Expanded multilayer cation exchange membrane

Abstract

The cation exchange membrane comprises a stretched film comprising at least two layers of a fluorinated ionomer comprising sulfonate or sulfonic acid groups. The layers have different ion exchange ratio values ​​defining one or more high ion exchange ratio layers and one or more low ion exchange ratio layers. The high ion exchange ratio layers and the low ion exchange ratio layers differ in ion exchange ratio by at least about 1. The process for making the cation exchange membrane comprises forming a film comprising at least two layers of a fluorinated ionomer comprising sulfonate or sulfonic acid groups to form a multilayer film and stretching the multilayer film. The electrochemical cell has an anode compartment and a cathode compartment and comprises a cation exchange membrane as a separator between the anode compartment and the cathode compartment, the membrane comprising a stretched film comprising at least two layers of a fluorinated ionomer comprising sulfonate or sulfonic acid groups.

Description

[Technical field]

[0001] (CROSS REFERENCE TO RELATED APPLICATIONS) This application claims priority to U.S. Provisional Patent Application No. 63 / 292,083, filed December 21, 2021, the disclosure of which is incorporated herein by reference in its entirety.

[0002] FIELD OF THEINVENTION The present invention relates to ion exchange membranes for electrochemical cells, and more particularly to expanded multi-layer cation exchange membranes for vanadium redox flow battery cells. [Background technology]

[0003] 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.

[0004] 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 the 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 only operates 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.

[0005] 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 family of vanadium redox flow batteries includes so-called "All Vanadium Redox Flow Batteries" (VRB), 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 / BrRB), 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 electric current to complete the circuit.

[0006] The V(V) ions in the VRB system and the polyhalide ions in the V / BrRB system are highly oxidizing, rapidly degrading most polymeric membranes during use and reducing their durability. As a result, potential materials for the membrane / separator are limited, which is the main obstacle to commercialization of these types of energy storage systems. Ideally, the membrane should be stable to the acidic environment of the electrolyte, 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 or polyhalide ions, high permeability to charge-carrying hydrogen ions, good mechanical properties, and be low cost. To date, it remains difficult to develop a suitable polymer system with respect to this balance of properties.

[0007] 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 with respect to resistance to acidic environments and highly oxidizing species, but there is room for further improvement in water and vanadium ion crossover resistance. High vanadium ion crossover not only reduces the coulombic efficiency, reduces capacity, and causes self-discharge of the battery, but also creates a continuous need to readjust the electrolyte concentration of the two half-cells. This undesirable capacity drop due to mixing of electroactive ions requires the overall battery to be made larger to meet the target discharge capacity when capacity is reduced. 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 a significant motivation to improve the coulombic efficiency of the cell, and the primary way to achieve this is through improved crossover resistance and improved ion selectivity of the charge carrying species relative to the electroactive species.

[0008] 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 uniaxial stretching to provide higher coulombic efficiency and longer self-discharge time, but also reduced proton conductivity.

[0009] Karpushkin et al. ("Effect of biaxial stretching on the ion-conducting properties of Nafion membranes", Mendeleev Commun., Vol. 26, pp. 117-118, 2016) disclose biaxial stretching at various stretch ratios that not only reduces vanadium permeability but also reduces the self-discharge time.

[0010] Grot (EP 0145426) describes the preparation of chloralkali membranes which are oriented in at least one planar direction by swelling or stretching. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] European Patent No. 0145426 [Non-patent literature]

[0012] [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 [Means for solving the problem]

[0013] There is a need for ion exchange membranes that have reduced vanadium crossover, improved energy efficiency, reduced self-discharge rates, and / or increased ion selectivity.

[0014] In one embodiment of the process, the cation exchange membrane comprises a stretched film comprising at least two layers of a fluorinated ionomer containing sulfonate or sulfonic acid groups. The layers have different ion exchange ratio (IXR) values ​​that define one or more high ion exchange ratio layers and one or more low ion exchange ratio layers. The high ion exchange ratio layers and the low ion exchange ratio layers differ in ion exchange ratio by at least about 1.

[0015] In another embodiment, the process produces a cation exchange membrane comprising a stretched film. The process includes forming a film comprising at least two layers of a fluorinated ionomer comprising sulfonate or sulfonic acid groups to form a multilayer film. The process also includes stretching the multilayer film. The layers have different ion exchange ratio values ​​that define one or more high ion exchange ratio layers and one or more low ion exchange ratio layers. The high ion exchange ratio layers and the low ion exchange ratio layers differ in ion exchange ratio by at least about 1.

[0016] In yet another embodiment, 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. The membrane includes a stretched film including at least two layers of a fluorinated ionomer including sulfonate or sulfonic acid groups. The layers have different ion exchange ratio values ​​that define one or more high ion exchange ratio layers and one or more low ion exchange ratio layers. The high ion exchange ratio layers and the low ion exchange ratio layers differ in ion exchange ratio by at least about 1.

[0017] 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 description of the drawings]

[0018] [Figure 1]FIG. 1 is a schematic diagram of an electrochemical cell in one embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Stretched multi-layer ion exchange membranes having low vanadium crossover, high energy efficiency, low self-discharge rate, high ion selectivity, increased mechanical strength, reduced in-plane dimensional expansion in the xy directions, or combinations thereof are provided.

[0020] In an exemplary embodiment, the cation exchange membrane comprises a stretched film comprising at least two layers of a fluorinated ionomer containing sulfonate or sulfonic acid groups. The layers have different ion exchange ratio (IXR) values ​​that define one or more high ion exchange ratio layers and one or more low ion exchange ratio layers. The high ion exchange ratio layers and the low ion exchange ratio layers differ in ion exchange ratio by at least about 1.

[0021] As used herein, a stretched film refers to a film that is stretched in at least one direction with a stretch ratio of greater than 1.0. In some embodiments, the at least one direction is the machine direction and / or the transverse direction.

[0022] As used herein, stretch ratio refers to the ratio of the stretched length of a film to the unstretched length of the film.

[0023] As used herein, 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.

[0024] As used herein, the cross direction refers to the in-plane direction of the film perpendicular to the machine direction.

[0025] As used herein, ion exchange ratio (IXR) refers to the number of carbon atoms in the ionomer backbone relative to the number of cation exchange groups. In some embodiments, the IXR of an ionomer is calculated according to the formula EW=(50×IXR)+MW scIt can be related to its equivalent weight (EW) by the formula: sc is the molecular weight of the ionomer side chain.

[0026] As used herein, equivalent weight (EW) refers to the weight of ionomer in proton form required to neutralize one equivalent of NaOH.

[0027] As used herein, sulfonate or sulfonic acid groups are 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 of the formula -SO 3 X, 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, H, CH 3 , or C 2 H 5 A preferred class of fluorinated ionomers containing sulfonate or sulfonic acid groups for one or more layers of the multilayer oriented film of the present invention includes a highly fluorinated, and most preferably perfluorinated, carbon backbone with side chains of the formula -(O-CF 2 CFR f ) a -O-CF 2 CFR ’f SO 3 X, where R f and R ’f are independently selected from F, Cl, or a perfluorinated alkyl group 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 R4 are the same or different and are H, CH3 or C 2 H 5 Preferred fluorinated ionomers containing sulfonate or sulfonic acid groups for one or more layers of the multilayer oriented film of the present invention can include, for example, the polymers disclosed in U.S. Pat. No. 3,282,875, U.S. Pat. No. 4,358,545, or U.S. Pat. No. 4,940,525. When used 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.

[0028] One preferred fluorinated ionomer containing sulfonate or sulfonic acid groups for one or more layers of the multilayer oriented film of the present invention has a perfluorocarbon backbone and a structure of the formula -O-CF 2 CF(CF 3 )-O-CF 2 CF 2 SO 3 and a side chain represented by X, where X is as defined above. When X is H, the side chain is -O-CF 2 CF(CF 3 )-O-CF 2 CF 2 SO 3 H. Fluorinated ionomers containing sulfonate or sulfonic acid groups of this type are disclosed in U.S. Pat. No. 3,282,875 and are based on tetrafluoroethylene (TFE) and perfluorinated vinyl ethers. 2 =CF-O-CF 2 CF(CF 3 )-O-CF 2 CF 2 SO 2 F, perfluoro(3,6-dioxa-4-methyl-7-octene sulfonyl fluoride) (PSEPVE), followed by hydrolysis of the sulfonyl fluoride groups to sulfonic acid groups, and conversion to the proton form if required for a particular application.

[0029] One preferred fluorinated ionomer containing sulfonate or sulfonic acid groups for one or more layers of the multilayer oriented film of the present invention of the type disclosed in U.S. Pat. Nos. 4,358,545 and 4,940,525 has the pendant chain -O-CF 2 CF 2 SO 3 X, where X is as defined above. The fluorinated ionomer containing sulfonate or sulfonic acid groups for one or more layers of the multilayer oriented film of the present disclosure comprises a mixture of TFE and a perfluorinated vinyl ether, CF 2 =CF-O-CF 2 CF 2 SO 2 F, 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-CF 2 CF 2 SO 3 It's H.

[0030] In an exemplary embodiment, the fluorinated ionomer containing sulfonate or sulfonic acid groups for one or more layers of the multilayer oriented film of the present invention is of the type available under the trade name Nafion™ (The Chemours Company FC, LLC, Wilmington, Del.).

[0031] In some embodiments, the stretched film has an apparent ion exchange ratio in the range of about 7.1 to about 25.2, alternatively about 9.1 to about 23.2, alternatively about 9.1 to about 21.2, alternatively about 11.1 to about 19.2, alternatively about 12.1 to about 19.2, alternatively about 12.1 to about 15.2, or any value, range, or subrange therebetween. As used herein, apparent ion exchange ratio refers to the layer thickness weighted average ion exchange ratio of a multilayer film based on the film thickness before stretching. In other words, the apparent ion exchange ratio is the sum of the thickness percentage multiplied by the ion exchange ratio for each of the layers.

[0032] In some embodiments, the stretched film has an apparent equivalent weight in the range of about 700 to about 1600, alternatively about 800 to about 1500, alternatively about 800 to about 1400, alternatively about 900 to about 1300, alternatively about 950 to about 1300, alternatively about 950 to about 1100, or any value, range, or subrange therebetween. As used herein, apparent ion exchange ratio refers to the layer thickness weighted average IXR of the multilayer film. As used herein, apparent EW refers to the layer thickness weighted average EW of the multilayer film based on the film thickness before stretching.

[0033] In an exemplary embodiment, a fluorinated ionomer containing sulfonate or sulfonic acid groups for one or more layers of the multilayer oriented film of the invention is prepared by copolymerization of TFE and PSEPVE, followed by hydrolysis and ion exchange to the proton form. Such a membrane has the formula -O-CF 2 CF(CF 3 )-O-CF 2 CF 2 SO 3 H and has an EW in the range of about 600 to about 1600, alternatively about 700 to about 1600, alternatively about 850 to about 1500, alternatively about 800 to about 1400, alternatively about 900 to about 1300, alternatively about 950 to about 1300, alternatively about 950 to about 1100, or any value, range, or subrange therebetween. 2 -CF(CF 3 )-O-CF 2 -CF 2 -SO 3 The IXR of the fluorinated ionomers of H, i.e., those made from copolymers of TFE and PSEPVE, can be related to EW using the equation 50 IXR+344=EW.

[0034] In an exemplary embodiment, a fluorinated ionomer containing sulfonate or sulfonic acid groups for one or more layers of the multilayer oriented film of the present invention is prepared by copolymerization of TFE and PFSVE, followed by hydrolysis and ion exchange to the proton form. Such membranes have the formula -O-CF 2CF 2 SO 3 H and has an EW in the range of about 400 to about 1600, alternatively about 530 to about 1440, alternatively about 630 to about 1340, alternatively about 630 to about 1240, alternatively about 730 to about 1140, alternatively about 780 to about 1140, alternatively about 780 to about 940, or any value, range, or subrange therebetween. Such ionomers are sometimes referred to as short side chain ionomers. 2 CF 2 SO 3 The IXR of the fluorinated ionomers of H, i.e., those formed from copolymers of TFE and PFSVE, can be related to equivalent weight using the formula 50 IXR+178=EW.

[0035] In some embodiments, the high and low ion exchange ratio layers of fluorinated ionomers containing sulfonate or sulfonic acid groups differ in ion exchange ratio by at least about 1, alternatively at least about 2, alternatively from about 1 to about 15, alternatively at least about 3, alternatively at least about 5, alternatively at least about 10, or any value, range, or subrange therebetween.

[0036] In some embodiments, the high ion exchange ratio layer and the preliminary low ion exchange ratio layer of fluorinated ionomers containing sulfonate or sulfonic acid groups differ in equivalent weight by at least about 50, alternatively at least about 100, alternatively at least about 200, or any value, range, or subrange therebetween.

[0037] In some embodiments, one or more of the low ion exchange ratio layers have an ion exchange ratio of at most about 17.2, alternatively at least about 9.1, alternatively in the range of about 9.1 to about 17.2, alternatively at most about 15.2, alternatively at most about 13.2, alternatively at most about 15.2, or any value, range, or subrange therebetween.

[0038] In some embodiments, one or more of the low ion exchange ratio layers have an equivalent weight of at most about 1200, alternatively at least about 800, alternatively in the range of about 800 to about 1200, alternatively at most about 1100, alternatively at most about 1000, alternatively at most about 900, or any value, range, or subrange therebetween.

[0039] In some embodiments, one or more of the high ion exchange ratio layers have an ion exchange ratio of at least about 11.1, alternatively up to about 23.2, alternatively in the range of about 11.1 to about 23.2, alternatively at least about 13.1, alternatively at least about 17.1, or any value, range, or subrange therebetween.

[0040] In some embodiments, one or more of the high ion exchange ratio layers have an equivalent weight of at least about 900, alternatively up to about 1500, alternatively in the range of about 900 to about 1500, alternatively at least about 1000, alternatively at least about 1200, or any value, range, or subrange therebetween.

[0041] In some embodiments, the stretched film is biaxially stretched, which as used herein refers to stretching in both the machine direction and a transverse direction perpendicular to the machine direction.

[0042] In other embodiments, the stretched film is uniaxially stretched. As used herein, uniaxially stretched refers to stretching in only a single direction, preferably the machine direction.

[0043] In some embodiments, the stretched film includes at least two high ion exchange ratio layers, hi some embodiments, the at least two high ion exchange ratio layers have comparable ion exchange ratios.

[0044] In some embodiments, the thickness of the one or more low ion exchange ratio layers is greater than or equal to the thickness of the one or more high ion exchange ratio layers.

[0045] In some embodiments, the membrane thickness after stretching ranges 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. As used herein, membrane thickness refers to the total dry thickness of the membrane after stretching.

[0046] In some embodiments, the membrane is not reinforced.

[0047] In an exemplary embodiment, the membrane is -6 cm 2 Vanadyl ion permeability in units of 1.5, alternatively less than about 1.4, alternatively less than about 1.2, alternatively less than about 1.0, alternatively less than about 0.8, alternatively less than about 0.6, or any value, range, or subrange therebetween. 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.

[0048] 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. As used herein, ion selectivity refers to the permeability of protons through the film in a direction perpendicular to the plane of the film for vanadyl ions, and is expressed as the through-plane conductivity divided by the vanadyl ion permeability. As used herein, through-plane conductivity refers to the through-plane proton conductivity of the film in a direction perpendicular to the plane of the film.

[0049] In some embodiments, a process for making a cation exchange membrane comprising a stretched film includes forming a film comprising at least two layers of a fluorinated ionomer comprising sulfonate or sulfonic acid groups to form a multilayer film, the process also including stretching the multilayer film.

[0050] In some embodiments, the forming comprises hot pressing at least two unhydrolyzed (sulfonyl fluoride form) films of fluorinated ionomers at a hot press temperature ranging from about 200° C. to about 250° C. under a pressure ranging from about 3500 to 5000 pounds of force for about 5 to 7 minutes. The individual unhydrolyzed (sulfonyl fluoride form) films may be formed by extruding a polymer film. In other embodiments, the forming comprises coextrusion of at least two unhydrolyzed (sulfonyl fluoride form) films of fluorinated ionomers containing sulfonate or sulfonic acid groups. In some embodiments, the forming comprises hydrolyzing and acid exchanging the hot pressed or coextruded multilayer film prior to stretching. In other embodiments, the multilayer film is produced by casting ionomer dispersions of different IXR ionomers, followed by a drying step.

[0051] In an exemplary embodiment, the backbone of the fluorinated ionomer containing sulfonate or sulfonic acid groups for one or more layers of the multilayer stretched film of the invention 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 process includes heating the multilayer film to a temperature within about 20° C. below the glass transition temperature of the backbone of the fluorinated ionomer, or within about 10° C. below the glass transition temperature of the backbone of the ionomer film, or above the glass transition temperature of the backbone of the ionomer film, or any value, range, or subrange therebetween. In an exemplary embodiment, the stretching is performed at a temperature greater than about 20° C. below the glass transition temperature of the backbone of the ionomer film, or greater than about 10° C. below the glass transition temperature of the backbone of the ionomer film, or any value, range, or subrange therebetween. In exemplary embodiments, stretching is carried out at a temperature ranging from about 70° C. to about 250° C., alternatively from about 75° C. to about 150° C., alternatively from about 80° C. to about 140° C., or any value, range, or subrange therebetween.

[0052] In exemplary embodiments, the stretching occurs at a rate in the range of about 1% per second (% / sec) to about 200% / sec, alternatively about 1% / sec to about 50% / sec, alternatively about 1% / sec to about 40% / sec, alternatively about 1% / sec to about 30% / sec, alternatively about 1% / sec to about 20% / sec, alternatively about 5% / sec to about 20% / sec, alternatively about 1% / sec to about 10% / sec, alternatively about 10% / sec to about 20% / sec, alternatively about 20% / sec to about 30% / sec, or any value, range, or subrange therebetween.

[0053] In some embodiments, stretching involves sequential stretching first in the machine direction and then in the transverse direction, in other embodiments stretching occurs simultaneously in the machine direction and the transverse direction, and in other embodiments stretching occurs only in the machine direction.

[0054] In an exemplary embodiment, stretching includes stretching in both the machine direction and the transverse direction, each at a stretch ratio in the range of 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.

[0055] In an exemplary embodiment, the process further comprises annealing the film after stretching. Annealing comprises heating the film to a temperature in the range of about 0° C. to about 300° C., alternatively about 25° C. to about 200° C., alternatively about 50° C. to about 200° C., alternatively about 85° C. to about 200° C., alternatively about 100° C. to about 190° C., alternatively about 125° C. to about 160° C., or any value, range, or subrange therebetween, for about 5 seconds to about 30 minutes 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.

[0056] In some embodiments, a film stretcher stretches the film. In some embodiments, the film stretcher is a biaxial film stretcher for simultaneously or sequentially stretching the film in the machine direction and / or the transverse direction. In some embodiments, the film is stretched at a temperature ranging from about 125° C. to about 150° C., alternatively about 140° C., or any value, range, or subrange therebetween. In some embodiments, the film is then annealed at a temperature ranging from about 140° C. to about 160° C., alternatively about 150° C., or any value, range, or subrange therebetween.

[0057] In some embodiments, the film stretcher stretches the film in the machine direction. The film is fed into the machine at a predetermined speed, such as 5 feet per minute. The stretching is accomplished by passing the film through two preheat rolls to heat the film, followed by a slow and fast roll to stretch the film. The slow and fast rolls provide a predetermined stretch ratio. The film can then pass through an annealing roll, 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 slow roll, about 225°F (about 107°C) for the fast roll, about 180°F (about 82°C) for the annealing roll, and about 82°F (about 28°C) for the cooling roll.

[0058] In some embodiments, the film is stretched in the transverse direction by a tenter process after being stretched in the longitudinal direction. The film is fed into a tenter oven and is tightly gripped by clips at both ends. The tenter oven includes three successive zones: preheating, stretching, and annealing. The temperature of each zone is controlled separately, 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 allowed to relax 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.

[0059] 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.

[0060] In some embodiments, 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. The membrane includes a stretched film including at least two layers of a fluorinated ionomer that includes sulfonate or sulfonic acid groups.

[0061] In some embodiments, the electrochemical cell is a redox flow battery cell, a fuel cell, a chlor-alkali cell, or a water electrolysis cell.

[0062] In some embodiments, the electrochemical cell is a redox flow battery cell. In some embodiments, the redox flow battery cell is a vanadium redox flow battery cell.

[0063] In some embodiments, the electrochemical cell is a fuel cell. In some embodiments, the fuel cell is a direct methanol fuel cell.

[0064] 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.

[0065] 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 flow.

[0066] 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.

[0067] Test Method Through-plane conductivity measurements To measure the transplane (proton) conductivity, the film was immersed in a deionized water bath at 60 °C for 6 h. The membrane was then immediately transferred to a covered container filled with the test electrolyte (2.5 M sulfuric acid) and left to soak overnight. A customized H-cell was used for the measurements. The electrical resistance was measured by electrochemical impedance spectroscopy (EIS) technique with a four-electrode setup on a BioLogic potentiostat (BioLogic Sciences Instruments, Seyssinet-Pariset, France).

[0068] The cell was first assembled without a membrane and filled with 2.5 M sulfuric acid to obtain 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 of the membrane R membrane (Ω) is the difference between the total resistance and the cell resistance.

[0069] The through-plane conductivity of the film σ T (mS / cm) was calculated using Equation 1.

[0070]

number

[0071] Vanadyl ion permeability measurement To measure vanadyl ion permeability, the film was immersed in a deionized water bath at 60 °C for 6 h. The membrane was then soaked in the test electrolyte (1.5 M MgSO in 2.5 M sulfuric acid). 4 The samples were immediately transferred to a covered container filled with 1.5 M MgSO in 2.5 M sulfuric acid electrolyte and left to soak overnight. A customized H cell was used for the measurements. One side of the cell was filled with 1.5 M MgSO in 2.5 M sulfuric acid electrolyte. 4 and the opposing compartment of the cell was filled with 1.5 M VOSO in 2.5 M sulfuric acid electrolyte. 4 The UV-Vis probe was filled with the same volume of MgSO4 Insert it into the electrolyte side and VOSO 4 VO diffused from the electrolyte 2+ The intensity of the absorption peak at 760 nm associated with the vanadyl ion was monitored. VO 2+ was calculated using Equation 2.

[0072]

number

[0073] Ion selectivity measurements Equation 3 was used to calculate ion selectivity from measurements of membrane transplane conductivity and vanadyl ion permeability.

[0074]

number

[0075] Comparative Example Four comparative examples were prepared and tested for comparison with the inventive examples. The first comparative example (C1) was a cast monolayer film of TFE / PSEPVE ionomer containing sulfonic acid groups. The film of C1 was cast from an ionomer dispersion. The second comparative example (C2) was a stretched monolayer film of TFE / PSEPVE ionomer containing sulfonic acid groups. C2 was extruded, then hydrolyzed and acid exchanged to convert to sulfonic acid groups, and then stretched. The third comparative example (C3) was a coextrusion of a first film of a first thickness (T1) of TFE / PSEPVE ionomer containing sulfonic acid groups with a first ion-exchange ratio (IXR1) and a second film of a second thickness (T2) of TFE / PSEPVE ionomer containing sulfonic acid groups with a second ion-exchange ratio (IXR2) less than the first ion-exchange ratio (IXR1). The fourth comparative example (C4) was an extruded monolayer film of TFE / PSEPVE ionomer containing sulfonic acid groups. Comparative examples C3 and C4 were hydrolyzed and acid exchanged to convert to sulfonic acid groups after extrusion. Film thickness and thickness were measured using a Mitutoyo ID-S1112EX Measurement Indicator (Mitutoyo Corporation, Kawasaki, Japan). Table 1 shows the equivalent weight, ion exchange ratio, and thickness for the layers of each comparative example. Table 1 also shows the apparent IXR (IXRA), which was calculated based on the thickness-weighted average IXR of the layers for C3.

[0076] Comparative Example C2 was biaxially stretched after hydrolysis and acid exchange with a stretch ratio in the machine direction and a stretch ratio in the transverse direction. Stretching was performed first in the machine direction and then in the transverse direction. The machine direction stretch ratio (SR) for C2 was MD ) and transverse stretch ratio (SR TD ) are listed in Table 1. The other comparative examples were unstretched.

[0077] [Table 1] N / A

[0078] The dry film thickness (TF) was measured for each comparative example and the measurements are shown in Table 4. For C2, the dry film thickness is after stretching. Each comparative example was then evaluated for through-plane conductivity and vanadyl ion permeability. Ion selectivity was calculated from the through-plane conductivity and vanadyl ion permeability values. Table 2 shows the through-plane conductivity (σ T ) and vanadyl ion permeability (P VO 2+ ) and calculated ion selectivity (IS).

[0079] [Table 2]

[0080] C1 is a conventional benchmark cast film for comparison. The stretched monolayer film of C2 shows a significant decrease in vanadyl ion permeability and only a slight decrease in through-plane conductivity compared to the cast film of C1, thus showing a significant increase in ion selectivity. The unstretched bilayer film of C3 has an even lower vanadyl ion permeability than the stretched monolayer film of C2, but also a significantly lower through-plane conductivity, and thus a lower ion selectivity than C2. The monolayer extruded film of C4 has a similar vanadyl ion permeability to the cast film of C1, but a lower through-plane conductivity than C1, and thus has the lowest ion selectivity of the four comparative examples.

[0081] Examples of the present invention Each inventive example included a first film of a first thickness (T1) of a TFE / PSEPVE ionomer containing sulfonate or sulfonic acid groups having a first ion-exchange ratio (IXR1) and a second film of a second thickness (T2) of a TFE / PSEPVE ionomer containing sulfonate or sulfonic acid groups having a second ion-exchange ratio (IXR2) less than the first ion-exchange ratio (IXR1). Inventive examples 19-22 were trilayers that also included a third film of a third thickness (T3) of a TFE / PSEPVE ionomer containing sulfonate or sulfonic acid groups and having a third ion-exchange ratio (IXR3). For the inventive examples, IXR values ​​of 10.1, 10.8, 11.5, 17.1, and 23.1 corresponded to EW values ​​of 850, 885, 920, 1200, and 1500, respectively. The films of inventive Examples 1-12, 17, and 19-22 were sized and hot pressed together. The films of inventive Examples 13-16 and 18 were melt coextruded. Table 3 shows the equivalent weight (EW), ion exchange ratio, and thickness of the layers of each inventive Example before hot pressing. Table 3 also shows the apparent IXR (IXRA) calculated based on the thickness weighted average IXR of the layers. The inventive Examples had a first film thickness ranging from 13 μm to 64 μm, a first ion exchange ratio ranging from 11.5 to 23.1, a second film thickness ranging from 64 μm to 178 μm, and a second ion exchange ratio ranging from 10.1 to 17.1. The difference between the first ion exchange ratio and the second ion exchange ratio ranged from 1.4 to 13.0.

[0082] Hot pressed examples of the invention were prepared by hot pressing two or three unhydrolyzed (sulfonyl fluoride form) films for about 5-7 minutes at hot press temperatures ranging from about 200° C. to about 250° C. under pressures ranging from about 3500-5000 pounds of force. The pressed films were then hydrolyzed and acid exchanged prior to stretching. The coextruded films were hydrolyzed and acid exchanged after coextrusion but prior to stretching.

[0083] [Table 3] N / A

[0084] After hydrolysis and ion exchange after film formation, each multilayer film, except for inventive example 17, was biaxially stretched with a longitudinal stretch ratio and a transverse stretch ratio. In the inventive examples, the multilayer films were stretched on a laboratory-scale biaxial Bruckner Karo VI stretcher (Bruckner Maschinenbau GmbH, Siegsdorf, Germany). Films were stretched at 140°C for all inventive examples, except for inventive examples 13 and 14, which were stretched at 125°C and 150°C, respectively. Films were stretched at a stretch rate of 5% / s for all inventive examples, except for inventive examples 4 and 9, which were stretched at 1% / s and 10% / s, respectively. All films were then annealed at a temperature 10°C higher than their respective stretch temperatures. Inventive example 17 was stretched only in the transverse direction.

[0085] The longitudinal stretch ratio (SR MD ) and transverse stretch ratio (SR TD ) are listed in Table 3. The inventive examples had stretch ratios in the machine direction ranging from 1.0 to 3.5 and in the transverse direction ranging from 1.7 to 3.5. The machine direction stretching and the transverse direction stretching were performed simultaneously for all biaxially stretched inventive examples, except for inventive example 11, which was stretched first in the machine direction and then in the transverse direction.

[0086] After stretching, the dry thickness of each of the inventive examples, except for inventive examples 14-16, was measured. The measured dry film thickness (TF) is shown in Table 4. The measured dry thicknesses were in the range of 24-112 μm. The thicknesses of inventive examples 14-16 were not measured until after the vanadium permeability test and were 45 μm, 79 μm, and 36 μm, respectively. Each of the inventive examples was then evaluated for through-plane conductivity and vanadyl ion permeability. The ion selectivity was calculated from the through-plane conductivity and vanadyl ion permeability values. Table 4 shows the through-plane conductivity (σ T ) and vanadyl ion permeability (P VO 2+) and calculated ion selectivity (IS).

[0087] All of the inventive examples had lower vanadyl ion permeability and higher ion selectivity than the cast film comparative example C1 and the extruded monolayer film comparative example C4. The through-plane conductivity of the inventive examples ranged from 25.3 to 131.3 mS / cm. The vanadyl ion permeability of the inventive examples ranged from 0.101 to 1.364 10 -6 cm 2 The ion selectivity of the examples of the present invention was in the range of 55 to 347 10 6 mS min / cm 3 was within the range.

[0088] [Table 4] N / D=Not decided

[0089] All of the above references are incorporated herein by reference.

[0090] Although the present invention has been described with reference to preferred embodiments, those skilled in the art will recognize that various changes can be made without departing from the scope of the invention and that equivalents can be substituted for its elements. 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 the invention is intended to include all embodiments falling within the scope of the appended claims.

Claims

1. A cation exchange membrane comprising a stretched film including at least two layers of a fluorinated ionomer containing sulfonate or sulfonic acid groups, the layers having different ion exchange ratio values ​​defining one or more high ion exchange ratio layers and one or more low ion exchange ratio layers, the high ion exchange ratio layers and the low ion exchange ratio layers differing in ion exchange ratio by at least one.

2. 10. The membrane of claim 1, wherein the stretched film has an apparent ion exchange ratio in the range of 7.1 to 25.

2.

3. 3. The membrane of claim 2, wherein the apparent ion exchange ratio is in the range of 9.1 to 23.

2.

4. 2. The membrane of claim 1, wherein the high ion exchange ratio layer of fluorinated ionomer and the low ion exchange ratio layer of fluorinated ionomer comprise sulfonate or sulfonic acid groups whose ion exchange ratios differ by at least two.

5. 10. The membrane of claim 1, wherein the one or more low ion exchange ratio layers have an ion exchange ratio of at most 17.

2.

6. 10. The membrane of claim 1, wherein the one or more high ion exchange ratio layers have an ion exchange ratio of at least 11.

1.

7. The membrane of claim 1 , wherein the stretched film is biaxially stretched.

8. The membrane of claim 1 , wherein the stretched film comprises at least two high ion exchange ratio layers.

9. 9. The membrane of claim 8, wherein the at least two high ion exchange ratio layers have equivalent ion exchange ratios.

10. The membrane of claim 1 , wherein the thickness of the one or more low ion exchange ratio layers is equal to or greater than the thickness of the one or more high ion exchange ratio layers.

11. The film of claim 1, having a thickness after stretching in the range of 10 μm to 200 μm.

12. The membrane of claim 1 , wherein the membrane is unreinforced.

13. 1. A process for producing a cation exchange membrane comprising a stretched film, the process comprising: forming a film comprising at least two layers of a fluorinated ionomer containing sulfonate or sulfonic acid groups to form a multilayer film; stretching the multilayer film; Including, The layers have different ion exchange ratio values ​​defining one or more high ion exchange ratio layers and one or more low ion exchange ratio layers, the high ion exchange ratio layers and the low ion exchange ratio layers differing in ion exchange ratio by at least one.

14. 14. The process of claim 13, further comprising heating the multilayer film to a temperature within 20°C below the glass transition temperature of the fluorinated ionomer.

15. The process of claim 13, wherein the stretching is performed at a rate of from 1% / sec to 200% / sec.

16. 14. The process of claim 13, wherein the apparent ion exchange ratio of the layer of the fluorinated ionomer containing sulfonate or sulfonic acid groups is from 7.1 to 25.

2.

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 stretched film including at least two layers of a fluorinated ionomer containing sulfonate or sulfonic acid groups, the layers having different ion exchange ratio values ​​defining one or more high ion exchange ratio layers and one or more low ion exchange ratio layers, the high ion exchange ratio layers and the low ion exchange ratio layers differing in ion exchange ratio by at least one.

18. 18. The electrochemical cell of claim 17, wherein the electrochemical cell is selected from the group consisting of a redox flow battery cell, a fuel cell, a chlor-alkali cell, and a water electrolysis cell.

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

20. 20. The electrochemical cell of claim 19, wherein the redox flow battery cell is a vanadium redox flow battery cell.

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