KR20090049193A - Cathode comprising two kinds of catalysts for direct methanol fuel cell and membrane electrode assembly and direct methanol fuel cell comprising the same - Google Patents

Abstract

The present invention relates to a cathode for a direct methanol fuel cell comprising two catalysts, a membrane electrode assembly using the same, and a direct methanol fuel cell. The cathode electrode for the direct methanol fuel cell of the present invention includes a gas diffusion layer provided on one side of an electrolyte membrane and provided on a surface facing the electrolyte membrane and a catalyst layer interposed between the electrolyte membrane and the gas diffusion layer. A platinum catalyst supported on a platinum catalyst or carbonaceous support; And a platinum-ruthenium alloy catalyst supported on a platinum-ruthenium alloy catalyst or a carbon-based support.

The cathode of the direct methanol fuel cell of the present invention includes two specific catalysts, thereby minimizing the effect of methanol crossover and reducing the amount of catalyst so that the fuel cell has excellent performance.

Direct Methanol Fuel Cell, Catalyst Layer, Cathode

Description

Cathode comprising two kinds of catalysts for direct methanol fuel cell and Membrane electrode assembly and Direct methanol fuel cell comprising the same}

The present invention relates to a cathode for a direct methanol fuel cell comprising two catalysts, a membrane electrode assembly using the same, and a direct methanol fuel cell. More particularly, the present invention relates to a cathode electrode for a direct methanol fuel cell, a membrane electrode assembly using the same, and a direct methanol fuel cell, which can be used for a distributed power plant, a cogeneration plant, a pollution-free automobile power source, a commercial power source, a household power source, a mobile power source, and the like. .

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.

Among them, the direct methanol fuel cell (DMFC) using methanol as fuel has a lower electrode output power than the fuel cell using hydrogen directly, but the output density is low. However, methanol, which is a fuel, has a high energy density and is easy to store. It is very advantageous to use.

In a direct methanol fuel cell, the most basic unit for generating electricity is a membrane electrode assembly (MEA), which is composed of an anode electrode and a cathode electrode formed on both sides of the electrolyte membrane and the electrolyte membrane. Referring to FIG. 1 and Scheme 1 illustrating the electricity generation principle of a direct methanol fuel cell, an oxidation reaction of a fuel occurs at the anode electrode to generate hydrogen ions and electrons, and the hydrogen ions move to the cathode electrode through the electrolyte membrane, and the cathode electrode. In the case of water, oxygen (oxidant) and hydrogen ions delivered through the electrolyte membrane react with electrons. This reaction causes the movement of electrons in the external circuit.

A fuel electrode _{(anode): CH 3 OH + H 2} O → CO 2 + 6H + + 6e -

An air electrode _{(cathode): 3 / 2O 2 + 6H} + + 6e - → 3H 2 O

Total reaction: CH ₃ OH + H ₂ O → CO ₂ + 3H ₂ O

Referring to FIG. 2 which shows a general configuration of a membrane electrode assembly for a fuel cell, the membrane electrode assembly of a fuel cell is composed of an anode electrode and a cathode electrode which are opposed to each other with an electrolyte membrane 201 and an electrolyte membrane 201 interposed therebetween. The anode electrode and the cathode electrode are composed of the catalyst layers 203 and 205 and the gas diffusion layer 208. The gas diffusion layer is composed of electrode substrates 209a and 209b and microporous layers 207a and 207b formed thereon.

The reaction is carried out in the catalyst layer of each electrode, the platinum catalyst is mainly used in the cathode catalyst layer, and the reactive activity point of the catalyst is easily poisoned due to the CO generated incidentally due to the oxidation reaction in the anode catalyst layer, so that the endothelial to the CO The toxic platinum-ruthenium alloy catalyst is mainly used.

However, methanol oxidation at the anode is relatively slow, and a large amount of platinum catalyst must be used to compensate for this. Therefore, it is necessary to reduce the amount of catalyst used. If the amount of catalyst at both electrodes is reduced, the absolute catalyst surface area is reduced at the anode electrode, thereby reducing the amount of oxidation of methanol and increasing the crossover phenomenon of residual methanol. In addition, there is a problem that the catalyst of the cathode electrode having a reduced total amount of catalyst has a larger CO poisoning area due to the increased methanol crossover, and gradually loses a reaction active point to cause a reduction reaction.

In order to solve this problem, research has been conducted on various catalysts for excellent endothelial toxicity to CO due to methanol crossover at the cathode and reducing the amount of catalyst used per unit area of the electrode. However, as far as is known, the effect of reducing the amount of catalyst used is often insufficient.

For example, Korean Patent Laid-Open Publication No. 2004-7854 discloses a fuel cell having a four-component or more catalyst formed of platinum, ruthenium, molybdenum, tungsten, gold, or the like. In this case, the activity of the catalyst is somewhat improved, but it does not show an improved effect on the reduction in the amount of the catalyst used.

In addition, Korean Patent Laid-Open Publication No. 2005-89324 discloses an electrode for a fuel cell using a promoter such as rhodium, ceria or ruthenium as a platinum-silver or platinum-gold main catalyst. Even in this case, the activity of the catalyst is somewhat improved, but it does not show an improved effect on the reduction in the amount of the catalyst used.

As such, there is an urgent need to develop a cathode electrode using a catalyst capable of providing a direct methanol fuel cell having excellent cell performance even though the amount of reduced cross-linking is excellent in methanol crossover.

Accordingly, an object of the present invention is to provide a cathode electrode for a direct methanol fuel cell including a catalyst which is resistant to methanol crossover and has excellent performance even when the amount of catalyst used is reduced.

In order to solve the above problems, the cathode for direct methanol fuel cell of the present invention is provided on one side of the electrolyte membrane, the gas diffusion layer is provided on the surface facing the electrolyte membrane and interposed between the electrolyte membrane and the gas diffusion layer. A cathode electrode for a direct methanol fuel cell comprising a catalyst layer, the catalyst layer comprising: a platinum catalyst supported on a platinum catalyst or a carbon-based support; And a platinum-ruthenium alloy catalyst supported on a platinum-ruthenium alloy catalyst or a carbon-based support. The catalyst layer of the cathode electrode of the present invention can increase the resistance to CO poisoning due to methanol crossover by using a platinum catalyst and a platinum-ruthenium alloy catalyst at the same time, thereby maintaining excellent catalyst activity and reducing the amount of catalyst used.

In the cathode of the present invention, the platinum catalyst and the platinum-ruthenium alloy catalyst supported on the platinum catalyst or the carbon-based support or the platinum-ruthenium alloy catalyst supported on the carbon-based support may be mixed with each other to form a catalyst layer as a single layer.

In another aspect of the present invention, the platinum catalyst and the platinum-ruthenium alloy catalyst supported on the platinum catalyst or carbon-based support or the platinum-ruthenium alloy catalyst supported on the carbon-based support each form a separate layer so that the catalyst layer is formed as a double layer. Can be.

The cathode electrode of the present invention described above can be used in the membrane electrode assembly for direct methanol fuel cell and direct methanol fuel cell.

The cathode electrode for direct methanol fuel cell of the present invention has a strong resistance to methanol crossover, maintains excellent fuel cell performance even when the amount of catalyst used is reduced, and shows a marked improvement in extending the life of the fuel cell.

Hereinafter, the present invention will be described in detail. The terms or words used in this specification and claims are not to be construed as limiting in their usual or dictionary meanings, and the inventors may appropriately define the concept of terms in order to best explain their invention in the best way possible. It should be interpreted as meaning and concept corresponding to the technical idea of the present invention based on the principle that the present invention.

As described above, the cathode electrode for direct methanol fuel cell is typically provided on one side of the electrolyte membrane, and includes a gas diffusion layer provided on the side opposite to the electrolyte membrane and a catalyst layer interposed between the electrolyte membrane and the gas diffusion layer. . In general, the catalyst layer is formed by applying an ink for forming a catalyst layer to an electrolyte membrane or a gas diffusion layer and then drying. Conventional catalyst layer inks include a metal catalyst polymer ionomer and a solvent supported on a metal catalyst or carbon-based support. In the present invention, an ink for forming a catalyst layer used in the art may be applied without limitation.

In the cathode of the direct methanol fuel cell of the present invention, the catalyst layer comprises a platinum catalyst or a platinum catalyst supported on a carbon-based support; And a platinum-ruthenium alloy catalyst supported on a platinum-ruthenium alloy catalyst or a carbon-based support.

The cathode of the direct methanol fuel cell of the present invention uses the platinum catalyst and the platinum-ruthenium alloy catalyst of the present invention at the same time to maintain excellent catalytic activity despite methanol crossover, thereby improving fuel cell performance even when the amount of catalyst used is reduced. It exhibits significant synergistic effects that can be sustained and exceeded to those skilled in the art.

The catalyst layer according to the present invention may further include a metal catalyst supported on a metal catalyst or a carbon-based support used in the art, and representative metal catalysts include ruthenium, osmium, platinum-osmium alloys, platinum-palladium alloys, and platinum-transitions. Any one or a mixture of two or more selected from the group consisting of metal alloys may be used, but is not limited thereto.

According to the present invention, a carbon-based support capable of supporting a platinum catalyst, a platinum-ruthenium alloy catalyst or another metal catalyst may be used without limitation in the art, for example, graphite (graphite), carbon black, Any one selected from the group consisting of acetylene black, denka black, canyon black, activated carbon, mesoporous carbon, carbon nanotube, carbon nanofiber, carbon nanohorn, carbon nano ring, carbon nanowire, fullerene (C60) and super P One or a mixture of two or more may be preferred examples.

As the polymer ionomer used in the ink for forming the catalyst layer, sulfonated polymers such as nafion ionomer or sulfonated polytrifluorostyrene may be representatively used.

The solvent used for the ink for forming the catalyst layer is preferably any one or a mixture of two or more selected from the group consisting of water, butanol, isopropanol, methanol, ethanol, n-propanol, n-butyl acetate and ethylene glycol. Can be used.

In one aspect of the present invention, the platinum catalyst and the platinum-ruthenium alloy catalyst supported on the platinum catalyst or carbon-based support or the platinum-ruthenium alloy catalyst supported on the carbon-based support are mixed with each other to form a catalyst layer as a single layer. Can be. 3 shows a platinum catalyst (Pt / C catalyst) 212 supported on a carbon-based support and a platinum-ruthenium alloy catalyst (PtRu / C catalyst) 213 supported on a carbon-based support together with the polymer ionomer 211. Shown is a catalyst layer that is mixed to form a single layer. In order to form the catalyst layer as a single layer as shown in the embodiment of the present invention shown in FIG. A platinum-ruthenium alloy catalyst (PtRu / C catalyst) 213 supported on a support is added together to prepare an ink for forming a catalyst layer. When the ink for forming a catalyst layer is applied to the electrolyte membrane 201 and dried, a catalyst layer of a single layer can be formed.

In another aspect of the present invention, the platinum catalyst and the platinum-ruthenium alloy catalyst supported on the platinum catalyst or the carbon-based support or the platinum-ruthenium alloy catalyst supported on the carbon-based support each form a separate layer to form a catalyst layer. It may be formed in a double layer. 4 shows a first layer comprising a platinum catalyst (Pt / C catalyst) 212 supported on a carbon-based support together with a polymer ionomer 211 and a platinum-ruthenium alloy catalyst (PtRu catalyst) 214. The catalyst layer which is a double layer formed in two layers is shown. In order to form the catalyst layer as a double layer as shown in the embodiment of the present invention shown in FIG. 4, a platinum catalyst (Pt / C catalyst) 212 supported on a carbon-based support is added when the catalyst layer forming ink is prepared. The first catalyst layer forming ink and the second catalyst layer forming ink prepared by adding a platinum-ruthenium alloy catalyst (PtRu catalyst) 214 are separately prepared. Among the catalyst layer forming inks thus prepared, the second catalyst layer forming ink is first applied to the electrolyte membrane 201 and dried, and then the first catalyst layer forming ink is applied thereon and dried to form a double catalyst layer. Can be formed.

The content ratio of the platinum catalyst and the platinum-ruthenium alloy catalyst forming the catalyst layer according to the present invention can be appropriately adjusted as necessary. For example, the weight ratio may be platinum catalyst: platinum-ruthenium alloy catalyst = 1: 0.2-0.7. have. Within this range, while having excellent endothelial toxicity against methanol crossover, it may have an appropriate electrode layer thickness that may exhibit an effect of reducing the amount of catalyst, and may not increase the electrical resistance and the material transfer resistance.

The catalyst layer included in the cathode electrode for direct methanol fuel cell of the present invention, unlike the catalyst used in the conventional catalyst layer, can maintain excellent battery performance even if the amount of catalyst used is small. Conventional catalysts used in conventionally known direct methanol fuel cell electrodes are used in an amount of 4 mg / cm ² or more per electrode, but those skilled in the art can adjust the amount of the catalyst of the present invention as needed, for example, performance of a fuel cell. It may be 1 to 2.5 mg / cm ² that can be best maintained, but is not limited thereto. The thickness of the catalyst layer of the cathode electrode formed according to the present invention may be preferably 5 ~ 100㎛.

The cathode electrode for direct methanol fuel cell of the present invention described above may be formed on an electrolyte membrane or a gas diffusion layer and used in the manufacture of the membrane electrode assembly for fuel cell of the present invention.

As shown in FIG. 2, the membrane electrode assembly for the direct methanol fuel cell of the present invention includes an electrolyte membrane 201; And an anode catalyst layer 203 and a cathode catalyst layer 205 positioned to face each other with the electrolyte membrane 201 interposed therebetween. The anode electrode and the cathode electrode may include a gas diffusion layer 208 and a catalyst layer (203, 205), the gas diffusion layer 208 for the fuel cell of the present invention is formed on the substrate (209a, 209b) and one surface of the substrate fine The pore layers 207a and 207b may be included.

As the electrolyte membrane of the present invention, an electrolyte membrane used in the art may be used without limitation, for example, perfluorosulfonic acid polymer, hydrocarbon polymer, polyimide, polyvinylidene fluoride, polyether sulfone, polyphenylene sulfide, A polymer selected from the group consisting of polyphenylene oxide, polyphosphazine, polyethylene naphthalate, polyester, doped polybenzimidazole, polyetherketone, polysulfone, and acids and bases thereof may be used, but is not limited thereto. Does not.

The gas diffusion layer of the present invention may be used without limitation the gas diffusion layer used in the art, it may be made of a conductive substrate selected from the group consisting of carbon paper, carbon cloth and carbon felt. The gas diffusion layer may further include a microporous layer formed on one surface of the conductive substrate, and the microporous layer may include a carbonaceous material and a fluorine resin.

Examples of the carbon-based material include graphite (graphite), carbon black, acetylene black, denka black, canyon black, activated carbon, mesoporous carbon, carbon nanotubes, carbon nanofibers, carbon nanohorns, carbon nanorings, carbon nanowires, One or a mixture of two or more selected from the group consisting of fullerene (C60) and super P may be used, but is not limited thereto.

The fluororesin may be polytetrafluoroethylene, polyvinylidene fluoride (PVdF), polyvinyl alcohol, cellulose acetate, copolymer of polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) or styrene-butadiene rubber One or a mixture of two or more selected from the group consisting of (SBR) may be used, but is not limited thereto.

At this time, the catalyst layer is formed between the gas diffusion layer and the electrolyte membrane.

The present invention also provides a direct methanol fuel cell comprising the membrane electrode assembly of the present invention. 5 is a view schematically showing a fuel cell according to an embodiment of the present invention. Referring to FIG. 5, 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 is electrically connected to the membrane electrode assemblies. It serves to prevent fuel loss and to deliver externally supplied fuel and oxidant 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 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 present invention will be described in detail with reference to Examples. However, embodiments according to the present invention can be modified in many different forms, the scope of the present invention should not be construed as limited to the embodiments described below. The embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Example 1

A mixed catalyst in which a Pt / C catalyst and a PtRu / C catalyst, both of which use carbon black, as a carrier of the catalyst, are mixed in a weight ratio of Pt: PtRu = 2: 1, 30 parts by weight of Nafion, and 100 parts by weight of the mixed catalyst; An ink for forming a catalyst layer containing water and isopropanol as a solvent was prepared.

The catalyst layer was formed by adjusting the application amount of the mixed catalyst per unit area to 1.5 mg / cm ² on the electrolyte membrane (Nafion 115) by dry spray coating, thereby forming a catalyst layer for the cathode electrode according to the present invention.

Meanwhile, a PtRu alloy catalyst was applied to the electrolyte membrane at a coating amount of 2 mg / cm ² to form a catalyst layer for the anode electrode to prepare a membrane electrode assembly.

Example 2

After preparing the first catalyst layer forming ink using only the Pt / C catalyst and the second catalyst layer forming ink using only the PtRu alloy catalyst as the cathode electrode catalyst, the second catalyst layer forming ink was first applied to the electrolyte membrane. Next, a cathode electrode catalyst layer, an anode electrode catalyst layer, and a membrane electrode assembly were prepared in the same manner as in Example 1 except that the catalyst layer was formed into a double layer by applying the ink for forming the first catalyst layer.

Comparative example

A cathode electrode catalyst layer, an anode electrode catalyst layer, and a membrane electrode assembly were prepared in the same manner as in Example 1, except that only a platinum catalyst was used as the cathode electrode catalyst.

6 and 7 show test results of unit cells manufactured using the membrane electrode assembly prepared in Examples 1 and 2 and Comparative Examples. 6 shows current-voltage characteristics, and FIG. 7 shows long-term performance test results.

6 and 7, it can be seen that the embodiments according to the present invention have better current-voltage characteristics than the comparative examples, and the battery life is extended by maintaining excellent performance over time even in a long-term performance test.

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

2 is a view schematically showing the structure of a membrane electrode assembly for a general direct methanol fuel cell.

3 is a view schematically showing a catalyst layer formed of a single layer according to an embodiment of the present invention.

4 is a view schematically showing a catalyst layer formed of a double layer according to an embodiment of the present invention.

5 is a view schematically showing an example of a fuel cell according to the present invention.

6 is a graph illustrating a test result of current-voltage characteristics of a unit cell manufactured according to an exemplary embodiment of the present invention.

7 is a graph showing the results of the unit long-term performance test prepared in accordance with an embodiment of the present invention.

Claims (8)

In the cathode electrode for the direct methanol fuel cell comprising a gas diffusion layer provided on one side of the electrolyte membrane, the gas diffusion layer provided on the surface facing the electrolyte membrane and the electrolyte membrane and the gas diffusion layer, The catalyst layer, A platinum catalyst supported on a platinum catalyst or carbonaceous support; And Platinum-ruthenium alloy catalyst or platinum-ruthenium alloy catalyst supported on carbon-based support Cathode electrode for direct methanol fuel cell comprising a. The method of claim 1, A platinum catalyst supported on the platinum catalyst or carbon-based support; And a platinum-ruthenium alloy catalyst supported on a platinum-ruthenium alloy catalyst or a carbon-based support is mixed with each other to form a catalyst layer in a single layer. The method of claim 1, A platinum catalyst supported on the platinum catalyst or carbon-based support; And a platinum-ruthenium alloy catalyst supported on a platinum-ruthenium alloy catalyst or a carbon-based support, each of which forms a separate layer so that a catalyst layer is formed of a double layer. The method of claim 1, A weight ratio of the platinum catalyst and the platinum-ruthenium alloy catalyst is platinum catalyst: platinum-ruthenium alloy catalyst = 1: 0.2 ~ 0.7 cathode for direct methanol fuel cell. The method of claim 1, The catalyst layer is a cathode of the direct methanol fuel cell, characterized in that the catalyst usage is 1 ~ 2.5 mg / cm ² . The method of claim 1, The catalyst layer further comprises the metal catalyst supported on a metal catalyst or a carbon-based support, which is any one or a mixture of two or more selected from the group consisting of ruthenium, osmium, platinum-osmium alloys, platinum-palladium alloys, and platinum-transition metal alloys. Cathode electrode for direct methanol fuel cell comprising a. In the fuel cell membrane electrode assembly formed between the electrolyte membrane and the electrolyte membrane, and including an anode electrode and a cathode electrode each comprising a catalyst layer and a gas diffusion layer, The anode electrode or cathode electrode is a membrane electrode assembly for a direct methanol fuel cell, characterized in that the electrode for fuel cells according to any one of claims 1 to 6. A stack comprising at least one membrane electrode assembly according to claim 7 and a separator interposed between the membrane electrode assemblies; A fuel supply unit supplying fuel to the stack; And Oxidizer supply unit for supplying an oxidant to the stack Direct methanol fuel cell comprising a.

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