EP1390996A1 - Structures d'electrodes derivees d'un silicium poreux et de sol-gel, et ensembles utilises avec des systemes de piles a combustible - Google Patents

Structures d'electrodes derivees d'un silicium poreux et de sol-gel, et ensembles utilises avec des systemes de piles a combustible

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Publication number
EP1390996A1
EP1390996A1 EP02731430A EP02731430A EP1390996A1 EP 1390996 A1 EP1390996 A1 EP 1390996A1 EP 02731430 A EP02731430 A EP 02731430A EP 02731430 A EP02731430 A EP 02731430A EP 1390996 A1 EP1390996 A1 EP 1390996A1
Authority
EP
European Patent Office
Prior art keywords
anode
cathode
electrode assembly
cathodic
anodic
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP02731430A
Other languages
German (de)
English (en)
Inventor
Leroy J. Ohlsen
Aaron M. Cooke
Jonathan C. Mallari
Chung M. Chan
Gordon Rice
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Neah Power Systems Inc
Original Assignee
Neah Power Systems Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from US09/839,950 external-priority patent/US20020041991A1/en
Priority claimed from US09/839,786 external-priority patent/US6808840B2/en
Priority claimed from US09/839,787 external-priority patent/US6720105B2/en
Priority claimed from US09/858,327 external-priority patent/US6924058B2/en
Application filed by Neah Power Systems Inc filed Critical Neah Power Systems Inc
Publication of EP1390996A1 publication Critical patent/EP1390996A1/fr
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/8605—Porous electrodes
    • H01M4/8626—Porous electrodes characterised by the form
    • 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
    • H01M4/9016—Oxides, hydroxides or oxygenated metallic salts
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/8605—Porous electrodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/90—Selection of catalytic material
    • H01M4/92—Metals of platinum group
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/02—Details
    • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
    • H01M8/0204—Non-porous and characterised by the material
    • H01M8/0206—Metals or alloys
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1004—Fuel cells with solid electrolytes characterised by membrane-electrode assemblies [MEA]
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1009—Fuel cells with solid electrolytes with one of the reactants being liquid, solid or liquid-charged
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells

Definitions

  • the present invention relates generally to fuel cell systems and, more specifically, to porous silicon and sol-gel derived fuel cell electrode structures and assemblies adapted for use with fuel cell systems.
  • a fuel cell is an energy conversion device that consists essentially of two opposing electrodes, an anode and a cathode, ionically connected together via an interposing electrolyte. Unlike a battery, fuel cell reactants are supplied externally rather than internally. Fuel c ells o perate by e onverting fuels, s uch as h ydrogen o r a hydrocarbon (e.g., methanol), to electrical power through an electrochemical process rather than combustion. It does so by harnessing the electrons released from controlled oxidation-reduction reactions occurring on the surface of a catalyst. A fuel cell c an produce electricity continuously so long as fuel is supplied from an outside source.
  • electrochemical fuel cells employing methanol as the fuel supplied to the anode (also commonly referred to as a "Direct Methanol Fuel Cell (DMFC)" system
  • the electrochemical reactions are essentially as follows: first, a methanol molecule's carbon-hydrogen, and oxygen-hydrogen bonds are broken to generate electrons and protons; simultaneously, a water molecule's oxygen-hydrogen bond is also broken to generate an additional electron and proton.
  • the carbon from the methanol and the oxygen from the water combine to form carbon dioxide.
  • Oxygen from air supplied to the c athode) is likewise simultaneously reduced at the cathode.
  • existing fuel cell systems are typically classified based on one or more criteria, such as, for example: (1) the type of fuel and/or oxidant used by the system, (2) the type of electrolyte used in the electrode stack assembly, (3) the steady-state operating temperature of the electrode stack assembly, (4) whether the fuel is processed outside (external reforming) or inside (internal reforming) the electrode stack assembly, and (5) whether the reactants are fed to the cells by internal manifolds (direct feed) or external manifolds (indirect feed).
  • the type of electrolyte i.e., ion conducting media
  • Alkaline fuel cells e.g., KO ⁇ electrolyte
  • Acid fuel cells e.g., phosphoric acid electrolyte
  • Molten carbonate fuel cells e.g., Li 2 CO 3 /K 2 CO electrolyte
  • Solid oxide fuel cells e.g., yttria-stabilized zirconia electrolyte
  • Proton exchange membrane fuel cells e.g., NAFION electrolyte
  • one type of common electrode structures consists essentially of a conductive substrate
  • Electrode structure consists essentially of a porous bulk matrix substrate (e.g., silicon and/or sol-gel) that has catalyst particles chemisorbed on the pore surfaces (see, e.g., International Publication No. WO 01/37357, which publication is incorporated herein by reference in its entirety).
  • a porous bulk matrix substrate e.g., silicon and/or sol-gel
  • Some of the problems associated with existing porous electrode structures include, for example: (1) poor catalyst utilization, (2) less than optimal electrical conductivity, and (3) mass transfer limitations associated with reactants reaching catalytic electrode surfaces.
  • the present invention fulfills these needs and provides for further related advantages.
  • the present invention relates generally to fuel cell systems and, more specifically, to porous silicon and sol-gel derived fuel cell electrode structures and assemblies adapted for use with fuel cell systems.
  • the present invention is directed to an electrode assembly adapted for use with a fuel cell system, comprising: an anode derived from a first silicon substrate (with an optional sol-gel region); an electrolyte; and a cathode derived from a second silicon substrate (with an optional sol-gel region); wherein the anode and the cathode are spaced apart and substantially parallel to each other so as to define a spaced apart region, and wherein the electrolyte is interposed between the anode and the cathode, and wherein at least (i) the anode comprises one or more .
  • each of the one or more discrete anodic porous regions is defined by a plurality of anodic acicular pores that extend through the anode, and wherein the plurality of anodic acicular pores define inner anodic pore surfaces
  • the cathode comprises one or more discrete cathodic porous regions disposed across a top surface of the cathode, wherein each of the one or more discrete porous cathodic porous regions is defined by a plurality of cathodic acicular pores that extend through the cathode, and wherein the plurality of cathodic acicular pores define inner cathodic pore surfaces.
  • the fuel cell systems of the present invention may be gaseous or direct liquid feed fuel cell systems such as, for example, a direct methanol fuel cell system.
  • a flowing fluid reactant/electrolyte may ionically connect the anode to the cathode, wherein suitable fluid reactant/electrolytes include, for example, an organic liquid fuel combined with an acid solution (i.e., a liquid aprotic organic electrolyte).
  • suitable fluid reactant/electrolytes include, for example, an organic liquid fuel combined with an acid solution (i.e., a liquid aprotic organic electrolyte).
  • Exemplary organic fuels comprise water and an alcohol such as ethanol, propanol, methanol, or a combination thereof; and the acid solution may be phosphoric acid, sulfuric acid, trifluoromethane sulfonic acid, or a combination thereof.
  • Figure 1 illustrates a fuel cell system in accordance with the prior art.
  • Figure 2A illustrates a top plan view of an electrode structure having a plurality of acicular pores in accordance with an embodiment ofthe present invention.
  • Figure 2B illustrates a cross-sectional view of the electrode structure of Figure 2 A, wherein the view is taken along line B-B of Figure 2 A.
  • Figure 2C illustrates a top perspective view of the electrode structure of Figures 2A and 2B.
  • Figure 3A illustrates a cross-sectional view of an exemplary electrode assembly in accordance with an embodiment of the present invention, wherein a planar anode and a planar cathode have porous silicon substrate regions, and wherein the planar anode and the planar cathode are attached to each other by a plurality of bridge members that span across a spaced apart region.
  • Figure 3B illustrates a top view ofthe electrode assembly of Figure 3 A.
  • Figure 4A illustrates a cross-sectional view of an exemplary electrode assembly in accordance with an embodiment ofthe present invention.
  • Figure 4B illustrates a top view ofthe electrode assembly of Figure 4 A.
  • Figure 5A illustrates a cross-sectional view of an exemplary electrode assembly in accordance with an embodiment of the present invention, wherein a planar anode and a planar cathode have sol-gel derived support structure regions, and wherein the planar anode and the planar cathode are attached to each other by a plurality of bridge members that span across a spaced apart region.
  • Figure 5B illustrates a top view ofthe electrode assembly of Figure 5 A.
  • Figure 6A illustrates a cross-sectional view of an exemplary electrode assembly in accordance with an embodiment ofthe present invention.
  • Figure 6B illustrates a top view ofthe electrode assembly of Figure 6 A.
  • Figure 7A illustrates a cross-sectional view of an exemplary electrode assembly in accordance with an embodiment ofthe present invention.
  • Figure 7B illustrates a top view ofthe electrode assembly of Figure 7 A.
  • Figure 8A illustrates a cross-sectional view of an exemplary electrode assembly in accordance with an embodiment ofthe present invention.
  • Figure 8B illustrates a top view ofthe electrode assembly of Figure 8 A.
  • Figure 9A illustrates a cross-sectional view of an exemplary electrode assembly in accordance with an embodiment ofthe present invention.
  • Figure 9B illustrates a top view ofthe electrode assembly of Figure 9 A.
  • Figure 10A illustrates a cross-sectional view of an exemplary electrode assembly in accordance with an embodiment ofthe present invention.
  • Figure 10B illustrates a top view of the electrode assembly of
  • a fuel cell system generally comprises a stack of electrode assemblies (referred to as an electrode stack assembly), wherein each individual electrode assembly consists essentially of two opposing electrode structures, an anode and a cathode, ionically connected together via an interposing electrolyte.
  • the electrode stack assembly of such fuel cell systems also generally includes a series of flow channels for flowing reactant streams adjacent to and/or through discrete regions of the electrode structures.
  • the inventive fuel cell systems, electrode assemblies, and electrode structures are based, in large part, on novel substrates and support structures that are particularly useful for carrying a catalyst, wherein the catalyst is readily accessible to flowing gaseous and/or liquid reactant streams.
  • the substrate and support structures disclosed herein principally include silicon substrates, sol-gel derived support structures, and combinations thereof.
  • these types of substrates and/or support structures are particularly useful as electrodes for fuel cell systems (especially for micro-scale direct methanol fuel cell systems), mainly because such substrates and/or support structures are able to provide a high surface area to bulk volume ratio, have good mechanical strength, and are compatible with thin/thick films that are often need for making selected electrical connections.
  • the electrode structures, electrode assemblies, and fuel cell systems of the present invention are useful for the manufacture of small-scale portable power generating devices - portable power devices capable of delivering up to 200 Watts of power.
  • novel silicon substrates disclosed herein may be made by utilizing standard microelectronic processes such as, for example, alkaline etching, plasma etching, lithography, electroplating, as well as electrochemical pore formation on silicon substrates and wafers.
  • a silicon substrate useful for carrying a catalyst may be p roduced, w herein t he s ilicon s ubstrate m ay h ave any n umber o f pores and pores sizes such as, for example, random or ordered pore arrays - including pore arrays having selected pore diameters, depths, and distances relative to one another.
  • novel sol-gel derived support structures may be made by conventional sol-gel processing techniques, wherein the sol-gel derived support structures may have any number of pores, pores sizes, and/or pore structures.
  • the present invention is inclusive of all silicon substrates and sol-gel derived support structures, including combinations thereof, that have any number of possible porosities and/or void spaces associated therewith.
  • FIG. 2A-C An exemplary embodiment of a porous silicon electrode structure of the present invention is shown in Figures 2A-C, which drawings show an isolated electrode structure 200 (which structure is adapted for use with a fuel cell system).
  • the electrode structure 200 comprises a silicon substrate 210 (thickness ranging from about 300 to about 500 microns) having one or more discrete porous regions 220 disposed across a top surface 230 of the substrate 210.
  • each of the one or more discrete porous regions 220 is defined by a plurality of acicular or columnar pores 240 that extend through the substrate 210 (diameter ranging from about 0.5 to about 10 microns).
  • the plurality of acicular or columnar pores define inner pore surfaces 250, and the inner pore surfaces 250 may have an optional conformal electrically conductive layer 270 thereon.
  • the pores are substantially perpendicular to the top and bottom surfaces 230, 235 of the substrate 210.
  • the pores each have a diameter of about 10 microns and are spaced apart from one another about 10 microns (from pore center axis to adjacent pore center axis) so as to yield substrate having an approximate 39% porosity.
  • Porous silicon substrates (and/or support structures) useful as electrode structures may be formed by silicon micro-machining and/or wet chemical techniques (employed by the semiconductor industry) such as, for example, anodic polarization of silicon in hydrofluoric acid.
  • anodic polarization of silicon in hydrofluoric acid is a chemical dissolution technique and is generally referred to as HF anodic etching; this technique has been used in the semiconductor industry for wafer thinning, polishing, and the manufacture of thick porous silicon films.
  • the porous silicon may be microporous silicon (i.e., average pore size ⁇ 2 nm), mesoporous silicon (i.e., average pore size of 2 nm to 50 nm), or microporous silicon (i.e., average pore size > 50 nm); the pores may also be a series of parallelly aligned acicular or columnar pores that extend into or through the silicon substrate. Although the pores may be angled, they are preferably substantially perpendicular to the surfaces ofthe substrate.
  • porous silicon substrates useful in the context of the present invention may be formed by a photoelectrochemical HF anodic etching technique, wherein selected oxidation-dissolution of silicon occurs under a controlled current density.
  • a photoelectrochemical HF anodic etching technique wherein selected oxidation-dissolution of silicon occurs under a controlled current density.
  • an electrode assembly adapted for use with a fuel cell system generally comprises: an anode derived from a first silicon substrate, an electrolyte, and a cathode derived from a second silicon substrate.
  • the anode is generally processed so as to have one or more discrete anodic porous regions disposed across a top surface of the anode, wherein each of the one or more discrete anodic porous regions is defined by a plurality of anodic acicular pores that extend through the anode, and wherein the plurality of anodic acicular pores define inner anodic pore surfaces.
  • the cathode is generally processed so as to have one or more discrete cathodic porous regions disposed across a top surface of the cathode, wherein each ofthe one or more discrete porous cathodic porous regions is defined by a plurality o f c athodic a cicular p ores t hat e xtend t hrough the cathode, and wherein the plurality of cathodic acicular pores define inner cathodic pore surfaces.
  • the anode and cathode are generally spaced apart and substantially parallel to each other so as to define a spaced apart region, and the electrolyte is generally interposed between the anode and the cathode.
  • the electrode assembly may further comprise a fluid delivery channel disposed across a first face of the anode or the cathode; and a fluid removal channel disposed across a second face of the anode or the cathode; wherein at least one of the one or more discrete anodic porous regions or at least one of the one or more discrete cathodic porous regions is (i) interposed between the fluid delivery channel and the fluid removal channel, and (ii) adapted to flow a liquid therebetween (e.g., the acicular or columnar pores serve as hydrodynamic transport channels or fluid flow through-holes).
  • a liquid therebetween e.g., the acicular or columnar pores serve as hydrodynamic transport channels or fluid flow through-holes.
  • a solid polymer electrolyte e.g., NAFION, E.I DuPont de Nemours, United States
  • a flowing fluid reactant/electrolyte may ionically connect the anode to the cathode.
  • suitable fluid reactant/electrolytes include, for example, an organic liquid fuel combined with an acid solution (i.e., a liquid aprotic organic electrolyte).
  • exemplary organic fuels comprise water and an alcohol such as ethanol, propanol, methanol, or a combination thereof; and the acid solution may be phosphoric acid, sulfuric acid, trifluoromethane sulfonic acid, or a combination thereof.
  • the fluid reactant/electrolyte comprises equal molar amounts of methanol and water together with an acid in an amount of about 0.01 to 3.0 M, and preferably in an amount of about 0.25 M.
  • the present invention is directed to an electrode assembly 300 that comprises a planar anode 302 made from a silicon substrate, an electrolyte layer 304, a planar cathode 306 made from a silicon substrate, and optionally a blocking layer 308 that is substantially impermeable to at least methanol and is substantially permeable to protons.
  • planar anode 302 and the planar cathode 306 are spaced apart and substantially parallel to each other so as to define a spaced apart region 310, wherein the electrolyte layer 304 and optional blocking layer 308 are interposed between the planar anode 302 and the planar cathode 306 and within at least a portion of the spaced apart region 310, and wherein the planar anode 302 and the planar cathode 306 are attached to each other by at least one bridge member 312 that spans across the spaced apart region 310.
  • fuel (optionally combined with a liquid electrolyte) flows through the anode and, in some embodiments, through the spaced apart region 310; an oxidant such as, for example, air, oxygen, or a hydrogen peroxide solution simultaneously flows through the cathode.
  • an oxidant such as, for example, air, oxygen, or a hydrogen peroxide solution simultaneously flows through the cathode.
  • FIGs 4 A through 10B Other exemplary electrode assemblies in accordance with the present invention are depicted in Figures 4 A through 10B.
  • the electrode assembly may still further comprise a conformal electrically conductive layer on at least one of the inner anodic pore surfaces or inner cathodic pore surfaces.
  • the conformal electrically conductive layer of the present invention may be selectively deposited on the one or more pore surfaces of the selected substrate (i.e., porous silicon and/or sol-gel derived support structure) by use of a sequential gas phase deposition technique such as, for example, atomic layer deposition (ALD) or atomic layer epitaxy (ALE).
  • ALD atomic layer deposition
  • ALE atomic layer epitaxy
  • CVD chemical vapor deposition
  • the reactants or precursors used with a sequential atomic deposition technique are introduced into a deposition or reaction chamber as gases.
  • the reactants or precursors used are supplied in pulses, separated from each other (in the flow stream) by an intervening purge gas.
  • Each reactant pulse chemically reacts with the substrate; and it is the chemical reactions between the reactants and the surface that makes sequential atomic deposition a self- limiting process that is inherently capable of achieving precise monolayer growth (see, e.g., Atomic Layer Deposition, T. Suntola and M. Simpson, Eds., Blackie and Sons (1990)).
  • solid thin films may be grown on heated substrates by exposing the heated substrate to a first evaporated gaseous element or compound, allowing a monolayer of the element to form on the surface of the substrate, and then removing the excess gas by evacuating the chamber with a vacuum pump (or by use of a purge gas such as Argon or Nitrogen).
  • a second evaporated gaseous element or compound may be introduced into the reaction chamber.
  • the first and second elements/compounds c an then combine to produce a solid thin compound monolayer film. Once the monolayer film has been formed, any excess second evaporated gaseous element or compound may be removed by again evacuating the chamber with the vacuum pump.
  • the desired film thickness may be built up by repeating the process cycle many (e.g., hundreds or thousands) of times. Accordingly, such an atomic deposition technique may be used to deposit on an electrode support structure (e.g., silicon or other appropriately selected substrate) a variety of materials, including group II-VI and m-V compound semiconductors, elemental silicon, SiO 2 , and various metal oxides and nitrides thereof.
  • an electrode support structure e.g., silicon or other appropriately selected substrate
  • materials including group II-VI and m-V compound semiconductors, elemental silicon, SiO 2 , and various metal oxides and nitrides thereof.
  • an atomic layer deposition (ALD) technique is used to selectively deposit on the pore surfaces of a porous silicon support structure a conformal electrically conductive layer that consists essentially of a first tungsten or ruthenium layer (about 2,000 A thick) together with a second platinum layer (about 100 A thick).
  • the conformal electrically conductive layer enhances electrical conductivity (between the electrons released on the catalyst as a result of electrochemical oxidation-reduction reactions), and also functions as a catalyst.
  • atomic deposition techniques may also be used to deposit a film or layer containing a metal and a non-metal such as, for example, O or N.
  • the first reactant pulse may contain the metal and the second reactant pulse the non-metal.
  • the first pulse deposits a metal-containing layer, and the reactant of the second pulse reacts with that layer to form the complete film of metal oxide or metal nitride.
  • Both reactants react chemically with the surface on which they deposit, and each reaction is self-limiting. In this way, film or layer thicknesses may be controlled to within a monolayer solely by limiting the number of pulses.
  • the deposited films may be amorphous, epitaxial or polycrystalline.
  • the deposition rate of the atomic deposition process is preferably about one atomic layer and the reaction temperature is preferably ranges from about 200-400°C.
  • the conformal electrically conductive layer may have deposited thereon a plurality of catalysts particles such as, for example, bi-metallic particles of platinum and ruthenium (i.e., chemisorbed bi-metallic catalysts particles derived from platinum and ruthenium precursors).
  • a noncontiguous bi-metallic layer of platinum and ruthenium may be chemisorbed on the exposed surfaces of the conformal electrically conductive layer by selective use of platinum and ruthenium precursors.
  • a conformally coated porous silicon substrate may be immersed, under basic conditions (pH 8.5), into an aqueous ammonia solution of tetraamineplatinum(II) hydroxide hydrate, [Pt(NH ) 4 ](OH) 2 -xH 2 O, and stirred for a selected period of time.
  • the various precursors listed above are generally available from Strem Chemicals, Inc., Newburyport, Maine.
  • noncontiguous layers may also be formed by the above described sequential atomic deposition techniques, wherein such layers comprise either islands of nanocrystalhtes or an interconnected network of nanocrystalhtes.
  • island formation may be controlled to some degree by increasing or decreasing the number of bonding sites on the surface of the underlying substrate or support structure.
  • metal concentration on the surface may be decreased by reducing the number of bonding sites by either dehydroxylation (heat treatment) or chemical blocking of the bonding sites with, for example, hexamethyldisilazane (HMDS) (E. Lakomaa, "Atomic Layer Epitaxy (ALE) on Porous Substrates," J. Applied Surface Science 75:185-196 (1994)).
  • HMDS hexamethyldisilazane
  • the electrode assembly further comprises a metallic blocking layer that is (1) integrally associated with either the anode or cathode, and (2) interposed between the anode and cathode.
  • the metallic blocking layer is generally a substantially voidless metallic membrane that is substantially impermeable to at least methanol and is substantially permeable to hydrogen atoms.
  • the metallic blocking layer is preferably a palladium thin film adhered to either the anode or cathode, wherein the blocking layer has a thickness of about 1 to 5 microns (blocking layer thickness depends upon pore diameter; smaller pore diameters may support thinner films).
  • the blocking layer may comprise palladium, niobium, tantalum, vanadium, or a combination thereof.
  • Such thin films may be deposited so as to cover a porous surface by, for example, electroplating, physical vapor deposition, sputtering, thermal evaporation, e-beam evaporation, or electroless deposition.
  • Another aspect of the present invention relates to a porous silicon electrode structure that has been selectively doped so as to form one or more selectively doped regions. The doped regions are each adapted to function as a current collector for the transmission of an electrical current.
  • silicon may be selectively doped (via appropriate masking of the silicon substrate) by high-temperature diffusion and/or ion implantation.
  • Boron is preferably used as a p- type dopant whereas antimony, phosphorous, and arsenic may be used as n-type dopants.
  • doping may adversely affect the HF anodic etching process (tends to mess up the pore morphology), it is generally preferable to selectively dope the silicon substrates of the present invention after the porous regions have been formed. In this way, the porous regions and surrounding interconnect channels may be made more conductive to electron transport, thereby facilitating electrical current removal and delivery to an outside source.
  • the present invention utilizes an electrode made with a sol-gel derived support structure (optionally integrated together with a silicon substrate).
  • sol-gel processes are a way to make dispersed ceramic materials through the growth of metal oxo polymers in a solvent, (see, e.g., Brinker et al., "Sol-Gel Science, the Physics and Chemistry of Sol- Gel Processing," Academic (1990).)
  • the chemistry associated with sol-gel processes is based on inorganic polymerization reactions.
  • M Si, Ti, Al, Zr, V, W, Ir, Mn, Mo, Re, Rh, Nb, Ni, Sr, Ba, Ta, Mg, Co
  • OR is an alkoxy group and Z is the valence or oxidation state of the metal
  • the mechanism occurs in three steps: (a) nucleophilic attack of the metal M by an oxygen atom of a water molecule; (b) transfer of a proton from the water to an OR group of the metal; and (c) release of the resulting ROH molecule (Livage et al., "sol-Gel Chemistry of Transition-Metal Oxides," Progress in Solid State Chemistry, 1 8(4):259-341 ( 1988)).
  • a) nucleophilic attack of the metal M by an oxygen atom of a water molecule (b) transfer of a proton from the water to an OR group of the metal; and (c) release of the resulting ROH molecule.
  • reactive hydroxy groups are generated, the formation of branched oligomers and polymers with a metal oxo based skeleton and reactive residual hydroxo and alkoxy groups occurs through a polycondensation process.
  • two competitive mechanism have been described, namely, oxolation and olation.
  • a sol-gel derived support structure may be cast into etched or micromachined trenches, channels, and/or pits of a silicon substrate (e.g., a silicon wafer with selectively placed trenches), wherein the sol- gel derived support structure combined with the silicon substrate (together with a catalyst) serves as an electrode of a fuel cell system.
  • a silicon substrate e.g., a silicon wafer with selectively placed trenches
  • a sol-gel derived support structure comprising platinum ruthenium dioxide (Pt-RuO 2 ) may be cast into etched or micromachined trenches, channels, and or pits of a silicon substrate, wherein the sol-gel derived support structure combined with silicon substrate and a catalyst serves as an electrode of a fuel cell.
  • Pt-RuO 2 platinum ruthenium dioxide
  • An exemplary platinum-ruthenium oxide precursor solution useful for this purpose may be prepared, for example, by mixing hexachloroplatinic acid (H PtCl 6 xH 2 O), ruthenium nitrosyl nitrate (Ru(NO)(NO 3 ) x (OH) 3-x ) with nitric acid (HNO 3 ), ethyl alchohol (C 2 H 5 OH), and DI water.
  • H PtCl 6 xH 2 O hexachloroplatinic acid
  • Ru(NO)(NO 3 ) x (OH) 3-x ) ruthenium nitrosyl nitrate
  • HNO 3 ruthenium nitrosyl nitrate
  • C 2 H 5 OH ethyl alchohol
  • the solution may be refluxed under vigorous stirring at ⁇ 60°C for ⁇ 1 hr to yield a nominal molar ratio of 1 : 0.5 : 5 : 0.08 : 20 of H 2 PtCl 6 xH 2 O : Ru(NO)(NO 3 ) x (OH) 3-x : H 2 O : HNO 3 : C 2 H 5 OH.
  • an exemplary ruthenium dioxide precursor solution may be prepared, for example, by dissolving ruthenium chloride hydrate RuCl 3 XH 2 O) in a mixture of ethyl alcohol, nitric acid, and DI water.
  • the solution may be refluxed under vigorous stirring at ⁇ 60°C for ⁇ 1 hr to yield a nominal molar ratio of 1 : 20 : 5 : 0.08 of RuC13Xh20 : C 2 H 5 OH : H 2 O : HNO 3 (optionally, a 10 wt% ruthenium (IV) oxide submicron particles may then be dispersed into the precursor solution).
  • RuC13Xh20 C 2 H 5 OH : H 2 O : HNO 3
  • a 10 wt% ruthenium (IV) oxide submicron particles may then be dispersed into the precursor solution.
  • an exemplary aluminum oxide precursor solution may be prepared by mixing aluminum sec-butoxide (AI[O(CH )CHC 2 H 5 ] 3 ), hydrochloric acid (HCl), ethyl alcohol (C 2 H 5 OH), and DI water.
  • AI[O(CH )CHC 2 H 5 ] 3 hydrochloric acid
  • C 2 H 5 OH ethyl alcohol
  • DI water DI water
  • the solution may be refluxed under vigorous stirring at ⁇ 60°C ⁇ 1 hr to yield a nominal molar ratio of 1 : 0.5 : 20 : 40 of AI[O(CH 3 )CHC 2 H 5 ] 3 : HCl : C 2 H 5 OH : H 2 O.
  • AI[O(CH 3 )CHC 2 H 5 ] 3 HCl : C 2 H 5 OH : H 2 O.
  • an exemplary vanadium pentoxide precursor solution may be prepared by mixing vanadyl triisopropoxide (VO(OC H 7 ) 3 ), ethyl alcohol (C 2 H 5 OH), and DI water. The solution will be refluxed under vigorous stirring at ⁇ 60°C for ⁇ lhr to yield a nominal molar ration of 1 : 15 : 30 of VO(OC 3 H 7 ) 3 : CH 3 OCH 3 : H 2 O. (Chemicals are commercially available from Aldrich Chemical Company, Inc., Milwaukee, Wisconsin.)
  • Examples 1-4 disclose, among other, methodologies for generating porous silicon substrate, where as Examples 5-10 disclose, among other things, general processing steps associated with making an electrode structure for use with a fuel cell system, wherein the electrode structure comprises a silicon substrate having a plurality of acicular or columnar p ores t hat e xtend t hrough t he s ubstrate, w herein e ach o f t he plurality of pores has one or more atomic layers of an electrically conductive material deposited thereon.
  • Ru and RuO 2 precursors are used in an ALD process to derive an electrode structure in accordance with certain embodiments ofthe present invention.
  • the silicon sample was pre-structured; KOH etched 10-micron diameter pits for pore initiation. It was a N-type silicon with resistivity of 100 ohm-cm, (100) crystal orientation and 525-micron thickness, the silicon sample was etched in 3 wt%HF-H2O solution with an additional 100 ml of ethanol at a current density of 1.00 mA/cm ⁇ 2. A potential of 6 volts was also applied. For the above sample, the etching time was done for about 3 hours. The resulting acicular or columnar pores were pe ⁇ endicular into the wafer with an average depth of 150 microns. Deeper pores may be obtained with longer etches.
  • a typical membrane with acicular or columnar through holes would require about 24 hours etching (-500 microns).
  • the final porous silicon membrane with acicular or columnar through holes was then made by grinding off the remaining 100 microns of silicon. Each pore was roughly 10 microns in diameter, which translates to about a 35% membrane porosity.
  • N-type silicon with resistivity of 23-27 ohm-cm, (100) crystal orientation and 525-micron thickness the silicon sample was etched in 4 wt%HF-H2O solution with an additional 1 vol% (of the HF solution) surfactant (NCW-1001) at a current density of 10.00 mA/cm ⁇ 2. A potential of 1.4 volts was also applied.
  • the etching time was done for about 5 hours.
  • the resulting acicular or columnar pores were pe ⁇ endicular into the wafer with an average depth of 150 microns. Deeper pores may be obtained with longer etches.
  • a typical membrane with the above shown pores would require about 24 hours etching (-500 microns). Each acicular or columnar pore was roughly 5 microns in diameter.
  • the porous membrane was of about 39% porosity.
  • the above porous silicon sample was generated with randomly distributed pores.
  • the etching conditions were: 1.5 mA/cm A 2 current density, 6 volts potentials and 20° C etching temperature.
  • the sample was of (100) crystal orientation, n-type with resistivity of 1-10 ohm-cm and 525 micron thickness.
  • the silicon sample was etched in 3 wt%HF-H2O solutions. A typical membrane with the above shown pores would require about 24 hours etching (-500 microns). Each pore was roughly 2 microns in diameter.
  • (100) oriented n-type silicon of 1-10 ohm-cm resistivity at 525 micron thickness was initially anodically etched in one surface at 4 wt%HF-H2O electrolyte with 1 vol% NCW-1001 surfactant.
  • the etching was done at 6 volts, 2 mA/cm ⁇ 2 and at 20°C.
  • the resulting acicular or columnar pores extended into each wafer with an average depth of 400 + microns.
  • the through holes were created by etching the opposite surface of the silicon wafer at 10 wt%HF-H2O electrolyte with 1 vol% NCW- 1001.
  • the etching conditions were at 6 volts, 2 mA/cm ⁇ 2 and at 20°C.
  • the resulting membrane had a through hole consisting of wavy columnar pores having two distinct pore diameter ranges. The first ranged from about 2-3 microns while the other ranged from about 100 to about 500 nm pores.
  • a Ruthenium precursor is selected from the group consisting of Ru 3 (CO) ⁇ 2 (Dodecacarbonyltriruthenium), (C n H 19 O 2 ) 2 (C 8 H ⁇ 2 )Ru (Bis(2,2,6,6- tetramethyl-3,5-heptanedionato)(l ,5-cyclooctadiene)ruthenium (II)), and
  • Ru 3 (CO) 1 is selected as the precursor and O 2 is used.
  • the Ru 3 (CO) ⁇ 2 precursor and O 2 are separately pulsed through the reaction chamber with intervening Argon purging steps.
  • the substrate/growth temperature is set to between 120-150°C.
  • the pulse duration is about 1.0 second for Ru 3 (CO) ⁇ 2 at 100 mtorr and about 1.0 second for O 2 at 300-400 mtorr, whereas the separating purging pulses of Argon at 100 mtorr are about 2.0 seconds in duration.
  • a pure Ruthenium film is first deposited onto the porous silicon substrate in the following steps.
  • a suitable Ruthenium precursor is thermostated between 100-250°C, which results in a vapor pressure of approximately 0.05-10 Torr.
  • Oxygen gas or water vapor is used as a second precursor if Ruthenium oxide is to be deposited.
  • Hydrogen gas is used as a second precursor if Ruthenium is to be deposited.
  • Argon or Nitrogen is used as both the precursor carrier and the purge gas at a pressure between 0.1-0.5Torr.
  • the substrate/growth temperature is varied between 100 and 300°C.
  • a cycle generally consists of alternating pulses of the two precursors separated by pulses of the purge gas, the pulse duration of the precursors is 0.5-20 seconds and the purge duration is 15-30 seconds.
  • Platinum nanocrystallites are subsequently deposited by limiting the number of reaction cycles to 100-200 cycles, yielding islands of platinum between 2-20nm in size.
  • Each cycle consists of alternating pulses of the selected platinum precursor and Hydrogen gas with durations of 0.5-20 seconds, separated by pulses ofthe purge gas with durations of 15-30 seconds.
  • Exposure of the silicon substrate to water vapor could also increase the presence of reactive surface sites such as hydroxyl groups, thereby increasing the surface saturation or coverage of the metal precursor.
  • the deposited Ruthenium oxide may then be reduced to pure metallic Ruthenium with Hydrogen gas or radicals.
  • Subsequent Ruthenium may then be deposited by direct layer-by-layer reduction of an adsorbed Ruthenium precursor using Hydrogen gas or radicals to increase the overall film thickness and uniformity.
  • Active catalyst surface area can be significantly increased by forming nanocrystallites of Platinum and Ruthenium instead of a continuous film.
  • ALD growth theoretically should proceed in a layer-by-layer fashion, phenomena of island formation or nucleation of agglomerates during early stages of ALD growth have been reported (T. Suntola, "Surface Chemistry of Materials Deposition at Atomic Layer Level," J. Applied Surface Science 100/101:391-398 (1996)). It is, therefore, possible to encourage island formation and thus growth of nanosize particles by controlling surface saturation density in three different ways.
  • One way is to increase the size ofthe precursor molecule, in other words, choosing a Ruthenium and/or Platinum metal precursor with bulky ligands to increase the spacing between the resulting metal atoms thus reducing the metal density.
  • One example of such a precursor is Bis (2,2,6,6- tetramethyl-3,5-heptanedionato) (1,5-cyclooctadiene) ruthenium (II),
  • Suitable Ruthenium and Platinum precursors are, for example, Ruthenium (III) acetylacetonate, Ru(CH 3 COCHCOCH 3 ) 3 and Platinum (H) acetylacetonate, Pt(CH 3 COCHCOCH 3 ) 2 .
  • substrate temperature may be increased to promote mobility of adsorbed metal atoms favoring nucleation and crystallization.
  • Catalyst activity and stability at the anode of a direct methanol fuel cell is a function of the Platinum to Ruthenium ratio. Therefore, it is important to be able to vary that ratio at the atomic level in a homogeneous manner. Concentrations of Platinum and Ruthenium may be adjusted accurately by mixing their respective inorganic salts in a mutual solvent. The homogeneous mixture may then be used as a single precursor to deposit an alloy of the two with the desired atomic ratio. For example, Ruthenium (HI) Chloride and Dihydrogen hexachloroplatinate (IV) may be mixed and dissolved in ethanol and used as a single precursor in an atomic deposition process.
  • HI Ruthenium
  • IV Dihydrogen hexachloroplatinate

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Abstract

L'invention concerne des systèmes de piles à combustible, des ensembles d'électrodes et des structures d'électrodes. Selon un mode de réalisation, l'invention se rapporte à un ensemble d'électrodes utilisé avec un ensemble de piles à combustible comprenant une anode dérivée d'un premier substrat de silicium (avec une zone sol-gel facultative); un électrolyte; et une cathode dérivée d'un second substrat de silicium (avec une zone sol-gel facultative), l'anode et la cathode étant séparées et pratiquement parallèles de manière à définir une zone séparée. L'électrolyte, quant à lui, vient entre l'anode et la cathode. L'anode comprend au moins (i) une ou plusieurs zones poreuses anodiques discrètes traversant une surface supérieure de l'anode, chaque zone poreuse anodique discrète étant définie par une pluralité de pores aciculaires anodiques s'étendant à travers l'anode et la pluralité de pores aciculaires anodique définissant des surfaces poreuses anodiques internes, ou la cathode comprend (ii) une ou plusieurs zones poreuses cathodiques discrètes traversant une surface supérieure de la cathode, chaque zone poreuse cathodique discrète étant constituée par une pluralité de pores aciculaires cathodiques s'étendant à travers la cathode et la pluralité de pores aciculaires cathodiques définissant des surfaces poreuses cathodiques internes.
EP02731430A 2001-04-19 2002-04-19 Structures d'electrodes derivees d'un silicium poreux et de sol-gel, et ensembles utilises avec des systemes de piles a combustible Withdrawn EP1390996A1 (fr)

Applications Claiming Priority (9)

Application Number Priority Date Filing Date Title
US839787 2001-04-19
US09/839,950 US20020041991A1 (en) 1999-11-17 2001-04-19 Sol-gel derived fuel cell electrode structures and fuel cell electrode stack assemblies
US09/839,786 US6808840B2 (en) 1999-11-17 2001-04-19 Silicon-based fuel cell electrode structures and fuel cell electrode stack assemblies
US839786 2001-04-19
US09/839,787 US6720105B2 (en) 1999-11-17 2001-04-19 Metallic blocking layers integrally associated with fuel cell electrode structures and fuel cell electrode stack assemblies
US839950 2001-04-19
US858327 2001-05-15
US09/858,327 US6924058B2 (en) 1999-11-17 2001-05-15 Hydrodynamic transport and flow channel passageways associated with fuel cell electrode structures and fuel cell electrode stack assemblies
PCT/US2002/012386 WO2002086994A1 (fr) 2001-04-19 2002-04-19 Structures d'electrodes derivees d'un silicium poreux et de sol-gel, et ensembles utilises avec des systemes de piles a combustible

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