JP2003077512A - Operating method for methanol direct supply type fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an operating method capable of obtaining higher output density, in a fuel cell provided with a cell wherein a perfluorocarbon sulfonic acid based ion exchange membrane is used and a positive electrode and a negative electrode are disposed on both sides of the exchange membrane, to generate power by supplying methanol aqueous solution to the negative electrode and by supplying oxidized gas to the positive electrode. SOLUTION: In this operating method for a methanol direct supply type fuel cell, when starting power generation, supply of methanol aqueous solution to a negative electrode is first started, and after that supply of oxidized gas to a positive electrode is started.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a direct methanol fuel cell for generating power while directly supplying an aqueous methanol solution as a fuel at a negative electrode.

[0002]

2. Description of the Related Art A fuel cell has a basic structure in which a pair of electrodes called a negative electrode and a positive electrode are provided at both ends of an electrolyte forming an ion passage, and a channel for passing a fuel or an oxidizing gas is provided. One cell is formed by sandwiching it with the formed separator.

Conventionally, as a fuel cell, a solid electrolyte type, a molten carbonate type, a phosphoric acid type and the like are known, but in recent years, in particular, a fuel cell using a perfluorocarbon sulfonic acid type membrane as an electrolyte (solid polymer) Type fuel cell), which can operate at room temperature and can obtain high output density, is considered to be most suitable as a small power source for next-generation automobiles and household cogeneration systems. Research and development is being conducted.

This solid polymer electrolyte fuel cell mainly uses hydrogen as a fuel. However, when pure hydrogen is used as a fuel, it will be stored in a high-pressure cylinder, a liquefied hydrogen cylinder, a hydrogen storage alloy, etc., but in any case, there are problems in practical application due to safety, cost, and energy density problems. . On the other hand, when methanol and gasoline are stored and reformed to take out hydrogen, a reformer is required, which complicates the equipment, and also poses safety issues for reactions at high temperatures, catalyst life, and startup time. There are still many problems to be solved for practical use, such as shortening

Therefore, methanol can be used to generate electricity with a simple device, which is easy to handle and has a high energy density in terms of weight and volume. Type fuel cells have received attention.

[0006]

While the direct methanol fuel cell has the above advantages, it has a lower output density than the case where hydrogen is used as a fuel, and therefore the electrode is required to obtain the same output as a power source. Need to increase the area,
In particular, it is a great disadvantage in terms of cost, which is the biggest problem for practical use of fuel cells. Therefore, at present, in the direct methanol fuel cell, improvement of the output density is the first issue. An object of the present invention is to provide an operating method capable of obtaining a higher power density in a direct methanol fuel cell.

[0007]

Means for Solving the Problems The inventors of the present invention have made earnest studies and found an operating method capable of obtaining a higher output density when power is generated by a direct methanol fuel cell,
The present invention has been reached. That is, the present invention includes (1) a cell in which a perfluorocarbon sulfonic acid-based ion exchange membrane is used as an electrolyte, and a negative electrode and a positive electrode are arranged on both sides of the exchange membrane. In a fuel cell for generating power by supplying to the positive electrode, at the start of power generation, the methanol aqueous solution is first supplied to the negative electrode, and then the oxidizing gas supply to the positive electrode is started. Direct fuel cell operating method,
(2) A cell in which a perfluorocarbon sulfonic acid ion exchange membrane is used as an electrolyte and a negative electrode and a positive electrode are arranged on both sides of the exchange membrane, an aqueous methanol solution of fuel is supplied to the negative electrode, and an oxidizing gas is supplied to the positive electrode. In a fuel cell that generates electric power by means of the above, a method for operating a direct methanol fuel cell, characterized in that power is temporarily generated using hydrogen as a fuel before power generation using an aqueous methanol solution as a fuel,
(3) A cell in which a perfluorocarbon sulfonic acid ion exchange membrane is used as an electrolyte and a negative electrode and a positive electrode are arranged on both sides of the exchange membrane, a methanol aqueous solution of fuel is supplied to the negative electrode, and an oxidizing gas is supplied to the positive electrode. In particular, the present invention relates to a method for operating a direct methanol fuel cell, characterized in that fuel or water is allowed to exist in the negative electrode passage during a temporary suspension of power generation operation.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The fuel cell used in the present invention is a polymer electrolyte fuel cell for direct methanol use, which uses a perfluorocarbon sulfonic acid type ion exchange membrane as an electrolyte. The fuel cell includes cells in which a negative electrode and a positive electrode are arranged on both sides of the ion exchange membrane, and a methanol aqueous solution of fuel is supplied to the negative electrode and an oxidizing gas is supplied to the positive electrode to generate power.
The oxidizing gas is not particularly limited as long as it can supply oxygen for oxidizing H ⁺ generated in the negative electrode, but a gas containing molecular oxygen is preferable and it is economically advantageous to use air. At this time, the following reactions proceed in the negative electrode. _{CH 3 OH + H 2 O →} CO 2 + 6H + + 6e - H + generated at the negative electrode reaches the positive electrode by moving the ion exchange membrane, reacts as follows with the oxygen supplied to the cathode. _{^{O 2 + 4H + + 4e -}} → 2H 2 O The ion exchange membrane is moisturized by of H ₂ O in methanol aqueous solution supplied to the negative electrode, ion conductivity is maintained.

In the first embodiment of the present invention, in the order of starting the supply of the aqueous methanol solution and the oxidizing gas at the time of starting power generation (at the time of starting) in the present fuel cell, the aqueous methanol solution is supplied to the negative electrode first. To start. In this case, a power density about twice that obtained when the supply is started in the reverse order is obtained. At this time, a sufficient effect can be obtained even if the time difference from the start of the supply of the fuel to the start of the supply of the oxidizing gas is about several minutes, but it is desirable that the time difference is 10 minutes or more to obtain the higher effect.

In the second embodiment of the present invention, when power generation is started (started), hydrogen is used as a fuel for a certain period of time to generate power (hydrogen power generation process), and then an aqueous methanol solution is used as fuel to generate power. The power density in this case is about 20 to 50% higher than that in the case where the hydrogen power generation process is not performed. When hydrogen power generation is performed, it is necessary to humidify both hydrogen (negative electrode) and oxidizing gas (positive electrode), as in the case of hydrogen-type polymer electrolyte fuel cells. The time required for the hydrogen power generation treatment is 10 minutes or longer, and more preferably 1 hour or longer is more effective. Further, during hydrogen power generation, it is desirable to operate so as to extract more current.

In the third embodiment of the present invention, when the power generation operation is temporarily stopped, it is advisable to allow a methanol aqueous solution or water, which is a fuel, to exist in the negative electrode flow path and keep it in a wet state. It is more preferable to put it in a state. In this case, the fuel or water may be in a flowing state or a stationary state. By doing so, the power density at the time of restart is higher than that in the case where the inside of the negative electrode flow channel is stored in a dry state during rest. In addition, it is preferable that the oxidizing gas for the positive electrode is not allowed to flow while the power generation is stopped.

[0012]

EXAMPLES The present invention will be specifically described below with reference to examples. However, the present invention is not limited to these examples.

Fuel cell production procedure: A perfluorocarbon sulfonic acid membrane Nafion ^™ -117 (manufactured by DuPont) was selected as the polymer membrane used as the electrolyte, and it was used after boiling and washing in hydrogen peroxide solution and dilute sulfuric acid. . The electrode is treated by water repellent treatment of carbon paper with an aqueous solution of tetrafluoroethylene, and then a slurry in which carbon, alcohol, and an aqueous solution of tetrafluoroethylene are mixed at a fixed ratio is spray-coated and dried to form a gas diffusion layer, on which It was prepared by applying a catalyst ink prepared by mixing a catalyst and a Nafion (perfluorocarbon sulfonic acid) solution at a constant ratio by the doctor blade method and drying. The catalyst used was a platinum-based catalyst for both electrodes. The positive electrode and the negative electrode thus produced were used on the respective surfaces of the electrolyte membrane and pressure-bonded to produce a membrane electrode assembly.

Power generation conditions In the following examples and comparative examples, fuel cells were prepared using the membrane electrode assembly obtained by the above operation, air was used as the oxidizing gas for the positive electrode, and aqueous methanol solution was used for the negative electrode. It was supplied to generate power, and the current density during discharge at a constant voltage was examined. The test conditions are shown below. Battery temperature: 80 ° C Positive air: Dry air, 4 ml ・ min ^-1・ cm ^-2 Negative electrode fuel: Methanol aqueous solution, 0.02 ml ・ min ^-1・ cm ^-2

Example 1 When starting power generation using the above-mentioned fuel cell (at start-up)
First, start supplying the aqueous methanol solution to the negative electrode, and
After a few minutes, the supply of air to the positive electrode was started. After that, after the open circuit voltage became stable, a load was applied to start power generation. The current density obtained at this time was 100 mA / cm ² .

COMPARATIVE EXAMPLE 1 In Example 1, the order of starting fuel and air supply was reversed, the air supply was started first, and the same operation was performed to start power generation, and the current density was 40 mA / cm ^2. Met.

By carrying out the first embodiment of the present invention, the current density is more than doubled.

Example 2 The produced fuel cell was first subjected to a power generation process using hydrogen as a fuel (hydrogen power generation process), and then an aqueous methanol solution and air were simultaneously supplied to generate power under the above conditions. . The current density obtained in this case was 80 mA / cm ² .
The conditions for hydrogen power generation treatment are shown below. Battery temperature: 50 ℃ Positive air and negative hydrogen: 8 ml ・ min ^-1・ cm ^-2 , humidification amount is 8
Saturated water vapor pressure discharge voltage at 0 ℃: 400 mV Discharge time: 1 hour

Comparative Example 2 In Example 2, power generation was performed using a methanol aqueous solution as a fuel without performing hydrogen power generation treatment. The current density obtained in this case was 50 mA / cm ² .

By carrying out the second embodiment of the present invention, the effect of improving the current density by 1.6 times was observed.

Example 3 After power generation was performed under the conditions where the current density of 100 mA / cm ² described in Example 1 was obtained, the load applied to the cell was released to stop the power generation and supply of fuel and air. Was stopped, and the cell was stored overnight while being cooled to room temperature. At this time, the anode flow channel was stored while being filled with fuel. After that, when the power was generated again, the current density was 100 mA / cm ² .

Comparative Example 3 In Example 3, the inside of the negative electrode flow path was replaced with dry air and the fuel was removed and stored. In this case, the current density when the power was generated again later was 70 mA / cm ² .

By carrying out the third embodiment of the present invention, it is possible to prevent a decrease in output density when power is generated again after the suspension, and it is possible to repeatedly obtain a high current density.

[0024]

As is apparent from the above embodiments, the output of the direct methanol fuel cell can be increased by implementing the operating method of the present invention. Therefore, it can be said that the contribution of the present invention to the industry is great.

Claims (4)

[Claims] 1. A perfluorocarbon sulfonic acid ion exchange membrane is used as an electrolyte, and a cell having a negative electrode and a positive electrode on both sides of the exchange membrane is provided, an aqueous methanol solution of fuel is supplied to the negative electrode, and an oxidizing gas is supplied to the positive electrode. In a fuel cell that generates electricity by supplying, a methanol direct type fuel characterized by starting supply of an aqueous methanol solution to a negative electrode first and then starting to supply an oxidizing gas to a positive electrode when starting power generation. How to operate the battery. 2. A cell comprising a perfluorocarbon sulfonic acid type ion exchange membrane as an electrolyte and a negative electrode and a positive electrode arranged on both sides of the exchange membrane, an aqueous methanol solution of fuel is supplied to the negative electrode, and an oxidizing gas is supplied to the positive electrode. A method for operating a direct methanol fuel cell, characterized in that, in a fuel cell that generates electric power by supplying it, hydrogen is temporarily used as a fuel to generate electric power before power generation using an aqueous methanol solution as a fuel. 3. A perfluorocarbon sulfonic acid ion exchange membrane is used as an electrolyte, and a cell having a negative electrode and a positive electrode on both sides of the exchange membrane is provided, an aqueous methanol solution of fuel is supplied to the negative electrode, and an oxidizing gas is supplied to the positive electrode. A method for operating a direct methanol fuel cell, characterized in that, in a fuel cell that generates electric power by supplying the fuel, a fuel or water is allowed to exist in the negative electrode passage during a temporary suspension of the power generation operation. 4. The method for operating a direct methanol fuel cell according to claim 3, wherein the anode flow path is filled with fuel or water during rest.