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
  • H01M4/88—Processes of manufacture
  • H01M4/8825—Methods for deposition of the catalytic active composition
  • H01M4/8842—Coating using a catalyst salt precursor in solution followed by evaporation and reduction of the precursor
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J35/00—Catalysts, in general, characterised by their form or physical properties
  • B01J35/40—Catalysts, in general, characterised by their form or physical properties characterised by dimensions, e.g. grain size
  • B01J35/45—Nanoparticles
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/90—Selection of catalytic material
  • H01M4/92—Metals of platinum group
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/90—Selection of catalytic material
  • H01M4/92—Metals of platinum group
  • H01M4/925—Metals of platinum group supported on carriers, e.g. powder carriers
  • H01M4/926—Metals of platinum group supported on carriers, e.g. powder carriers on carbon or graphite
• • 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
  • H01M2300/00—Electrolytes
  • H01M2300/0017—Non-aqueous electrolytes
  • H01M2300/0065—Solid electrolytes
  • H01M2300/0082—Organic polymers
• • 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 invention relates to metal colloid solutions which contain one or more platinum compounds and optionally one or more compounds of Rh, Ru, Ir or Pd and are stabilized by polymeric protective colloids, and a process for their preparation and their use for catalysts, in particular in fuel cells .
• heterogeneous catalysts the active centers of which consist of a metal, in particular a noble metal
• a sol of the relevant, catalytically active metal or possibly several metals is generated in a separate process step and the immobilized or solubilized nanoparticles are then immobilized on the support.
• This method can be found, for example, in (a) B.C. Gates, L. Guczi, H. Knözinger, Metal Clusters in Catalysis, Elsevier, Amsterdam, 1986; (b) J.S. Bradley in Clusters and Colloids, VCH, Weinheim 1994, pp. 459-544 or (c) B.C. Gates, Chem Rev. 1995, 95, 511-522.
• the brine is made using a stabilizer, especially when you need processable brine with a metal concentration of 0.1% or higher.
• the stabilizer envelops the metal particle and prevents the particles from agglomerating due to electrostatic or steric repulsion.
• the stabilizer affects the solubility of the particles to a certain extent.
• EP-A-0 672 765 describes the electrochemical production of platinum hydrosols using cationic and betaine stabilizers as well as catalysts produced therefrom which are said to be suitable, inter alia, for fuel cells.
• Shell catalysts of platinum which are produced via a cationically stabilized hydrosol and are suitable for fuel cells, are described, for example, in DE-A-44 43 701.
• the particles of a precious metal form a bowl reaching up to 200 nanometers deep.
• the polymeric stabilizers e.g. Containing polyacrylic acid, polyvinyl alcohol or poly- (N-vinylpyrrolidone), and their use for the production of catalysts, including for fuel cells, have also been described (J. Kiwi and M. Grätzel, J. Am. Chem. Soc. 101 ( 1979) 7214; N. Toshima et al., Chemistry Letters 1981 793). Apart from the purpose of stabilizing the sol in question, the polymers mentioned have no functional significance.
• a difficult task is to accomplish protic charge transport through the contact between the catalytically active platinum centers and the membrane.
• the platinum / carbon mixture is, for example, superficially worked into the membrane by rolling or pressing. This type of contacting is difficult, however control and there is a risk of excessive hindrance to the mass transport to and from the platinum centers. There is also the risk that some of the platinum particles will lose contact with the current collector.
• a certain amount of the polymeric cation exchange material is introduced into the platinum / carbon layer before the membrane is pressed, for example by impregnating the platinum / carbon mixture with a solution of the polymeric cation exchange material.
• This process has the disadvantage that the surface area of the catalyst can be reduced or the carbon particle is coated too much and is therefore electrically insulated.
• the board insert is about 0.5 to 4 mg / cm 2 membrane area. So far, this corresponds to several 100 g of platinum for a roadworthy vehicle with an engine output of 40-50 kW.
• Another major reason for the increased platinum requirement is based on the manufacturing process for the platinum / carbon mixture that has been predominantly used up to now.
• the solution of a reducible or precipitable platinum compound is applied to the carbon support by soaking or spraying.
• the platinum compound is then converted into finely dispersed platinum or platinum oxide particles by precipitation and / or chemical reduction, with it frequently forming larger particles of up to a few 10 to 100
• Nanometer diameter is coming. This leads to a weakening of the catalytic activity due to the reduction in the specific platinum surface.
• a cluster which is simplified as a cube and consists of atoms of a metal with an assumed diameter of 0.25 nanometers, contains approximately 87% with an edge length of 1 nanometer and with an edge length of 2.5 nanometers 49% and with an edge length of 10 nanometers 0.14% surface atoms.
• a platinum catalyst on a carbon carrier loses surface area under the usual operating conditions, ie at elevated temperature. This loss is due to the fact that the platinum particles migrate on the carrier surface and can unite with other particles, ie recrystallize to form larger particles. The smaller the platinum particles, the more pronounced this effect. From this point of view, it is desirable to reduce the rate of migration of the platinum particles by embedding them in a polar microenvironment that interacts more with the carbon support.
• the object of the present invention is to provide water-soluble, stabilized metal colloids which contain ultrafine particles of platinum or platinum metals, and to provide a process for their preparation, which are suitable for catalysts, in particular for fuel cells.
• the platinum particles should be in good proton-conducting contact with the membrane and show a reduced tendency towards recrystallization.
• the present invention solves this problem and thus relates to water-soluble metal colloids containing one or more platinum compounds and optionally one or more compounds of Rh, Ru, Ir or Pd, the metal colloids being stabilized by a proton-conducting protective colloid.
• water-soluble or solubilizable cation exchange polymers are used as protective colloids.
• the present invention further relates to a process for the preparation of these metal colloid solutions by reacting a platinum compound and, if appropriate, one or more compounds of Rh, Ru, Ir or Pd with a reducing agent.
• At least one cation exchange polymer is used, the reduction either taking place in the presence of the cation exchange polymer or the cation exchange polymer being added to the solution after the reduction step.
• the stabilized metal colloid (sol) can then be cleaned by falling over and / or concentrated by evaporation.
• the ultrafine particles containing a proton-conductive polymer and encased in a microstructure are immobilized in a fine, uniform distribution on the surface or in areas close to the surface of a carbon carrier in such a way that the platinum particles subsequently build up MEA can be brought into improved proton-conducting contact with the membrane and show a reduced tendency towards recrystallization.
• the metal compounds of platinum, rhodium, ruthenium, palladium and iridium used are used as starting materials in the form of soluble compounds, in particular water-soluble salts.
• these are hexachloroplatinic acid hydrate, hydroxy-disulfito-platinic acid, platinum nitrate, hexachloroiridium (IV) acid hydrate, palladium (II) acetate, iridium (III) acetylacetonate, ruthenium (III) acetylacetonate, ruthenium ( III) nitrate, rhodium (III) chloride hydrate, to name just a few.
• the metal compounds are used in a concentration of about 0.1 to 100 g per liter, preferably from 1 to 50 g per liter, based on the solvent.
• the ratio is Pt to others
• the cation exchange polymers used to prepare the metal colloid solutions have strongly acidic, easily dissociable groups, for example carboxylic acid groups, sulfonic acid groups or phosphonic acid groups. Characteristic of the polymers used is therefore their property, particularly in the swollen state, that cations and in particular protons are easily mobile in the polymer matrix.
• the cation exchanger polymers which can be used according to the invention can be selected from various chemical substance classes, such as, for example, sulfonated polyaryl ether ketones, sulfonated polyether sulfones, sulfonated
• ABS sulfonated acrylonitrile-butadiene-styrene copolymers
• poly- (styrene sulfonic acids) poly- ( ⁇ , ß, ⁇ -trifluorostyrene sulfonic acids) as well as poly- [perfluoroethylene-co-2- (1-methyl-2- vinyloxy-ethoxy) -ethanesulfonic acid] and similarly structured perfluor
• the polymers belonging to the group of the sulfonated polyaryl ether ketones are phenylene radicals which are linked via ether or keto groups and which preferably carry sulfo groups in the ether subunits.
• Examples of such polymers are the sulfonated polyether ketones (PEK) (1), sulfonated polyether ether ketones (PEEK) (2), sulfonated polyether ketone ketones (PEKK) (3), sulfonated polyether ether ketone ketones (PEEKK) (4) or sulfonated polyether ketone ether ketone ketones ( PEKEKK) (5):
• the underlying, non-sulfonated precursor polymers are known, for example, under the trade names Hostatec®, Victrex® or Ultrapek®.
• the polymers belonging to the group of the sulfonated polyaryl ether sulfones are phenylene radicals which are linked via ether or sulfone groups and which carry sulfonic acid groups in the ether subunits, for example sulfonated polyether sulfone (PES) (6):
• Such polymers are obtained from non-sulfonated base polymers, which are known for example under the trade names Polyethersulfon Victrex 200 P®, Polyethersulfon Victrex 720 P®, Polyarylsulfon Radel®, Polyethersulfon Astrel®, Polysulfon or Udel®.
• the sulfonic acid groups can be introduced by processes known per se by reacting the base polymers with sulfuric acid, oleum or chlorosulfonic acid in accordance with JP-A-043 107 732.
• the degree of sulfonation indicates the percentage of monomer unit units which carry a sulfonic acid group.
• Polyether ketones or polyether sulfones suitable for the process according to the invention preferably have a degree of sulfonation in the range from 20 and 95%, in particular in the range from 40 and 85%.
• Perfluorinated cation exchange resins which can be used according to the invention are, for example, copolymers of tetrafluoroethylene and perfluorinated vinyl ethers with a terminal sulfonic acid group, phosphonic acid group or carboxylic acid group.
• Formula (7) shows a typical structure of perfluorinated cation exchange resins without restricting the fluoropolymers which can be used according to the invention to the general formula:
• the products are available under the trade names Aciplex-S® (Asahi Chemical) or Nafion® (EI DuPont de Nemours) or as a test membrane (Dow Chemical).
• soluble or solubilizable cation exchange polymers can be used as protective colloids.
• the solubility of the polymers in water or lower aliphatic alcohols, for example methanol or ethanol, can be controlled via the degree of polymerization and via the number of acid groups.
• Aqueous alcoholic solutions or brine from Nafion® are commercially available.
• colloidal solutions can also be prepared from commercially available, perfluorinated cation exchange membranes by heating in N-methylpyrrolidone for several hours, which can then be diluted with water.
• Solutions of, for example, N-methylpyrrolidone, dimethyl sulfoxide, dimethylformamide or dimethylacetamide can be prepared from sulfonated polyether ketones, which can then be diluted with water or lower aliphatic alcohols.
• Cation exchange polymers suitable according to the invention have an ion exchange capacity in the range from 0.5 to 5 mA / g, in particular in the range from 0.8 and 3.5 me / g.
• polymers with higher ion exchange capacities which tend to have good solubility in water and / or polar solvents and have a high swelling capacity.
• the cation exchange polymers are used in amounts by weight of 5 to 4000%, preferably 10 to 2000%, based on the metal content (Pt, Ir, Rh, Ru and Pd).
• the reduction can be carried out in water or in a mixture of water and one or more water-miscible organic solvents or with the exclusion of water in an organic solvent.
• suitable solvents are methanol, ethanol, ethylene glycol, tetrahydrofuran, dimethoxyethane, acetone, N-methylpyrrolidone, dimethylformamide or dimethylacetamide. It is preferred to prepare the metal colloid solutions in water (hydrosols) or in water with the addition of 1 to 50% by weight, preferably 5 to 25%, of an organic solvent.
• Suitable reducing agents are all customary reducing agents which have a sufficiently negative reduction potential, such as, for example, hydrogen, sodium borohydride, mono- or dihydric alcohols, such as e.g. Ethanol, ethylene glycol, hydroxymethanesulfinic acid sodium salt. Hydroxymethanesulfinic acid sodium salt (Rongalit®) is used as the preferred reducing agent.
• the reducing agent is generally used in stoichiometric amounts, based on the metal compound (s), but preferably in a certain excess.
• the excess can be, for example, 10 to 100 mol%.
• the brine is preferably produced at temperatures in the range from 0 and 200 ° C., in particular at 20 and 100 ° C.
• the components can generally be added in any order. It is useful that
• the reducing agent is added last. If the cation exchange polymer is only added after the reduction, the addition must be made before the agglomeration begins.
• the soluble metal colloids according to the invention are soluble in water or an organic solvent, "soluble” also in the sense of "solubilizable", i.e. H. Forming brine means.
• the solubility is at least 50 g / l and is usually in the range from 50 to 200 g / l, in particular in the range from 70 to 150 g / l.
• the metal colloids stabilized with cation exchange polymers are new Compounds of relatively uniform composition. Based on transmission electron microscopy (TEM) investigations, the particles obtained are in a very narrow size distribution. Typically 90% of the particles deviate less than 20% from the mean diameter.
• the diameter of the metal core depends to a certain extent on the type and amount of the stabilizer used. It is usually less than 3 nanometers, mostly less than 2 nanometers. In many cases, the diameter of the metal core is about 1 nanometer or less.
• the total diameter R h of the particles, including the protective colloid shell, was determined with the aid of dynamic light scattering in the range from approximately 2 to 4 nanometers.
• the platinum cation exchange polymer complexes contain about 55-65% by weight of metal (s) (Pt, Pd, Ir, Rh, Ru) as a solid after falling over and isolating as a solid.
• the metal colloids according to the invention are suitable as catalysts, in particular for fuel cells.
• a finely dispersed carrier e.g. made of carbon, carbon black or graphite brought into contact with the metal colloid solution according to the invention and the catalyst is separated from the liquid phase in a manner known per se by filtering or centrifuging.
• Metal concentrations of at least 10 g / liter are generally desirable for producing the platinum / carbon black mixture.
• the metal colloid solutions (brine) obtained according to the invention can, if appropriate, be concentrated by gently distilling off water and / or the solvent. If necessary, the brine obtained according to the invention can be purified by falling over in a manner known per se and, if appropriate, concentrated at the same time.
• a colloidally dissolved platinum-cation exchanger polymer complex can be precipitated by adding acetone or isopropanol.
• the platinum cation exchange polymer gels obtained are soluble in water again, with metal concentrations of at least 50 g / liter being achieved.
• the aqueous metal colloid solutions prepared as described above are brought into contact with a finely powdered conductive support material and then the liquid phase is separated off.
• the platinum particles surrounded by an inherent proton-conducting sheath are immobilized on the carrier particles. It has been found that the platinum-cation exchanger polymer complexes according to the invention are preferably deposited on the surface or in regions near the surface of the support and have good adhesion to the support.
• the carrier consists in particular of finely dispersed carbon, carbon black or graphite.
• Special, electrically conductive carbons (carbon black) that are commercially available, for example ⁇ Vulcan XC 72R, are preferably used.
• the carbon carriers to be used can be additionally loaded with materials, for example with proton-conducting polymers (US Pat. No. 4,876,115), before or after loading with the platinum nanoparticles according to the invention.
• the loading of the carbon carrier can e.g. by adding the metal colloid solution to a suspension of the carrier in water or a water / alcohol mixture with thorough mixing, stirring the suspension and isolating the platinum / carbon mixture by filtration or centrifugation.
• the metal colloid solutions produced according to the invention are very stable and contain particles with a diameter of typically 1 nanometer or less. This results in an extraordinarily high dispersity of the expensive precious metals.
• microenvironment of a rigid, polar cation exchange polymer causes also a good stabilization of the catalytic centers on the support and makes it difficult to recrystallize the particles on the support.
• the cation exchange polymers used to coat the catalytically active centers in particular the perfluorinated cation exchange resins, have good solubility for the gaseous fuels or oxygen. Therefore, the transport of the reactants to the centers is not hindered.
• the cation exchange polymers are generally distinguished by a high level which can be adjusted via the molecular weight and the ion exchange capacity
• the TEM analysis of the particles (transmission electron microscope: Philips CM 30; a sample of the sol was applied to a copper network filmed with carbon) showed a particle size of 1 nanometer.
• the TEM analysis of the particles (transmission electron microscope: Philips CM 30; a sample of the sol was applied to a copper network filmed with carbon) showed a particle size of less than 1 nanometer.
• a sample of the sol obtained was precipitated with acetone, centrifuged and concentrated in a vacuum desiccator. Dried sulfuric acid.
• the centrifuge residue was dissolved in 50 ml of deionized water, precipitated by adding 100 ml of acetone and centrifuged again.
• the moist residue obtained was dissolved in 40 ml of water and, after adding 10 g of N-methylpyrrolidone, concentrated to 20 g.
• the TEM analysis of the particles (transmission electron microscope: Philips CM 30; a sample of the sol was applied to a copper network filmed with carbon) showed a particle size of less than 1 nanometer.
• a sample of the sol obtained is precipitated with acetone, centrifuged and in Vacuum desiccator over conc. Dried sulfuric acid. Analysis of the solid obtained revealed 63% platinum (ICP-OES) and 10.9% sulfur (combustion analysis / IR detection). The dried gel dissolves in water again.
• Nafion® 117 is a commercially available perfluorinated
• the TEM analysis of the particles (transmission electron microscope: Philips CM 30; a sample of the sol was applied to a copper network filmed with carbon) showed a particle size of less than 1 nanometer.
• Vulcan XC 72R manufactured by Cabot BV, Rozenburg, The Netherlands
• 25 ml of water and 5 ml of methanol were placed in a 100 ml round bottom flask equipped with 5 porcelain balls (diameter 10 mm) and 4 hours by rotating at 100 rpm Rotary evaporator mixed.
• the coated coal was sucked off (Blue band filter, Schleicher & Schüll) and in the vacuum desiccator via conc. Dried sulfuric acid. The weight was 2.24 g. Analysis of the catalyst obtained gave 66% platinum (ICP-OES). TEM analysis (transmission electron microscope: Philips CM 30; the particles were applied to a copper-filmed copper mesh) of the catalyst particles gave a uniform distribution of the platinum particles, which had a diameter of about 1 nanometer.
• Example 8 The procedure was as in Example 8. 2.00 g Vulcan XC 72R was suspended in 20 ml water and 5 ml methanol. For this purpose, 6.7 g of Nafion-stabilized platinum sol, which was prepared according to Example 5 with a protective colloid of perfluorinated cation exchange resin, was pumped in over the course of 1 h at 20-25 ° C. with continued rotation. The suspension was rotated for a further 2 hours and then centrifuged. The centrifuge residue was concentrated in the vacuum desiccator. Dried sulfuric acid. The weight was 2.20 g. The analysis of the catalyst obtained gave 8.1% platinum (ICP-OES). The TEM analysis (transmission electron microscope: Philips CM 30; the particles were applied to a copper-filmed copper mesh) of the catalyst particles revealed a fine coating of platinum particles had a diameter of about 1-2 nanometers.
• Example 8 The procedure was as in Example 8. 2.00 g Vulcan XC 72R was suspended in 20 ml water and 5 ml methanol. For this purpose, 6.7 g of platinum sol was pumped over a period of 1 h at 20-25 ° C. with continued rotation, which was produced according to Example 3 with a protective colloid made from sulfonated PEEK. The suspension was rotated for a further 2 hours and then centrifuged. The centrifuge residue was concentrated in the vacuum desiccator. Dried sulfuric acid. The weight was 2.30 g. Analysis of the catalyst obtained gave 13% platinum (ICP-OES)
• the TEM analysis (transmission electron microscope: Philips CM 30; the particles were applied to a copper filmed copper network) of the catalyst particles revealed a coating of very fine platinum particles, the diameter of which was max. Was 1 nanometer.
• Immobilization was carried out analogously to Example 8. 2.00 g Vulcan XC 72R (manufacturer: Cabot B.V., Rozenburg, Netherlands) was used as the carrier material, which had previously been treated with a solution of sulfonated Hostatec®. 6.6 g (approx. 0.33 g of platinum, stabilizer: suifonierter Hostatec®) sol concentrate which had been prepared in accordance with Example 1 were used. The weight was 1.97 g.
• Immobilization was carried out analogously to Example 8. 2.00 g Vulcan XC 72R (manufacturer: Cabot B.V., Rozenburg, Netherlands) was used as the carrier material, which had previously been treated with a solution of sulfonated Victrex®. 6.6 g (approx. 0.33 g platinum, stabilizer: sulfonated Victrex®) sol concentrate, which had been prepared according to Example 3, were used. The weight was 2.06 g. Analysis of the catalyst obtained showed 3.8% platinum (ICP-OES).

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• 1997
  • 1997-10-17 DE DE19745904A patent/DE19745904A1/de not_active Withdrawn
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