JP4969161B2 - ELECTRODE FOR DIRECT FUEL CELL AND MANUFACTURING METHOD THEREOF, AND MEMBRANE ELECTRODE ASSEMBLY, FUEL DIRECT FUEL CELL, ELECTRONIC DEVICE - Google Patents

Description

  The present invention relates to an electrode for a direct fuel cell, a manufacturing method thereof, a membrane electrode assembly using the electrode, a direct fuel cell, and an electronic device.

  In recent years, with the spread of small portable devices such as notebook PCs, small LCD TVs, small CD players, and small DVD players, and the growing demand for energy sources for outdoor work and leisure, there is a craving for portable and portable power sources that can be used for a long time. Has been. Under such circumstances, attention has been focused on direct fuel cells using liquids such as methanol and ethanol as fuel. These liquid fuels have high energy density and are effective in building a power supply for a long time.

  For example, a polymer electrolyte fuel cell has a membrane electrode assembly (Membrane Electrode Assembly) in which a fuel electrode (negative electrode) is provided on one surface of a polymer solid electrolyte membrane and an air electrode (positive electrode) is provided on the other surface. MEA) is the basic structure. Here, FIG. 6 is a cross-sectional view schematically showing a general membrane electrode assembly 51. In the membrane electrode assembly 51, the fuel electrode 52 and the air electrode 53 usually include catalyst layers 54 and 55 and gas diffusion layers 56 and 57, and the catalyst layers 54 and 55 are adjacent to the electrolyte membrane 58. Yes. The reaction mechanism in the fuel cell having such a configuration is such that when a fuel such as methanol is supplied to the fuel electrode 52 and an oxidant such as air is supplied to the air electrode 53, ) Moves to the air electrode 53 through the electrolyte membrane 58, and electric energy is taken out by using an electrochemical reaction in which the air electrode 53 becomes water.

  The electrode reaction of the fuel cell proceeds on the electrode catalyst. For example, in the case of a methanol-oxygen fuel cell, the following reaction occurs.

(Air electrode) 3/2 O ₂ + 6H ⁺ + 6e ⁻ → 3H ₂ O
(Fuel electrode) CH ₃ OH + H ₂ O → CO ₂ + 6H ⁺ + 6e ⁻
Here, the reaction of each electrode is carried out at the three-phase interface formed by the catalytically active substance, the ion conductive substance and the electron conductive substance, which can simultaneously exchange the reactants methanol, oxygen, electrons and protons. Only progress. Therefore, in order for the catalytically active material to function as a reaction field, the catalytically active material must be in contact with both the electron conductive material and the ion conductive material. Therefore, in the catalyst layer of a polymer electrolyte fuel cell, the size of the three-phase interface formed by the catalytically active substance, the ion conductive substance, and the electron conductive substance is the most important factor that affects the discharge performance of the battery. One.

  In recent years, in order to increase the three-phase interface area, studies have been made on how to utilize pores having a nanometer pore diameter in the primary particles of an electron conductive material. Generally, a polymer electrolyte solution of perfluorocarbon sulfonic acid ionomer is used as an ion conductive material for forming a catalyst layer. However, in this solution, the ionomer of the polymer electrolyte is not dissolved in the liquid but is dispersed. Therefore, the ionomer has a degree of polymerization of about 1000 in order to maintain physical self-supporting property, and cannot penetrate into the pores having a pore diameter of several nanometers of the primary particles of the electron conductive material. Therefore, in the prior art, the catalytically active substance generally having a particle size of 30 to 50 mm is also supported in the pores of the primary particles, but the ionomer cannot penetrate into the pores of the primary particles, In this pore, a three-phase interface was not formed, and the catalyst utilization efficiency was low.

As a means for solving this problem, in Japanese Patent No. 3690651 (Patent Document 1), the ionomer size is reduced by lowering the degree of polymerization of the ionomer, and the pores in the primary particles of the electron conductive material are reduced. Ionomer (ion-conducting substance) was introduced into and the inside of the pores was used as a reaction field to increase the three-phase interface. In Patent Document 1, since the ionomer is adsorbed to an electron conductive substance, it is considered that it is not necessary to have physical independence, and the degree of polymerization is lowered.

  The technique disclosed in Patent Document 1 can newly utilize the inside of the pores in the primary particles of the electron conductive substance as a reaction field. However, in this structure, the pore diameter in the pores of the primary particles is on the order of several nanometers, and the pores in the catalyst layer outside the pores of the primary particles are on the order of tens of nanometers to several tens of micrometers. It was. Here, a pore part refers to the hole (gap part) formed after solvent volatilization. The solvent is contained in a polymer electrolyte solution used as an ion conductive substance, and is usually used also when preparing a catalyst paste. In the following description, the pores in the pores of the primary particles are defined as “primary particle pores”, and the pores in the catalyst layer outside the pores of the primary particles are defined as “multiple particle pores”.

FIG. 7 is a diagram conceptually showing the structure of carbon particles generally used as an electron conductive substance. As shown in FIG. 7, the carbon particles have an aggregate structure (aggregate) 62 in which the primary particles 61 are bonded in a fused state, or an agglomerate structure (aggregate ) generated simply by being physically entangled. And form a structure. The structure is defined as a particulate structure called agglomerate particles 63 in which aggregate structures 62 are further aggregated. The pores 64 of the agglomerate particles 63 are the above-mentioned secondary particle pores, and the pore diameter is on the order of several tens of nanometers to several tens of micrometers.

  In the catalyst layer, the pore portion functions as a fuel supply and product discharge path, but when the pore diameters are greatly different, the path from the relatively large pore portion to the small pore portion does not function effectively. Therefore, in the structure disclosed in Patent Document 1, local fuel shortage, product retention, and the like have occurred.

  Here, FIG. 8 is a diagram conceptually showing the structure of a conventional catalyst layer. As shown in FIG. 8, the conventional catalyst layer includes an ion conductive material 71, an electron conductive material 72, and a catalytically active material 73. In the conventional catalyst layer structure, the catalyst layer matrix has the secondary particle pores 74 such as in the pores of the agglomerate particles of the electron conductive material 72. In the catalyst layer having the structure shown in FIG. 8, the ion conductive material 71 becomes a transfer path of protons generated in the reaction of the anode. Further, the electron conductive material 72 serves as an electron conduction path. Furthermore, the catalytically active substance 73 has a role of causing the reaction to proceed with less energy. The secondary particle pores 74 formed in the pores of the agglomerate particles of the electron conductive material 72 serve as a main diffusion path for fuel and products.

  Furthermore, conventionally, efforts have been widely made to improve any one of gas diffusivity, ion conductivity, and electron conductivity as a catalyst layer.

  As examples of improving the gas diffusibility of the catalyst layer, methods of mixing water-repellent particles such as PTFE (polytetrafluoroethylene) and attempts to control the ratio of the pores of the secondary particles in the catalyst layer have been made. For example, Japanese Patent Laid-Open No. 5-36418 (Patent Document 2) discloses a technique for producing a catalyst layer by dispersing and kneading carbon powder carrying PTFE powder and a catalyst in a polymer electrolyte solution. . However, since PTFE powder is a compound that disrupts the ion conduction path of the catalyst layer, the technique disclosed in Patent Document 2 has a performance problem that causes a decrease in battery performance.

  In JP 2002-110202 A (Patent Document 3), the ratio of the total volume in which the pore diameter of the secondary particle pores is 10 to 30 μm is 20 to 60% with respect to the total pore volume in the catalyst layer. Thus, a technique has been disclosed in which an allowable range of mechanical strength is provided and gas diffusibility is improved. However, when the volume ratio of the secondary particle pores is adjusted as in the technique disclosed in Patent Document 3, there is a concern that the ion conductivity is lowered with the improvement of gas diffusibility. This is because the pore portion does not have ionic conductivity, and therefore the volume proportion of the ion conductive material contained in the catalyst layer decreases with the increase in the proportion of the pore portion. Therefore, these conventional techniques cannot be said to be an essential improvement means.

On the other hand, an attempt to improve the ionic conductivity in the catalyst layer has been conventionally conducted. In general, the improvements include (1) changing the mixing ratio between the electron conductive material carrying the catalytically active material and the ion conductive material, and (2) improving the adhesion between the electrolyte membrane and the catalyst layer. There are two main categories: improving and reducing interface resistance. The former method for changing the mixing ratio is essentially an effective means because the gas diffusivity and the ionic conductivity are in a trade-off relationship as described above. The latter is an effective means as an example of improving ion conductivity, but is limited to the improvement in the vicinity of the surface of the catalyst layer, and the catalyst layer structure is still insufficient.
Japanese Patent No. 3690651 Japanese Patent Laid-Open No. 5-36418 JP 2002-110202 A

  The conventional structural design of the catalyst layer has the same structure regardless of whether the fuel is gas or liquid, and the emphasis has been on improving gas diffusivity by controlling the porous structure of the catalyst layer. It is important to control the ratio of the pores in the catalyst layer. However, in the conventional catalyst layer design concept, the fuel supply and product discharge are handled by the pores, and the ion conduction is performed by an ion conductive material. It was designed with roles and roles separated. Therefore, material diffusion and ionic conduction have a trade-off relationship due to the volume fraction in the catalyst layer between the pores and the ionic conductive material, and the method of finding the optimum value on the line was repeated. . Naturally, in this method, the positions where the pores are formed are random, so that not only a large number of pores that do not act effectively exist, but also a three-phase interface cannot be formed sufficiently. Pore portions that are not effectively utilized not only serve as places where fuel and exhausts stay, but also sometimes cut off the ion conduction and electron conduction networks.

  Further, in order to reduce the resistance of the catalyst layer, which is a resistance component for both material diffusion and ion conduction, the moving distance of the material and ions is shortened. Therefore, the catalyst layer thickness is preferably as thin as possible. From this point of view, it is understood that even if the pore volume fraction is increased in order to improve the material diffusivity while keeping the amount of the ion conductive material constant, there is a trade-off relationship after all. Due to these factors, the actual catalyst utilization rate of the conventional fuel cell electrode is limited to about 20 to 60%, and the original power generation capacity cannot be exhibited.

  The present invention has been made in order to solve the above-mentioned problems. The object of the present invention is to form a catalyst layer having a high ionic conductivity and a three-phase interfacial ratio and suitable for crossover suppression. It is intended to provide a fuel cell that achieves a reduction in resistance and an improvement in catalyst utilization rate and is excellent in output characteristics at low cost, an electrode used therefor, a manufacturing method thereof, and an electronic device including the electrode.

  The electrode for a direct fuel cell of the present invention includes a catalyst layer containing an ion conductive material, an electron conductive material, and a catalytically active material, and the pore diameter of the primary particles of the electron conductive material is 6 nm or less. Is characterized by the absence of an ion conductive substance.

  Here, it is preferable that the catalyst layer does not have a pore portion in a region other than the pore having the pore diameter of the primary particle of the electron conductive material of 6 nm or less.

  In the fuel direct fuel cell electrode of the present invention, it is preferable that the average particle diameter of the catalytically active substance is 50 mm or less.

  In the direct fuel cell electrode of the present invention, the electron conductive substance is preferably a carbon material, and more preferably carbon black or activated carbon subjected to activation treatment.

The present invention relates to a method for producing an electrode for a fuel cell comprising a catalyst layer containing an ion conductive material, an electron conductive material, and a catalytically active material, by heating the ion conductive material to a glass transition temperature or higher. melted, including the step of decreasing the porosity of the catalyst layer also provides method for producing a fuel direct type fuel cell electrode.

The present invention is a membrane electrode assembly comprising a fuel direct type fuel cell electrode of the present invention described above, the fuel direct type fuel cell, and also provides an electronic device.

According to the present invention, the ion conductivity and the three-phase interface rate is high, fuel direct type fuel cell electrode and a manufacturing method thereof comprising a catalyst layer structure suitable for crossover suppression is provided. By using the fuel directly fuel cell electrode of the present invention, to achieve improved reduction and catalyst utilization loss resistance, a membrane electrode assembly having excellent output characteristics at low cost, direct fuel type fuel Batteries and electronic devices can be provided.

  FIG. 1 is a sectional view conceptually showing a catalyst layer 1 in an electrode for a direct fuel cell of the present invention. FIGS. 2 and 3 are enlarged views of FIG. As shown in FIG. 1, the electrode for a direct fuel cell of the present invention basically includes a catalyst layer 1 including an ion conductive material 2, an electron conductive material 3, and a catalytically active material 4. As shown in FIG. 3, the ion conductive material 2 is not present in the pores of the primary particles of the electron conductive material 3 having a pore diameter of 6 nm or less. Here, the pore diameter of the primary particles of the electron conductive material 3 in which the ion conductive material 2 does not exist is 6 nm or less, which means that the catalyst particles are controlled to 20 cm or more, It is considered that the vicinity of the entrance is more reactive than the inside of the primary particle pores, and the manufacturing process includes (1) supporting catalyst particles on an electron conductive material, and (2) an electron conductive material loaded with catalyst particles. It can be concluded from the order of kneading of the ion conductive material. That is, after the catalyst particles are supported, the entrance of the primary particle pores has a size of about 3 nm at most, so that it is considered that the ion conductive substance cannot enter.

  As shown in FIG. 2, the electrode for a fuel direct fuel cell according to the present invention is a primary material of an electron conductive material 3 that supports a catalytically active material 4 without the presence of the ion conductive material 2 in the pores having the above pore diameters. The particles partially form an aggregate structure (aggregate) 5 as shown in FIG. 3 and a part further forms an agglomerate structure (aggregate) to form a catalyst layer 1 as shown in FIG. It becomes. Further, in the catalyst layer 1 of the present invention, as shown in FIG. 1, a matrix composed of pores in the agglomerate structure of the electron conductive material 3 is filled with the ion conductive material 2, and the electron conductive material 3 It is preferable that the pores of the primary particles have no pores in the region other than the pores having a pore diameter of 6 nm or less (that is, inside the pores of the agglomerate particles). Here, as described above, the “pore portion” refers to a void portion formed by volatilization of the solvent at the time of preparing the catalyst layer. Such a direct fuel cell electrode structure of the present invention is the structure in which pores are provided in the pores of the agglomerate particles of the electron conductive material described above with reference to FIG. 8 in the prior art. It is a distinctly different characteristic structure. Thus, the fact that the pores of the primary particles of the electron conductive material 3 in the catalyst layer 1 do not have pores in the region other than in the pores of 6 nm or less is, for example, the film at the closest packing calculated from the charged amount This can be confirmed by comparing the thickness value with the thickness value of the actually produced catalyst layer. In addition, in view of the fact that the ratio of the pore volume with a pore diameter of 10 to 30 μm, which is already known, is not 20% or more with respect to the total pore volume (see Patent Document 3), the gas diffusibility becomes poor. In the present invention, the volume of pores having a pore diameter larger than 6 nm in the catalyst layer is preferably less than 20%, more preferably 5% or less, based on the total pore volume.

In the present invention, with respect to the volume fraction in the catalyst layer of the pore portion and the ion conductive material, which had been in a trade-off relationship in the prior art (Patent Document 3), the improvement in characteristics was confirmed when the pore portion was small. Yes .

This factor is not clear, but I think as follows . When pores are reduced in the structure of the present invention, formation of a three-phase interface is sufficiently promoted . This increase in the three-phase interface area is a specific effect of the structure of the present invention, and is thought to have contributed to an improvement in output characteristics .

  In the present invention, there is an important feature in the utilization method of the pores of the agglomerate particles of the electron conductive substance. The catalyst layer 1 of the present invention having the structure shown in FIG. 1 differs from the conventional catalyst layer shown in FIG. 8 in the fuel supply and product discharge paths. That is, in the catalyst layer 1 of the present invention, the ion conductive substance 2 becomes not only a transfer path of protons generated in the reaction of the anode but also a substance diffusion path of liquid fuel. Of course, also in the conventional catalyst layer shown in FIG. 8, the ion conductive substance 2 can be a substance diffusion path of liquid fuel, but it is not a main path because a large number of pores are formed.

  In the catalyst layer 1 of the present invention, the aggregate structure of the electron conductive material 3 serves not only as a path for electron conduction but also as a discharge path for products (such as carbon dioxide). That is, as shown in FIG. 2, in the catalyst layer 1 according to the present invention, the primary particles of the electron conductive material 3 cause the product (gas) generated by the reaction on the surface of the catalytically active material 4 to be ion conductive. It is discharged not into the substance 2 but into the pores 3a in the primary particles. As shown in FIG. 3, the electron conductive substance 3 is discharged into the pores 3 a of the primary particles because the primary particles are three-dimensionally connected in a spherical shape and a chain shape to form the aggregate structure 5. The gas diffuses in the pores 3a that continuously form a three-dimensional network, and is finally discharged out of the system from the upper surface portion and the side surface portion of the catalyst layer. On the other hand, in the conventional catalyst layer shown in FIG. 8, the pores of the primary particles of the electron conductive material cannot function as a product discharge path. This is because the conventional catalyst layer has pores with a larger pore size in the agglomerate structure and cannot function in a path with a relatively small pore size. As described above, the direct fuel cell electrode of the present invention has a new concept that the fuel supply path and the product discharge path are separated as described above.

  In the catalyst layer 1 of the present invention, as described above, the product discharge path is a pore network of the pores in the primary particles of the electron conductive material and the aggregate particles formed by linking the primary particles. . The present invention is characterized in that the pore size distribution of the pores serving as gas escape routes is limited to a very narrow range of several nm by limiting the discharge path to only the pores of the primary particles. Furthermore, the fact that the product is discharged from the reaction field to the outside air is also an important factor for establishing the structure of the catalyst layer in the present invention.

  In the catalyst layer 1 having the above-described structure, the fuel is supplied only by diffusion in the ion conductive material 2. Therefore, in order to realize sufficient fuel supply in the direct fuel cell electrode of the present invention, the terminal concentration may be designed to be 0 or more in consideration of fuel consumption at the three-phase interface. There are two parameters that can be adjusted to realize this design: catalyst layer thickness and fuel concentration (initial concentration). In the present invention, since the fuel supply is only diffused in the ion conductive material 2, the material diffusivity is naturally lower than that of the conventional catalyst layer having pores. Therefore, in the present invention, either means for reducing the thickness of the catalyst layer or increasing the fuel concentration is required. However, it is not easy to simply reduce the thickness of the catalyst layer because the catalyst layer needs to contain a catalyst that can generate sufficient power. Therefore, in the present invention, it is necessary to increase the fuel concentration.

  Therefore, the present invention is used for a direct fuel cell, and it is preferable to use a liquid fuel such as methanol, ethanol, diethyl ether or the like as the fuel. When gaseous fuel is used, the gaseous fuel has a relatively low volumetric energy density (number of hydrogen atoms per unit volume), so it is difficult to achieve a fuel concentration that is satisfactory for application to the catalyst layer 1 having the above-described structure. This is because it is considered to be. In contrast, when a liquid fuel having a relatively much higher volumetric energy density is used, the fuel concentration can be adjusted so that the fuel can be sufficiently supplied.

  Specifically, it is possible to adjust the fuel concentration by producing a membrane electrode assembly (MEA) using an electrode having a catalyst layer formed in a desired thickness and examining the power generation characteristics at several fuel concentrations. Become. From the obtained power generation characteristic data, it is only necessary to confirm whether the fuel supply is sufficient even when the desired current value is set, and whether the influence of the crossover is suppressed within an allowable range.

  According to the direct fuel cell electrode of the present invention having the catalyst layer 1 having the structure as described above, ion conduction in the catalyst layer is greatly improved as compared with the catalyst layer as shown in FIG. be able to. In order to improve the ion conductivity, it is necessary to improve either the ion conductivity of the material or the ratio of the ion conductive substance contained in the catalyst layer. The structure of the catalyst layer 1 in the present invention shown in FIG. 1 has a structure in which the pores of the secondary particles that have been conventionally used as a fuel diffusion path are replaced with an ion conductive material, so that the structure shown in FIG. In spite of the same volume as the conventional catalyst layer, the ratio of the ion conductive material 2 can be greatly increased. Therefore, in the present invention, it is possible to increase the ratio of the ion conductive substance while maintaining the substance diffusibility by the external factor of the fuel concentration. Since this is not optimization on a trade-off line like the conventional improvement method, there is a special effect that the catalyst layer structure can be improved essentially.

  Further, in the catalyst layer in the present invention, as described above, the pore portion is preferably not formed in the pores of the agglomerate structure, and as a result, the electron conductive material 3 carrying the catalytically active material 4 and the ionic conductivity can be obtained. The nonuniformity of the contact degree with the substance 2 is eliminated. For this reason, an increase in the three-phase interface can also be achieved. By increasing the three-phase interface, the catalyst utilization rate is improved, and further, the output characteristics are greatly improved.

  The direct fuel cell electrode of the present invention can also exhibit the effect of suppressing crossover. Here, the crossover is a phenomenon in which, in a direct fuel cell, when high concentration fuel is used, the supplied fuel is not consumed at the fuel electrode but reaches the air electrode without being reacted. This is a major factor that degrades the characteristics. In the catalyst layer 1 in the present invention, as described above, the fuel is supplied only by diffusion in the ion conductive material 2, and the effect of suppressing the crossover is attributed to this. Usually, the diffusion in the ion conductive material 2 is governed by the diffusion coefficient and the concentration gradient. Further, the concentration gradient is determined by the thickness of the catalyst layer, the initial concentration (concentration at the outermost surface of the catalyst layer on the diffusion layer side), and the termination concentration (concentration at the outermost surface of the catalyst layer on the electrolyte membrane side). In the present invention, as described above, in consideration of the fuel consumption at the three-phase interface, it is possible to perform a quantitative design such that the terminal concentration is close to 0, and thereby the fuel concentration reaching the electrolyte membrane. Can be controlled, so that crossover is suppressed. The present invention also contributes to the downsizing of the apparatus because the improvement of the output characteristics is recognized and the use of high-concentration fuel is possible from such a viewpoint.

  The catalyst layer 1 in the present invention preferably has a structure in which pores are not formed by filling the matrix of the catalyst layer such as pores in the agglomerate structure of the electron conductive material 3 with the ion conductive material 2 as described above. Therefore, there is an advantage that the mechanical strength of the catalyst layer is also improved. Furthermore, in the present invention, the thickness of the catalyst layer 1 can be designed to be as thin as the volume fraction of the pores that has been conventionally formed. Decreasing the thickness of the catalyst layer leads to shortening of the material diffusion and ion conduction paths in the catalyst layer, thereby reducing the resistance loss of the catalyst layer and realizing improvement in output characteristics. .

  The ion conductive material 2 in the direct fuel cell electrode of the present invention not only serves as a transfer path for protons generated in the reaction of the anode, but also serves as a material diffusion path for liquid fuel. As such an ion conductive material, an ion conductive material that has been widely used in the art can be used without particular limitation, and among them, a fluorine-based or hydrocarbon-based ion conductive material is preferable. The ion conductive material is particularly preferable. Specifically, Nafion (manufactured by DuPont), which is an ion exchange resin of a perfluorosulfonic acid polymer, can be suitably used.

  The content of the ion conductive material 2 in the catalyst layer in the present invention is not particularly limited, but is preferably 0.1 or more (mass ratio) with respect to the electron conductive material 3. More preferably, it is 5 or more (mass ratio). When the content of the ion conductive material 2 is less than 0.1 (mass ratio) with respect to the electron conductive material 3, the formation ratio of the three-phase interface is low, and the catalyst tends not to be used effectively. The content of the ion conductive material 2 in the catalyst layer can be easily controlled by adjusting, for example, the charging ratio at the time of kneading the electron conductive material and the ion conductive material, and the amount of ion conductive material to be cast. It is possible to calculate.

The electron conductive material 3 in the direct fuel cell electrode of the present invention not only serves as an electron conduction path, but also has an infinite number of pores having a number of nanopores as shown in FIG. It also serves as a discharge path for products (such as carbon dioxide). As such an electron conductive substance 3, a material that has been widely used as an electron conductive substance in the art can be used without any particular limitation. However, the electron conductive substance 3 has a large surface area and has a pore size of several nanometers. It is preferable to use a carbon material because it has an infinite number. Of these, carbon blacks such as acetylene black and ketjen black, which have been activated, or activated carbon are more preferred because they preferably have a large surface area and a large number of nanopores. However, as described above, the pores 3a of the primary particles of the electron conductive material 3 serve as a discharge path for products (carbon dioxide and the like), so that the current is increased to a high current density region (200 mA / cm ² or more). When used in a fuel cell, a large total volume of pores in primary particles represented by activated carbon or activated carbon black (specifically, the total volume is 0.1 mL / g or more) Are preferably used. Here, the total volume of the pores in the primary particles is determined from the BET specific surface area, the measurement result by the mercury intrusion method, and the like. A semiconductor or metal particles can also be used as the electron conductive material 3 if the total pore volume on the order of several nanometers is equal to or greater than that of carbon black.

  The content of the electron conductive material 3 in the catalyst layer in the present invention is not particularly limited, but if the content of the electron conductive material 3 is too low, the amount of catalyst in the unit volume of the catalyst layer Therefore, if the content of the electron conductive material 3 is too high, the quantitative balance with the ion conductive material is poor and sufficient characteristics cannot be obtained. is there. In addition, the content rate of the electron conductive substance 3 in a catalyst layer can be controlled by the preparation ratio at the time of kneading | mixing an electron conductive substance and an ion conductive substance, for example, and can be calculated easily.

  Further, the catalytically active substance in the direct fuel cell electrode of the present invention has a role of causing the reaction to proceed with less energy, and as such a catalytically active substance, platinum, platinum and ruthenium, gold, Ordinary catalytic metal materials in the art such as alloys with rhenium, tin, rhodium, palladium, iridium, osmium, ruthenium, tin, rhenium, gold, silver, nickel, cobalt and alloys thereof can be used without any particular limitation. However, platinum or a platinum-based alloy (for example, platinum-ruthenium, platinum-tin, etc.) is preferable.

The particle diameter of the catalytically active substance 4 used in the present invention is preferably 50 mm or less in order to increase the surface area per weight and increase the activity per weight of the catalyst from the viewpoint of cost and the like. Furthermore, in the present invention, as shown in FIG. 1, since the primary particles of the electron conductive material 3 are not supported by the pores 3a having a pore diameter of 6 nm or less, the particle size of the catalytically active material 4 is more preferably 20 mm or more. preferable. As described above, in the present invention, preferably, the electron conductive material 3 having a large pore volume in the primary particles is used. In general, the catalytically active material 4 is supported on the electron conductive material 3 serving as a catalyst carrier by reduction deposition and colloidal adsorption. At this time, as a reaction field, the reaction occurs more preferentially at the electrochemically unstable edge portion than the flat portion of the electron conductive material 3. Therefore, as described above, by using the catalytically active substance 4 having a particle size of preferably 50 mm or less (more preferably 20 mm or more), the structure shown in FIG. 2 is obtained, and at least the pore diameter is 6 nm or less. The pores are selectively supported only near the entrance, and the catalytically active substance 4 cannot penetrate into the pores 3a. Moreover, the catalytically active material 4 having a particle size of 20 mm or more is also preferable from the viewpoint of preventing aggregation of the catalytically active material 4 after power generation. Note that the particle size of the catalytically active substance 4 in the direct fuel cell electrode uses Scherrer's equation based on, for example, a method of actual measurement from an image obtained with a transmission electron microscope or data obtained by X-ray diffraction analysis. Can be estimated. The particle size of the catalytically active substance 4 can be controlled by the reaction time, reaction temperature, and the solution state during the reaction (atmosphere control such as convection), and qualitatively promotes nucleation and suppresses nucleation. The parameters at the time of manufacture may be controlled.

  The content of the catalytically active substance 4 in the catalyst layer in the present invention is not particularly limited, but is within the range of 0.1 to 0.8 (mass ratio) with respect to the electron conductive substance 3. Is preferable, and it is more preferable to be within the range of 0.3 to 0.8 (mass ratio). When the content of the catalytically active material 4 is less than 0.1 (mass ratio) with respect to the electron conductive material 3, the amount of catalyst is small and it tends to be difficult to obtain desired output characteristics. Further, with the current technology, it is difficult to make the content of the catalytically active substance 4 with respect to the electron conductive substance 3 larger than 0.8 (mass ratio). Note that the content of the catalytically active substance 4 in the catalyst layer can be calculated from, for example, a change in weight before and after supporting the catalytically active substance, and the amount charged from the method of burning the carrier using a thermogravimetric analyzer, It can also be calculated by examining the remaining amount after combustion.

  The present invention also provides a method for producing an electrode for a direct fuel cell. The direct fuel cell electrode of the present invention having the catalyst layer 1 having the structure as described above is not particularly limited in its manufacturing method, but may be manufactured by the manufacturing method of the present invention. preferable. The production method of the present invention is characterized in that it includes a step of melting the ion conductive material by heating it to a glass transition temperature or more to reduce pores in the catalyst layer. In the manufacturing method of this invention, if such a process is included, another process will not be restrict | limited especially, Each process in the conventional method of manufacturing the electrode for fuel cells can be combined suitably. Hereinafter, a method for producing an electrode for a direct fuel cell of the present invention will be described in detail.

  For example, first, a catalyst paste containing the electron conductive material 3 supporting the catalytically active material 4 and the ion conductive material is prepared, and after applying this catalyst paste on the diffusion layer or the electrolyte membrane, a predetermined amount of the ion conductive material is added. It is cast on it and dried to form a layer. As a method for applying the catalyst paste, a screen printing method, a doctor blade method, a roll coater method, or the like can be used as in the past.

  Thereafter, the ion conductive material contained in the layered material is heated to a temperature higher than the glass transition point. Specifically, the ion conductive material is compressed and melted by pressurizing the catalyst layer on the fuel electrode side in a state of being heated at least to the glass transition temperature of the ion conductive material by hot pressing, for example. As a result, as shown in FIG. 1, the catalyst layer 1 in which no pores are formed in the pores of the agglomerate structure can be formed. FIG. 4 is a diagram schematically showing a material configuration at the time of hot pressing in the method for producing an electrode for a direct fuel cell of the present invention. For example, after the layered material 11 is applied on the diffusion layer 12 (carbon paper or carbon cloth) as described above, the hydrocarbon-based resin film 13 is disposed on the side in contact with the layered material 11 so as to be sandwiched therebetween. At the same time, a sheet 14 made of PET (polyethylene terephthalate) is disposed on the diffusion layer 12 side. In addition, the material selection of the film | membrane 13 and the sheet | seat 14 will not be restrict | limited especially if the layered material 11 and the diffused layer 12 do not adhere, The material generally used for the hot press method is applicable. In the example shown in FIG. 4, a sheet 15 made of PTFE (polytetrafluoroethylene), for example, is further arranged outside the membrane 13 and the sheet 14. With such a material structure, hot pressing as described above, which is an example of a process for reducing pores in the manufacturing method of the present invention, can be suitably performed.

  For the evaluation of the presence / absence of pores and the degree of reduction, first, the change in the weight of the catalyst layer before and after the casting step of the ion conductive material is measured. Next, it is confirmed that there is no change in the thickness of the catalyst layer before casting the ion conductive material, the thickness of the catalyst layer after passing through the two steps of casting the ion conductive material and hot pressing. The thickness of the catalyst layer can be made the same by adjusting the spacer thickness during hot pressing. From these two results, the presence / absence of the pores and the degree of reduction can be comprehensively determined. For example, if the catalyst layer weight is confirmed to increase even though the catalyst layer thickness has not changed, it can be said that the pores in the catalyst layer have decreased. Furthermore, if several types of data are collected, it is possible to estimate the degree of reduction of the pores semi-quantitatively.

  The present invention also provides a membrane electrode assembly comprising the above-described direct fuel cell electrode of the present invention. The membrane electrode assembly of the present invention is provided with two electrodes, a fuel electrode and an air electrode, on the solid electrolyte membrane, except that the electrode having the catalyst layer having the structure as described above is used, and further diffuses thereon. This is realized by a general structure including layers (see FIG. 6). The electrode of the present invention is preferably used for at least the fuel electrode of the fuel electrode and the air electrode, but may be used for both the fuel electrode and the air electrode. The solid polymer electrolyte membrane and diffusion layer used in the membrane electrode assembly of the present invention are not particularly limited, and a solid polymer electrolyte membrane and diffusion layer that have been widely used in the art can be appropriately used. it can.

  The present invention also provides a direct fuel cell fuel cell and an electronic device comprising the direct fuel cell electrode of the present invention described above. Such a direct fuel cell and electronic device according to the present invention use the direct fuel cell electrode according to the present invention to achieve a reduction in loss resistance and an improvement in the catalyst utilization rate, and at a low cost. Excellent output characteristics. Note that examples of the electronic device of the present invention include, but are not limited to, a charger, a notebook computer, and a portable game device.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated in detail, this invention is not limited to these.

<Example 1>
A platinum-ruthenium alloy particle having a particle diameter of 30 mm as a catalytically active material was supported on an electron conductive material obtained by activating Ketjen Black powder using a colloid supporting method. An alcohol solution of Nafion (made by DuPont), which is an ion exchange resin of perfluorosulfonic acid polymer as an ion conductive material (Nafion content 20% by weight, Aldrich Co. Manufactured), an inorganic solvent (such as pure water) for viscosity adjustment, and an organic solvent (such as isopropyl alcohol) were mixed well to prepare a paste.

  In order to form a catalyst layer on the fuel electrode side, the catalyst paste prepared above was uniformly applied by screen printing to the surface of a diffusion layer (carbon paper) as a 22.5 mm square electrode base material, and a dryer And dried at 60 ° C. for 15 minutes. Next, in order to form a catalyst layer in which no pores were formed, a predetermined amount of the above-mentioned solution of Nafion (manufactured by DuPont) was cast on the layered product after drying. Thereafter, as shown in FIG. 4, the PTFE sheet 15, the hydrocarbon-based film 13 for avoiding the adhesion of the catalyst layer, and the PET sheet 14 for avoiding the adhesion of the diffusion layer are arranged, and the diffusion in which the layered material 11 is applied The layer 12 is pressed and heated by a hot press method (130 ° C.), and the cast Nafion (manufactured by DuPont) is compressed and melted to melt the agglomerate structure of the electron conductive material so as not to form pores. A fuel electrode catalyst layer was produced by impregnating the pores with an ion conductive material.

  The catalyst paste for the air electrode uses TEC10E50E (46.5 wt% Pt / Ketjen Black, Tanaka Kikinzoku Kogyo Co., Ltd.) as the catalyst support material. An alcohol solution of Nafion (DuPont), which is an ion exchange resin of perfluorosulfonic acid polymer as a conductive substance (Nafion content 20% by weight, manufactured by Aldrich), an inorganic solvent (such as pure water) for adjusting viscosity, and organic A predetermined amount of a solvent (such as isopropyl alcohol) was measured and mixed well to prepare a paste. The air electrode catalyst paste prepared above was uniformly applied to the surface of a diffusion layer (carbon paper) as a 22.5 mm square electrode substrate by screen printing, and then dried at 60 ° C. for 15 minutes. It was made to dry and the air electrode catalyst layer was produced.

  Next, the fuel electrode and the air electrode, which are the two electrodes prepared as described above, are placed on both sides of a Nafion 117 membrane (manufactured by DuPont) as an ion exchange membrane, and the catalyst layer side is the Nafion 117 membrane (DuPont). The membrane electrode assembly (MEA) was produced by bonding with a hot press so as to face the product.

<Comparative Example 1>
The fuel electrode catalyst paste prepared in the same manner as in Example 1 was uniformly applied to the surface of the diffusion layer (carbon paper) by screen printing, and dried at 60 ° C. for 15 minutes in a dryer to form a fuel electrode catalyst layer. A membrane / electrode assembly was produced in the same manner as in Example 1 except that it was produced (that is, the step of reducing the pores was not performed).

<Example 2>
A catalyst paste for a fuel electrode was prepared in the same manner as in Example 1 except that TEC66E50 (49.7 wt%, Pt-Ru / Ketjen Black, Tanaka Kikinzoku Kogyo Co., Ltd.) was used as the catalyst support material. A fuel electrode catalyst layer and an air electrode catalyst layer were produced in the same manner as in Example 1 except that a catalyst paste containing such a catalyst-carrying material was used, and a membrane electrode assembly was produced.

<Comparative example 2>
Similarly to Example 2, a catalyst paste for a fuel electrode was prepared using TEC66E50 (49.7 wt% PtRu / Ketjen Black, Tanaka Kikinzoku Kogyo Co., Ltd.) as a catalyst support material, and a diffusion layer (carbon paper) Example 1 except that the surface was uniformly coated by screen printing and dried in a dryer at 60 ° C. for 15 minutes to produce a fuel electrode catalyst layer (ie, the step of reducing pores was not performed). In the same manner as above, a membrane electrode assembly was produced.

<Evaluation test>
The membrane electrode assemblies of Examples 1 and 2 and Comparative Examples 1 and 2 produced as described above were accommodated in a fuel cell container (size: 50 mm long, 100 mm wide, 100 mm high), and a single fuel cell Formed. Thereafter, air was supplied to the positive electrode and methanol (3M) was supplied to the negative electrode to generate power, and the relationship between current and voltage was examined. FIG. 5 is a graph showing voltage-current characteristics obtained in Examples 1 and 2 and Comparative Examples 1 and 2. The vertical axis represents voltage (V), and the horizontal axis represents current density (mA · cm ⁻² ). is there.

  It should be understood that the embodiments, examples and comparative examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is sectional drawing which shows notionally the catalyst layer 1 in the electrode for fuel direct fuel cells of this invention. It is a figure which expands FIG. 1 partially and shows the primary particle of an electron-conductive substance notionally. FIG. 2 is a partially enlarged view of FIG. 1 and conceptually showing an aggregate structure of an electron conductive material. It is a figure which shows typically the material structure at the time of the hot press in the manufacturing method of the electrode for fuel direct fuel cells of this invention. 4 is a graph showing voltage-current characteristics obtained in Examples 1 and 2 and Comparative Examples 1 and 2, where the vertical axis represents voltage (V) and the horizontal axis represents current density (mA · cm ⁻² ). It is sectional drawing which shows a typical membrane electrode assembly typically. It is the figure which showed notionally the structure of the carbon particle generally used as an electron-conductive substance. It is a figure which shows notionally the structure of the conventional catalyst layer.

Explanation of symbols

  1 catalyst layer, 2 ion conductive material, 3 electron conductive material, 3a pore, 4 catalytically active material, 5 aggregate structure of electron conductive material, 11 layered material, 12 diffusion layer, 13 made of hydrocarbon resin Membrane, 14 PET sheet, 15 PTFE sheet.

Claims (9)

E Bei a catalyst layer containing an ion conductive material and an electronically conductive material and the catalytically active material, the ion conductive material is not present in the pores pore size less 6nm primary particles before Symbol electron conductive substance A fuel cell electrode, wherein the volume of pores having a pore diameter larger than 6 nm in the catalyst layer is 5% or less with respect to the total pore volume . 2. The direct fuel cell electrode according to claim 1 , wherein the average particle diameter of the catalytically active substance is 50 kg or less. 3. The direct fuel cell electrode according to claim 1, wherein the electron conductive substance is a carbon material. 4. The direct fuel cell electrode according to claim 3 , wherein the carbon material is activated carbon black or activated carbon. The direct fuel cell electrode according to any one of claims 1 to 4 , wherein the fuel is a liquid. E Bei a catalyst layer containing an ion conductive material and an electronically conductive material and the catalytically active material, fuel pore diameter of the primary particles of the electron conductive material is the absence of the ion conductive material in the following pore 6nm A method for producing a direct fuel cell electrode for a battery, wherein the pore volume in the catalyst layer has a pore volume of more than 6 nm, the volume of the pore being 5% or less of the total pore volume , A method for producing an electrode for a direct fuel cell fuel, comprising a step of melting an ion conductive material by heating to a glass transition temperature or higher to reduce pores in a catalyst layer. A membrane electrode assembly comprising the direct fuel cell electrode according to any one of claims 1 to 4 . A direct fuel cell comprising the electrode for a direct fuel cell according to any one of claims 1 to 4 . Electronic apparatus, wherein a fuel is directly fuel cell electrode according mounted to one of the claims 1-4.

Images