Stretched, highly uniform cation exchange membrane and method for forming same

Abstract

The cation exchange membrane includes a film of a fluorinated ionomer containing sulfonic acid groups. The film has a machine direction and a cross direction perpendicular to the machine direction. The membrane has a water swelling of less than about 5% in both the machine direction and the cross direction. The membrane has a ratio of in-plane conductivity in the machine direction to in-plane conductivity in the cross direction of about 0.9 to about 1.1. The process produces a cation exchange membrane including a film of a fluorinated ionomer containing sulfonic acid groups. The process includes forming a film of the ionomer. The process also includes biaxially stretching the film in both the machine direction and the cross direction perpendicular to the machine direction. The electrochemical cell has an anode compartment and a cathode compartment and includes a cation exchange membrane as a separator between the anode compartment and the cathode compartment.

Description

[Technical Field]

[0001] The present invention relates to ion exchange membranes for flow batteries and other electrochemical applications, and more particularly to expanded, highly uniform cation exchange membranes for vanadium redox flow batteries. [Background technology]

[0002] A flow battery is a type of rechargeable battery in which an electrolyte containing one or more dissolved electroactive species flows through an electrochemical cell, converting chemical energy directly into electricity. Additional electrolyte is stored externally, typically in a tank, and is usually pumped through the reactor cell or cells, although gravity-fed systems are also possible. Flow batteries can be rapidly recharged by replacing the electrolyte while simultaneously recovering spent materials for re-energization.

[0003] The three main types of flow batteries are redox (reduction-oxidation) flow batteries, hybrid flow batteries, and fuel cells. In redox flow batteries, all electroactive components are dissolved or dispersed in an electrolyte. Hybrid flow batteries differ in that one or more of the electroactive components are deposited as a solid layer. Redox fuel cells have a conventional flow battery reactor, but the flow battery reactor operates only to generate electricity and is not electrically recharged. In the latter case, recharging is achieved by reducing the negative electrolyte using a fuel such as hydrogen and oxidizing the positive electrolyte using an oxidant such as air or oxygen.

[0004] Vanadium redox flow batteries are an example of a redox flow battery and generally involve the use of two redox pair electrolytes separated by an ion-exchange membrane. The vanadium redox flow battery family includes so-called "all-vanadium redox flow batteries" (VRBs), which use a V(II) / V(III) couple in the negative half-cell and a V(IV) / V(V) couple in the positive half-cell, and "vanadium bromide redox flow cells and flow batteries" (V / BrRBs), which use a V(II) / V(III) couple in the negative half-cell and a bromide / polyhalide couple in the positive half-cell. In both cases, the positive and negative half-cells are separated by a membrane / separator, which prevents cross-mixing of the positive and negative electrolytes and allows the transport of ions during the passage of current to complete the circuit.

[0005] V(V) ions in VRB systems and polyhalide ions in V / BrRB systems are highly oxidizing, rapidly degrading most polymer membranes during use and reducing their durability. As a result, potential materials for membranes / separators are limited, which is a major obstacle to the commercialization of these types of energy storage systems. Ideally, membranes should be stable in the acidic environment of electrolytes such as vanadium sulfate (often with excess free sulfuric acid) or vanadium bromide, exhibit good resistance to the highly oxidizing V(V) or polyhalide ions in the charged positive half-cell electrolyte, have low electrical resistance, low permeability to vanadium ions or polyhalide ions, high permeability to charge-carrying hydrogen ions, good mechanical properties, and be low cost. To date, developing an appropriate polymer system with this balance of properties remains challenging.

[0006] Certain perfluorinated ion-exchange polymers, such as perfluorosulfonic acid polymers (e.g., Nafion™ polymers available from The Chemours Company FC, LLC, Wilmington, DE), show great promise in terms of resistance to acidic environments and highly oxidizing species, but there is room for further improvement in resistance to water and vanadium ion crossover. High vanadium ion crossover not only reduces coulombic efficiency, reduces capacity, and leads to battery self-discharge, but also requires continuous readjustment of the electrolyte concentrations in the two half-cells. This undesirable capacity loss due to mixing of electroactive ions requires the overall battery to be oversized to meet the target discharge capacity when capacity decreases. Furthermore, crossover is typically suppressed by using thick membranes, which also suppresses proton conductivity and significantly increases costs. This puts flow battery manufacturers at a significant competitive disadvantage compared to manufacturers of other batteries with higher coulombic efficiency. Clearly, there is significant motivation to improve the coulombic efficiency of cells, and the primary ways to achieve this are through improved crossover resistance and improved ion selectivity of charge-carrying species over electroactive species.

[0007] So et al. ("Hydrophilic Channel Alignment of Perfluoronated Sulfonic-Acid Ionomers for Vanadium Redox Batteries", ACS Appl. Mater. Interfaces, Vol. 10, pp. 19689-19696, 2018) disclose that uniaxial stretching not only provides higher coulombic efficiency and longer self-discharge time, but also reduces proton conductivity.

[0008] Karpushkin et al. ("Effect of biaxial stretching on the ion-conducting properties of Nafion membranes", Mendeleev Commun., Vol. 26, pp. 117-118, 2016) disclose that biaxial stretching at various draw ratios not only reduces the vanadium permeability but also decreases the self-discharge time. [Prior art documents] [Non-patent literature]

[0009] [Non-Patent Document 1] So et al. ("Hydrophilic Channel Alignment of Perfluoronated Sulfonic-Acid Ionomers for Vanadium Redox Batteries", ACS Appl. Mater. Interfaces, Vol. 10, pp. 19689-19696, 2018) [Non-patent document 2] Karpushkin et al. (“Effect of biaxial stretching on the ion-conducting properties of Nafion membranes”, Mendeleev Commun., Vol. 26, pp. 117-118, 2016) Summary of the Invention [Problem to be solved by the invention]

[0010] There is a need for ion exchange membranes that have higher tensile strength, reduced wet swelling, reduced vanadium crossover, improved energy efficiency, reduced self-discharge rates, and / or increased ion selectivity. [Means for solving the problem]

[0011] In an exemplary embodiment, the cation exchange membrane comprises a film of a fluorinated ionomer containing sulfonic acid groups. The film has a longitudinal direction and a transverse direction perpendicular to the longitudinal direction. The membrane has a water swelling of less than about 5% in both the longitudinal and transverse directions. The ratio of the membrane's in-plane conductivity in the longitudinal direction to its in-plane conductivity in the transverse direction is in the range of about 0.9 to about 1.1.

[0012] In another exemplary embodiment, a process is provided for producing a cation exchange membrane comprising a film of a fluorinated ionomer containing sulfonic acid groups. The process includes forming a film of the ionomer. The process also includes biaxially stretching the film in both a machine direction and a transverse direction perpendicular to the machine direction, so that the membrane has a water swell of less than about 5% in both the machine direction and the transverse direction, and the ratio of the in-plane conductivity in the machine direction to the in-plane conductivity in the transverse direction of the membrane is in the range of about 0.9 to about 1.1.

[0013] In yet another exemplary embodiment, an electrochemical cell has an anode compartment and a cathode compartment and includes a cation exchange membrane as a separator between the anode compartment and the cathode compartment. The membrane includes a film of a fluorinated ionomer containing sulfonic acid groups. The film has a longitudinal direction and a transverse direction perpendicular to the longitudinal direction. The membrane has a water swelling of less than about 5% in both the longitudinal and transverse directions, and the ratio of the membrane's in-plane conductivity in the longitudinal direction to its in-plane conductivity in the transverse direction is in the range of about 0.9 to about 1.1.

[0014] Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic diagram of an all-vanadium redox flow battery in one embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0016] Expanded ion exchange membranes having high tensile strength, low wet swelling, low vanadium crossover, high energy efficiency, low self-discharge rate, high ion selectivity, or a combination thereof are provided.

[0017] In an exemplary embodiment, a stretched ion exchange membrane film is biaxially stretched in both the machine direction and the transverse direction at a predetermined stretch ratio, such that the stretched ion exchange membrane has a water swelling of less than about 5% in both the machine direction and the transverse direction, and a ratio of the in-plane conductivity in the machine direction to the in-plane conductivity in the transverse direction in the range of about 0.9 to about 1.1.

[0018] As used herein, machine direction refers to the in-plane direction of the film parallel to the direction of movement or winding of the membrane on the roll during the manufacture of the film.

[0019] As used herein, the cross direction refers to the in-plane direction of the film perpendicular to the machine direction.

[0020] As used herein, stretch ratio refers to the ratio of the stretched length of the film to the unstretched length of the film.

[0021] As used herein, in-plane conductivity refers to the proton conductivity of the film in the plane of the film.

[0022] As used herein, through-plane conductivity refers to the proton conductivity of the film in a direction perpendicular to the plane of the film.

[0023] As used herein, vanadyl ion (VO 2+ ) Permeability is the VO through the film in a direction perpendicular to the plane of the film. 2+ Refers to the permeability of vanadyl ions.

[0024] As used herein, ion selectivity refers to the permeability of protons through the film in a direction perpendicular to the plane of the film to vanadyl ions, and is expressed as the through-plane conductivity divided by the vanadyl ion permeability.

[0025] As used herein, tensile strength refers to the resistance of a film to break under tension in a given direction and is calculated as the maximum load divided by the minimum cross-sectional area before break.

[0026] As used herein, water swelling refers to the percent change in length of a film in a given in-plane direction from room temperature and 50% relative humidity to immediately after immersion in boiling water for 1 hour.

[0027] In exemplary embodiments, the film comprises a fluorinated ionomer containing sulfonic acid groups. In some embodiments, the membrane comprises a hydrolyzed melt-extruded film of the ionomer. In some embodiments, the membrane comprises a cast film of the ionomer.

[0028] The term "sulfonic acid group" is intended to refer to either a sulfonic acid group or a salt of sulfonic acid, preferably an alkali metal or ammonium salt. A preferred functional group is represented by the formula -SO3X, where X is H, Li, Na, K, or N(R 1 )(R 2 )(R 3 )(R 4 ) and R 1 , R 2 , R 3 , and R 4 are the same or different and are H, CH, or C2H5. A preferred class of fluorinated ionomers containing sulfonic acid groups for use in the present films includes a highly fluorinated, most preferably perfluorinated, carbon backbone with side chains of the formula -(O-CF2CFR f ) a -O-CF2CFR' f SO3X, where R f and R' f are independently selected from F, Cl, or perfluorinated alkyl groups having 1 to 10 carbon atoms, a=0, 1, or 2, and X is H, Li, Na, K, or N(R 1 )(R 2 )(R 3 )(R 4 ) and R 1 , R 2, R 3 , and R 4 are the same or different and are H, CH3, or C2H5. Preferred fluorinated ionomers containing sulfonic acid groups include, for example, the polymers disclosed in U.S. Pat. Nos. 3,282,875, 4,358,545, or 4,940,525. For use in vanadium redox flow batteries and fuel cells, the fluorinated ionomers in the membranes are typically used in the proton form, i.e., X is H.

[0029] One preferred fluorinated ionomer containing sulfonic acid groups comprises a perfluorocarbon backbone and side chains of the formula -O-CFCF(CF)-O-CFCFSOX, where X is as defined above. When X is H, the side chain is -O-CFCF(CF)-O-CFCFSOH. This type of fluorinated ionomer containing sulfonic acid groups is disclosed in U.S. Pat. No. 3,282,875 and can be prepared by copolymerizing tetrafluoroethylene (TFE) with a perfluorinated vinyl ether, CF=CF-O-CFCF(CF)-O-CFCFSOF, perfluoro(3,6-dioxa-4-methyl-7-octene sulfonyl fluoride) (PSEPVE), followed by hydrolysis of the sulfonyl fluoride groups to sulfonic acid groups and, if required for a particular application, conversion to the proton form.

[0030] One preferred fluorinated ionomer containing sulfonic acid groups of the type disclosed in U.S. Patent Nos. 4,358,545 and 4,940,525 has a side chain -O-CFCFSOX, where X is as defined above. This fluorinated ionomer containing sulfonic acid groups can be prepared by copolymerization of tetrafluoroethylene (TFE) with a perfluorinated vinyl ether, CF=CF-O-CFCFSOF, perfluoro(3-oxa-4-pentenesulfonyl fluoride) (PFSVE), followed by hydrolysis and conversion to the proton form if required for a particular application. When X is H, the side chain is -O-CFCFSOH.

[0031] In an exemplary embodiment, the fluorinated ionomer containing sulfonic acid groups is of the type available under the trade name Nafion™ (The Chemours Company FC, LLC, Wilmington, Del.).

[0032] In an exemplary embodiment, the fluorinated ionomer film is biaxially stretched to improve the in-plane conductivity uniformity of the fluorinated ionomer film in the biaxial directions, resulting in improved membrane ion selectivity and reduced self-discharge of the film compared to non-biaxially stretched fluorinated ionomer films.

[0033] In an exemplary embodiment, the membrane has a conductivity of (mS cm -1 ) / (10 -6 cm 2 min -1 ) of at least about 50, alternatively at least about 60, alternatively at least about 70, alternatively at least about 80, alternatively at least about 90, alternatively at least about 100, alternatively at least about 110, or any value, range, or subrange therebetween.

[0034] In exemplary embodiments, the membranes have an ion exchange ratio (IXR) of from about 7 to about 25, alternatively from about 10 to about 25, alternatively from about 9 to about 15, alternatively from about 11 to about 19, alternatively from about 11 to about 14, or any value, range, or subrange therebetween. As used herein, IXR refers to the number of carbon atoms in the ionomer backbone relative to the number of cation exchange groups.

[0035] In an exemplary embodiment, the membrane is prepared by copolymerization of TFE and PSEPVE, followed by hydrolysis and ion exchange to the proton form. Such membranes have side chains represented by the formula -O-CFCF(CF)-O-CFCFSOH and have an equivalent weight (EW) of about 600 to about 1600, alternatively about 700 to about 1600, alternatively about 850 to about 1430, alternatively about 850 to about 1200, alternatively about 900 to about 1100, or any value, range, or subrange therebetween. As used herein, EW refers to the weight of the ionomer in the proton form required to neutralize one equivalent of NaOH.

[0036] In an exemplary embodiment, the membrane is prepared by copolymerization of TFE and PSEPVE, followed by hydrolysis and ion exchange to the proton form. Such membranes have side chains of the formula —O—CFCFSOH and have an EW of about 400 to about 1600, alternatively about 500 to about 1430, alternatively about 600 to about 1200, alternatively about 760 to about 1100, alternatively about 850 to about 1100, or any value, range, or subrange therebetween. Such ionomers are sometimes referred to as short side chain ionomers.

[0037] The IXR of a fluoroionomer with side chains of -O-CF2-CF(CF3)-O-CF2-CF2-SO3H, i.e., formed from a copolymer of TFE and PSEPVE, can be related to EW using the formula 50 IXR + 344 = EW. The IXR of a fluoroionomer with side chains of -O-CF2CF2SO3H, i.e., formed from a copolymer of TFE and PSEPVE, can be related to EW using the formula 50 IXR + 178 = EW.

[0038] In exemplary embodiments, the membrane has a thickness ranging from about 10 μm to about 200 μm, alternatively from about 15 μm to about 100 μm, alternatively from about 20 μm to about 50 μm, or any value, range, or subrange therebetween.

[0039] In an exemplary embodiment, a process for producing a cation exchange membrane includes forming a film of a fluorinated ionomer containing sulfonic acid groups and biaxially stretching the film in both the machine direction and the transverse direction perpendicular to the machine direction, such that the membrane has a water swelling in both the machine direction and the transverse direction that is less than a predetermined value, and the ratio of the in-plane conductivity in the machine direction to the in-plane conductivity in the transverse direction is within a predetermined range.

[0040] In an exemplary embodiment, the forming includes extruding the sulfonyl fluoride form of the ionomer into a precursor film, followed by hydrolyzing the sulfonyl fluoride groups of the ionomer in the precursor film to form a hydrolyzed melt-extruded film.

[0041] In an exemplary embodiment, the ionomer film is in the proton form during biaxial stretching.

[0042] In an exemplary embodiment, biaxial stretching involves sequential stretching first in the machine direction and then in the transverse direction.

[0043] In some embodiments, a film stretching machine stretches a film in the machine direction. The film is fed into the machine at a predetermined speed, such as 5 feet per minute. Stretching is accomplished by passing the film through two preheat rolls to heat it, followed by low-speed and high-speed rolls to stretch it. The low-speed and high-speed rolls provide a predetermined stretch ratio. The film then passes through an annealing roll, which can be followed by a cooling roll. In some embodiments, the roll temperatures are about 150°F (about 66°C) for the first preheat roll, about 230°F (about 110°C) for the second preheat roll and the low-speed roll, about 225°F (about 107°C) for the high-speed roll, about 180°F (about 82°C) for the annealing roll, and about 82°F (about 28°C) for the cooling roll.

[0044] In some embodiments, the film is stretched in the machine direction and then stretched in the transverse direction by a tenter process. The film is fed into a tenter oven and firmly gripped by clips at both ends. The tenter oven includes three consecutive zones: preheating, stretching, and annealing. The temperature of each zone is independently controlled, e.g., about 300°F (about 149°C) in the preheating zone, about 290°F (about 143°C) in the stretching zone, and about 285°F (about 141°C) in the annealing zone. The transverse stretching is performed over a predetermined distance with a predetermined stretch ratio, e.g., a distance of about 9.5 feet and a stretch ratio of about 2.5. The film is relaxed in the annealing oven by a predetermined amount, e.g., about 0.01%. After annealing, the edges of the film may be trimmed and the film can be wrapped around a cardboard core.

[0045] In some embodiments, biaxial stretching occurs simultaneously in the machine and transverse directions.

[0046] In some embodiments, biaxially stretched membranes have a water swell in both the machine and cross directions of less than about 5%, alternatively less than about 4%, alternatively less than about 3%, alternatively less than about 2%, alternatively less than about 1%, alternatively less than about 0%, alternatively between about -2% and about -6%, or any value, range, or subrange therebetween.

[0047] In exemplary embodiments, biaxial stretching results in a film having a ratio of in-plane conductivity in the machine direction to in-plane conductivity in the transverse direction in a range of from about 0.8 to about 1.2, alternatively from about 0.9 to about 1.1, alternatively from about 0.95 to about 1.05, alternatively from about 0.96 to about 1.04, alternatively from about 0.98 to about 1.02, alternatively from about 0.99 to about 1.01, or any value, range, or subrange therebetween.

[0048] In exemplary embodiments, the rate of biaxial stretching is in the range of about 1% per second to about 50% per second, alternatively about 1% per second to about 40% per second, alternatively about 1% per second to about 30% per second, alternatively about 1% per second to about 20% per second, alternatively about 5% per second to about 20% per second, alternatively about 1% per second to about 10% per second, alternatively about 10% per second to about 20% per second, alternatively about 20% per second to about 30% per second, or any value, range, or subrange therebetween.

[0049] In an exemplary embodiment, the main chain of the fluorinated ionomer containing sulfonic acid groups has a glass transition temperature in the range of about 100°C to about 125°C, and the side chains have a glass transition temperature in the range of about 190°C to about 245°C. In an exemplary embodiment, the biaxial stretching is carried out at a temperature greater than about 20°C below the glass transition temperature of the main chain of the ionomer film, or greater than about 10°C below the glass transition temperature of the main chain of the ionomer film, or at a temperature greater than the glass transition temperature of the main chain of the ionomer film, or at any value, range, or subrange therebetween. In an exemplary embodiment, the biaxial stretching is carried out at a temperature in the range of about 70°C to about 250°C, or about 75°C to about 150°C, or about 80°C to about 140°C, or at any value, range, or subrange therebetween.

[0050] In an exemplary embodiment, biaxial stretching involves stretching in both the machine direction and the transverse direction, respectively, at stretch ratios ranging from about 1.1 to about 5, alternatively from about 1.2 to about 2, alternatively from about 1.2 to about 2.5, alternatively from about 2 to about 5, alternatively from about 2 to about 3.5, alternatively from about 2 to about 3, alternatively from about 2 to about 2.5, alternatively from about 1.7 to about 3, alternatively from about 1.7 to about 2, or any value, range, or subrange therebetween.

[0051] In exemplary embodiments, the process further comprises annealing the film after biaxial stretching. Annealing involves heating the film for about 5 seconds to about 30 minutes to a temperature ranging from about 0°C to about 300°C, alternatively from about 25°C to about 200°C, alternatively from about 50°C to about 200°C, alternatively from about 100°C to about 190°C, alternatively from about 125°C to about 160°C, or any value, range, or subrange therebetween, while providing sufficient tension to hold the film in a stretched state. In some embodiments, annealing further comprises partially releasing the tension in the transverse direction such that the width of the film in the transverse direction is reduced by about 10% or less. While stretched films without annealing may have better performance compared to comparable unstretched films, annealing has been found to provide even better performance in some embodiments.

[0052] The films and membranes of the present disclosure can be used in any of a number of different applications, including, but not limited to, electrochemical batteries, flow batteries, vanadium redox flow batteries, water electrolysis, direct methanol fuel cells, hydrogen fuel cells, or carbon dioxide electrolysis.

[0053] 1 shows an electrochemical cell 10 having an anode compartment 12 and a cathode compartment 14, and including a cation exchange membrane 16 as disclosed herein as a separator between the anode compartment 12 and the cathode compartment 14. In some embodiments, the electrochemical cell 10 is a flow battery. In some embodiments, the flow battery is a vanadium redox flow battery or an all-vanadium redox flow battery.

[0054] The anode compartment 12 contains an anode 20 and an anolyte 22. Additional anolyte 22 may be stored in an anolyte tank 24 and supplied to the anode compartment 12 by an anolyte pump 26 having an anolyte valve 28 that controls the direction of the flow.

[0055] The cathode compartment 14 contains a cathode 30 and a catholyte 32. Additional catholyte 32 may be stored in a catholyte tank 34 and supplied to the cathode compartment 14 by a catholyte pump 36 having a catholyte valve 38 that controls the direction of flow. [Example]

[0056] The invention is illustrated by the following examples, which do not limit the scope of the invention described in the claims.

[0057] Test Method In-plane conductivity measurements To measure the in-plane conductivity (IPC), the film was marked in the MD and TD. The film was then immersed in a boiling water bath for 1 hour and then immediately transferred to deionized water to form a membrane. The membrane was then assembled into a customized in-plane conductivity cell so that the desired measurement direction was perpendicular to the direction of the platinum wire. The in-plane conductivity cell containing the membrane was kept immersed in deionized water for the entire measurement. The electrical resistance R of the membrane was measured. MD or R TD (Ω) was measured by linear sweep voltammetry (LSV) with a four-electrode setup on a BioLogic potentiostat (BioLogic Science Instruments, Seyssinet-Pariset, France).

[0058] Therefore, the MD or TD conductivity of the membrane, σ MD or σ TD (mS / cm) was calculated using Equation 1.

[0059]

number

[0060] Through-plane conductivity measurements To measure the trans-plane (proton) conductivity, the film was immersed in a deionized water bath at 60 °C for 6 h to form a membrane. The membrane was then immediately transferred to a covered container filled with the test electrolyte (2.5 M sulfuric acid) and allowed to soak overnight. A customized H-cell was used for the measurements. The electrical resistance was measured by electrochemical impedance spectroscopy (EIS) using a four-electrode setup on a potentiostat (BioLogic).

[0061] The cell was first assembled without a membrane and filled with 2.5 M sulfuric acid to measure the non-membrane ohmic resistance or cell resistance R cell With the membrane fixed in the H-cell, equal volumes of test solution were added to both sides of the assembled cell to measure the total resistance R total The resistance (Ω) of the membrane was measured. membrane (Ω) is the difference between the total resistance and the cell resistance.

[0062] Trans-plane conductivity of the film σ T (mS / cm) was calculated using Equation 2.

[0063]

number

[0064] Vanadyl ion permeability measurement To measure vanadyl ion permeability, the film was immersed in a deionized water bath at 60 °C for 6 hours to form a membrane. The membrane was then immediately transferred to a lidded container filled with the test electrolyte (1.5 M MgSO4 in 2.5 M sulfuric acid) and allowed to soak overnight. A customized H-cell was used for the measurements. One side of the cell was filled with 1.5 M MgSO4 in 2.5 M sulfuric acid electrolyte, and the opposite compartment of the cell was filled with an equal volume of 1.5 M VO4 in 2.5 M sulfuric acid electrolyte. A UV-Vis probe was inserted into the MgSO4 electrolyte side to measure the VO4 diffused from the VO4 electrolyte. 2+ The intensity of the absorption peak at 760 nm associated with the vanadyl ion was monitored. VO2+ was calculated using Equation 3.

[0065]

number

[0066] Ion selectivity measurements Equation 4 was used to calculate the ion selectivity from the measurements of the membrane's transplane conductivity and vanadyl ion permeability.

[0067]

number

[0068] Tensile strength measurement To measure tensile strength, films were conditioned at 50% relative humidity (RH) and 22°C for at least 40 hours. Tensile strength was then measured according to international standard ASTM D882. Tensile strength was calculated by dividing the maximum load by the original minimum cross-sectional area of ​​the membrane.

[0069] Water swelling measurement To measure water swelling, the film was cut into 5cm x 5cm pieces. Marks were made on the film in the MD and TD at 4cm intervals, and then the film was transferred to a 50% RH chamber overnight. The film was then placed in boiling water for 1 hour. The distance between the marks on the film was then immediately measured as the swelling distance. The water swelling in each direction was calculated as a percentage based on the difference between the swelling distance and the original distance (4cm) divided by the original distance.

[0070] Examples of the present invention For each of Inventive Example 1, Inventive Example 2, and Inventive Example 3, a Nafion™ N1110 film (The Chemours Company FC, LLC, Wilmington, Del.) having a thickness of 254 microns was used as the starting film for the fluorinated ionomer containing sulfonic acid groups. The extruded film was a protonated TFE / PSEPVE copolymer with an EW of approximately 1000. The film had been pre-hydrolyzed and converted to the protonated form by the manufacturer. The film was stretched using a continuous film stretcher. The film was fed into the machine at a speed of 5 feet per minute. Machine direction (MD) stretching was achieved by passing the film through two preheated rolls, followed by a slow and fast roll for stretching. The film was then subjected to an annealing roll followed by a cooling roll. The MD stretch ratio was maintained at the value shown in Table 1. The two preheated rolls were maintained at temperatures of 150°F and 230°F, respectively. The temperature of the low speed roll was set at 230° F. and the temperature of the high speed roll was set at 225° F. The temperatures of the annealing and cooling rolls for machine direction orientation were 180° F. and 82° F., respectively.

[0071] After the MD stretching step, the film was stretched in the transverse direction (TD) using a tenter oven. The film was fed into a tenter oven and firmly gripped by clips on both ends. The tenter oven contained three segments: preheating, stretching, and annealing. The temperature of each segment was controlled separately. The preheating oven was maintained at 300°F, the stretching oven at 290°F, and the annealing oven at 285°F. The TD stretching was carried out over a distance of 9.5 feet. The TD stretch ratio was maintained at 2.5. The film was relaxed by 0.01% in the annealing oven. After the annealing step, the edges of the film were trimmed, and then the film was wrapped around a cardboard core as in Inventive Example 1, Inventive Example 2, and Inventive Example 3.

[0072] Inventive Examples 4, 5, 6, and 7 were formed similarly to Inventive Examples 1, 2, and 3. The starting extruded film for these comparative examples was Nafion™ N1110 film, a TFE / PSEPVE copolymer in the proton form, having an EW of about 1000 and a thickness of about 254 microns. The film had been previously hydrolyzed to the proton form by the manufacturer.

[0073] Examples of the present invention were tested for in-plane conductivity, trans-plane (proton) conductivity, vanadyl ion permeability, ion selectivity, tensile strength, and water swelling.

[0074] The values ​​for longitudinal in-plane conductivity, transverse in-plane conductivity, ratio of longitudinal to transverse in-plane conductivity, proton conductivity, vanadyl ion permeability, ion selectivity, longitudinal tensile strength, transverse tensile strength, longitudinal water swelling, and transverse water swelling obtained for the examples of the present invention are shown in Table 1. Negative water swelling values ​​indicate that the film decreased in length or contracted in the given direction during boiling conditions rather than swelling. N / D indicates that the parameter was not determined, and the standard deviation is provided for the particular measurement.

[0075] [Table 1]

[0076] Comparative Example Comparative Example 1 was a cast and unstretched film of Nafion™ NR212 having a thickness of about 50 microns. The film was cast from a dispersion of hydrolyzed TFE / PSEPVE copolymer in the proton form having an EW of about 1000.

[0077] Comparative Examples 2 and 3 were formed similarly to the inventive examples, with MD and TD stretch ratios as shown in Table 2. The starting extruded film for these comparative examples was Nafion™ N1110 film, a TFE / PSEPVE copolymer in the proton form, with an EW of about 1000 and a thickness of about 254 microns. The film had been previously hydrolyzed to the proton form by the manufacturer.

[0078] The comparative examples were tested for in-plane conductivity, trans-plane (proton) conductivity, vanadyl ion permeability, ion selectivity, tensile strength, and water swelling.

[0079] The values ​​of longitudinal in-plane conductivity, transverse in-plane conductivity, ratio of longitudinal to transverse in-plane conductivity, proton conductivity, vanadyl ion permeability, ion selectivity, longitudinal tensile strength, transverse tensile strength, longitudinal water swelling, and transverse water swelling obtained for the comparative examples are shown in Table 2. N / A indicates that the parameter is not applicable, N / D indicates that the parameter was not determined, and the standard deviation is provided for the particular measurement.

[0080] [Table 2]

[0081] Compared to the comparative examples, the examples of the present invention had a ratio of longitudinal to transverse in-plane conductivity closer to 1, reduced vanadyl permeability, and / or higher ion selectivity.

[0082] All of the above references are incorporated herein by reference.

[0083] While the present invention has been described with reference to preferred embodiments, those skilled in the art will recognize that various changes can be made and equivalents can be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications can be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is not intended that the invention be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but rather, the invention is intended to include all embodiments falling within the scope of the appended claims.

Claims

1. 1. A cation exchange membrane comprising a film of a fluorinated ionomer containing sulfonic acid groups, the film having a machine direction and a cross direction perpendicular to the machine direction, wherein the machine direction refers to the in-plane direction of the film parallel to the direction of movement or winding of the membrane on a roll during manufacture of the film, and the cross direction refers to the in-plane direction of the film perpendicular to the machine direction; the membrane has a water swelling of less than 5% in both the machine direction and the cross direction, wherein water swelling refers to the percent change in length of the film in a given in-plane direction from a condition of 50% relative humidity at room temperature to immediately after immersion in boiling water for 1 hour; and the ratio of the in-plane conductivity in the machine direction to the in-plane conductivity in the cross direction of the membrane is in the range of 0.9 to 1.1, wherein the in-plane conductivity refers to the proton conductivity of the film in the plane of the film measured by immersing the film in a boiling water bath for 1 hour, and then immediately transferring it to deionized water and leaving it immersed in the deionized water.

2. At least 60 (mS cm -1 ) / (10 -6 cm 2 min -1 ) ions (protons / VO 2+ ) selectivity, where ion selectivity refers to the permeability of protons relative to the permeability of vanadyl ions through the film in a direction perpendicular to the plane of the film.

3. 2. The cation exchange membrane according to claim 1, having an ion exchange ratio in the range of 7 to 25, wherein the ion exchange ratio refers to the number of carbon atoms in the ionomer backbone relative to the number of cation exchange groups.

4. 4. The cation exchange membrane according to claim 3, wherein the ion exchange ratio is in the range of 9 to 15.

5. The cation exchange membrane of claim 1, having a thickness in the range of 10 μm to 200 μm.

6. 1. A process for producing a cation exchange membrane comprising a film of a fluorinated ionomer containing sulfonic acid groups, said process comprising: forming a film of the ionomer; biaxially stretching the film in both a machine direction and a transverse direction perpendicular to the machine direction, so that the membrane has a water swell of less than 5% in both the machine direction and the transverse direction, where water swell refers to the percent change in length of the film in a given in-plane direction from a condition of 50% relative humidity at room temperature to immediately after being placed in boiling water for 1 hour, and such that the ratio of the in-plane conductivity in the machine direction to the in-plane conductivity in the transverse direction of the membrane is in the range of 0.9 to 1.1, where in-plane conductivity refers to the proton conductivity of the film in the plane of the film measured by immersing the film in a boiling water bath for 1 hour and then immediately transferring it to deionized water and leaving it immersed in the deionized water.

7. 7. The process of claim 6, wherein the biaxial stretching is carried out in the machine direction and the transverse direction at a rate ranging from 1% / sec to 30% / sec.

8. 7. The process of claim 6, wherein the film is heated to at least 20°C below the glass transition temperature of the ionomer during the biaxial stretching.

9. 7. The process of claim 6, further comprising the step of heating the film to a temperature in the range of 70°C to 250°C during the biaxial stretching.

10. The process of claim 6, wherein the film is stretched at a ratio of 1.2 to 5 in both the machine direction and the transverse direction.

11. 7. The process of claim 6, wherein said forming said film comprises extruding the sulfonyl fluoride form of said ionomer into a precursor film, followed by hydrolyzing the sulfonyl fluoride groups of said ionomer in said precursor film to form a hydrolyzed melt-extruded film.

12. 12. The process of claim 11, wherein the machine direction is the direction in which the film is extruded into a precursor film.

13. The process of claim 6 wherein the ionomer film is in the proton form during the biaxial stretching.

14. 7. The process of claim 6, wherein the ionomer film is sequentially stretched by first stretching in the machine direction and then stretching in the transverse direction.

15. 7. The process of claim 6, further comprising the step of annealing the film after biaxial stretching by heating the film to a temperature in the range of 85°C to 200°C for 5 seconds to 30 minutes while providing sufficient tension to the film to hold it in a stretched state.

16. 16. The process of claim 15, wherein the annealing comprises partially releasing tension in the transverse direction such that the width of the film in the transverse direction decreases by no more than 10%.

17. 1. An electrochemical cell having an anode compartment and a cathode compartment, comprising a cation exchange membrane as a separator between the anode compartment and the cathode compartment, wherein the membrane comprises a film of a fluorinated ionomer containing sulfonic acid groups, the film having a machine direction and a cross direction perpendicular to the machine direction, wherein the machine direction refers to the in-plane direction of the film that is parallel to the direction of movement or winding of the membrane on a roll during manufacture of the film, and the cross direction refers to the in-plane direction of the film that is perpendicular to the machine direction, and the membrane has a cation exchange membrane in both the machine direction and the cross direction. an electrochemical cell having a water swelling of less than 5% in a given in-plane direction, wherein water swelling refers to the percent change in length of the film in a given in-plane direction from a condition of 50% relative humidity at room temperature to immediately after immersion in boiling water for 1 hour, and wherein the ratio of the in-plane conductivity of the membrane in the machine direction to the in-plane conductivity in the cross direction is in the range of 0.9 to 1.1, wherein in-plane conductivity refers to the proton conductivity of the film in the plane of the film measured by immersing the film in a boiling water bath for 1 hour, and then immediately transferring the film to deionized water and leaving the film immersed in the deionized water.

18. 20. The electrochemical cell of claim 17, wherein the electrochemical cell is a flow battery.

19. 20. The electrochemical cell of claim 18, wherein the flow battery is an all-vanadium redox flow battery.

20. The membrane has a conductivity of at least 60 (mS cm -1 ) / (10 -6 cm 2 min -1 ) ions (protons / VO 2+ 20. The electrochemical cell of claim 19, wherein the ion selectivity refers to the permeability of protons relative to the permeability of vanadyl ions through the film in a direction perpendicular to the plane of the film.

Images