JP2005108546A - Method for producing electrode for fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent degradation of a solid polymer electrolyte by lowering reduction temperature. <P>SOLUTION: A method of manufacturing an electrode for a fuel cell has a reduction process treating a first material containing gas phase growth carbon fibers, the solid polymer electrolyte, and a catalyst precursor at temperature lower than the degradation temperature of the solid polymer electrolyte and in non-oxidizing atmosphere to reduce the catalyst precursor. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for producing a fuel cell electrode.

  In order to reduce air pollution as much as possible, measures for exhaust gas from automobiles are important, and electric vehicles are used as one of the countermeasures. However, they are not widely used due to problems such as charging facilities and mileage.

Air pollution, to deal with environmental and resource problems on a global scale, such as CO ₂ emission regulations and depletion of oil resources, high energy density clean, unnecessary fuel cell charging time attracting the most limelight, rapid worldwide Research and development is underway on the pitch. That is, a fuel cell is attracting attention as a clean power generation device that generates electricity by reverse reaction of electrolysis using hydrogen and oxygen and has no emissions other than water. Cars using this fuel cell are considered to be the most promising clean cars. Among fuel cells, solid polymer electrolyte fuel cells are most promising for automobiles because they operate at low temperatures. It is also promising for stationary use for home use and business use.

  A solid polymer electrolyte fuel cell is generally configured by laminating a large number of single cells. In a single cell, a membrane / electrode assembly formed by sandwiching and joining a solid polymer electrolyte membrane between two electrodes (a fuel electrode and an oxidant electrode) is sandwiched by a separator having a gas flow path of fuel gas or oxidant gas. Has a structure.

  Conventionally, as a general electrode, a catalyst layer made of carbon particles carrying a catalyst and a solid polymer electrolyte is provided on the plane of a porous electrode structure having both a gas diffusion layer capable of gas diffusion and a current collector. Is provided. As the catalyst, noble metals such as platinum and platinum-based alloys are used. In order to reduce the amount of expensive noble metal used, catalyst-supported carbon in which a noble metal is supported on carbon black having a very large specific surface area is generally used as carbon particles supporting the catalyst.

  This catalyst layer is opposed to the solid polymer electrolyte membrane side to form a membrane / electrode assembly. During power generation, a fuel gas containing hydrogen is supplied to the fuel electrode, an oxidant gas containing oxygen is supplied to the oxidant electrode, and the reaction of the equation (1) is performed at the fuel electrode, and the equation (2) at the oxidant electrode. The reaction occurs and power is generated.

H ₂ → 2H ⁺ + 2e ⁻ (1)
2H ⁺ + 2e ⁻ + (1/2) O ₂ → H ₂ O (2)
The reactions of formula (1) and formula (2) require supply of reaction gas (fuel gas or oxidant gas), exchange of protons (H ⁺ ) and electrons (e ⁻ ), which are satisfied simultaneously. It proceeds only at the three-phase interface in the electrode, that is, the reaction gas, catalyst, and solid polymer electrolyte interface.

  In the above-described method using carbon particles carrying a general catalyst, there is a problem that the ratio of the catalyst present at the three-phase interface and contributing to the reaction (catalyst utilization rate) is low. In order to solve this problem, a method for producing a fuel cell electrode has been reported in which a layer containing carbon particles and a solid polymer electrolyte is formed, and a catalyst precursor is adsorbed or impregnated thereon, followed by reduction treatment. However, the reduction of the catalyst precursor requires a high temperature of 180 ° C., which causes a problem that the solid polymer electrolyte is decomposed or deteriorated. When the solid polymer electrolyte is decomposed or deteriorated, the reaction at the three-phase interface is lowered, and the power generation performance is lowered.

Patent Document 1 discloses a fuel that is reduced by a pressurized gas containing hydrogen after adsorbing a cation containing a catalytic metal element to a solid polymer electrolyte containing a solid polymer electrolyte (cation exchange resin) and carbon particles. A method for manufacturing a battery electrode is disclosed.
However, even if the reduction temperature is lowered by increasing the pressure during the reduction treatment, the temperature lowering effect is not recognized up to about 150 ° C. In Example 1 of Patent Document 1, reduction treatment is performed in a hydrogen atmosphere of 10 atm (about 1 MPa) and 180 ° C. Even if the solid polymer electrolyte is lower than the decomposition temperature, the ion exchange groups and the like are partially deteriorated and the power generation performance is lowered. In order to prevent deterioration of the solid polymer electrolyte, the reduction temperature needs to be 100 ° C. or lower.

  The present invention solves the above-described problems, and provides a method for producing an electrode for a fuel cell that can reduce the reduction temperature and prevent the deterioration of the solid polymer electrolyte.

  In order to solve the above technical problem, according to the first aspect of the present invention, the first material containing the vapor-grown carbon fiber, the solid polymer electrolyte, and the catalyst precursor is heated to a temperature lower than the deterioration temperature of the solid polymer electrolyte and hydrogen. And a reduction step of reducing the catalyst precursor by treatment in a non-oxidizing atmosphere containing a fuel cell electrode.

  In the invention of claim 2, the first material is a second material mixing step of mixing a second material containing vapor grown carbon fiber and a solid polymer electrolyte, and a sheet forming step of forming the second material into a sheet. 2. The method for producing a fuel cell electrode according to claim 1, wherein the sheet is produced by a step provided with an impregnation step of impregnating the sheet with a solution containing the catalyst precursor.

  The invention according to claim 3 is the method for producing a fuel cell electrode according to claim 1 or 2, wherein the vapor-grown carbon fiber is a multi-walled carbon nanotube.

  According to a fourth aspect of the present invention, there is provided the method for producing a fuel cell electrode according to any one of the first to third aspects, wherein the catalyst precursor is hexachloroplatinic (IV) acid.

  According to the first aspect of the present invention, there is an effect that the reduction temperature of the catalyst precursor can be remarkably lowered by using the vapor growth carbon fiber. As a result, deterioration of the solid polymer electrolyte can be prevented, and a fuel cell excellent in power generation performance can be obtained.

  According to the invention of claim 2, since the catalyst precursor is adhered after the sheet is formed, the catalyst can be distributed in a large amount on the surface of the catalyst layer where the catalytic reaction occurs, and the fuel cell having excellent power generation performance Can do.

  According to the invention of claim 3, since the multi-walled carbon nanotube is used as the vapor growth carbon fiber, there is an effect that the reduction temperature can be remarkably lowered.

  According to the invention of claim 4, since hexachloroplatinic (IV) acid having a low reduction temperature of itself is used as the catalyst precursor, there is an effect that the original temperature can be brought to around 30 ° C. or less and a very low room temperature.

  As a result of diligent research to reduce the reduction temperature of the catalyst precursor, the present inventor has found that the reduction temperature of the catalyst precursor can be significantly reduced by using vapor-grown carbon fibers as the carbon material of the catalyst layer.

  Vapor-grown carbon fiber is a fibrous carbon material produced by a vapor phase method using a carbon-containing material such as hydrocarbon as a carbon source. The vapor phase method includes a chemical vapor deposition method and a liquid pulse injection method. Examples of vapor grown carbon fibers used in the present invention include carbon nanofibers and carbon nanotubes. Carbon nanofibers include single-walled carbon nanotubes composed of a single tube, multi-walled carbon nanotubes with multiple tubes (multi-walled carbon nanotubes), and nanofibers with a graphite structure whose crystal plane is perpendicular to the fiber axis direction. is there. Above all, when multi-walled carbon nanotubes are used, the catalyst precursor can be reduced at room temperature.

  Any catalyst can be used as long as it can function as the catalyst of formula (1) or formula (2). In general, a platinum catalyst or a platinum-ruthenium catalyst is used among noble metal catalysts. As the catalyst precursor, any substance that becomes a catalyst upon reduction can be used. For example, in the case of a platinum catalyst, an anionic complex such as hexachloroplatinum (IV) acid, tetrachloroplatinum (II) acid and its salts, positive ions such as hexaammineplatinum (IV) chloride, tetraammineplatinum (II) chloride, etc. Examples include ionic complex salts and neutral complexes such as dinitrodiammineplatinum (II). Hexachloroplatinic (IV) acid (commonly referred to as chloroplatinic acid) is desirable as a catalyst precursor for fuel cell electrodes because of its low reduction temperature.

  As the solid polymer electrolyte, various hydrocarbon electrolytes such as those having a fluorine-substituted hydrocarbon skeleton typified by Nafion (registered trademark of DuPont Co., Ltd.) and those sulfonated with polystyrene can be used. As an atmosphere gas for the reduction treatment, a gas containing hydrogen without containing an oxidizing gas such as pure hydrogen or a mixed gas of an inert gas and hydrogen can be used. The gas pressure at this time is not particularly limited, but a pressure of 0.2 MPa or less is sufficient. The reduction temperature may be a temperature lower than the deterioration temperature of the solid polymer electrolyte. Here, the deterioration temperature of the solid polymer electrolyte is a temperature at which the water content of the solid polymer electrolyte decreases. The reduction temperature is preferably 100 ° C. or lower, more preferably 60 ° C. or lower, 40 ° C. or lower, or 30 ° C. or lower. The lower the reduction temperature, the less the influence on the solid polymer electrolyte and the lower the processing cost.

  The second material includes at least a vapor-grown carbon fiber and a solid polymer electrolyte, and the first material further includes a catalyst precursor in addition to the second material. The first material and the second material may contain other materials as necessary. For example, a water repellent material such as tetrafluoroethylene may be added in order to control the water repellency of the catalyst layer and make it easy for the gas to reach the three-phase interface.

  Whether the second material is molded into a sheet and then impregnated with the catalyst precursor, or the second material is impregnated with the catalyst precursor and then molded into the sheet, the second material is impregnated with the catalyst precursor and reduced. You may shape | mold into a sheet | seat after processing. If the catalyst precursor is impregnated after forming the second material into a sheet, a relatively large amount of the catalyst substance is distributed on the surface of the sheet. As a result, a relatively large amount of the catalyst after the reduction treatment is distributed on the surface. When producing a membrane / electrode assembly for a fuel cell, if this surface is bonded to the solid polymer electrolyte membrane side, a relatively large amount of catalyst is present near the sea surface with the solid polymer electrolyte membrane where the electrode reaction takes place. Since it is distributed, the electrode reaction efficiency can be improved and the power generation performance can be improved.

  In the sheet forming step, the sheet may be formed on a gas diffusion layer having conductivity and gas permeability to form a gas diffusion layer with a catalyst layer, or may be formed on a polymer film such as an ETFE film. In the latter case, the sheet is impregnated with a catalyst precursor and reduced to form a catalyst layer, which is then joined to a solid polymer electrolyte membrane. After this joining, the polymer film is peeled off.

  Examples of the present invention will be described below.

  VGCF (registered trademark of Showa Denko KK) manufactured by Showa Denko Co., Ltd. was used as the vapor growth carbon fiber. VGCF is a multi-walled carbon nanotube in which tubes having a carbon network surface parallel to the fiber axis are multi-layered, and is a kind of vapor-grown carbon fiber produced by a vapor growth method. The carbon network surface is a surface in which six-membered rings formed by covalent bonds of six carbon atoms are continuous. The VGCF used has an average outer diameter of 120 nm and an average length of about 3 μm.

  Asiplex (registered trademark of Asahi Chemical Co., Ltd.) SS-1100 / 50 membrane solution manufactured by Asahi Chemical Co., Ltd. was used as the solid polymer electrolyte. Aciplex SS-1100 is a hydrogen fluoride-based solid polymer electrolyte. Aciplex SS-1100 / 50 (concentration 5 wt%) is obtained by dissolving this in an alcohol solvent. Hexachloroplatinum (IV) acid (Ishifuku Metal Industry Co., Ltd.) was used as a catalyst precursor.

  First, 1 g of VGCF and 24 g of Aciplex were mixed and treated with an ultrasonic homogenizer for 15 minutes to obtain a dispersion (second material mixing step). This dispersion is a paste.

  Next, this paste was applied uniformly on an ETEF (ethylene-tetrafluoroethylene) film having a thickness of 100 μm with an applicator to a thickness of 300 μm, and then allowed to stand at room temperature for about 24 hours and dried to give ETFE A sheet with film was formed (sheet forming step).

Thereafter, the sheet with the ETFE film was immersed for about 12 hours at room temperature in a catalyst precursor solution in which 1 g of hexachloroplatinic (IV) acid was dissolved in 500 cm ^{3 of a} 1 wt% aqueous ethanol solution (impregnation step). The reason why a slight amount of ethanol is contained in the catalyst precursor solution is to facilitate the penetration of the solution into the sheet. The sheet with the ETFE film was taken out from the catalyst precursor solution, and the solution adhering to the surface was removed by air blowing, then allowed to stand at room temperature for 24 hours and dried to produce a first material.

Thereafter, the first material was reduced for 1 hour in a hydrogen stream (flow rate: 1000 cm ³ / min) at 30 ° C. and 0.2 MPa (reduction process). Finally, in order to remove unreduced hexachloroplatinic (IV) acid and the like, it was immersed in a 1 wt% ethanol solution of 1N hydrochloric acid, left for 6 hours, taken out, washed with pure water and dried. Thus, the catalyst layer of the fuel cell electrode is produced on the ETFE film.

  A part of the prepared catalyst layer was sampled and subjected to powder X-ray diffraction analysis to investigate the reduction state of hexachloroplatinic (IV) acid. The X-ray source for powder X-ray diffraction analysis is CuKα rays.

Comparative Example 1

  Carbon black was used instead of vapor grown carbon fiber. Carbon black is a carbon powder produced by pyrolysis due to incomplete combustion of hydrocarbons. The carbon black used in Comparative Example 1 is Vulcan manufactured by Cabot Corporation (registered trademark manufactured by Cabot Corporation) XC-72R.

  A catalyst layer for a fuel cell electrode was prepared on an ETFE film in the same manner as in Example 1 except that Vulcan XC-72R was used instead of VGCF, and powder X-ray diffraction analysis was performed in the same manner as in Example 1.

  FIG. 1 is a powder X-ray diffraction chart of Example 1, and FIG. 2 is a powder X-ray diffraction chart of Comparative Example 1. Each horizontal axis represents an angle (unit: degree) of 2θ. The vertical axis represents the X-ray diffraction intensity. In Example 1, a clear diffraction peak of metallic platinum is observed. From the half-value width of the diffraction peak, it can be seen that the size of the metal platinum crystal particles is about 4 nm. On the other hand, in Comparative Example 1, only a slight diffraction peak of platinum metal is observed.

  Therefore, if VGCF is used, hexachloroplatinic (IV) acid can be reduced at a low temperature of 30 ° C., so that the catalyst can be reduced without any deterioration of the solid polymer electrolyte.

  The catalyst layer is prepared in the same manner for both the fuel electrode and the oxidant electrode. The ETFE film is peeled and removed after the solid polymer electrolyte is sandwiched between the fuel electrode catalyst layer and the oxidant electrode catalyst layer with hot press bonding. To do. Thereafter, a gas diffusion layer is bonded to each of the fuel electrode catalyst layer and the oxidant electrode catalyst layer to produce a membrane / electrode assembly. A single cell of the fuel cell is configured by sandwiching both sides of the membrane-electrode assembly with a separator having a fuel gas or oxidant gas passage. In general, a fuel cell is formed by stacking a plurality of single cells. By using the fuel cell electrode manufactured by the above-described manufacturing method, a fuel cell excellent in power generation performance can be obtained.

Powder X-ray diffraction chart of Example 1 Powder X-ray diffraction chart of Comparative Example 1

Claims (4)

Reduction in which a first material containing vapor-grown carbon fiber, a solid polymer electrolyte, and a catalyst precursor is treated in a non-oxidizing atmosphere at a temperature lower than the deterioration temperature of the solid polymer electrolyte and containing hydrogen to reduce the catalyst precursor. A method for producing an electrode for a fuel cell, comprising a step. The first material includes a second material mixing step of mixing a vapor-grown carbon fiber and a second material containing a solid polymer electrolyte, a sheet forming step of forming the second material into a sheet, and the sheet as the catalyst. 2. The method for producing a fuel cell electrode according to claim 1, wherein the electrode is produced by a step provided with an impregnation step of impregnating a solution containing a precursor. The method for producing an electrode for a fuel cell according to claim 1 or 2, wherein the vapor-grown carbon fiber is a multi-walled carbon nanotube. The method for producing a fuel cell electrode according to any one of claims 1 to 3, wherein the catalyst precursor is hexachloroplatinic (IV) acid.

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