JPS6125791B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing an ion exchange membrane-electrode catalyst layer assembly, and more particularly to a method for easily producing an ion exchange membrane-electrode catalyst layer assembly that has excellent electrolysis performance for alkali metal halides. Previously, the mercury method was the mainstream method for electrolyzing alkali metal halides, especially aqueous sodium chloride solutions in Japan, but environmental regulations forced a switch to the diaphragm method. However, with the diaphragm method, which uses asbestos membranes as its main component, it is inevitable that a large amount of sodium chloride is mixed into the produced caustic soda, and even after purification, the product has a low purity, which has the disadvantage that it cannot meet the demand for high purity. There is. Therefore, in order to overcome the above-mentioned drawbacks, an electrolytic method using an ion exchange membrane as a diaphragm, that is, an ion exchange membrane method, has been developed and is being put into practical use. The important point in the development and improvement of this ion exchange membrane method is to reduce the cell voltage, and losses other than the theoretical electrolysis voltage include solution resistance, increased solution resistance due to bubbles, hydrogen overvoltage, and low membrane resistance. . In particular, the increase in solution resistance due to air bubbles has a large effect, so even if the ion exchange membrane and electrode are brought into close contact for the purpose of reducing solution resistance, the cell voltage will increase. Therefore, it is necessary to provide an appropriate distance between the ion exchange membrane and the electrode to minimize the influence of bubbles. In recent years, solid polymer electrolytes have been developed to further improve the shortcomings of the ion exchange membrane method.
An electrolysis method using electrolyte (SPE) has been proposed. That is, in the SPE electrolysis method, since the electrode catalyst layer is bonded to the ion exchange membrane, gas generation due to the electrode reaction occurs from the surface of the electrode catalyst layer, and the solution resistance between the ion exchange membrane and the electrode and the solution resistance due to bubbles are generated. The increase in hydrogen can be eliminated, and the hydrogen overvoltage can also be lowered. On the other hand, methods for manufacturing such an ion exchange membrane-electrode catalyst layer assembly include a method of sintering and forming a mixture of precious metal salt catalyst particles and Teflon particles into a sheet shape and bonding it onto the ion exchange membrane; A method of directly embedding a catalyst into an ion exchange membrane by heat pressing,
Furthermore, a method has been proposed in which metal is deposited on an ion exchange membrane by electroless plating. However, in the former method, since the Teflon particles that act as a binder for the catalyst particles are hydrophobic, the generated air bubbles are adsorbed on the surface or inside of the electrode catalyst layer, which has the disadvantage of hindering the electrode reaction and increasing the electrolytic voltage.
Furthermore, the latter method using electroless plating requires large equipment for industrially etching the ion exchange membrane and severe operational management. Glow discharge or low-temperature plasma is generally used for etching, which damages the surface of the ion exchange membrane and reduces electrolytic performance. Therefore, gentle etching is required that maintains the adhesion between the ion exchange membrane and the electrode catalyst layer and does not reduce the electrolytic performance. As a result of intensive research into the development of ion exchange membrane-electrode catalyst layer assemblies, the present inventors discovered a simple method for bonding an electrode catalyst layer to an ion exchange membrane with good adhesion without using any binder, and developed the present invention. I came up with a proposal. That is, the present invention provides an ion exchange membrane characterized in that a solution containing a noble metal salt is present on at least one side of the ion exchange membrane, and then, after the solvent is removed, the noble metal salt is thermally decomposed.
This is a method for manufacturing an electrode catalyst layer. In the present invention, the cathode catalyst layer formed on at least one side of the ion exchange membrane, generally on the cathode side of the membrane, is a metal layer in which a noble metal salt is reduced by thermal decomposition. The ion exchange membrane used in the present invention is not particularly limited as long as it is a cation exchange membrane with excellent durability, but generally it is based on fluorocarbon having an exchange group such as a sulfonic acid group, a carboxylic acid group, or an amine group. In particular, perfluorocarbon-based cation exchange membranes are preferably used. In addition, the noble metal salts used in the present invention are platinum, iridium, palladium, ruthenium, rhodium, etc., which are dissolved in an organic solvent or a mixed system of water and an organic solvent, and have a reductive decomposition temperature of 60 to 300°C, preferably 200°C or less. and inorganic salts or organic complexes of osmium. Examples include chloroplatinic acid, platinum tetraiodide, platinum tetrabromide, iridium tetrachloride, iridium tetrabromide, iridium tetraiodide, sodium hexachloroiridate; palladium iodide, palladium nitrate, palladium dicarbonyl chloride; Examples include ruthenium pentafluoride, ruthenium pentanitrile, ruthenium monocarbonyl bromide; rhodium trifluoride, rhodium carbonyl, and the like. In the method of the present invention, the ion exchange membrane is swollen by first providing a solution containing a noble metal salt on at least one side of the ion exchange membrane, on the cathode side during use, to make it easier for the noble metal salt to penetrate into the membrane surface. As a result, it is possible to obtain a bonded body with good adhesion. The solvent is not particularly limited as long as it dissolves the noble metal salt and can be easily dried under reduced pressure, but organic solvents are generally used, and alcohols such as methanol, ethanol, propanol, and butanol are preferably used. Methods of applying a noble metal salt solution to one side of an ion exchange membrane include applying directly with a brush, spraying, or immersing an ion exchange membrane with massing on the other side in a noble metal salt solution. Ru. Next, according to the method of the present invention, the noble metal salt present on the cathode surface of the ion exchange membrane is thermally decomposed to form an electrode catalyst layer which is a reduction product of the noble metal salt. Therefore, in the present invention, thermal decomposition of the noble metal salt is carried out after sufficiently removing the solvent from the ion exchange membrane in order to ensure good adhesion of the noble metal cathode catalyst layer to one side of the ion exchange membrane. is important. In other words, especially when thermal decomposition is carried out with the organic solvent still present in the ion-exchange membrane due to insufficient removal, the ion-exchange membrane is likely to deteriorate, and the organic solvent is carbonized and the membrane is removed. Because it remains inside, the adhesion of the electrode catalyst layer decreases,
Its conductivity also decreases. As a means for removing the solvent, a method of drying under reduced pressure is generally employed. Thermal decomposition may be carried out at temperatures above the temperature at which the noble metal salt begins to undergo thermal decomposition reduction and below the deterioration temperature of the ion exchange membrane, and is generally carried out at a temperature of 60 to 300°C, preferably below 200°C, in a reducing atmosphere such as hydrogen. be exposed. The thermal decomposition time requires time for sufficient thermal decomposition of the noble metal salt to be completed, and generally takes 5 minutes to 5 hours, although it varies depending on the thermal decomposition temperature. Therefore, in the method of the present invention, a solution of a noble metal salt dissolved in a solvent is present on one side of the ion exchange membrane, and the solvent is removed.
It is extremely preferable to repeat the steps of pyrolyzing the noble metal salt several times in order to form the electrode catalyst layer on the ion exchange membrane with good adhesion. That is, forming an electrode catalyst layer on an ion exchange membrane by performing each of the above operations only once is a simple operation, and it is possible to obtain an ion exchange membrane-electrode catalyst layer assembly having usable electrolytic performance. . However, such an ion exchange membrane-electrode catalyst layer has insufficient adhesion and lacks long-term durability. Therefore, it is preferable to adjust the concentration of the noble metal salt solution used and repeat the operation generally 5 to 30 times to form an electrode catalyst layer on the ion exchange membrane to a desired thickness. In addition, when using the ion exchange membrane-electrode catalyst layer assembly of the present invention, since contact between the catalyst layer and the current collector is unavoidable, contact resistance between the current collector and the catalyst layer is reduced and the electrode The thickness of the catalyst layer required for uniform current flow with small ohmic drop to any point on the catalyst layer is 0.5 μm or more. However, if the above-mentioned catalyst layer is too thick, there is a tendency for the electrolytic voltage to increase and the current efficiency to decrease.Furthermore, in order to process expensive noble metals, from an economical point of view, the thinner the layer is, the more preferable it is, and the maximum thickness is about 5μ. The ion exchange membrane-electrode catalyst layer assembly obtained by the present invention has good adhesion without using Teflon particles as a binder.
This method has the advantage of superior electrolytic performance and no increase in electrolytic voltage compared to conventional bonded bodies. Further, the method of the present invention is extremely simple and allows the heat treatment temperature to be lowered, so it has the advantage that it can be particularly advantageously applied to thermally unstable carboxylic acid-based cation exchange membranes. Examples are shown below to specifically explain the present invention, but the present invention is not limited thereto. The cation exchange membrane-electrode catalyst layer assembly obtained in the example was set in the electrolytic cell described below and subjected to salt electrolysis. That is, a cation exchange membrane-electrode catalyst layer assembly is placed between the anode chamber and the cathode chamber,
An electrolytic cell with a current-carrying area of 0.5 dm ² (5×10 cm) was constructed in which an anode was crimped to each of the cation exchange membranes and a cathode current collector was crimped to the electrode catalyst layer. Supply 4N purified saline as the anolyte and increase the current density
A caustic soda aqueous solution of a predetermined concentration was obtained from the cathode chamber under the condition of 30 A/dm ² . Examples 1 to 5, Comparative Example 1 On one side of a perfluorocarbon cation exchange membrane of Neocepta FC-2000 (trade name, manufactured by Tokuyama Soda Co., Ltd.) having a current carrying area of 0.5 dm ² (5 cm x 10 cm),
Using butanol as a solvent, a solution of H ₂ PtC ₆ prepared to a predetermined concentration shown in Table 1 was uniformly applied with a brush. Next, the cation exchange membrane was air-dried and further dried under reduced pressure at a temperature of 50°C for 2 hours. After that, the ion exchange membrane was heated at a temperature of 100 °C in a _H2 stream for 1.5
I processed time. As shown in Table 1, each of the above operations was sequentially repeated a predetermined number of times using H ₂ PtC ₆ solutions of predetermined concentrations. The electrode catalyst layer bonded to the obtained cation exchange membrane has a thickness of approximately 1μ.
It was hot. After immersing each of the above cation exchange membrane-electrode catalyst layer assemblies in 6N-NaOH (80°C) and stretching them sufficiently, an aqueous NaC solution was prepared using an electrolytic bath set with the electrode catalyst layer on the cathode side. Electrolysis of 9N-
Obtained NaOH. The electrolytic results are also listed in Table 1.

[Table] In addition, as a result of examining the adhesion of the catalyst layer after 100 days of energization, some falling off was observed in the cases of Examples 1 and 2, but the adhesion was poor in the cases of Examples 3 to 5. It was good and warm. Example 6, Comparative Example 2 A 2% by weight solution of H ₂ PtC ₆ prepared using methanol was uniformly applied with a brush to one side of a perfluorinated cation exchange membrane (H ⁺ type) of Neocepta FC-2000. Next, the ion exchange membrane was air-dried and further dried under reduced pressure at a temperature of 50°C for 2 hours. Thereafter, thermal decomposition was carried out for 1.5 hours at a temperature of 100° C. in a H ₂ stream. The above treatment operations for the ion exchange membrane were sequentially repeated 15 times. The thus obtained ion exchange membrane-electrode catalyst layer assembly was mixed with 6N-NaOH
After being immersed at 80° C. and sufficiently stretched, it was set in an electrolytic bath in the same manner as in Example 1, and the saline solution was electrolyzed. For comparison, NaC
Electrolysis of aqueous solution was carried out. The results are shown in Table 2. In addition, in the example, even after 100 days of energization, the cathode catalyst layer did not fall off and stable electrolytic performance was exhibited.

[Table] Example 7, Comparative Example 3 The ion exchange membrane used has an energized area of 0.5 dm ² (5
perfluorinated cation exchange membrane (Na ⁺
IrC prepared using butanol on one side of the mold
A 2% by weight solution of _{No. 4} was applied uniformly with a brush. The ion exchange membrane was then air-dried and further heated to a temperature of 50°C.
Drying was carried out under reduced pressure at ℃ for 5 hours. After that, the temperature
Thermal decomposition was carried out at 150° C. in a stream of H ₂ for 1.5 hours. The above treatment operations for ion exchange membranes are performed sequentially.
I did it 30 times. The thus obtained ion exchange membrane-electrode catalyst layer assembly was dissolved in 6N-NaOH (80%
℃) for 4 hours until the elongation became steady. Thereafter, the ion exchange membrane was set in an electrolytic cell with the cathode catalyst layer facing the cathode, and NaC was electrolyzed. For comparison, NaC was similarly electrolyzed in an electrolytic cell set using an ion exchange membrane that was not subjected to any of the above treatments. The results are shown in Table 3. In addition, in the example, even after 100 days of energization, the cathode catalyst layer did not fall off and stable electrolytic performance was exhibited.

【table】

Claims (1)

[Scope of Claims] 1. An ion exchange membrane characterized in that a solution of a noble metal salt is present on at least one side of the ion exchange membrane, and then, after drying, the noble metal salt is thermally decomposed.
Method for manufacturing an electrode catalyst layer assembly. 2. A method for producing an ion exchange membrane-electrode catalyst layer assembly, which comprises repeating the treatment operation described in claim 1 a plurality of times.