Multi-phase reactor for hydrogen production

Abstract

The disclosure provides a method of producing hydrogen. The method comprises conducting a thermochemical reaction by contacting an active reagent and a basic aqueous solution, to thereby cause water from the basic aqueous solution to react with the active reagent and to produce hydrogen and a basic aqueous solution comprising an oxidised product. The method further comprises disposing the basic aqueous solution comprising the oxidised product in an electrochemical cell comprising an anode and a cathode, such that at least a portion of the cathode contacts the solution; and conducting an electrochemical reaction by applying a voltage across the anode and the cathode to produce hydrogen, oxygen and the active reagent. The active reagent comprises a metal or metal ion in a first oxidation state and the oxidised product comprises the metal or metal ion in a second oxidation state which is higher than the first oxidation state.

Description

[0001]Multi-Phase Reactor for Hydrogen Production The invention relates to a method of producing hydrogen gas. In particular embodiment of the invention, the method may be a sustainable cyclic process. The invention also extends to an apparatus for producing hydrogen gas. Hydrogen is an important energy carrier and has the potential to replace hydrocarbon based fuels for sustainable development. The current energy related problems with hydrocarbon fuels, such as air pollution, climate change and scarcity of the resource, are important motivations for exploring hydrogen. As an alternative fuel source, hydrogen has the highest specific energy content of all fuels, and can be used for clean power generation in fuel cells with limited or no net atmospheric emissions and is convenient for efficient energy storage. Hydrogen can be used directly as a transportation fuel yielding a higher energy efficiency which is receiving much favourable attention as a technical and political issue. Currently, several industrial methods of hydrogen production exist, and among these are reforming, photoconversion and electrolysis, which have gained prominence. Water electrolysis provides the cleanest solution for hydrogen production. Its advantages are (i) it gives zero carbon emissions; (ii) it produces pure hydrogen, influencing fuel cell technology which is heavily affected by impurities in the hydrogen feed; (iii) it is independent of hydrocarbon resources; (iv) it can be operated in small scale plants; and (v) renewable energy can be used to produce the hydrogen. WO 2020 / 016580 A2 describes a process which can be used to produce hydrogen in both an electrolytic process of water decomposition and a thermochemical reaction. The latter is carried out in an enclosed reactor where a metal, such as zinc, is reacted with steam at high temperature to produce hydrogen gas and a metal oxide. This method suffers from disadvantages including: (a) the method is difficult to run in a continuous mode since the solid metal has to be introduced to the thermochemical reactor chamber and the solid oxide product removed therefrom, and (b) during the thermochemical reaction, the metal surface quickly passivates with a layer of oxide, which retards the rate of hydrogen production. The present invention arose from our work in attempting to overcome the aforementioned problems associated with the prior art. We hereby disclose an invention enabling hydrogen production in a system which can operate in a continuous mode and is readily scalable. In accordance with a first aspect of the invention, there is provided a method of producing hydrogen, the method comprising: - conducting a thermochemical reaction by contacting an active reagent and a basic aqueous solution to produce hydrogen and a basic aqueous solution comprising an oxidised product; - disposing the basic aqueous solution comprising the oxidised product in an electrochemical cell comprising an anode and a cathode, such that at least a portion of the cathode contacts the solution; and - conducting an electrochemical reaction by applying a voltage across the anode and the cathode to produce hydrogen, oxygen and the active reagent; wherein the active reagent comprises a metal or metal ion in a first oxidation state and the oxidised product comprises the metal or metal ion in a second oxidation state which is higher than the first oxidation state. Advantageously, both the thermochemical and electrochemical reactions produce hydrogen. Additionally, both the thermochemical reaction and the electrochemical reaction occur in an aqueous solution. Accordingly, the solution can be readily circulated between a thermochemical reactor and the electrochemical cell without additional processing steps being required. Furthermore, since the thermochemical reaction is conducted in a solution, the oxidised product may dissolve as it is formed. Accordingly, the method overcomes the problem of passivation observed in the prior art and favours conversion of the active reagent into the oxidised product. By using an alkaline solutions and changing the reaction phase to the aqueous mode, the type of reactor can be simplified to a “hydrolyser” rather than a “fixed bed” reactor. Conducting the reaction step in an “aqueous phase” reaction mode offers a big advantage to the process operation by making it possible to run both the reactor and electrolyser in a “continuous” mode. In other words, the “hydrothermal reactor” and “the electrochemical cell” could be directly connected using a pump to recirculate a single liquid phase “basic aqueous solution” across these central units. The phrase “thermochemical reaction”, as recited herein may be replaced with the phrase “hydrothermal reaction”. Similarly, the phrase “thermochemical reactor” may be replaced by the phrase “hydrothermal reactor” or “hydrolyser”. It may be appreciated that the electrochemical reaction may be conducted before the thermochemical reaction. Accordingly, in accordance with a second aspect, there is provided a method of producing hydrogen, the method comprising: - disposing a basic aqueous solution comprising an oxidised product in an electrochemical cell comprising an anode and a cathode, such that at least a portion of the cathode contacts the solution; - conducting an electrochemical reaction by applying a voltage across the anode and the cathode to produce hydrogen, oxygen and an active reagent; and - conducting a thermochemical reaction by contacting the active reagent and a basic aqueous solution to produce hydrogen and a basic aqueous solution comprising an oxidised product, wherein the active reagent comprises a metal or metal ion in a first oxidation state and the oxidised product comprises the metal or metal ion in a second oxidation state which is higher than the first oxidation state. The electrochemical reaction may be conducted continuously or repeatedly. The thermochemical reaction may be conducted continuously or repeatedly. The methods of the first and second aspects may comprise discrete steps which are repeated multiple times, or continuous reactions which are run in parallel. In a preferred embodiment, the electrochemical reaction and thermochemical reaction are both conducted continuously and concurrently. Advantageously, hydrogen may be produced continuously from both reactions. The basic aqueous solution may be circulated in a cyclic manner or loop between the electrochemical cell and a thermochemical reactor. The basic aqueous solution may be circulated in a continuous manner. The method of producing hydrogen may be viewed as a thermochemical / electrochemical cycle. The only feedstock required for the continuous production of hydrogen gas is water. Accordingly, once an initial quantity of the active reagent and / or the oxidised product has been supplied this can be recycled continuously in the thermochemical / electrochemical cycle and further active reagent and / or the oxidised product will not be required. The active reagent may be or comprise a transition metal or an alloy thereof, a p-block metal or an alloy thereof or a reactivated spinel / perovskite composite with multiple oxidation states. The p-block metal, or the alloy thereof, may be selected from the group consisting of tin, lead, thallium, selenium and bismuth. Preferably, the p-block metal is lead or tin. In a most preferred embodiment, the p-block metal, or the alloy thereof, is tin. The transition metal, or the alloy thereof, may be selected from the group consisting of zinc, copper, iron, nickel, cobalt, manganese, titanium, molybdenum, cadmium, chromium, vanadium, silver, rhodium, platinum, palladium, iridium, osmium, rhenium, ruthenium, lanthanum and zirconium. Preferably, the transition metal, or the alloy thereof, is selected from the group consisting of zinc, copper, iron, nickel, cobalt, manganese, chromium and vanadium. More preferably, the transition metal, or the alloy thereof, is selected from the group consisting of zinc, iron, nickel, chromium and vanadium. In a most preferred embodiment, the transition metal, or the alloy thereof, is zinc. The reactivated spinel / perovskite composite with multiple oxidation states may have general formula M1xM2yOz, where M and M2 are both metals, O is oxygen and x, y and z are each an integer between 1 and 10. M1 may be an s-block metal, a transition metal or a p-block metal. For instance, M1 may be an alkali metal, an alkaline earth metal or a p-block metal. M1 may be sodium, potassium or zinc. M2 may be a transition metal or a p-block metal. When M2 is a transition metal or a p-block metal. the transition metal or p-block metal may be tin, iron, manganese, chromium, titanium or lead. x may be an integer between 1 and 5 or between 1 and 3, and is preferably 1 or 2. y may be an integer between 1 and 5 or between 1 and 3, and is preferably 1, 2 or 3. z may be 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, and is preferably, 1, 3, 4 or 8. Accordingly, the reactivated spinel / perovskite composite with multiple oxidation states may be a stannate, a ferrate, a ferrite, a manganate, a chromate, a titanate or a plumbate. The reactivated spinel / perovskite composite with multiple oxidation states may be zinc stannate, zinc ferrate, zinc ferrite, zinc manganate, zinc chromate, zinc titanate, zinc plumbate, sodium stannate or potassium stannate. It may be appreciated that the oxidised product may be or comprise a metal ion, and the metal ion may be a cation of a metal, where the metal is as defined above. The oxidised product may further comprise a hydroxide group. Accordingly, the oxidised product may be a metal hydroxide. The metal hydroxide may be produced directly by the reaction of the active reagent and water. Alternatively, the reaction of the active reagent and water may produce a metal oxide and the metal oxide may further react with the basic aqueous solution to produce the metal hydroxide. The oxidised product is preferably dissolved in the basic aqueous solution. For instance, the oxidised product may be or comprise zinc hydroxide, a zincate, a copper hydroxide, iron hydroxide, nickel hydroxide, cobalt hydroxide, manganese hydroxide, titanium hydroxide, molybdenum hydroxide, cadmium hydroxide, chromium hydroxide, vanadium hydroxide, silver hydroxide, rhodium hydroxide, platinum hydroxide, palladium hydroxide, iridium hydroxide, osmium hydroxide, rhenium hydroxide, ruthenium hydroxide, lanthanum hydroxide, zirconium hydroxide, tin hydroxide, lead hydroxide, thallium hydroxide, selenium hydroxide or bismuth hydroxide. Alternatively, or additionally, the oxidised product may be or comprise a reactivated spinel / perovskite composite with multiple oxidation states. The reactivated spinel / perovskite composite with multiple oxidation states may be as defined above. It may be appreciated that zinc hydroxide can exist in equilibrium with zincate in a basic aqueous solution. The method may comprise contacting the active reagent and the basic aqueous solution in the presence of a catalyst. The catalyst may be a metal oxide or a metal hydroxide. The catalyst may be or comprise an iron based catalyst, a nickel based catalyst, a stannate based catalyst and / or a copper based catalyst. Accordingly, the catalyst may comprise or be iron(III) oxide (Fe2O3), nickel hydroxide (Ni(OH)2), potassium stannate (K2SnO3), copper hydroxide (Cu(OH)2), or combinations thereof. In some embodiments, the catalyst is iron(III) oxide. The catalyst may be in the form of a powder. The molar ratio of active reagent to the catalyst may be between 1:0.001 and 1:1, between 1:0.005 and 1:0.75, between 1:0.01 and 1:0.5, between 1:0.02 and 1:0.3, between 1:0.04 and 1:0.2, between 1:0.06 and 1:0.15, between 1:0.08 and 1:0.12 or between 1:0.09 and 1:0.11. The concentration of the catalyst may be at least 0.01 mmol, at least 0.02 mmol, at least 0.04 mmol, at least 0.06 mmol, at least 0.08 mmol, at least 0.1 mmol, at least 0.3 mmol or at least 0.4 mmol. The concentration of the catalyst may be between 0.01 and 50 mmol, between 0.02 and 10 mmol, between 0.04 and 5 mmol, between 0.06 and 2 mmol, between 0.08 and 1.5 mmol, between 0.1 and 1 mmol, between 0.3 and 0.8 mmol or between 0.4 and 0.6 mmol. If the catalyst is insoluble or substantially insoluble, the concentration may be understood to be calculated as the number of moles of catalyst present as a suspension in the solution. Accordingly, the concentration may be calculated using the standard formula: Concentration of catalyst = Number of moles of catalyst * 1000 volume of basic aqueous solution (ml) In some embodiments, the catalyst may be magnetic. The method may comprise separating the catalyst from the basic aqueous solution and the catalyst prior to feeding the basic aqueous solution to the electrochemical cell. The method may comprise conducting the thermochemical reaction in a thermochemical reactor. The method may comprise holding the catalyst in the thermochemical reactor. The method may comprise using one or more magnets to separate the catalyst from the basic aqueous solution and / or hold the catalyst in the thermochemical reactor. In all cases, the thermochemical reaction is conducted in water. The thermochemical reaction may be conducted at a temperature of at least 0°C, at least 5°C, at least 10°C, at least 15°C or at least 20°C. In some embodiments, the thermochemical reaction may be conducted at a temperature of at least 30°C, at least 40°C, at least 50°C or at least 60°C, more preferably at a temperature of at least 70°C, at least 80°C, at least 90°C or at least 100°C, and most preferably at a temperature of at least 110°C, at least 120°C, at least 130°C or at least 135°C. In some embodiments, the thermochemical reaction is conducted at a temperature of at least 150°C, at least 200°C, at least 250°C, at least 300°C, at least 350°C, at least 400°C, at least 450°C, at least 500°C, at least 550°C, at least 600°C, at least 650°C, at least 700°C, at least 750°C, at least 800°C at least 850°C or at least 900°C. The thermochemical reaction may be conducted at a temperature of less than 1,000°C, least than 950°C, less than 900°C, less than 800°C or less than 850°C. In some embodiments, the thermochemical reaction is conducted at a temperature of less than 700°C, more preferably at a temperature of less than 650°C, less than 600°C or less than 550°C, and most preferably at a temperature of less than 500°C, less than 450°C, less than 400°C, less than 300°C, less than 250°C, less than 200°C, less than 175°C, less than 160°C or less than 150°C. In some embodiments, the thermochemical reaction may be conducted at a temperature of less than 130°C, less than 100°C, less than 80°C, less than 60°C or less than 40°C. In some embodiments, the thermochemical reaction is conducted at a temperature of between 0 and 1,000°C, between 5 and 950°C, between 10 and 900°C, between 15 and 850°C, between 20 and 800 or between 25 and 750°C. In some embodiments, the thermochemical reaction is conducted at a temperature of between 0 and 1,000°C or between 5°C and 700°C. In some embodiment, the thermochemical reaction is conducted at a temperature of between 30 and 600°C, between 40 and 500°C, between 60°C and 400°C, between 80 and 300°C or between 100 and 250°C, and most preferably at a temperature of between 110 and 200°C, between 120 and 175°C, between 130 and 160°C or between 135 and 150°C. In alternative embodiments, the thermochemical reaction is conducted at a temperature of between 0 and 130°C, between 5 and 100°C, between 10 and 80°C, between 15 and 60°C or between 20 and 40°C. Advantageously, a catalyst enables the reaction to proceed at a lower temperature. In some embodiments, the thermochemical reaction is conducted at a temperature of between 350 and 1,000°C, between 400 and 950°C, between 500 and 900°C, between 600 and 850°C or between 700 and 800°C. Advantageously, increasing the temperature increases the rate of the reaction. The method may comprise conducting the thermochemical reaction in a thermochemical reactor. The method may comprise controlling the temperature in the thermochemical reactor. The method may comprise causing the temperature in the thermochemical reactor to be as defined above. Controlling the temperature in the thermochemical reactor may comprise causing a first fluid to flow through a heat exchanger disposed adjacent the thermochemical reactor such that heat may be transferred between the thermochemical reactor to the first fluid and thereby maintaining the temperature in the thermochemical reactor. The method may comprise transferring heat from the thermochemical reactor to the first fluid. The first fluid may be or comprise water. The first fluid may be a liquid when it is fed into the heat exchanger. Transferring heat to the first fluid from the thermochemical reactor may cause the first fluid to evaporate and form a gas (e.g. steam). The method may subsequently comprise recovering heat from the first fluid. Recovering heat from the fluid may cause the first fluid to condense. Recovering heat from the fluid may comprise transferring heat from the gas stream to a cooling fluid. The cooling fluid may be or comprise water. Advantageously, recovering heat from the first fluid may heat the cooling fluid. In embodiments where the cooling fluid is or comprises water, the water may subsequently be used in the thermochemical reaction. Accordingly, cooling the gas stream may preheat water which is used in the thermochemical reaction. The thermochemical reaction may produce a gas stream comprising hydrogen. It may be appreciated that the gas stream may further comprise steam. Accordingly, the method may comprise cooling the gas stream produced in the thermochemical reaction to cause water to condense out of the gas stream. For instance, the thermochemical reaction may be conducted under reflux conditions. Advantageously, the condensation step will remove the steam from the gas stream and allow a user to obtain hydrogen gas. Cooling the gas stream may comprise transferring heat from the gas stream to a cooling fluid. The cooling fluid may be or comprise water. Advantageously, cooling the gas stream may heat the cooling fluid. In embodiments where the cooling fluid is or comprises water, the water may subsequently be used in the thermochemical reaction. Accordingly, cooling the gas stream may preheat water which is used in the thermochemical reaction. The thermochemical reaction may be conducted in a thermochemical reactor. The method may therefore comprise transferring heat from the gas stream to liquid comprising or consisting of water, and subsequently feeding the liquid into the thermochemical reactor. It may be appreciated that the pressure may be selected depending upon the temperature at which the thermochemical reaction is conducted. The pressure may be selected to be high to ensure basic aqueous solution does not boil at the temperature at which the thermochemical reaction is conducted. The thermochemical reaction may be conducted at a pressure of at least 10 kPa or at least 20 kPa, more preferably at a pressure of at least 40 kPa, at least 60 kPa or at least 80 kPa, and most preferably at a pressure of at least 90 kPa, at least 95 kPa or at least 100 kPa. In an embodiment, the thermochemical reaction is conducted at a pressure of at least 200 kPa, at least 400 kPa, at least 500 kPa, at least 600 kPa, at least 800 kPa or at least 1 MPa. In an embodiment, the thermochemical reaction is conducted at a pressure of at least 5 MPa, at least 10 MPa or at least 15 MPa. In an embodiment, the thermochemical reaction is conducted at a pressure of less than 5,000 kPa. In an embodiment, the thermochemical reaction is conducted at a pressure of less than 1,000 kPa, less than 500 kPa or less than 250 kPa, and most preferably at a pressure of less than 200 kPa, less than 150 kPa or less than 110 KPa. In an embodiment, the thermochemical reaction is conducted at a pressure of between 20 and 5,000 kPa, more preferably at a pressure of between 40 and 1,000 kPa, between 60 and 500 kPa or between 80 and 250 kPa, and most preferably at a pressure of between 90 and 200 kPa, between 95 and 150 kPa or between 100 and 110 kPa. In an alternative embodiment, the thermochemical reaction is conducted at a pressure of between 100 kPa and 75 MPa, between 200 kPa and 50 MPa, between 400 kPa and 40 MPa, between 600 kPa and 30 MPa, between 800 kPa and 25 MPa, between 1 and 20 MPa, between 5 and 17.5 MPa or between 7 MPa and 15 MPa. The active reagent may comprise a powder, pellets or flakes. Preferably, during the thermochemical reaction, the active reagent is suspended in the basic aqueous solution. The method may comprise placing the active reagent in a thermochemical reactor prior to conducting the thermochemical reaction. Alternatively, or additionally, the method may comprise feeding the active reagent into the thermochemical reactor while the thermochemical reaction is being conducted. The method may comprise continuously feeding the active reagent into the thermochemical reactor while the thermochemical reaction is being conducted. Preferably, the method comprises continuously feeding the basic aqueous solution comprising the active reagent suspended therein into the thermochemical reactor while the thermochemical reaction is being conducted. The method may comprise removing the basic aqueous solution comprising the oxidised product from the thermochemical reactor at the end of the thermochemical reaction. Alternatively, or additionally, the method may comprise removing the basic aqueous solution comprising the oxidised product from the thermochemical reactor while the thermochemical reaction is being conducted. Preferably, the method comprises continuously removing the basic aqueous solution comprising the oxidised product from the thermochemical reactor while the thermochemical reaction is being conducted. The method may comprise agitating the basic aqueous solution while the thermochemical reaction is conducted. It may be appreciated that the basic aqueous solution comprises a base. The base may be an organic or an inorganic base. The base may be an Arrhenius base, a Lewis base, and / or a Bronsted-Lowry base, more preferably a strong Arrhenius base and / or a Lewis superbase. The Arrhenius base may comprise an alkali metal hydroxide or alkaline earth metal hydroxide. The Arrhenius base may comprise potassium hydroxide, sodium hydroxide, barium hydroxide, caesium hydroxide, strontium hydroxide, calcium hydroxide, lithium hydroxide and / or rubidium hydroxide. Accordingly, the basic aqueous solution may comprise an aqueous alkaline solution. The Lewis base may comprise ammonia (NH3), butyllithium (n-BuLi), lithium diisopropylamide (LDA), lithium diethylamide (LDEA), sodium amide, sodium hydride (NaH) and / or lithium bis(trimethylsilyl)amide. The Bronsted-Lowry base may comprise ammonium hydroxide, an aliphatic amine, or an aromatic amine. The aliphatic amine may comprise methylamine, ethylamine or dimethylamine. The aromatic amine may comprise aniline, phenylenediamine or o- toludine. In a preferred embodiment, the basic aqueous solution comprises an Arrhenius base. The basic aqueous solution may comprise a concentration of at least 0.5 M, at least 1 M or at least 2 M of the base, more preferably at least 4 M, at least 6 M or at least 8 M, at least 10 M, at least 12M, at least 14 M or at least 15 M of the base. The basic aqueous solution may comprise a concentration of less than 8.5 M, less than 8 M or less than 7.5 M of the base, more preferably less than 7 M, less than 40 M or less than 30 M of the base, and most preferably less than 25 M or less than 20 M of the base. The basic aqueous solution may comprise a concentration of between 0.5 and 50 M, between 1 and 45 M or between 2 and 40 M of the base, more preferably between 4 and 35 M, between 6 and 30 M or between 8 and 28 M of the base. In some embodiments, the basic aqueous solution may comprise a concentration of between 10 and 26 M, between 12 and 24 M, between 14 and 22 M or between 15 and 20 M of the base. In alternative embodiments, the basic aqueous solution may comprise a concentration of between 6 and 20 M, between 8 and 15 M or between 10 and 12 M base. The concentration of the base in the basic aqueous solution may be substantially the same for the basic aqueous solution used in the thermochemical reaction and for the basic aqueous solution used in the electrochemical reaction. In the thermochemical reaction, the oxidised product may be present in the basic aqueous solution at a concentration of at least 0.001 M or at least 0.005 M, more preferably at least 0.01 M, at least 0.02 M or at least 0.06 M, and most preferably at least 0.08 M or at least 0.1 M. In some embodiment, the oxidised product is present at a concentration of at least 0.2 M or at least 0.6 M, and most preferably at least 0.8 M or at least 1 M. The oxidised product may be present at a concentration of less than 5 M, less than 4 M, less than 3 M, less than 2 M, less than 1.5 M, less than 1 M or less than 0.5 M, more preferably less than 0.3 M, less than 0.25 M or less than 0.2 M, and most preferably less than 0.17 M or less than 0.14 M. The oxidised product may be present at a concentration of between 0.001 and 5 M or between 0.005 and 4 M, more preferably between 0.02 and 3 M, between 0.06 and 2 M or between 0.1 and 1.5 M. In one embodiment, the oxidised product is present at a concentration of between 0.001 and 1 M or between 0.005 and 0.5 M, more preferably between 0.01 and 0.3 M, between 0.02 and 0.25 M or between 0.06 and 0.2 M, and most preferably between 0.08 and 1.7 M or between 0.1 and 0.14 M. In an alternative embodiment, the oxidised product is present at a concentration of between 0.2 and 5 M or between 0.4 and 4 M, more preferably between 0.6 and 3 M, between 1 and 1.5 M, between 1.1 and 1.3 M or between 1.15 and 1.25 M. The electrochemical cell may be an undivided cell. Accordingly, in this embodiment, disposing the basic aqueous solution comprising the oxidised product will cause at least a portion of the anode to contact the solution. In an alternative embodiment, the electrochemical cell comprises a divided cell. Accordingly, the cell may comprise a membrane disposed between the anode and the cathode dividing the cell into two portions. Preferably, the membrane is an anion exchange membrane, more preferably an alkaline anion exchange membrane (AAEM). Suitable AAEMs will be known to the skilled person, but could include a polymer anion exchange base membrane of a chloromethylated polysulfone, or a copolymer of chloromethylstyrene and divinylvenzene with polyethylene fabric. In this embodiment, the method may comprise disposing the basic aqueous solution comprising the oxidised product in a cathode portion of the cell, such that at least a portion of the cathode contacts the solution. The method may also comprise disposing a further electrolyte in an anode portion of the cell, such that at least a portion of the anode contacts the further electrolyte. The further electrolyte preferably comprises an aqueous solution, and more preferably a basic aqueous solution. The basic aqueous solution may be understood to comprise a base. The base may be as defined above. It may be appreciated that as the electrochemical reaction proceeds water will react to produce hydrogen and oxygen. Similarly, as the thermochemical reaction proceeds, water will react to produce hydrogen. Accordingly, the concentration of the base in the basic aqueous solution will increase. Similarly, in embodiments where the cell comprises a divided cell, the concentration of a base in the further electrolyte will also increase. Accordingly, the method may comprise adding water to the basic aqueous solution. The method may comprise adding water to the basic aqueous solution to maintain a desired concentration of the base in the basic aqueous solution and / or a desired pH of the basic aqueous solution. The method may comprise adding water to the basic aqueous solution when the pH of the basic aqueous solution rises above a predetermined maximum. Water may be added to the basic aqueous solution at any point in the method. For instance, water may be added to the basic aqueous solution in the thermochemical reactor and / or in the electrolysis cell. Alternatively, or additionally, water may be added to the basic aqueous solution downstream of the thermochemical reactor and upstream of the electrolysis cell. Alternatively, or additionally, water may be added to the basic aqueous solution upstream of the thermochemical reactor and downstream of the electrolysis cell. When water is added to the basic aqueous solution downstream of the thermochemical reactor and upstream of the electrolysis cell or upstream of the thermochemical reactor and downstream of the electrolysis cell the water may be added to the basic aqueous solution in a conduit between the thermochemical reactor and the electrolysis cell. In embodiments where the cell comprises a divided cell, the method may comprise contacting the further electrolyte with a sufficient quantity of water to cause the further electrolyte to have a desired concentration of base. The method may comprise monitoring the pH of the further electrolyte, and contacting the further electrolyte with a sufficient quantity of water to cause the further electrolyte to have a desired concentration of base when the pH rises above a predetermined maximum. The desired concentration of base may be as defined above. The method may comprise continuously feeding the basic aqueous solution comprising the oxidised product into the electrochemical cell while the electrochemical reaction is being conducted. The method may further comprise removing the basic aqueous solution from the electrochemical cell as the electrochemical reaction is being conducted. Advantageously, this step continuously refreshes the basic aqueous solution. Additionally, the inventors have also found that this is sufficient to cause the active reagent to be removed from the electrolysis cell. Alternatively, or additionally, the method may comprise mechanically separating the active reagent from the cathode. The active reagent may be removed after the electrochemical reaction has been completed. Alternatively, active reagent may be removed continuously or periodically as the electrochemical reaction is conducted. For instance, the electrochemical cell may comprise a blade configured to remove the active reagent the cathode. The blade may be configured to move across the cathode and thereby remove the active reagent from a surface of the cathode. The blade may be moved using a magnetic force or an electromotor. Accordingly, the method may comprise causing the basic aqueous solution to flow through the electrochemical cell and thereby removing the active reagent from the electrochemical cell due to the active reagent being suspended in the basic aqueous solution. The method may comprise causing the basic aqueous solution to flow through the electrochemical cell as the electrochemical reaction is being conducted. In embodiments where the cell is a divided cell, the method may comprise feeding the basic aqueous solution comprising the oxidised product into the cathode portion of the electrochemical cell while the electrochemical reaction is being conducted. The method may further comprise removing the basic aqueous solution from the cathode portion of the cell as the electrochemical reaction is being conducted. The method may comprise feeding the further electrolyte into the anode portion of the electrochemical cell while the electrochemical reaction is being conducted and / or removing the further electrolyte from the cathode portion of the cell as the electrochemical reaction is being conducted. The anode and cathode may independently comprise a carbon based electrode or a metal based electrode. The or each carbon based electrode may comprise graphite. The or each metal based electrode may comprise chromium, nickel, zinc, cadmium, copper, tin, lead, rhodium, platinum, gold, palladium, iridium, osmium, rhenium, ruthenium, germanium, beryllium, and / or silver. Alternatively, the or each metal based electrode may comprise an alloy, such as brass, bronze or steel. The steel may be stainless steel. In a preferred embodiment, the electrodes comprise graphite or steel. In an alternative preferred embodiment, the electrodes comprise the same metal as the active reagent. For instance, if the electrochemical reaction produces zinc, then the electrodes may comprise zinc. Preferably, the cathode comprises a substantially non-porous material. Advantageously, when the cathode is non-porous the active material would be electrodeposited on the surface of the structure and would not become trapped within pores thereof. The method may comprise applying a voltage of at least 1 V, at least 1.5 V, at least 1.75 V or at least 2 V across the anode and cathode, more preferably the method comprises applying a voltage of at least 2.5 V, at least 3 V or at least 3.5 V across the anode and cathode, and most preferably the method comprises applying a voltage of at least 4 V or at least 4.5 V across the anode and cathode. The method may comprise applying a voltage of less than 8 V or less than 7.5 V across the anode and cathode, more preferably the method comprises applying a voltage of less than 7 V, less than 6.5 V or less than 6 V across the anode and cathode, and most preferably the method comprises applying a voltage of less than 5.5 V or less than 5 V across the anode and cathode. In some embodiments, the method comprises applying a voltage of less than 4 V, less than 3 V or less than 2.5 V. The method may comprise applying a voltage of between 1 and 8 V or between 2 and 7.5 V across the anode and cathode. In some embodiments, the method comprises applying a voltage of between 2.5 and 7 V, between 3 and 6.5 V or between 3.5 and 6 V across the anode and cathode, and most preferably the method comprises applying a voltage of between 4 and 5.5 V or between 4.5 and 5 V across the anode and cathode. In alternative embodiment, the method comprises applying a voltage of between 1 and 4 V, between 1.5 and 3 V or between 1.75 and 2.5 V across the anode and cathode. The method may comprise causing a current of at least 0.5 A, at least 1 A or at least 1.5 A to flow through the anode, cathode and the solution comprising the metal ion, more preferably causing a current of at least 2 A, at least 2.5 A or at least 3 A to flow through the anode, cathode and the solution comprising the metal ion and most preferably causing a current of at least 3.5 A to flow through the anode, cathode and the solution comprising the metal ion. The method may comprise causing a current of less than 10 A, less than 8 A or less than 6 A to flow through the anode, cathode and the solution comprising the metal ion, more preferably causing a current of less than 5.5 A, less than 5 A or less than 4.5 A to flow through the anode, cathode and the solution comprising the metal ion and most preferably causing a current of less than 4 A to flow through the anode, cathode and the solution comprising the metal ion. The method may comprise causing a current of between 0.5 and 10 A, between 1 and 8 A or between 1.5 and 6 A to flow through the anode, cathode and the solution comprising the metal ion, more preferably causing a current of between 2 and 5.5 A, between 2.5 and 5 A or between 3 and 4.5 A to flow through the anode, cathode and the solution comprising the metal ion and most preferably causing a current of between 3.5 and 4 A to flow through the anode, cathode and the solution comprising the metal ion. Preferably, the electrochemical reaction is conducted at a temperature of at least 0°C, more preferably at a temperature of at least 10°C, at least 12.5°C, at least 15°C, at least 17.5°C or at least 20°C. In some embodiments, the electrochemical reaction is conducted at a temperature of at least 25°C, and most preferably at a temperature of at least 30°C or at least 55°C. Preferably, the electrochemical reaction is conducted at a temperature of less than 95°C, more preferably at a temperature of less than 90°C, less than 85°C or less than 80°C, and most preferably at a temperature of less than 70°C or less than 65°C. In some embodiments, the electrochemical reaction is conducted at a temperature of less than 50°C, less than 40°C, less than 30°C or less than 25°C. Preferably, electrochemical reaction is conducted at a temperature of between 0°C and 95°C, more preferably at a temperature of between 10°C and 90°C. In some embodiments, the electrochemical reaction is conducted at a temperature between 20°C and 85°C or between 25°C and 80°C, and most preferably at a temperature of between 30°C and 70°C or between 55°C and 65°C. In some embodiments, the electrochemical reaction is conducted at a temperature between 10°C and 50°C, between 12.5 and 40°C, between 15°C and 30°C or between 17.5°C and 25°C. It may be appreciated that the thermochemical reaction may be conducted at a higher temperature than the electrochemical reaction. Accordingly, subsequent to conducting the thermochemical reaction and prior to conducting the electrochemical reaction, the method may comprise cooling the basic aqueous solution. Similarly, subsequent to conducting the electrochemical reaction and prior to conducting the thermochemical reaction, the method may comprise heating the basic aqueous solution. It may be appreciated that in a preferred embodiment, where both the thermochemical reaction and the electrochemical reaction are conducted continuously, the method may comprise continuously circulating the basic aqueous solution between the thermochemical reactor and the electrochemical cell. Accordingly, the method may comprise transferring heat from a portion of the basic aqueous solution at a location upstream of the electrochemical cell and downstream of the thermochemical reactor to a further portion of the basic aqueous solution at a location upstream of the thermochemical reactor and downstream of the electrochemical cell. The electrochemical reaction may be conducted at the same or a different pressure to the thermochemical reaction. In some embodiments, the electrochemical reaction is conducted at the same pressure as the thermochemical reaction. In this embodiment the electrochemical cell and the thermochemical reactor may be maintained at the same pressure, which may be an elevated pressure. Any conduits extending between the electrochemical cell and the thermochemical reactor may also be maintained at the same pressure. The pressure may be as defined above. In some embodiments, the pressure may be between 100 kPa and 7 MPa, between 1 and 6 MPa, between 2 and 4 MPa or between 3 and 4 MPa. In an alternative embodiment, the thermochemical reaction is conducted at a pressure of between 100 kPa and 75 MPa, between 200 kPa and 50 MPa, between 400 kPa and 40 MPa, between 600 kPa and 30 MPa, between 800 kPa and 25 MPa, between 1 and 20 MPa, between 5 and 17.5 MPa or between 7 MPa and 15 MPa. In alternative embodiments, the electrochemical reaction is conducted at a lower pressure than the thermochemical reaction. In this embodiment, the thermochemical reaction may be conducted at a first pressure and the electrochemical reaction may be conducted at a second pressure. The first pressure may be a higher pressure than the second pressure. The method may comprise pressurising the basic aqueous solution to the first pressure prior to or at the same time as feeding it into the thermochemical reactor. The method may comprise pressurising the basic aqueous solution to the first pressure (a) as it is removed from the electrochemical cell, (b) while it transported between the electrochemical cell and the thermochemical reactor and / or (c) as it is fed into the thermochemical reactor. The method may comprise depressurising the basic aqueous solution to the second pressure prior to or at the same time as feeding it into the electrochemical cell. The method may comprise depressurising the basic aqueous solution to the second pressure (a) as it is removed from the thermochemical reactor, (b) while it transported between the thermochemical reactor and the electrochemical cell and / or (c) as it is fed into the electrochemical cell. The electrochemical reaction may be conducted at a pressure of at least 10 kPa or at least 20 kPa, more preferably at a pressure of at least 40 kPa, at least 60 kPa or at least 80 kPa, and most preferably at a pressure of at least 90 kPa, at least 95 kPa or at least 100 kPa. In an embodiment, the electrochemical reaction is conducted at a pressure of at least 200 kPa, at least 400 kPa, at least 1 MPa, at least 2 MPa, at least 3 MPa, at least 4 MPa or at least 5 MPa. In an embodiment, the electrochemical reaction is conducted at a pressure of less than 5 MPa or less than 3 MPa. In an embodiment, the electrochemical reaction is conducted at a pressure of less than 1 MPa, less than 500 kPa or less than 250 kPa, and most preferably at a pressure of less than 200 kPa, less than 150 kPa or less than 110 KPa. In an embodiment, the electrochemical reaction is conducted at a pressure of between 20 and 5,000 kPa, more preferably at a pressure of between 40 and 1,000 kPa, between 60 and 500 kPa or between 80 and 250 kPa, and most preferably at a pressure of between 90 and 200 kPa, between 95 and 150 kPa or between 100 and 110 kPa. In an alternative embodiment, the electrochemical reaction is conducted at a pressure of between 100 kPa and 30 MPa, between 250 kPa and 20 MPa, between 500 kPa and 15 MPa, between 750 kPa and 12 MPa, between 1 and 10 MPa or between 5 MPa and 7 MPa. The inventors believe that the apparatus used to conduct the methods of the first and second aspects is novel and inventive per se. Accordingly, in accordance with a third aspect there is provided an apparatus for producing hydrogen, the apparatus comprising: - a thermochemical reactor, configured to hold a basic aqueous solution and an active reagent therein and thereby allow a thermochemical reaction to proceed and produce a gas stream comprising hydrogen and a basic aqueous solution comprising an oxidised product; - an electrochemical cell comprising an anode and a cathode, and configured to receive the basic aqueous solution comprising the oxidised product from the thermochemical reactor, such that at least a portion of the cathode contacts the basic aqueous solution comprising the oxidised product, the electrochemical cell being configured to cause an electrochemical reaction to proceed and produce hydrogen gas and the active reagent at the cathode and oxygen at the anode; - a first conduit extending between the thermochemical reactor and the electrochemical cell, the first conduit being configured to feed the basic aqueous solution comprising the oxidised product from the thermochemical reactor to the electrochemical cell; and - a second conduit extending between the thermochemical reactor and the electrochemical cell, the second conduit being configured to feed the basic aqueous solution and the active reagent from the electrochemical cell to the thermochemical reactor. The term “thermochemical reactor” may be used interchangeably with “hydrolyser”, “hydrolysis reactor” or “hydrothermal reactor”. The thermochemical reactor may comprise an agitator configured to agitate the basic aqueous solution in the thermochemical reactor. Preferably, the apparatus comprises a temperature controller configured to control the temperature in the thermochemical reactor. The temperature controller may comprise a heater configured to heat the basic aqueous solution in the thermochemical reactor to an elevated temperature. Alternatively, or additionally, the temperature controller may comprise a heat exchanger, disposed adjacent to the thermochemical reactor, wherein the heat exchanger is configured to allow a first fluid to flow therethrough, such that heat may be exchanged between the first fluid and the thermochemical reactor. The heat exchanger may be a jacket disposed around the thermochemical reactor. Preferably, the heat exchanger is configured to maintain the thermochemical reactor at an elevated temperature. The elevated temperature may be as defined in relation to the first and second aspects. The apparatus may comprise a further heat exchanger configured to cool the first fluid. The further heat exchanger may be configured to transfer heat from the first fluid to a further fluid. The further fluid may be the basic aqueous solution or water. Accordingly, the further heat exchanger may be configured to pre-heat the basic aqueous solution or water prior to feeding it into the thermochemical reactor. The apparatus may comprise a condenser configured to cool the gas stream produced in the thermochemical reactor, and thereby cause water in the gas stream to condense. Advantageously, this will allow concentrated hydrogen gas to be obtained. The apparatus may be configured to return the condensed water to the thermochemical reactor. The condenser may be configured to transfer heat from the gas stream to a cooling fluid. The cooling fluid may be the basic aqueous solution or water. Preferably, the condenser is preferably configured to pre-heat the basic aqueous solution or water prior to it being fed into the thermochemical reactor. The apparatus may comprise a pressure controller configured to control the pressure in the thermochemical reactor. The pressure controller may comprise a first pressure sensor. The first pressure sensor may be disposed in the thermochemical reactor. The pressure controller may be configured to maintain the pressure in the thermochemical reactor at a first desired pressure. The first desired pressure may be a pressure as defined above in relation to the first aspect. The pressure controller may be configured to control the pressure in the electrochemical reactor. The pressure controller may comprise a second pressure sensor. The second pressure sensor may be disposed in the electrochemical reactor. The pressure controller may be configured to maintain the pressure in the electrochemical reactor at a second desired pressure. The second desired pressure may be a pressure as defined above in relation to the first aspect. Accordingly, the second desired pressure may be the same or different to the first desired pressure. The pressure controller may comprise a compressor configured to compress the basic aqueous solution to a first pressure. The compressor may be configured to compress the basic aqueous solution prior to it being fed into the thermochemical reactor. The compressor may be configured to compress the basic aqueous solution to the first pressure (a) as it is removed from the electrochemical cell, (b) while it transported between the electrochemical cell and the thermochemical reactor and / or (c) as it is fed into the thermochemical reactor. The pressure controller may comprise a decompressor configured to cause the basic aqueous solution to decompress to a second pressure. The decompressor may be configured to decompress the basic aqueous solution prior to it being fed into the electrochemical cell. The decompressor may be configured to decompress the basic aqueous solution to the first pressure (a) as it is removed from the thermochemical reactor, (b) while it transported between the thermochemical reactor and the electrochemical cell and / or (c) as it is fed into the electrochemical cell. The apparatus may comprise a pump configured to cause the basic aqueous solution comprising the oxidised product to flow along the first conduit from the thermochemical reactor to the electrochemical cell. The apparatus may comprise a pump configured to cause the basic aqueous solution and the active reagent to flow along the second conduit from the electrochemical cell to the thermochemical reactor. One pump could be used or the apparatus may comprise two or more separate pumps. The or each pump may be a peristaltic pump and / or a slurry pump. The apparatus may comprise a hydrogen collection vessel configured to receive hydrogen gas produced in the thermochemical reactor. The apparatus may comprise a conduit extending between the condenser and the hydrogen collection vessel. The electrochemical cell may be an undivided cell. Alternatively, the electrochemical cell may comprise a divided cell. Accordingly, the electrochemical cell may comprise a membrane disposed between the anode and the cathode dividing the cell into two portions. Preferably, the membrane is as defined in relation to the first and second aspects. In this embodiment, the first conduit may extend between the cathode portion of the electrochemical cell and the thermochemical reactor. Similarly, the second conduit may extend between the cathode portion of the electrochemical cell and the thermochemical reactor. An anode portion of the cell may be configured to receive a further electrolyte, such that at least a portion of the anode contacts the electrolyte. The further electrolyte may be as defined in relation to the first and second aspects. The apparatus may comprise a pH meter configured to monitor the pH of the solution comprising the metal ion. The pH meter may be disposed in the electrochemical cell, the thermochemical reactor, the first conduit and / or the second conduit. The apparatus may comprise one or more water inlets configured to feed water into the electrochemical cell, the thermochemical reactor, the first conduit and / or the second conduit. In some embodiments, the apparatus may comprise a further conduit extending between the condenser and a heated water inlet, and configured to transport water heated by the condenser to the heated water inlet. The heated water inlet may be configured to feed water into the thermochemical reactor and / or the second conduit. The apparatus may be configured to feed water into the reservoir or the electrochemical cell when the pH meter senses a pH which is above a predetermined maximum pH. The anode and cathode may be as defined in relation to the first and second aspects. The apparatus may comprise a remover configured to remove the active reagent from the electrochemical cell. The remover may be configured to remove the active reagent from the cathode. The remover may comprise a blade configured to remove the active reagent from the cathode. The blade may be configured to move across the cathode and thereby remove the active reagent from a surface of the cathode. The apparatus may be configured to move the blade using a magnetic force or an electromotor. The apparatus may comprise a power supply configured to apply a voltage across the anode and cathode. The power supply may comprise a battery, a generator, a renewable power source or it could comprise the national grid. Preferably, the power supply is a renewable power source. The renewable power source may comprise a solar power generator, wind power or a hydropower generator. The power supply may be configured to supply a direct current to the anode and cathode. The power supply may be configured to apply a voltage of at least 1 V or at least 2 V across the anode and cathode, more preferably a voltage of at least 2.5 V, at least 3 V or at least 3.5 V across the anode and cathode, and most preferably a voltage of at least 4 V or at least 4.5 V across the anode and cathode. The power supply may be configured to apply a voltage of less than 8 V or less than 7.5 V across the anode and cathode, more preferably a voltage of less than 7 V, less than 6.5 V or less than 6 V across the anode and cathode, and most preferably a voltage of less than 5.5 V or less than 5 V across the anode and cathode. The power supply may be configured to apply a voltage of between 1 and 8 V or between 2 and 7.5 V across the anode and cathode, more preferably a voltage of between 2.5 and 7 V, between 3 and 6.5 V or between 3.5 and 6 V across the anode and cathode, and most preferably a voltage of between 4 and 5.5 V or between 4.5 and 5 V across the anode and cathode. The power supply may be configured to cause a current of at least 0.5 A, at least 1 A or at least 1.5 A to flow through the anode, cathode and the solution comprising the metal ion, more preferably to cause a current of at least 2 A, at least 2.5 A or at least 3 A to flow through the anode, cathode and the solution comprising the metal ion and most preferably to cause a current of at least 3.5 A to flow through the anode, cathode and the solution comprising the metal ion. The power supply may be configured to cause a current of less than 10 A, less than 8 A or less than 6 A to flow through the anode, cathode and the solution comprising the metal ion, more preferably to cause a current of less than 5.5 A, less than 5 A or less than 4.5 A to flow through the anode, cathode and the solution comprising the metal ion and most preferably to cause a current of less than 4 A to flow through the anode, cathode and the solution comprising the metal ion. The power supply may be configured to cause a current of between 0.5 and 10 A, between 1 and 8 A or between 1.5 and 6 A to flow through the anode, cathode and the solution comprising the metal ion, more preferably to cause a current of between 2 and 5.5 A, between 2.5 and 5 A or between 3 and 4.5 A to flow through the anode, cathode and the solution comprising the metal ion and most preferably to cause a current of between 3.5 and 4 A to flow through the anode, cathode and the solution comprising the metal ion. The apparatus may comprise a hydrogen collection vessel configured to receive hydrogen gas produced in the electrochemical cell. The apparatus may comprise a conduit extending between the electrochemical cell and the hydrogen collection vessel. In embodiments where the cell is a divided cell, the conduit may extend between the cathode portion of the electrochemical cell and the hydrogen collection vessel. The hydrogen collection vessel configured to receive hydrogen gas produced in the thermochemical reactor may also be the hydrogen collection vessel configured to receive hydrogen gas produced in the electrochemical cell. Alternatively, the apparatus could comprise two separate hydrogen collection vessels. The apparatus may comprise an oxygen collection vessel configured to receive oxygen gas produced in the electrochemical cell. The apparatus may comprise a conduit extending between the electrochemical cell and the oxygen collection vessel. In embodiments where the cell is a divided cell, the conduit may extend between the anode portion of the electrochemical cell and the oxygen collection vessel. The apparatus may comprise a heat exchanger, and the first and second conduits may extend therethrough, wherein the heat exchanger is configured to transfer heat between the first and second conduits. The heat exchanger is preferably configured to transfer heat from the first conduit to the second conduit. All features described herein (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined with any of the above aspects in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- Figure 1 is a schematic showing a hydrogen production cycle comprising a thermal reactor and an electrolyser; Figure 2 is a lab-scale hydrolysis reactor setup; Figure 3 (a) shows hydrogen generation from zinc powder hydrolysis in alkaline solution KOH (8M) at 130°C; and (b) shows the kinetics of zinc hydrolysis at 130°C; Figure 4 (a) shows hydrogen generation from zinc hydrolysis in alkaline solution where Fe2O3was present at a molar ratio of Fe2O3 / zinc from 0.06 to 0.5; and (b) shows the kinetics of zinc hydrolysis in the presence of Fe2O3, wherein the molar ratio of Fe2O3 / zinc was 0.1; Figure 5 (a) shows hydrogen generation with different Fe2O3loading; and (b) shows the kinetics of zinc hydrolysis with different Fe2O3loading; Figure 6(a) shows hydrogen generation at different temperatures; and (b) shows the kinetics of zinc hydrolysis at different temperatures; Figure 7 (a) shows hydrogen generation at different KOH concentrations; and (b) shows the kinetics of zinc hydrolysis at different KOH concentrations; Figure 8 is a graph showing the conductivity and resistance of an electrolyte as a function of concentration; Figure 9 is a graph showing accumulated hydrogen production over time, for different electrolyte concentrations at 2V; Figure 10 provides an insight into the change of current and hydrogen production rate against time, at 1.2M and 2V, based on Figure 4; Figure 11 is a graph showing accumulated hydrogen production over time, for different voltages at 1.2M; and Figure 12 demonstrates the zinc conversion in the thermochemical reaction using (a) Gibbs minimization method, (b) conversion rates at various activation energy values based on experimental reaction rates where the reaction pressure is kept above the corresponding saturation temperature of water to prevent evaporation through passing the reactor cooling jacket and (c) effect of boosting the pressure at the reactor on conversion rate at E_a=35 kJ / mol. Examples WO 2020 / 016580 A2, the contents of which is incorporated herein by reference, describes a highly efficient and low-cost hybrid thermochemical cycle for hydrogen production. The present invention has arisen due to improvements made to the invention disclosed in WO 2020 / 016580 A2. A schematic diagram showing an apparatus configured to carry out the method of the invention is shown in Figure 1. In particular, in the present invention, the thermal reactor described in WO 2020 / 016580 A2 is replaced with a hydrolyser which facilitates the reaction between water and an active reagent in the presence of an alkali base solution to produce hydrogen. The active reagent can be a transition or p-block metal (zinc, iron, tin, etc.) or a reactivated spinel / perovskite (zinc ferrate, zinc ferrite, zinc manganate, zinc chromate, or zinc plumbate) composite with multiple oxidation states (e.g. zinc stannate, zinc ferrate, zinc ferrite, zinc manganate, zinc chromate, zinc plumbate or sodium stannate). In methods of the present invention the active reagent reacts with liquid water in the hydrolyser, and is thereby oxidised. For instance, in embodiments where the active reagent is zinc, the oxidation reaction which occurs may be written like so: Zn + H2O(l)⇌ ZnO + H2ΔH° = –62.45 KJ / mol (Oxidation) It will be noted that the above reaction is similar to the oxidation reaction described in WO 2020 / 016580 A2, except in the present application the water used in the reaction is liquid and in WO 2020 / 016580 A2 the water was provided in the form of steam. In WO 2020 / 016580 A2, the oxidised product of the oxidation reaction forms a passivation layer on the surface of the active reagent. For instance, in the example where the active reagent is zinc, the passivation layer will be formed from zinc oxide. In WO 2020 / 016580 A2, this would prevent the active reagent from reacting further. The rate of conversion in WO 2020 / 016580 A2 of zinc into zinc oxide would only be about 30-50%. However, in the present method, the alkali base solution helps refresh the surface of the active reagent by dissolving the oxidised product. In embodiments where the active reagent is zinc and the alkali base is potassium hydroxide, the reaction which occurs may be written like so: ZnO + H2O(l)+ 2KOH.xH2O(l)⇌ K2Zn(OH)4+ xH2O(l)ΔH° = –7.89 KJ / mol (Hydrolysis) The overall reaction in this embodiment may be written like so: Zn + 2H2O(l)+ 2KOH.xH2O(l)⇌ K2Zn(OH)4+ xH2O(l)+ H2ΔH° = –70.34 KJ / mol By dissolving the oxidised product, the surface of the active reagent will be freely exposed to water for further reaction until the active reagent has completely reacted. Hence, the alkaline solution allows efficient water conversion (for hydrogen production) in the hydrolyser by inhibiting the catalytic “surface deactivation”, which is a common problem in the oxidation step of the chemical looping processes. It will be appreciated that a gas stream comprising hydrogen gas will be produced in the above reaction. The gas stream will also comprise steam. Accordingly, the gas stream may be passed through a heat exchanger configured to cool the gas stream and thereby condense out the water. This will provide a gas with a high concentration of hydrogen. Additionally, it is noted that the thermodynamically favoured reaction between water and the active reagent generates a substantial amount of excess heat. This heat, in part, will be present in the product gas stream. Accordingly, the heat exchanger could be used to pre-heat water which is then either fed into the hydrolyser or into a jacket of the hydrolyser. Water fed to the jacket of the hydrolyser would also form stream, shown exiting the jacket of the hydrolyser on the right of the hydrolyser in Figure 1. It will be appreciated that the heat from this steam could also be captured using a heat exchanger. The excess heat recovered from the hydrolysis reactor can significantly increase the energy efficiency of the entire loop. In particular, the cycle consumes 48.5 kWh electrical energy for 1 kg H2production (39.4 kWh / kg). Dividing these, the efficiency is 39.4 / 48.5=81.2%. When heat recovery with steam is made, for each kg of hydrogen produced, 4.6 kWh worth of steam is produced. Now the efficiency becomes (39.4+4.6) / 48.5=90.7%. It may be appreciated that potassium hydroxozincate (K2Zn(OH)4) is entirely soluble in a basic solution. Accordingly, the solution may be pumped directly to an electrolyser to undergo electrochemical reduction reactions. These electrochemical reactions may be as described in WO 2020 / 016580 A2. In particular, it may be appreciated that potassium hydroxozincate may be in equilibrium as shown below: K2Zn(OH)4+ xH2O(l)⇌ Zn(OH)2+ 2KOH.xH2O(l)ΔH° ≈ 0 KJ / mol (Dissociation) It is denoted that the set of reactions above can be applied likewise for the other alkaline solutions (NaOH, LiOH, etc.), ammonia, and the other organic bases. Two electrochemical reduction reactions simultaneously happen at the cathode surface. These would be the electrochemical reduction of Zn(OH)2to recover zinc and the typical water-electrolysis in the alkaline environment to make hydrogen (so-called HER: hydrogen evolution reaction), as follows: Zn(OH)2+ 2 e–⇌ Zn + 2 OH–E° = –1.249 V 2 H2O + 2 e–⇌ H2+ 2 OH–E° = –0.828 V The cathode overall half-reaction (reduction) would be the sum of the above reactions, as follows: Zn(OH)2+ 2 H2O + 4 e–⇌ Zn + H2+ 4 OH–E° = –1.039 V For the anode half-reaction, similar to the other typical alkaline electrolysers, the oxygen reduction reaction (ORR) through the direct four-electron pathway would take place as follows: 4 OH–⇌ O2+ 2 H2O + 4 e–E° = –0.401 V The sum of the above half-reactions (anode and cathode) would make the overall electrolyser redox reaction: Zn(OH)2 ⇌ Zn + H2 + O2 E° = –1.44 V Accordingly, the dissolved metal-oxides produced in the hydrolyser are cathodically reduced to their initial state using the electrolyser. The inventors’ continuous electrochemical cell experiments have shown that the pressure forces of a flowing solution can easily remove the zinc particles from the cathodic surface. Accordingly, a solution comprising the zinc particles in suspension can be fed back to the hydrolyser. The process can thereby be conducted continuously producing hydrogen. It is noted that the only inputs required are water and energy. Example 1 – Hydrolysis of zinc in sodium hydroxide solution The inventors set up the lab-scale hydrolysis reactor shown in Figure 2. In brief, a round bottom flask 2 was disposed on a hot plate 1. A thermocouple 3 was provided and configured to measure the temperature in the flask 2. The apparatus further comprised a condenser 4 disposed on top of the round bottom flask and configured to cool any gas produced in the round bottom flask 2. A chiller 5 was provided to ensure a constant stream of chilled fluid would flow through the condenser 4. A rotameter 6 was provided downstream of the condenser to measure the flow of the gas which was produced in the reaction, and downstream of the rotameter a 1 Litre capacity syringe 7 was provided to capture the evolved gas. The round bottom flask was fitted with a nitrogen line (a stainless steel tube connected to the side of the round bottom flask), configured to allow nitrogen to be fed into the round bottom flask. The round bottom flask was also fitted with an input tube and an output tube configured to feed liquids into and remove liquids from the round bottom flask 2. A carbonate-free, clear solution of 180 g of NaOH in 250 ml of H2O was prepared and added to the round-bottom flask. Pure nitrogen gas was supplied (at the rate of 100 ml / min for 30 minutes) to purge the reactor before starting the reaction. 1.37 g of Zn was separately suspended in 50 ml of water and added to the reactor through the input tube. Inlet valves were then closed, and the hot plate heated the mixture while it was vigorously stirred using a magnetic stirrer. All the hydrogen gas product was collected in the syringe. The reaction system was refluxed for 4 hours at 135°C and atmospheric pressure to complete the reaction, producing 504 ml of hydrogen, equivalent to 96.8 % hydrolysis conversion percentage for the zinc powder initially added to the apparatus. Stoichiometric calculations show that the reaction yielded 3.68g of Na2Zn(OH)4. Table 1: Quantity of reactants provided and products obtained Reaction systemP, atm T, °C Na2Zn(OH)4, Zn Conv. Zn, g H2O, g NaOH, g H2(ml) g % 1 135 1.37 300 180 3.68 504 96.8 Example 2 – Hydrolysis of zinc in potassium hydroxide solution The hydrolysis reactor described above in example 1 and shown in Fig 2 was also used for the present example. A carbonate-free, clear solution of 337 g of KOH in 350 ml of H2O was prepared and added to the hydrolysis reactor. Pure nitrogen gas was supplied (at the rate of 100 ml / min for 30 minutes) to purge the reactor before starting the reaction. 1.93 g of Zn was separately suspended in 50 ml of water and added to the reactor. The reactor mixture was refluxed for 4 hours (at 150°C and atmospheric pressure) while stirring to complete the reaction.715 ml of hydrogen was collected by the end of the reaction, equivalent to 97.5 % hydrolysis conversion percentage for the zinc powder initially loaded to the reactor. Stoichiometric calculations showed the reaction yielded 6.08g of K2Zn(OH)4. Table 2: Quantity of reactants provided and products obtained Reaction systemP, atm T, °C Zn Conv. Zn, g H2O, g KOH, g K2Zn(OH)4, g H2(ml) % 1 150 1.93 400 337 6.08 715 97.5 The reaction was repeated at 130°C for 30 minutes using an 8M concentration of KOH, and the results are shown in Figure 3. In Figure 3b the results were fitted to the Avrami-Eroveef model was applied to fit the data: α(t) = 1 – exp(k.t-m) where α=is the fraction of reacted particles, t is the time for fraction of reacted particles, k is a specific rate constant and m is a parameter which depends on geometry. Example 3 – Improving Hydrolysis Reaction Efficiency The inventors wished to increase the rate of the hydrolysis reaction at lower temperatures. Accordingly, they looked into the use of a catalyst, hematite (Fe2O3). Without wishing to be bound by theory, the inventors believe that Fe2O3can catalyse the reaction as discussed below. Firstly, the inventors note that Fe2O3can form iron(III) oxide-hydroxide or ferric oxyhydroxide followed by iron(III) hydroxide in the presence of water and KOH solution, as follows: Fe2O3+ 3H2O + KOH ⇌ 2FeO.OH +2H2O + KOH ⇌ 2Fe(OH)3+ KOH There is then reduction of the iron(III) oxide, as follows: 3Zn + 2Fe(OH)3+KOH.xH2O ^ 3Zn(OH)2+ 2Fe + KOH.xH2O Finally, there is oxidation of the iron and production of hydrogen: 2Fe + 6H2O + KOH.xH2O ^ 2Fe(OH)3+ 3H2+ KOH.xH2O The inventors investigated how various factors effected the catalysed reaction, as discussed below. Effect of molar ratio of Fe2O3 / Zinc A series of tests were carried out with a constant amount of Fe2O3 (0.0005 mole) and different amount of zinc from 0.001 to 0.0125 mole. Accordingly, the molar ratio of zinc to Fe2O3varied from 1:0.5 to 1:0.06. In these experiments, the concentration of potassium hydroxide was 8M. The volume of the solution was 15 ml. The results are provided in Figure 4 and show two moles of zinc can generate one mole hydrogen when the mole ratio of Fe2O3 / Zn decreased from 0.5 to 0.1. Further decreasing the ratio to 0.06 decreased the hydrogen which was generated. Accordingly, the optimum molar ratio of F2O3 / zinc appears to be 0.5. Effect of concentration of Fe2O3The inventors then conducted a series of tests where the Fe2O3dosage varied from 0.0625 mmol to 0.5 mmol. It will be appreciated that Fe2O3is relatively insoluble in aqueous solutions. Accordingly, the Fe2O3was provided as a powder suspended in the KOH solution. The “concentrations” given may be understood to confirm the amount of the Fe2O3in the solution, as opposed to a true concentration. In these experiments, the amount of zinc present was 0.1 mole and the concentration of potassium hydroxide was 8M. The volume of the solution was 15 ml. As shown in Figure 5, an increase in Fe2O3dosage from 0.0625 mmol to 0.5 mmol cause the volume of hydrogen produced in 30 min to increase. This indicates an increase in the catalyst amount results in much chance for the reactant to contact the catalst. However, the hydrogen production yield decreased from 0.985 to 0.529 when the concentration of Fe2O3 increased from 0.5 mmol to 1.25 mmol. This is likely due to the increased concentration causing an observed increase in pH in the solution. The increase in pH may be due to converting Fe ions into Fe(OH)3. Effect of temperature The inventors then investigated how temperature affected the rate of the reaction. The inventors conducted a series of tests where the temperature varied from 4 to 36°C. In these experiments, the amount of zinc present was 0.1 mole, the concentration of Fe2O3was 0.5 mmol and the concentration of potassium hydroxide was 8M. The volume of the solution was 15 ml. As shown in Figure 6, an increase in temperature has a significant effect in zinc hydrolysis because of the increase in the speed of molecular movement. Accordingly, molecular collisions appear to be the most important factor in increasing the hydrogen generation rate. However, it should be noted that even at 4°C, a significant amount of hydrogen was produced. Effect of concentration of KOH The inventors then investigated how the concentration of KOH affected the rate of the reaction. The inventors conducted a series of tests where the concentration of KOH varied from 4 to 12 M. In these experiments, the amount of zinc present was 0.1 mole, the concentration of Fe2O3was 0.5 mmol and the volume of the solution was 15 ml. As shown in Figure 7, the generated hydrogen increased with increasing the KOH concentration from 4 to 12 M. However, only a slight enhancement was observed for the 12M solution compared to the 10M solution. It is thought that this is due to increasing viscosity of the solution. Analysis of experimental design using a general full factorial (Minitab) The study of the effect of individual factors and the influence of possible interactions between three factors in the experiments were conducted in the Minitab® software. A full factorial experimental design (DoE) in three levels was carried out to determine the relative contribution of three factors KOH concentration (8, 10 and 12 M), catalyst dosage (0.06, 0.08 and 0.12 g) and temperature (4, 21 and 36 °C) to find the most significant parameters that influence hydrogen production. The statistical significance of each factor was evaluated using P-values at a 95% confidence interval. The results are provided in table 3. Table 3: Results of analysis of experimental design L1 L2 L3 P-value KOH (M) 8 10 12 0.005 Catalyst (mmol) 0.37 0.5 0.75 0.004 Temp (°C) 4 21 36 0.000 The temperature had the lowest P-value (0.000) indicating the most influential parameter on hydrogen production. Besides, the low p-value of 0.000 indicates the high significance of the regression model. The KOH concentration had the highest P- value of (0.005) providing the least influential factor on the hydrogen production. In conclusion, the addition of a catalyst is highly advantageous as it allows the hydrolysis reaction to proceed at low temperatures. The catalyst can also easily be separated from the electrolyte. In particular, both iron and its oxides, including hematite (Fe2O3), have very good magnetic properties. The inventors noted that a magnet could easily separate all the catalyst particles very effectively. In the above described experiments, the inventors used a magnetic stirrer, and the catalyst only dispersed when a rigorous spinning rate above 150 rpm was used. At lower speeds, the iron catalyst particles were attached to the magnet. Example 4 – Electrolysis System Using Sodium Stannate Trihydrate The inventors wished to show that the hydrogen production system envisaged above can be used with other active agents than zinc. Accordingly, the inventors looked to demonstrate this by using sodium stannate trihydrate as an alternative active agent. Materials and Methods Apparatus An electrolysis cell was used to carry out the batch experiments. The cell comprised of a rectangular, open-top perspex acrylic vessel with two cylindrical graphite electrodes connected to the bottom, with part of the electrode was inside the vessel with the rest outside. The vessel had a height of 25cm, a length of 20cm and a width of 12cm. The electrodes were 6cm apart, disposed either side of the centre of the vessel. The part of the electrodes which remained inside the vessel had a length of 3cm and a diameter of 1cm. These values were constant since the electrodes used were unchanged. Electric wires connected the electrodes to the power supply. The power supply set the voltage (and current) prior to each experiment. Each electrode had cylinders (500ml) with fitted valves placed over them to collect any gas produced in the electrolysis reactions. Tubing was used to connect the cylinders to a vacuum pump. Prior to conducting the electrolysis reaction, the valves on the cylinders were opened and the vacuum pump engaged to create a partial vacuum, and thereby remove the air in the cylinder and replace it with an electrolyte solution (i.e. sodium stannate trihydrate solution). During the electrolysis reaction, the valves were closed, so that any gas present in the cylinders would have been produced in the reaction. Procedure Electrical Conductivity The electrolyte (sodium stannate trihydrate solution) was prepared by dissolving 55.4g of sodium stannate (42-45%) SnO2basis, into 200ml of distilled water. The solution produced had a concentration of 1.3M. This concentration was determined based on sodium stannate’s solubility limit in water. The reaction which occurred was exothermic: Na2SnO3(s)+ 3H2O ^ Na2[Sn(OH)6](aq)(1) The electrical conductivity of the electrolyte was measured using a conductivity measuring device. Then the solution was diluted with the addition of distilled water to the desired concentrations. These ranged from 1.3-0.1M, decreasing by 0.1M, at each interval. The conductivity was measured at every concentration. Using, the con- ductivity, the resistance was calculated using the equation: R = L / σ.S (2) Where R is electrical resistance (Ω), σ is electrical conductivity (mS / cm), L is the distance between the electrodes (cm) and S is electrode surface area (cm2), which was fixed at 11cm2. Hydrogen production The first step involved making the electrolyte to the required concentration. The electrolyte was mixed thoroughly, to allow for a constant concentration gradient. It was then poured carefully into the vessel and the air in the cylinders was evacuated (as described above) and the valves closed. The power supply was turned on. The current was fixed at 1A for all experiments. Two different experimental procedures were carried out. The first involved measuring the hydrogen production rate, over time, at three different concentrations around the optimum. The results from the electrical conductivity measurements determined these. The second involved measuring the hydrogen production rate, over time, at three different voltages: 2, 2.5, and 3V. Each experiment lasted for three hours, with the hydrogen production rates measured at five-minute intervals. The electrolysis which is desirable is that of sodium stannate trihydrate. However, it can exist in equilibrium with tin (IV) hydroxide and NaOH, like so: Na2[Sn(OH)6](aq) ⇌ 2NaOH(aq) + Sn(OH)4(aq) (3) The process is carried out at room temperature, which is beneficial as it tends to favour the dissociation of sodium stannate trihydrate at equilibrium. Even though the dissociation of water is relatively small in this process, it is still possible and will lead to the production of hydrogen gas at the cathode (equation 4). The more prominent reaction which occurs at the same time is tin(IV) hydroxide being reduced to tin and depositing on the surface of the cathode (equation 5). These reactions can be seen respectively as: 4H2O + 4e−⇌ 2H2(g) + 4OH-(aq) (4) The of oxygen gas: 8OH−(aq)⇌ 2O2(g)+ 4H2O + 8e− (6) By taking the summation of Eq.4,5 and 6, they simplify to: Sn(OH)4(aq)⇌ Sn(s)+ 2H2(g)+ 2O2(g)(7) ↔ The tin deposited on the cathode can now be oxidised in the presence of water to produce tin (IV) hydroxide and hydrogen gas: Sn(s)+ 4H2O ⇌ Sn(OH)4(aq)+ 2H2(g)(8) ↔ The tin (IV) hydroxide produced can be used to reproduce sodium stannate trihydrate, as a result of Eq.3. By taking the summation of Eq.8 and 9, the overall equation used to represent the process can simplify to: 4H2O ^ 4H2(g)+ 2O2(g)(9) Changing electrodes Due to the fact that a layer of tin is deposited as the reaction proceeds, the thickness of the layer of tin increases over time. This also increases the impedance at the electrode surface. Accordingly, there is a point that a drop in the electric current is observed and the electrodes had to be changed. The three indicators which could determine the time to change the electrodes were; a decrease in the current of the cell, a decrease in the hydrogen production rate and an increase in the oxygen production rate. The chosen indicator was the current, because this was something that could be seen visually as a decreasing of the power supply. Whereas, if a decrease in the hydrogen production rates were chosen, it would not be observed instantly. Changing the electrodes was achieved by changing the wires around in the power supply. Consequently, the anode will become the cathode, and the cathode will become the anode. Hydrogen production will now occur at the other electrode where no layer of tin has deposited, in the meantime. Therefore, this is a process which allows for the continuous production of hydrogen gas. It may be appreciated that in these experiments the inventors switched the electrodes. However, in their continuous process, they could overcome the problem by instead removing the deposited tin from the cathode. The tin could be removed using mechanical means (such as a blade) or due to the current of the electrolyte in the cell. The tin could be removed from the cathode continuously or at regular intervals. Cell efficiency The cell efficiency was calculated based on a combination of measurements taken from the experiments and parameters used to conduct the experiments. The equation used to determine the cell efficiency: n = q / (U.I.t) (10) Where n is cell efficiency based on the hydrogen production rate [m3m−3h−1(kWh)−1], q is hydrogen production rate per unit volume of electrolyte in electrolysis cell (m3m−3h−1), U is cell voltage (V), I is cell current (A) and t is time (hours). The term (kWh)−1, is a combination of the terms in the denominator of Eq.10. It is a representation of the amount of power consumed by electrolysis. During the experiments, the voltage varies (2, 2.5, and 3V) with a fixed electrolyte concentration, current and specified time. Results and Discussion Concentration and conductivity The conductivity of electrolyte was measured at various concentrations. The factors which affect the conductivity of a solution are the nature of the electrolyte, the electrode and the concentration of the solution. The electrolyte and the conditions it was made (room temperature) were constant. Hence, conductivity is a function of the concentration. When considering resistance, the distance between the electrodes factored. A 6cm gap was required to allow both of the cylinders to fit next to each other in the perspex acrylic vessel. A shorter distance between the electrodes would have reduced the resistance. The trend which can be seen in Fig.8 is that a decrease in the concentration of the electrolyte leads to a decrease in the conductivity. The number of ions per unit volume decreases as the solution is diluted (concentration decreases). Therefore, there are fewer ions to carry the electrical charge (per unit volume), which results in a decrease in conductivity. Based on this fact, it is expected that the electrolyte concentration of 1.3M, the maximum concentration used in the experiment, will have the highest conductivity. However, this was not the case. The optimum conductivity was at a concentration of 1.2M. The unexpected results could be explained by the electrolyte being at its solubility limit. The electrolyte solution is allowing for the maximum amount of sodium stannate particles to exist between the water molecules. As a result, this would lead to a decrease in the mobility of the ions, which could lead to a decrease in the conductivity of the solution. The resistance was determined by Eq. (2). As previously mentioned, the distance between the electrode and electrode surface area were constant. Therefore, resistance was a function of conductivity, based on Eq. (2). Based on Fig.8, the relationship between resistance and conductivity is reciprocal. Concentration and hydrogen production The three concentrations considered in Fig.9 were chosen based on being the optimum concentrations based on their conductivities. The hydrogen production was then measured. The most amount of hydrogen accumulated was when the concentration was 1.2M, which exemplifies the results shown in Fig.8. The optimum concentration based on the conductivity led to the most hydrogen produced over time. This can be explained using similar principles to those previously mentioned. Ions are required to flow freely. The freer the ions flow, the more mobile they will be. As a result, more ions will flow to the electrodes, increasing the rate of hydrogen production. Fig.10 is a graphical representation of when the electrodes were changed. To make this clearer, additional measurements were taken at 21 minutes, when the first drop in current was observed and 24 minutes when the electrode would have been changed due to drop in the current below the threshold. The hydrogen production rate decreased at a fast rate once the current started to decrease. However, both started to decrease at the same time, which suggests that they occur at the same time. The drop in current could be due to less tin deposited on the electrode, which leads to a decrease in the hydrogen production rate. Voltage and hydrogen production The three voltages considered in Fig.11 were chosen based on the minimum cell voltage used in water electrolysis for hydrogen production, which was 2V. The hydrogen production was then measured. The most amount of hydrogen accumulated was when the voltage was at its highest, i.e. 3V. This is expected since a larger voltage accelerates the rate at which electrolysis occurs, which leads to more ions flowing to the electrodes. Hence, more hydrogen accumulated over time. Comparing this to Fig.9, it can be seen that voltage has a more significant effect on hydrogen production than concentration. The difference in the hydrogen production rate between 2V and 2.5V was approximately 150cm3. This is a substantial change in hydrogen accumulated based on the scale of production for the relative change in voltage. The number of electrode changes decreased, as the voltage increased as seen in Table 4. This was part of the reason the value for the accumulated hydrogen production was bigger at higher voltages. The fewer times the electrodes were changed, the fewer times the hydrogen production rate dropped off. Ultimately, this led to more accumulated hydrogen over time. Table 4: The number of electrode changes for a 1.2M electrolyte solution Voltage (V) No. of electrode changes 2 7 2.5 5 3 3 Cell efficiency The cell efficiency was calculated based on a 1.2M electrolyte solution for reasons previously mentioned. The cell should be the most efficient at this concentration. Eq.10 was used to calculate the cell efficiencies, and the results are provided in Table 5. Table 5: The cell efficiency for a 1.2M electrolyte solution Voltage (V)Cell efficiency(m3 m−3 h−1 (kWh)−1)2 10.68 2.5 4.35 3 2.03 Based on literature, the cell efficiency is given as a value of about 2.3 m3m−3h−1(kWh)−1for a typical unipolar water electrolyser (see Santos D, C, Sequeira F, J. Hydrogen production by alkaline water electrolysis; 2019). It is noted that this value is given at a cell voltage of 2.2V. Accordingly, the present cell is more efficient than the typical water electrolyser at both 2 and 2.5 V. For this reason, a recommendation would be to use smaller voltages, as this is when the cells are the most efficient. Example 5 – Effect of temperature on the thermochemical reaction The inventors calculated the conversion rate of zinc to zinc oxide at various temperatures and pressures, and the results are shown in Figure 12. As shown in Figure 12a, low temperature is desired for good conversion. However, as shown in Figure 12b, at low temperatures the rate of reaction is low, resulting in a low yield. However, as shown in Figure 12c, if higher temperature are used along with higher pressures, thereby keeping the media in liquid form then a high conversion may be achieved. It is noted that the conversion rate was calculated on the basis that the catalyst was not present. It may be appreciated that lower temperatures and / or pressures could be used if the catalyst was present.

Claims

Claims 1. A method of producing hydrogen, the method comprising: - conducting a thermochemical reaction by contacting an active reagent and a basic aqueous solution, to thereby cause water from the basic aqueous solution to react with the active reagent and to produce hydrogen and a basic aqueous solution comprising an oxidised product; - disposing the basic aqueous solution comprising the oxidised product in an electrochemical cell comprising an anode and a cathode, such that at least a portion of the cathode contacts the solution; and - conducting an electrochemical reaction by applying a voltage across the anode and the cathode to produce hydrogen, oxygen and the active reagent; wherein the active reagent comprises a metal or metal ion in a first oxidation state and the oxidised product comprises the metal or metal ion in a second oxidation state which is higher than the first oxidation state.

2. A method of producing hydrogen, the method comprising: - disposing a basic aqueous solution comprising an oxidised product in an electrochemical cell comprising an anode and a cathode, such that at least a portion of the cathode contacts the solution; - conducting an electrochemical reaction by applying a voltage across the anode and the cathode to produce hydrogen, oxygen and an active reagent; and - conducting a thermochemical reaction by contacting the active reagent and a basic aqueous solution, to thereby cause water from the basic aqueous solution to react with the active reagent and to produce hydrogen and a basic aqueous solution comprising an oxidised product, wherein the active reagent comprises a metal or metal ion in a first oxidation state and the oxidised product comprises the metal or metal ion in a second oxidation state which is higher than the first oxidation state.

3. The method according to claim 1 or claim 2, wherein the electrochemical reaction is conducted continuously.

4. The method according to any preceding claim, wherein the thermochemical reaction is conducted continuously.

5. The method according to any preceding claim, wherein the active reagent is or comprises a transition metal or an alloy thereof, a p-block metal or an alloy thereof or a reactivated spinel / perovskite composite with multiple oxidation states.

6. The method according to claim 5, wherein the active reagent is or comprises a transition metal or an alloy thereof or a p-block metal or an alloy thereof, and the p- block metal, or the alloy thereof, is selected from the group consisting of tin, lead, thallium, selenium and bismuth and the transition metal, or the alloy thereof, is selected from the group consisting of zinc, copper, iron, nickel, cobalt, manganese, titanium, molybdenum, cadmium, chromium, vanadium, silver, rhodium, platinum, palladium, iridium, osmium, rhenium, ruthenium, lanthanum and zirconium.

7. The method according to claim 6, wherein the active reagent is or comprises a transition metal or an alloy thereof and is zinc.

8. The method according to claim 5, wherein the active reagent is or comprises a reactivated spinel / perovskite composite with multiple oxidation states having general formula ZnxMyOz, where Zn is zinc, M is a metal, O is oxygen and x, y and z are each an integer between 1 and 10, and the metal is a transition metal or a p-block metal.

9. The method according to any preceding claim, wherein the method comprises contacting the active reagent and the basic aqueous solution in the presence of a catalyst.

10. The method according to claim 10, wherein the catalyst comprises or is iron(III) oxide (Fe2O3), nickel hydroxide (Ni(OH)2), potassium stannate (K2SnO3), copper hydroxide (Cu(OH)2), or combinations thereof, and is preferably iron(III) oxide (Fe2O3).

11. The method according to any preceding claim, wherein the thermochemical reaction is conducted at an elevated temperature of less than 700°C, less than 650°C, less than 600°C, less than 550°C, less than 500°C, less than 450°C, less than 400°C, less than 300°C, less than 250°C, less than 200°C, less than 175°C, less than 160°C or less than 150°C. .

12. The method according to any preceding claim, wherein the thermochemical reaction produces a gas stream comprising hydrogen and steam, and the method comprises cooling the gas stream produced in the thermochemical reaction to cause water to condense out of the gas stream.

13. The method according to claim 12, wherein cooling the gas stream comprises transferring heat from the gas stream to a cooling fluid, and the cooling fluid is or comprises water, and is subsequently be used in the thermochemical reaction.

14. The method according to any preceding claim, wherein during the thermochemical reaction, the active reagent is suspended in the basic aqueous solution.

15. The method according to any preceding claim, wherein the basic aqueous solution comprises a base at a concentration of between 0.5 and 50 M, between 1 and 45 M, between 2 and 40 M, between 4 and 35 M, between 6 and 30 M, or between 8 and 28 M.

16. The method according to claim 13, wherein the method may comprises adding water to the basic aqueous solution to maintain a desired concentration of the base in the basic aqueous solution and / or a desired pH of the basic aqueous solution.

17. An apparatus for producing hydrogen, the apparatus comprising: - a thermochemical reactor, configured to hold a basic aqueous solution and an active reagent therein and thereby allow a thermochemical reaction to proceed and produce a gas stream comprising hydrogen and a basic aqueous solution comprising an oxidised product; - an electrochemical cell comprising an anode and a cathode, and configured to receive the basic aqueous solution comprising the oxidised product from the thermochemical reactor, such that at least a portion of the cathode contacts the basic aqueous solution comprising the oxidised product, the electrochemical cell being configured to cause an electrochemical reaction to proceed and produce hydrogen gas and the active reagent at the cathode and oxygen at the anode; - a first conduit extending between the thermochemical reactor and the electrochemical cell, the first conduit being configured to feed the basic aqueous solution comprising the oxidised product from the thermochemical reactor to the electrochemical cell; and- a second conduit extending between the thermochemical reactor and the electrochemical cell, the second conduit being configured to feed the basic aqueous solution and the active reagent from the electrochemical cell to the thermochemical reactor.

18. The apparatus according to claim 17, wherein the thermochemical reactor comprises an agitator configured to agitate the basic aqueous solution in the thermochemical reactor.

19. The apparatus according to claim 17 or claim 18, wherein the apparatus comprises a condenser configured to cool the gas stream produced in the thermochemical reactor, and thereby cause water in the gas stream to condense.

20. The apparatus according to any one of claims 17 to 19, wherein the apparatus comprises a heat exchanger, and the first and second conduits extend therethrough, wherein the heat exchanger is configured to transfer heat between the first and second conduits.

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