Method for manufacturing anode for fluorine gas electrolytics synthesis

Abstract

Provided is a method for manufacturing an anode where the anode effect is not likely to occur even when an electrolytic solution containing hydrogen fluoride and a metal fluoride is electrolyzed and used for electrolytically synthesizing fluorine gas. The method for manufacturing an anode for fluorine gas electrolytic synthesis includes an anodic treatment step. The anodic treatment step is a step of immersing not only an anode (21) that is an anode substrate including a carbon material but also a cathode (22) in a mixed liquid (10) for manufacturing an anode containing hydrogen fluoride, a metal fluoride, and potassium hexafluoronickelate (IV) and causing a current to flow between the anode (21) and the cathode (22) to perform an anodic treatment of the anode substrate such that nickel fluoride is attached to a surface of the anode substrate.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for manufacturing an anode for fluorine gas electrolytic synthesis.BACKGROUND ART

[0002] Fluorine gas can be synthesized by electrolyzing an electrolytic solution containing hydrogen fluoride and a metal fluoride. When fluorine gas is electrolytically synthesized, electrolysis is generally performed at an electrolysis temperature of 80° C. to 100° C. by using KF·2HF (which is a mixed molten salt of hydrogen fluoride and potassium fluoride, in which a molar ratio between the hydrogen fluoride and the potassium fluoride in the mixed molten salt is hydrogen fluoride:potassium fluoride=2:1) as an electrolytic solution and using a carbon electrode as an anode.

[0003] However, when the carbon electrode is used as the anode, a phenomenon (hereinafter, also referred to as “anode effect”) in which the anode voltage increases during the electrolysis such that the flow of a current decreases is likely to occur. Therefore, there is a problem in that electrolysis cannot be continuously and stably performed for a long period of time. This anode effect is a phenomenon in which, when a discharge reaction of fluoride ions occurs on a surface of the carbon electrode, graphite fluoride ((CF)n) or the like having a very low surface energy is formed on the surface of the carbon electrode such that the electrolytic solution and the surface of the carbon electrode are not likely to come into contact with each other to deteriorate the flow of a current.

[0004] As the carbon electrode where the anode effect is suppressed, PTL 1 discloses an electrode where a metal fluoride is supported on pores of porous carbon. In addition, to inhibit the formation of the graphite fluoride, PTL 2 discloses a technology of coating a surface of a carbon electrode with a film containing a potassium nickel fluoride compound.CITATION LISTPatent Literatures

[0005] PTL 1: JP 3037464 B

[0006] PTL 2: JP 5772102 BSUMMARY OF INVENTIONTechnical Problem

[0007] When the electrolysis of the electrolytic solution is performed using the carbon electrode as an anode, a technology of more sufficiently suppressing the occurrence of the anode effect has been desired.

[0008] An object of the present invention is to provide a method for manufacturing an anode where the anode effect is not likely to occur even when an electrolytic solution containing hydrogen fluoride and a metal fluoride is electrolyzed and used for electrolytically synthesizing fluorine gas.Solution to Problem

[0009] To solve the above objects, one aspect of the present invention is as the following [1] to [8].

[0010] [1] A method for manufacturing an anode for fluorine gas electrolytic synthesis, the anode being usable for electrolyzing an electrolytic solution containing hydrogen fluoride and a metal fluoride to electrolytically synthesize fluorine gas, and the method including:

[0011] an anodic treatment step of immersing not only an anode that is an anode substrate including a carbon material but also a cathode in a mixed liquid for manufacturing an anode containing hydrogen fluoride, a metal fluoride, and potassium hexafluoronickelate (IV) and causing a current to flow between the anode and the cathode to perform an anodic treatment of the anode substrate such that nickel fluoride is attached to a surface of the anode substrate.

[0012] [2] The method for manufacturing an anode for fluorine gas electrolytic synthesis according to [1], in which a concentration of the potassium hexafluoronickelate (IV) in the mixed liquid for manufacturing an anode is 500 ppm by mass or more and 5000 ppm by mass or less.

[0013] [3] The method for manufacturing an anode for fluorine gas electrolytic synthesis according to [1] or [2], in which in the anodic treatment, an anode current density is 0.01 A / cm2 or more and 0.8 A / cm2 or less, and an amount of current applied per unit surface area of the anode substrate is 100 coulomb / cm2 or more and 5000 coulomb / cm2 or less.

[0014] [4] The method for manufacturing an anode for fluorine gas electrolytic synthesis according to any one of [1] to [3], in which the nickel fluoride includes nickel ions having a valence of more than 2.

[0015] [5] The method for manufacturing an anode for fluorine gas electrolytic synthesis according to any one of [1] to [3], in which the nickel fluoride is at least one among nickel trifluoride, dinickel pentafluoride, and nickel tetrafluoride.

[0016] [6] The method for manufacturing an anode for fluorine gas electrolytic synthesis according to any one of [1] to [5], in which the metal fluoride is at least one of potassium fluoride and cesium fluoride.

[0017] [7] The method for manufacturing an anode for fluorine gas electrolytic synthesis according to any one of [1] to [6], in which a ratio of a molar amount of the hydrogen fluoride to a molar amount of the metal fluoride in the mixed liquid for manufacturing an anode is 1.6 or more and 3.2 or less.

[0018] [8] The method for manufacturing an anode for fluorine gas electrolytic synthesis according to any one of [1] to [7], in which the cathode is an electrode formed of at least one among iron, nickel, copper, and a copper nickel alloy.Advantageous Effects of Invention

[0019] According to the present invention, it is possible to manufacture an anode where the anode effect is not likely to occur even when an electrolytic solution containing hydrogen fluoride and a metal fluoride is electrolyzed and used for electrolytically synthesizing fluorine gas.BRIEF DESCRIPTION OF DRAWINGS

[0020] FIG. 1 is a view illustrating an example of an electrolytic cell that can perform a method for manufacturing an anode for fluorine gas electrolytic synthesis according to an embodiment of the present invention; and

[0021] FIG. 2 is a view illustrating a cyclic voltammogram obtained by cyclic voltammetry.DESCRIPTION OF EMBODIMENTS

[0022] Embodiments of the present invention will now be described. The embodiments are merely examples of the present invention, and the present invention is not limited to the embodiments. Various modifications or improvements can be made in the embodiments, and such modifications and improvements can be encompassed by the present invention.

[0023] A method for manufacturing an anode for fluorine gas electrolytic synthesis according to the present embodiment is a method for manufacturing an anode for fluorine gas electrolytic synthesis, the anode being usable for electrolyzing an electrolytic solution containing hydrogen fluoride (HF) and a metal fluoride to electrolytically synthesize fluorine gas (F2), the method including an anodic treatment step. The anodic treatment step is a step of immersing not only an anode that is an anode substrate including a carbon material but also a cathode in a mixed liquid for manufacturing an anode containing hydrogen fluoride, a metal fluoride, and potassium hexafluoronickelate (IV) (K2NiF6) and causing a current to flow between the anode (anode substrate) and the cathode to perform an anodic treatment of the anode substrate such that nickel fluoride is attached to a surface of the anode substrate.

[0024] In an anode manufactured using the method for manufacturing an anode for fluorine gas electrolytic synthesis (hereinafter, also referred to as “the anode for fluorine gas electrolytic synthesis according to the present embodiment”) according to the present embodiment, nickel fluoride is attached to a surface of the anode substrate that is anodized in the anodic treatment (for example, a film of nickel fluoride is formed on the surface of the anode substrate). Therefore, even when the anode for fluorine gas electrolytic synthesis according to the present embodiment is used for electrolyzing an electrolytic solution containing hydrogen fluoride and a metal fluoride to electrolytically synthesize fluorine gas, the anode effect is not likely to occur.

[0025] For example, by using the anode for fluorine gas electrolytic synthesis according to the present embodiment for electrolyzing an electrolytic solution (for example, KF·2HF) containing hydrogen fluoride and a metal fluoride and not containing potassium hexafluoronickelate (IV) to electrolytically synthesize fluorine gas, the electrolysis where the anode effect is suppressed such that the anode current density is 0.01 A / cm2 or more and 0.8 A / cm2 or less can be performed. As a result, the frequency of interruption or stop of the operation of electrolytically synthesizing fluorine gas can be reduced, and the electrolysis can be performed with a high current density. Therefore, the production cost of fluorine gas can be reduced.

[0026] Here, the anode effect will be described. When a carbon anode is anodized in an electrolytic solution containing hydrogen fluoride and a metal fluoride, typically, a discharge reaction of fluoride ions (hereinafter, also referred to as “HF2−”) occurs on a surface of the carbon anode to produce fluorine atoms. The produced fluorine atoms are coupled to produce fluorine gas, a part of the fluorine atoms are bonded to carbon of the carbon anode, and a water-repellent graphite fluoride film called a (CF)n film is formed on a surface of the carbon anode. As a result, the electrolytic solution is not likely to come into contact with the surface of the carbon anode during the electrolysis. Therefore, a discharge reaction of fluoride ions is not likely to occur on a surface of the carbon anode, the true current density increases, and thus the anode voltage increases such that the flow of a current decreases. This is the mechanism for the anode effect.

[0027] PTL 2 discloses a technology of coating the surface of the carbon electrode with a film other than the (CF)n film. That is, regarding an electrode having a configuration where a part of the surface of the carbon substrate is coated with a conductive diamond layer and the remaining part of the surface of the carbon substrate is not coated with the conductive diamond layer, a treatment of coating a potassium nickel fluoride compound-containing film is performed such that only the surface where the conductive diamond layer is not coated is coated with the potassium nickel fluoride compound-containing film.

[0028] Since the surface of the carbon substrate is coated with the conductive diamond layer and the potassium nickel fluoride compound-containing film, the (CF)n film is not likely to be formed on the surface. Examples of the potassium nickel fluoride compound include potassium hexafluoronickelate (IV) (K2NiF6).

[0029] PTL 2 describes forming conditions of the potassium nickel fluoride compound-containing film, and using a method of adding nickel fluoride to the electrolytic solution or a method of eluting nickel from a material of an electrolytic cell, the nickel ion concentration in the electrolytic solution is preferably 10 ppm to 5%, in particular, 30 ppm to 1000 ppm. In addition, electrolysis conditions of the electrolytic solution, the current density is 0.001 A / cm2 or more and 0.05 A / cm2 or less, and the electrolysis time is 0.1 hours or longer and 10 hours or shorter.

[0030] PTL 2 describes that, when the current density is higher than 0.05 A / cm2, a graphite fluoride layer is likely to be formed before forming the potassium nickel fluoride compound-containing film, which is not preferable. In addition, PTL 2 describes that, when the electrolysis time is longer than 10 hours, powder consumption or a decrease in productivity occurs, which is not preferable.

[0031] Accordingly, as a method of causing nickel ions to coexist in the electrolytic solution, a method of adding nickel fluoride (NiF2, molecular weight: 96.7 g / mol) to the electrolytic solution was attempted. 100 g of a KF·2HF solution was put into a Teflon (registered trademark) airtight container accommodated in a constant-temperature bath and held at 90° C., and 0.165 g of nickel fluoride powder was added to the KF·2HF solution such that the concentration of nickel (atomic weight: 58.7 g / mol) was 1000 ppm by mass. The solution in the airtight container was left to stand for 36 hours in a state where the solution was held at 90° C. while sometimes being stirred. The undissolved nickel fluoride powder was verified in the bottom portion of the airtight container after the standing. Therefore, after removing the undissolved nickel fluoride powder, the nickel concentration in the KF·2HF solution was measured by inductively coupled plasma optical emission spectroscopy (ICP optical emission spectroscopy), which was 164 ppm by mass.

[0032] In addition, as the method of causing nickel ions to coexist in the electrolytic solution, PTL 2 describes the method of eluting nickel from the material of the electrolytic cell. In a state where the material of the electrolytic cell is at a potential (high potential in the positive direction) more positive than the dissolution potential of nickel (about 0.2 to 0.3 V vs. SEP), nickel forming the electrolytic cell is ionized and dissolved in the electrolytic solution.

[0033] To facilitate the dissolution of nickel, by setting nickel forming the electrolytic cell as an anode, the KF·2HF electrolytic solution in the electrolytic cell at 85° C. was electrolyzed at a constant current. By regularly measuring the mass of the nickel anode during the electrolysis, the amount of decrease of nickel was measured. The electrolysis was continued until the amount of decrease of nickel reached an amount in which the concentration of nickel in the KF·2HF electrolytic solution was 1000 ppm by mass by the dissolution of nickel. Sludge-shaped deposits were verified in the bottom portion of the electrolytic cell. Therefore, after removing the sludge-shaped deposits, the nickel concentration in the KF·2HF electrolytic solution was measured by ICP optical emission spectroscopy, which was 282 ppm by mass.

[0034] This way, as the method of causing nickel ions to coexist in the electrolytic solution, the method of adding nickel fluoride to the electrolytic solution or the method of eluting nickel from the material of the electrolytic cell can be used. However, it was found that the concentration of nickel dissolved in the electrolytic solution has a limit of about 200 to 300 ppm by mass in these methods.

[0035] On the other hand, 10% by mass or more of the potassium hexafluoronickelate (IV) with respect to anhydrous hydrogen fluoride can be dissolved. Although there is a small change depending on the temperature, the solubility of potassium hexafluoronickelate (IV) in KF·2HF is about 0.5% by mass, and the concentration of nickel dissolved is about 1000 ppm by mass. Accordingly, a larger amount of nickel electrolyte (for example divalent, trivalent, tetravalent nickel ions) than that of the method described in PTL 2 can be dissolved in the KF·2HF electrolytic solution.

[0036] Using the method of adding nickel fluoride to the electrolytic solution described in PTL 2, nickel fluoride was dissolved in the KF·2HF solution up to 160 ppm by mass close to the saturated solubility, and the critical current density was measured. However, it was found that the effect of suppressing the occurrence of the anode effect was poor. The details will be described below in Comparative Example 1. The critical current density is a current density before the electrolysis voltage rapidly rises. The phenomenon in which the electrolysis voltage rapidly rises corresponds to the anode effect. Therefore, It can be determined that the electrode is in a state where, as the critical current density increases, the anode effect is not likely to occur.

[0037] Next, characteristics of the nickel electrolyte to be dissolved will be described. PTL 2 describes “In the coexistence of nickel ions in the molten salt, the nickel ions form high-valence metal ions . . . ”, which can be interpreted that nickel is dissolved in the electrolytic solution as divalent nickel ions.

[0038] In consideration of the mechanism where the electrode is coated with potassium hexafluoronickelate (IV) in the technology disclosed in PTL 2, it is considered that divalent nickel ions (NiF42−) are diffused on the surface of the electrode and react with fluorine gas or fluorine atoms produced on the surface of the electrode such that the divalent nickel ions are oxidized into tetravalent nickel ions (NiF62−) and diffused again on the surface of the electrode. It can be interpreted that potassium ions present in the vicinity of the electrode are bonded to the tetravalent nickel ions and deposited on the surface of the electrode such that the surface is coated with potassium hexafluoronickelate (IV).

[0039] The amount of dissolution of the divalent nickel ions in the KF·2HF electrolytic solution is about 200 to 300 ppm by mass. Therefore, it can be easily predicted that the amount of tetravalent nickel ions produced by reaction with fluorine gas or fluorine atoms is very small. Accordingly, it is considered that the potassium hexafluoronickelate (IV) film is not substantially formed by the technology disclosed in PTL 2.

[0040] In another mechanism, it is also considered that nickel fluoride (NiF2) solid may be formed due to the discharge reaction of divalent nickel ions on the carbon surface and the nickel fluoride on the carbon surface may react with produced fluorine gas to form a nickel fluoride compound in the high valence state. However, in general, the reaction of forming nickel tetrafluoride (NiF4) from nickel fluoride and fluorine gas requires a high reaction temperature of 250 to 450° C. Therefore, it is expected that nickel fluoride on the carbon electrode does not substantially react with fluorine gas in the electrode reaction.

[0041] Even in any of the reaction routes, to allow the dissolved divalent nickel ions to enter the high valence state, the divalent nickel ions need to interact with fluorine gas produced in the electrode reaction, and the nickel ions in the high valence state cannot be produced unless fluorine gas is produced on the electrode. The production of fluorine gas on the surface of the electrode also leads to formation of the (CF)n film.

[0042] The characteristics of the nickel electrolyte dissolved in the mixed liquid for manufacturing an anode in the present embodiment are different from those of the technology disclosed in PTL 2, and thus the action thereof on the surface of carbon electrode is also different from that of the technology disclosed in PTL 2. In the method for manufacturing an anode for fluorine gas electrolytic synthesis according to the present embodiment, the anodic treatment of the anode substrate is performed in the mixed liquid for manufacturing an anode where free hexafluoronickelate (IV) ions are present. Therefore, irrespective of the production of fluorine gas by the electrode reaction, hexafluoronickelate (IV) ions directly cause the electrode reaction to occur with the surface of the anode substrate. Accordingly, the film formed on the carbon surface exhibits characteristics completely different from those of the technology disclosed in PTL 2.

[0043] To cause the free hexafluoronickelate (IV) ions to be present in the mixed liquid for manufacturing an anode, it is preferable that the amount of easily oxidizable impurities present in the mixed liquid for manufacturing an anode is small. Examples of the easily oxidizable impurities include water, sulfuric acid derived from raw materials, and a silicon compound. For example, when water is present in the mixed liquid for manufacturing an anode, hexafluoronickelate (IV) ions oxidize water. Therefore, nickel of the hexafluoronickelate (IV) ions is reduced in the low valence state, and the effect of the present invention is not likely to be exhibited.

[0044] In the method for manufacturing an anode for fluorine gas electrolytic synthesis according to the present embodiment, the mixed liquid for manufacturing an anode contains potassium hexafluoronickelate (IV). However, instead of the potassium hexafluoronickelate (IV), a metal fluoride complex salt that can release the hexafluoronickelate (IV) ions can be used. Examples of the metal fluoride complex salt include cesium hexafluoronickelate (IV) and rubidium hexafluoronickelate (IV).

[0045] In a case where red crystal powder of potassium hexafluoronickelate (IV) is added to and dissolved in the transparent KF·2HF solution, when water is present in the KF·2HF solution, the potassium hexafluoronickelate (IV) reacts with water. Therefore, the red color of the potassium hexafluoronickelate (IV) disappears, and the mixed liquid for manufacturing an anode becomes cloudy. However, when the free hexafluoronickelate (IV) ions are present in the mixed liquid for manufacturing an anode, the mixed liquid for manufacturing an anode exhibits crimson that is deeper than light pink as the concentration of the hexafluoronickelate (IV) ions increases. When the free hexafluoronickelate (IV) ions are not present in the mixed liquid for manufacturing an anode, the effect of the present invention is not likely to be exhibited.

[0046] The method for manufacturing an anode for fluorine gas electrolytic synthesis according to the present embodiment can be performed, for example, using an electrolytic cell. Examples of the electrolytic cell include an electrolytic cell used for electrolyzing an electrolytic solution containing hydrogen fluoride and a metal fluoride to electrolytically synthesize fluorine gas. Although the details will be described in Examples below, the anode for fluorine gas electrolytic synthesis according to the present embodiment can be manufactured, for example, using a Teflon electrolytic cell illustrated in FIG. 1.

[0047] A structure of the Teflon electrolytic cell of FIG. 1 will be simply described. In the electrolytic cell of FIG. 1, a main body 11 accommodating a mixed liquid 10 for manufacturing an anode is formed of Teflon. In addition, a lid 12 of the main body 11 is formed of a transparent acrylic sheet, and the color of the mixed liquid 10 for manufacturing an anode in the main body 11 can be checked. In the electrolytic cell, a Teflon partition wall 13 is provided, and a gas phase portion in the electrolytic cell is divided into an anode-side gas phase portion and a cathode-side gas phase portion by the partition wall 13.

[0048] Although not illustrated in FIG. 1, an inert gas supply pipe through which inert gas such as nitrogen gas is supplied into the electrolytic cell is connected to the electrolytic cell such that the gas phase portion in the electrolytic cell can be diluted with the inert gas. In addition, a hydrogen fluoride supply pipe 31 for supplying hydrogen fluoride into the mixed liquid 10 for manufacturing an anode in the main body 11 is connected to the electrolytic cell. Further, a fluorine gas extraction pipe 33 and a hydrogen gas extraction pipe 34 for extracting fluorine gas and hydrogen gas from the electrolytic cell, respectively, are connected to the electrolytic cell, the fluorine gas and the hydrogen gas being produced from an anode 21 and a cathode 22, respectively.

[0049] Further, the electrolytic cell includes an external heater (not illustrated), and the mixed liquid 10 for manufacturing an anode in the main body 11 can be heated to a temperature of, for example, 85° C. In addition, the electrolytic cell includes a thermometer 32 such as a thermocouple, and the temperature of the mixed liquid 10 for manufacturing an anode in the main body 11 can be measured. Further, the electrolytic cell includes a stirring device (not illustrated), and the mixed liquid 10 for manufacturing an anode in the main body 11 can be stirred.

[0050] Further, the mixed liquid 10 for manufacturing an anode is a mixed liquid containing hydrogen fluoride, a metal fluoride, and potassium hexafluoronickelate (IV), and is, for example, KF·2HF in which potassium hexafluoronickelate (IV) is dissolved. The electrolytic cell includes the anode 21 and the cathode 22, and the anode 21 and the cathode 22 are immersed in the mixed liquid 10 for manufacturing an anode. The anode 21 is an anode substrate having a carbon material and is, for example, an amorphous carbon sheet. The cathode 22 is, for example, a metal sheet formed of a metal such as nickel. The electrolytic cell is provided in a dry box, and water in the atmosphere is suppressed from being dissolved in the mixed liquid 10 for manufacturing an anode in the main body 11.[Dehydrated Electrolytic Solution 1 Prepared by Performing Dehydration Electrolysis 1]

[0051] Using the electrolytic cell of FIG. 1, dehydration electrolysis of an electrolytic solution that is a mixture of hydrogen fluoride and potassium fluoride is performed. The anode current density is 0.3 A / cm2, and the electrolysis time is 2 hours. In addition, as the anode, a conductive diamond electrode coated with conductive diamond is used. The shape of the anode is a square shape having a length of 1 cm and a width of 1 cm.

[0052] The conductive diamond has a carbon bond of the sp3 orbital. Therefore, a (CF)n film formed by reaction of carbon of the sp2 orbital and fluorine gas is not formed on the surface of the conductive diamond electrode. Accordingly, the conductive diamond electrode is an electrode where the anode effect does not occur. Therefore, by using the conductive diamond electrode, even in the electrolytic solution where water is present, the dehydration electrolysis can be performed without the occurrence of the anode effect.

[0053] The gas produced from the anode is diluted with nitrogen gas, the fluorine gas is absorbed on a potassium iodide aqueous solution trap and removed, and the oxygen gas concentration in the gas exhausted from the potassium iodide aqueous solution trap is measured by gas chromatography. As a result, the oxygen gas is not detected, and thus it is determined that the dehydration electrolysis is completed.

[0054] Since hydrogen fluoride is consumed by the electrolysis, the hydrogen fluoride in the dehydration electrolysis is intermittently supplied to the electrolytic solution to adjust the hydrogen fluoride concentration in the electrolytic solution to be in a range of 40% by mass to 43% by mass. This operation is called a dehydration electrolysis 1, and the electrolytic solution prepared by performing the dehydration electrolysis 1 is called a dehydrated electrolytic solution 1.[Potassium Hexafluoronickelate (IV)-Added Electrode 1]

[0055] Crystal powder of potassium hexafluoronickelate (IV) is added to the dehydrated electrolytic solution 1, and the concentration of the potassium hexafluoronickelate (IV) in the dehydrated electrolytic solution 1 is 0.1% by mass. The nickel concentration in the dehydrated electrolytic solution 1 is 210 ppm by mass. When the potassium hexafluoronickelate (IV) is added, the potassium hexafluoronickelate (IV) slightly is reacted. However, the entire amount of the crystal powder of the potassium hexafluoronickelate (IV) is dissolved, and the dehydrated electrolytic solution 1 exhibits light crimson.

[0056] A part of the dehydrated electrolytic solution 1 (corresponding to the mixed liquid for manufacturing an anode) where the potassium hexafluoronickelate (IV) is dissolved is collected, and the nickel concentration in the dehydrated electrolytic solution 1 is measured by ICP optical emission spectroscopy. As a result, it is verified that the same amount of nickel as the addition amount is dissolved.

[0057] In the dehydrated electrolytic solution 1 where the potassium hexafluoronickelate (IV) is dissolved, not only as an anode that is the carbon electrode (corresponding to the anode substrate) having a square shape with a length of 1 cm and a width of 1 cm but also a metal cathode are immersed. By applying a current of 0.1 A between the anode and the cathode for 1 hour, a surface treatment (corresponding to the anodic treatment) of the carbon electrode is performed.

[0058] The amount of current applied per unit surface area of the carbon electrode in the surface treatment is 360 coulomb / cm2. In addition, the carbon electrode is grade ABR (manufactured by SGL carbon), and a new product (which is not used for electrolysis) is used. The carbon electrode on which the surface treatment is performed is called a potassium hexafluoronickelate (IV)-added electrode 1. Unless specified otherwise, “carbon electrode” refers to a new product (which is not used for electrolysis) of grade ABR (manufactured by SGL carbon).[PTL 2 Additional Test Electrode 1]

[0059] In the dehydrated electrolytic solution 1, not only an anode that is a nickel sheet having a rectangular shape with a length of 2 cm and a width of 3 cm but also a metal cathode are immersed. By applying a current of 0.6 A between the anode and the cathode for 16 hours, electrolysis is performed, and nickel is eluted from the nickel sheet to the dehydrated electrolytic solution 1. After the electrolysis, the dehydrated electrolytic solution 1 exhibits light yellow green.

[0060] The nickel concentration in the dehydrated electrolytic solution 1 where nickel is eluted is calculated as 1000 ppm by mass from the amount of decrease in the mass of the nickel sheet. However, a precipitate is formed in the dehydrated electrolytic solution 1 where nickel is eluted, and the nickel concentration in the dehydrated electrolytic solution 1 not containing the precipitate is 220 ppm by mass when measured by ICP optical emission spectroscopy.

[0061] In the dehydrated electrolytic solution 1 where nickel is eluted, not only an anode that is a carbon electrode having a square shape with a length of 1 cm and a width of 1 cm but also a metal cathode are immersed. By applying a current of 25 mA between the anode and the cathode for 3 hours, electrolysis is performed. Due to the electrolysis, a potassium nickel fluoride film is formed on the surface of the carbon electrode. The amount of current applied per unit surface area of the carbon electrode in the electrolysis is 270 coulomb / cm2. In addition, the current density in the electrolysis is 0.0025 A / cm2.

[0062] Since the carbon electrode where the potassium nickel fluoride film is formed is the electrode described in PTL 2, this carbon electrode is called a PTL 2 additional test electrode 1.[PTL 2 Additional Test Electrode 2]

[0063] In the dehydrated electrolytic solution 1, not only an anode that is a nickel sheet having a rectangular shape with a length of 2 cm and a width of 3 cm but also a metal cathode are immersed. By applying a current of 1 A between the anode and the cathode for 26 hours, electrolysis is performed, and nickel is eluted from the nickel sheet to the dehydrated electrolytic solution 1. After the electrolysis, the dehydrated electrolytic solution 1 exhibits light yellow green (which is slightly deeper green than the color of the dehydrated electrolytic solution 1 in the PTL 2 additional test electrode 1).

[0064] The nickel concentration in the dehydrated electrolytic solution 1 where nickel is eluted is calculated as 2700 ppm by mass from the amount of decrease in the mass of the nickel sheet. However, a precipitate is formed in the dehydrated electrolytic solution 1 where nickel is eluted, and the nickel concentration in the dehydrated electrolytic solution 1 not containing the precipitate is 280 ppm by mass when measured by ICP optical emission spectroscopy.

[0065] In the dehydrated electrolytic solution 1 where nickel is eluted, not only an anode that is a carbon electrode having a square shape with a length of 1 cm and a width of 1 cm but also a metal cathode are immersed. By applying a current of 25 mA between the anode and the cathode for 3 hours, electrolysis is performed. Due to the electrolysis, a potassium nickel fluoride film is formed on the surface of the carbon electrode. The amount of current applied per unit surface area of the carbon electrode in the electrolysis is 270 coulomb / cm2. In addition, the current density in the electrolysis is 0.0025 A / cm2.

[0066] Since the carbon electrode where the potassium nickel fluoride film is formed is the electrode described in PTL 2, this carbon electrode is called a PTL 2 additional test electrode 2.[Measurement of Blank Critical Current Density]

[0067] In the dehydrated electrolytic solution 1, not only an anode that is a carbon electrode having a square shape with a length of 1 cm and a width of 1 cm but also a metal cathode are immersed, and the critical current density is measured. Hereinafter, the critical current density measured using this method is called a blank critical current density, and which represents the critical current density in a state where the treatment is not performed on the electrode.

[0068] A method of measuring the critical current density is as follows. A direct current of 25 mA is initially applied between the anode and the cathode for 15 minutes, and subsequently a direct current of 50 mA is applied between the anode and the cathode for 15 minutes. Further, by increasing the current by 25 mA per 15-minute current application, the current density before the electrolysis voltage rapidly rises is set as the critical current density.

[0069] The phenomenon in which the electrolysis voltage rapidly rises corresponds to the anode effect. Therefore, It can be determined that the electrode is in a state where, as the critical current density increases, the anode effect is not likely to occur. Unless specified otherwise, the critical current density described below is a value obtained using the present measurement method. After the critical current density is measured, the anode is removed from the electrolytic cell.

[0070] Using the above-described measurement method, the critical current density (blank critical current density) of the dehydrated electrolytic solution 1 not containing nickel ion species is measured. The carbon electrode used for the measurement is not used for the measurement of the critical current density or the electrolysis. While replacing the carbon electrode with a new product six times, the critical current density is measured six times. Among these measured values, an average value is 0.29 A / cm2, a maximum value is 0.35 A / cm2, and a minimum value is 0.225 A / cm2.[Measurement of Critical Current Density of Treated Electrode]

[0071] A method of measuring the critical current density of the anode manufactured using the method for manufacturing an anode for fluorine gas electrolytic synthesis according to the present embodiment as it is without taking out the anode from the mixed liquid for manufacturing an anode used during the manufacturing is called a critical current density measurement method A.

[0072] In addition, a method of taking out the anode manufactured using the method for manufacturing an anode for fluorine gas electrolytic synthesis according to the present embodiment from the mixed liquid for manufacturing an anode used during the manufacturing, immersing the anode in the dehydrated electrolytic solution 1 to which the nickel electrolyte is not added or in the dehydrated electrolytic solution 1 where the nickel electrolyte is not present, and measuring the critical current density is called a critical current density measurement method B.

[0073] Regarding the above-described potassium hexafluoronickelate (IV)-added electrode 1, the critical current density is measured using the critical current density measurement method A. As a result, the critical current density is 0.575 A / cm2.

[0074] Further, by manufacturing the potassium hexafluoronickelate (IV)-added electrode 1 two times, the critical current density of each of the electrodes is measured using the critical current density measurement method A. As a result, the critical current densities are 0.575 A / cm2 and 0.675 A / cm2, respectively.

[0075] As described above, the critical current density of the potassium hexafluoronickelate (IV)-added electrode 1 is a value that is two to three times the above-described blank critical current density.

[0076] Next, regarding the above-described potassium hexafluoronickelate (IV)-added electrode 1, the critical current density is measured using the critical current density measurement method B. That is, the potassium hexafluoronickelate (IV)-added electrode 1 is taken out from the mixed liquid for manufacturing an anode used during the manufacturing of the potassium hexafluoronickelate (IV)-added electrode 1. In another electrolytic cell having completely the same structure as the electrolytic cell used during the manufacturing of the potassium hexafluoronickelate (IV)-added electrode 1, the dehydrated electrolytic solution 1 is prepared, the potassium hexafluoronickelate (IV)-added electrode 1 that is taken out as described above is taken into the other electrolytic cell, and the critical current density is measured.

[0077] The measured critical current density is 0.550 A / cm2. Accordingly, it can be said that the potassium hexafluoronickelate (IV)-added electrode 1 is maintained in the surface-modified state.

[0078] The above-described series of electrode replacement operations are performed in a glove box in a nitrogen atmosphere not containing water.

[0079] In addition, the critical current densities of the PTL 2 additional test electrode 1 and the PTL 2 additional test electrode 2 are measured using the critical current density measurement method A. The critical current density of the PTL 2 additional test electrode 1 is 0.300 A / cm2. The color of the electrolytic solution is light yellow green without any change, and it cannot be said that hexafluoronickelate (IV) ions are present.

[0080] The critical current density of the PTL 2 additional test electrode 2 is 0.350 A / cm2. The color of the electrolytic solution is light yellow green without any change, and it cannot be said that hexafluoronickelate (IV) ions are present. The critical current densities of the PTL 2 additional test electrode 1 and the PTL 2 additional test electrode 2 are slightly higher than the average value of the blank critical current density but do not exceed the maximum value of the blank critical current density.

[0081] As described above, it can be seen that, in the anode manufactured using the method for manufacturing an anode for fluorine gas electrolytic synthesis according to the present embodiment, the anode effect is not likely to occur. The mechanism for suppressing the anode effect is not clear but is presumed to relate to a configuration where a fluorinating agent is present on the surface of the electrode.

[0082] Hereinafter, the method for manufacturing an anode for fluorine gas electrolytic synthesis according to the present embodiment will be described in more detail.

[0083] Examples of a device for performing the method for manufacturing an anode for fluorine gas electrolytic synthesis according to the present embodiment include an electrolytic cell used for electrolytically synthesizing fluorine gas. An aspect of the electrolytic cell is not particularly limited, and any electrolytic cell can be used as long as it is an electrolytic cell where an electrolytic solution containing hydrogen fluoride and a metal fluoride can be electrolyzed to produce fluorine gas.

[0084] Typically, the inside of the electrolytic cell is divided by a separation member such as a partition wall into an anode chamber where the anode is disposed and a cathode chamber where the cathode is disposed, and thus fluorine gas produced from the anode and hydrogen gas produced from the cathode are not mixed with each other.

[0085] Although the inside of the electrolytic cell is divided into the anode chamber where the anode is disposed and the cathode chamber where the cathode is disposed, the mixed liquid for manufacturing an anode accommodated in the anode chamber and the mixed liquid for manufacturing an anode accommodated in the cathode chamber are not separated from each other. This way, the inside of the electrolytic cell is likely to have a structure where the mixed liquids for manufacturing an anode accommodated in both of the chambers can be freely mixed.

[0086] When the inside of the electrolytic cell has the structure where the mixed liquids for manufacturing an anode accommodated in both of the chambers can be freely mixed, the potassium hexafluoronickelate (IV) may be contained in any of the mixed liquid for manufacturing an anode accommodated in the anode chamber and the mixed liquid for manufacturing an anode accommodated in the cathode chamber.

[0087] As the anode, for example, a carbonaceous electrode formed of a carbon material such as diamond, diamond-like carbon, amorphous carbon, graphite, or glassy carbon can be used. As the cathode, for example, a metal electrode formed of a metal such as iron (Fe), nickel (Ni), copper (Cu), or a copper nickel alloy (for example, Monel (trademark)) can be used.

[0088] The mixed liquid for manufacturing an anode contains hydrogen fluoride, a metal fluoride, and potassium hexafluoronickelate (IV). The kind of the metal fluoride is not particularly limited, but is preferably an alkali metal fluoride and more preferably at least one of potassium fluoride (KF) and cesium fluoride (CsF). The metal fluoride may be used alone or in combination of two or more kinds thereof. That is, as the metal fluoride, potassium fluoride and cesium fluoride may be used in combination.

[0089] A ratio of a molar amount of the hydrogen fluoride to a molar amount of the metal fluoride ([molar amount of hydrogen fluoride] / [molar amount of metal fluoride]) in the mixed liquid for manufacturing an anode is preferably 1.6 or more and 3.2 or less and more preferably 1.9 or more and 3.0 or less.

[0090] As the mixed liquid for manufacturing an anode, for example, a mixed liquid where potassium hexafluoronickelate (IV) is dissolved in a mixed molten salt of hydrogen fluoride and potassium fluoride can be used. The molar ratio between hydrogen fluoride and potassium fluoride in the mixed molten salt of hydrogen fluoride and potassium fluoride can be set to, for example, hydrogen fluoride:potassium fluoride=1.5 to 2.5:1. KF·2HF where hydrogen fluoride:potassium fluoride=2:1 is a representative mixed molten salt, and the melting point of the mixed molten salt is about 72° C.

[0091] In addition, as the mixed liquid for manufacturing an anode, for example, a mixed liquid where potassium hexafluoronickelate (IV) is dissolved in a mixed molten salt of hydrogen fluoride and cesium fluoride can also be used. The molar ratio between hydrogen fluoride and cesium fluoride in the mixed molten salt of hydrogen fluoride and cesium fluoride can be set to, for example, hydrogen fluoride:cesium fluoride=1.8 to 3.1:1. CsF·2.4HF where hydrogen fluoride:cesium fluoride=2.4:1 is a representative mixed molten salt, and the melting point of the mixed molten salt is about 16° C.

[0092] Since this mixed liquid for manufacturing an anode is corrosive, it is preferable that a portion such as an inner surface of the electrolytic cell in contact with the mixed liquid for manufacturing an anode is formed of a metal such as iron, nickel, or Monel (trademark).

[0093] Since the hydrogen fluoride in the mixed liquid for manufacturing an anode is consumed by the electrolysis for manufacturing the anode, it is preferable that hydrogen fluoride is continuously or intermittently supplied to the mixed liquid for manufacturing an anode, for example, during the electrolysis for manufacturing the anode. The hydrogen fluoride may be supplied to the mixed liquid for manufacturing an anode on the cathode chamber side of the electrolytic cell or may be supplied to the mixed liquid for manufacturing an anode on the anode chamber side.

[0094] Assuming that the concentration of hydrogen fluoride when the mixed molten salt used in the mixed liquid for manufacturing an anode is KF·2HF (the concentration of hydrogen fluoride: 40.4% by mass) or CsF·2.4HF (the concentration of hydrogen fluoride: 24.0% by mass) is set as a standard concentration, it is preferable that the concentration of hydrogen fluoride in the mixed liquid for manufacturing an anode is controlled such that a variation is within a range of −5% by mass or more and +5% by mass or less with respect to the standard concentration, it is more preferable that the concentration of hydrogen fluoride in the mixed liquid for manufacturing an anode is controlled such that a variation is within a range of −2.5% by mass or more and +2.5% by mass or less with respect to the standard concentration, and it is still more preferable that the concentration of hydrogen fluoride in the mixed liquid for manufacturing an anode is controlled such that a variation is within a range of −1.5% by mass or more and +1.5% by mass or less with respect to the standard concentration.

[0095] The mixed liquid for manufacturing an anode can be obtained by mixing hydrogen fluoride, a metal fluoride, and potassium hexafluoronickelate (IV) (K2NiF6). As the potassium hexafluoronickelate (IV), a commercially available product may be used, or a prepared product may be used. The potassium hexafluoronickelate (IV) can be prepared by mixing potassium fluoride and nickel fluoride (NiF2) at a molar ratio of 2:1 and treating the mixture in a fluorine gas atmosphere at a temperature of 250° C. or higher and 450° C. or lower.

[0096] The potassium hexafluoronickelate (IV) is an oxidizable solid, and thus reacts with water to be changed into a potassium fluoride salt of trivalent nickel (K2NiF5) or a potassium fluoride salt of divalent nickel (K2NiF4). The solubility of potassium pentafluoridenickelate (III) (K2NiF5) or potassium tetrafluoridenickelate (II) (K2NiF4) in the mixed liquid for manufacturing an anode is lower than that of potassium hexafluoronickelate (IV). Accordingly, potassium hexafluoronickelate (IV) having a low content of water is preferably used. The water content in the potassium hexafluoronickelate (IV) is preferably 0.5% by mass or less and more preferably 0.3% by mass or less.

[0097] A method of causing the mixed liquid for manufacturing an anode to contain potassium hexafluoronickelate (IV) is not particularly limited. The mixed liquid for manufacturing an anode may be prepared by mixing the metal fluoride in the liquid hydrogen fluoride where potassium hexafluoronickelate (IV) is dissolved, and the mixed liquid for manufacturing an anode may be prepared by mixing solid potassium hexafluoronickelate (IV) in the mixture of hydrogen fluoride and the metal fluoride. In addition, a predetermined amount of potassium hexafluoronickelate (IV) may be added collectively or dividedly.

[0098] The concentration of the potassium hexafluoronickelate (IV) in the mixed liquid for manufacturing an anode is preferably 500 ppm by mass or more and 5000 ppm by mass or less and more preferably 1000 ppm by mass or more and 4000 ppm by mass or less.

[0099] The mixed liquid for manufacturing an anode may contain potassium hexafluoronickelate (IV) in an amount exceeding the saturated solubility. When potassium hexafluoronickelate (IV) is contained in the amount exceeding the saturated solubility, a solid of undissolved potassium hexafluoronickelate (IV) is deposited in the bottom portion of the electrolytic cell. In this case, by performing the method for manufacturing an anode for fluorine gas electrolytic synthesis according to the present embodiment, the nickel electrolyte in the mixed liquid for manufacturing an anode is attached to the surface of the anode substrate as nickel fluoride. Therefore, the concentration of the nickel electrolyte in the mixed liquid for manufacturing an anode decreases. When the mixed liquid for manufacturing an anode contains potassium hexafluoronickelate (IV) in the amount exceeding the saturated solubility, the solid of undissolved potassium hexafluoronickelate (IV) is dissolved along with the decreases in the concentration of the nickel electrolyte. Therefore, the state of the saturated solubility is maintained.

[0100] In the method for manufacturing an anode for fluorine gas electrolytic synthesis according to the present embodiment, by applying a current between the anode (anode substrate) and the cathode to perform the anodic treatment of the anode substrate, nickel fluoride is attached to the surface of the anode substrate. In the anodic treatment, the anode current density is preferably 0.01 A / cm2 or more and 0.8 A / cm2 or less, and the amount of current applied per unit surface area of the anode substrate is preferably 100 coulomb / cm2 or more and 5000 coulomb / cm2 or less.

[0101] The anode current density may be less than 0.01 A / cm2, which can be handled by increasing the treatment time of the anodic treatment. In addition, when the anode current density increases to be more than 0.8 A / cm2, the anodic treatment can be completed within a short period of time. In this case, when an increase in voltage is verified in the anodic treatment, it is preferable to take a countermeasure, for example, to quickly stop the supply of a current such that the anode effect does not occur.

[0102] The anode current density is more preferably more than 0.05 A / cm2 and 0.6 A / cm2 or less.

[0103] In addition, the anode current density may be a fixed value, may gradually increase from a low value, or may gradually decrease from a high value. A method of changing the anode current density is not particularly limited.

[0104] The amount of current applied per unit surface area of the anode substrate is an important numerical value because it relates to the amount of nickel fluoride deposited on the surface of the anode substrate, the nickel fluoride functioning as a catalyst. The amount of current applied per unit surface area of the anode substrate is preferably 100 coulomb / cm2 or more. The amount of current applied per unit surface area of the anode substrate may be more than 5000 coulomb / cm2, but the treatment time of the anodic treatment increases. The amount of current applied per unit surface area of the anode substrate is more preferably 1800 coulomb / cm2 or more.

[0105] It is preferable to adjust the amount of current applied per unit surface area of the anode substrate according to the concentration of the potassium hexafluoronickelate (IV) in the mixed liquid for manufacturing an anode. When the concentration of the potassium hexafluoronickelate (IV) is low, the amount of current applied may be increased, and when the concentration of the potassium hexafluoronickelate (IV) is high, the amount of current applied may be decreased.

[0106] The concentration of nickel electrolyte (IV) ions in the mixed liquid for manufacturing an anode in the anodic treatment can be measured by ICP optical emission spectroscopy. To release the hexafluoronickelate (IV) ions in the mixed liquid for manufacturing an anode, it is desirable that the amount of easily oxidizable impurities present in the mixed liquid for manufacturing an anode is small. Particularly when the water content in the mixed molten salt to which potassium hexafluoronickelate (IV) is not added yet is low, the addition amount of the potassium hexafluoronickelate (IV) can be reduced, which is preferable.

[0107] When water in the mixed molten salt is removed by electrolysis (dehydration electrolysis), water can be removed by performing electrolysis using an electrode such as a conductive diamond electrode, a nickel electrode, or a carbon electrode. When water in the mixed molten salt is removed by electrolysis using the carbon electrode, the carbon electrode used in the dehydration electrolysis can be used as the anode substrate in the method for manufacturing an anode for fluorine gas electrolytic synthesis according to the present embodiment.

[0108] In this case, the (CF)n film is formed on the surface of the carbon electrode during the dehydration electrolysis. Therefore, the effect of the anodic treatment in the presence of the hexafluoronickelate (IV) ions is lower than that of a case where a new product of the carbon electrode is used but still exhibits the effect.

[0109] In the method for manufacturing an anode for fluorine gas electrolytic synthesis according to the present embodiment, by causing a current to flow between the anode and the cathode to perform the anodic treatment of the anode substrate, nickel fluoride is attached to the surface of the anode substrate. Examples of the nickel fluoride include nickel fluorides that include nickel ions having a valence of more than 2. For example, at least one of nickel trifluoride (NiF3), dinickel pentafluoride (Ni2F5), and nickel tetrafluoride (NiF4) can be used.EXAMPLES

[0110] Hereinafter, the present invention will be described more specifically by describing Examples and Comparative Examples.Comparative Example 1

[0111] Using the above-described Teflon electrolytic cell of FIG. 1, an anode for fluorine gas electrolytic synthesis was manufactured. KF·2HF was used as the electrolytic solution, and 750 g of KF·2HF was put into a main body of the electrolytic cell. The electrolytic solution was heated to 85° C., and an anode and a cathode were immersed in the electrolytic solution.

[0112] The cathode was a nickel sheet having a square shape with a length of 5 cm and a width of 5 cm. The anode was a diamond electrode having a square shape with a length of 1 cm and a width of 1 cm, and the surface was coated with conductive diamond.

[0113] By causing a current to flow between the anode and the cathode, the dehydration electrolysis of the electrolytic solution was performed. The anode current density was 0.3 A / cm2, and the electrolysis time was 2 hours. The gas produced from the anode was diluted with nitrogen gas, the fluorine gas was absorbed on a potassium iodide aqueous solution trap and removed, and the oxygen gas concentration in the gas exhausted from the potassium iodide aqueous solution trap was measured by gas chromatography. As a result, the oxygen gas was not detected. Therefore, it was determined that the dehydration electrolysis was completed.

[0114] Since hydrogen fluoride in the electrolytic solution was consumed by the dehydration electrolysis, the hydrogen fluoride in the dehydration electrolysis was intermittently supplied to the electrolytic solution during the dehydration electrolysis to adjust the hydrogen fluoride concentration in the electrolytic solution to be in a range of 40% by mass to 43% by mass. The electrolytic solution prepared through this operation is called the dehydrated electrolytic solution 1.

[0115] Next, 0.2 g of nickel fluoride was added to and dissolved in the dehydrated electrolytic solution 1 in the electrolytic cell. The concentration of nickel fluoride in the dehydrated electrolytic solution 1 was 270 ppm by mass, and the amount of dissolution as nickel was 160 ppm by mass. The addition amount of the nickel fluoride was determined by measuring the amount of dissolution of nickel fluoride with respect to KF·2HF at 85° C. in advance (the details will be described below).

[0116] The color of the dehydrated electrolytic solution 1 where nickel fluoride was dissolved was transparent. The nickel concentration in the dehydrated electrolytic solution 1 was measured by ICP optical emission spectroscopy, and the result was the same as the above-described concentration.

[0117] A new product of the carbon electrode was immersed as an anode in the dehydrated electrolytic solution 1 where nickel fluoride was dissolved, and by applying a current of 25 mA between the anode and the cathode for 3 hours, electrolysis was performed. Due to the electrolysis, the potassium nickel fluoride film described in PTL 2 was formed on the surface of the carbon electrode. The shape of the carbon electrode was a square shape having a length of 1 cm and a width of 1 cm. In the electrolysis, the anode current density was 0.0025 A / cm2, and the amount of current applied per unit surface area of the carbon electrode was 270 coulomb / cm2.

[0118] Here, a method of measuring the above-described amount of dissolution of nickel fluoride will be described. 100 g of KF·2HF was put into a sealable Teflon container, a predetermined amount of nickel fluoride powder was added to and sealed in the container, and the container was left to stand in a constant-temperature bath at 85° C. for 36 hours. Note that the content in the container was sometimes stirred. After leaving to stand for 36 hours, the content of the container was observed by visual inspection to verify whether or not undissolved nickel fluoride powder was present.

[0119] As a result of investigating the solubility while changing the addition amount of the nickel fluoride powder to various values, when the nickel fluoride powder was added in an amount less than or equal to an amount corresponding to a concentration of 320 ppm by mass, the undissolved nickel fluoride powder was not able to be observed. On the other hand, when the nickel fluoride powder was added in an amount more than the amount corresponding to a concentration of 320 ppm by mass, the undissolved nickel fluoride powder was verified. After removing the undissolved nickel fluoride powder, the nickel concentration in the KF·2HF was measured by ICP optical emission spectroscopy, and the maximum value of the nickel concentration was 200 ppm by mass.Comparative Example 2

[0120] The critical current density of the anode according to Comparative Example 1 was 0.25 A / cm2 when measured in the electrolytic solution according to Comparative Example 1 using the critical current density measurement method A. This critical current density was a value in the range of the critical current density obtained in the above-described blank experiment. Therefore, it cannot be said that the anode effect was suppressed. In addition, it cannot be said from the color of the electrolytic solution that the hexafluoronickelate (IV) ions were present.Comparative Example 3

[0121] After completing the measurement of the critical current density in comparative Example 2, the anode was taken out from the electrolytic solution and was heated to 300° C. using a gas flow of inert gas at a high temperature without being cleaned with water. As a result, the hydrogen fluoride component adsorbed on the carbon electrode was volatilized. Next, by performing energy dispersive X-ray spectroscopy using a tabletop scanning electron microscope JCM-7000 (manufactured by JEOL Ltd.), whether or not nickel was present on the surface of the carbon electrode was verified, which was the lower detection limit or less. Therefore, nickel was not able to be verified.Comparative Example 4

[0122] The dehydrated electrolytic solution 1 obtained using the same method as that of Comparative Example 1 was accommodated in the electrolytic cell of FIG. 1, and the anode and the cathode were immersed in the dehydrated electrolytic solution 1. The anode was a nickel sheet having a rectangular shape with a length of 2 cm and a width of 3 cm, and the cathode was a nickel sheet. By applying a current of 1 A between the anode and the cathode for 26 hours, electrolysis was performed, and nickel was eluted from the nickel sheet to the dehydrated electrolytic solution 1.

[0123] The nickel concentration in the dehydrated electrolytic solution 1 was calculated as 2800 ppm by mass from the amount of decrease in the mass of the nickel sheet used for the anode. However, a precipitate was formed in the dehydrated electrolytic solution 1, and the nickel concentration in the dehydrated electrolytic solution 1 not containing the precipitate was 270 ppm by mass when measured by ICP optical emission spectroscopy. After the electrolysis, the dehydrated electrolytic solution 1 exhibited light yellow green.

[0124] A new product of the carbon electrode was immersed as an anode in the dehydrated electrolytic solution 1 where nickel was eluted, and by applying a current of 25 mA between the anode and the cathode for 10 hours, electrolysis was performed. Due to the electrolysis, the potassium nickel fluoride film described in PTL 2 was formed on the surface of the carbon electrode. The shape of the carbon electrode was a square shape having a length of 1 cm and a width of 1 cm. In the electrolysis, the anode current density was 0.0025 A / cm2, and the amount of current applied per unit surface area of the carbon electrode was 900 coulomb / cm2.

[0125] The carbon electrode on which the potassium nickel fluoride film was formed was taken out from the dehydrated electrolytic solution 1 and was heated to 300° C. in the inert gas without being cleaned with water. As a result, the hydrogen fluoride adsorbed on the carbon electrode was volatilized. Next, using the same method as that of Comparative Example 3, whether or not nickel was present on the surface of the carbon electrode was verified, which was the lower detection limit or less. Therefore, nickel was not able to be verified.Comparative Example 5

[0126] The dehydrated electrolytic solution 1 obtained using the same method as that of Comparative Example 1 was accommodated in the electrolytic cell of FIG. 1, and nickel fluoride was added to and dissolved in the dehydrated electrolytic solution 1. The anode was immersed in the dehydrated electrolytic solution 1 where nickel fluoride was dissolved. The anode was a carbon electrode having a square shape with a length of 1 cm and a width of 1 cm. This carbon electrode was formed of highly ordered pyrolytic graphite having a basal plane (HOPG ZYH, manufactured by Momentive).

[0127] By performing cyclic voltammetry using Ni / NiF2 as a reference electrode, a cyclic voltammogram was acquired. The scanning speed of the potential was 0.4 mV / sec. FIG. 2 illustrates the 1st run of the obtained cyclic voltammogram. As can be seen from FIG. 2, the current showed a peak value at a potential in the vicinity of 5.5 V vs. Ni / NiF2, and the current rapidly decreased at a higher potential. This behavior corresponds to the anode effect.Comparative Example 6

[0128] The dehydrated electrolytic solution 1 obtained using the same method as that of Comparative Example 1 was accommodated in the electrolytic cell of FIG. 1, and lithium fluoride (LiF) powder was added to and dissolved in the dehydrated electrolytic solution 1. Lithium fluoride is an additive that is considered to be effective for suppressing the anode effect. The concentration of lithium fluoride in the dehydrated electrolytic solution 1 was 1.0% by mass.

[0129] A new product of the carbon electrode was immersed as an anode in the dehydrated electrolytic solution 1 where lithium fluoride was dissolved, and by applying a current of 0.05 A between the anode and the cathode for 5 hours, electrolysis was performed. Due to this electrolysis, the surface treatment was performed on the surface of the carbon electrode. The shape of the carbon electrode was a square shape having a length of 1 cm and a width of 1 cm. The amount of current applied per unit surface area of the carbon electrode in the electrolysis was 900 coulomb / cm2. The dehydrated electrolytic solution 1 was stirred by a stirrer during the electrolysis to prevent lithium fluoride from settling.

[0130] After completing the surface treatment by the electrolysis, the critical current density was measured. That is, by initially applying a direct current of 25 mA between the anode and the cathode for 15 minutes, the current was increased by 25 mA per 15-minute current application, and the current density before the electrolysis voltage rapidly rose was obtained as the critical current density. As a result, the critical current density is 0.275 A / cm2.

[0131] When the critical current density was measured by performing completely the same operation again, the critical current density was 0.30 A / cm2.

[0132] It can be seen from these results that the addition of lithium fluoride cannot suppress the anode effect.Comparative Example 7

[0133] The dehydrated electrolytic solution 1 obtained using the same method as that of Comparative Example 1 was accommodated in the electrolytic cell of FIG. 1, and crystal powder of potassium hexafluoronickelate (IV) was added to and dissolved in the dehydrated electrolytic solution 1. The concentration of potassium hexafluoronickelate (IV) in the dehydrated electrolytic solution 1 was 0.3% by mass, and the amount of dissolution as nickel was 630 ppm by mass. After dissolving potassium hexafluoronickelate (IV), the dehydrated electrolytic solution 1 exhibited crimson.

[0134] In the dehydrated electrolytic solution 1 where the potassium hexafluoronickelate (IV) was dissolved, not only the anode that was a new product of the carbon electrode having a square shape with a length of 1 cm and a width of 1 cm but also a metal cathode were immersed and were left to stand for 6 hours without applying a current between the anode and the cathode. The critical current density of the anode was measured using the critical current density measurement method B. As a result, the critical current density is 0.250 A / cm2. It can be seen from the result that the effect of suppressing the anode effect cannot be obtained simply by immersing the carbon electrode in the dehydrated electrolytic solution 1 in which potassium hexafluoronickelate (IV) was dissolved.Example 1

[0135] The dehydrated electrolytic solution 1 obtained using the same method as that of Comparative Example 1 was accommodated in the electrolytic cell of FIG. 1, and crystal powder of potassium hexafluoronickelate (IV) was added to and dissolved in the dehydrated electrolytic solution 1. The concentration of potassium hexafluoronickelate (IV) in the dehydrated electrolytic solution 1 was 0.3% by mass, and the amount of dissolution as nickel was 630 ppm by mass. After dissolving potassium hexafluoronickelate (IV), the dehydrated electrolytic solution 1 exhibited crimson.

[0136] The dehydrated electrolytic solution 1 where the potassium hexafluoronickelate (IV) was dissolved was used as the mixed liquid for manufacturing an anode, and not only as an anode (anode substrate) that was a new product of the carbon electrode but also a metal cathode were immersed in the mixed liquid for manufacturing an anode. By applying a current of 0.1 A between the anode and the cathode for 6 hours, electrolysis was performed. Due to the electrolysis, nickel fluoride was formed on the surface of the carbon electrode as the anode substrate to manufacture an anode for fluorine gas electrolytic synthesis. The shape of the carbon electrode was a square shape having a length of 1 cm and a width of 1 cm. In the electrolysis, the anode current density was 0.1 A / cm2, and the amount of current applied per unit surface area of the carbon electrode was 2160 coulomb / cm2.

[0137] In the obtained anode for fluorine gas electrolytic synthesis, the critical current density was measured using the critical current density measurement method A. When the measurement was performed two times, the critical current densities were 0.875 A / cm2 and 0.825 A / cm2, respectively. Table 1 shows manufacturing conditions of the anode for fluorine gas electrolytic synthesis and the measurement results of the critical current densities.Examples 2 to 4

[0138] Anodes for fluorine gas electrolytic synthesis were manufactured using the same method as that of Example 1, except that the concentration of the potassium hexafluoronickelate (IV) in the dehydrated electrolytic solution 1, the time of the electrolysis (pretreatment), and the amount of current applied during the electrolysis were changed as shown in Table 1.

[0139] In the obtained anode for fluorine gas electrolytic synthesis, the critical current density was measured using the critical current density measurement method A. The measurement was performed once for each of the anodes. Table 1 collectively shows manufacturing conditions of the anodes for fluorine gas electrolytic synthesis and the measurement results of the critical current densities.TABLE 1Potassium Hexafluoronickelate (IV)-Added Electrode 1Ex. 1Ex. 2Ex. 3Ex. 4Ex. 7Electrolytic SolutionDehydratedDehydratedDehydratedDehydratedDehydratedDehydratedDehydratedElectrolyticElectrolyticElectrolyticElectrolyticElectrolyticElectrolyticElectrolyticSolution 1Solution 1Solution 1Solution 1Solution 1Solution 1Solution 1Concentration of1000100030002000200050005000Potassium Hexa-fluoronickelate(IV) (ppm by mass)Pretreatment0.1 × 10.1 × 10.1 × 60.1 × 40.1 × 70.1 × 50.1 × 5(A × h)Amount of Current36036021601440252018001800Applied (coulomb / cm2)Critical CurrentABAAAABDensity Measure-ment MethodCritical Current(1) 0.575(1) 0.550(1) 0.875(1) 0.725(1) 0.825(1) 0.850(1) 0.850Density(2) 0.575(2) 0.825(A / cm2)(3) 0.675

[0140] It can be seen from the results shown in Table 1 that, as the amount of dissolution of the potassium hexafluoronickelate (IV) increased, the critical current density tends to increase, but the increase of the critical current density is substantially saturated at a concentration of 5000 ppm by mass. In addition, it can be seen from the results of Examples 2 and 3 that, as the amount of current applied during the electrolysis increases, the critical current density increases, and the critical current density significantly increases as compared to the blank critical current density.Example 5

[0141] After the measurement of the critical current density of Example 4, the anode was taken out from the mixed liquid for manufacturing an anode and was heated to 300° C. in the inert gas. As a result, the hydrogen fluoride adsorbed on the carbon electrode was volatilized. Next, using the same method as that of Comparative Example 3, whether or not nickel was present on the surface of the carbon electrode was verified. Therefore, the presence of nickel was verified.Example 6

[0142] The dehydrated electrolytic solution 1 obtained using the same method as that of Comparative Example 1 was accommodated in the electrolytic cell of FIG. 1, and crystal powder of potassium hexafluoronickelate (IV) was added to and dissolved in the dehydrated electrolytic solution 1. The concentration of the potassium hexafluoronickelate (IV) in the dehydrated electrolytic solution 1 was 0.1% by mass. The anode was immersed in the dehydrated electrolytic solution 1 where the potassium hexafluoronickelate (IV) was dissolved. The anode was a carbon electrode having a square shape with a length of 1 cm and a width of 1 cm. This carbon electrode was formed of highly ordered pyrolytic graphite having a basal plane (HOPG ZYH, manufactured by Momentive).

[0143] By performing cyclic voltammetry using Ni / NiF2 as a reference electrode, a cyclic voltammogram was acquired. The scanning speed of the potential was 0.4 mV / sec. FIG. 2 illustrates the 1st run of the obtained cyclic voltammogram. As can be seen from FIG. 2, the current showed a peak value at a potential in the vicinity of 5.5 V vs. Ni / NiF2, and this peak current value was a value that was 2.7 times the peak current value of Comparative Example 5. It can be seen from this result that, although the anode effect occurred, the addition of the potassium hexafluoronickelate (IV) increased the current value that was able to be caused to flow until the anode effect occurred.

[0144] In addition, as a result of micro Raman spectroscopy, there was no difference in peak intensities of the D-band and the G-band between the potassium hexafluoronickelate (IV) and the nickel fluoride. Therefore, it is considered that the amount of the intercalation compound inserted has no change between the potassium hexafluoronickelate (IV) and the nickel fluoride. Accordingly, the reason for the difference between the peak current values is presumed to be that, due to the addition of the potassium hexafluoronickelate (IV), the nickel fluoride having a valence of more than 2 acted as a catalyst for the reaction of producing fluorine gas to suppress the formation of the (CF)n film on the surface of the carbon electrode.Example 7

[0145] An anode for fluorine gas electrolytic synthesis was manufactured using the same method as that of Example 4. In the obtained anode for fluorine gas electrolytic synthesis, the critical current density was 0.850 A / cm2 when measured using the critical current density measurement method B.Example 8

[0146] The dehydrated electrolytic solution 1 obtained using the same method as that of Comparative Example 1 was accommodated in the electrolytic cell of FIG. 1, and crystal powder of potassium hexafluoronickelate (IV) was added to and dissolved in the dehydrated electrolytic solution 1. The concentration of potassium hexafluoronickelate (IV) in the dehydrated electrolytic solution 1 was 0.4% by mass, and the amount of dissolution as nickel was 840 ppm by mass. After dissolving potassium hexafluoronickelate (IV), the dehydrated electrolytic solution 1 exhibited crimson.

[0147] The dehydrated electrolytic solution 1 where the potassium hexafluoronickelate (IV) was dissolved was used as the mixed liquid for manufacturing an anode, and not only as an anode (anode substrate) that was a new product of the carbon electrode but also a metal cathode were immersed in the mixed liquid for manufacturing an anode. By applying a current of 0.3 A between the anode and the cathode for 4.5 hours, electrolysis was performed. Due to the electrolysis, nickel fluoride was formed on the surface of the carbon electrode as the anode substrate to manufacture an anode for fluorine gas electrolytic synthesis. The shape of the carbon electrode was a square shape having a length of 1 cm and a width of 1 cm. In the electrolysis, the anode current density was 0.3 A / cm2, and the amount of current applied per unit surface area of the carbon electrode was 4860 coulomb / cm2.

[0148] After taking out the completed anode for fluorine gas electrolytic synthesis (the first anode for fluorine gas electrolytic synthesis) from the electrolytic cell, a new product of the carbon electrode was immersed as an anode in the dehydrated electrolytic solution 1 where potassium hexafluoronickelate (IV) was dissolved, and the same operation as described above was performed to manufacture an anode for fluorine gas electrolytic synthesis (the second anode for fluorine gas electrolytic synthesis). The same operation was further repeated to sequentially manufacture the third to sixth anodes for fluorine gas electrolytic synthesis. As a result, the six anodes for fluorine gas electrolytic synthesis in total were manufactured.

[0149] After taking out the completed six anodes for fluorine gas electrolytic synthesis from the electrolytic cell, crystal powder of potassium hexafluoronickelate (IV) was further added to and dissolved in the dehydrated electrolytic solution 1. The addition amount of the potassium hexafluoronickelate (IV) was an amount where the concentration of the potassium hexafluoronickelate (IV) in the dehydrated electrolytic solution 1 was increased by 0.1% by mass due to the addition.

[0150] A new product of the carbon electrode was immersed as an anode in the dehydrated electrolytic solution 1 where the added potassium hexafluoronickelate (IV) was dissolved, and by applying a current of 0.3 A between the anode and the cathode for 4.5 hours, electrolysis was performed. Due to the electrolysis, nickel fluoride was formed on the surface of the carbon electrode as the anode substrate to manufacture the seventh anode for fluorine gas electrolytic synthesis. The shape of the carbon electrode was a square shape having a length of 1 cm and a width of 1 cm. In the electrolysis, the anode current density was 0.3 A / cm2, and the amount of current applied per unit surface area of the carbon electrode was 4860 coulomb / cm2.

[0151] In each of the seven anodes for fluorine gas electrolytic synthesis manufactured as described above, the critical current density was measured using the critical current density measurement method B. Table 2 shows the results. As can be seen from the results shown in Table 2, it was verified that, regarding the first to sixth anodes for fluorine gas electrolytic synthesis, the critical current density tended to decrease in order of manufacturing. It is considered that the nickel electrolyte of the potassium hexafluoronickelate (IV) was attached to the surface of the electrode by the electrolysis to deal with the gradual decrease in the concentration of the hexafluoronickelate (IV) ions in the dehydrated electrolytic solution 1.TABLE 2FirstSecondThirdFourthFifthSixthSeventhElectrolytic SolutionDehydratedDehydratedDehydratedDehydratedDehydratedDehydratedDehydratedElectrolyticElectrolyticElectrolyticElectrolyticElectrolyticElectrolyticElectrolyticSolution 1Solution 1Solution 1Solution 1Solution 1Solution 1Solution 1Concentration of Potassium4000400040004000400040004000Hexafluoronickelate (IV) (ppm by mass)Pretreatment (A × h)0.3 × 4.50.3 × 4.50.3 × 4.50.3 × 4.50.3 × 4.50.3 × 4.50.3 × 4.5Amount of Current Applied4860486048604860486048604860(coulomb / cm2)Critical Current Density MeasurementBBBBBBBMethodCritical Current Density (A / cm2)0.8500.8000.7500.5750.5750.3500.825

[0152] Regarding the seventh anode for fluorine gas electrolytic synthesis, the critical current density was recovered again. It is considered from this result that the concentration of the hexafluoronickelate (IV) ions in the dehydrated electrolytic solution 1 was increased by re-addition of the potassium hexafluoronickelate (IV) such that the hexafluoronickelate (IV) ions were attached to the surface of the carbon electrode by the electrolysis.

[0153] After completing the manufacturing of the sixth anode for fluorine gas electrolytic synthesis, the color of the dehydrated electrolytic solution 1 was lighter than the initial crimson but was still crimson. Therefore, it is considered that, although the hexafluoronickelate (IV) ions were present, the concentration thereof decreased. Example 8 shows that, when the concentration of the hexafluoronickelate (IV) ions is high, the concentration of the hexafluoronickelate (IV) ions attached to the surface of the carbon electrode is high.REFERENCE SIGNS LIST10: mixed liquid for manufacturing an anode

[0155] 11: main body

[0156] 13: partition wall

[0157] 21: anode

[0158] 22: cathode

Claims

1. A method for manufacturing an anode for fluorine gas electrolytic synthesis, the anode being usable for electrolyzing an electrolytic solution containing hydrogen fluoride and a metal fluoride to electrolytically synthesize fluorine gas, and the method comprising:an anodic treatment step of immersing not only an anode that is an anode substrate including a carbon material but also a cathode in a mixed liquid for manufacturing an anode containing hydrogen fluoride, a metal fluoride, and potassium hexafluoronickelate (IV) and causing a current to flow between the anode and the cathode to perform an anodic treatment of the anode substrate such that nickel fluoride is attached to a surface of the anode substrate.

2. The method for manufacturing an anode for fluorine gas electrolytic synthesis according to claim 1, wherein a concentration of the potassium hexafluoronickelate (IV) in the mixed liquid for manufacturing an anode is 500 ppm by mass or more and 5000 ppm by mass or less.

3. The method for manufacturing an anode for fluorine gas electrolytic synthesis according to claim 1, wherein in the anodic treatment, an anode current density is 0.01 A / cm2 or more and 0.8 A / cm2 or less, and an amount of current applied per unit surface area of the anode substrate is 100 coulomb / cm2 or more and 5000 coulomb / cm2 or less.

4. The method for manufacturing an anode for fluorine gas electrolytic synthesis according to claim 1, wherein the nickel fluoride includes nickel ions having a valence of more than 2.

5. The method for manufacturing an anode for fluorine gas electrolytic synthesis according to claim 1, wherein the nickel fluoride is at least one among nickel trifluoride, dinickel pentafluoride, and nickel tetrafluoride.

6. The method for manufacturing an anode for fluorine gas electrolytic synthesis according to claim 1, wherein the metal fluoride is at least one of potassium fluoride and cesium fluoride.

7. The method for manufacturing an anode for fluorine gas electrolytic synthesis according to claim 1, wherein a ratio of a molar amount of the hydrogen fluoride to a molar amount of the metal fluoride in the mixed liquid for manufacturing an anode is 1.6 or more and 3.2 or less.

8. The method for manufacturing an anode for fluorine gas electrolytic synthesis according to claim 1, wherein the cathode is an electrode formed of at least one among iron, nickel, copper, and a copper nickel alloy.

9. The method for manufacturing an anode for fluorine gas electrolytic synthesis according to claim 2, wherein in the anodic treatment, an anode current density is 0.01 A / cm2 or more and 0.8 A / cm2 or less, and an amount of current applied per unit surface area of the anode substrate is 100 coulomb / cm2 or more and 5000 coulomb / cm2 or less.

10. The method for manufacturing an anode for fluorine gas electrolytic synthesis according to claim 2, wherein the nickel fluoride includes nickel ions having a valence of more than 2.

11. The method for manufacturing an anode for fluorine gas electrolytic synthesis according to claim 2, wherein the nickel fluoride is at least one among nickel trifluoride, dinickel pentafluoride, and nickel tetrafluoride.

12. The method for manufacturing an anode for fluorine gas electrolytic synthesis according to claim 2, wherein the metal fluoride is at least one of potassium fluoride and cesium fluoride.

13. The method for manufacturing an anode for fluorine gas electrolytic synthesis according to claim 2, wherein a ratio of a molar amount of the hydrogen fluoride to a molar amount of the metal fluoride in the mixed liquid for manufacturing an anode is 1.6 or more and 3.2 or less.

14. The method for manufacturing an anode for fluorine gas electrolytic synthesis according to claim 2, wherein the cathode is an electrode formed of at least one among iron, nickel, copper, and a copper nickel alloy.

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