JP2012001798A - Method for manufacturing electrode for electrolytic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing an electrode for electrolytic device, in which a high performance electrode can be obtained by appropriately controlling the termination on the electrode for electrolytic device.SOLUTION: A carbonaceous film is formed on a base material and then, a hydrogen termination rate of the carbonaceous film formed on the base material is increased. By such method for manufacturing the electrode for electrolytic device, the hydrogen termination rate of the carbonaceous film formed on the base material is increased, and thereby the high performance electrode can be obtained by appropriately controlling the termination on the electrode for electrolytic device.

Description

  The present invention relates to a method for producing an electrode for an electrolytic device used in an electrolytic fluorine generator.

  Japanese Patent No. 3893397 discloses a technique in which a diamond electrode is used as an electrode for electrolytic synthesis of a fluorine-containing substance using a molten salt electrolytic bath containing fluoride ions.

Japanese Patent No. 3893397

  However, the above publication does not describe the influence of the terminal state of the diamond electrode on the electrode performance. There is no description of a method for controlling the end of the diamond electrode. The present invention is based on the discovery that the terminal state of an electrode for an electrolysis device has a great influence on the performance of the electrode.

  The objective of this invention is providing the manufacturing method of the electrode for electrolyzers which can obtain a high performance electrode by controlling the termination | terminus of the electrode for electrolyzers appropriately.

The method for producing an electrode for an electrolysis apparatus according to the present invention is a method for producing an electrode for an electrolysis apparatus used in an electrolytic fluorine generator, and a step of forming a carbon film on a substrate and a film formed on the substrate Increasing the hydrogen termination coverage of the carbon film.
According to this method for producing an electrode for an electrolysis apparatus, the hydrogen termination coverage of the carbon film formed on the substrate is increased, so that a high-performance electrode is obtained by appropriately controlling the termination of the electrode for the electrolysis apparatus. be able to.

  The carbon film may be diamond (as-grown diamond).

  In the step of forming the carbon film, diamond (as-grown diamond) may be formed using a CVD method.

  The carbon film may be ECR sputtered carbon.

  In the step of increasing the hydrogen termination coverage, at least one of HF treatment, hydrogen plasma treatment, heat treatment in a hydrogen atmosphere, hydrogen radical treatment, and cathode reduction method may be used.

  According to the method for manufacturing an electrode for an electrolysis apparatus of the present invention, the hydrogen termination coverage of a carbon film formed on a substrate is increased. Therefore, a high-performance electrode can be obtained by appropriately controlling the termination of the electrode for an electrolysis apparatus. Can be obtained.

The figure which shows the manufacturing procedure of the electrode for electrolytic devices using a hot filament CVD method. It is a figure which shows the state of the termination | terminus of a conductive diamond electrode, (a) is the state of the termination | terminus of the conductive diamond electrode after film-forming of a diamond layer, (b) is the state of the conductive diamond electrode after a hydrogen termination process. The figure which shows the state of a termination | terminus, respectively. Sectional drawing which shows the structure of the electrolyzer using the electrode for electrolyzers (diamond electrode) produced through the hydrogen termination process.

  Hereinafter, an embodiment of a method for producing an electrode for an electrolysis device according to the present invention will be described.

  The surface condition of the wetted surface is specially controlled by using a diamond electrode with a high hydrogen termination coverage on the wetted surface of the electrolyte as the anode (working electrode) for generating fluorine gas in the electrolytic fluorine gas generator. Compared with the diamond electrode which is not made, the current density can be improved, and the electrode life can be extended by delaying the anodic oxidation during the electrolysis operation.

  Hereinafter, the principle will be described.

  An electrolytic fluorine gas generator using a molten salt KF · nHF as an electrolytic solution (n is not particularly limited, but is preferably 1 or more and 3 or less) is disclosed in, for example, Japanese Patent No. 3893397. As such, a diamond electrode is used for the anode. When a predetermined operating potential is applied to the electrode, fluorine is generated from the anode and hydrogen is generated from the cathode by the electrochemical reaction represented by the equations (1) and (2).

Anode: 2F ⁻ → F ₂ + 2e ⁻ (1) Formula Cathode: 2H ⁺ + 2e ⁻ → H ₂ (2) Formula

The inventors of the present application have found that, as a fluorine generating electrode, it is preferable from the viewpoint of improving the current density that the wetted surface of diamond is hydrogen-terminated compared to the case of carbon-terminated, oxygen-terminated and fluorine-terminated. Yes. In other words, the larger the ratio of hydrogen termination to the number of terminations on the wetted surface of the diamond electrode (hereinafter referred to as “hydrogen termination coverage”), the larger the current density (A / cm ² ), which is a guideline for electrode performance. Become.

  There are several reasons why a higher current density can be obtained as the hydrogen termination coverage on the wetted surface of the diamond electrode is higher. One of them will be described below.

  In an electrochemical reaction by an electrode, the chemical reaction proceeds on the surface of the electrode, so that the chemical state of the electrode surface is one of the factors that govern the electrode performance. As a terminal state of diamond, hydrogen termination (for example, C—H, etc.), oxygen termination (for example, C—O, etc.), fluorine termination (for example, C—F, etc.), carbon termination (for example, C═C, etc.), etc. It has been known. In hydrogen-terminated diamond, the electronegativity differs between hydrogen atoms (about 2.1) and carbon atoms (about 2.5), so the surface side (hydrogen atom side) has a positive charge (+ 0.05e per hydrogen atom). It will be. On the other hand, oxygen-terminated diamond has three forms: oxygen is an ether bond with two carbon atoms, or a carbonyl bond with a double bond with one carbon atom, and a hydroxy bond terminated with one carbon atom and OH. However, in either case, since the electronegativity is greatly different between oxygen atoms (about 3.5) and carbon atoms (about 2.5), negative charges (−0.000 per oxygen atom) on the surface side (oxygen atom side). 2e). Further, in the case of fluorine-terminated diamond, it is the same as hydrogen in that one fluorine atom terminates a dangling bond, but the electronegativity is fluorine (about 4.0) and carbon atom (about 2.5). ) Have a negative charge on the surface side.

  When using as an anode for an electrode for electrolysis, especially an electrolytic fluorine generator that uses a fluorine-based molten salt as a raw material liquid, the positive charge on the electrode surface is more negative than the negative charge when the electrochemical reaction proceeds. And as a result, the chemical reaction is further promoted. Therefore, the higher the hydrogen termination coverage of diamond, the more effectively the generation of fluorine gas.

  In addition, a diamond electrode having a high hydrogen termination coverage has an effect of delaying electrode deterioration due to anodic oxidation. The deterioration of the diamond electrode is mainly due to a deterioration mode caused by anodic oxidation, and the deterioration proceeds at a speed corresponding to the current value. If the ratio of oxygen termination or fluorine termination on the surface of the diamond electrode is increased, the proportion of the oxidized state occupying the wetted surface is increased, and electrode deterioration proceeds. Therefore, the electrode lifetime can be extended by increasing the hydrogen termination coverage on the wetted surface in the electrode manufacturing process.

  The diamond electrode is formed by forming a diamond thin film on a substrate (for example, Si substrate or metal substrate) by a method such as hot filament chemical vapor deposition (HFCVD) or plasma chemical vapor deposition (PCVD). Percentage of hydrogen termination in the as-grown diamond state (as grown: a state in which crystals are grown on the substrate and no post-treatment such as special surface treatment is applied after crystal growth) However, there are some element terminations other than hydrogen termination. For example, when methane is used as a carbon raw material, it occupies most of the two elements, hydrogen termination and carbon termination, and when alcohol is used as the carbon raw material, it occupies most of the hydrogen termination, carbon termination, and oxygen termination, When acetone is used as a carbon raw material, the hydrogen termination, carbon termination, oxygen termination, and nitrogen termination occupy most. In other words, two or more kinds of terminal ends exist on the surface of the as-grown diamond crystal.

  On the other hand, the electrode for an electrolysis apparatus according to the present invention is characterized in that the hydrogen termination coverage on the wetted surface is increased as compared with an as-grown diamond, thereby improving the current density. In addition, the life can be extended.

  In the present invention, the method for supporting the conductive diamond film on the conductive substrate is not limited, and any method can be used. As a typical production method, a vapor phase synthesis method can be used, and as a vapor phase synthesis method, a CVD (chemical vapor deposition) method, a physical vapor deposition (PVD) method, or a plasma arc jet method can be used. As the CVD method, a hot filament CVD method, a microwave plasma CVD method, or the like is used.

When carrying a conductive diamond film, a mixed gas of hydrogen gas and carbon source is used as a diamond raw material in any method, but in order to impart conductivity to diamond for electrolysis, an element (dopant) having a different valence is used. Is added in a small amount. As a dopant, boron, phosphorus, and nitrogen are preferable, and a preferable content rate is 1 to 100,000 ppm, and more preferably 100 to 10,000 ppm. In addition, the conductive diamond layer synthesized by any method is polycrystalline, and amorphous carbon and graphite components are mixed in the diamond layer. Amorphous carbon or graphite component from the viewpoint of the stability of the diamond layer is preferably lesser, in Raman spectroscopic analysis, and around 1332 cm ^-1 attributable to the diamond peak present in (1312～1352Cm range of ^-1) intensity I (D) , the ratio I (D) / I of the peak intensity near 1580 cm ^-1 attributable to the G band of graphite (range ^{1560~1600cm -1) I (G) (} G) is not less than 1, the content of diamond It is desirable that the content be higher than the graphite content.

  Hereinafter, a procedure for producing a diamond electrode having a high hydrogen-terminated coating rate using a hot filament CVD method, which is one of the most preferable methods for supporting a conductive diamond film on a conductive substrate, will be described.

  FIG. 1 is a diagram showing a procedure for manufacturing an electrode for an electrolysis apparatus using a hot filament CVD method. Hereinafter, step 1 to step 4 will be described.

(Process 1)
In step 1, the conductive substrate is polished. For example, a Si substrate can be used as the conductive substrate. The adhesion between the conductive substrate and the diamond layer of the diamond film can be improved by polishing. Polishing is preferably performed so that the arithmetic average roughness Ra is 0.1 to 15 μm and the maximum height Rz is 1 to 100 μm.

(Process 2)
In step 2, a diamond nucleation process is performed on the surface of the conductive substrate in order to grow a uniform diamond layer. As a nucleation method, a solution containing diamond fine particles can be applied to an electroconductive substrate by an ultrasonic method, a dipping method, or other methods, followed by solvent drying.

(Process 3)
In step 3, a diamond fine particle layer is formed on the conductive substrate by vapor deposition or the like. A carbon source, for example, a low molecular weight organic compound such as methane, alcohol, and acetone, and a dopant, for example, boron are supplied to the filament together with hydrogen gas. The filament is heated to a temperature range where hydrogen radicals and the like are generated (for example, 1800 to 2800 ° C.), and the conductive substrate is placed in this atmosphere so that a temperature range where diamond is deposited (for example, 750 to 1200 ° C.). Deploy. The supply rate of the mixed gas depends on the size of the reaction vessel, but the pressure is preferably 15 to 760 Torr. A diamond fine particle layer having a particle size of 0.001 to 2 μm is usually deposited on the conductive substrate. The thickness of the diamond fine particle layer can be adjusted by the deposition time, but is preferably 0.5 to 20 μm from the viewpoint of economy.

(Process 4)
In step 4, a terminal other than hydrogen (for example, carbon terminal or oxygen terminal) of as-grown diamond after the diamond fine particle layer is formed is subjected to hydrogen termination. FIG. 2A shows an example of the state of termination of the conductive diamond electrode after the formation of the diamond fine particle layer, and FIG. 2B shows the state of termination of the conductive diamond electrode after the hydrogen termination treatment. ing. As a method for hydrogen termination treatment, any of HF treatment, hydrogen plasma treatment, heat treatment in a hydrogen atmosphere, hydrogen radical treatment, and cathode reduction method can be applied. Of these methods, two or more types of hydrogen termination treatments can be combined to enhance the effect of the hydrogen termination treatment. As the cathode reduction method, for example, a method of applying a voltage of about −1.8 V to a conductive diamond electrode in an as-grown state and immersing it in a 0.1 M sulfuric acid aqueous solution (H ₂ SO ₄ ) for about 30 minutes. Conceivable.

  The surface state such as the hydrogen termination coverage of the diamond electrode manufactured through the above steps can be analyzed by a known technique such as Fourier transform infrared spectroscopy (FTIR).

  Note that Step 1 and Step 2 can be omitted.

  ECR sputtered carbon film formation may be applied as an alternative to step 3. ECR sputtered carbon is a carbon film partially having an sp3 structure. The film forming process of ECR carbon is disclosed in, for example, Japanese Patent Application Laid-Open No. 2006-090875. In ECR sputtered carbon film formation, a carbon target is generally used as a raw material target, and an argon gas is used as a plasma gas. The carbon surface of the as-grown film after film formation is carbon-terminated.

  In this case, the carbon terminal is hydrogen-terminated by performing hydrogenation on the as-grown ECR sputtered carbon film formation in a step corresponding to step 4. As a method for hydrogen termination treatment, any of HF treatment, hydrogen plasma treatment, heat treatment in a hydrogen atmosphere, hydrogen radical treatment, and cathode reduction method can be applied. Of these methods, two or more types of hydrogen termination treatments can be combined to enhance the effect of the hydrogen termination treatment. The hydrogen-terminated ECR sputtered carbon film can be expected to have improved durability, improved current density, and the like by suppressing anodization, as compared to an as-grown carbon-terminated ECR sputtered carbon electrode.

  The surface state of the ECR sputtered carbon film (hydrogen termination coverage, etc.) can be analyzed by a known technique such as Fourier transform infrared spectroscopy (FTIR).

  In addition to conductive diamond (polycrystalline, single crystal), conductive carbonaceous materials having a diamond structure used for diamond electrodes include conductive diamond-like carbon, ECR sputtered carbon, RF sputtered carbon, carbon nanotube, fullerene, and carbon nanohorn. Etc. and a conductive carbon material containing them as a main component can be used. A structure mainly having a high sp3 ratio (sp3 / sp2 ratio) to sp2, such as diamond, ECR sputtered carbon, and diamond-like carbon, is preferable because corrosion due to molten salt and an anodic effect are unlikely to occur. A diamond crystal body (polycrystal, single crystal) having the highest composition ratio is most suitable.

  In this specification, the diamond electrode refers to a conductive substrate (for example, a structure in which a conductive carbonaceous film having a diamond structure is formed on the surface of a conductive substrate (for example, Si, carbon). The electrode includes an electrode having a high hydrogen termination coverage on the surface of the diamond plate. In the present invention, the diamond film is not limited to a polycrystal but may be a single crystal.

  The form of the electrode according to the present invention is not limited to a flat plate shape. For example, in order to make it possible to efficiently separate the gas generated on the electrode surface from the electrolytic solution, a fine porosity can be provided in the diamond electrode or the like. In this case, if a porous diamond electrode or the like is used as the anode, it is effective in terms of improving the current density. For example, the porous electrode disclosed in Japanese Patent Application No. 2009-195488 can be configured as a diamond electrode.

  FIG. 3 is a cross-sectional view showing a configuration of an electrolysis apparatus using an electrode for an electrolysis apparatus (diamond electrode or the like) prepared through a hydrogen termination process.

  As shown in FIG. 3, the electrolyzer includes an electrolyzer electrode 11 as an anode that is set in an electrolyzer 10 containing a molten salt 10 </ b> A that is a fluorine raw material and is prepared through a hydrogen termination process, and a cathode 12. A gas separation skirt 13 for separating the generated gas is provided between the electrode 11 for an electrolysis device and the cathode 12. The electrode material of the cathode 12 is not limited. For example, Ni, carbon, conductive diamond, or the like can be used.

  When an operating voltage equal to or higher than the fluorine generation voltage is applied from the power source 21 via the wiring 22 between the electrode 11 for the electrolyzer and the cathode 12, the fluorine 15 is supplied from the electrode 11 for the electrolyzer as the anode, and the hydrogen 16 is supplied from the cathode 12. Each occurs.

  In addition, in the example of FIG. 3, although the example of constant potential operation is shown as an operation method, it is good also as constant current operation.

  As described above, according to the method for manufacturing an electrode for an electrolysis apparatus of the present invention, the hydrogen termination coverage of the carbon film formed on the substrate is increased, so that the termination of the electrode for the electrolysis apparatus is appropriately controlled. Thus, a high-performance electrode can be obtained.

  The scope of application of the present invention is not limited to the above embodiment. The present invention can be widely applied to a method for producing an electrode for an electrolytic device used in an electrolytic fluorine generator.

  11 Electrode electrode

Claims (5)

In the method for producing an electrode for an electrolysis apparatus used in an electrolytic fluorine generator,
Depositing a carbon film on the substrate;
Increasing the hydrogen termination coverage of the carbon film deposited on the substrate;
The manufacturing method of the electrode for electrolyzers characterized by providing.   The method for manufacturing an electrode for an electrolysis device according to claim 1, wherein the carbon film is as-grown diamond.   The method for producing an electrode for an electrolysis device according to claim 2, wherein in the step of forming the carbon film, an as-grown diamond film is formed using a CVD method.   The method for manufacturing an electrode for an electrolysis device according to claim 1, wherein the carbon film is ECR sputtered carbon.   5. The step of increasing the hydrogen termination coverage uses at least one of HF treatment, hydrogen plasma treatment, heat treatment in a hydrogen atmosphere, hydrogen radical treatment, and cathodic reduction method. The manufacturing method of the electrode for electrolytic devices of any one of these.

Images