JPWO2007072880A1 - Fuel cell - Google Patents

Abstract

The fuel cell includes a cathode, an anode, a solid electrolyte membrane 6 disposed between the cathode and the anode and having a methanol crossover value to water crossover value of 0.5 or less, and a liquid fuel containing methanol. The fuel chamber 9 accommodated therein and supply means for supplying at least the vapor of the liquid fuel to the anode are provided.

Description

  The present invention relates to a fuel cell.

  In recent years, various electronic devices such as personal computers and mobile phones have been downsized with the development of semiconductor technology. Attempts have been made to use fuel cells as power sources for these small devices. Fuel cells have the advantage that they can generate electricity simply by supplying fuel and oxidant, and can continuously generate electricity if only the fuel is replenished / replaced. For this reason, if the fuel cell can be miniaturized, it can be said that the system is extremely advantageous for the operation of the portable electronic device. In particular, direct methanol fuel cells (DMFCs) use methanol with high energy density as fuel, and can take out current directly from methanol on the electrode catalyst, enabling downsizing and handling of fuel. Since it is easier than hydrogen gas fuel, it is promising as a power supply for small equipment.

  DMFC fuel supply methods include a gas supply type DMFC in which liquid fuel is vaporized and then fed into the fuel cell by a blower or the like, and a liquid supply type DMFC in which liquid fuel is directly fed into the fuel cell by a pump or the like. However, in these fuel supply methods, the pump for supplying methanol and the blower for supplying air are required as auxiliary devices as described above, and the system has a complicated form and has a drawback that it is difficult to reduce the size.

  On the other hand, an internal vaporization type DMFC as shown in Japanese Patent Publication No. 3413111 is known as another fuel supply method. The internal vaporization type DMFC includes a fuel permeation layer that holds liquid fuel, and a fuel vaporization layer for diffusing a vaporized component of the liquid fuel held in the fuel permeation layer. In this internal vaporization type DMFC, vaporized liquid fuel is supplied from the fuel vaporization layer to the anode. In Japanese Patent No. 3413111, a methanol aqueous solution in which methanol and water are mixed at a molar ratio of 1: 1 is used as a liquid fuel, and both methanol and water are supplied to the anode in the form of vaporized gas.

  According to such an internal vaporization type DMFC shown in the patent publication, sufficiently high output characteristics cannot be obtained. Since water has a lower vapor pressure than methanol and the vaporization rate of water is slower than the vaporization rate of methanol, when both methanol and water are supplied to the anode by vaporization, the relative supply rate of water to the methanol supply rate Is lacking. As a result, since the reaction resistance of the reaction for internally reforming methanol is increased, sufficient output characteristics cannot be obtained.

  An object of the present invention is to provide a fuel cell with improved power density.

A fuel cell according to the present invention comprises a cathode,
An anode,
A solid electrolyte membrane disposed between the cathode and the anode and having a methanol crossover value to water crossover value ratio of 0.5 or less;
A fuel chamber containing a liquid fuel containing methanol;
And a supply means for supplying at least the vapor of the liquid fuel to the anode.

A fuel cell according to the present invention includes a cathode,
An anode,
A solid electrolyte membrane disposed between the cathode and the anode and having a methanol crossover value to water crossover value ratio of 0.5 or less;
A fuel chamber containing a liquid fuel containing methanol;
A gas-liquid separation layer is provided between the fuel chamber and the anode, and selectively transmits the vapor of the liquid fuel in the fuel chamber.

FIG. 1 is a schematic diagram of an apparatus used for measuring a ratio of a methanol crossover value to a water crossover value. FIG. 2 is a schematic cross-sectional view showing a direct methanol fuel cell according to an embodiment of the present invention.

  As a result of intensive research, the present inventors have found that in a fuel cell that supplies liquid fuel vapor to the anode, the ratio of the methanol crossover value to the water crossover value of the solid electrolyte membrane (hereinafter referred to as the crossover ratio) is It has been found that a high output density can be obtained when it is 0.5 or less. When the crossover ratio exceeds 0.5, since the water diffusion from the cathode to the anode is small, the reaction resistance of the reforming reaction is high and a high output density cannot be obtained. The smaller the crossover ratio, the greater the amount of water supplied to the anode, but the lower the methanol diffusivity, the lower the output at the initial stage of power generation. For this reason, since the amount of water generated at the cathode is small at the initial stage of power generation, the amount of water diffusing from the cathode to the anode is small. For this reason, if the crossover ratio is not set too low, methanol can be supplied to the cathode to generate water by the methanol oxidation reaction, and the generated water can be diffused to the anode through the solid electrolyte membrane. In addition, it is possible to make up for water shortage at the initial stage of power generation and to obtain high output from the initial stage of power generation.

  In a liquid-feed type fuel cell that supplies liquid fuel to the anode, if a solid electrolyte membrane having a high water permeability with a crossover ratio of 0.5 or less is used, gas diffusibility at the cathode is reduced or supplied to the anode. The output density is reduced by diluting the fuel to be diluted. In the liquid feed type fuel cell, a solid electrolyte membrane in which the methanol crossover value and the water crossover value are close to each other and approximately equal to each other is preferable. Nafion membranes frequently used as solid electrolyte membranes have a crossover ratio exceeding 0.5.

  The measurement of the crossover ratio is performed by the method described below. First, as shown in FIG. 1, the solid electrolyte membrane 22 is attached to the H-type cell 21. The H-type cell 21 includes two cylindrical tubes 24a and 24b each having connection tubes 23a and 23b on the peripheral surface. The solid electrolyte membrane 22 is disposed between the connecting tube 23a of the cylindrical tube 24a and the connecting tube 23b of the cylindrical tube 24b, and the upper end of the solid electrolyte membrane 22 and the ends of the connecting tubes 23a and 23b are sandwiched by the sandwiching member 25, and the solid The lower end of the electrolyte membrane 22 and the ends of the connecting pipes 23a and 23b are sandwiched between the sandwiching members 25. Examples of the clamping member 25 include a screw and a spring. Pure methanol is accommodated in the right cylindrical tube 24a, and the same amount of water is accommodated in the left cylindrical tube 24b. The temperature of pure methanol and water is kept at 25 ° C., and the liquid in each of the cylindrical tubes 24a and 24b is sampled every 6 hours up to 6 hours, and the water and methanol concentration analysis of each liquid is performed. For the right cylindrical tube 24a, the water concentration obtained by measurement every hour is averaged. For the left cylindrical tube 24b, the methanol concentration obtained by measurement every hour is averaged. The average water concentration is the water crossover value, and the average methanol concentration is the methanol crossover value.

The methanol crossover value of the solid electrolyte membrane is desirably 1 μM / cm ² · s or less. This is because if the methanol crossover value exceeds 1 μM / cm ² · s, the amount of fuel consumed by the methanol crossover increases and a high power density may not be obtained. As the proton conductive material contained in the solid electrolyte membrane, for example, a hydrocarbon resin can be used.

  Examples of the liquid fuel include pure methanol and aqueous methanol solution. The methanol concentration of the liquid fuel is preferably 64% by weight or more and 100% by weight or less. Since the solid electrolyte membrane with a crossover ratio of 0.5 or less has excellent water diffusibility, when the methanol concentration of the liquid fuel is low, water permeates through the solid electrolyte membrane and is supplied to the cathode. This is because there is a risk that the output density may be lowered due to a decrease in performance. A more preferable range of the methanol concentration is 65% by weight or more and 100% by weight or less.

  Hereinafter, a direct methanol fuel cell which is an embodiment of a fuel cell according to the present invention will be described with reference to the drawings.

  FIG. 2 is a schematic cross-sectional view showing a direct methanol fuel cell according to an embodiment of the present invention.

  As shown in FIG. 2, the membrane electrode assembly (MEA) 1 includes a cathode (for example, an oxidant electrode) including a cathode catalyst layer 2 and a cathode gas diffusion layer 4, and an anode catalyst layer 3 and an anode gas diffusion layer 5. An anode (for example, a fuel electrode) and a solid electrolyte membrane 6 disposed between the cathode catalyst layer 2 and the anode catalyst layer 3 are provided.

  Examples of the catalyst contained in the cathode catalyst layer 2 and the anode catalyst layer 3 include platinum group element simple metals (Pt, Ru, Rh, Ir, Os, Pd, etc.), alloys containing platinum group elements, and the like. be able to. Although it is desirable to use Pt-Ru which has strong resistance to methanol and carbon monoxide as the anode catalyst and platinum as the cathode catalyst, it is not limited to this. Further, a supported catalyst using a conductive support such as a carbon material may be used, or an unsupported catalyst may be used.

  The cathode catalyst layer 2 is laminated on the cathode gas diffusion layer 4, and the anode catalyst layer 3 is laminated on the anode gas diffusion layer 5. The cathode gas diffusion layer 4 plays a role of uniformly supplying the oxidant to the cathode catalyst layer 2, but also serves as a current collector for the cathode catalyst layer 2. On the other hand, the anode gas diffusion layer 5 serves to uniformly supply fuel to the anode catalyst layer 3 and also serves as a current collector for the anode catalyst layer 3. The cathode conductive layer 7a and the anode conductive layer 7b are in contact with the cathode gas diffusion layer 4 and the anode gas diffusion layer 5, respectively. For the cathode conductive layer 7a and the anode conductive layer 7b, for example, porous layers (for example, mesh) made of a metal material such as gold can be used.

  The rectangular frame-shaped cathode sealing material 8 a is located between the cathode conductive layer 7 a and the solid electrolyte membrane 6 and surrounds the cathode catalyst layer 2 and the cathode gas diffusion layer 4. On the other hand, the rectangular frame-shaped anode sealing material 8 b is located between the anode conductive layer 7 b and the solid electrolyte membrane 6 and surrounds the anode catalyst layer 3 and the anode gas diffusion layer 5. The cathode sealing material 8 a and the anode sealing material 8 b are O-rings for preventing fuel leakage and oxidant leakage from the membrane electrode assembly 1.

  A liquid fuel tank 9 is disposed below the membrane electrode assembly 1 as a fuel chamber. In the liquid fuel tank 9, liquid methanol or aqueous methanol solution is accommodated.

  The gas-liquid separation film 10 as a gas-liquid separation layer has a function of selectively allowing liquid fuel vapor to permeate and separating the liquid fuel vapor and the liquid component. Examples of the gas-liquid separation membrane 10 include a silicone rubber sheet. The gas-liquid separation membrane 10 is disposed at the open end of the liquid fuel tank 9.

  A resin frame 11 is laminated between the gas-liquid separation membrane 10 and the anode conductive layer 7b. The space surrounded by the frame 11 functions as a steam storage chamber 12 (so-called steam reservoir) that temporarily stores the steam diffused through the gas-liquid separation membrane 10. Due to the effect of suppressing the amount of methanol permeated by the vapor storage chamber 12 and the gas-liquid separation membrane 10, it is possible to avoid supplying a large amount of vapor to the anode catalyst layer 3 at one time, thereby suppressing the occurrence of methanol crossover. Is possible. The frame 11 is a rectangular frame, and is formed of a thermoplastic polyester resin such as PET.

  A moisturizing plate 13 is laminated on the cathode conductive layer 7a. The moisturizing plate 13 serves to suppress the transpiration of the water generated in the cathode catalyst layer 2 and uniformly introduces the oxidant into the cathode gas diffusion layer 4 so that the oxidant is uniformly diffused into the cathode catalyst layer 2. It also plays a role as an auxiliary diffusion layer.

  A cover 15 in which a plurality of air inlets 14 for taking in air as an oxidant is formed is laminated on the moisture retaining plate 13. Since the cover 15 also plays a role of pressurizing the stack including the membrane electrode assembly 1 to increase its adhesion, the cover 15 is formed of a metal such as SUS304, for example.

  According to the direct methanol fuel cell having the above-described configuration, the liquid fuel in the liquid fuel tank 9 is vaporized, and the vapor of the liquid fuel diffuses through the gas-liquid separation film 10 and is temporarily stored in the vapor storage chamber 12. From there, the anode gas diffusion layer 5 is gradually diffused and supplied to the anode catalyst layer 3. A part of the vapor is also supplied to the cathode through the solid electrolyte membrane 6. As a result, an internal reforming reaction of methanol shown in the following reaction formula (1) occurs.

CH ₃ OH + H ₂ O → CO ₂ + 6H ⁺ + 6e ⁻ (1)
Protons (H ⁺ ) generated by the internal reforming reaction diffuse through the solid electrolyte membrane 6 and reach the cathode catalyst layer 3. On the other hand, the air taken in from the air inlet 14 of the cover 15 diffuses through the moisture retention plate 13 and the cathode gas diffusion layer 4 and is supplied to the cathode catalyst layer 2. In the cathode catalyst layer 2, water is generated by the reaction shown in the following formula (2), that is, a power generation reaction occurs.

(3/2) O ₂ + 6H ⁺ + 6e ⁻ → 3H ₂ O (2)
When the power generation reaction proceeds, water generated in the cathode catalyst layer 2 by the reaction of the above-described formula (2) or the like diffuses in the cathode gas diffusion layer 4 and reaches the moisturizing plate 13, and transpiration is caused by the moisturizing plate 13. The water storage amount in the cathode catalyst layer 2 is increased due to inhibition. On the other hand, on the anode side, water vapor is supplied through the gas-liquid separation membrane 10 or is not supplied at all. For this reason, it is possible to create a state in which the moisture retention amount of the cathode catalyst layer 2 is larger than the moisture retention amount of the anode catalyst layer 3 as the power generation reaction proceeds.

  Since the crossover ratio of the solid electrolyte membrane 6 is 0.5 or less, water diffusion from the cathode catalyst layer 2 to the anode catalyst layer 3 can be promoted, and the internal reforming reaction of methanol shown in the above formula (1) Thus, a high power density can be obtained.

  In the embodiment described above, the moisturizing plate 13 is provided, but the present invention is not limited to the configuration having the moisturizing plate. According to the solid electrolyte membrane having a crossover ratio of 0.5 or less, water diffusion from the cathode to the anode can be promoted even when the moisture retention plate 13 is not provided, and a high output density can be obtained.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Example 1 to Example 5)
<Production of anode>
A perfluorocarbon sulfonic acid solution, water and methoxypropanol were added to an anode catalyst (Pt: Ru = 1: 1) supported carbon black, and the catalyst supported carbon black was dispersed to prepare a paste. The obtained paste was applied to porous carbon paper as an anode gas diffusion layer to obtain an anode having a thickness of 450 μm.

<Production of cathode>
A perfluorocarbon sulfonic acid solution, water and methoxypropanol were added to the cathode catalyst (Pt) -supported carbon black, and the catalyst-supported carbon black was dispersed to prepare a paste. The obtained paste was applied to porous carbon paper as a cathode gas diffusion layer to obtain a cathode having a thickness of 400 μm.

<Preparation of solid electrolyte membrane>
The crossover ratio of the solid electrolyte membrane having various crossover ratios was measured by the method shown in FIG. The crossover ratio is shown in Table 1 below.

  A solid electrolyte membrane was disposed between the anode catalyst layer and the cathode catalyst layer, and hot pressing was performed on these to obtain a membrane electrode assembly (MEA).

A polyethylene porous film having a thickness of 500 μm, a moisture permeability of 2 seconds / 100 cm ³ (JIS P-8117), and a moisture permeability of 4000 g / m ² 24h (JIS L-1099 A-1 method) as a moisture retaining plate. Prepared.

  The frame is made of PET and has a thickness of 25 μm. In addition, a silicone rubber sheet having a thickness of 200 μm was prepared as a gas-liquid separation membrane.

  An internal vaporization type direct methanol fuel cell having the structure shown in FIG. 2 was assembled using the obtained membrane electrode assembly, moisture retention plate, frame, and gas-liquid separation membrane. At this time, 2 mL of pure methanol having a purity of 99.9% by weight was stored in the fuel tank.

(Comparative Examples 1-3)
The internal vaporization type direct methanol type having the same configuration as that described in Example 1 except that a membrane having a crossover ratio outside the range of the crossover ratio defined in the present invention is used as the solid electrolyte membrane. A fuel cell was assembled.

As a result of measuring the output density change when the current density (mA / cm ² ) was increased for the obtained fuel cells, the results of Examples 1 to 5 provided with solid electrolyte membranes having a crossover ratio of 0.5 or less. According to the fuel cell, high power density was obtained.

  On the other hand, in the fuel cells of Comparative Examples 1 to 3 including the solid electrolyte membrane having a crossover ratio exceeding 0.5, the output density was lower than that of Examples 1 to 5.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

  For example, in the above description, as a configuration of the fuel cell, a fuel storage part (fuel chamber) is provided below the membrane electrode assembly (MEA), and a gas-liquid separation layer is provided as a supply means for supplying at least liquid fuel vapor to the anode. As described in the structure used, the fuel supply from the fuel storage section (fuel chamber) to the MEA is provided with a flow path between the fuel chamber and the MEA, and the liquid fuel and the liquid fuel are supplied to the MEA via this flow path. It may be performed by supplying steam. Further, although the description has been given by taking the passive fuel cell as an example of the configuration of the fuel cell body, the present invention can also be applied to a semi-passive fuel cell in which a pump or the like is partially used for fuel supply or the like. it can. Only the vapor of the liquid fuel may be supplied to the anode, but the present invention can be applied even when the vapor of the liquid fuel and the liquid fuel are supplied to the anode. Even if it is these structures, the effect similar to the above-mentioned description is acquired.

  According to the present invention, a fuel cell with improved power density can be provided.

Claims (5)

A cathode,
An anode,
A solid electrolyte membrane disposed between the cathode and the anode and having a methanol crossover value to water crossover value ratio of 0.5 or less;
A fuel chamber containing a liquid fuel containing methanol;
A fuel cell comprising supply means for supplying at least the vapor of the liquid fuel to the anode.   The fuel cell according to claim 1, wherein the liquid fuel has a methanol concentration of 64% or more. 3. The fuel cell according to claim 1, wherein the solid electrolyte membrane has a methanol crossover value of 1 μM / cm ² · s or less.   The fuel cell according to claim 1, wherein the solid electrolyte membrane contains a hydrocarbon-based resin.   The said supply means is a gas-liquid separation layer which is arrange | positioned between the said fuel chamber and the said anode, and selectively permeate | transmits the vapor | steam of the said liquid fuel of the said fuel chamber. Fuel cell.

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