JP2016100262A - Catalyst for solid polymer fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst for a solid polymer fuel cell, which enables the formation of a catalyst layer less in reduction of voltage even if a large current is passed therethrough when used as a catalyst for forming a catalyst layer of a solid polymer fuel cell, and especially a catalyst for a solid polymer fuel cell, which is less in reduction of voltage even when a large current is passed therethrough under a wet environment easy to cause a flooding.SOLUTION: A catalyst for a solid polymer fuel cell comprises a mixture of: a catalyst metal-supporting carbon material including a carrier carbon material consisting of a porous carbon material and a catalyst metal component supported by the carrier carbon material; and a catalyst metal-nonsupporting carbon material consisting of a dendritic graphite carbon material supporting no catalyst metal component. The carrier carbon material of the catalyst-supporting carbon material is 100 m/g or larger in specific surface area (S) of meso holes having a micropore diameter of 4 to less than 10 nm, which is measured by nitrogen absorption measurement. The dendritic graphite carbon material making the catalyst metal-nonsupporting carbon material is 80-220 m/g in BET specific surface area (S), and 80-170 mL/100 g in DBP oil absorption (O), and it has a crystallite size (Lc) of 5-10 nm according to X-ray diffraction.SELECTED DRAWING: None

Description

  The present invention relates to a catalyst for a polymer electrolyte fuel cell useful for producing a polymer electrolyte fuel cell, and in particular, by using it as a catalyst layer of a polymer electrolyte fuel cell, In particular, the present invention relates to a polymer electrolyte fuel cell catalyst capable of forming a fuel cell with little voltage drop even when a large current flows in a wet environment where flooding is likely to occur.

  In general, a polymer electrolyte fuel cell has a catalyst layer serving as an anode and a catalyst layer serving as a cathode with a proton conductive electrolyte membrane sandwiched therebetween, and a gas diffusion layer disposed on the outside with these sandwiched therebetween. Further, the basic structure is a structure in which a separator is arranged outside, and this basic structure is called a unit cell. A fuel cell is usually configured by stacking as many unit cells as necessary to develop a necessary output. In addition, the catalyst layer serving as an anode or a cathode is usually obtained by dispersing a catalyst obtained by supporting a catalytic metal component on a support carbon material and an electrolyte resin such as an ionomer in an appropriate dispersion medium such as ethanol or water. The obtained catalyst layer ink liquid is formed on a substrate such as a Teflon (registered trademark) sheet and dried.

When a current is taken out from the solid polymer fuel cell (unit cell) having such a basic structure, oxygen or air is supplied to the cathode side from the gas flow paths of the separators arranged on the anode side and the cathode side, respectively. In addition, a reducing gas such as hydrogen is supplied to the anode side, and the supplied oxidizing gas and reducing gas are respectively supplied to the catalyst layer through the gas diffusion layer. A current is taken out by utilizing an energy difference (potential difference) between a chemical reaction occurring in the anode catalyst layer and a chemical reaction occurring in the cathode catalyst layer. For example, when hydrogen gas and oxygen gas are used, a chemical reaction [oxidation reaction: H ₂ → 2H ⁺ + 2e ⁻ (E ₀ = 0 V)] that occurs on the catalyst metal of the anode catalyst layer and the catalyst of the cathode catalyst layer Electricity is generated using an energy difference (potential difference) from a chemical reaction occurring on the metal [reduction reaction: O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O (E ₀ = 1.23 V)].

  Here, various studies have been made on catalytic metal components having catalytic action so far, but operation in a strongly acidic environment such as a polymer electrolyte fuel cell is required, and oxidation reaction and reduction reaction. In both cases, Pt has the highest reaction activity as a pure metal. Currently, Pt or alloys mainly composed of Pt are also used as the catalyst metal used in catalysts for ENE-FARM (household fuel cell cogeneration system) that is currently in practical use and fuel cell automobiles that are close to practical use. It has been.

  Further, for the carrier carbon material supporting such a catalytic metal component, a carbon material is used from the viewpoints of electronic conductivity, chemical stability, electrochemical stability, and the like, and the reaction gas is contained in the catalyst layer. When a catalyst layer is formed so that it diffuses without resistance, it must have pores for the reaction gas to flow, and it has carbon that has a “three-dimensionally developed structure” called a “dendritic structure”. Material is used. At present, carbon black is most widely used as such a carrier carbon material. This carbon black has a structure in which particles of several tens of nanometers are connected in a dendritic shape (aggregate structure), and a catalyst layer. When formed, voids between the branches become gas diffusion pores, and excellent gas diffusion characteristics are exhibited. Representative examples of such carbon black include CABOT's trade name: Vulcan XC-72, Lion's trade name: EC600JD, and Lion's trade name: EC300.

By the way, in the polymer electrolyte fuel cell, in accordance with the power generation principle described above, proton conduction, electron conduction, and reaction gas (anode: hydrogen, cathode: oxygen) flow in order to proceed with the oxidation reaction and reduction reaction to generate power.・ Diffusion is essential. Specifically, it is generated on the gas diffusion path for oxygen gas or hydrogen gas to move from the gas flow path of the separator to the catalytic metal component in the catalyst layer on the cathode side or the anode side, or on the catalytic metal component on the anode side. Proton (H ⁺ ) travels through the proton-conducting electrolyte membrane to the cathode-side catalytic metal component, and the proton conduction path. Furthermore, the electrons (e ⁻ ) generated on the anode-side catalytic metal component gas diffuse. It is necessary that the electron transfer paths for moving to the catalytic metal component on the cathode side through the layer, the separator, and the external circuit are continuously connected without being divided.

  In such a catalyst layer, generally, pores serving as diffusion paths for oxygen gas or hydrogen gas formed in gaps between constituent materials, electrolyte materials serving as proton conduction paths, and carbon serving as electron conduction paths. It is important that conductive materials such as materials and separator metal materials form respective continuous networks. In addition, as an electrolyte resin serving as a proton conduction path in the proton conductive electrolyte membrane or the catalyst layer, an ion exchange resin typified by a perfluorosulfonic acid polymer is used as a polymer electrolyte material. These generally used polymer electrolyte materials have a proton hopping through a water molecule in a conductive form, and therefore exhibit high proton conductivity for the first time in a wet environment and decrease in proton conductivity in a dry environment. Therefore, in order to operate the fuel cell with the output voltage loss reduced as much as possible, it is necessary that the polymer electrolyte material is in a sufficiently wet state. At the same time, steam is supplied to make it humid.

However, in the catalyst layer on the cathode side, water molecules generated by the reduction reaction on the cathode side are added as water vapor in addition to water molecules moving accompanying protons from the anode side. Condensed in water beyond. The amount of water produced by this reduction reaction increases, for example, during high-current discharge exceeding 1.5 A / cm ² current density under high humidification conditions (saturated humidification conditions). In the catalyst layer, which also serves as a drainage path for the generated water, clogging occurs, causing gas supply and diffusion into the catalyst layer to become insufficient, and the voltage of the fuel cell decreases. ) Cause problems. The problem of flooding is that, for fuel cell vehicles, etc., the output voltage at the time of large current discharge is increased to increase the maximum output (current x maximum voltage), thereby reducing the amount of platinum used per output and reducing the cost. In many applications where down is required, it has become a major issue in the practical application of polymer electrolyte fuel cells. The large current in the present invention refers to power generation with a current density of 1.5 A / cm ² or more per unit area of the electrode. Regardless of the amount of platinum used as a catalyst, with the current technology, in order to exert an output voltage of 0.6 V or higher at 1.5 A / cm ² , the diffusion resistance of oxygen gas is reduced and the generated water is used. It is essential to suppress flooding.

  Therefore, in order to solve such a flooding problem, various conventional methods such as PTFE powder, PTFE colloid, PTFE water-repellent carbon powder, carbon material water-repellent treated with a silane coupling agent, etc. In which water generated in the catalyst layer is contained in the catalyst layer and the water repellency of the catalyst layer is increased to quickly discharge generated water to the outside of the system (Patent Documents 1 to 4). In which the ion exchange capacity of the ion exchange resin on the anode side is made larger than the ion exchange capacity of the ion exchange resin on the cathode side (Patent Document 5), and the perfluorosulfonic acid ion in the catalyst layer The sulfonic acid equivalent (EW) of the ion exchange resin on the anode side is made smaller than the sulfonic acid equivalent (EW) of the ion exchange resin on the cathode side. Method for despreading water generated at the cathode side to the anode side (Patent Document 6) have been proposed.

  However, in the method of incorporating a water repellent in the catalyst layer, the electrical resistance of the catalyst layer increases as the amount of the water repellent used increases, and the gas permeability decreases as the thickness of the catalyst layer increases. However, in the method of coating the catalyst in the catalyst layer with an ion exchange resin, it is difficult to control the coating thickness of the resin to be coated. The current density cannot be increased because the diffusion of oxygen on the catalyst coated with is deteriorated. In addition, since the ion-exchange resin has a high property of retaining moisture, coating of the catalyst with the resin makes it difficult for water generated on the catalyst to be discharged to the outside, resulting in flooding and difficult to take out a large current. Further, in the method of incorporating an ion exchange resin in the catalyst layer, there is a problem that it is difficult to take out a large current because it causes flooding for the same reason.

In Patent Document 7, a moisturizing carbon material and carbon black are mixed at a predetermined ratio, and the value of a specific water vapor adsorption amount (V _0.95 ) for the moisturizing carbon material is 1250 cm ³ / g ≦ V _0.95 ≦ The relative water vapor pressure (P _1/2 ) indicating 2500 cm ³ / g and half the water vapor adsorption amount of V _0.95 is P _1/2 ≦ 0.55, and for carbon black, the DBP oil absorption amount (O _DBP ) is O _DBP ≧ 100 mL / 100 g, V _0.95 ≧ 100 cm ³ / g, and for solid polymer fuel cells that exhibit performance equivalent to that of saturated humidification even in a low humidification environment with a relative humidity of 50% or less A catalyst carbon material for a catalyst layer has been proposed. However, in the fuel cell proposed in Patent Document 7, both the moisture-retaining carbon material and the carbon black are hydrophilic, and the discharge of generated water generated at a large current is not always sufficient, and particularly high humidification operation. There is a problem that the output decreases at the time of large current discharge due to flooding.

Further, in Patent Document 8, at least the carbon material in the catalyst layer on the cathode side is composed of a catalyst-carrying carbon material carrying a catalyst component and a gas diffusion carbon material not carrying a catalyst component, It has been proposed that by controlling the amount of water vapor adsorbed, the material mobility in the catalyst layer (gas diffusion, electron conduction, proton conduction, wet management) is excellent, and both cost reduction and output characteristic improvement are compatible. In Patent Document 9, at least the carbon material of the catalyst layer on the cathode side is composed of a catalyst metal carrier carbon material carrying a catalyst metal component and a gas diffusion carbon material not carrying a catalyst metal component, and the catalyst metal carrier In addition to a structure in which a catalyst aggregation phase mainly composed of a carbon material and an electrolyte material and a gas diffusion carbon material aggregation phase mainly composed of a gas diffusion carbon material are mixed, catalyst gold By controlling the water vapor adsorption amount of the carrier carbon material and the gas diffusion carbon material, and using a predetermined perfluorosulfonic acid proton conductive electrolyte resin as the electrolyte material, it is possible to operate under low and high humidification operating environments. Both have been proposed to exhibit sufficient power generation characteristics. However, the fuel cells proposed in Patent Documents 8 and 9 are aimed at optimizing the power generation characteristics assuming flooding suppression at a current density of 1000 mA / cm ² at most. On the other hand, in recent automobile applications, a high output is required at a current density exceeding 2000 mA / cm ² that causes flooding in the conventional catalyst layer, and higher flooding characteristics are essential. The catalyst layers proposed in Patent Documents 8 and 9 have a problem of output reduction due to flooding at a current density exceeding 2000 mA / cm ² . The present invention aims at suppressing flooding at such a large current density.

  Thus, in order to stably operate the polymer electrolyte fuel cell during a large current discharge, the inside of the catalyst layer on the cathode side or the anode side is sufficiently humidified, and at the same time, particularly in the catalyst layer on the cathode side. It is necessary to satisfy the mutually contradictory requirements of quickly discharging the generated water that is condensed and generated to the outside, and the methods proposed so far reliably prevent flooding during high current discharge and stabilize the fuel cell. In many applications where it is required to improve battery characteristics during high-current discharge and reduce costs, such as for fuel cell vehicles, it is a major issue in practical application. It was.

JP 05-036,418 Japanese Patent Laid-Open No. 04-264,367 Japanese Patent Laid-Open No. 07-183,035 Japanese Unexamined Patent Publication No. 2000-243,404 JP 09-213,350 A JP 2011-029,070 JP 2013-020,793 JP JP 2005-332,807 A JP 2013-225,433 A

Therefore, the present inventors aim to develop a polymer electrolyte fuel cell that can maintain excellent battery performance with little voltage drop even when a large current is passed in a wet environment that is usually easily flooded. Under the viewpoints shown, the inventors have intensively studied a new catalyst for forming the catalyst layer.
That is, the catalyst layer serving as the anode and the cathode is formed of the catalyst metal-supporting carbon material supporting the catalyst metal component and the proton conductive resin, and the proportion of the support carbon material forming the catalyst metal-supporting carbon material is Is large in both the weight ratio and the volume ratio, and the support carbon material supporting the catalytic metal component is basically required to be hydrophilic, especially on the cathode side catalyst layer during large current discharge. In order to quickly discharge this water vapor from the catalyst layer before the water vapor generated in the catalyst layer condenses in the catalyst layer, the volume ratio (porosity) of the pores of the catalyst layer is increased and at the same time high water repellency. In view of the fact that it is necessary to provide a carrier carbon material carrying a catalytic metal component, hydrophilicity can be ensured in order to ensure proton conductivity. The raw material is used, and the catalyst metal component having high porosity and water repellency is not supported near the support carbon material (catalyst metal support carbon material) supporting the catalyst metal component in the formed catalyst layer. A carbon material (catalyst metal non-supported carbon material) is arranged, and thereby, a new catalyst layer can be formed in which water vapor generated from the catalyst metal support carbon material is quickly discharged through the catalyst metal non-support carbon material. Has been intensively studied to develop a new catalyst.

The present inventors have obtained the following development guidelines in the process of developing this new catalyst.
That is, with respect to the support carbon material of the catalyst metal-supporting carbon material responsible for the oxidation reaction and the reduction reaction in the catalyst layer, in order to support the catalyst metal component not only on the surface but also inside the pores, the diameter is 4 nm or more and 10 nm. It is necessary that a large number of mesopores less than 10 nm be present, and it is preferable to adopt a mesopore specific surface area (S _{4-10 nm} ) having a pore diameter of 4 nm or more and less than 10 nm as the evaluation. In order to achieve high porosity and water repellency, the branch structure and crystallinity in the carbon material are important for the catalytic metal non-supporting carbon material that is responsible for the discharge of the generated water vapor. It is preferable to adopt the physical properties of specific surface area (S _BET ), DBP oil absorption (O _DBP ), and crystallite size (Lc: average size in the stacking direction) at which the 002 plane diffraction line appears most strongly in X-ray diffraction. I found something

  Further, the present inventors examined the above-mentioned catalytic metal non-supported carbon material, in order to prevent a voltage drop due to electric resistance at the time of large current discharge in the catalyst layer for this catalytic metal non-supported carbon material, It has been determined that a function as a conductive auxiliary is also required, and the characteristic as the conductive auxiliary is not related to the specific electrical resistance of the added carbon material, but the contact resistance when mixed is dominant. It is determined that the contact resistance is determined by the form of the carbon material, and the size of the particles of the non-catalyst metal-supported carbon material is made equal to the size of the particles of the support carbon material of the catalyst metal-supported carbon material. I found it important.

  From the viewpoint of gas diffusion in the catalyst layer, it is desirable that the size of the pores formed by the carrier carbon material is equal to or greater than the mean free path of oxygen (about 70 nm). On the other hand, in order to support catalytic metal fine particles of several nm, the particle diameter of the supported carbon material is practically essential at least 10 nm or more. The pores formed in the catalyst layer are formed by the particle diameter (primary particles) of the support carbon material, the voids of the aggregated structure (secondary particles) in which the primary particles are aggregated, and the voids between the secondary particles. However, the voids in the secondary particles are based on branches formed by aggregation of several primary particles. In addition, the size of the voids between the secondary particles can be regarded as the length of the above-mentioned branch, that is, the size of the pores formed in the catalyst layer is about the same as the size of the primary particles. It is reasonable to estimate that the size is several times at most. In view of these requirements, a particle size (primary particle size) of 10 nm or more is desirable for both the catalytic metal carrier and the non-catalyst metal carrier. The upper limit of the primary particles is 100 nm or less. With primary particles larger than 100 nm, it is difficult to secure a surface area on which catalytic metal fine particles necessary for power generation are to be supported.

As a result of intensive studies under the above-mentioned viewpoints, the present inventors have found that the supported carbon material of the catalyst-supporting carbon material has a mesopore specific surface area (pore diameter of 4 nm or more and less than 10 nm measured by nitrogen adsorption measurement). S _{4-10 nm} ) is 100 m ² / g or more, and the catalytic metal non-supporting carbon material has a BET specific surface area (S _BET ) of 80 m ² / g or more and 220 m ² / g or less, and DBP oil absorption amount (2) is 80 mL / 100 g or more and 170 mL / 100 g or less, and the crystallite size (Lc) by X-ray diffraction is required to be a dendritic graphitic carbon material of 5 nm or more and 10 nm or less. The solid polymer fuel cell catalyst of the present invention was completed.

  Accordingly, an object of the present invention is to prevent flooding reliably at the time of discharging a large current even if a large current flows in a wet environment where flooding is likely to occur, thereby enabling stable operation of the fuel cell. In addition, the present invention provides a catalyst for a polymer electrolyte fuel cell that is useful in manufacturing a polymer electrolyte fuel cell capable of reducing the cost by reducing the amount of expensive catalytic metal components such as platinum. is there.

That is, the present invention has the following configuration.
(1) a catalytic metal-supporting carbon material in which a catalytic metal component is supported on a support carbon material made of a porous carbon material, and a non-catalytic metal-supporting carbon material made of a dendritic graphitic carbon material not supporting a catalytic metal component; The support carbon material of the catalyst-supporting carbon material has a mesopore specific surface area (S _{4-10 nm} ) of 4 to 10 nm as measured by nitrogen adsorption measurement of 100 m ^2. The dendritic graphitic carbon material which is the catalyst metal non-supporting carbon material has a BET specific surface area (S _BET ) of 80 m ² / g or more and 220 m ² / g or less, and a DBP oil absorption amount ( A catalyst for a polymer electrolyte fuel cell, wherein O _DBP ) is 80 mL / 100 g or more and 170 mL / 100 g or less, and a crystallite size (Lc) by X-ray diffraction is 5 nm or more and 10 nm or less.

(2) The average particle diameter of the support carbon material of the catalyst metal-supported carbon material is 10 nm or more and 1100 nm or less, and the average particle diameter of the non-catalyst metal-supported carbon material is ± the average particle diameter of the support carbon material. The solid polymer fuel cell catalyst according to (1), wherein the catalyst is in the range of 50 nm and is in the range of 10 nm to 50 nm.
(3) The mixing ratio of the catalytic metal-carrying carbon material and the catalytic metal-noncarrying carbon material is 60% by mass to 99% by mass of the catalytic metal-carrying carbon material, and 1% by mass of the catalytic metal-noncarrying carbon material. The catalyst for a polymer electrolyte fuel cell according to the above (1) or (2), wherein the catalyst is in the range of% to 40% by mass.

(4) The dendritic graphitic carbon material which is the catalyst metal non-supporting carbon material has a BET specific surface area (S _BET ) of 90 m ² / g or more and 200 m ² / g or less, and DBP oil absorption (O _DBP ) Is 100 mL / 100 g or more and 150 mL / 100 g or less, The catalyst for a polymer electrolyte fuel cell according to any one of the above (1) to (3).
(5) The dendritic graphitic carbon material which is the catalyst metal non-supporting carbon material has a crystallite size (Lc) by X-ray diffraction of 6 nm or more and 10 nm or less. The catalyst for a polymer electrolyte fuel cell according to any one of 1).

In the present invention, the support carbon material for forming the catalytic metal-supporting carbon material has a mesopore specific surface area (S _{4-10 nm} ) of 4 to 10 nm as measured by nitrogen adsorption measurement of 100 m ² / g. As described above, the porous carbon material should preferably be 150 m ² / g or more, and if the mesopore specific surface area (S _{4-10 nm} ) is less than 100 m ² / g, the catalyst metal component shown below is desired. It becomes difficult to carry uniformly with a carrying amount, particularly 40% by mass or more.

  Here, “mesopores” are pores having a diameter of 2 nm to 50 nm according to IUPAC, but in the present invention, mesopores having a pore diameter of 4 nm or more and less than 10 nm are important. This is because a reduction reaction is considered to occur efficiently on the catalyst metal supported in mesopores having a pore diameter of 4 nm or more and less than 10 nm under the fuel cell operating conditions during large current discharge. The measurement of the mesopores employs so-called adsorption isotherm measurement of nitrogen gas, and the analysis employs the Dollimore-Heal method for the nitrogen adsorption isotherm of the adsorption process.

  As such a carrier carbon material, for example, porous carbon materials such as carbon black, graphite, carbon fiber, activated carbon and the like, pulverized products thereof, and carbon compounds such as carbon nanofiber can be used. May be used alone, or two or more may be used in combination. Here, commercially available products of carbon black include, for example, Furnace Black such as Vulcan XC-72, Vulcan P, Black Pearls 880, Black Pearls 1100, Black Pearls 1300, Black Pearls 2000, and Legal 400 manufactured by Cabot, Lion Ketjen Black EC and EC600JD manufactured by Mitsubishi Chemical, oil furnace black such as # 3150 and # 3250 manufactured by Mitsubishi Chemical, acetylene black such as Denka Black manufactured by Denki Kagaku Kogyo, and Printex XE2 and Printex XE2 manufactured by Degussa -B etc. can be mentioned, and further, those having pores introduced by activating them can be used. Examples of the activated carbon include YP and RP manufactured by Kuraray Chemical Co., and further include activated carbon fibers.

  Here, the catalyst metal component supported on the support carbon material in order to form the catalyst metal-supported carbon material has excellent catalytic activity for the oxidation reaction and reduction reaction described above in a strongly acidic environment. For example, metals such as platinum, palladium, ruthenium, gold, rhodium, osmium, iridium, tungsten, lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, etc. Examples include composites and alloys in which two or more types are combined, and other catalyst metals and promoter metals may be used in combination. Preferably, Pt has the highest reaction activity as a pure metal. Therefore, Pt or an alloy containing Pt as a main component is used. The amount of the catalytic metal component supported in the catalytic metal-supporting carbon material is usually 10% by mass or more, preferably 20% by mass or more and 80% by mass, and more preferably 40% by mass or more and 80% by mass. If the supported amount of the catalytic metal component is lower than 10% by mass, the catalyst layer becomes thick, and gas diffusion in the catalyst layer becomes rate-determining, resulting in a problem that large current characteristics are deteriorated.

The catalyst metal non-supporting carbon material not supporting the catalyst metal component has a BET specific surface area (S _BET ) of 80 m ² / g or more and 220 m ² / g or less, preferably 90 m ² / g or more and 200 m ² / g. The DBP oil absorption (O _DBP ) is 80 mL / 100 g to 170 mL / 100 g, preferably 100 mL / 100 g to 150 mL / 100 g, and the crystallite size (Lc) by X-ray diffraction is 5 nm to 10 nm. In the following, it is necessary that the dendritic graphitic carbon material is preferably 6 nm or more and 10 nm or less.

Here, the BET specific surface area (S _BET ) is an evaluation of the average value of the particle size from the surface area of the dendritic graphitic carbon material used as the catalyst metal non-supporting carbon material. _{If the BET} ) is less than 80 m ² / g, the primary particle size becomes too large, and uniform mixing with the catalytic metal-carrying carbon material becomes difficult, and contact with the catalytic metal-carrying carbon material is insufficient, resulting in electrical resistance. On the other hand, if it exceeds 220 m ² / g, the primary particle size becomes too small, or even in this case, uniform mixing with the catalyst metal-supporting carbon material becomes difficult, and the catalyst metal support Insufficient contact with the carbon material increases the electrical resistance, causing a voltage drop during large current discharge.

In addition, the DBP oil absorption (O _DBP ) determines the pore size necessary to give a desired porosity to the produced catalyst layer for the dendritic graphitic carbon material used as the catalyst metal non-supported carbon material. This is an evaluation of the branch structure, and is an indicator that correlates with the amount of voids (degree of structure development) in the secondary aggregate structure of the carbon material. The DBP oil absorption (O _DBP ) is less than 80 mL / 100 g. On the other hand, it becomes difficult to create the necessary voids in the catalyst layer, and conversely, if it exceeds 170 mL / 100 g, the mechanical strength of the branches is weakened, and the branches are destroyed during the preparation of the catalyst layer ink liquid. It becomes difficult to form necessary pores.

  Further, the crystallite size (Lc) by X-ray diffraction is an evaluation of crystallinity as an index of water repellency of a dendritic graphitic carbon material used as a catalyst metal non-supporting carbon material. Then, the crystallite size (Lc) measured by X-ray diffraction becomes large. If the crystallite size (Lc) by X-ray diffraction is less than 5 nm, the water repellency is low and the desired flooding suppression effect is not exhibited. Conversely, if the crystallite size (Lc) exceeds 10 nm, the dendritic graphitic carbon material The shape is not spherical but is a sharp polyhedron, the contact resistance with the catalytic metal-supporting carbon material is increased in the catalyst layer, the electrical resistance is increased, and a voltage drop occurs during large current discharge. As an evaluation index of water repellency, evaluation of water vapor adsorption amount is generally used. However, in the case of a carbon material specified by the present invention, since water is highly crystalline and has no pore structure, The absolute value of the quantity is very small and is not suitable as an index for distinguishing materials. As a result of intensive studies, it has been found that the crystallite size (Lc) representing the degree of crystal growth is suitable and adopted as an index. It is.

  Examples of the dendritic graphitic carbon material forming such a catalyst metal non-supporting carbon material include graphitized carbon black, carbon black, graphite, carbon fiber, activated carbon and the like, pulverized products thereof, carbon nanofiber, carbon nanotube Carbon compounds such as can be used, and only one of them may be used alone, or two or more may be mixed and used. Here, as a commercial product of graphitized carbon black, for example, Toka Black # 3855, Toka Black # 3845, Toka Black # 3800, which are graphitized carbon blacks manufactured by Tokai Carbon Co., and commercial carbon black are used as an inert atmosphere. Examples thereof include those heat-treated at a temperature of 2000 ° C. or higher.

  In the present invention, the carrier carbon material for forming the catalytic metal-supported carbon material may have an average particle diameter of 10 nm to 100 nm, preferably 15 nm to 90 nm, and the catalyst metal non-supported carbon. The material desirably has an average particle size within the range of ± 50 nm, preferably ± 45 nm, of the average particle size of the carrier carbon material. When the average particle diameter of the support carbon material is less than 10 nm, there is a problem that it becomes difficult to support the catalyst metal fine particles of 2 to 6 nm. On the contrary, when the average particle diameter exceeds 100 nm, the specific surface area of the carbon material is increased. A problem arises in that the catalyst layer becomes thick because it is too small, and the large current characteristic deteriorates due to diffusion control. Also, if the average particle diameter of the non-catalyst metal carbon material is out of the range of ± 50 nm of the average particle diameter of the support carbon material, the particle size differs when mixed with the catalytic metal carbon material. In this case, it is difficult to disperse microscopically, and as a result, the water repellency around the catalytic metal-supporting carbon material becomes insufficient, and as a result, there arises a problem that the large current characteristic is deteriorated due to flooding.

  In the present invention, the mixing ratio of the catalytic metal-supported carbon material and the catalytic metal-nonsupported carbon material is such that the catalytic metal-supported carbon material is 60% by mass to 99% by mass, The material is 1% by mass or more and 40% by mass or less, preferably, the catalyst metal-supported carbon material is 60% by mass or more and 95% by mass or less, and the catalyst metal non-supported carbon material is 5% by mass or more and 40% by mass or less. is there. In the mixing ratio of the catalytic metal-supporting carbon material and the non-catalytic metal-supporting carbon material, if the ratio of the catalytic metal-nonsupporting carbon material is lower than 1% by mass, the ratio existing around the catalytic metal-supporting carbon material is reduced. However, if the amount exceeds 40% by mass, the catalyst layer becomes too thick and the gas diffusion resistance is reduced. There arises a problem that it increases and causes a decrease in large current characteristics.

  In the present invention, the method for supporting the catalyst metal component on the support carbon material made of the porous carbon material is not particularly limited, and the conventional method can be employed as it is, and the obtained catalyst metal-supported carbon material is used. The method of mixing the catalyst metal non-supporting carbon material made of dendritic graphitic carbon material is not particularly limited as long as the two carbon materials can be mixed uniformly.

  In addition, a catalyst layer ink liquid is prepared using a catalyst obtained by mixing a catalyst metal-carrying carbon material and a catalyst metal non-carrying carbon material, and a catalyst layer is prepared using the catalyst layer ink liquid. Furthermore, the method for preparing a membrane electrode assembly (MEA) using the produced catalyst layer is not particularly limited, and a conventional method can be employed as it is.

  According to the catalyst for a polymer electrolyte fuel cell of the present invention, a catalyst layer that becomes an anode or a cathode is formed using this catalyst, so that even if a large current flows in a wet environment where flooding easily occurs, Solid polymer fuel that can prevent flooding reliably at the time of discharge, thereby enabling stable operation and reducing the cost by reducing the amount of expensive catalytic metals such as platinum A battery can be formed.

Hereinafter, preferred embodiments of the polymer electrolyte fuel cell catalyst of the present invention will be described based on Examples and Comparative Examples.
In the following Examples and Comparative Examples, the mesopore specific surface area (S _{4-10 nm} ) having a pore diameter of 4 nm or more and less than 10 nm in the support carbon material of the catalyst metal-supported carbon material, and the dendritic shape of the catalyst metal non-support carbon material The BET specific surface area (S _BET ), DBP oil absorption (O _DBP ), and crystallite size (Lc) by X-ray diffraction in the graphitic carbon material were measured by the following methods.

[Measurement of mesopore specific surface area (S _{4-10 nm} ) with a pore diameter of 4 nm or more and less than 10 nm]
About mesopore specific surface area (S _{4-10 nm} : m ² / g) having a pore diameter of 4 nm or more and less than 10 nm, about 50 mg of a sample was measured and dried in vacuo at 90 ° C. for 5 hours. Samples were measured by a gas adsorption method using nitrogen gas using an automatic specific surface area measuring device (BELSORP36, manufactured by Nippon Bell), and the nitrogen adsorption isotherm during the adsorption process was analyzed by the Dollimore-Heal method. By taking the difference between the cumulative value of the pore surface area when the diameter is 4 nm and the cumulative value of the pore surface area when the pore diameter is 10 nm, the specific surface area of the mesopores with a pore diameter of 4 nm or more and less than 10 nm (m ² / g )

[Measurement of BET specific surface area (S _BET )]
About BET specific surface area (BET: m ² / g), a sample of about 50 mg was measured, and this was vacuum-dried at 90 ° C. for 5 hours. , BELSORP36) was used and measured by a gas adsorption method using nitrogen gas, and the specific surface area was determined by a multipoint method based on the BET method.

[Measurement of DBP oil absorption (O _DBP )]
DBP oil absorption (O _DBP : mL / 100g) is a general property index of carbon black, measured according to JIS K 6217-4, measured 10-30g of sample, and vacuum dried at 90 ° C for 1 hour Then, for the obtained sample after drying, using an absorber meter (manufactured by Brabender), the amount of dibutyl phthalate (DBP) added at about 50% of the maximum torque is converted to the amount of DBP oil absorption per 100 g of sample. It is what I asked for.

[Measurement of crystallite size (Lc) by X-ray diffraction]
For the crystallite size (Lc: nm) by X-ray diffraction, Lc was calculated using the measurement method proposed by the Japan Society for the Promotion of Science 117 and the analysis method using CuKα rays as a radiation source.
(Measurement of average particle size)
The average particle diameter was calculated from an SEM (secondary electron image) image. The specific method is as follows. Twenty measurement particles capable of clearly discriminating the spherical shape were selected from an image with a magnification of 100,000 to 200,000, the diameter of the particle was calculated from the screen, and the arithmetic average was taken.

1. About catalytic metal-supported carbon materials
(1) Carrier carbon material As a carrier carbon material, a porous carbon material (carriers A to Q) having a mesopore specific surface area (S _{4-10 nm} ) having a pore diameter of 4 nm or more and less than 10 nm and an average particle diameter shown in Table 1 is used. used.
Carrier A: Ketjen Black (trade name: EC300, manufactured by Lion)
Carrier B: Ketjen Black (Lion's product name: EC600JD)
Carrier C: Ketjen black used as carrier A was heat-treated in an inert gas atmosphere at 2000 ° C. for 1 hour Carrier D: Ketjen black used as carrier A at 2200 ° C. in an inert gas atmosphere Heat-treated for 1 hour Carrier E: Coconut shell activated carbon (Kuraray Chemical Co., Ltd., trade name: YP80F)
Carrier F: Carbon black (trade name: SEAST G-FY, manufactured by Tokai Carbon Co., Ltd.)
Carrier G: Carbon black used as carrier F is made porous by activation under conditions of 1100 ° C. and 50 minutes under CO ₂ gas flow Carrier H: Carbon black used as carrier F under CO ₂ gas flow Activated at 1100 ° C. for 100 minutes and made porous by carrier I: carbon black used as carrier F is activated at 1100 ° C. for 200 minutes under a CO ₂ gas flow to make it porous Carrier J: Carbon black (trade name: Seast SP, manufactured by Tokai Carbon Co., Ltd.)
Carrier K: Carbon black used as carrier F was activated and made porous under the conditions of 1100 ° C. and 70 minutes under CO ₂ gas flow Carrier L: Carbon black used as carrier F under CO ₂ gas flow Activated at 1100 ° C. for 110 minutes and made porous by support M: carbon black used as support F was made porous by activation at 1100 ° C. for 200 minutes under CO ₂ gas flow Carrier N: Carbon black (trade name: SEAST TA, manufactured by Tokai Carbon Co., Ltd.)
Carrier O: Carbon black used as carrier F is activated and porous under conditions of 1100 ° C. and 80 minutes under CO ₂ gas flow Carrier P: Carbon black used as carrier F under CO ₂ gas flow Activated at 1100 ° C. for 120 minutes and made porous. Support Q: Graphitized carbon black (trade name: Toka Black TK # 3855 manufactured by Tokai Carbon Co., Ltd.)

(2) Preparation of catalytic metal-supported carbon material (platinum-supported carbon material) Each of the above-mentioned support carbon materials (supports A to Q) is dispersed in a nitric acid solution of a dinitrodiammine platinum complex and stirred while maintaining a temperature of 95 ° C. 1-propanol was added and held for 5 hours to obtain a catalyst precursor. This catalyst precursor is filtered, washed with water, dried, and then subjected to reduction treatment at 150 ° C. for 1 hour in an air stream in which 5 vol% -H ₂ is mixed with argon. Materials A to Q were prepared.

2. About non-catalyst metal supported carbon materials BET specific surface area (S _BET ), DBP oil absorption (O _DBP ), crystallite size (Lc) by X-ray diffraction, and average particle diameter as shown in Table 2 Dendritic graphitic carbon material (carbon materials a to k) having
Carbon material a: graphitized carbon black (trade name: Toka Black TK3855 manufactured by Tokai Carbon Co., Ltd.)
Carbon material b: Graphitized carbon black (trade name: Toka Black TK3845 manufactured by Tokai Carbon Co., Ltd.)
Carbon material c: Graphitized carbon black (trade name: Toka Black TK3800 manufactured by Tokai Carbon Co., Ltd.)
Carbon material d: Toka Carbon manufactured by Tokai Carbon Co., Ltd., which was made porous by activation under conditions of 1100 ° C. and 150 minutes under the flow of CO ₂ gas, and further at 2200 ° C. in an inert gas atmosphere Heat treated under conditions for 1 hour Carbon material e: Acetylene black (trade name: Denka Black, manufactured by Denki Kagaku Kogyo Co., Ltd.)
Carbon material f: Carbon black (trade name: Toka Black # 5500, manufactured by Tokai Carbon Co., Ltd.)
Carbon material g: Carbon material f heat-treated in an inert gas atmosphere at 2000 ° C. for 1 hour Carbon material h: Carbon material f heat-treated in an inert gas atmosphere at 2200 ° C. for 1 hour Carbon material i: Carbon material f heat-treated at 2400 ° C. for 1 hour in an inert gas atmosphere Carbon material j: Carbon black (trade name: SEAST 3H, manufactured by Tokai Carbon Co., Ltd.)
Carbon material k: Carbon material j heat-treated in an inert gas atmosphere at 2200 ° C. for 1 hour Carbon material l: Carbon black (trade name: Seast 300, manufactured by Tokai Carbon Co., Ltd.)
Carbon material m: Carbon material l heat-treated in an inert gas atmosphere at 2200 ° C. for 1 hour Carbon material n: Product name Toka Black SO manufactured by Tokai Carbon Co., Ltd. under CO ₂ gas flow at 1100 ° C. and 200 minutes What was further made porous by activation treatment under the conditions described above was further heat-treated in an inert gas atmosphere at 2200 ° C. for 1 hour. Carbon material o: carbon black (trade name: Toka Black # manufactured by Tokai Carbon Co., Ltd.) 6300)

3. Preparation of platinum catalyst [Examples 1-17 and Comparative Examples 1-23]
Platinum supported carbon materials A to Q are used as catalytic metal supported carbon materials, and carbon materials a to o are used as catalytic metal non-supported carbon materials. As shown in Table 3, each of these platinum supported carbon materials A to O is used. Q and carbon materials a to o were blended in such a ratio that the platinum-supporting carbon material was 80% by weight and the carbon material was 20% by weight, and then thoroughly mixed using a menor mortar. Platinum catalysts for polymer electrolyte fuel cells of Comparative Examples 1 to 23 were obtained.

4). Preparation of polymer electrolyte fuel cell and evaluation of battery performance
(1) Preparation of catalyst layer ink liquid The platinum catalysts of Examples 1 to 17 and Comparative Examples 1 to 23 prepared above were placed in a container, and a 5% -Nafion solution (DE521 manufactured by DuPont) was added thereto. After stirring, the container was placed in an ultrasonic cleaner to disperse the platinum catalyst. Further, 2-propanol was added while stirring to adjust the total solid concentration of the platinum catalyst and Nafion to 2% by mass, thereby preparing a catalyst layer ink liquid for forming a cathode-side catalyst layer. In preparing the catalyst layer ink, 1 part by mass of the carrier carbon material in each platinum catalyst was mixed at a ratio of Nafion of 0.8 part by mass.
The catalyst layer ink liquid for forming the anode catalyst layer was prepared in the same manner as the cathode catalyst layer ink liquid except that the platinum-supported carbon material B was used as a catalyst.

(2) Preparation of cathode-side and anode-side catalyst layers Using the cathode-side and anode-side catalyst layer ink solutions prepared above, this catalyst-layer ink solution was sprayed on a Teflon (registered trademark) sheet, The catalyst layer was dried at 120 ° C. for 60 minutes in an atmosphere to prepare a cathode side catalyst layer and an anode side catalyst layer, respectively.

The amount of platinum per unit area of the prepared catalyst layer on the cathode side and anode side was measured by cutting the catalyst layer on the prepared Teflon (registered trademark) sheet into a 3 cm square and measuring the mass, and then removing the catalyst layer with a scraper. Measure the mass of Teflon (registered trademark) sheet after peeling off, calculate the catalyst layer mass from the difference from the previous mass, the proportion of solid content in the catalyst ink liquid and the proportion of platinum in this solid content The amount of spray was adjusted so that the amount of platinum was 0.20 mg / cm ² .

(3) Production of Membrane Electrode Assembly (MEA) MEA was produced as follows using each of the catalyst layers on the cathode side and the anode side produced as described above.
The Nafion membrane (DuPont N112) was cut into a 6 cm square, and the cathode and anode catalyst layers coated on the Teflon (registered trademark) sheet were each 2.5 cm square with a cutter knife. Cut into squares. A Nafion membrane is sandwiched between the catalyst layer on the cathode side and the catalyst layer on the anode side, and at this time, there is no deviation in the positional relationship in which the catalyst layers face each other across the center of the Nafion membrane as the anode and cathode. And pressed under conditions of 120 ° C., 100 kg / cm ² and 10 minutes. Thereafter, after cooling to room temperature, only the Teflon (registered trademark) sheet was carefully peeled off on both the cathode side and the anode side to obtain a catalyst layer-fixed Nafion film in which the cathode and anode catalyst layers were fixed to the Nafion film.

Next, a commercially available carbon cloth (LT-1200W manufactured by E-TEK) was cut into a 2.5 cm square as the gas diffusion layer, and the positional relationship between the cathode and anode was shifted between the two carbon cloths obtained. The catalyst layer fixing Nafion membrane prepared earlier was sandwiched and pressed under the conditions of 120 ° C., 50 kg / cm ² and 10 minutes to prepare an MEA.
The mass of the fixed catalyst layer was determined from the difference between the mass of the Teflon (registered trademark) sheet with the catalyst layer before pressing and the mass of the Teflon (registered trademark) sheet peeled off after pressing, and from the mass ratio of the composition of the catalyst layer The platinum basis weight was calculated and confirmed to be 0.2 mg / cm ² .

(4) Evaluation of battery performance A unit cell (fuel cell) was constructed using the MEAs of Examples 1 to 17 and Comparative Examples 1 to 23 produced as described above, and each unit cell was incorporated into a fuel cell measurement device. The battery performance was evaluated according to the following procedure.
First, battery performance was evaluated under the following typical conditions of “highly humidified output”. That is, as the supply gas, air was supplied to the cathode and pure hydrogen was supplied to the anode so that the utilization rates were 30% and 60%, respectively. The supply gas was evaluated by a flow, and the pressure adjustment at a back pressure valve provided downstream of the cell was 0.05 MPa. The cell temperature was set to 80 ° C., and the supplied air and pure hydrogen were humidified under high humidification conditions in which bubbling was performed in distilled water kept at 80 ° C., respectively. After supplying a gas to the cell under these conditions, the load gradually increases over about 2 hours to 1000 mA / cm ² of target, the load is fixed at the time of reaching the 1000 mA / cm ² of the target, Thereafter, the voltage between the cell terminals after 30 minutes was measured and recorded as “highly humidified output”.

Next, the battery performance was evaluated under typical conditions of the following “low humidification output”. That is, while the operation of the above “high humidification output” evaluation was performed, the supplied air and pure hydrogen were switched to humidification under low humidification conditions in which bubbling was performed in distilled water kept at 65 ° C., respectively.
After supplying a gas to the cell under conditions such as where the load gradually increases over about 2 hours to 1000 mA / cm ² of target, to secure the load at the time of reaching the 1000 mA / cm ² of target Thereafter, the voltage between the cell terminals after 30 minutes was measured and recorded as “low humidified output”.
The results are shown in Table 3.

  In Examples 1 to 6 and Comparative Examples 1 to 9, a platinum-supported carbon material B obtained by supporting Pt on a support carbon material (support B) defined in the present invention was used, and various non-catalyst-supported carbon materials were used. (Carbon materials a to o) were mixed, and the results of examining the battery characteristics were summarized. The platinum catalysts (Examples 1 to 6) obtained by mixing a, d, g, h, k, and n that satisfy the provisions of the present invention have excellent power generation characteristics under both high and low humidification conditions. It can be seen that On the other hand, it was found that the power generation characteristics of a platinum catalyst mixed with a non-catalyst-supported carbon material that does not satisfy the provisions of the present invention are low.

  In Examples 7 to 11 and Comparative Examples 10 to 19, the catalyst-unsupported carbon material d satisfying the provisions of the present invention and various support carbon materials (supports A and C to Q) were used. The result of having examined the electric power generation characteristic of the platinum catalyst obtained by mixing each of platinum carrying | support carbon material A and each of C-Q was put together. The platinum catalysts of Examples 7 to 11 obtained using the carrier carbon materials (carriers C, H, I, L, and M) that satisfy the provisions of the present invention exhibited excellent high and low humidification characteristics. Is a platinum catalyst of Comparative Examples 10 to 19 using a carbon support material (supports A, D, E, F, G, J, K, L, M, N, O, and Q) that does not satisfy the provisions of the present invention Was found to have low power generation characteristics.

  Further, in Examples 12 to 17 and Examples 18 to 22, the carrier carbon materials (supports C, H, I, L, M, and P) that satisfy the provisions of the present invention used when preparing the platinum-supported carbon material were used. ) And the average particle size of the non-catalyst-supported carbon material h or n that satisfies the provisions of the present invention were examined. In the case of Examples 15 to 17, 20, and 21 in which the particle diameter of the support at the time of preparing the platinum-supporting carbon material is out of the range of ± 50 nm from the particle diameter of the non-catalyst-supported carbon material, power generation characteristics tend to decrease. It was seen.

5. Examination of blending ratio of catalytic metal-supporting carbon material and non-catalytic metal-supporting carbon material [Comparative Example 20 and Examples 23 to 31]
The platinum-supporting carbon material B is used as the catalytic metal-supporting carbon material, and the carbon material a is used as the non-catalytic metal-supporting carbon material. The blending ratios of these platinum-supporting carbon material B and carbon material a shown in Table 4 ( The ratio of the carbon material a) was mixed and mixed in the same manner as described above to obtain platinum catalysts for solid polymer fuel cells of Examples 23 to 31. For reference, the case where the ratio of the carbon material a was 0 mass% was referred to as Comparative Example 20.

Using the obtained platinum catalysts of Examples 23 to 31 and Comparative Example 20, as in the case of Examples 1 to 22 and Comparative Examples 1 to 19 above, In accordance with the evaluation of battery performance, a catalyst layer ink solution was prepared, cathode and anode catalyst layers were prepared, a membrane electrode assembly (MEA) was prepared, and battery performance was evaluated.
The results are shown in Table 4.

  As is clear from the results of Examples 23 to 31 shown in Table 4, when the ratio of the carbon material a is 1 to 40% by mass, excellent power generation characteristics were exhibited under both high humidification conditions and humidification resistance conditions. .

Claims (5)

A catalytic metal-supporting carbon material in which a catalytic metal component is supported on a support carbon material made of a porous carbon material and a non-catalytic metal-supporting carbon material made of a dendritic graphitic carbon material that does not support the catalytic metal component are mixed. A catalyst obtained by
The support carbon material of the catalyst-supported carbon material has a mesopore specific surface area (S _{4-10 nm} ) of 4 to 10 nm as measured by nitrogen adsorption measurement of 100 m ² / g or more,
The dendritic graphitic carbon material which is the catalyst metal non-supporting carbon material has a BET specific surface area (S _BET ) of 80 m ² / g or more and 220 m ² / g or less, and a DBP oil absorption (O _DBP ) of 80 mL / 100 g or more. A catalyst for a polymer electrolyte fuel cell, which is 170 mL / 100 g or less and has a crystallite size (Lc) by X-ray diffraction of 5 nm or more and 10 nm or less.   The average particle diameter of the support carbon material of the catalyst metal-supported carbon material is 10 nm or more and 100 nm or less, and the average particle diameter of the catalyst metal non-support carbon material is within a range of ± 50 nm of the average particle diameter of the support carbon material. 2. The catalyst for a polymer electrolyte fuel cell according to claim 1, wherein the catalyst is 10 nm or more and 50 nm or less.   The mixing ratio of the catalytic metal-supporting carbon material and the catalytic metal-nonsupporting carbon material is 60% by mass or more and 99% by mass or less of the catalytic metal-supporting carbon material, and 1% by mass or more of the catalytic metal-nonsupporting carbon material. The catalyst for a polymer electrolyte fuel cell according to claim 1 or 2, wherein the catalyst is in mass% or less. The dendritic graphitic carbon material which is the catalyst metal non-supporting carbon material has a BET specific surface area (S _BET ) of 90 m ² / g to 200 m ² / g and a DBP oil absorption (O _DBP ) of 100 mL. The solid polymer fuel cell catalyst according to any one of claims 1 to 3, wherein the catalyst is / 100g or more and 150mL / 100g or less.   The dendritic graphitic carbon material, which is the catalyst metal non-supporting carbon material, has a crystallite size (Lc) by X-ray diffraction of 6 nm or more and 10 nm or less. Catalyst for polymer electrolyte fuel cell.