JP2006172865A - Electrode for fuel cell, and fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for a fuel cell which includes as an electron conductor a carbon fiber having a large specific surface area, and in which a high output can be obtained. <P>SOLUTION: The electrode for the fuel cell comprises as an electron conductor a carbon fiber of which on the surface carbon is adhered. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electrode for a fuel cell and a fuel cell.

  A fuel cell is composed of a negative electrode and a positive electrode, and an electrolyte provided between them, and a fuel is supplied to the negative electrode and an oxidant is supplied to the positive electrode to generate electricity by an electrochemical reaction.

  In many of these positive electrodes and negative electrodes, a catalyst is supported on at least a portion in contact with the electrolyte.

  Also, carbon black is usually used as the electrode. However, carbon black has a problem that electric resistance is large, catalyst loading is not large, and output is small.

As a solution to this problem, Patent Document 1 discusses the use of nanocarbons such as carbon nanofibers.
International Publication No. 2002/027844 Pamphlet

  However, even if nanocarbon such as carbon nanofiber was used, the output improvement width was small.

  This invention is made | formed in view of said situation, The objective is to provide the electrode for fuel cells and fuel cell which can obtain high output.

The above object of the present invention is to
(Claim 1) An electrode for a fuel cell comprising carbon fiber having carbon adhered to the surface as an electron conductor.

  (Claim 2) The fuel cell electrode according to claim 1, wherein the electron conductor carries a catalyst.

(3) The fuel cell electrode according to (1) or (2), wherein the electron conductor has a specific surface area of 50 to 1000 m ² / g.

  (Claim 4) The carbon is fixed by firing after attaching an organic polymer to the surface of the carbon fiber, according to any one of claims 1 to 3. electrode.

(Claim 5) At least one of the negative electrode and the positive electrode uses the fuel cell electrode according to any one of claims 1 to 4.
Achieved by:

  That is, the present inventor uses carbon fibers whose specific surface area is increased by fixing carbon to the surface as an electron conductor for an electrode for a fuel cell, thereby increasing the electron conductivity due to an increase in the contact area between the carbon fibers. As a result of improvement and an increase in the amount of catalyst supported on the carbon fiber itself, the output of the fuel cell is considered to be improved, and the present invention has been achieved.

  The fuel cell electrode according to the present invention has a large specific surface area of the electron conductor, and can provide a high output when used in a fuel cell.

  The present invention is characterized in that an electrode containing a carbon fiber having carbon adhered to its surface as an electron conductor is used as at least one of a negative electrode and a positive electrode of a fuel cell.

  Examples of the fuel that can be used in the fuel cell of the present invention include hydrogen gas, methanol, ethanol, 1-propanol, dimethyl ether, ammonia, and the like, and methanol is preferable.

  The direct methanol fuel cell has a feature that it can generate electric power by directly supplying a liquid as it is without taking out hydrogen gas by reforming an aqueous methanol solution as a fuel. Therefore, the fuel is gasified or reformed and supplied. Compared to conventional polymer electrolyte fuel cells, the structure as a power generation system can be simplified, and the size and weight can be easily reduced, and the use as a distributed power source and a portable power source has attracted attention.

Such a direct methanol fuel cell uses a proton conductive solid polymer membrane as an electrolyte membrane, and a catalyst layer is applied to porous carbon paper serving as a diffusion layer through the electrolyte membrane, and a negative electrode and a positive electrode The negative electrode side separator having a flow channel for supplying a methanol aqueous solution as a fuel is provided on the negative electrode side, and the flow channel groove for supplying air as an oxidant gas is provided on the positive electrode side. When a positive electrode side separator is provided, a methanol aqueous solution is supplied to the negative electrode, and air is supplied to the positive electrode, carbon dioxide gas is generated in the negative electrode by the oxidation reaction of methanol and water, and hydrogen ions and electrons are released ( _{CH 3 OH + H 2 O →} CO 2 + 6H + + 6e -), a positive electrode was produced water by reduction reaction with the hydrogen ions and the air having passed through the electrolyte membrane (6H ⁺ + ^{_{3/2) O 2 + 6e - →}} 3H 2 O), it is possible to obtain electrical energy to an external circuit connecting the anode and the cathode. Therefore, the total reaction of the direct methanol fuel cell is a reaction in which water and carbon dioxide are generated from methanol and oxygen.

  As the solid polymer electrolyte membrane having proton conductivity, known ones such as a sulfonated polyimide polymer electrolyte membrane, a fluorine polymer electrolyte membrane, a hydrocarbon polymer electrolyte membrane, and a composite material can be employed.

  For example, as the hydrocarbon polymer electrolyte material, sulfonated engineering plastic electrolytes such as sulfonated polyetherketone, sulfonated polyethersulfone, sulfonated polyetherethersulfone, sulfonated polysulfone, sulfonated polysulfide, and sulfonated polyphenylene, Examples include sulfoalkylated engineering plastic electrolytes such as sulfoalkylated polyetheretherketone, sulfoalkylated polyethersulfone, sulfoalkylated polyetherethersulfone, sulfoalkylated polysulfone, sulfoalkylated polysulfide, and sulfoalkylated polyphenylene. The sulfonic acid equivalent of these electrolyte materials is about 0.5 to 2.0 meq / g dry resin, preferably 0.7 to 1.6 meq / g dry resin. When the sulfonic acid equivalent is less than 0.5 meq / g dry resin, the ionic conduction resistance is increased, and when it is greater than 2.0 meq / g dry resin, it is easily dissolved in water.

  The ionic conductor is not particularly limited as long as it is an electrolyte having an ionic conductivity as used in an electrolyte membrane, and examples thereof include a fluorine-based electrolyte material, a partial fluorine-based electrolyte material, and a hydrocarbon-based electrolyte material.

As the fluorine-based electrolyte material, conventionally known polymers are widely used. For example, the general formula CF ₂ = CF- (OCF ₂ CFX) m-Oq- (CF ₂ ) n-A
(Wherein, m = 0 to 3, n = 0 to 12, q = 0 or 1, X = F or CF ₃ , A = sulfonic acid type functional group), tetrafluoroethylene, hexafluoro And copolymers with perfluoroolefins such as propylene, chlorotrifluoroethylene or perfluoroalkyl vinyl ether. Preferable examples of the fluorovinyl compound include, for example, CF ₂ ═CFO (CF ₂ ) aSO ₂ F, CF ₂ ═CFOCF ₂ CF (CF ₃ ) O (CF ₂ ) aSO ₂ F, CF ₂ ═CF (CF ₂ ) bSO ₂ F, CF ₂ = CF (OCF ₂ CF (CF ₃ )) cO (CF ₂ ) ₂ SO ₂ F (where a = 1 to 8, b = 0 to 8, c = 1 to 5) Can be mentioned.

  As the carbon fibers before the surface treatment, known fibers can be used, but carbon fibers having a fiber diameter in the range of 50 to 200 nm and an aspect ratio in the range of 10 to 200 are particularly preferable. As such carbon fiber, a vapor grown carbon fiber “VGCF or VGNF (both manufactured by Showa Denko)” series can be used.

  After the organic polymer is attached to the surface of the carbon fiber, it is baked to fix the carbon to the surface of the carbon fiber to obtain an electronic conductor having an increased specific surface area. Examples are given in the examples below.

The specific surface area of the electron conductor after the surface treatment is preferably 50 to 1000 m ² / g. When the specific surface area is larger than 1000 m ² / g, the reaction efficiency and the generated substance of the fuel cell are difficult to move in the electrode, so that the reaction efficiency is lowered.

  Examples of organic polymers include polyacrylonitrile, phenol, furan, divinylbenzene, unsaturated polyester, polyimide, diallyl phthalate and system, vinyl ester, polyurethane, melamine, and urea. It is done.

  The amount ratio of carbon fiber to carbon is preferably 1:10 to 1: 5000 of carbon: carbon fiber. More preferably, it is 1: 100 to 1: 5000.

  As a method of attaching the organic polymer to the carbon fiber, a method of polymerizing after treating with a monomer is preferable.

  The firing atmosphere is preferably a non-oxidizing atmosphere, and the firing temperature is preferably 800 to 1600 ° C.

  Here, the fiber diameter is defined as the fiber diameter by observing the short diameter of the carbon fiber on the surface in the direction substantially perpendicular to the fiber length direction and measuring the length in the direction perpendicular to the short diameter. It can be measured with an electron microscope.

  The specific surface area can be measured by the BET method.

The specific resistance of the electron conductor of the present invention is preferably 10 ^-1 Ωcm or less.

  Moreover, the electrode in this invention should just contain the carbon fiber by which such surface treatment was carried out, and may mix with a carbon particle.

  As the carbon particles, activated carbon, carbon black, graphite and a mixture thereof can be preferably used.

  Examples of carbon black include acetylene black, ketjen black, furnace black, lamp black, and thermal black. Denka BLACK (manufactured by Denki Kagaku Kogyo Co., Ltd.), Valcan XC-72 (manufactured by Cabot Corporation), Black Pearl 2000 ( The same as before), commercially available products such as Ketjen Black EC300J (manufactured by Ketjenblack International Co., Ltd.) can be used.

  As the noble metal catalyst to be supported, platinum, ruthenium, rhodium, palladium, iridium, gold, silver, iron, cobalt, nickel, chromium, tungsten, magazine, vanadium, lanthanum, zirconium, titanium, magnesium or a multicomponent alloy thereof should be used. Preferably, it is at least one selected from platinum and platinum ruthenium alloys. The metal catalyst diameter is preferably 10 nm or less, and more preferably 5 nm or less.

The noble metal catalyst can be supported on the carbon fiber of the present invention by, for example, adding platinum or a ruthenium salt to the carbon fiber dispersion, reducing it with hydrazine, etc., filtering and drying. Further, heat treatment may be performed. The true specific gravity of the carbon fiber is preferably 1.0 to 2.1 g / cm ³ .

  As a method for producing a solid polymer electrolyte membrane and an electrode containing the electron conductor of the present invention by joining, for example, a platinum catalyst powder supported on a surface-treated carbon fiber is dispersed in a polytetrafluoroethylene suspension. And then apply to the carbon paper to be a diffusion layer, heat-treat to form a catalyst layer, then apply the same electrolyte solution as the electrolyte membrane to the catalyst layer, and hot press and integrate with the electrolyte membrane Yes. In addition, there is a method of applying the catalyst layer paste and the diffusion layer paste to the electrolyte membrane, but the method is not limited thereto.

  A fuel distribution plate (separator) in which a groove for forming a fuel flow path and an oxidant flow path is formed outside the joined body of the electrolyte membrane and the electrode manufactured as described above, and an oxidant flow distribution plate (separator) A fuel cell is constructed by stacking a plurality of single cells via a cooling plate or the like. Moreover, you may provide the electrode used as a current collection layer in the contact surface with the said conjugate | zygote of a separator.

  The electronic conductor of the present invention may constitute at least a part of an electrode for a fuel cell, and an effect can be obtained by using the electronic conductor of the present invention in at least one of a diffusion layer, a catalyst layer, and a current collecting layer. .

  Hereinafter, the fuel cell of the present invention will be described in more detail with reference to examples, but the present invention is not limited thereto.

[Carbon fiber treatment 1]
Vapor grown carbon fiber VGCF (Showa Denko) 10 g, aniline 0.02 g, H ₂ SO ₄ 0.5 mol / liter of aqueous solution containing 1 mol of ammonium persulfate 0.5 mol, under strong stirring The polymerization was carried out with stirring for 5 hours. The obtained suspension was filtered, suction filtered and washed with ion exchange water, and the solid phase was dried to obtain a polyaniline-treated VGCF powder. The obtained powder was heated to 800 ° C. at a temperature rising rate of 3 ° C./min in a nitrogen atmosphere and baked at 800 ° C. for 1 hour. The specific surface area of the obtained polyaniline-treated VGCF was 230 m ² / g.

[Carbon fiber treatment 2]
The same treatment as in treatment 1 was performed except that the amount of aniline was changed to 0.05 g. The specific surface area of the obtained polyaniline-treated VGCF was 500 m ² / g.

[Treatment of carbon fiber 3]
The same treatment as in treatment 1 was performed except that the amount of aniline was changed to 0.1 g. The specific surface area of the obtained polyaniline-treated VGCF was 900 m ² / g.

[Preparation of catalyst-supported carbon fiber 1]
Catalysts were loaded on the polyaniline-treated VGCF obtained in the above treatments 1 to 3 by the following methods.

  1 g of the obtained polyaniline-treated VGCF powder and 50 g of pure water are mixed. Next, 0.6 g of a chloroplatinic acid aqueous solution as platinum and 0.6 g of ruthenium chloride as ruthenium are added, and the temperature is raised to 60 ° C. After the temperature becomes constant, the pH is adjusted to 10 with a 2 mol / l sodium hydroxide solution, and a 3% by mass hydrazine solution is added dropwise for reduction. After completion of the reduction, it was filtered and washed with a glass filter and dried to obtain a platinum ruthenium-supported polyaniline-treated VGCF. The amount of platinum supported on this polyaniline-treated VGCF was 40% by mass.

[Production of catalyst-supported carbon fiber 2]
1 g of VGCF powder and 50 g of pure water are mixed. Next, 0.6 g of a chloroplatinic acid aqueous solution as platinum and 0.6 g of ruthenium chloride as ruthenium are added, and the temperature is raised to 60 ° C. After the temperature becomes constant, the pH is adjusted to 10 with a 2 mol / l sodium hydroxide solution, and a 3% by mass hydrazine solution is added dropwise for reduction. After completion of the reduction, the filter was washed with a glass filter and dried to obtain platinum ruthenium-supported VGCF. The platinum loading of this VGCF was 10% by mass.

(Preparation of electrode paste)
(Preparation of negative electrode paste 1)
Platinum ruthenium-supported polyaniline-treated VGCF powder (specific surface area is 230 m ² / g), distilled water, 5% by mass Nafion solution (manufactured by Aldrich) is VGCF: Nafion mass ratio is 4: 1, and the solid content is 10% by mass. The negative electrode paste 1 was prepared by uniformly mixing with ultrasonic waves.

(Preparation of negative electrode paste 2)
Platinum ruthenium-supported polyaniline-treated VGCF powder (specific surface area is 230 m ² / g), distilled water, 20% by mass Nafion solution (manufactured by Aldrich), VGCF: Nafion mass ratio is 4: 1, solid content is 30% by mass Thus, the mixture was uniformly dispersed with ultrasonic waves to prepare a negative electrode paste 2.

(Preparation of negative electrode paste 3)
A negative electrode paste 3 was prepared in the same manner as the negative electrode paste 1 except that platinum ruthenium-supported carbon supporting 50% platinum was used as a catalyst.

(Preparation of negative electrode paste 4)
Platinum ruthenium-supported VGCF powder, distilled water, 60% by mass of Teflon (registered trademark) dispersion, 20% by mass of Nafion solution (manufactured by Aldrich), VGCF: Nafion mass ratio of 2: 1, and 10% by mass as solid content The negative electrode paste 4 was prepared by uniformly mixing with ultrasonic waves and uniformly dispersing with ultrasonic waves.

(Preparation of negative electrode paste 5)
Platinum ruthenium-supported polyaniline-treated VGCF powder (specific surface area is 500 m ² / g), distilled water, 60% by mass Teflon (registered trademark) dispersion, 20% by mass Nafion solution (manufactured by Aldrich), VGCF: Nafion mass ratio The mixture was mixed so as to have a solid content of 2: 1 and 10% by mass, and dispersed uniformly with ultrasonic waves to prepare a paste 5 for negative electrode.

(Preparation of negative electrode paste 6)
Platinum ruthenium-supported polyaniline-treated VGCF powder (specific surface area is 900 m ² / g), distilled water, 60% by mass Teflon (registered trademark) dispersion, 20% by mass Nafion solution (manufactured by Aldrich), VGCF: Nafion mass ratio The mixture was mixed so as to have a solid content of 2: 1 and 10% by mass, and was uniformly dispersed with ultrasonic waves to prepare a negative electrode paste 6.

(Preparation of positive electrode paste)
A positive electrode paste was prepared in the same manner as in the preparation of the negative electrode paste 1 except that the platinum ruthenium-supported polyaniline-treated VGCF powder was changed to 40% platinum-supported carbon.

(Production of electrodes)
Example 1
A negative electrode paste 1 was uniformly applied to the surface of the water-repellent treated carbon paper so that the amount of platinum was 3.0 mg / cm ² and dried at 80 ° C. for 1 hour in a nitrogen atmosphere to prepare a negative electrode.

Similarly, a positive electrode paste was uniformly applied to the surface of carbon paper so that the amount of platinum was 3.0 mg / cm ^2, and a positive electrode was produced.

  Next, an electrolyte membrane Nafion 112 (manufactured by DuPont) was sandwiched between the positive electrode and the negative electrode, and a hot press treatment was performed to produce an electrolyte membrane-electrode assembly of Example 1.

Example 2
In the production of the negative electrode, an electrolyte membrane / electrode assembly of Example 2 was produced in the same manner as in Example 1 except that the negative electrode paste 2 was used instead of the negative electrode paste 1.

Example 3
In the production of the negative electrode, an electrolyte membrane / electrode assembly of Example 3 was produced in the same manner as in Example 1 except that the negative electrode paste 5 was used instead of the negative electrode paste 1.

Example 4
In the production of the negative electrode, an electrolyte membrane / electrode assembly of Example 4 was produced in the same manner as in Example 1 except that the negative electrode paste 6 was used instead of the negative electrode paste 1.

Comparative Example 1
In the production of the negative electrode, an electrolyte membrane / electrode assembly of Comparative Example 1 was produced in the same manner as in Example 1 except that the negative electrode paste 3 was used instead of the negative electrode paste 1.

Comparative Example 2
In the production of the negative electrode, an electrolyte membrane / electrode assembly of Comparative Example 2 was produced in the same manner as in Example 1 except that the negative electrode paste 4 was used instead of the negative electrode paste 1.

[Battery evaluation and results]
A unit cell of a direct methanol fuel cell was assembled using the assembly of the electrolyte membrane and electrode of the examples and comparative examples produced as described above, a fuel flow rate of 30 ml / min at a temperature of 25 ° C. and atmospheric pressure, and a flow rate of air of 100 ml / min. Under conditions of 2 minutes, a 2 mol aqueous methanol solution was supplied to the negative electrode side, and air was supplied to the positive electrode side, and current-voltage characteristics were measured.

  The results are shown in Table 1.

  Obviously, it can be seen that the current-voltage characteristics when the electrolyte membrane-electrode assembly of the present invention is used are better than those of the comparative example.

Claims (5)

An electrode for a fuel cell comprising carbon fiber having carbon adhered to the surface as an electron conductor. The fuel cell electrode according to claim 1, wherein the electron conductor carries a catalyst. 3. The fuel cell electrode according to claim 1, wherein the electron conductor has a specific surface area of 50 to 1000 m ² / g. 4. The fuel cell electrode according to any one of claims 1 to 3, wherein the carbon is fixed by firing after attaching an organic polymer to a surface of a carbon fiber. A fuel cell, wherein at least one of the negative electrode and the positive electrode uses the electrode for a fuel cell according to any one of claims 1 to 4.