HK1165510A - Gas diffusion electrode and process for production thereof - Google Patents

Description

Gas diffusion electrode and method for producing the same

Cross Reference to Related Applications

This application claims the benefit of german patent application No. 102010030203.1, filed 6/17/2010, which is incorporated by reference in its entirety for all useful purposes.

Technical Field

The present invention relates to gas diffusion electrodes, in particular to oxygen-consuming electrodes for the reduction of oxygen under alkaline conditions, in particular suitable for chlor-alkali electrolysis, having a new specific catalyst morphology, and to an electrolysis apparatus. The invention further relates to a method for producing an oxygen-consuming electrode and to its use in chlor-alkali electrolysis or fuel cell technology.

The invention proceeds from oxygen-consuming electrodes known per se, which are configured as gas diffusion electrodes and typically comprise an electrically conductive support and a gas diffusion layer and a catalytically active component.

Background

An oxygen consuming electrode, hereinafter referred to as OCE, is one form of gas diffusion electrode. A gas diffusion electrode is an electrode in which three states of matter-solid, liquid and gaseous-are in contact with each other, and a solid, electrically conductive catalyst catalyzes an electrochemical reaction between the liquid and gas phases. The solid catalyst is typically pressed into a porous membrane, typically having a thickness in excess of 200 μm.

Various proposals for the operation of oxygen-consuming electrodes in electrolysis cells on an industrial scale are known in principle from the prior art. The basic idea is to replace the hydrogen-evolving cathode of the electrolysis (e.g. in chlor-alkali electrolysis) with an oxygen-consuming electrode (cathode). An overview of possible cell designs and technical solutions can be found in the publication "chlorine-Alkali Electrolysis with Oxygen depolarised Cathodes: History, Present Status and Future Prospects", J. appl. electrochem. 38 (2008) 1177. 1194 by Moussalem et al.

Oxygen-consuming electrodes, hereinafter also abbreviated to OCE, have to meet a series of essential requirements in order to be usable in industrial electrolysis cells. For example, the catalyst and all other materials used must be chemically stable to about 32wt% sodium hydroxide solution and to pure oxygen at temperatures of typically 80-90 ℃. Similarly, a high degree of mechanical stability is required to enable the electrode to have a thickness typically greater than 2m²In an area sized cell (industrial scale) is installed and operated. Other properties are: high conductivity, low layer thickness, high internal surface area and high electrochemical activity of the electrocatalyst. Suitable hydrophobic and hydrophilic pores and corresponding pore structures for gas and electrolyte conduction are also required, it being this impermeability (impermeability) that keeps the gas and liquid spaces separate from each other. Long-term stability and low production costs are further special requirements for industrially usable oxygen-consuming electrodes.

Further research on the utilization of OCE technology in chlor-alkali electrolysis is directed to zero-gap technology. In this case, the OCE is in direct contact with an ion-exchange membrane, which separates the anode space and the cathode space in the electrolysis cell. There is no gap left for the sodium hydroxide solution. This arrangement is typically also used in fuel cell technology. The disadvantage here is that the sodium hydroxide solution formed needs to pass through the OCE to the gas side and then flow down on the OCE. In this process, it is necessary that the pores in the OCE are not subjected to any clogging by the sodium hydroxide solution or crystallization of the sodium hydroxide solution in the pores. It has been found that very high sodium hydroxide solution concentrations also occur therein, in which case the ion-exchange membranes do not have a long-term stability for these high concentrations (Lipp et al, J. appl. electrochem. 35 (2005)1015-Los Alamos National Laboratory "Peroxide formation reducing chloride-alkali electrolysis with carbon-based ODC").

An important requirement for the operation of gas diffusion electrodes is the presence of liquid and gas phases in the pore system of the electrode. How this is achieved is revealed by the Young-Laplace equation (Young-Laplace equation):

the gas pressure p is thus related to the liquid in the pore system by the pore radius r, the surface tension σ of the liquid and the wetting angle Θ. However, this equation should be understood only for reference, as too many parameters are unknown or difficult to determine:

for surface tension, the difference in surface tension of the solid and liquid needs to be taken into account. The surface tension of catalysts such as platinum-on-carbon or silver, however, is barely measurable.

Wetting angle can be determined on a flat surface. In contrast, a single pore cannot be examined, since in this case the pore system of the entire electrode would be determined.

The wetting angle also changes under the influence of the electric field and the temperature; nor inside the electrode.

To create gas and liquid spaces in an OCE, it is necessary to create pores with different pore radii or different surface tensions. In addition to this wettability, the OCE must have good conductivity so that electrons can be transported with the lowest ohmic resistance.

In chlor-alkali electrolysis in a limited gap arrangement, for example, OCE separates the electrolyte space from the gas space. In this case, as mentioned above, gas must not pass from the gas space into the electrolyte space, and electrolyte must not pass from the electrolyte space into the gas spaceA space. In industrial electrolyzers, the oxygen-consuming cathode should withstand the hydrostatic pressure present at the bottom of the industrial electrolyzer, for example 170 mbar. Since the gas diffusion electrode has a pore system, a small amount of liquid is always introduced into the gas space, and gas is introduced into the liquid space. The amount depends on the cell configuration of the cell. The OCE should be impermeable to pressure differences between the gas space and the liquid space in the range of 10-60 mbar. By "impermeable" is meant herein that the entry of gas bubbles into the electrolyte space is not observable with the naked eye. By "liquid-impermeable" is meant not more than 10 g/(h cm)²) Is passed through the OCE (where g represents the amount of liquid and h represents one hour and cm)²Representing geometric electrode surface area). However, when too much liquid passes through the OCE, it can only flow down on the side facing the gas side. In this case, a liquid film can be formed which hinders the gas from entering the OCE and therefore has an extremely adverse effect on the performance of the OCE (insufficient oxygen supply). When too much gas enters the electrolyte space, it must be possible to extract gas bubbles from the electrolyte space. In each case, the gas bubbles shield a portion of the electrode area and membrane area, which leads to a current density shift and, therefore, in electrostatic operation of the electrolysis cell, to a local increase in the current density and to an undesirable increase in the cell voltage across the electrolysis cell.

Another possibility is to use a sintered electrode solution. In this case, it is possible, for example, to use three different particle sizes in different layers of the OCE. For example a top layer consisting of a fine-grained material, a working layer of different grades, and a gas-conducting layer of a coarse material (DE 1219553).

The disadvantage of these electrodes is that the electrodes are relatively thick and heavy-typically about 2 mm thick-the individual layers must be very thin and defect free. The cost of the metal of this type of electrode is high and it is not possible to produce the electrode in a continuous production process.

A further disadvantage of gas diffusion electrodes of this type is that they are very sensitive to pressure variations and cannot be used, for example, in industrial electrolysis cells, since, due to the height of the construction, the electrolyte has a high static pressure here at the bottom of the electrolysis cell, which acts on the gas diffusion electrode and thus floods the pore system.

Such electrodes are produced by scattering application and subsequent sintering or hot pressing. To produce a multilayer electrode, the fine-grained material is therefore first dispersed into the template and then smoothed. Subsequently, the other materials are applied in layers on top of each other and then pressed. This production is not only error prone but also time consuming and difficult to automate.

EP797265(Degussa) describes a gas diffusion electrode and a method of producing a gas diffusion electrode which results in a bimodal pore distribution in the electrode layer. In this case, the catalyst is dispersed with a proton conductive ionomer. The total porosity of the electrode is 40-75% and consists of small pores with an average diameter of at most 500 nm and large pores with an average diameter of 1000-2000 nm. After spraying the coating dispersion onto the hot film, small pores are formed as the solvent evaporates. When the pore former added in advance is decomposed or leached, large pores are formed. The average diameter of the pores can thus be influenced by the particle size of the pore former used. The bimodal pore distribution is intended to be used to result in an improvement in mass transfer in the electrode layer. Through the large pores, the reaction gas can quickly penetrate into the electrode layer, and the formed reaction water can be removed. The small pores then begin to be transported in the ion-conducting polymer as far as the catalyst particles. The distance covered here is only short and the transport slowed down in the small pores does not significantly impair the performance of the electrode. A significant improvement in transport in the electrode layer over conventional coatings was only observed at over 40% total porosity. With increasing porosity, the supply of electrocatalyst with the reaction medium increases. However, as porosity increases, the amount of electrocatalyst and the amount of ionomer available in the coating decreases. Therefore, as the porosity is increased, the adhesion of the catalyst to the ionomer and the ionic conductivity of the coating become deteriorated, and thus the performance data of the electrode layer is again deteriorated at a porosity higher than 75%.

This applies to fuel cell electrodes in which an electrocatalyst is dispersed in a proton-conducting polymer.

Document US-A-6503655 describes A hydrophobic gas diffusion electrode with A smooth surface for use in PEM fuel cells, having A pore diameter of 10-10000 nm. The permeability of the electrode to nitrogen should be at standard pressure>10^-6 m²S, preferably>10^-5 m²And s. For this purpose, the largest pores should have a diameter of more than 100 nm; the diameter should preferably be 500-10000 nm. Also important is the hydrophobicity of the electrodes. This also prevents water formed in the electrochemical reaction between hydrogen and oxygen from accumulating in the pores and closing them. In order to meet the requirements mentioned, modified carbon papers are used in gas diffusion electrodes, i.e. carbon papers whose surface density has been increased with carbon black or graphite. However, these materials are inadequate in terms of surface smoothness and pore size. Us 6503655 makes no statement about pore volume or porosity.

Disclosure of Invention

An embodiment of the invention is a gas diffusion electrode comprising an electrically conductive support, and a porous coating based on an electrochemically active catalyst and a hydrophobic material. The electrode has a first side facing the oxygen-containing gas and a second side facing the alkaline electrolyte. The catalyst comprises a noble metal as a catalytically active component. The hydrophobic material comprises a hydrophobic polymer. The coating containing the catalyst has a thickness of 10 to 500 mm³Pore volume,/g, and pore diameter in the range of 100-10,000 nm.

Another embodiment of the present invention is the above gas diffusion electrode, wherein the noble metal is silver or platinum.

Another embodiment of the present invention is the above gas diffusion electrode, wherein the coating layer containing the catalyst has a thickness of 50 to 200 mm³(ii) a pore volume per gram of the polymer,

another embodiment of the present invention is the above gas diffusion electrode wherein the catalyst-containing coating has a pore diameter of 600-6,000 nm.

Another embodiment of the present invention is the above gas diffusion electrode wherein the coating has a monomodal pore distribution.

Another embodiment of the present invention is the above gas diffusion electrode, wherein the coating layer comprising the catalyst has a porosity of 10 to 70%.

Another embodiment of the present invention is the above gas diffusion electrode wherein the coating layer comprising the catalyst has a porosity of 20 to 60%.

Another embodiment of the present invention is the above gas diffusion electrode, wherein the catalyst-containing coating layer has a thickness of 20 to 1,000 μm.

Another embodiment of the present invention is the above gas diffusion electrode wherein the catalyst-containing coating has a thickness of 200-600 μm.

Another embodiment of the present invention is the above gas diffusion electrode, wherein the hydrophobic component comprises a hydrophobic polymer.

Another embodiment of the present invention is the above gas diffusion electrode, wherein the hydrophobic polymer comprises a fluorine substituted polymer.

Another embodiment of the present invention is the above gas diffusion electrode, wherein the hydrophobic polymer comprises Polytetrafluoroethylene (PTFE).

Another embodiment of the present invention is the above gas diffusion electrode, wherein the electrode has a thickness of at 5 mg/cm² - 300 mg/cm²Total loading of catalytically active component in the range.

Another embodiment of the present invention is the above gas diffusion electrode, wherein the electrode has a thickness of at least 10 mg/cm² - 250 mg/cm²Total loading of catalytically active components in the rangeAmount of the compound (A).

Another embodiment of the present invention is the above gas diffusion electrode, wherein the support is based on nickel, silver or a mixture thereof.

Another embodiment of the present invention is the above gas diffusion electrode, wherein the support is in a form selected from the group consisting of a mesh fabric, a woven fabric, a knitted fabric, a nonwoven fabric, an expanded metal, or a foam.

Another embodiment of the present invention is the above gas diffusion electrode, wherein the support is in the form of a woven fabric.

Yet another embodiment of the present invention is a chlor-alkali electrolysis plant comprising the above gas diffusion electrode as oxygen consuming cathode.

Yet another embodiment of the present invention is a fuel cell including the above gas diffusion electrode.

Yet another embodiment of the present invention is a metal/air battery comprising the above gas diffusion electrode.

Detailed Description

The object of the present invention is to provide an oxygen-consuming electrode for use in oxygen reduction under alkaline conditions, for example for use in chlor-alkali electrolysis, which overcomes the above disadvantages and allows a lower operating voltage in chlor-alkali electrolysis.

The invention, by means of which the above-mentioned objects are achieved, provides a gas diffusion electrode for oxygen reduction in an aqueous alkaline medium, comprising at least a support, which is in particular electrically conductive, and a porous coating based on an electrochemically active catalyst and a hydrophobic material, the electrode having a side facing an oxygen-containing gas and a side facing an alkaline electrolyte, characterized in that the catalyst comprises a noble metal (in particular silver or platinum, preferably silver) as catalytically active component, the hydrophobic material comprises a hydrophobic polymer, the coating comprising the catalyst has a thickness of 10-500 mm³Per g, youSelecting 20-300 mm³In g, more preferably from 50 to 200 mm³A pore volume/g, and a pore diameter in the range of 100-.

It has surprisingly been found that in addition to the partial hydrophilicity and partial hydrophobicity of the coating, the porosity, and in particular the pore diameter in combination with the pore volume of the gas diffusion electrode, is critical for the performance of the gas diffusion electrode.

Preferred is the design of the novel gas diffusion electrode, which is characterized in that the coating has a monomodal pore distribution.

Further preferred are gas diffusion electrodes wherein the porosity of the catalytically active coating is 10-70%, preferably 20-60%.

The thickness of the catalytically active coating is preferably 20-1000. mu.m, more preferably 100-800. mu.m, most preferably 200-600. mu.m.

Further preferred is the design of the novel gas diffusion electrode wherein the hydrophobic component comprises a hydrophobic polymer, preferably a fluorine substituted polymer, more preferably Polytetrafluoroethylene (PTFE).

A further preferred embodiment of the gas diffusion electrode is characterized in that the electrode has a thickness of between 5 mg/cm²-300 mg/cm²In the range of preferably 10 mg/cm²-250 mg/cm²Total loading of catalytically active component in the range.

The novel gas diffusion electrode preferably has a carrier consisting of a material selected from the group consisting of silver, nickel, coated nickel (e.g. silver coated nickel), plastic, copper-nickel alloy or coated copper-nickel alloy (e.g. silver plated copper-nickel alloy), from which a flat textile structure has been produced.

The conductive support can in principle be a wire mesh, a non-woven fabric, a foam, a woven fabric, a knitted strip, an expanded metal. The support is preferably composed of a metal, more preferably nickel, silver or silver-plated nickel. The carrier may have one or more layers. The multi-layer carrier may be formed of two or more wires, non-woven fabrics, foams, woven fabrics, braided strips or expanded metals arranged one on top of the other. The wire mesh, nonwoven fabric, foam, woven fabric, braided strips, expanded metal may be different. For example, they may have different thicknesses or different porosities or have different mesh sizes. Two or more wires, non-woven fabrics, foams, woven fabrics, braided strips, expanded metals may be bonded to each other, for example by sintering or welding. It is preferred to use a nickel wire mesh having a wire diameter of 0.04-0.4 mm and a mesh size of 0.2-1.2 mm.

The support of the gas diffusion electrode is preferably based on nickel, silver or a combination of nickel and silver.

Also preferred is a gas diffusion electrode in the form of a mesh, braid, knitted or non-woven fabric, an expanded metal or foam, preferably in the form of a woven fabric.

In principle, different forms of NaCl-OCE electrolysis can be distinguished by the way in which OCE is introduced and how this is set at the distance between the ion-exchange membrane and the OCE. Many cell designs allow for a gap, known as a limited gap arrangement, between the ion exchange membrane and the OCE. The gap may be 1-3 mm; sodium hydroxide solution flows through the gap. In an upright arrangement of the electrodes, the flow may be from the top downwards (falling film trough principle; see for example WO 2001/057290A2) or from the bottom upwards (gas pocket principle; see for example DE 4444114A 2).

A particular embodiment of the invention is a polymer-bonded electrode, in which case the gas diffusion electrode is provided with both hydrophilic and hydrophobic regions. These gas diffusion electrodes are chemically very stable, especially when PTFE (polytetrafluoroethylene) is used.

The regions with high PTFE content are hydrophobic; no electrolyte can penetrate here, but it can penetrate at sites with low or no PTFE content. In which case the catalyst itself must be hydrophilic.

Such PTFE-catalyst mixtures are in principle produced, for example, by the use of a dispersion of water, PTFE and catalyst. For the stabilization of the PTFE particles in aqueous solution, in particular the addition of emulsifiers, preference is given to using thickeners for the processing of the dispersion. An alternative to this wet production process is production from PTFE powder and catalyst powder by a dry mixing process.

The OCE of the present invention can be produced by either a wet process or a dispersion process and a dry process as described above. Particularly preferred is a dry production process.

The dispersion method was mainly chosen for electrodes with polymer electrolytes-for example, successfully incorporated in PEM (polymer-electrolyte-membrane) fuel cells or HCl-OCE membrane electrolysis (WO 2002/18675).

For the case where OCE is used in a liquid electrolyte, the dry process results in a more suitable OCE. In the wet or dispersion process, it is possible to dispense with the noteworthy mechanical pressing by evaporating the water and sintering the PTFE at 340 ℃. These electrodes typically have very open pores. On the other hand, however, incorrect drying conditions can quickly create cracks in the electrodes, through which the liquid electrolyte can pass. Thus, dry processes have been established for applications with liquid electrolytes, such as zinc-air batteries or alkaline fuel cells.

In the dry process, the catalyst is intimately mixed with the polymer component, preferably PTFE. The powder mixture can be shaped by pressing, preferably by compression using a roller method, to give a film-like structure which is subsequently applied to a carrier (see, for example, DE 3710168A 2; EP 144002A 2). A preferred alternative which can likewise be used is described by DE 102005023615 a 2; in this case, the powder mixture is dispersed onto the carrier and compressed together with the carrier.

In a dry process, in a particularly preferred embodiment, the electrode is produced from a powder mixture consisting of silver and/or silver oxide and PTFE. Doped silver and/or its oxides or a mixture of silver and/or its oxides with silver and a mixture of PTFE can likewise be used. The catalyst and PTFE are processed, for example, in a dry mixing process as described in US 6838408, the powder being compressed into tablets.

The sheet is then compressed together with a mechanical carrier (mechanical carrier). The sheet forming method and the pressing of the sheet and the support can be performed, for example, by a roll method. Compaction forces affect the pore diameter and porosity of the OCE, among other properties. The pore diameter and the porosity affect the performance of the OCE.

Alternatively, the OCE of the invention can be produced according to DE 10148599 by applying the catalyst powder mixture directly onto a support.

In this case, the powder mixture consists at least of the catalyst and the binder. The catalyst used is a metal, a metal compound, a non-metal compound, or a mixture of metals, metal compounds or non-metal compounds. The catalyst preferably comprises silver, silver (I) oxide, silver (II) oxide or mixtures thereof. The binder is preferably a hydrophobic polymer, more preferably Polytetrafluoroethylene (PTFE). It is particularly preferred to use a powder mixture consisting of 70 to 99.5% by weight of silver (I) oxide, 0 to 15% by weight of silver metal powder and 0.5 to 17% by weight of PTFE. The powder mixture used may also be a mixture known from DE 10130441 a, for example. In this case, the catalyst is produced in such a way that the catalyst is present on the surface of the PTFE particles.

The powder mixture may include additional other components, such as fillers including nickel metal powder, Raney nickel powder, Raney silver powder, or mixtures thereof. The powder mixture comprising the catalyst and the binder, after application to and compression with the support, forms an electrochemically active layer of OCE.

In a particularly preferred embodiment, the powder mixture is produced by mixing the catalyst powder and the binder powder and optionally further components. The mixing is preferably carried out in a mixer with rapidly rotating mixing elements, such as vanes. For the mixing of the components of the powder mixture, the mixing elements are preferably rotated at a speed of 10-30 m/s, or at a speed of 4000-8000 rpm. When a catalyst, for example silver (I) oxide, is mixed in such a mixer together with PTFE as binder, the PTFE is stretched to form a thread-like structure and in this way serves as binder for the catalyst. After mixing, the powder mixture is preferably sieved. The screening is preferably carried out with a screening device equipped with a wire mesh or the like having a mesh size of 0.04-2 mm.

Mixing in a mixer with rotating mixing elements introduces energy into the powder mixture, which greatly heats the powder mixture. In the case of overheating of the powder, deterioration of OCE properties is observed, so that the temperature in the mixing operation is preferably 35-80 ℃. This can be done by cooling during mixing, for example by the addition of a coolant such as liquid nitrogen or other inert endothermic substance. Further ways of temperature control may be to interrupt the mixing in order to let the powder mixture cool, or to select a suitable mixing device or to change the filling level in the mixer.

The powder mixture can be applied to the electrically conductive support, for example by dispersion. The powder mixture can be dispersed onto the support, for example, by means of a screen. Particularly advantageously, a frame-shaped template is placed onto the carrier, which template is preferably selected such that it only surrounds the carrier. Alternatively, the template selected may be smaller than the area of the support. In this case, the uncoated edge of the support remains free of electrochemically active coating after dispersion of the powder mixture and pressing together with the support. The thickness of the template can be selected according to the amount of powder mixture to be applied to the carrier. The template is filled with the powder mixture. Excess powder can be removed using a skimmer. The template is then removed.

In a further step, in a particularly preferred embodiment, the powder mixture is compressed together with the carrier. The pressing can be carried out in particular by means of rollers. Preferably, a pair of rollers is used. However, it is also possible to use a roller on a substantially flat substrate, in which case the roller or the substrate moves. In addition, the pressing can be performed by a compression mold (compression die). The compressive force is in particular 0.01 to 7 kN/cm.

The OCE of the present invention may in principle have a monolayer or multilayer structure. To produce a multilayer OCE, powder mixtures having different compositions and different properties are applied layer by layer to a support. The layers of the different powder mixtures are preferably not each compressed together with the carrier, but are first applied one after the other and then compressed together with the carrier in one step. For example, it is possible to apply a layer of a powder mixture having a higher content of binder, in particular a higher content of PTFE, than the electrochemically active layer. Such layers with a high PTFE content of 6-100% can be used as gas diffusion layers.

Alternatively or in addition, a gas diffusion layer of PTFE may also be applied. The layer with a high PTFE content can be applied directly on the support, for example as the lowest layer. Other layers having different compositions can be applied to produce the gas diffusion electrode. For multilayer OCEs, the desired physical and/or chemical properties can be established in a controlled manner. These include the hydrophobicity or hydrophilicity, conductivity, gas permeability of the layer. For example, a gradient in performance can be created in the performance by means of measurements of the performance that increases or decreases from one layer to another.

The thickness of the respective layers of OCE can be adjusted by the amount of powder mixture applied to the support and by the pressing force during compression. The amount of powder mixture applied can be adjusted, for example, by the thickness of a stencil which is placed on a carrier to disperse the powder mixture onto the carrier. According to the method of DE 10148599, tablets are produced from a powder mixture. In this case, the thickness or density of the sheet can be set independently of one another, since roll parameters such as roll diameter, roll distance, roll material, closing force and peripheral speed have a significant influence on these parameters.

The compression force in the pressing of the powder mixture or the layers of different powder mixtures with the carrier is applied, for example, by a roller press process with a linear pressing force in the range of 0.01 to 7 kN/cm.

The novel oxygen-consuming electrode is preferably connected as a cathode, in particular in an electrolysis cell for the electrolysis of alkali metal chlorides, preferably sodium chloride or potassium chloride, more preferably sodium chloride.

Alternatively, the oxygen-consuming electrode can be connected as a cathode in a fuel cell, preferably in an alkaline fuel cell.

The invention therefore further provides the use of a novel oxygen-consuming electrode for the reduction of oxygen in the presence of an alkaline electrolyte (e.g. a sodium hydroxide solution), in particular for alkaline fuel cells, in drinking water treatment, for example for the production of sodium hypochlorite (as bleaching liquor), or in chlor-alkali electrolysis, in particular for the electrolysis of LiCl, KCl or NaCl, or as an electrode in metal/air batteries.

The novel OCE is more preferably used in chlor-alkali electrolysis and here especially in sodium chloride (NaCl) electrolysis.

The invention further provides an electrolysis apparatus, in particular for chlor-alkali electrolysis, comprising the novel gas diffusion electrode of the invention as oxygen-consuming cathode.

The respective terms used in the description of the present invention are explained in detail below:

hg porosimetry

The pore analysis for determining the porosimetry and pore diameter is carried out by mercury porosimetry. The measuring instrument used is available from Quantachrome, Poremaster 60, with which pores of 3 nm to 950 μm can be analyzed.

The main advantage of mercury permeability assays is the detectable large pore range. The process is used as a reverse process for gas adsorption: mercury as a non-wetting liquid is forced into the pores, large pores being filled first and smaller pores being filled only at high pressure. The dependence of pressure and pore radius is traditionally described by the walsh equation. So-called intrusion and extrusion curves (intrusion and extrusion curves) are used to calculate the pore size distribution. Further information, surface area or bulk density can additionally be obtained by this test method.

Porosity of

Ratio of solid volume to empty volume in OCE. Mercury pycnometry was used to determine the apparent density (unit: g/cm) of OCE³). Mercury porosimetry gives the volume of mercury penetrated (unit: g/cm)³) This corresponds to the pore volume of the sample used. The apparent density and the volume of Hg penetrated can be used to calculate this porosity.

Porosity = volume of Hg penetrated/apparent density

When the calculated porosity is reported, it is the ratio of the sum of the volumes of the components added to the empty volume, which can be calculated from the density of the OCE.

Pore distribution

Various pore distributions are possible; the OCE of the present invention is notable for a monomodal pore distribution. "monomodal" as used herein means that the pore diameter has a maximum; for a bimodal distribution, two maxima will be obtained.

The present invention is explained in detail below by way of examples, which, however, do not constitute any limitation to the present invention.

All references mentioned above are incorporated by reference in their entirety for all useful purposes.

While certain specific configurations that embody the present invention have been shown and described, it will be obvious to those skilled in the art that various changes and rearrangements of the parts can be made without departing from the spirit and scope of the inventive concept set forth below, and the invention is not limited to the specific forms set forth and described herein.

Examples

The OCE produced according to the latter example was used in chlor-alkali electrolysis. For this purpose, laboratory cells are used, which consist of an anode space and a cathode space separated by an ion-exchange membrane. In the anode space, a sodium chloride solution was used at a concentration of 200-210 g/l, wherein chlorine gas was produced on an industrial DSA coated titanium electrode. The cathode space was separated from the anode space by commercially available cation exchange membranes obtained from DuPont, Nafion 982. There is an electrolyte gap between the OCE and the cation exchange membrane, into which a 32% sodium hydroxide solution is pumped in circulation. The OCE supplies oxygen via the gas space in a concentration greater than 99.5% (by volume). The anode, membrane and gas diffusion electrode areas were each 100 cm². The electrolyte temperature was 90 ℃. The current density of the electrolysis was 4 kA/m in all the experiments²。

OCE was produced as follows: 3.5 kg of a powder mixture consisting of 5-7 wt.% of PTFE powder, 88 wt.% of silver (I) oxide and 5-7 wt.% of silver powder (e.g. 331 from Ferro) were mixed in an Eirich R02 mixer equipped with a star fluidizer as mixing element at a speed of 6000 rpm in such a way that the temperature of the powder mixture did not exceed 55 ℃. This is achieved by stopping the mixing operation and cooling the powder mixture. The mixing was carried out three times for a mixing time of 50 seconds and three times for a mixing time of 60 seconds. After mixing, the powder mixture was sieved through a wire mesh with a mesh size of 1.0 mm. The sieved powder mixture is then applied to an electrically conductive carrier element. The carrier element is a nickel wire mesh having a wire thickness of 0.14 mm and a mesh size of 0.5 mm. The application is carried out by means of a2 mm thick template, the powder being applied with a screen having a mesh size of 1.0 mm. Excess powder that extends beyond the thickness of the die plate is removed using a skimmer. After the template has been removed, the carrier with the applied powder mixture is pressed with a roller press with a compression force of 0.45-0.55 kN/cm. The gas diffusion electrode was removed from the roll press.

The OCE thus produced was subjected to electrochemical reduction in a laboratory cell.

Examples 1 ( According to the invention OCE) BBS 3533-2 Dry process

OCE was produced by a dry process by mixing 5wt% of silver powder (SFQED) commercially available from Ferro, 7wt% of PTFE commercially available from DYNEON TF2053 and 88wt% of silver oxide commercially available from Umicore, followed by pressing with a roller press at a force of 0.46 kN/cm. The electrode was used in the above cell and was at 4 kA/m²In the lower operation, the cell voltage was 2.06V. The average pore diameter of the electrode was 2096 nm and the pore volume was 115 mm³(ii) in terms of/g. The porosity was 50%, the density without mechanical support was 4.21 g/cm³The thickness is 0.48 mm.

Examples 2 ( Different pore volume ) BBS 3543-2 Dry process

(N.B. : comparative example 1 Chinese character shao (a Chinese character of 'shao') 2wt% Is/are as follows PTFE And the pressing force is higher )

OCE was produced by a dry process by mixing 7wt% of silver powder (SFQED) commercially available from Ferro, 5wt% of PTFE commercially available from DYNEON TF2053 and 88wt% of silver oxide commercially available from Umicore, followed by pressing with a roller press at a force of 0.50 kN/cm. The electrode was used in the above cell and was at 4 kA/m²For the lower operation, the cell voltage was 2.18V. The average pore diameter of the electrode was 3042 nm and the pore volume was 78 mm³(ii) in terms of/g. The porosity was calculated to be 33.8% and the density without mechanical support was 4.33 g/cm³The thickness is 0.55 mm.

Examples 3( Comparative examples )

( And embodiments thereof 1 And 2 use of different silver powders )

OCE was produced by a dry process by mixing 7wt% of silver powder commercially available from Ferro ("311" product), 5wt% of PTFE commercially available from DYNEON TF2053 and 88wt% of silver oxide commercially available from Umicore, and then pressing with a roll press at a force of 0.48 kN/cm. The electrode was used in the above cell and was at 4 kA/m²For the lower operation, the cell voltage was 2.47V. The average pore diameter of the electrode was 9515 nm and the pore volume was 42 mm³(ii) in terms of/g. The porosity was 17%, the density without mechanical support was 3.81 g/cm³The thickness is 0.57 mm.

Claims (20)

1. A gas diffusion electrode comprising

An electrically conductive support, and

a porous coating based on an electrochemically active catalyst and a hydrophobic material,

wherein the electrode has a first side facing the oxygen-containing gas and a second side facing the alkaline electrolyte,

wherein the catalyst comprises a noble metal as a catalytically active component,

wherein the hydrophobic material comprises a hydrophobic polymer, and

wherein the coating comprising the catalyst has a thickness of 10-500 mm³Pore volume in g and pore diameter in the range of 100-10,000 nm.

2. The gas diffusion electrode according to claim 1, wherein the noble metal is silver or platinum.

3. Gas diffusion electrode according to claim 1, wherein said coating comprising said catalyst has a thickness of 50-200 mm³Pore volume in g.

4. The gas diffusion electrode according to claim 1, wherein said coating comprising said catalyst has a pore diameter of 600-6,000 nm.

5. The gas diffusion electrode according to claim 1, wherein said coating has a monomodal pore distribution.

6. The gas diffusion electrode according to claim 1, wherein said coating comprising said catalyst has a porosity of 10-70%.

7. A gas diffusion electrode according to claim 3, wherein said coating comprising said catalyst has a porosity of 20-60%.

8. The gas diffusion electrode according to claim 1, wherein said coating comprising said catalyst has a thickness of 20-1,000 μm.

9. The gas diffusion electrode according to claim 1, wherein said coating comprising said catalyst has a thickness of 200-600 μm.

10. The gas diffusion electrode of claim 1 wherein said hydrophobic component comprises a hydrophobic polymer.

11. The gas diffusion electrode according to claim 10, wherein said hydrophobic polymer comprises a fluorine substituted polymer.

12. The gas diffusion electrode of claim 10, wherein said hydrophobic polymer comprises Polytetrafluoroethylene (PTFE).

13. A gas diffusion electrode according to claim 10, wherein the electrode has a thickness of at 5 mg/cm² - 300 mg/cm²(ii) a total loading of said catalytically active component within the range.

14. A gas diffusion electrode according to claim 10, wherein the electrode has a thickness of at 10 mg/cm² - 250 mg/cm²(ii) a total loading of said catalytically active component within the range.

15. A gas diffusion electrode according to claim 1, wherein the support is based on nickel, silver or mixtures thereof.

16. The gas diffusion electrode according to claim 1, wherein said support is in a form selected from the group consisting of a mesh fabric, a woven fabric, a knitted fabric, a nonwoven fabric, an expanded metal, and a foam.

17. A gas diffusion electrode according to claim 1, wherein the support is in the form of a woven fabric.

18. Chlor-alkali electrolysis plant comprising as oxygen-consuming cathode a gas diffusion electrode according to claim 1.

19. A fuel cell comprising a gas diffusion electrode according to claim 1.

20. A metal/air battery comprising a gas diffusion electrode according to claim 1.