JP5417771B2 - Method for producing catalyst paste - Google Patents

Description

  The present invention relates to a method for producing a catalyst paste used for a catalyst electrode material of a fuel cell.

  Conventional carbon materials have been used as catalyst carriers, adsorbents, separating agents, inks, toners, etc. Recently, with the advent of nano-sized carbon materials such as carbon nanotubes and carbon nanohorns, their structures The feature as is attracting attention. In particular, the unique structure of carbon nanohorns has attracted attention as an industrial catalyst carrier, and has recently been used as a catalyst carrier for steam reforming catalysts in fuel cells and hydrogen production, and has been reported as one of the optimum catalyst carriers. Yes.

  A fuel cell is one that converts chemical energy of a fuel into electric energy by electrochemically oxidizing a fuel such as hydrogen or methanol, and has attracted attention as a new electric energy supply source.

  Conventional electrode catalyst pastes for anodes and cathodes of fuel cells are composed of a catalyst in which metal particles such as platinum or a platinum alloy are supported on the surface of a catalyst carrier such as carbon, and a fluorine resin having a sulfonic acid group, for example. A composition comprising a representative proton conductive material is used. In order to uniformly disperse the catalyst and the proton conductive material, the proton conductive material is usually mixed with carbon fine particles in a state of being dispersed in an organic solvent (Patent Document 1). However, a catalyst in which metal particles such as platinum and its alloy are supported on the carbon surface may ignite when it comes into contact with an organic solvent (especially alcohol) in an oxygen-containing atmosphere. Therefore, when preparing a conventional catalyst paste, first, the catalyst is dispersed in water so that the organic solvent is not brought into direct contact with the catalyst surface in the presence of air (oxygen), and the proton conductive material is dispersed. A catalyst paste was prepared by mixing with a solvent.

However, when a catalyst paste of a carbon nanohorn catalyst in which metal particles such as platinum or an alloy thereof are supported on a carbon nanohorn aggregate by a conventional method, carbon nanohorn has a particularly high water repellency among carbons with high water repellency. The fuel cell produced by using this catalyst paste is not able to generate power efficiently.
JP 2004-152489 A

  The catalyst electrode of a fuel cell is composed of a mixture of catalyst particles in which metal particles such as platinum or a platinum alloy are supported on a conductive carrier such as carbon and a proton conductive material. In order to efficiently exhibit the three characteristics of metal catalytic properties, carrier conductivity properties, and proton conductivity properties, it is necessary to uniformly disperse and mix metal particles, conductive carriers, and proton conductive materials.

  As described above, in the conventional catalyst paste manufacturing method, in the manufacturing process, carbon having high water repellency such as carbon nanohorn does not uniformly disperse even when stirred with water and becomes an agglomerate. The proton conductive material dispersed therein cannot enter the aggregate. Therefore, the contact rate between the catalyst particles (metal particles such as platinum or platinum alloy) in the obtained electrode and the proton conductive material is lowered, and protons generated in the catalyst particles cannot be transferred to the proton conductive membrane. .

  Then, this invention is made | formed in view of the above situation, and it aims at providing the manufacturing method of a catalyst paste with high dispersibility in a carbon nanohorn catalyst. It is another object of the present invention to provide a fuel cell capable of highly efficient power generation.

  The present invention is characterized by the following in order to solve the above problems.

That is, the method for producing a catalyst paste according to the present invention prepares a catalyst particle dispersion by dispersing a carbon nanohorn catalyst in which metal particles made of platinum or an alloy thereof are supported on a carbon nanohorn aggregate in an organic solvent under an oxygen removal atmosphere And a mixing step of mixing the catalyst particle dispersion with a solution obtained by dissolving a proton conductive material in an organic solvent.

  Further, in the catalyst particle dispersion step, ultrasonic dispersion treatment is performed when the carbon nanohorn catalyst is dispersed in an organic solvent in an oxygen removing atmosphere.

  The ultrasonic frequency in the ultrasonic dispersion treatment is 10 KHz to 100 KHz.

  In the catalyst particle dispersion step, the relative permittivity of the organic solvent in which the carbon nanohorn catalyst is dispersed is 40 or less at 25 ° C.

  In the catalyst particle dispersion step, the organic solvent in which the carbon nanohorn catalyst is dispersed is isopropanol.

  The catalyst paste according to the present invention is manufactured by the above manufacturing method.

  The catalyst electrode for a fuel cell according to the present invention is manufactured using the catalyst paste.

  The fuel cell according to the present invention includes a fuel electrode, an oxidant electrode, and a solid electrolyte membrane sandwiched between the fuel electrode and the oxidant electrode, wherein at least one of the fuel electrode and the oxidant electrode is The catalyst electrode for a fuel cell.

  According to the production method of the present invention, a catalyst paste in which the carbon nanohorn catalyst is not aggregated and the proton conductive material is uniformly dispersed can be produced. In addition, a fuel cell using an electrode produced using the catalyst paste is excellent in catalytic characteristics, conductive characteristics and proton conductive characteristics, and can generate power efficiently.

  Embodiments of the present invention will be described below.

  The method for producing a catalyst paste according to the present invention comprises a catalyst for preparing a catalyst particle dispersion by dispersing a carbon nanohorn catalyst in which metal particles comprising platinum or an alloy thereof are supported on a carbon nanohorn aggregate in an organic solvent in an oxygen-removing atmosphere. A particle dispersion step, and a mixing step of mixing the solution in which the proton conductive material is dispersed in an organic solvent and the catalyst particle dispersion.

(Catalyst particle dispersion process)
In the catalyst particle dispersion step, a carbon nanohorn catalyst in which metal particles made of platinum or an alloy thereof are supported on a carbon nanohorn aggregate is dispersed in an organic solvent in an oxygen removal atmosphere to prepare a catalyst particle dispersion.

  The carbon nanohorn aggregate is a tubular body in which one end of a carbon nanotube has a conical shape, and a plurality of ones are centered on the tube by van der Waals force acting between the conical portions, and the conical portion is a horn. As shown in the figure, they are assembled in a configuration that protrudes from the surface. FIG. 1 shows an example of a configuration of a catalyst paste including a carbon nanohorn aggregate 1 according to the present invention, metal particles 2 and a proton conductive material 3. The diameter of the carbon nanohorn aggregate 1 is 120 nm or less, and generally 30 nm or more and 200 nm or less. Each carbon nanohorn constituting the carbon nanohorn aggregate 1 has a tube diameter of about 2 nm, a general length of 30 nm or more and 50 nm or less, and the cone portion has an average inclination angle of the axial section of about 20 °. It is. In order to take such a unique structure, the carbon nanohorn aggregate 1 has a packing structure with a very large specific surface area. The carbon nanohorn aggregate 1 can be usually produced by a laser ablation method using a solid carbon simple substance such as graphite as a target at room temperature and in an inert gas atmosphere. Moreover, the size of the pores between the spherical particles of the carbon nanohorn aggregate 1 can be controlled by the production conditions by the laser ablation method, the oxidation treatment after the production, or the like. In the central part of the carbon nanohorn aggregate 1, there may be a shape in which carbon nanohorns are chemically bonded to each other, or the carbon nanotube is rounded like a kick, but this is limited by the structure of the central part. is not. Moreover, what has a hollow center part is also considered.

  The carbon nanohorn constituting the carbon nanohorn aggregate 1 may be closed at one end or may not be closed. Moreover, you may terminate in the shape where the vertex of the cone shape of the one end was round. When the carbon nanohorns constituting the carbon nanohorn aggregate 1 are terminated with a rounded conical apex at one end, the carbon nanohorns are gathered radially with the rounded apex outward. Further, a part of the structure of the carbon nanohorn may be incomplete and may have fine pores. Furthermore, the carbon nanohorn aggregate 1 can include carbon nanotubes.

  The carbon nanohorn aggregate 1 can be a single-layer carbon nanohorn. By carrying out like this, the hydrogen ion conductivity in the carbon nanohorn aggregate | assembly 1 can be improved. The carbon nanohorn aggregate 1 can be a single-layer carbon nanohorn aggregate composed of single-layer graphite nanohorns. By doing so, the electrical conductivity of the carbon nanohorn aggregate 1 is improved, so that the performance can be improved when it is used as a catalyst electrode for a fuel cell.

  The carbon nanohorn catalyst is obtained by supporting metal particles 2 made of platinum or an alloy thereof on the carbon nanohorn aggregate.

  Examples of the metal particles 2 include platinum, platinum alloys that are alloys of platinum, rhodium, palladium, iridium, osmium, ruthenium, rhenium, gold, silver, nickel, cobalt, lithium, lanthanum, strontium, yttrium, and the like. These metal particles can be used alone or in combination of two or more.

  The metal particles 2 can be supported on the carbon nanohorn aggregate 1 by a generally used impregnation method. This is a method of carrying out a reduction treatment after a colloidal catalyst substance obtained by dissolving or dispersing a metal salt of the metal particles 2 in a solvent is supported on the carbon nanohorn aggregate 1. By performing the reduction treatment at a low temperature of 100 ° C. or lower including room temperature, for example, 10 ° C. or higher and 100 ° C. or lower, extremely small spherical metal particles having an average particle diameter of 5 nm or less are formed on the surface of the carbon nanohorn aggregate 1. 2 can be carried. Moreover, it can also carry out by a general colloid method. In these methods, the metal particles 2 can be uniformly dispersed on the carbon nanohorn aggregate 1.

  The obtained carbon nanohorn catalyst is dispersed in an organic solvent under an oxygen removing atmosphere to prepare a catalyst particle dispersion.

  In the present invention, the carbon nanohorn catalyst is dispersed in an organic solvent in an atmosphere from which oxygen is removed in order to prevent ignition without adding water for preventing ignition.

  The organic solvent is preferably an organic solvent in which the carbon nanohorn catalyst is easily dispersed and has a relative dielectric constant of 40 or less (25 ° C.). Examples of the organic solvent having a relative dielectric constant of 40 or less (25 ° C.) include methanol, ethanol, isopropanol, cyclohexane, toluene and the like. Among these, isopropanol is preferable from the viewpoint of dispersibility of the carbon nanohorn catalyst. These organic solvents may use only 1 type, and may use 2 or more types together, if it does not phase-separate.

  In addition, when dispersing the carbon nanohorn catalyst in an organic solvent in an oxygen removing atmosphere, it is preferable to perform an ultrasonic dispersion treatment from the viewpoint of further improving the dispersibility of the carbon nanohorn catalyst.

  The ultrasonic frequency in the ultrasonic dispersion treatment is preferably 10 KHz to 100 KHz from the viewpoint of dispersibility of the carbon nanohorn catalyst.

  As will be described later, a catalyst paste in which the carbon nanohorn catalyst and the proton conductive material are uniformly dispersed can be produced by mixing the catalyst particle dispersion and the proton conductive material dispersed in the organic solvent. Since the proton conductive material may be aggregated by heat, the catalyst paste to which the proton conductive material is added must not be ultrasonically dispersed to generate heat. For this reason, it is effective to disperse the carbon nanohorn catalyst in the organic solvent by ultrasonic dispersion treatment before mixing the catalyst particle dispersion and the proton conductive material dispersed in the organic solvent. It is.

(Mixing process)
In the mixing step, a solution in which a proton conductive material is dispersed in an organic solvent and the catalyst particle dispersion are mixed. Thereby, a catalyst paste in which the carbon nanohorn catalyst and the proton conductive material are uniformly dispersed can be prepared.

  Although it does not specifically limit as a solution which disperse | distributed the proton-conductive substance in the organic solvent, Nafion solution (DuPont) etc. are mentioned in a commercial item.

(Catalyst electrode for fuel cell and fuel cell)
FIG. 2 is a cross-sectional view schematically showing an example of the structure of the fuel cell according to the present invention. The catalyst electrode-solid electrolyte membrane assembly 101 (hereinafter sometimes referred to as MEA) includes a fuel electrode 102, an oxidant electrode 108, and a solid electrolyte membrane 114. The fuel electrode 102 and the oxidant electrode 108 are collectively referred to as a catalyst electrode, and the fuel cell catalyst electrode according to the present invention is used as at least one of the fuel electrode 102 and the oxidant electrode 108. The fuel electrode 102 includes a substrate 104 and a catalyst layer 106. The oxidant electrode 108 includes a base 110 and a catalyst layer 112. One or a plurality of catalyst electrode-solid electrolyte membrane assemblies 101 are electrically connected via a fuel electrode side separator 120 and an oxidant electrode side separator 122.

  In the fuel cell 100 configured as described above, the fuel 124 is supplied to the fuel electrode 102 of each catalyst electrode-solid electrolyte membrane assembly 101 via the fuel electrode-side separator 120. Further, an oxidant 126 such as air or oxygen is supplied to the oxidant electrode 108 of each catalyst electrode-solid electrolyte membrane assembly 101 via the oxidant electrode side separator 122.

  The fuel electrode 102 and the oxidant electrode 108 include the carbon nanohorn catalyst and the proton conductive material, and have a configuration in which the catalyst layer 106 and the catalyst layer 112 are formed on the base 104 and the base 110. The substrate surface may be subjected to a water repellent treatment.

  As the substrate 104 and the substrate 110, porous substrates such as carbon paper, a carbon molded body, a carbon sintered body, a sintered metal, and a foam metal can be used. A water repellent such as polytetrafluoroethylene can be used for the water repellent treatment of the substrate.

  The fuel electrode 102 and the oxidant electrode 108 can be produced, for example, by applying the catalyst paste of the present invention on the substrate 104 and the substrate 110 by a screen printing method or the like, and drying by heating.

  The following examples illustrate and describe the invention in more detail. However, the present invention is not limited to the following examples.

Example 1
(Production of carbon nanohorn)
The carbon nanohorn aggregate was produced by a CO ₂ laser ablation method. That is, sintered round carbon as a solid carbon material is placed in a vacuum vessel, and the solid carbon material is irradiated with CO ₂ laser light under conditions of laser power density 30 kW / cm ² and target rotation speed ² rpm under Ar atmosphere. For 30 minutes at room temperature. When the soot-like substance thus obtained was observed with a transmission electron microscope (TEM), it was confirmed to have a carbon nanohorn aggregate structure.

(Catalyst loading)
The carbon nanohorn aggregate obtained above was used as carbon powder. 1N sodium hydroxide was added after sodium hydrogensulfite was added and reduced while maintaining the liquid temperature of the aqueous solution of chloroplatinic acid in an amount of 2: 3 so that the mass ratio of carbon and Pt (platinum) in the carbon nanohorn catalyst was 50 ° C. The pH was adjusted to 5 with the solution. The obtained solution was diluted with water, an amount of carbon powder satisfying the mass ratio was added, and the mixture was stirred for 30 minutes using a homogenizer. A 30% hydrogen peroxide solution was added little by little to change the platinum compound in the solution into a platinum oxide colloid and simultaneously adsorbed onto the carbon powder. While maintaining the liquid temperature at 75 ° C., the pH of the solution was adjusted to 5 and stirred for 12 hours. The solution was boiled for 10 minutes, and after natural cooling, unnecessary salts were removed by centrifugation and washing with water, and kept at 70 ° C. for 12 hours and dried to obtain a carbon powder adsorbed with platinum oxide. Thereafter, reduction was carried out in hydrogen for 2 days. The obtained carbon nanohorn catalyst was observed with a scanning transmission electron microscope image (STEM), and the black spot in the STEM image was analyzed by energy dispersive X-ray spectroscopy (EDS). As a result, it was confirmed to be Pt. . From the STEM image, it was estimated that the average Pt particle diameter of the sample was 4.5 nm. As a result of thermogravimetric analysis in an oxygen atmosphere, the Pt loading on the carbon nanohorn catalyst was confirmed to be 60% by mass with respect to the carbon nanohorn.

(Dispersibility evaluation of carbon nanohorn catalyst)
The carbon nanohorn 60% Pt-supported catalyst prepared above was examined for dispersibility in an organic solvent, and the dispersibility in isopropanol was particularly high.

(Production of carbon nanohorn catalyst paste)
18 ml of isopropanol was added to 6.25 g of the 60% Pt-supported catalyst of carbon nanohorn produced above in a nitrogen atmosphere from which oxygen was removed to disperse the carbon nanohorn catalyst. Further, in order to further improve the dispersibility of the carbon nanohorn catalyst, ultrasonic dispersion treatment at a frequency of 20 kHz was performed. 30 ml of 5% Nafion solution (manufactured by DuPont) was added to the carbon nanohorn catalyst dispersed in an organic solvent and kneaded to prepare a catalyst paste.

(MEA evaluation of catalyst)
The catalyst paste prepared above was applied and dried at 140 ° C. to prepare a fuel cell catalyst electrode. This electrode was pressed on both sides of the solid electrolyte membrane to produce an MEA. A direct methanol fuel cell was prepared using the MEA, and the battery characteristics were measured at a temperature of 40 ° C. The results are shown in FIG.

(Example 2)
A catalyst paste and MEA were prepared in the same manner as in Example 1 except that the ultrasonic dispersion treatment was not performed during the preparation of the catalyst paste, and the battery characteristics were evaluated. The results are shown in FIG.

(Comparative Example 1)
12 ml of water was added to 6.25 g of the 60% Pt-supported carbon nanohorn catalyst prepared in Example 1 to disperse the carbon nanohorn catalyst. To this, 30 ml of 5% Nafion solution (manufactured by DuPont) was added and kneaded to prepare a catalyst paste. Otherwise, an MEA was produced in the same manner as in Example 1, and the battery characteristics were evaluated. The results are shown in FIG.

As shown in FIG. 3, the MEA (Comparative Example 1) produced by the conventional manufacturing method had a battery output of 28 mW / cm ² , whereas that dispersed with isopropanol (Example 2) was 52 mW / cm ^2. It was confirmed that the battery output was improved at cm ² . In addition, when ultrasonic dispersion was applied to isopropanol (Example 1), the output was further improved to 60 mW / cm ² .

It is a figure which shows an example of a structure of the catalyst paste containing the carbon nanohorn aggregate | assembly which concerns on this invention, a metal particle, and a proton-conductive substance. It is sectional drawing which shows typically an example of a structure of the fuel cell concerning this invention. It is the figure which showed the output with respect to the electric current value of the cell using MEA produced in Example 1 and 2 and the comparative example 1. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Carbon nanohorn aggregate | assembly 2 Metal particle 3 Proton conductive substance 100 Fuel cell 101 Catalyst electrode-solid electrolyte membrane assembly (MEA)
102 Fuel electrode 104 Base 106 Catalyst layer 108 Oxidant electrode 110 Base 112 Catalyst layer 114 Solid electrolyte membrane 120 Fuel electrode side separator 122 Oxidant electrode side separator 124 Fuel 126 Oxidant

Claims (8)

A catalyst particle dispersion step of preparing a catalyst particle dispersion by dispersing a carbon nanohorn catalyst carrying metal particles made of platinum or an alloy thereof on a carbon nanohorn aggregate in an organic solvent under an oxygen removal atmosphere;
A mixing step of mixing a solution obtained by dissolving a proton conductive material in an organic solvent and the catalyst particle dispersion;
A method for producing a catalyst paste, comprising:   The method for producing a catalyst paste according to claim 1, wherein in the catalyst particle dispersion step, ultrasonic dispersion treatment is performed when the carbon nanohorn catalyst is dispersed in an organic solvent under an oxygen removal atmosphere.   The method for producing a catalyst paste according to claim 2, wherein an ultrasonic frequency in the ultrasonic dispersion treatment is 10 KHz to 100 KHz.   The method for producing a catalyst paste according to any one of claims 1 to 3, wherein in the catalyst particle dispersion step, a relative dielectric constant of an organic solvent in which the carbon nanohorn catalyst is dispersed is 40 or less at 25 ° C.   The method for producing a catalyst paste according to any one of claims 1 to 4, wherein in the catalyst particle dispersion step, an organic solvent in which the carbon nanohorn catalyst is dispersed is isopropanol.   The catalyst paste manufactured by the manufacturing method of any one of Claim 1 to 5.   A catalyst electrode for a fuel cell produced using the catalyst paste according to claim 6. In a fuel cell having a fuel electrode, an oxidant electrode, and a solid electrolyte membrane sandwiched between the fuel electrode and the oxidant electrode,
The fuel cell in which at least one of the fuel electrode and the oxidant electrode is the catalyst electrode for a fuel cell according to claim 7.

Images