JP2008218131A - Membrane-electrode assembly for polymer electrolyte fuel cell, and polymer electrolyte fuel cell using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent flooding of a cathode by carrying out moisture management of a membrane-electrode assembly of a PEFC, and to prevent an operation voltage of the PEFC from dropping in continuous operation. <P>SOLUTION: The membrane-electrode assembly for a polymer electrolyte fuel cell includes a catalyst layer containing a cation exchange resin, a carbonaceous material, a water-repellent resin and a catalyst metal in each of an anode and a cathode. When it is assumed that the ratio of the water-repellent resin included in the cathode catalyst layer and the ratio of the water-repellent resin included in the anode catalyst layer are X (mass%) and Y (mass%), respectively, Y/X<1 is satisfied. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a membrane / electrode assembly used in a polymer electrolyte fuel cell and a polymer electrolyte fuel cell using the same.

  A single cell of a polymer electrolyte fuel cell (hereinafter abbreviated as “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, power is obtained by supplying hydrogen as fuel to the anode and oxygen as oxidant to the cathode. In the PEFC anode and cathode, the following electrochemical reaction proceeds.

Anod: 2H ₂ → 4H ⁺ + 4e ⁻ (1)
Cathode: O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O (2)
The above-described electrochemical reaction proceeds at a region where hydrogen or oxygen and protons (H ⁺ ) are transmitted and an interface with the catalyst (hereinafter, this interface is referred to as a “reaction interface”). Since the catalyst is in contact with the electron conductive member, the electrons (e ⁻ ) are collected through the member.

  Conventionally, a mixture of a catalyst-supporting carbon powder in which a catalyst metal such as platinum is supported on carbon and a cation exchange resin has been used for a catalyst layer of a catalyst electrode of PEFC.

  Recently, the catalyst-supported powder includes a cation exchange resin, a carbonaceous material, and a catalyst metal, and 50% by mass or more of the catalyst metal is in contact with the proton conduction path of the cation exchange resin and the surface of the carbonaceous material. What was carried on the Since this catalyst-supported powder has a high utilization rate of the catalyst metal, it is possible to reduce the amount of the metal used, which is effective in reducing the cost of PEFC.

  Such a catalyst-supported powder is called “Ultra-Low Platinum Loading Carbon” (hereinafter abbreviated as “ULPLC”), and is disclosed in Patent Document 1 and Patent Document 2. This ULPLC is currently attracting attention as one of the technical elements that can reduce the cost of commercializing PEFC.

  Furthermore, Patent Document 3 shows that by adding a polymer material containing fluorine that does not have an ion exchange group as a water-repellent resin to ULPLC, flooding phenomenon during continuous operation can be suppressed and a decrease in PEFC operating voltage can be suppressed. Is disclosed.

Furthermore, Patent Documents 4 to 6 disclose techniques for adding a water-repellent resin to ULPLC.
Japanese Patent No. 3648898 JP 2003-257439 A International Publication No. 2006/082981 Pamphlet Japanese Patent Application No. 2005-030949 Japanese Patent Application No. 2005-168607 Japanese Patent Application No. 2005-140000

  It is known that moisture moves inside the PEFC during operation. For example, a phenomenon in which protons move from the anode to the cathode as entrained water when moving inside the cation exchange membrane from the anode to the cathode, a reverse diffusion phenomenon from the cathode to the anode caused by a difference in moisture concentration between the anode and the cathode, and There is a phenomenon that water is generated by the reaction of the cathode.

  The amount of entrained water and product water is determined by the current density, while the amount of water that moves by back diffusion is strongly influenced by the water repellency of the anode and cathode. Therefore, it is considered that the water content of the back diffusion can be controlled by appropriately adjusting the water repellency of the anode and the cathode, and the water management of the membrane / electrode assembly can be performed.

  In order to adjust the water repellency of the anode and the cathode (hereinafter collectively referred to as “electrode”), a water repellent resin such as polytetrafluoroethylene (PTFE) is added to the electrode. However, PEFC using an electrode equipped with a water-repellent resin has a problem that the operating voltage decreases in continuous operation, and the cause is found to be due to the “flooding phenomenon” at the cathode.

  That is, the catalyst does not participate in the reaction by covering the surface of the catalyst without draining the water produced by the reaction, or the hydrogen gas or oxygen gas, which is the active material, is blocked by blocking the gas diffusion path. As a result of the hindrance to reaching the reaction interface from the outside, no reaction occurs at the reaction interface where the gas does not reach, and the current density is biased, so that the operating voltage of the PEFC decreases.

  In addition, the catalyst layer provided with ULPLC is more susceptible to the porosity of the catalyst layer than the catalyst layer provided with the conventional catalyst-supporting powder. Therefore, for ULPLC that is easily affected by porosity, it is important to adjust water repellency more than conventional catalyst-supported powder.

  In addition, in the technology for adding a water-repellent resin to the ULPLC disclosed in Patent Documents 4 to 6, the same cathode and anode catalyst layers are used (Patent Document 4), and the anode does not use ULPLC. (Patent Documents 5 and 6), and the relationship between the ratio of the water-repellent resin contained in the cathode catalyst layer and the ratio of the water-repellent resin contained in the anode catalyst layer catalyst layer has not been studied.

  In view of such circumstances, the present invention was made in order to solve the problem that the operating voltage of PEFC decreases during continuous operation, and performs water management of the membrane / electrode assembly of PEFC, and flooding phenomenon at the cathode. It aims at suppressing the fall of the operating voltage of PEFC.

  The invention according to claim 1 is a membrane / electrode assembly for a polymer electrolyte fuel cell comprising a catalyst layer containing a cation exchange resin, a carbonaceous material, a water repellent resin, and a catalyst metal on both the anode and the cathode. When the ratio of the water-repellent resin contained in the cathode catalyst layer is X (mass%) and the ratio of the water-repellent resin contained in the anode catalyst layer is Y (mass%), Y / X <1 It is characterized by being.

  According to a second aspect of the present invention, a polymer electrolyte fuel cell includes the membrane / electrode assembly for a polymer electrolyte fuel cell.

  As in the present invention, in the membrane / electrode assembly for a polymer electrolyte fuel cell, by making the proportion of the water-repellent resin contained in the cathode catalyst layer larger than the proportion of the water-repellent resin contained in the anode catalyst layer, Since the back diffusion of moisture from the cathode to the anode can be promoted, the flooding of the cathode can be suppressed. In particular, when Y / X ≦ 0.9, the suppression is significant.

  Furthermore, when the polymer electrolyte fuel cell includes the membrane / electrode assembly of the present invention, the operating voltage in the continuous operation of the polymer electrolyte fuel cell can be improved.

  The membrane / electrode assembly for a polymer electrolyte fuel cell of the present invention has a catalyst layer formed on both sides of a cation exchange resin membrane, and a conductive porous body joined to the outside of the catalyst layer. is there. The catalyst layer in the membrane / electrode assembly for a polymer electrolyte fuel cell of the present invention contains a cation exchange resin, a carbonaceous material, a water repellent resin, and a catalyst metal, and a place where an electrochemical reaction occurs at the anode and the cathode. That is.

  In the present invention, when the ratio of the water-repellent resin contained in the cathode catalyst layer is X (mass%) and the ratio of the water-repellent resin contained in the anode catalyst layer is Y (mass%), Y / X <1. Thus, the reverse diffusion of moisture from the cathode to the anode can be promoted, and flooding of the cathode during continuous operation can be suppressed.

  In particular, when Y / X ≦ 0.9, the back diffusion of moisture from the cathode to the anode is promoted, and the effect of suppressing the cathode flooding in the continuous operation is increased. Further, by setting Y / X ≦ 0.8, the promotion of the back diffusion of moisture from the cathode to the anode becomes remarkable.

  On the other hand, when Y / X ≧ 1, the reverse diffusion of moisture from the cathode to the anode is suppressed, so that the flooding of the cathode in the continuous operation is easily promoted.

  In the membrane / electrode assembly for a polymer electrolyte fuel cell of the present invention, when the ratio X of the water repellent resin contained in the cathode catalyst layer is 6 to 42% by mass, the cathode flooding is suppressed, The electronic conductivity of the catalyst layer can be suitably maintained.

  When the ratio of the water-repellent resin contained in the cathode catalyst layer is less than 6% by mass, the effect of suppressing the flooding of the cathode becomes small because the water-repellent effect is not sufficiently exhibited. Further, when the proportion of the water repellent resin contained in the cathode catalyst layer is greater than 42% by mass, the water repellent resin is insulative, so that the internal resistance due to electron conduction is slightly increased.

  The cation exchange resin contained in the catalyst layer of the present invention is, for example, a perfluorocarbon sulfonic acid type, an acetylene-divinylbenzene sulfonic acid type cation exchange resin, or a cation exchange resin having a carboxyl group as an ion exchange group. Can be used. The ion exchange capacity of the cation exchange resin is preferably 0.90 meq / g to 1.40 meq / g because excellent proton conduction and durability can be obtained.

  As the cation exchange resin solution, a solution obtained by dissolving or / and dispersing a cation exchange resin in a solvent can be used. The solvent is preferably a mixed solvent of water and alcohol, and the alcohol preferably has 4 or less carbon atoms.

  The carbonaceous material contained in the catalyst layer of the present invention preferably has a high electron conductivity, such as carbon black such as acetylene black and furnace black, and activated carbon. Carbon black is particularly preferable because the supported catalyst is highly dispersed.

  The water-repellent resin contained in the catalyst layer of the present invention may be a polymer material containing fluorine having no ion exchange group. For example, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), polyvinylidene fluoride ( PVdF), polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), tetrafluoroethylene-perfluoroalkyl vinyl ether (PFA), polychlorotrifluoroethylene (PCTFE), ethylene-chloroalkylethylene copolymer (ECTFE) And ethylene-tetrafluoroethylene copolymer (ETFE).

  Since the catalytic metal contained in the catalyst layer of the present invention has high catalytic activity for oxygen reduction reaction and hydrogen oxidation reaction, platinum group metals such as platinum, rhodium, ruthenium, iridium, palladium, osmium, and these metals An alloy containing a plurality of is preferable. In particular, when hydrogen gas containing a small amount of carbon monoxide is used as the fuel gas, the catalyst metal is preferably an alloy of platinum and ruthenium.

  In addition to this, from the fact that the amount of platinum group metal used can be reduced and high activity against carbon monoxide poisoning resistance and oxygen reduction reaction, magnesium, aluminum, vanadium, chromium, manganese, iron, An alloy containing at least one element selected from the group consisting of cobalt, nickel, copper, zinc, silver, or tungsten and a platinum group metal can also be used.

  The catalyst layer containing the catalyst-supporting carbon in the membrane / electrode assembly for a polymer electrolyte fuel cell of the present invention can be produced, for example, as follows.

  Water and an organic solvent are added to the catalyst-supporting carbon and stirred to disperse the catalyst-supporting carbon in a water / organic solvent mixed solvent. A water-repellent resin dispersion is added to the mixture, and a cation exchange resin solution is further added. The catalyst dispersion containing catalyst-supporting carbon, water-repellent resin, and cation exchange resin is prepared, and this catalyst dispersion is applied to a solid polymer membrane, dried, and hot pressed to form a catalyst layer.

  In the present invention, as the catalyst, ULPLC can be used in place of the catalyst-supporting carbon. In order to increase the cost reduction effect by reducing the amount of catalyst metal used, ULPLC in which 50% by mass or more of the catalyst metal is supported on the contact surface between the cation exchange resin cluster and the surface of the carbonaceous material is used. It is preferable to do. In particular, when 80% by mass or more of the catalyst metal is supported on the contact surface, the effect becomes higher.

  The “cation exchange resin cluster” refers to a region formed by cation exchange groups of the cation exchange resin and capable of conducting protons.

  The ratio of the catalyst metal supported on the contact surface between the cation exchange resin cluster and the surface of the carbonaceous material with respect to the total amount of the catalyst metal can be calculated, for example, by the following procedure. First, the charge amount based on the hydrogen adsorption / desorption reaction in the catalyst metal of the catalyst layer is evaluated. This charge amount can be evaluated by cyclic voltammetry in argon gas. This adsorption / desorption reaction is a reaction on the catalyst metal contained in the catalyst layer.

  Next, the surface area involved in this reaction is calculated from this charge amount. For example, the calculation formula is as follows.

S = Q × a
In this formula, S is the surface area involved in the reaction (unit is cm ² ), Q is the amount of charge (unit is C), and a is the amount of electricity associated with the adsorption / desorption reaction of hydrogen on 1 cm ^{2 of the} catalyst surface (unit is C / Cm ² ). This coefficient is 210 × 10 ⁻⁶ when the catalyst involved in the reaction is platinum. Since the adsorption / desorption reaction of hydrogen occurs on the surface of the electrochemically active catalyst, the surface area obtained by this calculation is supported on the surface of the carbonaceous material and the cation exchange resin cluster. It means the surface area of the catalyst only. Let this surface area be S1.

  Next, by the same method, the amount of electricity by hydrogen adsorption / desorption reaction when the same electrode is immersed in sulfuric acid aqueous solution and cyclic voltammetry is calculated. In this case, since the electrode is immersed in an aqueous sulfuric acid solution, the catalyst metal supported on the surface of the carbonaceous material can exchange protons regardless of contact with the cation exchange resin cluster. Therefore, the surface areas of all the catalyst metals contained in the catalyst layer are calculated. Let this surface area be S2.

  By using the values of S1 and S2 calculated above, the ratio of the catalyst metal supported on the contact surface between the cluster of the cation exchange resin and the surface of the carbonaceous material in all the catalyst metals included in the catalyst layer (Z) can be calculated as follows.

Z = S1 / S2 × 100
In the present invention, the ULPLC can be manufactured, for example, by the following method. First, a dispersion obtained by mixing a carbonaceous material and a cation exchange resin solution is dried and granulated using a spray dryer to obtain a mixture of the cation exchange resin and the carbonaceous material, and the cation in the mixture is obtained. The cation of the catalytic metal is adsorbed on the ion exchange resin, and the cation is reduced to form the catalytic metal.

  Catalytic metal cations that can be used in this method include, for example, cations including platinum, rhodium, ruthenium, iridium, palladium and osmium. Further, instead of the cation of the platinum group metal, a cation that can be used as a catalyst by reducing the cation can be used. Such cations include cations containing metal elements such as magnesium, aluminum, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, silver, and tungsten. In particular, a cation containing platinum, rhodium, ruthenium, iridium, palladium, or osmium is preferred because a catalytic metal having a high catalytic activity for an electrochemical oxygen reduction reaction or hydrogen oxidation reaction can be obtained.

The cation of the catalytic metal element takes precedence over the ion exchange group of the cation exchange resin compared to the carbon surface not coated with the cation exchange resin by an ion exchange reaction with the counter ion of the ion exchange group of the cation exchange resin. It is preferable to adsorb | suck. As a cation containing a platinum group metal having such an adsorption property, a platinum group metal complex ion, for example, platinum ammine complex cation such as [Pt (NH ₃ ) ₄ ] ²⁺ and [Pt (NH ₃ ) ₆ ] ⁴⁺ Ions, or ruthenium ammine complex cations such as [Ru (NH ₃ ) ₄ ] ²⁺ and [Ru (NH ₃ ) ₆ ] ³⁺ .

  In ULPLC, the average particle size of the catalyst-supported powder is preferably 0.1 to 200 μm. When the particle size is 0.1 μm or less, the scattering property is high, so that the handling is inferior. On the other hand, when the particle size is 200 μm or more, the roughness of the catalyst layer is remarkably increased. A catalyst-supported powder is preferred. The average particle size of the catalyst-supported powder can be measured using a particle size measuring device such as a laser diffraction particle size distribution measuring device or a microscope such as a scanning electron microscope.

  In ULPLC, the average particle diameter of the catalyst metal may be 0.6 to 10 nm, and is particularly preferably 2 to 4 nm because the catalyst metal is active in the oxygen reduction reaction. The particle diameter of the catalyst metal can be estimated by observing the catalyst-supported powder with an electron microscope.

  In ULPLC, the ratio of the mass of the catalyst metal to the mass of the catalyst metal and the carbonaceous material is preferably 5% by mass or more because the thickness of the catalyst layer can be reduced. However, when the ratio is 30% by mass or more, the catalyst metal particles become large, and the specific surface area decreases. Therefore, it is preferable that the ratio is 5-30 mass%.

  The ULPLC can contain a water-repellent resin, and the amount of the water-repellent resin contained in the ULPLC is 10 to 120% by mass with respect to the carbonaceous material. From the viewpoint of water repellency.

  In the ULPLC production method, a reducing agent can be used as a method for chemically reducing a cation of a metal element. In this method, the cation of the metal element present near the surface of the carbonaceous material can be preferentially reduced by adjusting the type of reducing agent, reducing pressure, reducing agent concentration, reducing time, and reducing temperature. preferable. For example, when hydrogen is used as the reducing agent, by adjusting the reduction temperature, it is possible to selectively reduce metal element cations near the surface of the carbonaceous material. This phenomenon utilizes the fact that the carbonaceous material has catalytic activity for the reduction reaction of the cation of the metal element.

  In the production method of ULPLC, usable reducing agents include non-metallic ions such as hydrogen, hydrogen sulfide, hydrogen iodide and iodine ions, lower oxides such as carbon monoxide and sulfur dioxide, sodium phosphinate, sodium thiosulfate Hydrides such as sodium borohydride and lithium tetrahydroaluminate, hydrazine, diimide, formic acid, aldehyde, saccharides and the like. In particular, a gas containing hydrogen or hydrazine is preferable because it is suitable for mass production. The hydrogen is preferably used as a mixed gas (hydrogen mixed gas) with an inert gas such as nitrogen, helium or argon, and the hydrogen content of the hydrogen mixed gas is 10% by volume or more. preferable.

  Further, in the ULPLC production method, the reduction temperature is preferably lower than the decomposition temperature of the cation exchange resin in order to prevent the deterioration of the cation exchange resin. For example, when the cation exchange resin is a perfluorocarbon sulfonic acid type cation exchange resin, deterioration of the resin can be suppressed by reducing the cation exchange resin at a temperature lower than the decomposition temperature of 280 ° C. When hydrogen gas or a hydrogen mixed gas is used as the reducing agent, the reduction temperature condition is preferably 100 ° C. or higher and 200 ° C. or lower.

  In the membrane / electrode assembly for a polymer electrolyte fuel cell of the present invention, a catalyst layer is formed by applying and drying a catalyst paste on the surface of a substrate such as a metal sheet or a polymer sheet. The catalyst layer and the cation exchange membrane can be transferred to the exchange resin membrane, or the catalyst layer can be applied directly to the cation exchange resin membrane and dried to form the catalyst layer on the surface of the cation exchange resin membrane. A membrane / electrode assembly can be manufactured by manufacturing a bonded body with an ion exchange resin membrane and bonding a conductive porous body to the catalyst layer of the bonded body.

  When a conductive porous body such as a porous carbon plate is used as the base material, the catalyst paste is directly applied to one side of the conductive porous body and dried, so that one side of the conductive porous body is formed. A membrane / electrode assembly can be produced by forming a catalyst layer and bonding this catalyst layer to a cation exchange resin membrane.

  It is preferable that the base material on which the catalyst layer is formed and the cation exchange resin membrane are pressure-welded while heating. The bonding conditions are preferably performed under conditions of a temperature of 50 ° C. or higher and 200 ° C. or lower and a pressure of 0.1 MPa or higher and 30 MPa or lower in order to maintain sufficient strength of the bonded body and reduce internal resistance. Further, the joining is preferably performed for 1 minute or more and 20 minutes or less in order to sufficiently heat the catalyst layer and the cation exchange resin membrane. This pressure contact can be performed in a wet state or a dry state of the cation exchange resin membrane.

  The conductive porous body functions as a gas diffusion layer. The conductive porous body preferably includes a carbonaceous material having high electron conductivity and corrosion resistance. For example, carbon paper made of carbon fiber, carbon felt, or the like can be used. The porosity of the conductive porous body is preferably 40 to 95% by volume in order to improve the gas diffusibility, and preferably 80 to 95% by volume in order to improve the gas diffusibility. . Yes.

  Moreover, in order to prevent flooding due to generated water, these conductive porous bodies preferably have water repellency imparted by a water repellent resin. The mass ratio of the water-repellent resin to the conductive porous body is preferably 2 to 30% by mass in order to improve the water repellency.

  In the membrane / electrode assembly for a polymer electrolyte fuel cell of the present invention, by forming an intermediate layer between the conductive porous body and the catalyst layer, the surface smoothness is improved and the current density distribution is uniform. And improvement of water repellency can be achieved. The intermediate layer is a porous layer containing a carbonaceous material, a binder, and a water repellent resin, and is a layer having water repellency, gas permeability, and electron conductivity. The mass ratio of the water-repellent resin in the intermediate layer is preferably 2 to 40% by mass in order to improve the water repellency.

  The amount of the cation exchange resin contained in the catalyst layer of the present invention is preferably 25 to 150% by mass with respect to the carbonaceous material from the viewpoint of the electron conductivity and proton conductivity of the catalyst layer. In particular, the amount of the cation exchange resin is preferably 25 to 100% by mass with respect to the carbonaceous material because the gas diffusibility of the catalyst layer is increased.

  Further, the mass of the liquid used for the preparation of the catalyst layer paste is preferably 60 to 95% by mass, particularly 80 to 90% by mass with respect to the mass of the paste containing the liquid and the catalyst-supporting powder. Preferably there is. When the liquid has a mass in this range, the dispersibility of the catalyst-supported powder contained in the paste is optimized, and as a result, the moldability of the catalyst layer is improved. As a result, the performance of the polymer electrolyte fuel cell including that layer Will improve.

  The type of liquid used for the catalyst paste is not particularly limited. For example, inorganic compounds such as water, hydrocarbon liquids such as hexane, pentane, cyclohexane, octane, and benzene, dichloromethane, 1,1,2- Halogen compounds such as trichloro-1,2,2-trifluoroethane, alcohols such as methanol, ethanol, ethylene glycol and glycerin, ethers such as anisole, ketones such as acetone, N, N-dimethylformamide, dimethyl sulfoxide, N- Nitrogen-containing compounds or sulfur-containing compounds such as methyl-2-pyrrolidone and pyridine, or a mixture of these liquids can be used.

  Among them, a liquid having a viscosity of 10 mPas or more is preferable because mixing of the liquid and its powder is good. More preferably, the boiling point of the liquid is 60 ° C. or higher. When the boiling point of the liquid is 60 ° C. or less, the concentration of the paste is likely to increase due to the evaporation of the liquid, so that the handling becomes difficult.

  In addition, it is preferable that the amount of ion exchange resin taken in as a liquid is 50 mass% or more and 200 mass% or less. By using a material in this range, the cation exchange resin swells with the liquid and the binding property between the catalyst-supported powder particles is improved. As a result, the ionic conductivity of the catalyst layer is improved, and the performance is improved.

  The amount M (mass%) of ion exchange resin taken up is defined by the following equation (1).

Ｍ：取り込み量（質量％）
Ｗ_ｗｅｔ：液体に接触させた後の陽イオン交換樹脂膜の質量（ｇ）
Ｗ_ｄｒｙ：陽イオン交換樹脂膜の乾燥質量（ｇ）
このような条件を満たす液体として、Ｎ，Ｎ−ジメチルホルムアミド（ＤＭＦ）またはＮ−メチル−２−ピロリドン（ＮＭＰ）などがある。これを用いて製造した触媒層は、性能が著しく高い。

M: Uptake amount (% by mass)
W _wet : Mass of cation exchange resin membrane after contact with liquid (g)
W _dry : dry mass of cation exchange resin membrane (g)
Examples of the liquid that satisfies such conditions include N, N-dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP). The catalyst layer produced using this has remarkably high performance.

  In order to improve the proton conductivity of the catalyst layer, the catalyst layer paste may contain a cation exchange resin solution. The cation exchange resin contained in the catalyst layer paste is preferably 5 to 400% by mass relative to the mass of the carbonaceous material contained in the catalyst-supporting powder.

  The catalyst layer paste may contain a water-repellent resin such as FEP in order to increase the strength of the catalyst layer. The water-repellent resin contained in the catalyst layer paste is 10 to 120% by mass relative to the mass of the carbonaceous material contained in the catalyst-supporting powder, and is effective in suppressing high water retention in the catalyst layer and high electrons. This is preferable because it provides conductivity.

  In the catalyst layer paste, a pore-forming agent that can improve the porosity of the catalyst layer by removing after forming the catalyst layer, such as zinc, aluminum, calcium, chromium, cobalt, tin, iron, copper, nickel, magnesium, etc. Metal particles or alloys containing at least one of these metal elements, inorganic salts such as calcium carbonate or sodium chloride, or water-soluble polymers such as polyvinyl alcohol may also be included. In that case, it is preferable to perform a step of removing the substance after forming the catalyst layer. For example, when nickel particles are used, the particles can be eluted by dipping in dilute sulfuric acid. The substance is preferably one that is sparingly soluble in the liquid contained in the paste.

  In the present invention, the thickness of the catalyst layer is preferably 1 to 150 μm from the viewpoint of improving catalyst activity and maintaining high gas diffusibility of the catalyst layer. Furthermore, in order to reduce the electric resistance of the layer, the thickness is preferably 4 to 30 μm.

  In the present invention, the porosity of the catalyst layer is preferably 60 to 85% by volume which can achieve both high gas diffusibility and proton conductivity. The reason for this is as follows. By forming pores in the catalyst layer, water generated by the oxygen reduction reaction at the cathode can be sufficiently discharged. This effect of water discharge is remarkably exhibited when the porosity is 60% by volume or more. Furthermore, when the porosity is 85% by volume or less, the proton conductivity and electron conductivity of the catalyst layer can be maintained.

  In the present invention, when the porosity of the catalyst layer is less than 60% by volume, gas permeation performance and water discharge performance are suppressed, and the value of current density decreases as the porosity decreases, and 85 When the volume% is exceeded, the porosity of the mixture decreases due to an increase in porosity, and the electronic conductivity and proton conductivity of the catalyst layer decrease, resulting in a rapid decrease in current density. Therefore, the porosity of the catalyst layer is preferably 60 to 85% by volume. The porosity of the catalyst layer can be determined using, for example, a mercury intrusion method.

In the catalyst layer of the present invention, the mass of the catalyst metal contained in the catalyst layer per unit area is preferably 0.01 to 1.00 mg / cm ² from the viewpoint of improving the catalyst activity and reducing the cost. The mass per unit area of the catalyst metal can be obtained, for example, by extracting the catalyst metal in the catalyst layer and then quantifying the amount of the catalyst metal in the extract by ICP emission analysis.

  The present invention will be described below with reference to preferred embodiments.

[Examples 1 to 15 and Comparative Examples 1 to 5]
[Preparation of cathode catalyst layer]
A cathode catalyst layer comprising a cation exchange resin, a carbonaceous material, a water repellent resin, and a catalyst metal, wherein 50% by mass or more of the catalyst metal is supported on the contact surface between the cation exchange resin cluster and the surface of the carbonaceous material. Was prepared as follows.

  First, a carbonaceous material (Vulcan XC-72, manufactured by Cabot), a cation exchange resin solution (Nafion, 5% by mass solution, manufactured by Aldrich) and an FEP dispersion (54% by mass, manufactured by Mitsui DuPont Fluorochemical, FEP120) -J) was spray dried with a spray dryer (Fujisaki Electric Co., Ltd.) to produce a mixed powder containing a carbonaceous material, a cation exchange resin, and FEP (water repellent resin).

Next, this mixed powder was impregnated with [Pt (NH ₃ ) ₄ ] Cl ₂ aqueous solution (50 mol / l) to adsorb [Pt (NH ₃ ) ₄ ] ²⁺ to the ion exchange group of the cation exchange resin. . Furthermore, the mixed powder was reduced under a hydrogen atmosphere to produce a catalyst powder containing Pt (catalytic metal), Nafion, FEP, and a carbonaceous material.

  Next, the catalyst powder and N-methyl-2-pyrrolidone (NMP) are mixed to form a catalyst layer paste. The catalyst layer paste is applied onto a polymer film and then dried to obtain a cathode catalyst. Layered.

  The composition of the obtained cathode catalyst layer was Nafion: carbonaceous material: Pt = 0.4: 1.0: 0.28 (mass ratio). And when the ratio of FEP (water-repellent resin) contained in the cathode catalyst layer is X (mass%), five types of cathode catalyst layers A1 to A5 where X is 3 to 47 mass% were produced. The compositions (mass ratio) of the cathode catalyst layers A1 to A5 are summarized in Table 1.

[Preparation of anode catalyst layer]
An anode catalyst powder was produced in the same manner as the cathode catalyst powder. And using this catalyst powder, the anode catalyst layer was produced like the cathode catalyst layer.

  The composition of the obtained anode catalyst layer was Nafion: carbonaceous material: Pt = 0.4: 1.0: 0.22 (mass ratio). And when it is set as the ratio Y (mass%) of FEP (water-repellent resin) contained in an anode catalyst layer, 20 types of anode catalyst layers B1-B15 and R1-R5 whose Y is 0.6-47 mass% are produced. did. Table 2 summarizes the compositions (mass ratio) of the anode catalyst layers B1 to B15 and R1 to R5.

[Example 1]
After the cathode catalyst layer A1 and the anode catalyst layer B1 are cut into a square having a side of 5 cm, the cathode catalyst layer A1, the anode catalyst layer B1, and the solid polymer membrane (Nafion 112, manufactured by DuPont) are hot pressed, The cathode catalyst layer A1 and the anode catalyst layer B1 were transferred to both surfaces of the solid polymer membrane. Furthermore, the membrane / electrode assembly of Example 1 was manufactured by disposing carbon paper (gas diffusion layer) outside the catalyst layer. Since X of the cathode catalyst layer A1 is 3% and Y of the anode catalyst layer B1 is 0.6%, in the obtained membrane / electrode assembly of Example 1, Y / X = 0.2.

  The PEFC provided with this joined body was used as the PEFC of Example 1. Using cyclic voltammetry, the ratio of the catalyst metal supported on the contact surface between the cluster of the cation exchange resin and the surface of the carbonaceous material with respect to the amount of the catalyst metal contained in the cathode catalyst layer A1 and the anode catalyst layer B11 Were found to be 81% by mass and 80% by mass, respectively.

[Examples 2 and 3 and Comparative Example 1]
Membrane / electrode assemblies of Examples 2, 3 and Comparative Example 1 were produced in the same manner as in Example 1 by combining the cathode catalyst layer A1 and the anode catalyst layers B2, B3 and R1, and these assemblies A PEFC equipped with was prepared. Y / X of the membrane / electrode assembly of Example 2 is 0.5, Y / X of the membrane / electrode assembly of Example 3 is 0.9, and Y / X of the membrane / electrode assembly of Comparative Example 1 is 1.0.

[Examples 4 to 15 and Comparative Examples 2 to 5]
Membrane / electrode assemblies of Examples 4 to 6 and Comparative Example 2 were produced in the same manner as in Example 1 by combining the cathode catalyst layer A2 and the anode catalyst layers B4 to B6 and R2, and these assemblies A PEFC equipped with was prepared.

  Further, the membrane / electrode assemblies of Examples 7 to 15 and Comparative Examples 3 to 5 were combined with cathode catalyst layers A3 to A5 and anode catalyst layers B7 to B15 and R3 to R5 in the same manner as in Example 1. A PEFC equipped with these assemblies was prepared.

  The cathode catalyst layer and anode catalyst layer used in the produced PEFC membrane / electrode assembly and the Y / X values are shown in Table 3.

[Characteristic measurement]
Example under conditions where cell temperature is 70 ° C., anode gas is pure hydrogen, anode utilization is 80%, anode humidification temperature is 70 ° C., cathode gas is air, cathode utilization is 40%, and cathode humidification temperature is 70 ° C. The PEFCs 1 to 15 and Comparative Examples 1 to 5 were operated at a current density of 500 mA / cm ² for 500 hours, the change in operating voltage with time was measured, and the operating voltage per hour decreased from the change in operating voltage over time. The rate (μV / h) was determined.

PEFC under conditions of cell temperature 70 ° C., anode gas pure hydrogen, anode utilization 80%, anode humidification temperature 70 ° C., cathode gas air, cathode utilization 40%, cathode humidification temperature 70 ° C. The voltage-current characteristics were measured, and the operating voltage at a current density of 500 mA / cm ² was determined.

The measurement results are summarized in Table 4. In Table 4, “Voltage reduction rate” represents the rate of decrease in operating voltage per hour of PEFC, the unit is μV / h, and “Operating voltage” has a current density of 500 mA / cm ^2. Is expressed in mV, and the unit is mV.

  From Table 4, the rate of decrease in the working voltage of the PEFCs of Examples 1 to 15 where Y / X is in the range of 0.9 or less is compared to the case of Y / X = 1 of the PEFCs of Comparative Examples 1 to 5. I found it low. In Comparative Examples 1 to 5, this is considered to be due to the fact that the cathode catalyst layer and the anode catalyst layer have no difference in water repellency, so that cathode flooding is likely to occur. On the other hand, in Examples 1 to 15, since there is a difference in water repellency between the cathode catalyst layer and the anode catalyst layer, the reverse diffusion of moisture from the cathode to the anode is promoted. Is considered to be suppressed.

  Moreover, from Table 4, the rate of decrease in the operating voltage of PEFC in Examples 4 to 15 in which the ratio of FEP contained in the cathode catalyst layer is 6% by mass or more is compared with the PEFCs of Examples 1 to 3. I found it low. This is because in Examples 1 to 3, the addition of FEP is insufficient, so that the water repellency of the cathode catalyst layer is low. On the other hand, in Examples 4 to 15, the cathode catalyst layer has sufficient water repellency. This is considered to be due to the grant.

  From Table 4, it turned out that the working voltage of PEFC of Examples 1-12 whose ratio of FEP contained in a cathode catalyst layer is 42 mass% or less is higher than the case of PEFC of Examples 13-15. This is considered to be due to the fact that in Examples 13 to 15 a large amount of insulating FEP was contained, the electron conductivity of the catalyst layer was slightly lowered and the internal resistance was increased. In the case of PEFCs of Examples 1 to 12, it is considered that the electronic conductivity of the catalyst layer is suitably maintained.

  From the above results, including cation exchange resin, carbonaceous material, water repellent resin, and catalyst metal, 50% by mass or more of the catalyst metal is supported on the contact surface between the cation exchange resin cluster and the surface of the carbonaceous material. In the membrane / electrode assembly using the cathode catalyst layer and the anode catalyst layer, Y / X <1, and the ratio of FEP (water repellent resin) contained in the cathode catalyst layer is 6 to 42% by mass It was found that a PEFC having a high operating voltage and a small reduction rate of the operating voltage can be obtained.

[Examples 16 to 30 and Comparative Examples 6 to 10]
[Preparation of cathode catalyst layer]
A cathode catalyst layer using platinum-supported carbon was produced as follows. First, purified water is added to a platinum-supporting carbon (made by Tanaka Kikinzoku Co., Ltd., 10V50E: Vulcan XC-72, platinum 50% by mass) with a platinum loading rate of 50% by mass, and then 2-propanol is added while gradually stirring, Carbon was dispersed in a water / 2-propanol mixed solvent.

  FEP dispersion (54% by mass, manufactured by Mitsui DuPont Fluorochemicals, FEP120-J) is gradually added to this mixture while stirring, and further, a cation exchange resin solution (Nafion, 5% by mass, Aldrich) is added. The catalyst dispersion was prepared by gradually adding with stirring.

  In this catalyst dispersion, the ratio of Nafion to the carbonaceous material was 40% by mass, and the ratio of Pt to carbon was 50% by mass. This catalyst dispersion was applied to one side of a solid polymer membrane (Nafion 112, manufactured by DuPont), dried and hot pressed to form a cathode catalyst layer.

  The composition of the obtained cathode catalyst layer was Nafion: carbon: Pt = 0.4: 0.5: 0.5 (mass ratio). And when the ratio of FEP (water-repellent resin) contained in the cathode catalyst layer is X (mass%), five types of cathode catalyst layers A6 to A10 with X of 3 to 47 mass% were produced. Table 5 summarizes the composition (mass ratio) of the cathode catalyst layers A6 to A10.

[Preparation of anode catalyst layer]
Apply, dry, and hot-press the solid polymer membrane (Nafion 112, manufactured by DuPont) on the opposite side of the cathode catalyst layer using the same material as that used to create the cathode catalyst layer. Thus, an anode catalyst layer was produced.

    The composition of the obtained anode catalyst layer was Nafion: carbon: Pt = 0.4: 0.5: 0.5 (mass ratio). And when the ratio of FEP (water-repellent resin) contained in the anode catalyst layer is Y (mass%), 20 kinds of anode catalyst layers B16 to B30 and R6 to R10 having Y of 0.6 to 47 mass% are Produced. Table 6 summarizes the composition (mass ratio) of the anode catalyst layers B16 to B30 and R6 to R10.

[Example 16]
A square cathode catalyst layer A6 and an anode catalyst layer B16 each having a side of 5 cm are hot-pressed with a solid polymer membrane (Nafion 112, manufactured by DuPont), and the cathode catalyst layer A6 and the anode catalyst are formed on both sides of the solid polymer membrane. Layer B16 was joined so as to face each other with the solid polymer film interposed therebetween. Furthermore, the membrane / electrode assembly of Example 16 was manufactured by disposing carbon paper (gas diffusion layer) outside the catalyst layer. Since X of the cathode catalyst layer A6 is 3% and Y of the anode catalyst layer B16 is 0.6%, in the obtained membrane / electrode assembly of Example 16, Y / X = 0.2. The PEFC provided with this joined body was referred to as PEFC of Example 16.

[Examples 17 and 18 and Comparative Example 6]
Membrane / electrode assemblies of Examples 17 and 18 and Comparative Example 6 were produced in the same manner as in Example 16 by combining the cathode catalyst layer A6 and the anode catalyst layers B17, B18, and R6. A PEFC equipped with was prepared. Y / X of the membrane / electrode assembly of Example 17 is 0.5, Y / X of the membrane / electrode assembly of Example 18 is 0.9, and Y / X of the membrane / electrode assembly of Comparative Example 6 is 1.0.

[Examples 19 to 30 and Comparative Examples 6 to 10]
Membrane / electrode assemblies of Examples 19 to 30 and Comparative Examples 7 to 10 were produced in the same manner as Example 16 by combining the cathode catalyst layers A7 to A10 and the anode catalyst layers B19 to B30 and R7 to R10. And PEFC provided with these joined bodies was produced.

  The cathode catalyst layer and anode catalyst layer used in the produced PEFC membrane / electrode assembly and Y / X values are shown in Table 7.

[Characteristic measurement]
The PEFCs of Examples 16 to 30 and Comparative Examples 6 to 10 were operated for 500 hours at a current density of 500 mA / cm ^{2 under} the same conditions as in Example 1, the change in operating voltage over time was measured, and the change in operating voltage over time was measured. From this, the rate of decrease in operating voltage per hour (μV / h) was determined. Further, voltage-current characteristics were measured under the same conditions as in Example 1, and the operating voltage at a current density of 500 mA / cm ² was determined.

The measurement results are summarized in Table 8. In Table 8, “Voltage reduction rate” represents the rate of decrease in operating voltage per hour of PEFC, the unit is μV / h, and “Operating voltage” has a current density of 500 mA / cm ^2. Is expressed in mV, and the unit is mV.

  From Table 8, when Y / X is in the range of 0.9 or less, the reduction rate of the working voltage of the PEFC of Examples 16 to 30 is compared with the case of Y / X = 1 of the PEFC of Comparative Examples 6 to 10. I found it low. Furthermore, when Y / X was 0.8 or less, the rate of decrease in operating voltage of PEFC was lower.

  Moreover, the rate of decrease in the working voltage of the PEFC in Examples 19 to 30 in which the proportion of FEP contained in the catalyst layer of the cathode is 6% by mass or more is lower than that in the PEFCs of Examples 16 to 18. I understood.

  From the above results, even in a membrane / electrode assembly using a cathode catalyst layer and an anode catalyst layer containing platinum-supported carbon, Nafion (cation exchange tree) and FEP (water repellent resin), 50% by mass of the catalyst metal The above is the same reason as in the case of the membrane / electrode assembly using the cathode catalyst layer and the anode catalyst layer supported on the contact surface between the cation exchange resin cluster and the surface of the carbonaceous material, and Y / X <1. In addition, when the ratio of the FEP (water repellent resin) contained in the cathode catalyst layer is in the range of 6 to 42% by mass, it is found that a PEFC having a high operating voltage and a small reduction rate of the operating voltage can be obtained. It was.

[Example 31]
First, a mixture of a carbonaceous material (Vulcan XC-72, manufactured by Cabot) and a cation exchange resin solution (Nafion, 5 mass% solution, manufactured by Aldrich) is sprayed with a spray dryer (manufactured by Fujisaki Electric). It dried and produced the mixed powder containing a carbonaceous material and a cation exchange resin. In this mixed powder, the ratio of the cation exchange resin to the carbonaceous material was 40% by mass.

Next, this mixed powder was impregnated with [Pt (NH ₃ ) ₄ ] Cl ₂ aqueous solution (50 mol / l) to adsorb [Pt (NH ₃ ) ₄ ] ²⁺ to the ion exchange group of the cation exchange resin. . Further, the mixed powder was reduced in a hydrogen atmosphere to produce catalyst-supported powder C containing a catalyst metal, a cation exchange resin, and a carbonaceous material.

  Next, a catalyst layer paste was prepared by mixing the catalyst-supporting powder C, a liquid, and a water-repellent resin, and a catalyst layer was formed using the catalyst layer paste. The method is as follows. First, catalyst-supporting powder C, N-methyl-2-pyrrolidone (NMP) and FEP dispersion (54% by mass, manufactured by Mitsui DuPont Fluorochemical, FEP120-J) are mixed to prepare a catalyst layer paste. did. Then, the catalyst layer paste was applied on a polymer film and then dried to form a cathode catalyst layer C. The proportion of FEP contained in the cathode catalyst layer was 20% by mass.

  The composition of the obtained cathode catalyst layer C was Nafion: carbon material: FEP: Pt = 0.40: 1.00: 0.42: 0.28 (mass ratio), and the FEP contained in the cathode catalyst layer ( The ratio X of the water repellent resin was 20% by mass.

  Further, an anode catalyst layer D was produced in the same manner as the cathode catalyst layer C. However, the composition of the obtained anode catalyst layer D is Nafion: carbon material: FEP: Pt = 0.40: 1.00: 0.18: 0.22 (mass ratio) and is included in the anode catalyst layer. The ratio Y of FEP (water repellent resin) was 10% by mass.

  A membrane / electrode assembly of Example 31 and a PEFC using the same were prepared in the same manner as in Example 1 except that the cathode catalyst layer C and the anode catalyst D were used. In this membrane / electrode assembly, Y / X is 0.5.

And the time-dependent change of the working voltage of PEFC was measured on the same conditions as Example 1, the decreasing rate of the working voltage per hour was calculated | required, and the voltage-current characteristic was measured. As a result, the rate of decrease in operating voltage per hour was 11 μV / h, and the operating voltage at a current density of 500 mA / cm ² was 672 mV. The same results as in Example 8 were obtained.

  Therefore, even when the manufacturing method of the cathode catalyst layer and the anode catalyst layer is different, when Y / X <1 and the ratio of FEP contained in the cathode catalyst layer is 20% by mass, a high operating voltage and an operating voltage of It was found that a PEFC with a small reduction rate can be obtained.

  Although data is omitted, the same results as in Example 1 were obtained when PTFE, PVdF, PVF, PFA, PCTFE, ECTFE, and ETFE were used as the water repellent resin instead of FEP.

Claims (2)

In the membrane / electrode assembly for a polymer electrolyte fuel cell comprising a catalyst layer containing a cation exchange resin, a carbonaceous material, a water repellent resin, and a catalyst metal on both the anode and the cathode, A solid, wherein Y / X <1, where X is a mass ratio of the water-repellent resin and Y is a mass ratio of the water-repellent resin contained in the anode catalyst layer. A membrane / electrode assembly for a polymer fuel cell. A polymer electrolyte fuel cell comprising the membrane / electrode assembly for a polymer electrolyte fuel cell according to claim 1.