Composite proton exchange membranes, methods of preparing them, and redox flow batteries comprising them

Abstract

A composite proton exchange membrane is provided, comprising a membrane substrate in the form of a sheet and comprising i) a polyolefin matrix, the polyolefin matrix defining a network of interconnecting pores communicating throughout the polyolefin matrix, and substantially water-insoluble particulate filler distributed throughout the polyolefin matrix; and ii) an at least partially crystalline proton exchange ionomer that is impregnated into and / or coated on a surface of the membrane substrate. Also provided are methods for preparing a composite proton exchange membrane, and redox flow batteries comprising the composite proton exchange membrane either as described above or the composite proton exchange membrane comprising only i) above.

Description

COMPOSITE PROTON EXCHANGE MEMBRANES, METHODS OF PREPARING THEM, AND REDOX FLOW BATTERIES COMPRISING THEMCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is related to provisional U.S. Patent Application Serial No. 63 / 725,267, filed November 26, 2024, and titled “COMPOSITE PROTON EXCHANGE MEMBRANES, METHODS OF PREPARING THEM, AND REDOX FLOW BATTERIES COMPRISING THEM”, and to provisional U.S. Patent Application Serial No. 63 / 839,771 , filed July 7, 2025, and titled “COMPOSITE PROTON EXCHANGE MEMBRANES, METHODS OF PREPARING THEM, AND REDOX FLOW BATTERIES COMPRISING THEM”, both of which are incorporated herein by reference in their entireties.FIELD OF THE DISCLOSURE

[0002] The present disclosure relates to composite proton exchange membranes suitable for use in redox flow batteries, methods for preparing composite proton exchange membranes, and redox flow batteries comprising composite proton exchange membranes.BACKGROUND OF THE DISCLOSURE

[0003] A redox flow battery (RFB) is a type of rechargeable electrochemical cell that converts chemical energy into electrical energy via reversible oxidation and reduction of working fluids. Chemical energy is stored in and provided by two electrolytes dissolved in liquids that are pumped through the system on separate sides of an ion- selective membrane. Ion transfer inside the cell occurs across the membrane to charge-balance the electrochemical reactions while the liquids circulate in their respective spaces. The membrane also inhibits the transport of electroactive species, which may lead to capacity decay. Current flows through an external circuit. Redox flow batteries are being aggressively developed because they can transform energy through electrochemical processes and store it in external tanks. As a result, RFBs hold significant potential for large-scale stationary applications in various energy storage systems. They offer a readily scalable design for grid scale energy storage.

[0004] At this stage, the widespread adoption of RFBs is hindered by the high cost and limited functionality of the current state-of-the-art proton exchange membraneseparators. Many currently available options also often have limited lifetimes due to high electrolyte crossover (i. e., low selectivity from protons).

[0005] It would be desirable to provide safe, cost-effective and durable composite proton exchange membranes for use in redox flow batteries that demonstrate acceptable conductivity and selectivity (and thus longer lifetime), minimizing cationic crossover.SUMMARY OF THE DISCLOSURE

[0006] The present disclosure is directed to composite proton exchange membranes comprising:1 ) a membrane substrate comprising:(a) a polyolefin matrix, the polyolefin matrix defining a network of interconnecting pores communicating throughout the polyolefin matrix; and(b) substantially water-insoluble particulate filler comprising silica distributed throughout the polyolefin matrix, wherein the membrane substrate comprises 5 to 90 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler; wherein the polyolefin matrix is present in the membrane substrate in an amount of 5 to 90 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler; and wherein the membrane substrate is in the form of a sheet; and2) an at least partially crystalline proton exchange ionomer that is at least partially impregnated into and / or at least partially coated on a surface of the membrane substrate.

[0007] The present disclosure is further directed to redox flow batteries comprising the composite proton exchange membranes described above, or composite proton exchange membranes comprising only the membrane substrate 1 ).

[0008] Also provided is a method of preparing a composite proton exchange membrane, comprising:(1 ) rinsing a membrane substrate sequentially with organic solvent and deionized water to form a rinsed membrane substrate, wherein the membrane substrate comprises:(a) a polyolefin matrix, the polyolefin matrix defining a network of interconnecting pores communicating throughout the polyolefin matrix; and(b) particulate filler comprising silica distributed throughout the polyolefin matrix, wherein the membrane substrate comprises 10 to 90 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler; wherein the polyolefin is present in the membrane substrate in an amount of 10 to 90 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler; and wherein the membrane substrate is in the form of a sheet;(2) optionally hydrating the rinsed membrane substrate with deionized water to form a hydrated membrane substrate;(3) applying a proton exchange ionomer to the membrane substrate to form a composite membrane, wherein the proton exchange ionomer is at least partially impregnated into and / or at least partially coated on a surface of the membrane substrate;(4) drying the composite membrane; and(5) annealing the composite membrane to at least partially crystallize the proton exchange ionomer and form a composite proton exchange membrane.DETAILED DESCRIPTION

[0009] Other than in any operating examples, or where otherwise indicated, all numbers expressing, for example, quantities of ingredients used in the specification and claims are to be understood as being modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties to be obtained by the disclosed membranes. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0010] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard variation found in their respective testing measurements.

[0011] Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.

[0012] In this application, the use of the singular includes the plural and plural encompasses singular, unless specifically stated otherwise. Further, in this application, the use of “a” or “an” means “at least one” unless specifically stated otherwise. For example, “an” additive, “a” silica, and the like refer to one or more of these items. Also, as used herein, the term “polymer” is meant to refer to ionomers, prepolymers, oligomers, and both homopolymers and copolymers. The term “resin” is used interchangeably with “polymer.”

[0013] The present disclosure is directed to a composite proton exchange membrane comprising a membrane substrate, which may be microporous. As used herein, “microporous material” or “microporous membrane” or “microporous sheet” means a material having a network of interconnecting pores, wherein, on a treatment- free, coating-free, printing ink-free, impregnant-free, and pre-bonding basis, the pores typically have a volume average diameter ranging from 0.001 to 1.0 micrometer, and may constitute at least 5 percent by volume of the microporous material as discussed herein below. In other words, the volume average diameter above is reported for virgin microporous material that has not been coated, impregnated, or otherwise treated. The volume average diameter of the pores may be determined, for example, using a gas adsorption or bubble point method as known in the art.

[0014] The polyolefin matrix may comprise ultrahigh molecular weight (LIHMW) polyethylene as defined in the art (typically 2 million to 6 million Da). Non-limiting examples of ultrahigh molecular weight (UHMW) polyethylene can include essentially linear UHMW polyethylene (PE). Inasmuch as UHMW polyolefins are not thermoset polymers having an infinite molecular weight, they are technically classified as thermoplastic materials.

[0015] While there is no particular restriction on the upper limit of the intrinsic viscosity of the UHMW polyethylene, in one non-limiting example, the intrinsic viscosity can range from at least 6 deciliters / gram, or at least 7 deciliters / gram, or at least 18 deciliters / gram, to at most 50 deciliters / gram, or at most 45 deciliters / gram, or at most 18 deciliters / gram, or at most 16 deciliters / gram. Thus, the intrinsic viscosity of theUHMW may be, for example, 6 to 50 deciliters / gram, or 6 to 45 deciliters / gram, or 6 to 18 deciliters / gram, or 6 to 16 deciliters / gram, or 7 to 50 deciliters / gram, or 7 to 45 deciliters / gram, or 7 to 18 deciliters / gram, or 7 to 16 deciliters / gram, or 18 to 50 deciliters / gram, or 18 to 45 deciliters / gram.

[0016] For purposes of the present disclosure, intrinsic viscosity may be determined by extrapolating to zero concentration the reduced viscosities or the inherent viscosities of several dilute solutions of the UHMW polyolefin where the solvent is freshly distilled decahydronaphthalene to which 0.2 percent by weight, 3,5-di-tert- butyl-4-hydroxyhydrocinnamic acid, neopentanetetrayl ester [CAS Registry No. 6683- 19-8] has been added. The reduced viscosities or the inherent viscosities of the UHMW polyolefin are ascertained from relative viscosities obtained at 135°C using an Ubbelohde No. 1 viscometer in accordance with the general procedures of ASTM D 4020-81 , except that several dilute solutions of differing concentration are employed.

[0017] The nominal molecular weight of UHMW polyethylene is empirically related to the intrinsic viscosity of the polymer in accordance with the following equation:M=5.37x104[q]1 37wherein M is the nominal molecular weight and [q] is the intrinsic viscosity of the UHMW polyethylene expressed in deciliters / gram. Similarly, the nominal molecular weight of UHMW polypropylene is empirically related to the intrinsic viscosity of the polymer according to the following equation:M=8.88x104[q]1 25wherein M is the nominal molecular weight and [q] is the intrinsic viscosity of the UHMW polypropylene expressed in deciliters / gram.

[0018] Often, the polyolefin matrix further comprises high density polyethylene (HDPE), which usually has a density of at least 900 kg / m3, such as 930 to 940 kg / m3. In certain examples, other thermoplastic organic polymers also may be present in the polyolefin matrix, provided that their presence does not materially affect the properties of the membrane substrate in an adverse manner. The amount of the other thermoplastic polymer which may be present depends upon the nature of such polymer. Non-limiting examples of thermoplastic organic polymers that optionally may be present in the matrix include low density polyethylene (i. e., polyethylene having a density of 0.91 -0.94 g / cm3, as defined in the art), poly(tetrafluoroethylene), polypropylene, copolymers of ethylene and propylene, copolymers of ethylene and butylene, copolymers of ethylene and (meth)acrylic (i. e., acrylic and / or methacrylic)acid, polyetherketone, polyvinylidene fluoride (PVDF), polysulfone, and / or polyethersulfone. Note that the phrase “and / or” when used in a list is meant to encompass alternative examples including each individual component in the list as well as any combination of components. For example, the list “A, B, and / or C” is meant to encompass seven separate examples that include A, or B, or C, or A + B, or A +C, or B + C, or A + B + C.

[0019] The polyolefin may be present in the membrane substrate in an amount of at least 5 percent by weight, and at most 90 percent by weight, based on the total weight of the polyolefin matrix and particulate filler, described below. Often the polyolefin matrix comprises a mixture of high density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (UHMWPE). When this mixture is used, the polyolefin matrix is typically present in the membrane substrate in an amount of 5 to 50 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler.

[0020] The membrane substrates used to prepare the composite proton exchange membranes of the present disclosure further comprise finely divided, particulate, substantially water-insoluble filler comprising silica, distributed throughout the polyolefin matrix.

[0021] The filler can include any of a number of additional fillers known in the art. The filler should be finely divided and substantially water insoluble to permit uniform distribution throughout the polyolefin matrix during manufacture of the membrane substrate. Generally, the filler comprises silica, such as precipitated silica, talc, carbon black, charcoal, graphite, titanium oxide, iron oxide, copper oxide, zinc oxide, antimony oxide, zirconia, magnesia, alumina, molybdenum disulfide, zinc sulfide, barium sulfate, strontium sulfate, and / or magnesium carbonate. Often the filler comprises silica, and may further comprise an antioxidant and / or a peroxide scavenger. Anti-oxidants such as those sold by BASF under the names IRGANOX® and IRGAFOS®, and those sold by Solvay under the name CYANOX®, are suitable for use.

[0022] The substantially water-insoluble filler may be in the form of ultimate particles, aggregates of ultimate particles, or a combination of both. With respect to silica particles, by “finely divided” is meant that the silica used in preparing the membrane substrate has a number average particle size D50 of from 5 to 100 microns, or 5 to 75 microns, or 5 to 50 microns, as determined by the use of a laser diffraction particle size instrument, such as LS230 available from Beckman Coulton, capable of measuring particle diameters as small as 0.04 micron using RIDS (polarizationintensity differential scattering). The D50 is selected such that average particle size of the silica is smaller than the intended thickness of the membrane substrate, discussed below. Usually at least 90 percent by weight of the silica used in preparing the membrane substrate has gross particle sizes in the range of from 5 to 40 microns; i. e., usually no larger than the thickness of the membrane. The sizes of the filler agglomerates may be reduced during processing of the ingredients used to prepare the membrane substrate. Accordingly, the distribution of particle sizes in the membrane substrate may be smaller than in the raw filler itself. By “substantially insoluble” is meant that less than 3 percent by weight, or less than 1 percent by weight of the filler particles, based on the total weight of the filler particles, dissolves into the liquid phase at 25°C when dispersed therein. This can facilitate retention of the filler in the microporous material.

[0023] The filler typically has a high surface area, which may be influenced by particle size and / or porosity, allowing the filler to carry much of the processing plasticizer used to form the membrane substrate. The surface area of the filler particles can range from at least 20 square meters per gram, or at least 25 square meters per gram, to at most 900 square meters per gram, or at most 850 square meters per gram; e. g., from 20 to 900 square meters per gram, or from 20 to 850 square meters per gram, or from 25 to 900 square meters per gram, or from 25 to 850 square meters per gram, as determined by the Brunauer, Emmett, Teller (BET) method according to ASTM C 819-77 using nitrogen as the adsorbate but modified by outgassing the system and the sample for one hour at 130°C. Prior to nitrogen sorption, filler samples are dried by heating to 160°C in flowing nitrogen (PS) for 1 hour.

[0024] In particular examples, the inorganic filler comprises at least one of precipitated silica, silica gel, and fumed silica. Such fillers may demonstrate a BET of 125 to 700 m2 / g, determined as noted above.

[0025] Precipitated silica powders differ from silica gels that have been pulverized in that the precipitated silica powders generally have a more open structure, that is, a higher specific pore volume. However, the specific surface area of precipitated silica, as measured by the Brunauer, Emmet, Teller (BET) method using nitrogen as the adsorbate, is often lower than that of silica gel.

[0026] Many different precipitated silicas can be employed as the filler used to prepare the membrane substrate. Precipitated silicas are well-known commercial materials, and processes for producing them are described in detail in many UnitedStates patents, including United States Patent Numbers 2,940,830 and 4,681 ,750. The average ultimate particle size (irrespective of whether or not the ultimate particles are agglomerated) of precipitated silicas used is generally less than 0.1 micrometer, e.g., less than 0.05 micrometer or less than 0.03 micrometer, as determined by transmission electron microscopy. Non-limiting examples of suitable precipitated silicas include those sold under the HI-SIL tradename by PPG (Pittsburgh, PA).

[0027] The membrane substrate typically comprises at least 5 percent by weight, or at least 30 percent by weight, or at least 50 percent by weight, and at most 90 percent by weight, or at most 80 percent by weight, or at most 75 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler. For example, the membrane substrate may comprise 5 to 90 percent by weight, or 5 to 80 percent by weight, or 5 to 75 percent by weight, or 30 percent to 90 percent by weight, or 30 to 80 percent by weight, or 30 to 75 percent by weight, or 50 percent to 90 percent by weight, or 50 to 80 percent by weight, or 50 to 75 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler.

[0028] Typically, the membrane substrate does not contain any sulfonic acidfunctional groups, carboxylic acid-functional groups, or phosphonic acid-functional groups bonded thereto, such as by introduction onto the silica. However, in certain examples, the membrane substrate may contain sulfonic acid-functional groups, carboxylic acid-functional groups, or phosphonic acid-functional groups bonded thereto, such as by introduction onto the silica.

[0029] The membrane substrate may further comprise a network of interconnecting pores communicating throughout the polyolefin matrix of the membrane substrate. In certain examples, especially prior to incorporation of an ionomer, at least a portion of the interconnecting pores communicate from the first surface of the membrane through the polyolefin matrix to the opposing surface; i. e., a path is provided through the polyolefin matrix from the first surface to the opposing surface through at least a portion of the network of interconnecting pores.

[0030] On a treatment-free, coating free, and impregnant-free basis (i. e., prior to any treatment, application of any coatings, and impregnating any additives), such pores can make up at least 5 percent by volume, or at least 15 percent by volume, or at least 20 percent by volume, or at least 25 percent by volume, or at least 35 percent by volume, or at least 45 percent by volume, and at most 95 percent by volume, or at most 75 percent by volume. Thus, the pores can make up 5 to 95 percent by volume,or 15 to 95 percent by volume, or 20 to 95 percent by volume, or 25 to 95 percent by volume, or 35 to 95 percent by volume, or 45 to 95 percent by volume, or 5 to 70 percent by volume, or 15 to 70 percent by volume, or 20 to 70 percent by volume, or 25 to 70 percent by volume, or 35 to 70 percent by volume, or 45 to 70 percent by volume of the membrane substrate. Often, the pores comprise at least 35 percent by volume, or even at least 45 percent by volume of the membrane substrate.

[0031] Porosity can be measured using a Gurley Densometer, model 4340, manufactured by GPI Gurley Precision Instruments (Troy, NY). The porosity values reported are a measure of the rate of air flow through a sample or it's resistance to an air flow through the sample. The unit of measure for this method is a “Gurley second” and represents the time in seconds to pass 100 cc of air through a 1 -inch square area using a pressure differential of 4.88 inches of water. Lower values equate to less air flow resistance (more air is allowed to pass freely). For purposes of the present disclosure, the measurements are completed using the procedure listed in the manual for MODEL 4340 Automatic Densometer. The membrane substrate may demonstrate a Gurley porosity of at least 1 second, or at least 10 seconds, and at most 1000 seconds, or at most 750 seconds. Typically, the membrane substrate demonstrates a Gurley porosity of 1 to 1000 seconds, or 10 to 1000 seconds, or 1 to 750 seconds, or 10 to 750 seconds.

[0032] The membrane substrate sheet typically has a thickness of 10 to 500 microns, or 150 to 300 microns, or 150 to 275 microns.

[0033] The composite proton exchange membranes of the present disclosure further comprise 2) an at least partially crystalline proton exchange ionomer that is at least partially impregnated into and / or at least partially coated on a surface of the membrane substrate. By “ionomer” is meant a polymer that contains ionic groups covalently bonded to the structure, such that the polymer exhibits ionic conductive behavior. The at least partial crystallinity is of the ionomer itself, excluding crystallinity from inorganic filler present in the membrane substrate.

[0034] The at least partially crystalline proton exchange ionomer may comprise a phosphonic acid-functional polymer, a phosphonate-functional polymer, a sulfonate- functional polymer, and / or a sulfonic acid functional polymer, such as a phosphonic acid-, phosphonate-, sulfonate- and / or sulfonic acid-functional fluoropolymer. Examples of suitable proton exchange ionomers include NAFION™, available from the Chemours Company, 800 EW Ionomer, available from 3M Advanced MaterialsDivision, Aquivion PFSA, available from Solvay Specialty Polymers, and FORBLUE, available from AGC Chemicals Company. They are typically supplied as aqueous dispersions of the ionomer in a concentration of 5 to 20 percent by weight.

[0035] The at least partially crystalline proton exchange ionomer is typically present in the composite proton exchange membrane in an amount of 2 to 70 percent by total weight of the composite membrane, such as 2 to 67 percent by weight (i. e., at 67 percent by weight, there is about twice as much proton exchange ionomer as membrane substrate by weight), or 2 to 50 percent by weight, or 2 to 35 percent by weight, or 2 to 20 percent by weight, or 5 to 70 percent by weight, 5 to 67 percent by weight, or 5 to 50 percent by weight, or 5 to 35 percent by weight, or 5 to 20 percent by weight, based on the total weight of the composite proton exchange membrane. Even at low loadings of the ionomer, such as 2 to 20 percent by weight, conductivity and selectivity (i. e., ratio of conductivity to permeability) of the composite proton exchange membrane in a redox flow battery are still acceptable.

[0036] In certain examples, the at least partially crystalline proton exchange ionomer is uncrosslinked. It is believed that crosslinking increases the acid equivalent weight of the ionomer and may lower the ion exchange capacity of the proton exchange membrane (due to fewer acid groups available for ion exchange), which may in turn decrease the efficiency of the membrane when used in a redox flow battery.

[0037] Composite proton exchange membranes such as those disclosed above may be prepared by a process comprising:(1 ) rinsing a membrane substrate sequentially with organic solvent such as methanol, ethanol, n-propanol, or isopropanol, followed by deionized water to form a rinsed membrane substrate, wherein the membrane substrate is in the form of a sheet and comprises a polyolefin matrix and particulate filler, each as described above;(2) optionally hydrating the rinsed membrane substrate with deionized water to form a hydrated membrane substrate;(3) applying a proton exchange ionomer to the membrane substrate, which may be hydrated, to form a composite membrane, wherein the proton exchange ionomer is at least partially impregnated into and / or at least partially coated on a surface of the membrane substrate;(4) drying the composite membrane; and(5) annealing the composite membrane to at least partially crystallize the proton exchange ionomer and form a composite proton exchange membrane. Note that incertain examples, incorporation of ionomer is sufficient to provide some semicrystallinity, such that annealing may not be necessary and may be an optional step.

[0038] Rinsing is typically performed in step (1 ) first with an organic solvent as noted above, typically isopropanol, followed by deionized water. Hydrating, which is optional, may be done by immersion in or spraying with deionized water or any other suitable method. Note that rinsing and hydrating may be done simultaneously in a single, combined step of the process.

[0039] In step (3), the proton exchange ionomer, which may comprise any of those disclosed above, may be applied to the membrane substrate by immersion, vacuum coating, inkjet printing, screen printing, spin coating, solution exchange, draw down, doctor blade application, casting, pressured flow cell impregnation, chemical vapor deposition or physical vapor deposition.

[0040] Drying may be done using any acceptable method such as by exposure to still air or forced air at ambient or elevated (i. e., above ambient) temperatures. Ambient temperature typically ranges from 60 to 90 °F (15.6 to 32.2 °C), such as a typical room temperature, 72°F (22.2°C).

[0041] Annealing of the composite membrane in step (5) may be performed by heating the composite membrane to a temperature of 120 to 175°C, such as 140 to 175°C, for at least 1 hour, such as 2 to 3 hours, under dynamic vacuum such as up to 30 inches Hg, or up to 28 inches Hg, followed by cooling to ambient temperature under static vacuum. Static vacuum (as opposed to dynamic) during the cooling phase helps to avoid quenching the ionomer. Annealing of the composite membrane imparts an at least partially crystalline structure to the ionomer. Prior to testing or use such as in a redox flow battery, the composite proton exchange membrane may be soaked in an aqueous medium such as deionized water, usually for at least 24 hours to ensure saturation.

[0042] The membrane substrates used to prepare the composite proton exchange membranes described above may be prepared by a method comprising:(1 ) mixing a filler comprising silica, a polyolefin polymer, a processing plasticizer, a lubricant, optionally a free radical scavenger, and optionally an antioxidant until a substantially uniform mixture is obtained;(2) introducing the mixture to a heated barrel of a screw extruder, to which is attached a sheeting die to form a sheet;(3) forwarding the sheet to a pair of heated calender rolls acting cooperatively to form a continuous sheet of lesser thickness than the sheet exiting from the die;(4) optionally stretching the sheet in at least one stretching direction to form a stretched sheet; and(5) extracting the processing plasticizer from the sheet.

[0043] In an exemplary process, the components of the polyolefin matrix (typically in solid form such as powder or pellets), filler, processing plasticizer, and essential amounts of lubricant, antioxidant, and free radical scavenger are mixed until a substantially uniform mixture is obtained. The weight ratio of filler to polymer employed in forming the mixture is essentially the same as that of the membrane substrate to be produced. The mixture, together with additional processing plasticizer as needed, is introduced to the heated barrel of a screw extruder. Attached to the extruder is a die, such as a sheeting die, to form the desired end shape.

[0044] In an exemplary manufacturing process, when the material is formed into a sheet or film, a continuous sheet or film formed by a die is forwarded to a pair of heated calender rolls acting cooperatively to form a continuous sheet of lesser thickness than the continuous sheet exiting from the die. The final thickness after all processing steps, including any optional stretching and extraction, may depend on the desired end-use application. The membrane substrate in the form of a sheet may have a thickness ranging from 10 to 300 microns, or 150 to 300 microns, or 150 to 275 microns.

[0045] A noted in step (4), the sheet may optionally be stretched in at least one stretching direction. Stretching may take place before extraction, after extraction, or both. Stretched microporous material substrate may be produced by stretching the intermediate product in at least one stretching direction, often above the elastic limit. Usually, the stretch ratio is at least 1 .2. In many cases, the stretch ratio is at least 1 .5. Often it is at least 2. Frequently, the stretch ratio is in the range of from 1 .2 to 15. Often, the stretch ratio is in the range of from 1 .5 to 10. Usually, the stretch ratio is in the range of from 2 to 6.

[0046] The temperatures at which stretching is accomplished may vary widely. Stretching may be accomplished at ambient temperature, such as room temperature, but usually elevated temperatures are employed. The intermediate product may be heated by any of a wide variety of techniques prior to, during, and / or after stretching. Examples of these techniques include radiative heating, such as that provided by electrically heated or gas fired infrared heaters; convective heating, such as thatprovided by recirculating hot air; and conductive heating, such as that provided by contact with heated rolls. The temperatures which are measured for temperature control purposes may vary according to the apparatus used and personal preference. For example, temperature-measuring devices may be placed to ascertain the temperatures of the surfaces of infrared heaters, the interiors of infrared heaters, the air temperatures of points between the infrared heaters and the intermediate product, the temperatures of circulating hot air at points within the apparatus, the temperature of hot air entering or leaving the apparatus, the temperatures of the surfaces of rolls used in the stretching process, the temperature of heat transfer fluid entering or leaving such rolls, or film surface temperatures. In general, the temperature or temperatures are controlled such that the intermediate product is stretched about evenly to minimize any variations in film thickness of the stretched microporous material. It will be apparent that the temperatures used for control purposes may or may not be close to those of the intermediate product itself since they depend upon the nature of the apparatus used, the locations of the temperature-measuring devices, and the identities of the substances or objects whose temperatures are being measured.

[0047] In view of the locations of the heating devices and the line speeds usually employed during stretching, gradients of varying temperatures may or may not be present through the thickness of the intermediate product. Also, because of such line speeds, it is impractical to measure these temperature gradients. The presence of gradients of varying temperatures, when they occur, makes it unreasonable to refer to a singular film temperature. Accordingly, film surface temperatures, which can be measured, are best used for characterizing the thermal condition of the intermediate product.

[0048] The film surface temperatures at which stretching is accomplished may vary widely, but in general they are such that the intermediate product is stretched about evenly, as explained above. In most cases, the film surface temperatures during stretching are in the range of from 20°C to 220°C. Often, such temperatures are in the range of from 50°C to 200°C, such as from 75°C to 180°C.

[0049] Stretching may be accomplished in a single step or a plurality of steps as desired. For example, when the intermediate product is to be stretched in a single direction (uniaxial stretching), the stretching may be accomplished by a single stretching step or a sequence of stretching steps until the desired final stretch ratio is attained. Similarly, when the intermediate product is to be stretched in two directions(biaxial stretching), the stretching can be conducted by a single biaxial stretching step or a sequence of biaxial stretching steps until the desired final stretch ratios are attained. Biaxial stretching may also be accomplished by a sequence of one of more uniaxial stretching steps in one direction and one or more uniaxial stretching steps in another direction. Biaxial stretching steps where the intermediate product is stretched simultaneously in two directions and uniaxial stretching steps may be conducted in sequence in any order. Stretching in more than two directions is within contemplation. It may be seen that the various permutations of steps are quite numerous. Other steps, such as cooling, heating, sintering, annealing, reeling, unreeling, and the like, may optionally be included in the overall process as desired.

[0050] Various types of stretching apparatus are well known and may be used to accomplish stretching of the intermediate product. Uniaxial stretching is usually accomplished by stretching between two rollers, wherein the second or downstream roller rotates at a greater peripheral speed than the first or upstream roller. Uniaxial stretching can also be accomplished on a standard tentering machine. Biaxial stretching may be accomplished by simultaneously stretching in two different directions on a tentering machine. More commonly, however, biaxial stretching is accomplished by first uniaxially stretching between two differentially rotating rollers as described above, followed by either uniaxially stretching in a different direction using a tenter machine or by biaxially stretching using a tenter machine. The most common type of biaxial stretching is where the two stretching directions are approximately at right angles to each other. In most cases where the continuous sheet is being stretched, one stretching direction is at least approximately parallel to the long axis of the sheet (machine direction) and the other stretching direction is at least approximately perpendicular to the machine direction and is in the plane of the sheet (transverse direction).

[0051] While optional, stretching the sheets prior to extraction of the processing plasticizer allows for thinner films with larger pore sizes than in microporous materials conventionally processed. It is also believed that stretching of the sheets prior to extraction of the processing plasticizer minimizes thermal shrinkage after processing.

[0052] The product passes to a first extraction zone where the processing plasticizer is substantially removed by extraction with an organic liquid, which is a better solvent for the processing plasticizer than for the organic polymer, and more volatile than the processing plasticizer. The product then passes to a second extraction zone wherethe residual organic extraction liquid is substantially removed by steam and / or water. The product is then passed through a forced air dryer for substantial removal of residual water and remaining residual organic extraction liquid. From the dryer, the microporous material may be passed to a take-up roll, when it is in the form of a sheet.

[0053] The processing plasticizer is a liquid at room temperature and usually it is a processing oil, such as paraffinic oil, naphthenic oil, or aromatic oil. Suitable processing oils include those meeting the requirements of ASTM D 2226-82, Types 103 and 104. Those oils which have a pour point of less than 22°C, or less than 10°C, according to ASTM D 97-66 (reapproved 1978) are used most often. Examples of suitable oils include SHELLFLEX 412 and SHELLFLEX 371 oil (Shell Oil Co. (Houston, TX)), which are solvent refined and hydrotreated oils derived from naphthenic crude. It is expected that other materials, including the phthalate ester plasticizers such as dibutyl phthalate, bis(2-ethylhexyl) phthalate, diisodecyl phthalate, dicyclohexyl phthalate, butyl benzyl phthalate, and ditridecyl phthalate will function satisfactorily as processing plasticizers.

[0054] There are many organic extraction liquids that can be used in the process of manufacturing the membrane substrate. Examples of suitable organic extraction liquids include, but are not limited to, 1 ,1 ,2-trichloroethylene; perchloroethylene; 1 ,2- dichloroethane; 1 ,1 ,1 -trichloroethane; 1 ,1 ,2-trichloroethane; methylene chloride; chloroform; 1 ,1 ,2-trichloro-1 ,2,2-trifluoroethane; isopropyl alcohol; diethyl ether; acetone; hexane; heptane and toluene. One or more azeotropes of halogenated hydrocarbons selected from trans-1 ,2-dichloroethylene, 1 ,1 ,1 ,2,2,3, 4,5, 5,5- decafluoropentane, and / or 1 ,1 ,1 ,3,3-pentafluorobutane also can be employed. Such materials are available commercially as VERTREL MCA (a binary azeotrope of 1 ,1 ,1 ,2,2,3,4,5,5,5-dihydrodecafluoropentane and trans-1 ,2-dichloroethylene: 62% / 38%) and VERTREL CCA (a ternary azeotrope of 1 ,1 ,1 ,2, 2, 3, 4, 5,5,5- dihydrodecafluorpentane, 1 ,1 ,1 ,3,3-pentafluorbutane, and trans-1 ,2-dichloroethylene: 33% / 28% / 39%); VERTREL SDG (80-83% trans-1 ,2-dichloroethylene, 17-20% hydrofluorocarbon mixture), all available from MicroCare Corporation (New Britain, CT).

[0055] In the above-described process for producing a membrane substrate, extrusion and calendering are facilitated when the filler carries much of the processing plasticizer. The capacity of the filler particles to absorb and hold the processing plasticizer is a function of the surface area of the filler. Therefore, the filler typically hasa high surface area as discussed above. The residual processing plasticizer content is usually less than 15 percent by weight of the resulting microporous material and this may be reduced even further to levels, such as less than 5 percent by weight, by additional extractions using the same or a different organic extraction liquid.

[0056] The composite proton exchange membranes described above, which include an at least partially crystalline proton exchange ionomer impregnated into and / or coated onto the membrane substrate, are particularly suitable for use as a component of a redox flow battery. Thus, the present disclosure is further drawn to a redox flow battery comprising any of the composite proton exchange membranes described above. The composite proton exchange membrane is typically immersed in an aqueous medium, and in rare instances a non-aqueous medium, within the redox flow battery.

[0057] Alternatively, the redox flow battery may comprise a proton exchange membrane that does not include an at least partially crystalline proton exchange ionomer. For example, the redox flow battery may comprise a proton exchange membrane wherein the proton exchange membrane comprises:(a) a polyolefin matrix such as any of those described above, the polyolefin matrix defining a network of interconnecting pores as described above communicating throughout the polyolefin matrix; and(b) finely divided, substantially water-insoluble particulate filler comprising silica distributed throughout the polyolefin matrix, wherein the proton exchange membrane comprises 5 to 90 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler; wherein the polyolefin is present in the proton exchange membrane in an amount of 5 to 90 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler; and wherein the proton exchange membrane is in the form of a sheet having a first surface and an opposing surface. At least a portion of the interconnecting pores may communicate from the first surface of the membrane through the polyolefin matrix to the opposing surface. The silica and any other particulate filler may be any of those described above.

[0058] A typical redox flow battery of the present disclosure may further comprise a cathodic flow system and an anodic flow system separated by any of the composite proton exchange membranes disclosed above, and electrolyte compositions (catholyte and anolyte) that flow through the respective flow systems. The cathodicflow system houses a circulating catholyte solution and comprises a storage tank and a flow field (often following a tortuous path such as serpentine), connected by conduits. Similarly, the anodic flow system houses the circulating anolyte solution and comprises a storage tank and a flow field, connected by conduits. The flow fields in both flow systems often comprise carbon and serve as current collectors as the electrolytes are oxidized and reduced. Both flow systems further comprise porous carbon electrodes adjacent to the flow fields. The porosity maximizes the contact area with the liquid electrolyte. The composite proton exchange membrane is situated between the flow fields.

[0059] The electrolyte compositions that flow through the respective flow systems are almost always aqueous and may comprise any of a number of different pairs of electrolyte compositions with specific chemistries. For example, the pair of electrolyte compositions may comprise an iron-chromium system, such as comprising an Fe2+ / 3+catholyte and a Cr2+ / 3+anolyte. The pair of electrolyte compositions may comprise an all-vanadium system with vanadium as catholyte and anolyte at two different oxidation states, such as comprising a V5+catholyte and a V2+anolyte. Note that the V5+catholyte is typically an oxo-complex such as C>2+. The pair of electrolyte compositions may comprise an all-iron system with iron as catholyte and anolyte at two different oxidation states, such as an Fe2+ / 0or Fe2+ / 3+catholyte and an Fe2+ / 3+anolyte. Note that when the catholyte comprises Fe2+ / 3+it is complexed with different ligands from those in the anolyte. The pair of electrolyte compositions may comprise a zinc-bromine system such as comprising a Br2 catholyte and a Zn anolyte. The pair of electrolyte compositions may comprise organic redox systems comprising synthesized organic molecules such as quinones or viologens. Such systems allow for tunable redox potentials and sustainability. The pair of electrolyte compositions may comprise a polysulfide-bromine system. For example, a sodium polysulfide / bromine RFB utilizes aqueous sodium polysulfide and sodium bromide solution as anolyte and catholyte, respectively. Different electrolyte pairs will provide different cell voltage, energy density, and operating temperature range to the redox flow battery, and may be selected according to an intended purpose. Likewise, different electrolyte pairs may dictate the nature of the carrier medium (aqueous or organic solvent) as well as the concentration range of each electrolyte in the medium.

[0060] The present disclosure is further drawn to the following aspects:1. A composite proton exchange membrane comprising:1 ) a membrane substrate comprising:(a) a polyolefin matrix, the polyolefin matrix defining a network of interconnecting pores communicating throughout the polyolefin matrix; and(b) substantially water-insoluble particulate filler comprising silica distributed throughout the polyolefin matrix, wherein the membrane substrate comprises 5 to 90 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler; wherein the polyolefin matrix is present in the membrane substrate in an amount of 5 to 90 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler; and wherein the membrane substrate is in the form of a sheet; and2) an at least partially crystalline proton exchange ionomer that is at least partially impregnated into and / or at least partially coated on a surface of the membrane substrate.2. The composite proton exchange membrane of aspect 1 , wherein the polyolefin matrix is present in the membrane substrate in an amount of 20 to 90 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler.3. The composite proton exchange membrane of any preceding aspect , wherein the polyolefin matrix is present in the membrane substrate in an amount of 5 to 50 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler.4. The composite proton exchange membrane of aspect 1 , wherein the polyolefin matrix is present in the membrane substrate in an amount of 20 to 50 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler.5. The composite proton exchange membrane of any preceding aspect, wherein the membrane substrate comprises 5 to 80 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler.6. The composite proton exchange membrane of any preceding aspect, wherein the membrane substrate comprises 5 to 75 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler.7. The composite proton exchange membrane of aspect 1 , wherein the membrane substrate comprises 30 to 90 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler.8. The composite proton exchange membrane of any preceding aspect, wherein the membrane substrate comprises 30 to 80 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler.9. The composite proton exchange membrane of any preceding aspect, wherein the membrane substrate comprises 30 to 75 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler.10. The composite proton exchange membrane of aspect 7, wherein the membrane substrate comprises 50 to 90 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler.1 1 . The composite proton exchange membrane of aspect 10, wherein the membrane substrate comprises 50 to 80 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler.12. The composite proton exchange membrane of any preceding aspect, wherein the membrane substrate comprises 50 to 75 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler.13. The composite proton exchange membrane of any preceding aspect, wherein the at least partially crystalline proton exchange ionomer comprises a polymer having phosphonic acid-, phosphonate-, sulfonate-, and / or sulfonic acid-functional groups.14. The composite proton exchange membrane of any preceding aspect, wherein the at least partially crystalline proton exchange ionomer comprises a fluoropolymer having phosphonic acid-, phosphonate-, sulfonate-, and / or sulfonic acid-functional groups.15. The composite proton exchange membrane of any preceding aspect, wherein the at least partially crystalline proton exchange ionomer is present in the composite proton exchange membrane in an amount of 2 to 70 percent by weight, based on the total weight of the composite proton exchange membrane.16. The composite proton exchange membrane of any preceding aspect, wherein the at least partially crystalline proton exchange ionomer is uncrosslinked.17. The composite proton exchange membrane of any preceding aspect, wherein the polyolefin matrix comprises high density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (UHMWPE), and wherein the polyolefin matrix is present in the membrane substrate in an amount of 5 to 50 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler.18. The composite proton exchange membrane of any preceding aspect, wherein the filler further comprises an antioxidant and / or a peroxide scavenger.19. The composite proton exchange membrane of any preceding aspect, wherein the silica demonstrates an average particle size D50 of from 5 to 100 microns, as determined by use of a laser diffraction particle size instrument.20. Use of the composite proton exchange membrane of any preceding aspect as a component of a redox flow battery.21 . A redox flow battery comprising the composite proton exchange membrane of any of aspects 1 to 19.22. A redox flow battery comprising a proton exchange membrane, wherein the proton exchange membrane comprises:(a) a polyolefin matrix, the polyolefin matrix defining a network of interconnecting pores communicating throughout the polyolefin matrix; and(b) substantially water-insoluble particulate filler comprising silica distributed throughout the polyolefin matrix, wherein the proton exchange membrane comprises 5 to 90 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler; wherein the polyolefin is present in the proton exchange membrane in an amount of 5 to 90 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler; and wherein the proton exchange membrane is in the form of a sheet.23. The redox flow battery of any of aspects 21 and 22, further comprising a pair of electrolyte compositions comprising Fe and Cr, such as an Fe2+ / 3+catholyte and a Cr2+ / 3+anolyte.24. The redox flow battery of any of aspects 21 and 22, further comprising an all-vanadium system with vanadium as catholyte and anolyte at two different oxidation states, such as a V5+catholyte and a V2+anolyte.25. The redox flow battery of any of aspects 21 and 22, further comprising an all-iron system with iron as catholyte and anolyte at two different oxidation states, such as an Fe2+ / 0or Fe2+ / 3+catholyte and an Fe2+ / 3+anolyte.26. The redox flow battery of any of aspects 21 and 22, further comprising a pair of electrolyte compositions comprising Zn and Br, such as a Br2catholyte and a Zn anolyte.27. The redox flow battery of any of aspects 21 and 22, further comprising a pair of electrolyte compositions comprising an organic redox system comprising quinones or viologens.28. The redox flow battery of any of aspects 21 and 22, further comprising a pair of electrolyte compositions comprising a sodium polysulfide and bromine, such as a Br - catholyte and a polysulfide anolyte.29. A method for preparing a composite proton exchange membrane, comprising:(1 ) rinsing a membrane substrate sequentially with organic solvent such as methanol, ethanol, n-propanol, or isopropanol, followed by deionized water to form a rinsed membrane substrate, wherein the membrane substrate comprises:(a) a polyolefin matrix, the polyolefin matrix defining a network of interconnecting pores communicating throughout the polyolefin matrix; and(b) particulate filler comprising silica distributed throughout the polyolefin matrix, wherein the membrane substrate comprises 10 to 90 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler; wherein the polyolefin is present in the membrane substrate in an amount of 10 to 90 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler; and wherein the membrane substrate is in the form of a sheet;(2) optionally hydrating the rinsed membrane substrate with deionized water to form a hydrated membrane substrate;(3) applying a proton exchange ionomer to the membrane substrate, which may be hydrated, to form a composite membrane, wherein the proton exchange ionomer is at least partially impregnated into and / or at least partially coated on a surface of the membrane substrate;(4) drying the composite membrane; and(5) annealing the composite membrane to at least partially crystallize the proton exchange ionomer and form a composite proton exchange membrane.30. The method of aspect 29, wherein the membrane substrate is prepared by:(1 ) mixing a filler comprising silica, a polyolefin polymer, a processing plasticizer, a lubricant, optionally a free radical scavenger, and optionally an antioxidant until a substantially uniform mixture is obtained;(2) introducing the mixture to a heated barrel of a screw extruder, to which is attached a sheeting die to form a sheet;(3) forwarding the sheet to a pair of heated calender rolls acting cooperatively to form a continuous sheet of lesser thickness than the sheet exiting from the die;(4) optionally stretching the sheet in at least one stretching direction to form a stretched sheet; and(5) extracting the processing plasticizer from the sheet, which may be stretched.31 . The method of aspect 29 or 30, wherein the polyolefin matrix comprises high density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (LIHMWPE), and wherein the polyolefin matrix is present in the membrane substrate in an amount of 5 to 50 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler.32. The method of any of aspects 29 to 31 , wherein the filler further comprises an antioxidant and / or a peroxide scavenger.33. The method of any of aspects 29 to 32, wherein the silica demonstrates a number average particle size D50 of from 5 to 100 microns, as determined by use of a laser diffraction particle size instrument.34. The method of any of aspects 29 to 33, wherein the proton exchange ionomer is applied to the membrane substrate in step (3) of the method for preparing a composite proton exchange membrane by immersion, vacuum coating, inkjet printing, screen printing, spin coating, solution exchange, draw down, doctor blade application, casting, pressured flow cell impregnation, chemical vapor deposition or physical vapor deposition.35. The method of any of aspects 29 to 34, wherein the proton exchange ionomer comprises a polymer having phosphonic acid-, phosphonate-, sulfonate-, and / or sulfonic acid-functional groups.36. The method of any of aspects 29 to 35, wherein the proton exchange ionomer comprises a fluoropolymer having phosphonic acid-, phosphonate-, sulfonate-, and / or sulfonic acid-functional groups.37. The method of any of aspects 29 to 36, wherein the sheet has a thickness of 10 to 300 microns, such as 150 to 300 microns.38. The method of any of aspects 29 to 37, wherein the sheet has a thickness of 150 to 275 microns.39. A composite proton exchange membrane prepared according to the method of any of aspects 29 to 38.

[0061] The following examples are intended to illustrate various aspects of the disclosure, and should not be construed as limiting the disclosure in any way. Components that are mentioned elsewhere in the specification as suitable alternative materials for use, but which are not demonstrated in the working Examples below, are expected to provide results comparable to their demonstrated counterparts. Unless otherwise indicated, all parts are by weight.EXAMPLESExample 1 Preparation of composite ionomer membranes

[0062] A 20 wt%, dispersion of a proton exchange ionomer (NAFION™ D2021 CS, commercially available from Ion Power) in proprietary low molecular weight alcohols was further diluted with 85% denatured ethanol (commercially available from Sigma Aldrich) to achieve a 5 wt% dispersion and a 10 wt% dispersion.

[0063] A polyolefin membrane substrate (TESLIN® SP600 or PSP10, both available from PPG) was washed in 100% isopropyl alcohol for 30 minutes and then rinsed with deionized water for 30 minutes to form a hydrated substrate. To the hydrated substrate, the ionomer dispersion described above at various concentrations was applied in excess and left to agitate for various lengths of time. Small binder clips were attached to the edges of the wetted substrate to keep it elevated and flat. The substrate was allowed to sit at ambient conditions for 24 hours to dry, forming composite membranes.

[0064] After the composite membranes were dried, the weight uptake of the proton exchange ionomer was measured for each sample and calculated on the basis of total weight of the composite proton exchange membrane.Preparation of partially crystalline composite ionomer membranes

[0065] After drying, the ionomer on several samples of the composite ionomer membranes described above was annealed by the following process. Dried composite membranes were placed in a vacuum oven at 140 °C for two hours under dynamic vacuum of ~27 inHg. After two hours, the heating on the oven was turned off and the vacuum value shut to maintain a static vacuum while the samples cooled to room temperature. After cooling to room temperature under static vacuum, the oven was slowly purged and samples removed to yield annealed composite membranes. Details of sample substrate, ionomer dispersion concentration, duration of incorporation, weight uptake (i. e., percent of weight increase), and annealed / unannealed are described in Table 1 .Table 11Extruded ionomer membrane sheet (7 mil thick), prepared from perfluorosulfonic acid-PTFE copolymers with equivalent weight of 1 100, available from Ion Power.2Extruded ionomer membrane sheet (5 mil thick), prepared from perfluorosulfonic acid-PTFE copolymers with equivalent weight of 1 100, available from Ion Power.3Weight uptake % is an average calculated for replicate samples.Example 2 H-cell Testing

[0066] Procedure - Crossover and conductivity measurements were performed in a standard H-cell with a 16 mm diameter exposed membrane area. The H-cell is divided into two compartments, A and B, which are separated by the composite ionomermembranes described in Example 1 . For conductivity measurements, compartments A and B were filled with enough 1 M H2SO4 such that the entire surface area of the composite ionomer membrane was submerged. Potentiostatic impedance spectroscopy was performed with a BioLogic potentiostat using platinum electrodes placed on either side of the composite ionomer membrane. Conductivity measurements were then calculated by subtracting the resistance obtained by repeating the same procedure while excluding the composite ionomer membrane. For crossover and permeability determination, compartment A was filled with 20 mL of a 1 M solution of H2SO4 containing 1 M of VOSO4, while compartment B was filled with 20 mL of a 1 M solution of H2SO4 containing 1 M of MgSC . The H-cell was of a size such that adding 20 mL to both compartment A and B completely covered the surface area of the composite ionomer. Crossover and permeability were determined by quantifying the amount of VOSO4 that had migrated over to compartment B overtime. This quantification was performed using cyclic voltammetry. The maximum voltage obtained during this process was linearly correlated with the concentration of the vanadyl ion. All measurements were normalized to the hydrated thickness of the proton exchange membrane.

[0067] The conductivity, vanadyl permeability, and selectivity are shown in Table 2. Selectivity was calculated as a ratio between conductivity and vanadyl permeability.

[0068] As shown in Table 2 below, the conductivity did not change significantly from the extruded NAFION 117 sample (Sample 1 , described in Table 1 ) to any of the modified membranes. However, the vanadyl permeability was reduced with the composite proton exchange membranes, notably Samples 3 and 4.Table 2Example 3 Mechanical Testing Procedure

[0069] Sample substrates as described in Example 1 were punched out in the shape of “dog bones” with dimensions specified in the ASTM D638 Type IV (2022) standard. Dog bones were soaked and optionally annealed using the method described in Example 1 . The distance between clamps on an Instron tensiometer was adjusted to match the dog bone length. Samples were clamped at each end while dry, and its dimensions were specified in the test method built in Instron software. The test method, once run, pulled samples at a constant rate of 50 mm / min until failure, which is where the sample strain was recorded.Table 3Example 4 Flow Cell Battery Performance Testing

[0070] The membranes described in T able 1 were cut similarly to an area of 1 .7 cm2. Flow battery parts (cell components, glass vials, graphite current collector) were dried before the experiment and the proton exchange membrane was stored in water. The flow battery was assembled in the following order: end plate, O-rings, graphite flow field current collector, a gasket, carbon paper, proton exchange membrane, carbon paper, a gasket, graphite flow field current collector, and the end plate. The assembly was fastened using four threaded bolts against the two end plates using a torque wrench. PFA compression fittings were used to connect the tubes and the electrolyte reservoir, where the electrolyte was pumped through the flow battery assembly with a peristaltic pump.

[0071] The positive and negative current collectors of the flow cell were connected to a battery cycler. The impedance of the flow cell was measured at the open circuit potential. The area-specific resistivity (ASR) of the proton exchange membranes islisted in Table4. The flow battery was repeatedly charged / discharged at the constant current setting. Further, coulombic efficiency (CE), energy efficiency (EE), capacity (Q), and capacity fade are also listed in Table 4.

[0072] Before electrochemical testing, the vanadium electrolyte must first be activated. Commercial vanadium electrolytes are typically supplied with an average vanadium oxidation state of +3.5, corresponding to an equimolar mixture of V(lll) and V(IV) species. Once both the positive and negative electrolyte tanks are filled with this solution, a splitting procedure is carried out to generate the redox couples required for cell operation. This process involves charging the cell at a low current density (typically 10 mA cm-2). At the end of this initial step, the positive electrolyte consists primarily of V(IV) cations, while the negative electrolyte contains V(lll) cations. Subsequently, to reach the desired state of charge (SOC) for testing, the cell is further charged until the target SOC is achieved (determined by coulomb counting). Here, the electrolyte was charged to SOC = 50%, corresponding to an open-circuit voltage (OCV) of 1 .38 V. All tests were conducted at room temperature with a volumetric flow rate of 20 mL-min’1for both electrolytes. After electrolyte splitting, we start with polarization measurements and then proceed to charge-discharge cycling in the same cells. The compression ratio for all experiments was 25%, with the number of gaskets adjusted to achieve this value.

[0073] To obtain area-specific resistance (ASR), a specific charge discharge routine is applied, starting from a defined state of charge (SOC), typically SOC = 50%, corresponding to a cell voltage of approximately 1 .38 V. The cell is alternately charged and discharged by applying positive (charging) and negative (discharging) current densities for a fixed time interval. This interval must be long enough for the cell voltage to approach a steady-state condition; in practice, one minute was generally sufficient. For this study, seven current density values were applied: 10, 20, 30, 40, 50, 80, and 100 mA-cm"2. The last current value (100 mA-cm’2) approaches the potentiostat’s limit, resulting in a slightly lower actual value. During each step, the corresponding cell voltage was recorded. The voltage values reported in the polarization curve represent the average voltage over the last ten seconds of each current step, corresponding to the steady-state portion of the signal. The measured ASR depends on several operating parameters, including electrolyte flow rate, temperature, and electrode compression ratio, he ASR [Q cm2] was calculated using the following equation:v-ovc

[0074] ASR = where OCV is the open-circuit voltage [V], V is the measured cell voltage [V], and j is the applied current density (positive during charge, negative during discharge) [A cm-2], Thus, for each membrane sample and test condition, two ASR values were obtained: one under charging (ASRC) and one under discharging (ASRd) conditions.

[0075] To estimate the coulombic efficiency (CE), energy efficiency (EE), capacity (Q) and electrolyte utilization (EU) of the VFB single-cell with different membranes, a series of charge-discharge cycles were performed. The test consists of alternating galvanostatic charging and discharging steps of the cell. The procedure terminates when the maximum charge voltage of 1 .6 V and the minimum discharge voltage of 0.9 V are reached. The test can be initiated at any SOC, as it begins with an automatic discharge of the cell to the set minimum voltage. During each cycle, the cell voltage and current are continuously recorded, enabling calculation of CE, EE, Q, and EU via the following equations:where i = current [A], Vceii = cell voltage [V], c = charge, d = discharge, ( ai is the actual charge (or discharge) capacity obtained from the battery [Ah or C], and ^theoretical is the theoretical capacity based on the total amount of redox-active species present in the electrolyte solution [Ah or C]. For all membrane samples tested in this study, a constant current density of 80 mA-cm’2was applied during the charge-discharge cycling. Each charge-discharge cycle lasted ca. 10-12 h, depending on the electrochemical performance of the specific membrane.Table 4Example 5 Semi-Crystalline Domain Evaluation

[0081] Porous membrane samples, approximately 1 inch2were mounted in a sample holder using clay. Data was collected using a PANalytical X’Pert MPD diffractomer with a 9-20 goniometer configuration using Cu Ko radiation at an accelerating voltage of 40 kV and current of 50 mA. The scan rate was 0.05° perl 0s over a range of 5-75° which resulted in a 3.75 h scan per sample. Optics included a 5 mm beam mask, fixed1 / 2° divergence slit, fixed1 / 2° anti-scatter slit, and 0.5 mm receiving slit. Patterns were indexed in JADE software using the PDF2+ ICDD / MDI database. Measured data was fit using a linear background, a Pseudo-Voigt profile shape function, and initialized using the diffraction peaks identified in the measured data. Ionomer peaks were manually added from 15-20° and 35-45°. An amorphous peak was added from 15-30°. Patterns were refined so that residual error was minimized. To quantify the semicrystalline polymeric phase present in each sample, the peaks centered at 17° and 41 ° were summed and divided by the total peak area. Total peak area includes areas of both crystalline and non-crystalline peaks.Table 5

[0082] The ionomer incorporation to the composite membranes increased the semicrystalline component of the composite membranes, where the annealing process further increased the semi-crystalline contribution. Note that there is still crystallinity observed in Sample 3, but it is not polymeric semi-crystallinity but rather crystallinity afforded by the inorganic filler of the membrane sheet. The increase in total crystallinity from Sample 3 to Samples 8 and 8 shows that the semi-crystalline portion of theionomer has increased the amount of crystallinity of the composite membranes. As shown in Table 4, the performance in the redox flow battery of all inventive examples showed approximately equivalent %CE and capacity fade relative to the comparative Nation 115 (Sample 2). The ASRCof Samples 6 and 8 were slightly higher than Sample2, while Sample 3 was measurably higher. Samples 6 and 8 both contribute semicrystalline nature to the composite membrane and show similar performance, with slightly better performance when there is slightly higher semi-crystallinity. For Sample3, the higher ASRCis still within a range of reasonable performance for the RFB, especially considering the comparable %CE and capacity fade.Example 6 Preparation of a composite membrane

[0083] An ionomer dispersion (Aquivion D79-25BS, commercially available from Sigma Aldrich) was further diluted with 85% denatured ethanol (commercially available from Sigma Aldrich) to achieve a 5 wt% dispersion and a 10 wt% dispersion.

[0084] A polyolefin membrane substrate (TESLIN® SP600) was washed in 100% isopropyl alcohol for 30 minutes and then rinsed with deionized water for 30 minutes to from a hydrated substrate. To the hydrated substrate, an ionomer dispersion at various concentrations was applied in excess and left to agitate for various lengths of time. Small binder clips were attached to the edges of the wetted substrate to keep it elevated and flat. The substrate was allowed to sit at ambient conditions for 24 hours to dry. Further, the annealing process described in Example 1 was utilized to prepare an annealed composite proton exchange membrane.

[0085] Whereas particular examples of this disclosure have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present disclosure may be made without departing from the disclosure as defined in the appended claims.

Claims

What is claimed is:1 . A composite proton exchange membrane comprising:1 ) a membrane substrate comprising:(a) a polyolefin matrix, the polyolefin matrix defining a network of interconnecting pores communicating throughout the polyolefin matrix; and(b) substantially water-insoluble particulate filler comprising silica distributed throughout the polyolefin matrix, wherein the membrane substrate comprises 5 to 90 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler; wherein the polyolefin matrix is present in the membrane substrate in an amount of 5 to 90 percent by weight, based on total weight of the polyolefin matrix and the particulate filler; and wherein the membrane substrate is in the form of a sheet; and2) an at least partially crystalline proton exchange ionomer that is at least partially impregnated into and / or at least partially coated on a surface of the membrane substrate.

2. The composite proton exchange membrane of claim 1 , wherein the at least partially crystalline proton exchange ionomer comprises a polymer having phosphonic acid-, phosphonate-, sulfonate-, and / or sulfonic acid-functional groups.

3. The composite proton exchange membrane of claim 1 or 2, wherein the at least partially crystalline proton exchange ionomer comprises a fluoropolymer having phosphonic acid-, phosphonate-, sulfonate-, and / or sulfonic acid-functional groups.

4. The composite proton exchange membrane of any preceding claim, wherein the at least partially crystalline proton exchange ionomer is present in the composite proton exchange membrane in an amount of 2 to 70 percent by weight, based on the total weight of the composite proton exchange membrane.

5. The composite proton exchange membrane of any preceding claim, wherein the at least partially crystalline proton exchange ionomer is uncrosslinked.

6. The composite proton exchange membrane of any preceding claim, wherein the polyolefin matrix comprises high density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (UHMWPE), and wherein the polyolefin matrix is present in the membrane substrate in an amount of 5 to 50 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler.

7. The composite proton exchange membrane of any preceding claim, wherein the filler further comprises an antioxidant and / or a peroxide scavenger.

8. The composite proton exchange membrane of any preceding claim, wherein the silica demonstrates a number average particle size D50 of from 5 to 100 microns, as determined by use of a laser diffraction particle size instrument.

9. Use of the composite proton exchange membrane of any preceding claim as a component of a redox flow battery.

10. A redox flow battery comprising the composite proton exchange membrane of any of claims 1 to 8.1 1 . A redox flow battery comprising a proton exchange membrane, wherein the proton exchange membrane comprises:(a) a polyolefin matrix, the polyolefin matrix defining a network of interconnecting pores communicating throughout the polyolefin matrix; and(b) substantially water-insoluble particulate filler comprising silica distributed throughout the polyolefin matrix, wherein the proton exchange membrane comprises 5 to 90 percent by weight silica, based on the total weight of the polyolefin matrix and the particulate filler; wherein the polyolefin matrix is present in the proton exchange membrane in an amount of 5 to 90 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler; and wherein the proton exchange membrane is in the form of a sheet having a first surface and an opposing surface, and wherein at least a portion of the interconnecting pores communicate from the first surface of the membrane through the polyolefin matrix to the opposing surface.

12. The redox flow battery of claim 10 or 11 , further comprising a pair of electrolyte compositions comprising iron and chromium, vanadium as catholyte and anolyte at two different oxidation states, iron as catholyte and anolyte at two different oxidation states, a Br2 catholyte and a zinc anolyte, an organic redox system comprising quinones or viologens, or a Br_catholyte and a polysulfide anolyte.

13. A method for preparing a composite proton exchange membrane, comprising:(1 ) rinsing a membrane substrate sequentially with organic solvent and deionized water to form a rinsed membrane substrate, wherein the membrane substrate comprises:(a) a polyolefin matrix, the polyolefin matrix defining a network of interconnecting pores communicating throughout the polyolefin matrix; and(b) particulate filler comprising silica distributed throughout the polyolefin matrix, wherein the membrane substrate comprises 10 to 90 percent by weight silica, based on total weight of the polyolefin matrix and the particulate filler; wherein the polyolefin matrix is present in the membrane substrate in an amount of 10 to 90 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler; and wherein the membrane substrate is in the form of a sheet;(2) optionally hydrating the rinsed membrane substrate with deionized water to form a hydrated membrane substrate;(3) applying a proton exchange ionomer to the membrane substrate to form a composite membrane, wherein the proton exchange ionomer is at least partially impregnated into and / or at least partially coated on a surface of the membrane substrate;(4) drying the composite membrane; and(5) optionally annealing the composite membrane if necessary to at least partially crystallize the proton exchange ionomer and form a composite proton exchange membrane.

14. The method of claim 13, wherein the membrane substrate is prepared by:(1) mixing a filler comprising silica, a polyolefin polymer, a processing plasticizer, a lubricant, optionally a free radical scavenger, and optionally an antioxidant until a substantially uniform mixture is obtained;(2) introducing the mixture to a heated barrel of a screw extruder, to which is attached a sheeting die to form a sheet;(3) forwarding the sheet to a pair of heated calender rolls acting cooperatively to form a continuous sheet of lesser thickness than the sheet exiting from the die;(4) optionally stretching the sheet in at least one stretching direction to form a stretched sheet; and(5) extracting the processing plasticizer from the sheet.

15. The method of claim 13 or 14, wherein the polyolefin matrix comprises high density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (LIHMWPE), and wherein the polyolefin matrix is present in the membrane substrate in an amount of 5 to 50 percent by weight, based on the total weight of the polyolefin matrix and the particulate filler.

16. The method of any of claims 13 to 15, wherein the filler further comprises an antioxidant and / or a peroxide scavenger.

17. The method of any of claims 13 to 16, wherein the silica demonstrates a number average particle size D50 of from 5 to 100 microns, as determined by use of a laser diffraction particle size instrument.

18. The method of any of claims 13 to 17, wherein the proton exchange ionomer is applied to the membrane substrate by immersion, vacuum coating, inkjet printing, screen printing, spin coating, solution exchange, draw down, doctor blade application, casting, pressured flow cell impregnation, chemical vapor deposition or physical vapor deposition.

19. The method of any of claims 13 to 18, wherein the proton exchange ionomer comprises a polymer having phosphonic acid-, phosphonate-, sulfonate-, and / or sulfonic acid-functional groups.

20. The method of any of claims 13 to 19, wherein the proton exchange ionomer comprises a fluoropolymer having phosphonic acid-, phosphonate-, sulfonate-, and / or sulfonic acid-functional groups.21 . The method of any of claims 13 to 20, wherein the sheet has a thickness of 10 to 500 microns.

22. A composite proton exchange membrane prepared according to the method of any of claims 13 to 21 .

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