JPH07166389A - Gas electrode structure - Google Patents

Abstract

PURPOSE:To obtain a gas electrode structure almost preventing the residence of a gaseous by-product on the catalyst layer and capable of maintaining high electrolytic efficiency. CONSTITUTION:This gas electrode structure 1 contains an ion exchange membrane 2 and a catalyst layer 3 formed by chemical plating on one side of the membrane 2 or further contains a current collector 4 having water repellency and electric conductivity kept in press contact with the catalyst layer 3. Since the catalyst layer 3 is thin and adheres to the ion exchange membrane 2, a gaseous by-product does not stay on the surface of the catalyst layer 3 even if it is produced and electrolytic efficiency is not reduced. The transfer of a hydrophobia gaseous by-product to the direction of the current collector 4 is accelerated owing to the hydrophilic property of the catalyst layer 3 and the hydrophobic property of the current collector 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas electrode structure used for electrolytically recovering an acid and an alkali from a neutral salt, and more specifically, to efficiently electrolyze with or without side reaction. The present invention relates to a gas electrode structure that can be advanced.

[0002]

2. Description of the Related Art In recent years, salt electrolysis using the ion exchange membrane method has become widespread due to the remarkable progress and improvement of the fluorine ion exchange membrane. This method is a method in which hydrogen gas and caustic soda are co-produced in the cathode chamber and chlorine is co-produced in the cathode chamber by electrolysis using saline as a raw material. And in this ion exchange membrane electrolysis,
In addition to the two-chamber method in which the electrolysis tank is divided into two chambers by one ion exchange membrane and electrolysis is performed, the electrolysis tank is divided into three chambers by the cation exchange membrane and the anion exchange membrane, and the neutral salt aqueous solution is placed in the intermediate chamber. Liquid is passed to recover alkali in the cathode chamber side and acid in the anode chamber side 3
Muroha is also known since ancient times. However, few examples have been put to practical use due to the corrosion resistance of the membrane material or the high cell voltage, that is, the energy unit.

As a means for reducing energy consumption, a gas cathode is used as a cathode, and electrolysis is performed while supplying oxygen to the cathode chamber to suppress the generation of hydrogen gas and greatly reduce the cell voltage, or a cathode is used as a gas anode. A method of circulating the hydrogen gas generated in the chamber to the anode chamber to suppress oxygen generation in the anode chamber to reduce the cell voltage has been proposed and studied (US Pat. No. 4,561,945, European Patent Publication No. 0552382 A1). issue). That is, at the cathode, 2H ₂ 0 + 2e ⁻
→ H ₂ + 2OH ⁻ produces hydrogen and alkali by the reaction, while H ₂ → 2H ⁺ + 2e at the anode.
^- reaction hydrogen oxygen evolution is suppressed circulating is oxidized by the. In this electrolytic reaction, the cell voltage of 1 to 1.5 V can be reduced as compared with the reaction in which oxygen is generated without circulating hydrogen, and the deterioration of the film due to the oxidizing power of oxygen is also eliminated.

3 described in European Patent Publication No. 0552382 A1
In order to confirm the effectiveness of the room method electrolysis, actual electrolysis was performed under the following conditions. A cation exchange membrane was arranged on the cathode side and an anion exchange membrane was arranged on the anode side, and an intermediate chamber was formed between both exchange membranes. An activated nickel cathode was used as a cathode, and a cation exchange membrane, a gas electrode having a catalyst supported on a carbon cloth, and a gas electrode structure formed by pressing a metal current collector were used as an anode. An aqueous sodium nitrate or sodium sulfate solution was passed through the intermediate chamber and electrolysis was performed while supplying hydrogen gas to the current collector side of the gas electrode structure to generate caustic soda in the cathode chamber.

There is no problem with sodium sulfate electrolysis.
At a current density of A / dm ^2, the cell voltage was 5 V and stable operation was possible, but in the case of sodium nitrate, it was 5 A / dm ^2.
The cell voltage already exceeded 5 V at dm ² , and the cell voltage gradually increased as the current density increased. Observation of the anode during electrolysis revealed that gas was retained between the cation exchange membrane and the gas electrode, resulting in insufficient contact between the two. As described above, the conventional three-chamber method is not without problems, and considering that the drawback of sodium nitrate electrolysis does not occur in sodium sulfate electrolysis, it is presumed that it is due to a side reaction unique to sodium nitrate electrolysis. When the substance of this side reaction was examined, the nitrate ions reaching the anode gas electrode through the ion exchange membrane reacted with hydrogen, and for example, 2NO ₃ ⁻ + 6H ₂
It was found that a reaction such as N ₂ + 6H ₂ O + 2e ⁻ produces nitrogen and a trace amount of NO _x , and this by-product inhibits the operation of the gas electrode.

[0006]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems of the prior art and reduces the electrolysis efficiency even when used in an electrolytic system containing ions that generate gas that adversely affects the reaction by electrolysis such as nitrate ions. It is an object of the present invention to provide a gas electrode structure that can be operated without operating the gas electrode structure.

[0007]

SUMMARY OF THE INVENTION The present invention is a gas electrode structure comprising an ion exchange membrane and a catalyst layer such as a platinum group metal chemically plated on one side thereof, the gas electrode structure further comprising: A water-repellent and conductive current collector may be brought into pressure contact with the catalyst layer side.

The present invention will be described in detail below. As a result of further diligent examination of the behavior of the above-described by-products such as nitrogen, the present inventors have made it easier for the by-products to pass through the gas electrode structure, that is, nitrogen and NO _x are removed from the gas electrode structure by diffusion. It has been found that by devising such a method, problems such as a decrease in electrolysis efficiency due to the by-product can be solved. That is, the catalyst layer of the gas electrode is formed on the surface of the ion exchange membrane, which is the base material of the gas electrode, by chemical plating to make the thickness of the catalyst layer as thin as possible and to form the voids in which the gas is retained in the catalyst layer. It is set to almost zero, and the gap between the catalyst layer and the ion exchange membrane is also eliminated to prevent the retention of gas which similarly lowers the electrolysis efficiency.

Furthermore, when a current collector having water repellency and conductivity is brought into pressure contact with the catalyst layer side of the gas electrode substrate obtained by chemically plating the catalyst layer on the ion exchange membrane, the produced gas which is hydrophobic is hydrophilic. It becomes easy to transfer from the catalyst layer to the hydrophobic current collector, and it becomes possible to perform gas escape more favorably.
As the ion exchange membrane of the gas electrode structure of the present invention, a fluorine-based ion exchange membrane (typically a product name of Nafion manufactured by DuPont, USA), and a hydrocarbon-based ion exchange membrane (for example, Tokuyama Soda Co., Ltd. Neoceptor, Any of Asahi Glass Co., Ltd. Selemion, Asahi Kasei Co., Ltd. Aciplex etc.) can be used. Of course, it is desirable to use a membrane having good ion selectivity.

The metal constituting the catalyst layer is not limited as long as it is a metal that can be chemically plated and is active in electrolysis, but a platinum group metal is most suitable, and platinum is particularly active in oxidizing hydrogen. The means for chemically plating this metal is not particularly limited (for example, the method described in JP-B-58-47471 can be used), and one side of the ion exchange membrane is preferably about 1 to several μm by chemical plating. The first gas electrode structure of the present invention can be manufactured by forming the catalyst layer of 1. This gas electrode structure may be used for an electrolytic reaction, but more preferably, a current collector having water repellency and conductivity is used in pressure contact with the catalyst layer of this gas electrode structure. The base body of this current collector is preferably a metal, and a corrosion-resistant metal or alloy such as titanium or stainless steel is used.
The shape is not particularly limited, but it is preferable to use a porous body such as an expanded mesh or a punch plate. The finer the mesh of the substrate is, the more uniform the current can be, which is preferable, but it is necessary to avoid clogging of the mesh during the subsequent water repellent treatment.
A length of 8 mm and a minor axis of 1 to 4 mm are suitable.

Next, the current collector is hydrophobized, but if only the hydrophobic particles such as polytetrafluoroethylene (PTFE) are coated on the surface of the current collector, the power feeding function which is the original function of the current collector is impaired. A mixture of hydrophobic particles and conductive particles is applied to the surface of the current collector. As the hydrophobic particles, for example, a PTFE suspension commercially available under the trade name Nafion 30J (manufactured by Mitsui DuPont Fluorochemicals) can be preferably used. As the conductive particles, carbon powder, platinum powder, iridium oxide powder, or other corrosion resistant powder is used. The suspension or the mixture of particles is applied to the current collector and fired to obtain the current collector of the present invention. The current collector is brought into pressure contact with the catalyst layer side of the ion exchange membrane having the catalyst layer formed on one surface to form the second gas electrode structure of the present invention.

The gas electrode structure according to the present invention thus constructed is widely used as a gas electrode for various electrolytic reactions such as salt electrolysis and sodium sulfate electrolysis as an anode or a cathode of a two-chamber method and a three-chamber electrolytic cell. In particular, it is useful for electrolysis of an electrolyte that produces nitrogen or NO _x, which is difficult to release gas from a gas electrode such as sodium nitrate.

Next, a gas electrode structure according to the present invention and an example of its application to a three-chamber electrolytic cell will be described with reference to the accompanying drawings. FIG. 1 is a partial vertical cross-sectional view showing an embodiment of the gas electrode structure according to the present invention, and FIG. 2 shows the gas electrode structure of FIG.
FIG. 3 is a vertical cross-sectional front view showing an example in which it is incorporated as an anode in a room-method electrolytic cell. The gas electrode structure 1 is a fluorine-based ion exchange membrane 2
The catalyst layer 3 made of platinum group metal is chemically plated on one side of
The current collector 4 is pressed against the surface of the catalyst layer 3. The current collector 4 is formed by covering the surface of an expanded mesh with a hydrophobic layer 5 which is a mixture of hydrophobic particles and conductive particles.

A gas electrode structure 1 having such a structure
Is a three-chamber electrolytic cell divided into an anode chamber 8, an intermediate chamber 9 and a cathode chamber 10 by an anion exchange membrane 6 and a cation exchange membrane 7.
The current collector 4 is installed in the anode chamber 8 of 11 so as to face the surface opposite to the anion exchange membrane 6, and the anode chamber 8 is divided into a solution chamber 12 and a gas chamber 13. 14 is a cathode disposed in the cathode chamber 10, 15 and 16 are hydrogen gas inlet and exhaust gas outlet, 17 and 18 are anolyte inlet and outlet, 19 and 20 are electrolyte inlet and Outlet ports 21 and 22 are a catholyte inlet port and an outlet port, respectively.

An electrolytic solution, for example, a sodium nitrate aqueous solution is passed through the electrolytic solution inlet 19 to the intermediate chamber 9 of the electrolytic cell 11, and hydrogen circulated from the cathode chamber 10 is supplied to the gas chamber 13 from the hydrogen gas inlet 15. While energizing. Sodium ions of sodium nitrate in the intermediate chamber 9 permeate the cation exchange membrane 7 and move to the cathode chamber 10 to produce caustic soda, and
Nitrate ions of sodium nitrate therein penetrate the anion exchange membrane 6 and move to the solution chamber 12. Oxidation reaction of hydrogen occurs on the anode catalyst layer 3, and the generated hydrogen ions permeate the ion exchange membrane 2 and reach the solution chamber 12 to generate the nitrate ions and nitric acid. A small amount of nitrogen or NO _X is these product gases generated thickness is very thin and the catalyst layer 3 of catalyst layer 3 trace of nitrate when this reaches the catalyst layer 3 passes through the ion exchange membrane 2 Since there is no space between the ion-exchange membrane 2 and the ion-exchange membrane 2, the gas electrode structure 1 does not diffuse into the anode gas chamber 13 to adversely affect a desired electrolytic reaction, and high electrolysis efficiency can be maintained.

[0016]

EXAMPLES Next, examples of a method for producing a gas electrode structure according to the present invention and an electrolysis method using the gas electrode structure will be described, but the examples do not limit the present invention.

[0017]

Example 1 A gas electrode structure according to the present invention was produced as follows. Ion-exchange membrane Nafion 117 is immersed in 3% ammonia water in which chloroplatinic acid is dissolved at a concentration of 5%, and then taken out and only one side of the ion-exchange membrane is contacted with a 2% sodium borohydride aqueous solution for initial platinum plating. Was applied. Then, only this initial plated surface was contacted with 3% ammonia water in which chloroplatinic acid and dimethylamine borane were dissolved to be 1% and 2%, respectively, and platinum plating of 3 μm was performed to form a catalyst layer. Long diameter 3 as a base material for current collectors
mm, a short diameter of 1.5 mm and a plate thickness of 0.2 mm, using a titanium expanded mesh having a thickness of 30 mm, a slurry of Teflon (manufactured by DuPont) 30J and iridium oxide powder was applied on the base material, and baked and coated at 350 ° C. in air. A current collector was produced. This current collector was arranged so as to be in contact with the catalyst layer of the ion exchange membrane to form a gas electrode structure.

Using a cation exchange membrane (Nafion 427) and an anion exchange membrane (Aciplex A200), the interior of the electrolytic cell is divided into an anode chamber, an intermediate chamber and a cathode chamber to assemble a three-chamber electrolytic cell, and the anode chamber is assembled. The gas electrode structure as an anode and a nickel expanded mesh as a cathode in the cathode chamber, respectively, 25% aqueous sodium nitrate solution is passed through the intermediate chamber, and hydrogen generated in the cathode chamber is circulated to the anode gas chamber. However, when electrolysis was performed under the conditions of 60 ° C. and 30 A / dm ² , 15% nitric acid was used in the anode solution chamber and 15% in the cathode chamber.
% Caustic soda was recovered. The cell voltage is stable at 5.0 V,
The current efficiencies of caustic soda and nitric acid production were 84% and 47%, respectively.

[0019]

[Comparative Example 1] An ion exchange membrane Nafion 117, a gas electrode having a platinum catalyst supported on a carbon cloth, and a titanium expanded mesh current collector were brought into close contact with each other to form an anode structure. Using this structure, a three-chamber process electrolytic cell was assembled in the same manner as in Example 1. While passing a 25% sodium nitrate aqueous solution through the intermediate chamber
When electrolysis was performed while maintaining the temperature at 60 ° C., the electrolysis voltage reached 5 V at a current density of 5 A / dm ² , and the electrolysis was stopped because the voltage further increased as the current density increased.

[0020]

The present invention is firstly a gas electrode structure comprising an ion exchange membrane and a metal catalyst layer chemically plated on one side thereof. In this gas electrode structure, the adhesion between the ion exchange membrane and the catalyst layer is good, there are no voids between them, the catalyst layer is extremely thin, and a dense layer is formed by plating. Even when a gas that adversely affects a predetermined electrolytic reaction is generated by a side reaction when electrolyzing using the gas, the side-produced gas does not stay in the catalyst layer of the gas electrode structure and immediately flows toward the current collector. And then diffuses into the electrolysis chamber. Therefore, unlike the conventional gas electrode, the gas electrode structure according to the present invention does not reduce the electrolysis efficiency due to the by-products, and the desired electrolysis product can be obtained in a high yield.

A second aspect of the present invention is a gas electrode structure comprising a gas electrode substrate having a metal catalyst layer chemically plated on one side of an ion exchange membrane and a current collector having water repellency and conductivity pressed against the gas electrode substrate. is there. In this gas electrode structure, the same effect as that of the gas electrode structure is obtained, and in addition, since the hydrophilic catalyst layer and the hydrophobic current collector are brought into pressure contact with each other, the hydrophilic by-product gas that is hydrophobic is hydrophilic. Of the by-product gas to the hydrophobic current collector is promoted, the gas escape of the by-product gas from the catalyst layer proceeds more smoothly, and the electrolysis efficiency is maintained high.

[Brief description of drawings]

FIG. 1 is a partial vertical sectional view showing an embodiment of a gas electrode structure according to the present invention.

FIG. 2 is a vertical sectional front view showing an example in which the gas electrode structure of FIG. 1 is incorporated as an anode in a three-chamber method electrolytic cell.

[Explanation of symbols]

1 ... Gas electrode structure 2 ... Ion exchange membrane 3.
..Catalyst layer 4 ... Current collector 5 ... Hydrophobic layer 6 ...
・ Anion exchange membrane 7 ・ ・ ・ Cation exchange membrane 8 ・ ・ ・ Anode chamber 9 ・ ・ ・ Intermediate chamber 10 ・ ・ ・ Cathode chamber 11 ・ ・ ・ Electrolytic cell 14 ・ ・ ・ Cathode

Claims (2)

[Claims] 1. A gas electrode structure comprising an ion exchange membrane and a metal catalyst layer chemically plated on one side thereof. 2. A gas electrode structure comprising a gas electrode substrate having a metal catalyst layer chemically plated on one surface of an ion exchange membrane, and a current collector having water repellency and conductivity.