JP5960795B2 - Method for producing oxygen gas diffusion electrode - Google Patents

Description

  The present invention relates to an alkaline fuel cell, a metal-air battery, and a gas diffusion electrode for salt electrolysis. More specifically, the present invention relates to a corrosion-resistant carbon-based catalyst having almost the same electrolysis performance without using an expensive platinum catalyst. The present invention relates to a gas diffusion electrode.

[Oxygen gas diffusion electrode for salt electrolysis]
Caustic soda (sodium hydroxide) and chlorine, which are important as industrial raw materials, are mainly produced by the salt electrolysis method. This electrolysis process has gone through the mercury method using a mercury cathode and the diaphragm method using an asbestos diaphragm and a soft iron cathode, and then has shifted to an ion exchange membrane method using an active cathode with a small overvoltage using an ion exchange membrane as a diaphragm. . During this time, the power consumption required to produce 1 ton of caustic soda has decreased to 2000 kWh. However, since caustic soda production is a power-intensive industry, there is a demand for further reduction in power consumption.

The anode and cathode reactions in the conventional electrolysis method are as shown in formulas (1) and (2), respectively, and the theoretical decomposition voltage is 2.19V.
2Cl ⁻ → Cl ₂ + 2e ⁻ (1.36V) (1)
2H ₂ O + 2e ⁻ → 2OH ⁻ + H ₂ (−0.83 V) (2)
If an oxygen gas diffusion electrode is used instead of performing a hydrogen generation reaction at the cathode, the reaction is as shown in Equation (3), theoretically 1.23V, and a cell voltage of about 0.88V even in a practical current density range. It is possible to reduce the power intensity of 700 kWh per ton of sodium hydroxide.
O ₂ + 2H ₂ O + 4e ⁻ → 4OH ⁻ (0.40V) (3)

For this reason, the practical application of a salt electrolysis process using a gas diffusion electrode has been studied since the 1980s. In order to realize this process, an oxygen gas diffusion electrode that requires high performance and sufficient stability in the electrolytic system is required. Development is essential.
Conventionally, as this type of oxygen gas diffusion electrode for salt electrolysis, Patent Document 1 discloses a technique for reducing the resistance of a power feeding unit, and Patent Document 2 discloses an appropriate porosity in an oxygen gas diffusion electrode. Patent Document 3 discloses a gas diffusion electrode using palladium and silver. In Non-Patent Documents 1 and 2, the recent development status of gas diffusion electrodes is reported.

[Oxygen gas diffusion electrode for alkaline fuel cells]
A fuel cell is a clean and highly efficient power generation system that can convert chemical energy into electrical energy. By combining the oxidation reaction of hydrogen and organic carbon raw materials with the reduction reaction of oxygen in the air, electric energy is obtained from the electromotive force. After attracting attention for practical use as a space battery in the 1960s, Recently, it has attracted attention again as a power storage leveling power source for fuel cell vehicles and small portable power sources, households and power plants.
When hydrogen is used as the fuel, the reaction of the formula (4) proceeds at the anode (fuel electrode) in the alkaline aqueous solution.
H ₂ + 2OH ⁻ → 2H ₂ O + 2e ⁻ (−0.83 V) (4)
When oxygen is used as the oxidizing agent, the reaction of the formula (3) proceeds at the cathode (oxygen electrode).

The alkaline fuel cell is composed of an aqueous solution such as potassium hydroxide as an electrolytic solution that separates the anode and cathode electrodes made of metal or carbon, which are porous electrode constituent materials, and both electrodes. In the alkaline fuel cell as described above, when carbon dioxide enters the electrolytic solution, it reacts with the electrolyte as shown by the following formula (5) to generate carbonate ions, and when the carbonate ions further increase in concentration, There arises a problem that a carbonate with an alkali metal in the electrolyte is formed and deposited on the electrode to inhibit the electrode reaction.
CO ₂ + 2KOH → CO ₃ ²⁻ + 2K ⁺ + H ₂ O → K ₂ CO ₃ ↓ (5)
Accordingly, it has been essential for an alkaline fuel cell to supply pure hydrogen gas as a fuel gas to the fuel electrode. On the other hand, air from which pure oxygen gas or carbon dioxide has been removed has also been supplied to the oxidant gas electrode. It is necessary to supply.

As described above, in the alkaline fuel cell, the clogging of the cell member due to the absorption of carbon dioxide is regarded as a problem in the air raw material. However, in the battery system using the anion exchange membrane, the carbonate ion is expressed by the formula (5) in the anode chamber. Thus, the alkali is consumed, the pH shifts to the acidic side, the carbonate ion is gasified again, is released to the outside of the cell, and it has been found that the accumulation does not exceed a certain level. Even when KOH is used as the electrolyte, if an anion membrane is used as a diaphragm, the precipitation of carbonate is suppressed.
Conventionally, as this type of oxygen gas diffusion electrode for an alkaline fuel cell, as described in Patent Documents 4, 5, and 6, a low resistance membrane and a binder in which an anion exchange resin component is dissolved have been developed. Improvements have been achieved.

[Oxygen gas diffusion electrode for metal-air batteries]
Development of a new battery using a metal such as lithium, zinc, and aluminum as an anode and an air electrode as a cathode has been under development, and has attracted attention not only for use in fuel cells but also for storage of renewable energy. Compared to a lithium ion battery, the active material of the cathode is oxygen in the air, and the weight and volume can be reduced.
The cathode reaction in the discharge reaction of the lithium-air battery is represented by formula (3), and the anode side is represented by the following formula (6).
Li → Li ⁺ + E ⁻ (−3.04 V) (6)
The anode discharge reaction of the zinc-air battery is represented by the following formula (7).
Zn + 2OH ⁻ → ZnO + H ₂ O + 2e ⁻ (−1.25 V) (7)
The charge reaction is the reverse of these reactions.

The cathode in these batteries is expected to develop an oxygen gas diffusion electrode that can be used in an alkaline system from the viewpoint of material cost.
Conventionally, as this type of oxygen gas diffusion electrode for metal-air batteries, Patent Document 7 discloses a lithium ion conductive material, and Patent Document 8 discloses a catalyst material. Patent Document 9 discloses a metal-air battery system, and Patent Document 10 discloses a nonaqueous electrolyte. Regarding the present state of the present technology, details are reported in Non-Patent Document 3, and the performance of various metal-air batteries is compared.

[Catalyst of gas diffusion electrode for fuel cell]
Although conventional platinum-based catalysts have high performance but have a very small amount of resources and cannot be used practically, alternative catalysts are indispensable, and development of non-platinum-based catalysts is important. As materials that are particularly attracting attention are metal oxide materials and carbon materials. Regarding the former, as described in Non-Patent Document 4, group 4 and group 5 metal oxides have attracted attention, and it has been reported that their carbonitride partial oxidation catalysts exhibit good performance. Has been. The development status of the latter is described below.

[Nitrogen-containing carbon-based catalyst]
Since the reduction characteristics of oxygen in a Co-phthalocyanine complex were confirmed by Jasinski in 1964, various complexes, polymers, and thermal decomposition products of these complexes have been studied (Non-Patent Document 5). Recently, a nanoshell structure having a graphite component around a metal particle has been found (Non-Patent Document 6), a carbon material catalyst containing a hetero element, or a carbon-based catalyst containing a metal component in a nitrogen-containing polymer. Have been reported to have high performance (Non-Patent Documents 7 and 8).
It is estimated that high oxygen reduction activity is expressed by containing nitrogen. A catalyst in which a metal such as iron is introduced into a carbon-based pyrolysis product is particularly excellent in activity. It is said that a specific electronic structure of carbon, a lattice disorder / defect of the carbon network structure, a remaining or introduced complex structure contributes as an expression mechanism of the catalyst. Catalytic activity is also related to electrical conductivity.

  When a complex or nitrogen-containing polymer is pyrolyzed under an inert gas flow, a new carbon network structure is formed. The metal contained as a raw material is finely divided and coated with a graphite layer, and even a trace amount has a great influence on the catalytic activity. As a technique for increasing the nitrogen content, it is known to use a raw material selection or an existing synthesis process such as ammonia. The former can be fixed more stably. At present, no method has been established for producing a nitrogen-containing carbon-based catalyst having high activity and excellent stability.

[Synthesis of silk-derived activated carbon]
Yarn (raw silk) has been produced since ancient times. Silk fabric is made of long fiber among silk fibroin from which sericin is removed from raw silk, and short fiber is called “waste silk”, which is cheap. Silk fibroin includes proteins composed of amino acids such as glycine, alanine, and tyrosine as proteins other than sericin. The silk-derived activated carbon of the present invention can use all proteins obtained from straw as raw materials.
In the present invention, the silk-derived activated carbon refers to a product obtained by synthesizing silk fibroin obtained by removing sericin from raw silk through carbonization treatment, heat treatment and activation treatment in an inert air stream. The activation treatment is not limited to steam activation, but can also be performed in a gas containing carbon dioxide, alkali, and ammonia.

The carbon-based electrode catalyst made of silk-derived activated carbon is disclosed in Non-Patent Documents 9, 10, 11, 12 and Patent Document 11 that the carbon-based electrode catalyst made of silk-derived activated carbon is sulfuric acid. It is known to exhibit excellent performance in acidity. However, the suitability when the carbon-based electrode catalyst made of silk-derived activated carbon is used in an alkaline solution is not disclosed in any of Patent Document 11 and Non-Patent Documents 9, 10, 11, and 12.
That is, there has been no report on the performance of a carbon-based electrode catalyst made of silk-derived activated carbon in an alkaline solution, and its activity and stability have not been known.

Japanese Patent Laid-Open No. 11-050289 JP 2006-219694 A JP 2008-127631 A JP 2010-045024 A JP 2010-1113889 A JP 2009-129881 A JP 2007-294429 A JP 2008-198590 A JP 2009-032399 A JP 2009-093983 A JP 2010-063952 A

Soda and chlorine, 45, 85 (1994) Journal of Applied Electrochemistry, 38, 1177 (2008) Electrochemistry, Volume 78, 529 (2010) Electrochemistry, Volume 78, 970 (2010) Nature, 201, 1212 (1964) J. Appl. Electrochem., 36, 239 (2006) J. Power Source, 196, 1006 (2011) Nature, 443, 63 (2006) Journal of Applied Electrochemistry, 38, 507 (2008) Journal of Applied Electrochemistry, 40, 675 (2010) Electrochemistry Communications, 11, 376 (2009) Journal of Power Sources, 195, 5840 (2010)

  The present invention eliminates the problems of the prior art as described above, and what kind of performance the carbon-based electrode catalyst comprising silk-derived activated carbon containing silk-derived nitrogen and formed into a powder is in an alkaline solution. As an alkaline fuel cell, metal-air battery, and salt electrolysis gas diffusion electrode, the amount of expensive platinum catalyst used is reduced in more detail, and it has almost the same electrolytic performance as before. An object of the present invention is to provide a gas diffusion electrode excellent in durability and long-term stability of an electrode during electrolysis or emergency stop in an alkaline solution.

  In order to solve the above-mentioned problems, the present invention provides, as a first solution, a porous electroconductive substrate having a carbon-based electrode catalyst comprising silk-derived activated carbon that contains silk-derived nitrogen and formed into a powder form. An object of the present invention is to provide an oxygen gas diffusion electrode for an alkaline fuel cell, a metal-air battery or a salt electrolysis used in an alkaline aqueous solution.

  In order to solve the above problems, the present invention provides, as a second solution, an oxygen gas diffusion characterized in that an N / C (atomic ratio) in the carbon-based electrode catalyst is 0.004 to 0.07. It is to provide an electrode.

  In order to solve the above-mentioned problems, the present invention provides, as a third solution, an oxygen gas diffusion electrode characterized in that a metal catalyst is contained in the carbon-based electrode catalyst.

  In order to solve the above problems, the present invention provides, as a fourth solution, the metal catalyst contained in the carbon-based electrode catalyst is a noble metal comprising any one or more of Pt, Ir, Ru, Ag, and Pd. Another object of the present invention is to provide an oxygen gas diffusion electrode.

  In order to solve the above-mentioned problems, the present invention provides, as a fifth solution, an oxygen gas diffusion electrode characterized in that a metal oxide catalyst is contained in the carbon-based electrode catalyst.

  In order to solve the above problems, the present invention provides, as a sixth solution, that the metal oxide catalyst contained in the carbon-based electrode catalyst is titanium oxide, zirconium oxide, niobium oxide, tin oxide, tungsten oxide, or tantalum oxide. An object of the present invention is to provide an oxygen gas diffusion electrode characterized by being a metal oxide comprising any one or more of them.

  In order to solve the above-mentioned problems, the present invention provides, as a seventh solution, carbon made of silk-derived activated carbon that is calcined with silk fibroin at 500 ° C. to 1500 ° C., contains silk-derived nitrogen, and is molded into powder. An object of the present invention is to provide a method for producing an oxygen gas diffusion electrode, characterized in that a system electrode catalyst is produced.

  According to the present invention, an oxygen gas diffusion electrode for an alkaline fuel cell, a metal-air battery, or a salt electrolysis that is highly active in an alkaline aqueous solution using a silk-derived carbon-based electrode catalyst containing nitrogen, It can provide a gas diffusion electrode with excellent durability and long-term stability of the electrode during charge / discharge and emergency stop of the battery, and furthermore, a four-electron reduction reaction by coexisting a noble metal catalyst or metal oxide catalyst. By performing only this, the production of hydrogen peroxide can be suppressed, and the deterioration of the electrode and the electrochemical cell can be suppressed.

The carbon nitrogen graphite structure corresponding to the XPS spectrum of the silk origin activated carbon of this invention. The schematic longitudinal cross-sectional view which illustrates the gas diffusion electrode of this invention. 1 is a schematic longitudinal sectional view of an alkaline fuel cell equipped with a gas diffusion electrode of the present invention. The schematic longitudinal cross-sectional view of the three chamber method electrolysis cell equipped with the gas diffusion electrode of this invention. The schematic longitudinal cross-sectional view of the two-chamber method electrolysis cell equipped with the gas diffusion electrode of this invention. The schematic longitudinal cross-sectional view of the lithium air battery equipped with the gas diffusion electrode of the present invention. The graph which shows the electric current-potential of the oxygen reduction | restoration of the gas diffusion electrode of this invention which consists of a silk origin activated carbon manufactured by heat-processing 1200 degreeC. The graph which shows the relationship of the reaction order of the gas diffusion electrode of this invention which consists of a silk origin activated carbon manufactured by heat-processing 1200 degreeC. The graph which shows the electric current-potential of oxygen reduction of the gas diffusion electrode of this invention which consists of a silk origin activated carbon manufactured by heat-processing 900 degreeC. The graph which shows the relationship of the reaction order of the gas diffusion electrode of this invention which consists of a silk origin activated carbon manufactured by heat-processing 900 degreeC.

Hereinafter, the constituent members of the oxygen gas diffusion electrode according to the present invention will be described in detail with reference to the drawings.
[Carbon-based electrode catalyst composed of silk-derived activated carbon]
Patent Document 11 discloses a method for producing silk fibroin-derived carbon-based electrode catalyst. The production method of the carbon-based electrode catalyst made of the silk-derived activated carbon of the present invention is almost the same.
(1) Carbonization The raw silk material is fired in an inert gas atmosphere such as nitrogen at 500 ° C. for several hours. After cooling to room temperature, it is powdered using a ball mill or the like. The particle size is not particularly limited, but about 10 μm is preferable. When the firing temperature is lower than 500 ° C., carbonization is insufficient, which is not preferable.
(2) Heat treatment Next, it is fired in an inert gas atmosphere at a firing temperature of 700 to 1500 ° C. for several hours. It is particularly preferable to perform firing at a temperature of 1200 ° C. When the calcination temperature is higher than 1500 ° C., no nitrogen component remains in the powder and the catalytic activity becomes insufficient.
(3) Activation treatment Next, the fired powder is activated. In the activation process, the temperature is raised to, for example, 850 ° C. in an inert gas atmosphere, and steam is activated by blowing steam and holding it for several hours. The activation treatment is not limited to steam activation, and can be performed even in a gas containing carbon dioxide, alkali, and ammonia. The activation treatment temperature is preferably about 700 ° C to 1000 ° C. By the activation treatment, the surface area of the powdery substance, particularly the mesopore volume effective for the catalytic reaction, is remarkably increased, and the catalytic activity is increased.

As described above, by performing firing in multiple stages and by heating at a moderate temperature increase rate and firing, a rapid increase in protein higher order structure in which an amorphous structure and a crystalline structure are complicated Decomposition is avoided and a flexible fired body is obtained.
X-ray diffraction measurement of silk activated carbon that was heat-treated at 700 ° C., 900 ° C., and 1200 ° C. and steam activated at 850 ° C. confirmed that all were amorphous carbon. It is similar. Each resistivity was 3.2 × 10 ⁻² Ωm, 1.4 × 10 ⁻³ Ωm, and 5.1 × 10 ⁻⁴ Ωm in this order. Further, as a result of component analysis, it is confirmed that the activated carbon contains nitrogen and oxygen in addition to carbon, and the N / C ratio (atomic ratio) decreases when the heat treatment temperature is raised. The steam activation treatment increases the BET specific surface area by about two orders of magnitude, and the higher the heat treatment temperature and the longer the steam activation treatment, the larger the mesopore volume.
FIG. 1 shows an example of a carbon-nitrogen graphite structure corresponding to an XPS (X-ray photoelectron spectroscopy) spectrum of silk-derived activated carbon. (A) 700 ° C. heat treatment, (b) 900 ° C. heat treatment, (c) It shows 1200 ° C. heat treatment, (d) activated carbon (RP-20) and (e) furnace black (Vulcan XC-72). FIG. 1 compares XPS spectra according to the present invention. The vertical axis indicates the spectrum intensity, and the horizontal axis indicates the bond energy value, and different spectra are obtained depending on the bond energy of nitrogen atoms present in the carbon-nitrogen structure in the right figure. As a result of confirming the presence of N1s in FIG. 1, pyridine type nitrogen species (398.6 eV), pyrrole type nitrogen species (400.5 eV), and oxidized nitrogen species (402 to 405 eV) are heated at 900 ° C. when heated at 1200 ° C. Although it remained in the treatment, the stability of the graphite type nitrogen species (401.3 eV) was confirmed while it disappeared by the heat treatment at 1200 ° C.
The amount of remaining nitrogen component is not particularly limited, but it has been confirmed that the catalyst has sufficient catalytic activity when the N / C ratio is 0.004 to 0.07.

[Precious metal / metal oxide catalyst]
A catalyst having a further excellent activity is obtained by using a catalyst obtained by supporting a noble metal composed of one or more of Pt, Ir, Ru, Ag, and Pd or an alloy thereof on the powdered activated carbon catalyst. be able to. Since the powdered carbide itself has catalytic activity, the amount of noble metal used can be reduced. This catalyst metal loading method can be carried out in a normal process. For example, in the case of Pt, platinum can be supported by applying a Pt-containing solution to the activated carbon, or immersing the activated carbon in a Pt-containing solution, and reducing with heat treatment, hydrogen, or the like.
Moreover, the catalyst which has the outstanding activity by using what carried metal oxide components, such as titanium oxide, zirconium oxide, niobium oxide, tin oxide, tungsten oxide, and tantalum oxide, on the above-mentioned powdery activated carbon catalyst as a catalyst It can be. This catalyst metal loading method can be carried out in the usual steps as described above. In the case of Ti, by applying a Ti-containing solution to the activated carbon, or immersing the activated carbon in the Ti-containing solution and heat-treating it. Titanium oxide can be supported. Alternatively, high activation can be achieved by preparing a powdered metal oxide in advance and mixing it with the activated carbon. As a cause of high activation, it is assumed that the pore structure of the electrode (the void formed between the activated carbon and the oxide fine particles) acts effectively.

[Porous conductive substrate]
Materials such as a cloth made of nickel, stainless steel or carbon, a fiber / powder fired body, or the like can be used as the porous conductive substrate. It is preferable that the porous conductive substrate has an appropriate porosity and keeps sufficient conductivity for supply and removal of gas and solution. It is preferable that the thickness is 0.01 to 5 mm, the porosity is 30 to 95%, and the typical pore diameter is 0.001 to 1 mm.

  The carbon cloth is made of a bundle of several hundreds of fine carbon fibers having a diameter of several μm, which is used as a woven fabric. However, the carbon cloth is a material excellent in gas-liquid permeability and is preferably used as the substrate. Carbon paper is obtained by using carbon raw material fibers as a precursor of a thin film by a papermaking method and firing it. This is also a material suitable for use. Direct power supply to this carbon conductive substrate causes local concentration of current due to insufficient conductivity, and locally concentrated current is also supplied to the gas diffusion layer and reaction layer, thereby reducing electrolysis efficiency. However, a uniform current is supplied to the conductive substrate by the formation of the conductive layer described below. Accordingly, a uniform current is also supplied to the gas diffusion layer and the reaction layer, so that an improvement in performance is achieved.

[Conductive layer]
For the purpose of improving the electrical conductivity inside the back surface and inside the electrode, it is preferable to mix the metal powder with a solvent such as hydrophobic resin, water, naphtha, etc. to form a paste, and apply and fix it to the back surface of the gas diffusion layer. The particle size of the hydrophobic resin (fluorine resin component) powder is preferably 0.005 to 10 μm. As the metal powder for salt electrolysis, it is preferable that it is stable in a high-temperature alkaline solution and inexpensive, and silver or a silver alloy (containing a small amount of copper, platinum, or palladium) is preferably selected. You may synthesize | combine by dry methods, such as vapor deposition and a sputter | spatter.

[Catalyst application method]
The activated carbon and the catalytic metal / metal oxide particles are mixed with a solvent such as a hydrophobic resin, water, and naphtha to form a paste, which is applied and fixed to the gas diffusion layer. The particle size of the hydrophobic resin (fluorine resin component) powder is preferably 0.005 to 10 μm. Moreover, in order to perform application | coating easily, it is preferable to melt | dissolve thickeners, such as carboxymethylcellulose, and to adjust a viscosity. It is particularly preferable that the coating, drying, and firing are performed in several steps because a uniform catalyst layer can be obtained. Hydrophobic resins provide sufficient gas permeability and prevent wetting by alkaline solutions.

The powdery silk-derived activated carbon and the catalyst metal / metal oxide particles obtained as described above are kneaded with an ion exchange resin solution (Nafion (registered trademark) solution, etc.) to form a paste, and the substrate described above It can also be applied and dried. The resin liquid is suitable because it is a sticking agent for the catalyst and at the same time is added with ionic conductivity and can contribute to performance improvement. When an ion exchange resin liquid is used as a binder, heat treatment is performed in the range of 60 to 140 ° C. in consideration of the glass transition temperature. In the case of high temperature treatment, an inert gas atmosphere is particularly good. The amount of the activated carbon is preferably 10 to 1000 g / m ² .
FIG. 2 is a schematic longitudinal sectional view illustrating the gas diffusion electrode of the present invention, in which 16 is an electrode substrate, 17 is a catalyst layer, and 18 is a conductive layer.

[Gas diffusion electrode molding]
Since this gas diffusion electrode is used by applying pressure in the thickness direction, it is not preferable that the conductivity in the thickness direction is changed by this. For the purpose of improving the performance and forming a cathode having a filling rate of 20 to 50%, it is preferable to perform press working. The press working is performed to increase the conductivity by compressing the carbon material and to stabilize the filling rate and the change in conductivity when used under pressure. An improvement in the degree of bonding between the catalyst and the substrate also contributes to an improvement in conductivity. Further, the supply capacity of the raw material oxygen gas is improved by compressing the base and the reaction layer and improving the degree of bonding between the catalyst and the base. As the press working apparatus, a known apparatus such as a hot press or a hot roller can be used. As pressing conditions, room temperature to 360 ° C. and a pressure of 0.1 to 5 MPa are desirable.
As described above, a gas diffusion electrode having high conductivity and catalytic properties is manufactured.

Next, application examples of the gas diffusion electrode according to the present invention will be described.
[Gas diffusion electrode in fuel cell]
FIG. 3 is a schematic longitudinal sectional view of an alkaline fuel cell equipped with the gas diffusion electrode of the present invention.
Reference numeral 1 denotes an ion exchange membrane (anion selective exchange property) functioning as a polymer solid electrolyte. On both surfaces of the ion exchange membrane 1, a plate-like oxygen electrode (cathode) 2 and a hydrogen electrode (gas diffusion electrodes) are provided. Anode) 3 has a structure (membrane-electrode assembly, MEA) in which each reaction layer side is in close contact with ion exchange membrane 1 and ion exchange membrane 1 is sandwiched between both electrodes.

The oxygen electrode 2 and the hydrogen electrode 3 are configured by coating silk-derived activated carbon and catalyst particles such as metal or metal oxide on an electrode substrate such as carbon paper together with a binder such as a hydrophobic resin and firing.
A frame-shaped oxygen electrode gasket 4 and a hydrogen electrode gasket 5 are in close contact with the peripheral edges of the oxygen electrode 2 and the hydrogen electrode 3 opposite to the ion exchange membrane 1. A porous oxygen electrode current collector 6 and a hydrogen electrode current collector 7 are in contact with the oxygen electrode 2 and the hydrogen electrode 3 on the inner edge sides of the oxygen electrode gasket 4 and the hydrogen electrode gasket 5, respectively. It is installed as follows.
The oxygen electrode gasket 4 is in contact with the periphery of an oxygen electrode frame 8 having a plurality of concave surfaces on the side facing the ion exchange membrane, and an oxygen electrode chamber 9 is formed between the oxygen electrode frame 8 and the oxygen electrode 2. Has been.

On the other hand, the peripheral edge of the hydrogen electrode frame 10 having a plurality of concave surfaces formed on the side facing the ion exchange membrane is in contact with the hydrogen electrode gasket 5, and a hydrogen electrode chamber 11 is formed between the hydrogen electrode frame 10 and the hydrogen electrode 3. Is formed.
Reference numeral 12 denotes an oxygen gas supply port opened laterally at the upper part of the oxygen electrode frame 8, 13 denotes an unreacted oxygen gas and generated water outlet opened laterally to the lower part of the oxygen electrode frame 8, and 14 denotes a hydrogen electrode. A hydrogen gas supply port 15, which opens laterally at the upper part of the frame 10, is an unreacted hydrogen gas outlet that opens laterally at the lower part of the hydrogen electrode frame 10. An aqueous solution such as potassium hydroxide is supplied to each electrode chamber as necessary.

  The oxygen-containing gas and the hydrogen of the fuel are respectively supplied to the oxygen electrode 2 and the hydrogen electrode 3 of the fuel cell having such a configuration. The supply amount of hydrogen is preferably about 1 to 2 times the theoretical amount. As the raw material hydrogen gas, natural gas or hydrogen gas generated by petroleum reforming may be used, but the CO contamination rate is as small as possible and is allowed to be about 10 ppm. The supply gas is wetted as necessary. The supply amount of oxygen is also preferably about 1 to 2 times the theoretical amount. In general, the larger the oxygen concentration, the larger the current density that allows a current to flow.

  By the gas supply, electrons, oxygen and water react with each other on the oxygen electrode side to generate hydroxide ions. Hydroxide ions pass through the membrane to the hydrogen electrode, react with hydrogen and dissociate into water and electrons. After these electrons are supplied to the external load from the hydrogen electrode terminal to give energy, the electrons reach the oxygen electrode through the oxygen electrode terminal and are used for the reaction at the oxygen electrode.

[Gas diffusion electrode in salt electrolysis cell]
FIG. 4 is a schematic longitudinal sectional view illustrating a three-chamber electrolysis cell equipped with the gas diffusion electrode of the present invention.
The three-chamber electrolysis cell 21 is divided into an anode chamber 23 and a cathode chamber 24 by a perfluorosulfonic acid-based cation exchange membrane 22. A porous chlorine generating anode DSE (registered trademark of Permelec Electrode) 25 is in close contact with the anode chamber 23 side of the cation exchange membrane 22, and gas diffusion is performed on the cathode chamber side of the cation exchange membrane 22 with an interval. An electrode (cathode) 26 is installed, and the gas diffusion electrode 26 divides the cathode chamber 24 into a cathode gas chamber 28 opposite to the catholyte chamber 27 on the cation exchange membrane 22 side. The gas diffusion electrode 26 is formed by coating an electrode substrate such as carbon paper with a silk-derived activated carbon and catalyst particles such as metal or metal oxide together with a binder such as a hydrophobic resin and firing.

  When the saline solution is supplied to the anode chamber 23 of the three-chamber electrolysis cell 21, the diluted sodium hydroxide aqueous solution is supplied to the catholyte chamber 27, and the oxygen-containing gas is supplied to the cathode gas chamber 28, the anode chamber 23 The generated sodium ions pass through the cation exchange membrane 22 and reach the catholyte chamber 27. On the other hand, oxygen in the oxygen-containing gas supplied to the cathode gas chamber 28 diffuses in the gas diffusion cathode 26, reacts with water by the catalyst particles in the electrode catalyst layer, and is reduced to hydroxide ions to be catholyte chamber 27. To form sodium hydroxide by combining with the sodium ions.

FIG. 5 is a schematic longitudinal sectional view illustrating a two-chamber (zero gap type) electrolysis cell equipped with the gas diffusion cathode of the present invention.
The two-chamber electrolysis cell 31 is divided into an anode chamber 33 and a cathode gas chamber 34 by a perfluorosulfonic acid cation exchange membrane 32. A chlorine generating anode DSE 35 is in close contact with the anode chamber 33 side of the cation exchange membrane 32, and a gas diffusion cathode 36 having the same configuration as in FIG. 4 is in close contact with the cathode gas chamber 34 side of the cation exchange membrane 32. Has been.

When a saline solution is supplied to the anode chamber 33 of the electrolysis cell 31 and a wet oxygen-containing gas is supplied to the cathode gas chamber 34 between the two electrodes, sodium ions generated in the anode chamber 33 permeate the cation exchange membrane 32. The gas diffusion cathode 36 in the cathode gas chamber 34 is reached. On the other hand, oxygen in the oxygen-containing gas supplied to the cathode gas chamber 34 is reduced to hydroxide ions by the catalyst in the electrode catalyst layer of the gas diffusion cathode 36 and combined with the sodium ions to generate sodium hydroxide. Then, it is dissolved in water supplied together with the oxygen-containing gas to form an aqueous sodium hydroxide solution.
In the electrolytic cell 31 of FIG. 5, a hydrophilic layer may be disposed between the cation exchange membrane 32 and the gas diffusion electrode 36.

[Gas diffusion electrode in metal-air battery cell]
As an example, a gas diffusion electrode in a lithium air battery is shown.
FIG. 6 is a schematic longitudinal sectional view of a lithium air battery as an example of a metal air battery equipped with the gas diffusion electrode of the present invention.
The lithium-air battery cell 41 is partitioned into an anode chamber (hydrogen electrode chamber) 43 and a cathode chamber (oxygen electrode chamber) 44 by a solid electrolyte 42 that is selectively permeable to Li ions. The anode chamber side of the solid electrolyte 42 is filled with a lithium negative electrode 45 and a non-aqueous organic electrolyte solvent 47, and an alkaline electrolyte 48 and a gas diffusion electrode (oxygen electrode) 46 are installed on the cathode chamber side of the solid electrolyte 42. ing. The gas diffusion electrode 46 is constructed by coating silk activated carbon and catalyst particles such as metal or metal oxide on an electrode substrate such as carbon paper together with a binder such as a hydrophobic resin and firing.

  When both electrodes are electrically connected while supplying an oxygen-containing gas to the oxygen electrode chamber 28 of the battery cell 41, Li ions generated in the hydrogen electrode chamber 43 permeate the solid electrolyte 42 and reach the oxygen electrode chamber 44. . On the other hand, oxygen in the oxygen-containing gas supplied to the oxygen electrode chamber 44 diffuses in the gas diffusion electrode 46, reacts with water by the catalyst particles in the electrode catalyst layer, and is reduced to hydroxide ions to be oxygen electrode chamber. 44 and combines with the Li ions to produce lithium hydroxide.

  Examples relating to production and use of the gas diffusion electrode of the present invention will be described below, but the present invention is not limited thereto.

  Examples of the present invention and comparative examples are shown below.

[Example 1]
Silk raw cotton was calcined for 6 hours at 500 ° C. in a nitrogen atmosphere, and then pulverized to a particle size of about 10 μm with a ball mill. The pulverized silk powder was fired at 1200 ° C. for 7 hours in a nitrogen atmosphere. Thereafter, steam activation was carried out at 850 ° C. for 3 hours to produce a silk-derived activated carbon catalyst. This was fixed on a glassy carbon substrate (6 mmφ) with a Nafion resin solution to obtain an electrode. The counter electrode is a glassy carbon plate, and the electrode is mounted on a rotating electrode device at 1 ° C NaOH, 30 ° C. and operated at 2200 rpm. The relationship between voltage and current was measured as s, and the results are shown in FIGS. 7a and 7b. 7a and 7b are graphs showing the relationship between the oxygen reduction current-potential and reaction order of a carbon-based electrode catalyst made of silk-derived activated carbon produced by heat treatment at 1200 ° C. In FIG. 7a, an oxygen reduction current was observed from around 0.8 V, and it was confirmed that it has oxygen reducing ability.
It is known that the reduction of oxygen proceeds not only by the 4-electron reduction of formula (3) but also by formula (8).
O ₂ + H ₂ O + 2e ⁻ → OH ⁻ + HO ₂ ⁻ (8)
The reaction order n is calculated from the generation current efficiency of hydrogen peroxide in equation (8), and if n = 2, it indicates that everything proceeds with equation (8) including the generation of hydrogen peroxide, and n = 4 If there is, it indicates that the process proceeds according to the formula (3), which is the generation of hydroxide ions.
In FIG. 7b, the reaction order n of the product was about 3.8, and the production efficiency of hydrogen peroxide was about 10%.

[Example 2]
An electrode was produced in the same manner as in Example 1 except that the heat treatment was performed at 900 ° C. And the result of having performed evaluation similar to Example 1 was shown to FIG. 8 a and FIG. 8 b. 8a and 8b are graphs showing the relationship between the oxygen reduction current-potential and the reaction order of a carbon-based electrode catalyst made of silk-derived activated carbon produced by heat treatment at 900 ° C. In FIG. 8a, an oxygen reduction current was observed from around 0.8 V, and it was confirmed that it has oxygen reducing ability. In FIG. 8b, the reaction order n of the product was about 3.8, and the production efficiency of hydrogen peroxide was about 10%.

[Example 3]
An electrode was produced in the same manner as in Example 1 except that the particles of silk-derived activated carbon and ZrO _{2 in} Example 2 were mixed so that the apparent volume ratio was 1: 1. And when the same evaluation as Example 1 was performed, the oxygen reduction current comparable as Example 1 was observed, and it was confirmed that it has the ability to reduce oxygen. The reaction order n of the product was about 4, and the production of hydrogen peroxide was suppressed.

Using the electrode of Example 3, cyclic voltammogram was repeatedly measured 6000 times in 1M HClO ₄ at 30 ° C. in the RHE potential range at 0 to 1.2 V and a scanning speed of 100 mV / s. A slight decrease in current was confirmed. This shows that it is excellent in corrosion resistance.

[Example 4]
A slurry in which an ion exchange resin liquid and fluororesin fine particles are added to a water solvent in which a trace amount of a silk-derived activated carbon and a surfactant prepared in the same manner as in Example 1 is added is prepared, and a porous woven fabric substrate made of carbon fiber is prepared. An oxygen gas diffusion electrode (cathode) coated on top to form a catalyst was produced. The amount of catalyst was set to 100 g / m ² . A commercially available gas diffusion electrode with a Pt / C catalyst was used as the hydrogen anode for the counter electrode.
An anion exchange membrane was sandwiched between two porous electrodes, and integrated by performing a hot press at 130 ° C. for 5 minutes. Nickel foams were placed as the respective current collectors on the back side of the electrodes, sandwiched between the grooved graphite power feeding bodies, the cell was assembled, and the cathode chamber was filled with 2M KOH.
Hydrogen and oxygen were supplied to each electrode chamber at 0.2 MPa. The temperature was 80 ° C., was measured the relationship between the voltage and the current, the open circuit voltage is 0.92 V, 0.2 A / cm ^2, the cell voltage at 0.4A / cm ^2, 0.6V, 0.4V The maximum power density was 0.14 W / cm ² .

[Example 5]
The silk raw material was calcined at 500 ° C. for 6 hours in a nitrogen atmosphere, and then pulverized with a ball mill to a particle size of about 10 μm. The pulverized silk powder was fired at 900 ° C. for 7 hours under a nitrogen atmosphere. Thereafter, steam activation was carried out at 850 ° C. for 3 hours to produce a silk-derived activated carbon catalyst. The prepared carbon-based catalyst and PTFE water suspension (Mitsui Fluorochemical Co., Ltd. 31JR) were mixed, and after sufficiently stirring in water in which Triton corresponding to 20 wt% and carboxymethylcellulose corresponding to 1.5 wt% were dissolved, The mixed suspension was applied to a carbon cloth having a thickness of 0.4 mm so that the weight of activated carbon per projected area was 100 g / m ² . Dried at 60 ° C.

Next, a conductive layer was produced as follows.
Silver particles (AgC-H manufactured by Fukuda Metal Foil Powder Co., Ltd.) and PTFE water suspension (31JR manufactured by Deyupon / Mitsui Fluorochemical Co., Ltd.) are mixed. Triton equivalent to 20 wt% and 1.5 wt% equivalent After sufficiently stirring in water in which carboxymethyl cellulose was dissolved, the mixed suspension was applied to the back surface of the gas diffusion layer so that the weight of silver particles per projected area was 100 g / m ² .
After drying at 60 ° C., it was baked at 305 ° C. for 15 minutes in an electric furnace and pressed at a pressure of 0.6 MPa so that the filling rate of the gas diffusion cathode was 40%.

  The electrode characteristics were measured as follows.

(1) Steady test Anode DSE for chlorine generation mainly composed of ruthenium oxide as an anode (Permelec Electrode Co., Ltd.) and Flemion F8020 (Asahi Glass Co., Ltd.) as an ion exchange membrane are used to make the thickness 0.4 mm hydrophilic. The treated carbon cloth is used as a hydrophilic layer, the hydrophilic layer is sandwiched between the gas diffusion cathode and the ion exchange membrane, the anode and the gas diffusion cathode are pressed inward, and the ion exchange membrane is positioned in the vertical direction. As described above, the electrolytic cell was configured by closely fixing each member.

The sodium chloride concentration in the anode chamber was adjusted so that the sodium hydroxide concentration in the cathode chamber was 32 wt%, and oxygen gas was supplied to the cathode at a rate of about 1.2 times the theoretical amount. The liquid temperature of the anolyte was 90 ° C., Electrolysis was performed at a current density of 60 A / dm ² . The initial cell voltage was 2.13V. When electrolysis was continued for 150 days, the cell voltage was 2.15 V, the voltage increase was small, and the current efficiency was maintained at about 95%.

(2) Short-circuit test After operating continuously for 10 days under the above-mentioned steady-state test conditions, the current was turned off, the cathode chamber was not replaced with nitrogen, and the salt water was not replaced and left for one day and night. Thereafter, the temperature was lowered to room temperature, the current was passed, the cell was operated, this short-circuiting operation was repeated three times, and the cell voltage was measured. As a result, the voltage increase was 10 mV.

[Example 6]
An electrode was produced in the same manner as in Example 1 except that the particles of silk-derived activated carbon and ZrO _{2 in} Example 1 were mixed so that the apparent volume ratio was 1: 1 and the amount of activated carbon was 80 g / m ² . And when the electrolytic cell similar to Example 1 was assembled and the steady test was done, the cell voltage was 2.14V also in the initial stage and after 150 days of electrolysis. The voltage increase after the short circuit test was 0 mV.

[Example 7]
A silk-derived activated carbon catalyst was prepared in the same manner as in Example 1. When the same electrolytic cell as in Example 1 was assembled and a steady test was performed, the cell voltage was 2.17 V at the initial stage and after 150 days of electrolysis. The voltage increase after the short circuit test was 0 mV.

[Example 8]
An electrode was prepared in the same manner as in Example 1 except that the silk-derived activated carbon catalyst of Example 1 and each particle of Ag were mixed so that the apparent volume ratio was 1: 1 and the amount of activated carbon was 80 g / m ² . And when the electrolytic cell similar to Example 1 was assembled and the steady test was done, the cell voltage was 2.14V also in the initial stage and after 150 days of electrolysis. The voltage increase after the short circuit test was 0 mV.

[Comparative Example 1]
Furnace black particles are used as catalyst particles, and the silver particles and the PTFE water suspension are mixed so that the apparent volume ratio of the particles to the resin is 1: 1, and then the carbon cloth is 0.4 mm thick. After coating and drying at 60 ° C., baking was performed at 305 ° C. for 15 minutes in an electric furnace, and press working was performed at a pressure of 0.2 MPa to obtain an oxygen gas diffusion cathode. When the same electrolysis test as in Example 1 was performed, the cell voltage increased from 2.16 V to 2.20 V in 150 days from the beginning. The voltage after the short-circuit test increased by 70 mV and could not be used as an electrode.

  The present invention is an oxygen gas diffusion electrode for alkaline fuel cells, metal-air batteries, or salt electrolysis that is highly active in an alkaline aqueous solution using a silk-derived carbon-based electrode catalyst containing nitrogen. It can be used in the field of gas diffusion electrodes excellent in durability and long-term stability of electrodes during charge / discharge and emergency stop.

1: Ion exchange membrane 2: Oxygen electrode 3: Hydrogen electrode 4: Gasket for oxygen electrode 5: Gasket for hydrogen electrode 6: Current collector for oxygen electrode 7: Current collector for hydrogen electrode 8: Oxygen electrode frame 9: Oxygen electrode Chamber 10: Hydrogen electrode frame 11: Hydrogen electrode chamber 12: Oxygen gas supply port 13: Unreacted oxygen gas and generated water outlet 14: Hydrogen gas supply port 15: Unreacted hydrogen gas outlet 16: Electrode base material 17: Catalyst Layer 18: Conductive layer 21: Three-chamber electrolytic cell 22: Cation exchange membrane 23: Anode chamber 24: Cathode chamber 25: Anode DSE for chlorine generation
26: gas diffusion cathode 27: catholyte chamber 28: cathode gas chamber 31: two-chamber electrolytic cell 32: cation exchange membrane 33: anode chamber 34: cathode gas chamber 35: anode DSE for chlorine generation
36: Gas diffusion cathode 41: Lithium air battery cell 42: Solid electrolyte 43: Hydrogen electrode (anode) chamber 44: Oxygen electrode (cathode) chamber 45: Metallic lithium 46: Gas diffusion electrode 47: Nonaqueous organic electrolyte solvent 48: Alkaline Electrolyte

Claims (2)

Silk fibroin is baked at 500 ° C. to 1500 ° C. to form a carbon-based electrode catalyst composed of silk-derived activated carbon containing nitrogen derived from silk and molded into powder, and together with the carbon-based electrode catalyst, titanium oxide, oxidation Alkaline fuel for use in an alkaline aqueous solution characterized in that a metal oxide comprising at least one of zirconium, niobium oxide, tin oxide, tungsten oxide, and tantalum oxide is supported on the surface of a porous conductive substrate. Manufacturing method of oxygen gas diffusion electrode for battery, metal-air battery or salt electrolysis . The method for producing an oxygen gas diffusion electrode according to claim 1, wherein N / C (atomic ratio) in the carbon-based electrode catalyst is 0.004 to 0.07.

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