Ion-conducting thin-film composite membranes for energy storage applications.

Abstract

An ion-conducting thin-film composite (TFC) membrane is described. The low-cost, high-performance TFC membrane includes a first microporous support membrane, a hydrophilic ionomer polymer coating layer on a first surface of the microporous support membrane, and a second microporous support membrane on a surface of the hydrophilic ionomer polymer coating layer opposite the first microporous support membrane. The hydrophilic ionomer polymer coating is ion-conductive. The ionomer polymer can also be present within the pores of the first microporous support membrane. Methods for fabricating the TFC membrane and redox flow battery systems incorporating the TFC membrane are also described.

Description

[Technical Field]

[0001] (Statement of priority) This application claims priority to U.S. Provisional Patent Application No. 63 / 477,591, filed December 29, 2022, which is incorporated herein by reference in its entirety. [Background technology]

[0002] Energy storage systems have played an important role in collecting energy from various sources. Energy storage systems can be used to store energy and convert it for use in many different applications, such as buildings, transportation, utilities, and industry. Various energy storage systems are in commercial use, and new systems are currently being developed. Energy storage system types can be categorized as electrochemical and battery, thermal, thermochemical, flywheel, compressed air, pumped hydro, magnetic, biological, chemical, and hydrogen energy storage. To solve the energy crisis and overcome the mismatch between power generation and end use, the development of cost-effective and environmentally friendly energy storage systems is necessary.

[0003] Renewable energy sources, such as wind and solar power, have transient characteristics that require energy storage. Renewable energy storage systems, such as redox flow batteries (RFBs), have attracted significant attention for power grids, electric vehicles, and other large-scale stationary applications. RFBs are electrochemical energy storage systems that reversibly convert chemical energy directly into electricity. Converting electricity into hydrogen as an energy carrier through water electrolysis without producing carbon monoxide or carbon dioxide as byproducts enables the intersection of electrical, chemical, mobility, and heating fields. Water electrolysis produces high-quality hydrogen by electrochemically splitting water into hydrogen and oxygen. When the process is powered by renewable power sources, such as wind, solar, or geothermal energy, water electrolysis has a zero carbon footprint. Major water electrolysis technologies include alkaline electrolysis, polymer electrolyte membrane (PEM) electrolysis, and solid oxide electrolysis. PEM water electrolysis is one of the preferred methods for converting renewable energy into high-purity hydrogen, and has the advantages of compact design, high current density, high efficiency, fast response, small footprint, low temperature (20-90°C) operation, and high-purity oxygen by-product.

[0004] An RFB consists of two tanks filled with active materials containing metal ions that can be in different valence states, two circulation pumps, and a flow cell with a separation membrane. The separation membrane is placed between the anode and cathode and serves to separate the anolyte and catholyte, while also allowing the movement of equilibrium ions to utilize the current circuit. Among all redox flow batteries developed to date, the all-vanadium redox flow battery (VRFB) has been the most widely studied. VRFBs use the same vanadium element in both half-cells, preventing electrolyte crossover contamination from one half-cell to the other. However, VRFBs are inherently expensive due to the use of high-cost vanadium and expensive membranes. All-iron redox flow batteries (IFBs) are particularly attractive for grid-scale storage applications because they use low-cost iron, salt, and water as the electrolyte.

[0005] Membranes are one of the key materials that make up batteries or electrolysis cells as they are a key factor for safety and performance. Some important properties of membranes for flow batteries, fuel cells, and membrane electrolysis include high conductivity, high ion permeability (porosity, pore size, and pore size distribution), high ion exchange capacity (for ion exchange membranes), high ion / electrolyte selectivity (low permeability / crossover to electrolyte), and low cost (150-200 USD / m 2 These include low area resistance to minimize efficiency loss due to ohmic polarization, high resistance to oxidizing and reducing conditions, chemical inertness over a wide pH range, high thermal stability (above 120°C for fuel cells) along with high proton conductivity, high proton conductivity at high T without HO, high proton conductivity at high T while maintaining high RH, and high mechanical strength (thickness, low swelling).

[0006] The two main types of membranes for redox flow batteries, fuel cells, and electrolysis applications are polymeric ion exchange membranes and microporous separators. Polymeric ion exchange membranes are used to - , -COO - , -PO3 2- , -PO3H - , or -C6H4O - Cation exchange membrane containing cation exchange functional groups, -NH3 + , -NRH2 + , -NR2H + , or -NR3 + The membranes can be anion-exchange membranes containing anion-exchange functional groups, or bipolar membranes containing both cation-exchange and anion-exchange polymers. Polymers for preparing ion-exchange membranes can be perfluorinated ionomers, such as Nafion®, Flemion®, and NEOSEPTA®-F, partially fluorinated polymers, non-fluorinated hydrocarbon polymers, non-fluorinated polymers with aromatic backbones, or acid-base blends. Perfluorosulfonic acid (PFSA)-based membranes, such as Nafion® and Flemion®, are commonly used in vanadium redox flow battery (VRFB) systems due to their oxidative stability, good ionic conductivity, unique morphology, mechanical strength, and high electrochemical performance. However, these membranes suffer from low equilibrium ion / electrolyte ion selectivity and high electrolyte metal ion crossover, which leads to reduced capacity in VRFBs and are expensive.

[0007] Microporous and nanoporous membrane separators can be inert microporous / nanoporous polymer membrane separators, inert nonwoven porous films, or polymer / inorganic material coated / impregnated separators. Inert microporous / nanoporous polymer membrane separators can be microporous polyethylene (PE), polypropylene (PP), PE / PP, or composite inorganic / PE / PP membranes, inert nonwoven porous films, nonwoven PE, PP, polyamide (PA), polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), polyethylene terephthalate (PET), or polyester porous films. For example, microporous Daramic® membrane separators and Celgard® membrane separators made from PE polymer or PP polymer are commercially available. They typically have high ionic conductivity but also high electrolyte crossover for RFB applications.

[0008] Despite significant research efforts, widespread adoption of redox flow batteries for grid energy storage applications remains a challenge.

[0009] Therefore, there is a need for reliable, high performance (low electrolyte or gas crossover and excellent conductivity), low cost membranes for energy storage applications such as redox flow batteries, fuel cells, and electrolysis applications. DETAILED DESCRIPTION OF THE INVENTION

[0010] Low-cost, high-performance ion-conducting thin film composite (TFC) membranes have previously been developed for energy storage applications such as redox flow batteries, fuel cells, and electrolysis applications. These TFC membranes provide an ion-conducting membrane that combines a size-exclusion ion-conducting separation mechanism derived from the hydrophilicity of the polymer with an ion-exchange ion-conducting separation mechanism derived from the ionomeric nature of the polymer. The ion-conducting TFC membrane exhibits improved performance compared to conventional polymer ion-exchange membranes with an ion-exchange ion-conducting separation mechanism and microporous membrane separators with a size-exclusion ion-conducting separation mechanism. The TFC membrane includes a microporous support membrane and a hydrophilic ionomer polymer coating layer on the surface of the microporous support membrane. The ionomer polymer can also be present within the micropores of the support membrane. The hydrophilic ionomer polymer coating layer is ion-conducting. This is because the hydrophilic ionomer polymer coating has ion conductivity, allowing the transport of protons or chloride ions (Cl). - This means that charge-carrying ions, such as protons, chloride ions, potassium ions, or sodium ions in an all-iron redox flow battery system, can be transported from one side of the membrane to the other side of the membrane to maintain an electrical circuit. Electrical balance is achieved by the transport of charge-carrying ions in the electrolyte (such as protons, chloride ions, potassium ions, or sodium ions in an all-iron redox flow battery system) across the membrane containing the hydrophilic ionomer polymer coating layer during operation of the battery cell.

[0011] The ionic conductivity (σ) of a membrane is a measure of its ability to conduct charge-carrying ions, and the unit of measure for conductivity is siemens per meter (S / m). The ionic conductivity (σ) of an ion-conducting TFC membrane can be measured by determining the resistance (R) of the membrane between two electrodes separated by a fixed distance. The resistance is determined by electrochemical impedance spectroscopy (EIS), and the unit of measure for resistance is ohms (Ω). The membrane area specific resistance (RA) is the product of the membrane resistance (R) and the membrane active area (A), and the unit of measure for membrane area specific resistance is (Ω cm 2The membrane ionic conductivity (σ, S / cm) is proportional to the membrane thickness (L, cm) and the membrane area specific resistance (RA, Ω cm 2 The performance of ion-conducting TFC membranes for RFB applications is evaluated by several parameters, including membrane solubility and stability in the electrolyte, area-specific resistivity, the number of battery charge / discharge cycles, electrolyte crossover through the membrane, voltage efficiency (VE), coulombic efficiency (CE), and energy efficiency (EE) of RFB cells. CE is the ratio of a cell's discharge capacity divided by its charge capacity. A higher CE, indicating lower capacity loss, is primarily due to a lower crossover rate of electrolyte ions, such as ferric and ferrous ions, in iron redox flow battery systems. VE is defined as the ratio of a cell's average discharge voltage divided by its average charge voltage (see M. Skyllas-Kazacos, C. Menictas, and T. Lim, Chapter 12 on Redox Flow Batteries for Medium- to Large-Scale Energy Storage in Electricity Transmission, Distribution and Storage Systems, A volume in Woodhead Publishing Series in Energy, 2013). The higher VE, indicating higher ionic conductivity, is mainly due to the lower area-specific resistance of the membrane. EE, the product of VE and CE, is an indicator of the energy loss in the charge-discharge process. EE is an important parameter for evaluating energy storage systems.

[0012] By incorporating low-cost, high-performance hydrophilic ionomeric polymers into TFC membranes, we have provided ion-conductive membranes that combine the size-exclusion ion-conductive separation mechanism derived from the polymer's hydrophilicity with the ion-exchange ion-conductive separation mechanism derived from the polymer's ionomeric nature. Therefore, compared with conventional polymer ion-exchange membranes with ion-exchange ion-conductive separation mechanisms and microporous membrane separators with size-exclusion ion-conductive separation mechanisms, the ion-conductive TFC membranes exhibit improved performance for energy storage applications, such as redox flow battery applications. The ion-conductive TFC membranes exhibited excellent membrane stability in electrolytes, low area-specific resistance, high battery charge / discharge cycle counts, low electrolyte crossover through the membrane, and high VE, CE, and EE for redox flow battery applications.

[0013] However, it has been found that the hydrophilic ionomer polymer coating layer on the surface of the microporous support membrane near the sealing area of ​​the battery cell can be damaged during stack assembly. The coating layer can also be damaged by fibers on the carbon felt electrode during battery stack assembly or battery operation. This damage can lead to reduced server performance.

[0014] To avoid this potential damage, a new low-cost, high-performance ion-conducting TFC membrane has been developed. The new TFC membrane includes a first microporous support membrane, a hydrophilic ionomer polymer coating layer on one surface of the first microporous support membrane, and a second microporous support membrane on the opposite surface of the hydrophilic ionomer polymer coating layer from the first microporous support membrane. This forms a sandwich structure in which the hydrophilic ionomer polymer coating is positioned between and protected by the two microporous support membranes. The second microporous support membrane can be bonded to the second surface of the hydrophilic ionomer polymer coating layer. Alternatively, the second microporous support membrane can be positioned next to the hydrophilic ionomer polymer coating layer without being bonded to the hydrophilic ionomer polymer coating layer.

[0015] Hydrophilic ionomeric polymers are composed of hydrophilic ionomeric polymers or electrically neutral repeating units and -SO3 - Group, -COO - Group, -PO3 2- group, -PO3H - group, -C6H4O - group, -O4B - group, -NH3 + group, -NRH2 + Group, -NR2H + group, -NR3 + group, or -SR2 - Crosslinked hydrophilic ionomeric polymers contain repeat units containing both ionic functional groups and small amounts of ionic functional groups such as -SO3 - Group, -COO - group, or -NH3 + The crosslinked hydrophilic polymers contain polar or charged functional groups with high water affinity, such as groups. Crosslinked hydrophilic polymers include hydrophilic polymers complexed with a complexing agent, such as polyphosphate, boric acid, metal ions, or mixtures thereof. Hydrophilic ionomeric polymers not only have high stability in aqueous electrolyte solutions due to their insolubility in aqueous electrolyte solutions, but also have high reactivity with water and HO due to the hydrophilic and ionomeric nature of the polymers. + or Cl - and therefore has high ionic conductivity and low membrane area resistance.

[0016] The hydrophilic ionomer polymer coating layer on the ion-conducting TFC membrane typically comprises a dense layer having a thickness ranging from 1 micrometer to 100 micrometers, or from 5 micrometers to 50 micrometers. The dense hydrophilic ionomer polymer coating layer forms very small nanopores with pore sizes less than 0.5 nm in the presence of liquid water or water vapor, which, in some cases, combined with the presence of a crosslinked polymer structure via a complexing agent to control the degree of swelling of the polymer, results in high selectivity for charge-carrying ions such as protons, hydrated protons, chloride ions, potassium ions, hydrated potassium ions, sodium ions, and hydrated sodium ions over electrolytes such as ferric ions, hydrated ferric ions, ferrous ions, and hydrated ferrous ions.

[0017] Suitable hydrophilic ionomeric polymers include, but are not limited to, polyphosphate-complexed polysaccharide polymers, polyphosphate- and metal ion-complexed polysaccharide polymers, metal ion-complexed polysaccharide polymers, borate-complexed polysaccharide polymers, alginate polymers such as sodium alginate, potassium alginate, calcium alginate, ammonium alginate, alginate polymers, hyaluronic acid polymers, borate-complexed polyvinyl alcohol polymers, polyphosphate-complexed polyvinyl alcohol polymers, polyphosphate- and metal ion-complexed polyvinyl alcohol polymers, metal ion-complexed polyvinyl alcohol polymers, metal ion-complexed poly(acrylic acid) polymers, borate-complexed poly(acrylic acid) polymers, metal ion-complexed poly(methacrylic acid), borate-complexed poly(methacrylic acid), or combinations thereof.

[0018] Various types of polysaccharide polymers can be used, including, but not limited to, chitosan, sodium alginate, potassium alginate, calcium alginate, ammonium alginate, alginic acid, sodium hyaluronate, potassium hyaluronate, calcium hyaluronate, ammonium hyaluronate, hyaluronic acid, dextran, pullulan, carboxymethyl curdlan, sodium carboxymethyl curdlan, potassium carboxymethyl curdlan, calcium carboxymethyl curdlan, ammonium carboxymethyl curdlan, κ-carrageenan, λ-carrageenan, ι-carrageenan, carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, calcium carboxymethyl cellulose, ammonium carboxymethyl cellulose, pectinic acid, chitin, chondroitin, xanthan gum, or combinations thereof.

[0019] In some embodiments, the hydrophilic ionomeric polymer is a polyphosphate-complexed chitosan polymer, a polyphosphate- and metal ion-complexed chitosan polymer, a metal ion-complexed alginate polymer, or a combination thereof.

[0020] In some embodiments, the hydrophilic ionomeric polymer is a borate-complexed polyvinyl alcohol polymer, a borate-complexed alginic acid, or a blend of borate-complexed polyvinyl alcohol and alginic acid polymer.

[0021] In some embodiments, the metal ion complexing agent is a ferric ion, a ferrous ion, or a vanadium ion.

[0022] In some embodiments, the hydrophilic ionomeric polymer is present within the pores of the first microporous support membrane.

[0023] The first and second microporous support membranes should have good thermal stability (stable up to at least 100°C), high aqueous and organic solution resistance (insoluble in aqueous and organic solutions) under low pH conditions (e.g., pH less than 6), high resistance to oxidizing and reducing conditions (insolubility and no performance degradation under oxidizing and reducing conditions), high mechanical strength (no dimensional change under system operating conditions), and other factors determined by the operating conditions for the energy storage application. The first and second microporous support membranes should be compatible with the cell chemistry and meet the mechanical demands of the cell stacking or wound assembly operation. The microporous support membranes have high ionic conductivity but low selectivity for charge-carrying ions such as protons, hydrated protons, chloride ions, potassium ions, sodium ions, hydrated potassium ions, and hydrated sodium ions over electrolytes such as ferric ions, hydrated ferric ions, ferrous ions, and hydrated ferrous ions.

[0024] The first microporous support membrane and the second microporous support membrane can be made from the same material or different materials.

[0025] Suitable polymers for preparing the first and second microporous support membranes can be selected from, but are not limited to, polyolefins such as polyethylene and polypropylene, mixtures of polyethylene and silica particles, mixtures of polypropylene and silica particles, polyamides such as nylon 6 and nylon 6,6, polyacrylonitrile, polyethersulfone, sulfonated polyethersulfone, polysulfone, sulfonated polysulfone, poly(ether ether ketone), sulfonated poly(ether ether ketone), polyester, cellulose acetate, cellulose triacetate, polybenzimidazole, polyimide, polyvinylidene fluoride, polycarbonate, cellulose, or combinations thereof. These polymers offer a range of properties, including low cost, high stability in water and electrolytes over a wide pH range, good mechanical stability, and easy processability for membrane formation.

[0026] The first and second microporous support membranes can have either symmetric or asymmetric pore structures. Asymmetric microporous support membranes can be formed by a phase inversion membrane formation approach followed by direct air drying or by phase inversion followed by solvent exchange. The first and second microporous support membranes can also be produced by dry processing of thermoplastic polyolefins or wet processing of thermoplastic olefins. Dry processing of thermoplastic polyolefins utilizes extrusion to raise the polymer above its melting point and form it into the desired shape. Subsequent annealing and stretching processes may be performed to increase the crystallinity, orientation, and size of the pores. Wet processing of polyolefin separators is carried out with the aid of a hydrocarbon liquid or low molecular weight oil mixed with a polymer resin or a mixture of polymer resin and inorganic nanoparticles in the molten phase. The molten mixture is extruded through a die similar to that used for dry-processed separators. The thickness of the microporous support membrane can be in the range of 10 to 1000 micrometers, or 10 to 900 micrometers, or 10 to 800 micrometers, or 10 to 700 micrometers, or 10 to 600 micrometers, or 10 to 500 micrometers, or 20 to 500 micrometers. The pore size of the microporous membrane can be in the range of 10 nanometers to 50 micrometers, or 50 nanometers to 10 micrometers, or 0.2 micrometers to 1 micrometer.

[0027] Another aspect of the invention is a method of making a TFC membrane. In one embodiment, the method includes applying a layer of an aqueous solution containing a hydrophilic ionomeric polymer to one surface of a first microporous support membrane, drying the coated membrane, optionally complexing the hydrophilic ionomeric polymer using a complexing agent to form a crosslinked hydrophilic ionomeric polymer, and applying a second microporous support membrane to the coated membrane on the opposite surface of the first microporous support membrane.

[0028] In some embodiments, the coated membrane is dried before complexing the hydrophilic ionomeric polymer. In other embodiments, the coated membrane is dried after complexing the hydrophilic polymer. In other embodiments, the coated membrane is dried before complexing the hydrophilic ionomeric polymer and again after complexing the hydrophilic polymer. The coated membrane may be dried for a time ranging from 5 minutes to 5 hours, or from 5 minutes to 4 hours, or from 5 minutes to 3 hours, or from 10 minutes to 2 hours, or from 30 minutes to 1 hour, at a temperature ranging from 40°C to 100°C, or from 40°C to 80°C, or from 55°C to 65°C.

[0029] In some embodiments, a second microporous membrane is applied to the coated membrane after the coated membrane has dried.

[0030] In some embodiments, the complexing agent is selected from polyphosphate, boric acid, metal ions selected from ferric ions, ferrous ions, or vanadium ions, or combinations thereof.

[0031] In some embodiments, complexing the hydrophilic ionomeric polymer comprises immersing the dried, coated membrane in a second aqueous solution of polyphosphoric acid, boric acid, a metal salt, hydrochloric acid, or a combination thereof.

[0032] In some embodiments, complexing the hydrophilic ionomeric polymer comprises complexing the dried, coated membrane together with a second microporous support membrane in situ within a redox flow battery cell with a complexing agent.

[0033] In some embodiments, the aqueous solution comprises acetic acid or other inorganic or organic acids.

[0034] In some embodiments, the hydrophilic ionomeric polymer on the coated membrane is treated in an aqueous solution of hydrochloric acid prior to complexing the hydrophilic polymer.

[0035] In some embodiments, the hydrophilic polymer layer on the coated membrane is immersed in a second aqueous solution of polyphosphoric acid or boric acid for a time ranging from 5 minutes to 24 hours, or from 5 minutes to 12 hours, or from 5 minutes to 8 hours, or from 10 minutes to 5 hours, or from 30 minutes to 1 hour, and then immersed in an aqueous metal salt solution or an aqueous hydrochloric acid solution for a time ranging from 5 minutes to 24 hours, or from 5 minutes to 12 hours, or from 5 minutes to 8 hours, or from 10 minutes to 5 hours, or from 30 minutes to 1 hour.

[0036] In other embodiments, the hydrophilic polymer is complexed in situ with a complexing agent in the negative electrolyte, the positive electrolyte, or both the negative and positive electrolytes in the redox flow battery cell.

[0037] In some embodiments, the hydrophilic ionomeric polymer comprises a polysaccharide polymer, a poly(acrylic acid) polymer, a poly(methacrylic acid), or a combination thereof.

[0038] In some embodiments, the polysaccharide polymer comprises chitosan, sodium alginate, potassium alginate, calcium alginate, ammonium alginate, alginic acid, sodium hyaluronate, potassium hyaluronate, calcium hyaluronate, ammonium hyaluronate, hyaluronic acid, dextran, pullulan, carboxymethyl curdlan, sodium carboxymethyl curdlan, potassium carboxymethyl curdlan, calcium carboxymethyl curdlan, ammonium carboxymethyl curdlan, κ-carrageenan, λ-carrageenan, ι-carrageenan, carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, calcium carboxymethyl cellulose, ammonium carboxymethyl cellulose, pectinic acid, chitin, chondroitin, xanthan gum, or a combination thereof.

[0039] Another aspect of the present invention is a redox flow battery system. In one embodiment, the redox flow battery system includes at least one rechargeable cell including a positive electrolyte, a negative electrolyte, and an ion-conducting thin film composite (TFC) membrane disposed between the positive electrolyte and the negative electrolyte, the TFC membrane including a first microporous support membrane, a hydrophilic ionomer polymer coating layer on a first surface of the microporous support membrane, and a second microporous support membrane on a surface of the hydrophilic ionomer polymer coating layer opposite the first microporous support membrane, wherein the hydrophilic ionomer polymer coating layer is ion-conducting.

[0040] In some embodiments, the negative electrolyte, the positive electrolyte, or both the negative and positive electrolytes comprise a boric acid additive that can complex with a hydrophilic polymer on the surface of the first microporous support membrane to form a crosslinked hydrophilic ionomeric polymer coating layer.

[0041] In some embodiments, the negative electrolyte, the positive electrolyte, or both the negative and positive electrolytes comprise ferrous chloride.

[0042] In some embodiments, the positive electrolyte comprises ferrous chloride and hydrochloric acid.

[0043] In some embodiments, the hydrophilic ionomeric polymer coating layer is formed in situ by complexing the hydrophilic polymer on the surface of a first microporous support membrane with a complexing agent in a negative electrolyte, a positive electrolyte, or both a negative and a positive electrolyte, typically in the presence of a second microporous support membrane. [Example]

[0044] The following examples are provided to illustrate, but not to limit, one or more preferred embodiments of the invention. Many variations can be made to the following examples that remain within the scope of the invention.

[0045] Comparative Example 1. Preparation of an iron flow battery cell containing an alginate / Daramic® thin film composite (TFC) membrane (abbreviated as AD-IFB) An iron flow battery cell containing an alginate / Daramic® thin film composite (TFC) membrane, carbon felt positive and negative electrodes, positive and negative electrolytes was prepared as follows.

[0046] A 9.0 wt% sodium alginate aqueous solution was prepared by dissolving sodium alginate polymer in deionized (DI) water. One surface of a Daramic® microporous support membrane (purchased from Daramic, LLC) was coated with a thin layer of the 9.0 wt% sodium alginate aqueous solution and dried at 60°C for 2 hours to form a sodium alginate layer on the surface of the Daramic® support membrane. The dried sodium alginate / Daramic® TFC membrane was assembled with a carbon felt cathode, a carbon felt anode, two plastic flow frames, two graphite bipolar plates, and two copper current collectors to form an iron flow battery cell. A positive electrolyte tank containing a positive electrolyte and a negative electrolyte tank containing a negative electrolyte were connected to the positive and negative sides of the iron flow battery cell, respectively. The positive electrolyte was circulated in the battery cell for 2 hours before the battery was operated to form the final iron flow battery cell containing the alginate / Daramic® TFC membrane (abbreviated as AD-IFB).

[0047] Example 1. Preparation of an iron flow battery cell containing a sandwiched Daramic® / alginate / Daramic® TFC membrane (abbreviated as DAD-IFB) An ion flow battery cell containing a sandwich of Daramic® / alginate / Daramic® TFC membrane, carbon felt positive and negative electrodes, and positive and negative electrolytes was prepared as follows. A 9.0 wt% sodium alginate solution was prepared by dissolving sodium alginate polymer in deionized (DI) water. One surface of a Daramic® microporous support membrane (purchased from Daramic, LLC) was coated with a thin layer of the 9.0 wt% sodium alginate solution and dried at 60°C for 2 hours to form a sodium alginate layer on the surface of the Daramic® support membrane. A second Daramic® microporous support membrane was added to the surface of the dried sodium alginate coating layer to form a sandwiched Daramic® / alginic acid / Daramic® TFC membrane. The membrane was assembled with a carbon felt cathode, a carbon felt anode, two plastic flow frames, two graphite bipolar plates, and two copper current collectors to form an iron flow battery cell. A positive electrolyte tank containing a positive electrolyte and a negative electrolyte tank containing a negative electrolyte were connected to the positive and negative sides of the iron flow battery cell, respectively. The positive electrolyte was circulated in the battery cell for 2 hours before the battery was operated to form the final iron flow battery cell (abbreviated as DAD-IFB) containing the sandwiched Daramic® / alginate / Daramic® TFC membrane.

[0048] Example 2. All-Iron Redox Flow Battery Performance Study of AD-IFB and DAD-IFB Batteries The battery performance of the all-iron redox flow battery AD-IFB described in Comparative Example 1 and the DAD-IFB described in Example 1 was evaluated at 40°C using an Arbin RBT battery tester (Arbin Instruments, USA). Both batteries used the same electrolyte formulation, including a positive electrolyte solution and a negative electrolyte solution. Both the positive and negative electrolyte solutions contained FeCl2, NH4Cl, HCl, and glycine in ultrapure water (18.2 MΩ·cm). The results show that the initial CE of DAD-IFB is 2% higher than that of AD-IFB due to the addition of the second microporous support membrane, but the VE of DAD-IFB is 2.8% lower than that of AD-IFB. The initial overall performance (EE) of DAD-IFB and AD-IFB is comparable. However, more importantly, long-term testing revealed that the EE of DAD-IFB is much more stable than that achieved by AD-IFB. These results demonstrated that for the new TFC membrane, the hydrophilic ionomer polymer coating layer sandwiched between two microporous support membranes can be effectively protected from breakage near the sealing area during stack assembly, as well as damage by fibers on the carbon felt during stack assembly or battery operation.

[0049] Specific Embodiments While the following will be described in conjunction with specific embodiments, it will be understood that this description is illustrative, but not intended to limit the scope of the preceding description and appended claims.

[0050] A first embodiment of the present invention is a composition comprising a first microporous support membrane, a hydrophilic ionomer polymer coating layer on a surface of the first microporous support membrane, the hydrophilic ionomer polymer coating layer being ion-conductive, and a second microporous support membrane on a surface of the hydrophilic ionomer polymer coating layer opposite the first microporous support membrane. An embodiment of the present invention is one, any, or all of the previous embodiments of this paragraph, including through the first embodiment of this paragraph, wherein the hydrophilic ionomeric polymer comprises a polyphosphate-complexed polysaccharide polymer, a polyphosphate- and metal ion-complexed polysaccharide polymer, a metal ion-complexed polysaccharide polymer, a borate-complexed polysaccharide polymer, an alginate polymer, an alginate polymer, a hyaluronic acid polymer, a borate-complexed polyvinyl alcohol polymer, a polyphosphate-complexed polyvinyl alcohol polymer, a polyphosphate- and metal ion-complexed polyvinyl alcohol polymer, a metal ion-complexed polyvinyl alcohol polymer, a metal ion-complexed poly(acrylic acid) polymer, a borate-complexed poly(acrylic acid) polymer, a metal ion-complexed poly(methacrylic acid), a borate-complexed poly(methacrylic acid), or a combination thereof. An embodiment of the present invention is one, any, or all of the preceding embodiments of this paragraph, including through the first embodiment of this paragraph, wherein the polysaccharide polymer comprises chitosan, sodium alginate, potassium alginate, calcium alginate, ammonium alginate, alginic acid, sodium hyaluronate, potassium hyaluronate, calcium hyaluronate, ammonium hyaluronate, hyaluronic acid, dextran, pullulan, carboxymethyl curdlan, sodium carboxymethyl curdlan, potassium carboxymethyl curdlan, calcium carboxymethyl curdlan, ammonium carboxymethyl curdlan, κ-carrageenan, λ-carrageenan, ι-carrageenan, carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, calcium carboxymethyl cellulose, ammonium carboxymethyl cellulose, pectinic acid, chitin, chondroitin, xanthan gum, or a combination thereof.An embodiment of the present invention is one, any, or all of the preceding embodiments in this paragraph, including up to the first embodiment of this paragraph, wherein the metal ion is a ferric ion, a ferrous ion, or a vanadium ion. An embodiment of the present invention is one, any, or all of the preceding embodiments in this paragraph, including up to the first embodiment of this paragraph, wherein the hydrophilic ionomeric polymer is a polyphosphate-complexed chitosan polymer, a polyphosphate- and metal ion-complexed chitosan polymer, a metal ion-complexed alginate polymer, a sodium alginate polymer, an alginate polymer, a hyaluronic acid polymer, or a combination thereof. An embodiment of the present invention is one, any, or all of the preceding embodiments in this paragraph, including up to the first embodiment of this paragraph, wherein the metal ion is a ferric ion, a ferrous ion, or a vanadium ion. An embodiment of the present invention is one, any, or all of the preceding embodiments of this paragraph, including through the first embodiment of this paragraph, wherein the hydrophilic ionomeric polymer is a borate-complexed polyvinyl alcohol polymer, a borate-complexed alginic acid, or a blend of borate-complexed polyvinyl alcohol and an alginic acid polymer.An embodiment of the present invention is one, any, or all of the preceding embodiments of this paragraph, including through the first embodiment of this paragraph, wherein the first microporous support membrane, the second microporous support membrane, or both, comprise polyethylene, polypropylene, a mixture of polyethylene and silica particles, a mixture of polypropylene and silica particles, polyamide, polyacrylonitrile, polyethersulfone, sulfonated polyethersulfone, polysulfone, sulfonated polysulfone, poly(ether ether ketone), sulfonated poly(ether ether ketone), polyester, cellulose acetate, cellulose triacetate, polybenzimidazole, polyimide, polyvinylidene fluoride, polycarbonate, cellulose, or a combination thereof. One embodiment of the present invention is one, any, or all of the preceding embodiments of this paragraph, including the first embodiment of this paragraph, wherein the hydrophilic ionomeric polymer is present within the pores of the first microporous support membrane.

[0051] A second embodiment of the present invention is a method for preparing an ion-conducting thin film composite (TFC) membrane, comprising applying a layer of an aqueous solution containing a hydrophilic ionomer polymer to one surface of a first microporous support membrane, drying the coated membrane, optionally complexing the hydrophilic ionomer polymer using a complexing agent to form a crosslinked hydrophilic ionomer polymer, and applying a second microporous support membrane to the coated membrane on the opposite surface of the first microporous support membrane. An embodiment of the present invention is one, any, or all of the previous embodiments of this paragraph, including the first and second embodiments of this paragraph, wherein the hydrophilic ionomer polymer on the coated membrane is dried before complexing the hydrophilic ionomer polymer. An embodiment of the present invention is one, any, or all of the previous embodiments of this paragraph, including the first and second embodiments of this paragraph, wherein the second microporous support membrane is applied to the coated membrane after drying the coated membrane. An embodiment of the present invention is one, any, or all of the preceding embodiments of this paragraph, including through the second embodiment of this paragraph, wherein the complexing agent is selected from polyphosphoric acid, boric acid, a metal ion selected from ferric ions, ferrous ions, or vanadium ions, or a combination thereof. An embodiment of the present invention is one, any, or all of the preceding embodiments of this paragraph, including through the second embodiment of this paragraph, wherein complexing the hydrophilic ionomeric polymer comprises immersing the dried, coated membrane in a second aqueous solution of polyphosphoric acid, boric acid, a metal salt, hydrochloric acid, or a combination thereof. An embodiment of the present invention is one, any, or all of the preceding embodiments of this paragraph, including through the second embodiment of this paragraph, wherein complexing the hydrophilic ionomeric polymer comprises in situ complexing the dried, coated membrane together with a second microporous support membrane with a complexing agent within a redox flow battery cell. An embodiment of the present invention is one, any, or all of the preceding embodiments of this paragraph, including through the second embodiment of this paragraph, wherein the hydrophilic ionomeric polymer comprises a polysaccharide polymer, a poly(acrylic acid) polymer, a poly(methacrylic acid), or a combination thereof.An embodiment of the present invention is one, any, or all of the previous embodiments of this paragraph, including through the second embodiment of this paragraph, wherein the polysaccharide polymer comprises chitosan, sodium alginate, potassium alginate, calcium alginate, ammonium alginate, alginic acid, sodium hyaluronate, potassium hyaluronate, calcium hyaluronate, ammonium hyaluronate, hyaluronic acid, dextran, pullulan, carboxymethyl curdlan, sodium carboxymethyl curdlan, potassium carboxymethyl curdlan, calcium carboxymethyl curdlan, ammonium carboxymethyl curdlan, κ-carrageenan, λ-carrageenan, ι-carrageenan, carboxymethylcellulose, sodium carboxymethylcellulose, potassium carboxymethylcellulose, calcium carboxymethylcellulose, ammonium carboxymethylcellulose, pectinic acid, chitin, chondroitin, xanthan gum, or a combination thereof.

[0052] A third embodiment of the present invention is a system including at least one rechargeable cell including a positive electrolyte, a negative electrolyte, and an ion-conducting thin film composite (TFC) membrane disposed between the positive and negative electrolytes, the TFC membrane including a first microporous support membrane, a hydrophilic ionomer polymer coating layer on a surface of the first microporous support membrane, and a second microporous support membrane on a surface of the hydrophilic ionomer polymer coating layer opposite the first microporous support membrane, the hydrophilic ionomer polymer coating layer being ion-conductive. An embodiment of the present invention is one, any, or all of the previous embodiments in this paragraph, including the first to third embodiments in this paragraph, wherein the negative electrolyte, the positive electrolyte, or both the negative and positive electrolytes include a boric acid additive capable of complexing with the hydrophilic ionomer polymer on the surface of the microporous support membrane to form a crosslinked hydrophilic ionomer polymer coating layer. An embodiment of the present invention is one, any, or all of the previous embodiments of this paragraph, including up to the third embodiment of this paragraph, wherein the hydrophilic ionomeric polymer coating layer is formed in situ by complexing the hydrophilic polymer on the surface of the first microporous support membrane with a complexing agent in a negative electrolyte, a positive electrolyte, or both the negative and positive electrolytes.

[0053] Without further elaboration, it is believed that, using the preceding description, one skilled in the art can utilize the present invention to its fullest extent and easily ascertain the essential characteristics of the present invention, and can make various changes and modifications to the present invention to adapt it to various uses and conditions, without departing from the spirit and scope of the present invention. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way, and are intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

[0054] Above, all temperatures are set forth in degrees Celsius and all parts and percentages are by weight unless otherwise indicated.

Claims

1. An ion-conducting thin film composite (TFC) membrane, comprising: a first microporous support membrane; a hydrophilic ionomer polymer coating layer on a surface of the first microporous support membrane, the hydrophilic ionomer polymer coating layer being ionically conductive; and a second microporous support membrane on a surface of said hydrophilic ionomeric polymer coating layer opposite said first microporous support membrane.

2. 10. The TFC membrane of claim 1, wherein the hydrophilic ionomeric polymer comprises a polyphosphate-complexed polysaccharide polymer, a polyphosphate- and metal ion-complexed polysaccharide polymer, a metal ion-complexed polysaccharide polymer, a borate-complexed polysaccharide polymer, an alginate polymer, an alginic acid polymer, a hyaluronic acid polymer, a borate-complexed polyvinyl alcohol polymer, a polyphosphate-complexed polyvinyl alcohol polymer, a polyphosphate- and metal ion-complexed polyvinyl alcohol polymer, a metal ion-complexed polyvinyl alcohol polymer, a metal ion-complexed poly(acrylic acid) polymer, a borate-complexed poly(acrylic acid) polymer, a metal ion-complexed poly(methacrylic acid), borate-complexed poly(methacrylic acid), or a combination thereof.

3. 3. The TFC membrane of claim 1 or 2, wherein the hydrophilic ionomeric polymer is a polyphosphate-complexed chitosan polymer, a polyphosphate- and metal ion-complexed chitosan polymer, a metal ion-complexed alginate polymer, a sodium alginate polymer, an alginate polymer, a hyaluronic acid polymer, or a combination thereof.

4. 3. The TFC membrane of claim 1, wherein the hydrophilic ionomeric polymer is a borate-complexed polyvinyl alcohol polymer, a borate-complexed alginic acid, or a blend of a borate-complexed polyvinyl alcohol and an alginic acid polymer.

5. 3. The TFC membrane of claim 1, wherein the first microporous support membrane, the second microporous support membrane, or both comprise polyethylene, polypropylene, a mixture of polyethylene and silica particles, a mixture of polypropylene and silica particles, polyamide, polyacrylonitrile, polyethersulfone, sulfonated polyethersulfone, polysulfone, sulfonated polysulfone, poly(ether ether ketone), sulfonated poly(ether ether ketone), polyester, cellulose acetate, cellulose triacetate, polybenzimidazole, polyimide, polyvinylidene fluoride, polycarbonate, cellulose, or a combination thereof.

6. 3. The TFC membrane of claim 1 or 2, wherein the hydrophilic ionomeric polymer is present within the pores of the first microporous support membrane.

7. 1. A method for preparing an ion-conducting thin film composite (TFC) membrane, comprising: applying a layer of an aqueous solution comprising a hydrophilic ionomeric polymer to one surface of a first microporous support membrane; drying the coated membrane; Optionally, complexing the hydrophilic ionomeric polymer using a complexing agent to form a crosslinked hydrophilic ionomeric polymer; applying a second microporous support membrane to the coated membrane on a surface opposite the first microporous support membrane.

8. 8. The method of claim 7, wherein the complexing agent is selected from polyphosphate, boric acid, metal ions selected from ferric ions, ferrous ions, or vanadium ions, or combinations thereof.

9. 9. The method of claim 7 or 8, wherein complexing the hydrophilic ionomeric polymer comprises immersing the dried, coated membrane in a second aqueous solution of polyphosphoric acid, boric acid, a metal salt, hydrochloric acid, or a combination thereof.

10. 1. A redox flow battery system including at least one rechargeable cell including a positive electrolyte, a negative electrolyte, and an ion-conducting thin film composite (TFC) membrane disposed between the positive electrolyte and the negative electrolyte, wherein the TFC membrane includes a first microporous support membrane, a hydrophilic ionomer polymer coating layer on a surface of the first microporous support membrane, and a second microporous support membrane on a surface of the hydrophilic ionomer polymer coating layer opposite the first microporous support membrane, wherein the hydrophilic ionomer polymer coating layer is ion-conducting.

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