BACKGROUND OF THE INVENTION

Salt splitting has several applications in water treatment and purification (e.g., demineralization (e.g., of industrial waste streams, and desalination, e.g., of brackish water, e.g., sea water), production of commodities (e.g., sodium and chlorine), processing of minerals, recovery of materials, etc. Salt splitting using electrodialysis typically involves concomitant evolution of oxygen and hydrogen gases at the electrodes. The formation of oxygen and hydrogen at the electrodes represents both safety challenges and an energy cost in the energy wasted in producing these side products.

Thus, there is a need for improved salt splitting.

SUMMARY OF THE INVENTION

The invention provides methods and systems for salt splitting employing redox active species at each electrode. Advantageously, cycling of the redox active species does not require electrolyzing water to form hydrogen and oxygen gas.

The invention provides a method for splitting salts. The method includes providing a device including an anode and a cathode and a membrane stack disposed between the anode and cathode. The membrane stack includes a cathodic cation exchange membrane adjacent the cathode and a first unit cell. The first unit cell includes a first fluid compartment disposed between an anodic cation exchange membrane and a first bipolar membrane; a second fluid compartment disposed between the first bipolar membrane and a first anion exchange membrane; and a third fluid compartment disposed between the first anion exchange membrane and the cathodic cation exchange membrane. The device further includes an anodic redox solution including a first anionic redox active species dissolved or dispersed in solution (e.g., neutral or basic) and in contact with the membrane stack and the anode; and a cathodic redox solution including a second anionic redox active species dissolved or dispersed in solution (e.g., neutral or basic) and in contact with the membrane stack and the cathode. The method further includes providing a first salt solution in the first compartment, a second salt solution in the second compartment, and a third salt solution in the third compartment. The method further includes applying an electrical potential between the anode and cathode that oxidizes the first anionic redox active species and reduces the second anionic redox active species and drives cations from the anodic redox solution across anodic cation exchange membrane into the first fluid compartment, anions of the third salt solution from the third fluid compartment across the first anion exchange membrane into the second fluid compartment, and cations of the third salt solution from the third fluid compartment toward the cathodic redox solution and induces water dissociation in the first bipolar membrane to insert hydroxide ions into the first fluid compartment and protons into the second fluid compartment. The method thus splits the third salt solution in the third fluid compartment while creating acid in the second fluid compartment and base in the first fluid compartment.

In some embodiments, the membrane stack further includes a second unit cell. The second unit cell includes a fourth fluid compartment disposed between a second cation exchange membrane adjacent the third fluid compartment and a second bipolar membrane; a fifth fluid compartment disposed between the second bipolar membrane and a second anion exchange membrane; and a sixth fluid compartment disposed between the second anion exchange membrane and the cathodic cation exchange membrane. The method further includes providing a fourth salt solution in the fourth fluid compartment, a fifth salt solution in the fifth fluid compartment, and a sixth salt solution in the sixth fluid compartment. The electrical potential drives cations of the third salt solution across the second cation exchange membrane from the third fluid compartment into the fourth fluid compartment, anions of the sixth salt solution across the second anion exchange membrane from the sixth fluid compartment into the fifth fluid compartment, and cations of the sixth salt solution toward the cathodic redox solution and induces water to dissociate at the second bipolar membrane to insert hydroxide ions into the fourth fluid compartment and protons into the fifth fluid compartment. The method thus splits the sixth salt solution while creating acid in the fifth fluid compartment and base in the fourth fluid compartment. Additional unit cells may be included.

In some embodiments, the first anionic redox active species is a reduced form of the second anionic redox active species. In some embodiments, the method further includes contacting the anodic redox solution with the cathode after oxidation at the anode and cathodic redox solution with the anode after reduction at the cathode. In particular embodiments, the first anionic redox active species includes ferrocyanide or a viologen, e.g., 1 ,1 '-bis(3-phosphonopropyl)-[4,4'-bipyridine]-1 ,T-diium dibromide.

Another aspect of the invention provides a device for splitting salt. The device includes an anode and a cathode, and a membrane stack disposed therebetween. The membrane stack includes a cathodic cation exchange membrane adjacent the cathode and a first unit cell. The first unit cell includes a first fluid compartment disposed between an anodic cation exchange membrane and a first bipolar membrane; a second fluid compartment disposed between the first bipolar membrane and a first anion exchange membrane; and a third fluid compartment disposed between the first anion exchange membrane and the cathodic cation exchange membrane. The device further includes an anodic redox solution including a first anionic redox active species dissolved or dispersed in solution (e.g., neutral or basic) and in contact with the membrane stack and the anode; and a cathodic redox solution including a second anionic redox active species dissolved or dispersed in solution (e.g., neutral or basic) and in contact with the membrane stack and the cathode.

In some embodiments, the membrane stack further includes a second unit cell. The second unit cell includes a fourth fluid compartment disposed between a second cation exchange membrane adjacent the third compartment and a second bipolar membrane; a fifth fluid compartment disposed between the second bipolar membrane and a second anion exchange membrane; and a sixth fluid compartment disposed between the second anion exchange membrane and the cathodic cation exchange membrane. Additional unit cells may be included.

In some embodiments, the first anionic redox active species is a reduced form of the second anionic redox active species. In particular embodiments, the first anionic redox active species includes ferrocyanide or a viologen, e.g., 1 ,1 '-bis(3-phosphonopropyl)-[4,4'-bipyridine]-1 ,T-diium dibromide.

In another aspect, the invention provides a method for splitting salts. The method includes providing a device including an anode and a cathode and a membrane stack disposed between the anode and cathode. The membrane stack includes an anodic anion exchange membrane adjacent the anode and a first unit cell. The first unit cell includes a first fluid compartment disposed between the anodic anion exchange membrane and a first cation exchange membrane; a second fluid compartment disposed between the first cation exchange membrane and a first bipolar membrane; and a third fluid compartment disposed between the first bipolar membrane and a first anion exchange membrane. The device further includes an anodic redox solution including a first cationic redox active species dissolved or dispersed in solution (e.g., acidic or neutral) and in contact with the membrane stack and the anode; and a cathodic redox solution including a second cationic redox active species dissolved or dispersed in solution (e.g., acidic or neutral) and in contact with the membrane stack and the cathode. The method further includes providing a first salt solution in the first fluid compartment, a second salt solution in the second fluid compartment, and a third salt solution in the third fluid compartment. The method further includes applying an electrical potential between the anode and cathode that oxidizes the first cationic redox active species and reduces the second cationic redox active species and drives anions of the cathodic redox solution toward the anodic redox solution, anions of the first salt solution from the first fluid compartment across the anodic anion exchange membrane into the anodic redox solution, and cations of the first salt solution from the first fluid compartment across the first cation exchange membrane into the second fluid compartment, and induces water to dissociate in the first bipolar membrane to insert hydroxide ions into the second fluid compartment and protons into the third fluid compartment. The method thus splits the first salt solution in the first fluid compartment while creating acid in the third fluid compartment and base in the second fluid compartment.

In some embodiments, the membrane stack further includes a second unit cell. The second unit cell includes a fourth fluid compartment disposed between the first anion exchange membrane and a second cation exchange membrane; a fifth fluid compartment disposed between the second cation exchange membrane and a second bipolar membrane; and a sixth fluid compartment disposed between the second bipolar membrane and a second anion exchange membrane. The method may further include providing a fourth salt solution in the fourth fluid compartment, a fifth salt solution in the fifth fluid compartment, and a sixth salt solution in the sixth fluid compartment. The electrical potential drives anions of the fourth salt solution across the first anion exchange membrane from the fourth fluid compartment into the third fluid compartment, cations of the fourth salt solution across the second cation exchange membrane from the fourth fluid compartment into the fifth fluid compartment, and induces water to dissociate in the second bipolar membrane to insert hydroxide ions into the fifth fluid compartment and protons into the sixth fluid compartment. The method thus splits the fourth salt solution while creating acid in the sixth fluid compartment and base in the fifth fluid compartment. Additional unit cells may be included.

In some embodiments, the first cationic redox active species is a reduced form of the second cationic redox active species. In certain embodiments, where the method further includes contacting the anodic redox solution with the cathode after oxidation at the anode and cathodic redox solution with the anode after reduction at the cathode. In particular embodiments, the first cationic redox active species includes an Fe" species, e.g., a ferrocene species, e.g., 1 ,T-bis-[(3-trimethylammonio)propyl]-ferrocene dichloride.

In another aspect, the invention provides a device for splitting salt. The device includes an anode, a cathode, and a membrane stack disposed between the anode and cathode. The membrane stack includes an anodic anion exchange membrane adjacent the anode and a first unit cell. The first unit cell includes a first fluid compartment disposed between the anodic anion exchange membrane and a first cation exchange membrane; a second fluid compartment disposed between the first cation exchange membrane and a first bipolar membrane; and a third fluid compartment disposed between the first bipolar membrane and a first anion exchange membrane. The device further includes an anodic redox solution including a first cationic redox active species dissolved or dispersed in solution (e.g., acidic or neutral) and in contact with the membrane stack and the anode; and a cathodic redox solution including a second cationic redox active species dissolved or dispersed in solution (e.g., acidic or neutral) and in contact with the membrane stack and the cathode.

In some embodiments the membrane stack further includes a second unit cell. The second unit cell includes a fourth fluid compartment disposed between the first anion exchange membrane and a second cation exchange membrane; a fifth fluid compartment disposed between the second cation exchange membrane and a second bipolar membrane; and a sixth fluid compartment disposed between the second bipolar membrane and a second anion exchange membrane. Additional unit cells may be included.

In some embodiments, the first cationic redox active species is a reduced form of the second cationic redox active species. In particular embodiments, the first cationic redox active species includes an Fe" species, e.g., a ferrocene species, e.g., 1 ,T-bis-[(3-trimethylammonio)propyl]-ferrocene dichloride.

In another aspect, the invention provides a method for splitting salts. The method includes providing a device including an anode, a cathode, and a membrane stack disposed between the anode and cathode. The membrane stack includes an anodic cation exchange membrane adjacent the anode and a first unit cell. The first unit cell includes a first fluid compartment disposed between the anodic cation exchange membrane and a first anion exchange membrane; a second fluid compartment disposed between the first anion exchange membrane and a first cation exchange membrane; and a third fluid compartment disposed between the first cation exchange membrane and a first bipolar membrane. The device further includes an anodic redox solution including a first anionic redox active species dissolved or dispersed in solution (e.g., acidic) and in contact with the membrane stack and the anode; and a cathodic redox solution including a second anionic redox active species dissolved or dispersed in solution (e.g., acidic) and in contact with the membrane stack and the cathode. The method further includes providing a first salt solution in the first fluid compartment, a second salt solution in the second fluid compartment, and a third salt solution in the third fluid compartment. The method further includes applying an electrical potential between the anode and cathode that oxidizes the first anionic redox active species and reduces the second anionic redox active species and drives protons of the anodic redox solution across the anodic cation exchange membrane into the first fluid compartment, anions of the second salt solution from the second fluid compartment across the first anion exchange membrane into first fluid compartment, and cations of the second salt solution from the second fluid compartment across the first cationic exchange membrane into the third fluid compartment, and induces water to dissociate in the first bipolar membrane to insert hydroxide ions into the third fluid compartment and protons toward the cathodic redox solution. The method thus splits the second salt solution in the second fluid compartment while creating acid in the first fluid compartment and base in the third fluid compartment. In some embodiments of the method, the membrane stack further includes a second unit cell. The second unit cell includes a fourth fluid compartment disposed between the first bipolar membrane and a second anion exchange membrane; a fifth fluid compartment disposed between the second anion exchange membrane and a second cation exchange membrane; and a sixth fluid compartment disposed between the second cation exchange membrane and a second bipolar membrane. The method may further include providing a fourth salt solution in the fourth fluid compartment, a fifth salt solution in the fifth fluid compartment, and a sixth salt solution in the sixth fluid compartment. The electrical potential drives anions of the fifth salt solution across the second anion exchange membrane from the fifth fluid compartment into the fourth fluid compartment, cations of the fifth salt solution across the second cation exchange membrane from the fifth fluid compartment into the sixth fluid compartment, and induces water to dissociate in the second bipolar membrane to insert hydroxide ions into the sixth fluid compartment and protons toward the cathodic redox solution. The method thus splits the fifth salt solution while creating acid in the fourth fluid compartment and base in the sixth fluid compartment. Additional unit cells may be included.

In some embodiments, the first anionic redox active species is a reduced form of the second anionic redox active species. In some embodiments, the method further includes contacting the anodic redox solution with the cathode after oxidation at the anode and cathodic redox solution with the anode after reduction at the cathode. In certain embodiments, the first anionic redox active species includes an anthraquinone species, e.g., 9,10-anthraquinone-2,6-disulfonic acid.

In another aspect, the invention provides a device for splitting salt. The device includes an anode, a cathode, and a membrane stack disposed between the anode and cathode. The membrane stack includes an anodic cation exchange membrane and a first unit cell. The first unit cell includes a first fluid compartment disposed between the anodic cation exchange membrane and a first anion exchange membrane; a second fluid compartment disposed between the first anion exchange membrane and a first cation exchange membrane; and a third fluid compartment disposed between the first cation exchange membrane and a first bipolar membrane. The device further includes an anodic redox solution including a first anionic redox active species dissolved or dispersed in solution (e.g., acidic) and in contact with the membrane stack and the anode; and a cathodic redox solution including a second anionic redox active species dissolved or dispersed in solution (e.g., acidic) and in contact with the membrane stack and the cathode.

In some embodiments of the device, the membrane stack further includes a second unit cell. The second unit cell includes a fourth fluid compartment disposed between the first bipolar membrane and a second anion exchange membrane; a fifth fluid compartment disposed between the second anion exchange membrane and a second cation exchange membrane; and a sixth fluid compartment disposed between the second cation exchange membrane and a second bipolar membrane. Additional unit cells may be included.

In some embodiments, the first anionic redox active species is a reduced form of the second anionic redox active species. In certain embodiments, the first anionic redox active species includes an anthraquinone species, e.g., 9,10-anthraquinone-2,6-disulfonic acid. Another aspect of the invention provides a system for splitting salts. The system includes any device of the preceding aspects or described herein and a first salt solution in the first fluid compartment; a second salt solution in the second fluid compartment; and a third salt solution in the third fluid compartment.

In certain embodiments, the system further includes a second unit cell and a fourth salt solution in the fourth fluid compartment; a fifth salt solution in the fifth fluid compartment; and a sixth salt solution in the sixth fluid compartment. Additional unit cells may be included.

In some embodiments of the system, the first redox active species is a reduced form of the second redox active species. In certain embodiments, the system further includes a voltage source to cycle the first anionic or cationic redox active species oxidized at the anode to the cathode and the second anionic or cationic redox active species reduced at the cathode to the anode. In particular embodiments, the system further includes one or more electrochemical cells configured to cycle oxidized first anionic or cationic redox active species and/or reduced second anionic or cationic redox active species.

In embodiments of any method, device, or system described herein, the anode and/or cathode includes any chemically inert, conductive material, e.g., carbon (e.g., carbon cloth, carbon paper, or carbon felt), e.g., an inert metal (e.g., gold).

Definitions

The term “about,” as used herein, refers to ±10% of a recited value.

The term “anodic,” as used herein, denotes a component (e.g., a cationic exchange membrane or redox solution) that is proximal to (e.g., adjacent to, e.g., separated by a fluid or a single fluid compartment from) the anode.

The term “cathodic,” as used herein, denotes a component (e.g., a cationic exchange membrane or redox solution) that is proximal to (e.g., adjacent to, e.g., separated by a fluid or a single fluid compartment from) the cathode.

The term “redox active species,” as used herein denotes a species that can accept or lose one or more electrons by reduction or oxidation at an electrode. A redox active species of the invention is a species for which water is not an oxidized or reduced form thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a device of the invention with a two-unit cell membrane stack and basic redox solutions with anionic redox active species therein.

FIG. 2 illustrates a device of the invention with a two-unit cell membrane stack and neutral redox solutions with anionic redox active species therein.

FIG. 3 illustrates a device of the invention with a two-unit cell membrane stack and neutral redox solutions with cationic redox active species therein. FIG.4 illustrates a device of the invention with a two-unit cell membrane stack and acidic redox solutions with cationic redox active species therein.

FIG. 5 illustrates a device of the invention with a two-unit cell membrane stack and acidic redox solutions with anionic redox active species therein.

DETAILED DESCRIPTION OF THE INVENTION

The invention employs redox active species disposed either side of a membrane stack between electrodes to achieve salt splitting with greater energy efficiency than existing techniques. The reduction and oxidation of the redox active species at the electrodes replaces the energy inefficient and problematic water splitting redox reactions previously employed in electrodialytic salt splitting.

The invention includes a membrane stack disposed between electrodes (e.g., an anode and a cathode), from with it is separated by solutions containing redox active species. The membrane stack includes one or more anion exchange membranes (AEM), one or more bipolar membranes (BPM), and one or more cation exchange membranes (CEM), with compartments for fluid flow disposed therebetween.

Depending on the membrane stack layout and redox solutions used, the membrane stack may include one more AEM or one more CEM than there are of the other membranes. The membrane stack may start and end with a cation exchange membrane (e.g., an anodic cation exchange membrane and a cathodic cation exchange membrane, e.g., FIGs. 1 and 2) or an anion exchange membrane (e.g., an anodic anion exchange membrane and a first or second anion exchange membrane, e.g., FIGs. 3 and 4). Alternatively, the membrane stack may include an anodic cation exchange membrane proximal to the anode (e.g., adjacent the anodic redox solution) and a bipolar membrane proximal to the cathode (e.g., adjacent the cathodic redox solution, e.g., FIG. 5). A membrane at the beginning or end of a membrane stack is separated from its proximal electrode (e.g., an anode or cathode) by a solution containing a redox active species (e.g., an anodic redox solution or cathodic redox solution) and/or is in contact with a porous proximal electrode containing a solution of redox active species. The inclusion of the redox active species in contact with the electrodes can allow for more efficient electrodialysis, e.g., by reducing the energy cost relative to water spitting at the electrodes, since oxidation and reduction of the redox active species takes the place of water electrolysis at the electrodes.

An exemplary system is shown in FIG. 1 . Here, the cathode is separated from the membrane stack by a solution (e.g., a basic solution, e.g., a flowing solution) of an anionic redox active species, e.g., ferricyanide and NaOH, and the anode is separated from the membrane stack by a solution (e.g., a basic solution, e.g., a flowing solution) of an anionic redox active species, e.g., ferrocyanide in NaOH. The membrane stack of FIG. 1 has two unit-cells. Each unit cell includes a cation exchange membrane (CEM) on the side of the unit cell proximal to the anode (e.g., an anodic cation exchange membrane), a bipolar membrane (BPM), and an anion exchange membrane (AEM). The membrane stack further includes a cathodic cation exchange membrane between the anionic exchange membrane of a unit cell and the cathode. The bipolar membrane is between the anion exchange membrane and the cation exchange membrane, and the anion exchange membrane is between the bipolar membrane and the cathodic cation exchange membrane or the cation exchange membrane of another unit cell. Salt solutions (e.g., flowing salt solutions, e.g., 1 M NaCI) separate each membrane from its neighboring membranes in the stack. When a sufficient electrical potential is applied across the electrodes, the redox active species (e.g., ferrocyanide) at the anode is oxidized (e.g., to ferricyanide), and the redox active species (e.g., ferricyanide) at the cathode is reduced (e.g., to ferrocyanide). Each redox active species oxidized at the anode (e.g., ferrocyanide) results in an unbalanced positive charge, and each redox active species reduced at the cathode (e.g., ferricyanide) has an unbalanced negative charge. Under the electric field between the electrodes, cations are driven toward the cathode, and anions are driven toward the anode. During this process, water is dissociated into hydroxide and protons (e.g., water-complexed protons, e.g., hydronium ions or [HsC>2] ⁺ ) in bipolar membranes. Unbalanced charges can be balanced by counterions from adjacent salt solutions crossing ion exchange membranes in the direction dictated by the electric filed and the type of ion exchange membrane(s) adjacent the fluid compartments. The process results in streams of acid, base, and desalinated liquids. Ion flow is illustrated in FIG. 1 . A corresponding process using neutral anodic and cathodic redox solutions is illustrated in FIG. 2.

Another exemplary system is shown in FIG. 3. Here, the cathode is separated from the membrane stack by a solution (e.g., a neutral solution, e.g., a flowing solution) of a cationic redox active species, e.g., 1 ,1 bis-[(3-trimethylammonio)propyl]-ferrocenium (BTMAP-Fc ³⁺ ), and the anode is separated from the membrane stack by a solution (e.g., an acidic solution, e.g., a flowing solution) of a cationic redox active species, e.g., 1 ,1 ’-bis-[(3-trimethylammonio)propyl]-ferrocene (BTMAP-Fc ²⁺ ). The membrane stack of FIG. 3 has two unit-cells. Each unit cell includes an anion exchange membrane, which in a device with a single unit cell would be the cathodic anion exchange membrane, a bipolar membrane, and a cation exchange membrane. The membrane stack further includes an anodic anion exchange membrane between the cation exchange membrane of the first unit cell and the anode. The bipolar membranes are between the anion exchange membranes and the cation exchange membranes of each unit cell, and the cation exchange membranes are between the bipolar membrane and the anodic anion exchange membrane or the anion exchange membrane of another unit cell. Salt solutions (e.g., flowing salt solutions, e.g., 1 M NaCI) separate each membrane from its neighboring membranes in the stack. When a sufficient electrical potential is applied across the electrodes, the redox active species (e.g., BTMAP- Fc ²⁺ ) at the anode is oxidized (e.g., to BTMAP-Fc ³⁺ ), and the redox active species at the cathode (e.g., BTMAP-Fc ³⁺ ) is reduced (e.g., to BTMAP-Fc ²⁺ ). Each redox active species oxidized at the anode (e.g., BTMAP-Fc ²⁺ ) results in an unbalanced positive charge, and each redox active species reduced at the cathode (e.g., BTMAP-Fc ³⁺ ) has an unbalanced negative charge. Under the electric field between the electrodes, cations are driven toward the cathode, and anions are driven toward the anode. During this process, water is dissociated into hydroxide and protons (e.g., water-complexed protons, e.g., hydronium ions or [Hdq2] ⁺ ) in bipolar membranes. Unbalanced charges can be balanced by counterions from adjacent salt solutions crossing ion exchange membranes in the direction dictated by the electric filed and the type of ion exchange membrane(s) adjacent the fluid compartments. The process results in streams of acid, base, and desalinated liquids. Ion flow is illustrated in FIG. 3. A corresponding process using acid anodic and cathodic redox solutions with, e.g., inorganic Fe ²⁺ and Fe ³⁺ species (e.g., inorganic Fe ²⁺ /Fe ³⁺ species, e.g., iron (ll/lll) halides, e.g., FeC^/FeCh) is illustrated in FIG. 4.

Another exemplary system is shown in FIG. 5. Here, the cathode is separated from the membrane stack by a solution (e.g., an acidic solution, e.g., a flowing solution) of an anionic redox active species (e.g., 9.10-anthraquinone-2,6-disulfonic acid (AQDS ² )), and the anode is separated from the membrane stack by a solution (e.g., an acidic solution, e.g., a flowing solution) of an anionic redox active species (e.g.,

9.10-anthrahydroquinone-2,6-disulfonic acid (AQDS ⁴ )). The membrane stack of FIG. 5 has two unit- cells. Each unit cell includes an anion exchange membrane, a bipolar membrane, and a cation exchange membrane. The membrane stack further includes an anodic cation exchange membrane between the anion exchange membrane of the first unit cell and the anode. The bipolar membranes are terminal to the unit cells in this embodiment, e.g., adjacent the anion exchange membranes of the next unit cell or the cathode. The cation exchange membranes of each unit cell are between the bipolar membrane and the anion exchange membrane of the unit cell. Salt solutions (e.g., flowing salt solutions, e.g., 1 M NaCI) separate each membrane from its neighboring membranes in the stack. When a sufficient electrical potential is applied across the electrodes, the redox active species (e.g., AQDS ⁴ ) at the anode is oxidized (e.g., to the anthraquinone, AQDS ² ), and the redox active species at the cathode (e.g., AQDS ² ) is reduced (e.g., to AQDS ⁴ ). Each oxidized redox active species at the anode (e.g., AQDS ² ) results in unbalanced positive charge, and each reduced redox active species at the cathode (e.g., AQDS ⁴ ) has unbalanced negative charge. Under the electric field between the electrodes, cations are driven toward the cathode, and anions are driven toward the anode. During this process, water is dissociated into hydroxide and protons (e.g., water-complexed protons, e.g., hydronium ions or [Hs02] ⁺ ) in the bipolar membranes. Unbalanced charges can be balanced by counterions from adjacent salt solutions crossing ion exchange membranes in the direction dictated by the electric filed and the type of ion exchange membrane(s) adjacent the fluid compartments. The process results in streams of acid, base, and desalinated liquids. Ion flow is illustrated in FIG. 5.

The membrane stacks of the invention may include any integer number of unit cells, e.g., 1 ; up to 2, 5, 10, 50, 100, 500, 1000 or 10,000; or more than 10,000.

Membrane Stacks

Methods of the invention include stacks of membranes, i.e. , ion exchange (e.g., anion exchange membranes and cation exchange membranes) and bipolar membranes. Membranes in a membrane stack may include, e.g., ionomers, e.g., polymers containing anionic groups (e.g., polysulfonated fluoropolymers, e.g., NAFION®, e.g., as cation exchange membranes) or polymers containing cationic groups (e.g., polymers containing a plurality of tertiary ammonium groups, e.g., as anion exchange membranes). Bipolar membranes (e.g., FUMASEP® FBM) may include both polyanionic and polycationic ionomers. Membranes may include polymers with hydrocarbon or fluorocarbon repeat units, or both. Membranes may be inorganic, e.g., including graphene, oxides (e.g., metal or semimetal oxides), silicates (e.g., metal or semimetal silicates), nitrides (e.g., metal or semimetal oxides), etc. A membrane stack may include a plurality of unit cells (e.g., 2, 3, 4, 5, 6, 7, 8, 9, 10, or more unit cells).

Redox Active Species

Methods, devices, and systems of the invention include one or more redox active species. Examples of suitable redox active species may include ferricyanide/ferrocyanide, quinones/hydroquinones (e.g., as described in WO 2014/052682, WO 2016/144909, or WO 2015/048550 and are incorporated by reference), ferrocenes/ferroceniums (e.g., those described in WO 2018/032003 and are incorporated by reference), bipyridines (e.g., viologens (e.g., BBP-Vi, e.g., in a device such as FIG. 2, e.g., those described in WO 2018/032003 and are incorporated by reference), alloxazines (e.g., those described in WO 2016/144909 and are incorporated by reference), or phenazines. Methods, systems, and devices of the invention may employ an oxidized form of a redox active species on one side of the stack and its reduced form on the other. An advantage of such an arrangement is in recirculating the redox active species between the two electrodes, such that a redox active species oxidized at the anode is directed to the cathode to be reduced. Recirculating the redox active species between the electrodes may save energy that would otherwise be needed to regenerate the redox active species. Alternatively, a redox active species may be regenerated in an external electrochemical cell. Alternatively, redox active species that are not oxidized and reduced forms of the same species, or are separated by changes of more than one or two units of formal charge, may be used at each electrode, which may confer an advantage of widening the potential window of the device or system. Examples of redox active species may include bromide (Br), tribromide ((B^)-), 4,4’-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyrate (2,6-DBEAQ), 2,6- DPPEAQ, (((9,10-dioxo-9,10-dihydroanthracene-2,6-diyl)bis(oxy))bis(p ropane-3,1 -diyl))bis(phosphonic acid), 3,3'-(9,10-anthraquinone-diyl)bis(3-methylbutanoic acid) (DPivOHAQ), or 4,4'-(9,10-anthraquinone- diyl)dibutanoic acid (DBAQ). A benzoquinone or naphthoquinone may also be used as the redox active species. The membrane stacks described herein may be employed with any pH of anionic or cationic redox solution. In certain embodiments, the pH of solutions containing the redox active species (e.g., the anodic and cathodic redox solutions) may be >7, e.g., at least 8, 9, 10, 11 , 12, 13, or 14, 8-14, 9-14, IQ- 14, 11-14, 12-14, 13-14, or above 14. In other embodiments, the pH of solutions containing the redox active species (e.g., the anodic and cathodic redox solutions) may be <7, e.g., at less than 6, 5, 4, 3, 2, 1 or 0, e.g., 7-0, 6-0, 5-0, 4-0, 3-0, 2-0, or below 0. In some embodiments, the pH of solutions containing the redox active species (e.g., the anodic and cathodic redox solutions) may be neutral, e.g., pH 6.5-7.5. Redox active species may be in a solution containing hydroxide ions. The hydroxide ions may have cations that are the same as or different to the cations of the salt to be split. The redox active species may include anionic/cationic counterions that are the same as or different to the anions/cations of the salt to be split. The pH of the redox solutions may be selected for stability of the anionic or cationic redox active species.

Salt Solutions

Methods and systems of the invention include one or more salt solutions. The salt solution may have an initial concentration of, e.g., at least about 0.1 M, e.g., about 0.1 to 0.5 M (e.g., about 0.1 -0.2 M, 0.2-0.3 M, 0.3-0.4 M, or 0.4-0.5 M), or, e.g., about 0.5-1 M (e.g., about 0.5-0.6 M, 0.6-0.7 M, 0.7-0.8 M, 0.8-0.9 M or 0.9-1 M), or, e.g., about 1 -2 M (e.g., about 1-1.1 M, 1 .1 -1 .2 M, 1 .2-1 .3 M, 1 .3-1 .4 M, 1 .4-1 .5 M, 1 .5-1 .6 M, 1 .6-1 .7 M, 1 .7-1 .8 M, 1 .8-1 .9 M, or 1 .9-2 M), or about 1 -3 M (e.g., about 1 -1 .5 M, 1 .5-2 M, 2-2.5 M, or 2.5-3 M), or at least about 2 M, e.g., about 2-5 M (e.g., about 2-2.5 M, 2.5-3 M, 3.5-4 M, or 4.5-5 M), or at least 5 M, e.g., about 5 M, 6 M, 7 M, or 8 M or higher. Cations in the salt solution may be, e.g., any metal (e.g., alkali metals, e.g., lithium, sodium, potassium, or rubidium, or alkaline earth metals, e.g., beryllium, magnesium, calcium, strontium, and radium), a non-metal, a cationic metal complex, or a cationic small molecule. Anions of the salt may be, e.g., halides (e.g., fluoride, chloride, bromide, or iodide), sulfate, phosphate, nitrate, mesylate, tosylate, perchlorate, or triflate., anionic metal complex, an anionic small molecule, etc.

Electrodes

Electrodes of the invention may include any conductive material that is chemically inert to the redox solutions under operating conditions of the device. Examples include carbon electrodes, e.g., glassy carbon electrodes, carbon paper electrodes, carbon felt electrodes, or carbon nanotube electrodes.

Other suitable electrodes may include metals, e.g., any metal that is chemically stable to components of the redox solutions (e.g., acids, bases, salts, and redox active species), examples include noble metals (e.g., gold, silver, iridium, platinum, etc.). Depending on the redox solutions, non-noble metals may also be suitable. Titanium electrodes may also be employed. Electrodes can also be made of a high specific surface area conducting material, such as a nanoporous metal sponge (T. Wada, A.D. Setyawan, K. Yubuta, and H. Kato, Scripta Materialia 65, 532 (2011 )), which has been synthesized previously by electrochemical dealloying (J.D. Erlebacher, M.J. Aziz, A. Karma, N. Dmitrov, and K. Sieradzki, Nature 410, 450 (2001 )), or a conducting metal oxide, which has been synthesized by wet chemical methods (B.T. Huskinson, J.S. Rugolo, S.K. Mondal, and M.J. Aziz, arXiv:1206.2883 [cond-mat.mtrl-sci]; Energy & Environmental Science 5, 8690 (2012); S.K. Mondal, J.S. Rugolo, and M.J. Aziz, Mater. Res. Soc. Symp. Proc. 1311 , GG10.9 (2010)). Electrodes suitable for other redox active species are known in the art. Electrodes may be porous structures into which the redox solutions can enter or flow.

Methods

Methods of the invention may include splitting one or more salt solutions (e.g., 2, 3, 4, 5, 6, 7, 8, 9, 10, etc. salt solutions). Methods of the invention split salt by applying an electrical potential across a membrane stack disposed between two electrodes and separated therefrom by solutions containing redox active species. The stacks contain one or more (e.g., 1 , 2, 3, 4, or 5 or more) anion exchange membranes, one or more (e.g., 2, 3, 4, 5, or 6 or more) cation exchange membranes, and one or more (e.g., 1 ,2, 3, 4, or 5 or more) bipolar membranes. Depending on the redox solutions, the number of anion exchange or cation exchange membranes is equal to the number of unit cells plus one. Salt solutions (e.g., 1 M NaCI) flow between the membranes. Under the electric field generated by the electrical potential between the electrodes, cations move toward the cathode, and anions move toward the anode, with the anion and cation exchange membranes selectively permitting transfer. Water is dissociated at the bipolar membrane, maintaining charge balance. Thus, a flowing salt solution bounded by a bipolar membrane and an anion exchange membrane receives salt anions, e.g., chloride ions, from the salt solution on the other side of the anion exchange membrane which charge balances protons generated at the bipolar membrane. A salt solution bounded by a cation exchange membrane and a bipolar membrane receives salt cations, e.g., sodium cations, from, e.g., a salt solution or a solution containing a redox active species (e.g., ferrocyanide in sodium or potassium hydroxide solution) on the other side of the cation exchange membrane, which charge balances hydroxide ions generated at the bipolar membrane. Methods may include splitting the salt solutions continuously (e.g., with salt solutions (and redox solutions) flowing into the compartments continuously) or in a batchwise manner (e.g., providing salt solutions to the compartments (and redox solutions into contact with the electrodes), performing the salt splitting without flow, and then replenishing the salt (and redox) solutions).

The acid and/or base solutions may then be used for any suitable purpose. In one example, the basic solution is used to absorb gaseous CO2, e.g., from air or flue gas, e.g., for subsequent sequestration or utilization.

Systems and Additional Components

Devices and systems of the invention may include a device of the invention and one or more salt solutions. Systems of the invention may include a plurality of devices of the invention. Devices or systems of the invention may include additional electrical components, e.g., electrodes, electrochemical cells, etc., e.g., to provide a potential to drive the salt splitting process or to regenerate the redox active species externally or fully convert one redox active species to its oxidized or reduced form such that it may be used in a redox solution used at the other electrode. Alternatively, devices or systems may include components to separate oxidized and reduced forms of a redox active species in a fluid flow, such that reduced forms can be returned to the anode and oxidized forms directed to the cathode. Devices or systems of the invention may include pumps, liquid storage components, transport components, etc. Systems of the invention may include various kinds of liquid storage and transport components, e.g., tanks, ponds, reservoirs, pipes, etc. Devices or systems may also include components for monitoring and controlling, e.g., pH, pressure, temperature, etc. (e.g., pH sensors, thermocouples, pressure gauges, valves, actuators, switches, computers, heaters, chillers, etc.). Devices or systems may include components to recycle or redistribute heat produced. Devices or systems of the invention may include components to remove water vapor, e.g., condensers (e.g., chilled high surface area structures or membrane condensers). Devices or systems may include components to collect and convert renewable energy (e.g., wind, solar, or tidal energy) into electrical energy (e.g., photovoltaic cells, fuel cells, solar concentrators, etc.). Devices or systems of the invention may be configured to allow the splitting to be performed in a continuous manner.

Other embodiments are in the claims.