WO2011097286A2 - Electrodes for metal-air batteries and fuel cells - Google Patents

Electrodes for metal-air batteries and fuel cells Download PDF

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Publication number
WO2011097286A2
WO2011097286A2 PCT/US2011/023459 US2011023459W WO2011097286A2 WO 2011097286 A2 WO2011097286 A2 WO 2011097286A2 US 2011023459 W US2011023459 W US 2011023459W WO 2011097286 A2 WO2011097286 A2 WO 2011097286A2
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Prior art keywords
polymer
conductive
electrode
carbon
conductive polymer
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PCT/US2011/023459
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French (fr)
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WO2011097286A3 (en
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Amy Marschilok
Esther Takeuchi
Kenneth Takeuchi
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The Research Foundation Of State University Of The New York
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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
    • H01M12/00—Hybrid cells; Manufacture thereof
    • H01M12/04—Hybrid cells; Manufacture thereof composed of a half-cell of the fuel-cell type and of a half-cell of the primary-cell type
    • H01M12/06—Hybrid cells; Manufacture thereof composed of a half-cell of the fuel-cell type and of a half-cell of the primary-cell type with one metallic and one gaseous electrode
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
    • H01B1/02—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors mainly consisting of metals or alloys
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
    • H01B1/02—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors mainly consisting of metals or alloys
    • H01B1/026—Alloys based on copper
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
    • H01B1/04—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors mainly consisting of carbon-silicon compounds, carbon or silicon
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
    • H01B1/06—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors mainly consisting of other non-metallic substances
    • H01B1/12—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors mainly consisting of other non-metallic substances organic substances
    • H01B1/124—Intrinsically conductive polymers
    • 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
    • H01M4/92—Metals of platinum group
    • H01M4/921—Alloys or mixtures with metallic elements

Definitions

  • the present invention generally relates to electrodes and methods of making the same. More particularly, the present invention relates to electrodes for use in applications such as metal-air batteries and fuel cells.
  • Future energy storage solutions face many challenges that include selecting composites and components that can provide the following properties: lightweight, long life, high energy, low cost, sustainable (low environmental impact), flexible to achieve multiple design geometries.
  • Lithium based metal-air batteries are a theoretically appealing solution due to their high practical operating voltage and fundamentally high capacity of the lithium metal electrode.
  • Fuel cells are also a desirable energy solution because of their characteristics as an environmentally friendly, long life, and reusable energy source.
  • improving the performance and durability of lithium air batteries and fuel cells remain significant challenges, as these products have not yet realized widespread commercial use.
  • Metal-air batteries are unusual because the only electroactive material contained within metal-air batteries is the anode, since the electroactive cathode material (0 2 ) is provided by ambient air. Even after accounting for the mass of the physical structure at the cathode, and the mass gained at the cathode during the oxygen reduction process, lithium air batteries compare favorably to existing battery technologies, offering the opportunity for significant improvements in energy density over current lithium-ion chemistries.
  • the present invention provides an electrode or composite electrode.
  • the present invention provides a metal-air battery or fuel cell comprising an electrode or composite electrode of the present invention.
  • the composite electrode comprises: a conductive substrate; a conductive polymer or conductive co-polymer disposed on the conductive substrate; and, optionally, a catalyst disposed on the conductive polymer or conductive co-polymer.
  • the electrode comprises: a conductive polymer or co-polymer; and, optionally, a catalyst disposed on the conductive polymer or co-polymer.
  • the conductive substrate can, for example, be selected from the group consisting of carbon-based material, metal-material (such as, for example, stainless steel or copper) or metalized conductive polymer or co-polymer.
  • the carbon-based material can be, for example, carbon black, glassy carbon, graphite, carbon fiber, reticulated vitreous carbon, carbon felt, carbon nanotubes, carbon paper, or a combination thereof.
  • the conductive polymer can, for example, be selected from the group consisting of polypyrrole, polyaniline, polyacetylene, polyparaphenylene, polyparaphenylene sulfide, polyparaphenylenevinylene, polythiophene, polyisothianapthene and combinations thereof.
  • the catalyst can, for example, be selected from the group consisting of silver, platinum, gold, copper, iron, metal oxides and combinations thereof.
  • the present invention provides a method of making an electrode or composite electrode.
  • the method for making a composite electrode comprises the steps of: a) providing a conductive substrate; b) electrochemically or chemically depositing a conductive polymer or co-polymer layer on the substrate; and c) optionally, electrochemically or chemically depositing a catalyst on the conductive polymer or co-polymer layer, thereby forming a composite electrode.
  • the method for making an electrode comprises the steps of: a) electrochemically or chemically depositing a conductive polymer or co-polymer layer on the substrate; and c) optionally,
  • the catalyst is disposed on only a portion of the conductive polymer or co-polymer.
  • Figure 1 A schematic showing that a composite electrode of the current disclosure can have a 2D or 3D geometry.
  • Figure 2 A depiction of a 2D deposition of three electrode set-up using Pt as common (c), Ag/Ag + as reference (r) and carbon as working (w).
  • Figures 3A-D Graphical depictions of the cyclic voltammetry (CV) (at cycle scans ranging from 1-20) deposition of Layer (1) - conductive polymer on a carbon (C) substrate.
  • CV cyclic voltammetry
  • FIG. 4 A graph of quartz crystal microbalance (QCM) data) for the deposition of Layer (1) - conductive polymer on a gold (Au) substrate. Scan rate (mV/s) correlates negatively with conductive polymer mass deposited.
  • Figure 5 A graph of deposition of Layer (1) - conductive polymer on a gold
  • Figure 6 A graph of deposition of Layer (1) - conductive polymer on a carbon substrate: potentiostatic; demonstrated consistent mass / charge relationship.
  • Figure 7 A graph of deposition of Layer (2) - silver (Ag) on a conductive polymer coated carbon substrate: linear sweep; achieved well controlled, variable Ag deposition using linear sweep method.
  • Figure 8 A graph of deposition of Layer (2) - silver deposited onto conductive polymer coated carbon substrates (P/C): potentiostatic; after deposition, Ag layer was dissolved and quantitatively analyzed; calculated mass (based on Faraday's law) and measured mass (based on ICP spectroscopy) showed excellent correspondence.
  • Figure 9 An example of Ag / P composite morphology shown using SEM images.
  • Figure 10 An example of Ag/P composite morphology shown using additional SEM images.
  • Figures 12 A and B An example of 0 2 reduction using composite electrodes of the present invention - (A) Ag vs. (B) C. Flat Ag showed significantly enhanced oxygen reduction activity over Flat Carbon electrode. Comparative analysis was based on maximum Coulomb flux for the reduction peak.
  • Figures 13 A and B Graphs of data for 0 2 reduction using examples of composite electrodes of the present invention -
  • A Ag vs.
  • B C.
  • the reduction was 3.3x higher for Ag vs. C based on peak current.
  • FIG. 14 A and B Graphs of data for 0 2 reduction using examples of composite electrodes of the present invention -
  • A Ag / P / C vs.
  • B Ag.
  • Ag / P / C composite shows 75% of activity of pure Ag, with a loading of only 1 mg/cm 2 .
  • FIG. 15 A and B Graphs of data for 0 2 reduction capability retention test using examples of composite electrodes of the present invention.
  • Ag / P / C composite electrode retains high oxygen reduction capability on multiple high rate (100 mV/s) cycling in air.
  • Figures 18 A and B Example of 3D composite electrode morphology at 1.6x
  • Figures 19 A- D Magnified images of example of 3D composite electrode morphology.
  • Figure 20 Example of Ag deposition onto GC electrode and PPy/GC electrode via constant potential method.
  • Figure 21 SEM image of steel substrate/conductive polymer/silver composite electrode.
  • FIG. 24 Silver quantification on example of silver-polypyrrole coated glassy carbon (Ag/PPy/GC) composite electrodes.
  • FIG. 26 Scanning electron micrograph showing example of silver- polypyrrole (Ag/PPy) composite morphology.
  • Figure 27 Example of oxygen reduction activity by electrode type for oxygen- saturated solution.
  • FIG. 28 Oxygen reduction activity as a function of oxygen concentration for example of silver-polypyrrole coated glassy carbon (Ag/PPy/GC) composite electrode.
  • Ag/PPy/GC silver-polypyrrole coated glassy carbon
  • Figure 29 Example of oxygen reduction activity by electrode type as a function of oxygen concentration.
  • FIG. 30 Multiple cycling of example of silver-polypyrrole coated glassy carbon (Ag/PPy/GC) composite electrode in air.
  • FIG. 31 Oxygen reduction activity of example of carbon (C) composite electrode in nitrogen, air, and oxygen.
  • FIG. 32 Oxygen reduction activity of example of gold (Au) composite electrode in nitrogen, air, and oxygen.
  • FIG. 34 Oxygen reduction activity of examples of carbon-conductive polymer (C-cp) and gold-conductive polymer (Au-cp) composite electrodes in nitrogen, air, and oxygen.
  • FIG. 34 Oxygen reduction activity of examples of carbon-conductive polymer-silver (C-cp-Ag) and gold-conductive polymer-silver (Au-cp-Ag) composite electrodes in nitrogen, air, and oxygen.
  • C-cp-Ag polymer-free carbon-silver
  • C-Ag polymer-free carbon-silver
  • FIG. 36 Oxygen reduction activity of example of carbon-conductive polymer-silver (C-cp-Ag) composite electrode before and after electrode durability test.
  • FIG 39 Oxygen reduction activity of examples of carbon-conductive polymer-silver (C-cp-Ag) composite electrodes in pure oxygen as a function of Ag loading, with representative Ag, C-cp and C electrode data shown as reference.
  • the present invention provides electrodes (e.g., a multi-component composite electrode) having a conductive polymer or co-polymer and a method of making such electrodes.
  • the present invention also provides devices (e.g., a metal-air battery or a fuel cell) comprising such electrodes.
  • the present invention is based on the surprising result that composite electrodes comprising a conductive polymer or co-polymer as described herein exhibit improved properties relative to electrodes that do not have such polymers.
  • the present invention provides a 3 -component composite electrode structure that can be used to prepare porous electrodes with high oxygen reduction activity.
  • the oxygen reduction catalyst is a key component for the effective functioning of devices such as, for example, metal-air batteries and fuel cell electrodes.
  • the processes disclosed herein allow the porosity of the electrode to be tuned. This tunability is desirable as different porosities may be needed, for example, for different battery and fuel cell applications. For example, high porosity promotes fast electron transfer (high current), while low porosity promotes higher energy density (high capacity).
  • Metal-air batteries In metal-air batteries, oxidation occurs at the anode according to M ⁇ M n+ + ne " .
  • Metal-air batteries have 0 2(g) as the active material so the chemistry and structure of the cathode (reduction) can be varied: 0 2 ⁇ (superoxide) 0 2 + e ⁇ ⁇ 0 2 2 ⁇ (peroxide)
  • Aprotic oxygen reduction is a 2-step process.
  • the 1 st step can be reversible, quasi-reversible, or irreversible depending on the electrolyte solvent, electrolyte salt, and nature of the electrode surface:
  • the 2 nd step is typically irreversible, due to reactivity of 0 2 2 ⁇ :
  • Catalysts promote 0 2 reduction in nonaqueous electrolyte. Mechanism of activity is not yet fully understood. Catalyst can impact voltage, current, and reversibility.
  • Li-air batteries have their own set of challenges which include, for example: better understanding of the 0 2 reduction mechanism and finding new strategies for improving 0 2 reduction capability.
  • the present invention provides improved cathode oxygen reduction activity and optimization of cathode porosity resulting in increased current capability and power output of, for example, lithium-air batteries.
  • the present invention can facilitate development of small, lightweight, long-life power sources for deployment in currently inaccessible locations.
  • the process used to make the electrodes of the present invention allows the electrode porosity to be tuned.
  • the composite electrode concept has the arrangement Ag/P/C.
  • the Ag coated electrodes have improved oxygen reduction activity versus uncoated C electrodes.
  • Ag/P/C electrodes have improved robustness versus Ag/C electrodes.
  • Ag coated electrodes are lower cost option relative to pure Ag electrodes.
  • the present invention provides electrodes (e.g., multi-component porous electrode) with high oxygen reduction activity which can be used in devices such as, for example, metal-air batteries and fuel cells.
  • the present invention provides a composite electrode comprising a substrate and a conductive polymer or co- polymer disposed on the substrate.
  • the present invention provides a composite electrode comprising a substrate, a conductive polymer or co-polymer disposed on the substrate, and a catalyst disposed on the conductive polymer or co-polymer.
  • the present invention provides a composite electrode comprising a conductive polymer or co-polymer.
  • the present invention provides an electrode comprising a conductive polymer or co-polymer, and a catalyst disposed on the conductive polymer or co-polymer. Examples of data obtained using examples of electrodes of the present invention can be found in Figures 12-16.
  • the conductive substrate provides structural support and conductivity to the cathode current collector (also referred to as an electrode). It is desirable that the substrate has sufficient conductivity such that it provides electrodes suitable for use in devices such as, for example, metal-air batteries and fuel cells.
  • suitable substrate materials include, but are not limited to, carbon-based materials such as, for example, carbon black, glassy carbon, graphite, carbon fiber, reticulated vitreous carbon (RVC) (a glass-like and rigid material), carbon felt (C-felt) (a cloth-like and pliable material), carbon nanotubes, carbon paper, and the like, and a metal-material (e.g., copper, stainless steel, and the like).
  • RVC reticulated vitreous carbon
  • C-felt carbon felt
  • carbon nanotubes carbon paper, and the like
  • metal-material e.g., copper, stainless steel, and the like
  • it is desirable that the carbon-based materials have high conductivity, high porosity (high void volume) and high surface area,
  • carbon shows adequate oxygen reduction activity in the absence of metal or metal oxide based catalysts, demonstrating reversible behavior in some electrolyte systems.
  • the conductive polymer or conductive co-polymer promotes contact (e.g., electrical contact) between the substrate and the catalyst. It is desirable to select a conductive polymer or co-polymer that exhibits high conductivity and high stability.
  • the conductivity of the polymer or co-polymer is at least 10 ⁇ 5 S/cm. In various embodiments, the conductivity of the polymer or co-polymer is 10 ⁇ 3 S/cm or 10 1 S/cm.
  • the polymer or co-polymer is stable, e.g., losing less than 10% of its mass after storage in aqueous or nonaqueous electrolyte for 5 days.
  • the polymer loses less than 5%, 4%, 3%, 2% or 1% of its mass after storage in aqueous or non-aqueous electrolyte for 5 days.
  • suitable conductive polymers include, but are not limited to, polyaniline, polyacetylene, polyparaphenylene, polyparaphenylene sulfide, polyparaphenylenevinylene, polythiophene, polyisothianapthene and the like.
  • Suitable conductive co-polymers include, but are not limited to, co-polymers comprising polyaniline, polyacetylene, polyparaphenylene, polyparaphenylene sulfide, polyparaphenylenevinylene, polythiophene, polyisothianapthene and the like.
  • polypyrrole is the conductive polymer.
  • the conductive polymer or co-polymer is disposed on the substrate.
  • the conductive polymer or co-polymer covers 100% of the substrate surface.
  • the conductive polymer or co-polymer covers at least 50%>, 60%>, 70%>, 80%), 90%), 95%o or 99% of the substrate surface, including all integer percentages between 50%) and 100%.
  • the conductive polymer or co-polymer can be deposited by any of the methods known in the art. In an embodiment, the conductive polymer or co-polymer is deposited by an electrochemical or chemical method.
  • the catalyst is any material that promotes the reduction of oxygen (0 2 ). It is desirable that the catalyst promote the reduction of oxygen in aqueous and non-aqueous environments (e.g., the supporting electrolyte in a metal-air battery or fuel cell).
  • An ionic liquid is an example of a non-aqueous environment.
  • suitable catalyst materials include, but are not limited to, metals and conductive metal oxides. Suitable metals include, for example, silver, platinum, gold, copper, iron, and the like. Suitable metal oxides include, for example, iron oxide, manganese oxide, copper oxide, ruthenium oxide, palladium oxide, iridium oxide, and the like.
  • the catalyst is silver (Ag) (e.g., polycrystalline silver).
  • Suitable metal oxide catalysts include, but are not limited to, Mn x O y , Cu x O y , Fe x O y , CoFe 2 C"4, Co x Oy, NiO, La 0 .8Sr 0 . 2 MnO3 and the like.
  • the catalyst when present, is disposed on the surface of the conductive polymer or co-polymer.
  • the amount of catalyst can be from 0.01 mg/cm 2 to 500 mg/cm 2 . In an embodiment, it is desirable that the amount of catalyst is at least 0.08 mg/cm 2 .
  • the present invention provides, in an embodiment, a composite electrode that addresses the challenges and shortcomings of current metal-air batteries utilizing an Ag / P /
  • a composite electrode can have a 2D or 3D geometry (see Figure
  • the present invention provides a 3D Ag/P/C composite structure where the composite electrode has the following components: (0) Carbon (C) - substrate - provides structure, conductivity porosity to cathode current collector; (1)
  • Conductive Polymer (P) - Layer #1 - promotes good physical and electrical contact between the silver and carbon; (2) Silver (Ag) - Layer #2 - provides conductivity and catalyzes the oxygen reduction reaction.
  • Polymer (P) - Layer #1 - promotes good physical and electrical contact between the silver and carbon.
  • Silver (Ag) - Layer #2 - provides conductivity and catalyzes the oxygen reduction reaction.
  • Component #1 (substrate) provides structural support and high conductivity.
  • Component #2 conductive polymer promotes good contact between Components #1 and #3 and efficient coating of Component #3 onto Component #1.
  • Component #3 acts to promote the oxygen reduction reaction.
  • Component #3 as part of electrode process will catalyze oxygen reduction, providing higher battery current than batteries based on catalyst-free electrodes. However, only small quantities of Component #3 can be used in this process. This is desirable, as Component #3 could be an expensive material, such as silver metal.
  • the present invention provides a method for making electrodes of the present invention.
  • a composite electrode according to the present invention is generally formed by providing a suitable substrate, depositing a conductive polymer or copolymer onto the substrate, and then depositing a catalyst onto the conductive polymer (or co-polymer)-coated- substrate. Examples of composite electrodes disclosed herein made using the methods of the present invention are shown in Figures 9-11.
  • the present invention provides a method of making a composite electrode comprising the steps of: a) providing a substrate; b) electrochemically or chemically depositing a conductive polymer or co-polymer layer on the substrate; c) electrochemically or chemically depositing a catalyst on the conductive polymer or copolymer layer thereby forming a composite electrode.
  • the present invention provides a method for making an electrode comprising the steps of: a) electrochemically or chemically depositing a conductive polymer or co-polymer; b) optionally, electrochemically or chemically depositing a catalyst on the conductive polymer or co-polymer thereby forming an electrode.
  • the conductive polymer or co-polymer can deposited on the substrate (e.g., using electrochemical or chemical methods) or adhered onto the substrate using physical compression.
  • the conductive polymer or co-polymer can be electrochemically deposited on the substrate by a constant potential method (see Figure 17), a constant current method, and using cyclic voltammetry (see Figures 3A-D, 4, 11 and 25), where an
  • the conductive polymer or co-polymer can be chemically deposited on the substrate, where a second chemical, an oxidizing agent, is added to the monomer-containing solution and initiates deposition on the substrate surface.
  • the thickness of the polymer layer can be from 0.1 ⁇ to 10 mm, including all values to the 0.1 ⁇ and ranges therebetween.
  • the conductive polymer or co-polymer has a thickness of at least 0.5 ⁇ . It is desirable that the polymer layer provides a conformal coating of the substrate.
  • An advantage of the method of the present invention is the ability to control the porosity and surface area of the composite electrode.
  • the porosity can be controlled by, for example, the choice of substrate, thickness of the polymer layer, morphology of the catalyst, or composition of the polymer. For example, a RVC substrate with 10 pores per inch would result in a surface area of -150 ft 2 /ft 3 , while a RVC substrate with 80 pores per linear inch would result in a surface area of -1500 ft 2 /ft 3 .
  • the catalyst is, optionally, electrochemically or chemically deposited onto the conductive polymer (or co-polymer)-coated-substrate or the catalyst could be adhered onto the substrate and conductive polymer or co-polymer using physical compression.
  • the catalyst can be electrochemically deposited potentiostatically and using linear sweep voltammetry (see Figures 7-8).
  • the catalyst can be chemically deposited, where the catalyst forms as a precipitate in the presence of the conductive polymer or co-polymer coated substrate. In an embodiment, it is desirable that the catalyst has a loading of at least 0.08 mg/cm 2 .
  • the present invention provides an electrode comprising a conductive polymer or conductive co-polymer disclosed herein.
  • the electrode can, optionally, further comprise a catalyst disclosed herein.
  • the electrode consists essentially of the conductive polymer or co-polymer.
  • the present invention provides devices comprising electrodes of the present invention.
  • devices such as metal-air batteries and fuel cells can comprise the electrodes.
  • Metal-air batteries and fuel cell devices are well-known in the art.
  • a metal-air battery typically has an air electrode (cathode) which is the site of the reduction reaction, an electropositive metal based anode which is the site of the oxidation reaction, an ion conducting/isolating separator material, and a supporting electrolyte.
  • the present invention provides a primary (single-use) metal-air battery comprising a electrode (e.g., a composite electrode) of the present invention, a metal based electrode, an ion conducting/isolating separator material and a supporting electrolyte.
  • the electrode of the present invention functions as an air electrode or cathode, which takes oxygen and reduces it to form superoxide, peroxide, or oxide as products.
  • the metal anode can be any combination of the present invention.
  • electropositive metal such as lithium, zinc, sodium, etc.
  • the present invention provides a secondary
  • an (rechargeable) metal-air battery comprising an electrode (e.g., a composite electrode) of the present invention, a metal based electrode, an ion conducting/isolating separator material and a supporting electrolyte.
  • an electrode of the present invention functions as the reduction site during the discharge process of the battery, and functions as the oxidation site during the charge process of the battery.
  • a fuel cell is an electrochemical device designed for continuous replenishment of the reactants consumed and continuous removal of the products.
  • Fuel cell components and architectures are well-known in the art.
  • a fuel cell comprises an anode (site for hydrogen oxidation), a cathode (site for oxygen reduction), an electrolyte (which can be ion conducting but not electron conducting).
  • the present invention provides a fuel cell comprising an electrode (e.g., a composite electrode) of the present invention.
  • an electrode of the present invention functions as the cathode, while a second electrode containing, for example, platinum or another suitable catalyst functions as the anode.
  • EXAMPLE 1 EXAMPLE 1
  • This example demonstrates electrochemical reduction of 0 2 at a silver- polymer-carbon electrode in a nonaqueous cell.
  • the preparation, characterization, and electrochemical activity of a novel composite electrode containing silver on a polypyrrole (PPy)-coated carbon substrate are described here.
  • An enhanced oxygen reduction activity for the composite electrode is observed relative to uncoated glassy carbon (GC) or silver disk electrodes.
  • GC uncoated glassy carbon
  • the improvement of the cathode oxygen reduction activity increases the current capability and power output of the air electrode, facilitating future development of small, lightweight, long-life power sources.
  • Oxygen reduction was measured using an electrolyte of 0.1 M
  • TAPF 6 tetrabutylammonium hexafluorophosphate
  • CH3CN acetonitrile
  • Nitrogen:oxygen gas ratios were controlled using Matheson Trigas flowmeters, where CH 3 CN-saturated gas at the desired ratio was bubbled through the electrolyte solution before measurement, and flowed over the electrolyte solution during measurement.
  • Oxygen concentration was calculated using Henry's law assuming an 8.1 mM oxygen solubility at standard pressure and was adjusted using local atmospheric pressures obtained from the National Oceanic and Atmospheric Administration's National Data Buoy Center. Oxygen reduction data were collected using linear sweep voltammetry at 0.1 V/s.
  • a composite electrode was developed where PPy and then silver were deposited on a GC substrate.
  • GC was selected as it is a readily available electrode material suitable for use in electrochemical investigations.
  • Carbon is relevant to battery applications as a low cost, environmentally sustainable, lightweight, and low resistivity (10 5 - 10 2 ⁇ cm) material, with a wealth of carbon substrates of differing geometry and a porosity available for future composite electrode studies.
  • PPy was selected to promote good physical and electrical contact between the carbon substrate and the silver deposit.
  • the oxygen reduction activity of the Ag, GC, and Ag/PPy/GC electrode types was assessed at different oxygen concentrations, ranging from 1 to 9 mM.
  • An example is shown for one Ag/PPy/GC composite electrode, with a low silver loading of ⁇ 0.3 mg/cm 2 (Figure 28).
  • the peak coulomb flux of oxygen reduction increased linearly with oxygen concentration for all three electrode types, consistent with a first-order mechanism for oxygen reduction ( Figure 29).
  • the Ag/PPy/GC composite electrode showed the highest oxygen reduction activity at all concentrations.
  • This example describes development of novel current cc-cp-Ag composite electrodes for nonaqueous metal-air batteries.
  • the contribution of each individual component toward the activity of the composite electrode is assessed.
  • the role of the chemical identity of the current collector (cc) substrate is investigated by examining the behavior of carbon versus gold (C versus Au) toward the electrochemical reduction of oxygen, in conjunction with a conductive polymer (cp) deposition study.
  • the structural role of the cp in improving the physical strength of the composite electrode is assessed.
  • a systematic study of the Ag loading effect is undertaken to determine the minimum silver loading required for a significant enhancement in the oxygen reduction activity of the cccp-Ag composite. Improvement of oxygen reduction activity at the air electrode will increase the current capability and the power output of the metal-air battery, facilitating future development of small, lightweight, and long-life power sources.
  • Oxygen reduction was measured using an electrolyte of 0.1 M
  • tetrabutylammonium hexafluorophosphate (TBAPF 6 ) in acetonitrile (CH 3 CN), at a scan rate of 100 mV 1 .
  • Acetonitrile was selected as it is an aprotic protophobic solvent with the same solvent classification as conventional lithium battery electrolytes, which has also been used in other published studies relevant to lithium metal-air batteries.
  • the oxygen reduction analysis was based on peak coulomb flux, based on the planar geometric area of the substrate of each electrode. Nitrogen:oxygen gas ratios were controlled using Matheson Trigas (Pennsylvania, USA) flowmeters.
  • Oxygen concentration was calculated using Henry's law assuming an 8.1 mM oxygen solubility at the standard pressure and adjusted using local atmospheric pressures obtained from the National Oceanic and Atmospheric Administration's National Data Buoy Center.
  • coated electrodes were horizontally dragged a distance of 5.6 cm over a Buehler polishing microcloth, while exerting a 4 N downward force.
  • the peak reduction potentials were significantly different for the two electrode types, -1.3 V for C and -1.5 V for Au.
  • the C and Au disk electrodes were each coated with a thin polypyrrole cp layer using a previously described method.
  • the carbonconductive polymer (C-cp) and gold- conductive polymer (Au-cp) electrodes were subjected to the same oxygen reduction tests.
  • both types of polymer coated electrodes showed quasireversible oxygen reduction behavior at the same potentials and similar magnitude, regardless of the substrate used ( Figure 33).
  • the C-cp and Au-cp polymer coated electrodes were each coated with Ag, at similar loadings of 0.5 and 0.4 mg cm 2 , respectively.
  • the oxygen reduction activities of the C-cp-Ag and Au-cp-Ag composite electrodes were measured (Figure 34). As with the silver-free polymer coated electrodes, both types of silver-polymer coated electrodes showed quasireversible oxygen reduction behavior at the same potentials and similar magnitude, regardless of the substrate used. This indicates that the oxygen reduction activity of the composite electrodes is dictated by the conductive polymer and not by the underlying substrate, opening the possibility for use for diverse types of current collector substrates for silver-polymer composite electrodes in the future. Also notably, the oxygen reduction activity of the C-cp-Ag composite electrode was comparable to that of the gold disk (Au) electrode, at a low Ag loading of 0.5 mg cm 2 . Our composite electrode concept shows a new pathway for the development of low cost, high functioning electrodes for metal-air batteries.
  • Electrode durability test Polypyrrole was selected as a cp for the composite electrode to promote good physical and electrical contact between the current collector and silver deposit.
  • An electrode durability test was designed to assess the structural role of the polymer in improving the physical strength of the composite electrode.
  • Carbon-silver (C-Ag) composite electrodes were prepared with and without cp coatings, and both types of electrodes were subjected to an aggressive physical abuse test. Optical images were acquired before and after the test ( Figure 35). The C-Ag electrode showed virtually complete Ag loss, while the C-cp-Ag composite showed good Ag retention. To quantify the activity retained, three C-cp-Ag composite electrodes were prepared, and the oxygen reduction activity of each was assessed before and after durability testing.
  • the oxygen reduction activity of the C-cp-Ag composite electrodes was measured in nonaqueous electrolyte. Comparative data was collected with uncoated C, an uncoated solid silver disk (Ag), and silver-free conductive polymer coated glassy carbon (C- cp) electrodes. Because oxygen partial pressure can significantly affect the performance of the air electrode, measurements were made under both air ( Figure 38) and pure oxygen ( Figure 39). The relative activities of the various electrodes were consistent in both gases. The measured activity in air was 20-40% of the measured activity in pure oxygen. The C-cp electrodes and uncoated C electrodes showed very similar activity, consistent with our previous results.
  • the Ag disk electrodes showed 3 X higher activity in air (Figure 38) and 2 X higher activity in oxygen (Figure 39) relative to the C-cp and C electrodes.
  • the activity of the C-cp-Ag composite electrodes with high silver loading was typically slightly higher than that of a solid Ag disk electrode.
  • 0.08 ⁇ is the minimum Ag thickness required to maximize oxygen reduction activity.
  • the Ag content of a composite electrode is measured by inductively coupled plasma (ICP) mass spectrometry for three Ag/P/C composite electrodes.
  • ICP inductively coupled plasma

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Abstract

An electrode or composite electrode comprising a conductive polymer or co-polymer for use in devices such as, for example, metal-air batteries or fuel cells. Also, methods of making such electrodes or composite electrodes. The electrode or composite electrode can, optionally, further comprise a catalyst. For example, electrochemically deposited polypyrrole can be used as the polymer and electrochemically deposited silver can be used as the catalyst.

Description

ELECTRODES FOR METAL-AIR BATTERIES AND FUEL CELLS
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. provisional patent application no.
61/300,719 filed February 2, 2010, the disclosures of which is incorporated herein by reference.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with government support under contract no.
1R01HL093044-01 Al awarded by Air Force Office of Scientific Research. The government has certain rights in the invention.
FIELD OF THE INVENTION
[0003] The present invention generally relates to electrodes and methods of making the same. More particularly, the present invention relates to electrodes for use in applications such as metal-air batteries and fuel cells.
BACKGROUND OF THE INVENTION
[0004] Future energy storage solutions face many challenges that include selecting composites and components that can provide the following properties: lightweight, long life, high energy, low cost, sustainable (low environmental impact), flexible to achieve multiple design geometries. Lithium based metal-air batteries are a theoretically appealing solution due to their high practical operating voltage and fundamentally high capacity of the lithium metal electrode. Fuel cells are also a desirable energy solution because of their characteristics as an environmentally friendly, long life, and reusable energy source. However, improving the performance and durability of lithium air batteries and fuel cells remain significant challenges, as these products have not yet realized widespread commercial use.
[0005] Metal-air batteries are unusual because the only electroactive material contained within metal-air batteries is the anode, since the electroactive cathode material (02) is provided by ambient air. Even after accounting for the mass of the physical structure at the cathode, and the mass gained at the cathode during the oxygen reduction process, lithium air batteries compare favorably to existing battery technologies, offering the opportunity for significant improvements in energy density over current lithium-ion chemistries.
[0006] Both metal-air batteries and fuel cells use oxygen from the air to generate electricity. One of the major limitations to current lithium air battery and fuel cell technologies is the oxygen reduction reaction, which is a critical component to the functioning of both systems. In particular, slow oxygen reduction kinetics at the air electrode has been a significant challenge to the development of practical metal-air batteries. The slow kinetics result in high cathode polarization, causing substantial voltage drop on load. To address this issue, the search for new oxygen reduction catalysts has been an area of research interest, in conjunction with investigations of the mechanism of catalyst activity. The primary focus to date has been metal oxide catalysts, with some studies involving metallic gold, platinum, and bimetallic gold-platinum catalysts.
[0007] Based on the foregoing, there exists an ongoing and unmet need for an electrode that will improve the oxygen reduction capabilities of metal- air batteries and fuel cells, and address other limitations of currently available technologies.
BRIEF SUMMARY OF THE INVENTION
[0008] In an aspect, the present invention provides an electrode or composite electrode. In another aspect, the present invention provides a metal-air battery or fuel cell comprising an electrode or composite electrode of the present invention.
[0009] In an embodiment, the composite electrode comprises: a conductive substrate; a conductive polymer or conductive co-polymer disposed on the conductive substrate; and, optionally, a catalyst disposed on the conductive polymer or conductive co-polymer. In another embodiment, the electrode comprises: a conductive polymer or co-polymer; and, optionally, a catalyst disposed on the conductive polymer or co-polymer.
[0010] The conductive substrate can, for example, be selected from the group consisting of carbon-based material, metal-material (such as, for example, stainless steel or copper) or metalized conductive polymer or co-polymer. The carbon-based material can be, for example, carbon black, glassy carbon, graphite, carbon fiber, reticulated vitreous carbon, carbon felt, carbon nanotubes, carbon paper, or a combination thereof. The conductive polymer can, for example, be selected from the group consisting of polypyrrole, polyaniline, polyacetylene, polyparaphenylene, polyparaphenylene sulfide, polyparaphenylenevinylene, polythiophene, polyisothianapthene and combinations thereof. The catalyst can, for example, be selected from the group consisting of silver, platinum, gold, copper, iron, metal oxides and combinations thereof.
[0011] In another aspect, the present invention provides a method of making an electrode or composite electrode. In an embodiment, the method for making a composite electrode comprises the steps of: a) providing a conductive substrate; b) electrochemically or chemically depositing a conductive polymer or co-polymer layer on the substrate; and c) optionally, electrochemically or chemically depositing a catalyst on the conductive polymer or co-polymer layer, thereby forming a composite electrode. In an embodiment, the method for making an electrode comprises the steps of: a) electrochemically or chemically depositing a conductive polymer or co-polymer layer on the substrate; and c) optionally,
electrochemically or chemically depositing a catalyst on the conductive polymer or copolymer layer, thereby forming an electrode. In an embodiment, the catalyst is disposed on only a portion of the conductive polymer or co-polymer.
BRIEF DESCRIPTION OF THE FIGURES
[0012] Figure 1. A schematic showing that a composite electrode of the current disclosure can have a 2D or 3D geometry.
[0013] Figure 2. A depiction of a 2D deposition of three electrode set-up using Pt as common (c), Ag/Ag+ as reference (r) and carbon as working (w).
[0014] Figures 3A-D. Graphical depictions of the cyclic voltammetry (CV) (at cycle scans ranging from 1-20) deposition of Layer (1) - conductive polymer on a carbon (C) substrate.
[0015] Figure 4. A graph of quartz crystal microbalance (QCM) data) for the deposition of Layer (1) - conductive polymer on a gold (Au) substrate. Scan rate (mV/s) correlates negatively with conductive polymer mass deposited.
[0016] Figure 5. A graph of deposition of Layer (1) - conductive polymer on a gold
(Au) substrate: potentiostatic w/ QCM; demonstrated reproducible mass / charge relationship for potentiostatic deposition.
[0017] Figure 6. A graph of deposition of Layer (1) - conductive polymer on a carbon substrate: potentiostatic; demonstrated consistent mass / charge relationship.
[0018] Figure 7. A graph of deposition of Layer (2) - silver (Ag) on a conductive polymer coated carbon substrate: linear sweep; achieved well controlled, variable Ag deposition using linear sweep method.
[0019] Figure 8. A graph of deposition of Layer (2) - silver deposited onto conductive polymer coated carbon substrates (P/C): potentiostatic; after deposition, Ag layer was dissolved and quantitatively analyzed; calculated mass (based on Faraday's law) and measured mass (based on ICP spectroscopy) showed excellent correspondence.
[0020] Figure 9. An example of Ag / P composite morphology shown using SEM images. [0021] Figure 10. An example of Ag/P composite morphology shown using additional SEM images.
[0022] Figure 11. An example of Ag/P composite morphology shown using additional SEM images.
[0023] Figures 12 A and B. An example of 02 reduction using composite electrodes of the present invention - (A) Ag vs. (B) C. Flat Ag showed significantly enhanced oxygen reduction activity over Flat Carbon electrode. Comparative analysis was based on maximum Coulomb flux for the reduction peak.
[0024] Figures 13 A and B. Graphs of data for 02 reduction using examples of composite electrodes of the present invention - (A) Ag vs. (B) C. The reduction was 3.3x higher for Ag vs. C based on peak current.
[0025] Figures 14 A and B. Graphs of data for 02 reduction using examples of composite electrodes of the present invention - (A) Ag / P / C vs. (B) Ag. Ag / P / C composite shows 75% of activity of pure Ag, with a loading of only 1 mg/cm2.
[0026] Figures 15 A and B. Graphs of data for 02 reduction capability retention test using examples of composite electrodes of the present invention. Ag / P / C composite electrode retains high oxygen reduction capability on multiple high rate (100 mV/s) cycling in air.
[0027] Figures 16 A and B. Ag loading effect on 02 reduction in (A) air and (B) pure
02 using examples of composite Ag / P / C electrodes of the present invention, with comparative flat Ag, P/C, and C data shown as reference.
[0028] Figure 17. Graph of data for P deposition on 2D (glassy carbon) and 3D
(RVC, carbon felt) C substrates. Method demonstrated consistent charge / mass relationship for potentiostatic PPy deposition on 2D and 3D C electrodes.
[0029] Figures 18 A and B. Example of 3D composite electrode morphology at 1.6x
(26A) and at higher magnification (26B).
[0030] Figures 19 A- D. Magnified images of example of 3D composite electrode morphology.
[0031] Figure 20. Example of Ag deposition onto GC electrode and PPy/GC electrode via constant potential method.
[0032] Figure 21. SEM image of steel substrate/conductive polymer/silver composite electrode.
[0033] Figure 22. Cyclic voltammogram for example of deposition of polypyrrole
(PPy) on glassy carbon (GC) electrode. [0034] Figure 23. Linear sweep voltammogram for example of deposition of silver
(Ag) on polypyrrole coated glassy carbon (PPy/GC) electrode.
[0035] Figure 24. Silver quantification on example of silver-polypyrrole coated glassy carbon (Ag/PPy/GC) composite electrodes.
[0036] Figure 25. AC impedance of example of silver-polypyrrole-glassy carbon
(Ag/PPy/GC) composite electrode.
[0037] Figure 26. Scanning electron micrograph showing example of silver- polypyrrole (Ag/PPy) composite morphology.
[0038] Figure 27. Example of oxygen reduction activity by electrode type for oxygen- saturated solution.
[0039] Figure 28. Oxygen reduction activity as a function of oxygen concentration for example of silver-polypyrrole coated glassy carbon (Ag/PPy/GC) composite electrode.
[0040] Figure 29. Example of oxygen reduction activity by electrode type as a function of oxygen concentration.
[0041] Figure 30. Multiple cycling of example of silver-polypyrrole coated glassy carbon (Ag/PPy/GC) composite electrode in air.
[0042] Figure 31. Oxygen reduction activity of example of carbon (C) composite electrode in nitrogen, air, and oxygen.
[0043] Figure 32. Oxygen reduction activity of example of gold (Au) composite electrode in nitrogen, air, and oxygen.
[0044] Figure 33. Oxygen reduction activity of examples of carbon-conductive polymer (C-cp) and gold-conductive polymer (Au-cp) composite electrodes in nitrogen, air, and oxygen.
[0045] Figure 34. Oxygen reduction activity of examples of carbon-conductive polymer-silver (C-cp-Ag) and gold-conductive polymer-silver (Au-cp-Ag) composite electrodes in nitrogen, air, and oxygen.
[0046] Figure 35. Optical images of examples of carbon-conductive polymer-silver
(C-cp-Ag) and polymer-free carbon-silver (C-Ag) composite electrodes before and after electrode durability tests.
[0047] Figure 36. Oxygen reduction activity of example of carbon-conductive polymer-silver (C-cp-Ag) composite electrode before and after electrode durability test.
[0048] Figure 37. Ag loading of example of carbon-conductive polymer- silver (C-cp-
Ag) electrodes as a function of scan rate. [0049] Figure 38. Oxygen reduction activity of examples of carbon-conductive polymer-silver (C-cp-Ag) composite electrodes in air as a function of Ag loading, with representative Ag, C-cp and C electrode data shown as reference.
[0050] Figure 39. Oxygen reduction activity of examples of carbon-conductive polymer-silver (C-cp-Ag) composite electrodes in pure oxygen as a function of Ag loading, with representative Ag, C-cp and C electrode data shown as reference.
DETAILED DESCRIPTION OF THE INVENTION
[0051] The present invention provides electrodes (e.g., a multi-component composite electrode) having a conductive polymer or co-polymer and a method of making such electrodes. The present invention also provides devices (e.g., a metal-air battery or a fuel cell) comprising such electrodes. The present invention is based on the surprising result that composite electrodes comprising a conductive polymer or co-polymer as described herein exhibit improved properties relative to electrodes that do not have such polymers.
[0052] The present invention, in an embodiment, provides a 3 -component composite electrode structure that can be used to prepare porous electrodes with high oxygen reduction activity. The oxygen reduction catalyst is a key component for the effective functioning of devices such as, for example, metal-air batteries and fuel cell electrodes. The processes disclosed herein allow the porosity of the electrode to be tuned. This tunability is desirable as different porosities may be needed, for example, for different battery and fuel cell applications. For example, high porosity promotes fast electron transfer (high current), while low porosity promotes higher energy density (high capacity).
[0053] In a conventional electrode battery the specific energy is dictated by the mass of the electrode. Li / 02 has the highest theoretical specific capacity and specific energy. See Table 1 below.
Table 1. Theoretical specific capacity and specific energy for battery materials.
Figure imgf000007_0001
[0054] In metal-air batteries, oxidation occurs at the anode according to M→ Mn+ + ne". Metal-air batteries have 02(g) as the active material so the chemistry and structure of the cathode (reduction) can be varied: 02 ~ (superoxide) 02 + e~→ 02 2~ (peroxide)
Figure imgf000008_0001
02~ (oxide)
[0055] 02 reduction in nonaqueous media. Aprotic oxygen reduction is a 2-step process. The 1st step can be reversible, quasi-reversible, or irreversible depending on the electrolyte solvent, electrolyte salt, and nature of the electrode surface:
02 + e ^O2 "
The 2nd step is typically irreversible, due to reactivity of 02 2~:
02 " + e"→ 02 2"
[0056] Role of the Catalyst. Catalysts promote 02 reduction in nonaqueous electrolyte. Mechanism of activity is not yet fully understood. Catalyst can impact voltage, current, and reversibility.
[0057] Li-air batteries have their own set of challenges which include, for example: better understanding of the 02 reduction mechanism and finding new strategies for improving 02 reduction capability.
[0058] The present invention provides improved cathode oxygen reduction activity and optimization of cathode porosity resulting in increased current capability and power output of, for example, lithium-air batteries. Thus, the present invention can facilitate development of small, lightweight, long-life power sources for deployment in currently inaccessible locations.
[0059] The process used to make the electrodes of the present invention allows the electrode porosity to be tuned. In an embodiment, the composite electrode concept has the arrangement Ag/P/C. The Ag coated electrodes have improved oxygen reduction activity versus uncoated C electrodes. Also, Ag/P/C electrodes have improved robustness versus Ag/C electrodes. Ag coated electrodes are lower cost option relative to pure Ag electrodes.
[0060] In an aspect, the present invention provides electrodes (e.g., multi-component porous electrode) with high oxygen reduction activity which can be used in devices such as, for example, metal-air batteries and fuel cells. In an embodiment, the present invention provides a composite electrode comprising a substrate and a conductive polymer or co- polymer disposed on the substrate. In another embodiment, the present invention provides a composite electrode comprising a substrate, a conductive polymer or co-polymer disposed on the substrate, and a catalyst disposed on the conductive polymer or co-polymer. In yet another embodiment, the present invention provides a composite electrode comprising a conductive polymer or co-polymer. In yet another embodiment, the present invention provides an electrode comprising a conductive polymer or co-polymer, and a catalyst disposed on the conductive polymer or co-polymer. Examples of data obtained using examples of electrodes of the present invention can be found in Figures 12-16.
[0061] The conductive substrate provides structural support and conductivity to the cathode current collector (also referred to as an electrode). It is desirable that the substrate has sufficient conductivity such that it provides electrodes suitable for use in devices such as, for example, metal-air batteries and fuel cells. Examples of suitable substrate materials include, but are not limited to, carbon-based materials such as, for example, carbon black, glassy carbon, graphite, carbon fiber, reticulated vitreous carbon (RVC) (a glass-like and rigid material), carbon felt (C-felt) (a cloth-like and pliable material), carbon nanotubes, carbon paper, and the like, and a metal-material (e.g., copper, stainless steel, and the like). In some cases, it is desirable that the carbon-based materials have high conductivity, high porosity (high void volume) and high surface area, for example, when used as the substrate layer in a 3D electrode. Carbon is a desirable substrate material because it is low in cost,
environmentally sustainable, lightweight (d = 1.3 - 3.5 g/cm3-*, has a low electrical resistivity (10~5 - 10~2 Ω-cm), is active for aqueous and nonaqueous oxygen reduction, and is a suitable substrate for aqueous oxygen reduction. Carbon is also a useful substrate material because of the availability of a variety of carbon substrates of differing geometry and porosity.
Additionally, carbon shows adequate oxygen reduction activity in the absence of metal or metal oxide based catalysts, demonstrating reversible behavior in some electrolyte systems.
[0062] The conductive polymer or conductive co-polymer promotes contact (e.g., electrical contact) between the substrate and the catalyst. It is desirable to select a conductive polymer or co-polymer that exhibits high conductivity and high stability. The conductivity of the polymer or co-polymer is at least 10~5 S/cm. In various embodiments, the conductivity of the polymer or co-polymer is 10~3 S/cm or 10 1 S/cm. The polymer or co-polymer is stable, e.g., losing less than 10% of its mass after storage in aqueous or nonaqueous electrolyte for 5 days. In various embodiments, the polymer loses less than 5%, 4%, 3%, 2% or 1% of its mass after storage in aqueous or non-aqueous electrolyte for 5 days. Examples of suitable conductive polymers include, but are not limited to, polyaniline, polyacetylene, polyparaphenylene, polyparaphenylene sulfide, polyparaphenylenevinylene, polythiophene, polyisothianapthene and the like. Examples of suitable conductive co-polymers include, but are not limited to, co-polymers comprising polyaniline, polyacetylene, polyparaphenylene, polyparaphenylene sulfide, polyparaphenylenevinylene, polythiophene, polyisothianapthene and the like.
[0063] In an embodiment, polypyrrole (PPy) is the conductive polymer. PPy is desirable because it is lightweight (d = 1.5 g/cm3), has low electrical resistivity (10~4- 10° Ω-cm), is functional for oxygen reduction, is stable, and is electroactive in aqueous and nonaqueous solvents.
[0064] The conductive polymer or co-polymer is disposed on the substrate. In an embodiment, the conductive polymer or co-polymer covers 100% of the substrate surface. In various embodiments, the conductive polymer or co-polymer covers at least 50%>, 60%>, 70%>, 80%), 90%), 95%o or 99% of the substrate surface, including all integer percentages between 50%) and 100%. The conductive polymer or co-polymer can be deposited by any of the methods known in the art. In an embodiment, the conductive polymer or co-polymer is deposited by an electrochemical or chemical method.
[0065] The catalyst is any material that promotes the reduction of oxygen (02). It is desirable that the catalyst promote the reduction of oxygen in aqueous and non-aqueous environments (e.g., the supporting electrolyte in a metal-air battery or fuel cell). An ionic liquid is an example of a non-aqueous environment. Examples of suitable catalyst materials include, but are not limited to, metals and conductive metal oxides. Suitable metals include, for example, silver, platinum, gold, copper, iron, and the like. Suitable metal oxides include, for example, iron oxide, manganese oxide, copper oxide, ruthenium oxide, palladium oxide, iridium oxide, and the like. In an embodiment, the catalyst is silver (Ag) (e.g., polycrystalline silver). Suitable metal oxide catalysts include, but are not limited to, MnxOy, CuxOy, FexOy, CoFe2C"4, CoxOy, NiO, La0.8Sr0.2MnO3 and the like.
[0066] The catalyst, when present, is disposed on the surface of the conductive polymer or co-polymer. The amount of catalyst can be from 0.01 mg/cm2 to 500 mg/cm2. In an embodiment, it is desirable that the amount of catalyst is at least 0.08 mg/cm2.
[0067] The present invention provides, in an embodiment, a composite electrode that addresses the challenges and shortcomings of current metal-air batteries utilizing an Ag / P /
C composite. For example, a composite electrode can have a 2D or 3D geometry (see Figure
1). [0068] In an embodiment, the present invention provides a 3D Ag/P/C composite structure where the composite electrode has the following components: (0) Carbon (C) - substrate - provides structure, conductivity porosity to cathode current collector; (1)
Conductive Polymer (P) - Layer #1 - promotes good physical and electrical contact between the silver and carbon; (2) Silver (Ag) - Layer #2 - provides conductivity and catalyzes the oxygen reduction reaction. (1) Polymer (P) - Layer #1 - promotes good physical and electrical contact between the silver and carbon. (2) Silver (Ag) - Layer #2 - provides conductivity and catalyzes the oxygen reduction reaction. In this embodiment, Component #1 (substrate) provides structural support and high conductivity. Characteristics of layer (0): C = carbon substrate include: low cost, environmentally sustainable, lightweight (d = 1.3 - 3.5 g/cm3), low electrical resistivity 10~5 - 10~2 Ω-cm, active for aqueous and nonaqueous oxygen reduction, and suitable substrate for aqueous oxygen reduction. Component #2 (conductive polymer) promotes good contact between Components #1 and #3 and efficient coating of Component #3 onto Component #1. Characteristics of Layer (1) (Component # 2): P = conductive polymer include: polypyrrole (PPy) is made via oxidizing (anodic) process, lightweight d = 1.5 g/cm3, low electrical resistivity 10~4- 10° Ω·αη, functional for oxygen reduction in aqueous solvent, stable, and electroactive in nonaqueous solvent.
[0069] Component #3 (catalyst) acts to promote the oxygen reduction reaction. Use of
Component #3 as part of electrode process will catalyze oxygen reduction, providing higher battery current than batteries based on catalyst-free electrodes. However, only small quantities of Component #3 can be used in this process. This is desirable, as Component #3 could be an expensive material, such as silver metal. Characteristics of Layer (2) (Component # 3) include: Ag = Active catalyst (e.g., polycrystalline silver and supported silver).
[0070] In another aspect, the present invention provides a method for making electrodes of the present invention. A composite electrode according to the present invention is generally formed by providing a suitable substrate, depositing a conductive polymer or copolymer onto the substrate, and then depositing a catalyst onto the conductive polymer (or co-polymer)-coated- substrate. Examples of composite electrodes disclosed herein made using the methods of the present invention are shown in Figures 9-11.
[0071] In an embodiment, the present invention provides a method of making a composite electrode comprising the steps of: a) providing a substrate; b) electrochemically or chemically depositing a conductive polymer or co-polymer layer on the substrate; c) electrochemically or chemically depositing a catalyst on the conductive polymer or copolymer layer thereby forming a composite electrode. In another embodiment, the present invention provides a method for making an electrode comprising the steps of: a) electrochemically or chemically depositing a conductive polymer or co-polymer; b) optionally, electrochemically or chemically depositing a catalyst on the conductive polymer or co-polymer thereby forming an electrode.
[0072] The conductive polymer or co-polymer can deposited on the substrate (e.g., using electrochemical or chemical methods) or adhered onto the substrate using physical compression. For example, the conductive polymer or co-polymer can be electrochemically deposited on the substrate by a constant potential method (see Figure 17), a constant current method, and using cyclic voltammetry (see Figures 3A-D, 4, 11 and 25), where an
appropriately oxidizing potential is applied to the monomer-containing solution. As another example, the conductive polymer or co-polymer can be chemically deposited on the substrate, where a second chemical, an oxidizing agent, is added to the monomer-containing solution and initiates deposition on the substrate surface. The thickness of the polymer layer can be from 0.1 μιη to 10 mm, including all values to the 0.1 μιη and ranges therebetween. In an embodiment, it is desirable the conductive polymer or co-polymer has a thickness of at least 0.5 μιη. It is desirable that the polymer layer provides a conformal coating of the substrate.
[0073] An advantage of the method of the present invention is the ability to control the porosity and surface area of the composite electrode. The porosity can be controlled by, for example, the choice of substrate, thickness of the polymer layer, morphology of the catalyst, or composition of the polymer. For example, a RVC substrate with 10 pores per inch would result in a surface area of -150 ft2/ft3, while a RVC substrate with 80 pores per linear inch would result in a surface area of -1500 ft2/ft3.
[0074] After depositing the conductive polymer or co-polymer onto the substrate, the catalyst is, optionally, electrochemically or chemically deposited onto the conductive polymer (or co-polymer)-coated-substrate or the catalyst could be adhered onto the substrate and conductive polymer or co-polymer using physical compression. For example, the catalyst can be electrochemically deposited potentiostatically and using linear sweep voltammetry (see Figures 7-8). As another example, the catalyst can be chemically deposited, where the catalyst forms as a precipitate in the presence of the conductive polymer or co-polymer coated substrate. In an embodiment, it is desirable that the catalyst has a loading of at least 0.08 mg/cm2.
[0075] In an embodiment, the present invention provides an electrode comprising a conductive polymer or conductive co-polymer disclosed herein. In another embodiment, the electrode can, optionally, further comprise a catalyst disclosed herein. In yet another embodiment, the electrode consists essentially of the conductive polymer or co-polymer.
[0076] In another aspect, the present invention provides devices comprising electrodes of the present invention. For example, devices such as metal-air batteries and fuel cells can comprise the electrodes. Metal-air batteries and fuel cell devices are well-known in the art.
[0077] Metal-air battery components and architectures are well-known in the art. A metal-air battery typically has an air electrode (cathode) which is the site of the reduction reaction, an electropositive metal based anode which is the site of the oxidation reaction, an ion conducting/isolating separator material, and a supporting electrolyte. In an embodiment, the present invention provides a primary (single-use) metal-air battery comprising a electrode (e.g., a composite electrode) of the present invention, a metal based electrode, an ion conducting/isolating separator material and a supporting electrolyte. The electrode of the present invention functions as an air electrode or cathode, which takes oxygen and reduces it to form superoxide, peroxide, or oxide as products. The metal anode can be any
electropositive metal such as lithium, zinc, sodium, etc.
[0078] In another embodiment, the present invention provides a secondary
(rechargeable) metal-air battery comprising an electrode (e.g., a composite electrode) of the present invention, a metal based electrode, an ion conducting/isolating separator material and a supporting electrolyte. In this embodiment, an electrode of the present invention functions as the reduction site during the discharge process of the battery, and functions as the oxidation site during the charge process of the battery.
[0079] A fuel cell is an electrochemical device designed for continuous replenishment of the reactants consumed and continuous removal of the products. Fuel cell components and architectures are well-known in the art. Typically, a fuel cell comprises an anode (site for hydrogen oxidation), a cathode (site for oxygen reduction), an electrolyte (which can be ion conducting but not electron conducting). In an embodiment, the present invention provides a fuel cell comprising an electrode (e.g., a composite electrode) of the present invention. In this embodiment, an electrode of the present invention functions as the cathode, while a second electrode containing, for example, platinum or another suitable catalyst functions as the anode. EXAMPLE 1
[0080] This example demonstrates electrochemical reduction of 02 at a silver- polymer-carbon electrode in a nonaqueous cell. The preparation, characterization, and electrochemical activity of a novel composite electrode containing silver on a polypyrrole (PPy)-coated carbon substrate are described here. An enhanced oxygen reduction activity for the composite electrode is observed relative to uncoated glassy carbon (GC) or silver disk electrodes. The improvement of the cathode oxygen reduction activity increases the current capability and power output of the air electrode, facilitating future development of small, lightweight, long-life power sources.
[0081] Experimental. CH Instruments potentiostats and electrodes were used for the deposition, oxygen reduction, and ac impedance experiments. Platinum auxiliary electrodes were used for all experiments. Reference electrodes were purchased from CH Instruments. For aqueous measurements, a silver/silver chloride reference was used, whereas for nonaqueous measurements, a silver/silver nitrate reference electrode was used. All potentials reported are relative to the reference electrodes used. A Thermo Fisher Scientific ICAP 6000 inductively coupled plasma spectrophotometer was used for silver analysis. Scanning electron microscopy (SEM) images were recorded using a Hitachi S-800. Temperature was maintained at 25°C throughout all electrochemical experiments. Silver deposition was conducted in a method consistent with that described by Palomar-Pardave and co-workers.14 PPy was deposited using a methodology similar to that described by Wallace, Ralph, and coworkers.
[0082] Oxygen reduction was measured using an electrolyte of 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile (CH3CN).
Nitrogen:oxygen gas ratios were controlled using Matheson Trigas flowmeters, where CH3CN-saturated gas at the desired ratio was bubbled through the electrolyte solution before measurement, and flowed over the electrolyte solution during measurement. Oxygen concentration was calculated using Henry's law assuming an 8.1 mM oxygen solubility at standard pressure and was adjusted using local atmospheric pressures obtained from the National Oceanic and Atmospheric Administration's National Data Buoy Center. Oxygen reduction data were collected using linear sweep voltammetry at 0.1 V/s.
[0083] Results and Discussion. Composite electrode concept. Silver metal promotes oxygen reduction in an aqueous alkaline electrolyte. In addition, polycrystalline silver and deposited silver particles have shown feasibility as aqueous gas diffusion electrodes for fuel cells and metal-air batteries in a basic aqueous electrolyte. Due to its successful utilization for aqueous oxygen reduction, silver was identified to be a promising material for
nonaqueous oxygen reduction and was selected as the focus of this study.
[0084] To investigate the activity of silver deposits, a composite electrode was developed where PPy and then silver were deposited on a GC substrate. For this study, GC was selected as it is a readily available electrode material suitable for use in electrochemical investigations. Carbon is relevant to battery applications as a low cost, environmentally sustainable, lightweight, and low resistivity (10 5 - 10 2 Ω cm) material, with a wealth of carbon substrates of differing geometry and a porosity available for future composite electrode studies. PPy was selected to promote good physical and electrical contact between the carbon substrate and the silver deposit.
[0085] Composite electrode preparation and characterization. PPy was deposited on
GC electrodes using cyclic voltammetry (Figure 22). PPy polymerizes oxidatively at +0.6 V. A semireversible reduction-oxidation couple appeared with the oxidative peak potential at +0.4 V and the reductive peak potential at -0.25 V, where both waves grew in magnitude and the reduction peak broadened as the deposition progressed. This couple is attributed to the switching of the PPy film between oxidized (PPy ) and neutral (PPy) forms. A broader reduction peak with a narrower oxidation peak has been observed for other PPy films.
[0086] Silver was electrochemically deposited on PPy-coated GC (PPy/GC) electrodes (Figure 23). The peak potential for silver deposition was -0.1 V. Masses of silver deposited were calculated from the total cumulative charge using Faraday's law, assuming one electron reduction per formula unit for Ag+→ Ag°. To independently quantify the amount of silver deposited, a series of Ag/PPy/GC composite electrodes was analyzed using inductively coupled plasma spectroscopy. The calculated and measured silver showed excellent correspondence (Figure 24). AC impedance measurements were taken to assess the conductivity of the Ag/PPy/GC composite electrode near the open-circuit potential (-0.5 V) and at an appropriate potential for oxygen reduction (-1.6 V) The diameter of the semicircle in the Nyquist plot was ~100 Ω in both cases and did not change significantly at the lower potential (Figure 25). This showed that the composite electrode maintained its conductivity at a negative potential appropriate for oxygen reduction. Scanning electron micrographs showed a regular arrangement of Ag nanoparticles deposited on the smooth PPy surface, with a low silver loading of < 0.3 mg/cm2 (Figure 26). The Ag coating adhered well to the PPy substrate, without noticeable Ag loss during the bending of the Ag/PPy composite for the preparation of the SEM sample. This holds promise for future development of a composite freestanding Ag/PPy electrode.
[0087] Electrochemical evaluation. Oxygen reduction was evaluated for the
Ag/PPy/GC composite electrode and compared with a silver free PPy/GC composite an uncoated GC substrate electrode, a solid silver disk electrode (Ag), and a solid platinum disk electrode (Pt). The evaluation under pure oxygen atmosphere with the five electrode types is shown (Figure 27). Reductive waves appeared at peak potentials near -1.7, - 1.7, - 1.3, - 1.4, and - 1.5 V for the Ag, Pt, GC, PPy/GC, and Ag/PPy/GC electrodes, respectively. The PPy/GC composite showed little enhancement in oxygen reduction activity over the uncoated GC electrode. The peak coulomb flux (mA/cm2) for the Ag/PPy/GC composite electrode was significantly higher than the uncoated Ag, Pt, and GC electrodes, with a low silver loading of < 0.3 mg/cm2.
[0088] The oxygen reduction activity of the Ag, GC, and Ag/PPy/GC electrode types was assessed at different oxygen concentrations, ranging from 1 to 9 mM. An example is shown for one Ag/PPy/GC composite electrode, with a low silver loading of < 0.3 mg/cm2 (Figure 28). The peak coulomb flux of oxygen reduction increased linearly with oxygen concentration for all three electrode types, consistent with a first-order mechanism for oxygen reduction (Figure 29). The Ag/PPy/GC composite electrode showed the highest oxygen reduction activity at all concentrations. For the preliminary assessment of the oxygen reduction activity retained upon multiple cycling, a Ag/PPy/GC composite electrode with a low silver loading of < 0.3 mg/cm2 was subjected to 20 consecutive cycles in ambient air at 100 mV/s (Figure 30). The composite electrode showed a quasireversible behavior, retaining high oxygen reduction activity over all 20 cycles.
[0089] Conclusions. The preparation, characterization, and electrochemical activity of a novel composite electrode containing silver (Ag) on a PPy-coated carbon (C) substrate has been described here, including the first demonstration of the enhanced oxygen reduction activity for the Ag/PPy/C composite electrode relative to an uncoated GC and a silver metal disk electrode. The results reported here have relevance toward future development of small, lightweight, long-life metal-air batteries.
EXAMPLE 2
[0090] This example describes development of novel current cc-cp-Ag composite electrodes for nonaqueous metal-air batteries. The contribution of each individual component toward the activity of the composite electrode is assessed. The role of the chemical identity of the current collector (cc) substrate is investigated by examining the behavior of carbon versus gold (C versus Au) toward the electrochemical reduction of oxygen, in conjunction with a conductive polymer (cp) deposition study. The structural role of the cp in improving the physical strength of the composite electrode is assessed. A systematic study of the Ag loading effect is undertaken to determine the minimum silver loading required for a significant enhancement in the oxygen reduction activity of the cccp-Ag composite. Improvement of oxygen reduction activity at the air electrode will increase the current capability and the power output of the metal-air battery, facilitating future development of small, lightweight, and long-life power sources.
[0091] Experimental. CH Instruments (Texas, USA) potentiostats and electrodes were used for the deposition and oxygen reduction experiments. Platinum auxiliary electrodes were used for all experiments. Reference electrodes were purchased from CH Instruments. For aqueous measurements, a silver/silver-chloride reference was used, while for nonaqueous measurements a silver-silver nitrate reference electrode was used. Potentials are reported versus the reference electrodes used. Glassy carbon and gold disk working electrodes obtained from CH Instruments were used as substrates for the depositions.
[0092] All electrochemical experiments were conducted at room temperature. Silver deposition was conducted using linear sweep voltammetry in a method consistent with that described by Batina and co-workers. Potential windows for silver deposition and typical deposition voltammograms were previously reported. Polypyrrole was deposited using cyclic voltammetry with a methodology similar to that described by Wallace, Ralph, and coworkers. Potential windows for polypyrrole deposition were +0.65 to -0.65 V versus Ag/Ag+ reference, and typical deposition voltammograms are shown in Figure 3. For some glassy carbon cc-cp-Ag composite electrodes, optical images of the cc-cp-Ag composite electrodes were acquired, and then the conductive polymer-and silver (cp-Ag) coatings were physically removed from the glassy carbon (C) substrate and subjected to scanning electron microscopy. Surface areas of the silver deposits were estimated using IMAGE -J software, after measurements of < 100 particles per image. Increases in surface area upon Ag deposition were estimated assuming the surface area prior to Ag deposition was equivalent to the planar geometric area.
[0093] Oxygen reduction was measured using an electrolyte of 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile (CH3CN), at a scan rate of 100 mV 1. Acetonitrile was selected as it is an aprotic protophobic solvent with the same solvent classification as conventional lithium battery electrolytes, which has also been used in other published studies relevant to lithium metal-air batteries. The oxygen reduction analysis was based on peak coulomb flux, based on the planar geometric area of the substrate of each electrode. Nitrogen:oxygen gas ratios were controlled using Matheson Trigas (Pennsylvania, USA) flowmeters. Oxygen concentration was calculated using Henry's law assuming an 8.1 mM oxygen solubility at the standard pressure and adjusted using local atmospheric pressures obtained from the National Oceanic and Atmospheric Administration's National Data Buoy Center. For the electrode durability test, coated electrodes were horizontally dragged a distance of 5.6 cm over a Buehler polishing microcloth, while exerting a 4 N downward force.
[0094] Results and Discussion. Substrate functionality assessment. C was selected as a substrate for our composite electrode because it is an inexpensive, lightweight, and high conductivity material. In order to assess the contribution of the current collector substrate toward the overall activity of the cc-cp-Ag composite, comparative electrodes were prepared and tested using a Au disk electrode as a current collector substrate in place of the C electrode, where the resulting composites were identified as Au-cp-Ag and C-cp-Ag, respectively. Tests were typically conducted in a nonaqueous electrolyte of
tetrabutylammonium hexafluorophosphate in acetonitrile over five reduction-oxidation cycles each in pure nitrogen, air, and pure oxygen atmospheres. Each electrode type showed consistent behavior over all five cycles. Representative data from the first cycle for each electrode type are shown in Figures 31-34. Similar and consistent activity was observed in oxygen reduction tests conducted in the tetrabutylammonium tetrafluoroborate based electrolyte.
[0095] Carbon shows adequate oxygen reduction activity in the absence of metal or metal oxide based catalysts, demonstrating reversible behavior in some electrolyte systems. We tested the C electrode under pure nitrogen, air, and pure oxygen atmospheres and observed good activity and quasireversible behavior (Figure 31). Au has been established to be a highly active metal catalyst for oxygen reduction; however, it shows poor reversibility for the reverse oxidation process. Our test of a Au disk electrode (Figure 32) showed significantly higher reduction activity than the C electrode, by a factor of 2.5 X in air and 1.9 X in pure oxygen. Although the Au electrode showed repeatable behavior with no loss in activity over multiple cycling, no oxidation peak was visible, consistent with previous literature. Notably, the peak reduction potentials were significantly different for the two electrode types, -1.3 V for C and -1.5 V for Au. [0096] The C and Au disk electrodes were each coated with a thin polypyrrole cp layer using a previously described method. The carbonconductive polymer (C-cp) and gold- conductive polymer (Au-cp) electrodes were subjected to the same oxygen reduction tests. Notably, both types of polymer coated electrodes showed quasireversible oxygen reduction behavior at the same potentials and similar magnitude, regardless of the substrate used (Figure 33). Next, the C-cp and Au-cp polymer coated electrodes were each coated with Ag, at similar loadings of 0.5 and 0.4 mg cm 2, respectively. The oxygen reduction activities of the C-cp-Ag and Au-cp-Ag composite electrodes were measured (Figure 34). As with the silver-free polymer coated electrodes, both types of silver-polymer coated electrodes showed quasireversible oxygen reduction behavior at the same potentials and similar magnitude, regardless of the substrate used. This indicates that the oxygen reduction activity of the composite electrodes is dictated by the conductive polymer and not by the underlying substrate, opening the possibility for use for diverse types of current collector substrates for silver-polymer composite electrodes in the future. Also notably, the oxygen reduction activity of the C-cp-Ag composite electrode was comparable to that of the gold disk (Au) electrode, at a low Ag loading of 0.5 mg cm 2. Our composite electrode concept shows a new pathway for the development of low cost, high functioning electrodes for metal-air batteries.
[0097] Polymer functionality assessment: Electrode durability test. Polypyrrole was selected as a cp for the composite electrode to promote good physical and electrical contact between the current collector and silver deposit. An electrode durability test was designed to assess the structural role of the polymer in improving the physical strength of the composite electrode. Carbon-silver (C-Ag) composite electrodes were prepared with and without cp coatings, and both types of electrodes were subjected to an aggressive physical abuse test. Optical images were acquired before and after the test (Figure 35). The C-Ag electrode showed virtually complete Ag loss, while the C-cp-Ag composite showed good Ag retention. To quantify the activity retained, three C-cp-Ag composite electrodes were prepared, and the oxygen reduction activity of each was assessed before and after durability testing. A representative data set is shown in Figure 36. The C-cp-Ag composite consistently retained >70% of its original activity. For the 12 measurements, the average activity retained was 72.5%, the median activity retained was 71.9%, and the standard deviation was 8.7%. In contrast, for the polymer- free electrode, typical activity retained was 50%. This demonstrates that the polymer layer plays an important role in improving the robustness of the composite electrode. [0098] Catalyst functionality assessment: Silver loading test. Silver was selected as a catalyst to promote the oxygen reduction reaction. Our previous study demonstrated enhanced oxygen reduction activity for an C-cp-Ag composite electrode relative to the uncoated glassy carbon or silver disk electrodes, with a low Ag loading of < 0.3 mg cm 2. In order to determine the minimum Ag loading required for significantly increased activity, a series of C-cp-Ag composite electrodes was prepared with differing Ag content. Ag was deposited using linear sweep voltammetry, in a consistent potential window. Masses of silver deposited were calculated from the total cumulative charge using Faraday's law, assuming one electron reduction per formula unit for Ag+→ Ag°. Using a range of scan rates for the depositions provided a well controlled, highly tunable process (Figure 37).
[0099] The oxygen reduction activity of the C-cp-Ag composite electrodes was measured in nonaqueous electrolyte. Comparative data was collected with uncoated C, an uncoated solid silver disk (Ag), and silver-free conductive polymer coated glassy carbon (C- cp) electrodes. Because oxygen partial pressure can significantly affect the performance of the air electrode, measurements were made under both air (Figure 38) and pure oxygen (Figure 39). The relative activities of the various electrodes were consistent in both gases. The measured activity in air was 20-40% of the measured activity in pure oxygen. The C-cp electrodes and uncoated C electrodes showed very similar activity, consistent with our previous results. The Ag disk electrodes showed 3 X higher activity in air (Figure 38) and 2 X higher activity in oxygen (Figure 39) relative to the C-cp and C electrodes. Notably, the activity of the C-cp-Ag composite electrodes with high silver loading was typically slightly higher than that of a solid Ag disk electrode. For the C-cp-Ag composite electrodes with lower silver loading (> 0.08 mg cm 2), a strong dependence was observed where oxygen reduction activity increased linearly with Ag loading. With higher silver loading (< 0.08 mg cm 2), the oxygen reduction activity remained relatively constant, showing no consistent increase with increased Ag loading. Using the density of silver metal (10.5 g cm 3) and assuming an even Ag distribution, these data suggest that 0.08 μιη is the minimum Ag thickness required to maximize oxygen reduction activity.
[0100] Scanning electron micrographs were acquired and analyzed for several cp-Ag coatings, including electrodes with Ag loadings ranging from 0.07 to 0.4 mg crrf 2. The corresponding increases in surface area upon Ag deposition were estimated to range from 70 to 300%), where surface area increased with increasing mass loading. Thus, the contribution of effective electrode surface area is likely a factor in the increased current observed for oxygen reduction for the electrodes with silver loading. [0101] Summary. Progress toward the development of novel cc-cp-Ag composite electrodes for nonaqueous metal-air batteries was presented herein. The contribution of each individual component toward the activity of the multifunctional composite was assessed. First, the role of the chemical identity of the cc substrate was investigated, demonstrating that a conductive polymer deposit can eliminate any competitive electrochemistry due to the current collector. Stepwise preparation and electrochemical characterization of C-cp-Ag and gold conductive polymer-silver (Au-cp-Ag) electrodes demonstrated that under the appropriate conditions, the oxygen reduction activity could be determined predominantly by the cp layer or conductive polymer-silver (cc-Ag) layers, independent of the Au or C cc. Second, the structural role of the cp in improving the physical strength of the composite electrode was assessed, where a conductive polymer containing cc-cp-Ag composite was found to provide favorable robustness compared to that of a polymer-free cc-Ag electrode. Finally, a systematic study of the Ag loading effect was undertaken, where a silver loading of 0.08 mg cm 2 (equivalent thickness of 0.08 μιη was determined to be the minimum silver loading required for a significant enhancement in the oxygen reduction activity of the cc-cp- Ag composite. Improvement of oxygen reduction activity at the air electrode will increase current capability and power output of the metal-air battery, facilitating future development of small, lightweight, and long-life power sources.
EXAMPLE 3
[0102] In this example, the Ag content of a composite electrode is measured by inductively coupled plasma (ICP) mass spectrometry for three Ag/P/C composite electrodes.
Table 2. ICP quantification of Ag content.
Date 1/20/2010 1/20/2010 ! 1/20/2010
W.E. GC#12 GC#13 GC#14
R.E. Ag/AgCI #C Ag/AgCI #D : Ag/AgCI #E
A.E. Pt wire #E Pt wire #F i Pt wire #G
P deposition A05080A, B, C \ A05080A, B, C \ A05080A, B, C
Ag deposition A05082B,C A05082D,E ! A05083A, B
Mass of Ag based on
0.087 0.115 0.171
Faraday's law (mg)
Mass of Ag based on
0.104 0.122 0.189
ICP (mg)
% difference 16.6 5.3 9.5 [0103] While the invention has been particularly shown and described with reference to specific embodiments (some of which are preferred embodiments), it should be understood by those having skill in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present invention as disclosed herein.

Claims

What is claimed is:
1. A composite electrode comprising:
a conductive substrate;
a conductive polymer or conductive co-polymer disposed on the conductive substrate; and
optionally, a catalyst disposed on the conductive polymer or conductive co-polymer.
2. The composite electrode of claim 1, wherein the conductive substrate is selected from the group consisting of carbon-based material and metal material.
3. The composite electrode of claim 2, wherein the metal material is stainless steel or copper.
4. The composite electrode of claim 2, wherein the carbon-based material is selected from the group consisting of carbon black, glassy carbon, graphite, carbon fiber, reticulated vitreous carbon, carbon felt, carbon nanotubes, carbon paper and combinations thereof.
5. The composite electrode of claim 1, wherein the conductive polymer is selected from the group consisting of polypyrrole, polyaniline, polyacetylene, polyparaphenylene,
polyparaphenylene sulfide, polyparaphenylenevinylene, polythiophene, polyisothianapthene and combinations thereof.
6. The composite electrode of claim 1, wherein the catalyst is selected from the group consisting of silver, platinum, gold, copper, iron, metal oxides and combinations thereof.
7. A method of making a composite electrode comprising the steps of:
a) providing a substrate;
b) electrochemically or chemically depositing a conductive polymer or
conductive co- polymer layer on the substrate; and
c) optionally, electrochemically or chemically depositing a catalyst on the
conductive polymer or conductive co-polymer layer,
thereby forming a composite electrode.
8. The method of claim 7, wherein the conductive substrate is selected from the group consisting of carbon-based material and metal-material.
9. The method of claim 8, wherein the metal material is stainless steel or copper.
10. The method of claim 8, wherein the carbon-based material is selected from the group consisting of carbon black, glassy carbon, graphite, carbon fiber, reticulated vitreous carbon, carbon felt, carbon nanotubes, carbon paper and combinations thereof.
11. The method of claim 7, wherein the conductive polymer is selected from the group consisting of polypyrrole, polyaniline, polyacetylene, polyparaphenylene, polyparaphenylene sulfide, polyparaphenylenevinylene, polythiophene, polyisothianapthene and combinations thereof.
12. The method of claim 7, wherein the catalyst is selected from the group consisting of silver, platinum, gold, copper, iron, metal oxides and combinations thereof.
13. The method of claim 7, wherein the catalyst is disposed on only a portion of the conductive polymer or conductive co-polymer.
14. The method of claim 7, wherein the conductive polymer or co-polymer is deposited by a constant current method, a constant potential method or cyclic voltammetry.
15. The method of claim 7, wherein the catalyst is deposited by a constant current method, a potentiostatic method or linear sweep voltammetry.
16. The method of claim 7, wherein the catalyst is deposited on only a portion of the conductive polymer or conductive co-polymer.
17. An electrode comprising
a conductive polymer or conductive co-polymer; and
optionally, a catalyst disposed on the conductive polymer or co-polymer.
18. The electrode of claim 17, wherein the conductive polymer is selected from the group consisting of polypyrrole, polyaniline, polyacetylene, polyparaphenylene, polyparaphenylene sulfide, polyparaphenylenevinylene, polythiophene, polyisothianapthene and combinations thereof.
19. The electrode of claim 17, wherein the catalyst is selected from the group consisting of silver, platinum, gold, copper, iron, metal oxides and combinations thereof.
20. A metal-air battery or fuel cell comprising:
I. )
a composite electrode comprising:
a conductive substrate;
a conductive polymer or conductive co-polymer disposed on the conductive substrate; and
optionally, a catalyst disposed on the conductive polymer or conductive copolymer;
or
II. )
an electrode comprising:
a conductive polymer or a conductive co-polymer; and
optionally, a catalyst disposed on the conductive polymer or conductive copolymer.
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