DE1546718A1 - Gas diffusion electrode with an electrically insulating intermediate layer - Google Patents

Description

Gas diffusion electrode with an electrically insulating intermediate layer.

There are for the electrochemical dissolution and deposition of Gases Gas diffusion electrodes have become known which consist of an electrically conductive and catalytically active porous working layer on the side facing the electrolyte a catalytically inactive porous cover layer is applied.

If this cover layer consists, for example, of a metal that has a high overvoltage for the gas to be separated, then it is possible these electrodes when used as deposition electrodes in an electrolyzer to deliver the gases formed under excess pressure on the side facing away from the electrolyte, so that a such an electrode can be referred to as a "gas valve electrode" can * The deposition under excess pressure is possible because the capillary pressure of the electrolyte in the pores of the top layer is greater than the capillary pressure in the pores of the working layer.

The use of a porous cover layer made of metal, however, brings Disadvantages, because the potential difference between this layer and the electrolyte due to the electrochemically active Working shift is determined. As a result, it cannot be completely avoided that when the electrode is loaded, the cover layer is also affected electrochemical reactions take place. When used as a deposition electrode, a certain part of the gas formed is always used also deposited on the top layer and escapes into the electrical space. In addition, with anodic load, such a strong

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Oxidation of the cover material occur that the pores of this Layer become clogged by the oxide layers formed and the transport of electricity into the working layer is no longer possible.

The object of the invention is to avoid the disadvantages mentioned such gas diffusion electrodes by introducing a porous, electrically non-conductive layer, hereinafter "intermediate layer" called, between the top layer, which now serves primarily as a pressure layer, and the working layer.

If the capillary pressure in the pores of this intermediate layer is greater than the capillary pressure in the pores of the working layer, then - as with the gas valve electrode mentioned above - during electrolysis, the gases produced under excess pressure in the gas space behind the electrode. In addition to avoiding the disadvantages already mentioned, the electrode according to the invention is also used nor the distribution of being able to quantitatively deposit either hydrogen or oxygen on the same electrode, since the material no longer has to meet the condition for the pressure switch, have a high overvoltage compared to the respective gas. Since the contact pressure is usually no longer electrochemical must be effective, it can, apart from special cases, consist of any material, but that of the electrolyte must be wetted and must not be attacked by him or only very insignificantly. The Andrückschieh only needs to be like that have great mechanical stability so that the load on the intermediate layer is absorbed when the gases are deposited under excess pressure and thus the intermediate layer is prevented from lifting off. Because of this one is preferably required mechanical Stability choose a metallic material for the pressure layer, for example as a sintered layer, perforated disc or sieve can be formed.

Of course, a metallic pressure layer can also be used give a potential different from the work shift and thus the resistance for the transport of electricity and thus the electrochemical Influence the properties of the electrode.

By installing suitable catalysts in the work shift, in particular of Raney and finely divided precious metals

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reduce the electrochemical losses considerably. This is particularly the case when the working layer material is the Electrode at or near the respective reversible gas potential is working. This can be done for the hydrogen electrode, e.g. by incorporating Raney nickel into the work shift. Other catalysts for the hydrogen electrode are above all the noble metals of the 8th subgroup of the periodic system, like palladium and fiatin, also in their Raney form. The best so far known catalyst for the dissolution of oxygen is silver or Raney silver. For the separation of oxygen with low overvoltage is nickel or Raney nickel. Therefore, the catalytically active material is advantageously the Working shift of an electrode on which both hydrogen and Oxygen can also be separated out if alkaline is used Electrolytes made from Raney nickel or a mixture of Raney nickel and silver. Separation electrodes for the elements of the halogen group are known from electrolysis technology. For example, chlorine is used as a catalytically active material porous carbon or platinum in question.

The electrode according to the invention can be produced in such a way that that the zuB structure of the working layer, intermediate layer and Preformed or powdered materials provided for the pressure layer, if necessary with the addition of pore-forming substances, introduced into the press die in the required order and be pressed; then the compact can be sintered. The solidification of powders can also be achieved by hot pressing happen.

Instead of powdery material for the layers can be porous Membranes, in particular asbestos membranes for the intermediate layer and / or prefabricated porous pressure layers, e.g. screens pressed in the same way.

Example:

8 g of a mixture of carbonyl nickel powder with a grain diameter * knife of 5 / U and KCl powder with a grain diameter of 200, u as a pore-forming agent in a weight ratio of 3: 1, the top layer was filled into a die with a diameter of 40 mm. Then a

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Asbestos membrane with a thickness of 0.52 mm and pore diameters of less than 2 / rev is placed and a further 8 g of the same powder mixture is poured into the die as a working layer. These three layers were then pressed together at 4-00 ° C. with 9 tons of pressure. Was obtained a round 3 _t 6 mm thick electrode. The bond between the pressure and working layer and the intermediate layer is so strong that the layers can only be separated from one another by force. The electrode was also provided with a border made of plexigum; this fixes the intimate contact of the pressure and working layer with the intermediate layer.

This electrode was operated as a deposition electrode for oxygen under positive pressure. Fig. 1 shows schematically the arrangement, 2 shows the dependence of the potential on the current density, measured against a saturated calomel electrode.

In Fig. 1, the reference number 1 denotes the electrolyte-resistant, the container is insensitive to the selected overpressure and has two outlet openings for oxygen gas 8 and hydrogen gas 9 has und.die enclosed by the frame 3 made of insulating material Oxygen separation electrode, consisting of pressure layer 5 »electrically insulating fine-pored intermediate layer 6 and coarser, porous, catalytically active and electrically conductive working layer 7fderart takes up that one of the oxygen gas space 2 completely separate electrolyte space 4- is present, the hydrogen on the wall opposite the oxygen separation electrode from the separation electrode 10 picks up.

The layer thicknesses shown in FIG. 1 are by no means true to scale. The pressure layer is only a mechanical support for the intermediate layer, which is only in close contact with the work shift must stand. The pore diameter of the pressure layer is therefore irrelevant. It is more important that the largest possible areas of the intermediate layer are not covered by the pressure layer will. All ionic current threads that end in the working layer also pass through the pressure and intermediate layers. To the The voltage drop that occurs when the current flows, as small as possible keep these two layers as thin as possible.

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But here are due to the requirement for mechanical stability Limits set; the required minimum thickness is approximately in the order of 1 mm. For the work shift is also a minimum thickness is required, since here too a certain mechanical There must be stability and, in addition, the current has a certain penetration depth into the electrode. This depends on the catalytic Activity of the electrode and is between 0.1 and 1 mm. The work shift can, however, have no effect on the electrochemical one Properties as well as the pressure layer are made much stronger when it comes to large-area electrodes.

In addition to asbestos, all non-conductive oxides (eg, Bo Al ₂ O, and MgO) or sparingly soluble salts (eg BaSO ^) that are not attacked by the electrolyte are suitable as material for the electrically insulating intermediate layer. The non-conductive oxides can also be produced by themselves by means of the known flame spraying process or applied to the working or pressure layer.

The use of electrically insulating plastic layers is also possible if one can accept certain disadvantages. So it is hardly possible to use the electrodes in one operation to manufacture; the sintering of the plastic layer must be in the Usually after the work shift has been completed, as the working temperature required is normally below that during production the temperature applied during the work shift. It should also be noted that the capillary pressure in the pores except the pore radius also depends on the specific wetting angle of the electrolyte. Because plastics compared with metals or metal oxides are poorly wettable, an electrode with a plastic layer can only be used with relatively little pressure otherwise the electrolyte will be squeezed out of the pores .

Plastics are also used as the material for the pressure layer their mostly inadequate mechanical stability hardly taken into account . They generally have a low softening point; the temperature of the electrolyte may therefore have a certain value do not exceed, do not disturb the electrode. '· Continue it is often desirable to press the electrode into a plastic frame after manufacture using pressure and heat,

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which encloses the electrode in a gas-tight manner. This is also because of the temperature sensitivity of a plastic electrode layer to be realized only under special circumstances.

If the pressure layer also consists of electrochemically active material, the gas diffusion electrode can also be used as a complete electrochemical cell use; it has been observed that cells of this type are very well suited for pressure electrolysis suitable.

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Claims (5)

Seκ.-Nr .. BSF 26a * patent claims 1. J Gas diffusion electrode for electrochemical dissolution or separation of gases, characterized by an electrochemical active porous working layer, an inactive porous pressure layer and a porous, electrically insulating intermediate layer between pressure and working layer, the mean pore radius of which is smaller than the mean pore diameter of the working layer, with the pressure and intermediate layer cover the work shift in such a way that all ion current filaments ending in the work shift and penetrate the intermediate layer. 2. Gas diffusion electrode according to claim 1, characterized in that that the pressure layer is preferably made of porous or perforated metal. ?, Gas diffusion electrode according to claim 1, characterized in that, that the electrically insulating intermediate layer consists of lyophilic material. 4. Gas diffusion electrode according to claims 1 to 5, characterized characterized in that the working layer contains Raney nickel as the catalytically active material. 5. gas diffusion electrode according to claims 1 to 4-, characterized characterized in that the ratio of the cosine of the wetting angle of the electrolyte to the mean pore radius for the intermediate layer is larger, preferably twice as large, than for the work shift is. BAD ORlOMNAl. . _A ", L e r s e i t e