DE1421355A1 - Process for the manufacture of porous electrodes for electrochemical devices - Google Patents

Description

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Public limited company Brown, Boveri & Cie., Baden (Switzerland)

Process for the manufacture of porous electrodes for electrochemical devices

The invention relates to a method for producing a porous electrode for electrochemical devices by Combining a catalytically active metal with a catalytically inactive metal that is soluble in an alkali or acid Metal and then dissolving out the soluble component.

For making an electrode of this type for fuel elements a method has been proposed in which these catalytically active metals in powder form with a powder of said catalytically inactive metals are mixed and pressed and sintered to form a solid shaped body and the shaped body is then treated with alkali to dissolve out the inactive component. Such electrodes are hereinafter referred to as monoskeleton catalyst electrodes.

In this proposed process, mixing of the metal powder between the active and the inactive _np , _{G (NAL}

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Metal grains form an interfacial compound, which, as a result of the subsequent heat treatment, forms a layer on the active Can transfer metal grains located phases between the two metals. Such phase mixtures from which the inactive metal is dissolved out, lead to active metal surfaces that are particularly good catalytic and electrochemical Have properties. It turns out, however, that after this treatment the mixed phases are only slight occur so they for the catalytic properties and thus hardly any advantages for the electrochemical usability of the electrode material compared to the active metal because when the inactive metal is dissolved out of the ordered mixed phases, the active metal atoms regroup.

The invention is intended to provide a monoskeleton catalyst electrode are created whose catalytic and electrochemical properties due to multiphase layers the surface of the catalytically active metal grains are particularly advantageous.

The method of making a monoskeleton catalyst electrode for electrochemical devices according to the invention is characterized in that the catalytic active metal powder is mixed with the catalytically inactive metal powder using mechanical pressure and that the foMnköiv ^ r bet a temperau * tempered> rd, at which no glowing occurs.

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Mixing under mechanical pressure is different from conventional mixing by simply stirring, shaking, etc., for example in a cube-shaped container, in that the powder mixture is pressed at the same time during the mixing process. This happens, for example, in a rotating Vessel in which

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the powder is pressed simultaneously through a roller-like device. Here, prints are used, which cause a plastic deformation of the inactive metal grains.

A powder mixture that has been mixed under mechanical pressure will form on the surface after heat treatment of the active metal grains to a large extent multiphase layers, which, for example, when using nickel as catalytic active and aluminum as inactive metal in the composition NiAl and NIgAl. are present. After releasing the inactive metal components, in the present example aluminum, they form Layers create an extremely active surface.

Another benefit of mixing under mechanical pressure is the simultaneous tearing of a possibly existing oxide layer on the surfaces of the inactive metal grains. This significantly improves the interaction between the active and the inactive metal grains, so that after the mixing and the heat treatment, the active metal grains have the desired mixing phases over the entire surface. This advantage is particularly large with inactive nets such as aluminum, which is very easily oxidize.

Finally, there is the advantage that the sintering is the active Metal grains in the pellet to create a mechanically strong skeleton with the formation of bridges between the majority of the active metal grains even at relatively low temperatures made below the lowest melting temperature of the system

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can be. In the example of a powder mixture composed of nickel and aluminum, sintering at 460 ° C to 600 ° C Mechanically stable Ni / Ni bridges are created, which are also produced by chemical activation, the leaching of the aluminum, in contrast to pure Raney nickel cannot be destroyed, so a mechanically stable electrode body is obtained.

There is the possibility that the molded body, which is present after mixing, under mechanical pressure and seizure, will open up when heated the sintering temperature can burn up and melt together, so that Finally, a Raney alloy is disadvantageously present, which can no longer be processed into a shaped gas diffusion electrode. This disadvantageous phenomenon can easily be avoided if the shaped body is tempered before sintering at a temperature at which no annealing occurs. Tempering is more advantageous in several steps, each time increased Temperature made below the glowing temperature. On that the sintering can be carried out without disadvantageous consequences and finally the catalytically inactive metal component can be dissolved out by treatment with alkali or acid. With the already mentioned use of nickel and aluminum, it is two-fold to carry out the tempering between 350 ° C and 450 ° C.

The production of a monoskeleton catalyst electrode according to the invention will be described using an example.

B * is based on a powder mixture, yz percent by weight of nickel and 68 percent by weight of aluminum (corresponding

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accordingly each to about 50 atomic percent) exists. The grain size should between about 10 and 40 fly. The powder is during a Thoroughly mixed in a pressure mixing device for an hour. Afterwards send molded articles with a diameter of 40 mm are produced from each 1J g of the powder mixture by pressing. The pressure is 4 to / cm, with which a volume filling of about 8θ £ is achieved. Afterwards, the shaped bodies are placed on a nickel base in a quartz tube with the purest hydrogen atmosphere and for the time being for 2 hours at 400 ° C and then for 2 hours at 440 ° C annealed. The temperature is then increased to 550 ° C. and sintered at this temperature for half an hour. the Cooling takes place for 1 hour to a temperature below 50 ° C. The aluminum is now turned from the sintered bodies Activation of the inner surfaces of the electrode released. For this purpose, the sintered body is treated as follows:

24 hours with n / 10 KOH at 25 ° C, 24 hours with 2n KOH at 25 ° C, 24 hours with 2N KOH at 50 ° C, 24 hours with 6N KOH at 50 ° C and 24 hours with 6n KOH at 80 ° C.

It is useful to add tartaric acid to the potassium hydroxide solution as a complexing agent.

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Claims (3)

- -6- 145/62 Patent claims: 1 · Method of making a porous electrode for electrochemical devices, in which catalytically active Metals in powder form with powder catalytically inactive, Me that is soluble in a certain alkali or acid talle mixed, pressed into a shaped body and sintered, or hot-pressed, which is then used for Dissolving out the inactive component is treated with this alkali or acid, characterized in that the catalytically active metal powder with the catalytically inactive metal powder using mechanical Pressure is mixed and that the shaped body is tempered before sintering at a temperature at which still no glowing occurs. 2. Electrode according to claim 1, characterized in that in several steps at an elevated temperature in each case is tempered below the glowing temperature. 3. Electrode according to claim 1, characterized in that Nickel and aluminum powder are mixed under mechanical pressure and pressed into a molded body, which is used in Tempered 350 ° C to 450 ° C and sintered at 46o ° to 600 ° C, and then with an alkali solution with increasing concentration and temperature. 009841/1386