Low transmembrane permeable ion exchange membranes for redox flow batteries

Abstract

Ion exchange membranes and methods for forming the membranes are described. The ion exchange membrane may be incorporated into a redox flow battery. The membrane may exhibit a desirable high conductivity of ions (e.g., protons or hydroxide ions) as well as stability and durability in redox flow battery environments while also exhibiting low permeability to redox couple species. The film is formed according to a polyphosphoric acid forming technique and subjected to densification and heat treatment.

Description

Cross-references to related applications

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

[0002] The increasing demand in the energy sector has created a need for large-scale energy storage devices that can provide backup power and improve grid management, while also being able to integrate seamlessly with renewable energy devices. Redox flow batteries can meet this need. Redox flow batteries are charged and discharged using positive and negative electrode electrolyte solutions separated by an ion exchange membrane. Each electrolyte solution contains metal ions (active materials) that form redox pairs (also known as redox couples), whose valence state is changed through oxidation-reduction.

[0003] Despite the potential of redox flow batteries, their widespread adoption has been limited, primarily due to the high cost of device manufacturing. For example, in vanadium redox flow batteries, a major portion of the cost is attributed to the vanadium electrolyte. Such costs can be reduced by developing systems capable of operating for many cycles under high current loads. To meet these requirements, the system will need a highly efficient ion exchange membrane, as it is a key component of redox flow batteries and significantly impacts the battery's output, capacity, lifespan, and cost.

[0004] Ideally, the ion exchange membrane in a redox flow battery should exhibit high conductivity for desired cations or anions (e.g., protons or hydroxide ions) while exhibiting low crossover permeation between redox molecules to function under high current loads. Furthermore, the ion exchange membrane in a redox flow battery is immersed in an electrolyte solution and therefore must be resistant to mechanical and chemical degradation caused by oxidation, etc. The stability and durability of the membrane are key factors determining the lifespan of a redox flow battery.

[0005] Unfortunately, the conventional membranes used in redox flow batteries (typically perfluorosulfonic acid membranes such as Nafion) ®Due to their high transmembrane permeation characteristics and the resulting membrane poisoning, as well as lack of durability, these requirements are not met. Recently, polybenzimidazole (PBI) membranes have been considered for use in redox flow batteries. Conventional PBI membranes, prepared by casting a dense membrane in a solution of N,N'-dimethylacetamide (DMAc) and then absorbing the desired electrolyte into the formed membrane, have been shown to exhibit extremely low ionic conductivity (less than 20 mS·cm) when absorbing the electrolyte solution. -1 And cannot exceed approximately 100 mA cm -2 The current load operation is limited. However, PBI films formed by directly casting a composition of PBI polymer contained in a polyphosphoric acid (PPA) solvent, followed by hydrolysis of the PPA solvent to phosphoric acid (PA) and resulting in the curing of the formed PA-absorbing PBI film have shown more promise for use in redox flow batteries.

[0006] While improvements in the art have been described above, there is room for further improvements. What is needed in the art is a method for forming ion exchange membranes and the resulting membranes that exhibit low transmembrane permeation of redox substances from one side to the other, while also exhibiting high ion permeability and stability and durability in redox flow battery environments. Overview

[0007] According to one embodiment, a method for forming an ion exchange membrane is disclosed. The method may include shaping a polymerization solution to form a membrane precursor. The polymerization solution comprises polybenzimidazole (PBI) dissolved in a polyphosphoric acid solvent. The method may further include hydrolyzing at least a portion of the polyphosphoric acid in the polymerization solution to form phosphoric acid and water, thereby inducing sol-gel transfer and solidification of the polybenzimidazole and forming a gel membrane containing the polybenzimidazole. Subsequently, the gel membrane may be densified to form a densified membrane. After densification, the membrane may be heat-treated. The heat treatment includes subjecting the densified membrane to a temperature of about 350°C or higher for about 30 minutes or longer in an inert atmosphere.

[0008] Also disclosed are ion exchange membranes that can be formed according to the disclosed methods. For example, the ion exchange membrane may include a densified PBI gel membrane and a redox flow battery supporting electrolyte absorbed within the PBI gel membrane. The ion exchange membrane can exhibit approximately 50 mS / cm in 2.6 M sulfuric acid solution. 2 Or even greater in-plane ionic conductivity; and can exhibit approximately 2 × 10⁻⁶. -9 cm 2 / s or lower permeability to redox pairs of substances.

[0009] A redox flow battery incorporating the disclosed ion exchange membrane was also disclosed. Brief description of the attached diagram

[0010] The full and practicable disclosure of the subject matter of the invention, including its best mode of presentation to those skilled in the art, is set forth in more detail in the remainder of this specification (including with reference to the accompanying drawings), in which: Figure 1 The dimensions of the membranes used in this disclosure are defined.

[0011] Figure 2 The schematic diagram illustrates a redox flow cell that can be incorporated into a membrane as described herein.

[0012] Figure 3 The schematic diagram illustrates a multi-cell stack that can be included in a redox flow cell as described. Detailed Explanation

[0013] Reference will now be made in detail to various embodiments of the disclosed subject matter, one or more of which are set forth below. Each embodiment is provided by way of explanation rather than limitation of the subject matter. Indeed, it will be apparent to those skilled in the art that various modifications and variations can be made to this disclosure without departing from the scope or spirit of the subject matter. For example, features shown or described as part of one embodiment may be used in another embodiment to produce yet another embodiment.

[0014] Typically, this disclosure relates to ion exchange membranes and methods for forming said membranes. The disclosed membranes can exhibit high conductivity for desired ions (e.g., protons or hydroxide ions) and stability and durability in redox flow battery environments, while also exhibiting resistance to redox couples, such as VOCs. 2+ and VO 4+ Vanadium redox couple materials have low permeability. For example, redox flow cell membranes as described can be used for redox couple members such as VO₂. 2+ or VO 4+ It presents approximately 2×10 -9 cm 2 / s or lower, such as about 10 in some implementations -10 cm 2 / s or approximately 10 11 cm 2 / s, such as approximately 1×10 in some implementations -9 cm 2 / s or lower, approximately 5×10 -10 cm 2 / s or lower, approximately 3×10 -10 cm 2 / s or lower or about 9×10 -11 cm2 / s or lower, such as from about 1×10 -11 cm 2 / s to approximately 2×10 -9 cm 2 / s of penetration.

[0015] Combined with their low permeability to redox couples, ion exchange membranes can exhibit excellent stability and durability, as well as desirable electrochemical activity, in redox flow battery environments. For example, redox flow battery membranes as described can exhibit in-plane ionic conductivity of about 50 mS / cm in 2.6 M sulfuric acid solution, or even higher in some embodiments, such as about 100 mS / cm or about 200 mS / cm in some embodiments.

[0016] Furthermore, in some embodiments, the redox flow cell incorporating the disclosed membrane can operate at high current densities, such as approximately 50 mA / cm². 2 Or higher, for example from about 100 mA / cm 2 Approximately 500 mA / cm 2 Operation. Furthermore, redox flow batteries with integrated membranes can operate at high efficiency. By way of example, at 242 mA / cm²... 2 The current density, when incorporated into the membrane as described, allows the redox flow cell to exhibit a coulombic efficiency (CE) of about 90%, for example, from about 94% to about 99% in some embodiments; an energy efficiency (EE) of about 70%, for example, from about 73% to about 84% in some embodiments; and a voltage efficiency (VE) of about 75%, for example, from about 78% to about 80%. At 483 mA / cm² 2 The current density of the redox flow cell incorporated into the membrane as described can exhibit 85% or greater, for example from about 87% to about 99% in some embodiments; about 50% or greater, for example from about 54% to about 75% in some embodiments; and about 60% or greater, for example from about 62% to about 77% in embodiments.

[0017] Ion exchange membranes are based on PBI membranes formed according to PPA formation technology, in which a polymer composition containing a PBI polymer in a PPA solvent is cast and then hydrolyzed to form a cured PBI gel membrane. Combining this basic membrane formation technology, it has been observed that by heat-treating the densified membrane formed via PPA technology, the beneficial properties of the membrane (electrochemical properties, physical and chemical durability, etc.) can be retained while reducing the membrane's permeability to redox substances, providing excellent ion exchange membranes with very low transmembrane permeation suitable for use in redox flow batteries.

[0018] To form an ion exchange membrane, a polymeric composition can be formed comprising a PPA solvent and a selected PBI-forming compound, such as a PBI-forming monomer. In some embodiments, the monomer content of the polymeric composition can typically be low, for example, about 10 wt.% or less, about 8 wt.% or less, or about 5 wt.% or less.

[0019] The PBI polymer of the membrane can have any PBI structure as generally known in the art, and is formed by the polymerization of a PBI forming compound comprising at least one aromatic or heteroaromatic tetraamino compound and at least one aromatic or heteroaromatic polycarboxylic acid or its ester, anhydride or acyl chloride, or at least one aromatic or heteroaromatic diaminocarboxylic acid. Heteroaromatic compounds covered herein include aromatic systems containing at least one nitrogen, oxygen, sulfur or phosphorus atom in the aromatic ring.

[0020] Examples of aromatic and heteroaromatic tetraamino compounds that can be used to form films may include, but are not limited to, 2,3,5,6-tetraaminopyridine; 3,3',4,4'-tetraaminodiphenyl sulfone; 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 and their salts, such as monohydrochloride, dihydrochloride, trihydrochloride and tetrahydrochloride, and any combination of aromatic or heteroaromatic tetraamino monomers.

[0021] In one embodiment, the aromatic polycarboxylic acid may include a dicarboxylic acid. The dicarboxylic acid may be used alone or in combination with one or more other polycarboxylic acid compounds, such as tricarboxylic acids and / or tetracarboxylic acids. When incorporated, the content of the tricarboxylic acid or tetracarboxylic acid is typically about 30 mol% or less, for example, from about 0.1 mol% to about 20 mol% or from about 0.5 mol% to about 10 mol%, depending on the amount of the dicarboxylic acid compound used. Esters of the polycarboxylic acid, such as C1-C20 alkyl esters or C5-C12 aryl esters of the polycarboxylic acid, may be used. Anhydrides of the polycarboxylic acid or acyl chlorides of the polycarboxylic acid may be polymerized according to the disclosed methods.

[0022] Examples of aromatic dicarboxylic acids may 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; 5-hydroxyisophthalic acid; 4-hydroxy Isophthalic 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-dihydroxy Naphthalene-3,6-dicarboxylic acid; diphenyl sulfone-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; tetrasulfoisophthalic acid; tetrasulfo Terephthalic acid; 1,4-naphthalenedicarboxylic acid; 1,5-naphthalenedicarboxylic acid; 2,6-naphthalenedicarboxylic acid; 2,7-naphthalenedicarboxylic acid; biphenylcarboxylic acid; diphenyl ether 4,4'-dicarboxylic acid; benzophenone-4,4'-dicarboxylic acid; biphenyl-4,4'-dicarboxylic acid; 4-trifluoromethyl phthalic acid; 2,2-bis(4-carboxyphenyl)hexafluoropropane; 4,4'-stilbenedicarboxylic acid; and 4-carboxycinnamic acid or any combination thereof.

[0023] Examples of aromatic tricarboxylic acids and their esters, anhydrides and acyl chlorides include, but are not limited to, 1,3,5-benzenetricarboxylic acid (triphenylcarboxylic acid); 1,2,4-benzenetricarboxylic acid (triphenylcarboxylic acid); (2-carboxyphenyl)iminodiacetic acid; 3,5,3'-biphenyltricarboxylic acid; and 3,5,4'-biphenyltricarboxylic acid; or any combination thereof.

[0024] Examples of aromatic tetracarboxylic acids and their esters, anhydrides and acyl chlorides include, but are not limited to, 3,5,3',5'-biphenyltetracarboxylic acid; benzene-1,2,4,5-tetracarboxylic acid; benzophenone tetracarboxylic acid; 3,3',4,4'-biphenyltetracarboxylic acid; 2,2',3,3'-biphenyltetracarboxylic acid; 1,2,5,6-naphthotetracarboxylic acid; and 1,4,5,8-naphthotetracarboxylic acid; or any combination thereof.

[0025] Heteroaromatic carboxylic acids can include heteroaromatic dicarboxylic acids, heteroaromatic tricarboxylic acids, and heteroaromatic tetracarboxylic acids, including their respective esters such as C1-C20 alkyl esters, C5-C12 aryl esters, or anhydrides or acyl chlorides of heteroaromatic carboxylic acids. Examples of heteroaromatic carboxylic 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 their C1-C20 alkyl esters or their C5-C12 aryl esters, or their anhydrides or their acyl chlorides, or any combination thereof.

[0026] In one embodiment, the polymeric composition may include diaminocarboxylic acids, examples of which include, but are not limited to, diaminobenzoic acid and monohydrochloride and dihydrochloride derivatives of said acid, as well as 4,4'-diphenyl ether of 1,2-diamino-3'-carboxylic acid, or any combination thereof.

[0027] The PPA that can be used in the polymerization composition can be a commercial PPA, such as that available, for example, from Riedel-de Haen. The PPA can comprise more than 100% concentrated grade PA (H3PO4). At high concentrations, individual H3PO4 units are polymerized through dehydration, and the PPA can be produced from formula H... n+2 P n O 3n+1 (n>1) represents the expression.

[0028] In some embodiments, the PPA may have a P2O5 content of about 70 wt.% or more, such as about 75 wt.% or more, or about 82 wt.% or more, calculated by acid titration, for example, from 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, which generally depends on the nature of the compounds to be polymerized and any other components of the polymerization solution.

[0029] Polymerization can be carried out at a certain temperature and continued for a period of time until suitable polymerization of the compound has occurred, which can typically be determined by an increase in the viscosity of the polymeric composition. The increase in viscosity can be determined by visual inspection, by determination of intrinsic viscosity, or by any other suitable means. For example, in some embodiments, polymerization can continue until the polymeric composition exhibits an intrinsic viscosity of about 0.8 dL / g or greater, such as about 1.0 dL / g or greater, or about 1.5 dL / g or greater. In some embodiments, the polymerization temperature can typically be about 220°C or lower, such as about 200°C or lower, like about 100°C to 195°C. Polymerization can be carried out over a period of time ranging from a few minutes (e.g., about 5 minutes) to several hours (e.g., about 100 hours). In one embodiment, the polymeric composition can be heated in a stepwise manner, for example in three or more steps, each step lasting from about 10 minutes to about 5 hours, and the temperature is increased by about 15°C or more for each step. Of course, specific polymerization conditions can vary, usually depending on the reactivity and concentration of the particular monomer, as will be apparent to those skilled in the art, and specific polymerization conditions are not required in the formation of redox flow cell films.

[0030] Exemplary PBI polymer repeating units of a PBI gel membrane may include polymer repeating units as previously identified in the art, including but not limited to polymer repeating units disclosed in U.S. Patent Nos. 11,482,721, 11,302,948, and 11,180,621, all of which are incorporated herein by reference.

[0031] Following polymerization, the polymer solution can be processed to form a membrane precursor with a desired thickness. The membrane precursor can be formed according to any suitable formation process, such as, but not limited to, casting, spraying, and blade coating. For example, in one embodiment, the gel membrane precursor can be formed to a thickness from about 20 micrometers (μm) to about 4,000 μm, such as from about 30 μm to about 3,500 μm or from about 50 μm to about 1,000 μm in some embodiments.

[0032] To solidify the PBI polymer 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. Following hydrolysis, the PPA will be hydrolyzed to form PA and water, thereby causing sol-gel transfer of the PBI polymer solution and polymer solidification, since the PBI polymer is less soluble in PA compared to PPA.

[0033] The hydrolysis treatment can be carried out at a temperature sufficient to solidify the gel membrane and for a certain duration so that it is self-supporting and can be manipulated without damage, while incorporating a high liquid content (e.g., about 60 wt.% or more of the total solids and liquid content of the membrane). By way of example, the hydrolysis treatment can be carried out at temperatures ranging from about 0°C to about 150°C, for example from about 10°C to about 120°C or from about 20°C to about 90°C, for example at ambient temperature in some embodiments.

[0034] Hydrolysis can be carried out by contacting the membrane precursor with H2O, for example in liquid, steam, or vapor form, 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 vapor and / or aqueous PA (e.g., a PA solution having a PA concentration from about 10 wt.% to about 90 wt.%, such as about 30 wt.% to about 70 wt.%, or about 45 wt.% to about 55 wt.%). The treatment can be carried out at standard pressures, but this is not a requirement for the process, and in some embodiments, the hydrolysis treatment can be carried out at modified pressures.

[0035] In one embodiment, hydrolysis can be carried out in a climate-controlled environment where the H2O content can be strictly controlled. For example, the moisture content of the local environment can be controlled by controlling the temperature or saturation of the fluid contacting the precursor membrane. For example, a carrier gas such as air, nitrogen, carbon dioxide, or other suitable gas can carry H2O (e.g., vapor) in a controlled amount for contacting the precursor membrane. In one embodiment, hydrolysis can be carried out in an environment with a relative humidity of about 40% or higher, about 50% or higher, or about 70% or higher, such as from about 20% to 100%, from about 40% to about 95%, or from about 50% to about 90% in some embodiments.

[0036] Hydrolysis treatment time can vary depending on parameters such as, for example, H2O content and the form of contact, film thickness, contact temperature, etc. Typically, hydrolysis can be carried out over a period of time between seconds and minutes, for example when superheated steam is used, or optionally over a period of several days, for example when hydrolysis is carried out at ambient temperature and relatively low atmospheric humidity. In some embodiments, hydrolysis can be carried out over a period of time between about 10 seconds and about 300 hours, for example from about 1 minute to about 200 hours. By way of example, in embodiments where at least partial hydrolysis of PPA in the PBI polymer solution is carried out at room temperature (e.g., about 20°C) in ambient air with a relative atmospheric humidity (i.e., relative humidity) content of about 20% to 100%, for example from about 40% to about 80%, the treatment time can typically be between about 5 hours and about 200 hours.

[0037] Following the hydrolysis of at least a portion of the PPA in the 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 from about 15 μm to about 3000 μm, for example from about 20 μm to about 2000 μm or from about 20 μm to about 1500 μm, although any particular film thickness is not critical. In some embodiments, the PBI gel film can have a thickness less than that of the membrane precursor. The PBI gel film can be self-supporting after hydrolysis, even at high liquid contents, which is believed to be due to the intramolecular and intermolecular polymer structures present in the solidified polymer matrix.

[0038] In one embodiment, the gel membrane may have a PBI solids content of from about 5 wt.% to about 40 wt.%, for example from about 8 wt.% to about 30 wt.%, or from about 10 wt.% to about 25 wt.%, of the total weight of the membrane (including liquid content). The formed PBI gel membrane may be self-supporting, for example having a Young's modulus of about 2.0 MPa or greater, such as about 3.0 MPa or greater, or about 4.5 MPa or greater, as determined for a PBI gel membrane (e.g., polybenzimidazole) having a thickness of 0.43 mm and a PBI content of 5 wt.%.

[0039] Optionally, the PBI of the gel membrane can be crosslinked. There are no particular limitations on the manner of crosslinking or the timing of the crosslinking process. For example, the gel membrane can undergo crosslinking after the hydrolysis of the PPA and before other processing steps as further described herein. In other embodiments, the gel membrane can be crosslinked after one or more of the additional processing steps further described herein, such as after the removal of residual PA and PPA and before or after membrane densification.

[0040] In one embodiment, the PBI polymer of the membrane can be crosslinked simply by heating in the presence of atmospheric oxygen. Crosslinking can also be achieved by the action of radiation, such as infrared (IR) radiation (with wavelengths from about 700 nm to about 1 mm), including near-IR (radiation with wavelengths from about 700 nm to about 2000 nm or energies in the range of about 0.6 eV to about 1.75 eV).

[0041] To crosslink the PBI polymer in the membrane, reactive functional groups can be incorporated into the polymer chain to crosslink with itself, or optionally with a crosslinking agent (i.e., a multifunctional compound that can react with one or more functional groups (e.g., amines) of the PBI polymer). The crosslinking agent can include any suitable functional group to achieve crosslinking. Suitable crosslinking agents are not particularly limited, and examples include, but are not limited to, epichlorohydrins, diepoxides, diisocyanates, α,ω-dihaloalkanes, diacrylates, and bisacrylamide; specific examples include, but are not limited to, α,α'-dichloro-p-xylene, chloromethyl methyl ether, bis(chloromethyl) ether, terephthaloyl chloride, succinoyl chloride, and dimethyl succinate, as well as combinations of crosslinking agents. In one embodiment, each available aromatic ring can use a crosslinking agent in amounts from 1 to 20 equivalents, but the implementation of membrane crosslinking is not limited to any particular crosslinking density.

[0042] Optionally, after the hydrolysis of PPA, the PBI gel membrane can be processed to remove residual PA and PPA from the gel membrane. When included in the formation process, this removal can be performed at any point in the formation process after the hydrolysis of PPA. For example, residual PA and PPA in the gel membrane 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.

[0043] In some implementations, the removal (e.g., rinsing) process can be performed more than once. For example, the gel membrane can be processed to remove PA and PPA residues in the formed gel membrane, and at a later time, it can undergo one or more additional processes to remove materials previously incorporated into the membrane, such as PA or another electrolyte that the membrane has absorbed or reabsorbed.

[0044] There are no particular limitations on the method of removing compounds from the membrane. For example, the membrane can simply be washed several times with water to remove unwanted materials from the substrate. For example, a PBI gel membrane can be immersed in one or more water baths, each holding the PBI membrane for a duration ranging from a few minutes (e.g., about 5 minutes) to several hours (e.g., about 24 hours). Optionally, the baths can be heated, for example to a temperature ranging from about 20°C to about 150°C, or from about 25°C to about 90°C, although in other embodiments, the membrane can be rinsed at ambient temperature without particular temperature control. To confirm the removal of materials such as acidic materials like PA and PPA, the pH of the washing solution can be determined, and washing / rinsing can continue until the pH of the washing solution is neutral.

[0045] Optionally, such as in embodiments where any remaining PA and PPA have been pre-removed from the membrane, the membrane may absorb a supporting electrolyte. When included, the electrolyte absorption step can be performed at any point after the hydrolysis of the precursor membrane. For example, the membrane may absorb the supporting electrolyte before or after a crosslinking step, before or after a densification step, before or after a drying step, or before or after a heat treatment step. The selected supporting electrolyte may be the same as or different from the supporting electrolyte used in a redox flow battery. For example, the membrane may absorb a first supporting electrolyte, such as reabsorbing a PA supporting electrolyte, before a heat treatment step, and then absorb a different supporting electrolyte after the heat treatment step and before use in a redox flow battery. For example, but not limited to, the membrane may absorb inorganic 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, such as combinations of inorganic and organic acids. In one embodiment, the membrane may reabsorb a phosphoric acid solution. Other examples of supporting electrolytes that can be absorbed in the membrane include, but are not limited to, sodium chloride, potassium chloride, sodium hydroxide, potassium hydroxide, sodium sulfide, potassium sulfide, and combinations thereof. By way of example, supporting electrolytes may include H₂SO₄, HBr, HBr / HCl mixtures, HCl, NaS₂, NaS₂ / NaBr mixtures, Br₂ in HBr, Br₂ in H₂SO₄, Br₂ in HBr / H₂SO₄ mixtures, etc. In one embodiment, a tetraalkylammonium supporting cation can be absorbed in the membrane, wherein Et₄N + and Bu4N + These are two non-limiting examples. Tetrafluoroborate (BF) 4- ), perchlorate (ClO) 4- ) or hexafluorophosphate (PF6) 6- Solutions of (or combinations thereof) are another example of supporting electrolytes that can be absorbed into a membrane.

[0046] The membrane can absorb the supporting electrolyte using any suitable method. For example, the membrane can absorb the supporting electrolyte by immersing it in a solution of the supporting electrolyte for a period ranging from several minutes to several hours or days, optionally in an elevated temperature environment. In another embodiment, a solution containing the supporting electrolyte, such as a solution heated to about 100°C or higher, for example, a solution heated to about 130°C in some embodiments, can be poured onto the membrane, and the membrane can be immersed in the resulting bath for a period of time. In some embodiments, the membrane can absorb the supporting electrolyte once or more, and the membrane can be rinsed as described above and optionally dried before subsequent absorption processes.

[0047] Prior to heat treatment, PBI gel membranes can be densified. As used herein, the term "densification" generally refers to a process that reduces the molecular porosity of the membrane. Densification processes are typically accompanied by a permanent reduction in at least one dimension of the membrane after re-swelling to a similar liquid content following the densification process. For example, after densification, a fully liquid-incorporated membrane (i.e., one that cannot absorb any additional liquid) can exhibit a reduction in size (e.g., thickness) compared to a fully liquid-incorporated membrane before densification. As used herein, the term "thickness" generally refers to... Figure 1 The dimensions of membrane 12 in the z-direction are shown in the figure.

[0048] Advantageously, the gel membrane can be densified without altering the basic morphology of the polymer matrix of the membrane formed according to the PPA formation process.

[0049] In one embodiment, the densification process may include one or more planar directions on the membrane surface (i.e., as by means of...). Figure 1 The gel membrane is actively stretched along the x and y planes (as illustrated in the figure). In the stretch densification process, the gel membrane can be actively stretched to any amount until the membrane structure ruptures. For example, in some embodiments, the gel membrane can be stretched to about 200%, about 190%, about 180%, about 170%, about 160%, or about 150% of its original size in one or more directions of the x and y planes, for example, from about 110% to about 200% or from about 120% to about 190%.

[0050] Membranes can be densified without actively stretching them. For example, when liquid is removed from the membrane, the membrane can be densified in one or more dimensions (e.g., as shown in the image) on the membrane surface. Figure 1 The membrane can be compacted by being constrained in the x-direction, y-direction, or any combination of angles or surface dimensions in the x and y planes as defined herein. By way of example, when liquid is removed from the membrane, it can be constrained on two opposite sides or optionally on all four sides, which can compact the membrane.

[0051] In one embodiment, the membrane may be in the form of a substantially continuous roll, such as on a roller, and the membrane may be constrained on opposite sides (e.g., in a cross direction) and optionally held under tension along at least a portion of the length of the sheet (e.g., in a mechanical direction) while liquid is removed from the membrane, which can densify the membrane.

[0052] In some embodiments, pressure can be applied to the membrane surface (in the z-direction) during the densification process. For example, pressure can be applied to the membrane surface by sandwiching the membrane between two substrates, allowing liquid to be removed from the membrane while constraining it in at least one of the x or y directions to densify the membrane. In such embodiments, the sandwiched substrates can allow liquid to drain from the membrane while it is densified. For example, one or both substrates can define pores, channels, etc., to allow liquid to drain from the membrane.

[0053] Liquid removal from the gel membrane during the densification process can be active liquid removal, passive liquid removal, or some combination of both. For example, simply holding the membrane in an ambient atmosphere while constraining it across one or more surface dimensions can passively remove liquid and densify the membrane. Similarly, in those embodiments where the membrane is stretched across one or more surface dimensions and / or where pressure is applied across the membrane's surface, liquid will be removed from the gel membrane solely by stretching / compression actions performed on the membrane.

[0054] In some implementations, the membrane can be processed by adding heat, contacting it with a relatively dry gas stream such as air, nitrogen, argon, or a combination thereof, to actively remove liquid from the membrane. Furthermore, active liquid removal from the membrane can be combined with stretching / pressing the membrane or simply constraining the membrane simultaneously in one or more directions on the membrane surface.

[0055] In some embodiments, the forming process may include one or more drying steps in which the membrane can be dried without constraining or stretching it in one or more directions. For example, the membrane can be actively or passively dried by clamping it between substrates (one or two of which may be porous) without constraining or stretching it in one or both of the x and y directions, and liquid can be removed from the membrane as discussed above.

[0056] The amount of liquid removed from the membrane during the densification or drying steps can make the membrane dry, i.e., retaining no additional liquid besides that due to the surrounding atmosphere, thus forming a PBI membrane. As used herein, the term "film" generally refers to a polymer membrane that is significantly thin (z-direction) compared to its surface dimensions (x, y plane) and incorporates little or no moisture within the polymer matrix, for example, about 5 wt.% or less of moisture. Therefore, the term "membrane" as used herein refers to both membranes containing a large amount of moisture and substantially dry membranes. In one embodiment, densification of the membrane may include removing a significant portion of the liquid from the gel membrane, but not necessarily to the extent that it is considered "dry." For example, 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 membrane may be removed during the densification process.

[0057] Densification processes can be performed as single-step or multi-step processes. By way of example, a multi-step process may include a first step during which the membrane may be stretched in one or more directions on its surface, and optionally, pressure may be applied across the membrane thickness to the surface region during the first step; followed by a second step during which the membrane may be constrained at one or more dimensions while liquid is actively or passively removed from the membrane. A single-step process may include stretching or constraining the membrane, optionally simultaneously pressing the membrane between substrates, along with active or passive removal of liquid from the membrane. Any combination of liquid removal, membrane stretching, and membrane constraining at one or more dimensions, optionally along with membrane pressing between substrates, may be used individually or in combination and in a single step or in multiple steps to densify the membrane.

[0058] Following densification, the membrane may undergo heat treatment, which reduces the material's permeability to redox pairs while maintaining its permeability to desired ionic substances such as protons, and preserves other desirable membrane properties. Heat treatment typically involves subjecting the densified membrane to temperatures of about 350°C or higher, such as about 375°C or higher in some embodiments, or about 400°C or higher, such as from about 350°C to about 475°C in some embodiments, or from about 400°C to about 450°C. In some embodiments, the heat treatment can last from several minutes to several hours, such as about 30 minutes or longer, or about 1 hour or longer, such as a period from about 30 minutes to about 2 hours. In embodiments, the heat treatment can be performed in an inert atmosphere, such as nitrogen. Typically, the heat treatment can be performed at atmospheric pressure, although this is not required for the process, and in some embodiments, the heat treatment can be performed at increased or decreased pressures.

[0059] Following heat treatment, the resulting membrane can be suitable for use as an ion exchange membrane in a redox flow battery. Typically, the membrane can absorb a supporting electrolyte after heat treatment and before use in the redox flow battery. In those embodiments where the membrane absorbs a supporting electrolyte before the heat treatment step, the supporting electrolyte absorbed into the membrane after the heat treatment step can be the same as or different from the previously absorbed supporting electrolyte. The final supporting electrolyte can generally depend on the specific characteristics of the redox flow battery in which the membrane will be used and can include acidic supporting electrolytes, alkaline supporting electrolytes, and neutral substances (e.g., water), including but not limited to the supporting electrolytes described above.

[0060] There is no particular limitation on the concentration of the supporting electrolyte in the membrane, and in some embodiments, it can typically be up to about 25 mol / L (M), for example from about 0.1 M to about 25 M, from about 0.5 M to about 10 M, or from about 1 M to about 5 M.

[0061] Redox flow battery membranes may include one or more additives, which may be incorporated into the membrane during membrane formation, during the processing steps described above, or in combination with the final supporting electrolyte. By way of example, small organic molecules, such as small C1-C4 alcohols (e.g., glycerol), small organic acids, urea, etc., may be incorporated into the redox flow battery membrane in combination with the supporting electrolyte.

[0062] In one embodiment, the redox flow battery membrane can typically incorporate microparticles, such as titanium dioxide or PBI microparticles, in an amount of about 2 wt.% or less, which can reduce the membrane porosity. For example, nano-sized PBI microparticles can be incorporated into the polymer matrix during the curing of the PBI gel membrane by adding the microparticles to the polymer solution during hydrolysis.

[0063] Membranes can be incorporated into redox flow batteries for any intended purpose and can be combined with any suitable electrolyte solution and redox material. For example, redox flow battery membranes can be incorporated into batteries for use in renewable energy sectors and / or current power grids to back up / reduce energy outages during peak usage periods.

[0064] One embodiment of the redox flow battery 10 is in Figure 2As shown in the diagram, the battery can be in liquid communication with a first tank 100 that can hold a first electrolyte solution and a second tank 200 that can hold a second electrolyte solution. Tanks 100 and 200 can be in liquid communication with either side of the redox flow battery ion exchange membrane 12 of the battery 10 via conduits 110 and 210, pumps 112 and 212, valves, control systems, etc. The electrolyte solution stored in tanks 100 and 200 can be circulated to either side of the battery 10 to contact either side of the membrane 12 during charging and discharging via pumps 112 and 212, respectively.

[0065] As is known, the electrolyte solutions of a battery can each incorporate one member of a redox pair. In a particular embodiment, a redox flow battery membrane can be used in a VRB. The VRB comprises a vanadium-based compound in the first electrolyte solution, wherein vanadium alternates between +5 (pentavalent) and +4 (tetravalent) vanadium, such as (VO₂)₂SO₄, VO(SO₄), or combinations thereof. The second electrolyte solution may comprise a vanadium-based compound as an active material, wherein vanadium alternates between +2 (divalent) and +3 (trivalent) vanadium, such as VSO₄, V₂(SO₄)₃, or combinations thereof.

[0066] The charging / discharging chemical reaction of one implementation of VRB can be represented as: Positive electrode:

[0067]

[0068] E 0 = +1.00 V relative to the standard hydrogen electrode (SHE) negative electrode

[0069]

[0070] E 0 = -0.26 V relative to SHE Overall chemical reaction:

[0071]

[0072] E 0 电池 = 1.26 V relative to SHE Of course, the redox flow battery described herein is not limited to VRB, and other batteries including other redox pairs are covered herein. Exemplary redox pairs may include, but are not limited to, Zn / Br2; Zn / Fe; Fe / Cr; polysulfide / Br2; polysulfide / I2; 9,10-anthraquinone-2,7-disulfonic acid (AQDS) / Br2; poly(methyl viologen)(poly(MV)) / poly(2,2,6,6-tetramethylpiperidinoxy-4-ylmethacrylate)(poly(TEMPO)); bis-(trimethylammonium)propyltetrachloride viologen (BTMAP-Vi) / BTMAP-ferrocene dichloride (BTMAP-Fc); 2,6-dihydroxyanthraquinone (2,6-DHAQ) / ferrocyanide; and pyrrolizidine 7 / 8-carboxylic acid (ACA) / ferrocyanide.

[0073] By way of example, a redox flow battery may include an electrolyte system comprising ferrocyanide such as [Fe(CN)6]3 / [Fe(CN)6]4 as the active anode electrolyte material and Fe as the active cathode electrolyte material. 2+ and Fe 3+ The cathode electrolyte in such a system may include an iron / ligand complex, examples of which may include, but are not limited to, triethanolamine, diethanolamine, ethanolamine, N,N-bis-(2-hydroxyethyl)-(iminotri)-(hydroxymethyl)-methane and mixtures thereof, wherein the cathode electrolyte may have a ligand to iron ratio from about 3:1 to about 10:1.

[0074] Electrolyte solutions typically include active materials (e.g., vanadium ions, iron ions, etc.) at concentrations ranging from about 0.5 M to about 10 M. For example, electrolyte solutions may include active materials at concentrations of about 0.5 M or more, about 0.6 M or more, or about 0.7 M or more, such as from about 1 M to about 3 M.

[0075] In one embodiment, the electrolyte solution may include corresponding active materials at concentrations ranging from 1 M to 10 M. In one embodiment, when the active materials have concentrations within this range, it can promote high energy density and high power density operation of the redox flow battery membrane. In one embodiment, when the active materials have a concentration less than 1 M, the amount of active material included per unit volume in the liquid may be too small, thereby reducing the energy density. In one embodiment, when the active materials have a concentration greater than 10 M, the electrolyte solution may have a sharply increased viscosity and therefore a significantly reduced redox reaction rate, thereby reducing the power density. Pairs of electrolyte solutions for redox flow batteries may include their respective redox pair active materials at the same or different concentrations, wherein the preferred concentrations typically depend on the specific redox pair to be used, the application of the battery, and the presence of any additional additives in the electrolyte solution.

[0076] The electrolyte solution of the battery may include additives, such as one or more redox flow battery support electrolytes as previously discussed. In one embodiment, the electrolyte solution of the battery may include a support electrolyte already absorbed in a redox flow battery membrane.

[0077] In one embodiment, the electrolyte solution may include a sulfuric acid-supported electrolyte. For example, the electrolyte solution may include a mixture of sulfuric acid and water, i.e., an aqueous sulfuric acid solution, along with the active material of the solution, for example, as a solvent. In one embodiment, the mixture of the supporting electrolyte and water, such as the aqueous sulfuric acid solution, may include a supporting electrolyte at a concentration from 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 solution. Therefore, the solution can exhibit the desired ionic conductivity and viscosity, and overvoltage problems in the battery can be avoided.

[0078] like Figure 2As indicated, each side of battery 10 may include additional components adjacent to membrane 12 as known in the art, including but not limited to conductive separator 14, such as porous carbon paper, carbon cloth, carbon felt, or metal cloth (a porous membrane made of fibrous metal or a metal film formed on the surface of polymer fiber cloth) and others. As known, the battery may also include electrodes 16, which may be the same or different from each other and may be made of a conductive substrate (e.g., graphite) suitable for a corresponding electrolyte solution of the battery. Current collector 18 (e.g., gold-plated copper) may be electrically connected to electrodes 16, and the battery may include end plates 20 (e.g., stainless steel end plates) on either side of the battery and opposite to the separator. Current collector 18 provides electrical communication between battery 10 and external circuitry, as shown.

[0079] Figure 3 The diagram illustrates more than one battery 10 arranged in a typical battery stack 150 of a redox flow battery. As shown, a first circulation path 300 may pass through one side of each of the batteries 10 in the stack 150, such that the electrolyte solution of that portion of the battery flows through path 300 and returns to a first tank 100. A second circulation path 400 passes through the other side of each of the batteries 10 in the stack 150, such that the electrolyte solution of that portion of the battery flows through path 400 and returns to a second tank 200. The redox flow battery may also include corresponding charge / discharge circuitry, as well as converters, controllers, etc., to collect and supply power using the battery, as is known in the art.

[0080] The redox flow battery membranes described herein can enable high-performance flow batteries operating under high current loads with low transmembrane permeation. Such improved materials can alleviate the need for large electrochemical stacks and thus reduce the overall cost of commercial flow battery devices.

[0081] The present disclosure can be better understood by referring to the embodiments described below. Example

[0082] PBI gel film (Celtec) formed using PPA technology and doped with phosphoric acid. ® -P) Sourced from BASF Corporation. The membranes were modified into dense membranes by first washing them with water to remove all phosphoric acid. Washing was completed by immersing each membrane in a DI water bath for up to 24 hours, with the water changed periodically 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 any shrinkage in these directions, and then the membrane collapsed in the Z direction as it dried.

[0083] The n-PBI membrane is also formed internally according to the PPA scheme. A typical polymerization consists of the following: 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) are added to 243 g of polyphosphoric acid, mixed with a top-mounted stirrer, and purged with dry nitrogen. The contents are heated in a high-temperature silicone oil bath, and the temperature is controlled by a programmable temperature controller with ramp and soaking features. The reaction temperature is gradually increased from room temperature to 120°C, then to 150°C, then to 170°C, and then to 190°C. In a typical polymerization, the final reaction temperature is approximately 190°C and maintained for 12 hours. After the reaction is confirmed to be complete by visual inspection of viscosity, the polymer solution is cast onto a transparent glass plate using a scraper with a controlled gate thickness of 15 mils. The cast solution was hydrolyzed into a gel film at 25°C in a humidity chamber adjusted to 55% RH. The intrinsic viscosity, measured in sulfuric acid at 30°C, yielded a value of approximately 6 dL / g.

[0084] After formation, the n-PBI gel membrane interacts with Celtec. ® -P membranes are rinsed and densified in the same way.

[0085] Based on a processed film from two subsets. The first subset undergoes reabsorption after densification and before heat treatment, and the second subset undergoes heat treatment after densification.

[0086] The first subset of membranes is reabsorbed by attaching a densified membrane to a glass plate. A reactive oligomer with 0.25 wt.% ( p,p -OPBI)(See, for example, Rohlfing, Liu, Daryaei, Benicewicz, Polymer 2023 280 A phosphoric acid solution (126034) was heated to 130°C to completely dissolve the oligomers. The hot solution was then gently poured onto the membrane and allowed to stand and permeate the dense membrane overnight. The membrane was then placed in a DI water bath, with the water changed periodically until the pH of the water was neutral (up to 24 hours). The washed membrane was then dried overnight between porous sheets until all the water had evaporated.

[0087] The heat treatment of all membranes is the same, whether they absorb oligomers and are then washed and dried, or they are simply washed and dried membranes without further reabsorption. For heat treatment, the membrane is inserted into a preheated tube furnace at the desired temperature. The tube furnace is preheated under N2 gas, and an inert atmosphere is maintained during treatment. The membrane treatment is continued for a specific duration, up to 1 hour, and then the membrane is removed from the tube furnace for cooling.

[0088] Vanadium (VOSO4) permeability measurements were performed using a PermeGear "side-by-side" direct permeation cell. This cell has two electrolyte chambers, each with a capacity of 45 mL, separated by the membrane or film being tested. The temperature was maintained at 25°C throughout the test using a circulating water bath. A typical experiment consisted of two electrolytes: a donor chamber containing 1.6 M VOSO4 in 2 M H2SO4, and an acceptor chamber containing 1.6 M MgSO4 in 2 M H2SO4. Vanadium (IV) exhibits detectable absorption at 248 nm. Aliquots of the acceptor solution were periodically collected over time to measure the vanadium salt permeability. A Shimadzu UV-2450 UV-Vis was used to measure VOSO4. 2+ Permeability, calculated using Fick's diffusion law:

[0089] Where P s It is the calculated salt permeability, c r (t) is the concentration of VOSO4 in the acceptor 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 membrane thickness, and A is the effective area of ​​the membrane.

[0090] Multiple samples were processed as follows: Sample 1: As a comparison, the measurement of the densified Celtec, as previously described, is as follows. ® - The permeability of the P membrane (without heat treatment) was determined to be 5.87 × 10⁻⁶. -09 cm 2 / s.

[0091] Sample 2: As described, the densified Celtec ® The -P membrane was heat-treated at 300°C for 1 hour under inert conditions. Permeability was measured as described, and the permeability was determined to be 6.54 × 10⁻⁶. -09 cm 2 / s.

[0092] Sample 3: As described, the densified Celtec ® The -P membrane was heat-treated at 400°C for 1 hour under inert conditions. Permeability was measured as described, and the permeability was determined to be 1.07 × 10⁻⁶. -09 cm 2 / s. This permeability is higher than the control Celtec. ®-P membranes have a permeability that is one-fifth lower.

[0093] Sample 4: As described, the densified Celtec ® The -P membrane was heat-treated at 425°C for 1 hour under inert conditions. Permeability was measured as described, and the permeability was determined to be 4.10 × 10⁻⁶. -10 cm 2 / s. This permeability is compared to Celtec. ® -P membranes have approximately 1 / 14 the permeability.

[0094] Sample 5: As described, the densified Celtec®-P membrane was heat-treated at 450°C for 30 minutes under inert conditions. Permeability was measured as described, and the permeability was determined to be 1.42 × 10⁻⁶. -10 cm 2 / s. This permeability is less than 1 / 40th of that of the control Celtec®-P membrane.

[0095] Sample 6: As described, the densified Celtec®-P membrane was heat-treated at 450°C for 1 hour under inert conditions. Permeability was measured as described, and the permeability was determined to be 1.02 × 10⁻⁶. -11 cm 2 / s. This permeability is approximately 1 / 575 of that of the control Celtec®-P membrane.

[0096] Example 7: As a second control, the permeability of the densified n-PBI membrane (without heat treatment) was measured as described, and the permeability was determined to be 1.35 × 10⁻⁶. -08 cm 2 / s.

[0097] Example 8: As described, the densified n-PBI membrane was heat-treated at 450°C for 1 hour under inert conditions. Permeability was measured as described, and the permeability was determined to be 3.00 × 10⁻⁶. -10 cm 2 / s. This permeability is 1 / 45 of that of the control n-PBI membrane.

[0098] Example 9: As described, the densified Celtec®-P membrane was reabsorbed. Then, as described, the membrane was heat-treated under inert conditions at 450°C for 1 hour. Permeability was measured as described, and the permeability was determined to be 9.14 × 10⁻⁶. -11 cm 2 / s. This permeability is approximately 1 / 64th of that of the control Celtec®-P membrane.

[0099] Although certain embodiments of the disclosed subject matter have been described using specific terminology, such description is for illustrative purposes only, and it should 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, comprising: shaping a polymeric solution to form a membrane precursor, the polymeric solution comprising a polybenzimidazole dissolved in a polyphosphoric acid solvent; hydrolyzing at least a portion of the polyphosphoric acid of the polymeric solution to form phosphoric acid and water, thereby causing sol-gel transition and solidification of the polybenzimidazole, and forming a gel membrane comprising the polybenzimidazole; densifying the gel membrane to form a densified membrane; and heat treating the densified membrane, the heat treating comprising subjecting the densified membrane to a temperature of about 350 °C or greater in an inert atmosphere for a period of about 30 minutes or greater.

2. The method of claim 1, further comprising removing the phosphoric acid and any remaining polyphosphoric acid from the gel membrane prior to densifying the gel membrane.

3. The method of claim 2, further comprising absorbing a supporting electrolyte into the densified gel membrane prior to heat treating the densified membrane.

4. The method of claim 1, further comprising drying the densified membrane prior to heat treating the densified membrane.

5. The method of claim 1, further comprising absorbing a supporting electrolyte into the membrane after the step of heat treating the densified membrane.

6. The method of claim 1, further comprising crosslinking the gel membrane.

7. The method of claim 1, the step of densifying the gel membrane comprising stretching the gel membrane in one or more planar directions of the membrane surface.

8. The method of claim 1, the step of densifying the gel membrane comprising constraining the membrane in one or more dimensions of the membrane surface.

9. The method of claim 1, the step of densifying the gel membrane comprising pressing the gel membrane between two substrates.

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

11. The method of claim 1, the hydrolyzing being performed at a relative humidity of about 40% or greater.

12. An ion exchange membrane, comprising: a densified polybenzimidazole gel membrane; and a redox flow battery supporting electrolyte absorbed within the densified polybenzimidazole gel membrane; wherein The ion exchange membrane exhibits an in-plane ion conductivity of about 50 mS / cm 2 or greater; and wherein The ion exchange membrane exhibits a permeability to redox-active species of about 2 x 10 -9 cm 2 / s or less.

13. The ion exchange membrane of claim 12, wherein the supporting electrolyte comprises an inorganic acid, a strong organic acid, or a mixture thereof.

14. The ion exchange membrane of claim 13, wherein the supporting electrolyte comprises a strong inorganic acid.

15. The ion exchange membrane of 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 triflic acid, or any combination thereof.

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

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

18. The redox flow battery of claim 17, wherein the redox flow battery is a vanadium redox flow battery.

19. The redox flow battery of claim 17, comprising a redox couple selected from the group consisting of: Zn / Br2; Zn / Fe; Fe / Cr; polysulfide / Br2; polysulfide / I2; 9,10-anthraquinone-2,7-disulfonic acid (AQDS) / Br2; poly(methyl viologen) (poly(MV)) / poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (poly(TEMPO)); bis-(trimethylammonio)propylviologen tetrachloride (BTMAP-Vi) / BTMAP-ferrocenium dichloride (BTMAP-Fc); 2,6-dihydroxyanthraquinone (2,6-DHAQ) / ferrocyanide; and porphyrin 7 / 8-carboxylic acid (ACA) / ferrocyanide.

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