CN119234057A

CN119234057A - Method for manufacturing anode for electrolytic synthesis of fluorine gas

Info

Publication number: CN119234057A
Application number: CN202480002276.5A
Authority: CN
Inventors: 福地阳介; 藤克昭; 小林浩; 小黑慎也
Original assignee: Resonac Corp
Current assignee: Resonac Corp
Priority date: 2023-04-27
Filing date: 2024-03-11
Publication date: 2024-12-31
Also published as: TW202444969A; WO2024224841A1

Abstract

A method for manufacturing an anode that is not prone to produce an anode effect even when an electrolyte containing hydrogen fluoride and a metal fluoride is electrolyzed to electrolytically synthesize fluorine gas is provided. The method for manufacturing an anode for electrolytic synthesis of fluorine gas comprises an anode polarization treatment step. The anode polarization treatment step is a step of immersing an anode substrate having a carbon material as an anode (21) and a cathode (22) in a mixed solution (10) for manufacturing an anode containing hydrogen fluoride, a metal fluoride and potassium hexafluoronickel (IV) acid as an anode, and immersing a cathode (22), passing a current between the anode (21) and the cathode (22) to perform an anode polarization treatment on the anode substrate, and attaching nickel fluoride to the surface of the anode substrate.

Description

Method for manufacturing anode for electrolytic synthesis of fluorine gas

Technical Field

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

Background

Fluorine gas can be synthesized by electrolyzing an electrolyte containing hydrogen fluoride and a metal fluoride. In the case of synthesizing fluorine gas by electrolysis, 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 being hydrogen fluoride: potassium fluoride=2:1) is generally used as an electrolyte, and electrolysis is performed at an electrolysis temperature of 80 ℃ to 100 ℃ using a carbon electrode as an anode.

However, when a carbon electrode is used as the anode, there is a problem in that the anode voltage increases and the flow of current decreases during electrolysis (hereinafter, sometimes referred to as "anode effect"), and thus, the electrolysis may not be performed continuously and stably for a long period of time. This anode effect is a phenomenon in which, when a discharge reaction of fluorine ions (fluorine-containing ions) occurs on the surface of the carbon electrode, graphite fluoride ((CF) n) or the like having extremely low surface energy is formed on the surface of the carbon electrode, and therefore, the electrolyte and the surface of the carbon electrode are difficult to contact, and the flow of current is deteriorated.

Patent document 1 discloses an electrode in which a metal fluoride is supported in pores of porous carbon as a carbon electrode for suppressing an anode effect. Patent document 2 discloses a technique of coating a film containing a nickel potassium fluoride compound on the surface of a carbon electrode in order to prevent the formation of graphite fluoride.

Prior art literature

Patent document 1 Japanese patent publication No. 3037464

Patent document 2 Japanese patent publication No. 5772102

Disclosure of Invention

In the case of performing electrolysis of an electrolyte using a carbon electrode as an anode, a technique of more sufficiently suppressing the occurrence of an anode effect is desired.

The present invention addresses the problem of providing a method for producing an anode which is less likely to cause an anode effect even when used in the electrolysis of fluorine gas by the electrolysis of an electrolyte containing hydrogen fluoride and a metal fluoride.

In order to solve the above-described problems, one embodiment of the present invention is as described in the following [1] to [8 ].

[1] A method for producing an anode for electrolytic synthesis of fluorine gas, which can be used when an electrolytic solution containing hydrogen fluoride and a metal fluoride is electrolyzed to synthesize fluorine gas, comprising an anodic polarization treatment step,

In the anodizing treatment step, an anode substrate having a carbon material is immersed in a mixed solution for anode production containing hydrogen fluoride, a metal fluoride, and potassium hexafluoro-nickel (IV) acid as an anode, and a cathode is immersed, and a current is passed between the anode and the cathode, so that the anode substrate is anodized, and nickel fluoride is adhered to the surface of the anode substrate.

[2] The method for producing an anode for electrolytic synthesis of fluorine gas according to [1], wherein the concentration of potassium hexafluoronickel (IV) acid in the mixed solution for producing an anode is 500 mass ppm or more and 5000 mass ppm or less.

[3] The method for producing an anode for electrolytic synthesis of fluorine gas according to [1] or [2], wherein in the anodic polarization treatment, the anode current density is 0.01A/cm ² or more and 0.8A/cm ² or less, and the current flow per unit surface area of the anode substrate is 100 coulomb/cm ² or more and 5000 coulomb/cm ² or less.

[4] The method for producing an anode for electrolytic synthesis of fluorine gas according to any one of [1] to [3], wherein the nickel fluoride has nickel ions having a valence of more than 2.

[5] The method for producing an anode for electrolytic synthesis of fluorine gas according to any one of [1] to [3], wherein the nickel fluoride is at least 1 of nickel trifluoride, nickel pentafluoride and nickel tetrafluoride.

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

[7] The method for producing an anode for electrolytic synthesis of fluorine gas according to any one of [1] to [6], wherein a ratio of a molar amount of hydrogen fluoride contained in the anode production mixture to a molar amount of the metal fluoride is 1.6 or more and 3.2 or less.

[8] The method for producing an anode for electrolytic synthesis of fluorine gas according to any one of [1] to [7], wherein the cathode is an electrode formed of at least 1 of iron, nickel, copper and copper-nickel alloy.

According to the present invention, an anode which is less likely to cause an anode effect even when an electrolyte containing hydrogen fluoride and a metal fluoride is electrolyzed to synthesize fluorine gas by electrolysis can be produced.

Drawings

Fig. 1 is a diagram illustrating an example of an electrolytic cell in which the method for manufacturing an anode for electrolytic synthesis of fluorine gas according to the present embodiment can be performed.

Fig. 2 is a graph showing a cyclic voltammogram obtained by cyclic voltammetry.

Detailed Description

An embodiment of the present invention will be described below. The present embodiment shows an example of the present invention, and the present invention is not limited to the present embodiment. Various changes and modifications may be made to the present embodiment, and those modifications and improvements are also included in the present invention.

The method for producing an anode for electrolytic synthesis of fluorine gas according to the present embodiment is a method for producing an anode that can be used when electrolyzing an electrolyte containing Hydrogen Fluoride (HF) and a metal fluoride to synthesize fluorine gas (F ₂) by electrolysis, and includes an anodic polarization treatment step. The anodic polarization treatment step is a step of immersing an anode substrate having a carbon material as an anode in a mixed solution for anode production containing hydrogen fluoride, a metal fluoride and potassium hexafluoro (IV) nickel (K ₂NiF₆), immersing a cathode in the mixed solution, and conducting anodic polarization treatment of the anode substrate by passing a current between the anode (anode substrate) and the cathode to adhere the nickel fluoride to the surface of the anode substrate.

The anode produced by the method for producing an anode for electrolytic synthesis of fluorine gas according to the present embodiment (hereinafter also referred to as "anode for electrolytic synthesis of fluorine gas according to the present embodiment") is anodized in an anodizing treatment, whereby nickel fluoride (e.g., a film of nickel fluoride is formed on the surface of an anode substrate) adheres to the surface of the anode substrate. Therefore, the anode for electrolytic synthesis of fluorine gas according to the present embodiment is less likely to cause an anode effect even when used for electrolytic synthesis of fluorine gas by electrolysis of an electrolyte solution containing hydrogen fluoride and a metal fluoride.

For example, when an electrolytic solution (for example, kf·2hf) containing hydrogen fluoride and a metal fluoride and containing no potassium hexafluoro-nickel (IV) acid is electrolyzed to synthesize fluorine gas, if the anode for fluorine gas electrolytic synthesis of the present embodiment is used, electrolysis in which the anode effect is suppressed, such as an anode current density of 0.01A/cm ² or more and 0.8A/cm ² or less, can be performed. As a result, the frequency of interruption and stoppage of the operation of electrolyzing the synthetic fluorine gas can be reduced, and the electrolysis can be performed at a high current density, so that the production cost of the fluorine gas can be reduced.

Here, the anode effect will be described. If the carbon anode is anodized in an electrolyte containing hydrogen fluoride and a metal fluoride, a discharge reaction of fluorine ions (hereinafter also sometimes referred to as "HF ₂ ^－") generally occurs on the surface of the carbon anode to generate fluorine atoms. The generated fluorine atoms are coupled to generate fluorine gas, but a part of the fluorine atoms may be bonded to carbon of the carbon anode, and a hydrophobic graphite fluoride film called a (CF) n film may be formed on the surface of the carbon anode. As a result, the electrolyte is less likely to contact the surface of the carbon anode during electrolysis, so that a discharge reaction of fluoride ions is less likely to occur on the surface of the carbon anode, and the true current density increases, so that the anode voltage increases and the flow of current decreases. This is the mechanism of the anodic effect.

Patent document 2 discloses a technique of coating a surface of a carbon electrode with a film other than a (CF) n film. Specifically, an electrode having a structure in which a part of the surface of a carbon substrate is coated with a conductive diamond layer and the other part is not coated with a conductive diamond layer is treated by coating a film containing a nickel potassium fluoride compound, and only the surface not coated with a conductive diamond layer is coated with a film containing a nickel potassium fluoride compound.

Since the surface of the carbon substrate is coated with the conductive diamond layer and the film containing the nickel potassium fluoride compound, it becomes difficult to form a (CF) n film on the surface. Examples of the potassium nickel fluoride compound include potassium hexafluoro nickel (IV) acid (K ₂NiF₆).

Patent document 2 describes a method of adding nickel fluoride to an electrolytic solution or a method of eluting nickel from the material of an electrolytic cell under the conditions of forming a film containing a potassium nickel fluoride compound, and the concentration of nickel ions in the electrolytic solution is 10ppm to 5%, particularly preferably 30ppm to 1000ppm. In addition, the current density is not less than 0.001A/cm ² and not more than 0.05A/cm ², and the electrolysis time is not less than 0.1 hours and not more than 10 hours.

Patent document 2 describes that if the current density is higher than 0.05A/cm ², a graphite fluoride layer is easily formed before the film formation of the nickel potassium fluoride-containing compound, and that if the electrolysis time is longer than 10 hours, the consumption of electric power and the reduction of productivity are generated, which is not preferable.

Therefore, as a method for allowing nickel ions to coexist in an electrolyte, a method of adding nickel fluoride (NiF ₂, molecular weight 96.7 g/mol) to an electrolyte has been attempted. 100g of KF.2HF solution was added to a Teflon (registered trademark) closed container stored in a constant temperature bath and kept at 90℃and 0.165g 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 ppm by mass. Then, the liquid in the closed vessel was stirred from time to time and left to stand at 90℃for 36 hours. Since the dissolved residual nickel fluoride powder was confirmed at the bottom of the closed vessel after the storage, the concentration of nickel in the KF 2HF solution was measured by inductively coupled plasma emission spectrometry (ICP emission spectrometry) after the removal of the dissolved residual nickel fluoride powder, and as a result, it was 164 mass ppm.

Patent document 2 describes a method of eluting nickel from the material of an electrolytic cell as a method of allowing nickel ions to coexist in an electrolytic solution. If the material of the electrolytic cell is in a state of having a potential (potential greater in the forward direction) higher than the dissolution potential of nickel (about 0.2 to 0.3v vs. sep), nickel forming the electrolytic cell becomes ion-dissolved in the electrolyte.

In order to facilitate dissolution of nickel, the KF.2HF electrolyte at 85 ℃ in the electrolytic tank was electrolyzed with a constant current using nickel forming the electrolytic tank as an anode. The mass of the nickel anode in the electrolysis was periodically measured, and the decrease in nickel was measured. The electrolysis was continued until the nickel reduction amount reached an amount such that the concentration of nickel in the KF 2HF electrolyte reached 1000 mass ppm due to the dissolution of nickel. Since a sludge-like deposit was observed at the bottom of the electrolytic cell, the nickel concentration in the KF 2HF electrolyte was measured by ICP emission spectrometry after removing the sludge-like deposit, and found to be 282 mass ppm.

As described above, as a method of allowing nickel ions to coexist in the electrolytic solution, there are a method of adding nickel fluoride to the electrolytic solution and a method of eluting nickel from the material of the electrolytic tank, but in these methods, the limit of the concentration of nickel dissolved in the electrolytic solution is about 200 to 300 mass ppm.

In contrast, potassium hexafluoro-nickel (IV) acid can be dissolved in anhydrous hydrogen fluoride by 10 mass% or more. The solubility of potassium hexafluoro-nickel (IV) acid with respect to kf.2hf was about 0.5 mass% with a slight change in temperature, and the concentration of dissolved nickel was about 1000 mass ppm. Therefore, more nickel electrolyte (nickel ions of 2, 3,4, etc.) can be dissolved in KF 2HF electrolyte than in the method described in patent document 2.

By the method of adding nickel fluoride to an electrolyte described in patent document 2, it was found that the effect of suppressing the occurrence of the anode effect was poor by measuring the critical current density by dissolving nickel fluoride in KF 2HF solution until 160 mass ppm close to the saturation solubility. Details are described later in comparative example 1. Further, the critical current density is the current density before the electrolytic voltage sharply rises. Since the phenomenon in which the electrolytic voltage sharply rises corresponds to the anode effect, it can be explained that the larger the critical current density is, the more difficult the electrode state causing the anode effect is.

Next, the properties of the dissolved nickel electrolyte will be described. Patent document 2 describes "nickel ions form high-valence metal ions by allowing nickel ions to coexist in molten salt". Therefore, it can be interpreted that nickel is dissolved as 2-valence nickel ions in an electrolyte.

In the technique disclosed in patent document 2, if the mechanism of coating the electrode with potassium hexafluoronickel (IV) is considered, it is considered that nickel ions of valence 2 (NiF ₄ ^2-) diffuse down on the surface of the electrode, react with fluorine gas or fluorine atoms generated on the surface of the electrode, oxidize nickel ions of valence 2 into nickel ions of valence 4 (NiF ₆ ^2-), and diffuse down again on the surface of the electrode. It can be explained that potassium ions and 4-valent nickel ions existing in the vicinity of the electrode combine and precipitate on the surface of the electrode, thereby coating potassium hexafluoro-nickel (IV) acid.

It can be easily predicted that the amount of the 2-valent nickel ions dissolved in the KF.2HF electrolyte is about 200 to 300 mass ppm, and therefore the amount of the 4-valent nickel ions generated by the reaction with fluorine gas or fluorine atoms is very small. Therefore, it is considered that according to the technique disclosed in patent document 2, formation of a potassium hexafluoronickel (IV) acid film hardly occurs.

In addition, in other mechanisms, there is considered a possibility that a solid of nickel fluoride (NiF ₂) is formed by a discharge reaction of 2-valent nickel ions on the carbon surface, and nickel fluoride on the carbon surface reacts with generated fluorine gas to form a nickel fluoride compound in an expensive state. However, in general, it is expected that the reaction of nickel fluoride and fluorine gas to form nickel tetrafluoride (NiF ₄) requires a high reaction temperature of 250 to 450 ℃.

In either reaction route, in order to bring the dissolved 2-valent nickel ions into a high-valent state, it is necessary to interact with fluorine gas generated in the electrode reaction, and the high-valent nickel ions cannot be generated unless fluorine gas is generated on the electrode. The formation of fluorine gas on the surface of the electrode means that a (CF) n film is also formed.

In this embodiment, the properties of the nickel electrolyte dissolved in the mixed solution for anode production are different from those of the technique disclosed in patent document 2, and therefore the effect on the carbon electrode surface is also different. In the method for producing an anode for electrolytic synthesis of fluorine gas according to the present embodiment, since the anode substrate is anodized in the mixed liquid for anode production in which free nickel (IV) hexafluorooxide ions are present, the nickel (IV) hexafluorooxide ions directly react with the surface of the anode substrate regardless of the generation of fluorine gas by the electrode reaction. Thus, the film formed on the carbon surface shows a completely different behavior from the case of the technique disclosed in patent document 2.

In order to make free nickel (IV) hexafluorooxide ions exist in the mixed solution for anode production, it is preferable that the amount of impurities which are easily oxidized in the mixed solution for anode production be small. Examples of impurities which are easily oxidized include water, sulfuric acid derived from a raw material, and silicon compounds. For example, if water is present in the mixed solution for anode production, nickel (IV) hexafluorooxide ions oxidize water, and thus nickel in the nickel (IV) hexafluorooxide ions is reduced to a low-cost state, and it is difficult to exhibit the effect of the present invention.

In the method for producing an anode for electrolytic synthesis of fluorine gas according to the present embodiment, potassium hexafluoro-nickel (IV) acid is contained in the mixture liquid for producing an anode, but a metal fluoride complex salt capable of dissociating hexafluoro-nickel (IV) ions may be used instead of potassium hexafluoro-nickel (IV) acid. Examples of such metal fluoride complex salts include cesium hexafluoro-nickel (IV) acid and rubidium hexafluoro-nickel (IV) acid.

When potassium hexafluoro (IV) acid of red crystal powder is added to and dissolved in a transparent KF 2HF solution, if water is present in the KF 2HF solution, the potassium hexafluoro (IV) acid reacts with water, and thus the red color of the potassium hexafluoro (IV) acid disappears, and the mixed solution for anode production shows a cloudy color. However, if free nickel (IV) hexafluoroions are present in the mixed liquid for anode production, the mixed liquid for anode production changes from pale pink to dark red as the concentration of nickel (IV) hexafluoroions increases. If free nickel (IV) hexafluoro ions are not present in the mixed solution for anode production, the effect of the present invention is hardly exhibited.

The method for producing the anode for electrolytic synthesis of fluorine gas according to the present embodiment may be carried out using an electrolytic cell, for example. Examples of the electrolytic cell include an electrolytic cell used when an electrolytic solution containing hydrogen fluoride and a metal fluoride is electrolyzed to synthesize fluorine gas. The details are described in the examples, and for example, the anode for electrolytic synthesis of fluorine gas according to the present embodiment can be produced using an electrolytic cell made of teflon as shown in fig. 1.

The structure of the teflon-made electrolytic cell of fig. 1 will be briefly described. In the electrolytic cell of fig. 1, a main body 11 for accommodating the anode-producing mixed solution 10 is made of teflon. The cover 12 of the main body 11 is made of a transparent acrylic plate, and the color of the anode manufacturing liquid mixture 10 in the main body 11 can be checked. A teflon partition wall 13 is provided in the electrolytic cell, 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.

Although not shown in fig. 1, an inert gas supply pipe for supplying inert gas into the electrolytic cell is connected to the electrolytic cell so that the gas phase portion in the electrolytic cell can be diluted with inert gas such as nitrogen gas. A hydrogen fluoride supply pipe 31 for supplying hydrogen fluoride to the mixed solution 10 for anode production 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 generated by the anode 21 and the cathode 22 from the electrolytic cell are connected to the electrolytic cell, respectively.

The electrolytic cell is provided with an external heater (not shown), and the anode manufacturing liquid mixture 10 in the main body 11 can be heated to a temperature of 85 ℃. The electrolytic cell is provided with a thermometer 32 such as a thermocouple, and the temperature of the anode manufacturing liquid mixture 10 in the main body 11 can be measured. The electrolytic cell is provided with a stirring device, not shown, and can stir the anode manufacturing mixed solution 10 in the main body 11.

The mixed solution 10 for producing an anode is a mixed solution containing hydrogen fluoride, a metal fluoride, and potassium hexafluoro-nickel (IV) acid, and is KF 2HF in which potassium hexafluoro-nickel (IV) acid is dissolved, for example. The electrolytic cell includes an anode 21 and a cathode 22, and both the anode 21 and the cathode 22 are immersed in the mixed solution 10 for anode production. The anode 21 is an anode substrate having a carbon material, and is, for example, a plate made of amorphous carbon. The cathode 22 is a metal plate made of a metal such as nickel. The electrolytic cell is provided in the drying box, and prevents water in the atmosphere from being dissolved in the mixed liquid 10 for anode production in the main body 11.

[ Post-dehydration electrolyte 1 prepared by carrying out dehydration electrolysis 1]

The electrolytic bath of fig. 1 was used to carry out dehydration electrolysis of an electrolyte solution which is a mixture of hydrogen fluoride and potassium fluoride. The anode current density was 0.3A/cm ² and the electrolysis time was 2 hours. Further, as the anode, a conductive diamond electrode coated with a conductive diamond was used. The anode was square with a length of 1cm and a width of 1 cm.

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

The gas generated from the anode was diluted with nitrogen gas, and the fluorine gas was absorbed and removed by a potassium iodide aqueous solution trap, and the oxygen concentration in the gas discharged from the potassium iodide aqueous solution trap was measured by gas chromatography. As a result, since oxygen was not 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, and the concentration of hydrogen fluoride in the electrolyte is set to a range of 40 mass% to 43 mass%. Such an operation is referred to as dehydration electrolysis 1, and an electrolyte prepared by performing dehydration electrolysis 1 is referred to as post-dehydration electrolyte 1.

[ Potassium hexafluoro Nickel (IV) acid Potassium addition electrode 1]

The dehydrated electrolyte 1 was added with crystal powder of potassium hexafluoro-nickel (IV) acid so that the concentration of potassium hexafluoro-nickel (IV) acid in the dehydrated electrolyte 1 was 0.1 mass%. The concentration of nickel in the dehydrated electrolyte 1 was 210 mass ppm. When potassium hexafluoro-nickel (IV) acid is added, a small amount of reaction occurs in potassium hexafluoro-nickel (IV) acid, but the crystal powder of potassium hexafluoro-nickel (IV) acid is completely dissolved, and the electrolyte 1 turns light red after dehydration.

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

A square carbon electrode (corresponding to an anode base material) having a length of 1cm and a width of 1cm was immersed in the dehydrated electrolyte 1 in which the potassium hexafluoro-nickel (IV) acid was dissolved, to serve as an anode, and a metal cathode was immersed. Then, a current of 0.1A was applied between the anode and the cathode for 1 hour, thereby performing surface treatment (corresponding to anodic polarization treatment) of the carbon electrode.

The current flow per unit surface area of the carbon electrode in the surface treatment was 360 coulomb/cm ². The Carbon electrode was a grade ABR manufactured by SGL Carbon corporation, and a new product (product not used for electrolysis) was used. The carbon electrode subjected to such surface treatment is referred to as a potassium hexafluoro nickel (IV) acid addition electrode 1. The "Carbon electrode" hereinafter is a new product of grade ABR (product not used for electrolysis) manufactured by SGL Carbon company unless otherwise specified.

Patent document 2 additionally provided with a test electrode 1

A rectangular nickel plate having a length of 2cm and a width of 3cm was immersed in the dehydrated electrolyte 1 as an anode, and a metal cathode was immersed. Then, electrolysis was performed by passing a current of 0.6A for 16 hours between the anode and the cathode, and nickel was eluted from the nickel plate into the dehydrated electrolyte 1. The dehydrated electrolyte 1 after electrolysis is light yellow green.

The nickel concentration in the dehydrated electrolyte 1 in which nickel was eluted was 1000 mass ppm based on the mass reduction amount of the nickel plate, but a precipitate was generated in the dehydrated electrolyte 1 in which nickel was eluted, and the nickel concentration in the dehydrated electrolyte 1 containing no precipitate was measured by ICP emission spectrometry, and was 220 mass ppm.

A square carbon electrode 1cm long and 1cm wide was immersed in the nickel-eluted electrolyte 1 as an anode, and a metal cathode was immersed. Then, electrolysis was performed by passing a current of 25mA between these anodes and cathodes for 3 hours. By this electrolysis, a nickel potassium fluoride film was formed on the surface of the carbon electrode. The current flow per unit surface area of the carbon electrode in the electrolysis was 270 coulomb/cm ². The current density in the electrolysis was 0.0025A/cm ².

Since the carbon electrode having the nickel potassium fluoride film formed in this way is the electrode described in patent document 2, this carbon electrode is referred to as an additional test electrode 1 in patent document 2.

Patent document 2 additionally provided with a test electrode 2

A rectangular nickel plate having a length of 2cm and a width of 3cm was immersed in the dehydrated electrolyte 1 as an anode, and a metal cathode was immersed. Then, the electrolysis was performed by passing a current of 1A between these anode and cathode for 26 hours, and nickel was eluted from the nickel plate into the dehydrated electrolyte 1. The dehydrated electrolyte 1 after electrolysis exhibits yellowish green (green is slightly more intense than the color of the dehydrated electrolyte 1 in the case where the test electrode 1 is added to the electrolyte in patent document 2).

The nickel concentration in the dehydrated electrolyte 1 in which nickel was eluted was 2700 mass ppm based on the mass reduction amount of the nickel plate, but a precipitate was generated in the dehydrated electrolyte 1 in which nickel was eluted, and the nickel concentration in the dehydrated electrolyte 1 containing no precipitate was measured by ICP emission spectrometry, and as a result, was 280 mass ppm.

A square carbon electrode 1cm long and 1cm wide was immersed in the dehydrated electrolyte 1 to obtain an anode, and a metal cathode was immersed. Then, electrolysis was performed by passing a current of 25mA between these anodes and cathodes for 3 hours. By this electrolysis, a nickel potassium fluoride film was formed on the surface of the carbon electrode. The current flow per unit surface area of the carbon electrode in the electrolysis was 270 coulomb/cm ². The current density in the electrolysis was 0.0025A/cm ².

Since the carbon electrode having the nickel potassium fluoride film formed in this way is the electrode described in patent document 2, this carbon electrode is referred to as a test electrode 2 added to patent document 2.

[ Measurement of blank critical current Density ]

A square carbon electrode 1cm long and 1cm wide was immersed in the dehydrated electrolyte 1 as an anode, and a metal cathode was immersed, and the critical current density was measured. Hereinafter, the critical current density measured by this method is referred to as a blank critical current density, and represents a critical current density in a state where no treatment is performed on the electrode.

The critical current density was measured as follows. A direct current of 25mA was initially applied between the anode and the cathode for 15 minutes, and a direct current of 50mA was applied thereafter for 15 minutes. Further, the current was increased by 25mA every 15 minutes of energization, and the current density before the electrolytic voltage was rapidly increased was set as the critical current density.

Since the phenomenon in which the electrolytic voltage rapidly increases corresponds to the anode effect, it can be determined that the electrode state is such that the anode effect is less likely to occur as the critical current density increases. The critical current density shown below is a value obtained by the present measurement method unless otherwise specified. After measuring the critical current density, the anode is removed from the cell.

By the above measurement method, the critical current density (blank critical current density) was measured for the dehydrated electrolyte 1 containing no nickel ion species. The carbon electrode used in the measurement is a carbon electrode which is not used for measurement of critical current density and electrolysis. The carbon electrode was replaced with a new one 6 times, and the critical current density was measured 6 times, wherein the average value of the measured values was 0.29A/cm ², the maximum value was 0.35A/cm ², and the minimum value was 0.225A/cm ².

[ Measurement of critical current Density of treatment electrode ]

The method of directly measuring the critical current density without taking out the anode produced by the method of producing an anode for electrolytic synthesis of fluorine gas according to the present embodiment from the anode production mixed liquid used at the time of production is referred to as a critical current density measurement method a.

The anode produced by the method for producing an anode for electrolytic synthesis of fluorine gas according to the present embodiment is taken out of the mixed solution for anode production used at the time of production, immersed in the dehydrated electrolyte 1 without nickel electrolyte added or the dehydrated electrolyte 1 without nickel electrolyte added, and the method for measuring the critical current density is referred to as a critical current density measurement method B.

To the potassium hexafluoro (IV) acid addition electrode 1, the critical current density was measured by critical current density measurement method a. As a result, the critical current density was 0.575A/cm ².

Then, the production of the potassium hexafluoro nickel (IV) acid addition electrode 1 was performed 2 times, and the critical current density was measured for each electrode by the critical current density measurement method a. As a result, the critical current densities were 0.575A/cm ² and 0.675A/cm ².

As described above, the critical current density of the potassium hexafluoro (IV) acid addition electrode 1 is a value 2 to 3 times higher than the blank critical current density.

Next, the electrode 1 was added to the potassium hexafluoro-nickel (IV), and the critical current density was measured by the critical current density measurement method B. That is, the potassium hexafluoro (IV) acid addition electrode 1 was taken out from the anode manufacturing mixed solution used in manufacturing the potassium hexafluoro (IV) acid addition electrode 1. Then, the dehydrated electrolyte 1 was prepared in another electrolytic cell having the same structure as that used for producing the potassium hexafluoro (IV) acid addition electrode 1, and the removed potassium hexafluoro (IV) acid addition electrode 1 was attached to the other electrolytic cell to measure the critical current density.

The critical current density was measured to be 0.550A/cm ². Thus, it can be said that the potassium hexafluoro (IV) acid addition electrode 1 maintains a surface-modified state.

The above-described series of electrode replacement operations were performed in a glove box under a nitrogen atmosphere containing no water.

Next, the critical current density of the additional test electrode 1 of patent document 2 and the critical current density of the additional test electrode 2 of patent document 2 were measured by the critical current density measurement method a. Patent document 2 discloses that the critical current density of the additional test electrode 1 is 0.300A/cm ². The color of the electrolyte was pale yellow green, and it could not be said that nickel (IV) hexafluoro ions were present.

Patent document 2 discloses that the critical current density of the additional test electrode 2 is 0.350A/cm ². The color of the electrolyte was pale yellow green, and it could not be said that nickel (IV) hexafluoro ions were present. Patent document 2 the critical current density of the additional test electrode 1 and the additional test electrode 2 of patent document 2 is a value 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 is found that the anode produced by the method for producing an anode for electrolytic synthesis of fluorine gas according to the present embodiment is extremely unlikely to cause an anode effect. The mechanism of inhibition of this anodic effect is not well defined, but is presumably related to the presence of a fluorinating agent on the surface of the electrode.

Hereinafter, the method for producing the anode for electrolytic synthesis of fluorine gas according to the present embodiment will be described in more detail.

As an example of an apparatus for carrying out the method for producing an anode for electrolytic synthesis of fluorine gas according to the present embodiment, an electrolytic cell used for electrolytic synthesis of fluorine gas is given. The form of the electrolytic cell is not particularly limited, and various electrolytic cells may be used as long as they are capable of generating fluorine gas by electrolysis of an electrolyte solution containing hydrogen fluoride and a metal fluoride.

In general, the interior of an electrolytic cell is divided into an anode chamber in which an anode is disposed and a cathode chamber in which a cathode is disposed by partition members such as partition walls, so that fluorine gas generated at the anode and hydrogen gas generated at the cathode are not mixed.

The interior of the electrolytic cell is divided into an anode chamber in which an anode is disposed and a cathode chamber in which a cathode is disposed, but the anode-producing mixed solution stored in the anode chamber and the anode-producing mixed solution stored in the cathode chamber are not isolated, and in many cases, the anode-producing mixed solution stored in the two-stage chamber is capable of freely mixing.

In the case where the inside of the electrolytic cell has a structure in which the anode-producing mixed solution stored in the bipolar chamber can be freely mixed, potassium hexafluoronickel (IV) acid may be contained in either one of the anode-producing mixed solution stored in the anode chamber and the anode-producing mixed solution stored in the cathode chamber.

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.

The mixture for producing an anode contains hydrogen fluoride, a metal fluoride, and potassium hexafluoro-nickel (IV) acid. The kind of the metal fluoride is not particularly limited, but alkali metal fluoride is preferable, and at least one of potassium fluoride (KF) and cesium fluoride (CsF) is more preferable. The metal fluoride may be used alone or in combination of 2 or more. That is, as the metal fluoride, potassium fluoride and cesium fluoride may be used in combination.

The ratio of the molar amount of hydrogen fluoride to the molar amount of metal fluoride (i.e., [ molar amount of hydrogen fluoride ]/[ molar amount of metal fluoride ]) in the mixture liquid for anode production is preferably 1.6 to 3.2, more preferably 1.9 to 3.0.

As the mixed liquid for producing the anode, for example, a mixed liquid in which potassium hexafluoronickel (IV) acid is 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 of hydrogen fluoride and potassium fluoride may be, for example, hydrogen fluoride: potassium fluoride=1.5 to 2.5:1. Kf.2hf when potassium fluoride=2:1 is a representative mixed molten salt, which has a melting point of about 72 ℃.

As an example of the mixed liquid for producing an anode, a mixed liquid in which potassium hexafluoronickel (IV) acid is dissolved in a mixed molten salt of hydrogen fluoride and cesium fluoride may be used. The molar ratio of hydrogen fluoride to cesium fluoride in the mixed molten salt of hydrogen fluoride and cesium fluoride may be, for example, hydrogen fluoride: cesium fluoride=1.8:1 to 3.1:1. CsF 2.4HF with cesium fluoride=2.4:1 is a representative mixed molten salt having a melting point of about 16 ℃.

Since this mixed solution for anode production is corrosive, the parts of the inner surface of the electrolytic cell and the like that are in contact with the mixed solution for anode production are preferably made of metal such as iron, nickel, MONEL (trademark).

Since hydrogen fluoride in the mixed solution for anode production is consumed by electrolysis at the time of anode production, for example, in electrolysis at the time of anode production, hydrogen fluoride is preferably continuously or intermittently supplied to the mixed solution for anode production. The hydrogen fluoride may be supplied to the mixed liquid for anode production in the cathode chamber side of the electrolytic cell, or may be supplied to the mixed liquid for anode production in the anode chamber side.

The concentration of hydrogen fluoride in the mixed liquid for anode production is preferably controlled to fall within a range of-5 mass% to +5 mass%, more preferably controlled to fall within a range of-2.5 mass% to +2.5 mass%, and even more preferably controlled to fall within a range of-1.5 mass% to +1.5 mass%, based on the concentration of hydrogen fluoride when the mixed molten salt used in the mixed liquid for anode production is kf·2hf (concentration of hydrogen fluoride: 40.4 mass%) or csf·2.4HF (concentration of hydrogen fluoride: 24.0 mass%), as a reference concentration.

The mixed solution for anode production can be obtained by mixing hydrogen fluoride, a metal fluoride, and potassium hexafluoro-nickel (IV) acid (K ₂NiF₆). The potassium hexafluoro-nickel (IV) may be used as a commercially available product or as a product obtained by the preparation. Potassium hexafluoro-nickel (IV) acid can be prepared by mixing potassium fluoride and nickel fluoride (NiF ₂) in a molar ratio of 2:1 and treating at a temperature of 250 ℃ to 450 ℃ under a fluorine atmosphere.

Potassium hexafluoro (IV) is a solid that exhibits oxidizing properties, and thus changes to potassium fluoride salt of nickel 3 (K ₂NiF₅) or potassium fluoride salt of nickel 2 (K ₂NiF₄) upon reaction with water. Regarding the solubility in the mixed solution for anode production, potassium pentafluoride (K ₂NiF₅) and potassium tetrafluoro nickel (II) acid (K ₂NiF₄) are lower than potassium hexafluoro nickel (IV) acid. Thus, potassium hexafluoronickelate (IV) with a low water content is preferably used. The water content of potassium hexafluoro-nickel (IV) acid is preferably 0.5 mass% or less, more preferably 0.3 mass% or less.

The method for adding potassium hexafluoro-nickel (IV) acid to the mixed solution for anode production is not particularly limited, and the mixed solution for anode production may be prepared by mixing a metal fluoride with liquid hydrogen fluoride in which potassium hexafluoro-nickel (IV) acid is dissolved, or may be prepared by mixing solid potassium hexafluoro-nickel (IV) acid with a mixture of hydrogen fluoride and a metal fluoride. The potassium hexafluoro-nickel (IV) acid may be added in a predetermined amount at one time or separately.

The concentration of potassium hexafluoronickel (IV) acid in the mixture liquid for anode production is preferably 500 mass ppm or more and 5000 mass ppm or less, more preferably 1000 mass ppm or more and 4000 mass ppm or less.

The mixed solution for anode production contains potassium hexafluoronickelate (IV) in an amount exceeding the saturation solubility. If the potassium hexafluoro (IV) acid is contained in an amount exceeding the saturation solubility, the solid of undissolved potassium hexafluoro (IV) acid is deposited on the bottom of the electrolytic cell, and if the method for producing an anode for electrolytic synthesis of fluorine gas according to the present embodiment is carried out, the nickel electrolyte in the mixed solution for producing an anode becomes nickel fluoride to adhere to the surface of the anode base material, and therefore the concentration of the nickel electrolyte in the mixed solution for producing an anode decreases. If the anode-producing mixed solution contains potassium hexafluoro (IV) ate in an amount exceeding the saturation solubility, the undissolved solid matter of potassium hexafluoro (IV) ate is dissolved with a decrease in the concentration of the nickel electrolyte, and thus the state of the saturation solubility is maintained.

In the method for producing an anode for electrolytic synthesis of fluorine gas according to the present embodiment, an anode substrate (anode substrate) is subjected to anodic polarization treatment in which a current is passed between the anode and the cathode to adhere nickel fluoride to the surface of the anode substrate, and in the anodic polarization treatment, the anode current density is preferably 0.01A/cm ² to 0.8A/cm ², and the current flow per unit surface area of the anode substrate is preferably 100 coulomb/cm ² to 5000 coulomb/cm ².

Even if the anodic current density is less than 0.01A/cm ², it is possible to cope with this by extending the treatment time of the anodic polarization treatment without any particular obstacle. If the anodic current density is set to be greater than 0.8A/cm ², the anodic polarization treatment can be terminated in a short time, but if a voltage increase is confirmed in the anodic polarization treatment, it is preferable to rapidly stop the current supply or the like so as not to cause the anodic effect.

Further, the anode current density is more preferably more than 0.05A/cm ² and not more than 0.6A/cm ².

The anode current density may be a constant value, but may be gradually increased from a low value or gradually decreased from a high value. The method of changing the anode current density is not particularly limited.

The amount of electricity supplied per unit surface area of the anode substrate is an important value because it relates to the amount of nickel fluoride deposited on the surface of the anode substrate, which acts as a catalyst. The current flow per unit surface area of the anode base material is preferably 100 coulomb/cm ² or more, and even if the current flow rate is more than 5000 coulomb/cm ², the treatment time of the anodic polarization treatment is long. The specific surface area of the anode substrate is more preferably 1800 coulombs/cm ² or more.

The amount of electricity to be supplied per unit surface area of the anode substrate is preferably adjusted according to the concentration of potassium hexafluoronickel (IV) acid in the mixed solution for anode production, but the amount of electricity to be supplied may be increased when the concentration of potassium hexafluoronickel (IV) acid is low, or may be decreased when the concentration of potassium hexafluoronickel (IV) acid is high.

The concentration of nickel electrolyte (IV) ions in the anode manufacturing mixture in the anodic polarization treatment can be measured by ICP emission spectrometry. In order to free nickel (IV) hexafluorooxide ions in the mixed solution for anode production, the amount of impurities that are easily oxidized and are present in the mixed solution for anode production is preferably small. In particular, if the amount of water in the mixed molten salt before adding potassium hexafluoronickel (IV) acid is small, the amount of potassium hexafluoronickel (IV) acid to be added can be reduced, which is preferable.

In the case of removing moisture in the mixed molten salt by electrolysis (dehydration electrolysis), the moisture can be removed by electrolysis using an electrode such as a conductive diamond electrode, a nickel electrode, or a carbon electrode. In the case where the water in the mixed molten salt is removed by electrolysis using a carbon electrode, the carbon electrode used in dehydration electrolysis can be used as an anode base material in the method for producing an anode for electrolytic synthesis of fluorine gas according to the present embodiment.

In this case, since the (CF) n film is formed on the surface of the carbon electrode during dehydration electrolysis, the effect of the anodic polarization treatment in the presence of nickel (IV) hexafluoro ions is smaller than that in the case of using a new carbon electrode, but it is not completely ineffective.

In the method for producing an anode for electrolytic synthesis of fluorine gas according to the present embodiment, an anode substrate is anodized by passing a current between an anode and a cathode, and a nickel fluoride is attached to the surface of the anode substrate, and as the nickel fluoride, a nickel fluoride having nickel ions with a valence of more than 2 is exemplified. For example, at least 1 of nickel trifluoride (NiF ₃), nickel pentafluoride (Ni ₂F₅), and nickel tetrafluoride (NiF ₄) can be cited.

Examples

The present invention will be described in more detail below with reference to examples and comparative examples.

Comparative example 1

An anode for electrolytic synthesis of fluorine gas was produced using the teflon-made electrolytic cell of fig. 1. Using KF.2HF as the electrolyte, 750g of KF.2HF was added to the body of the electrolytic cell. The electrolyte was heated to 85 ℃ and the anode and cathode were immersed therein.

The cathode was a square nickel plate 5cm long and 5cm wide. The anode was a square diamond electrode 1cm long and 1cm wide, and the surface was coated with conductive diamond.

An electric current is passed between the anode and the cathode to perform dehydration electrolysis of the electrolyte. The anode current density was 0.3A/cm ² and the electrolysis time was 2 hours. The gas generated from the anode was diluted with nitrogen gas, and the fluorine gas was absorbed and removed by a potassium iodide aqueous solution trap, and the oxygen concentration in the gas discharged from the potassium iodide aqueous solution trap was measured by gas chromatography. As a result, no oxygen was detected. And judging that the dehydration electrolysis is completed.

Further, since hydrogen fluoride in the electrolyte is consumed by dehydration electrolysis, hydrogen fluoride is intermittently supplied to the electrolyte during dehydration electrolysis, and the concentration of hydrogen fluoride in the electrolyte is set to be in the range of 40 mass% to 43 mass%. The electrolyte prepared by such an operation is referred to as post-dehydration electrolyte 1.

Next, 0.2g of nickel fluoride was added to the dehydrated electrolyte 1 in the electrolytic cell to dissolve the nickel fluoride. The concentration of nickel fluoride in the dehydrated electrolyte 1 was 270 mass ppm, and the dissolved amount of nickel was 160 mass ppm. The amount of nickel fluoride to be added is determined by measuring the amount of nickel fluoride dissolved in KF 2HF at 85 ℃.

The color of the dehydrated electrolyte 1 in which nickel fluoride is dissolved is transparent. The nickel concentration in the dehydrated electrolyte 1 was also measured by ICP emission spectrometry, and as a result, the same concentration as described above was obtained.

A new carbon electrode was immersed in the dehydrated electrolyte 1 in which nickel fluoride was dissolved as an anode, and electrolysis was performed by passing a current of 25mA 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 was square with a length of 1cm and a width of 1 cm. The anode current density in this electrolysis was 0.0025A/cm ², and the current flow per unit surface area of the carbon electrode was 270 coulomb/cm ².

Here, a method for measuring the solubility of nickel fluoride will be described. 100g of KF.2HF was charged into a sealable Teflon-made vessel, a predetermined amount of nickel fluoride powder was added thereto, and the vessel was sealed and left in a thermostatic bath at 85℃for 36 hours. However, the contents of the vessel were stirred from time to time. After leaving for 36 hours, the contents of the container were visually observed to confirm whether or not the nickel fluoride powder remained dissolved.

The solubility was examined by varying the amount of nickel fluoride powder added, and as a result, dissolution residue was not observed when nickel fluoride powder was added in an amount equivalent to a concentration of 320 mass ppm or less. On the other hand, when the nickel fluoride powder was added in an amount larger than the amount corresponding to the concentration of 320 mass ppm, the dissolution residual was confirmed. After removing the dissolved residual nickel fluoride powder, the nickel concentration in kf.2hf was measured by ICP emission spectrometry, and as a result, the maximum value of the 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 by critical current density measurement method a, and as a result, it was 0.25A/cm ². This critical current density is a value within the range of the critical current density obtained in the blank experiment, and thus it cannot be said that the anode effect is suppressed. In addition, from the viewpoint of the color of the electrolyte, it cannot be said that nickel (IV) hexafluoroion is present.

Comparative example 3

In comparative example 2, after the measurement of the critical current density was completed, the anode was taken out of the electrolyte, and the anode was heated to 300 ℃ with a high-temperature inert gas flow without washing, thereby volatilizing the hydrogen fluoride component adsorbed on the carbon electrode. Then, energy dispersive X-ray analysis was performed using a desktop scanning electron microscope JCM-7000 manufactured by japan electronics corporation, and the presence of nickel on the carbon electrode surface was confirmed, and as a result, nickel could not be confirmed since it was not more than the lower detection limit.

Comparative example 4

The dehydrated electrolyte 1 obtained in the same manner as in comparative example 1 was stored in the electrolytic cell of fig. 1, and the anode and the cathode were immersed in the dehydrated electrolyte 1. The anode was a rectangular nickel plate having a length of 2cm and a width of 3cm, and the cathode was a nickel plate. Then, the electrolysis was performed by passing a current of 1A between these anode and cathode for 26 hours, and nickel was eluted from the nickel plate into the dehydrated electrolyte 1.

The nickel concentration in the dehydrated electrolyte 1 was 2800 mass ppm based on the mass reduction of the nickel plate used for the anode, but the nickel concentration in the dehydrated electrolyte 1 containing no precipitate was measured by ICP emission spectrometry, and found to be 270 mass ppm. The dehydrated electrolyte 1 after electrolysis is light yellow green.

A new carbon electrode was immersed in the dehydrated electrolyte 1 after the nickel elution as an anode, and electrolysis was performed by passing a current of 25mA between the anode and the cathode for 10 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 was square with a length of 1cm and a width of 1 cm. The anodic current density in this electrolysis was 0.0025A/cm ², and the specific surface area of the carbon electrode was 900 coulomb/cm ².

The carbon electrode having the nickel potassium fluoride film formed thereon was taken out of the dehydrated electrolyte 1, and the carbon electrode was heated to 300 ℃ in an inert gas without washing with water, to volatilize hydrogen fluoride adsorbed on the carbon electrode. Thereafter, the presence of nickel on the carbon electrode surface was confirmed in the same manner as in comparative example 3, but nickel could not be confirmed since the detection limit was lower than the detection limit.

Comparative example 5

The dehydrated electrolyte 1 obtained in the same manner as in comparative example 1 was stored in the electrolytic cell of fig. 1, and nickel fluoride was added thereto and dissolved. Then, the anode was immersed in the dehydrated electrolyte 1 in which nickel fluoride was dissolved. The anode was a square carbon electrode 1cm long and 1cm wide. The carbon electrode is formed of highly oriented pyrolytic graphite (HOPG ZYH manufactured by Momentive Performance Materials company) having a basal plane.

Then, a reference electrode was set to Ni/NiF ₂, and cyclic voltammetry was performed to obtain a cyclic voltammogram. The potential was scanned at a rate of 0.4 mV/sec. The 1st Run (1 st Run) of the resulting cyclic voltammogram is shown in fig. 2. As is clear from fig. 2, the current shows a peak at a potential in the vicinity of 5.5v vs. ni/NiF ₂, and the current drops sharply at a potential higher than that. This behavior corresponds to the anodic effect.

Comparative example 6

The dehydrated electrolyte 1 obtained in the same manner as in comparative example 1 was stored in the electrolytic cell of fig. 1, and lithium fluoride (LiF) powder was added thereto and dissolved. Lithium fluoride is an additive that is believed to be effective in inhibiting the anodic effect. The concentration of lithium fluoride in the dehydrated electrolyte 1 was 1.0 mass%.

A new carbon electrode was immersed in the dehydrated electrolyte 1 in which the lithium fluoride was dissolved as an anode, and an electric current of 0.05A was applied between the anode and the cathode for 5 hours to perform electrolysis. By this electrolysis, the surface of the carbon electrode is subjected to surface treatment. The carbon electrode was square with a length of 1cm and a width of 1 cm. The specific surface area of the carbon electrode in the electrolysis was 900 coulomb/cm ². Further, the dehydrated electrolyte 1 in the electrolysis was stirred with a stirrer so that lithium fluoride did not settle.

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

The same operation was repeated, and the critical current density was measured, so that the critical current density was 0.30A/cm ².

From these results, it was found that the anode effect could not be suppressed by adding lithium fluoride.

Comparative example 7

The dehydrated electrolyte 1 obtained in the same manner as in comparative example 1 was stored in the electrolytic cell of fig. 1, and crystal powder of potassium hexafluoro nickel (IV) acid was added thereto and dissolved. The concentration of potassium hexafluoro-nickel (IV) acid in the dehydrated electrolyte 1 was 0.3 mass%, and the amount of nickel dissolved was 630 mass ppm. The dehydrated electrolyte 1 after dissolution of potassium hexafluoro-nickel (IV) acid is red.

A square new carbon electrode 1cm long and 1cm wide was immersed as an anode in the dehydrated electrolyte 1 in which potassium hexafluoro-nickel (IV) acid was dissolved, and a metal cathode was immersed, and left for 6 hours without passing a current. Then, the critical current density of the anode was measured by the critical current density measurement method B. As a result, the critical current density was 0.250A/cm ². From the results, it was found that the carbon electrode was immersed in the dehydrated electrolyte 1 in which potassium hexafluoro-nickel (IV) acid was dissolved, and the effect of suppressing the anode effect was not obtained.

Example 1

The dehydrated electrolyte 1 in which potassium hexafluoro nickel (IV) acid is dissolved is used as a mixed solution for anode production, and a new carbon electrode is immersed in the mixed solution for anode production as an anode (anode base material), and a metal cathode is immersed. Then, electrolysis was performed by passing a current of 0.1A between the anode and the cathode for 6 hours. By this electrolysis, nickel fluoride is formed on the surface of the carbon electrode as the anode base material, and an anode for electrolytic synthesis of fluorine gas is produced. The carbon electrode was square with a length of 1cm and a width of 1 cm. The anode current density in this electrolysis was 0.1A/cm ², and the current flow per unit surface area of the carbon electrode was 2160 coulomb/cm ².

The critical current density of the obtained anode for electrolytic synthesis of fluorine gas was measured by a critical current density measurement method a. 2 determinations were made with critical current densities of 0.875A/cm ² and 0.825A/cm ². The production conditions and the measurement results of the critical current density of the anode for electrolytic synthesis of fluorine gas are shown in table 1.

Examples 2 to 4

An anode for electrolytic synthesis of fluorine gas was produced in the same manner as in example 1, except that the concentration of potassium hexafluoro-nickel (IV) acid in the dehydrated electrolyte 1, the time of electrolysis (pretreatment), and the amount of electricity charged during electrolysis were changed as shown in table 1.

Then, the critical current density of the obtained anode for electrolytic synthesis of fluorine gas was measured by a critical current density measurement method a. The measurements were performed 1 time each. The production conditions of the anode for electrolytic synthesis of fluorine gas and the measurement results of the critical current density are summarized in the following table

Table 1.

As is clear from the results shown in table 1, the critical current density tends to be higher as the amount of potassium hexafluoronickel (IV) acid dissolved increases, but the increase in critical current density becomes substantially saturated at a concentration of 5000 mass ppm. Further, as is clear from the results of examples 2 and 3, the larger the amount of electricity supplied during electrolysis, the higher the critical current density, and the critical current density is greatly increased compared to the blank critical current density.

Example 5

The anode of example 4, in which the critical current density was measured, was taken out of the anode production mixture, and heated to 300 ℃ in an inert gas to volatilize hydrogen fluoride adsorbed to the carbon electrode. Thereafter, the presence of nickel on the surface of the carbon electrode was confirmed in the same manner as in comparative example 3, and as a result, the presence of nickel was confirmed.

Example 6

The dehydrated electrolyte 1 obtained in the same manner as in comparative example 1 was stored in the electrolytic cell of fig. 1, and crystal powder of potassium hexafluoro nickel (IV) acid was added thereto and dissolved. The concentration of potassium hexafluoro-nickel (IV) acid in the dehydrated electrolyte 1 was 0.1 mass%. Then, the anode was immersed in the dehydrated electrolyte 1 in which potassium hexafluoro-nickel (IV) acid was dissolved. The anode was a square carbon electrode 1cm long and 1cm wide. The carbon electrode is formed of highly oriented pyrolytic graphite (HOPG ZYH manufactured by Momentive Performance Materials company) having a basal plane.

Then, a reference electrode was set to Ni/NiF ₂, and cyclic voltammetry was performed to obtain a cyclic voltammogram. The potential was scanned at a rate of 0.4 mV/sec. The 1 st run of the resulting cyclic voltammogram is shown in fig. 2. As can be seen from fig. 2, the current shows a peak value at a potential in the vicinity of 5.5v vs. ni/NiF ₂, and the peak current value is a value 2.7 times larger than that of comparative example 5. From the results, it was found that the anode effect was generated, but the current value flowing before the anode effect was generated was increased by adding potassium hexafluoronickel (IV) acid.

Further, as a result of the microscopic raman spectrum analysis, the peak intensities of the D band and the G band were not different in potassium hexafluoro (IV) and nickel fluoride, and therefore, it was considered that the intercalation amount of the interlayer compound was not changed in both. Therefore, it is considered that this difference in peak current value is due to the fact that the addition of potassium hexafluoronickel (IV) acid causes a fluoride of nickel having a valence higher than 2 to act as a catalyst for the fluorine gas generation reaction, and as a result, formation of a (CF) n film on the surface of the carbon electrode is suppressed.

Example 7

An anode for electrolytic synthesis of fluorine gas was produced in the same manner as in example 4. Then, the critical current density of the obtained anode for electrolytic synthesis of fluorine gas was measured by the critical current density measurement method B, and as a result, it was 0.850A/cm ².

Example 8

The dehydrated electrolyte 1 obtained in the same manner as in comparative example 1 was stored in the electrolytic cell of fig. 1, and crystal powder of potassium hexafluoro nickel (IV) acid was added thereto and dissolved. The concentration of potassium hexafluoro-nickel (IV) acid in the dehydrated electrolyte 1 was 0.4 mass%, and the amount of nickel dissolved was 840 mass ppm. The dehydrated electrolyte 1 after dissolution of potassium hexafluoro-nickel (IV) acid is red.

Then, the dehydrated electrolyte 1 in which potassium hexafluoro nickel (IV) acid is dissolved was used as a mixed solution for anode production, and a new carbon electrode was immersed in the mixed solution for anode production as an anode (anode base material), and a metal cathode was immersed. Then, electrolysis was performed by passing a current of 0.3A between the anode and the cathode for 4.5 hours. By this electrolysis, nickel fluoride is formed on the surface of the carbon electrode as the anode base material, and an anode for electrolytic synthesis of fluorine gas is produced. The carbon electrode was square with a length of 1cm and a width of 1 cm. The anode current density in this electrolysis was 0.3A/cm ², and the current flow per unit surface area of the carbon electrode was 4860 coulomb/cm ².

After the produced anode for electrolytic synthesis of fluorine gas (anode for electrolytic synthesis of 1 st fluorine gas) was taken out of the electrolytic bath, a new carbon electrode was immersed as an anode in the dehydrated electrolyte 1 in which potassium hexafluoro-nickel (IV) acid was dissolved, and the same procedure as described above was performed to produce the anode for electrolytic synthesis of fluorine gas (anode for electrolytic synthesis of 2 nd fluorine gas). The same procedure was repeated to sequentially produce the 3 rd to 6 th anodes for electrolytic synthesis of fluorine gas, and a total of 6 anodes for electrolytic synthesis of fluorine gas were produced.

After the produced anode for electrolytic synthesis of 6 th fluorine gas was taken out of the electrolytic cell, the crystal powder of potassium hexafluoro nickel (IV) acid was further added to the dehydrated electrolyte 1 and dissolved. The amount of potassium hexafluoro (IV) acid added is an amount such that the concentration of potassium hexafluoro (IV) acid in the electrolyte 1 after dehydration increases by 0.1 mass% by the addition.

A new carbon electrode was immersed in the dehydrated electrolyte 1 in which additional potassium hexafluoro-nickel (IV) acid was dissolved as an anode, and a current of 0.3A was applied between the anode and the cathode for 4.5 hours to perform electrolysis. By this electrolysis, nickel fluoride was formed on the surface of the carbon electrode as the anode base material, and the 7 th anode for electrolytic synthesis of fluorine gas was produced. The carbon electrode was square with a length of 1cm and a width of 1 cm. The anode current density in this electrolysis was 0.3A/cm ², and the current flow per unit surface area of the carbon electrode was 4860 coulomb/cm ².

The critical current density of the 7 anodes for electrolytic synthesis of fluorine gas produced as described above was measured by the critical current density measurement method B. The results are shown in Table 2. From the results shown in table 2, it was confirmed that the critical current density tended to be lower for the 1 st to 6 th anodes for electrolytic synthesis of fluorine gas according to the order of production. This is thought to be because the nickel electrolyte of potassium hexafluoro (IV) acid adheres to the surface of the electrode by electrolysis, and thus corresponds to a case where the concentration of hexafluoro (IV) ions in the electrolyte 1 gradually decreases after dehydration.

For the 7 th anode for electrolytic synthesis of fluorine gas, the critical current density was again recovered. Thus, it is considered that the concentration of hexafluoronickel (IV) ions in the electrolyte 1 increases after dehydration by adding potassium hexafluoronickel (IV) acid again, and that the hexafluoronickel (IV) ions adhere to the surface of the carbon electrode by electrolysis.

At the end of the production of the 6 th anode for electrolytic synthesis of fluorine gas, the color of the dehydrated electrolyte 1 is lighter than the initial red color, but it is still red, and therefore it is considered that the concentration of nickel (IV) hexafluoro ions is reduced although it is present. Example 8 shows that when the concentration of nickel (IV) hexafluoroion is high, the amount of nickel (IV) hexafluoroion adhering to the carbon electrode surface increases.

Description of the reference numerals

Mixed solution for anode production

Main body

Partition wall

Anode

Cathode

Claims

1. A method for producing an anode for electrolytic synthesis of fluorine gas, wherein the anode can be used when electrolyzing an electrolyte containing hydrogen fluoride and a metal fluoride to electrolytically synthesize fluorine gas, the method comprising an anode polarization treatment step,

In the anode polarization treatment step, an anode substrate having a carbon material is immersed as an anode, and a cathode is immersed in a mixed solution for anode manufacturing containing hydrogen fluoride, metal fluoride and potassium hexafluoronickel (IV) oxide, and an electric current is passed between the anode and the cathode to perform an anode polarization treatment on the anode substrate, thereby attaching nickel fluoride to the surface of the anode substrate.

2 . The method for producing an anode for fluorine gas electrolytic synthesis according to claim 1 , wherein the concentration of the potassium hexafluoronickel(IV)ate in the mixed solution for producing the 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 or 2, wherein in the anode polarization treatment, the anode current density is greater than 0.01 A/ ^cm2 and less than 0.8 A/ ^cm2 , and the amount of current per unit surface area of the anode substrate is greater than 100 coulombs/ ^cm2 and less than 5000 coulombs/ ^cm2 .

4 . The method for producing an anode for fluorine gas electrolytic synthesis according to claim 1 , wherein the nickel fluoride has nickel ions having a valence greater than 2.

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

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

8. The method for producing an anode for fluorine gas electrolytic synthesis according to claim 1 or 2, wherein the cathode is an electrode formed of at least one of iron, nickel, copper and a copper-nickel alloy.