DE1421359A1 - Process for the production of porous oxygen electrodes - Google Patents

Description

Robert Bosch GmbH, Stuttgart, 2nd year 1968

Process for the production of porous oxygen electrodes

The invention relates to a method for producing porous Oxygen electrodes with silver catalyst, especially for electrochemical fuel cells, in which a porous electrically conductive electrode body with the solution of a reducible to silver Silver salt soaked, dried and the super compound at temperatures below their thermal decomposition temperature Catalyst is reduced.

It is known to electrochemically apply oxygen to a gas electrode to reduce. To produce such an electrode, for example a porous charcoal impregnated with the solutions of at least two sehwermetal salts. These are then thermally at higher Temperature decomposes to the metal oxides or metals, which together form an effective catalyst.

Silver is also known as a catalyst for electrochemical Use reduction of oxygen. A suitable electrode with Raney silver as a catalyst is obtained if For example, mixing a powdery silver-aluminum alloy as a procatalyst with carbonyl nickel powder as a framework material, an electrode is formed from the mixture by sintering and from the Alloy that dissolves aluminum. Because of the low melting temperature With the silver-aluminum alloy, carbon powder cannot be used as a framework material.

For the catalytic effect of silver, the grain size is the individual silver crystals are of vital importance. Decomposed

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if the silver salt is thermally, then the necessary for this are necessary Temperatures so high that they are released from the silver salt Silver atams, as far as they somehow touch one another, to small, well-ordered ones Find single crystals together. The formation of single crystals, however, reduces the surface area of the silver distributed in the carrier and considerably reduces the catalytic activity. To prevent For single crystal formation, one of the known electrodes therefore involved the use of at least two different heavy metals required. With the known electrode, which contains Raney silver, ' is the consideration of the temperature level during sintering of the electrode body is not required, as the aluminum is only is subsequently dissolved out of the alloy at normal temperatures and there is therefore no risk of single crystal formation.

The invention was based on the object of a method for production of oxygen electrodes with silver catalyst, especially for electrochemical fuel cells, in which the use of a second heavy metal salt can be dispensed with and the silver nevertheless in a fine distribution on the surface of the Electrode is present.

The solution to the problem consists in using a porous, electrically conductive electrode body, which with the solution of one to silver reducible silver salt soaked ^ dried and the silver compound is then reduced to the catalyst at temperatures below its thermal decomposition temperature, according to the invention therein, 'That silver carbonate deposited in the pores of the electrode body will. <; ·:>

For this purpose, the silver carbonate is dissolved in ammoniacal ammonium carbonate solution and a porous graphite disk is soaked in the heat with the solution. In order to prevent the silver carbonate from creeping out of the pores during drying, the soaked body is freeze-dried, with finely divided silver carbonate and some ammonium carbonate remaining in the pores. The graphite body is then heated ^{to a temperature of 150 ° C. in hydrogen.}

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The ammonium carbonate and the silver carbonate evaporate is reduced. Because of the low production temperature, the finely divided silver produced is a very active catalyst The porosity of the electrode body can be chosen so that the pressure under which one passes the oxygen or the air through the Let the electrode flow through it, is at most 0.5 atmospheres.

'In the case of such electrodes coated with oxygen, in 6 normal potassium hydroxide solution at a temperature of 80 ° C has a resting potential against a hydrogen electrode in the same Solution of 1080 mV measured. When exposed to a current density of 100 mA / cm, the potential drops to 880 mV and is at 200 mA / cm current density still 730 mV. The polarization is determined by the Affects the porosity of the electrode body used.

In addition to graphite, metallic electrodes can also be used as porous electrode bodies Use sintered bodies. These sintered bodies have to be made of fine powders of e.g. nickel, cobalt or steel with a high porosity getting produced. You can also use a porous plastic framework in which nickel is first deposited in order to achieve the to generate electrical conductivity.

In the following the method according to the invention is based on a Execution examples explained.

In hot ammoniacal ammonium carbonate solution, enough silver carbonate is dissolved that the solution has a concentration of about 500 g / l. A> mm thick graphite disc with 60% porosity, which has previously been boiled with moderately concentrated nitric acid, watered and dried is immersed in this solution. After pulling it out, the soaked graphite body is briefly rinsed with water and frozen for freeze-drying. The dry electrode body is then heated in hydrogen to a temperature of 150 ⁰ C to reduce the silver carbonate in the pores and to volatilize the ammonium carbonate also known. The electrode is now glued into a flat holder, for example made of Plexiglas, provided with a current discharge wire and a gas supply tube.

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The holder is shaped so that only one side of the electrode with comes into contact with the electrolyte and air or oxygen can then be forced through it. In the manner described an electrode has been obtained that only contains about 30 mg of silver each

ρ
cm cross-sectional area and which can be operated with air or oxygen at a pressure of about 0.15 atm. It is excellently suited for the use / your fuel cell with alkaline electrolyte.

Another method that results in even more active silver is to soak the dry electrode body containing silver nitrate in a sodium carbonate solution. The silver nitrate is thereby converted into silver carbonate. You can now dry the electrode body and then reduce the silver carbonate with hydrogen at a low temperature. However, it is easier to leave the electrode body in the sodium carbonate solution and to reduce the silver carbonate electrochemically. Here it is cathodically loaded with such a current density that the potential always remains above the hydrogen potential. After installing in a
Holder, the electrode is now ready for use for the electrochemical reduction of oxygen under the slight overpressure of about 0.5 atü in an alkaline electrolyte.

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Claims (5)

New demands fl.i process for the production of porous oxygen electrodes with Silver catalyst, especially for electrochemical fuel cells, in which a porous, electrically conductive electrode body with the solution of a silver salt reducible to silver soaked, dried and the silver compound then at temperatures below its thermal decomposition temperature Catalyst is reduced, characterized in that silver carbonate is deposited in the pores of the electrode body. 2. The method according to claim 1, characterized in that the The electrode body is soaked in silver nitrate, then immersed in a sodium carbonate solution and the resulting SilberkarborEt is then reduced. 3. The method according to claim 1 and 2, characterized by the use of porous graphite bodies as electrode bodies. 4. The method according to claim 1 and 2, characterized by the use of porous, metallized plastic bodies as electrode bodies. 5. The method according to claim 1 and 2, characterized by the use of porous, metallic sintered bodies as electrode bodies. 909808/0951