WO2019180524A1

WO2019180524A1 - Membrane

Info

Publication number: WO2019180524A1
Application number: PCT/IB2019/051734
Authority: WO
Inventors: Gregory M. Haugen; Matthew J. LINDELL; Marina M. KAPLUN
Original assignee: 3M Innovative Properties Company
Priority date: 2018-03-21
Filing date: 2019-03-04
Publication date: 2019-09-26

Abstract

Membrane assembly comprising an electrolyte diffusion layer having first and second major surfaces and a thickness between the first and second major surfaces, wherein at least 1 percent of the thickness from the first major surface comprises ionomer, wherein the electrolyte diffusion layer has a length and a width, wherein the electrolyte diffusion layer has a planar volume defined by the average length of the electrolyte diffusion layer, the average width of the average electrolyte diffusion layer, and the average thickness from the first major surface to the second major surface, wherein at least 10 percent of the planar volume comprises ionomer, wherein the electrolyte diffusion layer has a planar volume, wherein the electrolyte diffusion layer absent any ionomer has an open area porosity in a range from 5 to 95 percent of the planar volume of the electrolyte diffusion layer, and wherein the first major surface has an electrical resistivity of at least 2,500 ohm-centimeters-squared. Membrane assemblies described herein are useful, for example, to make electrode assemblies, electrochemical cells, and liquid redox flow batteries.

Description

MEMBRANE

Cross Reference To Related Application

This application claims the benefit of U.S. Provisional Patent Application Number 62/645848, filed March 21, 2018, the disclosure of which is incorporated by reference herein in its entirety.

Background

[0001] Cost reduction in redox flow batteries can be obtained by decreasing the membrane thickness to lower the voltage drop, thus enabling higher current density operation and requiring fewer cells and less ionomer for the same power. Redox flow batteries operating with thin membranes can lose coulombic efficiency at low current densities due to electronic“soft” shorts and/or charged species crossover. A “soft short” can be thought of as a leakage current stemming from two points in a conductive mesh being in closest proximity to two respective conductive planes held at a potential difference and separated by an electronic insulator, where the electric field will be the highest (see e.g., Modem Topics in Polymer Electrolyte Fuel Cell Degradation, Chapter: Membrane Durability: Physical and Chemical Degradation, Publisher: Elsevier, Editors: M. Mench, E. C. Kumbur, T. N. Veziroglu, pp. 15-88, 2012).

[0002] For crossover reduction, the selectivity of the membrane can be increased by either increasing the size of the ions in the electrolyte or changing the properties of the membrane. Redox flow batteries typically use carbon papers or felts at much higher compression (about 20-50%) than do lower compression fuel cells (about 10-20%), increasing the chance for a high electronic shorting current. Further, redox flow batteries do not typically have the thick and smooth coated catalyst layers on the membrane that can provide a buffer to the roughness of the carbon paper electrodes. Redox flow batteries electrolytes are typically either highly acidic or basic, but some may operate near neutral pH.

[0003] It is desired to have alternative membranes for electronic devices such as redox flow batteries, particularly that offer desired properties such as thin membranes that do not undesirably short during use.

Summary

[0004] In one aspect, the present disclosure describes a membrane assembly comprising an electrolyte diffusion layer having first and second major surfaces and a thickness between the first and second major surfaces, wherein at least 1 (in some embodiments, at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 97, 98, or even 99) percent of the thickness from the first major surface comprises ionomer, wherein the electrolyte diffusion layer has a length and a width, wherein the electrolyte diffusion layer has a planar volume defined by the average length of the electrolyte diffusion layer, the average width of the electrolyte diffusion layer, and the average thickness from the first major surface to the second major surface, wherein at least 10 (in some embodiments, at least 15, 20, 25, 30,

40, 50, 60, 70, 75, 80, 90, 95, 96, 97, 98, or even at least 99; in some embodiments, in a range from 10 to 99, 15 to 95, 20 to 90, or even 25 to 75) percent of the volume comprises ionomer, based on the defined planar volume of the electrolyte diffusion layer, wherein the electrolyte diffusion layer absent any ionomer has an open area porosity in a range from 5 to 95 (in some embodiments, in a range from 10 to 95, 10 to 90, 10 to 80, 10 to 75, 10 to 50, 20 to 50, or even 25 to 50) percent of the planar volume of the electrolyte diffusion layer, and wherein the first major surface has an electrical resistivity of at least 2,500 (in some embodiments, greater than 5,000, 7,500, or even greater than 10,000) ohm-centimeters- squared.

[0005] In this application:

[0006] “electrically non-conductive” refers to having an electrical conductivity of less than 5x10 ⁵ Siemens-per-centimeter as measured by the“Redox Flow Battery Screening Test” in the Examples;

[0007] “electrical resistivity” is the resistance to an electronic current measured as measured by the “Electronic Short Test” described in the Examples; and

[0008] “porous electrodes” refers to electrodes that are at least 60 percent by volume porous.

[0009] In another aspect, the present disclosure describes a method of making a membrane assembly described herein, the method comprising at least partially impregnating ionomer into a porous layer to provide an electrolyte diffusion layer having first and second major surfaces and a thickness between the first and second major surfaces, wherein at least 1 (in some embodiments, at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 97, 98, or even 99) percent of the thickness from the first major surface comprises ionomer, wherein the electrolyte diffusion layer has a length and a width, wherein the electrolyte diffusion layer has a planar volume defined by the average length of the electrolyte diffusion layer, the average width of the electrolyte diffusion layer, and the average thickness from the first major surface, wherein at least 10 (in some embodiments, at least 15, 20, 25, 30, 40, 50, 60, 70, 75, 80, 90, 95, 96, 97, 98, or even at least 99; in some embodiments, in a range from 10 to 99, 15 to 95, 20 to 90, or even 25 to 75) percent by the volume comprises ionomer, based on the defined planar volume of electrolyte diffusion layer, wherein the electrolyte diffusion layer absent any ionomer has an open area porosity in a range from 5 to 95 (in some embodiments, in a range from 10 to 95, 10 to 90, 10 to 80, 10 to 75, 10 to 50, 20 to 50, or even 25 to 50) percent of the planar volume of the electrolyte diffusion layer, and wherein the first major surface has an electrical resistivity of at least 2,500 (in some embodiments, greater than 5,000, 7,500, or even greater than 10,000) ohm-centimeters- squared.

[0010] Membrane assemblies described herein are useful, for example, to make membrane electrode assemblies, for example, for electrochemical cells and redox flow batteries. Brief Description of the Drawings

[0011] FIG. 1 is a cross-sectional view of an exemplary membrane assembly described herein.

[0012] FIG. 2 is a schematic of another exemplary membrane assembly described herein.

[0013] FIGS. 3A and 3B are scanning electron microscope (SEM) digital images of the Example 4 ion exchange membrane.

[0014] FIG. 4 is block diagram of an exemplary redox flow battery.

Detailed Description

[0015] The present disclosure describes a membrane assembly comprising an electrolyte diffusion layer having first and second major surfaces and a thickness between the first and second major surfaces, wherein at least 1 (in some embodiments, at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 97, 98, or even 99) percent of the thickness from the first major surface comprises ionomer, wherein the electrolyte diffusion layer has a length and a width, wherein the electrolyte diffusion layer has a planar volume defined by the average length of the electrolyte diffusion layer, the average width of the electrolyte diffusion layer, and the average thickness from the first major surface to the second major surface, wherein at least 10 (in some embodiments, at least 15, 20, 25, 30,

40, 50, 60, 70, 75, 80, 90, 95, 96, 97, 98, or even at least 99; in some embodiments, in a range from 10 to 99, 15 to 95, 20 to 90, or even 25 to 75) percent by the volume comprises ionomer, wherein the electrolyte diffusion layer has a planar volume, wherein the electrolyte diffusion layer absent any ionomer has an open area porosity in a range from 5 to 95 (in some embodiments, in a range from 10 to 95, 10 to 90, 10 to 80, 10 to 75, 10 to 50, 20 to 50, or even 25 to 50) percent of the planar volume of the electrolyte diffusion layer, and wherein the first major surface has an electrical resistivity of at least 2,500 (in some embodiments, greater than 5,000, 7,500, or even greater than 10,000) ohm-centimeters- squared.

[0016] In some embodiments, the first major surface is free of ionomer (i.e., the first 1 percent of the thickness from the first major surface contains no ionomer). In some embodiments, the second major surface is free of ionomer (i.e., the first 1 percent of the thickness from the second major surface contains no ionomer). In some embodiments, both the first and second major surfaces are free of ionomer. In some embodiments, the first 1 (in some embodiments, the first 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, or even the first 75) percent of the thickness from the first major surface does not contain ionomer. In some embodiments, the first 1 (in some embodiments, the first 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, or even the first 75) percent of the thickness from the second major surface does not contain ionomer. In some embodiments, the first 1 (in some embodiments, the first 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, or even the first 35) percent of the thickness from the first major surface and the first 1 (in some embodiments, the first 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, or even the first 35) percent of the thickness from the second major surface do not contain ionomer.

[0017] Referring to FIG. 1, exemplary membrane assembly 100 has electrolyte diffusion layer 101 having first and second major surfaces 102, 103 and thickness t’ between the first and second major surfaces 102, 103. At least 1 percent of thickness t’ from first major surface 102 includes ionomer. The electrolyte diffusion layer absent any ionomer has an open area porosity in a range from 5 to 95 percent of the total planar volume of the electrolyte diffusion layer. The first major surface has an electrical resistivity of at least 2500 ohm-centimeters-squared, and first major surface 102 includes ionomer.

[0018] Referring to FIG. 2, exemplary membrane assembly 200 has electrolyte diffusion layers 201 and 202 having first and second major surfaces 203, 204 and thickness t” between the first and second major surfaces 203 and 204. At least 1 percent of thickness t” has its pores filled with ionomer. The electrolyte diffusion layers absent any ionomer have an open area porosity in a range from 5 to 95 percent of the total planar volume of the electrolyte diffusion layer. The at least 1 percent of thickness t” that has its pores filled with ionomer has an electrical resistivity of at least 2500 ohm-centimeters- squared.

[0019] Membrane assembly embodiments are best paired with highly ionically conductive electrolytes to maximize voltage efficiency. In the case of electrolytes where either posolyte or negolyte is substantially less ionically conductive, it would be best to have the low conductivity electrolyte in contact with a major surface that includes ionomer.

[0020] Exemplary ionomers include perfluorosulfonic acid (PFSA), perfluoroimide acid (PFIA), perfluoroimide ionene chain extended (PFICE), sulfonated poly ether sulfone (SPES), sulfonated polyether ether ether ketone (SPEEK), and perfluorosulfonamide (including blends thereof). Such ionomers are commercially available, for example, under the trade designations“NAFION” from DuPont, Wilmington, DE, and“AQUIVION” from Solvay, Brussels, Belgium. In some embodiments, the ionomers are short side chained.

[0021] In some embodiments, the electrolyte diffusion layer comprises at least one of woven or nonwoven electrically non-conductive fibers. In some embodiments, the electrolyte diffusion layer comprises electrically non-conductive polymeric fibers. In some embodiments, the electrically non- conductive polymeric fibers comprise at least one of a polyurethane, a polyester, a polyamide, a polyether, a polycarbonate, a polyimide, a polysulfone, a polyphenylene oxide, a polyacrylate, a polymethacrylate, a polyolefin, a polystyrene (including a styrene-based random or block copolymer), a polyvinyl chloride, or a fluorinated polymer. In some embodiments, the electrically non-conductive fibers are inorganic fibers. In some embodiments, the non-conductive fibers are inorganic fibers comprising a ceramic (including a glass, a crystalline ceramic, or a glass-ceramic) (e.g., alumina, borides, zirconia, silica, magnesium silicate, calcium silicate, and rock wool). Exemplary fibers are commercially available, for example, from U.S. Composites, West Palm Beach, FL, and ACP

Composites Inc., Livermore, CA. Layers can be made from the fibers using techniques known in the art, including chopped, wet laid and micro entanglement.

[0022] In some embodiments, the thickness of the electrolyte diffusion layer is in a range from 5 micrometers to 100 micrometers (in some embodiments, in a range from 10 micrometers to 50 micrometers, 20 micrometers to 40 micrometers, or 25 micrometers to 35 micrometers).

[0023] Ionomer can be impregnated into the electrolyte diffusion layer using impregnation techniques known in the art such as pressing the lower portion of an electrolyte diffusion layer into a wet layer of an ionomer coating solution, then allowing the solvent of the solution to dry, resulting in a porous electrolyte diffusion layer having its lower surface and an adjacent portion filled with dried ionomer. Membranes can be made, for example, by solution casting ionomer onto a liner, and then while the solution is still wet, an electrolyte diffusion layer is at least partial placed into the solution. The electrolyte diffusion layer materials may have differing degrees of hydrophilicity that can be accounted for by varying, for example, the viscosity and solid content of the ionomer solution.

[0024] Membrane assemblies described herein can be made, for example, by a method comprising at least partially impregnating ionomer into a porous layer to provide an electrolyte diffusion layer having first and second major surfaces and a thickness between the first and second major surfaces, wherein at least 1 (in some embodiments, at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 97, 98, or even 99) percent of the thickness from the first major surface comprises ionomer, wherein the electrolyte diffusion layer has a length and a width, wherein the electrolyte diffusion layer has a volume defined by the length of the electrolyte diffusion layer, the width of the electrolyte diffusion layer, and the thickness from the first major surface to the second major surface, wherein at least 10 (in some embodiments, at least 15, 20, 25, 30, 40, 50, 60, 70, 75, 80, 90, 95, 96, 97, 98, or even at least 99; in some embodiments, in a range from 10 to 99, 15 to 95, 20 to 90, or even 25 to 75) percent of the volume comprises ionomer, based on the defined volume of the electrolyte diffusion layer, wherein the electrolyte diffusion layer has a planar volume, wherein the electrolyte diffusion layer absent any ionomer has an open area porosity in a range from 5 to 95 (in some embodiments, in a range from 10 to 95, 10 to 90, 10 to 80, 10 to 75, 10 to 50, 20 to 50, or even 25 to 50) percent of the planar volume of the electrolyte diffusion layer, and wherein the first major surface has an electrical resistivity of at least 2,500 (in some embodiments, greater than 5,000, 7,500, or even greater than 10,000) ohm- centimeters-squared.

[0025] Membrane assemblies described herein are useful, for example, to make electrode assemblies for, for example, electrochemical cells and redox flow batteries. Membrane assemblies can be assembled as membrane electrode assemblies as is known in the art (see, e.g., PCT Pub. No.

W02017160972 Al, published September 17, 2017.) [0026] For example, referring to FIG. 4, exemplary membrane assembly 401 of redox flow battery system 400 is disposed between first and second porous electrodes 402, 403. In some embodiments, first and second porous electrodes 402, 403 each independently comprise carbon fibers. In some

embodiments, first and second porous electrode 402, 403 each independently comprise at least one of carbon paper, carbon felt, or carbon cloth. In some embodiments, at least one of the first or second porous electrodes 402, 403 is hydrophilic. Referring to again to FIG. 4, exemplary embodiment of a single cell redox flow battery system, 400, includes anolyte storage tank 411 for containing an anolyte, current collector plates 412 and 415, anode 413, membrane electrode assembly 401, cathode 414, and catholyte storage tank 416 for containing a catholyte

Exemplary Embodiments

1A. A membrane assembly comprising an electrolyte diffusion layer having first and second major surfaces and a thickness between the first and second major surfaces, wherein at least 1 (in some embodiments, at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85,

90, 95, 97, 98, or even 99) percent of the thickness from the first major surface comprises ionomer, wherein the electrolyte diffusion layer has a length and a width, wherein the electrolyte diffusion layer has a planar volume defined by the average length of the electrolyte diffusion layer, the average width of the electrolyte diffusion layer, and the average thickness from the first major surface to the second major surface, wherein at least 10 (in some embodiments, at least 15, 20, 25, 30, 40, 50, 60, 70, 75, 80, 90, 95, 96, 97, 98, or even at least 99; in some embodiments, in a range from 10 to 99, 15 to 95, 20 to 90, or even 25 to 75) percent of the volume comprises ionomer, wherein the electrolyte diffusion layer has a planar volume, wherein the electrolyte diffusion layer absent any ionomer has an open area porosity in a range from 5 to 95 (in some embodiments, in a range from 10 to 95, 10 to 90, 10 to 80, 10 to 75, 10 to 50, 20 to 50, or even 25 to 50) percent of the planar volume of the electrolyte diffusion layer, and wherein the first major surface has an electrical resistivity of at least 2,500 (in some embodiments, greater than 5,000, 7,500, or even greater than 10,000) ohm-centimeters-squared.

2A. The membrane assembly of Exemplary Embodiment 1A, wherein the first major surface is free of ionomer (i.e., the first 1 percent of the thickness from the first major surface contains no ionomer).

3A. The membrane assembly of any preceding A Exemplary Embodiment, wherein the second major surface is free of ionomer (i.e., the first 1 percent of the thickness from the second major surface contains no ionomer). 4A. The membrane assembly of Exemplary Embodiment 1A, wherein both the first and second major surfaces are free of ionomer.

5A. The membrane assembly of Exemplary Embodiment 1A, wherein the first 1 (in some embodiments, the first 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, or even the first 75) percent of the thickness from the first major surface does not contain ionomer.

6A. The membrane assembly of Exemplary Embodiment 1A, wherein the first 1 (in some embodiments, the first 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, or even the first 75) percent of the thickness from the second major surface does not contain ionomer.

7A. The membrane assembly of Exemplary Embodiment 1A, wherein the first 1 (in some embodiments, the first 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, or even the first 35) percent of the thickness from the first major surface and the first 1 (in some embodiments, the first 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, or even the first 35) percent of the thickness from the second major surface do not contain ionomer.

8A. The membrane assembly of any preceding A Exemplary Embodiment, wherein the electrolyte diffusion layer comprises at least one of woven or nonwoven electrically non-conductive fibers.

9A. The membrane assembly of any preceding A Exemplary Embodiment, wherein the electrolyte diffusion layer comprises electrically non-conductive polymeric fibers.

10A. The membrane assembly of Exemplary Embodiment 9 A, wherein the electrically non- conductive polymeric fibers comprise at least one of a polyurethane, a polyester, a polyamide, a polyether, a polycarbonate, a polyimide, a polysulfone, a polyphenylene oxide, a polyacrylate, a polymethacrylate, a polyolefin, a polystyrene (including a styrene-based random or block copolymer), a polyvinyl chloride, or a fluorinated polymer.

11 A. The membrane assembly of Exemplary Embodiment 9A, wherein the electrically non- conductive fibers are inorganic fibers.

12A. The membrane assembly of Exemplary Embodiment 9 A, wherein the electrically non- conductive fibers are inorganic fibers comprising at least one of a ceramic (including a glass, a crystalline ceramic, or a glass-ceramic) (e.g., alumina, borides, zirconia, silica, magnesium silicate, calcium silicate, and rock wool).

13 A. The membrane assembly of any preceding A Exemplary Embodiment, wherein the ionomer comprises at least one of perfluorosulfonic acid (PFSA), perfluoroimide acid (PFIA), perfluoroimide ionene chain extended (PFICE), sulfonated poly ether sulfone (SPES), sulfonated polyether ether ether ketone (SPEEK), or perfluorosulfonamide (including blends thereof).

14A. The membrane assembly of any preceding A Exemplary Embodiment, wherein the thickness of the electrolyte diffusion layer is in a range from 10 micrometers to 100 micrometers (in some embodiments, in a range from 10 micrometers to 60 micrometers, 20 micrometers to 50 micrometers, or even 25 micrometers to 40 micrometers).

15 A. An electrode assembly comprising first and second porous electrodes with at least one membrane assembly of any preceding A Exemplary Embodiment disposed between the first and second porous electrodes.

16A. The electrode assembly of Exemplary Embodiment 15 A, wherein the first and second porous electrodes each independently comprise carbon fibers.

17A. The electrode assembly of Exemplary Embodiment 15 A, wherein the first and second porous electrodes each independently comprise at least one of carbon paper, carbon felt, or carbon cloth.

18A. The electrode assembly of any of Exemplary Embodiments l5Ato 17A, wherein at least one of the first or second porous electrodes is hydrophilic.

19A. An electrochemical cell comprising an electrode assembly of any of Exemplary Embodiments l5A to 18A.

20A. A liquid redox flow battery comprising an electrode assembly of any of Exemplary

Embodiments l5A to 18A.

1B. A method of making a membrane assembly of any of Exemplary Embodiments 1 A to 14 A, the method comprising:

at least partially impregnating ionomer into a porous layer to provide an electrolyte diffusion layer having first and second major surfaces and a thickness between the first and second major surfaces, wherein at least 1 (in some embodiments, at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 97, 98, or even 99) percent of the thickness from the first major surface comprises ionomer, wherein the electrolyte diffusion layer has a length and a width, wherein the electrolyte diffusion layer has a planar volume defined by the average length of the electrolyte diffusion layer, the average width of the electrolyte diffusion layer, and the average thickness from the first major surface, wherein at least 10 (in some embodiments, at least 15, 20, 25, 30, 40, 50, 60, 70, 75, 80, 90, 95, 96, 97, 98, or even at least 99; in some embodiments, in a range from 10 to 99, 15 to 95, 20 to 90, or even 25 to 75) percent of the volume comprises ionomer, wherein the electrolyte diffusion layer has a planar volume, wherein the electrolyte diffusion layer absent any ionomer has an open area porosity in a range from 5 to 95 (in some embodiments, in a range from 10 to 95, 10 to 90, 10 to 80, 10 to 75, 10 to 50, 20 to 50, or even 25 to 50) percent of the planar volume of the electrolyte diffusion layer, and wherein the first major surface has an electrical resistivity of at least 2,500 (in some embodiments, greater than 5,000, 7,500, or even greater than 10,000) ohm-centimeters-squared.

1C. A method of making a membrane assembly of any of Exemplary Embodiments 1A to 14 A, the method comprising:

providing first and second electrolyte diffusion layers each having first and second major surfaces and a thickness between the first and second major surfaces, wherein for each of the first and second electrolyte diffusion layers at least a portion of the thickness from their respective first major surfaces comprises ionomer; and

bonding the first major surfaces of each of the first and second electrolyte diffusion layers together.

[0027] Advantages and embodiments of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention. All parts and percentages are by weight unless otherwise indicated.

Redox Flow Battery Test Assembly

[0028] Membranes or membrane assemblies were tested as a redox flow battery cell assembly. Each membrane assembly was first assembled into a unitized electrode assembly (UEA.) The UEA construction consisted of two pieces of heat treated carbon paper (obtained under the trade designation “39AA” from SGL Carbon SE, Wiesbaden, Germany) per side of the membrane assembly (for a total of four carbon paper sheets). The heat treatment of the“39AA” carbon paper was done in a tube furnace at 400°C for 48 hours under a constant air flow. The heat treated“39AA” electrodes (-250 micrometer thickness per piece) had been die cut into four 2.5 x 2.5 cm squares. Two pieces of 25 micrometer thick polyethylene naphthalate (PEN) subgasket (obtained under the trade designation“TEONEX 1MIL” from Du Pont Teijin Films, Chester, VA) were die cut in 7.6 cm x 7.6 cm squares with a 2.21 cm x 2.21 cm hole in the center. Two-single 375 micrometer thick polytetrafluoroethylene (PTFE) sheets (obtained under the trade designation“TFV-.015-R12” from Plastics International, Eden Prairie, MN) were die cut into gaskets to frame the membrane, one per side, resulting in an approximate compression in the 5 cm² active area of 25% in the assembled redox flow battery cell.

[0029] The UEA was hand assembled in the cell between the two bipolar plates in the following order: PTFE gasket, two pieces of heat treated electrodes (“39AA”) in the square hole in the gasket, then a piece of the PEN subgasket, the membrane under test, another PEN subgasket, and finally the second PTFE gasket framing two more carbon papers. The test fixtures were 5 cm² cells (obtained under the trade designation“5 cm² CELL HARDWARE” from Fuel Cell Technologies, Albuquerque, NM) and modified for redox flow battery testing by drilling holes through the end plate to facilitate the use of plastic tubing connection. The graphite flow field components in the Fuel Cell Technologies test fixture were replaced with the same components machined from graphite (obtained under the trade designation “TOKAI G347B” from MWI Carbon & Graphite Solutions, Rochester, NY). After assembly, a digital volt meter (DVM) was used (obtained under the trade designation“FLUKE MODEL 73 III

MULTIMETER” from Fluke Corporation, Everett, WA) to measure the electrical resistivity of the UEA. This is done by measuring the resistance with leads on the cell’s collector plates. This procedure is referred to as the Electronic Short Test. The value, in ohms, was multiplied by the active area of the cell to give the electronic shorting resistance in ohm-centimeters squared. At high values of resistance (i.e., >50,000 ohm-centimeters squared) the UEA acted more like a charging capacitor and the resistance changed as a function of time, but usually stabilized after a minute of measurement. It is this value that is reported.

Redox Flow Battery Screening Test

[0030] The UEAs were tested at room temperature (i.e., about 25°C) in the modified Fuel Cell

Technologies test cell (described above in“Redox Flow Battery Test Assembly”) with a single dual head pump (obtained under the trade designation“KNF 5 KTDCB-4” from KNF Neuberger, Inc., Trenton, NJ) for pumping the negolyte and posoltye. A block diagram of a redox flow battery is shown in FIG. 4. A multi-channel potentiostat (obtained under the trade designation“MPG-205” from

BioLogic Science Instrument, Seyssinet-Pariset, France) was used to carry out the test protocol and control the pump. The flow rate of 20 cmVmin. per side was maintained, with the negolyte side being inerted by a low nitrogen flow in the negolyte containers. Negolyte and posoltye containers started with 50 cm³ of a vanadium V(4) solution each. [0031] The vanadium V(4) solution was 1.5 M VOSO4, 2.6 M H2SO4 prepared by dissolving 676.19 grams of VOSO4 * xThO powder (obtained as vanadyl sulfate hydrate from Sigma Aldrich, St. Louis, MO) and 287.2 milliliters of 96.5% sulfuric acid in sufficient 18 mega ohm deionized water to make 2 liters of solution.

[0032] A vanadium V(5) solution was made from a portion of the V(4) solution by first charging the battery to 1.8 volt at a current density of 80 mA/cm², at which point the posolyte (the

chargeable/dischargeable electrolyte solution for the positive side of the redox flow battery) was removed, having been charged to 90% V(5) and 10% V(4).

[0033] Upon the first charging to 1.8 volt at a current density of 80 mA/cm², the negolyte and posolyte were mixed back together. After allowing the mixed solution to equilibrate for at least two hours, it was poured back into the negolyte and posolyte containers in equal portions.

[0034] Next a test script was run to first recharge the electrolyte again, then a loop was performed twice with the following performance metrics tested in each loop: ten charge/discharge cycles at each of three different current densities (i.e., 80 mA/cm², 160 mA/cm², and 400 mA/cm²) (yielding coulombic and voltage efficiency values), full spectrum impedance at open circuit at top of charge and after discharging to 1 volt, bottom of charge, and power curves for determining cell resistance as a function of current density at top of charge and bottom of charge. The potentiostat (“MPG-205”) was used to carry out impedance measurements over a frequency range of 10,000 Hz to 100 Hz. The highest frequency point at which the imaginary component of the impedance goes to zero is referred to as the high frequency resistance (HFR). The HFR resistance of the membrane or membrane electrode assembly is the HFR tested at top of charge. At the completion of the test metric loops of the script, the electrolyte was charged to 1.8 volt across the cell and then the pumps were shut off to record the open circuit voltage decay as the cell was allowed to self-discharge by crossover of charged species and/or electronic shorting. After discharging to 0.9 volt, with the pumps still off, the electrolyte in the cell alone was recharged to 1.8 volt then allowed to decay to 0.9 volt again while recording the voltage as a function of time of the decay. This was repeated two more times for a total of four open cell voltage (OCV) decay measurements so that an average value and error bar could be ascribed. The self-discharge time is proportional to the coulombic efficiency and electronic short resistance.

[0035] The thicknesses of the membrane assembly’s electrolyte diffusion layer prior to coating, and of the membrane/electrolyte diffusion layer, were measured using a micrometer (obtained under the trade designation“TMI 49-16-01 PRECISION MICROMETER” from Testing Machines Inc., Ronkonkoma, NY), with a dead weight pressure of 50 kPa (7.3 psi) and a diameter of standard anvil of 0.63 inch (1.6 cm). The reported values are an average of 5 individual measurements. The equivalent thickness of ionomer coating itself was measured using the same method. Comparative Example A

[0036] Comparative Example A (CEx A) was a 50-micrometer membrane (obtained under the trade designation“NAFION NR-212” from Ion Power Incorporated, New Castle, DE).

[0037] The membrane was tested using the Redox Flow Battery Screening Test. The results of these tests are reported in Table 1, below.

Table 1

Comparative Example B

[0038] Comparative Example B (“CEx B”) was made on a roll-to-roll line (obtained under the trade designation“ML200” from Hirano Entec Ltd., Nara, Japan). The roll-to-roll line had four drying zones arranged sequentially in the down-web direction, set to 50°C, l00°C, l20°C, and l45°C, respectively.

The coating solution was a perfluorosulfonic acid (PFSA) ionomer having an equivalent weight of 825 grams per equivalent (obtained under the trade designation“825 EW PFSA” from 3M Company, St. Paul, MN) dissolved at 35% solids by weight in a mixture of ethanol/water (75/25 by weight). The coating solution was coated onto a polyimide liner (obtained under the trade designation“KAPTON HN200” from DuPont, Wilmington, DE) at a constant flow rate using a notch bar coating die and a line speed of about 2 meters per minute, with a target dry thickness of 20 micrometers.

[0039] The coated material was annealed at 200°C, over a conventional heated drum with a residence time of 2 minutes.

[0040] The resulting membrane was tested using the Redox Flow Battery Screening Test. The results of these tests are reported in Table 1, above.

Example 1 [0041] Example 1 was prepared as described for Comparative Example B, except a porous web was fed into the wet coated solution before drying. More specifically, a fiberglass tissue (obtained under the trade designation“MF-03” from ACP Composites Incorporated, Livermore, CA) was used. The porous fiber mat, thicker than the coated solution, was fed into the wet ionomer solution coated on a liner prior to the drying ovens. The tension of the porous support layer was kept taunt to facilitate pulling the support into the solution with the liner serving as the hard stop. The resultant film was one whereby the liner side was support filled with ionomer and the other side was support free of ionomer, as shown in FIG. 1. The liner and annealing conditions were as described in CEx 2. The thickness of the free standing ionomer was measured from the portion of the coating located on the liner before and after the support fed portion, after drying. Further, basis weights taken of the coated membrane/electrolyte diffusion layer minus uncoated support/electrolyte diffusion layer were in agreement with the thickness measured, assuming a dried coating density of 2 g/cm³. The membrane assembly was tested in the Redox Flow Battery Screening Test, the results of which are reported in Table 1, above.

Example 2

[0042] Example 2 was prepared as described for Example 1, except a lower concentration of the PFSA ionomer was used in the coating solution, such that the target coating thickness of the dried unsupported ionomer coating was 10 micrometers. The ionomer-only free-standing thickness was measured from the portion of the ionomer-coated liner before and after the supported membrane region. Further, basis weights taken of the coated membrane/electrolyte diffusion layer minus uncoated support/electrolyte diffusion layer were in agreement with the thickness measured, assuming a dried coating density of 2 g/cm³.

[0043] The resulting membrane assembly was tested using Redox Flow Battery Screening Test. The results of these tests are reported in Table 1, above.

Example 3

[0044] Example 3 consists of two lO-cm-wide pieces of the membrane assembly prepared in Example 1 that were removed from their liners and bonded together, liner side to liner side, using a laminator (obtained under the trade designation“HL-101” from Cheminstruments, West Chester Township, OH). Lamination was conducted with pressure setpoint of 1.1 Megapascal applied to two 10-centimeter diameter cylinders heated to a setpoint temperature of l77°C, with a drive rate of 30 centimeters per minute. The two pieces of membrane were passed through the laminator with ionomer side to ionomer side together sandwiched between two oversized pieces of 50-micrometer thick polyimide (“KAPTON HN200”). [0045] The resulting membrane assembly was tested using the Redox Flow Battery Screening Test. The results of these tests are reported in Table 1, above.

Example 4

[0046] Example 4 was a hand spread made as follows. An applicator (obtained under the trade designation“GARDCO SQUARE APPLICATOR” from Paul N. Gardner Company, Inc., Pompano Beach, FL) was used to apply coating solution onto a polyimide liner (obtained under the trade designation“KAPTON HN200” from DuPont, Wilmington, DE). The fiber mat used for the support/electrolyte diffusion material was 101.7 g/m² glass (obtained under the trade designation“3 OUNCE E GLASS STYLE 120” from U.S. Composites Incorporated, West Palm Beach, FL). The ionomer used for coating material was obtained under the trade designation“3M 825EW PFSA” from 3M Company. The weight percent solids of the PFSA in an ethanohwater blend of 75:25 was 30%.

[0047] Referring to FIGS. 3A and 3B, scanning electron microscope (SEM) digital images of the membrane side and liner side, respectively of Example 4, taken with an SEM (obtained under the trade designation“JEOL 6010LA” from JEOL Ltd., Japan, Tokyo) are shown. The planar view samples were prepared by gold sputter coating with a sputter coater (obtained under the trade designation“SC7620” from Quorum Technologies, East Sussex, Great Britain).

[0048] The resulting membrane was tested using the Redox Flow Battery Screening Test. The results of these tests are reported in Table 1, above.

[0049] Foreseeable modifications and alteration of this disclosure will be apparent to those skilled in the art without departing from the scope and spirit of this invention. This invention should not be restricted to the embodiments that are set forth in this application for illustrative purposes.

Claims

What is claimed is:

1. A membrane assembly comprising an electrolyte diffusion layer having first and second major surfaces and a thickness between the first and second major surfaces, wherein at least 1 percent of the average thickness from the first major surface comprises ionomer, wherein the electrolyte diffusion layer has a length and a width, wherein the electrolyte diffusion layer has a planar volume defined by the average length of the electrolyte diffusion layer, the average width of the electrolyte diffusion layer, and the average thickness from the first major surface to the second major surface, wherein at least 10 percent of the planar volume comprises ionomer, wherein the electrolyte diffusion layer has a planar volume, wherein the electrolyte diffusion layer absent any ionomer has an open area porosity in a range from 5 to 95 percent of the planar volume of the electrolyte diffusion layer, and wherein the first major surface has an electrical resistivity of at least 2,500 ohm-centimeters-squared.

2. The membrane assembly of claim 1, wherein, the first major surface is free of ionomer.

3. The membrane assembly of any preceding claim, wherein the second major surface is free of ionomer.

4. The membrane assembly of any preceding claim, wherein the electrolyte diffusion layer comprises at least one of woven or nonwoven electrically non-conductive fibers.

5. The membrane assembly of any preceding claim, wherein the electrolyte diffusion layer comprises electrically non-conductive polymeric fibers.

6. The membrane assembly of claim 4, wherein the electrically non-conductive polymeric fibers comprise at least one of a polyurethane, a polyester, a polyamide, a polyether, a polycarbonate, a polyimide, a polysulfone, a polyphenylene oxide, a polyacrylate, a polymethacrylate, a polyolefin, a polystyrene, a polyvinyl chloride, or a fluorinated polymer.

7. The membrane assembly of claim 4, wherein the electrically non-conductive fibers are inorganic fibers.

8. The membrane assembly of any preceding claim, wherein the ionomer comprises at least one of perfluorosulfonic acid, perfluoroimide acid, perfluoroimide ionene chain extended, sulfonated poly ether sulfone, sulfonated polyether ether ether ketone, or perfluorosulfonamide.

9. The membrane assembly of any preceding claim, wherein the thickness of the electrolyte diffusion layer is in a range from 5 micrometers to 500 micrometers.

10. An electrode assembly comprising first and second porous electrodes with at least one membrane assembly of any preceding claim disposed between the first and second porous electrodes.

11. The electrode assembly of claim 10, wherein the first and second porous electrodes each independently comprise at least one of carbon paper, carbon felt, or carbon cloth.

12. The electrode assembly of either claims 10 or 11, wherein at least one of the first or second porous electrodes is hydrophilic.

13. An electrochemical cell comprising an electrode assembly of any of claims 10 to 12.

14. A liquid redox flow battery comprising an electrode assembly of any of claims 10 to 12.

15. A method of making a membrane assembly of any of claims 1 to 9, the method comprising: at least partially impregnating ionomer into a porous layer to provide an electrolyte diffusion layer having first and second major surfaces and a thickness between the first and second major surfaces, wherein at least 1 percent of the thickness from the first major surface comprises ionomer, wherein the electrolyte diffusion layer has a length and a width, wherein the electrolyte diffusion layer has a planar volume defined by the average length of the electrolyte diffusion layer, the average width of the electrolyte diffusion layer, and the average thickness from the first major surface, wherein at least 10 percent of the planar volume comprises ionomer, wherein the electrolyte diffusion layer has a planar volume, wherein the electrolyte diffusion layer absent any ionomer has an open area porosity in a range from 5 to 95 percent of the planar volume of the electrolyte diffusion layer, and wherein the first major surface has an electrical resistivity of at least 2,500 ohm-centimeters-squared.