GAS DIFFUSION ELECTRODE FOR CARBON DIOXIDE TREATMENT, METHOD FOR PRODUCTION THEREOF, AND ELECTROLYSIS CELL HAVING A GAS DIFFUSION ELECTRODE

Abstract

A gas diffusion electrode for carbon dioxide treatment includes a metal substrate and an electrically conductive catalyst layer which is applied thereto and has hydrophilic pores and/or channels and hydrophobic pores and/or channels, the catalyst layer including metal particles which are coated at least in regions with a polymeric binder material. A method produces a gas diffusion electrode for CO2 treatment, and an electrolysis cell has a corresponding gas diffusion electrode.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2019/062595 filed 16 May 2019, and claims the benefit thereof. The International Application claims the benefit of German Application No. DE 10 2018 210 458.1 filed 27 Jun. 2018. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The present invention relates to a gas diffusion electrode for utilization of carbon dioxide and also a process for producing a gas diffusion electrode. The invention further relates to an electrolysis system comprising a corresponding gas diffusion electrode.

BACKGROUND OF INVENTION

At present, about 80% of worldwide energy requirements are covered by the combustion of fossil fuels. As a result of these combustion processes, about 34 000 million metric tons of the greenhouse gas carbon dioxide (CO₂) are emitted into the atmosphere every year worldwide. The major part of the carbon dioxide is disposed of by this liberation into the atmosphere (in the case of large brown coal power stations, more than 50 000 metric tons per day).

Owing to the increasing scarcity of fossil fuel resources and the volatile availability of renewable energy sources, research into the reduction of CO₂ is of ever increasing interest. In this way CO₂ emissions would be decreased and the CO₂ could be utilized as inexpensive carbon source.

The discussion about the adverse effects of CO₂ on the climate has led to reutilization of CO₂ being considered. However, CO₂ is thermodynamically in a very low position and can therefore be reduced again to give usable products only with difficulty.

A natural degradation of carbon dioxide occurs, for example, by means of photosynthesis. Here, carbon dioxide is converted into carbohydrates in a process divided into many substeps over time and spatially on a molecular level. However, this process cannot readily be carried over to an industrial scale. A copy of the natural photosynthesis process using industrial photocatalysis has hitherto not been sufficiently efficient.

A further method is the electrochemical reduction of carbon dioxide. Systematic studies on the electrochemical reduction of carbon dioxide are still a relatively young field of development. Only since a few years ago have efforts been made to develop an electrochemical system which can reduce an acceptable amount of carbon dioxide.

Research studies on a laboratory scale have shown that metals are preferably to be used as catalysts for electrolysis of carbon dioxide. Faraday efficiencies at various metal cathodes are known from the publication Electrochemical CO₂ reduction on metal electrodes by Y. Hori, published in: C. Vayenas, et al. (Eds.), Modern Aspects of Electrochemistry, Springer, N.Y., 2008, pp. 89-189.

The Faraday efficiencies (FE [%]) reported in table 1 below apply to products which are formed in the reduction of carbon dioxide at various metal electrodes. The values indicated are for a 0.1 M potassium hydrogencarbonate solution as electrolyte.

TABLE 1
Faraday efficiencies for the conversion of CO2 into products at various metal electrodes
Electrode	CH4	C2H4	C2H5OH	C3H7OH	CO	HCOO−	H2	Total
Cu	33.3	25.5	5.7	3.0	1.3	9.4	20.5	103.5
Au	0.0	0.0	0.0	0.0	87.1	0.7	10.2	98.0
Ag	0.0	0.0	0.0	0.0	81.5	0.8	12.4	94.6
Zn	0.0	0.0	0.0	0.0	79.4	6.1	9.9	95.4
Pd	2.9	0.0	0.0	0.0	28.3	2.8	26.2	60.2
Ga	0.0	0.0	0.0	0.0	23.2	0.0	79.0	102.0
Pb	0.0	0.0	0.0	0.0	0.0	97.4	5.0	102.4
Hg	0.0	0.0	0.0	0.0	0.0	99.5	0.0	99.5
In	0.0	0.0	0.0	0.0	2.1	94.9	3.3	100.3
Sn	0.0	0.0	0.0	0.0	7.1	88.4	4.6	100.1
Cd	1.3	0.0	0.0	0.0	13.9	78.4	9.4	103.0
Tl	0.0	0.0	0.0	0.0	0.0	95.1	6.2	101.3
Ni	1.8	0.1	0.0	0.0	0.0	1.4	88.9	92.4
Fe	0.0	0.0	0.0	0.0	0.0	0.0	94.8	94.8
Pt	0.0	0.0	0.0	0.0	0.0	0.1	95.7	95.8
Ti	0.0	0.0	0.0	0.0	0.0	0.0	99.7	99.7

While carbon dioxide is, for example, reduced virtually exclusively to carbon monoxide at silver, gold, zinc, palladium and gallium cathodes, many hydrocarbons are formed as reaction products at a copper cathode. Thus, for example, predominantly carbon monoxide and little hydrogen would be formed at a silver cathode. The reactions at anode and cathode can be represented by the following reaction equations:

Cathode: 2 CO₂+4 e⁻⁺4 H⁺→2 CO+2 H₂O

Anode: 2 H₂O→O₂+4 H⁺+4 e⁻

The electrochemical production of, for example, carbon monoxide, methane, ethene and also ethylene is of particular economic interest. The corresponding overall reaction equations are shown below:

Carbon monoxide: CO₂+2e⁻+H₂O→CO+2 OH⁻
Ethylene: 2 CO₂+12 e⁻+8 H₂O→C₂H₄₊₁₂ OH⁻
Methane: CO₂+8 e⁻+6 H₂O→CH₄+8 OH⁻
Ethanol: 2 CO₂+12 e⁻+9 H₂O→C₂H₅OH+12 OH⁻
Monoethylene glycol: 2 CO₂+10 e⁻+8 H₂O→HOC₂H₄OH+10 OH⁻

Only in recent years have there been an increased number of systematic studies on the electrochemical reduction of CO₂. Despite many efforts, there has hitherto not been any success in developing an electrochemical system by means of which CO₂ could be reduced to competitive energy carriers in a long-term stable manner and energetically favorably at a sufficiently high current density and acceptable yield.

Owing to the increasing scarcity of fossil fuel resources and the volatile availability of renewable energy sources, research into the reduction of CO₂ is of ever increasing interest. In this way CO₂ emissions would be decreased and the CO₂ could be utilized as inexpensive carbon source. However, to ensure a high current density or in attempts to increase this further, only the carbon dioxide reduction occurring at the catalytically active cathode surface of the electrolysis cell has hitherto been examined.

Electrolysis cells which are suitable for the electrochemical reduction of carbon dioxide usually consist of a cathode space and an anode space. To achieve effective reaction of the CO₂ introduced, the cathode is ideally configured as a porous gas diffusion electrode. Gas diffusion electrodes (GDEs) are porous electrodes in which liquid, solid and gaseous phases are present and the electrically conductive catalyst catalyzes the electrochemical reaction between the liquid phase and the gaseous phase.

Catalyst-based gas diffusion electrodes which are known in a similar way from industrial chloralkali electrolysis are preferably used for the electrochemical utilization of carbon dioxide. The catalyst-based gas diffusion electrode can either be in contact with a liquid, salt-containing electrolyte or in a special case can rest directly against the separator membrane. In the latter case, ionic attachment of the catalyst particles to the membrane is necessary since the membrane is used as solid electrolyte in this mode of operation.

The gas diffusion electrodes used in CO₂ reduction usually consist of a mixture of an inorganic metal catalyst (Ag, Au, Cu, Pb, etc.) and an organic binder (PTFE, PVDF, PFA, FEP, PFSA). The prepared electrodes are characterized by a high connectivity of the pores and a broad pore radius distribution. The use of gas diffusion electrodes in the electroreduction of CO₂ in aqueous electrolyte solutions is possible within a relatively narrow process window over a time of >1000 hours.

Anode space and cathode space of electrolysis cells suitable for the electrochemical reduction of carbon dioxide are typically kept separate from one another in a CO₂ electrolyzer by means of a cation-selective membrane, an anion-selective membrane or a diaphragm. This prevents undesirable mixing of the gaseous materials of value formed at the cathode and at the anode.

Passage of the electrolyte used through the cathode occurs as a result of two driving forces. One driving force is the electrostatic attraction of the electrolyte cations. Secondly, anionic species (generally hydrogencarbonate ions) are generated at the cathode and require a cation for balancing the charge. This results in a concentration gradient which leads to penetration of cations into the electrode.

However, the extent of this passage frequently goes beyond the degree necessary for ionic attachment. Owing to electroosmosis, the electrolyte also gets to the side of the electrode facing away from the electrolyte chamber. This leads in a limiting case to blockage of the pores of the electrode, which results in an undesirable undersupply of CO₂ to the catalyst.

As a further limiting case, substantial passage of the aqueous medium through the pores, which contributes to flooding of the pore system and likewise to an undersupply of CO₂ to the catalyst, can be observed. These problems are frequently observed in the case of electrolyzer constructions in which the cathode is in direct contact with a liquid, salt-containing electrolyte.

A further problem associated with this variant is flooding of the pores with electrolyte. A known cause of the penetration of electrolyte into the pores of the electrode is the hydrostatic pressure of the water column in the electrolyte gap, which limits the industrial construction height of the electrolysis cells. Furthermore, increasing salt crystallization can be observed in the region of the side facing away from the electrolyte during operation, and this leads firstly to blockage of the pores of the electrode, so that an undersupply of CO₂ to the catalyst is also the consequence here. Furthermore, substantial passage of the aqueous medium through the pores, which contributes to flooding of the pore system and likewise to an undersupply of CO₂ to the catalyst, is observed.

A stable operating state is achieved when the limiting cases mentioned are avoided. Consequently, it is technically necessary to widen the stable operating window for industrial use of the technology in order to ensure more efficient conversion of the CO₂ in long-term operation of large cells and to avoid the above-described problems.

In the case of cell constructions without an electrolyte gap or when using an MEA (membrane electrode assembly) having a gas diffusion electrode as cathode, severe salt formation in the region of the interface between gas diffusion electrode (cathode) and the separator membrane can occur during electrolysis operation, so that stable electrolysis operation is not ensured.

A cause of this is the above-described formation of hydrogencarbonate salts from the cations transported through the membrane and the hydrogencarbonate ions formed at the cathode. Without liquid electrolytes, these salts cannot be removed. The accumulation of the electrolyte cations in the region of the interface is attributable to electroosmosis. The concentration gradient cannot be dissipated on the electrode side since a catalyst-based gas diffusion electrode has only a very poor ionic conductivity.

In summary, the current densities of previously known methods without gas diffusion electrodes are far below the values of <100 mA/cm² relevant for economical use. Industrially relevant current densities can be achieved using gas diffusion electrodes. This is known from the existing prior art, for example for industrially operated chloralkali electrolyses.

Thus, silver/silver oxide/PTFE-based gas diffusion electrodes have been used industrially in recent times for the production of sodium hydroxide in the existing chloralkali electrolysis process (oxygen-depolarized electrodes). The efficiency of the chloralkali electrolysis process was able to be increased by 30-40% compared to a conventional electrode. The methodology of embedding catalyst in PTFE is known from many publications and patents.

The method of embedding catalyst in PTFE is known from many publications. The methodology of the “dry process” is based on a roller calendering process of PTFE/catalyst powders. The technique underlying the method is attributable to EP 0 297 377 A2, in which Mn₂O₃-based electrodes for batteries were produced.

In DE 3 710 168 A1, reference is made for the first time to the use of the dry process in the preparation of metallic electrocatalyst electrodes. The technique was also used in patents for producing silver-based (silver(I) oxide or silver(II) oxide) gas diffusion electrodes (oxygen-depolarized electrodes).

EP 2 444 526 A2 and DE 10 2005 023 615 A1 disclose mixtures having a binder content of 0.5-7%. As carrier, mention was made of Ag or nickel gauzes having a wire diameter of 0.1-0.3 mm and a mesh opening of 0.2-1.2 mm. The powder is applied directly to the gauze before it is fed to the roller calender.

DE 10 148 599 A1 and EP 0 115 845 B1 describe a similar process in which the powder mixture is firstly extruded to give a sheet or film which is pressed onto the gauze in a further step.

DE 10 2015 215 309 A1 relates to a gas diffusion electrode comprising a copper-containing support and a first layer comprising at least copper and at least one binder, wherein the layer comprises hydrophilic and hydrophobic pores and/or channels. Here, a plurality of layers are applied individually in the form of mixtures to the support and then rolled on together. The particle sizes of copper and binder are set suitably. The mechanical stressing of the binder by the rolling process leads to cros slinking of the powder by formation of binder channels, for example PTFE fibrils. The degree of fibrillation of the binder correlates directly with an applied shear rate.

Owing to the lower mechanical stability, the latter method is less suitable than the above-described single-stage process. EP 2 410 079 A2 describes the single-stage process for producing a silver-based oxygen-depolarized electrode with addition of metal oxide additives such as TiO₂, Fe₃O₄, Fe₂O₃, NiO₂, Y₂O₃, Mn₂O₃, Mn₅O₈, W0₃, CeO₂ and also spinels such as CoAl₂O₄, Co(AlCr)₂O₄ and also inverse spinels such as (Co,Ni,Zn)₂ (Ti,Al)O₄, perovskites such as LaNiO₃, ZnFe₂O₄.

Additions of silicon nitride, boron nitride, TiN, AN, SiC, TiC, CrC, WC, Cr₃C₂, TiCN have likewise been found to be suitable, and oxides of the type ZrO₂, WO₃ were identified as being particularly suitable. The materials are expressly said to be fillers without catalytic activity. The objective here is expressly the reduction of the hydrophobic character of the electrode.

DE 10 335 184 A1 discloses further catalysts which can alternatively be used for oxygen-depolarized electrodes: noble metals, e.g. Pt, Rh, Ir, Re, Pd, noble metal alloys, e.g. Pt-Ru, noble metal-containing compounds, e.g. noble metal-containing sulfides and oxides, and also Chevrel phases, e.g. Mo₄Ru₂Se₈ or Mo₄Ru₂S₈, with these also being able to contain Pt, Rh, Re, Pd, etc.

Known Cu-based gas diffusion electrodes for producing hydrocarbons on the basis of CO₂ are mentioned, for example, in the studies by R. Cook [J. Electrochem. Soc., Vol. 137, No. 2, 1990]. There, mention is made of a wet chemical process based on a PTFE 30B (suspension)/Cu(OAc)₂/Vulkan XC 72 mixture. The method describes how a hydrophobic gas transport layer is applied by means of three coating cycles and a catalyst-comprising layer is applied by means of three further coating operations.

After each layer, a drying phase (325° C.) with a subsequent static pressing operation (1000-5000 Psi) is carried out. A Faraday efficiency of >60% and a current density of >400 mA/cm² were reported for the electrode obtained. However, reproduction experiments indicated subsequently as comparative examples demonstrate that the static pressing process described does not lead to stable electrodes. Likewise, an adverse influence of the added Vulcan XC 72 was likewise established, so that once again no hydrocarbons were obtained.

The calendering methods described lead to highly porous single-layer electrodes which are characterized by a low flow resistance. As a result of the high porosity (50-70%) and the large pore opening radii brought about by such a method of production, the gas diffusion electrodes prepared in this way have a very narrow operating window when used for CO₂ electrolysis in aqueous electrolytes. This is usually characterized by cations such as Li+, K+, Na+ and Cs+ of the electrolyte having penetrated into the porous structure due to the electric attraction of the cathode and there forming hydrogen carbonates with OH ions formed and absorbed CO₂ according to the following reaction equation, which hydrogen carbonates usually precipitate because of the high salt content of the electrolyte:

M⁺+CO₂+OH⁻→MHCO₃↓

As a further undesirable consequence, passive penetration of water as a result of diffusion along the concentration gradient (osmosis) is observed. This effect is also known as the “water entry pressure effect”. As a result of the salt formation described, complete blockage of the pore structure can occur, depending on the amount of salt and moisture content. Apart from complete blockage by salt crystals, there is the possibility of complete flooding with electrolyte, so that this exits continuously at the rear side.

Both limiting states lead to breakdown of stable operation and therefore have a direct effect on the product Faraday efficiencies determined and on the achievable current density. The latter phenomenon is also known from the field of chloralkali electrolysis and has been considered to be critical in DE 10 2010 054 643 A1 and EP 2 398 101 A1 since the liquid passing through can form a continuous film on the rear side, which film prevents further passage of gas into the pore system.

In order to ensure an ideal operating state of a gas diffusion electrode, stable formation of the three-phase boundary (catalyst/electrolyte/gas) has to be ensured. In the case of complete flooding with electrolyte, CO₂ mass transfer is virtually impossible, so that the formation of hydrogen predominates in this operating state. In the case of partial flooding with electrolyte, only a partial undersupply with CO₂ is present. The two states can be converted into one another by pressure influences (differential pressure between the electrolyte and the gas space). Complete suppression of flooding of the electrode, i.e. passage of electrolyte, cannot be prevented by means of the two above-described systems.

A further criterion for operation of a gas diffusion electrode is the bubble formation point which because of the high porosity is very low with values in the range from 5 mbar to 20 mbar in the case of calendered electrodes. Electrodes having low bubble formation points react relatively strongly to pressure fluctuations, so that regulation of the differential pressure (buildup pressure of the CO₂ behind the electrode) by means of a differential pressure regulator is complicated in an industrial application. As in DE 10 2013 011 298 A1, a complicated regulating loop with the regulation parameters gas composition, pressure and volume flow becomes necessary.

SUMMARY OF INVENTION

It is therefore an object of the invention to provide a possibility by means of which a targeted electrochemical utilization of CO₂ can be implemented in a widened operating window.

This object is achieved according to the invention by the features of the independent claims. Advantageous embodiments of the invention are set forth in the dependent claims and the following description.

The gas diffusion electrode of the invention is used for utilization of carbon dioxide and comprises a metallic support and an electrically conductive catalyst layer which has been applied to this metallic support and has hydrophilic pores and/or channels and hydrophobic pores and/or channels, wherein the catalyst layer comprises metallic particles which are coated at least in subregions with a polymeric binder.

The gas diffusion electrode of the invention is both CO-selective and C₂H₄-selective. Due to the at least partially precoated metallic particles, the penetration of the electrolyte is prevented by a very strongly pronounced hydrophobic character of the electrode. The hydrophobicization of the catalyst itself is critical to this.

The metallic particles are for this purpose coated with binder fibers or binder material fibrils before application to the support, as a result of which the hydrophobicity of the pores in the catalyst layer is increased. The penetration of the electrolyte into the electrode is prevented in this way and stable formation of the three-phase boundary between the catalyst layer, the electrolyte and the respective gas is ensured.

Overall, coating of the metallic particles (advantageously silver particles) with the binder material (advantageously PTFE) offers the following advantages:—Hydrophobicization of the catalyst and thus suppression of electroosmosis.—The hydrophobic and hydrophilic regions make the presence of a three-phase boundary at each metallic particle possible, as a result of which precise localization of the reaction zone is not absolutely necessary.—The long-term stability of the gas diffusion electrode is significantly improved by the inert binder polymer.—The fibrillation allows a high loading with the polymeric binder material above 20% by weight. This is not possible in the case of purely random mixing of metallic particles and binder material since the particles are insulated here.

In other words, the binder material in the present case functions not only as an “adhesive” but, as a result of the at least partial coating of the metallic particles (catalyst particles), also prevents undesirable electroosmosis in a targeted manner, so that no electrolyte gets to the side of the gas diffusion electrode facing away from the electrolyte chamber. The at least partially coated catalyst particles are advantageously part of a mixture which together with the binder material not adhering to catalyst particles form the catalyst layer of the gas diffusion electrode. The coating on the metallic particles is advantageously formed by fibrils formed as a result of the process.

The CO- and C₂H₄-selective gas diffusion electrode also satisfies the following requirements which block the passage of electrolyte through the gas diffusion electrode and are accordingly necessary for the selective product formation according to the invention.—Accessibility of the catalyst particles for the feedgas CO₂ via the hydrophobic pores.—Hydrophilic regions in the catalyst layer which allow contact between the electrolyte and the catalyst particles.—High electrical conductivity of the gas diffusion electrode or the catalyst layer formed, and also homogeneous distribution of the potential over the entire electrode area (potential-dependent product selectivity).—High chemical and mechanical stability in electrolysis operation (suppression of crack formation and corrosion).—Defined porosity with a suitable ratio between hydrophilic and hydrophobic channels and pores in the direct vicinity of one another to ensure CO₂ availability in the simultaneous presence of H⁺ ions.

Preference is given to all particles present being part of the three-phase boundary, so as to be able to achieve high current densities. Furthermore, the pore system in the catalyst layer displays sufficient absorption of intermediates to ensure further reaction or dimerization/oligomerization. The reaction zone is advantageously located directly on the side of the gas diffusion electrode facing the electrolyte.

Furthermore, the catalyst layer satisfies, in particular, the following requirements in order to ensure the electrochemical reduction of CO₂ to ethylene:—Uniform particle size with high specific surface area—Dendritic morphology without isolated centers or clusters—High purity without traces of foreign transition metals or carbon constituents such as soot or carbonized material—Use of electrochemically stable oxides for stabilizing the structural defects and high selectivity and long-term stability—Low overvoltage for the reduction of CO₂.

In a particularly advantageous embodiment of the invention, the catalyst layer has a bubble formation point (“bubble point”) above 40 mbar, in particular in the range from 80 mbar to 150 mbar. The value of the bubble formation point indicates the pressure which is necessary to push liquid out of the pores of the catalyst layer. The larger the pore, the smaller the pressure required to make it free. Air which travels through the empty pore is detected as bubbles. The differential pressure necessary to push out the first bubble is defined as the bubble formation point.

The flooding pressure (“wetting point”) of the catalyst layer is advantageously above 150 mbar, advantageously in the range from 200 mbar to 1000 mbar. The penetration of fluids into the catalyst layer can thus occur only at pressures which are significantly higher compared to the prior art because of the precoated metallic particles. Flooding of the gas diffusion electrode is effectively prevented in this way.

Preference is given to using silver particles as metallic particles. As an alternative, copper particles are advantageous as metallic particles. Regardless of the type of metallic particles used, these have such a nature that the binder material used, in particular in the form of fibers or fibrils, at least partially wraps around the particles during production of a particle/binder mixture (for formation of the catalyst layer).

The average particle diameter d₅₀ of the metallic particles is advantageously in the range from 1 μm to 10 μm and more advantageously from 2 μm to 5 μm. Spherical particles are advantageously used as metallic particles. The metallic particles advantageously have a BET surface area in the range from 0.1 m²/g to 10 m²/g. The BET surface area is calculated according to BET=6/(δ(Ag)/d(metallic particles).

The catalyst layer advantageously comprises promoters which interact with the metallic particles to improve the catalytic activity of the gas diffusion electrode. The catalyst layer advantageously contains at least one metal oxide which advantageously has a lower reduction potential than the evolution of ethylene, so that the formation of ethylene from CO₂ by means of the gas diffusion electrode of the invention is made possible. Furthermore, the metal oxides are advantageously not inert but instead should advantageously represent hydrophilic reaction sites which can serve to provide protons.

The polymeric binder material advantageously has a strongly pronounced shear-thinning behavior, so that fiber formation takes place during the mixing process. The fibers or fibrils of the polymeric binder material which are formed during the mixing process wrap around or become laid around the metallic particles without completely enclosing the surface. The binder polymer is advantageously stable in a strongly alkaline environment.

PTFE (polytetrafluoroethylene) is advantageously used as polymeric binder (binder polymer). As powders, Dyneon® TF 9205 and Dyneon TF 1750 have been found to be particularly useful. Particular advantage is given to using from 0.1 to 30% by weight of the polymeric binder material, advantageously from 5 to 25% by weight and more advantageously from 15 to 20% by weight. The average particle diameter (d₅₀) of the polymeric binder material is advantageously in the range from 0.5 μm to 20 μm.

After production of the appropriate catalyst/binder mixture, this is applied to the metallic support and subsequently consolidated. The catalyst layer having the appropriate pores or channels is formed here. The porosity of the catalyst layer here is advantageously in the range from 60% to 80%. The values of the porosity of the catalyst layer here relate to the proportion of the free spaces (pores and/or channels) within the catalyst layer relative to the volume of the catalyst layer.

The ratio of hydrophilic pores and/or channels and hydrophobic pores and/or channels in the catalyst layer is advantageously in the range from 50:50 to 20:80. Here, the ratio within the catalyst layer can have been shifted in favor of the hydrophilic pores. The base layer advantageously has exclusively hydrophobic regions.

As metallic support, advantage is given to using a gauze having a mesh opening in the range from 0.3 mm to 1.4 mm. This makes it possible to ensure both a satisfactory mechanical stability and also the functionality as gas diffusion electrode, for example in respect of a high electrical conductivity. As an alternative, the support can also be in the form of parallel wires for the purposes of the invention.

Depending on the metallic particles used, a silver-containing or copper-containing gauze (or a corresponding sheet-like structure made of wire) is advantageously used as support. The gauze used as metallic support, in particular the silver gauze, advantageously has a wire diameter in the range from 0.1 mm to 0.25 mm.

The process of the invention serves to produce a gas diffusion electrode for utilization of CO₂. The process comprises production of a mixture of metallic particles and at least one binder to form a mixture, application of the mixture to a metallic support and embedding of the applied mixture into a metallic support. According to the invention, the metallic particles are coated at least in subregions with the polymeric binder during the production of the mixture.

As a result of the production of a mixture of metallic particles and binder material before application of the mixture, the desired partial coating of the metallic particles with fibers or fibrils is achieved, so that an increased hydrophobicity of the boundary layer of the gas diffusion electrode is achieved. The mixing procedure and the at least partial coating of the metallic particles associated therewith is the property-dominating process step for the gas diffusion electrode.

The mixing time depends critically on the properties of the catalyst powder, i.e. the metallic particles. The hardness and the specific surface area of the metallic particles are of particular significance here. The catalyst powder and the binder material are advantageously present in a homogeneous mixture before application of high shear forces.

The catalyst layer is advantageously produced with a bubble formation point above 40 mbar and in particular in the range from 80 mbar to 150 mbar. Further advantage is given to the catalyst layer being produced in such a way that the flooding pressure of the catalyst layer is above 150 mbar, advantageously in the range from 200 mbar to 1000 mbar.

For the purposes of the invention, the application of the mixture can be carried out in various ways. Suitable methods are, for example, sprinkling on, sieving on, doctor blade coating or the like.

In an advantageous embodiment, the mixture is embedded by means of an extraction process into the support. The extraction process leads to electrodes having a bubble point of 100-250 mbar. In the case of calendered (rolled) electrodes, the bubble point of the hydrophobic base layer is in the range from 10 mbar to 20 mbar. Application of a hydrophilic catalyst layer makes it possible to increase the bubble point by up to 200 mbar.

In the extraction process, a suspension is produced from the metallic, at least partially coated particles, the polymeric binder and a solvent and this suspension is applied to the support. Preference is given to using a fine-meshed polymer gauze composed of PP as support. The use of metal gauzes is also possible as an alternative. Suitable solvents have been found to be, in particular, N-methyl-2-pyrrolidone, dimethyl sulfoxide and dimethylformamide. As an alternative, the use of γ-butyrolactone is also possible. The support laden with the suspension is then dipped into a precipitation bath for the polymeric binder, filled with a nonsolvent for the polymeric binder. There, replacement of the solvent of the suspension by nonsolvent occurs as a result of diffusion and phase separation thus occurs. The polymeric binder solidifies and forms a porous matrix.

As an alternative, advantage is also given to the mixture being embedded by dry rolling-on into the metallic support. The mechanical stressing of the binder material by the rolling process leads to cros slinking of the mixture as a result of formation of binder channels, for example PTFE fibrils. The attainment of this state is particularly important in order to ensure a suitable porosity and mechanical stability of the electrode.

The mixture is advantageously rolled on with a ratio between the exit thickness H and gap width H₀ in the range from 1 to 1.5, with the rate of rotation of the roller being in the range from 1.2 to 2. The roller advantageously rotates at a rate of rotation in the range from 0.5 rpm to 2 rpm. Application is advantageously carried out at a flow rate Q in the range from 0.07 m/min to 0.3 m/min. In addition, heating of the rollers can assist the flow process. The advantageous temperature range is from room temperature to 200° C. and more advantageously 40-100° C.

The degree of fibrillation of the binder material (structural parameter) correlates directly with the applied shear rate since the binder material, in particular a polymer, behaves as shear-thinning (non-Newtonian) fluid on rolling out during application. After application, the layer obtained has an elastic character due to the fibrillation. This structural change is irreversible, so that this effect can no longer be increased subsequently by further rolling-out, but instead the layer is damaged on further action of shear forces due to the elastic behavior. A particularly high degree of fibrillation can disadvantageously lead to a layer-side rolling together of the electrode, so that excessively high contents of binder should be avoided.

The advantages and advantageous embodiments described for the gas diffusion electrode of the invention apply equally to the process of the invention and can accordingly be carried over analogously to this.

The electrolysis cell of the invention comprises a gas diffusion electrode as per one of the above-described embodiments. The gas diffusion electrode is advantageously used as cathode here. The gas diffusion electrode is advantageously configured specifically for operation in plate electrolyzers. The electrolysis cell is advantageously configured on the cathode side for the reduction of carbon dioxide.

The further constituents of the electrolysis cell, for instance the anode, optionally one or more membranes, feed conduit(s) and discharge conduit(s), the voltage source and also further optional facilities such as cooling or heating devices, are basically variable according to the invention. The same applies to the anolytes and/or catholytes which are used in such an electrolysis cell.

The advantages and advantageous embodiments described for the gas diffusion electrode of the invention and the process of the invention apply equally to the electrolysis cell of the invention and can accordingly be carried over analogously to this.

In the following, the production of a CO-selective gas diffusion electrode is explained in more detail.

Firstly, the catalyst precursor silver(I) oxide Ag₂O was produced by precipitation (2 AgNO₃+2 NaOH→Ag₂O+2 NaNO₃+H₂O). For this purpose, 200 g (1.177 mol) of silver nitrate dissolved in water were heated to 60° C. A 4 M sodium hydroxide solution (160 g/l) was added dropwise while stirring. The precipitated silver(I) oxide was centrifuged off as product, washed until neutral and subsequently dried.

Mixing of the catalyst particles (metallic particle) and the binder and thus the at least partial coating of the particles were carried out both in an Eirich mixer EL1 and also in an IKA-A10 cutter mill using cemented carbide (WC) cutters. 35 g of dry silver(I) oxide which had been precipitated by the above-described procedure, 10 g of purified silver powder (d₅₀=2μm-3 μm) and then 15 g of PTFE (Dyneon TF 2021) were introduced in each case. The total loading was 60 g.

In the Eirich mixer, mixing was carried out at a speed of rotation of the star swirler of from 2 to 7 m/sec. for 5 minutes with the same direction of rotation of mixing vessel and swirler. A mixing operation in the IKA cutter mill took 15 seconds and was repeated 7 times with a pause of 15 seconds each time. After each passage, manual stirring was carried out. The resulting powder mixtures were stored airtight in closed containers.

Table 2 shows the relationships between various mixing times and the respective Vickers hardness for the Eirich mixer and the IKA cutter mill.

TABLE 2
Mixing time for PTFE (Dyneon TF 2021) as a function of the Vickers
hardness of the metallic particles used
Vickers hardness	Mixing time in IKA A10	Mixing time in EL1 Eirich
(kp/mm2)	(widia)	(widia)
5-30	7 * 15 sec.	30 * 15 sec.
50-100	6 * 15 sec.	20 * 15 sec.
1000-2000	3 * 15 sec.	10 * 15 sec.

The dry calendering process was used here for the subsequent production of the gas diffusion electrode. The premixed power mixture obtained (composed of the at least partially coated metallic particles and the binder material) was applied to a metal gauze. A woven silver wire mesh (mesh: 60×60) having a mesh opening of 0.296 mm and a wire diameter of 0.127 mm was used for this purpose. The weight of the gauze was 0.48 kg/m² and it had a size of 2.13 m×305 mm.

The woven silver wire mesh was firstly degreased by means of acetone and the surface was pickled by means of 2N HNO₃ (30-60 min). The woven silver wire mesh was blown dry with compressed air and provided on one side with a stuck-on Capton adhesive tape. The woven wire mesh provided with the adhesive tape was fixed flat by means of a vacuum plate and a stainless steel frame having a thickness of 0.5 mm and an open area of L_x=120 mm*60 mm was placed on top.

The application of the premixed powder was carried out by sieving-on through a PP sieve having a mesh opening of w=1 mm, so that only a thin uniform covering was firstly achieved. The powder was then stirred up in the sieve using a plastic spatula and applied continuously until the height of the frame had been reached. The excess amount of powder on the frame periphery is removed completely.

A striking tool having a rounded striking edge was used for this purpose. This tool was held at an angle of 10° to the substrate and quickly moved back and forth during the striking operation. After striking, further sieving with repetition of the striking operation was carried out in loosely packed regions until a uniform surface had been obtained.

To embed the mixture and thus form a corresponding gas diffusion electrode, both the extraction process (a) and also the dry calendering process (b) were used. The two processes are explained separately below.

(a) Production of a Gas Diffusion Electrode by Means of the Extraction Process

To produce a gas diffusion electrode by means of the extraction process, 48 g of silver powder (2-3 μm) were mixed with 2 g of PTFE Dyneon TF 2021 7 times for 15 seconds each time in an IKA A10 cutter mill. Furthermore, 14 g of PVDF were dissolved in 68 ml of N-methyl-2-pyrrolidone (NMP) at 80° C. with stirring (20% by weight). The mixture of 48 g of silver power and 2 g of PTFE was added to this solution and dispersed twice for 30 seconds using an Ultraturrax. The resulting dispersion was applied by means of doctor blade coating to a PP gauze and the NMP was extracted in a water/isopropanol bath (50% by volume of isopropanol). Curing was then carried out in a second water bath, with the oligomers finally being linked together.

(b) Production of a Gas Diffusion Electrode by Means of the Dry Calendering Process

To embed the mixture by means of the dry calendering process, the mixture was rolled into the gauze structure. In a two-roll calender model Dima B64E having a roll width of 130 mm and a roll diameter of 64 mm, film extrusion is carried out at a tape speed of 30 cm/min at a gap thickness of 0.6 mm. The Capton film is then removed and the roller gap is reduced to 0.3 mm and the electrode is rolled again.

Rolling of the coated particles into the gauze structure is expressly desired in order to ensure a high mechanical stability of the electrode, which is not the case in the abovementioned two-stage process where the pre-extruded film only rests on the gauze. The mechanical stressing of the polymer particles by the rolling process leads to cros slinking of the powder by formation of PTFE fibrils. This ensures the suitable porosity and mechanical stability of the electrode.

The electrochemical activation of the electrode was carried out in 2.5 M KOH at a current density of 200 mA/cm² using a platinum counterelectrode. The electrode spacing was 2 cm. After switching on the current, an immediate reduction of the silver(I) oxide over the full area occurred. The clamping voltage rose from 3.8 V to 5.8 V and remained constant at this value. The electrode was taken out after an activation time of 30 minutes and the surface was rinsed under flowing deionized water with rubbing, so that the dark veil formed could be removed. The electrode was washed with isopropanol and ether to simplify drying.

BRIEF DESCRIPTION OF THE DRAWINGS

Working examples of the invention will be described in more detail below with the aid of a drawing. The drawing shows:

FIG. 1 a schematic depiction of a section of a catalyst layer as per the prior art,

FIG. 2 a schematic depiction of a section of a further catalyst layer as per the prior art,

FIG. 3 a schematic depiction of a section of a catalyst layer according to the invention as per the prior art,

FIG. 4 a representation of the course over time of the Faraday efficiency of a gas diffusion electrode,

FIG. 5 a representation of the course over time of the Faraday efficiency of a further gas diffusion electrode,

FIG. 6 a schematic depiction of an electrolysis cell having a gas diffusion electrode,

FIG. 7 a schematic depiction of an alternative electrolysis cell having a gas diffusion electrode, and

FIG. 8 a schematic depiction of a further electrolysis cell having a gas diffusion electrode.

DETAILED DESCRIPTION OF INVENTION

FIGS. 1 to 3 each schematically show sections of gas diffusion electrodes 1, 3, 5 with corresponding catalyst layers 7, 9, 11. Each of the catalyst layers has been applied to a metallic support 12 (in the present case merely indicated by an arrow) and embedded therein.

The catalyst layers 7 and 9 of the gas diffusion electrodes 1, 3 as per FIGS. 1 and 2 show examples of the prior art; the section as per FIG. 3 concerns a gas diffusion electrode 5 according to the invention.

It can be seen from FIG. 3 that the gas diffusion electrode 5 produced by the process of the invention comprises a catalyst layer 11 with at least partially coated metallic particles 13. The particles are coated in subregion 16 with PTFE as polymeric binder material 15 which partially encloses the particles 13 in the form of fibrils 17.

In contrast thereto, the fibrils 17 of FIG. 1 are arranged between mixed molecules of the binder material 15 and the metallic particles 13. The metallic particles are not fibrilated. The depiction here is of a single-layer gas diffusion electrode 1.

In FIG. 2, the gas diffusion electrode 3 has two separate layers 19, 21. Fibrils 17 are in each case likewise arranged between the metallic particles 13 in the first layer 19 and between the binder material molecules 15 in the second layer 21.

FIGS. 4 and 5 show representations 23, 26 of the course over time of the Faraday efficiencies which were obtained in the electrochemical characterization of a gas diffusion electrode 5 according to the invention. The gas diffusion electrode 5 was produced by means of the dry calendering process.

In FIGS. 4 and 5, the Faraday efficiency [%] is in each case shown as a function of the current density [J/mA*cm⁻²]. A Faraday efficiency for CO₂ of 100% (curve 24) and for H₂ of 0% (curve 25) is obtained at 30° C. in 0.5 M K₂SO₄ and 1 M KHCO₃ at 250 mA/cm² (FIG. 4). At 30° C. in 0.5 M K₂SO₄ and 1 M KHCO₃ at 300 mA/cm², a Faraday efficiency for CO₂ of 80% (curve 27) and for H₂ of about 20% (curve 28) is obtained (FIG. 5).

Furthermore, the following was observed: the production of the powder mixture containing 25% by weight of PTFE required a stronger mixing apparatus than the IKA A10 because of the high viscosity. Sieving-on of the powder mixture was associated with greater difficulty than in the case of comparable mixtures containing 5% by weight to 10% by weight of PTFE. The total porosity of the electrode did not increase significantly at a higher PTFE content.

The following relationships were able to be identified: at a constant roller gap, the total porosity of the gas diffusion electrode 5 could be influenced by the activation so that when the current density was halved in the range from 50 to 400 mA/cm² the porosity increased by about 50%. PTFE contents of 5% by weight and 10% by weight were not sufficiently high to prevent permeation of the electrolyte. At a PTFE content of 25%, flooding of the gas diffusion electrode 11 in electrolysis operation could be prevented.

FIGS. 6 to 8 schematically show various electrolysis cells 31, 33, 35 which are suitable in principle for electrochemical reduction of CO₂ using in each case a gas diffusion electrode 5 according to the invention.

The electrolysis cell 31 of FIG. 6 displays a 3-chamber structure having an anode space I and a cathode space II which are separated from one another by a membrane M. The cathode space II of FIG. 6 is, by way of example, configured so that a catholyte is fed in from below and then leaves the cathode space II in an upward direction. As an alternative, the catholyte can also be fed in from the top, as in the case of, for example, falling film electrodes. At the anode A which is electrically connected to the cathode K (gas diffusion electrode 5) by means of a current source for providing the voltage for the electrolysis, the oxidation of a material which is fed in from the bottom, for example with an anolyte, takes place in the anode space I. The anolyte leaves the anode space together with the oxidation product.

In addition, a reaction gas such as, in particular, carbon dioxide can be conveyed through the gas diffusion electrode into the cathode space II for reduction in the electrolysis cell 31 of FIG. 6. Although not shown, embodiments having a porous anode are also conceivable.

In contrast thereto, the cathode K (gas diffusion electrode 5) and a porous anode A lie directly against the membrane M in the PEM (proton- or ion-exchange membrane) structure of the electrolysis cell 33 shown in FIG. 7, so that the anode space I is separated from the cathode space II.

The structure of the electrolysis cell 35 of FIG. 8 corresponds to a mixed form of the structures of FIGS. 6 and 7, with a structure having the gas diffusion electrode 5 (as per FIG. 6) being provided on the catholyte side and a structure as per FIG. 7 being provided on the anolyte side. Of course, mixed forms or other variants of the electrode spaces presented by way of example are also conceivable. Embodiments without a membrane are also conceivable. In particular embodiments, the cathode-side electrolyte and the anode-side electrolyte can thus be identical, so that the respective electrolysis cell/electrolysis unit can be constructed without a membrane M. It is equally possible for the respective electrolysis cell in such embodiments to have a membrane M.

In particular embodiments, the distance between electrode and membrane is very small or 0 when the membrane has a porous configuration and includes an introduction facility for the electrolyte. The membrane can also have a multilayer configuration so that separate introductions of anolyte and catholyte are made possible. Separation effects are achieved in the case of aqueous electrolytes by, for example, the hydrophobicity of intermediate layers. Conductivity can nevertheless be ensured when conductive groups are integrated into such separation layers. The membrane can be an ion-conducting membrane or a separator which effects only mechanical separation and is permeable to cations and anions.

As a result of the use of the gas diffusion electrode 5 accordig to the invention, it is possible to construct a three-phase electrode. For example, a gas can be supplied from behind to the electrically active front side of the gas diffusion electrode 5 in order to carry out an electrochemical reaction there. As an alternative, flow can also occur only to the rear side of the gas diffusion electrode 5, with a gas such as, in particular CO₂ being conveyed past the rear side of the gas diffusion electrode 5 relative to the electrolyte. The gas then penetrates through the pores of the gas diffusion electrode 5 and the product is discharged at the rear side.

The gas flow in the case of flow on the rear side is advantageously in the direction opposite to the flow of the electrolyte, so that any liquid which has been pushed through can be transported away. Here too, a gap between the gas diffusion electrode and the membrane is advantageous as electrolyte reservoir.

Further details regarding particular embodiments of electrolysis cells according to the invention having gas diffusion electrodes for utilization of carbon dioxide may also be found in DE 10 2015 215 309 A1.

The above embodiments, configurations and further developments can, if it serves a purpose, be combined with one another in any way. Further possible configurations, further developments and implementations of the invention also encompass combinations not explicitly mentioned of features of the invention described above or in the following in respect of the working examples. In particular, a person skilled in the art will also add individual aspects as improvements or supplementations to the respective basic form of the present invention.

LIST OF REFERENCE NUMERALS

1 Gas diffusion electrode (as per the prior art)

3 Gas diffusion electrode (as per the prior art)

5 Gas diffusion electrode

7 Catalyst layer

9 Catalyst layer

11 Catalyst layer

12 Metallic support

13 Metallic particles

15 Binder material

16 Coated subregions

17 Fibrils

19 First layer of gas diffusion electrode

21 Second layer of gas diffusion electrode

23 Representation of Faraday efficiency

24 Faraday efficiency for CO₂

25 Faraday efficiency for H₂

26 Representation of Faraday efficiency

27 Faraday efficiency for CO₂

28 Faraday efficiency for H₂

31 Electrolysis cell

33 Electrolysis cell

35 Electrolysis cell

Claims

1-16. (canceled)

17. A gas diffusion electrode for utilization of carbon dioxide, comprising:

a metallic support, and

an electrically conductive catalyst layer which has been applied to this metallic support and has hydrophilic pores and/or channels and hydrophobic pores and/or channels,

wherein the catalyst layer comprises metallic particles which are coated at least in subregions with a polymeric binder,

wherein, in the production of the gas diffusion electrode, the metallic particles are coated with fibers of the binder material before application to the support.

18. The gas diffusion electrode as claimed in claim 17,

wherein the catalyst layer has a bubble formation point above 40 mbar.

19. The gas diffusion electrode as claimed in claim 17,

wherein the flooding pressure of the catalyst layer is above 150 mbar.

20. The gas diffusion electrode as claimed in claim 17,

wherein silver particles have been used as metallic particles.

21. The gas diffusion electrode as claimed in claim 17,

wherein the average particle diameter of the metallic particles is in the range from 1 μm to 10 μm.

22. The gas diffusion electrode as claimed in claim 17,

wherein the metallic particles have a specific BET surface area in the range from 0.1 m2/g to 10 m2/g.

23. The gas diffusion electrode as claimed in claim 17,

wherein from 0.1 to 30% by weight of the polymeric binder in each case based on a catalyst/binder mixture from which the catalyst layer is formed, have been used.

24. The gas diffusion electrode as claimed in claim 17,

wherein the average particle diameter of the polymeric binder is in the range from 0.5 μm to 20 μm.

25. The gas diffusion electrode as claimed in claim 17,

wherein the porosity of the catalyst layer is in the range from 60% to 80%.

26. The gas diffusion electrode as claimed in claim 17,

wherein the ratio of hydrophilic pores and/or channels and hydrophobic pores and/or channels in the catalyst layer is in the range from 50:50 to 20:80.

27. The gas diffusion electrode as claimed in claim 17,

wherein a gauze, preferably a silver gauze having a mesh opening in the range from 0.3 mm to 1.4 mm, has been used as metallic support.

28. The gas diffusion electrode as claimed in claim 17,

wherein the gauze has a wire diameter in the range from 0.1 mm to 0.25 mm.

29. A process for producing a gas diffusion electrode for utilization of CO2, comprising:

producing a mixture of metallic particles and at least one binder material to form a mixture,

applying the mixture to a metallic support, and

embedding the applied mixture into the metallic support,

wherein the metallic particles are coated at least in subregions with the polymeric binder material during the production of the mixture and the metallic particles are coated with fibers of the binder material before application to the support and wherein an electrically conductive catalyst layer having hydrophilic pores and/or channels and hydrophobic pores and/or channels is produced by:

embedding the mixture by means of an extraction process into the metallic support, or

embedding the mixture by dry rolling-on into the metallic support.

30. An electrolysis cell, comprising:

a gas diffusion electrode as claimed in claim 17.

31. The gas diffusion electrode as claimed in claim 18,

wherein the catalyst layer has a bubble formation point is in the range from 80 mbar to 150 mbar.

32. The gas diffusion electrode as claimed in claim 19,

wherein the flooding pressure of the catalyst layer is in the range from 200 mbar to 1000 mbar.

33. The gas diffusion electrode as claimed in claim 21,

wherein the average particle diameter of the metallic particles is in the range from 2 μm to 5 μm.

34. The gas diffusion electrode as claimed in claim 23,

wherein from 5 to 25% by weight of the polymeric binder, in each case based on a catalyst/binder mixture from which the catalyst layer is formed, have been used.

35. The gas diffusion electrode as claimed in claim 23,

wherein from 15 to 20% by weight of the polymeric binder, in each case based on a catalyst/binder mixture from which the catalyst layer is formed, have been used.