WO2013018843A1

WO2013018843A1 - Oxygen gas diffusion electrode and method of making the same

Info

Publication number: WO2013018843A1
Application number: PCT/JP2012/069634
Authority: WO
Inventors: Yoshio Takasu; Wataru Sugimoto; Ryojin OBINATA; Yoshinori Nishiki; Kazuhiro Hirao; Masaharu Uno; Takaaki NAKAI; Koji Nakano
Original assignee: Shinshu University; Permelec Electrode Ltd.
Priority date: 2011-07-29
Filing date: 2012-07-24
Publication date: 2013-02-07
Also published as: JP2014522076A; JP5960795B2

Abstract

The present invention aims to provide a gas diffusion electrode and a method of making the same for alkaline fuel cells, metal-air batteries or brine electrolysis cells which, more in detail, can provide electrolysis performance almost equivalent to the conventional one without using expensive platinum catalyst, and is superior in durability as electrode in electrolysis in alkaline solution or at the time of emergency shut down and in stability in a long time operation. The present invention aims to provide an oxygen gas diffusion electrode for alkaline fuel cells, metal-air batteries or brine electrolysis cells used in an alkaline aqueous solution, characterized in that powder state carbon-based electrode catalyst comprising silk-derived activated carbon containing silk-derived nitrogen is supported on the surface of a porous conductive substrate.

Description

DESCRIPTION

Title of Invention

OXYGEN GAS DIFFUSION ELECTRODE AND METHOD OF MAKING THE SAME

Technical Field

The present invention relates to gas diffusion electrodes for alkaline fuel cells, metal-air batteries and brine electrolysis; more in detail, relates to gas diffusion electrodes having corrosion resistant carbon-based catalyst, which achieves electrolysis performance corresponding to that by the conventional ones without using expensive platinum catalyst.

Background Art

[Oxygen gas diffusion electrodes for brine electrolysis]

Caustic soda (sodium hydroxide) and chlorine, industrially essential materials, are produced by the brine electrolysis process. This electrolysis process has been shifted to the ion exchange membrane process with an ion exchange membrane as diaphragm applying the activated cathode with a small overvoltage, via the mercury process applying mercury cathodes and the diaphragm process applying asbestos diaphragms and soft iron cathodes. While these periods of shifting, the unit power consumption to produce one ton of caustic soda has been reduced to 2000 kWh. However, in view that the production of caustic soda belongs to the heavy power consuming industries, further reduction of the unit power consumption is required.

The anode and cathode reactions by the conventional electrolysis method are as shown by the equations (1) and (2), and the theoretical decomposition voltage is 2.19 V.

2C1^" →C1₂ + 2e^" (1.36V) (1)

2H₂0 + 2e^" →20H^~ + ¾ (-0.83 V) (2)

If oxygen gas diffusion electrode is used at the cathode instead of hydrogen generation reaction, the reaction will be the equation (3), and the cell voltage will be reduced by 1.23 V theoretically, and by around 0.88 within the practical range of electric current density. The reduction of 700 kWh per ton of sodium hydroxide can be expected.

0₂ + 2H₂0 + 4e^~ →40H^" (0.40 V) (3)

For this reason, commercialization of the brine electrolysis process applying gas diffusion electrodes has been discussed since 1980s. But, to realize this process, it is essential that the oxygen gas diffusion electrodes with high performance and sufficient stability in this particular electrolysis system are developed.

Formerly, as an oxygen gas diffusion electrode for this kind of brine electrolysis, PTL 1 has disclosed the technology which lessens the resistance of a power supply part, PTL 2 has disclosed an optimum porosity in the oxygen gas diffusion electrode, and PTL 3 has disclosed the gas diffusion electrode applying palladium and silver. In addition, NPLs 1 and 2 report the recent development status of the gas diffusion electrode.

[Oxygen gas diffusion electrode for alkaline fuel cells]

Fuel cells are clean power generation systems which can convert chemical energy into electrical energy at a high efficiency. By combining oxidation reaction of hydrogen or organic carbon materials with reduction reaction of oxygen in the air, electrical energy is obtained from the electromotive force and the commercialization of the process was highlighted as a battery for the space exploration in 1960s. Recently, the process has been focused, again, for fuel cell vehicles, a small portable power source, or a power source for load leveling of power storage for household and power station.

In case that hydrogen is applied as fuel, the reaction of the equation (4) proceeds at the anode (fuel electrode) in an alkaline aqueous solution.

H₂ + 20FT →2H₂0 + 2e^" (-0.83 V) (4)

When oxygen is applied as oxidant, the reaction of the equation (3) proceeds at the cathode (oxygen electrode). The alkaline fuel cells include the anode electrode comprising metals as a construction material of the porous electrode or carbon, the cathode electrode and an aqueous solution such as of potassium hydroxide as electrolyte which separates these electrodes. In the alkaline fuel cell as above-mentioned, if carbon dioxide enters the electrolyte, it reacts with the electrolyte to form carbonate ion, as shown by the equation (5). Further, if the carbonate ion concentrates, it, together with alkali metal in the electrolyte, forms carbonate and deposits on the electrode, impeding the electrode reaction.

C0₂ + 2KOH→C0₃ ^2~ + 2K⁺ + H₂0 -→K₂C0₃| (5)

Therefore, it has been essential in the alkaline fuel cell that pure hydrogen gas is supplied as fuel gas to the fuel electrode. Also to the oxidant gas electrode, it is necessary that pure oxygen gas or air without carbon dioxide is supplied.

Thus, the alkaline fuel cell has been regarded as having a potential problem of blocking of the cell member by absorbing carbon dioxide in case of air source material. However, for the battery system applying anion exchange membrane, alkali of carbonate ion is consumed in the anode compartment as shown in the equation (5), and the carbonate ion, the pH of which has shifted on the acid side is again gasified and discharged outside the cell, proving that accumulation will not exceed a certain level. Such finding is attracting renewed interest. In case that KOH is applied as electrolyte, if an anion membrane is chosen as diaphragm membrane, deposition of carbonate can be suppressed.

Conventionally, as the oxygen gas diffusion electrode for this sort of alkaline fuel cells, a membrane with a small resistance or a binder with dissolved anion exchange resin component has been developed, as described in PTLs 4, 5, and 6, having been achieving improved cell characteristics.

[Oxygen gas diffusion electrode for metal-air battery]

A new battery applying such metals as lithium, zinc, and aluminum as the anode and an air electrode as the cathode is being developed, and is attracting attentions not only as the fuel cells but also as the storages of renewable energy. Compared with the lithium ion battery, the metal-air battery, which applies oxygen in air as active material of the cathode can reduce the weight and the volume.

The cathodic reaction in the discharge reaction of the lithium-air battery is as shown in the equation (3), and the anodic reaction is as per the equation (6).

Li →Li⁺ + e^" (-3.04 V) (6)

The anodic discharge reaction of the zinc-air battery is as per the equation (7).

Zn+ 20H^" →ZnO + H₂0 + 2e^" (- 1.25 V) (7)

Charge reactions are reverse reactions of these.

As the cathode for these batteries, development of oxygen gas diffusion electrodes, applicable in the alkaline system, is regarded as promising, in view of raw material cost.

So far, as this kind of the oxygen gas diffusion electrode for the metal-air battery, PTL 7 discloses lithium ion conductive material, and PTL 8 discloses catalyst material. Also, PTL 9 discloses the metal-air battery system and PTL 10 discloses non-aqueous electrolyte. The status quo of the present technology is reported in detail in NPL 3, giving comparisons of performance by different kinds of metal-air batteries.

[Catalyst of gas diffusion electrode for fuel cell]

Former platinum group catalyst has given a high performance, but the material resources are extremely limited for practical use, and it is essential to develop non-platinum group catalysts as an alternative catalyst. Among them, metal oxide materials and carbon-based materials are attracting special attention. In the former materials, metal oxides of Group 4 metals and Group 5 metals are regarded as promising, as described in NPL 4, in which the desirable performance by the partial oxidation catalyst of their carbonitride has been reported. For the latter, the following describes the present development state.

[Nitrogen-containing carbon-based catalyst]

Since the verification of oxygen reduction property of Co-phthalocyanine complex by Jasinski in 1964, studies have been made for verious complexes, polymers and pyrolysate catalysts of them (NPL 5).

Recently, nanoshell structure, which has graphite component around metal particles, has been found (NPL 6) and it is reported that carbon material (carbon alloy) catalyst containing hetero elements or carbon-based catalyst having metal elements in nitrogen- containing polymer gives high performance. (NPLs 7 and 8)

It is presumed that containing nitrogen manifests high oxygen reduction activity. The catalyst in which metals, such as iron, are introduced in the carbon-based thermal decomposition product is especially superior in the activity. As the manifestation mechanism of the above-mentioned catalyst, such stuructures are reported, as contributive, as the electronic structure specific to carbon, lattice disorder or defect of carbon network structure, and remaining or introduced complex structure. Catalyst activity is related also to electric conductivity.

When complex or nitrogen-containing polymer is subject to thermal decomposition in the presence of inert gas flow, a new carbon network structure is formed. Metals contained as raw materials are microparticulated and coated on the graphite layer, giving a significant effect on the catalyst activity even with a trace of content. Nitrogen content can be increased through selection of raw materials or application of the existing synthesis processes of materials including ammonia. The former can fix more stably. Manufacturing methods of nitrogen-containing carbon-based catalyst with high activity and stability have not yet established.

[Synthesis of silk-derived activated carbon]

Since ancient times, raw silk has been produced from silkworm cocoon. For silk fabric, long fibers among silk fibroin with sericin removed from raw silk are used, while short fibers are inexpensive and called "silk waste". Silk fibroin contains proteins comprising amino acids including glycine, alanine, and tyrosine, other than sericin. The silk-derived activated carbon by the present invention can utilize all proteins obtained from cocoon as raw material. The silk-derived activated carbon under the present invention is defined as the synthetic silk fibroin with sericin removed from raw silk subjected to carbonization in the inert gas flow, heat treatment and activation treatment. Activation treatment is not limited to steam activation, but can be conducted in gases containing carbon dioxide, alkali and ammonia.

NPLs 9, 10, 1 1, 12 and PTL 1 1 disclose that carbon-based electrode catalyst comprising silk-based activated carbon can be applied to acid fuel cells; and it is known that carbon- based electrode catalyst comprising silk-based activated carbon expresses superior performance in acidity by sulfuric acid. However, none of PTL 1 1 and NPLs 9, 10, 1 1, 12 discloses suitability of carbon-based electrode catalyst comprising silk-based activated carbon when used in an alkaline solution. In other words, there has been no report as to what performance carbon-based electrode catalyst comprising silk-based activated carbon achieves in an alkaline solution, and the activity or stability of it remain unclear. Citation List

Patent Literature

PTL 1 : JP11-050289A

PTL 2: JP2006-219694A

PTL 3: JP2008-127631A

PTL 4: JP2010-045024A

PTL 5: JP2010-1 13889A

PTL 6: JP2009-129881A

PTL 7: JP2007-294429A

PTL 8: JP2008-198590A

PTL 9: JP2009-032399A

PTL 10: JP2009-093983A

PTL 1 1 : JP2010-063952 A Non Patent Literature

NPL 1 : Soda and Chlorine, Vol.45, 85(1994)

NPL 2: J. Appl. Electrochem., 38, 1 177(2008)

NPL 3: Electrochemistry, Vol.78 529(2010)

NPL 4: Electrochemistry, Vol.78 970(2010)

NPL 5: Nature, 201, 1212 (1964)

NPL 6: J. Appl. Electrochem., 36, 239 (2006)

NPL 7: J. Power Sources, 196, 1006 (201 1)

NPL 8: Nature, 443, 63 (2006)

NPL 9: J. Appl. Electrochem., 38, 507(2008)

NPL 10: J. Appl. Electrochem., 40, 675(2010)

NPL 1 1 : Electrochemistry Communications, 1 1, 376(2009)

NPL 12: J. Power Sources, 195, 5840(2010) Summary of Invention

Technical Problem

The present invention aims to solve the problems of the conventional technologies; to elucidate what performance the powder state carbon-based electrode catalyst comprising silk-derived activated carbon containing silk-derived nitrogen has in an alkaline solution; and to provide a gas diffusion electrode for alkaline fuel cells, metal-air batteries or brine electrolysis cells which, more in detail, can reduce consumption of expensive platinum catalyst and provide electrolysis performance almost equivalent to the conventional one, and is superior in durability as electrode in electrolysis in alkaline solution or at the time of emergency shut down and in stability in a long time operation.

Solution to Problem

As the first solution to solve the above-mentioned problems, the present invention provides an oxygen gas diffusion electrode for alkaline fuel cells, metal-air batteries or brine electrolysis cells used in an alkaline aqueous solution, characterized in that powder state carbon-based electrode catalyst comprising silk-derived activated carbon containing silk-derived nitrogen is supported on a porous conductive substrate. As the second solution to solve the above-mentioned problems, the present invention provides an oxygen gas diffusion electrode, characterized in that the N/C (atomic ratio) in the carbon-based electrode catalyst is in a range of 0.004 - 0.07.

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

As the fourth solution to solve the above-mentioned problems, the present invention provides an oxygen gas diffusion electrode, characterized in that the metal catalyst contained in the carbon-based electrode catalyst is precious metal comprising any one or more of Pt, Ir, Ru, Ag, and Pd.

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

As the sixth solution to solve the above-mentioned problems, the present invention provides an oxygen gas diffusion electrode, characterized in that the metal oxides catalyst contained in the carbon-based electrode catalyst comprises any one or more of titanium oxide, zirconium oxide, niobium oxide, tin oxide, tungsten oxide and tantalum oxide.

As the seventh solution to solve the above-mentioned problems, the present invention provides a manufacturing method for an oxygen gas diffusion electrode, characterized in that the powder state carbon-based electrode catalyst comprising silk-derived activated carbon containing silk-derived nitrogen is manufactured by baking silk fibroin at a temperature of 500 - 1500 degrees Celsius.

Advantageous Effects of Invention

The oxygen gas diffusion electrode by the present invention applying silk-derived carbon-based electrode catalyst containing nitrogen is highly active in an alkaline aqueous solution for the use of alkaline fuel cells, metal-air batteries or brine electrolysis cells, and is durable and stable for a long time as electrode at charging and discharging of electrolysis and battery as well as at an emergency shutdown. In addition, formation of hydrogen peroxide is suppressed by making precious metal catalyst or metal oxides catalyst coexist, which allows four-electron reduction only, leading to less deterioration of electrodes and electrochemical cells.

Brief Description of Drawings

Fig. l shows XPS spectra and corresponding graphite-like structures with carbon- nitrogen materials of the silk-derived activated carbon by the present invention.

Fig.2 shows a schematic view of longitudinal section of the gas diffusion electrode by the present invention

Fig.3 shows a schematic view of longitudinal section of an alkaline fuel cell equipped with the gas diffusion electrode by the present invention

Fig.4 shows a schematic view of longitudinal section of a three-chamber electrolytic cell equipped with the gas diffusion electrode by the present invention

Fig.5 shows a schematic view of longitudinal section of a two-chamber electrolytic cell equipped with the gas diffusion electrode by the present invention

Fig.6 shows a schematic view of longitudinal section of a lithium-air battery equipped with the gas diffusion electrode by the present invention

Fig.7a shows a graph showing electric current-potential of oxygen reduction of the gas diffusion electrode by the present invention comprising the silk-derived activated carbon treated at 1200 degrees Celsius. Fig.7b shows a graph showing the relation of the order of reaction of the gas diffusion electrode by the present invention comprising the silk-derived activated carbon treated at 1200 degrees Celsius.

Fig.8a shows a graph showing electric current-potential of oxygen reduction of the gas diffusion electrode by the present invention comprising the silk-derived activated carbon treated at 900 degrees Celsius.

Fig.8b shows a graph showing the relation of the order of reaction of the gas diffusion electrode by the present invention comprising the silk-derived activated carbon treated at 900 degrees Celsius.

Description of Embodiments

The following explains construction members of the oxygen gas diffusion electrode by the present invention, more in detail, in reference to the figures.

[Carbon-based electrode catalyst comprising silk-derived activated carbon]

PTL 1 1 discloses the preparation method of the silk fibroin-derived carbon-based electrode catalyst. The preparation method for the carbon-based electrode catalyst comprising silk-derived activated carbon by the present invention is almost equivalent to the method in PTL 1 1.

(1) Carbonization

Raw material silk is baked in the atmosphere of inert gases including nitrogen for several hours at 500 degrees Celsius. After cooling down to room temperature, the baked raw material silk is pulverized by a ball mill. The grain size is not necessarily specified, but that around ΙΟμπι is suitable. Baking temperature below 500 degrees Celsius is not preferable, leading to an insufficient carbonization.

(2) Heat treatment

Then, the powdered material is baked in an inert gas atmosphere for several hours at the baking temperature of 700-1500 degrees Celsius, most suitably at 1200 degrees Celsius. If the baking temperature is over 1500 degrees Celsius, nitrogen component will not remain in the powdered material, leading to an insufficient catalyst activity. (3) Activation treatment

The baked powder material is, then, subject to activation treatment. Steam activation is performed in such a manner that the baked powder material is heated to, for instance, 850 degrees Celsius in an inert gas atmosphere and left for several hours with steam being supplied. Activation treatment is not limited to steam activation, but can be performed in the gases including carbon dioxide, alkali, and ammonia. The suitable temperature for the activation treatment will be 700 -1000 degrees Celsius. Activation treatment increases the surface area of the powder material, especially the mesopore volume, which is effective to catalyst reaction, leading to an increase of catalyst activity.

Multiple steps of baking at a moderate rate of temperature rise for baking, as mentioned above, can prevent rapid decomposition of the superstructure of protein, in which non- crystal structure and crystal structure are mingled, resulting in a preparation of a flexible baked material. Amorphous carbon, similar to carbon black, is confirmed by the X-ray diffraction measurements in all cases of the silk activated carbons treated with the heat treatment at 700 degrees Celsius, 900 degrees Celsius, and 1200 degrees Celsius followed by the steam activation at 850 degrees Celsius. Resistances are 3.2xlO^"2 Q/m, 1.4xlO^"3Q/m and 5.1χ10^"4Ω/ιη, respectively. Further as the results of the composition analysis, the activated carbon contains nitrogen and oxygen in addition to carbon and with a rise of heat treatment temperature, N/C ratio (atomic ratio) decreases. By the steam activation treatment, the BET specific surface area increases by two digits and the mesopore volume increases with a higher temperature of heat treatment and a longer time of the steam activation treatment.

Fig.l gives XPS (X-ray photoelectron spectroscopy) spectra and corresponding graphitelike structures with carbon-nitrogen materials of the silk-derived activated carbon by the present invention, showing (a) 700 degrees Celsius heat treatment, (b) 900 degrees Celsius heat treatment, (c) 1200 degrees Celsius heat treatment, (d) activated carbon (RP-20) and , (e) furnace black (Vulcan XC-72). Fig.1 compares the XPS spectra related with the present invention, in which the vertical axis gives spectra intensity, and the horizontal axis gives the binding energy value. Different spectra are obtained depending of the binding energy of nitrogen atoms present in the carbon-nitrogen structure illustrated in the right figure. From Fig.l, the presence of Nls is examined and it is confirmed that pyridine-type nitrogen species (398.6eV) remains in the heating at 1200 degrees Celsius; pyrrole-type nitrogen species (400.5eV) and oxidation- type nitrogen species (402-405eV) remain in the heating at 900 degrees Celsius but disappear in the heating at 1200 degrees Celsius; and that graphite-type nitrogen species (401.3eV) shows stability.

Remaining amount of nitrogen component is not always restrictively specified, but it is confirmed that sufficient catalyst activity is kept in the range of 0.004-0.07 of N/C ratio.

[Precious metal/metal oxides catalyst]

The above-mentioned powder state activated carbon catalyst can obtain further improved activity as catalyst by supporting one or more of precious metal or its alloy from among Pt, Ir, Ru, Ag, and Pd. Since the above-mentioned powder state carbide intrinsically has catalytic activity, the amount of precious metal to be applied can be reduced. The supporting of the catalyst metals can be carried out by an ordinary process. For instance, in case of Pt, the activated carbon is coated with Pt containing solution, or is immersed in Pt containing solution, followed by a heat treatment, reduction with hydrogen, etc. for platinum supporting.

Also, the powder state activated carbon catalyst can obtain superior activity as catalyst through applying materials which support metal oxides as catalyst including titanium oxide, zirconium oxide, niobium oxide, tin oxide, tungsten oxide, tantalum oxide. The supporting of catalyst metal is performed in an ordinary process, as above-mentioned. In case of Ti, the activated carbon is coated with Ti containing solution, or the activated carbon is immersed in Ti containing solution, followed by a heat treatment for supporting titanium oxide. Alternatively, the powder state metal oxides is prepared in advance and mixed with the activated carbon for achieving a high activation. Presumably, high activation may be attributed to the porous construction of the electrode, which is the gap formed between the activated carbon and oxide particles. [Porous conductive substrate]

For a porous conductive substrate, such raw materials as cloth and fiber with baked powder comprising nickel, stainless steel and carbon are applicable. The porous conductive substrate desirably has a suitable porosity together with sufficient conductivity for supply and removal of gas and solution. It is preferable to have 0.01-5 mm in thickness, 30-95 % in porosity, and a typical pore diameter of 0.001-1 mm.

The carbon cloth is a woven fabric of several hundreds of fine carbon fibers with a few μπι in diameter, which is superior in gas-liquid permeability and is preferable as a raw material of the substrate. Another applicable raw material is the carbon paper, which is manufactured in such a manner that a thin membrane, as precursor, is prepared with a carbon material fiber by the paper making process, followed by baking. It should be noted that if power is directly supplied to the carbon-made conductive substrate, electric current locally concentrates for its insufficient conductivity, and such locally concentrated current flows also to the gas diffusion layer or the reaction layer, causing a poor electrolytic efficiency. However, if an additional conductive layer to be described is provided, electric current is supplied uniformly to the conductive substrate, which flows uniformly also to the gas diffusion layer and the reaction layer to achieve enhanced performance.

[Conductive layer]

In order to enhance conductivity of the rear face and the electrode inside, it is preferable to coat or fix metal powder paste prepared with solvents including hydrophobic resin, water, naphtha, etc., on the rear face of the gas diffusion layer. The hydrophobic resin (fluororesin component) has preferably 0.005- ΙΟμιη in grain diameter. As the metal powder for the brine electrolysis, which is required to be stable in the alkaline solution at a high temperature and also inexpensive, silver or silver alloy (containing a small amount of copper, platinum, and palladium) is desirable. For the synthesis, dry processes, such as vapor deposition and spattering are applicable. [Catalyst coating method]

The activated carbon and the catalyst metal/metal oxides particles are prepared to be a paste with solvents including hydrophobic resin, water, and naphtha and coated or fixed on the gas diffusion layer. The hydrophobic resin (fluororesin component) has preferably 0.005- ΙΟμιη in grain diameter. To facilitate the coating, the viscosity is preferably controlled with such thickeners as carboxy methyl cellulose. For providing a uniform catalyst layer, coating, drying and baking are most preferably repeated at several times. Hydrophobic resin provides sufficient gas permeability and at the same time, prevents wetting by alkaline solution.

The powder state silk-derived activated carbon and the catalyst metal/metal oxides particles obtained in the above-mentioned manner are mixed with ion exchange resin solution (such as a solution of Nafion -registered trade name) to prepare paste and can be applied to the substrate by coating and drying. The resin solution works as binder for the catalyst and simultaneously provides ion conductivity, contributing to the enhancement of performance. In case that ion exchange resin solution is applied as binder, heat treatment is conducted at a temperature of 60-140 degrees Celsius, considering the glass transition temperature and in an inert gas atmosphere, especially preferable at a high temperature treatment. Suitable amount of the activated carbon is in a range of 10-1000 g/m².

Fig.2 shows a schematic view of longitudinal section of the gas diffusion electrode by the present invention, illustrating the electrode substrate 16, the catalyst layer 17 and the conductive layer 18. [Manufacture of gas diffusion electrode]

The present gas diffusion electrode is used under pressure in thickness direction and it is not desirable that the conductivity in thickness direction changes because of the pressure. Press working is preferably performed to prepare a cathode with improved performance and a packing ratio of 20-50 %. The press working is performed to enhance the conductivity through compressing the carbon material and also to stabilize the packing ratio and the conductivity when pressure is applied. Improved bonding effect of catalyst to the substrate also contributes to enhancement of the conductivity. Also, compression of the substrate and the reaction layer and improved bonding effect of the catalyst and the substrate lead to an increase in supply capacity of raw material oxygen gas. As the press working equipment, well known units including a hot press and a hot roller are applicable. Preferable press conditions include a temperature range from room temperature-360 degrees Celsius, and a pressure range of 0.1-5 MPa. According to these methods, a gas diffusion electrode with high conductivity and high catalytic activity is manufactured.

The following explain application examples of the gas diffusion electrode by the present invention.

[Gas diffusion electrode in fuel cells]

Fig.3 shows a schematic view of longitudinal section of an alkaline fuel cell equipped with the gas diffusion electrode by the present invention.

The figure shows the ion exchange membrane 1 (anion-selectively exchangeable) working as solid polymer electrolyte, the oxygen electrode plate (cathode) 2 and the hydrogen electrode plate (anode) 3, which are both the gas diffusion electrode adhering respectively to the ion exchange membrane 1 with each reaction layer positioned inside, constituting membrane-electrode assembly (MEA) with the ion exchange membrane 1 tightly sandwiched by the both electrodes.

The oxygen electrode 2 and the hydrogen electrode 3 are prepared in such a manner that the silk-derived activated carbon and the catalyst particles comprising metal or metal oxides are coated and baked, together with a binder of hydrophobic resin, on the electrode substrate of the carbon paper, etc.

At the periphery of the oxygen electrode 2 and the hydrogen electrode 3 on the respective faces opposite to the ion exchange membrane 1 , the frame-shaped gasket for oxygen electrode 4 and the frame-shaped gasket for hydrogen electrode 5 are tightly adhered. The porous current collector for oxygen electrode 6 and the porous current collector for hydrogen electrode 7 are provided at the internal edges of the gasket for oxygen electrode 4 and the gasket for hydrogen electrode 5, respectively so as to contact the oxygen electrode 2 and the hydrogen electrode 3.

The gasket for oxygen electrode 4 is in contact with the periphery of the oxygen electrode frame 8 having a multiple number of concaves on the side of the ion exchange membrane, constituting the oxygen electrode compartment 9 between the oxygen electrode frame 8 and the oxygen electrode 2. On the other hand, the gasket for hydrogen electrode 5 is in contact with the periphery of the hydrogen electrode frame 10 having a multiple number of concaves on the side of the ion exchange membrane, constituting the hydrogen electrode compartment 1 1 between the hydrogen electrode frame 10 and the hydrogen electrode 3.

The oxygen gas inlet 12 opens laterally at the upper part of the oxygen electrode frame 8, the unreacting oxygen gas and produced water outlet 13 opens laterally at the lower part of the oxygen electrode frame 8, the hydrogen gas inlet 14 opens laterally at the upper part of the hydrogen electrode frame 10, and the unreacting hydrogen gas outlet 15 opens laterally at the lower part of the hydrogen electrode frame 10. Aqueous solutions such as of potassium hydroxide are supplied to each compartment as required.

Oxygen-containing gas and hydrogen as fuel are supplied to the oxygen electrode 2 and the hydrogen electrode 3, respectively, of the fuel cell having the above-mentioned construction. The supply amount of hydrogen should be 1-2 times the theoretical one. Hydrogen gas, as raw material, can be procured from natural gas or hydrogen gas generated by the petroleum reforming, but the mix rate of CO should be as low as possible, with an allowable level of below 10 ppm. Supply gases are subject to moisturizing treatment as required. The supply amount of oxygen also should be 1 -2 times the theoretical one. In general, the larger the oxygen concentration, the larger electric density is applicable. With the above-mentioned feed gas supply, hydroxide ion is formed from the reaction of electron and oxygen and water at the cathode. The hydroxide ion permeates the membrane to the anode, where it reacts with hydrogen to dissociate into water and electron. This electron is supplied to the external load from the anode terminal, transfers energy, reaches via the cathode terminal to the cathode, and is utilized for the reaction at the cathode.

[Gas diffusion electrodes in the brine electrolysis cells]

The three-chamber electrolytic cell 21 is separated into the anode compartment 23 and the cathode compartment 24 by the cation exchange membrane 22 of perfluorosulfonic acid group. A porous DSE (Registered trademark of Permelec Electrode Ltd.) anode 25 for chlorine generation is closely attached to the cation exchange membrane 22 on the side of the anode compartment 23; the gas diffusion electrode (cathode ) 26 is positioned on the side of the cathode compartment of the cation exchange membrane 22 with a gap; and by the gas diffusion electrode 26, the cathode compartment 24 is separated into the catholyte compartment 27 on the side of the cation exchange membrane 22 and the cathode gas compartment 28 on the opposite side. The gas diffusion electrode 26 is prepared in such a manner that the silk-derived activated carbon and the catalyst particles comprising metal or metal oxides are coated and baked, together with a binder of hydrophobic resin, on the electrode substrate of the carbon paper, etc.

If electric power is supplied to the both electrodes, while brine is supplied to the anode compartment 23, diluted aqueous solution of sodium hydroxide is supplied to the catholyte compartment 27, and oxygen-containing gas is supplied to the cathode gas compartment 28, of the three-chamber electrolytic cell 21, respectively, sodium ion formed at the anode compartment 23 permeates through the cation exchange membrane 22 to the catholyte compartment 27. On the other hand, oxygen in the oxygen-containing gas supplied to the cathode gas compartment 28 diffuses in the gas diffusion cathode 26, reacts with water to be reduced to hydroxide ion by the help of catalyst particles in the electrode catalyst layer, moves to the catholyte compartment 27, and bonds with the sodium ion to form sodium hydroxide.

Fig.5 shows a schematic view of longitudinal section of a two-chamber (zero gap type) electrolytic cell equipped with the gas diffusion electrode by the present invention. The two-chamber electrolytic cell 31 is divided into the anode compartment 33 and the cathode gas compartment 34 by the cation exchange membrane 32 of peril uorosulfonic acid group. On the side of the anode compartment 33 of the cation exchange membrane 32, the DSE anode 35 for chlorine generation is closely adhered and on the cathode gas compartment 34 of the cation exchange membrane 32, the gas diffusion cathode 36 with the same structure as in Fig.4 is installed in tight contact. If electric power is supplied to the both electrodes, while brine is supplied to the anode compartment 33 and wet oxygen-containing gas is supplied to the cathode gas

compartment 34, of the two-chamber electrolytic cell 31, respectively, sodium ion formed at the anode compartment 33 permeates through the cation exchange membrane 32 to the gas diffusion cathode 36 in the cathode gas compartment 34. On the other hand, oxygen in the oxygen-containing gas supplied to the cathode gas compartment 34 is reduced to hydroxide ion by the help of catalyst in the electrode catalyst layer of the gas diffusion cathode 36, bonds with the sodium ion to form sodium hydroxide, and dissolves in moisture supplied together with oxygen-containing gas to form sodium hydroxide aqueous solution.

In the two-chamber 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 for metal-air battery cell]

As an example, the gas diffusion electrode for a lithium-air battery is explained. Fig.6 shows a schematic view of longitudinal section of a lithium-air battery equipped with the gas diffusion electrode by the present invention, as an example.

The lithium air battery cell 41 is divided into the anode compartment (hydrogen electrode compartment) 43 and the cathode compartment (oxygen electrode compartment) 44 by the solid electrolyte 42 having Li ion-selective permeability. On the side of the anode compartment of the solid electrolyte 42, the lithium anode 45 and the non-aqueous organic electrolyte solvent 47 are filled, and on the side of the cathode compartment, the alkaline electrolyte 48 and the gas diffusion electrode (oxygen electrode) 46 are provided. The gas diffusion electrode 46 is prepared in such a manner that silk-derived activated carbon and catalyst particles of metal or metal oxides are coated together with the binder of

hydrophobic resin, etc. on the electrode substrate of carbon paper, etc., followed by baking.

If electric power is supplied to the both electrodes, while oxygen-containing gas is supplied to the cathode gas compartment 28 of the battery cell 41, Li ion formed at the hydrogen electrode compartment 43 permeates through the solid electrolyte 42 to the oxygen electrode compartment 44. On the other hand, oxygen in the oxygen-containing gas supplied to the oxygen electrode compartment 44 diffuses in the gas diffusion electrode 46, reacts with water to be reduced to hydroxide ion by the help of catalyst particles in the electrode catalyst layer, moves to the oxygen electrode compartment 44 and bonds with the Li ion to form lithium hydroxide.

The following explains examples relating to the manufacture and application of the gas diffusion electrode by the present invention; provided, however, the present invention shall not be limited to these examples.

Examples

The following show examples of the present invention together with a comparative example. [Example 1]

Spongiform silk material was baked at 500 degrees Celsius in nitrogen atmosphere for 6 hours for carbonization and crushed by a ball mill to approx. ΙΟμηι in grain diameter. The crushed silk powder was baked at 1200 degrees Celsius in nitrogen atmosphere for 7 hours. Then, it was treated at 850 degrees Celsius for 3 hours for steam activation to prepare silk- derived activated carbon catalyst. Thus prepared silk-derived activated carbon catalyst was fixed with Nation resin liquid on a glassy carbon substrate (6 mm φ) as an electrode. The electrode was installed on a rotary electrode apparatus with a glassy carbon plate as counter electrode. The relation of voltage vs. electric current was measured under the conditions of 1M NaOH at 30 degrees Celsius, the scan rate at 10 mV/s, within the potential scan range of 0.2-1.2 V, at 2200 rpm. The results are shown in Fig.7a and Fig.7b. Fig.7a and Fig.7b show the relation of electric current-potential of oxygen reduction and the relation of the order of reaction, respectively, of the carbon-based electrode catalyst comprising silk- derived activated carbon, prepared by the heat treatment at 1200 degrees Celsius. In Fig.7a, oxygen reduction current was confirmed from around 0.8V, proving the possession of reducibility to oxygen.

The reduction of oxygen is not only 4-electron reduction by Equation (3), but also is known by Equation (8).

0₂ + H₂0 + 2e^" →OFT + H0₂ ^" (8)

The order of reaction n is calculated from the current efficiency of forming hydrogen peroxide by Equation (8). When n=2, all proceeds by Equation (8) involving formation of hydrogen peroxide. If n=4, all proceed by Equation (3) involving formation of hydroxide ions. In Fig.7b, the order of reaction n of formed substance was almost 3.8, and the formation efficiency of hydrogen peroxide was around 10%.

[Example 2]

The electrode was manufactured in the same manner as with Example 1 , except that the heat treatment was 900 degrees Celsius. The same evaluation was conducted as with Example 1, and the results are shown in Fig.8a and Fig.8b. Fig.8a and Fig.8b show a 1 graph showing electric current-potential of oxygen reduction and the relation of the order of reaction, respectively, of the carbon-based electrode catalyst comprising silk-derived activated carbon, prepared by the heat treatment at 900 degrees Celsius. In Fig.8a, oxygen reduction current was confirmed from around 0.8 V, proving the possession of reducibility to oxygen. In Fig.8b, the order of reaction n of the product is approx. 3.8 and the generation efficiency of hydrogen peroxide was approx. 10%.

[Example 3]

The electrode was manufactured in the same manner as with Example 1 , except that the particles of silk-derived activated carbon of Example 2 and Zr0₂ were mixed so as to be 1 : 1 in apparent volume ratio. The same evaluation was conducted as with Example 1. As a result, equivalent amount of oxygen reduction current to Example 1 was observed, proving the possession of reducibility to oxygen. The order of reaction n of the product is approx. 4 and the generation of hydrogen peroxide was suppressed.

Using the electrode of Example 3, cyclic voltammogram was repeated 6000 times at the scanning rate of lOOmV/s within 0-1.2 V RHE potential range at 30 degrees Celsius in 1M HCIO4. Measurement showed a reduction of electric current by approx. 5%, proving superiority in corrosion resistance.

[Example 4]

A slurry prepared in such a manner that ion exchange resin solution and fluororesin particles were added to an aqueous solution of silk-derived activated carbon formed similarly to Example 1 and a trace amount of surfactant was coated on the carbon-fiber made porous woven fabrics substrate to make an oxygen gas diffusion electrode (cathode) with catalyst being formed. The amount of catalyst was controlled to 100 g/m². For a hydrogen anode as counter electrode, a commercially available gas diffusion electrode with Pt/C catalyst was applied.

An anion exchange membrane was interleaved between two porous electrodes and treated with hot-press at 130 degrees Celsius for 5 minutes to unify them. Nickel foams were provided on the back side of the electrodes as a respective current collector and pinched by the graphite-made current distributers with a groove processed. The cells were assembled and 2M KOH was filled in the cathode compartment.

Hydrogen and oxygen were supplied to each electrode compartment at 0.2 MPa. At 80 degrees Celsius, relation of voltage vs. electric current was measured. The open circuit voltage was 0.92 V, the cell voltage at 0.2A/cm² and 0.4 A/cm² was 0.6 V and 0.4 V, and the maximum output density was 0.14 W/cm². [Example 5]

Silk raw material was baked at 500 degrees Celsius in nitrogen atmosphere for 6 hours for carbonization and crushed by a ball mill to approx. ΙΟμιη in grain diameter. The crushed silk powder was baked at 900 degrees Celsius in nitrogen atmosphere for 7 hours. Then, it was treated at 850 degrees Celsius for 3 hours for steam activation to prepare silk- derived activated carbon catalyst. Prepared carbon-based catalyst was mixed with PTFE aqueous suspension (31JR manufactured by Du Pont-Mitsui Fluorochemicals Company, Ltd.) and sufficiently stirred in the water with triton, corresponding to 20 wt% and carboxy methyl cellulose, corresponding to 1.5 wt%. The mixed suspension was coated on the 0.4mm thick carbon cloth so that the weight of activated carbon per projected area becomes 100 g/m², followed by drying at 60 degrees Celsius.

Then, the conductive layer was prepared as follows.

Silver particles (AgC-H manufactured by Fukuda Metal Foil & Powder Co., LTD.) and PTFE aqueous suspension (31 JR manufactured by Du Pont-Mitsui Fluorochemicals Company, Ltd.) were mixed and sufficiently stirred in the water with triton, corresponding to 20 wt% and carboxy methyl cellulose, corresponding to 1.5 wt%. The mixed suspension was coated on the rear face of the gas diffusion layer so that the weight of silver particle per projected area becomes 100g/m², followed by drying at 60 degrees Celsius, baking at 305 degrees Celsius for 15 minutes in the electric furnace, and press working at 0.6MPa so that the packing ratio of gas diffusion cathode becomes 40%.

The characteristics of the electrode were measured as follows. (1) Routine Test

An electrolysis cell was constructed in the following manner.

As anode, DSE anode for chlorine generation (manufactured by Permelec Electrode Co., Ltd.) with ruthenium oxide as chief element was applied and, as ion exchange membrane, Flemion F8020 (manufactured by Asahi Glass Co., Ltd.) was applied. Hydrophilic layer, which was carbon cloth of 0.4mm in thickness with hydrophilic treatment was interleaved between the gas diffusion cathode and the ion exchange membrane. The anode and the gas diffusion cathode are pressed, facing inward and respective members were tightly adhered so that the ion exchange membrane positions in vertical direction. Brine concentration in the anode compartment was controlled so that the concentration of the sodium hydroxide in the cathode compartment becomes 32 wt% and electrolysis operation was conducted at a current density of 60A/dm² at 90 degrees Celsius supplying oxygen gas to the cathode at approx. 1.2 times the theoretical amount. The initial cell voltage was 2.13 V. In the continuous electrolysis operation for 150 days, the cell voltage was as low as 2.15 V and the electric current efficiency was maintained at approx. 95 %.

(2) Short-circuiting test

Under the routine test conditions, operation was continued for 10 days, then the electric current was cut off. The electrolytic cell in the state of short-circuiting was left for 24 hours without nitrogen replacement in the cathode compartment, and without brine feed. Then, the cell temperature, which had dropped to the room temperature, was raised and the cell was started by supplying electric current. Such short-circuiting operation was repeated three times and the cell voltage was measured. The increase of voltage was 10 mV. [Example 6]

The same electrode was prepared as with Example 1 , except that the particles of silk- derived activated carbon and Zr0₂ were mixed to be 1 : 1 as the apparent volume ratio, and the activated carbon amount was 80 g/m . The electrolytic cell similar to that of Example 1 was constructed for the routine test. The initial cell voltage was 2.14 V and the same value was given in the electrolytic operation in 150 days. The voltage increase after the short- circuiting test was 0 mV.

[Example 7]

Silk-derived activated carbon catalyst was prepared as with Example 1. The electrolytic cell similar to that of Example 1 was constructed for the routine test. The initial cell voltage was 2.17 V and the same value was given in the electrolytic operation in 150 days. The voltage increase after the short-circuiting test was 0 mV. [Example 8]

The same electrode was prepared as with Example 1 , except that the particles of silk- derived activated carbon and Ag were mixed to be 1 :1 as the apparent volume ratio, and the activated carbon amount was 80 g/m . The electrolytic cell similar to that of Example 1 was constructed for the routine test. The initial cell voltage was 2.14 V and the same value was given in the electrolytic operation in 150 days. The voltage increase after the short- circuiting test was 0 mV.

[Comparative Example 1]

The oxygen gas diffusion cathode was prepared in such a manner that furnace black particle was applied as catalyst particle, silver particle and PTFE aqueous suspension was mixed to be 1 : 1 of the apparent volume ratio between the particle and the resin, and the suspension was coated on the 0.4 mm carbon cloth and dried at 60 degrees Celsius and baked for 15 minutes at 305 degrees Celsius in an electric furnace, followed by the press working at 0.2 MPa. Electrolysis test as with Example 1 was conducted and it was found that the cell voltage increased from the initial voltage at 2.16 V to 2.20 V in 150 days operation. After the short-circuiting test, the voltage increased by 70 mV, showing dysfunction as an electrode.

Industrial Applicability

The present invention relates to the gas diffusion electrode applying the silk-derived carbon-based electrode catalyst containing nitrogen, which is highly active in an alkaline aqueous solution for the use of alkaline fuel cells, metal-air batteries or brine electrolysis cells and durable as electrode at charging and discharging of electrolysis and battery with long time stability.

Such electrode is widely applied in the above fields of gas diffusion electrode in various industries.

Reference Signs List

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 compartment

10 : hydrogen electrode frame

11 : hydrogen electrode compartment

12 : oxygen gas inlet

13 : unreacting oxygen gas and produced water outlet

14 : hydrogen gas inlet 15 : unreacting hydrogen gas outlet

16 : electrode substrate

17 : catalyst layer

18 : conductive layer

21 : three-chamber electrolytic cell

22 : cation exchange membrane

23 : anode compartment

24 : cathode compartment

25 : DSE anode for chlorine generation

26 : : gas diffusion cathode

27 : : catholyte compartment

28 : : cathode gas compartment

31 : two-chamber electrolytic cell

32 : : cation exchange membrane

33 : : anode compartment

34 : : cathode gas compartment

35 : DSE anode for chlorine generation

36 : gas diffusion cathode

41 : : lithium air battery cell

42 : : solid electrolyte

43 : : hydrogen electrode compartment

44 : : oxygen electrode compartment

45 : : metal lithium

46 : : gas diffusion electrode

47 : : Non-aqueous organic electrolyte solvent

48 : : alkaline electrolyte

Claims

[Claim 1]

An oxygen gas diffusion electrode for alkaline fuel cells, metal-air batteries or brine electrolysis cells used in an alkaline aqueous solution, characterized in that powder state carbon-based electrode catalyst comprising silk-derived activated carbon containing silk- derived nitrogen is supported on the surface of a porous conductive substrate.

[Claim 2]

The oxygen gas diffusion electrode as in claim 1, characterized in that the N/C (atomic ratio) in the carbon-based electrode catalyst is in a range of 0.004 - 0.07.

[Claim 3]

The oxygen gas diffusion electrode as in claim 1 or 2, characterized in that metal catalyst is contained in the carbon-based electrode catalyst.

[Claim4]

The oxygen gas diffusion electrode as in claim 3, characterized in that the metal catalyst is precious metal comprising any one or more of Pt, Ir, Ru, Ag, and Pd.

[Claim5]

The oxygen gas diffusion electrode as in claim 1 or 2, characterized in that metal oxides catalyst is contained in the carbon-based electrode catalyst. [Claim 6]

The oxygen gas diffusion electrode as in claim 5, characterized in that the metal oxides catalyst contained in the carbon-based electrode catalyst comprises any one or more of titanium oxide, zirconium oxide, niobium oxide, tin oxide, tungsten oxide and tantalum oxide. [Claim7]

A manufacturing method of an oxygen gas diffusion electrode, characterized in that the powder state carbon-based electrode catalyst comprising silk-derived activated carbon containing silk-derived nitrogen is manufactured by baking silk fibroin at a temperature of 500 -1500 degrees Celsius.