DE1272902B - Process for the production of porous oxygen electrodes with a silver catalyst - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. Cl .:

German AI .:

Number:
File number:
Registration date:
Display day:

BOIk

HOIm

12h-2
21b -14/01

P 12 72 902.1-45 (B 69204)

October 12, 1962

July 18, 1968

The invention relates to a method for producing a porous oxygen electrode with Silver catalyst, in particular for galvanic fuel cells, the electrode being made of a silver-containing one powdery compound, mixed with a pore former that can later be removed again, first prefabricated and then reduced the connection in hydrogen to the metallic silver catalyst will.

It is known to use silver as a catalyst for an oxygen electrode. Suitable for this Porous electrodes can be called Raney silver electrodes or by electrolytic deposition of silver on a porous electrode body. It is also already known to produce porous electrode plates for accumulators in that a plasticized thermoplastic resin is mixed with metal oxide powder and rolled out in the form of a band Mixture is pressed into grid plates, from which one after electrolytic reduction of the metal compound the resin components then dissolve out again using a solvent.

However, the production of electrodes with built-in Raney catalyst requires a variety of Operations and is therefore time consuming and expensive. The electrolytic silver deposition on porous Electrode bodies produced electrodes have the disadvantage that the silver catalyst located only on the outer surface of the electrode body. In the case of a gas electrode with a large catalytic Activity, however, it is necessary that the catalyst also covers the inner walls of the pores. Catalyst also covered the inner walls of the pores.

Such electrodes are also, if their electrode body is made of graphite, against mechanical ones Sensitive to stress. Also the known ones made using plasticized synthetic resin The electrode plates of the accumulators do not have sufficient porosity for fuel cells and sufficient strength only when using lattice scaffolding panels.

From the German patent specification 946 071 it is known, electrodes for accumulators with increased Establish strength by having a powder mixture before pressing and sintering Nickel powder added to the electrode blank.

In the documents of an earlier right, the German patent specification 1197 941, a method for producing electrodes for fuel elements from silver carbonate by electrochemical reduction is described. However, this method is ver-procedural for the production of porous
Oxygen electrodes with silver catalyst

Applicant:

Robert Bosch G. m. B. H.,

7000 Stuttgart 1, Breitscheidstr. 4th

Named as inventor:
AIfons Köhling, 6231 Niederhöchststadt;
Dr. Helmar Krupp, 6083 Walldorf;
Kurt Richter, Agrapatne (Ceylon);
Dr. Gerd Sandstede, 6000 Frankfurt;
Dr. Gerhard Walter, 6374 Steinbach

equally expensive, since a larger number of work steps and a higher expenditure of time are required is.

Finally, in the documents of an older patent application, the German Auslegeschrift 1174 861, a manufacturing method for fuel and air oxygen cells explained, in which from a or a mixture of several easily decomposable compounds of one or more catalytically active compounds and a solid body is pressed against alkaline solutions of resistant metals, whereupon the connections decomposed at a temperature below the melting point of the metals in question will.

In electrodes manufactured according to this method, the porosity is important for gas electrodes difficult and then variable within narrow limits.

The invention was therefore based on the object of providing a method which can be carried out as simply as possible to develop porous oxygen electrodes with silver catalyst, which have a large catalytic Have effectiveness and mechanical stability.

The object is achieved according to the invention in that the powder mixture, optionally in a manner known per se after the addition of nickel powder or fine steel filings, is compressed to a compact ^{at a pressure of about 5 to 10 t / cm 2 and that the compact is then reduced} the silver compound is ^{heated to a maximum of 200 °} C. and exposed to a stream of hydrogen gas.

As silver compounds such silver salts come into question, which already at temperatures up to about 200 ° C can be reduced to metallic silver

809 570/535

nen. These are in particular the compounds silver carbonate, silver sulfate, organic silver salts and Silver oxide.

For the catalytic effect of the silver is namely the grain size of the individual silver crystals of crucial. The temperature at which the silver salt is reduced into hydrogen becomes too high, the silver crystallites released during the reduction combine as far as they touch, to larger single crystals, which considerably reduces the attainable porosity. Will the On the other hand, if temperatures are kept sufficiently low, the silver crystallites grow together in the case of the invention Reduction with hydrogen gas only at the points of contact and thus only form the structural bridges required for power conduction and strength. This leaves the large surface area the numerous and as small as possible individual crystallites are almost completely preserved.

The well-known electrochemical reduction of the SiI over compound in dilute sulfuric acid or alkaline solution therefore does not have to be carried out and can, as well as the known reduction with chemical reducing agents, in the invention Process advantageously replaced by reduction with gaseous hydrogen will.

The purpose of increasing the porosity inside the electrode of the silver compound prior to the formation of the Pore-forming substances mixed in with compacts are dissolved out again before or after the compacts are reduced by suitable measures such as evaporation or washing out.

Electrodes produced by the method according to the invention are characterized by a large surface effect and high porosity, that is to say in particular by the fact that they only require a slight overpressure of at most 0.5 atmospheres. Almost the reversible oxygen potential was measured with such an electrode. In 66% potassium hydroxide solution, at a temperature of 160 ° C with oxygen, a potential against a hydrogen electrode in the same solution of 113OmV was established. This value is only 20 mV lower than the reversible oxygen potential at this temperature. The electrodes are so active that the oxygen is converted at them with a current density of 100 mA / cm ² with a polarization of only 150 mV. At a temperature of 70 ° C, the current density is only half lower at the same potential.

It should also be mentioned that the electrodes according to the invention are also used for water electrolysis Oxygen evolution are suitable. Furthermore, the electrode is also used for the reduction of dissolved or bound oxygen applicable, e.g. B. for the reduction of hydrogen peroxide. Because that's in the electrodes If the silver contained is in a finely divided state, it can easily be oxidized electrochemically. As a result, the electrodes can also be used as silver electrodes in primary cells.

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

example 1

65

A small excess of sodium carbonate solution is quickly added to a silver nitrate solution, and silver carbonate is thereby precipitated in very fine crystalline form. The silver carbonate is intimately mixed with 40 percent by volume of ammonium carbonate by grinding in a mill. The mixture is pressed in a pressing tool with a pressure of 5 t / cm ² to form a 2 mm thick disc. The compact is then slowly heated to a temperature of 100 ° C. in a water jet pump vacuum. After the atomic carbonate has evaporated, the yellow electrode blank is reduced in a stream of hydrogen at a temperature of 150 ° C. A white disk is obtained which still has to be inserted into an electrode holder. This serves to divert the current and enables air to be pushed through the electrode from the rear with a pressure of about 0.3 atmospheres. For working temperatures up to 100 ° C, e.g. B. Plexiglas can be used with an appropriate adhesive. The electrode can now be used in an alkaline electrolyte fuel cell.

Example 2

From a silver nitrate solution, fine-grained silver oxide is precipitated by means of a sodium hydroxide solution by quickly adding it. The dried silver oxide is mixed with 30 percent by volume of dry sodium chloride powder with a grain size of 10 to 30 μ and with 40 percent by volume of nickel powder with a grain size of 20 to 50 μ. You can also use fine steel filings. The metal powder must not be oxidized, otherwise it must first be reduced with hydrogen. A thin disc is produced from the mixture in a press tool with a pressure of ^{10 t / cm 2.} This electrode blank is then heated to a temperature of 200 ° C in a stream of hydrogen, during which the silver oxide is reduced. The sodium chloride is then dissolved out with water. After being installed in a holder, the electrode is ready for use. It also only needs to be operated with the low air pressure of 0.3 atü.

Claims (1)

Claim: A process for the production of a porous oxygen electrode with a silver catalyst, in particular for galvanic fuel cells, wherein the electrode is first prefabricated from a powdery compound containing silver, mixed with a pore former that can later be removed again, and then the compound is reduced in hydrogen to the metallic silver catalyst, characterized in that the powder mixture, to 10 t, if appropriate, in a known manner after the addition of nickel powder or fine steel filings, at a pressure of about 5 / cm is compressed ³ to a compact and that the compact is then heated to a maximum of 200 ⁰ C to reduce the silver compound and is exposed to a stream of hydrogen gas. Considered publications:
German Patent No. 946 071;
German interpretative document No. 1103 415. Legacy Patents Considered:
German Patent No. 1197 941. 809 570/535 7.68 © Bundesdruckerei Berlin