Production method for hydrogen

Abstract

A production method for hydrogen according to the present invention includes a step for electrolyzing an electrolytic solution that has been heated to a temperature between a lower limit temperature that is at least 100°C and at least the melting point and an upper limit temperature that is less than the boiling point, the electrolytic solution being composed of sodium hydroxide, potassium hydroxide, and water and satisfying expressions (1)–(3).　(1) 4≤x≤14. (2) 51≤y≤71. (3) 15≤z≤45.

Description

Hydrogen production method

[0001] The present disclosure relates to a method for producing hydrogen.

[0002] Because renewable energy sources such as solar and wind power are intermittent power sources, their large-scale introduction makes it difficult to balance supply and demand. A promising solution is the so-called "hydrogen energy system," in which large amounts of surplus electricity are converted into hydrogen by water electrolysis, stored and transported, and then reconverted into electricity using fuel cells and / or hydrogen turbines when and where needed.

[0003] Alkaline water electrolysis is one type of water electrolysis method. For example, as described in Patent Document 1, conventional alkaline water electrolysis industrially uses an aqueous potassium hydroxide (KOH) solution with a concentration of approximately 30% by weight, and is operated at approximately 80°C. Considering overvoltage, electrolysis at higher temperatures is desirable, but this poses problems such as the need for a pressurized, sealed structure to suppress evaporation of the electrolyte, resulting in large-scale equipment. Patent Document 2 also discloses an organic hydride production apparatus that generates hydrogen by electrolyzing an electrolyte and produces organic hydrides from the generated hydrogen. Patent Document 2 discloses that operation is performed in a temperature range of approximately 100 to 200°C to reduce the overvoltage required for water electrolysis, that the electrolyte is preferably an alkaline aqueous solution such as sodium hydroxide or potassium hydroxide, and that when using an alkaline aqueous solution such as sodium hydroxide or potassium hydroxide as the electrolyte and operating at 100°C or higher, the concentration of sodium hydroxide or potassium hydroxide needs to be increased to 50 to 90% by weight.

[0004] JP 2013-189699 A JP 2009-7647 A

[0005] In the conventional techniques disclosed in Patent Documents 1 and 2, the oxygen overvoltage and hydrogen overvoltage in water electrolysis remain high, and further improvements are desired.

[0006] The present disclosure has been made in light of these circumstances, and one of its objectives is to provide a method for producing hydrogen that can further reduce the total value of oxygen overvoltage and hydrogen overvoltage compared to conventional techniques.

[0007] A first aspect of the present invention is a method for producing hydrogen, comprising a step of electrolyzing an electrolyte solution heated to a temperature between a lower limit temperature that is 100°C or higher and equal to or higher than the melting point and an upper limit temperature that is lower than the boiling point, wherein the electrolyte solution is composed of sodium hydroxide, potassium hydroxide, and water, and satisfies the following formulas (1) to (3): 4≦x≦14 (1) 51≦y≦71 (2) 15≦z≦45 (3) where x is the molar ratio (%) of the sodium hydroxide to the total of the sodium hydroxide, the potassium hydroxide, and the water, y is the molar ratio (%) of the potassium hydroxide to the total of the sodium hydroxide, the potassium hydroxide, and the water, and z is the molar ratio (%) of the water to the total of the sodium hydroxide, the potassium hydroxide, and the water.

[0008] A second aspect of the present invention is the production method according to the first aspect, which satisfies the following formulas (4) to (6): 7≦x≦11 (4) 56≦y≦65 (5) 24≦z≦36 (6)

[0009] A third aspect of the present invention is the production method according to the second aspect, which satisfies the following formulas (7) to (9): 8≦x≦10 (7) 59≦y≦63 (8) 27≦z≦33 (9)

[0010] A fourth aspect of the present invention is the production method according to any one of the first to third aspects, wherein the electrolysis step uses an anode containing: Ni: 25% by mass or more and 95% by mass or less; at least one selected from the group consisting of Cr, Mn, Fe, Co, and Cu: 1% by mass or more in total; and W: 0% by mass or more and less than 8% by mass.

[0011] A fifth aspect of the present invention is the manufacturing method according to the fourth aspect, wherein the anode comprises: Ni: 25% by mass or more and 95% by mass or less; Co: 5% by mass or more; and W: 0% by mass or more and less than 8% by mass.

[0012] Aspect 6 of the present invention is characterized in that the current density in the electrolysis step is 0.5 to 3.0 A / cm ２ 6. The method according to any one of Aspects 1 to 5, wherein

[0013] Aspect 7 of the present invention is 1 × 10 ５ ~2 x 10 ７ The method according to any one of Aspects 1 to 6, wherein the electrolysis step is carried out under a pressure of 1000 Pa.

[0014] According to the embodiments of the present invention, the total value of oxygen overvoltage and hydrogen overvoltage can be further reduced compared to the prior art, and as a result, a method for producing hydrogen with improved energy efficiency in water electrolysis can be provided.

[0015] FIG. 1 is a schematic diagram of an experimental device for verifying the effects of an embodiment of the present invention.

[0016] The present inventors have conducted investigations from various angles in order to realize a hydrogen production method that can reduce the total value of oxygen overvoltage and hydrogen overvoltage compared to conventional techniques in which, for example, water electrolysis is performed using a KOH or other suitable solution at a concentration of 50 to 90 wt % at a temperature of 100°C to 200°C.

[0017] In terms of cost, supplyability, and the like, sodium hydroxide and potassium hydroxide are preferred as electrolytes in addition to water, and the inventors investigated whether these components could be used to reduce overvoltage compared to conventional methods. However, while it is generally conceivable to mix the components in equal molar ratios in a typical mixed system, it was found that even if sodium hydroxide and potassium hydroxide were mixed in a ratio of approximately 1:1 while adjusting the remaining water ratio, the heating temperature in the electrolysis process would be significantly restricted due to a narrow melting point-to-boiling point temperature range (i.e., the temperature range at which the solution exists as a liquid) and a high melting point (e.g., 150°C or higher at atmospheric pressure). Further investigations by the inventors revealed that by mixing NaOH, KOH, and water in a specific ratio (particularly, a molar ratio x of NaOH of 4 to 14, significantly lower than a molar ratio y of KOH of 51 to 71), a composition with a wide melting point-to-boiling point temperature range and a low melting point could be obtained, thereby reducing the constraints on the heating temperature in the electrolysis process. Furthermore, they found that by using the composition as an electrolyte and performing electrolysis at 100° C. or higher (and above the melting point and below the boiling point of the electrolyte), the total value of oxygen overvoltage and hydrogen overvoltage can be further reduced.

[0018] The details of each requirement defined by the embodiment of the present invention are shown below. As will be described later, the embodiment of the present invention is not limited to being carried out at atmospheric pressure, and may be carried out, for example, at a pressure higher than atmospheric pressure. In this case, if the melting point and boiling point of the electrolyte under the pressure at the time of carrying out are known, those values ​​may be used. If the melting point and boiling point under the pressure at the time of carrying out are unknown, the melting point and boiling point at atmospheric pressure may be used.

[0019] A method for producing hydrogen according to an embodiment of the present invention includes a step of electrolyzing an electrolyte solution heated to a temperature between a lower limit temperature that is 100°C or higher and equal to or higher than the melting point and an upper limit temperature that is lower than the boiling point, wherein the electrolyte solution is composed of sodium hydroxide, potassium hydroxide, and water, and satisfies the following formulas (1) to (3): 4≦x≦14 (1) 51≦y≦71 (2) 15≦z≦45 (3) where x is the molar ratio (%) of the sodium hydroxide to the total of the sodium hydroxide, the potassium hydroxide, and the water, y is the molar ratio (%) of the potassium hydroxide to the total of the sodium hydroxide, the potassium hydroxide, and the water, and z is the molar ratio (%) of the water to the total of the sodium hydroxide, the potassium hydroxide, and the water. This allows the total value of oxygen overvoltage and hydrogen overvoltage to be further reduced compared to conventional techniques.

[0020] In an embodiment of the present invention, the electrolyte is a mixture of sodium hydroxide (NaOH), potassium hydroxide (KOH), and water (H ２ The electrolyte may contain unavoidable impurities other than sodium hydroxide, potassium hydroxide, and water. Examples of the unavoidable impurities include hydroxides of alkali metals and alkaline earth metals, and the total amount of the impurities may be about 1% by mass. The electrolyte may be an alkaline aqueous solution, and NaOH and KOH are present in the electrolyte as M ＋ (M is Na and K) and OH － can be dissociated into

[0021] In an embodiment of the present invention, the electrolytic solution satisfies the above formulas (1) to (3). This allows the total value of oxygen overvoltage and hydrogen overvoltage to be further reduced. It also allows the melting point to be low (for example, less than 150°C under atmospheric pressure) while widening the temperature range from the melting point to the boiling point (for example, a temperature range from the melting point to the boiling point of 40°C or more under atmospheric pressure), thereby reducing the constraints on the heating temperature in the electrolysis process.

[0022] The electrolyte preferably satisfies the following formulas (4) to (6): 7≦x≦11 (4) 56≦y≦65 (5) 24≦z≦36 (6) By satisfying the above formulas (4) to (6), the boiling point can be made to exceed 200°C under normal pressure, while the melting point can be made to be 130°C or lower under normal pressure.

[0023] The electrolyte more preferably satisfies the following formulas (7) to (9): 8≦x≦10 (7) 59≦y≦63 (8) 27≦z≦33 (9) By satisfying the above formulas (7) to (9), the boiling point can be made to exceed 200°C under normal pressure, while the melting point can be made to be 100°C or lower under normal pressure.

[0024] In the embodiment of the present invention, the electrolytic solution may have a concentration gradient within the solution, as long as the ratio is adjusted to the above-mentioned range on average.

[0025] In an embodiment of the present invention, the electrolyte may be in a liquid state after heating (i.e., when electrolyzing the electrolyte), for example, a solidified state before heating. Furthermore, the electrolyte only needs to be adjusted to the above ratio after heating (i.e., when electrolyzing the electrolyte), and does not necessarily need to be adjusted to the above ratio before heating. For example, even during electrolysis, the ratio may be continuously or intermittently adjusted by adding NaOH, KOH, and / or water. The electrolyte (or solidified state) before heating may be prepared by a known method, for example, by adding NaOH, KOH, or a hydrate thereof to water as needed to achieve the above ratio, and further stirring and mixing as needed.

[0026] When preparing a heated electrolyte solution, the lower limit of the heating temperature must be 100 ° C. or higher and above the melting point of the electrolyte solution. This allows the electrode reaction to be sufficiently promoted. When the melting point of the electrolyte solution is higher than 100 ° C., setting the heating temperature below 100 ° C. may result in the solid phase remaining undissolved, which may adversely affect, for example, electrolysis. Preferably, the lower limit of the heating temperature is 100 ° C. or higher and the melting point of the electrolyte solution + 20 ° C. or higher. Since a higher heating temperature can further reduce overvoltage, the lower limit of the heating temperature is preferably 120 ° C. or higher, 140 ° C. or higher, 160 ° C. or higher, 180 ° C. or higher, or 200 ° C. or higher and above the melting point of the electrolyte solution, and more preferably 120 ° C. or higher, 140 ° C. or higher, 160 ° C. or higher, 180 ° C. or higher, or 200 ° C. or higher and above the melting point of the electrolyte solution + 20 ° C. or higher. This heating temperature can be achieved by utilizing various waste heat sources (for example, from various factories and waste incineration plants), so no particular energy consumption is required for heating.

[0027] The melting point of the electrolyte can be determined by actual measurement. Specifically, the melting point of the electrolyte can be determined by the rise temperature of the endothermic peak of melting when a small amount of the electrolyte (which may be solid at room temperature depending on the MOH concentration) is placed in a platinum sample container and heated from a sufficiently low temperature (solid state) using a differential scanning calorimeter (DSC). The pressure during the melting point measurement can be the pressure during the electrolysis process.

[0028] The upper limit of the heating temperature needs to be below the boiling point. If the heating temperature is set to the boiling point or higher, the water content of the electrolytic solution will evaporate and a solid phase will precipitate, which may prevent the total value of the oxygen overvoltage and the hydrogen overvoltage from being sufficiently reduced or may even make electrolysis impossible. Preferably, the upper limit of the heating temperature is lower than the boiling point of the electrolytic solution minus 20°C.

[0029] The boiling point of the electrolyte can be determined by actual measurement. Specifically, the boiling point of the electrolyte can be determined by the following method. The electrolyte (which may be solid at room temperature depending on the MOH concentration) is placed in a sealed container, and the atmosphere is evacuated by evacuation at a low temperature where the vapor pressure is sufficiently low. Thereafter, the entire container is heated in a sealed state until the internal pressure reaches the pressure during the electrolysis process (1×10 at normal pressure). ５The temperature at which the temperature (°C) becomes 0.05 Pa can be regarded as the boiling point of the electrolyte.

[0030] The electrolysis process was carried out at atmospheric pressure (approximately 1×10 ５ The hydrogen may be pressurized in advance in consideration of storing the hydrogen under pressure later. For example, the pressure may be 1×10 ５ ~2 x 10 ７ From the viewpoint of productivity, it is preferable that the upper limit of the pressure is low. Specifically, the upper limit of the pressure is 1×10 ７ Pa or less, 5×10 ６ Pa or less, 1×10 ６ Pa or less, 5×10 ５ It is further preferable that the pressure is 2×10 Pa or less. ５ By carrying out electrolysis at a pressure of less than 1000 kJ / cm², the hydrogen obtained can be handled as not being within the range of "high pressure gas" under the Globally Harmonized System of Classification and Labelling of Chemicals (GHS), and this is more preferable.

[0031] Known conductive materials can be used as the anode material for the electrolysis process. In an embodiment of the present invention, an anode containing 25 to 95 mass% Ni, at least one of Cr, Mn, Fe, Co, and Cu, and at least one selected from the group consisting of Al, Ti, Zn, Nb, Mo, Ta, and W is preferably used. The Ni content is more preferably 50 to 75 mass%. The content of at least one of Cr, Mn, Fe, Co, and Cu (the total content of Cr, Mn, Fe, Co, and Cu) is preferably 1.0 mass% or more, and more preferably 5.0 mass% or more. The content of at least one selected from the group consisting of Al, Ti, Zn, Nb, Mo, Ta, and W (the total content of Al, Ti, Zn, Nb, Mo, Ta, and W) is preferably 0.1 mass% or more, and more preferably 3.0 mass% or more. The use of such a material, among known conductive materials, for the anode can further reduce the oxygen overvoltage. In a preferred embodiment of the present invention, the anode material may contain Ni, at least one of Cr, Mn, Fe, Co, and Cu, and at least one selected from the group consisting of Al, Ti, Zn, Nb, Mo, Ta, and W, in the above-mentioned contents, with the remainder being inevitable impurities. The inevitable impurities refer to elements other than those specified above that are introduced due to the conditions of raw materials, materials, production facilities, etc., and may, for example, be 1.0 mass% or less in total. Known conductive materials, such as platinum and nickel, can be used as the cathode material. Known diaphragm materials for alkaline water electrolysis, such as porous insulating materials, can be used as the diaphragm material. Known materials can also be used appropriately for other components (such as the tank and packing). For example, fluororesins can be used for the tank and packing.

[0032] In another embodiment of the present invention, preferably, Ni: 25 mass % or more and 95 mass % or less and M Ａ (M ＡThe anode contains at least one element selected from the group consisting of Cr, Mn, Fe, Co, and Cu (at least 1 mass % in total) and W (at least 0 mass % but less than 8 mass %). This makes it possible to further reduce the oxygen overvoltage. Although the detailed mechanism is unclear, it is believed that the anode contains M in addition to Ni. Ａ It is believed that this is because the oxides and / or oxyhydroxides thereof have an oxygen generating catalytic function when they contain M. Ａ The total content of M is more preferably 5% by mass or more, even more preferably 10% by mass or more, and even more preferably 15% by mass or more. Ａ There is no particular upper limit to the total amount of Ni and M, but for example, the total amount may be 75 mass % or less. Ａ The total content of Ni and M is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. Ａ The upper limit of the total content of M is not particularly limited and may be 100% by mass. Furthermore, it is considered that the smaller the W content, the better in terms of reducing the oxygen overvoltage, and the W content is more preferably 0% by mass or more and 5% by mass or less, and even more preferably 0% by mass. In another embodiment of the present invention, Ａ is more preferably at least one selected from the group consisting of Cr and Co, and even more preferably Co. Ａ Among these, oxides and / or oxyhydroxides of Cr and Co are considered to have particularly excellent oxygen generation catalytic function, and further, oxides and / or oxyhydroxides of Co are considered to have even more excellent oxygen generation catalytic function. Ｂ (M Ｂ In another embodiment of the present invention, the balance may be unavoidable impurities.

[0033] In the hydrogen production method according to the embodiment of the present invention, the total value of the oxygen overvoltage and the hydrogen overvoltage can be further reduced compared to the conventional method, and therefore the amount of hydrogen generated can be increased by increasing the current density. For example, when the current density is set to 0.5 A / cm ２ It is preferable to perform electrolysis at a current density of 0.7 A / cm or more, as this will result in a significant effect of the present invention. ２ More preferably, the current density is 1.0 A / cm or more. ２ On the other hand, although there is no particular upper limit to the current density, if the current density is increased too much, voltage loss other than overvoltage may increase, so the upper limit is set to 3.0 A / cm ２ It is preferable to set it to 2.0 A / cm or less, and more preferably 2.0 A / cm ２ The following is the result.

[0034] The following examples are provided to more specifically describe the embodiments of the present invention. The embodiments of the present invention are not limited to the following examples, and may be modified as appropriate within the scope of the above-described and below-described aims, and all such modifications are within the technical scope of the embodiments of the present invention.

[0035] In Example 1, the relationship between the mixing ratio of the electrolyte solution used in the embodiment of the present invention and the melting point and boiling point was investigated. ２ The melting point and boiling point of each composition were measured by the above-mentioned method under a pressure of 1.0 × 10 ５ ~1.1 x 10 ５ The boiling point was measured in units of Pa. The results are shown in Table 1. In the "Boiling point" column, "-" indicates that the value was not measured.

[0036]

[0037] ​The following can be seen from Table 1. Compositions Nos. 1 to 4, which are composed of sodium hydroxide, potassium hydroxide, and water and satisfy the above formulas (1) to (3) as requirements for an electrolyte solution in an embodiment of the present invention, were able to achieve a wide temperature range from the melting point to the boiling point (for example, a temperature range from the melting point to the boiling point of 40°C or more) while maintaining a low melting point (for example, less than 150°C). Compositions Nos. 1 to 4 also satisfied the above formulas (4) to (6), which are preferred requirements for an embodiment of the present invention, and were able to achieve a boiling point of more than 200°C while maintaining a melting point of 130°C or less. Compositions Nos. 1 to 3 also satisfied the above formulas (7) to (9), which are more preferred requirements for an embodiment of the present invention, and were able to achieve a boiling point of more than 200°C while maintaining a melting point of 100°C or less. On the other hand, Compositions Nos. Compositions No. 5 to 7 are electrolytes prepared by mixing sodium hydroxide and potassium hydroxide in a 1:1 molar ratio with the remaining water content adjusted, but they do not meet the above-mentioned requirements of the present invention, having a narrow temperature range between the melting point and boiling point (for example, a temperature range of less than 40°C) or a high melting point (for example, 150°C or higher under normal pressure). Composition No. 8 is an electrolyte prepared by mixing sodium hydroxide and potassium hydroxide in a 1:1 mass ratio with the remaining water content adjusted, but it does not meet the above-mentioned requirements of the present invention, having a high melting point.

[0038] Example 2 shows that the hydrogen production method according to the embodiment of the present invention can reduce the overvoltage compared to the conventional method.

[0039] Fig. 1 is a schematic diagram of an experimental device for verifying the effects of an embodiment of the present invention. As shown in Fig. 1, a tank 1 is filled with an electrolyte 2. A working electrode 3, a counter electrode 4, a reference electrode 5, and a thermometer 6 are disposed in the electrolyte 2. Fluororesin is used for the tank 1 and packing (not shown). A heater 7 is disposed on the outside of the tank 1. Ar gas is flowing in the tank 1, and the pressure inside the tank 1 is 1.0 x 10 ５ ~1.1 x 10 ５ The pressure was adjusted to 0.157 Pa. A Ni flag electrode was used as the working electrode, and the electrode area was 0.157 cm. ２ The counter electrode was a platinum wire electrode (Φ2 mm) or a Ni wire electrode (Φ2 mm), with an electrode area of ​​1.92 cm ２A palladium hydride (Pd—H) electrode was used as the reference electrode, and the potential was calibrated using a reversible hydrogen electrode (RHE). The equilibrium potential of the Pd—H electrode relative to the RHE was determined by thermodynamic calculation. In Examples 1 and 2 of the present invention, the electrolyte was prepared using Composition 1 in Example 1 (the molar ratio of sodium hydroxide:potassium hydroxide:water was 9:61:30). In the prior art example, the electrolyte was prepared using a composition (Composition No. 9) composed of potassium hydroxide and water, with a molar ratio of potassium hydroxide:water of 65:35. The heating temperature was 150°C or 200°C. Linear sweep voltammetry was performed at a scan rate of 10 mV / s, and polarization curves for hydrogen evolution and oxygen evolution were generated.

[0040] In the polarization curve for hydrogen evolution, the current density was −0.5 A / cm ２ and −1.0 A / cm ２ The electrode potential after IR correction at 1000 kJ / cm was determined, and the hydrogen overvoltage was calculated from the difference between the electrode potential and the equilibrium potential of hydrogen generation. The equilibrium potential of hydrogen generation at each heating temperature was set to 0 V vs. RHE. In the polarization curve for oxygen generation, the current density was 0.5 A / cm. ２ and 1.0 A / cm ２ The electrode potential after IR correction at 150°C was determined, and the oxygen overvoltage was calculated from the difference between the electrode potential and the equilibrium potential of oxygen generation. The equilibrium potential of oxygen generation at each heating temperature was 1.155 V vs. RHE at 150°C and 1.142 V vs. RHE at 200°C. The equilibrium potential was determined from the standard theoretical decomposition voltage calculated from the standard Gibbs energy change of the water electrolysis reaction at each heating temperature. The results are shown in Table 2.

[0041]

[0042] As shown in Table 2, when compared at the same heating temperature (150°C), Invention Example 2, which satisfied the requirements of the embodiment of the present invention, was able to reduce the total value of the hydrogen overvoltage and the oxygen overvoltage (total overvoltage) compared to the conventional technology example. Furthermore, Invention Example 1, which satisfied the preferable requirement of a heating temperature of 200°C or higher (and above the melting point of the electrolyte), was able to further reduce the total overvoltage.

[0043] The material of the working electrode was changed from Ni in Example 2 to the following materials A to J, and the oxygen overvoltage was evaluated in the same manner as in Example 2. The composition of each material was analyzed by energy dispersive X-ray spectroscopy under conditions of a magnification of 500 times and an accelerating voltage of 15 kV. Material A: Kovar (registered trademark) (Fe: 56 mass%, Ni: 28 mass%, and Co: 16 mass%) Material B: Hastelloy (registered trademark) C-276 (Ni: 55 mass%, Mo: 17 mass%, Cr: 16 mass%, Fe: 6 mass%, W: 4 mass%, and Al: 2 mass%) Material C: Inconel (registered trademark) 600 (Ni: 73 mass%, Cr: 17 mass%, and Fe: 10 mass%) Material D: Inconel (registered trademark) 625 (Ni: 60 mass%, Cr: 22 mass%, Mo: 10 mass%, Nb: 5 mass%, and Fe: 3 mass%) Material E: Nichrome (registered trademark) (Ni: 79 mass%, and Cr: 21 mass%) Material F: Hastelloy (registered trademark) B-2 (Ni: 69% by mass, Mo: 29% by mass, and Fe: 2% by mass) Material G: Invar (registered trademark) 42 (Fe: 62% by mass, and Ni: 38% by mass) Material H: Permalloy (registered trademark) 45 (Fe: 57% by mass, and Ni: 43% by mass) Material I: Monel (registered trademark) (Ni: 62% by mass, Cu: 36% by mass, and Mn: 2% by mass) Material J: Permalloy (registered trademark) 78 (Ni: 76% by mass, Fe: 15% by mass, Mo: 5% by mass, and Cu: 4% by mass) The results are shown in Table 3.

[0044]

[0045] The results in Tables 2 and 3 reveal the following. When the heating temperature was 150°C, Invention Examples 4 to 13, which satisfied the preferred requirements of the embodiment of the present invention (i.e., the anode contained 25% by mass or more and 95% by mass or less of Ni, at least one selected from the group consisting of Cr, Mn, Fe, Co, and Cu: 1% by mass or more in total, and 0% by mass or more and less than 8% by mass of W), were able to reduce the oxygen overvoltage more than Invention Example 2 shown in Table 2. Furthermore, Invention Example 4, which satisfied the more preferred requirements of the embodiment of the present invention (i.e., the anode contained 25% by mass or more and 95% by mass or less of Ni, 5% by mass or more of Co, and 0% by mass or more and less than 8% by mass of W), was able to further reduce the oxygen overvoltage. Furthermore, Invention Example 3, which satisfied the preferred requirement of a heating temperature of 200°C or more (and above the melting point of the electrolyte), was able to further reduce the oxygen overvoltage than Invention Example 4.

[0046] This application claims priority from Japanese Patent Application No. 2023-191687, filed November 9, 2023. Japanese Patent Application No. 2023-191687 is incorporated herein by reference.

[0047] 1 Tank 2 Electrolyte 3 Working electrode 4 Counter electrode 5 Reference electrode 6 Thermometer 7 Heater

Claims

1. A method for producing hydrogen, comprising a step of electrolyzing an electrolyte solution heated to a temperature between a lower limit temperature that is 100°C or higher and equal to or higher than the melting point and an upper limit temperature that is lower than the boiling point, the electrolyte solution being composed of sodium hydroxide, potassium hydroxide and water, and satisfying the following formulas (1) to (3): 4≦x≦14 ... (1) 51≦y≦71 ... (2) 15≦z≦45 ... (3) where x is the molar ratio (%) of the sodium hydroxide to the total of the sodium hydroxide, the potassium hydroxide and the water, y is the molar ratio (%) of the potassium hydroxide to the total of the sodium hydroxide, the potassium hydroxide and the water, and z is the molar ratio (%) of the water to the total of the sodium hydroxide, the potassium hydroxide and the water.

2. The method according to claim 1, which satisfies the following formulas (4) to (6): 7≦x≦11 (4) 56≦y≦65 (5) 24≦z≦36 (6) 3. The manufacturing method according to claim 2, which satisfies the following formulas (7) to (9): 8≦x≦10 (7) 59≦y≦63 (8) 27≦z≦33 (9) 4. A manufacturing method according to any one of claims 1 to 3, wherein in the electrolysis step, an anode is used that contains: Ni: 25% by mass or more and 95% by mass or less; at least one selected from the group consisting of Cr, Mn, Fe, Co and Cu: 1% by mass or more in total; and W: 0% by mass or more and less than 8% by mass.

5. The method according to claim 4, wherein the anode comprises: Ni: 25% by mass or more and 95% by mass or less; Co: 5% by mass or more; and W: 0% by mass or more and less than 8% by mass.

6. The current density in the electrolysis step is 0.5 to 3.0 A / cm ２ The method according to any one of claims 1 to 5, 7. 1×10 ５ ~2×10 ７ The method according to any one of claims 1 to 6, wherein the electrolysis step is carried out under a pressure of 100 Pa.

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