Low crossover ion exchange membrane for redox flow batteries

Abstract

An ion exchange membrane and a method for forming the membrane are described. This ion exchange membrane can be incorporated into a redox flow battery. In a redox flow battery environment, the membrane can exhibit high conductivity, stability, and durability for desired ions (e.g., protons or hydroxide ions), while also exhibiting low permeability to redox couple species. The membrane is formed by polyphosphate formation technology and subjected to densification and heat treatment.

Description

Technical Field

[0001] Cross-reference to Related Applications

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 518618, filed on August 10, 2023, and U.S. Provisional Patent Application No. 63 / 609596, filed on December 13, 2023, which is incorporated herein by reference for all purposes. Background Art

[0002]

[0002] With the increasing demand in the energy field, a need has arisen for large-scale energy storage devices that can provide backup power, improve grid management, and also have the capability to be seamlessly integrated with renewable energy devices. Redox flow batteries can meet these needs. Redox flow batteries perform charging and discharging by using a positive electrode electrolyte and a negative electrode electrolyte separated by an ion exchange membrane. Each electrolyte contains metal ions (active materials) that form a redox couple (also referred to as a redox pair), whose valence changes through oxidation-reduction.

[0003]

[0003] Despite their potential, the widespread adoption of redox flow batteries has been limited primarily due to the high manufacturing cost of the devices. For example, in vanadium redox flow batteries, most of the cost is attributed to the vanadium electrolyte. Such costs can be reduced through the development of systems capable of multi-cycle operation under high current loads. To meet these requirements, systems require high-efficiency ion exchange membranes, since ion exchange membranes are a key component of redox flow batteries and have a significant impact on the output, capacity, service life and cost of batteries.

[0004]

[0004] Ideally, the ion exchange membrane of a redox flow battery exhibits high conductivity for the desired cation or anion (e.g., proton or hydroxide ion) while simultaneously showing low crossover between redox pairs to function under high current loads. In addition, the ion exchange membrane of a redox flow battery is immersed in the electrolyte and therefore must be able to withstand mechanical and chemical degradation caused by oxidation, etc. The stability and durability of the membrane are important factors in determining the lifespan of a redox flow battery.

[0005]

[0005] Unfortunately, conventional membranes used in redox flow batteries (generally perfluorosulfonic acid membranes such as Nafion®) do not meet these requirements due to high crossover characteristics and resulting membrane poisoning and lack of durability. More recently, polybenzimidazole (PBI) membranes have been investigated for use in redox flow batteries. Conventional PBI membranes, prepared by solution casting in N,N'-dimethylacetamide (DMAc) to form a high-density film and then immersing the formed film in the desired electrolyte, exhibit very low ionic conductivity (20 mScm) when in bibe. -1 It shows less than 100 mAcm², which is approximately 100 mAcm². -2 This indicates that operation is impossible under extremely high current loads. However, a PBI film formed by directly casting a composition containing a PBI polymer into a polyphosphate (PPA) solvent, then hydrolyzing the PPA solvent to phosphoric acid (PA), and solidification resulting from the formation of a PA-absorbing PBI film, has been shown to be more promising for use in redox flow batteries. [Overview of the project] [Problems that the invention aims to solve]

[0006]

[0006] The above describes improvements in the art, but there is room for further improvement. What is needed in the art is a method for forming an ion exchange membrane that exhibits high ion permeability, stability and durability in a redox flow cell environment while showing low crossover of redox species from one side to the other, and a membrane manufactured thereby. [Means for solving the problem]

[0007]

[0007] According to one embodiment, a method for forming an ion exchange membrane is disclosed. The method may include the step of preparing a polymerization solution to form a membrane precursor. The polymerization solution comprises polybenzimidazole (PBI) dissolved in a polyphosphate solvent. The method may also include hydrolyzing at least a portion of the polyphosphate in the polymerization solution to form phosphoric acid and water, thereby causing a sol-gel transition and solidification of polybenzimidazole, and the formation of a gel membrane containing polybenzimidazole. The gel membrane can then be densified to form a densified membrane. After densification, the membrane can be heat-treated. The heat treatment includes subjecting the densified membrane to a temperature of about 350°C or higher in an inert atmosphere for a period of about 30 minutes or more.

[0008]

[0008] Furthermore, ion exchange membranes that can be formed by the disclosed method are disclosed. For example, the ion exchange membrane may include a densified PBI gel membrane and a support electrolyte for a redox flow battery absorbed within the PBI gel membrane. The ion exchange membrane has a flow rate of about 50 mS / cm in a 2.6 M sulfuric acid solution. 2 The above in-plane ionic conductivity can be observed, and for redox couple species, it can be approximately 2 × 10⁻⁶ -9 cm 2 It can exhibit transparency of less than / s.

[0009]

[0009] Furthermore, a redox flow battery incorporating the ion exchange membrane of the present disclosure is disclosed.

[0010] A full and implementable disclosure of this subject matter, including the best mode for those skilled in the art, is described in more detail in the remainder of the specification with reference to the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] [Figure 1]

[0011] FIG. 1 is a diagram defining dimensions of a membrane used in the present disclosure. [Figure 2]

[0012] FIG. 2 is a schematic diagram showing a redox flow battery into which the membrane described in the present specification can be incorporated. [Figure 3]

[0013] FIG. 3 is a schematic diagram showing a multi-cell stack that can include the described redox flow battery. MODE FOR CARRYING OUT THE INVENTION

[0011]

[0014] Reference will now be made in detail to various embodiments of the subject matter of the present disclosure, one or more examples of which are set forth below. Each example is provided to explain the subject matter, and not for the purpose of limiting the same. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the subject matter. For example, features illustrated or described as part of one embodiment can be used in another embodiment to yield a still further embodiment.

[0012]

[0015] In general, the present disclosure is directed to ion exchange membranes and methods of forming the membranes. The disclosed membranes exhibit high conductivity for desired ions (e.g., protons or hydroxide ions) as well as stability and durability in redox flow battery environments, while maintaining redox couple species such as VO 2+ and VO 4+ can also exhibit low permeability to vanadium redox couple species. For example, the described redox flow battery membrane can have a permeability of about 2×10 -9 cm 2 / s or less, for example, in some embodiments, on the order of about 10 -10 cm 2 / s or on the order of about 10 11 cm 2 / s, for example about 1×10 -9 cm 2 / s or less, approximately 5×10 -10 cm 2 / s or less, approximately 3×10 -10 cm 2 / s or less, or in some embodiments about 9 × 10 -11 cm 2 Less than / s, for example, about 1 × 10 -11 cm 2 / s~approx. 2×10 -9 cm 2 The components of an oxidation-reduction couple are / s, for example, VO 2+ or VO 4+ It can demonstrate permeability to [something].

[0013]

[0016] Along with low permeability to redox couple species, ion exchange membranes can exhibit excellent stability and durability, as well as desirable electrochemical activity, in redox flow cell environments. For example, the described redox flow cell membrane can exhibit in-plane ion conductivity in a 2.6 M sulfuric acid solution of about 50 mS / cm, or even higher in some embodiments, for example, about 100 mS / cm or about 200 mS / cm in some embodiments.

[0014]

[0017] In addition, a redox flow battery incorporating the disclosed film has a flow rate of approximately 50 mA / cm², for example. 2 As described above, for example, in some embodiments, the current is approximately 100 mA / cm². 2 ~about 500mA / cm 2 It can operate at high current densities. Furthermore, redox flow batteries incorporating this film can operate with high efficiency. For example, 242 mA / cm². 2 At a current density of 483 mA / cm², a redox flow battery incorporating the described film can exhibit a Coulomb efficiency (CE) of approximately 90% or more, for example, approximately 94% to approximately 99% in some embodiments; an energy efficiency (EE) of approximately 70% or more, for example, approximately 73% to approximately 84% in some embodiments; and a voltage efficiency (VE) of approximately 75% or more, for example, approximately 78% to approximately 80%. 2At a current density, a redox flow battery incorporating the described film can exhibit a CE of 85% or more, for example, about 87% to about 99% in some embodiments; an EE of about 50% or more, for example, about 54% to about 75% in some embodiments; and an VE of about 60% or more, for example, about 62% to about 77%.

[0015]

[0018] The ion exchange membrane is based on a PBI membrane formed by PPA formation technology, in which a polymer composition containing PBI polymer in a PPA solvent is cast and then hydrolyzed to form a solidified PBI gel membrane. Along with the basic membrane formation technology, it has been found that through heat treatment of the densified membrane formed via PPA technology, it is possible to provide an excellent ion exchange membrane with very low crossover, which can be used in redox flow batteries, while retaining the membrane's advantageous properties (electrochemical properties, physical and chemical durability, etc.) and reducing the membrane's permeability to redox species.

[0016]

[0019] To form an ion exchange membrane, a polymerization composition can be formed containing a PPA solvent and a selected PBI-forming compound, such as a PBI-forming monomer. The monomer content of the polymerization composition is generally low, for example, about 10 wt% or less, about 8 wt% or less, or in some embodiments, about 5 wt% or less.

[0017]

[0020] The PBI polymer of the film may have any PBI structure generally known in the art, formed by polymerization of a PBI-forming compound comprising at least one aromatic or heterocyclic aromatic tetraamino compound and at least one aromatic or heterocyclic aromatic polycarboxylic acid or its ester, anhydride, or acid chloride, or at least one aromatic or heterocyclic aromatic diaminocarboxylic acid. Heterocyclic aromatic compounds included herein include aromatic systems containing at least one nitrogen, oxygen, sulfur, or phosphorus atom in the aromatic ring.

[0018]

[0021] Examples of aromatic and heterocyclic aromatic tetraamino compounds that can be used to form films include, but are not limited to, 2,3,5,6-tetraaminopyridine; 3,3',4,4'-tetraaminodiphenylsulfone; 3,3',4,4'-tetraaminodiphenyl ether; 3,3',4,4'-tetraaminobiphenyl; 1,2,4,5-tetraaminobenzene; 3,3',4,4'-tetraaminobenzophenone; 3,3',4,4'-tetraaminodiphenylmethane; and 3,3',4,4'-tetraaminodiphenyldimethylmethane, as well as their salts, e.g., mono-, di-, tri- and tetrahydrochloride salts, and any combination of aromatic or heterocyclic aromatic tetraamino monomers.

[0019]

[0022] In one embodiment, the aromatic polycarboxylic acid may include a dicarboxylic acid. The dicarboxylic acid can be used alone or in combination with one or more additional polycarboxylic acid compounds, such as tricarboxylic acid and / or tetracarboxylic acid. When incorporated, the content of the tricarboxylic acid or tetracarboxylic acid may generally be about 30 mol% or less, for example, about 0.1 mol% to about 20 mol%, or about 0.5 mol% to about 10 mol%, relative to the amount of the dicarboxylic acid compound used. Esters of polycarboxylic acids, such as C1-C20 alkyl esters or C5-C12 aryl esters of polycarboxylic acids, can be used. Polycarboxylic anhydrides or polycarboxylic acid acid chlorides can be polymerized by the disclosed method.

[0020]

[0023] Examples of aromatic dicarboxylic acids include, but are not limited to, pyridine-2,5-dicarboxylic acid; pyridine-3,5-dicarboxylic acid; pyridine-2,6-dicarboxylic acid; pyridine-2,4-dicarboxylic acid; 4-phenyl-2,5-pyridinedicarboxylic acid; 3,5-pyrazoledicarboxylic acid; 2,6-pyrimidinedicarboxylic acid; 2,5-pyrazinedicarboxylic acid; 2,4,6-pyridinetricarboxylic acid; benzimidazole-5,6-dicarboxylic acid; and 5-hydroxyisophthalic acid. Acids; 4-hydroxyisophthalic acid; 2-hydroxyterephthalic acid; 5-aminoisophthalic acid; 5-N,N-dimethylaminoisophthalic acid; 5-N,N-diethylaminoisophthalic acid; 2,5-dihydroxyterephthalic acid; 2,6-dihydroxyisophthalic acid; 4,6-dihydroxyisophthalic acid; 2,3-dihydroxyphthalic acid; 2,4-dihydroxyphthalic acid; 3,4-dihydroxyphthalic acid; 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid; diph Examples include phenylsulfone-4,4'-dicarboxylic acid; isophthalic acid; terephthalic acid; phthalic acid; 3-fluorophthalic acid; 5-fluoroisophthalic acid; 2-fluoroterephthalic acid; tetrafluorophthalic acid; tetrafluoroisophthalic acid; tetrafluoroterephthalic acid; 3-sulfophthalic acid; 5-sulfoisophthalic acid; 2-sulfoterephthalic acid; tetrasulfophthalic acid; tetrasulfisophthalic acid; tetrasulfoterephthalic acid; 1,4-naphthalenedicarboxylic acid; 1,5-naphthalenedicarboxylic acid; 2,6-naphthalenedicarboxylic acid; 2,7-naphthalenedicarboxylic acid; diphenic acid; diphenyl ether 4,4'-dicarboxylic acid; benzophenone-4,4'-dicarboxylic acid; biphenyl-4,4'-dicarboxylic acid; 4-trifluoromethylphthalic acid; 2,2-bis(4-carboxyphenyl)hexafluoropropane; 4,4'-stilbenidicarboxylic acid; and 4-carboxycinnamic acid or any combination thereof.

[0021]

[0024] Examples of aromatic tricarboxylic acids, as well as their esters, acid anhydrides, and acid chlorides, include, but are not limited to, 1,3,5-benzenetricarboxylic acid (trimesic acid); 1,2,4-benzenetricarboxylic acid (trimellitic acid); (2-carboxyphenyl)iminodiacetic acid; 3,5,3'-biphenyltricarboxylic acid; and 3,5,4'-biphenyltricarboxylic acid; or any combination thereof.

[0022]

[0025] Examples of aromatic tetracarboxylic acids and their esters, acid anhydrides, and acid chlorides include, but are not limited to, 3,5,3',5'-biphenyltetracarboxylic acid; benzene-1,2,4,5-tetracarboxylic acid; benzophenonetetracarboxylic acid; 3,3',4,4'-biphenyltetracarboxylic acid; 2,2',3,3'-biphenyltetracarboxylic acid; 1,2,5,6-naphthalenetetracarboxylic acid; and 1,4,5,8-naphthalenetetracarboxylic acid; or any combination thereof.

[0023]

[0026] Examples of heterocyclic aromatic carboxylic acids include heterocyclic aromatic dicarboxylic acids, heterocyclic aromatic tricarboxylic acids, and heterocyclic aromatic tetracarboxylic acids, which include esters such as C1-C20 alkyl esters and C5-C12 aryl esters of heterocyclic aromatic carboxylic acids, or acid anhydrides or acid chlorides. Examples of heterocyclic aromatic carboxylic acids, but not limited to, include pyridine-2,5-dicarboxylic acid; pyridine-3,5-dicarboxylic acid; pyridine-2,6-dicarboxylic acid; pyridine-2,4-dicarboxylic acid; 4-phenyl-2,5-pyridinedicarboxylic acid; 3,5-pyrazoledicarboxylic acid; 2,6-pyrimidinedicarboxylic acid; 2,5-pyrazinedicarboxylic acid; 2,4,6-pyridinetricarboxylic acid; benzimidazole-5,6-dicarboxylic acid; and further, their C1-C20 alkyl esters or their C5-C12 aryl esters, or their acid anhydrides or their acid chlorides, or any combination thereof.

[0024]

[0027] In one embodiment, the polymerization composition may contain diaminocarboxylic acids, including, but are not limited to, diaminobenzoic acid and its hydroxychloride derivatives, as well as 1,2-diamino-3'-carboxylate 4,4'-diphenyl ether, or any combination thereof.

[0025]

[0028] PPA available for use in polymerization compositions may be, for example, commercially available PPA from Riedel-de Haen. PPA can include concentrated grades of PA(H3PO4) exceeding 100%. At high concentrations, individual H3PO4 units polymerize by dehydration, and PPA becomes H3PO4. n+2 P n O 3n+1 It can be expressed as (n>1).

[0026]

[0029] The PPA may have a P2O5 content calculated by acid titration of about 70 wt% or more, for example, about 75 wt% or more, or about 82 wt% or more, and in some embodiments, for example, about 70 wt% to about 86 wt%. The polymerization composition may be in the form of a solution of monomers and other compounds, or a dispersion / suspension of monomers / compounds in the PPA, depending generally on the properties of the compound being polymerized and any additional components of the polymerization solution.

[0027]

[0030] Polymerization can be carried out at a temperature and time at which a suitable polymerization of the compound occurs, which can generally be determined by the increase in viscosity of the polymerized composition. The increase in viscosity can be determined by visual inspection, through determination of intrinsic viscosity, or by other preferred means. For example, polymerization can be continued until the polymerized composition exhibits an intrinsic viscosity of about 0.8 dL / g or more, for example, about 1.0 dL / g or more in some embodiments, or about 1.5 dL / g or more. The polymerization temperature can generally be about 220°C or less, for example, about 200°C or less, such as about 100°C to 195°C in some embodiments. Polymerization can be carried out over a period of time from a few minutes (e.g., about 5 minutes) to several hours (e.g., about 100 hours). In one embodiment, the polymerized composition can be heated in a stepwise manner, for example, in three or more steps, with each step lasting about 10 minutes to about 5 hours, and the temperature increasing by about 15°C or more for each step. Naturally, as will be obvious to those skilled in the art, specific polymerization conditions can generally be varied depending on the reactivity and concentration of a particular monomer, and specific polymerization conditions are not required for the formation of redox flow cell films.

[0028]

[0031] Typical PBI polymer repeating units for PBI gel films include, but are not limited to, those identified in the art to date, the polymer repeating units disclosed in U.S. Patent No. 11,482,721; U.S. Patent No. 11,302,948; and U.S. Patent No. 11,180,621, all of which are incorporated herein by reference.

[0029]

[0032] After polymerization, the polymer solution can be treated to form a film precursor having a desired thickness. The film precursor can be formed by any suitable formation process, but is not limited to casting, spray coating, knife coating, etc. For example, in some embodiments, a gel film precursor can be formed to a thickness of about 30 micrometers (μm) to about 3,500 μm, or about 50 μm to about 1,000 μm, and in one embodiment, a thickness of about 20 μm to about 4,000 μm.

[0030]

[0033] To solidify the PBI polymer membrane precursor, the membrane precursor can be treated in the presence of water and / or moisture to hydrolyze at least a portion of the PPA in the membrane precursor. During hydrolysis, PPA hydrolyzes to form PA and water, and since the PBI polymer is less soluble in PA than PPA, hydrolysis causes a sol-gel transition of the PBI polymer solution and solidification of the polymer.

[0031]

[0034] The hydrolysis treatment can be carried out at a temperature and time sufficient for the gel film to solidify, such that it is self-supporting, can be manipulated without destruction, and incorporates a high liquid content (e.g., a liquid content of about 60 wt% or more of the total solid-liquid content of the film). For example, the hydrolysis treatment can be carried out at a temperature of about 0°C to about 150°C, for example, about 10°C to about 120°C, or about 20°C to about 90°C, for example, at room temperature in some embodiments.

[0032]

[0035] Hydrolysis can be carried out by contacting the membrane precursor with H2O in the form of, for example, liquid, steam, or vapor, and / or in the presence of other components. For example, the gel membrane precursor can be contacted with water vapor and / or liquid water and / or steam and / or aqueous PA (e.g., a PA solution having a PA concentration of about 10 wt% to about 90 wt%, for example, about 30 wt% to about 70 wt%, or about 45 wt% to about 55 wt%). The process can be carried out under standard pressure, but this is not a requirement of the formation process, and in some embodiments, the hydrolysis can be carried out under modified pressure.

[0033]

[0036] In one embodiment, hydrolysis can be carried out in a climate-controlled environment, in which case the H2O content can be precisely controlled. For example, the moisture content of the local environment can be controlled by controlling the temperature or saturation of the fluid in contact with the precursor membrane. For example, a carrier gas such as air, nitrogen, carbon dioxide, or other suitable gas can carry H2O (e.g., steam) in a controlled amount for contact with the precursor membrane. In one embodiment, hydrolysis can be carried out in an environment with a relative humidity of about 40% or more, about 50% or more, or about 70% or more, such as about 20% to 100%, about 40% to about 95%, or about 50% to about 90% in some embodiments.

[0034]

[0037] The hydrolysis treatment time can generally be varied depending on parameters such as H2O content, contact method, film thickness, and contact temperature. Generally, the hydrolysis treatment can last from a few seconds to a few minutes if, for example, the hydrolysis treatment utilizes superheated steam, or over several days if, for example, the hydrolysis treatment is carried out at room temperature and under relatively low relative atmospheric moisture. In some embodiments, the hydrolysis treatment can last from about 10 seconds to about 300 hours, for example, from about 1 minute to about 200 hours. For example, in one embodiment in which at least partial hydrolysis of PPA in a PBI polymer solution is carried out at room temperature (e.g., about 20°C) using ambient air with relative atmospheric moisture (i.e., relative humidity) of about 20% to 100%, for example, about 40% to about 80%, the treatment time can generally be from about 5 hours to about 200 hours.

[0035]

[0038] Upon hydrolysis of at least a portion of the PPA in a PBI polymer solution, the polymer can solidify to form a self-supporting gel film. In one embodiment, the PBI gel film can have a thickness of about 15 μm to about 3000 μm, for example, about 20 μm to about 2000 μm, or about 20 μm to about 1500 μm, but the specific film thickness is not critical. In some embodiments, the PBI gel film may have a thickness thinner than the thickness of the film precursor. After hydrolysis, the PBI gel film can be self-supporting even with a high liquid content, which is thought to be due to the intramolecular and intermolecular polymer structures present within the solidified polymer matrix.

[0036]

[0039] In one embodiment, the gel membrane may have a PBI solid content of about 5 wt% to about 40 wt%, for example, about 8 wt% to about 30 wt%, or about 10 wt% to about 25 wt%, of the total weight of the membrane including the liquid content. The formed PBI gel membrane may be self-supporting and have a Young's modulus of, for example, about 2.0 MPa or more, for example, about 3.0 MPa or more, or in some embodiments, about 4.5 MPa or more, and the Young's modulus is determined for a PBI gel membrane (e.g., polybenzimidazole) with a thickness of 0.43 mm and a PBI content of 5 wt%.

[0037]

[0040] Optionally, the PBI of the gel film can be crosslinked. The method of crosslinking and the point in time at which the polymer is crosslinked in the formation process are not particularly limited. For example, the gel film can be crosslinked after hydrolysis of the PPA and before other processing steps further described herein. In other embodiments, the gel film can be crosslinked after one or more additional processing steps further described herein, for example, after removal of residual PA and PPA and before or after densification of the film.

[0038]

[0041] In one embodiment, the PBI polymer of the film can be crosslinked simply by heating in the presence of atmospheric oxygen. Crosslinking may also be affected by the action of radiation, such as infrared (IR) radiation (having wavelengths of about 700 nm to about 1 mm), including near-infrared radiation (radiation having wavelengths of about 700 to about 2000 nm or energies in the range of about 0.6 to about 1.75 eV).

[0039]

[0042] To crosslink the PBI polymer of the membrane, the PBI polymer can incorporate reactive functional groups on its polymer chains to crosslink with itself, or instead to crosslink with a crosslinking agent, i.e., a polyfunctional compound (e.g., an amine) that can react with one or more functional groups of the PBI polymer. The crosslinking agent can include any suitable functionality that affects crosslinking. Suitable crosslinking agents are not particularly limited, and examples include, but are not limited, epichlorohydrins, diexoxides, diisocyanates, α,ω-dihaloalkanes, diacrylates, and bisacrylamides. Specific examples, but are not limited, include α,α'-dichloro-p-xylene, chloromethyl methyl ether, bis(chloromethyl) ether, terephthaloyl chloride, succinyl chloride, and dimethyl succinate, as well as combinations of crosslinking agents. In one embodiment, 1 to 20 equivalents of crosslinking agent per available aromatic ring may be used, but the crosslinking embodiment of the membrane is not limited to a specific crosslinking density.

[0040]

[0043] Optionally, after hydrolysis of PPA, the PBI gel film may be treated to remove any remaining PA and PPA from the gel film. If included in the formation process, this removal can be performed at any point in the formation process following the hydrolysis of PPA. For example, any remaining PA and PPA in the gel film can be removed before or after the crosslinking step, before or after the densification step, before or after the drying step, or before or after the heat treatment step.

[0041]

[0044] In some embodiments, the removal (e.g., rinsing) process can be performed two or more times. For example, the gel film can be treated to remove any remaining PA and PPA from the formed gel film, and then subjected to one or more additional processes to remove any materials subsequently incorporated into the film, such as PA or other electrolytes absorbed or reabsorbed by the film.

[0042]

[0045] The method for removing compounds from the membrane is not particularly limited. For example, the membrane can be simply washed several times with water to remove undesirable materials from the substrate. For example, the PBI gel membrane can be immersed in one or more water baths, and the PBI membrane can be kept in each bath for a period of several minutes (e.g., about 5 minutes) to several hours (e.g., about 24 hours). Optionally, the baths can be heated to temperatures such as about 20°C to about 150°C, or about 25°C to about 90°C, although in other embodiments, the membrane can be rinsed at room temperature without particular temperature control. To confirm the removal of materials, such as acidic materials such as PA and PPA, the pH of the washing solution can be determined, and washing / rinsing can be continued until the pH of the washing solution becomes neutral.

[0043]

[0046] Optionally, in one embodiment where any remaining PA and PPA have been pre-removed from the membrane, a supporting electrolyte can be absorbed into the membrane. If the step of electrolyte absorption is included, it can be performed at any point following the hydrolysis of the precursor membrane. For example, the supporting electrolyte can be absorbed into the membrane before or after the crosslinking step, before or after the densification step, before or after the drying step, or before or after the heat treatment step. The selected supporting electrolyte may be the same as or different from the supporting electrolyte used in the redox flow battery. For example, the membrane can be made to absorb a first supporting electrolyte before the heat treatment step, then re-absorb, for example, a PA supporting electrolyte, and then absorb a different supporting electrolyte after the heat treatment step before being used in the redox flow battery. For example, but not limited to, the membrane can absorb mineral acids (e.g., strong inorganic acids) such as hydrochloric acid, nitric acid, fluorosulfonic acid, or sulfuric acid, or mixtures thereof, or strong organic acids such as acetic acid, formic acid, p-toluenesulfonic acid, or trifluoromethanesulfonic acid, or mixtures thereof, and mixtures of different types of acids, for example, a combination of mineral and organic acids. In one embodiment, a phosphoric acid solution can be reabsorbed into the membrane. Other examples of supporting electrolytes that can be absorbed into the membrane include, but are not limited to, sodium chloride, potassium chloride, sodium hydroxide, potassium hydroxide, sodium sulfide, potassium sulfide, and combinations thereof. Examples of supporting electrolytes include H2SO4, HBr, HBr / HCl mixture, HCl, NaS2, NaS2 / NaBr mixture, Br2 in HBr, Br2 in H2SO4, and Br2 in HBr / H2SO4 mixture. In one embodiment, a tetraalkylammonium supporting cation can be absorbed into the membrane, and Et4N + and Bu4N + These are two non-restrictive examples. Tetrafluoroborate (BF 4- ), perchlorate (ClO 4- ), or hexafluorophosphate (PF 6- Solutions of these, or combinations thereof, are examples of additional supporting electrolytes that can be absorbed into the membrane.

[0044]

[0047] The membrane can be made to absorb the supporting electrolyte by any preferred method. For example, the membrane can be made to absorb the supporting electrolyte by immersing it in a solution of the supporting electrolyte in an optionally elevated temperature environment for a period of several minutes to several hours or several days. In another embodiment, a solution containing the supporting electrolyte, for example, a heated solution heated to a temperature of about 100°C or higher, for example, about 130°C in some embodiments, can be poured onto the membrane, and the membrane can be immersed in the bath thus formed for a certain period of time. In some embodiments, the membrane can be made to absorb the supporting electrolyte once or multiple times, the membrane can be rinsed as described above, and optionally dried before subsequent absorption processes.

[0045]

[0048] PBI gel films can be densified before heat treatment. As used herein, the term “densified” generally refers to a process that reduces the molecular porosity of a film. The densification process generally involves a permanent reduction in at least one dimension of the film when the film is swollen again to the same liquid content after the densification process. For example, a film that is fully liquid-incorporated after densification (i.e., unable to absorb any additional liquid) may show a reduction in dimension (e.g., thickness) compared to a film that was fully liquid-incorporated before densification. As used herein, the term “thickness” generally refers to the dimension of film 12 in the z-direction as shown in Figure 1.

[0046]

[0049] Beneficially, gel films can be densified without altering the fundamental morphology of the polymer matrix of the film formed by the PPA formation process.

[0050] In one embodiment, the densification process may include actively stretching the gel film in one or more planar directions of the film surface (i.e., defined by the x,y planes shown in Figure 1). In the stretch densification process, the gel film can be actively stretched to an amount that destroys the film structure. For example, the gel film can be stretched to about 200%, about 190%, about 180%, about 170%, about 160%, or about 150% of its original dimensions in one or more directions of the x,y planes of the film, for example, about 110% to about 200%, or about 120% to about 190% in some embodiments.

[0047]

[0051] The membrane can be densified without actively stretching it. For example, the membrane can be constrained on one or more dimensions of its surface (e.g., the x-direction, y-direction, or any angle or combination of surface dimensions in the x,y plane as defined in Figure 1) while the liquid is removed from the membrane, thereby densifying it. For instance, the membrane can be constrained on two opposing sides or, instead, on all four sides, while the liquid is removed from it, thereby densifying it.

[0048]

[0052] In one embodiment, the membrane can be in the form of an essentially infinite sheet, for example, a roll, and while the liquid is removed from the membrane, the membrane can be constrained on opposing sides (e.g., transversely) and optionally held under tension along at least a portion of the length of the sheet (e.g., in the machine direction), thereby densifying the membrane.

[0049]

[0053] In some embodiments, pressure can be applied to the film surface (in the z-direction) during the densification process. For example, pressure can be applied to the film surface by sandwiching the film between two substrates, allowing liquid to be removed from the film while constraining it in at least one of the x or y directions in order to densify the film. In such embodiments, as the film is densified, the sandwiched substrates allow the liquid to flow out of the film. For example, one or both of the substrates can define pores, channels, or similar structures that allow the liquid to flow out of the film.

[0050]

[0054] Removing liquid from a gel membrane during the densification process can be active, passive, or a combination of both. For example, the membrane can be passively removed and densified simply by holding it in the ambient atmosphere while it is constrained by one or more surface dimensions. Similarly, in embodiments where the membrane is stretched by one or more surface dimensions, and / or pressure is applied across the surface of the membrane, the liquid is removed from the gel membrane solely by the stretching / pressure action performed on the membrane.

[0051]

[0055] In some embodiments, the film can be treated by applying heat, contacting a relatively dry gas such as air, nitrogen, or argon, or a combination thereof, to actively remove liquid from the film. Furthermore, the active removal of liquid from the film can be carried out in conjunction with stretching / pressuring the film, or simply while the film is constrained in one or more directions on its surface.

[0052]

[0056] In some embodiments, the forming process may include one or more drying steps that allow the film to be dried without constraining or stretching the film in one or more directions. For example, the film may be actively or passively dried by sandwiching it between substrates (one or both of which may be porous) without constraining or stretching the film in one or both of the x and y directions, thereby removing the liquid from the film as discussed above.

[0053]

[0057] Whether in the densification or drying step, the amount of liquid removed from the film may result in the film being in a dry state, i.e., not retaining any additional liquid beyond the liquid due to the surrounding atmosphere, thereby forming a PBI film. As used herein, the term “film” generally refers to a polymer film that has a significantly small thickness (in the z direction) compared to its surface dimensions (in the x, y planes) and contains little to no moisture, for example, a polymer film with a moisture content of about 5 wt% or less in the polymer matrix. Thus, as used herein, the term “film” refers to both films containing a large amount of moisture and films that are essentially dry. In one embodiment, film densification may involve removing a substantial portion of the liquid in the gel film, but not necessarily to the point of being considered “dry.” For example, during the densification process, about 50 wt% or more, about 60 wt% or more, about 70 wt% or more, or about 80 wt% or more of the liquid content of the gel film may be removed.

[0054]

[0058] The densification process can be carried out as a single-step or multi-step process. For example, a multi-step process may include a first step in which the film can be stretched in one or more directions on the film surface and optionally pressure can be applied to the surface region over the thickness of the film, followed by a second step in which the film can be constrained in one or more dimensions, during which liquid is actively or passively removed from the film. A single-step process may include stretching or constraining the film and optionally, during which liquid is actively or passively removed from the film, along with pressurizing the film between substrates. Any combination of liquid removal, stretching, and constraining the film in one or more dimensions, along with optionally pressurizing the film between substrates, can be used individually or in combination, and in one or more steps, to densify the film.

[0055]

[0059] After densification, the film can be subjected to heat treatment, which reduces the permeability of the material to redox couple species while maintaining the permeability of the material to desired ionic species, such as protons, and preserves other desirable properties of the film. The heat treatment generally involves subjecting the densified film to a temperature of about 350°C to about 475°C, or about 400°C to about 450°C, in some embodiments, or about 375°C or higher. The heat treatment can be carried out for a period of several minutes to several hours, or about 30 minutes to about 2 hours, in some embodiments, or about 30 minutes to about 2 hours, in some embodiments. In some embodiments, the heat treatment can be carried out in an inert atmosphere, for example, under nitrogen gas. Generally, although this is not a process requirement, the heat treatment can be carried out at atmospheric pressure, and in some embodiments, the heat treatment can be carried out under high or low pressure.

[0056]

[0060] After heat treatment, the resulting membrane may be suitable for use as an ion exchange membrane in a redox flow battery. Generally, the membrane can absorb a support electrolyte after heat treatment before being used in a redox flow battery. In embodiments where the membrane absorbs the support electrolyte before the heat treatment step, the support electrolyte absorbed by the membrane after the heat treatment step may be the same as or different from the support electrolyte absorbed beforehand. The final support electrolyte may generally depend on the specific characteristics of the redox flow battery using the membrane and may include acidic support electrolytes, basic support electrolytes, and neutral chemical species (e.g., water), including, but not limited to, the support electrolytes described above.

[0057]

[0061] The concentration of the supporting electrolyte in the membrane is not particularly limited, but is generally about 25 moles / liter (M) or less, for example, about 0.1 M to about 25 M, about 0.5 M to about 10 M, or in some embodiments about 1 M to about 5 M.

[0058]

[0062] The redox flow cell membrane may contain one or more additives that can be incorporated into the membrane during membrane formation, during the processing steps described above, or in conjunction with the final supporting electrolyte. For example, small organic molecules such as small C1-C4 alcohols (e.g., glycerol), small organic acids, or urea can be incorporated into the redox flow cell membrane in conjunction with the absorption of the supporting electrolyte.

[0059]

[0063] In one embodiment, the redox flow cell membrane can incorporate fine particles, such as titanium dioxide or PBI fine particles, in amounts generally of about 2 wt% or less, thereby reducing the porosity of the membrane. For example, by adding particulate matter to a polymer solution during hydrolysis, nano-sized PBI particles can be incorporated into the polymer matrix while the PBI gel membrane is solidifying.

[0060]

[0064] The membrane can be incorporated into redox flow batteries intended for any application, in combination with any suitable electrolyte and redox species. For example, redox flow battery membranes can be incorporated into batteries for use in renewable energy sectors and / or in existing power grids for backup / reduced energy interruption during peak use.

[0061]

[0065] One embodiment of the redox flow battery cell 10 is shown in Figure 2. As shown, the cell can be liquid-communicated with a first tank 100 that can hold a first electrolyte and a second tank 200 that can hold a second electrolyte. Tanks 100 and 200 can be liquid-communicated with either side of the redox flow battery ion exchange membrane 12 of the cell 10 by the use of conduits 110, 210, pumps 112, 212, valves, control systems, etc. The electrolytes stored in tanks 100 and 200 can be circulated to either side of the cell 10 by pumps 112 and 212, respectively, during charging and discharging, and brought into contact with either side of the membrane 12.

[0062]

[0066] As is well known, the electrolytes of batteries can each incorporate one component of an oxidation-reduction pair. In one particular embodiment, a redox flow battery film can be used in a VRB. The VRB contains, in the first electrolyte, an alternate vanadium compound in which vanadium alternates between +5 (pentavalent) and +4 (tetravalent) vanadium, such as (VO2)2SO4, VO(SO4), or a combination thereof. The second electrolyte may contain, as an active material, a vanadium compound in which vanadium alternates between +2 (divalent) and +3 (trivalent) vanadium, such as VSO4, V2(SO4)3, or a combination thereof.

[0063]

[0067] The charge / discharge chemical reaction of one embodiment of the VRB can be expressed as follows: Positive electrode: VO 2+ +H2O-e - →VO2 + +2H + (charging) VO 2+ +H2O-e - ←VO2 + +2H + (discharge) E 0 = +1.00V relative to standard hydrogen electrode (SHE) negative electrode V 3+ +e - →V 2+ (charging) V 3+ +e - ←V 2+ (discharge) E 0 =-0.26 V vs SHE Overall chemical reaction: VO 2+ +V 3+ +H2O→VO2 + +2H + +V 2+ (charging) VO 2+ +V 3+ +H2O←VO2 + +2H + +V 2+ (discharge) E 0 cell = 1.26V vs. SHE.

[0064]

[0068] Naturally, the redox flow batteries described herein are not limited to VRBs, and other batteries containing other redox pairs are also included herein. Representative redox pairs include, but are not limited to, Zn / Br2; Zn / Fe; Fe / Cr; polysulfide / Br2; polysulfide / I2; 9,10-anthraquinone-2,7-sulfonic acid (AQDS) / Br2; poly(methyl viologen) (Poly(MV)) / poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (Poly(TEMPO)); bis-(trimethylammonio)propyl viologen tetrachloride (BTMAP-Vi) / BTMAP-ferrocened dichloride (BTMAP-Fc); 2,6-dihydroxyanthraquinone (2,6-DHAQ) / ferrocyanide; and alloxazine 7 / 8-carboxylic acid (ACA) / ferrocyanide.

[0065]

[0069] For example, a redox flow battery uses ferrocyanides such as [Fe(CN)6]3 / [Fe(CN)6]4 as the active anode liquid material, and Fe as the active cathode liquid material. 2+ and Fe 3+ The system may include an electrolyte system containing iron / ligand complexes. The cathode solution in such a system may contain iron / ligand complexes, including, but are not limited to, triethanolamine, diethanolamine, ethanolamine, N,N-bis-(2-hydroxyethyl)-(iminotris)-(hydroxymethyl)-methane and mixtures thereof, and the cathode solution may have a ligand-to-iron ratio of about 3:1 to about 10:1.

[0066]

[0070] Electrolytes can generally contain active materials (e.g., vanadium ions, iron ions, etc.) at concentrations of approximately 0.5 M to 10 M. For example, an electrolyte can contain active materials at concentrations of approximately 0.5 M or higher, approximately 0.6 M or higher, or 0.7 M or higher, for example, approximately 1 M to 3 M.

[0067]

[0071] In one embodiment, the electrolyte can contain each active material at concentrations ranging from 1 M to 10 M. In one embodiment, when the active material is at a concentration within this range, it can promote a high energy density and high power density for the redox flow battery film to operate. In one embodiment, if the active material is at a concentration of less than 1 M, the amount of active material per unit volume in the liquid may be too small, potentially reducing the energy density. In one embodiment, if the active material is at a concentration exceeding 10 M, the viscosity of the electrolyte will increase rapidly, and therefore the rate of the oxidation / reduction reaction will decrease significantly, potentially reducing the power density. The paired electrolytes of a redox flow battery can contain each oxidation-reduction pair active material at the same or different concentrations, and the preferred concentrations generally depend on the specific oxidation-reduction pair used, the application of the battery, and the presence of additional additives in the electrolyte.

[0068]

[0072] The battery electrolyte may contain additives such as one or more redox flow battery support electrolytes, as discussed previously. In one embodiment, the battery electrolyte may contain a support electrolyte absorbed by the redox flow battery membrane.

[0069]

[0073] In one embodiment, the electrolyte may include a sulfuric acid-supported electrolyte. For example, the electrolyte may include a mixture of sulfuric acid and water, i.e., an aqueous sulfuric acid solution, as a solvent, along with the active material of the solution. In one embodiment, the mixture of the supporting electrolyte and water, for example, the aqueous sulfuric acid solution, may contain the supporting electrolyte at a concentration of about 1 M to about 5 M. In one embodiment, the concentration of the supporting electrolyte can be selected to provide suitable solubility for the active material of the electrolyte. As a result, the solution can exhibit desirable ionic conductivity and viscosity, and the occurrence of overvoltage problems in the battery can be avoided.

[0070]

[0074] As shown in Figure 2, each side of the cell 10 may include additional components known in the art adjacent to the film 12, which include, but are not limited to, conductive separators 14, such as, among other things, porous carbon paper, carbon cloth, carbon felt, or metal cloth (a porous film made of fibrous metal or a metal film formed on the surface of a polymer fiber cloth). The cell may further include electrodes 16, which may be identical or different from each other and may be made of a conductive substrate (e.g., graphite) suitable for each electrolyte of the cell. A current collector 18 (e.g., gold-plated copper) may be electrically connected to the electrodes 16, and the cell may include one end plate 20 (e.g., a stainless steel end plate) on either side of the 1 / 2 cell, facing away from the separator. The current collector 18 provides an electrical connection between the cell 10 and an external circuit, as shown in the figure.

[0071]

[0075] Figure 3 shows a plurality of cells 10 arranged in a typical cell stack 150 of a redox flow battery. As shown, a first circulation path 300 can pass through one side of each cell 10 in the stack 150 so that the electrolyte of this part of the battery flows through path 300 and returns to the first tank 100. A second circulation path 400 passes through the opposite side of each cell 10 in the stack 150 so that the electrolyte of this part of the battery flows through path 400 and returns to the second tank 200. The redox flow battery may further include respective charge / discharge circuits, and converters, controllers, etc., to collect and provide power through the use of the battery, as is known in the art.

[0072]

[0076] The redox flow cell films described herein enable high-performance flow cells that operate under low crossover and high current loads. Improvements in these materials can alleviate the need for large electrochemical stacks, thereby reducing the overall cost of commercially available flow cell devices.

[0073]

[0077] The present disclosure may be better understood by referring to the embodiments described below. [Examples]

[0074]

[0078] PBI gel membranes formed using PPA technology and doped with phosphoric acid (Celtec®-P) were obtained from BASF Ltd. The membranes were first converted to high-density films by washing them with water to remove all the phosphoric acid. Washing was achieved by immersing each membrane in a bath of deionized water for up to 24 hours, with the water periodically changed until a neutral pH was measured. Each membrane was then densified in a controlled manner between porous sheets. Each membrane was fixed in the X and Y directions to prevent shrinkage in those directions, and then, as it dried, the membranes shrunk in the Z direction.

[0075]

[0079] The n-PBI film was also formed in-house using the PPA procedure. Typical polymerization involves adding 243 g of polyphosphate to 3.11 g of tetraaminobiphenyl (TAB (14.5 mmol)) and 3.14 g of naphthalene-1,4-dicarboxylic acid (1,4-ndc 14.5 mmol), mixing with an overhead stirrer, and purging with dry nitrogen. The contents were heated in a high-temperature silicone oil bath, and the temperature was controlled by a programmable temperature controller with lamp and soak functions. The reaction temperature was gradually increased from room temperature to 120°C, then 150°C, then 170°C, and then 190°C. In typical polymerization, the final reaction temperature was approximately 190°C and held for 12 hours. Once the completion of the reaction was determined by visual inspection of viscosity, the polymer solution was cast onto a clear glass plate using a doctor blade with a controlled gate thickness of 15 mils. The cast solution was hydrolyzed into a gel film in a humidity chamber adjusted to 55% RH at 25°C. Intrinsic viscosity was measured in sulfuric acid at 30°C, yielding a value of approximately 6 dL / g.

[0076]

[0080] After formation, the n-PBI gel film was rinsed and densified in the same manner as the Celtec®-P film.

[0081] The film was treated with one of two subsets. The first subset was reabsorbed after densification and before heat treatment, while the second subset was heat-treated after densification.

[0077]

[0082] The first subset of films was reabsorbed by taping the densified film onto a glass plate. A phosphoric acid solution containing 0.25 wt% of the reactive oligomer (p,p-OPBI) (see, e.g., Rohlfing, Liu, Daryaei, Benicewicz, Polymer, 2023, 280, 126034) was heated to 130°C to completely dissolve the oligomer. The hot solution was then gently poured onto the film and left overnight to permeate the high-density film. The films were then placed in a bath of deionized water, with the water being changed periodically until the water pH became neutral (up to 24 hours). The films, washed with water, were then dried overnight between porous sheets until all the water evaporated.

[0078]

[0083] The heat treatment was the same for all films, regardless of whether they were films that had absorbed oligomers and then washed and dried, or films that were simply washed and dried without additional reabsorption. For the heat treatment, the films were placed in a tubular furnace preheated to the desired temperature. The tubular furnace was preheated under N2 gas and an inert atmosphere was maintained during the treatment. The films were treated for a specific time of up to one hour, and then removed from the tubular furnace for cooling.

[0079]

[0084] Vanadium (VOSO4) permeability measurements were completed using a PermeGear "Side-by-Side" direct permeation cell. The cell had two 45 mL electrolyte chambers separated by the membrane or film to be tested. The temperature during the test was maintained at 25°C using a recirculating water bath. A typical experiment consisted of two electrolytes: a donor compartment with 1.6 M VOSO4 in 2 M H2SO4 and an acceptor compartment with 1.6 M MgSO4 in 2 M H2SO4. Vanadium(IV) has an absorption property detectable at 248 nm. Aliquots of the acceptor solution were taken periodically over time, and the permeability of the vanadium salt was measured. VOSO4 was measured using a Shimadzu UV-2450 UV-Vis. 2+ Measure permeability and Fick's law of diffusion:

[0080]

number

[0081] (In the formula, P s This is the calculated salt permeability, c r (t) is the concentration of VOSO4 in the receptor chamber at time t, c r (0) is the initial concentration of VOSO4 in the donor chamber, V is the volume of solution added to each chamber, d is the film thickness, and A is the effective area of ​​the membrane. The calculation was performed using [this method].

[0082] Multiple samples were subjected to the following procedures.

[0085] Sample 1

[0086] For control, the transmittance of the densified Celtec®-P film (unheat-treated) was measured as described above, and the transmittance was 5.87 × 10⁻⁶. -09 cm 2 The decision was made to use / s.

[0083]

[0087] Sample 2

[0088] The densified Celtec®-P film was heat-treated at 300°C for 1 hour under inert conditions as described. The transmittance was measured as described, and the transmittance was 6.54 × 10⁻⁶.-09 cm 2 The decision was made to use / s.

[0084]

[0089] Sample 3

[0090] The densified Celtec®-P film was heat-treated at 400°C for 1 hour under inert conditions as described. The permeability was measured as described, and the permeability was 1.07 × 10⁻⁶. -09 cm 2 The result was determined to be / s. This is more than 5 times lower than the control Celtec(registered trademark)-P film.

[0085]

[0091] Sample 4

[0092] The densified Celtec®-P film was heat-treated at 425°C for 1 hour under inert conditions as described. The transmittance was measured as described, and the transmittance was 4.10 × 10⁻⁶. -10 cm 2 The result was determined to be / s. This is approximately 14 times lower than the control Celtec(registered trademark)-P film.

[0086]

[0093] Sample 5

[0094] The densified Celtec®-P film was heat-treated at 450°C for 30 minutes under inert conditions as described. The permeability was measured as described, and the permeability was 1.42 × 10⁻⁶. -10 cm 2 The result was determined to be / s. This is more than 40 times lower than the control Celtec(registered trademark)-P film.

[0087]

[0095] Sample 6

[0096] The densified Celtec®-P film was heat-treated at 450°C for 1 hour under inert conditions as described. The permeability was measured as described, and the permeability was 1.02 × 10⁻⁶. -10 cm 2 The value was determined to be / s. This is approximately 575 times lower than the control Celtec(registered trademark)-P film.

[0088]

[0097] Example 7

[0098] As a second control, the permeability of a densified n-PBI film (not heat-treated) was measured as described, and the permeability was determined to be 1.35×10 -08 cm 2 / s.

[0089]

[0099] Example 8

[0100] A densified n-PBI film was heat-treated as described at 450°C for 1 hour under inert conditions. The permeability was measured as described, and the permeability was determined to be 3.00×10 -10 cm 2 / s. This is 45 times lower than that of the control n-PBI film.

[0090]

[0101] Example 9

[0102] A densified Celtec®-P film was reabsorbed as described. The film was then heat-treated as described at 450°C for 1 hour under inert conditions. The permeability was measured as described, and the permeability was determined to be 9.14×10 -11 cm 2 / s. This is approximately 64 times lower than that of the control Celtec®-P film.

[0091]

[0103] While certain embodiments of the subject matter of the present disclosure have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the subject matter.

Claims

1. A method for forming an ion exchange membrane, The step involves preparing a polymerization solution to form a membrane precursor, wherein the polymerization solution contains polybenzimidazole dissolved in a polyphosphate solvent. A step of hydrolyzing at least a portion of the polyphosphate in the polymerization solution to form phosphoric acid and water, thereby causing sol-gel transition and solidification of polybenzimidazole, and the formation of a gel film containing polybenzimidazole, The steps include: densifying the gel film to form a densified film, The step of heat-treating the densified film, the heat treatment comprising subjecting the densified film to a temperature of approximately 350°C or higher in an inert atmosphere for a period of approximately 30 minutes or more. Methods that include...

2. The method according to claim 1, further comprising the step of removing phosphoric acid and any residual polyphosphate from the gel film before the step of densifying the gel film.

3. The method according to claim 2, further comprising the step of allowing a supporting electrolyte to be absorbed into the densified gel film before the step of heat-treating the densified film.

4. The method according to claim 1, further comprising the step of drying the densified film before the step of heat-treating the densified film.

5. The method according to claim 1, further comprising the step of heat-treating the densified film, followed by the step of allowing the film to absorb a supporting electrolyte.

6. The method according to claim 1, further comprising the step of crosslinking a gel film.

7. The method according to claim 1, wherein the step of densifying the gel film includes stretching the gel film in one or more planar directions on the film surface.

8. The method according to claim 1, wherein the step of densifying the gel film includes constraining the film by one or more dimensions of the film surface.

9. The method according to claim 1, wherein the step of densifying the gel film includes pressurizing the gel film between two substrates.

10. The method according to claim 9, wherein at least one of the substrates is porous.

11. The method according to claim 1, wherein hydrolysis is carried out at a relative humidity of approximately 40% or higher.

12. Densified polybenzimidazole gel membrane; and Redox flow cell support electrolyte absorbed within a densified polybenzimidazole gel membrane An ion exchange membrane comprising, Approximately 50 mS / cm in a 2.6 M sulfuric acid solution ２ The above in-plane ionic conductivity was observed, Approximately 2 × 10 for redox couple species －９ cm ２ It exhibits permeability of / s or less. Ion exchange membrane.

13. The ion exchange membrane according to claim 12, wherein the supporting electrolyte comprises a mineral acid, a strong organic acid, or a mixture thereof.

14. The ion exchange membrane according to claim 13, wherein the supporting electrolyte contains a strong inorganic acid.

15. The ion exchange membrane according to claim 13, wherein the strong inorganic acid comprises hydrochloric acid, hydrobromic acid, nitric acid, fluorosulfonic acid, sulfuric acid, acetic acid, formic acid, p-toluenesulfonic acid, phosphoric acid, or trifluoromethanesulfonic acid or any combination thereof.

16. The ion exchange membrane according to claim 12, wherein the supporting electrolyte comprises sodium chloride, potassium chloride, sodium hydroxide, potassium hydroxide, sodium sulfide, potassium sulfide, tetraalkylammonium cation, tetrafluoroborate, perchlorate, or hexafluorophosphate.

17. A redox flow battery comprising the ion exchange membrane described in claim 12.

18. The redox flow battery according to claim 17, which is a vanadium redox flow battery.

19. Zn / Br ２ ;Zn / Fe;Fe / Cr;Polysulfide / Br ２ ; Polysulfide / I ２ ; 9,10-Anthraquinone-2,7-disulfonic acid (AQDS) / Br ２ A redox flow battery according to claim 17, comprising a redox pair selected from the group consisting of: poly(methyl viologen)(poly(MV)) / poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate)(poly(TEMPO)); bis-(trimethylammonio)propyl viologen tetrachloride (BTMAP-Vi) / BTMAP-ferrocened dichloride (BTMAP-Fc); 2,6-dihydroxyanthraquinone (2,6-DHAQ) / ferrocyanide; and alloxazine 7 / 8-carboxylic acid (ACA) / ferrocyanide.

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