WO2007061248A1 - Catalyst for fuel cell electrode and method of preparing the same - Google Patents

Catalyst for fuel cell electrode and method of preparing the same Download PDF

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Publication number
WO2007061248A1
WO2007061248A1 PCT/KR2006/004971 KR2006004971W WO2007061248A1 WO 2007061248 A1 WO2007061248 A1 WO 2007061248A1 KR 2006004971 W KR2006004971 W KR 2006004971W WO 2007061248 A1 WO2007061248 A1 WO 2007061248A1
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Prior art keywords
catalyst
carbon
fuel cell
transition metal
cell electrode
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PCT/KR2006/004971
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French (fr)
Inventor
Jong-Ho Park
Won Shim
Eun-Sook Lee
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Jinwoo Engineering Co., Ltd.
Skc Co., Ltd.
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Publication of WO2007061248A1 publication Critical patent/WO2007061248A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/90—Selection of catalytic material
    • H01M4/92—Metals of platinum group
    • H01M4/925—Metals of platinum group supported on carriers, e.g. powder carriers
    • H01M4/926—Metals of platinum group supported on carriers, e.g. powder carriers on carbon or graphite
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/8605—Porous electrodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/8663—Selection of inactive substances as ingredients for catalytic active masses, e.g. binders, fillers
    • H01M4/8668—Binders
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/8663—Selection of inactive substances as ingredients for catalytic active masses, e.g. binders, fillers
    • H01M4/8673—Electrically conductive fillers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/88—Processes of manufacture
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/90—Selection of catalytic material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/90—Selection of catalytic material
    • H01M4/9075—Catalytic material supported on carriers, e.g. powder carriers
    • H01M4/9083—Catalytic material supported on carriers, e.g. powder carriers on carbon or graphite
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/90—Selection of catalytic material
    • H01M4/92—Metals of platinum group
    • H01M4/921—Alloys or mixtures with metallic elements
    • 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
    • H01M8/1007—Fuel cells with solid electrolytes with both reactants being gaseous or vaporised
    • 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 present invention relates to a catalyst for a fuel cell electrode and a method of preparing the same, and more particularly, to a catalyst for a fuel cell electrode which has excellent catalyst activity, can be manufactured at low cost, and is highly resistant to CO poisoning, and a method of preparing the same.
  • a fuel cell is a type of electrical energy generating system in which energy from an electrochemical reaction between fuel and oxygen is directly converted into electrical energy.
  • the fuel cell is also an energy generating system in which the heat efficiency is not involved unlike the Carnot cycle.
  • Fuel cells are an environment-friendly energy generation technology since they are free of such problems as pollutant gases NOx and SOx, and emit little carbon dioxide compared to thermal power generation systems.
  • Fuel cells are classified into proton exchange membrane fuel cells (PEMFC), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), etc. according to the type of electrolyte.
  • the proton exchange membrane fuel cells are classified into polymer electrolyte membrane fuel cells (PEMFCs) using hydrogen and oxygen as reactant gases and direct methanol fuel cells (DMFCs) using methanol and oxygen as reactants.
  • PEMFCs polymer electrolyte membrane fuel cells
  • DMFCs direct methanol fuel cells
  • the polymer electrolyte membrane fuel cells are very useful as portable power sources since the operation temperature of the polymer electrolyte membrane fuel cells is relatively low and the polymer electrolyte membrane fuel cells offer the advantage of low weight and volume compared to other fuel cells.
  • a supported catalyst in which a Pt or Pt/Ru catalyst is supported on a carbon support is generally used as a catalyst for fuel cells.
  • materials such as carbon nanotubes which have large surface area and excellent electrical conductivity have been used as a support instead of conventional carbon black.
  • the catalyst can be supported on the carbon nanotubes in a high concentration using a Bonnenman method.
  • the manufacturing process is i complex and difficult, and thus the supported catalyst using the carbon nanotubes cannot be mass-produced easily.
  • a supported catalyst in which catalyst is supported on carbon nanotubes in an amount of 40% by weight of the carbon nanotubes or greater cannot be easily realized using a bulk-loading method that is commonly used in a catalyst preparation.
  • an electrode in which platinum nano particles are uniformly dispersed on a polypyrrole film has been introduced by Rajeshwar, etc. in U.S. Patent No. 5,334,292. Additionally, an electrode in which platinum nano particles are uniformly dispersed on a polyaniline film has been introduced by Finkelshtain, etc. in U.S. Patent No. 6,380,126.
  • conductivities of such films depend on the type of dopants, and the films are not suitable for a catalyst support since the conductivity drastically decreases without the dopants.
  • the manufacturing costs are high in consideration of the efficiencies of the electrodes since considerable amount of noble metal catalyst particles are dispersed inside the polymer films. Further, properties of CO poisoning resistance of such electrodes are not described.
  • the present invention provides a catalyst for a fuel cell electrode which has excellent catalyst activity, can be manufactured at low cost, and is highly resistant to carbon monoxide (CO) poisoning.
  • CO carbon monoxide
  • the present invention also provides a catalyst for a fuel cell electrode which has excellent catalyst activity and excellent electrical conductivity, and is highly resistant to CO poisoning.
  • the present invention also provides a method of preparing a catalyst for a fuel cell electrode which has excellent catalyst activity and is highly resistant to CO poisoning.
  • the present invention also provides an electrode for a fuel cell which has excellent catalyst activity and is highly resistant to CO poisoning.
  • the present invention also provides a membrane electrode assembly for a fuel cell which has excellent catalyst activity and is highly resistant to CO poisoning.
  • the present invention also provides a fuel cell which has excellent catalyst activity and is highly resistant to CO poisoning.
  • a catalyst for fuel cell electrode including: a carbon-based support; an electrically conductive polymer at least partially coating the carbon-based support; a transition metal particle distributed on the surface of the electrically conductive polymer; and a catalyst metal precursor or atoms of the transition metal distributed inside the electrically conductive polymer.
  • a catalyst for fuel cell electrode including: a carbon-based support; a carbon layer formed in a different phase from the carbon-based support and at least partially coating the carbon-based support; a transition metal particle distributed on the surface of the carbon layer; and a catalyst metal precursor or atoms of the transition metal distributed inside the carbon layer.
  • a method of preparing a catalyst for fuel cell electrode including: a) providing a mixture in which a carbon-based support, a transition metal catalyst precursor, a monomer which can be polymerized to form a conductive polymer are dispersed in a dispersion medium; b) polymerization of the monomers in the mixture; and c) reduction of the catalyst metal precursor.
  • the method of preparing the catalyst for fuel cell electrode may be performed at a temperature in the range of 1 to 30 0 C .
  • the method of preparing the catalyst for fuel cell electrode may further include heat-treating the reduced catalyst metal precursor obtained from operation c).
  • an electrode for fuel cells including the catalyst for fuel cell.
  • a membrane electrode assembly for fuel cells including: a cathode including a catalyst layer and a diffusion layer; an anode including a catalyst layer and a diffusion layer; and an electrolyte membrane interposed between the cathode and the anode, wherein the cathode or the anode includes the catalyst for fuel cell electrode of the present invention.
  • a fuel cell including: a cathode including a catalyst layer and a diffusion layer; an anode including a catalyst layer and a diffusion layer; and an electrolyte membrane interposed between the cathode and the anode, wherein the cathode or the anode includes the catalyst for fuel cell electrode.
  • a fuel cell which has excellent catalyst activity and is highly resistant to CO poisoning can be prepared using the catalyst for fuel cell electrode of the present invention.
  • FIG. 1 is a conceptual diagram illustrating a method of preparing a catalyst for a fuel cell electrode according to an embodiment of the present invention
  • FIG. 2 is a flow chart illustrating a method of preparing a catalyst for a fuel cell electrode according to an embodiment of the present invention
  • FIGS. 3A and 3B are TEM images illustrating supported catalysts prepared according to Examples 1 and 7 of the present invention.
  • FIG. 4 is a graph illustrating the result of an X-ray diffraction (XRD) analysis of a supported catalyst prepared according to Example 1 of the present invention
  • FIG. 5 is a graph illustrating the result of fuel cell performance tests of membrane electrode assemblies (MEAs) prepared according to Examples 3 and 4 and Comparative Example 1 ;
  • FIG. 6 is a graph illustrating the result of evaluation tests on carbon monoxide (CO) poisoning resistance of membrane electrode assemblies (MEAs) prepared according to Example 3 and Comparative Example 1 ;
  • FIG. 7 is a graph illustrating the result of fuel cell performance tests of membrane electrode assemblies (MEAs) prepared according to Examples 5 and 6 and Comparative Example 2.
  • a catalyst for a fuel cell electrode includes: a carbon-based support; an electrically conductive polymer at least partially coating the carbon-based support; transition metal particles distributed on the surface of the electrically conductive polymer; and a catalyst metal precursor or atoms of the transition metal distributed inside the electrically conductive polymer.
  • the carbon-based support may be formed of any material which is mainly composed of carbon and commonly used as a material for forming a support.
  • the carbon-based support may be formed of graphite, carbon powder, acetylene black, carbon black, activated carbon, mesoporous carbon, carbon nanotubes, carbon nanofibers, carbon nanohorns, carbon nanorings, carbon nanowires, or fullerene (Ceo), but is not limited thereto.
  • the electrically conductive polymer may entirely or partially coat the carbon-based support.
  • the thickness of the coating may be in the range of 0.1 to 1000 nm, and more preferably 0.5 to 100 nm. When the thickness of the coating is greater than 1000 nm, the electrical conductivity may decrease.
  • the electrically conductive polymer may be polypyrrole, polyaniline, polyacetylene, polythiophene, polyethylenedioxythiophene, polythienylenevinylene, poly(p-phenylenesulfide), or a mixture thereof, but is not limited thereto.
  • the transition metal may be Pt, Ru, Ir, W, Co, Ni, Fe, Os, Rh, Re or an alloy thereof, but is not limited thereto.
  • the transition metal included in the transition metal may be Pt or an alloy of Pt and Ru.
  • the average particle size of the transition metal may be in the range of 1 to 500 nm, and more preferably 1 to 10 nm. When the average particle size of the transition metal is less than 1 nm, catalytic properties of the transition metal is not fully activated. On the other hand, when the average particle size of the transition metal is greater than 500 nm, the efficiency may decrease due to reduced specific surface area.
  • the active transition metals are distributed on the surface of the electrically conductive polymer. That is, the electrically conductive polymer is coated on the surface of the carbon-based support and the active transition metals are distributed on the outer surface of the electrically conductive polymer.
  • a catalyst metal precursor or atoms of the transition metals may be distributed inside the electrically conductive polymer. This is because the outer surface of the electrically conductive polymer is advantageous to mass transfer, and thus the transition metal particles easily grow during the reduction of the catalyst metal precursor, and the inside of the electrically conductive polymer is disadvantageous to mass transfer, and thus the transition metals particles cannot grow during the reduction of the catalyst metal precursor.
  • a catalyst for a fuel cell electrode includes: a carbon-based support; a carbon layer formed in a different phase from the carbon-based support and at least partially coating the carbon-based support; a active transition metals distributed on the surface of the carbon layer; and a catalyst metal precursor or atoms of the active transition metal distributed inside the carbon layer.
  • the carbon-based support and the active transition metals are as described in the previous embodiment.
  • the carbon layer may be formed as a result of a carbonization of a conducting polymer, and is formed in a different phase from the carbon-based support.
  • the carbon layer may be amorphous or have certain crystalline.
  • the carbon layer may entirely or partially coat the carbon-based support.
  • the thickness of the coating may be in the range of 0.1 to 1000 nm, and more preferably 0.5 to 100 nm. When the thickness of the coating is greater than 1000 nm, the electrical conductivity may decrease. On the other hand, when the thickness of the coating is less than 0.1 nm, the binding force to the carbon-based support may decrease, and thus doping of a transition metal on the carbon layer cannot be easily achieved.
  • the active transition metals are distributed on the surface of the carbon layer.
  • the carbon layer is coated on the surface of the carbon-based support and the active transition metals are distributed on the outer surface of the carbon layer.
  • a catalyst metal precursor or atoms of the transition metal may be distributed inside the carbon layer.
  • a method of preparing a catalyst for a fuel cell electrode according to another embodiment of the present invention includes: a) providing a mixture in which a carbon-based support, a transition metal catalyst precursor, and a monomer which can be polymerized to form a conductive polymer are dispersed in a dispersion medium; b) polymerization of the monomers in the mixture; and c) reduction of the transition metal catalyst precursor.
  • the amount of the carbon-based support may be in the range of 1 to 60% by weight
  • the amount of the transition metal catalyst precursor may be in the range of 20 to 90% by weight
  • the amount of the monomer which can be polymerized to form a conductive polymer may be in the range of 10 to 60% by weight based on the total amount of the mixture including the carbon-based support, the transition metal catalyst precursor, and the monomer.
  • the amount of the carbon-based support is less than 1% by weight, a supported catalyst cannot be sufficiently formed.
  • the amount of the carbon-based support is greater than 60% by weight, the amounts of the electrically conductive polymer and the transition metals are too low, and thus the efficiency decreases.
  • the amount of the transition metal catalyst precursor is less than 20% by weight, a supported catalyst having a high metal content cannot be easily formed.
  • the amount of the transition metal catalyst precursor is greater than 90% by weight, the size of the active transition metal increase and thus the specific surface area may decrease.
  • the amount of the conductive polymer monomer is less than 10% by weight, the active transition metals cannot be sufficiently dispersed.
  • the amount of the conductive polymer monomer is greater than 60% by weight, the electrical conductivity of conductive polymer decrease due to the increased thickness of the conductive polymer, which is less electrically conductive than the carbon-based support.
  • the transition metal catalyst precursor is ionized, and uniformly distributed by being doped inside and outside the electrically conductive polymer as a result of an electrostatic interaction between the ionized metal ion and functional groups of the electrically conductive polymer.
  • the doped transition metal catalyst precursor ions provide a nucleation site on the surface of the conductive polymer on which the transition metal can grow in a reduction process, and the nucleation site on the surface of the conductive polymer having facilitated mass transfer grows to be transition metal particles.
  • the amount of the transition metal catalyst precursor corresponding to a doping level of conductive polymer is used for the doping according to the amount of the electrically conductive polymer and the remaining amount of the transition metal catalyst precursor is used to increase the particle size of the transition metal.
  • the concentration of the transition metal catalyst precursor may be 1/4 to 2 times that of the monomer which can be polymerized to form a conductive polymer. That is, if the concentration of the transition metal catalyst precursor is less than 1/4 times the concentration of the monomer, the active transition metal may not sufficiently grow on the surface, thereby decreasing the catalyst activity. When the concentration of the transition metal catalyst precursor is greater than 2 times the concentration of the monomer, the surface area may decrease, and thus, is inefficient.
  • the transition metal catalyst precursor may be a chloride, a nitrate, or a sulfate of the transition metal, but is not limited thereto.
  • Examples of the transition metal catalyst precursor are H 2 PtCI 6 , H 2 IrCI 6 , IrCI 3 , PtCI 2 , RuCI 3 , RhCI 3 , NiCI 2 , and WCI 6 .
  • the conductive polymer monomer may be any monomer which can be polymerized to form polypyrrole, polyaniline, polyacetylene, polythiophene, polyethylenedioxythiophene, polythienylenevinylene, poly(p-phenylenesulfide) or a mixture thereof.
  • the dispersion medium may be an alcohol-based dispersion medium such as methanol, ethanol, or propanol; a non-polar solvent such as tetrahydrofuran (THF) or acetone; a polar solvent such as benzene or toluene; or water.
  • a non-polar solvent such as tetrahydrofuran (THF) or acetone
  • a polar solvent such as benzene or toluene
  • water water
  • the mixture may further include a surfactant.
  • the surfactant assists the transition metal catalyst precursor and the monomer to be uniformly distributed on the surface of the carbon-based support.
  • examples of the surfactant are nonionic surfactants, cationic surfactants, anionic surfactants, and zwitterionic surfactants.
  • the zwitterionic surfactants may be an alanine-based compound, an imidazolium betaine-based compound, or an aminodipropionate salt, and the nonionic surfactants may be an • alkylarylpolyetheralcohol-based compound, but is not limited thereto.
  • the zwitterionic surfactants may be N-n-dodecyl-N,N-dimethyl-3-amino-1 -propane sulfonate, and the nonionic surfactant may be a material such as Triton X-100.
  • the concentration of the surfactants may be 2 to 30 times the critical micelle concentration
  • CMC CMC
  • the CMC indicates the minimal concentration of surfactants which can form a micelle.
  • the mixture may further include an acidic solution.
  • the acidic solution is added to assist the polymerization of the monomer to form a conductive polymer, and may be an inorganic acid such as a sulfuric acid, a hydrochloric acid, or a nitric acid.
  • the method of mixing the carbon-based support, the transition metal catalyst precursor, and the monomer to form a mixture is not limited.
  • the carbon-based support can be dispersed in a dispersion medium, and then the transition metal catalyst precursor and the monomer can be added thereto.
  • the transition metal catalyst precursor and the conductive polymer monomer may be mixed while being stirred at room temperature for 30 minutes to 8 hours, or using ultrasonic waves for 20 minutes to 1 hour.
  • the method of preparing a catalyst for fuel cell electrode may further include adsorption of the transition metal catalyst precursor and the monomer in the carbon-based support after forming the mixture described above.
  • the adsorptions of the transition metal catalyst precursor and the monomer may be performed, for example, using ultrasonic waves for 2 to 6 hours.
  • the carbon-based support may be coated by polymerizing the monomer.
  • An initiator may be added to the mixture to polymerize the monomer.
  • the initiator may be FeCb, [NH 4 J 2 SaO 8 , or Na 2 S 2 O 8 , and more particularly [NH- J ] 2 S 2 O 8 which does not influence an electrochemical activity of a produced catalyst.
  • the concentration of the initiator may be in the range of 1 to 10% by weight based on the amount of the monomer. When the concentration of the initiator is less than 1 % by weight, the polymerization of the monomer may be slow. When the concentration of the initiator is greater than 10% by weight, the initiator, which is added to polymerize the monomer, may act as an impurity, thereby decreasing the degree of polymerization of the conductive polymer.
  • the polymerization may be performed at a temperature in the range of about 1 to 30 0 C , and more preferably about 1 to 6 "C .
  • the temperature is less than about 1 ° C , the reaction rate is too fast, nucleation rate increases, the amount of the oligomers having low molecular weight increases, and thus the conductive polymer cannot be uniformly coated on the carbon-based support.
  • the temperature is higher than about 30 ° C, the polymerization is carried out too slowly, and thus, is inefficient.
  • the polymerization may be performed for 30 minutes to 4 hours. When the polymerization is finished within 30 minutes, the polymerization is not sufficiently performed, thereby obtaining unsatisfactory formation of the electrically conductive polymer. When the polymerization is performed over 4 hours, the polymerization is sufficiently performed, and thus additional reaction is insignificant.
  • a catalyst for fuel cell electrode having an activity can be prepared by polymerizing the monomer to form an electrically conductive polymer as described above and then reducing the catalyst precursor.
  • a reducing agent may directly be added as in Method 1 below, or the dispersion medium in the mixture may be removed and a reductive gas may be supplied in an enclosed heating space such as an oven or a furnace as in Method 2 below.
  • the reducing agent may be sodium borohydride (NaBH 4 ), methanol, ethanol, formic acid, ethylene glycol, hydrazine, or the like.
  • the catalyst precursor can be reduced by being stirred at room temperature for 1 to 2 hours after adding the reducing agent.
  • an alcohol reducing agent is preferably not used.
  • Method 2 for example, a reductant gas containing hydrogen is supplied into an enclosed heating space and the heating space is heated at a temperature in the range of 100 to 1000°C to reduce the catalyst precursor.
  • the catalyst for fuel cell electrode prepared according to the methods described above may include impurities such as surfactants, metal ions, or the like, and thus the catalyst may be washed or filtered using a method that is commonly used in the art.
  • the prepared catalyst for a fuel cell electrode may be washed using a liquid such as deionized water, filtered, and dried at a temperature of 60 °C or less.
  • the reduced catalyst for a fuel cell electrode may be heat-treated to carbonize the electrically conductive polymer thereby forming a carbon layer.
  • the carbon layer generated by carbonizing the electrically conductive polymer has increased the activity of catalysts due to higher electrical conductivity since the carbon layer retains higher electrical conductivity than the electrically conductive polymer although it is formed in a different phase from the carbon-based support.
  • the heat-treatment may be performed at a temperature in the range of about 300 to about 1000°C for about 10 minutes to about 1 hour.
  • the heat-treating temperature is less than 300 ° C , the carbonization is not sufficiently performed.
  • the heat-treating temperature is greater than about 1000 0 C, the particle size of the active transition metal is to aggregate in size, and the specific surface area thereof decreases, and thus the activity of catalysts may decrease.
  • the carbonization is not sufficiently performed.
  • the heat-treatment is performed for longer than 1 hour, the particle size of the transition metal particles is to enlarge, and the specific surface area thereof decreases, and thus the activity of catalysts may decrease.
  • the heat-treatment may be performed under an inert gaseous atmosphere such as nitrogen, argon, neon, or helium.
  • an inert gaseous atmosphere such as nitrogen, argon, neon, or helium.
  • a hydrogen atmosphere and the inert gaseous atmosphere may be alternated in the heat-treatment to prevent the growth of the catalyst particles.
  • FIGS. 1 and 2 A conceptual diagram and a flow diagram illustrating a method of preparing a catalyst for fuel cell electrode according to an embodiment of the present invention are illustrated in FIGS. 1 and 2.
  • a monomer 10 which can be polymerized to form a conductive polymer and a transition metal precursor 20 are adsorbed on the surface of a carbon-based support.
  • the monomer 10 is polymerized to form an electrically conductive polymer, and a transition metal nucleation site 30' is formed through the reduction process.
  • the active transition metal rapidly grows on the nucleation site to form the transition metal catalyst particle 30.
  • the transition metal catalyst particle 30 may be filtered, washed, dried, and annealed.
  • an electrode for fuel cells including the catalyst for a fuel cell electrode.
  • the electrode may be prepared using a well known method in the art without limitation.
  • powder of a catalyst for a fuel cell electrode and a binder are dispersed in a solvent, the dispersion is coated on a diffusion layer, preferably a porous diffusion layer, and the resultant is dried to prepare an electrode.
  • a membrane electrode assembly for fuel cells including the catalyst for a fuel cell electrode.
  • the membrane electrode assembly includes a cathode including a catalyst layer and a diffusion layer; an anode including a catalyst layer and a diffusion layer; and an electrolyte membrane interposed between the cathode and the anode, wherein the cathode and/or the anode includes a catalyst for a fuel cell electrode prepared according to an embodiment of the present invention.
  • the diffusion layer included in the cathode and the anode, and the electrolyte membrane may be any diffusion layer and electrolyte membrane that are commonly used in the art.
  • the catalyst layer of the cathode and/or the anode may include the catalyst for a fuel cell electrode prepared according to an embodiment of the present invention.
  • a fuel cell including the catalyst a for fuel cell electrode.
  • the fuel cell includes: a cathode including a catalyst layer and a diffusion layer; an anode including a catalyst layer and a diffusion layer; and an electrolyte membrane interposed between the cathode and the anode, wherein the cathode and/or the anode includes a catalyst for a fuel cell electrode prepared according to an embodiment of the present invention.
  • the diffusion layer included in the cathode and the anode, and the electrolyte membrane may be any diffusion layer and electrolyte membrane that are commonly used in the art.
  • the catalyst layer of the cathode and/or the anode may include the catalyst for a fuel cell electrode prepared according to an embodiment of the present invention.
  • the fuel cell can be prepared using a well known method, and thus detailed description on the method of preparing the fuel cell is not disclosed herein.
  • the present invention will now be described in more detail with reference to following Examples and Comparative Examples. The following examples are for illustrative purposes and are not intended to limit the scope of the invention.
  • Example 1 Preparation of a supported catalyst including an electrically conductive polymer layer The temperature was maintained at 5 0 C during the whole process of the reaction by connecting a cooling circulation water bath to an annular pipe reactor and circulating cooling water. 30 ml of 20 mM Triton X-100 (Aldrich Corporation) as a surfactant was added to 0.1 g of carbon nanotubes (lljin Nanotech Co., Ltd.) and sufficiently stirred. 10 ml of 0.25 mM pyrrole (Aldrich Corporation) solution was added thereto and sufficiently stirred, and 10 ml of 0.1 M platinum chloride solution was added thereto and sufficiently stirred.
  • Triton X-100 Aldrich Corporation
  • FIG. 3A is a TEM image illustrating the surface of the supported catalyst prepared according to Example 1. As shown in FIG. 3A, a metal catalyst having a particle size of about 3 nm or less was uniformly distributed and supported.
  • FIG. 4 is a graph illustrating the result of an X-ray diffraction (XRD) analysis of a supported catalyst prepared according to Example 1 of the present invention.
  • XRD X-ray diffraction
  • Example 2 Preparation of a supported catalyst in which an electrically conductive polymer is carbonized
  • the catalyst prepared according to Example 1 was heat-treated in a tube furnace under an argon atmosphere at the temperature of 350 °C for 30 minutes to prepare a supported catalyst having 78% of platinum.
  • Example 3 Preparation of a membrane electrode assembly (MEA) using the supported catalyst prepared according to Example 1
  • the supported catalyst prepared according to Example 1 was coated on a gas diffusion layer for an anode such that the amount of supported Pt is 0.1 mg/cm 2 .
  • a commercially available Pt/C catalyst (E-TEK Corporation, HP 60% by weight Pt/C) was coated on a gas diffusion layer for a cathode such that the amount of supported Pt is 1 mg/cm 2 .
  • a commercially available carbon paper (SGL-10BC, SGL Corporation) was used for the gas diffusion layer for the anode and the cathode.
  • a solution of Pt/C and Nafion (SE5012, Dupont Corporation) was added to a solvent in which isopropylalcohol and water was mixed in the ratio of 1 :1 , and dispersed in an ultrasonic wave tank for about 10 minutes or more to obtain a catalyst ink.
  • the obtained catalyst ink was coated on a gas diffusion layer using a spray coating, and dried to obtain a cathode and an anode.
  • the amount of Nafion in the dried catalyst layer of the anode was 15% by weight of the anode, and the amount of Nafion in the dried catalyst layer of the cathode was 10% by weight of the cathode.
  • Nafion 117 (Dupont Corporation) was used for a polymer electrolyte membrane.
  • the polymer electrolyte membrane was interposed between the anode and the cathode, and the resultant was treated under the pressure of 1 ton at about 135°C for about 3 minutes to prepare a membrane electrode assembly.
  • Example 4 Preparation of a MEA using the supported catalyst prepared according to Example 2
  • a MEA was prepared in the same manner as in Example 3, except that a supported catalyst prepared according to Example 2 was used instead of a supported catalyst prepared according to Example 1.
  • Comparative Example 1 Preparation of an MEA using a commercially available Pt/C catalyst
  • An MEA was prepared in the same manner as in Example 3, except that the anode was prepared according to the method of preparing the cathode in which a commercially available Pt/C catalyst was used.
  • Comparative Example 1 were tested. Humidified hydrogen (99.9%) was supplied to an anode, and humidified oxygen (99.9%) was supplied to a cathode. Voltages of the MEAs were measured according to current density at the operation temperature of 50 ° C, and the results are shown in FIG. 5. As shown in FIG. 5, although the amount of supported platinum catalyst prepared according to Examples 3 and 4 was about 1/10 of the amount of supported platinum catalyst prepared according to Comparative Example 1 , the MEAs of Examples 3 and 4 had similar performance to, or more excellent performance than, the MEA of Comparative Example 1. The results are considered to be caused since the catalyst having small particle size was uniformly dispersed and supported, and thus the catalyst availability was increased.
  • Evaluation Example 2 Evaluation test on carbon monoxide (CO) poisoning resistance
  • Example 3A Evaluation tests on carbon monoxide (CO) poisoning resistance of the MEAs prepared according to Example 3 and Comparative Example 1 were performed.
  • the test was performed in the same manner as in Evaluation Example 1 , except that humidified hydrogen including 100 ppm of CO was supplied to an anode of each MEA.
  • the results are shown in FIG. 6.
  • the test result of the MEA of Example 3 obtained in Evaluation Example 1 (Example 3A) was illustrated in FIG. 6 to compare the decreased performance with CO to the test result without CO.
  • the MEA of Example 3 is suitable for a catalyst for a fuel cell in which a reformed hydrogen enriched gas is used as a fuel.
  • the principle of having the property of CO poisoning resistance has not been clearly revealed.
  • the supported amount of the catalyst of Example 3 was 1/10 of the supported amount of the catalyst of Comparative Example 1 , it can be considered that the property of CO poisoning resistance has been drastically increased.
  • Example 5 Preparation of a Pt/Ru supported catalyst including an electrically conductive polymer layer A supported catalyst was prepared in the same manner as in Example 1 , except that 5 ml of 0.133 M platinum chloride solution and 5 ml of 0.133 M ruthenium chloride were added instead of 10 ml of 0.1 M platinum chloride.
  • Example 6 Preparation of an MEA using the supported catalyst prepared according to Example 5 An MEA was prepared in the same manner as in Example 3, except that the supported catalyst prepared according to Example 5 was coated on a gas diffusion layer for an anode with the concentration of 1.0 mg/cm 2 instead of the supported catalyst prepared according to Example 1.
  • An MEA was prepared in the same manner as in Example 3, except that a commercially available Pt/Ru catalyst (E-TEK Corporation) was coated on a gas diffusion layer for an anode with the concentration of 4.0 mg/cm 2 instead of the supported catalyst prepared according to Example 1. Evaluation Example 3
  • a supported catalyst was prepared in the same manner as in Example 1 , except that the amount of the 0.1 M platinum chloride solution was increased to 30 ml.
  • the amount of platinum in the supported catalyst was 78% by weight of the supported catalyst.
  • TEM images of the supported catalyst are illustrated in FIG. 3B. As shown in FIG. 3B, it was identified that the metal catalyst having a particle size of about 3 nm or less was uniformly dispersed and supported. While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.
  • the present invention provides a supported catalyst in which a active transition metal is supported on a support in a high content, and is uniformly dispersed, and thus the catalyst for a fuel cell electrode of the present invention has excellent catalyst activity, can be manufactured at low cost, and is highly resistant to CO poisoning.

Abstract

Provided are a catalyst for a fuel cell electrode and a method of preparing the same, and more particularly, to a catalyst for a fuel cell electrode including a carbon-based support, an electrically conductive polymer at least partially coating the carbon-based support; transition metal particles distributed on the surface of the electrically conductive polymer, and a method of preparing the same. The catalyst for a fuel cell electrode, in which a transition metal is supported on a support in high contents and is uniformly dispersed, has excellent catalyst activity, can be manufactured at low cost, and is highly resistant to CO poisoning.

Description

CATALYST FOR FUEL CELL ELECTRODE AND METHOD OF PREPARING THE
SAME
TECHNICAL FIELD
The present invention relates to a catalyst for a fuel cell electrode and a method of preparing the same, and more particularly, to a catalyst for a fuel cell electrode which has excellent catalyst activity, can be manufactured at low cost, and is highly resistant to CO poisoning, and a method of preparing the same.
BACKGROUND ART
A fuel cell is a type of electrical energy generating system in which energy from an electrochemical reaction between fuel and oxygen is directly converted into electrical energy. The fuel cell is also an energy generating system in which the heat efficiency is not involved unlike the Carnot cycle. Fuel cells are an environment-friendly energy generation technology since they are free of such problems as pollutant gases NOx and SOx, and emit little carbon dioxide compared to thermal power generation systems. Fuel cells are classified into proton exchange membrane fuel cells (PEMFC), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), etc. according to the type of electrolyte.
The proton exchange membrane fuel cells are classified into polymer electrolyte membrane fuel cells (PEMFCs) using hydrogen and oxygen as reactant gases and direct methanol fuel cells (DMFCs) using methanol and oxygen as reactants. The polymer electrolyte membrane fuel cells are very useful as portable power sources since the operation temperature of the polymer electrolyte membrane fuel cells is relatively low and the polymer electrolyte membrane fuel cells offer the advantage of low weight and volume compared to other fuel cells.
A supported catalyst in which a Pt or Pt/Ru catalyst is supported on a carbon support is generally used as a catalyst for fuel cells. Recently, to increase the catalyst efficiency, materials such as carbon nanotubes which have large surface area and excellent electrical conductivity have been used as a support instead of conventional carbon black. The catalyst can be supported on the carbon nanotubes in a high concentration using a Bonnenman method. However, the manufacturing process is i complex and difficult, and thus the supported catalyst using the carbon nanotubes cannot be mass-produced easily. In addition, it is known that a supported catalyst in which catalyst is supported on carbon nanotubes in an amount of 40% by weight of the carbon nanotubes or greater cannot be easily realized using a bulk-loading method that is commonly used in a catalyst preparation.
Meanwhile, as a method of using an electrical conductive polymer in a catalyst, an electrode in which platinum nano particles are uniformly dispersed on a polypyrrole film has been introduced by Rajeshwar, etc. in U.S. Patent No. 5,334,292. Additionally, an electrode in which platinum nano particles are uniformly dispersed on a polyaniline film has been introduced by Finkelshtain, etc. in U.S. Patent No. 6,380,126. However, conductivities of such films depend on the type of dopants, and the films are not suitable for a catalyst support since the conductivity drastically decreases without the dopants. The manufacturing costs are high in consideration of the efficiencies of the electrodes since considerable amount of noble metal catalyst particles are dispersed inside the polymer films. Further, properties of CO poisoning resistance of such electrodes are not described.
DETAILED DESCRIPTION OF THE INVENTION
TECHNICAL PROBLEM The present invention provides a catalyst for a fuel cell electrode which has excellent catalyst activity, can be manufactured at low cost, and is highly resistant to carbon monoxide (CO) poisoning.
The present invention also provides a catalyst for a fuel cell electrode which has excellent catalyst activity and excellent electrical conductivity, and is highly resistant to CO poisoning.
The present invention also provides a method of preparing a catalyst for a fuel cell electrode which has excellent catalyst activity and is highly resistant to CO poisoning.
The present invention also provides an electrode for a fuel cell which has excellent catalyst activity and is highly resistant to CO poisoning. The present invention also provides a membrane electrode assembly for a fuel cell which has excellent catalyst activity and is highly resistant to CO poisoning.
The present invention also provides a fuel cell which has excellent catalyst activity and is highly resistant to CO poisoning. TECHNICAL SOLUTION
According to an aspect of the present invention, there is provided a catalyst for fuel cell electrode including: a carbon-based support; an electrically conductive polymer at least partially coating the carbon-based support; a transition metal particle distributed on the surface of the electrically conductive polymer; and a catalyst metal precursor or atoms of the transition metal distributed inside the electrically conductive polymer.
According to another aspect of the present invention, there is provided a catalyst for fuel cell electrode including: a carbon-based support; a carbon layer formed in a different phase from the carbon-based support and at least partially coating the carbon-based support; a transition metal particle distributed on the surface of the carbon layer; and a catalyst metal precursor or atoms of the transition metal distributed inside the carbon layer. According to another aspect of the present invention, there is provided a method of preparing a catalyst for fuel cell electrode including: a) providing a mixture in which a carbon-based support, a transition metal catalyst precursor, a monomer which can be polymerized to form a conductive polymer are dispersed in a dispersion medium; b) polymerization of the monomers in the mixture; and c) reduction of the catalyst metal precursor.
The method of preparing the catalyst for fuel cell electrode may be performed at a temperature in the range of 1 to 300C .
The method of preparing the catalyst for fuel cell electrode may further include heat-treating the reduced catalyst metal precursor obtained from operation c).
According to another aspect of the present invention, there is provided an electrode for fuel cells including the catalyst for fuel cell. According to another aspect of the present invention, there is provided a membrane electrode assembly for fuel cells including: a cathode including a catalyst layer and a diffusion layer; an anode including a catalyst layer and a diffusion layer; and an electrolyte membrane interposed between the cathode and the anode, wherein the cathode or the anode includes the catalyst for fuel cell electrode of the present invention.
According to another aspect of the present invention, there is provided a fuel cell including: a cathode including a catalyst layer and a diffusion layer; an anode including a catalyst layer and a diffusion layer; and an electrolyte membrane interposed between the cathode and the anode, wherein the cathode or the anode includes the catalyst for fuel cell electrode.
A fuel cell which has excellent catalyst activity and is highly resistant to CO poisoning can be prepared using the catalyst for fuel cell electrode of the present invention.
DESCRIPTION OF THE DRAWINGS
The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:
FIG. 1 is a conceptual diagram illustrating a method of preparing a catalyst for a fuel cell electrode according to an embodiment of the present invention;
FIG. 2 is a flow chart illustrating a method of preparing a catalyst for a fuel cell electrode according to an embodiment of the present invention;
FIGS. 3A and 3B are TEM images illustrating supported catalysts prepared according to Examples 1 and 7 of the present invention;
FIG. 4 is a graph illustrating the result of an X-ray diffraction (XRD) analysis of a supported catalyst prepared according to Example 1 of the present invention; FIG. 5 is a graph illustrating the result of fuel cell performance tests of membrane electrode assemblies (MEAs) prepared according to Examples 3 and 4 and Comparative Example 1 ;
FIG. 6 is a graph illustrating the result of evaluation tests on carbon monoxide (CO) poisoning resistance of membrane electrode assemblies (MEAs) prepared according to Example 3 and Comparative Example 1 ; and
FIG. 7 is a graph illustrating the result of fuel cell performance tests of membrane electrode assemblies (MEAs) prepared according to Examples 5 and 6 and Comparative Example 2.
BEST MODE
Hereinafter, the present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.
A catalyst for a fuel cell electrode according to an embodiment of the present invention includes: a carbon-based support; an electrically conductive polymer at least partially coating the carbon-based support; transition metal particles distributed on the surface of the electrically conductive polymer; and a catalyst metal precursor or atoms of the transition metal distributed inside the electrically conductive polymer.
The carbon-based support may be formed of any material which is mainly composed of carbon and commonly used as a material for forming a support. The carbon-based support may be formed of graphite, carbon powder, acetylene black, carbon black, activated carbon, mesoporous carbon, carbon nanotubes, carbon nanofibers, carbon nanohorns, carbon nanorings, carbon nanowires, or fullerene (Ceo), but is not limited thereto. The electrically conductive polymer may entirely or partially coat the carbon-based support. The thickness of the coating may be in the range of 0.1 to 1000 nm, and more preferably 0.5 to 100 nm. When the thickness of the coating is greater than 1000 nm, the electrical conductivity may decrease. On the other hand, when the thickness of the coating is less than 0.1 nm, the binding force to the carbon-based support may decrease, and thus doping of a transition metal on the electrically conductive polymer cannot be easily achieved. The electrically conductive polymer may be polypyrrole, polyaniline, polyacetylene, polythiophene, polyethylenedioxythiophene, polythienylenevinylene, poly(p-phenylenesulfide), or a mixture thereof, but is not limited thereto.
The transition metal may be Pt, Ru, Ir, W, Co, Ni, Fe, Os, Rh, Re or an alloy thereof, but is not limited thereto. In particular, the transition metal included in the transition metal may be Pt or an alloy of Pt and Ru.
The average particle size of the transition metal may be in the range of 1 to 500 nm, and more preferably 1 to 10 nm. When the average particle size of the transition metal is less than 1 nm, catalytic properties of the transition metal is not fully activated. On the other hand, when the average particle size of the transition metal is greater than 500 nm, the efficiency may decrease due to reduced specific surface area.
The active transition metals are distributed on the surface of the electrically conductive polymer. That is, the electrically conductive polymer is coated on the surface of the carbon-based support and the active transition metals are distributed on the outer surface of the electrically conductive polymer. In addition, a catalyst metal precursor or atoms of the transition metals may be distributed inside the electrically conductive polymer. This is because the outer surface of the electrically conductive polymer is advantageous to mass transfer, and thus the transition metal particles easily grow during the reduction of the catalyst metal precursor, and the inside of the electrically conductive polymer is disadvantageous to mass transfer, and thus the transition metals particles cannot grow during the reduction of the catalyst metal precursor.
A catalyst for a fuel cell electrode according to another embodiment of the present invention includes: a carbon-based support; a carbon layer formed in a different phase from the carbon-based support and at least partially coating the carbon-based support; a active transition metals distributed on the surface of the carbon layer; and a catalyst metal precursor or atoms of the active transition metal distributed inside the carbon layer.
Here, the carbon-based support and the active transition metals are as described in the previous embodiment.
The carbon layer may be formed as a result of a carbonization of a conducting polymer, and is formed in a different phase from the carbon-based support. The carbon layer may be amorphous or have certain crystalline. The carbon layer may entirely or partially coat the carbon-based support. Here, the thickness of the coating may be in the range of 0.1 to 1000 nm, and more preferably 0.5 to 100 nm. When the thickness of the coating is greater than 1000 nm, the electrical conductivity may decrease. On the other hand, when the thickness of the coating is less than 0.1 nm, the binding force to the carbon-based support may decrease, and thus doping of a transition metal on the carbon layer cannot be easily achieved. The active transition metals are distributed on the surface of the carbon layer. That is, the carbon layer is coated on the surface of the carbon-based support and the active transition metals are distributed on the outer surface of the carbon layer. In addition, a catalyst metal precursor or atoms of the transition metal may be distributed inside the carbon layer. This is because the outer surface of the electrically conductive polymer, which corresponds to the precursor of the carbon layer, is advantageous to mass transfer, and thus the transition metal easily grow during the reduction of the catalyst metal precursor, and the inside of the electrically conductive polymer, which corresponds to the precursor of the carbon layer is disadvantageous to mass transfer, and thus the active transition metal cannot grow during the reduction of the catalyst metal precursor.
A method of preparing a catalyst for a fuel cell electrode according to another embodiment of the present invention includes: a) providing a mixture in which a carbon-based support, a transition metal catalyst precursor, and a monomer which can be polymerized to form a conductive polymer are dispersed in a dispersion medium; b) polymerization of the monomers in the mixture; and c) reduction of the transition metal catalyst precursor. The amount of the carbon-based support may be in the range of 1 to 60% by weight, the amount of the transition metal catalyst precursor may be in the range of 20 to 90% by weight, and the amount of the monomer which can be polymerized to form a conductive polymer may be in the range of 10 to 60% by weight based on the total amount of the mixture including the carbon-based support, the transition metal catalyst precursor, and the monomer.
When the amount of the carbon-based support is less than 1% by weight, a supported catalyst cannot be sufficiently formed. When the amount of the carbon-based support is greater than 60% by weight, the amounts of the electrically conductive polymer and the transition metals are too low, and thus the efficiency decreases.
When the amount of the transition metal catalyst precursor is less than 20% by weight, a supported catalyst having a high metal content cannot be easily formed. When the amount of the transition metal catalyst precursor is greater than 90% by weight, the size of the active transition metal increase and thus the specific surface area may decrease.
When the amount of the conductive polymer monomer is less than 10% by weight, the active transition metals cannot be sufficiently dispersed. When the amount of the conductive polymer monomer is greater than 60% by weight, the electrical conductivity of conductive polymer decrease due to the increased thickness of the conductive polymer, which is less electrically conductive than the carbon-based support.
The transition metal catalyst precursor is ionized, and uniformly distributed by being doped inside and outside the electrically conductive polymer as a result of an electrostatic interaction between the ionized metal ion and functional groups of the electrically conductive polymer. The doped transition metal catalyst precursor ions provide a nucleation site on the surface of the conductive polymer on which the transition metal can grow in a reduction process, and the nucleation site on the surface of the conductive polymer having facilitated mass transfer grows to be transition metal particles. The amount of the transition metal catalyst precursor corresponding to a doping level of conductive polymer is used for the doping according to the amount of the electrically conductive polymer and the remaining amount of the transition metal catalyst precursor is used to increase the particle size of the transition metal. Accordingly, the concentration of the transition metal catalyst precursor may be 1/4 to 2 times that of the monomer which can be polymerized to form a conductive polymer. That is, if the concentration of the transition metal catalyst precursor is less than 1/4 times the concentration of the monomer, the active transition metal may not sufficiently grow on the surface, thereby decreasing the catalyst activity. When the concentration of the transition metal catalyst precursor is greater than 2 times the concentration of the monomer, the surface area may decrease, and thus, is inefficient.
The transition metal catalyst precursor may be a chloride, a nitrate, or a sulfate of the transition metal, but is not limited thereto. Examples of the transition metal catalyst precursor are H2PtCI6, H2IrCI6, IrCI3, PtCI2, RuCI3, RhCI3, NiCI2, and WCI6. The conductive polymer monomer may be any monomer which can be polymerized to form polypyrrole, polyaniline, polyacetylene, polythiophene, polyethylenedioxythiophene, polythienylenevinylene, poly(p-phenylenesulfide) or a mixture thereof. The dispersion medium may be an alcohol-based dispersion medium such as methanol, ethanol, or propanol; a non-polar solvent such as tetrahydrofuran (THF) or acetone; a polar solvent such as benzene or toluene; or water.
Optionally, the mixture may further include a surfactant. The surfactant assists the transition metal catalyst precursor and the monomer to be uniformly distributed on the surface of the carbon-based support. Examples of the surfactant are nonionic surfactants, cationic surfactants, anionic surfactants, and zwitterionic surfactants.
However, when the cationic and anionic surfactants are used, interactions between the surfactant and the transition metal catalyst precursor ions and compatibility with the dispersion medium must be considered. The zwitterionic surfactants may be an alanine-based compound, an imidazolium betaine-based compound, or an aminodipropionate salt, and the nonionic surfactants may be an • alkylarylpolyetheralcohol-based compound, but is not limited thereto. In more particular, the zwitterionic surfactants may be N-n-dodecyl-N,N-dimethyl-3-amino-1 -propane sulfonate, and the nonionic surfactant may be a material such as Triton X-100. The concentration of the surfactants may be 2 to 30 times the critical micelle concentration
(CMC). The CMC indicates the minimal concentration of surfactants which can form a micelle.
Optionally, the mixture may further include an acidic solution. The acidic solution is added to assist the polymerization of the monomer to form a conductive polymer, and may be an inorganic acid such as a sulfuric acid, a hydrochloric acid, or a nitric acid. The method of mixing the carbon-based support, the transition metal catalyst precursor, and the monomer to form a mixture is not limited. For example, the carbon-based support can be dispersed in a dispersion medium, and then the transition metal catalyst precursor and the monomer can be added thereto. The transition metal catalyst precursor and the conductive polymer monomer may be mixed while being stirred at room temperature for 30 minutes to 8 hours, or using ultrasonic waves for 20 minutes to 1 hour. The method of preparing a catalyst for fuel cell electrode may further include adsorption of the transition metal catalyst precursor and the monomer in the carbon-based support after forming the mixture described above. The adsorptions of the transition metal catalyst precursor and the monomer may be performed, for example, using ultrasonic waves for 2 to 6 hours.
The carbon-based support may be coated by polymerizing the monomer.
An initiator may be added to the mixture to polymerize the monomer. The initiator may be FeCb, [NH4J2SaO8, or Na2S2O8, and more particularly [NH-J]2S2O8 which does not influence an electrochemical activity of a produced catalyst. The concentration of the initiator may be in the range of 1 to 10% by weight based on the amount of the monomer. When the concentration of the initiator is less than 1 % by weight, the polymerization of the monomer may be slow. When the concentration of the initiator is greater than 10% by weight, the initiator, which is added to polymerize the monomer, may act as an impurity, thereby decreasing the degree of polymerization of the conductive polymer.
The polymerization may be performed at a temperature in the range of about 1 to 300C , and more preferably about 1 to 6 "C . When the temperature is less than about 1 °C , the reaction rate is too fast, nucleation rate increases, the amount of the oligomers having low molecular weight increases, and thus the conductive polymer cannot be uniformly coated on the carbon-based support. When the temperature is higher than about 30 °C, the polymerization is carried out too slowly, and thus, is inefficient.
The polymerization may be performed for 30 minutes to 4 hours. When the polymerization is finished within 30 minutes, the polymerization is not sufficiently performed, thereby obtaining unsatisfactory formation of the electrically conductive polymer. When the polymerization is performed over 4 hours, the polymerization is sufficiently performed, and thus additional reaction is insignificant.
A catalyst for fuel cell electrode having an activity can be prepared by polymerizing the monomer to form an electrically conductive polymer as described above and then reducing the catalyst precursor. In order to reduce the catalyst precursor, a reducing agent may directly be added as in Method 1 below, or the dispersion medium in the mixture may be removed and a reductive gas may be supplied in an enclosed heating space such as an oven or a furnace as in Method 2 below. In Method 1 , the reducing agent may be sodium borohydride (NaBH4), methanol, ethanol, formic acid, ethylene glycol, hydrazine, or the like. The catalyst precursor can be reduced by being stirred at room temperature for 1 to 2 hours after adding the reducing agent. When the nonionic surfactant is used, an alcohol reducing agent is preferably not used.
In Method 2, for example, a reductant gas containing hydrogen is supplied into an enclosed heating space and the heating space is heated at a temperature in the range of 100 to 1000°C to reduce the catalyst precursor.
The catalyst for fuel cell electrode prepared according to the methods described above may include impurities such as surfactants, metal ions, or the like, and thus the catalyst may be washed or filtered using a method that is commonly used in the art. For example, the prepared catalyst for a fuel cell electrode may be washed using a liquid such as deionized water, filtered, and dried at a temperature of 60 °C or less.
Optionally, the reduced catalyst for a fuel cell electrode may be heat-treated to carbonize the electrically conductive polymer thereby forming a carbon layer. The carbon layer generated by carbonizing the electrically conductive polymer has increased the activity of catalysts due to higher electrical conductivity since the carbon layer retains higher electrical conductivity than the electrically conductive polymer although it is formed in a different phase from the carbon-based support. The heat-treatment may be performed at a temperature in the range of about 300 to about 1000°C for about 10 minutes to about 1 hour.
When the heat-treating temperature is less than 300 °C , the carbonization is not sufficiently performed. On the other hand, when the heat-treating temperature is greater than about 10000C, the particle size of the active transition metal is to aggregate in size, and the specific surface area thereof decreases, and thus the activity of catalysts may decrease.
When the heat-treatment is performed for less than 10 minutes, the carbonization is not sufficiently performed. When the heat-treatment is performed for longer than 1 hour, the particle size of the transition metal particles is to enlarge, and the specific surface area thereof decreases, and thus the activity of catalysts may decrease.
Here, the heat-treatment may be performed under an inert gaseous atmosphere such as nitrogen, argon, neon, or helium. Alternatively, a hydrogen atmosphere and the inert gaseous atmosphere may be alternated in the heat-treatment to prevent the growth of the catalyst particles.
A conceptual diagram and a flow diagram illustrating a method of preparing a catalyst for fuel cell electrode according to an embodiment of the present invention are illustrated in FIGS. 1 and 2. In FIG. 1 , a monomer 10, which can be polymerized to form a conductive polymer and a transition metal precursor 20 are adsorbed on the surface of a carbon-based support. The monomer 10 is polymerized to form an electrically conductive polymer, and a transition metal nucleation site 30' is formed through the reduction process. In particular, on the surface in which materials are relatively easily transported, the active transition metal rapidly grows on the nucleation site to form the transition metal catalyst particle 30.
Optionally, the transition metal catalyst particle 30 may be filtered, washed, dried, and annealed.
According to another embodiment of the present invention, there is provided an electrode for fuel cells including the catalyst for a fuel cell electrode.
The electrode may be prepared using a well known method in the art without limitation. For example, powder of a catalyst for a fuel cell electrode and a binder are dispersed in a solvent, the dispersion is coated on a diffusion layer, preferably a porous diffusion layer, and the resultant is dried to prepare an electrode. According to another embodiment of the present invention, there is provided a membrane electrode assembly for fuel cells including the catalyst for a fuel cell electrode.
The membrane electrode assembly includes a cathode including a catalyst layer and a diffusion layer; an anode including a catalyst layer and a diffusion layer; and an electrolyte membrane interposed between the cathode and the anode, wherein the cathode and/or the anode includes a catalyst for a fuel cell electrode prepared according to an embodiment of the present invention.
The diffusion layer included in the cathode and the anode, and the electrolyte membrane may be any diffusion layer and electrolyte membrane that are commonly used in the art. In addition, the catalyst layer of the cathode and/or the anode may include the catalyst for a fuel cell electrode prepared according to an embodiment of the present invention. According to another embodiment of the present invention, there is provided a fuel cell including the catalyst a for fuel cell electrode.
The fuel cell includes: a cathode including a catalyst layer and a diffusion layer; an anode including a catalyst layer and a diffusion layer; and an electrolyte membrane interposed between the cathode and the anode, wherein the cathode and/or the anode includes a catalyst for a fuel cell electrode prepared according to an embodiment of the present invention.
The diffusion layer included in the cathode and the anode, and the electrolyte membrane may be any diffusion layer and electrolyte membrane that are commonly used in the art. In addition, the catalyst layer of the cathode and/or the anode may include the catalyst for a fuel cell electrode prepared according to an embodiment of the present invention.
The fuel cell can be prepared using a well known method, and thus detailed description on the method of preparing the fuel cell is not disclosed herein. The present invention will now be described in more detail with reference to following Examples and Comparative Examples. The following examples are for illustrative purposes and are not intended to limit the scope of the invention.
Example 1 : Preparation of a supported catalyst including an electrically conductive polymer layer The temperature was maintained at 50C during the whole process of the reaction by connecting a cooling circulation water bath to an annular pipe reactor and circulating cooling water. 30 ml of 20 mM Triton X-100 (Aldrich Corporation) as a surfactant was added to 0.1 g of carbon nanotubes (lljin Nanotech Co., Ltd.) and sufficiently stirred. 10 ml of 0.25 mM pyrrole (Aldrich Corporation) solution was added thereto and sufficiently stirred, and 10 ml of 0.1 M platinum chloride solution was added thereto and sufficiently stirred. 1 ml of 0.5 M ammonium peroxydisulfate as an initiator was added to the mixture and polymerization was performed for 1 hour. Then, 1 M sodium borohydride solution was added thereto and the platinum catalyst was reduced. The resultant was washed with deionized water, filtered and dried to obtain about 0.64 g of supported catalyst having a platinum-polypyrrole complex coated on the carbon nanotubes. As a result of thermogravi metric analysis (TGA) analysis, the amount of platinum in the supported catalyst was found to be 61% by weight. FIG. 3A is a TEM image illustrating the surface of the supported catalyst prepared according to Example 1. As shown in FIG. 3A, a metal catalyst having a particle size of about 3 nm or less was uniformly distributed and supported.
FIG. 4 is a graph illustrating the result of an X-ray diffraction (XRD) analysis of a supported catalyst prepared according to Example 1 of the present invention. Referring to FIG. 4, a crystalline peak of the carbon nanotubes for which 2Θ is observed in the vicinity of 25 degree is not present, and thus it was identified that an electrically conductive polymer was polymerized on the surface of the carbon nanotubes. It was also identified that a crystalline peak of the platinum particles is formed in the vicinity of 40 degree.
Example 2: Preparation of a supported catalyst in which an electrically conductive polymer is carbonized
The catalyst prepared according to Example 1 was heat-treated in a tube furnace under an argon atmosphere at the temperature of 350 °C for 30 minutes to prepare a supported catalyst having 78% of platinum.
Example 3: Preparation of a membrane electrode assembly (MEA) using the supported catalyst prepared according to Example 1
The supported catalyst prepared according to Example 1 was coated on a gas diffusion layer for an anode such that the amount of supported Pt is 0.1 mg/cm2. In addition, a commercially available Pt/C catalyst (E-TEK Corporation, HP 60% by weight Pt/C) was coated on a gas diffusion layer for a cathode such that the amount of supported Pt is 1 mg/cm2. A commercially available carbon paper (SGL-10BC, SGL Corporation) was used for the gas diffusion layer for the anode and the cathode. A solution of Pt/C and Nafion (SE5012, Dupont Corporation) was added to a solvent in which isopropylalcohol and water was mixed in the ratio of 1 :1 , and dispersed in an ultrasonic wave tank for about 10 minutes or more to obtain a catalyst ink. The obtained catalyst ink was coated on a gas diffusion layer using a spray coating, and dried to obtain a cathode and an anode. The amount of Nafion in the dried catalyst layer of the anode was 15% by weight of the anode, and the amount of Nafion in the dried catalyst layer of the cathode was 10% by weight of the cathode.
Nafion 117 (Dupont Corporation) was used for a polymer electrolyte membrane. The polymer electrolyte membrane was interposed between the anode and the cathode, and the resultant was treated under the pressure of 1 ton at about 135°C for about 3 minutes to prepare a membrane electrode assembly.
Example 4: Preparation of a MEA using the supported catalyst prepared according to Example 2
A MEA was prepared in the same manner as in Example 3, except that a supported catalyst prepared according to Example 2 was used instead of a supported catalyst prepared according to Example 1.
Comparative Example 1 : Preparation of an MEA using a commercially available Pt/C catalyst
An MEA was prepared in the same manner as in Example 3, except that the anode was prepared according to the method of preparing the cathode in which a commercially available Pt/C catalyst was used.
Evaluation Example 1 Performance of MEAs prepared according to Example 3, Example 4, and
Comparative Example 1 were tested. Humidified hydrogen (99.9%) was supplied to an anode, and humidified oxygen (99.9%) was supplied to a cathode. Voltages of the MEAs were measured according to current density at the operation temperature of 50 °C, and the results are shown in FIG. 5. As shown in FIG. 5, although the amount of supported platinum catalyst prepared according to Examples 3 and 4 was about 1/10 of the amount of supported platinum catalyst prepared according to Comparative Example 1 , the MEAs of Examples 3 and 4 had similar performance to, or more excellent performance than, the MEA of Comparative Example 1. The results are considered to be caused since the catalyst having small particle size was uniformly dispersed and supported, and thus the catalyst availability was increased.
Evaluation Example 2: Evaluation test on carbon monoxide (CO) poisoning resistance
Evaluation tests on carbon monoxide (CO) poisoning resistance of the MEAs prepared according to Example 3 and Comparative Example 1 were performed. The test was performed in the same manner as in Evaluation Example 1 , except that humidified hydrogen including 100 ppm of CO was supplied to an anode of each MEA. The results are shown in FIG. 6. In addition, the test result of the MEA of Example 3 obtained in Evaluation Example 1 (Example 3A) was illustrated in FIG. 6 to compare the decreased performance with CO to the test result without CO.
As shown in FIG. 6, when a fuel including CO was used, the performance decreased compared with a fuel without CO1 and the property of CO poisoning resistance of the MEA of Example 3 was more excellent than that of the MEA of Comparative Example 1. Such results indicate that the MEA of Example 3 is suitable for a catalyst for a fuel cell in which a reformed hydrogen enriched gas is used as a fuel. The principle of having the property of CO poisoning resistance has not been clearly revealed. However, since the supported amount of the catalyst of Example 3 was 1/10 of the supported amount of the catalyst of Comparative Example 1 , it can be considered that the property of CO poisoning resistance has been drastically increased.
Example 5: Preparation of a Pt/Ru supported catalyst including an electrically conductive polymer layer A supported catalyst was prepared in the same manner as in Example 1 , except that 5 ml of 0.133 M platinum chloride solution and 5 ml of 0.133 M ruthenium chloride were added instead of 10 ml of 0.1 M platinum chloride.
Example 6: Preparation of an MEA using the supported catalyst prepared according to Example 5 An MEA was prepared in the same manner as in Example 3, except that the supported catalyst prepared according to Example 5 was coated on a gas diffusion layer for an anode with the concentration of 1.0 mg/cm2 instead of the supported catalyst prepared according to Example 1.
Comparative Example 2: Preparation of an MEA using a commercially available Pt/Ru catalyst
An MEA was prepared in the same manner as in Example 3, except that a commercially available Pt/Ru catalyst (E-TEK Corporation) was coated on a gas diffusion layer for an anode with the concentration of 4.0 mg/cm2 instead of the supported catalyst prepared according to Example 1. Evaluation Example 3
Performance of MEAs prepared according to Example 6 and Comparative Example 2 were tested. 2 M methanol solution was supplied to an anode, and dry air was supplied to a cathode. Voltages of the MEAs were measured according to current density at the operation temperature of 500C , and the results are shown in FIG. 7.
As shown in FIG. 7, although the amount of supported catalyst prepared according to Example 6 was about 1/4 of the amount of the supported catalyst prepared according to Comparative Example 2, the performances of both MEAs were almost the same. Such results indicate that the catalyst for fuel cell electrode has excellent efficiency in methanol oxidation.
Example 7
A supported catalyst was prepared in the same manner as in Example 1 , except that the amount of the 0.1 M platinum chloride solution was increased to 30 ml.
As a result of a TGA analysis, the amount of platinum in the supported catalyst was 78% by weight of the supported catalyst. TEM images of the supported catalyst are illustrated in FIG. 3B. As shown in FIG. 3B, it was identified that the metal catalyst having a particle size of about 3 nm or less was uniformly dispersed and supported. While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.
INDUSTRIAL APPLICABILITY
The present invention provides a supported catalyst in which a active transition metal is supported on a support in a high content, and is uniformly dispersed, and thus the catalyst for a fuel cell electrode of the present invention has excellent catalyst activity, can be manufactured at low cost, and is highly resistant to CO poisoning.

Claims

1. A catalyst for a fuel cell electrode comprising: a carbon-based support; an electrically conductive polymer at least partially coating the carbon-based support; transition metal particles distributed on the surface of the electrically conductive polymer; and a catalyst metal precursor or atoms of the transition metal distributed inside the electrically conductive polymer.
2. The catalyst for a fuel cell electrode of claim 1 , wherein the electrically conductive polymer is selected from the group consisting of polypyrrole, polyaniline, polythiophene, polyethylenedioxythiophene, polythienylenevinylene, poly(p-phenylenesulfide and a mixture thereof.
3. The catalyst for a fuel cell electrode of claim 1 , wherein the thickness of the coating formed of the electrically conductive polymer is in the range of 0.1 to 1000 nm.
4. The catalyst for a fuel cell electrode of claim 1 , wherein the thickness of the coating formed of the electrically conductive polymer is in the range of 0.5 to 100 nm.
5. The catalyst for a fuel cell electrode of claim 1 , wherein the transition metal is selected from the group consisting of Pt, Ru, Ir, W, Co, Ni, Fe, Os, Rh, Re and an alloy thereof.
6. The catalyst for a fuel cell electrode of claim 1 , wherein the average particle size of the transition metal is in the range of 1 to 500 nm.
7. The catalyst for a fuel cell electrode of claim 1 , wherein the carbon-based support is formed of one from the group consisting of graphite, carbon powder, acetylene black, carbon black, activated carbon, mesoporous carbon, carbon nanotubes, carbon nanofibers, carbon nonohorns, carbon nanorings, carbon nanowires, and fullerene (Ceo)-
8. A catalyst for a fuel cell electrode comprising: a carbon-based support; a carbon layer formed in a different phase from the carbon-based support and at least partially coating the carbon-based support; transition metal particles distributed on the surface of the carbon layer; and a catalyst metal precursor or atoms of the transition metal distributed inside the carbon layer.
9. A method of preparing a catalyst for a fuel cell electrode, the method comprising: a) providing a mixture in which a carbon-based support, a transition metal catalyst precursor, a monomer, which can be polymerized to form a conductive polymer are dispersed in a dispersion medium; b) polymerization of the monomers in the mixture; and c) reduction of the catalyst metal precursor.
10. The method of claim 9, wherein the amount of the carbon-based support is in the range of 1 to 60% by weight, the amount of the transition metal catalyst precursor is in the range of 20 to 90% by weight, and the amount of the monomer is in the range of 10 to 60% by weight based on the total amount of the mixture comprising the carbon-based support, the transition metal catalyst precursor, and the monomer.
11. The method of claim 9, wherein the concentration of the transition metal catalyst precursor is 1/4 to 2 times that of the monomer.
12. The method of claim 9, wherein the polymerization is performed at a temperature in the range of 1 to 30 °C .
13. The method of claim 9, wherein the polymerization is performed at a temperature in the range of 1 to 6 °C.
14. The method of claim 9, wherein the mixture in operation a) further comprises a surfactant.
15. The method of claim 14, wherein the surfactant is selected from the group consisting of an alanine-based compound, an imidazolium betaine-based compound, an aminodipropionate salt, and an alkylarylpolyetheralcohol-based compound.
16. The method of claim 9, wherein the reduction in operation c) is performed under a hydrogen atmosphere at a temperature in the range of 100 to 1000°C .
17. The method of claim 9, further comprising heat-treating the reduced catalyst metal precursor obtained from operation c).
18. The method of claim 17, wherein the heat-treatment is performed at a temperature in the range of 300 to 10000C.
19. An electrode for fuel cells comprising a catalyst for a fuel cell electrode of any one of claims 1 through 8.
20. A membrane electrode assembly for a fuel cell comprising: a cathode comprising a catalyst layer and a diffusion layer; an anode comprising a catalyst layer and a diffusion layer; and an electrolyte membrane interposed between the cathode and the anode, wherein the cathode or the anode comprises a catalyst for a fuel cell electrode of claim 1 or 8.
21. A fuel cell comprising: a cathode comprising a catalyst layer and a diffusion layer; an anode comprising a catalyst layer and a diffusion layer; and an electrolyte membrane interposed between the cathode and the anode, wherein the cathode or the anode comprises a catalyst for a fuel cell electrode of claim 1 or 8.
PCT/KR2006/004971 2005-11-25 2006-11-24 Catalyst for fuel cell electrode and method of preparing the same WO2007061248A1 (en)

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