DE1952915A1 - Porous electrodes for gas cells - Google Patents

Abstract

Powdered catalyst material e.g. Raney nickel or like metal-aluminium catalyst, is deposited electrophoretically on a mesh, of mesh size between 500 and 5 mu and bonded by a metal deposit applied with(out) the use od electrolysis but not for so long that the catalyst particles are completely covered. Nickel-chrom steel, pre-nickeled may be used for the mesh. The catalyst particle size is less than the mesh size and it may be deposited as a number of layers of different particle sizes.

Description

Process for the production of porous electrodes for gas elements The invention relates to a method for producing porous electrodes for gas elements and especially on the way the porous body interacts with one Catalyst is connected.

The negative electrodes for gas elements are mostly used in the prepared in the manner described below.

In a suitable form, carbonyl nickel powder and a catalyst powder, for example Raney nickel, filled in approximately equal parts, pressed and in Sintered hydrogen at 800 ° C. Similarly, the positive electrodes are also made Made of silver powder. Very high pressures are used to press the electrodes required (30 kg / mm2). To compensate for the low strength of the pellets the thickness of the pellets can be chosen to be quite large. But this requires a high Weight and thus a high price of the electrodes because of their manufacture that expensive metals, nickel or silver, must be chosen. Because of the low mechanical Strength of the electrodes, these can only be produced in small dimensions, once, because otherwise the expenses for the required presses would be too high, then ar, he also because of the stress on the electrodes due to the unidirectional gas pressure grows in the fuel element in proportion to the square of the diameter of the electrode.

These circumstances make the construction of large area electrodes and thus the development of larger fuel cell units difficult.

It is now to save on expensive, metallic materials already known (US Pat. No. 3,351,492) instead of sintered electrode compacts particularly thin, with a large number of fine holes due to special measures Different sieves and foils can be used.

Catalytically körinen readily on these porous sieves or foils active substances such as platinum, palladium or silver are discharged. It was assumed that the diameter of the holes in the foils for the Maximum allowable gas pressures would be the same. You are determined by that Equation p = 2 # 2 r where p is the maximum usable gas pressure up to the passage of the gas through the electrodes, = the surface tension of the electrolyte and r = the radius of the capillaries. When using such thin, porous sieves or However, difficulties arise due to the low strength of the 29 screens on and also because the catalysts, for example black or platinum Palladium black, on the sieves or on the foils only with low adhesive strength be liable.

The invention; deals with the task of using porous electrodes on metal wire meshes, Seven or finely perforated foils of firmly applied, powdery catalyst, especially Raney nickel, with improved strength, so that the The catalyst present in powder form is no longer wiped off the porous substrate can be, but the required porosity is retained.

In the following the simplification '; half of the Elelztrode basic body only called "sieves".

According to the invention, a method is proposed for this purpose, which consists of several procedural steps.

The first stage of the process is to add the catalyst particles electrophoretically deposited on the screen to be coated and through a suitable Firmly connect the process to the sieve.

The mesh size of the sieves is generally between 500M and 5M lie. The sieves with larger mesh sizes have the advantage that they are substantial are cheaper than screens with smaller mesh sizes.

The electrophoretic coating of sieves with metal powder can but can also be done with powders whose grain size is less than the mesh size of the sieve lies; For example, a Raney nickel steel wire screen can reduce the grain size of 75 µ to a Chromium-nickel steel wire screen with a width of 200 meters in the case of the invention Procedures are applied.

The adhesive strength of the applied metal powder particles on the However, the lower sieve is then not sufficient.

In the further process stage, therefore, the electrophoresis coated screens subjected to a treatment to increase the adhesive strength. According to the invention, the catalyst particles applied to the sieves are under Use of a metal which is electroless or from a solution of this metal is deposited with electricity, firmly connected to the sieve. The powder layer is then so firmly attached to the substrate and to the metal deposited in the pores, that even with very strong mechanical stress no separation the shift can be brought about. A galvanic nickel bath is preferred used. It is also useful to nickel-plate the screen before coating. In some cases, a bath can be used with advantage, which is a wetting agent contains.

After switching on the current, the powder layer shows which directly adjacent to the electrolyte, no or no significant metal deposition instead of; the first metal deposition takes place almost exclusively on the metallic carrier, i.e. on the sieve, and proceeding therefrom in dc-n pores of the powder layer instead of dignity al '. after the pores in the metal powder are filled with deposited nickel are to be electroplated further, a coherent one that excludes the porosity would result Spread the metal layer on the powder layer. The electroplating must therefore at the latest be terminated after filling the pores in the coating powder.

After this solid anchoring of the catalyst, for example. of Raney nickel, the aluminum contained in the Raney nickel is then carried out in the usual way by means of potassium hydroxide solution detached.

This creates a fine-pored electrode that has porosity in the is essentially based on the pores resulting from the dissolution of the aluminum. Instead of Raney nickel, metal-aluminum catalysts of a similar type can also be used be applied to the sieves, for example those that replace the nickel Contains cobalt, copper or iron. To prevent the between the pores located space almost entirely or completely with the electrodeposited metal felt and thus cut off the gas supply to the grains of the catalyst metal this can be done before the electrophoretic application of the powder particles a cover layer of inorganic or organic nature can be provided. Will the #o pre-treated powder particles in the ### way described above electrophoretically applied to the screen surface and those present between the particles Interstices filled with the electrodeposited metal are solid Connection established between the sieve as a carrier and the powder particles. In such Trap leaves the area surrounding the catalyst with a suitable solvent Remove the top layer or, in the case of evaporable top layers made of paraffin, for example Eliminate heating or burning. That flowing towards the fuel cell electrode Gas can then through the resulting from the dissolution of the cover layers Gaps are brought up to the catalyst grain surface and there the electrochemical bring about the desired conversions.

While with the sintered powder electrodes the reactions between gas, The main electrolyte and catalyst are located in the pores of the sintered electrode play, the reaction occurs to a large extent in the case of electrodes produced according to the invention in the sponge-like chambers of the catalyst, the aluminum is extracted from the grid became.

Because the mesh of the wire screen is taken up by the catalyst grains is for the highest permissible gas.

print the packing of the catalyst grains and the filling: the pore spaces with deposited nickel is decisive.

In the manner according to the invention, layers of Separate catalyst grains of different grain sizes. That's for the manufacture of double-layer electrodes.

Such double-layer electrodes are often used in fuel cells today. They have a coarser structure on the gas side and a finer structure on the electrolyte side, on. They are so far made of coarser and layered on top finer Carbon or metal powders, with the addition of appropriate catalysts. by Pressed and manufactured internally.

If the gas pressure in the cell is set so that .erccle no gas through the fine, electrolyte-sided gaps: ì pores can escape, so will the boundary layer -le'-trolyte / gas on the boundary between the finer ullci -ro.oeren Adjust powder particles. In this way there is greater activity of the electrodes achieved.

According to the invention, one can be made up of coarser and finer powder particles A built-up layer on the sieve by first electrophoretically coarser and then finer powder on the sieve serving as a base applies the coated in this way The sieve is then placed in a Dalvan bath and the galvanic deposition of the for the connection with the sieve base serving metal in the pores of the powder particles undertakes. But you can also first electrophoretically apply the coarser powder to the Apply the base and galvanically connect it to the base and then that finer powders electrophoretically onto the coarser powder layer in the same way Apply around the two layers, one below the other and with the sieve underlay by the galvanic deposition of the material used for the connection with the base To connect metal.

The sieves are made from acid-resistant chrome-nickel steel wire used, you can, if necessary, the applied Raney nickel including the on the sieve wires previously deposited nickel layers, for example in Remove nitric acid and then recoat the sieves. That is particular important if as a result of poisoning or as a result of relocation reactions at higher levels Temperatures the effect of the catalyst wears off.

Using an example, the coating of a screen with a Mesh size of 200μ described with Raney nickel powder of Krom size of about 75μ will: A sieve made of chrome-nickel steel wire with a mesh size of 200µ and a wire thickness of 130µ is given for 30 seconds at a current density of 1 / A dm2 in a bath consisting of 240 g / l, nickel chloride and 45 ml / l hydrochloric acid (D = 1.19) pre-nickel-plated and then in an electrolyte containing 296 g / l nickel sulfate, 40 g / l boric acid, 10 g / l nickel chloride, 4 g / l potassium acetate, 0.5 g / l wetting agent (Sinpor DeWeKa Doniplat of the applicant), as well as 0.25 rr / l of a gloss additive (E-solution DeWeKa Applicant's Duoplat) and after-treated for 5 minutes at a current density of 2 A / dm2. The sieve, provided with a thin layer of nickel, was then immersed in methyl alcohol introduced containing 1 mmol of nickel chloride. Also were in the alcohol 100: Raney nickel powder with a bulk density of approx. 1.0 kg / dm3 and a grain size of 90% smaller than 60 µ and 10% smaller than 95 µ. The mixing the powder with the alcohol was done with a magnetic stirrer.

The electrolytic coating then took place in this suspension taken with the screen forming the cathode. Al anode served a nickel sheet, whose distance from the cathode was about 50 mm.

A voltage of 300 volts was applied to the two electrodes. The coating lasted 4 minutes. After the dryer: the applied layer it was sprinkled with a @@ igen alcoholic shellac solution and so solidified, that during electroplating as a result of the, albeit mostly small, hydrogen formation do not dissolve any powder particles. The sieve was then opened after the drying introduced a thin layer of lacquer into the nickel bath already mentioned above. The nickel plating takes place at a current density of 2 A / dm2. The treatment time was 30 minutes.

As stated above, the nickel practically did not separate on the metal powder, but in the pores and on the surface. I had it like that If the layer applied to the base was not yet thick enough, werm necessary, the whole process of coating and metal deposition is repeated, before the coated screen is reinserted, the thin layer of lacquer is sprayed on was removed in a paint solvent. After sufficient filling of the pores was made the coated sieve in 6n KOH-Lau ': e of the aluminum present in the Raney nickel freed When the electrode was tested, the maximum pore size was found of too µ a sufficient pore sity0 claims

Claims (10)

Claims: 1. A method for producing porous electrodes for gas elements, in which a metal screen or the like with a catalyst is firmly connected, characterized in that the catalyst particles on the The sieve to be coated is deposited electrophoretically and then the catalyst particles be firmly connected to the sieve using a metal, which consists of a Solution of this metal is deposited electrolessly or with electricity, and the metal deposition is terminated before the powder layer of the deposited metal from the solution is covered. 2. The method according to claim 1, characterized in that a sieve with a mesh size between 500 µ and 5 µ is used. 3. The method according to claim 1 or 2, characterized in that the sieve is coated with catalizer powders, the grain size of which is below the mesh size of the sieve. 4. The method according to claim 1, characterized in that powder with different grain sizes are applied in layers one after the other. 5. Ancestors nacri one of claims 1-4, characterized in that that the deposit is used to firmly anchor the catalyst on the sieve Metal is made from an electrolyte that disintegrates an amount of a wetting agent contains. 6. The method according to any one of the preceding claims, characterized in, that Raney nickel is used as the catalyst powder and that it contains Aluminum dissolved in a known manner after the catalyst was firmly anchored will. 7. The method according to claim 6, characterized in that instead Raney nickel metal-aluminum catalysts of a similar type are used, for example those that. contain cobalt, copper or iron instead of nickel. S '. Method according to one of the preceding claims, characterized in that that when using a sieve made of passive metals, e.g. chrome-nickel steel, pre-nickel plating is carried out before coating with catalyst powder. 9. The method according to any one of claims 1 -, characterized in that that the powder particles to be applied electrophoretically with a top layer of an inorganic or organic nature that can be coated after attaching Remove on deifl sieve by solvent or evaporation or incineration leaves. 10. The method according to claim 9, characterized in that as a cover layer Paraffin is used.