DESALTING DEVICE AND METHOD FOR DESALTING WATER

Abstract

Desalination device, comprising: a container configured to contain a saltwater solution with an increased concentration of dissolved salts; a first electrode and a second electrode, arranged in a first compartment and a second compartment of the container respectively, wherein the first and second electrodes are configured to be in liquid communication with the saltwater solution; an anion exchange membrane (AEM) separating the first and second compartments, the AEM having a porous membrane structure and several negatively charged oxygen functional groups coupled to the porous membrane structure; and an energy source configured to selectively apply a voltage to one of the first or second electrodes, wherein the AEM has a selective permeability when the voltage is applied such that cations in the salt water solution have a first diffusion rate d1 through the membrane and anions in the salt water solution have a second diffusion rate d2 through the membrane, where 0 ≤ d1 < d2, and wherein the energy source is configured to apply a membrane voltage to the AEM via an external circuit, and the membrane voltage is configured to decrease the first diffusion rate d1 of the cations through the AEM and to increase the second diffusion rate d2 of the anions through the AEM.

Description

FIELD OF THE INVENTION

[0001] The disclosure relates to a desalination apparatus and a method for desalinating water. GENERAL STATE OF THE ART

[0002] The need for fresh drinking water is essential, and securing fresh and potable water sources for a growing global population is one of the most important challenges of the twenty-first century. Increasing amounts of freshwater are needed not only for human consumption but also for industrial and agricultural activities. Desalination is an emerging technology used to extract freshwater from otherwise non-freshwater sources and involves the removal of salt and other contaminants from saltwater. The desalination process can be implemented in different ways, such as thermally (e.g., heating and drying, thermal distillation), applying electrical energy (i.e., charge-based separation of ions), and via mechanical energy / pressure (e.g., reverse osmosis (RO), membrane-based filtration).

[0003] Capacitive deionization (CDI) is a technology that uses electrical energy to perform charge-based separation of ions. A CDI device typically comprises a cathode and an anode. During a purification cycle, cations are added to the cathode and anions are added to the anode. The next cycle applies a voltage to flush the ions from the electrodes and expel wastewater with high contaminant content. The electrodes are then available for the next purification cycle. In some implementations, a CDI device comprises at least two cation-intercalation host electrodes separated by an anion-exchange membrane (AEM) for the water desalination and purification processes. This technique is sometimes referred to as hybrid capacitive deionization (HCDI) and has been discussed in recent literature, including Energy Environ. Sci., 2014, 7, 3683; J. Electrochem. Soc., 2016, 163, A530; Electrochim.Acta, 2016, 203, 265; and ACS Omega, 2017, 2, 1653.

[0004] Fig. Figure 5 illustrates a conventional electrochemical device 500 that implements the HCDI technique to desalinate salt water from a water source. The device 500 includes a first electrode 504, a second electrode 508 spaced from the first electrode 504, and an exchange membrane 512 disposed between the first and second electrodes. The membrane 512 is typically an anion exchange membrane (AEM) that divides the device 500 into a first salt water compartment 516 with the first electrode 504 and a second salt water compartment 520 with the second electrode 508. In some devices, intercalation host materials (typically such as Na 0,44MnO2 or similar oxides) are used as the electrode components (i.e., cathode and anode). In other devices, the electrode 504 is a cation intercalation host electrode, and the second electrode 508 is a carbon-based electrode (e.g., porous carbon, activated carbon, etc.) coupled to the first electrode on the other side.

[0005] Fig. Figure 5 illustrates the device 500 in a first operating state in which a positive voltage V IN is applied to the first electrode 504 (anode), and the second electrode 508 (cathode) is connected to ground. In the first operating state, first target ions 524 such as Na + -cations are released from the first electrode 504 and distributed in the first compartment 516, which in this operating state is sometimes referred to as the "brine compartment." When the Na +-Concentration in the first compartment 516 increases, the electrostatics generates an electric field that attracts second target ions 528, such as the anion Cl - from the second compartment 520, which is sometimes referred to as the "clean compartment" or "fresh compartment" in the first operating state. When Cl - Ions 528 move from the second compartment 520 into the first compartment 516, intercalate Na + Ions 524 from the second compartment 520 into the second electrode 508 to obtain a neutral electrostatic charge, which completes the water desalination cycle. Through this desalination cycle, the device 500, in the first operating state, increases the salt concentration in the first compartment 516 and decreases the salt concentration in the second compartment 520.

[0006] The brine water and the clean water produced in the first operating state are extracted separately after the water in the second compartment 520 has been sufficiently purified by the device 500. To perform another purification cycle, the first and second compartments 516, 520 are filled with new salt water from the water supply. The device 500 is then operated in a second operating state (not shown) in which a positive voltage is applied to the second electrode 508 and the first electrode 504 is connected to ground. In the second operating state, the device 500 produces fresh water in the first compartment 516 (the "fresh water compartment" in the second operating state) and produces brine water in the second compartment 520 (the "brine compartment" in the second operating state). The alternating operation of the device 500 helps to reduce the amount of Na +-ion ​​concentration in the intercalation host electrode materials within a desired range. It can be seen that this process only works if the tendency for Na + diffused from the brine compartment to the clean compartment, is slowed down or blocked by the anion exchange membrane (AEM) 512, which prevents the diffusion of Na + cations, while allowing the unhindered diffusion of anions (e.g. Cl - ). Therefore, the selective ion permeability of the AEM 512 of the device 500 strongly influences the overall efficiency of the water desalination process.

[0007] The typical anion exchange membrane (AEM) used for HCDI technology, such as the AEM 512, contains "fixed" positively charged groups that prevent cation transport through the AEM while allowing anion transport. It is desirable to produce a thin, scalable, low-resistance AEM for the water desalination process that allows selective ion diffusion. For example, graphene and graphene oxide (GO) are electrically conductive 2D materials that have been proposed for use in water filtration applications (Nano Lett. 2012, 12, 3602 and Energy Environ. Sci. 2014, 7, 1134). In these proposed applications, the "applied pressure" allows water molecules to permeate, while salts from the brine water remain in the graphene filter.Recent literature has also demonstrated that applying an electrical voltage to GO modifies water transport and diffusion properties across the GO sheet (Nature, 2018, 559, 236). Appl. Surf. Sci., 2018, 442, 700 discloses a heterogeneous anion exchange membrane modified with graphene oxide by electroplating.

[0008] Therefore, an AEM that improves the efficiency of water desalination via an HCDI device is needed. An AEM that improves efficiency via a specifically controlled chemical functional group configured to encapsulate and confine ions at a controlled voltage would also be advantageous. SUMMARY OF THE INVENTION

[0009] An anion exchange membrane for a desalination device comprises a membrane support having a porous first side, a porous second side, and a continuous porous structure extending from the first side to the second side. The continuous porous structure comprises graphene oxide (GO) sheets and a plurality of negatively charged oxygen functional groups coupled to the GO sheets.

[0010] A desalination device according to the present invention comprises a container, a first electrode, a second electrode, an anion exchange membrane (AEM), and a power source. The container is configured to contain a saltwater solution having an elevated concentration of dissolved salts. The AEM separates the container into a first compartment and a second compartment. The first and second electrodes are disposed in the first and second compartments, respectively, and are configured to be in contact with the saltwater solution. The AEM has a porous membrane structure and a plurality of negatively charged oxygen functional groups coupled to the porous membrane structure. The power source is configured to selectively apply a voltage to one of the first and second electrodes while the other of the first and second electrodes is connected to ground.The AEM exhibits selective permeability when the voltage is applied, such that cations in the saltwater solution have a first diffusion rate d1 through the saltwater solution and anions in the saltwater solution have a second diffusion rate d2 through the saltwater solution, where 0 ≤ d1 < d2. The energy source is configured to apply a membrane voltage to the AEM via an external circuit. The membrane voltage is configured to decrease the first diffusion rate d1 of the cations through the AEM and increase the second diffusion rate d2 of the anions through the AEM.

[0011] A method for desalinating water, in one embodiment, comprises filling the respective first and second compartments of a container with a saltwater solution having an elevated concentration of dissolved salts. The container comprises first and second electrodes disposed in the first and second compartments, respectively, and an anion exchange membrane (AEM) separating the first and second compartments. The method further comprises operating the first and second electrodes in a forward or reverse direction to release cations into one of the first and second compartments and remove cations from the other of the first and second compartments. The method further comprises applying a membrane voltage to the AEM during operation of the first and second electrodes.The membrane voltage decreases a first diffusion rate of cations through the AEM and increases a second diffusion rate of anions through the AEM relative to the first and second diffusion rates when the membrane voltage is not applied to the AEM. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a simplified schematic of a hybrid capacitive deionization (HCDI) device configured to remove target ions from a water supply, the device being shown in a first operation in the state where a first voltage is applied to one of two electrodes and a second voltage is applied to an anion exchange membrane (AEM) disposed between the electrodes; Fig. 2 is a schematic diagram showing a design strategy for using the AEM from Fig. 1 illustrated with a controlled redox voltage; Fig. 3 illustrates the adsorption of a Na +-ions in different materials of the AEM from Fig. 1; Fig. 4 illustrates the adsorption of a Cl - -ions in different materials of the AEM from Fig. 1; Fig. Figure 5 is a simplified schematic of an HCDI device with a known AEM; and Fig. Figure 6 shows a method for desalinating water using the device of Fig. 1. DETAILED DESCRIPTION

[0012] For the purpose of promoting an understanding of the principles of the disclosure, reference is now made to the embodiments illustrated in the drawings and described in the following written specification. It is to be understood that no limitation on the scope of the disclosure is intended. It is further understood that the disclosure also includes all changes and modifications to the illustrated embodiments and includes other applications of the principles of the disclosure that would normally occur to one skilled in the art to whom this disclosure is directed.

[0013] Fig. 1 illustrates a non-limiting first embodiment of a device 100 configured to remove ions from a water source via a hybrid capacitive deionization (HCDI) process. The device 100 includes a container 102 configured to contain a liquid solution, such as a saltwater solution having an elevated concentration of dissolved salts 124, 128. The container 102 may be a container, compartment, enclosure, receptacle, canister, tank, or the like of any shape, size, or configuration capable of capturing, retaining, holding, and / or releasing a liquid solution such as saltwater, brackish water, seawater, ocean water, freshwater, freshwater, potable water, desalinated water, contaminated water, industrial water, etc.The container is large enough to accommodate a sufficient amount of water solution undergoing desalination; therefore, the dimensions are based on a specific application. The container can be large enough to serve industrial applications.

[0014] The container can be made of various materials capable of resisting corrosion, temperature fluctuations, changing pH, varying pressure, and other chemical, mechanical, and / or physical conditions. The container can be made of gas, plastic, composite, metal, ceramic, or a combination of materials. The container can have one or more protective coatings. The container can be made of a material that minimizes the occurrence of water contamination. The container can be made of one or more materials that are non-toxic and meet drinking water standards.

[0015] The device 100 further includes a first electrode 104, a second electrode 108 spaced apart from the first electrode 104, and an anion exchange membrane (AEM) 112 disposed between the first electrode 104 and the second electrode 108. The AEM 112 divides the container 102 into a first compartment 113 containing the first electrode 104 and a second compartment 115 containing the second electrode 108. The first and second electrodes 104, 108 are disposed within the container 102 to be in fluid communication with the saltwater solution in the first and second compartments 113, 115. The first and second electrodes 104, 108 are at least partially submerged in the saltwater solution. The first and second electrodes 104, 108 may, in some embodiments, be fully submerged in the saltwater solution.The first and second electrodes 104, 108 may be placed on opposite sides of the container 102 and separated by a distance as shown in the figures. The distance may be 1 mm or more, 1 cm or more, 10 cm or more, 20 cm or more, or 30 cm or more, depending on the dimensions of the container 102 and the first and second electrodes 104, 108. Other distances between the first and second electrodes are also contemplated.

[0016] The first and second electrodes 104, 108 in the illustrated embodiment comprise an intercalation host material 116 that reversibly removes and releases first target ions 124 in various operating states of the device 100. Intercalation refers to the reversible inclusion of one or more ions in materials with layered structures. The spaces between the layers can serve as a temporary storage for one or more types of ions. The first target ions 124 in the illustrated embodiment include the cation Na + , but in other embodiments, the first target ions 124 may be the cations K + , Mg 2+ , Ca 2+ , Pb 2+ and others. The intercalation host material 116 of the first and second electrodes 104, 108, in some embodiments, comprises similar alkali metal oxides or phosphates, such as Na x MnO2 and Na xMPO4, with variations in the alkali metal concentration, such as by changing the composition of Na by varying x. In other embodiments, carbon-based and other oxide-based electrode materials may be used to fabricate the electrode materials in device 100 over other carbon and PVDF binders.

[0017] In further embodiments, the intercalation host material 116 of the first and second electrodes 104, 108 is at least one of doped or undoped cubic spinel MnO2, Na4Mn9O 18 (or equivalent, Na 0.44MnO2) tunnel-structured orthorhombic materials and NaM2(PO4)3 (where M = Ti, Mn, Fe, Ni, Cu, or a combination thereof), where the precise composition of the Na can be controlled by thoroughly mixing various starting amounts of Na2CO3 with metal oxide precursors, followed by heat treatment at high temperature (e.g., 800 °C). In these further embodiments, it is permissible to partially replace Na (if present) with Li, Mg, Ca, and / or K.

[0018] The AEM 112 in the illustrated embodiment includes a membrane material 120 identified using Detailed First-Principles Density Functional Theory (DFT) calculations. The membrane material 120 of the AEM 112, in some embodiments, includes single- or few-layer graphene and graphene oxide (GO) sheets with at least one chemically added functional group configured to encapsulate and confine the first target ions 124 in at least one operating state of the device 100. As used herein, "single-layer" means that the graphene and GO can each be configured with a single layer, and "few layers" means that the graphene and GO can each be configured with up to 8 layers. The chemical functional group is a negatively charged oxygen-containing functional group that allows for slow diffusion of the first target ions 124 present in the water source (e.g.,Seawater or industrial water). The first target ions 124 in this embodiment include, but are not limited to, sodium, magnesium, calcium, potassium, strontium, silicon, carbon, and aluminum ions. The chemical functional group in the AEM 112 also allows for very rapid diffusion of second target ions 128 between the electrodes 104, 108 through the repulsive force between the negatively charged AEM 112 and the second target ions 128 in the water supply. The second target ions 128 in this embodiment include, among others, chloride, sulfur, bromine, nitrate, and ammonia.

[0019] The membrane material 120 of the AEM 112, in some embodiments, comprises a carbon- or polymer-based porous membrane structure and one or more hydroxyl (-OH) oxygen functional groups and carboxyl (-COOH) oxygen functional groups coupled to the structure to allow selective cation (de)adsorption processes in the AEM. In other embodiments, the chemically added oxygen functional groups may include carbonyl (=O) oxygen functional groups, epoxy (-O) groups, or combinations thereof. In one example, hydroxyl (-OH)-rich GO samples are generated by the Hummers method described in Carbon, 2013, 64, 225 & J. Am. Chem. Soc., 1958, 6, 1339. First, a graphite plate and NaNO3 can be suspended in H2SO4 at low temperature (e.g., 5 °C), and KMnO4 can be added to the suspension. After further stirring and dilution with water, the mixture can be heated to approximately 100 °C and treated with H2O2.The resulting powder can be rinsed with HCl and deionized water. Since the inclusion of the first target ions 124 depends on the number of oxygen functional groups present in the GO, the control of ion transport can be adjusted via the processing method. In particular, further heat treatment can control the functional group density, followed by filtration, washing, and drying under vacuum.

[0020] In some embodiments, the AEM 112 further comprises a mixture of the membrane material 120 and other electronically conductive polymers, such as polyethylene oxide (PEO), poly(pyrrole)ene (PPY), polyaniline (PANI), poly(thiophene)ene (PT), poly(3,4-ethylenedioxythiophene) (PEDOT), poly(p-phenylene sulfide) (PPS), poly(acetylene)ene (PAC), and poly(p-phenylenevinylene) (PPV). In another embodiment, the AEM 112 comprises the membrane material 120 and other polymers that are not electronically conductive, such as cross-linked polyvinyl alcohol (PVA), cross-linked polymethyl methacrylate (PMMA), and polyphenylenevinylene (PPV). Further, electronically conductive material, such as graphite, hard carbon, soft carbon, carbon black, and others, may be added as needed.

[0021] Fig. 3 shows structurally the Na + -Ion adsorption in the membrane material 120 disclosed herein. The structures disclosed in Fig. 3 include sodium (Na), carbon (C), oxygen (O), and hydrogen (H). These elements are distinguished from each other in the figure by using different fill patterns in the circles representing the elements according to the corresponding legend. The membrane material 120 is shown as: pure graphene in panel a, GO with an epoxy (-O) group in panel b, GO with a carbonyl (=O) group in panel c, GO with a hydroxyl (-OH) group in panel d, and GO with a carboxyl (-COOH) group in panel e. Fig. 3 further illustrates the calculated Na +Ion adsorption voltage for each case illustrated in panels a to e. Pure graphene (panel a), GO with hydroxyl (panel d), and GO with carboxyl (panel e) meet the target voltage criteria within the water stability window (i.e., -0.401 to +1.23 V vs. standard hydrogen electrode (SHE), which can vary slightly with the solution pH). Epoxide or carbonyl functional groups can be incorporated into GO (panels b and c), however, more applied voltage may be required to reversibly (remove or insert) the cations.

[0022] Further calculations estimate the adsorption voltage of the first target ions 124 except Na +-ions on some of the GO functional groups of the membrane material 120 disclosed herein. For example, Table 1 identifies the calculated potential of K, Mg, and Ca adsorption on hydroxyl and carboxyl GO functional groups using first-principles density functional theory (DFT) calculations. TABLE 1 Spannung vs. SHE K Mg Ca Hydroxyl (-OH) -0,908 -0,915 -1,119 Carboxyl (-COOH) -0,865 -0,897 -0,832

[0023] The results disclosed herein indicate that cations other than Na, Mg, K, and Ca ions can also be attracted by the GO-based AEM 112 within (or slightly outside) the water stability window.

[0024] Fig. 4 shows structurally the Cl - -Ion adsorption in the membrane material 120 disclosed herein. The structures disclosed in Fig. 4 include chlorine (Cl), carbon (C), oxygen (O), and hydrogen (H). These elements are distinguished from each other in the figure by using different fill patterns in the circles representing the elements according to the corresponding legend. The membrane material 120 is shown as: pure graphene in panel a, GO with an epoxy (-O) group in panel b, GO with a carbonyl (=O) group in panel c, GO with a hydroxyl (-OH) group in panel d, and GO with a carboxyl (-COOH) group in panel e. Fig. Figure 4 further illustrates the calculated Cl - -ion ​​adsorption voltage for each case illustrated in panels a to e. As shown, all GO oxygen functional groups (panels b to e) repel the Cl - ions. In other words, these GO oxygen functional groups require a very high voltage (more than 3 eV) to detach the Cl - -ions. In contrast, it is possible that Cl --ions to pure graphene, such as when approximately 0.4 V is applied, forming a C-Cl-C epoxy-like compound, as described in panel a. To reduce anion desorption in AEM 112, the GO functional group populations can be maximized as in the GO functional group density control method mentioned above.

[0025] Further calculations also estimate the DFT binding energy of water to the GO functional groups of the membrane material 120 disclosed herein. For example, Table 2 identifies the binding energy of H2O in eV / H2O for the different GO functional groups. TABLE 2 GO-Funktionsgruppen Bindungsenergie von H 2 O [eV / Haq 2 O] Epoxid (-O-) +1,191 Carbonyl (=O) +1,184 Carboxyl (-COOH) +1,273 Hydroxyl (-OH) +1,285

[0026] Based on these calculations, at least approximately 1 eV is necessary for H2O to bind to the GO functional groups. Therefore, the use of the GO-based AEM 112 disclosed herein does not affect H2O transport through the AEM. In other words, H2O molecules 132 penetrate the AEM 112 as in Fig. 1 illustrated.

[0027] With a view to Fig. 3 and Fig. 4, the (de)sorption voltage of the first target ions 124 in the membrane material 120 of the AEM 112 disclosed herein is approximately within the water stability window (i.e., approximately -1.0 to 1.5 V vs. standard hydrogen electrode (SHE)). In some embodiments, the measured (de)sorption voltage of the first target ions 124 may slightly exceed the water stability voltage window, possibly due to one or more of kinetically limited ion diffusion, activation overpotential, ohmic resistance, and passivation layer formation. In contrast, very high energy is required to attract the second target ions 128 to the AEM 112, such as the single anion species F - , Cl - , Br - , I - , and S 2- and anion complexes such as ClO4 - , ClO3 - , ClO2 - , BrO4 - , BrO3 - , SO4 2- , SiO3 2- , and CN -. Therefore, the second target ions 128 freely permeate the AEM 112. In other words, the controlled modification of the oxygen functional groups in graphene oxide (GO) allows a reversible cation (e.g., Na, Mg, K, and Ca) (de-)sorption within the water stability window by inclusion of cations in the hydroxyl (-OH) and carboxyl (-COOH) groups of the membrane material 120. The anions such as Cl - -Ion are not bound by the oxygen functional groups present on the GO, which allows for rapid anion diffusion through the GO.

[0028] Again with reference to Fig. 1, in some embodiments, the device 100 further includes an external circuit on the AEM 112 configured to apply a voltage bias to the AEM. An HCDI device with an AEM that does not have a voltage output (such as the device 500 of Fig. 5), experiences an effective “natural” voltage V N ,eff , which depends on the applied voltage, V IN , to the ground at the electrodes. To enable more efficient and faster ion exchange, the device 100 is further configured to provide a new membrane voltage V M compared to ground to the AEM 112. In some embodiments, the membrane voltage V M in a range of 0 to ±0.5 V. In some embodiments, one or more of the natural voltage V N,eff . and the membrane voltage V M controlled and adjusted to control the adsorption and desorption rate of certain cations and anions in the AEM 112.

[0029] Fig. Figure 2 is a schematic diagram illustrating the design strategy for using the AEM 112 of the device 100 with a controlled redox voltage. Sodium ions are the most dominant species in seawater, with slow diffusion due to their tendency to be absorbed by the negatively charged species on the AEM, while anions (e.g., Cl - ) penetrate the AEM. Other cations such as K + , Ca 2+ and Mg 2+ require a lower potential for a reversible reaction that can compete within (or slightly outside) the water stability window (i.e., -0.401 to +1.23 V, depending on the pH). Lower concentrations of such cations (e.g., K + , Ca 2+ and Mg 2+) in the seawater, although the reversible extraction of these cations is still highly desirable. In some embodiments, the electrodes 104, 108 comprise other oxide materials, discussed herein, to adjust the overall voltage window. The voltage in the plotted y-axis is not to scale.

[0030] The basic operating principles of the device 100 of Fig. 1 for the alternating production of fresh water and brine water in the first compartment 113 and the second compartment 115 respectively are similar to that of the device 500 of Fig. 5. However, the device 100 comprises the graphene oxide (GO)-based AEM 112, which maximizes oxygen functional groups such as hydroxyl (-OH) and carboxyl (-COOH) to enable the inclusion of cations (e.g., Na, Mg, Ca, and K). Furthermore, the device 100 applies a membrane voltage V Mto the AEM 112 to allow more effective cation entrapment and improved anion diffusion through the AEM.

[0031] The device 100 in the illustrated embodiment includes symmetrical electrodes 104, 108 intended for use in the AEM 112 disclosed herein. The symmetrical electrodes 104, 108 allow continuous collection of clean water in successive water purification cycles. In other embodiments, the device includes the AEM 112 disclosed herein used in conjunction with asymmetrical electrodes (not shown) when the concentration of alkali metal within the metal oxide may differ between the electrodes. In further embodiments, the device includes two different electrode materials installed at an anode and a cathode in each of two compartments. One compartment is connected to a reservoir for collecting and / or separating salt, and the other compartment is connected to the clean water stream and alternately dispensed purified water.In these further embodiments, the device may require an additional cycle to regenerate the electrodes.

[0032] Fig. 6 shows a method 600 for desalinating water with an HCDI device, such as the device 100 described with reference to Fig. 1 to 4. The method 600 includes filling the first and second compartments 113, 115 of the container 102 with a salt water solution having an increased concentration of dissolved salts 124, 128 (block 602). The first and second electrodes 104 are operated in a forward direction or a reverse direction, depending on the direction in which the first and second electrodes were previously operated (block 604). When the first and second electrodes 104, 108 are operated in the forward direction, the voltage V INapplied to the first electrode 104, and the second electrode 108 is connected to ground. The first electrode 104 is configured to deintercalate cations 124 from the first electrode 104 into the saline solution in the first compartment 113 when operation occurs in the forward direction. The second electrode 108 is configured to intercalate cations 124 from the saline solution in the second compartment 115 into the second electrode 108 when operation occurs in the forward direction.

[0033] When the first and second electrodes 104, 108 are operated in the reverse direction, the voltage V INapplied to the second electrode 108 and the first electrode 104 is connected to ground. The second electrode 104 is configured to deintercalate cations 124 from the second electrode 104 into the saline solution in the second compartment 115 when operating in the reverse direction. The first electrode 104 is configured to intercalate cations 124 from the saline solution in the first compartment 113 into the first electrode 104 when operating in the reverse direction. While the first and second electrodes 104, 108 are operated in the forward or reverse direction (block 604), a membrane voltage V M applied to the AEM 112 via an external circuit (block 606). While the membrane voltage V Mis applied to the AEM (block 606), a first diffusion rate d1 of the cations 124 through the AEM 112 decreases and a second diffusion rate d2 of anions 128 through the AEM increases relative to the first and second diffusion rates, respectively, when the membrane voltage V M is not connected to the AEM.

[0034] The method 600 produces a brine water solution in the first compartment 113 and a fresh water solution in the second compartment 115 when blocks 604 and 606 are performed simultaneously and the first and second electrodes 104, 108 are operated in the forward direction. The brine and fresh water solutions are produced in the second compartment 115 and the first compartment 113, respectively, when blocks 604 and 606 are performed simultaneously and the first and second electrodes 104, 108 are operated in the reverse direction. The brine water solution has a higher concentration of dissolved salts 124, 128 than the concentration of dissolved salts in the salt water solution initially charged to the first compartment 113. The fresh water solution has a lower concentration of dissolved salts 124, 128 than the concentration of dissolved salts in the salt water solution initially charged to the second compartment 115.

[0035] The method 600 further includes separately extracting the brine water solution and the freshwater solution from the first and second compartments 113, 115 (block 608). Extracting the brine and freshwater solutions from the container 102 may occur when an amount of the cations 124 introduced into the first or second electrodes 104, 108 approaches or reaches the capacity of the electrode. If another cleaning cycle is to be performed (block 610), the method 600 returns to block 602, where the first and second compartments 113, 115 of the container 102 are filled with the salt water solution. If another cleaning cycle is not performed (block 610), the method 600 ends (block 612).

[0036] In some embodiments, the method 600 may include a cleaning operation after extracting the brine and freshwater solutions (block 608) and before refilling the first and second compartments 113, 115 in preparation for a subsequent cleaning cycle (block 602). The cleaning operation cleans the first and second electrodes 104, 108 and the AEM 112 by immersing the electrodes and the AEM in clean water, acid (i.e., HCl), or alkali (i.e., NaOH). The cleaning operation may occur at regular intervals or as needed. Upon completion of the cleaning operation, the method 600 may end (block 612) or perform another cleaning cycle following blocks 602 through 608.

Claims

[1] Desalination device comprising: a container configured to contain a salt water solution having an increased concentration of dissolved salts; a first electrode and a second electrode disposed in a first compartment and a second compartment of the container, respectively, the first and second electrodes being configured to be in fluid communication with the salt water solution; an anion exchange membrane (AEM) separating the first and second compartments, the AEM having a porous membrane structure and a plurality of negatively charged oxygen functional groups coupled to the porous membrane structure; and an energy source configured to selectively apply a voltage to one of the first or second electrodes, wherein the AEM has a selective permeability when the voltage is applied such that cations in the salt water solution have a first diffusion rate d1 through the membrane and anions in the salt water solution have a second diffusion rate d2 through the membrane, where 0 ≤ d1 < d2, and wherein the energy source is configured to apply a membrane voltage to the AEM via an external circuit, and the membrane voltage is configured to decrease the first diffusion rate d1 of the cations through the AEM and increase the second diffusion rate d2 of the anions through the AEM. [2] The desalination device according to claim 1, wherein the porous membrane structure comprises graphene oxide sheets. [3] The desalination device according to claim 2, wherein the plurality of negatively charged oxygen functional groups comprise one or more hydroxyl groups and carboxyl groups. [4] The desalination device according to claim 2, wherein the plurality of negatively charged oxygen functional groups comprise one or more epoxy groups and carbonyl groups. [5] The desalination device according to claim 2, wherein the porous membrane structure comprises a mixture of the graphene oxide sheets and at least one other polymer that is electrically conductive. [6] The desalination device of claim 5, wherein the at least one further polymer comprises one or more of polyethylene oxide (PEO), poly(pyrrole)ene (PPY), polyaniline (PANI), poly(thiophene)ene (PT), poly(3,4-ethylenedioxythiophene) (PEDOT), poly(p-phenylene sulfide) (PPS), poly(acetylene)ene (PAC), and poly(p-phenylenevinylene) (PPV). [7] The desalination device according to claim 2, wherein the porous membrane structure comprises a mixture of the graphene oxide sheets and at least one other polymer that is ionically conductive and not electronically conductive. [8] The desalination device of claim 7, wherein the at least one further polymer comprises one or more of cross-linked polyvinyl alcohol (PVA), cross-linked polymethyl methacrylate (PMMA), and polyphenylene vinylene (PPV). [9] The desalination apparatus according to claim 7, wherein the porous membrane structure comprises one or more of graphite, hard carbon, soft carbon and carbon black. [10] Desalination device according to claim 1, wherein the membrane voltage is applied in a range of 0 to ± 0.5 V. [11] Desalination device according to claim 1, wherein a (de)sorption voltage of the cations in and out of the AEM is in a range of -1.0 to 1.5 V relative to a standard hydrogen electrode. [12] The desalination device according to claim 1, wherein the porous membrane structure comprises graphene with one or a few layers, the layers being in the range of 1 to 8 layers. [13] Desalination device according to claim 1, wherein the first and second electrodes are symmetrical electrodes containing an identical intercalation material. [14] Desalination device according to claim 13, wherein the intercalation material is one of doped or undoped cubic spinel MnO2, tunnel-structured orthorhombic Na4Mn9O 18 , or NaM2(PO4)3, where M = Ti, Mn, Fe, Ni, Cu, or combinations thereof. [15] The desalination apparatus according to claim 1, wherein the first and second electrodes are asymmetric electrodes each including an electrode material consisting of a metal oxide, wherein a concentration of alkali metal within the metal oxide is configured to differ between the electrode materials. [16] A process for desalinating water, comprising: Filling the first and second compartments of a container with a salt water solution having an increased concentration of dissolved salts, the container comprising first and second electrodes disposed in the first and second compartments, respectively, and an anion exchange membrane (AEM) separating the first and second compartments; Operating the first and second electrodes in a forward direction or a reverse direction to release cations into one of the first and second compartments and to remove cations from the other of the first and second compartments; and Applying a membrane voltage to the AEM during operation of the first and second electrodes, wherein the membrane voltage decreases a first diffusion rate of cations through the AEM relative to the first and second diffusion rates, respectively, and increases a second diffusion rate of anions through the AEM when the membrane voltage is not applied to the AEM.

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