TW202444969A - Method for manufacturing anode for fluorine gas electrolytic synthesis - Google Patents

Abstract

Provided is a method for manufacturing an anode in which an anode effect does not easily occur, even if used when electrolyzing an electrolytic solution containing hydrogen fluoride and a metal fluoride to electrolytically synthesize fluorine gas. The method for manufacturing an anode for electrolytic synthesis of fluorine gas comprises a positive polarization treatment step. In the positive polarization treatment step, an anode base material having a carbon material is immersed as an anode (21) in a mixed liquid (10) for anode production, the mixed liquid (10) containing hydrogen fluoride, a metal fluoride, and potassium hexafluoronickelate(IV) together with immersing a cathode (22) therein, a current is passed between the anode (21) and the cathode (22) to subject the anode base material to a positive polarization treatment, and nickel fluoride is caused to adhere to the surface of the anode base material.

Description

Method for manufacturing anode for fluorine gas electrolytic synthesis

The present invention relates to a method for manufacturing an anode for fluorine gas electrolytic synthesis.

Fluorine gas can be synthesized by electrolyzing an electrolyte containing hydrogen fluoride and metal fluoride. When electrolyzing fluorine gas, KF･2HF (a mixed molten salt of hydrogen fluoride and potassium fluoride, the molar ratio of hydrogen fluoride to potassium fluoride in the mixed molten salt is hydrogen fluoride: potassium fluoride = 2:1) is generally used as the electrolyte, and a carbon electrode is used as the anode, and the electrolysis is performed at an electrolysis temperature of 80℃ to 100℃.

However, when using a carbon electrode as an anode, the anode voltage tends to rise and the current flow decreases during electrolysis (hereinafter sometimes referred to as the "anodic effect"), so there is a problem that stable electrolysis may not be able to be performed continuously for a long time. The anodic effect is a phenomenon in which when the discharge reaction of fluoride ions occurs on the surface of the carbon electrode, graphite fluoride ((CF)n) with extremely low surface energy is formed on the surface of the carbon electrode, making it difficult for the electrolyte and the carbon electrode surface to contact, thereby deteriorating the current flow. Patent document 1 discloses an electrode that holds a metal fluoride in the pores of porous carbon as a carbon electrode that suppresses the anodic effect. Patent document 2 discloses a technique for coating the surface of a carbon electrode with a film containing a nickel-potassium fluoride compound in order to prevent the formation of graphite fluoride. [Prior technical document] [Patent document]

[Patent document 1] Japanese Patent Gazette No. 3037464 [Patent document 2] Japanese Patent Gazette No. 5772102

[Problems to be solved by the invention]

When a carbon electrode is used as an anode for electrolysis of an electrolyte, it is desirable to have a technique that can more effectively suppress the occurrence of the anodic effect. The purpose of the present invention is to provide a method for manufacturing an anode that is less likely to cause the anodic effect even when used to electrolyze an electrolyte containing hydrogen fluoride and a metal fluoride to synthesize fluorine gas. [Means for Solving the Problem]

In order to solve the above-mentioned problems, one aspect of the present invention is as follows [1]~[8]. [1] A method for producing an anode for electrolytic synthesis of fluorine gas, which can be used for electrolytic synthesis of fluorine gas by electrolyzing an electrolyte containing hydrogen fluoride and a metal fluoride, and comprises an anodic treatment step, wherein an anode substrate having a carbon material as an anode and a cathode are immersed in a mixed solution for producing an anode containing hydrogen fluoride, a metal fluoride and potassium hexafluoronickel (IV) acid, and a current is passed between the anode and the cathode to perform an anodic treatment on the anode substrate, thereby attaching nickel fluoride to the surface of the anode substrate.

[2] A method for producing an anode for fluorine gas electrolytic synthesis as described in [1], wherein the concentration of the potassium hexafluoronickel (IV) oxide in the anode production mixed solution is 500 mass ppm or more and 5000 mass ppm or less. [3] A method for producing an anode for fluorine gas electrolytic synthesis as described in [1] or [2], wherein in the anode polarization treatment, the anode current density is 0.01 A/ ^cm2 or more and 0.8 A/ ^cm2 or less, and the current per unit surface area of the anode substrate is 100 coulomb/ ^cm2 or more and 5000 coulomb/cm2 ^or less.

[4] A method for producing an anode for fluorine gas electrolytic synthesis as described in any one of [1] to [3], wherein the nickel fluoride has nickel ions with a valence greater than 2. [5] A method for producing an anode for fluorine gas electrolytic synthesis as described in any one of [1] to [3], wherein the nickel fluoride is at least one of nickel trifluoride, nickel pentafluoride and nickel tetrafluoride.

[6] A method for producing an anode for fluorine gas electrolysis synthesis as described in any one of [1] to [5], wherein the metal fluoride is at least one of potassium fluoride and cesium fluoride. [7] A method for producing an anode for fluorine gas electrolysis synthesis as described in any one of [1] to [6], wherein the ratio of the molar amount of the hydrogen fluoride to the molar amount of the metal fluoride contained in the mixed solution for producing the anode is 1.6 to 3.2. [8] A method for producing an anode for fluorine gas electrolysis synthesis as described in any one of [1] to [7], wherein the cathode is an electrode formed of at least one of iron, nickel, copper and copper-nickel alloy. [Effect of the invention]

According to the present invention, a method for manufacturing an anode that is not prone to anodic effect even when used to electrolyze an electrolyte containing hydrogen fluoride and metal fluoride to electrolytically synthesize fluorine gas can be achieved.

The following is a description of an embodiment of the present invention. This embodiment is an example of the present invention and does not limit the present invention to this embodiment. In addition, various changes or improvements can be made to this embodiment, and such changes or improvements can also be included in the present invention.

The method for manufacturing an anode for electrolytic synthesis of fluorine gas of this embodiment can be used for electrolyzing an electrolyte containing hydrogen fluoride (HF) and a metal fluoride to electrolytically synthesize fluorine gas (F ₂ ), and has an anode polarization treatment step. The anodic treatment step is to immerse an anodic substrate having a carbon material as an anode and a cathode in an anode manufacturing mixed solution containing hydrogen fluoride, metal fluoride and potassium nickel( _IV ) hexafluorophosphate ( _K2NiF6 ), and to flow a current between the anode (anodic substrate) and the cathode to perform anodic treatment on the anodic substrate, thereby attaching nickel fluoride to the surface of the anodic substrate.

The anode manufactured by the manufacturing method of the anode for fluorine electrolytic synthesis of the present embodiment (hereinafter sometimes referred to as "the anode for fluorine electrolytic synthesis of the present embodiment") is anodic polarized during the anodic polarization process, and nickel fluoride is attached to the surface of the anode substrate (for example, a nickel fluoride film is formed on the surface of the anode substrate). Therefore, the anode for fluorine electrolytic synthesis of the present embodiment is not likely to cause the anodic effect even when it is used for electrolysis of an electrolyte containing hydrogen fluoride and metal fluoride to electrolytically synthesize fluorine.

For example, when electrolyzing an electrolyte solution containing hydrogen fluoride and metal fluoride and not containing potassium nickel hexafluoro(IV)ate (e.g., KF･2HF) to electrolytically synthesize fluorine gas, if the anode for electrolytic synthesis of fluorine gas of this embodiment is used, electrolysis with suppressed anodic effect can be performed at an anode current density of 0.01A/ ^cm2 or more and 0.8A/ ^cm2 or less. As a result, the frequency of interruption or stoppage of the electrolytic synthesis of fluorine gas can be reduced, and electrolysis can be performed at a high current density, thereby reducing the production cost of fluorine gas.

Here, the anodic effect is explained. If a carbon anode is subjected to anodic polarization in an electrolyte containing hydrogen fluoride and metal fluoride, a discharge reaction of fluoride ions (hereinafter sometimes referred to as "HF ₂ ^- ") usually occurs on the surface of the carbon anode to generate fluorine atoms. The generated fluorine atoms couple to form fluorine gas, but sometimes a part of the fluorine atoms is combined with the carbon of the carbon anode to form a hydrophobic graphite fluoride film called (CF)n film on the surface of the carbon anode. As a result, the electrolyte has difficulty in contacting the carbon anode surface during electrolysis, so it is not easy for the fluoride ion discharge reaction to occur on the carbon anode surface. As the true current density increases, the anode voltage increases and the current flow decreases. This is the mechanism of the anode effect.

Patent document 2 discloses a technology for coating the surface of a carbon electrode with a film other than a (CF)n film. That is, a process for coating an electrode having a structure in which a portion of the surface of a carbon substrate is coated with a conductive diamond layer and the other portion is not coated with the conductive diamond layer is performed to coat the electrode with a film containing a nickel-potassium fluoride compound, so that only the surface not coated with the conductive diamond layer is coated with the film containing a nickel-potassium fluoride compound.

Since the surface of the carbon substrate is coated with a conductive diamond layer and a film containing a nickel potassium fluoride compound, it is difficult to form a (CF)n film on the surface. An example of a nickel potassium fluoride compound is potassium hexafluoronickel (IV) acid (K ₂ NiF ₆ )

Patent document 2 describes the conditions for forming a film containing a nickel potassium fluoride compound, and the nickel ion concentration in the electrolyte is preferably set to 10ppm~5%, particularly preferably 30ppm~1000ppm by adding nickel fluoride to the electrolyte or dissolving nickel from the material of the electrolytic cell. In addition, the electrolysis conditions of the electrolyte are set to a current density of 0.001A/ ^cm2 to 0.05A/ ^cm2 , and an electrolysis time of 0.1 hours to 10 hours.

Patent Document 2 states that if the current density is higher than 0.05 A/cm ² , a graphite fluoride layer is easily formed before the film containing the nickel potassium fluoride compound is formed, which is not preferable. Also, if the electrolysis time is longer than 10 hours, power consumption or a decrease in productivity occurs, which is not preferable.

Therefore, as a method for coexisting nickel ions in the electrolyte, a method of adding nickel fluoride (NiF ₂ , molecular weight 96.7 g/mol) to the electrolyte was tried. 100 g of KF･2HF solution was placed in a sealed container made of Teflon (registered trademark) in a thermostat and maintained at 90°C. 0.165 g of nickel fluoride powder was added to the KF･2HF solution so that the concentration of nickel (atomic weight 58.7 g/mol) in the KF･2HF solution became 1000 mass ppm. Then, the liquid in the sealed container was occasionally stirred and kept at 90°C for 36 hours. Since undissolved nickel fluoride powder was found at the bottom of the sealed container after standing, the undissolved nickel fluoride powder was removed and the nickel concentration in the KF･2HF solution was measured by inductively coupled plasma optical emission spectrometry (ICP optical emission spectrometry), and the result was 164 mass ppm.

Furthermore, Patent Document 2 describes a method for eluting nickel from the material of an electrolytic cell as a method of making nickel ions coexist in an electrolyte. If the material of the electrolytic cell is in a state of higher potential (larger potential in the positive direction) than the dissolution potential of nickel (approximately 0.2 to 0.3 V relative to SEP), the nickel forming the electrolytic cell becomes ions and dissolves in the electrolyte.

In order to make nickel dissolve easily, the nickel forming the electrolytic cell is used as the anode, and the KF･2HF electrolyte at 85℃ is electrolyzed at a constant current in the electrolytic cell. The mass of the nickel anode during electrolysis is measured regularly, and the amount of nickel reduction is measured. The electrolysis is continued until the amount of nickel reduction becomes the amount at which the nickel concentration in the KF･2HF electrolyte reaches 1000 mass ppm due to the dissolution of nickel. Since sludge-like deposits are confirmed at the bottom of the electrolytic cell, the sludge-like deposits are removed and the nickel concentration in the KF･2HF electrolyte is measured by ICP emission spectrometry, which results in 282 mass ppm.

Thus, as a method for making nickel ions coexist in the electrolyte, there are a method of adding nickel fluoride to the electrolyte and a method of eluting nickel from the material of the electrolytic cell. However, it is known that with these methods, the concentration limit of nickel dissolved in the electrolyte is about 200 to 300 mass ppm.

In contrast, potassium hexafluoronickel (IV) acid can dissolve more than 10% by mass in anhydrous hydrogen fluoride. Although it varies somewhat depending on the temperature, the solubility of potassium hexafluoronickel (IV) acid in KF･2HF is about 0.5% by mass, and the concentration of dissolved nickel is about 1000 ppm by mass. Therefore, compared with the method described in Patent Document 2, more nickel electrolyte (divalent, trivalent, tetravalent nickel ions, etc.) can be dissolved in KF･2HF electrolyte.

By adding nickel fluoride to the electrolyte as described in Patent Document 2, nickel fluoride was dissolved in KF･2HF solution until it reached 160 mass ppm, which is close to the saturated solubility, and the critical current density was measured, but it is known that there is no effect of suppressing the occurrence of the anodic effect. The details will be described later in Comparative Example 1. The critical current density is the current density before the electrolytic voltage rises sharply. Since the phenomenon of the electrolytic voltage rising sharply is equivalent to the anodic effect, it can be judged that the larger the critical current density is, the more difficult it is for the electrode state to cause the anodic effect.

Next, the properties of the dissolved nickel electrolyte are explained. Since Patent Document 2 states that "nickel ions are made to coexist in molten salt, and the nickel ions are made to form higher metal ions...", it can be interpreted that nickel is dissolved in the electrolyte as divalent nickel ions.

If we consider the mechanism of coating the electrode with potassium hexafluoronickel (IV) oxide in the technology disclosed in Patent Document 2, it is believed that the divalent nickel ions (NiF ₄ ^2- ) diffuse to the electrode surface, react with the fluorine gas or fluorine atoms generated on the electrode surface, oxidize the divalent nickel ions to tetravalent nickel ions (NiF ₆ ^2- ), and diffuse to the electrode surface again. Therefore, it can be explained that potassium hexafluoronickel (IV) oxide is coated by combining the potassium ions existing near the electrode with the tetravalent nickel ions and precipitating on the electrode surface.

Since the amount of divalent nickel ions dissolved in KF･2HF electrolyte is about 200~300 mass ppm, it can be easily predicted that the amount of tetravalent nickel ions generated by the reaction with fluorine gas or fluorine atoms is very small. Therefore, according to the technology disclosed in Patent Document 2, it is believed that the film formation of potassium hexafluoronickel (IV) acid is almost not caused.

In another mechanism, it is believed that the nickel fluoride (NiF ₂ ) solid is formed by the discharge reaction of the divalent nickel ions on the carbon surface, and the nickel fluoride on the carbon surface reacts with the generated fluorine gas to form a nickel fluoride compound in a high valence state. However, since the reaction of nickel fluoride and fluorine gas to form nickel tetrafluoride (NiF ₄ ) generally requires a high reaction temperature of 250~450℃, it is predicted that the nickel fluoride on the carbon electrode in the electrode reaction will hardly react with fluorine gas.

Regardless of the reaction pathway, in order for the dissolved divalent nickel ions to be in a high-valence state, they must interact with the fluorine gas generated in the electrode reaction. As long as fluorine gas is not generated on the electrode, nickel ions in a high-valence state cannot be generated. The generation of fluorine gas on the electrode surface means that a (CF)n film is also formed.

Since the properties of the nickel electrolyte dissolved in the mixed solution for anode production in this embodiment are different from those disclosed in Patent Document 2, the effect on the surface of the carbon electrode is also different. The method for producing an anode for fluorine gas electrolytic synthesis in this embodiment is to perform an anodic polarization treatment of the anode substrate in the mixed solution for anode production in which free nickel hexafluoride (IV) ions exist, so that the nickel hexafluoride (IV) ions and the surface of the anode substrate directly react with each other independently of the generation of fluorine gas due to the electrode reaction. Therefore, the film formed on the carbon surface becomes a film showing completely different properties from those in the technology disclosed in Patent Document 2.

In order to allow the free nickel hexafluoride (IV) ions to exist in the mixed solution for manufacturing the anode, the amount of impurities that are easily oxidized in the mixed solution for manufacturing the anode is preferably as small as possible. Examples of impurities that are easily oxidized are water, sulfuric acid derived from the raw materials, silicon compounds, etc. For example, when water is present in the mixed solution for manufacturing the anode, the nickel hexafluoride (IV) ions will oxidize the water, causing the nickel in the nickel hexafluoride (IV) ions to be reduced to a low atomic valence state, making it difficult to exhibit the effect of the present invention.

In the method for producing an anode for fluorine gas electrolytic synthesis of this embodiment, potassium hexafluoronickel (IV) oxide is contained in the mixed solution for producing the anode, but a metal fluoride complex salt that can release hexafluoronickel (IV) ions may be used instead of potassium hexafluoronickel (IV) oxide. Examples of such metal fluoride complex salts include cesium hexafluoronickel (IV) oxide and cadmium hexafluoronickel (IV) oxide.

When potassium hexafluoronickel (IV) acid is added to a transparent KF･2HF solution and dissolved in red crystalline powder, if there is water in the KF･2HF solution, the red color of potassium hexafluoronickel (IV) acid disappears due to the reaction between potassium hexafluoronickel (IV) acid and water, and the mixed solution for anode production becomes turbid. However, if there are free hexafluoronickel (IV) ions in the mixed solution for anode production, the mixed solution for anode production changes from light pink to dark red as the concentration of hexafluoronickel (IV) ions increases. If free hexafluoronickel (IV) ions do not exist in the mixed solution for anode production, the effect of the present invention is not easily exhibited.

The method for manufacturing the anode for electrolytic synthesis of fluorine gas of the present embodiment can be implemented using, for example, an electrolytic cell. An example of an electrolytic cell is an electrolytic cell used when electrolyzing an electrolyte containing hydrogen fluoride and a metal fluoride to electrolytically synthesize fluorine gas. The details will be described in the embodiment, but for example, the Teflon electrolytic cell shown in FIG. 1 can be used to manufacture the anode for electrolytic synthesis of fluorine gas of the present embodiment.

The structure of the Teflon electrolytic cell of FIG1 is briefly described. The electrolytic cell of FIG1 has a body 11 containing an anode manufacturing mixed solution 10 made of Teflon. In addition, the cover 12 of the body 11 is made of a transparent acrylic plate, and the color of the anode manufacturing mixed solution 10 in the body 11 can be confirmed. A Teflon partition wall 13 is provided inside the electrolytic cell, and the gas phase part inside the electrolytic cell is divided into an anode side gas phase part and a cathode side gas phase part by the partition wall 13.

Although not shown in FIG. 1 , an inert gas supply pipe for supplying inert gas to the inside of the electrolytic cell is connected to the electrolytic cell in such a manner that the gas phase inside the electrolytic cell can be diluted by an inert gas such as nitrogen. Also, a hydrogen fluoride supply pipe 31 for supplying hydrogen fluoride to the anode manufacturing mixed solution 10 in the body 11 is connected to the electrolytic cell. Furthermore, a fluorine gas extraction pipe 33 and a hydrogen gas extraction pipe 34 for extracting fluorine gas and hydrogen gas generated from the anode 21 and the cathode 22 from the electrolytic cell are connected to the electrolytic cell, respectively.

In addition, the electrolytic cell is provided with an external heater (not shown) to heat the anode manufacturing mixed solution 10 in the body 11 to a temperature of, for example, 85° C. In addition, the electrolytic cell is provided with a thermometer 32 such as a thermocouple to measure the temperature of the anode manufacturing mixed solution 10 in the body 11. Furthermore, the electrolytic cell is provided with a stirring device (not shown) to stir the anode manufacturing mixed solution 10 in the body 11.

Furthermore, the mixed solution 10 for producing the anode is a mixed solution containing hydrogen fluoride, metal fluoride and potassium hexafluoronickel (IV) acid, for example, KF.2HF in which potassium hexafluoronickel (IV) acid is dissolved. The electrolytic cell has an anode 21 and a cathode 22, and both the anode 21 and the cathode 22 are immersed in the mixed solution 10 for producing the anode. The anode 21 is an anode substrate having a carbon material, for example, an amorphous carbon plate. The cathode 22 is a metal plate formed of a metal such as nickel. The electrolytic cell is set in a drying box to suppress water in the atmosphere from dissolving into the mixed solution 10 for producing the anode in the body 11.

[Dehydrated electrolyte 1 prepared by performing dehydration electrolysis 1] Using the electrolytic cell of FIG. 1 , dehydration electrolysis of an electrolyte of a mixture of hydrogen fluoride and potassium fluoride was performed. The anode current density was set to 0.3 A/cm ² , and the electrolysis time was set to 2 hours. Furthermore, the anode used a conductive diamond electrode coated with conductive diamond. The anode shape was a square with a length of 1 cm and a width of 1 cm.

Since conductive diamond has sp3 carbon bonds, the (CF)n film formed by the reaction of sp2 carbon and fluorine gas will not form on the surface of the conductive diamond electrode. Therefore, the conductive diamond electrode is an electrode that does not cause the anodic effect. Therefore, if a conductive diamond electrode is used, dehydration electrolysis can be performed without causing the anodic effect even in the presence of water in the electrolyte.

The gas generated from the anode was diluted with nitrogen, the fluorine gas was absorbed and removed by the potassium iodide aqueous solution collector, and the oxygen concentration in the gas discharged from the potassium iodide aqueous solution collector was measured by gas chromatography. As a result, since no oxygen was detected, it was judged that the dehydration electrolysis was completed.

Since hydrogen fluoride is consumed by electrolysis, hydrogen fluoride is intermittently supplied to the electrolyte during dehydration electrolysis so that the concentration of hydrogen fluoride in the electrolyte is in the range of 40 mass % to 43 mass %. This operation is called dehydration electrolysis 1, and the electrolyte prepared by dehydration electrolysis 1 is called dehydrated electrolyte 1.

[Potassium hexafluoronickel (IV) acid added electrode 1] For the addition of potassium hexafluoronickel (IV) acid crystalline powder to the dehydrated electrolyte 1, the potassium hexafluoronickel (IV) acid concentration in the dehydrated electrolyte 1 was set to 0.1 mass %. The nickel concentration in the dehydrated electrolyte 1 was 210 mass ppm. When potassium hexafluoronickel (IV) acid was added, although potassium hexafluoronickel (IV) acid slightly reacted, the potassium hexafluoronickel (IV) acid crystalline powder was completely dissolved, making the dehydrated electrolyte 1 appear light red.

A portion of the dehydrated electrolyte 1 (corresponding to the mixed solution for anode production) in which potassium hexafluoronickel (IV) oxide was dissolved was taken and the nickel concentration in the dehydrated electrolyte 1 was measured by ICP emission spectrometry. As a result, it was confirmed that nickel was dissolved in an amount equivalent to the added amount.

A square carbon electrode with a length of 1 cm and a width of 1 cm (equivalent to the anode substrate) is immersed in a dehydrated electrolyte 1 in which potassium hexafluoronickel (IV) is dissolved, and a metal cathode is immersed at the same time. Then, a current of 0.1 A is passed between the anode and the cathode for 1 hour to perform a surface treatment of the carbon electrode (equivalent to anode polarization treatment).

The amount of electricity carried per unit surface area of the carbon electrode during the surface treatment is 360 coulombs/ ^cm2 . The carbon electrode used is a new product of ABR grade (not used in electrolysis) manufactured by SGL Carbon Co., Ltd. The carbon electrode subjected to this surface treatment is called potassium hexafluoronickel (IV) acid added electrode 1. Unless otherwise specified, the "carbon electrode" hereafter is a new product of ABR grade (not used in electrolysis) manufactured by SGL Carbon Co., Ltd.

[Patent Document 2 Additional Test Electrode 1] A rectangular nickel plate with a length of 2 cm and a width of 3 cm was immersed in the dehydrated electrolyte 1, and a metal cathode was immersed at the same time. Then, a current of 0.6A was passed between the anode and cathode for 16 hours to perform electrolysis, and nickel was dissolved from the nickel plate into the dehydrated electrolyte 1. The dehydrated electrolyte 1 after electrolysis showed a light yellow-green color.

Based on the mass reduction of the nickel plate, the nickel concentration in the dehydrated electrolyte 1 from which nickel was eluted was calculated to be 1000 mass ppm. However, a precipitate was generated in the dehydrated electrolyte 1 from which nickel was eluted, and the nickel concentration in the dehydrated electrolyte 1 without the precipitate was measured by ICP emission spectrometry, and the result was 220 mass ppm.

A square carbon electrode with a length of 1 cm and a width of 1 cm is immersed in the dehydrated electrolyte 1 from which nickel is eluted, and a metal cathode is immersed at the same time. Then, electrolysis is performed by passing a current of 25 mA between the anode and the cathode for 3 hours. By the electrolysis, a nickel-potassium fluoride film is formed on the surface of the carbon electrode. The amount of electricity passed per unit surface area of the carbon electrode during the electrolysis is 270 coulombs/ ^cm2 . And the current density during the electrolysis is 0.0025A/ ^cm2 . Since the carbon electrode having the nickel potassium fluoride film formed thereon is the electrode described in Patent Document 2, the carbon electrode is referred to as Patent Document 2 Additional Test Electrode 1.

[Patent Document 2 Additional Test Electrode 2] A rectangular nickel plate with a length of 2 cm and a width of 3 cm was immersed in the dehydrated electrolyte 1 as an anode, and a metal cathode was immersed at the same time. Then, by passing a current of 1A between the anode and the cathode for 26 hours, nickel was dissolved from the nickel plate into the dehydrated electrolyte 1. The dehydrated electrolyte 1 after electrolysis showed a light yellow-green color (slightly darker green than the color of the dehydrated electrolyte 1 in the case of the additional test electrode 1 of Patent Document 2).

Based on the mass reduction of the nickel plate, the nickel concentration in the dehydrated electrolyte 1 from which nickel was eluted was calculated to be 2700 mass ppm. However, a precipitate was generated in the dehydrated electrolyte 1 from which nickel was eluted, and the nickel concentration in the dehydrated electrolyte 1 without the precipitate was measured by ICP emission spectrometry, and the result was 280 mass ppm.

A square carbon electrode with a length of 1 cm and a width of 1 cm is immersed in the dehydrated electrolyte 1 from which nickel is eluted, and a metal cathode is immersed at the same time. Then, electrolysis is performed by passing a current of 25 mA between the anode and the cathode for 3 hours. By the electrolysis, a nickel-potassium fluoride film is formed on the surface of the carbon electrode. The amount of electricity passed per unit surface area of the carbon electrode during the electrolysis is 270 coulombs/ ^cm2 . And the current density during the electrolysis is 0.0025A/ ^cm2 . Since the carbon electrode having the nickel potassium fluoride film formed thereon is the electrode described in Patent Document 2, the carbon electrode is referred to as Patent Document 2 Additional Test Electrode 2.

[Determination of blank critical current density] A square carbon electrode with a length of 1 cm and a width of 1 cm is immersed in the dehydrated electrolyte 1 as an anode, and a metal cathode is immersed at the same time, and the critical current density is measured. Hereinafter, the critical current density measured by this method is referred to as the blank critical current density, which means the critical current density in the state where no treatment is applied to the electrode.

The method for determining the critical current density is as follows. A 25mA DC current is first applied between the anode and cathode for 15 minutes, followed by a 50mA DC current for 15 minutes. The current is then increased by 25mA every 15 minutes, and the current density before the electrolytic voltage rises sharply is taken as the critical current density.

Since the phenomenon of rapid rise in electrolytic voltage is equivalent to the anodic effect, it can be judged that the larger the critical current density value, the more difficult it is for the electrode state to cause the anodic effect. Unless otherwise specified, the critical current density shown below is the value obtained by this measurement method. After measuring the critical current density, remove the anode from the electrolytic cell.

By the above-mentioned determination method, the critical current density (blank critical current density) was determined for the dehydrated electrolyte 1 that did not contain nickel ion species. The carbon electrode used in the determination was not used for critical current density determination and electrolysis. After the carbon electrode was replaced with a new one 6 times and the critical current density was measured 6 times, the average value of the measured values was 0.29A/ ^cm2 , the maximum value was 0.35A/ ^cm2 , and the minimum value was 0.225A/ ^cm2 .

[Determination of critical current density of treatment electrode] The method of directly measuring the critical current density of the anode manufactured by the manufacturing method of the anode for fluorine gas electrolytic synthesis of this embodiment without taking it out from the anode manufacturing mixed liquid used in the manufacturing is called critical current density measurement method A.

Furthermore, the method of measuring the critical current density by taking out the anode produced by the method for producing the anode for fluorine gas electrolytic synthesis of the present embodiment from the mixed solution for producing the anode used in the production and immersing it in the dehydrated electrolyte 1 to which the nickel electrolyte is not added or in the dehydrated electrolyte 1 without the nickel electrolyte is referred to as the critical current density measurement method B. The critical current density of the potassium hexafluoronickel (IV) acid added electrode 1 was measured by the critical current density measurement method A. As a result, the critical current density was 0.575 A/cm ² .

Furthermore, the production of the potassium hexafluoronickel (IV) acid added electrode 1 was carried out twice, and the critical current density of each electrode was measured by the critical current density measurement method A. As a result, the critical current density was 0.575A/ ^cm2 and 0.675A/ ^cm2 . As shown above, the critical current density of the potassium hexafluoronickel (IV) acid added electrode 1 is 2 to 3 times higher than the critical current density of the blank.

Next, the critical current density of the potassium hexafluoronickel (IV) acid-added electrode 1 is measured by the critical current density measurement method B. That is, the potassium hexafluoronickel (IV) acid-added electrode 1 is taken out from the anode manufacturing mixed solution used in the manufacture of the potassium hexafluoronickel (IV) acid-added electrode 1. Then, a dehydrated electrolyte 1 is prepared in another electrolytic cell having the same structure as the electrolytic cell used in the manufacture of the potassium hexafluoronickel (IV) acid-added electrode 1, and the potassium hexafluoronickel (IV) acid-added electrode 1 taken out is installed in the other electrolytic cell to measure the critical current density.

The critical current density was measured to be 0.550 A/cm ² . Therefore, it can be said that the potassium hexafluoronickel (IV) acid added electrode 1 maintained the surface modified state. The above series of electrode replacement operations were performed in a glove box under a nitrogen environment without water.

Next, the critical current density of the additional test electrode 1 of Patent Document 2 and the additional test electrode 2 of Patent Document 2 was measured by critical current density measurement method A. The critical current density of the additional test electrode 1 of Patent Document 2 was 0.300A/cm ² . The color of the electrolyte was light yellow-green and did not change, and it could not be said that nickel hexafluoride (IV) ions were present.

The critical current density of the additional test electrode 2 of Patent Document 2 is 0.350A/cm ² . The color of the electrolyte is light yellow-green and does not change, and it cannot be said that nickel hexafluoride (IV) ions exist. The critical current density of the additional test electrode 1 of Patent Document 2 and the additional test electrode 2 of Patent Document 2 is slightly higher than the average value of the blank critical current density, but does not exceed the maximum value of the blank critical current density.

As described above, it can be seen that the anode manufactured by the manufacturing method of the anode for fluorine gas electrolytic synthesis of this embodiment is very difficult to cause the anodic effect. The mechanism of suppressing the anodic effect is not clear, but it is speculated to be related to the presence of a fluorinating agent on the electrode surface.

The following is a further detailed description of the method for manufacturing the anode for electrolytic synthesis of fluorine gas according to the present embodiment. As an example of an apparatus for implementing the method for manufacturing the anode for electrolytic synthesis of fluorine gas according to the present embodiment, an electrolytic cell for electrolytic synthesis of fluorine gas is given. The type of the electrolytic cell is not particularly limited, and any electrolytic cell can be used as long as it can electrolyze an electrolyte containing hydrogen fluoride and metal fluoride to produce fluorine gas.

Usually, the inside of the electrolytic cell is divided into an anode chamber where the anode is arranged and a cathode chamber where the cathode is arranged by a partition member such as a partition wall, so that the fluorine gas generated in the anode and the hydrogen gas generated in the cathode are not mixed. Although the inside of the electrolytic cell is divided into an anode chamber where the anode is arranged and a cathode chamber where the cathode is arranged, most of them are designed to not separate the anode manufacturing mixed liquid contained in the anode chamber and the anode manufacturing mixed liquid contained in the cathode chamber, but to allow the anode manufacturing mixed liquid contained in the two electrode chambers to mix freely.

The interior of the electrolytic cell is configured so that the mixed solution for producing the anode contained in the two electrode chambers can be mixed freely, and potassium hexafluoronickel (IV) acid can be contained in either the mixed solution for producing the anode contained in the anode chamber or the mixed solution for producing the anode contained in the cathode chamber.

As the anode, for example, a carbon electrode made of a carbon material such as diamond, diamond-like carbon, amorphous carbon, graphite, glassy carbon, etc. can be used. As the cathode, for example, a metal electrode made of a metal such as iron (Fe), nickel (Ni), copper (Cu), copper-nickel alloy (such as monel alloy (trademark)) can be used.

The mixed solution for manufacturing the anode contains hydrogen fluoride, metal fluoride and potassium nickel hexafluoride (IV). The type of 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. That is, potassium fluoride and cesium fluoride may be used in combination as the metal fluoride.

Moreover, the ratio of the molar amount of hydrogen fluoride contained in the mixed solution for anode production to the molar amount of metal fluoride ([molar amount of hydrogen fluoride]/[molar amount of metal fluoride]) is preferably 1.6 to 3.2, and more preferably 1.9 to 3.0. As the mixed solution for anode production, for example, potassium hexafluoronickel (IV) acid dissolved in a mixed molten salt of hydrogen fluoride and potassium fluoride can be used. The molar ratio of hydrogen fluoride to potassium fluoride in the mixed molten salt can be, for example, hydrogen fluoride:potassium fluoride=1.5~2.5:1. KF･2HF, which is a representative mixed molten salt with a ratio of hydrogen fluoride to potassium fluoride = 2:1, has a melting point of about 72°C.

As an example of a mixed solution for manufacturing an anode, a mixed molten salt of hydrogen fluoride and csF2 in which potassium nickel hexafluoride (IV) is dissolved can also be used. The molar ratio of hydrogen fluoride to csF2 in the mixed molten salt of hydrogen fluoride and csF2 can be, for example, hydrogen fluoride: csF2.4HF at a ratio of hydrogen fluoride: csF2.4HF = 2.4:1 is a representative mixed molten salt, and the melting point of the mixed molten salt is about 16°C.

Since the anode manufacturing mixed solution is corrosive, the parts of the electrolytic cell that come into contact with the anode manufacturing mixed solution, such as the inner surface, are preferably formed of metals such as iron, nickel, and monel (trademark). Since the hydrogen fluoride in the anode manufacturing mixed solution is consumed by electrolysis during anode manufacturing, it is preferable to supply hydrogen fluoride to the anode manufacturing mixed solution continuously or intermittently during electrolysis during anode manufacturing. The supply of hydrogen fluoride can be supplied to the anode manufacturing mixed solution on the cathode chamber side of the electrolytic cell, or to the anode manufacturing mixed solution on the anode chamber side.

The concentration of hydrogen fluoride in the mixed solution for producing an anode is preferably controlled within a range of -5 mass % to +5 mass % of the reference concentration when the mixed molten salt used in the mixed solution for producing an anode is KF･2HF (hydrogen fluoride concentration: 40.4 mass %) or CsF･2.4HF (hydrogen fluoride concentration: 24.0 mass %), more preferably within a range of -2.5 mass % to +2.5 mass % of the reference concentration, and even more preferably within a range of -1.5 mass % to +1.5 mass % of the reference concentration.

The mixed solution for manufacturing the anode can be obtained by mixing hydrogen fluoride, metal fluoride and potassium hexafluoronickel (IV) acid (K ₂ NiF ₆ ). Potassium hexafluoronickel (IV) acid can be used in commercial form or in a prepared form. Potassium hexafluoronickel (IV) acid can be prepared by mixing potassium fluoride and nickel fluoride (NiF ₂ ) in a molar ratio of 2:1 and treating the mixture at a temperature of 250°C to 450°C in a fluorine gas environment.

Since potassium hexafluoronickel(IV)ate is a solid that exhibits oxidizing properties, it reacts with water to become trivalent nickel potassium fluoride salt (K ₂ NiF ₅ ) or divalent nickel potassium fluoride salt (K ₂ NiF ₄ ). The solubility of potassium pentafluoronickel(III)ate (K ₂ NiF ₅ ) and potassium tetrafluoronickel(II)ate (K ₂ NiF ₄ ) in the mixed solution for manufacturing the anode is lower than that of potassium hexafluoronickel(IV)ate. Therefore, it is preferred to use potassium hexafluoronickel(IV)ate with a low water content. The water content of potassium hexafluoronickel(IV)ate is preferably 0.5% by mass or less, and more preferably 0.3% by mass or less.

The method of including potassium hexafluoronickel (IV) acid in the mixed solution for anode production is not particularly limited, but the mixed solution for anode production can be prepared by mixing metal fluoride in hydrogen fluoride in which potassium hexafluoronickel (IV) acid is dissolved, or by mixing solid potassium hexafluoronickel (IV) acid in a mixture of hydrogen fluoride and metal fluoride. Potassium hexafluoronickel (IV) acid can be added in a specific amount at one time or in batches. The concentration of potassium hexafluoronickel (IV) acid in the mixed solution for anode production is preferably 500 mass ppm or more and 5000 mass ppm or less, and more preferably 1000 mass ppm or more and 4000 mass ppm or less.

The mixed solution for producing the anode may contain potassium hexafluoronickel (IV) acid in an amount exceeding the saturated solubility. When potassium hexafluoronickel (IV) acid is contained in an amount exceeding the saturated solubility, the undissolved solid matter of potassium hexafluoronickel (IV) acid accumulates at the bottom of the electrolytic cell. However, if the method for producing cations for fluorine gas electrolytic synthesis of this embodiment is implemented, the nickel electrolyte in the mixed solution for producing the anode becomes nickel fluoride and adheres to the surface of the anode substrate, thereby reducing the concentration of the nickel electrolyte in the mixed solution for producing the anode. If the mixed solution for producing the anode contains potassium hexafluoronickel (IV) oxide in an amount exceeding the saturated solubility, the undissolved solid potassium hexafluoronickel (IV) oxide dissolves as the nickel electrolyte concentration decreases, thereby maintaining the saturated solubility.

In the method for manufacturing an anode for fluorine gas electrolytic synthesis of the present embodiment, a current is passed between the anode (anodole substrate) and the cathode to perform an anodic polarization treatment on the anode substrate, thereby attaching nickel fluoride to the surface of the anode substrate. However, during the anodic polarization treatment, the anode current density is preferably not less than 0.01 A/ ^cm2 and not more than 0.8 A/ ^cm2 , and the amount of current per unit surface area of the anode substrate is not less than 100 coulombs/ ^cm2 and not more than 5000 coulombs/ ^cm2 .

Even if the anodic current density is less than 0.01A/ ^cm2 , there is no particular problem, and it can be dealt with by extending the treatment time of the anodic polarization treatment. In addition, when the anodic current density is greater than 0.8A/ ^cm2 , the anodic polarization treatment can be completed in a short time, but when the voltage rise is confirmed during the anodic polarization treatment, it is better to take measures such as quickly stopping the current supply in a way that the anodic effect does not occur.

Furthermore, the anode current density is more preferably greater than 0.05 A/cm ² and less than 0.6 A/cm ^2. Furthermore, the anode current density is preferably a constant value, but may be gradually increased from a low value, or may be gradually decreased from a high value. The method for changing the anode current density is not particularly limited.

The amount of current per unit surface area of the anodic substrate is an important value because it is related to the amount of nickel fluoride that functions as a catalyst and is deposited on the surface of the anodic substrate. The amount of current per unit surface area of the anodic substrate is preferably 100 coulombs/ ^cm2 or more. It is not particularly problematic if it is greater than 5000 coulombs/ ^cm2 , but it will increase the processing time of the anodic treatment. Furthermore, the amount of current per unit surface area of the anodic substrate is more preferably 1800 coulombs/ ^cm2 or more.

It is preferred to adjust the current per unit surface area of the anode substrate according to the concentration of potassium hexafluoronickel (IV) oxide in the anode manufacturing mixture. However, when the concentration of potassium hexafluoronickel (IV) oxide is low, the current can be increased, and when the concentration of potassium hexafluoronickel (IV) oxide is high, the current can be reduced.

The concentration of nickel electrolyte (IV) ions in the mixed solution for anode production during the anodic treatment can be measured by ICP emission spectrometry. In order to dissociate nickel hexafluoride (IV) ions in the mixed solution for anode production, it is desirable that the amount of impurities that are easily oxidized in the mixed solution for anode production is small. In particular, if the amount of water in the mixed molten salt before adding potassium hexafluoride (IV) oxide is small, the amount of potassium hexafluoride (IV) oxide added can be reduced, so it is better.

When removing water from the mixed molten salt by electrolysis (dehydration electrolysis), water can be removed by using an electrode such as a conductive diamond electrode, a nickel electrode, or a carbon electrode. When removing water from the mixed molten salt by electrolysis using a carbon electrode, the carbon electrode used in the dehydration electrolysis can be used as an anode substrate in the manufacturing method of the anode for fluorine gas electrolytic synthesis of this embodiment. In this case, since a (CF)n film is formed on the surface of the carbon electrode during dehydration electrolysis, the effect of the cation treatment in the presence of nickel hexafluoride (IV) ions will be less than that when a new carbon electrode is used, but it is not completely ineffective.

In the method for manufacturing an anode for fluorine gas electrolytic synthesis of this embodiment, a current is passed between the anode and the cathode to perform an anodic polarization treatment on the anode substrate, so that nickel fluoride is attached to the surface of the anode substrate. Examples of the nickel fluoride include nickel ions having a valence greater than 2. For example, at least one of nickel trifluoride (NiF ₃ ), nickel pentafluoride (Ni ₂ F ₅ ) and nickel tetrafluoride (NiF ₄ ) can be cited. [Example]

The following examples and comparative examples are presented to more specifically illustrate the present invention. [Comparative Example 1] The Teflon electrolytic cell of FIG. 1 is used to manufacture an anode for electrolytic synthesis of fluorine gas. KF･2HF is used as an electrolyte, and 750 g of KF･2HF is placed in the body of the electrolytic cell. The electrolyte is heated to 85°C, and the anode and cathode are immersed therein. The cathode is a square nickel plate with a length of 5 cm and a width of 5 cm. The anode is a square diamond electrode with a length of 1 cm and a width of 1 cm, and the surface is coated with conductive diamond.

A current was passed between the anode and cathode to perform dehydration electrolysis of the electrolyte. The anode current density was 0.3A/ ^cm2 and the electrolysis time was 2 hours. The gas generated from the anode was diluted with nitrogen, and the fluorine gas was absorbed and removed by a potassium iodide aqueous solution trap. The oxygen concentration in the gas discharged from the potassium iodide aqueous solution trap was measured by gas chromatography. As a result, since no oxygen was detected, it was judged that the dehydration electrolysis was terminated.

Furthermore, since hydrogen fluoride in the electrolyte is consumed by dehydration electrolysis, hydrogen fluoride is intermittently supplied to the electrolyte during dehydration electrolysis so that the concentration of hydrogen fluoride in the electrolyte is within the range of 40 mass % to 43 mass %. The electrolyte prepared by this operation is referred to as dehydrated electrolyte 1.

Next, 0.2 g of nickel fluoride was added to the dehydrated electrolyte 1 in the electrolytic cell and dissolved. The concentration of nickel fluoride in the dehydrated electrolyte 1 was 270 mass ppm, and the amount dissolved in nickel was 160 mass ppm. The amount of nickel fluoride added was determined by measuring the amount of nickel fluoride dissolved in KF･2HF at 85°C in advance (details will be described later).

The color of the dehydrated electrolyte 1 in which nickel fluoride is dissolved is transparent. The nickel concentration in the dehydrated electrolyte 1 is also measured by ICP emission spectrometry, but the result is the same concentration as above. A new carbon electrode serving as an anode is immersed in the dehydrated electrolyte 1 in which nickel fluoride is dissolved, and electrolysis is performed by passing a current of 25 mA between the anode and the cathode for 3 hours. By this electrolysis, a nickel-potassium fluoride film described in patent document 2 is formed on the surface of the carbon electrode. The carbon electrode is a square with a length of 1 cm and a width of 1 cm. The anode current density in the electrolysis was 0.0025A/ ^cm2 , and the current per unit surface area of the carbon electrode was 270 coulombs/ ^cm2 .

Here, the method for determining the amount of nickel fluoride dissolved is described. 100g of KF･2HF is placed in a sealable Teflon container, a specific amount of nickel fluoride powder is added thereto, the container is sealed, and placed in a constant temperature bath at 85°C for 36 hours. However, the contents of the container are occasionally stirred. After 36 hours, the contents of the container are visually observed to confirm whether there is any undissolved nickel fluoride powder.

The results of investigating the solubility by changing the amount of nickel fluoride powder added in various ways showed that when the nickel fluoride powder was added in an amount equivalent to or less than 320 mass ppm, no undissolved matter was observed. In contrast, when the nickel fluoride powder was added in an amount greater than 320 mass ppm, undissolved matter was observed. After removing the undissolved nickel fluoride powder, the nickel concentration in KF･2HF was measured by ICP emission spectrometry, and the maximum nickel concentration was 200 mass ppm.

[Comparative Example 2] The critical current density of the anode of Comparative Example 1 was measured in the electrolyte of Comparative Example 1 using the critical current density measurement method A, and was 0.25 A/cm ² . Since this critical current density is within the range of the critical current density obtained in the blank experiment above, it cannot be said that the anode effect is suppressed. In addition, judging from the color of the electrolyte, it cannot be said that nickel hexafluoride (IV) ions are present.

[Comparative Example 3] After the critical current density measurement in Comparative Example 2 was completed, the anode was removed from the electrolyte and heated to 300°C using a high-temperature inert gas stream without water washing to volatilize the hydrogen fluoride component adsorbed on the carbon electrode. Afterwards, energy dispersive X-ray analysis was performed using a JCM-7000 desktop scanning electron microscope manufactured by JEOL Ltd. to confirm the presence of nickel on the surface of the carbon electrode. However, nickel was not confirmed because it was below the detection limit.

[Comparative Example 4] The dehydrated electrolyte 1 obtained in the same manner as in Comparative Example 1 was placed in the electrolytic cell of Figure 1, and the anode and cathode were immersed in the dehydrated electrolyte 1. The anode was a rectangular nickel plate with a length of 2 cm and a width of 3 cm, and the cathode was a nickel plate. Then, a current of 1A was passed between the anode and the cathode for 26 hours to perform electrolysis, so that nickel was dissolved from the nickel plate into the dehydrated electrolyte 1.

Based on the mass reduction of the nickel plate used for the anode, the nickel concentration in the dehydrated electrolyte 1 was calculated to be 2800 mass ppm, but a precipitate was generated in the dehydrated electrolyte 1. The nickel concentration in the dehydrated electrolyte 1 without the precipitate was determined by ICP emission spectrometry to be 270 mass ppm. The dehydrated electrolyte 1 after electrolysis showed a light yellow-green color.

A new carbon electrode as an anode was immersed in the dehydrated electrolyte 1 from which nickel was leached, and electrolysis was performed by passing a current of 25 mA between the anode and the cathode for 10 hours. By the electrolysis, a nickel-potassium fluoride film described in patent document 2 was formed on the surface of the carbon electrode. The carbon electrode was a square with a length of 1 cm and a width of 1 cm. The anode current density in the electrolysis was 0.0025 A/cm ² , and the current per unit surface area of the carbon electrode was 900 coulombs/cm ² .

The carbon electrode with the nickel-potassium fluoride film formed was taken out from the dehydrated electrolyte 1, and heated to 300°C in an inert gas without washing to volatilize the hydrogen fluoride adsorbed on the carbon electrode. Subsequently, as in Comparative Example 3, the presence of nickel on the surface of the carbon electrode was confirmed, but nickel was not confirmed because it was below the detection limit.

[Comparative Example 5] The dehydrated electrolyte 1 obtained in the same manner as in Comparative Example 1 is placed in the electrolytic cell of FIG. 1, and nickel fluoride is added thereto to dissolve it. Then, an anode is immersed in the dehydrated electrolyte 1 in which nickel fluoride is dissolved. The anode is a square carbon electrode having a length of 1 cm and a width of 1 cm. The carbon electrode is formed of highly oriented thermally decomposed graphite (HOPG ZYH manufactured by Momentive) having a basal surface.

Next, the reference electrode was set to Ni/ _NiF2 and cyclic voltammetry was performed to obtain a cyclic voltammogram. The potential scanning rate was set to 0.4mV/sec. The first run of the obtained cyclic voltammogram is shown in Figure 2. As can be seen from Figure 2, at a potential of 5.5V relative to Ni/ _NiF2 , the current shows a peak value, and at a potential higher than that, the current drops sharply. This behavior is equivalent to the anodic effect.

[Comparative Example 6] The dehydrated electrolyte 1 obtained in the same manner as in Comparative Example 1 was placed in the electrolytic cell of FIG1 , and lithium fluoride (LiF) powder was added thereto and dissolved. Lithium fluoride is an additive believed to have an effect of suppressing the anodic effect. The concentration of lithium fluoride in the dehydrated electrolyte 1 was 1.0 mass %.

A new carbon electrode as an anode is immersed in the dehydrated electrolyte 1 in which lithium fluoride is dissolved, and electrolysis is performed by passing a current of 0.05A between the anode and the cathode for 5 hours. The surface of the carbon electrode is surface treated by this electrolysis. The carbon electrode is a square with a length of 1 cm and a width of 1 cm. The amount of electricity passed per unit surface area of the carbon electrode during the electrolysis is 900 coulombs/ ^cm2 . The dehydrated electrolyte 1 during the electrolysis is stirred with a stirrer to prevent the precipitation of lithium fluoride.

After the surface treatment was completed by electrolysis, the critical current density was measured. That is, a 25 mA direct current was first applied between the anode and cathode for 15 minutes, and then the current was increased by 25 mA every 15 minutes, and the current density before the electrolytic voltage increased sharply was set as the critical current density. As a result, the critical current density was 0.275 A/cm ² .

The critical current density was measured again by performing the exact same operation, and the critical current density was 0.30 A/cm ² . These results show that the addition of lithium fluoride cannot suppress the anodic effect.

[Comparative Example 7] The dehydrated electrolyte 1 obtained in the same manner as in Comparative Example 1 was placed in the electrolytic cell of FIG. 1, and the crystalline powder of potassium hexafluoronickel (IV) acid was added thereto and dissolved. The concentration of potassium hexafluoronickel (IV) acid in the dehydrated electrolyte 1 was 0.3 mass %, and the dissolved amount in terms of nickel was 630 mass ppm. The dehydrated electrolyte 1 after the potassium hexafluoronickel (IV) acid was dissolved was red.

A new square carbon electrode with a length of 1 cm and a width of 1 cm, which is used as an anode, was immersed in a dehydrated electrolyte 1 in which potassium hexafluoronickel (IV) was dissolved, and a metal cathode was immersed at the same time, and left for 6 hours without flowing a current. Then, the critical current density of the anode was measured by critical current density measurement method B. As a result, the critical current density was 0.250 A/cm ² . From this result, it can be seen that the anodic effect cannot be suppressed by simply immersing the carbon electrode in a dehydrated electrolyte 1 in which potassium hexafluoronickel (IV) was dissolved.

[Example 1] The dehydrated electrolyte 1 obtained in the same manner as in Comparative Example 1 was placed in the electrolytic cell of FIG. 1, and the crystalline powder of potassium hexafluoronickel (IV) acid was added thereto and dissolved. The concentration of potassium hexafluoronickel (IV) acid in the dehydrated electrolyte 1 was 0.3 mass %, and the dissolved amount in terms of nickel was 630 mass ppm. The dehydrated electrolyte 1 after the potassium hexafluoronickel (IV) acid was dissolved was red.

A dehydrated electrolyte 1 in which potassium hexafluoronickel (IV) acid is dissolved is used as a mixed solution for anode production. A new carbon electrode (anode substrate) is immersed in the mixed solution for anode production, and a metal cathode is immersed at the same time. Then, a current of 0.1A is passed between the anode and the cathode for 6 hours for electrolysis. By this electrolysis, nickel fluoride is formed on the surface of the carbon electrode of the anode substrate, and an anode for fluorine gas electrolytic synthesis is produced. The carbon electrode is a square with a length of 1 cm and a width of 1 cm. The anode current density in the electrolysis was 0.1A/ ^cm2 , and the current per unit surface area of the carbon electrode was 2160 coulombs/ ^cm2 .

The critical current density of the obtained anode for fluorine gas electrolysis synthesis was measured by critical current density measurement method A. After two measurements, the critical current density was 0.875A/ ^cm2 and 0.825A/ ^cm2 . The manufacturing conditions of the anode for fluorine gas electrolysis synthesis and the measurement results of the critical current density are shown in Table 1.

[Examples 2 to 4] Except that the concentration of potassium hexafluoronickel (IV) oxide in the dehydrated electrolyte 1, the electrolysis (pretreatment) time, and the amount of current applied during electrolysis were changed as shown in Table 1, the anode for fluorine gas electrolysis synthesis was manufactured in the same manner as in Example 1. Then, the critical current density of the obtained anode for fluorine gas electrolysis synthesis was measured by critical current density measurement method A. The measurement was performed once each time. The manufacturing conditions of the anode for fluorine gas electrolysis synthesis and the measurement results of the critical current density are summarized in Table 1.

From the results shown in Table 1, it can be seen that the more potassium hexafluoronickel (IV) acid is dissolved, the greater the critical current density tends to be, but at a concentration of 5000 mass ppm, the increase in critical current density is approximately saturated. In addition, from the results of Examples 2 and 3, it can be seen that the more the amount of current applied during electrolysis, the greater the critical current density, and the critical current density increases more significantly than the blank critical current density.

[Example 5] The anode of Example 4 with the critical current density measured was taken out from the mixed solution for anode production and heated to 300°C in an inert gas to volatilize the hydrogen fluoride adsorbed on the carbon electrode. Then, as in Comparative Example 3, the presence of nickel on the surface of the carbon electrode was confirmed.

[Example 6] The dehydrated electrolyte 1 obtained in the same manner as in Comparative Example 1 is placed in the electrolytic cell of FIG. 1, and the crystalline powder of potassium hexafluoronickel (IV) acid is added thereto to dissolve it. The concentration of potassium hexafluoronickel (IV) acid in the dehydrated electrolyte 1 is 0.1 mass %. Then, the anode is immersed in the dehydrated electrolyte 1 in which potassium hexafluoronickel (IV) acid is dissolved. The anode is a square carbon electrode having a length of 1 cm and a width of 1 cm. The carbon electrode is formed of highly oriented thermally decomposed graphite (HOPG ZYH manufactured by Momentive) having a basal surface.

Next, the reference electrode was set to Ni/ _NiF2 and cyclic voltammetry was performed to obtain a cyclic voltammogram. The potential scanning rate was set to 0.4mV/sec. The first run of the obtained cyclic voltammogram is shown in Figure 2. As can be seen from Figure 2, the current shows a peak value at a potential of 5.5V relative to Ni/ _NiF2 , and the peak current value is 2.7 times greater than the peak current value of Comparative Example 5. From this result, it can be seen that although the anodic effect occurs, by adding potassium hexafluoronickel (IV) acid, the current value that can flow until the anodic effect occurs is larger.

Furthermore, the results of micro-Raman spectroscopy showed that there was no difference in the peak intensity of the D band and the G band of potassium hexafluoronickel (IV) oxide and nickel fluoride, so the amount of intercalation compound inserted did not change between the two. Therefore, the difference in the peak current value is considered to be due to the fact that the nickel fluoride with a valence higher than 2 due to the addition of potassium hexafluoronickel (IV) oxide acts as a catalyst for the fluorine gas generation reaction, thereby inhibiting the formation of a (CF)n film on the carbon electrode surface.

[Example 7] An anode for fluorine gas electrolytic synthesis was produced in the same manner as in Example 4. Next, the critical current density of the obtained anode for fluorine gas electrolytic synthesis was measured by critical current density measurement method B, and was found to be 0.850 A/cm ² .

[Example 8] The dehydrated electrolyte 1 obtained in the same manner as in Comparative Example 1 was placed in the electrolytic cell of FIG. 1 , and the crystalline powder of potassium hexafluoronickel (IV) acid was added thereto and dissolved. The concentration of potassium hexafluoronickel (IV) acid in the dehydrated electrolyte 1 was 0.4 mass %, and the dissolved amount in terms of nickel was 840 mass ppm. The dehydrated electrolyte 1 after the potassium hexafluoronickel (IV) acid was dissolved was red.

Next, the dehydrated electrolyte 1 in which potassium hexafluoronickel (IV) acid is dissolved is used as a mixed solution for anode manufacturing, and a new carbon electrode as an anode (anode substrate) is immersed in the mixed solution for anode manufacturing, and a metal cathode is immersed at the same time. Then, a current of 0.3A is passed between the anode and the cathode for 4.5 hours for electrolysis. By this electrolysis, nickel fluoride is formed on the surface of the carbon electrode of the anode substrate, and an anode for fluorine gas electrolytic synthesis is manufactured. The carbon electrode is a square with a length of 1 cm and a width of 1 cm. The anode current density in the electrolysis was 0.3A/ ^cm2 , and the current per unit surface area of the carbon electrode was 4860 coulombs/ ^cm2 .

After the completed fluorine gas electrolytic synthesis anode (the first fluorine gas electrolytic synthesis anode) is taken out from the electrolytic cell, the new carbon electrode serving as the anode is immersed in the dehydrated electrolyte 1 in which potassium hexafluoronickel (IV) acid is dissolved, and the same operation as above is performed to manufacture the fluorine gas electrolytic synthesis anode (the second fluorine gas electrolytic synthesis anode). The same operation is then repeated to manufacture the third to sixth fluorine gas electrolytic synthesis anodes in sequence, and a total of six fluorine gas electrolytic synthesis anodes are manufactured.

After the completed sixth fluorine gas electrolytic synthesis anode is taken out from the electrolytic cell, the crystalline powder of potassium hexafluoronickel (IV) acid is further added to the dehydrated electrolyte 1 and dissolved. The amount of potassium hexafluoronickel (IV) acid added is such that the concentration of potassium hexafluoronickel (IV) acid in the dehydrated electrolyte 1 increases by 0.1 mass %.

A new carbon electrode as an anode was immersed in a dehydrated electrolyte 1 in which potassium hexafluoronickel (IV) was dissolved, and electrolysis was performed by passing a current of 0.3A between the anode and the cathode for 4.5 hours. By the electrolysis, nickel fluoride was formed on the surface of the carbon electrode of the anode substrate, and the seventh anode for fluorine gas electrolysis synthesis was manufactured. The carbon electrode was a square with a length of 1 cm and a width of 1 cm. The anode current density in the electrolysis was 0.3A/ ^cm2 , and the current per unit surface area of the carbon electrode was 4860 coulombs/ ^cm2 .

The critical current density of the seventh fluorine gas electrolytic synthesis anode manufactured as described above was measured by critical current density measurement method B. The results are shown in Table 2. As can be seen from the results in Table 2, for the first to sixth fluorine gas electrolytic synthesis anodes, there is a tendency for the critical current density to decrease with the manufacturing order. This is considered to be because the nickel electrolyte of potassium hexafluoronickel (IV) acid adheres to the electrode surface by electrolysis, which corresponds to the gradual decrease in the concentration of hexafluoronickel (IV) ions in the electrolyte 1 after dehydration.

For the seventh fluorine gas electrolytic synthesis anode, the critical current density is restored again. It can be considered that the nickel hexafluoride (IV) ion concentration in the electrolyte 1 after dehydration is increased by adding potassium nickel hexafluoride (IV) acid, and the nickel hexafluoride (IV) ions are attached to the carbon electrode surface by electrolysis.

The color of the electrolyte 1 after dehydration at the end of the production of the sixth fluorine gas electrolytic synthesis anode is lighter than the initial red color, but still red, so it is believed that hexafluoronickel (IV) ions still exist, but their concentration is decreasing. Example 8 shows that the higher the hexafluoronickel (IV) ion concentration, the more hexafluoronickel (IV) ions are attached to the surface of the carbon electrode.

1: Dehydrated electrolyte 2: Additional test electrode 10: Anode manufacturing mixture 11: Main body 12: Cover 13: Isolation wall 21: Anode 22: Cathode 32: Thermometer 33: Fluorine gas extraction piping 34: Hydrogen gas extraction piping

[Figure 1] is a diagram illustrating an example of an electrolytic cell in which the present embodiment can be implemented in the method for manufacturing an anode for electrolytic synthesis of fluorine gas. [Figure 2] is a diagram showing a cyclic voltammogram obtained by cyclic voltammography.

10: Mixed liquid for anode manufacturing

11: Body

12: Cover

13: Isolation wall

21: Yang pole

22: cathode

31: Hydrogen fluoride supply piping

32: Thermometer

33: Fluorine gas extraction piping

34: Hydrogen extraction piping

Claims (8)

A method for producing an anode for electrolytic synthesis of fluorine gas can be used for electrolyzing an electrolyte containing hydrogen fluoride and a metal fluoride to electrolytically synthesize fluorine gas, and the method comprises an anodic treatment step, wherein an anodic substrate having a carbon material as an anode and a cathode are immersed in a mixed solution for producing an anode containing hydrogen fluoride, a metal fluoride and potassium hexafluoronickel (IV) acid, and a current is passed between the anode and the cathode to perform an anodic treatment on the anode substrate, thereby attaching nickel fluoride to the surface of the anode substrate. A method for producing an anode for fluorine gas electrolytic synthesis as claimed in claim 1, wherein the concentration of the potassium hexafluoronickel (IV) oxide in the mixed solution for producing the anode is 500 mass ppm or more and 5000 mass ppm or less. The method for manufacturing an anode for fluorine gas electrolytic synthesis as claimed in claim 1 or 2, wherein in the aforementioned anode polarization treatment, the anode current density is not less than 0.01A/ ^cm2 and not more than 0.8A/ ^cm2 , and the current per unit surface area of the aforementioned anode substrate is not less than 100 coulomb/ ^cm2 and not more than 5000 coulomb/ ^cm2 . A method for producing an anode for fluorine gas electrolytic synthesis as claimed in claim 1 or 2, wherein the nickel fluoride has nickel ions with a valence greater than 2. A method for producing an anode for fluorine gas electrolytic synthesis as claimed in claim 1 or 2, wherein the nickel fluoride is at least one of nickel trifluoride, nickel pentafluoride and nickel tetrafluoride. A method for producing an anode for fluorine gas electrolytic synthesis as claimed in claim 1 or 2, wherein the metal fluoride is at least one of potassium fluoride and cesium fluoride. A method for producing an anode for fluorine gas electrolytic synthesis as claimed in claim 1 or 2, wherein the ratio of the molar amount of the aforementioned hydrogen fluoride contained in the aforementioned mixed liquid for producing the anode to the molar amount of the aforementioned metal fluoride is not less than 1.6 and not more than 3.2. A method for manufacturing an anode for fluorine gas electrolytic synthesis as claimed in claim 1 or 2, wherein the cathode is an electrode formed of at least one of iron, nickel, copper and copper-nickel alloy.

Images