JP2008235156A - Electrode catalyst layer for fuel cell, and fuel cell using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode catalyst layer for a fuel cell which is improved in utilization rate of catalyst and discharge performance of water and has an excellent power generation characteristics and is of low cost. <P>SOLUTION: The catalyst carrier is a composite carbon material in which carbon fiber is grown from the surface of a carbon base material, and the carbon base material and the carbon fiber are a carbon material one of the carbon crystal plane of which is oriented in parallel to the surface of the particles or fiber, and the other carbon crystal plane is not oriented in parallel to the surface of the particles or fiber. The carbon material the carbon crystal plane of which is not oriented in parallel to the surface is hydrophilic treated. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a fuel cell, and more particularly to a fuel cell having a suitable configuration of its catalyst layer.

  High energy density of on-board battery to cope with increase in power consumption and increase in continuous use time due to multi-functionalization of portable electronic devices such as mobile phones, personal digital assistants (PDAs), notebook PCs, and video cameras There is a strong demand for it. Currently, lithium-ion secondary batteries are mainly used as these power sources, but it is predicted that they will reach the limit in the near future in terms of energy density, and solid polymer electrolyte membranes are used as alternative power sources. It is expected that the fuel cell used will be put to practical use at an early stage.

  A polymer electrolyte fuel cell has a membrane electrode assembly (MEA) bonded to both sides of a polymer electrolyte membrane by a catalyst layer and a gas diffusion layer, and a pair of separators on both sides. The fuel cell has a sandwiched cell structure and generates electricity by supplying a fuel such as hydrogen or methanol to the anode side and an oxidant such as oxygen or air to the cathode side.

  Among these, the direct fuel cell that generates electricity by directly supplying the inside of the cell without reforming it into hydrogen and generating electrode power is the high theoretical energy density of organic fuel, simplification of the system, It is attracting attention from the viewpoint of ease of fuel storage, and active research and development is underway. For example, a direct methanol fuel cell supplies methanol or an aqueous methanol solution directly as a fuel to the anode.

  However, there are some problems in putting these solid polymer fuel cells into practical use.

  One of them is the low utilization rate of the catalyst due to the low contact rate between the catalyst and the electrolyte in the catalyst layer. Catalysts that are not in contact with the electrolyte are inactive because there is no proton conduction path. If the catalyst utilization rate is low, an expensive noble metal catalyst such as platinum is wasted, and the cost of the fuel cell increases. In general, the catalyst utilization is said to be about 30%, and various techniques have been proposed to improve this.

  An example of the technique is to improve the affinity with the electrolyte by hydrophilizing the surface of the catalyst or the catalyst carrier that supports the catalyst. The catalyst carrier is generally made of a carbon material such as carbon black, and is originally hydrophobic. For this reason, the affinity with the electrolyte is low, causing a low contact rate between the catalyst and the electrolyte. In the prior art, the catalyst utilization rate is improved by hydrophilizing the surface of the supported catalyst.

  However, if the surface of the supported catalyst is hydrophilic, the water discharging ability from the catalyst layer is lowered, and the gas diffusibility in the catalyst layer is lowered. In the direct methanol fuel cell, an aqueous methanol solution is supplied to the anode, and in the polymer electrolyte fuel cell, the fuel is supplied after being humidified. Furthermore, water is generated by an electrode reaction at the cathode. If water discharge is not ensured, these waters accumulate in the catalyst layer and block the gas diffusion path.

  As a technique for solving such problems, a catalyst layer is proposed in which a catalyst carrier is made of a hydrophilic carbon material and further contains a water-repellent carbon material. The power generation characteristics are improved by improving the contact rate between the catalyst and the electrolyte and ensuring the water discharge of the catalyst layer.

In addition to the hydrophilization technique, a technique using nanocarbon grown from a carbon material has been proposed as a technique related to a catalyst carrier (for example, Patent Document 1 or 2).
JP 2006-156366 A JP 2006-156387 A

  Even in the configuration such as the technique using the hydrophilic material and the water repellent material described above, a certain improvement in characteristics can be expected with respect to the conventional configuration due to the compatibility of the hydrophilicity and water repellency of the catalyst layer. However, since the hydrophilic carbon material and the water-repellent carbon material have poor affinity, they are unlikely to be uniformly dispersed in the catalyst layer. In order to form a catalyst layer, generally, a catalyst layer paste in which a supported catalyst and an electrolyte are mixed in a dispersion medium is prepared, and this is applied and dried. In preparing the catalyst layer paste, it is difficult to uniformly mix the hydrophilic carbon material and the water repellent carbon material. For this reason, in the vicinity of the catalyst, since the catalyst carrier is hydrophilic, it is locally hydrophilic, and the problem of low water discharge and low gas diffusibility has not been solved.

  Also, Patent Documents 1 and 2 do not recognize this problem.

  Accordingly, in the present invention, in order to solve the conventional problems as described above, the hydrophilic carbon material and the water-repellent carbon material are uniformly dispersed in the catalyst layer to improve the contact ratio between the catalyst and the electrolyte and to discharge water. An object of the present invention is to provide a fuel cell electrode catalyst layer capable of achieving both improvement in performance locally.

  The present invention is a fuel cell electrode catalyst layer comprising a catalyst, a catalyst support made of a carbon material, and a polymer electrolyte, wherein the catalyst support is a composite carbon material in which carbon fibers are grown from the surface of a carbon substrate. The carbon base material and the carbon fiber are carbon materials in which one carbon crystal plane is oriented parallel to the surface and the other carbon crystal plane is not oriented parallel to the surface. A carbon material in which the carbon crystal plane is not oriented parallel to the surface is subjected to a hydrophilic treatment.

  According to the configuration of the present invention, since both the hydrophilic portion and the water-repellent portion are present in one catalyst carrier, the hydrophilic carbon material and the hydrophobic carbon material are homogeneously prepared in the preparation of the catalyst layer paste. It becomes possible to mix. Thereby, hydrophilicity and hydrophobicity can be reconciled locally in the catalyst layer.

  According to the present invention, since the hydrophilic carbon material is present in the vicinity of the catalyst, the contact ratio with the electrolyte can be improved and the catalyst utilization can be improved, and at the same time, the water-repellent carbon material is also present in the vicinity of the catalyst. Therefore, it is possible to improve water discharge and improve gas diffusibility. From these effects, the cost can be reduced by reducing the amount of catalyst used, and the overvoltage can be reduced by reducing the diffusion resistance of the gas.

The fuel cell electrode catalyst layer according to the embodiment of the present invention includes a catalyst, a catalyst carrier made of a carbon material, and a polymer electrolyte. As the catalyst, a highly active noble metal or the like is used, and platinum is particularly preferable. In particular, as an anode catalyst, an alloy catalyst of platinum and ruthenium is preferably used in order to reduce poisoning of the catalyst by carbon monoxide. As the catalyst carrier, a carbon material such as carbon black is preferably used because of its high electron conductivity and acid resistance. As the polymer electrolyte, a perfluorosulfonic acid polymer is preferable because it is excellent in proton conductivity, heat resistance, and oxidation resistance.

  In the catalyst layer according to the present invention, the catalyst support is a composite carbon material in which carbon fibers are grown from the surface of the carbon substrate, and the carbon substrate and the carbon fibers are in the form of carbon crystal planes with respect to their surfaces that are particles or fibers. The orientation is different.

  As the carbon substrate, particulate carbon black is preferably used, but the orientation of the carbon crystal plane can be controlled by the synthesis method, heat treatment temperature, and the like. Carbon fiber can also be used as a carbon substrate. As the carbon fiber, a carbon nanofiber or a carbon nanotube having a small fiber diameter and a large specific surface area is preferable. These carbon fibers can be controlled in fiber diameter, fiber length, orientation of the carbon crystal plane, and the like depending on the synthesis conditions, and have various shapes.

  In order to synthesize carbon nanofibers and carbon nanotubes, the temperature of a substrate carrying a compound containing a catalytic element on the surface is raised in an inert gas, and then a mixed gas of carbon-containing gas and hydrogen gas is used. There is a method to react by inflow. Although it does not specifically limit as a catalyst element, Mn, Fe, Co, Ni, Cu, Mo etc. are mentioned. These may be used alone or in combination of two or more. Further, the carbon-containing gas is not particularly limited, and examples thereof include methane, ethane, butane, ethylene, acetylene, carbon monoxide, benzene, toluene and the like. Depending on the selection of the catalytic element, the carbon-containing gas, and the reaction temperature, the shape of the carbon nanofiber to be synthesized is different.

  When particulate carbon black is used for the carbon substrate, the orientation is low if the carbon crystal plane is not grown if it is normal, such as acetylene black or ketjen black, and is oriented parallel to the particle surface. Not. On the other hand, when carbon black is heat-treated at 1500 to 2500 ° C. in an inert atmosphere, graphitization of the carbon crystal occurs, and the carbon crystal plane grows and is oriented parallel to the particle surface. These can be confirmed by observation with a transmission electron microscope (TEM).

  In the present invention, the “particle surface” refers to the surface when the particle is captured as a substantially spherical body without considering minute pores or irregularities, and the “alignment in parallel” refers to the particle as the substantially spherical body. When a cross section is considered, it means that the carbon crystal plane is oriented along the particle surface. In the case of carbon black, when heat treatment as described above is performed, fine pores and irregularities are reduced, and particles are more appropriately represented as polyhedrons than spheres. At this time, when the cross section of the particle is considered, the carbon crystal plane is oriented in parallel along the particle surface so as to form a polyhedral plane.

  Carbon nanofibers include plate shapes, herringbone shapes, cup stack shapes, etc. due to the orientation of the carbon crystal plane, and tube-shaped ones with the carbon crystal plane oriented parallel to the fiber plane are particularly Called carbon nanotube. In the plate-shaped, herringbone-shaped, and cup-stacked carbon nanofibers, the carbon crystal plane is oriented with a certain angle with respect to the fiber plane. These can also be confirmed from observation with a TEM.

  “Fiber surface” in the present invention refers to a wall surface in the length direction when a fiber is captured as a substantially cylinder without considering minute pores or irregularities, and includes the upper and lower surfaces of the cylinder at the end. Absent. “Oriented in parallel” means that the carbon crystal plane is oriented along the fiber surface when a cross section parallel to the length direction of the fiber as the substantially cylindrical shape is considered. In the case of carbon nanofibers, the tube-like one has parallel orientation as described above.

By growing the carbon fiber from the surface of the carbon base as described above, composite carbon materials having different orientations of the carbon crystal plane with respect to the surface of the particle or fiber can be obtained. Carbon substrate,
When carbon black whose carbon crystal plane is not oriented parallel to the particle surface or carbon nanofibers that are not oriented parallel to the fiber surface is used, the carbon crystal plane is the carbon fiber to be grown. Carbon nanotubes that are oriented parallel to the fiber surface are preferred. In addition, when carbonized carbon black whose carbon crystal plane is oriented parallel to the particle surface or carbon nanotubes oriented parallel to the fiber surface is used as the carbon substrate, the carbon fiber to be grown is used. As the carbon nanofiber, the carbon crystal plane is not oriented parallel to the fiber surface.

  The carbon crystal plane forms a six-membered ring by a covalent bond and is in a chemically stable state. On the other hand, the edge portion of the carbon crystal plane is chemically unstable because the covalent bond is broken. When the carbon material is hydrophilized, the hydrophilization preferentially occurs from the edge portion of the unstable carbon crystal plane. For this reason, in the case of a composite carbon material having a different orientation of the carbon crystal plane relative to the particle or fiber surface as described above, the carbon crystal plane is not oriented parallel to the particle or fiber surface. The carbon material is preferentially hydrophilized. The term “hydrophilization” as used herein means that a hydrophilic functional group adheres to an edge portion of a carbon crystal plane exposed on the surface of particles or fibers. Examples of the hydrophilic functional group include a hydroxyl group, a sulfonic acid group, and a carboxyl group.

  The description that “the carbon crystal planes are oriented in parallel” in the present invention includes not only the case where all the carbon crystal planes are oriented in parallel, but also there are places that are not partially oriented in parallel. Including cases. In addition, the description “the carbon crystal planes are not oriented in parallel” in the present invention includes not only a case where all the carbon crystal planes are not oriented in parallel, but also a place where the carbon crystal planes are partially oriented in parallel. Including the case where exists.

  In the hydrophilization treatment as described above, it is possible to give a clear difference in the priority of hydrophilization if the orientation of most carbon crystal planes of the carbon substrate and the carbon fiber to be grown is different. Therefore, the effect of the present invention can be obtained. For example, a carbon material in which 70% or more of the carbon crystal planes that can be confirmed by TEM observation are oriented in parallel to the particle or fiber surface, and a carbon material in which more than 70% are not oriented in parallel The effect of the present invention can be obtained by a combination.

  In addition, since the amorphous carbon material in which the carbon crystal plane cannot be clearly confirmed is poor in orientation itself, hydrophilic functional groups are easily attached to the surface. It is included in a carbon material whose planes are not oriented in parallel.

  FIG. 1 is an example of a fuel cell catalyst carrier according to the present invention. Carbon black 1 whose carbon crystal plane is not oriented parallel to the particle surface is used as the carbon substrate, and carbon nanotube 2 whose carbon crystal plane is oriented parallel to the fiber surface is used as the carbon fiber. It is what was used. FIG. 2 is also an example of a fuel cell catalyst carrier according to the present invention. Carbon nanofibers 3 whose carbon crystal planes are not oriented parallel to the particle surface are used as the carbon substrate, and carbon nanotubes 4 whose carbon crystal planes are oriented parallel to the fiber surface are used as carbon fibers. Is used.

  The method for the hydrophilic treatment is not particularly limited, and various known methods can be used. There are various methods, such as a method of modifying the surface by mixing with an acid or an oxidizing agent, a method of modifying the surface by irradiating with plasma, or a method of attaching a silane coupling agent having a hydrophilic group to the surface. . Among these, the method of attaching sulfonic acid groups to the surface by treatment with fuming sulfuric acid is simple and preferable.

  As a hydrophilic evaluation, there is a method in which a hydrophilic functional group attached to the surface is quantified as an ion exchange amount. In order to achieve both hydrophilicity and water repellency of the composite carbon material of the present invention, the hydrophilic functional group is preferably contained in an amount of 10 meq / g or more. When the content of the hydrophilic functional group is less than 10 meq / g, the affinity with the electrolyte may decrease, and the effect of improving the catalyst utilization rate may decrease. Moreover, it is preferable that content of a hydrophilic functional group is 100 meq / g or less. When the content of the hydrophilic functional group exceeds 100 meq / g, the water discharging property is lowered, and the effect of improving the gas diffusibility may be lowered.

  Examples of the quantitative method as the ion exchange amount of the hydrophilic functional group include a method by neutralization titration. The composite carbon material is dispersed in a sodium chloride aqueous solution and stirred, and then the filtrate is neutralized and titrated with an aqueous sodium hydroxide solution to quantify the proton content of the hydrophilic functional group exchanged with sodium. Various methods can be used.

  In the composite carbon material of the present invention, the weight ratio of the carbon material in which the carbon crystal plane is not oriented parallel to the particle or fiber surface is preferably 20 to 80% by weight. If it is not within this range, it may be difficult to adjust the hydrophilic functional group to 10 to 100 meq / g.

  The structure of the fuel cell in the present invention is not particularly limited as long as it is known as a polymer electrolyte fuel cell or a direct methanol fuel cell, and a known structure can be adopted. In general, a membrane electrode assembly (MEA) comprising a polymer electrolyte membrane, a pair of catalyst layers arranged so as to sandwich the polymer electrolyte membrane, and a pair of gas diffusion layers arranged so as to sandwich them is constituted. Become an element.

  The polymer electrolyte membrane is not particularly limited as long as it is excellent in proton conductivity, heat resistance, and oxidation resistance. In general, a perfluorosulfonic acid polymer membrane is used, but in a direct methanol fuel cell or the like, a hydrocarbon polymer membrane that can reduce methanol crossover is also preferable.

  As a catalyst layer preparation process, first, a catalyst layer paste is prepared by mixing a supported catalyst powder and a dispersion liquid in which a polymer electrolyte is dispersed in a solvent. Next, this is directly applied to the surface of the electrolyte membrane using a spray or a doctor blade, and dried to form a catalyst layer. Alternatively, the catalyst layer paste is applied on a support such as polytetrafluoroethylene (PTFE) sheet, dried to form a catalyst layer on the support, and then hot pressed on the surface of the electrolyte membrane. There is also a method of thermal transfer by a method.

  As the gas diffusion layer, it is possible to use a conductive porous material such as carbon paper or carbon cloth having both diffusibility of fuel and oxidant, discharge of water generated by power generation, and electronic conductivity. Moreover, in order to improve water discharge | emission property, these electroconductive porous materials may be immersed in PTFE dispersion etc., and you may dry and provide water repellency.

  Further, a porous conductive water-repellent layer may be provided on the surface of the gas diffusion layer that contacts the catalyst layer in order to improve the binding property with the catalyst layer and the water discharging property. . The conductive water repellent layer is composed of an electron conductive carbon powder such as carbon black and a water repellent material such as PTFE or tetrafluoroethylene-hexafluoropropylene copolymer. The conductive water repellent layer is manufactured by preparing a paste in which carbon black and water repellent material dispersion are mixed in a solvent, and applying this to the surface of the gas diffusion layer using a doctor blade, etc. There are ways to do it.

  As a method for producing the MEA, there is a method in which both sides of an electrolyte membrane with a catalyst layer (CCM) in which a catalyst layer is formed on both sides of a polymer electrolyte membrane are sandwiched between a pair of gas diffusion layers and thermally welded by a hot press method or the like. is there. In general, both the catalyst layer and the gas diffusion layer of the MEA have the same area and only the electrolyte membrane is enlarged. However, only the electrolyte membrane contains fuel and oxidant. In order to prevent leakage to the outside of the electrode, a pair of gaskets such as rubber sheets are arranged so as to sandwich the electrolyte membrane.

  The separator is a gas impermeable material that separates the fuel and the oxidant, processability capable of forming a flow path for the fuel and the oxidant, electron conductivity, acid resistance, etc., and generally from a carbon material such as graphite. Composed. A flow path for supplying fuel or an oxidant is formed on the surface of the separator that contacts the MEA. Moreover, when using a metal as a separator material, the coating which prevents corrosion is given.

  Each MEA is sandwiched between a pair of gaskets and separators, and if necessary, a plurality of MEAs and separators are alternately stacked, and further sandwiched between a pair of current collector plates, heaters, insulating plates, and end plates, respectively. A battery cell is used.

  The method for supporting the catalyst, the method for preparing the catalyst layer paste, the method for joining the electrolyte membrane and the catalyst layer, etc. in the fuel cell electrode catalyst layer of the present invention are not particularly limited. Can be used. Furthermore, the constituent members such as current collector plates and separators in the polymer electrolyte fuel cell of the present invention, the structure of the cell, and the system structure including auxiliary devices are not particularly limited, and various known ones Can be used.

  Specific embodiments of the present invention will be described, but the present invention is not limited only to the following examples.

(Example 1)
(Synthesis of composite carbon materials)
As the carbon substrate, acetylene black (Denka Black, manufactured by Denki Kagaku Kogyo Co., Ltd.), which is carbon black, was used. As a result of observation by TEM, the carbon crystal plane was very small and the orientation was poor.

  Iron nitrate (manufactured by Kanto Chemical Co., Inc., reagent) was dissolved in ion exchange water, and acetylene black was mixed and dispersed therein. After stirring this for 1 hour, the water was removed by an evaporator, whereby iron nitrate was supported on acetylene black.

  This carbon substrate is set in a ceramic reaction vessel, heated to 800 ° C. in a helium gas atmosphere, then replaced with a mixed gas of 50% hydrogen gas and 50% acetylene gas, and held at 800 ° C. for 10 minutes. By doing so, carbon nanotubes were grown. This was treated with hydrochloric acid to remove iron and washed with ion-exchanged water to obtain a composite carbon material.

  The weight ratio of carbon nanotubes in the composite carbon material was 50% by weight. As a result of observing the carbon nanotubes by TEM, all the carbon crystal planes were oriented parallel to the fiber surface.

  The composite carbon material was dispersed in a 5 mol / L sulfuric acid aqueous solution and stirred for 1 hour to make the surface hydrophilic. The amount of the hydrophilic functional group of the composite carbon material by the titration method was 20 meq / g.

(Catalyst layer formation)
A cathode catalyst prepared by supporting 30% by weight of platinum having an average particle diameter of 5 nm on the above composite carbon material is similarly 30% by weight of a platinum ruthenium alloy having an average particle diameter of 5 nm (atomic ratio Pt: Ru = 1: 1). The supported catalyst was used as an anode catalyst.

  Mix each of the above supported catalysts dispersed in an isopropanol aqueous solution and a solution in which the polymer electrolyte Nafion (registered trademark) is dispersed in a solvent (Aldrich, reagent, 5 wt% solution) and mix well. Thus, catalyst layer pastes were respectively prepared. At this time, the weight ratio of the catalyst carrier to the polymer electrolyte in the catalyst layer paste was 1: 1. These catalyst layer pastes were applied on a PTFE sheet (manufactured by Nichias, Naflon PTFE sheet) with a gap of 200 μm using a doctor blade, and then dried at room temperature in the atmosphere for 6 hours to form a catalyst layer sheet.

(Production of MEA)
Each of the catalyst layer sheets is cut into a size of 6 cm × 6 cm, and each catalyst layer sheet is centered on Nafion (Nafion 112, manufactured by DuPont), which is a polymer electrolyte membrane cut into a size of 12 cm × 12 cm. Was laminated such that the coated surface was in contact with the electrolyte membrane, and was thermally transferred by a hot press method.

  A gas diffusion layer cut to a size of 6 cm × 6 cm is stacked around an electrolyte membrane having a catalyst layer formed on both sides, and bonded by a hot press method to obtain an MEA.

(Production of fuel cell)
A pair of rubber gas seals cut into 12 cm x 12 cm and provided with a 6 cm x 6 cm hole in the center are laminated so as to sandwich the MEA obtained above, and further laminated with a pair of separators and current collectors, respectively. It laminated | stacked so that it might be inserted | pinched in order with a board, a heater, an insulating board, and an end plate, and it fixed with the fastening rod. At this time, the separator is formed with a serpentine type flow path having a width of 1.5 mm and a depth of 1 mm, and the current collector plate and the end plate are made of a stainless steel plate subjected to gold plating.

(Example 2)
The acetylene black used in Example 1 was set in a ceramic reaction vessel, heated to 2000 ° C. in a helium gas atmosphere, and subjected to heat treatment for 5 hours to obtain graphitized carbon black. As a result of observing this with TEM, the carbon crystal plane was oriented parallel to the particle surface.

  Nickel nitrate (manufactured by Kanto Chemical Co., Inc., reagent) was dissolved in ion exchange water, and the graphitized carbon black was mixed and dispersed therein. After stirring this for 1 hour, the water was removed by an evaporator, thereby supporting nickel nitrate on graphitized acetylene black.

  This carbon substrate is set in a ceramic reaction vessel, heated to 500 ° C. in a helium gas atmosphere, then replaced with a mixed gas of 50% hydrogen gas and 50% ethylene gas, and held at 500 ° C. for 10 minutes. By doing so, carbon nanofibers were grown. This was treated with hydrochloric acid to remove nickel and washed with ion-exchanged water to obtain a composite carbon material.

  The weight ratio of carbon nanofibers in the composite carbon material was 50% by weight. As a result of observing the carbon nanofibers with a TEM, all the carbon crystal planes were in the form of a herringbone with a certain angle to the fiber surface.

  The composite carbon material was dispersed in a 5 mol / L sulfuric acid aqueous solution and stirred for 1 hour to make the surface hydrophilic. The amount of the hydrophilic functional group of the composite carbon material by the titration method was 20 meq / g.

  An MEA was produced in the same manner as in Example 1 except that the composite carbon material was used as a catalyst carrier. Using this MEA, the same polymer electrolyte fuel cell as in Example 1 was produced.

(Example 3)
The silicon wafer was set in a substrate in a chamber of a sputtering apparatus, after the pressure in the chamber reaches 1 × 10 ^-6 Pa, argon gas was introduced a sputtering gas into the chamber. By performing glow discharge on a nickel target, a nickel thin film having a thickness of 4 nm was formed on the silicon wafer.

  A silicon wafer on which a nickel thin film was formed was set in a ceramic reaction vessel, and carbon nanofibers were synthesized in the same manner as in Example 2. The carbon nanofibers were scraped off from the silicon wafer, pulverized, treated with hydrochloric acid to remove nickel, and washed with ion-exchanged water to obtain carbon nanofibers.

  As a result of observing this carbon nanofiber by TEM, all the carbon crystal planes were in a herringbone shape oriented with a certain angle with respect to the fiber surface.

  A composite carbon material was produced in the same manner as in Example 1 except that the carbon nanofiber was used as the carbon substrate. The weight ratio of carbon nanotubes in the composite carbon material was 50% by weight. As a result of observing the carbon nanotube by TEM, all the carbon crystal planes were oriented in parallel to the particle surface.

  The composite carbon material was dispersed in a 5 mol / L sulfuric acid aqueous solution and stirred for 1 hour to make the surface hydrophilic. The amount of the hydrophilic functional group of the composite carbon material by the titration method was 20 meq / g.

  An MEA was produced in the same manner as in Example 1 except that this composite carbon material was used as a catalyst carrier, and a polymer electrolyte fuel cell similar to that in Example 1 was produced.

Example 4
The silicon wafer was set in a substrate in a chamber of a sputtering apparatus, after the pressure in the chamber reaches 1 × 10 ^-6 Pa, argon gas was introduced a sputtering gas into the chamber. By performing glow discharge on an iron target, an iron thin film having a thickness of 4 nm was formed on the silicon wafer.

  The silicon wafer on which the iron thin film was formed was set in a ceramic reaction vessel, and carbon nanotubes were synthesized in the same manner as in Example 1. The carbon nanotubes were scraped off from the silicon wafer, pulverized, treated with hydrochloric acid to remove iron, and washed with ion exchange water to obtain carbon nanotubes.

  As a result of observing the carbon nanotube by TEM, all the carbon crystal planes were oriented parallel to the particle surface.

A composite carbon material was produced in the same manner as in Example 2 except that the above carbon nanotube was used as the carbon substrate. The weight ratio of carbon nanofibers in the composite carbon material was 50% by weight. As a result of observing the carbon nanofibers with a TEM, all the carbon crystal planes were in the form of a herringbone with a certain angle with respect to the fiber surface.

The composite carbon material was dispersed in a 5 mol / L sulfuric acid aqueous solution and stirred for 1 hour to make the surface hydrophilic. The amount of the hydrophilic functional group of the composite carbon material by the titration method was 20 meq / g. An MEA was prepared in the same manner as in Example 1 except that this composite carbon material was used as a catalyst carrier. A similar polymer electrolyte fuel cell was produced.

(Comparative Example 1)
The surface was hydrophilized by dispersing acetylene black used in Example 1 in a 5 mol / L sulfuric acid aqueous solution and stirring for 1 hour. The amount of the hydrophilic functional group by the titration method was 30 meq / g.

  An MEA was produced in the same manner as in Example 1 except that this acetylene black was used as a catalyst carrier, and a polymer electrolyte fuel cell similar to that in Example 1 was produced.

(Comparative Example 2)
The surface was hydrophilized by dispersing acetylene black used in Example 1 in a 5 mol / L sulfuric acid aqueous solution and stirring for 1 hour. Further, in the same manner as in Example 4, carbon nanotubes were produced, and it was confirmed that all the carbon crystal planes were in the form of a herringbone oriented with a certain angle with respect to the fiber surface.

  A cathode catalyst comprising 30% by weight of platinum having an average particle diameter of 5 nm supported on the above acetylene black and similarly supporting 30% by weight of a platinum ruthenium alloy having an average particle diameter of 5 nm (atomic ratio Pt: Ru = 1: 1). This was used as the anode catalyst.

  A catalyst layer paste was prepared by mixing the above-mentioned supported catalyst in an isopropanol aqueous solution, a solution in which Nafion was dispersed in a solvent, and the above-mentioned carbon nanotubes and mixing them sufficiently. At this time, the weight ratio of acetylene black, carbon nanotube, and polymer electrolyte in the catalyst layer paste was 1: 1: 2. These catalyst layer pastes were applied on a PTFE sheet (manufactured by Nichias, Naflon PTFE sheet) with a gap of 200 μm using a doctor blade, and then dried at room temperature in the atmosphere for 6 hours to form a catalyst layer sheet.

  An MEA was produced in the same manner as in Example 1 except that the catalyst layer sheet was used, and a polymer electrolyte fuel cell similar to that in Example 1 was produced.

(Comparative Example 3)
A composite carbon material was produced in the same manner as in Example 1 except that the hydrophilic treatment was not performed. An MEA was produced in the same manner as in Example 1 except that this composite carbon material was used as a catalyst carrier, and a polymer electrolyte fuel cell similar to that in Example 1 was produced.

(Evaluation of contact area of electrolyte)
For the polymer electrolyte fuel cells of Examples 1 to 4 and Comparative Examples 1 to 3 prepared above, the contact area between the catalyst and the electrolyte was evaluated.

Humidified nitrogen was supplied to the working electrode of each fuel cell at 150 cm ³ / min, and humidified hydrogen was supplied to the counter electrode at 150 cm ³ / min. The amount of electricity of 0.4 V to 0.05 V at the working electrode was measured by cyclic voltammetry, and the contact area of the electrolyte was calculated therefrom.

Incidentally, the supported catalysts of Examples 1 to 4 and Comparative Examples 1 to 3, none of the specific surface area of the catalyst with CO gas adsorption amount measurement, the cathode at 70m ² / g, was the anode in 55m ² / g.
(Evaluation of power generation characteristics)
The power generation characteristics of the polymer electrolyte fuel cells of Examples 1 to 4 and Comparative Examples 1 to 3 prepared above were evaluated.

The temperature of each fuel cell was controlled at 60 ° C. A 2 mol / L aqueous methanol solution was supplied at 2 cm ³ / min to the anode of each fuel cell, and non-humidified air was supplied to the cathode at 400 cm ³ / min. The current density was set to 150 mA / cm ^{2 by} constant current control, and the effective voltage after power generation for 8 hours was measured.

The data thus obtained is shown in Table 1. In the table, carbon black is indicated as CB, carbon nanotubes as CNT, and carbon nanofibers as CNF.

  In Examples 1 to 4 and Comparative Examples 1 and 2 in which the catalyst carrier was hydrophilized, the contact area with the electrolyte was improved as compared with Comparative Example 3 in which the hydrophilization was not performed. This is because the hydrophilicity has improved the affinity with the electrolyte. However, in Comparative Example 1 using only a carbon material that has been subjected to a hydrophilic treatment, and in Comparative Example 2 in which only a water-repellent carbon material is mixed, the generated voltage is reduced. This is because the water-discharging property in the catalyst layer is lowered due to the hydrophilization and the gas diffusibility is lowered. On the other hand, in Examples 1 to 4, in which the catalyst carrier is a composite carbon material and partially hydrophilized, the improvement of the contact area with the electrolyte and the improvement of the water discharge performance are both achieved, and good power generation is achieved. Characteristics are obtained.

  From the above results, it was found that the catalyst layer of the present invention can improve the utilization rate of the catalyst, reduce the overvoltage, and obtain good power generation characteristics. By using the fuel cell membrane electrolyte assembly provided with the catalyst layer, it is possible to obtain a solid polymer fuel cell with excellent power generation characteristics and low cost.

  The polymer electrolyte fuel cell of the present invention has excellent power generation characteristics and is useful as a power source for portable small electronic devices such as a mobile phone, a personal digital assistant (PDA), a notebook PC, and a video camera. It can also be applied to a power source for electric scooters.

Schematic diagram of an example of a fuel cell catalyst carrier according to the present invention. Schematic view of another example of a fuel cell catalyst carrier according to the present invention.

Explanation of symbols

1 carbon black 2 carbon nanotube 3 carbon nanofiber 4 carbon nanotube

Claims (8)

  A fuel cell electrode catalyst layer comprising a catalyst, a catalyst carrier supporting the catalyst, and a polymer electrolyte, the catalyst carrier comprising a composite carbon material obtained by growing carbon fibers from the surface of a carbon substrate; The carbon base material is a carbon material in which a carbon crystal plane in the carbon base material is oriented parallel to the surface of the carbon base material, and the carbon fiber has a carbon crystal plane in the carbon fiber. An electrode catalyst layer for a fuel cell, which is a carbon material that is not oriented parallel to the surface of the carbon fiber, and in which the carbon fiber is hydrophilized.   A fuel cell electrode catalyst layer comprising a catalyst, a catalyst carrier supporting the catalyst, and a polymer electrolyte, the catalyst carrier comprising a composite carbon material obtained by growing carbon fibers from the surface of a carbon substrate; The carbon fiber is a carbon material in which a carbon crystal plane in the carbon fiber is oriented in parallel to the surface of the carbon fiber, and the carbon base material has a carbon crystal plane in the carbon base material. An electrode catalyst layer for a fuel cell, which is a carbon material that is not oriented parallel to the surface of the carbon substrate, and wherein the carbon substrate is hydrophilized.   The carbon base material is a particulate carbon material, and the carbon crystal plane is carbon black whose orientation is not parallel to the particle surface. The carbon fiber has a carbon crystal plane with respect to the fiber surface. The electrode catalyst layer for a fuel cell according to claim 2, wherein the carbon nanotubes are oriented in parallel with each other.   The carbon base material is a particulate carbon material, and the carbon crystal plane is graphitized carbon black oriented parallel to the particle surface, and the carbon fiber has the carbon crystal plane on the fiber surface. The electrode catalyst layer for a fuel cell according to claim 1, wherein the carbon nanofibers are not oriented parallel to each other.   The carbon base material is a fibrous carbon material, and the carbon crystal plane is a carbon nanofiber whose orientation is not parallel to the fiber surface, and the carbon fiber has a carbon crystal plane with respect to the fiber surface. The electrode catalyst layer for a fuel cell according to claim 2, wherein the carbon nanotubes are oriented in parallel with each other.   The carbon base material is a fibrous carbon material, and the carbon crystal plane is a carbon nanotube oriented parallel to the fiber surface, and the carbon fiber has the carbon crystal plane with respect to the fiber surface. The electrode catalyst layer for a fuel cell according to claim 1, wherein the carbon nanofibers are not oriented in parallel.   The fuel cell electrode catalyst layer according to any one of claims 1 to 6, wherein the composite carbon material contains 10 meq / g or more of a hydrophilic functional group.   A fuel cell comprising the fuel cell electrode catalyst layer according to claim 1.

Images