DE1272277B - Process for the production of porous oxygen electrodes with a silver catalyst - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY GERMAN VWWW ^ PATENT OFFICE Int. Cl .:

BOIk

EDITORIAL

HOIm German class: 12h-2 21b-14/01 Number: 1272277 File number: P 12 72 277.9-45 (B 85800) Registration date: October 12, 1962 Ausleeetetae: July 11, 1968

The invention relates to a method for producing a porous oxygen electrode with Silver catalyst, in particular for galvanic fuel cells, the electrode from a powdery silver compound, mixed with a pore former that can later be removed again, first prefabricated and then the connection, preferably electrochemically, to the metallic silver catalyst is reduced.

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 by 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 bond the resin components then dissolve out again using a solvent. After all, it belongs to the State of the art, silver electrodes for alkaline batteries by sintering nickel powder and To produce silver oxide powder and subsequent reduction of the silver oxide to metallic silver.

However, the production of electrodes with a built-in Raney catalyst requires a lot of operations and is therefore time consuming and expensive. The by electrolytic silver deposition Electrodes produced on porous electrode bodies have the disadvantage that the silver catalyst located only on the outer surface of the electrode body. With a gas electrode with high catalytic activity, it is necessary that the catalyst also the inner walls of the pores covered. 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 produced electrode plates for accumulators do not have the sufficient for fuel cells Porosity and sufficient strength only when using lattice scaffolding panels.

The above-mentioned accumulator electrodes made using nickel powder likewise do not have the porosity required for fuel cells. At the specified high Furthermore, sintering temperatures of 700 to 900 ° C result in a considerable loss of catalytic properties active surface of the silver is a practically homogeneous mixture of nickel and silver.

Method of making 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,

6700 Ludwigshafen-Rheingönheim;
Dr. Gerd Sandstede, 6000 Frankfurt;
Dr. Gerd Walter, 6374 Steinbach

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 mixture of the pore former with the silver compound, the decomposition temperature of which is higher than the intended sintering temperature, is mixed with a sinterable, pulverized metal compound or, in a manner known per se, with a sinterable, pulverized metal and with a pressure from about 3 to 5 t / cm ^{2 is} pressed together to form a compact, the compact is sintered and only then is the silver compound reduced.

The sintering of powder mixtures for the production of electrodes for fuel cells is so far only in connection with so-called Raney alloys made of catalytically active metals known with ineffective metals such as aluminum. In the present case, however, the sintering fulfills one other purpose:

You can namely even without highly porous electrodes for fuel cells from powder compacts Produce sintering process in hot hydrogen gas from silver compounds or those silver salts that can be reduced to metallic silver at temperatures of up to around 200 ° C. In contrast to such silver compounds as silver carbonate, silver sulfate, organic silver salts and silver oxide, However, there is the following disadvantage with other silver compounds:

809 569/542

3 4

The evolution of oxygen is suitable for the catalytic effect of silver. Furthermore, the Grain size of the individual silver crystallites of ent- electrode also for the reduction of dissolved or crucial importance. If the temperature is applicable to bound oxygen, e.g. B. for the Reder The silver salt can be reduced to hydrogen by means of hydrogen peroxide. Since that is in the can, too high, the silver contained in the Reduk- 5 electrodes combine in finely divided addition released silver crystallites, insofar as they exist, can easily be electrochemically stir, to form larger single crystals, which oxidize. As a result, the electrodes achievable porosity considerably reduced. At tem- perature also as silver electrodes in primary cells from more than 200 ° C the SiI turns grow together.

overcrystallites not only at the points of contact, io In the following the method according to the invention form with a considerable loss of effectiveness on the basis of two exemplary embodiments Correspondingly larger crystallites on the surface. purifies.

Silver compounds of this type, such as the silver example 1

halides and silver phosphate, which are only

Allow higher temperatures to be reduced in hydrogen, 15 From a silver nitrate solution, according to the invention, silver phosphate is formed by means of sodium before the phosphate solution is rapidly added to form the compact with a sinterable, pulpate in extremely fine-grained form. The dried silver-verified metal or a metal compound phosphate is mixed with 40 percent by volume of Monel metal. The compact is first sintered and first powder, a copper-nickel alloy, the grain then the silver compound is reduced. The size 20 to 40 μΐη and 30 percent by volume of ammonia in the sintered metal powder prevents carbonate of grain size smaller than 20 μΐη so that the rubbing resulting from the reduction is intimately mixed. The mixture is closed in metallic silver to form larger crystallites using a pressing tool with a pressure of 3 t / cm ² and the catalytic effectiveness is thus pressed into a thin disk. The compact is lowered. 25 then slowly heated in an inert gas atmosphere,

Such scaffolding metal powder, which when pressed so that the ammonium carbonate without splitting does not yet adhere firmly to one another, the pressed part oxidizes and evaporates. Then the one beforehand superficially and then takes the sintering compact in the inert gas atmosphere at one time Hydrogen before. The metal oxide is sintered at a temperature of around 700 ° C. Hydrogen is allowed which reduces, and the crystallizing metal must not be present, as otherwise the silver phosphate will bind each other. The silver halides would be reduced.

Nickel is particularly suitable as a framework metal powder. The electrode blank is now thinned into a

in the case of silver phosphate, a sodium hydroxide solution is preferably used dipped and as the cathode Copper-nickel alloy. switched into the circuit. The electricity to reduce

The known electrochemical reduction 35 tion of the silver phosphate is regulated so that The silver compound can then be prevented from developing either. "After finishing thinner Sulfuric acid as well as a dilute alkali of the reduction is the electrode in one carry out a lischer solution. The reduction must holder, z. B. made of plexiglass, glued in that of they do not necessarily have to be carried out electrochemically; the reverse side does not bind oxygen or air through them; you can the silver connection in the electro 40 can be pushed through. The keeper is out The blank also contains chemical reducing agents for gas supply and for current dissipation aqueous solution or with gaseous water-directed. The electrode can now be placed in a fuel to reduce. Substance cell with alkaline electrolyte used

Which are used to increase the porosity inside the.

Silver Compound Electrode Before Forming 45 Example 2

pore-forming substances mixed in with the compact

are before or after the sintering of the compact In another embodiment of the method

If removed again by suitable measures, silver iodide is assumed. This will be off like evaporation or washing out. a silver nitrate solution by quickly precipitating with

Potassium iodide solution obtained as a fine powder by the method according to the invention. The presented electrodes are characterized by large upper silver iodide with 45 volume percent nickel powder surface effect and high porosity, thus the grain size 20 to 40 μm, which is characterized by the fact that they heat only a slight overpressure in air with an oxide layer at most Need 0.5 atm. With such a den ist, and 25 volume percent finely powdered Na electrode, almost the reversible oxygen trium chloride with a grain size of 20 to 40 μm was mixed, potential measured. In 66% potassium hydroxide, a disk-shaped compact is produced with oxygen in a pressing tool with solution at a temperature of 160 ° C. 5 t / cm ^{2 of pressure.} This is then converted into hydrogen at an electrode in the same solution of 1130 mV. Annealed temperature of about 520 ° C. This value is only 20 mV smaller than the reversible 60 reduced nickel oxide, and the nickel grains sinsible oxygen potential at this temperature. The tern together this way. The electrodes are then so active that the acidic sodium chloride is dissolved out of the electrode material with water at a current density of 100 mA / cm ² in the case of the blank. This is then converted into a dilute polarization of only 150 mV. ter sulfuric acid cathodically charged with current, where-At a temperature of 70 ° C, the current density is 65 when the silver iodide is reduced. After installation at the same potential, only half the size. The electrode for the electrical

It should also be mentioned that the electrodes after the chemical reduction of oxygen under the geErfindung also ready for operation for water electrolysis to a slight overpressure of about 0.3 atü.

Claims (3)

Patent claims: 1. 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 silver compound mixed with a pore former that can later be removed again and then the compound is reduced, preferably electrochemically, to the metallic silver catalyst, characterized in that the mixture of the pore-forming agent with the silver compound, the decomposition temperature of which is higher than the intended sintering temperature, with a sinterable, powdered metal compound or with a sinterable, powdered metal added in a known manner, and with a pressure of about 3 to 5 t / cm ² is pressed together to form a compact, the compact is sintered and only then is the silver compound reduced. 2. The method according to claim 1, characterized in that a powdered nickel-copper alloy is used as a sinterable metal. 3. The method according to claim 1 and 2, characterized in that a silver halide is used as the silver salt is used. Considered publications:
German Patent No. 1 103 415;
German interpretative document No. 946 071.