JP2009193910A - Membrane-electrode assembly and solid polymer fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a membrane-electrode assembly and a solid polymer fuel cell excellent in durability and indicating high-power generation characteristics, under low humidity condition. <P>SOLUTION: In the membrane-electrode assembly having a structure wherein a polymer electrolyte membrane is held by a pair of electrode catalyst layers, at least one of the electrode catalyst layers has a polymer electrolyte, a catalyst material, and a carbon carrier for carrying the catalyst material, and the carbon carrier has at least two types or more of carbon carriers which are different on the degree of graphitization. Furthermore, the carbon carrier in the membrane-electrode assembly is selected from among graphite carbon, carbon nanotube, nanohorn, and fullerene. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a membrane electrode assembly used for a polymer electrolyte fuel cell and a polymer electrolyte fuel cell using the same. More specifically, the present invention relates to a membrane electrode assembly having excellent durability and high power generation characteristics under low humidification conditions, and a polymer electrolyte fuel cell using the membrane electrode assembly.

  A fuel cell is a power generation system that generates electricity by using hydrogen and oxygen as fuel and causing reverse reaction of water electrolysis. This has features such as high efficiency, low environmental load and low noise compared with the conventional power generation method, and is attracting attention as a clean energy source in the future. Fuel cells can be classified according to their electrolyte, and include molten carbonate fuel cells, phosphoric acid fuel cells, solid oxide fuel cells, solid polymer fuel cells, and the like.

  Among the fuel cells, the polymer electrolyte fuel cell can be operated in a low temperature region, and is generally used at an operating temperature of 80 to 100 ° C., and is used for an in-vehicle power source or a household stationary power source. The use of is promising. The polymer electrolyte fuel cell supplies a fuel gas containing hydrogen to one of the electrodes, and uses a joined body in which a pair of catalyst electrode layers are arranged on both sides of a polymer electrolyte membrane called a membrane electrode assembly (MEA). The battery is sandwiched between a pair of separator plates formed with a gas flow path for supplying an oxidant gas containing oxygen to the other electrode. Here, the electrode for supplying the fuel gas is called a fuel electrode, and the electrode for supplying the oxidant is called an air electrode.

  In the polymer electrolyte fuel cell, it is necessary to humidify the MEA in order to ensure the conductivity of the electrolyte membrane. However, in order to humidify, an auxiliary machine is required, which leads to high cost of the entire fuel cell system. The operation with low humidification is preferable, and the non-humidification operation is more preferable.

On the other hand, solid polymer fuel cells are desired to have improved durability along with cost. When the fuel cell is started and stopped, a small amount of air is mixed into the fuel electrode, and as a result, the potential of the air electrode becomes a high potential of 1.2 V or more. Therefore, an oxidation reaction (C + 2H ₂ O → CO ₂ + 4H ++ 4e ⁻ ) that generates carbon dioxide using water present in the catalyst layer as an oxidizing agent occurs, which may promote and deteriorate the oxidation of the carrier carbon.

  Also, when hydrogen supply becomes insufficient at the fuel electrode, in order to maintain a predetermined current density, there is a possibility that an electrolysis reaction of water or an oxidation reaction of the carbon support may occur instead of the oxidation reaction of hydrogen. . As described above, when the oxidation reaction of the carbon carrier proceeds and the carbon carrier is oxidatively corroded, the supported catalyst substance is dissolved and re-precipitated in the polymer electrolyte of the catalyst layer. The power generation characteristics of the are reduced.

  As a means for preventing the oxidation of carbon and improving the durability of the electrode catalyst layer as described above, it has been proposed to graphitize the support by high-temperature heat treatment in advance. It has been disclosed that the water repellency and corrosion resistance of a catalyst layer are improved by subjecting carbon to heat treatment at high temperature to cause graphitization (see Patent Documents 1 to 6).

JP 2000-268828 A JP 2001-357857 A JP 2002-015745 A JP 2003-036859 A JP 2005-26174 A JP 20064662 A

  However, graphitized carbon tends to be hydrophobic on the surface by heat treatment. When the carrier surface is hydrophobic, the affinity with the polymer electrolyte having a sulfonic acid group is lowered. In addition, there is a problem in that sufficient power generation characteristics cannot be obtained due to a large voltage drop under low humidification operation or in a low load region where water generated by power generation is low.

  Accordingly, it is an object of the present invention to provide a membrane electrode assembly and a polymer electrolyte fuel cell that are excellent in durability and exhibit high power generation characteristics under low humidification conditions.

  As a result of intensive studies, the present inventor has been able to solve the above-mentioned problems and has completed the present invention.

  That is, the invention according to claim 1 is a membrane electrode assembly having a structure in which a polymer electrolyte membrane is sandwiched between a pair of electrode catalyst layers, wherein at least one of the electrode catalyst layers includes a polymer electrolyte and a catalyst substance. And a carbon support for supporting the catalyst substance, and the carbon support includes at least two kinds of carbon supports having different degrees of graphitization.

  As an invention according to claim 2. 2. The membrane electrode assembly according to claim 1, wherein the carbon support is selected from graphitic carbon, carbon fiber, carbon nanotube, nanohorn, and fullerene.

  According to a third aspect of the invention, the membrane electrode assembly according to the first or second aspect is sandwiched between a pair of gas diffusion layers, and the membrane electrode is sandwiched between the pair of gas diffusion layers. A solid polymer fuel cell having a structure in which the joined body is sandwiched between a pair of separators was obtained.

  The membrane electrode assembly according to claim 1 of the present invention is a membrane electrode assembly comprising a polymer electrolyte membrane sandwiched between a pair of electrode catalyst layers, the electrode catalyst layer comprising a polymer electrolyte, a catalyst substance and A carbon carrier carrying a catalyst substance is provided, and the carbon carrier uses at least two kinds of carbon carriers having different degrees of graphitization. One type of carrier prevents oxidation of the carbon carrier by using a carrier having a high degree of graphitization, and the other type of carrier has a sulfonic acid group by using a carrier having a lower degree of graphitization than the carrier. The affinity with the polymer electrolyte could be improved, the membrane electrode assembly having excellent durability and high power generation characteristics under low humidification conditions could be obtained.

  The membrane electrode assembly according to claim 2 is characterized in that the carbon support used in the electrode catalyst layer is selected from graphitic carbon, carbon fiber, carbon nanotube, nanohorn, and fullerene. By using a material having high oxidation resistance, a membrane electrode assembly having further excellent durability could be obtained.

  A fuel cell according to claim 3 is provided with a structure in which the membrane electrode assembly according to claim 1 or 2 is sandwiched between a pair of gas diffusion layers and sandwiched between a pair of separators. Thus, a fuel cell having excellent durability and sufficient power generation characteristics even during operation under low humidification conditions could be obtained.

  Below, the membrane electrode assembly (MEA) and fuel cell of this invention are demonstrated. Note that the present invention is not limited to the embodiments described below, and modifications such as design changes can be made based on the knowledge of those skilled in the art, and such modifications are added. The embodiments may be included in the scope of the present invention.

  FIG. 1 shows a schematic cross-sectional view of the membrane electrode assembly of the present invention. The membrane electrode assembly (MEA) 12 of the present invention has a structure in which an electrode catalyst layer 2 and an electrode catalyst layer 3 are bonded to both surfaces of a solid polymer electrolyte membrane 1 and sandwiched.

  In the membrane / electrode assembly of the present invention, at least one of the electrode catalyst layers includes a polymer electrolyte, a catalyst material, and a carbon support that supports the catalyst material, and the carbon support has a different degree of graphitization. It is characterized by comprising at least two kinds of carbon carriers. At this time, one of the carbon supports having a high degree of graphitization is not easily oxidized, and the durability of the formed catalyst electrode layer can be improved. The other carbon support with low graphitization has high affinity with the polymer electrolyte and can exhibit sufficient power generation performance. In particular, sufficient power generation performance can be exhibited in the formed membrane electrode assembly and the fuel cell during low humidification operation and in a low load region.

  In the carbon support, the higher the graphitization, the higher the crystallinity and the better the carbon support. However, a carbon support with a high degree of graphitization has a low affinity with a polymer electrolyte, and cannot be a membrane electrode assembly or a fuel cell having a sufficient power generation performance particularly at low humidification. The present inventor succeeded in achieving both durability and power generation performance during low humidification by using two or more types of carbon supports having different degrees of graphitization.

  In addition, the membrane electrode assembly of the present invention has fewer manufacturing steps, lower cost, durability, and power generation performance at low humidification compared to, for example, a method of imparting a hydrophilic functional group to a carbon support to make it hydrophilic. A membrane electrode assembly and a fuel cell that are compatible with each other can be manufactured.

The G / D ratio can be used as an index representing the degree of graphitization of the carbon support of the present invention. Here, G is the peak intensity of a band derived from graphite that is found in the vicinity of 1590 cm ⁻¹ in the spectrum obtained by the resonance Raman scattering measurement method. D is a peak intensity of a band seen in the vicinity of 1350 cm ⁻¹ in a spectrum obtained by a resonance Raman scattering measurement method, and this band is derived from a defect of graphite or amorphous carbon. That is, by obtaining G / D from each peak intensity of the spectrum obtained by the resonance Raman scattering measurement method, it can be used as an index of the degree of graphitization of the carbon support.

  In the present invention, in the carbon support having a higher degree of graphitization among two or more types of carbon supports having different degrees of graphitization, the G / D is preferably 10 or more. It is preferable that it is 20 or more. In addition, in the carbon support having a lower degree of graphitization among two or more types of carbon supports having different degrees of graphitization, the G / D is preferably 5 or less. By making G / D of two or more types of carbon supports having different degrees of graphitization within the above ranges, the effects of the present invention can be made more remarkable.

  Furthermore, in the present invention, the weight ratio of the carbon support having a high graphitization degree having a G / D of 10 or more and the carbon support having a low graphitization degree having a G / D of 5 or less is (a high graphitization degree). Method): (the one having a lower degree of graphitization) = 30: 70 to 95: 5 is preferable. By making the weight ratio of two or more kinds of carbon supports having different degrees of graphitization within the above range, the effect of the present invention can be made more remarkable.

  In addition, the carbon support of the present invention is preferably selected from graphitic carbon, carbon fiber, carbon nanotube, nanohorn, and fullerene in terms of oxidation resistance.

  FIG. 2 shows an exploded schematic view of the polymer electrolyte fuel cell of the present invention. In the polymer electrolyte fuel cell of the present invention, the air electrode side gas diffusion layer 4 and the fuel electrode side gas diffusion layer 5 are opposed to the electrode catalyst layer 2 and the electrode catalyst layer 3 of the membrane electrode assembly 12. Be placed. Thereby, the air electrode 6 and the fuel electrode 7 are comprised, respectively. Then, a set of separators 10 made of a conductive and impermeable material, which is provided with a gas flow path 8 for gas flow and is provided with a cooling water flow path 9 for cooling water flow on the opposing main surface, is disposed. For example, hydrogen gas is supplied as a fuel gas from the gas flow path 8 of the separator 10 on the fuel electrode 7 side. On the other hand, a gas containing oxygen, for example, is supplied as an oxidant gas from the gas flow path 8 of the separator 10 on the air electrode 6 side.

  In the solid polymer fuel cell shown in FIG. 2, the solid polymer electrolyte membrane 1, the electrode catalyst layers 2, 3, and the gas diffusion layers 4, 5 are sandwiched between a pair of separators. Although it is a solid polymer fuel cell having a so-called single cell structure, in the present invention, a plurality of cells can be stacked via a separator 10 to form a fuel cell.

  Below, the manufacturing method of the membrane electrode assembly (MEA) of this invention and the manufacturing method of a fuel cell are demonstrated.

  The solid polymer electrolyte membrane used in the present invention is not particularly limited as long as it is made of a material that is excellent in proton conductivity and does not flow electrons. In particular, a perfluoro type sulfonic acid membrane such as a membrane such as Nafion (registered trademark of DuPont), Flemion (registered trademark of Asahi Glass Co., Ltd.), Aciplex (registered trademark of Asahi Kasei Co., Ltd.) or the like is used. Other examples include hydrocarbon resins such as polyimide having a proton conductive group.

  The polymer electrolyte membrane used in the present invention is preferably made of the same material as the polymer electrolyte used for the electrode catalyst layer.

  Next, an electrode catalyst layer is formed on both surfaces of the prepared solid polymer electrolyte membrane. In forming the catalyst electrode layer, a catalyst ink containing a polymer electrolyte, a catalyst material, a carbon carrier carrying the catalyst material, and a dispersion medium is prepared.

  Various polymer electrolytes are used in the catalyst ink, but the same material as the solid polymer electrolyte membrane to be used can be used, and the same material as the solid polymer electrolyte membrane is preferably used. When Nafion is used as the solid polymer electrolyte membrane, it is preferable to use Nafion as the polymer electrolyte contained in the catalyst ink. When a material other than Nafion is used for the solid polymer electrolyte membrane, it is preferable to optimize such as dissolving the same components as the electrolyte membrane in the catalyst ink.

  As the catalyst used in the present invention, those generally used can be used and are not particularly limited. Specifically, the platinum-supported carbon platinum may be carbon particles carrying a platinum simple substance or a platinum alloy. Examples of the alloy include palladium, ruthenium, and molybdenum, and ruthenium is particularly desirable. Moreover, tungsten, tin, rhenium, etc. may be contained as an additive in the platinum alloy. If the additive is contained, CO poisoning resistance can be increased. The additive metal may exist as a platinum alloy intermetallic compound or may form an alloy. Further, the catalyst particle diameter is preferably 0.5 to 20 nm because the activity of the catalyst is too large, and if it is too small, the stability of the catalyst is lowered. More preferably, 1-5 nm is good. The catalyst loading rate is preferably 40 to 60% by weight. When the catalyst loading rate is less than 40% by weight, the battery characteristics are deteriorated by increasing the thickness of the battery. It becomes worse.

  These catalytic materials are supported on a carbon support. The type of the carbon carrier may be any fine particle that is electrically conductive and is not affected by the catalyst. Graphite carbon, carbon fiber, carbon nanotube, nanohorn, and fullerene are preferably used. Can do. At this time, in the present invention, it is necessary to use two or more types of carbon supports having different degrees of graphitization. If the particle size of the carbon support is too small, it becomes difficult to form an electron conduction path. If the particle size is too large, the gas diffusibility of the electrode catalyst layer is reduced or the utilization factor of the catalyst is reduced. Is preferred. More preferably, 10-100 nm is preferable.

  The solvent used as the dispersion medium for the catalyst ink is not particularly limited as long as it does not erode the catalyst particles or the hydrogen ion conductive resin, and can dissolve or disperse the proton conductive polymer in a highly fluid state as a fine gel. Although there is no limitation, it is desirable to include at least a nascent liquid organic solvent, and although not particularly limited, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutyl alcohol , Tert-butyl alcohol, pentanol, 2-heptanol, benzyl alcohol, and the like, acetone, methyl ethyl ketone, methyl propyl ketone, methyl butyl ketone, methyl isobutyl ketone, methyl amyl ketone, pentanone, heptanone, cyclohexanone, methylcyclohexane S , Acetonyl acetone, diethyl ketone, dipropyl ketone, diisobutyl ketone and other ketones, tetrahydrofuran, tetrahydropyran, dioxane, diethylene glycol dimethyl ether, anisole, methoxytoluene, diethyl ether, dipropyl ether, dibutyl ether and other ethers, isopropyl Amines such as amine, butylamine, isobutylamine, cyclohexylamine, diethylamine, aniline, propyl formate, isobutyl formate, amyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, isobutyl acetate, pentyl acetate, isopentyl acetate, propionic acid Esters such as methyl, ethyl propionate, butyl propionate, etc., acetic acid, propionic acid, dimethylformamide, dimethyl ester Polar solvents such as acetoamide, N-methylpyrrolidone, ethylene glycol, diethylene glycol, propylene glycol, ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diacetone alcohol, 1-methoxy-2-propanol are used. . Moreover, what mixed 2 or more types of these solvents can also be used.

  By using two kinds of solvents having different dielectric constants among these solvents, it is possible to control the dispersion state of the polymer electrolyte in the catalyst ink. These solvents and those using lower alcohols as the solvent have a high risk of ignition, and when using such a solvent, it is preferable to use a mixed solvent with water. Further, water that is compatible with the polymer electrolyte may be contained. The amount of water added is not particularly limited as long as the proton conductive polymer is not separated to cause white turbidity or gelation.

  Further, the catalyst ink may contain a dispersant in order to disperse the carbon carrier carrying the catalyst substance. As the dispersant, an anionic surfactant, a cationic surfactant, an amphoteric surfactant, a nonionic surfactant, or the like can be used.

  The catalyst ink may contain a pore forming agent. By removing the pore-forming agent after the formation of the electrode catalyst layer, pores can be formed. Examples include substances that are soluble in acids, alkalis, and water, substances that sublime such as camphor, and substances that thermally decompose. If the substance is soluble in hot water, it may be removed with water generated during power generation.

  Examples of pore-forming agents that are soluble in acids, alkalis, and water include acid-soluble inorganic salts such as calcium carbonate, barium carbonate, magnesium carbonate, magnesium sulfate, and magnesium oxide, inorganic salts that are soluble in alkaline aqueous solutions such as alumina, silica gel, and silica sol, aluminum Metals soluble in acids or alkalis such as zinc, tin, nickel and iron, water-soluble inorganic salts such as sodium chloride, potassium chloride, ammonium chloride, sodium carbonate, sodium sulfate and monosodium phosphate, polyvinyl alcohol, polyethylene glycol It is also effective to use two or more types in combination.

  The optimum value of the viscosity of the catalyst ink varies depending on the coating method. For example, in the case of application by a screen printing method or a doctor blade method, the viscosity of the ink is preferably 50 to 500 cP. Whether the viscosity is higher or lower than this range of viscosity makes it difficult to apply ink. On the other hand, when spraying on a base material by a spray method, it is preferable that the viscosity of an ink is 0.1-100 cP. If the ink viscosity is higher than this range, spraying becomes difficult. If the ink viscosity is too low, the film formation rate is very slow and the productivity is lowered. The viscosity is optimized by changing the type of solvent and the solid content concentration. Further, the viscosity can be controlled by adding a dispersing agent when the ink is dispersed.

  The catalyst ink containing the polymer electrolyte, the catalyst material, the carbon carrier carrying the catalyst material, and the dispersion medium is appropriately dispersed by a known method.

  The prepared catalyst ink is applied onto the solid polymer electrolyte membrane or the gas diffusion layer by using a doctor blade method, a dipping method, a screen printing method, a roll coating method, a coating method such as a spray method, or a spraying method. Is formed. Also, using a transfer substrate, applying a catalyst ink on the transfer substrate, forming an electrode catalyst layer on the transfer substrate, and then forming an electrode catalyst layer on the solid polymer electrolyte membrane by a transfer method. Also good.

  When a method of spraying catalyst ink such as a spray method is used, the ink is atomized and sprayed onto the surface of the gas diffusion layer or solid polymer electrolyte membrane. Most of the dispersion medium evaporates before adhering to the surface. Therefore, it is preferable that the evaporation rate of the dispersion medium is high because there is less aggregation of particles due to the flow of droplets after coating, and a homogeneous film can be produced.

  The electrode catalyst layer and the solid polymer electrolyte membrane are joined by thermocompression bonding. Further, in order to improve the bonding property between the electrode catalyst layer and the proton conductive polymer, it is preferable to apply a solution containing the proton conductive polymer as a binder, which is the same as the solid polymer electrolyte membrane. It is more preferable to use these materials.

  Furthermore, as the gas diffusion layer and separator used in the present invention, those used in ordinary fuel cells can be used. Specifically, porous carbon materials such as carbon cloth, carbon paper, and nonwoven fabric are used as the gas diffusion layer. As the separator, a carbon type metal type or the like can be used. The fuel cell is manufactured by assembling other accompanying devices such as a gas supply device and a cooling device.

[Example]
[Preparation of catalyst ink]
After heat-treating Ketjen Black to increase the degree of graphitization, a carrier carrying platinum and a carrier carrying platinum on Ketjen Black, an amorphous carbon with a low degree of graphitization (Tanaka Kikinzoku Kogyo Co., Ltd.) are carbonized. 1: 1 by weight, pure water and a Nafion (DuPont) solution were mixed and dispersed to obtain a catalyst ink.

[Comparative example]
After performing heat treatment and increasing the degree of graphitization, using only the carrier supporting platinum, except that the carrier supporting platinum was not mixed with ketjen black, which is an amorphous carbon having a low degree of graphitization, It was produced in the same manner as in Example 1.
[Method for producing electrode catalyst layer]
The catalyst inks thus prepared (Example) and (Comparative Example) were applied onto a transfer substrate to prepare an electrode catalyst layer. At this time, the catalyst ink application conditions were adjusted so that the Pt mass per unit area was 0.3 mg / cm ² or less. After the drying process, it was punched into a predetermined electrode size.

[Membrane electrode assembly production]
As the solid polymer electrolyte membrane, a proton conductive polymer membrane, Nafion 212 (manufactured by DuPont) was used. By sandwiching the electrode catalyst layer on both surfaces of the electrolyte membrane with the previously prepared electrode catalyst layer and performing hot pressing under conditions of 130 ° C. and 6.0 MPa, only the transfer substrate is peeled off, so that the solid catalyst with electrode catalyst layer is removed. A molecular electrolyte membrane and a membrane electrode assembly were obtained.

  Carbon paper was disposed as a gas diffusion layer on both surfaces of the obtained membrane electrode assembly, and was further sandwiched between a pair of calcined carbon separators to produce a single-cell solid polymer fuel cell.

[Evaluation]
[Power generation characteristics]
The power generation characteristics were evaluated with a fuel cell measurement device (GFT-SG1 manufactured by Toyo Technica Co., Ltd.). Using hydrogen gas as the fuel and air as the oxidant, the cell voltage values were evaluated at a current density of 0.5 A / cm ² and 1.0 A / cm ² under a cell temperature of 80 ° C. and low humidification conditions.

FIG. 3 shows the results of power generation characteristics of the polymer electrolyte fuel cells produced using the electrode catalyst layers of Example 1 and Comparative Example 1, and current densities of 0.5 A / cm ² and 1.0 A / cm ^{2 are shown.} It is a graph which shows the cell voltage value in. It was confirmed that the cell voltage of Example 1 was higher than that of Comparative Example 1 and the performance was improved.

FIG. 1 is a schematic cross-sectional view of a membrane electrode assembly of the present invention. FIG. 2 is an exploded schematic view of the polymer electrolyte fuel cell of the present invention. FIG. 3 is a graph showing the results of power generation characteristics of the polymer electrolyte fuel cells of Example 1 and Comparative Example 1.

Explanation of symbols

DESCRIPTION OF SYMBOLS 12 Membrane electrode assembly 1 Solid polymer electrolyte membrane 2 Electrode catalyst layer 3 Electrode catalyst layer 4 Air electrode side gas diffusion layer 5 Fuel electrode side gas diffusion layer 6 Air electrode 7 Fuel electrode 8 Gas flow path 9 Cooling water flow path 10 Separator

Claims (3)

A membrane electrode assembly having a structure in which a polymer electrolyte membrane is sandwiched between a pair of electrode catalyst layers,
At least one of the electrode catalyst layers includes a polymer electrolyte, a catalyst substance, and a carbon support that supports the catalyst substance, and the carbon support includes at least two kinds of carbon supports having different degrees of graphitization. Membrane electrode assembly.   The membrane electrode assembly according to claim 1, wherein the carbon support is selected from graphitic carbon, carbon fiber, carbon nanotube, nanohorn, and fullerene. The membrane electrode assembly according to claim 1 or 2 is sandwiched between a pair of gas diffusion layers, and
A solid polymer fuel cell comprising a structure in which a membrane electrode assembly sandwiched between a pair of gas diffusion layers is sandwiched between a pair of separators.