WO2012053561A1

WO2012053561A1 - Electrode material and method for producing same

Info

Publication number: WO2012053561A1
Application number: PCT/JP2011/074069
Authority: WO
Inventors: 優吉武; 木原　直人; 敦義竹中; 陽介網野; 洋一世良; 佳孝西條; 川本　昌子
Original assignee: 旭硝子株式会社
Priority date: 2010-10-19
Filing date: 2011-10-19
Publication date: 2012-04-26
Also published as: JPWO2012053561A1

Abstract

An electrode material with a high support rate, good durability, and high activity is provided. The electrode material is formed by supporting platinum on carbon nanotubes in the gas phase, and the peak ratio of the D band and the G band (D/G) of the carbon nanotubes, as determined by Raman spectroscopy, is not more than 0.3. The carbon nanotubes are preferably heat treated at not less than 2000°C. Furthermore, the diameter of the carbon nanotubes is preferably not more than 200 nm, and the aspect ratio of the carbon nanotubes is preferably not less than 10.

Description

Electrode material and manufacturing method thereof

The present invention relates to an electrode material suitable for a fuel cell, an air cell, and the like, and a manufacturing method thereof.

Electrode materials in which a catalyst such as platinum is supported on carbon are often used for electrodes of fuel cells and the like. In particular, the electrode characteristics of the electrode material vary greatly depending on the catalyst loading method. In a polymer electrolyte fuel cell (PEFC) characterized by a high power density, it is necessary to make the catalyst layer thin in order to ensure the diffusibility of the reaction gas and product, and a high loading rate catalyst is used. Therefore, conventionally, a carrier such as carbon black having a high specific surface area that can ensure a large distance between catalyst particles has been used. For example, proposals such as supporting platinum fine particles in a highly dispersed manner have been made (see Patent Documents 1 and 2).

However, PEFCs for fuel cell vehicles are required to have durability against frequent start and stop. High power density is also required. Compared to a carbon alloy catalyst, oxide catalyst, or enzyme catalyst, the advantage of the platinum catalyst is that it is easy to obtain a higher output density in addition to high activity and high stability. In order to obtain a high output density, the catalyst is required to have a high loading rate. Conventionally, carbon catalysts having a high specific surface area such as Vulcan and Ketjen Black have been used as platinum-based catalysts. However, these high specific surface area carbon-based carriers are insufficient in durability, particularly oxidation resistance. In particular, the supported platinum particles themselves are catalysts for carrier oxidation. For this reason, metal oxide carriers such as graphitized carbon and titania have been studied, but there is a problem that the specific surface area is small and it is difficult to increase the catalyst loading.

On the other hand, carbon nanotubes are carriers with excellent oxidation resistance and electron conductivity, but have problems such as water repellency and low specific surface area. For this reason, it has been difficult to support the catalyst using the untreated carbon nanotubes as they are. Therefore, it is often performed that carbon nanotubes are treated with a mixed acid or the like to form a functional group. However, in this case, the activity was not always sufficient. Moreover, introduction of a functional group impairs the stability of the carbon nanotube.

Japanese Unexamined Patent Publication No. 2009-255058 Japanese Unexamined Patent Publication No. 2007-179963

An object of the present invention is to provide an electrode material having a high catalyst loading ratio, good durability, and high activity.

In order to solve the above problems, the present inventor provides the following inventions.
(1) An electrode material in which platinum is supported on a carbon nanotube in a gas phase, and the peak ratio (D / G) between the D band and the G band of the carbon nanotube by Raman spectroscopy is 0.3 or less. An electrode material characterized by.
(2) The electrode material according to (1) above, wherein the carbon nanotubes are heat-treated at 2000 ° C. or higher.
(3) The electrode material according to (1) or (2), wherein the carbon nanotube has a diameter of 200 nm or less.
(4) The electrode material according to any one of (1) to (3), wherein the carbon nanotube has an aspect ratio of 10 or more.
(5) The electrode material according to any one of (1) to (4), wherein the carbon nanotubes are not subjected to a hydrophilic treatment.
(6) A method for producing an electrode material, wherein platinum is supported in a gas phase on carbon nanotubes having a peak ratio (D / G) of D band to G band of 0.3 or less by Raman spectroscopy.
(7) The method for producing an electrode material according to (6), wherein the carbon nanotubes are supported with platinum without being subjected to a hydrophilic treatment.
(8) The method for producing an electrode material according to the above (6) or (7), wherein platinum is supported on the carbon nanotubes by a vacuum deposition method, a sputtering method, or an arc plasma method.
(9) The method for producing an electrode material according to any one of (6) to (8), wherein the carbon nanotube is heat-treated at 2000 ° C. or higher.
(10) The method for producing an electrode material according to any one of (6) to (9), wherein the carbon nanotube has a diameter of 200 nm or less.
(11) The method for producing an electrode material according to any one of (6) to (10), wherein the carbon nanotube has an aspect ratio of 10 or more.
(12) An electrode using the electrode material according to any one of (1) to (5) above.

The electrode material obtained in the present invention has high activity and durability, and can be used for an electrode for a fuel cell having a high power density.

It is a cyclic voltammogram (CV curve) measured using the electrode material of the present invention. It is the CV curve measured using the electrode material which carry | supported the platinum catalyst on the commercially available carbon black. It is a CV curve measured using commercially available carbon black. It is a CV curve measured using commercially available carbon black. It is a CV curve measured using cup lamination type carbon nanofiber. It is the CV curve measured using the multi-walled carbon nanotube. It is a conceptual diagram which estimates the correlation of a carbon nanotube and a platinum atom.

In this specification, underpotential deposition (hereinafter also referred to as “UPD”) is a potential higher than the theoretically calculated deposition potential (noble potential) when the interaction between ions and substrate atoms is strong. ) Refers to the phenomenon of metal and hydrogen deposition.

<Carbon nanotube>
In the electrode material of the present invention, a carbon nanotube (hereinafter sometimes abbreviated as “CNT”) is used as a carrier. CNT is superior in oxidation resistance to other carbon-based materials (carbon black, carbon nanohorn, etc.).
3 to 6 are cyclic voltammograms (CV curves) showing the comparison. The potential is shown based on the Ag / AgCl electrode. In all of FIGS. 3 to 6, the broken line is the case of the gold (Au) substrate only, and the solid line is the CV curve when the carbon-based material is applied. The measurement conditions for the CV curve are the same as in the examples described later. The method for producing the sample electrode was in accordance with the example, and only the carbon-based material was supported on the gold substrate. In FIG. 3, commercially available carbon black (Ketjen Black having a specific surface area of 1200 m ² / g) was used. In FIG. 4, commercially available carbon black (Vulcan XC-72 having a specific surface area of 250 m ² / g) was used. In FIG. 5, cup-stacked carbon nanofibers were used. In FIG. 6, the same CNT used in Example 1 of the example was used.

For carbon-based materials other than CNTs in FIGS. 3, 4 and 5, in particular, 0.6 V (standard hydrogen electrode standard. In the figure, since it is displayed with a silver / silver chloride electrode standard, it corresponds to about 0.4 V) It shows that the oxidation current is large at a high potential (noble potential) and is easily oxidized. On the other hand, as shown in FIG. 6, the CNT has almost the same characteristics as the CV curve of gold used for the substrate. Thereby, it was confirmed that CNT was excellent in oxidation resistance.
This is probably because most of the surface of the CNT is a surface corresponding to the basal plane of the graphite structure. An electrode material using CNTs has high electron conductivity and high gas permeability. For this reason, it is suitable as a material for an electrode for PEFC (gas diffusion electrode). Furthermore, unlike carbon black with fine pores such as ketjen black, CNT is easy to coat ionomers such as ion exchange resins when applying an electrode catalyst to a membrane electrode assembly (MEA). preferable.

The CNT may be a single-wall CNT or a multi-wall CNT, but is not easily oxidized as a material, and is easy to disperse and handle during electrode production (the multi-wall CNT is less likely to aggregate than the single-wall CNT. Therefore, multilayer CNT is preferable.

In CNT, it is considered that platinum is supported as highly active fine particles as compared with carbon nanohorn or the like. This is probably because platinum supported on the CNT surface composed of carbon sp ² hybrid orbitals exhibits high activity. On the other hand, in the edge portion, sp ³ hybrid orbitals increase, and the activity of platinum existing in this portion is considered to decrease.
The interatomic distance of platinum as a general metal is 0.277 nm. On the other hand, the distance between the 6-membered rings of the graphene structure composed of carbon atoms on the CNT surface is 0.246 nm. For this reason, when a strong bond is formed between platinum and CNT, platinum is subjected to compressive strain.
In a core-shell catalyst in which a thin layer of platinum is formed on the surface of core particles such as palladium and gold, the d-band structure of the surface platinum layer is changed due to lattice mismatch with the underlying core, and the oxygen reduction activity changes. It is considered. (See VR Stamenkovic, et al., Science, 315 (2007) 493.)
Also in the case of CNT, when a strong bond is formed between carbon and platinum, it is considered that the electronic structure of platinum is changed, the center of the d band is shifted in the deep direction, and the catalytic activity is improved.
There are many unclear points about the causal relationship between the UPD of hydrogen and the electronic structure of the electrode surface. However, as shown in the CV diagram of FIG. 1, in the electrode material of the present invention, the potential at which discharge / adsorption of hydrogen ions corresponding to hydrogen ion UPD occurs is significantly higher than that on a general platinum surface. It was found that it was biased toward a nasty direction. This reason is presumed to be based on the interaction between the CNT surface and platinum atoms.

The CNT used in the present invention has a peak ratio (D / G) of D band and G band by Raman spectroscopy of 0.3 or less, more preferably 0.25 or less, further preferably 0.17 or less, and 10 or less is particularly preferable.
However, the peak of the D band by Raman spectroscopy is a peak near 1350 cm ⁻¹ , which is caused by a point defect or a crystal edge defect. The G band peak is a peak in the vicinity of 1580 cm ⁻¹ and is a peak commonly observed in graphite. A small value of D / G indicates that there are few defects on the surface or end of the CNT. That is, it means a long CNT with few defects and many good surfaces. A small number of defects means that there are few defects that are the starting point of the oxidation reaction, which means that the durability is excellent.

The peak ratio between the D band and the G band by the above Raman spectroscopy is a value obtained when Raman spectroscopy is measured under the following conditions.
Semiconductor laser Wavelength: 532 nm, Output: 100 mW, Dimming rate: 10% When the laser power on the sample is about 0.6 mW,
Objective lens: 100 times,
Time: 30-180 seconds.

The heat treatment temperature of the CNT used in the present invention is preferably 2000 ° C. or higher. This is because the aforementioned D / G can be reduced, and the oxidation resistance is improved together with the mechanical strength. In addition, it is known that heat treatment at high temperature is also effective for removing the catalyst component used during CNT production. As this heat processing temperature, 2200 degreeC or more is more preferable, and 2400 degreeC or more is especially preferable. Although there is no upper limit on the heat treatment temperature, 3000 ° C. or less is economically preferable. (See J. Chen et al., Carbon, 45 (2007) 274.)

The diameter of the CNT is preferably 200 nm or less, more preferably 150 nm or less, further preferably 10 to 100 nm, and particularly preferably 10 to 60 nm. If the diameter is in the above range, it is presumed that the electrode activity increases because platinum fine particles are supported on the surface of the CNT while being dispersed as fine particles while maintaining a highly active state.
The diameter of the CNT is obtained from the result of image analysis of a FE-SEM (Field Emission-Scanning Electron Microscope) or TEM (Transmission Electron Microscope) photograph. The aspect ratio of CNT is preferably 10 or more, and more preferably 50 or more. Although there is no particular upper limit value for the aspect ratio, it is generally 1000. If the aspect ratio is large, the D / G value tends to be small. The aspect ratio of CNT can be obtained from the result of image analysis of FE-SEM or TEM photograph.

In the present invention, platinum is preferably supported directly on CNT. Here, directly supporting means that the surface of the CNT is not modified. That is, the surface of the CNT is not modified with, for example, an organic group. In particular, chemical treatment for the purpose of hydrophilization is not performed.
Although the details of the mechanism of action are not necessarily clear, it is thought that the atoms present on the supported platinum surface can have a less disturbed structure by the strong bonding of platinum fine particles to the CNT surface with particularly high crystallinity. It is done. On the other hand, the distance between carbon atoms constituting the CNT surface is not the same as the distance between closest atoms (2.77 angstroms) of the platinum crystal.
FIG. 7 shows a conceptual diagram for estimating the correlation between carbon nanotubes and platinum atoms. When platinum atoms form a bond on CNT as shown in FIG. 7, the distance between AB of corresponding carbon atoms (corresponding to the vertices of an equilateral triangle composed of every other vertex of the hexagon); Since it is smaller than the closest interatomic distance of the platinum single crystal for 46 angstroms, the interatomic distance of platinum is reduced. As a result, the distance between atoms changes, which is considered to express a predetermined activity by changing the electronic structure.
On the other hand, when the support surface is disordered or there are bulky compounds such as polybenzimidazole or the like, such as carboxyl groups or phenolic hydroxyl groups on the surface, the supported platinum atoms and the carbon in the support CNT It is thought that the atomic bond is not sufficient or the platinum bond is disturbed. In that case, it seems that the structure pattern of the supported platinum particles is not so different from that of the unsupported platinum particles.
For example, when a chemical hydrophilization treatment by a mixed acid treatment is performed, the D / G of CNT is increased, and the catalytic activity tends to be lowered. This is presumably because a part of the graphene structure is converted into a chemically active functional group and inhibits the formation of a strong bond between the carbon atom and platinum.
In the present invention, an appropriate amount of super strong acid such as perfluorosulfonic acid (PFSA) may be adsorbed on the surface. By adsorbing an appropriate amount of PFSA, it becomes easy to disperse even if a raw material with low dispersibility such as single-walled CNT is used. In PFSA, due to the strong electronegativity of its constituent fluorine atom, the bond with the carrier is not as strong as a general hydrocarbon compound, and when the amount of adsorption is not excessive, strong bond formation between platinum and CNT is formed. It is thought that it does not inhibit so much. However, it is not preferable to adsorb a large amount of PFSA before supporting platinum in terms of supporting platinum as fine particles. The adsorption amount of PFSA is preferably 10 parts by mass or less with respect to 100 parts by mass of CNT.

<Catalyst fine particles>
In the present invention, it is preferable to support platinum as a catalyst on the CNT as an electrode material, that is, use platinum fine particles as catalyst fine particles. Here, as the catalyst, only platinum may be used, or platinum and a metal other than platinum may be used in combination. Examples of the metal other than platinum used in combination include nickel, palladium, silver, and gold.

<Vapor phase support>
In the present invention, platinum is supported on CNTs in a gas phase (gas phase method). Although the details of the phenomenon are not necessarily clear, it is considered that when platinum is supported in a gas phase, it is likely to be supported as fine particles on the surface of the CNT. As a specific method of supporting in the gas phase, a vacuum deposition method, a sputtering method, or an arc plasma method is preferable.
In the vacuum deposition method, platinum as a target is heated, and platinum is deposited on CNTs under vacuum. Examples of the method for heating platinum include a resistance heating method, an electron beam method, a high frequency induction method, and a laser method. Among these, the resistance heating vapor deposition method or the electron beam vapor deposition method is preferable because the apparatus has a relatively simple configuration. The degree of vacuum is preferably about 10 ⁻³ to 10 ⁻⁴ Pa.
In the sputtering method, platinum as a target is placed in a vacuum chamber, and a rare gas element (generally using argon) ionized by applying a high voltage is collided. The platinum atoms of the target are repelled by the collision, reach the CNTs, and can carry platinum.
In the arc plasma method, plasma is formed by arc discharge in vacuum, platinum (evaporation material) provided near the cathode or in the vicinity of the cathode is evaporated, and platinum is supported on the surface of the CNT.

Conventionally, platinum particles are supported on carbons by a liquid phase method (liquid phase hydrogen) in which carbons are dispersed in water, platinum salts such as chloroplatinic acid aqueous solution are added, dried and then reduced. Reduction method) is known. In order to increase the loading of carbon with a low specific surface area by the liquid phase method, it was necessary to prevent aggregation of platinum salt particles. For this reason, a colloid protection method in which colloidal particles of platinum are generated using a protective agent and simultaneously supported has been adopted. The protective agent can be removed by treatment at a high temperature, but in the course of the high temperature treatment, the platinum fine particles aggregate and tend to grow, resulting in a decrease in activity. In the colloid protection method, it has been difficult to carry a large amount of platinum fine particles on a CNT having a small specific surface area and a surface with high water repellency without performing a hydrophilic treatment. CNT has a small specific surface area and high water repellency compared to carbon black having a high specific surface area such as Vulcan and Ketjen Black. For this reason, it has been performed that a CNT is subjected to a chemical treatment to introduce a polar functional group to make the surface hydrophilic.

However, the activity of electrode materials using CNTs introduced with polar functional groups was low. Its activity was equal to or about half that of the conventional carbon black. Furthermore, by introducing a polar functional group, it is easily oxidized at a high potential, resulting in a decrease in oxidation resistance of the CNT at a high potential, which is inappropriate. Table 1 shows the melting points of platinum and platinum compounds, the temperature at which lattice atoms easily move (Tamman temperature), and the temperature at which surface atoms move easily (Huetig temperature).

Platinum has a very low melting point for compounds such as oxides (PtO, PtO ₂ ) and chlorides (PtCl ₂ , PtCl ₄ ) compared to metal (Pt). In particular, in the case of fine particles, it can be seen that they tend to aggregate even at about 100 to 200 ° C. The temperature of the process of carrying these on a carrier such as carbon and carrying out the drying treatment or reduction is usually 150 to several hundred degrees C., and the aggregation of fine particles tends to occur. In particular, in the case of CNT having a small specific surface area, special measures are required. On the other hand, in the case of metal platinum, the Huettig temperature is about 300 ° C. or higher, and it can be said that the aggregation of fine particles is less likely to occur compared to oxides and chlorides.

In the present invention, platinum is supported by a vapor phase method. Thereby, platinum fine particles as a catalyst are supported on the CNT surface. According to this method, it is not necessary to introduce a special polar functional group into the CNT. For this reason, CNTs are hardly oxidized at a high potential, and a desirable electrode material can be obtained. Further, since a strong bond is formed between the platinum atom and the carbon atom on the surface of the CNT, it is considered that a predetermined activity is expressed.

In the gas phase method of the present invention, no aggregation inhibitor is used. Anti-agglomeration agents are generally used in colloid protection methods. Since no anti-aggregation agent is used, the removal step is also unnecessary. Heating, which is a general anti-aggregating agent removal step, causes aggregation of platinum fine particles and causes a decrease in the degree of dispersion. Further, the gas phase method has an advantage that no waste liquid or the like is generated.

<Electrode>
The electrode material of the present invention uses the above electrode material. Specifically, for example, an electrode material is kneaded with a binder and formed into a necessary shape to obtain an electrode. Examples of the binder include a fluorinated resin having a sulfonic acid group. Examples of the fluorine-containing resin having a sulfonic acid group include a resin (trade name: Flemion) manufactured by Asahi Glass Co., Ltd. and a resin (trade name: Nafion) manufactured by DuPont.

The electrode of the present invention is suitable for a fuel cell electrode (hydrogen electrode or air electrode). It can also be applied to the air electrode of an air battery. Furthermore, it is expected to be applied as a cathode for FED (Field Emission Display), flat fluorescent tubes, and cold cathode tubes.

The present invention will be described in the following examples, but the present invention is not limited to these examples. Examples 1 to 6 and 10 are examples, and examples 7 to 9 and 11 are comparative examples.

(Preparation of electrode material)
(Example 1: EB deposition / CNT)
As the CNT, MWNT-7 manufactured by Hodogaya Chemical Co., Ltd. was used. It is manufactured through a heat treatment at about 2200 ° C. It is a multi-walled CNT having a D / G of 0.08, a diameter of 60 nm, and an aspect ratio of about 120. 10 mg of the CNT was put into 50 mL of the mixed solvent A and dispersed by irradiating ultrasonic waves for 15 minutes to obtain a solution 1A. However, the mixed solvent A is a solvent obtained by mixing tetrahydrofuran and HFE-347pc-f (CF ₃ CH ₂ OCF ₂ CF ₂ H, manufactured by Asahi Glass Co., Ltd.) at 1: 1 (mass ratio). 5 mL of this solution 1A was pipetted and dropped onto an ethylene-tetrafluoroethylene copolymer resin film (hereinafter also referred to as ETFE film) and dried to deposit CNTs. The ETFE film on which the CNTs were deposited was placed in an electron beam (EB) vapor deposition apparatus (manufactured by Showa Vacuum Co., Ltd.) and maintained at a vacuum degree of about 8 × 10 ⁻⁴ Pa. The acceleration voltage was 10 kV, the current value was 260 mA, and an electron beam was applied to the platinum crucible. By opening and closing the shutter and controlling the film formation time, platinum particles were deposited 1 nm on the ETFE film carrying CNTs.
From the ETFE film, CNT supporting platinum was scraped off. The loading ratio of platinum was measured by the following method. Aqua regia was added to the weighed catalyst (CNT supporting platinum) to dissolve platinum. Platinum was quantified by ICP emission spectroscopy, and the loading rate was calculated therefrom.
5 mg of this CNT carrying platinum was put into 50 mL of the mixed solvent A and dispersed by irradiating with ultrasonic waves for 15 minutes to obtain a solution AC-1. This solution AC-1 was dropped on the rotating electrode with a pipette to deposit CNTs. Furthermore, an ionomer dispersion solution (manufactured by DuPont, Nafion solution) was added dropwise and dried to obtain a sample electrode 1. This sample electrode 1 was subjected to electrode characteristic evaluation.
FIG. 1 is a CV curve in the case of using the sample electrode 1 measured according to the conditions described later.
As shown in FIG. 1, from the CV curve, the hydrogen adsorption / desorption peak was observed at a considerably lower potential than that of a general catalyst, that is, a portion close to the equilibrium potential of hydrogen. Furthermore, the characteristic characteristic of the electrode material of the present invention that the oxidation-reduction potential of platinum was higher than that of a general catalyst was confirmed.

(Example 2: Sputtering / CNT)
The ETFE film on which CNTs obtained in the same manner as in Example 1 were deposited was placed in a sputtering apparatus (manufactured by Tokki Co., Ltd.) and kept at a vacuum of about 6.7 × 10 ⁻⁴ Pa. Argon (Ar) gas was introduced into the chamber at a rate of 50 sccm to 0.3 Pa. However, 1 sccm means a flow rate of 1 cm ³ per second in terms of the standard state. Subsequently, a plasma was generated by applying a DC voltage with a power of 50 W. Platinum was deposited to 1 nm on CNT by using platinum as a target, opening and closing the shutter, and controlling the film formation time.
From the ETFE film, CNT supporting platinum was scraped, and 3 mg of this material was put into 30 mL of the mixed solvent A and dispersed by irradiating with ultrasonic waves for 15 minutes to obtain a solution AC-2. This solution AC-2 was dropped onto the rotating electrode with a pipette to deposit CNTs. Further, the same ionomer dispersion solution as in Example 1 was dropped and dried to obtain a sample electrode 2. When this sample electrode 2 was subjected to electrode characteristic evaluation, the CV curve was almost the same as in Example 1.

(Example 3: Arc plasma / CNT)
The ETFE film on which CNTs having different diameters obtained using the same preparation conditions as in Example 1 were deposited was placed in an arc plasma apparatus (manufactured by ULVAC-RIKO), and the degree of vacuum was about 1 × 10 ⁻³ Pa. Retained. Platinum was deposited on the CNTs with a repetitive pulse having a pulse width of 200 μs. Thereafter, a sample electrode 3 was obtained in the same manner as in Example 1, and the electrode characteristics were evaluated. As in the case of Example 1, the CV curve was such that the hydrogen UPD was biased toward a base potential.

(Example 4: EB deposition / hydrophilized CNT)
The same CNT as in Example 1 was hydrophilized by immersing it in a mixed acid (sulfuric acid: nitric acid = 3: 1 (volume ratio)) at 40 ° C. for 24 hours. As a result, BET surface area becomes ^{(25 m} 2 / g in ^{untreated) 37m 2 / g, D /} G was about 3-fold compared to that of untreated becomes 0.23. Using this carrier, an electrode material of 30% by mass Pt / CNT (calculated value) was prepared in the same manner as in Example 1, sample electrode 4 was obtained, and electrode characteristics were evaluated. The CV curve showed a general shape in which hydrogen adsorption / desorption occurs at a relatively high potential.

(Example 5: Sputtering / hydrophilic CNT)
The same CNT as in Example 1 was hydrophilized in the same manner as in Example 4, and platinum was supported under the same conditions as in Example 2. The obtained sample electrode 5 was evaluated for electrode characteristics in the same manner as in Example 1. The CV curve showed a general shape in which hydrogen adsorption / desorption occurs at a relatively high potential.

(Example 6: Arc plasma / hydrophilic CNT)
The same CNT as in Example 1 was hydrophilized in the same manner as in Example 4, and platinum was supported under the same conditions as in Example 3. The obtained sample electrode 6 was evaluated for electrode characteristics in the same manner as in Example 1. The CV curve showed a general shape in which hydrogen adsorption / desorption occurs at a relatively high potential.

(Example 7: Sputtering / Vulcan)
Carbon black (Vulcan) was used as a carrier, and platinum was supported in the same manner as in Example 1. The obtained sample electrode 7 was evaluated for electrode characteristics in the same manner as in Example 1. As in FIG. 2 of Example 9, the CV curve showed a general shape in which hydrogen adsorption / desorption occurs at a relatively high potential.

(Example 8: Arc plasma / Ketjen Black)
Carbon black (Ketjen Black) was used as a carrier, and platinum was supported in the same manner as in Example 3. The obtained sample electrode 8 was similarly evaluated for characteristics. As in FIG. 2 of Example 9, the CV curve showed a general shape in which hydrogen adsorption / desorption occurs at a relatively high potential.

(Example 9: Commercial Catalyst / Catalyst Society Fuel Cell Related Catalysts Research Committee Reference Catalyst / Evaluation Method Study Group Common Catalyst FC-I1)
The common catalyst FC-I1 (manufactured by Ishifuku Metal Industry Co., Ltd.) proposed by the Catalysis Society of Japan's Fuel Cell Related Catalyst Study Group was evaluated in the same manner as in Example 1. This catalyst is prepared by supporting a platinum salt on a Vulcan carrier, drying it, and then reducing it. The CV curve (FIG. 2) showed a general shape in which hydrogen adsorption / desorption occurs at a relatively high potential.

<Electrode characteristic evaluation>
The electrode characteristics were determined by the following electrochemical measurement. A cell manufactured by Hokuto Denko Co., Ltd. was used as the cell. As the working electrode, each sample electrode prepared in each of the above examples was used. Glassy carbon was used for the counter electrode. As the reference electrode, a double junction type silver / silver chloride electrode was used. The cell temperature was 60 ° C., 0.5 M sulfuric acid aqueous solution was used, and nitrogen gas was bubbled for measurement. The redox activity was measured while rotating the sample electrode at 1000 rpm. The sweep speed is 50 mV / s, and the sweep range is 0.05 V to 1.2 V (standard hydrogen electrode standard) (The description of FIGS. 1 and 2 is −0.17 V to 0.98 V based on the silver / silver chloride electrode standard). is there. In this state, cyclic voltammetry (CV) measurement was performed. The results are shown in Table 2. Samples 1 to 9 correspond to Examples 1 to 9 above.

The mass activity ratio was determined as follows. When the commercially available catalyst (Example 9) was 1, the ratio of the current value per the same platinum amount at 0.85 V was determined as the activity ratio.
As shown in Examples 1 to 3, the electrode material of the present invention using CNTs that had not been subjected to a hydrophilization treatment exhibited a mass activity that was more than four times that of a commercially available catalyst. This is probably because the D / G was as small as 0.1 or less, and the platinum fine particles were directly supported on the CNT without performing the hydrophilic treatment.
Further, as shown in Examples 4 to 6, even when the hydrophilization treatment is performed, if D / G is 0.3 or less, the mass activity is about 6 to 10 times that of a commercially available catalyst. was gotten.

(Example 10)
In the same manner as in Example 3, 15% by mass of platinum was supported on the CNT obtained in the same manner as in Example 1. Using this electrode, a potential step cycle test was conducted in a 0.5 M sulfuric acid aqueous solution at 60 ° C. under the following conditions.
[1.3 V, hold for 30 seconds: 0.9 V, hold for 30 seconds] was repeated 300 times.
The mass activity (A / mgPt) before and after this potential step cycle test was compared, and the reduction rate of mass activity at 0.8 V and 0.9 V was calculated.

(Example 11)
An electrode was prepared and a potential step cycle test was conducted in the same manner as in Example 10 except that ketjen black having a large specific surface area was used instead of CNT as the support.

Platinum has a small distance between particles on a carrier having a small specific surface area and is likely to deteriorate. Therefore, the potential fluctuation resistance of the electrode material in which platinum was supported on carbon black (KB) having a very large specific surface area by the liquid phase hydrogen reduction method and the electrode material of the present invention were compared by a potential step cycle test. As a result, as shown in Table 3, it was confirmed that the decrease rate of mass activity was small in CNT, although the specific surface area was overwhelmingly small. That is, it was shown that the mass activity of the electrode material is not easily lowered and the durability is excellent. Each sample 10 to 11 corresponds to the above examples 10 to 11.

The electrode material of the present invention is suitable for an electrode (hydrogen electrode or air electrode) of a fuel cell and an air electrode of an air cell, and can also be used as a cathode of an FED, a flat fluorescent tube and a cold cathode tube.

The entire contents of the specification, claims, drawings and abstract of Japanese Patent Application No. 2010-234294 filed on October 19, 2010 are cited herein as disclosure of the specification of the present invention. Incorporated.

Claims

An electrode material in which platinum is supported in a gas phase on carbon nanotubes,
An electrode material, wherein a peak ratio (D / G) between a D band and a G band of a carbon nanotube by Raman spectroscopy is 0.3 or less.
The electrode material according to claim 1, wherein the carbon nanotube is heat-treated at 2000 ° C or higher.
The electrode material according to claim 1 or 2, wherein the carbon nanotube has a diameter of 200 nm or less.
4. The electrode material according to claim 1, wherein the carbon nanotube has an aspect ratio of 10 or more.
The electrode material according to any one of claims 1 to 4, wherein the carbon nanotubes are not subjected to a hydrophilic treatment.
A method for producing an electrode material, characterized in that platinum is supported in a gas phase on carbon nanotubes having a peak ratio (D / G) of D band to G band of 0.3 or less by Raman spectroscopy.
The method for producing an electrode material according to claim 6, wherein platinum is supported on the carbon nanotube without subjecting it to a hydrophilic treatment.
The method for producing an electrode material according to claim 6 or 7, wherein platinum is supported on the carbon nanotubes by a vacuum deposition method, a sputtering method, or an arc plasma method.
The method for producing an electrode material according to any one of claims 6 to 8, wherein the carbon nanotubes are heat-treated at 2000 ° C or higher.
The method for producing an electrode material according to any one of claims 6 to 9, wherein the carbon nanotube has a diameter of 200 nm or less.
The method for producing an electrode material according to any one of claims 6 to 10, wherein the carbon nanotube has an aspect ratio of 10 or more.
An electrode using the electrode material according to any one of claims 1 to 5.