JP2007234589A - Direct oxidation fuel cell and method for operating direct oxidation fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a direct oxidation fuel cell in which an diffusion layer of an anode is improved to provide uniform supply of fuel to a catalyst layer and reduce fuel crossover, the fuel cell exhibiting excellent power generating characteristics without reducing fuel utilization efficiency even under an operating condition that uses high concentration fuel. <P>SOLUTION: The direct oxidation fuel cell includes at least one unit cell including an anode, a cathode, and a hydrogen ion conductive polymer electrolyte membrane interposed between the anode and the cathode. The anode includes: a catalyst layer in contact with the polymer electrolyte membrane; and a diffusion layer. The diffusion layer includes: a porous composite layer containing a water-repellent binding material and an electron-conductive material; a first conductive porous substrate provided on the anode-side separator side of the porous composite layer; and a second conductive porous substrate provided on the catalyst layer side of the porous composite layer. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fuel cell, and more particularly to a solid polymer electrolyte fuel cell that directly uses a fuel without reforming it to hydrogen, and a method for operating the system.

  Installed in these small electronic devices in order to cope with the increase in power consumption and increase in continuous use time due to the multi-functionalization of portable small electronic devices such as mobile phones, personal digital assistants (PDAs), notebook PCs, and video cameras. There is a strong demand for higher energy density of batteries. Currently, lithium secondary batteries are mainly used as power sources for these small electronic devices. Lithium secondary batteries are expected to reach a limit at an energy density of about 600 Wh / L soon, and early commercialization of fuel cells using solid polymer electrolyte membranes is expected as an alternative power source.

  Among the solid polymer electrolyte fuel cells, the direct energy fuel cell of the type that generates electricity without directly reforming the fuel into hydrogen and supplying it directly to the inside of the cell and oxidizing it with the electrode is the theoretical energy of organic fuel. Active research and development has been conducted with a focus on high density, system simplification, and ease of fuel storage.

  For example, a direct methanol fuel cell includes a solid polymer electrolyte membrane, a membrane-electrode assembly (MEA) including an anode and a cathode sandwiching the polymer electrolyte membrane, an anode separator and a cathode separator sandwiching the MEA. It has at least one unit cell. The anode and the cathode are each composed of a catalyst layer and a diffusion layer. This fuel cell generates electricity by supplying methanol or an aqueous methanol solution as fuel to the anode side and supplying an oxidant gas, usually air, to the cathode side.

  The electrode reaction of the direct methanol fuel cell is as follows.

Anode: CH ₃ OH + H ₂ O → CO ₂ + 6H ⁺ + 6e ⁻
Cathode: 3/2 O ₂ + 6H ⁺ + 6e ⁻ → 3H ₂ O
That is, at the anode, methanol and water react to generate carbon dioxide, protons, and electrons, and the protons reach the cathode through the electrolyte membrane. At the cathode, oxygen, protons, and electrons via an external circuit combine to produce water.

However, there are several problems in putting this direct methanol fuel cell into practical use.
One of the phenomena is a so-called “methanol crossover” in which methanol supplied to the anode side passes through the electrolyte membrane unreacted and reaches the cathode side. As an electrolyte membrane of a direct methanol fuel cell, an ion exchange membrane made of perfluoroalkylsulfonic acid is used from the viewpoint of proton conductivity, heat resistance, and oxidation resistance. This type of electrolyte membrane consists of a hydrophobic polytetrafluoroethylene (PTFE) main chain and a side chain with a hydrophilic sulfonic acid group fixed to the end of the perfluoro group. Methanol having both parts becomes a good solvent for the electrolyte membrane and easily passes through the electrolyte membrane.

  This methanol crossover not only reduces the fuel utilization rate, but also causes a decrease in the electrode potential on the cathode side, leading to a significant deterioration in power generation characteristics. Since this methanol crossover tends to increase as the methanol concentration increases, at present, the methanol concentration is diluted to about 2 to 4 M, which is a major factor in reducing the size of the fuel cell system. It is an obstacle.

  Another problem is related to concentration polarization on the anode side. The direct methanol fuel cell uses an aqueous methanol solution, which is a liquid fuel, as a fuel. Therefore, the fuel diffusion rate on the anode side is slower than the hydrogen supply type fuel cell, causing the power generation characteristics to deteriorate. It has become. In particular, on the downstream side of the fuel flow path, methanol, which is fuel, is consumed, so that the shortage of fuel supply to the catalyst layer becomes significant, leading to an increase in concentration polarization of methanol. On the other hand, in order to avoid this, if the methanol concentration is increased, excess methanol is supplied to the catalyst layer upstream of the fuel flow path, resulting in an increase in methanol crossover, and power generation characteristics and fuel utilization rate are reduced. descend.

Therefore, in order to solve these problems, many proposals have been made to improve the structure of the anode side diffusion layer itself.
For example, Patent Document 1 discloses that the methanol crossover in the first half of the fuel flow path and the shortage of methanol supply in the second half of the fuel flow path are suppressed, and the fuel supply to the anode-side catalyst layer is made uniform. A direct methanol fuel cell is disclosed in which the methanol permeability coefficient of the anode-side diffusion layer increases toward the downstream side of the fuel flow path. As the diffusion layer on the anode side, a mixed layer containing carbon black and polytetrafluoroethylene is formed on the surface of a substrate made of carbon paper or the like. And along the direction of fuel flow, the thickness of the mixed layer is reduced, the weight ratio of polytetrafluoroethylene is reduced, the water repellency of carbon black is lowered, the porosity and pore diameter of carbon black are increased The method of doing etc. is described.

  Further, Patent Document 2 discloses a liquid that connects the catalyst layer and the base material of the diffusion layer in order to make the fuel supply to the anode-side catalyst layer uniform and improve the elimination of carbon dioxide as a reaction product. An electrode diffusion layer is disclosed in which a fuel supply passage (hydrophilic passage) and a gas product discharge passage (hydrophobic passage) exist independently and randomly. The hydrophilic passage is composed of a porous aggregate containing electron conductive particles and forming a three-dimensional network structure that plays a role of a polar liquid transmission passage. On the other hand, the hydrophobic passage is composed of a porous aggregate containing electron conductive particles and a hydrophobic binder resin and forming a three-dimensional network structure that does not get wet with the polar liquid and plays the role of a gas transmission passage.

Patent Document 3 discloses a structure in which a conductive porous support coated with a hydrophilic material is filled with conductive powder as a diffusion layer on the anode side in order to reduce concentration polarization in the anode of liquid fuel. Is disclosed.
JP 2002-110191 A JP 2005-108837 A JP 2004-342489 A

  The use of high-concentration fuel is an effective means for reducing the size and weight of the fuel cell and extending the driving time. However, in the conventional configuration, it is difficult to provide a direct oxidation fuel cell having excellent power generation characteristics without lowering the fuel utilization rate under operating conditions using high-concentration fuel. The problem exists.

    In the case of the technique represented by Patent Literature 1, sufficient consideration is not given to the influence of the methanol concentration and the influence of the power generation operating temperature on the methanol permeability coefficient of the diffusion layer on the anode side. For this reason, for example, when high-concentration methanol is used or when the power generation operating temperature is set high, there is a problem that the methanol crossover increases and the power generation characteristics are greatly deteriorated.

  In the case of the technique represented by Patent Document 2 or 3, although the elimination property of carbon dioxide as a reaction product is improved, the diffusion layer itself is perpendicular to the fuel flow path surface (the thickness of the diffusion layer). Direction) and methanol diffusivity in the horizontal direction (plane direction of the diffusion layer) with respect to the fuel flow path surface. For this reason, for example, when a small amount of high-concentration methanol is supplied close to the power generation consumption, the supply of methanol fuel to the catalyst layer becomes non-uniform, resulting in deterioration of power generation characteristics.

  In view of the above-mentioned problems, the present invention realizes the uniformity of the fuel supply to the entire catalyst layer and the reduction of the fuel crossover even when the high concentration fuel is directly supplied, thereby reducing the fuel utilization rate. It is an object of the present invention to provide a direct oxidation fuel cell having excellent power generation characteristics without being reduced.

  The present invention is a fundamental role of the anode diffusion layer, a role of uniformly supplying fuel from the fuel flow path to the catalyst layer, and a role of quickly discharging carbon dioxide generated in the catalyst layer to the fuel flow path In addition to the role of quickly transmitting electrons generated in the catalyst layer to the separator, a new function such as control of the permeation flux of the fuel is added to the anode diffusion layer. This makes it possible to supply an appropriate amount of fuel uniformly to the anode catalyst layer, and to reduce concentration polarization due to insufficient fuel supply while reducing fuel crossover.

  The present invention is a direct oxidation fuel cell including at least one unit cell, wherein the at least one unit cell includes an anode, a cathode, and a hydrogen ion conductive polymer electrolyte inserted between the anode and the cathode. The present invention relates to a direct oxidation fuel cell comprising a membrane, an anode separator having a flow path for supplying and discharging fuel to the anode, and a cathode separator having a gas flow path for supplying and discharging oxidant gas to the cathode.

  A feature of the present invention is that the anode includes a catalyst layer in contact with the polymer electrolyte membrane and a diffusion layer, and the diffusion layer includes a porous composite layer including a water-repellent binding material and an electron conductive material, and the porous layer. A first conductive porous substrate disposed on the anode separator side of the porous composite layer, and a second conductive porous substrate disposed on the catalyst layer side of the porous composite layer It is.

The present invention also provides a method for operating a fuel cell system including the fuel cell.
The fuel cell system includes: a fuel tank connected to an anode inlet of the fuel cell by a fuel supply passage; a fuel discharge passage connected to an anode outlet of the fuel cell; and a cathode of the fuel cell. An oxidant supply source connected to the inlet by an oxidant supply path, and an oxidant discharge path connected to the outlet of the cathode of the fuel cell, the operation method of which is the fuel supplied to the anode of the fuel cell The supply amount is controlled to be 1.1 to 2.2 times the fuel consumption by power generation of the fuel cell.

  According to the present invention, when supplying high-concentration fuel to the anode, the fuel blocking property in the thickness direction of the diffusion layer can be controlled, and the fuel diffusibility in the surface direction of the diffusion layer can be improved. Is possible. As a result, it is possible to simultaneously ensure the uniformity of fuel supply to the entire surface of the catalyst layer and reduce the crossover of the fuel.

The direct oxidation fuel cell of the present invention includes at least one unit cell, and the at least one unit cell includes an anode, a cathode, and a hydrogen ion conductive polymer electrolyte membrane inserted between the anode and the cathode, An anode side separator having a flow path for supplying and discharging fuel to the anode, and a cathode side separator having a gas flow path for supplying and discharging oxidant gas to the cathode;
The anode includes a catalyst layer in contact with the polymer electrolyte membrane and a diffusion layer, and the diffusion layer includes a porous composite layer including a water-repellent binding material and an electron conductive material, and the anode of the porous composite layer A first conductive porous substrate disposed on the side separator side, and a second conductive porous substrate disposed on the catalyst layer side of the porous composite layer.

A schematic configuration of the anode diffusion layer is shown in FIG. 1A.
The diffusion layer includes a porous composite layer 1 containing a water-repellent binding material and an electron conductive material, a first conductive porous substrate 2 disposed on the anode separator side of the porous composite layer 1, And a laminate including at least three layers of the second conductive porous substrate 3 disposed on the catalyst layer side of the porous composite layer 1.

  When fuel, for example, an aqueous solution of methanol is supplied to the diffusion layer in the direction of the arrow A, the fuel diffuses into the first conductive porous substrate 2 as indicated by arrows a1, a2, a3,. To do. In the porous substrate 2, the fuel diffuses not only in the thickness direction but also in the surface direction. However, the fuel concentration in the porous base material is generally higher on the upstream side of the supplied fuel than on the downstream side. The fuel diffused in the porous substrate 2 is then diffused in the thickness direction of the porous composite layer 1 as shown by the arrows b1, b2, b3,. The base material 3 is reached. In the porous base material 3, the fuel diffuses also in the surface direction of the porous base material 3. Therefore, the fuel that has diffused as shown by b <b> 1 is c1-1, c1-2, and c1-3. Spread. Similarly, the fuel diffused as shown by b2, b3,... diffuses toward the back side of the base material while diffusing in the surface direction of the base material, and eventually the catalyst layer. To reach.

FIG. 1B shows a configuration of a diffusion layer as a comparative example. This diffusion layer is composed of two layers: a porous composite layer 1 similar to the above, and a porous substrate 4 disposed on the anode separator side.
When the fuel is supplied to the diffusion layer in the direction of arrow A, similarly to the above, it diffuses in the porous substrate 4, reaches the porous composite layer 1, and diffuses in the layer in the thickness direction.
In the configuration of FIG. 1B, if the fuel permeation flux of the porous substrate 4 is increased, the fuel cannot be sufficiently diffused in the surface direction of the diffusion layer. Therefore, the fuel concentration reaching the catalyst layer has a non-uniform portion, and good power generation characteristics cannot be obtained. Moreover, when the fuel permeation flux of the porous base material 4 is reduced, the fuel supply to the catalyst layer becomes insufficient, the concentration polarization increases, and the power generation characteristics deteriorate.

  On the other hand, in the configuration of FIG. 1A, the porous composite layer 1 interposed between the porous base materials 2 and 3 appropriately blocks the diffusion of fuel in the thickness direction of the diffusion layer, and is porous. In the base materials 2 and 3, the diffusion of fuel in the surface direction of the diffusion layer is promoted. Therefore, it is possible to suppress the crossover of the fuel, make the fuel supply to the catalyst layer substantially uniform, and improve the power generation characteristics.

It is preferable that the fuel permeation flux of the layer composed of the first conductive porous substrate and the porous composite layer is smaller than the fuel permeation flux of the second conductive porous substrate.
With this configuration, when supplying high-concentration fuel, it is possible to uniformly control the diffusibility of the fuel that permeates and moves across the entire diffusion layer. As a result, it is possible to simultaneously solve the problem of fuel crossover due to excessive fuel supply on the upstream side of the fuel flow path and the concentration polarization problem due to insufficient fuel supply on the downstream side.

In the anode diffusion layer of the present invention, the porous composite layer has a substantially flat surface in contact with the second conductive porous substrate, and the repellent material constituting the porous composite layer. It is preferable that the aqueous binder material and the electron conductive material do not enter the gap between the second conductive porous substrate. The reason is that if the water-repellent binding material and the electron conductive material of the porous composite layer enter the gap between the second conductive porous base materials, the diffusibility of the fuel in the surface direction of the diffusion layer decreases. Because. In order to prevent the constituent material of the porous composite layer from entering the gap between the second conductive porous base materials, the porous composite layer is formed on the first conductive porous base material. At this time, the surface of the porous composite layer to which the second conductive porous substrate is bonded has a substantially flat surface. And it is good to take the assembly method which laminates | stacks a 2nd electroconductive porous base material on the flat surface of this porous composite layer.
Here, the water-repellent binding material and the electron conductive material constituting the porous composite layer do not enter the gap between the second conductive porous base material. This means that the water-repellent binding material and the electron conductive material have not penetrated into the layer to such an extent that the diffusion of the fuel in the surface direction is affected.

In the anode diffusion layer of the present invention, the water-repellent binding material in the porous composite layer is preferably mainly composed of a fluororesin.
By using a fluororesin having a chemically stable C—F bond as the water-repellent binding material, a surface with a small interaction with other kinds of molecules, a so-called water-repellent surface can be formed. Examples of fluororesins include polytetrafluoroethylene resin (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), polyvinyl fluoride resin (PVF), polyvinylidene fluoride resin (PVDF), tetrafluoroethylene-perfluoro (Alkyl vinyl ether) copolymer (PFA) and the like can be mentioned.

In the anode diffusion layer of the present invention, the electron conductive material in the porous composite layer is preferably mainly composed of conductive carbon black.
As an electron conductive material, the structure (primarily agglomerated permanently agglomerated) is highly developed, and conductive carbon black with a large specific surface area is used to ensure the electron conductivity and within the structure. The carbon dioxide generated in the catalyst layer can be quickly discharged through the pores.

In the anode diffusion layer of the present invention, the fuel permeation flux can be controlled by adhering a water-repellent binding material to the first and second conductive porous substrates and adjusting the amount of adhesion. it can.
The water-repellent binding material is deposited on the surfaces of the first and second conductive porous substrates and the inner walls of the pores to form a number of irregular shapes (fractal surfaces) derived from the particle shape of the water-repellent binding material. This makes it possible to increase the water repellency of the substrate itself. Therefore, by adjusting the amount of the water-repellent binding material attached to the first and second conductive porous base materials, the water repellency of the base material can be changed and the fuel permeation flux can be controlled.

  In the anode diffusion layer of the present invention, it is preferable that the water-repellent binding material to be adhered to the first and second conductive porous substrates is mainly composed of a fluororesin. As the fluororesin, the fluororesin exemplified as the water-repellent binding material in the porous composite layer can be used.

The fuel cell system of the present invention is preferably operated with the amount of fuel supplied to the fuel cell being 1.1 to 2.2 times the amount of fuel consumed by power generation.
Thus, when the amount of fuel supplied to the fuel cell is made as close as possible to the amount consumed during power generation, the amount of fuel crossover based on surplus fuel can be greatly reduced. When the fuel supply amount is set to a value that exceeds 2.2 times the fuel consumption by power generation, the power generation characteristics are significantly reduced due to fuel crossover.
The fuel consumption of the fuel cell is obtained from a desired output. Further, the fuel supply amount is controlled by adjusting the concentration of fuel supplied to the fuel cell and the supply speed thereof.

In a preferred embodiment of the operation method of the fuel cell system of the present invention, the fuel is methanol or a methanol aqueous solution, and the fuel permeation flux of two layers of the first conductive porous substrate and the porous composite layer is , 0.6 × 10 ^{−4 to} 1.5 × 10 ⁻⁴ mol / (cm ² · min) at a fuel concentration and a cell temperature.

By setting the methanol permeation flux values of the two layers of the first conductive porous substrate and the porous composite layer on the fuel flow path side within the specified range, the fuel in the thickness direction of the diffusion layer can be obtained. It is possible to optimize the blocking property and solve the problem of fuel crossover due to excessive supply of fuel on the upstream side of the fuel flow path. When the methanol permeation flux exceeds 1.5 × 10 ⁻⁴ mol / (cm ² · min), the methanol permeation rate in the thickness direction of the diffusion layer is remarkably increased. The methanol supply to the tank becomes uneven, and the power generation characteristics are deteriorated. On the other hand, if the methanol permeation flux is less than 0.6 × 10 ⁻⁴ mol / (cm ² · min), the amount of fuel supplied to the catalyst layer cannot be secured, resulting in an increase in concentration polarization and a deterioration in power generation characteristics. Invite.

In another preferred embodiment of the operation method of the fuel cell system of the present invention, the fuel is methanol or a methanol aqueous solution, and the fuel permeation flux of the second conductive porous substrate is 4.5 × 10 ^−. Electric power is generated at a fuel concentration and cell temperature that is ^{4 to} 8.0 × 10 ⁻⁴ mol / (cm ² · min).

By setting the methanol permeation flux value of the second conductive porous base material within the specified range described above, it becomes possible to uniformly diffuse an appropriate amount of methanol in the surface direction of the diffusion layer. The problem of concentration polarization due to insufficient fuel supply on the downstream side can be solved. When the methanol permeation flux exceeds 8.0 × 10 ⁻⁴ mol / (cm ² · min), the methanol permeation rate in the thickness direction of the second conductive porous substrate is the surface direction. Thus, the methanol permeation rate becomes larger than that of the catalyst layer, so that the methanol supply to the entire surface of the catalyst layer becomes uneven, and the power generation characteristics deteriorate. On the contrary, if the methanol permeation flux is less than 4.5 × 10 ⁻⁴ mol / (cm ² · min), the amount of fuel supplied to the catalyst layer cannot be secured and concentration polarization increases, resulting in deterioration of power generation characteristics. Invite.

As described above, according to the present invention, even when high-concentration fuel is directly supplied, it is possible to simultaneously ensure fuel supply uniformity over the entire catalyst layer and reduce fuel crossover. In addition, a direct oxidation fuel cell having excellent power generation characteristics can be provided without reducing the fuel utilization rate.
Hereinafter, embodiments of the present invention will be described with reference to the drawings.

Embodiment 1
FIG. 2 is a schematic longitudinal sectional view showing the structure of the fuel cell in one embodiment of the present invention. In this example, the fuel cell is composed of one unit cell. The unit cell 10 includes a hydrogen ion conductive electrolyte membrane 11, a membrane-electrode assembly (MEA) composed of an anode 12 and a cathode 13 sandwiching the electrolyte membrane 11, and an anode side separator 14 and a cathode side separator 15 sandwiching the MEA. The anode and the cathode each include a catalyst layer in contact with the electrolyte membrane and a diffusion layer on the separator side. The anode-side separator 14 has a flow path 16 for supplying and discharging fuel on the surface facing the anode, and the cathode-side separator 15 has a gas flow path 17 for supplying and discharging oxidant gas on the surface facing the cathode. Have. 18 and 19 are gaskets sandwiching the electrolyte membrane around the anode and the cathode.
The unit cell 10 has current collector plates 20 and 21, planar heaters 22 and 23, insulating plates 24 and 25, and end plates 26 and 27 stacked on both sides thereof, and are integrally coupled by fastening means.

The electrolyte membrane 11 may be any material as long as it has hydrogen ion (proton) conductivity and excellent heat resistance and chemical stability, and the material thereof is not particularly limited.
The catalyst layers of the anode and the cathode are thin films having a thickness of about 10 to 100 μm mainly composed of conductive carbon particles or catalyst metal fine particles supporting a catalyst metal and a polymer electrolyte. Platinum (Pt) -ruthenium (Ru) alloy fine particles are used as the catalyst metal of the catalyst layer of the anode. On the other hand, Pt fine particles are used as the catalyst metal of the cathode catalyst layer. As the polymer electrolyte, it is preferable to use the same material as the electrolyte membrane 11.

  As shown in FIG. 3, the anode 12 includes a diffusion layer 30 and a catalyst layer 34. The diffusion layer 30 includes a porous composite layer 31 including a water-repellent binding material and an electron conductive material, a first conductive porous substrate 32 disposed on the anode separator side of the porous composite layer 31, And a laminate including at least three layers of the second conductive porous base material 33 disposed on the catalyst layer side of the porous composite layer 31.

The water-repellent binding material constituting the porous composite layer 30 may be any material mainly composed of a fluororesin. Examples of the fluororesin include polytetrafluoroethylene resin (PTFE) and tetrafluoroethylene-hexafluoropropylene. It is preferable to use a polymer (FEP), a polyvinyl fluoride resin (PVF), a polyvinylidene fluoride resin (PVDF), a tetrafluoroethylene-perfluoro (alkyl vinyl ether) copolymer (PFA), or the like. The electron conductive material constituting the porous composite layer 30 may be any material mainly composed of conductive carbon black. The conductive carbon black has a highly developed structure and has a specific surface area of 200 m. It is preferable to use a material that is ² / g or more.

As the first and second conductive porous substrates, a water-repellent binding material is attached to a conductive porous material having both diffusibility of fuel, elimination of carbon dioxide generated by power generation, and electronic conductivity. Can be used. Examples of such conductive porous materials include carbon paper and carbon cloth. Examples of such water-repellent binding materials include polytetrafluoroethylene resin and tetrafluoroethylene-hexafluoropropylene copolymer. , Polyvinyl fluoride resin, polyvinylidene fluoride resin, and tetrafluoroethylene-perfluoro (alkyl vinyl ether) copolymer.
The porous composite layer 10 does not enter the gap between the second conductive porous substrate on the catalyst layer side.

  For the diffusion layer of the cathode 13, for example, a conductive porous substrate such as carbon paper or carbon cloth having both air diffusibility, drainage of water generated by power generation, and electronic conductivity can be used.

  FIG. 4 shows an embodiment of a system using the fuel cell of the present invention as described above. This system is a non-circulating fuel cell system, and does not collect the liquid and gas discharged from the anode side of the fuel cell and reuse them for power generation. That is, the amount of fuel supplied is as close as possible to the amount consumed during power generation, and the amount of fuel discharged from the anode side is reduced as much as possible. Devices such as a cooler and a gas-liquid separator become unnecessary. .

  Reference numeral 40 denotes a fuel cell. The fuel cell 40 includes a cell stack in which one or a plurality of unit cells as described above are stacked, and a current collector plate, a planar heater, an insulating plate, and an end plate sandwiching the cell stack. The planar heater is used to control the cell temperature. The fuel in the fuel tank 49 is supplied to the anode 43 of the fuel cell 40 through a fuel supply path 47 provided with a fuel pump 48. On the other hand, oxidant gas air is supplied to the cathode 42 of the fuel cell 40 through an air supply path 44 having an air pump 45. The fuel discharged from the anode 43 of the fuel cell 40 is sent to the catalyst combustor 51 through the fuel discharge path 50. Air discharged from the cathode 42 through the air discharge path 46 is sent to the catalyst combustor 51. Then, the fuel discharged from the fuel cell is oxidized and purified by this catalytic combustor and released into the atmosphere as air containing water and carbon dioxide. The catalyst combustor 51 is composed of two combustion chambers divided by a porous sheet with a catalyst layer, and one combustion chamber is provided with only an inlet for introducing fuel discharged from the fuel cell 40. The other combustion chamber is provided with an introduction port through which air is introduced and a discharge path 52 through which air containing water and carbon dioxide after catalytic combustion is discharged.

  Next, the steps for obtaining the fuel concentration and the cell temperature for operating the fuel cell system under conditions that provide the preferred methanol permeation flux of the anode diffusion layer of the present invention will be described.

(1) First step:
The component of the diffusion layer, that is, the first conductive porous member and the porous composite under the conditions in which the concentration and temperature of the methanol aqueous solution are changed using the apparatus for measuring the methanol permeation flux shown in FIG. The methanol permeation flux of the combined body with the member and the second conductive porous member is measured. A method for measuring the permeation flux of methanol using the apparatus of FIG. 5 will be described in a later example.
The measurement temperature is in the range of 20 to 80 ° C. assuming the actual use temperature. At this time, the apparatus is left in a constant temperature bath for 60 minutes, and the measurement is performed after the temperature of the aqueous methanol solution is stabilized.

(2) Second step:
Based on the above measurement results, the methanol concentration and temperature are determined so as to achieve the preferred permeation flux.
At this time, methanol concentration and temperature for satisfying the preferable permeation flux of the combined body of the first conductive porous member and the porous composite member, and the preferable permeation of the second conductive porous member. When the methanol concentration and temperature for satisfying the flux are different, the thickness and composition of the porous composite layer are changed based on the methanol concentration and temperature in the second conductive porous member. It adjusts so that it may become the same methanol concentration and temperature conditions also about the coupling body of 1 electroconductive porous member and porous composite member.

(3) Third step:
While supplying the fuel having the methanol concentration determined in the second step to the anode, the temperature of the fuel cell is set so as to be the methanol aqueous solution temperature determined in the second step, and the fuel cell is caused to generate power.

Hereinafter, the present invention will be described in detail with reference to examples.
Example 1
Carbon black, which is conductive carbon particles having an average primary particle size of 30 nm (Ketjen Black EC manufactured by Mitsubishi Chemical Corporation), and 30% by weight of Pt and Ru having an average particle size of 3 nm supported on each anode Side catalyst-carrying particles were obtained. Further, a catalyst-supporting particle on the cathode side was prepared by supporting 50% by weight of Pt having an average particle diameter of 3 nm on the same ketjen black EC as described above. The anode-side catalyst-carrying particles and the cathode-side catalyst-carrying particles are each ultrasonically dispersed in an aqueous solution of isopropanol, a polymer electrolyte is added to these dispersions, and the particles are highly dispersed by a bead mill, whereby an anode catalyst paste and a cathode catalyst Each paste was prepared. At this time, the weight ratio of the conductive carbon particles to the polymer electrolyte in each catalyst paste was 1: 1. As the polymer electrolyte, perfluorocarbon sulfonic acid ionomer (Flemion manufactured by Asahi Glass Co., Ltd.) was used.

After coating these anode catalyst paste and cathode catalyst paste on a polytetrafluoroethylene sheet (Naflon PTFE sheet manufactured by Nichias Co., Ltd.) using a doctor blade, the anode catalyst layer and the anode catalyst layer were dried at room temperature in the atmosphere for 6 hours. A cathode catalyst layer was formed. Next, the sheet on which each catalyst layer was formed was cut into a size of 6 cm × 6 cm. An electrolyte membrane is sandwiched between a sheet having an anode catalyst layer and a sheet having a cathode catalyst layer so that each catalyst layer is inside, and each catalyst layer is formed into an electrolyte membrane by hot pressing (130 ° C., 7 MPa, 5 minutes). Joined. The electrolyte membrane used here is an ion exchange membrane (Nafion 117 manufactured by DuPont) made of perfluoroalkylsulfonic acid. The polytetrafluoroethylene sheet was peeled off from the joined body thus obtained, thereby forming an anode catalyst layer and a cathode catalyst layer on the electrolyte membrane. The amount of Pt catalyst on the anode side and the cathode side was 1.8 mg / cm ² .

  As the first conductive porous substrate, carbon paper (TGP-H090 manufactured by Toray Industries, Inc.) was used, and this was dispersed in a 7% by weight polytetrafluoroethylene resin (PTFE) (Daikin Industries, Ltd. D -1E diluted with ion-exchanged water) for 1 minute, and then dried in air at room temperature for 3 hours. Thereafter, the surfactant was removed by baking at 360 ° C. for 1 hour in nitrogen gas. Thus, water repellency was imparted to the first conductive porous substrate. The amount of PTFE adhering to the first conductive porous substrate was 11.5% by weight.

  Next, the porous composite layer 31 was formed on the surface of the first conductive porous substrate 32 as follows. First, conductive carbon black (Vulcan XC-72R manufactured by CABOT) was ultrasonically dispersed in an aqueous solution to which a surfactant (Triton X-100 manufactured by Aldrich) was added, and then Hibismix (manufactured by Primix Co., Ltd.) was used. High dispersion was used. Next, after adding a PTFE dispersion (D-1E manufactured by Daikin Industries, Ltd.), the dispersion was again highly dispersed to prepare a porous composite layer paste. The porous composite layer paste was uniformly applied to the entire surface of the first conductive porous substrate 32 by a doctor blade, and then dried at room temperature in the atmosphere for 8 hours. Thereafter, the surfactant was removed by baking at 360 ° C. for 1 hour in nitrogen gas. The weight ratio of conductive carbon black / PTFE in the porous composite layer 31 thus obtained was 3/5, and its thickness was about 50 μm.

  Next, the first conductive porous substrate 32 on which the porous composite layer 31 was formed and the second conductive porous substrate 33 described below were each cut into a size of 6 cm × 6 cm. Next, the second conductive porous substrate 33 is overlaid on the porous composite layer 31 formed on the first conductive porous substrate 32, and hot pressing (130 ° C., 4 MPa, 3 MPa The diffusion layer 30 was fabricated by bonding together. The second conductive porous substrate 33 used here is carbon paper (TGP-H060 manufactured by Toray Industries, Inc.), and the same procedure as in the case of the first conductive porous substrate 32 described above is used. The PTFE was deposited by 5.5% by weight by performing a water repellent treatment with a 3% by weight dispersion of PTFE.

By producing the diffusion layer 30 as described above, the porous composite layer 31 is laminated with the second conductive porous base material 33 without entering the gap between the second conductive porous base material 33. I was able to.
Next, the anode side diffusion layer 30 and the cathode side diffusion layer cut into a size of 6 cm × 6 cm are laminated on the electrolyte membrane 11 on which the anode catalyst layer and the cathode catalyst layer are formed, and hot pressing (130 ° C., 4 MPa) is performed. For 3 minutes). Carbon cloth (LT2500W manufactured by E-TEK) was used for the cathode side diffusion layer.

Further, the MEA 1 was manufactured by thermally welding the gaskets 18 and 19 around the anode 12 and the cathode 13 with the electrolyte membrane 11 interposed therebetween (130 ° C., 4 MPa, 5 minutes).
The MEA was sandwiched from both sides by a separator, a current collector plate, a planar heater, an insulating plate, and an end plate having an outer dimension of 10 cm × 10 cm, and fixed with a fastening rod. The fastening pressure at this time was 20 kgf / cm ² per area of the separator. The anode-side separator and the cathode-side separator are made of a classy carbon plate having a thickness of 4 mm, and serpentine-type channels having a width of 1.5 mm and a depth of 1 mm are formed on the surfaces facing the anode and the cathode, respectively. A stainless steel plate subjected to gold plating was used for the current collector plate, and a stainless steel plate was used for the end plate.
The fuel cell assembled as described above is denoted by A.

Example 2
A fuel cell B was produced in the same manner as in Example 1 except that the weight ratio of conductive carbon black / PTFE in the porous composite layer 31 was 5/3 and the thickness thereof was about 30 μm.

Example 3
The first conductive porous substrate 32 was subjected to water repellency treatment using 13 wt% PTFE dispersion to deposit 20.5 wt% of PTFE, and the thickness of the porous composite layer 31 was about 60 μm. A fuel cell C was produced by the same method as in Example 1 except that.

Example 4
By using carbon paper (TGP-H030 manufactured by Toray Industries, Inc.) as the second conductive porous base material 33 and subjecting it to a water repellent treatment using a 7 wt% PTFE dispersion, PTFE was 11.5. A fuel cell D was produced in the same manner as in Example 1 except that the weight% was adhered.

Example 5
The second conductive porous base material 33 was subjected to a water repellent treatment using a 7 wt% PTFE dispersion, whereby a fuel was produced in the same manner as in Example 1 except that 11.5 wt% of PTFE was deposited. Battery E was produced.

Example 6
The first conductive porous substrate 32 was subjected to a water repellent treatment using 3 wt% PTFE dispersion, thereby adhering 5.5 wt% of PTFE, and the conductivity in the porous composite layer 31. A fuel cell F was produced in the same manner as in Example 1 except that the weight ratio of carbon black / PTFE was 5/3 and the thickness was about 20 μm.

Example 7
The first conductive porous substrate A11 was subjected to a water repellent treatment using a 13% by weight PTFE dispersion, so that 20.5% by weight of PTFE was adhered, and the thickness of the porous composite layer 31 was reduced to about A fuel cell G was produced in the same manner as in Example 1 except that the thickness was 80 μm.

Example 8
As the second conductive porous base material 33, a carbon paper (TGP-H030 manufactured by Toray Industries, Inc.) was used, and the fuel cell H was manufactured in the same manner as in Example 1 except that this water-repellent treatment was not performed. Was made.

Example 9
The second conductive porous substrate 33 was subjected to a water repellent treatment using a 13 wt% PTFE dispersion, so that 20.5 wt% of PTFE was adhered to the fuel by the same method as in Example 1. Battery I was produced.

Example 10
A method similar to that of Example 1 except that the paste for the porous composite layer was uniformly applied on the second conductive porous substrate 33 instead of the first conductive porous substrate 32 using a doctor blade. Thus, a fuel cell J was produced.

Example 11
As the first conductive porous substrate 32, carbon paper (TGP-H060 manufactured by Toray Industries, Inc.) is used, and this is subjected to a water repellent treatment using 3% by weight PTFE dispersion, whereby PTFE is treated with 5. 5% by weight and carbon paper (TGP-H090 manufactured by Toray Industries, Inc.) was used as the second conductive porous substrate 33, and this was water repellent using 7% by weight PTFE dispersion. By processing, a fuel cell K was produced in the same manner as in Example 1 except that 11.5% by weight of PTFE was deposited.

<< Comparative Example 1 >>
The anode diffusion layer has a single-layer structure composed of only the first conductive porous substrate 32, and carbon paper (TGP-H120 manufactured by Toray Industries, Inc.) is used in the same manner as in Example 1. A fuel cell 1 was produced.

<< Comparative Example 2 >>
A fuel cell 2 was produced in the same manner as in Example 1 except that the anode diffusion layer had a two-layer structure of the porous composite layer 31 and the first conductive porous substrate 32.

<< Comparative Example 3 >>
The anode diffusion layer has a two-layer structure of a porous composite layer 31 and a first conductive porous substrate, and carbon paper (TGP-H120 manufactured by Toray Industries, Inc.) is used as the first conductive porous substrate. This was carried out except that a water repellent treatment was carried out using a 13 wt% PTFE dispersion to deposit 20.5 wt% of PTFE and the thickness of the porous composite layer 31 was about 80 μm. A fuel cell 3 was produced in the same manner as in Example 1.

The first conductive porous base material 32 and the second conductive porous material, which are combined with the porous composite layer 10 and constitute the anode-side diffusion layer 4 used in Examples 1 to 11 and Comparative Examples 1 to 3. The methanol permeation flux of each of the base materials 33 was measured using the following method. The results are shown in Table 1.
In the fuel cells A to J, the methanol permeation flux of the first conductive porous base material is larger than the methanol permeation flux of the second conductive porous base material. The methanol permeation flux of the conductive porous substrate is greater than the methanol permeation flux of the first conductive porous substrate.

(1) Methanol permeation flux A glass H-type cell 60 as shown in FIG. 5 was used. The cell 60 has a glass tank 61, a glass tank 62, and a connecting portion 63 that connects the two. The first conductive porous substrate 32 or the second conductive porous substrate 33 in which the porous composite layer 31 is bonded as the sample 66 to the connecting portion 63 (cross-sectional area: 3.14 cm ² ) is used as a rubber. It was placed between rings 67 and 68. The cell 60 was placed in a constant temperature bath at 60 ° C. Then, 50 cc of a 6M aqueous methanol solution was placed in one glass tank 61 and 200 cc of ion-exchanged water was placed in the other glass tank 62, respectively, and stirred at a constant speed by stirring bars 64 and 65. Thereafter, about 1 cc of the aqueous solution in the glass tank 62 was sampled at regular intervals, and the methanol concentration was quantified by gas chromatography. Based on the increase amount of methanol per unit time, the methanol permeation flux value was calculated.

  Next, a fuel cell system as shown in FIG. 4 was configured using the fuel cells A to K manufactured in Examples 1 to 11 and the fuel cells 1 to 3 manufactured in Comparative Examples 1 to 3. The fuel cell system was operated to evaluate the current-voltage characteristics and continuous power generation characteristics of each fuel cell. The evaluation method is shown below. The evaluation results are shown in Table 1.

(2) Current-voltage characteristics A 6M aqueous methanol solution was supplied to the anode side at a flow rate of 0.14 cc / min, air was supplied to the cathode side at a flow rate of 0.3 L / min, and a current density of 150 mA / The effective voltage at the time of generating power for 15 minutes at cm ² was measured. Under these evaluation conditions, the fuel supply amount was set to 1.5 times the fuel consumption amount by power generation, and the air supply amount was set to 3.1 times the air consumption amount by power generation.

(3) Continuous power generation characteristics The effective voltage when the fuel cell was generated for 15 minutes under the above conditions was used as the initial voltage. Thereafter, the effective voltage at the time when power was continuously generated for 100 hours under the same conditions was measured, and the ratio of the voltage to the initial voltage (voltage maintenance rate) was calculated.

  As is clear from Table 1, it can be seen that the fuel cells A to K have excellent power generation characteristics under operating conditions using high-concentration methanol. This is because the diffusion layer on the anode side is made into a three-layer laminated structure composed of a conductive porous substrate / porous composite layer / conductive porous substrate imparted with water repellency. This is because it is possible to simultaneously realize the control of the blocking property of the fuel in the thickness direction and the improvement of the diffusibility of the fuel in the surface direction. In particular, the batteries A to E have greatly improved power generation characteristics. This is probably due to the following reasons. The first is that the methanol permeation flux of the first conductive porous substrate and the second conductive porous substrate, which constitute the anode-side diffusion layer and bonded with the porous composite layer, is appropriate. It is controlled within the range. The second is appropriate because the porous composite layer has a structure in which the porous conductive layer is laminated with the second conductive porous substrate without entering the gap between the second conductive porous substrates. The amount of methanol can be uniformly diffused in the plane direction of the diffusion layer.

  On the other hand, in the fuel cell 1 of the comparative example, the anode-side diffusion layer has a single-layer structure of only a conductive porous base material imparted with water repellency. As a result, the blocking property of the fuel becomes insufficient, the methanol crossover on the upstream side of the fuel flow path is significantly increased, and the power generation characteristics are greatly deteriorated.

  In the fuel cell 2 of the comparative example, the anode-side diffusion layer has a two-layer structure including a conductive porous base material imparted with water repellency and a porous composite layer, so that an appropriate amount of methanol is diffused. It is assumed that it is difficult to diffuse uniformly in the surface direction of the layer, and the power generation characteristics are deteriorated.

  In the fuel cell 3, the anode-side diffusion layer has a large two-layer structure composed of a conductive porous base material imparted with water repellency and a porous composite layer, so that fuel is supplied to the catalyst layer. The amount of polarization could not be ensured and the concentration polarization increased, resulting in deterioration of power generation characteristics.

  The diffusion layer in the embodiment of the present invention is a laminated structure composed of three layers, but is not limited to this, for example, a porous layer composed of two layers in which the weight ratio of conductive carbon black / PTFE is changed. A composite layer of quality may be used.

  The fuel cell of the present invention can be used directly as fuel without reforming methanol, dimethyl ether or the like into hydrogen, and is used for portable small electronic devices such as mobile phones, personal digital assistants (PDAs), notebook PCs, video cameras and the like. It is useful as a power source. Further, the present invention can be applied to an electric scooter, an automobile power source and the like.

1 is a schematic diagram illustrating a state of fuel diffusion in an anode diffusion layer according to the present invention. It is the schematic explaining the diffusion condition of the fuel in the diffusion layer of the anode by a comparative example. 1 is a schematic longitudinal sectional view of a unit cell of a fuel cell according to an embodiment of the present invention. It is a schematic cross section which shows the structure of the principal part of the diffusion layer by the side of the anode of a unit cell. 1 is a block diagram showing a schematic configuration of a fuel cell system according to an embodiment of the present invention. It is a longitudinal cross-sectional schematic diagram which shows schematic structure of the apparatus for measuring the permeation | transmission flux of methanol.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 31 Porous composite layer 2, 32 1st electroconductive porous base material 3, 33 2nd electroconductive porous base material 11 Electrolyte membrane 12 Anode 13 Cathode 14 Anode side separator 15 Cathode side separator 16 Flow of fuel Channel 17 Oxidant gas channel 18, 19 Gasket 30 Anode-side diffusion layer 34 Catalyst layer

Claims (10)

A direct oxidation fuel cell comprising at least one unit cell, the at least one unit cell comprising an anode, a cathode, a hydrogen ion conductive polymer electrolyte membrane inserted between the anode and the cathode, and the anode An anode side separator having a flow path for supplying and discharging fuel, and a cathode side separator having a gas flow path for supplying and discharging oxidant gas to the cathode,
The anode includes a catalyst layer in contact with the polymer electrolyte membrane and a diffusion layer, and the diffusion layer includes a porous composite layer including a water-repellent binding material and an electron conductive material, and the anode of the porous composite layer Direct oxidation comprising a first conductive porous substrate disposed on the side separator side and a second conductive porous substrate disposed on the catalyst layer side of the porous composite layer Type fuel cell.   The fuel permeation flux of two layers consisting of the first conductive porous substrate and the porous composite layer is smaller than the fuel permeation flux of the second conductive porous substrate. Direct oxidation fuel cell.   The surface where the second conductive porous substrate is joined in the porous composite layer is a substantially flat surface, and the water-repellent binding material and the electron conductive material constituting the porous composite layer are 2. The direct oxidation fuel cell according to claim 1, wherein the direct oxidation fuel cell does not enter the gap between the second conductive porous substrates.   2. The direct oxidation fuel cell according to claim 1, wherein the water-repellent binding material is mainly composed of a fluororesin.   2. The direct oxidation fuel cell according to claim 1, wherein the electron conductive material is mainly composed of conductive carbon black.   2. The direct oxidation fuel cell according to claim 1, wherein the first and second conductive porous substrates include a water-repellent binding material.   The direct oxidation fuel cell according to claim 6, wherein the water-repellent binding material mainly comprises a fluororesin.   The fuel cell according to claim 1, a fuel tank connected to an inlet of an anode of the fuel cell by a fuel supply path, a fuel discharge path connected to an outlet of the anode of the fuel cell, and an oxidation to an inlet of a cathode of the fuel cell An operating method of a fuel cell system comprising an oxidant supply source connected by an agent supply passage and an oxidant discharge passage connected to an outlet of a cathode of the fuel cell, wherein the fuel cell system is supplied to the anode of the fuel cell A method for operating a direct oxidation fuel cell system, wherein the fuel supply amount is controlled to be 1.1 to 2.2 times the fuel consumption by power generation of the fuel cell. The fuel is methanol or a methanol aqueous solution, and the two-layer fuel permeation flux comprising the first conductive porous substrate and the porous composite layer is 0.6 × 10 ^{−4 to} 1.5 × 10 6. The operation method of the direct oxidation fuel cell system according to claim 8, wherein the fuel cell is caused to generate electric power at a fuel concentration and a cell temperature that are ^-4 mol / (cm ² · min). The fuel is methanol or an aqueous methanol solution, and the fuel permeation flux of the second conductive porous substrate is 4.5 × 10 ^{−4 to} 8.0 × 10 ⁻⁴ mol / (cm ² · min) The method for operating a direct oxidation fuel cell system according to claim 8, wherein the fuel cell is generated at a fuel concentration and a cell temperature such that

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