JP2007305502A - Direct type liquid fuel battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a direct type liquid fuel battery, of which supply of air will not be interrupted by waterdrops produced at power generation during cleaning of air supplied to a cathode electrode by an air purification filter. <P>SOLUTION: In the direct type fuel battery provided with a fuel battery cell equipped with an anode electrode and a cathode electrode through an electrolyte membrane, with the anode electrode pinched by an anode end plate having a collecting function and a fuel supply port, and a cathode electrode pinched by a cathode end plate having a collecting function and a supply/exhaust port of air and product water, and a means for supplying liquid fuel to the anode electrode, an air purification filter is fitted to the supply/exhaust port of the cathode end plate via a moisture-absorption frame. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention is a fuel in which liquid fuel is oxidized at the anode electrode of a membrane electrode assembly (MEA) composed of an anode electrode, an electrolyte membrane, a cathode electrode, and a diffusion layer, and oxygen is reduced at the cathode electrode. It relates to batteries.

  A fuel cell is composed of at least a solid or liquid electrolyte and two electrodes (anode electrode and cathode electrode) that induce a desired electrochemical reaction, and converts the chemical energy of the fuel directly into electric energy with high efficiency. It is. A polymer electrolyte fuel cell (PEM-FC) system generally includes unit cells in which a porous anode electrode and a cathode electrode are arranged on both sides of a solid polymer electrolyte membrane in series and as required. And connected in parallel to each other, a fuel container, a fuel supply device, and an air or oxygen supply device.

Among PEM-FCs, direct methanol fuel cells using liquid fuel (DMFC:
Direct Methanol Fuel Cell), metal hydride, and hydrazine fuel cells are effective as small portable or portable power sources because of their high volumetric energy density. Among these direct liquid fuel cells, DMFCs that use methanol as fuel, which are easy to handle and are expected to be produced from biomass in the near future, are ideal power systems.

In the case of DMFC, an oxidation reaction of methanol occurs at the anode electrode, and carbon dioxide (CO ₂ ) is generated. On the other hand, at the cathode electrode, hydrogen ions generated by the reaction at the anode electrode react with oxygen in the air on the cathode electrode to generate water. When the generated water stays on the surface of the cathode electrode, supply of oxygen to the cathode electrode is interrupted, and the output voltage is greatly reduced. In addition, when impurity elements entering from the outside are mixed in the accumulated product water and ionized, various adverse effects are exerted on the DMFC. For example, the catalytic activity is weakened by reacting or adsorbing with the cathode catalyst or the electrolyte membrane, and the proton conductivity of the electrolyte membrane is reduced. Therefore, in order for the DMFC to maintain a stable output voltage, it is important not to retain the generated water on the surface of the cathode electrode, to prevent the entry of impurity elements into the cathode electrode, and the like.

  In recent years, with the rapid spread of portable information terminal devices, the usage environment has diversified, and power generation in a harsher environment than DMFC must be considered. For example, it is necessary to maintain a stable output even in an environment in which dust containing a large amount of impurity elements such as the coast, sea, indoor stadium, outdoor ground, streets, and trunk roads cannot be avoided. Therefore, in such an environment, it is necessary to prevent dust from entering the cathode electrode. Among the PEM-FCs, in the case of an installation type power source using hydrogen gas as a fuel, a method of removing dust and organic substances in the air by providing an air purification device in the air supply path as represented by Patent Document 1 is known. ing. Since such an installation type power supply is relatively large in size, it has a blower auxiliary machine and the like along with an air supply path so that sufficient air can be sent to the cathode electrode. It can be said that the method of attaching purification devices is an effective means. However, in order to be a portable power source such as a DMFC, it is essential to reduce the size and power consumption of the battery itself. I have to refrain. Further, Patent Document 2 describes a method for reducing the activity of various intruding impurity elements by including a weakly acidic substance in the electrolyte membrane as a measure for improving the durability of the electrolyte membrane instead of the air purification filter. However, in this case, in order to achieve both the output and durability of the DMFC, it is important to optimize the type and addition amount of the weakly acidic substance to be used, and the appearance of a simpler measure is desired.

The power generation operation system of the direct liquid fuel cell includes an active system in which fuel supply, air supply, and generated water treatment are forcibly operated by an auxiliary machine, and a passive system in which these are operated by natural convection. The present invention is particularly effective in the latter passive system having a simple battery configuration. The basic cell configuration in the passive system is a structure in which each end plate (referred to as an anode extreme plate and a cathode extreme plate) having a current collecting function is sandwiched between both sides of the MEA, and liquid fuel is supplied to the anode end plate. The supply port for supplying and the cathode end plate are usually provided with a plurality of supply / discharge ports for supplying air and discharging generated water at a portion in contact with the diffusion layer. Patent Document 3 describes a method of positively removing the generated water on the cathode side so as not to hinder the supply of air. Here, a water absorption frame is installed on the outer periphery of the surface of the cathode electrode of the MEA, and the water absorbed through the surface of the cathode electrode is absorbed by the absorption frame by pressing the water absorption frame with the cathode end plate. It is characterized by discharging water to the outside of the battery cell. In other words, the water absorption frame here serves to remove the generated water on the cathode electrode side to the outside. By this action, an effect of preventing a decrease in the output of the fuel cell can be obtained. However, when the air purification filter is directly attached to the cathode electrode side, the cathode end plate and the air purification filter are in close contact with each other, so that the generated water near the center of the MEA passes through the cathode end plate. It may be easily absorbed and transferred to the purification filter, which may reduce the function and air supply capability of the air purification filter.

  On the other hand, as a method for removing impurities in place of the air purification filter, as represented by Patent Document 4, in order to prevent the air flowing from the air supply port from directly hitting the cathode end plate, a contact plate is applied to the supply port. Proposals have been made for ventilation structures with an arranged structure. In this case, depending on the thickness of the plate, the cathode end plate and the air supply port of the ventilation structure can be prevented from being in direct contact with each other. It can be prevented from occurring. However, it cannot be denied that the removal performance of fine dust and ionic substances is inferior to that of an air purification filter.

Japanese Patent Application Laid-Open No. 2004-273311 (Abstract) JP 2003-282092 A (summary) JP 2004-165002 A (summary) JP 2000-268835 A (summary)

  One of the causes of the decrease in the power generation performance of the direct liquid fuel cell is that various impurity elements entering from the cathode electrode side decrease the catalytic activity or proton conductivity of MEA. As an effective means for preventing the intrusion of impurities, a method of attaching an air purification filter to the cathode electrode side can be considered, but it is necessary to improve the shortage of air supply due to moisture newly generated at the cathode electrode.

  An object of the present invention is to provide a direct type liquid fuel cell that prevents the supply of air from being blocked by generated water when supplying air purified by an air purification filter to a cathode electrode.

  The present invention has an anode electrode for oxidizing fuel on the first surface of an electrolyte membrane having proton conductivity, and a cathode electrode for reducing oxygen on the other surface. The anode electrode has a current collecting function and a fuel supply port. An anode end plate, a fuel cell sandwiched between a cathode end plate having a current collecting function and a supply and discharge port for air and generated water, and a means for supplying liquid fuel to the anode electrode. The direct type liquid fuel cell is a direct type liquid fuel cell in which an air purification filter is attached to the supply / exhaust port of the cathode end plate through a water absorption frame. The water absorption frame is a porous body.

  The direct liquid fuel cell of the present invention can include a fuel cartridge for sending liquid fuel to the fuel tank in addition to the fuel cell and the fuel tank. This fuel cartridge is preferably removable.

  According to the present invention, the air purification filter is attached to the supply / exhaust port of the cathode end plate via a porous water absorption frame to prevent the phenomenon that the air purification filter absorbs the cathode generated water, It is possible to prevent a decrease in output voltage due to insufficient air supply.

  In general, in a direct liquid fuel cell, moisture generated by a chemical reaction at the cathode electrode penetrates from the catalyst layer at the cathode electrode into the diffusion layer and reaches the surface of the diffusion layer in contact with the outside air. A portion of the water that reaches the surface of the diffusion layer is diffused into the air as water vapor, and the remaining portion remains as water droplets in the air intake port of the cathode end plate. Therefore, when the air purification filter is directly attached to the cathode end plate, the accumulated water droplets accumulate in the gap between the cathode end plate supply / discharge port and the air purification filter, or the water droplets absorbed by the air purification filter are contained in the filter. The air supply to the cathode electrode is shut off, for example, by closing the air holes.

  In the direct liquid fuel cell of the present invention, the air purification filter is attached to the supply / discharge port of the cathode end plate via a porous water absorption frame, so that the distance between the cathode end plate and the air purification filter is widened. Even if water droplets stay in the supply / discharge port of the end plate, it is possible to prevent the air purification filter air holes from being blocked in order to prevent contact between the water droplets and the air purification filter. At the same time, the accumulated water droplets naturally flow out from the supply / discharge port of the cathode end plate and are absorbed by the water absorption frame as the amount increases, so that it is possible to prevent the generated water from leaking to the surroundings. The absorbed moisture moves to the side surface of the water absorption frame by capillary force and can be gradually evaporated. Therefore, the role of the porous water-absorbing frame serves to prevent the air purification filter from closing the pores and to prevent leakage of the cathode-generated water to the surroundings.

  As described above, these actions are performed by attaching a water absorption frame to the cathode end plate, thereby creating a gap between the cathode end plate and the air purification filter, and water droplets staying on the cathode end plate move on the cathode end plate. It is thought that it flows out and is absorbed by the water absorption frame. Therefore, since the attitude of the fuel cell always changes depending on the portable power supply for the portable electronic device, the water absorption frame preferably has a shape that surrounds the four sides of the supply / discharge port of the cathode end plate. On the other hand, the position of the cathode generation water in a stationary type such as an emergency power source that flows out from the cathode end plate is specified, and therefore, the water absorption frame may be attached only in one direction of flowing out.

  The porous water-absorbing frame in the present invention is appropriately selected from plastic, metal, paper, etc. having micron-order pores, and usually has a thickness of 20 mm or less, and at least a portion corresponding to the supply / discharge port of the cathode end plate is opened. Yes. On the other hand, the air purification filter preferably has a structure that combines a membrane-like material with micron-order pores and a chemical adsorbent, which removes dust with a membrane-like material with pores and fine ions with a chemical adsorbent. It becomes possible to capture the sex substance at the same time. Here, the film-like material is appropriately selected from a plastic film, natural fiber, synthetic fiber, paper, and the like, but by using a material having excellent water repellency, it is possible to prevent the adsorbent from absorbing water. The chemical adsorbent is appropriately selected from, for example, activated carbon, zeolite, ion exchange resin, and the like.

  Hereinafter, DMFC will be described as an example, but it goes without saying that the present invention can be widely applied to other liquid fuel cells.

  In DMFC, an aqueous solution having a methanol concentration of several percent to several tens of percent (weight ratio) is used. This fuel cell generates electricity in such a way that the chemical energy of methanol is directly converted into electrical energy by the electrochemical reaction described below. On the anode electrode side, the supplied aqueous methanol solution reacts according to the equation (1) to dissociate into carbon dioxide, hydrogen ions, and electrons (oxidation reaction of methanol).

CH ₃ OH + H ₂ O → CO ₂ + 6H ⁺ + 6e ⁻ (1)
The generated hydrogen ions move from the anode side to the cathode side in the electrolyte membrane, and react with the oxygen gas diffused from the air on the cathode electrode and the electrons on the electrode according to the formula (2) to generate water. (Reduction reaction of oxygen).

6H ⁺ + 3 / 2O ₂ + 6e ⁻ → 3H ₂ O (2)
Therefore, as shown in the equation (3), the total chemical reaction accompanying power generation is that methanol is oxidized by oxygen to generate carbon dioxide gas and water, and the chemical reaction equation is the same as that of methanol flame combustion.

CH ₃ OH + 3 / 2O ₂ → CO ₂ + 3H ₂ O (3)
In addition to the main reaction related to power generation shown above, as a reaction that produces by-products, a reaction in which methanol shown in formula (4) is oxidized to formaldehyde, and formaldehyde shown in formula (5) is oxidized. A reaction for producing formic acid, a reaction in which formic acid and methanol shown in the formula (6) react to produce methyl formate, and the like occur.

CH ₃ OH → HCHO + 2H ⁺ + 2e (4)
HCHO + 1 / 2O ₂ → HCOOH (5)
HCOOH + CH ₃ OH → HCOOCH ₃ + H ₂ O (6)
These by-products can also be removed according to the present invention when contained in the cathode product water.

  Next, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the embodiments described below.

  A cross-sectional view of the basic configuration of a DMFC cell according to the present invention using an aqueous methanol solution as a fuel is shown in FIG. The MEA 2 (membrane electrode assembly) is sandwiched and tightened between the anode end plate 1 and the cathode end plate 3 having a current collecting function, and the air purification filter 5 is attached to the cathode end plate 3 via the water absorption frame 4. is there. Here, as in the example shown in FIG. 2, the water absorption frame 4 has an opening of the water absorption frame that is larger than the supply / discharge port of the cathode end plate (opening for supplying air and discharging generated water) near the center. 7. The air purification filter 5 has an adsorbent portion 6.

  An example when the DMFC cell having the above basic configuration is used as a power source for electronic equipment is shown in FIG. A fuel tank 8 is attached to the anode end plate side, and a methanol aqueous solution as fuel is supplied from a fuel cartridge 9. An example of the emission of carbon dioxide generated at the anode is shown in FIG. Carbon dioxide permeates through a gas-liquid separation membrane (a membrane having a property that gas permeates but liquid does not permeate) attached to the carbon dioxide outlet 11 of the fuel tank lid 10 and is released to the outside air.

  The fuel cartridge 9 is of a type that sends out an aqueous methanol solution by pressure such as high-pressure gas or a spring, and supplies an aqueous methanol solution as a fuel into the fuel tank, and at a pressure higher than atmospheric pressure with liquid fuel in the fuel tank It has a structure to maintain. By keeping the inside of the fuel tank higher than atmospheric pressure with liquid fuel, the carbon dioxide generated at the anode electrode is guided to the carbon dioxide outlet 11 provided in the fuel tank lid 10 and permeates the gas-liquid separation membrane to the outside. Released. On the other hand, on the cathode electrode side, oxygen in the air that has passed through the air purification filter 5 reacts with hydrogen ions that have passed through the electrolyte membrane from the anode to generate water and electrons.

  As the power is generated, the generated water gradually adheres to the cathode end plate as water droplets. However, the dropped generated water is naturally absorbed by the water absorption frame 4 and evaporated from the side surface of the water absorption frame 4 into the atmosphere. Further, although the aqueous methanol solution in the fuel tank 8 is consumed, the fuel is replenished from the fuel cartridge 9, so that a substantially constant amount of aqueous methanol solution can always be supplied to the anode electrode. When the fuel in the fuel cartridge 9 runs out, the fuel tank can be continuously replenished by replacing it with a new fuel cartridge, and continuous power generation becomes possible.

(Test Example 1)
In the DMFC having the configuration shown in FIG. 3, a metal (Fe—Ni sintered body) porous body having an average pore size of 40 μm and a porosity of 40% is used as the water absorption frame, and the average pore diameter is 10 μm made of polytetrafluoroethylene. And an air purification filter using pulverized activated carbon (particle size 2 to 5 mm) as an adsorbent was attached. A fuel tank having a capacity of 9 ml was used, and a fuel cartridge containing 10 ml of a 15 wt% aqueous methanol solution was used. A continuous power generation test was conducted for 200 hours under a load current density of 50 mA / cm ^2. During this time, when the methanol aqueous solution became 1 ml or less, a new fuel cartridge was replaced. The output voltage drop rate at this time was 0.2 mV / h.

  For comparison, a power generation test similar to the above was performed by directly attaching an air purification filter to the cathode extreme plate without providing a water absorption frame. As a result, the output voltage suddenly decreased from around 10 h after the start of the test, and the power generation test could not be continued. When the air purification filter was peeled off from the cathode extreme plate and observed, a large amount of water adhered to the supply / exhaust port of the cathode extreme plate. Therefore, it was found that the water generated by power generation stayed as water droplets in the gap between the supply / discharge port of the cathode end plate and the air purification filter, shutting off the air supply to the cathode electrode and lowering the output voltage.

  In this test example, by attaching an air purification filter to the anode extreme plate via a water absorption frame, it is possible to prevent the moisture generated between the anode end plate and the air purification filter from staying, and the output voltage due to insufficient air supply is reduced. It became clear that the decrease could be suppressed.

(Test Example 2)
In the DMFC having the basic configuration of FIG. 3, the water absorption frame is made of a plastic (polyurethane) porous body having an average pore size of 100 μm and the porosity is 50%, and the adsorbent of the air purification filter is made of zeolite (artificial zeolite, average particle size of about A DMFC power generation test was conducted in the same manner as in Test Example 1 except that the thickness was changed to 100 μm. The output voltage drop rate at this time was 0.2 mV / h.

  According to this test example, it has been clarified that a reduction in output voltage due to insufficient air supply can be suppressed by attaching an air purification filter via a water absorption frame.

(Test Example 3)
In Test Example 2, a power generation test of DFMC was performed in the same manner except that the air purification filter was removed. The output voltage reduction rate at this time was 1.0 mV / h, and the output voltage reduction rate was increased by not using the air purification filter. During this time, as a result of observing the state of water droplet adhesion at the air supply port of the cathode end plate, it was confirmed that the generated water droplets flowed out and absorbed by the water absorption frame, and it is not considered that air supply is insufficient due to water droplet adhesion. It was speculated that the cathode electrode deteriorated faster due to impurities in the air.

  According to this test example, it was found that unless the air purification filter is used, the rate of decrease of the output voltage becomes large and the effect of suppressing the decrease cannot be obtained sufficiently.

It is sectional drawing which shows the basic composition of DMFC in this invention. It is the schematic which shows an example of the water absorption frame in this invention. It is the schematic seen from the cathode pole side which shows the Example of this invention. It is the schematic seen from the anode pole side which shows the Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Anode end plate, 2 ... MEA, 3 ... Cathode end plate, 4 ... Water absorption frame, 5 ... Air purification filter, 6 ... Adsorbent part, 7 ... Opening part, 8 ... Fuel tank, 9 ... Fuel cartridge, 10 ... Fuel tank lid, 11 ... carbon dioxide outlet.

Claims (2)

  An anode end plate having an anode electrode for oxidizing fuel on the first surface of the electrolyte membrane having proton conductivity and a cathode electrode for reducing oxygen on the other surface, the anode electrode having a current collecting function and a fuel supply port A direct form comprising a fuel cell in which the cathode electrode is sandwiched between a current collecting function and a cathode end plate having supply and discharge ports for air and product water, and means for supplying liquid fuel to the anode electrode A direct type liquid fuel cell, wherein an air purification filter is attached to a supply / exhaust port of the cathode end plate through a water absorption frame.   2. The direct liquid fuel cell according to claim 1, wherein the water absorption frame is made of a porous body.

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