Use of a membrane (m) comprising a non-crosslinked sulfonated polyarylene ether sulfone polymer (SP) in an aqueous redox flow battery (ARFB)

Abstract

The present invention relates to the use of a membrane (M) in an aqueous redox flow battery (ARFB), wherein the membrane (M) comprises a non-crosslinked sulfonated polyarylene ether sulfone polymer (sP). The non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) is prepared by a process comprising the step I) converting a reaction mixture (RG) comprising as components (A1) at least one non-sulfonated aromatic dihalogen sulfone, (A2) at least one sulfonated aromatic dihalogen sulfone, (B) at least one aromatic dihydroxy component, (C) at least one carbonate component, and (D) at least one aprotic polar solvent.

Description

[0001] Use of a membrane (M) comprising a non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) in an aqueous redox flow battery (ARFB)

[0002] Description

[0003] The present invention relates to the use of a membrane (M) in an aqueous redox flow battery (ARFB), wherein the membrane (M) comprises a non-crosslinked sulfonated polyarylene ether sulfone polymer (sP). The non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) is prepared by a process comprising the step I) converting a reaction mixture (RG) comprising as components (A1) at least one non-sulfonated aromatic dihalogen sulfone, (A2) at least one sulfonated aromatic dihalogen sulfone, (B) at least one aromatic dihydroxy component, (C) at least one carbonate component, and (D) at least one aprotic polar solvent.

[0004] Redox flow batteries (RFBs) are researched and commercialized as a promising energy storage system (ESS) to integrate renewable energies produced from intermittent sources into the electricity grid, and to solve the environmental problems caused by sources emitting carbon dioxide. The active materials contained in liquid electrolytes of RFB systems store and release electrical energy by undergoing reversible redox reactions, wherein the performance of RFB systems depends on the properties of their main components such as electrolyte, redox active materials, membrane, and electrode. To make cost-effective RFB systems which do not depend on the cost condition of active materials, organic materials are suggested as redox-active materials. Aqueous redox flow batteries (ARFB) using organic materials have clear benefits over the most widely developed vanadium redox flow batteries (VRFBs). Firstly, the abundance of the metals on earth plays an important role for the costs. Organic compounds are generally cheaper than vanadium metal, which affects the overall costs of the battery. In addition, metals often show a higher toxicity and a higher risk for the environment. Thus, aqueous redox flow batteries using organic materials are developed to decrease the overall costs, the hazard potential of the batteries and to obtain sustainable alternatives. Furthermore, organic compounds show a highly structural diversity and adjustments can often easily be made.

[0005] The pH of supporting electrolytes for ARFB using organic materials systems can be acidic, neutral, or alkaline. Among them, alkaline pH based ARFB systems are most common. However, because the active materials used in catholyte are mostly ferrocyanide, iodide, or ferrocene derivatives that do not involve protons in their redox reactions, their redox reactions and potentials are independent of pH. On the other hand, the active materials used in anolyte are mostly phenazine, alloxazine or quinone derivatives whose redox reactions depend on pH. Therefore, their redox potential decreases with increasing pH. Based on that, to achieve a high cell voltage of ARFB

[0006] EB23-1211 PC December 13, 2024 using organic materials, the redox potential of active material used in anolyte should be low, and most ARFB systems use alkaline supporting electrolytes such as KOH or NaOH.

[0007] The two half-cells in redox flow batteries are separated by membranes. Crucial properties of the membranes are the prevention of the active species crossover but also the permeability towards the supporting electrolyte to ensure the charge balance. The crossover of the active anolyte and catholyte species leads in the most redox flow batteries to a decrease of efficiencies and of the capacity. Thus, in general low permeation rates of the redox active species are needed to minimize self-discharge. In addition, the membrane should exhibit a low resistance to suppress as much as possible ohmic polarization effects leading to low efficiencies. Furthermore, a good chemical stability of the membrane to improve the lifetime of the whole battery and low materials costs are required. Most redox flow batteries utilize ion exchange membranes, which only transport certain dissolved ions and block other and neutral molecules. In aqueous RFBs, that are operated at high pH values of more than 12, predominantly cation exchange membranes (CEMs) are used, since AEMs are generally not stable at high pH values. Moreover, CEMs are highly favorable due to the fact that on both sides only anionic active species are found. Therefore, anolyte or catholyte crossover can be prevented by the negatively charged functional groups of the CEM but the supporting electrolyte containing K+or Na+ions can ensure the charge balance.

[0008] The most often used membrane is the commercially available Nation® membrane, that is based on a perfluorinated polymer containing sulfonic acid ionic groups. This type shows usually a high chemical stability and conductivity, but with thinner Nation® membranes serious active species crossover is observed. In addition to the low ion selectivity, high costs cause problems. The Fumasep® E-620 membrane containing a sulfonated poly(ether ether ketone) (sPEEK) is also commercially available. However, it also suffers from low long-time stability at high pH values and relatively high costs.

[0009] Membranes based on sulfonated polyarylene ether sulfones are known from fuel cells. Polyarylene ether sulfone polymers are high-performance thermoplastics in that they feature high heat resistance, good mechanical properties and inherent flame retardancy E.M. Koch, H.-M. Walter, Kunststoffe 80 (1990) 1146; E. Doring, Kunststoffe 80, (1990) 1149, N. Inchaurondo-Nehm, Kunststoffe 98, (2008) 190).

[0010] In the article “Alkaline naphthoquinone-based redox flow batteries with a crosslinked sulfonated polyphenylsulfone membrane” by W. Lee et al. (Int. J. Energy Res., 2022, 1- 15), the performance of an alkaline aqueous redox flow battery using an isomeric mixture of 1 ,2-naphthoquinone-4-sulfonic acid sodium salt and 2-hydroxy-1 ,4-naphthoquinone (NQSO) and potassium ferrocyanide (FeCN) as active materials dissolved in potassium hydroxide (KOH) is enhanced with replacing a commercial Nation membrane by a thermally crosslinked sulfonated polyphenylsulfone membrane.

[0011] However, membranes comprising a crosslinked sulfonated polyphenylsulfone are often very brittle. In addition, the preparation of these membranes is very inconvenient and thus not suitable for series production.

[0012] The object of the present invention therefore was to provide an improved membrane (M) which can be used in an aqueous redox flow battery (ARFB). The membrane (M) should be chemically and mechanically stable and easy to produce at relatively low costs. In addition, the membrane should show a good conductivity.

[0013] This object is achieved by the use of a membrane (M) in an aqueous redox flow battery (ARFB), wherein the membrane (M) comprises a non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) and the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) is prepared by a process comprising the step

[0014] I) converting a reaction mixture (RG) comprising as components

[0015] (A1) at least one non-sulfonated aromatic dihalogen sulfone,

[0016] (A2) at least one sulfonated aromatic dihalogen sulfone,

[0017] (B) at least one aromatic dihydroxy component,

[0018] (C) at least one carbonate component, and

[0019] (D) at least one aprotic polar solvent.

[0020] It has surprisingly been found that a membrane (M) comprising a non-crosslinked sulfonated polyarylene ether sulfone polymer (sP), wherein the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) is prepared by a process comprising step I), can successfully be used in an aqueous redox flow battery (ARFB), especially in an alkaline aqueous redox flow battery (ARFB) using organic redox-active materials or in an alkaline zinc-iron redox-flow battery.

[0021] The membrane (M) shows a higher conductivity and a higher energy efficiency (EE) in aqueous redox flow batteries (ARFB) compared to the conductivity and energy efficiency (EE) of commercial membranes. In addition, aqueous redox flow batteries (ARFB) using the membrane (M) show a better long-time stability compared to aqueous redox flow batteries (ARFB) using commercial sPEEK membranes. A further advantage of the membranes (M) is their much lower production costs compared to the production costs of, for example, Nation® membranes.

[0022] The present invention will be described in more detail hereinafter.

[0023] Process

[0024] The process for preparing the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) comprises step I) converting a reaction mixture (RG) comprising the components (A1), (A2), (B), (C) and (D) described above.

[0025] The components (A1), (A2) and (B) enter a polycondensation reaction.

[0026] Component (D) acts as a solvent and component (C) acts as a base to deprotonate component (B) during the condensation reaction.

[0027] The reaction mixture (RG) is understood to mean the mixture that is used according to the present invention for preparing the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP). In the present case all details given with respect to the reaction mixture (RG) thus, relate to the mixture that is present prior to the polycondensation. The polycondensation takes place during the process in which the reaction mixture (RG) reacts by polycondensation of components (A1), (A2) and (B) to give the target product, the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP). The mixture obtained after the polycondensation which comprises the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) target product is also referred to as product mixture (PG). The product mixture (PG) usually furthermore comprises the at least one aprotic polar solvent (component (D)) and a halide compound. The halide compound is formed during the conversion of the reaction mixture (RG). During the conversion first, component (C) reacts with component (B) to deprotonate component (B). Deprotonated component (B) then reacts with components (A1) and / or (A2) wherein the halide compound is formed. This process is known to the person skilled in the art.

[0028] The components of the reaction mixture (RG) are generally reacted concurrently. The individual components may be mixed in an upstream step and subsequently be reacted. It is also possible to feed the individual components into a reactor in which these are mixed and then reacted.

[0029] In the process, the individual components of the reaction mixture (RG) are generally reacted concurrently in step I). This reaction is preferably conducted in one stage. This means, that the deprotonation of component (B) and also the condensation reaction between components (A1), (A2) and (B) take place in a single reaction stage without isolation of the intermediate products, for example the deprotonated species of component (B).

[0030] The process according to step I) of the invention is carried out according to the so called “carbonate method”. The process is not carried out according to the so called “hydroxide method”. This means, that the process according to the invention is not carried out in two stages with isolation of phenolate anions. Therefore, in a preferred embodiment, the reaction mixture (RG) is essentially free from sodium hydroxide and potassium hydroxide. More preferably, the reaction mixture (RG) is essentially free from alkali metal hydroxides and alkali earth metal hydroxides.

[0031] The term “essentially free” in the present case is understood to mean that the reaction mixture (RG) comprises less than 100 ppm, preferably less than 50 ppm of sodium hydroxide and potassium hydroxide, preferably of alkali metal hydroxides and alkali earth metal hydroxides, based on the total weight of the reaction mixture (RG).

[0032] It is furthermore preferred that the reaction mixture (RG) does not comprise toluene. It is particularly preferred that the reaction mixture (RG) does not comprise any substance which forms an azeotrope with water.

[0033] Another object of the present invention is therefore also a use wherein the reaction mixture (RG) does not comprise any substance which forms an azeotrope with water.

[0034] The ratio of component (A1), component (A2) and component (B) derives in principle from the stoichiometry of the polycondensation reaction which proceeds with theoretical elimination of hydrogen chloride and is established by the person skilled in the art in a known manner.

[0035] Preferably, the ratio of halogen end groups derived from components (A1) and (A2) to phenolic end groups derived from component (B) is adjusted by controlled establishment of an excess of components (A1) and (A2) in relation to component (B) as starting compound.

[0036] More preferably, the molar ratio of components (A1) and (A2) to component (B) is from 1 to 1.08, especially from 1 to 1.06, most preferably from 1 to 1.05.

[0037] Preferably, the conversion in the polycondensation reaction is at least 0.9.

[0038] Process step I) for the preparation of the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) is typically carried out under conditions of the so called “carbonate method”. This means that the reaction mixture (RG) is reacted under the conditions of the so called “carbonate method”. The reaction (polycondensation reaction) is generally conducted at temperatures in the range from 80 to 250 °C, preferably in the range from 100 to 220 °C. The upper limit of the temperature is determined by the boiling point of the at least one aprotic polar solvent (component (D)) at standard pressure (1013.25 mbar). The reaction is generally carried out at standard pressure. The reaction is preferably carried out over a time interval of 2 to 12 h.

[0039] The isolation of the obtained non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) obtained in the process according to the present invention in the product mixture (PG) may be carried out for example by precipitation of the product mixture (PG) in water, in an alcohol or mixtures of water with an alcohol. Preferably, the isolation of the obtained non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) is carried out by precipitation of the product mixture (PG) in isopropanol. The precipitated non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) can subsequently be extracted with water and then be dried. In one embodiment of the invention, the precipitate can also be taken up in an acidic medium. Suitable acids are for example organic or inorganic acids for example carboxylic acid such as acetic acid, propionic acid, succinic acid or citric acid and mineral acids such as hydrochloric acid, sulfuric acid or phosphoric acid.

[0040] It is possible to filter the product mixture (PG) after step I). The halide compound can thereby be removed.

[0041] The present invention therefore also provides a process wherein the process furthermore comprises step

[0042] II) filtration of the product mixture (PG) obtained in step I).

[0043] Component (A1)

[0044] The reaction mixture (RG) comprises at least one non-sulfonated aromatic dihalogen sulfone as component (A1). Preferably, the reaction mixture (RG) comprises from 40 to 70 mol-% and most preferably from 45 to 65 mol-% of at least one non-sulfonated aromatic dihalogen sulfone as component (A1), based on the sum of the mol-% of components (A1) and (A2).

[0045] The term “at least one non-sulfonated aromatic dihalogen sulfone” in the present case, is understood to mean exactly one non-sulfonated aromatic dihalogen sulfone and also mixtures of two or more non-sulfonated aromatic dihalogen sulfones.

[0046] The at least one non-sulfonated aromatic dihalogen sulfone (component (A1)) is preferably at least one non-sulfonated aromatic dihalodiphenyl sulfone. The present invention therefore also relates to a use wherein the reaction mixture (RG) comprises at least one non-sulfonated dihalodiphenyl sulfone as component (A1).

[0047] “Non-sulfonated” within the context of the present invention means that the aromatic dihalogen sulfone does not comprise groups resulting from the sulfonation of the aromatic dihalogen sulfone. Processes for the sulfonation are known to the skilled person. In particular, “non-sulfonated” within the context of the present invention means that the aromatic dihalogen sulfone does not comprise any -SO2X group wherein X is selected from the group consisting of OH, O and one cation equivalent and a halogen such as Cl, Br or I.

[0048] “One cation equivalent” within the context of the present invention means one cation of a single positive charge or one charge equivalent of a cation with two or more positive charges, for example Li+, Na+, K+, Mg2+, Ca2+or NH4+.

[0049] Component (A1) is preferably used as a monomer. This means that the reaction mixture (RG) comprises component (A1) preferably as a monomer and not as a prepolymer.

[0050] Preferred non-sulfonated aromatic dihalogen sulfones are non-sulfonated 4,4‘-dihalodiphenyl sulfones. Particular preference is given to 4,4‘-dichlorodiphenyl sulfone, 4,4‘-difluorodiphenyl sulfone and / or 4,4‘-dibromodiphenyl sulfone. 4,4‘-dichlorodiphenyl sulfone and 4,4‘-difluorodiphenyl sulfone are particularly preferred, while 4,4‘-dichlorodiphenyl sulfone is most preferred.

[0051] Another object of the present invention is therefore also a use wherein component (A1) is selected from the group consisting of 4,4’-dichlorodiphenyl sulfone and 4,4’- difluorodiphenyl sulfone.

[0052] The present invention therefore also relates to a use wherein component (A1) comprises at least 40 % by weight of at least one non-sulfonated aromatic dihalogen sulfone selected from the group consisting of 4,4‘-dichlorodiphenyl sulfone and 4,4‘-difluorodiphenyl sulfone, based on the total weight of component (A1) in the reaction mixture (RG).

[0053] In a particularly preferred embodiment, component (A1) comprises at least 80 % by weight, preferably at least 90 % by weight, more preferably at least 98 % by weight, of at least one non-sulfonated aromatic dihalogen sulfone selected from the group consisting of 4,4‘-dichlorodiphenyl sulfone and 4,4‘-difluorodiphenyl sulfone, based on the total weight of component (A1) in the reaction mixture (RG). In a further particularly preferred embodiment, component (A1) consists essentially of at least one non-sulfonated aromatic dihalogen sulfone selected from the group consisting of 4,4‘-dichlorodiphenyl sulfone and 4,4‘-difluorodiphenyl sulfone.

[0054] “Consisting essentially of”, in the present case is understood to mean that component (A1) comprises more than 99 % by weight, preferably more than 99.5 % by weight, particularly preferably more than 99.9 % by weight of at least one non-sulfonated aromatic dihalogen sulfone compound selected from the group consisting of 4,4‘-dichlorodiphenyl sulfone and 4,4‘-difluorodiphenyl sulfone, based in each case on the total weight of component (A1) in the reaction mixture (RG). In these embodiments, 4,4‘-dichlorodiphenyl sulfone is particularly preferred as component (A1).

[0055] In a further preferred embodiment, component (A1) consists of 4,4‘-dichlorodiphenyl sulfone.

[0056] Component (A2)

[0057] The reaction mixture (RG) comprises at least one sulfonated aromatic dihalogen sulfone as component (A2).

[0058] The term “at least one sulfonated aromatic dihalogen sulfone” in the present case, is understood to mean exactly one sulfonated aromatic dihalogen sulfone and also mixtures of two or more sulfonated aromatic dihalogen sulfones.

[0059] “Sulfonated” within the context of the present invention means that the aromatic dihalogen sulfone comprises at least one group resulting from the sulfonation of the aromatic dihalogen sulfone. The sulfonation of aromatic dihalogen sulfones is known to the skilled person. In particular, “sulfonated” means that the aromatic dihalogen sulfone comprises at least one -SO3Y group wherein Y is hydrogen or a cation equivalent.

[0060] “Cation equivalent” within the context of the present invention means a cation of a single positive charge or one charge equivalent of a cation with two or more positive charges, for example Li+, Na+, K+, Mg2+, Ca2+, NH4+, preferably Na+, K+.

[0061] “At least one -SO3Y group” within the context of the present invention means precisely one -SO3Y group and also two or more -SO3Y groups. Preferred are precisely two -SO3Y groups. This means that the at least one sulfonated aromatic dihalogen sulfone is preferably at least one disulfonated aromatic halogen sulfone.

[0062] Another object of the present invention is therefore also a use wherein component (A2) is at least one disulfonated aromatic dihalogen sulfone. The reaction mixture (RG) comprises preferably from 30 to 60 mol-% and more preferably from 35 to 55 mol-% of at least one sulfonated aromatic dihalogen sulfone as component (A2) based on the sum of the mol-% of components (A1) and (A2).

[0063] The sum of the mol-% of components (A1) and (A2) usually is 100 mol-%.

[0064] In a preferred embodiment, the reaction mixture (RG) comprises component (A1) in an amount of from 40 to 70 mol-% and component (A2) in an amount of from 30 to 60 mol- %, based on the sum of the mol-% of components (A1) and (A2).

[0065] Therefore, another object of the present invention is also a use wherein the reaction mixture (RG) comprises component (A1) in an amount of from 40 to 70 mol-% and component (A2) in an amount of from 30 to 60 mol-%, based on the sum of the mol-% of components (A1) and (A2).

[0066] Component (A2) is preferably selected from the group consisting of 4,4‘-dichlorodiphenyl sulfone-3,3’-disulfonic acid, 4,4‘-difluorodiphenylsulfone-3,3’-disulfonic acid, 4,4'- dichloro-diphenylsulfone-3,3'-disulfonic acid disodium salt, 4,4'-dichlorodiphenylsulfone- 3,3'-disulfonic acid dipotassium salt, 4,4'-difluorodiphenylsulfone-3,3'-disulfonic acid disodium salt and 4,4'-difluorodiphenylsulfone-3,3'-disulfonic acid dipotassium salt.

[0067] It is furthermore preferred that component (A2) comprises at least 50 % by weight of at least one sulfonated aromatic dihalogen sulfone selected from the group consisting of 4,4‘-dichlorodiphenyl sulfone-3,3’-disulfonic acid, 4,4‘- difluorodiphenylsulfone-3,3’-disulfonic acid, 4,4'-dichloro-diphenylsulfone-3,3'-disulfonic acid disodium salt, 4,4'-dichlorodiphenylsulfone-3,3'-disulfonic acid dipotassium salt, 4,4'-difluorodiphenylsulfone-3,3'-disulfonic acid disodium salt and 4,4'- difluorodiphenylsulfone-3,3'-disulfonic acid dipotassium salt, based on the total weight of component (A2).

[0068] The present invention therefore also relates to a use wherein component (A2) comprises at least 50 % by weight of at least one sulfonated aromatic dihalogen sulfone selected from the group consisting of 4,4‘-dichlorodiphenyl sulfone-3,3’-disulfonic acid, 4,4‘- difluorodiphenylsulfone-3,3’-disulfonic acid, 4,4'-dichloro-diphenylsulfone-3,3'-disulfonic acid disodium salt, 4,4'-dichlorodiphenylsulfone-3,3'-disulfonic acid dipotassium salt, 4,4'-difluorodiphenylsulfone-3,3'-disulfonic acid disodium salt and 4,4'- difluorodiphenylsulfone-3,3'-disulfonic acid dipotassium salt, based on the total weight of component (A2) in the reaction mixture (RG).

[0069] In a particularly preferred embodiment component (A2) comprises at least 80 % by weight, preferably at least 90 % by weight, more preferably at least 98 % by weight of at least one sulfonated aromatic dihalogen sulfone selected from the group consisting of 4,4‘-dichlorodiphenyl sulfone-3,3’-disulfonic acid, 4,4‘- difluorodiphenylsulfone-3,3’-disulfonic acid, 4,4'-dichloro-diphenylsulfone-3,3'-disulfonic acid disodium salt, 4,4'-dichlorodiphenylsulfone-3,3'-disulfonic acid dipotassium salt, 4,4'-difluorodiphenylsulfone-3,3'-disulfonic acid disodium salt and 4,4'- difluorodiphenylsulfone-3,3'-disulfonic acid dipotassium salt, based on the total weight of component (A2) in the reaction mixture (RG).

[0070] The terms “sulfonic acid” and “-SO3Y group” in the context of component (A2) are used synonymously and have the same meaning. The term “sulfonic acid” in the 4,4’-dichlorodiphenyl sulfone-3,3’-disulfonic acid and 4,4‘-difluorodiphenyl sulfone-3,3’-disulfonic acid therefore means “-SO3Y group” wherein Y is hydrogen or a cation equivalent.

[0071] In a further particularly preferred embodiment component (A2) consists essentially of at least one sulfonated aromatic dihalogen sulfone selected from the group consisting of 4,4‘-dichlorodiphenyl sulfone-3,3’-disulfonic acid, 4,4‘-difluorodiphenyl sulfone-3,3’-disulfonic acid, 4,4'-dichloro-diphenylsulfone-3,3'-disulfonic acid disodium salt, 4,4'-dichlorodiphenylsulfone-3,3'-disulfonic acid dipotassium salt, 4,4'- difluorodiphenylsulfone-3,3'-disulfonic acid disodium salt and 4,4'- difluorodiphenylsulfone-3,3'-disulfonic acid dipotassium salt.

[0072] “Consisting essentially of” in the present case is understood to mean that component (A2) comprises more than 95 % by weight, preferably more than 97 % by weight, particularly preferably more than 99 % by weight of at least one sulfonated aromatic dihalogen sulfone selected from the group consisting of 4,4‘-dichlorodiphenyl sulfone-3,3’-disulfonic acid, 4,4‘-difluorodiphenyl sulfone-3,3’-disulfonic acid, 4,4'- dichloro-diphenylsulfone-3,3'-disulfonic acid disodium salt, 4,4'-dichlorodiphenylsulfone- 3,3'-disulfonic acid dipotassium salt, 4,4'-difluorodiphenylsulfone-3,3'-disulfonic acid disodium salt and 4,4'-difluorodiphenylsulfone-3,3'-disulfonic acid dipotassium salt, based on the total weight of component (A2) in the reaction mixture (RG).

[0073] 4,4'-dichlorodiphenylsulfone-3,3'-disulfonic acid dipotassium salt and 4,4'- dichlorodiphenylsulfone-3,3'-disulfonic acid disodium salt are particularly preferred as component (A2).

[0074] In a further particularly preferred embodiment, component (A2) consists of 4,4'- dichlorodiphenylsulfone-3,3'-disulfonic acid dipotassium salt or 4,4'- dichlorodiphenylsulfone-3,3'-disulfonic acid disodium salt.

[0075] Component (B) The reaction mixture (RG) comprises at least one aromatic dihydroxy component (B). The term “at least one aromatic dihydroxy component”, in the present case, is understood to mean exactly one aromatic dihydroxy component and also mixtures of two or more aromatic dihydroxy components. Preferably, component (B) is precisely one aromatic dihydroxy component or a mixture of precisely two aromatic dihydroxy components. Most preferred component (B) is precisely one aromatic dihydroxy component.

[0076] The aromatic dihydroxy components used are typically components having two phenolic hydroxyl groups. Since the reaction mixture (RG) comprises at least one carbonate component, the hydroxyl groups of component (B) in the reaction mixture (RG) may be present partially in deprotonated form.

[0077] Component (B) is preferably used as a monomer. This means that the reaction mixture (RG) comprises component (B) preferably as monomer and not as prepolymer.

[0078] Preferably, component (B) is selected from the group consisting of 4,4’- dihydroxybiphenyl, 4,4'-dihydroxydiphenylsulfone, bisphenol A (2,2-bis(4- hydroxyphenyl)propane), 4,4'-dihydroxybenzophenone and hydroquinone. From among the aforementioned aromatic dihydroxy components, 4,4’-dihydroxybiphenyl, 4,4'- dihydroxydiphenyl sulfone and bisphenol A are preferable, while 4,4’-dihydroxybiphenyl is particularly preferable.

[0079] The present invention accordingly also provides a use wherein component (B) is selected from the group consisting of 4,4‘-dihydroxybiphenyl, 4,4'-dihydroxydiphenyl sulfone, bisphenol A, 4,4'-dihydroxybenzophenone and hydroquinone.

[0080] Said 4,4’-dihydroxybiphenyl, said 4,4'-dihydroxydiphenyl sulfone, said bisphenol A (2,2- bis(4-hydroxyphenyl)propane), said 4,4'-dihydroxybenzophenone and said hydroquinone may here be used in pure form or as a technical-grade product, which may comprise up to 2 wt%, preferably up to 1 wt% and more preferably up to 0.5 wt% of impurities, all based on the overall weight of the 4,4’-dihydroxybiphenyl, 4,4'- dihydroxydiphenyl sulfone used, the bisphenol A (2,2-bis(4-hydroxyphenyl)propane) used, the 4'4-dihydroxybenzophenone used and the hydroquinone used. Any impurities present are included in the wt% percentages relating to component (B).

[0081] Preferably, component (B) comprises not less than 80 wt%, preferably not less than 90 wt% and more preferably not less than 98 wt% of 4,4'-dihydroxybiphenyl, based on the overall weight of component (B) in reaction mixture (RG).

[0082] The weight percentages here in relation to component (B) further relate to the sum total of the 4,4’-dihydroxybiphenyl, 4,4'-dihydroxydiphenyl sulfone, bisphenol A (2,2-bis-(4- hydroxyphenyl)propane), 4,4'-dihydroxybenzophenone and hydroquinone used. In a further particularly preferred embodiment, component (B) consists essentially of at least one aromatic dihydroxy component selected from the group consisting of 4,4’- dihydroxybiphenyl, 4,4'-dihydroxydiphenylsulfone, bisphenol A (2,2-bis(4-hydroxy- phenyl)propane), 4,4’-dihydroxybenzophenone and hydroquinone. What is meant herein by "consisting essentially of" is that component (B) comprises more than 99 wt%, preferably more than 99.5 wt% and more preferably more than 99.9 wt% of at least one aromatic dihydroxy component selected from the group consisting of 4,4’- dihydroxybiphenyl, 4,4'-dihydroxydiphenylsulfone, bisphenol A (2,2-bis(4- hydroxyphenyl)propane), 4,4'-dihydroxybenzophenone and hydroquinone, all based on the overall weight of component (B) in reaction mixture (RG). In these embodiments, 4,4’- dihydroxybiphenyl, bisphenol A and 4,4'-dihydroxydiphenylsulfone are particularly preferable for use as component (B), while 4,4'-dihydroxybiphenyl is most preferable.

[0083] It is preferable that the non-crosslinked sulfonated polyarylene ether sulfone polymers (sP) have halogen groups, in particular terminal chlorine groups.

[0084] The reaction mixture (RG) comprises at least one carbonate component as component (C). The term “at least one carbonate component” in the present case, is understood to mean exactly one carbonate component and also mixtures of two or more carbonate components. The at least one carbonate component is preferably at least one metal carbonate. The metal carbonate is preferably anhydrous.

[0085] Preference is given to alkali metal carbonates and / or alkaline earth metal carbonates as metal carbonates. At least one metal carbonate selected from the group consisting of sodium carbonate, potassium carbonate and calcium carbonate is particularly preferred as metal carbonate. Potassium carbonate is most preferred.

[0086] For example, component (C) comprises at least 50 % by weight, more preferred at least 70 % by weight and most preferred at least 90 % by weight of potassium carbonate based on the total weight of the at least one carbonate component in the reaction mixture (RG).

[0087] Another object of the present invention is therefore also a use wherein component (C) comprises at least 50 % by weight of potassium carbonate, based on the total weight of component (C).

[0088] In a preferred embodiment component (C) consists essentially of potassium carbonate. “Consisting essentially of” in the present case is understood to mean that component (C) comprises more than 99 % by weight, preferably more than 99.5 % by weight, particular preferably more than 99.9 % by weight of potassium carbonate based in each case on the total weight of component (C) in the reaction mixture (RG).

[0089] In a particularly preferred embodiment component (C) consists of potassium carbonate.

[0090] Potassium carbonate having a volume weighted average particle size of less than 200 pm is particularly preferred as potassium carbonate. The volume weighted average particle size of the potassium carbonate is determined in a suspension of potassium carbonate in N-methylpyrrolidone using a particle size analyser.

[0091] In a preferred embodiment, the reaction mixture (RG) does not comprise any alkali metal hydroxides or alkaline earth metal hydroxides.

[0092] Component (D)

[0093] The reaction mixture (RG) comprises at least one aprotic polar solvent as component (D). “At least one aprotic polar solvent”, according to the invention, is understood to mean exactly one aprotic polar solvent and also mixtures of two or more aprotic polar solvents.

[0094] Suitable aprotic polar solvents are, for example, selected from the group consisting of anisole, dimethylformamide, dimethyl sulfoxide, N-methylpyrrolidone, N-ethylpyrrolidone, sulfolane and N-dimethylacetamide.

[0095] Preferably, component (D) is selected from the group consisting of N-methylpyrrolidone, N-dimethylacetamide, dimethyl sulfoxide, sulfolane and dimethylformamide. N-methylpyrrolidone is particularly preferred as component (D).

[0096] Another object of the present invention is therefore also a use wherein component (D) is selected from the group consisting of N-methylpyrrolidone, N-dimethylacetamide, dimethyl sulfoxide, sulfolane and dimethylformamide.

[0097] It is preferred that component (D) comprises at least 50 % by weight of at least one solvent selected from the group consisting of N-methylpyrrolidone, N- dimethylacetamide, dimethyl sulfoxide, sulfolane and dimethylformamide, based on the total weight of component (D) in the reaction mixture (RG). N-methylpyrrolidone is particularly preferred as component (D).

[0098] In a further preferred embodiment, component (D) consists essentially of N-methylpyrrolidone. “Consist essentially of’, in the present case, is understood to mean that component (D) comprises more than 98 % by weight, particularly preferably more than 99 % by weight, more preferably more than 99.5 % by weight, of at least one aprotic polar solvent selected from the group consisting of N-methylpyrrolidone, N-dimethylacetamide, dimethylsulfoxide, sulfolane and dimethylformamide with preference given to N- methylpyrrolidone.

[0099] In a preferred embodiment, component (D) consists of N-methylpyrrolidone. N-methylpyrrolidone is also referred to as NMP or N-methyl-2-pyrrolidone.

[0100] Non-crosslinked sulfonated polyarylether sulfone polymer (sP)

[0101] The non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) obtainable by the above-described process comprises units that are derived from component (A1) and from component (B), as well as units that are derived from component (A2) and from component (B). In a preferred embodiment, the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) consists of units that are derived from component (A1) and from component (B), as well as units that are derived from component (A2) and from component (B).

[0102] Preferred non-crosslinked sulfonated polyarylene ether sulfone polymers (sP) comprise repeating units of the general formula I: where t and q : are each independently 0, 1 , 2 or 3,

[0103] Q, T and Y: are each independently a chemical bond or selected from -O-, -S-, -SO2-, -S(=O)-, -C(=O)-, -N=N-, and -CRaRb-, wherein Raand Rbare each independently a hydrogen atom or a C C12- alkyl, C C^-alkoxy or C6-C18-aryl group, and wherein at least one of Q, T and Y is -SO2-,

[0104] Ar and Ar1: are each independently C6-C18aryl, wherein said C6-C18aryl is unsubstituted or substituted with at least one substituent selected from Cr C12alkyl, C C12alkoxy, C6-C18aryl, halogen and -SO3X, p, m, n, and k:are each independently 0, 1 , 2, 3 or 4, with the proviso that the sum total of p, m, n and k is not less than 1 , and

[0105] X: is hydrogen or one cation equivalent.

[0106] If Q, T, or Y, with the abovementioned preconditions, is a chemical bond, this means that the adjacent group on the left-hand side and the adjacent group on the right-hand side have direct linkage to one another by way of a chemical bond.

[0107] Raand Rbare preferably each independently hydrogen or CrC12alkyl.

[0108] Preferred CrC12alkyl groups include linear and branched, saturated alkyl groups of 1 to 12 carbon atoms. The following moieties are suitable in particular: C Ce alkyl, such as methyl, ethyl, n-propyl, i-propyl, n-butyl, sec-butyl, 2- or 3-methylpentyl or comparatively long-chain moieties such as unbranched heptyl, octyl, nonyl, decyl, undecyl, lauryl, and the branched analogs thereof.

[0109] Alkyl moieties in the CrC12alkoxy groups used include the above-defined alkyl groups of 1 to 12 carbon atoms. Preferably used cycloalkyl moieties include in particular C3-C12cycloalkyl moieties, e.g., cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclopropylmethyl, cyclopropylethyl, cyclopropylpropyl, cyclobutylmethyl, cyclobutylethyl, cyclopentylethyl, -propyl, -butyl, -pentyl, -hexyl, -cyclohexylmethyl, -dimethyl, -trimethyl.

[0110] Ar and Ar1are each independently C6-C18aryl. Proceeding from the starting materials hereinbelow, Ar preferably derives from an electron-rich aromatic substance very susceptible to electrophilic attack, preferably selected from the group consisting of sulfonated or unsulfonated hydroquinone, resorcinol, dihydroxynaphthalene, in particular 2,7-dihydroxynaphthalene and 4,4'-bisphenol. Ar1is preferably an unsubstituted C6or C12arylene group.

[0111] Ar and Ar1in the preferred embodiment of formula (I) are each preferably selected independently from sulfonated or unsulfonated 1 ,4-phenylene, 1 ,3-phenylene, naphthylene, in particular 2,7-dihydroxynaphthalene and 4,4'-bisphenylene.

[0112] The membrane (M) of the present invention preferably comprises at least one noncrosslinked sulfonated polyarylene ether sulfone polymer (sP) having the following structural units (la) to (Io):

[0113] where

[0114] I, k, m, n, o, p are each independently 0, 1 , 2, 3 or 4 subject to the proviso that the sum total of I, k, m, n, o and p is >1 , and

[0115] X is hydrogen or one cation equivalent.

[0116] By "one cation equivalent" in the context of the present invention is meant one cation of a single positive charge or one charge equivalent of a cation with two or more positive charges, for example Li, Na, K, Mg, Ca, NH4, preferably Na, K.

[0117] In addition to the preferred building blocks (la) to (Io), preference is also given to those structural units in which one or more unsulfonated 1 ,4-dihydroxyphenyl units are replaced by resorcinol or dihydroxynaphthalene.

[0118] Copolymers constructed of the various structural units in combination or of sulfonated and non-sulfonated structural units are also usable.

[0119] Structural units (la), (lb), (Ig) and (Ik) or copolymers thereof are used with particular preference as repeat unit of general formula (I).

[0120] In one particularly preferred embodiment, Ar is 1 ,4-phenylene, t is 1 , T is a chemical bond, Y is -SO2-, q is 0, p is 0, m is 0, n is 1 and k is 1. Non-crosslinked sulfonated polyarylene ether sulfone polymers (sP) constructed of this recited structural repeat unit are denoted sPPSU.

[0121] In a particularly preferred embodiment, Ar is 1 ,4-phenylene, t is 0, Y is -SO2-, q is 0, n is 1 and k is 1. Non-crosslinked sulfonated polyarylene ether sulfone polymers (sP) constructed of this recited structural repeat unit are denoted sulfonated polyether ether sulfones (sPEES). The non-crosslinked sulfonated polyarylene ether sulfone polymers (sP) used according to the present invention preferably have a viscosity number of 20 ml / g to 150 ml / g, preferably of 20 ml / g to 120 ml / g. This viscosity number is quantified according to DIN EN ISO 1628-1 in a 1% solution of N-methylpyrrolidone (NMP) at 25°C.

[0122] The weight average molecular weight (Mw) of the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) used in the method of the present invention is generally in the range from 10 000 to 250 000 g / mol, preferably in the range from 15 000 to 200 000 g / mol and more preferably in the range from 18 000 to 180 000 g / mol. The weight average molecular weights (Mw) are measured using gel permeation chromatography (GPC). Dimethylacetamide (DMAc) was used as solvent and narrowly distributed polymethyl methacrylate was used as standard in the measurement.

[0123] Preferably, the sulfonation degree of the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) is at least 30 %, more preferably at least 35 %. The sulfonation degree of the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) is determined by1H-NMR-spectroscopy.

[0124] Membrane (M)

[0125] The non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) obtained by the process described above is comprised in a membrane (M).

[0126] “Non-crosslinked” in the present case is understood to mean that the chains of the sulfonated polyarylene ether sulfone polymer (sP) are not linked to another. To avoid cross-linking of the sulfonated polyarylene ether sulfone polymer (sP), the membrane is after its preparation not thermally treated.

[0127] The membrane (M) comprises preferably at least 50 % by weight of the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP), more preferably at least 70 % by weight and most preferably at least 90 % by weight of the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) based on the total weight of the membrane (M).

[0128] In a further preferred embodiment, the membrane (M) consists essentially of the non- crosslinked sulfonated polyarylene ether sulfone polymer (sP).

[0129] “Consisting essentially of” means that the membrane (M) comprises more than 93% by weight, preferably more than 95% by weight and most preferably more than 97% by weight of the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) based on the total weight of the membrane (M). During the formation of the membrane (M) the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) is separated from at least one solvent. Therefore, the obtained membrane (M) is essentially free from the at least one solvent.

[0130] “Essentially free” within the context of the present invention means that the membrane (M) comprises at most 7 % by weight, preferably at most 5 % by weight and particularly preferably at most 3 % by weight of the at least one solvent based on the total weight of the membrane (M). The membrane (M) comprises at least 0.0001 % by weight, preferably at least 0.001 % by weight and particularly preferably at least 0.01 % by weight of the at least one solvent based on the total weight of the membrane (M).

[0131] To the person skilled in the art, it is clear that if in one embodiment of the present invention additives for the membrane preparation are used in the preparation of the membrane (M) then the membrane (M) usually furthermore comprises the additives for the membrane preparation. For example, the membrane (M) then comprises in the range from 0.1 to 10 % by weight, preferably in the range from 0.15 to 7.5 % by weight and most preferably in the range from 0.2 to 5 % by weight of the additives for membrane preparation, based on the total weight of the membrane (M).

[0132] Preferably, the membrane (M) is a dense membrane.

[0133] If the membrane (M) is a dense membrane, then the membrane (M) typically comprises virtually no pores.

[0134] A dense membrane is typically obtained by a solution casting process in which a solvent comprised in the casted solution is evaporated. Usually the solution (S) is casted on a support, which might be another polymer like polysulfone, cellulose acetate or polyethylene terephthalate.

[0135] The membrane might also be reinforced by a non-woven, the non-woven preferably has a thickness of 15 to 70 pm and preferably consists of polyphenylensulfide fibers, polyetheretherketone fibers, LC polyester fibers or polyester fibers.

[0136] The membrane (M) can have any thickness. For example, the thickness of the membrane (M) is in the range from 2 to 1000 pm, preferably in the range from 3 to 300 pm and most preferably in the range from 5 to 150 pm.

[0137] The membrane (M) can be used in an aqueous redox flow battery (ARFB), preferably in an alkaline aqueous redox flow battery (ARFB). Examples for alkaline aqueous redox flow batteries (ARFB) are alkaline aqueous redox flow batteries (ARFB) using organic redox-active materials or an alkaline zinc-iron redoxflow battery.

[0138] Preferably, the aqueous redox flow battery (ARFB) comprises an anolyte, wherein in discharged state, the anolyte comprises derivatives of anthraquinone, naphthoquinone or phenazine, dissolved in an alkaline or neutral solution.

[0139] Another object of the present invention is therefore also a use wherein the aqueous redox flow battery (ARFB) comprises an anolyte, wherein in discharged state, the anolyte comprises derivatives of anthraquinone, naphthoquinone or phenazine, dissolved in an alkaline or neutral solution.

[0140] Further, it is preferred that the aqueous redox flow battery (ARFB) comprises a catholyte, wherein in discharged state, the catholyte comprises at least one compound selected from the group consisting of potassium hexacyanoferrate (II), sodium hexacyanoferrate (II), lithium hexacyanoferrate (II), ammonium hexacyanoferrate (II), potassium iodide, sodium iodide, lithium iodide and ammonium iodide, wherein the at least one compound is dissolved in an alkaline or neutral solution.

[0141] Another object of the present invention is therefore also a use wherein the aqueous redox flow battery (ARFB) comprises a catholyte, wherein in discharged state, the catholyte comprises at least one compound selected from the group consisting of potassium hexacyanoferrate (II), sodium hexacyanoferrate (II), lithium hexacyanoferrate (II), ammonium hexacyanoferrate (II), potassium iodide, sodium iodide, lithium iodide and ammonium iodide, wherein the at least one compound is dissolved in an alkaline or neutral solution.

[0142] The alkaline or neutral solution preferably comprises water and optionally, alkali or ammonium hydroxide.

[0143] Another object of the present invention is therefore also a use wherein the alkaline or neutral solution comprises water and optionally, alkali or ammonium hydroxide.

[0144] For example, a neutral solution is prepared by dissolving an organic compound in stoichiometric alkaline to reach pH 7, and adding some salt like sodium chloride (NaCI) or potassium chloride (KCI) to get appropriate conductivity.

[0145] Preferably, the aqueous redox flow battery (ARFB) comprises a first half-cell comprising a first electrode and a second half-cell comprising a second electrode, wherein the first electrode is in contact with the anolyte and the second electrode is in contact with the catholyte. Therefore, another object of the present invention is also a use wherein the aqueous redox flow battery (ARFB) comprises a first half-cell comprising a first electrode and a second half-cell comprising a second electrode, wherein the first electrode is in contact with the anolyte and the second electrode is in contact with the catholyte.

[0146] In case the anolyte comprises 2,6-dihydroxyanthraquinone dissolved in an alkaline solution and the catholyte comprises potassium hexacyanoferrate (II) dissolved in an alkaline solution, the following reactions take place during charging and discharging, respectively (Equations (4) and (5)):

[0147] Equation (4):

[0148] Equation (5):

[0149] Membrane preparation

[0150] A membrane (M) can be prepared from the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) according to the present invention by any method known to the skilled person.

[0151] Preferably, the membrane (M) comprising the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) is prepared by a method comprising the steps i) providing a solution (S) which comprises the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) and at least one solvent, ii) separating the at least one solvent from the solution (S) to obtain the membrane (M).

[0152] Another object of the present invention is therefore a use wherein the membrane (M) is prepared by a method comprising the steps i) providing a solution (S) which comprises the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) and at least one solvent, ii) separating the at least one solvent from the solution (S) to obtain the membrane (M).

[0153] Step i)

[0154] In step i) a solution (S) is provided which comprises the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) and at least one solvent.

[0155] “At least one solvent” within the context of the present invention means precisely one solvent and also a mixture of two or more solvents.

[0156] The solution (S) can be provided in step i) by any method known to the skilled person. For example, the solution (S) can be provided in step i) in customary vessels which may comprise a stirring device and preferably a temperature control device. Preferably, the solution (S) is provided by dissolving the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) in the at least one solvent.

[0157] The dissolution of the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) in the at least one solvent to provide the solution (S) is preferably effected under agitation.

[0158] Step i) is preferably carried out at elevated temperatures, especially in the range from 20 to 120 °C, more preferably in the range from 40 to 100 °C. A person skilled in the art will choose the temperature in accordance with the at least one solvent.

[0159] The solution (S) preferably comprises the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) completely dissolved in the at least one solvent. This means that the solution (S) preferably comprises no solid particles of the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP). Therefore, the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) preferably cannot be separated from the at least one solvent by filtration.

[0160] The solution (S) preferably comprises from 0.001 to 50 % by weight of the non- crosslinked sulfonated polyarylene ether sulfone polymer (sP) based on the total weight of the solution (S). More preferably, the solution (S) in step i) comprises from 0.1 to 30 % by weight of the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) and most preferably the solution (S) comprises from 0.5 to 25 % by weight of the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) based on the total weight of the solution (S). Another object of the present invention is therefore also a use, wherein the solution (S) in step i) comprises from 0.1 to 30 % by weight of the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP), based on the total weight of the solution (S).

[0161] As the at least one solvent, any solvent known to the skilled person for the noncrosslinked sulfonated polyarylene ether sulfone polymer (sP) is suitable. Preferably, the at least one solvent is soluble in water. Therefore, the at least one solvent is preferably selected from the group consisting of N-methylpyrrolidone, dimethylacetamide, dimethylsulfoxide, dimethyllactamide, dimethylformamide and sulfolane. N- methylpyrrolidone and dimethyllactamide are particularly preferred.

[0162] Another object of the present invention is therefore also a use wherein the at least one solvent is selected from the group consisting of N-methylpyrrolidone, dimethylacetamide, dimethyl sulfoxide, dimethylformamide, dimethyllactamide and sulfolane.

[0163] The solution (S) preferably comprises in the range from 50 to 99.999 % by weight of the at least one solvent, more preferably in the range from 70 to 99.9 % by weight and most preferably in the range from 75 to 99.5 % by weight of the at least one solvent based on the total weight of the solution (S).

[0164] The solution (S) provided in step i) can furthermore comprise additives for the membrane preparation.

[0165] Suitable additives for the membrane preparation are known to the skilled person and are, for example, polyvinylpyrrolidone (PVP), polyethylene oxide (PEO), polyethylene oxide-polypropylene oxide copolymer (PEO-PPO) and poly(tetrahydrofurane) (poly- THF). Polyvinylpyrrolidone (PVP) and polyethylene oxide (PEO) are particularly preferred as additives for the membrane preparation.

[0166] The additives for membrane preparation can, for example, be comprised in the solution (S) in an amount of from 0.01 to 20 % by weight, preferably in the range from 0.1 to 15 % by weight and more preferably in the range from 1 to 10 % by weight based on the total weight of the solution (S).

[0167] To the person skilled in the art, it is clear that the percentages by weight of the noncrosslinked sulfonated polyarylene ether sulfone polymer (sP), the at least one solvent and the optionally comprised additive for membrane preparation comprised in the solution (S) typically add up to 100 % by weight.

[0168] The duration of step i) may vary between wide limits. The duration of step i) is preferably in the range from 10 min to 48 h (hours), especially in the range from 10 min to 24 h and more preferably in the range from 15 min to 12 h. A person skilled in the art will choose the duration of step i) so as to obtain a homogeneous solution of the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) in the at least one solvent.

[0169] For the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) comprised in the solution (S) the embodiments and preferences given for the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) obtained in the process described above hold true.

[0170] Step ii)

[0171] In step ii) the at least one solvent is separated from the solution (S) to obtain the membrane (M). It is possible to filter the solution (S) provided in step i) before the at least one solvent is separated from the solution (S) in step ii) to obtain a filtered solution (fS). The following embodiments and preferences for separating the at least one solvent from the solution (S) apply equally for separating the at least one solvent from the filtered solution (fS) which is used in this embodiment of the invention.

[0172] Moreover, it is possible to degas the solution (S) in step i) before the at least one solvent is separated from the solution (S) in step ii) to obtain a degassed solution (dS). This embodiment is preferred. The following embodiments and preferences for separating the at least one solvent from the solution (S) apply equally for separating the at least one solvent from the degassed solution (dS) which is used in this embodiment of the invention.

[0173] The degassing of the solution (S) in step i) can be carried out by any method known to the skilled person, for example via vacuum or by allowing the solution (S) to rest.

[0174] The separation of the at least one solvent from the solution (S) can be performed by any method known to the skilled person which is suitable to separate solvents from polymers.

[0175] Preferably, the separation of the at least one solvent from the solution (S) is carried out via evaporation.

[0176] Another object of the present invention is therefore also a use, wherein the separation of the at least one solvent in step ii) is carried out via evaporation.

[0177] Step ii) usually comprises a provision of the solution (S) in a form that corresponds to the form of the membrane (M) which is obtained in step ii).

[0178] Therefore, in one embodiment of the present invention step ii) comprises a casting of the solution (S) to obtain a film of the solution (S). Therefore, in one preferred embodiment of the present invention, step ii) comprise the following steps: ii-1) casting the solution (S) provided in step i) to obtain a film of the solution (S), ii-2) evaporating the at least one solvent from the film of the solution (S) obtained in step ii-1) to obtain the membrane (M) which is in the form of a film.

[0179] This means that the membrane (M) is formed by evaporating the at least one solvent from a film of the solution (S).

[0180] In step ii-1) the solution (S) can be cast by any method known to the skilled person. Usually, the solution (S) is cast with a casting knife that is heated to a temperature in the range from 20 to 150 °C, preferably in the range from 40 to 100°C.

[0181] The solution (S) is usually cast on a substrate that does not react with the noncrosslinked sulfonated polyarylene ether sulfone polymer (sP) or the at least one solvent comprised in the solution (S).

[0182] Suitable substrates are known to the skilled person and are, for example, selected from other polymers like polysulfones, cellulose acetate, polyolefins, polyacrylonitrile, and polyesters like polyethylene terephthalate.

[0183] To obtain a dense membrane, the separation in step ii) is typically carried out by evaporation of the at least one solvent comprised in the solution (S).

[0184] The present invention is further elucidated by the following examples without limiting it thereto.

[0185] Components used

[0186] Component (A1): DCDPS: 4,4'-dichlorodiphenyl sulfone

[0187] Component (A2): sDCDPS: 3,3’-Disodiumdisulfone-4,4’-dichlorodiphenyl sulfone

[0188] Component (B): 4,4'-dihydroxybiphenyl

[0189] Component (C): Potassium carbonate: K2CO3; anhydrous; volume-average particle size of 32.6 pm Component (D): NMP: N-methylpyrrolidone

[0190] General procedures

[0191] The viscosity number of the polymers is determined in a 1 % solution in NMP at 25 °C.

[0192] The sulfonation degree of the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) is determined by1H-NMR-spectroscopy.

[0193] In a 4 liter glass reactor fitted with a thermometer, a gas inlet tube and a Dean-Stark- trap, 267.06 g (0.930 mol) of DCDPS, 294.75 g (0.60 mol) of sDCDPS, 279.315 g (1.50 mol) 4,4'-dihydroxybiphenyl and 310.97 g (2.25 mol) of potassium carbonate were suspended in 938 ml NMP in a nitrogen atmosphere.

[0194] The mixture was heated to 190°C within one hour. In the following, the reaction time shall be understood to be the time during which the reaction mixture was maintained at 190 °C. The water that was formed in the reaction was continuously removed by distillation.

[0195] After a reaction time of 9.75 hours, the reaction was stopped by the addition of 2312 ml NMP and cooling down to room temperature (within one hour). The potassium chloride formed in the reaction was removed by filtration. The obtained polymer solution was then precipitated in isopropanol, the resulting polymer precipitate was separated and then extracted with hot water (85°C) for 20 h. Then the material was dried at 120°C for 24 h at reduced pressure (< 100 mbar). The properties of the obtained non-crosslinked sulfonated polyphenylenesulfone sPPSU 1 are summarized in table 1.

[0196] Example 2: sPPSU 2

[0197] In a 4 liter glass reactor fitted with a thermometer, a gas inlet tube and a Dean-Stark- trap, 288.61 g (1.005 mol) of DCDPS, 257.90 g (0.525 mol) of sDCDPS, 279.315 g (1.50 mol) 4,4'-dihydroxybiphenyl and 362.80 g (2.625 mol) of potassium carbonate were suspended in 938 ml NMP in a nitrogen atmosphere.

[0198] The mixture was heated to 190°C within one hour. In the following, the reaction time shall be understood to be the time during which the reaction mixture was maintained at 190 °C. The water that was formed in the reaction was continuously removed by distillation.

[0199] After a reaction time of 6.5 hours, the reaction was stopped by the addition of 2312 ml NMP and cooling down to room temperature (within one hour). The potassium chloride formed in the reaction was removed by filtration. The obtained polymer solution was then precipitated in isopropanol, the resulting polymer precipitate was separated and then extracted with hot water (85°C) for 20 h. Then the material was dried at 120°C for 24 h at reduced pressure (< 100 mbar). The properties of the obtained non-crosslinked sulfonated polyphenylenesulfone sPPSU 2 are summarized in table 1.

[0200] Example 3: sPPSU 3

[0201] In a 4 liter glass reactor fitted with a thermometer, a gas inlet tube and a Dean-Stark- trap, 245.53 g (0.855 mol) of DCDPS, 331 .60 g (0.675 mol) of sDCDPS, 279.315 g (1.50 mol) 4,4'-dihydroxybiphenyl and 321.33 g (2.325 mol) of potassium carbonate were suspended in 938 ml NMP in a nitrogen atmosphere.

[0202] The mixture was heated to 190°C within one hour. In the following, the reaction time shall be understood to be the time during which the reaction mixture was maintained at 190 °C. The water that was formed in the reaction was continuously removed by distillation.

[0203] After a reaction time of 11 .5 hours, the reaction was stopped by the addition of 2312 ml NMP and cooling down to room temperature (within one hour). The potassium chloride formed in the reaction was removed by filtration. The obtained polymer solution was then precipitated in isopropanol, the resulting polymer precipitate was separated and then extracted with hot water (85°C) for 20 h. Then the material was dried at 120°C for 24 h at reduced pressure (< 100 mbar). The properties of the obtained non-crosslinked sulfonated polyphenylenesulfone sPPSU 3 are summarized in table 1.

[0204] Example 4: sPPSU 4

[0205] In a 4 liter glass reactor fitted with a thermometer, a gas inlet tube and a Dean-Stark- trap, 267.06 g (0.93 mol) of DCDPS, 294.75 g (0.600 mol) of sDCDPS, 279.315 g (1.50 mol) 4,4'-dihydroxybiphenyl and 310.97 g (2.25 mol) of potassium carbonate were suspended in 938 ml NMP in a nitrogen atmosphere.

[0206] The mixture was heated to 190°C within one hour. In the following, the reaction time shall be understood to be the time during which the reaction mixture was maintained at 190 °C. The water that was formed in the reaction was continuously removed by distillation.

[0207] After a reaction time of 10.3 hours, the reaction was stopped by the addition of 2312 ml NMP and cooling down to room temperature (within one hour). The potassium chloride formed in the reaction was removed by filtration. The obtained polymer solution was then precipitated in isopropanol, the resulting polymer precipitate was separated and then extracted with hot water (85°C) for 20 h. Then the material was dried at 120°C for 24 h at reduced pressure (< 100 mbar). The properties of the obtained non-crosslinked sulfonated polyphenylenesulfone sPPSU 4 are summarized in table 1.

[0208] Example 5: sPPSU 5

[0209] In a 4 liter glass reactor fitted with a thermometer, a gas inlet tube and a Dean-Stark- trap, 223.99 g (0.78 mol) of DCDPS, 368.43 g (0.75 mol) of sDCDPS, 279.315 g (1.50 mol) 4,4'-dihydroxybiphenyl and 366.25 g (2.65 mol) of potassium carbonate were suspended in 938 ml NMP in a nitrogen atmosphere.

[0210] The mixture was heated to 190°C within one hour. In the following, the reaction time shall be understood to be the time during which the reaction mixture was maintained at 190 °C. The water that was formed in the reaction was continuously removed by distillation.

[0211] After a reaction time of 9.7 hours, the reaction was stopped by the addition of 2312 ml NMP and cooling down to room temperature (within one hour). The potassium chloride formed in the reaction was removed by filtration. The obtained polymer solution was then precipitated in isopropanol, the resulting polymer precipitate was separated and then extracted with hot water (85°C) for 20 h. Then the material was dried at 120°C for 24 h at reduced pressure (< 100 mbar). The properties of the obtained non-crosslinked sulfonated polyphenylenesulfone sPPSU 5 are summarized in table 1.

[0212] Table 1 As can be seen from table 1 , all sPPSU products have a high viscosity number and the required degree of sulfonated units.

[0213] Preparation of membranes

[0214] Membrane preparation method A

[0215] The respective polymers were dissolved in NMP at a concentration of 20 wt.%. The obtained solutions were then filtered using a PA-filter with a pore size of 0.8 pm applying a pressure of 2 bar. Finally, the solutions were allowed to rest for 24 h in the dark for degassing.

[0216] Membranes from the solutions were prepared by casting these solutions with a doctor blade at a speed of 5 mm / s at a temperature of 60°C onto a glass plate. The glass plate was transferred into a vacuum oven and the temperature was gradually increased to 100°C and kept there for 12 h. After cooling to room temperature, the plates were put into a water bath, which led to a detachment of the membrane from the glass plate. The wet membrane was then fixed and dried in the vacuum for 12 h at 120°C.

[0217] Membrane preparation method B

[0218] Membranes from the solutions were also prepared, wherein casting was also done according to the following procedure:

[0219] To produce a single layer film, an automatic film applicator and a universal applicator from Zehntner were used. The application solutions were applied to a PET substrate (Hostaphan®, Mitsubishi Polyester Film). The gap width of the doctor blade was chosen so that the layer after drying at 70°C had a thickness of 60 pm. Subsequently the film was delaminated from the PET carrier for further processing.

[0220] The membranes prepared as described were cut into the required size (5x5 cm).

[0221] The membranes according to examples M1a, M1b and M1c were produced according to membrane preparation method A, all other membranes were produced according to membrane preparation method B.

[0222] Reference membrane for the membrane trials

[0223] Comparative example M6: Fumasep® E-620 (K)

[0224] As reference membrane, a commercially available Fumasep® E-620 (K) (fumatech) membrane was used. The Fumasep® E-620 (K) membrane is a non-fluorinated cation exchange membrane and composed of a sulfonated polyaryletherketone-copolymer backbone.

[0225] Membrane characterization

[0226] • Ex-situ conductivity measurements

[0227] Since redox flow battery cell tests are time intensive, first, ex-situ electrochemical impedance (EIS) measurements were performed using a sandwich cell, in which the wet membrane (0.25 cm2active area) is clamped between two electrodes. The cell is tightened by 0.3 Nm. In table 2, the different membranes are listed with their thickness after activation in 1 M KOH and their ex-situ impedances. Since the thickness of the membranes was in some cases inhomogeneous, the impedance shows some deviations as well. Therefore, the measurement was repeated at least three times and the standard deviation was calculated.

[0228] Table 2

[0229] From table 2, it is clearly visible that within the row of membranes of the same production route and with sulfonation degrees up to 32%, the resistance increases with higher thickness. With higher sulfonation ratios, the conductivity gets more independent from the thickness. Here, the sulfonation degree has a higher influence on the conductivity leading to lower impedances with higher sulfonation ratios. Since all the membranes show lower ex-situ impedances than the Fumasep® E-620(K) membrane, all should be suitable for an aqueous, preferably alkaline, redox flow battery. Electrochemical redox flow battery measurements

[0230] All electrochemical cell tests were performed with a Scribner redox flow cell connected to two electrolyte reservoirs via a peristaltic pump (ISMATEC MCP-process IP65). For the testing of the membranes in a redox flow battery cell, the cell was equipped with 33 mL of 2,6-dihydroxyanthraquinone (2,6-DHAQ) solution (0.2M in 1 M KOH) as anolyte (97% purity) and 132 mL of potassium hexacyanoferrate (II) (K4[Fe(CN)6]*3 H2O) solution (0.2M in 1 M KOH) as catholyte. To balance the redox equivalents and losses due to air intake, the catholyte solution was used with 4 mol-equivalents. Prior use, the electrolyte solutions were degassed with argon for 2 h and a constant flow of inert gas (argon) was ensured during the electrochemical experiments to avoid decomposition. The electrolytes were pumped between the cell and the reservoirs with a flow rate of 26 mL min-1, which was calibrated with water prior every experiment. All used membranes were cut to squares (5 cm x 5 cm) and activated prior use for at least 24 h in 1 M KOH. SGL 39AA electrodes were thermally activated at 400 °C for 20 h under air to decrease the hydrophobic properties, cut to squares with an area of 5 cm2and used with 72% compression in the cell (3 felts were stacked on each side).

[0231] For the electrochemical measurements, a Gamry Interface 1000 potentiostat / galvanostat with Gamry Framework 7.8.2 software was utilized, sequences were programmed with the integrated sequence wizard. The obtained data was processed with the Gamry Analyst software (version 7.10.0) and summarized in excel sheets. Normal membrane testing experiments consist of several electrochemical measurements. First, the cell was charged to a state of charge (SOO) = 50% to measure the ohmic resistance and the total resistance (ohmic and polarization resistance) of the cell before cycling it via a potentiostatic EIS experiment and an L / - / -curve, respectively. In addition, the electrolyte utilization was measured by potentiostatic and galvanostatic charging and discharging. The lower and upper cutoffs voltages were set to 0.7 V and 1.6 V, respectively. Afterwards, charge-discharge cycles were performed using different current densities, 5 cycles were performed at every current density (cycle 1 to 5: 30 mA cm-2, 6 to 10: 50 mA cm-2, 11 to 15: 80 mA cm-2, 16 to 20: 100 mA cm-2). Therefore, the mode was fixed to galvanostatic (dis)charging. From this data, the coulombic and energy efficiencies were extracted, and the voltage efficiency was calculated to evaluate the performance of the cell.

[0232] To evaluate the performance of a redox flow battery, various parameters are used to describe the cell. Common experiments are charge and discharge cycles to determine different efficiencies. The coulombic efficiency (CE) gives information about the storage capacity losses during charge / discharge processes, for example, due to active species crossover and side reaction. It describes the ratio between the total electric charge drawn during the discharge (Qd°sCharge)anc* charge (Qcharge)as seen'nEquation (1).

[0233] In addition, the voltage efficiency (VE) describes the losses due to resistance, overpotential and polarization and can be calculated by the ratio of the discharge voltage (vdischarge)overthe charge voltage (V^arge)asshown in Equation (2).

[0234] We calculated energy efficiency (EE) from the experimental data as a ratio of energy at discharge (EdisCharge) over the energy at charge (E^arge) (Equation (3)):

[0235] For each current density, the average value was determined from the second to the fifth cycle to determine the efficiencies. The results are summarized in tables 3, 4 and 5.

[0236] Table 3 Table 4

[0237] Table 5 As can be seen from tables 3, 4 and 5, the sulfonation degree as well influences the properties of the RFB. Higher sulfonation ratios also lead to higher conductivities in RFB measurements. Therefore, the efficiencies increase slightly compared to M1b and M1c when M3b is used with a sulfonation ratio of 38.2%. The M3b shows even slightly better efficiencies than the M6. Furthermore, long-term cycling with a constant current density (100 mA / cm2), at which the electrolyte utilization was over 50%, was performed for several days (100 cycles) to investigate the long-term stability of the membrane. After the cycling experiments, a control sequence with resistance and electrolyte utilization measurements was used to check the performance of the cell. In case of some instability of the electrolyte solutions, they were exchanged and degassed. The results for Ex. M3b and Comp. Ex. M6 are shown in table 6.

[0238] Table 6

[0239] As can be seen from table 6, the electrolyte utilization of the M3b membrane is significantly higher than the electrolyte utilization of the M6 membrane, both before and after 100 cycles at 100 mA / cm2.

Claims

Claims1. The use of a membrane (M) in an aqueous redox flow battery (ARFB), wherein the membrane (M) comprises a non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) and the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) is prepared by a process comprising the stepI) converting a reaction mixture (RG) comprising as components(A1) at least one non-sulfonated aromatic dihalogen sulfone,(A2) at least one sulfonated aromatic dihalogen sulfone,(B) at least one aromatic dihydroxy component,(C) at least one carbonate component, and(D) at least one aprotic polar solvent.

2. The use according to claim 1, wherein component (A1) is selected from the group consisting of 4,4’-dichlorodiphenyl sulfone and 4,4’-difluorodiphenyl sulfone.

3. The use according to claim 1 or claim 2, wherein component (A2) is at least one disulfonated aromatic dihalogen sulfone.

4. The use according to any of claims 1 to 3, wherein component (B) is selected from the group consisting of 4,4‘-dihydroxybiphenyl, 4,4'-dihydroxydiphenyl sulfone, bisphenol A, 4,4'-dihydroxybenzophenone and hydroquinone.

5. The use according to any of claims 1 to 4, wherein component (C) comprises at least 50 wt.-% of potassium carbonate, based on the total weight of component (C).

6. The use according to any of claims 1 to 5, wherein component (D) is selected from the group consisting of N-methylpyrrolidone, N-dimethylacetamide, dimethyl sulfoxide, sulfolane and dimethylformamide.

7. The use according to any of claims 1 to 6, wherein the reaction mixture (RG) comprises component (A1) in an amount of from 40 to 70 mol-% andEB23-1211 PCcomponent (A2) in an amount of from 30 to 60 mol-%, based on the sum of the mol-% of components (A1) and (A2).

8. The use according to any of claims 1 to 7, wherein the membrane (M) is prepared by a method comprising the steps i) providing a solution (S) which comprises the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP) and at least one solvent, ii) separating the at least one solvent from the solution (S) to obtain the membrane (M).

9. The use according to claim 8, wherein the at least one solvent is selected from the group consisting of N-methylpyrrolidone, N-dimethylacetamide, dimethyl sulfoxide, dimethylformamide, dimethyllactamide and sulfolane.

10. The use according to claim 8 or claim 9, wherein the solution (S) provided in step i) comprises in the range from 0.1 to 30 wt.-% of the non-crosslinked sulfonated polyarylene ether sulfone polymer (sP), based on the total weight of the solution (S).

11. The use according to any of claims 8 to 10, wherein the separation in step ii) is performed by evaporation.

12. The use according to any of claims 1 to 11 , wherein the aqueous redox flow battery (ARFB) comprises an anolyte, wherein in discharged state, the anolyte comprises derivatives of anthraquinone, naphthoquinone or phenazine, dissolved in an alkaline or neutral solution.

13. The use according to any of claims 1 to 12, wherein the aqueous redox flow battery (ARFB) comprises a catholyte, wherein in discharged state, the catholyte comprises at least one compound selected from the group consisting of potassium hexacyanoferrate (II), sodium hexacyanoferrate (II), lithium hexacyanoferrate (II), ammonium hexacyanoferrate (II), potassium iodide, sodium iodide, lithium iodide and ammonium iodide, wherein the at least one compound is dissolved in an alkaline or neutral solution.

14. The use according to claim 12 or claim 13, wherein the alkaline or neutral solution comprises water and optionally, alkali or ammonium hydroxide.

15. The use according to any of claims 12 to 14, wherein the aqueous redox flow battery (ARFB) comprises a first half-cell comprising a first electrode and asecond half-cell comprising a second electrode, wherein the first electrode is in contact with the anolyte and the second electrode is in contact with the catholyte.

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