WO2008029259A2 - Proton- conductive material based on alklysulfonic acid containing polysiloxanes, method for producing same material, solid polymer electrolyte membrane, and fuel cell - Google Patents

Proton- conductive material based on alklysulfonic acid containing polysiloxanes, method for producing same material, solid polymer electrolyte membrane, and fuel cell Download PDF

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WO2008029259A2
WO2008029259A2 PCT/IB2007/002565 IB2007002565W WO2008029259A2 WO 2008029259 A2 WO2008029259 A2 WO 2008029259A2 IB 2007002565 W IB2007002565 W IB 2007002565W WO 2008029259 A2 WO2008029259 A2 WO 2008029259A2
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proton
conductive material
solid polymer
polymer electrolyte
electrolyte membrane
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PCT/IB2007/002565
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French (fr)
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WO2008029259A3 (en
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Kohei Hase
Tomohiro Nakano
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Toyota Jidosha Kabushiki Kaisha
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
    • H01B1/06—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors mainly consisting of other non-metallic substances
    • H01B1/12—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors mainly consisting of other non-metallic substances organic substances
    • H01B1/122—Ionic conductors
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0002—Organic membrane manufacture
    • B01D67/0009—Organic membrane manufacture by phase separation, sol-gel transition, evaporation or solvent quenching
    • B01D67/0011—Casting solutions therefor
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/06—Organic material
    • B01D71/70—Polymers having silicon in the main chain, with or without sulfur, nitrogen, oxygen or carbon only
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/06—Organic material
    • B01D71/76—Macromolecular material not specifically provided for in a single one of groups B01D71/08 - B01D71/74
    • B01D71/82—Macromolecular material not specifically provided for in a single one of groups B01D71/08 - B01D71/74 characterised by the presence of specified groups, e.g. introduced by chemical after-treatment
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
    • C08G77/00—Macromolecular compounds obtained by reactions forming a linkage containing silicon with or without sulfur, nitrogen, oxygen or carbon in the main chain of the macromolecule
    • C08G77/04—Polysiloxanes
    • C08G77/38—Polysiloxanes modified by chemical after-treatment
    • C08G77/382—Polysiloxanes modified by chemical after-treatment containing atoms other than carbon, hydrogen, oxygen or silicon
    • C08G77/392—Polysiloxanes modified by chemical after-treatment containing atoms other than carbon, hydrogen, oxygen or silicon containing sulfur
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J5/00—Manufacture of articles or shaped materials containing macromolecular substances
    • C08J5/20—Manufacture of shaped structures of ion-exchange resins
    • C08J5/22—Films, membranes or diaphragms
    • C08J5/2206—Films, membranes or diaphragms based on organic and/or inorganic macromolecular compounds
    • C08J5/2218—Synthetic macromolecular compounds
    • C08J5/2256—Synthetic macromolecular compounds based on macromolecular compounds obtained by reactions other than those involving carbon-to-carbon bonds, e.g. obtained by polycondensation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • H01M8/1018—Polymeric electrolyte materials
    • H01M8/102—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer
    • H01M8/1027—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer having carbon, oxygen and other atoms, e.g. sulfonated polyethersulfones [S-PES]
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • H01M8/1018—Polymeric electrolyte materials
    • H01M8/102—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer
    • H01M8/1037—Polymeric electrolyte materials characterised by the chemical structure of the main chain of the ion-conducting polymer having silicon, e.g. sulfonated crosslinked polydimethylsiloxanes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • H01M8/1018—Polymeric electrolyte materials
    • H01M8/1069—Polymeric electrolyte materials characterised by the manufacturing processes
    • H01M8/1072—Polymeric electrolyte materials characterised by the manufacturing processes by chemical reactions, e.g. insitu polymerisation or insitu crosslinking
    • H01M8/1074—Sol-gel processes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/24—Mechanical properties, e.g. strength
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/26—Electrical properties
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2383/00—Characterised by the use of macromolecular compounds obtained by reactions forming in the main chain of the macromolecule a linkage containing silicon with or without sulfur, nitrogen, oxygen, or carbon only; Derivatives of such polymers
    • C08J2383/02—Polysilicates
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the invention relates to a proton-conductive material, a method for preparing the same material, a solid polymer electrolyte membrane, and a fuel cell including the same material or membrane. More specifically, the invention relates to a proton-conductive material that exhibits a high proton conductivity even when it is not highly moisturized and thus can be advantageously used as the material for an electrolyte membrane of a fuel cell, etc., and to a solid polymer electrolyte membrane.
  • Solid polymer electrolyte is a solid polymer material having electrolyte groups, such as sulfonic acid groups, in its polymeric chains. Therefore, solid polymer electrolyte is firmly bonded with specific ions, and positive ions and negative ions selectively permeate solid polymer electrolyte.
  • solid polymer electrolyte is provided in various forms, such as particles, fibers, and membranes, and is widely used for various applications, such as electrodialysis, diffusion dialysis, and barrier membranes for batteries.
  • Fuel cells directly convert the chemical energy of fuel (e.g., hydrogen, methanol) into electric energy by oxidizing the fuel electrochemically.
  • fuel cells have been attracting much attention for their utilization as a clean source of electric energy.
  • solid polymer fuel cells using, as electrolyte, proton exchange membranes have a high output density and thus can operate at a low temperature, and therefore their utilization as power sources for electric vehicles is anticipated.
  • the basic structure of such a solid polymer fuel cell is comprised of an electrolyte membrane, a pair of gas diffusion electrodes attached on both sides of the electrolyte membrane and each having a catalytic layer, and a pair of current collectors provided on the outer sides of the respective gas diffusion electrodes.
  • fuel e.g., hydrogen, methanol
  • an oxidizer e.g., oxygen, air
  • external load circuits are then connected to between the gas diffusion electrodes, whereby the fuel cell starts its powering operation.
  • the electrolyte membranes serve as diaphragms for hydrogen gas and oxygen gas.
  • the electrolyte membranes are required to have a high proton conductivity, a high strength, and a high chemical stability.
  • the catalyst of each gas diffusion electrode is, typically, formed of electron-conductive carriers (e.g., carbon) on each of which noble metal (e.g., platinum) is supported.
  • proton-conductive polymer electrolyte is used as an electrode catalyst binder through which protons moves to the catalyst.
  • fluorine-containing polymer having sulfonic acid groups e.g., perfluorosulfonic acid polymer
  • the fluorine-containing polymer having sulfonic acid groups can be used also as a binder for the catalyst on each gas diffusion electrode or as a bonding agent for improving the adhesion between the ion-exchange membrane and each gas diffusion electrode.
  • fluorine-based electrolyte which is typified by perfluorosulfonic acid membranes, has C-F bond and thus has a very high chemical stability. Therefore, fluorine-based electrolyte is used for solid polymer electrolyte membranes for electrolyzing hydrohalic acid, as well as solid polymer electrolyte membranes for fuel cells and as solid polymer electrolyte membranes for electrolyzing water and sodium chloride. Further, fluorine-based electrolyte is widely used for humidity sensors, gas sensors, oxygen condensers, etc.
  • Fluorine-based electrolyte membranes each having perfluoroalkylene as the main skeleton and having ion-exchange groups (e.g., sulfonic acid groups, carboxylic acid groups) at the ends of some of perfluorovinylether side chains are typically used as electrolyte membranes for fuel cells.
  • fluorine-based electrolyte membranes which are typified by perfluorosulfonic acid membranes, have a very high chemical stability, they have been commercially used as electrolyte membranes for the use under severe conditions.
  • fluorine-based electrolyte membranes are, for example, Nafion membranes (registered trademark, E. I.
  • the present solid polymer fuel cells are normally operated in a relatively low temperature range from the room temperature to approx. 80 0 C.
  • the reason of this temperature limitation is as follows. (1) Because water is used as a proton-conductive medium, if the fuel cells is used at a temperature above 100 0 C, which is the boiling point of water, pressurization is needed and it requires a major system. (2) Because a glass transition temperature Tg of the fluorine-based electrolyte membranes used in the fuel cells is around 130 °C, if these membranes are heated exceeding this temperature, the ion channel structures, which contribute to the proton conduction, are destroyed. Thus, in fact, such fluorine-based electrolyte membranes can be used only in a temperature range up to 100 0 C.
  • the lower the operation temperature of the fuel cells the lower their power generation efficiency, which is not desirable. If the operation temperature is made equal to or higher than 100 0 C, it increases the power generation efficiency and enables the utilization of the waste heat, leading to a more efficient use of the energy. Further, if the operation temperature is increased up to 120 °C, it significantly broadens the scope of selection of the catalyst material and thus reduces the cost of fuel cells, as well as increasing the efficiency and enabling the utilization of the waste heat.
  • the proton-conductive membranes are essentially contain water as the substance for enabling the proton conduction.
  • the proton conductivity of a proton-conductive membrane which is typified by National (registered trademark), largely depends on the amount of water contained in the membrane. When no water is contained, the membrane exhibits no proton conductivity. Therefore, at a high temperature exceeding 100 0 C, pressurization is needed and the load on the system therefore increases. Especially, when the temperature is higher than 150 °C, a very high pressure is needed, which increases the cost of the fuel cells. On the other hand, the water contained in the proton-conductive membrane is frozen below the freezing point, and it may destroy the proton-conductive membrane.
  • perfluorosulfonic-acid-based solid electrolyte membranes require the use of water to enable the proton conduction, it is necessary to moisturize the fuel and the oxidizer.
  • the perfluorosulfonic-acid-based solid electrolyte membranes may emit acid substances that are produced as a result of the electrolyte membranes being decomposed due to various deterioration and degradation factors, and the produced acid substances may affect the peripheral parts and components.
  • the molecular structure of each perfluorosulfonic-acid-based solid electrolyte membrane is made flexible to increase the freedom of sulfonic acid groups, and therefore its stability is not sufficient.
  • perfluorosulfonic-acid-based electrolyte has drawbacks of being not suitable for the high temperature operation of fuel cells, etc., being difficult to be produced, and being very expensive. Therefore, it has been desired to develop a new ion-conductive/ion-exchange material as an alternative to perfluorosulfonic-acid-based electrolyte.
  • electrolyte membranes having a high ion conductivity are preferably used.
  • the ion conductivity of a membrane largely depends on the number of ion-exchange groups.
  • fluorine-based ion-exchange resin membranes having a dry weight of approx. 950 to 1200 per equivalent (equivalent weight) are used in fuel cells.
  • Fluorine-based ion-exchange resin membranes having an equivalent weight of less than 950 exhibit a higher ion conductivity. However, they easily dissolve in water and warm water, and therefore their durability is not high when used in fuel cells.
  • JP-2002-352819 recites a fluorine-based ion-exchange resin membrane having a low equivalent weight, which can be used in fuel cells. More specifically, the dry weight of this membrane per equivalent of ion-exchange group is from 250 to 940. The amount of decrease in the weight of this membrane after heated in a boiled water for 8 hours is 5 wt % as measured with respect to its dry weight before being heated in the boiled water.
  • the fluorine-based ion-exchange resin membrane recited in Japanese Patent Application Publication No. JP-2002-352819 is an ion-conductive membrane formed of conventional perfluorosulfonic-acid-based electrolyte, although its equivalent weight is relatively small, it needs to be moisturized when used, and therefore it is difficult to raise the operation temperature of the membrane to 100 0 C or higher. Further, although this membrane is claimed to have an equivalent weight of 250 to 940, its equivalent weight was only 614 when it is actually produced.
  • a new proton-conductive material which has a low equivalent weight, which exhibits a good proton conductivity even when it is not moisturized or when it contains only a small amount of water, which has a high strength, a high thermal stability, and a high chemical stability, and which can be produced in a simple procedure and at a low cost, and for the purpose of providing fuel cells that can operate at a high temperature even when they are not moisturized or when they contain only a small amount of water
  • the present inventors have invented a proton-conductive material that has a dry weight of 250 or less, preferably 200 or less, per equivalent of ion-exchange group (equivalent weight) (Japanese Patent Application Publication No. 2006-114277 (JP-A-2006-114277)).
  • the first aspect of the invention relates to a proton-conductive material in which the distance between adjacent clusters is 5 to 20 angstroms.
  • the distance between adjacent clusters represents the average distance between clusters each formed by ion-exchange groups of polymer electrolyte and water molecules, that is, the average distance between the centers of adjacent proton conduction paths.
  • the distance between adjacent clusters is 5 to 20 angstroms, and it means that in the proton conductive material according to the first aspect of the invention, each cluster is thin and clusters are located close to each other as compared to in conventional proton-conducive materials such as Naffion (registered trademark). As such, the proton conductive material according to the first aspect of the invention exhibits a high proton conductivity even when it is not highly moisturized.
  • the proton-conductive material according to the first aspect of the invention may have a basic skeleton expressed by the following structural formula:
  • n 100 : 0 to 80 : 20.
  • the second aspect of the invention relates to a solid polymer electrolyte membrane having a proton-conductive material in which the distance between adjacent clusters is 5 to 20 angstroms.
  • a proton-conductive material having a basic skeleton expressed by the above structural formula may be used as the proton-conductive material for the solid polymer electrolyte membrane according to the second aspect of the invention.
  • the above polymer electrolyte membrane exhibits a sufficiently high proton conductivity even when it contains only a small amount of water or no water.
  • the third aspect of the invention relates to a method for producing the above proton-conductive material.
  • This method includes: obtaining trialkoxysilane-alkylsulfonic acid by oxidizing mercapto group of mercaptoalkyl trialkoxysilane; making an alkoxy group of the trialkoxysilane-alkylsulfonic acid a hydroxyl group; and obtaining a proton-conductive material in which the distance between adjacent clusters is 5 to 20 angstroms by using, as a starting material, the trialkoxysilane-alkylsulfonic acid in which the alkoxy group has been made the hydroxyl group and which is a monomer compound.
  • a proton-conductive material having a basic skeleton expressed by the above structural formula is one example of the proton-conductive material that can be produced by the method according to the third aspect of the invention.
  • the sol-gel method may be used to produce the proton-conductive material.
  • solid polymer electrolyte membrane can be produced while producing the proton-conductive material without any additional membrane-forming process.
  • the method for forming the membrane is not limited to any specific one.
  • the membrane can be formed as follows after mixing powders of the solid polymer electrolyte of the invention and an appropriate binder(s).
  • the membrane may be formed using various other methods, such as the casting method in which membrane is formed by casting a solution onto a flat plate, a method in which a solution is applied onto a flat plate using a die coater, a comma coater, etc. Further, the membrane may be formed by spreading melted polymer material.
  • the method for producing the above proton-conductive material may include: a process of oxidizing a mercapto group of mercaptoalkyl trialkoxysilane to a sulfonic acid group, a process of converting an alkoxy group of trialkoxysilane-alkylsulfonic acid to a hydroxyl group, and, optionally, converting an alkoxy group of tetraalkoxysilane to a hydroxyl group, and a process of condensing these monomer compounds.
  • Preferred examples of the starting material are 3-mercaptopropyl trimethoxysilane (MePTMS) as the mercaptoalkyl trialkoxysilane and tetramethoxysilane (TMOS) as the tetraalkoxysilane.
  • MePTMS 3-mercaptopropyl trimethoxysilane
  • TMOS tetramethoxysilane
  • the fourth aspect of the invention relates to a fuel cell using the proton-conductive material described above. More specifically, this fuel cell is a solid polymer fuel cell having a membrane-electrode assembly (MEA) constituted of a solid polymer electrolyte membrane (a) and gas diffusion layers (b) each including, as the main material, electrode catalyst made of conductive carriers on each of which catalytic metal is supported and a proton-exchange material.
  • MEA membrane-electrode assembly
  • the solid polymer electrolyte membrane is the one recited in the second aspect of the invention
  • the proton-exchange material is the proton-conductive material recited in the first or third aspect of the invention.
  • the above fuel cells can operate properly even when they are not moisturized or even when they are not highly moisturized and even at a high temperature, has a high mechanical strength, and can be produced in a simple procedure and at a low cost.
  • the proton-conductive material of the invention exhibits a sufficiently high proton conductivity even when it is not moisturized or when it is not highly moisturized. That is, by making the distance between adjacent clusters 5 to 20 angstroms, it is possible to obtain a high proton conductivity of a 10 "2 S/cm order in a low humidity state where the temperature is 80 0 C and the humidity is 5 %RH. Having such a high proton conductivity at a low humidity, the fuel cells can operate at 100 0 C or higher.
  • the invention therefore, it is possible to provide fuel cells that can operate at 100 °C or higher and thus have a higher operation efficiency and a higher capacity. Further, according to the invention, the components and parts related to the low-temperature operation and the moisturization can be removed, and therefore the system can be made small in size.
  • FIG. 1 is a graph illustrating the relation between the cluster-to-cluster distance and the proton conductivity for each of the proton-conductive materials of the example of the invention and the first to third comparative examples, which was found at 80 °C and at 20 %RH;
  • FIG.2 is a graph illustrating how the proton conductivity of the proton-conductive material of the example of the invention and the proton conductivity of Nafion (registered trademark) 112 changed depending upon the relative humidity in a low humidity region at 80 0 C.
  • FIG. 3 is a graph illustrating how the cluster-cluster distance in the proton-conductive material of the example of the invention and the cluster-cluster distance in Nafion (registered trademark) 112 changed depending upon the relative humidity in a low humidity region at 80 0 C.
  • polymer electrolyte membranes used in solid polymer fuel cells are required to have a high proton conductivity.
  • a solid polymer electrolyte membrane is constituted of a hydrophobic main chain skeleton and a hydrophilic side chain having a sulfonic acid group, forming a phase-separated structure. Therefore, the cluster structure is formed.
  • the cluster structure of a solid polymer electrolyte membrane is regarded as being suitable for proton conduction.
  • the diameter of each cluster in a polymer electrolyte membrane can be measured using various other measurement methods.
  • a measurement method may be used in which ion exchange is performed between H + ion and metal cation of each sulfonic acid group and the cluster structures are then observed using a transmission electron microscope (TEM).
  • TEM transmission electron microscope
  • a measurement method may be used in which the diameter of each cluster is measured using a differential scanning calorimeter (DSC) based on the fact that the melting temperature of the ices retained in a nanometer order in the pores and the clusters is lower than the melting temperature of a bulky ice.
  • DSC differential scanning calorimeter
  • the small angle X-ray scattering method is used to measure the diameter of each cluster because it allows the observation under the in-situ state of a polymer electrolyte membrane and allows the measurement under various ambient atmospheres.
  • Example of Invention Synthesis of Proton-conductive Material
  • MePTMS 3-mercaptopropyl-trimethoxysilane
  • TMOS tetramethoxysilane
  • a proton conductive material having an equivalent weight of 175 was synthesized by the following reaction scheme.
  • Nafion (registered trademark) 112 was used as the first comparative example.
  • Nafion (registered trademark) 1 15 was used as the second comparative example.
  • Nafion (registered trademark) having an equivalent weight of 1500 was used as the third comparative example.
  • the cluster-to-cluster distance and the proton conductivity of each of the proton-conductive materials of the example of the invention and the first to third comparative examples were measured.
  • the cluster-to-cluster distance was measured using the in-situ small angle X-ray scattering (SAXS) method, and the proton conductivity was calculated through an impedance analysis using the AC two-terminal method.
  • SAXS small angle X-ray scattering
  • FIG. 1 shows the relation between the cluster-to-cluster distance and the proton conductivity for each of the proton-conductive materials of the example of the invention and the first to third comparative examples, which was found at 80 0 C and at 20 %RH.
  • "+" represents the data of the example of the invention
  • the black square represents the data of the first comparative example
  • "x" represents the data of the second comparative example
  • the empty square represents the data of the third comparative example.
  • FIG. 1 the proton conductivity of the proton-conductive material of the example of the invention (designated by "+” in FIG 1) and the proton conductivity of Nafion 112 (designated by the black square in FIG. 1 ), which were measured at 80 0 C and at 20 %RH, were high.
  • FIG2 illustrates how the proton conductivity of the proton-conductive material of the example of the invention and the proton conductivity of Nafion (registered trademark) 112 changed depending upon the relative humidity in a low humidity region at 80 °C.
  • FIG 3 illustrates how the cluster-cluster distance in the proton-conductive material of the example of the invention (designated by "+ " in FIG. 1 ) and the cluster-cluster distance in Nafion (registered trademark) 112 (designated by the black square in FIG. 1 ) changed depending upon the relative humidity in a low humidity region at 80 0 C.
  • the cluster-to-cluster distance of Nafion (registered trademark) 112 in a humidity range of 30 % or less was 27 to 29 angstroms, and its proton conductivity significantly decreased with a decrease in the humidity.
  • the cluster-to-cluster distance of the proton-conductive material of the example of the invention in a humidity range of 30 % or less was 18 to 19 angstroms, and its proton conductivity was 10 "2 S/cm, which is relatively high, in a low humidity state where the temperature was 80 0 C and the humidity was 5 %RH.
  • the proton-conductive material of the example of the invention can be used also as an electrode catalyst binder, as well as the material of an electrolyte membrane.
  • proton-conductive materials according to the invention exhibit a sufficiently high proton conductivity even when they are not highly moisturized and also when they are not moisturized. That is, a high proton conductivity of a IO 2 S/cm order can be obtained even at an extremely low temperature by reducing the cluster-to-cluster distance to 20 angstroms or less.
  • the proton conductivity of the proton-conductive material can be maintained high even if it is not highly moisturized, it is possible to provide fuel cells that can operate at 100 0 C or higher and thus have a higher operation efficiency and a higher capacity.
  • the use of the proton-conductive material of the invention makes it possible to remove the components and parts related to the low-temperature operation and the moisturization. Therefore, the system can be made small in size.
  • the proton-conductive material of the invention exhibits an extremely high proton conductivity at a high temperature and has a high heat resistance, it is possible to provide fuel cells that can operate at a higher operation temperature and thus have a higher power generation efficiency. Thus, the production cost of fuel cells can be reduced.
  • Solid polymer electrolyte membranes according to the invention may be widely used as an electrolyte membrane for electrolyzing hydrohalic acid, an electrolyte membrane for electrolyzing water, an electrolyte membrane for electrolyzing sodium chloride, an oxygen condenser, a humidity sensor, a gas sensor, and so on, as well as an electrolyte membrane for fuel cells.

Abstract

The invention provides a proton-conductive material in which the distance between adjacent clusters is 5 to 20 angstroms.

Description

PROTON-CONDUCTIVE MATERIAL, METHOD FOR PRODUCING SAME
MATERIAL, SOLID POLYMER ELECTROLYTE MEMBRANE, AND FUEL CELL
FIELD OF THE INVENTION
[0001] The invention relates to a proton-conductive material, a method for preparing the same material, a solid polymer electrolyte membrane, and a fuel cell including the same material or membrane. More specifically, the invention relates to a proton-conductive material that exhibits a high proton conductivity even when it is not highly moisturized and thus can be advantageously used as the material for an electrolyte membrane of a fuel cell, etc., and to a solid polymer electrolyte membrane.
BACKGROUND OF THE INVENTION
[0002] Solid polymer electrolyte is a solid polymer material having electrolyte groups, such as sulfonic acid groups, in its polymeric chains. Therefore, solid polymer electrolyte is firmly bonded with specific ions, and positive ions and negative ions selectively permeate solid polymer electrolyte. Thus, solid polymer electrolyte is provided in various forms, such as particles, fibers, and membranes, and is widely used for various applications, such as electrodialysis, diffusion dialysis, and barrier membranes for batteries.
[0003] Fuel cells directly convert the chemical energy of fuel (e.g., hydrogen, methanol) into electric energy by oxidizing the fuel electrochemically. Thus, in recent years, fuel cells have been attracting much attention for their utilization as a clean source of electric energy. In particular, solid polymer fuel cells using, as electrolyte, proton exchange membranes have a high output density and thus can operate at a low temperature, and therefore their utilization as power sources for electric vehicles is anticipated.
[0004] The basic structure of such a solid polymer fuel cell is comprised of an electrolyte membrane, a pair of gas diffusion electrodes attached on both sides of the electrolyte membrane and each having a catalytic layer, and a pair of current collectors provided on the outer sides of the respective gas diffusion electrodes. In operation, fuel (e.g., hydrogen, methanol) is supplied to one of the gas diffusion electrodes (anode) while an oxidizer (e.g., oxygen, air) is supplied to the other (cathode), and external load circuits are then connected to between the gas diffusion electrodes, whereby the fuel cell starts its powering operation. During this time, the protons produced at the anode move to the cathode through the electrolyte membrane, and they react with oxygen at the cathode, whereby water is produced. Here, the electrolyte membranes serve as diaphragms for hydrogen gas and oxygen gas. Thus, the electrolyte membranes are required to have a high proton conductivity, a high strength, and a high chemical stability.
[0005] Meanwhile, the catalyst of each gas diffusion electrode is, typically, formed of electron-conductive carriers (e.g., carbon) on each of which noble metal (e.g., platinum) is supported. For the purpose of improving the usage efficiency of the catalyst supported on each gas diffusion electrode, proton-conductive polymer electrolyte is used as an electrode catalyst binder through which protons moves to the catalyst. As the material of this proton-conductive electrolyte, fluorine-containing polymer having sulfonic acid groups (e.g., perfluorosulfonic acid polymer) can be used, which can be used also as the material of the ion-exchange membrane. The fluorine-containing polymer having sulfonic acid groups can be used also as a binder for the catalyst on each gas diffusion electrode or as a bonding agent for improving the adhesion between the ion-exchange membrane and each gas diffusion electrode.
[0006] In the meantime, fluorine-based electrolyte, which is typified by perfluorosulfonic acid membranes, has C-F bond and thus has a very high chemical stability. Therefore, fluorine-based electrolyte is used for solid polymer electrolyte membranes for electrolyzing hydrohalic acid, as well as solid polymer electrolyte membranes for fuel cells and as solid polymer electrolyte membranes for electrolyzing water and sodium chloride. Further, fluorine-based electrolyte is widely used for humidity sensors, gas sensors, oxygen condensers, etc. [0007] Fluorine-based electrolyte membranes each having perfluoroalkylene as the main skeleton and having ion-exchange groups (e.g., sulfonic acid groups, carboxylic acid groups) at the ends of some of perfluorovinylether side chains are typically used as electrolyte membranes for fuel cells. Because fluorine-based electrolyte membranes, which are typified by perfluorosulfonic acid membranes, have a very high chemical stability, they have been commercially used as electrolyte membranes for the use under severe conditions. Known examples of such fluorine-based electrolyte membranes are, for example, Nafion membranes (registered trademark, E. I. du Pont de Nemours and Company), Dow membranes (Dow Chemical Company), Aciplex membranes (registered trademark, Asahi Kasei Corporation), Flemion membranes (registered trademark, Asahi Glass. Co., Ltd.), and so on.
[0008] The present solid polymer fuel cells are normally operated in a relatively low temperature range from the room temperature to approx. 80 0C. The reason of this temperature limitation is as follows. (1) Because water is used as a proton-conductive medium, if the fuel cells is used at a temperature above 100 0C, which is the boiling point of water, pressurization is needed and it requires a major system. (2) Because a glass transition temperature Tg of the fluorine-based electrolyte membranes used in the fuel cells is around 130 °C, if these membranes are heated exceeding this temperature, the ion channel structures, which contribute to the proton conduction, are destroyed. Thus, in fact, such fluorine-based electrolyte membranes can be used only in a temperature range up to 100 0C.
[0009] The lower the operation temperature of the fuel cells, the lower their power generation efficiency, which is not desirable. If the operation temperature is made equal to or higher than 100 0C, it increases the power generation efficiency and enables the utilization of the waste heat, leading to a more efficient use of the energy. Further, if the operation temperature is increased up to 120 °C, it significantly broadens the scope of selection of the catalyst material and thus reduces the cost of fuel cells, as well as increasing the efficiency and enabling the utilization of the waste heat.
[0010] On the other hand, one of the reasons for the difficulty in using the present proton-conductive membranes at a high temperature is that they essentially contain water as the substance for enabling the proton conduction. The proton conductivity of a proton-conductive membrane, which is typified by Nation (registered trademark), largely depends on the amount of water contained in the membrane. When no water is contained, the membrane exhibits no proton conductivity. Therefore, at a high temperature exceeding 100 0C, pressurization is needed and the load on the system therefore increases. Especially, when the temperature is higher than 150 °C, a very high pressure is needed, which increases the cost of the fuel cells. On the other hand, the water contained in the proton-conductive membrane is frozen below the freezing point, and it may destroy the proton-conductive membrane.
[0011] Further, even in the case where the fuel cells are operated in a temperature range from the room temperature to approx. 80 0C as they are conventionally, the essentiality of water is one of major concerns. To maintain water in the membranes continuously, it is necessary to moisturize the fuel (e.g., hydrogen) before distributing it into each membrane, for example. Performing such fuel moisturization requires an accurate and complicated water amount management, therefore the structures of the fuel cells become complicated and the possibility of operation failures and malfunctions increases accordingly.
[0012] As such, because perfluorosulfonic-acid-based solid electrolyte membranes require the use of water to enable the proton conduction, it is necessary to moisturize the fuel and the oxidizer. Also, the perfluorosulfonic-acid-based solid electrolyte membranes may emit acid substances that are produced as a result of the electrolyte membranes being decomposed due to various deterioration and degradation factors, and the produced acid substances may affect the peripheral parts and components. Further, the molecular structure of each perfluorosulfonic-acid-based solid electrolyte membrane is made flexible to increase the freedom of sulfonic acid groups, and therefore its stability is not sufficient.
[0013] Namely, perfluorosulfonic-acid-based electrolyte has drawbacks of being not suitable for the high temperature operation of fuel cells, etc., being difficult to be produced, and being very expensive. Therefore, it has been desired to develop a new ion-conductive/ion-exchange material as an alternative to perfluorosulfonic-acid-based electrolyte.
[0014] In the case where proton-conductive membranes are used as solid polymer electrolyte membranes in fuel cells, in order to minimize the electric resistance during power generation, electrolyte membranes having a high ion conductivity are preferably used. The ion conductivity of a membrane largely depends on the number of ion-exchange groups. Typically, fluorine-based ion-exchange resin membranes having a dry weight of approx. 950 to 1200 per equivalent (equivalent weight) are used in fuel cells. Fluorine-based ion-exchange resin membranes having an equivalent weight of less than 950 exhibit a higher ion conductivity. However, they easily dissolve in water and warm water, and therefore their durability is not high when used in fuel cells.
[0015] In view of the above, Japanese Patent Application Publication No. JP-2002-352819 (JP-A-2002-352819) recites a fluorine-based ion-exchange resin membrane having a low equivalent weight, which can be used in fuel cells. More specifically, the dry weight of this membrane per equivalent of ion-exchange group is from 250 to 940. The amount of decrease in the weight of this membrane after heated in a boiled water for 8 hours is 5 wt % as measured with respect to its dry weight before being heated in the boiled water.
[0016] As such, because the fluorine-based ion-exchange resin membrane recited in Japanese Patent Application Publication No. JP-2002-352819 (JP-A-2002-352819) is an ion-conductive membrane formed of conventional perfluorosulfonic-acid-based electrolyte, although its equivalent weight is relatively small, it needs to be moisturized when used, and therefore it is difficult to raise the operation temperature of the membrane to 100 0C or higher. Further, although this membrane is claimed to have an equivalent weight of 250 to 940, its equivalent weight was only 614 when it is actually produced. The reason why the equivalent weight of such a membrane made of perfluorosulfonic-acid-based electrolyte can not be made lower than 600 is that the molecular weight of the unit having sulfonic-acid groups is large and that copolymer units having no sulfonic-acid groups, such as tetrafluoroethylene, are essentially used to synthesize polymers.
DISCLOSURE OF THE INVENTION
[0017] For the purpose of providing, as an alternative to the conventional perfluorosulfonic-acid-based electrolyte, a new proton-conductive material which has a low equivalent weight, which exhibits a good proton conductivity even when it is not moisturized or when it contains only a small amount of water, which has a high strength, a high thermal stability, and a high chemical stability, and which can be produced in a simple procedure and at a low cost, and for the purpose of providing fuel cells that can operate at a high temperature even when they are not moisturized or when they contain only a small amount of water, the present inventors have invented a proton-conductive material that has a dry weight of 250 or less, preferably 200 or less, per equivalent of ion-exchange group (equivalent weight) (Japanese Patent Application Publication No. 2006-114277 (JP-A-2006-114277)).
[0018] During the study on the factors of a proton-conductive material that exhibits a high proton conductivity even when it is not highly moisturized, the present inventors tried to obtain a proton-conductive material that exhibits a high proton conductivity even when the degree of moisturization is further low.
[0019] As a result of their trials, the present inventors discovered that the aforementioned problem can be solved by addressing the factors related to the distance between the clusters in high polymer electrolyte and thereby reached this invention.
[0020] In view of the above, the first aspect of the invention relates to a proton-conductive material in which the distance between adjacent clusters is 5 to 20 angstroms. "The distance between adjacent clusters" represents the average distance between clusters each formed by ion-exchange groups of polymer electrolyte and water molecules, that is, the average distance between the centers of adjacent proton conduction paths. [0021] In the proton conductive material according to the first aspect of the invention, as described above, the distance between adjacent clusters is 5 to 20 angstroms, and it means that in the proton conductive material according to the first aspect of the invention, each cluster is thin and clusters are located close to each other as compared to in conventional proton-conducive materials such as Naffion (registered trademark). As such, the proton conductive material according to the first aspect of the invention exhibits a high proton conductivity even when it is not highly moisturized.
[0022] The proton-conductive material according to the first aspect of the invention may have a basic skeleton expressed by the following structural formula:
Figure imgf000008_0001
where p is 1 to 10, preferably 1 to 5, and m : n is 100 : 0 to 80 : 20.
[0023] The second aspect of the invention relates to a solid polymer electrolyte membrane having a proton-conductive material in which the distance between adjacent clusters is 5 to 20 angstroms. A proton-conductive material having a basic skeleton expressed by the above structural formula may be used as the proton-conductive material for the solid polymer electrolyte membrane according to the second aspect of the invention.
[0024] The above polymer electrolyte membrane exhibits a sufficiently high proton conductivity even when it contains only a small amount of water or no water.
[0025] The third aspect of the invention relates to a method for producing the above proton-conductive material. This method includes: obtaining trialkoxysilane-alkylsulfonic acid by oxidizing mercapto group of mercaptoalkyl trialkoxysilane; making an alkoxy group of the trialkoxysilane-alkylsulfonic acid a hydroxyl group; and obtaining a proton-conductive material in which the distance between adjacent clusters is 5 to 20 angstroms by using, as a starting material, the trialkoxysilane-alkylsulfonic acid in which the alkoxy group has been made the hydroxyl group and which is a monomer compound.
[0026] A proton-conductive material having a basic skeleton expressed by the above structural formula is one example of the proton-conductive material that can be produced by the method according to the third aspect of the invention. Further, in the method according to the third aspect of the invention, the sol-gel method may be used to produce the proton-conductive material. In this case, solid polymer electrolyte membrane can be produced while producing the proton-conductive material without any additional membrane-forming process. On the other hand, in the case where the membrane is directly formed, the method for forming the membrane is not limited to any specific one. For example, the membrane can be formed as follows after mixing powders of the solid polymer electrolyte of the invention and an appropriate binder(s). The membrane may be formed using various other methods, such as the casting method in which membrane is formed by casting a solution onto a flat plate, a method in which a solution is applied onto a flat plate using a die coater, a comma coater, etc. Further, the membrane may be formed by spreading melted polymer material.
[0027] For example, as illustrated in the reaction scheme indicated below, the method for producing the above proton-conductive material may include: a process of oxidizing a mercapto group of mercaptoalkyl trialkoxysilane to a sulfonic acid group, a process of converting an alkoxy group of trialkoxysilane-alkylsulfonic acid to a hydroxyl group, and, optionally, converting an alkoxy group of tetraalkoxysilane to a hydroxyl group, and a process of condensing these monomer compounds. Hi(R1O)3Si-R2Smn(R3O)4Si
H2O2 water solution m(R ' O)3Si-R2SO3H+n(R3O)4S i t-butanol
-> m(HO)3Si-R2SO3H+n(HO)4Si
Figure imgf000010_0001
In the above scheme, "R1" and "R3" represent alkyl groups, and "R2" represents an alkylene group.
[0028] The hydrogen peroxide and the t-butanol used in the process of oxidizing the mercapto group of mercaptoalkyl trialkoxysilane to the sulfonic acid group easily evaporate and are therefore removed from the reaction system. Further, the sulfonic acid group (- SO3H) obtained by the above-described process serves as catalyst in the process in which the alkoxy group is converted to a hydroxyl group, and therefore no secondary reactants and impurities are produced. Thus, the production method of the invention is significantly rational.
[0029] Preferred examples of the starting material are 3-mercaptopropyl trimethoxysilane (MePTMS) as the mercaptoalkyl trialkoxysilane and tetramethoxysilane (TMOS) as the tetraalkoxysilane.
[0030] The fourth aspect of the invention relates to a fuel cell using the proton-conductive material described above. More specifically, this fuel cell is a solid polymer fuel cell having a membrane-electrode assembly (MEA) constituted of a solid polymer electrolyte membrane (a) and gas diffusion layers (b) each including, as the main material, electrode catalyst made of conductive carriers on each of which catalytic metal is supported and a proton-exchange material. In this solid polymer fuel cell, the solid polymer electrolyte membrane is the one recited in the second aspect of the invention, and/or the proton-exchange material is the proton-conductive material recited in the first or third aspect of the invention.
[0031] Being made of the solid polymer electrolyte and/or the solid polymer electrolyte membranes according to the invention, the above fuel cells can operate properly even when they are not moisturized or even when they are not highly moisturized and even at a high temperature, has a high mechanical strength, and can be produced in a simple procedure and at a low cost.
[0032] Conventional perfluorosulfonic-acid-based solid electrolyte membranes require the use of water to enable proton conduction, and therefore the fuel and the oxidizer need to be moisturized. On the other hand, the proton-conductive material of the invention exhibits a sufficiently high proton conductivity even when it is not moisturized or when it is not highly moisturized. That is, by making the distance between adjacent clusters 5 to 20 angstroms, it is possible to obtain a high proton conductivity of a 10"2 S/cm order in a low humidity state where the temperature is 80 0C and the humidity is 5 %RH. Having such a high proton conductivity at a low humidity, the fuel cells can operate at 100 0C or higher. According to the invention, therefore, it is possible to provide fuel cells that can operate at 100 °C or higher and thus have a higher operation efficiency and a higher capacity. Further, according to the invention, the components and parts related to the low-temperature operation and the moisturization can be removed, and therefore the system can be made small in size.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] The foregoing and further objects, features and advantages of the invention will become apparent from the following description of exemplary embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: FIG. 1 is a graph illustrating the relation between the cluster-to-cluster distance and the proton conductivity for each of the proton-conductive materials of the example of the invention and the first to third comparative examples, which was found at 80 °C and at 20 %RH;
FIG.2 is a graph illustrating how the proton conductivity of the proton-conductive material of the example of the invention and the proton conductivity of Nafion (registered trademark) 112 changed depending upon the relative humidity in a low humidity region at 80 0C.
FIG. 3 is a graph illustrating how the cluster-cluster distance in the proton-conductive material of the example of the invention and the cluster-cluster distance in Nafion (registered trademark) 112 changed depending upon the relative humidity in a low humidity region at 80 0C.
DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
[0034] Generally, polymer electrolyte membranes used in solid polymer fuel cells are required to have a high proton conductivity. A solid polymer electrolyte membrane is constituted of a hydrophobic main chain skeleton and a hydrophilic side chain having a sulfonic acid group, forming a phase-separated structure. Therefore, the cluster structure is formed. The cluster structure of a solid polymer electrolyte membrane is regarded as being suitable for proton conduction.
[0035] Owing to such cluster structures in each polymer electrolyte membrane, it is not necessary to incorporate much of proton conductive substituent, such as sulfonic acid groups, in the membrane and therefore a good proton conductivity can be obtained without sacrificing the characteristics of the membrane, such as the durability and the ease of handling.
[0036] It is known that if a small angle X-ray scattering measurement is performed to a solid polymer electrolyte membrane, a peak or shoulder is found near q = 1.5 - 2.5 nm"1. Note that "q" is expressed by the following expression. q = 4 πsinq / λ In the above expression, "q" represents the Bragg angle and "λ" represents the wavelength of the incident X ray. The peak or shoulder depends on the distribution of cluster structures, each of which measures several run or so, in the membrane.
[0037] As well as the small angle X-ray scattering method (SAXS), the diameter of each cluster in a polymer electrolyte membrane can be measured using various other measurement methods. For example, a measurement method may be used in which ion exchange is performed between H+ ion and metal cation of each sulfonic acid group and the cluster structures are then observed using a transmission electron microscope (TEM). Further, a measurement method may be used in which the diameter of each cluster is measured using a differential scanning calorimeter (DSC) based on the fact that the melting temperature of the ices retained in a nanometer order in the pores and the clusters is lower than the melting temperature of a bulky ice. However, preferably, the small angle X-ray scattering method is used to measure the diameter of each cluster because it allows the observation under the in-situ state of a polymer electrolyte membrane and allows the measurement under various ambient atmospheres.
[0038] Hereinafter, an example of the invention will be described in comparison with comparative examples.
[0039] (Example of Invention: Synthesis of Proton-conductive Material) 3-mercaptopropyl-trimethoxysilane (MePTMS) and tetramethoxysilane (TMOS) were used as the starting material, and the proton source was densified using the sol-gel method. A proton conductive material having an equivalent weight of 175 was synthesized by the following reaction scheme.
m(CH3O)3Si(CH2)3SH MePTMS
H2O2 30% water solution
-> Hi(CH3O)3Si(CH2)JSO3H t-butanol 70 0C, 1 hr
-> m(HO)3Si(CH2)3SO3H
O
O Si- ■o-
(CH2)3 m SO3H
[0040] The detail of each reaction is as follows. ( I ) A solution A was prepared by mixing MePTMS and butyl alcohol (t-BuOH). (2) A solution B was prepared by mixing oxygenated water and t-BuOH. The amount of the oxygenated water was five times the amount of MePTMS (mole ratio). The mixture ratio was H2O2 : t-BuOH = 1 : 4 (mole ratio). (3) The solution B was slowly dripped while stirring the solution A. After dripped, it was subjected to a 70 °C heat stirring process for an hour. (4) The resultant solution was put in a petri dish and dried, whereby electrolyte was obtained.
[0041] Note that it is also possible to synthesize electrolyte in the same manner by using, as the synthetic material, 3-mercaptomethyl-trimethoxysilane in place of MePTMS.
[0042] (Comparative Example) Nafion (registered trademark) 112 was used as the first comparative example. Nafion (registered trademark) 1 15 was used as the second comparative example. Nafion (registered trademark) having an equivalent weight of 1500 was used as the third comparative example.
[0043] (Proton Conductivity) The cluster-to-cluster distance and the proton conductivity of each of the proton-conductive materials of the example of the invention and the first to third comparative examples were measured. The cluster-to-cluster distance was measured using the in-situ small angle X-ray scattering (SAXS) method, and the proton conductivity was calculated through an impedance analysis using the AC two-terminal method.
[0044] FIG. 1 shows the relation between the cluster-to-cluster distance and the proton conductivity for each of the proton-conductive materials of the example of the invention and the first to third comparative examples, which was found at 80 0C and at 20 %RH. In FIG. 1, "+" represents the data of the example of the invention, the black square represents the data of the first comparative example, "x" represents the data of the second comparative example, and the empty square represents the data of the third comparative example.
[0045] As is evident from the result shown in FIG. 1 , there is a negative correlation between the cluster-cluster distance and the proton conductivity in the same humidity range. Even in a low humidity state where the temperature is 80 0C and the humidity is 20 %RH, by making the cluster-to-cluster distance 20 angstroms or less, the proton conductivity can be increased to a level close to that obtained under a fully moisturized state.
[0046] Referring to FIG. 1 , the proton conductivity of the proton-conductive material of the example of the invention (designated by "+" in FIG 1) and the proton conductivity of Nafion 112 (designated by the black square in FIG. 1 ), which were measured at 80 0C and at 20 %RH, were high. FIG2 illustrates how the proton conductivity of the proton-conductive material of the example of the invention and the proton conductivity of Nafion (registered trademark) 112 changed depending upon the relative humidity in a low humidity region at 80 °C. FIG 3 illustrates how the cluster-cluster distance in the proton-conductive material of the example of the invention (designated by "+ " in FIG. 1 ) and the cluster-cluster distance in Nafion (registered trademark) 112 (designated by the black square in FIG. 1 ) changed depending upon the relative humidity in a low humidity region at 80 0C.
[0047] As evident from the results shown in FIG. 2 and FIG. 3, the cluster-to-cluster distance of Nafion (registered trademark) 112 in a humidity range of 30 % or less was 27 to 29 angstroms, and its proton conductivity significantly decreased with a decrease in the humidity. On the other hand, the cluster-to-cluster distance of the proton-conductive material of the example of the invention in a humidity range of 30 % or less was 18 to 19 angstroms, and its proton conductivity was 10"2 S/cm, which is relatively high, in a low humidity state where the temperature was 80 0C and the humidity was 5 %RH.
[0048] As such, it was discovered that it is possible to obtain a high proton conductivity of a 10'2 S/cm order even at an extremely low humidity by reducing the cluster-to-cluster distance to 20 angstroms or less.
[0049] Note that the proton-conductive material of the example of the invention can be used also as an electrode catalyst binder, as well as the material of an electrolyte membrane.
[0050] As such, proton-conductive materials according to the invention, including the one presented above as the example of the invention, exhibit a sufficiently high proton conductivity even when they are not highly moisturized and also when they are not moisturized. That is, a high proton conductivity of a IO 2 S/cm order can be obtained even at an extremely low temperature by reducing the cluster-to-cluster distance to 20 angstroms or less. Thus, because the proton conductivity of the proton-conductive material can be maintained high even if it is not highly moisturized, it is possible to provide fuel cells that can operate at 100 0C or higher and thus have a higher operation efficiency and a higher capacity. Further, the use of the proton-conductive material of the invention makes it possible to remove the components and parts related to the low-temperature operation and the moisturization. Therefore, the system can be made small in size. As such, because the proton-conductive material of the invention exhibits an extremely high proton conductivity at a high temperature and has a high heat resistance, it is possible to provide fuel cells that can operate at a higher operation temperature and thus have a higher power generation efficiency. Thus, the production cost of fuel cells can be reduced.
[0051] Solid polymer electrolyte membranes according to the invention, including the one made of the proton-conductive material presented above as the example of the invention, may be widely used as an electrolyte membrane for electrolyzing hydrohalic acid, an electrolyte membrane for electrolyzing water, an electrolyte membrane for electrolyzing sodium chloride, an oxygen condenser, a humidity sensor, a gas sensor, and so on, as well as an electrolyte membrane for fuel cells.

Claims

1. A proton-conductive material in which the distance between adjacent clusters is 5 to 20 angstroms.
2. The proton-conductive material according to claim 1 having a basic skeleton expressed by the following structural formula:
O O
O Si O- O Si O-
(CH2)P O m SO3H
where p is 1 to 10 and m : n is 100 : 0 to 80 : 20.
3. The proton-conductive material according to claim 2, wherein p is 1 to 5.
4. A solid polymer electrolyte membrane having a proton-conductive material in which the distance between adjacent clusters is 5 to 20 angstroms.
5. The solid polymer electrolyte membrane according to claim 4, wherein the proton-conductive material has a basic skeleton expressed by the following structural formula: O O
O Si O- O Si- ■o-
(CH2)P 0 m SO3H
where p is 1 to 10 and m : n is 100 : 0 to 80 : 20.
6. The solid polymer electrolyte membrane according to claim 5, wherein p is 1 to 5.
7. A method for producing a proton-conductive material, comprising: obtaining trialkoxysilane-alkylsulfonic acid by oxidizing mercapto group of mercaptoalkyl trialkoxysilane; converting an alkoxy group of the trialkoxysilane-alkylsulfonic acid to a hydroxyl group; obtaining a proton-conductive material in which the distance between adjacent clusters is 5 to 20 angstroms and a basic skeleton thereof expressed by the following structural formula, by polymerization using a sol-gel method, wherein the trialkoxysilane-alkylsulfonic acid in which the alkoxy group has been converted to the hydroxyl group is used as a starting material which is a monomer compound,
Figure imgf000019_0001
where p is 1 to 10 and m : n is 100 : 0 to 80 : 20.
8. The method according to claim 7, wherein p is 1 to 5.
9. The method according to claim 7 or 8, wherein the alkoxy group of the tetraalkoxysilane is converted to a hydroxyl group and is used as the monomer compound.
10. The method according to any one of claims 7 to 9, wherein the mercaptoalkyl trialkoxysilane is 3-mercaptopropyl trimethoxysilane (MePTMS), and the tetraalkoxysilane is tetramethoxysilane (TMOS).
11. A fuel cell including the proton-conductive material according to any one of claims 1 to 3.
12. A fuel cell comprising the solid polymer electrolyte membrane according to any one of claims 4 to 6.
PCT/IB2007/002565 2006-09-07 2007-09-06 Proton- conductive material based on alklysulfonic acid containing polysiloxanes, method for producing same material, solid polymer electrolyte membrane, and fuel cell WO2008029259A2 (en)

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US20040062970A1 (en) * 2001-10-30 2004-04-01 Shigeki Nomura Proton conducting membrane, process for its production, and fuel cells made by using the same
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US20040062970A1 (en) * 2001-10-30 2004-04-01 Shigeki Nomura Proton conducting membrane, process for its production, and fuel cells made by using the same
US20030144450A1 (en) * 2001-12-21 2003-07-31 Fraunhofer-Gesellschaft Zur Forderung Der Angewandten Forschung E.V. Proton-conductive membranes and layers and methods for their production
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