Low crossover ion exchange membranes for redox flow batteries

Abstract

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

Description

LOW CROSSOVER ION EXCHANGE MEMBRANES FOR REDOX FLOW BATTERIESCross Reference to Related Application

[0001] This application claims filing benefit of United States Provisional Patent Application Serial No. 63 / 518618 having a filing date of 08 / 10 / 2023, and United States Provisional Patent Application Serial No. 63 / 609596 having a filing date of 12 / 13 / 2023, which are incorporated herein by reference for all purposes.Background

[0002] Increasing demands on the energy sector have created a need for large- scale energy storage devices that can provide back-up power and improve grid management while also having capability for seamless integration with renewable energy devices. Redox flow batteries can meet such needs. Redox flow batteries perform charging and discharging by using a positive electrode electrolyte solution and a negative electrode electrolyte solution separated by an ion exchange membrane. Each electrolyte solution contains metal ions (active materials) that form a redox pair (also referred to as a redox couple) by which valence is changed by oxidation-reduction.

[0003] In spite of its potential, the widespread adoption of redox flow batteries has been limited, primarily due to the high cost of device fabrication. For instance, in a vanadium redox flow battery a major portion of the cost is attributed to the vanadium electrolyte. Such costs could be mitigated by the development of systems able to operate under high current loads for many cycles. To meet these demands, systems will require highly effective ion-exchange membranes, as the ion exchange membrane is a primary component of a redox flow battery and has an important effect upon the output, capacity, lifespan, and cost of the battery.

[0004] Ideally, the ion-exchange membrane of a redox flow battery will exhibit high conductivity of the desired cation or anion (e.g., protons or hydroxide ions) while simultaneously exhibiting low crossover of redox pair species so as to function at high current loads. In addition, the ion exchange membrane of a redox flow battery is immersed in an electrolyte solution, and thus must be able to resist mechanical and chemical degradation due to oxidation or the like. The stability and durability of the membrane is a key factor determining the lifespan of a redox flow battery.

[0005] Unfortunately, traditional membranes used in redox flow batteries (generally perfluorosulfonic acid membranes such as Nation®) do not meet these requirements due to high crossover characteristics and resulting poisoning of the membrane as well as a lack of durability. More recently, polybenzimidazole (PBI) membranes have been considered for use in redox flow batteries. Traditional PBI membranes prepared by solution casting in N,N’-dimethylacetamide (DMAc) to form a dense film followed by imbibing the formed film in the desired electrolyte have been shown to exhibit extremely low ionic conductivities when imbibed with electrolyte solutions (less than 20 mS-cm-1) and an inability to operate at current loads above about 100 mA cm-2. However, PBI membranes formed by direct casting of a composition comprising the PBI polymer in polyphosphoric acid (PPA) solvent, with subsequent hydrolysis of the PPA solvent to phosphoric acid (PA) and resulting solidification of the as-formed PA-imbibed PBI membrane have shown more promise for utilization in redox flow batteries.

[0006] While the above describes improvement in the art, room for further improvement exists. What are needed in the art are methods for forming ionexchange membranes and membranes produced thereby that exhibit low crossover of redox species from one side to the other, while exhibiting high ionic permeability as well as stability and durability in the redox flow battery environment.Summary

[0007] According to one embodiment, disclosed is a method for forming an ionexchange membrane. A method can include shaping a polymerization solution to form a membrane precursor. The polymerization solution includes a polybenzimidazole (PBI) dissolved in a polyphosphoric acid solvent. A method can also include hydrolyzing at least a portion of the polyphosphoric acid of the polymerization solution to form phosphoric acid and water, thereby causing a sol-gel transfer and solidification of the polybenzimidazole and formation of a gel membrane comprising the polybenzimidazole. Following, the gel membrane can be densified to form a densified membrane. Following densification, the membrane can be thermally treated. The thermal treatment including subjecting the densified membrane to a temperature of about 350°C or greater for a period of time of about 30 minutes or greater in an inert atmosphere.

[0008] Also disclosed are ion exchange membranes as may be formed according to disclosed methods. For instance, an ion exchange membrane can include a densified FBI gel membrane and a redox flow battery supporting electrolyte imbibed within the PBI gel membrane. An ion exchange membrane can exhibit an in-plane ionic conductivity in a 2.6 M sulfuric acid solution of about 50 mS / cm2or greater; and can exhibit a permeability to a redox couple species of about of about 2x1 O'9cm2 / s or less.

[0009] Also disclosed are redox flow batteries incorporating the disclosed ion exchange membranes.Brief Description of the Figures

[0010] A full and enabling disclosure of the present subject matter, including the best mode thereof to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures in which:

[0011] FIG. 1 defines dimensions of a membrane utilized in this disclosure.

[0012] FIG. 2 schematically illustrates a redox flow battery as may incorporate a membrane as described herein.

[0013] FIG. 3 schematically illustrates a multi-cell stack as may be included in a redox flow battery as described.Detailed Description

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

[0015] In general, the present disclosure is directed to ion exchange membranes and methods for forming the membranes. Disclosed membranes can exhibit high conductivity of desired ions (e.g., protons or hydroxide ions) as well as stability and durability in the redox flow battery environment while also exhibiting low permeability to redox couple species, e.g., VO2+and VO4+vanadium redox couple species. Forinstance, a redox flow battery membrane as described can exhibit a permeability to a member of a redox couple, e.g., VO2+or VO4+, of about 2x10-9cm2 / s or less, such as on the order of 10'10cm2 / s or on the order of 1011cm2 / s in some embodiments, such as about 1x1 O'9cm2 / s or less, about 5 x 10'10cm2 / s or less, about 3 x 10'10cm2 / s or less, or about 9 x 10-11cm2 / s or less in some embodiments, such as from about 1 x 10’11' cm2 / s to about 2x1 O'9cm2 / s.

[0016] In conjunction with the low permeability to redox couple species, the ion exchange membranes can exhibit excellent stability and durability in the redox flow battery environment as well as desirable electrochemical activity. For instance, a redox flow battery membrane as described can exhibit an in-plane ionic conductivity in a 2.6 M sulfuric acid solution of about 50 mS / cm, or even higher in some embodiments, e.g., about 100 mS / cm or about 200 mS / cm in some embodiments.

[0017] In addition, redox flow batteries incorporating disclosed membranes can operate at high current density, for instance about 50 mA / cm2or higher, e.g., from about 100 mA / cm2to about 500 mA / cm2in some embodiments. Moreover, redox flow batteries incorporating the membranes can operate at high efficiency. By way of example, at a current density of 242 mA / cm2a redox flow battery incorporating a membrane as described can exhibit a coulombic efficiency (CE) of about 90% or greater, for instance from about 94% to about 99% in some embodiments; an energy efficiency (EE) of about 70% or greater, for instance from about 73% to about 84% in some embodiments; and a voltage efficiency (VE) of about 75% or greater, for instance from about 78% to about 80%. At a current density of 483 mA / cm2a redox flow battery incorporating a membrane as described can exhibit a CE of 85% or greater, for instance from about 87% to about 99% in some embodiments; an EE of about 50% or greater, for instance from about 54% to about 75% in some embodiments; and a VE of about 60% or greater, for instance from about 62% to about 77%.

[0018] The ion exchange membranes are based upon FBI membranes formed according to a PPA formation technique, in which a polymer composition including a PBI polymer in a PPA solvent is cast and then hydrolyzed to form a solidified PBI gel membrane. In conjunction with the basic membrane formation techniques, it has been discovered that through thermal treatment of a densified membrane formed via the PPA technique, the beneficial characteristics of the membranes can be retained (electrochemical characteristics, physical and chemical durability, etc.), whiledecreasing the permeability of the membrane to redox species, providing an excellent ion exchange membrane with very low crossover as may be utilized in a redox flow battery.

[0019] To form an ion-exchange membrane, a polymerization composition can be formed that includes a PPA solvent and the PBI-forming compounds of choice, e.g., PBI-forming monomers. The monomer content of the polymerization composition can generally be low, for instance about 10 wt.% or less, about 8 wt.% or less, or about 5 wt.% or less in some embodiments.

[0020] The PBI polymer of the membrane can have any PBI structure as is generally known in the art formed by polymerization of PBI-forming compounds including at least one aromatic or heteroaromatic tetraamino compound and at least one aromatic or heteroaromatic polycarboxylic acid or ester, anhydride, or acid chloride thereof or at least one aromatic or heteroaromatic diaminocarboxylic acid. Heteroaromatic compounds encompassed herein include aromatic systems that contain at least one nitrogen, oxygen, sulfur, or phosphorus atom in an aromatic ring.

[0021] Examples of aromatic and heteroaromatic tetraamino compounds as may be utilized in forming the membrane can include, without limitation, 2, 3,5,6- tetraminopyridine; 3,3',4,4'-tetraminodiphenylsulfone; 3,3',4,4'-tetraminodiphenyl ether; 3,3',4,4'-tetraminobiphenyl; 1 ,2,4,5-tetraminobenzene; 3, 3’, 4,4'- tetraminobenzophenone; 3,3',4,4'-tetraminodiphenylmethane; and 3, 3', 4,4'- tetraminodiphenyldimethyl-methane and the salts thereof, e.g., the mono-, di-, tri- and tetrahydrochloride salts, as well as any combination of aromatic or heteroaromatic tetraamino monomers.

[0022] In one embodiment, an aromatic polycarboxylic acid can include a dicarboxylic acid. A dicarboxylic acid can be utilized alone or in combination with one or more additional polycarboxylic acid compounds, e.g., tricarboxylic acids and / or tetracarboxylic acids. When incorporated, the content of tricarboxylic acid or tetracarboxylic acids can generally be about 30 mol % or less, for instance from about 0.1 mol% to about 20 mol %, or from about 0.5 mol% to about 10 mol % based on the amount of the dicarboxylic acid compound used. An ester of a polycarboxylic acid can be utilized such as C1-C20-alkyl esters or C5-C12-aryl esters of a polycarboxylic acid. An anhydride of a polycarboxylic acid or an acid chloride of a polycarboxylic acid can be polymerized according to disclosed methods.

[0023] Examples of aromatic dicarboxylic acids can include, without limitation, 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-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; diphenylsulfone-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; 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 - stilbenedicarboxylic acid; and 4-carboxycinnamic acid or any combination thereof.

[0024] Examples of aromatic tricarboxylic acids and esters, acid anhydrides, and acid chlorides thereof include, without limitation, 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.

[0025] Examples of aromatic tetracarboxylic acids and esters, acid anhydrides, and acid chlorides thereof include, without limitation, 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.

[0026] 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 the acid anhydrides or the acid chlorides of the heteroaromatic carboxylic acids.Examples of heteroaromatic carboxylic acids include, without limitation, 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 also their C1-C20-alkyl esters or their C5-C12-aryl esters, or their acid anhydrides or their acid chlorides, or any combination thereof.

[0027] In one embodiment, the polymerization composition can include a diaminocarboxylic acid, examples of which include, without limitation, diaminobenzoic acid and the mono and dihydrochloride derivatives of said acid, as well as 1 ,2-diamino-3'-carboxy acid 4,4'-diphenyl ether, or any combination thereof.

[0028] PPA as can be utilized in the polymerization composition can be commercial PPA as obtainable, for example, from Riedel-de Haen. PPA can include concentrated grades of PA (H3PO4) above 100%. At high concentrations, the individual H3PO4 units are polymerized by dehydration and the PPA can be expressed by the formula Hn+2PnO3n+i (n>1).

[0029] The PPA can have a P2O5 content as calculated by acidimetry of about 70 wt.% or more, for instance about 75 wt.% or more, or about 82 wt.% or more, for instance from about 70 wt.% to about 86 wt.% in some embodiments. The polymerization composition can be in the form of a solution of the monomers and other compounds, or a dispersion / suspension of the monomers / compounds in the PPA, generally depending upon the nature of the compounds to be polymerized and any additional components of the polymerization solution.

[0030] The polymerization can be carried out at a temperature and for a time until suitable polymerization of the compounds has taken place, which can generally be determined by an increase in viscosity of the polymerization composition. The increase in viscosity can be determined by visual inspection, through determination of the intrinsic viscosity, or by any other suitable means. For instance, the polymerization can continue until the polymerization composition exhibits an intrinsic viscosity of about 0.8 dL / g or greater, for instance about 1.0 dL / g or greater, or about 1.5 dL / g or greater, in some embodiments. The polymerization temperature can generally be about 220° C or less, for instance about 200° C or less, such as about100° C to 195° C in some embodiments. The polymerization can be carried out over a time of from a few minutes (e.g., about 5 minutes) up to several hours (e.g., about 100 hours). In one embodiment, the polymerization composition can be heated in a stepwise fashion, for instance in three or more steps, each step lasting from about 10 minutes to about 5 hours and increasing the temperature by about 15° C or more for each step. Of course, the particular polymerization conditions can be varied, depending generally upon the reactivity and concentration of the particular monomers, as would be evident to one of skill in the art, and no particular polymerization conditions are required in formation of the redox flow battery membranes.

[0031] Exemplary FBI polymer repeating units of a FBI gel membrane can include those as have been previously identified in the art, including, without limitation, polymer repeating units as disclosed in US Patent Nos. 11 ,482,721 ; 11 ,302,948; and 11 ,180,621 , all of which being incorporated herein by reference thereto.

[0032] Following polymerization, the polymer solution can be processed to form a membrane precursor having a desired thickness. The membrane precursor can be formed according to any suitable formation process, such as, and without limitation to, casting, spray coating, knife coating, etc. For instance, the gel membrane precursor can be formed to a thickness of from about 20 micrometers (pm) to about 4,000 pm in one embodiment, such as from about 30 pm to about 3,500 pm, or from about 50 pm to about 1 ,000 pm, in some embodiments.

[0033] To solidify the FBI polymer of the 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 of the membrane precursor. Upon hydrolysis, the PPA will hydrolyze to form PA and water, thereby causing a sol-gel transfer of the FBI polymer solution and solidification of the polymer, as the PBI polymer is less soluble in PA as compared to PPA.

[0034] The hydrolysis treatment can be carried out at temperatures and for a time sufficient for the gel membrane to solidify so as to be self-supporting and capable of being manipulated without destruction while incorporating high liquid content (e.g., about 60 wt.% or higher liquid content of the total solid and liquid content of the membrane). By way of example, the hydrolysis treatment can be carried out at atemperature of from about 0°C to about 150°C, for instance from about 10°C to about 120°C, or from about 20°C to about 90°C, e.g., at ambient temperature in some embodiments.

[0035] The hydrolysis can be carried out by contact of the membrane precursor with H2O, for instance in the form of a liquid, steam, or vapor, and / or in the presence of other components. For instance, 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 from about 10 wt.% to about 90 wt.%, e.g., about 30 wt.% to about 70 wt.% or about 45 wt.% to about 55 wt.%). The treatment can be carried out under standard pressure, but this is not a requirement of a formation process, and in some embodiments, the hydrolysis treatment can be carried out under modified pressure.

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

[0037] The hydrolysis treatment time can generally vary depending upon parameters such as, e.g., H2O content and form of the contact, membrane thickness, contact temperature, etc. In general, the hydrolysis treatment can be carried for a period of time between a few seconds to a few minutes, for instance when the hydrolysis treatment utilizes superheated steam, or alternatively over a period of several days, for example when the hydrolysis treatment is carried out at ambient temperature and relatively low relative atmospheric moisture. In some embodiments, the hydrolysis treatment can be carried out over a period of time between about 10 seconds and about 300 hours, for instance from about 1 minute to about 200 hours. By way of example, in an embodiment in which the at least partial hydrolysis of the PPA of the PBI polymer solution is carried out at room temperature (e.g., about 20° C) with ambient air of relative atmospheric moisture (i.e. , relativehumidity) content of from about 20% to 100%, for instance from about 40% to about 80%, the treatment time can generally be between about 5 hours and about 200 hours.

[0038] Upon hydrolysis of at least a portion of the PPA of the PBI polymer solution, the polymer can solidify to form a self-supporting gel membrane. A PBI gel membrane can in one embodiment have a thickness of from about 15 pm to about 3000 pm, for instance from about 20 pm to about 2000 pm, or from about 20 pm to about 1500 pm, though any particular membrane thickness is not critical. In some embodiments, the PBI gel membrane can have a thickness that is less than that of the membrane precursor. Following hydrolysis, the PBI gel membrane can be self- supporting, even at high liquid content, which is believed to be due to the intra- and intermolecular polymer structures present in the solidified polymeric matrix.

[0039] The gel membrane can in one embodiment have a PBI solids content of from about 5 wt.% to about 40 wt.%, for instance 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 as-formed PBI gel membrane can be self-supporting, for instance having a Young's modulus of about 2.0 MPa or greater, for instance about 3.0 MPa or greater, or about 4.5 MPa or greater in some embodiments as determined for a PBI gel membrane having a thickness of 0.43 mm and a PBI content of 5 wt.% (e.g., polybenzimidazole).

[0040] Optionally, the PBI of the gel membrane can be crosslinked. The manner of crosslinking as well as the point in the formation process at which the polymer is crosslinked is not particularly limited. For instance, a gel membrane can be subjected to crosslinking following hydrolysis of the PPA and prior to other processing steps as described further herein. In other embodiments, a gel membrane can be crosslinked following one or more of the additional processing steps described further herein, e.g. following removal of remaining PA and PPA and prior to or following densification of the membrane.

[0041] 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 affected by the action of radiation, e.g., infrared (IR) radiation (having a wavelength of from about 700 nm to about 1 mm) including near IR (radiation having awavelength of from about 700 to about 2000 nm or an energy in the range from about 0.6 to about 1 .75 eV).

[0042] In order to crosslink the FBI polymer of the membrane, the FBI polymer can incorporate reactive functionality on the polymer chains so as to crosslink with itself or alternatively in conjunction with a crosslinking agent, i.e. , a polyfunctional compound that can react with one or more functionalities of the FBI polymer (e.g., amines). Crosslinking agents can include any suitable functionality to effect crosslinking. Suitable crosslinking agents are not particularly limited, examples of which can include, without limitation, epichlorohydrin, diepoxides, diisocyanates, a,w-dihaloalkanes, diacrylates, and bisacrylamides, particular examples of which can include, without limitation, a,a'-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, from 1 to 20 equivalents of crosslinking agent can be utilized per available aromatic ring, but crosslinked embodiments of the membranes are not limited to any particular crosslink density.

[0043] Optionally, following hydrolysis of the PPA, the PBI gel membrane can be processed to remove PA and PPA remaining in the gel membrane. When included in a formation process, this removal can take place at any point in a formation process following the hydrolysis of the PPA. For instance, remaining PA and PPA of a gel membrane can be removed prior to or following a crosslinking step, prior to or following a densification step, prior to or following a drying step, or prior to or following a thermal treatment step.

[0044] In some embodiments, a removal (e.g., rinsing) process can be carried out more than once. For instance, a gel membrane can be processed to remove PA and PPA remaining in the as-formed gel membrane and at a later time, can be subjected to one or more additional processes to remove a material that has been since incorporated into the membrane, e.g., PA or another electrolyte with which the membrane has been imbibed or re-imbibed.

[0045] The manner of removing a compound from a membrane is not particularly limited. For instance, the membrane can be simply washed with water several times to remove an undesired material from the substrate. For example, a PBI gel membrane can be soaked in one or more water baths, each bath retaining the PBI membrane for a period of time from a few minutes (e.g., about 5 minutes) to severalhours (e.g., about 24 hours). Optionally, a bath can be heated, for instance to a temperature of from about 20° C to about 150° C, for instance from about 25° C to about 90° C, though in other embodiments, a membrane can be rinsed at ambient temperature, with no particular temperature control. To confirm removal of the material, e.g., an acidic material such as PA and PPA, the pH of the wash solution can be determined, and washing / rinsing can continue until the pH of the wash solution is neutral.

[0046] Optionally, for instance in an embodiment in which any remaining PA and PPA has been previously removed from the membrane, a membrane can be imbibed with a supporting electrolyte. When included, an electrolyte imbibing step can be carried out at any point following the hydrolysis of the precursor membrane. For instance, a membrane can be imbibed with a supporting electrolyte prior to or following a crosslinking step, prior to or following a densification step, prior to or following a drying step, or prior to or following a thermal treatment step. The supporting electrolyte of choice can be the same or different than a supporting electrolyte for use in a redox flow battery. For instance, a membrane can be imbibed with a first supporting electrolyte prior to a thermal treatment step, e.g, re-imbibed with a PA supporting electrolyte, and then imbibed with a different supporting electrolyte following a thermal treatment step and prior to utilization in a redox flow battery. For instance, and without limitations, a membrane can be imbibed with a mineral acid (e.g., a strong inorganic acid) such as hydrochloric acid, nitric acid, fluorosulfonic acid, or sulfuric acid, or a mixture thereof, or a strong organic acid such as acetic acid, formic acid, p-toluene sulfonic acid, or trifluoromethane sulfonic acid or mixtures thereof, as well as mixtures of different types of acids, e.g., a combination of a mineral acid and an organic acid. In one embodiment, a membrane can be re-imbibed with a phosphoric acid solution. Other examples of supporting electrolytes that can be imbibed in the membrane can include, without limitation, sodium chloride, potassium chloride, sodium hydroxide, potassium hydroxide, sodium sulfide, potassium sulfide, and combinations thereof. By way of example, a supporting electrolyte can include H2SO4, HBr, HBr / HCI mixtures, HCI, NaS2, NaS2 / NaBr mixtures, Br2 in HBr, Br2 in H2SO4, Br2 in HBr / H2SO4 mixtures, etc. Tetraalkylammonium supporting cations can be imbibed in the membranes in one embodiment, with Et4N+and Bu4N+being two non-limiting examples. A solution of a tetrafluoroborate (BF4-), perchlorate (CIO4-), or hexafluorophosphate (PF6-), or acombination thereof are additional examples of supporting electrolytes that can be imbibed in the membranes.

[0047] A membrane can be imbibed with a supporting electrolyte according to any suitable methodology. For example, a membrane can be imbibed with the supporting electrolyte by soaking the membrane in a solution of the supporting electrolyte for a period of time from a few minutes up to hours or days, optionally in an environment of increased temperature. In another embodiment, a solution containing the supporting electrolyte, e.g., a heated solution heated to a temperature of about 100°C or greater, e.g., about 130°C in some embodiments, can be poured over the membrane and the membrane can the soak in the bath thus formed for a period of time. In some embodiments, the membrane can be imbibed with a supporting electrolyte one or more times, with rinsing of the membrane as described above and optionally drying of the membrane carried out prior to a follow-on imbibing process.

[0048] Prior to a thermal treatment, a PBI gel membrane can be densified. As utilized herein, the term “densify” generally refers to a process that decreases a molecular porosity of the membrane. A densification process will generally be accompanied by a permanent decrease in at least one dimension of the membrane upon re-swelling of the membrane to a similar liquid content following the densification process. For example, following densification, a membrane that is fully incorporated with a liquid (i.e., cannot absorb any additional liquid) can exhibit a decrease in a dimension (e.g., thickness) as compared to the fully liquid incorporated membrane prior to densification. As utilized herein, the term “thickness” generally refers to the dimension of a membrane 12 in the z direction as illustrated in FIG. 1 .

[0049] Beneficially, a gel membranes can be densified without altering the basic morphology of the polymeric matrix of the membrane that is formed according to the PPA formation process.

[0050] In one embodiment, a densification process can include actively stretching a gel membrane in one or more planar directions of the membrane surface (i.e., as defined by the x, y plane as illustrated in FIG. 1). In a stretching densification process, the gel membrane can be actively stretched to any amount up to rupture of the membrane structure. For instance, a gel membrane can be stretched to about 200%, to about 190%, to about 180%, to about 170%, to about 160%, or to about 150% of the original dimension in one or more directions of the x, y plane of themembrane, for instance, from about 110% to about 200%, or from about 120% to about 190%, in some embodiments.

[0051] A membrane can be densified without active stretching of the membrane. For instance, a membrane can be restrained in one or more dimensions of the membrane surface (e.g., the x direction, the y direction or any angle of the x, y plane as defined in FIG. 1 or combination of surface dimensions) while liquid is removed from the membrane, which can densify the membrane. By way of example, a membrane can be restrained on two opposite sides or alternatively on all four sides while liquid is removed from the membrane, which can densify the membrane.

[0052] In one embodiment, a membrane can be in the form of an essentially endless sheet, e.g., on a roll, and the membrane can be restrained on the opposing sides (e.g., in the cross direction) and optionally held under tension along at least a portion of the length of the sheet (e.g., in the machine direction) while liquid is removed from the membrane, which can densify the membrane.

[0053] In some embodiments, pressure can be applied to a membrane surface (in the z direction) during a densification process. For instance, pressure can be applied to a membrane surface by sandwiching the membrane between two substrates and liquid can be removed from the membrane while the membrane is restrained in at least one of the x or y direction so as to densify the membrane. In such embodiments, the sandwiching substrates can allow for liquid to drain away from the membrane as it is densified. For instance, one or both of the substrates can define pores, channels, or the like to allow liquid to drain from the membrane.

[0054] Removal of liquid from a gel membrane during a densification process can be an active or passive liquid removal or some combination of both. For example, simply holding a membrane in an ambient atmosphere while the membrane is restrained in one or more surface dimensions can passively remove liquid and densify the membrane. Similarly, in those embodiments in which a membrane is stretched in one or more surface dimensions and / or when pressure is applied across a surface of a membrane, liquid will be removed from the gel membrane merely by the stretching / pressing action carried out on the membrane.

[0055] In some embodiments, a membrane can be processed through the addition of heat, contact with a relatively dry gaseous stream, e.g., air, nitrogen, argon, etc. or combinations thereof, to actively remove liquid from the membrane. Moreover, active removal of liquid from the membrane can be carried out inconjunction with stretching / pressing the membrane or simply while the membrane is restrained in one or more directions of the membrane surface.

[0056] In some embodiments, a formation process can include one or more drying steps in which a membrane can be dried without restraining or stretching the membrane in one or more directions. For instance, a membrane can be actively or passively dried by sandwiching the membrane between substrates (one or both of which can be porous) without restraining or stretching the membrane in one or both of the x and y directions, and liquid can be removed from the membrane as discussed above.

[0057] The amount of liquid to be removed from a membrane in either a densification or a drying step can be such that the membrane becomes dry, i.e. , retaining no additional liquid beyond that due to the surrounding atmosphere, thereby forming a FBI film. As utilized herein, the term “film” generally refers to a polymeric membrane that is significantly smaller in thickness (the z direction) as compared to the surface dimensions (the x, y plane) and that incorporates little or no moisture, e.g., about 5 wt.% or less moisture within the polymeric matrix. Thus, the term “membrane” as utilized herein refers to both a membrane that includes high quantities of moisture as well as a film that is essentially dry. In one embodiment, densification of the membrane can include removal of a significant portion of the liquid of the gel membrane, but not necessarily to the point of being considered “dry.” For instance, 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 can be removed during a densification process.

[0058] A densification process can be carried out as a single-step or a multi-step process. By way of example, a multi-step process can include a first step during which a membrane can be stretched in one or more directions of a membrane surface and optionally during which pressure can be applied to a surface area across the membrane thickness followed by a second step during which a membrane can be restrained in one or more dimensions while liquid is actively or passively removed from the membrane. A single step process can include stretching or restraining a membrane, optionally while pressing the membrane between substrates in conjunction with active or passive removal of liquid from the membrane. Any combination of liquid removal, stretching, and restraining a membrane in one or more dimensions optionally in conjunction with pressing of the membrane betweensubstrates can be utilized individually or in combination and in a single or in multiple steps to densify a membrane.

[0059] Following densification, a membrane can be subjected to a thermal treatment, which can decrease the permeability of the material to redox couple species while retaining permeability of the material to desired ionic species, e.g., protons, as well as retaining other desirable characteristics of the membrane. A thermal treatment can generally include subjecting the densified membrane to a temperature of about 350°C or greater, such as about 375°C or greater, or about 400°C or greater in some embodiments, such as from about 350°C to about 475°C, or from about 400°C to about 450°C in some embodiments. The thermal treatment can be carried out for a period of time of from a few minutes to several hours, such as about 30 minutes or greater, or about 1 hour or greater, such as from about 30 minutes to about 2 hours in some embodiments. In embodiments, the thermal treatment can be carried out in an inert atmosphere, e.g., under nitrogen gas or the like. In general, a thermal treatment can be carried out at atmospheric pressure, though this is not a requirement of the process, and in some embodiments, a thermal treatment can be carried out at increased or decreased pressure.

[0060] Following the thermal treatment, the membrane thus formed can be suitable for use as a redox flow battery ion exchange membrane. In general, a membrane can be imbibed with a supporting electrolyte following the thermal treatment and prior to utilization in a redox flow battery. In those embodiments in which the membrane was imbibed with a supporting electrolyte prior to a thermal treatment step, the supporting electrolyte imbibed in the membrane following the thermal treatment step can be the same or different than the previously imbibed supporting electrolyte. The final supporting electrolyte can generally depend upon the particular characteristics of the redox flow battery in which the membrane is to be employed, and can include acidic supporting electrolytes, basic supporting electrolytes, as well as neutral species (e.g., water) including, without limitation, supporting electrolytes described previously.

[0061] The concentration of a supporting electrolyte in the membrane is not particularly limited, and in general can be up to about 25 moles / liter (M), for instance 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 in some embodiments.

[0062] A redox flow battery membrane can include one or more additives that can be incorporated in the membrane at the time of membrane formation, during a processing step as describe above, or in conjunction with the final supporting electrolyte. By way of example, an organic small molecule, such as small C1-C4 alcohols (e.g., glycerol), small organic acids, urea, etc. can be incorporated in the redox flow battery membrane in conjunction with the imbibing of the supporting electrolyte.

[0063] In one embodiment, a redox flow battery membrane can incorporate a particulate, e.g., a titanium dioxide or a PBI particulate, generally in an amount of about 2 wt.% or less, which can decrease the porosity of the membranes. For instance, nano-sized particulates of PBI can be incorporated into the polymeric matrix during solidification of the PBI gel membrane by addition of the particulate to the polymer solution during hydrolysis.

[0064] A membrane can be incorporated in a redox flow battery intended for any use and in conjunction with any suitable electrolyte solutions and redox species. For instance, redox flow battery membranes can be incorporated in batteries for use in the renewable energy sector and / or in current power grids for backup / reducing energy interruption during peak usage times.

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

[0066] The electrolyte solutions of a battery can each incorporate one member of a redox pair, as is known. In one particular embodiment, a redox flow battery membrane can be utilized in a VRB. A VRB includes in a first electrolyte solution a vanadium-based compound in which the vanadium alternates between a +5-valent (pentavalent) and a +4-valent (tetravalent) vanadium such as, for example, (VO2)2SC>4, VO(SC>4), or a combination thereof. The second electrolyte solution can include as active material vanadium-based compound in which the vanadiumalternates between a +2-valent (divalent) to +3-valent (trivalent) vanadium, such as, for example, VSO4, V2(SC>4)3, or a combination thereof.

[0067] The charge / discharge chemical reactions of one embodiment of a VRB can be represented as: Positive electrode:VO2++ H2O - e- VO2++ 2H+(charge)VO2++ H2O - e- «— VO2++ 2H+(discharge)E° = +1 .00 V vs. standard hydrogen electrode (SHE) Negative electrodeV3++ e- — ► V2+(charge)V3++ e- «— V2+(discharge)E° = -0.26 V vs. SHE Overall chemical reaction:VO2++ V3++ H2O VO2++ 2H++ V2+(charge) VO2++ V3++ H2O VO2++ 2H++ V2+(discharge) E°ceii = 1.26 V vs. SHE

[0068] Of course, the redox flow batteries described herein are not limited to VRB, and other batteries including other redox pairs are encompassed herein. Exemplary redox pairs can include, without limitation, Zn / Br2; Zn / Fe; Fe / Cr; polysulfide / Br2; polysulfide / 12; 9,10-anthraquinone-2,7-disulphonic 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- ferrocene dichloride (BTMAP-Fc); 2,6-dihydroxyanthraquinone (2,6 - DHAQ) / ferrocyanide; and alloxazine7 / 8-carboxylic acid (ACA) / ferrocyanide.

[0069] By way of example, a redox flow battery can include an electrolyte system that includes as an active anolyte material a ferrocyanide such as [Fe(CN)s]31 [Fe(CN)e]4 and as an active catholyte material Fe2+and Fe3+. The catholyte in such a system can include an iron / ligand complex, examples of which can include, without limitation, triethanolamine, diethanolamine, ethanolamine, N,N-bis-(2-hydroxyethyl)- (iminotris)-(hydroxymethyl)-methane and mixtures thereof in which the catholyte may have a ligand-to-iron ratio of from about 3:1 to about 10:1.

[0070] The electrolyte solutions can generally include the active material (e.g., vanadium ion, iron ion, etc.) in a concentration of from about 0.5 M to about 10 M. For instance, an electrolyte solution can include an active material in a concentrationof at about 0.5M or more, about 0.6M or more, or about 0.7M or more, for instance from about 1 M to about 3 M.

[0071] In one embodiment, the electrolyte solutions can include the respective active material at a concentration of the active material in a range from 1 M to 10M. In one embodiment, when the active material has a concentration within this range, it can encourage the high-energy density and high-power density under which the redox flow battery membranes are capable of operating. In one embodiment, when the active material has a concentration of less than 1 M, the active material included in the liquid can be too little of an amount per unit volume, thereby decreasing energy density. In one embodiment, when the active material has a concentration of more than 10M, the electrolyte solution can have a sharply increased viscosity and thus, a remarkably decreased oxidation / reduction reaction speed, thereby decreasing power density. The paired electrolyte solutions of a redox flow battery can include their respective redox pair active materials in the same concentration as one another or in different concentrations, with the preferred concentrations generally depending upon the particular redox pair to be utilized, the application of the battery, and the presence of any additional additives in the electrolyte solutions.

[0072] The electrolyte solutions of a battery can include additives, such as one or more redox flow battery supporting electrolytes as discussed previously. In one embodiment, the electrolyte solutions of a battery can include the supporting electrolyte that has been imbibed in the redox flow battery membrane.

[0073] An electrolyte solution can include a sulfuric acid supporting electrolyte in one embodiment. For instance, an electrolyte solution can include a mixture of sulfuric acid and water, that is, a sulfuric acid aqueous solution, in conjunction with the active material of the solution, for instance as a solvent. In one embodiment, a mixture of a supporting electrolyte and water, e.g., a sulfuric acid aqueous solution, can include a supporting electrolyte in a concentration of from about 1 M to about 5M. The concentration of the supporting electrolyte can be selected in one embodiment so as to provide suitable solubility for the active material of the electrolyte solution. As such, the solution can exhibit desirable ion conductivity and viscosity and can avoid creating an overvoltage issue in the battery.

[0074] As indicated in FIG. 2, each side of a cell 10 can include additional components adjacent the membrane 12 as are known in the art including, without limitation, a conductive separator 14, e.g., a porous carbon paper, carbon cloth,carbon felt, or metal cloth (a porous film made of fiber-type metal or a metal film formed on the surface of a polymer fiber cloth), among others. The cell can also include electrodes 16, as are known, which may be the same or different from one another and may be made of a conductive substrate appropriate for the respective electrolyte solution of the cell (e.g., graphite). Current collectors 18 (e.g., gold-plated copper) can be in electrical communication with the electrodes 16, and a cell can include end plates 20 (e.g., stainless steel end plates), one on either side of the / cell, and facing oppositely away from a separator. The current collectors 18 provide electrical communication between the cell 10 and an exterior circuit, as shown.

[0075] FIG. 3 illustrates 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 of the cells 10 of the stack 150 such that the electrolyte solution of this portion of the battery flows through the path 300 and returns to the first tank 100. A second circulation path 400 passes through the other side of each of the cells 10 of the stack 150 such that the electrolyte solution of this portion of the battery flows through the path 400 and returns to the second tank 200. A redox flow battery can further include respective charging / discharging circuits, as well as converters, controllers, etc. as are known in the art to collect and supply power by use of the battery.

[0076] Redox flow battery membranes as described herein can allow for higher performing flow batteries operating under high current loads with low crossover. Such improved materials can mitigate the need for large electrochemical stacks, and thereby can reduce the overall cost of commercial flow battery devices.

[0077] The present disclosure may be better understood with reference to the Examples set forth below.Example

[0078] PBI gel membranes formed according to the PPA technique and doped with phosphoric acid (Celtec®-P) were obtained from BASF Corporation. The membranes were modified into dense films by first washing the membranes with water to remove all phosphoric acid. Washing was accomplished by submerging each membrane in a DI water bath for up to 24 hours and periodically changing the water until a neutral pH was measured. Each membrane was then densified in a controlled manner between porous sheets. Each membrane was secured in the Xand Y directions to prevent any shrinkage in those directions, and the membrane then collapsed in the Z direction as it was dried.

[0079] n-PBI membranes were also formed in-house according to a PPA protocol. A typical polymerization consisted of 3.11 g tetraaminobiphenyl (TAB, 14.5 mmol), and 3.14 g naphthalene-1 ,4-dicarboxylic acid (1 ,4-ndc 14.5 mmol) which were added to 243 g polyphosphoric acid, mixed with an overhead stirrer, and purged 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 ramp and soak features. The reaction temperature was increased stepwise 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 was approximately 190°C and held for 12 hours. Once the reaction was completed, 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 membrane in a humidity chamber regulated to 55% R.H. at 25°C. Inherent viscosity measurements conducted in sulfuric acid at 30 °C resulted in values of approximately 6 dL / g.

[0080] Following formation, the n-PBI gel membranes were rinsed and densified in the same fashion as the Celtec®-P membranes.

[0081] The membranes were processed according to one of two subsets. A first subset was re-imbibed following densification and before thermal treatment, and a second subset was thermally treated following densification.

[0082] The membranes of the first subset were re-imbibed by taping the densified films onto glass plates. A phosphoric acid solution with 0.25 wt.% reactive oligomer (p,p-OPBI) (see e.g., Rohlfing, Liu, Daryaei, Benicewicz, Polymer, 2023, 280, 126034) was heated to 130°C to fully dissolve the oligomers. The hot solution was then gently poured over the film, and allowed to sit and penetrate the dense film overnight. The membrane was then put in a DI water bath and the water was changed periodically until the pH of the water was neutral (up to 24 hours). The water washed membrane was then dried between porous sheets overnight until all of the water was evaporated.

[0083] The thermal treatment was the same for all membranes, whether they were imbibed with oligomers and then washed and dried or whether they were simply washed and dried films with no additional re-imbibing. For thermal treatment,the films were inserted into a preheated tube furnace at the desired temperature. The tube furnace was pre-heated while under N2 gas and the inert atmosphere was maintained during treatment. The films were treated for a specific amount of time, up to 1 hour and then removed from the tube furnace to cool.

[0084] Measurements for vanadium (VOSO4) permeability were completed with the PermeGear “side-by-side” direct permeation cell. The cell had two electrolyte chambers with a 45mL capacity and were separated by the membrane or film being tested. The temperature was maintained at 25°C throughout testing with a recirculating water bath. A typical experiment consisted of two electrolytes, the donor compartment with a 1.6 M VOSO4 in 2 M H2SO4, and the acceptor compartment with 1.6 M MgSC>4 in 2 M H2SO4. Vanadium (IV) has a detectable absorption characteristic at 248 nm. Aliquots of the acceptor solutions were periodically taken over time to measure the vanadium salt permeability. A Shimadzu UV-2450 UV-Vis was used to measure the VO2+permeability and calculated using Ficks Law of diffusion:Pstwhere Pswas the calculated salt permeability, cr(t) was the concentration of VOSO4 in the receptor chamber at time t, cr(0) was the initial concentration of VOSO4 in the donor chamber, V was the volume of solution added to each chamber, d was the membrane thickness, and A was the active area of the membrane.Multiple Samples were carried out as follows:

[0085] Sample 1 :

[0086] As a control, a densified Celtec®-P film (not heat treated) was measured for permeability as previously described and the permeability was determined to be 5.87 x 10-09cm2 / s.

[0087] Sample 2:

[0088] A densified Celtec®-P film was thermally treated at 300°C for 1 hour under inert conditions as described. The permeability was measured as described and the permeability was determined to be 6.54 x 10-09cm2 / s.

[0089] Sample 3:

[0090] A densified Celtec®-? film was thermally treated at 400°C for 1 hour under inert conditions as described. The permeability was measured as described and thepermeability was determined to be 1.07 x 10-09cm2 / s. This is more than 5 times lower than the control Celtec®-P film.

[0091] Sample 4:

[0092] A densified Celtec®-P film was thermally treated at 425°C for 1 hour under inert conditions as described. The permeability was measured as described and the permeability was determined to be 4.10 x 10-10cm2 / s. This is approximately 14 times lower than the control Celtec®-P film.

[0093] Sample 5:

[0094] A densified Celtec®-P film was thermally treated at 450°C for 30 minutes under inert conditions as described. The permeability was measured as described and the permeability was determined to be 1 .42 x 10‘10cm2 / s. This is more than 40 times lower than the control Celtec®-P film.

[0095] Sample 6:

[0096] A densified Celtec®-P film was thermally treated at 450°C for 1 hour under inert conditions as described. The permeability was measured as described and the permeability was determined to be 1.02 x 10’11cm2 / s. This is approximately 575 times lower than the control Celtec®-P film.

[0097] Example 7:

[0098] As a second control, a densified n-PBI film (not heat treated) was measured for permeability described and the permeability was determined to be 1 .35 x 10‘08cm2 / s.

[0099] Example 8:

[0100] A densified n-PBI film was thermally treated at 450°C for 1 hour under inert conditions as described. The permeability was measured as described and the permeability was determined to be 3.00 x 10'10cm2 / s. This is 45 times lower than the control n-PBI film.

[0101] Example 9:

[0102] A densified Celtec®-P film was re-imbibed as described. The film was then thermally treated at 450°C for 1 hour under inert conditions as described. The permeability was measured as described and the permeability was determined to be 9.14 x 10'11cm2 / s. This is approximately 64 times lower than the control Celtec®-P film.

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

Claims

WHAT IS CLAIMED IS:

1. A method for forming an ion-exchange membrane, comprising: shaping a polymerization solution to form a membrane precursor, the polymerization solution comprising a polybenzimidazole dissolved in a polyphosphoric acid solvent; hydrolyzing at least a portion of the polyphosphoric acid of the polymerization solution to form phosphoric acid and water, thereby causing a sol-gel transfer and solidification of the polybenzimidazole and formation of a gel membrane comprising the polybenzimidazole; densifying the gel membrane to form a densified membrane; and thermally treating the densified membrane, the thermal treatment comprising subjecting the densified membrane to a temperature of about 350°C or greater for a period of time of about 30 minutes or greater in an inert atmosphere.

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 imbibing the densified gel membrane with a supporting electrolyte prior to thermally treating the densified membrane.

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

5. The method of claim 1 , further comprising imbibing the membrane with a supporting electrolyte following the step of thermally 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 a membrane surface.

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

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 hydrolysis being carried out at a relative humidity of about 40% or higher.

12. An ion exchange membrane comprising: a densified polybenzimidazole gel membrane; and a redox flow battery supporting electrolyte imbibed within the densified polybenzimidazole gel membrane; wherein the ion exchange membrane exhibits an in-plane ionic conductivity in a 2.6 M sulfuric acid solution of about 50 mS / cm2or greater; and wherein the ion exchange membrane exhibits a permeability to a redox couple species of about of about 2x1 O'9cm2 / s or less.

13. The ion exchange membrane of claim 12, wherein the supporting electrolyte comprises a mineral 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-toluene sulfonic acid, phosphoric acid, or trifluoromethane sulfonic 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, a perchlorate, or a hexafluorophosphate.

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 pair selected from the group consisting of Zn / Br?; Zn / Fe; Fe / Cr; polysulfide / Br?; polysulfide / l?; 9,10- anthraquinone-2,7-disulphonic 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- ferrocene dichloride (BTMAP-Fc); 2,6-dihydroxyanthraquinone (2,6 - DHAQ) / ferrocyanide; and alloxazine7 / 8-carboxylic acid (ACA) / ferrocyanide.

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