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
  • H01M4/90—Selection of catalytic material
  • H01M4/92—Metals of platinum group
  • H01M4/925—Metals of platinum group supported on carriers, e.g. powder carriers
• • 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
  • H01M4/90—Selection of catalytic material
  • H01M4/92—Metals of platinum group
• • 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
  • H01M4/96—Carbon-based electrodes
• • 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
  • H01M8/08—Fuel cells with aqueous electrolytes
  • H01M8/083—Alkaline 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
  • H01M2004/8678—Inert electrodes with catalytic activity, e.g. for fuel cells characterised by the polarity
  • H01M2004/8684—Negative 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
  • H01M2004/8678—Inert electrodes with catalytic activity, e.g. for fuel cells characterised by the polarity
  • H01M2004/8689—Positive electrodes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M2300/00—Electrolytes
  • H01M2300/0002—Aqueous electrolytes
  • H01M2300/0014—Alkaline electrolytes
• • 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
• • 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

Definitions

• the present invention pertains to fuel cell electrodes, and particularly to electrodes for fuel cells having an alkaline electrolyte.
• a fuel cell is a device for converting chemical energy into electricity.
• An alkaline fuel cell comprises an anode, a cathode and an alkaline
• hydroxide is typically used as the alkaline
• the anode and cathode each comprise a porous substrate and a porous catalyst layer supported on the substrate.
• a hydrogen containing gas is fed to the anode and an oxygen containing gas is fed to the cathode.
• the reactant gases diffuse through the electrodes to react with the electrolyte in the presence of the catalysts to produce water, heat and electricity.
• the hydrogen is electrochemically oxidized and gives up electrons according to the reaction:
• the electrons so generated are conducted from the anode through an external circuit to the cathode.
• electrons are electrochemically combined with the oxidant according to the reaction:
• a flow of ions through the electrolyte completes the electrical circuit.
• the catalyst layer comprises a polymeric hydrophobic phase and a wettable catalyst phase. Particles of catalyst form a network of electrolyte-filled channels through the catalyst layer.
• the porous hydrophobic phase binds the electrode together and provides a network of channels through which reactant gases gain access to the catalytic surfaces and through which gaseous reaction products escape from the catalyst layer.
• water-producing electrode i.e. the anode of an alkaline electrolyte fuel cell.
• plating refers to bulk movement of electrolyte from one side of a fuel cell to the other due to the electromigration of nonelectroactive ions.
• electromigration For example, in an alkaline fuel cell having a KOH electrolyte nonelectroactive potassium ions migrate toward the cathode.
• pressure may be built up that is sufficient to force electrolyte out of the gas side of the cathode. Accumulation of electrolyte on the gas side of the electrode restricts the supply of reactant gas to the catalyst layer of the electrode and may lead to severe concentration polarization of the electrode.
• the conventional approach to the problem of flooding has been to make the catalyst layer of the electrode more hydrophobic.
• the catalyst layer can be made more hydrophobic by increasing the amount of hydrophobic binder in the layer or by increasing the temperature or process time during sintering of the electrode.
• the pumping phenomenon has been addressed by applying a thin layer of a porous hydrophobic polymer to the gas side of the electrode.
• What is needed in the art is a way to provide electrodes that are resistant to flooding and pumping and to thereby provide an alkaline fuel cell that may be operated continuously at high current density.
• a gas diffusion electrode for conducting an electrochemical reaction in an alkaline fuel cell comprises a porous substrate and a catalyst layer supported on the substrate.
• the catalyst layer comprises a catalytically effective amount of catalyst particles for promoting the
• a porous hydrophobic binder for providing a network of hydrophobic gas pathways communicating with the catalyst particles and
• hydrophilic particles in an amount effective to provide a network of liquid transport pathways throughout the catalyst layer.
• the hydrophilic particles are not active in promoting the
• a method for making a gas diffusion electrode comprises preparing an aqueous dispersion of catalyst particles and
• hydrophobic polymer particles flocculating the dispersion to form a flocculent suspension, mixing hydrophilic particles with the flocculent suspension to form a mixture, filtering the mixture to form a catalyst layer, disposing the catalyst layer on a porous substrate and sintering the catalyst layer to form the electrode.
• Figure 1 shows a schematic cross section of an alkaline fuel cell
• Figure 2 shows a schematic diagram of a portion of a catalyst layer of a conventional electrode
• Figure 3 shows a schematic diagram of a portion of a catalyst layer of an electrode of the present invention.
• the fuel cell includes an anode 1, a cathode 2, an electrolyte-filled matrix 3 between the anode 1 and the cathode 2, an anode gas flow field 4 and a cathode gas flow field 5.
• the anode 1 comprises a substrate 6 and a catalyst layer 7.
• the cathode 2 comprises a substrate 8 and a catalyst layer 9.
• the catalyst layers 7 and 9 are each oriented toward the matrix 3.
• the matrix 3 is a thin porous sheet of an electrically insulating material which holds the alkaline electrolyte within its pores by capillary forces.
• the anode 1 and cathode 2 are electrically connected by an external circuit (not shown). Gas flow passages 10 and 11 distribute reactant gases over the electrodes.
• the electrodes of the present invention each include an electrically conductive porous substrate.
• the porous substrate may be any substrate
• the porous substrate may comprise gold screen, gold plated perforated metal foil, porous sintered nickel, carbon paper or graphite paper.
• the porous substrate of the electrode of the present invention comprises a gold plated
• the electrodes of the present invention each include a catalyst layer supported on the porous substrate.
• the catalyst layer includes a hydrophobic binder, catalyst particles, and hydrophilic particles.
• the hydrophobic binder may comprise any one of
• the hydrophobic binder of the present invention comprises a fluoropolymer.
• the hydrophobic binder comprises polytetrafluoroethylene (PTFE).
• the catalyst layer may comprise between about 10 weight percent and about 50 weight percent of hydrophobic binder.
• the catalyst layer comprises about 20 weight percent hydrophobic binder.
• the catalyst layer of the anode of the present invention includes the particles that catalyze the electrochemical oxidation of hydrogen in an alkaline electrolyte.
• the catalyst particles of the anode catalyst layer may comprise any material
• the catalyst conventionally used as a catalyst in the anode of an alkaline fuel cell.
• the catalyst is conventionally used as a catalyst in the anode of an alkaline fuel cell.
• the catalyst is conventionally used as a catalyst in the anode of an alkaline fuel cell.
• the catalyst is conventionally used as a catalyst in the anode of an alkaline fuel cell.
• the catalyst is conventionally used as a catalyst in the anode of an alkaline fuel cell.
• the catalyst is conventionally used as a catalyst in the anode of an alkaline fuel cell.
• particles of the anode catalyst layer may comprise platinum, palladium, rhodium, iridium and mixtures or alloys thereof, either as metal blacks or supported on a chemically inert support.
• the catalyst particles of the anode catalyst layer comprise
• platinum black or a mixture of platinum black and palladium black.
• the anode catalyst particles have a mean specific surface area between about 5 m 2 /g and 200 m 2 /g.
• the anode catalyst particles are metal blacks having a surface area between
• the anode catalyst layer may comprise between 25 weight percent and 90 weight percent catalyst particles.
• the catalyst layer of the anode comprises between 50 weight percent and 70 weight percent catalyst particles.
• the catalyst layer of the cathode of the present invention includes particles that catalyze the
• the catalyst particles of the cathode catalyst layer may be particles of any material conventionally used as a catalyst in the cathode of an alkaline fuel cell.
• the catalyst may be particles of any material conventionally used as a catalyst in the cathode of an alkaline fuel cell.
• the particles of the catalyst layer may comprise platinum, gold, silver, iridium, rhodium and mixtures or alloys thereof, as metal blacks or supported on an inert support.
• the cathode catalyst particles have a mean specific surface area between about
• the cathode catalyst particles are metal blacks and have a surface area between about 10 m 2 /g and about 40 m 2 /g.
• the cathode catalyst particles comprise a platinum-iridium alloy black comprising 90 weight percent platinum and 10 weight percent iridium.
• the cathode catalyst layer may comprise between 25 weight percent and about 90 weight percent catalyst
• the catalyst layer of the cathode comprises between about 50 weight percent and about 70 weight percent catalyst particles.
• the catalyst layer of the electrode of the present invention comprises between about 2.5 weight percent and about 45 weight percent hydrophilic particles.
• the hydrophilic particles of the anode of the present invention may be any particles that have a hydrophilic, i.e. wettable by water, surface and that are stable in an alkaline electrolyte under the operating conditions of the anode.
• Suitable hydrophilic particles include, e.g.
• the hydrophilic particles of the anode catalyst layer comprise carbon black or a gold-platinum alloy black.
• the hydrophilic particles of the anode catalyst layer comprise a gold-platinum alloy black comprising about 90 weight percent gold and about 10 weight percent platinum.
• the hydrophilic particles of the cathode catalyst layer may comprise any particles that have hydrophilic surfaces and that are stable in an alkaline
• Suitable hydrophilic particles include, e.g. certain ceramic materials, spinels, (e.g.
• the hydrophilic particles of a particular cathode catalyst layer of the present invention comprise the same material as the catalyst particles of the particular cathode catalyst layer.
• a schematic diagram of a portion of the catalyst layer 12 of a conventional electrode is shown in
• the layer 12 includes catalyst particles 13 and a hydrophobic binder 14 which provides a network of gas pathways communicating with the catalyst particles 13.
• FIG. 3 A schematic diagram of a portion of a catalyst layer 15 of an electrode of the present invention is shown in Figure 3.
• the layer 15 includes catalyst particles 16 associated with a porous hydrophobic binder 17 which provides a network of hydrophobic gas pathways communicating with the catalyst particles 16.
• Hydrophilic particles 18 are distributed throughout the layer 15 to form an interpenetrating network of liquid transport pathways throughout the catalyst layer 15.
• the surfaces of the catalyst particles of the electrode of the present invention are associated with the
• hydrophobic polymeric binder to control the
• this is accomplished by flocculating a dispersion of the hydrophobic polymer and catalyst particles.
• hydrophilic particles of the present invention are incorporated into the catalyst layer in a manner which maintains the wettability of the surfaces of the hydrophilic particles.
• the process of the present invention is one method of providing an electrode having the desired composition and structure.
• catalyst particles and hydrophobic polymer particles are dispersed in water and the dispersion is caused to flocculate to form a
• hydrophilic particles are stirred rapidly into the flocculent aqueous suspension and the mixture so formed is immediately filtered to form a catalyst layer.
• the catalyst layer is transferred to a suitable porous substrate.
• the catalyst layer and substrate are dried and sintered to form the electrode.
• a small amount e.g. from about 5 to about 10 weight percent based on the combined weight of added hydrophobic polymer and hydrophilic particles, of polymer i.e. binder, may be added to the flocculent suspension along with the hydrophilic particles to more securely bind the hydrophilic particles into the catalyst layer without rendering the surfaces of the hydrophilic particles hydrophobic.
• hydrophilic particles of the present invention are present.
• hydrophilic particles form a network of liquid
• the hydrophilic particles are substantially inactive in the electrochemical reaction occurring at the electrode, even in embodiments of the present invention wherein the hydrophilic particles comprise a material that is catalytically active in the reaction occurring at an electrode that includes the
• the hydrophilic particles of the electrode of the present invention provide a network of wettable surfaces throughout the catalyst layer. Since the hydrophilic particles of the electrode are not active in promoting the electrochemical reaction occurring at the surfaces of the catalyst particles of the
• the hydrophilic particles make no substantial contribution to water generation at the anode or to hydroxyl ion generation at the cathode, the network of wettable surfaces provided by the hydrophilic particles functions primarily as a passive network of liquid transport pathways.
• An anode (Al) according to the present invention was fabricated according to the method of the present invention.
• a catalyst suspension was formed by ultrasonically codispersing 1.92g platinum black (fuel cell grade from Englehard Corp.) and 0.22g palladium black (Johnson Matthey Inc.) in distilled water.
• a hydrophobic polymer 0.715g polytetrafluoroethylene was added to the catalyst suspension in the form of a diluted aqueous dispersion of TFE-3170 (DuPont). The catalyst/PTFE mixture so formed was blended
• a conventional anode (A2) was fabricated by a process analogous to the process of the present invention, except that the step of adding hydrophilic particles to the flocculated catalyst particle/PTFE suspension was omitted.
• Catalyst particles (1.92g platinum black and 0.22g palladium black) were
• the electrode was transferred onto a 100 mesh gold plated nickel screen to form an electrode.
• the electrode was sintered at 590°F for 5 minutes.
• Anodes A1 and A2 were operated for over 2 hours at 200 ps ⁇ a,and 3500 amp/ft 2 in identical H 2 /O 2 fuel cells, using 40 weight percent KOH as the electrolyte.
• the cells were disassembled and the polarizations and limiting currents of the anodes were determined in a half cell apparatus as a function of current density.
• the values obtained and comparable results obtained from testing unused portions of the same electrodes are given in Table 1.
• the anode (Al) of the present invention exhibited no increase in polarization following operation in the fuel cell whereas the conventional anode (A2)
• the limiting current of the anode of the present invention was substantially unchanged, while the post test limiting current of the conventional electrode was substantially lower than its pretest limiting current. The results indicate that the anode of the present invention offers improved resistance to flooding.
• a cathode (Cl) of the present invention was fabricated according to the method of the present invention.
• Catalyst particles (2.80g of 90:10/Pt:Ir alloy black) were dispersed ultrasonically in 75 ml
• a hydrophobic polymer (0.70g PTFE in the form of a diluted aqueous dispersion of TFE-30 (DuPont) was added to the catalyst dispersion and blended ultrasonically. Aluminum ions were added and the pH of the catalyst/PTFE dispersion was adjusted to flocculate the catalyst/PTFE dispersion. Hydrophilic particles (0.65g of 90:l ⁇ /Pt:Ir alloy black) were rapidly stirred into the flocculated catalyst/PTFE suspension. The catalyst particle/PTFE/hydrophilic particle mixture was vacuum filtered and the damp filter cake was press transferred on to a 0.003 inch thick, gold-plated, perforated nickel foil to form an electrode having an active area of 131 cm. The electrode was vacuum dried, heated to volatilize the surfactant introduced with the TFE-30 dispersion and then sintered at 610"F for 10 minutes.
• a conventional cathode (C2) was fabricated by a conventional process analogous to the process of the present invention, except that the step of adding the hydrophilic particles to the flocculated catalyst particle/PTFE suspension was omitted.
• Cathodes Cl and C2 were operated in identical H 2 /O 2 fuel cells at 200 psia and 250°F using 40 weight percent KOH as the electrolyte.
• Each of the cells included anodes of the present invention as described in Example 1, i.e. identical to electrode Al. The results of the testing are given in TABLE II.
• the cell including the cathode (C1) of the present invention exhibited higher performance and substantially improved stability compared to the cell including the conventional cathode (C2).
• Electrodes are able to transport liquids.
• Flooding occurs when the rate at which water is generated in the anode reaction exceeds the rate at which the water can be transported away from the catalytic surfaces of the anode catalyst layer.
• hydrophilic particles of the particles of the electrode of the present invention provide
• catalytically inactive conduits for liquid transport through the catalyst layer Improving the ability of the fuel cell electrode to transport liquid directly addresses the causes of the flooding and pumping problems experienced with conventional electrodes and results in substantial improvement in electrode performance.
• Anodes of the present invention are resistant to flooding, even when operated at high current densities.
• Cathodes of the present invention are resistant to pumping, even when operated at high current densities.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• General Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Life Sciences & Earth Sciences (AREA)
• Manufacturing & Machinery (AREA)
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• 1991
  • 1991-07-26 WO PCT/US1991/005320 patent/WO1993003506A1/en not_active Application Discontinuation
  • 1991-07-26 EP EP91919623A patent/EP0600882A1/de not_active Withdrawn

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