KR19990020912A - A fuel cell having a composition for forming a catalyst layer and a catalyst layer formed using the same - Google Patents

Abstract

The present invention relates to a composition for forming a catalyst layer and a fuel cell having a catalyst layer formed using the same. Using a composition comprising a platinum / carbon powder, a binder, and a hydrogen ion conductive polymer coated on a silica surface, a catalyst layer excellent in oxidation / reduction reaction and ion conductivity is easily formed even if a small amount of platinum catalyst is contained. . In addition, the fuel cell having such a catalyst layer has an advantage that the output density is excellent.

Description

A fuel cell having a composition for forming a catalyst layer and a catalyst layer formed using the same

The present invention relates to a composition for forming a catalyst layer and a fuel cell having a catalyst layer formed using the same. Specifically, even when a small amount of platinum catalyst is contained, a catalyst layer excellent in oxidation / reduction reaction and ion conductivity can be easily formed. The present invention relates to a fuel cell having a high catalyst density comprising a catalyst layer composition and a catalyst layer formed using the catalyst layer composition.

The Proton Exchange Membrane Fuel Cell is a future clean energy source that can replace fossil energy with excellent power density and energy conversion efficiency. In addition, it can operate at room temperature, and can be miniaturized and encapsulated, so there are many applications such as pollution-free automobiles, household power generation systems, mobile communication equipment, medical devices, military use, and space business.

The hydrogen ion exchange membrane fuel cell is a power generation system that generates direct current electricity from an electrochemical reaction between hydrogen and oxygen, and its basic structure is shown in FIG. 1. In Fig. 1, reference numerals 1 and 2 are gas diffusion electrodes composed of a backing layer for supplying fuel gas and a catalyst layer for causing an oxidation-reduction reaction of the fuel gas, respectively.

In the gas diffusion electrodes 1 and 2, oxidation / reduction reactions of the reaction gas, that is, hydrogen and oxygen, occur. Schemes 1 and 2 show oxidation / reduction reactions occurring in a fuel cell. That is, in the anode 1 of the gas diffusion electrode, an oxidation reaction as in Scheme 1 occurs, and hydrogen molecules are converted into protons and electrons. Protons are transferred to the cathode 2 via the proton conductive film 3. In cathode (2), the same reduction reaction as in Scheme 1 takes place. That is, oxygen molecules receive electrons and are converted into oxygen ions, which react with protons from the anode 1 and are converted into water molecules.

In the fuel cell operating on the principle as described above, in the hydrogen ion exchange membrane fuel cell, the catalyst layer is formed on an electrode support made of carbon cloth or carbon paper. A proton conductive film 3 having a thickness of 50 to 200 µm is interposed between the gas diffusion electrodes 1 and 2 made of such a catalyst layer and a support.

In general, a proton conductive film that transfers hydrogen ions is composed of a solid polymer. In this case, since the bonding interface between the electrode and the proton conductive film is two-dimensional, there is a problem that the utilization of the catalyst is lower than that of the case where the electrolyte is a liquid. Accordingly, it is necessary to increase the utilization of the catalyst by making the interface between the electrode and the conductive film into a three-dimensional structure.

In the initial hydrogen ion exchange membrane fuel cell, the catalyst layer was formed using a powder of platinum-coated carbon (platinum / carbon) powder and polytetrafluoroethylene as a binder. In such a fuel cell, since the redox reaction of the gas is performed only at the interface between the electrode and the conductive film, the catalyst utilization rate is very low. Therefore, the amount of platinum loading in the catalyst layer must be increased to 4 mg / cm 2 to obtain a practical output density. Could get As described above, the fuel cell of the related art has a high platinum content and a high manufacturing cost, thus making it difficult to use the fuel cell except for some special applications such as space shuttles.

In order to solve this problem, US Pat. No. 4,876,115 uses platinum / carbon powder coated with 3 nm size fine platinum powder on the carbon black surface as a catalyst layer material to maximize platinum surface area, and uses polytetrafluoro as a binder. A method of forming a catalyst layer using ethylene is disclosed. In addition, a method of forming a three-dimensional interface between the platinum catalyst and the Nafion electrolyte by impregnation of the liquid Nafion polymer electrolyte on the surface of the catalyst layer in contact with the hydrogen ion exchange membrane and then drying to form a Nafion coating Started. In this case, it is reported that the platinum loading amount can be reduced to 1/10 level compared with the conventional one. However, when employing such a method, there are the following problems.

First, a process for forming a Nafion coating on the catalyst layer is added, which makes the manufacturing process complicated.

Second, when impregnating the liquid Nafion polymer on the catalyst layer, some liquid Nafion electrolyte is not limited to the electrode catalyst layer but penetrates to the support layer to generate unnecessary loss of the Nafion polymer.

To solve this problem, US Patent No. 5,234, 777 discloses a method of preparing a mixture in the form of an ink by mixing a platinum / carbon powder and a liquid Nafion polymer electrolyte, and then spraying and drying the mixture directly on a hydrogen ion exchange membrane. . In this method, Nafion itself was used as a binder to suppress the use of polytetrafluoroethylene to minimize the internal resistance of the electrode and to increase the density of the catalyst contained in the electrode catalyst layer.

The catalyst layer composition used in the above-mentioned method does not contain polytetrafluoroethylene which has strong hydrophobicity, and thus has high hydrophilicity. Therefore, the catalyst layer must be formed by coating an extremely thin catalyst layer on a decal plate and then thermally bonding the gas diffusion layer or directly on the Nafion polymer membrane. For this reason, there is still a problem that the electrode manufacturing process is complicated.

The technical problem to be achieved by the present invention is to provide a composition for forming a catalyst layer which can easily form a catalyst layer excellent in oxidation / reduction reaction and ion conductivity even when a small amount of platinum catalyst is contained.

Another object of the present invention is to provide a fuel cell having an excellent power density.

1 is a schematic cross-sectional view of a typical hydrogen ion exchange membrane fuel cell.

Figure 2 is a schematic diagram for explaining the catalyst layer formed in accordance with an embodiment of the present invention.

3 is a graph showing output characteristics of a fuel cell manufactured according to the present invention and a fuel cell manufactured according to a conventional method.

* Brief description of symbols for the main parts of the drawing

1,2. Gas diffusion electrode 3. conductive film

11. Platinum / Carbon Powder 12. Binder

14. Silica 15. Hydrogen Conductive Polymer

In order to achieve the above technical problem, in the present invention, in the composition for forming a catalyst layer of a hydrogen ion exchange membrane fuel cell comprising a platinum / carbon powder, a binder, and a hydrogen ion conductive polymer, the hydrogen ion conductive polymer is coated on a silica surface. A composition for forming a catalyst layer is provided.

In order to achieve the above technical problem, the present invention provides a fuel cell including two gas diffusion electrodes composed of a support layer and a catalyst layer, and a proton conductive film interposed between the two gas diffusion electrodes, wherein the catalyst layer is platinum / carbon. A fuel cell is provided comprising a powder, a binder, and silica coated with a hydrogen ion conductive polymer.

In the catalyst layer composition and the catalyst layer formed therefrom, the silica has a particle size of 7 to 50 nm, and the weight ratio of the hydrogen ion conductive polymer and the silica is preferably 3: 7 to 7: 3. In addition, the weight ratio of the total weight of the platinum / carbon powder and silica and the hydrogen ion conductive polymer is preferably 90:10 to 50:50.

The hydrogen ion conductive polymer is preferably perfluorocarbon sulfonic acid or perfluorocarbon sulfonic acid ionomer.

Hereinafter, the present invention will be described in detail with reference to the drawings.

2 is a schematic diagram illustrating a catalyst layer of a gas diffusion electrode formed according to an embodiment of the present invention, wherein reference numeral 11 is a platinum / carbon powder, reference numeral 12 denotes a binder, reference numeral 13 The silica 14 is coated with a hydrogen ion conductive polymer 15. As shown in FIG. 2, a hydrogen ion conductive polymer coated silica is present between the carbon particles, and a contact is made between the platinum catalyst and the hydrogen ion conductive polymer coated on the carbon particles.

When the catalyst layer is formed as shown in FIG. 2, the interface between the electrode and the conductive film has a three-dimensional structure, thereby increasing the utilization of the catalyst. For example, when hydrogen is supplied to the anode 1, protons are produced by an oxidation reaction of hydrogen on a platinum catalyst, and are transferred to the proton conductive film 3 through a hydrogen ion conductive polymer channel on the silica surface, which is a proton conductor ( See FIG. 1). The hydrogen ion conductive polymer is coated on the silica powder having a large specific surface area, and thus the platinum utilization rate is improved due to the large number of interface formation points between the platinum catalyst and the hydrogen ion conductive polymer. In addition, since the silica also has a moisture effect to suppress evaporation of moisture essential for maintaining the ion conductivity of the hydrogen ion conductive polymer, the composition for forming a catalyst layer of the present invention can be applied to a non-humidified fuel cell.

According to the present invention, a fuel cell is manufactured by a conventional method except for the manufacturing method and components of the gas diffusion electrode. That is, after the slurry is prepared by adding and mixing platinum / carbon powder and hydrogen ion conductive polymer-coated silica to the binder dispersion, the doctor blade method is used to form a catalyst layer on the support layer. Through this, a fuel cell as shown in FIG. 1 is manufactured.

In the hydrogen ion conductive polymer-coated silica used in the present invention, the weight ratio of the hydrogen ion conductive polymer and the silica is preferably 3: 7 to 7: 3. If the silica content exceeds the above range, it is difficult to form the polymer channel, so that the proton conductivity is not good. If the silica content is below the above range, the effect of increasing the interface formation point between the platinum catalyst / hydrogen ion conductive polymer is too small.

In addition, the hydrogen ion conductive polymer content in the catalyst layer should be maintained in an appropriate range. If the content of the conductive polymer is too small, it is difficult to form a channel between the hydrogen-ion conductive polymer coated on the silica, so that the proton conductivity is not good. If the content of the conductive polymer is too high, the proton conductivity improvement effect does not improve and the manufacturing cost increases. Because of this. In view of this, the ratio between the total weight of components other than the hydrogen ion conductive polymer, namely silica, platinum / carbon powder and binder, and the weight of the hydrogen ion conductive polymer is preferably 90:10 to 50:50.

Hereinafter, the effects of the present invention will be described in detail through Examples and Comparative Examples, but the present invention is not necessarily limited thereto.

Example 1

1 g of fumed silica and 10 ml of a 5 wt% Nafion solution were mixed, and then placed in a vacuum drying oven and dried at 100 ° C. Subsequently, silica was ground using a ball mill to prepare Nafion coated silica powder (weight ratio of silica and Nafion = 1: 0.5). 1 g of platinum / carbon powder (trade name: Vulcan XC72R) coated with platinum particles on carbon black, 0.3 g of Nafion coated silica powder, and 0.5 g of binder (polytetrafluoroethylene) After addition to the jar, a homogeneous mixture (A) was obtained by using a mixer and an ultrasonic dispenser.

The mixture (A) was placed on a support layer, cast using a doctor-blade, and then placed in a vacuum oven and dried at 180 ° C. to prepare an electrode coated with a catalyst layer on the support layer. At this time, a wet-proof carbon cloth (manufactured by E-Tek) loaded with 4 mg / cm 2 carbon black was used as the support layer.

The electrodes prepared in the above steps were superimposed on both surfaces of Nafion 117 (manufactured by DuPont) as a hydrogen ion exchange membrane. Subsequently, the electrode layer / polymer / electrode layer assembly was prepared by pressing at a pressure of 80 atm at 130 ° C. using a hot press.

After the single cell for fuel cell measurement was constructed using the manufactured conjugate, the output characteristics were measured according to the following conditions, and are shown in FIG. 3 (curve a).

Output characteristic measurement condition

Cell temperature: 80 ℃

Humidifier Temperature: 90 ℃

Hydrogen pressure: 3 atm

Oxygen pressure: 5 atm

Comparative example

A catalyst layer was formed on a carbon cloth (manufactured by E-Tek Co., Ltd.) using only polytetrapolyethylene as a binder, and then impregnated and dried with a 5 wt% Nafion solution to prepare an Nafion-loaded electrode having the same content as in Example It was. Subsequently, an electrode / polymer / electrode assembly was prepared, mounted in a single cell for fuel cell measurement, and output characteristics were measured in the same manner as in Example, and are shown in FIG. 3 (curve b).

As can be seen from the above, the fuel cell produced according to the present invention has the following advantages.

First, the interface between the catalyst layer and the proton conductive film has a three-dimensional structure, and the platinum utilization rate of the catalyst layer is improved, thereby reducing the manufacturing cost.

Second, since the proton formed at the portion other than the interface between the catalyst layer and the proton conductive film is also smoothly transferred, the output density is improved.

Third, the hydrogen ion conductive polymer content is reduced, thereby reducing the manufacturing cost.

Fourth, the electrode manufacturing process using the catalyst layer composition of the present invention is simple, which is advantageous for mass production.

Claims (10)

In the composition for forming a catalyst layer of a hydrogen ion exchange membrane fuel cell comprising a platinum / carbon powder, a binder and a hydrogen ion conductive polymer, The hydrogen ion conductive polymer is a composition for forming a catalyst layer, characterized in that the coating on the silica surface. The composition of claim 1, wherein the silica has a particle diameter of 7 to 50 nm. The composition of claim 1, wherein the weight ratio of the hydrogen ion conductive polymer and the silica is 3: 7 to 7: 3. According to claim 1, wherein the ratio of the total weight of the platinum / carbon powder and silica and the weight of the hydrogen ion conductive polymer is 90:10 to 50:50 composition for forming a catalyst layer, characterized in that The composition of claim 1, wherein the hydrogen ion conductive polymer is selected from the group consisting of perfluorocarbon sulfonic acid and perfluorocarbon sulfonic acid ionomer. Two gas diffusion electrodes composed of a support layer and a catalyst layer; And A fuel cell comprising a proton conductive film interposed between two gas diffusion electrodes, The catalyst layer comprises a platinum / carbon powder, a binder, and silica coated with a hydrogen ion conductive polymer. The fuel cell according to claim 6, wherein the silica has a particle diameter of 7 to 50 nm. The fuel cell of claim 6, wherein a weight ratio of the hydrogen ion conductive polymer to the silica is 3: 7 to 7: 3. The fuel cell of claim 6, wherein the ratio of the total weight of the platinum / carbon powder and the silica to the weight of the hydrogen ion conductive polymer is 90:10 to 50:50. 7. The fuel cell of claim 6, wherein the hydrogen ion conductive polymer is selected from the group consisting of perfluorocarbonsulfonic acid and perfluorocarbonsulfonic acid ionomers.