invention further relates to the use of the ion exchange membrane according to the invention in a redox flow cell, a fuel cell or a galvanic cell to separate a cathode compartment from an anode compartment. In a redox flow cell, the ion exchange membrane is preferably used as a separating wall between the two compartments and their electrolytes, as already mentioned. It is also an ion conductor and an electrical insulator. In a fuel cell, the energy of the chemical reaction between a fuel and an oxidant is converted into electrical energy. Thus, the fuel cell functions as an energy converter. A fuel cell consists of electrodes separated by an ion exchange membrane. A galvanic cell is a device for converting chemical energy into electrical energy. Any combination of electrodes and an electrolyte constitutes a galvanic cell.

[0046] In the redox flow cell of the present invention, ionic liquids can be used as redox flow battery electrolytes. They can be used as supporting electrolytes, additives, reaction media, or active species in redox flow systems. Their use as electrolyte additives aims to improve the overall performance of the system. Ionic liquids used as electrolyte additives can improve system and cycling stability, species diffusion, and active species solubility, widening the electrochemical window of the media. In redox flow cells, the potential difference is limited by the stability of water at 1.23 V under standard conditions. Using ionic liquids, the electrochemical window can be extended to 6.0 V. As active species, ionic liquids can be used as stand-alone solutions or in combination with other species. When used as stand-alone solutions, they act as both electrolytes and active species.

[0047] These and further advantageous embodiments of the invention are explained on the basis of the following description, and those skilled in the art will understand that the various embodiments can be combined.

[0048] Embodiment 1. A method for producing an ion exchange membrane, the method comprising: a. applying an ionic liquid containing at least one polymerizable and / or crosslinkable group on the cation and / or anion onto a cellulosic substrate; b. polymerizing and / or crosslinking at least one polymerizable and / or crosslinkable group to form a polymer or copolymer layer on the cellulosic substrate.

[0049] Embodiment 2. A method for preparing an ion exchange membrane according to embodiment 1, wherein the polymerizable and / or crosslinkable group is an alkenyl group or an alkynyl group.

[0050] Embodiment 3. The method for preparing an ion exchange membrane according to embodiment 1 or 2, wherein the cation of the ionic liquid is selected from the group consisting of imidazole salt cations and derivatives thereof, quaternary ammonium salt cations, pyrrole salt cations, quaternary phosphine salt cations, and pyridine salt cations.

[0051] Embodiment 4. The method for preparing an ion exchange membrane according to embodiment 3, wherein the salt cation is selected from the group consisting of 1-ethyl-3-methylimidazolium, 1-allyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-vinyl-3-butylimidazolium, triethyl(4-vinylbenzyl)phosphonium tetrafluoroborate, 1-ethyl-3-methylimidazolium, 1-hexyl-3-ethylimidazolium, 1-methylimidazolium, 1-hexyl-3-methylimidazolium, 1-butyl-1-methylpiperidinium, 2-hydroxyethyl-trimethylammonium, trihexyltetradecylphosphonium, or 1-butyl-1-methylpyrrolidinium.

[0052] Embodiment 5. A method for preparing an ion exchange membrane according to any one of embodiments 1 to 4, wherein the anion of the ionic liquid is selected from the group consisting of acrylate, dicyanamide, acetate, preferably vinyl acetate, phosphonate, preferably vinyl phosphonate, bis((trifluoromethyl)sulfonyl)imide, bis((pentafluoromethyl)sulfonyl)imide, hexafluorophosphate, tetrafluoroborate, methyl sulfate, triflate, thiocyanate, trifluoroacetate, hydrogen sulfate or a halide.

[0053] Embodiment 6. A method for preparing an ion exchange membrane according to any one of embodiments 1 to 5, wherein the polymerization is initiated by a radical reaction induced by a chemical method, and / or a photochemical method, and / or a thermochemical method, and / or a plasma method.

[0054] Embodiment 7. A method for preparing an ion exchange membrane according to any one of embodiments 1 to 6, wherein polymerization is initiated by vapor deposition, including oxidative chemical vapor deposition, chemically initiated vapor deposition, and plasma-enhanced chemical vapor deposition, or by the use of UV radiation, thermal activation, and the formation of radicals by a transition metal complex using an alkyl halide as an initiator.

[0055] Embodiment 8. A method for preparing an ion exchange membrane according to any one of embodiments 1 to 7, wherein the cellulosic substrate is contacted with at least one polymerizable monomer and / or at least one cross-linking agent after step a.

[0056] Embodiment 9. A method for preparing an ion exchange membrane according to embodiment 8, wherein at least one polymerizable monomer and / or at least one crosslinker comprises at least one alkenyl or alkynyl group or ring structure capable of ring-opening during the reaction.

[0057] Embodiment 10. A method for preparing an ion exchange membrane according to embodiment 8 or 9, wherein the at least one polymerizable monomer and / or the at least one crosslinker is selected from the group consisting of methacrylic acid, methacrylate, divinylbenzene, hexavinyldisiloxane, 1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane, or ethylene glycol dimethacrylate.

[0058] Embodiment 11. A method for preparing an ion exchange membrane according to any one of embodiments 1 to 10, wherein the cellulosic substrate is a paper derived from a cellulosic source that can be used in acidic or alkaline environments without decomposition, can act as a support material for polymers, and allows for covalent or physical bonding of polymers to cellulose or cellulose derivatives.

[0059] Embodiment 12. A method for preparing an ion exchange membrane according to any one of embodiments 1 to 11, wherein the application of the ionic liquid onto the cellulosic substrate is carried out by a gas phase deposition method using a solution-based coating technique, such as blade casting, drop casting, spin coating, dip coating, spray coating, and / or vapor deposition in a vacuum.

[0060] Embodiment 13. An ion exchange membrane obtained by the method according to any one of embodiments 1 to 12.

[0061] Embodiment 14. A redox flow cell comprising the ion exchange membrane of embodiment 13 between a cathode cell and an anode cell.

[0062] Embodiment 15. Use of an ion exchange membrane according to embodiment 13 in a redox flow cell, a fuel cell, or a galvanic cell to separate a cathode compartment from an anode compartment.

[0063] The present invention is further illustrated by the following examples, but is not limited thereto.

[0064] Example material Three different base substrates were tested in the following examples: paper filters (general-purpose filter paper DP 400 150, ALBET LabScience, average thickness 140 μm), PES filters (Millipore Express PLUS hydrophilic PES filters, pore size 0.22 μm, φ47 mm, thickness 175 μm, Sigma-Aldrich), and PVDF filters (Durapore® hydrophilic PVDF filters, pore size 0.22 μm, φ47 mm, thickness 115 μm, Sigma-Aldrich). Five different ionic liquids were analyzed in the examples: 1-allyl-3-methylimidazolium dicyanamide (≥98%, Sigma-Aldrich, AMIM-DCA), 1-ethyl-3-methylimidazolium acrylate (≥90%, proionic GmbH, EMIM-AC), 1-ethyl-3-methylimidazolium vinyl acetate (≥90%, proionic GmbH, EMIM-VA), 1-allyl-3-methylimidazolium methanesulfonate (≥90%, proionic GmbH, AMIM-MS), and 1-ethyl-3-methylimidazolium vinylphosphonate (≥90%, proionic GmbH, EMIM-VP). The combination of three base layers and five different ionic liquids resulted in 15 different membranes.

[0065] [Table 1]

[0066] The monomers used were 2-hydroxyethyl methacrylate (Sigma-Aldrich, HEMA) and ethylene glycol dimethacrylate (Sigma-Aldrich, EGDMA). The initiator used was tert-butyl peroxide (Sigma-Aldrich, TBPO). The monomers and initiator were used without further treatment.

[0067] Hydrochloric acid (HCl, 0.1 mol / l, Carl Roth GmbH+Co.KG), hydrochloric acid (HCl, 1 mol / l, Carl Roth GmbH+Co.KG), sulfuric acid (H2SO4, Merck Millipore), phosphoric acid (H3PO4, Carl Roth GmbH+Co.KG), sodium hydroxide (NaOH, 0.1 mol / l, Honeywell International Inc), sodium hydroxide (NaOH, 1 mol / l, Honeywell International Inc), phenolphthalein (Reag.Ph Eur, Merck Millipore, 0.1 wt% solution in ethanol), ethanol (EtOH, ≥998%, denatured, Carl Roth GmbH+Co.KG), acetic acid (≥959%, Carl Roth GmbH+Co.KG), p-anisaldehyde (4-methoxybenzaldehyde, Alfa Aesar), cyclohexane (Carl Roth GmbH+Co.KG), and ethyl acetate (Carl Roth GmbH+Co.KG). GmbH+Co.KG were used as received without further treatment. For zinc determination, zincon monosodium salt (C 20 H 15N4NaO6S, MQ 100, Sigma-Aldrich) were used. HPTLC silica gel 60 F254 plates on aluminum supports (Merk Millipore) and glass ring-cap capillaries (10 μl, Hirschmann) were used for TLC. 2-Methoxyhydroquinone (MHQ, batch synthesis used by Schlemmer et al.), zinc sulfate heptahydrate (ZnSO4*7H2O, Sigma-Aldrich), and para-benzoquinone (pBQ, Sigma-Aldrich) were used as active materials.

[0068] Four different types of commercially available membranes were purchased for comparison with the currently used Nafion™ N211 (25.4 μm) membrane. Nafion™ N117 (183 μm), Fumasep E 630 K (30 μm), Fumasep F 950 (50 μm), and Vanadion (254 μm) were used. The Nafion™ membrane and two Fumasep membranes were purchased from Fuel Cell Store. The Vanadion membrane was provided by Ion-Power. The membranes differed in thickness and chemical composition.

[0069] Example 1: Preparation of ion exchange membrane For the production of ion exchange membranes, an adapted process of Krauter et al. was used (Macromolecules 2020, 53, 7962), consisting of the following steps:

[0070] 1. The ionic liquid was dropped onto the substrate (i.e., the membrane).

[0071] 2. The ionic liquid is degassed in the reaction chamber for at least 2 hours and a clean silicon wafer is added to the reaction chamber.

[0072] 3. Heat to a temperature high enough to vaporize the monomer.

[0073] 4. The monomer and initiator were adjusted to base pressure.

[0074] 5. The chamber pressure was reduced to the desired level.

[0075] 6. A cooling phase was initiated inside the bottom of the reaction chamber where the filament was heated.

[0076] 7. The copolymer layer was grown in the chamber until a thickness on the silicon wafer of approximately 250 nm was achieved.

[0077] EGDMA and HEMA were used as the polymerizable and crosslinkable groups at standard flow rates of 0.17 sccm and 0.42 sccm, respectively. To initiate deposition, tert-butyl peroxide was used at a flow rate of 0.76 sccm. During deposition, the chamber pressure was kept constant at 26.66 Pa. The filament was heated to approximately 250 °C, and the bottom stage was cooled to 30 °C. For the formation of monomer vapor, EGDMA and HEMA were heated to 85 °C and 75 °C, respectively. For in-situ thickness analysis, a Thor-Lab laser with a wavelength of 633 nm was used. The peak-to-peak amplitude corresponded to a polymer thickness of approximately 80 nm on the silicon wafer. Polymerization was carried out until the ionic liquid was visibly polymerized, and the thickness of the polymer layer was varied.

[0078] Depending on the ionic liquid, monomer and crosslinker used to coat the membrane, the following structures were exemplarily obtained:

[0079] 1-Ethyl-3-methylimidazolium-based ionic liquids:

[0080] [ka]

[0081] 1-Allyl-3-methylimidazolium-based ionic liquids:

[0082] [ka]

[0083] Example 2: Contact angle as an indicator of diffusion and conductivity The contact angle between the droplet and the substrate was measured using an STFI contact angle tester. The volume of the droplet was also measured. Both measurements were performed over a period of 60 seconds. MilliQ® water was used in combination with 1-ethyl-3-methylimidazolium acrylate or 1-ethyl-3-methylimidazolium vinyl acetate to form the droplet. Paper was used as the substrate.

[0084] The measurement results are shown in Figure 1.

[0085] The contact angle of the paper / 1-ethyl-3-methylimidazolium acrylate (EMIM-AC) film was approximately 85° and remained nearly constant over 60 seconds (see Figure 1a). Furthermore, the volume of the droplet remained constant because the water did not penetrate the matrix.

[0086] Similar results were obtained for the paper / 1-ethyl-3-methylimidazolium vinyl acetate (EMIM-VA) combination. Here, the contact angle decreases from 95 to 85 degrees within the first 2 seconds of measurement. After that, the angle remains constant (see Figure 1b). The volume also remains constant.

[0087] The measurements show that the contact angle is beneficial for the transport of ions towards the membrane. The constant volume of the droplet indicates that water cannot permeate the membrane quickly, allowing the membrane to be used in applications such as redox flow batteries.

[0088] Example 3: Diffusivity of active species in novel membranes compared to Nafion™ 211 The diffusion rate of the active species was analyzed using a setup consisting of two pipes with an inner diameter of 25.4 mm filled with 2-methoxyhydroquinone (MHQ) and zinc solution. The two compartments were separated by a membrane obtained according to Example 1. 150 mL of the corresponding solution with an active material concentration of 2.5 g / L was filled into the compartment and sealed to prevent evaporation loss of the chemical. After 24 h, samples were taken from the solution. The zinc concentration was measured using Zincon (2-carboxy-2'-hydroxy-5'-sulfoformadylbenzene monosodium salt) as a complexing agent using UV-Vis measurements and a calibration curve on a Shimadzu UV-1900i photometer. Quinone evaluation was performed by thin-layer chromatography coupled with colorimetry. A calibration curve was also used.

[0089] In the table below, the percent loss of 2-methoxyhydroquinone (MHQ) and zinc within 24 hours is shown for different paper / ionic liquid combinations and the reference membrane Nafion™ 211. The abbreviations in the table stand for 1-ethyl-3-methylimidazolium acrylate (EMIM-AC), 1-ethyl-3-methylimidazolium vinyl acetate (EMIM-VA), and 1-ethyl-3-methylimidazolium vinyl phosphonate (EMIM-VP).

[0090] [Table 2]

[0091] The results show that the combination of ionic liquid and paper results in low diffusivity of the active materials present in the fuel cell. All combinations show significantly lower losses of MHQ and zinc compared to the reference membrane Nafion™ 211.

[0092] Example 4: Ionic conductivity of novel membranes compared to Nafion™ 211 Electrochemical impedance spectroscopy was performed to measure the ionic conductivity of the membrane. To do so, a cell and corresponding equivalent circuit were constructed as described by Muller et al. (J.Phys.Chem.B 2014, 118, 1102) and shown in Figure 2. Prior to the measurements, the membrane was immersed in 0.5 M phosphoric acid for 24 hours to simulate actual operating conditions. Measurements were performed using a Gamry 600 potentiostat over a frequency range of 1 MHz to 10 kHz with a voltage amplitude of 10 mV. After fitting to the equivalent circuit, the membrane resistance was calculated. To allow for comparison, the membrane resistance was normalized to the proton conductivity. The cell had a volume of 1.77 cm. 2 The cross-sectional area of the

[0093] In the table below, the calculated ionic conductivities are listed for different paper / ionic liquid combinations and the reference membrane Nafion™ 211. The abbreviations in the table stand for 1-ethyl-3-methylimidazolium acrylate (EMIM-AC), 1-ethyl-3-methylimidazolium vinyl acetate (EMIM-VA), and 1-allyl-3-methylimidazolium methanesulfonate (EMIM-MS).

[0094] [Table 3]

[0095] As can be seen from the results in the table above, the membranes prepared as described in Example 1 have higher ionic conductivity than the reference membrane. High ionic conductivity is an important property for ion exchange membranes.

[0096] Example 5: Battery testing of paper / EMIM-AC membranes Battery testing was performed in a LAB-1x1 cell from C-Tech Innovation Ltd. (Chester, UK) using the redox couple 2-MHQ / MQ vs. p-BQ (parabenzoquinone), BHQ. A homemade pumping system was adjusted to a minimum flow rate of approximately 40 mL / min on both sides of the cell. Flow rate regulation was achieved using an Arduino Uno ATmega328 microcontroller, a Recom LED driver, an LCD display, an RS Pro flow sensor (0.05–10 L / min), an average well power adapter (32 W, 6 A), a Fibox Euronorm cask, a MH Connector Sub-D, and an RS Pro Sub-D Print Connector form RS Components (Gmund, Austria). This setup was used to control an accompanying WPDC 0.45 L-3.1 M-12 rotary pump purchased from Rotek Handels GmbH (Austria). 0.02 g of each active material mixed with 40 mL of 0.5 M H3PO4 was used as the electrolyte. A constant voltage of 0.5 V / -0.5 V was applied during 30 minute charge / discharge cycles. 50 cycles were performed to test the membranes.

[0097] The measured data is shown in Figure 3 and can be divided into three main regions: a start-up phase where the battery needs to stabilize; a stable phase where the battery is performing well; and a breakdown phase where problems occur in the system. The battery is charged and discharged to approximately 4.5 mAh of its calculated capacity. Stable phase 2 is 20 cycles long, which confirms that the battery can be applied to a redox flow system.

[0098] Example 6: Membrane of the present invention in a vanadium redox flow battery A typical commercial vanadium redox flow battery electrolyte (1.6 M in 6 M H2SO4) was deposited into the redox flow battery tank, and the battery was fitted with a paper membrane (fabricated by polymerizing 1-allyl-3-methylimidazolium acetate into a paper matrix) and carbon felt. The battery delivered 50–100 mA / cm 2 The current-voltage curve is shown in Figure 4.

Claims

1. 1. A method for producing an ion exchange membrane, the method comprising: a) applying an ionic liquid containing at least one polymerizable and / or crosslinkable group in its cation and / or anion onto a cellulosic substrate, wherein the cation of the ionic liquid is a heterocyclic aromatic ring or derivative thereof containing at least one nitrogen atom as a heteroatom; b) polymerizing and / or crosslinking said at least one polymerizable and / or crosslinkable group to form a polymer or copolymer layer on said cellulosic substrate.

2. 2. The method for preparing an ion exchange membrane according to claim 1, wherein the polymerizable and / or crosslinkable group is an alkenyl group or an alkynyl group.

3. 3. The method for preparing an ion exchange membrane according to claim 1 or 2, wherein the cation of the ionic liquid is a heterocyclic aromatic containing one, two or three nitrogen atoms as heteroatoms, preferably selected from the group consisting of imidazole salt cations, pyrrole salt cations, pyridine salt cations and derivatives thereof.

4. The method for preparing an ion exchange membrane according to any one of claims 1 to 3, wherein the cation is selected from the group consisting of 1-ethyl-3-methylimidazolium, 1-allyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-vinyl-3-butylimidazolium, 1-ethyl-3-methylimidazolium, 1-hexyl-3-ethylimidazolium, 1-methylimidazolium, 1-hexyl-3-methylimidazolium, 1-butyl-1-methylpiperidinium, 2-hydroxyethyl-trimethylammonium, or 1-butyl-1-methylpyrrolidinium.

5. 5. The method for preparing an ion exchange membrane according to claim 1, wherein the anion of the ionic liquid is selected from the group consisting of acrylate, dicyanamide, acetate, preferably vinyl acetate, phosphonate, preferably vinyl phosphonate, bis((trifluoromethyl)sulfonyl)imide, bis((pentafluoromethyl)sulfonyl)imide, hexafluorophosphate, tetrafluoroborate, methyl sulfate, triflate, thiocyanate, trifluoroacetate, hydrogen sulfate or a halide.

6. 6. The method for preparing an ion exchange membrane according to claim 1, wherein the polymerization is initiated by a radical reaction induced by a chemical method, and / or a photochemical method, and / or a thermochemical method, and / or a plasma method.

7. 7. The method for preparing an ion exchange membrane according to claim 1, wherein the polymerization is initiated by vapor deposition, including oxidative chemical vapor deposition, chemically initiated vapor deposition, and plasma-enhanced chemical vapor deposition, or by the use of UV radiation, thermal activation, and formation of radicals by transition metal complexes using alkyl halides as initiators.

8. 8. The method for preparing an ion exchange membrane according to claim 1, wherein the cellulosic substrate is contacted with at least one polymerizable monomer and / or at least one cross-linking agent after step a.

9. 9. The method for preparing an ion exchange membrane according to claim 8, wherein the at least one polymerizable monomer and / or the at least one cross-linking agent comprises at least one ring structure or at least one alkenyl or alkynyl group that can be opened during the reaction.

10. 10. The method for preparing an ion exchange membrane according to claim 8 or 9, wherein the at least one polymerizable monomer and / or the at least one crosslinking agent is selected from the group consisting of methacrylic acid, methacrylate, divinylbenzene, hexavinyldisiloxane, 1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane, or ethylene glycol dimethacrylate.

11. 11. The method for preparing an ion exchange membrane according to claim 1, wherein the cellulosic substrate is paper derived from a cellulosic raw material that can be used in acidic or alkaline environments without decomposition, can act as a support material for polymers, and allows for covalent or physical bonding of polymers to cellulose or cellulose derivatives.

12. 12. The method for preparing an ion exchange membrane according to any one of claims 1 to 11, wherein the application of the ionic liquid onto the cellulosic substrate is carried out by a gas phase deposition method using a solution-based coating technique, such as blade casting, drop casting, spin coating, dip coating, spray coating, and / or vapor deposition in a vacuum.

13. An ion exchange membrane obtainable by the method according to any one of claims 1 to 12.

14. A redox flow cell comprising the ion exchange membrane of claim 13 between a cathode cell and an anode cell.

15. 14. Use of the ion exchange membrane of claim 13 in a redox flow cell, a fuel cell or a galvanic cell to separate a cathode compartment from an anode compartment.

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