JP3264535B2 - Gas electrode structure and electrolysis method using the gas electrode structure - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power-saving gas electrode structure used in an electrochemical process and an electrolysis method using the gas electrode structure.

[0002]

2. Description of the Related Art Conventionally, as an electrode for an electrochemical process, a conductive material having sufficient durability when used as an electrode has been used. That is, iron or nickel which is stable and easily available at the time of negative polarization is used as a cathode, and graphite carbon is used as an anode instead of a usual metal which dissolves. In addition, a valve metal such as titanium is passivated as it is, but is extremely stable against anodic polarization, and is used on the valve metal as an electrode substrate to prevent passivation of the valve metal and as an electrode material. Electrodes prepared by coating a functional platinum group metal or its oxide have been used. In addition, lead oxide and manganese oxide, which are conductive oxides, themselves have insufficient physical strength and conductivity, but have been put into practical use as electrodes by coating the above substance on a substrate such as titanium. I have.

[0003] In these electrodes, a current is caused to flow in an electrolytic solution while generating hydrogen at the cathode and oxygen and halogen are generated at the anode, and a desired electrolytic reaction proceeds or electrolytic salt separation is performed. In these anodes and cathodes, the generated gas is often not a target product, and there is a problem in that the energy required for generating the gas is extremely large, resulting in energy loss. In order to solve this drawback, for example, an oxygen-containing gas is supplied to the cathode chamber and reacted with hydrogen generated in the cathode chamber to avoid wasting energy required for hydrogen gas generation, thereby reducing energy consumption or recovering energy. There is known a so-called gas electrode for performing electrolysis while performing hydrogenation, or circulating hydrogen generated in a cathode chamber to react with oxygen generated in the anode chamber to similarly perform electrolysis while reducing energy consumption.

The gas electrode has a porous structure which is usually made of a conductive material mainly composed of carbon, and has one surface made hydrophobic (water repellent) to prevent the permeation of the electrolytic solution and to effectively supply the gas over the entire surface. The other surface is a hydrophilic layer carrying an electrode catalyst, and the gas infiltrating the hydrophobicity and the electrolyte in the electrolyte react on the surface of the hydrophilic layer to depolarize, and the electrolysis is performed with a small amount of power. To do. This electrode has no problem under ideal electrolysis conditions with few impurities as in electrolysis experiments, but when it is used industrially, impurities are usually contained in the electrolytic solution and these impurities form the electrode catalyst. If the electrolyte used is poisoned or corrosive, the electrode catalyst is consumed. Conventionally, there has been proposed a method of obtaining salt and caustic soda without generating hydrogen by industrial salt electrolysis, that is, chloralkali electrolysis, to lower the electrolysis voltage, but it has not been put to practical use. This is thought to be due to the fact that the electrode catalyst does not have sufficient durability in the conditions of the current ion exchange membrane method in the cathode chamber, that is, in 30 to 35% caustic soda at 90 ° C or higher. It has to be said that it is extremely difficult to produce an electrode having both the activity as a catalyst and the performance that can be maintained for several years or more even with the current technology.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and to provide a gas electrode structure capable of sufficiently withstanding long-term industrial use even if the electrolyte is corrosive, and an electrolytic solution using the gas electrode structure. The aim is to provide a method.

[0006]

The gas electrode structure according to the present invention has an ion exchange membrane substrate and a platinum group metal or an oxide thereof formed on one surface of the ion exchange membrane substrate as an electrode catalyst. A gas electrode structure according to the present invention, comprising: a conductive carbon thin layer; and a porous current collector disposed in close contact with the surface of the thin layer opposite to the ion exchange membrane metal. This is an electrolysis method in which the gas electrode structure is used as an anode or a cathode or as both an anode and a cathode. Hereinafter, the present invention will be described in detail.

A feature of the gas electrode in the present invention is that, unlike the above-mentioned conventional gas electrode, an ion exchange membrane substrate is used as a substrate, and the ion exchange membrane substrate is placed on the side in contact with an electrolyte, and a conductive electrode having an electrode catalyst is provided. The point is that the conductive carbon thin layer and the electrolytic solution are blocked by a substantially liquid-impermeable and conductive ion exchange membrane. When a conventional porous substrate is used as the substrate instead of the ion exchange membrane substrate, the electrolytic solution reaches the conductive carbon thin layer having the electrode catalyst as in the conventional case, and the corrosiveness of the electrolytic solution and impurities contained in the electrolytic solution are used. This leads to consumption and deterioration of the electrode catalyst. However, in the electrolytic reaction using a gas electrode, the electrolytic solution does not need to contact the electrode catalyst, and the electrolyte in the electrolytic solution and the gas supplied separately from the electrolyte contact the electrode catalyst to form predetermined ions. All you need is enough. In other words, hydrogen ions are generated in the anode chamber according to the formula H ₂ → 2H ⁺ + 2e ⁻ and O ₂ + 2H ₂ 0+ in the cathode chamber.
It is sufficient that hydroxyl ions are generated according to the formula of 4e ⁻ → 4OH ⁻ .

In the gas electrode structure of the present invention, since a substantially liquid-impermeable and conductive ion-exchange membrane substrate is used as the substrate, the gas electrode structure allows the anode chamber and / or the cathode chamber to be used. Is divided into an electrolyte solution chamber and a gas chamber, and only a desired electrolyte that can permeate the ion exchange membrane substrate in the electrolyte solution on the electrolyte solution side penetrates the substrate to reach the conductive carbon thin layer, and the gas is generated by the electrode catalyst. The desired electrolytic reaction is performed by the reaction, and the product returns to the electrolyte chamber through the ion exchange membrane substrate again by the electric field. Therefore, in the gas electrode structure according to the present invention, the electrode material in the conductive carbon thin layer is basically gaseous water (steam), hydrogen and oxygen gas (or air).
And the corrosive substances and impurities in the electrolyte rarely permeate through the ion exchange membrane substrate and reach the conductive carbon thin layer to deteriorate the electrode catalyst. A voltage drop can be achieved, and a long-life gas electrode can be provided. On the other hand, it is conceivable that the use of the ion exchange membrane substrate causes a resistance loss and accordingly increases the voltage and power consumption. However, the ion exchange membrane substrate is mainly for the purpose of preventing the diffusion of the electrolyte, In this case, there is almost no resistance due to only the flow of protons (H ⁺ ), and there is a voltage rise of 3 to 400 mV depending on the type of the ion exchange membrane of the ion exchange membrane base due to the flow of hydroxyl ions on the cathode chamber side. However, this is only a fraction of the total power consumption, and this level is not so large considering the presence of the overvoltage of the generated gas in the conventional method, and is sufficient as an effect.

The gas electrode structure according to the present invention can be used as either an anode or a cathode, and the type of the ion exchange membrane substrate used in the gas electrode structure according to the present invention is It is preferable to use a cation exchange membrane when used as an anode, and to use an anion exchange membrane when used as a cathode. However, when used as an anode, protons are small in terms of proton permeation and easily penetrate freely regardless of the type of ion exchange membrane. Therefore, either a cation exchange membrane or an anion exchange membrane may be used. It is desirable to use an anion exchange membrane when it is desired to eliminate the permeation of a cation having a large value, but in other cases, a cation exchange membrane having a low resistance is generally used. The cation exchange membrane not only suppresses the permeation of anions, but also the protons generated during electrolysis and the accompanying water that migrates from the membrane into the electrolytic solution by the force of the conductive carbon thin layer of other ions. The transfer in the direction is hindered, and the cations are substantially prevented from transferring to the thin conductive carbon layer supporting the electrocatalyst, whereby the electrocatalyst is hardly deteriorated as described above.

On the other hand, when the above-mentioned gas electrode structure is used as a cathode, an anion exchange membrane is used as the ion exchange membrane substrate since hydroxyl ions must be supplied to the electrolyte through the ion exchange membrane substrate. It is necessary.
The material of the anion exchange membrane may be appropriately selected according to electrolysis conditions such as an electrolytic solution. When the material is used for chloralkali electrolysis, an ion exchange membrane that is resistant to a 30 to 35% aqueous solution of caustic soda is selected. It is necessary to select a fluororesin-based ion exchange membrane instead of a hydrocarbon-based ion exchange membrane used for ordinary electrodialysis. Although the ion exchange capacity of the ion exchange membrane of these ion exchange membrane substrates is not particularly limited, since the anion exchange membrane has a large electric resistance, it is desirable to select an ion exchange membrane having a small electric resistance. It has been pointed out that the use of an anion exchange membrane usually lowers the current efficiency, but since an electric field is applied and a gas layer is formed on one surface of the anion exchange membrane,
Almost no problem. The conductive carbon thin layer formed on one side of the ion exchange membrane substrate has a contact surface area between a suitable moisture and a sufficient gas because gas is converted into ions on an electrode catalyst supported on the thin layer. It is necessary to have The thin layer may be separately prepared and adhered to the ion exchange membrane substrate, but is preferably formed directly on the surface area of the ion exchange membrane substrate. The material of the thin layer is any carbon-based material having conductivity, and the material is desirably graphite carbon from the viewpoint of high surface area and high electrical conductivity, and the thickness thereof is preferably 30 to 100 μm. preferable. This is 100
If the thickness exceeds 30 μm, the conductivity may decrease.
If it is less than m, a sufficient surface area may not be able to be secured. The material and forming method of the conductive carbon thin layer may be the same or different for the anode and the cathode. The method of forming the conductive carbon thin layer is not particularly limited. For example, graphite fine powder, preferably fine powder containing submicron size, or pitch-based carbon fiber is kneaded with a fluororesin and baked or chemically vapor-deposited on the ion exchange membrane substrate. (CV
The fine graphite powder can be attached by a method D) or a spray coating method, but a CVD method which can be formed at a relatively low temperature is desirable. Although the operating conditions of the CVD method are not particularly limited, the conventional method can be modified and implemented for producing the gas electrode structure of the present invention. For example, after the surface of the ion exchange membrane substrate is roughened and sanded in advance by sanding or PVD,
Heating the methane or methyl alcohol to about 700 ° C. while maintaining a temperature of 0 to 200 ° C. and flowing a little argon to adhere the decomposed carbon while decomposing to form the conductive carbon thin layer on the ion exchange membrane substrate. Can be. The conductive carbon thin layer may contain a fluorine resin as a binder.

Next, an electrode catalyst is supported on the conductive carbon thin layer. As the electrode catalyst, a platinum group metal or an oxide thereof is used for both the anode and the cathode. Platinum black and ruthenium black are particularly effective, and ruthenium oxide and iridium oxide are particularly effective as the anode material. These electrocatalyst components may be selected in consideration of the type of reaction, economy, and ease of preparation. For example, in order to carry platinum, after activating the surface of the conductive carbon thin layer, an aqueous solution of a platinum group metal compound such as chloroplatinic acid, an alcohol solution or a dilute hydrochloric acid aqueous solution is applied to the conductive carbon thin layer surface. And
Decompose by heating at 200 to 300 ° C, or apply the aqueous solution of the platinum group metal compound on a conductive carbon thin layer and dry.Apply an aqueous solution of a reducing agent such as hydrazine, heat to about 100 ° C, and then wash and dry as appropriate. And so on.

The gas electrode structure carrying the electrode catalyst prepared as described above may be used as it is. However, when the gas electrode structure is exposed to a wet gas, the entire gas electrode structure is eventually covered with a water layer and the supply gas and the supply gas are removed. Contact may not be perfect and durability may be insufficient. For this purpose, it is preferable to impart a certain degree of hydrophobicity to the surface of the gas electrode structure. Hydrophobicity of the electrode surface can be achieved by applying and drying a fluororesin liquid or a Nafion (trade name) liquid which is a fluororesin-based ion exchange resin liquid. In particular, Nafion liquid is particularly desirable because it has a hydrophilic surface and a large protective effect. Thereafter, heat treatment may be performed at a temperature of about 100 to 250 ° C., if necessary. Although the surface area is reduced by the coating with the Nafion solution, the reactant is a gas, becomes an ion upon contact with the catalyst, passes through the ion exchange membrane substrate, and is quickly removed together with the electrolytic solution, so that there is practically no problem.

[0014] A current collector is connected to the conductive carbon thin layer side of the gas electrode structure thus prepared. The current collector may be a porous body such as a mesh having metal fine holes or a perforated plate. The material is desirably negatively polarized when used on the cathode side, so iron, nickel and stainless steel, which are corrosion-resistant on the negative side, are desirable. And since little potential is applied, there are few stable metal conductors, and valve metals such as titanium, tantalum, zirconium, and niobium are usually used, but they are not always satisfactory, and niobium, tantalum, or their base alloys are used as they are. Although it can be used, in the case of titanium or zirconium, it is desirable that its surface be plated with platinum or the like. In particular, in the case of titanium, the durability in acid is not sufficient, so that it is necessary to perform platinum or gold plating on the surface. Iron, nickel and stainless steel with negative polarity and corrosion resistance

Next, the gas electrode structure of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic vertical sectional view showing a state in which a gas electrode structure according to the present invention is incorporated in an electrolytic cell as an anode. The box-type electrolytic cell 1 is divided into an anode chamber 3 and a cathode chamber 4 by an ion exchange membrane 2, and the anode chamber 3 is further divided by a gas electrode structure 5 into an electrolyte chamber 6 and a gas chamber 7. The gas electrode structure 5 carries, from the side of the ion exchange membrane 2, a perfluorosulfonic acid-based ion exchange membrane base 9 whose lower end is in contact with the electrolytic cell bottom plate 8-an electrode catalyst comprising a platinum group metal or an oxide thereof. The conductive carbon thin layer 10 made of graphite or the like and the current collector 11 are laminated in this order, and the ion exchange membrane substrate 9 prevents the electrolyte in the electrolyte chamber 6 from migrating into the gas chamber 7. I have. In the cathode chamber 4, a cathode 12 made of a nickel plate or the like immersed in an electrolytic solution is provided.

When the electrolytic cell 1 having the gas electrode structure 5 is used as a sodium sulfate electrolytic cell, an aqueous solution of sodium sulfate is used in the electrolyte chamber 6 and the cathode chamber 4 of the anode chamber 3 and the gas chamber 7 of the anode chamber 3 is used.
Is supplied with electricity through the current collector 11 while supplying hydrogen gas. The water in the electrolyte chamber 6 permeates the ion exchange resin thin layer 9 and reaches the conductive carbon thin layer 10 supporting the electrode catalyst,
Electrolyzed to generate oxygen ions. Since the oxygen ions react with the supplied hydrogen gas and are converted into water, the electrolysis can be advanced with energy lower than that required for gas generation, that is, at a low electrolysis voltage. In this electrolysis, the permeation of sulfate ions and other impurities contained in the electrolytic solution in the electrolytic solution chamber 6 is suppressed by the ion-exchange membrane substrate 9, and these substances which may deteriorate the electrode catalyst are converted into conductive materials having the electrode catalyst. Since the electrode catalyst does not reach the thin carbon layer 10, deterioration of the electrode catalyst is suppressed, and the life of the electrode catalyst can be extended.

[0017]

EXAMPLES Next, examples in which the gas electrode structure of the present invention is applied to Glauber's salt electrolysis will be described, but the gas electrode structure of the present invention and the electrolysis method of the present invention are not limited thereto.

Example 1 The surface of Nafion 117, which is a perfluorosulfonic acid type cation exchange membrane, was blasted with glass beads and then immersed in a 5% aqueous solution of caustic soda at 40 ° C. for 24 hours. Further, a thin conductive carbon layer was formed on the surface by a chemical vapor deposition method using a mixed gas of 10% by weight of methane and 90% by weight of methyl alcohol and adhering carbon powder generated by decomposition at about 700 ° C. The film formation rate was 5 μm / min, and the treatment was performed until the thickness of the thin layer became 40 μm.

The ion exchange membrane substrate on which the conductive carbon thin layer was formed was used as an electrode in a chloroplatinic acid aqueous solution and treated with a direct current with an alternating current to coat platinum black at a rate of about 10 g / m ² .
After washing and drying the ion-exchange membrane substrate, a commercially available Nafion solution diluted with water was applied to the conductive carbon thin layer, and the solution was heated at 150 ° C.
Bake for 15 minutes. After washing with water again, an expanded mesh having a size of 4 × 2 mm and a thickness of 0.2 mm was attached as a current collector to obtain a gas electrode of this example. 20% of the theoretical amount of hydrogen gas is passed through the aqueous layer and then separated by a cation exchange membrane and supplied to the anode chamber of a two-chamber electrolytic cell provided with the gas electrode while supplying 20% of anolyte to the anode chamber. Of sodium sulfate (Glauber's salt) solution at a temperature of 70 ° C and a current density of
Electrolysis was performed at dm ² to obtain a mixed aqueous solution of sulfuric acid and sodium sulfate in the anode chamber, and a 15% aqueous solution of caustic soda in the cathode chamber. Tank voltage is 1.9 V
And the current efficiency is 88%, and the potential of the anode part is 0.
3 V.

[0019]

COMPARATIVE EXAMPLE 1 The procedure of Example 1 was repeated except that the gas electrode of Example 1 was replaced with a conventional oxygen-generating anode formed of a composite oxide of iridium oxide and tantalum oxide on the surface of a titanium substrate. When the Glauber's salt electrolysis was performed under the same conditions as those described above, the anode potential was 1.55 V and the cell voltage was 3.2 V.

[0020]

Example 2 A conductive carbon thin layer and an electrode catalyst were supported under the same conditions as in Example 1 except that Toflex IE-SA-48 manufactured by Tosoh Corporation was used as the ion exchange membrane substrate. A nickel porous plate in which holes having a diameter of 2 mm and a diameter of 2 mm were staggered at a pitch of 2.5 mm was attached.
This ion exchange membrane substrate with a current collector was installed as a cathode in the cathode chamber of an ion exchange membrane electrolytic cell for caustic alkali electrolysis. Electrolysis was performed with a current density of 3 kA / m ² , a temperature of 90 ° C., and an anolyte solution of 200 g / liter of sodium chloride. As a result, 33% of sodium hydroxide was obtained in the cathode chamber, and the cell voltage was 2.4 V.

[0021]

Comparative Example 2 Instead of the ion-exchange membrane substrate of Example 2, electrolysis was carried out while generating hydrogen in the cathode chamber using a 1 mm thick nickel expanded mesh coated with Raney nickel as a normal activated cathode. When the sodium sulfate electrolysis was performed under the same conditions as in Example 2 except that the cell voltage was 3.0 V, it was 60% when the gas electrode of Example 2 was used.
A voltage drop of 0 mV was observed. It is considered that the reason why the voltage drop is smaller than when the gas electrode is used on the anode side (Example 1 and Comparative Example 1) is that the resistance of the anion exchange membrane is large and the overvoltage of the gas electrode is large.

[0022]

The gas electrode structure according to the present invention comprises an ion-exchange membrane substrate, a thin conductive carbon layer formed on one surface of the ion-exchange membrane substrate and carrying a platinum group metal or an oxide thereof as an electrode catalyst. And a porous current collector disposed in close contact with a surface of the thin layer opposite to the metal of the ion exchange membrane. In the gas electrode structure of the present invention having such a configuration, the ion exchange membrane substrate is provided on the electrolyte side from the conductive carbon thin layer supporting the electrode catalyst. And / or the cathode compartment is divided into an electrolyte compartment and a gas compartment,
Only the desired electrolyte that can permeate the ion-exchange membrane substrate in the electrolyte on the electrolyte solution chamber side penetrates the substrate, reaches the conductive carbon thin layer, reacts with the gas by the electrode catalyst, and the desired electrolytic reaction is performed. And the product returns to the electrolyte chamber through the substrate again by the electric field. Therefore, corrosive substances and impurities in the electrolyte rarely permeate through the ion exchange membrane substrate and reach the conductive carbon thin layer to deteriorate the electrode catalyst. A gas electrode that can be achieved and has a long life can be provided.

The gas electrode of the present invention can be used both as an anode and a cathode. The preparation is carried out in the same process except that the type of the electrode catalyst and, if necessary, the ion exchange resin of the ion exchange membrane substrate are appropriately selected. Can be prepared. The thin conductive carbon layer on the surface of the ion exchange membrane substrate is preferably a graphite porous body formed by chemical vapor deposition. According to the chemical vapor deposition, even if the ion exchange membrane substrate has a complicated shape, a high surface area and a high Graphite having electrical conductivity can be formed.

The conductive carbon thin layer has a thickness of 30 to 100 to secure a sufficient surface area and to prevent a decrease in conductivity.
μm is desirable. The method of the present invention is a method for performing various electrolysis using the above-mentioned gas electrode as an anode and / or a cathode. As in the case of the above-mentioned gas electrode, corrosive substances and impurities in the electrolytic solution are ion-exchanged. There is almost no deterioration of the electrode catalyst after reaching the conductive carbon thin layer through the membrane substrate, and sufficient depolarization can be performed to reduce the electrolysis voltage.

[Brief description of the drawings]

FIG. 1 is a schematic longitudinal sectional view showing a state in which a gas electrode structure according to the present invention is incorporated in an electrolytic cell as an anode.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Electrolysis tank 2 ... Ion exchange membrane 3 ... Anode chamber 4 ... Cathode chamber 5 ... Gas electrode 6 ... Electrolyte chamber 7 ... Gas chamber 8 ... Bottom plate 9 ... Ion exchange membrane substrate 10 ...
Conductive carbon thin layer 11 ・ ・ ・ Current collector 12 ・ ・ ・ Cathode

Continuation of front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C25B 1/00-15/08

Claims (5)

(57) [Claims] 1. An ion-exchange membrane substrate, a conductive carbon thin layer formed on one surface of the ion-exchange membrane substrate and carrying a platinum group metal or an oxide thereof as an electrode catalyst, and the ion-exchange membrane of the thin layer A gas electrode structure comprising a porous current collector disposed in close contact with a surface opposite to a metal. 2. The gas electrode structure according to claim 1, wherein the conductive carbon thin layer is a graphite porous body formed by chemical vapor deposition. 3. The gas electrode structure according to claim 1, wherein the conductive carbon thin layer has a thickness of 30 to 100 μm. 4. A conductive carbon in which a platinum group metal or its oxide is supported as an electrode catalyst on one surface of an ion exchange membrane substrate in the anode chamber of an electrolytic cell partitioned into an anode chamber and a cathode chamber by an ion exchange membrane. A gas electrode structure formed by forming a thin layer and placing a porous current collector in close contact with the surface of the thin layer opposite to the ion exchange membrane substrate is provided as an anode, and hydrogen gas is supplied from the current collector side. An electrolysis method, wherein a positive potential is applied to the current collector while being supplied to perform electrolysis. 5. A conductive material in which a platinum group metal or an oxide thereof is supported as an electrode catalyst on one surface of an anion exchange membrane substrate in the cathode chamber of an electrolytic cell partitioned into an anode chamber and a cathode chamber by an ion exchange membrane. A gas electrode structure formed by forming a thin carbon layer and closely contacting a porous current collector on the surface of the thin layer opposite to the ion-exchange membrane substrate is provided as a cathode, and wet oxygen is supplied from the current collector side. An electrolysis method comprising applying a negative potential to the current collector while supplying a contained gas to perform electrolysis.