Hydrogen extraction system and method

Abstract

A hydrogen extraction system for extracting hydrogen from a liquid electrolyte 102 comprising at least one isotopologue of lithium hydride (LiH), the system including an electrolysis cell 100 comprisi

Description

Technical Field The present invention relates to extracting hydrogen isotopes, such as tritium, from a liquid electrolyte comprising one or more lithium hydride isotopologues (e.g., LiT), such as may be produced in a tritium breeder blanket. Background Hydrogen has three isotopes, protium (H), deuterium (D) and tritium (T), which comprise a single proton in combination with respectively 0, 1 or 2 neutrons. Molecules or compounds comprising different hydrogen isotopes are referred to as hydrogen isotopologues. The challenge of producing fusion power is hugely complex. One route to fusion involves confining a deuterium-tritium (D-T) plasma at temperatures that are sufficiently high that the nuclei fuse together, releasing highly energetic neutrons. The deuterium-tritium reaction is summarised in the following reaction: + |T ^He (3.52 MeV) + n° (14.06 MeV). The deuterium and tritium fuel may be injected into the plasma at high speeds in a frozen pellet or as a pulsed molecular beam. Tritium is difficult to obtain in the quantities needed for fusion because it is radioactive (with a half-life of 12.3 years) and there are no readily extractable sources of tritium that exist naturally on Earth. One of the challenges for economical power generation from fusion is therefore the need to regenerate the tritium consumed in the above reaction. To generate the necessary quantities of tritium to be self-sufficient, many fusion device concepts will include a tritium generation apparatus such as tritium breeding blankets as part of the fuel cycle. One family of breeder blanket concepts react energetic neutrons generated from fusion reactions (such as the reaction above) with lithium nuclei to breed tritium. For example, the breeder blanket may comprise lithium-containing material, e.g., liquid-phase lithium containing material (often referred to as ‘liquid lithium’), that is irradiated with neutrons generated by a fusion reactor to breed tritium. However, tritium generated in this way can be trapped within the lithium-containing material, e.g., by chemical reaction with lithium to form lithium hydride, or more specifically, lithium tritide, LiT (an isotopologue of lithium hydride). It is therefore generally necessary to extract the tritium from the lithium-containing material so that the tritium can be reintroduced into the fuel cycle. Efficient extraction of the tritium is important to minimise the time before which the tritium can be provided to the fusion reactor. A hydrogen extraction system for extracting tritium and other hydrogen isotopes from their lithium hydride isotopologues is therefore an important component for the safe and sustainable operation of a fusion reactor. One extraction approach is electrolytic decomposition of lithium hydride, as described in, WO2016 / 073434, for example. However, sustained and reliable operation of electrochemical cells is a significant challenge because of high operating temperatures and corrosion in the presence of liquid lithium materials. For example, parts of the electrochemical cells, such as those made from lithium lanthanum zirconate (LLZO). may need frequent replacement or repair as a result of the harsh operating conditions. Summary According to a first aspect of the present disclosure there is provided a hydrogen extraction system for extracting hydrogen from a liquid electrolyte comprising at least one isotopologue of lithium hydride (e.g., LiH, LiD, LiT or mixtures thereof, e.g., dissociated lithium hydride or lithium hydride in molecular form in the liquid electrolyte). The system includes an electrolysis cell having an anode electrode for generating hydrogen (e.g., molecular hydrogen, e.g., H2, HD, D2, DT orT2) from the liquid electrolyte; a cathode electrode spaced apart from the first electrode; and a solid-state electrolyte comprising a lithium-containing high entropy oxide (HEO) material. The solid-state electrolyte is for physically isolating the cathode from the liquid electrolyte (e.g., by providing a barrier between the cathode and the liquid electrolyte which is impermeable to the liquid electrolyte) and conducting lithium ions between the liquid electrolyte and the cathode thus physically isolated. In these examples and more generally, the HEO material that makes up the solid-state electrolyte can be selected so as to provide lithium ion conductivity properties (e.g., diffusion constant, permeability, lithium-ion conductance, electron conductance) suitable for the electrolysis cell in which the solidstate electrolyte is deployed. As noted below, there may be trade-offs between the ion conductivity of the selected HEO material and its thermomechanical properties. The lithium hydride isotopologue(s) can be present in the liquid electrolyte in a molecular or dissociated form. As used herein, a “lithium-containing” HEO is an HEO that is a compound of lithium. In some implementations, the HEO material comprises a solid solution of at least five metal cations other than lithium. As discussed below, in some (but not all) implementations, the HEO material comprises substantially equimolar amounts of the at least five metal cations other than lithium. Some particular examples of the lithium-containing HEO material include: (Mg, Co, Ni, Cu, Zn)o.9Lio.20; and Li7La3Zro.5Nbo.5Tao.5Hfo.5O12. A high entropy oxide (HEO) material refers to an oxide material that comprises a plurality of metal cations that are randomly distributed throughout a single-phase crystal structure to form a solid solution. That is, the single-phase crystal structure comprises lattice sites that are each occupied by (a random) one of the plurality of metal cations. The singlephase crystal structure of the HEO material may be stabilised during its formation by the configurational entropy associated with the random arrangement of the metal cations over the lattice sites. For example, the molar configurational entropy, Sconfig, for an HEO material may be calculated from: N ^config — — ' xi xi i = l in which R is the molar gas constant, N is the number of different metal cations and xt is the respective mole fraction of the j-th metal cation. For example, in some instances, the HEO material may comprise five or more metal cations. In some implementations, the five or more metal cations may comprise cations of Mg, Co, Ni, Cu, and Zn. Whilst in general, the mole fractions of the metal cations may be different, the maximum configurational entropy is achieved when the mole fractions are equal. For a five-cation HEO material the maximum molar configurational entropy is therefore 1.61 R. The HEO materials considered in this specification generally have a molar configurational entropy greater than or equal to 1.5 R. HEO materials are typically formed at high temperatures, such that the entropy of forming the solid solution (the entropy of mixing, which is increased by the configurational entropy) is able to overcome a typically unfavourable change in enthalpy (the enthalpy of mixing), which allows a single-phase crystal structure to form, which persists on cooling because of its kinetic stability. The lithium-ion conductivity of the lithium-containing HEO can be increased compared to conventional oxide materials as a result of structural distortions introduced into the structure of the HEO by the inclusion of the plurality of metal cations, which can provide percolating pathways for the lithium ions through the structure. The plurality of metal cations can be selected to have similar ionic radii to avoid excess structural distortions, which can e.g., lead to phase separation. The at least five metal cations other than lithium can be selected so that the HEO material is adapted for particular operating conditions of the electrolysis cell. For example, the proportions and / or identities of least five metal cations other than lithium can be varied to optimize oxygen vacancy concentrations and ionic mobility within the HEO material. The at least five metal cations other than lithium can comprise a combination of divalent cations and trivalent cations. For example, in some implementations, the at least five metal cations other than lithium can comprise divalent cations and one or more trivalent cations (e.g., one or more of: Al, Ga or In) to reduce the concentration of oxygen vacancies in the HEO structure and thereby tune diffusion pathways for lithium ions within the HEO material. In some implementations, the HEO material has a cubic or pseudocubic crystal structure. The phase of the HEO material having the cubic or pseudocubic crystal structure can be stabilised by the formation of the solid solution of the at least five metal cations other than lithium. That is, the proportions of the at least five metal cations other than lithium can be selected to ensure phase stability of the phase of the HEO material having the cubic or pseudocubic crystal structure at the operating temperature of the electrolysis cell, e.g., at an operating temperature in a range from 350°C to 750°C, or from 400°C to 700°C, or from 450°C to 650°C. A pseudocubic crystal structure can, for example, be a rhombohedral structure that is almost cubic. In some implementations, the HEO material is a lithium-doped HEO having a singlephase rock salt structure. For example, the HEO material can comprise (M)i.xLixO or (M)i-2XLixGaxO, in which M denotes the at least five metal cations other than lithium collectively. (M)i.x or (M)i-2X can be written in an expanded form as M1aM2bM3cM4dM5e in which M1, M2, M3, M4 and M5 denote the respective at least five metal cations other than lithium, (e.g., M1 = Mg, M2 = Co, M3 = Ni, M4 = Cu and M5 = Zn), and “a”, “b”, “c”, “d” and “e” denote the respective proportions of the at least five metal cations other than lithium (e.g., a = b = c = d = ein some implementations). The stoichiometric parameter x for the lithium doping can be from 0.02 to 0.50, e.g., from 0.05 to 0.40, or from 0.1 to 0.30, or from 0.15 to 0.25. As one example, the at least five metal cations other than lithium can comprise cations of each of: Mg, Co, Ni, Cu, and Zn. In some implementations, the at least five metal cations other than lithium comprise cations of magnesium. The proportion of magnesium cations can be optimized to improve the ionic conductivity of the HEO material, e.g., by introducing structural distortions into the HEO material that provide lower energy pathways for lithium ion transport. Including magnesium cations can also improve the stability of the HEO material, particularly at higher temperatures, e.g., at temperatures of 350 degrees Celsius and above. As one particular example, the HEO material can comprise (Mg, Co, Ni, Cu, Zn)o.9Lio.20. As another example, the HEO material can be (Mg, Co, Ni, Cu ,Zn)o.67Lio.330. The HEO materials can be synthesized by, for example, solid state reaction, e.g., mechanically grinding a mixture of the corresponding binary oxides and carbonates, followed by pressure and heat treatment, e.g., as described in Berardan et al. (2016). Journal of Materials Chemistry A, 4(24), 9536-9541. In some implementations, the respective atomic proportions of the at least five metal cations other than lithium can be substantially equimolar. For example, the highest and lowest values of the respective atomic proportions may be equal to within 20% of the lowest value, or within 10% of the lowest value, or within 5% of the lowest value. Substantially equimolar atomic proportions for the at least five metal cations other than lithium can help to stabilise the (e.g., rock-salt) phase of the HEO material by increasing configurational entropy of the solid solution. In some implementations, the HEO material comprises a lithium garnet. For example, the HEO material can comprise lithium lanthanum zirconium oxide (LLZO, LiyLasZ^O^) doped with at least three metal cations other than cations of lithium, lanthanum and zirconium. Many different metal cations can be used, e.g., any suitable metal cation having an appropriate valency and ionic size. For example, the at least three metal cations can comprise cations of three or more of: Nb, Ta, Hf, Ce, Ti, Nd, Sc, Sn, and Sb. The doping can, for example, be selected to stabilize a cubic phase of the lithium garnet, e.g., such that the cubic phase remains stable at temperatures of 350°C and above, e.g., greater than or equal to 400, or 450, or 500, or 550, or 600, or 700, or 750 degrees Celsius. In some implementations, the at least three metal cations can comprise one or more dopant cations for zirconium that replace a proportion of the zirconium cations present in undoped LLZO. For example, the one or more dopant cations for zirconium can comprise one or more of: Ta, Nb, Hf, Sb, and Al. For example, the proportion of the zirconium cations present in undoped LLZO replaced by the one or more dopant cations for zirconium can be from 0.02 to 0.3. By way of example, the HEO can comprise Li7La3Zr2.x AxO12, where A denotes one or more of Ta, Nb, Hf, Sb, and Al, and the stoichiometric parameter x is from 0.1 to 0.5. The proportion of Ta can be optimized to stabilise the cubic phase of the lithium garnet, whilst the proportions of Nb and HF can be optimized to improve the ionic conductivity and / or stability of the HEO material. For example, these properties can be optimized (e.g., using experimental measurements and / or computer simulations of different HEO compositions) according to the operating parameters of the electrolysis cell. In some implementations, the at least three metal cations can comprise one or more dopant cations for lanthanum that replace a proportion of the lanthanum cations present in undoped LLZO, the one or more dopant cations for lanthanum comprising one or more of: Pr, Nd, and Y. For example, the proportion of the lanthanum cations present in undoped LLZO replaced by the one or more dopant cations for lanthanum is from 0.02 to 0.20. Rare earth substitutions such as these can reduce grain boundary resistance and improve the phase stability of the HEO material. In some implementations, the molar ratio of lithium to oxygen in the lithium garnet is from 6.4 to 7.2 lithium atoms for every 12 oxygen atoms. As one particular example, the HEO material can comprise Li7La3Zro.5Nbo.5Tao.5Hfo.5O12. In some implementations, the HEO material can comprise a compound such as Lix(CoCrFeMnNi)3-xO4-y, where the stoichiometric parameter x for lithium is from 0.02 to 1.2 and y is a parameter indicates an amount of oxygen vacancies in the structure of the HEO material. The proportions of oxygen vacancies to oxygen atoms in the structure can, for example, be from around 18% to 40% in some cases. In some implementations, the lithium ion conductivity of the HEO material is greater than 0.1 x 10'3 S / cm, or greater than 1 x 10'3 S / cm, at a temperature of approximately 350 degrees Celsius or greater, e.g., greater than or equal to 400, or 450, or 500, or 550, or 600, or 700, or 750 degrees Celsius. In some implementations, the flexural strength of the solid-state electrolyte is greater than 50 MPa at a temperature of approximately 350 degrees Celsius or greater, e.g., greater than or equal to 400, or 450, or 500, or 550, or 600, or 700, or 750 degrees Celsius. The HEO materials can generally be synthesized by, for example, standard solid state reactions, e.g., mechanically grinding a mixture of the corresponding binary oxides and carbonates, followed by pressure and heat treatment. Although the solid-state electrolyte is impermeable to the liquid electrolyte it is nevertheless able to conduct lithium ions from the liquid electrolyte to the physically isolated electrode when a potential gradient is applied across the solid-state electrolyte. Thus, in use, the electrolysis cell can be used to extract hydrogen isotopes and lithium from the lithium hydride by electrolysis. In some other implementations, the HEO material can have a single-phase crystal structure that is one of: a fluorite structure; a perovskite structure; and a spinel structure. The use of an HEO material, rather than a conventional oxide material such as an undoped (i.e., non-HEO) lithium lanthanum zirconium oxide (LLZO, LiyLasZ^O^), can increase the thermochemical stability of the electrolyte as a result of the increased configurational entropy of the HEO material. Improving the stability of the solid-state electrolyte allows the surface area of the solid-state electrolyte to be increased and / or the thickness of the solid-state electrolyte to be decreased, which improves the performance of the electrolysis cell. In particular, conventional oxide materials such as LLZO can be prone to cracking or fracturing when immersed in the liquid electrolyte, especially at operating temperatures above 350 or 400 degrees Celsius, leading to a reduction in performance and potentially requiring the electrolysis cell to be taken offline for repair or replacement of the LLZO electrolyte. By contrast, use of the HEO material may avoid or reduce the need for replacement or repair of the solid-state electrolyte and simultaneously improve the performance of the electrolysis cell. The HEO material can be used without significant cracking or fracturing when immersed in the liquid electrolyte, at liquid electrolyte temperatures ranging from approximately 300 degrees Celsius to liquid electrolyte temperatures above approximately 400 degrees Celsius, or above approximately 500, 600, 700 or 750 degrees Celsius. Also, the HEO material can provide better ion conductivity than LLZO. For example, the HEO material can have a lithium-ion conductivities of around 10'4 to 10'3 S / cm, which is higher than typical lithium-ion conductivities of LLZO-based solid-state electrolytes and improves the operating efficiency and the hydrogen extraction rate of the electrolysis cell. A trade-off between ion conductivity and durability is possible with HEOs; for example, an HEO material of higher ion conductivity may provide more efficient tritium extraction but relatively less durability against cracking and fracturing, while an HEO material of lower ion conductivity may provide relatively greater durability against cracking and fracturing, but less efficient tritium extraction. Thus, the HEO material may be optimised for particular operating conditions of the electrolysis cell, e.g., operating conditions determined by the temperature of the liquid electrolyte and the required tritium extraction efficiency. In some implementations, the solid-state electrolyte can be mechanically stable in contact with the liquid electrolyte (e.g., liquid lithium) at a temperature of at least 350, 400, 500, 600, 700 or 750 degrees Celsius. In particular, the solid-state electrolyte may comprise an HEO material that durably retains substantial structural integrity (e.g., resists cracking or fracture) during operation of the electrolysis cell at these temperatures for a prolonged period, e.g., more than 50 hours, 100 hours, or more than 1000 hours, at an operating temperature of at least 350, 400, 500, 600, 700 or 750 degrees Celsius. As an example, the solid-state electrolyte may maintain its structural integrity when in contact with the liquid electrolyte at a temperature of 400 degrees Celsius under operating conditions that would cause failure of a corresponding solid-state electrolyte in which the HEO material is replaced with another solid-state electrolyte material, such as undoped lithium lanthanum zirconium oxide (LLZO, e.g., LiyLasZ^O^). In some examples, the liquid electrolyte comprises liquid (i.e., molten) lithium and / or a lithium-lead eutectic, e.g., in which the lithium hydride is dissolved or suspended, which may for example cause degradation or cracking of LLZO-based solid-state electrolytes. The liquid electrolyte may in some cases comprise a molten salt, such as one or more of FLiBe or FliNaK. In use, a potential applied across the anode and cathode causes electrolytic decomposition of the lithium hydride (e.g., LiH, LiD, or LiT). For example, the anode may generate tritium gas by oxidising the tritium present in the liquid electrolyte, e.g., according to a half-reaction, 2T' —> T2 (g> + 2e; whilst lithium metal is formed at the cathode by reducing lithium ions transferred to the cathode through the solid-state electrolyte, e.g., according to a half-reaction. Li+ + e_ Li. The lithium metal can be formed as a liquid. For example, the cathode may comprise liquid lithium in contact with the solid-state electrolyte such that reduction of lithium ions transferred to the cathode through the solid-state electrolyte generates additional liquid lithium at the cathode. The solid-state electrolyte physically isolates the liquid electrolyte from the cathode such that electric current (i.e., electrons) cannot flow between the anode and the cathode via the liquid electrolyte, i.e., the solid-state electrolyte has a very low conductivity for electrons but a much higher conductivity for lithium ions. Thus, the solid-state electrolyte physically separates the cathode from the liquid electrolyte whilst still allowing an ionic current to flow to the cathode from the liquid electrolyte. In some implementations, in which the cathode comprises liquid lithium and the solid-state electrode is configured as a container or vessel for containing the liquid lithium of the cathode. The vessel may be at least partially submerged in the liquid electrolyte, for example. The solid-state electrolyte may be configured such that gas generated within the electrolyte is not trapped beneath the solid-state electrolyte. For example, a lower outer surface of the solid-state electrolyte may be convex to allow gas to travel upwards over the surface to escape the liquid electrolyte. In general, the lithium hydride can include one or more hydrogen isotopes, i.e., protium (H), deuterium (D) and tritium (T). Thus, references to hydrogen in this specification can include one or more or each of the hydrogen isotopes. Similarly, references to lithium can include either or both lithium-6 and lithium-7. In some implementations, the liquid electrolyte comprises liquid lithium and lithium hydride present in either a liquid or solution phase, or as a suspension of solid particles (e.g., granular particles) in the liquid lithium. The lithium hydride may comprise more than one isotopologue of lithium hydride (i.e., two or more of LiH, LiD, and LiT). In some cases, the temperature of the liquid electrolyte may be greater than 350°C or greater than 400°C, e.g., around 450°C. In some implementations, the concentration of lithium-hydride isotopologues in the liquid-phase lithium may be in the range of 0.01 ppm (parts per million) to 10000 ppm, or more specifically 0.1 ppm to 1000 ppm, or yet more specifically below 1 ppm. For example, the liquid electrolyte can include tritium formed by irradiating lithium-containing material with neutrons generated by a fusion reactor, e.g., a reactor comprising a plasma confinement device, such as a tokamak or stellarator. Tritium extracted from the liquid electrolyte using the hydrogen extraction system can be provided to the fusion reactor as a fuel. The hydrogen extraction system can, for example, form part of the tritium fuel cycle for the fusion reactor in which tritium is consumed by the fusion reactor, regenerated in one or more breeder blankets using neutrons generated by the fusion reactor, extracted from the breeder blanket material by the hydrogen extraction system and then re-introduced into the fusion reactor as fuel such that the cycle can be repeated. In some implementations, the hydrogen extraction system comprises a gas extraction subsystem comprising a source of sparging gas connected to an inlet of the electrolysis cell and a gas collector connected to an outlet of the electrolysis cell. The inlet and the outlet are arranged so that sparging gas from the source passes through the liquid electrolyte before being received by the outlet. The sparging gas can therefore be bubbled through the liquid electrolyte to facilitate removal of hydrogen gas that is dissolved or otherwise trapped in the liquid electrolyte. The sparging gas may comprise one or more inert gases such as noble gases, e.g., argon and / or helium. The sparging gas may also be used to control the temperature of the liquid electrolyte. For example, the hydrogen extraction system may comprise a flow controller for controlling a flow rate of the sparging gas through the electrolysis cell-based temperature readings provided by one or more temperature sensors in the electrolysis cell, e.g., in the liquid electrolyte. In some implementations, the hydrogen extraction system further comprises one or more additional electrolysis cells, each additional electrolysis cell comprising: a respective first electrode for generating hydrogen gas from the liquid electrolyte; a respective second electrode; and a respective solid-state electrolyte comprising the high entropy oxide. The solid-state electrolyte is for physically isolating one of the respective first and second electrodes from the liquid electrolyte and conducting ions from the liquid electrolyte to the respective electrode thus physically isolated. The system can be configured for conveying (i.e., transporting) liquid electrolyte from a source of the liquid electrolyte to the electrolysis cell and the additional one or more electrolysis cells. The system can be configured to return the liquid electrolyte to the source of the liquid electrolyte after the liquid electrolyte has passed through the electrolysis cell and the additional electrolysis cells. Each of the electrolysis cell and the one or more additional electrolysis cells may provide a different respective flow path for the liquid electrolyte. Such “parallel” processing of the liquid electrolyte may increase the throughput of the system, e.g., to achieve a high tritium recovery rate in order to sustain the operation of a fusion reactor. Each electrolysis cell may comprise multiple anodes and cathodes to further increase the rate of extraction. In some cases, the solid-state electrolyte may be arranged to isolate each of the cathodes physically from the liquid electrolyte and, optionally, from the other cathodes as well, e.g., such that the solid-state electrolyte forms, for each of the cathodes, a respective vessel comprising a respective volume of liquid lithium. In a second aspect of the present invention, there is provided a method of extracting hydrogen from a liquid electrolyte comprising lithium hydride. The method comprises: providing a liquid electrolyte comprising at least one isotopologue of lithium hydride to the hydrogen extraction system of the first aspect described above; and applying a potential difference between the first and second electrodes of at least one electrolysis cell of the system (i.e., the electrolysis cell and / or one or more of the additional electrolysis cells, when present) to electrolyse the lithium hydride. The method may be carried out with the liquid electrolyte at an elevated temperature of up to approximately 350, 400, 500, 600, 700 or 750 degrees Celsius. A third aspect of the present invention comprises use of a solid-state electrolyte comprising a high entropy oxide in the electrolytic extraction of hydrogen from a liquid electrolyte comprising at least one isotopologue of lithium hydride. The liquid electrolyte may comprise liquid lithium, for example. According to a fourth aspect of the present invention, there is provided a tritium breeding system comprising the hydrogen extraction system of the first aspect described above and a breeder blanket. The tritium breeding system is configured to supply liquid electrolyte comprising at least one isotopologue of lithium hydride to the electrolysis cell from the breeder blanket and to return liquid electrolyte to the breeder blanket from the electrolysis cell following electrolysis of the at least one isotopologue of lithium hydride. The tritium breeding system may comprise a plasma confinement device for generating a flux of neutrons for breeding tritium in the breeder blanket. The plasma confinement device may be a magnetic plasma confinement device, e.g., a tokamak or a stellarator, or an inertial confinement device. Brief description of the drawings FIG. 1 is a schematic view of a vertical cross section of an electrolysis cell; FIG. 2 is a schematic diagram of a hydrogen-isotope extraction system; FIG. 3 is a schematic view of a vertical cross section of another electrolysis cell; and FIG. 4 is a schematic view of a power generation system. Detailed description FIG. 1 shows an electrolysis cell for extracting hydrogen from a liquid electrolyte 102, in this case liquid lithium, comprising lithium hydride, in this case a mixture of isotopologues comprising LiH, LiD and LiT. The cell 100 comprises a tank or vessel 104 into which the liquid electrolyte 102 is introduced through an inlet 106, which may for example be connected to a breeder blanket or breeder module (see FIG. 2). The vessel 102 may comprise a ceramic material, such as a silicon carbide or aluminium nitride, e.g., as a lining on the interior walls of the vessel 102. The exterior surfaces of the vessel 102 can optionally include thermal insulation, e.g., provided as a coating, to reduce heat transfer to / from the vessel 102. The cell 100 comprises an anode 108 that is at least partially immersed in (or otherwise in contact with) the liquid electrolyte 102 for generating hydrogen from the liquid electrolyte 102, and a cathode 110, which is spaced apart from the anode 108 and is physically isolated from the liquid electrolyte 102 by a solid-state electrolyte 112. In this example, the solid-state electrolyte 112 forms a container or vessel that retains a volume of liquid lithium 114 around the cathode 110. When the cell 100 is in use, the solid-state electrolyte 112 conducts lithium ions from the liquid electrolyte 102 to the volume of liquid lithium 114 where the lithium ions are reduced to form additional lithium metal. The solidstate electrolyte 112 is a high entropy oxide (HEO) material, in this case, (Mg, Co, Ni, Cu, Zn)i-xLixO, where x is 0.33, with a single-phase rock salt crystal structure. The proportions of the Mg, Co, Ni, Cu, and Zn cations are substantially equimolar (e.g., to within 5% of one another). Other compositions of HEOs can be used alternatively or in addition, depending on the thermomechanical and thermochemical requirements of the electrolysis cell. The exterior of the solid-state electrolyte 112 is convex from below such that isotopologues of hydrogen (and other gases) do not become trapped beneath the solid-state electrolyte 112, which may compromise the efficiency of the electrolysis cell. The cell 100 further comprises a power source 116 connected across the anode 108 and the cathode 110 to provide a potential difference across the anode 108 and the cathode 110 to drive the electrolysis of the lithium hydride. A variety of different materials can be used to form the cathode 110 and anode 108. For example, the cathode 110 and / or anode 108 can comprise one or more materials selected from: tungsten; nickel, cobalt; molybdenum; niobium; tantalum; rhenium, alloys of two or more of these materials; carbon, e.g., graphite or a composite carbon material; a conductive ceramic; and so on. The cell 100 further comprises a gas extraction subsystem comprising a gas inlet 118 and a gas outlet 120. In use, the gas inlet 118 introduces bubbles or a stream of a sparging gas 122 (e.g., an inert gas, such as argon or helium) into the liquid electrolyte adjacent to the anode 108 to remove hydrogen gas generated at the anode, i.e., H2, D2, T2, HD, HT, or DT, depending on the lithium hydride isotopologues present in the liquid electrolyte 202. The sparging gas and the hydrogen gas are removed from the vessel 104 through the gas outlet 120, for subsequent processing, e.g., to extract the tritium containing isotopologues from the hydrogen gas, or the gas extraction subsystem may direct the sparging gas and hydrogen gas through one or more other electrolysis cells (i.e., cells that are the same as or similar to the electrolysis cell 100) to facilitate extraction of the hydrogen gas generated in those cells. The gas extraction subsystem can be configured to supply the sparging gas 122 to facilitate generation of hydrogen at the anode 108. The cell 100 also comprises an outlet 124 for the liquid electrolyte 102, which allows the liquid electrolyte to be returned to the breeder blanket, for example. FIG. 2 shows a hydrogen extraction system 200 comprising two electrolysis cells 100A, 100B (i.e., two of the electrolysis cell 100 described above with respect to Figure 1) that are each configured to receive liquid electrolyte 202 comprising lithium hydride from a breeder blanket 204. The breeder blanket 204 comprises a lithium-containing material for generating tritium within the breeder blanket 205 via the reaction of the lithium with neutrons emitted from a plasma confinement device 205, which in this example is a tokamak, e.g., a spherical tokamak. The lithium-containing material can be provided in liquid form in the breeder blanket 204, e.g., the lithium-containing material can be provided as liquid lithium, so that it can be easily transported from the breeder blanket 204. Alternatively, some or all of the lithium-containing material can be provided as a solid phase during tritium breeding, in which case, the system 200 may be configured to melt the lithium-containing material to allow it to be transported in liquid form, i.e., as the liquid electrolyte 202, to the electrolysis cells 100A, 100B and then electrolysed. The hydrogen extraction system 200 is configured to extract and combine the lithium 206 generated at the cathodes 110 of each of the electrolysis cells 100A, 100B and return the lithium 206 to the breeder blanket 204. The hydrogen extraction system 200 may comprise various ancillary systems and / or components for this purpose, including pumps, pipes and other vessels and control systems (not shown). Thus, the liquid electrolyte 202 is circulated around a closed loop comprising the breeder blanket 204 and the two electrolysis cells 100A, 100B. The electrolysis cells 100A, 100B are arranged in parallel with one another, i.e., the system comprises alternative flow paths for the liquid electrolyte 202 that each pass through a respective one of the electrolysis cells 100A, 100B. By using multiple electrolysis cells 100A, 100B to process the liquid electrolyte 202 in parallel, the rate at which tritium can be recovered from the breeder blanket 204 can be increased, which reduces the overall amount of tritium that needs to be stored in the system (i.e., the tritium inventory), and increases the rate at which the tritium can be supplied to the plasma confinement system 205 as fuel, which may for example allow the system to be self-sustaining in terms of tritium. It is also possible to have the electrolysis cells 100A, 100B arranged in series, such that the liquid electrolyte 202 passes through each electrolysis cell 100A, 100B in turn, which may for example increase the total amount of tritium recovered by increasing the amount of time for which the liquid electrolyte is electrolysed before being returned to the breeder blanket 204. It will be appreciated that although FIG. 2 illustrates an implementation with two electrolysis cells, in other implementations, more than two cells can be used, in different combinations of series and parallel arrangements. The hydrogen extraction system 200 comprises a gas extraction subsystem 208 comprising a source of sparging gas 210 (e.g., an inert gas such as helium or argon), which is introduced into each of the electrolysis cells 100A, 100B to extract hydrogen isotopologues generated at the cathodes 108 of the cells. In the present example, the gas extraction subsystem 208 is configured such that the sparging gas 210 passes through each of the electrolysis cells 100A, 100B in turn, i.e., the outlet 120 of the first electrolysis cell 100A is connected to the inlet of the second electrolysis cell 100B. The mixture of gases 212 exiting the electrolysis cells 100A, 100B comprise the sparging gas 210 and extracted hydrogen isotopologues (e.g., H2, D2, T2, DT, etc.), which are then introduced into fuel cycle 214 for processing, such that the recovered tritium and / or deuterium can be injected into the plasma confinement device 205. The processing of the mixture of hydrogen isotopologues 212 may, for example, adjust the ratio of hydrogen isotopologues, such as by cryogenic distillation. Thus, in some implementations (not shown here), the hydrogen isotopologues 212 may pass through, as part of the fuel cycle 214, one or more intermediate systems such as one or more of a separation system (e.g., to separate the various hydrogen isotopes), a purification system (to remove unwanted gases) and / or a fuel distribution system (e.g., to control a ratio of deuterium and tritium that is injected into the plasma). Ionization and subsequent trapping of the deuterium and tritium fuel within a plasma confined by the plasma confinement device 205 under sufficient pressure and / or temperature cause the deuterium and tritium to fuse to release energetic helium ions and neutrons, which are then used to generate tritium within the breeder blanket 204. Thus, the plasma confinement device 205, the breeder blanket 204, the hydrogen extraction system 200 and the fuel cycle 214 cooperate to consume and regenerate tritium whilst generating energy through fusion in the plasma confinement device 205. The fuel cycle may comprise additional elements, such as a cooling mechanism to cool the flowing liquid lithium before it enters the hydrogen isotope extraction system 200 and / or an exhaust system for processing waste gas from the plasma confinement device 205. FIG. 3 shows an another electrolysis cell 300 that can also be used in addition or as an alternative to the electrolysis cell(s) 100, 100A, 100B described above with reference to FIGS. 1 and 2. The electrolysis cell 300 comprises a tank or vessel 304 into which the liquid electrolyte 102 is introduced through an inlet 306, which may for example be connected to a breeder blanket or breeder module, e.g., the breeder blanket 204 of FIG. 2. The cell 300 also comprises an outlet 324 for the liquid electrolyte 102, which allows the liquid electrolyte to be returned to the breeder blanket, for example. The electrolysis cell 300 comprises a cathode 310 that is at least partially immersed in (or otherwise in contact with) the liquid electrolyte 102 for generating lithium from the liquid electrolyte 102, and an anode 308 which is spaced apart from the cathode 310 and is physically isolated from the liquid electrolyte 102 by a solid-state electrolyte 312. The solid-state electrolyte 312 is a high entropy oxide (HEO), which may be the same as or different from the HEO used for the solid-state electrolyte 112 described above in connection with the electrolysis cell 100 of FIG. 1. In this case, the HEO is (Mg, Co, Ni, Cu, Zn)i-xLixO, where x is 0.33, with a single-phase rock salt crystal structure. Other compositions of HEOs can be used alternatively or in addition, depending on the thermomechanical and thermochemical requirements of the electrolysis cell 300. Alternatively, the HEO can comprise a lithium garnet HEO material. When the cell 300 is in use, the solid-state electrolyte 312 conducts hydride ions, e.g., tritide ions (T-), from the liquid electrolyte 102 to a hydride-conducting electrolyte 314 in which the anode 308 is at least partially immersed. The hydride-conducting electrolyte 314 can, for example, comprise one or more hydride salts (e.g., CaH2, CaHBr, SrHBr, etc.) and may be in liquid form, e.g., as a molten salt. The cell 300 further comprises a power source 116 connected across the anode 308 and the cathode 310 to provide a potential difference across the anode 308 and the cathode 310 to drive the electrolysis of the lithium hydride. The cathode 310 and / or anode 308 can comprise one or more materials selected from: tungsten; nickel, cobalt; molybdenum; niobium; tantalum; rhenium, alloys of two or more of these materials; carbon, e.g., graphite or a composite carbon material; a conductive ceramic; and so on. The electrolysis cell 300 additionally comprises a gas extraction subsystem as described above for the electrolysis cell 100 of FIG. 1. The gas extraction subsystem can be configured to supply sparging gas 122 to facilitate generation and liberation of hydrogen at the anode 108. FIG. 4 shows a power generation system 400 comprising the plasma confinement device 205, the breeder blanket 204, the hydrogen-isotope extraction system 208 (referred to here as a tritium extraction system) and the fuel cycle 214. The power generation system 400 further comprises a heat exchanger 402 that is configured to extract heat from the breeder blanket 204, e.g., generated as a result of the neutron flux from the plasma confinement device 205. The heat exchanger 402 receives breeder blanket material, in this case liquid lithium (which may include lithium hydride isotopologues) from the breeder blanket 204 at a first temperature and returns the material to the breeder blanket 204 at a second temperature that is lower than the first temperature. The extracted heat is then used for power (e.g., electricity) generation (not shown). The breeder blanket 204 may, for example, supply the lithium at a temperature of around 750 degrees Celsius and the heat exchanger 402 may return the lithium to the breeder blanket 204 at a temperature of around 300 degrees Celsius. To generate power efficiently, the temperature of the breeder blanket material entering the heat exchanger 402 should be as high as possible, within appropriate operating parameters. As the breeder blanket 204 also supplies the breeder blanket material, i.e., liquid lithium and lithium hydride isotopologues, to the tritium extraction system 208, this requirement means that the tritium extraction system 208 should be capable of operating at high temperatures, e.g., around 750 degrees Celsius, which places demanding requirements on the materials used in the tritium extraction system 208. For example, existing solid-state electrolyte materials, such as LLZO, may be unable to operate for prolonged periods at the required temperatures without mechanical or structural failure. The performance of the power generation system 400 is also affected by the efficiency with which tritium can be extracted from the breeder blanket 204. The tritium extraction system 208 must therefore maximise tritium extraction efficiency whilst also being able to withstand high liquid electrolyte temperatures. These competing requirements may result in a trade-off having to be made in terms of the material(s) used for the solid-state electrolyte 112, 312. For example, solid-state electrolytes 112, 312 with higher ionic conductivities, which are needed for efficient tritium extraction, may have relatively poorer thermomechanical stability, such that they are more prone to structural / mechanical failure. Using HEO material(s) for the solid-state electrolyte 112, 312 can allow the properties of the solid-state electrolyte 112, 312 to be tuned by adjusting the composition of the HEO material(s). For example, the concentration of oxygen vacancies in the HEO material can be tuned by varying the proportions of trivalent cations to divalent cations in the HEO material. The use of HEO material(s) therefore provides an effective and flexible way of achieving an appropriate balance between ionic conductivity and thermomechanical stability. Thus, the overall performance of the power generation system 400 can be improved by suitable optimisation of the solid-state electrolyte 112, 312 for a given operating temperature. It will be appreciated by those skilled in the art that the specific embodiments of the invention described herein provide a method and system for hydrogen extraction from liquid lithium, wherein the extracted hydrogen isotopes may be utilised as part of fuelling system for a fusion device. This may provide significant benefits over known systems, including increasing the throughput and speed of hydrogen isotope extraction. In particular, the use of a high entropy oxide (HEO) solid-state electrolyte may allow the hydrogen extraction system to be operated for long periods and / or high throughputs before the solid-state electrolyte needs to be repaired or replaced. It will also be 5 appreciated that many variations of the specific systems described are possible within the scope of the invention.

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