DE1172650B - Process for the manufacture of electrodes for electrochemical reactions - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Boarding school KI .: BOIk

German class: 12 h-2

Number: 1 172 650

File number: B 68757IV a / 12 h

Filing date: September 7, 1962

Opening day: June 25, 1964

The invention relates to a method for producing electrodes with Raney catalysts, preferably for electrochemical fuel cells.

For the catalytic hydrogenation and dehydrogenation of organic substances are among other things Raney catalysts used. They consist of nickel, cobalt or iron in a microporous form. To obtain the Raney catalysts, the metal is first treated with a less noble metal, e.g. B. Aluminum, tin, zinc, etc. are alloyed, the alloy is crushed, and the less noble metal is then dissolved out of the alloy with an alkali or acid. It is known in a corresponding manner also produce catalysts from platinum metals. These platinum metal catalysts are also used Called Raney catalysts.

There are also so-called double skeleton catalyst electrodes known to contain Raney nickel or Raney silver. Such an electrode is used for example by mixing powdered nickel-aluminum alloy with nickel powder as Scaffolding material and pressing the mixture into a tablet, which is then heated so that the Nickel powder sintered together to form a framework. The aluminum is finally made from the electrode blank the Raney alloy dissolved out, whereby the active Raney nickel in the pores of the nickel framework remains behind. This electrode is only for the electrochemical oxidation of hydrogen or Glycol as fuel in potassium hydroxide solution suitable as electrolyte. A double skeleton catalyst electrode with Raney silver as the catalyst was in the same manner as the electrode with Raney nickel catalyst manufactured. It is used for the electrochemical reduction of gaseous oxygen in the alkaline electrolyte. In addition, they already have powdered Raney alloys made from platinum metals used to manufacture electrodes for fuel cells.

These electrodes are expensive, however, since large quantities of noble metals are required for their manufacture will.

It is therefore already known to use other production processes for the production of such electrodes, which produce practically equivalent Raney electrode plates with less effort. For this purpose, in view of the fact that the catalytic effectiveness of the electrons is essentially due to the nature of the layers close to the surface, only surface layers made of Raney metals have been applied to prefabricated electrically conductive supports. Processes for the production of electrodes are suitable for this purpose
for electrochemical reactions

Applicant:

Robert Bosch G. m. B. H.,

Stuttgart W, Breitscheidstr. 4th

Named as inventor:

Alfons Rohling,

Dr. Helmar Krupp,

Kurt Richter,

Dr. Gerd Sandstede,

Dr. Gerhard Walter, Frankfurt / M.

wire meshes or foils with a surface roughened beforehand by etching, but preferably Sintered body. The Raney metals are either cold welding, pressing or rolling Applied in a powdery state to the carrier bodies and followed by a The sintering process is firmly bonded to this, or the metallic surface layers are applied by electrolysis on the carriers. Then in a known manner on chemical Ways the catalytically inactive Raney components out of the surface layers again detached. However, these methods have the inevitable disadvantages that that by pressing or rolling on with subsequent sintering layers that can be produced unevenly and not homogeneously fail, while the electrolytically produced layers form crystallites that are too large, as a result of which the catalytically active surface is considerably reduced. You can also do this in a known manner also porous electrode carriers by soaking in a molten metal or in a salt solution with surface layers coating, but only those metals can be used for the melt that are mutually exclusive can be alloyed, and only certain metal compounds are available for the metal-salt solutions to disposal. Another disadvantage in this case is the inevitable formation of large Crystallites instead of the desired fine crystalline form.

The aim of the invention is to avoid the disadvantages mentioned and to provide a particularly uniform, homogeneous one

409 627/329

To manufacture surface layers of any kind, which are composed of Raney metals, which neither have to be alloyable with one another nor be represented in the form of complex metal salts and thereby the formation to avoid large crystallites and, if possible, the pore structure of the Raney surface layers to influence in an advantageous manner.

The solution to the problem is according to the invention that an electrically conductive base with the

vapor deposited alloy layer changes completely when the aluminum dissolves, because now that Palladium released has to adhere to the substrate again in the presence of water. Here-5 a thin, fine-pored layer in the Raney shape of the metal is created.

The alloy components can also be vapor-deposited one after the other. This has the advantage that one the substrate is vaporized at a small distance from the adjacent components of a Raney alloy and then past the evaporation sources set up the less noble alloy component can lead, as z. B. for continuous with an acid or alkali from the vapor-deposited tape vapor deposition is appropriate. But you have to Layer is detached. If necessary, the vaporized layer will subsequently heat what the vapor deposited layer to form the Raney ζ. Β. is possible through radiant heating. Through this Alloy still heated. For the production of gas electrodes, the greater part of the base would be on a wetrode if used as a base a porous significantly lower temperature will remain. Other material. on the other hand, of course, you can also use the entire

This method of manufacturing electrodes has to heat up the electrode blank. The temperature for the advantage that the Raney catalyst in very low formation must be selected in this case higher than during small amounts, but in coherent thinner 20 of the common vapor deposition, because and strongly disordered layer on the electrodes - those in different layers existing alloy surface is generated. In order to equip one of the components, they have to diffuse into one another. Electrode suitable graphite disk with Raney. The heating must be carried out in an inert gas atmosphere or in palladium if it is attached via the evaporation vacuum, so that the alloy sources in a vacuum bell. Do not oxidize one of the components,
The source of the vapor is palladium, in which the electrode materials are suitable for vapor deposition.

other z. B. aluminum. After evacuation, graphite disks, graphite fabric, graphite die will appear Graphite disc z. B. by radiant heating on felt, metallic sintered bodies, metal wire nets, etc. heated to a temperature of about 300 ° C. You can also z. B. also ceramic sintered bodies or The initially still covered porous plastics or other sources of acid or alkali vapor are used heated up. The temperature-resistant fabrics or felts will be used for this. These melt temperatures are regulated in such a way that they must be electrically metallized beforehand vaporization speeds of the aluminum can be made conductive. This can be B. also through twice as large as that of the palladium. Then vapor deposition of a suitable metal will take place, the substrate, the graphite disk, vaporized so long, 35 One can also use thin layers of tissue or until the layer thickness is about 1 μ. It is advisable to steam felting with the Raney alloy and the substrate in succession in all four closing several layers to form a porous electrode To incline directions from the horizontal, compress. In a similar way one can so that the walls of the electrodes on the surface of the electrodes are made of powdered materials, Substrate opening pores are vaporized, which is 40 by using the powder with the Raney alloy happens to a depth of 10 μ. steams and then z. B. together with an art

Depending on the resistance of the catalyst or material or metal powder pressed to create the framework, of the electrode materials, the electrodes can be made in Of the many possible, less noble alloys

An alkaline or acidic electrolyte are used as examples only aluminum. They are suitable on the one hand for the electro- 45 minium and tin. As catalyst metals chemical reduction of gaseous oxygen are z. B. silver and the metals of the eighth group and on the other hand also as fuel electrodes. These are suitable for the Periodic Table of the Elements. They can either be used as gas electrodes, e.g. B. for the catalytic activity can be increased if oxidation of hydrogen or propane, or as mixtures of catalysts or additives immersed electrodes, z. B. used for the oxidation of 50, which are not very active on their own, such as methanol dissolved in the electrolyte, are used. Gold, vanadium, molybdenum, chrome. The temperature of the substrate is still far removed from the Raney alloy by vapor deposition of all of the melting temperature of the alloy. Components of the catalyst are produced.
Nonetheless, when the joint For the electrochemical oxidation of water

Vaporization of the components of the alloy, as the 55 substance, methanol or other alcohols in one Metals as atomic beams on the surface of an alkaline electrolyte - e.g. B. in aqueous hydroxide solution - The Raney catalysts made of nickel and palladium have proven to be the best. Im sour Electrolyte, e.g. B. sulfuric acid, methanol is best made on electrodes with Raney catalysts Platinum, rhodium, iridium and ruthenium implemented. These catalysts are also suitable for the electrochemical Reduction of oxygen in acidic electrolyte, while doing so in alkaline electrolyte

dilute lye and finish the dissolution with 65 silver is best.
hot 20% lye. The platinum metals have the advantage that they

There is no floating of palladium crystallites, they are acid-resistant. For this reason can also as long as the loosening is not stormy. The an acidic electrolyte used in the fuel cell

come and as such are very reactive. The vacuum must be good or at least free of the residual gas Oxygen so that the aluminum does not oxidize and thereby disrupt the formation of the alloy.

After the vapor deposition, the electrode blank is immersed in lye to remove the aluminum from the To dissolve the alloy layer. In order for this process to proceed slowly, one starts with cold, very

will. This proves to be advantageous when methanol or a hydrocarbon on the fuel electrode is implemented; because then the resulting carbon dioxide can escape in gaseous form. In a lye, on the other hand, would dissolve the carbon dioxide and thereby the hydroxide with the formation of Consume carbonate so that the electrolyte must be renewed.

It should also be mentioned that the electrodes can also be used for the electrolysis of e.g. B. water are well usable. You do not need to vaporize porous substances, but can, for example, metal sheets use.

In the following, the method according to the invention is explained using three exemplary embodiments.

example 1

To produce an electrode with Raney silver as a catalyst for the electrochemical reduction of oxygen in the alkaline electrolyte, a porous graphite disk with a diameter of 5 cm is used as the electrode body. Aluminum is used as a less noble alloy component. Under the bell of a high-vacuum vapor deposition, which is up to a pressure of 10 ^-5 Torr evacuated, the two electrically heatable crucible, the evaporation sources for the silver and aluminum, at a distance of 5 cm side by side. Above this there is a holder at a distance of 10 cm, which can be rotated by about 10 ° in each direction. This is used to hold the graphite disc. First, however, a slide is attached in their place. The heating currents for the crucibles are now determined, with which a 1 μ thick layer can be vapor-deposited in one minute, in which the atomic ratio of silver and aluminum is about 1: 3. For this purpose, each source is vaporized one after the other and the vaporized slide is weighed each time. After the heaters have been adjusted, the substrate to be vaporized is attached to the holder, the bell is evacuated again and the heaters are switched on with the crucibles covered. The radiant heater located above the substrate is also switched on, which heats the substrate to a temperature of around 300 ° C. As soon as the crucibles are heated up, the substrate is steamed for 1 minute and inclined in all directions so that the steam jets penetrate a little into the pores.

After the vapor deposition, the electrode blank is immersed in dilute, cold lye. This dissolves the aluminum with evolution of hydrogen. Finally, to end the dissolving process heated in 5 normal lye. This way the electrode is equipped with about 1 mg of silver catalyst each Square centimeters of cross-sectional area. It is now boiled with water and dried and in a holder z. B. glued in plexiglass. The holder comes with a current drain wire as well as provided with a gas supply tube, thus through the electrode immersed in the electrolyte Oxygen or air can be pushed through.

In a corresponding manner, a porous graphite disc can also be equipped with other catalysts and thereby z. B. a fuel electrode for hydrogen or propane can be produced.

Example 2

For the production of an immersed electrode with a palladium catalyst for the oxidation of in the electrolyte dissolved methanol, thin graphite fabric is vaporized on both sides with palladium and aluminum. Proceed according to the first example. After steaming one side, it will The fabric is turned and immediately afterwards the other side is also steamed. Should the electrical Power of the evaporation system is not sufficient, the palladium with sufficient speed from one To evaporate crucible, it can also be evaporated from a tungsten filament. This can the higher temperature required for the evaporation of the palladium compared to the silver can be reached faster. After the vapor deposition, the aluminum is made from the resulting alloy dissolved out again with lye. The Raney palladium now remains in a thin layer on both sides of the Graphite fabric back. This is stretched in a frame with a current drain wire and can now immersed in the electrolyte as a methanol electrode

will. _. . , "

Example 3

For the production of an electrode with a platinum catalyst for use in a hydrogen cell with Cation exchange membrane as a quasi-solid acidic electrolyte is thin graphite felt with platinum and a less noble alloy component, e.g. B. tin, vaporized. One goes according to that first example. However, platinum can only work at sufficient speed with electron beam heating be vaporized. For this purpose, the platinum in a crucible is exposed to an electron beam shot at. With this device, platinum layers of Vaporize 1000 Å thick. The tin is again vapor-deposited onto the heated substrate at the same time, so that the alloy forms immediately. After steaming, the tin is made from the alloy with lye dissolved out, so that the Raney platinum in a very thin layer on the surface of the graphite felt remains behind. A cation exchange membrane is now placed between two such electrodes placed that the platinum layers adjoin the membrane. Finally, with two stable sieve plates the pack compressed and provided with a gas inlet on each side. The cell can now be operated with hydrogen and oxygen.

Claims (13)

Patent claims: 1. Process for the production of electrodes with layers applied to substrates Raney catalysts, preferably for electrochemical fuel cells, characterized in that that an electrically conductive pad with the components of a Raney alloy vaporized and then the less noble alloy component with an acid or Lye is dissolved out of the vapor-deposited layer. 2. The method according to claim 1, characterized in that the electrode, if necessary is heated after vapor deposition to form an alloy. 3. The method according to claim 1, characterized in that the alloy components vaporized simultaneously from separate evaporation sources onto a heated surface. 4. The method according to claim 1 to 3, characterized in that silver is used as a catalyst will. 5. The method according to claim 1 to 3, characterized in that nickel is used as a catalyst will. 6. The method according to claim 1 to 3, characterized in that one of the platinum metals as Catalyst is used. 7. The method according to claim 1 to 3, characterized in that several catalyst metals evaporated to obtain mixed catalysts. 8. The method according to claim 7, characterized by the simultaneous use of nickel and palladium as a catalyst. 9. The method according to claim 7, characterized by the simultaneous use of several Platinum metals as a catalyst. 10. The method according to claim 1 to 9, characterized in that the base of porous Material. 11. The method according to claim 10, characterized in that the surface of the porous During the steaming process is inclined at different angles to the steam jet. 12. The method according to claim 1 to 9, characterized in that the electrodes are pressed together can be made of several vapor-deposited layers or networks. 13. The method according to claim 1 to 9, characterized in that the base is powdery Material is used, which is then brought into the desired shape after steaming. Considered publications:
Austrian Patent No. 206 867;
Zeitschrift für Instrumentenkunde (1959), pp. 154 to 156. 409 627/320 6.64 © Bundesdruckerei Berlin