TECHNICAL FIELD

The present disclosure broadly relates to porous electrodes, methods of making them, and devices including them.

BACKGROUND

A redox flow battery is an electrochemical energy storage device that contains at least one active redox couple that is either dissolved in a solvent or suspended in a liquid phase. The individual cells that are present in a redox flow battery contain two or more electrodes (typically carbon fiber electrodes) that have catalytic surface sites for the redox reactions, an ion-exchange or non-ionic nanoporous membrane, and bipolar plates that allow for current transport through the cell stack (i.e., the battery).

The core components of electrodes, membrane disposed between the electrodes, and optional transport protection layers disposed between each electrode and the membrane define a membrane electrode assembly (MEA). Since the electrochemically active species are present in fluid media, it is possible to alter the energy (volume of the electrolyte tanks) and power (area of the electrodes) independently for an installation. This decoupling of energy and power is a significant advantage over other forms of electrochemical energy storage.

Some commercial redox flow battery cells use a cell configuration containing thick (>l-2 mm) graphitic carbon felts that sandwich a thick cation exchange membrane (greater or equal to 50 pm). Electrolyte is injected through the felts using ports in the bipolar plate that line one side of the outer periphery of the felt, and the electrolyte flows through the felt to ports on the opposite side of plate. Since the electrolyte flows through the felt, this configuration is called "flow-through" . In contrast, the higher power "flow-by" configuration uses flow channels that are machined into the bipolar plates. By switching to this configuration, thinner graphitic carbon papers (<500 pm) and a thinner membrane can be used, which lowers the resistance of individual cells. Rapid flow rates can also be maintained with certain flow field architectures for the flow-by configuration, while high pressure drops would be encountered in the flow through configuration for a similar flow rate.

In order to decrease the cost of MEAs, the papers currently used as electrodes in commercial installations are carbon-carbon composites (carbon papers). These papers are typically made by impregnating a pre-formed mat of graphitic carbon fibers with phenolic resin or another carbonizable polymer, and then bringing the web to temperatures >1500 °C in a non-oxidizing environment to graphitize the polymer. This processing step is expensive, and the resulting carbon paper is brittle and difficult to manipulate in a roll format without cracking the paper, leading to yield loss. This process results in a practical limitation on the thickness of the carbon paper that can be made, and so in some applications multiple carbon papers are used. WO 2016/154171 Al describes a method for making carbon paper without having to conduct the final carbonization step. Instead, the particulate -based electrode is produced by making a wet-laid paper that contains carbon particulates (e.g., fibers, nanofibers, graphite flakes) all bound together with a polyolefinic binder. Aside from reducing cost by avoiding additional carbonization steps, this wet-laid electrode is significantly more flexible than incumbent carbon papers and does not fracture when a roll of the material is wound or unwound from around a core. While the electrodes described in WO

2016/154171 Al are flexible and can be manipulated in roll form, they may deform under a mechanical load and collapse into the flow fields inscribed in the bipolar plates of the MEA. This results in a narrowing of the flow channels, and an increase in pressure in the fluid lines that feed into the cell. Two negative effects occur because of this. First, the pressure increase leads to increased energy losses due to a greater load on the pumping system to maintain flowrates. Second, electrode collapse causes an increase in the overpotential related to mass-transport and degrades the voltaic efficiency of the cell. In severe cases, the combined action of the collapse of the electrodes and the continual flow of electrolyte can rip the electrodes along the edge of the flow channels.

SUMMARY

The present disclosure overcomes the problem of structural failure of the electrodes during use by incorporating electrically-insulating inorganic fibers into the electrode structure. Unexpectedly, it is presently discovered that electrodes highly loaded with electrically-nonconductive reinforcing fibers (e.g., 20 percent by weight) still maintain through-plane resistance and cell resistance in a catholyte double half-cell, performing similarly to carbon paper electrodes used in commercial devices, but with improved durability and/or reduced manufacturing cost. Further, electrodes according to the present disclosure can be made thicker than carbon paper electrodes while achieving useful levels of mechanical and electrochemical performance.

While the mechanical properties of the graphitic carbon fibers and the electrically-nonconductive reinforcing fibers are similar, unexpectedly when a blend of graphitic carbon fibers and electrically- nonconductive reinforcing fibers is used according to the present disclosure, the mechanical properties of the resultant electrode are better than one made using only the graphitic carbon fibers.

In one aspect, the present disclosure provides a porous electrode having first and second opposed major surfaces and comprising, based on the total weight on the porous electrode:

60 to 92.5 percent by weight of graphitic carbon fibers having an average fiber diameter of 5 to 10 microns and an average length of 6 to 25 mm;

5 to 25 percent by weight of electrically-nonconductive reinforcing fibers having an average fiber diameter of 5 to 25 microns and an average length of 3 to 50 mm, wherein the electrically-nonconductive reinforcing fibers are ceramic, glass, glass-ceramic, or a combination thereof; 2.5 to 15 percent by weight of polymer fibers selected from hydrocarbon polyolefin fibers, perfhiorinated polymer fibers, partially fluorinated polymer fibers, chlorinated polyolefin fibers, polyvinyl chloride fibers, polysulfone fibers, polyaryletherketone fibers, and combinations thereof,

wherein at least some of the polymer fibers proximal to the first and second major surfaces are melt-bonded to each other.

In another aspect, the present disclosure provides a membrane-electrode assembly for a redox flow battery comprising:

first and second porous electrodes according to the present disclosure; and

an ion exchange membrane disposed between the first and second porous electrodes.

In yet another aspect, the present disclosure provides a redox flow battery comprising at least one porous electrode according to the present disclosure.

In yet another aspect, the present disclosure provides a method of making a porous electrode, the method comprising:

providing a composition comprising, based on the total weight on the porous electrode:

60 to 92.5 parts by weight of graphitic carbon fibers having an average fiber diameter of 5 to 10 microns and an average length of 6 to 25 mm;

5 to 25 parts by weight of electrically-nonconductive reinforcing fibers having an average fiber diameter of 5 to 25 microns and an average length of 3 to 50 mm, wherein the electrically- nonconductive reinforcing fibers are ceramic, glass, glass-ceramic, or a combination thereof;

2.5 to 15 parts by weight of polymer fibers selected from hydrocarbon polyolefin fibers, perfhiorinated polymer fibers, partially fluorinated polymer fibers, chlorinated polyolefin fibers, polyvinyl chloride fibers, polysulfone fibers, polyaryletherketone fibers, and combinations thereof; and

a liquid vehicle;

depositing the composition on a porous substrate;

removing at least a portion of the liquid vehicle through the porous substrate to make a nonwoven fibrous web; and

melt-bonding at least some of the polymer fibers proximal to the first and second major surfaces to each other the polymer fibers and consolidating the nonwoven fibrous web to make the porous electrode.

As used herein, "electrically-nonconductive" means having a through-plane electrical conductivity of less than 0.01 Siemens/meter, and "electrically-conductive" means not electrically- nonconductive.

As used herein, when a surface of one substrate is in "contact" with the surface of another substrate, there are no intervening layer(s) between the two substrates and at least a portion of the surfaces of the two substrates are in physical contact. As used herein, if a surface of a first substrate is "adjacent" to a surface of a second substrate, the two surfaces are considered to be facing one another. They may be in contact with one another or they may not be in contact with one another, an intervening third substrate or substrates being disposed between them.

As used herein, if a surface of a first substrate is "proximate" a surface of a second substrate, the two surfaces are considered to be facing one another and to be in close proximity to one another, i.e., to be within less than 1000 microns, less than 500 microns, less than 100 microns or even in contact with one another. However, there may be one or more intervening substrates disposed between the substrate surfaces.

As used herein, an electrode is considered "porous", and an electrode material is considered "porous", if it allows a liquid to flow from one exterior surface of a 3-dimensional porous electrode structure containing the porous electrode material to the exterior of an opposing surface of the 3- dimensional structure.

Features and advantages of the present disclosure will be further understood upon consideration of the detailed description as well as the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective drawing of an exemplary porous electrode 100 according to the present disclosure.

FIG. 2 is a micrograph showing an exemplary porous electrode 200 according to the present disclosure.

FIG. 3 is a schematic view of an exemplary membrane electrode assembly 300 according to the present disclosure.

FIG. 4 is a schematic cross-sectional side view of exemplary electrochemical cell 400 according to the present disclosure.

FIG. 5 is a schematic cross-sectional side view of electrochemical cell stack 500 according to the present disclosure.

FIG. 6 shows a schematic view of an exemplary single-cell, redox flow battery 600 according to the present disclosure.

FIG. 7 is a plot of applied pressure and through-plane resistance of the various porous electrodes of Examples 1-4 and Comparative Examples A-C.

FIG. 8 is a plot of applied pressure and thickness of the porous electrodes of Examples 1-4 and Comparative Examples A-C.

FIGS. 9A and 9B are digital images of a cutaway cell used to analyze channel intrusion and materials compressed within the cell.

FIG. 10 is an image of two Comparative Example 2 electrodes and a membrane after compression at 14 mils (360 microns) for 30 minutes. FIG. 11 is a schematic diagram showing the flow cell design used in the Catholyte Double Half- Cell Test according to the present disclosure.

FIG. 12 is a schematic diagram illustrating the process used in the Catholyte Double Half-Cell Test according to the present disclosure.

Repeated use of reference characters in the specification and drawings is intended to represent the same or analogous features or elements of the disclosure. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of the principles of the disclosure. The figures may not be drawn to scale.

DETAILED DESCRIPTION

A single electrochemical cell, which may be used in the fabrication of a redox flow battery (e.g., a redox flow battery) generally includes: two porous electrodes, an anode and a cathode; an ion- permeable membrane disposed between the two electrodes, providing electrical insulation between the electrodes and providing a path for one or more select ionic species to pass between the anode and cathode half-cells; anode and cathode flow plates, the former positioned adjacent the anode and the later positioned adjacent the cathode, each containing one or more channels which allow the anolyte and catholyte electrolytic solutions to contact and penetrate into the anode and cathode, respectively. In a redox flow battery containing a single electrochemical cell, for example, the cell would also include two current collectors, one adjacent to and in contact with the exterior surface of the anode flow plate and one adjacent to and in contact with the exterior surface of the cathode flow plate. The current collectors allow electrons generated during cell discharge to connect to an external circuit and do useful work. A functioning redox flow battery or electrochemical cell also includes an anolyte, anolyte reservoir and corresponding fluid distribution system (piping and at least one or more pumps) to facilitate flow of anolyte into the anode half-cell, and a catholyte, catholyte reservoir and corresponding fluid distribution system to facilitate flow of catholyte into the cathode half-cell. Although pumps are typically employed, gravity feed systems may also be used. During discharge, active species (e.g., cations) in the anolyte are oxidized and the corresponding electrons flow though the exterior circuit and load to the cathode where they reduce active species in the catholyte. As the active species for electrochemical oxidation and reduction are contained in the anolyte and catholyte, redox flow cells and batteries have the unique feature of being able to store their energy outside the main body of the electrochemical cell, i.e., in the anolyte. The amount of storage capacity is mainly limited by the amount of anolyte and catholyte and the concentration of active species in these solutions. As such, redox flow batteries may be used for large scale energy storage needs associated with wind farms and solar energy plants, for example, by scaling the size of the reservoir tanks and active species concentrations, accordingly. Redox flow cells also have the advantage of having their storage capacity being independent of their power. The power in a redox flow battery or cell is generally determined by the size and number of electrode-membrane assemblies along with their corresponding flow plates (sometimes referred to in total as a "stack") within the battery. Additionally, as redox flow batteries are being designed for electrical grid use, the voltages must be high. However, the voltage of a single redox flow electrochemical cell is generally less than 3 volts (difference in the potential of the half-cell reactions making up the cell). As such, hundreds of cells may be required to be connected in series to generate voltages great enough to have practical utility and a significant amount of the cost of the cell or battery relates to the cost of the components making an individual cell.

At the core of the redox flow electrochemical cell and battery is the membrane-electrode assembly (anode, cathode, and ion-permeable membrane disposed therebetween). Hundreds of MEAs may be required per cell stack and battery. The design of the MEA is critical to the power output of a redox flow cell and battery. Accordingly, the materials selected for these components are critical to performance. Materials used for the electrodes may be based on carbon, which provides desirable catalytic activity for the oxidation/reduction reactions to occur and is electrically-conductive to provide electron transfer to the flow plates. The electrode materials may be porous, to provide greater surface area for the oxidation/reduction reactions to occur. Porous electrodes may include carbon fiber-based papers, felts, and cloths. When porous electrodes are used, the electrolytes may penetrate into the body of the electrode, access the additional surface area for reaction and thus increase the rate of energy generation per unit volume of the electrode. Also, as one or both of the anolyte and catholyte may be water-based, there may be a need for the electrode to have a hydrophilic surface, to facilitate electrolyte permeation into the body of a porous electrode. Surface treatments may be used to enhance the hydrophilicity of the redox flow electrodes. This is in contrast to fuel cell electrodes, which typically are designed to be hydrophobic to prevent moisture from entering the electrode and corresponding catalyst layer/region, and to facilitate removal of moisture from the electrode region in, for example, a hydrogen/oxygen-based fuel cell.

Materials used for the ion-permeable membrane are required to be good electrical insulators while enabling one or more select ions to pass through the membrane. These materials are often fabricated from polymers and may include ionic species to facilitate ion transfer through the membrane. Thus, the material making up the ion-permeable membrane may be an expensive specialty polymer.

Referring now to FIG. 1, porous electrode 100 has first and second opposed major surfaces 110, 112. Porous electrode 100 comprises graphitic carbon fibers 120, electrically-nonconductive reinforcing fibers 130, and polymer fibers 140. At least some of the polymer fibers 140 proximal to first and second major surfaces 110, 112 are melt-bonded to each other at one or more points of contact 150 between the polymer fibers.

FIG. 2 shows an exemplary porous electrode 200 in greater detail, wherein graphitic carbon fibers 220, electrically-nonconductive reinforcing fibers 230, and fibrillated melt-bonded polymer fiber 240 are visible.

Graphitic carbon fibers are electrically-conductive carbon fibers made from organic precursor fiber by heating it in the absence of oxygen at a temperature above 2000 °C in an oxygen-depleted atmosphere. Exemplary organic precursor fibers include fibers of polyacrylonitrile (PAN), isotropic pitch, mesophase pitch, lignin, rayon, polyethylene, cellulose, or combinations thereof. Combinations of these fibers may also be used. Useful graphitic carbon fibers are typically 5 to 10 microns in average diameter, although other diameters may also be used. Useful graphitic carbon fibers have an average length of 3 to 25 millimeters (mm), preferably 6 to 25 mm, more preferably 4 to 18 mm, and even more preferably 6 to 12 mm. In some embodiments, graphitic carbon fibers may have a single-filament tensile modulus of 350 to 900 gigapascals (GPa), more preferably 500 to 900, and/or a single-filament tensile strength of preferably 2.5 to 5 GPa, more preferably 3 to 4 GPa, although other values may also be used.

In some embodiments, the graphitic carbon fibers may be surface-treated. Surface treatment may enhance the wettability of the electrode to a given anolyte or catholyte or to provide or enhance the electrochemical activity of the electrode relative to the oxidation-reduction reactions associated with the chemical composition of a given anolyte or catholyte. Surface treatments include, but are not limited to, at least one of chemical treatments, thermal treatments and plasma treatments. In some embodiments, the graphitic carbon fibers have enhanced electrochemical activity, produced by at least one of chemical treatment, thermal treatment and plasma treatment. The term "enhanced" means that the electrochemical activity of the graphitic carbon fibers is increased after treatment relative to the electrochemical activity of the graphitic carbon fibers prior to treatment.

Enhanced electrochemical activity may include at least one of increased current density, reduced oxygen evolution and reduced hydrogen evolution. The electrochemical activity can be measured by fabricating a porous electrode from the graphitic carbon fibers (prior to and after treatment) and comparing the current density generated in an electrochemical cell by the electrode, higher current density indicating enhancement of the electrochemical activity. Electrochemical impedance spectroscopy can be used to measure activity improvement. In some embodiments, the electrically-conductive fiber is hydrophilic.

Electrically-nonconductive reinforcing fibers may comprise ceramic and/or glass fibers such as, for example, low boron or boron-free E-CR (electrical/chemical resistant) glass, a-alumina fiber, mullite fiber, calcined amorphous silica fiber, zirconium oxide fiber, silicon carbide fiber, yttrium oxide fiber, cerium oxide fiber. Any doped or mixed oxide fibers of the previous list (e.g., yttrium-stabilized zirconium oxide fiber) can be included. Preferably, the electrically-nonconductive reinforcing fibers have a single-filament tensile modulus of 50 to 400 gigapascals (GPa) and/or a single-filament tensile strength of 1.5 to 4 GPa, preferably 3 to 4 GPa, although other values may also be used. In some embodiments, useful electrically-nonconductive reinforcing fibers may have an average fiber width of 5 to 25 microns, preferably 5 to 15 microns, and more preferably 8 to 12 microns, and/or average length of 3 to 50 mm, preferably 12 to 25 mm, preferably 8 to 12 microns. Preferably, the electrically-nonconductive reinforcing fibers are able to span the flow channels (these should be at least double the length of the channels to compensate for any orientational variation), collapse is significantly curtailed, and the rupture of the electrodes is prevented. Beyond the mechanical reinforcement of the porous electrodes, the electrically -nonconductive reinforcing fibers can also enhance the hydrophilicity of the network and help with wicking of the electrolyte through the porous electrode.

The polymer fibers should generally be unreactive to the chemical environment during use, and may comprise, for example, linear and cyclic polyolefins (includes polyethylene, polypropylene, and copolymers such as TOPAS Cyclic Olefin Copolymer from TOPAS Advanced Polymers, Florence, Kentucky, perfluorinated and fluorinated polymers, chlorinated polyethylene, polyvinyl chloride, polysulfones, and polyaryletherketones. Core-shell and block copolymers may also be used, provided that the polymers are adequately unreactive to the chemical environment during use. Blends of the aforementioned polymer fibers may also be used. In some embodiments, the polymer fibers are fibrillated, preferably microfibrillated.

During manufacture of the porous electrode, the polymer fibers are heated so as to soften them and cause bonding at points of contact with other fibers, especially at or near the surface of the porous electrode. In some embodiments, the polymer fibers may be heated at a temperature that is not less than 30 °C, not less than 20 °C or even not less than 10 °C lower than the lowest glass lowest transition temperature of the polymer fiber. The polymer fiber may have more than one glass transition temperature, if, for example, it is a block copolymer or a core-shell polymer. In some embodiments, the polymer fiber may be bonded at points of contact to other fibers at a temperature that is below the highest melting temperature of the polymer fiber or, when the polymer fiber is an amorphous polymer, no greater than 50 °C, no greater than 30 °C or even no greater than 10 °C above the highest glass transition temperature of the polymer fiber.

The polymer fiber may be selected to facilitate the transfer of select ion(s) of the electrolytes through the electrode. This may be achieved by allowing the electrolyte to easily wet a given polymer fiber. The material properties, particularly the surface wetting characteristics of the polymer may be selected based on the type of anolyte and catholyte solution, i.e., whether they are aqueous or non- aqueous. As disclosed herein, an aqueous solution is defined as a solution wherein the solvent includes at least 50 percent water by weight. A non-aqueous solution is defined as a solution wherein the solvent contains less than 50 percent water by weight. In some embodiments, the polymer of the electrode may be hydrophilic. This may be particularly beneficial when the electrode is to be used in conjunction with aqueous anolyte and/or catholyte solutions. In some embodiments the polymer may have a surface contact angle with water, catholyte and/or anolyte of less than 90°. In some embodiments, the polymer may have a surface contact with water, catholyte and/or anolyte of between 85° and 0°, between 70° and 0°, between 50° and 0°, between 30° and 0°, between 20° and 0°, or even between 10° and 0°.

In some embodiments, the polymer fibers have a softening temperature (e.g., the glass transition temperature and/or the melting temperature) of between 20 °C and 400 °C, between 20 °C and 350 °C, between 20 °C and 300 °C, between 20 °C and 250 °C, between 20 °C and 200 °C, between 20 °C and 150 °C, between 35 °C and 400 °C, between 35 °C and 350 °C, between 35 °C and 300 °C, between 35 °C and 250 °C, between 35 °C and 200 °C, between 35 °C and 150 °C, between 50 °C and 400 °C, between 50 °C and 350 °C, between 50 °C and 300 °C, between 50 °C and 250 °C, between 50 °C and 200 °C, between 50 °C and 150 °C, between 75 °C and 400 °C, between 75 °C and 350 °C, between 75 °C and 300 °C, between 75 °C and 250 °C, between 75 °C and 200 °C, or even between 75 °C and 150 °C.

In some embodiments, the polymer fiber is composed of two or more polymers selected as described above and has a core-shell structure, i.e., an inner core comprising a first polymer and an outer shell comprising a second polymer. In some embodiments the polymer of the outer shell, e.g., second polymer, has a softening temperature, e.g., the glass transition temperature and/or the melting temperature that is lower than softening temperature of the first polymer. In some embodiments, the second polymer has a softening temperature (e.g., the glass transition temperature and/or the melting temperature) of between 20 °C and 400 °C, between 20 °C and 350 °C, between 20 °C and 300 °C, between 20 °C and 250 °C, between 20 °C and 200 °C, between 20 °C and 150 °C, between 35 °C and 400 °C, between 35 °C and 350 °C, between 35 °C and 300 °C, between 35 °C and 250 °C, between 35 °C and 200 °C, between 35 °C and 150 °C, between 50 °C and 400 °C, between 50 °C and 350 °C, between 50 °C and 300 °C, between 50 °C and 250 °C, between 50 °C and 200 °C, between 50 °C and 150 °C, between 75 °C and 400 °C, between 75 °C and 350 °C, between 75 °C and 300 °C, between 75 °C and 250 °C, between 75 °C and 200 °C, or even between 75 °C and 150 °C.

In some embodiments, the electrodes of the present disclosure may contain a non-electrically conductive, inorganic particulate. Non-electrically conductive, inorganic particulate include, but is not limited to, minerals and clays known in the art. In some embodiments the non-electrically conductive inorganic particulate may be a metal oxide. In some embodiments the non-electrically conductive, inorganic particulate include at least one of silica, alumina, titania, and zirconia.

The porous electrode may be in the form of a sheet, which may be flat or textured (e.g., corrugated or otherwise embossed). After drying or during drying, the temperature may be such that the temperature is near, at or above the softening temperature of the polymer, e.g., the glass transition temperature and/or the melting temperature of the polymer, which may aid in the adhering of carbon particulate to the polymer and/or further fuse the polymer.

In one embodiment, polymer fiber, electrically-nonconducting reinforcing fiber, and graphitic carbon fibers may be mixed together in a liquid vehicle, along with any optional ingredients, forming a slurry. Examples of suitable liquid vehicles include water, alcohols (e.g., methanol, ethanol, propanol, butanol), ethers (e.g., tetrahydrofuran, tetrahydropyran, glyme, diglyme), and combinations of the foregoing, although any solvent that can suspend the fibers may be used. The slurry may then be formed into a fiber web or mat by casting it onto a porous substrate (e.g., a screen) and then removing some or preferably substantially all, or the liquid vehicle using wet-laid fiber web making techniques. Suitable wet-laid techniques are well-known and described in, for example, WO 2016/154171 Al (Y ORDEM et ak). The fiber web may then be heat treated and/or consolidated at temperatures near, at, or above, the softening temperature of the polymer fiber, e.g., the glass transition temperature and/or the melting temperature of the polymer fiber, to fuse at least a portion of the polymer fiber/carbon particulate dry blend into a unitary, porous material, thereby forming a porous electrode. The porous electrode may be in the form of a sheet. The thermal treatment may also aid in adhering the graphitic carbon fibers to the surface of the polymer fiber. The thermal treatment may be conducted under pressure, e.g., in a heated press or between heated rolls (e.g., heated calender rolls). The press and or heated rolls may be set to provide a specific desired gap, which will facilitate obtaining a desired electrode thickness.

The porous electrodes of the present disclosure may be washed using conventional techniques to remove loose particulate debris. The washing technique may include and appropriate solvent, e.g., water, and/or surfactant to aid in the removal of loose carbon particulate. The porous electrodes of the present disclosure may be made by a continuous roll to roll process, the electrode sheet being wound to form a roll good.

In some embodiments, the porous electrode may be hydrophilic. This may be particularly beneficial when the porous electrode is to be used in conjunction with aqueous anolyte and/or catholyte solutions. Uptake of a liquid, e.g., water, catholyte and/or anolyte, into the pores of a redox flow battery electrode may be considered a key property for optimal operation of a redox flow battery. In some embodiments, 100 percent of the pores of the porous electrode may be filled by the liquid, creating the maximum interface between the liquid and the electrode surface. In other embodiments, between 30 percent and 100 percent, between 50 percent and 100 percent, between 70 percent and 100 percent or even between 80 percent and 100 percent of the pores of the electrode may be filled by the liquid. In some embodiments, the porous electrode may have a static surface contact angle with water, catholyte and/or anolyte of less than 90°.

In some embodiments, the electrode may be surface-treated to enhance the wettability of the electrode to a given anolyte or catholyte or to provide or enhance the electrochemical activity of the electrode relative to the oxidation-reduction reactions associated with the chemical composition of a given anolyte or catholyte. Surface treatments include, but are not limited to, at least one of chemical treatments, thermal treatments and plasma treatments.

Surfactants may be used in the electrode dispersion/coating solutions, for example, to improve wetting and/or aid in dispersing of the graphitic carbon fibers. Surfactants may include cationic, anionic and nonionic surfactants. Surfactants useful in the porous electrode dispersion/coating solutions include, but are not limited to TRITON X-100, available from Dow Chemical Company, Midland, Michigan; DISPERSBYK 190, available from BYK Chemie GMBH, Wesel, Germany; amines, e.g., olyelamine and dodecylamine; amines with more than 8 carbons in the backbone, e.g., 3-(/V./V-dimethyldodecyl- ammoniojpro panes ulfonate (SB 12); SMA 1000, available from Cray Valley USA, UUC, Exton,

Pennsylvania; 1, 2-propanediol, triethanolamine, dimethylaminoethanol; quaternary amine and surfactants disclosed in U.S. Pat. Publ. No. 2013/0011764 Al (Okada et ak), which is incorporated herein by reference in its entirety. If one or more surfactants are used in the dispersions/coating solutions, the surfactant may be removed from the electrode by a thermal process, wherein the surfactant either volatilizes at the temperature of the thermal treatment or decomposes and the resulting compounds volatilize at the temperature of the thermal treatment. In some embodiments, the electrode is substantially free of surfactant. By "substantially free" it is meant that the electrodes contain, by weight, from 0 percent to 0.5 percent, from 0 percent to 0.1 percent, from 0 percent to 0.05 percent or even from 0 percent to 0.01 percent surfactant. In some embodiments, the electrode layer contains no surfactant. The surfactant may be removed from the electrode by washing or rinsing with a solvent of the surfactant. Solvents include, but are not limited to water, alcohols (e.g., methanol, ethanol and propanol), acetone, ethyl acetate, alkyl solvents (e.g., pentane, hexane, cyclohexane, heptane and octane), methyl ethyl ketone, diethyl ketone, dimethyl ether, petroleum ether, toluene, benzene, xylenes, dimethylformamide, dimethyl sulfoxide, chloroform, carbon tetrachloride, chlorobenzene, and mixtures thereof.

The thickness of the porous electrode may be, for example, from 10 microns to 5000 microns, from 10 microns to 1000 microns, from 10 microns to 500 microns, from 10 microns to 250 microns, from 10 microns to 100 microns, from 25 microns to 5000 microns, from 25 microns to 1000 microns, from 25 microns to 500 microns, from 25 microns to 250 microns, or even from 25 microns to 100 microns. The porosity of the porous electrodes, on a volume basis, may be from 5 percent to 95 percent, from 5 percent to 90 percent, from 5 percent to 80 percent, from 5 percent to 70 percent, from 10 percent to 95 percent, from 10 percent to 90 percent, from 10 percent to 80 percent, from 10 percent to 70 percent, from 10 percent to 70 percent, from 20 percent to 95 percent, from 20 percent to 90 percent, from 20 percent to 80 percent, from 20 percent to 70 percent, from 20 percent to 70 percent, from 30 percent to 95 percent, from 30 percent to 90 percent, from 30 percent to 80 percent, or even from 30 percent to 70 percent.

The porous electrode may be a single layer or multiple layers. When the porous electrode includes multiple layers, there is no particular limit as to the number of layers that may be used.

However, as there is a general desire to keep the thickness of the porous electrode and membrane assembly as thin as possible, the electrode may include from 2 to 20 layers, from 2 to 10 layers, from 2 to 8 layer, from 2 to 5 layers, from 3 to 20 layers, from 3 to 10 layers, from 3 to 8 layers, or even from 3 to 5. In some embodiments, when the porous electrode includes multiple layers, the electrode material of each layer may be the same electrode material, i.e., the composition of the electrode material of each layer is the same. In some embodiments, when the electrode includes multiple layers, the electrode material of at least one, up to including all of the layers, may be different, i.e., the composition of the electrode material of at least one, up to and including all layers, differs from the composition of the electrode material of another layer. In some embodiments, the porous electrodes may have a basis weight of 30 to

300 g/m , preferably 40 to 200 g/m , and more preferably 50 to 150 g/m . Advantageously, porous electrodes according to the present disclosure have good through-plane electrical resistance while simultaneously achieving good structural integrity. In some embodiments, the porous electrode has a through-plane resistance lower than 0.4 ohnrcm (preferably lower than 0.35 ohnrcm ) at an applied pressure of 0.6 megapascals (MPa).

Porous electrodes of the present disclosure may have an electrical resistivity, at 1.00 atmosphere (0.101 MPa) of pressure of from 0.1 pohnrm (microohm meter) to 10000 pohnrm, from 1 pohnrm to 10000 pohnrm. from 10 pohnrm to 10000 pohnrm, from 0.1 pohnrm to 1000 pohnrm, from 1 pohnrm to 1000 pohnrm, from 10 pohnrm to 1000 pohnrm, from 0.1 pohnrm to 100 pohnrm, from 1 pohnrm to 100 pohnrm, or even from 10 pohnrm to 100 pohnrm, for example.

In another embodiment, of the present disclosure, the porous electrodes of the present disclosure may be used to form membrane-electrode assemblies, for use in, for example, liquid flow batteries. A membrane-electrode assembly includes an ion exchange membrane, having a first surface and an opposed second surface, and a porous electrode according to any one of the embodiments of the present disclosure, wherein a major surface of the porous electrode is adjacent the first surface of the ion exchange membrane. In some embodiments a major surface of the porous electrode is proximate the first surface of the ion exchange membrane. In some embodiments a major surface of the porous electrode is in contact with the first surface of the ion exchange membrane. The membrane-electrode assembly may further include a second porous electrode according to any one of the porous electrodes of the present disclosure, wherein a major surface of the second porous electrode is adjacent the opposed second surface of the ion exchange membrane.

An exemplary membrane-electrode assembly 300 is shown in FIG. 3, wherein membrane- electrode assembly 300 comprises a stack of porous electrodes (lOOa, lOOb), optional transport protection layers (370a, 370b), and ion-exchange membrane 310. Optional release liners (320a, 320b) may be present to protect the membrane electrode assembly from damage and/or provide stiffness until it is used, e.g., in a redox flow battery.

The membrane-electrode assemblies of the present disclosure include an ion exchange membrane. Ion exchange membranes known in the art may be used. In some embodiments, the ion exchange membranes may include a fluorinated ion exchange resin. Ion exchange membranes useful in the embodiments of the present disclosure may be fabricated from ion exchange resins known in in the art or be commercially available as membrane films and include, but are not limited to, NAFION PFSA

MEMBRANES, available from DuPont, Wilmington, Delaware; AQUIVION PFSA, a perfluorosulfonic acid, available from SOLVAY, Brussels, Belgium; FLEMION and SELEMION, fhioropolymer ion exchange membranes, available from Asahi Glass, Tokyo, Japan; FUMASEP ion exchange membranes, including FKS, FKB, FKL, FKE cation exchange membranes and FAB, FAA, FAP and FAD anionic exchange membranes, available from Fumatek, Bietigheim-Bissingen, Germany and ion exchange membranes and materials described in U.S. Pat. No. 7,348,088 (Hamrock et al.), incorporated herein by reference in its entirety.

The ion exchange membranes of the present disclosure may be obtained as free-standing fdms from commercial suppliers or may be fabricated by coating a solution of the appropriate ion exchange membrane resin in an appropriate solvent, and then heating to remove the solvent. The ion exchange membrane may be formed from an ion exchange membrane coating solution by coating the solution on a release liner and then drying the ion exchange membrane coating solution coating to remove the solvent. The first surface of the resulting ion exchange membrane can then be laminated to a first surface of an electrode using conventional lamination techniques, which may include at least one of pressure and heat, forming membrane -electrode assembly. A first surface of a second electrode may then be laminated to the second surface of the ion exchange membrane, forming a membrane-electrode assembly.

Optional release liners may remain with the ion exchange membrane until it is used to fabricate a membrane-electrode assembly, in order to protect the outer surface of the electrode from dust and debris. The release liners may also provide mechanical support and prevent tearing of electrode and/or marring of its surface, prior to fabrication of the membrane-electrode assembly. The ion exchange membrane coating solution may be coated directly on a surface of an electrode. The ion exchange membrane coating solution coating is then dried to form an ion exchange membrane and the corresponding membrane- electrode assembly. If a second electrode is laminated or coated on the exposed surface of the formed ion exchange membrane, a membrane -electrode assembly with two electrodes may be formed. In another embodiment, the ion exchange membrane coating solution may be coated between two electrodes and then dried to form a membrane-electrode assembly.

Any suitable method of coating may be used to coat the ion exchange membrane coating solution on either a release liner or an electrode. Typical methods include both hand and machine methods, including hand brushing, notch bar coating, fluid bearing die coating, wire-wound rod coating, fluid bearing coating, slot-fed knife coating, and three-roll coating. Most typically three-roll coating is used. Advantageously, coating is accomplished without bleed-through of the ion exchange membrane coating from the coated side of the electrode to the uncoated side. Coating may be achieved in one pass or in multiple passes. Coating in multiple passes may be useful to increase coating weight without

corresponding increases in cracking of the ion exchange membrane.

The amount of solvent, on a weight basis, in the ion exchange membrane coating solution may be from 5 to 95 percent, from 10 to 95 percent, from 20 to 95 percent, from 30 to 95 percent, from 40 to 95 percent, from 50 to 95 percent, from 60 to 95 percent, from 5 to 90 percent, from 10 to 90 percent, from 20 percent to 90 percent, from 30 to 90 percent, from 40 to 90 percent, from 50 to 90 percent, from 60 to 90 percent, from 5 to 80 percent, from 10 to 80 percent from 20 percent to 80 percent, from 30 to 80 percent, from 40 to 80 percent, from 50 to 80 percent, from 60 to 80 percent, from 5 percent to 70 percent, from 10 percent to 70 percent, from 20 percent to 70 percent, from 30 to 70 percent, from 40 to 70 percent, or even from 50 to 70 percent. The amount of ion exchange resin, on a weight basis, in the ion exchange membrane coating solution may be from 5 to 95 percent, from 5 to 90 percent, from 5 to 80 percent, from 5 to 70 percent, from 5 to 60 percent, from 5 to 50 percent, from 5 to 40 percent, from 10 to 95 percent, from 10 to 90 percent, from 10 to 80 percent, from 10 to 70 percent, from 10 to 60 percent, from 10 to 50 percent, from 10 to 40 percent, from 20 to 95 percent, from 20 to 90 percent, from 20 to 80 percent, from 20 to 70 percent, from 20 to 60 percent, from 20 to 50 percent, from 20 to 40 percent, from 30 to 95 percent, from 30 to 90 percent, from 30 to 80 percent, from 30 to 70 percent, from 30 to 60 percent, or even from 30 to 50 percent.

The electrodes, membranes, e.g., ion exchange membranes, membrane-electrode assemblies and the electrochemical cells and liquid flow batteries of the present disclosure may include one or more transport protection layers. The membrane-electrode assemblies of the present disclosure may further include a transport protection layer disposed between the porous electrode and the ion exchange membrane. Transport protection layers are layers that may be coated or laminated on at least one of the electrode and membrane or may be place between the membrane and electrode for the purpose of preventing puncture of the membrane by the materials of the electrode. WO 2018/029617 Al (Weber et al.) discloses details concerning various transport protection layers, the disclosure of which is incorporated herein by reference. By preventing puncture of the membrane by the conductive electrode, the corresponding localized shorting of a cell or battery may be prevented.

Transport protection layers, electrode assemblies, and methods of making them are also disclosed in U. S. Pat. Appln. Publ. No. 2018/0053955 Al (Weber et al.), incorporated herein by reference in its entirety. Electrode assemblies may be fabricated, for example, by laminating a major surface of a previously formed porous electrode to a previously formed surface of a transport protection layer, heat and or pressure may be used to facilitate the laminating process) or by coating at least one major surface of a porous electrode with a transport protection layer coating, then curing and/or drying the coating to form a transport protection layer and, subsequently, an electrode assembly.

Transport protection layers may comprise (or in some embodiments, consist essentially of) a polymer resin, polymer fibers, and/or graphitic carbon fibers and, optionally, a non-electrically conductive particulate material. The composition of the transport protection layer differs from the composition of the porous electrodes. In some embodiments, the polymer resin of the first transport protection layer and second transport protection layer, if present, includes an ionic resin.

Any of the membrane assemblies of the present disclosure may include one or more transport protecting layers disposed between the ion exchange membrane and the porous electrode. In membrane- electrode assemblies that include a first porous electrode and a second porous electrode, the membrane electrode assembly may further include a first transport protection layer disposed between the ion exchange membrane and the first porous electrode and a second transport protection layer disposed between the ion exchange membrane and the second porous electrode. The first and second transport protection layers may each comprise a polymer resin and graphitic carbon fibers and, optionally, a non- electrically conductive particulate. In some embodiments, the polymer resin of the first transport protection layer and second transport protection layer is an ionic resin. The composition of the first and second transport protection layers may be the same or may differ.

The present disclosure further provides an electrode assembly for a redox flow battery. The electrode assembly includes a first porous electrode according to any one of the porous electrodes of the present disclosure and a first transport protection layer. The first electrode includes a first major surface and an opposed second major surface, and the first transport protection layer includes a first surface and an opposed second surface. A major surface of the first porous electrode is adjacent, proximate or in contact with the second surface of the first transport protection layer. In some embodiments, the first major surface of the first porous electrode is adjacent, proximate or in contact with the second surface of the first transport protection layer. In some embodiments, the second major surface of the first porous electrode is adjacent, proximate or in contact with the second surface of the first transport protection layer. In some embodiments, the first transport protection layer comprises polymer fibers (e.g., a polypropylene nonwoven fabric) and, optionally, electrically conductive or non-electrically conductive particulates. In some embodiments, the first transport protection layer comprises a polymer resin and graphitic carbon fibers and, optionally, a non-electrically conductive particulate. The composition of the transport protection layer differs from the composition of the porous electrode. In some embodiments, the polymer resin of the first transport protection is an ionic resin, the ionic resin may be as previously described with respect to the ionic resin of the polymer of the porous electrode material. The transport protection layer may include at least one of include particles, flakes, fibers, dendrites and the like.

Electrically-conductive particulate of the transport protection layers may include metals, metalized dielectrics, e.g., metallized polymer fibers or metalize glass particulates, conductive polymers and carbon, including but not limited to, glass like carbon, amorphous carbon, graphene, graphite, carbon nanotubes and carbon dendrites, e.g., branched carbon nanotubes, for example, carbon nanotrees. In some embodiments, the transport protection layer is free of metal particulate.

In some embodiments, the electrically-conductive particulate of the transport protection layer may be surface treated to enhance the wettability of the transport protection layer to a given anolyte or catholyte or to provide or enhance the electrochemical activity of the transport protection layer relative to the oxidation-reduction reactions associated with the chemical composition of a given anolyte or catholyte. Surface treatments include, but are not limited to, at least one of chemical treatments, thermal treatments and plasma treatments. In some embodiments, the electrically-conductive particulate of the transport protection layer is hydrophilic.

Non-electrically conductive particulate of the transport protection layer may include, for example, non-electrically conductive inorganic particulate and non-electrically conductive polymeric particulate.

In some embodiments, the non-electrically conductive particulate of the transport protection layer comprises a non-electrically conductive inorganic particulate. Non-electrically conductive inorganic particulates include, for example, minerals and clays known in the art. In some embodiments the non- electrically conductive inorganic particulate includes at least one of silica, alumina, titania, ceria, yttria, and zirconia. In some embodiments, the non-electrically conductive particulate may be ionically conductive, e.g., a polymeric ionomer. In some embodiments, the non-electrically conductive particulate comprises a non-electrically conductive polymeric particulate. In some embodiments, the non-electrically conductive polymeric particulate is a non-ionic polymer, i.e., a polymer free of repeat units having ionic functional groups. Non-electrically conductive polymers include, for example, polysulfones, hydrocarbon polyolefins (e.g., polyethylene and polypropylene), and fluorinated polymers, e.g., polyvinylidene fluoride and polytetrafluoroethylene. In some embodiments, the non-electrically conductive particulate is substantially free of a non-electrically conductive polymeric particulate. By substantially free it is meant that the non-electrically conductive particulate contains, by weight, between 0 percent and 5 percent, between 0 percent and 3 percent, between 0 percent and 2 percent, between 0 percent and 1 percent, or even between 0 percent and 0.5 percent of a non-electrically conductive polymeric particulate.

In some embodiments of the present disclosure, the redox flow battery may be a redox flow battery, for example, a vanadium redox flow battery (VRFB), wherein a V / V sulfate solution serves as the negative electrolyte ("anolyte") and a V ³⁺ /V ⁴⁺ sulfate solution serves as the positive electrolyte ("catholyte"). It is to be understood, however, that other redox chemistries are contemplated and within the scope of the present disclosure, including, but not limited to, V /V vs. Br / Br ₂ , Br ₂ /Br vs.

S/S ₂ , Br /Br ₂ vs. Zn ²⁺ /Zn, Ce ⁴⁺ /Ce ³⁺ vs. V ²⁺ /V ³⁺ , Fe ³⁺ /Fe ²⁺ vs. Br ₂ /Br , Mn ²⁺ /Mn ³⁺ vs.

Br ₂ /Br ^_ , Fe ³⁺ /Fe ²⁺ vs. Ti ²⁺ /Ti ⁴⁺ and Cr ³⁺ /Cr ²⁺ , acidic/basic chemistries. Other chemistries useful in liquid flow batteries include coordination chemistries, for example, those disclosed in U.S. Pat. Appl.

Nos. 2014/028260, 2014/0099569, and 2014/0193687 and organic complexes, for example, U.S. Pat.

Publ. No. 2014/370403 and PCT Publ. No. WO 2014/052682, all of which are incorporated herein by reference in their entirety.

Methods of making membrane-electrode assemblies include laminating the exposed surface of a membrane, e.g., and ion exchange membrane, to a first major surface of a porous electrode according to any one of the porous electrode embodiments of the present disclosure. This may be conducted by hand or under heat and/or pressure using conventional lamination equipment. Additionally, the membrane- electrode assembly may be formed during the fabrication of an electrochemical cell or battery. The components of the cell may be layered on top of one another in the desired order, for example, a first porous electrode, membrane, i.e., an ion exchange membrane, and a second porous electrode. The components are then assembled between, for example, the end plates of a single cell or bipolar plates of a stack having multiple cells, along with any other required gasket/sealing material. The plates, with membrane assembly there between, are then coupled together, usually by a mechanical means, e.g., bolts, clamps or the like, the plates providing a means for holding the membrane assembly together and in position within the cell. In another embodiment, the present disclosure provides an electrochemical cell including at least one porous electrode according to any one of the porous electrodes of the present disclosure. In yet another embodiment, the present disclosure provides an electrochemical cell including a membrane- electrode assembly according to any one of the membrane -electrode assemblies of the present disclosure. In another embodiment, the present disclosure provides an electrochemical cell including at least one electrode assembly according to any one of the electrode assemblies of the present disclosure.

FIG. 4 shows exemplary electrochemical cell 400, which includes membrane-electrode assembly 300, end plates 450 and 450' having fluid inlet ports, 45 la and 45 la', respectively, and fluid outlet ports, 45 lb and 45 lb', respectively, flow channels 455 and 455', respectively and first surface 450a and 452a respectively. Electrochemical cell 400 also includes current collectors 460 and 462. Membrane-electrode assembly 300 is as described in FIG. 3, without optional release liners 330 and 332. Electrochemical cell 400 includes electrodes 440 and 442, optional transport protection layers 470 and 472, and ion exchange membrane 420, all as previously described. End plates 450 and 450' are in electrical communication with electrodes 440 and 442, through surfaces 450a and 452a, respectively. Electrodes 440 or 442 may be replaced with at least one membrane -electrode assembly 300 arranged in alternating fashion (e.g., electrode-membrane-electrode- membrane, etc.) according to any one of the membrane -electrode assemblies of the present disclosure, producing an electrochemical cell which includes multiple electrode assemblies. Support plates, not shown, may be placed adjacent to the exterior surfaces of current collectors 460 and 462. The support plates are electrically isolated from the current collector and provide mechanical strength and support to facilitate compression of the cell assembly. End plates 450 and 450' include fluid inlet and outlet ports and flow channels that allow anolyte and catholyte solutions to be circulated through the electrochemical cell. Assuming the anolyte is flowing through plate 450 and the catholyte is flowing through plate 450', the flow channels 455 allow the anolyte to contact and flow into porous electrode 440, facilitating the oxidation-reduction reactions of the cell. Similarly, for the catholyte, the flow channels 455' allow the catholyte to contact and flow into porous electrode 442, facilitating the oxidation-reduction reactions of the cell. The current collectors may be electrically connected to an external circuit.

FIG. 5 shows an exemplary electrochemical cell stack 500 including membrane -electrode assembly 300 (as previously described), for example, separated by bipolar plates 550" and end plates 550 and 550' having flow channels 555 and 555'. Bipolar plates 550" allow anolyte to flow through one set of channels, 555 and catholyte to flow through a second set of channels, 555', for example. Cell stack 500 includes multiple electrochemical cells, each cell represented by a membrane-electrode assembly and the corresponding adjacent bipolar plates and/or end plates. Support plates, not shown, may be placed adjacent to the exterior surfaces of current collectors 560 and 562. The support plates are electrically isolated from the current collector and provide mechanical strength and support to facilitate compression of the cell assembly. The anolyte and catholyte inlet and outlet ports and corresponding fluid distribution system is not show. These features may be provided as known in the art. The porous electrodes of the present disclosure may be used to fabricate a redox flow battery (e.g., a redox flow battery). In some embodiments, the present disclosure provides a redox flow battery that include at least one porous electrode according to any one of the porous electrode embodiments of the present disclosure. The number of porous electrode of the redox flow battery, which may correlate to the number of cells in a stack, is not particularly limited. In some embodiments, the redox flow battery includes at least 1, at least 2, at least 5, at least 10 or even at least 20 porous electrodes. In some embodiments the number of porous electrodes of the redox flow battery ranges from 1 to 500, 2 to 500, from 5 to 500, from 10 to 500 or even from 20 to 500. In another embodiment, the present disclosure provides a redox flow battery including at least one membrane-electrode assembly according to any one of the membrane-electrode assembly embodiments of the present disclosure. The number of membrane- electrode assemblies of the redox flow battery, which may correlate to the number of cells in a stack, is not particularly limited. In some embodiments, the redox flow battery includes at least 1, at least 2, at least 5, at least 10 or even at least 20 membrane-electrode assemblies. In some embodiments the number of membrane -electrode assemblies of the redox flow battery ranges from 1 to 500, 2 to 500, from 5 to 500, from 10 to 200 or even from 20 to 500.

FIG. 6 shows an exemplary single-cell, redox flow battery 600 including membrane-electrode assembly 300, which includes ion exchange membrane 320, transport protection layers 370 and 372, and porous electrodes 100, and end plates 650 and 650', current collectors 660 and 662, anolyte reservoir 680 and anolyte fluid distribution 680', and catholyte reservoir 682 and catholyte fluid distribution system 682'. Pumps for the fluid distribution system are not shown. Porous electrodes 640 and 642 may be replaced with an electrode assembly 300, producing redox flow battery which includes an electrode assembly of the present disclosure. If an electrode assembly is used, the transport protection layer of the electrode assembly is adjacent, proximate or in contact with the ion exchange membrane 620. Current collectors 660 and 662 may be connected to an external circuit which includes an electrical load (not shown). Although a single cell redox flow battery is shown, it is known in the art that liquid flow batteries may contain multiple electrochemical cells, i.e., a cell stack. Further multiple cell stacks may be used to form a redox flow battery, e. g. multiple cell stacks connected in series. The porous electrodes, the ion exchange membranes, and their corresponding membrane-electrode assemblies of the present disclosure may be used to fabricate liquid flow batteries having multiple cells, for example, multiple cell stack of FIG. 5. Flow fields may be present, but this is not a requirement. SELECT EMBODIMENTS OF THE PRESENT DISCLOSURE

In a first embodiment, the present disclosure provides a porous electrode having first and second opposed major surfaces and comprising, based on the total weight on the porous electrode:

60 to 92.5 percent by weight of graphitic carbon fibers having an average fiber diameter of 5 to 10 microns and an average length of 6 to 25 mm;

5 to 25 percent by weight of electrically -nonconductive reinforcing fibers having an average fiber diameter of 5 to 25 microns and an average length of 3 to 50 mm, wherein the electrically-nonconductive reinforcing fibers are ceramic, glass, glass-ceramic, or a combination thereof;

2.5 to 15 percent by weight of polymer fibers selected from hydrocarbon polyolefin fibers, perfhiorinated polymer fibers, partially fluorinated polymer fibers, chlorinated polyolefin fibers, polyvinyl chloride fibers, polysulfone fibers, polyaryletherketone fibers, and combinations thereof,

wherein at least some of the polymer fibers proximal to the first and second major surfaces are melt-bonded to each other.

In a second embodiment, the present disclosure provides a porous electrode according to the first embodiment, wherein the graphitic carbon fibers have a single -filament tensile modulus of 350 to 900 GPa.

In a third embodiment, the present disclosure provides a porous electrode according to the first or second embodiment, wherein the graphitic carbon fibers have a single-filament tensile strength of 2.5 to 5 GPa.

In a fourth embodiment, the present disclosure provides a porous electrode according to any one of the first to third embodiments, wherein the graphitic carbon fibers have an average length of 6 to 12 mm .

In a fifth embodiment, the present disclosure provides a porous electrode according to any one of the first to fourth embodiments, wherein at least some of the polymer fibers are microfibrillated.

In a sixth embodiment, the present disclosure provides a porous electrode according to any one of the first to fifth embodiments, wherein the polymer fibers are present in an amount of 2.5 to 15 percent by weight.

In a seventh embodiment, the present disclosure provides a porous electrode according to any one of the first to sixth embodiments, wherein the polymer fibers are present in an amount of 5 to 7.5 percent by weight.

In an eighth embodiment, the present disclosure provides a porous electrode according to any one of the first to seventh embodiments, wherein the electrically-nonconductive reinforcing fibers are present in an amount of 10 to 20 percent by weight.

In a ninth embodiment, the present disclosure provides a porous electrode according to any one of the first to eighth embodiments, wherein the electrically-nonconductive reinforcing fibers have an average length of 6 to 25 mm. In a tenth embodiment, the present disclosure provides a porous electrode according to any one of the first to eighth embodiments, wherein the electrically-nonconductive reinforcing fibers have an average length of 12 to 25 mm.

In an eleventh embodiment, the present disclosure provides a porous electrode according to any one of the first to tenth embodiments, wherein the electrically-nonconductive reinforcing fibers have an average fiber diameter of 8 to 12 microns.

In a twelfth embodiment, the present disclosure provides a porous electrode according to any one of the first to eleventh embodiments, wherein the electrically-nonconductive reinforcing fibers have a single-filament tensile modulus of 50 to 400 GPa.

In a thirteenth embodiment, the present disclosure provides a porous electrode according to any one of the first to twelfth embodiments, wherein the electrically-nonconductive reinforcing fibers have a single-filament tensile strength of 1.5 to 4 GPa.

In a fourteenth embodiment, the present disclosure provides a porous electrode according to any one of the first to twelfth embodiments, wherein the electrically-nonconductive reinforcing fibers have a single-filament tensile strength of 3 to 4 GPa.

In a fifteenth embodiment, the present disclosure provides a porous electrode according to any one of the first to fourteenth embodiments, wherein the porous electrode has a through-plane conductivity lower than 0.4 ohm-cm at an applied pressure of 0.6 MPa.

In a sixteenth embodiment, the present disclosure provides a porous electrode according to any one of the first to fourteenth embodiments, wherein the porous electrode has a through-plane conductivity lower than 0.35 ohm-cnr· at an applied pressure of 0.6 MPa.

In a seventeenth embodiment, the present disclosure provides a porous electrode according to any one of the first to sixteenth embodiments, wherein the porous electrode has a basis weight of 30 to 300 g/m ² .

In an eighteenth embodiment, the present disclosure provides a membrane-electrode assembly for a redox flow battery comprising: a first porous electrode according to any one of first to seventeenth embodiments;

a second porous electrode according to any one of first to seventeenth embodiments; and an ion exchange membrane disposed between the first and second porous electrodes.

In a nineteenth embodiment, the present disclosure provides a membrane-electrode assembly for a redox flow battery according to the eighteenth embodiment, further comprising:

a first transport protection layer disposed between the ion exchange membrane and the first porous electrode; and a second transport protection layer disposed between the ion exchange membrane and the second porous electrode, wherein the first and second transport protection layers each comprise a polymer resin and graphitic carbon fibers.

In a twentieth embodiment, the present disclosure provides a redox flow battery comprising at least one porous electrode according to any one of the first to seventeenth embodiments.

In a twenty-first embodiment, the present disclosure provides a method of making a porous electrode, the method comprising:

providing a composition comprising, based on the total weight on the porous electrode:

60 to 92.5 parts by weight of graphitic carbon fibers having an average fiber diameter of 5 to 10 microns and an average length of 6 to 12 mm;

5 to 25 parts by weight of electrically-nonconductive reinforcing fibers having an average fiber diameter of 5 to 25 microns and an average length of 3 to 50 mm, wherein the electrically- nonconductive reinforcing fibers are ceramic, glass, glass-ceramic, or a combination thereof;

2.5 to 15 parts by weight of polymer fibers selected from hydrocarbon polyolefin fibers, perfhiorinated polymer fibers, partially fluorinated polymer fibers, chlorinated polyethylene fibers, polyvinyl chloride fibers, polysulfone fibers, polyaryletherketone fibers, and combinations thereof; and

a liquid vehicle;

depositing the composition on a porous substrate;

removing at least a portion of the liquid vehicle through the porous substrate to make a nonwoven fibrous web; and

melt-bonding at least some of the polymer fibers proximal to the first and second major surfaces to each other the polymer fibers and consolidating the nonwoven fibrous web to make the porous electrode.

In a twenty-second embodiment, the present disclosure provides a method according to the twenty-first embodiment, wherein at least some of the polymer fibers are microfibrillated.

In a twenty-third embodiment, the present disclosure provides a method according to the twenty- first or twenty-second embodiment, wherein prior to said softening the polymer fibers, separating at least a portion of the nonwoven fibrous web from the porous substrate.

Objects and advantages of this disclosure are further illustrated by the following non-limiting 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 disclosure.

EXAMPLES

Unless otherwise noted, all parts, percentages, ratios, etc. in the Examples and the rest of the specification are by weight. COMPARATIVE EXAMPLE A

A roll of Mitsubishi Chemical Carbon Fiber and Composites (Tokyo, Japan) U105 carbon paper was heated in air in a furnace at 400 °C for 48 h. After the roll of U 105 cooled and was removed from the furnace, this material was cut into pieces needed for the various tests described below.

COMPARATIVE EXAMPLE B

A batch of GRANOC CN-90C-6Z chopped graphitic fiber (diameter = 10 microns, length = 6 mm, Nippon Graphite Fiber Corporation, Tokyo, Japan) was calcined under static air at 575 °C for 12 hours. The Mitsui Fybrel E 620 fibrillated polyolefin fiber (Mitsui Chemicals, Inc., Tokyo, Japan) was used as received.

A 4-liter Waring heavy-duty laboratory blender (Waring Products, Torrington, Connecticut) was filled with tap water. To this solution, 0.47 g of Mitsui Fybrel E 620 polyolefin synthetic pulp highly fibrillated fiber was added, and it was blended for 120 seconds on the "High" setting. After that step, 8.84 g of the GRANOC CN-90C-6Z carbon fiber was added to the slurry, and the slurry was blended for 15 seconds on the "Low" setting. This slurry was then poured into a Williams Standard Pulp Testing Apparatus (Williams Apparatus Co., Watertown, New York) that already contained deionized water, so that the total mass of water was 22.01 kg. The slurry was briefly agitated, the water was drained, and the solids were collected on a stainless steel screen. The wet paper was transferred onto a polymer scrim, excess water was removed, and it was dried for two hours at 80 °C.

Dry felts comprising graphitic fibers, and polymer fibers were consolidated by briefly exposing them to heat to soften polymer fibers. Consolidation was achieved by passing the felt between heated nip rolls, maintained at a fixed gap, of a calendering machine. The diameters of the bottom and top stainless steel roll were 13 inches (33 cm) and 8 inches (20 cm), respectively. Both rotating rolls were heated at a temperature of 135 °C and had an l8-mil (0.46-mm) gap maintained between them. Dry felts were placed between two 3 -mil (76-micron) thick Kapton films and manually hand fed through the gap between the nip rolls at a rate of 3 ft/min (0.9 m/min). The consolidation process provided enough heat to soften the polymer fibers without melting them to cover the graphitic fibers that can lead to increase the resistance.

After calendering, each side of the integrated electrode composite sheet was plasma treated following the protocol as disclosed in Kim et al. "The effects of modification on carbon felt electrodes for use in vanadium redox flow batteries" Mater. Chem. Phys. (2011), 131, 547-553.

COMPARATIVE EXAMPLE C

GRANOC CN-90C-6Z chopped graphitic fiber was calcined under static air at 575 °C for 12 hours. Mitsui Fybrel E 620 was used as received. Nextel 610 (3M Company, Saint Paul, Minnesota) was milled to a nominal length of 1 mm by Engineered Fibers Technology, Shelton, Connecticut. Prior to milling, the sizing on the Nextel 610 was removed via a heat treatment under static air for 1 hour at 750 °C.

A 4-liter Waring heavy-duty laboratory blender was filled with tap water. To this solution, 0.47 g of Mitsui Fybrel E 620 was added, and it was blended for 120 seconds on the "High" setting. After that step, 6.98 g of the GRANOC CN-90C-6Z carbon fiber and 1.86 g of the 1 mm Nextel 610 was added to the slurry, and the slurry was blended for 15 seconds on the "Low" setting. This slurry was then poured into a Williams Standard Pulp Testing Apparatus that already contained deionized water, so that the total mass of water was 22.01 kg. The slurry was briefly agitated, the water was drained, and the solids were collected on a stainless steel screen. The wet paper was transferred onto a polymer scrim, excess water was removed, and it was dried for two hours at 80 °C.

Dry felts comprising graphitic fibers, inorganic fibers, and polymer fibers were consolidated by briefly exposing them to heat to soften polymer fibers. Consolidation was achieved by passing the felt between heated nip rolls, maintained at a fixed gap, of a calendering machine. The diameters of the bottom and top stainless steel roll were 13 inches (33 cm) and 8 inches (20 cm), respectively. Both rotating rolls were heated at a temperature of 135 °C and had an l8-mil (0.46-mm) gap maintained between them. Dry felts were placed between two 3 -mil (76-micron) thick Kapton films and manually hand fed through the gap between the nip rolls at a rate of 3 ft/min (0.9 m/min). The consolidation process provided enough heat to soften the polymer fibers without melting them to cover the graphitic fibers that can lead to increase the resistance.

After calendering, each side of the integrated electrode composite sheet was plasma treated following the protocol as disclosed in Kim et al. "The effects of modification on carbon felt electrodes for use in vanadium redox flow batteries" Mater. Chem. Phys. (2011), 131, 547-553.

COMPARATIVE EXAMPLE D

A batch of GRANOC CN-90C-6Z chopped graphitic fiber was calcined under static air at 575 °C for 12 hours. A batch of Nextel 720 (3M Company, Saint Paul, Minnesota) chopped to 6 mm was calcined under static air at 750 °C for 1 hour. Mitsui Fybrel E 620 synthetic pulp was used as received.

A 4-liter Waring heavy-duty laboratory blender was filled with tap water. To this solution, 0.47 g of Mitsui Fybrel E 620 was added, and it was blended for 120 seconds on the“High” setting. After that step, 6.05 g of the GRANOC CN-90C-6Z carbon fiber and 2.79 g of the Nextel 710 was added to the slurry, and the slurry was blended for 15 seconds on the“Low” setting. This slurry was then poured into a Williams Standard Pulp Testing Apparatus that already contained deionized water, so that the total mass of water was 22.01 kg. The slurry was briefly agitated, the water was drained, and the solids were collected on a stainless steel screen. The wet paper was transferred onto a polymer scrim, excess water was removed, and it was dried for two hours at 80 °C.

Dry felts comprising graphitic fibers, inorganic fibers, and polymer fibers were consolidated by briefly exposing them to heat to soften polymer fibers. Consolidation was achieved by passing the felt between heated nip rolls, maintained at a fixed gap, of a calendering machine. The diameters of the bottom and top stainless steel roll were 13 inches (33 cm) and 8 inches (20 cm), respectively. Both rotating rolls were heated at a temperature of 135 °C and had an l8-mil (0.46-mm) gap maintained between them. Dry felts were placed between two 3 -mil (76-micron) thick Kapton films and manually hand fed through the gap between the nip rolls at a rate of 3 ft/min (0.9 m/min). The consolidation process provided enough heat to soften the polymer fibers without melting them to cover the graphitic fibers that can lead to increase the resistance.

After calendering, each side of the integrated electrode composite sheet was plasma treated following the protocol as disclosed in Kim et al. "The effects of modification on carbon felt electrodes for use in vanadium redox flow batteries" Mater. Chem. Phys. (2011), 131, 547-553.

COMPARATIVE EXAMPLE E

A batch of GRANOC CN-90C-6Z chopped graphitic fiber was calcined under static air at 575 °C for 12 hours. A batch of Nextel 720 (3M Company, Saint Paul, Minnesota) chopped to 6 mm was calcined under static air at 750 °C for 1 hour. Mitsui Fybrel E 620 synthetic pulp was used as received.

A 4-liter Waring heavy-duty laboratory blender was filled with tap water. To this solution, 0.47 g of Mitsui Fybrel E 620 was added, and it was blended for 120 seconds on the“High” setting. After that step,, 8.38 g of the GRANOC CN-90C-6Z carbon fiber and 0.466 g of the Nextel 710 was added to the slurry , and the slurry was blended for 15 seconds on the“Low” setting. This slurry was then poured into a Williams Standard Pulp Testing Apparatus that already contained deionized water, so that the total mass of water was 22.01 kg. The slurry was briefly agitated, the water was drained, and the solids were collected on a stainless steel screen. The wet paper was transferred onto a polymer scrim, excess water was removed, and it was dried for two hours at 80 °C.

Dry felts comprising graphitic fibers, inorganic fibers, and polymer fibers were consolidated by briefly exposing them to heat to soften polymer fibers. Consolidation was achieved by passing the felt between heated nip rolls, maintained at a fixed gap, of a calendering machine. The diameters of the bottom and top stainless steel roll were 13 inches (33 cm) and 8 inches (20 cm), respectively. Both rotating rolls were heated at a temperature of 135 °C and had an l8-mil (0.46-mm) gap maintained between them. Dry felts were placed between two 3 -mil (76-micron) thick Kapton films and manually hand fed through the gap between the nip rolls at a rate of 3 ft/min (0.9 m/min). The consolidation process provided enough heat to soften the polymer fibers without melting them to cover the graphitic fibers that can lead to increase the resistance.

After calendering, each side of the integrated electrode composite sheet was plasma treated following the protocol as disclosed in Kim et al. "The effects of modification on carbon felt electrodes for use in vanadium redox flow batteries" Mater. Chem. Phys. (2011), 131, 547-553. EXAMPLE 1

GRANOC CN-90C-6Z chopped graphitic fiber was calcined under static air at 575 °C for 12 hours. A batch of Advantex EC-R glass fibers (Owens-Coming, Toledo, Ohio) was chopped to 0.5 inch (2.77 cm) length, and then calcined under static air at 575 °C for 12 hours. Mitsui Fybrel E 620 synthetic pulp was used as received.

A 4-liter Waring heavy-duty laboratory blender was filled with tap water. To this solution, 0.47 g of Mitsui Fybrel E 620 was added, and it was blended for 120 seconds on the "High" setting. After that step, 7.91 g of GRANOC CN-90C-6Z carbon fiber and 0.93 g of the chopped Advantex EC-R glass fibers was added to the slurry, and the slurry was blended for 15 seconds on the "Low" setting. This slurry was then poured into a Williams Standard Pulp Testing Apparatus (Williams Apparatus Co., Watertown, New York) that already contained deionized water, so that the total mass of water was 22.01 kg. The slurry was briefly agitated, the water was drained, and the solids were collected on a stainless steel screen with dimensions of 0.291 meters by 0.291 meters. The wet paper was transferred onto a polymer scrim, excess water was removed, and it was dried for two hours at 80 °C.

Dry felts comprising graphitic fibers, inorganic fibers, and polymer fibers were consolidated by briefly exposing them to heat to soften polymer fibers. Consolidation was achieved by passing the felt between heated nip rolls, maintained at a fixed gap, of a calendering machine. The diameters of the bottom and top stainless steel roll were 13 inches (33 cm) and 8 inches (20 cm), respectively. Both rotating rolls were heated at a temperature of 135 °C and had an l8-mil (0.46-mm) gap maintained between them. Dry felts were placed between two 3 -mil (76-micron) thick Kapton films and manually hand fed through the gap between the nip rolls at a rate of 3 ft/min (0.9 m/min). The consolidation process provided enough heat to soften the polymer fibers without melting them to cover the graphitic fibers that can lead to increase the resistance.

After calendering, each side of the integrated electrode composite sheet was plasma treated following the protocol as disclosed in Kim et al. "The effects of modification on carbon felt electrodes for use in vanadium redox flow batteries" Mater. Chem. Phys. (2011), 131, 547-553.

EXAMPLE 2

A batch of GRANOC CN-90C-6Z chopped graphitic fiber was calcined under static air at 575 °C for 12 hours. A batch of Advantex E-CR glass fibers was chopped to 0.5 inch (1.27 cm) and then calcined under static air at 575 °C for 12 hours. The Mitsui Fybrel E 620 synthetic pulp was used as received.

A 4-liter Waring heavy-duty laboratory blender was filled with tap water. To this solution, 0.47 g of Mitsui Fybrel E 620 was added, and it was blended for 120 seconds on the "High" setting. After that step, 6.98 g of the GRANOC CN-90C-6Z carbon fiber and 1.86 g of the chopped Advantex glass fiber was added to the slurry, and the slurry was blended for 15 seconds on the "Low" setting. This slurry was then poured into a Williams Standard Pulp Testing Apparatus that already contained deionized water, so that the total mass of water was 22.01 kg. The slurry was briefly agitated, the water was drained, and the solids were collected on a stainless steel screen with dimensions of 0.291 meters by 0.291 meters. The wet paper was transferred onto a polymer scrim, excess water was removed, and it was dried for two hours at 80 °C.

Dry felts comprising graphitic fibers, inorganic fibers, and polymer fibers were consolidated by briefly exposing them to heat to soften polymer fibers. Consolidation was achieved by passing the felt between heated nip rolls, maintained at a fixed gap, of a calendering machine. The diameters of the bottom and top stainless steel roll were 13 inches (33 cm) and 8 inches (20 cm), respectively. Both rotating rolls were heated at a temperature of 135 °C and had an l8-mil (0.46-mm) gap maintained between them. Dry felts were placed between two 3 -mil (76-micron) thick Kapton films and manually hand fed through the gap between the nip rolls at a rate of 3 ft/min (0.9 m/min). The consolidation process provided enough heat to soften the polymer fibers without melting them to cover the graphitic fibers that can lead to increase the resistance.

After calendering, each side of the integrated electrode composite sheet was plasma treated following the protocol as disclosed in Kim et al. "The effects of modification on carbon felt electrodes for use in vanadium redox flow batteries" Mater. Chem. Phys. (2011), 131, 547-553.

EXAMPLE 3

A batch of GRANOC CN-90C-6Z chopped graphitic fiber was calcined under static air at 575 °C for 12 hours. A batch of Advantex EC-R glass fibers was chopped to 1 inch (2.54 cm), and then calcined under static air at 575 °C for 12 hours. The Mitsui Fybrel E 620 synthetic pulp was used as received.

A 4-liter Waring heavy-duty laboratory blender was filled with tap water. To this solution, 0.47 g of Mitsui Fybrel E 620 was added, and it was blended for 120 seconds on the "High" setting. After that step, 6.98 g of the GRANOC CN-90C-6Z carbon fiber and 1.86 g of the chopped Advantex glass fiber was added to the slurry, and the slurry was blended for 15 seconds on the "Low" setting. This slurry was then poured into a Williams Standard Pulp Testing Apparatus that already contained deionized water, so that the total mass of water was 22.01 kg. The slurry was briefly agitated, the water was drained, and the solids were collected on a stainless steel screen with dimensions of 0.291 meters by 0.291 meters. The wet paper was transferred onto a polymer scrim, excess water was removed, and it was dried for two hours at 80 °C.

Dry felts comprising graphitic fibers, inorganic fibers, and polymer fibers were consolidated by briefly exposing them to heat to soften polymer fibers. Consolidation was achieved by passing the felt between heated nip rolls, maintained at a fixed gap, of a calendering machine. The diameters of the bottom and top stainless steel roll were 13 inches (33 cm) and 8 inches (20 cm), respectively. Both rotating rolls were heated at a temperature of 135 °C and had an l8-mil (0.46-mm) gap maintained between them. Dry felts were placed between two 3 -mil (76-micron) thick Kapton films and manually hand fed through the gap between the nip rolls at a rate of 3 ft/min (0.9 m/min). The consolidation process provided enough heat to soften the polymer fibers without melting them to cover the graphitic fibers that can lead to increase the resistance.

After calendering, each side of the integrated electrode composite sheet was plasma treated following the protocol as disclosed in Kim et al. "The effects of modification on carbon felt electrodes for use in vanadium redox flow batteries" Mater. Chem. Phys. (2011), 131, 547-553.

EXAMPLE 4

A batch of GRANOC CN-90C-6Z chopped graphitic fiber was calcined under static air at 575 °C for 12 hours. A batch of Nextel 720 chopped to 6 mm was calcined under static air at 750 °C for 1 hour. Mitsui Fybrel E 620 synthetic pulp was used as received.

A 4-liter Waring heavy-duty laboratory blender was filled with tap water. To this solution, 0.47 g of Mitsui Fybrel E 620 was added, and it was blended for 120 seconds on the "High" setting. After that step, 6.98 g of the GRANOC CN-90C-6Z carbon fiber and 1.86 g of the Nextel fiber was added to the slurry, and the slurry was blended for 15 seconds on the "Low" setting. This slurry was then poured into a Williams Standard Pulp Testing Apparatus that already contained deionized water, so that the total mass of water was 22.01 kg. The slurry was briefly agitated, the water was drained, and the solids were collected on a stainless steel screen with dimensions of 0.291 meters by 0.291 meters. The wet paper was transferred onto a polymer scrim, excess water was removed, and it was dried for two hours at 80 °C.

Dry felts comprising graphitic fibers, inorganic fibers, and polymer fibers were consolidated by briefly exposing them to heat to soften polymer fibers. Consolidation was achieved by passing the felt between heated nip rolls, maintained at a fixed gap, of a calendering machine. The diameters of the bottom and top stainless steel roll were 13 inches (33 cm) and 8 inches (20 cm), respectively. Both rotating rolls were heated at a temperature of 135 °C and had an l8-mil (0.46-mm) gap maintained between them. Dry felts were placed between two 3 -mil (76-micron) thick Kapton films and manually hand fed through the gap between the nip rolls at a rate of 3 ft/min (0.9 m/min). The consolidation process provided enough heat to soften the polymer fibers without melting them to cover the graphitic fibers that can lead to increase the resistance.

After calendering, each side of the integrated electrode composite sheet was plasma treated following the protocol as disclosed in Kim et al. "The effects of modification on carbon felt electrodes for use in vanadium redox flow batteries" Mater. Chem. Phys. (2011), 131, 547-553.

Table 1, below, summarizes the composition and basis weight of the above examples. TABLE 1

CONDUCTIVITY TESTING

A custom through-plane conductivity tester was made using an Instron 5965 load frame and a TDK-Lambda ZUP10-40 power supply (TDK-Lambda Americas, National City, California). For this type of testing, 25 cm squares were die cut out of the aforementioned examples. These electrodes were loaded into a gold-plated holder. A current was passed through the electrodes at a set of pressures less than or equal to 1 MPa. After compressing to 1 MPa, the pressure was reduced back to zero. The through-plane resistivity was determined via the measured voltage drop using a Fluke 8845A multimeter (Fluke Corp. Everett, Washington) across the contacts and compressed electrodes for the various currents applied at a given pressure. The thickness was determined using a linear variable differential transformer. All electrodes were tested as a single layer, except for Comparative Example A, which was tested as two layers.

FIG. 7 shows the relationship between the applied pressure and the through-plane resistance of the various porous electrodes of Examples 1-4 and Comparative Examples A-C.

FIG. 8 shows the relationship between the applied pressure and thickness of the porous electrodes of Examples 1-4 and Comparative Examples A-C.

DETERMINATION OF CHANNEL COLLAPSE AND INTRUSION

A flow cell and two bipolar plates was machined so that it was possible to view a cross section of the flow field; this is known as a cutaway cell. FIG. 9A shows a diagram of the configuration of a cutaway cell used to analyze channel intrusion and materials compressed within the cell. This cell contains eight channels 0.5 mm in width and six channels 1 mm in width. FIG. 9B shows a micrograph of four pieces of Comparative Example 1 (two on each side) and an ion-exchange membrane compressed to 14 mils for 30 minutes. Little intrusion into the flow channels occurs for this material.

For all examples, the cutaway cell was assembled by placing 14 mils of Teflon gaskets, electrodes on each side of a cation exchange membrane, and tightening the bolts of the cell using a torque wrench. However, for Comparative Example 1, two electrodes were stacked on top of each other on each side of the cell (four pieces of porous membrane total), while only one electrode was placed on each side of the cell for the other examples. The bolts of cell were then tightened, and the electrode was maintained in a compressed state for 30 minutes. After 30 minutes, the cell was brought to a Keyence VHX microscope (Keyence Corp., Osaka, Japan) and imaged using a lOOx objective lens. Images were taken across the flow field and these were subsequently analyzed in using the program ImageJ. Distance measurements were taken for one side of the cell from the point of maximum penetration of the electrode to the line parallel with the "land" in-between the flow channels. These measurements can be used to determine how prone the electrode is toward collapse into the flow channels. Averages and standard deviations for both the 1 mm and 0.5 mm channels were calculated.

Table 2 reports Average Flow Channel Penetration Depth of the porous electrodes of Examples 1- 4 and Comparative Examples A-C. In Table 2, below, ± indicates one standard deviation.

TABLE 2

In contrast, FIG. 10 is an image of two Comparative Example 2 electrodes and a membrane after compression at 14 mils (360 microns) for 30 minutes. Considerable intrusion into the flow channels is evident in FIG. 10.

CATHOLYTE DOUBLE HALF-CELL TEST

In-house machined flow cells with serpentine channels were used for double half-cell testing. A diagram of the flow cell is shown in FIG. 11 and includes working electrode 1132 having a gasket 1138, a counter electrode 1142 having a gasket 1148, and a membrane 1152 separating the electrode. The unloaded cell contains several other parts. First, there are two graphite/polymer composite bipolar plates 1136 and 1146 that contain machined flow channels that allow entry and exit of electrolyte through two machined through holes in the bipolar end plates. Second, there are two gold-plated copper current collectors 1134 and 1144 that are placed in contact with the bipolar plates 1136 and 1146. The current collectors allow for attachment of the leads of the potentiostat 1260 (see FIG. 12) to apply desired potential. Third, a polymeric, electrically-insulating separator 1140 and 1150 is placed on the outer surface of the current collector. Finally, there are two aluminum end plates 1130 and 1131 with eight aligned machined holes in each.

Porous electrode samples were evaluated using a Catholyte Double Half-cell Test following standard test protocol (see e.g., Darling, R. M.; Perry, M. L. J. Electrochem. Soc. 2014, 161, A1381). In this test, a steel die was used to hand cut electrodes 5 cm ² in area. These composite electrodes were weighed, their thicknesses were measured, and they were loaded into a flow cell. To assemble the cell, a composite electrode was placed over the serpentine flow field (750 pm wide lands and channels) inscribed into graphite/polymer bipolar plate. One electrode was placed on top of the flow field. A 14 mils of Teflon gaskets (2 gaskets of 2 mils in thickness and 2 gaskets of 5 mils in thickness) with a 5 cm ² opening aligned to the electrode was then placed around the electrode. A 3M perfluorosulfonic acid (PFSA) membrane with an equivalent weight of 825 and a thickness of 50 micrometers was used as a separator for the two sides. In reverse order, the same configuration of gaskets, and electrodes were stacked on the other side. This cell is symmetric about the membrane separator. The cell was then compressed by tightening eight bolts that were threaded into the aluminum endplates using a torque of 110 in-lb. For the test of Comparative Example 1, two electrodes were used on each side of the membrane.

A 50% state of charge (SOC) electrolyte catholyte solution was made for this test by flowing a 50:50 mix of vanadium (III) and vanadium (IV) sulfate in sulfuric acid (received from Riverside Specialty Chemical with a vanadium concentration of 1.5 M and a sulfuric acid contraction of 2.6 M) through a carbon paper-loaded flow cell with a 50 micrometers PFSA membrane. The electrolyte was charged to 1.41 V in this separate charging cell. The solution from the working side of the cell, the catholyte, was recovered after charging.

During half-cell test, the 50% SOC catholyte was placed in a glass reservoir and fed into each side of the flow cell using a dual -headed corrosion resistant diaphragm pump (received as NFB 25, KNF Neuberger Inc., Trenton, New Jersey) This is shown in FIG. 12 where pump 1262 transports the fluid into the serpentine flow field of the bipolar plates 1136 and 1146 that are proximate to the electrodes and then exiting in an outlet. . The two outlet streams recombined in the reservoir, maintaining the state of charge at 50% SOC. Electrolyte flow on both sides of the cell was maintained at roughly 22 mL per minute for the duration of the test.

For the half-cell test, the flow cell was connected to a four-channel Arbin battery tester (received as Model BT-2000 from Arbin Instruments, College Station, TX). The assembled cell was discharged at 50 mV, 100 mV, 150 mV, 200 mV, 250 mV, and 300 mV for 5 minutes each. The cell was then kept at open-circuit voltage for 10 minutes and discharged at 200 mV for 30 minutes, and this process was repeated two more times. This procedure constitutes one cycle of the test. Eighteen cycles were performed and then the cell was charged (polarity reversed for the electrodes) using the same procedure as above for two cycles. A final discharge cycle was performed after the charging cycles. In total, 21 cycles were performed.

Upon the completion of the half-cell testing protocols, a potentiostat (received as Model SP-300 BioLogic from Science Instruments, France) with an integrated frequency response analyzer was connected to the test cell. An impedance spectrum was taken while the electrolyte continued to flow through the cell. The spectra were taken in the frequency range from 50 mHz to 20 kHz using a 10 mV signal amplitude. Measurements were taken at open circuit voltage. The high frequency resistance (HFR), which is taken as the X-axis intercept for the impedance spectra taken from the cells provides the sum of all the ohmic resistances in the cell, and varies largely due to the electrode alone, since all other components of the cell and the electrolyte composition are fixed from test to test. Finally, the electrodes were recovered from the test cell and visually inspected for any macroscopic tears.

Test results are reported in Table 3, below.

TABLE 3

Patents and patent applications in the above application that are incorporated by reference in their entirety, are incorporated in a consistent manner. In the event of inconsistencies or contradictions between portions of the incorporated references and this application, the information in the preceding description shall control. The preceding description, given in order to enable one of ordinary skill in the art to practice the claimed disclosure, is not to be construed as limiting the scope of the disclosure, which is defined by the claims and all equivalents thereto.