Bipolar membrane electrodialysis apparatus and method

Abstract

An electrochemical apparatus comprising: an anode; a bipolar membrane; an anion exchange membrane; a cation exchange membrane; and a cathode; wherein: the anode at least partially defines a first fluid flow path; the anode and / or the first fluid flow path is in contact with the anion exchange membrane of the bipolar membrane; a second fluid flow path is defined between the cation exchange membrane of the bipolar membrane and the anion exchange membrane; a third fluid flow path is defined between the anion exchange membrane and the cation exchange membrane; the cathode at least partially defines a fourth fluid flow path; and the cation exchange membrane is in contact with the cathode and / or the fourth fluid flow path.

Description

[0001] BIPOLAR MEMBRANE ELECTRODIALYSIS APPARATUS AND METHOD

[0002] Field of the Invention

[0003] The present invention relates to an electrochemical apparatus that can be used to produce sodium hydroxide, hydrochloric acid, partially de-ionized water, hydrogen and oxygen from a brine stream. The present invention also relates to a method of electrodialysis employing the electrochemical apparatus.

[0004] Background of the Invention

[0005] Marine carbon dioxide removal (mCDR) technologies such as ocean alkalinity enhancement (OAE), direct ocean capture (DOC), and brine mineralization require NaOH and / or HC1. In order to make mCDR technologies sustainable, the limitations of traditional electrochemical technologies need to be addressed in order to efficiently produce appropriate amounts of NaOH and / or HC1.

[0006] Bipolar membrane electrodialysis (ED) is commercially deployed to generate an acid, base, and potable water stream from a salt-water feed. ED apparatuses use many ionexchange membrane pairs. The use of many membranes increases cell resistance, requiring high operating voltages and allowing low limiting current densities. The slow rate of base generation can mean that several systems must be operated simultaneously to maintain a useable output feed alkalinity and / or acidity for mCDR applications.

[0007] Chlor-alkali electrolysis (CA) is the predominant technology for global NaOH production. CA uses a high-current saline electrolysis system which most commonly involves a single cation exchange membrane separating anode from cathode. NaOH can be efficiently produced via CA. However, CA also generates chlorine gas in large quantities, which necessitates expensive specialised equipment for handling and monitoring chlorine gas. Furthermore, CA typically requires expensive “dimensionally stable” anodes based on IrOx and / or RuOx coated on platinized titanium due to the high acidity and chloride content that CA anodes are exposed to. As result, CA plants require high capital expenditure, and plant sizes must be sufficiently large to unlock economic viability. This required economy of scale is inappropriate in many use-cases, such as mCDR where alkalinity addition must be carefully controlled. High-current water electrolysis (WE) produces hydrogen and oxygen and therefore cannot support mCDR with its outputs alone. Current densities up to two orders of magnitude higher than ED can be achieved with high-purity feed water, with operating voltages lower than CA. However, WE systems are incredibly sensitive to the introduction of oxidizable anions like chloride to the feed, as rapid generation of corrosive CE, HOC1, and / or OCf can quickly corrode anode and membrane constituents — deactivating the device and increasing costs. Furthermore, high-current WE systems suffer from nearly immediate failure when operating with naturally extracted NaCl solutions due to the deposition of magnesium hydroxide and calcium hydroxide on the cathode surface. Water softening is therefore commonly required in saline water treatment technologies.

[0008] Membranes conventionally used in electrochemical cells include sulfonated tetrafluoroethylene based fluoropolymer-copolymer (e.g. Nafion) membranes. Alternative solutions that can avoid these polymers are desirable due to these polymers being fluorinated, which provides environmental and legislative concerns, as well as their high cost of production relative to non-fluorinated alternatives.

[0009] The present invention has been devised with the foregoing in mind.

[0010] Summary of the Invention

[0011] According to a first aspect, the claimed invention provides an electrochemical apparatus comprising: an anode; a bipolar membrane; an anion exchange membrane; a cation exchange membrane; and a cathode. The anode at least partially defines a first fluid flow path; the anode and / or the first fluid flow path is in contact with the anion exchange membrane of the bipolar membrane; a second fluid flow path is defined between the cation exchange membrane of the bipolar membrane and the anion exchange membrane; a third fluid flow path is defined between the anion exchange membrane and the cation exchange membrane; the cathode at least partially defines a fourth fluid flow path; and the cation exchange membrane is in contact with the cathode and / or the fourth fluid flow path. As such, in the electrochemical apparatus of the first invention, one side of the cation exchange membrane is in contact with the third fluid flow path and the other side of the cation exchange membrane is in contact with the fourth fluid flow path and / or the cathode.

[0012] According to a second aspect, the claimed invention provides a method of electrodialysis, the method comprising: providing the electrochemical apparatus of the first aspect; applying a positive charge to the anode and a negative charge to the cathode; providing a first aqueous stream to the first fluid flow path, providing a second aqueous stream to the second fluid flow path, providing a third aqueous stream to the third fluid flow path and providing a fourth aqueous stream to the fourth fluid flow path; wherein at least the third aqueous stream is a brine stream.

[0013] The present invention provides many wide-ranging benefits compared to conventional electrochemical apparatuses.

[0014] Surprisingly, the present invention can provide as high as 25 times increased current density compared to commercial ED apparatuses. Therefore, sodium hydroxide and / or hydrochloric acid can also be produced at 25 times higher rate compared to conventional ED apparatuses.

[0015] The bipolar membrane, the anion exchange membrane (AEM) and the cation exchange membrane (AEM) may be thought of as a set of membranes, or two AEM-CEM pairs of membranes. The present invention can operate with fewer membranes than conventional ED apparatuses. Compared to traditional ED apparatuses, which use tens or hundreds of ion-exchange membrane pairs, this can reduce the ohmic resistance of the apparatus of the invention - allowing for higher current density and product output. Preferably the apparatus of the invention comprises from 2 to 6 membrane pairs.

[0016] Furthermore, the increased efficiency obtainable with the present invention avoids the need for multiple systems to be operated simultaneously, as with conventional ED apparatuses.

[0017] It has been found that commercial cation exchange membrane (CEM, such as proton exchange membrane (PEM)), and anion exchange membrane (AEM) configurations fail within minutes with saline feeds due to high free chlorine generation and / or rapid anolyte pH swings and subsequent anode corrosion. However, surprisingly, the present invention exhibits dramatically improved stability of the membranes, the electrodes, and the overall membrane-electrode assembly to saline feeds, allowing chloride impurities to be well-tolerated.

[0018] Unlike conventional electrodialysis apparatuses, which do not produce hydrogen or oxygen in appreciable quantities, the present invention can produce significant amounts of hydrogen and oxygen through catalysed hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode. These gaseous products can provide afford energy savings (theoretically up to around 30%, using a hydrogen fuel cell and / or conversion to heat energy) and / or additional revenue for commercial operations. The present invention can provide oxygen, from the first fluid flow path. The present invention can provide hydrogen, from the fourth fluid flow path. Therefore, the method of the second aspect may additionally be termed a method of electrolysis (e.g. electrolytic electrodialysis).

[0019] In addition to an electric polarization gradient moving ions to form acid and base via migration, the present invention leverages the large concentration gradient of OH" at the cathode to drag Na+ions from the third flow path across the cation exchange membrane more quickly. Therefore, without being bound by theory, the generation of hydrogen and / or oxygen may be significantly driving the electrodialysis processes of the present invention.

[0020] The present invention uses the effective depletion of protons at the cathode via HER (i.e. consumption of protons directly, or separation of H+from H2O to yield OH ) as a main driver for ion transport. This proton depletion requires sodium ions to move to the catholyte to charge balance the resulting hydroxide ions, as well as the migration / diffusion of chloride from the brine feed to the acid compartment (i.e. the second fluid path) to charge balance the protons that originate from the bipolar membrane junction. Compared to traditional ED, which uses a more exclusively voltametric mechanism, the present invention uses a combined voltametric and catalytic mechanism for forcing ion separation.

[0021] The present invention provides benefits in terms of electrode cost and / or lifespan. Specifically, the present invention can avoid expensive high-stability anodes, for example based on iridium and / or ruthenium, conventionally required for ED, CA and / or WE. Therefore, the present invention can employ more Earth -abundant metal oxide catalysts, such as Fe-, Ni-, and Co-based oxides and oxy-hydroxides that have a far lower capital cost.

[0022] Furthermore, the present invention can avoid producing significant amounts of chlorine gas, avoiding the requirement for systems to manage the production of this byproduct.

[0023] The benefits discussed in the two paragraphs above are a result of the configuration of the apparatus of the invention. The configuration avoids high acidity and / or chloride concentration around the electrodes due to the natural alkalinity of the AEM and hydroxide ions being actively shuttled from the bipolar membrane (BPM) junction to the anode interface when the device is running. The present invention achieves this by firstly limiting the access of chloride ions to the anode due to the orientation of the bipolar membrane, and secondly due to the alkaline nature of the anion exchange membrane side of the bipolar membrane (interfaced to the anode) thermodynamically favouring oxygen evolution (i.e. OER) over chloride oxidation. Interfacing the BPM to the anode and operating at high current density drives water dissociation at the BPM junction to produce OH" that provides alkaline interface for OER.

[0024] The present invention can operate using sulfonated polystyrenes and / or sulfonated (poly)ether ether ketone (sPEEK) membranes, which are effective, cheap, and can avoid fluorinated ion-exchange membranes.

[0025] The present invention is also thought to reduce or avoid failure seen in high-current WE systems due to the deposition of divalent cation impurities (e.g. magnesium ions and calcium ions) on the cathode. As monovalent ions transport more readily through the CEM than divalent (scaling) ions, the feed configuration of the present invention is expected to prevent the precipitation of magnesium hydroxide and calcium hydroxide on the cathode. Thus, the apparatus of the invention has a higher tolerance for divalent cations than traditional WE systems, allowing the softening of the feed prior to entry into the apparatus to be avoided in many instances.

[0026] The method of the second aspect may be a method of brine valorisation, wherein the method comprises contacting the output from the second and / or fourth fluid flow paths with seawater or desalination waste brine. The system of the first aspect and the method of the second aspect find particular application in performing ocean alkalinity enhancement (OAE).

[0027] The method of the second aspect may be a method of performing OAE. Such a method may comprise (a) contacting at least a portion of an output from the fourth fluid flow path with seawater; and / or (b) reacting at least a portion of the output from the fourth fluid flow path with carbon dioxide so as to provide a carbonate solution (e.g. sodium carbonate), and contacting the carbonate solution with seawater.

[0028] According to a third aspect, the claimed invention provides an OAE system. The system of the third aspect may comprise an electrochemical apparatus according to the first aspect, wherein the system is configured to (a) contact at least a portion of an output from the fourth fluid flow path with seawater; and / or (b) react at least a portion of the output from the fourth fluid flow path with carbon dioxide so as to provide a carbonate solution, and contact the carbonate solution with seawater.

[0029] OAE allows for net introduction of alkalinity (e.g. NaOH) to seawater and / or net removal of acidity (e.g. HC1) from seawater. Alkalinity and acidity can be produced by the system of the first aspect and the method of the second aspect.

[0030] In use, the fourth fluid flow path generates alkalinity (especially NaOH) that is present in the output from the fourth fluid flow path. Embodiment (a) relates to directly contacting (a portion or all of) the output of the fourth fluid flow path with seawater. Embodiment (b) relates to contacting, and so reacting the output of the fourth fluid flow path with carbon dioxide (e.g. atmospheric carbon dioxide) so as to produce a carbonate solution. The solvent of the carbonate solution is preferably the liquid from the output from the fourth fluid flow path. The carbonate solution is then contacted with seawater. The contacting of the carbonate solution and / or the output of the output of the fourth fluid flow path increases the pH of the seawater, thereby enhancing the alkalinity of the seawater. Output from the second fluid flow path, which typically comprises HC1, may be neutralised, for example by reaction with alkaline minerals, such as olivine, that may be sourced from nearby. The system of the first aspect and the method of the second aspect find particular application in performing direct ocean capture (DOC).

[0031] The method of the second aspect may be a method of performing DOC. Such a method may comprise contacting at least a portion of an output from the second fluid flow path with seawater to provide acidified seawater, encouraging removal of carbon dioxide from the acidified seawater to provide decarbonated seawater, and contacting at least a portion of an output from the fourth fluid flow path with the decarbonated seawater so as to increase the pH of the decarbonated seawater. Increasing the pH of the decarbonated seawater can make the decarbonated seawater safe for release into an ocean. Encouraging removal of carbon dioxide from the acidified seawater may remove some or all of the carbon dioxide from the acidified seawater.

[0032] According to a fourth aspect, the claimed invention provides a DOC system. The system of the fourth aspect may comprise an electrochemical apparatus according to the first aspect, wherein the system is configured to contact at least a portion of an output from the second fluid flow path with seawater to provide acidified seawater, encourage the removal of carbon dioxide from the acidified seawater to provide decarbonated seawater, and contact at least a portion of an output from the fourth fluid flow path with the decarbonated seawater so as to increase the pH of the decarbonated seawater.

[0033] WO 2022 / 079043 Al provides an ambiguous disclosure relating to an anion exchange membrane, cation exchange membrane and bipolar membrane; however, there is no disclosure of the specific membranes as required by the claimed invention being present in the order required by the claimed invention, and in particular in the required orientation in relation to the anode and cathode.

[0034] Song et al., Chemical Engineering Journal 423 (2021) 130179, discloses the separation of sodium hydroxide from post-carbonation brines by bipolar membrane electrodialysis.

[0035] Chen et al., Water Research 226 (2022) 119274, discloses a review of the application of bipolar membrane electrodialysis (BMED) for simultaneous recovery of high-value acid / alkali from saline wastewater. Song et al. and Chen et al. disclose a three-chamber BMED devices. However, these do not have a cation exchange membrane with a third fluid flow path on one side and a fourth fluid flow path on the other side, where the cathode at least partially defines the fourth fluid flow path.

[0036] Detailed Description of the Invention

[0037] Electrodes

[0038] The skilled person will appreciate that a wide variety of metal and / or conductive carbonbased electrode materials can be used for the electrodes of the apparatus.

[0039] Each electrode may independently comprise one or more material selected from the list consisting of nickel (e.g. nickel foam), a conductive allotrope of carbon (e.g. graphite), steel (e.g. stainless steel), titanium, and platinum, such as IrOx-decorated platinized titanium and / or platinum on carbon. Each electrode may independently comprise one or more catalysts (e.g. on the surface thereof) selected from the list consisting of RuOx, CrOx, IrOx, Pt, MoS2, NiOOH, FeOOH, and NiO.

[0040] The use of Ni foam as the anode and platinum on carbon as the cathode has been found to provide very good current density compared to IrOx-decorated platinized titanium as the anode and stainless steel as the cathode. However, both of these pairs of electrodes work.

[0041] Preferably each electrode comprises a catalyst on the surface of the electrode. It will be understood that the catalyst needs to be appropriate for the reaction of interest at each electrode. Thus, the anode preferably comprises an oxygen evolution reaction catalyst, and the cathode preferably comprises a hydrogen evolution reaction catalyst.

[0042] Suitable cathode catalyst materials include Pt (e.g. platinum on carbon) and MoS2.

[0043] Suitable anode catalyst materials include IrOx, Ni, NiOOH, FeOOH, and NiO. Nickelbased and / or iron-based catalysts are preferred due to their lower cost. Preferably each electrode is porous, allowing the aqueous stream to access a high surface area of the electrode, and also allowing faster egress of gaseous products from the system.

[0044] The apparatus may comprise one or more current collectors. Each electrode may independently contact a current collector. The current collector may be made of a conductive metal (e.g. brass, gold, copper, stainless steel, and / or aluminium) or a conductive form of carbon (e.g. graphite).

[0045] Membranes

[0046] The skilled person will appreciate that a wide variety of cation exchange, anion exchange and bipolar membranes can be used for the apparatus.

[0047] Suitable cation exchange membranes include fluorinated, sulfonated polystyrene, sulfonated PEEK (s-SPEEK), Nafion (sulfonated tetrafluoroethylene based fluoropolymer-copolymer), Fumapem (per-fluorinated sulfonic acid / PTFE copolymer), and Aquivion (Short Side Chain (SSC) perfluorosulfonic acid (PFSA) ionomer).

[0048] Suitable anion exchange membranes include Fumasep (brominated polysulfone backbone with quaternary ammonium side chain groups), Sustainion (IH-Imidazole, 1,2,4,5-tetramethyl-, compd. with l(chloromethyl)-4-ethenylbenzene polymer), PiperlON (functionalized poly(aryl piperidinium) resin), Pention (poly(norbornene) based resin), and Durion (polyphenylene backbone with benzyl trimethyl ammonium functional groups).

[0049] It will be understood that the cation exchange membrane is spaced (physically separated) from the anion exchange membrane unless otherwise stated (e.g. in relation to a bipolar membrane).

[0050] Suitable bipolar membranes (BPM) typically include an anion exchange membrane and a cation exchange membrane. BPMs may comprise a water dissociation catalyst such as TiCE, reduced graphene oxide, Sb-SnOxand / or SnCE. Suitable bipolar membranes include interfaced combinations one or more CEMs with one or more AEMs, with one or more water dissociation catalysts at the interface. Suitable dissociation catalysts include cationic microbeads with tertiary and quaternary amines, poly acrylic acid / poly vinylpyridine salt complex, Fe(III) ions, chromium(III) hydroxide, zirconium oxide, zirconium aluminosilicate, TiCE, reduced graphene oxide, antimony tin oxide, and SnCh. The anion permeable layer of a BPM may comprise one or more materials selected from the list consisting of: a sulfonated polymer (e.g. a sulfonated polyolefin, a sulfonated poly(ethersulfone) or a sulfonated polyethylene); poly-styrene / vinylbenzoyl- chloride co-polymer; polysulfone (for example aminated, e.g. with bicyclic amines); Pall / Raipore R1030 (e.g. a membrane comprising perfluorinated vinyl chloride); styrene / divinylbenzene co-polymer with quaternary amines; perfluorinated polymer with quaternary and secondary amines; and ion exchange resin with secondary to quaternary amine groups or with quaternary amines. The anion permeable layer of a BPM may comprise one or more materials selected from the list consisting of: sulfonated polystyrene and Kraton G (styrenic block copolymer (SBC) comprising polystyrene blocks and rubber (polybutadiene, polyisoprene, and / or their hydrogenated equivalents) blocks); sulfonated, crosslinked poly-ether ether ketone; CM-1 Membrane; Pall / Raipore R1010 (e.g. a membrane comprising perfluorinated vinyl chloride); perfluorinated polymer with sulfonic acid groups; and ion exchange resin with sulfonic acid groups or with phosphoric acid groups. Membranes may comprise a polymer (e.g. polyolefin) binder and / or support.

[0051] Each pair of anion exchange membrane and cation exchange membrane may be considered a membrane pair (even if they are not touching one another and / or in a bipolar membrane). The minimum number of membrane pairs that the present invention can operate with is two. It will be understood that the number of electrode pairs should be increased by two each time. Therefore, the system of the present invention preferably has an even number of membrane pairs (e.g. 2, 4, 6 or 8 membrane pairs). The addition of an extra two membrane pairs provides an extra bipolar membrane, an extra anion exchange membrane, and an extra cation exchange membrane. Where the apparatus comprises four membrane pairs, the electrodes and membranes of the apparatus will be ordered: anode - BPM (AEM-CEM) - AEM - CEM - BPM (AEM-CEM) - AEM - CEM - cathode.

[0052] Where the apparatus comprises further membrane units, the apparatus may comprise: an anode, a bipolar membrane, an nthanion exchange membrane, an nthcation exchange membrane, an nthbipolar membrane, an anion exchange membrane, a cation exchange membrane, and a cathode, n represents the number of the repeating membrane unit (e.g. the 1strepeating membrane unit, 2ndrepeating membrane unit, 3rdrepeating membrane unit and so on). The second fluid flow path is defined between the cation exchange membrane of the bipolar membrane and the nthanion exchange membrane; a fifth fluid flow path is defined between the nthanion exchange membrane and the nthcation exchange membrane; a sixth fluid flow path is defined between the nthcation exchange membrane and the anion exchange membrane of the nthbipolar membrane; and a seventh fluid flow path is defined between the cation exchange membrane of the nthbipolar membrane and the anion exchange membrane.

[0053] The addition of further membranes may be represented as follows: anode - BPM (AEM- CEM) - X(AEM - CEM - BPM (AEM-CEM)) - AEM - CEM - cathode, where X represents the number of repeating membrane units. The number of repeating membrane units (X) may be 0 or higher, such as from 0 to 10, or 1, 2 or 3.

[0054] It will be appreciated that including membranes other than those defined herein may inhibit the performance of the apparatus.

[0055] It will be understood that additional pairs of membranes will provide additional base, acid, and dilute brine streams in the same order as described for the initial pairs of membranes (i.e. acid output between the BPM and the AEM, dilute brine output between the AEM and the CEM, and base output between the CEM and the BPM). The additional pairs of membranes will draw hydroxide ions from within the BPM junction of the added BPM. The additional pairs of membranes will also draw protons from the BPM junction of the added BPM. This is an analogous mode of operation to traditional electrodialysis, where the BPM is doing the work of acid and base generation for all of the additional pairs. Despite the additional membrane pairs, hydrogen and oxygen would only be generated at the cathode and anode, respectively. When taking into account possible additional membranes and flow paths, the apparatus would be configured follows: the anode at least partially defines a first fluid flow path; the anode and / or the first fluid flow path is in contact with the anion exchange membrane of the bipolar membrane; a second fluid flow path (in contact with the cation exchange membrane of the bipolar membrane); nthanion exchange membrane; an nthfifth fluid flow path; nthcation exchange membrane; an nthsixth fluid flow path; nthbipolar membrane (AEM of the BPM in contact with the nthsixth fluid flow path); an nthseventh fluid flow path (in contact with the cation exchange membrane of the nthbipolar membrane); anion exchange membrane; a third fluid flow path; cation exchange membrane; the cathode at least partially defines a fourth fluid flow path; and the cation exchange membrane is in contact with the cathode and / or the fourth fluid flow path. n represents the number of the repeating membrane unit. The number of repeating membrane units may be 0 or higher, as described above. Where there are two or more repeating membrane units, one or more nthseventh fluid flow path is in contact with one or more nthanion exchange membrane.

[0056] The or each fifth fluid flow path is preferably configured as with the third fluid flow path, and features disclosed in relation to the third fluid flow path apply equally to the or each fifth fluid flow path. The or each sixth fluid flow path is preferably configured as with the fourth fluid flow path, except that hydrogen is not generated. Features disclosed in relation to the fourth fluid flow path except those relating to hydrogen (e.g. generation / collection of hydrogen) apply equally to the or each sixth fluid flow path. The or each seventh fluid flow path is preferably configured as with the second fluid flow path. Features disclosed in relation to the second fluid flow path apply equally to the or each seventh fluid flow path.

[0057] Preferably the number of membrane pairs is from 2 to 20, such as from 2 to 10, for example from 2 to 6, or from 2 to 4, most preferably 2. Reducing the requisite number of membrane pairs drops the required input voltage by ~0.6 V per cell pair, thereby reducing the energy input required per unit of NaOH produced compared to conventional electrodialysis that typically uses 10-100 membrane pairs. Streams

[0058] The method of the invention defines providing a first aqueous stream to the first fluid flow path, providing a second aqueous stream to the second fluid flow path, providing a third aqueous stream to the third fluid flow path and providing a fourth aqueous stream to the fourth fluid flow path. At least the third aqueous stream is a brine stream.

[0059] It will be appreciated that the below discussion of controlling the composition of the aqueous streams typically relates to controlling the composition of such streams when it enters its corresponding flow path. The composition of each aqueous stream can vary as it passes along the length of its corresponding flow path.

[0060] Each of the first, second and fourth aqueous streams may have a low ionic content. For example, the first and fourth aqueous streams may have a low ionic content. Preferably the first aqueous stream has a low ionic content. Preferably the fourth aqueous stream has a low ionic content. Each aqueous streams that has a low ionic content may independently be tap water, potable water, and / or deionised water. Each aqueous stream that has a low ionic content may comprise water in an amount of 90 wt% or more, such as 95 wt% or more, or 98 wt% or more, for example 99 wt% or more, or 99.9 wt% or more, for example 99.99 wt% or more. Each aqueous stream that has a low ionic content may comprise a total dissolved solids of 500ppm or less, such as 200ppm or less, or lOOppm or less, for example 20ppm or less, or Ippm or less.

[0061] Preferably the first aqueous stream has a low chloride content, such as 500ppm or less, such as 200ppm or less, or lOOppm or less, for example 20ppm or less, or Ippm or less, for example no chloride. Chloride is oxidisable, and produces damaging and / or harmful substances such as chlorine and / or hypochlorite salts (e.g. hypochlorous acid) upon oxidation. It will be appreciated that not all oxidisable anions produce damaging and / or harmful oxidation products. The first aqueous stream may have a low oxidisable ion content, such as 500ppm or less, such as 200ppm or less, or lOOppm or less, for example 20ppm or less, or Ippm or less, for example no oxidisable ions.

[0062] Preferably the first aqueous stream is not highly acidic. Preferably the pH of the first aqueous stream has a pH of 5 or higher, such as 6 or higher, or 7 or higher, such as from 5 to 11, or from 6 to 9. This can avoid degrading acid-sensitive components such as oxygen evolution catalysts and / or electrode supports.

[0063] The first aqueous stream may comprise anions other than chloride, such as non- oxidisable anions (such as sulfate ions, for example in the form of sodium sulfate) in an amount of 500ppm or more, such as 0.1 wt% or more, or 1 wt% or more. The ionic content of the first aqueous stream can increase the conductivity of the first aqueous stream, which may be particularly helpful where the anode is not (directly) interfaced with (the AEM side of) the bipolar membrane. Anions that are not oxidisable, such as sulfate, reduce or avoid the formation of damaging and / or harmful species such as chlorine gas and / or hypochlorite salts.

[0064] A brine stream is provided to the third fluid flow path. It will be understood that the brine is an aqueous salt solution.

[0065] The brine may comprise desalination brine, seawater or mine tailing water. The brine may have been treated, such as to remove magnesium and / or calcium ions. The brine may comprise magnesium and / or calcium ions in an amount of 200ppm or less, or lOOppm or less, for example 20ppm or less, or Ippm or less. The seawater may have been that has been treated to remove sulfate ions. The brine may comprise sulfate in an amount of 200ppm or less, or lOOppm or less, for example 20ppm or less, or Ippm or less.

[0066] The brine may comprise sodium chloride and / or potassium chloride. The dissolved solids content of the brine may comprise sodium, potassium and chloride in a proportion of 50wt% or more, such as 70 wt% or more, or 90 wt% or more, for example 95 wt% or more, or 98 wt% or more, or 99 wt% or more. The concentration of sodium and / or potassium ions, or of chloride ions (e.g. the concentration of sodium chloride) in the brine may be 0.05M or more, such as 0. IM or more, or 0.2M or more, for example 0.4M or more, or 0.45M or more. The concentration of sodium and / or potassium ions, or of chloride ions (e.g. the concentration of sodium chloride) in the brine may be 6.2M or less, such as 6. IM or less, for example 5M or less, or 3M or less, preferably 2M or less, such as 1.5M or less, or IM or less. The concentration of sodium and / or potassium ions, or of chloride ions (e.g. the concentration of sodium chloride) in the brine may be from 0.05M to 6.2M, such as from 0.2M to 2M. The brine preferably comprises sodium chloride. The dissolved solids content of the brine may comprise sodium and chloride in a proportion of 50wt% or more, such as 70 wt% or more, or 90 wt% or more, for example 95 wt% or more, or 98 wt% or more, or 99 wt% or more.

[0067] The output from the third fluid flow path may be more and more dilute brine as the process continues. As the concentration of the brine in the third fluid flow path decreases, the output of the third fluid flow path may be suitable for use in the first, second or fourth aqueous streams.

[0068] The second aqueous stream may be acidic. For example, the second aqueous stream may comprise an acid selected from the list consisting of hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid and boric acid. The second aqueous stream may be a brine stream, which may be the same as or different from the third aqueous stream. This may reduce the “start-up” time of the apparatus by allowing the apparatus to start at a high Faradaic efficiency for NaOH production through preventing water from dissociating outside of the bipolar membrane. Aqueous streams with low ionic content, such as tap water, have a high resistance, and would cause relatively large resistive loss and a high voltage drop across a flow path, which could drive water dissociation as a source of deleterious current. Furthermore, the provision of acidic or brine as the second aqueous stream may reduce or eliminate the difference in conductivity along the length of the second flow path, thereby reducing or eliminating uneven degradation of membranes, catalysts and / or electrodes.

[0069] Method

[0070] The method comprises providing a potential difference across the anode and the cathode. The potential difference may be 0.5V or more, or IV or more, such as 1.5V or more, or 2V or more, or 3V or more, such as 5V or more. The potential difference may be 100V or less, such as 50V or less, or 30V or less, or 20V or less, such as 15V or less, preferably 10V or less, such as 8V or less, or 5V or less. The potential difference may be from 0.5 to 100V, such as from 1 to 10V. The method typically generates oxygen in the first flow path; hydrochloric acid in the second flow path; dilute brine in the third flow path; and aqueous sodium hydroxide with gaseous hydrogen in the fourth flow path. Each flow path may be recirculated, for example to increase the concentration of products in the outputs of each flow path.

[0071] The method may comprise separating oxygen from the output of the first fluid flow path and / or separating hydrogen from the output of the fourth fluid flow path. The separation of gases from the outputs of flow paths may be performed in a settling tank and / or using a gas-permeable membrane.

[0072] Apparatus

[0073] Preferably one or more (e.g. all) of the flow paths are curved and / or serpentine. Curved / serpentine flow paths are not conventionally used in electrodialysis because gasses are not formed. Such configurations improve mass transport. Such configurations can increase the linear velocity of the stream through a flow path while being able to maintain the residence time of the stream within the flow path.

[0074] For example, in relation to the first and fourth flow paths, a curved and / or serpentine flow path allows product gasses formed at either electrode to be quickly removed from the electrode surface (especially the active catalyst sites on the electrode surface). Such configurations can therefore enhance the efficiency of the electrode (and so the entire apparatus) and allow the product gasses to be used in downstream applications such as a hydrogen / oxygen fuel cell. Curved / serpentine flow paths may provide improved product stream quality.

[0075] In relation to the second and third flow paths, a curved and / or serpentine flow path can allow an even current distribution to be attained and provide efficient use of membrane material within the apparatus. Flow path design may allow for up to 50% reduction in required membrane surface area (per unit area of apparatus).

[0076] The thickness (in the direction between the electrodes) of each flow path may be 0.1mm or more, such as 0.2mm or more, or 0.5mm or more, such as 0.8mm or more. The thickness of each flow path may be 10mm or less, such as 5mm or less, or 2mm or less, such as 1.5mm or less, or 1.2mm or less, such as 1.0mm or less. The thickness may be from 0.1mm to 10mm, such as from 0.2 to 1.2mm.

[0077] The thickness (in the direction between the electrodes) of each flow path may be tapered. This can minimise ohmic resistance losses and / or increase the efficiency of the apparatus. This can reduce or eliminate changes in ohmic resistance along the length of one or more of the flow paths, especially the second and / or third flow paths. This can prevent local hotspots in current that have, in some circumstances, been shown to lead prematurely to component failure.

[0078] Preferably the thickness of the second flow path increases from the input end to the output end, as the ionic concentration of the stream will increase along the course of the second flow path. Preferably the thickness of the third flow path decreases from the input end to the output end, as the ionic concentration of the stream will decrease along the course of the third flow path. Preferably the thickness of the second flow path tapers, and the thickness of the third flow path tapers, and the taper of the first plane is opposite to that of the second plane. In this way, as the second flow path becomes more thick, the third flow path becomes less thick.

[0079] The input of each flow path may be defined as where a stream passing through a flow path first contacts adjacent membranes and / or electrodes. The output of each flow path may be defined as where a stream passing through a flow path last contacts adjacent membranes and / or electrodes.

[0080] A tapered flow path may change in thickness by 10% or more, such as 20% or more, or 50% or more, such as 80% or more, such as 90% or more. A tapered flow path may change in thickness by 500% or less, such as 300% or less, or 200% or less, for example 150% or less, or 120% or less. A tapered flow path may change in thickness by from 10 to 500%, such as from 50 to 150%.

[0081] The apparatus may comprise plates to press components of the apparatus (e.g. the anode, bipolar membrane, anion exchange membrane, cathode and optional current collector(s)) together. This can be used to hold the components together, and optionally to compress the components together. The plates may be made of a metal (e.g. aluminium) or plastics material. The apparatus and / or method of the invention may comprise a hydrogen / oxygen fuel cell. The fuel cell may be configured to generate power from hydrogen generated in the fourth flow path, and optionally oxygen generated in the first flow path. Preferably the power generated by the fuel cell is used as part of the power applied across the anode and cathode. It will be appreciated that the remainder of the power applied across the anode and the cathode will be made up from another power source.

[0082] Detailed Description of the Drawings

[0083] An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

[0084] Figure 1 is a schematic representation of a traditional electrodialysis bipolar membrane configuration of the prior art;

[0085] Figure 2 is a schematic representation of an electrochemical apparatus according to the invention;

[0086] Figure 3 is a schematic representation of the streams used for testing the membrane configuration shown in Figure 2;

[0087] Figure 4 shows computer aided design (CAD) drawings of electrolysis apparatus of the invention;

[0088] Figure 5 shows a schematic depicting how the invention can be integrated into a system to perform ocean alkalinity enhancement (OAE);

[0089] Figure 6 shows a schematic depicting how the invention can be integrated into a system to perform direct ocean capture (DOC);

[0090] Figure 7 is a graph showing polarisation curves for a comparative membrane configuration and a membrane configuration of the invention;

[0091] Figure 8 shows pH and current profiles over time during chronoamperometry experiments for a comparative membrane configuration and a membrane configuration of the invention; and

[0092] Figure 9 shows graphs of current density against cell voltage for different electrode materials. Figure 1 of the accompanying drawings shows a conventional electrodialysis bipolar membrane configuration of the prior art. The cell of the prior art includes: anode 10 made of titanium / RuOx, fluid flow path 15, separated from anode 10 by a gasket, anion exchange membrane 20 between fluid flow path 15 and fluid flow path 25; cation exchange membrane 30 between fluid flow path 25 and fluid flow path 35; bipolar membrane 40 between fluid flow path 35 and fluid flow path 45, with its anion exchange membrane side facing fluid flow path 35 and its cation exchange membrane side facing fluid flow path 45; anion exchange membrane 50 between fluid flow path 45 and fluid flow path 55; a gasket between fluid flow path 55 and cathode 60, which is made of stainless steel.

[0093] The cell of the prior art is equipped to take a brine (e.g. aqueous NaCl) feed and produce alkaline (e.g. NaOH) and acidic (e.g. HC1) streams.

[0094] In use, brine (MX) is introduced to fluid flow path 25. Water is introduced to fluid flow path 35. Metal (e.g. sodium) ions from the brine feed are biased towards cathode 60 to pass through membrane 30 to enter flow path 35. Hydroxide ions from the junction of bipolar membrane 40 are biased towards anode 10 to also pass into flow path 35 to generate a base (MOH) stream as the output of flow path 35.

[0095] Anions (e.g. chloride ions) from brine feed in flow path 25 are biased towards anode 10 to pass through anion exchange membrane 20 and into flow path 15. Flow path 15 is circulated into flow path 55. The anions in flow path 55 are biased towards anode 10 to pass through anion exchange membrane 50, into flow path 45. Protons (hydronium ions, i.e. HsO+) from the junction of bipolar membrane 40 are biased towards cathode 60 to pass into flow path 45 to generate an acid (HX) stream as the output of flow path 45.

[0096] The configuration of the prior art shown in Figure 1 requires at least five exchange membranes (including two for the bipolar membrane). The membrane configuration described may be repeated one or more times to provide additional streams. Prior art configurations such as this typically achieve a current density of around 10 mA / cm2. Figure 2 of the accompanying drawings shows a schematic representation of an electrochemical cell according to the invention. The cell of the invention includes: anode 110; bipolar membrane 130 comprising anion exchange membrane 132 and cation exchange membrane 134; fluid flow path 105 that is in contact with anode 110 where anode 110 also contacts anion exchange membrane 132, and / or that is positioned between anode 110 and anion exchange membrane 132; anion exchange membrane 140, separated from cation exchange membrane 134 by fluid flow path 135; cation exchange membrane 150, separated from anion exchange membrane 140 by fluid flow path 145; cathode 180; and fluid flow path 185 that is in contact with cathode 180 where cathode 180 is also in contact with cation exchange membrane 150 and / or that is positioned between cathode 180 and cation exchange membrane 150.

[0097] While not shown in Figure 2, in this example, anode 110 and cathode 180 are made of porous materials, thereby allowing the fluid flow paths 105 and 185 to flow through and around anode 110 and cathode 180 respectively.

[0098] In use, brine (MX, e.g. concentrated aqueous NaCl solution) is introduced to fluid flow path 145. Water (e.g. deionised water) is introduced to fluid flow paths 105, 185. Either water (e.g. deionised water) or brine is introduced to flow path 135. Metal (e.g. sodium) ions from flow path 145 are biased towards cathode 180 to pass through membrane 150 to enter flow path 185. Reduction at the cathode generates hydrogen and hydroxide ions in flow path 185, generating hydrogen and a base (MOH, e.g. NaOH) stream as the output of flow path 185.

[0099] Hydroxide ions from the junction of bipolar membrane 130 are biased towards anode 110 to pass into flow path 105 to be oxidised on the anode surface, producing oxygen as the output of flow path 105.

[0100] Anions (e.g. chloride ions) from brine feed in flow path 145 are biased towards anode 110 to pass through anion exchange membrane 140 and into flow path 135. Protons (hydronium ions, i.e. HsO ) from the junction of bipolar membrane 130 are biased towards cathode 180 to pass into flow path 135 to generate an acid (HX) stream as the output of flow path 135. The concentration of brine in flow path 145 is reduced as ions are removed from it, providing an output of dilute brine stream from flow path 145.

[0101] The configuration of the invention shown in Figure 2 can operate with as few as four exchange membranes (including two for the bipolar membrane). Therefore, the resistance of the cell is naturally reduced compared to traditional systems.

[0102] Further, compared to conventional electrodialysis, the system of the invention produces hydrogen and oxygen that can be used to reduce the energy consumption of the system and / or generate additional revenue. This difference means that it is preferable for flow paths 105, 185 not to be contacted with one another.

[0103] The present invention relates to a high-current BPM configuration that encourages the intentional transport of Na+ions to generate a basic output stream. Hydrogen / oxygen generating water electrolysers would naturally avoid sodium ions by using pure water feeds.

[0104] The separation of brine stream from the cathode helps to prevent scaling by divalent hydroxide species in the brine stream, allowing the cathode life to be extended and / or brine streams including divalent species to be better tolerated.

[0105] Figure 3 of the accompanying drawings shows a schematic representation of the streams used for testing the membrane configuration shown in Figure 2. Flow paths 105, 135, 145, 185 are recirculated via reservoirs 190 using pumps 195.

[0106] The cell of the present invention has been shown to successfully produce hydrogen, oxygen, HC1, NaOH and dilute brine while maintaining current densities of 250 mA / cm2, or even higher. Thus, the present invention can enable the rapid generation of a more dilute brine stream (which may be potable), an acidic (HC1) stream, and an alkaline (NaOH) stream. The alkaline stream may be used for applications such as the crystallisation of Mg(OH)2. Figure 4 of the accompanying drawings shows computer aided design (CAD) drawings of electrolysis apparatus 200 of the invention. (A) is an exploded view; (B) is a sandwich view; and (C) shows some individual components. Electrolysis apparatus 200 comprises electrodes 210, membrane 220, and flow paths 230. Electrolysis apparatus 200 also comprises two perforated gold-plated brass current collectors 240, to supply electricity to the cell, and two aluminium back plates 250, to sandwich the cell together under even compression.

[0107] Figure 5 of the accompanying drawings shows a schematic depicting how the invention can be integrated into a system to perform ocean alkalinity enhancement (OAE). Seawater 50 is softened in water softener 52. The softened seawater is used in electrochemical apparatus 100, which produces oxygen (typically from the first fluid flow path), HC1 (typically from the second fluid flow path), and hydrogen and NaOH (typically from the fourth fluid flow path). The NaOH (i.e. base) stream is passed through a pH regulation buffer tank 54 to control the pH to be from 8 to 9. This can reduce or prevent ecological impact from discharging more highly alkaline materials into the ocean. The output from buffer tank 54 is combined with seawater, allowing for some of the effects of carbon dioxide that has been passively absorbed into seawater (i.e. absorbed by air-sea gas (ASG) exchange) to be counteracted. The HC1 (i.e. acid) stream is passed into mineral neutralisation unit 56 where it is combined with an alkaline mineral for neutralisation. As an example, olivine (magnesium iron silicate) may be used as the alkaline mineral to prepare SiCE, MgCOs and MgCh. The hydrogen and oxygen can be stored, transported and / or used for other purposes, providing an additional revenue stream.

[0108] Preferably the output of the fourth fluid flow path (or at least part thereof) is diluted, for example with fresh seawater, before discharge to the ocean. This can reduce or prevent ecological impact from discharging more highly alkaline materials into the ocean. The output of the fourth fluid flow path may be diluted (e.g. with seawater) until the pH is 7.5 to 10, preferably from 8 to 9. The pH of the fourth fluid flow path may additionally or alternatively be lowered by contacting the fourth fluid flow path with atmospheric carbon dioxide to produce a carbonate solution (e.g. sodium carbonate), and the carbonate solution is combined with seawater. Contacting seawater with (at least part of) the output of the fourth fluid flow path and / or the carbonate solution increases the pH of the seawater. Figure 6 of the accompanying drawings shows a schematic depicting how the invention can be integrated into a system to perform direct ocean capture (DOC). Seawater 60 is softened in water softener 62. The softened seawater is used in electrochemical apparatus 100, which produces oxygen (typically from the first fluid flow path), HC1 (typically from the second fluid flow path), and hydrogen and NaOH (typically from the fourth fluid flow path). Another portion of seawater is also combined with the HC1 (i.e. acid) stream in mixing chamber 64. This acidifies the seawater, shifting the equilibrium toward dissolved CO2, allowing carbon dioxide to be more easily removed from solution. The acidified seawater is transferred to CO2 stripper unit 66, where the carbon dioxide can be removed from the solution, for example using a vacuum membrane contactor and / or a vacuum tower. The acidification of the seawater allows the carbon dioxide to be more easily removed from that portion of seawater. The carbon dioxide is collected, and can be transported, stored and / or used for other purposes, providing a potential additional revenue stream. The NaOH (base) stream may be used to perform direct atmospheric CO2 uptake, for example by exposing the NaOH (base) stream to atmospheric CO2 and thereby producing a sodium carbonate stream. Either the sodium carbonate stream or the NaOH are combined with the decarbonated seawater from CO2 stripper 66 in mixing chamber 68 to neutralise the decarbonated seawater (i.e. control the pH of the water to be 6-8, preferably 6.5-7.5). Atmospheric oxygen (e.g. air) may also be combined with the water in mixing chamber 68 to reoxygenate the water. An air contactor may be used for this purpose. The neutralised and optionally reoxygenated seawater is combined with seawater (e.g. by discharge into the ocean). This allows for some of the effects of carbon dioxide that has been passively absorbed into seawater (i.e. absorbed by air-sea gas (ASG) exchange) to be counteracted. The hydrogen and oxygen can be stored, transported and / or used for other purposes, providing an additional revenue stream. The DOC system may allow the ratio of forced (active) to passive recarbonation of the seawater to be controlled, depending on for example the ocean mixing conditions and water temperature. This can provide a balance between reuptake rate and process energy cost.

[0109] The present invention may find application in brine valorisation (e.g. brine mining). The acid and / or alkali streams from the second and / or fourth fluid flow paths may be combined with seawater or desalination waste brine. Such methods may precipitate or otherwise extract minerals from the seawater or brine. For example, contacting the output of the fourth fluid flow path with the seawater or brine causes alkalinisation of the liquid, forming magnesium and / or calcium hydroxides and / or carbonates. Magnesium chloride and / or calcium chloride may be prepared from the output of the second fluid flow path and a magnesium and / or calcium salt.

[0110] The present invention may find application in the generation of electrical power by generating electrical power from hydrogen produced in the fourth fluid flow path and / or oxygen produced in the first fluid flow path. The generation of electrical power may be realised by a hydrogen fuel cell and / or a internal combustion generator. The electrical power may be used to power a secondary process such as the chloralkali process.

[0111] Membrane Configuration Polarisation Curves

[0112] The membrane configuration according to the invention was tested within a commercial EDBPM device architecture via stepped chronopotentiometry to evaluate its intrinsic effect on current density. Comparative results were obtained for a commercial membrane configuration. The same device chassis, electrodes, and membrane compositions were used in each configuration.

[0113] Figure 7 of the accompanying drawings shows the polarization curve results of these tests, showing lines for a commercial ED system membrane configuration (“Commercial EDBPM”) and the membrane configuration of the invention (“Brineworks Electrolyzer”).

[0114] The resulting polarization curves indicated that, for all applied voltages, the membrane configuration of the invention yielded significantly higher current, while using ~5x lower membrane surface area (electrode surface areas and compositions were held constant). Compared to the commercial membrane configuration, the membrane configuration of the invention was able to produce NaOH up to 27 times faster despite using a membrane area that was 5 times smaller.

[0115] This illustrates the inherent advantage for base production of the membrane configuration of the invention, with lower material requirement. pH and Current Profiles

[0116] Commercial ED hardware was compared to a cell according to the invention that integrated the same membranes as in the commercial ED hardware, but in the membrane configuration of the invention and in the setup as illustrated by Figures 2-4. In these tests, a constant voltage was applied across the electrodes to determine the rate of NaOH production across the two devices. These experiments where performed under identical conditions where brine, acid, and base streams were all circulated into and out of their respective compartments at 20 litres per hour, under an applied voltage of 4 V between the anode and cathode.

[0117] Figure 8 shows pH and current profiles over time during chronoamperometry experiments where an as-assembled commercial ED system (Figure 8A) and an electrolyser of the invention with the same membrane compositions (Figure 8B) separated acid and base from synthetic NaCl brine streams having similar total dissolved solids (TDS) of -21-28 g / L.

[0118] From the graphs in Figure 8, the current density and the rate of sodium hydroxide production were calculated. These results are shown in the table below.

[0119] The results indicate that, compared to the commercial system, the membrane configuration of the invention provided:

[0120] 1) a significant increase in electrode area-normalized production of NaOH, and

[0121] 2) a higher current density. This illustrates the benefit of thrifting membrane pairs, re-constructing the membrane assembly, and enhancing mass transport by way of accelerating linear flow while maintaining long residence time for brine streams.

[0122] It can be seen that the electrolyser with the membrane configuration of the invention achieved 95 times higher current density than the comparative membrane configuration.

[0123] The rate of NaOH production, normalised for current density, was 318 times higher for the electrolyser with the membrane configuration of the invention, compared to the comparative membrane configuration.

[0124] The projected rate of NaOH production at the m2membrane scale, at a current density of 15 mA / cm2, was 4.5 times higher for the electrolyser with the membrane configuration of the invention, compared to the comparative membrane configuration. Achieving such high current densities for the comparative membrane would require extreme voltages to be applied, leading to other safety and efficiency concerns.

[0125] Electrode materials

[0126] The current density obtained for different electrode materials was investigated. Specifically, the current density from 0 V to 15 V was determined for a cell including a stainless steel cathode with an IrOx / Pt / Ti anode, which are typical electrode materials used in electrodialysis, and for a cell including a Pt / C cathode with a Ni foam anode, which are traditional electrode materials used in water electrolysis.

[0127] Figure 9 of the accompanying drawings shows graphs of cell density against cell voltage. Figure 9A shows the graph for the cell including a stainless steel cathode with an IrOx / Pt / Ti anode. Figure 9B shows the graph for the cell including a Pt / C cathode with a Ni foam anode.

[0128] At 10V the current density for the cell including a Pt / C cathode with a Ni foam anode was double that of the cell including a stainless steel cathode with an IrOx / Pt / Ti anode. At 15V the current density for the cell including a Pt / C cathode with a Ni foam anode was five times that of the cell including a stainless steel cathode with an IrOx / Pt / Ti anode. This shows that, while both sets of electrode materials worked, in the present invention the electrode materials typically used in water electrolysis provide enhanced current density compared to the electrode materials typically used in electrodialysis.

[0129] Less resistant electrode materials, such as those based on iron, nickel and / or cobalt, that can be used in water electrolysis would be unsuitable for use in conventional electrodialysis setups due to the corrosive nature of the electrode solution if an apparatus is subjected to high current densities or high voltages. However, the present invention averts this because there is not a high concentration of chloride ions or protons at either of the electrodes, and because the membrane configuration at the anode side of the apparatus maintains an alkaline pH relative to a water electrolysis apparatus.

Claims

CLAIMS1. An electrochemical apparatus comprising: an anode; a bipolar membrane; an anion exchange membrane; a cation exchange membrane; and a cathode; wherein: the anode at least partially defines a first fluid flow path; the anode and / or the first fluid flow path is in contact with the anion exchange membrane of the bipolar membrane; a second fluid flow path is defined between the cation exchange membrane of the bipolar membrane and the anion exchange membrane; a third fluid flow path is defined between the anion exchange membrane and the cation exchange membrane; the cathode at least partially defines a fourth fluid flow path; and the cation exchange membrane is in contact with the cathode and / or the fourth fluid flow path.

2. The electrochemical apparatus of claim 1, wherein the thickness of one or more of the flow paths in the direction between the electrodes is tapered.

3. The electrochemical apparatus of claim 2, wherein the thickness of the second flow path increases from the input end to the output end.

4. The electrochemical apparatus of claim 2 or claim 3, wherein the thickness of the third flow path decreases from the input end to the output end.

5. The electrochemical apparatus of any preceding claim, wherein number of membrane pairs is from 2 to 6.

6. The electrochemical apparatus of any preceding claim, wherein the apparatus further comprises:- an nthanion exchange membrane;- an nthcation exchange membrane; and- an nthbipolar membrane; wherein:- the second fluid flow path is further defined between the cation exchange membrane of the bipolar membrane and the nthanion exchange membrane;- a fifth fluid flow path is defined between the nthanion exchange membrane and the nthcation exchange membrane;- a sixth fluid flow path is defined between the nthcation exchange membrane and the anion exchange membrane of the nthbipolar membrane; and- a seventh fluid flow path is defined between the cation exchange membrane of the nthbipolar membrane and the anion exchange membrane.

7. The electrochemical apparatus of any preceding claim, wherein each electrode is porous.

8. The electrochemical apparatus of any preceding claim, wherein the anode comprises a catalyst that is an iron-based, nickel-based, and / or cobalt-based oxide or oxy-hydroxide.

9. The electrochemical apparatus of any preceding claim, wherein the anode comprises Ni foam, and wherein the cathode comprises platinum on carbon.

10. The electrochemical apparatus of any preceding claim, wherein the cation exchange membrane is a sulfonated polystyrene membrane and / or a sulfonated (poly)ether ether ketone membrane.

11. The electrochemical apparatus of any preceding claim, wherein the apparatus further comprises a hydrogen fuel cell configured to generate power from hydrogen generated in the fourth flow path.

12. A method of electrodialysis, the method comprising: providing the electrochemical apparatus of any one of claims 1 to 11; applying a positive charge to the anode and a negative charge to the cathode; providing a first aqueous stream to the first fluid flow path; providing a second aqueous stream to the second fluid flow path; providing a third aqueous stream to the third fluid flow path; and providing a fourth aqueous stream to the fourth fluid flow path; wherein at least the third aqueous stream is a brine stream.

13. The method of claim 12, wherein at least the first aqueous stream and the fourth aqueous stream contain total dissolved solids in a concentration of 500ppm or less.

14. The method of claim 12 or claim 13, wherein the second aqueous stream is a brine stream.

15. The method of any one of claims 12 to 14, wherein the concentration of sodium and / or potassium ions, or of chloride ions in each brine stream is 0.05M or more.

16. The method of any one of claims 12 to 15, wherein the dissolved solids content of the brine may comprise sodium and chloride in a proportion of 70 wt% or more.

17. The method of any one of claims 12 to 16, wherein method further comprises providing a potential difference of 0.5V or more across the anode and the cathode.

18. The method of any one of claims 12 to 17, wherein the method further comprises separating oxygen from the output of the first fluid flow path and / or separating hydrogen from the output of the fourth fluid flow path.

19. The method of any one of claims 12 to 18, wherein the method is a method of performing ocean alkalinity enhancement, and wherein the method comprises (a) contacting at least a portion of an output from the fourth fluid flow path with seawater; and / or (b) reacting at least a portion of the output from the fourth fluid flow path with carbon dioxide so as to provide a carbonate solution, and contacting the carbonate solution with seawater.

20. The method of any one of claims 12 to 19, wherein the method is a method of performing direct ocean capture, and wherein the method comprises contacting at least a portion of an output from the second fluid flow path with seawater to provide acidified seawater, removing carbon dioxide from the acidified seawater to provide decarbonated seawater, and contacting the decarbonated seawater with at least a portion of an output from the fourth fluid flow path.

21. The method of any one of claims 12 to 20, wherein the method is a method of brine valorisation, and wherein the method comprises contacting the output from the second and / or fourth fluid flow paths with seawater or desalination waste brine.

22. The method of any one of claims 12 to 21, wherein the method is a method of generating electrical power, and wherein the method comprises generating electrical power from hydrogen produced in the fourth fluid flow path and / or oxygen produced in the first fluid flow path.

23. The method of claim 22, wherein the method comprises performing the chloralkali process with a membrane cell comprising electrodes, and applying the electrical power across the electrodes.

24. An ocean alkalinity enhancement system comprising an electrochemical apparatus according to any one of claims 1 to 11, wherein the system is configured to (a) contact at least a portion of the output from the fourth fluid flow path with seawater; and / or (b) react at least a portion of the output from the fourth fluid flow path with carbon dioxide so as to provide a carbonate solution, and contact the carbonate solution with seawater.

25. A direct ocean capture system comprising an electrochemical apparatus according to any one of claims 1 to 11, wherein the system is configured to contact at least a portion of an output from the second fluid flow path with seawater to provide acidified seawater, remove carbon dioxide from the acidified seawater to provide decarbonated seawater, and contact the decarbonated seawater with at least a portion of an output from the fourth fluid flow path.

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