JPH10158878A - Aqueous alkali metal chloride solution electrolytic cell using oxygen-cathode gas diffusion electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an aq. alkali metal chloride soln. electrolytic cell by which an electrolyzing voltage is remarkably lowered in the electrolysis of an aq. alkali metal chloride soln., and electrolysis is performed at a low electrolyzing voltage even if a dil. gaseous oxygen such as air is supplied. SOLUTION: This aq. alkali metal chloride soln. electrolytic cell 1 consists of an ion-exchange membrane 7, the anode compartment 9 and cathode compartment 5 separated from each other by a gas diffusion electrode 2 and a gas chamber 4. The porous bodies of sponge metal through which a gas is three- dimensionally circulated and having an average pore diameter extending from 70ppi (0.36mm) to 6ppi (4.2mm) are packed in the gas chamber to constitute an oxygen-cathode gas diffusion electrode which is used. A porous body with sponge nickel as the sponge metal is preferably used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolytic cell using an oxygen cathode gas diffusion electrode in the electrolysis of an alkali metal chloride aqueous solution by an ion exchange membrane method, and more particularly, to an oxygen supply efficiency which can be increased and a dilute oxygen can be used. The present invention relates to an alkali metal chloride aqueous solution electrolytic cell using an oxygen cathode gas diffusion electrode capable of performing electrolysis at a low electrolysis voltage.

[0002]

2. Description of the Related Art In an alkali metal chloride aqueous solution electrolytic cell provided with an anode, a cation exchange membrane and a gas diffusion electrode, for example, an aqueous solution of sodium chloride is placed in an anode compartment, a diluted aqueous solution of caustic soda is placed in a catholyte compartment, and a gas containing oxygen is placed in the cathode compartment. A method of obtaining a concentrated aqueous solution of caustic soda and chlorine by performing electrolysis while supplying the gas to a gas chamber is known. The structure of an alkali metal chloride aqueous solution electrolytic cell provided with a cathode part using a gas diffusion electrode will be described below with reference to FIG. As shown in FIG. 4, an anode chamber 9 having an anode 8 and containing salt water and a cathode chamber 5 containing water or an aqueous caustic soda solution are connected to an ion exchange membrane 7 having a cation exchange ability.
A gas diffusion electrode 2 (so-called oxygen cathode) to which an oxygen-containing gas is supplied is used as a cathode. A catholyte 5 (hereinafter also referred to as a “catholyte compartment”) containing a catholyte (dilute caustic soda aqueous solution) by supplying an oxygen-containing gas (hereinafter simply referred to as a gas) to the gas diffusion electrode 2 and performing electrolysis as the cathode 2. ) To obtain an aqueous solution of caustic soda in which caustic soda is concentrated, and obtain chlorine on the anode 8 side.

In the above-mentioned salt cell provided with an anode, a cation exchange membrane and a gas diffusion electrode, usually, a cation exchange membrane is used for an anode chamber and a catholyte chamber, and a gas diffusion electrode is used for a catholyte chamber and an oxygen gas chamber. The following reaction proceeds at the gas diffusion electrode, which is a cathode, which is also referred to as a “gas chamber”. ４O ₂ + ／ H ₂ O + e ⁻ → OH ⁻ (1) At the anode, the following reaction proceeds as in the normal electrolysis method of the hydrogen generation type. _{^{Cl - → 1 / 2Cl 2 +}} e - ···· (2) gas diffusion electrode provided on the cathode portion has a structure in which laminated a normal reaction layer and a gas diffusion layer, the reaction layer is aqueous sodium hydroxide solution and gas The diffusion layer is in contact with oxygen gas. In the formula (1), H ₂ O (water) is supplied from the surface of the reaction layer in contact with the diluted aqueous sodium hydroxide solution, and O ₂ (oxygen gas) is supplied from the surface of the gas diffusion layer in contact with the gas chamber. The reaction represented by the formula (1) proceeds at the interface where the particles come into contact with each other.

[0004] Since it is desired that the cost of producing caustic soda be reduced as much as possible, when producing caustic soda by the salt electrolysis method, it is necessary to lower the electrolysis voltage and reduce the power consumption for electrolysis. is necessary. Since salt gas electrolysis using a gas diffusion electrode does not generate hydrogen gas, there is an advantage that the electrolysis voltage can be reduced as compared with a normal hydrogen generation type electrolysis method. The cathode part of the salt electrolysis cell having the gas diffusion electrode is compared with the cathode part of the ordinary salt electrolysis cell in which a cathode having a simple structure is used and a gas chamber is unnecessary.
Although there is a factor that causes energy loss as described below, the reaction can proceed at a low voltage as a whole because no hydrogen gas is generated at the cathode during electrolysis. However, there are other factors that cause energy loss in the cathode portion of the salt electrolytic cell having the gas diffusion electrode, and the energy loss due to these factors increases as the electrolytic cell becomes larger. In a salt electrolytic cell having a gas diffusion electrode, which is desired to be practically used after being enlarged to such an extent that the electrolytic cell can be carried out on an industrial scale, further improvement is required particularly for the cathode portion.

In the improvement of the ion exchange membrane method electrolysis using the gas diffusion electrode, attention has hitherto been paid only to the production method and the performance improvement of the gas diffusion electrode. Little consideration has been given to improvements in the tank structure. As for the structural problems of such a salt cell, regarding the cathode part, the energy loss caused by passing the catholyte through the catholyte compartment, the energy loss caused by the electric conduction between the gas diffusion electrode and the external cathode terminal, and the gas loss Problems such as energy loss caused by supplying a uniform and sufficient gas from the chamber to the gas diffusion electrode can be cited, and by improving such problems, the electrolytic voltage can be further reduced compared to the current situation, It is considered that the gas diffusion electrode can be applied to the salt electrolysis apparatus.

[0006] The structure of a gas diffusion electrode used in a salt electrolysis tank or the like has, for example, a two-layer structure in which a reaction layer 16 and a gas diffusion layer 17 are laminated as shown in FIG. Both layers are formed from a mixture of graphitic carbon and Teflon particles. Of these two layers, the reaction layer 16 faces the catholyte chamber 5 and is in contact with the electrolytic solution, and is mainly composed of a mixture of hydrophilic carbon fine particles and water-repellent carbon fine particles. A catalyst made of a noble metal such as platinum is supported on the fine particles. The gas diffusion layer 17 is mainly composed of a mixture of fluororesin fine particles and water-repellent carbon, and a small amount of hydrophilic carbon fine particles and a binder particle connecting these fine particles are mixed with each other. It is molded into a body. When the reaction layer 16 and the gas diffusion layer 17 are used in the electrolytic cell shown in FIG. 4, a hydrophilic reaction layer 16 is provided on the catholyte chamber 5 side, and a hydrophobic gas diffusion layer is provided on the gas chamber 4 side. The gas diffusion electrode 2 is obtained by arranging and laminating in the order of 17. Electrical continuity between the inside and outside of the gas diffusion electrode 2 is maintained by the conductivity of the conductive mesh and carbon inserted in and between the reaction layer 16 and the gas diffusion layer 17.

In order to reinforce the gas diffusion electrode, conventionally, as shown in FIG. 5, for example, an oxygen passage 18 is provided in the gas chamber 4 and the gas diffusion electrode 2 is supported. There is an example in which a net is provided on the back. However, in the prior art, the purpose of inserting the support is mainly to reinforce the gas diffusion electrode in order to compensate for the disadvantage that the strength of the gas diffusion electrode is low. , The flow of gas becomes non-uniform, and the supply of gas is insufficient, resulting in an increase in electrolysis voltage. Therefore, it cannot be said that the above-mentioned reinforcement measures can sufficiently cope with concerns about deformation of the gas diffusion electrode. Also, the previously attempted materials for supporting and reinforcing the gas diffusion electrode were not joined to the gas diffusion electrode, so the support was incomplete, and the assembly of the cathode of the gas diffusion electrode was troublesome. Was.

The inventor of the present invention has been mainly concerned with improving the structure of a salt electrolyzer using an ion-exchange membrane method for reducing the electrolysis voltage and increasing the size of the electrolyzer in salt electrolysis using a gas diffusion electrode. After careful examination of
A conductive rigid gas-permeable porous body, preferably a metal-made rigid gas-permeable porous body, is provided in the gas chamber 4 of the gas diffusion electrode in FIG.
To prevent deformation of the gas diffusion electrode 2, improve the uniformity of gas supply from the gas chamber 4 to the gas diffusion electrode 2, and improve the conventional method for improving the catholyte supply means. By removing the supply port and discharge port outside the cathode frame), the gap between the ion exchange membrane and the gas diffusion electrode can be reduced, and the distance from the gas diffusion electrode to the cathode terminal can be reduced. It has been found that the gas diffusion electrode can be applied to a practical soda electrolytic cell having an electrolytic device of 1 m square or more.

[0009]

SUMMARY OF THE INVENTION In electrolysis using a gas diffusion electrode, how much an element relating to the supply of oxygen to the gas diffusion electrode contributes to the electrolysis voltage, and what kind of element is required to reduce the electrolysis overvoltage. The systematic study on whether there is any means has not been sufficiently conducted. An object of the present invention is to clarify the structure of a gas chamber and the supply state of oxygen (gas) in the gas chamber depending on the structure, and to reduce the influence of the supply state and the like on the electrolysis voltage. . Among the factors related to the reduction of the electrolysis voltage in the electrolysis of alkaline chloride aqueous solution, diligent studies on the effect of the supply of oxygen in the gas chamber on the reduction of the electrolysis voltage, especially the effect of the structure of the gas permeable porous body provided in the gas chamber However, the present inventors have found that the structural improvement of the gas-permeable porous body provided in the oxygen gas chamber significantly contributes to the voltage drop related to the supply of oxygen to the gas diffusion electrode, and reached the present invention.

[0010]

The above object is achieved by the following alkaline chloride aqueous solution electrolytic cell of the present invention. (1) In an alkaline chloride aqueous solution electrolysis tank composed of an anode chamber, a catholyte chamber, and a gas chamber separated from each other by an ion exchange membrane and a gas diffusion electrode, gas can flow three-dimensionally into the gas chamber. An alkaline chloride aqueous solution electrolytic cell using an oxygen cathode gas diffusion electrode, characterized by being filled with a porous body made of a sponge metal having a pore diameter of 70 ppi or more (0.36 mm or more) and 6 ppi or less (4.2 mm or less). (2) The alkaline chloride aqueous solution electrolytic cell using the oxygen cathode gas diffusion electrode according to (1), wherein the sponge metal is sponge nickel. (3) The alkaline metal chloride aqueous solution electrolyzer according to (2), wherein the sponge nickel is coated with silver.

The gist of the present invention is to fill a gas chamber of a gas diffusion electrode with a porous body made of a sponge metal having a specific porous structure and supply a gas to the gas diffusion electrode via the porous body made of a sponge metal. Then, it is possible to maximize the voltage drop that is considered to be based on the gas supply state. According to the present invention, when a porous body made of a sponge metal is filled and an oxygen-containing gas is supplied, the degree of voltage drop can be largely maintained even when a dilute oxygen such as air is supplied.
Further, the porous body made of sponge metal filled in the gas chamber also has a function of reinforcing the gas diffusion electrode. The porous body made of sponge metal has an average pore diameter of 70 p as described above.
pi or more (0.36 mm or more), 6 ppi or less (4.2
mm or less).

If the distance between the electrodes is reduced to about 2 mm in the conventional electrolytic cell not filled with the gas-permeable porous body shown in FIG. 3 kA / m ² using oxygen, 2.1 V at 90 ° C.
A very low electrolysis voltage is obtained. It is preferable that air can be used as an oxygen source for the oxygen cathode reaction. However, when oxygen is diluted with an inert gas such as nitrogen, the overvoltage of the oxygen cathode further increases, and it is difficult to use air. For example, FIG.
In the case of the gas chamber having the above structure, the voltage was further increased by 1.0 V when air was used.

[0013] It was not previously known what caused such a large overvoltage of the oxygen cathode. The present inventor has found that in the course of studying the structure of the current collector in the gas chamber, the structure of the oxygen passage has a significant effect on the “oxygen overvoltage”. The filling effect of these porous bodies is understood as follows. It is considered that oxygen in the gas chamber is diluted with water vapor and nitrogen, and a film having an extremely low oxygen concentration is formed on the surface of the gas diffusion electrode.
Due to the presence of the packing, the flow of the oxygen-containing gas becomes turbulent,
It is considered that the film becomes thinner, the supply of oxygen is promoted, and the overvoltage is reduced. Therefore, it is considered that it is more suitable to fill a wire having a large porosity such as a wire mesh or foamed nickel and having a large turbulence effect than a simple groove-shaped passage.

The preferred form of the gas chamber in the cathode portion of the electrolytic cell of the present invention will be described. That is, the gas chamber of the cathode of the electrolytic cell of the present invention is filled with a porous body made of sponge metal which allows gas to flow three-dimensionally in contact with the gas diffusion electrode, and the gas is diffused toward the gas diffusion layer of the gas diffusion electrode. In addition, the gas supply is made uniform, thereby making the gas supply in the gas chamber uniform, and preventing the gas diffusion electrode from being deformed. Since the porous body in the present invention is in contact with the gas diffusion electrode on the entire surface, by connecting a cathode terminal to the porous body, the porous body becomes a current collector having a large area. Thus, the current distribution can be made more uniform. Furthermore, if the porous body and the gas diffusion electrode are joined and molded integrally, a special current collector as in the prior art is not required. At this time, the current applied to the gas diffusion electrode is also uniformly applied to the entire surface of the gas diffusion electrode as compared with the current flowing when the current is applied from a terminal provided at the peripheral end of the gas diffusion electrode. can do.

Side longitudinal section of a gas chamber in which a gas diffusion electrode is arranged, wherein a sponge metal is filled as a rigid porous body capable of uniformly supplying gas three-dimensionally to a gas chamber in a cathode portion of an electrolytic cell. The figure is shown as FIG. In FIG. 2, 2 is a gas diffusion electrode, 4 is a gas chamber, 5 is a porous body made of sponge metal, and 6 is a cathode terminal.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION A preferred embodiment of a porous body made of sponge metal will be described in more detail. The sponge metal has a name such as a foamed metal or a porous metal body, and can be obtained by plating a foamed polyurethane with a metal, mixing with a metal powder, and firing. To further explain an example of the production method, sponge metal is immersed in open-cell foamed polyurethane in a medium in which fine metal powder is dispersed, and the fine metal powder is deposited inside the foamed polyurethane foam, Thereafter, the foamed polyurethane on which the metal fine powder is deposited is subjected to a centrifugal separation treatment to remove the medium, and then the surface is preferably metal-plated. At this time, by repeating the process of squeezing the foamed polyurethane, removing the pushing force, expanding the polyurethane to absorb the liquid, the metal fine powder can be sufficiently deposited inside. The treated polyurethane foam containing fine metal powder is placed in a reduction furnace and fired. The organic portion is completely burned off, and the metal fine powder is also partially melt-bonded to each other to obtain sponge metal as a sponge-like porous body. The porosity of the obtained sponge metal is as high as about 95%,
And it has the characteristic of open cells. Nickel, copper, silver and the like are suitable as the metal.

When using sponge metal as a filling material for a gas chamber, an important point is firstly a pore diameter. The average pore diameter is represented by ppi (average number of holes per inch) in relation to the raw material foamed polyurethane. However, since this “ppi” is represented by the average number of holes per inch, it does not directly indicate the diameter of the holes of the “ppi”. Since it is difficult to calculate the hole diameter, such a display method is used. Those having an average pore diameter of 70 ppi or more (0.36 mm or more) and 6 ppi or less (4.2 mm or less) are suitable. If the average pore diameter exceeds 6 ppi, the supply capacity of oxygen is inferior, and when dilute oxygen (for example, air) is used, the overvoltage of the cathode increases. On the other hand, when the pore diameter is 70 ppi or less, the pressure loss accompanying the gas supply increases, and a high pressure is applied to the gas diffusion electrode from the back surface, which is not practically preferable. The second important point is that the conductive resistance must be low. The conductive resistance is determined by the material, the bonding method with the gas diffusion electrode, the bonding method with the back cathode terminal, and the like. Preferred materials include copper, nickel, and silver. In particular, nickel is preferable in terms of corrosion resistance and economy. In addition, when the surface of the sponge nickel is coated with a silver material, the corrosion resistance is further improved. Plating, powder coating and the like are preferred as the coating method. In particular, it is preferable to cover the surface in contact with the gas diffusion electrode with silver. By coating with silver, the voltage does not increase even if the electrode is used for a long time. For bonding with gas diffusion electrode,
Pressing by pressure through a good conductor such as silver may be used,
Preferred results are obtained by bonding silver or a silver alloy as the bonding material.

The thickness of the gas chamber is also a condition to be considered. Although the appropriate thickness varies depending on the size of the electrolytic cell, it is preferably about 0.5 mm to 3 mm for a normal-sized electrolytic cell. When the thickness is 3 mm or more, the flow velocity decreases, the oxygen supply capacity decreases, and as a result, when the oxygen concentration decreases, the overvoltage of the cathode increases. On the other hand, when the thickness is 0.5 mm or less, the oxygen supply capacity is increased, but the pressure loss accompanying the gas flow becomes large, which is not practical. Gas chamber thickness 1
The structure completely filled with sponge metal having a thickness of about 1 mm and a thickness of about 1 mm is extremely good. By using the electrolytic cell having such a configuration, the cell voltage can be reduced to about 2V. Further, even if air is used as the oxygen source, the rise in voltage can be suppressed to 0.2 V or less. The conductor resistance can be reduced to 10 mV or less. Even if such sponge metal is filled, the pressure loss can be reduced to 2 m or less. Regarding the connection with the electrode and the terminal, fusion, paste bonding, pressure welding, solder bonding, welding, screwing, and the like are possible according to the combination.

[0019]

The present invention will be described below in detail with reference to examples. However, the present invention is not limited to only this embodiment. Example 1 The electrolytic cell of FIG. 1 was assembled. Gas diffusion electrode 2 is disclosed in
It was prepared according to the description in Japanese Patent Application Laid-Open No. 3-245683. The thickness of the reaction layer is 100 μm, and the thickness of the gas diffusion layer is 400 μm
This is an electrode having a two-layer structure. The reaction layer has a structure in which a hydrophilic portion and a water-repellent portion are entangled. The reaction layer supports platinum as a catalyst. The porous body 3 made of sponge metal to be filled in the gas chamber 4 having a thickness of 1 mm has an average pore diameter of 50 ppi (the number of holes per inch),
Sponge nickel (manufactured by Nippon Heavy Industries, Ltd.) having a thickness of 1 mm was used. This porous body 3 is sponge nickel 3
Also called. The gas diffusion electrode 2 was pressed against the sponge nickel 3 at a temperature of 350 ° C. at a pressure of 50 kg / cm ² . In the assembled state as shown in FIG. 1, the sponge nickel 3 is strongly pressed against the cathode terminal 6 on the back surface.

An electrode tank 1 was constructed by stacking an anode 8, called a DSA (registered trademark), a fluorine-based cation exchange membrane 7, and the cathode portion. The distance between the anode 8 and the gas diffusion electrode 2 is 2 mm
And The effective area of the electrode and the ion exchange membrane 7 is 10 × 10
cm ² . 310 g / l of purified saline is supplied to the anode compartment 9 while heating the catholyte compartment 5 (ion exchange membrane 7).
And a cathode 2), a 32% aqueous solution of sodium hydroxide was heated and supplied at a flow rate of 200 ml / min.
A mixed gas of oxygen and nitrogen was supplied to the gas chamber 4 (oxygen chamber) while changing the oxygen concentration and changing the overall flow rate so that the oxygen flow rate was the same. Electrolysis was performed while adjusting the temperature of the entire electrolytic cell to 90 ° C. The voltage between the terminals (voltage between the cathode terminal and the anode) at that time was as shown in Table 1 below.

[0021]

[Table 1]

When the ohmic loss and overvoltage analysis were performed on the voltage, all the ohmic losses were 0.13 V / (kA / m ² ).
It was found that the voltage change in Table 1 was a change in overvoltage. According to Table 1, even if the oxygen flow rate is the same, the voltage increases as the oxygen concentration decreases, but the voltage is maintained in a practical range even when the oxygen concentration is as low as 20%, such as air. It turned out to be.

Comparative Example 1 An electrode equivalent to that of Example 1 was used as a gas diffusion electrode. Gas chamber 4 not filled with a porous body having the structure of FIG.
The electrolytic cell 1 using the (oxygen chamber) was assembled. Groove width is 7
mm. The gas electrode is 310 through a silver mesh.
Pressure bonding was performed at a temperature of ° C. Thickness of gas chamber 4 (depth of groove)
Was 1 mm. The distance between the electrodes was 2 mm. The material is nickel. An electrolysis experiment similar to that of the example was performed. The voltage between the terminals at that time was as shown in Table 2 below.

[0024]

[Table 2]

When oxygen is reduced to a concentration equivalent to the air composition,
The voltage rises up to 1V.

Example 2 An electrolysis experiment was carried out in the same manner as in Example 1 except that the pore size of the sponge nickel 3 was different from 13 ppi.
The change in voltage when the oxygen concentration was changed was as shown in Table 3 below.

[0027]

[Table 3]

[0028]

According to the present invention, in an alkaline chloride aqueous solution electrolysis tank comprising an anode chamber, a catholyte chamber and a gas chamber separated from each other by an ion exchange membrane and a gas diffusion electrode,
By filling the gas chamber with sponge metal, preferably sponge nickel: Under conditions where the oxygen concentration is insufficient, for example, under conditions where air is used, an increase in electrode overvoltage in the cathodic reaction of oxygen can be minimized. 2. The electrolysis voltage of the aqueous alkali chloride solution electrolysis performed by supplying oxygen to the gas diffusion electrode via the gas chamber in the aqueous alkali chloride solution electrolyzer can be significantly lower than the electrolysis voltage of the conventional alkali chloride aqueous solution electrolysis. 3. When the catholyte is passed through the catholyte compartment, distortion occurring in the gas diffusion electrode can be prevented. 4. By integrally fixing the sponge metal to the outer frame of the gas chamber and the gas diffusion electrode, operability such as assembly of the cathode portion of the alkaline chloride aqueous solution electrolytic cell having the gas diffusion electrode can be simplified. .

[Brief description of the drawings]

FIG. 1 is an explanatory sectional view showing an example of an alkaline chloride aqueous solution electrolytic cell provided with a gas diffusion electrode having a gas chamber filled with a sponge metal of the present invention.

FIG. 2 is an explanatory sectional view showing an example of a gas chamber filled with a porous body of the present invention.

FIG. 3 is an explanatory sectional view showing an example of the gas diffusion electrode of the present invention.

FIG. 4 is a cross-sectional explanatory view showing an example of an alkaline chloride aqueous solution electrolytic cell provided with a conventional gas diffusion electrode.

FIG. 5 is an explanatory cross-sectional view showing an example of a conventional gas chamber in which a gas passage is provided in a room.

[Explanation of symbols]

 Reference Signs List 1 electrolytic cell 2 gas diffusion electrode 3 porous metal sponge 4 gas chamber 5 cathode chamber (catholyte chamber) 6 cathode terminal 7 ion exchange membrane 8 anode 9 anode chamber 10 gas inlet 11 gas outlet 12 caustic soda liquid inlet 13 caustic soda liquid outlet 14 Salt water inlet 15 Salt water outlet 16 Reaction layer 17 Gas diffusion layer 18 Oxygen passage 19 Current collector

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Choichi Furiya 2-14 Nakamuracho, Kofu City, Yamanashi Prefecture

Claims (3)

[Claims] 1. An alkali metal chloride aqueous solution electrolysis tank comprising an anode exchange chamber, a catholyte compartment, and a gas compartment separated from each other by an ion exchange membrane and a gas diffusion electrode, whereby gas can flow three-dimensionally into the gas compartment. The average pore size is 70
ppi or more (0.36 mm or more), 6 ppi or less (4.
An alkali metal chloride aqueous solution electrolytic cell using an oxygen cathode gas diffusion electrode, wherein a porous material made of sponge metal (2 mm or less) is filled in contact with the gas diffusion electrode. 2. The electrolytic solution of an alkali metal chloride aqueous solution using an oxygen cathode gas diffusion electrode according to claim 1, wherein the sponge metal is sponge nickel. 3. The electrolytic solution of an alkali metal chloride aqueous solution according to claim 2, wherein said nickel sponge is coated with silver.