KR100842298B1 - Preparation of pt-co electrode catalysts for polymer-electrolyte membrane and direct-methanol fuel cells - Google Patents

Abstract

A preparation method of an electrode catalyst with high activity for polymer electrolyte fuel cells and direct methanol fuel cells by supporting Pt particles with a small particle size and a low oxidation degree onto a surface of a Co-supported carbon at a high dispersion degree through the chemical vapor deposition process is provided. A preparation method of an electrode catalyst for polymer electrolyte fuel cells and direct methanol fuel cells comprises: a cobalt supporting step(A) of adding an aqueous cobalt precursor solution to a carbon support to support a cobalt precursor with the carbon support, and heating the cobalt precursor-supported carbon support to remove water, decompose the cobalt precursor, and reduce a cobalt surface; and a platinum supporting step(B) of injecting a platinum precursor vapor onto a cobalt catalyst supported onto carbon by a pulse to perform a chemical vapor deposition of the platinum precursor vapor on the cobalt catalyst, and decomposing a platinum precursor by heating the platinum precursor-deposited cobalt catalyst while flowing a gas in which hydrogen is mixed with nitrogen at a mixing ratio of 1:3 to 1:5 onto a platinum precursor-deposited cobalt catalyst. The preparation method further comprises a pre-treatment step of heat-treating the carbon support in an air atmosphere and cleaning the heat-treated carbon support in prior to the cobalt supporting step(A).

Description

Preparation of Pt-Co electrode catalysts for polymer-electrolyte membrane and direct-methanol fuel cells

Figure 1 shows the platinum particle size of the platinum-cobalt catalyst prepared in Example 2 and Comparative Example 1 by X-ray diffraction (X-ray diffraction).

       FIG. 2 shows the platinum surface areas of the platinum-cobalt catalysts prepared in Example 2 and Comparative Example 1 as hydrogen adsorption regions measured by cyclic-voltammetry.

       Figure 3 shows the activity per platinum mass of the platinum-cobalt catalyst prepared in Examples and Comparative Examples.

       Figure 4 shows the concentration of platinum exposed to the catalyst surface in the platinum-cobalt catalyst prepared in Example 2 and Comparative Example 1 by X-ray photoelectron spectroscopy (X-ray photoelectron spectroscopy).

FIG. 5 shows the change in the surface area of platinum by heat in the platinum-cobalt catalysts prepared in Example 2 and Comparative Example 1 as a hydrogen adsorption region measured by cyclic-voltammetry.

The present invention relates to a method for producing a carbon-supported platinum-cobalt electrode catalyst for use in a polymer electrolyte fuel cell and a direct methanol fuel cell.

Fuel cells are divided into various types according to operating temperatures and types of main fuels. The polymer electrolyte fuel cells and the direct methanol fuel cells operated at low temperatures are particularly dependent on the activity of the electrode catalyst.

Platinum has been used as an electrode catalyst since the development of polymer electrolyte fuel cells and direct methanol fuel cells. However, platinum has high activity but high cost. Therefore, alloy catalysts that can increase the activity of platinum while reducing the amount of platinum has been steadily researched.

Platinum-cobalt alloy catalysts for fuel cells are generally used because of their higher activity than platinum and the highest activity among alloy catalysts ( J. Electrochem. Soc., 1994 , 141, 2659-2668). However, since most platinum-cobalt catalysts are prepared by impregnation, most of the added cobalt is present on the surface of carbon rather than platinum. Recent studies have shown that cobalt present in carbon easily elutes under fuel cell operating conditions, contaminating the membrane used in the fuel cell, and the contaminated membrane causes a sharp drop in electrical conductivity, resulting in a decrease in electrode activity. ( J. Power Sources, 2005 , 144, 11-20).

In order to make up for the disadvantages of impregnation, cobalt added through chemical vapor deposition, which is one of the semiconductor processes, is not present on the carbon surface but selectively deposited only on the platinum surface. Cases have been reported to reduce the chemical properties and increase the catalytic performance ( J. Power Sources, 2006, Avaliable online 27 ).

However, the platinum-cobalt catalyst prepared by the impregnation method or chemical vapor deposition method has better activity than the platinum catalyst, but as the amount of added cobalt increases, the surface of platinum is gradually increased by cobalt. The surface area of the catalyzed platinum is reduced, resulting in a decrease in activity.

An object of the present invention is to prepare a highly active polymer electrolyte fuel cell and direct methanol fuel cell electrode catalyst by supporting a small dispersion of platinum particles of low size and low oxidation rate through chemical vapor deposition on the surface of cobalt supported on carbon. To provide a way.

Method for producing a polymer electrolyte fuel cell and a direct methanol fuel cell electrode catalyst of the present invention for achieving the above object,

A cobalt supporting step (A) in which a cobalt precursor aqueous solution is added to the carbon carrier to support the cobalt precursor, and the temperature is increased to remove water, decompose the cobalt precursor, and reduce the cobalt surface;

Platinum supporting step (B) to decompose the platinum precursor by heating the gas while the gas is hydrogen 1: nitrogen ratio 1: 3 ~ 1: 5 by injecting a platinum precursor vapor in a pulse to the cobalt-containing carbon pulse It includes.

Preferably, prior to the cobalt loading step (A) may comprise a pre-treatment step of washing by heating the carbon carrier in an air atmosphere.

Cobalt precursor is used by selecting one or two or more from cobaltacetylacetonate hydrate (cobaltacetylacetonatehydrate), cobalt chloride hexahydrate (cobaltchloridehexahydrate), cobalt nitrate hexahydrate (cobaltnitratehexahydrate),

Platinum precursors include platinum acetylacetonate, tetrakis (trifluorophosphine) platinum, dichloro (dicarbonyl) platinum, (trimethyl) methylcyclo 1 or 2 or more selected from (trimethyl) methylcyclopentadienylplatinum, trimethylcyclopentadienylplatinum, and trimethylacetylacetonate platinum.

Hereinafter, the configuration of the present invention in more detail step by step.

First, pretreatment of the carbon carrier is performed by heat treating the carbon carrier at an air temperature of 400 to 600 ° C. for 1 to 3 hours and then washing with water. This increases the surface area per unit mass of the carbon carrier.

The cobalt loading step (A) is to immerse the carbon carrier in an aqueous solution of cobalt precursor, to support the cobalt precursor in a carbon carrier, to raise the temperature to remove water, to decompose the cobalt precursor, and to reduce the cobalt surface. This is a known method.

The amount of cobalt precursor used varies depending on the amount of cobalt supported on the electrode catalyst to be prepared, but is preferably mixed at a ratio of 2 to 20 parts by weight based on 100 parts by weight of carbon carrier, and water removal is performed at 100 to 150 ° C. for 10 to 30 minutes. The decomposition of the cobalt precursor proceeds for 1 to 2 hours at 220 to 370 ° C, and the reduction of the cobalt surface is carried out for 1 to 5 hours at 375 to 600 ° C.

The water removal, decomposition and reduction process proceeds by flowing a gas with a hydrogen to nitrogen ratio of 1: 5.

Platinum supporting step (B) is a chemical vapor deposition by injecting a platinum precursor vapor on the surface of the cobalt catalyst supported on carbon in a pulse, and then heated up to decompose the platinum precursor to support the platinum, put the platinum precursor in a vaporizer and the platinum precursor Depending on the type of while maintaining at 5 ~ 150 ℃ flow to the reactor containing the cobalt-carbon catalyst prepared in step (A).

Here, the concentration of the platinum precursor, the number of pulses, and the injection amount per pulse vary depending on the platinum loading amount of the electrode catalyst to be prepared, and the platinum precursor is preferably injected by 0.1 to 5 cc using a sampling loop.

       Subsequently, the adsorbed platinum precursor is decomposed to proceed for 1 to 2 hours at 200 to 300 ° C. while flowing a gas having a ratio of hydrogen: nitrogen 1: 3 to 1: 5.

The cobalt catalyst supported on the carbon prepared in step (A) is present in the metal state instead of the cobalt in the oxidation state through the reduction process, and the reduced cobalt-carbon catalyst and the platinum precursor including hydrogen react in step (B). When the platinum precursor is reacted only with cobalt adsorbed hydrogen, not carbon. Subsequently, it is completely reduced to platinum particles through decomposition and reduction to exist on the cobalt surface.

Chemical vapor deposition by injecting the vapor of the platinum precursor into the pulse is the most characteristic step in the present invention, in the case of the conventional impregnation method, while the platinum precursor added to the cobalt-carbon catalyst is present on the surface of carbon as well as cobalt In the present invention, the biggest difference is that the platinum precursor is present only on the surface of cobalt.

In addition, there are two main effects resulting from such a difference.

(1) The dispersion of platinum is excellent.

In the conventional impregnation method, the aqueous solution of the platinum precursor is supported on the cobalt-carbon catalyst. In this process, the aqueous solution of the platinum precursor is difficult to evenly disperse in the cobalt-carbon catalyst. It is present in carbon. After that, since the decomposition and reduction process at a relatively high temperature of 300 ~ 400 ℃ has a disadvantage that the platinum particles are relatively large by heat.

However, in the present invention, since the platinum precursor in the gaseous state reacts with cobalt, the platinum particles can be supported by smaller particles than the impregnation method. In addition, since the decomposition and reduction process is less than 300 ℃ and relatively lower than the processing temperature of the catalyst prepared by the impregnation method, the platinum particles are less affected by heat. As a result, the platinum surface is small because the platinum particles are small and platinum is present only on the cobalt surface.

As shown in Figures 1 and 2, in the case of the platinum-cobalt catalyst prepared by chemical vapor deposition method, the size of the platinum particles is very small compared to the impregnation method so that the observation of the platinum (111) peak in XRD is very small, and the circulating voltage The platinum surface area observed by the ampere method is also relatively large.

(2) The thermal stability of platinum particles is excellent.

The platinum-cobalt catalyst is heat treated at 500 to 600 ° C. for 1 to 2 hours after the preparation of the catalyst. In this process, an alloy is formed between platinum and cobalt to increase activity. However, due to the high temperature heat, the platinum particles of the platinum-cobalt catalyst become large due to the sintering phenomenon, thereby reducing the active surface area.

In the method of the present invention, since the platinum particles are selectively highly dispersed only on the cobalt surface, the interaction between platinum and cobalt becomes stronger, and the sintering of platinum is suppressed.

The construction of the present invention will be further clarified by the following examples, and the effect thereof will be demonstrated in comparison with the comparative example.

<Example 1>

Carbon was pretreated, and the cobalt precursor was supported by impregnation, and then the platinum precursor was supported by chemical vapor deposition to prepare a catalyst.

A. Sample

(1) Carbon carrier: Carbon black (Vulcan XC-72) with surface area of 220m2 / g

(2) Cobalt precursor: cobalt nitrate hexahydrate (Co (NO ₃ ) _{2 ˙} 6H ₂ O)

(3) Platinum precursors: tetrakis (trifluorophosphine) platinum (Pt (PF ₃ ) ₄ )

B. Preparation of Catalyst

(1) pretreatment of carbon carrier

Under air atmosphere, 2 g of the carbon carrier was heat treated at 500 ° C. for 2 hours, washed 5 times with water, dried at 100 ° C., and then ground with a glass rod.

The surface area of the carbon carrier after pretreatment is 303 m 2 / g.

(2) cobalt support

2 g of the carbon carrier after the pretreatment was immersed in an aqueous solution of 1.1 g of cobalt nitrate hexahydrate dissolved in 40 ml of water. The cobalt precursor was dipped into the carbon carrier, the water was removed at 120 ° C. for 30 minutes, and the cobalt precursor was heated at 220 ° C. for 60 minutes. The cobalt surface was reduced by decomposition and maintained at 500 ° C. for 60 minutes.

Here, the water removal, decomposition and reduction process was performed while flowing a gas of the ratio of hydrogen: nitrogen 1: 5.

(3) platinum support

0.1 g of cobalt-carrying carbon was placed in a reactor. Meanwhile, 5 g of tetrakis (trifluorophosphine) platinum as a platinum precursor was placed in a vaporizer and vaporized at 5 ° C. Then, the vapor of the platinum precursor thus obtained was 1 cc each. The reactor was injected through a sampling loop 60 times at minute intervals. Hydrogen was used as a carrier gas of the platinum precursor, and after the injection was completed, the temperature of the reactor was raised to 200 ° C. and maintained for 1 hour.

The amount of platinum supported on the platinum-cobalt electrocatalyst prepared through the above process was 4 wt.%.

      <Example 2>

A catalyst was prepared in the same manner as in Example 1 except that the amount of tetrakis (trifluorophosphine) platinum injected into the reactor was changed from 60 times to 120 times.

The amount of platinum supported on the platinum-cobalt electrocatalyst was 6.7 wt.%.

Comparative Example 1

The carbon carrier was pretreated (1) in the same manner as in Example 1, and cobalt was supported (2) in the same manner as in Example 1, and then platinum was supported by impregnation (prior art) to form a platinum-cobalt electrode catalyst. Was prepared.

(1) pretreatment of carbon carrier

Same as Example 1

(2) cobalt support

Same as Example 1

(3) platinum support

2g of cobalt-containing carbon prepared in step (2) was added to an aqueous solution of 0.29g of hexachloroplatinic acid (H ₂ PtCl ₆ ) as a platinum precursor, and 40ml of water was used to support the platinum precursor. The gas was flowed at a ratio of 1: 5 and decomposed at 350 ° C. for 60 minutes.

The amount of platinum supported on the platinum-cobalt electrocatalyst through the above process was 10wt.%.

      Comparative Example 2

Commercial platinum-carbon catalysts with 10 wt.% Platinum (Johnson & Matthey Co.)

      <Example 3>

The physical properties of the catalysts prepared in Examples 1 and 2 and Comparative Example 1 and the catalyst of Comparative Example 2 were compared. The result is as follows.

       (1) Oxygen reduction reaction activity

3 is a result of measuring the oxygen reduction reaction activity of the catalyst prepared in Examples 1 and 2 and Comparative Examples 1 and 2 through a half-cell experiment.

Although the platinum-cobalt catalysts prepared in Examples 1 and 2 had less platinum than the catalysts prepared in Comparative Examples 1 and 2, it can be seen that the activity per mass is high. In particular, the catalyst prepared in Example 2 is about 42% higher than the catalyst prepared in Comparative Example 1.

(2) Size, surface area and dispersion of platinum particles

The size of the platinum particles can be calculated from the half width of the platinum (111) peak in the spectrum obtained by X-ray diffraction.

According to the results of FIG. 1, the size of the platinum particles of the platinum-cobalt catalyst prepared in Example 2 is very small so that the platinum (111) peak cannot be distinguished, and the cyclic voltammetry hydrogen adsorption shown in FIG. It can be seen that the platinum surface area measured by the amount is larger than that of the catalyst prepared by Comparative Example 1.

FIG. 4 shows the concentration of platinum exposed on the catalyst surface measured by X-ray photoelectron spectroscopy. In the case of the platinum-cobalt catalyst prepared by Example 2, the amount of platinum exposed on the surface of the catalyst is approximately two times higher than that of the catalyst prepared by Comparative Example 1.

Therefore, it can be seen that the platinum-cobalt catalyst prepared according to the example has a small particle size of platinum and a high degree of dispersion. This highly dispersed platinum has high catalytic activity because of its large area of contact with the reactants.

(3) thermal stability of platinum particles

FIG. 5 shows the platinum surface area before and after applying a heated catalyst at 500 ° C. to measure the thermal stability of the catalyst by cyclic voltammetry.

      In Example 2, the platinum surface area before and after applying the heat did not show a large change in platinum-cobalt, but in Comparative Example 1, the platinum surface area decreased by about 50% after applying the heat. In the examples, since the platinum particles are selectively and highly dispersed only in the cobalt, it is determined that the interaction between the platinum and the cobalt becomes stronger and the sintering of the platinum is suppressed.

According to the present invention, an electrode catalyst for a highly active polymer electrolyte fuel cell and a direct methanol fuel cell can be produced by supporting small platinum particles on a cobalt-carbon carrier with high dispersion.

Since polymer electrolyte fuel cells and direct methanol fuel cells operate at low temperatures, it is necessary to use platinum catalysts in which a large amount of platinum is supported at high dispersion. Particularly, the oxygen reduction reaction occurring at the cathode is more slow because the reaction rate is slower than the hydrogen oxidation reaction occurring at the anode. According to the present invention, the platinum particles are small in size and have a high dispersion so that a catalyst having a large platinum surface area can be prepared. Since the exposed platinum concentration is relatively high, a highly efficient fuel cell electrode can be manufactured even with a small amount of platinum.

Claims (4)

A cobalt supporting step (A) in which a cobalt precursor aqueous solution is added to the carbon carrier to support the cobalt precursor, and the temperature is increased to remove water, decompose the cobalt precursor, and reduce the cobalt surface; Platinum precursor vapor is injected into a cobalt catalyst supported on carbon by pulse injection, followed by chemical vapor deposition. A platinum supporting step of decomposing a platinum precursor by heating up while flowing a gas having a hydrogen: nitrogen ratio of 1: 3 to 1: 5 (B) Method for producing a polymer electrolyte fuel cell and an electrocatalyst for direct methanol fuel cell comprising a). The method according to claim 1, wherein a pretreatment step of heat treating and washing the carbon carrier in an air atmosphere prior to the cobalt supporting step (A) is provided. The electrode for a polymer electrolyte fuel cell and a direct methanol fuel cell according to claim 1 or 2, wherein the cobalt precursor is selected from cobalt acetylacetonate hydrate, cobalt chloride hexahydrate, and cobalt nitrate hexahydrate. Method for preparing a catalyst. 3. The platinum precursor of claim 1, wherein the platinum precursor is platinum acetylacetonate, tetrakis (trifluorophosphine) platinum, dichloro (dicarbonyl) platinum, (trimethyl) methylcyclopentadienyl platinum, trimethyl A method for producing an electrode catalyst for a polymer electrolyte fuel cell and a direct methanol fuel cell, characterized in that at least one selected from cyclopentadienyl platinum and trimethylacetylacetonate platinum.

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