Multiphase reactor for producing hydrogen

Abstract

The present disclosure provides a method of producing hydrogen. The method includes performing a thermochemical reaction by contacting an active reagent with an alkaline aqueous solution, thereby reacting water in the alkaline aqueous solution with the active reagent and producing hydrogen and an alkaline aqueous solution comprising an oxidation product. The method further comprises placing the alkaline aqueous solution comprising the oxidation product in an electrochemical cell comprising an anode and a cathode such that at least a portion of the cathode contacts the solution; and performing an electrochemical reaction by applying a voltage between the anode and the cathode to produce hydrogen, oxygen and an active reagent. The active agent comprises a metal or metal ion in a first oxidation state and the oxidation product comprises a metal or metal ion in a second oxidation state higher than the first oxidation state.

Description

[0001] This invention relates to a method for producing hydrogen. In a specific embodiment of the invention, the method can be a sustainable cyclic process. The invention also relates to an apparatus for producing hydrogen.

[0002] Hydrogen is an important energy carrier and has the potential to replace hydrocarbon-based fuels for sustainable development. Current energy issues related to hydrocarbon fuels, such as air pollution, climate change, and resource scarcity, are significant drivers 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 zero net atmospheric emissions, and it is also conducive to efficient energy storage. Hydrogen can be used as a direct transport fuel with high energy efficiency, which is receiving widespread attention as a technological and political issue.

[0003] Currently, several industrial methods exist for hydrogen production, among which reforming, photoconversion, and electrolysis have gained significant traction. Water electrolysis offers the cleanest solution for hydrogen production. Its advantages include: (i) zero carbon emissions; (ii) production of pure hydrogen, which is crucial for fuel cell technology, which is heavily reliant on impurities in the hydrogen feedstock; (iii) independence from hydrocarbon resources; (iv) operability in small-scale installations; and (v) the ability to produce hydrogen using renewable energy sources.

[0004] WO 2020 / 016580 A2 describes a method for producing hydrogen in both an electrolytic process of water splitting and a thermochemical reaction. The latter is carried out in a closed reactor, in which a metal (e.g., zinc) reacts with steam at high temperatures to produce hydrogen and metal oxides. This method has several drawbacks, including: (a) the method is difficult to operate in continuous mode because a solid metal must be introduced into the thermochemical reactor chamber and the solid oxide products must be removed from it; and (b) during the thermochemical reaction, the metal surface is rapidly passivated by a layer of oxide, which hinders the rate of hydrogen production.

[0005] This invention stems from our research work aimed at overcoming the aforementioned problems related to the prior art. We hereby disclose an invention capable of producing hydrogen in a system that can operate in continuous mode and is easily scalable.

[0006] According to a first aspect of the present invention, a method for generating hydrogen gas is provided, the method comprising: - A thermochemical reaction is carried out by contacting an active reagent with an alkaline aqueous solution to produce hydrogen gas and an alkaline aqueous solution containing oxidation products; - Placing an alkaline aqueous solution containing the oxidation product in an electrochemical cell comprising an anode and a cathode, such that at least a portion of the cathode is in contact with the solution; and - An electrochemical reaction is carried out by applying a voltage between the anode and cathode to produce hydrogen, oxygen and active reagents; The active reagent contains a metal or metal ion in a first oxidation state, and the oxidation product contains a metal or metal ion in a second oxidation state higher than the first oxidation state.

[0007] Advantageously, both the thermochemical and electrochemical reactions produce hydrogen gas. Furthermore, both reactions are carried out in an aqueous solution. Therefore, the solution can be readily recycled between the thermochemical reactor and the electrochemical cell without additional processing steps. Moreover, since the thermochemical reaction occurs in solution, the oxidation products can dissolve upon formation. Thus, this method overcomes the passivation problem observed in the prior art and facilitates the conversion of active reagents into oxidation products.

[0008] By using an alkaline solution and changing the reaction phase to an aqueous phase, the reactor type can be simplified to a "hydrolyzer" instead of a "fixed-bed" reactor. Conducting the reaction steps in an "aqueous phase" mode provides significant advantages for process operation by allowing both the reactor and the electrolyzer to operate in a "continuous" mode. In other words, the "hydrothermal reactor" and the "electrochemical cell" can be directly connected using pumps, allowing a single liquid phase "alkaline aqueous solution" to be recycled between these core units.

[0009] The term "thermochemical reaction" as used herein may be replaced by the term "hydrothermal reaction." Similarly, the term "thermochemical reactor" may be replaced by the terms "hydrothermal reactor" or "hydrolyzer."

[0010] It is understandable that electrochemical reactions can occur before thermochemical reactions.

[0011] Therefore, according to a second aspect, a method for generating hydrogen gas is provided, the method comprising: - Place an alkaline aqueous solution containing oxidation products in an electrochemical cell containing an anode and a cathode, such that at least a portion of the cathode is in contact with the solution; - An electrochemical reaction is carried out by applying a voltage between the anode and cathode to produce hydrogen, oxygen, and active reagents; and - A thermochemical reaction is carried out by contacting an active reagent with an alkaline aqueous solution to produce hydrogen gas and an alkaline aqueous solution containing oxidation products. The active reagent contains a metal or metal ion in a first oxidation state, and the oxidation product contains a metal or metal ion in a second oxidation state higher than the first oxidation state.

[0012] Electrochemical reactions can occur continuously or repeatedly. Thermochemical reactions can occur continuously or repeatedly.

[0013] The methods of the first and second aspects may include discrete steps repeated multiple times, or continuous reactions running in parallel. In a preferred embodiment, both the electrochemical and thermochemical reactions are continuous and simultaneous. Advantageously, hydrogen can be continuously produced from the two reactions. The alkaline aqueous solution can be circulated between the electrochemical cell and the thermochemical reactor in a loop or cyclic manner. The alkaline aqueous solution can be circulated continuously.

[0014] This method of producing hydrogen can be viewed as a thermochemical / electrochemical cycle. The only feedstock required for continuous hydrogen production is water. Therefore, once an initial amount of active reagent and / or oxidation product is provided, this can be continuously cycled in a thermochemical / electrochemical cycle without the need for additional active reagent and / or oxidation product.

[0015] The active agent may be or contains a transition metal or its alloy, a p-block metal or its alloy, or a reactivated spinel / perovskite complex with multiple oxidation states.

[0016] The p-block metal or its alloy 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 its alloy is tin.

[0017] The transition metal or its alloy 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 its alloy is selected from the group consisting of zinc, copper, iron, nickel, cobalt, manganese, chromium, and vanadium. More preferably, the transition metal or its alloy is selected from the group consisting of zinc, iron, nickel, chromium, and vanadium. In a most preferred embodiment, the transition metal or its alloy is zinc.

[0018] Reactivated spinel / perovskite composites with multiple oxidation states can have the general formula M1 x M2 yOz, where M1 and M2 are both metals, O is oxygen, and x, y, and z are each integers between 1 and 10. M1 can be an s-block metal, a transition metal, or a p-block metal. For example, M1 can be an alkali metal, an alkaline earth metal, or a p-block metal. M1 can be sodium, potassium, or zinc. M2 can 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 can be tin, iron, manganese, chromium, titanium, or lead. x can be an integer between 1 and 5 or between 1 and 3, and preferably 1 or 2. y can be an integer between 1 and 5 or between 1 and 3, and preferably 1, 2, or 3. z can be 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, and preferably 1, 3, 4, or 8. Therefore, the reactivated spinel / perovskite composite having multiple oxidation states can be stannate, ferrite, ferrous ferrate, manganate, chromate, titanate, or leadate. The reactivated spinel / perovskite composite having multiple oxidation states can be zinc stannate, zinc ferrite, zinc ferrous ferrite, zinc manganate, zinc chromate, zinc titanate, zinc lead oxide, sodium stannate, or potassium stannate.

[0019] It is understood that the oxidation product may be or contains metal ions, and said metal ions may be metal cations, wherein said metal is as defined above. The oxidation product may further contain a hydroxide group. Therefore, the oxidation product may be a metal hydroxide. The metal hydroxide can be directly generated by the reaction of an active reagent with water. Alternatively, the reaction of an active reagent with water can generate a metal oxide, and said metal oxide can further react with an alkaline aqueous solution to generate a metal hydroxide. The oxidation product is preferably dissolved in an alkaline aqueous solution.

[0020] For example, the oxidation product may be or may contain zinc hydroxide, zincate, 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.

[0021] Alternatively, the oxidation product may be or comprise a reactivated spinel / perovskite composite having multiple oxidation states. The reactivated spinel / perovskite composite having multiple oxidation states may be as defined above.

[0022] It is understandable that zinc hydroxide can be in equilibrium with zincates in alkaline aqueous solutions.

[0023] The method may include contacting an active reagent with an alkaline 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. Therefore, the catalyst may comprise iron(III) oxide (Fe₂O₃), nickel hydroxide (Ni(OH)₂), potassium stannate (K₂SnO₃), copper hydroxide (Cu(OH)₂), or a combination thereof. In some embodiments, the catalyst is iron(III) oxide.

[0024] The catalyst may be in powder form.

[0025] The molar ratio of the active reagent to the catalyst can be between 1:0.001 and 1:1, 1:0.005 and 1:0.75, 1:0.01 and 1:0.5, 1:0.02 and 1:0.3, 1:0.04 and 1:0.2, 1:0.06 and 1:0.15, 1:0.08 and 1:0.12, or 1:0.09 and 1:0.11.

[0026] The concentration of the catalyst can 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 can be between 0.01 and 50 mmol, 0.02 and 10 mmol, 0.04 and 5 mmol, 0.06 and 2 mmol, 0.08 and 1.5 mmol, 0.1 and 1 mmol, 0.3 and 0.8 mmol, or 0.4 and 0.6 mmol. If the catalyst is insoluble or substantially insoluble, the concentration can be understood as calculating the number of moles of catalyst present in suspension in the solution. Therefore, the concentration can be calculated using the standard formula: In some implementations, the catalyst may be magnetic.

[0027] The method may include separating the catalyst from the alkaline aqueous solution before feeding the alkaline aqueous solution into the electrochemical cell. The method may include carrying out a thermochemical reaction in a thermochemical reactor. The method may include holding the catalyst in the thermochemical reactor. The method may include using one or more magnets to separate the catalyst from the alkaline aqueous solution and / or hold the catalyst in the thermochemical reactor.

[0028] In all cases, the thermochemical reaction is carried out in water. The thermochemical reaction can be carried out at temperatures 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 can be carried out at temperatures of at least 30°C, at least 40°C, at least 50°C, or at least 60°C, more preferably at at least 70°C, at least 80°C, at least 90°C, or at least 100°C, and most preferably at at least 110°C, at least 120°C, at least 130°C, or at least 135°C. In some embodiments, the thermochemical reaction is carried out 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.

[0029] Thermochemical reactions can be carried out at temperatures below 1,000°C, below 950°C, below 900°C, below 800°C, or below 850°C. In some embodiments, the thermochemical reactions are carried out at temperatures below 700°C, more preferably below 650°C, below 600°C, or below 550°C, and most preferably below 500°C, below 450°C, below 400°C, below 300°C, below 250°C, below 200°C, below 175°C, below 160°C, or below 150°C. In some embodiments, the thermochemical reactions can be carried out at temperatures below 130°C, below 100°C, below 80°C, below 60°C, or below 40°C.

[0030] In some embodiments, the thermochemical reaction is carried out at temperatures 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°C, or between 25 and 750°C.

[0031] In some embodiments, the thermochemical reaction is carried out at a temperature between 0 and 1,000°C or between 5°C and 700°C. In some embodiments, the thermochemical reaction is carried out at a temperature between 30 and 600°C, 40 and 500°C, 60 and 400°C, 80 and 300°C, or 100 and 250°C, most preferably between 110 and 200°C, 120 and 175°C, 130 and 160°C, or 135 and 150°C. In alternative embodiments, the thermochemical reaction is carried out at a temperature between 0 and 130°C, 5 and 100°C, 10 and 80°C, 15 and 60°C, or 20 and 40°C. Advantageously, a catalyst enables the reaction to proceed at a lower temperature.

[0032] In some embodiments, the thermochemical reaction is carried out at temperatures between 350 and 1,000°C, 400 and 950°C, 500 and 900°C, 600 and 850°C, or 700 and 800°C. Advantageously, increasing the temperature increases the reaction rate.

[0033] The method may include carrying out a thermochemical reaction in a thermochemical reactor. The method may include controlling the temperature within the thermochemical reactor. The method may include bringing the temperature within the thermochemical reactor to the temperature defined above. Controlling the temperature within the thermochemical reactor may include flowing a first fluid through a heat exchanger located adjacent to the thermochemical reactor, such that heat can be transferred between the thermochemical reactor and the first fluid, thereby maintaining the temperature within the thermochemical reactor. The method may include transferring heat from the thermochemical reactor to the first fluid. The first fluid may be or contain water. The first fluid may be a liquid when it is fed into the heat exchanger. Transferring heat from the thermochemical reactor to the first fluid may cause the first fluid to evaporate and form a gas (e.g., steam).

[0034] The method may then include recovering heat from a first fluid. Recovering heat from the fluid can cause the first fluid to condense. Recovering heat from the fluid may include transferring heat from the gas stream to a cooling fluid. The cooling fluid may be or contain water. Advantageously, recovering heat from the first fluid can heat the cooling fluid. In embodiments where the cooling fluid is or contains water, the water can then be used for a thermochemical reaction. Therefore, cooling the gas stream can preheat the water used for the thermochemical reaction.

[0035] Thermochemical reactions can produce a gaseous stream containing hydrogen. It is understood that this gaseous stream may further contain vapor. Therefore, the method may include cooling the gaseous stream produced in the thermochemical reaction to allow water to condense from the gaseous stream. For example, the thermochemical reaction may be carried out under reflux conditions. Advantageously, the condensation step removes vapor from the gaseous stream and allows the user to obtain hydrogen.

[0036] Cooling the gas stream may include transferring heat from the gas stream to a cooling fluid. The cooling fluid may be or contain water. Advantageously, cooling the gas stream may heat the cooling fluid. In embodiments where the cooling fluid is or contains water, the water may then be used for a thermochemical reaction. Therefore, cooling the gas stream may preheat the water used for the thermochemical reaction.

[0037] Thermochemical reactions can be carried out in a thermochemical reactor. Therefore, the method may include transferring heat from a gaseous feed stream to a liquid containing or composed of water, and subsequently feeding the liquid into the thermochemical reactor.

[0038] Understandably, the pressure can be selected based on the temperature at which the thermochemical reaction takes place. A higher pressure can be selected to ensure that the alkaline aqueous solution does not boil at the temperature at which the thermochemical reaction takes place. The thermochemical reaction can be carried out at a pressure of at least 10 kPa or at least 20 kPa, more preferably at least 40 kPa, at least 60 kPa or at least 80 kPa, and most preferably at at least 90 kPa, at least 95 kPa or at least 100 kPa. In one embodiment, the thermochemical reaction is carried out 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 one embodiment, the thermochemical reaction is carried out at a pressure of at least 5 MPa, at least 10 MPa or at least 15 MPa.

[0039] In one embodiment, the thermochemical reaction is carried out at a pressure below 5,000 kPa. In another embodiment, the thermochemical reaction is carried out at a pressure below 1,000 kPa, below 500 kPa, or below 250 kPa, most preferably below 200 kPa, below 150 kPa, or below 110 kPa.

[0040] In one embodiment, the thermochemical reaction is carried out at a pressure between 20 and 5,000 kPa, more preferably between 40 and 1,000 kPa, 60 and 500 kPa, or 80 and 250 kPa, and most preferably between 90 and 200 kPa, 95 and 150 kPa, or 100 and 110 kPa.

[0041] In an alternative embodiment, the thermochemical reaction is carried out at pressures 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.

[0042] The active reagent may be in the form of powder, granules, or flakes. Preferably, the active reagent is suspended in an alkaline aqueous solution during the thermochemical reaction.

[0043] The method may include placing the active reagent in the thermochemical reactor prior to the thermochemical reaction. Alternatively, the method may include feeding the active reagent into the thermochemical reactor simultaneously with the thermochemical reaction. The method may include continuously feeding the active reagent into the thermochemical reactor simultaneously with the thermochemical reaction. Preferably, the method includes continuously feeding an alkaline aqueous solution in which the active reagent is suspended into the thermochemical reactor simultaneously with the thermochemical reaction.

[0044] The method may include removing an alkaline aqueous solution containing oxidation products from the thermochemical reactor at the end of the thermochemical reaction. Alternatively, the method may include removing an alkaline aqueous solution containing oxidation products from the thermochemical reactor while the thermochemical reaction is taking place. Preferably, the method includes continuously removing an alkaline aqueous solution containing oxidation products from the thermochemical reactor while the thermochemical reaction is taking place.

[0045] The method may include stirring an alkaline aqueous solution while a thermochemical reaction is being carried out.

[0046] It is understood that alkaline aqueous solutions contain a base. The base can be an organic or inorganic base. The base can 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 can include alkali metal hydroxides or alkaline earth metal hydroxides. The Arrhenius base can include potassium hydroxide, sodium hydroxide, barium hydroxide, cesium hydroxide, strontium hydroxide, calcium hydroxide, lithium hydroxide, and / or rubidium hydroxide. Therefore, the alkaline aqueous solution can include an aqueous alkaline solution.

[0047] The Lewis base may include ammonia (NH3), butyllithium (n-BuLi), lithium diisopropylamino (LDA), lithium diethylamino (LDEA), sodium amide, sodium hydride (NaH), and / or bis(trimethylsilyl)aminolithium.

[0048] The Brønsted-Laurent base may include ammonium hydroxide, aliphatic amines, or aromatic amines. The aliphatic amine may include methylamine, ethylamine, or dimethylamine. The aromatic amine may include aniline, phenylenediamine, or o-toluidine.

[0049] In a preferred embodiment, the alkaline aqueous solution contains Arrhenius base.

[0050] The concentration of alkali in the alkaline aqueous solution can be at least 0.5 M, at least 1 M, or at least 2 M, more preferably at least 4 M, at least 6 M, or at least 8 M, at least 10 M, at least 12 M, at least 14 M, or at least 15 M. The concentration of alkali in the alkaline aqueous solution can be less than 8.5 M, less than 8 M, or less than 7.5 M, more preferably less than 7 M, less than 40 M, or less than 30 M, and most preferably less than 25 M or less than 20 M. The concentration of alkali in the alkaline aqueous solution can be between 0.5 and 50 M, between 1 and 45 M, or between 2 and 40 M, more preferably between 4 and 35 M, between 6 and 30 M, or between 8 and 28 M. In some embodiments, the concentration of alkali in the alkaline aqueous solution can be between 10 and 26 M, between 12 and 24 M, between 14 and 22 M, or between 15 and 20 M. In alternative embodiments, the concentration of alkali in the alkaline aqueous solution can be between 6 and 20 M, between 8 and 15 M, or between 10 and 12 M.

[0051] The concentration of alkali in this alkaline aqueous solution can be basically the same as the concentration of alkali in alkaline aqueous solutions used for thermochemical reactions and alkaline aqueous solutions used for electrochemical reactions.

[0052] In the thermochemical reaction, the oxidation product may be present in an alkaline 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 embodiments, the oxidation 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 oxidation 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 oxidation product may be present at a concentration 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.

[0053] In one embodiment, the concentration of the oxidation product is 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.

[0054] In an alternative embodiment, the concentration of the oxidation product is 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.

[0055] The electrochemical cell may be an unseparated cell. Therefore, in this embodiment, placing an alkaline aqueous solution containing oxidation products into the cell will cause at least a portion of the anode to contact the solution.

[0056] In an alternative embodiment, the electrochemical cell includes a separator cell. Therefore, the cell may include a membrane disposed between the anode and cathode, separating the cell into two parts. Preferably, the membrane is an anion exchange membrane, more preferably an alkaline anion exchange membrane (AAEM). Suitable AAEMs are known to those skilled in the art, but may include polymeric anion exchange base membranes of chloromethylated polysulfone, or copolymers of chloromethylstyrene and divinylbenzene with polyethylene fabric.

[0057] In this embodiment, the method may include placing an alkaline aqueous solution containing the oxidation product into the cathode portion of the battery, such that at least a portion of the cathode is in contact with the solution. The method may also include placing another electrolyte into the anode portion of the battery, such that at least a portion of the anode is in contact with the other electrolyte. The other electrolyte preferably comprises an aqueous solution, more preferably an alkaline aqueous solution. It is understood that the alkaline aqueous solution contains a base. The base may be as defined above.

[0058] Understandably, as an electrochemical reaction proceeds, water will react to produce hydrogen and oxygen. Similarly, as a thermochemical reaction proceeds, water will react to produce hydrogen.

[0059] Therefore, the concentration of alkali in an alkaline aqueous solution will increase. Similarly, in embodiments where the battery includes a separator cell, the concentration of alkali in the other electrolyte will also increase.

[0060] Therefore, the method may include adding water to an alkaline aqueous solution. The method may include adding water to an alkaline aqueous solution to maintain a desired concentration of alkali and / or a desired pH value in the alkaline aqueous solution. The method may include adding water to the alkaline aqueous solution when the pH value rises above a predetermined maximum value.

[0061] Water can be added to the alkaline aqueous solution at any stage of the method. For example, water can be added to the alkaline aqueous solution in the thermochemical reactor and / or electrolytic cell. Alternatively, water can be added to the alkaline aqueous solution downstream of the thermochemical reactor and upstream of the electrolytic cell. When adding water to the alkaline aqueous solution downstream of the thermochemical reactor and upstream of the electrolytic cell, or vice versa, water can be added to the alkaline aqueous solution through a conduit between the thermochemical reactor and the electrolytic cell.

[0062] In embodiments where the battery includes a separator battery, the method may include contacting another electrolyte with a sufficient amount of water to give the other electrolyte a desired alkaline concentration. The method may include monitoring the pH of the other electrolyte and, when the pH rises above a predetermined maximum value, contacting the other electrolyte with a sufficient amount of water to give the other electrolyte a desired alkaline concentration.

[0063] The desired alkali concentration can be defined as described above.

[0064] The method may include continuously feeding an alkaline aqueous solution containing oxidation products into an electrochemical cell while the electrochemical reaction is taking place. The method may also include removing the alkaline aqueous solution from the electrochemical cell while the electrochemical reaction is taking place. Advantageously, this step continuously replenishes the alkaline aqueous solution. Furthermore, the inventors have found that this is sufficient to remove the active reagent from the electrolytic cell.

[0065] Alternatively, the method may include mechanically separating the active reagent from the cathode. The active reagent may be removed after the electrochemical reaction is complete. Alternatively, the active reagent may be removed continuously or periodically during the electrochemical reaction. For example, the electrochemical cell may include a blade configured to remove the active reagent from the cathode. The blade may be configured to move across the cathode, thereby removing the active reagent from the cathode surface. The blade may be driven by magnetic force or an electric motor.

[0066] Therefore, the method may include flowing an alkaline aqueous solution through the electrochemical cell, thereby removing the active reagent from the electrochemical cell because it is suspended in the alkaline aqueous solution. The method may also include flowing the alkaline aqueous solution through the electrochemical cell while an electrochemical reaction is taking place.

[0067] In embodiments where the battery is a separated battery, the method may include feeding an alkaline aqueous solution containing oxidation products into the cathode portion of the electrochemical battery while the electrochemical reaction is taking place. The method may also include removing the alkaline aqueous solution from the cathode portion of the battery while the electrochemical reaction is taking place. The method may also include feeding another electrolyte into the anode portion of the electrochemical battery while the electrochemical reaction is taking place, and / or removing another electrolyte from the anode portion of the battery while the electrochemical reaction is taking place.

[0068] The anode and cathode can independently comprise carbon-based electrodes or metal-based electrodes. The carbon-based electrode, or each carbon-based electrode, can comprise graphite. The metal-based electrode, or each metal-based electrode, can comprise chromium, nickel, zinc, cadmium, copper, tin, lead, rhodium, platinum, gold, palladium, iridium, osmium, rhenium, ruthenium, germanium, beryllium, and / or silver. Alternatively, the metal-based electrode, or each metal-based electrode, can comprise an alloy, such as brass, bronze, or steel. The steel can be stainless steel. In a preferred embodiment, the electrode comprises graphite or steel. In another preferred embodiment, the electrode comprises the same metal as the active reagent. For example, if the electrochemical reaction produces zinc, then the electrode can comprise zinc.

[0069] Preferably, the cathode comprises a substantially non-porous material. Advantageously, when the cathode is non-porous, the active material will be electrodeposited on the surface of the structure without being trapped inside its pores.

[0070] The method may include applying a voltage of at least 1 V, at least 1.5 V, at least 1.75 V, or at least 2 V between the anode and cathode. More preferably, the method includes applying a voltage of at least 2.5 V, at least 3 V, or at least 3.5 V between the anode and cathode. Most preferably, the method includes applying a voltage of at least 4 V or at least 4.5 V between the anode and cathode. The method may also include applying a voltage less than 8 V or less than 7.5 V between the anode and cathode. More preferably, the method includes applying a voltage less than 7 V, less than 6.5 V, or less than 6 V between the anode and cathode. Most preferably, the method includes applying a voltage less than 5.5 V or less than 5 V between the anode and cathode. In some embodiments, the method includes applying a voltage less than 4 V, less than 3 V, or less than 2.5 V. The method may also include applying a voltage between 1 and 8 V or between 2 and 7.5 V between the anode and cathode. In some embodiments, the method includes applying a voltage between 2.5 and 7 V, 3 and 6.5 V, or 3.5 and 6 V between the anode and cathode. Most preferably, the method includes applying a voltage between 4 and 5.5 V or 4.5 and 5 V between the anode and cathode. In alternative embodiments, the method includes applying a voltage between 1 and 4 V, 1.5 and 3 V, or 1.75 and 2.5 V between the anode and cathode.

[0071] The method may include flowing a current of at least 0.5 A, at least 1 A, or at least 1.5 A through an anode, a cathode, and a solution containing metal ions; more preferably, flowing a current of at least 2 A, at least 2.5 A, or at least 3 A through an anode, a cathode, and a solution containing metal ions; and most preferably, flowing a current of at least 3.5 A through an anode, a cathode, and a solution containing metal ions. The method may also include flowing a current of less than 10 A, less than 8 A, or less than 6 A through an anode, a cathode, and a solution containing metal ions; more preferably, flowing a current of less than 5.5 A, less than 5 A, or less than 4.5 A through an anode, a cathode, and a solution containing metal ions; and most preferably, flowing a current of less than 4 A through an anode, a cathode, and a solution containing metal ions. The method may include flowing a current between 0.5 and 10 A, between 1 and 8 A, or between 1.5 and 6 A through an anode, a cathode, and a solution containing metal ions; more preferably, flowing a current between 2 and 5.5 A, between 2.5 and 5 A, or between 3 and 4.5 A through an anode, a cathode, and a solution containing metal ions; and most preferably, flowing a current between 3.5 and 4 A through an anode, a cathode, and a solution containing metal ions.

[0072] Preferably, the electrochemical reaction is carried out at a temperature of at least 0°C, more preferably 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 carried out at a temperature of at least 25°C, most preferably at least 30°C, or at least 55°C. Preferably, the electrochemical reaction is carried out at a temperature below 95°C, more preferably below 90°C, below 85°C, or below 80°C, and most preferably below 70°C or below 65°C. In some embodiments, the electrochemical reaction is carried out at a temperature below 50°C, below 40°C, below 30°C, or below 25°C. Preferably, the electrochemical reaction is carried out at a temperature between 0°C and 95°C, more preferably between 10°C and 90°C. In some embodiments, the electrochemical reaction is carried out at a temperature between 20°C and 85°C or between 25°C and 80°C, most preferably between 30°C and 70°C or between 55°C and 65°C. In some embodiments, the electrochemical reaction is carried out at a temperature between 10°C and 50°C, between 12.5°C and 40°C, between 15°C and 30°C, or between 17.5°C and 25°C.

[0073] It is understood that thermochemical reactions can be carried out at higher temperatures than electrochemical reactions. Therefore, the method may include cooling an alkaline aqueous solution after a thermochemical reaction and before an electrochemical reaction. Similarly, the method may include heating an alkaline aqueous solution after an electrochemical reaction and before a thermochemical reaction.

[0074] It is understood that, in a preferred embodiment, when both the thermochemical and electrochemical reactions are carried out continuously, the method may include continuously circulating the alkaline aqueous solution between the thermochemical reactor and the electrochemical cell. Therefore, the method may include transferring heat from a portion of the alkaline aqueous solution located upstream of the electrochemical cell and downstream of the thermochemical reactor to another portion of the alkaline aqueous solution located upstream of the thermochemical reactor and downstream of the electrochemical cell.

[0075] Electrochemical reactions can be carried out at the same or different pressures as thermochemical reactions. In some embodiments, the electrochemical reaction is carried out at the same pressure as the thermochemical reaction. In this embodiment, the electrochemical cell and the thermochemical reactor can be maintained at the same pressure, which can be an elevated pressure. Any conduits extending between the electrochemical cell and the thermochemical reactor can also be maintained at this same pressure. This pressure can be as defined above. In some embodiments, the pressure can be between 100 kPa and 7 MPa, between 1 and 6 MPa, between 2 and 4 MPa, or between 3 and 4 MPa.

[0076] In an alternative embodiment, the thermochemical reaction is carried out at pressures 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.

[0077] In an alternative embodiment, the electrochemical reaction is carried out at a lower pressure than the thermochemical reaction. In this embodiment, the thermochemical reaction can be carried out at a first pressure, and the electrochemical reaction can be carried out at a second pressure. The first pressure can be higher than the second pressure.

[0078] The method may include pressurizing the alkaline aqueous solution to a first pressure before or simultaneously with feeding it into the thermochemical reactor. The method may include pressurizing the alkaline aqueous solution to the first pressure when (a) removing it from the electrochemical cell, (b) transferring it between the electrochemical cell and the thermochemical reactor, and / or (c) feeding it into the thermochemical reactor.

[0079] The method may include depressurizing the alkaline aqueous solution to a second pressure before or simultaneously with feeding the alkaline aqueous solution into the electrochemical cell. The method may include depressurizing the alkaline aqueous solution to a second pressure when (a) removing the alkaline aqueous solution from the thermochemical reactor, (b) transferring the alkaline aqueous solution between the thermochemical reactor and the electrochemical cell, and / or (c) feeding the alkaline aqueous solution into the electrochemical cell.

[0080] The electrochemical reaction can be carried out at a pressure of at least 10 kPa or at least 20 kPa, more preferably at least 40 kPa, at least 60 kPa or at least 80 kPa, and most preferably at least 90 kPa, at least 95 kPa or at least 100 kPa. In one embodiment, the electrochemical reaction is carried out 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.

[0081] In one embodiment, the electrochemical reaction is carried out at a pressure below 5 MPa or below 3 MPa. In another embodiment, the electrochemical reaction is carried out at a pressure below 1 MPa, below 500 kPa, or below 250 kPa, and most preferably below 200 kPa, below 150 kPa, or below 110 kPa.

[0082] In one embodiment, the electrochemical reaction is carried out at a pressure between 20 and 5,000 kPa, more preferably between 40 and 1,000 kPa, 60 and 500 kPa, or 80 and 250 kPa, and most preferably between 90 and 200 kPa, 95 and 150 kPa, or 100 and 110 kPa.

[0083] In an alternative embodiment, the electrochemical reaction is carried out at pressures 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.

[0084] The inventors believe that the apparatus used to implement the methods of the first and second aspects is novel and inventive.

[0085] Therefore, according to a third aspect, an apparatus for generating hydrogen is provided, the apparatus comprising: - A thermochemical reactor configured to contain an alkaline aqueous solution and an active reagent, thereby allowing thermochemical reactions to occur and producing a gaseous stream containing hydrogen and an alkaline aqueous solution containing oxidation products. - An electrochemical cell comprising an anode and a cathode, and configured to receive an alkaline aqueous solution containing oxidation products from the thermochemical reactor, such that at least a portion of the cathode is in contact with the alkaline aqueous solution containing oxidation products, the electrochemical cell being configured to induce an electrochemical reaction and generate hydrogen and the active reagent at the cathode, and generate oxygen at the anode. - A first conduit extending between the thermochemical reactor and the electrochemical cell, the first conduit being configured to feed an alkaline aqueous solution containing oxidation products 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 supply an alkaline aqueous solution and an active reagent from the electrochemical cell to the thermochemical reactor.

[0086] The term "thermochemical reactor" may be used interchangeably with "hydrolyzer", "hydrolysis reactor" or "hydrothermal reactor".

[0087] The thermochemical reactor may include a stirrer configured to stir an alkaline aqueous solution in the thermochemical reactor.

[0088] Preferably, the device includes a temperature controller configured to control the temperature in the thermochemical reactor. The temperature controller may include a heater configured to heat the alkaline aqueous solution in the thermochemical reactor to an elevated temperature. Alternatively, the temperature controller may include a heat exchanger disposed adjacent to the thermochemical reactor, wherein the heat exchanger is configured to allow a first fluid to flow through it, thereby enabling heat exchange 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 the first and second aspects.

[0089] The apparatus may include another heat exchanger configured to cool the first fluid. The other heat exchanger may be configured to transfer heat from the first fluid to another fluid. The other fluid may be an alkaline aqueous solution or water. Therefore, the other heat exchanger may be configured to preheat the alkaline aqueous solution or water before feeding it into the thermochemical reactor.

[0090] The apparatus may include a condenser configured to cool the gas stream generated in the thermochemical reactor, thereby condensing water in the gas stream. Advantageously, this makes it possible to obtain concentrated hydrogen.

[0091] The device can be configured to return condensed water to the thermochemical reactor.

[0092] The condenser may be configured to transfer heat from the gaseous feed stream to a cooling fluid. The cooling fluid may be an alkaline aqueous solution or water. Preferably, the condenser is configured to preheat the alkaline aqueous solution or water before feeding it into the thermochemical reactor.

[0093] The device may include a pressure controller configured to control the pressure in the thermochemical reactor. The pressure controller may include 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 the pressure defined above in conjunction with the first aspect.

[0094] The pressure controller may be configured to control the pressure in the electrochemical reactor. The pressure controller may include 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 the pressure defined above in conjunction with the first aspect. Therefore, the second desired pressure may be the same as or different from the first desired pressure.

[0095] The pressure controller may include a compressor configured to compress the alkaline aqueous solution to a first pressure. The compressor may be configured to compress the alkaline aqueous solution before feeding it into the thermochemical reactor. The compressor may be configured to compress the alkaline aqueous solution to the first pressure when (a) removing it from the electrochemical cell, (b) transferring it between the electrochemical cell and the thermochemical reactor, and / or (c) feeding it into the thermochemical reactor.

[0096] The pressure controller may include a pressure reducer configured to reduce the pressure of the alkaline aqueous solution to a second pressure. The pressure reducer may be configured to reduce the pressure of the alkaline aqueous solution before feeding it into the electrochemical cell. The pressure reducer may be configured to reduce the pressure to the second pressure when (a) removing the alkaline aqueous solution from the thermochemical reactor, (b) transferring the alkaline aqueous solution between the thermochemical reactor and the electrochemical cell, and / or (c) feeding the alkaline aqueous solution into the electrochemical cell.

[0097] The apparatus may include a pump configured to flow an alkaline aqueous solution containing oxidation products from a thermochemical reactor to an electrochemical cell via a first conduit. The apparatus may also include a pump configured to flow an alkaline aqueous solution and an active reagent from the electrochemical cell to the thermochemical reactor via a second conduit. One pump may be used, or the apparatus may include two or more independent pumps. The one or more pumps may be peristaltic pumps and / or slurry pumps.

[0098] The apparatus may include a hydrogen collection container configured to receive hydrogen generated in a thermochemical reactor. The apparatus may include a conduit extending between a condenser and the hydrogen collection container.

[0099] The electrochemical cell may be an unseparated cell.

[0100] Alternatively, the electrochemical cell may include a separator cell. Therefore, the electrochemical cell may include a membrane disposed between the anode and cathode, which separates the cell into two parts. Preferably, the membrane is as defined in the first and second aspects.

[0101] 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.

[0102] The anode portion of the battery may be configured to receive another electrolyte, such that at least a portion of the anode contacts the electrolyte. The other electrolyte may be as defined in the first and second aspects.

[0103] The device may include a pH meter configured to monitor the pH value of a solution containing metal ions. The pH meter may be disposed in an electrochemical cell, a thermochemical reactor, a first conduit, and / or a second conduit.

[0104] The device may include one or more inlets configured to feed water into an electrochemical cell, a thermochemical reactor, a first conduit, and / or a second conduit. In some embodiments, the device may include another conduit extending between a condenser and a heated water inlet, configured to deliver 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 device may be configured to feed water into a storage tank or electrochemical cell when a pH meter senses a pH value higher than a predetermined maximum pH value.

[0105] The anode and cathode may be as defined in the first and second aspects.

[0106] The device may include a remover configured to remove an active reagent from an electrochemical cell. The remover may be configured to remove the active reagent from the cathode. The remover may include a scraper configured to remove the active reagent from the cathode. The scraper may be configured to move across the cathode to remove the active reagent from the cathode surface. The device may be configured to use magnetic force or an electric motor to move the scraper.

[0107] The device may include a power source configured to apply a voltage between the anode and cathode. The power source may include a battery, a generator, a renewable energy source, or may include the national power grid. Preferably, the power source is a renewable energy source. The renewable energy source may include a solar generator, a wind generator, or a hydroelectric generator. The power source may be configured to supply direct current (DC) to the anode and cathode.

[0108] The power supply can be configured to apply a voltage of at least 1 V or at least 2 V between the anode and cathode, more preferably at least 2.5 V, at least 3 V, or at least 3.5 V between the anode and cathode, and most preferably at least 4 V or at least 4.5 V between the anode and cathode. The power supply can be configured to apply a voltage lower than 8 V or lower than 7.5 V between the anode and cathode, more preferably lower than 7 V, lower than 6.5 V, or lower than 6 V between the anode and cathode, and most preferably lower than 5.5 V or lower than 5 V between the anode and cathode. The power supply can be configured to apply a voltage between 1 and 8 V or between 2 and 7.5 V between the anode and cathode, more preferably between 2.5 and 7 V, between 3 and 6.5 V, or between 3.5 and 6 V between the anode and cathode, and most preferably between 4 and 5.5 V or between 4.5 and 5 V between the anode and cathode.

[0109] The power supply can be configured to allow 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 solution containing metal ions; more preferably, to allow a current of at least 2 A, at least 2.5 A, or at least 3 A to flow through the anode, cathode, and solution containing metal ions; and most preferably, to allow a current of at least 3.5 A to flow through the anode, cathode, and solution containing metal ions. The power supply can also be configured to allow a current of less than 10 A, less than 8 A, or less than 6 A to flow through the anode, cathode, and solution containing metal ions; more preferably, to allow 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 solution containing metal ions; and most preferably, to allow a current of less than 4 A to flow through the anode, cathode, and solution containing metal ions. The power supply can be configured to allow a current between 0.5 and 10 A, 1 and 8 A, or 1.5 and 6 A to flow through the anode, cathode, and solution containing metal ions; more preferably, a current between 2 and 5.5 A, 2.5 and 5 A, or 3 and 4.5 A to flow through the anode, cathode, and solution containing metal ions; and most preferably, a current between 3.5 and 4 A to flow through the anode, cathode, and solution containing metal ions.

[0110] The device may include a hydrogen collection container configured to receive hydrogen generated in the electrochemical cell. The device may include a conduit extending between the electrochemical cell and the hydrogen collection container. In embodiments where the cell is a separate cell, the conduit may extend between the cathode portion of the electrochemical cell and the hydrogen collection container.

[0111] The hydrogen collection container can be configured to receive hydrogen generated in a thermochemical reactor, or it can be configured to receive hydrogen generated in an electrochemical cell. Alternatively, the device may include two separate hydrogen collection containers.

[0112] The device may include an oxygen collection container configured to receive oxygen generated in the electrochemical cell. The device may include a conduit extending between the electrochemical cell and the oxygen collection container. In embodiments where the cell is a separate cell, the conduit may extend between the anode portion of the electrochemical cell and the oxygen collection container.

[0113] The device may include a heat exchanger, and the first conduit and the second conduit may extend through the heat exchanger, wherein the heat exchanger is configured to transfer heat between the first conduit and the second conduit. The heat exchanger is preferably configured to transfer heat from the first conduit to the second conduit.

[0114] All features described herein (including any accompanying claims, abstract, and drawings), and / or all steps of any method or process disclosed, may be combined with any of the foregoing aspects in any combination, except for at least some such features and / or steps in mutually exclusive combinations.

[0115] To better understand the present invention and to illustrate how to implement its embodiments, reference will now be made to the accompanying drawings by way of example, wherein: - Figure 1 This is a schematic diagram showing the hydrogen production cycle, including a thermal reactor and an electrolyzer; Figure 2 It is a laboratory-scale hydrolysis reactor device; Figure 3 (a) shows the hydrogen gas produced by the hydrolysis of zinc powder in an alkaline solution of KOH (8M) at 130°C; and (b) shows the kinetics of zinc hydrolysis at 130°C. Figure 4 (a) shows the hydrogen produced by zinc hydrolysis in an alkaline solution in the presence of Fe2O3 and a Fe2O3 / zinc molar ratio of 0.06 to 0.5; and (b) shows the kinetics of zinc hydrolysis in the presence of Fe2O3, where the Fe2O3 / zinc molar ratio is 0.1. Figure 5 (a) shows the hydrogen production under different Fe2O3 loadings; and (b) shows the kinetics of zinc hydrolysis under different Fe2O3 loadings; Figure 6 (a) shows the hydrogen production at different temperatures; and (b) shows the kinetics of zinc hydrolysis at different temperatures; Figure 7 (a) shows the hydrogen production at different KOH concentrations; and (b) shows the kinetics of zinc hydrolysis at different KOH concentrations; Figure 8 It is a graph showing the conductivity and resistance of an electrolyte as a function of concentration; Figure 9 This is a graph showing the cumulative hydrogen production over time at different electrolyte concentrations under a voltage of 2V. Figure 10 Based on Figure 4, the changes of current and hydrogen production rate over time are shown in detail under the conditions of 1.2M and 2V. Figure 11 This is a graph showing the cumulative hydrogen production over time at different voltages at a concentration of 1.2 M; and Figure 12 shows the zinc conversion in the thermochemical reaction, using (a) the Gibbs minimization method, (b) the conversion at various activation energies based on the experimental reaction rate, where the reaction pressure is kept above the corresponding saturation temperature of water to prevent evaporation through the reactor cooling jacket, and (c) the effect of increasing reactor pressure on the conversion at E_a = 35 kJ / mol.

[0116] Example WO 2020 / 016580 A2 (the contents of which are incorporated herein by reference) describes an efficient and low-cost hybrid thermochemical cycle for the production of hydrogen.

[0117] This invention is derived from improvements made to the invention disclosed in WO 2020 / 016580 A2. A schematic diagram of an apparatus configured to perform the method of the invention is shown. In particular, in this invention, the thermal reactor described in WO 2020 / 016580 A2 is replaced by a hydrolyzer that facilitates the reaction of water with an active reagent in the presence of an alkaline solution to produce hydrogen. The active reagent may be a transition metal or a p-block metal (zinc, iron, tin, etc.) or a reactivated spinel / perovskite (zinc ferrite, zinc ferrous oxide, zinc manganate, zinc chromate, or zinc lead oxide) complex having multiple oxidation states (e.g., zinc stannate, zinc ferrite, zinc ferrous oxide, zinc manganate, zinc chromate, zinc lead oxide, or sodium stannate).

[0118] In the method of this invention, the active reagent reacts with liquid water in a hydrolyzer, thereby being oxidized. For example, in an embodiment where the active reagent is zinc, the oxidation reaction can be written as: It should be noted that the above reaction is similar to the oxidation reaction described in WO 2020 / 016580 A2, the difference being that the water used in the reaction in this application is in liquid form, while in WO 2020 / 016580 A2, the water is provided in the form of steam.

[0119] In WO 2020 / 016580 A2, the oxidation products of the oxidation reaction form a passivation layer on the surface of the active reagent. For example, in an instance where the active reagent is zinc, the passivation layer will be formed of zinc oxide. In WO 2020 / 016580 A2, this will prevent the active reagent from reacting further. In WO 2020 / 016580 A2, the conversion rate of zinc to zinc oxide is only about 30-50%.

[0120] However, in the method of this invention, the alkaline solution helps renew the surface of the active reagent by dissolving the oxidation products. In an embodiment where the active reagent is zinc and the base is potassium hydroxide, the reaction that occurs can be written as: The overall reaction in this embodiment can be written as: By dissolving the oxidation products, the surface of the active reagent is freely exposed to water for further reaction until the active reagent has completely reacted. Therefore, the alkaline solution allows for the efficient conversion of water in the hydrolyzer (for hydrogen production) by inhibiting catalytic "surface deactivation" (a common problem in the oxidation step of a chemical looping process).

[0121] Understandably, the above reaction will produce a gaseous stream containing hydrogen. This gaseous stream will also contain vapor. Therefore, the gaseous stream can be passed through a heat exchanger configured to cool the gaseous stream, thereby condensing water. This will provide a gas with a high concentration of hydrogen.

[0122] Furthermore, it should be noted that the thermodynamically favorable reaction between water and the active reagent generates a significant amount of residual heat. This heat will partially remain in the product gas stream. Therefore, this heat exchanger can be used to preheat water subsequently fed into the hydrolyzer or into the hydrolyzer jacket.

[0123] The water fed into the hydrolyzer jacket will also generate steam, such as Figure 1 The steam discharged from the hydrolyzer jacket is shown on the right side of the hydrolyzer. It is understood that the heat from this steam can also be captured using a heat exchanger.

[0124] The waste heat recovered from the hydrolysis reactor can significantly improve the overall energy efficiency of the loop. Specifically, this cycle consumes 48.5 kWh of electricity (39.4 kWh / kg) to produce 1 kg of hydrogen. Dividing these two figures, the efficiency is 39.4 / 48.5 = 81.2%. When heat recovery with steam is performed, 4.6 kWh of steam is generated for every 1 kg of hydrogen produced. The efficiency then becomes (39.4 + 4.6) / 48.5 = 90.7%.

[0125] It is understood that potassium hydroxozincate (K₂Zn(OH)₄) is completely soluble in alkaline solutions. Therefore, this solution can be directly pumped into an electrolyzer for electrochemical reduction reactions. These electrochemical reactions can be described as in WO 2020 / 016580 A2.

[0126] In particular, it is understandable that potassium hydroxyzincate may exist in an equilibrium as shown below: It should be noted that the above series of reactions can also be applied to other alkaline solutions (NaOH, LiOH, etc.), ammonia, and other organic bases.

[0127] Two electrochemical reduction reactions occur simultaneously at the cathode surface: the electrochemical reduction of Zn(OH)₂ to recover zinc, and the typical water electrolysis in an alkaline environment to produce hydrogen (the so-called HER: hydrogen evolution reaction), as shown below: The total half-reaction (reduction) at the cathode will be the sum of the above reactions, as shown below: For the anodic half-reaction, similar to other typical alkaline electrolyzers, the oxygen reduction reaction (ORR) will occur via a direct four-electron pathway, as shown below: The sum of the above half-reactions (anode and cathode) constitutes the redox reaction of the entire electrolyzer: Therefore, the dissolved metal oxides produced in the hydrolyzer are reduced back to their initial state through the cathode when the electrolyzer is used.

[0128] The inventors' continuous electrochemical cell experiments demonstrated that the pressure (force) of the flowing solution can easily remove zinc particles from the cathode surface. Therefore, the solution containing suspended zinc particles can be returned to the hydrolyzer. Thus, the process can proceed continuously, producing hydrogen gas. It is important to note that the only required inputs are water and energy.

[0129] Example 1 – Hydrolysis of Zinc in Sodium Hydroxide Solution The inventor built Figure 2 The laboratory-scale hydrolysis reactor is shown. In short, a round-bottom flask 2 is placed on a heating plate 1. A thermocouple 3 is provided, configured to measure the temperature inside the flask 2.

[0130] The apparatus also includes a condenser 4 positioned at the top of the round-bottom flask, configured to cool any gases produced in the round-bottom flask 2. A cooler 5 is provided to ensure a constant flow of cooling fluid through the condenser 4. A rotameter 6 is positioned downstream of the condenser to measure the flow rate of gases produced in the reaction, and a 1-liter capacity syringe 7 is positioned downstream of the rotameter to collect the produced gases.

[0131] The round-bottom flask is equipped 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 is also equipped with an inlet and an outlet tube configured to feed liquid into and remove liquid from the round-bottom flask.

[0132] A carbonate-free, clear solution of 180 g NaOH dissolved in 250 ml H2O was prepared and added to a round-bottom flask. Before starting the reaction, the reactor was purged with pure nitrogen gas (at a rate of 100 ml / min for 30 minutes).

[0133] 1.37 g of Zn was suspended separately in 50 ml of water and added to the reactor through the inlet tube. The inlet valve was then closed, and the mixture was heated with a heating plate while being vigorously stirred with a magnetic stirrer. All hydrogen products were collected in a syringe. The reaction system was refluxed at 135°C and atmospheric pressure for 4 hours to complete the reaction, producing 504 ml of hydrogen gas, equivalent to a 96.8% hydrolysis conversion of the zinc powder initially added to the equipment. Stoichiometric calculations indicate that the reaction produced 3.68 g of Na₂Zn(OH)₄.

[0134] Example 2 – Hydrolysis of Zinc in Potassium Hydroxide Solution The above embodiment 1 is described and shown in Figure 2 The hydrolysis reactor is also used in this embodiment.

[0135] A carbonate-free clear solution of 337 g KOH dissolved in 350 ml H2O was prepared and added to the hydrolysis reactor. Before starting the reaction, the reactor was purged with pure nitrogen gas (at a rate of 100 ml / min for 30 minutes).

[0136] 1.93 g of Zn was suspended separately in 50 ml of water and added to the reactor. The reaction mixture was refluxed with stirring for 4 hours (150°C, atmospheric pressure) to complete the reaction. At the end of the reaction, 715 ml of hydrogen gas was collected, which corresponds to a hydrolysis conversion of 97.5% of the zinc powder initially added to the reactor. Stoichiometric calculations indicate that the reaction produced 6.08 g of K₂Zn(OH)₄.

[0137] The reaction was repeated for 30 minutes at 130°C using 8M KOH, and the results are shown in Figure 3. Figure 3b In this process, the Avrami-Eroveef model is used to fit the results to match the data: Where α is the fraction of reacted particles, t is the time to reach that fraction, k is the specific rate constant, and m is a parameter that depends on the geometry.

[0138] Example 3 – Improving the efficiency of hydrolysis reaction The inventors wanted to increase the rate of the hydrolysis reaction at lower temperatures. Therefore, they investigated the use of hematite (Fe2O3) as a catalyst.

[0139] Unwilling to be bound by theory, the inventors believe that Fe2O3 can catalyze reactions in the manner discussed below.

[0140] First, the inventors noted that in the presence of water and KOH solution, Fe2O3 can form iron(III) oxide-hydroxide or ferric oxyhydroxide, subsequently forming ferric(III) hydroxide, as shown below: Then reduction of iron(III) occurs, as follows: Finally, iron oxidizes and produces hydrogen gas: The inventors investigated how various factors affect catalytic reactions, as described below.

[0141] Fe 2 O 3 Effect of zinc molar ratio A series of experiments were conducted in which the amount of Fe₂O₃ was kept constant (0.0005 mol), while the amount of zinc varied from 0.001 mol to 0.0125 mol. Therefore, the molar ratio of zinc to Fe₂O₃ varied from 1:0.5 to 1:0.06. In these experiments, the concentration of potassium hydroxide was 8 M, and the solution volume was 15 ml. The results, shown in Figure 4, indicate that when the molar ratio of Fe₂O₃ / Zn decreased from 0.5 to 0.1, two moles of zinc produced one mole of hydrogen gas. Further reducing this ratio to 0.06 resulted in a decrease in the amount of hydrogen gas produced. Therefore, the optimal molar ratio of Fe₂O₃ / Zn appears to be 0.5.

[0142] Fe2 O 3 Effect of concentration Subsequently, the inventors conducted a series of experiments, in which the amount of Fe₂O₃ used varied from 0.0625 mmol to 0.5 mmol. It is understandable that Fe₂O₃ is relatively insoluble in aqueous solution. Therefore, Fe₂O₃ was suspended in KOH solution in powder form. The given "concentration" should be understood as confirming the amount of Fe₂O₃ in the solution, rather than its actual concentration.

[0143] In these experiments, the amount of zinc present was 0.1 mol, and the concentration of potassium hydroxide was 8 M. The solution volume was 15 ml. As shown in Figure 5, increasing the amount of Fe₂O₃ from 0.0625 mmol to 0.5 mmol resulted in an increase in the volume of hydrogen produced within 30 minutes. This indicates that increasing the amount of catalyst leads to greater contact between the reactants and the catalyst. However, when the Fe₂O₃ concentration increased from 0.5 mmol to 1.25 mmol, the hydrogen yield decreased from 0.985 to 0.529. This is likely due to the observed increase in solution pH caused by the increased concentration. The increase in pH may be due to the conversion of iron ions to Fe(OH)₃.

[0144] The effect of temperature The inventors then investigated how temperature affects the reaction rate. They conducted a series of experiments with temperatures ranging from 4°C to 36°C. In these experiments, the amount of zinc present was 0.1 mol, the Fe₂O₃ concentration was 0.5 mmol, and the potassium hydroxide concentration was 8 M. The solution volume was 15 ml. As shown in Figure 6, the increase in temperature significantly affected zinc hydrolysis due to the increased molecular velocity. Therefore, molecular collisions appear to be the most important factor in increasing the hydrogen production rate. However, it should be noted that even at 4°C, a considerable amount of hydrogen gas was produced.

[0145] Effect of KOH concentration Subsequently, the inventors investigated how the KOH concentration affected the reaction rate. They conducted a series of experiments in which the KOH concentration varied from 4 M to 12 M. In these experiments, the amount of zinc present was 0.1 mol, the Fe₂O₃ concentration was 0.5 mmol, and the solution volume was 15 ml.

[0146] As shown in Figure 7, the amount of hydrogen produced increases with increasing KOH concentration from 4M to 12M. However, compared to the 10M solution, only a slight increase was observed in the 12M solution. This is believed to be due to the increase in solution viscosity.

[0147] Experimental design analysis using full factorial general analysis (Minitab) The effects of individual factors and potential interactions among the three factors were investigated using Minitab® software. A three-level full factorial experimental design (DoE) was performed to determine the relative contributions of the 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), thereby identifying the most significant parameters affecting hydrogen production. The statistical significance of each factor was assessed using p-values ​​with 95% confidence intervals. The results are presented in Table 3.

[0148] Temperature had the lowest p-value (0.000), indicating it had the greatest impact on hydrogen production. Furthermore, the p-value as low as 0.000 indicated high significance of the regression model. KOH concentration had the highest p-value (0.005), indicating it had the least impact on hydrogen production.

[0149] In summary, the addition of a catalyst is highly advantageous because it allows the hydrolysis reaction to proceed at low temperatures.

[0150] The catalyst can also be easily separated from the electrolyte. In particular, iron and its oxides (including hematite (Fe2O3)) have very good magnetic properties. The inventors noted that magnets can very effectively and easily separate all catalyst particles. In the above experiments, the inventors used a magnetic stirrer, and the catalyst was only dispersed when a vigorous rotation speed above 150 rpm was used. At lower speeds, the iron catalyst particles adhered to the magnet.

[0151] Example 4 – Electrolysis system using sodium stannate trihydrate The inventors hope to demonstrate that the proposed hydrogen-generating system can be used for other active reagents besides zinc. Therefore, the inventors attempted to demonstrate this by using sodium stannate trihydrate as an alternative active agent.

[0152] Materials and Methods equipment Batch experiments were conducted using an electrolytic cell. This cell consisted of a rectangular, open-topped plexiglass container with two cylindrical graphite electrodes connected to the bottom, part of which was inside the container and the remainder outside. The container was 25 cm high, 20 cm long, and 12 cm wide. The electrodes were spaced 6 cm apart and positioned on either side of the center of the container. The portion of the electrode held inside the container was 3 cm long and 1 cm in diameter. These values ​​were constant as the electrodes used remained unchanged. Wires connected the electrodes to a power source. Before each experiment, the voltage (and current) of the power source were set. A 500 ml cylindrical container with a valve was placed above each electrode to collect any gases produced during the electrolysis reaction. The cylindrical containers were connected to a vacuum pump using tubing. Before the electrolysis reaction, the valves on the cylindrical containers were opened, and the vacuum pump was started to create a partial vacuum, removing air from the cylindrical containers and filling them with an electrolyte solution (i.e., sodium stannate trihydrate solution). During the electrolysis reaction, the valves were closed, so any gases present in the cylindrical containers would be those produced during the reaction.

[0153] program electrical conductivity An electrolyte (sodium stannate trihydrate solution) was prepared by dissolving 55.4 g of sodium stannate (42-45%, calculated as SnO2) in 200 ml of distilled water. The resulting solution had a concentration of 1.3 M. This concentration was determined based on the solubility limit of sodium stannate in water. The reaction that occurred was exothermic. The conductivity of the electrolyte was measured using a conductivity meter. The solution was then diluted to the desired concentration by adding distilled water. These concentrations ranged from 1.3–0.1 M, decreasing by 0.1 M at each interval. The conductivity was measured at each concentration. Using the conductivity, the resistance was calculated using the following equation: Where R is resistance (Ω), σ is conductivity (mS / cm), L is the distance between electrodes (cm), and S is the electrode surface area (cm²). 2 ), fixed at 11 cm 2 .

[0154] Hydrogen production The first step is to prepare the electrolyte to the desired concentration. Mix the electrolyte thoroughly to maintain a constant concentration gradient. Then carefully pour it into a container, evacuate the air from the cylindrical container (as described above), and close the valve.

[0155] Turn on the power. The current was kept constant at 1 A for all experiments. Two different experimental procedures were performed. The first measured the hydrogen production rate over time at three different concentrations near the optimum concentration. This was determined by conductivity measurements. The second measured the hydrogen production rate over time at three different voltages: 2, 2.5, and 3 V. Each experiment lasted three hours, with the hydrogen production rate measured every five minutes.

[0156] The desired electrolysis is that of sodium stannate trihydrate. However, it can be in equilibrium with tin(IV) hydroxide and NaOH, as shown below: This process is carried out at room temperature, which is advantageous because it tends to cause sodium stannate trihydrate to dissociate at equilibrium. Although the dissociation of water is relatively small in this process, it is still possible and results in the production of hydrogen gas at the cathode (Equation 4). A more significant reaction occurs simultaneously: tin hydroxide (IV) is reduced to tin and deposited on the cathode surface (Equation 5). These reactions are shown below: The reaction at the anode involves the production of oxygen: Adding equations 4, 5, and 6 together, they simplify to: Now, the tin deposited on the cathode can be oxidized in the presence of water, producing tin hydroxide (IV) and hydrogen gas: According to Equation 3, the generated tin hydroxide (IV) can be used to regenerate sodium stannate trihydrate. Adding Equations 8 and 9, the overall equation representing this process can be simplified to: Replace the electrode As the reaction proceeds, a layer of tin is deposited, and the thickness of this tin layer increases over time. This also increases the impedance of the electrode surface. Therefore, at a certain point, a decrease in current is observed, at which point the electrode must be replaced. Three indicators can be used to determine when to replace the electrode: a decrease in battery current, a decrease in the hydrogen production rate, and an increase in the oxygen production rate. Current was chosen as the indicator because it can be visually observed through a decrease in the power supply reading. If a decrease in the hydrogen production rate were chosen, it would not be immediately observable.

[0157] The electrodes are swapped by switching the wires on the power supply. Thus, the anode becomes the cathode, and the cathode becomes the anode. Hydrogen is then produced on the other electrode, where no tin layer is deposited. Therefore, this is a process that allows for continuous hydrogen production.

[0158] Understandably, the inventors switched electrodes in these experiments. However, in their continuous process, they could overcome this problem by removing the deposited tin from the cathode. The tin can be removed mechanically (e.g., by scraping) or due to the flow of electrolyte in the electrolytic cell. The tin can be removed from the cathode continuously or periodically.

[0159] Battery efficiency Battery efficiency is calculated based on a combination of experimental measurements and the parameters used in the experiment. The equation used to determine battery efficiency is: Where n is the battery efficiency based on the hydrogen production rate [m] 3 m -3 h -1 (kWh) -1 ], q is the hydrogen production rate per unit volume of electrolyte in the electrolytic cell (m 3 m -3 h -1 U is the battery voltage (V), I is the battery current (A), and t is the time (hours).

[0160] Item (kWh) -1 It is a combination of the terms in the denominator of the equation. It represents the electrical energy consumed in electrolysis.

[0161] During the experiment, the voltage varied (2, 2.5, and 3 V), while the electrolyte concentration, current, and specified time were kept constant.

[0162] Results and Discussion Concentration and conductivity The conductivity of the electrolyte was measured at different concentrations. Factors affecting the solution conductivity are the properties of the electrolyte, the electrodes, and the concentration of the solution. The electrolyte and its preparation conditions (room temperature) were constant. Therefore, conductivity is a function of concentration. When considering resistance, the distance between the electrodes must also be taken into account. A 6 cm gap is required to allow two cylindrical containers to be mounted side-by-side in an acrylic container. A shorter distance between the electrodes can reduce resistance.

[0163] Figure 8The observable trend is that decreasing electrolyte concentration leads to decreasing conductivity. As the solution is diluted (concentration decreases), the number of ions per unit volume decreases. Therefore, there are fewer charged ions (per unit volume), which leads to lower conductivity. Based on this fact, it was expected that the electrolyte at the maximum concentration of 1.3 M used in the experiment would have the highest conductivity. However, this was not the case. The optimal conductivity occurred at a concentration of 1.2 M. This unexpected result can be explained by the electrolyte reaching its solubility limit. The electrolyte solution allows the maximum amount of sodium stannate particles to exist between water molecules. Therefore, this leads to reduced ion mobility, resulting in lower solution conductivity.

[0164] Resistance is determined by equation (2). As mentioned earlier, the distance between the electrodes and the electrode surface area are constant. Therefore, based on equation (2), resistance is a function of conductivity. Figure 8 The relationship between resistance and conductivity is reciprocal.

[0165] Concentration and hydrogen production Figure 9 The three concentrations considered were selected as the optimal concentrations based on their conductivity. Hydrogen production was then measured.

[0166] The highest cumulative hydrogen production was observed at a concentration of 1.2 M, which confirms... Figure 8 The results shown indicate that the optimal concentration based on conductivity results in the production of the most hydrogen over time. This can be explained using a principle similar to that mentioned earlier. Ions need to flow freely. The freer the ions flow, the higher their mobility. Therefore, more ions will flow towards the electrode, thus increasing the hydrogen production rate.

[0167] Figure 10 This is a graphical representation of the electrode replacement process. For clarity, measurement points were added at 21 minutes (when the first current drop was observed) and 24 minutes (when electrode replacement was necessary due to the current falling below the threshold). Once the current began to decrease, the hydrogen production rate dropped rapidly. However, both the current and current began to decrease simultaneously, indicating that they occurred concurrently. The current drop is likely due to a reduction in the amount of tin deposited on the electrode, leading to a decrease in the hydrogen production rate.

[0168] Voltage and hydrogen production Figure 11 The three voltages considered were selected based on the minimum battery voltage (i.e., 2V) used in the electrolysis of water to produce hydrogen. Hydrogen production was then measured.

[0169] The highest cumulative hydrogen production occurs at the highest voltage, i.e., 3V. This is expected because higher voltage accelerates the electrolysis process, causing more ions to flow to the electrodes. Therefore, more hydrogen accumulates over time. Figure 9 In comparison, it can be seen that voltage has a more significant effect on hydrogen production than concentration. The difference in hydrogen production rate between 2V and 2.5V is approximately 150 cm⁻¹. 3 Based on production scale, this represents a significant change in cumulative hydrogen production relative to the corresponding change in voltage.

[0170] As shown in Table 4, the number of electrode replacements decreases with increasing voltage. This is partly why the cumulative hydrogen production value is greater at higher voltages. Fewer electrode replacements mean fewer drops in the hydrogen production rate. Ultimately, this leads to the accumulation of more hydrogen over time.

[0171] Battery efficiency For the reasons stated above, a 1.2M electrolyte solution was used to calculate the battery efficiency. The battery should have the highest efficiency at this concentration. The battery efficiency was calculated using equation (10), and the results are provided in Table 5.

[0172] According to the literature, the typical cell efficiency of a unipolar water electrolyzer is approximately 2.3 m. 3 m -3 h -1 (kWh) -1 (See Santos D, C, Sequeira F, J. Hydrogen production by alkaline waterelectrolysis; 2019). It should be noted that this value is given at a battery voltage of 2.2V.

[0173] Therefore, the battery of the present invention is more efficient than a typical water electrolyzer at both 2V and 2.5V voltages. Based on this, it is recommended to use a lower voltage, as the battery efficiency is highest at this voltage.

[0174] Example 5 – Effect of Temperature on Thermochemical Reactions The inventors calculated the conversion rate of zinc to zinc oxide under different temperatures and pressures, and the results are shown in Figure 12. Figure 12a As shown, a lower temperature is desired to achieve a high conversion rate. However, as... Figure 12b As shown, the reaction rate is lower at low temperatures, resulting in a lower yield. However, as... Figure 12c As shown, a high conversion rate can be achieved by using higher pressure at higher temperatures to keep the medium in a liquid state.

[0175] It should be noted that the conversion rate calculation is based on the absence of a catalyst. Understandably, if a catalyst is present, lower temperatures and / or pressures can be used.

Claims

1. A method of producing hydrogen gas, the method comprising: - performing a thermochemical reaction by contacting an active agent with an aqueous alkaline solution, thereby reacting water in the aqueous alkaline solution with the active agent and producing hydrogen gas and an aqueous alkaline solution comprising an oxidation product; - placing the aqueous alkaline solution comprising the oxidation product in an electrochemical cell comprising an anode and a cathode, such that at least a portion of the cathode contacts the solution; and - performing an electrochemical reaction by applying a voltage between the anode and the cathode to produce hydrogen gas, oxygen gas, and the active agent; wherein the active agent comprises a metal or metal ion in a first oxidation state, and the oxidation product comprises a metal or metal ion in a second oxidation state higher than the first oxidation state.

2. A method of producing hydrogen gas, the method comprising: - placing an aqueous alkaline solution comprising an oxidation product in an electrochemical cell comprising an anode and a cathode, such that at least a portion of the cathode contacts the solution; - performing an electrochemical reaction by applying a voltage between the anode and the cathode to produce hydrogen gas, oxygen gas, and an active agent; and - performing a thermochemical reaction by contacting the active agent with an aqueous alkaline solution, thereby reacting water in the aqueous alkaline solution with the active agent and producing hydrogen gas and an aqueous alkaline solution comprising an oxidation product, wherein the active agent comprises a metal or metal ion in a first oxidation state, and the oxidation product comprises a metal or metal ion in a second oxidation state higher than the first oxidation state.

3. The method of claim 1 or 2, wherein, The electrochemical reaction is performed continuously.

4. The method according to any of the preceding claims, wherein, The thermochemical reaction is performed continuously.

5. The method according to any of the preceding claims, wherein, The active agent is a transition metal or alloy thereof, a p-block metal or alloy thereof, or a reactivated spinel / perovskite composite having multiple oxidation states, or comprises a transition metal or alloy thereof, a p-block metal or alloy thereof, or a reactivated spinel / perovskite composite having multiple oxidation states.

6. The method of claim 5, wherein, The active agent is or comprises a transition metal or alloy thereof or a p-block metal or alloy thereof, and the p-block metal or alloy thereof is selected from the group consisting of tin, lead, thallium, selenium, and bismuth, and the transition metal or 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 of claim 6, wherein, The active agent is or comprises a transition metal or alloy thereof, and is zinc.

8. The method of claim 5, wherein, The active agent is or comprises a reactivated spinel / perovskite composite having multiple oxidation states, having the general formula Zn x M y O z wherein 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 of any of the preceding claims, wherein, The method comprises contacting the active agent with the aqueous alkaline solution in the presence of a catalyst.

10. The method of 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 a combination thereof, and is preferably iron(III) oxide (Fe2O3).

11. The method of any of the preceding claims, wherein, The thermochemical reaction is performed 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 of any of the preceding claims, wherein, The thermochemical reaction produces a gaseous stream comprising hydrogen and steam, and the method comprises cooling the gaseous stream produced in the thermochemical reaction to condense water out of the gaseous stream.

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

14. The method of any of the preceding claims, wherein, During the thermochemical reaction, the active agent is suspended in the aqueous alkaline solution.

15. The method of any of the preceding claims, wherein, The aqueous alkaline 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 of claim 13, wherein, The method can comprise adding water to the aqueous alkaline solution to maintain a desired concentration of base in the aqueous alkaline solution and / or a desired pH of the aqueous alkaline solution.

17. An apparatus for producing hydrogen, the apparatus comprising: - a thermochemical reactor configured to contain an aqueous alkaline solution and an active agent therein, thereby allowing a thermochemical reaction to proceed and produce a gaseous stream comprising hydrogen and an aqueous alkaline solution comprising oxidation products; - an electrochemical cell comprising an anode and a cathode, and configured to receive the aqueous alkaline solution comprising oxidation products from the thermochemical reactor such that at least a portion of the cathode contacts the aqueous alkaline solution comprising oxidation products, the electrochemical cell being configured to cause an electrochemical reaction to proceed and produce hydrogen at the cathode and the active agent at the anode, oxygen at the anode; - a first conduit extending between the thermochemical reactor and the electrochemical cell, the first conduit being configured to feed the aqueous alkaline solution comprising oxidation products 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 aqueous alkaline solution and the active agent from the electrochemical cell to the thermochemical reactor. The thermochemical reactor comprises an agitator configured to agitate the aqueous alkaline solution in the thermochemical reactor.

18. The apparatus of claim 17, wherein, The apparatus comprises a condenser configured to cool a gaseous stream produced in the thermochemical reactor, thereby condensing water in the gaseous stream.

19. The apparatus of claim 17 or 18, wherein, The apparatus comprises a heat exchanger, and the first conduit and the second conduit extend through the heat exchanger, wherein the heat exchanger is configured to transfer heat between the first conduit and the second conduit.

20. The apparatus of any one of claims 17-19, wherein, ​

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