Classifications

• • 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
  • H01M8/12—Fuel cells with solid electrolytes operating at high temperature, e.g. with stabilised ZrO2 electrolyte
  • H01M8/124—Fuel cells with solid electrolytes operating at high temperature, e.g. with stabilised ZrO2 electrolyte characterised by the process of manufacturing or by the material of the electrolyte
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/24—Halogens or compounds thereof
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/32—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by electrical effects other than those provided for in group B01D61/00
  • B01D53/326—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by electrical effects other than those provided for in group B01D61/00 in electrochemical cells
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2257/00—Components to be removed
  • B01D2257/20—Halogens or halogen compounds
  • B01D2257/202—Single element halogens
  • B01D2257/2027—Fluorine
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/50—Fuel cells
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
  • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

• the present invention relates generally to the field of fluorine separation and fluorine generation devices, and more particularly to a novel electrolytic device having unusual and unexpected electrochemical performance.
• Electrolytic separation offers the potential advantages of producing high purity fluorine at room temperature at high flux in a compact unit.
• Methods are known for the electrochemical separation of gas mixtures.
• One technique, described in U.S. Patent No. 5,618,405 teaches the separation of halides from high temperature gas mixtures using an electrochemical cell, the contents of which are hereby incorporated by reference in its entirety.
• Another technique known as the "outer-cell” method the gaseous component of a waste gas to be stripped are first absorbed in an absorption column in a wash solution; then the wash solution containing the polluting component is cathodically reduced or anodically oxidized in a connected electrolysis cell.
• This arrangement requires two different devices, namely one for the absorption and one for the electrolysis.
• Another technique is the “inner-cell” method in which absorption and electrochemical conversion take place in an electrolysis cell, and because the concentration of pollutants is always kept low by electrochemical conversion.
• Yet another method is the "indirect” electrolysis processes where the oxidizing or reducing agent used in a wet-chemical waste-gas treatment is regenerated by electrolysis of the wash solution used.
• U.S. Patent Nos. 6,071,401 and 5,840,174 the contents of which are incorporated herein by reference in their entirety disclose an electrolysis cell with a fixed-bed electrode for the purification of waste gases. In reductive purification hydrogen is supplied to the gas diffusion electrode and in oxidative purification oxygen is used.
• U.S. Patent No. 6,030,591 the contents of which are incorporated herein by reference in their entirety discloses the separation of fluorocompounds by cryogenic processing, membrane separation and/or adsorption.
• U.S. Patent Nos. 6,514,314 and 5,820,655 the contents of which are incorporated herein by reference in their entirety disclose a ceramic membrane structure an oxygen separation method.
• One disadvantage of the inner cell method is the high residual content of impurities in the purified gas.
• the residual content is approximately a factor often above the limit value of 5 ppm .
• the purity of gases generated by solid state devices is much higher than that of liquid (or melt) containing cells.
• Solid-state electrochemical devices are often implemented as cells including two porous electrodes, the anode and the cathode, and a dense solid electrolyte and/or membrane, which separate the electrodes.
• electrolyte should be understood to include solid oxide membranes used in electrochemical devices, whether or not potential is applied or developed across them during operation of the device.
• the solid membrane is an electrolyte composed of a material capable of conducting ionic species, such as fluorine ions, yet has a low electronic conductivity.
• the solid membrane may be composed of a mixed ionic electronic conducting material ("MIEC").
• MIEC mixed ionic electronic conducting material
• the electrolyte/membrane must be dense and as pinhole free as possible("gas-tight") to prevent mixing of the electrochemical reactants. In all of these devices a lower total internal resistance of the cell improves performance.
• Solid-state electrochemical devices are typically based on electrochemical cells with ceramic electrodes and electrolytes and have two basic designs: tubular and planar.
• Tubular designs have traditionally been more easily implemented than planar designs, and thus have been proposed for commercial applications.
• tubular designs provide less power density than planar designs due to their inherently relatively long current path that results in substantial resistive power loss.
• Planar designs are theoretically more efficient than tubular designs, but are generally recognized as having significant safety and reliability issues due to the complexity of sealing and manifolding a planar stack.
• the present invention describes an electrolytic cell for the electrolytic removal of fluorine from gas mixtures. Another object of the present invention is the separation of fluorine from fluorine/fluoride containing sources (gases, liquids or solids), such as HF, NF 3 , CF , SF 6 , etc.
• the present invention describes the electrolytic cell to accomplish the aforementioned and novel methods for making improved cells over the prior art.
• FIG 1 depicts a planar design solid state electrochemical device of the prior art.
• FIG 2 depicts a fluorine separation device in accordance with one embodiment of the present invention.
• FIG 3 depicts a fluorine separation device in accordance with one embodiment of the present invention
• FIG 4 shows the current/voltage relationship for Pt electrodes.
• FIG 5 shows the current/voltage relationship for Pt electrodes.
• a solid oxide fuel cell SOFC
• a similar device designs is also used for fluorine separation is useful depending on the selection of materials used as the electrodes and separators, the environment in which the device is operated (gases supplied at each electrode), pressures or electrical potentials applied, and the operation of the device.
• a hydrogen-based fuel typically methane that is reformed to hydrogen during operation of the device
• air is provided at the air electrode.
• Oxygen ions (O 2' ) formed at the air electrode/electrolyte interface migrate through the electrolyte and react with the hydrogen at the fuel electrode/electrolyte interface to form water, thereby releasing electrical energy that is collected by the interconnect/current collector.
• FIG 1 illustrates a basic planar design for a solid-state electrochemical device, for example, a solid oxide fuel cell (SOFC).
• the cell 100 includes an anode 102 (the "fuel electrode”) and a cathode 104 (the “air electrode”) and a solid electrolyte 106 separating the two electrodes.
• the electrodes and electrolytes are typically formed from ceramic materials, since ceramics are able to withstand the high temperatures at which the devices are operated.
• SOFCs are conventionally operated at about 950°C. This operating temperature is determined by a number of factors, in particular, the temperature required for the reformation of methane to produce hydrogen and reaction efficiency considerations.
• typical solid-state ionic devices such as SOFCs have a structural element onto which the SOFC is built.
• the structural element is a thick solid electrolyte plate; the porous electrodes are then screen-printed onto the electrolyte.
• the porous electrodes are of low strength and are not highly conductive.
• a thick porous electrode and a thin electrolyte membrane can be co-fired, yielding an electrode/electrolyte bilayer.
• electrolyte/membrane conductivity Another consideration governing the temperature at which the solid-state electrochemical device described herein is operated is the electrolyte/membrane conductivity. Conventional devices must be operated at a high enough temperature to make the ceramic electrolyte sufficiently ionically conductive for the energy producing reactions (in the case of a SOFC; other reactions for gas separators or generators).
• Typical devices described in accordance with the present invention operate at temperatures of approximately 120 °C, but that temperature ranges from between 100- 300 °C, and preferably between 120-150 °C, depending partially upon the choice of electrolyte.
• the operating temperature is about 120°C.
• the temperature is between 300-500 °C, because of the conductivity of the LaF 3 . If the electrolytic cell is run at a temperature of less than 150 °C, a teflon or teflon based material may be used in the device, for seals and the like.
• EVD/CVD Enhanced Metal-organic chemical vapor deposition
• EVD/CVD is a complex and expensive technique, and the ceramic-based devices to which the technique has been applied still require high operating temperatures to be at all efficient.
• most metals are not stable at this temperature in an oxidizing environment and very quickly become converted to brittle oxides.
• the present invention contemplates sputtering as a method of forming thin film electrolytes on substrates. Sputtering contemplated by this method is taught in the art, see for instance P. Hagenmuller, A. Levasseur, C. Lucat, J. M. Reau, and G. Villeneuve, in "Fast ion transport in solids. Electrodes and electrolytes.
• the cell 100 is depicted in the form in which it could be stacked with other like cells 110, as it typically would be to increase the capacity of the device.
• This embodiment is contemplated in the present invention.
• the cells require bipolar interconnects 108 adjacent to each electrode that are electrically, but not ionically, conductive, in the present invention.
• the interconnects 108 allow current generated in the cells to flow between cells and be collected for use. These interconnects are typically formed into manifolds through which the source gas and carrier gas may be supplied to the respective electrodes (allow lateral movement of gas in channels; but not allow intermixing of gas (vertical movement).
• interconnect Due to the corrosive nature of F 2) materials for the interconnect must be corrosion resistant. Teflon and teflon based materials are contemplated.
• the interconnect may also be a F 2 corrosion resistant materials such as Ni or a Ni alloy, or, preferably, stainless steel. Electrically conductive interconnects in the present invention may be used to separate the anode and cathode chambers and to apply current to the electrodes. Aluminum and aluminum alloys may be used if the device of the present invention is to be operated at or below 300 °C. The choice of interconnect material is readily determinable by one of ordinary skill depending on the temperatures of use.
• the electrode materials may be different between cathode and anode.
• the anode material should have a low overpotential for electrochemical fluorine generation
• the cathode should have a high overpotential for electrolyte reduction, i.e. electrodeposition of Pb and Sn, if PbSnF 4 is used.
• Pt is most preferred for the anode
• graphite the most preferred for the cathode.
• the overpotential is the applied potential less the initial or equilibrium potential and the TR drop on the electrolyte. In operation the reference electrode may not be necessary, and one having skill in the art will be able to optimize the overpotential for any particular electrolyte.
• This value will enable operation of the device inside the potential range or stability window for the particular electrolyte chosen.
• the window would be approximately 0 to about 5-6 volts, vs Sn/SnF reference.
• the electrodes (anode and cathode) used in accordance with the fluorine generation or separation device described herein are preferably materials which do not produce highly volatile or electrically insulating fluorides in and under the electrical potentials applied to the device.
• electrode materials must be chosen that do not have adverse reactions with the thin-film electrolyte.
• Non-limiting examples are metals such as platinum, gold, nickel, palladium, copper, silver, alloys of these metals, and graphitic carbon.
• a preferred material for the cathode is carbon, preferably graphitic carbon.
• a preferred material for the anode is Pt.
• the anode and cathode may both be of the same material, depending on the choice of electrolyte.
• a triple phase boundry with a high surface area of the electrolyte material, the electrode material and the gas being used in the gas separation or generation device. This is accomplished by providing small particles (high surface area) of Pt powder, i.e. Pt black (.05 microns to 20 microns, preferably .7 microns to 2 microns) and pressing at 1000 psi. . During operation, all three phases are interpenetrating, resulting in three phase boundaries throughout.
• a carbon electrode This may be prepared from petroleum cokes containing coal tar pitch as binder.
• the carbon anode is formed with 40 wt% of coal tar pitch as a binder, which will lead to the increase in the effective internal surface due to the proper size and distribution of pores on the carbon anode.
• Similar anodes are known in the art and described in A n et al. Journal of the Korean Chemical Society, 2001, Vol. 45, No. 5, the contents of which are hereby incorporated by reference in its entirety for all purposes.
• a dense membrane that is conductive to fluoride ion, with reasonably high ionic conductivity at ambient or slightly elevated temperatures.
• Such materials are known.
• Two non-limiting examples are PbSnF 4 and LaF 3 , which both have high ionic conductivity for fluoride ion.
• the ionic conductivity of PbSnF 4 is about 10 "3 ⁇ "1 cm "1 at room temperature; therefore resistivity (p) is 10 3 ⁇ cm at room temperature.
• R p(length/Area)
• R 1000 (length/1 cm 2 ).
• ASR 1 ⁇ cm 2 at room temperature. This means that in a device run at a current density of 1 amp/cm 2 , the electrolyte resistance will only contribute 1 volt in iR loss at room temperature.
• fluoride ion conductors such as LaF 3 , which has an ionic conductivity of LaF 3 is 5 x 10 "5 S/cm at 20 °C.
• the overpotential of the F 2 /F " electrodes as a function of current density is important.
• there is used use carbon-based electrodes as is done for HF oxidation, however, those electrodes are known to become passivated giving rise to large overpotentials.
• Groult et al. showed this to be due to the formation of CF X on the carbon electrode surface which inhibits wetting of the electrode, see H. Groult, D. Devilliers, S. Durand-Vidal, F. Nicolas, and M. Combest, Electrochimica Ada, 44, 2793 (1999), the contents of which are incorporated herein by reference.
• the present invention is not particularly concerned with wetting of a carbon electrode with molten KF-HF, so this phenomenon may not be an issue for redox of F 2 mixed with impurity gases.
• Pletcher showed that the overpotentials for the F 2 /F " redox reaction are much lower on Pt electrodes than on carbon, see A.G. Doughty, M. Fleischmann, and D. Pletcher, Eleciroanal. Chem. And Interfacial Electrochem., 51, 329 (1974), the contents of which are incorporated herein by reference.
• the current voltage relationship for Pt electrodes are shown in the FIGs 1 and 2. Given the low overpotential for F 2 evolution, very low overpotentials for porous Pt electrodes having high surface area are possible.
• the present invention contemplates a thick PbSnF disk (.1-2.5 mm, preferably 1-2 mm thick) coated with thin (50 microns) Pt PbSnF composite electrodes.
• the invention contemplates that the electrode thickness may be 2 microns to 100 microns, preferably between 10 and 50 microns.
• APt reference electrode can be used to monitor anode and cathode overpotentials.
• the fluorine separation and generation device contemplates the structure shown in FIG 3.
• the thickness of the electrolyte is minimized to eliminate iR losses, and the membrane is supported on a conductive substrate.
• the substrate will be a stainless steel porous support onto which a Pt PbSnF slurry is deposited followed by deposition of a dense PbSnF film (2-200 ⁇ m, preferably ⁇ 10-20 ⁇ m) onto which the second Pt PbSnF 4 electrode is deposited.
• the invention contemplates that the fluorine separation device is functional when fabricated in flat plates (smaller footprint device), or a tubular shape (simplified seals).
• the electrolyte membrane of the present invention comprises a material capable of conducting fluorine.
• the material must be a solid, and not be porous such that there is no gas movement through the membrane.
• the electrolyte must be gas tight.
• the material is PbSnF 4 .
• the PbF 2 / SnF 2 system is very rich in new materials. These include a wide Pb ⁇ . x Sn x F2 solid solution (0x0.50, cubic -PbF 2 fluorite-type for 0x0.30, tetragonal. -PbSnF fluorite-type for 0.30 ⁇ x0.50) and stoichiometric Pb 2 SnF 6 , PbSnF 4 and PbSn-JFio.
• -PbF 2 like behavior is observed, while for the highest x values, it behaves like -PbSnF 4 , with a slowing down of the transformation as x moves towards the center of the solid solution, where no change is observed.
• the particle size obtained at a given ball-milling time is a function of the fractional amount x of tin in the samples.
• Electrolyte materials contemplated for the electrolyte are LaF 3 , doped or undoped with
• rare earth metals preferably Er.
• the electrolyte is a ceramic electrolyte which is substantially impermeable to the passage of gases but permeable to fluoride- ions.
• An example of such an electrolyte and method of making is known in the art.
• U.S. Patent No. 4,707,224 the contents of which are hereby incorporated by reference in its entirety for all purposes.
• An unexpected advantage to the present invention over this patent is that electrodes was made by sputtering and/or evaporation and there is not a triple phase boundry uniformly present, except perhaps in film pinholes and other defects.
• porous metals such as the transition metals chromium, silver, copper, iron and nickel, or a porous alloy such as low-chromium ferritic steels, such as type 405 and 409 (11-15% Cr), intermediate-chromium ferritic steels, such as type 430 and 434, (16-18% Cr), high- chromium ferritic steels, such as type 442, 446 and E-Brite (19-30% Cr), chrome-based alloys such as Cr5Fel Y and chrome-containing nickel-based Inconel alloys including Inconel 600 (Ni 76%, Cr 15.5%, Fe 8%, Cu 0.2%, Si 0.2%, Mn 0.5%, and C 0.08%).
• the substrate may be a porous cermet incorporating one or more of the transition metals Ni, Cr, Fe, Cu and Ag, or alloys thereof.
• a protective layer for either or both electrodes is further contemplated.
• the protective layer should conduct ions generated during discharge of the negative electrode.
• the protective layer may be deposited by sputtering or evaporation.
• Materials for the protective layer may include alkali and alkali earth metal fluorides, such as CaF 2 , MgF 2 or KF.
• doped or undoped LaF 3 is also, contemplated by this invention and preferred. This layer will separate the electrode from the electrolyte. The thickness of the layer is less than 1 micron.
• the present invention also contemplates that the solid state electrochemical device described herein is also useful as a fluorine generation device.
• gasses such as HF, NF 3 , CF 4 and SF 6 may be electrochemically converted to fluorine gas.
• the invention contemplates that a mixture of gases may also be used as input gases, to generate fluorine gases.
• Example 4 describe one embodiment in accordance with this concept. Also contemplated is the use of liquids (nonlimiting examples of which include KF*HF melts) and solids (nonlimiting examples which include KF, PBF 2 , CoF 3 ) as input materials.
• the cathode may be different than the cathode for a fluorine separation device dependent on the source of fluorine, but the anode and electrolyte can be as described for the separation device, because the chemistry is the same.
• Preferred cathode materials for a fluorine generation device are Pt, if one is using as input material NF 3 , for example.
• suitable materials for the cathode material depending on the choice of input material. The choice is based on minimizing the overpotential for the reaction which is the reduction of gases to fluoride ion F " and a residual gas (N 2 in the case of NF 3 source gas).
• a thin film electrolyte cell can be prepared by melt solidification.
• a Pt-PbSnF powder mixture is uniformly spread on top of a porous stainless steel support and pressed in a die at 1000 PSI to form a pressed film of 1-20 microns, preferably 5-10 microns.
• PbSnF 4 powder is approximately uniformly spread on the bilayer structure and die pressed at 5000 PSI.
• the cell is placed in a closed cylinder which can be purged with F 2 orF 2 in an inert gas (He, Ar or N 2 ) and heated to 390 °C, the melting point of PbSnF .
• the second Pt-PbSnF 4 electrode is sprayed on the electrolyte film to a thickness of 1-20 microns, preferably 5- 10 microns.
• Example 2 An electrolyte supported cell can be prepared by the following method. A saturated aqueous solution of Pb(NO 3 ) 2 is added drop-wise to a saturated aqueous solution of SnF 2 , acidified by 5% HF, under stirring. The white precipitate formed is filtered by vacuum filtration, and dried in vacuum oven at room temperature overnight. The resulted powder is die pressed uniaxially at 50 KPSI and room temperature for 5 min. The disc resulted has an electrical conductivity of 3xl0 "2 S/cm at 100 °C, measured by impedance spectroscopy.
• Pt or Pt- PbSnF 4 powder mixture is spread on both faces of the disc, and pressed at 1000 PSI, to form porous electrodes.
• Example 3 Operation of a fluorine separation device in accordance with one embodiment of this invention.
• a membrane electrode assembly may be sandwiched between two aluminum blocks (not shown), separated by a Teflon sheet (not shown).
• F2 refers to a fluorine/fluoride containing compound, or a gas mixture, containing fluorine gas compound as well as other gases.
• the anode and cathode chambers can be provided with gas inlets and outlets. Gas tight seals are enabled on each face (not shown) and between blocks by Viton or Kalrez O-rings.
• Power is applied to the electrodes directly through the Al blocks, from a power source (not shown), either at a constant voltage (potentiostatic mode), or at a constant current (galvanostatic mode).
• the fluorine source can be F 2 in N 2 , or other F 2 containing gas mixtures. Unreacted F 2 from the cathode chamber can be neutralized in a scrubber, or can be recirculated for increased efficiency.
• the anode chamber (not shown) can be purged with an inert carrier gas, or can produce pure F 2 if the corresponding mass flow controller is stopped.
• Example 4 Operation of a fluorine generation device in accordance with one embodiment of the present invention.
• the process of operation is similar to that of Example 3, however the cathode material has to be electrochemically active for the reduction of the fluorine precursor.
• a preferred chemical for storing fluorine is NF 3 . This can be reduced at the cathode according to the reaction
• the cathode material can be a metal such as that used in previous example, preferably Pt powder, most preferably activated Pt (black Pt).

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• 2003
  • 2003-10-03 US US10/678,428 patent/US7090752B2/en not_active Expired - Fee Related
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