JPH11269687A - Electrolytic electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress damages of a base material under conditions of high current density and high voltage and to improve the production efficiency of electrolytic products by successively forming coating layers of platinum, iridium and iridium oxide on an electrode base body such as titanium, niobium and tantalum. SOLUTION: A platinum coating layer, an iridium coating layer and an iridium oxide coating layer are successively formed on an electrode base body containing one or more kinds of metal selected from titanium, niobium and tantalum to form an electrode. The platinum coating layer and the iridium coating layer are formed to each 0.1 to 5 μm thickness, while the iridium oxide coating layer is formed to 0.1 to 0.5 μm thickness. The platinum coating layer and the iridium coating layer are formed by dry plating such as a platinum- iridium cladding method to laminate thin plates under pressure, and vacuum vapor deposition or sputtering, or by wet electric plating. The iridium oxide layer is preferably formed by a pyrolyzing method such as by applying an iridium compd. soln. on the iridium coating layer, drying and heating.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode for electrolyzing a dilute saline solution to produce a strongly acidic water having a bactericidal effect.
The present invention relates to an insoluble electrode used for applications such as electrolytic cleaning of wastewater containing organic substances.

[0002]

2. Description of the Related Art Strongly acidic water is a water having a bactericidal effect.
A diluted saline solution having a concentration of about 1000 mg / l to about 1000 mg / l is obtained from the anolyte by electrolysis with a diaphragm. In the electrolysis of strongly acidic water,
A method is used in which city water and concentrated saline are mixed and electrolyzed. Since city water contains calcium and magnesium, calcium salts and magnesium salts adhere to the electrodes as the strongly acidic water production device operates.
In order to remove these, a reverse electrolysis operation for inverting the electrode polarity is performed. Examples of the electrode used for the production of strongly acidic water include a baked electrode formed by firing an electrode active material composed of iridium oxide alone or iridium oxide and metal platinum together with a binder on a valve metal base material such as titanium and a valve metal such as titanium. Electrodes or the like in which a substrate is plated with platinum have been used. Of these electrodes, platinum-plated electrodes have the advantage that electrode manufacturing conditions can be manufactured more stably than baked electrodes, exhibit stable characteristics over their lifetime, and cause a gradual rise in voltage. It has the advantage that it can be predicted in advance, and the advantage that it is robust to reverse electrolysis. However, compared to a baked electrode, there is a drawback that chlorine generation efficiency in dilute saline is inferior, and more current must be supplied to the diaphragm electrolysis cell, resulting in a shorter electrode life. There was a problem.

[0003] In a platinum-plated electrode, since the catalytically active material layer is made of metallic platinum, the difference between the voltage applied to the nonconductive electrode substrate and the voltage applied to the platinum-plated portion, even when consumed, is baked. Larger than electrodes. The substrate is protected, and the current flowing in the spalled portion flows to the non-peeled catalytically active material layer. Therefore, damage to the substrate is suppressed even under high current density conditions and high voltage conditions. Since the damage between the platinum layer and the electrode base material also progresses gradually, the consumption of the electrode can be determined in advance by increasing the voltage. Since the consumption gradually proceeds, it is easier to make a replacement plan as compared with the above-mentioned burnt electrode containing iridium oxide. However, taking seawater electrolysis for cooling water in thermal power generation as an example, there is a disadvantage that when used as an anode, a large amount of oxygen is generated and a small amount of chlorine is generated. That is, the generation of oxygen is dominant, which is disadvantageous when an electrolytic product other than oxygen is obtained. Also, in the case of generating oxygen, it is necessary to supply a large amount of electric power with a high oxygen overvoltage. This tendency becomes even more pronounced when the current density is high. Therefore, in an electrolytic operation using an insoluble anode for industrial use, a baked electrode containing iridium oxide, electrolysis of platinum, and the like are appropriately used in consideration of life determination and electrolytic conditions.

Further, as an improvement of these electrodes, there has been proposed an electrode in which a platinum group metal oxide such as iridium oxide is three-dimensionally supported on a porous platinum layer (Japanese Patent Laid-Open No. 58-1).
No. 71589). In a typical embodiment of the present invention, a solution containing a compound forming a platinum group metal oxide is applied to a porous platinum coating layer formed by electroplating, and the surface of the platinum coating layer and the pores are formed by pyrolysis. It is a three-dimensional electrode structure obtained by forming and supporting a metal oxide. By taking this structure, the advantages of two electrochemically active components, platinum and a platinum group metal oxide, are maximized, It is characterized in that an effect of the invention of obtaining an electrode having low overvoltage and high durability is obtained.

[0005] However, even in the case of using the electrode having the three-dimensional structure described above, a situation arises in which the durability cannot always be satisfied under severe use conditions in recent years, that is, under high current density conditions and high voltage conditions. May be.
For example, in long-term use, there is an example in which the platinum group metal oxide layer peels off from the platinum layer, and a remarkable deterioration in characteristics is observed. As long as it is in the reaction field, it is at risk of physical separation due to the flow of liquid, and the local electric field formed by the voltage applied to the electrode surface during the reaction triggers the separation of the platinum group metal oxide layer It is supposed to be. In this case, it is presumed that the platinum group metal oxide layer formed by the above-mentioned pyrolysis method is likely to be easily peeled off due to insufficient bonding strength with the platinum layer.

[0006]

SUMMARY OF THE INVENTION The present invention is capable of suppressing damage to a substrate under high current density conditions and high voltage conditions, which are the drawbacks of the above-mentioned conventional electrodes, and at the same time, is capable of electrolytically removing chlorine and other substances other than oxygen. An object of the present invention is to provide an electrode having excellent product generation efficiency.

[0007]

According to the present invention, there is provided an electrode substrate comprising at least one metal selected from titanium, niobium and tantalum, wherein (a) a platinum coating layer, (b) an iridium coating layer, (C) An electrode in which an iridium oxide coating layer is sequentially formed. It is desirable that the thickness of the platinum coating layer and the iridium coating layer be 0.1 to 5 μm, and the thickness of the iridium oxide coating layer be 0.1 to 0.5 μm. By adopting such a structure, the platinum coating layer and the iridium coating layer have high bonding strength because they are metals, and the iridium coating layer and the iridium oxide coating layer are the same element component system. It is considered that the film strength of the entire electrode is improved, and as a result, high durability as described later can be maintained.

According to the present invention, damage to a substrate under high current density conditions or high voltage conditions of 5 A / dm ² or more and 200 A / dm ² or less is suppressed, and a catalyst activity similar to that of an iridium oxide electrode or the like is obtained. The obtained electrode is obtained. In the vicinity of the electrode life point,
Since the increase in the electrolytic voltage or the decrease in the electrolytic current starts gradually, the life of the electrode can be determined in advance based on the change in the voltage value or the current value. Since the disadvantages of the iridium oxide electrode and the disadvantages of the platinum-plated electrode can be compensated for, the range of application in the field of use of the electrode is widened.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Valve metals such as titanium, niobium, and tantalum used for a base material are easily available. Alloys such as niobium and tantalum, and niobium and titanium are also preferred. This is because the surface of the substrate is passivated after the electrochemically active component layer is peeled off, so that the electrode substrate is protected. In the pretreatment of the substrate, if the surface of the substrate is roughened by blasting by emery or chemical etching, the adhesion between the plated electrodeposit and the substrate is improved. In the chemical etching method, an etching solution using a fluoride is often used.
In addition, a solution of concentrated sulfuric acid, hydrochloric acid, oxalic acid, or the like is used, and may be used by being mixed with fluoride. In addition to these treatments, known pretreatment methods such as a sulfuric acid immersion step, a water washing step, and an oxide removal step in a fluoride solution for adjusting the condition of the base material before plating may be combined.

The method for forming the platinum coating layer and the iridium coating layer is not particularly limited, and various methods such as a so-called platinum iridium clad in which thin plates are pressure-bonded, dry plating such as vacuum evaporation and sputtering, and wet electroplating can be applied. It is. In the case of electroplating, a neutral bath or an alkaline bath other than a sulfuric acid bath can be appropriately selected and used. A diffusion layer of platinum-oxide may be formed between iridium and platinum.

For forming the iridium oxide layer, a so-called "pyrolysis method" is preferable. According to the "pyrolysis method", the iridium compound solution is applied to the iridium coating layer and dried,
By heating, a thin and uniform iridium oxide film is formed on the iridium coating layer. The heat treatment is performed at a temperature of about 400 ° C. to 800 ° C. in the air or a simple oxidizing atmosphere. If the temperature is lower than 400 ° C., there is not much effect on the extension of the service life. When performing heat treatment at a temperature exceeding 800 ° C, in order to prevent damage to the base material,
The treatment is preferably performed in a nitrogen atmosphere or an argon atmosphere, but equipment costs and costs are high. The processing time is generally from several minutes to several hours, but a low temperature may be used for a long time, and a high temperature may be used for a short time. The heat treatment conditions may be appropriately selected, but in the air, the temperature is preferably from 450 ° C. to 650 ° C. for about 10 minutes to 60 minutes. Since iridium generated by the thermal decomposition is easily oxidized in the above temperature range, it is considered that an almost complete iridium oxide film is formed. Since the heat treatment is performed when forming the iridium oxide coating layer, the electrode life is increased. In the case where the heat treatment is performed in this manner, the effect on the life when the heat treatment is performed on a single-layer electrode formed by platinum plating is 1.
Although it was about three times, in the multilayer electrode of the present invention, the effect of the heat treatment was 2.5 times. This effect is considered to be due to the interaction between the crystal growth direction by electroplating and the generation of particles and the heat treatment.

In the electrode according to the present invention, a coating layer of iridium oxide, iridium, and platinum is formed in order from the surface. Even when iridium oxide on the electrode surface is peeled or consumed, it is considered that iridium changes to iridium oxide by anodic oxidation with electrolysis. Taking seawater electrolysis as an example, during positive electrolysis, the action of iridium oxide
Although a high chlorine generation efficiency can be obtained, as described above, in the electrode of the present invention, iridium oxide always takes a form exposed on the surface unless iridium in the second layer is exhausted, so that the electrode can be stably formed. Chlorine generation efficiency is obtained.

Further, since the platinum coating layer is provided between the substrate and the iridium coating layer, there is an effect that the recovery of iridium is facilitated and an effect that the electrode substrate is protected. In order to recover iridium from an electrode using used iridium, there is a method of dissolving with chlorinated water. However, when recovering a small amount of iridium on the surface,
Since the base material is also dissolved, a large amount of the chemical solution is used, and the base material cannot be reused. When a platinum coating layer is provided in the middle, the platinum portion is dissolved by aqua regia and iridium can be decomposed from the base material as a foil and a fine powder. Since a metal or an alloy such as titanium, niobium, or tantalum is used for the base material, it is not dissolved in aqua regia, so that it can be recovered with a small amount of chemical solution, and the base material can be reused. The separated iridium powder, foil, and platinum solution can be easily recovered as compared with the above-described method. Since the platinum coating layer is formed under the iridium coating layer, the substrate is not exposed even if the iridium coating layer is consumed. Since the exposed platinum coating layer contributes to the electrolysis, even if the iridium coating layer peels off, the deterioration of the electrolytic performance can be suppressed. Since the voltage rise gradually progresses during this period, the electrodes may be replaced when the platinum coating layer starts to be exposed.

When heat treatment is applied, an iridium diffusion layer is formed on the platinum coating layer. For this reason, when the platinum layer is initially exposed, it is in a state close to a platinum-iridium alloy, so that the electrode subjected to the heat treatment exhibits more excellent characteristics.

The thickness of the iridium oxide coating layer is 0.1 to 0.5
The thickness of the iridium coating layer is preferably about 0.2 to 5.0 μm in consideration of economy and workability. The thickness of platinum is preferably about 0.1 to 2.0 μm. In particular, for platinum, it can be set in consideration of the time from when the iridium coating layer of the electrode starts to be consumed and when it is replaced.

The electrode according to the present invention can be used from a low current density to a high current density, and shows excellent performance under conditions of 5 A / dm ² or more. The upper limit is 200A for practical use
/ dm ² or less.

Hereinafter, embodiments of the present invention will be described.

[0018]

[Example 1] 40 mm long with a lead of 10 mm in width and 100 mm in length
An example in which an electrode of the present invention was prepared using a titanium plate having a width of 50 mm and a thickness of 1 mm as an electrode substrate will be described. First, a titanium plate was roughened by a sandblast method to prepare a pretreatment for plating. The pretreatment of plating is performed using an acidic degreasing solution (manufactured by Nippon Electroplating Engineers Co., Ltd .:
After being immersed in Eatrex 15) for 30 seconds, it was washed with water, immersed in an alkaline degreasing solution (manufactured by Nippon Electroplating Engineers Co., Ltd .: Eatrex 11) for 1 minute under ultrasonic waves, and washed with water. Further, in order to remove the titanium oxide film, the film was immersed in a 5% acidic ammonium fluoride solution containing 1% nitric acid for 1 minute, washed with water, immersed in a 5% sulfuric acid solution for 30 seconds, and washed with water. After pretreatment, bath temperature 50 ℃, current density 0.5A
Platinum plating was performed under stirring conditions of / dm ² to form a platinum coating layer having a thickness of 1.4 μm. The platinum plating bath was performed with a sulfuric acid bath based on dinitrodiammine platinum salt. Subsequently, iridium plating was carried out at a bath temperature of 80 ° C., a current density of 0.15 A / dm ² and stirring conditions to form an iridium coating layer having a thickness of 0.6 μm. The iridium plating bath was performed by a bath using sodium hexabromoiridate (III) and oxalic acid. Finally, a solution prepared by dissolving 10 g of sodium chloride iridate in 20 ml of butanol was applied and dried on the platinum-iridium coating layer, and then baked at 600 ° C. for 5 hours in the atmosphere to form an iridium oxide coating layer. The overall coating thickness is 2.15
μm.

(Evaluation 1, Effective Chlorine Concentration Measurement) A test method for measuring chlorine generation efficiency assuming seawater electrolysis or soda electrolysis will be described. Using a 6-electrode cell with a width of 60 mm and a length of 100 mm and a 5-cell bipolar electrode with a gap of 3 mm and a gap of 3 mm, the flow rate of catholyte to anolyte was set to 1: 1 at a rate of 2 l / min. , 20 ° C sodium chloride solution (concentration 100
0 ppm). As the diaphragm, a plastic frame with a flow straightening bar and a fluorine-based cation exchange membrane was used. A current of 7.5 A / dm ² was applied at a current density per cell, and after 5 minutes, the effective chlorine concentration in the liquid flowing out from the anode side was measured by iodine titration to be 116 ppm.

(Evaluation 2, Life Test) This embodiment shows a method for performing a life test of an electrode. 20mm width, vertical width
Accelerated life test in a non-diaphragm electrolytic cell equipped with an inlet for feeding sodium chloride solution from the lower part in a water tank made of vinyl chloride with a depth of 80 mm and a depth of 100 mm, and a liquid outlet for overflowing the liquid from a depth of 20 mm Was carried out. An electrode having a length of 40 mm, a width of 50 mm, and a thickness of 1 mm having a lead of 10 mm in width and 100 mm in length, which is the electrode dimension shown in Example 1, was used as a cathode and an anode, and was disposed at the center of the cell at a distance of 3 mm between the electrodes. A constant current electrolysis was performed at a current density of 200 A / dm ² for 700 hours, and the results are shown in FIG. The sodium chloride solution having a concentration of 5 g / 1 at room temperature was continuously fed into the test cell at a rate of 50 ml / min. In this test, the current density is increased to perform an accelerated test. A life test was performed according to this test method. As a result, although the surface of the electrode according to the present invention was slightly darkened, there was almost no change in the thickness of the coating layer, and the consumption after 700 hours was only 0.05 μm (about 2.3 μm). %)
Met.

[0021]

Example 2 A titanium plate having the dimensions of the electrode shown in Example 1 was immersed in 5% hydrofluoric acid for 1 minute to roughen the surface. Separately,
A platinum foil and an iridium foil are stacked and pressed together to prepare a clad metal foil having a platinum thickness of 100 μm and an iridium thickness of 300 μm, placed on the titanium plate with the platinum side down, and pressed against the iridium coating. A titanium plate coated with platinum was formed, and finally iridium oxide was formed on the iridium coating layer in the same manner as in Example 1. When six samples were prepared in this way and subjected to a life test according to the procedure of Example 1, the surface of the electrode was slightly darkened, but the electrode thickness was hardly changed.

[0022]

EXAMPLE 3 A titanium plate whose surface was roughened with hydrofluoric acid was prepared in the same manner as in Example 2, and a platinum coating layer, an iridium coating layer, and an iridium oxide coating layer were formed in this order according to the procedure of Example 1. According to the procedure of Example 1, another iridium coating layer and another iridium oxide coating layer were sequentially formed. When this was subjected to a life test according to the procedure of Example 1, the surface of the electrode was slightly darkened, but the thickness of the coating layer was hardly changed, and the consumption at the time of 700 hours was only 0.05 μm (about 1.8 μm). %), Which only decreased from 2.85 μm before the life test to 2.80 μm.

[0023]

Conventional Example 1 Six electrodes having the dimensions of the electrodes shown in Example 1 were prepared by forming only a platinum coating layer having an average thickness of 2.05 μm on a titanium plate by platinum plating. When the effective chlorine concentration of this electrode was measured according to the procedure of Example 1, the concentration was only 45 ppm, which was considerably lower than that of Example 1. A life test was performed according to the procedure of Example 1, and
The results are shown in FIG. As can be seen from FIG. 1, remarkable wear of the electrode was observed, and the thickness of the coating layer was reduced by the life test.
At the elapse of time, about 1.00 μm, that is, almost half of the coating layer had been consumed.

[0024]

Conventional Example 2 A sample having the dimensions of the electrode shown in Conventional Example 1 and having only a platinum coating layer formed on a titanium plate by platinum plating and having an iridium oxide coating layer formed by the procedure of Example 1 was used. Created. The effective chlorine concentration was measured by the procedure of Example 1 to be 80 ppm,
It was slightly below Example 1. Further, a life test was performed according to the procedure of Example 1. As a result, electrode wear was observed. The thickness of the coating layer was reduced from 2.15 μm before the life test to 1.95 μm, and the consumption was 0.20 μm (corresponding to about 9.3%). ).

As described above, the electrodes according to the present invention and the conventional example 2 have a high effective chlorine concentration due to the function of the iridium oxide coating layer provided as the outermost layer. As shown, it can be seen that the efficiency of chlorine generation is low and the generation of oxygen increases only with platinum plating. Also,
As shown in the life test, the electrode according to the present invention has a reduced film thickness when the accelerated electrolytic test is performed,
It can be seen that the life is much less.

[0026]

According to the electrode for producing acidic water according to the present invention, (1) the configuration of the electrode by iridium electroplating can extend the life of the electrode and obtain advantages in maintenance. (2) The electrode consumption under high current density conditions is suppressed, and the device incorporating the present invention can withstand long-term use. (3) The expensive precious metal can be easily recovered, and the base material can be reused.

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[Procedure amendment]

[Submission date] June 15, 1998

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Brief description of drawings

[Correction method] Change

[Correction contents]

[Brief description of the drawings]

FIG. 1 is a diagram showing the results of an electrode life test performed for comparative evaluation of the present invention and a conventional example.

Claims (4)

[Claims] 1. An electrode substrate comprising at least one metal selected from titanium, niobium and tantalum, wherein (a) a platinum coating layer, (b) an iridium coating layer and (c) an iridium oxide coating layer. Electrodes formed in order. 2. The electrode according to claim 1, wherein the thickness of the platinum coating layer and the iridium coating layer is 0.1 to 5 μm, and the thickness of the iridium oxide coating layer is 0.1 to 0.5 μm. 3. The method according to claim 1, wherein the platinum coating layer and the iridium coating layer are formed by plating, and the iridium oxide coating layer is formed by thermally decomposing an iridium compound solution. Item 3. The electrode according to item 1 or 2. 4. The electrode according to claim 1, wherein the three layers (a), (b), and (c) are repeatedly formed.