DE102018210458A1 - Gas diffusion electrode for carbon dioxide utilization, process for its production and electrolysis cell with gas diffusion electrode - Google Patents

Abstract

The invention relates to a gas diffusion electrode (5) for utilizing carbon dioxide, comprising a metallic support (12) and an electrically conductive catalyst layer (11) with hydrophilic pores and / or channels and hydrophobic pores and / or channels applied thereon, the catalyst layer ( 11) comprises metallic particles (13) which are coated at least in partial areas (16) with a polymeric binding material (15). The invention further relates to a method for producing a gas diffusion electrode (5) for CO utilization, and an electrolysis cell (31, 33, 35) with a corresponding gas diffusion electrode (5).

Description

The present invention relates to a gas diffusion electrode for utilizing carbon dioxide and a method for producing a gas diffusion electrode. The invention further relates to an electrolysis system with a corresponding gas diffusion electrode.

Around 80% of global energy requirements are currently met by burning fossil fuels. These combustion processes emit around 34,000 million tons of the greenhouse gas carbon dioxide (carbon dioxide, CO ₂ ) into the atmosphere annually. As a result of this release into the atmosphere, most of the carbon dioxide is disposed of (for large lignite-fired power plants over 50,000 t per day).

Due to the growing scarcity of resources in fossil fuels and the volatile availability of renewable energy sources, research on CO ₂ reduction is increasingly becoming the focus of interest. This would reduce CO ₂ emissions and CO ₂ could be used as an inexpensive source of carbon.

The discussion about the negative effects of CO ₂ on the climate has led to the consideration of a recycling of CO ₂ . From a thermodynamic point of view, however, CO ₂ is very low and can therefore only be reduced to usable products with difficulty.

Natural carbon dioxide degradation takes place, for example, through photosynthesis. In this process, carbon dioxide is converted into carbohydrates in a process that is divided into many sub-steps in terms of time and on a molecular level.

However, this process is not readily transferable to an industrial scale. A copy of the natural photosynthesis process with large-scale photo catalysis has not been sufficiently efficient so far.

Another method is the electrochemical reduction of carbon dioxide. Systematic studies of the electrochemical reduction of carbon dioxide are still a relatively young field of development. Only a few years ago there have been efforts to develop an electrochemical system that can reduce an acceptable amount of carbon dioxide.

Laboratory-scale research has shown that metals are preferred as catalysts for the electrolysis of carbon dioxide. From the publication Electrochemical CO ₂ reduction on metal electrodes by Y. Hori, published in: C. Vayenas, et al. (Eds.), Modern Aspects of Electrochemistry, Springer, New York, 2008, pp. 89-189, Faraday efficiencies on different metal cathodes are known.

The Faraday efficiencies (FE [%]) given in Table 1 below apply to products that are formed during the carbon dioxide reduction on various metal electrodes. The values given apply to a 0.1 M potassium bicarbonate solution as the electrolyte. Table 1: Faraday efficiencies for the conversion of CO ₂ into products on different metal electrodes electrode CH ₄ C ₂ H ₄ C ₂ H ₅ OH C ₃ H ₇ OH CO HCOO ^- H ₂ 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

If, for example, carbon dioxide is almost exclusively reduced to carbon monoxide on silver, gold, zinc, palladium and gallium cathodes, a large number of hydrocarbons are formed as reaction products on a copper cathode. For example, carbon monoxide and little hydrogen would predominantly be produced on a silver cathode. The reactions at the anode and cathode can be represented with the following reaction equations: Cathode: 2 CO ₂ + 4 e ^- + 4 H ⁺ → 2 CO + 2 H ₂ O Anode: 2 H ₂ O → O ₂ + 4 H ⁺ + 4 e ^-

The electrochemical production of carbon monoxide, methane, ethene and also ethylene is of particular economic interest. The corresponding sum reaction equations are shown below Carbon monoxide: CO ₂ + 2e + H ₂ O → CO + 2 OH ^- ethylene: 2 CO ₂ + 12 e ^- + 8 H ₂ O → C ₂ H ₄ + 12 OH ^- Methane: CO ₂ + 8 e ^- + 6 H ₂ O → CH ₄ + 8 OH ^- ethanol: 2 CO ₂ + 12 e ^- + 9 H ₂ O → C2H ₅ OH + 12 OH ^- Monoethylene glycol: 2 CO ₂ + 10 e ^- + 8 H ₂ O → HOC ₂ H ₄ OH + 10 OH ^-

Systematic studies of the electrochemical reduction of CO _{2 have} only started to take place in recent years. Despite many efforts, it has so far not been possible to develop an electrochemical system which, with a sufficiently high current density and an acceptable yield, has been able to reduce CO _{2 in a} long-term and energetically favorable manner to competitive energy sources.

Due to the growing scarcity of resources in fossil fuels and the volatile availability of regenerative energy sources, research into CO ₂ reduction is becoming increasingly popular. This would reduce CO ₂ emissions and CO ₂ could be used as an inexpensive source of carbon. To ensure a high current density or in attempts to increase it further, only the carbon dioxide reduction taking place on the catalytically active cathode surface of the electrolytic cell has so far been considered.

Electrolysis cells that are suitable for the electrochemical reduction of carbon dioxide usually consist of a cathode compartment and an anode compartment. In order to achieve an effective conversion of the CO _{2 used} , the cathode is ideally designed as a porous gas diffusion electrode. Gas diffusion electrodes (GDE) are porous electrodes in which there are liquid, solid and gaseous phases and the electrically conductive catalyst catalyzes the electrochemical reaction between the liquid and the gaseous phase.

For electrochemical carbon dioxide utilization, catalyst-based gas diffusion electrodes are preferably used, which are similarly known from large-scale chloralkali electrolysis. The catalyst-based gas diffusion electrode can either be in contact with a liquid, salt-containing electrolyte or, in a special case, can be in direct contact with the separator membrane. In the latter case, an ionic connection of the catalyst particles to the membrane is required, since the membrane is used as a solid electrolyte in this mode of operation.

The gas diffusion electrodes used for CO ₂ reduction mostly 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 wide pore radius distribution. The use of gas diffusion electrodes within the electroreduction of CO ₂ in aqueous electrolyte solutions is possible in a relatively narrow process window over a period of> 1000 h.

The anode compartment and cathode compartment of electrolysis cells suitable for the electrochemical reduction of carbon dioxide are typically kept separate from one another in a CO ₂ electrolyzer with a cation-selective membrane, an anion-selective membrane or a diaphragm. This prevents undesired mixing of the gaseous materials formed on the cathode and on the anode.

The penetration of the cathode with the electrolyte used takes place here by two driving forces. The electrostatic attraction of the electrolyte cations should be named as a driving force. On the other hand, anionic species (usually hydrogen carbonate ions) are generated on the cathode, which need a cation to balance the charge. This results in a concentration gradient that leads to the penetration of cations into the electrode.

However, the extent of this penetration often goes beyond what is necessary for ionic binding. Due to electro-osmosis, the electrolyte also reaches the side of the electrode facing away from the electrolyte chamber. In a borderline case, this leads to pore clogging of the electrode, which results in an undesirable undersupply of the catalyst with CO ₂ .

As a further limit case, a strong passage of the aqueous medium through the pores can be observed, which contributes to a flooding of the pore system and also to a CO ₂ undersupply of the catalyst. This problem is frequently observed in electrolyzer assemblies in which the cathode is in direct contact with a liquid, saline electrolyte.

Another problem with this variant is the flooding of the pores with electrolyte. A known cause for 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 structural height of the electrolytic cells. In addition, increasing salt crystallization can be observed in operation in the area on the side facing away from the electrolyte, which on the one hand leads to pore clogging of the electrode, so that the catalyst is also undersupplied with CO ₂ . Furthermore, a strong passage of the aqueous medium through the pores is observed, which contributes to a flooding of the pore system and also to a CO ₂ undersupply of the catalyst.

A stable operating state is achieved while avoiding the limit cases mentioned. It is therefore technically necessary to widen the stable operating window for an industrial application of the technology in order to ensure a more efficient implementation of CO ₂ in the long-term operation of large cells and to avoid the problems described above.

With cell structures without an electrolyte gap or when using a MEA (membrane electrode assembly) with a gas diffusion electrode as the cathode, strong salt formation can occur in the area of the interface between the gas diffusion electrode (cathode) and the separator membrane in electrolysis operation, so that stable electrolysis operation is not guaranteed.

The reason for this is the previously described formation of hydrogen carbonate salts from the cations transported through the membrane and the hydrogen carbonate ions formed at the cathode. These salts cannot be removed without liquid electrolytes. The accumulation of the electrolyte cations in the area of the interface is due to electroosmosis. The concentration gradient cannot be reduced on the electrode side, since a catalyst-based gas diffusion electrode has only very poor ion conductivity.

In summary, the current densities of previously known methods without gas diffusion electrodes are far below the values <100 mA / cm ² relevant for economic use. Technically relevant current densities can be achieved using gas diffusion electrodes. This is known from the existing state of the art, for example, for large-scale chloralkali electrolysis.

In recent times, silver / silver oxide / PTFE-based gas diffusion electrodes for the production of sodium hydroxide solution in the existing chlor-alkali electrolysis process (oxygen consumption electrodes) have become large-scale used. The efficiency of the chlor-alkali electrolysis process could be increased by 30-40% in contrast to the conventional electrode. The methodology of catalyst embedding with PTFE is known from a large number of publications and patents.

The method of catalyst embedding with PTFE is known from a large number of publications. The method of the so-called “drying process” is based on a roller calendar process of PTFE / catalyst powders. The technique on which the method is based is based on EP 0 297 377 A2 attributed, according to which electrodes were made on the basis of Mn ₂ O ₃ for batteries.

In the DE 3 710 168 A1 For the first time, attention is drawn to the application of the drying process with regard to the preparation of metallic electrocatalyst electrodes. The technology was also used in patents for the production of silver-based (silver (I) or silver (II) oxide) gas diffusion electrodes (oxygen consumption electrodes).

The EP 2 444 526 A2 and the DE 10 2005 023 615 A1 show mixtures that have a binder content of 0.5-7%. Ag or nickel nets with a wire diameter of 0.1-0.3 mm and a mesh width of 0.2-1.2 mm were mentioned as carriers. The powder is fed directly onto the network before it is fed to the roller calender.

The DE 10 148 599 A1 and the EP 0 115 845 B1 describe a similar process in which the powder mixture is first extruded into a fur or film, which is pressed onto the mesh in a further step.

The latter method is less suitable than the one-step process described above due to the lower mechanical stability. The EP 2 410 079 A2 describes the one-step process for producing a silver-based oxygen consumption electrode with the addition of metal oxide additives such as TiO ₂ , Fe ₃ O ₄ , Fe ₂ O ₃ , NiO ₂ , Y ₂ O ₃ , Mn ₂ O ₃ , Mn ₅ O ₈ , WO ₃ , CeO ₂ and spinels such as CoAl ₂ O ₄ , Co (AlCr) ₂ O ₄ and inverse spinels such as (Co, Ni, Zn) ₂ (Ti, Al) O ₄ , perovskites such as LaNiO ₃ , ZnFe ₂ O ₄ .

Supplements of silicon nitride, boron nitride, TiN, AlN, SiC, TiC, CrC, WC, Cr ₃ C ₂ , TiCN were also found to be suitable and oxides of the type ZrO ₂ , WO ₃ were identified as particularly suitable. The materials are expressly declared as fillers with no catalytic effect. The aim here is explicitly to reduce the hydrophobic character of the electrode.

From the DE 10 335 184 A1 further catalysts are known which can alternatively be used for oxygen-consuming 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 chevrel phases, e.g. Mo ₄ Ru ₂ Se ₈ or Mo ₄ Ru ₂ S ₈ , which may also contain Pt, Rh, Re, Pd etc.

Known Cu-based gas diffusion electrodes for producing hydrocarbons based on CO ₂ are, for example, in the work of R. Cook [J. Electrochem.Soc., Vol. 137, No. 2, 1990 ] mentioned. There is a wet chemical process based on a PTFE 30B (Suspension) / Cu (OAc) ₂ / Vulkan XC 72 Mixture mentioned. The method describes how a hydrophobic gas transport layer is applied using three coating cycles and a catalyst-containing layer using three further coatings.

After each shift there is a drying phase (325 ° C) followed by a static pressing process ( 1000 - 5000 Psi). A Faraday efficiency of> 60% and a current density of> 400 mA / cm ^{2 were} stated for the electrode obtained. Reproduction experiments, which are given below as comparative examples, prove, however, that the static pressing process described does not lead to stable electrodes. The admixed volcano XC also became a disadvantage 72 found that no hydrocarbons were obtained either.

The calendaring methods described lead to highly porous single-layer electrodes which are characterized by low flow resistances. Due to the high porosity (50-70%) or large pore opening radii, which are caused by this type of production, the correspondingly prepared gas diffusion electrodes have a very narrow operating stability when used for CO ₂ electrolysis in aqueous electrolytes. This is usually characterized by the fact that cations such as Li +, K +, Na + and Cs + of the electrolyte penetrate into the porous structure through the electrical attraction of the cathode and there after the following Form the reaction equation with the OH ions and absorbed CO ₂ hydrogen carbonates, which usually fail due to the high salt content of the electrolyte. M ⁺ + CO ₂ + OH ^- → MHCO ₃ ↓

Another undesirable consequence is the passive penetration of water by diffusion along the concentration gradient (osmosis). This effect is also known as the "water inlet pressure" effect. As a result of the described salinization, depending on the amount of salt and moisture content, the pore structure can be completely blocked. In addition to complete blockage from salt crystals, there is the possibility of complete flooding with electrolyte so that it continuously escapes from the back.

Both limit states lead to the breakdown of stable operation and have a direct effect on the determined Faraday efficiencies or on the achievable current density. The latter phenomenon is also known from the field of chlor-alkali electrolysis and was described in DE 10 2010 054 643 A1 as in EP 2 398 101 A1 regarded as critical, since the liquid passing through can form a continuous film on the back, which prevents further gas entry into the pore system.

To ensure an ideal operating state of a gas diffusion electrode, the three-phase boundary (catalyst / electrolyte / gas) must be stable. When the electrolyte is completely flooded, almost no CO ₂ mass transport is possible, so that the formation of hydrogen predominates in this operating state. With partial flooding with electrolyte, there is only a partial undersupply of CO ₂ . Both states can be converted into one another via pressure influences (differential pressure between the electrolyte and the gas space). A complete suppression of the flooding of the electrode, i.e. the passage of electrolyte, cannot be prevented by both of the systems described above.

Another criterion for the operation of a gas diffusion electrode is the bubble formation point, which is very low due to the high porosity in calendered electrodes with values in the range from 5 mbar to 20 mbar. Electrodes with low bubble formation points react relatively strongly to pressure fluctuations, so that regulating the differential pressure (back pressure of the CO ₂ behind the electrode) by a differential pressure regulator is complicated in a technical application. Like in the DE 10 2013 011 298 A1 a more complicated control loop with the control parameters gas composition, pressure and volume flow is required.

The invention is therefore based on the object of specifying a possibility by means of which a targeted electrochemical CO ₂ utilization can be implemented in a widened operating window.

This object is achieved according to the invention by the features of claims 1, 13 and 16. Advantageous refinements of the invention are set out in the subclaims and the description below.

The gas diffusion electrode according to the invention is used for carbon dioxide utilization and comprises a metallic support and an electrically conductive catalyst layer with hydrophilic pores and / or channels and hydrophobic pores and / or channels applied thereon, the catalyst layer comprising metallic particles which contain at least in some areas with a polymeric binding material are coated.

The gas diffusion electrode according to the invention is both CO and C ₂ H ₄ -selective. The at least partially precoated metallic particles prevent the electrolyte from penetrating due to the very pronounced hydrophobic character of the electrode. The hydrophobization of the catalyst itself is crucial for this.

For this purpose, the metallic particles are already 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 and a stable formation of the three-phase boundary between the catalyst layer, the electrolyte and the respective gas is ensured.

• - Hydrophobierung des Katalysators und damit eine Unterdrückung der Elektroosmose.
• - Die hydrophoben und hydrophilen Bereiche ermöglichen das Vorliegen einer Dreiphasengrenze an jedem metallischen Partikel, wodurch eine präzise Lokalisierung der Reaktionszone nicht zwingend nötig ist.
• - Die Langzeitstabilität der Gasdiffusionselektrode wird durch das inerte Binderpolymer deutlich verbessert.
• - Die Fibrillierung ermöglicht eine hohe Beladung mit dem polymeren Bindermaterial oberhalb von 20 Gew.-%. Dies ist bei einer reinen Zufallsmischung von metallischen Partikeln und Bindermaterial nicht möglich, da die Partikel hier isoliert werden.

Overall, the coating of the metallic particles (preferably silver particles) with the binder material (preferably PTFE) offers the following advantages:

• - Hydrophobization of the catalyst and thus suppression of electroosmosis.
• - The hydrophobic and hydrophilic areas allow the presence of a three-phase boundary on each metallic particle, which means that 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 enables a high loading with the polymeric binder material above 20% by weight. This is not possible with a pure random mixture of metallic particles and binder material, since the particles are isolated here.

In other words, the binding material not only functions in the sense of an “adhesive”, but also prevents the undesired electro-osmosis by the at least partial coating of the metallic particles (catalyst particles), so that no electrolyte reaches the side of the gas diffusion electrode facing away from the electrolyte chamber. The at least partially coated catalyst particles are expediently part of a mixture which, together with the binding material which does not adhere to the catalyst particles, form the catalyst layer of the gas diffusion electrode. The coating of the metallic particles is preferably formed by fibrils formed due to the process.

• - Zugänglichkeit der Katalysatorpartikel für das Eduktgas CO₂ über die hydrophobe Poren.
• - Hydrophile Bereiche in der Katalysatorschicht, die einen Kontakt zwischen dem Elektrolyt und den Katalysatorpartikeln ermöglichen.
• - Hohe elektrische Leitfähigkeit der Gasdiffusionselektrode bzw. der ausgebildeten Katalysatorschicht, sowie eine homogene Potentialverteilung über die gesamte Elektrodenfläche (potentialabhängige Produktselektivität).
• - Hohe chemische und mechanische Stabilität im Elektrolysebetrieb (Unterdrückung von Rissbildung und Korrosion).
• - Definierte Porosität mit einem geeigneten Verhältnis zwischen hydrophilen und hydrophoben Kanälen bzw. Poren in unmittelbarer Nachbarschaft zueinander zur Sicherstellung von CO₂-Verfügbarkeit bei gleichzeitigem Vorhandensein von H⁺-Ionen.

The CO or C ₂ H ₄ -selective gas diffusion electrode further fulfills the requirements shown below, which block the gas diffusion electrode for the electrolyte passage and are accordingly required for the selective product formation according to the invention.

• - Accessibility of the catalyst particles for the starting gas CO ₂ via the hydrophobic pores.
• - Hydrophilic areas in the catalyst layer, which allow contact between the electrolyte and the catalyst particles.
• - High electrical conductivity of the gas diffusion electrode or the formed catalyst layer, as well as a homogeneous potential distribution over the entire electrode area (potential-dependent product selectivity).
• - High chemical and mechanical stability in electrolysis operation (suppression of cracking and corrosion).
• - Defined porosity with a suitable ratio between hydrophilic and hydrophobic channels or pores in the immediate vicinity of one another in order to ensure availability of CO ₂ with simultaneous presence of H ⁺ ions.

All of the particles contained are preferably part of the three-phase limit in order to be able to achieve high current densities. Furthermore, the pore system in the catalyst layer has sufficient absorption of intermediate products to ensure a further reaction or dimerization / oligomerization. The reaction zone is advantageously located directly on the side of the gas diffusion electrode facing the electrolyte.

• - Einheitliche Partikelgröße mit hoher spezifischer Oberfläche
• - Dendritische Morphologie ohne isolierte Zentren oder Cluster
• - Hohe Reinheit ohne Fremdmetallspuren von Übergangsmetallen oder Kohlenstoffbestandteilen wie Ruße oder Kohlen
• - Verwendung von elektrochemisch stabilen Oxide zur Stabilisierung der Strukturdefekte hohe Selektivität und Langzeitstabilität
• - Geringe Überspannung gegenüber der CO₂-Reduktion.

In addition, the catalyst layer particularly meets the following requirements in order to ensure the electrochemical reduction of CO ₂ to ethylene:

• - Uniform particle size with a high specific surface
• - Dendritic morphology with no isolated centers or clusters
• - High purity without traces of foreign metal from transition metals or carbon components such as carbon black or carbon
• - Use of electrochemically stable oxides to stabilize the structural defects, high selectivity and long-term stability
• - Low overvoltage compared to the CO ₂ reduction.

In a particularly advantageous embodiment of the invention, the catalyst layer has a bubble point above 40 mbar, in particular in a range between 80 mbar and 150 mbar. The value of the bubble point indicates the pressure which is necessary to press liquid out of the pores of the catalyst layer. The larger the pore, the lower the pressure to clear it. Air that passes through the empty pore is identified as bubbles. The differential pressure required to squeeze out the first bubble is defined as the bubble point.

The flooding pressure (“wetting point”) of the catalyst layer is preferably above 150 mbar, preferably in a range between 200 mbar and 1000 mbar. Because of the precoated metallic particles, the penetration of fluids into the catalyst layer is therefore only possible at pressures which are significantly higher than in the prior art. This effectively prevents flooding of the gas diffusion electrode.

Silver particles are preferably used as metallic particles. Alternatively, copper particles are expedient as metallic particles. Regardless of the type of metal particles used, these are such that during the production of a particle-binder mixture (to form the catalyst layer) the binding material used - in particular in the form of fibers or fibrils - at least partially wraps around the particles.

The average particle diameter d ₅₀ the metallic particles are preferably in a range between 1 μm and 10 μm, and preferably between 2 μm and 5 μm. Spherical particles are preferably used as metallic particles. The metallic particles preferably have a BET surface area in a range between 0.1 m ² / g and 10 m ² / g. The BET surface area is calculated according to BET = 6 / (δ (Ag) / d (metallic particles).

The catalyst layer expediently comprises promoters which, in cooperation with the metallic particles, improve the catalytic activity of the gas diffusion electrode. The catalyst layer preferably contains at least one metal oxide, which preferably has a lower reduction potential than the ethylene evolution, so that the formation of ethylene from CO _{2 can} be implemented by means of the gas diffusion electrode according to the invention. Furthermore, the metal oxides are preferably not inert, but should preferably be hydrophilic reaction centers which can be used to provide protons.

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

PTFE (polytetrafluoroethylene) is preferably used as the polymeric binder material (binder polymer). Dyneon® TF 9205 and Dyneon TF 1750 especially proven. In particular, 0.1 to 30% by weight of the polymeric binding material, preferably 5 to 25% by weight, and more preferably 15 to 20% by weight, are used. The average particle diameter ( d ₅₀ ) of the polymeric binding material is preferably in a range between 0.5 microns and 20 microns.

After the corresponding catalyst-binder mixture has been produced, it is applied to the metallic support and then solidified. The catalyst layer with the corresponding pores or channels is formed. The porosity of the catalyst layer is preferably in a range between 60% and 80%. The values of the porosity of the catalyst layer relate to the proportion of free spaces (pores and / or channels) within the catalyst layer in comparison 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 preferably in a range between 50:50 and 20:80. The ratio within the catalyst layer can be shifted in favor of the hydrophilic pores. The base layer preferably has only hydrophobic areas.

A mesh with a mesh size between 0.3 mm and 1.4 mm is preferably used as the metallic carrier. This can ensure both adequate mechanical stability and functionality as a gas diffusion electrode, for example with regard to high electrical conductivity. Alternatively, the carrier can also be designed in the form of parallel wires in the context of the invention.

Depending on the metallic particles used, a silver-containing or copper-containing mesh (or a corresponding sheet of wire) is expediently used as the carrier. The mesh used as the metallic carrier, in particular the silver mesh, preferably has a wire diameter in a range between 0.1 mm and 0.25 mm.

The method according to the invention is used to produce a gas diffusion electrode for CO ₂ utilization. The method comprises the production of a mixture of metallic particles and at least one binding material to form a mixture, the application of the mixture to a metallic carrier, and the embedding of the applied mixture in a metallic carrier. According to the invention, the metallic particles are coated with the polymeric binding material at least in some areas during the production of the mixture.

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

The mixing time depends largely on the properties of the catalyst powder, i.e. the metallic particles. In particular, the hardness and the specific surface of the metallic particles have to be taken into account. The catalyst powder and the binding material are preferably present in a homogeneous mixture before the action of large shear forces.

The catalyst layer is preferably produced with a bubble point above 40 mbar, and in particular in a range between 80 mbar and 150 mbar. The catalyst layer is more preferably produced in such a way that the flooding pressure of the catalyst layer is above 150 mbar, preferably in a range between 200 mbar and 1000 mbar.

The mixture can be applied in different ways within the scope of the invention. For example, sprinkling, sieving, knife coating or the like are suitable.

In an advantageous embodiment, the mixture is embedded in the carrier by an extraction process. The extraction process leads to electrodes with a bubble point of 100-250mbar. In the case of calendered (rolled) electrodes, the bubble point of the hydrophobic base layer is in a range between 10 mbar and 20 mbar. The bubble point can be increased by up to 200 mbar by applying a hydrophilic catalyst layer.

In the extraction process, a suspension is produced from the metallic, at least partially coated particles, the polymeric binding material and a solvent, and these are applied to the carrier. A fine-mesh polymer network made of PP is preferably used as the carrier. Alternatively, the use of metal nets is also possible. In particular, N-methyl-2-pyrrolidone, dimethyl sulfoxide and dimethylformamide have proven to be suitable solvents. Alternatively, the use of γ-butyrolactone is also possible. The carrier loaded with the suspension is then immersed in a precipitation bath for the polymeric binding material - filled with a so-called non-solvent for the polymeric binding material. There diffusion leads to an exchange of the solvent of the suspension with non-solvent and thus to phase separation. The polymeric binding material solidifies and forms a porous matrix.

Alternatively, the mixture is preferably embedded in the metallic carrier by dry rolling. The mechanical loading of the binding material by the rolling process leads to the cross-linking of the mixture through the formation of binding channels, for example PTFE fibrils. Reaching this state is particularly important in order to ensure a suitable porosity or mechanical stability of the electrode.

Preferably the mixture is made with a ratio between the exit thickness H and gap width H ₀ in a range between 1 and 1 . 5 rolled on. Where the roller speed range from 1.2 lie Second The roller expediently rotates at a roller speed in a range between 0.5 rpm and 2 rpm. The application is preferably carried out with a flow rate Q in a range between 0.07 m / min and 0.3 m / min. Temperature control of the rollers can also support the flow process. The preferred temperature range is between room temperature and 200 ° C and more preferably between 40-100 ° C.

The degree of fibrillation of the binding material (structural parameter ζ) correlates directly with the applied shear rate, since the binding material, in particular a polymer, behaves as a shear-thinning (pseudoplastic) fluid when it is rolled out. 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 reinforced by further rolling out, but the layer by the elastic Behavior with further exposure to shear forces is damaged. A particularly strong fibrillation can disadvantageously lead to the electrode rolling up on the layer side, so that excessive binder contents should be avoided.

The advantages and preferred embodiments described for the gas diffusion electrode according to the invention apply equally to the method according to the invention and can be applied accordingly to this.

The electrolysis cell according to the invention comprises a gas diffusion electrode according to one of the above-described configurations. The gas diffusion electrode is preferably used here as a cathode. The gas diffusion electrode is preferably designed specifically for operation in plate electrolyzers. The electrolysis cell is expediently designed on the cathode side for reducing carbon dioxide.

The other constituents of the electrolytic cell, such as the anode, possibly one or more membranes, supply line (s) and discharge line (s), the voltage source and further optional devices such as cooling or heating devices are fundamentally 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 preferred embodiments described for the gas diffusion electrode according to the invention and the method according to the invention apply equally to the electrolysis cell according to the invention and can be correspondingly transferred to it accordingly.

The production of a CO-selective gas diffusion electrode is explained in more detail below.

At the beginning, the catalyst precursor (catalyst precursor) silver (I) oxide Ag ₂ O was prepared by precipitation (2 AgNO ₃ + 2 NaOH → Ag ₂ O + 2 NaNO ₃ + H ₂ O). For this purpose, 200 g (1.177 mol) of silver nitrate dissolved in water were heated to 60 ° C. A 4M sodium hydroxide solution (160 g / l) was added dropwise with stirring. The precipitated silver (I) oxide, the product was centrifuged off, washed neutral and then dried.

The mixing of the catalyst particles (metallic particles) and the binding material and thus the at least partial coating of the particles took place both in an Eirich mixer EL1 and in an IKA-A10 knife mill with a hard metal knife (WC). 35 g of dry silver (I-) oxide precipitated according to the procedure described above, 10 g of purified silver powder (d ₅₀ = 2 μm-3 μm) and then 15 g of PTFE (Dyneon TF 2021) were used in each case. The total load was 60g.

Mixing took place in the Eirich mixer at a rotational speed of the star whirler of 2 to 7 m / sec. for 5 minutes with the same direction of rotation of the mixing container and swirler. A mixing process in the IKA knife mill lasted 15 seconds and was repeated 7 times with a 15 second pause each. Manual stirring was carried out after each run. The resulting powder mixtures were kept airtight in closed containers.

Table 2 shows the relationships between different mixing times and the respective Vickers hardness for the Eirich mixer and the IKA knife mill. Table 2: Mixing time for PTFE (Dyneon TF 2021) depending on the Vickers hardness of the metallic particles used Vickers hardness Mixing time IKA A10 Mixing time EL1 [kp / mm ² ) (Wida) Eirich (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 for the subsequent production of the gas diffusion electrode. The premixed powder mixture obtained (from the at least partially coated metallic particles and the binding material) was applied to a metal mesh. For this purpose, a silver wire mesh (mesh: 60 x 60) with a mesh size of 0.296 mm and a wire diameter of 0.127 mm was used.

The weight of the network was 0.48 kg / m ² and had a size of 2.13 mx 305 mm.

The silver wire mesh was first degreased with acetone and the surface was etched using 2N HNO ₃ (30 to 60 min). The silver wire mesh was blown off with compressed air and glued on one side with a Capton adhesive tape. Using a vacuum plate, the glued wire mesh was fixed flat and a stainless steel frame with a thickness of 0.5mm and an open area of L _x = 120 mm * 60 mm was placed.

The premixed powder was applied by sieving through a PP sieve with a mesh size of w = 1 mm, so that initially only a thin, uniform covering was achieved. The powder was then stirred in the sieve with a plastic spatula and applied continuously until the frame height was reached. The excess amount of powder on the frame edge is completely removed.

For this purpose, a peeling tool with a rounded peeling edge was used. This was held at a 10 ° angle to the surface and quickly moved back and forth during the pulling process. After the removal, loosely packed areas were screened with repetition of the removal process until a uniform surface was reached.

Both the extraction process (a) and the dry calendaring process (b) were used to embed the mixture and thus to form a corresponding gas diffusion electrode. Both methods are explained separately below.

Production of a gas diffusion electrode using 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 in an IKA A10 knife mill 7 times for 15 seconds each. 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 powder and 2 g of PTFE was added to this solution and dispersed twice for 30 seconds with an Ultraturraxx. The resulting dispersion was knife-coated onto a PP network and the NMP was extracted in a water / isopropanol bath (50% by volume isopropanol). Curing was then carried out in a second water bath, in which the oligomers are ultimately linked.

Production of a gas diffusion electrode using the dry calendaring process

To embed the mixture using the dry calendaring process, it was rolled into the network structure. In a two-roll calender of the Dima B64E type with a roll width of 130 mm and a roll diameter of 64 mm, the film is extruded at a belt speed of 30 cm / min with a gap thickness of 0.6 mm. The Capton film is then removed and the nip is reduced to 0.3 mm and the electrode is rolled again.

Rolling the coated particles into the mesh structure is expressly desired in order to ensure a high mechanical stability of the electrode, this is not the case with the previously mentioned two-stage process, here the pre-extruded film rests only on the mesh. The mechanical stress on the plastic particles by the rolling process leads to the crosslinking of the powder through the formation of PTFE fibrils. This ensures the appropriate porosity or 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 / cm ² using a platinum counter electrode. The distance between the electrodes was 2 cm. After the current was switched on, the silver (I) oxide was immediately reduced across the entire surface. The clamping voltage rose from 3.8 V to 5.8 V and remained constant at this value. After an activation time of 30 minutes, the electrode was removed and the surface was washed off under running deionized water with rubbing off, so that the dark veil formed could be removed. The electrode was washed with isopropanol and ether for easier drying.

• 1 in schematischer Darstellung einen Ausschnitt einer Katalysatorschicht gemäß Stand der Technik,
• 2 in schematischer Darstellung einen Ausschnitt einer weiteren Katalysatorschicht gemäß Stand der Technik,
• 3 in schematischer Darstellung einen Ausschnitt einer erfindungsgemäßen Katalysatorschicht gemäß Stand der Technik,
• 4 eine Auftragung des zeitlichen Verlaufs der Faraday-Effizienz einer Gasdiffusionselektrode,
• 5 eine Auftragung des zeitlichen Verlaufs der Faraday-Effizienz einer weiteren Gasdiffusionselektrode,
• 6 eine schematische Darstellung einer Elektrolysezelle mit einer Gasdiffusionselektrode,
• 7 eine schematische Darstellung einer alternativen Elektrolysezelle mit einer Gasdiffusionselektrode, sowie
• 8 eine schematische Darstellung einer weiteren Elektrolysezelle mit einer Gasdiffusionselektrode.

Exemplary embodiments of the invention are explained in more detail below with reference to a drawing. Show:

• 1 a schematic representation of a section of a catalyst layer according to the prior art,
• 2 a schematic representation of a section of a further catalyst layer according to the prior art,
• 3 a schematic representation of a section of a catalyst layer according to the prior art,
• 4 a plot of the time course of the Faraday efficiency of a gas diffusion electrode,
• 5 a plot of the time course of the Faraday efficiency of a further gas diffusion electrode,
• 6 1 shows a schematic illustration of an electrolysis cell with a gas diffusion electrode,
• 7 a schematic representation of an alternative electrolytic cell with a gas diffusion electrode, and
• 8th is a schematic representation of another electrolytic cell with a gas diffusion electrode.

In the 1 to 3 are each a schematic representation of sections of gas diffusion electrodes 1 . 3 . 5 with appropriate catalyst layers 7 . 9 . 11 shown. Each of the catalyst layers is on a metallic support 12 (in the present case only indicated by an arrow) applied or embedded in this.

The catalyst layers 9 and 9 the gas diffusion electrodes 1 . 3 according to the 1 and 2 show examples of the prior art, with the cutout according to 3 it is a gas diffusion electrode according to the invention 5 ,

Based on 3 it can be seen that the gas diffusion electrode produced by the method according to the invention 5 a catalyst layer 11 with at least partially coated metallic particles 13 having. The particles are with PTFE as a polymeric binding material 15 in some areas 16 coated, which the particles 13 in the form of fibrils 17 partially encloses.

The fibrils are different 17 according to 1 between mixed molecules of the binding material 15 and the metallic particles 13 arranged. The metallic particles are not fibrillated. This is a single-layer gas diffusion electrode 1 ,

In 2 there is the gas diffusion electrode 3 with two separate layers 19 . 21 , Between the metallic particles 13 in the first shift 19 and the binding material molecules 15 in the second shift 21 are also fibrils 17 arranged.

The 4 and 5 show applications here 23 . 26 the time course of the Faraday efficiencies, which are used in the electrochemical characterization of a gas diffusion electrode according to the invention 5 were obtained. The gas diffusion electrode 5 was made using the dry calendaring process.

In the 4 and 5 the Faraday efficiency [%] is plotted depending on the current density [J / mA * cm ^-2 ]. The result is a Faraday efficiency for CO ₂ of 100% (curve 24) and for H ₂ of 0% (curve 25) at 30 ° C in 0.5MK ₂ SO ₄ and 1M KHCO ₃ at 250mA / cm ² ( 4 ). At 30 ° C in 0.5MK ₂ SO ₄ and 1M KHCO ₃ at 300mA / cm ² , a Faraday efficiency for CO ₂ of 80% (curve 27), for H ₂ of about 20% (curve 28) results ( 5 ).

The following could also be observed: The production of the powder mixture with 25% by weight of PTFE required a stronger mixing device than the IKA A10 due to the high viscosity. The sieving of the powder mixture was associated with more effort than with comparable mixtures with 5% by weight to 10% by weight of PTFE. The overall porosity of the electrode did not increase significantly with a higher PTFE content.

The following correlations could be identified: With the roll gap remaining the same, the total porosity of the gas diffusion electrode 5 are influenced by the activation, so that when the current density is halved in a range from 50 to 400 mA / cm ^2, the porosity increases by approximately 50%. PTFE contents of 5% by weight and 10% by weight were not sufficiently high to prevent permeation of the electrolyte. With a PTFE content of 25% it was possible to flood the gas diffusion electrode 11 can be prevented in electrolysis operation.

In the 6 to 8th are various electrolysis cells in a schematic representation 31 . 33 . 35 shown, which are in principle for an electrochemical reduction of CO ₂ , each with a gas diffusion electrode according to the invention 5 suitable.

The electrolytic cell 31 according to 6 shows a 3-chamber structure with an anode compartment I and a cathode compartment II, which are separated from each other by a membrane M. The cathode compartment II according to FIG 6 designed so that a catholyte is supplied from below and then leaves the cathode compartment II upwards. Alternatively, the catholyte can also be supplied from above, for example in the case of falling film electrodes. At the anode A, which is connected to the cathode K (gas diffusion electrode 5 ) is electrically connected by means of a current source to provide the voltage for the electrolysis, the oxidation of a substance takes place in the anode compartment I, which is supplied from below, for example, with an anolyte. The anolyte leaves the anode compartment together with the oxidation product.

In addition, in the electrolysis cell 31 according to 6 a reaction gas, in particular carbon dioxide, is conveyed through the gas diffusion electrode into the cathode compartment II for reduction. Although not shown, embodiments with a porous anode are also conceivable.

In contrast, the PEM (proton or ion exchange membrane) structure of the electrolysis cell 33 according to 7 the cathode K (gas diffusion electrode 5 ) and a porous anode A directly on the membrane M, whereby the anode compartment I is separated from the cathode compartment II.

The structure of the electrolytic cell 35 according to 8th corresponds to a mixed form of the structures according to the 6 and 7 , on the catholyte side a structure with the gas diffusion electrode 5 (according to 6 ) and on the anolyte side a structure according to 7 is provided. Mixed forms or other configurations of the electrode spaces shown as examples are of course also conceivable. Embodiments without a membrane are also conceivable. According to certain embodiments, the cathode-side electrolyte and the anode-side electrolyte can thus be identical, so that the respective electrolysis cell / electrolysis unit can be formed without a membrane M. It is equally possible that the respective electrolysis cell has a membrane M in such embodiments.

According to certain embodiments, the distance between the electrode and the membrane is very small or 0 if the membrane is porous and contains a supply of the electrolyte. The membrane can also have a multi-layer design, so that separate feeds of anolyte or catholyte are made possible. Separation effects are achieved in aqueous electrolytes, for example, by the hydrophobicity of intermediate layers. Conductivity can nevertheless be guaranteed if conductive groups are integrated in such separating layers. The membrane can be an ion-conducting membrane or a separator which only brings about a mechanical separation and is permeable to cations and anions.

By using the gas diffusion electrode according to the invention 5 it is possible to build a three-phase electrode. For example, a gas can flow from behind to the electrically active front side of the gas diffusion electrode 5 be carried out in order to carry out an electrical-chemical reaction there. Alternatively, the gas diffusion electrode 5 also only flow behind, a gas such as in particular CO ₂ on the rear side of the gas diffusion electrode 5 in relation to the electrolyte. The gas then penetrates the pores of the gas diffusion electrode 5 and the product is discharged at the rear.

The gas flow when flowing behind is preferably reversed to the flow of the electrolyte, so that any liquid that has been pressed through can be removed. Here too, a gap between the gas diffusion electrode and the membrane is advantageous as an electrolyte reservoir.

Further details on preferred embodiments of electrolysis cells according to the invention with gas diffusion electrodes for carbon dioxide utilization are also from the DE 10 2015 215 309 A1 refer to.

The above embodiments, refinements and developments can, if appropriate, be combined with one another as desired. Further possible refinements, developments and implementations of the invention also include combinations of features of the invention described above or below with reference to the exemplary embodiments, which are not explicitly mentioned. In particular, the person skilled in the art will also add individual aspects as improvements or additions to the respective basic form of the present invention.

LIST OF REFERENCE NUMBERS

1

Gas diffusion electrode (according to the prior art)

3

Gas diffusion electrode (according to the prior art)

5

Gas diffusion electrode

7

catalyst layer

9

catalyst layer

11

catalyst layer

12

metallic support

13

metallic particles

15

binding material

16

coated sections

17

fibril

19

first layer gas diffusion electrode

21

second layer gas diffusion electrode

23

Plotting Faraday efficiency

24

Faraday efficiency for CO ₂

25

Faraday efficiency for H ₂

26

Plotting Faraday efficiency

27

Faraday efficiency for CO ₂

28

Faraday efficiency for H ₂

31

electrolysis cell

33

electrolysis cell

35

electrolysis cell

QUOTES INCLUDE IN THE DESCRIPTION

This list of documents listed by the applicant has been generated automatically and is only included for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• EP 0297377 A2 [0027]
• DE 3710168 A1 [0028]
• EP 2444526 A2 [0029]
• DE 102005023615 A1 [0029]
• DE 10148599 A1 [0030]
• EP 0115845 B1 [0030]
• EP 2410079 A2 [0031]
• DE 10335184 A1 [0033]
• DE 102010054643 A1 [0038]
• EP 2398101 A1 [0038]
• DE 102013011298 A1 [0040]
• DE 102015215309 A1 [0109]

Non-patent literature cited

• R. Cook [J. Electrochem.Soc., Vol. 137, No. 2, 1990 [0034]

Claims (16)

Gas diffusion electrode (5) for carbon dioxide utilization, comprising a metallic support (12) and an electrically conductive catalyst layer (11) with hydrophilic pores and / or channels and hydrophobic pores and / or channels applied thereon, the catalyst layer (11) being metallic particles (13), which are coated at least in partial areas (16) with a polymeric binding material (15). Gas diffusion electrode (5) after Claim 1 , wherein the catalyst layer (11) has a bubble formation point above 40 mbar, in particular in a range between 80 mbar and 150 mbar. Gas diffusion electrode (5) after Claim 1 or 2 , wherein the flooding pressure of the catalyst layer (11) is above 150 mbar, preferably in a range between 200 mbar and 1000 mbar. Gas diffusion electrode (5) according to any one of the preceding claims, wherein silver particles are used as metallic particles (13). Gas diffusion electrode (5) according to one of the preceding claims, wherein the mean particle diameter (d ₅₀ ) of the metallic particles (13) is in a range between 1 µm and 10 µm, and preferably between 2 µm and 5 µm. Gas diffusion electrode (5) according to any one of the preceding claims, wherein the metallic particles (13) have a specific BET surface area in a range between 0.1 m ² / g and 10 m ² / g. Gas diffusion electrode (5) according to any one of the preceding claims, wherein 0.1 to 30 wt .-% of the polymeric binding material (15), in particular 5 to 25 wt .-%, and more preferably 15 to 20 wt .-% are used. Gas diffusion electrode (5) according to any one of the preceding claims, wherein the mean particle diameter (d ₅₀ ) of the polymeric binding material (15) is in a range between 0.5 µm and 20 µm. Gas diffusion electrode (5) according to any one of the preceding claims, wherein the porosity of the catalyst layer (11) is in a range between 60% and 80%. Gas diffusion electrode (5) according to one of the preceding claims, wherein the ratio of hydrophilic pores and / or channels and hydrophobic pores and / or channels in the catalyst layer (11) is in a range between 50:50 and 20:80. Gas diffusion electrode (5) according to one of the preceding claims, wherein a mesh, preferably a silver mesh with a mesh size between 0.3 mm and 1.4 mm, is used as the metallic carrier (12). Gas diffusion electrode (5) according to any one of the preceding claims, wherein the network has a wire diameter in a range between 0.1 mm and 0.25 mm. Method for producing a gas diffusion electrode (5) for CO ₂ utilization, comprising - producing a mixture of metallic particles (13) and at least one binding material (15) while forming a mixture, - applying the mixture to a metallic carrier (12), and - Embedding the applied mixture in the metallic carrier (12), the metallic particles (13) being coated with the polymeric binding material (15) at least in partial areas (16) during the production of the mixture. Procedure according to Claim 13 , wherein the mixture is embedded in the metallic carrier (12) by means of an extraction process. Procedure according to Claim 13 , the mixture being embedded in the metallic carrier (12) by dry rolling. Electrolysis cell (31, 33, 35), comprising a gas diffusion electrode (5) according to one of the Claims 1 to 12 ,

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