Method for splitting salts and electrodialysis cell

Abstract

A method for splitting salts comprises the steps of: (i) providing an electrodialysis cell comprising an anode compartment housing an anode and a solution of a redox-active electrolyte material which can exist as a reduced and oxidized species; a cathode compartment housing a cathode and a solution of the redox-active electrolyte material; at least one unit cell in sequence from a first to a last unit cell, each unit cell comprising a feed compartment, an acid compartment housing an aqueous acid and a base compartment housing an aqueous base, the feed compartment separated from the compartment adjacent to the feed compartment in a direction towards the anode by an anion exchange membrane, and separated from the compartment adjacent to the feed compartment in a direction towards the cathode by a cation exchange membrane; a bipolar membrane disposed between the base compartment and the acid compartment of the unit cell; (II) introducing an aqueous solution of a salt having a cation and an anion component into the feed compartments; introducing a dilute aqueous acid solution into the acid compartment; and introducing a dilute aqueous base solution into the base compartment; (ill) imposing a voltage across the anode and the cathode to generate an oxidized first redox-active electrolyte material at the anode, and a reduced second redox- active electrolyte material at the cathode; and (iv) withdrawing a concentrated aqueous acid solution from the acid compartment; and withdrawing a concentrated aqueous base solution from the base compartment; wherein the anion component of the salt is transported across the anion exchange membranes from the feed compartments into the acid compartments adjacent to the feed compartments to form an acid of the anion component of the salt by protons supplied across the bipolar membranes; and the cation component of the salt is transported across the cation exchange membranes from the feed compartments to the base compartments adjacent to the feed compartments to form a base of the cation component of the salt by hydroxyl groups supplied across the bipolar membranes. The energy consumption of the process is lower than with state-of-the-art electrodialysis techniques as it avoids hydrogen and oxygen evolution reactions on the electrodes. The salt splitting process also dispenses with the use of expensive electrocatalysts.

Description

[0001] Method for Splitting Salts and Electrodialysis Cell

[0002] The present invention relates to a method for splitting salts by electrodialysis and an electrodialysis cell for splitting salts.

[0003] Salt splitting by electrodialysis is a useful technique for recovering acid and base values from salt streams. Currently, electrodialysis is carried out in stacks with alternating ion-selective membranes, sandwiched between electrodes. A hydrogen evolution reaction proceeds on the anode, and oxygen evolution reaction proceeds on the cathode. This creates a charge imbalance at the electrodes that is balanced by the movement of ions through strategically placed ion-selective membranes. However, water splitting involves an energetic penalty because energy is required to do so. Due to non-zero overpotentials for both reactions, an overall cell potential is higher than the thermodynamic value for water splitting (1.23 V). Therefore, the current electrodialysis process has a relatively high energy demand. Moreover, oxygen gas generated at the anode is highly destructive and requires the use of platinum / iridium-plated electrodes.

[0004] According to Kuldeep et al., Membranes 2021 , 11 (9), 718, in the splitting of sodium sulfate into sodium hydroxide and sulfuric acid by bipolar membrane electrodialysis, roughly 1.4 kWh of electrical energy is required to split 1 kg of Na2SC>4 (https: / / doi.org / 10.3390 / membranes11090718). This high energy consumption leads to high operating expense costs. Moreover, expensive platinum coated titanium is used as electrode material for the water splitting, which increases capital expenditure costs as well.

[0005] Digdaya et al., Nature Communications 2020, 11 , 4412, describe that using redox couple (such as hexacyanoferrate) for the electrochemical recovery of CO2 from seawater allows to significantly decrease energy consumption, compared to state-of-the-art electrodialysis techniques (https: / / doi.org / 10.1038 / s41467-020-18232-y). Moreover, US 2019 / 240623 describes an electrochemical cell, containing cation and anion exchange membranes for the desalination of salt water. By using redox couples, i.e., iron complexes, such as ferrocene derivatives, energy consumption for the desalination can be significantly lowered.

[0006] US 2022 / 0144673 discloses electrodialyzers suitable for capturing CO2 from ocean water. In an embodiment the electrodialyzer includes a stack having one or more multi-compartment cells. Each of the cells includes a first compartment (compartment A), a second compartment (compartment B), and a third compartment (compartment C). An anion exchange membrane (AEM) separates the first compartment and the second compartment, and a bipolar membrane (BPM) separates the second compartment and the third compartment. The electrodialyzer further includes end electrodes at either end of the cell stack. A first monovalent cation exchange membrane (M-CEM) separates the catholyte compartment and the first compartment of cell 1 . A second M-CEM separates the anolyte compartment and the third compartment of the nth cell. One or more intermediate M-CEMs separate the cells from their adjacent neighboring cells, provided there is more than one cell in the electrodialyzer. At the end electrodes, instead of a water-splitting reaction, one-electron, reversible redox couple electrolytes may be used.

[0007] The operating principle for ocean water capture of CO2 is to push the CO2 / bicarbonate equilibrium toward dissolved CO2 by acidifying the ocean water via electrodialysis. At the same time, a solution in the second compartment is basified. Therefore, electrodialysis in US 2022 / 0144673 is used to perform a process known as "pH swing”. Low operational current densities have been used to that effect.

[0008] As ion transport rate is directly proportional to the current density, sufficient high current densities are desirable in salt splitting by electrodialysis. However, it has not yet been established whether high current densities are compatible with the use of reversible redox couple electrolytes in view of their cyclic stability and redox kinetics.

[0009] The invention seeks to provide a salt splitting process for which energy consumption is lower than with state- of-the-art electrodialysis techniques. The invention further seeks to provide a salt splitting process which can dispense with the use of expensive electrocatalysts, thus lowering stack costs.

[0010] The invention provides a method for splitting salts, which comprises the steps of:

[0011] (I) providing an electrodialysis cell comprising an anode compartment housing an anode and a solution of a redox-active electrolyte material which can exist as a reduced and oxidized species; a cathode compartment housing a cathode and a solution of the redox-active electrolyte material; at least one unit cell in sequence from a first to a last unit cell, each unit cell comprising a feed compartment, an acid compartment housing an aqueous acid and a base compartment housing an aqueous base, the feed compartment separated from the compartment adjacent to the feed compartment in a direction towards the anode by an anion exchange membrane, and separated from the compartment adjacent to the feed compartment in a direction towards the cathode by a cation exchange membrane; a bipolar membrane disposed between the base compartment and the acid compartment of the unit cell; the electrodialysis cell having either configuration A or C:

[0012] A: the compartment adjacent to the feed compartment in a direction towards the anode is an acid compartment; and the compartment adjacent to the feed compartment in a direction towards the cathode is a base compartment for all unit cells but the last and is the cathode compartment for the last unit cell; the anode compartment is adjacent to the base compartment of the first unit cell, and the anode compartment is separated from the base compartment by a cation exchange membrane;

[0013] C: the compartment adjacent to the feed compartment in a direction towards the anode is an acid compartment for all unit cells but the first and is the anode compartment for the first unit cell; and the compartment adjacent to the feed compartment in a direction towards the cathode is a base compartment; the cathode compartment is adjacent to the acid compartment of the last unit cell, and the acid compartment is separated from the cathode compartment by an anion exchange membrane; (ii) introducing an aqueous solution of a salt having a cation and an anion component into the feed compartments; introducing a dilute aqueous acid solution into the acid compartment; and introducing a dilute aqueous base solution into the base compartment;

[0014] (ill) imposing a voltage across the anode and the cathode to generate an oxidized first redox-active electrolyte material at the anode, and a reduced second redox-active electrolyte material at the cathode; and

[0015] (iv) withdrawing a concentrated aqueous acid solution from the acid compartment; and withdrawing a concentrated aqueous base solution from the base compartment; wherein the anion component of the salt is transported across the anion exchange membranes from the feed compartments into the acid compartments adjacent to the feed compartments to form an acid of the anion component of the salt by protons supplied across the bipolar membranes; and the cation component of the salt is transported across the cation exchange membranes from the feed compartments to the base compartments adjacent to the feed compartments to form a base of the cation component of the salt by hydroxyl groups supplied across the bipolar membranes.

[0016] The invention further relates to electrodialysis cells useful in the aspects of the invention.

[0017] In a still further aspect the method comprises providing a plurality of electrodialysis cells, wherein the cathode and the anode of two juxtaposed electrodialysis cells constitute a bipolar electrode, electrically joining an electrodialysis cell to the other electrodialysis cells in said plurality of electrodialysis cells. The bipolar plates are externally connectable intermediate taps between the terminal anode and cathode. Voltage is imposed across the anode and the first bipolar electrode, two consecutive bipolar electrodes, and the last bipolar electrode and the cathode. In this aspect, the individual electrodialysis cells preferably comprise only one unit cell each.

[0018] Advantages of the new process include:

[0019] - lower energy consumption;

[0020] - less expensive electrode material, e.g., graphite instead of platinum; and

[0021] - no handling of the hydrogen gas byproduct is required.

[0022] A "unit cell” denotes a membrane assembly which may be stacked in a repetitive manner defining compartments between membranes. The proposed electrodialysis cell comprises repeating unit cell comprising a feed compartment, an acid compartment housing an aqueous acid and a base compartment housing an aqueous base. The feed compartment is separated from the adjacent compartments by ion exchange membranes. More specifically, the feed compartment is separated from the compartment adjacent to the feed compartment in a direction towards the anode by an anion exchange membrane, the adjacent compartment being either one of an acid compartment or the anode compartment, and separated from the compartment adjacent to the feed compartment in a direction towards the cathode by a cation exchange membrane, the adjacent compartment being either one of a base compartment or the cathode compartment. A bipolar membrane is disposed between the base compartment and the acid compartment of the unit cell. An arbitrary number of repeating unit cells can be employed, such as 1 to 200. Preferably 5 to 100 of repeating unit cells are used. The spaces between membranes form compartments. An aqueous salt solution, base solution or acid solution is flowed through the respective compartments.

[0023] A solution of a redox-active electrolyte material is used as anolyte and catholyte in the anode compartment or cathode compartment, respectively. A redox-active electrolyte material or redox couple can exist as a reduced and oxidized species.

[0024] The repeating unit cells are grouped together in an electrodialysis cell with a single pair of electrodes at the outer ends. The electrodialysis cell comprises an anode compartment and a cathode compartment. In order to accommodate whether a positively charged redox shuttle or a negatively charged redox shuttle is used, the compartments adjacent to the anode compartment and cathode compartment, respectively, are preferably different.

[0025] In an embodiment, the anode compartment is adjacent to the base compartment of the first unit cell, the anode compartment separated from the base compartment by a cation exchange membrane; the cathode compartment is adjacent to the feed compartment of the last unit cell; and the redox-active electrolyte material is configured to drive a cation component from the anode compartment and to accept a cation component in the cathode compartment.

[0026] Useful redox-active electrolyte materials may be selected from Fe2+ / Fe3+redox couple, such as hexacyanoferrate(ll) I hexacyanoferrate(lll); Zn2+ / Zn redox couple; p-quinone compounds, such as quinones, naphthaquinones, anthraquinones; o-quinone compounds; thiazine compounds; NO radicals, such as (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, 5, 5-dimethyl-1 -pyrroline N-oxide, phthalimide N-oxyl; alloxane compounds; flavine compounds; phenazine compounds.

[0027] In another embodiment, the anode compartment is adjacent to the feed compartment of the first unit cell; the cathode compartment is adjacent to the acid compartment of the last unit cell, the acid compartment separated from the cathode compartment by an anion exchange membrane; and the redox-active electrolyte material is configured to drive an anion component from the cathode compartment and to accept an anion component in the anode compartment.

[0028] Useful redox-active electrolyte materials may be a selected from ferrocene derivatives; pyridinium compounds; FeCl2 / FeCh redox couple; pyridinium derivatives, 4,4'-bipyridyl compounds such as viologens; thiazine compounds; flavine compounds; NO radicals, such as (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, 5, 5-dimethyl-1 -pyrroline N-oxide, phthalimide N-oxyl; alloxane compounds; flavine compounds; quinone compounds, such as p-quinones, naphthaquinones, anthraquinones; o-quinone compounds; phenazine compounds.

[0029] Examples of ferrocene derivatives include (bis(q5-cyclopentadienyl)iron) or a derivative thereof, including, e.g., functionalised derivatives such as ferrocene monocarboxylic acid, 1 ,1 '-bis(trimethylammoniopropyl)- ferrocene dichloride (BTMAP-Fc) or polymeric forms (polyferrocenes). Viologens are derivatives of 4,4'-bipyridyl. Examples of viologen are methyl viologen, ethyl viologen, and benzyl viologen. These viologens have been isolated as, e.g., chloride, acetate, tetraflurorborate, perchlorate, triflurormethanesulfonate, tetraphenylborate, and hexaflurophosphate salts.

[0030] A preferred redox-active electrolyte material is a hexacyanoferrate(ll) and hexacyanoferrate(lll) mixture. The redox-active electrolyte material is dissolved in water in overall concentration (reduced and oxidized form) 0.1 M to 0.6 M, but preferably 0.1 M to 0.4 M. In order to minimize concentration overpotentials, the overall concentration of the redox-active electrolyte material of the solution housed in the anode compartment and cathode compartment, respectively, is preferably at least 0.2 M, preferably at least 0.4 M.

[0031] Useful cation exchange membranes may comprise, e.g., sulfonic acid groups. The cation exchange membrane should be stable and have a low resistance to the cation being transported. Representative examples of useful cation exchange membranes, in addition to the sulfonic acid membranes, include perfluorinated radiation grafted materials, such as Pall Raipore or Solvay Morgane products and fully perfluorinated sulfonic acid cation exchange membranes, such as those available from DuPont under the Nation Trademark, such as the Nation 300 and 400 series membranes. Cation exchange membranes for use herein also include membranes under the FUMASEP® trade designation, such as FUMASEP® FKS and FKE. Suitable cation exchange membranes can be obtained from Tokuyama such as Tokuyama CMX, CMS and CMB.

[0032] Representative anion exchange membranes include polystyrene-polydivinyl-benzene polymeric base materials, such as Tokuyama Neosepta AMH or Asahi Glass Selemion AMV, and perfluorinated radiation grafted materials, such as Pall Raipore. Solvay Morgane products may also be used. Anion exchange membranes for use herein include membranes under the FUMASEP® trade designation, such as FUMASEP® FAS and FAB. Suitable anion exchange membranes also include Tokuyama NEOSEPTA® membranes such as Tokuyama AMX, AMH and ACM.

[0033] The term "bipolar membrane” as used in this application means a functional membrane through which only hydrogen ions are permeable and other cations or anions are impermeable, and which is a hybrid membrane composed of laminated cation-exchange membrane and anion-exchange membrane. When an electric potential gradient is applied to the bipolar membrane, water is decomposed to form protons and hydroxyl groups, and the protons and hydroxyl groups move towards the cathode side and the anode side, respectively. The protons make the aqueous solution in the acid compartment (more) acidic, and the hydroxyl groups make the aqueous solution in the base compartment (more) alkaline. As examples of commercially available bipolar membranes may be mentioned Selemion HSV (manufactured by Asahi Glass Co., Ltd.), Neosepta BP1 E (manufactured by Tokuyama Corporation), and Fumasep FBM (manufactured by FuMA-Tech GmbH).

[0034] Electrodes, i.e., anode and cathode, can be made of different materials, such as platinum, stainless steel, titanium, platinum on carbon / graphite, graphite or iron. In view of the lower capital expenditure costs, electrodes made of graphite or carbon are preferably used.

[0035] During the electrodialysis process, electrolyte solution which is a solution of redox-active electrolyte material is flowed through the electrode compartments. Generally, the first solution and the second solution of redox- active electrolyte material are the same and the first and second solutions are circulated between the anode compartment and the cathode compartment during the operation mode.

[0036] During the electrodialysis process, an aqueous solution of a salt having a cation and an anion component is introduced into the feed compartments. The aqueous solution may contain a plurality of cation species and a plurality of anion species. Alternatively, the aqueous solution may contain substantially only a single cation species paired with a plurality of anion species, or vice versa. However, aqueous solutions containing substantially only a single cation species paired with a substantially only a single anion species are generally preferred. "Substantially only a single species” means that the species accounts for at least 95% by charge of the cation or anion species, respectively, present in the solution. Preferably, the aqueous solution of a salt is essentially neutral, meaning that its pH value is in the range from 4 to 10, preferably, 5 to 9, in particular 6 to 8. In certain embodiments, the aqueous solution of a salt is essentially free of dissolved CO2 and / or HCOr.

[0037] Preferably, the cation component is monovalent and does not form an insoluble base. Preferred cation components are ammonium and alkali metal ions, in particular lithium, sodium, potassium, rubidium and cesium. Preferred anion components are sulfate, phosphate and halogenides, in particular chloride, bromide and iodide. In certain embodiments, the salt is selected from sodium chloride and sodium sulfate.

[0038] From the feed compartment, a solution of reduced or zero salinity is withdrawn.

[0039] Diluted base (NaOH, KOH, etc.) and acid (H2SO4, HOI, etc.) solutions are used as a feed for basic and acidic compartment, respectively. In order to produce a commercially useful concentrated aqueous base solution, a dilute aqueous base solution containing substantially only a single cation species (which cation species corresponds to the cation component of the salt solution) is used. Similarly, a dilute aqueous acid solution containing substantially only a single anion species (which anion species corresponds to the anion component of the salt solution) is preferably used. Suitably, the concentration of the dilute aqueous acid solution or dilute aqueous base solution, respectively, is 0.1 to 5 wt.-%, preferably 0.5 to 2 wt.-%.

[0040] A concentrated aqueous acid solution is withdrawn from the acid compartment; and a concentrated aqueous base solution is withdrawn from the base compartment. A "concentrated aqueous acid solution” is intended to mean that the molar concentration of anions (and H+counterion) is larger than in the dilute solution fed to the acid compartment. Likewise, a "concentrated aqueous base solution” is intended to mean that the concentration of cations (and OH’ counterion) is larger than in the dilute solution fed to the base compartment. Suitably, the concentration of the concentrated aqueous acid solution or concentrated aqueous base solution, respectively, is 5 to 20 wt.-%, preferably 5 to 10 wt.-%.

[0041] An appropriate current is applied to the cell, voltage is measured. Energy consumption may be calculated from the measured voltage value. Generally, the current density is at least 10 mA / cm2, preferably at least 20 mA / cm2, for example at least 40 mA / cm2, and may be up to 80 mA / cm2or to 100 mA / cm2.

[0042] The invention is further illustrated by the appended drawing and the examples that follow.

[0043] Fig. 1 illustrates a redox-assisted electrodialysis system useful in the process of the invention. Fig. 2 illustrates the ion movements across membranes in the electrodialysis system of Fig. 1.

[0044] Fig. 1 illustrates a redox-assisted electrodialysis system with a negatively charged redox shuttle (i.e., hexacyanoferrate (II) and hexacyanoferrate (III) mixture).

[0045] The system comprises an anode compartment (An.), a cathode compartment (Cat.) and two repeating unit cells. Repeating unit cells comprise a feed compartment (F), an acid compartment (A) and a base compartment (B). Bipolar membranes (BPM) separate an acid compartment (A) from an adjacent base compartment (B).

[0046] The anode compartment (An.) and cathode compartment (Cat.) house an anode or cathode, respectively, and a solution of a redox-active electrolyte material. Movement of the redox-active electrolyte material from the anode compartment (An.) to the cathode compartment (Cat.) is not shown.

[0047] The feed compartment (F) adjacent to the cathode compartment is separated from the cathode compartment (Cat.) by a cation exchange membrane (CEM) and is separated from the adjacent acid compartment (A) by an anion exchange membrane (AEM). The base compartment (B) adjacent to the anode compartment (An) is separated from the anode compartment (An.) by a cation exchange membrane (CEM). The feed compartment (F) between an acid compartment (A) and a base compartment (B) is separated from the acid compartment (A) by an anion exchange membrane (AEM) and is separated from the base compartment (B) by a cation exchange membrane (CEM).

[0048] Fig. 2 illustrates the ion movements across membranes in the electrodialysis system. Aqueous salt solution (i.e., sodium sulfate) is flowed into the feed compartments (F). Sulfate and sodium ions cross membranes AEM and CEM to enter the acid stream or base stream, respectively, in the acid compartments (A) or base compartments (B) while protons or hydroxyl groups are supplied across the bipolar membranes.

[0049] Examples

[0050] In the following examples, energy consumption between a comparative process (examples 1 and 3) and the process of the invention (examples 2 and 4) is compared.

[0051] Example 1 : Electrodialysis with sodium sulfate as electrolyte and metal electrodes

[0052] An electrodialysis cell is assembled with 10 repeating unit cells, analogously to Figure 1. The anode is made of nickel, the cathode is made of stainless steel. The anion-exchange membranes are Fumasep FAB-PK-130, the cation-exchange membranes are Fumasep FKB-PK-130 and the bipolar membranes are Fumasep FBM-PK. The effective membrane area is 100 cm2and the overall membrane area is 1000 cm2.

[0053] A solution of 5 wt.-% Na2SO4 is flowed through the cathode and anode compartments. A Na2SO4 solution (15 wt.-%) is used as feed for the feed compartment, whereas diluted NaOH and H2SO4 solutions (0.5 wt.- %) are used as feed solutions which were cycled through the basic and acidic compartments, respectively. All solutions were kept at constant temperature of 30 °C. A constant current of 40 mA / cm2is applied to the cell, and the voltage is measured. Conductivities and pH values of basic, acidic and salt solutions are continuously monitored. Aliquots from the basic and acidic compartments are regularly taken and titrated. Energy consumption is calculated from the averaged voltage.

[0054] Example 2: Electrodialysis with hexacyanoferrate redox couple as electrolyte and metal electrodes

[0055] An electrodialysis cell is assembled in the same way as described in Example 1. A solution of 0.2 M K3[Fe(CN)e] and 0.2M K4[Fe(CN)e] is used as an electrolyte for the electrode cycle, instead of Na2SO4 solution. All solutions were kept at a constant temperature of 30 °C. A constant current of 40 mA / cm2is applied to the cell, and voltage is measured. Conductivities and pH values of basic, acidic and salt solutions are continuously monitored. Aliquots from the basic and acidic compartments are regularly taken and titrate. Energy consumption is calculated from the averaged voltage.

[0056] The pH value of the K3[Fe(CN)6] / K4[Fe(CN)e] electrolyte after the experiment was 3.4.

[0057] The results of Examples 1 and 2 are shown in Figs. 3 to 6 (Example 1 : dotted line or dashed bar; Example 2: solid line or solid bar).

[0058] Fig. 3 shows the required voltage to attain a current density of 40 mA / cm2. Evidently, a considerably lower voltage is required with K3[Fe(CN)6] / K4[Fe(CN)e] as electrolyte.

[0059] Figs. 4A and 4B show the concentration development of H2SO4 and NaOH in the acid or base compartment. No significant differences are seen between Examples 1 and 2.

[0060] Fig. 5 shows the current yields. Higher current yields are observed with the K3[Fe(CN)e] / K4[Fe(CN)6 electrolyte. Current yields and conversion to NaOH are higher than to H2SO4. This is due to loss of H2SO4 via the salt cycle (2.0 or 1.0 wt.-% in Example 1 and 2, respectively).

[0061] Fig. 6 shows the energy consumption per kg of split Na2SO4. Evidently, energy consumption after 3.5 h is about 20% lower with K3[Fe(CN)e] / K4[Fe(CN)6 electrolyte.

[0062] Example 3: Electrodialysis with sodium sulfate as electrolyte and graphite electrodes

[0063] An electrodialysis cell is assembled in the same way as described in Example 1 , except electrodes are made of graphite.

[0064] Example 4: Electrodialysis with hexacyanoferrate redox couple as electrolyte and graphite electrodes

[0065] An electrodialysis cell is assembled in the same way as described in Example 2, except electrodes are made of graphite. A solution of 0.2 M Na3[Fe(CN)e] and 0.2M Na4[Fe(CN)e] is used as an electrolyte for the electrode cycle, instead of 0.5 M Na2SC>4 solution.

Claims

Claims1 . A method for splitting salts, which comprises the steps of:(I) providing an electrodialysis cell comprising an anode compartment housing an anode and a solution of a redox-active electrolyte material which can exist as a reduced and oxidized species; a cathode compartment housing a cathode and a solution of the redox-active electrolyte material; at least one unit cell in sequence from a first to a last unit cell, each unit cell comprising a feed compartment, an acid compartment housing an aqueous acid and a base compartment housing an aqueous base, the feed compartment separated from the compartment adjacent to the feed compartment in a direction towards the anode by an anion exchange membrane, and separated from the compartment adjacent to the feed compartment in a direction towards the cathode by a cation exchange membrane; a bipolar membrane disposed between the base compartment and the acid compartment of the unit cell; the electrodialysis cell having either configuration A or C:A: the compartment adjacent to the feed compartment in a direction towards the anode is an acid compartment; and the compartment adjacent to the feed compartment in a direction towards the cathode is a base compartment for all unit cells but the last and is the cathode compartment for the last unit cell; the anode compartment is adjacent to the base compartment of the first unit cell, and the anode compartment is separated from the base compartment by a cation exchange membrane;C: the compartment adjacent to the feed compartment in a direction towards the anode is an acid compartment for all unit cells but the first and is the anode compartment for the first unit cell; and the compartment adjacent to the feed compartment in a direction towards the cathode is a base compartment; the cathode compartment is adjacent to the acid compartment of the last unit cell, and the acid compartment is separated from the cathode compartment by an anion exchange membrane;(II) introducing an aqueous solution of a salt having a cation and an anion component into the feed compartments; introducing a dilute aqueous acid solution into the acid compartment; and introducing a dilute aqueous base solution into the base compartment;(ill) imposing a voltage across the anode and the cathode to generate an oxidized first redox-active electrolyte material at the anode, and a reduced second redox-active electrolyte material at the cathode; and(iv) withdrawing a concentrated aqueous acid solution from the acid compartment; and withdrawing a concentrated aqueous base solution from the base compartment; wherein the anion component of the salt is transported across the anion exchange membranes from the feed compartments into the acid compartments adjacent to the feed compartments to form an acid of the anion component of the salt by protons supplied across the bipolar membranes; and the cation component of the salt is transported across the cation exchange membranes from the feed compartments to the base compartments adjacent to the feed compartments to form a base of the cation component of the salt by hydroxyl groups supplied across the bipolar membranes.

2. The method of claim 1 , wherein the electrodialysis cell has configuration A and the redox-active electrolyte material is configured to drive a cation component from the anode compartment and to accept a cation component in the cathode compartment.

3. The method of claim 2, wherein the redox-active electrolyte material is selected from Fe2+ / Fe3+redox couple, such as hexacyanoferrate(ll) / hexacyanoferrate(lll); Zn2+ / Zn redox couple; p-quinone compounds, such as quinones, naphthaquinones, anthraquinones; o-quinone compounds; thiazine compounds; NO radicals, such as (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, 5, 5-dimethyl-1 -pyrroline N-oxide, phthalimide N-oxyl; alloxane compounds; flavine compounds; phenazine compounds.

4. The method of claim 1, wherein the electrodialysis cell has configuration C and the redox-active electrolyte material is configured to drive an anion component from the cathode compartment and to accept an anion component in the anode compartment.

5. The method of claim 4, wherein the redox-active electrolyte material is selected from ferrocene derivatives, FeC^ / FeCh redox couple, pyridinium compounds, such as pyridinium derivatives, 4,4'-bipy ridy I compounds (viologens); thiazine compounds; NO radicals, such as (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, 5, 5-dimethyl-1 -pyrroline N-oxide, phthalimide N-oxyl; alloxane compounds; flavine compounds; quinone compounds, such as p-quinones, naphthaquinones, anthraquinones; o-quinone compounds; phenazine compounds.

6. The method of any one of the preceding claims, wherein the solution of the redox-active electrolyte material is circulated between the anode compartment and the cathode compartment during the operation mode.

7. The method of any one of the preceding claims, comprising providing a plurality of electrodialysis cells, wherein the cathode and the anode of two juxtaposed electrodialysis cells constitute a bipolar electrode, electrically joining an electrodialysis cell to the other electrodialysis cells in said plurality of electrodialysis cells.

8. The method of any one of the preceding claims, wherein the cation component of the salt is selected from alkali metal ions, in particular from lithium, sodium, potassium, rubidium and cesium.

9. The method of any one of the preceding claims, wherein the anion component of the salt is selected from halogenides, sulfate and phosphate.

10. The method of any one of the preceding claims, wherein the anode and the cathode are made of carbon.11 . The method of any one of the preceding claims, wherein the concentration of the redox-active electrolyte material of the solution housed in the anode compartment housing and cathode compartment, respectively, is at least 0.2 M.

12. The method of any one of the preceding claims, wherein the current density is at least 20 mA / cm2.

13. An electrodialysis cell for splitting salts, comprising an anode compartment housing an anode and a solution of a redox-active electrolyte material; a cathode compartment housing a cathode and a solution of the redox-active electrolyte material; at least one unit cell in sequence from a first to a last unit cell, each unit cell comprising a feed compartment, an acid compartment housing an aqueous acid and a base compartment housing an aqueous base, the feed compartment separated from the compartment adjacent to the feed compartment in a direction towards the anode by an anion exchange membrane, the adjacent compartment being either one of an acid compartment or the anode compartment, and separated from the compartment adjacent to the feed compartment in a direction towards the cathode by a cation exchange membrane, the adjacent compartment being either one of a base compartment or the cathode compartment; and a bipolar membrane disposed between the base compartment and the acid compartment of the unit cell the electrodialysis cell having either configuration A or C:A: the compartment adjacent to the feed compartment in a direction towards the anode is an acid compartment; and the compartment adjacent to the feed compartment in a direction towards the cathode is a base compartment for all unit cells but the last and is the cathode compartment for the last unit cell; the anode compartment is adjacent to the base compartment of the first unit cell, and the anode compartment is separated from the base compartment by a cation exchange membrane;C: the compartment adjacent to the feed compartment in a direction towards the anode is an acid compartment for all unit cells but the first and is the anode compartment for the first unit cell; and the compartment adjacent to the feed compartment in a direction towards the cathode is a base compartment; the cathode compartment is adjacent to the acid compartment of the last unit cell, and the acid compartment is separated from the cathode compartment by an anion exchange membrane.

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