System and Process for Improving the Electrolysis of Saltwater

Abstract

A system and process for facilitating the direct electrolysis of saltwater, such as seawater, is described. The system comprises an acid-base flow battery (ABFB) 230 with an acid solution outlet 403,

Description

Technical field The present invention relates generally to systems and processes for improving the electrolysis of saltwater, for example direct electrolysis of saltwater, e.g. seawater. Background to the invention The hydrogen economy is one important tool in the fight against climate change. Using electricity generated by renewable sources such as wind and solar, hydrogen can be generated by splitting water into hydrogen and oxygen using electrolysis. Given the relative scarcity of fresh water and abundance of seawater, research is ongoing into ways to optimise the direct electrolytic splitting of seawater, so that hydrogen may be produced even in arid coastal regions. The hydrogen produced by electrolysis is usually stored for a period of time before being used either as a fuel for combustion or to power fuel cells, for example in vehicles. A significant problem associated with the direct electrolysis of saltwater, such as seawater, is the concurrent evolution of chlorine gas. Such chlorine evolution is desirable in some circumstances, for instance in the chlor-alkali process, during which brine is electrolysed on an industrial scale to produce chlorine and sodium hydroxide (and hydrogen by-product). However, where the primary aim is to produce hydrogen as a means to store energy from the electrolysis of saltwater, the evolution of chlorine as a byproduct may present an unwanted burden. If seawater is electrolysed for the purpose of producing hydrogen, then without a way to prevent chlorine evolution it would be necessary to store the chlorine by-product and find a suitably sized market for its disposal. During the electrolysis of water, including saltwater, the hydrogen and oxygen evolution reactions (HER and OER) occur at the negative electrode (cathode) and positive electrode (anode), respectively (reactions (1) and (2)): 2H*E° = 0 V vs SHE (1) 02 + 4H^ + 4e”^2H2:0 E° = 1.23 V vs SHE (2) The presence of dissolved chloride in saltwater, such as seawater, causes the oxidation of chloride to chlorine at the anode according to reaction (3): Cb+2e-#2£F E°= 1.4 V vs SHE (3) Since the standard equilibrium potential of the CI7Ch couple (1.4 V vs SHE) is more positive than the standard equilibrium potential of the H2O / O2 couple (1.23 V vs SHE), it is theoretically thermodynamically possible to electrolyse chloride solutions to generate hydrogen without the evolution of chlorine. However, because of sluggish oxygen evolution kinetics, the reaction (2) typically occurs at 200-300 mV over the equilibrium potential, meaning that, especially at low pH where the equilibrium potential of the OER is more positive, there is overlap with the chlorine evolution reaction which has faster kinetics. Furthermore, in the presence of chloride, dissolution of the active OER catalyst (typically Pt- or Pt-group metals-based) may occur at the anode due to formation of soluble chlorocomplexes at the very positive potentials required for the OER. Aside from chlorine, bromine evolution is another problem often overlooked, since seawater contains bromide alongside chloride (albeit in lower concentrations that chloride). The Br / Bra couple has a significantly lower standard equilibrium potential than the CI' / Ch couple, at 1.1 V vs SHE, which is also lower than the H2O / O2 couple: Br2 + # 2Br- E° = 1.1 V vs SHE (4) Hence bromine evolution will also be a problem during the electrolysis of seawater. However the kinetics of reaction (4) are slow unless in alkaline medium. This suggests that in near neutral conditions, seawater electrolysis without oxidising bromide may be possible. However, as reactions (1) and (2) proceed the initially neutral solutions at the anode and cathode will become increasingly acidic and alkaline, respectively, which will increase the voltage needed for water electrolysis as the equilibrium potentials of reactions (1) and (2) will become more negative and positive, respectively. This will cause an increase in energy needed per gram of H2 and greater tendency to oxidise halogens such as chloride and bromide at the anode. This issue is typically addressed by adding buffers to limit pH changes during operation and maintain favourable conditions for oxygen evolution over chloride / bromide oxidation. However, such addition of additives may prevent the release of the electrolyte back to its source without treatment, which is problematic if the electrolyte is seawater which is intended to be discharged back into the sea after electrolysis. Given all of the above, it is often necessary to first desalinate saltwater before subjecting it to electrolysis, to remove dissolved chloride and eliminate the risk of chlorine evolution. This adds a costly and energy intensive preliminary step of desalination to the process. It would be desirable to provide a process which is able to reliably electrolyse saltwater, without the need for a preliminary desalination step and without the evolution of chlorine or bromine during electrolysis. The present invention was developed with a view to addressing these issues. Disclosure of the invention The invention concerns systems and processes for facilitating the direct electrolysis of saltwater, such as seawater, without the need for a preliminary desalination step and without the generation of chlorine during electrolysis. At its most general, the invention provides a system comprising an acid-base flow battery upstream of a water electrolyser, wherein the acid-base flow battery is configured to generate an acidic solution and an alkaline solution from a saltwater (e.g. seawater) feed and supply the acidic solution and alkaline solution to the water electrolyser to electrolyse the solutions and generate hydrogen. At its most general, the invention also provides a process comprising generating an acidic solution and an alkaline solution from a saltwater (e.g. seawater) feed in an acid-base flow battery upstream of a water electrolyser, and supplying the acidic solution and alkaline solution to a water electrolyser to electrolyse the solutions and generate hydrogen. More specifically, the invention provides a system and process for the electrolysis of saltwater, the system comprising an acid-base flow battery comprising an acid solution outlet, an alkaline solution outlet and a saltwater inlet; and a water electrolyser downstream of acid-base flow battery for producing hydrogen, comprising a negative electrode and a positive electrode. The process comprises producing acid and base from saltwater in an acid-base flow battery; and feeding the acid and base to a downstream water electrolyser. In one aspect, there is provided a system for the electrolysis of saltwater, comprising: an acid-base flow battery comprising an acid solution outlet, an alkaline solution outlet and a saltwater inlet; and a water electrolyser downstream of acid-base flow battery for producing hydrogen, comprising a negative electrode and a positive electrode; wherein the acid-base flow battery is in fluid communication with the water electrolyser, such that, in use, an alkaline solution from the alkaline solution outlet of the acid-base flow battery passes into a positive electrode channel of the water electrolyser proximal the positive electrode. By coupling a water electrolyser with an upstream acid-base flow battery (ABFB) in this way, the base solution by-product from the ABFB is fed into the positive (anode) channel of the electrolyser. In this way, the pH proximal the pH proximal the positive electrode is increased. As a result, saltwater such as seawater is subjected to electrolysis without the evolution of chlorine or bromine at the positive electrode. Furthermore, since no addition of buffers is necessary, the electrolyte after electrolysis can be more safely returned to source, e.g. discharged directly into the sea or ocean. Hence the system can operate at a location near the coast and function continuously, using seawater as a feed to the ABFB and discharging the effluent from the water electrolyser back into the sea in an environmentally friendly manner. Since the use of the base from the ABFB enables electrolysis without the evolution of halogens, there is no need for any additional desalination step prior to electrolysis. The overall process is therefore more straightforward and more energy efficient. A second aspect of the invention is a process for the electrolysis of saltwater, comprising: providing an acid-base flow battery comprising an acid solution outlet, an alkaline solution outlet and a saltwater inlet; feeding an aqueous solution of sodium chloride into the saltwater inlet of the acid-base flow battery; applying a voltage to the acid-base flow battery to produce both an acidic solution and an alkaline solution within the acid-base flow battery; providing a water electrolyser downstream of the acid-base flow battery for producing hydrogen, comprising a negative electrode, a positive electrode, a negative electrode channel proximal the negative electrode and a positive electrode channel proximal the positive electrode, wherein the acid-base flow battery is in fluid communication with the water electrolyser; feeding at least a portion of the alkaline solution from the alkaline solution outlet of the acid-base flow battery into the positive electrode channel of the water electrolyser; and applying a voltage to the water electrolyser to produce hydrogen gas. The process generates hydrogen gas from a saltwater source (e.g. seawater) without the need for prior desalination and with negligible or no evolution of chlorine or bromine, due to the increased pH at the positive electrode of the water electrolyser, resulting from the use of an upstream ABFB to generate acidic and alkaline solutions from the saltwater source and the feeding the alkaline solution to the positive channel of the electrolyser. Preferred and / or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention unless the context demands otherwise. Any of the preferred and / or optional features of any aspect may be combined, either singly or in combination, with any aspect of the invention unless the context demands otherwise. The invention couples an acid-base flow battery (ABFB) with a water electrolyser. ABFBs are devices which convert the Gibbs free energy of acid-base neutralisation to electricity, or use electricity to generate acid and base from a neutral solution (depending on whether the ABFB is being charged or discharged). In the present invention, a voltage is applied across the ABFB to convert a neutral saltwater feed into acid and alkaline solutions (i.e. charging of the ABFB). Saez etal. (A. Saez, V. Montiel, and A. Aldaz, ‘An Acid-Base Electrochemical Flow Battery as energy storage system’, I nt J Hydrogen Energy, vol. 41, no. 40, pp. 17801-17806, Oct. 2016, doi: 10.1016 / j.ijhydene.2016.08.141) were the first to show the potential of utilising hydrogen gas as a charge carrier in an ABFB with a single cation exchange membrane separating the acid and base channel. Because the equilibrium potential of the H7H2 couple depends on pH, separating the acid and base in a hydrogen atmosphere generates a potential difference between the two electrodes which will result in an external current when connected. During discharge of an ABFB, H2 is produced at the positive electrode (acidic compartment, Reaction 1) and oxidised in exactly the same amount at the negative electrode (alkaline compartment, Reaction 2): 2H+ + 2e- H2 H2 + 2OH~ -+ 2H2O + 2e~ (1) (2) During charge the reverse happens - a voltage applied across the battery consumes H2 in the acidic compartment and produces H2 in the alkaline compartment, resulting in acidic and basic solutions as products. ABFBs combine the benefits of a flow battery with relatively safe and cheap electrolytes. ABFBs are a specific case of concentration gradient batteries, in which the difference in electrochemical potential across a membrane separating two different solutions is converted into electricity (or vice versa) by coupling the process to suitable redox reactions at the negative and positive electrodes. The hydrogen evolution and oxidation reactions (HER and HOR, respectively) are ideally suited to couple trans-membrane ion transfer due to acid-base neutralisation with electron transfer across the electrodeelectrolyte interface, necessary for conversion of the Gibbs free energy of neutralisation to electricity. The negative Gibbs free energy of acid-base neutralisation can also be used to drive water desalination in neutralization dialysis (ND), by simply separating the acid and base in the ABFB by a neutral salt channel (Igawa et al., Neutralization Dialysis for Deionization', Bull. Chern. Soc. Jpn, vol 60, pp. 381-383, Jan. 1987). Neutralisation in the central channel causes ions to migrate outwards from it to maintain charge balance in all three compartments. However, in ND cells the Gibbs free energy used in desalination is not converted to an external electrical current and instead released simply as heat (although the energy required for desalination is lower than the total energy obtainable from the Gibbs free energy of acid-base neutralisation, such that electricity can be generated by the ABFB simultaneously with desalination). Acid-base flow battery (ABFB) The ABFB comprises a saltwater inlet. In some embodiments the saltwater inlet is coupled to a source of saltwater, for example a natural source of saltwater. Herein, “saltwater” refers to an aqueous solution of sodium chloride, which may be natural or synthetic, or a derivative of a natural source (e.g. seawater which has undergone treatment or processing). “Seawater” refers to naturally-occurring saltwater which has been obtained from a natural source of saltwater, such as a sea or ocean. Both saltwater and seawater specifically contain dissolved sodium chloride, but may also contain further dissolved ions including cations such as potassium, calcium and magnesium, and anions such as sulphate, phosphate, bromide and fluoride. The seawater may be used directly in the system or process described herein, or may be subjected to one or more preliminary treatment steps before feeding to the system, for example filtration and / or the removal of organic matter, as described in more detail below. In some embodiments, the saltwater used as a feed to the ABFB in the system and process of the present invention is a brine containing one or more metal ions used in the manufacture of battery materials. In some embodiments, the saltwater used as a feed to the ABFB in the system and process of the present invention is a brine containing one or more of lithium ions, magnesium ions, nickel ions, cobalt ions and manganese ions. Such ions are used in the preparation of battery materials, e.g. electrode materials. As explained in more detail below, certain embodiments of the invention result in product and by-product brines which are more concentrated than the initial saltwater input to the ABFB. As such, when a brine containing one or more of lithium ions, magnesium ions, nickel ions, cobalt ions and manganese ions is used as the saltwater input to the ABFB, in certain embodiments a product brine results which contains even higher concentrations of such ions. Since the recovery of such ions for use in the manufacture of battery materials often involves steps of concentrating brines containing the ions, the present system and process therefore provide a way to achieve this concentration of brine as part of a process of recovering one or more of lithium, magnesium, nickel, cobalt and manganese from the brines to use in the manufacture of battery materials. In some embodiments, the saltwater used as a feed to the ABFB in the system and process of the present invention is a brine containing lithium ions, thereby enabling the system and process to facilitate the concentration of a lithium-containing brine for use in manufacturing battery materials. In some embodiments, the ABFB comprises first and second compartments separated by an ion-permeable membrane or a non-permeable separator; the first compartment comprising the alkaline solution outlet, and the second compartment comprising the acid solution outlet. In some embodiments, the ABFB comprises first and second compartments separated by an ion-permeable membrane or a non-permeable separator; the first compartment comprising the alkaline solution outlet and a first liquid inlet, and the second compartment comprising the acid solution outlet and a second liquid inlet. The first compartment thereby provides an alkaline channel of the ABFB and the second compartment provides an acidic channel. In this way, the ABFB is configured to receive saltwater (e.g. into both the first liquid inlet and the second liquid inlet), before flowing the saltwater through the first compartment on one side of the ion-permeable membrane or non-permeable separator, and flowing the saltwater through the second compartment on another side of the ion-permeable membrane or non-permeable separator. When the ABFB is connected to an electricity supply, the saltwater flowing through the first compartment will become more alkaline and will eventually be discharged through the alkaline solution outlet, and the saltwater flowing through the second compartment will become more acidic and will eventually be discharged through the acid solution outlet. In some embodiments, the ABFB comprises first and second compartments separated by an ion-permeable membrane. The ion-permeable membrane may permit the passage of ions but prevent the passage of liquid, i.e. prevent the mixing of liquids between the first and second compartments. The ion-permeable membrane may permit the passage of a single polarity of ion. The ion-permeable membrane may be a cation exchange membrane (CEM) or an anion exchange membrane (AEM). In some embodiments, the ABFB comprises first and second compartments separated by a cation exchange membrane (CEM). In some embodiments, the CEM comprises a polymeric film comprising perfluorosulfonic acid polymer or copolymer. In other embodiments, the ABFB comprises first and second compartments separated by an anion exchange membrane (AEM). In some embodiments, the AEM comprises a poly ether ether ketone reinforced film. The choice of either CEM or AEM to separate the first and second compartments does not affect the ability of the ABFB to produce the alkaline and acidic solutions in the first and second compartments, but the composition of these alkaline and acidic solutions would be different, which would in turn influence the composition of the product from the downstream electrolyser. In a system comprising a CEM separating the first and second compartments, assuming a 1.0 M concentration of NaCI in the saltwater feed to each of the first and second compartments, a solution of 1.0 M NaOH and 1.0 M NaCI would be produced in the first compartment and a solution of 1.0 M HCI would be produced in the second compartment. In other words, the alkaline solution would also contain NaCI. In a system comprising an AEM separating the first and second compartments, assuming a 1.0 M concentration of NaCI in the saltwater feed to each of the first and second compartments, a solution of 1.0 M NaOH would be produced in the first compartment and a solution of 1.0 M HCI and 1.0 M NaCI would be produced in the second compartment. In other words, the acidic solution would also contain NaCI. In some cases, the ABFB comprises first and second compartments separated by a single ion-permeable membrane, i.e. the first and second compartments are directly adjacent one another and ions pass directly from the first compartment into the second compartment through the single ion-permeable membrane (or vice versa). In such cases, the ABFB functions to use electricity to generate the alkaline and acidic solutions for use in the water electrolyser. This has the effect of reducing the net electricity consumption of the water electrolysis. However, in other cases the acid-base flow battery comprises first and second compartments separated by more than one ion-permeable membrane, for example separated by two distinct ion-permeable membranes. In some embodiments, the acid-base flow battery comprises a third compartment between the first and second compartments, wherein the third compartment is separated from the first compartment by a first ion-permeable membrane, and the third compartment is separated from the second compartment by a second ion-permeable membrane. Thus in some embodiments the first and second compartments are separated by two distinct ion-permeable membranes, and a third compartment is defined between the two membranes. In some embodiments the first and second compartments are sealed with respect to one another to prevent the passage of liquid between them (other than the passage of ions through the membranes). In some embodiments the first, second and third compartments are sealed with respect to one another to prevent the passage of liquid between any of them (other than the passage of ions through the membranes). Such a third compartment may be used to contain or allow the passage of a solution, for example a saline solution (e.g. seawater), which will either (a) undergo desalination, or (b) become a concentrated brine, as the ABFB charges (depending on the arrangement of ion-permeable membranes, as discussed in more detail below). Thus, in such embodiments the ABFB will both generate the alkaline and acidic solution, and perform desalination of a saline solution contained within the third compartment, or produce a brine from it. In some embodiments, the third compartment comprises a saline solution inlet and a third liquid outlet. In some embodiments, the ABFB comprises a third compartment between the first and second compartments, wherein the third compartment is separated from the first compartment by a cation exchange membrane (CEM) and the third compartment is separated from the second compartment by an anion exchange membrane (AEM). In some embodiments, the third compartment comprises a saline solution inlet and a third liquid outlet. In such embodiments, when saltwater (e.g. seawater) is fed into the saline solution inlet of the third compartment, the saltwater in the third compartment will be desalinated during operation of the ABFB as it flows through the third compartment, and desalinated water (e.g. freshwater) will be discharged from the third liquid outlet. As such, with this arrangement of CEM and AEM in the ABFB, the ABFB produces the alkaline solution and acidic solution in the first and second compartments respectively (which are then fed to the water electrolyser), and simultaneously desalinates water in the third compartment. In this way the system is configured to produce desalinated water which may find use e.g. as drinking water or for irrigation, which could be especially desirable for a system located in an arid coastal region. In other embodiments, the ABFB comprises a third compartment between the first and second compartments, wherein the third compartment is separated from the first compartment by an anion exchange membrane (AEM) and the third compartment is separated from the second compartment by a cation exchange membrane (CEM). In some embodiments, the third compartment comprises a saline solution inlet and a third liquid outlet. In such embodiments, when saltwater (e.g. seawater) is fed into the saline solution inlet of the third compartment, the saltwater in the third compartment will be concentrated during operation of the ABFB as it flows through the third compartment, and a concentrated brine with a NaCI concentration higher than the saltwater feed will be discharged from the third liquid outlet. As such, with this arrangement of CEM and AEM in the ABFB, the ABFB produces the alkaline solution and acidic solution in the first and second compartments respectively (which are then fed to the water electrolyser), and simultaneously produces a concentrated brine in the third compartment, having a NaCI concentration higher that of the saltwater feed. This brine may also contain more highly concentrated levels of other cations than the initial saltwater feed, including lithium ions (Li+). Thus such an ABFB may generate a Li-rich brine in the third compartment, which may be useful for recovering lithium for use in the downstream manufacture of lithium-containing materials (e.g. lithium hydroxide, lithium carbonate) useful in the manufacture of components of lithium-ion batteries. In this way the system is configured to produce brine, which may be discharged back to source (e.g. into the sea or ocean), or recycled for use elsewhere in the process (e.g. as an input in the 3-channel electrolyser discussed in more detail below, which will simultaneously reduce the NaCI concentration in the brine) or in a different process (e.g. in the chlor-alkali process for production of chlorine and sodium hydroxide, or in the manufacture of battery materials). In some embodiments, the CEM comprises a polymeric film comprising perfluorosulfonic acid polymer or copolymer. In some embodiments, the AEM comprises a poly ether ether ketone reinforced film. Such cation and anion exchange membranes are commercially available and known to the skilled person. In the above embodiments comprising an AEM and / or CEM (whether or not the third compartment is present), the membrane may extend across the entire internal area of the ABFB such that there is no fluid communication between the compartments which lie on either side of the membrane. In this way, physical mixing of the liquids is prevented and only ions are able to pass between them through the membrane, preserving the compositional integrity and pH of the acidic and alkaline solutions produced in the ABFB. Without wishing to be bound by theory, it is believed that a thicker CEM or AEM may lead to lower ion flux across the membrane. In contrast, a thinner CEM or AEM would tend to increase ion flux across the membrane, thereby increasing current density. In some embodiments, a CEM is chosen which is thicker than the AEM, reducing proton flux across the CEM relative to hydroxide flux across the AEM. In this way, faster hydroxide ion flux improves separation efficiency by balancing excess acidification which occurs because spontaneous proton flux is faster than hydroxide flux. However in alternative embodiments, the ABFB may be “membraneless”, i.e. lack a CEM or AEM between the compartments. In such embodiments, the compartments may be physically separated in a way which prevents or substantially prevents physical mixing of liquids between compartments, thus again preserving the compositional integrity and pH of the acidic and alkaline solutions produced in the ABFB. However ionic conductivity must be maintained between the liquids within the ABFB to ensure that the ABFB functions when connected to an electricity supply. Thus in some embodiments, the ABFB comprises first and second compartments separated by a separator which substantially prevents mixing of liquid contents of the first and second compartments but maintains ionic conductivity between the liquid contents of the first and second compartments. In some embodiments, the separator comprises a membrane comprising pores which maintain ionic conductivity across the membrane. Such membranes are not ionexchange membranes, since they are uncharged, but instead allow ions to migrate across the membrane through the existence of pores which provide physical channels through which ions are able to pass. In some embodiments, the separator comprises a non-permeable membrane which extends across a first portion of the ABFB and provides fluid communication between the first and second compartments across a second portion of the ABFB. In some embodiments, the non-permeable membrane is not permeable to the passage of molecules or ions, i.e. does not allow the liquids within the first and second compartments to mix and also does not allow migration of ions across the non-permeable membrane between the first and second compartments. Instead, when such non-permeable membranes are used, ionic conductivity is maintained within the ABFB by ensuring that the non-permeable membrane does not extend across the entire ABFB and that a region remains where the liquids within the first and second compartments are in physical contact with one another. In some embodiments, the separator comprises a non-permeable membrane which extends from a downstream end of the ABFB in an upstream direction, terminating within the ABFB to leave an upstream portion of the ABFB where the first and second compartments are in fluid communication with one another. Put otherwise, in some embodiments the ABFB comprises an upstream region comprising a single, integral “mixing” compartment, and a downstream region where the first and second compartments are separated by a non-permeable separator which substantially prevents mixing of liquid contents of the first and second compartments, wherein each of the first and second compartments are in fluid communication with the mixing compartment. Although in such embodiments the separator will prevent both the physical mixing of liquids and the passage of ions, ionic conductivity is maintained due to the upstream region where liquids and ions can flow across the ABFB. In such a “membraneless” design, sodium ions will tend to migrate into the first compartment and chloride ions will tend to migrate into the second compartment (due to the polarisation of the ABFB during charging), thereby producing an alkaline sodium hydroxide solution in the first compartment and an acidic HCI solution in the second compartment. Neither of the compartments will include a substantial NaCI concentration in such embodiments, since ions are able to migrate in both directions. Such a “membraneless” design also has the advantage that, in theory, it provides reduced energy consumption of the ABFB to produce the alkaline and acidic solutions from saltwater. Assuming neutral seawater feed with a 1.0 M concentration of NaCI, a minimum voltage of 0.83 V would be required to produce 1.0 M solutions of acid and base in the “membraneless” design including only a separator which allows some mixing. Replacing the “membraneless” separator with an ion-permeable membrane (CEM or AEM) as discussed above increases this minimum voltage slightly to 0.84 V, which may be worthwhile depending on the desired ABFB design and desired products. The embodiment discussed above which comprises a third compartment in which a concentrated brine is produced will theoretically require a minimum voltage of 0.86 V, but again this may be worthwhile where the brine product is desired for downstream use or recycling. The embodiment discussed above which comprises a third compartment in which freshwater is produced will theoretically require a minimum voltage of 1.1 V which, although higher than the other embodiments described, may be worthwhile in embodiments where the valuable freshwater byproduct is desirable (e.g. where the ABFB is operating in arid coastal regions where the freshwater could be used as drinking water or for irrigation). In some embodiments, the ABFB comprises a connection to a source of hydrogen gas. As such, in some embodiments the ABFB is a hydrogen-coupled ABFB, where hydrogen acts as the charge carrier during charging. In some embodiments, the connection to a source of hydrogen gas comprises first and second gas diffusion electrodes (GDE) configured to provide diffusion of hydrogen gas out of a first electrode of the ABFB and into a second electrode of the ABFB. In some embodiments, the acid-base flow battery comprises means to remove hydrogen gas from the first compartment and means to introduce hydrogen gas into the second compartment. This provides the supply of H2 to the second (acidic) compartment necessary to drive the hydrogen oxidation reaction, and removes the H2 generated by reduction in the first (alkaline) compartment In some embodiments, the means to remove hydrogen gas from the first compartment comprises a first gas diffusion electrode, and the means to introduce hydrogen gas into the second compartment comprises a second gas diffusion electrode. First and second GDEs may be independently in contact with a supply of hydrogen gas. In some embodiments, first and second GDEs are in contact with hydrogen gas at a pressure of from 80 kPa to 150 kPa, for example about 100 kPa. In some embodiments, the first and second GDEs each comprise carbon electrodes. In some embodiments, the first and second GDEs each comprise a carbon cloth substrate containing a platinum-based catalyst. In some embodiments, the first and second GDEs each comprise a platinum loading of from 0.4 to 0.6 mg cm-2, for example about 0.5 mg cm-2. In some embodiments, the first and second GDEs each comprise the carbon cloth substrate laminated with a layer of hydrophobic carbon paper. It was found that this further layer of hydrophobic carbon paper reduced the amount of liquid leaking through the GDE from the solution-containing compartments of the ABFB, thereby improving the functioning of the system. In some embodiments, the first and second GDEs each comprise the carbon cloth substrate laminated with a layer of hydrophobic carbon paper, further laminated onto a current collector layer, for example graphite foil current collector layer. It is not necessary to cycle hydrogen around the ABFB, it is sufficient to connect both the anode and the cathode to the same hydrogen reservoir, or hydrogen reservoirs of the same or similar pressure. In the absence of leaks, the pressure in the reservoir will remain the same no matter how long the ABFB is in operation, because the same amount of hydrogen produced in one electrode is consumed in the other. However in some embodiments, the system comprises means to transport hydrogen gas from the first compartment of the ABFB to the second compartment of the ABFB. In this way, the H2 produced in the first compartment is transported to the second compartment, and is able to provide all the H2 needed for oxidation in the second compartment. H2 thereby functions purely as a charge carrier which is recycled during the ABFB charge cycle, without being consumed. In this way, the system provides a way for H2 (for example, H2 which has been generated by electrolysis of water) to perform a useful function as an alternative to idle storage. In some embodiments, the means to transport hydrogen gas facilitates constant H2 pressure and full saturation at both gas diffusion electrodes. Although in theory the production of hydrogen should make up for the consumption on the other side, such that hydrogen gas could flow externally to the ABFB from the first compartment to the second compartment, in practice some leaking of hydrogen will likely occur through the electrodes into the solution or simply through the cell as hydrogen gas easily permeates most materials. So, in some embodiments the external volume adjacent each gas diffusion electrode is pressurised with hydrogen (e.g. at a pressure of from 80 kPa to 150 kPa, for example about 100 kPa) from an external source. Hydrogen gas is then able to flow from the gas field adjacent the gas diffusion electrode into the second compartment, and from the first compartment through the associated gas diffusion electrode into the adjacent hydrogen gas field. As an alternative to the use of GDEs, the solution within the second compartment may be saturated with hydrogen by bubbling hydrogen gas through the solution. Hence in some embodiments, the system comprises a hydrogen saturation device to dissolve hydrogen in the saltwater within the second compartment of the ABFB. In some embodiments, the hydrogen saturation device may comprise a gas sparger connected to a supply of H2 to bubble H2 into the saltwater. In some embodiments, each of the first and second compartments comprises a H2 gas sparger. In some embodiments, the system comprises a hydrogen saturation device located upstream of the ABFB to dissolve H2 in the saltwater feed before it is introduced into the second compartment of the ABFB. For example, the system may include a separate upstream hydrogen saturation module, comprising a vessel through which the saltwater passes, wherein the vessel comprises a hydrogen saturation device such as a gas sparger connected to a H2 supply. It is only necessary to bubble hydrogen through the solution within the second compartment, to facilitate the oxidation reaction there. Hydrogen bubbles would then form in the first compartment as a result of the reduction reaction. In some embodiments, the first compartment comprises a hydrogen gas collection device, e.g. a hydrogen gas outlet to collect the bubbles of hydrogen gas which form in the first compartment during charging of the ABFB. In some embodiments, the system comprises means to transport hydrogen gas from the first electrode of the ABFB to the second electrode of the ABFB. During operation of the system, saltwater (e.g. seawater) is fed into each of the first and second compartments. An alkaline solution is discharged from the first compartment and an acidic solution is discharged from the second compartment. In some embodiments, the system is adapted to provide a continuous flow of solution through the first compartment and a continuous flow of solution through the second compartment. When the third compartment is present, the system may be adapted to provide a continuous flow of solution through the third compartment. In some embodiments, the system comprises a first pump to control the flow rate of saltwater into the first compartment. In some embodiments, the first pump comprises a peristaltic pump. In some embodiments, the system comprises a second pump to control the flow rate of saltwater into the second compartment. In some embodiments, the second pump comprises a peristaltic pump. In some embodiments, the system comprises a third pump to control the flow rate of saltwater into the third compartment. In some embodiments, the third pump comprises a peristaltic pump. In some embodiments, the system comprises a fourth pump to control the flow rate of alkaline solution out of the first compartment. In some embodiments, the fourth pump comprises a peristaltic pump. In some embodiments, the system comprises a fifth pump to control the flow rate of acidic solution out of the second compartment. In some embodiments, the fifth pump comprises a peristaltic pump. In some embodiments, the system comprises a sixth pump to control the flow rate of liquid out of the third compartment. In some embodiments, the sixth pump comprises a peristaltic pump. Solutions may be fed into the first, second and third compartments through any suitable conduits. In some cases, the system may comprise flexible tubing, such as PVC tubing, to transport the solutions to and from the ABFB. In some embodiments, the system further comprises one or more rubber gaskets separating adjacent components within the ABFB, thereby defining an inner surface of a compartment. For example, there may be a first rubber gasket between the first ion-permeable membrane and the first gas diffusion electrode, such that the first compartment interior volume is defined by the first gas diffusion electrode, the first ion-permeable membrane and the first rubber gasket. In some embodiments there is a second rubber gasket between the second ion-permeable membrane and the second gas diffusion electrode, such that the second compartment interior volume is defined by the second gas diffusion electrode, the second ion-permeable membrane and the second rubber gasket. In some embodiments there is a third rubber gasket between the first ion-permeable membrane and the second ion-permeable membrane, such that the third compartment interior volume is defined by the first ion-permeable membrane, the second ion-permeable membrane and the third rubber gasket. In some embodiments, each of the first, second and third rubber gaskets independently comprise a fluorocarbon-based fluoroelastomer (also known as fluorine rubber or fluoro-rubber). In some embodiments, each of the first, second and third rubber gaskets independently comprise an FKM rubber. Water electrolyser The design of the water electrolyser in the system of the invention is not particularly limited. Water electrolysers are well-known devices and the invention is applicable to any known water electrolyser in order to use it to generate hydrogen from a saltwater source without the concurrent evolution of chlorine or bromine, and avoid the need for the addition of buffers or prior desalination. In some embodiments the water electrolyser is adapted for the direct electrolysis of seawater or brine. In some embodiments, the water electrolyser is an alkaline water electrolyser (AWE) or a proton exchange membrane water electrolyser (PEMWE). In some embodiments, the water electrolyser is an electrolyser adapted or configured for the electrolysis of near-neutral saltwater, for example the electrolysis of seawater. In some embodiments, the water electrolyser is an electrolyser adapted or configured for the electrolysis of an aqueous solution of a compound comprising a conjugate acid and a conjugate base of a strong acid / base pair, for example an aqueous solution of one or more of Na2SO4, NaCI, KCI and K2SO4. During the electrolysis of water, as discussed in the background section above, protons are reduced at the cathode (negative electrode) and hydroxide anions are oxidised at the anode (positive electrode). As a result of this, water in the vicinity of the cathode becomes basic as electrolysis progresses (due to the consumption of H+ there) and water in the vicinity of the anode becomes acidic (due to the consumption of OH'). If the electrolyte in the vicinity of the cathode is kept separated from the electrolyte in the vicinity of the anode and they are not allowed to mix, water electrolysers fed with neutral water produce acid and base waste solutions as a by-product. However, in the present invention, the alkaline solution from the ABFB is fed to the positive electrode, effectively “fuelling” the hydroxide oxidation at the anode. In some embodiments, the acidic solution from the ABFB is fed to the negative electrode, effectively “fuelling” the HER at the cathode. The resultant products from the electrolyser are neutral or near-neutral solutions. In some embodiments, the alkaline solution outlet of the acid-base flow battery is in fluid communication with the positive electrode channel of the water electrolyser. In some embodiments, the acid solution outlet of the acid-base flow battery is in fluid communication with the negative electrode channel of the water electrolyser. In some embodiments, the water electrolyser comprises first and second compartments separated by an ion-permeable membrane or non-permeable separator. The first compartment may be a compartment in which the OER occurs (proximal the positive electrode) and the second compartment may be the compartment in which the HER occurs (proximal the negative electrode). The ion-permeable membrane or non-permeable separator may permit the passage of ions but prevent or substantially prevent the passage of liquid, i.e. prevent the mixing of liquids between the first and second compartments. The ion-permeable membrane may permit the passage of a single polarity of ion. The ion-permeable membrane may be a cation exchange membrane (CEM) or an anion exchange membrane (AEM). In some embodiments, during use of the system, an acidic solution from the acid solution outlet of the acid-base flow battery passes into a negative electrode channel of the water electrolyser proximal the negative electrode. Although such feeding of acidic solution from the acid solution outlet of the acid-base flow battery into the negative electrode channel of the water electrolyser is not essential to reduce or eliminate the evolution of Ch at the positive electrode, it may help reduce the operation voltage of the electrolyser, thereby providing a more efficient system. It also makes use of the acidic solution byproduct of the ABFB, which would otherwise go to waste or require a separate application. The first compartment (i.e. positive electrode channel) may comprise a positive electrode channel inlet, e.g. an alkaline solution inlet, for the passage of alkaline solution into the electrolyser from the ABFB. The second compartment (i.e. negative electrode channel) may comprise a negative electrode channel inlet, e.g. an acidic solution inlet, for the passage of acidic solution into the electrolyser from the ABFB. The water electrolyser may comprise one or more liquid outlets at a downstream end for the passage of one or more respective liquid products out of the water electrolyser after electrolysis. In some embodiments, the water electrolyser comprises a first liquid outlet at a downstream end of the first compartment (i.e. positive electrode channel), and a second liquid outlet at a downstream end of the second compartment (i.e. negative electrode channel). The water electrolyser comprises a negative electrode channel proximal the negative electrode (cathode), and a positive electrode channel proximal the positive electrode (anode). In some embodiments, the positive electrode channel comprises a conduit for the passage of liquid from an alkaline solution inlet to a liquid outlet, e.g. a first liquid outlet. In some embodiments, the negative electrode channel comprises a conduit for the passage of liquid from an acidic solution inlet to a liquid outlet, e.g. a second liquid outlet. In some embodiments, the positive electrode channel comprises the positive electrode, which is immersed in liquid contained within the positive electrode channel during use of the electrolyser. In some embodiments, the negative electrode channel comprises the negative electrode, which is immersed in liquid contained within the negative electrode channel during use of the electrolyser. In some embodiments, the water electrolyser comprises first and second compartments separated by an ion-permeable membrane. In some embodiments, the water electrolyser comprises first and second compartments separated by a cation exchange membrane (CEM). In some embodiments, the CEM comprises a polymeric film comprising perfluorosulfonic acid polymer or copolymer. In other embodiments, the water electrolyser comprises first and second compartments separated by an anion exchange membrane (AEM). In some embodiments, the AEM comprises a poly ether ether ketone reinforced film. The choice of either CEM or AEM to separate the first and second compartments does not affect the ability of the water electrolyser to produce hydrogen, but the composition of the residual solutions after electrolysis would be different. In a system comprising a CEM separating the first and second compartments of the water electrolyser, with alkaline solution fed to the first compartment and acidic solution fed to the second compartment, freshwater (desalinated water) would be produced in the first compartment and a concentrated brine would be produced in the second compartment. Possible uses of these products are discussed above. In a system comprising an AEM separating the first and second compartments of the water electrolyser, with alkaline solution fed to the first compartment and acidic solution fed to the second compartment, saltwater of a concentration substantially equal to that of the initial feed to the ABFB would be produced in both compartments. This can be easily and safely discharged back to the original source, e.g. the sea or the ocean. In some cases, the water electrolyser comprises first and second compartments separated by a single ion-permeable membrane, i.e. the first and second compartments are directly adjacent one another and ions pass directly from the first compartment into the second compartment through the single ion-permeable membrane (or vice versa). In such cases, the water electrolyser functions to use electricity to generate hydrogen and produce the solutions described immediately above. However, in other cases the water electrolyser comprises first and second compartments separated by more than one ion-permeable membrane, for example separated by two distinct ion-permeable membranes. In some embodiments, the water electrolyser comprises a third compartment between the first and second compartments, wherein the third compartment is separated from the first compartment by a first ion-permeable membrane, and the third compartment is separated from the second compartment by a second ion-permeable membrane. Thus in some embodiments the first and second compartments are separated by two distinct ion-permeable membranes, and a third compartment is defined between the two membranes. In some embodiments the first and second compartments are sealed with respect to one another to prevent the passage of liquid between them (other than the passage of ions through the membranes). In some embodiments the first, second and third compartments are sealed with respect to one another to prevent the passage of liquid between any of them (other than the passage of ions through the membranes). Such a third compartment may be used to contain or allow the passage of a solution, for example a saline solution or brine, which will either (a) undergo desalination, or (b) become a concentrated brine, as the water electrolyser operates (depending on the arrangement of ion-permeable membranes, as discussed in more detail below). Thus, in such embodiments the water electrolyser will both generate hydrogen, and perform desalination of a saline solution contained within the third compartment, or produce a brine from it. Furthermore, the products produced in the first and second compartments will also depend upon the arrangement of the ion-permeable membranes in the water electrolyser. In some embodiments, the third compartment comprises a saline solution inlet and a third liquid outlet. In some embodiments, the water electrolyser comprises a third compartment between the first and second compartments, wherein the third compartment is separated from the first compartment by an anion exchange membrane (AEM) and the third compartment is separated from the second compartment by a cation exchange membrane (CEM). In some embodiments, the third compartment comprises a saline solution inlet and a third liquid outlet. In such embodiments, when saltwater (e.g. seawater) or brine (e.g. having an NaCI concentration double that of seawater) is fed into the saline solution inlet of the third compartment, the saltwater or brine in the third compartment will be desalinated during operation of the water electrolyser as it flows through the third compartment, and desalinated water (e.g. freshwater, or a saline solution of lower NaCI concentration than that fed to the third compartment) will be discharged from the third liquid outlet. As such, with this arrangement of CEM and AEM in the water electrolyser, the water electrolyser produces hydrogen, and simultaneously desalinates water in the third compartment. The first and second compartments in this arrangement will produce neutral solutions from the acid and alkaline feed solutions, and the NaCI concentration of these solutions will depend on the NaCI concentration of the feed solutions. When a brine having an NaCI concentration approximately double that of the original saltwater feed to the ABFB is fed to the third compartment of the water electrolyser, neutral saltwater having an NaCI concentration approximately equal to that of the original saltwater feed to the ABFB will be produced in both the first and second compartments, and these solutions can be safely discharged to the original source, e.g. sea or ocean. In this way the system is configured to produce desalinated water which may find use e.g. as drinking water or for irrigation, or can reduce the concentration of a brine to facilitate its discharge back into the original saltwater source, e.g. the sea or ocean. In other embodiments, the water electrolyser comprises a third compartment between the first and second compartments, wherein the third compartment is separated from the first compartment by a cation exchange membrane (CEM) and the third compartment is separated from the second compartment by an anion exchange membrane (AEM). In some embodiments, the third compartment comprises a saline solution inlet and a third liquid outlet. In such embodiments, when saltwater (e.g. seawater) is fed into the saline solution inlet of the third compartment, the saltwater in the third compartment will be concentrated during operation of the water electrolyser as it flows through the third compartment, and a concentrated brine with a NaCI concentration higher than the saltwater feed will be discharged from the third liquid outlet. As such, with this arrangement of CEM and AEM in the water electrolyser, the water electrolyser produces hydrogen, and simultaneously produces a concentrated brine in the third compartment, having a NaCI concentration around double that of the saltwater feed to the water electrolyser. The first and second compartments in this arrangement will produce neutral freshwater from the acid and alkali feed solutions when a brine having an NaCI concentration approximately double that of the original saltwater feed to the ABFB is fed to the third compartment of the water electrolyser. This freshwater from the first and second compartments can be used as drinking water, or for irrigation. In this way the system is configured to produce brine, which may be discharged back to source (e.g. into the sea or ocean), or recycled for use elsewhere in the process, or in a different process (e.g. in the chlor-alkali process for production of chlorine and sodium hydroxide, or in the manufacture of battery materials); alongside freshwater, which can be used as drinking water, or for irrigation. In some embodiments, the CEM in the water electrolyser comprises a polymeric film comprising perfluorosulfonic acid polymer or copolymer. In some embodiments, the AEM in the water electrolyser comprises a poly ether ether ketone reinforced film. Such cation and anion exchange membranes are commercially available and known to the skilled person. In the above embodiments comprising an AEM and / or CEM (whether or not the third compartment is present), the membrane may extend across the entire internal area of the water electrolyser such that there is no fluid communication between the compartments which lie on either side of the membrane. In this way, physical mixing of the liquids is prevented and only ions are able to pass between them through the membrane, preserving the compositional integrity and pH of the solutions produced in the water electrolyser. However in alternative embodiments, the water electrolyser may be “membraneless”, i.e. lack a CEM or AEM between the compartments. In such embodiments, the compartments may be physically separated in a way which prevents or substantially prevents physical mixing of liquids between compartments, thus again preserving the compositional integrity and pH of the solutions produced in the water electrolyser. However ionic conductivity must be maintained between the liquids within the water electrolyser to ensure that the water electrolyser functions when connected to an electricity supply. Thus in some embodiments, the water electrolyser comprises first and second compartments separated by a separator which substantially prevents mixing of liquid contents of the first and second compartments but maintains ionic conductivity between the liquid contents of the first and second compartments. In some embodiments, the separator comprises a membrane comprising pores which maintain ionic conductivity across the membrane. Such membranes are not ionexchange membranes, since they are uncharged, but instead allow ions to migrate across the membrane through the existence of pores which provide physical channels through which ions are able to pass. In some embodiments, the separator comprises a non-permeable membrane which extends across a first portion of the water electrolyser and provides fluid communication between the first and second compartments across a second portion of the water electrolyser. In some embodiments, the non-permeable membrane is not permeable to the passage of molecules or ions, i.e. does not allow the liquids within the first and second compartments to mix and also does not allow migration of ions across the non-permeable membrane between the first and second compartments. Instead, when such non-permeable membranes are used, ionic conductivity is maintained within the water electrolyser by ensuring that the non-permeable membrane does not extend across the entire water electrolyser and that a region remains where the liquids within the first and second compartments are in physical contact with one another. In some embodiments, the separator comprises a non-permeable membrane which extends from an upstream end of the water electrolyser in a downstream direction, terminating within the water electrolyser to leave a downstream portion of the water electrolyser where the first and second compartments are in fluid communication with one another. Put otherwise, in some embodiments the water electrolyser comprises a downstream region comprising a single, integral “mixing” compartment, and an upstream region where the first and second compartments are separated by a non-permeable separator which substantially prevents mixing of liquid contents of the first and second compartments, wherein each of the first and second compartments are in fluid communication with the mixing compartment. Although in such embodiments the separator will prevent both the physical mixing of liquids and the passage of ions, ionic conductivity is maintained due to the downstream region where liquids and ions can flow across the water electrolyser. In such a “membraneless” design, chloride ions will tend to migrate into the first compartment and sodium ions will tend to migrate into the second compartment (due to the polarisation of the water electrolyser), balancing the sodium and chloride concentrations across the water electrolyser, thereby producing neutral sodium chloride solutions in both first and second compartments, with a concentration approximately equal to the original saltwater feed to the upstream ABFB. Such sodium chloride solutions produced by the water electrolyser can then be safely discharged back to the original source, e.g. the sea or ocean. In some embodiments, the ABFB comprises first and second compartments separated by a separator which substantially prevents mixing of liquid contents of the first and second compartments but maintains ionic conductivity between the liquid contents of the first and second compartments (i.e. the above-described “membraneless” design); and the water electrolyser comprises first and second compartments separated by a separator which substantially prevents mixing of liquid contents of the first and second compartments but maintains ionic conductivity between the liquid contents of the first and second compartments (i.e. the above-described “membraneless” design); wherein the first and second compartments of the ABFB are in fluid communication with the respective first and second compartments (positive and negative electrode channels) of the water electrolyser, such that liquid discharged from the first compartment of the ABFB passes into the first compartment of the water electrolyser, and liquid discharged from the second compartment of the ABFB passes into the second compartment of the water electrolyser. In some embodiments of this arrangement, alkaline solution is generated in the first compartment of the ABFB (proximal the negative electrode of the ABFB) when the ABFB is connected to an electricity supply; acid solution is generated in the second compartment of the ABFB (proximal the positive electrode of the ABFB); oxygen is evolved in the first compartment of the water electrolyser (proximal the positive electrode of the water electrolyser) when the water electrolyser is connected to an electricity supply; and hydrogen is evolved in the second compartment of the water electrolyser (proximal the negative electrode of the water electrolyser). In some embodiments, the ABFB comprises first and second compartments separated by an AEM; the water electrolyser comprises first and second compartments separated by an AEM; wherein the first and second compartments of the ABFB are in fluid communication with the respective first and second compartments of the water electrolyser, such that liquid discharged from the first compartment of the ABFB passes into the first compartment of the water electrolyser, and liquid discharged from the second compartment of the ABFB passes into the second compartment of the water electrolyser. In some embodiments of this arrangement, alkaline solution is generated in the first compartment of the ABFB (proximal the negative electrode of the ABFB) when the ABFB is connected to an electricity supply; acid solution is generated in the second compartment of the ABFB (proximal the positive electrode of the ABFB); oxygen is evolved in the first compartment of the water electrolyser (proximal the positive electrode of the water electrolyser) when the water electrolyser is connected to an electricity supply; and hydrogen is evolved in the second compartment of the water electrolyser (proximal the negative electrode of the water electrolyser). In such embodiments where both the ABFB and the water electrolyser have a 2-channel design with AEM separating the channels, the final product from the water electrolyser will be saltwater with an NaCI concentration approximately equal to that of the saltwater feed to the upstream ABFB. In some embodiments, the ABFB comprises first and second compartments separated by an AEM; the water electrolyser comprises first and second compartments separated by a CEM; wherein the first and second compartments of the ABFB are in fluid communication with the respective first and second compartments of the water electrolyser, such that liquid discharged from the first compartment of the ABFB passes into the first compartment of the water electrolyser, and liquid discharged from the second compartment of the ABFB passes into the second compartment of the water electrolyser. In some embodiments of this arrangement, alkaline solution is generated in the first compartment of the ABFB (proximal the negative electrode of the ABFB) when the ABFB is connected to an electricity supply; acid solution is generated in the second compartment of the ABFB (proximal the positive electrode of the ABFB); oxygen is evolved in the first compartment of the water electrolyser (proximal the positive electrode of the water electrolyser) when the water electrolyser is connected to an electricity supply; and hydrogen is evolved in the second compartment of the water electrolyser (proximal the negative electrode of the water electrolyser). In such embodiments where the ABFB has a 2-channel design with AEM separating the channels, and the water electrolyser has a 2-channel design with CEM separating the channels, the final product from the water electrolyser will be freshwater (desalinated water) in the first compartment and a concentrated brine in the second compartment. Possible uses of these products are discussed above. In some embodiments, the ABFB comprises a third compartment between the first and second compartments, wherein the third compartment is separated from the first compartment by a CEM and the third compartment is separated from the second compartment by an AEM; and the water electrolyser comprises any of the designs described herein. In some embodiments of this arrangement, alkaline solution is generated in the first compartment of the ABFB (proximal the negative electrode of the ABFB) when the ABFB is connected to an electricity supply; acid solution is generated in the second compartment of the ABFB (proximal the positive electrode of the ABFB); freshwater is generated in the third compartment of the ABFB; and hydrogen and oxygen are evolved in the water electrolyser. The freshwater generated in the third compartment of the ABFB can find use as drinking water or for irrigation. In some embodiments, the ABFB comprises a third compartment between the first and second compartments, wherein the third compartment is separated from the first compartment by an AEM and the third compartment is separated from the second compartment by a CEM; and the water electrolyser comprises a third compartment between the first and second compartments, wherein the third compartment is separated from the first compartment by a first ion-permeable membrane and the third compartment is separated from the second compartment by a second ion-permeable membrane; wherein one of the first and second ion-permeable membranes is a CEM and the other is an AEM. In some embodiments of this arrangement, alkaline solution is generated in the first compartment of the ABFB (proximal the negative electrode of the ABFB) when the ABFB is connected to an electricity supply; acid solution is generated in the second compartment of the ABFB (proximal the positive electrode of the ABFB); brine is generated in the third compartment of the ABFB; and hydrogen and oxygen are evolved in the water electrolyser. The brine generated in the third compartment of the ABFB is fed to the third compartment of the water electrolyser, which thereby produces either (a) freshwater, or (b) a more concentrated brine, depending on the arrangement of ion-permeable membranes in the water electrolyser. In some embodiments, the ABFB is connected to an external power supply, e.g. mains electricity. In some embodiments, the water electrolyser is connected to an external power supply, e.g. mains electricity. In some embodiments, one or both of the ABFB and the water electrolyser are powered at least in part by electricity generated by one or more renewable sources. In some embodiments the renewable sources comprise one or more of wind and solar. In some embodiments, the water electrolyser comprises a negative electrode channel inlet, a positive electrode channel inlet and one or more electrolyser liquid outlets. In some embodiments, the water electrolyser comprises first and second compartments separated by an ion-permeable membrane or a non-permeable separator; the first compartment comprising the positive electrode channel inlet and a first electrolyser liquid outlet, and the second compartment comprising the negative electrode channel inlet and a second electrolyser liquid outlet. In some embodiments, the negative electrode channel inlet is an acid solution inlet and the positive electrode channel inlet is an alkaline solution inlet. Process The second aspect of the invention is a process for the electrolysis of saltwater, comprising: providing an acid-base flow battery comprising an acid solution outlet, an alkaline solution outlet and a saltwater inlet; feeding an aqueous solution of sodium chloride into the saltwater inlet of the acid-base flow battery; applying a voltage to the acid-base flow battery to produce both an acidic solution and an alkaline solution within the acid-base flow battery; providing a water electrolyser downstream of the acid-base flow battery for producing hydrogen, comprising a negative electrode, a positive electrode, a negative electrode channel proximal the negative electrode and a positive electrode channel proximal the positive electrode, wherein the acid-base flow battery is in fluid communication with the water electrolyser; feeding at least a portion of the alkaline solution from the alkaline solution outlet of the acid-base flow battery into the positive electrode channel of the water electrolyser; and applying a voltage to the water electrolyser to produce hydrogen gas. In some embodiments, the process of the second aspect uses the system of the first aspect. All of the options and preferences discussed under the first aspect apply to the second aspect mutatis mutandis. In some embodiments, the process of the second aspect comprises feeding at least a portion of the acidic solution from the acid solution outlet of the acid-base flow battery into the negative electrode channel of the water electrolyser. As explained above, this is not necessary for the reduction or elimination of Ch evolution, but may reduce the operation voltage of the electrolyser, thereby improving the efficiency of the system. In some embodiments, the acidic solution from the acid solution outlet of the acid-base flow battery is not fed into the negative electrode channel of the water electrolyser. In such embodiments, an alternative liquid or solution such as water, saltwater (e.g. seawater) or brine may be fed into the negative electrode channel of the water electrolyser. The acidic solution from the acid solution outlet of the acid-base flow battery may be disposed of, or used as a feed to a separate process. In some embodiments, the aqueous solution of sodium chloride is a saltwater solution as described above. In some embodiments, the aqueous solution of sodium chloride comprises or consists of seawater, or a derivative thereof. In some embodiments, the aqueous solution of sodium chloride is a brine containing one or more metal ions used in the manufacture of battery materials. In some embodiments, the aqueous solution of sodium chloride is a brine containing one or more of lithium ions, magnesium ions, nickel ions, cobalt ions and manganese ions. In some embodiments, the aqueous solution of sodium chloride is a brine containing lithium ions. For the reasons already discussed above, this enables the process to produce a more concentrated brine containing such ions, in certain embodiments described in more detail below, and such a more concentrated brine may then be used to recover the ions for use in the manufacture of battery materials. As explained under the first aspect, the design of the water electrolyser in the system and process of the invention is not particularly limited. Provided that the water electrolyser produces hydrogen gas, any type of water electrolyser may be used in the process. In some embodiments, the acidic and alkaline solutions flow continuously from the ABFB to the water electrolyser as the ABFB operates. In some embodiments, the hydrogen produced by the electrolyser is stored for future use. In some embodiments, the hydrogen produced by the electrolyser is transported to the ABFB for use as a charge carrier during cycling of the ABFB. When the hydrogen used by the ABFB is obtained from the electrolyser, purification of the hydrogen should not be necessary because electrolysis tends to produce very high purity hydrogen. If the hydrogen originates from other sources then one or more purification steps may be necessary to remove impurities. A common impurity is carbon monoxide, which may poison a Pt catalyst in the ABFB over time and so would ideally be removed from the H2. In some embodiments, hydrogen gas is fed into the second compartment of the acid-base flow battery and removed from the first compartment of the acid-base flow battery. In some embodiments, the acid-base flow battery comprises means to introduce hydrogen gas into the second compartment and means to remove hydrogen gas from the first compartment; and hydrogen gas is fed into the second compartment via the means to introduce hydrogen gas into the second compartment; and hydrogen gas is removed from the first compartment via the means to remove hydrogen gas from the first compartment. Hydrogen gas may be fed to the second compartment via a gas diffusion electrode and removed from the first compartment via a gas diffusion electrode. The gas diffusion electrodes may be as described under the first aspect. In some embodiments the gas diffusion electrodes are pressurised with hydrogen at a pressure of from 80 kPa to 150 kPa, for example about 100 kPa. In some embodiments, hydrogen gas from the first compartment of the acid-base flow battery is transported to the second compartment of the acid-base flow battery. In some embodiments, hydrogen gas from the first compartment of the acid-base flow battery is transported to the second compartment of the acid-base flow battery through a suitable gas transport conduit, for example flexible metal or alloy tubing. In some embodiments, flexible stainless steel tubing is used. In some embodiments, hydrogen gas is bubbled through the aqueous solution of sodium chloride within the second compartment to dissolve hydrogen into the aqueous solution of sodium chloride within the second compartment Such bubbling may be achieved by features as described under the first aspect, e.g. a hydrogen saturation device such as a gas sparger. Such bubbling is an alternative to delivering hydrogen through a GDE. In some embodiments, hydrogen gas is bubbled through the aqueous solution of sodium chloride within the second compartment to saturate the aqueous solution of sodium chloride with hydrogen. Hydrogen bubbles will form in the first compartment and in some embodiments these hydrogen bubbles are collected and used to supply the hydrogen bubbles to the second compartment. The process comprises feeding at least a portion of the alkaline solution from the alkaline solution outlet of the acid-base flow battery into the positive electrode channel of the water electrolyser. In some embodiments the process may also comprise feeding at least a portion of the acidic solution from the acid solution outlet of the acid-base flow battery into the negative electrode channel of the water electrolyser. In some embodiments, solutions flow continuously through the ABFB, flow continuously from the ABFB to the water electrolyser and flow continuously through the water electrolyser. Hence in some embodiments, the entire process in continuous. In some embodiments, the process comprises one or more preliminary treatment steps before feeding the aqueous solution of sodium chloride to the ABFB. In some embodiments, the process comprises filtration of the aqueous solution of sodium chloride and / or the removal of organic matter from the aqueous solution of sodium chloride. The presence of organic matter in the aqueous solution of sodium chloride (which would be inevitable if the aqueous solution of sodium chloride comprises seawater) would risk fouling the membrane(s) within the ABFB. Although this problem would be circumvented by the “membraneless” ABFB design discussed above, the removal of organic matter may be desirable when an ABFB is used which includes membranes. In some embodiments, the aqueous solution of sodium chloride is irradiated with UV before feeding to the ABFB. Irradiation with UV will remove or reduce the amount of organic matter in the feed. In some embodiments, the process comprises filtration of the aqueous solution of sodium chloride before feeding to the ABFB. Such a filtration step will remove solid material from the feed which could cause blockages within the system. In some embodiments, the process comprises filtration of the alkaline solution generated by the ABFB before feeding to the water electrolyser. In some embodiments, the process comprises filtration of the acidic solution and / or the alkaline solution generated by the ABFB before feeding to the water electrolyser. The operation of the ABFB may produce some precipitates, especially within the first compartment where the alkaline solution is generated. A filtration step between the ABFB and the water electrolyser will remove any such precipitates, which could cause blockages within the water electrolyser. In some embodiments, the ABFB comprises first and second compartments separated by a single ion-permeable membrane (i.e. the first and second compartments are directly adjacent one another); the first compartment comprising the alkaline solution outlet and a first liquid inlet, and the second compartment comprising the acid solution outlet and a second liquid inlet; and the process comprises feeding the aqueous solution of sodium chloride into both the first liquid inlet and the second liquid inlet. In some embodiments, the process comprises flowing the aqueous solution of sodium chloride through the first compartment while applying a voltage to the ABFB to generate the alkaline solution in the first compartment, and flowing the aqueous solution of sodium chloride through the second compartment while applying a voltage to the ABFB to generate the acidic solution in the second compartment. The process may then comprise passing the alkaline solution out of the first compartment through the alkaline solution outlet and into the water electrolyser, and passing the acidic solution out of the second compartment through the acid solution outlet and into the water electrolyser. In some embodiments, the ABFB comprises first and second compartments separated by a single CEM (i.e. the first and second compartments are directly adjacent one another); the first compartment comprising the alkaline solution outlet and a first liquid inlet, and the second compartment comprising the acid solution outlet and a second liquid inlet; and the process comprises feeding the aqueous solution of sodium chloride into both the first liquid inlet and the second liquid inlet. In some embodiments, the process comprises flowing the aqueous solution of sodium chloride through the first compartment while applying a voltage to the ABFB to generate an alkaline sodium chloride solution in the first compartment, and flowing the aqueous solution of sodium chloride through the second compartment while applying a voltage to the ABFB to generate the acidic solution in the second compartment. The process may then comprise passing the alkaline sodium chloride solution out of the first compartment through the alkaline solution outlet and into the water electrolyser, and passing the acidic solution out of the second compartment through the acid solution outlet and into the water electrolyser. In some embodiments, the ABFB comprises first and second compartments separated by a single AEM (i.e. the first and second compartments are directly adjacent one another); the first compartment comprising the alkaline solution outlet and a first liquid inlet, and the second compartment comprising the acid solution outlet and a second liquid inlet; and the process comprises feeding the aqueous solution of sodium chloride into both the first - - liquid inlet and the second liquid inlet. In some embodiments, the process comprises flowing the aqueous solution of sodium chloride through the first compartment while applying a voltage to the ABFB to generate the alkaline solution in the first compartment, and flowing the aqueous solution of sodium chloride through the second compartment while applying a voltage to the ABFB to generate an acidic sodium chloride solution in the second compartment. The process may then comprise passing the alkaline solution out of the first compartment through the alkaline solution outlet and into the water electrolyser, and passing the acidic sodium chloride solution out of the second compartment through the acid solution outlet and into the water electrolyser. In some embodiments, the ABFB comprises a third compartment between the first and second compartments, wherein the third compartment is separated from the first compartment by a first ion-permeable membrane, and the third compartment is separated from the second compartment by a second ion-permeable membrane; the first compartment comprising the alkaline solution outlet and a first liquid inlet, the second compartment comprising the acid solution outlet and a second liquid inlet, and the third compartment comprises a saline solution inlet and a third liquid outlet. In some embodiments, the process comprises flowing the aqueous solution of sodium chloride through the first compartment while applying a voltage to the ABFB to generate the alkaline solution in the first compartment, flowing the aqueous solution of sodium chloride through the second compartment while applying a voltage to the ABFB to generate the acidic solution in the second compartment, and flowing the aqueous solution of sodium chloride through the third compartment while applying a voltage to the ABFB to generate either freshwater or brine in the third compartment. The process may then comprise passing the alkaline solution out of the first compartment through the alkaline solution outlet and into the water electrolyser, passing the acidic solution out of the second compartment through the acid solution outlet and into the water electrolyser, and passing the freshwater or brine out of the third compartment through the third liquid outlet. In some embodiments, the ABFB comprises a third compartment between the first and second compartments, wherein the third compartment is separated from the first compartment by a CEM, and the third compartment is separated from the second compartment by an AEM; the first compartment comprising the alkaline solution outlet and a first liquid inlet, the second compartment comprising the acid solution outlet and a second liquid inlet, and the third compartment comprises a saline solution inlet and a third liquid outlet. In some embodiments, the process comprises flowing the aqueous solution of sodium chloride through the first compartment while applying a voltage to the ABFB to generate the alkaline solution in the first compartment, flowing the aqueous solution of sodium chloride through the second compartment while applying a voltage to the ABFB to generate the acidic solution in the second compartment, and flowing the aqueous solution of sodium chloride through the third compartment while applying a voltage to the ABFB to generate freshwater in the third compartment. The process may then comprise passing the alkaline solution out of the first compartment through the alkaline solution outlet and into the water electrolyser, passing the acidic solution out of the second compartment through the acid solution outlet and into the water electrolyser, and passing the freshwater out of the third compartment through the third liquid outlet. The freshwater may then be stored or transported for future use e.g. as drinking water or for irrigation. In some embodiments, the ABFB comprises a third compartment between the first and second compartments, wherein the third compartment is separated from the first compartment by an AEM, and the third compartment is separated from the second compartment by a CEM; the first compartment comprising the alkaline solution outlet and a first liquid inlet, the second compartment comprising the acid solution outlet and a second liquid inlet, and the third compartment comprises a saline solution inlet and a third liquid outlet. In some embodiments, the process comprises flowing the aqueous solution of sodium chloride through the first compartment while applying a voltage to the ABFB to generate the alkaline solution in the first compartment, flowing the aqueous solution of sodium chloride through the second compartment while applying a voltage to the ABFB to generate the acidic solution in the second compartment, and flowing the aqueous solution of sodium chloride through the third compartment while applying a voltage to the ABFB to generate brine in the third compartment. The process may then comprise passing the alkaline solution out of the first compartment through the alkaline solution outlet and into the water electrolyser, passing the acidic solution out of the second compartment through the acid solution outlet and into the water electrolyser, and passing the brine out of the third compartment through the third liquid outlet. The process may then comprise feeding the brine from the third liquid outlet to the water electrolyser, as described in more detail below. In some embodiments, the ABFB comprises first and second compartments separated by a separator which substantially prevents mixing of liquid contents of the first and second compartments but maintains ionic conductivity between the liquid contents of the first and second compartments, wherein the ABFB comprises an upstream region comprising a mixing compartment which extends across the ABFB between two electrodes, and a downstream region where the first and second compartments are separated by a non-permeable separator which substantially prevents mixing of liquid contents of the first and second compartments; the first compartment comprising the alkaline solution outlet, the second compartment comprising the acid solution outlet, and the mixing compartment comprising one or more liquid inlets; and the process comprises feeding the aqueous solution of sodium chloride into the one or more liquid inlets in the mixing compartment. In some embodiments, the process comprises flowing the aqueous solution of sodium chloride through the mixing compartment and into the first compartment and second compartments while applying a voltage to the ABFB to generate the alkaline solution in the first compartment and the acidic solution in the second compartment. The process may then comprise passing the alkaline solution out of the first compartment through the alkaline solution outlet and into the water electrolyser, and passing the acidic solution out of the second compartment through the acid solution outlet and into the water electrolyser. In some embodiments, the alkaline solution is fed directly from the alkaline solution outlet of the ABFB to the water electrolyser, i.e. without any interruption in flow of the solution but optionally through one or more processing steps e.g. filtration and / or UV treatment. In some embodiments, the acidic solution is fed directly from the acid solution outlet of the ABFB to the water electrolyser, i.e. without any interruption in flow of the solution but optionally through one or more processing steps e.g. filtration and / or UV treatment. In some embodiments, the feed to the positive electrode channel of the electrolyser consists of the effluent from the alkaline solution outlet of the ABFB, i.e. no mixing or dilution of the effluent from the alkaline solution outlet takes place before feeding to the water electrolyser. This ensures maximum increase in pH to ensure minimum risk of evolution of halogens at the positive electrode. However, dilution of the effluent from the alkaline solution outlet of the ABFB or mixing with one or more other liquids may be performed before feeding to the water electrolyser, if desired (e.g. to increase the throughput of liquid through the system). In some embodiments the amount of liquid added to the effluent from the alkaline solution outlet of the ABFB during any such dilution or mixing is such that the risk of evolution of halogens at the positive electrode is kept to a suitably low level. In some embodiments, the acidic solution comprises a solution of hydrochloric acid (HCI). In some embodiments, the pH of the acidic solution on leaving the ABFB is from 0 to 3, for example about 1. In some embodiments, the acidic solution comprises a solution of HCI having a concentration of 0.2 to 1.5 M, for example from 0.25 to 1.0 M. The process comprises feeding at least a portion of the alkaline solution from the ABFB into the first compartment of the water electrolyser. In some embodiments, the alkaline solution comprises a solution of sodium hydroxide (NaOH). In some embodiments, the pH of the alkaline solution on entering the ABFB is from 11 to 14, for example about 14. A higher pH for the solution in the first compartment of the water electrolyser means that electrolysis can continue for longer without the evolution of Cl2. In some embodiments, the alkaline solution comprises a solution of NaOH having a concentration of 0.2 to 1.5 M, for example from 0.25 to 1.0 M. In some embodiments, the acidic solution fed into the second compartment of the water electrolyser and the alkaline solution fed into the first compartment of the water electrolyser have concentrations of acid and base of equal or substantially equal molarity. For example, in some embodiments the acidic solution comprises a solution of HCI having a concentration of 1.0 M and the alkaline solution comprises a solution of NaOH having a concentration of 1.0 M. In some embodiments, the difference in molarity between the acidic solution fed into the second compartment of the water electrolyser and the alkaline solution fed into the first compartment of the water electrolyser is less than 0.2 M, for example less than 0.1 M or less than 0.05 M. In some embodiments, the aqueous solution of sodium chloride fed into the saltwater inlet of the acid-base flow battery comprises or consists of seawater, or a derivative thereof. A derivative of seawater may include seawater which has undergone a preliminary processing step, for example one or more of preliminary desalination, filtration, UV irradiation and heat-treatment. In some embodiments, the aqueous solution of sodium chloride fed into the saltwater inlet of the acid-base flow battery comprises NaCI at a concentration of from 0.005 to 1.0 M, for example from 0.005 to 0.8 M, from 0.005 to 0.6 M or from 0.01 to 0.6 M. 0.01 M NaCI solution is considered to be potable “fresh water”, whereas 0.6 M NaCI solution corresponds to the salinity of seawater. In some embodiments, the process further comprises taking a brine by-product from either (a) the ABFB, or (b) the water electrolyser, and using it to produce one or more of an acid solution and an alkaline solution, wherein one or both of the acid solution and alkaline solution so produced are fed to the water electrolyser for use in further hydrogen generation. The one or more of an acid solution and an alkaline solution may be generated from the brine by-product by, for example, performing electrolysis of the brine. In some embodiments, the brine by-product of the process (e.g. from the 3-channel ABFB, the 2-channel CEM-containing water electrolyser or the 3-shannel water electrolyser) is subjected to freshwater-saltwater reverse electrodialysis (RED) to convert dilution energy to usable electricity. In this way, a by-product of the process which may be considered to be a pollutant is put to good use. In some embodiments, the water electrolyser comprises the positive and negative electrode channels (first and second compartments, respectively) separated by a single ion-permeable membrane (i.e. the positive and negative electrode channels are directly adjacent one another); the positive electrode channel comprising an alkaline solution inlet and a first liquid outlet, and the negative electrode channel comprising an acid solution inlet and a second liquid outlet; and the process comprises feeding at least a portion of the acidic solution from the acid solution outlet of the ABFB into the negative electrode channel of the water electrolyser, and feeding at least a portion of the alkaline solution from the alkaline solution outlet of the ABFB into the positive electrode channel of the water electrolyser. In some embodiments, the process comprises flowing said solutions through the channels of the water electrolyser while applying a voltage to the water electrolyser to generate hydrogen in the negative electrode channel (second compartment). The process may then comprise passing first and second product solutions out of the positive and negative electrode channels through the first and second liquid outlets respectively. The composition of these product solutions depends on the type of ion-permeable membrane used in the water electrolyser. In some embodiments, the water electrolyser comprises the positive and negative electrode channels (first and second compartments, respectively) separated by a single CEM (i.e. the positive and negative electrode channels are directly adjacent one another); the positive electrode channel comprising an alkaline solution inlet and a freshwater outlet, and the negative electrode channel comprising an acid solution inlet and a brine outlet; and the process comprises feeding at least a portion of the acidic solution from the acid solution outlet of the ABFB into the negative electrode channel of the water electrolyser, and feeding at least a portion of the alkaline solution from the alkaline solution outlet of the ABFB into the positive electrode channel of the water electrolyser. In some embodiments, the process comprises flowing said solutions through the channels of the water electrolyser while applying a voltage to the water electrolyser to generate hydrogen in the negative electrode channel (second compartment). The products from the electrolyser in such embodiments depends upon the type of ionexchange membrane used in the ABFB, as follows: When the ABFB comprises first and second compartments separated by an AEM, the CEM-containing electrolyser will produce desalinated water (e.g. freshwater) in the positive electrode channel and concentrated saltwater (e.g. brine) in the negative electrode channel. The process may then comprise passing freshwater out of the positive electrode channel through the freshwater outlet and passing brine out of the negative electrode channel through the brine outlet. The process may then comprise storing the freshwater from the freshwater outlet for future use. In some embodiments, the process comprises using the freshwater from the freshwater outlet as drinking water or for irrigation. The process may comprise storing the brine from the brine outlet for future use. In some embodiments, the process comprises using the brine from the brine outlet by either (a) recycling the brine for use in a subsequent electrolysis process, (b) discharging the brine back to the original saltwater source, e.g. the sea or ocean, or (c) using the brine in a parallel process such as a chlor-alkali process (or in the manufacture of battery materials, as explained above, when the initial saltwater feed to the ABFB contains one or more ions, such as lithium, which find use in battery materials). However when the ABFB comprises first and second compartments separated by a CEM, the CEM-containing electrolyser will produce saltwater (e.g. seawater) in both positive and negative electrode channels. The process may then comprise passing saltwater out of the positive electrode channel through the first saltwater outlet and passing saltwater out of the negative electrode channel through the second saltwater outlet. The saltwater streams from the first and second saltwater outlets may each have an NaCI concentration substantially the same as the original saltwater source fed to the ABFB. The process may then comprise discharging the saltwater streams back to the original saltwater source, e.g. the sea or ocean. In some embodiments, the water electrolyser comprises the positive and negative electrode channels (first and second compartments, respectively) separated by a single AEM (i.e. the positive and negative electrode channels are directly adjacent one another); the positive electrode channel comprising an alkaline solution inlet and a first saltwater outlet, and the negative electrode channel comprising an acid solution inlet and a second saltwater outlet; and the process comprises feeding at least a portion of the acidic solution from the acid solution outlet of the ABFB into the negative electrode channel of the water electrolyser, and feeding at least a portion of the alkaline solution from the alkaline solution outlet of the ABFB into the positive electrode channel of the water electrolyser. In some embodiments, the process comprises flowing said solutions through the channels of the water electrolyser while applying a voltage to the water electrolyser to generate hydrogen in the negative electrode channel (second compartment). The products from the electrolyser in such embodiments depends upon the type of ion-exchange membrane used in the ABFB, as follows: When the ABFB comprises first and second compartments separated by an AEM, the AEM-containing electrolyser will produce saltwater (e.g. seawater) in both positive and negative electrode channels. The process may then comprise passing saltwater out of the positive electrode channel through the first saltwater outlet and passing saltwater out of the negative electrode channel through the second saltwater outlet. The saltwater streams from the first and second saltwater outlets may each have an NaCI concentration substantially the same as the original saltwater source fed to the ABFB. The process may then comprise discharging the saltwater streams back to the original saltwater source, e.g. the sea or ocean. However when the ABFB comprises first and second compartments separated by a CEM, the AEM-containing electrolyser will produce desalinated water (e.g. freshwater) in the positive electrode channel and concentrated saltwater (e.g. brine) in the negative electrode channel. The process may then comprise passing freshwater out of the positive electrode channel through the freshwater outlet and passing brine out of the negative electrode channel through the brine outlet. The process may then comprise storing the freshwater from the freshwater outlet for future use. In some embodiments, the process comprises using the freshwater from the freshwater outlet as drinking water or for irrigation. The process may comprise storing the brine from the brine outlet for future use. In some embodiments, the process comprises using the brine from the brine outlet by either (a) recycling the brine for use in a subsequent electrolysis process, (b) discharging the brine back to the original saltwater source, e.g. the sea or ocean, or (c) using the brine in a parallel process such as a chlor-alkali process, or in the manufacture of battery materials, as explained above, when the initial saltwater feed to the ABFB contains one or more ions, such as lithium, which find use in battery materials. In some embodiments, the water electrolyser comprises a third compartment between the positive and negative electrode channels (first and second compartments respectively), wherein the third compartment is separated from the positive electrode channel by a first ion-permeable membrane, and the third compartment is separated from the negative electrode channel by a second ion-permeable membrane; the positive electrode channel comprising an alkaline solution inlet and a first liquid outlet, the negative electrode channel comprising an acid solution inlet and a second liquid outlet, and the third compartment comprising a saline solution inlet and a third liquid outlet; wherein the process comprises feeding at least a portion of the acidic solution from the acid solution outlet of the ABFB into the negative electrode channel of the water electrolyser, feeding at least a portion of the alkaline solution from the alkaline solution outlet of the ABFB into the positive electrode channel of the water electrolyser, and feeding an aqueous solution of sodium chloride into the third compartment of the water electrolyser. This aqueous solution of sodium chloride may be seawater or a solution with an NaCI concentration higher than seawater, e.g. brine. In some embodiments, the process comprises flowing said solutions through the channels of the water electrolyser while applying a voltage to the water electrolyser to generate hydrogen in the negative electrode channel (second compartment). The process may then comprise passing first and second product solutions out of the positive and negative electrode channels through the first and second liquid outlets respectively; and passing a third product solution out of the third compartment through the third liquid outlet. The composition of these product solutions depends on the type of ion-permeable membranes used in the water electrolyser. In some embodiments, the water electrolyser comprises a third compartment between the positive and negative electrode channels (first and second compartments respectively), wherein the third compartment is separated from the positive electrode channel by a CEM, and the third compartment is separated from the negative electrode channel by an AEM; the positive electrode channel comprising an alkaline solution inlet and a first liquid outlet, the negative electrode channel comprising an acid solution inlet and a second liquid outlet, and the third compartment comprising a saline solution inlet and a third liquid outlet; wherein the process comprises feeding at least a portion of the acidic solution from the acid solution outlet of the ABFB into the negative electrode channel of the water electrolyser, feeding at least a portion of the alkaline solution from the alkaline solution outlet of the ABFB into the positive electrode channel of the water electrolyser, and feeding an aqueous solution of sodium chloride into the third compartment of the water electrolyser. This aqueous solution of sodium chloride may be seawater or a solution with an NaCI concentration higher than seawater, e.g. brine. In some embodiments, the process comprises flowing said solutions through the channels of the water electrolyser while applying a voltage to the water electrolyser to generate hydrogen in the negative electrode channel (second compartment). The process may then comprise passing freshwater out of the positive and negative electrode channels through the first and second liquid outlets respectively; and passing a concentrated brine out of the third compartment through the third liquid outlet. In some embodiments, the water electrolyser comprises a third compartment between the positive and negative electrode channels (first and second compartments respectively), wherein the third compartment is separated from the positive electrode channel by an AEM, and the third compartment is separated from the negative electrode channel by a CEM; the positive electrode channel comprising an alkaline solution inlet and a first liquid outlet, the negative electrode channel comprising an acid solution inlet and a second liquid outlet, and the third compartment comprising a saline solution inlet and a third liquid outlet; wherein the process comprises feeding at least a portion of the acidic solution from the acid solution outlet of the ABFB into the negative electrode channel of the water electrolyser, feeding at least a portion of the alkaline solution from the alkaline solution outlet of the ABFB into the positive electrode channel of the water electrolyser, and feeding an aqueous solution of sodium chloride into the third compartment of the water electrolyser. This aqueous solution of sodium chloride may be seawater or a solution with an NaCI concentration higher than seawater, e.g. brine. In some embodiments, the process comprises flowing said solutions through the channels of the water electrolyser while applying a voltage to the water electrolyser to generate hydrogen in the negative electrode channel (second compartment). The process may then comprise passing an aqueous solution of sodium chloride out of the positive and negative electrode channels through the first and second liquid outlets respectively; and passing an aqueous solution of sodium chloride out of the third compartment through the third liquid outlet. Each of the aqueous solutions of sodium chloride may have an NaCI concentration substantially the same as the concentration in the original feed to the ABFB, e.g. substantially the same as seawater. In some embodiments, the water electrolyser comprises positive and negative electrode channels (first and second compartments respectively) separated by a separator which substantially prevents mixing of liquid contents of the positive and negative electrode channels but maintains ionic conductivity between the liquid contents of the positive and negative electrode channels, wherein the water electrolyser comprises a downstream region comprising a mixing compartment which extends across the water electrolyser between two electrodes, and an upstream region where the positive and negative electrode channels are separated by a non-permeable separator which substantially prevents mixing of liquid contents of the positive and negative electrode channels; the positive electrode channel comprising the alkaline solution inlet, the negative electrode channel comprising the acid solution inlet, and the mixing compartment comprising one or more liquid outlets; and the process comprises feeding at least a portion of the acidic solution from the acid solution outlet of the ABFB into the negative electrode channel of the water electrolyser and feeding at least a portion of the alkaline solution from the alkaline solution outlet of the ABFB into the positive electrode channel of the water electrolyser. In some embodiments, the process comprises flowing the solutions through the positive and negative electrode channels and into the mixing compartment while applying a voltage to the water electrolyser to generate hydrogen. The process may then comprise passing an aqueous solution of sodium chloride through the one or more liquid outlets. In some embodiments the aqueous solution of sodium chloride passed through the one or more liquid outlets is then discharged back to the original saltwater source, e.g. the sea or ocean. In some embodiments, at least a portion of the hydrogen gas generated by the water electrolyser is passed to the second compartment of the acid-base flow battery. Another aspect of the invention is the use of the system according to the first aspect. In some embodiments, the use of the system comprises use of the system to produce one or more of (a) hydrogen, (b) oxygen, (c) desalinated water, such as freshwater, and (d) brine. Another aspect of the invention is the use of the system according to the first aspect to reduce or eliminate the evolution of gaseous halogen species during the direct electrolysis of seawater. In some embodiments, the use of the system is to reduce or eliminate the evolution of chlorine during the direct electrolysis of seawater. Another aspect is the use of the system according to the first aspect to generate hydrogen and simultaneously do one or more of (a) produce freshwater from a saltwater feed, (b) produce brine from a saltwater feed, and (c) regenerate saltwater of substantially the same NaCI concentration as the original saltwater feed to the ABFB. Figures Figure 1 shows (a) a perspective exploded view of an electrolytic cell used in an ABFB within the system according to the invention, and (b) a perspective assembled view of the same electrolytic cell. Figure 2 shows a further exploded representation of the same cell showing the flow of different reagents through the system. Figure 3 shows a side view of the cell of Figures 1 and 2. Figure 4 shows the FKM rubber gaskets used within the cell to create conduits and flow fields for different reagents. These include (a) a gasket creating a flow field and compartment for the acidic solution; (b) a gasket creating a flow field and compartment for the alkaline solution; (c) a gasket creating a flow field and compartment for the hydrogen gas; and (d) a gasket creating a flow field and compartment for the saline solution. Figure 5 shows a schematic cross-sectional representation of a 2-channel acid-base flow battery. Figure 6 shows a schematic cross-sectional representation of a 2-channel acid-base flow battery. Figure 7 shows a schematic cross-sectional representation of a 2-channel acid-base flow battery. Figure 8 shows a schematic cross-sectional representation of a 3-channel acid-base flow battery. Figure 9 shows a schematic cross-sectional representation of a 3-channel acid-base flow battery. Figure 10 shows a schematic cross-sectional representation of a 2-channel water electrolyser. Figure 11 shows a schematic cross-sectional representation of a 2-channel water electrolyser. Figure 12 shows a schematic cross-sectional representation of a 2-channel water electrolyser. Figure 13 shows a schematic cross-sectional representation of a 3-channel water electrolyser. Figure 14 shows a schematic cross-sectional representation of a 3-channel water electrolyser. Figure 15 shows a schematic representation of a process according to the invention. Examples An ABFB was assembled utilising an electrochemical cell made in-house with an active geometrical area of 9 cm2. A schematic (exploded) diagram of the cell 100 is shown in Figure 1(a). The assembled cell 100 is shown in Figure 1(b). The cell 100 is made up of layers sandwiched between two end plates 101 and 102. The end plates 101, 102 are each milled out of steel with a PVC core for chemical compatibility and physical durability. The layers are held together by passing four pins 103 (only one pin labelled for clarity) through the end plates and all intervening layers, and securing the pins to each end plate 101, 102. End plate 102 includes H2 outlet 104a, saline solution outlet 104b, acidic solution outlet 104c, H2 inlet 104d, saline solution inlet 104e and saline solution inlet 104f. These allow for passing H2, neutral solution (e.g. H2O or salt solution) and acidic solutions into and out of the cell. End plate 101 includes a H2 outlet, an alkaline solution outlet, a H2 inlet and a saline solution inlet, none of which are visible in Figure 2. A first FKM rubber gasket of 3 mm thickness 105 creates a H2 gas flow field between the end plate 102 and a gas diffusion electrode 111. A similar second FKM rubber gasket of 3 mm thickness 106 creates a H2 gas flow field between the end plate 101 and a gas diffusion electrode 112. H2 outlet 104a and H2 inlet 104d are each connected to an external H2 gas reservoir. The H2 flows into the gas field created by the FKM gasket 105. A similar arrangement at the end plate 101 allows H2 to enter the gas field created by the FKM gasket 106. Each of the gas diffusion electrodes 111, 112 were custom ordered from Fuel Cell Store with platinum loading of 0.5 mg cm-2 and hydrophobic treatment on a carbon cloth substrate. The electrodes were then backed with two hydrophobic carbon paper layers to form gas diffusion electrode layers 111, 112. Gas diffusion electrode 111 is sandwiched between a current collector layer 107 which is a laminate of a graphite foil with a mylar layer, and a 1 mm thick FKM rubber gasket 113 which acted as a membrane cap to prevent leaking. A copper strip 109 is attached to the current collector layer 107 to provide an electrical contact. Gas diffusion electrode 112 is sandwiched between a current collector layer 108 which is a laminate of a graphite foil with a mylar layer, and a 1 mm thick FKM rubber gasket 114 which acted as a membrane cap to prevent leaking. A copper strip 110 is attached to the current collector layer 108 to provide an electrical contact. Adjacent the gasket 113 is a further FKM rubber gasket of 3 mm thickness 115 which creates a second compartment for the introduction and residence of a saltwater solution (e.g. seawater) which will be converted into an alkaline solution during charging of the cell. The saltwater solution flows into the inlet 104f and then through aligned apertures formed in each of the layers 105, 107 and 113 in turn. These aligned apertures form a conduit for the saline solution. Due to the design of the FKM rubber gasket 115, this conduit terminates at the FKM rubber gasket 115 such that the saline solution enters and occupies the first compartment created by the FKM rubber gasket 115. Acidic solution generated during charging the cell can then leave the ABFB through the outlet 104c through a similar aligned set of apertures in each of the layers 113, 107 and 105 in turn. Similarly, adjacent the gasket 114 is a further FKM rubber gasket of 3 mm thickness 116 which creates a first compartment for the introduction and residence of a saltwater solution (e.g. seawater) which will be converted into an alkaline solution during charging of the cell. The saline solution flows into an inlet (not shown) in the end plate 101 and then through aligned apertures formed in each of the layers 106, 108 and 114 in turn. These aligned apertures form a conduit for the saline solution. Due to the design of the FKM rubber gasket 116, this conduit terminates at the FKM rubber gasket 116 such that the saline solution enters and occupies the first compartment created by the FKM rubber gasket 116. Alkaline solution generated during charging the cell can then leave the ABFB through the alkaline solution outlet (not shown) in end plate 101 through a similar aligned set of apertures in each of the layers 105, 107 and 113 in turn. A cathode exchange membrane (CEM) 119 (Nation 1110, purchased from Fuel Cell Store) is sandwiched between two 1 mm thick FKM rubber gaskets 117 and 121 and placed adjacent the acidic solution flow field FKM gasket 115. Thus acidic solution within the acidic solution flow field FKM gasket 115 makes contact with the CEM 119 such that cations within the acidic solution can pass through the CEM 119. Similarly, an anion exchange membrane (AEM) 120 (Fumasep FAB-PK-130, purchased from Fuel Cell Store) is sandwiched between two 1 mm thick FKM rubber gaskets 118 and 122 and placed adjacent the alkaline solution flow field FKM gasket 116. Thus alkaline solution within the alkaline solution flow field FKM gasket 116 makes contact with the AEM 120 such that cations within the alkaline solution can pass through the AEM 120. Finally, between the CEM 119 on one side and the CEM 120 on the other side lies a further FKM rubber gasket of 3 mm thickness 123 which creates a third compartment for the introduction and residence of another saline solution (e.g. seawater). The saline solution flows into a saline solution inlet 104b in the end plate 102 and then through aligned apertures formed in each of the layers 105, 107, 113, 115, 117 and 121 in turn. These aligned apertures form a conduit for the saline solution. Due to the design of the FKM rubber gasket 123, this conduit terminates at the FKM rubber gasket 123 such that the saline solution enters and occupies the third compartment created by the FKM rubber gasket 123. Saline solution can then leave the ABFB through the saline solution outlet 104e through a similar aligned set of apertures in each of the layers 121, 117, 115, 113, 107 and 105 in turn. Figure 1b shows the ABFB 100 after all the layers are assembled and fixed together to create a sealed unit. Figure 2 demonstrates the flow of H2, acidic (HCI) solution and saline (NaCI) solution into and out of one half of the ABFB, through the inlets and outlets 104a-f which are shown in Figure 1a. FKM rubber gaskets 105,115,123, 116 and 106 are shown in more detail in Figure 4. Figure 4(c) shows the hydrogen gas flow field gasket which is used for gaskets 105 and 106 (which are identical). Figure 4(a) shows the acidic solution flow field gasket which is used for gasket 115. Figure 4(b) shows the alkaline solution flow field gasket which is used for gasket 116. Figure 4(d) shows the saline solution flow field gasket which is used for gasket 123. The active area (flow area) was 3x3 cm2, the width was 3 mm, so the total active volume was 2.7 cm3. The membranes and GDEs were cut out to be 4 x 4 cm2 to fully cover the active area. The membranes and GDEs were sandwiched between membrane caps which were 1 mm thick FKM rubber sheeting. An Ismatec Reglo 4-channel peristaltic pump was used to provide individual flow rates with chemically durable pump tubing and PVC tubing used to transport solution to and from the cell. High purity hydrogen gas (N4) was flowed through at 30 mL min’1. A CHI600C potentiostat was used for potentiostatic discharge and charge experiments while galvanostatic experiments were done on a B&K-Precision instrument. The cell was stacked and compressed by tightening four bolts through the end plates with a torque wrench. Figure 5 shows a schematic cross-sectional representation of an ABFB 200 which may be used as part of the present system. The ABFB 200 includes a first compartment 201 intended to receive a flow of saltwater, e.g. seawater, and a second compartment 202 intended to receive a flow of saltwater, e.g. seawater. The first compartment 201 includes a first liquid inlet 203 into which the saltwater will flow during use, and an alkaline solution outlet 204, out of which alkaline solution will flow during use after being generated within the first compartment 201 during operation of the ABFB. The second compartment 202 includes a second liquid inlet 205 into which the saltwater will flow during use, and an acid solution outlet 206, out of which acidic solution will flow during use after being generated within the second compartment 202 during operation of the ABFB. The first compartment 201 is separated from the second compartment 202 by a cation exchange membrane (CEM) 211 which permits the flow of cations between the first and second compartments. Since during use both first and second compartments will contain a solution of NaCI, Na+ ions will pass through the CEM 211 from the second compartment 202 into the first compartment 201, due to the polarisation of the ABFB during charging. The direction of flow of solutions through the ABFB 200 in Figure 5 is shown by block arrows within the first and second compartments. A first gas diffusion electrode (GDE) 209 separates the first compartment 201 from a first hydrogen gas field 207, and a second gas diffusion electrode (GDE) 210 separates the second compartment 202 from a second hydrogen gas field 208. The first and second GDEs are connected to one another via an external power source (not shown). Both first and second hydrogen gas fields are kept pressurised with hydrogen during use of the ABFB to enable the passage of hydrogen into the second compartment 202 through the second GDE 210 to allow hydrogen to be consumed within the second compartment 202 to produce an acidic solution, and to enable the passage of hydrogen out of the first compartment 201 through the first GDE 209 when hydrogen is produced within the first compartment 201 during production of an alkaline solution. During use of the ABFB shown in Figure 5, seawater (which may have been subjected to a pre-treatment step such as filtration to remove solid matter and / or UV irradiation to remove organic matter) is fed into each of the first liquid inlet 203 and second liquid inlet 205, the hydrogen gas fields 207 and 208 are pressurised with hydrogen and a voltage is applied across the GDEs 209, 210 of the ABFB. This causes the ABFB to “charge”, hydrogen is oxidised in the second compartment 202 to generate protons and produce an acidic solution, and water is reduced in the first compartment 201 to generate hydrogen and hydroxide ions and produce an alkaline solution. The so-produced acidic solution passes out of the acid solution outlet 206 and the so-produced alkaline solution passes out of the alkaline solution outlet 204. The alkaline solution is then fed to a downstream water electrolyser as described below. The acidic solution may also be fed to the downstream water electrolyser as described below. Due to the presence of the CEM 211, sodium ions will flow from the acidic solution in the second compartment 202 into the alkaline solution in the first compartment 201, such that the alkaline solution will contain dissolved NaCI. The ABFB 220 shown in Figure 6 is almost identical to that of Figure 5, except that the first compartment 201 is separated from the second compartment 202 by an anion exchange membrane (AEM) 221 which permits the flow of anions between the first and second compartments (instead of the CEM in Figure 5). Since during use both first and second compartments will contain a solution of NaCI, Cl’ ions will pass through the AEM 221 from the first compartment 201 into the second compartment 202, due to the polarisation of the ABFB during charging. The result is that the acidic solution produced in the second compartment 202 will contain dissolved NaCI. Otherwise the structure and function of the ABFB 220 in Figure 6 is the same as ABFB 200 in Figure 5. The ABFB 230 in Figure 7 is very similar to those shown in Figures 5 and 6 and like features are given the same numbering in the Figure. The only difference is that, instead of a CEM or AEM separating the first and second compartments 201,202, the ABFB 230 includes a non-permeable separator 231 which extends part-way into the ABFB 230, extending from the downstream end of the ABFB and extending in an upstream direction before terminating within the ABFB. This creates a mixing compartment 232 at the upstream end of the ABFB, providing fluid communication between the first compartment 201 and second compartment 202. This means that, as the saltwater solutions enter the first liquid inlet 203 and second liquid inlet 205, ionic conductivity across the cell is maintained and ions are able to pass freely in the region between the first GDE 209 and the second GDE 210. Eventually the flow within the cell separates the solutions into the first compartment 201 and the second compartment 202, where any further physical mixing between the two solutions is prevented by the non-permeable separator 231 and the flow of the solutions which prevents any return of the solutions to the mixing compartment 232. As a result, the ABFB 230 is able to be charged in a similar way to those shown in Figures 5 and 6 due to the ionic conductivity maintained by the mixing compartment 232, however the physical integrity of the acidic and alkaline solutions produced during charging of the ABFB 230 is also preserved due to the prevention of mixing by the non-permeable separator 231 and the flow of the solutions. Since both sodium and chloride ions are able to move freely across the cell within the mixing compartment, Na+ ions will tend to pass from the second compartment 202 into the first compartment 201, and Cl’ ions will tend to pass from the first compartment 201 into the second compartment 202, due to the polarisation of the ABFB during charging. The result is acidic and alkaline solutions which do not contain any appreciable concentration of NaCI. An alternative 3-channel ABFB 240 is shown in schematic cross-section in Figure 8. The ABFB 240 includes a first compartment 241 intended to receive a flow of saltwater, e.g. seawater, a second compartment 242 intended to receive a flow of saltwater, e.g. seawater, and a third compartment 251 intended to receive a flow of saltwater, e.g. seawater. The first compartment 241 includes a first liquid inlet 243 into which the saltwater will flow during use, and an alkaline solution outlet 244, out of which alkaline solution will flow during use after being generated within the first compartment 241 during operation of the ABFB. The second compartment 242 includes a second liquid inlet 245 into which the saltwater will flow during use, and an acid solution outlet 246, out of which acidic solution will flow during use after being generated within the second compartment 242 during operation of the ABFB. The first compartment 241 is separated from the third compartment 251 by a CEM 254 which permits the flow of cations between the first and third compartments. Since during use both first and third compartments will contain a solution of NaCI, Na+ ions will pass through the CEM 254 from the third compartment 251 into the first compartment 241, due to the polarisation of the ABFB during charging. The second compartment 242 is separated from the third compartment 251 by an AEM 255 which permits the flow of anions between the second and third compartments. Since during use both second and third compartments will contain a solution of NaCI, Ch ions will pass through the AEM 255 from the third compartment 251 into the second compartment 242, due to the polarisation of the ABFB during charging. The third compartment 251 includes a third liquid inlet 252 into which saltwater will flow during use, and a third liquid outlet 253, out of which freshwater will flow during use after being desalinated within the third compartment 251 during operation of the ABFB (due to movement of both Na+ and Cl’ ions out of the third compartment 251 as described above). Hence the 3-channel ABFB shown in Figure 8 can be used to simultaneously produce the alkaline and acidic solutions in the first and second compartments respectively to be used in the downstream electrolyser, and desalinate seawater in the third compartment. The direction of flow of solutions through the ABFB 240 in Figure 8 is shown by block arrows within the first and second compartments. A first GDE 249 separates the first compartment 241 from a first hydrogen gas field 247, and a second GDE 250 separates the second compartment 242 from a second hydrogen gas field 248. Both first and second hydrogen gas fields are kept pressurised with hydrogen during use of the ABFB to enable the passage of hydrogen into the second compartment 242 through the second GDE 250 to allow hydrogen to be consumed within the second compartment 242 to produce an acidic solution, and to enable the passage of hydrogen out of the first compartment 241 through the first GDE 249 when hydrogen is produced within the first compartment 202 during production of an alkaline solution. During use of the ABFB shown in Figure 8, seawater (which may have been subjected to a pre-treatment step such as filtration to remove solid matter and / or UV irradiation to remove organic matter) is fed into each of the first liquid inlet 243, second liquid inlet 245 and third liquid inlet 252, the hydrogen gas fields 247 and 248 are pressurised with hydrogen and a voltage is applied across the GDEs 249, 250 of the ABFB. This causes the ABFB to “charge”, hydrogen is oxidised in the second compartment 242 to generate protons and produce an acidic solution, and water is reduced in the first compartment 241 to generate hydrogen and hydroxide ions and produce an alkaline solution. Simultaneously the seawater in the third compartment 251 is desalinated. The so-produced acidic solution passes out of the acid solution outlet 246 and the so-produced alkaline solution passes out of the alkaline solution outlet 244. The desalinated water passes out of the third liquid outlet 253 (which in this embodiment may be referred to as a freshwater outlet). The alkaline solution (and optionally also the acidic solution) is then fed to a downstream water electrolyser as described below. The freshwater may be used for e.g. drinking water or irrigation. The ABFB 260 shown in Figure 9 is almost identical to that of Figure 8, except that the positions of the CEM and AEM are exchanged. This means that, instead of the seawater within the third compartment 251 becoming desalinated, it will become more concentrated (since Na+ ions will move into the third compartment 251 from the second compartment 242 and Cl’ ions will move into the third compartment 251 from the first compartment 241). As a result, a brine will pass out of the third liquid outlet 253 (which in this embodiment may be referred to as a brine outlet). The brine may be discharged back into the sea or ocean, or fed to an electrolyser as discussed below. Alternatively, it may be used as a feed for a different process, e.g. a chlor-alkali process, or in the manufacture of battery materials, as explained above, when the initial saltwater feed to the ABFB contains one or more ions, such as lithium, which find use in battery materials. Figure 10 shows a schematic cross-sectional representation of a water electrolyser 300 which may be used as part of the present system, downstream of the ABFB. The water electrolyser includes a first compartment 301 (also referred to as a positive electrode channel 301) which receives a feed of alkaline solution from the ABFB and a second compartment 302 (also referred to as a negative electrode channel 302) which receives a feed of liquid to enable electrolysis to occur, e.g. acidic solution from the ABFB. The water electrolyser 300 includes positive electrode channel inlet 306 into which alkaline solution from the first compartment of the ABFB passes, and negative electrode channel inlet 308 into which solution passes, e.g. acidic solution from the second compartment of the ABFB. In the embodiments described in detail below, acidic solution is fed into the negative electrode channel inlet of the various electrolysers described; however it will be understood that this is not necessary to achieve the reduced or eliminated evolution of halogens at the positive electrode, but is one possible embodiment of the invention. The first compartment 301 and second compartment 302 are separated by a CEM 305, which allows cations to pass between the first compartment 301 and second compartment 302. Na+ ions will therefore tend to pass from the first compartment 301 into the second compartment 302, due to the polarisation of the water electrolyser during operation. The result is that freshwater will be produced during electrolysis in the first compartment 301 and will pass out of a first liquid outlet 307 and a concentrated brine will be produced in the second compartment 302 and will pass out of a second liquid outlet 309. The electrolyser includes a positive electrode 303 and a negative electrode 304, connected to an external power source (not shown). When a voltage is applied to the electrolyser, electrolysis of the acidic and alkaline solutions will occur, producing hydrogen gas at the negative electrode (second compartment 302) and oxygen gas at the positive electrode (first compartment 301). The freshwater and brine products can be used in applications already described above. The hydrogen and oxygen gases can be collected using methods known to the skilled person and stored or transported for future use. The hydrogen can be stored and used e.g. as a fuel for combustion to generate heat or electricity, or as a fuel for a hydrogen fuel cell. The water electrolyser 320 shown in Figure 11 is almost identical to that of Figure 10, except that the first compartment 301 is separated from the second compartment 302 by an AEM 325 which permits the flow of anions between the first and second compartments (instead of the CEM in Figure 10). Cl’ ions will pass through the AEM 221 from the second compartment 302 into the first compartment 301, due to the polarisation of the water electrolyser during electrolysis. The result is that both the first and second compartments will produce a saltwater product, both of which can be safely returned to the original source, e.g. the sea or ocean. The water electrolyser 340 in Figure 12 is very similar to those shown in Figures 10 and 11 and like features are given the same numbering in the Figure. The only difference is that, instead of a CEM or AEM separating the first and second compartments 301,302, the water electrolyser 340 includes a non-permeable separator 345 which extends partway into the water electrolyser 340, extending from the upstream end of the water electrolyser 340 and extending in a downstream direction before terminating within the water electrolyser 340. This creates a mixing compartment 342 at the downstream end of the water electrolyser 340, providing fluid communication between the first compartment 301 and second compartment 302. This means that, as the alkaline and acidic solutions enter the alkaline solution inlet 306 and acidic solution inlet 308, ionic conductivity across the electrolyser is maintained and ions are able to pass freely in the region between the positive electrode 303 and the negative electrode 304. The alkaline and acidic solutions initially remain separate, but eventually the flow within the electrolyser brings the two solutions into contact in the region defined as the mixing compartment 342, ensuring ionic conductivity between the electrodes and functioning of the electrolyser. As a result, the water electrolyser 340 is able to operate in a similar way to those shown in Figures 10 and 11 due to the ionic conductivity maintained by the mixing compartment 342, however since both sodium and chloride ions are able to move freely across the cell within the mixing compartment 342, Na+ ions will tend to pass from the first compartment 301 into the second compartment 302, and Cl’ ions will tend to pass from the second compartment 302 into the first compartment 301, due to the polarisation of the water electrolyser 340 during operation. The result is a homogeneous saltwater product from the first and second liquid outlets 307,309. Note that since the product in the mixing compartment 342 is homogenous, a single liquid outlet could replace the first and second liquid outlets. An alternative 3-channel water electrolyser 360 is shown in schematic cross-section in Figure 13. The water electrolyser 360 includes a first compartment 361 intended to receive a flow of alkaline solution from the ABFB, a second compartment 362 intended to receive a flow of acidic solution from the ABFB, and a third compartment 363 intended to receive a flow of saltwater, e.g. seawater or brine, from either a natural source or from effluent of a parallel process or another part of the present process. The first compartment 361 includes an alkaline solution inlet 364 into which the alkaline solution will flow during use, and a first liquid outlet 365, out of which product solution will flow during use after being generated within the first compartment 361 during operation of the water electrolyser 360. The second compartment 362 includes an acidic solution inlet 366 into which the acidic solution will flow during use, and a second liquid outlet 367, out of which product solution will flow during use after being generated within the second compartment 362 during operation of the water electrolyser 360. The electrolyser 360 includes a positive electrode 372 and a negative electrode 373, connected to an external power source (not shown). When a voltage is applied to the electrolyser, electrolysis of the acidic and alkaline solutions will occur, producing hydrogen gas at the negative electrode 373 (second compartment 362) and oxygen gas at the positive electrode 372 (first compartment 361). The first compartment 361 is separated from the third compartment 363 by a CEM 368 which permits the flow of cations between the first and third compartments. Since during use both first and third compartments will contain Na+ ions (NaOH in the first compartment and NaCI in the third compartment), those ions will pass through the CEM 368 from the first compartment 361 into the third compartment 363, due to the polarisation of the water electrolyser 360 during operation. The second compartment 362 is separated from the third compartment 363 by an AEM 369 which permits the flow of anions between the second and third compartments. Since during use both second and third compartments will Cl’ ions (HCI in the second compartment and NaCI in the third compartment), those ions will pass through the AEM 369 from the second compartment 362 into the third compartment 363, due to the polarisation of the water electrolyser 360 during operation. The third compartment 363 includes a third liquid inlet 370 into which saltwater will flow during use, and a third liquid outlet 371, out of which more concentrated saltwater (brine) will flow during use after being concentrated within the third compartment 363 during operation of the water electrolyser 360 (due to movement of both Na+ and Cl’ ions into the third compartment 363 as described above). The same movement of ions out of the first and second compartments means that freshwater will be produced in both the first compartment 361 and second compartment 362, passing out of the first liquid outlet 365 and second liquid outlet 367. Hence the 3-channel water electrolyser 360 shown in Figure 13 can be used to simultaneously produce hydrogen, produce a concentrated brine in the third compartment, and produce freshwater (desalinated water) in the first and second compartments. The brine can be disposed of or used as feed in a separate process, e.g. a chlor-alkali process to produce Ch and NaOH, or in the manufacture of battery materials, as explained above, when the initial saltwater feed to the ABFB contains one or more ions, such as lithium, which find use in battery materials. The desalinated water can be used for drinking water or irrigation. The hydrogen can be used in the ways discussed above. Hence, by coupling the ABFB 260 of Figure 9 with the water electrolyser 360 of Figure 13, the initial seawater input to the ABFB 260 of Figure 9 is converted into both freshwater streams and a more concentrated brine by the water electrolyser 360 of Figure 13. As a result, the system comprising the 3-channel ABFB 260 of Figure 9 coupled with the 3-channel water electrolyser 360 of Figure 13 can operate in a location proximal a natural source of saltwater, e.g. a coastal location, and function to generate hydrogen with reduced or eliminated evolution of chlorine or bromine (and without the need to desalinate the saltwater before electrolysis), and also produce freshwater from the saltwater source, which would be very valuable in e.g. arid coastal regions where the freshwater could be used for drinking or irrigation. The brine by-product can also find use as a feed in other processes, or could be discharged back into the environment. The direction of flow of solutions through the water electrolyser 360 in Figure 13 is shown by block arrows within the first and second compartments. The water electrolyser 380 shown in Figure 14 is almost identical to that of Figure 13, except that the positions of the CEM and AEM are exchanged. This means that, instead of the saltwater within the third compartment 363 becoming more concentrated to produce a brine, it will become less concentrated (since Na+ ions will move into the second compartment 362 from the third compartment 363 and Cl' ions will move into the first compartment 361 from the third compartment 363). As a result, assuming that a brine with a concentration of NaCI approximately double that of seawater is fed into the third liquid inlet 370 of the third compartment 363 (e.g. the brine product from the third compartment 251 of the ABFB 260 of Figure 9), then the products from each of the first, second, and third compartments of the water electrolyser 380 will be saltwater with an NaCI concentration substantially the same as that of seawater. Hence, by coupling the ABFB 260 of Figure 9 with the water electrolyser 380 of Figure 14, the initial seawater input to the ABFB 260 of Figure 9 is effectively regenerated by the water electrolyser 380 of Figure 14 and can be safely discharged to the original source, e.g. the sea or ocean. As a result, the system comprising the 3-channel ABFB 260 of Figure 9 coupled with the 3-channel water electrolyser 380 of Figure 14 can operate in a location proximal a natural source of saltwater, e.g. a coastal location, and function to generate hydrogen with reduced or eliminated evolution of chlorine or bromine (and without the need to desalinate the saltwater before electrolysis), and with only environmentally friendly by-products which can be discharged safely back into the environment. Alternatively, the 3-channel ABFB 240 of Figure 8 could be coupled with any of the water electrolysers 300, 320, 340 or 360 of Figures 10-14. The 3-channel ABFB 240 of Figure 8 will produce acidic and alkaline solutions to feed the electrolyser, and will also produce freshwater (desalinated water) from the saltwater feed into the third compartment 251. As a result, the system comprising the 3-channel ABFB 240 of Figure 8 coupled with any of the water electrolysers 300, 320, 340 or 360 of Figures 10-14 can operate in a location proximal a natural source of saltwater, e.g. a coastal location, and function to generate hydrogen with reduced or eliminated evolution of chlorine or bromine (and without the need to desalinate the saltwater before electrolysis), and also produce freshwater from the saltwater source, which would be very valuable in e.g. arid coastal regions where the freshwater could be used for drinking or irrigation. Figure 15 shows schematic diagrams of five possible arrangements (a)-(e) of the ABFB and water electrolysers described above. Figure 15(a) shows ABFB 230 coupled with water electrolyser 340. Saltwater feed 401 into the ABFB is converted into alkaline solution 402 and acidic solution 403. The alkaline solution 402 and acidic solution 403 are fed into the water electrolyser 340 which performs electrolysis to produce hydrogen 408 and oxygen 409 along with saltwater effluent 404 which is of substantially the same NaCI concentration as feed 401, and can be released back into the environment. Figure 15(b) shows ABFB 220 coupled with water electrolyser 320. Saltwater feeds 401a and 401b into the ABFB are converted into alkaline solution 402 and acidic solution 403. The alkaline solution 402 and acidic solution 403 are fed into the water electrolyser 320 which performs electrolysis to produce hydrogen 408 and oxygen 409 along with saltwater effluent streams 404a and 404b, each of substantially the same NaCI concentration as feed 401, so can be released back into the environment. Figure 15(c) shows ABFB 220 coupled with water electrolyser 300. Saltwater feeds 401a and 401b into the ABFB are converted into alkaline solution 402 and acidic solution 403. The alkaline solution 402 and acidic solution 403 are fed into the water electrolyser 300 which performs electrolysis to produce hydrogen 408 and oxygen 409 along with freshwater effluent stream 405a (having an NaCI concentration lower than that of feed 401a, at potable levels) and brine effluent stream 405b (having an NaCI concentration higher than that of feed 401a). The freshwater stream 405a can be used for drinking water or irrigation, and the brine stream 405b can be recycled or used elsewhere as described herein. Figure 15(d) shows ABFB 240 coupled with water electrolyser 320. Saltwater feeds 401a and 401b into the ABFB are converted into alkaline solution 402 and acidic solution 403. Saltwater feed 401c into the ABFB is converted to desalinated water effluent stream 406, which can be used for drinking water or irrigation. The alkaline solution 402 and acidic solution 403 are fed into the water electrolyser 320 which performs electrolysis to produce hydrogen 408 and oxygen 409 along with saltwater effluent streams 404a and 404b, each of substantially the same NaCI concentration as feed 401, so can be released back into the environment. Figure 15(e) shows ABFB 260 coupled with water electrolyser 380. Saltwater feeds 401a and 401b into the ABFB are converted into alkaline solution 402 and acidic solution 403. Saltwater feed 401c into the ABFB is converted to brine 407 which is fed to the water electrolyser. The alkaline solution 402 and acidic solution 403 are fed into the water electrolyser 320 which performs electrolysis to produce hydrogen 408 and oxygen 409 along with saltwater effluent streams 404a and 404b, each of substantially the same NaCI concentration as feed 401, so can be released back into the environment. The brine stream 407 is fed to the third compartment of the electrolyser 380 and is desalinated to produce a third saltwater effluent stream 404c having an NaCI concentration substantially the same as feed 401, so this can also be released back into the environment. In another embodiments (not shown in the Figures), the ABFB 200 which contains a CEM feeds the water electrolyser 300 which also contains a CEM. The result will be a saltwater product from both channels of the electrolyser 300. In another embodiments (not shown in the Figures), the ABFB 200 which contains a CEM feeds the water electrolyser 320 which contains an AEM. The result will be desalinated water from one channel of the electrolyser and concentrated saltwater (brine) from the 5 other. 10 References: A. Saez, V. Montiel, and A. Aldaz, ‘An Acid-Base Electrochemical Flow Battery as energy storage system’, Int J Hydrogen Energy, vol. 41, no. 40, pp. 17801-17806, Oct. 2016, 5 doi: 10.1016 / j.ijhydene.2016.08.141. Z. M. Bhat et al., ‘An Electrochemical Neutralization Cell for Spontaneous Water Desalination’, Joule, vol. 4, no. 8, pp. 1730-1742, Aug. 2020, doi: 10.1016 / j.joule.2020.07.001. 10 Igawa et al., Neutralization Dialysis for Deionization', Bull. Chem. Soc. Jpn, vol 60, pp. 381-383, Jan. 1987, doi: https: / / doi.org / 10.1246 / bcsj.60.381

Claims

1. A system for the electrolysis of saltwater, comprising:an acid-base flow battery comprising an acid solution outlet, an alkaline solution outlet and a saltwater inlet; anda water electrolyser downstream of acid-base flow battery for producing hydrogen, comprising a negative electrode and a positive electrode;wherein the acid-base flow battery is in fluid communication with the water electrolyser, such that, in use, an alkaline solution from the alkaline solution outlet of the acid-base flow battery passes into a positive electrode channel of the water electrolyser proximal the positive electrode.

2. The system according to claim 1, wherein the acid-base flow battery comprises first and second compartments separated by an ion-permeable membrane; the first compartment comprising the alkaline solution outlet and a first liquid inlet, and the second compartment comprising the acid solution outlet and a second liquid inlet.

3. The system according to claim 2, wherein the acid-base flow battery comprises a third compartment between the first and second compartments, wherein either:(a) the third compartment is separated from the first compartment by a cation exchange membrane and the third compartment is separated from the second compartment by an anion exchange membrane; or(b) the third compartment is separated from the first compartment by an anion exchange membrane and the third compartment is separated from the second compartment by a cation exchange membrane.

4. The system according to claim 3, wherein the third compartment comprises a saline solution inlet and a third liquid outlet.

5. The system according to claim 3 or 4, wherein the cation exchange membrane comprises a polymeric film comprising perfluorosulfonic acid polymer or copolymer.

6. The system according to any one of claims 3 to 5, wherein the anion exchange membrane comprises a poly ether ether ketone reinforced film.

7. The system according to claim 1, wherein the acid-base flow battery comprises first and second compartments separated by a separator which substantially prevents mixing of liquid contents of the first and second compartments but maintains ionic conductivity between the liquid contents of the first and second compartments.

8. The system according to claim 7, wherein the separator comprises a non-permeable membrane which extends across a first portion of the acid-base flow battery and provides fluid communication between the first and second compartments across a second portion of the acid-base flow battery.

9. The system according to any one of the preceding claims, wherein the acid-base flow battery comprises a connection to a source of hydrogen gas.

10. The system according to claim 9, wherein the connection to a source of hydrogen gas comprises first and second gas diffusion electrodes configured to provide diffusion of hydrogen gas out of a first electrode of the acid-base flow battery and into a second electrode of the acid-base flow battery.

11. The system according to claim 10, comprising means to transport hydrogen gas from the first electrode of the acid-base flow battery to the second electrode of the acidbase flow battery.

12. The system according to any one of the preceding claims, wherein the water electrolyser comprises a negative electrode channel inlet, a positive electrode channel inlet and one or more electrolyser liquid outlets.

13. The system according to claim 12, wherein the water electrolyser comprises first and second compartments separated by an ion-permeable membrane or a non-permeable separator; the first compartment comprising the positive electrode channel inlet and a first electrolyser liquid outlet, and the second compartment comprising the negative electrode channel inlet and a second electrolyser liquid outlet.

14. The system according to any one of the preceding claims, wherein one or both of the acid-base flow battery and the water electrolyser are powered at least in part by electricity generated by one or more renewable sources.

15. The system according to any one of the preceding claims, wherein, in use, an acidic solution from the acid solution outlet of the acid-base flow battery passes into the negative electrode channel of the water electrolyser proximal the negative electrode.

16. A process for the electrolysis of saltwater, comprising:providing an acid-base flow battery comprising an acid solution outlet, an alkaline solution outlet and a saltwater inlet;feeding an aqueous solution of sodium chloride into the saltwater inlet of the acid-base flow battery;applying a voltage to the acid-base flow battery to produce both an acidic solution and an alkaline solution within the acid-base flow battery;providing a water electrolyser downstream of the acid-base flow battery for producing hydrogen, comprising a negative electrode, a positive electrode, a negative electrode channel proximal the negative electrode and a positive electrode channel proximal the positive electrode, wherein the acid-base flow battery is in fluid communication with the water electrolyser;feeding at least a portion of the alkaline solution from the alkaline solution outlet of the acid-base flow battery into the positive electrode channel of the water electrolyser; andapplying a voltage to the water electrolyser to produce hydrogen gas.

17. The process according to claim 16, comprising feeding at least a portion of the acidic solution from the acid solution outlet of the acid-base flow battery into the negative electrode channel of the water electrolyser.

18. The process according to claim 16 or 17, wherein the aqueous solution of sodium chloride fed into the saltwater inlet of the acid-base flow battery comprises or consists of: (a) seawater, or a derivative thereof; or(b) a brine comprising one or more ions used in the manufacture of battery materials, such as one or more of lithium, magnesium, cobalt, nickel and manganese ions.

19. The process according to any one of claims 16 to 18, wherein the alkaline solution flows continuously from the ABFB to the water electrolyser as the ABFB operates.

20. The process according to any one of claims 16 to 19, wherein hydrogen gas is fed into the second compartment of the acid-base flow battery and removed from the first compartment of the acid-base flow battery.

21. The process according to any one of claims 16 to 20, wherein solutions flow continuously through the ABFB, flow continuously from the ABFB to the water electrolyser and flow continuously through the water electrolyser.

22. The process according to any one of claims 16 to 21, wherein the process comprises one or more preliminary treatment steps before feeding the aqueous solution of sodium chloride to the ABFB.

23. The process according to any one of claims 16 to 22, wherein the ABFB comprises:(a) first and second compartments separated by a CEM; or(b) first and second compartments separated by an AEM; or(c) a third compartment between the first and second compartments, wherein the third compartment is separated from the first compartment by a first ion-permeable membrane, and the third compartment is separated from the second compartment by a second ion-permeable membrane; or(d) first and second compartments separated by a separator which substantially prevents mixing of liquid contents of the first and second compartments but maintains ionic conductivity between the liquid contents of the first and second compartments, wherein the ABFB comprises an upstream region comprising a mixing compartment which extends across the ABFB between two electrodes, and a downstream region where the first and second compartments are separated by a non-permeable separatorwhich substantially prevents mixing of liquid contents of the first and second compartments.

24. The process according to any one of claims 16 to 23, wherein the process is 5 performed using the system of any one of claims 1 to 15.

25. The use of the system according to any one of claims 1 to 15 to reduce or eliminate the evolution of gaseous halogen species during the direct electrolysis of seawater.53

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