Classifications

• • 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
  • H01M4/8803—Supports for the deposition of the catalytic active composition
  • H01M4/881—Electrolytic membranes
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D7/00—Electroplating characterised by the article coated
• • 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
  • H01M4/88—Processes of manufacture
  • H01M4/8817—Treatment of supports before application of the catalytic active composition
• • 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
  • H01M4/8825—Methods for deposition of the catalytic active composition
  • H01M4/8853—Electrodeposition
• • 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
  • H01M8/1004—Fuel cells with solid electrolytes characterised by membrane-electrode assemblies [MEA]
• • 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
  • H01M2008/1095—Fuel cells with polymeric electrolytes
• • 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 method of fabricating membrane-electrode assemblies, particularly for PEM fuel cells, which comprise catalytically active electrodes.
• Fuel cells are energy converters that convert chemical energy into electrical energy.
• the electrolytic principle is inverted.
• various types of fuel cells are known which generally differ from one another with respect to their operating temperature.
• the design of the cells is in principle the same for all types. They generally comprise two electrodes, an anode and a cathode, at which the reactions proceed, and an electrolyte between the two electrodes.
• the electrolyte used is a polymer membrane which conducts ions (especially H + ions).
• the electrolyte has three functions.
• the electrodes are supplied with gases which are reacted as part of a redox reaction.
• the electrodes have the functions of feeding in the gases (e.g. hydrogen or methanol and oxygen), of removing reaction products such as water or CO 2 , of catalytically reacting the starting materials and of drawing off or supplying electrons.
• the conversion of chemical into electrical energy takes place at the three-phase boundary of catalytically active sites (e.g. platinum), ion conductors (e.g. ion exchange polymers), electron conductors (e.g. graphite) and gases (e.g. H 2 and O 2 ).
• the catalysts should have as large an active surface area as possible.
• a method of electrolytically depositing metals on a solid electrolyte is disclosed by DE-A 28 21 271, wherein the solid electrolyte, in the dried state, treated in a solution containing the metal as a salt, is inserted into an electrolytic cell and is subjected to an electrolytic process. In so doing, the cell is kept at a constant current density over a predetermined period. This results in a continuous surface cutting of this electrolyte, which means that catalyst atoms are present not only at those sites which are catalytically active under fuel cell conditions, but on the entire solid electrolyte surface.
• MEA membrane-electrode assembly
• PEM polymer-electrolyte membrane
• the polymer-electrolyte membrane in the context of the present invention is to be understood as meaning either a polymer membrane serving as the electrolyte or a polymer membrane whose pores are filled with a substance serving as an electrolyte (e.g. with an ionomer, acid).
• the at least one membrane-electrode assembly composed of the components electrode/membrane/electrode arranged like a sandwich, represents the central element of the PEM fuel cell.
• a PEM fuel cell usually comprises a stack-like arrangement of a multiplicity of membrane-electrode assemblies.
• Each electrode usually comprises a reaction layer and, in the case of fuel cells that run on gases, a gas distribution layer.
• the gas distribution layer can serve as a mechanical support for the electrode and ensures that the respective gas is properly distributed across the reaction layer and that the electrons are collected.
• a gas distribution layer is required, in particular, for fuel cells operating with hydrogen on the one hand and oxygen or air on the other hand.
• the reaction layer is where the electrochemical reaction proper takes place during fuel cell operation. At least one of the reaction layers contains at least one catalytic component which catalytically supports e.g. the reaction of oxidation of hydrogen or of reduction of oxygen. Alternatively, however, the reaction layers may contain a plurality of catalytic substances having different functions. In addition, the reaction layer may contain a functionalized polymer (ionomer) or a nonfunctionalized polymer.
• an electron conductor in the reaction layers serves, inter alia, to conduct the electric current which flows during the fuel cell reaction, and as a support material for catalytic substances.
• ions of a catalytic component are first of all introduced into the polymer-electrolyte membrane.
• the ions can additionally be introduced into the ionomer which may, if required, have been incorporated into the reaction layer.
• the polymer-electrolyte membrane consists of cation-conductive polymer materials which hereinafter are referred to as ionomer.
• ionomer cation-conductive polymer materials
• Such a material is commercially available, for example, under the trade name Nafion® by E. I. du Pont.
• Examples of ionomer materials that can be used in the present invention are the following polymer materials or mixtures thereof:
• perfluorinated and/or partially fluorinated polymers such as “Dow experimental membrane” (Dow Chemicals, USA),
• fluorine-free, ionomer materials can be used, e.g. sulfonated phenol formaldehyde resins (linear or crosslinked); sulfonated polystyrene (linear or crosslinked); sulfonated poly-2,6-diphenyl-1,4-phenylene oxides, sulfonated polyarylethersulfones, sulfonated polyarylethersulfones, sulfonated polyaryletherketones, phosphonated poly-2-6-dimethyl-1,4-phenyl oxides, sulfonated polyetherketones, sulfonated polyetheretherketones, arylketones or polybenzimidazoles.
• sulfonated phenol formaldehyde resins linear or crosslinked
• sulfonated polystyrene linear or crosslinked
• polymer materials which include the following components (or mixtures thereof):
• the ion exchange materials used may include further inorganic and/or organic components (e.g. silicates, minerals, clays, silicones) which have a positive effect on the properties of the ionic exchange material (e.g. conductivity).
• further inorganic and/or organic components e.g. silicates, minerals, clays, silicones
• porous non-conductive polymers which require their conductivity by the pores being filled with e.g. an ionomer (for example Goreselect, Gore, USA) or an acid (for example H 3 PO 4 , H 2 SO 4 , methanesulfonic acid, . . . ).
• an ionomer for example Goreselect, Gore, USA
• an acid for example H 3 PO 4 , H 2 SO 4 , methanesulfonic acid, . . .
• the introduction of the ions of a catalytic substance into the polymer-electrolyte membrane is effected by a technique known in the prior art.
• the catalytic substance is present ionically in a solution with which the polymer-electrolyte membrane is impregnated.
• ion exchange causes the ions of the catalytic substance to bind to the membrane, e.g. in Nafion® to bind to ionic SO 3 H groups.
• the diffusion of the ions of the catalytic substance into the polymer-electrolyte membrane is promoted by an external electric field being applied.
• the next step B) in the method according to the invention is the application of the electron conductor to both sides of the polymer-electrolyte membrane.
• This purpose can be served by a technique known in the prior art, for example a dry or wet spray technique with the aid of which the electron conductor present as a powder or possibly dissolved in an ionomer solution is sprayed directly onto the membrane or onto a support, followed by optional hot compression bonding to the membrane.
• Further options of applying include e.g. screen printing or sintering followed by optional hot compression-bonding to the membrane.
• the introduction of ions of the catalytic component into an ionomer incorporated into the electron conductor layer are examples of the introduction of the catalytic component into an ionomer incorporated into the electron conductor layer.
• the at least one membrane-electrode assembly Prior to the next step C) of the method according to the invention, the at least one membrane-electrode assembly is largely finished and installed in an apparatus which allows an electric current to be impressed or reactants (e.g. H 2 /O 2 ) to be fed in while at the same time an electric current is tapped off. Also conceivable would be a continuous procedure, in which step C) of the method according to the invention is carried out, and the at least one membrane-electrode unit as processed is then installed in a PEM fuel cell.
• an electric current to be impressed or reactants e.g. H 2 /O 2
• the catalytic substance introduced into the membrane in step A) is present in the form of ions bound within the membrane (for example to its negatively charged sulfone groups). These are deposited electrochemically, in step C) of the method according to the invention, from the polymer-electrolyte membrane onto the electron conductor on at least one side of the polymer-electrolyte membrane.
• the electrochemical deposition is to be understood, in this context, as the deposition of the catalytic components while chemical energy is converted into electrical energy or vice versa, the mechanism being ion migration within the membrane and an electrode reaction taking place.
• An advantage of the method according to the invention is that as a result of the electrochemical deposition of the catalytic component from the membrane, said catalytic component can be deposited only where the electrochemically active three-phase boundary is also present.
• the catalytic component is therefore deposited specifically onto the electron conductor in those locations where the ion channels of the membrane terminate.
• a continuous layer of the catalytic component is not formed, the catalyst instead being deposited only at those plates where it is optimally utilized.
• the result is an effective reduction in catalytic loading without a decrease in the fuel cell performance. For example it is possible to operate fuel cells having a Pt loading of the electron conductor of less than 1 mg/cm 2 .
• the reduction in the catalytic loading advantageously means a cost reduction for MEA fabrication, since the catalytic components used are often metals. Since the catalytic component in the present invention is present in finely dispersed form as ions in the interior of the polymer-electrolyte membrane, another point is that, in contrast to the deposition of catalyst ions from a solution, with the method according to the invention no impurities in the form of other undesirable ions are deposited on the electron conductor, but only the catalyst ions present in the membrane.
• a further advantage of the method according to the invention is the small number of procedural steps for fabricating a polymer-electrolyte membrane fuel cell. This too has a positive effect on costs.
• the electrochemical deposition of the ions of the catalytic component in step C) of the method according to the invention is effected by operating an apparatus which allows an electrical current to be tapped off and fuel cell reactants to be fed in under fuel cell conditions, e.g. by operating a PEM fuel cell on the fuel conditions.
• an apparatus which allows an electrical current to be tapped off and fuel cell reactants to be fed in under fuel cell conditions, e.g. by operating a PEM fuel cell on the fuel conditions.
• gas distribution layers are required which must be applied to the reaction layers or to the respective electron conductor before step C) of the method according to the invention is carried out.
• the gas distribution layers for example bonded carbon fiber web (E-Tek carbon cloth) or carbon paper (e.g. Toray carbon paper (Electrochem. Inc.), Spectracorp carbon paper (Spectracorp), Sicracet Gas Diffusion Media (SGL Carbon)) are applied, prior to the electrochemical deposition (step C)) by laying on, rolling, hot pressing or other techniques known to those skilled in the art. Then the anode of the apparatus is fed with e.g.
• the cathode is fed with e.g. oxygen.
• H + ions and electrons are produced by oxidation of the hydrogen.
• the H + ions, together with the ions of the catalytic component introduced into the membrane in step A) migrate through the membrane to the cathode which preferably already includes a small amount of the catalytic component and at which the reduction of oxygen to water and the deposition of the catalyst cations takes place.
• the electrons required for the reduction flow through an external electric circuit from the anode to the cathode.
• the catalyst cations are advantageously, without an additional procedural step, deposited at precisely those locations on the electron conductor so as to be firmly attached thereto, where they are optimally utilized for the fuel cell.
• electrochemical deposition of the at least one catalytic component onto the electron conductor in a liquid milieu is possible, for example in a directoxidation fuel cell such as a direct-methanol fuel cell.
• the electrochemical deposition of the ions of the catalytic component in step C) of the method according to the invention is effected by operating an apparatus which allows an electric current to be impressed for the electrolytic deposition of the catalytic component. This is done, for example, by operating the apparatus (e.g. a PEM fuel cell) under electrolytic conditions.
• the apparatus e.g. a PEM fuel cell
• the ions present in the membrane are deposited electrolytically on the electron conductor.
• the catalyst ions thus deposited are located precisely as targeted at the end of ion-conducting regions of the membrane and are thus wholly active.
• the electrolysis can be fed out by a DC voltage being applied to the electrodes of the apparatus (e.g. fuel cell).
• the electrodes of the apparatus e.g. fuel cell.
• the metal ions which are present in uniform dispersion in the polymer-electrolyte membrane are deposited cathodically on the electron conductor.
• the polarity of the electrodes during the electrolysis is chosen accordingly.
• the electrolysis for depositing the catalytic component is carried out by applying a time-variant, e.g. pulsed DC voltage or a time-variant DC current to the electrodes of the fuel cell, with the resultant advantage that control of the particle size of the deposited particles and of the surface morphology of the catalyst (e.g. the noble metal) becomes possible.
• a time-variant e.g. pulsed DC voltage or a time-variant DC current
• the electrolysis for depositing the catalytic component from the membrane onto the respective electron conductor is carried out by applying an AC voltage (alternatively a DC voltage whose polarity is periodically reversed) or a DC voltage with an AC voltage superimposed thereon to the electrode in the fuel cell.
• an AC voltage alternatively a DC voltage whose polarity is periodically reversed
• a DC voltage with an AC voltage superimposed thereon to the electrode in the fuel cell.
• step C) at least one element from the 3 rd to 14 th group of the periodic table of the elements (PTE), equally preferably from the 8 th to 14 th group of the PTE is deposited as the catalytic component onto the electron conductor on at least one side of the polymer-electrolyte membrane.
• PTE periodic table of the elements
• These electrocatalysts promote the fuel cell reaction (oxidation of hydrogen or reduction of oxygen) catalytically.
• these catalytically active components are applied in highly dispersed form to the surface of the electron conductor serving as a support.
• the abovementioned catalytically active components are introduced into the polymer-electrolyte membrane, in step A) of the method according to the invention, in a concentration of preferably 0.000005 to 0.05 mmol/cm 2 .
• step C) at least one of the elements Pt, Co, Fe, Cr, Mn, Cu, V, Ru, Pd, Ni, Mo, Sn, Zn, Au, Ag, Rh, Ir or W is deposited as the catalytic component on the cathode-side electron conductor in the fuel cell. While this is done, a further catalytic substance required for lowering the activation energy for the fuel cell reaction may already be present on the electron conductor.
• a further catalytic substance required for lowering the activation energy for the fuel cell reaction may already be present on the electron conductor.
• copper can be deposited as a second catalytically active substance on an electron conductor which already supports platinum as the first catalytically active substance.
• catalytically active components Pt, Co, Fe, Cr, Mn, Cu, V, Ru, Pd, Ni, Mo, Sn, Zn, Au, Ag, Rh, Ir or W to be deposited on the cathode side of the fuel cell are introduced, in step A) of the method according to the invention, into the polymer-electrolyte membrane in an amount/concentration of preferably 0.000005 to 0.05 mmol/cm 2 .
• R is preferably H.
• HOO. is a peroxidic radical corresponding to H 2 O 2 (hydrogen peroxide).
• deperoxidation-active additives Only by introducing further deperoxidation-active additives is it possible to decompose the peroxides formed on the platinum in the fuel cell or to inhibit their formation.
• the active components to be mentioned for such elements or compounds acting as deperoxidation-active additives are primarily the metals Co, Fe, Cr, Mn, Cu, V, Ru, Pd, Ni, Mo, Sn, Zn, Au, Ag, Rh, Ir or W.
• the said metals are therefore deposited as catalytic components, preferably by means of the method according to the invention, on the cathode-side electron conductor. They are introduced, in step A) of the method according to the invention, into the polymer-electrolyte membrane in an amount/concentration of preferably 0.000005 to 0.05 mmol/cm 2 .
• catalysts comprising a plurality of active components.
• a plurality of catalytic components are therefore deposited on the electron conductor on the cathode side in the fuel cell.
• step A) of the method according to the invention to be introduced into the polymer-electrolyte membrane (PEM), which are then, in step C), jointly deposited on the electron conductor, and/or, on the other hand, where components that are already catalytic to be applied together with the electron conductor onto the PEM in step B), followed by the deposition thereonto, in step C) of additional catalytic components.
• PEM polymer-electrolyte membrane
• step C) at least one of the elements Co, Fe, Cr, Mn, Cu, V, Ru, Pd, Ni, Mo, Sn, Zn, Au, Ag, Rh, Ir or W is deposited as the catalytic component on the anode-side electron conductor in the fuel cell.
• Base cocatalysts e.g. ruthenium
• H 2 O oxidation of the CO on the CO-loaded catalyst by H 2 O it is possible to effect a “desorption” of the CO molecules.
• Base cocatalysts e.g. ruthenium
• such catalytic components can be deposited precisely on target on the electron conductor on the anode side, for example ruthenium on a Pt-C electron conductor, thereby reducing the risk of CO poisoning of the fuel cell.
• step A) of the method according to the invention into the polymer-electrolyte membrane in an amount/concentration of preferably 0.000005 to 0.5 mmol/cm 2 .
• the electron conductor used preferably contains at least one metallic element in the form of bonded fiber web, fibers or powder.
• the use of electron-conducting polymers as electron conductors Particular preference is given to the use of finely dispersed C blacks or graphite powders as electron conductors.
• the carbon black or the graphite by means of the large surface area of their particles, serve as electrically conductive gas-porous supports for at least one catalytic component.
• said catalytic component can be applied to the electron conductor which has previously been bonded to a polymer-electrolyte membrane.
• the electron conductor applied in step B) of the method according to the invention comprises at least one catalytic component from the group consisting of Pt, Co, Fe, Cr, Mn, Cu, V, Ru, Pd, Ni, Mo, Sn, Zn, Au, Ag, Rh, Ir or W.
• the electron conductor when it is bonded in step B) to the membrane, already serves as a support for at least one catalytic component (e.g. platinum), and at least one further catalytic component (e.g. Ru or Cu) or the catalytic component already present (e.g. additional Pt) is deposited, in step C) of the method according to the invention, on said catalyst-containing electron conductor.
• a catalytic component which enhances CO tolerance to be deposited on the anode side onto an electron conductor on which a catalytically active component which catalytically promotes the fuel cell reaction is already present.
• a deperoxidation-active component can be deposited on an electron conductor/catalyst combination.
• an ion conductor e.g. an ionomer solution or suspension
• an ion conductor e.g. an ionomer solution or suspension
• the joint application of ionomer and electron conductor advantageously results in the electron conductor being made accessible to a high degree by means of ionomer, which means a large 3-phase interface area.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Engineering & Computer Science (AREA)
• General Chemical & Material Sciences (AREA)
• Manufacturing & Machinery (AREA)
• Materials Engineering (AREA)
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• Metallurgy (AREA)
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• Composite Materials (AREA)
• Inert Electrodes (AREA)
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• 2002
  • 2002-12-10 DE DE10257643A patent/DE10257643A1/de not_active Withdrawn
• 2003
  • 2003-12-08 JP JP2003408661A patent/JP2004288620A/ja active Pending
  • 2003-12-09 EP EP03028202A patent/EP1437787A3/de not_active Withdrawn
  • 2003-12-10 US US10/731,168 patent/US20040112754A1/en not_active Abandoned

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