Multilayer cation exchange chlor-alkali membrane

Abstract

To provide a multilayer cation exchange chloralkali membrane capable of reducing a battery voltage without spoiling current efficiency when used in a chloralkali process.SOLUTION: A multilayer cation exchange chloralkali membrane for use in a chloralkali process includes: a carboxylic acid layer including a fluorinated ionomer containing a carboxylic acid group on one side of the membrane; an external sulfonic acid layer including a fluorinated ionomer containing a sulfonic acid group on the surface of the membrane opposite the carboxylic acid layer; and an internal sulfonic acid layer including a fluorinated ionomer containing a sulfonic acid group between the carboxylic acid layer and the external sulfonic acid layer. The external sulfonic acid layer has a resistivity greater than about 68.1 Ω-cm. The internal sulfonic acid layer has a resistivity less than about 60.3 Ω-cm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a multilayer cation exchange membrane for chlor-alkali electrolysis. [Background technology]

[0002] Chlorine and alkali metal hydroxides, typically sodium or potassium hydroxide, are commercially produced in an electrolytic process from aqueous sodium or potassium chloride solutions, i.e., the chlor-alkali process. The state-of-the-art in the chlor-alkali process employs membranes to separate the anode and cathode compartments made from fluorinated ionomers containing cation-exchange ionic groups, i.e., cation exchange membranes.

[0003] Cation exchange membranes for chlor-alkali processes often employ a layer of fluorinated ionomer containing carboxylic acid groups facing the cathode compartment. The carboxylic acid layer typically has a coating of inorganic particles, such as zirconium dioxide, optionally in a polymer binder, to provide bubble release. The other side of the chlor-alkali membrane, facing the anode compartment, is a layer of fluorinated ionomer containing sulfonic acid groups, typically with an inorganic particle coating similar to that facing the cathode compartment. Membranes often contain a woven fabric reinforcement made from a fluoropolymer, such as polytetrafluoroethylene (PTFE), or a combination of fluoropolymer fibers and "sacrificial" polymer fibers (such as polyethylene terephthalate or polyvinyl alcohol) that dissolve in alkali metal hydroxide solutions. The reinforcing fabric is typically at least partially embedded in the membrane.

[0004] In a chlor-alkali battery, the membrane provides a physical barrier between the alkali metal hydroxide solution and hydrogen present in the cathode compartment and the chlorine and alkali metal chloride solution present in the anode compartment. Additionally, another primary function of the ionomer in a chlor-alkali membrane is the transport of alkali metal ions through the membrane from the anode to the cathode. The carboxylic acid ionomer also provides selectivity by reducing the migration of hydroxide ions from the cathode to the anode, improving the current efficiency of the cell over the use of sulfonic acid ionomer alone. The sulfonic acid ionomer provides more effective ion transport, i.e., lower resistance, than the carboxylic acid layer, thereby reducing the cell voltage.

[0005] The capacity of carboxylic and sulfonic acid ionomers to transport alkali metal ions is often expressed as equivalent weight or ion exchange ratio (IXR). Equivalent weight (EW) is defined as the weight of the ionomer in acid form required to neutralize one equivalent of NaOH. Ion exchange ratio (IXR) is defined as the number of carbon atoms in the polymer backbone relative to the cation exchange groups. Resistivity is another useful measure for describing the ability to transport alkali metal ions, particularly sulfonic acid ionomers. Resistivity is measured using a set of specific conditions, such as those described in the test methods of this application, and is a measure of the intrinsic properties of fluoroionomers with respect to their ability to transport alkali metal ions.

[0006] In an attempt to optimize the operating efficiency of a chlor-alkali battery, the EW or IXR of the carboxylic acid and sulfonic acid ionomers is selected to achieve a balance between current efficiency and battery voltage characteristics. A bilayer membrane with a carboxylic acid ionomer having an EW of 1050 and an IXR of 14.8 and a sulfonic acid ionomer having an EW of 920 and an IXR of 11.5 provides a good balance of properties. However, further reduction in the EW or IXR of the sulfonic acid polymer to further reduce battery voltage results in a decrease in current efficiency.

[0007] Chlor-alkali membranes are known that have a layer of carboxylic acid ionomer and two layers of sulfonic acid ionomers with different ion exchange capacities. For example, U.S. Patent Application Publication No. 2017 / 0218526 discloses a membrane comprising a layer of carboxylic acid ionomer and at least two layers of sulfonic acid ionomer. One of the sulfonic acid ionomer layers is adjacent to the carboxylic acid layer, and the other is not adjacent to the carboxylic acid layer. The membrane also contains a reinforcing material. The EW of the sulfonic acid layer adjacent to the carboxylic acid layer is higher than the EW of the sulfonic acid layer not adjacent to the carboxylic acid layer. (U.S. Patent Application Publication No. 2017 / 0218526 does not describe EW, but instead uses the term "ion exchange capacity" in meq / g, from which EW can be calculated by dividing 1000 by ion exchange capacity. U.S. Patent Application Publication No. 2017 / 0218526 uses a transmission IR technique calibrated against a known standard to determine ion exchange capacity.) U.S. Patent Application Publication No. 2017 / 0218526 describes the use of a three-layer structure to prevent delamination between the carboxylic acid layer and the sulfonic acid layer adjacent to it, which can occur when the sulfonic acid layer has a low EW. However, the three-layer structure of U.S. Patent Application Publication No. 2017 / 0218526 does not result in a significant voltage drop compared to a bilayer membrane with an optimized EW. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] U.S. Patent Application Publication No. 2017 / 0218526 Summary of the Invention [Means for solving the problem]

[0009] The present invention provides a multilayer cation exchange membrane for use in a chlor-alkali process, the multilayer cation exchange membrane comprising: a carboxylic acid layer on one surface of the membrane comprising a fluorinated ionomer containing carboxylic acid groups; an outer sulfonic acid layer on the surface of the membrane opposite the carboxylic acid layer comprising a fluorinated ionomer containing sulfonic acid groups; and an inner sulfonic acid layer between the carboxylic acid layer and the outer sulfonic acid layer comprising a fluorinated ionomer containing sulfonic acid groups, the outer sulfonic acid layer having an ion exchange ratio greater than about 11.3 and the inner sulfonic acid layer having an ion exchange ratio less than about 11.

[0010] The multilayer cation exchange membranes according to the present invention, when used in a chlor-alkali process, advantageously provide a reduction in cell voltage without compromising current efficiency.

[0011] Preferably, the inner sulfonic acid layer has an ion exchange ratio that is at least about 0.6 less than the ion exchange ratio of the outer sulfonic acid layer.

[0012] Preferably, the carboxylic acid layer has an IXR of about 13.8 to about 16.

[0013] Preferably, the outer sulfonic acid layer has an ion exchange ratio of greater than about 11.5, more preferably from about 11.3 to about 17.5, even more preferably from about 11.3 to about 15.5, even more preferably from about 11.3 to about 13.5, and most preferably from about 11.3 to about 12.4.

[0014] Preferably, the inner sulfonic acid layer has an ion exchange ratio of less than about 10.9, more preferably from about 9.3 to about 11, more preferably from about 10 to about 11, and even more preferably from about 10 to about 10.9.

[0015] Preferably, the membrane according to the present invention further comprises a woven fabric reinforcement, preferably at least partially embedded in the membrane. In a preferred embodiment, the woven fabric comprises fluoropolymer yarns having a denier of less than about 100.

[0016] Preferably, the inner sulfonic acid layer has a thickness of at least about 40 micrometers, more preferably from about 50 micrometers to about 200 micrometers, and most preferably from about 60 micrometers to about 100 micrometers. Membranes according to the present invention preferably have an outer sulfonic acid layer with a thickness of less than about 30 micrometers, more preferably from about 5 micrometers to about 30 micrometers, and most preferably from about 7 micrometers to about 25 micrometers.

[0017] According to another embodiment of the present invention, there is provided a multi-layer cation exchange membrane for use in a chlor-alkali process, the multi-layer cation exchange membrane comprising: a carboxylic acid layer comprising a fluorinated ionomer containing carboxylic acid groups on one surface of the membrane, an outer sulfonic acid layer comprising a fluorinated ionomer containing sulfonic acid groups on the surface of the membrane opposite the carboxylic acid layer, and an inner sulfonic acid layer between the carboxylic acid layer and the outer sulfonic acid layer comprising a fluorinated ionomer containing sulfonic acid groups, wherein the outer sulfonic acid layer has a resistivity greater than about 68.1 Ω cm and the inner sulfonic acid layer has a resistivity less than about 60.3 Ω cm.

[0018] Preferably, the inner sulfonic acid layer of the membrane has a resistivity that is at least about 15.5 Ω·cm less than the resistivity of the outer sulfonic acid layer.

[0019] Preferably, the carboxylic acid layer has an IXR of about 13.8 to about 16.

[0020] Preferably, the outer sulfonic acid layer has a resistivity of greater than about 73.2 Ω·cm, more preferably from about 68.1 Ω·cm to about 186.3 Ω·cm, even more preferably from about 68.1 Ω·cm to about 155.4 Ω·cm, still more preferably from about 68.1 Ω·cm to about 118.3 Ω·cm, and most preferably from about 68.1 Ω·cm to about 78.1 Ω·cm.

[0021] Preferably, the inner sulfonic acid layer has a resistivity of less than about 57.7 Ω·cm, more preferably from about 10.7 Ω·cm to about 60.3 Ω·cm, even more preferably from about 32.3 Ω·cm to about 60.3 Ω·cm, and most preferably from about 32.3 Ω·cm to about 57.7 Ω·cm.

[0022] Preferably, the membranes according to the present invention comprise a woven fabric reinforcement, more preferably a woven fabric reinforcement at least partially embedded in the membrane. Preferably, the woven fabric comprises fluoropolymer yarns having a denier of less than about 100.

[0023] In preferred membranes according to the present invention, the inner sulfonic acid layer has a thickness of at least about 40 micrometers, more preferably from about 50 micrometers to about 200 micrometers, and most preferably from about 60 micrometers to about 100 micrometers.

[0024] Preferably, the outer sulfonic acid layer has a thickness of less than about 30 micrometers, more preferably from about 5 micrometers to about 30 micrometers, and most preferably from about 7 micrometers to about 25 micrometers.

[0025] According to another embodiment of the present invention, there is provided a multilayer cation exchange membrane for use in a chlor-alkali process, the multilayer cation exchange membrane comprising: a carboxylic acid layer comprising a fluorinated ionomer containing carboxylic acid groups on one surface of the membrane; an outer sulfonic acid layer comprising a fluorinated ionomer containing sulfonic acid groups on the surface of the membrane opposite the carboxylic acid layer; and first and second inner sulfonic acid layers comprising a fluorinated ionomer containing sulfonic acid groups between the carboxylic acid layer and the outer sulfonic acid layer, the first inner sulfonic acid layer being between the carboxylic acid layer and the second inner sulfonic acid layer and the second inner sulfonic acid layer being between the first and outer sulfonic acid layers, the outer sulfonic acid layer having an ion exchange ratio greater than about 11.3, the first inner sulfonic acid layer having an ion exchange ratio greater than about 11.3, and the second inner sulfonic acid layer having an ion exchange ratio less than about 11.

[0026] Preferably, the second, inner sulfonic acid layer has an ion exchange ratio that is at least about 0.6 less than the ion exchange ratio of the outer sulfonic acid layer.

[0027] Preferably, the first inner sulfonic acid layer has an ion exchange ratio that is at least about 0.6 greater than the ion exchange ratio of the second inner sulfonic acid layer.

[0028] Preferably, the carboxylic acid layer has an ion exchange ratio of about 13.8 to about 16.

[0029] Preferably, the first inner sulfonic acid layer has an ion exchange ratio that differs from the ion exchange ratio of the carboxylic acid layer by no more than about 3.3.

[0030] According to another embodiment of the present invention, there is provided a multilayer cation exchange membrane for use in a chlor-alkali process, the multilayer cation exchange membrane comprising: a carboxylic acid layer comprising a fluorinated ionomer containing carboxylic acid groups on one surface of the membrane; an outer sulfonic acid layer comprising a fluorinated ionomer containing sulfonic acid groups on a surface of the membrane opposite the carboxylic acid layer; and first and second inner sulfonic acid layers comprising a fluorinated ionomer containing sulfonic acid groups between the carboxylic acid layer and the outer sulfonic acid layer, the first inner sulfonic acid layer being between the carboxylic acid layer and the second inner sulfonic acid layer and the second inner sulfonic acid layer being between the first and outer sulfonic acid layers, the outer sulfonic acid layer having a resistivity greater than about 68.1 Ω cm, the first inner sulfonic acid layer having a resistivity greater than about 68.1 Ω cm, and the second inner sulfonic acid layer having a resistivity less than 60.3 Ω cm.

[0031] Preferably, the second inner sulfonic acid layer has a resistivity that is at least about 15.5 Ω·cm less than the resistivity of the outer sulfonic acid layer.

[0032] Preferably, the first inner sulfonic acid layer has a resistivity that is at least about 15.5 Ω·cm greater than the resistivity of the second inner sulfonic acid layer.

[0033] Preferably, the carboxylic acid layer has an IXR of about 13.8 to about 16.

[0034] Preferably, the first inner sulfonic acid layer has an ion exchange ratio that differs from the ion exchange ratio of the carboxylic acid layer by no more than about 3.3.

[0035] The multilayer cation exchange membranes according to the present invention, when used in a chlor-alkali process, advantageously provide a reduction in cell voltage without compromising current efficiency. [Brief explanation of the drawings]

[0036] [Figure 1] 1 is a schematic partial cross-sectional view of one embodiment of a multi-layer cation exchange membrane according to the present invention. [Figure 2] 2 is a schematic partial cross-sectional view of another embodiment of a multilayer cation exchange membrane according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0037] Fluorinated Ionomer The membranes of the present invention use fluorinated ionomers. The term "fluorinated ionomer" refers to a polymer that is at least partially fluorinated and contains ionic groups capable of ion exchange, i.e., cation exchange in the chlor-alkali process. Preferably, the ionomer is "highly fluorinated," meaning that at least 90% of the total number of monovalent atoms in the polymer are fluorine atoms. Most preferably, the ionomer is perfluorinated.

[0038] Preferably, the fluorinated ionomer comprises a polymer backbone with recurring side chains attached thereto, the side chains carrying cation exchange groups. Possible fluorinated ionomers include homopolymers or copolymers of two or more monomers. Copolymers are typically formed from one monomer, which is a non-functional monomer and provides the carbon atoms of the polymer backbone. A second monomer provides both carbon atoms of the polymer backbone and also contributes side chains carrying cation exchange groups or cation exchange group precursors, which can then be hydrolyzed to form functional groups. For example, a copolymer of a first fluorinated olefin monomer and a second fluorinated vinyl monomer has side chains containing cation exchange groups or precursors. The first monomer can also have side chains that do not interfere with the ion exchange function of the functional group. Possible first monomers include tetrafluoroethylene (TFE), hexafluoropropylene, vinyl fluoride, vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro(alkyl vinyl ether), and mixtures thereof. Possible second monomers include various fluorinated vinyl ethers with desired side chains containing functional groups or precursor groups.If desired, additional monomers can also be incorporated into these polymers.For example, two or more first fluorinated olefin monomers can be used, and similarly, two or more second monomers can be used.

[0039] The term "carboxylic acid group" is intended to refer to either a carboxylic acid group or a salt of a carboxylic acid, preferably an alkali metal or ammonium salt. Fluorinated ionomers containing carboxylic acid groups are referred to in this application as "carboxylic acid ionomers." Preferred functional groups are those of the formula -COX, 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 4are the same or different and are H, CH3, or C2H5. When used in a chlor-alkali process, the carboxylic acid groups are in the alkali metal form, e.g., sodium or potassium, corresponding to the salt being electrolyzed. A preferred class of polymers for use in the present invention includes a highly fluorinated, most preferably perfluorinated, carbon backbone with recurring side chains attached to the backbone, the side chains carrying carboxylic acid functionality. This type of ionomer is disclosed in U.S. Pat. No. 4,552,631 and preferably has the side chain -O-CF2CF(CF3)-O-CF2CF2CO2X. This ionomer can be made by copolymerizing tetrafluoroethylene (TFE), the perfluorinated vinyl ether CF2=CF-O-CF2CF(CF3)-O-CF2CF2CO2CH3, and the methyl ester of perfluoro(4,7-dioxa-5-methyl-8-nonenecarboxylic acid) (PDMNM), followed by hydrolysis to convert the methyl carboxylic acid groups to carboxylic acid groups. Another preferred carboxylic acid ionomer has the side chain -O-CFCFCFCOX and can be made by copolymerization of tetrafluoroethylene (TFE) with the perfluorinated vinyl ether CF=CF-O-CFCFCFCOCH. While other esters can be used to make films or bifilms, the methyl ester is preferred because it is sufficiently stable during typical extrusion conditions.

[0040] 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. Fluorinated ionomers containing sulfonic acid groups are referred to in this application as "sulfonic acid ionomers." Preferred functional groups are those of 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 4are the same or different and are H, CH3 or C2H5. When used in a chlor-alkali process, the sulfonic acid groups are in the alkali metal form, e.g., sodium or potassium, corresponding to the salt being electrolyzed. A preferred class of polymers for use in the present invention includes a highly fluorinated, most preferably perfluorinated, carbon backbone, with side chains of the formula -(O-CF2CFR f ) a -O-CF2CFR' f SO3X [wherein, 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 R 4are the same or different and are H, CH, or C H . Preferred polymers include those disclosed in U.S. Pat. Nos. 3,282,875, 4,358,545, and 4,940,525. One preferred polymer comprises a perfluorocarbon backbone and side chains represented by the formula -O-CFCF(CF)-O-CFCFSOX, where X is as defined above. This type of polymer is disclosed in U.S. Pat. No. 3,282,875 and can be prepared by copolymerizing tetrafluoroethylene (TFE) and the perfluorinated vinyl ether CF=CF-O-CFCF(CF)-O-CFCFSOF, perfluoro(3,6-dioxa-4-7-methyl-octene sulfonyl fluoride) (PDMOF), followed by hydrolysis of the sulfonyl fluoride groups to convert them to sulfonic acid groups. One preferred polymer of the type disclosed in U.S. Patent Nos. 4,358,545 and 4,940,525 has the side chain —O—CFCFSOX, where X is as defined above. This polymer can be made by copolymerizing tetrafluoroethylene (TFE) and the perfluorinated vinyl ether CF═CF—O—CFCFSOF, perfluoro(3-oxa-4-pentenesulfonyl fluoride) (POPF), followed by hydrolysis.

[0041] For the above-mentioned types of fluorinated ionomers, the capacity for cation exchange capacity is often expressed in equivalent weight (EW). Equivalent weight (EW) is defined as the weight of the ionomer in acid form required to neutralize one equivalent of NaOH. However, since fluorinated ionomers have side chains with different chemical structures and chain lengths, the equivalent weight value is ionomer-specific and is not a desirable measure of chlor-alkali membrane performance for different fluorinated ionomers with different side chains, especially considering the selectivity of carboxylic acid ionomers, i.e., their resistance to hydroxyl group crossover.

[0042] The "ion exchange ratio," or "IXR," is defined as the number of carbon atoms in the polymer backbone relative to the cation exchange group and is therefore a desirable value for describing or comparing the ion exchange capacity of different fluorinated ionomers, particularly carboxylic acid ionomers. In the case of a sulfonic acid polymer, where the polymer comprises a perfluorocarbon backbone and the side chains are -O-CF2-CF(CF3)-O-CF2-CF2-SO3X, as disclosed in U.S. Pat. No. 3,282,875, the IXR of this polymer can be related to its equivalent weight using the following formula: 50IXR + 344 = EW. While roughly the same IXR range is useful for the sulfonic acid polymers disclosed in U.S. Pat. Nos. 4,358,545 and 4,940,525, the equivalent weight corresponding to that IXR range is lower, i.e., provides shorter side chains, due to the lower molecular weight of the monomer units containing the cation exchange group. The IXR of this polymer can be related to its equivalent weight using the following formula: 50IXR + 178 = EW. For a carboxylic acid polymer having side chains -O-CF2CF(CF3)-O-CF2CF2CO2X, the IXR of the polymer can be related to equivalent weight using the following formula: 50 IXR + 308 = EW. For a carboxylic acid polymer having side chains -O-CF2CF2CF2CO2X, the IXR of the polymer can be related to equivalent weight using the following formula: 50 IXR + 192 = EW.

[0043] IXR is used in this application to describe either hydrolyzed ionomers containing functional groups in the acid or salt form, or non-hydrolyzed polymers containing precursor groups that are subsequently converted to functional groups during membrane fabrication, i.e., sulfonic acid ionomers, typically in the form of sulfonyl chlorides or sulfonyl fluorides, and carboxylic acid ionomers, typically in the form of methyl esters.

[0044] Reinforcement material The membranes of the present invention may be unreinforced or reinforced, but it is preferred to incorporate a reinforcement material into the membrane for dimensional stability and greater tear resistance. For this purpose, fluoropolymer reinforcements are preferably used, most preferably perfluoropolymer reinforcements. Perhalogenated polymers such as polychlorotrifluoroethylene may also be used, but perfluoropolymer reinforcements are preferred due to their better resistance to the chemicals in chlor-alkali batteries. Suitable perfluoropolymers include polytetrafluoroethylene or melt-processable copolymers of tetrafluoroethylene and hexafluoropropylene and / or tetrafluoroethylene and perfluoro(propyl vinyl ether). While porous fluoropolymer sheets or fluoropolymer fibrils or staple fibers may be used as reinforcements, woven fabric reinforcements are preferred. Fluoropolymer fibers may be woven into fabrics using various weaves, such as plain weave, basket weave, and leno weave. Relatively coarse weaves are preferred because coarse fabrics have lower membrane resistance. The preferred center-to-center fiber spacing is in the range of about 200 to about 500 micrometers.

[0045] The fluoropolymer fibers used in the woven fabric may be in the form of monofilament or multifilament yarns. The monofilaments may be circular or have a special cross-section. An oval cross-section, when properly oriented in the membrane, can provide more reinforcement with a thinner overall membrane. Some woven fabrics also desirably include sacrificial yarns with fluoropolymer fibers such as polyethylene terephthalate or polyvinyl alcohol soluble in alkali metal hydroxide solution. Various fiber deniers and fabric weights can be used. Preferred fluoropolymer fibers have a denier of less than about 100, more preferably less than about 90, and most preferably less than about 70. Deniers as low as about 5 can be used, but preferably are at least about 20. The fabrics used can be calendered to reduce their thickness before lamination.

[0046] The most preferred fluoropolymer fiber for use in the woven reinforcement is expanded polytetrafluoroethylene (ePTFE) monofilament. Suitable monofilaments are commercially available from WL Gore & Associates, Inc. (Newark, DE 19711).

[0047] Membrane structure Referring to Figure 1, a preferred multilayer cation exchange membrane 10 according to the present invention is shown in a schematic partial cross-sectional view. Membrane 10 includes a carboxylic acid layer 12 of a carboxylic acid ionomer on one side of the membrane for facing the cathode compartment of a chlor-alkali battery (not shown). Membrane 10 includes an outer sulfonic acid layer 14 of a sulfonic acid ionomer on the side of the membrane opposite the carboxylic acid layer for facing the anode compartment of a chlor-alkali battery (not shown). Between carboxylic acid layer 12 and the outer sulfonic acid layer, membrane 10 includes an inner sulfonic acid layer 16.

[0048] Membrane 10 includes a woven reinforcement material comprising expanded polytetrafluoroethylene (ePTFE) fiber monofilaments 18 and sacrificial multifilament yarns including polyethylene terephthalate (PET) filaments 20. The ePTFE monofilaments 18 and PET filaments 20 are at least partially embedded in the membrane for the purpose of increasing membrane strength. It will be understood that the ePTFE monofilaments 18 and PET filaments 20 of the woven reinforcement material are shown primarily embedded in the inner sulfonic acid layer. However, the woven reinforcement material can be embedded in any of layers 12, 14, or 16, or in the membrane, or at the interface of an adjacent layer, so as to be partially embedded in two or more layers. Preferably, the woven reinforcement material is at least partially embedded in the membrane. For example, the woven reinforcement material can be suitably embedded in one or both of the outer sulfonic acid layer 14 and the inner sulfonic acid layer 16.

[0049] In accordance with the present invention, the carboxylic acid layer 12 of membrane 10 has an IXR of about 13.8 to about 16.0. This range has been found to be desirable for achieving high current efficiencies, e.g., ≥96%, in chlor-alkali batteries. IXR ranges higher than about 16.0 generally result in increased cell voltage, which cannot be compensated for by using more conductive, i.e., lower IXR, sulfonic acid ionomers in the other layers 14 and 16. On the other hand, IXR ranges for the carboxylic acid layer lower than about 13.8 generally do not provide optimal selectivity and result in decreased current efficiency.

[0050] In accordance with the present invention, the outer sulfonic acid layer 14 of membrane 10 has an ion exchange ratio greater than about 11.3 (resistivity greater than about 68.1 Ω·cm), and the inner sulfonic acid layer 16 has an ion exchange ratio less than about 11 (resistivity less than about 60.3 Ω·cm). Preferably, the inner sulfonic acid layer 16 has an ion exchange ratio at least about 0.6 less than that of the outer sulfonic acid layer 14. Preferably, the inner sulfonic acid layer has a resistivity at least about 15.5 Ω·cm less than that of the outer sulfonic acid layer.

[0051] In a preferred embodiment of the present invention, the external sulfonic acid layer 14 has an ion exchange ratio of greater than about 11.5 (resistivity greater than about 73.2 Ω cm). Preferably, the external sulfonic acid layer 14 has an ion exchange ratio of about 11.3 to about 17.5 (resistivity of about 68.1 Ω cm to about 186.3 Ω cm), more preferably an ion exchange ratio of about 11.3 to about 15.5 (resistivity of about 68.1 Ω cm to about 155.4 Ω cm), even more preferably an ion exchange ratio of about 11.3 to about 13.5 (resistivity of about 68.1 Ω cm to about 118.3 Ω cm), and most preferably an ion exchange ratio of about 11.3 to about 12.4 (resistivity of about 68.1 Ω cm to about 94.6 Ω cm).

[0052] In a preferred embodiment of the present invention, the internal sulfonic acid layer 16 has an ion exchange ratio of less than about 10.9 (resistivity less than about 57.7 Ω cm). Preferably, the internal sulfonic acid layer 16 has an ion exchange ratio of about 9.3 to about 11 (resistivity of about 10.7 Ω cm to about 60.3 Ω cm), more preferably about 10 to about 11 (resistivity of about 32.3 Ω cm to about 60.3 Ω cm), and most preferably about 10 to about 10.9 (resistivity of about 32.3 Ω cm to about 57.7 Ω cm).

[0053] In a preferred form of membrane 10 according to the present invention, inner sulfonic acid layer 16 has a thickness of at least about 40 micrometers. Preferably, inner sulfonic acid layer 16 has a thickness of from about 50 micrometers to about 200 micrometers, more preferably from about 60 micrometers to about 100 micrometers.

[0054] In another preferred form of the invention, the outer sulfonic acid layer 16 has a thickness of less than about 30 micrometers, preferably from 5 micrometers to about 30 micrometers, and most preferably from about 7 micrometers to about 25 micrometers.

[0055] While not intending to limit the present invention to any theory or mode of operation, it has been discovered that an outer sulfonic acid layer 14 having a higher IXR than the inner layer 16 allows the present invention to take advantage of the reduced alkali metal ion tolerance and reduced cell voltage provided by the lower IXR inner sulfonic acid layer 16 without compromising current efficiency.

[0056] Referring to FIG. 2, another preferred multilayer cation exchange membrane 110 according to the present invention is shown in a schematic, partial cross-sectional view. The membrane 110 includes a carboxylic acid layer 112 on one side of the membrane facing the cathode compartment of a chlor-alkali battery (not shown). The multilayer membrane 110 includes an external sulfonic acid layer 114 on the side of the membrane opposite the carboxylic acid layer 112. Between the carboxylic acid layer 112 and the external sulfonic acid layer 114, the membrane 110 includes a first internal sulfonic acid layer 117 and a second internal sulfonic acid layer 116 comprising a sulfonic acid ionomer. The first internal sulfonic acid layer 117 is between the carboxylic acid layer 112 and the second internal sulfonic acid layer 116, and the second internal sulfonic acid layer 116 is between the first internal sulfonic acid layer 117 and the external sulfonic acid layer 114.

[0057] 1, the multilayer membrane 110 preferably includes a woven reinforcement material comprising expanded polytetrafluoroethylene (ePTFE) fiber monofilaments 118 and sacrificial multifilament yarns including polyethylene terephthalate (PET) filaments 120. The ePTFE monofilaments 118 and PET filaments 120 are at least partially embedded in the membrane for the purpose of enhancing membrane strength, and are embedded in the membrane 110 in locations similar to those described above for the multilayer membrane 10 shown in FIG.

[0058] In the multilayer membrane 110 according to the present invention shown in Figure 2, the outer sulfonic acid layer 114 has an ion exchange ratio greater than about 11.3 (resistivity greater than about 68.1 Ω·cm), the first inner sulfonic acid layer 117 has an ion exchange ratio greater than about 11.3 (resistivity greater than about 68.1 Ω·cm), and the second inner sulfonic acid layer 116 has an ion exchange ratio less than about 11 (resistivity less than about 60.3 Ω·cm). Preferably, the second inner sulfonic acid layer 116 has an ion exchange ratio at least about 0.6 less than that of the outer sulfonic acid layer 114. Preferably, the second inner sulfonic acid layer has a resistivity at least about 15.5 Ω·cm less than that of the outer sulfonic acid layer.

[0059] In accordance with a preferred embodiment of the present invention, first inner sulfonic acid layer 117 has an ion exchange ratio that is at least about 0.6 greater than the ion exchange ratio of second inner sulfonic acid layer 116. Preferably, first inner sulfonic acid layer has a resistivity that is at least about 15.5 Ω·cm greater than the resistivity of second inner sulfonic acid layer.

[0060] In membrane 110 of FIG. 2, the carboxylic acid layer preferably has an ion exchange ratio of about 13.8 to about 16.

[0061] It is also preferred that first inner sulfonic acid layer 117 have an ion exchange ratio that differs from that of carboxylic acid layer 112 by no more than about 3.3.

[0062] While not intending to limit the present invention to any theory or mode of operation, it has been discovered that a higher ion exchange ratio for the outer sulfonic acid layer 114 than for the second inner sulfonic acid layer 116 allows the present invention to take advantage of the reduced alkali metal ion tolerance and reduced battery voltage provided by the lower ion exchange ratio for the second inner sulfonic acid layer 116 without sacrificing current efficiency. It has also been discovered that a higher ion exchange ratio for the first inner layer 117 does not impair the reduced alkali metal ion tolerance and reduced battery voltage provided by the lower ion exchange ratio for the second inner sulfonic acid layer 116 without sacrificing current efficiency. The higher ion exchange ratio for the first inner sulfonic acid layer 116 preferably has an IXR that is no more than about 3.3 different from that of the carboxylic acid layer 112, which can advantageously provide resistance to delamination at the interface between the first inner sulfonic acid layer 116 and the carboxylic acid layer 112.

[0063] While the membranes shown in Figures 1 and 2 have three and four layers, respectively, it will be understood that membranes according to the present invention may have additional layers of the same or different fluorinated ionomers, additional layers of the same or different ion exchange ratios or resistivity values, and additional layers of the same or different thicknesses, provided that the additional layers do not interfere with the voltage reduction and maintenance of high current efficiency provided by the present invention. The total layer thickness is preferably about 250 micrometers or less. Preferably, the total layer thickness is from about 75 micrometers to about 150 micrometers.

[0064] manufacturing Known manufacturing methods for chlor-alkali membranes can be adapted to produce multilayer cation exchange membranes according to the present invention. Manufacturing can be carried out using fluorinated ionomers in melt-processable precursor forms, e.g., carboxylic acid ionomer precursors containing methyl ester groups and sulfonic acid ionomers containing sulfonyl fluoride groups. Multilayer membranes can be produced by laminating separate extruded ionomer precursor films, which can be assembled by laminating at elevated temperatures. Alternatively, coextruded multilayer films can be used. For example, a bifilm of carboxylic acid ionomer precursor and sulfonic acid ionomer precursor can be first produced and then laminated to multiple monolayer films of extruded sulfonic acid ionomer precursor, or to a multilayer coextruded sulfonic acid ionomer precursor film. The layers or multilayer films can then be laminated at elevated temperatures to fuse the polymer layers with the woven fabric reinforcement between the desired layers or with the woven fabric reinforcement on the surface of the membrane where it is embedded in the outer layers. The membrane is then hydrolyzed in an aqueous alkali metal hydroxide solution, optionally containing an organic solvent such as dimethyl sulfoxide (DMSO), to convert the ionomer precursor to its ionic form.

[0065] Test Method Chlorine alkaline battery performance: Zero gap, activated cathode, and 100 cm 2An experimental chlor-alkali electrolysis cell employing an active area of ​​1000 kJ / cm is used in the following examples to demonstrate the operation and performance of a membrane according to the present invention. Testing was carried out by mounting a membrane sample in the experimental cell and operating at a nominal temperature of 90°C, a 32 wt% NaOH catholyte, a 17.7 wt% (200 grams / liter) NaCl anolyte, and a current of 6 kA / m. 2 The test is performed by operating the battery under a current density load of 1000 kA / cm for seven days. The voltage result is the battery voltage on the last day, and the current efficiency result is the average of the current efficiencies over the last three days. The battery voltage measurements (CV) are performed using a Moore Industries Model SPT Programmable Signal Converter, which converts the voltage signal to digital and feeds it into the distributed control system. The distributed control system averages the voltage every 24 hours to generate a daily average battery voltage. This voltage is corrected for caustic concentration, temperature, and excess cathode overpotential to generate a reference voltage according to industry standard methods. The reference voltage is always measured at a battery temperature of 90°C, a 32% by weight NaOH catholyte, and a current density of 6 kA / cm. 2 The values ​​are given for the cathodic overpotential at a current density of 0.01 sq. m / s. Corrections are made for small variations from these conditions. Current efficiency (CE) measurements using this cell are made by measuring the total weight and caustic concentration of the liquid output from the cathode compartment over a 14-hour period. Caustic concentration is measured using a calibrated concentration meter and checked daily with a 30% NaOH solution. The total NaOH production (weight x concentration) is divided by the theoretical production calculated from the collection time and average current density, i.e., the total current flowing through the system. For each example reported below, two to eight identical membrane samples were tested. The values ​​reported for voltage and current efficiency in Table 1 are the average of two to eight tests.

[0066] Membrane resistance in sodium ion form The membrane is preconditioned in water at 60°C for 6 hours. After preconditioning, the membrane is transferred to a 24% NaCl / 1% NaOH solution and soaked overnight. The next morning, the solution is changed. The membrane is soaked in the solution until the time of measurement.

[0067] The membrane resistance in a 24% NaCl / 1% NaOH liquid electrolyte is measured using a four-probe impedance technique in a two-chamber cell fixture. The impedance spectrum is performed using constant current mode at 20 mA. Each compartment contains 50 mL of solution. The test is performed at 21°C. The resistance is 0.785 cm. 2 The membrane is placed between a chamber with an active area of ​​1000 sq. m. The sensing probes are thin platinum wires placed 1 mm from both surfaces of the membrane. The current electrodes are approximately 2 cm. 2 and placed 2 cm away from the membrane.

[0068] Resistance is determined by the high frequency intercept on the Nyquist diagram. The baseline resistance R is obtained for an electrolyte solution without a membrane between the chambers. B The membrane is then placed between two chambers with electrolyte solutions on both sides to measure the total resistance R T The membrane resistance R M is calculated from the difference between the total resistance and the baseline resistance using the following formula: R M =R T -R B

[0069] In that case, the sheet resistance (Ω cm 2 ) is the battery area R M times, or 0.785 cm 2 Furthermore, resistivity (Ω·cm) is the sheet resistance divided by the average thickness of the film sample in the active area. Sheet resistance is a property of the film dimensions, while resistivity at a specific set of conditions is an intrinsic property. Often, those skilled in the art use conductivity (Siemens / cm, also intrinsic) instead of resistance. Conductivity is simply the reciprocal of resistivity. [Example]

[0070] The multilayer membranes shown in the examples were fabricated by the following procedure, using specific fluorinated ionomer precursor layers identified in Table 1 and processed as described below. The carboxylic acid ionomer precursor used is a copolymer of tetrafluoroethylene (TFE) and the perfluorinated vinyl ether CF2=CF-O-CF2CF(CF3)-O-CF2CF2CO2CH3. The sulfonic acid ionomer precursor is a copolymer of TFE and the perfluorinated vinyl ether CF2=CF-O-CF2CF(CF3)-O-CF2CF2SO2F. Table 1 includes examples of both three-layer membrane structures (Examples 1-10) and four-layer membrane structures (Examples 11-12). In all cases, the SR layers are numbered, with SR1 closest to the CR layer, SR2 on the side of SR1 opposite the CR layer, and SR3 (if present) on the side of SR2 opposite the CR layer. For the three-layer structure shown in FIG. 1 (Examples 1-10), CR indicates the carboxylic acid ionomer layer 12, and SR1 and SR2 indicate the inner and outer sulfonic acid layers, i.e., sulfonic acid layer 16 and sulfonic acid layer 14, respectively, as shown in FIG. 1. For the membrane with three sulfonic acid layers, as shown in FIG. 2, CR indicates the carboxylic acid ionomer layer 112, and SR3 indicates the outer sulfonic acid layer 114. Referring to FIG. 2, SR1 and SR2 indicate the two inner sulfonic acid layers 117 and 116, respectively, i.e., SR1 is the inner sulfonic acid layer 117 between the CR layer 112 and the second inner SR layer 116, and SR2 is the second inner sulfonic acid layer 116 between the first inner sulfonic acid layer 117 and the outer sulfonic acid layer 114. The thicknesses, EW, and IXR of all layers, as well as the resistivity and area resistance of the sulfonic acid ionomer layers, are listed in Table 1.

[0071] First, a 4-foot (1.2 meter) wide bifilm of the carboxylic acid ionomer and sulfonic acid ionomer precursors is extruded. The extrusion is carried out at 270°C using two single-screw extruders, a die block, a film die, a chill roll, and a take-up roll. Additional membrane layers are produced by extruding single-layer sulfonic acid ionomer films of the same width using the same equipment with a single extruder.

[0072] A reinforcing fabric containing expanded tetrafluoroethylene (ePTFE) and polyethylene terephthalate fibers in a plain weave, with two PET fibers and one ePTFE fiber alternating in both directions, is used. The reinforcing fabric has a center-to-center fiber spacing of approximately 350 micrometers. The fiber weights are varied as listed in Table 1.

[0073] For lamination, the film, if used, is fed into a vacuum laminator over fabric reinforcement. The fabric is adjacent to a porous release paper, which is then adjacent to a vacuum source. The carboxylic acid layer is positioned away from the vacuum source, and the sulfonic acid layer is positioned toward the vacuum source. The conditions used for lamination are 200°C, a vacuum of -70 kPa (-70 kPa), and a feed rate of 1 to 2 ft / min (0.3 to 0.6 m / min).

[0074] The laminated film is hydrolyzed with 25% sodium hydroxide and 10% DMSO at 75°C for 25 minutes. A solution of sulfonic acid ionomer in acid form and zirconium dioxide (0.2-0.23 ratio, polymer to zirconium dioxide) is added to the film at approximately 0.3 mg / m 2 The dry film is spray coated to a loading of 0.015g. The anode and cathode are similarly coated.

[0075] The cell voltages and current efficiencies of the membranes listed in Table 2 are measured in laboratory chlor-alkali cells using the test methods described above. The multilayer cation exchange membranes according to the present invention provide improved cell voltages over bilayer membranes while maintaining high current efficiencies.

[0076] [Table 1]

[0077] [Table 2]

[0078] [Table 3]

[0079] Comparative Examples 1 and 2 represent typical commercially available bilayer membranes and serve as a baseline comparison to illustrate the improvements provided by multilayer membranes in accordance with the present invention. The polymers and reinforcements are similar, but the overall thicknesses are different. This difference in thickness means that the cell voltage of Example 1, at 127 μm (5 mils), is approximately 50 mV higher than that of Example 2, at 102 μm (4 mils). Because some examples are 127 μm and others are 102 μm, we compare either Example 1 or Example 2 to the corresponding thickness as a baseline comparison. Examples 3-6 are 127 μm thick and are compared to Comparative Example 1.

[0080] Comparative Examples 3 and 4 show the results obtained by attempting to reduce the overall membrane voltage by using a sulfonic acid ionomer with lower IXR and resistivity. The effect on voltage is as expected, improving voltage by slightly more than 20 mV over Comparative Example 1, which has the same overall thickness. While this is the desired effect on voltage, the current efficiency also decreases by about 1%, a significant reduction that generally renders the membrane unviable for commercial use.

[0081] Examples 5 and 6 illustrate the embodiment of the invention shown in Figure 1 by reducing the thickness of the low IXR sulfonic acid ionomer layer used in Comparative Examples 3 and 4 by 25 μm and adding a 25 μm layer of a sulfonic acid ionomer with higher IXR and resistivity for the same total thickness. These changes result in comparable voltage improvements as seen in Examples 3 and 4, but surprisingly, current efficiencies within the measurement error of Example 1. These results demonstrate that voltage can be reduced and current efficiency maintained by lowering the IXR and resistivity of most sulfonic acid ionomers, as long as the sulfonic acid ionomer for the anode compartment has an IXR or resistivity within commercially typical ranges.

[0082] Examples 7 and 8 also illustrate the embodiment of the invention shown in Figure 1, demonstrating that the same effect occurs on thinner membranes. Here, the sulfonic acid ionomer of similar structure to Example 2 is replaced by two layers: an inner layer with lower IXR and resistivity, and an outer layer with higher IXR and resistivity. As with Examples 5 and 6, we observe a voltage drop of over 20 mV and a current efficiency that is essentially the same as Example 2.

[0083] Comparative Example 9 shows the results obtained by attempting to reduce the voltage of a multilayer film by using a sulfonic acid ionomer with a lower IXR in the outer layer. Here, the sulfonic acid ionomer of similar structure to Example 2 is replaced by two layers: an outer layer with a lower IXR and resistivity, and an inner layer with a higher IXR and resistivity. The effect on voltage is as expected, improving the voltage over Comparative Example 2, which has the same total thickness. However, the current efficiency is reduced by about 1%, which is undesirable.

[0084] Comparative Example 9 also shows a significant decrease in current efficiency when a low IXR sulfonic acid ionomer is used in the outer layer, in contrast to Example 8. Both examples are made with the same sulfonic acid ionomer, one high IXR and one low IXR, but in the opposite order. They show comparable voltage improvements compared to Example 2. However, current efficiency is maintained in Example 8, which used a high IXR sulfonic acid ionomer in the outer layer, while current efficiency is reduced in Comparative Example 9.

[0085] Examples 10 and 11 illustrate the present invention, with a structure similar to that of Example 8, but with a modified reinforcement. The 90 denier ePTFE fiber is replaced with a 50 denier and a 70 denier, respectively, with no other changes to the fabric or membrane. In this case, the voltage is further reduced, but again the current efficiency remains essentially unchanged. This demonstrates that further voltage reductions to the structure, such as by reducing the resistance of the reinforcement, do not change the essential characteristics of the present invention.

[0086] Examples 12 and 13 illustrate the embodiment of the invention shown in Figure 2, where three sulfonic acid layers are used, but with a similar total thickness as Comparative Example 2. Both Examples 12 and 13 show an improvement in voltage due to a low IXR sulfonic acid replacing the high IXR sulfonic acid ionomer of Example 2 at a similar thickness. Surprisingly, current efficiency is maintained here as the higher IXR sulfonic acid is used in the outer layer adjacent the anode surface.

Claims

1. 1. A multi-layer cation exchange membrane for use in a chlor-alkali process, said multi-layer cation exchange membrane comprising: a carboxylic acid layer on one side of the membrane, the carboxylic acid layer comprising a fluorinated ionomer containing carboxylic acid groups; an outer sulfonic acid layer comprising a fluorinated ionomer containing sulfonic acid groups on the face of the membrane opposite the carboxylic acid layer; an inner sulfonic acid layer between the carboxylic acid layer and the outer sulfonic acid layer, the inner sulfonic acid layer comprising a fluorinated ionomer containing sulfonic acid groups; wherein the outer sulfonic acid layer has a resistivity greater than 68.1 Ω cm and the inner sulfonic acid layer has a resistivity less than 60.3 Ω cm.

2. 2. The multilayer cation exchange membrane of claim 1, wherein the inner sulfonic acid layer has a resistivity that is at least 15.5 ohm-cm less than the resistivity of the outer sulfonic acid layer.

3. 3. The multilayer cation exchange membrane according to claim 1, wherein the carboxylic acid layer has an IXR of 13.8 to 16.

4. 4. The multilayer cation exchange membrane according to claim 1, wherein the outer sulfonic acid layer has a resistivity greater than 73.2 Ω·cm.

5. 4. The multilayer cation exchange membrane according to claim 1, wherein the outer sulfonic acid layer has a resistivity of 68.1 Ω·cm to 186.3 Ω·cm.

6. The multilayer cation exchange membrane of claim 5, wherein the outer sulfonic acid layer has a resistivity of 68.1 Ω·cm to 155.4 Ω·cm.

7. The multilayer cation exchange membrane of claim 6, wherein the outer sulfonic acid layer has a resistivity of 68.1 Ω·cm to 118.3 Ω·cm.

8. The multilayer cation exchange membrane of claim 7, wherein the outer sulfonic acid layer has a resistivity of 68.1 Ω·cm to 78.1 Ω·cm.

9. 9. The multilayer cation exchange membrane according to claim 1, wherein the inner sulfonic acid layer has a resistivity of less than 57.7 Ω cm.

10. 9. The multilayer cation exchange membrane according to claim 1, wherein the inner sulfonic acid layer has a resistivity of 10.7 Ω·cm to 60.3 Ω·cm.

11. The multilayer cation exchange membrane of claim 10, wherein the inner sulfonic acid layer has a resistivity of 32.3 Ω·cm to 60.3 Ω·cm.

12. The multilayer cation exchange membrane of claim 11, wherein the inner sulfonic acid layer has a resistivity of 32.3 Ω·cm to 57.7 Ω·cm.

13. 13. The multilayer cation exchange membrane according to claim 1, further comprising a woven fabric reinforcement material.

14. 14. The multilayer cation exchange membrane of claim 13, wherein the woven fabric reinforcement is at least partially embedded in the membrane.

15. 15. The multilayer cation exchange membrane of claim 13 or 14, wherein the woven fabric reinforcement comprises fluoropolymer yarns having a denier of less than 100.

16. 16. The multilayer cation exchange membrane according to claim 1, wherein the inner sulfonic acid layer has a thickness of at least 40 micrometers.

17. The multilayer cation exchange membrane of claim 16, wherein the inner sulfonic acid layer has a thickness of 50 micrometers to 200 micrometers.

18. The multilayer cation exchange membrane of claim 17, wherein the inner sulfonic acid layer has a thickness of 60 micrometers to 100 micrometers.

19. 16. The multilayer cation exchange membrane according to claim 1, wherein the outer sulfonic acid layer has a thickness of less than 30 micrometers.

20. The multilayer cation exchange membrane of claim 19, wherein the outer sulfonic acid layer has a thickness of 5 micrometers to 30 micrometers.

21. The multilayer cation exchange membrane of claim 20, wherein the outer sulfonic acid layer has a thickness of 7 micrometers to 25 micrometers.

22. 1. A multi-layer cation exchange membrane for use in a chlor-alkali process, said multi-layer cation exchange membrane comprising: a carboxylic acid layer on one side of the membrane, the carboxylic acid layer comprising a fluorinated ionomer containing carboxylic acid groups; an outer sulfonic acid layer comprising a fluorinated ionomer containing sulfonic acid groups on the face of the membrane opposite the carboxylic acid layer; a first inner sulfonic acid layer and a second inner sulfonic acid layer between the carboxylic acid layer and the outer sulfonic acid layer, the first inner sulfonic acid layer and the second inner sulfonic acid layer comprising a fluorinated ionomer containing sulfonic acid groups; the first inner sulfonic acid layer is located between the carboxylic acid layer and the second inner sulfonic acid layer, the second inner sulfonic acid layer is located between the first inner sulfonic acid layer and the outer sulfonic acid layer, the outer sulfonic acid layer having a resistivity greater than 68.1 Ω cm, the first inner sulfonic acid layer having a resistivity greater than 68.1 Ω cm, and the second inner sulfonic acid layer having a resistivity less than 60.3 Ω cm. A multilayer cation exchange membrane characterized by:

23. 23. The multilayer cation exchange membrane of claim 22, wherein the second inner sulfonic acid layer has a resistivity at least 15.5 Ω cm less than the resistivity of the outer sulfonic acid layer and / or the first inner sulfonic acid layer has a resistivity at least 15.5 Ω cm greater than the resistivity of the second inner sulfonic acid layer.

24. 24. The multilayer cation exchange membrane according to claim 22 or 23, wherein the carboxylic acid layer has an IXR of 13.8 to 16, and the first inner sulfonic acid layer has an IXR that differs from the IXR of the carboxylic acid layer by 3.3 or less.

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