EP2385996A2

EP2385996A2 - Structured gas diffusion electrode for electrolysis cells

Info

Publication number: EP2385996A2
Application number: EP09796978A
Authority: EP
Inventors: Thomas Turek; Imad Moussallem
Original assignee: Bayer Technology Services GmbH
Current assignee: Bayer Intellectual Property GmbH
Priority date: 2009-01-08
Filing date: 2009-12-29
Publication date: 2011-11-16
Also published as: US20120000789A1; WO2010078952A3; WO2010078952A2; TW201042801A; CN102301037A; DE102009004031A1

Abstract

The invention relates to a novel gas diffusion electrode for oxygen reduction, preferably used in chlorine-alkali electrolysis.

Description

Structured gas diffusion electrode for electrolysis cells

The invention relates to a novel gas diffusion electrode for the oxygen reduction, which is preferably used in the chlor-alkali electrolysis.

The reduction of oxygen by means of electrochemical processes and devices is an essential process, for example in the production of chlorine. In this case, in particular gas diffusion electrodes are used for the reduction of oxygen, which are generally known under the term of the oxygen-consuming cathode.

In generally known electrolysis cells, on the anode side usually under application of a voltage chloride (from sodium chloride solutions) is oxidized to chlorine according to formula (I), while on the cathode side water to hydrogen according to formula (II) is reduced.

2 - CΪ → Cl ₂ + 2 - e ^" E ⁰ = +1.36 V (I)

2 ^• H ₂ O + 2 ^• e ^" → 2 ^• OH ^' + H ₂ E ⁰ = - 0.83 V (II)

The thermodynamic decomposition voltage for the oxidation and reduction according to the combination of formulas (I) and (II) to be applied at least over the electrolysis cell is then 2.19 V.

If, by using an oxygen-consuming cathode on the cathode side, it is possible to prevent the undesired hydrogen formation according to formula (II) and to carry out a reaction according to formula (III) in which oxygen is reduced electrochemically to hydroxide ions, O ₂ + 2 ^• H ₂ O + 4e ^~ → 4 ^• OH ^" ; E ⁰ = +0.4 V (III) can be the thermodynamic decomposition voltage to be applied across the electrolytic cell for the oxidation and reduction according to the combination of formulas (I) and (III) to 0.96 Thus, it is theoretically possible to reduce the necessary decomposition voltage by 1.23 V. Since, as just described, the abovementioned potentials and reactions according to the formulas (I) to (III) are thermodynamically ideal limiting cases and thus The thermodynamic decomposition voltage to be applied via the electrolysis cell forms a thermodynamically ideal limiting case, the physicochemical effects such No account is taken of overvoltages on electrode surfaces, the aforementioned 1.23 V saved voltage can never be achieved in real processes.

Any savings of such necessary voltages lead in large-scale processes to considerable savings in terms of the electrical energy supplied to the process, the price of which is subject to a steady increase, especially in recent years. The aforementioned overvoltages are caused to a certain extent by the design of the electrodes used, which cause about unnecessary ohmic resistance.

Thus, US Pat. No. 5,733,430 discloses a gas diffusion electrode for use in sodium chloride electrolysis, in which case oxygen may be present in an alkaline solution, which is reduced by means of the disclosed electrode. The disclosed gas diffusion electrode consists of a gas- and liquid-permeable layer of metal, on the surface of which a catalyst material is applied, and a gas- and liquid-permeable metal collector, which is connected to the gas- and liquid-permeable layer of metal. The catalyst material is a mixture of silver and / or gold particles, as well as particles containing a fluorine compound to increase the hydrophobicity of the surface. The aforementioned gas and liquid permeability is achieved according to US 5,733,430 by forming the layer of metal and the metal collector as a porous material, such as a screen or grid, or as a sintered structure. For the metal collector hole diameter of 1 to 55 mm are disclosed. Metal collector and layer of metal are usually in surface contact with each other and consist at least on their respective surface of silver and / or gold.

US 5,733,430 does not disclose structuring the surface of the resulting electrodes. The embodiments suggest that it is a flat, flat embodiment because of the concurrent compression of the metal and metal collector layer.

Such an electrode is disadvantageous because it can be assumed by an areal embodiment of an optionally lower chemical attack surface on the electrode material, but at the same time thereby a structurally disadvantageous, because necessarily large-area electrode results. Thus, a possible reduction of oxygen is possible only with the use of a plurality of large-area electrodes according to the disclosure according to US Pat. No. 5,733,430, or alternatively under disproportionately increased current flow. US 2007/0095676 A1 discloses another electrode and electrolysis cell construction in which the reactions according to formulas (I) and (II) can be carried out under the application of voltage. The cathode is designed as an oxygen-consuming cathode similar to that disclosed in US 5,733,430. The electrolytic cell according to US 2007/0095676 A1 using such oxygen-consuming cathodes can be designed in two types. Either there is a gap between the cathode and the polymer membrane located between cathode and anode, through which water and / or alkali is passed, or the cathode is directly on the aforementioned polymer membrane, on which in turn rests directly on the anode. The electrode surfaces of US 2007/0095676 A1 are likewise not disclosed as being deliberately structured. The first embodiment according to US 2007/0095676 Al is disadvantageous, because in this way a further ohmic resistance in the form of the aforementioned gap, through the water and / or Lye is guided, is present, but must be overcome, so that a current flow for the reduction of oxygen takes place.

The second embodiment according to US 2007/0095676 A1 is disadvantageous, because although the aforementioned ohmic resistance is no longer present, on the other hand, however, the oxygen-consuming cathode can no longer operate as efficiently, since now the influx of molecularly dissolved oxygen in the liquid, which is to be reduced, an electrolyte flow precludes that could be formerly behind the oxygen-consuming cathode. For example, US 2007/0095676 A1 discloses that in this embodiment the hydroxide ions formed from the oxygen have to be conducted away from the electrode surface in the form of sodium hydroxide solution. In order for a stronger driving force of the oxygen to the cathode surface is present, which may compensate for this disadvantage, in turn, a higher voltage must be applied, which in turn, as described above, is disadvantageous.

US Pat. No. 6,117,286, which refers to the aforementioned US 2007/0095676 A1 and in particular seeks to solve the technical disadvantage of the first embodiment with a gap between the oxygen-consuming cathode and the polymer membrane, discloses an arrangement which is similar to the second embodiment of US 2007/0095676 Al is, but differs in that there is another layer of a hydrophilic, liquid-permeable material between the oxygen-consuming cathode and the polymer membrane.

Although this can reduce the distance between the anode and the cathode and at the same time also the disadvantages of the second embodiment according to US 2007/0095676

Al are overcome, since now the hydroxide ions can be removed via the layer of a hydrophilic, liquid-permeable material. On the other hand, this introduces another material between the anode and the cathode, which is another

Ohmic resistance causes. Therefore, even after the disclosure of US 6,117,286, the aforementioned disadvantages, albeit possibly in a reduced extent, continue.

US Pat. No. 6,117,286 further discloses that the surface geometry of the cathode and anode sides can also be non-planar. However, the variation in surface geometry of US 6,117,286 is performed to allow for better drainage of hydroxide ion-comprising liquid from the layer of a hydrophilic, liquid-permeable material. Therefore, the cathode surface is provided with recesses through which the layer of hydrophilic, liquid-permeable material passes. This embodiment is disadvantageous in that it requires sacrificing electrode surface of the oxygen-consuming cathode, which, in turn, if one wishes to recover the original performance of the uninterrupted oxygen-consuming cathode, either in a necessary increase in the energy used (in the form of increased voltage and / or current) or in one necessary larger design results. Both variants are economically disadvantageous. A certain advantage of structured surfaces in connection with electrolytic processes is disclosed in US Pat. No. 3,493,487. US Pat. No. 3,493,487 discloses an electrolysis device which consists of anodes and cathodes which can also be separated from one another by means of coating the cathode surface. The structuring of the cathode surface is done according to the US 3,493,487 by metal plates having small openings, which in turn are supported by a corrugated structure in their interior in terms of stability. However, the disclosed arrangement clearly does not relate to membrane-supported electrolysis processes.

The arrangement disclosed according to US Pat. No. 3,493,487 is disadvantageous because either no separation between the anode space and the cathode space is provided by means of a membrane, so that a

Mixing of anolyte and catholyte can not be prevented; or if a coating of the

Cathode is provided, this is fully occupied, so that a supply of oxygen only through the coating is possible and thus the reduction of oxygen, if at all can be carried out very slowly. This results from the fact that the scope of the arrangement disclosed in US Pat. No. 3,493,487 is clearly not membrane-based electrolysis, as described, for example, in US Pat. No. 5,733,430, US 2007/0095676 A1 or US Pat

US 6,117,286 is disclosed as field of application of the devices.

Starting from the prior art, there is still the task of providing an electrolysis cell for membrane-supported electrolysis comprising an oxygen-consuming cathode, which no longer has the disadvantages of the prior art.

It has now surprisingly been found, as a first subject of the present invention, that a device for the electrolytic reduction of oxygen, comprising a first electrode (1) as anode, a polymer membrane (2) and a second gas and liquid-permeable electrode (3) as cathode, characterized in that the second electrode (3) rests on the polymer membrane (2) at punctiform and / or linear contact points, this task can be solved.

The first electrode (1) is usually designed in the form of a plate, a flat mesh, a flat grid or a flat fabric.

In alternative embodiments of the device according to the invention, the first electrode (1) may also rest on punctiform and / or linear contact points on the polymer membrane (2).

If the first electrode (1) is in the form of a plate, it may be porous or non-porous.

Preferably, the first electrode (1) is designed in the form of a porous plate, a flat mesh, a flat grid or a flat fabric.

Particularly preferably, the first electrode (1) is designed in the form of a porous plate. If the first electrode (1) is made porous, it is at the same time also permeable to gas and liquids.

In the context of the present invention, gas and liquid-permeable designates the property of the respective electrode that it does not prevent permeation of gas when high overpressures are applied. Usually, however, these electrodes counteract such a permeation by Kapilardrücke counterpressure, so that no gas permeation takes place in the process according to the invention for chloralkali electrolysis. At the same time, however, there is the possibility of diffusion of substances dissolved molecularly in the surrounding fluids.

The preferred and particularly preferred embodiments of the first electrode (1) are advantageous because they have a particularly large specific surface area per component volume due to the porous or open-pored design. The particularly preferred embodiment of the first electrode (1) is even more advantageous because, in addition to the above-mentioned particularly large specific surface area per component volume, it additionally has a high mechanical stability which stabilizes the entire device. The first electrode (1) usually consists of a material selected from the list consisting of carbon black, graphite, carbon nanotubes, titanium, titanium alloys and special metal alloys.

Preferred materials of which the first electrode (1) consists are those selected from the list consisting of graphite, titanium, titanium alloy and special metal alloys well known to those skilled in the art by the names Hastelloy and Incolloy.

The aforementioned first electrode (1) may also be coated. In preferred embodiments, this first electrode (1) is coated with a material selected from the list consisting of ruthenium oxide (RuO ₂ ) and iridium oxide (IrO ₂ ).

The aforementioned materials are advantageous because they are generally characterized by a good electrical conductivity, but at the same time by a good chemical stability to the electrolyte solutions comprising sodium chloride and / or hydrogen chloride and / or chlorine on the anode side, which is the use of the device in the In connection with the chlorine electrolysis is of particular advantage.

As chemically stable, in the context of the present invention is meant a material which, under the operating conditions of the device, does not chemically react with the surrounding electrolyte solutions, e.g. comprising sodium chloride and / or hydrogen chloride and / or chlorine.

The polymer membrane (2) is usually a polymer membrane, which the person skilled in the art generally knows by the name cation exchange membrane. Preferred polymer membranes (2) include polymeric perfluorosulfonic acids. The polymer membrane (2) may also comprise reinforcing fabrics of other chemically stable materials, preferably fluorinated polymers, and more preferably polytetrafluoroethylene.

The preferred polymer membranes (2) are advantageous because they are cation exchange membranes and thus promote the permeation of ions through the membrane. This in turn leads to a lower ohmic resistance across the membrane, which reduces the voltage necessary to achieve a current flow.

The thickness of the polymer membrane (2) is usually less than 1 mm. Preferably, the thickness of the membrane is less than 500 microns, more preferably less than 400 microns, most preferably less than 250 microns.

The small thicknesses of the membrane are particularly advantageous, because in this way the necessary voltage in the device can be chosen to be lower, since the ohmic resistance is reduced.

The second electrode (3) is gas- and liquid-permeable and can be designed for this purpose in the form of a porous material, a mesh, a grid or a fabric.

Preferably, the second electrode (3) is designed in the form of a net, a grid or a fabric.

Particularly preferably, the second electrode (3) is designed in the form of a net or a grid. The preferred and particularly preferred embodiments as mesh or grid of the second electrode (3) are advantageous because such embodiments can be obtained easily from the materials of the second electrode (3) described below, and because they can easily be deformed such that the punctiform and / or linear contact points according to the invention with the polymer membrane (2) can be obtained when in contact.

The second electrode (3) usually consists of a material selected from the list consisting of carbon black, graphite, carbon nanotubes, nickel, silver, titanium, titanium alloys and special metal alloys.

Preferably, the material of the second electrode (3) is nickel, silver and / or titanium, and their alloys.

On the second electrode (3) may be a coating or not. Preferably, a coating is located on the second electrode (3). In preferred developments of the device described here, the first electrode (1) may also be coated in the same or similar manner as the second electrode (3). If a coating is present on the second electrode (3), it is usually a coating comprising a conductive metal and a fluorinated polymer.

Conductive metals which are preferably present in the coating on the second electrode (3) are those selected from the list consisting of iron, manganese, cobalt, gold, iridium, copper, platinum, palladium, osmium, iridium, rhodium, ruthenium and Silver.

The oxides of the abovementioned conductive metals and / or their alloys and / or mixtures thereof may likewise preferably be present in the coating of the second electrode (3).

Particularly preferred is silver. A preferred fluorinated polymer is polytetrafluoroethylene (PTFE).

The preferred coating of the second electrode (3) is advantageous because it is possible by means of the ratios of conductive metal and fluorinated polymer, as well as by suitable choice of the conductive metal and the fluorinated polymer in a simple manner to those skilled in the second electrode (3) the hydrophilicity / hydrophobicity, as well as in terms of chemical stability to adapt to the requirements of the method in which the device according to the invention is to be used, while at the same time the Ohmic resistance generated by the additional coating on the second electrode (3) can be kept small.

For example, in the case of using the device in conjunction with chloralkali electrolysis, by coating with polytetrafluoroethylene in minor proportions and a higher level of silver, which is chemically stable to attack by chloride ions and chlorine in such a coating, those skilled in the art may desire low levels Ohmic resistors and achieve a substantially hydrophilic surface, so that the electrochemical reduction of oxygen succeeds in an advantageous manner.

In connection with the use of the apparatus for the chloralkali electrolysis, it is therefore preferred if the coating on the second electrode (3) has a proportion of more than 80% by weight of silver, particularly preferably 90% by weight of silver, particularly preferably more than 95% by weight of silver.

It is also preferred in this context if the coating on the second electrode (3) has a proportion of polytetrafluoroethylene of less than 20% by weight, particularly preferably less than 10% by weight, very particularly preferably from 0.2 to 7 % By weight.

The second electrode (3) is according to the invention at point and / or line-shaped contact points on the polymer membrane (2).

In a first preferred embodiment of the device according to the invention for the electrolytic reduction of oxygen, the second electrode (3) is in a spatial direction wavy or serrated and is located at line-shaped contact points on the polymer membrane (2).

In a second preferred embodiment of the device according to the invention for the electrolytic reduction of oxygen, the second electrode (3) is corrugated or serrated in two spatial directions and is located at punctiform contact points on the polymer membrane (2).

In a third preferred embodiment of the device according to the invention for the electrolytic reduction of oxygen, the second electrode (3) is corrugated or jagged in a first region in a spatial direction and lies in this first region at linear contact points on the polymer membrane (2) and is in a second region in two spatial directions wavy or serrated and is located at point-shaped contact points on the polymer membrane (2).

Wavy or serrated in one or in two spatial directions, in the context of the present invention, means that the second electrode (3) is formed in spatial directions perpendicular to the surface of the polymer membrane (2) on which it rests in a punctiform or linear manner. Particularly preferred embodiments of such in one and / or two spatial directions corrugated and / or serrated second electrodes (3) are characterized in that the formation relative to the point and / or line-shaped contact points with the polymer membrane a distance of 0.1 to 5 mm at the furthest point of the polymer membrane of a respective formation. Smaller than the abovementioned distances are often very costly in terms of tolerances and thus not economically advantageous, larger than the aforementioned distances lead to average distances between the second electrode (3) and polymer membrane (2), which cause high mean ohmic resistances which is unfavorable.

An example of a corrugated in a spatial direction embodiment is the execution of the second electrode (3) in the form of a corrugated sheet of the erfmdungemäßen porous materials, nets, meshes or fabrics.

An example of an embodiment jagged in a spatial direction is the execution of the second electrode (3) in the form of a zigzag sheet of the porous materials, meshes, meshes or fabrics according to the invention. An example of a corrugated embodiment in two directions is the execution of the second electrode (3) in the form of a periodically bulging sheet of the porous materials, nets, meshes or fabrics according to the invention.

An example of an embodiment jagged in two spatial directions is the execution of the second electrode (3) in the form of a periodically pyramidal plate made of the porous materials, nets, meshes or fabrics of the invention. The embodiment of the invention of the second electrode (3) in which this rests on the polymer membrane (2) at point and / or linear contact points, is particularly advantageous because by direct contact with the polymer membrane (2) of the ohmic resistance at such locations becomes minimal and at the same time at the points where the direct contact is not established, a space is formed through which the hydroxide ions formed can be removed without being countercurrent to the oxygen, which is to be brought into contact with the second electrode of the second electrode (3) must be removed. As a result, both a low ohmic resistance over the direct contact, and at the same time a lower ohmic resistance is achieved by eliminating a diffusive inhibition of the transport of oxygen. Overall, therefore, the necessary voltage is minimized.

In connection with this, the embodiment of the second electrode (3) according to the first preferred embodiment is particularly preferred as wavy or serrated in a spatial direction.

Most preferably, the embodiment of the second electrode (3) according to the first preferred embodiment is wavy or serrated in one spatial direction, the angle between the polymer membrane (2) and each jagged embodiment being from 5 ° to 80 ° from 20 ° to 75 °, more preferably from 30 ° to 70 °.

This is particularly advantageous because this results in comparatively more linear contact points between the polymer membrane (2) and the second electrode (3) and at the same time defined channels are formed between the polymer membrane (2) and the second electrode (3) through which the hydroxide ions formed can be selectively removed in a simple manner, if desired.

Finally, in alternative embodiments of the device according to the invention, the first electrode (1) may also be configured as has just been disclosed in connection with the second electrode.

Another object of the present invention is a method for producing an apparatus for the electrolytic reduction of oxygen, in which a first electrically conductive material is fixed at a distance of up to 5 mm to a polymembrane or connected to it, and in which Polymer polymer a second electrically conductive, porous material is punctiform and / or linearly connected, characterized in that the second electrically conductive, porous material, before it is connected to the polymer membrane, in one or two spatial directions wavy or serrated is formed.

The first electrically conductive material is that which has already been disclosed in connection with the device according to the invention as the material of the first electrode (1). The polymer membrane is that already disclosed in connection with the device according to the invention as a polymembrane (2).

The second electrically conductive, porous material is that which has already been disclosed in connection with the device according to the invention as material of the second electrode (3). Here, the aforementioned materials have an advantageous effect, since they all have a high specific conductivity, but at the same time are easily deformable, so that the method according to the invention can be carried out in a simple manner.

The corrugated or serrated shaping in one or two spatial directions can be carried out by means of generally known methods from the processing technology of metals. Such processes include deep drawing, bending, stretching, (hot) pressing, etc. of metal materials.

In a preferred further development of the method according to the invention, the second electrically conductive, porous material is subjected to a coating before it is brought into contact with the polymer membrane (2).

Coating takes place in the context of the preferred embodiment, first by treating the second electrically conductive, porous material with a mixture comprising at least a proportion of metal powder and a proportion of particles of a fluorinated polymer.

In further preferred embodiments of the method according to the invention, the mixture is a suspension. The treatment may be immersion in or spraying with the suspension. The liquid in which the suspension of the aforementioned particles is present may be water or else an organic solvent. When an organic solvent is used, it is an organic solvent which can not dissolve the fluorinated polymer.

However, the use of water as liquid for preparing the suspension is preferred.

The suspension may also comprise a thickener. Thickeners are those substances which can be dissolved in the liquid used to prepare the suspension and which significantly increase the dynamic viscosity of the liquid even in small amounts.

Non-exhaustive examples of such thickeners are, for example, the derivatives of cellulose, such as hydroxypropylcellulose, methylcellulose, ethylcellulose etc.

Preferably, a thickener is used and this is particularly preferably methylcellulose. The suspension may further comprise a detergent.

Detergents are, for example, ionic or nonionic surfactants, such as the substances commonly known by the trade name family Tween or the substances known by the trade name family Triton. _ _

Preferably, a detergent is used, and this is particularly preferably Triton-X 100.

The suspension used for coating the second electroconductive porous material usually comprises a proportion of 10 to 70% by weight of the metal powder and a proportion of 0.1 to 20% by weight of the particles of the fluorinated polymer. In preferred embodiments of the method according to the invention, the suspension contains from 30 to 80% by weight of water, from 10 to 70% by weight of the metal powder, from 0.1 to 20% by weight of the particles of the fluorinated polymer, of 0.05 to 1.5% by weight of the thickener and from 0.1 to 2% by weight of the detergent, the proportions adding to 100% by weight.

After treating the second electrically conductive, porous material with one of the abovementioned suspensions, the material obtained is usually dried within the preferred further development and subsequently sintered.

In a further preferred development, the drying can also take place in the sense of a hot pressing together with the shaping according to the method according to the invention. The drying is usually carried out at temperatures of 60 ⁰ C to 200 ⁰ C. When the drying according to the further preferred development is carried out as hot pressing with simultaneous shaping, the drying is carried out under elevated pressure (1013 hPa), wherein the pressure through a mechanical device comprising a negative mold to the formation of the resulting second electrically conductive, porous Material is applied. The drying is advantageous because it allows the residues of liquid in the suspension to be coated to coat the second electrically conductive, porous material, so that these residues can not remain as a film on the surface and thus increase the ohmic resistance.

The sintering is usually carried out at temperatures of 200 ⁰ C to 400 ⁰ C. The sintering is advantageous because it allows the remainders to be removed from thickeners and / or detergents which may still be present on the surface of the second electrically conductive, porous material by being converted into gaseous compounds, for example carbon dioxide and water vapor. Thus, they can not remain as a film on the surface and thus increase the ohmic resistance, which would be disadvantageous. At the same time, the fluorinated polymers usually do not yet have significant vapor pressure at these temperatures, or decompose at these temperatures to a considerable extent, so that they merely soften and, together with the metal powder, already at the interfaces with the second electrically conductive, porous Material as well as self-sintered results in a conductive, but chemically stabilizing film. After coating in the sense of a treatment with the suspension and subsequent drying and sintering, a coating is thus obtained on the second electrically conductive, porous material which essentially consists only of the material of the metal powder and small amounts of fluorinated polymer. Such a layer is advantageous because it also has high conductivity and favorable chemical stability together with a desired hydrophilicity.

The joining of the first electrically conductive material with the polymembrane and the optionally coated second electrically conductive, porous material can be achieved by mechanical elements, such as frames or clamping elements, but also by a joining in the sense of a pressing (hot) according to the further described above Further development takes place during coating.

Another object of the present invention is the use of the device according to the invention or the devices obtained according to the inventive method in processes for the electrochemical reduction of oxygen and / or to electrochemical oxidation of chloride to chlorine.

A final object of the present invention is a process for chloralkali electrolysis in which two reaction zones chlorine and sodium hydroxide separated in two from each other by a polymer membrane (2) surrounded by two electrodes (1 and 3) are formed electrochemically, wherein in the first reaction zone a sodium chloride solution is present and the other reaction zone is a solution comprising molecularly dissolved oxygen, characterized in that the reaction zone, in which the solution comprising molecularly dissolved oxygen is brought into contact with a gas and liquid-permeable electrode (3), which corrugated in one and / or two spatial directions and / or jagged against the polymer membrane (2) on which it rests, and in that the sodium chloride solution is brought into contact with a gas- and liquid-permeable electrode (1) and wherein between the two electrodes a voltage of less than 2.3 V is created.

Preferred embodiments of the method according to the invention with regard to the electrodes (1 and 3) used in the method, or with regard to the polymer membrane (2) used in the method according to the invention, include the embodiments described within the scope of the device according to the invention.

The erfmdungsgemäße method is usually operated by a current density of 2 to 10 kA / m ^{2 is} provided at the voltage described above.

In further preferred embodiments of the method according to the invention, a voltage of less than 2 V is applied at a current density of 2 to 6 kA / m ² . Such methods are particularly advantageous because at the same time a reduction of oxygen and an oxidation of chloride to chlorine is possible by the reduced voltage at the aforementioned current densities with the use of reduced power compared to the prior art. The invention will be explained in more detail below with reference to figures and examples, without, however, restricting them thereto.

FIGS. 1 to 3 show preferred embodiments of an electrode of the device according to the invention using a grid.

FIGS. 4 to 6 show preferred embodiments of the device according to the invention using the electrodes illustrated in FIGS. 1 to 3.

FIG. 7 shows a comparison of the course of cell voltage over current density for a method according to the invention and a method not according to the invention.

Fig. 1 shows specifically a serrated formation in a spatial direction of an electrode of the device according to the invention of a grid material. In (a), a plan view of an electrode surface is shown, and in (b), a sectional view taken along the line A-A of FIG. 1 (a) of the same electrode is shown.

Fig. 2 shows specifically a corrugated shape in a spatial direction of an electrode of the device according to the invention of a grid material. In (a), a plan view of an electrode surface is shown, and in (b), a sectional view taken along the line A-A of FIG. 2 (a) of the same electrode is shown.

3 specifically shows a serrated formation in two spatial directions of an electrode of the device according to the invention of a grid material. In (a), a plan view of an electrode surface is shown, and (b) is a sectional view taken along the line AA of FIG. 3 (a) of the same electrode, while FIG. 3 (c) is a sectional view taken along the line BB ,

4 shows in a side view in particular a first embodiment of the device according to the invention, comprising a first electrode (1) in the form of a fabric which is flat and rests with the entire surface of one side on a polymer membrane (2), and a second electrode (3 ), according to the embodiment shown in Fig. 1, which rests in line-shaped contact points on the polymer membrane (2).

5 shows, in particular in a side view, a second embodiment of the device according to the invention, comprising a first electrode (1) in the form of a net, according to the embodiment shown in FIG. 2, which rests in linear contact points on the polymer membrane (2) and a second one Electrode (3), according to the embodiment shown in Fig. 1, which also rests in linear contact points on the polymer membrane (2). 6 shows in particular in (a) and (b) the two side views of a third embodiment of the device according to the invention comprising a first electrode (1) in the form of a fabric which is planar and with the entire surface of one side on a polymer membrane (2). rests, and a second electrode (3), according to the embodiment shown in Fig. 3, which rests in punctiform contact points on the polymer membrane (2).

7 shows the profile of the cell voltage (S) in volts as a function of the current density (A) in kA / m ² for the chloralkali electrolysis according to the inventive method, in particular according to the data from Example 7 (dots and thick dashed line), as well as for the Chloralkalielektrolyse according to a method not according to the invention, in particular according to the data from comparison in game 7 (triangles and thin solid line).

- -

Examples:

Examples 1-3: Manufacture of electrodes according to the invention

Level nickel nets (Haver and Boecker) with a mesh size of 0.5 mm and a wire thickness of 0.1 mm were coated with various suspensions according to Table 1 coated by first sprayed with the suspensions 1 to 3, then at 130 ⁰ C in 60 ° angles at intervals of 3 mm to form a planar surface formed by bending, and then sintered at 34O ⁰ C.

Table 1: Composition of the suspensions for coating the second electrodes

Three electrodes according to the invention are obtained. Comparative Examples 1-3: Manufacture of non-inventive electrodes

There are three electrodes prepared analogously to Examples 1-3, with the only difference that they are not transformed, but remain in a flat form.

EXAMPLES 4-6: Production of Inventive Devices with Electrodes According to Examples 1-3 The electrodes according to Examples 1-3 were used as cathodes with a standard electrode (DeNora Deutschland GmbH) made of titanium as an anode and a polymer membrane made of Nafion 982WX (Fa. DuPont) were tentered together to form an electrolytic cell and the electrodes were further electrically connected via a voltage and current source. The free projection surface, which in the case of the planar anode is the same as the active one - -

Surface of the electrode was set by the frame to 25 cm ² . The active surface of the electrodes according to Examples 1-3 was 48 cm ² each due to the shaping.

Comparative Examples 4-6: Fabrication of Inventive Devices with Electrodes According to Comparative Examples 1-3 Electrolysis cells were prepared according to those of Examples 4-6, with the only difference being that instead of the electrodes of Examples 1-3, now those of Comparative Examples 1-3 were used and that a gap of 3 mm was provided between the electrodes of Comparative Examples 1-3 and the polymer membrane. These were flat and therefore had an active surface area of 25 cm ² each. Example 7 Chlor-alkali electrolysis process with apparatus according to Example 4

The electrolysis cell according to Example 4 (electrode according to Example 1) was used in a vessel, so that two reaction zones formed separately through the electrolysis cell. On the side of the electrode according to Example 1, a 30 wt .-%, aqueous sodium hydroxide solution was filled while a 20 wt .-% aqueous Natπumchloπdlösung was filled on the side of the standard electrode. Both solutions were recirculated against a large receiver tank into the respective reaction zones to achieve an approximately constant concentration of ingredients over an operating period of 100 hours.

The overpressure on the side of the soda lye solution was about 180 mbar, the oxygen overpressure of the oxygen introduced into the sodium hydroxide solution constituting an excess pressure of 30 mbar and the remaining overpressure of 150 mbar resulting from the elevated master vessel of the sodium hydroxide solution. The process was carried out at a temperature of 90 ^° C.

It was measured over a period of 100 hours at various applied cell voltages and current densities. The results are time-independent within the 100 hours and are shown in Fig. 7 with respect to the cell voltage plotted in relation to the current density.

Comparative Example 7 Chlor-alkali electrolysis process with apparatus according to Comparative Example 4

An experiment similar to that in Example 7 was carried out, with the only difference that now the electrolysis cell according to Comparative Example 4 (electrode according to Comparative Example 1) was used.

The results are also time-independent and are shown in Fig. 7 with respect to the cell voltage plotted in relation to the current density.

The comparison of Example 7 with Comparative Example 7 shows that at the same current densities, the device according to the invention does not show a significant increase of 130 mV at 4 kA / m2 Device according to the invention requires reduced cell voltage. To rule out that the advantageous effect of the reduced cell voltage at a disadvantageously selected gap between not inventive second electrode and polymer membrane, the gap was halved to only 1.5 mm. However, only a reduction of the cell voltage of about 60 mV at 4 kA / m ^{2 was} measured. This ruled out that the advantageous effect of the device according to the invention was attributable only to an average reduction in the gap width between the second electrode and the polymer membrane.

Claims

claims:

1. A device for the electrolytic reduction of oxygen, comprising a first electrode (1) as an anode, a polymer membrane (2) and a second gas and liquid-permeable electrode (3) as a cathode, characterized in that the second electrode (3) on the Polymer membrane (2) rests on point and / or linienföπnigen contact points.

2. Device according to claim 1, characterized in that the first electrode (1) in the form of a plate, a flat mesh, a flat grid or a flat fabric is executed.

3. A device according to claim 1 or claim 2, characterized in that the second electrode (3) is corrugated or jagged in a spatial direction.

4. The device according to claim 3, characterized in that the second electrode (3) is designed serrated and wherein the angle between the polymer membrane (2) and the serrated design of 5 ° to 80 °, preferably from 20 ° to 75 °, particularly preferred from 30 ° to 70 °.

5. A process for producing an apparatus for the electrolytic reduction of oxygen, in which a first electrically conductive material is fixed either at a distance of up to 5 mm to a polymer membrane or is connected to this and in which with the polymer membrane a second electrically conductive , Porous material is punctiform and / or linearly connected, characterized in that the second electrically conductive, porous material before it is connected to the polymer membrane is corrugated or serrated in one or two spatial directions.

6. The method according to claim 5, characterized in that the second electrically conductive, porous material prior to contacting with the polymer membrane (2) nor a coating by treating the second electrically conductive, porous material with a mixture comprising at least a proportion of metal powder and a share in

Particles of a fluorinated polymer is subjected.

7. The method according to claim 6, characterized in that the mixture is a suspension.

8. The method according to claim 7, characterized in that after treating the second electrically conductive, porous material with the aforementioned suspension, the resulting material is dried and subsequently sintered, wherein the drying is carried out as hot pressing together with the molding.

9. Use of a device according to one of claims 1 to 4, or according to obtained from a method according to claims 5 to 7 devices in methods of electrochemical reduction of oxygen and / or to electrochemical oxidation of chloride to chlorine.

10. A process for chloralkali electrolysis in which in two by a two of two electrodes (1 and 3) surrounded polymer membrane (2) separate reaction zones chlorine and caustic soda are formed electrochemically, wherein in the first reaction zone

Sodium chloride solution is present and in the other reaction zone is a solution comprising molecularly dissolved oxygen, characterized in that the reaction zone in which the solution comprising molecularly dissolved oxygen is brought into contact with a gas- and liquid-permeable electrode (3) in one and / or or two spatial directions wavy and / or jagged against the polymer membrane (2) on which it rests, is formed and that the sodium chloride solution with a gas- and liquid-permeable electrode (1) is brought into contact and between the two electrodes, a voltage of less than 2.3 V is applied.