KR20050047913A - Pt-based alloy catalyst for use of direct formic acid fuel cell - Google Patents

Abstract

The present invention provides a cathode catalyst based on platinum-gold. The negative electrode catalyst of the present invention preferably contains platinum and gold as a binary metal catalyst, and is not particularly limited and preferably contains 10 to 90 mol% of platinum and 10 to 90 mol% of gold. In the negative electrode catalyst according to the present invention, an appropriate solvent (for example, water) is selected to dissolve a metal salt that is a raw material of the catalyst, and then a raw material of each catalyst is prepared by a liquid reduction method in which a predetermined reducing agent is used. It is prepared by lyophilization.

Description

Platinum-based alloy catalyst for direct formic acid fuel cell {Pt-based Alloy Catalyst for Use of Direct Formic Acid Fuel Cell}

The present invention relates to a composite catalyst for direct formic acid fuel cells based on platinum, and more particularly to the development of a cathode (anode) material, which is a key material that influences the performance of a direct formic acid fuel cell (DFAFC). Is directed to a cathode catalyst composed of platinum (Pt) and gold (Au) as a cathode catalyst of a formic acid fuel cell (DFAFC).

Recently, due to the rapid spread of portable electronic devices and wireless communication devices, much attention and research is being conducted on the development of fuel cells as a battery as a portable power supply device, a fuel cell for a pollution-free automobile, and a power generation fuel cell as a clean energy source.

A fuel cell is a power generation system that converts chemical energy of fuel gas (hydrogen, methanol, or other organic substance) and oxidant (oxygen or air) directly into electrical energy through an electrochemical reaction. Battery, molten carbonate electrolyte fuel cell, phosphate electrolyte fuel cell and polymer electrolyte fuel cell.

The polymer electrolyte fuel cell (Polymer Electrolyte Membrane Fuel Cell) is subdivided directly into a hydrogen ion exchange membrane fuel cell (Proton Exchange Membrane Fuel Cell: PEMFC) that uses hydrogen gas as fuel and liquid methanol directly supplied as a fuel Methanol Fuel Cell (DMFC). Recently, fuel cells using various fuels such as ethanol, propanol, and formic acid have been researched and developed to solve problems of direct methanol fuel cells. Polymer electrolyte fuel cell is a clean energy source that can replace fossil energy. It has high power density and energy conversion efficiency, can operate at room temperature, and can be miniaturized and encapsulated. It can be widely used in fields such as portable power supply and military equipment.

In general, a gaseous fuel cell using hydrogen as a fuel has an advantage of high energy density, but requires attention to handling hydrogen gas and a fuel for reforming methane, methanol, and natural gas to produce hydrogen, which is a fuel gas. There is a problem that requires additional equipment such as a reforming device.

In addition, liquid fuel cells using liquids have lower energy densities than gas types, but are easy to handle in a liquid state, have a low operating temperature, and do not require a fuel reforming device. However, a platinum-ruthenium (Pt-Ru) binary catalyst has been developed and commercialized to improve the slow reaction rate of the methanol oxidation reaction and resistance to the catalytic poisoning effect of the carbon monoxide on the platinum catalyst. Nevertheless, research on the three- or four-way catalyst based on platinum-ruthenium is still underway due to its low activity against methanol oxidation. In addition, the methanol cross-over phenomenon, in which the methanol to be oxidized at the cathode, is passed through the polymer electrolyte to the cathode (cathode), leads to poisoning of the anode catalyst and deterioration of overall fuel cell performance and fuel utilization. For this reason, there are restrictions on the methanol concentration available.

On the other hand, in a direct formic acid fuel cell using formic acid as a fuel, formic acid is a strong electrolyte, which facilitates the transfer of electrons and hydrogen ions from the cathode, and has a lower output density per weight than methanol. In addition, the crossover of the fuel can be drastically reduced, and the solution can be used up to 20 molar concentration (20 M), which is about 70% by weight in the solution.

Referring to FIG. 1, a fuel cell has a structure in which a hydrogen ion exchange membrane 11 is interposed between a cathode and an anode. The hydrogen ion exchange membrane has a thickness of 30 to 300 µm and is made of a solid polymer electrolyte, and the cathode and the anode each support the support layers 14 and 15 for the supply of reactants and the catalyst layer 12 in which the oxidation / reduction reactions of the reactants occur. And a gas diffusion electrode (collectively referred to as a gas diffusion electrode), which is referred to as (13), and a current collector (16) and (17).

At the cathode of the formic acid fuel cell, a formic acid oxidation reaction occurs and the generated hydrogen ions and electrons move to the anode. The hydrogen ions moved to the anode are combined with oxygen to cause oxidation, and the electromotive force generated by the oxidation / reduction reaction of the hydrogen ions becomes a fuel cell energy generating source. At this time, the reaction occurring at the cathode and the anode is as follows.

Cathode (anode): HCOOH → CO ₂ + 2H ⁺ + 2e ^- E _a = -0.22 V

A positive electrode _{(cathode): 1 / 2O 2 + 2H} + + 2e - → H 2 O E c = 1.23 V

Total reaction: HCOOH + 1 / 2O ₂ → CO ₂ + H ₂ OE _cell = 1.45 V

As can be seen from the above equation, since the performance of the negative electrode material has a large effect on the overall performance of the fuel cell, the development of a catalyst for excellent formic acid oxidation is a core technology of direct commercialization of the formic acid fuel cell.

The oxidation reaction of formic acid with pure platinum catalysts is explained by a dual pathway mechanism (C. Rice, S. Ha, RI Masel, P. Waszczuk, A. Wieckowski, T. Barnard, J. Power Sources 2002, 11, 83). The most preferred reaction route is the direct generation of carbon dioxide (CO ₂ ) through a dehydrogenation reaction that does not produce carbon monoxide (CO), the reaction intermediate.

^{_{HCOOH → CO 2 + 2H + +}} 2e - (1)

The carbon dioxide produced by the above reaction prevents the production of carbon monoxide and thus increases the overall reaction rate. The second reaction, which takes place through the dehydration reaction, leaves carbon monoxide adsorbed on the surface of the platinum catalyst, similar to the methanol oxidation reaction.

HCOOH + Pt → Pt-CO + H ₂ O (2)

_{Pt + H 2 O → Pt-} OH + H + + e - (3)

Pt-CO + Pt-OH → 2Pt + CO 2 + 2H + + 2e - (4)

Total ^{_{reaction: HCOOH → CO 2 + 2H +}} + 2e - (5)

By the above reaction, formic acid is adsorbed on the surface of the platinum catalyst to form carbon monoxide, a reaction intermediate, and adsorption of OH groups is required for complete oxidation to carbon dioxide. The platinum catalyst under the operating voltage region of the fuel cell is difficult to adsorb OH groups, so it is preferable to proceed along the first reaction path, that is, dehydrogenation reaction.

It has been reported that the activity of platinum catalysts can be increased by synthesizing palladium on platinum to enhance the activity of formic acid oxidation. This increase in activity is understood that the oxidation reaction of formic acid under platinum-palladium catalyst is directly converted to carbon dioxide without the production of an intermediate carbon monoxide as in reaction (1). That is, the addition of palladium is known to act as a third body promotor that selectively blocks the active site of the platinum surface and inhibits the adsorption of carbon monoxide. In addition, the synthesis of ruthenium into platinum in direct formic acid fuel cells, as in direct methanol fuel cells, has been reported to improve the resistance to the catalytic poisoning effect of carbon monoxide in platinum catalysts. It is understood that the improvement in carbon monoxide resistance is due to the ability of ruthenium to adsorb water molecules in the chemical potential to which formic acid is adsorbed, and thus the activity is enhanced by the interaction between metals through this bifunctional mechanism. Eventually it improves reaction (4) (P. Waszczuk, TM Barnard, C. Rice, RI Masel, A. Wieckowski, Electrochem. Comm. 2002, 4, 599).

The platinum-gold cathodic catalyst developed by the present invention has a high activity against the formic acid oxidation reaction by interacting with the role of the third promoter and induction of a selective reaction route, and also has the ability to suppress the formation of by-products generated during the oxidation reaction. Have That is, gold is not a substance that satisfies both functional effects, but it functions to induce an oxidation reaction of formic acid to a reaction that is directly converted to carbon dioxide without producing carbon monoxide.

Knowledge of the relationship between phase equilibrium, interatomic bond strengths, and catalyst activity may be a guide in determining the component selection and combination ratios of catalyst compositions, but the results of the studies to date indicate that catalyst components make up the catalyst. No general law has been found to calculate the component selection and combination ratio of.

Therefore, the present inventors experimented with various transition metals to develop a cathode catalyst having better performance than the anode catalyst of the prior art, and as a result, the platinum-gold anode catalysts of the present invention suppressed the production of carbon monoxide acting as a catalyst poison. It was confirmed that the activity can be kept high, and that the improvement of the activity is maintained even in a high concentration of formic acid, thereby obtaining a fuel cell having excellent performance and completing the present invention.

Accordingly, an object of the present invention is to provide a synthetic catalyst for direct formic acid fuel cell based on platinum-gold, which has the advantage of resistance to carbon monoxide and high formic acid oxidation activity.

Another technical problem of the present invention is to obtain each catalyst raw material by a liquid phase reduction method in which the platinum-based direct formic acid fuel cell synthesis catalyst is reduced using water as a solvent and a metal salt serving as a catalyst as a reducing agent. It is to provide a method for producing a cathode catalyst comprising a method for lyophilization.

In order to achieve the above technical problem, the present invention provides a cathode catalyst based on platinum-gold.

The negative electrode catalyst of the present invention is preferably a binary metal catalyst composed of platinum and gold, and is not particularly limited and preferably contains 10 to 90 mol% of platinum and 10 to 90 mol% of gold.

The present invention preferably provides a multi-component synthesis catalyst further comprising at least one metal element in the platinum-gold binary synthesis catalyst. Examples of metals that may be included in the multicomponent synthesis catalyst include one or two or more metals selected from the group VIIIA of the periodic table, preferably Fe, Co, Ni, Ru, Rh, Pd, and Ir. have. The content of the third additional metal component is not particularly limited and is preferably included in the range of 5 to 90 mol%. In this case, 5 to 90 mol% of platinum and 5 to 90 mol% of gold are contained preferably.

The present invention provides binary catalysts in which the platinum-gold synthesis catalyst is supported on a catalyst carrier material. The material that can be used as the carrier is porous carbon, conductive polymers, metal oxides and the like. Examples of porous carbons include activated carbon, carbon fibers, graphite fibers and carbon nanotubes. Examples of the conductive polymer include polyvinylcarbazole and its derivatives, polyaniline and its derivatives, polypyrrole and its derivatives, and the like. Metal oxides include tungsten oxide, titanium oxide, nickel oxide, ruthenium oxide, tantalum oxide, cobalt oxide, and the like.

The catalyst carrier material is preferably used in an amount of 2 to 90% by weight based on the platinum-gold synthesis catalyst.

The present invention provides a fuel cell electrode including each synthesis catalyst and a direct formic acid fuel cell including the same fuel cell electrode.

Hereinafter, the cathode catalyst manufacturing method of the present invention will be described in more detail.

In order to prepare a cathode catalyst according to the present invention, a raw material of each catalyst is prepared by a liquid phase reduction method in which a suitable solvent (for example, water) is selected to dissolve a metal salt that is a raw material of the catalyst and then reduced with a predetermined reducing agent. There is a method for producing the obtained precipitate by lyophilization.

The metal salts of the platinum and gold are not particularly limited, but chlorides, nitrides, sulfates and the like containing the metals may be used. Specific examples of the metal salts include metal salts of platinum (H ₂ PtCl ₆ · xH ₂ O) and metal salts of gold (HAuCl ₄ · xH ₂ O). In the present invention, Aldrich's metal salt is used, but other metal salts of the same composition may be used, and other types of metal salts such as K ₂ PtCl ₆ may be used.

An appropriate amount is taken so that the above-mentioned metal salts become the molar ratios to be the targets of the respective implementations, each of which is added to distilled water and stirred at room temperature.

The pH of the mixed metal salt solution is preferably adjusted to 7 to 8, more preferably 8, and then a predetermined reducing agent is added thereto at a time to reduce the metal salts to obtain a precipitate. The obtained precipitate is washed with distilled water and then lyophilized to finally obtain a synthetic catalyst, and X-ray diffraction analysis is performed on the metal catalyst prepared by the above method. In the X-ray diffraction analysis, it was confirmed that the binary catalyst was properly synthesized by confirming the peak position for each metal element. The reducing agent is not particularly limited, nor is, using preferably sodium borohydride (NaBH _4), hydrazine (N ₂ H _4), sodium formate (HCOONa) and the like.

In order to compare the activity of the formic acid oxidation reaction, a 0.5 mole sulfuric acid solution and 2 moles were prepared by using a three-electrode cell having Pt wire as a counter electrode and Ag / AgCl as a reference electrode at room temperature. The activity of the catalyst is determined by measuring the oxidation current density in a molar formic acid / 0.5 molar sulfuric acid solution. Even when the same metal is used, the activity against formic acid is different depending on the synthesized composition ratio.

The above-described method for preparing a negative electrode catalyst may be applied to a method of preparing a catalyst by supporting the catalyst on a conductive polymer or a porous metal oxide as well as porous carbon such as activated carbon, carbon fiber, and carbon nanotubes in order to reduce the amount of catalyst used. Examples of the conductive polymer include polyvinylcarbazole and its derivatives, polyaniline and its derivatives, polypyrrole and its derivatives, and the like. Metal oxides include tungsten oxide, titanium oxide, nickel oxide, ruthenium oxide, tantalum oxide, cobalt oxide, and the like. The catalyst carrier material is preferably used in an amount of 2 to 90% by weight based on the platinum-gold synthesis catalyst.

Hereinafter, the present invention will be described in more detail with reference to Examples. These examples are only for illustrating the present invention, and the protection scope of the present invention is not limited by these examples.

Example 1 Preparation and Performance Test of PtAu Binary Cathode Catalyst

0.510 g and 0.402 g of the metal salt of platinum (H ₂ PtCl ₆ .xH ₂ O) and the metal salt of gold (HAuCl ₄ .xH ₂ O), respectively, were taken so as to be the molar ratio of the subject. Each of the metal salts was added to 510 ml of distilled water, and then stirred at room temperature (25 ° C.) for 3 hours, and then mixed with each of the metal salt solutions, and stirred for 3 hours. In the present embodiment, Aldrich's metal salt is used, but metal salts such as Sigma, Samhwa Industrial Co., Ltd. of the above composition may be used, and other types of metal salts such as K ₂ PtCl ₆ may be used.

After adjusting the pH of the mixed metal salt solution to 8, an excess of 2 molar sodium borohydride (NaBH ₄ ) aqueous solution (3 times the stoichiometric requirement) as a reducing agent was added to the mixed metal salt solution at a time to reduce the metal salts. A precipitate was obtained. The precipitate obtained was washed three times with distilled water and lyophilized for 12 hours to finally obtain a synthetic catalyst, and a performance measurement experiment was performed on the catalyst thus prepared.

2 shows the results of X-ray diffraction analysis for various molar compositions made of the above metals. As a result, the synthetic catalyst did not show a single peak for platinum and gold, shifted to a lower angle than the peak position in the presence of platinum alone, and a higher angle than the peak position in the presence of gold alone. It was confirmed that the movement toward). This confirmed that platinum and gold were properly synthesized.

3 is a graph in which a change in current is measured by sequentially applying a range of voltages in a 2 mole formic acid / 0.5 mole sulfuric acid solution, and comparing the measured current divided by the amount of catalyst used for relative comparison. This electrochemical analysis was performed using a three-electrode cell having a Pt wire as a counter electrode and Ag / AgCl as a reference electrode at room temperature. Determination of catalytic activity was made to compare the activity for formic acid oxidation in a 0.5 molar sulfuric acid solution and a 2 molar formic acid / 0.5 molar sulfuric acid solution. Even though the same metal was used, the formic acid oxidation reaction was different according to the synthesized composition ratio, and as shown in FIG. 3, the binary catalyst of the present example had a higher oxidation current density at the same voltage than that of pure platinum. It showed improved catalytic activity than the existing catalyst.

4 is a graph comparing unit cell performance test results of the PtAu binary catalyst and the pure platinum catalyst of this example in a 5 molar formic acid solution and oxygen supply conditions. The amount of each cathodic catalyst was 5 mg / cm 2, and the cathode catalysts were all 5 mg / cm 2 of Johnson Matthey. Nation Pt black ^? It was used to bond with 117 to form a MEA (membrane electrode assembly). The unit cell tested was 2㎠ It was sized and fed a formic acid solution and oxygen through a graphite channel at a flow rate of 0.2 to 2 cc / min and 300 to 1000 cc / min. As a result of the test, when pure Pt catalyst was used at 0.3V, a current density of 212 mA / cm 2 was obtained, but when the PtAu binary catalyst prepared in Example 1 was used, 243 mA / cm 2 showed an improvement of about 15%.

FIG. 5 is a graph comparing unit cell performance tests when 10M formic acid solution is used under the same experimental conditions as FIG. 4. As a result of the test, when pure Pt catalyst was used at 0.3V, a current density of 288 mA / cm 2 was obtained. However, when the PtAu binary catalyst prepared in Example 1 was used, the performance was improved by about 438 mA / cm 2 to about 52%. In addition, when 10M formic acid was used than when 5M formic acid was used as a fuel, the performance was improved by 25% for pure Pt catalyst but about 80% for PtAu catalyst. This showed that the higher the concentration of fuel (formic acid) used, the greater the increase in activity by the PtAu catalyst.

The cathode catalyst manufacturing method may be applied to a method of preparing by supporting a support on a porous carbon, a conductive polymer and a porous metal oxide such as activated carbon, carbon fiber and carbon nanotubes.

<Comparative Example 1> Comparison of unit cell manufacturing and performance test using PtRu and PtPd binary anode catalyst

Based on the results measured in Example 1, a performance comparison test with a unit cell using a conventional binary catalyst was performed using a PtAu catalyst (mol composition 1: 1) having excellent oxidation activity of formic acid.

Platinum metal salts (H ₂ PtCl ₆ ·) are used to prepare platinum-palladium catalysts with a high molar ratio 1: 1 platinum-ruthenium catalyst and direct formic acid fuel cell anode catalyst. xH ₂ O) and ruthenium metal salt (RuCl ₃ xH ₂ O) were 0.675 g and 0.280 g in 676 ml, respectively, or platinum metal salt (H ₂ PtCl ₆ xH ₂ O) and palladium metal salt so as to be the molar ratio of the test subject. 0.663 g and 0.235 g of PdCl ₂ were added to 664 ml of distilled water, respectively, and stirred at room temperature (25 ° C.) for 3 hours, followed by mixing the two metal salt solutions, respectively, for another 3 hours. Stirred.

After adjusting the pH of the metal salt mixed solutions to 8, an excess of 2 molar sodium borohydride (NaBH ₄ ) aqueous solution (3 times the stoichiometric requirement) as a reducing agent was added to the mixed solution of the metal salts at a time to reduce the metal salts. To obtain a precipitate.

The precipitate obtained was washed three times with distilled water and lyophilized for 12 hours to finally obtain a synthetic catalyst, and performance measurement experiments were performed on the catalyst thus prepared.

6 is a graph comparing unit cell performance test results of the PtAu binary catalyst of the present invention and the PtRu and PtPd binary catalysts prepared according to Comparative Example 1 in a 10 M formic acid solution and oxygen supply conditions. The amount of each cathodic catalyst was 5 mg / cm 2, and the cathode catalysts were all 5 mg / cm 2 of Johnson Matthey. Nation Pt black ^? It was used to bond with 117 to form a MEA (membrane electrode assembly). The unit cell tested was 2㎠ It was sized and fed a formic acid solution and oxygen through a graphite channel at a flow rate of 0.2 to 2 cc / min and 300 to 1000 cc / min. As a result, when the PtRu and PtPd synthetic catalysts prepared by Comparative Example 1 were used at 0.3 V, the current density was 266 and 200 mA / cm 2, respectively.However, when the PtAu binary catalyst prepared in Example 1 was used, 438 mA / cm 2 was used. The performance difference was about 65 and 119%, respectively.

As described above, according to the present invention, a platinum-based binary synthesis catalyst, in particular a platinum-gold synthesis catalyst, provides an oxygen source for the formic acid oxidation reaction to remove the adsorbed catalyst poison, but not to generate the catalyst poison carbon monoxide. There is an advantage of increasing the intrinsic catalytic activity that promotes the reaction to directly produce carbon dioxide without. In addition, the production of carbon monoxide is significantly reduced, so that there is no performance reduction due to the high concentration of the fuel, which allows the use of concentrated fuel. By using such a catalyst in a carrier having a large surface area, the catalyst can be applied to increase the effective surface area of the catalyst and to reduce the amount of the catalyst used. In addition, the performance of the fuel cell manufactured by the present invention is improved as the concentration of the fuel (formic acid) to be used increases, so that it is possible to manufacture a high-efficient direct formic acid fuel cell.

Therefore, when comparing the platinum-ruthenium catalyst commercialized for the direct methanol fuel cell and the activity of the formic acid oxidation reaction, the binary catalyst composed of platinum-gold can obtain high power in the direct formic acid fuel cell (DFAFC). It is expected to be commercialized.

1 is a schematic diagram of a direct formic acid fuel cell.

FIG. 2 shows the results of X-ray diffraction (XRD) analysis of the platinum-gold alloy binary catalysts prepared by Example 1 of the present invention. The mole percent composition of the platinum-gold catalysts is (a) 6: 1, (b) 4: 1, (c) 3: 1, (d) 2: 1, (e) 1: 1.

Figure 3 shows a comparison of formic acid oxidation current by the platinum-gold alloy binary catalyst prepared by Example 1 of the present invention.

FIG. 4 shows the results of comparing the fuel cell performance curves of the platinum-gold binary catalyst and the pure platinum catalyst under 5 molar concentration (5 M) formic acid conditions.

FIG. 5 shows a comparison of fuel cell performance curves of a platinum-gold binary catalyst and a pure platinum catalyst under 10 molar concentration (10 M) formic acid conditions.

FIG. 6 shows the results of comparing the fuel cell performance curves of the platinum-gold binary catalyst with the platinum-ruthenium and platinum-palladium binary catalyst under 10 molar concentration (10 M) formic acid conditions.

Claims (11)

In a cathode catalyst of a direct formic acid fuel cell (DFAFC), the catalyst is a cathode catalyst of a direct formic acid fuel cell (DFAFC) containing platinum (Pt) and gold (Au) A cathode catalyst according to claim 1, wherein platinum comprises 10 to 90 mol% and gold comprises 10 to 90 mol%. The cathode catalyst of claim 1, wherein the platinum-gold synthesis catalyst is supported on a catalyst carrier material. The anode catalyst of claim 3, wherein the catalyst carrier material is a porous carbon selected from activated carbon, carbon fiber, graphite fiber, and carbon nanotubes. 4. The catalyst of claim 3, wherein the catalyst carrier material is selected from polyvinylcarbazole, polyaniline, polypyrrole and derivatives thereof. 4. The catalyst of claim 3, wherein the catalyst carrier material is a negative electrode catalyst selected from tungsten oxide, titanium oxide, nickel oxide, ruthenium oxide, tantalum oxide, cobalt oxide 4. A cathode catalyst according to claim 3, characterized in that the catalyst carrier material is comprised between 2 and 90% by weight relative to the platinum-gold synthesis catalyst. The negative electrode catalyst according to any one of claims 1 to 7, wherein at least one metal element is further added to the platinum-gold binary synthesis catalyst.  9. A cathode catalyst according to claim 8, wherein the metal element is a Group VIIIA element of the Periodic Table of the Elements. 9. The anode catalyst according to claim 8, wherein the metal element is one or two or more metals selected from the group of Fe, Co, Ni, Ru, Rh, Pd, and Ir. Direct formic acid fuel cell comprising a fuel cell electrode prepared by claim 10