IONICALLY CONDUCTIVE THIN FILM COMPOSITE MEMBRANES FOR ENERGY STORAGE APPLICATIONS

Abstract

An ionically conductive thin film composite (TFC) membrane is described. The low cost, high performance TFC membrane comprises a micropous support membrane, and a hydrophilic ionomeric polymer coating layer on a surface of the microporous support membrane. The hydrophilic ionomeric polymer coating layer is ionically conductive. The ionomeric polymer can also be present in the micropores of the support membrane. Methods of making the membrane and redox flow battery system incorporating the TFC membrane are also described.

Claims

1. An ionically conductive thin film composite (TFC) membrane comprising: a microporous support membrane; a hydrophilic ionomeric polymer coating layer on a surface of the microporous support membrane, the hydrophilic ionomeric polymer coating layer is ionically conductive.

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

3. The TFC membrane of claim 2 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, .Math.-carrageenan, carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, calcium carboxymethyl cellulose, ammonium carboxymethyl cellulose, pectic acid, chitin, chondroitin, xanthan gum, or combinations thereof.

4. The TFC membrane of claim 2 wherein metal ion is ferric ion, ferrous ion, or vanadium ion.

5. The TFC membrane of claim 1 wherein the hydrophilic ionomeric polymer is a polyphosphoric acid-complexed chitosan polymer, a polyphosphoric acid and metal ion-complexed chitosan polymer, a metal ion-complexed alginic acid polymer, a sodium alginate polymer, an alginic acid polymer, a hyaluronic acid polymer, or combinations thereof.

6. The TFC membrane of claim 5 wherein the metal ion is ferric ion, ferrous ion, or vanadium ion.

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

8. The TFC membrane of claim 1 wherein the support membrane comprises polyethylene, polypropylene, 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 combinations thereof.

9. The TFC membrane of claim 1 wherein the hydrophilic ionomeric polymer is present in the micropores of the support membrane.

10. A method of preparing an ionically conductive thin film composite (TFC) membrane comprising: applying a layer of an aqueous solution comprising a hydrophilic ionomeric polymer to one surface of a microporous support membrane; drying the coated membrane; and optionally complexing the hydrophilic ionomeric polymer using a complexing agent to form a cross-linked hydrophilic ionomeric polymer.

11. The method of claim 10 wherein the hydrophilic ionomeric polymer on the coated membrane is dried before complexing the hydrophilic ionomeric polymer.

12. The method of claim 10 wherein the coated membrane is dried after complexing the hydrophilic ionomeric polymer.

13. The method of claim 10 wherein the complexing agent is selected from polyphosphoric acid, boric acid, a metal ion selected from ferric ion, ferrous ion, or vanadium ion, or combinations thereof.

14. The method of claim 10 wherein complexing the hydrophilic ionomeric polymer comprises immersing the dried coated membrane in a second aqueous solution of polyphosphoric acid, boric acid, metal salt, hydrochloric acid, or combinations thereof.

15. The method of claim 10 wherein complexing the hydrophilic ionomeric polymer comprises complexing the dried coated membrane with a complexing agent in situ in a redox flow battery cell.

16. The method of claim 10 wherein the hydrophilic ionomeric polymer comprises a polysaccharide polymer, a poly(acrylic acid) polymer, a poly(methacrylic acid), or combinations thereof.

17. The method of claim 16 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, .Math.-carrageenan, carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, calcium carboxymethyl cellulose, ammonium carboxymethyl cellulose, pectic acid, chitin, chondroitin, xanthan gum, or combinations thereof.

18. A redox flow battery system, comprising: at least one rechargeable cell comprising a positive electrolyte, a negative electrolyte, and an ionically conductive thin film composite (TFC) membrane positioned between the positive electrolyte and the negative electrolyte, the positive electrolyte in contact with a positive electrode, and the negative electrolyte in contact with a negative electrode, wherein the TFC membrane comprises a microporous support membrane and a hydrophilic ionomeric polymer coating layer on a surface of the microporous support membrane, wherein the hydrophilic ionomeric polymer coating layer is ionically conductive.

19. The redox flow battery system of claim 18 wherein the negative electrolyte, the positive electrolyte, or both the negative electrolyte and the positive electrolyte comprises a boric acid additive capable of complexing with a hydrophilic ionomeric polymer on the surface of the microporous support membrane to form a cross-linked hydrophilic ionomeric polymer coating layer.

20. The redox flow battery system of claim 18 wherein the hydrophilic ionomeric polymer coating layer is formed in situ by complexing a hydrophilic ionomeric polymer on the surface of the microporous support membrane with a complexing agent in the negative electrolyte, the positive electrolyte, or both the negative electrolyte and the positive electrolyte.

Description

EXAMPLES

Comparative Example 1

Preparation of Chitosan/Daramic® TFC Membrane

[0040] A 6.5 wt % chitosan aqueous solution was prepared by dissolving chitosan polymer in a 2 wt % acetic acid aqueous solution. One surface of a Daramic® microporous support membrane purchased from Daramic, LLC was coated with a thin layer of the 6.5 wt % chitosan aqueous solution and dried at 60° C. for 12 h in an oven to form a thin, nonporous, chitosan layer with a thickness of about 30 micrometers on the surface of the Daramic® support membrane. The coated membrane was treated with a basic sodium hydroxide solution, and washed with water to form a thin, nonporous, chitosan layer with a thickness of about 30 micrometers on the surface of the Daramic® support membrane.

Comparative Example 2

Preparation of Polyvinyl Alcohol (PVA)/Daramic® TFC Membrane

[0041] A 10.0 wt % polyvinyl alcohol (PVA) aqueous solution was prepared by dissolving PVA polymer with an average M.sub.w of 130,000 in deionized (DI) water. One surface of a Daramic® microporous support membrane purchased from Daramic, LLC was coated with a thin layer of the 10.0 wt % PVA aqueous solution and dried at 60° C. for 12 h in an oven to form a thin, nonporous, PVA layer with a thickness of about 30 micrometers on the surface of the Daramic® support membrane.

Example 1

Preparation of Polyphosphous Acid (PPA) and Ferric Ion (Fe.SUP.3+.) Complexed Chitosan/Daramic® TFC Membrane (Abbreviated as PPA-Fe-Chitosan/Daramic®)

[0042] A 6.5 wt % chitosan aqueous solution was prepared by dissolving chitosan polymer in a 2 wt % acetic acid aqueous solution. One surface of a Daramic® microporous support membrane purchased from Daramic, LLC was coated with a thin layer of the 6.5 wt % chitosan aqueous solution and dried at 60° C. for 2 h in an oven to form a thin, nonporous, chitosan layer with a thickness of about 30 micrometers on the surface of the Daramic® support membrane. The coated membrane was treated with a 10.0 wt % PPA aqueous solution for 30 min, rinsed with DI water, then treated with a 1.5 M FeCl.sub.3 aqueous solution for another 30 min, and finally rinsed with DI water to form PPA-Fe-Chitosan/Daramic® TFC membrane.

Example 2

Preparation of Boric Acid (BA) Complexed Polyvinyl Alcohol (PVA)/Daramic® TFC Membrane (Abbreviated as BA-PVA/Daramic®)

[0043] A 10.0 wt % polyvinyl alcohol (PVA) aqueous solution was prepared by dissolving PVA polymer with an average M.sub.w of 130,000 in DI water. One surface of a Daramic® microporous support membrane purchased from Daramic, LLC was coated with a thin layer of the 10.0 wt % PVA aqueous solution and dried at 60° C. for 2 h in an oven to form a thin, nonporous, PVA layer with a thickness of about 30 micrometers on the surface of the Daramic® support membrane. The dried TFC membrane was treated with a 0.5 M boric acid aqueous solution for 30 min and dried at 60° C. for 1 h to form the dried BA-PVA/Daramic® TFC membrane.

Example 3

Preparation of Ferric Ion (Fe.SUP.3+.) Complexed Alginic Acid (AA)/Daramic® TFC Membrane (Abbreviated as Fe-AA/Daramic®)

[0044] A 8.0 wt % sodium alginate aqueous solution was prepared by dissolving sodium alginate polymer in DI water. One surface of a Daramic® microporous support membrane purchased from Daramic, LLC was coated with a thin layer of the 8.0 wt % sodium alginate aqueous solution and dried at 60° C. for 2 h in an oven to form a thin, nonporous, sodium alginate layer with a thickness of about 30 micrometers on the surface of the Daramic® support membrane. The dried TFC membrane was treated with a 1.0 M hydrochloric acid aqueous solution for 30 min to convert sodium alginate coating layer to alginic acid coating layer, then treated with a 1.5 M FeCl.sub.3 aqueous solution for another 30 min, and finally dried at 60° C. for 1 h to form the dried Fe-AA/Daramic® TFC membrane.

Example 4

Preparation of Boric Acid Complexed Alginic Acid (AA) and PVA Blend Polymer/Daramic® TFC Membrane (Abbreviated as BA-AA-PVA/Daramic®)

[0045] An aqueous solution comprising 6.0 wt % of PVA and 4 wt % of sodium alginate was prepared by dissolving sodium alginate and PVA polymers in DI water. One surface of a Daramic® microporous support membrane purchased from Daramic, LLC was coated with a thin layer of the aqueous solution comprising 6.0 wt % of PVA and 4 wt % of sodium alginate and dried at 60° C. for 2 h in an oven to form a thin, nonporous, sodium alginate/PVA blend polymer layer with a thickness of about 30 micrometers on the surface of the Daramic® support membrane. The dried TFC membrane was treated with a 1.0 M hydrochloric acid aqueous solution for 30 min, then treated with a 0.5 M boric acid aqueous solution for another 30 min, and finally dried at 60° C. for 1 h to form the dried BA-AA-PVA/Daramic® TFC membrane.

Example 5

Preparation of Boric Acid Complexed Alginic Acid (AA)/Daramic® TFC Membrane (Abbreviated as BA-AA/Daramic®)

[0046] A 8.0 wt % sodium alginate aqueous solution was prepared by dissolving sodium alginate polymer in DI water. One surface of a Daramic® microporous support membrane purchased from Daramic, LLC was coated with a thin layer of the 8.0 wt % sodium alginate aqueous solution and dried at 60° C. for 2 h in an oven to form a thin, nonporous, sodium alginate layer with a thickness of about 30 micrometers on the surface of the Daramic® support membrane. The dried TFC membrane was treated with a 1.0 M hydrochloric acid aqueous solution for 30 min to convert sodium alginate coating layer to alginic acid coating layer. The alginic acid coating layer on the TFC membrane was complexed with boric acid in-situ during the IFB performance study in a BCS-810 battery cycling system (Biologic, FRANCE) comprising boric acid additive in the negative electrolyte solution.

Example 6

Preparation of Alginic Acid (AA)/Daramic® TFC Membrane (Abbreviated as AA/Daramic®)

[0047] A 8.0 wt % sodium alginate aqueous solution was prepared by dissolving sodium alginate polymer in DI water. One surface of a Daramic® microporous support membrane purchased from Daramic, LLC was coated with a thin layer of the 8.0 wt % sodium alginate aqueous solution and dried at 60° C. for 2 h in an oven to form a thin, nonporous, sodium alginate layer with a thickness of about 30 micrometers on the surface of the Daramic® support membrane. The dried TFC membrane was treated with a 1.0 M hydrochloric acid aqueous solution for 30 min to convert sodium alginate coating layer to alginic acid coating layer.

Example 7

Ferric Ion Crossover Study on Various Membranes

[0048] The low cost high performance hydrophilic ionomeric polymer coated TFC membranes are suitable for RFB applications. To compare the battery performance of these new membranes with the commercially available membranes, electrochemical impedance spectroscopy (EIS) was used to measure the ionic conductivity, the numbers of battery charge/discharge cycles, VE, CE, and EE of a IFB cell and the electrolyte crossover through the membranes were also measured.

[0049] Ferric ion crossover studies on a commercially available perfluorosulfonic acid (PFSA)-based Nafion® 117 cation exchange membrane, a microporous Daramic® membrane, the Chitosan/Daramic® TFC membrane prepared in Comparative Example 1, the PVA/Daramic® TFC membrane prepared in Comparative Example 2, the PPA-Fe-Chitosan/Daramic® TFC membrane prepared in Example 1, the BA-PVA/Daramic® TFC membrane prepared in Example 2, the Fe-AA/Daramic® TFC membrane prepared in Example 3, and the BA-AA-PVA/Daramic® TFC membrane prepared in Example 4 were conducted. The ferric ion crossover studies were conducted using a H-cell comprising two chambers with one chamber filled with 1.5 M FeCl.sub.2 and the other chamber filled with 1.5 M FeCl.sub.3. The concentration of Fe.sup.3+ in the 1.5 M FeCl.sub.2 chamber was measured using DR6000 UV-vis (HACH, US) over time at room temperature. The Fe.sup.3+ crossover was calculated based on the slope of Fe.sup.3+ concentration vs time and the results were summarized in Table 1.

[0050] It can be seen from Table 1 that the Nafion® 117 membrane showed much lower Fe.sup.3+ crossover than the microporous Daramic® membrane, suggesting that the Nafion® membrane will have higher proton/Fe.sup.3+ selectivity and therefore higher CE in IFB than a Daramic® membrane. The Chitosan/Daramic® TFC membrane prepared in Comparative Example 1 and the PVA/Daramic® TFC membrane prepared in Comparative Example 2 showed lower Fe.sup.3+ crossover than the microporous Daramic® membrane due to the incorporation of a chitosan or PVA layer on Daramic® membrane. All of the new membranes including PPA-Fe-Chitosan/Daramic® TFC membrane prepared in Example 1, the BA-PVA/Daramic® TFC membrane prepared in Example 2, the Fe-AA/Daramic® TFC membrane prepared in Example 3 and the BA-AA-PVA/Daramic® TFC membrane prepared in Example 4 showed significantly reduced Fe.sup.3+ crossover compared to the microporous Daramic® support membrane and even lower than Nafion® 117 membrane. These results demonstrated that the hydrophilic ionomeric polymer coated TFC membranes exhibited desired low Fe.sup.3+ crossover for IFB applications and better crossover performance than commercially available membranes. The crossover performance was also better than the hydrophilic polymer coated TFC membranes without iononic functionality.

TABLE-US-00001 TABLE 1 Ferric Ion Crossover Study on Various Membranes Membrane Fe.sup.3+ Crossover (mmol/h) Daramic ® 10.2 Nafion ® 117 0.38 Chitosan/Daramic ® 5.5 (Comparative Example 1) PVA/Daramic ® 4.5 (Comparative Example 2) PPA-Fe-Chitosan/ 0.1 Daramic ® (Example 1) BA-PVA/Daramic ® 0.05 (Example 2) Fe-AA/Daramic ® 0.25 (Example 3) BA-AA-PVA/Daramic ® 0.15 (Example 4)

Example 8

IFB Performance Study on Various Membranes

[0051] The ionic conductivity, number of battery charge/discharge cycles, VE, CE, and EE of the hydrophilic ionomeric polymer coated TFC membranes were measured using EIS with a BCS-810 battery cycling system (Biologic, FRANCE) at room temperature, and the results were shown in Table 2. It can be seen from Table 2 that all the new hydrophilic ionomeric polymer coated Daramic® TFC membranes showed lower area specific resistance, much longer battery cycles, and higher EE than the microporous Daramic® support membranes. These new membranes also showed much lower area specific resistance, longer battery cycles, and much higher EE than Nafion® 117 membrane. Furthermore, the new TFC membranes with hydrophilic ionomeric polymer coating layers having both hydrophilicity and ionomeric properties showed much longer battery cycles and higher EE than the corresponding TFC membranes with a hydrophilic non-ionomeric polymer coating layer. This demonstrates that the combination of the size-exclusion ion-conducting separation mechanism derived from the hydrophilic property of the polymer combined with the ion-exchange ion-conducting separation mechanism derived from the ionomeric property of the polymer in the new hydrophilic ionomeric polymer coated TFC membranes significantly improved the membrane performance compared to commercially available membranes with either a size-exclusion ion-conducting separation mechanism such as microporous membranes or an ion-exchange ion-conducting separation mechanism such as Nafion® membrane.

TABLE-US-00002 TABLE 2 IFB Performance Measurement on Various Membranes .sup.a Area Specific Resistance # VE CE EE Membrane (Ω .Math. cm.sup.2) Cycles (%) (%) (%) Daramic ® 3.5 10 69 70 48 Nafion ® 117 6.25 28 51 81 41 Chitosan/Daramic ® 1.1 15 68 72 49 (Comparative Example 1) PVA/Daramic ® 1.4 17 67 73 49 (Comparative Example 2) PPA-Fe-Chitosan/ 3.4 31 60 84 50 Daramic ® (Example 1) BA-PVA/Daramic ® 2.6 34 65 83 54 (Example 2) Fe-AA/Daramic ® 1.55 34 66 85 56 (Example 3) BA-AA-PVA/Daramic ® 2.25 33 62 85 52 (Example 4) BA-AA/Daramic ® 1.38 36 67 88 59 (Example 5) AA/Daramic ® 1.25 38 67 93 62 (Example 6) .sup.a Negative electrolyte solution: 1.5M FeCl.sub.2, 2M KCl, 0.3M boric acid; positive electrolyte solution: 1.5M FeCl.sub.2, 1.5M KCl, 0.3M ascorbic acid, 0.5M KOH; charge current density: 30 mA/cm.sup.2; charge time: 4 h; discharge current density: 30 mA/cm.sup.2; discharge time: 4 h; # of cycles were counted with ≥70% CE.

Specific Embodiments

[0052] While the following is described in conjunction with specific embodiments, it will be understood that this description is intended to illustrate and not limit the scope of the preceding description and the appended claims.

[0053] A first embodiment of the invention is an ionically conductive thin film composite (TFC) membrane comprising a microporous support membrane; a hydrophilic ionomeric polymer coating layer on a surface of the microporous support membrane, the hydrophilic ionomeric polymer coating layer is ionically conductive. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the hydrophilic ionomeric polymer comprises a polyphosphoric acid-complexed polysaccharide polymer, a polyphosphoric acid and metal ion-complexed polysaccharide polymer, a metal ion-complexed polysaccharide polymer, a boric acid-complexed polysaccharide polymer, an alginate polymer such as sodium alginate, potassium alginate, calcium alginate, ammonium alginate, an alginic acid polymer, a hyaluronic acid polymer, a boric acid-complexed polyvinyl alcohol polymer, polyphosphoric acid-complexed polyvinyl alcohol polymer, a polyphosphoric acid and metal ion-complexed polyvinyl alcohol polymer, a metal ion-complexed polyvinyl alcohol polymer, a metal ion-complexed poly(acrylic acid) polymer, a boric acid-complexed poly(acrylic acid) polymer, a metal ion-complexed poly(methacrylic acid), a boric acid-complexed poly(methacrylic acid), or combinations thereof. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in 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, .Math.-carrageenan, carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, calcium carboxymethyl cellulose, ammonium carboxymethyl cellulose, pectic acid, chitin, chondroitin, xanthan gum, or combinations thereof. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein metal ion is ferric ion, ferrous ion, or vanadium ion. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the hydrophilic ionomeric polymer is a polyphosphoric acid-complexed chitosan polymer, a polyphosphoric acid and metal ion-complexed chitosan polymer, a metal ion-complexed alginic acid polymer, or combinations thereof. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the metal ion is ferric ion, ferrous ion, or vanadium ion. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the hydrophilic ionomeric polymer is a boric acid-complexed polyvinyl alcohol polymer, a boric acid-complexed alginic acid, or a blend of boric acid-complexed polyvinyl alcohol and alginic acid polymer. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the support membrane comprises polyethylene, polypropylene, polyamide, polyacrylonitrile, polyethersulfone, sulfonated polyethersulfone, polysulfone, sulfonated polysulfone, poly(ether ether ketone), sulfonated poly(ether ether ketone), polyester, cellulose acetate, cellulose triacetate, polyimide, polyvinylidene fluoride, polycarbonate, cellulose, or combinations thereof. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the hydrophilic ionomeric polymer is present in the micropores of the support membrane.

[0054] A second embodiment of the invention is a method of preparing an ionically conductive thin film composite (TFC) membrane comprising applying a layer of an aqueous solution comprising a hydrophilic polymer to one surface of a microporous support membrane; drying the coated membrane; and optionally complexing the hydrophilic polymer using a complexing agent to form a cross-linked hydrophilic ionomeric polymer. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the hydrophilic polymer on the coated membrane is dried before complexing the hydrophilic polymer. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the coated membrane is dried after complexing the hydrophilic polymer. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the complexing agent is selected from polyphosphoric acid, boric acid, a metal ion selected from ferric ion, ferrous ion, or vanadium ion, or combinations thereof. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein complexing the hydrophilic polymer comprises immersing the dried coated membrane in a second aqueous solution of polyphosphoric acid, boric acid, metal salt, hydrochloric acid, or combinations thereof. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein complexing the hydrophilic polymer comprises complexing the dried coated membrane with a complexing agent in situ in a redox flow battery cell. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the hydrophilic polymer comprises a polysaccharide polymer, a poly(acrylic acid) polymer, a poly(methacrylic acid), or combinations thereof. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in 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, .Math.-carrageenan, carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, calcium carboxymethyl cellulose, ammonium carboxymethyl cellulose, pectic acid, chitin, chondroitin, xanthan gum, or combinations thereof.

[0055] A third embodiment of the invention is a redox flow battery system, comprising at least one rechargeable cell comprising a positive electrolyte, a negative electrolyte, and an ionically conductive thin film composite (TFC) membrane positioned between the positive electrolyte and the negative electrolyte, wherein the TFC membrane comprises a microporous support membrane and a hydrophilic ionomeric polymer coating layer on a surface of the microporous support membrane, wherein the hydrophilic ionomeric polymer coating layer is ionically conductive. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the third embodiment in this paragraph wherein the negative electrolyte, the positive electrolyte, or both the negative electrolyte and the positive electrolyte comprises a boric acid additive capable of complexing with a hydrophilic polymer on the surface of the microporous support membrane to form the cross-linked hydrophilic polymer coating. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the third embodiment in this paragraph wherein the hydrophilic ionomeric polymer coating layer is formed in situ by complexing a hydrophilic polymer on the surface of the microporous support membrane with a complexing agent in the negative electrolyte, the positive electrolyte, or both the negative electrolyte and the positive electrolyte.

[0056] Without further elaboration, it is believed that using the preceding description that one skilled in the art can utilize the present invention to its fullest extent and easily ascertain the essential characteristics of this invention, without departing from the spirit and scope thereof, to make various changes and modifications of the invention and to adapt it to various usages and conditions. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limiting the remainder of the disclosure in any way whatsoever, and that it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

[0057] In the foregoing, all temperatures are set forth in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.