KR20060034939A - Method for preparing electrode for fuel cell - Google Patents

Abstract

The present invention relates to a method for manufacturing an electrode for a fuel cell, wherein the electrode is prepared by impregnating a poly (2,5-benzimidazole) polymer in phosphoric acid having a concentration of 85% or more; Adding a metal catalyst to the gel to prepare a slurry composition; Coating the composition on a conductive substrate and then performing a heat treatment to form a catalyst layer.

In the fuel cell electrode manufactured according to the present invention, the poly (2,5-benzimidazole) polymer in a gel state can be used as a catalyst layer binder to increase the doping amount of phosphate ions, thereby improving conductivity of the catalyst layer.

Fuel Cell Electrode, Conductivity, Phosphoric Acid

Description

Manufacturing method of electrode for fuel cell {METHOD FOR PREPARING ELECTRODE FOR FUEL CELL}

1 is a cross-sectional view schematically showing an operating state of a fuel cell including a polymer electrolyte membrane.

<Description of Symbols for Main Parts of Drawings>

1: fuel cell 10a: anode catalyst layer (anode electrode)

10b: air cathode catalyst layer (cathode electrode) 15: polymer electrolyte membrane

20: membrane / electrode assembly

[Industrial use]

The present invention relates to a method for manufacturing a fuel cell electrode, and more particularly to a method for manufacturing a fuel cell electrode that can improve the conductivity of the catalyst layer by increasing the doping amount of phosphate ions.

[Prior art]

A fuel cell is a power generation system that directly converts the chemical reaction energy of hydrogen and oxygen contained in hydrocarbon-based materials such as methanol, ethanol and natural gas into electrical energy.

Fuel cells are classified into phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, polymer electrolyte batteries, or alkaline fuel cells according to the type of electrolyte used. Each of these fuel cells operates on essentially the same principle, but differs in the type of fuel used, operating temperature, catalyst, and electrolyte.

Among these, polymer electrolyte fuel cells (PEMFCs), which are being developed recently, have much higher output characteristics, lower operating temperatures, faster start-up and pressure characteristics than other fuel cells, As well as transportable power supply, there is a wide range of applications such as distributed power supply such as homes, public buildings and small power supply such as for electronic devices.

The polymer electrolyte fuel cell as described above basically includes a stack, a reformer, a fuel tank, a fuel pump, and the like to construct a system. The stack forms the body of the fuel cell, and the fuel pump supplies the fuel in the fuel tank to the reformer. The reformer reforms the fuel to generate hydrogen gas and supplies the hydrogen gas to the stack. Accordingly, the polymer electrolyte fuel cell supplies fuel in a fuel tank to a reformer by operation of a fuel pump, reforms the fuel in the reformer to generate hydrogen gas, and electrochemically reacts the hydrogen gas and oxygen in a stack. Generates electrical energy.                         

On the other hand, the fuel cell may employ a direct methanol fuel cell (DMFC) method that can supply a liquid methanol fuel directly to the stack. Such a direct methanol fuel cell fuel cell, unlike the polymer electrolyte fuel cell, the reformer is excluded.

In such a fuel cell system, a stack that substantially generates electricity is composed of a number of unit cells consisting of a membrane / electrode assembly (MEA) and a separator (also called a bipolar plate). It has a stacked structure of several tens. The membrane / electrode assembly has a structure in which an anode electrode (also called "fuel electrode" or "oxide electrode") and a cathode electrode (also called "air electrode" or "reduction electrode") are attached with a polymer electrolyte membrane interposed therebetween. Have

In this case, the polymer membrane / electrode assembly includes a solid polymer electrolyte membrane and a carbon supported catalyst electrode layer. Examples of the polymer electrolyte membrane in which Sach acts as an electrolyte include Nafion (Nafion, a brand name of DuPont), Premion (Flemion, a brand name of Asahi Glass), Asifrex (Asiplex, a brand name of Asahi Chemical), and Perfluorosulfonate ionomer membranes, such as Dow XUS (Dow XUS, Dow Chemical Co., Ltd.) electrolyte membrane, are used a lot. The carbon-supported catalyst electrode layer combines a carbon powder carrying fine catalyst particles such as platinum (Pt) or ruthenium (Ru) on an electrode support such as porous carbon paper or carbon cloth as a waterproof binder. I'm using it.

On the other hand, poly (2,5-benzimidazole) (ABPBI) polymers have recently been used as polymer films for fuel cells.

The poly (2,5-benzimidazole) polymer is generally produced by a condensation reaction by adding a dehydrating agent such as P ₂ O ₅ and CH ₃ SO ₃ H to the monomer. By the condensation reaction, two water molecules are released from the monomer, and the water reacts with P ₂ O ₅ used as a dehydrating agent, as shown in Scheme 1 below, to generate phosphoric acid, which is doped with four phosphoric acids per three molecules. The imidazole polymer is produced.

Scheme 1

2P ₂ O ₅ + 6H ₂ O → 4H ₃ PO ₄

By the way, when the electrode is manufactured using the poly (2,5-benzimidazole) polymer, it was difficult to find a solvent in which the polymer is dissolved in the prior art, and thus it is difficult to use the electrode.

In order to solve the problems in the prior art, an object of the present invention is to use the poly (2,5-benzimidazole) which is a polymer membrane used as a membrane as it is, as well as to increase the doping amount of phosphate ions through gelation catalyst layer To provide a method for manufacturing an electrode for a fuel cell that can increase the conductivity of, and further prevent the release of phosphoric acid (release).

The present invention to achieve the above object,

A gel was prepared by impregnating a poly (2,5-benzimidazole) polymer in phosphoric acid having a concentration of at least 85%; Adding a metal catalyst to the gel to prepare a slurry composition; It provides a method for producing an electrode for a fuel cell comprising the step of coating the composition on a conductive substrate and heat treatment to form a catalyst layer.

The present invention also provides a membrane / electrode assembly comprising an anode electrode and a cathode electrode positioned opposite each other, and a polymer membrane positioned between the anode electrode and the cathode electrode; And a separator for sandwiching the membrane / electrode assembly, the at least one electricity generating unit generating electricity through an electrochemical reaction between hydrogen and oxygen; A fuel supply unit supplying hydrogen to the electricity generation unit; And an oxygen supply unit supplying oxygen to the electricity generation unit, and at least one of the anode electrode and the cathode electrode includes an electrode manufactured by the above method.

Hereinafter, the present invention will be described in more detail.

In the fuel cell electrode manufacturing method of the present invention, a poly (2,5-benzimidazole) polymer is used as a catalyst layer binder, and the phosphoric acid per monomer is impregnated by impregnating it with phosphoric acid having a concentration of 85% or more, preferably 85 to 98%. By increasing the doping amount of the ions it is possible to improve the conductivity of the catalyst layer.

That is, according to the present invention, the poly (2,5-benzimidazole) polymer is immersed in 85% or more of phosphoric acid to gel the polymer to obtain improved ion conductivity, thereby forming an electrode rather than forming a catalyst layer using other conventional materials. Can improve the performance. The above% means v / v%.

The poly (2,5-benzimidazole) polymer of the present invention is included in the anode electrode and the cathode electrode layer to improve the adhesion between the electrode and the electrolyte membrane. Accordingly, according to the present invention, at least one of the anode electrode, the cathode electrode, and the polymer electrolyte membrane uses a poly (2,5-benzimidazole) polymer to reduce the interfacial resistance that may occur at the electrode and the polymer electrolyte membrane interface. Can be attached firmly.

In the method of manufacturing an electrode for a fuel cell in the present invention, the poly (2,5-benzimidazole) polymer is impregnated with phosphoric acid at a concentration of 85% or more to prepare a gel. Thereafter, a slurry is prepared by adding a metal catalyst thereto, and the slurry is coated on one surface of the conductive substrate with a predetermined thickness. After the coating process, it is possible to produce a fuel cell electrode having a catalyst layer by heat treatment in a conventional manner.

The poly (2,5-benzimidazole) polymer does not exist in a dissolved state but exists in a dispersed state. The coating process may be a screen printing method, a spray coating method or a coating method using a doctor blade according to the viscosity of the dispersion, but is not limited thereto.

The conductive substrate serves as a gas diffusion layer, which supports the fuel cell electrode and diffuses the reaction gas into the catalyst layer, thereby easily accessing the reaction gas to the catalyst layer. The gas diffusion layer may be carbon paper or carbon cloth, but is not limited thereto. In addition, the gas diffusion layer using a water-repellent treatment of carbon paper or carbon cloth with a fluorine-based resin such as polytetrafluoroethylene can prevent the gas diffusion efficiency from being lowered by water generated when the fuel cell is driven. desirable.

In the present invention, the metal catalyst is preferably to use a metal catalyst supported on carbon. In the electrode of the present invention, the catalyst layer includes a so-called metal catalyst which catalyzes the related reactions (oxidation of hydrogen and reduction of oxygen), and includes a platinum group metal of the periodic table of elements, for example, Pt or Ru, transitions such as Fe or Co Metals and the like can be used. Also generally, those supported on a carrier are used. As the carrier, carbon such as acetylene black or graphite may be used, or inorganic fine particles such as alumina or silica may be used. In the case of using the noble metal supported on the carrier as a catalyst, a commercially available commercially available product may be used, or the noble metal supported on the carrier may be prepared and used. Since the process of supporting the precious metal on the carrier is well known in the art, detailed descriptions thereof will be readily understood by those skilled in the art even if the detailed description is omitted. The metal catalyst used in the present invention is preferably selected from the group consisting of Pt, Pt / Ru, and Pt / Fe / Co.

In addition, the poly (2,5-benzimidazole) (ABPBI) polymer may be prepared by condensation polymerization by adding P ₂ O ₅ as a dehydrating agent to a 3,4-diaminobenzoic acid monomer.

In the preparation of the polymer, the dehydrating agent may be mixed with CX ₃ SO ₃ H (where X is H or F) in addition to P ₂ O ₅ .

In the present invention, the time when the poly (2,5-benzimidazole) polymer is impregnated with phosphoric acid is preferably carried out for at least 24 hours.

In addition, in the present invention, if the concentration of phosphoric acid is less than 85%, there is a problem in that the release of phosphoric acid occurs because gelation does not occur well.

The 3,4-diaminobenzoic acid is preferably used after being purified by reacting with a base or an acid before mixing with a dehydrating agent.

The polycondensation reaction is preferably made for 30 minutes to 1 hour at a temperature of 160 ℃ to 170 ℃.

In addition, the polycondensation reaction may be carried out in a nitrogen atmosphere or vacuum, water generated by the polycondensation reaction can be removed by using a device such as Dean Stark installed separately connected to the reactor.

In the present invention, when the condensation polymerization, a dehumidifying agent such as silica gel can be used if necessary.

In addition, the present invention can provide a membrane / electrode assembly using the electrode. The membrane / electrode assembly is formed by at least one of an anode electrode and a cathode electrode prepared above, by placing a polymer film between them, and then firing the hot film.

The polymer electrolyte membrane may include a conventional hydrogen ion conductive polymer in addition to the poly (2,5-benzimidazole) (ABPBI) polymer used in the present invention, which is connected three-dimensionally inside the micropores to transfer ions. Form a path. The hydrogen ion conductive polymer may be used as long as it has hydrogen ion conductivity, and for example, a perfluoro polymer, a ketone polymer, a polyether polymer, a polyester polymer, a polyamide polymer, a polyimide polymer, or the like. This can be used. Specific examples of the hydrogen ion conductive polymer include poly (perfluorosulfonic acid), poly (perfluorocarboxylic acid), Tetrafluoroethylene and fluorovinyl ether copolymers containing sulfonic acid groups, defluorinated sulfide polyether ketones, aryl ketones, and the like may be used, but are not limited thereto.

In addition, the present invention includes an electric generator, a fuel supply source and an oxygen supply source having at least one unit cell manufactured by inserting the membrane / electrode assembly between a gas flow channel channel and a separator (or bipolar plate) on which a cooling channel is formed. It provides a fuel cell system.

The electricity generator serves to generate electricity through an electrochemical reaction of hydrogen and oxygen. The fuel supply unit serves to supply fuel containing hydrogen to the electricity generator, and the oxygen supply unit serves to supply oxygen to the electricity generator.

1 is a view schematically showing an operating state of a fuel cell 1 including a membrane / electrode assembly 20. In the fuel cell 1, the membrane / electrode assembly 20 includes an anode electrode 10a serving as an anode catalyst layer, a cathode electrode 10b serving as an anode catalyst layer, and a polymer electrolyte membrane 15. Referring to FIG. 1, when a fuel supply source including hydrogen gas is supplied to the fuel electrode catalyst layer 10a, an electrochemical oxidation reaction occurs and ionizes to hydrogen ions H ⁺ and electrons e ⁻ . The ionized hydrogen ions move to the cathode catalyst layer 10b by the oxygen source through the polymer electrolyte membrane 15 and electrons move through the anode catalyst layer 10a. Hydrogen ions transferred to the cathode catalyst layer 10b cause an electrochemical reduction reaction with oxygen supplied to the cathode catalyst layer 10b to generate heat of reaction and water, and electrical energy is generated by the movement of electrons. This electrochemical reaction can be represented by the following scheme.

Scheme 1

The ^{_{anode: H 2 → 2H + + 2e}} -

_{^{Cathode: 2H + + 1/2 O 2 +}} 2e - → H 2 O

The electricity generating unit includes a stack in which unit cells are stacked, and a stack may be inserted between two end plates to manufacture a fuel cell, and the fuel cell may be all manufactured by conventional techniques in the art. The membrane / electrode assembly of the present invention can be applied to both low temperature humidification type, low temperature no humidification type, and high temperature no humidification type batteries.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only one preferred embodiment of the present invention and the present invention is not limited to the following examples.

(Example 1)                     

A gel was prepared by impregnating 1 g of poly (2,5-benzimidazole) polymer in phosphoric acid having a concentration of 85% for 24 hours.

Thereafter, the gel was mixed with Pt / C 3 g to prepare a catalyst slurry, and then coated on the water-repellent treated carbon paper to prepare an electrode including a catalyst layer.

The electrode prepared above is an anode electrode and a cathode electrode, and a poly (2,5-benzimidazole) polymer is used as an electrolyte membrane, and the anode electrode and the cathode electrode are laminated on both surfaces thereof, and then pressurized to form a membrane / electrode assembly. Was prepared.

The prepared membrane / electrode assembly is inserted between two gaskets, and then inserted into two separators (or bipolar plates) in which a gas channel and a cooling channel of a predetermined shape are formed, and then between the copper end plates. It was compressed in the unit cell was prepared.

(Comparative Example 1)

In the same manner as in Example 1, an electrode was prepared without impregnating poly (2,5-benzimidazole) with phosphoric acid.

(Comparative Example 2)

In the same manner as in Example 1, poly (2,5-benzimidazole) was impregnated with phosphoric acid at a 65% concentration to prepare an electrode.

Experimental Example

For the electrodes of Example 1 and Comparative Examples 1 and 2, the fuel cell performance test was carried out in a state of high temperature and no humidity, and the results are shown in Table 1 below.                     

Table 1

Current at 0.6V (Current at 0.6V) Power density Example 1 230 mA 138 mW / ㎠ Comparative Example 1 50 mA 30 mW / ㎠ Comparative Example 2 150 mA 90 mW / ㎠

As shown in Table 1, in Example 1 of the present invention, since the current value and power density at 0.6V are superior to those of Comparative Examples 1 and 2 by using 85% phosphoric acid, the conductivity of the catalyst electrode layer is improved to improve the fuel cell. Can improve efficiency.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited thereto, and various modifications and changes can be made within the scope of the claims and the detailed description of the invention and the accompanying drawings. Naturally, it belongs to the scope of the invention.

The present invention uses a poly (2,5-benzimidazole) polymer as a catalyst layer binder in the production of an electrode for a fuel cell, and impregnates it with 85% or more of phosphoric acid, thereby increasing the doping amount of phosphate ions to the monomer. By improving the conductivity of the fuel cell can improve the efficiency.

Claims (8)

A gel was prepared by impregnating a poly (2,5-benzimidazole) polymer in phosphoric acid having a concentration of at least 85%; Adding a metal catalyst to the gel to prepare a slurry composition; Coating the composition on a conductive substrate and then heat treatment to form a catalyst layer electrode comprising the steps of forming a catalyst layer. The method of claim 1, wherein the time of impregnation with phosphoric acid is for at least 24 hours. The method of claim 1, wherein the poly (2,5-benzimidazole) polymer is prepared by the condensation reaction of a 3,4-diaminobenzoic acid monomer and a dehydrating agent by adding P ₂ O ₅ to a reaction tank. Way. The method of claim 3, wherein the polycondensation reaction is carried out for 30 minutes to 1 hour at a temperature of 160 ℃ to 170 ℃. The method of claim 1, wherein the conductive substrate is made of water-repellent treated carbon paper or carbon cloth. The method of claim 1, wherein the metal catalyst is a metal catalyst supported on carbon. The method of claim 6, wherein the metal catalyst is platinum, ruthenium, or a mixed catalyst thereof. A membrane / electrode assembly including an anode electrode and a cathode electrode disposed to face each other, and a polymer membrane positioned between the anode electrode and the cathode electrode; And a separator for sandwiching the membrane / electrode assembly, the at least one electricity generating unit generating electricity through an electrochemical reaction between hydrogen and oxygen; A fuel supply unit supplying hydrogen to the electricity generation unit; And An oxygen supply unit for supplying oxygen to the electricity generating unit, At least one of the anode electrode and the cathode electrode is a fuel cell system fuel cell system comprising an electrode manufactured according to any one of claims 1 to 7.

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