KR20010004405A - Method for manufacturing electrode for direct methanol feul cell - Google Patents

Abstract

PURPOSE: A method of making an electrode of a direct methanol fuel cell is provided to improve a performance of the fuel cell. CONSTITUTION: A method of making an electrode of a direct methanol fuel cell comprises coating a slurry on a carbon fabric and drying a result during one to twenty-four hours in a vacuum oven maintained at a temperature of 20 to 200 deg.C. The slurry is obtained by mixing an active carbon and a Nafion solution according to a weight rate of 1:1-50 and agitating a mixed result during 10 minutes to three hours. A catalyst is obtained by adding water to a catalyst of Pt-Ru/C of 5-60 wt% and adding and mixing isopropanol and Nafion solution to the added result. The catalyst is coated on a carbon fabric on which an active carbon is coated, and is dried in a vacuum oven.

Description

Method for manufacturing electrode for direct methanol fuel cell {Method for manufacturing electrode for direct methanol feul cell}

The present invention relates to a method for manufacturing an electrode of a direct methanol fuel cell (DMFC), and more particularly, to form an activated carbon layer by coating an activated carbon powder, a hydrophilic material, on a carbon cloth as an electrode support when manufacturing an anode. A method of manufacturing an electrode of a direct methanol fuel cell in which an electrode catalyst layer is coated thereon to manufacture a fuel electrode, thereby allowing methanol, which is a liquid fuel, to be easily reached to the electrode catalyst layer by a hydrophilic activated carbon layer, thereby improving performance of the battery. will be.

A fuel cell is a high-efficiency clean power generation technology that converts hydrogen and oxygen contained in hydrocarbon fuels such as methanol and natural gas into electrical energy directly by electrochemical reactions. Research has been conducted to use this power supply developed for power supply for general power supply. Currently, developed countries for the practical use are being actively developed in developed countries such as the United States and Japan.

On the other hand, fuel cells are classified into gaseous fuel cells and liquid fuel cells according to the type of fuel used, and largely alkaline type, phosphoric acid type, molten carbonate type, solid oxide type and solid polymer electrolyte type depending on the type of electrolyte. Are classified as.

In general, gaseous fuel cells that use hydrogen as a fuel have the advantage of high energy density, but require attention to handling hydrogen gas and fuel reformers using methane or methanol to produce hydrogen gas, which is fuel gas. There is a problem that needs additional equipment.

In contrast, liquid fuel cells using liquid as fuel have lower energy density than gas type, but are easy to handle fuel, have low operating temperature, and do not require fuel reforming device. It is known as a suitable system.

Due to the above advantages of liquid fuel cells, many studies on direct methanol fuel cells (DMFCs), a representative form of liquid fuel cells, have been conducted. Recently, the Los Alamas National Laboratory, USA, used a solid polymer membrane as an electrolyte. was used to release the development of high-performance, the performance of 670mA / cm ² at 0.5V of the unit cell DMFC, Jet Propulsion Laboratory in the fuel for direct development of a liquid fuel is methanol released by the 180mA / cm ² of a stack developed by 0.6V It has come to prove the possibility of practical use of the fuel cell.

The direct methanol fuel cell is based on the power of the electromotive force obtained from the cathode reaction in which the oxidation reaction of methanol takes place and the cathode reaction in which the reduction reaction of oxygen occurs, and the reaction occurring in the anode and the cathode is as follows.

Fuel electrode: CH ₃ OH + H ₂ O → CO ₂ + H ⁺ + 6e ^- E _a = 0.04 V

^{_{Cathode: 3 / 2O 2 + 6H +}} + 6e - → 3H 2 OE c = 1.23V

Total reaction: CH ₃ OH + 3 / 2O ₂ → CO ₂ + 3H ₂ OE _cell = 1.19 V

As can be seen from the reactions occurring at the anode and the cathode, a highly hydrophilic electrode is required in the anode because a liquid phase methanol (CH ₃ OH) and water (H ₂ O) react with a solid phase electrode. . On the other hand, in the air electrode, hydrogen ions (H ⁺ ) and oxygen (O ₂ ) react with the fish (H ₂ O), oxygen (O ₂ ) in the air, and the electrode catalyst layer exist in three phases. Since the generated water must be discharged to the outside quickly, a strong hydrophobic electrode is required.

As such, in order to satisfy the requirements of the anode and the cathode, the amount of the hydrophobic binder used to form the electrode catalyst layer should be minimized in the case of the anode, and the dispersibility and the bonding property of the catalyst should be increased. In order to allow sufficient solid, liquid, and gas phase reactions to occur, sufficient dispersibility and binding property of the catalyst should be increased.

However, in the conventional fuel cell electrode manufacturing method, the dispersibility and the bondability of the catalyst are not normally controlled due to poor control of the addition amount of the binder during preparation of the slurry for forming the electrode catalyst layer and insufficient stirring of the slurry. It is difficult to expect satisfactory fuel cell performance due to the fall.

In view of the above problems, the present applicant has filed a patent application No. 1998-22324 for "Direct Methanol Fuel Cell Electrode Manufacturing Method" which improves the dispersibility and binding property of the catalyst through controlling the amount of binder added and stirring the catalyst slurry at low temperature. There is a bar.

The electrode manufacturing method of the prior application is largely divided into two methods, a cathode manufacturing method and a cathode manufacturing method. Among them, the anode manufacturing method includes water and 5 to 60wt% Pt-Ru / C catalyst as shown in the manufacturing process diagram of FIG. 1 to 1 isopropyl alcohol

After the slurry was prepared to complete dispersion of the catalyst particles by stirring in a ball mill over a long period of time at a volume ratio of 100, the slurry was stirred at a low temperature (0 to 10 ° C) and PTFE (Polytetrafluoroethylene) at a concentration of 30 to 60wt%. ) The solution obtained by gradually adding the solution and stirring for a long time is applied to the carbon paper and dried at a temperature of 50 to 200 ° C. until the predetermined catalyst content is reached. Then, PTFE of the electrode coated with the catalyst on the carbon paper is applied. By sintering and passing the rolling machine several times, the adhesion between the carbon paper and the catalyst is increased to produce the anode.

In addition, the cathode manufacturing process in the above method has a difference in that 20 to 80wt% Pt / C is used as a catalyst in performing the overall process as in the anode manufacturing process as shown in FIG.

In the method of manufacturing a cathode and an anode of a direct methanol fuel cell of such an earlier source, when a slurry is prepared by mixing a metal catalyst and a solvent, sufficient agitation is performed by ball milling to decrease the bonding force between catalyst particles, and through the above agitation. By adding a Teflon binder to the obtained slurry and performing a stirring operation at a low temperature, it can be seen that the heat generated during stirring is suppressed so that the binder is evenly dispersed in the catalyst layer.

However, as described above, there is a limit in improving dispersibility and bonding property by sufficient stirring between the catalyst and the solvent in the preparation of the electrode, and thus the performance improvement effect of the fuel cell manufactured using the electrode is also expected to be above a certain level. It is difficult to do.

The present invention was devised to improve the performance of the fuel cell by solving the above problems raised in the electrode manufacturing method of the conventional direct methanol fuel cell, and activated carbon which is a hydrophilic material on the carbon cloth as an electrode support when manufacturing the anode. The electrode of the direct methanol fuel cell which improves the performance of the battery by applying the powder and then applying the electrode catalyst layer thereon to produce a fuel electrode so that methanol, which is a liquid fuel, can be easily reached by the hydrophilic activated carbon layer to the electrode catalyst layer. The aim is to provide a method of manufacture.

1 is a fuel electrode manufacturing process diagram of a direct methanol fuel cell by a conventional method.

Figure 2 is a cathode manufacturing process of a direct methanol fuel cell by a conventional method.

3 is a manufacturing process of the anode according to the method of the present invention.

4 is a cross-sectional structural view of a fuel electrode manufactured by the method of the present invention.

5 is a cell performance comparison graph between a unit cell using a fuel electrode according to the method of the present invention and a unit cell according to a conventional method.

6 is a unit cell performance curve according to the operating temperature of the unit cell using the anode according to the method of the present invention

7 is a unit cell performance curve according to the methanol concentration of the unit cell using the anode according to the method of the present invention

8 is a unit cell performance curve according to the operating pressure of the unit cell using the anode according to the method of the present invention

The method of manufacturing a cathode of a direct methanol fuel cell of the present invention comprises the steps of applying a slurry of activated carbon powder and a Nafion solution on a carbon cloth as an electrode support to form a hydrophilic activated carbon layer, and a 5 to 60 wt% Pt-Ru / C catalyst. And a catalyst slurry obtained by mixing and stirring a 5-20 wt% Nafion solution and repeatedly applying it to a predetermined content on the electrode support to which the activated carbon layer is applied to dry in a vacuum oven and a carbon cloth. Passing the calcined electrode through a rolling machine to increase the adhesion between the catalyst and the catalyst.

On the other hand, in the direct methanol fuel cell electrode manufacturing method of the present invention, the cathode is manufactured through the same process as the cathode manufacturing method previously filed by the present applicant as described above.

3 is a process diagram of manufacturing a cathode of a direct methanol fuel cell of the present invention.

First, in order to form a hydrophilic activated carbon layer as a diffusion layer of the anode, a slurry obtained by mixing an activated carbon powder and a 5-20 wt% Nafion solution on a carbon cloth at a weight ratio of 1: 1-50 was stirred for 10 minutes to 3 hours. After coating, it is dried for about 1 to 24 hours in a vacuum oven maintained at a temperature of 20 to 200 ℃.

Next, an appropriate amount of water is added to the 5 to 60 wt% Pt-Ru / C catalyst, then isopropanol and 5 to 20 wt% Nafion solution are added and mixed, followed by stirring to form a catalyst slurry. The activated carbon layer is applied onto the coated carbon cloth and dried in a vacuum oven.

The coating and drying process of the catalyst layer is repeated until the content of the catalyst reaches a predetermined value, and finally, the adhesive force between the catalyst layer and the electrode support is increased by passing the rolling machine about 2 to 5 times. As a result, the anode of the present invention is obtained.

4 is a cross-sectional structure diagram of an anode of a direct methanol fuel cell of the present invention obtained through the above manufacturing process. As shown in the drawing, the anode of the present invention comprises an electrode support 1 made of carbon cloth and an ion exchange membrane ( Between the Proton Exchange Membrane (2), a metal catalyst layer (3) having a catalyst (Pt-Ru) particle (3b) attached to a surface of a carbon black as a carbon support (3a) is interposed. In a conventional fuel electrode, a diffusion layer 4 made of active carbon having a hydrophilic group (Hydrophilic Group) is interposed between the electrode support 1 and the metal catalyst layer 3.

In the diffusion layer 4 as the hydrophilic activated carbon layer, methanol (CH ₃ OH + H ₂ O) as anode fuel (CH ₃ OH + H ₂ O) flowing from the outside of the electrode support 1 easily reaches the metal catalyst layer 3 on the electrode surface. By doing so, the reaction at the anode is actively performed, thereby improving the performance of the fuel cell.

Embodiments of the present invention are as follows.

Anode production

The diffusion layer of the anode was prepared using a surface area (surface area) of Kuray Chemical Co., Ltd. 300 ~ 2000m ² / g activated carbon. First, after applying the appropriate amount of slurry obtained by mixing the activated carbon and 5-20 wt% Nafion solution in a weight ratio of 1: 1-50, stirring for 10 minutes to 3 hours, and then maintained at a temperature of 20 ~ 200 ℃ It dried for about 1 to 24 hours in the vacuum oven.

Next, an appropriate amount of water is added to the 5 to 60 wt% Pt-Ru / C catalyst, then isopropanol and 5 to 20 wt% Nafion solution are added and mixed, followed by ultrasonic stirring and mechanical stirring. After the slurry was prepared, it was applied on the carbon cloth to which the activated carbon layer was applied and dried in a vacuum oven maintained at a temperature of 20 to 200 ° C. for 1 to 24 hours until the predetermined catalyst content was reached.

Subsequently, the electrode molded body in which the diffusion layer and the metal catalyst layer were sequentially coated on the carbon cloth was passed through a rolling machine about 2 to 5 times to increase the adhesion between the catalyst layer and the carbon cloth to obtain the fuel electrode of the present invention.

On the other hand, the conventional anode as a conventional example specimen for comparison with the anode obtained by the method of the present invention was produced by the manufacturing method of FIG.

In the conventional anode, the content of the metal catalyst was 0.1 to 3 mg / cm ² based on the platinum content of the Pt-Ru / C catalyst, and the final dried catalyst layer was heated to 200 to 250 ° C. at a heating rate of 2 to 4 ° C./min. After heating up to 20 to 60 minutes at this temperature, it was heated at a temperature of 300 to 380 ℃ at a temperature increase rate of 2 to 4 ℃ / min to achieve the sintering of PTFE.

Air cathode manufacturers

The cathode was fabricated in accordance with the method of producing a prior application cathode, that is, the cathode manufacturing process diagram of FIG. 2.

First, as the cathode catalyst, a catalyst in which 20 to 80 wt% Pt / C (molar ratio 1: 1) is supported on a carbon black having a large specific surface area was used. Pure water and isopropyl alcohol were added to the metal catalyst in a volume ratio of 1 to 100, followed by stirring using a ball mill for 1 to 24 hours or, if possible, to achieve complete dispersion of the catalyst particles.

Subsequently, as little as possible, a PTFE solution having a concentration of 30 to 60 wt% was stirred in an ice-bath kept at a temperature of 0 ° C. to less than 10 ° C. or at the lowest temperature without freezing. The slurry was first prepared by slowly adding and stirring for about 30 minutes to 1 hour or, if possible, for a long time.

The slurry thus prepared was coated on carbon paper and dried for 10 to 24 hours at a temperature in the range of 50 to 200 ° C., and the weight was repeatedly measured until the desired catalyst content was repeatedly applied to the slurry evenly on the carbon paper.

The electrode coated with the catalyst on the carbon paper was dried at room temperature for about one day, and then heated to 230 to 250 ° C at a heating rate of 3 ° C / min to sinter PTFE, and then maintained at this temperature for 20 to 60 minutes, and maintained at 3 ° C / The catalyst layer was manufactured by heating to the temperature of 320-380 degreeC at the temperature increase rate of min, hold | maintaining at this temperature for 20 to 60 minutes. At this time, the atmosphere of the oven was nitrogen atmosphere.

The calcined electrode was passed through a rolling machine about 2 to 5 times to increase the adhesion between carbon paper and the catalyst to obtain an air electrode.

Performance Evaluation of Unit Battery

Cell voltage according to the current density between the unit cell in which the electrode having the hydrophilic activated carbon layer manufactured by the method of the present invention is used as the anode and the unit cell in which the anode manufactured by the conventional method is used. ), The results are shown in FIG.

At this time, the concentration of the fuel methanol was used 0.5M (mol) to 4.0M, Nafion (117) was used as the polymer membrane. In addition, 5 to 60 wt% Pt-Ru / C was used as the anode catalyst, and 20 to 80 wt% Pt / C was used as the cathode catalyst.

As shown in FIG. 5, the battery having the conventional anode shows a current density value of 336 mA / cm ² at 0.4 ° C. at 100 ° C., whereas the battery having the anode of the present invention has 427 mA / cm at 0.4 V. It can be seen that the current density value of ² .

That is, it can be seen that there is a difference in current density of about 91 mA / cm ² between the two electrodes, which is due to the presence of a hydrophilic activated carbon layer in the anode prepared by the method of the present invention. Reaching the electrode more easily translates into improved electrode performance.

Battery Performance Evaluation According to Operating Conditions

The performance of the fuel cell manufactured according to the method of the present invention, the air electrode manufactured by the conventional method, and the unit cell made of the Nafion 117 polymer membrane, the performance according to the operating temperature, methanol concentration and pressure were observed.

First, FIG. 6 is a graph showing voltage-current characteristics of a direct methanol fuel cell according to a change in operating temperature. Based on the output voltage 0.4V as shown in Figure 6, the current density at the operating temperature 100 ℃, 110 ℃ and 120 ℃ was 423mA / cm ² , 470 mA / cm ² and 513mA / cm ² .

From this, it can be seen that the voltage-current characteristics of the methanol fuel cell directly depend on the operating temperature. As the operating temperature increases, the oxidation reaction rate of the anode methanol increases and the hydrogen ions passing through the electrolyte membrane from the anode are increased. This is due to the increase in the rate of reduction reaction and the decrease in electrode resistance in the cathode.

Next, FIG. 7 is a current density characteristic curve at a constant cell voltage when the concentration of fuel methanol is changed to 0.1 M, 0.5 M, 1 M, 2.5 M, and 4 M, and the operating temperatures are 90 ° C., 100 ° C., and 110 ° C. FIG. As shown in Figure 7, it can be seen that the concentration of methanol is closely related to the performance of the unit cell. When methanol concentration is 2.5M, it shows the best current characteristic value, and it can be seen that the lower the concentration of fuel, the current density decreases at a constant voltage, which is caused by concentration polarization due to the diffusion rate of hydrogen ions. to be.

On the other hand, Figure 8 is a voltage-current characteristic curve according to the operating differential pressure (air cathode-anode) when operating at 110 ℃ using a fuel having an arbitrary methanol concentration.

It can be seen that the higher the differential pressure, the higher the voltage-current characteristic can be obtained.The differential pressure was increased to 1kgf / cm ² , 2kgf / cm ² , 3kgf / cm ² , 4kgf / cm ² and 5kgf / cm ² . case, each of ^{206mA / cm 2, 322mA / cm} 2, 396mA / cm 2, 434mA / cm 2 and 450mA / cm ² at a current density value in the constant voltage of 0.4V was increased.

This phenomenon is caused by the crossover of methanol passing from the anode to the cathode due to the pressure rise of the cathode and the reaction of hydrogen ions and oxygen in the cathode due to the increase in oxygen pressure. .

As described above, the anode prepared by the method of manufacturing the anode of the present invention direct methanol fuel cell is methanol introduced through the electrode support by a hydrophilic activated carbon layer as a diffusion layer existing between the electrode support (carbon cloth) and the electrode catalyst layer. It is expected that the performance of the fuel cell will be improved since the catalyst is easily diffused and reached to the coated electrode surface.

Claims (2)

In the method of manufacturing an electrode of a direct methanol fuel cell, the manufacturing of the anode is performed by applying a slurry obtained by mixing and stirring an activated carbon powder and a 5-20 wt% Nafion solution on a carbon cloth as an electrode support to form a hydrophilic activated carbon layer. And a catalyst slurry obtained by mixing and stirring a 5 to 60 wt% Pt-Ru / C catalyst and a 5 to 20 wt% Nafion solution on the electrode support coated with the activated carbon layer and drying the carbon cloth. And passing the dried electrode through a rolling machine to increase adhesion between the catalyst and the catalyst. 2. The method of claim 1, wherein the electrode molded body in which the activated carbon layer and the catalyst layer are sequentially coated on the carbon cloth is dried in a vacuum oven at 20 to 200 ° C. for 1 to 24 hours and dried.