JP2003331880A - Fuel cell and its power generation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell, particularly a direct methanol fuel cell capable of solving an accompanying crossover problem to realize high output, and of immediately following the output variation while a fuel cell is fed with a liquid fuel directly to an anode, particularly a direct methanol fuel cell. <P>SOLUTION: As a result of analyzing a cause of the crossover problem, it is found that a high-concentration fuel can be fed without causing the crossover by employing a feed structure for fuel and water capable of avoiding a condition where the surface of the anode is always wet. This fuel cell includes the anode, a cathode and an electrolyte interposed between them. The fuel cell is characteristically provided with a means for feeding the fuel and water in the form of a liquid phase to the surface of the anode without bringing a storage solution including the fuel into contact with the anode. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel cell, particularly a direct methanol fuel cell, and more particularly to a fuel cell with improved crossover, and more particularly to a direct methanol fuel cell.

[0002]

2. Description of the Related Art Direct methanol fuel cell (DMFC)
Is a power generator that operates at a relatively low temperature (normal temperature to 120 ° C.). Since this fuel cell has been researched for a long time, it supplies methanol fuel directly to the anode, so that a reformer for taking out hydrogen from alcohol is not required, and the device itself can be miniaturized and inexpensive, as well as the entire device. The driving means can also be simplified. Liquid fuels are greatly expected from the standpoint of safety as a combustible material and easy miniaturization as compared with the case of using hydrogen gas as a fuel.

In the reaction in which methanol is oxidized at the anode of this battery, the involvement of water molecules is indispensable as shown in the following formula when completely oxidized by six electrons. CH ₃ OH + H ₂ O → CO ₂ + 6H ⁺ + 6e ⁻ (1) That is, water molecules are indispensable as an oxygen source for completely oxidizing methanol to carbon dioxide. According to the equation (1), one molecule of water is required for the reaction for one molecule of methanol, and this composition is considered to be the best fuel composition ratio. However, in order for the protons generated by the formula (1) to move in the electrolyte membrane toward the cathode, it is necessary to supply water molecules for hydrating the protons, preferably from the anode side. Further, even when the electrolyte membrane having the highest performance at the present time is used, the phenomenon that various fuels such as methanol permeate through the membrane and significantly deteriorate the output characteristics of the fuel cell is a problem (crossover). . Therefore, as a practical matter, the concentration of methanol constantly in contact with the fuel electrode must be at most about 1.5 mol / liter.

Various methods have been proposed to solve this problem. For example, a method for improving the electrolyte membrane between the anode and the cathode and the structure thereof has been adopted (JP-A-11-26005, JP-A-2002-83612, etc.).
Further, a method has been proposed in which liquid fuel is once vaporized by using a vaporizer or a heater and supplied to the anode (for example, JP 2001-93541 A). However, it is not possible to supply high-concentration fuel in the vapor phase, and it requires energy to heat the carburetor, and in the case of a carburetor that does not heat it, it takes time for the fuel to reach the anode. Causes the problem of. In addition, it has been proposed that a conductive porous material layer is provided on the anode and a catalyst that promotes the oxidation of fuel is supported on this layer (Table 2000-502205), but the structure is rather complicated and the cost is high. I think it will be.

[0005]

SUMMARY OF THE INVENTION The present invention solves the problem of the crossover associated with the fuel cell which supplies the liquid fuel directly to the anode as described above, in particular, the direct methanol fuel cell. It is an object of the present invention to provide a fuel cell that can be realized and can immediately follow output fluctuations, in particular, a direct methanol fuel cell.

[0006]

DISCLOSURE OF THE INVENTION The inventors of the present invention have analyzed the cause of the accompanying crossover problem even though it is a fuel cell for supplying a liquid fuel directly to the anode, particularly a direct methanol fuel cell. The present invention has been completed by finding that a high-concentration fuel can be supplied without causing crossover by adopting a fuel and water supply structure capable of avoiding the always wet state. That is,
The present invention is a fuel cell including an anode, a cathode, and an electrolyte sandwiched between the anode and the cathode. The fuel cell includes a means for supplying a fuel and water in a liquid phase to the surface of the anode without contacting the storage liquid containing the fuel with the anode. It is a fuel cell characterized by the above. Here, as the anode, the cathode and the electrolyte, those normally used in a fuel cell may be used. For example, a metal such as platinum is used as the anode and the cathode and Du is used as the electrolyte.
A proton conductive solid polymer electrolyte such as Nafion (registered trademark) manufactured by Pont Co. can be used.

A normal fuel cell is of a type in which a stored liquid composed of fuel and water comes into contact with the anode directly or through a flow path or the like. As a result, this storage solution always wets the anode surface. As long as such a structure is adopted, in order to avoid the problem of crossover, the concentration of the fuel (methanol) in the stored liquid must be lowered, and as a result, high output cannot be obtained. The present invention is characterized in that such a structure that a storage liquid containing a fuel is in contact with such an anode is not adopted, but a structure is provided with a means for supplying a liquid phase of fuel and water to the surface of the anode. The means for supplying the fuel and water in the liquid phase to the surface of the anode comprises a syringe for injecting the fuel and water, and the injection speed is preferably variable.

The fuel and water may be mixed and supplied by one supply means, or the fuel and water may be independently supplied by separate supply means. The latter can be said to be preferable in such a case because the ratio of fuel to water can be changed. In this case, the means for supplying the fuel and water in the liquid phase to the surface of the anode is preferably composed of two types of injectors for injecting the fuel or water, and each injection rate is independently variable. Here, as the injector, for example, a combination of an electromagnetic valve, a liquid feed pump, a pressurizing device, an injector (for example, an inkjet printer head (piezo type, thermal type, etc.)), a surface acoustic wave element, or the like can be used. By adopting such a structure, it is possible to supply only the required amount of fuel and / or water to the anode surface, and as a result, it is possible to supply a high-concentration fuel without causing crossover, It is possible to supply fuel and water at an appropriate ratio and flow rate according to the operating conditions of the battery.

The means for supplying the fuel and water in the liquid phase to the surface of the anode is preferably controlled so as to produce a desired operating condition while detecting the operating condition of the fuel cell.
As such an operation status detecting means, it is possible to use a voltage and current measuring means of the battery unit, a fuel / water ratio detecting sensor on the anode surface, or a fuel / water ratio detecting sensor in the solid electrolyte. it can. Furthermore, M
It is preferable to supply the fuel and water to the anode at an appropriate ratio and flow rate according to the operating condition by using a PU arithmetic element or the like.

The present invention also provides a method for generating power in any of the above fuel cells, wherein the surface of the anode consumes fuel and water in an amount such that the anode surface is not always wet, and the fuel is consumed at a fuel supply rate. So that the speed is almost the same as
A method of generating power from a fuel cell, including a step of supplying in a liquid phase. It is preferable to change the supply rate depending on the operating condition of the fuel cell, and it is more preferable to change the supply rates of fuel and water independently. By such a method, fuel and water can be supplied to the anode surface at a controlled ratio and flow rate according to the operating condition of the fuel cell. As mentioned above, supplying fuel and water in an amount such that the anode surface is always wet causes a crossover problem, so that the supply of the fuel and water to the anode surface is such that the anode surface does not always wet. The water is supplied such that the fuel supply rate is approximately the same as the fuel consumption rate. Such a supply rate is, in other words, such a supply rate as to supply the fuel and water in an amount that does not cause crossover to the anode surface.

Further, by using such a method, the ratio of the fuel supply speed to the water supply speed is increased when the power generation amount is large, and the fuel ratio to the water supply speed is increased when the power generation amount is small. By reducing the ratio of the supply rates, it is possible to supply an appropriate amount of fuel and water according to the required amount of power generation. Since the appropriate supply speed of fuel and water depends on the configuration and operating condition of the fuel cell, it is possible to grasp the optimum value by feeding back the operating condition and problems of the fuel cell while operating the fuel cell. it can.

Further, when the fuel cell is suddenly stopped, if the supply of the mixture of fuel and water is suddenly stopped, the material of the anode is deteriorated. In order to prevent this, if only water is supplied to the anode, the surface of the anode is in a low fuel concentration state where crossover does not occur, and a sudden stop can be endured. That is, when stopping the power generation, it is preferable to stop the fuel supply and supply only water. On the other hand, when the operation is rapidly started from the stopped state, it is possible to realize the high fuel concentration state by supplying only the fuel. Further, in the case of shifting to a steady state, the ratio of fuel and water may be set to a constant composition.

Normally, when the anode is placed horizontally and the fuel and water are supplied to the surface of the anode, it is naturally diffused. However, even if a suitable auxiliary means for evenly diffusing the fuel and water to the surface of the anode is used. Good. For example, a plurality of fuel and water supply means may be used for more uniform diffusion or when the surface is large. Additionally, a liquid fuel absorber plate may be used to more evenly diffuse the fuel and water to the surface of the anode, which may be a porous material such as glass fiber mat, carbon foam, expanded polytetrafluoroethylene,
It is preferably composed of a reticulated metal or the like. Further, as the fuel, a lower alcohol having 1 to 4 carbon atoms, glycol, ether or the like, for example, dimethyl ether, diethyl ether, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, secondary butanol, tertiary Butanol, ethylene glycol, diethylene glycol and the like can be used, but methanol is preferably used.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described, but it is not intended to limit the present invention. An example of the fuel cell of the present invention is shown in FIGS. This fuel cell has an anode (fuel electrode) 2 and a cathode (air electrode)
3 and a solid electrolyte membrane 1 sandwiched therebetween, a liquid fuel absorption plate 7 for uniformly diffusing fuel and water is arranged on the anode 2 in contact therewith. If the uniformity of fuel and water is ensured on the anode surface, this liquid fuel absorption plate 7 may be omitted. The fuel and water may be mixed and supplied by one supply means 4 (FIG. 1), or the fuel and water may be separately supplied by two supply means (fuel supply means 9 and water supply means 10, FIG. 2). May be supplied independently. When the fuel and water are supplied at a constant ratio, the former method may be used. In this case, only the fuel and water supply rate can be changed while keeping the ratio constant. In the latter case, fuel or water can be supplied at independent supply rates. The fuel and water are supplied onto the anode 2 or the liquid fuel absorption plate 7 by the fuel supply means 9 and the water supply means 10 or the supply means 4 of the mixed liquid of fuel and water at the set ratio and supply speed. By such a supply method, an amount of fuel and water that the anode surface does not always wet the surface of the anode,
The fuel can be supplied so that the fuel supply speed is almost the same as the fuel consumption speed. Therefore, crossover can be prevented on the anode 7, and fuel can be supplied according to the operating condition.

Another embodiment of the fuel cell of the present invention is shown in FIG. The fuel cell 20 is formed by stacking a plurality of layers (3 layers in FIG. 3) of unit cells (MEA), and each unit cell has an anode (fuel electrode) 12, a cathode (air electrode) 14 and a solid sandwiched between them. It is composed of three layers of an electrolyte membrane 13, and is further arranged so that the liquid fuel absorption plate 11 is in contact with the anode 12 and the air flow path 15 is in contact with the cathode 14. Fuel and water are separately stored in the fuel storage unit 16 and the water storage unit, and these are separately supplied from the fuel water supply unit 17 to the liquid fuel absorption plate 11 through the air gap, and the liquid fuel (fuel and The water) immediately diffuses uniformly to the surface of the anode 12 by the liquid fuel absorption plate 11 and reaches. The three cell units may be connected in series or in parallel by a known means such as a bipolar plate.

Another embodiment of the fuel cell of the present invention is shown in FIG. The fuel cell 20 is an improved version of the fuel cell shown in FIG. 4 and has a plurality of water accommodating portions, a fuel accommodating portion 16, and a fuel water supply means 17 in one. Yet another embodiment of the fuel cell of the present invention is shown in FIG. A so-called flat stack cell in which four sets of anodes (fuel electrodes) 12 and cathodes (air electrodes) 14 are arranged on one solid electrolyte membrane 13 so as to sandwich the solid electrolyte membrane 13 is a liquid fuel absorption plate 11
Are shared, and two are arranged above and below it.
Similar to the above, the fuel and water are supplied from the fuel water supply means 17 to the liquid fuel absorption plate 11, and the liquid fuel (fuel and water) is uniformly diffused on the surface of the anode 12. The eight cell units may be connected in series or in parallel by known means.

Another embodiment of the fuel cell of the present invention is shown in FIG. The fuel cell main body 30 has a cylindrical shape, and a plurality of cell units 21 are arranged therein and connected in series or in parallel. The structure of the cell unit 21 is shown in FIG.
The fuel 24 and water 25 are mixed and sprayed as a mixture 27 of fuel and water onto the inner wall of the cylindrical fuel cell body 30 by the fuel water supply means 26. The anode 32 of the cell unit 21 is exposed to this inner wall, or the liquid fuel absorption plate 34 disposed in contact with the anode is exposed, and the liquid fuel supplied to these is uniformly dispersed in the anode. .

[0018]

The fuel cell of the present invention has various advantages. That is, since the required amount of fuel is appropriately supplied to the anode, crossover does not occur even if high-concentration fuel is supplied. Further, since high concentration fuel can be supplied, high output can be taken out. In addition, fuel and water are supplied to the anode at an appropriate ratio according to operating conditions, so it is possible to follow sudden output fluctuations (for example, laptop computer use mode / sleep mode; mobile phone standby mode / talk mode). ). Furthermore, since it does not use a vaporizer,
Does not require extra space or heating source.

[0019]

【Example】Example 1 Commercial direct methanol fuel cell (H-TEC
Manufactured) and take out the membrane electrode structure (MEA) shown in FIG.
I remodeled it. That is, G2 with a thickness of 2 mm on the anode
Attach the glass filter closely to prevent the fuel from volatilizing.
Polyethylene terephthalate with a 5 mm diameter hole on the top
It was covered with a sheet of film and used as a single cell in the test.
Electrochemical measurement of the anode and cathode terminals of this cell
For equipment (galvanostat: HA301 manufactured by Hokuto Denko)
Connected Syringe (Muromachi Kikai: KDS)
30:70 volume ratio of methanol: water (methanol concentration 7.4
Equivalent to M. ) Put the mixed solution of
Injected up. Keep the current value (I) constant and
Do not determine the feed rate of the liquid fuel mixture that does not cause the bar.
The maximum output (I × V)
The voltage (V) at which the can be taken out was recorded. Also the current
Change the value and plot the resulting voltage
It was The result is shown in FIG. As a result, Comparative Example 1 described later
A current-voltage characteristic of over 2 was observed.

[0020]Example 2 Using the same cell and apparatus as in Example 1, methanol and water were used.
The mixing ratio (volume ratio) was 45:55 (methanol concentration 11
Equivalent to M. ) Except that the same as in Example 1.
The current-voltage characteristics of the battery cell were measured. Figure 8 shows the measurement results.
Shown in. Also in this example, the results were higher than those in Comparative Examples 1 and 2 described later.
Current-voltage characteristics were observed.

[0021]Comparative Example 1 Direct methanol fuel cell modified and used in Example 1
The membrane electrode structure for ponds (MEA) is used without modification
Using a 0.5-10 M concentration aqueous methanol solution.
To generate electric power, and to determine the current-voltage characteristics of this battery cell by electrochemical
It measured using the measuring device (galvanostat). Use
The structure of the fuel cell is shown in Fig. 9, and the measurement results are shown in Fig. 10.
You When the methanol concentration is 0.5M to 5M,
The current-voltage characteristic also shows an increase in output with an increase. Only
However, when the methanol concentration is 7M or more, a decrease in output is observed.
The methanol crossed over the solid electrolyte membrane.
And indicates.

[0022]Comparative example 2 A mixed liquid 6 of fuel and water was supplied from the cell and apparatus used in Comparative Example 1.
Remove it, and replace it with a gas bottle containing methanol and water.
Saturated gas of methanol and water with dry nitrogen gas aeration 4
The ratio of 00 ml / min and 100 ml / min
While mixing with, led to the fuel electrode. Similar to the first embodiment,
The current-voltage characteristics of the pond cell were measured. Figure of this measurement result
8 shows. In this comparative example, you get very low output
I couldn't. In gas-phase fuel supply, high-concentration fuel
It is thought that it is not possible to send the fee.

From the results of FIG. 8, it is possible to operate at a fuel concentration higher than the fuel concentration at which crossover occurred in Comparative Example 1 by adjusting the supply rates of methanol and water. As a result, Comparative Example It was possible to obtain a current-voltage characteristic higher than 1 or 2. That is, it was shown that, when the liquid fuel supply system of the present invention is used, high output can be obtained without causing crossover even when supplying a high-concentration aqueous methanol solution.

[0024]Example 3 Using the same fuel cell as in Example 1, the structure of the fuel cell
Can supply fuel and water independently, as shown in FIG.
Was changed. That is, 2 syringes (Muromachi Kikai: K
DS), put methanol in one and water in the other
And change the mixing ratio of methanol and water as shown below.
While supplying the power to the battery cell under constant current output of 20mA
The voltage change was recorded. The measurement result is shown in FIG. In the figure
The arrow symbol indicates the time when the following ratio of liquid fuel was injected.
You A: Methanol: water (volume ratio) = 30: 70 (methanone
(Equivalent to a concentration of 7.4M) B: Methanol: water (volume ratio) = 0: 100 C; methanol: water (volume ratio) = 2: 98 (methanol
Equivalent to a concentration of 0.5M) D; methanol: water (volume ratio) = 0.5: 99.5 (medium
Equivalent to a tanol concentration of 0.12M) From this result, using the liquid fuel supply system of the present invention,
The output can be made to follow even a sudden load change.
I understand. Load fluctuation due to such liquid fuel supply system
A fuel cell system that can handle the above has become possible.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an embodiment of a fuel cell of the present invention. Fuel and water are mixed and injected.

FIG. 2 is a cross-sectional view showing an embodiment of the fuel cell of the present invention. Inject fuel and water independently. Dark color represents fuel and colorless color represents water.

FIG. 3 is a cross-sectional view showing an embodiment of the fuel cell of the present invention. Dark color represents fuel and colorless color represents water.

FIG. 4 is a cross-sectional view showing an embodiment of the fuel cell of the present invention. Dark color represents fuel and colorless color represents water.

FIG. 5 is a cross-sectional view showing an embodiment of the fuel cell of the present invention. Dark color represents fuel and colorless color represents water.

FIG. 6 is a diagram showing an embodiment of the present invention. Dark color represents fuel and colorless color represents water.

7 is a cross-sectional view showing a part 21 of the cell of FIG.

8 is a diagram showing current-voltage characteristics of Examples 1 and 2 and Comparative Example 2. FIG.

9 is a view showing a conventional direct methanol fuel cell used in Comparative Example 1. FIG.

10 is a diagram showing current-voltage characteristics of the fuel cell (FIG. 9) of Comparative Example 1. FIG.

11 is a diagram showing voltage-time characteristics of Example 3. FIG.

[Explanation of symbols]

1, 13, 31 Solid electrolyte membrane 2, 12, 32 Fuel electrode 3, 14, 33 Air electrode 4 Fuel and water mixture supply 5 Fuel container 6,27 Fuel and water mixture 7, 11, 34 Liquid fuel absorption plate 8 Fuel volatility prevention cover 9 Fuel supply means 10 Water supply means 15 Air flow path 16 Fuel storage 17, 26 Fuel water supply means 20, 30 Fuel cell body 21 Part of fuel cell 23a Fuel storage section 23b Water storage section 24 fuel 25 water

Claims (12)

[Claims] 1. A fuel cell comprising an anode, a cathode, and an electrolyte sandwiched between the anode and the cathode, and means for supplying a fuel and water in a liquid phase to the surface of the anode without the storage liquid containing the fuel coming into contact with the anode. A fuel cell characterized by being provided. 2. The fuel cell according to claim 1, wherein the means for supplying the fuel and water in a liquid phase to the surface of the anode comprises a syringe for injecting the fuel and water, and the injection speed is variable. 3. The fuel cell according to claim 1, wherein the means for supplying the fuel and water in a liquid phase to the surface of the anode can supply the fuel and water independently. 4. The means for liquid-phase supplying fuel and water to the surface of the anode comprises two kinds of injectors for injecting fuel or water, and each injection rate is independently variable.
The fuel cell described in 1. 5. The fuel cell according to claim 1, further comprising means for uniformly diffusing fuel and water on the surface of the anode. 6. The fuel cell according to claim 5, wherein the means for uniformly diffusing the fuel and water on the surface of the anode is a porous material layer. 7. The fuel according to claim 1, wherein the fuel is methanol.
6. The fuel cell according to any one of 6 above. 8. The fuel cell power generation method according to claim 1, wherein the surface of the anode is supplied with fuel and water in an amount such that the surface of the anode is not always wet. A method for power generation in a fuel cell, which includes the step of supplying in a liquid phase so that the fuel consumption rate is substantially the same as the fuel consumption rate. 9. The method of claim 8, wherein the feed rate is varied. 10. The method of claim 9, wherein the fuel and water feed rates are varied independently. 11. The ratio of the fuel supply speed to the water supply speed is increased when the power generation amount is high, and the ratio of the fuel supply speed to the water supply speed is decreased when the power generation amount is low. 10. The method according to 10. 12. The method according to claim 10, wherein when the power generation is stopped, the fuel supply is stopped and only water is supplied.