WO2024211335A1

WO2024211335A1 - Methods of producing caustic reagents from brines

Info

Publication number: WO2024211335A1
Application number: PCT/US2024/022727
Authority: WO
Inventors: David Jassby; Eric Hoek; Fan Yang; Dante SIMONETTI; Gaurav SANT
Original assignee: The Regents Of The University Of California
Priority date: 2023-04-04
Filing date: 2024-04-03
Publication date: 2024-10-10

Abstract

Disclosed herein are methods and systems for producing caustic brines (e.g., aqueous sodium hydroxide).

Description

METHODS OF PRODUCING CAUSTIC REAGENTS FROM BRINES

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. Provisional Application No. 63/456,971, filed April 4, 2023, the entire contents of which are incorporated by reference herein.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under 1926360 awarded by the National Science Foundation. The government has certain rights in the invention.

BACKGROUND

The energy-efficient production of caustic (e.g., NaOH) is critical to multiple industrial efforts, including traditional chemical production, food processing, and water treatment. In addition, caustic solutions are emerging as a key component in enabling the transition towards carbon-free industrial processes, such as CO2-free cement manufacturing, and CO2 capture and sequestration. Traditional caustic manufacturing revolves around the chlor-alkali process, of which there are several versions, such as the membrane, diaphragm, and oxygen depolarized cathode electrolysis. In all these process configurations, an anode and cathode are separated by a hydraulic separator (diaphragm or ion exchange membrane), and immersed into a high- salinity environment (typically a 25% NaCl aqueous solution). When the electrodes are polarized, chloride ions are oxidized on the anode to generate chlorine, while water is reduced on the cathode to generate hydroxide ions and hydrogen gas. In some configurations, the water is heated to 90° C to increase process efficiency. Typical NaOH concentrations resulting from traditional chlor-alkali reactors range between 10%-30%, and further concentration (if desired) is achieved through water evaporation and NaOH precipitation.¹

In all chlor-alkali configurations, the main cost components are: i) the energy (electricity) needed to drive the electrolysis reactions (-70% of total OpEx); and ii) the membrane/electrode cost, which varies with system size, with an average of $4,500/m² of electrode area. In terms of electricity, typical energy requirements of traditional chlor-alkali processes range between 2-3 kWh/kg NaOH. Therefore, there is a large incentive to minimize both the energy consumption associated with caustic production, as well as the size of the electrodes in the electrochemical reactor. Unfortunately, there is a tension between these two goals, as smaller electrode areas result in higher current densities, which can lead to reduced energy efficiencies and overall higher energy consumption. One of the main challenges in water electrolysis systems is the formation of hydrogen gas bubbles that “stick” to the surface of the cathode, a process known as “gas masking”. These gas bubbles reduce the water/cathode interfacial area, increase ohmic losses, and lead to current “hot spots” and reduced cathode efficiency. Thus, there is an ongoing, unmet need for new methods of producing caustic (e.g., aqueous sodium hydroxide) from brines.

SUMMARY OF THE INVENTION

In certain aspects, the present disclosure provides methods of producing an alkali hydroxide comprising passing an aqueous solution between an anode and a cathode, wherein the cathode is permeable; the aqueous solution comprises dissolved ions; and the aqueous solution permeates the cathode, thereby producing the alkali hydroxide.

In some aspects, the present disclosure provides electrodes comprising the anodes and cathodes disclosed herein.

In certain aspects, the present disclosure provides systems for performing the methods disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of caustic production process. Pressurized feed (brine) flowing between the anode and cathode is forced through the porous cathode, generating a high-pH permeate that contains H2 that can be further collected and utilized. The remaining feed, known as the retentate, becomes acidified and contains Ch that also can be collected. The physical flexibility of the materials enable the efficient packaging of the assembly into a spiral-wound configuration - the image depicts an assembly with >1 m² of cathode area.

FIG 2A. shows the Specific Energy Consumption (SEC) of caustic production as a function of membrane flux, with a minimum SEC measured at 1,300 LMH, with a 3% NaCl solution and 2 V potential applied.

FIG. 2B shows the SEC of caustic solution production as a function of applied potential with a 3% NaCl solution and a flux of 1,300 LMH.

FIG. 3 shows a process flow diagram of a proposed caustic production and Ca(OH)2 and Mg(OH)2 production system. Brine (e.g., seawater, groundwater, produced water, or leachate) is first softened using NF to produce a Ca- and Mg-rich solution and aNaCl solution that is fed to the chlor-alkali reactor. There, the stream is split again to yield an acidic stream that contains CI2, and a caustic stream that contains H2. The H2 is recovered, while the caustic stream is returned to the Ca- and Mg-rich stream from the NF to induce precipitation and recovery of Mg(0H)2 and Ca(0H)2 solids.

DETAILED DESCRIPTION OF THE INVENTION

Disclosed herein is an electrolysis system capable of generating caustic brines at a specific energy consumption (SEC) of 1.7 kWh/kg NaOH. In the system, pressurized water (100 psi) is passed between an anode (Pt-coated Ti) and porous cathode (composed of Ni- coated carbon nanotubes deposited on a porous polymeric support) (FIG. 1). Feed water (containing dissolved ions) flows between the anode and cathode, with a portion of the feed being pushed through the cathode (the permeate) at a rate that is dependent on the membrane (cathode) permeability and the applied pressure (FIG. 1). Water flowing through the polarized cathode “picks up” hydroxide ions generated on the cathode surface (a result of electrolysis), yielding a high pH stream (the caustic stream). Water staying in the feed stream (the retentate) accumulates protons and/or chlorine, yielding an acidic stream. Among the advantages of the presently disclosed systems and methods is that any gas bubbles evolving on the anode and cathode are separated - hydrogen from the cathode is swept away into the permeate stream, while oxygen and chlorine remain in the retentate (FIG. 1). Due to the physical (e.g, mechanical) flexibility of the materials, the entire configuration can be packaged as a spiralwound element, similar to that used in desalination plants, which dramatically reduces the physical footprint of the process (FIG. 1).

Data showing the SEC of caustic production is dependent on membrane flux, with the SEC minimized at a flux of 1,300 L/m²/hr (FIG. 2A). The SEC is impacted by the applied voltage, with the minimum SEC value (1.7 kWh/kg NaOH) achieved at a low voltage of 2.5 V (A/m²) (FIG. 2B). This SEC is far below the value reported for commercial chlor-alkali reactors. In addition, the electrolysis reaction generates appreciable amounts of hydrogen (estimated at 25 kg H2/ton NaOH), which can be harvested from the permeate stream (FIG. 1), and used to further reduce energy consumption (e.g, by powering a fuel cell) or sold to increase the economic merits of the process.

It is hypothesized that the flow of water through the membrane/cathode effectively strips any hydrogen gas bubbles from the surface, which minimizes gas masking and current “hot spots” on the cathode surface. In addition, since the system configuration does not rely on a hydraulic separator between the anode and cathode, the overall electrical resistance of the system is lower. Ultimately, by forcing the feed through the cathode at high rates the system enables effective separation between the anode and cathode that prevents the mixing of gasses (chlorine/oxygen and hydrogen) while minimizing the electrical resistance of the circuit.

While in certain embodiments, the system configuration generates a permeate stream with a pH of 12.5 (with a feed pH of 6), it is likely that looping the permeate into the feed could yield dramatically higher pH values in the permeate, albeit with the tradeoff of reducing SEC. Therefore, it is predicted that the value of the proposed system is in the generation of dilute caustic solutions from low-quality feed stock (e.g., seawater). However, to prevent mineral scaling on the membrane/cathode by Ca and Mg (e.g, CaCOs and Mg(0H)2), in certain embodiments, a softening step may be needed to remove divalent cations from the feed. Nanofiltration (NF) can readily achieve >99% removal of such cations from seawater, at an energy intensity of between 0.4 - 0.6 kWh/m³ of treated water. Considering seawater as a feed stock, and a final product pH of 12.5, it is estimated that the additional energy needed to desalinate seawater is 0.4 kWh/kgNaOH, bringing the overall SEC of certain embodiments of the present disclosure to 2.1 kWh/kgNaOH. Importantly, the SEC reported by traditional chloralkali processes assume the use of deionized water and pure salts, the provision of which is not included in the SEC values (2-3 kWh/kg NaOH) that are reported; in other words, the actual energy demand of the process is higher.

Dilute caustic streams have many potentially useful applications, as they can facilitate the precipitation of solids such as Mg(OH)2 (useful for subsequent CO2 capture and sequestration) and Ca(OH)2 (a promising carbon-free cement precursor). An example of a process train that utilizes the proposed electrolysis system and a generic brine to generate these solids is presented in FIG. 3. Here, NF is used to separate Ca and Mg from a brine to form 2 streams: i) a stream enriched in Ca and Mg, and ii) a high-salinity stream containing primarily NaCl. The second stream is fed to the electrolyzer producing a high pH caustic stream (the permeate) and a chlorine-containing stream (the retentate). The permeate is then mixed (titrated) into the Ca and Mg rich stream from the NF process to sequentially precipitate first CaCCh and Mg(OH)2 (at pH 10.5), and then Ca(OH)2 (at pH 12). The amounts of recoverable Ca and Mg solids are dependent on their initial concentrations in the brine, as well as the availability of caustic from the electrolysis reactor. Methods of Producing Alkali Hydroxides

In one aspect, the present disclosure provides methods of producing an alkali hydroxide comprising passing an aqueous solution between an anode and a cathode, wherein: the cathode is permeable; the aqueous solution comprises dissolved ions; and the aqueous solution permeates the cathode, thereby producing the alkali hydroxide.

In certain embodiments, the alkali hydroxide is sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, or magnesium hydroxide. In certain preferred embodiments, the alkali hydroxide is sodium hydroxide.

In some embodiments, the aqueous solution is brine, seawater, produced water, grey water (e.g., water that already has been used domestically, commercially, or industrially), a hot stream (e.g., frack water), a saline stream, water from mine drainage, or industrial cleaning. In further embodiments, the aqueous solution is brine (e.g., an aqueous solution of Na2SO4, K2SO4, NaF, KF, Nal, KI, NaNCh, KNO3, Ca(NO₃)₂, Mg(NO₃)₂, or MgSC ).

In certain embodiments, the aqueous solution is an aqueous solution of 3% sodium chloride. In some embodiments, the aqueous solution is seawater. In other embodiments, the aqueous solution is produced water.

In certain embodiments, the aqueous solution has a flow that is perpendicular to the cathode and the anode.

In some embodiments, the aqueous solution is pressurized. In further embodiments, the aqueous solution has a pressure of about 50 psi, about 75 psi, about 100 psi, about 125 psi, about 150 psi, about 175 psi, or about 200 psi. In certain embodiments, the aqueous solution has a pressure of about 100 psi. In some embodiments, the aqueous solution has a pressure greater than 90 psi.

In certain embodiments, the anode comprises a transition group metal (e.g., Ni, such as Ni(OH)₂), Co, NiCo alloy, or Pt). In certain such embodiments, the transition group metal is a noble metal, preferably platinum. In certain embodiments, the anode comprises titanium. In some such embodiments, the anode comprises platinum and titanium.

In some embodiments, the cathode comprises a first layer (e.g., a conductive layer) and a porous support. In certain such embodiments, the first layer is disposed (e.g., deposited) on the porous support.

In some embodiments, the first layer comprises a transition group metal, e.g., a group 10 metal. In certain such embodiments, the first layer comprises nickel. In certain embodiments, the first layer further comprises a carbonaceous material, e.g., carbon nanotubes, carbon fiber, or carbon felt. In certain preferred embodiments, the first layer further comprises carbon nanotubes. In some embodiments, the first layer comprises nickel coated carbon nanotubes.

In some embodiments, the porous support is a porous polymeric support. In certain such embodiments, the polymeric support comprises polysulfone, polyethersulfone, PVFD, PTFE, PAN, PEEK, polyimide, polyamide, or cellulose acetate. In some embodiments, the porous support has pores which have an average diameter of 0.5 nm to 10,000 nm.

In certain embodiments, the cathode is permeable to H2. In some embodiments, the cathode is permeable to water.

In some embodiments, the method produces Ch and the Ch does not permeate the cathode. In certain embodiments, the method produces O2 and the O2 does not permeate the cathode.

In certain embodiments, the alkali hydroxide is produced at a specific energy consumption (SEC) of about 0.5 kWh/kg of alkali hydroxide, about 0.6 kWh/kg of alkali hydroxide, about 0.7 kWh/kg of alkali hydroxide, about 0.8 kWh/kg of alkali hydroxide, about 0.9 kWh/kg of alkali hydroxide, about 1.0 kWh/kg of alkali hydroxide, about 1.1 kWh/kg of alkali hydroxide, about 1.2 kWh/kg of alkali hydroxide, about 1.3 kWh/kg of alkali hydroxide, about 1.4 kWh/kg of alkali hydroxide, about 1.5 kWh/kg of alkali hydroxide, about 1.6 kWh/kg of alkali hydroxide, about 1.7 kWh/kg of alkali hydroxide, about 1.8 kWh/kg of alkali hydroxide, or about 1.9 kWh/kg of alkali hydroxide. In some embodiments, the alkali hydroxide is produced at a specific energy consumption (SEC) of about 1.1 kWh/kg of alkali hydroxide, about 1.2 kWh/kg of alkali hydroxide, about 1.3 kWh/kg of alkali hydroxide, about 1.4 kWh/kg of alkali hydroxide, about 1.5 kWh/kg of alkali hydroxide, about 1.6 kWh/kg of alkali hydroxide, or about 1.7 kWh/kg of alkali hydroxide. In certain embodiments, the alkali hydroxide is produced at a specific energy consumption (SEC) of about 1.2 kWh/kg of alkali hydroxide. In some embodiments, the alkali hydroxide is produced at a specific energy consumption (SEC) of about 1.7 kWh/kg of alkali hydroxide. In certain embodiments, the alkali hydroxide is produced at a specific energy consumption (SEC) of below 2.0 kWh/kg of alkali hydroxide. In some embodiments, the alkali hydroxide is produced at a specific energy consumption (SEC) of below 2.5 kWh/kg of alkali hydroxide. In certain embodiments, the alkali hydroxide is produced at a specific energy consumption (SEC) of below 3.0 kWh/kg of alkali hydroxide. In some embodiments, the method is performed at a voltage of about 1 V to about 10 V. In certain such embodiments, the method is performed at a voltage of about 2.0 V, about 2.1 V, about 2.2 V, about 2.3 V, about 2.4 V, about 2.5 V, about 2.6 V, about 2.7V, about 2.8 V, about 2.9 V, or about 3.0V. In some embodiments, the method is performed at a voltage of about 2.5 V.

In certain embodiments, the method further comprises treating the aqueous solution to remove divalent cations (e.g., divalent alkali metal cations, such as CaCOs and Mg(OH)2) prior to passing the aqueous solution between the anode and the cathode. In some embodiments, treating the aqueous solution to remove divalent cations comprises filtering the aqueous solution using a membrane (e.g, a nanofiltration membrane). In some such embodiments, the membrane is a nanofiltration membrane. In certain such embodiments, the membrane is a reverse osmosis membrane. In certain embodiments, the anode is an oxygen selective anode.

Electrode Systems and Electrolysis Systems

In certain aspects, the present disclosure provides an electrode system comprising the anode and cathodes disclosed herein. In certain embodiments, the electrode is configured as a spiral-wound electrode.

In some aspects, the present disclosure provides an electrolysis system comprising: an inlet in fluid communication with a first outlet and a second outlet; a junction that connects each of the first and second outlets with the inlet; a power source; an anode as defined herein, the anode coupled to the power source and disposed in the junction; and a cathode as defined herein, the cathode coupled to the power source and disposed in the junction so as to separate the second outlet from a fluid path connecting the inlet and the first outlet.

In certain embodiments, the anode and the cathode are disposed facing one another.

In certain aspects, the present disclosure provides a system as set forth in FIG. 3.

In certain embodiments, systems and methods of the present disclosure comprise an oxygen-selective anode that is selective for the Oxygen Evolution Reaction (OER) over the Chlorine Evolution Reaction (C1ER) pathway, for example, in embodiments wherein the aqueous solution comprises chloride (C1‘) ions. In some such embodiments, the oxygen- selective anode comprises double-layered coatings, e.g., by overlaying a Cl'-blocking on an OER-catalyzing layer. The Cl'-blocking outer layer usually comprises negatively charged materials that repel the negatively charged Cl" ion, while letting water, oxygen, and cationic species (Na⁺, H⁺, etc.) pass through the outer layer to the active layers beneath. A number of anodes have been demonstrated to possess high OER-selectivity, most of which contain a Cl'- blocking overlayer composed of Manganese oxides (MnOx). Others use Nafion or Si- and Ni- hydroxides which play a similar Cl" blocking role to MnOx. As will be apparent to one of ordinary skill in the art, any suitable oxygen-selective anodes may be used in the systems and methods described herein. Certain exemplary configurations are described below, which are suitable for use in systems and methods of the present disclosure.

Table 1. Compositions and relevant properties of other known anodes.

Although the compatibility of the Cl'-blocking layer and the OER-catalyzing layer is important, the anode’s durability and overpotential are based on the underlying OER catalyst. As indicated in Table 1, the most durable anodes generally rely on a heavy loading of IrOx, which can lastingly endure acidic and chlorinated environments. For instance, either pure IrCh or a IrCh contents > 80 at.% (> 90 wt.%) is needed to ensure the longevity of the anodes. However, the tightening of global Ir production and price presents a barrier to large-scale production and use of these anodes. Exemplary PGM-free anodes are provided below, suitable for use in certain embodiments of the systems and methods described herein. Table 2. PGM-free, ER-selective catalysts from literature reports

Definitions

Unless otherwise defined herein, scientific and technical terms used in this application shall have the meanings that are commonly understood by those of ordinary skill in the art. Generally, nomenclature used in connection with, and techniques of, chemistry, chemical engineering, electrical engineering and civil engineering described herein, are those well- known and commonly used in the art.

The methods and techniques of the present disclosure are generally performed, unless otherwise indicated, according to conventional methods well known in the art and as described in various general and more specific references that are cited and discussed throughout this specification.

Chemistry terms used herein, unless otherwise defined herein, are used according to conventional usage in the art, as exemplified by “The McGraw-Hill Dictionary of Chemical Terms”, Parker S., Ed., McGraw-Hill, San Francisco, C.A. (1985).

All publications, patents and published patent applications referred to in this application are specifically incorporated by reference herein. In case of conflict, the present specification, including its specific definitions, will control.

As used herein, the terms “optional” or “optionally” mean that the subsequently described event or circumstance may occur or may not occur, and that the description includes instances where the event or circumstance occurs as well as instances in which it does not. For example, “optionally substituted alkyl” refers to the alkyl may be substituted as well as where the alkyl is not substituted.

The term “alkalinizing” as used herein refers to a process of increasing the pH of a given solution, e.g, alkalinizing the first solution to prepare an alkaline solution with a higher pH.

The term “acidifying” or “acidification” as used herein refers to a process of decreasing the pH of a given solution. The given solution may be of any starting pH before undergoing the acidifying, e.g. the solution may already have a pH below 7 before a step of acidifying the solution is performed.

The term “ionic communication” as used herein refers to the ability for ions to freely flow between two objects or regions of an object, e.g, between the cathodic chamber and anodic chamber of an electrochemical cell, in accordance with local chemical gradients. Nonlimiting examples of such gradients include flow of ions from an area of high electrical potential to low electrical potential, from high ion concentration to low ion concentration, and from high chemical potential to low chemical potential. In certain embodiments, two objects or regions may be physically separated by a semi-permeable barrier (e.g., not in fluid communication) but still be in ionic communication, e.g., by virtue of ion diffusion or transport through the barrier.

The term “caustic” refers to compositions comprising basic components, preferably sodium hydroxide.

The term “caustic brine” or “caustic solution” refers to an aqueous solution comprising basic components, preferably sodium hydroxide.

INCORPORATION BY REFERENCE

All publications and patents mentioned herein are hereby incorporated by reference in their entirety as if each individual publication or patent was specifically and individually indicated to be incorporated by reference. In case of conflict, the present application, including any definitions herein, will control.

EQUIVALENTS

While specific embodiments of the subject invention have been discussed, the above specification is illustrative and not restrictive. Many variations of the invention will become apparent to those skilled in the art upon review of this specification and the claims below. The full scope of the invention should be determined by reference to the claims, along with their full scope of equivalents, and the specification, along with such variations.

Claims

CLAIMS We claim:

1. A method of producing an alkali hydroxide comprising passing an aqueous solution between an anode and a cathode, wherein the cathode is permeable; the aqueous solution comprises dissolved ions; and the aqueous solution permeates the cathode, thereby producing the alkali hydroxide.

2. The method of claim 1, wherein the alkali hydroxide is sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, or magnesium hydroxide.

3. The method of claim 1, wherein the alkali hydroxide is sodium hydroxide.

4. The method of any one of claims 1-4, wherein the aqueous solution is brine, seawater, produced water, grey water (e.g., water that already has been used domestically, commercially, or industrially), a hot stream (e.g., frack water), a saline stream, water from mine drainage, or industrial cleaning.

5. The method of any one of claims 1-3, wherein the aqueous solution is brine (e.g., an aqueous solution of Na2SO4, K2SO4, NaF, KF, Nal, KI, NaNCh, KNO3, Ca(NOs)2, Mg(NO₃)₂, or MgSC ).

6. The method of any one of claims 1-5, wherein the aqueous solution is an aqueous solution of 3% sodium chloride.

7. The method of any one of claims 1-5, wherein the aqueous solution is seawater.

8. The method of any one of claims 1-5, wherein the aqueous solution is produced water.

9. The method of any one of claims 1-8, wherein the aqueous solution has a flow that is perpendicular to the cathode and the anode.

10. The method of any one of claims 1-9, wherein the aqueous solution is pressurized.

11. The method of any one of claims 1-10, wherein the aqueous solution has a pressure of about 50 psi, about 75 psi, about 100 psi, about 125 psi, about 150 psi, about 175 psi, or about 200 psi.

12. The method of any one of claims 1-10, wherein the aqueous solution has a pressure of about 100 psi.

13. The method of any one of claims 1-10, wherein the aqueous solution has a pressure of greater than 90 psi.

14. The method of any one of claims 1-13, wherein the anode comprises a transition group metal (e.g., Ni, such as Ni(0H)2), Co, NiCo alloy, or Pt).

15. The method of claim 14, wherein the transition group metal is a noble metal.

16. The method of any one of claims 1-13, wherein the anode comprises platinum.

17. The method of any one of claims 1-14, wherein the anode comprises titanium.

18. The method of any one of claims 1-14, wherein the anode comprises platinum and titanium.

19. The method of any one of claims 1-18, wherein the cathode comprises a first layer (e.g., a conductive layer) and a porous support.

20. The method of any one of claims 1-18, wherein the first layer is disposed (e.g, deposited) on the porous support.

21. The method of claim 19 or 20, wherein the first layer comprises a transition group metal.

22. The method of any one of claims 19-21, wherein the first layer comprises a group 10 metal.

23. The method of any one of claims 19-22, wherein the first layer comprises nickel.

24. The method of any one of claims 19-23, wherein the first layer further comprises a carbonaceous material.

25. The method of any one of claims 19-23, wherein the first layer further comprises carbon nanotubes, carbon fiber, or carbon felt.

26. The method of any one of claims 19-23, wherein the first layer further comprises carbon nanotubes.

27. The method of any one of claims 19-23, wherein the first layer comprises nickel coated carbon nanotubes.

28. The method of any one of claims 19-27, wherein the porous support is a porous polymeric support.

29. The method of claim 28, wherein the polymeric support comprises polysulfone, polyethersulfone, PVFD, PTFE, PAN, PEEK, polyimide, polyamide, or cellulose acetate.

30. The method of any one of claims 19-29, wherein the porous support has pores which have an average diameter of 0.5 nm to 10,000 nm.

31. The method of any one of claims 1-30, wherein the cathode is permeable to H2.

32. The method of any one of claims 1-31, wherein the cathode is permeable to water.

33. The method of any one of claims 1-32, wherein the method produces Ch and the CI2 does not permeate the cathode.

34. The method of any one of claims 1-33, wherein the method produces O2 and the O2 does not permeate the cathode.

35. The method of any one of claims 1-34, wherein the alkali hydroxide is produced at a specific energy consumption (SEC) of about 1.0 kWh/kg of alkali hydroxide, about 1.1 kWh/kg of alkali hydroxide, about 1.2 kWh/kg of alkali hydroxide, about 1.3 kWh/kg of alkali hydroxide, about 1.4 kWh/kg of alkali hydroxide, about 1.5 kWh/kg of alkali hydroxide, about 1.6 kWh/kg of alkali hydroxide, about 1.7 kWh/kg of alkali hydroxide, about 1.8 kWh/kg of alkali hydroxide, or about 1.9 kWh/kg of alkali hydroxide.

36. The method of any one of claims 1-34, wherein the alkali hydroxide is produced at a specific energy consumption (SEC) of about 1.3 kWh/kg of alkali hydroxide, about 1.4 kWh/kg of alkali hydroxide, about 1.5 kWh/kg of alkali hydroxide, about 1.6 kWh/kg of alkali hydroxide, or about 1.7 kWh/kg of alkali hydroxide.

37. The method of any one of claims 1-34, wherein the alkali hydroxide is produced at a specific energy consumption (SEC) of about 1.5 kWh/kg of alkali hydroxide.

38. The method of any one of claims 1-34, wherein the alkali hydroxide is produced at a specific energy consumption (SEC) of about 1.7 kWh/kg of alkali hydroxide.

39. The method of any one of claims 1-34, wherein the alkali hydroxide is produced at a specific energy consumption (SEC) of below 2.0 kWh/kg of alkali hydroxide.

40. The method of any one of claims 1-34, wherein the alkali hydroxide is produced at a specific energy consumption (SEC) of below 2.5 kWh/kg of alkali hydroxide.

41. The method of any one of claims 1-40, wherein the alkali hydroxide is produced at a specific energy consumption (SEC) of below 3.0 kWh/kg of alkali hydroxide.

42. The method of any one of claims 1-41, wherein the method is performed at a voltage of about 1 V to about 10 V.

43. The method of any one of claims 1-41, wherein the method is performed at a voltage of about 2.0 V, about 2.1 V, about 2.2 V, about 2.3 V, about 2.4 V, about 2.5 V, about 2.6 V, about 2.7V, about 2.8 V, about 2.9 V, or about 3.0V.

44. The method of any one of claims 1-41, wherein the method is performed at a voltage of about 1.8 V.

45. The method of any one of claims 1-44, wherein the method further comprises treating the aqueous solution to remove divalent cations (e.g., divalent alkali metal cations, such as CaCOs and Mg(OH)2) prior to passing the aqueous solution between the anode and the cathode.

46. The method of claim 45, wherein treating the aqueous solution to remove divalent cations comprises filtering the aqueous solution using a membrane (e.g., a nanofiltration membrane).

47. The method of any one of claims 1-46, wherein the anode is an oxygen selective anode.

48. The method of claim 47, wherein the oxygen selective anode is selected from the following:

49. The method of claim 47, wherein the oxygen selective anode is selected from the following:

50. An electrode comprising the anode and cathode defined in any one of claims 1-49.

51. The electrode of claim 50, wherein the electrode is configured as a spiral-wound electrode.

52. A system comprising: an inlet in fluid communication with a first outlet and a second outlet; a junction that connects each of the first and second outlets with the inlet; a power source; an anode as defined in any one of claims 1-49, the anode coupled to the power source and disposed in the junction; and a cathode as defined in any one of claims 1-49, the cathode coupled to the power source and disposed in the junction so as to separate the second outlet from a fluid path connecting the inlet and the first outlet.

53. The system of claim 52, wherein the anode and the cathode are disposed facing one another.

54. A system as set forth in FIG. 3.