WO2014175101A1 - Method for producing catalyst, electrode catalyst layer using said catalyst, membrane-electrode assembly, and fuel cell - Google Patents

Abstract

The objective of the present invention is to provide a method that is for producing a catalyst and that can obtain a catalyst having superior catalytic activity. This method is for producing a catalyst such that a catalyst metal is carried on a carrier having a mode radius in the distribution of pores that is at least 1 nm and less than 5 nm and a pore volume of the pores that is at least 0.3 cc per gram of the carrier, wherein the method for producing a catalyst contains: a step for impregnating the pores within the carrier with a constituent of the catalyst metal, and a step for thermal processing after the impregnation step; or a step for impregnating the pores within the carrier with a precursor of the catalyst metal, a step for reducing the precursor of the catalyst metal, and a step for thermal processing after the reducing step.

Description

[Title of Invention Determined by ISA Based on Rule 37.2] Catalyst Production Method, Electrocatalyst Layer, Membrane Electrode Assembly, and Fuel Cell Using the Catalyst

The present invention relates to a method for producing a catalyst, particularly to a method for producing an electrode catalyst used in a fuel cell (PEFC).

A solid polymer fuel cell using a proton conductive solid polymer membrane operates at a lower temperature than other types of fuel cells such as a solid oxide fuel cell and a molten carbonate fuel cell. For this reason, the polymer electrolyte fuel cell is expected as a stationary power source or a power source for a moving body such as an automobile, and its practical use has been started.

In such a polymer electrolyte fuel cell, generally, an expensive metal catalyst represented by platinum (Pt) or a Pt alloy is used, which is a high cost factor of such a fuel cell. . For this reason, development of a technique capable of reducing the cost of fuel cells by reducing the amount of noble metal catalyst used is required.

For example, in Japanese Patent Application Laid-Open No. 2007-250274 (US Patent Application Publication No. 2009/047559), in an electrode catalyst in which catalyst metal particles are supported on a conductive carrier, the average particle diameter of the catalyst metal particles is conductive. Electrocatalysts larger than the average pore size of the support pores are disclosed. Japanese Patent Application Laid-Open No. 2007-250274 (US Patent Application Publication No. 2009/047559) discloses a catalyst used for a three-phase interface by preventing the catalyst metal particles from entering the micropores of the support. It is described that the utilization efficiency of expensive noble metals can be improved by improving the ratio of metal particles.

However, the catalyst layer containing the catalyst described in Japanese Patent Application Laid-Open No. 2007-250274 (US Patent Application Publication No. 2009/047559) has a problem that the catalytic activity is lowered.

Therefore, the present invention has been made in view of the above circumstances, and an object thereof is to provide a method for producing a catalyst capable of obtaining a catalyst having excellent catalytic activity.

As a result of intensive studies to solve the above problems, the present inventors include a heat treatment step after the step of impregnating the pores inside the support with the constituent components of the catalyst metal or the precursor of the catalyst metal. The inventors have found that the above problems can be solved by a catalyst production method, and have completed the present invention.

It is a schematic sectional drawing which shows the basic composition of the polymer electrolyte fuel cell which concerns on one Embodiment of this invention. 1 in FIG. 1 is a polymer electrolyte fuel cell (PEFC), 2 is a solid polymer electrolyte membrane, 3a is an anode catalyst layer, 3c is a cathode catalyst layer, and 4a is an anode gas diffusion layer. Yes, 4c is a cathode gas diffusion layer, 5a is an anode separator, 5c is a cathode separator, 6a is an anode gas flow path, 6c is a cathode gas flow path, and 7 is a refrigerant flow path. Reference numeral 10 denotes a membrane electrode assembly (MEA). It is a schematic sectional explanatory drawing which shows an example of the shape and structure of the catalyst obtained by the manufacturing method of this invention. In FIG. 2, 20 is a catalyst, 22 is a catalyst metal, 23 is a support, 24 is a mesopore, and 25 is a micropore. It is a schematic diagram which shows the relationship between the catalyst and electrolyte in the catalyst layer which concerns on one Embodiment of this invention. In FIG. 3, 22 is a catalyst metal, 23 is a support, 24 is a mesopore, 25 is a micropore, and 26 is an electrolyte. 4 is a graph showing a pore size distribution of a carrier B used in Comparative Example 1.

The catalyst obtained by the production method of the present invention comprises a catalyst carrier and a catalyst metal supported on the catalyst carrier. And the manufacturing method of the catalyst of this invention (it is also called "electrode catalyst" in this specification) has the following processes:
(A) a step of impregnating the pores inside the carrier with the constituent components of the catalyst metal; and (b) a step of heat treatment after the impregnation step.

Moreover, the manufacturing method of the catalyst of this invention has the following process:
(C) impregnating pores inside the support with a catalyst metal precursor;
(D) reducing the catalyst metal precursor; and (b) heat-treating after the reducing step.

The carrier has a pore radius mode radius of 1 nm or more and less than 5 nm, and the pore volume of the pore is 0.3 cc / g carrier or more.

In the present specification, pores having a radius of less than 1 nm are also referred to as “micropores”. In the present specification, holes having a radius of 1 nm or more are also referred to as “meso holes”.

In the catalyst described in the above-mentioned Japanese Patent Application Laid-Open No. 2007-250274 (US Patent Application Publication No. 2009/047559), the electrolyte (electrolyte polymer) is on the surface of the catalyst as compared with a gas such as oxygen. Since it is easy to adsorb | suck, when the catalyst metal contacted electrolyte (electrolyte polymer), it discovered that the reaction active area of the catalyst surface decreased. In contrast, the present inventors have found that even when the catalyst does not contact the electrolyte, the catalyst can be effectively used by forming a three-phase interface with water. For this reason, catalyst activity can be improved by taking the structure which carries a catalyst metal inside the void | hole (mesopore) into which electrolyte cannot enter.

On the other hand, when the catalyst metal is supported inside pores (mesopores) into which the electrolyte cannot enter, the distance between the catalyst metal and the inner wall surface of the pores of the carrier is relatively large, and the amount of water adsorbed on the catalyst metal surface Will increase. Since water acts as an oxidant on the catalyst metal to generate a metal oxide, the activity of the catalyst metal is lowered and the catalyst performance is lowered. On the other hand, in the catalyst obtained by the production method of the present invention, the distance between the catalyst metal and the inner wall surface of the pore of the support is reduced, and the space where water can exist is reduced, that is, the water adsorbed on the surface of the catalyst metal. The amount is reduced. Further, water is subjected to the interaction of the inner wall surface of the hole, and the water is easily held on the inner wall surface of the hole. Therefore, the formation reaction of the metal oxide is delayed, and the metal oxide is hardly formed. As a result, the deactivation of the catalytic metal surface is suppressed. Therefore, the catalyst obtained by the production method of the present invention can exhibit high catalytic activity, that is, can promote catalytic reaction. For this reason, the membrane electrode assembly and fuel cell which have a catalyst layer using the catalyst obtained by the manufacturing method of this invention are excellent in electric power generation performance.

Furthermore, since the average particle radius of the catalyst metal is larger than the mode radius of the pore distribution of the catalyst pores, in addition to the above effects, the catalyst metal is less likely to be detached even when mechanical stress is applied. Since waste is reduced, the catalyst metal is easily used effectively.

Hereinafter, an embodiment of a method for producing a catalyst of the present invention, and an embodiment of a catalyst obtained by the production method, a catalyst layer using the catalyst, a membrane electrode assembly (MEA), and a fuel cell, with appropriate reference to the drawings A form is demonstrated in detail. However, the present invention is not limited only to the following embodiments. Each drawing is exaggerated for convenience of explanation, and the dimensional ratio of each component in each drawing may be different from the actual one. Further, in the case where the embodiment of the present invention is described with reference to the drawings, the same reference numerals are given to the same elements in the description of the drawings, and a duplicate description is omitted.

In the present specification, “X to Y” indicating a range means “X or more and Y or less”, “weight” and “mass”, “weight%” and “mass%”, “part by weight” and “weight part”. “Part by mass” is treated as a synonym. Unless otherwise specified, measurement of operation and physical properties is performed under conditions of room temperature (20 to 25 ° C.) / Relative humidity 40 to 50%.

[Fuel cell]
A fuel cell includes a membrane electrode assembly (MEA), a pair of separators including an anode side separator having a fuel gas flow path through which fuel gas flows and a cathode side separator having an oxidant gas flow path through which oxidant gas flows. Have. The fuel cell of this embodiment is excellent in durability and can exhibit high power generation performance.

FIG. 1 is a schematic diagram showing a basic configuration of a polymer electrolyte fuel cell (PEFC) 1 according to an embodiment of the present invention. The PEFC 1 first includes a solid polymer electrolyte membrane 2 and a pair of catalyst layers (an anode catalyst layer 3a and a cathode catalyst layer 3c) that sandwich the membrane. The laminate of the solid polymer electrolyte membrane 2 and the catalyst layers (3a, 3c) is further sandwiched between a pair of gas diffusion layers (GDL) (anode gas diffusion layer 4a and cathode gas diffusion layer 4c). Thus, the polymer electrolyte membrane 2, the pair of catalyst layers (3a, 3c), and the pair of gas diffusion layers (4a, 4c) constitute a membrane electrode assembly (MEA) 10 in a stacked state.

In PEFC1, the MEA 10 is further sandwiched between a pair of separators (anode separator 5a and cathode separator 5c). In FIG. 1, the separators (5 a, 5 c) are illustrated so as to be positioned at both ends of the illustrated MEA 10. However, in a fuel cell stack in which a plurality of MEAs are stacked, the separator is generally used as a separator for an adjacent PEFC (not shown). In other words, in the fuel cell stack, the MEAs are sequentially stacked via the separator to form a stack. In an actual fuel cell stack, a gas seal portion is disposed between the separator (5a, 5c) and the solid polymer electrolyte membrane 2, or between the PEFC 1 and another adjacent PEFC. These descriptions are omitted in FIG.

The separators (5a, 5c) are obtained, for example, by forming a concavo-convex shape as shown in FIG. 1 by subjecting a thin plate having a thickness of 0.5 mm or less to a press treatment. The convex part seen from the MEA side of the separator (5a, 5c) is in contact with the MEA 10. Thereby, the electrical connection with MEA10 is ensured. Further, a recess (space between the separator and the MEA generated due to the concavo-convex shape of the separator) viewed from the MEA side of the separator (5a, 5c) is a gas for circulating gas during operation of the PEFC 1 Functions as a flow path. Specifically, a fuel gas (for example, hydrogen) is circulated through the gas flow path 6a of the anode separator 5a, and an oxidant gas (for example, air) is circulated through the gas flow path 6c of the cathode separator 5c.

On the other hand, the recess viewed from the side opposite to the MEA side of the separator (5a, 5c) serves as a refrigerant flow path 7 for circulating a refrigerant (for example, water) for cooling the PEFC during operation of the PEFC 1. . Further, the separator is usually provided with a manifold (not shown). This manifold functions as a connection means for connecting cells when a stack is formed. With such a configuration, the mechanical strength of the fuel cell stack can be ensured.

In the embodiment shown in FIG. 1, the separators (5a, 5c) are formed in an uneven shape. However, the separator is not limited to such a concavo-convex shape, and may be any form such as a flat plate shape and a partially concavo-convex shape as long as the functions of the gas flow path and the refrigerant flow path can be exhibited. Also good.

The fuel cell having the MEA of the present invention as described above exhibits excellent power generation performance. Here, the type of the fuel cell is not particularly limited. In the above description, the polymer electrolyte fuel cell has been described as an example. However, in addition to the above, an alkaline fuel cell and a direct methanol fuel cell are used. And a micro fuel cell. Among them, a polymer electrolyte fuel cell (PEFC) is preferable because it is small and can achieve high density and high output. The fuel cell is useful as a stationary power source in addition to a power source for a moving body such as a vehicle in which a mounting space is limited. Among them, it is particularly preferable to use as a power source for a mobile body such as an automobile that requires a high output voltage after a relatively long time of operation stop.

The fuel used when operating the fuel cell is not particularly limited. For example, hydrogen, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, secondary butanol, tertiary butanol, dimethyl ether, diethyl ether, ethylene glycol, diethylene glycol and the like can be used. Of these, hydrogen and methanol are preferably used in that high output is possible.

Further, the application application of the fuel cell is not particularly limited, but it is preferably applied to a vehicle. The electrolyte membrane-electrode assembly of the present invention is excellent in power generation performance and durability, and can be downsized. For this reason, the fuel cell of this invention is especially advantageous when this fuel cell is applied to a vehicle from the point of in-vehicle property.

[Catalyst (Electrocatalyst)]
FIG. 2 is a schematic cross-sectional explanatory diagram showing an example of the shape and structure of a catalyst obtained by the production method of the present invention. As shown in FIG. 2, the catalyst 20 obtained by the production method of the present invention includes a catalytic metal 22 and a support 23. Further, the catalyst 20 has pores (mesopores) 24. Here, the catalytic metal 22 is carried inside the mesopores 24. Further, it is sufficient that at least a part of the catalyst metal 22 is supported inside the mesopores 24, and a part of the catalyst metal 22 may be provided on the surface of the carrier 23. However, from the viewpoint of preventing contact between the electrolyte and the catalyst metal in the catalyst layer, it is preferable that substantially all of the catalyst metal 22 is supported inside the mesopores 24. Here, “substantially all catalytic metals” is not particularly limited as long as it is an amount capable of improving sufficient catalytic activity. “Substantially all catalyst metals” are present in an amount of preferably 50 wt% or more (upper limit: 100 wt%), more preferably 80 wt% or more (upper limit: 100 wt%) in all catalyst metals.

[Production method of catalyst]
The material of the carrier according to the present invention can form the pores (primary pores) having the above-described pore volume and mode radius inside the carrier, and the catalyst component inside the pores (mesopores). There is no particular limitation as long as it has a sufficient specific surface area and sufficient electron conductivity to be supported in a dispersed state. Preferably, the main component is carbon. Specific examples include carbon particles made of carbon black (Ketjen black, oil furnace black, channel black, lamp black, thermal black, acetylene black, etc.), activated carbon, and the like. “The main component is carbon” means that the main component contains carbon atoms, and is a concept that includes both carbon atoms and substantially carbon atoms. It may be included. “Substantially consists of carbon atoms” means that contamination of impurities of about 2 to 3% by weight or less can be allowed.

More preferably, since it is easy to form a desired pore region inside the carrier, it is desirable to use carbon black, and particularly preferably, so-called mesoporous carbon having many vacancies with a radius of 5 nm or less is used.

In addition to the above carbon materials, porous metals such as Sn (tin) and Ti (titanium), and conductive metal oxides can also be used as carriers.

The pore volume of the carrier pores is 0.3 cc / g or more, preferably 0.4 to 3 cc / g carrier, more preferably 0.4 to 1.5 cc / g carrier. If the void volume is in the above range, a large amount of catalyst metal can be stored (supported) in the mesopores, and the catalyst and the catalyst metal in the catalyst layer are physically separated (contact between the catalyst metal and the electrolyte is prevented). It can be suppressed and prevented more effectively). Therefore, the activity of the catalytic metal can be utilized more effectively. In addition, the presence of many mesopores can more effectively promote the catalytic reaction by exerting the effects and advantages of the present invention more remarkably.

The mode radius (most frequent diameter) of the pore distribution of the carrier is from 1 nm to less than 5 nm, preferably from 1 nm to 4 nm, more preferably from 1 nm to 3 nm, still more preferably from 1 nm to 2 nm. It is as follows. If the mode radius of the pore distribution is as described above, a sufficient amount of catalyst metal can be stored (supported) in the mesopores, and the electrolyte and catalyst metal in the catalyst layer are physically separated (the catalyst metal and electrolyte are separated from each other). Can more effectively suppress and prevent contact). Therefore, the activity of the catalytic metal can be utilized more effectively. Further, the presence of a large volume of pores (mesopores) can more effectively promote the catalytic reaction by exerting the effects and effects of the present invention more remarkably.

Furthermore, since the average particle radius of the catalyst metal is larger than the mode radius of the pore distribution of the carrier pores, in addition to the above effects, the catalyst metal is less likely to be detached even when mechanical stress is applied. Since waste is reduced, the catalyst metal is easily used effectively.

In this specification, the mode radius of the pore distribution of the mesopores is also simply referred to as “mode diameter of the mesopores”.

The BET specific surface area of the support may be a specific surface area sufficient to support the catalyst component in a highly dispersed state. The BET specific surface area of the support is substantially equivalent to the BET specific surface area of the catalyst. The BET specific surface area of the support is preferably 1000 to 3000 m ² / g, more preferably 1000 to 1800 m ² / g. If the specific surface area is as described above, sufficient pores (mesopores) can be secured, so that a large amount of catalyst metal can be stored (supported) in the mesopores. In addition, the electrolyte and the catalyst metal in the catalyst layer are physically separated (contact between the catalyst metal and the electrolyte can be more effectively suppressed / prevented). Therefore, the activity of the catalytic metal can be utilized more effectively. In addition, the presence of many pores (mesopores) can more effectively promote the catalytic reaction by exerting the effects and advantages of the present invention more remarkably. In addition, the balance between the dispersibility of the catalyst component on the catalyst carrier and the effective utilization rate of the catalyst component can be appropriately controlled.

The average particle size of the carrier is preferably 20 to 2000 nm. Within such a range, the mechanical strength can be maintained and the thickness of the catalyst layer can be controlled within an appropriate range even when the support is provided with the above-described pore structure. The value of the “average particle diameter of the carrier” is observed in several to several tens of fields using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM) unless otherwise specified. The value calculated as the average value of the particle diameter of the particles shall be adopted. The “particle diameter” means the maximum distance among the distances between any two points on the particle outline.

The method for producing the carrier having the pore distribution as described above is not particularly limited, but specifically, the methods described in JP 2010-208887 A, International Publication No. 2009/75264, etc. are preferably used. Is done.

In the present invention, it is not always necessary to use the granular porous carrier as described above as long as the catalyst has the pore distribution as described above.

That is, examples of the carrier include a non-porous conductive carrier, a non-woven fabric made of carbon fibers constituting a gas diffusion layer, carbon paper, and carbon cloth. At this time, the catalyst can be supported on these non-porous conductive carriers, or directly attached to a non-woven fabric made of carbon fibers, carbon paper, carbon cloth, etc. constituting the gas diffusion layer of the membrane electrode assembly. It is.

The catalytic metal that can be used in the present invention has a function of catalyzing an electrochemical reaction. The catalyst metal used in the anode catalyst layer is not particularly limited as long as it has a catalytic action in the oxidation reaction of hydrogen, and a known catalyst can be used in the same manner. The catalyst metal used in the cathode catalyst layer is not particularly limited as long as it has a catalytic action for the oxygen reduction reaction, and a known catalyst can be used in the same manner. Specifically, metals such as platinum, ruthenium, iridium, rhodium, palladium, osmium, tungsten, lead, iron, copper, silver, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, and alloys thereof Can be selected.

Among these, those containing at least platinum are preferably used in order to improve catalytic activity, poisoning resistance to carbon monoxide, heat resistance, and the like. That is, the catalyst metal is preferably platinum or contains a metal component other than platinum and platinum, and more preferably platinum or a platinum-containing alloy. Such a catalytic metal can exhibit high activity. Although the composition of the alloy depends on the type of metal to be alloyed, the content of platinum is preferably 30 to 90 atomic%, and the content of the metal to be alloyed with platinum is preferably 10 to 70 atomic%. In general, an alloy is a generic term for a metal element having one or more metal elements or non-metal elements added and having metallic properties. The alloy structure consists of a eutectic alloy, which is a mixture of the component elements as separate crystals, a component element completely melted into a solid solution, and a component element composed of an intermetallic compound or a compound of a metal and a nonmetal. There is what is formed, and any may be used in the present application. At this time, the catalyst metal used for the anode catalyst layer and the catalyst metal used for the cathode catalyst layer can be appropriately selected from the above. In the present specification, unless otherwise specified, the description of the catalyst metal for the anode catalyst layer and the cathode catalyst layer has the same definition for both. However, the catalyst metals of the anode catalyst layer and the cathode catalyst layer do not have to be the same, and can be appropriately selected so as to exhibit the desired action as described above.

The shape and size of the catalyst metal (catalyst component) are not particularly limited, and the same shape and size as known catalyst components can be adopted. As the shape, for example, a granular shape, a scale shape, a layered shape, and the like can be used, but a granular shape is preferable.

[Step of impregnating the pores inside the support with the components of the catalyst metal]
In this step, the pores inside the carrier are impregnated with the constituent components of the catalyst metal as described above. Specific examples of the impregnating agent containing the catalyst metal component include, for example, platinum nanocolloid solution, dinitrodiammine platinum nitrate solution, platinum chloride (IV) acid hydrate, platinum chloride (IV) acid ammonium, tetraammine. Examples thereof include a solution containing a precursor of a catalytic metal such as platinum (II) dichloride hydrate and hexahydroxoplatinum (IV) nitric acid solution. These impregnating agents may be used alone or in admixture of two or more.

The average particle radius of the constituent components of the catalyst metal contained in the impregnating agent (the sum of the average particle radii when two or more constituent components of the catalyst metal are included) is the pore distribution mode of the pores of the support. It is preferably smaller than the radius. With such a configuration, the constituent components of the catalyst metal are easily impregnated by the pores inside the support.

The temperature at which the carrier is immersed in the impregnating agent (immersion temperature) is not particularly limited, but is preferably room temperature (25 ° C.) to 100 ° C. The time for immersion (immersion time) is not particularly limited, but is preferably 1 to 10 hours, and more preferably 2 to 8 hours.

During the immersion, it is preferable to perform stirring using ultrasonic waves or the like. Further, in order to degas the liquid, it is preferable to perform a depressurization process in order to remove bubbles remaining in the pores inside the carrier and to make it easier for the liquid to enter the inside of the carrier.

After completion of the impregnation step, the support is filtered and dried.

[Step of reducing precursor]
In the impregnation step, when a catalyst metal precursor is used as the impregnation agent, it is preferable to perform a step of reducing the catalyst metal precursor before filtration and drying. The reducing agent used in the reduction is not particularly limited, and examples thereof include hydrogen, sodium borohydride, sodium thiosulfate, citric acid, sodium citrate, L-ascorbic acid, acetic acid and other organic acids or salts thereof, Examples thereof include sodium borohydride, aldehydes such as formic acid, acetaldehyde and formaldehyde, alcohols such as methanol, ethanol and propanol, ethylene and carbon monoxide. These reducing agents can be used alone or in admixture of two or more.

The temperature during the reduction is not particularly limited, but is preferably room temperature (25 ° C.) to 100 ° C. The time for reduction is not particularly limited, but is preferably 1 to 10 hours, and more preferably 2 to 8 hours.

[Step of heat treatment]
After the impregnation step or the precursor reduction step, the support is heat-treated. Thereby, the distance between the catalyst metal and the inner wall surface of the pore of the carrier is reduced, and the space where water can exist is reduced, that is, the amount of water adsorbed on the surface of the catalyst metal is reduced. Further, water is subjected to the interaction of the inner wall surface of the hole, and the water is easily held on the inner wall surface of the hole. Therefore, the formation reaction of the metal oxide is delayed, and the metal oxide is hardly formed. As a result, the deactivation of the catalytic metal surface is suppressed, and high catalytic activity can be exhibited, that is, the catalytic reaction can be promoted.

The atmosphere during the heat treatment is not particularly limited, and examples thereof include a hydrogen atmosphere, a nitrogen atmosphere mixed with a small amount of hydrogen, and an argon atmosphere.

Examples of the apparatus used for the heat treatment include a heating apparatus such as a firing furnace.

The temperature of the heat treatment is not particularly limited, but is preferably 300 to 1200 ° C, more preferably 500 to 1150 ° C, and still more preferably 700 to 1100 ° C. Further, the heat treatment time is not particularly limited, but is preferably 0.02 to 3 hours, and more preferably 0.1 to 2 hours.

The mode radius of the pore distribution of the catalyst obtained as described above is preferably not more than the average particle radius of the catalyst metal. At this time, the average particle radius of the catalyst metal (catalyst metal particles) is preferably 1 nm to 3.5 nm, more preferably 1.5 nm to 2.5 nm. If the mode radius is equal to or less than the average particle radius of the catalyst metal, the distance between the catalyst metal and the inner wall surface of the pores of the support is reduced, and the space where water can exist is further reduced, that is, The amount of less. Further, water is subjected to the interaction of the inner wall surface of the hole, and the water is easily held on the inner wall surface of the hole. Therefore, the formation reaction of the metal oxide becomes slower and the metal oxide is more difficult to be formed. As a result, the deactivation of the catalytic metal surface is further suppressed, and a high catalytic activity can be exhibited, that is, the catalytic reaction can be further promoted. Further, the catalyst metal is supported relatively firmly in the pores (mesopores), and the contact with the electrolyte in the catalyst layer is more effectively suppressed / prevented. In addition, elution due to potential change can be prevented, and deterioration in performance over time can be suppressed. For this reason, the catalytic activity can be further improved, that is, the catalytic reaction can be promoted more efficiently. The “average particle radius of the catalyst metal particles” in the present invention is the crystallite radius determined from the half-value width of the diffraction peak of the catalyst metal component in X-ray diffraction, or the catalyst metal particles examined by a transmission electron microscope (TEM). It can be measured as the average value of the particle radii. In the present specification, the “average particle radius of the catalyst metal” is an average value of the particle radii of the catalyst component examined from a transmission electron microscope image of a statistically significant number (for example, at least 203 samples).

The relationship in which the mode radius of the pore distribution of the catalyst is equal to or less than the average particle radius of the catalyst metal as described above can be realized by controlling time, temperature, atmosphere, and the like in the heat treatment step. Under such manufacturing conditions, the catalyst metal grows and the distance between the catalyst metal and the inner wall surface of the pore of the carrier is reduced, so that the space where water can exist is further reduced, that is, the amount of water adsorbed on the catalyst metal surface. Is more reduced. Furthermore, since the average particle radius of the catalyst metal is larger than the mode radius of the pore distribution of the catalyst pores, in addition to the above effects, the catalyst metal is less likely to be detached even when mechanical stress is applied. Since waste is reduced, the catalyst metal is easily used effectively.

Further, the control such that the average particle radius of the catalyst metal is 1.5 nm or more and 2.5 nm or less can be performed by controlling the temperature, time, atmosphere and the like in the heat treatment step. In particular, it can be controlled by increasing the temperature of the heat treatment or by lengthening the heat treatment time.

In the present embodiment, the catalyst content (mg / cm ² ) per unit catalyst application area is not particularly limited as long as sufficient catalyst dispersion and power generation performance can be obtained. For example, 0.01 to 1 mg / Cm ² . However, when the catalyst contains platinum or a platinum-containing alloy, the platinum content per unit catalyst coating area is preferably 0.5 mg / cm ² or less. The use of expensive noble metal catalysts typified by platinum (Pt) and platinum alloys has become a high cost factor for fuel cells. Therefore, it is preferable to reduce the amount of expensive platinum used (platinum content) to the above range and reduce the cost. The lower limit is not particularly limited as long as power generation performance is obtained, and is, for example, 0.01 mg / cm ² or more. More preferably, the platinum content is 0.02 to 0.4 mg / cm ² . In this embodiment, since the activity per catalyst weight can be improved by controlling the pore structure of the carrier, the amount of expensive catalyst used can be reduced.

In this specification, inductively coupled plasma emission spectroscopy (ICP) is used for measurement (confirmation) of “catalyst (platinum) content per unit catalyst application area (mg / cm ² )”. A person skilled in the art can easily carry out a method of making the desired “catalyst (platinum) content per unit catalyst coating area (mg / cm ² )”, and control the slurry composition (catalyst concentration) and coating amount. You can adjust the amount.

The amount of the catalyst supported on the carrier (sometimes referred to as the loading ratio) is preferably 10 to 80% by weight, more preferably 20 to 70% by weight, based on the total amount of the catalyst carrier (that is, the carrier and the catalyst). % Is good. If the loading is within the above range, it is preferable because a sufficient degree of dispersion of the catalyst components on the carrier, improvement in power generation performance, economic advantages, and catalytic activity per unit weight can be achieved.

The pore volume of the pores (of the catalyst after supporting the catalyst metal) is preferably 0.3 cc / g or more, more preferably 0.4 to 3 cc / g, and even more preferably 0.4 ~ 1.5cc / g carrier. If the void volume is in the above range, a large amount of catalyst metal can be stored (supported) in the mesopores, and the catalyst and the catalyst metal in the catalyst layer are physically separated (contact between the catalyst metal and the electrolyte is prevented). It can be suppressed and prevented more effectively). Therefore, the activity of the catalytic metal can be utilized more effectively. In addition, the presence of many mesopores can more effectively promote the catalytic reaction by exerting the effects and advantages of the present invention more remarkably.

The mode radius (most frequent diameter) of the pore distribution (of the catalyst after supporting the catalyst metal) is preferably 1 nm or more and less than 5 nm, more preferably 1 nm or more and 4 nm or less, and further preferably 1 nm or more. It is 3 nm or less, and particularly preferably 1 nm or more and 2 nm or less. If the mode radius of the pore distribution is as described above, a sufficient amount of catalyst metal can be stored (supported) in the mesopores, and the electrolyte and catalyst metal in the catalyst layer are physically separated (the catalyst metal and electrolyte are separated from each other). Can more effectively suppress and prevent contact). Therefore, the activity of the catalytic metal can be utilized more effectively. Further, the presence of a large volume of pores (mesopores) can more effectively promote the catalytic reaction by exerting the effects and effects of the present invention more remarkably. Furthermore, since the average particle radius of the catalyst metal is larger than the mode radius of the pore distribution of the catalyst pores, in addition to the above effects, the catalyst metal is less likely to be detached even when mechanical stress is applied. Since waste is reduced, the catalyst metal is easily used effectively. In the present specification, the mode radius of the pore distribution of mesopores is also simply referred to as “mode diameter of mesopores”.

The BET specific surface area (of the catalyst after supporting the catalyst metal) [the BET specific surface area of the catalyst per 1 g of support (m ² / g support)] is not particularly limited, but is preferably 1000 m ² / g or more, more The carrier is preferably 1000 to 3000 m ² / g, and more preferably 1000 to 1800 m ² / g. With the specific surface area as described above, a large amount of catalyst metal can be stored (supported) in the mesopores. In addition, the electrolyte and the catalyst metal in the catalyst layer are physically separated (contact between the catalyst metal and the electrolyte can be more effectively suppressed / prevented). Therefore, the activity of the catalytic metal can be utilized more effectively. In addition, the presence of many pores (mesopores) can more effectively promote the catalytic reaction by exerting the effects and advantages of the present invention more remarkably.

In the present specification, the “BET specific surface area (m ² / g support)” of the catalyst is measured by a nitrogen adsorption method. Specifically, about 0.04 to 0.07 g of catalyst powder is precisely weighed and sealed in a sample tube. This sample tube is preliminarily dried at 90 ° C. for several hours in a vacuum dryer to obtain a measurement sample. For weighing, an electronic balance (AW220) manufactured by Shimadzu Corporation is used. In the case of a coated sheet, a net weight of about 0.03 to 0.04 g of the coated layer obtained by subtracting the weight of Teflon (registered trademark) (base material) of the same area from the total weight is used as the sample weight. . Next, the BET specific surface area is measured under the following measurement conditions. On the adsorption side of the adsorption / desorption isotherm, a BET specific surface area is calculated from the slope and intercept by creating a BET plot from a relative pressure (P / P ₀ ) range of about 0.00 to 0.45.

“Void radius (nm)” means the radius of a pore measured by the nitrogen adsorption method (DH method). Further, “mode radius (nm) of pore distribution” means a pore radius at a point where a peak value (maximum frequency) is obtained in a differential pore distribution curve obtained by a nitrogen adsorption method (DH method). Here, the upper limit of the radius of the hole is not particularly limited, but is 100 nm or less.

“The pore volume of pores” means the total volume of pores present in the catalyst, and is expressed as the volume per gram of carrier (cc / g carrier). The “pore volume of vacancies (cc / g carrier)” is calculated as the area (integrated value) below the differential pore distribution curve obtained by the nitrogen adsorption method (DH method).

The “differential pore distribution” is a distribution curve in which the pore diameter is plotted on the horizontal axis and the pore volume corresponding to the pore diameter in the catalyst is plotted on the vertical axis. That is, when the pore volume of the catalyst obtained by the nitrogen adsorption method (DH method) is V and the pore diameter is D, the differential pore volume dV is divided by the logarithmic difference d (log D) of the pore diameter. A value (dV / d (logD)) is obtained. A differential pore distribution curve is obtained by plotting this dV / d (logD) against the average pore diameter of each section. The differential hole volume dV refers to an increase in the hole volume between measurement points.

The method of measuring the mesopore radius and pore volume by the nitrogen adsorption method (DH method) is not particularly limited. For example, “Science of adsorption” (2nd edition, written by Seiichi Kondo, Tatsuo Ishikawa, Ikuo Abe, Maruzen Stock Company), “Fuel cell analysis method” (Yoshio Takasu, Yuu Yoshitake, Tatsumi Ishihara, edited by Chemistry), D. Dollion, G. R. Heal: J. Appl. Chem. Methods described in known literature can be employed. In this specification, the mesopore radius and pore volume by nitrogen adsorption method (DH method) are described in D. Dollion, G. R. Heal: J. Appl. Chem., 14, 109 (1964). The value measured by the method.

[Catalyst layer]
As described above, the catalyst obtained by the production method of the present invention can exhibit high catalytic activity, that is, can promote catalytic reaction. Therefore, the catalyst obtained by the production method of the present invention can be suitably used for an electrode catalyst layer for a fuel cell.

FIG. 3 is a schematic diagram showing the relationship between the catalyst and the electrolyte in the catalyst layer according to one embodiment of the present invention. As shown in FIG. 3, in the catalyst layer according to the present invention, the catalyst is covered with the electrolyte 26, but the electrolyte 26 does not enter the mesopores 24 of the catalyst (support 23). For this reason, the catalyst metal 22 on the surface of the carrier 23 is in contact with the electrolyte 26, but the catalyst metal 22 supported in the mesopores 24 is not in contact with the electrolyte 26. The catalytic metal in the mesopores forms a three-phase interface between oxygen gas and water in a non-contact state with the electrolyte, thereby ensuring a reaction active area of the catalytic metal.

The catalyst obtained by the production method of the present invention may be present in either the cathode catalyst layer or the anode catalyst layer, but is preferably used in the cathode catalyst layer. As described above, the catalyst obtained by the production method of the present invention can effectively utilize the catalyst by forming a three-phase interface with water without contact with the electrolyte. It is because it forms.

The electrolyte is not particularly limited, but is preferably an ion conductive polymer electrolyte. Since the polymer electrolyte plays a role of transmitting protons generated around the catalyst active material on the fuel electrode side, it is also called a proton conductive polymer.

The polymer electrolyte is not particularly limited, and conventionally known knowledge can be appropriately referred to. Polymer electrolytes are roughly classified into fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes depending on the type of ion exchange resin that is a constituent material.

Examples of ion exchange resins constituting the fluorine-based polymer electrolyte include Nafion (registered trademark, manufactured by DuPont), Aciplex (registered trademark, manufactured by Asahi Kasei Co., Ltd.), Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.), and the like. Perfluorocarbon sulfonic acid polymer, perfluorocarbon phosphonic acid polymer, trifluorostyrene sulfonic acid polymer, ethylene tetrafluoroethylene-g-styrene sulfonic acid polymer, ethylene-tetrafluoroethylene copolymer, polyvinylidene fluoride-per Examples thereof include fluorocarbon sulfonic acid polymers. From the viewpoint of excellent heat resistance, chemical stability, durability, and mechanical strength, these fluorine-based polymer electrolytes are preferably used, and particularly preferably fluorine-based polymer electrolytes composed of perfluorocarbon sulfonic acid polymers. Is used.

Specific examples of the hydrocarbon electrolyte include sulfonated polyethersulfone (S-PES), sulfonated polyaryletherketone, sulfonated polybenzimidazole alkyl, phosphonated polybenzimidazole alkyl, sulfonated polystyrene, sulfonated poly Examples include ether ether ketone (S-PEEK) and sulfonated polyphenylene (S-PPP). These hydrocarbon polymer electrolytes are preferably used from the viewpoint of production such that the raw material is inexpensive, the production process is simple, and the selectivity of the material is high. In addition, as for the ion exchange resin mentioned above, only 1 type may be used independently and 2 or more types may be used together. Moreover, it is not restricted only to the material mentioned above, Other materials may be used.

Proton conductivity is important in polymer electrolytes responsible for proton transmission. Here, when the EW of the polymer electrolyte is too large, the ionic conductivity in the entire catalyst layer is lowered. Therefore, it is preferable that the catalyst layer of this embodiment contains a polymer electrolyte having a small EW. Specifically, the catalyst layer of this embodiment preferably has an EW of 1500 g / eq. The following polymer electrolyte is contained, More preferably, it is 1200 g / eq. The following polymer electrolyte is included, and particularly preferably 1000 g / eq. The following polymer electrolytes are included.

On the other hand, if the EW is too small, the hydrophilicity is too high and it becomes difficult to smoothly move water. From such a viewpoint, the EW of the polymer electrolyte is preferably 600 or more. Note that EW (Equivalent Weight) represents an equivalent weight of an exchange group having proton conductivity. The equivalent weight is the dry weight of the ion exchange membrane per equivalent of ion exchange group, and is expressed in units of “g / eq”.

Further, the catalyst layer includes two or more types of polymer electrolytes having different EWs in the power generation surface. At this time, the polymer electrolyte having the lowest EW among the polymer electrolytes has a relative humidity of 90% or less of the gas in the flow path. It is preferable to use in the region. By adopting such a material arrangement, the resistance value becomes small regardless of the current density region, and the battery performance can be improved. The EW of the polymer electrolyte used in the region where the relative humidity of the gas in the flow channel is 90% or less, that is, the polymer electrolyte having the lowest EW is 900 g / eq. The following is desirable. Thereby, the above-mentioned effect becomes more reliable and remarkable.

Furthermore, it is desirable to use a polymer electrolyte with the lowest EW in a region higher than the average temperature of the cooling water inlet and outlet. As a result, the resistance value is reduced regardless of the current density region, and the battery performance can be further improved.

Furthermore, from the viewpoint of reducing the resistance value of the fuel cell system, the polymer electrolyte having the lowest EW is within 3/5 from the gas supply port of at least one of the fuel gas and the oxidant gas with respect to the channel length. It is desirable to use it in the range area.

The catalyst layer of this embodiment may include a liquid proton conductive material that can connect the catalyst and the polymer electrolyte (solid proton conductive material) in a proton conductive state between the catalyst and the polymer electrolyte. By introducing a liquid proton conductive material, a proton transport path through the liquid proton conductive material is secured between the catalyst and the polymer electrolyte, and protons necessary for power generation are efficiently transported to the catalyst surface. Is possible. Thereby, since the utilization efficiency of a catalyst improves, it becomes possible to reduce the usage-amount of a catalyst, maintaining electric power generation performance. The liquid proton conductive material only needs to be interposed between the catalyst and the polymer electrolyte, and the pores (secondary pores) between the porous carriers in the catalyst layer and the pores (micropores) in the porous carrier. Or mesopores: primary vacancies).

The liquid proton conductive material is not particularly limited as long as it has ion conductivity and can exhibit a function of forming a proton transport path between the catalyst and the polymer electrolyte. Specific examples include water, protic ionic liquid, aqueous perchloric acid solution, aqueous nitric acid solution, aqueous formic acid solution, and aqueous acetic acid solution.

When water is used as the liquid proton conductive material, water as the liquid proton conductive material is introduced into the catalyst layer by moistening the catalyst layer with a small amount of liquid water or humidified gas before starting power generation. Can do. Moreover, the water produced by the electrochemical reaction during the operation of the fuel cell can be used as the liquid proton conductive material. Therefore, it is not always necessary to hold the liquid proton conductive material when the fuel cell is in operation. For example, the surface distance between the catalyst and the electrolyte is preferably 0.28 nm or more, which is the diameter of oxygen ions constituting water molecules. By maintaining such a distance, water (liquid proton conductive material) is interposed between the catalyst and the polymer electrolyte (liquid conductive material holding part) while maintaining a non-contact state between the catalyst and the polymer electrolyte. Therefore, a proton transport route by water between them can be secured.

When a material other than water, such as an ionic liquid, is used as the liquid proton conducting material, it is desirable to disperse the ionic liquid, the polymer electrolyte, and the catalyst in the solution when preparing the catalyst ink. An ionic liquid may be added when applying to the layer substrate.

In the catalyst obtained by the production method of the present invention, the total area in contact with the polymer electrolyte of the catalyst is smaller than the total area where the catalyst is exposed to the liquid conductive material holding part.

These areas are compared, for example, with the capacity of the electric double layer formed at the catalyst-polymer electrolyte interface and the catalyst-liquid proton conducting material interface in a state where the liquid conducting material holding portion is filled with the liquid proton conducting material. This can be done by seeking a relationship. In other words, since the electric double layer capacity is proportional to the area of the electrochemically effective interface, the electric double layer capacity formed at the catalyst-electrolyte interface is the electric double layer capacity formed at the catalyst-liquid proton conducting material interface. If it is smaller, the contact area of the catalyst with the electrolyte is smaller than the area exposed to the liquid conductive material holding part.

Here, the measurement method of the electric double layer capacity formed at the catalyst-electrolyte interface and the catalyst-liquid proton conducting material interface, in other words, the contact area between the catalyst and electrolyte and between the catalyst and liquid proton conducting material ( A method for determining the relationship between the contact area of the catalyst with the electrolyte and the exposed area of the liquid conductive material holding portion will be described.

That is, in the catalyst layer of this embodiment,
(1) Catalyst-Polymer electrolyte (CS)
(2) Catalyst-Liquid proton conductive material (CL)
(3) Porous carrier-polymer electrolyte (Cr-S)
(4) Porous carrier-liquid proton conducting material (Cr-L)
These four types of interfaces can contribute as electric double layer capacitance (C _dl ).

Since the electric double layer capacity is directly proportional to the area of the electrochemically effective interface as described above, C _{dl CS} (electric double layer capacity at the catalyst-polymer electrolyte interface) and C _{dl CL} ( The electric double layer capacity at the catalyst-liquid proton conductive material interface may be obtained. The contribution of the four types of interfaces to the electric double layer capacity (C _dl ) can be separated as follows.

First, for example, the electric double layer capacity is measured under a high humidification condition such as 100% RH and a low humidification condition such as 10% RH or less. In addition, examples of the measurement method of the electric double layer capacitance include cyclic voltammetry and electrochemical impedance spectroscopy. From these comparisons, the contribution of the liquid proton conducting material (in this case “water”), that is, the above (2) and (4) can be separated.

Further, when the catalyst is deactivated, for example, when Pt is used as the catalyst, the catalyst is deactivated by supplying CO gas to the electrode to be measured and adsorbing CO on the Pt surface. The contribution to the multilayer capacity can be separated. In such a state, as described above, the electric double layer capacity under high and low humidification conditions is measured by the same method, and the contribution of the catalyst, that is, the above (1) and (2) is separated from these comparisons. be able to.

As described above, all the contributions (1) to (4) can be separated, and the electric double layer capacity formed at the interfaces of the catalyst, the polymer electrolyte, and the liquid proton conducting material can be obtained.

That is, the measured value (A) in the highly humidified state is the electric double layer capacity formed at all the interfaces (1) to (4), and the measured value (B) in the lowly humidified state is the above (1) and (3). The electric double layer capacity formed at the interface. Further, the measured value (C) in the catalyst deactivation / highly humidified state is the electric double layer capacity formed at the interface of the above (3) and (4), and the measured value (D) in the catalyst deactivated / lowly humidified state is the above It becomes an electric double layer capacity formed at the interface of (3).

Therefore, the difference between A and C is the electric double layer capacity formed at the interface of (1) and (2), and the difference between B and D is the electric double layer capacity formed at the interface of (1). Then, by calculating the difference between these values, (AC)-(BD), the electric double layer capacity formed at the interface of (2) can be obtained. In addition to the above, the contact area of the catalyst with the polymer electrolyte and the exposed area of the conductive material holding part can be obtained by, for example, TEM (transmission electron microscope) tomography.

In the catalyst layer according to the present invention, the coverage of the electrolyte with respect to the catalyst metal is preferably 0.45 or less, preferably 0.35, and more preferably 0.25 (lower limit: 0). . When the electrolyte coverage is in the above range, the catalytic activity is further improved.

The coverage of the electrolyte can be calculated from the electric double layer capacity, and specifically can be calculated by the method described in the examples.

For the catalyst layer, a water repellent such as polytetrafluoroethylene, polyhexafluoropropylene, tetrafluoroethylene-hexafluoropropylene copolymer, a dispersing agent such as a surfactant, glycerin, ethylene glycol (EG), as necessary. ), A thickener such as polyvinyl alcohol (PVA) and propylene glycol (PG), and an additive such as a pore-forming agent may be contained.

The thickness (dry film thickness) of the catalyst layer is preferably 0.05 to 30 μm, more preferably 1 to 20 μm, still more preferably 2 to 15 μm. In addition, the said thickness is applied to both a cathode catalyst layer and an anode catalyst layer. However, the thickness of the cathode catalyst layer and the anode catalyst layer may be the same or different.

(Method for producing catalyst layer)
Hereinafter, although preferable embodiment for manufacturing a catalyst layer is described, the technical scope of this invention is not limited only to the following form. Since the manufacturing method of the catalyst and various conditions such as the material of each component of the catalyst layer are as described above, the description thereof is omitted here.

A catalyst ink containing the catalyst powder, polymer electrolyte, and solvent obtained by the above-described catalyst production method of the present invention is prepared. The solvent is not particularly limited, and ordinary solvents used for forming the catalyst layer can be used in the same manner. Specifically, water such as tap water, pure water, ion exchange water, distilled water, cyclohexanol, methanol, ethanol, n-propanol (n-propyl alcohol), isopropanol, n-butanol, sec-butanol, isobutanol And lower alcohols having 1 to 4 carbon atoms such as tert-butanol, propylene glycol, benzene, toluene, xylene and the like. Besides these, butyl acetate alcohol, dimethyl ether, ethylene glycol, and the like may be used as a solvent. These solvents may be used alone or in the form of a mixture of two or more.

The amount of the solvent constituting the catalyst ink is not particularly limited as long as it is an amount capable of completely dissolving the electrolyte. Specifically, the solid content concentration of the catalyst powder and the polymer electrolyte is preferably 1 to 50% by weight, more preferably about 5 to 30% by weight in the electrode catalyst ink.

In addition, when additives such as a water repellent, a dispersant, a thickener, and a pore-forming agent are used, these additives may be added to the catalyst ink. At this time, the amount of the additive added is not particularly limited as long as it is an amount that does not interfere with the effects of the present invention. For example, the amount of the additive added is preferably 5 to 20% by weight with respect to the total weight of the electrode catalyst ink.

Next, a catalyst ink is applied to the surface of the substrate. The application method to the substrate is not particularly limited, and a known method can be used. Specifically, it can be performed using a known method such as a spray (spray coating) method, a gulliver printing method, a die coater method, a screen printing method, or a doctor blade method.

At this time, a solid polymer electrolyte membrane (electrolyte layer) or a gas diffusion substrate (gas diffusion layer) can be used as the substrate on which the catalyst ink is applied. In such a case, after forming the catalyst layer on the surface of the solid polymer electrolyte membrane (electrolyte layer) or the gas diffusion base material (gas diffusion layer), the obtained laminate can be used for the production of the membrane electrode assembly as it is. Can be used. Alternatively, a peelable substrate such as a polytetrafluoroethylene (PTFE) [Teflon (registered trademark)] sheet is used as the substrate, and after the catalyst layer is formed on the substrate, the catalyst layer portion is peeled from the substrate. Thus, a catalyst layer may be obtained.

Finally, the coating layer (film) of the catalyst ink is dried at room temperature to 150 ° C. for 1 to 60 minutes in an air atmosphere or an inert gas atmosphere. Thereby, a catalyst layer is formed.

(Membrane electrode assembly)
According to a further embodiment of the present invention, the solid polymer electrolyte membrane 2, a cathode catalyst layer disposed on one side of the electrolyte membrane, an anode catalyst layer disposed on the other side of the electrolyte membrane, There is provided a membrane electrode assembly for a fuel cell having an electrolyte membrane 2 and a pair of gas diffusion layers (4a, 4c) sandwiching the anode catalyst layer 3a and the cathode catalyst layer 3c. In this membrane electrode assembly, at least one of the cathode catalyst layer and the anode catalyst layer is the catalyst layer of the embodiment described above.

However, in consideration of the necessity for improvement of proton conductivity and improvement of transport characteristics (gas diffusibility) of the reaction gas (especially O ₂ ), at least the cathode catalyst layer may be the catalyst layer of the embodiment described above. preferable. However, the catalyst layer according to the above embodiment may be used as an anode catalyst layer, or may be used as both a cathode catalyst layer and an anode catalyst layer, and is not particularly limited.

According to a further embodiment of the present invention, there is provided a fuel cell having the membrane electrode assembly of the above form. That is, one embodiment of the present invention is a fuel cell having a pair of anode separator and cathode separator that sandwich the membrane electrode assembly of the above-described embodiment.

Hereinafter, the components of the PEFC 1 using the catalyst layer of the above embodiment will be described with reference to FIG. However, the present invention is characterized by the catalyst layer. Therefore, the specific form of the members other than the catalyst layer constituting the fuel cell can be appropriately modified with reference to conventionally known knowledge.

(Electrolyte membrane)
The electrolyte membrane is composed of a solid polymer electrolyte membrane 2 as shown in FIG. The solid polymer electrolyte membrane 2 has a function of selectively permeating protons generated in the anode catalyst layer 3a during operation of the PEFC 1 to the cathode catalyst layer 3c along the film thickness direction. The solid polymer electrolyte membrane 2 also has a function as a partition wall for preventing the fuel gas supplied to the anode side and the oxidant gas supplied to the cathode side from being mixed.

The electrolyte material constituting the solid polymer electrolyte membrane 2 is not particularly limited, and conventionally known knowledge can be appropriately referred to. For example, the fluorine-based polymer electrolyte or hydrocarbon-based polymer electrolyte described above as the polymer electrolyte can be used. At this time, it is not always necessary to use the same polymer electrolyte used for the catalyst layer.

The thickness of the electrolyte layer may be appropriately determined in consideration of the characteristics of the obtained fuel cell, and is not particularly limited. The thickness of the electrolyte layer is usually about 5 to 300 μm. When the thickness of the electrolyte layer is within such a range, the balance of strength during film formation, durability during use, and output characteristics during use can be appropriately controlled.

(Gas diffusion layer)
The gas diffusion layers (anode gas diffusion layer 4a, cathode gas diffusion layer 4c) are catalyst layers (3a, 3c) of gas (fuel gas or oxidant gas) supplied via the gas flow paths (6a, 6c) of the separator. ) And a function as an electron conduction path.

The material which comprises the base material of a gas diffusion layer (4a, 4c) is not specifically limited, A conventionally well-known knowledge can be referred suitably. For example, a sheet-like material having conductivity and porosity such as a carbon woven fabric, a paper-like paper body, a felt, and a non-woven fabric can be used. The thickness of the substrate may be appropriately determined in consideration of the characteristics of the obtained gas diffusion layer, but may be about 30 to 500 μm. If the thickness of the substrate is within such a range, the balance between mechanical strength and diffusibility such as gas and water can be appropriately controlled.

The gas diffusion layer preferably contains a water repellent for the purpose of further improving water repellency and preventing flooding. The water repellent is not particularly limited, but fluorine-based high repellents such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyhexafluoropropylene, and tetrafluoroethylene-hexafluoropropylene copolymer (FEP). Examples thereof include molecular materials, polypropylene, and polyethylene.

In order to further improve the water repellency, the gas diffusion layer has a carbon particle layer (microporous layer; MPL, not shown) made of an aggregate of carbon particles containing a water repellent agent on the catalyst layer side of the substrate. You may have.

The carbon particles contained in the carbon particle layer are not particularly limited, and conventionally known materials such as carbon black, graphite, and expanded graphite can be appropriately employed. Among them, carbon black such as oil furnace black, channel black, lamp black, thermal black, acetylene black and the like can be preferably used because of excellent electron conductivity and a large specific surface area. The average particle size of the carbon particles is preferably about 10 to 100 nm. Thereby, while being able to obtain the high drainage property by capillary force, it becomes possible to improve contact property with a catalyst layer.

Examples of the water repellent used for the carbon particle layer include the same water repellents as described above. Among these, fluorine-based polymer materials can be preferably used because of excellent water repellency, corrosion resistance during electrode reaction, and the like.

The mixing ratio of the carbon particles to the water repellent in the carbon particle layer is about 90:10 to 40:60 (carbon particles: water repellent) by weight in consideration of the balance between water repellency and electronic conductivity. It is good. In addition, there is no restriction | limiting in particular also about the thickness of a carbon particle layer, What is necessary is just to determine suitably in consideration of the water repellency of the gas diffusion layer obtained.

(Method for producing membrane electrode assembly)
A method for producing the membrane electrode assembly is not particularly limited, and a conventionally known method can be used. For example, a catalyst layer is transferred or applied to a solid polymer electrolyte membrane by hot pressing, and this is dried, and a gas diffusion layer is bonded to the gas diffusion layer, or a microporous layer side (a microporous layer is attached to the gas diffusion layer). When not included, two gas diffusion electrodes (GDE) are prepared by applying a catalyst layer on one side of the base material layer in advance and drying, and hot pressing the gas diffusion electrodes on both sides of the solid polymer electrolyte membrane. The application and joining conditions such as hot press are appropriately determined depending on the type of polymer electrolyte in the solid polymer electrolyte membrane or catalyst layer (perfluorosulfonic acid type or hydrocarbon type). Adjust it.

(Separator)
The separator has a function of electrically connecting each cell in series when a plurality of single cells of a fuel cell such as a polymer electrolyte fuel cell are connected in series to form a fuel cell stack. The separator also functions as a partition that separates the fuel gas, the oxidant gas, and the coolant from each other. In order to secure these flow paths, as described above, each of the separators is preferably provided with a gas flow path and a cooling flow path. As a material constituting the separator, conventionally known materials such as dense carbon graphite, carbon such as a carbon plate, and metal such as stainless steel can be appropriately employed without limitation. The thickness and size of the separator and the shape and size of each flow path provided are not particularly limited, and can be appropriately determined in consideration of the desired output characteristics of the obtained fuel cell.

The manufacturing method of the fuel cell is not particularly limited, and conventionally known knowledge can be appropriately referred to in the field of the fuel cell.

Furthermore, a fuel cell stack having a structure in which a plurality of membrane electrode assemblies are stacked and connected in series via a separator may be formed so that the fuel cell can exhibit a desired voltage. The shape of the fuel cell is not particularly limited, and may be determined as appropriate so that desired battery characteristics such as voltage can be obtained.

The above-mentioned PEFC and membrane electrode assembly use a catalyst layer having excellent power generation performance and durability. Therefore, the PEFC and the membrane electrode assembly are excellent in power generation performance and durability.

The PEFC of this embodiment and the fuel cell stack using the same can be mounted on a vehicle as a driving power source, for example.

The effect of the present invention will be described using the following examples and comparative examples. However, the technical scope of the present invention is not limited only to the following examples.

Synthesis example 1
Support A was prepared having a pore volume of 1.56 cc / g; a pore mode radius of 1.65 nm; and a BET specific surface area of 1773 m ² / g. Specifically, the carrier A was produced by the method described in International Publication No. 2009/075264.

Synthesis example 2
As the carrier B, Ketjen Black EC300J (manufactured by Ketjen Black International Co., Ltd.) having a pore volume of 0.69 cc / g and a BET specific surface area of 790 m ² / g was prepared.

Synthesis example 3
Support C was prepared with a pore volume of 2.16 cc / g; a pore mode radius of 2.13 nm; and a BET specific surface area of 1596 m ² / g. Specifically, the carrier C was produced by the method described in JP2009-35598A.

(Example 1)
Using the carrier A prepared in Synthesis Example 1 above, platinum (Pt) having an average particle radius of 1.8 nm as a catalyst metal was supported so that the supporting rate was 30% by weight, and catalyst powder A was obtained. That is, 46 g of carrier A was immersed in 1000 g (platinum content: 46 g) of a dinitrodiammine platinum nitric acid solution having a platinum concentration of 4.6% by mass, and 100 ml of 100% ethanol was added as a reducing agent. This solution was stirred and mixed at the boiling point for 7 hours, platinum was supported on the carrier A, filtered and dried. Thereafter, the catalyst powder A was held at a temperature of 900 ° C. for 1 hour in a hydrogen atmosphere to obtain a catalyst powder A having a loading rate of 31.1% by weight.

For the catalyst powder A thus obtained, the pore volume of the pores and the mode radius of the pores were measured. The results are shown in Table 2 below.

The weight ratio of the carbon support and the ionomer in the catalyst powder A prepared above and an ionomer dispersion (Nafion (registered trademark) D2020, EW = 1100 g / mol, manufactured by DuPont) as a polymer electrolyte is 0.9. Mixed. Further, an n-propyl alcohol solution (50%) was added as a solvent so that the solid content (Pt + carbon carrier + ionomer) was 7% by weight to prepare a cathode catalyst ink.

Ketjen black (particle size: 30 to 60 nm) is used as a carrier, and platinum (Pt) with an average particle size of 2.5 nm is supported on the catalyst metal so that the loading ratio is 50% by weight as catalyst metal. Got. This catalyst powder was mixed with an ionomer dispersion (Nafion (registered trademark) D2020, EW = 1100 g / mol, manufactured by DuPont) as a polymer electrolyte so that the weight ratio of the carbon support to the ionomer was 0.9. Further, an anode catalyst ink was prepared by adding an n-propyl alcohol solution (50%) as a solvent so that the solid content (Pt + carbon carrier + ionomer) was 7% by weight.

Next, a gasket (manufactured by Teijin DuPont Films, Teonex (registered trademark), thickness: 25 μm (adhesive layer: 25 μm) around both sides of a polymer electrolyte membrane (Dupont, Nafion (registered trademark) NR211, thickness: 25 μm). 10 μm)). Next, the catalyst ink was applied to a size of 5 cm × 2 cm by spray coating on the exposed portion of one side of the polymer electrolyte membrane. The catalyst ink was dried by keeping the stage for spray coating at 60 ° C. to obtain an electrode catalyst layer. The amount of platinum supported at this time is 0.15 mg / cm ² . Next, similarly to the cathode catalyst layer, spray coating and heat treatment were performed on the electrolyte membrane to form an anode catalyst layer, thereby obtaining a membrane electrode assembly of this example.

(Comparative Example 1)
Except that the carrier B prepared in Synthesis Example 2 was used instead of the carrier A, platinum (Pt) having an average particle radius of 2.25 nm was used as a catalyst metal, and heat treatment was not performed in a hydrogen atmosphere. Performed the same operation as in Example 1 to obtain catalyst powder B. With respect to the catalyst powder B thus obtained, the pore volume of the pores and the mode radius of the pores were measured. The results are shown in Table 2 below. Further, a membrane electrode assembly of this example was obtained in the same manner as in Example 1.

(Comparative Example 2)
Except that the carrier C produced in the above Synthesis Example 3 was used in place of the carrier A, platinum (Pt) having an average particle radius of 1.15 nm was used as the catalyst metal, and heat treatment was not performed in a hydrogen atmosphere. Performed the same operation as Example 1, and obtained catalyst powder C. With respect to the catalyst powder C thus obtained, the pore volume of the pores and the mode radius of the pores were measured. The results are shown in Table 2 below. Moreover, the membrane electrode assembly of this example was obtained in the same manner as in Example 1.

[Cover rate of electrolyte]
The coverage of the electrolyte with respect to the catalyst metal was calculated by measuring the electric double layer capacity formed at the interface of the catalyst with the solid proton conducting material and the liquid proton conducting material. In calculating the coverage, it was calculated from the ratio of the electric double layer capacity in the low humidified state to the high humidified state, and the measured values under the conditions of 5% RH and 100% RH were used as representatives of the humidity state. .

<Measurement of electric double layer capacity>
The obtained MEA was measured by electrochemical impedance spectroscopy to measure the electric double layer capacity in the highly humidified state, the lowly humidified state, and the catalyst deactivation and the highly humidified and lowly humidified states, respectively. The contact areas with both proton conducting materials were compared.

As the equipment used, an electrochemical measurement system HZ-3000 manufactured by Hokuto Denko Corporation and a frequency response analyzer FRA5020 manufactured by NF Circuit Design Block Co., Ltd. were used, and the measurement conditions shown in Table 1 below were adopted.

First, each battery was heated to 30 ° C. with a heater, and the electric double layer capacity was measured in a state where nitrogen gas and hydrogen gas adjusted to the humidified state shown in Table 1 were supplied to the working electrode and the counter electrode, respectively.

When measuring the electric double layer capacity, as shown in Table 1, it was held at 0.45 V, and the working electrode potential was oscillated with an amplitude of ± 10 mV and a frequency range of 20 kHz to 10 mHz.

That is, the real part and imaginary part of the impedance at each frequency are obtained from the response when the working electrode potential vibrates. Since the relationship between the imaginary part (Z ″) and the angular velocity ω (converted from the frequency) is expressed by the following equation, the reciprocal of the imaginary part is arranged with respect to −2 to the angular velocity, and The electric double layer capacitance C _dl is obtained by extrapolating the value.

Such measurement was sequentially performed in a low humidified state and a high humidified state (5% RH → 10% RH → 90% RH → 100% RH condition).

Furthermore, after deactivating the Pt catalyst by flowing nitrogen gas containing CO at a concentration of 1% (volume ratio) to the working electrode at 1 NL / min for 15 minutes or more, the above high humidification and low humidification conditions are as described above. The electric double layer capacity was measured in the same manner. These results are shown in Table 2. In addition, the obtained electric double layer capacity was shown in terms of a value per area of the catalyst layer.

Based on the measured values, the electric double layer capacity formed at the catalyst-solid proton conductive material (CS) interface and the catalyst-liquid proton conductive material (CL) interface was calculated.

In the calculation, the measured values under the conditions of 5% RH and 100% RH were used as representatives of the electric double layer capacity in the low and high humidification states. The results are shown in Table 2.

[Evaluation of power generation performance]
The fuel cell is kept at 80 ° C., and oxygen gas conditioned to 100% RH is circulated through the oxygen electrode and hydrogen gas conditioned to 100% RH is circulated through the fuel electrode. Water was introduced, and this water functions as a liquid proton conducting material), and the electronic load was set so that the current density was 1.0 A / cm ² and held for 15 minutes.

Thereafter, the current density was gradually reduced until the cell voltage became 0.9 V or higher. At this time, each current density was held for 15 minutes to obtain the relationship between the current density and the potential. And it converted into the current density per catalyst surface area using the catalyst effective surface area acquired on 100% RH conditions, and compared the current density in 0.9V. The results are shown in Table 2 below.

From Table 2 above, it was found that MEA using the catalyst obtained by the production method of the present invention was superior in power generation performance compared to MEA using the catalyst obtained by the production method outside the scope of the present invention.

The pore size distribution of the carrier B used in Comparative Example 1 is shown in FIG. In the pore diameter distribution of Comparative Example 1 shown in FIG. 4, the pore volume tends to increase up to 1 nm, and in the mesopore region (pore radius is 1 nm or more), a clear mode radius is obtained. It was confirmed that it does not have.

Note that this application is based on Japanese Patent Application No. 2013-92925 filed on April 25, 2013, the disclosure of which is incorporated by reference in its entirety.

Claims (12)

1. A method for producing a catalyst carrying a catalyst metal on a carrier having a pore radius distribution mode of 1 nm or more and less than 5 nm and a pore volume of the pores of 0.3 cc / g or more. There,
   Impregnating pores inside the carrier with the components of the catalytic metal;
   A heat treatment step after the impregnation step;
   A method for producing a catalyst, comprising:
2. A method for producing a catalyst carrying a catalyst metal on a carrier having a pore radius distribution mode of 1 nm or more and less than 5 nm and a pore volume of the pores of 0.3 cc / g or more. There,
   Impregnating pores inside the carrier with the catalyst metal precursor;
   Reducing the catalyst metal precursor;
   A heat treatment step after the reducing step;
   A method for producing a catalyst, comprising:
3. The method for producing a catalyst according to claim 1 or 2, wherein the heat treatment step is controlled such that an average grain radius of the catalyst metal is larger than a mode radius of a pore distribution of the pores of the catalyst.
4. The method for producing a catalyst according to any one of claims 1 to 3, wherein a mode radius of pore distribution of the pores of the catalyst is 1 nm or more and 2 nm or less.
5. The method for producing a catalyst according to claim 4, further comprising a step of controlling the catalyst metal so that an average particle radius is 1.5 nm or more and 2.5 nm or less.
6. The method for producing a catalyst according to any one of claims 1 to 5, wherein the catalyst metal is platinum or contains a metal component other than platinum and platinum.
7. The method for producing a catalyst according to claim 6, wherein an average particle radius of the constituent component of the catalyst metal is smaller than a mode radius of a pore distribution of the pores of the support.
8. An electrode catalyst layer for a fuel cell comprising a catalyst and an electrolyte obtained by the production method according to any one of claims 1 to 7.
9. The fuel cell electrode catalyst layer according to claim 8, comprising a liquid proton conducting material that couples the catalyst metal in the catalyst and the electrolyte in a state capable of proton conduction.
10. The electrode catalyst layer for a fuel cell according to claim 8 or 9, wherein a coverage of the catalyst metal by the electrolyte is 0.45 or less.
11. A fuel cell membrane electrode assembly comprising the fuel cell electrode catalyst layer according to any one of claims 8 to 10.
12. A fuel cell comprising the membrane electrode assembly for a fuel cell according to claim 11.

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