WO2013101299A1

WO2013101299A1 - Flow battery comprising a composite polymer separator membrane

Info

Publication number: WO2013101299A1
Application number: PCT/US2012/042788
Authority: WO
Inventors: Ralph Birchard Lloyd; Biswajit Choudhury; Murat ÜNLÜ
Original assignee: E. I. Du Pont De Nemours And Company
Priority date: 2011-12-29
Filing date: 2012-06-15
Publication date: 2013-07-04

Abstract

The invention relates to flow batteries having improved crossover resistance to electroactive species, excellent coulombic and voltage efficiency and durability, which batteries comprise a composite reinforced polymer separator membrane layer to achieve these performance benefits. The composite polymer separator membrane comprises an ePTFE reinforced ionomer. The ionomer is a perfluorosulfonic acid ionomer which has substantially all of the functional groups being represented by the formula SO₃X wherein X is H, Li, Na, K or N(R¹)(R²)(R³)(R⁴) and R¹, R², R³, and R⁴ are the same or different and are H, CH₃ or C₂H₅. Preferably, substantially all of the functional groups are represented by the formula SO₃X wherein X is H.

Description

TITLE

FLOW BATTERY COMPRISING A COMPOSITE POLYMER SEPARATOR MEMBRANE

FIELD OF THE INVENTION

The invention relates to flow batteries having improved crossover resistance to electroactive species, excellent coulombic and voltage efficiency and excellent durability, which utilize a composite reinforced polymer separator membrane layer to achieve these performance benefits. This invention provides flow batteries comprising an ePTFE reinforced ionomer separator membrane.

BACKGROUND OF THE INVENTION

One of the most promising approaches toward large scale energy storage and power output is the flow battery. A flow battery is a form of rechargeable battery in which electrolyte containing one or more dissolved electroactive species flows through an electrochemical cell that converts chemical energy directly to electricity. Additional electrolyte is stored externally, generally in tanks, and is usually pumped through the cell, or cells, of the reactor, although gravity feed systems are also possible. Flow batteries can be rapidly recharged by replacing the electrolyte liquid while simultaneously recovering the spent material for re-energization.

Three main classes of flow batteries are the redox (reduction-oxidation) flow battery, the hybrid flow battery, and the redox fuel cell. In the redox flow battery, all of the electroactive components are dissolved in the electrolyte. The hybrid flow battery is differentiated in that one or more of the electroactive components is deposited as a solid layer. The redox fuel cell has a conventional flow battery reactor, but only operates to produce electricity; it is not electrically recharged. In the latter case, recharge occurs by reduction of the negative electrolyte using a fuel, such as hydrogen, and oxidation of the positive electrolyte using an oxidant, such as air or oxygen. Commercially, the most promising flow battery systems for large scale energy storage and power output are the vanadium redox battery and the zinc flow battery.

The vanadium redox battery (VRB) is an example of a redox flow battery, which, in general, involves the use of two redox couple

electrolytes separated by an ion exchange membrane. The vanadium redox batteries include so-called "All-Vanadium Redox Cells and Batteries" which employ a V(II)A/(III) couple in the negative half-cell and a

V(IV)A/(V) couple in the positive half-cell, (sometimes referred to as VA/RB); and "Vanadium Bromide Redox Cells and Batteries" which employ the V(II)A/(III) couple in the negative half-cell and a

bromide/polyhalide couple in the positive half-cell, (sometimes referred to as V/BrRB). In either case, the positive and negative half-cells are separated by a membrane/separator which prevents cross mixing of the positive and negative electrolytes, whilst allowing transport of ions to complete the circuit during passage of current. The V(V) ions (in the VA/RB system) and the polyhalide ions (in the V/BrRB system) are highly oxidizing and result in rapid deterioration of most polymeric membranes during use leading to poor durability. Consequently, potential materials for the membrane/separator have been limited and this remains the main obstacle to commercialization of these types of energy storage systems. Ideally, the membrane should be stable to the acidic environments of electrolytes such as vanadium sulphate or vanadium bromide; show 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 the vanadium ions or polyhalide ions; high permeability to charge carrying hydrogen ions; good mechanical properties; and low cost. To date, no single polymer system has been found to be suitable with respect to this property balance. Perfluorinated ion exchange polymers such as the perfluorosulfonate polymers (for example, Nafion™ polymers, available from DuPont) show exceptional promise in terms of resistance to acidic environments and highly oxidizing species, but have been found to be deficient in water and vanadium ion crossover resistance. Low water crossover resistance or low vanadium ion crossover resistance results in low coulombic efficiencies and even self-discharge of the battery, as well as a continuing need to rebalance the electrolyte concentrations in the two half cells. Several zinc flow batteries exist, of which the zinc-bromine flow battery, and zinc-air flow battery are the most common. The zinc-bromine flow battery is a hybrid flow battery. A solution of zinc bromide is stored in two tanks. When the battery is charged or discharged the electrolyte solutions are pumped through a reactor stack and back into the tanks. One tank is used to store the electrolyte for the positive electrode reactions and the other for the negative electrode reactions. The predominantly aqueous electrolyte is composed of zinc bromide salt dissolved in water. During charge, metallic zinc is plated from the electrolyte solution onto the negative electrode surfaces in the cell stacks, and bromide ions are converted to bromine at the positive electrode surfaces. Some such systems utilize complexing agents to capture the bromine. The reverse process occurs upon discharge, with the metallic zinc dissolving in the electrolyte. Similar to the VRB, the two electrode chambers of each cell are separated by a membrane, in this case helping to prevent bromine from reaching the positive electrode, where it would react with zinc causing self-discharge of the battery. Separators, such as Daramic™ polyethylene or polypropylene membranes, have been found to suffer energy-efficiency losses in the battery due to the low resistance to bromine transport through the membrane, and, as for the VRB case, no polymeric system has yet been found to have the appropriate balance of properties. Accordingly, improved separators with lower bromine mass transport and higher electrolytic conductivity would be beneficial for improving the energy efficiency of zinc-bromine batteries.

Other combinations of reactants in a flow battery include Sn

(anolyte)/Fe (catholyte), Mn (anolyte)/Fe (catholyte), V (anolyte)/Ce (catholyte), V (anolyte)/Br₂ (catholyte), Fe (anolyte)/Br₂ (catholyte), and S (anolyte)/Br₂ (catholyte). Further examples of a workable redox flow battery chemistry and system are provided in U.S. Pat. No. 6,475,661 , the entire contents of which are incorporated herein by reference.

Electrolytic membranes used in fuel cell applications have typically employed membranes with a thickness of 50-175 μιτι depending on the nature of the application. In order to achieve higher power density and to reduce membrane resistance, thinner membranes (<25 μιτι) are

increasingly being used. Thin membranes offer substantial performance enhancement in fuel cells, but they reduce the mechanical strength of the membrane and hence make the membrane weak and subject to breaking during use.

In order to solve the above-mentioned problem of mechanical stability, practitioners have proposed the use of a mechanically strong and chemically stable porous reinforcement support material in conjunction with the ionomer membrane. Composite polymer electrolyte membranes are known, for example in U.S. Patent No. 5,547,551 (to Bahar et al.) a thin composite membrane is described which includes a base material and an ion exchange resin. Furthermore, there have been attempts to incorporate composite polymer electrolyte membranes in vanadium redox batteries. Tian and coworkers disclose the use of a Daramic™/Nafion™ composite membrane in a vanadium redox battery, Journal of Membrane Science, 234, (2004), 51 -54. However, the Daramic™ polypropylene support has insufficient durability properties over time in the presence of high concentration vanadium electrolyte to be commercially useful; specifically, the Daramic™ polymer supports deteriorate and are destroyed.

Furthermore, although the porous reinforcement matrix helps in improving the mechanical properties of the membrane, the presence of a non-conducting support layer in the membrane reduces the conductivity of the membrane.

The present invention provides flow batteries with a durable composite polymer separator membrane having improved crossover resistance to electroactive species, and resultant improved lifetime free of electrolyte maintenance.

SUMMARY OF THE INVENTION

The invention provides flow batteries comprising a composite polymer separator membrane which latter accomplishes improved crossover resistance to electroactive species, greater durability and longer life of the battery. Specifically, the invention provides a flow battery comprising an ePTFE reinforced ionomer separator membrane. In an embodiment, the reinforced ionomer separator membrane comprises a perfluorosulfonic acid (or perfluorosulfonate) ionomer or a highly fluorinated sulfonic acid (or highly fluorinated sulfonate) ionomer which has substantially all of the functional groups being represented by the formula -SO3X wherein X is H, Li, Na, K or N(R¹)(R²)(R³)(R⁴) and R¹, R², R³, and R⁴ are the same or different and are H, CH₃ or C2H₅. Preferably, substantially all of the functional groups are represented by the formula -SO3X wherein X is H.

In an embodiment, the reinforced ionomer separator membrane of the flow battery has a pore size no greater than 10 microns or no greater than 5 microns, or even no greater than 2 microns.

In an embodiment, the reinforced ionomer separator membrane of the flow battery comprises: (a) an ePTFE reinforcement layer having a porosity of at least about 45%, preferably at least 65%, and a mean pore size no greater than 10 μιτι, and having opposing surfaces, (b) at least one perfluorosulfonic acid (or perfluorosulfonate) ionomer, or highly fluorinated sulfonic acid (or highly fluorinated sulfonate) ionomer, impregnated between said opposing surfaces of said ePTFE reinforcement layer such that said at least one ionomer has a volume fraction of at least 40 percent, preferably at least 60% or even at least 80%, at a midpoint between the opposing surfaces.

In an embodiment, the reinforced ionomer separator membrane of the flow battery has a combined thickness of 150 microns or less, or 30 microns or less, or preferably 25 microns or less, or even 10 microns or less.

In an embodiment, the separator membrane of the flow battery comprises two or more reinforcement layers. One aspect of the invention provides a vanadium redox battery comprising an ePTFE reinforced ionomer separator membrane. In one such embodiment, the ionomer separator membrane comprises a perfluorosulfonic acid (or perfluorosulfonate) ionomer or a highly

fluorinated sulfonic acid (or highly fluorinated sulfonate) ionomer which has substantially all of the functional groups being represented by the formula -SO3X wherein X is H, Li, Na, K or N(R¹)(R²)(R³)(R⁴) and R¹ , R², R³, and R⁴ are the same or different and are H, CH₃ or C₂H₅. Preferably, substantially all of the functional groups are represented by the formula -SO3X wherein X is H. In an embodiment, the reinforced ionomer separator membrane of the vanadium redox battery has a pore size no greater than 10 microns or no greater than 5 microns, or even no greater than 2 microns.

In an embodiment, the ePTFE reinforced ionomer separator membrane is an asymmetric ePTFE reinforced ionomer separator membrane. In an embodiment, the reinforced ionomer separator membrane of the vanadium redox battery comprises: (a) an ePTFE reinforcement layer having a porosity of at least about 45%, preferably at least 65%, and a mean pore size no greater than 10 μιτι, and having opposing surfaces, (b) at least one perfluorosulfonic acid (or perfluorosulfonate) ionomer, or highly fluorinated sulfonic acid (or highly fluorinated sulfonate) ionomer, impregnated between said opposing surfaces of said ePTFE

reinforcement layer such that said at least one ionomer has a volume fraction of at least 40 percent, preferably at least 60% or even at least 80%, at a midpoint between the opposing surfaces. In an embodiment, the reinforced ionomer separator membrane of the vanadium redox battery has a combined thickness of 150 microns or less, or 30 microns or less, or preferably 25 microns or less, or even 10 microns or less. In an embodiment, the separator membrane of the vanadium redox battery comprises two or more reinforcement layers.

Another aspect of the invention provides a zinc flow battery, such as a zinc-bromine flow battery or a zinc-air flow battery, comprising an ePTFE reinforced ionomer separator membrane. In one such embodiment, the ionomer membrane comprises a perfluorosulfonic acid (or

perfluorosulfonate) ionomer or a highly fluorinated sulfonic acid (or a highly fluorinated sulfonate) ionomer which has substantially all of the functional groups being represented by the formula -SO3X wherein X is H, Li, Na, K or N(R¹)(R²)(R³)(R⁴) and R¹, R², R³, and R⁴ are the same or different and are H, CH₃ or C2H₅. Preferably, substantially all of the functional groups are represented by the formula -SO3X wherein X is H.

In an embodiment, the reinforced ionomer separator membrane of the zinc flow battery has a pore size no greater than 10 microns or no greater than 5 microns, or even no greater than 2 microns. In an embodiment, the reinforced ionomer separator membrane of the zinc flow battery comprises: (a) an ePTFE reinforcement layer having a porosity of at least about 45%, preferably at least 65%, and a mean pore size no greater than 10 μιτι, and having opposing surfaces, (b) at least one perfluorosulfonic acid (or perfluorosulfonate) ionomer, or highly fluorinated sulfonic acid (or highly fluorinated sulfonate) ionomer, impregnated between said opposing surfaces of said ePTFE reinforcement layer such that said at least one ionomer has a volume fraction of at least 40%, preferably at least 60% or even at least 80%, at a midpoint between the opposing surfaces. In an embodiment, the reinforced ionomer separator membrane of the zinc flow battery has a combined thickness of 150 microns or less, or 30 microns or less, or preferably 25 microns or less, or even 10 microns or less.

In an embodiment, the separator membrane of the zinc flow battery comprises two or more reinforcement layers. Embodiments of the present invention as described in the Summary of the Invention, and any other embodiments described herein, can be combined in any manner. Accordingly, the invention also includes embodiments which result from combinations of the elements described in each of the above embodiments. DESCRIPTION OF THE FIGURES

Figure 1 shows the average % swelling (at 50% relative humidity and 23°C) for the ePTFE-reinforced Nafion^® separator membrane for various membrane thicknesses.

Figure 2 shows the in-plane proton conductivity (at 100% relative humidity and 23°C) for the ePTFE-reinforced Nafion^® separator membrane for various membrane thicknesses, and the in-plane proton conductivity of the Nafion^® dense membrane (ionomer with no reinforcement).

Figure 3 illustrates the apparatus for the measurement of the permeability of vanadium ion, i.e. V(IV) across a polymer separator membrane. The left reservoir (1 ) contains 2 M VOSO₄ solution in 2.5 M H₂SO₄; the right reservoir (2) contains 2 M MgSO₄ solution in 2.5 M H₂SO₄; two sampling ports are labeled (3); the two solutions are separated by the test membrane (4).

Figure 4 illustrates the Diffusion Coefficient for various membranes. Figure 5 illustrates the selectivity for hydrogen ions (protons) through the membrane compared to vanadium IV ions for various membranes.

DETAILED DESCRIPTION OF THE INVENTION

Herein, when an amount, concentration, or other value or parameter is given as either a range, preferred range, or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values recited when describing a range.

For the purposes of the present invention, the term "membrane," a term of art in common use in the flow battery art is synonymous with the terms "film" or "sheet " which are terms of art in more general usage but refer to the same articles.

The porosity of the nonwoven web material is equivalent to 100 x (1 .0 - solidity) and is expressed as a percentage of free volume in the nonwoven web material structure wherein solidity is expressed as a fraction of solid material in the nonwoven web material structure.

"Mean pore size" is measured according to ASTM Designation E 1294- 89, "Standard Test Method for Pore Size Characteristics of Membrane Filters Using Automated Liquid Porosimeter." Individual samples of different size (8, 20 or 30 mm diameter) are wetted with a low surface tension fluid (1 ,1 ,2,3,3,3-hexafluoropropene, or "Galwick," having a surface tension of 16 dyne/cm) and placed in a holder, and a differential pressure of air is applied and the fluid removed from the sample. The differential pressure at which wet flow is equal to one-half the dry flow (flow without wetting solvent) is used to calculate the mean pore size using supplied software.

The Volume Fraction lonomer is the volume fraction of ionomer in the composite membrane at a given location (i.e., midpoint) and is equal to volume of ionomer/ (volume of ionomer + volume occupied by the fibers in the non-woven reinforcement substrate + volume of air + volume, if any, of additives) = the volume fraction of ionomer in the composite membrane at a given location. The volume fraction ionomer has no units as it is volume/volume which cancels, i.e., it is "unit-less".

The volume fraction ionomer is measured by considering volume elements as averages in the x,y plane over an area which has a

statistically significant number of reinforcement fibers. As can be determined by the worker with ordinary skill, the above-referenced statistically significant area will depend on the fiber diameter and other characteristics and may need to be adjusted to account for same, depending upon the particular sample. For example, if an area that is too small is chosen, e.g., equidistant between two fibers, it might only encompass ionomer and no fibers, and again give a misleading result of 100% ionomer. Accordingly, the chosen area for analysis should contain numerous fibers, and also be representative of the number of fibers in a similar area in another portion of the composite. Specifically, the volume fraction is visually analyzed from the pictures and graphs generated by using a Scanning Electron Microscope (SEM) [Hitachi S-4700 Cold

Cathode Field Emission] with energy-dispersive X-ray spectroscopy (EDS) and Mapping capability. Preparation of the sample entailed embedding films in epoxy and cutting, grinding, and polishing once cured. Fluorine and sulphur elemental line-scans and elemental mapping were used.

Herein, the phrase (and similar phrases): "substantially all of the functional groups are represented by the formula -SO3X wherein X is H" means that the percentage of such functional groups in the -SO3H form approaches, or is, 100%, such as, for example, at least 98%. Herein, conductivity refers to proton conductivity.

Ion Exchange Polymers

The compositions and method in accordance with the present invention employ composite polymer electrolyte membranes comprising highly fluorinated sulfonate polymer, i.e., having sulfonate functional groups in the resulting composite membrane. "Highly fluorinated" means that at least 90% of the total number of univalent atoms in the polymer are fluorine atoms. Most preferably, the polymer is perfluorinated. Herein, the term "sulfonate functional groups" means either sulfonic acid groups or salts of sulfonic acid groups, preferably alkali metal or ammonium salts. Most preferably, the functional groups are represented by the formula -SO3X wherein X is H, Li, Na, K or N(R¹)(R²)(R³)(R⁴) and R¹ , R², R³, and R⁴ are the same or different and are H, CH₃ or C₂H₅. In some

embodiments of the invention, the sulfonic acid form of the polymer is preferred, i.e., where X is H in the formula above. In further embodiments of the invention, substantially all of the functional groups (i.e., approaching and/or achieving 100%, such as, for example, at least 98%) are

represented by the formula -SO3X wherein X is H. Highly fluorinated sulfonate polymers of this type are known as ion exchange polymers (ionomers).

Preferably, the ion exchange polymer comprises a polymer backbone with recurring side chains attached to the backbone with the side chains carrying the cation exchange groups. Possible polymers and precursor polymers include homopolymers as well as copolymers of two or more monomers. Copolymers may be formed from one monomer which is a nonfunctional monomer and which provides carbon atoms for the polymer backbone. A second monomer provides both carbon atoms for the polymer backbone and also contributes the side chain carrying the cation exchange group or its precursor, e.g., a sulfonyl halide group such a sulfonyl fluoride (-SO₂F), which may optionally be subsequently

hydrolyzed to a sulfonate functional group. For example, copolymers of a first fluorinated vinyl monomer together with a second fluorinated vinyl monomer having a sulfonyl fluoride group (-SO₂F) can be used. Possible first monomers include tetrafluoroethylene (TFE), hexafluoropropylene, vinyl fluoride, vinylidine fluoride, trifluorethylene, chlorotrifluoroethylene, perfluoro (alkyl vinyl ether), and mixtures thereof. Possible second monomers include a variety of fluorinated vinyl ethers with sulfonate functional groups or precursor groups which can provide the desired side chain in the polymer. The first monomer may also have a side chain which does not interfere with the ion exchange function of the sulfonate functional group. Additional monomers can also be incorporated into these polymers if desired. Polymer membranes may be prepared by extrusion or may be cast from solvent, solvent/water mixtures or aqueous dispersions. A class of preferred polymers for use in the present invention include a highly fluorinated, most preferably perfluorinated, carbon backbone and the side chain is represented by the formula

-(O-CF₂CFRf)a-O-CF2CFR'fSO₃X, wherein R_f and R'_f are independently selected from F, CI 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(R¹)(R²)(R³)(R⁴) and R¹ , R², R³, and R⁴ are the same or different and are H, CH₃ or C₂H₅. The preferred polymers include, for example, polymers disclosed in U.S.

Patent 3,282,875 and in U.S. Patents 4,358,545 and 4,940,525. One preferred polymer comprises a perfluorocarbon backbone and the side chain is represented by the formula -O-CF₂CF(CF3)-O-CF2CF₂SO3X, wherein X is as defined above. Polymers of this type are disclosed in U.S. Patent 3,282,875 and can be made by copolymerization of

tetrafluoroethylene (TFE) and the perfluorinated vinyl ether

CF₂=CF-O-CF₂CF(CF₃)-O-CF₂CF₂SO₂F, perfluoro(3,6-dioxa- 4-methyl-7-octenesulfonyl fluoride) (PDMOF), followed by conversion to sulfonate groups by hydrolysis of the sulfonyl halide groups and ion exchanging if needed to convert to the desired form. One preferred polymer of the type disclosed in U.S. Patents 4,358,545 and 4,940,525 has the side chain -O-CF2CF2SO3X, wherein X is as defined above. This polymer can be made by copolymerization of tetrafluoroethylene (TFE) and the perfluorinated vinyl ether CF₂=CF-O-CF₂CF₂SO₂F,

perfluoro(3-oxa-4-pentenesulfonyl fluoride) (POPF), followed by hydrolysis and acid exchange if needed.

In embodiments of the present invention, highly fluorinated carboxylate polymer, i.e., having carboxylate functional groups in the resulting composite membrane, may be employed as will be discussed in more detail hereinafter. Herein, the term "carboxylate functional groups" means either carboxylic acid groups or salts of carboxylic acid groups, preferably alkali metal or ammonium salts. Most preferably, the functional groups are represented by the formula -CO₂X wherein X is H, Li, Na, K or

N(R¹)(R²)(R³)(R⁴) and R¹ , R², R³, and R⁴ are the same or different and are H, CH₃ or C2H₅. The polymer may comprise a polymer backbone with recurring side chains attached to the backbone with the side chains carrying the carboxylate functional groups. Polymers of this type are disclosed in U.S. Patent 4,552,631 and most preferably have the side chain -O-CF₂CF(CF3)-O-CF2CF₂CO2X. This polymer can be made by copolymerization of tetrafluoroethylene (TFE) and the perfluorinated vinyl ether CF₂=CF-O-CF2CF(CF3)-O-CF2CF2CO₂CH3, methyl ester of perfluoro(4,7-dioxa-5-methyl-8-nonenecarboxylic acid) (PDMNM), followed by conversion to carboxylate groups by hydrolysis of the methyl carboxylate groups and ion exchanging if needed to convert to the desired form. While other esters can be used for film or bifilm fabrication, the methyl ester is preferred since it is sufficiently stable during normal extrusion conditions. Such polymers also may be cast from solvent, solvent/water mixtures or aqueous dispersions.

Herein, "ion exchange ratio" or "IXR" is defined as number of carbon atoms in the polymer backbone in relation to the cation exchange groups. A wide range of IXR values for the polymer are possible. Typically, however, the IXR range used for layers of the membrane may be from about 7 to about 33. For perfluorinated polymers of the type described above, the cation exchange capacity of a polymer is often expressed in terms of equivalent weight (EW). Herein, equivalent weight (EW) is defined to be the weight of the polymer in acid form required to neutralize one equivalent of NaOH. In the case of a sulfonate polymer where the polymer comprises a perfluorocarbon backbone and the side chain is -O-CF₂-CF(CF3)-O-CF2-CF2-SO₃H (or a salt thereof), the equivalent weight range which corresponds to an IXR of about 7 to about 33 is about 700 EW to about 2000 EW. IXR for this polymer can be related to equivalent weight using the following formula: 50 IXR + 344 = EW. While generally the same IXR range is used for sulfonate polymers disclosed in U.S. Patents 4,358,545 and 4,940,525, the equivalent weight is somewhat lower because of the lower molecular weight of the monomer unit containing a cation exchange group. For the IXR range of about 7 to about 33, the corresponding equivalent weight range is about 500 EW to about 1800 EW. IXR for this polymer can be related to equivalent weight using the following formula: 50 IXR + 178 = EW. For carboxylate polymers having the side chain -O-CF₂CF(CF3)-O-CF₂CF₂CO₂X, a useful IXR range is about 12 to about 21 which corresponds to about 900 EW to about 1350 EW. IXR for this polymer can be related to equivalent weight using the following formula: 50 IXR + 308 = EW. IXR is used in this application to describe either hydrolyzed polymer which contains functional groups or unhydrolyzed polymer which contains precursor groups which will subsequently be converted to the functional groups during the manufacture of the membranes.

Microporous Supports

The microporous supports useful in a process of the invention are made of highly fluorinated nonionic polymers. As for the ion exchange polymers, "highly fluorinated" means that at least 90% of the total number of halogen and hydrogen atoms in the polymer are fluorine atoms. The pore size of the support is a key parameter. For increased resistance to thermal and chemical degradation, and good resistance to crossover of water and electroactive species, the microporous support preferably is made of a perfluorinated polymer. For example, the porous support can be a specially prepared polymer of polytetrafluoroethylene (PTFE) or a copolymer of tetrafluoroethylene with CF₂ = CFC_nF₂n₊i (n = 1 to 5) or

CF₂ = CFO— CF₂ CFO) _mC_nF_2n+i

CF₃

(m = O to 15, n = 1 to 15).

Microporous PTFE sheeting is known and is particularly suitable for use as the microporous support. One support having a small enough pore size to be effective in the present invention is expanded polytetrafluoroethylene polymer (ePTFE) having a microstructure of polymeric fibrils, or a microstructure of nodes interconnected by the fibrils. Films having a microstructure of polymeric fibrils with no nodes present are also useful. The preparation of such suitable supports is described in U.S. patents 3,593,566 and U.S. 3,962,153. These patents disclose the extruding of dispersion-polymerized PTFE in the presence of a lubricant into a tape and subsequently stretching under conditions which make the resulting material more porous and stronger. Heat treatment of the expanded PTFE under restraint to above the PTFE melting point

(approximately 342°C) increases the amorphous content of the PTFE. Films made in this manner can have a variety of pore sizes and void volumes. U.S. Patents 3,953,566, 3,962,153 and 4,187,390 disclose porous PTFE films having at least 35% voids. Pore size can vary but is typically at least about 0.2 μηη. In embodiments of the invention the microporous support has a mean flow pore size of between about 0.01 μιτι and about 20 μιτι, even between about 0.1 μιτι and about 10 μιτι, even between about 0.1 μιτι and about 5 μιτι, and even between about 0.01 μιτι and about 5 μιτι, or between about 0.01 μιτι and about 1 μιτι. These mean pore size values may be obtained after lightly calendaring the material, or in embodiments where no calendaring occurs, before imbibing with the ionomer occurs.

In embodiments of the invention the microporous support has a porosity of no less than 50%, and in other embodiments no less than 65%, and in other embodiments no less than 80%. These porosity values may be obtained after lightly calendaring the material, or in embodiments where no calendaring occurs, before imbibing with the ionomer occurs. The high porosity of the microporous support also provides for good ionomer absorption to provide a composite polymer separator membrane.

The thickness of the porous support can be varied depending on the type of composite to be made. The thickness may be from about 20 μηη to about 400 μητι, or from 30 μηη to about 60 μηη. However, more preferably it is less than or equal to 30 μητι, or less than or equal to 25 μηη in thickness, such as, for example, 5-30 μητι, or from 10-25 μηη in thickness, or even between about 5 μηη and 10 μηη. The microporous support is thick enough to provide good mechanical properties while allowing good flow of ions.

Suitable microporous ePTFE supports are available commercially from W. L. Gore & Associates, Elkton Maryland, under the trademark

GORE-TEX^® and from Tetratec, Feasterville, Pennsylvania, under the trademark TETRATEX^®.

Microporous supports made using other manufacturing processes with other highly fluorinated nonionic polymers may also be used in the process of the invention, although the pore size and porosity limitations discussed above apply. Such polymers may be selected from the broad spectrum of homopolymers and copolymers made using fluorinated monomers. Possible fluorinated monomers include vinyl fluoride;

vinylidene fluoride; trifluoroethylene; chlorotrifluoroethylene (CTFE);

1 ,2-difluoroethylene; tetrafluoroethylene (TFE); hexafluoropropylene (HFP); perfluoro(alkyl vinyl) ethers such as perfluoro(methyl vinyl) ether (PMVE), perfluoro(ethyl vinyl) ether (PEVE), and perfluoro(propyl vinyl) ether (PPVE); perfluoro(1 ,3-dioxole); perfluoro(2,2-dimethyl-1 ,3-dioxole) (PDD); F(CF₂)nCH₂OCF=CF2 wherein n is 1 , 2, 3, 4 or 5; R¹CH₂OCF=CF₂ wherein R¹ is hydrogen or F(CF₂)_m- and m is 1 , 2 or 3; and R³OCF=CH₂ wherein R³ is F(CF₂)_Z- and z is 1 , 2, 3 or 4; perfluorobutyl ethylene

(PFBE); 3,3,3-trifluoropropene and 2-trifluoromethyl-3,3,3-trifluoro-1 - propene.

If desired, the microporous support may also include an attached fabric, preferably a woven fabric. Most preferably, such fabrics are made of a yarn of a highly fluorinated polymer, preferably PTFE. If such fabrics are to be used, they are preferably securely attached to the ePTFE support as supplied for use in the process. Suitable woven fabrics include scrims of woven fibers of expanded PTFE, webs of extruded or oriented fluoropolymer or fluoropolymer netting, and woven materials of

fluoropolymer fiber. Nonwoven materials including spun-bonded

fluoropolymer may also be used if desired. The reinforced composite membrane in accordance with the invention may be assembled from the ion exchange polymers and microporous supports described above in any manner of art recognized methods, so long as the resultant reinforced composite membrane results in the ion exchange polymer present in the reinforced composite membrane having substantially all of the functional groups (i.e., approaching and/or achieving 100%) being represented by the formula -SO3X wherein X is H, Li, Na, K or N(R¹)(R²)(R³)(R⁴) and R¹, R², R³, and R⁴ are the same or different and are H, CH₃ or C2H₅. Preferably, substantially all of the functional groups are represented by the formula -SO3X wherein X is H.

For example, a coating of the ion exchange polymer may be applied to just one side of the ePTFE support (an asymmetric membrane); or applied to one side, optionally dried, and then a second coating applied on the other side of the support. Alternatively, the microporous support may be laid in a wet sample of the ion exchange polymer and have the latter soak throughout the support. Other techniques, such as transfer coating, also can be used.

Advantageously, very thin films of the composite structure may be obtained. The ePTFE substrate improves the physical durability of the thin films of the composite while allowing lower resistance. In an embodiment, the ePTFE is only partially filled with the ion exchange polymer, such as a Nafion® polymer, to create a very thin (<0.5 mil) supported film. To ensure a continuous coating, this very thin film may have to be laid down in two or more very thin passes since as less dispersion is applied, the ability to fill the pores and form a continuous film decreases. In an embodiment of this type, the wet ion exchange polymer coating may be applied to a Mylar^® film base and the ePTFE is laid into the wet coating from the top. The composite film, optionally dried, may then be removed from the Mylar^® film base and, if needed, a very thin second pass of ion exchange polymer solution or dispersion may be laid down on the same side of the ePTFE as the first thin film. The resulting structure will have a continuous thin film of ion exchange polymer on one side of the ePTFE, but the other side of the ePTFE will be unfilled and open. Another perhaps more efficient means of accomplishing this is to first coat a very thin film of ion exchange polymer onto Mylar^® and dry, then in a second pass apply fresh dispersion on the dried film and then lay in the ePTFE substrate. The second pass of dispersion then acts as the adhesive to bond the continuous film to the substrate. The composite can then be removed from the Mylar^®.

In embodiments of the invention, the flow battery comprises a reinforced composite separator membrane in accordance with the invention, which separator may be prepared by taking a microporous support such as ePTFE and imbibing it with an ionomer solution or dispersion, such as a Nafion^® solution or dispersion, which already has all of the functional groups (i.e., approaching and/or achieving 100%) being represented by the formula -SO3X wherein X is H, then drying the imbibed support, and then annealing the dried imbibed support. It is also possible to imbibe the polymer in the -SO₂F form and then carry out hydrolysis of the SO2F groups to SO3H in situ, for example hydrolysis of the reinforced composite membrane.

In an embodiment of the invention, the flow battery comprises a composite polymer separator membrane in accordance with the invention, which separator comprises a Nafion^® XL™ 100 ePTFE reinforced membrane which has a thickness of about 30 microns or less (1 .25 mils or less), or 30-25 microns (1 .25 to 1 .0 mils), or 25-20 microns (1 .0 to

0.8 mils) or as thin as 20-15 microns (0.8 to 0.6 mils) or 15-10 microns (0.6 mils to 0.4 mils), and which is a reinforced composite polymer membrane made of perfluorosulfonic acid ("PFSA") ionomer in the proton form and ePTFE support.

Experimental

Herein, unless otherwise stated, the composite polymer separator membrane is a symmetric structure, coated evenly and essentially the same on both sides of the reinforcement layer. Composite Polymer Separator Membranes:

The composite polymer separator membranes of Example 1 in accordance with the invention were Nafion^® XL™ 100 reinforced composite polymer membranes in the sulfonic acid form and having a thickness of about 25 microns (1 mil) and a size of about 10 cm x 10 cm (4 inch x 4 inch). A piece of dry membrane was used.

Testing

Membrane Swelling Measurement:

The swelling value for the reinforced membrane was determined using membrane strips punched out from the membrane using a 1 " χ 3" mm die along the direction parallel to MD and TD direction of the membrane. A punched out strip from MD was taken and it was conditioned in a humidity room (22°C, 50% RH) for 24 hrs. After conditioning the membrane strip, it was placed between polyethylene (PE) sheets and the length of the membrane strip along the long direction was marked on the PE sheet. The distance between these two marks was measured as the dry length L_d. After measurement of L_d, the membrane strip was boiled in deionized (Dl) water for one hour and then it was cooled to ambient temperature by placing it between polyethylene (PE) sheets to prevent water evaporation during the cooling. The length of the membrane strip along the long direction was marked on the PE sheet and the distance between these two marks was measured as the wet (or swollen) length L_w. The membrane swelling was calculated using the formula below.

Figure 1 shows the comparative swelling (at 50% relative humidity and 23°C) for the ePTFE-reinforced Nafion^® separator membrane for various membrane thicknesses. These can be compared with the swelling of the Nafion^® dense membrane (ionomer with no reinforcement).

The ePTFE reinforced membrane shows improved resistance to swelling (i.e. -3-5% for a 5 mil thickness) compared to the non-reinforced membrane, which has average swelling of approximately 10% for a 5 mil thickness.

Flow Batteries:

The ePTFE-reinforced Nafion^® separator membrane was assessed directly in Flow Battery applications, including all-vanadium VRB systems using sulfuric acid electrolyte throughout; all-vanadium VRB systems using a mixed sulfuric acid / hydrochloric acid electrolyte throughout; an Fe/V system using the same mixed acid electrolyte; and a Zn/Fe zinc flow battery system based on the disclosure of United States Patent Number 4,180,623 (to Gordon et a/.). Additionally, asymmetric ePTFE-reinforced Nafion^® separator membranes were also assessed in the same systems. In each case, the flow battery was operable with the ePTFE-reinforced Nafion^® separator membrane between the anolyte and catholyte solutions. In each case, it was found that the performance was comparable to that of the dense Nafion^® control membrane (same ionomer but without the reinforcement material), but the reinforced membrane was easier to handle and with less issues from swelling of the membrane. Typical performance characteristics included a Coulombic Efiiciency of -96%, Voltage Efficiency of -87%, and total Energy Efficiency of -84% (although the zinc flow battery was not run long enough to establish meaningful efficiency data).

Conductivity Measurement:

Conductivity measurements were made through-plane (current flows perpendicular to the plane of the membrane) and in-plane (current flows over the plane of the membrane) using the technique described below.

In-Plane (X-Y Direction) Conductivity

A rectangular sample of 1 .6 cm χ 3.0 cm was cut from the

preconditioned membrane sample and placed in the conductivity fixture. The fixture was placed into a glass beaker filled with Dl water. The membrane impedance was measured using Solartron SI-1260 Impedance Analyzer, The conductivity (k) was determined using the following equation,

1.00 cm

K =

(R X I X W) where, R is the membrane impedance, T is the membrane thickness and "w" is the membrane width. Both "Γ and "w" are in cm.

Figure 2 shows the in-plane conductivity (at 100% relative humidity) for the ePTFE-reinforced Nafion^® separator membrane for various membrane thicknesses, and compares them to the in-plane conductivity of the Nafion^® dense membrane (ionomer with no reinforcement). The ePTFE reinforced membranes, labeled PFSA, show comparable or slightly higher in-plane conductivity than the non-reinforced membranes of similar thickness, labeled N1 17 and N1 15 (Nafion^® N1 17 and N1 15, commercially available from DuPont), although this difference has an insignificant effect on the voltaic and coulombic efficiency in the Flow battery.

In-Plane (X-Y Direction) Conductivity

The experimental composite membrane sample was boiled in Dl water for one hour and then a rectangular sample of 1 .6 cm χ 3.0 cm was cut from the swollen membrane sample and placed in the conductivity fixture. The fixture was placed into a glass beaker filled with Dl water. The membrane impedance was measured using Solotron SI-1260 Impedance Analyzer. The conductivity (k) was determined using the following equation,

/

(R X A) where, R is the membrane impedance, "t" is the membrane thickness and "w" is the membrane width. Both "t" and "w" are in cm.

Ion Exchange Capacity (lEC) Measurement:

The IEC of the membranes may be determined using a titration method. The fixed ions such as sulfonic acid groups (SO₃ ^") are titrated with 1 .0 M NaOH. The membrane in the Na⁺ form is soaked in 1 .0 M HCI solution to convert to the H⁺ form of the membrane. The membrane is then immersed in a known volume of 1 .0 M NaOH solution and soaked overnight at room temperature. The amount of H⁺ (in millimoles) is determined by back titration with a 1 .0 M HCI. The membrane is then washed with distilled water and dried under vacuum. The IEC is calculated using the following formula.

1000

IEC

Dry Membrane Weight (g)

V_Na0H n x [NaOH] (M)

The units are shown in parentheses after each term in the equation: the dry membrane weight is in grams; the volume, V, of NaOH is in liters and the concentration of NaOH is a molarity (M) or moles per liter.

Permeability Measurement to Assess Vanadium Crossover:

Figure 3 illustrates the equipment used for the measurement of the permeability of vanadium ion, i.e. V(IV) across a polymer separator membrane, which, for example, can be used to assess vanadium ion crossover in VRB redox flow batteries. All membrane samples were preconditioned by soaking in 2.5 M H₂SO for 3 hours then rinsed throughly. The samples were kept hydrated until the test.

The V(IV) solution was prepared by dissolving VOSO₄ (Aldrich) in 2.5 M H₂SO₄. In Figure 3, the left reservoir (1 ) was filled with 2 M VOSO₄ solution in 2.5 M H₂SO₄, and the right reservoir (2) was filled with 2 M MgSO solution in 2.5 M H₂SO ; two sampling ports are labeled (3); and the test membrane is labeled (4). MgSO₄ was used to equalize the ionic strengths of two solutions and to minimize the osmotic pressure effect. (Vanadium ion crossover is easier to detect in a solution that initially has no vanadium ion present; this allows a comparative study of the effect of different membranes on vanadium crossover). The two solutions were separated by a membrane (4) and continuously stirred using magnetic stir bars during experiments at room temperature. The geometrical area of the exposed membrane was 5.7 cm² and the volume of the solution in each reservoir was 100 imL Samples of solution from the right reservoir were taken at regular intervals and analyzed for vanadium ion

concentration by UV spectrophotometer. The absorption values were collected at 760 nm. The rate of change in the solution absorbance was used to calculate the diffusion coefficient for vanadium ions. The measurements were performed at room temperature. The vanadium ion concentration obtained using the UV spectrophotometer was also validated by inductively coupled plasma spectroscopy (ICP). Experiments were performed for separator membranes with and without ePTFE reinforcement (Table 1 ).

The ePTFE-reinforced Nafion® separator membrane was evaluated for use as a polymer separator in VRB flow batteries and compared with both the equivalent dense membrane (ionomer with no reinforcement), as well as a polymer separator membrane comprising the same ionomer and a hydrocarbon membrane as reinforcement which is commercially available and has been considered for use in VRB type flow batteries. Two grades of Daramic® (ultra high molecular weight polyethylene, available from Daramic LLC, Polypore International Inc., Owensboro, KY, USA) were evaluated. In each case, the Daramic® reinforced ionomer separator was prepared by soaking the Daramic® membrane in a 20% Nafion® solution for 1 hour, followed by drying, and annealing the Daramic®/Nafion® composite membrane at 165°C for 45 seconds. For each membrane, the vanadium crossover was measured as described above and shown in the data table (Table 1 ) as Diffusion Coefficient, although perhaps better represented by Flux Factor (which is the diffusion coefficient normalized to a constant diffusion path length by dividing by the membrane thickness - shown in parentheses next to the Diffusion Coefficient). The conductivity reported in Table 1 is thru-plane conductivity, measured as described above. Figure 4 graphically compares the Diffusion Coefficient for each of the membranes and Figure 5 graphically compares the selectivity of allowing passage of hydrogen ions (protons) through the membrane compared to vanadium IV ions. Table 1 . V Diffusion Coefficient and H⁺A/ Selectivity of Separator

Membranes

Areal

Resistanc Diffusion

4+

Membrane Thickness Conductivity e coefficient of V Selectivity

μιη mS/cm ΠΊΩ cm2 x10-6 cm2 min-1 H⁺/V⁴⁺

D 165/250 255 4.5 5536.0 2.72 (1 .1 ) 1 .6544 D165/175 221 14.9 1537.0 1 .20 (0.5) 12.4167

NR212 AT 52 70.6 77.2 1 .20 (2.3) 58.8333

201 B1 -1 1041 1 -2 AT 57.6 53.4 106.6 0.89 (1 .5) 60.0000

In Table 1 , D165/250 and D165/175 are acid treated Daramic^®/Nafion^® composite membranes formed from 250 micron and 175 micron Daramic^® 5 membranes, respectively, each one imbibed with Nafion^® solution and

annealed at 165°C for 45 seconds; NR212 AT is an acid treated dense Nafion^® membrane (with no reinforcement); and 201 B1 -1 1041 1 -2 AT is an acid treated ePTFE-reinforced Nafion^® separator membrane. NR212 AT and 201 B1 -1 1041 1 -2 AT were also annealed at 165°C for 45 seconds.

10 The data show that the ePTFE-reinforced Nafion^® separator

membrane has a lower vanadium Diffusion Coefficient (and Flux Factor) than the dense Nafion^® membrane (with no reinforcement) and therefore shows reduced vanadium crossover compared to the dense Nafion membrane; and, furthermore, shows improved selectivity compared to the

15 Daramic^®/Nafion^® composite membranes in favoring proton conductivity over vanadium ion diffusion across the membrane.

Vanadium ion crossover does not immediately affect the measured performance efficiency characteristics in VRB redox flow batteries, but causes significant problems over longer periods of operation as would be 20 required in commercial uses. Vanadium ion crossover over longer periods of VRB operation causes capacity fade and accelerates the self-discharge rate of the battery. The latter would require periodic electrolyte

maintenance over the lifetime use of the battery, with the associated costs and down-time issues.

Claims

CLAIMS What is claimed is:

1 . A flow battery comprising an ePTFE reinforced ionomer separator membrane.

2. The flow battery of claim 1 wherein said reinforced ionomer

separator membrane comprises a perfluorosulfonic acid or perfluorosulfonate ionomer or a highly fluorinated sulfonic acid or highly fluorinated sulfonate ionomer which has substantially all of the functional groups being represented by the formula -SO₃X wherein X is H, Li, Na, K or N(R¹)(R²)(R³)(R⁴) and R¹, R², R³, and R⁴ are the same or different and are H, CH₃ or C2H₅.

3. The flow battery of claim 2 wherein X is H.

4. The flow battery of claim 2 or claim 3 wherein the ePTFE

reinforcement has a pore size no greater than 10 microns.

5. The flow battery of claim 2 or claim 3 wherein the ePTFE

reinforcement has a pore size no greater than 5 microns.

6. The flow battery of claim 2 or claim 3 wherein said reinforced

ionomer separator membrane comprises:

(a) an ePTFE reinforcement layer having a porosity of at least about 45% and a mean pore size no greater than 10 μιτι, and having opposing surfaces,

(b) at least one perfluorosulfonic acid or perfluorosulfonate

ionomer, or highly fluorinated sulfonic acid or highly fluorinated sulfonate ionomer, impregnated between said opposing surfaces of said ePTFE reinforcement layer such that said at least one ionomer has a volume fraction of at least 40 percent at a midpoint between the opposing surfaces.

7. The flow battery of claim 2 or claim 3 wherein said separator

membrane has a thickness of 150 microns or less.

8. The flow battery of claim 2 or claim 3 wherein said separator membrane has a thickness of 25 microns or less.

9. The flow battery of claim 2 or claim 3 wherein said separator

membrane comprises two or more reinforcement layers.

10. The flow battery of any of claims 1 to 12 wherein the flow battery is a vanadium redox battery.

1 1 . The flow battery of any of claims 1 to 12 wherein the flow battery is a zinc flow battery.

12. The flow battery of claim 1 wherein the ePTFE reinforced ionomer separator membrane is an asymmetric ePTFE reinforced ionomer separator membrane.