DE1921157B2 - Porous electrode for the separation and dissolution of gases in electrochemical cells - Google Patents

Description

The invention relates to an electrode for the deposition and dissolution of gases in electrochemical Cells with liquid electrolytes with two porous, electronically conductive layers and a process for operating a cell with such an electrode.

Electrochemical cells containing such electrodes can not only be used to generate electrical Energy, but also for electrolysis and for storing electrical energy. The electrodes can thus be used in metal / air cells and in gas accumulators, in which the functions of the Electrolyser and the power-supplying cell are combined in one unit, can be used.

From DE-AS 12 00 903 a method for bo Storage of electrical energy through pressure electrolysis of water, separate collection and Saving and, if necessary, electrochemical reunification of the electrolysis gases is known at the gas diffusion electrodes with a double skeleton catalyst structure are used. The oxygen electrode, which is used to separate and dissolve oxygen, has carbonyl nickel and Raney nickel and

Raney silver on.

The AT-PS 2 65 388, which also relates to electrochemical cells in which a water _{decomposer is combined with an H 2} / Or fuel element, can be seen, however, that with electrodes of the type mentioned the silver - despite the low oxygen overvoltage at the nickel during the oxygen separation - is dissolved in the form of silver oxide in the alkaline electrolyte, which makes the oxygen electrodes unusable over time. The oxyhydrogen cell known from this Austrian patent for storing electrical energy through water electrolysis, storage and subsequent recombination of the electrolysis gases with recovery of electrical energy therefore consists of a double-sided hydrogen electrode with alternately loaded oxygen electrodes as counter-electrodes, one of which contains nickel and is operated as an oxygen anode (deposition), while the other contains silver and serves exclusively as an oxygen cathode (dissolution)

In DE-OS 15 96 230 is a cell for storage electrical energy through electrolysis of water and recovery of the same through electricity-supplying recombination of that obtained during electrolysis Proposed hydrogen and oxygen, a valve electrode for hydrogen separation or dissolution, a gas diffusion electrode for dissolving the Oxygen and an ion-permeable oxygen separation electrode arranged in the intermediate electrolyte space. A disadvantage of this embodiment is seen in the fact that the Volume and weight of the electrochemical cells due to the introduction of the auxiliary electrodes and i.e. the additional electrolyte space that becomes necessary is increased. Also in this case the voltage drop in the cell is undesirably increased.

It is also already known in electrochemical Cells to use gas diffusion electrodes that can be loaded both anodically and cathodically (DE-PS 12 41 812). Such gas diffusion electrodes exist from a catalytically active porous working layer and a catalytically connected firmly to this inactive cover layer, the working layer having pores whose mean diameter is greater than the the top layer, and works as a reversible gas electrode. The sum of the overvoltages of the respective gas on the top layer is greater than that Sum of the overvoltages in the active work shift. On such electrodes, the gas separation and dissolution takes place only in the catalytically active working layer. Since the pores in the top layer are smaller than those in the working layer, the gases formed during gas separation cannot flow back into the electrolyte and are in stored under pressure in the adjoining gas spaces. However, such an electrode system works not optimal in both reaction directions, as the gases formed are present during electrolysis or recombination Electrode structure and catalyst each have different requirements. Also can during the course of the electrochemical reaction of the catalyst or the support material, which for one Direction of reaction are advantageous, then corrode if they are associated with the counter-reaction Electrode potential are brought. This will determine the range of usable materials and

Catalysts crucially restricted

The object of the invention is to provide an electrode of the type mentioned at the beginning with two porous, electronic To find conductive layers that do not have the aforementioned disadvantages and in with liquid electrolytes operated electrochemical cells, in particular metal / air cells, have a long service life, and a method for operating a cell with such an electrode.

This is achieved according to the invention in that the to electrolyte-side layer is hydrophilic and at least partially made of a catalyst for gas separation consists that the gas-side layer is either hydrophobic or hydrophilic and at least partially of one There is a catalyst for the gas dissolution and that, in the case of a hydrophilic gas-side layer, the pore radius in the gas-side layer is larger than the pore radius in the electrolyte-side layer

In the method for operating an.- Such an electrode is in the presence of a hydrophilic gas-side layer in this layer set a higher pressure than in the gas separation Gas dissolution.

In contrast to the known valve electrode, in the electrode according to the invention there is resolution and Separation of the gases in separate layers instead of during the gas dissolution is only the gas side Layer involved in the electrochemical process, only it contains what is necessary for the gas dissolution to occur Catalyst, for example carbon powder, graphite, metals of the 8th subgroup, polyphthalocyanines, silver and metal oxides, the metallic, oxidic and organic catalysts also reacting to an inactive or only a little active carrier material, such as carbonyl nickel, can be applied. There can also be two or several of the catalysts known per se can be used in a mixture. For an electrode that is used at the charge is anodically loaded and cathodically loaded during the discharge, combinations of carbon with Silver, charcoal with nickel and cobalt oxide, charcoal with cobalt and aluminum oxide, and charcoal with manganese oxide well proven The layer intended for gas dissolution can also contain a hydrophobic binder, such as polytetrafluoroethylene or polyethylene.

The catalysts and the carrier material must be in 4 be corrosion-resistant to the potential that occurs during gas dissolution, but not to the Potential of the electrode reaction taking place in the opposite direction.

The layer adjacent to the electrolyte does not take part in the gas dissolution process, it contains the for the gas separation effective catalyst, for example graphite or nickel powder, and activated - im In contrast to the top layer of known valve electrodes - gas separation. The electrolyte-side layer can contain several catalysts like the gas-side layer, the catalyst can also be on one Be applied carrier material. Support and catalyst must be at the potential for gas separation be corrosion resistant. Since this requirement is not addressed to bo the layer of the electrode which catalyzes the gas dissolution process is provided, the area for the Gas dissolving layer in question catalysts and support materials expanded significantly. the This is because pores of the gas-side layer are completely or partially with the reaction gas during the gas separation filled so that the gas-side layer is not involved in the gas separation. When operating the As already mentioned, the gas pressure in the gas-side electrode turned out to be an advantage Layer set higher during the gas deposition than during the gas dissolution and so the To move the electrolyte / gas phase boundary in the direction of the electrolyte side.

Another advantage of the invention is seen in the fact that the gas separation at the to Electrolyte adjacent surface of the catalyst-containing layer takes place mechanical stability of the electrode compared to known two-layer electrodes, in which gas bubble formation and a vigorous gas flow within the porous structure of the work layer occurs substantially improved

In a particularly favorable embodiment of the electrode according to the invention, the gas-side layer of the electrode is less wettable by the electrolyte than the electrolyte-side layer. This means that the pore radius in the gas-side layer is smaller than the pore radius in the electrolyte-side layer however, it must also be the same as that in the layer on the electrolyte side. However, if both layers are hydrophilic, the pore radius in the gas-side layer must, as already mentioned, be larger than that in the electrolyte-side layer

The invention is to be explained in more detail on the basis of exemplary embodiments.

In an electrochemical cell where the reaction

H ₂ + Cl ₂

2HCI

to run alternately in both directions, it is extremely difficult to provide an electrode that is anodically loaded during charging and cathodically loaded during discharging, since an electrode that is stable during chlorine deposition is not suitable for cathodic chlorine dissolution. Conversely, an electrode that is active for the chlorine dissolution is destroyed in most cases during the chlorine separation. As has now been shown, the cited reaction can take place over a very long period of time in both directions without difficulties if an electrode according to the invention is used as the chlorine electrode. The layer on the gas side, which is active for chlorine dissolution, consists in this case of a carbon layer activated with platinum and hydrophobized with polytetrafluoroethylene, and the layer on the electrolyte side, which is active for chlorine separation, consists of porous graphite. The preparation of the electrode takes place in such a manner that a 0.3 mm thick porous graphite layer with a pore diameter <5μπι with a mixture of occupied with 2 wt .-% of platinum activated carbon and 25 wt .-% of polytetrafluoroethylene after drying at 300 ⁰ C connected. A known noble metal electrode, which consists of a porous carbon layer coated with platinum black, is used as the hydrogen separation or hydrogen dissolution electrode.

When combining a water decomposer with an H2 / O2 fuel element, the hydrogen electrode a known valve electrode is used. The cover layer of the valve electrode consists of one sintered, fine-pored, 0.5 mm thick copper layer and the working layer made of bonded with carbonyl nickel Raney nickel. An electrode is used as the oxygen electrode, which consists of a hydrophilic.

fine-pored nickel layer and a hydrophobized, coarse-pored carbon layer. The waterproofing the carbon layer is made with an aqueous polytetrafluoroethylene emulsion. During the cathodic During the process, only the coarse-pored carbon layer of the hybrid electrode is active. The gas pressure is about 1.2 bar. The gas separation takes place on the finely porous nickel layer adjoining the electrolyte at a pressure of 2 bar in the carbon layer.

The electrode according to the invention is also suitable as an air electrode in metal / air cells. In metal / Air cells are made with reversible metal electrodes, as they are known from accumulators also reversible working oxygen electrodes combined The reactions taking place here can be represented in a simplified way by the following equations:

Zn + - O ₂ (air) ζ = ^ ZnO,

3 Fe + 2 O ₂ (air) «== ^ Fe ₃ O ₄

Cd + IO ₂ (air)

CdO.

During the current-supplying process, the metal of the negative electrode is oxidized while at the same time the reduction of oxygen takes place at the positive electrode. When charging will be the metal oxides formed are reduced again, with oxygen developing anodically at the counter electrode will.

As with the reversible H ₂ ZO ₂ cell already described, no suitable oxygen electrode has yet been found for metal / air cells. In a metal / air cell, the oxygen in the air is cathodically converted so that the operating pressure is close to atmospheric pressure. _{In order to be able to operate an O 2} electrode under these conditions, the porous electrode, which in most cases consists of carbon, must be made hydrophobic. In this way, the necessary three-phase boundary can develop, but even with such an electrode the oxygen separation and dissolution does not take place separately dissolve in the electrolyte. At the same time, the electrode is mechanically stressed to such an extent during the evolution of gas that it becomes unusable after a few charging and discharging cycles.

An electrode is therefore used instead of the previously customary hydrophobic single-layer electrode of the invention used. To make them, use 25 g of coal powder with 600 ml of water and 10 ml of one

ίο Polytetrafluoräthylendispersion well mixed After separation of the solid fraction from the liquid by a suction filter, which consists of carbon powder and polytetrafluoroethylene mixture at 150 ⁰ C in a drying cabinet is dried. The solidified powder is then crushed in a mill and sieved out.2 g of the mixture produced in this way are then placed on a fine-pored, 03 mm thick, sintered nickel layer with a diameter of 4 cm and covered with a nickel mesh. The thickness of the nickel mesh is about 03 mm mm and the mesh size 0.19 mm. The layers of the electrode can be pressed with 16 N / mm ² and at 380 ⁰ C The sintered Polytetrafluoräthylenanteil in the coal layer is 26.5 wt ° / o.

In the figure, the anodic and cathodic voltage profile of the electrode according to the invention is plotted against an Hg / HgO reference electrode over time, namely in curve 1 the voltage with cathodic loading and in curve 2 the voltage with anodic loading. The current density was 30 mA / cm ² in both cases. The electrolyte consisted of 6 η KOH, the temperature was 20 ⁰ C. As shown in the curves can be seen, the stresses remain constant over the Yersuchszeit. No mechanical changes could be determined on the electrode.

For comparison, the figure also shows the voltage profile of a single-layer electrode consisting only of carbon powder and 26.5% by weight of polytetrafluoroethylene. However, the load was only 20 mA / cm ² . Curve 3 shows the voltage curve in the anodic process and curve 4 in the cathodic process. As can be seen from curve 4, the voltage drops abruptly after about 250 hours of operation. As more detailed investigations have shown, the deterioration in the single-layer electrode is due to mechanical destruction of the electrode structure: the electrode was severely cracked, parts of the electrode had even been broken off.

1 sheet of drawings

Claims (6)

Patent claims: 1. Electrode for the separation and dissolution of gases in electrochemical cells with liquids Electrolytes with two porous, electronically conductive layers, characterized in that that the electrolyte-side layer is hydrophilic and at least partially from a catalyst for the Gas separation is that the gas-side layer is either hydrophobic or hydrophilic and at least partially consists of a catalyst for the gas dissolution and that in the case of a hydrophilic one gas-side layer, the pore radius in the gas-side layer is greater than the pore radius in the electrolyte-side layer 2. Electrode according to claim 1, characterized in that with a hydrophobic gas side Layer the pore radius in the gas-side layer is smaller than the pore radius in the electrolyte-side layer 3. Electrode according to claim 1 or 2, characterized in that the catalyst in itself applied to a carrier material in a known manner is 4. Electrode according to one of claims 1 to 3, characterized in that the catalyst in the The electrolyte-side layer is sintered nickel or porous graphite 5. Electrode according to one or more of claims 1 to 4, characterized in that the The catalyst in the gas-side layer is carbon powder, which is optionally silver, a mixture of Contains nickel and cobalt oxide or cobalt and aluminum oxide or manganese oxide 6. A method for operating an electrochemical cell having an electrode according to one or more of claims 1 to 5, characterized in that in the presence of a hydrophilic gas-side Layer in this layer, a higher pressure is set during the gas deposition than during the gas dissolution.