JP2001185185A - Fuel cell and its operating method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an excellent fuel cell which directly supplies DMFC and other organic fuels to an anode, by having a relatively simple configuration and by restricting a crossover in which an organic fuel is permeated through to the cathode for reaction. SOLUTION: CO2 isolated from a tank 41 mixed with the methanol supplied from a fuel tank 21 is supplied to a channel 14a on the anode. A channel 15a on a cathode is supplied with dampened air. Thereby, the partial pressure of water vapor in the channel 14a on the anode becomes lower than the one in the cathode, thus in the electrolytic membrane 11, a moisture concentration gradient is formed such that the concentration on the anode is higher than the concentration on the cathode, and crossover is restricted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a fuel cell,
In particular, the present invention relates to a fuel cell that generates power while directly supplying an organic fuel such as methanol to an anode.

[0002]

2. Description of the Related Art A fuel cell has a basic structure in which a cell having a cathode disposed on one side of an electrolyte membrane and an anode disposed on the other side is sandwiched between separator members provided with ribs and gas channels. Most of the fuel cells put into practical use have a structure in which a large number of such basic structures are stacked so that a high voltage can be taken out.

[0003] As a fuel cell, a solid electrolyte type, a molten carbonate type, a phosphoric acid type and the like have been conventionally known. In recent years, a solid polymer membrane made of an ion exchange resin has been used as an electrolyte membrane. A polymer electrolyte fuel cell (PEFC) that can be operated at a relatively low temperature has been developed. This PEFC is generally operated while reforming a fuel gas such as methanol or methane into a hydrogen-rich reformed gas in a reformer and sending it to the anode side, while sending air as an oxidant to the cathode side. However, recently, Direct Methanol Fuel Cell (DMFC)
In addition, fuel cells that operate while supplying organic fuel directly to the anode without reforming the fuel have been developed.

In a DMFC, an aqueous methanol solution is usually supplied to the anode side, and the following (1)
React like. CH ₃ OH + H ₂ O → CO ₂ + 6H ⁺⁺ 6e ⁻ (1) Then, H + generated on the anode side passes through the ion exchange membrane and moves to the cathode side. It reacts with oxygen like.

O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O (2) Further, the ion exchange membrane is kept moist by the water in the aqueous methanol solution supplied to the anode side, so that its ion conductivity is ensured. Is done. Such a type of fuel cell can use a liquid fuel at room temperature and does not require a reformer, so that the configuration of the device for supplying the fuel can be simplified, and the fuel cell can be used as an inexpensive fuel cell. Expected.

[0006]

In a fuel cell in which such an organic fuel is directly supplied to the anode side, a part of the organic fuel supplied to the anode side passes through the electrolyte membrane to reach the cathode, and However, there is a problem of crossover in which a combustion reaction occurs. This crossover causes a decrease in the cathode potential, that is, a voltage loss. Therefore, in order to realize a cell having excellent cell performance, it is desired to suppress the crossover as much as possible.

[0007] In order to solve such a crossover problem, for example, in a DMFC, the concentration of an aqueous methanol solution to be supplied to the anode side is set to be low.
Attempts have also been made to reduce the amount of methanol permeating to the cathode side. However, in this method, the crossover cannot be sufficiently suppressed unless the methanol concentration supplied to the anode side is set to a considerably low level, so that it is difficult to obtain high cell performance.

Although crossover can be suppressed by using a thick solid polymer film, it is difficult to secure ion conductivity when the solid polymer film is thickened.
It becomes difficult to obtain high cell performance. On the other hand, JP-A-11-26
No. 005, the electrolyte membrane has a structure in which an intermediate layer containing a proton-type ion exchange resin powder and water is interposed between two solid polymer electrolyte membranes, so that water in the intermediate layer flows. A DMFC is also disclosed. In this DMFC, methanol flowing from the anode side to the cathode side is dissolved in water flowing through the intermediate layer and is discharged out of the cell, so that the amount of methanol permeating to the cathode side can be kept low, but the cell structure is complicated. Problem.

The present invention has been made in view of the above problems, and
In a fuel cell that supplies an organic fuel directly to the anode side, such as C, an excellent cell performance is provided with a relatively simple configuration by suppressing the organic fuel from permeating and reacting to the cathode side. The purpose is to:

[0010]

In order to achieve the above object, the present invention relates to a fuel cell for supplying an organic fuel such as methanol or ethanol or cimethyl ether directly to an anode. The operation was performed while forming a water concentration gradient in the electrolyte membrane. Such a gradient of the water concentration can be formed by adjusting at least one of the water in the organic fuel and the water in the oxidizing gas.

In a conventional DMFC that supplies water together with an organic fuel to the anode side, the water moves from the anode side to the cathode side in the electrolyte membrane.
Accordingly, the organic fuel is also easily moved from the anode side to the cathode side (that is, there is an action of promoting the movement of the organic fuel). However, as described above, when the water concentration gradient in the electrolyte membrane is formed, Since water moves from the cathode side to the anode side in the electrolyte membrane, it becomes difficult for the organic fuel to move from the anode side to the cathode side (namely,
It has the effect of suppressing the movement of organic fuel. ).

Therefore, even when the organic fuel is sufficiently supplied to the anode, the movement of the organic fuel to the cathode is suppressed. That is, it is possible to suppress crossover and obtain excellent cell performance. Such a water gradient in the electrolyte membrane can be formed by adjusting at least one of the amount of water contained in the organic fuel supplied to the anode side and the amount of water contained in the oxidizing gas supplied to the cathode side. .

In other words, the partial pressure of the oxidant gas supplied to the cathode side may be adjusted so as to be higher than the partial pressure of the organic fuel supplied to the anode side. here,
"Steam partial pressure" refers to the partial pressure of steam at the battery operating temperature. Alternatively, by adjusting the dew point of the oxidizing gas supplied to the cathode side to be higher than the dew point of the organic fuel supplied to the anode side, a water gradient in the electrolyte membrane can be similarly formed. it can.

Such water content adjustment can be easily performed by, for example, supplying humidified air to the cathode side and mixing and supplying non-humidified carbon dioxide to the organic fuel to the anode side. it can. As described above, the fuel cell of the present invention does not need to particularly complicate the cell structure, and can obtain excellent cell performance by suppressing cross leak.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (Configuration of Fuel Cell) FIG. 1 is a configuration diagram of a fuel cell according to the present embodiment. The fuel cell includes an anode 12 on one side of a solid polymer electrolyte membrane 11.
Has a cell in which the cathode 13 is arranged on the other surface side,
A fuel cell body 10 for generating electricity by supplying methanol to the anode 12 and air to the cathode 13, a methanol supply mechanism 20 for supplying methanol gas to the anode 12, and supplying air to the cathode 13 An air supply device 30 and a CO ₂ mixing mechanism 40 for mixing CO ₂ into methanol gas supplied to the anode 12
And a humidifier 5 for humidifying the air supplied to the cathode 13
0 is a DMFC.

FIG. 2 is an exploded perspective view of a main part of the fuel cell main body 10, showing one cell unit. The fuel cell body 10 is similar to a general polymer electrolyte fuel cell,
The cell having the above configuration is constituted by a cell unit sandwiched between an anode side plate 14 and a cathode side plate 15.

The electrolyte membrane 11 is an ion exchange membrane made of a perfluorosulfonic acid-based resin, and the anode 12 and the cathode 13 are each made of a mixture of a carbon powder carrying a noble metal catalyst and a perfluorosulfonic acid-based resin.
It is formed in a film shape on carbon paper. Platinum-ruthenium is used as the noble metal catalyst on the anode side, and platinum is used as the noble metal catalyst on the cathode side.

An anode-side channel 14a is formed on the surface of the anode-side plate 14 facing the anode 12.
On the surface of the cathode plate 15 facing the cathode 13, a cathode channel 15a is formed. In addition,
Although only one cell unit is shown in FIG. 2, the fuel cell main body 10 generally has a stack structure in which a plurality of such cell units are stacked so as to output a high voltage.

The methanol supply mechanism 20 includes a fuel tank 21 for storing methanol (having a water content of several percent or less), a pump 22 for delivering methanol from the fuel tank 21,
An evaporator 23 is configured to heat and evaporate methanol sent from the pump 22 to generate methanol gas. The CO ₂ mixing mechanism 40 is connected to the anode-side channel 14.
a tank 41 for separating the recovered exudates discharged CO ₂ from a, a pump 42 for delivering the separated CO _2, the CO ₂ sent from the pump 42, the methanol gas generated by the evaporator 23 And a merging pipe 43 for mixing. The inside of the tank 41 is maintained at a temperature close to room temperature so that the CO ₂ separated in the tank 41 does not contain much water.

The air supply device 30 takes in and sends out outside air, and is, for example, an air pump. The humidifier 50 humidifies the air sent from the air supply device 30 by bringing the air into contact with hot water. For example, as shown in FIG. It is structured to humidify by blowing into warm water.

(Operating operation of fuel cell)
The driving operation is performed in the pond as follows. Fuel electricity
In the pond main body 10, the anode-side channel 14a includes:
CO is added to methanol gas from the methanol supply mechanism 20._Two
Mixed gas (CH_ThreeOH + CO _Two) Supplied
Humidified air is supplied to the cathode side channel 15a.
(Air + H_TwoO) is supplied.

In the anode 12, the anode-side channel 1
Using the methanol contained in the mixed gas flowing through 4a and water (this water passes through the electrolyte membrane 11 from the cathode 13 side as described later), the reaction represented by the above formula (1) Perform an electrochemical reaction in the same manner as described above. Protons (H ⁺ ) generated by this reaction move to the cathode 13 through the electrolyte membrane 11.

[0023] Although the mixed gas supplied to the anode channel 14a contains CO _2, does not electrochemical reaction at the anode 12 by CO ₂ is inhibited. At the cathode 13, oxygen (O ₂ ) contained in the humidified air flowing through the cathode side channel 15a and protons (H ⁺ ) moving from the anode 12, as in the reaction represented by the above formula (2). Using the above (2)
An electrochemical reaction is performed in the same manner as the reaction shown by the formula.

The CO ₂ produced by the reaction in the CO ₂ and the anode 12 in the mixed gas supplied to the anode channel 14a, the mixture with of CH ₃ OH moisture and unreacted anode-side channel 14a (CO ₂ + CH ₃ OH
+ H ₂ O) and collected in the tank 41.
Then, a part of the CO ₂ separated in the tank 41 is reused.

(Regarding the Crossover Suppressing Effect) The methanol supplied from the fuel tank 21 has a small water content of several percent or less, and the CO ₂ separated from the tank 41 does not contain much water. While the water content contained in the mixed gas supplied to the cathode is relatively small, the humidified air supplied to the cathode side channel 15a contains a large amount of water. In addition, a reaction that consumes water is performed at the anode 12, and a reaction that generates water is performed at the cathode 13.

Accordingly, the partial pressure of water vapor in the anode side channel 14a is lower than the partial pressure of water vapor in the cathode side channel 15a. In this state, it can be said that the dew point of the air passing through the cathode channel 15a is higher than the dew point of the methanol gas passing through the anode channel 14a. Therefore, in the electrolyte membrane 11, a water concentration gradient is formed such that the concentration on the cathode side is higher than that on the anode side.

The electrolyte membrane 11 is formed by the water concentration gradient.
Moisture moves through the inside from the cathode side to the anode side. When the water moves in the electrolyte membrane 11, the methanol also moves in the same direction as the water in accordance therewith, so that the crossover in which the methanol moves from the anode side channel 14a through the electrolyte membrane 11 to the cathode side is suppressed. Will be.

(Regarding the Mixing Ratio of CO ₂ to Methanol) In order to lower the water vapor partial pressure of the gas flowing through the anode-side channel 14a, it is considered advantageous to set the ratio of CO _{2 to} methanol to a higher value. However, from the viewpoint of sufficiently supplying methanol to the anode 12, C
It is desirable that the proportion of O ₂ not be too high.

(Regarding Modification of the Present Embodiment) In the above embodiment, methanol is vaporized by an evaporator and then mixed with CO ₂ and introduced into the fuel cell body. It is also possible to mix CO ₂ and introduce the gas-liquid mixture into the fuel cell body. In the above embodiment, CO ₂ separated from the mixture discharged from the anode side channel 14a is mixed with methanol and supplied to the anode side channel 14a. However, CO ₂ which separately supplies unhumidified CO ₂ is supplied. _{Even if two} supply sources (for example, CO ₂ cylinders) are provided, CO ₂ from the _two sources is mixed with methanol and supplied to the anode side channel 14a, the effect of suppressing crossover is similarly exerted.

The gas mixed with methanol is CO 2
_It is not limited to ₂ but may be used as long as it does not inhibit the electrochemical reaction at the anode 12 and is non-humidified (having a low water content), and has the same effect. For example, even if an inert gas such as a nitrogen gas is mixed with methanol without being humidified and supplied to the anode-side channel 14a, the effect of suppressing crossover is similarly exerted.

In order to obtain the crossover suppression effect,
As in the above embodiment, it is preferable to mix a gas such as CO _{2 with} a low water content with methanol, but only the methanol from the fuel tank 21 is supplied to the anode side channel 1.
Even if the water is supplied to 4a, a water concentration gradient is formed in the electrolyte membrane 11 so that the concentration on the cathode side is higher than that on the anode side, so that the effect of suppressing crossover can be obtained. .

In the above embodiment, the DMFC that operates while supplying methanol as an organic fuel to the anode has been described. However, a fuel cell that operates while directly supplying ethanol or cimethyl ether as the organic fuel to the anode is also described. , And it is expected that an effect of suppressing crossover can be obtained.

[0033]

[Example] [Example] Based on the above embodiment, the DM
An FC was prepared and operated using pure methanol as fuel. However, rather than using the recovered CO ₂ as CO ₂ to be mixed with methanol, it was used CO ₂ supplied from the CO ₂ cylinder.

The operating conditions of the DMFC of this example were set as follows. Fuel utilization rate: 50% Air utilization rate: 20% Air humidification temperature: 70 ° C. Flow rate of CO ₂ : 1/2 of air flow rate [Comparative Example] As shown in FIG.
A fuel cell main body 110; a methanol supply mechanism 120 for supplying a mixed gas of methanol gas and water vapor to an anode 112 of the fuel cell main body 110;
And an air supply device 130 that supplies air to the DMFC.

The fuel cell body 110 is the same as the fuel cell body 10 of the above embodiment. The methanol supply mechanism 120 includes a tank 121 for storing an aqueous methanol solution.
And a pump 122 for sending an aqueous methanol solution from the tank 121, and an evaporator 123 for heating and evaporating the aqueous methanol solution sent from the pump 122 to generate a mixed gas (methanol gas + steam).

In the tank 121, the mixture (CO ₂ + CH ₃ O) discharged from the anode side channel 114a
H + H ₂ O) is recovered, CO ₂ is separated, and the aqueous methanol solution (CH ₃ OH + H ₂ O) is reused. The air supply device 130 is similar to the air supply device 30 of the first embodiment, but air from the air supply device 130 is directly sent to the cathode side channel 15a without being humidified.

In the DMFC of this comparative example, a mixed gas of methanol and water vapor is supplied to the anode side channel 114a, whereas the air supplied to the cathode side channel 115a does not contain much moisture. ,
The partial pressure of water vapor in the anode side channel 14a is higher than the partial pressure of water vapor in the cathode side channel 115a (the dew point of the air passing through the cathode side channel 15a is the dew point of the methanol gas passing through the anode side channel 114a). Lower state).

Therefore, in the electrolyte membrane 111,
A water concentration gradient is formed so that the concentration on the anode side is higher than that on the cathode side. Due to this water concentration gradient, water moves from the anode side to the cathode side in the electrolyte membrane 111. When water moves in the electrolyte membrane 111, methanol also moves in the same direction as the water along with the water, so that a crossover in which methanol moves from the anode-side channel 114a through the electrolyte membrane 111 to the cathode side easily occurs. You will be in a state.

The operating conditions of this comparative example DMFC were set as follows. Methanol aqueous solution concentration: 2 mol / L (2 mol of methanol is present in 1 L of solution) Air utilization: 20% [Experiment] The cell voltage was measured while operating the DMFCs of the above Examples and Comparative Examples under the above operating conditions.

In measuring the cell voltage, the current density (current per unit area of the electrode) is 0, 0.05, 0.1,
It was set to 0.2, 0.3 (A / cm ² ). FIG. 4 is a characteristic diagram showing the relationship between the current density and the cell voltage, which is the result of this experiment. From this figure, the DMFC of the embodiment is
It can be seen that a cell voltage higher than that of the DMFC of the comparative example was obtained. This result is considered to support that the crossover is suppressed in the DMFC of the example as compared with the DMFC of the comparative example.

[0041]

As described above, according to the present invention, in a fuel cell in which an organic fuel is directly supplied to an anode, a method of adjusting the water content of the organic fuel and the water content of the oxidizing gas from the anode side to the cathode side is achieved. By operating while forming a water concentration gradient in the electrolyte membrane so that the concentration becomes high on the side, it is not necessary to particularly complicate the cell structure, and it is possible to suppress crossover and obtain excellent cell performance.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a fuel cell according to an embodiment.

FIG. 2 is an exploded perspective view of a main part of a fuel cell main body.

FIG. 3 is a configuration diagram of a fuel cell according to a comparative example.

FIG. 4 is a characteristic diagram showing the results of a cell voltage comparison test between an example and a comparative example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Fuel cell main body 11 Solid polymer electrolyte membrane 12 Anode 13 Cathode 14 Anode side plate 14a Anode side channel 15 Cathode side plate 15a Cathode side channel 20 Methanol supply mechanism 22 Pump 23 Evaporator 30 Air supply device 40 CO2 mixing mechanism 41 Tank 42 Pump 43 Confluence pipe 50 Humidifier

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Yoshito Chino 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (72) Inventor Ikuro Yonezu 2-chome Keihanhondori, Moriguchi-shi, Osaka No.5 Sanyo Electric Co., Ltd. F-term (reference) 5H026 AA06 CC03 5H027 AA06 BA19 KK31 MM02 MM04

Claims (8)

[Claims] 1. A fuel cell, comprising: a cell in which an anode and a cathode are provided in an electrolyte membrane; and an organic fuel is supplied to the anode and an oxidizing gas is supplied to a cathode to generate power. A fuel cell, comprising: water adjusting means for adjusting at least one of water in an organic fuel and water in an oxidizing gas so that the water concentration is higher on the cathode side than on the anode side. 2. A fuel cell, comprising: a cell in which an anode and a cathode are disposed on an electrolyte membrane; and an organic fuel is supplied to the anode and an oxidizing gas is supplied to the cathode to generate power. Moisture adjusting means for adjusting at least one of the moisture in the organic fuel and the moisture in the oxidant gas so that the steam partial pressure of the oxidant gas becomes higher than the steam partial pressure of the organic fuel supplied to the anode. A fuel cell, characterized by: 3. The fuel cell according to claim 1, wherein said moisture adjusting means includes carbon dioxide gas mixing means for mixing carbon dioxide gas with the organic fuel supplied to the anode side. 4. A cell in which an anode and a cathode are arranged on an electrolyte membrane; an organic fuel supply unit for supplying an organic fuel to the anode; an oxidant gas supply unit for supplying an oxidant gas to the cathode; A fuel cell, comprising: humidifying means for humidifying an oxidizing gas supplied from an oxidizing gas supplying means to a cathode such that a moisture concentration in the membrane is higher on a cathode side than on an anode side. 5. A cell in which an anode and a cathode are disposed on an electrolyte membrane; an organic fuel supply unit for supplying an organic fuel to the anode; an oxidant gas supply unit for supplying an oxidant gas to the cathode; Humidifying means for humidifying the oxidizing gas supplied from the oxidizing gas supplying means to the cathode such that the water vapor partial pressure of the oxidizing gas supplied to the anode is higher than the water vapor partial pressure of the organic fuel supplied to the anode. A fuel cell, comprising: 6. The fuel cell according to claim 4, further comprising carbon dioxide mixing means for mixing carbon dioxide into the organic fuel supplied to the anode side. 7. A method for operating a fuel cell, comprising operating a fuel cell including a cell having an anode and a cathode disposed on an electrolyte membrane by supplying an organic fuel to the anode and an oxidant gas to the cathode. A fuel cell characterized in that it is operated while adjusting at least one of the moisture in the organic fuel and the moisture in the oxidizing gas so that the moisture concentration in the electrolyte membrane is higher on the cathode side than on the anode side. how to drive. 8. A method for operating a fuel cell, comprising: operating a fuel cell including a cell in which an anode and a cathode are disposed on an electrolyte membrane by supplying an organic fuel to the anode and supplying an oxidizing gas to the cathode. And operating the fuel cell while humidifying the oxidizing gas supplied to the cathode such that the water concentration in the electrolyte membrane is higher on the cathode side than on the anode side.