JP3149629B2 - Oxygen generating electrode - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an insoluble anode used in an electrolysis process involving the generation of oxygen, mainly for electroplating zinc, tin or chromium, or for surface treatment of stainless steel sheets.

[0002]

2. Description of the Related Art Lead or a lead alloy is currently used as an electrogalvanizing anode of a steel sheet. Lead is consumed relatively quickly, and the dissolved lead contaminates a plating solution and deteriorates a plating film. There is a problem. Electrodes obtained by plating platinum on a valve metal substrate or platinum foil clad electrodes are being studied as alternative anodes, but platinum has been exhausted greatly and has not been solved yet. For this purpose, various insoluble anodes in which a noble metal and its compound with low consumption are coated on a valve metal substrate as an electrode catalyst material have been proposed, and a film made of a mixed oxide of tantalum oxide and iridium oxide is promising as an electrode catalyst material. is there.

For example, US Pat. No. 4,437,948 and JP-A-63-203800 describe an electrode in which a mixture film of iridium oxide and tantalum oxide is formed on a titanium base material. However, the film is formed by applying a chloride solution of each of iridium and tantalum on a substrate and thermally decomposing it in an oxidizing atmosphere. Mixed coatings by this method have numerous cracks in the layer. Therefore, there is a disadvantage that the electrolyte penetrates between the coating and the metal substrate (for example, titanium metal) through cracks during use, and an electric insulating layer is formed on the surface of the titanium metal substrate. In other words, when an electric insulating layer is formed between the mixed coating of tantalum oxide and iridium oxide and the titanium metal surface, the electrolysis voltage increases even though the electrode catalyst layer still remains in a sufficient amount for electrolysis, so that electrolysis is performed. Impossible.
The state of this crack is shown in FIG. Considering the use of expensive noble metals as electrode catalyst materials, their cost efficiency is far from good.

In order to improve such disadvantages, various types of electrodes having an intermediate layer containing a valve metal or an oxide thereof between a substrate and an electrode catalyst layer have been proposed. For example, JP-A-63-235493 describes an electrode provided with an intermediate layer composed of iridium oxide and tantalum oxide and an electrode catalyst layer of iridium oxide. However, this intermediate layer is formed by a thermal decomposition method by baking after applying a solution like the electrode catalyst layer, and its effect is not sufficient. In addition to such a thermal decomposition method, a thermal spraying method, an ion plating method, a sputtering method, and the like are known. For example, JP-A-2-247393 discloses an intermediate layer formed of a valve metal oxide by a vacuum sputtering method. An electrode in which an electrode catalyst layer is provided thereon by a pyrolysis method is described. Since cracks are present in the catalyst layer of this electrode, infiltration of the electrolyte cannot be suppressed. In addition, JP
In JP-A-214899, a valve metal layer was formed on the surface of an electrode base material, a sputtered film of iridium oxide was formed on the surface, and an iridium oxide film was formed thereon by coating and baking an iridium compound (pyrolysis method). An electrode or an electrode whose formation order is reversed is described. However, this electrode also has a two-layer structure using both the sputtering method and the thermal decomposition method, and there is still a problem in terms of electrode life.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a long-life electrode which prevents passivation of a metal substrate which is a problem in an insoluble electrode for oxygen generation which is mainly used as an anode for electroplating. Is to do.

[0006]

Means for Solving the Problems The present inventors have conducted various studies to solve the above-mentioned problems, and as a result, compared with a conventional oxygen generating electrode having an electrode catalyst layer formed by a pyrolysis method, In particular, the present inventors have found that an electrode having an electrode catalyst layer formed on the surface of a substrate by a sputtering method improves the utilization efficiency of the electrode catalyst substance and prolongs the life of the electrode, thereby completing the present invention.

According to the present invention, there is provided an oxygen generation method comprising providing a mixed oxide film made of iridium oxide and tantalum oxide as an electrode catalyst layer on a conductive metal substrate made of a valve metal or an alloy thereof by a sputtering method. Electrode.

The metal substrate used for the electrode substrate of the present invention includes a valve metal (titanium, tantalum, zirconium,
Niobium or the like or an alloy thereof is used, but from the economical viewpoint, titanium metal or an alloy thereof, for example, titanium-tantalum, titanium-niobium, or titanium-palladium is preferable.
The shape can be various shapes such as a plate shape, a rod shape, an expanded shape, a perforated plate shape, and the like.

A sputtering method for forming a mixed oxide of tantalum oxide and iridium oxide can be performed by both high frequency sputtering and direct current bipolar sputtering.
For example, when forming a film by high frequency sputtering, a target is prepared by mixing tantalum and iridium metal powder and processing by hot pressing or the like, or several small iridium metal plates are placed on a large tantalum metal plate. Is used. Reactive sputtering is performed on the surface of the valve metal substrate by performing high-frequency discharge in a high vacuum of 1 Pa or less under a mixed gas atmosphere of argon and oxygen. At this time, by appropriately roughening the substrate surface by a method such as dry blasting, the adhesion strength of tantalum oxide or iridium oxide is improved.

[0010] The electrode catalyst layer thus formed on the substrate contains at least 20 mol% of iridium oxide, and the remainder is made of tantalum oxide, preferably 20 to 9 iridium oxide.
5 mol%, 80 to 5 mol% of tantalum oxide, more preferably 30 to 90 mol% of iridium oxide, and 70 to 10 mol% of tantalum oxide. Within this range, an electrode having the longest electrode life can be obtained.

The thickness of the electrode catalyst layer formed by the above-mentioned sputtering method is a thin film of about 0.3 to 10 μm. When this coating film is observed with a scanning electron microscope, no cracks are observed as shown in FIG. 1 ( In FIG. 1, dotted lines show the state in which each metal oxide fine particle on the electrode surface grows and protrudes in a disk shape). Therefore, when this coating film is used as an electrode catalyst, the electrolyte does not easily penetrate between the coating film and the electrode substrate unless most of the catalyst layer is consumed, and the life of the electrode can be extended.

[0012]

EXAMPLE An electrolytic life test was performed using the electrode of the present invention and an electrode according to a conventional method. The amount of the iridium element on the electrode surface was fixed at 30 g / m ² to keep the test conditions constant. For the determination of iridium, an energy dispersive X-ray microanalyzer (EDX) and a fluorescent X-ray analyzer were used. The tip of the electrode was cut into a size of 10 × 10 mm, and the other part sealed was used as an anode for a life test. The electrolysis method is a constant current electrolysis method, the current density is 200 A / dm ² , and the electrolyte is pH = 1.
In step 2, Na ₂ SO ₄ was adjusted to 100 g / L. When the electrolysis voltage increased by 5 V from the initial voltage, the life of the electrode was determined to be reached.

Example 1 A 50 × 10 × 1.5 mm titanium plate is degreased by ultrasonic cleaning in acetone. Next, using # 30 alumina,
Dry blasting was performed on both sides of titanium at 4 kgf / cm ² for about 10 minutes. This titanium plate was washed with water and then ultrasonically washed again in acetone, and used as an electrode substrate.
After drying this titanium plate, it was mounted on a high frequency sputtering device. A target was prepared by placing three iridium plates each having a diameter of 20 mm and a thickness of 1 mm on a tantalum plate having a diameter of 100 mm and a thickness of 3 mm. The electrode substrate was placed at a distance of 25 mm above this target. A mixed gas of argon and oxygen (mixing ratio: 1: 1) is blown into the chamber to 1 Pa, and then a high frequency of 13.56 MHz is applied to start sputtering. After continuing the sputtering for 4 hours, the discharge was stopped. By this operation, a mixed film of tantalum oxide-iridium oxide having a thickness of 5 μm was formed. When the surface of the mixed film thus obtained was analyzed by an energy dispersive X-ray microanalyzer (EDX), the molar ratio of iridium to tantalum was 7: 3, and the amount of iridium element was 30.
It was found to be g / m ^2. The life of the electrode was 151 days.

Example 2 An iridium oxide-tantalum oxide catalyst layer formed by a sputtering method in which the molar ratio of iridium to tantalum is 3: 7 in the same manner as in Example 1 except that one iridium plate of Example 1 is used. A titanium electrode provided with was prepared and the same test was performed. The life of the electrode was 122 days.

Example 3 An iridium oxide-tantalum oxide catalyst layer having a molar ratio of iridium to tantalum of 6: 4 was provided in exactly the same manner as in Example 1 except that two iridium plates of Example 1 were used. When a titanium electrode was prepared and subjected to the same test, the life of the electrode was 140 days.

Example 4 An iridium oxide-tantalum oxide catalyst layer having a molar ratio of iridium to tantalum of 85:15 was provided in exactly the same manner as in Example 1 except that four iridium plates of Example 1 were used. When a titanium electrode was prepared and subjected to the same test, the life of the electrode was 135 days.

Example 5 The same procedure as in Example 1 was repeated except that a tantalum plate was used instead of a titanium plate as an electrode substrate and three iridium plates serving as targets were used, and the molar ratio of iridium to tantalum was 7: 3. A tantalum electrode provided with an iridium oxide-tantalum oxide catalyst layer was prepared and subjected to the same test. The life of the electrode was 209 days.

COMPARATIVE EXAMPLE 1 In order to form an electrode catalyst layer on a titanium electrode substrate by a conventional pyrolysis method, an electrode coating solution having the following liquid composition was prepared and treated in the same manner as in Example 1. 50 × 10 ×
It was applied on a 1.5 mm titanium plate. _{TaCl 5 0.32 g H 2 IrCl 6} · 6H 2 O 1.00 g conc.HCl 1.0 ml n-CH 3 (CH 2) 3 OH 10.0 ml After it was dried for 10 minutes at 120 ° C., an electric furnace kept at 490 ° C. For 20 minutes. This coating operation of the electrode catalyst material was repeated 25 times to produce an electrode using iridium oxide-tantalum oxide as the catalyst material (the molar ratio of the catalyst layer was Ir / Ta = 7 /
3. The amount of catalyst is 30 g / m ^{2 in} terms of iridium element (see FIG. 2). The tip of this electrode was cut out to 10 × 10 mm, and the other part sealed was used as an anode for a life test.
As a result of conducting an electrolysis test under the same conditions as in Example 1, a sharp rise in the cell voltage was observed after 95 days. The electrode was taken out of the electrolytic cell, and the amount of the remaining catalyst was measured by a fluorescent X-ray spectrometer. As a result, about 20 g / m ^{2 of the} catalyst in terms of iridium element remained. That is, it is considered that the increase in the electrolysis voltage was not due to the disappearance of the catalyst, but the permeation of the electrolyte near the boundary between the underlying titanium surface and the catalyst layer, the oxidation of the titanium surface, and the formation of an insulating layer, thereby preventing the passage of electricity.

Comparative Example 2 An IrO ₂ film was coated on a 50 × 10 × 1.5 mm titanium electrode substrate under the same sputtering conditions as in Example 1 using an iridium plate having a diameter of 100 mm and a thickness of 2 mm as a target. It was formed so that the amount of elements was 30 g / m ² . The electrode life was 37 days. When Ta ₂ O ₅ is not contained, the service life is shortened.

Comparative Example 3 An iridium oxide-tantalum oxide catalyst layer having the same composition was formed by the same thermal decomposition method by coating and baking as in Comparative Example 1, and the thickness was further increased by sputtering under the same conditions as in Comparative Example 2. An iridium oxide layer of 0.1 μm was formed (electrode A).
In addition, a thickness of 0.1 μm was formed on the titanium electrode substrate by sputtering.
m after forming an iridium oxide layer of Comparative Example 1
An iridium oxide-tantalum oxide catalyst layer of the same composition was formed by the same thermal decomposition method as in (1) above (electrode B). The amount of the iridium element in the iridium oxide-tantalum oxide catalyst layer formed by baking the electrodes A and B is 30 g / m ² . When the same electrolytic life test as in Example 1 was performed, the life was 95 days for the electrode A and 97 days for the electrode B.

[0021]

The electrode for oxygen generation according to the present invention is obtained by forming a mixed layer of iridium oxide-tantalum oxide having a high electrode activity on a substrate by a sputtering method, so that a dense electrode catalyst film having no cracks or pores at all. Is formed, and the electrode base of the valve metal can be protected from the electrolytic solution, and the effect of extending the electrode life can be obtained. In addition, the electrode can be manufactured by a relatively simple process without providing an intermediate layer as in the related art.

[Brief description of the drawings]

FIG. 1 is a schematic view of a state where the surface of an electrode catalyst layer of the present invention is observed with a scanning electron microscope.

FIG. 2 is a schematic view of a state in which the surface of an electrode catalyst layer manufactured by a conventional method is observed with a scanning electron microscope.

[Explanation of symbols]

 1 Raised part showing crystal growth 2 Crack (width: 0.3 μm)

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-B-46-21884 (JP, B1) (58) Field surveyed (Int. Cl. ⁷ , DB name) C25D 17/10-17/12

Claims (2)

(57) [Claims] An oxygen generating electrode comprising a conductive metal substrate made of a valve metal or an alloy thereof and a mixed oxide film of iridium oxide and tantalum oxide provided as an electrode catalyst layer by a sputtering method. 2. The oxygen generating electrode according to claim 1, wherein the composition of the mixed oxide film is 20 to 95 mol% of iridium oxide and 80 to 5 mol% of tantalum oxide.