JPH108283A - Liquid permeation type gas diffusion cathode structural body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable stable electrolysis for a long term even under severe conditions by disposing a metal mesh body between a gas diffusion cathode and a cathode power feeding body. SOLUTION: A metal mesh body 7 is interposed between a power feeding body 8 and a gas diffusion cathode 6 in such a manner that the mesh body 7 is in contact with the cathode 6 and that a cathode liquid generating on the cathode 6 and passing through the mesh to the cathode room touches the metal mesh body 7. Thereby, the metal mesh body 7 diffuses the cathode liquid in a three-dimensional direction, namely, the mesh body 7 has a function to diffuse the liquid not only in the parallel direction to the surface of the gas diffusion cathode 6 but in the direction to the power feeding body 8. Therefore, clogging of the gas diffusion cathode 6 or changes of the cathode surface into hydrophilic one which are caused when the cathode liquid is held on the surface of the gas diffusion cathode can be prevented. Thus, stable electrolysis can be performed for a long term without deteriorating the function of the gas diffusion cathode 6.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas diffusion cathode structure capable of efficiently removing an electrolytic product, and more particularly to a gas diffusion cathode structure which can be preferably used for soda electrolysis and can easily remove caustic soda from its surface. The present invention relates to a cathode structure.

[0002]

2. Description of the Related Art Electrolysis industry represented by chloralkali electrolysis plays an important role as a material industry. Although having such an important role, the energy consumption required for chloralkali electrolysis is large, and energy saving is a major problem in countries with high energy costs such as Japan. For example, in chlor-alkali electrolysis, in order to solve environmental problems and achieve energy saving, the mercury method was switched to the ion exchange membrane method via the diaphragm method, and about 25%.
Annual energy savings of about 40% have been achieved. However, even this energy saving is not enough, and the power cost, which is energy, accounts for 50% of the total manufacturing cost. However, no further power saving is possible if the current method is used. In order to achieve more energy savings, drastic changes must be made, such as correcting the electrode reaction. As an example, the use of a gas diffusion electrode employed in a fuel cell or the like is the most likely and possible means of saving electric power.

The anodic reaction using a conventional metal electrode is
When a gas diffusion electrode is used as an anode, it is converted into an anodic reaction. The _{2NaCl + 2H 2 0 → Cl 2} + 2NaOH + H 2 E O = 2.21V 2NaCl + 1 / 2O 2 + H 2 O → Cl 2 + 2NaOH E O = 0.96V clogging metal electrodes by converting the gas diffusion electrode,
The potential is reduced from 2.21 V to 0.96 V, and theoretically about 65% energy saving is possible. Therefore, various studies have been made toward the practical use of chloroalkali by using this gas diffusion electrode. The structure of the gas diffusion electrode is generally called a semi-hydrophobic (water-repellent) type, and has a structure in which a hydrophilic reaction layer carrying a catalyst such as platinum on the surface and a water-repellent gas diffusion layer are joined. I have. Water-repellent polytetrafluoroethylene (PT) is used as a binder for both the reaction layer and gas diffusion layer.
FE) resin is used, and by using the characteristics of the PTFE resin, the ratio is increased in the gas diffusion layer and reduced in the reaction layer to form both layers.

[0004] The use of such gas diffusion electrodes for chloralkali electrolysis has several problems. For example, in high-concentration caustic soda, PTFE resin as a water-repellent material becomes hydrophilic and easily loses water repellency. In order to prevent this, it has been attempted to attach a thin porous PTFE sheet to the gas diffusion layer on the gas chamber side. Electrolysis proceeds while supplying oxygen and air to the gas diffusion electrode. However, hydrogen peroxide is partially generated as a side reaction, which may corrode carbon as a constituent material to generate sodium carbonate. In an alkaline solution, the sodium carbonate precipitates and may block the gas diffusion layer or make the surface hydrophilic, thereby deteriorating the function of the gas diffusion electrode. It has also been observed that even if this sodium carbonate is not generated, only the catalyst is supported on the carbon surface, and that the catalyst causes carbon corrosion.

[0005] Conventionally, in order to eliminate such disadvantages,
Studies have been made on the selection of carbon to be used, the production method thereof, and the control of the mixing ratio of carbon and resin. However, these methods are not fundamental solutions, and can only stop the corrosion of carbon but cannot stop it. Since such a corrosion problem does not occur unless carbon is used, an attempt has been made to use silver, which is a metal, instead of carbon. However, the gas diffusion electrode using this metal is manufactured by a sintering method unlike the gas diffusion electrode using carbon as a constituent material, and the manufacturing method becomes extremely complicated.In addition, the gas diffusion electrode using a metal has a hydrophilic portion. There is a problem that it is difficult to control the hydrophobic portion.

As a solution to these problems and as a method for further reducing the electrolysis voltage, a gas diffusion electrode is adhered to or adhered to an ion exchange membrane to substantially eliminate the cathode chamber. A method of configuring a room has been proposed. When chlor-alkali electrolysis is performed using an electrolytic cell employing this method, the generated caustic soda reaches the gas chamber, which is the cathode chamber, through the reaction layer and the gas diffusion layer. In this method, since there is no catholyte, the influence of the pressure difference in the height direction of the gas chamber is eliminated and it is not necessary to consider the pressure distribution even when the size is increased, and the electric resistance is minimized because the catholyte is substantially absent. While having the advantage that the electrolysis voltage can be kept to a minimum, the size and distribution of the through holes in the gas diffusion layer must be controlled in order to promote the permeation of the generated caustic soda toward the gas chamber. Moreover, the caustic soda taken out to the gas chamber side easily blocks the through hole of the gas diffusion layer, and if the blockage occurs, the smooth progress of the electrolysis is hindered. In such a large electrolytic cell, problems such as non-uniform current distribution and an increase in electrolytic voltage due to the blockage are likely to occur, and the blockage of the through-hole is the biggest obstacle to achieving a large-sized electrolytic cell. Similar problems have been pointed out in soda electrolysis such as sodium sulfate electrolysis in addition to ordinary salt electrolysis.

[0007]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems of the prior art, that is, the drawback that the gas diffusion electrode cannot be used at a practical level for electrochemical reactions such as salt electrolysis and sodium sulfate electrolysis. It is an object of the present invention to provide a liquid-permeable gas diffusion cathode structure that is stable for a long period of time even under various conditions and can be substantially used for salt electrolysis and the like.

[0008]

The liquid-permeable gas diffusion cathode according to the present invention comprises a gas diffusion cathode in contact with an ion-exchange membrane defining an anode chamber and a cathode gas chamber, and a metal mesh in contact with the gas diffusion cathode. A liquid-permeable gas diffusion cathode structure comprising: a body; and a power supply connected to the metal network and configured to supply power to the gas diffusion cathode through the network.

Hereinafter, the present invention will be described in detail. In the present invention, in a soda electrolysis such as salt electrolysis and sodium sulfate electrolysis using a gas diffusion cathode or other electrolysis reaction, a cathode product such as caustic soda taken out to a cathode chamber side constituting a gas chamber is subjected to gas diffusion of the gas diffusion cathode. A gas diffusion cathode structure that can be quickly removed from the surface of the cathode to suppress instability of electrolysis conditions due to blockage and hydrophilicity of the gas diffusion cathode, and to perform electrolysis under stable conditions even after long-term use Can be provided. It is considered that releasing the caustic soda solution obtained from the surface of the gas diffusion cathode can be performed smoothly by making the surface of the gas diffusion cathode water-repellent, that is, by deteriorating the wettability of the liquid.

However, although the surface wettability of the surface of the gas diffusion cathode can be reduced only by making the surface of the gas diffusion cathode water-repellent, the solution that has passed through the gas diffusion cathode and reaches the gas chamber side is formed as a polka-dot droplet. There is a problem that the droplet remains on the surface of the cathode and does not separate from the surface unless the droplet becomes considerably large. Usually, a power supply for supplying electricity to the gas diffusion cathode is connected to the surface of the gas diffusion cathode opposite to the ion exchange membrane. In the present invention, a metal mesh is sandwiched between the power supply and the gas diffusion cathode so as to be in contact with the gas diffusion cathode, and the catholyte is generated by the gas diffusion cathode, passes through the gas diffusion cathode and reaches the cathode chamber side. Is brought into contact with the metal mesh. This catholyte is likely to remain on the gas diffusion cathode surface as droplets without a metal network, whereas the metal network diffuses the catholyte three-dimensionally, i.e., the gas in the metal network. The above-described gas diffusion caused by the catholyte having a function of diffusing the catholyte solution not only in the direction parallel to the surface of the diffusion cathode but also in the direction of the power feeder, and the catholyte solution is held on the surface of the gas diffusion cathode. This prevents the cathode from being clogged or the surface of the gas diffusion cathode from becoming hydrophilic, and enables long-term stable electrolysis without deteriorating the function of the gas diffusion cathode.

The metal mesh rapidly diffuses and removes the catholyte from the gas diffusion cathode surface so that the supply of the cathode gas such as oxygen and air and the removal of the produced gas can be performed smoothly. Further, when the metal net has elasticity, the gas diffusion cathode is pressed in the direction of the ion exchange membrane by the elasticity so that the gas diffusion cathode is uniformly adhered to the ion exchange membrane. Becomes possible. The mesh used in the present invention needs to be made of metal. This is because the power supply and the gas diffusion cathode are electrically connected, and the gas diffusion cathode of the present invention is often used for producing corrosive catholyte such as caustic soda by salt electrolysis. In so-called soda electrolysis, about 30 to 40% of caustic soda is usually obtained, and about 10 to 100 ppm of sodium chloride is mixed therein. Such a solution is highly corrosive and corrodes many metals. In particular, the reticulated body during electrolysis is not in a negatively polarized state at a position in contact with the catholyte, and is more susceptible to corrosion. Therefore, it is preferable to use a metal having high corrosion resistance, for example, nickel or silver, and it is preferable to use an alloy of these metals having high corrosion resistance. It is also possible to use a composite in which a relatively low-corrosive metal or alloy such as copper is plated with nickel or silver on the surface.

The net is preferably formed by knitting a metal wire into a net shape. For example, a net having a diameter of about 0.02 to 0.2 mm is knitted, and the net is superposed or three-dimensionally knitted. Is configured. In addition, a plurality of long wires can be randomly entangled with each other to form a felt-like net, but in this case, it is relatively difficult to form a gap between the wires and the surface is wet. Even if the properties are improved, the liquid may be clogged between the wires and hinder the gas flow. Therefore, when using the felt-like net, a relatively thick wire having a diameter of 0.1 to 0.2 mm is used, the packing density of the net is reduced to, for example, less than about 10%, and the distance between the wires is reduced. It is preferable to prevent liquid clogging.

The most preferred form of the mesh is a laminate of a knitted mesh having an uneven surface and elasticity. In this case, the mesh has an opening of about 0.5 to 1 mm and an apparent thickness of about 0.1 mm.
It is preferable that 5 to 20 layers of mesh of about 5 to 2 mm are laminated. The mesh size and apparent thickness are the same in the case of a three-dimensional mesh. In addition to the above-described laminate and three-dimensional body, a metal foam can also be used as the net of the present invention. However, since the metal foam has relatively low elasticity, a metal foam having a relatively large aperture and a large wire diameter is selected. The apparent thickness of the reticulated body including the reticulated body having the optimum shape is not particularly limited, but is preferably in a range of 5 to 10 mm in a natural state where no pressure is applied and about 1 to 3 mm in an installed state.

The metal or metal alloy constituting these nets usually has sufficient wettability with caustic soda or other aqueous solutions, but the wettability is insufficient and droplets are formed on the surface of the metal or the like. In this case, the reticulated body may be treated at a high temperature in an oxidizing atmosphere such as air to form an oxide on the surface of the reticulated body, thereby improving the wettability. The reticulated body is inserted between the gas diffusion cathode and the power supply, and is pressure-applied to the entire gas diffusion cathode to be integrated to be fixed at a predetermined position while being connected to the gas diffusion cathode and the power supply. Is done.

In this state, when electricity is applied between the two electrodes, a cathode product such as caustic soda is generated at the gas diffusion cathode, and the caustic soda passes through the gas diffusion cathode and reaches the gas diffusion cathode surface. The caustic soda or the like comes into contact with the mesh located on the surface, spreads three-dimensionally along the wire of the mesh, etc., and easily separates from the gas diffusion cathode surface. Therefore, only a minimal amount of caustic soda or the like is present on the gas diffusion cathode surface, without blocking the through holes of the gas diffusion cathode,
Supply and desorption of gas can also be easily performed, and the reticulated body is compressed by the pressure at the time of installation and has elasticity,
Due to this elasticity, the gas diffusion cathode is pressed in the direction of the ion exchange membrane to uniformly adhere the gas diffusion cathode to the ion exchange membrane, so that electrolysis at a low voltage is possible.

The accompanying drawings illustrate a two-chamber type electrolytic cell for soda electrolysis according to the present invention. FIG. 1 is a schematic longitudinal sectional view, and FIG. 2 is an enlarged view of a main part of FIG. The electrolytic cell body 1 is
An ion exchange membrane 2 separates an anode chamber 3 and a cathode chamber (gas chamber) 4. A mesh-shaped insoluble anode 5 is in close contact with the ion exchange membrane 2 on the anode chamber 3 side, and the cathode chamber of the ion exchange membrane 2 is formed. A gas diffusion cathode 6 is in close contact with the fourth side. On the cathode chamber 4 side of the gas diffusion cathode 6, a metal net 7 formed by three-dimensionally knitting a metal wire is located, and a cathode current collector 8 is connected to the net 7. Reference numeral 9 denotes an anolyte inlet formed in the bottom plate of the anode compartment, 10 denotes an anolyte and gas outlet formed in the top plate of the anode compartment, 11 denotes an oxygen-containing gas inlet formed in the top plate of the cathode compartment, and 12 denotes a gas inlet. A caustic soda outlet formed in the cathode chamber bottom plate. When an anolyte, for example, a saline solution is supplied to the anode chamber 3 of the electrolytic cell main body 1 and an oxygen-containing gas is supplied to the cathode chamber 4 and a current is supplied between the two electrodes 5 and 6, the surface of the ion exchange membrane 2 on the side of the cathode chamber 4 To form caustic soda, which passes through the gas diffusion cathode 6 and reaches the surface of the metal mesh 7.
Since the metal network 7 has a three-dimensional spread, the caustic soda does not form droplets, but separates from the surface of the metal network along the wires and the like of the metal network 7 and the surface of the gas diffusion cathode 6. Caustic soda does not remain in the gas, the gas can be supplied and taken out smoothly, and the gas diffusion cathode can be prevented from being clogged. Although the attached drawings show a two-chamber electrolytic cell for soda electrolysis, the present invention is also applicable to a three-chamber electrolytic cell for soda electrolysis.

[0017]

Next, examples of the gas diffusion cathode and electrolysis using the electrode according to the present invention will be described, but the examples do not limit the present invention.

[0018]

EXAMPLE 1 A nickel foam having a porosity of 90% manufactured by Eltec Co., Ltd. and having a thickness of 1 mm was crushed to a thickness of 0.2 mm by a press and used as a substrate. Nickel powder having an average particle size of 30 μm, and a PTFE dispersant J30 manufactured by DuPont, which is an aqueous dispersant for a fluororesin.
Was mixed with nickel and PTFE in a volume ratio of 1: 1 to impregnate the substrate. A paste prepared by dispersing silver powder having an average particle size of 0.1 μm in the above-mentioned PTFE dispersant J30 so that silver: PTFE = 9: 1 was applied to one surface of the substrate so that silver was 20 g / m ^2. This was applied, and this was heated and sintered at 300 ° C. for 15 minutes under a pressure of 0.2 kg / cm ² to obtain a liquid-permeable gas diffusion cathode main body.

The electrode material side of the cathode is brought into close contact with an ion-exchange membrane Nafion 90209 manufactured by DuPont. On the opposite side of the ion-exchange membrane of the gas diffusion cathode, a wire diameter of 0.1 mm, an aperture of 10 mm, and irregularities are formed. A nickel metal net having an apparent thickness of 10 mm and a mesh of about 6 to 7 mm laminated thereon was positioned, and a nickel power feeder having a wire diameter of 1 mm was connected to the net opposite to the cathode. An insoluble anode coated with a mixture of ruthenium oxide and tantalum oxide is closely adhered to a titanium mesh on the anode chamber side of the ion exchange membrane, and pressure is applied and fixed between the cathode power supply and the insoluble anode to increase the electrolytic area. An electrolytic cell for soda electrolysis having a length of 25 cm and a width of 10 cm was constructed. The filling rate of the mesh is 55%, and the apparent thickness after installation is 2-3mm
Met. The anode chamber of this electrolytic cell was supplied with 180 g / liter of a saline solution, and the cathode chamber was supplied with oxygen-enriched air having an oxygen concentration of 90% through an aqueous layer to supply a moistened gas at 120% of the theoretical amount. Electrolysis was performed at 90 ° C. and a current density of 30 A / dm ² . The initial cell voltage was 2.06 V, and 32% caustic soda was obtained from the cathode chamber. A few drops of caustic soda were found on the back of the feeder in the cathode compartment, but most had been removed downward through the mesh. Tank voltage after continuous operation for 30 days is 2.08
V, and the liquid removal was performed smoothly.

[0020]

Comparative Example 1 The same electrolytic cell as in Example 1 was assembled except that the metal net was not installed, and electrolytic production of caustic soda was performed under the same conditions. The initial cell voltage was 2.48 V, but after 1 hour, the voltage rose to 2.8 V, and slight hydrogen evolution was observed, so the electrolysis was stopped. From the viewpoint of the cathode power supply, it was observed that caustic soda droplets, which are catholyte, adhered to the entire back surface of the gas diffusion cathode. Since the catholyte removal was not performed smoothly and sufficient oxygen could not be supplied, it can be assumed that the cell voltage increased.

[0021]

Example 2 The same electrolytic cell as in Example 1 was constructed except that a stainless steel net-plated silver was plated between a power supply and a gas diffusion cathode. The reticulated body is formed by heating and oxidizing a three-dimensional knitted fabric having an apparent thickness of 10 mm, formed of SUS310S having a wire diameter of 0.15 mm, at 450 ° C. for 2 hours and hydrophilizing the surface.
mm. Electrolysis was performed under the same conditions as in Example 1 except that the amount of the saline solution supplied to the anode chamber was 200 g / liter and the oxygen-enriched air was not wetted. The initial cell voltage was 2.20 V, and 40-42% caustic soda was obtained from the cathode compartment. There was no change in cell voltage after 100 days of continuous operation.

[0022]

Example 3 The same electrolytic cell as in Example 2 was constructed except that a stainless steel net having no silver plating applied to the surface of the net was used, and electrolysis was performed under the same conditions. After a lapse of about 100 days, a slight amount of corrosion began to appear on the surface of the mesh. This is presumed to be due to a trace amount of chlorine contained in the electrolyte.

[0023]

Comparative Example 2 The same electrolytic cell as in Example 2 was assembled except that the stainless steel mesh was not installed, and electrolytic production of caustic soda was performed under the same conditions. The initial cell voltage was 2.55 V, and when the electrolysis was continued with the maximum voltage set to 2.8 V, the electrolysis was stopped because the current density became 10 A / dm ² after 30 minutes and hydrogen generation was observed. .

[0024]

The gas diffusion cathode according to the present invention comprises a gas diffusion cathode in contact with an ion exchange membrane defining an anode chamber and a cathode gas chamber, a metal mesh in contact with the gas diffusion cathode, and a connection to the metal mesh. And a feeder for feeding power to the gas diffusion cathode through the mesh. The gas diffusion cathode structure is such that caustic soda and the like generated and transmitted by the gas diffusion cathode come into contact with a metal network having a three-dimensional structure located on the gas diffusion cathode surface and from the gas diffusion cathode through the network. Spreads three-dimensionally and detaches from its surface. Therefore, the gas diffusion cathode does not block the through-holes due to the caustic soda or the like, thereby making the electrolysis conditions unstable, and unlike the conventional soda electrolysis using the gas diffusion cathode, the gas diffusion cathode can be used even if the operation is continued for a long time. The caustic soda and the like do not remain on the surface of the metal, and the generated caustic soda and the like can be immediately taken out from the cathode chamber side. Further, this makes it possible to smoothly supply and take out the gas, and it is possible to reduce the cell voltage.

When the size of the electrolytic cell is increased, separation of the liquid from the gas diffusion cathode surface described above tends to be a serious problem.
Solving this problem often becomes a bottleneck in increasing the size of the electrolytic cell. According to the present invention, even if the size of the electrolytic cell is increased, a large amount of liquid can be smoothly separated simply by increasing the size of the metal mesh to correspond to the size. Further, the metal mesh used in the gas diffusion cathode according to the present invention preferably has elasticity, and the elasticity makes the gas diffusion cathode uniformly adhere to the ion exchange membrane, thereby further reducing the cell voltage. Can be achieved. Examples of the form of the metal net include a layered metal wire mesh, a three-dimensional knitted metal wire, and a metal foam.Either form can easily remove caustic soda generated from the gas diffusion cathode surface. And it can be removed smoothly.

[Brief description of the drawings]

FIG. 1 is a schematic longitudinal sectional view illustrating an electrolytic cell for soda electrolysis according to the present invention.

FIG. 2 is an enlarged view of a main part of FIG. 1;

[Explanation of symbols]

1 ・ ・ ・ Electrolyzer main body 2 ・ ・ ・ Ion exchange membrane 3 ・ ・ ・
Anode chamber 4 ・ ・ ・ Cathode chamber (gas chamber) 5 ・ ・ ・ Insoluble anode 6 ・ ・ ・ Gas diffusion cathode 7 ・ ・ ・ Metal net 8.
·· Cathode feeder 9 ··· Anolyte inlet 10 ··· Anode solution and gas outlet 11 ··· Oxygen-containing gas inlet 12
..Caustic soda outlet

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[Procedure amendment]

[Submission date] August 1, 1996

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] All figures

[Correction method] Change

[Correction contents]

FIG.

FIG. 2

Claims (5)

[Claims] 1. A gas diffusion cathode in contact with an ion exchange membrane defining an anode chamber and a cathode gas chamber, a metal mesh contacting the gas diffusion cathode, and the gas diffusion connected to the metal mesh through the mesh. A liquid-permeable gas diffusion cathode structure comprising a power supply for supplying power to a cathode. 2. The gas diffusion cathode according to claim 1, wherein the metal mesh has elasticity and the gas diffusion cathode is uniformly adhered to the ion exchange membrane by the elasticity.
3. The gas diffusion cathode structure according to item 1. 3. The gas diffusion cathode structure according to claim 1, wherein the metal mesh is formed by laminating a mesh of metal wires. 4. The gas diffusion cathode structure according to claim 1, wherein the metal mesh is a three-dimensional braided metal wire. 5. The gas diffusion cathode structure according to claim 1, wherein the metal network is a metal foam.