KR20080050872A - Membrane-electrode assembly of fuel cell and fuel cell - Google Patents

Abstract

A membrane-electrode assembly for a fuel cell is provided to prevent or reduce a crossover phenomenon caused by fuel passing through an electrolyte membrane to a cathode, thereby improving the quality of a fuel cell. A membrane-electrode assembly for a fuel cell comprises: an ion conductive membrane(201); and an anode and a cathode facing to each other with the ion conductive membrane interposed between both electrodes, wherein each of the anode and the cathode comprises a diffusion layer(209) and a catalyst layer(203,205) formed on the diffusion layer, and the ion conductive membrane includes a crossover-preventing layer(201b) formed of a catalyst for oxidation of fuel, and polymer electrolyte membranes facing to each other with the crossover-preventing layer interposed therebetween.

Description

Membrane-electrode assembly of fuel cell and fuel cell

The following drawings attached to this specification are illustrative of preferred embodiments of the present invention, and together with the detailed description of the invention to serve to further understand the technical spirit of the present invention, the present invention is a matter described in such drawings It should not be construed as limited to.

1 is a view for explaining the principle of electricity generation of the fuel cell.

2 is a view schematically showing a fuel cell membrane electrode assembly according to an embodiment of the present invention.

3 is a view schematically showing a fuel cell according to an embodiment of the present invention.

4 is a performance evaluation graph of a unit cell manufactured using the membrane-electrode assembly prepared according to Examples 1 and 2 and Comparative Example 1. FIG.

The present invention relates to a membrane-electrode assembly and a fuel cell of a fuel cell, wherein in a fuel cell using a liquid fuel, a fuel cell is prevented or reduced by passing the electrolyte membrane along with water to the cathode electrode. A fuel cell membrane-electrode assembly and a fuel cell can improve performance.

Recently, as the depletion of existing energy resources such as oil and coal is predicted, interest in energy that can replace them is increasing. As one of the alternative energy sources, the fuel cell is particularly attracting attention due to its advantages such as high efficiency, no pollutants such as NO _x and SO _x , and abundant fuel used.

A fuel cell is a power generation system that converts chemical reaction energy of a fuel and an oxidant into electrical energy. Hydrogen, a hydrocarbon such as methanol, butane, and the like are typically used as an oxidant.

In a fuel cell, the most basic unit for generating electricity is a membrane-electrode assembly (MEA), which consists of an electrolyte membrane and anode and cathode electrodes formed on both sides of the electrolyte membrane. Referring to FIG. 1 and Reaction Formula 1 (Reaction formula of a fuel cell when hydrogen is used as a fuel) showing the electricity generation principle of a fuel cell, an oxidation reaction of a fuel occurs at an anode electrode, and hydrogen ions and electrons are generated. The electrolyte moves through the electrolyte membrane to the cathode electrode, where water is generated by reaction between oxygen (oxidant) and hydrogen ions transferred through the electrolyte membrane and electrons. This reaction causes the movement of electrons in the external circuit.

The ^{_{anode: H 2 → 2H + + 2e}} -

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

Total Reaction Formula: H ₂ + 1 / 2O ₂ → H ₂ O

Fuel cells include polymer electrolyte fuel cells (PEMFC), direct methanol fuel cells (DMFC), phosphoric acid fuel cells (PAFC), alkaline fuel cells (AFC), molten carbonate fuel cells (MCFC), solid oxide fuels Batteries (SOFC) and the like. Among them, the polymer electrolyte fuel cell has advantages of high energy density and high output, but has a problem of requiring additional equipment such as a reforming device for reforming fuel such as methane or methanol. On the contrary, the direct methanol fuel cell has a disadvantage of lower energy density, lower power, and a larger amount of catalyst than the polymer electrolyte fuel cell due to the slow reaction rate, but does not require a separate reforming device. The liquid fuel is easy to handle and has an advantage of low operating temperature. Therefore, recently, researches on fuel cells using liquid fuels including methanol have been actively conducted.

However, in the fuel cell using the liquid fuel, there is a problem in that a so-called crossover occurs in which the fuel passes through the electrolyte membrane along with water to the cathode electrode, thereby lowering the power density of the fuel cell. Therefore, in a fuel cell using liquid fuel, preventing or reducing fuel crossover is recognized as one of the most important factors for improving fuel cell performance. Therefore, efforts to solve such problems have been made steadily in the related field, and the present invention has been devised under such technical background.

The present invention has been made to solve the above-mentioned problems of the prior art, and the technical problem to be achieved in the present invention is a crossover phenomenon in which fuel passes through an electrolyte membrane together with water to a cathode electrode in a fuel cell using liquid fuel. The purpose of the present invention is to provide a fuel cell membrane-electrode assembly and a fuel cell that can prevent or reduce the performance of the fuel cell and improve the performance of the fuel cell.

The present invention to achieve the technical problem to be achieved by the present invention, the ion conductive film; And an anode electrode and a cathode electrode disposed to face each other with the ion conductive film interposed therebetween, wherein the anode electrode and the cathode electrode include a diffusion layer and a catalyst layer formed on the diffusion layer, wherein the ion conductive film prevents oxidation of fuel. It provides a membrane-electrode assembly for a fuel cell comprising a crossover prevention layer formed by including a catalyst and a polymer electrolyte membrane positioned to face each other with the crossover prevention layer therebetween.

In the fuel cell membrane electrode assembly of the present invention, the amount of catalyst loading in the crossover prevention layer is preferably 0.1 to 0.5 mg / cm ² , and the crossover prevention layer is platinum, ruthenium, osmium, platinum-ruthenium alloy, It is preferable to include a catalyst selected from the group consisting of a platinum-osmium alloy, a platinum-palladium alloy and a platinum-transition metal alloy, and the thickness of the crossover prevention layer is preferably 1 to 30 µm.

The gas diffusion layer may include one selected from the group consisting of carbon paper, carbon cloth and carbon felt, and the polymer electrolyte membrane is typically a perfluorosulfonic acid polymer, a hydrocarbon-based polymer, polyimide, polyvinylidene fluoride, Polyethersulfone, polyphenylenesulfide, polyphenylene oxide, polyphosphazine, polyethylene naphthalate, polyester, doped polybenzimidazole, polyetherketone, polysulfone, acids and bases thereof It may comprise a polymer.

The anode catalyst layer is preferably made of a catalyst selected from the group consisting of platinum, ruthenium, osmium, platinum-ruthenium alloy, platinum-osmium alloy, platinum-palladium alloy and platinum-transition metal alloy, the catalyst is It can be supported on a carbon-based carrier. Preferably, the catalyst layer of the cathode electrode comprises platinum, and the platinum may be supported on a carbon-based carrier.

The membrane-electrode assembly may be used in a fuel cell using a liquid fuel to effectively prevent crossover of the fuel over the electrolyte membrane. Representative liquid fuels used in a fuel cell include methanol, ethanol, butanol, propanol, or formic acid. Can be mentioned. The membrane-electrode assembly can typically be used in direct methanol fuel cells.

The invention also comprises a stack comprising one or more membrane-electrode assemblies according to the invention and a separator interposed between the membrane-electrode assemblies; A fuel supply unit supplying fuel to the stack; And an oxidant supply unit supplying an oxidant to the electricity generating unit.

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

2 is a view schematically showing a membrane-electrode assembly for a fuel cell according to an embodiment of the present invention. With reference to Figure 2 looks at with respect to the fuel cell membrane electrode assembly of the present invention.

In a fuel cell membrane electrode assembly, a polymer electrolyte membrane is positioned between an anode electrode and a cathode electrode. However, the membrane-electrode assembly of the present invention prevents crossover of fuel by placing a catalyst layer, that is, a crossover prevention layer 201b, containing a catalyst for oxidation of fuel between two layers of the polymer electrolyte membrane 201a. That is, the membrane-electrode assembly of the present invention improves the structure of a conventional polymer electrolyte membrane, so as to face each other with a crossover prevention layer 201b including a catalyst for oxidation of fuel and the crossover prevention layer 201b therebetween. An ion conductive membrane 201 including a polymer electrolyte membrane 201a is positioned. An anode electrode and a cathode electrode are positioned on both surfaces of the ion conductive film 201 having such a structure, and the anode electrode and the cathode electrode include the catalyst layers 203 and 205 and the gas diffusion layer 209, respectively.

In the membrane-electrode assembly of the present invention, since the liquid fuel that is not oxidized at the anode electrodes 203 and 209 and passes through the polymer electrolyte membrane 201a with water is oxidized by the catalysts of the crossover prevention layer 201b, the fuel Can prevent or significantly reduce crossover.

Catalysts for oxidizing fuel may be used for the crossover prevention layer 201b, and any catalysts used for the anode electrode of the fuel cell may be used without limitation. Typically, platinum, ruthenium, osmium, platinum-ruthenium alloy, platinum-osmium alloy, platinum-palladium alloy or platinum-transition metal alloy may be used, and these may be used on their own, but may be supported on a carrier. Representative carriers include carbon-based carriers such as denka black, acetylene black and graphite.

It is preferable that the amount of catalyst loading of the crossover prevention layer 201b is 0.1 to 0.5 mg / cm ^{2. If the} amount of catalyst loading falls below the lower limit, a desired degree of crossover prevention cannot be expected. It is not preferable because the ion conductivity of the electrolyte membrane is dropped. Preferably, the crossover prevention layer 201b has a thickness of 1 to 30 µm. When the thickness falls below the lower limit, a desired degree of crossover prevention cannot be expected, and when the upper limit is exceeded, the ion conductivity of the electrolyte membrane is lowered. It is not desirable to drop it.

The method of introducing the crossover prevention layer 201b may be performed in the same manner as the method of forming the catalyst layer on the electrolyte membrane. For example, the crosslinking prevention layer 201b may be prepared by directly coating the catalyst ink on the electrolyte membrane and then attaching the electrolyte membrane thereon. . At this time, the coating method of the catalyst ink is not particularly limited, but spray coating, tape casting, screen printing, blade coating, die coating or spin coating may be used. The catalyst ink may consist of a catalyst, a polymer ionomer and a solvent.

Polymer ionomers provide a path for ions produced by the reaction between a fuel such as hydrogen or methanol to the catalyst to move into the electrolyte membrane, such as nafion ionomers and sulfonated polytrifluorostyrenes. Fonned polymers include, but are not limited to.

Examples of the solvent that can be used include water, butanol, isopropanol, methanol, ethanol, n-propanol, n-butyl acetate, ethylene glycol, and the like, and these solvents may be used alone or in combination of two or more thereof. .

The electrolyte membrane 201a serves as a transfer passage through which hydrogen ions generated from the anode electrode are transferred to the cathode electrode, and the electrolyte membrane includes perfluorosulfonic acid polymer, hydrocarbon-based polymer, polyimide, polyvinylidene fluoride, polyether sulfone, Polyphenylene sulfides, polyphenylene oxides, polyphosphazines, polyethylene naphthalates, polyesters, doped polybenzimidazoles, polyetherketones, polysulfones, acids or bases thereof may be preferably used.

The anode electrode is where the oxidation of the fuel occurs, the catalyst layer 203 is selected from the group consisting of platinum, ruthenium, osmium, platinum-ruthenium alloy, platinum-osmium alloy, platinum-palladium alloy and platinum-transition metal alloy. The catalyst to be used may be preferably used, and the cathode electrode is where the reduction reaction of the oxidant occurs, and platinum may be used as the catalyst in the catalyst layer 205. The catalysts may be supported on a carbon-based carrier, and representative carriers include carbon-based carriers such as denca black, acetylene black, and graphite.

In the process of introducing the catalyst layers 203 and 205, conventional methods known in the art may be employed, and a method of forming a crossover prevention layer on the electrolyte membrane may also be employed.

The gas diffusion layer 209 moves and diffuses the reaction gas and water together with a role as a current conductor, and may include carbon paper, carbon cloth, or carbon felt, and is in contact with the catalyst layer of the gas diffusion layer. The microporous layer may be formed.

The membrane-electrode assembly may be used in a fuel cell using a liquid fuel to effectively prevent the crossover of the fuel over the electrolyte membrane 201a. Representative liquid fuels used in the fuel cell include methanol, ethanol, butanol, Propanol or formic acid. The most representative fuel cell using liquid fuel is a direct methanol fuel cell.

The present invention also provides a fuel cell comprising the membrane-electrode assembly of the present invention. 3 is a view schematically showing a fuel cell according to an embodiment of the present invention. Referring to FIG. 3, the fuel cell of the present invention includes a stack 200, a fuel supply unit 400, and an oxidant supply unit 300.

The stack 200 includes one or more membrane-electrode assemblies of the present invention, and when two or more membrane-electrode assemblies are included, the stack 200 includes a separator interposed therebetween. The separator prevents the membrane-electrode assemblies from being electrically connected to each other and delivers fuel and oxidant supplied from the outside to the membrane-electrode assembly.

The fuel supply unit 400 serves to supply fuel to the stack, and includes a fuel tank 410 for storing fuel and a pump 420 for supplying fuel stored in the fuel tank 410 to the stack 200. Can be. The fuel may be a gas or liquid hydrogen or hydrocarbon fuel, examples of the hydrocarbon fuel may be methanol, ethanol, propanol, butanol or natural gas.

The oxidant supply unit 300 serves to supply an oxidant to the stack. Oxygen is typically used as the oxidant, and may be used by injecting oxygen or air into the pump 300.

Hereinafter, the Example and comparative example of this invention are described. However, the following examples are only preferred embodiments of the present invention and the present invention is not limited to the following examples.

1) Preparation of membrane-electrode assembly

Example  One

In order to manufacture a double-membrane membrane-electrode assembly according to the present invention, the electrolyte membrane used on the anode facing surface is Nafion 115 (Dupont, 125 µm), and the electrolyte membrane used on the cathode facing surface is a carrier web (solluper, 25 µm). ) to, and on one side of the Nafion 115 PtRu was loaded into a black catalyst and Nafion ah mixing 1.4 _Pt mg / cm ² ionomer, the carrier web film on one side, the mixing Pt black catalyst and Nafion ionomer 4 _Pt mg Loaded at / cm ² . The crossover protection layer was loaded with PtRu black catalyst at 0.2 _Pt mg / cm ² , and the thickness of the crossover prevention layer was about 15 μm. The crossover prevention layer was coated on the other side of the anode catalyst layer of the Nafion 115 electrolyte membrane, which was not coated. The coating used was spray coating. Thereafter, the two membranes were bonded together and maintained at a weight of 1 ton at 140 ^° C. for 3 minutes, followed by hot pressing to prepare a membrane-electrode assembly.

Example  2

A membrane-electrode assembly was prepared in the same manner as in Example 1, except that 0.6 _Pt mg / cm ² of the PtRu black catalyst of the crossover prevention layer was loaded and the thickness of the crossover prevention layer was about 35 μm.

Comparative example  One

Using Nafion 115 membrane (125 μm), PtRu black catalyst and Nafion ionomer were mixed on one side to load 2 _Pt mg / cm ² , and on the other side, Pt black catalyst was loaded with 4 _Pt mg / cm ² . To prepare a membrane-electrode assembly without a crossover prevention layer. The remaining conditions are the same as in Example 1.

2) Performance Evaluation

Unit cells were prepared using the membrane-electrode assemblies of Examples 1 and 2 and Comparative Example 1, and performance evaluations thereof were performed. The effective area of the unit cell was 50 cm ² , and methanol was flowed into the anode and air was flowed into the cathode while maintaining a normal pressure of 70 ^° C. during the performance evaluation. The performance evaluation result is shown in FIG.

Referring to FIG. 4, as a result of Example 1, it was confirmed that the crossover prevention layer showed higher performance even though the amount of catalyst loading of the anode was smaller than that of Comparative Example 1. When comparing Comparative Example 1 without the crossover protection layer and Example 1 with the crossover protection layer, the current density obtained at 0.4V was about 50 mA / cm ² .

Example 2 also showed superior performance compared to Comparative Example 1, but showed much less results than Example 1. This is considered to be because the thickness of the crossover prevention layer is excessively formed and the ion conductivity of the hydrogen ions is lowered, which adversely affects the performance.

The terms or words used in this specification and claims should not be construed as being limited to the common or dictionary meanings, and the inventors can appropriately define the concept of terms in order to best describe their invention. Based on the principle, it should be interpreted as meaning and concept corresponding to the technical idea of the present invention.

Therefore, the exemplary embodiments described herein are only exemplary embodiments of the present invention and do not represent all of the technical ideas of the present invention, and various equivalents and modifications that may substitute them at the time of the present application may be used. It should be understood that there may be.

According to the fuel cell membrane electrode assembly of the present invention, a crossover prevention layer including a catalyst for oxidation of fuel is provided in the polymer electrolyte membrane so that the liquid fuel passes through the electrolyte membrane along with water to the cathode electrode (air electrode). By preventing or reducing the phenomenon, the performance of the fuel cell can be improved.

Claims (14)

Ion conductive membrane; And And an anode electrode and a cathode electrode disposed to face each other with the ion conductive layer interposed therebetween. The anode electrode and the cathode electrode includes a diffusion layer and a catalyst layer formed on the diffusion layer, The ion conductive membrane is a fuel cell membrane-electrode assembly, characterized in that it comprises a crossover prevention layer formed including a catalyst for the oxidation of the fuel and polymer electrolyte membranes positioned opposite to each other with the crossover prevention layer therebetween. The method of claim 1, Membrane-electrode assembly for a fuel cell, characterized in that the amount of catalyst loading in the crossover prevention layer is 0.1 to 0.5mg / cm ² . The method of claim 1, The crossover prevention layer is a fuel cell membrane electrode comprising a catalyst selected from the group consisting of platinum, ruthenium, osmium, platinum-ruthenium alloy, platinum-osmium alloy, platinum-palladium alloy and platinum-transition metal alloy Conjugate. The method of claim 1, The thickness of the crossover prevention layer is a fuel cell membrane-electrode assembly, characterized in that 1 to 30㎛. The method of claim 1, The gas diffusion layer is a fuel cell membrane-electrode assembly, characterized in that comprising a carbon paper, carbon cloth and carbon felt selected from the group consisting of. The method of claim 1, The polymer electrolyte membrane is perfluorosulfonic acid polymer, hydrocarbon-based polymer, polyimide, polyvinylidene fluoride, polyether sulfone, polyphenylene sulfide, polyphenylene oxide, polyphosphazine, polyethylene naphthalate, polyester, doped poly A membrane-electrode assembly for a fuel cell, comprising a polymer selected from the group consisting of benzimidazole, polyether ketone, polysulfone, and acids and bases thereof. The method of claim 1, The anode electrode catalyst layer is a fuel cell membrane comprising a catalyst selected from the group consisting of platinum, ruthenium, osmium, platinum-ruthenium alloy, platinum-osmium alloy, platinum-palladium alloy and platinum-transition metal alloy. Electrode assembly. The method of claim 7, wherein The catalyst is a membrane-electrode assembly for a fuel cell, characterized in that supported on a carbon-based carrier. The method of claim 1, Membrane-electrode assembly for a fuel cell, characterized in that the catalyst layer of the cathode electrode comprises platinum. The method of claim 9, Membrane electrode assembly for a fuel cell, characterized in that the platinum is supported on a carbon-based carrier. The method of claim 1, The membrane-electrode assembly is a fuel cell membrane-electrode assembly, characterized in that used for a fuel cell using a liquid fuel. The method of claim 11, The fuel cell membrane-electrode assembly, characterized in that selected from the group consisting of methanol, ethanol, butanol, propanol and formic acid. The method of claim 12, The membrane electrode assembly is a fuel cell membrane electrode assembly, characterized in that used in direct methanol fuel cell. A stack comprising a membrane-electrode assembly according to any one of claims 1 to 13 and a separator interposed between the membrane-electrode assemblies; A fuel supply unit supplying fuel to the stack; And And an oxidant supply unit for supplying an oxidant to the electricity generating unit.

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