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
  • H01M4/94—Non-porous diffusion electrodes, e.g. palladium membranes, ion exchange membranes
• • 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

• semipermeable membranes are often used, which selectively let a gas pass through and retain the remaining components.
• pore-free palladium / silver foils or tubes are still of great importance for the purification of hydrogen because of their ideal selectivity, the palladium content normally being between 75 and 80% and use in the temperature range between 350 and 400 ° C. Since palladium and many of its alloys have an extraordinarily good dissolving power for hydrogen, the use of such membranes is also possible at a lower temperature, the hydrogen adsorption or desorption being substantially improved by coating the surfaces with suitable catalysts, in particular finely divided palladium or platinum can be.
• suitable catalysts in particular finely divided palladium or platinum can be.
• the selectivity for hydrogen permeation is not achieved here by the (non-non-porous) Pd layer, but by the organosilicon film.
• this membrane is particularly unsuitable for use in fuel cells because it conducts the electrical current in the extremely thin Pd / Ag layer insufficiently and because it is unstable in the presence of strong alkalis (electrolyte).
• a Pd / Ag membrane from Johnson-Matthey (European Pat. 106,523 (1984)) connected to a perforated metal support or a metal net does not have this disadvantage.
• the Pd / Ag film has to be produced conventionally by rolling, there are limits to its thickness and thus to the need for palladium, as already mentioned.
• a palladium-based membrane can be produced directly on a porous metal body.
• the electrochemical deposition of palladium or palladium alloys is preferred as the deposition method over more complex and less efficient methods such as vapor deposition.
• the electrochemical deposition of such alloys on smooth metal surfaces, especially on copper, is already known. There are various methods for this, the metal ions in the electrolyte always being present as complexes. Examples of complexing agents are glycinate (HJ Schuster and KDHeppner, Manual Off. 26 57 925 (1978)), ammonia (B. Sturzenegger and J.Cl. Puippe, Plat.Met.Rev.
• a porous metal body would, for example, be a sintered body, preferably a sintered film.
• a method is proposed, such as an initially solid, non-porous metal body in a porous one Metal body can be converted.
• foil-shaped alloy consisting of a nobler and a less noble metal.
• foils made of a copper / zinc alloy (brass), copper / tin (bronze), copper / aluminum, nickel / aluminum etc. are particularly suitable.
• Brass foils with a relatively high zinc content are particularly suitable.
• the porosity is now created by the less noble metal, in this case the zinc, being selectively and practically quantitatively extracted from the foil-like alloy without this changing its external dimensions.
• the process is known per se and is used, for example, in Raney nickel, «where the less noble aluminum is selectively extracted from an aluminum / nickel powder by concentrated lye and highly porous, catalytically active nickel remains.
• the less noble component can be extracted in this way.
• the potential measured in relation to a reference electrode is cyclically varied over a longer period between a more positive and a less positive (or a less negative and a more negative) range, the more positive (or the less negative) potential range being chosen such that the less noble and also the nobler metal dissolve, and the less positive (or the more negative) area so that only the nobler metal can separate again.
• This method can be carried out both in an acidic and in an alkaline electrolyte, in the latter case the ions of the less noble metal must be (excess) soluble, which is the case when using conc. Potash or sodium hydroxide solution is often the case.
• the ions in solution of the nobler alloy component can migrate as little as possible so that the outer shape of the foil is not impaired by the pore formation.
• This can be achieved by adding anions to the electrolyte, which selectively precipitate the more noble metal, which is also oxidized when the potential is positive, and thus convert it into an insoluble or only slightly soluble form, the precipitated or not at all dissolved solution must be reducible in the more negative potential range.
• This function can be performed when using an acidic electrolyte such as 1 molar sulfuric acid or 1 molar sulfamic acid in the case of the use of copper alloys such as brass, halide ions, preferably chloride ions, which are best added in the form of hydrogen chloride, ammonium chloride or an alkali chloride.
• an acidic electrolyte such as 1 molar sulfuric acid or 1 molar sulfamic acid
• copper alloys such as brass
• halide ions preferably chloride ions, which are best added in the form of hydrogen chloride, ammonium chloride or an alkali chloride.
• the halide ions can also affect the counterelectrode, for example by forming complexes with platinum and partially solve continuous cycling. However, if platinum gets into solution, it can deposit undesirably on the working electrode, i.e. on the foil-like alloy. This can be remedied by installing a commercially available semipermeable membrane, which keeps the electrolyte space of the counter electrode free of halide ions.
• the use of sulfuric acid, slightly chloride-containing electrolytes is recommended.
• the electrochemical cyclization at pH 0 advantageously takes place in the range between 0 mV and -200 mV compared to a KCl-saturated calomel electrode, the cycle duration being in the second range and the temperature of the electrolyte being approximately 50 ° C.
• the brass foils should be chemically polished using conventional methods before use. The duration of the entire process is based on the thickness of the foils used and is in the range of several hours.
• the zinc loss can be determined by weighing the film and / or by using conventional analysis methods such as EDAX or AAS determination.
• a foil-like alloy consisting of a nobler and a less noble metal, preferably a brass foil
• a palladium alloy preferably with a palladium / silver alloy
• in a second step is converted into the porous form by means of the electrochemical cyclization described above, the electrolyte used for the first process being held in place by a holder which is sealed at the edges of the film should be separated from the electrolyte used for the second process.
• the foil-shaped alloy can at most before the galvanic deposition of the Palladium / silver alloy can be chemically or in particular electrochemically treated by superficially removing the less noble component according to the described method, which in this case is carried out only briefly.
• a catalyst such as palladium black or platinum black can also preferably be deposited galvanically.
• a thermal sintering process can be carried out using conventional methods for stabilization after the electrochemical production, with the benefit that the linear thermal expansion coefficient of copper (16.6-10 -6 ° C -1 ) lies between that of palladium (11.7 x 10 -6 ° C -1 ) and that of silver (19.7 - 10 -6 ° C -1 ), which is only a slight stress due to the different ones occurring during heating Expansions of the porous copper carrier on the one hand and the palladium / silver layer on the other hand can be expected.
• the proposed method is of course also applicable to bodies other than flat bodies, e.g. applicable to tubes.
• Such pore-free, palladium-based membranes produced in this way and supported by a porous metal body can be used instead of rolled Pd / Ag membranes in the manner mentioned at the outset for hydrogen separation and as electrodes in fuel cells such as in electrolyzers.
• porous metal bodies without the Pd / Ag layer for example as hydrogenation catalysts, particularly for viscous liquids or solutions which are difficult to filter, or as porous electrode materials in fuel cells or in electrolyzers, the metal optionally being additionally provided with suitable catalysts.
• the water-rinsed film is then electrochemically cycled in 1 M sulfuric acid / 0.004 M HCl at 50 ° C. for 12 hours in a cell separated by a semipermeable membrane using a potentiostat and a function generator.
• the 1 M sulfuric acid in the electrolyte area of the Pt counter electrode must be free of chloride.
• An SCE with saturated potassium sulfate solution as the intermediate electrolyte is used as the reference electrode.
• the potential of the working electrode is varied cyclically in the following way: 1) 500 ms at a potential of 0 mV
• the porous copper foil is (due to chemical etching) at the end approx. 85 ⁇ m thick, the Pd / Ag membrane contains 77% by weight palladium and is approx. 1 ⁇ m thick. If necessary, it can also be electrochemically coated with catalytically active Pd black or the like.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• General Chemical & Material Sciences (AREA)
• Inert Electrodes (AREA)
• Separation Using Semi-Permeable Membranes (AREA)
• Catalysts (AREA)
• Electroplating And Plating Baths Therefor (AREA)

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Citations (2)

• 1987
  • 1987-11-07 CH CH4326/87A patent/CH675843A5/de not_active IP Right Cessation
• 1988
  • 1988-11-04 JP JP63508389A patent/JPH02502320A/ja active Pending
  • 1988-11-04 WO PCT/CH1988/000205 patent/WO1989004556A1/de not_active Ceased
  • 1988-11-04 EP EP88909193A patent/EP0358727A1/de not_active Withdrawn

Patent Citations (2)

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