JP2006331845A - Catalyst powder for polymer electrolyte fuel cell and its manufacturing method, and electrode for polymer electrolyte fuel cell containing catalyst powder - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance the output performance and durability performance of a polymer electrolyte fuel cell (PEFC) equipped with an ultra-low platinum loading electrode (ULPLE) by controlling the water content of cation exchange resin in the catalyst powder contained in the ULPLE. <P>SOLUTION: In the catalyst powder for the polymer electrolyte fuel cell in which the catalyst metal is mainly carried on the contact surface of a proton conductive passage of the cation exchange resin and the surface of carbon, moisture percentage of the cation exchange resin in saturated steam atmosphere at 80°C is set to 18.5 mass% or lower. The electrode for the polymer electrolyte fuel cell contains the catalyst powder for the polymer electrolyte fuel cell, and also contains fluorine resin, and the polymer electrolyte fuel cell is equipped with the electrode for the polymer electrolyte fuel cell. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a catalyst powder for a polymer electrolyte fuel cell, a method for producing the same, and an electrode for a polymer electrolyte fuel cell containing the catalyst powder.

  A single cell of a polymer electrolyte fuel cell (PEFC) has a structure in which a membrane / electrode assembly is sandwiched between a pair of gas flow plates. The membrane / electrode assembly is obtained by bonding an anode to one surface of a cation exchange membrane and a cathode to the other surface. A gas flow path is processed in the gas flow plate. For example, electric power can be obtained by supplying hydrogen as fuel to the anode and oxygen as oxidant to the cathode. In the anode and cathode, the following electrochemical reaction proceeds.

Anod: 2H ₂ → 4H ⁺ + 4e ⁻
Cathode: O ₂ + 4H ⁺ + 4e ⁻ → H ₂ O
The electrochemical reaction as described above is performed at an interface where a reactant such as an oxidant or a fuel, protons (H ⁺ ), and electrons (e ⁻ ) exist (hereinafter, this interface is referred to as a reaction interface). proceed.

  The electrode of the polymer electrolyte fuel cell includes a catalyst layer containing a catalyst metal and a conductive porous body. As the conductive porous body, porous carbon paper, carbon felt, carbon cloth or the like imparted with water repellency is used. The catalyst layer of the electrode contains a cation exchange resin that is a polymer electrolyte, carbon, and a catalyst metal.

  As disclosed in Patent Document 1, a method for producing a membrane / electrode assembly in a conventional polymer electrolyte fuel cell is a catalyst-polymer composition in which a cation exchange resin solution (Nafion solution) and platinum-supported carbon are mixed. The product is applied to a water-repellent electrode substrate (carbon paper) and dried to prepare an electrode substrate with an electrode catalyst layer, and a cation exchange membrane is placed between the two electrode substrates with an electrode catalyst layer. The electrode catalyst layer was sandwiched so as to be in contact with the cation exchange membrane and integrated by hot pressing.

  In a conventional polymer electrolyte fuel cell, Patent Document 2 discloses that in a polymer electrolyte fuel cell, protons move in a hydrated state when moving through an ion exchange membrane from an anode toward a cathode. Near the anode, the water content decreases and the ion exchange membrane dries. Therefore, if water is not supplied near the anode, it becomes difficult to move protons. In addition, when air is used as the oxidizer, the air in the ion exchange membrane is sent to the air several times the theoretical consumption, so that the moisture in the ion exchange membrane is taken out into the air, so that the membrane does not dry out. A specific amount of water vapor is mixed into the fuel gas or oxidant gas and sent to the fuel cell to saturate the solid polymer electrolyte membrane so that the specific resistance of the solid polymer electrolyte membrane is 20 Ω · cm or less at room temperature. Technology is disclosed.

  Further, in Patent Document 3, the solid polymer electrolyte membrane includes water in its inside, and functions as an electrolyte, and at the same time, has a function of preventing cross leak in which fuel gas and oxidant gas are mixed with each other. In a polymer electrolyte fuel cell, when dry reaction gas is supplied, the polymer membrane is dried, and the internal resistance is increased due to the decrease in ionic conductivity. A technique for supplying humidified fuel gas and oxidant gas is disclosed.

  In the conventional polymer electrolyte fuel cell, Patent Document 4 describes that the water content of the ion exchange membrane after the swelling treatment with water is preferably 40% or more and 300% or less. 5 describes that the water content of the ion exchange membrane can be arbitrarily controlled at 10 to 150 wt%, and the example of Patent Document 6 describes using an ion exchange membrane with a water content of 30 to 45 wt%. ing.

  These conventional polymer electrolyte fuel cell electrodes are produced, for example, by applying a mixture of carbon powder supporting a catalytic metal and a solution of a cation exchange resin to a cation exchange membrane or a conductive porous body. Therefore, it is shown in Non-Patent Document 1 that the utilization rate of the catalyst metal is as low as about 10% because the catalyst metal is present at other than the reaction interface.

  Patent Document 7 discloses a polymer electrolyte fuel cell electrode including a catalyst powder in which a catalyst metal is mainly supported on a contact surface between a proton conduction path of a cation exchange resin and a carbon surface. Non-patent document 2 shows that the utilization rate of the catalyst metal of the electrode is extremely high. Hereinafter, the electrode is referred to as an ultra-small catalyst supporting electrode (hereinafter referred to as “ULPLE”).

  In ULPLE, since the catalyst metal is mainly supported on the contact surface between the proton conduction path of the cation exchange resin and the surface of the carbon, the output performance is remarkably deteriorated due to the accumulation of excess water in the catalyst layer. It was a problem to be strongly affected by so-called flooding. With respect to this problem, Patent Document 8 discloses a technique for suppressing the retention of water by adding a water-repellent material such as a fluororesin to the catalyst layer.

JP 2002-093424 A Japanese Patent Laid-Open No. 05-047394 JP 07-288134 A Japanese Patent Application Laid-Open No. 06-251782 JP 2001-348439 A JP 2003-077492 A JP 2000-012041 A Japanese Patent Application No. 2005-030949 Edson A. Ticianelli, J. et al. Electroanal. Chem, 251, 275 (1988) Shuji Hitomi et al., 40th Battery Discussion Meeting Abstract, 2B15, P167-168, (1999)

  Even when a water repellent material was added to ULPLE, a phenomenon was observed in which the battery voltage gradually decreased when a PEFC equipped with the electrode was continuously operated for a long time. One reason for this phenomenon is that the catalytic metal is mainly supported on the contact surface between the proton conduction path of the cation exchange resin and the surface of the carbon. The results of the study revealed that the amount of reactant supplied increased or decreased depending on the water content of the resin. That is, in order to improve the output performance and durability performance of the PEFC provided with this electrode, there has been a problem that it is necessary to control the water content of the cation exchange resin contained in the electrode.

  However, in the conventional polymer electrolyte fuel cell electrodes described in Patent Documents 4 to 6, there is a description of the water content of the cation exchange membrane, but the cation exchange membrane contained in the catalyst layer of the electrode has a description. The water content was not examined.

  Further, in the catalyst layer of the polymer electrolyte fuel cell electrode containing ULPLE described in Patent Document 7 and Patent Document 8, the water content of the cation exchange membrane contained in ULPLE has not been studied.

  Therefore, an object of the present invention is to improve the output performance and durability performance of the PEFC equipped with ULPLE by controlling the water content of the cation exchange resin in the catalyst powder contained in ULPLE within an optimal range.

  The invention according to claim 1 is the catalyst powder for a polymer electrolyte fuel cell in which the catalyst metal is mainly supported on the contact surface between the proton conduction path of the cation exchange resin and the carbon surface. The water content in a saturated water vapor atmosphere at 1 ° C. is 18.5% by mass or less.

  Invention of Claim 2 uses the cation exchange resin whose water content in a 80 degreeC saturated water vapor atmosphere is 18.5 mass% or less in the manufacturing method of the said catalyst powder for polymer electrolyte fuel cells, The said cation A first step of mixing and drying a solution of an exchange resin and carbon to obtain a mixture of the cation exchange resin and carbon; and a step of adsorbing a cation of a catalytic metal on fixed ions of the cation exchange resin. The second step and the third step of obtaining a powder containing the catalytic metal by chemically reducing the cation of the catalytic metal are characterized.

  According to a third aspect of the present invention, there is provided an electrode for a polymer electrolyte fuel cell, comprising the catalyst powder for a polymer electrolyte fuel cell according to the first aspect.

  According to a fourth aspect of the present invention, the electrode for a polymer electrolyte fuel cell further includes a water repellent resin having no ion exchange group.

  In the catalyst powder in which the catalyst metal of the present invention is mainly supported on the contact surface between the proton conduction path of the cation exchange resin and the carbon surface, the water content in an 80 ° C. saturated steam atmosphere is 18.5% by mass or less. By using a certain cation exchange resin, the water content of the cation exchange resin is reduced, so that an excessive increase in the volume of the cation exchange resin can be prevented. Therefore, in ULPLE containing the catalyst powder of this invention, the increase in the oxygen diffusion distance resulting from the volume increase of a cation exchange resin can be suppressed. As a result, it became possible to improve the output performance and durability performance of PEFC equipped with ULPLE of this invention.

  Further, as in the invention of claim 3, in the PEFC using the electrode for the polymer electrolyte fuel cell containing the catalyst powder of claim 1 and the water repellent resin having no ion exchange group, Since the water repellency of the catalyst layer is maintained, the durability performance is further improved.

  FIG. 1 shows a schematic diagram of a catalyst powder in which a catalyst metal is supported mainly on a contact surface between a proton conduction path of a cation exchange resin and a carbon surface. In FIG. 1, 11 is a carbon particle, 12 is a cation exchange resin, 13 is a proton conduction path (hydrophilic region) of the cation exchange resin, 14 is a hydrophobic region of the cation exchange resin, and 15 is a catalyst metal.

  In FIG. 1, the surfaces of carbon particles 11 are covered with a cation exchange resin 12. The cation exchange resin 12 includes a proton conduction path (hydrophilic region) 13 and a hydrophobic region 14. A catalytic metal 15 is mainly supported on the contact surface between the proton conduction path 13 of the cation exchange resin and the surface of the carbon particles 11.

For example, cation exchange resins such as perfluorocarbon sulfonic acid resins are described in H.C. L. As described in the report of Yeager et al. (J. Electrochem. Soc., 128 , 1880, (1981)) and Kukumi et al. (J. Electrochem. Soc., 132 , 2601 (1985)), the main chain is assembled. It is known that the structure is microphase-separated into a hydrophobic region and a hydrophilic region in which side chains are assembled.

  Since the hydrophobic region of the cation exchange resin has a structure similar to polytetrafluoroethylene, there is significantly less permeation of reactants and water. On the other hand, in the proton conduction path of the cation exchange resin, the ion exchange group bonded to the tip of the side chain forms a cluster, and water is taken into the cluster, so that the counter ion can move. Become. That is, water, protons and reactants (hydrogen or oxygen) can move through the proton conduction path.

  Accordingly, since the catalytic metal 15 supported on the contact surface between the proton conduction path 13 of the cation exchange resin 12 and the surface of the carbon particles 11 can form a reaction interface, it is active against an electrochemical reaction. is there. On the other hand, although not shown in FIG. 1, the catalytic metal present on the surface of the hydrophobic region 14 of the cation exchange resin 11 and the carbon particles 11 does not supply reactants and protons. Inactive to the reaction. In an electrode using a conventional catalyst, there are many such inert catalytic metals.

  It is essential that the catalytic metal contained in the catalyst powder of the present invention is mainly supported on the contact surface between the carbon and the proton conduction path of the cation exchange resin. Since the contact surface between the carbon and the proton conduction path of the cation exchange resin is a place where electrons and protons can be exchanged simultaneously, the catalytic metal supported on this contact surface is involved in the electrode reaction. Therefore, the amount of catalyst metal used can be reduced by increasing the ratio of the catalyst metal supported on the contact surface.

  In the catalyst layer of the PEFC electrode of the present invention, “the catalyst metal is mainly provided on the contact surface between the proton conduction path of the cation exchange resin and the carbon” means that the carbon in contact with the proton conduction path of the cation exchange resin It means that the amount of catalyst metal supported on the particle surface is 50% by mass or more of the total amount of catalyst metal supported. That is, 50% by mass or more of the total amount of the catalytic metal supported is a catalytic metal active for the electrode reaction, so that the utilization rate of the catalytic metal is remarkably increased.

  In the present invention, the ratio of the amount of the catalyst metal supported on the surface of the carbon particles in contact with the proton conduction path of the cation exchange resin to the total amount of the catalyst metal supported is preferably as high as possible, particularly exceeding 80% by mass. preferable. In this way, the electrode is highly activated by supporting the catalytic metal at a high rate on the contact surface between the proton conduction path and the carbon particles.

  In the catalyst powder of the present invention, the catalyst metal is mainly supported on the contact surface between the carbon and the proton conduction path of the cation exchange resin. This can be seen from the mass activity in the high current density region of the fuel cell equipped with this powder and the volume ratio between the proton conduction path of the cation exchange resin and the hydrophobic skeleton (M. Komomoto et.al., GS). Yuasa Technical Report, 1, 48 (2004)). This mass activity is obtained by dividing the current density at a certain voltage by the amount of catalyst metal supported per unit area.

  The ULPLE of the present invention has a special structure in which the catalytic metal is mainly supported on the contact surface between the proton conduction path of the cation exchange resin and the surface of the carbon, so that the influence of flooding due to water retention is reduced. Receive strongly. It has become clear from research results that the output performance and durability performance of PEFC with ULPLE are reduced by the flooding.

  In order to suppress flooding, a water repellent material (fluorine resin or the like) was added to ULPLE to obtain an appropriate porous structure, thereby improving its performance. However, when the battery was operated for a long time, the cell voltage gradually decreased. As a result of examining the cause of the decrease in detail, it was revealed that an increase in the water content of the cation exchange resin contained in ULPLE is one factor causing the decrease.

  The present invention is based on the discovery that the water content of the cation exchange resin contained in the ULPLE catalyst significantly affects the output performance and durability performance of the PEFC equipped with ULPLE.

  As the cation exchange resin, a fluororesin resin such as perfluorocarbon sulfonic acid resin is often used because it is chemically stable and has high proton conductivity. The perfluorocarbon sulfonic acid resin has a special structure in which a phase separation into a hydrophobic region in which main chains of fluorocarbons are aggregated and a hydrophilic region in which side chains having sulfone groups are associated.

  The hydrophilic region has a property of taking water from the outside. This water uptake causes protons to ionize from the sulfone group and move through the hydrophilic region. At this time, since the water is swollen by the uptake of the water, the volume of the resin increases. The amount of water taken up tends to increase as the number of sulfone groups contained in the resin increases.

  The amount of sulfone group contained in the perfluorocarbon sulfonic acid resin is indicated by a value called ion exchange capacity. This value is defined as the number of moles of sulfone groups contained per gram of dried resin. That is, by increasing or decreasing the ion exchange capacity, the water content of the ion exchange resin in an 80 ° C. saturated water vapor atmosphere can be controlled.

  There are several commercially available perfluorocarbon sulfonic acid resins, all of which can control the water content of the ion exchange resin in an 80 ° C. saturated water vapor atmosphere by increasing or decreasing the ion exchange capacity. However, the relationship between the ion exchange capacity and the water content of the ion exchange resin differs depending on the type of perfluorocarbon sulfonic acid resin.

  Therefore, in the catalyst powder for a polymer electrolyte fuel cell of the present invention, the catalyst metal is mainly supported on the contact surface between the proton conduction path of the cation exchange resin and the surface of the carbon, and the cation in an 80 ° C. saturated water vapor atmosphere. The water content of the exchange resin is 18.5% by mass or less. Here, “the water content of the cation exchange resin” represents the ratio (mass%) of the water content to the mass of the cation exchange resin in the dry state.

  The preferred electrode of the present invention has a special structure in which the catalytic metal is mainly supported on the contact surface between the proton conduction path of the cation exchange resin and the surface of the carbon. By reducing the water content, an excessive increase in the volume of the cation exchange resin can be prevented, so that an increase in the diffusion distance of oxygen due to the volume increase can be suppressed. As a result of the suppression, the PEFC equipped with the electrode The output performance and durability performance can be improved.

  The reason for the improvement of the output performance and durability performance of PEFC using the electrode containing ULPLE of the present invention can be explained as follows. That is, an important characteristic of the cation exchange resin is that its volume is increased by incorporating water into the proton conduction path of the cation exchange resin. In other words, the thickness of the cation exchange resin that coats the carbon surface increases due to the incorporation of water inside the electrode.

  FIG. 2 shows a schematic diagram of a catalyst powder containing a cation exchange resin having an increased thickness. The symbols in FIG. 2 represent the same as those in FIG. The magnitude of the thickness change depends on the water content of the cation exchange resin. For example, when the water content is high, the thickness of the cation exchange resin increases.

  Reactive substances such as oxygen or hydrogen move from the surface of the cation exchange resin to the catalyst metal, but the moving distance becomes longer as the thickness of the cation exchange increases. As a result, the supply amount of the reactant to the catalyst metal is reduced.

  Conversely, as in the present invention, when the water content of the cation exchange resin in an 80 ° C. saturated water vapor atmosphere is 18.5% by mass or less, excessive swelling of the resin can be suppressed, and Since the thickness can be maintained thin, it is considered that the reactant can be supplied smoothly and the output performance and durability performance of PEFC are improved.

  The cation exchange resin, when containing water, exhibits proton conductivity and can be used as an electrolyte membrane or catalyst powder for a polymer electrolyte fuel cell. When used as an electrolyte membrane, it is necessary to keep the specific resistance at a certain value or higher by saturating the solid polymer electrolyte membrane or the like, but the cation exchange resin in the catalyst powder contains a small amount of water. If proton is contained, proton conduction is possible. For this reason, the lower limit of the water content of the cation exchange resin in the catalyst powder of the present invention is not particularly limited unless it is in a completely dry state.

  Further, the water content of the cation exchange resin can be reduced by reducing the ion exchange capacity of the cation exchange resin, but as a result, the proton conductivity in the catalyst powder is lowered, which is not practical. .

  In the catalyst powder of the present invention, the ratio of the mass of the cation exchange resin to the mass of the catalyst powder is preferably 30% by mass to 50% by mass, more preferably 35% by mass to 45% by mass. It is to be.

  Furthermore, when producing a polymer electrolyte fuel cell electrode using the catalyst powder of the present invention, the electrode may contain only the catalyst powder of the present invention, but a fluororesin having no ion exchange group, etc. The water repellent material is preferably contained, and the ratio of the mass of the water repellent material to the mass of the catalyst powder is preferably 10% by mass or more and 35% by mass or less. As the water repellent material having no ion exchange group, a fluororesin such as polytetrafluoroethylene (PTFE) or hexafluoropropylene (FEP) can be used.

  The solid polymer fuel cell catalyst powder production procedure of the present invention uses a cation exchange resin having a water content of 18.5% by mass or less in an 80 ° C. saturated steam atmosphere, A first step of mixing and drying carbon to obtain a mixture of the cation exchange resin and carbon; a second step of adsorbing a cation of a catalytic metal on fixed ions of the cation exchange resin; A third step of obtaining a powder containing the catalytic metal by chemically reducing the cation of the catalytic metal is provided.

  First, in the first step, after preparing a dispersion containing a cation exchange resin solution and carbon, a solvent is removed from the dispersion to produce a mixture containing the cation exchange resin and carbon. .

  Next, in the second step, after preparing a solution in which a compound containing a catalyst metal cation is dissolved in water or water containing an alcohol, a mixture containing a cation exchange resin and carbon is added to the solution. Immerse and adsorb the catalyst metal cation to the fixed ion of the cation exchange resin, and then wash and dry with deionized water. Finally, in the third step, the catalyst metal is mainly supported on the contact surface between the carbon surface and the proton conduction path of the cation exchange resin by reducing the adsorbed cation of the catalyst metal in a hydrogen atmosphere. A catalyst powder for a polymer electrolyte fuel cell is obtained.

  The preferred conditions in the production process of the polymer electrolyte fuel cell catalyst of the present invention will be described below. First, a slurry-like dispersion is prepared by mixing a cation exchange resin solution and carbon particles. The ion exchange capacity of the cation exchange resin is preferably lower than 1.1 mmol / l.

  The carbon particles may be in the form of powder, granules or fibers, and a mixture thereof may be used. For the carbon particles, it is preferable to use various carbon materials such as carbon black, acetylene black, and furnace black having high electron conductivity and a large surface area.

  The dispersion containing the cation exchange resin solution and the carbon particles is prepared, for example, by adding carbon to the cation exchange resin solution and then stirring, but the mixing method is not particularly limited, and the solution and carbon are not limited. Any method can be used as long as it can be mixed. In this mixing process, the mixed solution and carbon can be more effectively mixed by irradiating the dispersion with ultrasonic waves.

  Next, a mixture containing a cation exchange resin and carbon particles is prepared. The mixture can be prepared by removing the solvent from the dispersion of the cation exchange resin solution and the carbon particles described above. The mixture is obtained in the form of a sheet, lump or powder. When the mixture is in the form of a sheet, it can be obtained by applying a dispersion containing a cation exchange resin solution and carbon to a base material such as a polymer sheet or a metal foil and then drying. When the mixture is in the form of a lump, it can be obtained by drying the dispersion in a container or the like. Further, when the mixture is in the form of powder, it can be obtained by spray-drying the dispersion, or by pulverizing a sheet-like or lump-like mixture.

  In the second step, a catalytic metal is imparted to the mixture containing the cation exchange resin and carbon. In this step, first, a solution in which a compound containing a catalyst metal cation is dissolved in water or water containing an alcohol is prepared. As the compound, a cation containing a platinum group metal or a complex ion of a platinum group metal can be used.

For example, as the complex ions, [Pt (NH ₃ ) ₄ ] ²⁺ and [Pt (NH ₃ ) ₆ ] ⁴⁺ can be represented as platinum ammine complex ions, or [Ru (NH ₃ ) ₆ ] ²⁺ and [Pt Ru (NH ₃ ) ₆ ] ³⁺ is preferred. Alternatively, in addition to the ammine complex ion, a complex ion of a platinum group metal coordinated with a nitrate group or a nitroso group can be used.

  Next, after immersing a mixture containing a cation exchange resin and carbon particles in a solution containing a compound containing a cation of a catalytic metal, the mixture is washed with deionized water and dried.

  By this immersion, the cation of the catalytic metal is adsorbed on the fixed ions of the cation exchange resin contained in the mixture by an ion exchange reaction. At that time, by using two or more types of cations to be ion-exchanged, two or more types of catalytic metal cations can be adsorbed to the mixture.

  By washing the mixture with deionized water, other than the cations adsorbed on the cation exchange resin contained in the mixture are removed. The cations to be removed include those adsorbed on carbon particles, for example.

  Finally, in the third step, the cation adsorbed on the cation exchange resin is chemically reduced. Since the reduction is suitable for mass production, a method using a reducing agent is preferable. In particular, a method in which gas phase reduction is performed with hydrogen gas or a gas containing hydrogen or a method in which gas phase reduction is performed with an inert gas containing hydrazine is preferable. Here, the gas containing hydrogen gas is preferably a mixed gas of hydrogen gas and an inert gas such as nitrogen, helium, or argon, and preferably contains 10 vol% or more of hydrogen gas.

[Examples 1 to 4 and Comparative Examples 1 and 2]
Hereinafter, specific examples of the present invention will be described using preferred embodiments.

[Example 1]
First, a catalyst powder in which a catalytic metal is mainly supported on the contact surface between the proton conduction path of the cation exchange resin and the carbon surface is prepared, and then the electrode containing the catalyst powder is bonded to the cation exchange membrane. A solid polymer fuel cell equipped with this membrane / electrode assembly was further evaluated. The specific procedure is as follows.

  The procedure for producing the catalyst powder is as follows. After mixing 320 g of a cation exchange resin solution A (Nafion 5 mass% solution, manufactured by Aldrich Chemical) having an ion exchange capacity of 0.92 mmol / g, 160 g of ethanol and 160 g of deionized water, carbon black (Vulcan) was added to the solution. XC-72, manufactured by Cabot Corporation) is added and mixed with a vacuum mixer to prepare a dispersion. Further, the dispersion is irradiated with ultrasonic waves using an ultrasonic irradiation device, and then the dispersion is spray-dried. By doing so, a mixture containing carbon black and a cation exchange resin was obtained.

Next, 35 g of the mixture was immersed in 250 ml of a 50 mmol / l concentration [Pt (NH ₃ ) ₄ ] Cl ₂ aqueous solution for 6 hours or more, so that the platinum ammine complex was immobilized on the fixed ion of the cation exchange resin contained in the mixture. Cations [Pt (NH ₃ ) ₄ ] ²⁺ were adsorbed, and then the mixture was thoroughly washed with deionized water and then dried.

Finally, the mixture is placed in a reducer and held in a hydrogen atmosphere at 150 ° C. for 6 hours to reduce the cation [Pt (NH ₃ ) ₄ ] ²⁺ to the catalytic metal (Pt), A catalyst powder in which a catalytic metal was mainly supported on the contact surface between the proton conduction path of the cation exchange resin and the carbon surface was obtained. In this catalyst powder, the weight of platinum supported was about 4.7% by mass with respect to the weight of carbon black.

  Next, a conductive porous layer having an electron conductive microporous layer on one surface of carbon paper (360 μm thick) was manufactured. The manufacturing procedure is as follows. After immersing the carbon paper in a PTFE dispersion having a solid content of 10% by mass, the excess dispersion was removed with a filter paper, followed by drying for 2 hours at room temperature and 1 hour using a dryer at 80 ° C. The carbon paper was coated and dried with a paste-like mixture for the electron conductive microporous layer, and then dried with a dryer at 350 ° C. for 1 hour. A mixture of 200 g of water, 15 g of carbon black (Vulcan XC-72, manufactured by Cabot) and 16.7 g of a PTFE dispersion having a solid content of 60% by mass was used as the mixture for the electron conductive microporous layer. .

  Further, an electrode was manufactured by forming a catalyst layer on the conductive porous layer. The manufacturing procedure is as follows. After applying and drying the paste-like mixture for the catalyst layer on the surface of the conductive porous body layer on which the electron conductive microporous layer was formed, it was vacuum dried at 100 ° C. for 2 hours.

The catalyst layer mixture was prepared by mixing 15 g of catalyst powder and 32 g of 2-methylethylpyrrolidone (NMP) in a twin-screw rotary mixer for 30 minutes. The obtained catalyst layer had a platinum loading of 0.06 mg / cm ² , and the electrode provided with this catalyst layer had a size of 25 cm ² and a thickness of 460 μm.

  Finally, a membrane / electrode assembly (MEA) was manufactured by heat-pressing the electrode and the cation exchange membrane. The joining procedure is as follows. First, after stacking in the order of electrode / cation exchange membrane / electrode, the laminate was pressed with a flat press and further heated to 145 ° C. and held for 5 minutes. The same specification manufactured in Example 1 was used for each electrode. As the cation exchange membrane, a Nafion 115 membrane manufactured by DuPont was used. The membrane was pretreated by boiling with dilute sulfuric acid at a concentration of 0.5 mol / l for 1 hour and then washing 5 times with deionized water. This MEA was used as the MEA of Example 1, and a polymer fuel cell (PEFC) of Example 1 having the MEA was manufactured.

  The water content of the cation exchange resin in the catalyst powder composed of the catalyst metal, carbon black, and cation exchange resin was measured at 70 ° C. and 80 ° C. saturated water vapor pressure, respectively. This measurement was performed according to the following procedure. First, the catalyst powder whose cation exchange resin content was known was vacuum-dried at 80 ° C. for 2 hours, and then the mass of the dried catalyst powder was measured. Next, after holding the catalyst powder in a saturated steam atmosphere at 70 ° C. or 80 ° C. for 12 hours or more, the mass of the catalyst powder in the water-containing state is measured, and the water content is determined from the difference between the mass in the water-containing state and the mass in the dry state. Asked. From the water content and the mass of the cation exchange resin contained in the catalyst powder, the water content of the cation exchange resin in a saturated steam atmosphere at 70 ° C. and 80 ° C. was determined. As a result, the moisture content in the 70 ° C. saturated steam atmosphere (the ratio of the moisture content to the mass of the cation exchange resin) is 11.0% by mass, and the moisture content in the 80 ° C. saturated steam atmosphere is 14.0% by mass. there were.

[Example 2]
For the catalyst layer mixture, 15 g of the catalyst powder prepared in Example 1 and 32 g of 2-methylethylpyrrolidone (NMP) were mixed for 30 minutes with a twin-screw rotary mixer, and then the mixture had a solid content of 60% by mass. An electrode of Example 2 was produced in the same manner as Example 1 except that 6.4 g of PTFE dispersion was added and then mixed for 30 minutes with a twin-screw rotary mixer. The weight ratio of PTFE to the total weight of the electrode was 20% by mass. The platinum carrying amount of the catalyst layer and the size and thickness of the electrode were the same as in Example 1.

  Next, in the same manner as in Example 1, a membrane / electrode assembly (MEA) was manufactured by heating and pressing the electrode and the cation exchange membrane. This MEA was used as the MEA in Example 2, and the MEA was provided. A polymer fuel cell (PEFC) of Example 2 was produced.

  The catalyst powder used in Example 2 had a moisture content of 11.0% by mass in a 70 ° C. saturated steam atmosphere, and a moisture content of 14.0% by mass in an 80 ° C. saturated steam atmosphere.

[Example 3]
An MEA of Example 3 was produced under the same conditions as in Example 2 except that the cation exchange resin solution B (concentration 5 mass%) having an ion exchange capacity of 0.99 mmol / g was used. The platinum loading, size and thickness of the electrode were 0.06 mg / cm ² , 25 cm ² and 460 μm, respectively. A PEFC of Example 3 having the MEA was produced.

  In the catalyst powder used in Example 3, the moisture content in a 70 ° C. saturated steam atmosphere was 12.9 mass%, and the moisture content in an 80 ° C. saturated steam atmosphere was 15.8 mass%.

[Example 4]
An MEA of Example 4 was produced under the same conditions as in Example 2 except that the cation exchange resin solution C (concentration 5 mass%) having an ion exchange capacity of 1.11 mmol / g was used. The platinum loading, size and thickness of the electrode were 0.06 mg / cm ² , 25 cm ² and 460 μm, respectively. A PEFC of Example 4 having the MEA was produced.

  In the catalyst powder used in Example 4, the moisture content in a 70 ° C. saturated steam atmosphere was 15.8 mass%, and the moisture content in an 80 ° C. saturated steam atmosphere was 18.5 mass%.

[Comparative Example 1]
An MEA of Comparative Example 1 was produced under the same conditions as in Example 2 except that the cation exchange resin solution D (concentration: 5% by mass) having an ion exchange capacity of 1.22 mmol / g was used. The platinum loading, size and thickness of the electrode were 0.06 mg / cm ² , 25 cm ² and 460 μm, respectively. A PEFC of Comparative Example 1 having the MEA was produced.

  The catalyst powder used in Comparative Example 1 had a moisture content of 18.5% by mass in a 70 ° C. saturated steam atmosphere, and a moisture content of 20.2% by mass in an 80 ° C. saturated steam atmosphere.

[Comparative Example 2]
An MEA of Comparative Example 2 was produced under the same conditions as in Example 2 except that the cation exchange resin solution E (concentration 5 mass%) having an ion exchange capacity of 1.37 mmol / g was used. The platinum loading, size and thickness of the electrode were 0.06 mg / cm ² , 25 cm ² and 460 μm, respectively. A PEFC of Comparative Example 2 having the MEA was produced.

  The catalyst powder used in Comparative Example 2 had a moisture content of 21.1% by mass in a 70 ° C. saturated steam atmosphere, and a moisture content of 22.8% by mass in an 80 ° C. saturated steam atmosphere.

[Continuous operation test]
About PEFC of Example 1 and Example 2, the continuous driving | running test was done on the following conditions. The cell temperature was set to 70 ° C., air and hydrogen humidified by a bubbler humidifier at 70 ° C. were supplied to the cathode and the cathode at a flow rate of a stoichiometric ratio of 2.5 and 1.25, respectively. Continuous operation was performed at a density of 300 mA / cm ² , and the PEFC voltage was measured.

  The measurement results are shown in FIG. In FIG. 3, curve 1 shows the time-dependent change of the battery voltage of PEFC of Example 1, and curve 2 shows the time-dependent change of the battery voltage of PEFC of Example 2. From FIG. 3, it was found that the battery voltage of the PEFC of Example 2 was stable for a long time compared to the PEFC of Example 1.

  The reason is that in the PEFC in which the electrode for the polymer electrolyte fuel cell includes the catalyst powder of the present invention and PTFE (water repellent resin having no ion exchange group), the water repellency of the electrode catalyst layer is maintained for a long time. Therefore, it is considered that the durability performance is further improved.

  It has been confirmed that the same durability performance is improved when FEP is used instead of PTFE.

[Relationship between water content and volume of cation exchange resin in catalyst powder]
FIG. 4 shows the relationship between the cation exchange capacity and the water content of the cation exchange resin in the catalyst powder held in a 70 ° C. and 80 ° C. saturated water vapor atmosphere. FIG. 4 shows that the water content increases as the ion exchange capacity increases.

  Next, the relationship between the volume of the cation exchange resin in a water-containing state and the water content was determined. The measuring method is as follows. A cast film having a size of 20 mm × 20 mm was prepared from the cation exchange resin solution, and the thickness of the cast film in a dry state was first measured. Then, the cast film was placed in a saturated water vapor pressure atmosphere at 70 ° C. and 80 ° C. After maintaining for more than an hour, the thickness of the hydrous cast film was measured.

  FIG. 5 shows the relationship between the water content of the cast membrane and the volume in the water-containing state. The vertical axis in FIG. 5 represents the ratio of the volume of the cast membrane in the dry state with the volume of the cast membrane in the dry state as “1”. In FIG. 5, the symbol ◯ indicates a value at 70 ° C., and the symbol Δ indicates a value at 80 ° C. FIG. 5 shows that the volume of the cast membrane increases as the water content increases. From FIG. 5, the relationship between the water content of the cation exchange resin and the volume became clear.

[Relationship between moisture content of cation exchange resin in catalyst powder and characteristics of PEFC]
The output performances of the PEFCs of Examples 2 to 4 and Comparative Examples 1 and 2 were evaluated. By this evaluation, the influence of the water content of the cation exchange resin contained in the ultra-small platinum-supported electrode on the output performance of PEFC was investigated. The evaluation condition is that the battery temperature is 70 ° C., and air and hydrogen humidified by a bubbler humidifier at 70 ° C. are supplied to the cathode and the cathode at a flow rate of a stoichiometric ratio of 2.5 and 1.25, respectively. . The value of the battery voltage at a current density of 300 mA / cm ² when each PEFC was operated for 1 hour under these conditions was measured. Further, after changing the battery temperature to 80 ° C. and the bubbler humidifier temperature to 80 ° C., the same measurement was performed.

  FIG. 6 shows the relationship between the water content of the cation exchange resin contained in the electrode in a 70 ° C. saturated steam atmosphere and the battery voltage, and FIG. 7 shows the same relationship in the 80 ° C. saturated steam atmosphere. FIG. 6 shows that the battery voltage is a substantially constant value regardless of the water content of the cation exchange resin. From FIG. 7, the water content of the cation exchange resin increases as the temperature of the battery and the humidifier increases. Increased, but the battery voltage was found to be nearly constant. That is, it was found that the battery voltage immediately after the start of power generation is not affected by the moisture content of the cation exchange resin.

  Next, FIG. 8 shows the relationship between the water content of the cation exchange resin contained in the electrode and the battery voltage after 100 hours and 3000 hours under the conditions of a battery temperature of 70 ° C. and a bubbler humidifier temperature of 70 ° C. In FIG. 8, symbol Δ indicates the battery voltage after 100 hours, and symbol □ indicates the battery voltage after 3000 hours. FIG. 8 shows that when the water content is 18.5% by mass or less, the decrease in battery voltage is very small even after 3000 hours.

  Further, FIG. 9 shows the relationship between the water content of the cation exchange resin contained in the electrode and the battery voltage after 100 hours and 3000 hours under the conditions of a battery temperature of 80 ° C. and a bubbler humidifier temperature of 80 ° C. In FIG. 9, symbol Δ indicates the battery voltage after 100 hours, and symbol □ indicates the battery voltage after 3000 hours. From FIG. 9, when the water content of the cation exchange resin contained in the electrode exceeds 18.5% by mass, the battery voltage after 100 hours tends to decrease, and the battery voltage significantly decreases after 3000 hours. I understood it.

  In other words, in a polymer electrolyte fuel cell having an electrode containing a cation exchange resin having a water content of 18.5% by mass or less in a 70 ° C. or 80 ° C. saturated water vapor atmosphere, the cell voltage when operated continuously for a long time. It can be seen that the decrease in the is suppressed. The suppression of the decrease in the battery voltage seems to be due to the suppression of the decrease in the oxygen supply amount accompanying the swelling of the cation exchange resin.

  That is, when the water content of the cation exchange resin is high, the thickness of the cation exchange resin film covering the carbon increases due to the water content, so that the distance that oxygen diffuses to the catalyst metal becomes long. Because of this extension, the amount of oxygen supplied to the catalytic metal per hour is reduced. Conversely, when the water content is small, the thickness of the resin due to the water content is kept thin, so that the supply amount is maintained in a sufficient state.

  Furthermore, in the electrode of the fuel cell in operation, it is surmised that the increase in the film thickness accompanying the improvement of the water content gradually proceeds with time. In other words, the ultra-small platinum-supported electrode has a special structure in which the catalytic metal is mainly supported on the contact surface between the proton conduction path of the cation exchange resin and the surface of the carbon. The decrease in the oxygen supply amount due to the increase in thickness due to the swelling of the exchange resin causes not only the output performance of the fuel cell including the electrode including the catalyst but also the durability performance to be significantly lowered.

  However, by suppressing the water content of the cation exchange resin contained in the electrode in an 80 ° C. saturated water vapor atmosphere to 18.5% by mass or less, the output performance and durability performance of the polymer electrolyte fuel cell equipped with the electrode can be reduced. It was confirmed that it could be improved.

[Examples 5 to 7 and Comparative Examples 3 and 4]
Next, the mixing ratio of the water-repellent resin having no ion exchange group in the polymer electrolyte fuel cell positive electrode was examined.

[Example 5]
For the catalyst layer mixture, 15 g of the catalyst powder prepared in Example 1 and 32 g of 2-methylethylpyrrolidone (NMP) were mixed for 30 minutes with a twin-screw rotary mixer, and then the mixture had a solid content of 60% by mass. An electrode of Example 5 was produced in the same manner as Example 2 except that 2.8 g of PTFE dispersion was added and then mixed by a twin-screw rotary mixer for 30 minutes. The weight ratio of PTFE to the total weight of the electrode was 10% by mass. The amount of platinum supported on the catalyst layer and the size and thickness of the electrode were the same as in Example 1.

  Next, in the same manner as in Example 2, a membrane / electrode assembly (MEA) was manufactured by heating and pressing the electrode and the cation exchange membrane. This MEA was used as the MEA of Example 5, and the MEA was provided. A polymer fuel cell (PEFC) of Example 5 was produced.

  The catalyst powder used in Example 5 had a moisture content of 11.0% by mass in a 70 ° C. saturated steam atmosphere, and a moisture content of 14.0% by mass in an 80 ° C. saturated steam atmosphere.

[Example 6]
An electrode of Example 6 was produced in the same manner as Example 5 except that 10.7 g of PTFE dispersion having a solid content of 60% by mass was added to the catalyst layer mixture. The weight ratio of PTFE to the total weight of the electrode was 30% by mass.

  Next, in the same manner as in Example 5, a membrane / electrode assembly (MEA) was manufactured by heating and pressing the electrode and the cation exchange membrane. This MEA was used as the MEA in Example 6, and the MEA was provided. A polymer fuel cell (PEFC) of Example 6 was produced.

[Example 7]
An electrode of Example 7 was produced in the same manner as Example 5 except that 13.5 g of PTFE dispersion having a solid content of 60% by mass was added to the catalyst layer mixture. The weight ratio of PTFE to the total weight of the electrode was 35% by mass.

  Next, in the same manner as in Example 5, a membrane / electrode assembly (MEA) was manufactured by heating and pressing the electrode and the cation exchange membrane. This MEA was used as the MEA of Example 7, and the MEA was provided. A polymer fuel cell (PEFC) of Example 7 was produced.

[Comparative Example 3]
An electrode of Example 7 was produced in the same manner as Example 5 except that 1.3 g of PTFE dispersion having a solid content of 60% by mass was added to the catalyst layer mixture. The weight ratio of PTFE to the total weight of the electrode was 5% by mass.

  Next, in the same manner as in Example 5, a membrane / electrode assembly (MEA) is manufactured by heating and pressing the electrode and the cation exchange membrane, and this MEA is used as the MEA of Comparative Example 3, and the comparison including the MEA is performed. A polymer fuel cell (PEFC) of Example 3 was produced.

[Comparative Example 4]
An electrode of Example 7 was produced in the same manner as Example 5 except that 16.7 g of PTFE dispersion having a solid content of 60% by mass was added to the catalyst layer mixture. The weight ratio of PTFE to the total weight of the electrode was 40% by mass.

  Next, in the same manner as in Example 5, a membrane / electrode assembly (MEA) is manufactured by heating and pressing the electrode and the cation exchange membrane, and this MEA is used as the MEA of Comparative Example 4, and the comparison including the MEA is performed. A polymer fuel cell (PEFC) of Example 4 was produced.

For the PEFCs of Examples 5 to 7 and Comparative Examples 3 and 4, the cell temperature was set to 80 ° C., and air and hydrogen humidified by a bubbler humidifier at 80 ° C. were respectively supplied to the cathode and the cathode in a stoichiometric ratio of 2.5 and 1. It was supposed to be supplied at a flow rate of .25. Under this condition, the value of the battery voltage at a current density of 300 mA / cm ² when each PEFC was operated for 1 hour and the battery voltage after 100 hours were measured. The measurement results are summarized in Table 1. Table 1 also shows the results of Example 2 for comparison.

From the results of Table 1, the battery voltage at 300 mA / cm ² when each PEFC was operated for 1 hour showed a high voltage of 0.67 V or more in Examples 2, 5 to 7 and Comparative Example 3, but the comparative example 4 was found to be a low value of 0.66 V or less. This is thought to be because the PTFE content in the electrode was large and the conductivity of the electrode was lowered.

  In addition, in Examples 2, 5 to 7, and Comparative Example 4, the battery voltage after 100 hours was only a few mV lower than that in the first hour, but in Comparative Example 3, it was found that the battery voltage was also reduced by 40 mV. . This is probably because the PTFE content in the electrode is too small, and the water repellency of the electrode was lowered during continuous operation.

[Examples 8 and 9 and Comparative Example 5]
[Example 8]
Instead of cation exchange resin solution A (Nafion 5 mass% solution, manufactured by Aldrich Chemical Co.), the same procedure as in Example 2 was used except that Aciplex having an ion exchange capacity of 0.91 mmol / g was used. An electrode was produced. The weight ratio of PTFE to the total weight of the electrode was 20% by mass. The platinum carrying amount of the catalyst layer and the size and thickness of the electrode were the same as in Example 2.

  Next, in the same manner as in Example 2, a membrane / electrode assembly (MEA) was manufactured by heating and pressing the electrode and the cation exchange membrane, and this MEA was used as the MEA of Example 8 and the MEA was provided. A polymer fuel cell (PEFC) of Example 8 was produced. The water content in the 80 ° C. saturated water vapor atmosphere of the catalyst powder used in Example 8 was 13.9% by mass.

[Example 9]
An MEA of Example 9 was produced under the same conditions as in Example 8 except that Aciplex having an ion exchange capacity of 1.08 mmol / g was used, and a PEFC of Example 9 having the MEA was produced. The water content in the 80 ° C. saturated water vapor atmosphere of the catalyst powder used in Example 9 was 18.5% by mass.

[Comparative Example 5]
A MEA of Comparative Example 5 was manufactured under the same conditions as in Example 8 except that Aciplex having an ion exchange capacity of 1.35 mmol / g was used, and a PEFC of Comparative Example 5 including the MEA was manufactured. The catalyst powder used in Comparative Example 5 had a water content of 22.6% by mass in an 80 ° C. saturated steam atmosphere.

  For the PEFCs of Examples 8 and 9 and Comparative Example 5, the battery voltage after 100 hours was measured under the conditions of a battery temperature of 80 ° C. and a bubbler humidifier temperature of 80 ° C. And the relationship between the moisture content of the cation exchange resin contained in an electrode and the relationship between battery voltage is shown in FIG.

  From FIG. 10, even in the case of using Aciplex as the cation exchange resin, in the case of Examples 8 and 9 where the water content of the cation exchange resin contained in the catalyst powder is 18.5% by mass or less, 100 hours later It was found that the decrease in battery voltage was very small. On the other hand, in the case of Comparative Example 5 in which the water content of the cation exchange resin contained in the catalyst powder was 22.6% by mass, it was found that the battery voltage decreased greatly after 100 hours.

  From this result, it is understood that PEFC having excellent durability performance can be obtained when the water content of the cation exchange resin contained in the catalyst powder is 18.5% by mass or less even when the type of cation exchange resin is different. It was.

The schematic diagram of the catalyst powder by which the catalyst metal was mainly carry | supported by the contact surface of the proton conduction path | route of cation exchange resin, and the surface of carbon. The schematic diagram of the catalyst powder containing the cation exchange resin whose thickness increased by water content. The figure which shows the time-dependent change of a battery voltage in the continuous driving | running test about PEFC of Example 1 and Example 2. FIG. The figure which shows the relationship between the ion exchange capacity | capacitance and the water content of the cation exchange resin in a catalyst powder. The figure which shows the relationship between the water content of a cast film | membrane, and the volume in a water-containing state. The figure which shows the relationship between the moisture content in the 70 degreeC saturated water vapor | steam atmosphere of the cation exchange resin contained in an electrode, and a battery voltage. The figure which shows the relationship between the water content in the 80 degreeC saturated water vapor | steam atmosphere of the cation exchange resin contained in an electrode, and a battery voltage. The figure which shows the relationship between the water content in 70 degreeC saturated water vapor | steam atmosphere of cation exchange resin, and the battery voltage after 100 hours and 3000 hours continuous operation on the conditions of battery temperature 70 degreeC and bubbler type humidifier temperature 70 degreeC. The figure which shows the relationship between the water content in 80 degreeC saturated water vapor | steam atmosphere of a cation exchange resin, and the battery voltage after 100 hours and 3000 hours continuous operation on the conditions of battery temperature 80 degreeC and bubbler humidifier temperature 80 degreeC. The battery voltage after 100 hours of continuous operation under the conditions of the moisture content of the cation exchange resin in Examples 8 and 9 and Comparative Example 5 in an 80 ° C. saturated steam atmosphere, the battery temperature of 80 ° C., and the bubbler humidifier temperature of 80 ° C. FIG.

Explanation of symbols

11 Carbon particles 12 Cation exchange resin 13 Proton conduction path of cation exchange resin (hydrophilic region)
14 Hydrophobic region of cation exchange resin 15 Catalytic metal

Claims (4)

In the catalyst powder for a polymer electrolyte fuel cell in which the catalytic metal is mainly supported on the contact surface between the proton conduction path of the cation exchange resin and the carbon surface, the water content of the cation exchange resin in a saturated steam atmosphere at 80 ° C. A catalyst powder for a polymer electrolyte fuel cell, wherein the rate is 18.5% by mass or less. Using a cation exchange resin having a water content of 18.5% by mass or less in an 80 ° C. saturated water vapor atmosphere, the cation exchange resin solution and carbon are mixed and dried, and the cation exchange resin and carbon are mixed. A first step of obtaining a mixture of: a second step of adsorbing a cation of a catalytic metal on a fixed ion of the cation exchange resin; and a catalytic metal containing a catalytic metal by chemically reducing the cation of the catalytic metal The method for producing a catalyst powder for a polymer electrolyte fuel cell according to claim 1, wherein the method comprises a third step of obtaining a powder. An electrode for a polymer electrolyte fuel cell comprising the catalyst powder for a polymer electrolyte fuel cell according to claim 1. 3. The polymer electrolyte fuel cell electrode according to claim 2, comprising a water repellent resin having no ion exchange group.

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