DE102018210303A1 - Low temperature electrochemical reverse water gas shift reaction - Google Patents

Abstract

The present invention relates to a method for the electrochemical conversion of a gas comprising CO and a device for the electrochemical conversion of a gas comprising CO, on a cathode of an electrolysis cell which comprises a metal which is selected from the group comprising Ag, Au, Zn, Pd, Cu, and / or alloys and / or mixtures thereof, a gas comprising CO is converted, preferably to a product gas comprising CO, and wherein a gas comprising H is converted to a product comprising protons at an anode of the electrolytic cell.

Description

The present invention relates to a method for the electrochemical conversion of a gas comprising CO ₂ and a device for the electrochemical conversion of a gas comprising CO ₂ .

The burning of fossil fuels currently covers around 80% of the world's energy needs. In 2011, around 34,032.7 million tons of carbon dioxide (CO ₂ ) were emitted into the atmosphere as a result of these combustion processes. This release is the easiest way to dispose of large amounts of CO ₂ (lignite-fired power plants over 50,000 tonnes per day).

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

In nature, CO _{2 is} converted to carbohydrates by photosynthesis. This process, which is divided into many sub-steps in terms of time and at the molecular level, is very difficult to copy on an industrial scale. The electrochemical reduction of CO ₂ s is currently the more efficient way compared to pure photocatalysis. A mixed form is light-assisted electrolysis or electrically assisted photocatalysis. Both terms are to be used synonymously, depending on the perspective of the viewer.

As with photosynthesis, this process transforms CO ₂ into an energetically higher-quality product (such as CO, CH ₄ , C ₂ H ₄ ) by supplying electrical energy (possibly photo-supported), which is preferably obtained from renewable energy sources such as wind or sun. etc.) converted. The amount of energy required for this reduction ideally corresponds to the combustion energy of the fuel and should only come from renewable sources. However, an overproduction of renewable energies is not continuously available, but currently only at times with strong sunshine and / or strong winds. However, this will intensify with the further expansion of facilities for the generation of renewable energy in the near future.

Some possible ways of producing energy sources and / or chemical raw materials based on renewable energies are currently being discussed. The direct electrochemical or photochemical conversion of CO ₂ into hydrocarbons and / or their oxygen derivatives is particularly desirable.

However, no industrial-grade catalysts are currently available for these direct routes. Multi-stage routes are therefore also under discussion, which promise a timely solution due to the higher degree of technical maturity of the individual steps.

The most important intermediate in these multi-level value chains is CO. It is generally considered the most important C1 building block in synthetic chemistry. In the synthesis gas mixture (H ₂ / CO> 2/1) with hydrogen, it can be used, for example, via the Fischer-Tropsch process to build up hydrocarbons and / or for methanol synthesis. CO-rich gas mixtures or pure CO can also be used for carbonylation reactions such as hydroformylation or for carboxylic acid synthesis (carbonylation of alcohols) in which the primary carbon chain is extended.

Efficient ways of generating CO from CO ₂ using renewable energy sources open up a multitude of possibilities to partially or completely replace fossil raw materials as a carbon source in many chemical products.

For example, the use of H ₂ consumable anodes in combination with electrochemical CO ₂ reduction at low temperatures for formate synthesis is described in Sen et al. MRS Advances 2016, 2 (8) pp. 451-458.

There are currently three routes under discussion for the sustainable production of CO from CO ₂ .

CO ₂ electrolysis:

A first route is the electrochemical decomposition of CO ₂ into CO and O ₂ . The advantage of the method is that it is a one-step process. In addition, no high temperatures or overpressure are required. A disadvantage, however, is that it is a relatively complex electrolysis process, since a gaseous substrate has to be added. Furthermore, the substrate CO ₂ can react with the ionic charge carriers generated in the electrolysis (eg OH ^- with CO ₂ to CO ₃ ^2- or HCO ₃ ^- ), and can then be reacted in the electrolytes used. CO ₂ + 2e ^- + H ₂ O → CO + 2 OH ^- respectively. 2 CO ₂ + 2e ^- → CO + CO ₃ ^2- CO ₂ + 2OH ^- → CO ₃ ^2- + H ₂ O

During the process, these carbonates can be broken down again as a result of proton generation, for example at the anode. 2 H ₂ O → O ₂ + 4H ⁺ + 4e ^- 4 H ⁺ + 2 CO ₂ ^3- → 2 CO ₂ + 2 H ₂ O

Depending on the structure of the cell, this release can e.g. in the electrolyte, a membrane contact surface (DE102017208610.6, DE102017211930.6) or - if no measures are taken to avoid this - take place directly on the anode. In the first two cases, gas bubbles are released in the ionic current path, which can lead to increased cell voltages if the cell geometry is unfavorable and thus to a loss in energy efficiency. Appropriate solutions to circumvent this are described, for example, in DE102017223521.7.

If no measures are taken to regulate the charge exchange between the electrodes, this is done primarily by hydrogen carbonate and / or carbonate ions. In this case, a mixture of CO ₂ and O _{2 is} formed on the anode. However, there are currently no possible uses for such mixtures, and separation would be very costly because, for safety reasons, classic CO ₂ separation processes such as amine or methanol washing cannot be used, since explosive gas mixtures of organic substances with O _{2 can} form.

Another problem is that CO ₂ electrolysis cannot easily be carried out in strongly basic form, since the CO ₂ would otherwise react with the electrolyte to carbonates and the thermodynamically given buffer equilibria (CO ₂ / HCO ₃ ^- / CO ₃ ^{2 -} ) can adjust. Therefore, only the expensive and poorly available IrO _{2 can be used} as the anode catalyst, since otherwise conventional oxidation catalysts, for example based on NiO _x , are only stable in strongly basic media. The use of Ir-free catalysts can only be used in CO ₂ electrolysis if it is coupled to an electrodialysis, for example when using a bipolar membrane, which can maintain a high pH at the anode, as described, for example, in DE102017208610.6. DE102017211930.6 and DE102017223521.7 is described.

• Die zweite Option für die nachhaltige Erzeugung von CO aus CO₂ ist die thermische Reduktion von CO₂ mit H₂, genannt die „Reverse Wassergas-Shift-Reaktion“.

H ₂ O electrolysis + reverse water gas shift reaction (rWGS):

• The second option for the sustainable production of CO from CO ₂ is the thermal reduction of CO ₂ with H ₂ , called the “reverse water gas shift reaction”.

This process initially involves the electrolytic generation of hydrogen by electrochemical water splitting. In contrast to CO ₂ electrolysis, the surrounding medium of the electrodes is decomposed, which enables much simpler and more efficient cell structures. In addition, the electrolysis can be carried out in strongly basic media, which makes it possible, for example, to use the cheaper, more available and more active nickel catalysts at the anode. This hydrogen is then converted in a thermal process, the reverse water gas shift reaction (rWGS), with CO ₂ to CO and H ₂ O.

The process is strongly inspired by the Haber-Bosch synthesis of ammonia.

Ammonia synthesis: 1.5 H ₂ + 0.5 N ₂ → NH ₃ ΔH = -45.9 kJ / mol

rWGS: CO ₂ (g) + H ₂ (g) → CO (g) + H ₂ O (g) ΔH = +41.2 kJ / mol

Unfortunately, the rWGS is not exothermic like ammonia synthesis, but slightly endothermic. The reaction therefore does not proceed automatically after sufficient activation energy has been supplied, but the chemical equilibrium has to be shifted to the product side by supplying thermal energy. This means that very high reactor temperatures are required. Since this is not a kinetic but a thermodynamic problem, the required temperatures cannot be reduced by suitable catalysts.

For a CO ₂ / H ₂ 1: 1 feed mixture, just 50% conversion is achieved at approx. 1000K. In order to achieve acceptable CO ₂ conversions at low temperatures, a high excess of hydrogen is therefore usually used.

The method can therefore only be used to produce very hydrogen-rich synthesis gas mixtures. However, the endothermic nature of the rWGS also means that the back reaction starts when the product gas mixture cools down. The product gas mixture must therefore be quenched, which makes heat integration significantly more difficult. An exemplary rWGS is in "TOWARD EFFICIENT REDUCTION OF CO2 TO CO FOR RENEWABLE FUELS", F. David Doty, PhD, Glenn N Doty, John P Staab, and Laura L Holte, PhD, Proceedings of ES2010; Energy Sustainability 2010; May 17-22, 2010, Phoenix, AZ USA; ES2010-90362).

High temperature H ₂ O / CO ₂ co-electrolysis:

The third option is the high-temperature co-electrolysis of water vapor and CO ₂ . Water vapor is reduced to H ₂ at a cathode. The O ² ions formed in the process are transported away through a ceramic oxide conductor membrane and oxidized to O ₂ at the anode. H ₂ O + 2e ^- → H ₂ + O ^2- 2 O ^2- → O ₂ + 4e ^-

Since the kermic membranes only have a high oxide conductivity at high temperatures, the process is generally carried out at high temperatures> 900K. If CO _{2 is} added to the water vapor, the rWGS is used at the required process temperatures. Since the resulting water is removed from the equilibrium by electrolysis, the equilibrium shifts to the product side of the rWGS reaction.

As a result, high CO ₂ conversions can be achieved even at lower temperatures than with purely thermal rWGS.

Nevertheless, the required process temperatures are very high. Since nickel-based electrodes are generally used, temperatures around 1100K are usually used to avoid the formation of methane from the Sabatier reaction and the associated coking of the catalysts. CO ₂ + 4H ₂ → CH ₄ + 2H ₂ O CH ₄ → C + 2 H ₂

In addition, the mechanical stability and scalability of the ceramic membranes are currently not available.

Nickel-based catalysts can also react with the carbon monoxide formed to form nickel carbonyl, which then leads to the discharge of the nickel catalyst.

• Nachteile Niedertemperatur CO₂-Elektrolyse:
  • - Inkompatibilität mir Ir-freinen Anodenkatalysatoren
  • - CO₂/O₂ Gemische als Anoden-Produkt oder CO₂-Blasen im Strompfad
  • - Hohe Zellspannung

In summary, there are the following disadvantages for the three methods discussed:

• Disadvantages of low temperature CO ₂ electrolysis:
  • - Incompatibility with Ir-free anode catalysts
  • - CO ₂ / O ₂ mixtures as an anode product or CO ₂ bubbles in the current path
  • - High cell voltage

• - Hoher erforderlicher Wärmeeintrag mit aufwändigen Wärmeintegrationsverfahren
• - Nur für H₂-reiche Synthesegas-Gemische anwendbar
• - Abschrecken des Produktgases erforderlich (irreversible Anteile steigen)
• - Hohe Temperaturen und Drücke erforderlich

Disadvantages of thermal rWGS:

• - High heat input required with complex heat integration processes
• - Can only be used for H ₂ -rich synthesis gas mixtures
• - Quenching of the product gas required (irreversible proportions increase)
• - High temperatures and pressures required

• - Fragile keramische Trennmembranen
• - Hohe Betriebstemperatur
• - Schlechte Skalierbarkeit

Disadvantages of H ₂ O / CO ₂ high-temperature CO electrolysis:

• - Fragile ceramic separation membranes
• - High operating temperature
• - Poor scalability

Since all three of the methods listed have disadvantages, the task of this invention report is to find new low-temperature solutions.

The inventors have found a new, efficient method, in particular using renewable energy, in which CO ₂ reduction, and in particular rWGS, can be carried out electrochemically at low temperatures, for example <100 ° C. The advantage of electrochemical processes is that the thermodynamic equilibrium constant of reactions can be shifted by applying a potential. The problems caused by the endothermic nature of the rWGS can therefore be overcome. The release of the individual products at different locations (cathode, anode) can also provide an additional degree of freedom to optimize the balance. Such a process can also set a low temperature so that the temperature is not high enough to provide the activation energy for the back reaction, and the water formed can be in the liquid phase. It is then not necessary to quench the product gas.

In a first aspect, the present invention relates to a method for converting a gas comprising CO ₂ , preferably a method for producing a product gas comprising CO from a gas comprising CO ₂ , with a cathode of an electrolysis cell which comprises a metal which is selected from the group comprising Ag, Au, Zn, Pd, Cu, and / or alloys and / or mixtures thereof, a gas comprising CO ₂ is preferably converted to a product gas comprising CO, and
wherein a gas comprising H _{2 is converted} to a product comprising protons at an anode of the electrolytic cell.

In addition, the present invention relates to a device for converting a gas comprising CO ₂ to a product gas, preferably for producing a product gas comprising CO from a gas comprising CO ₂
a first electrolytic cell comprising a first cathode space comprising a first cathode which comprises a metal which is selected from the group comprising Ag, Au, Zn, Pd, Cu, and / or alloys and / or mixtures thereof, and which is designed to convert a gas comprising CO ₂ , preferably to a product gas comprising CO, and a first anode space comprising a first anode which is designed to convert a gas comprising H ₂ to a product comprising protons;
a first supply device for the gas comprising CO ₂ , which is designed to supply the gas comprising CO ₂ to the first cathode compartment of the first electrolytic cell;
a second supply device for the gas comprising H ₂ , which is designed to supply the gas comprising H ₂ to the first anode space of the first electrolytic cell;
a first discharge device for the product gas, preferably the product gas comprising CO, which is designed to discharge the product gas, preferably the product gas comprising CO, from the first cathode compartment; and
a second discharge device for a gas comprising CO ₂ , which is designed to discharge a gas comprising CO ₂ from the first anode space.

Further aspects of the present invention can be found in the dependent claims and the detailed description.

list of figures

• 1 bis 4 zeigen schematisch beispielhafte erfindungsgemäße Vorrichtungen.

The accompanying drawings are intended to illustrate and provide further understanding of embodiments of the present invention. In connection with the description, they serve to explain concepts and principles of the invention. Other embodiments and many of the advantages mentioned result from the drawings. The elements of the drawings are not necessarily drawn to scale with respect to one another. Identical, functionally identical and identically acting elements, features and components are provided with the same reference numerals in the figures of the drawings, unless stated otherwise.

• 1 to 4 schematically show exemplary devices according to the invention.

Beyond show 5 to 7 schematically further exemplary devices according to the invention, in which a water electrolysis is additionally provided.

Detailed description of the invention

definitions

Unless otherwise defined, technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art.

Quantities in the context of the present invention relate to% by weight, unless stated otherwise or can be seen from the context.

In the context of the present invention, hydrophobic is understood to be water-repellent. Hydrophobic pores and / or channels are therefore those that repel water. In particular, according to the invention, hydrophobic properties are associated with substances or molecules with non-polar groups.

In contrast, the ability to interact with water and other polar substances is understood to be hydrophilic.

Gas diffusion electrodes (GDE) in general are electrodes in which there are liquid, solid and gaseous phases, and in particular where a conductive catalyst can catalyze an electrochemical reaction between the liquid and the gaseous phase.

The design can be of different types, for example as a porous “full material catalyst” with possibly auxiliary layers to adjust the hydrophobicity; or as a conductive porous support onto which a catalyst can be applied in a thin layer.

For the purposes of this invention, a gas diffusion electrode (GDE) is in particular a porous electrode, inside which gases can move through diffusion. For example, it can be designed to separate a gas and an electrolyte space from one another. According to certain embodiments, it can be characterized in that product gases can emerge from these gas diffusion electrodes on the side facing away from the counterelectrode, that is to say, for example, into a gas space to which a gas is supplied for conversion. Liquid and / or dissolved products and / or by-products of the electrochemical conversion, in particular the charge carriers generated in the process, can be released, for example, from the electrode into the electrolyte.

The normal pressure is 101325 Pa = 1.01325 bar.

Electro-osmosis:

Electro-osmosis is an electrodynamic phenomenon in which a force acts towards the cathode on particles in solution with a positive zeta potential and a force acts towards the anode on all particles with a negative zeta potential. If there is a turnover at the electrodes, i.e. If a galvanic current flows, there is also a mass flow of particles with a positive zeta potential to the cathode, regardless of whether the species is involved in the conversion or not. The same applies to a negative zeta potential and the anode. If the cathode is porous, the medium is also pumped through the electrode. One speaks also of an electro-osmotic pump.

The material flows caused by electro-osmosis can also flow in the opposite direction to concentration gradients. Diffusion-related currents that compensate for the concentration gradients can thus be overcompensated. In the present case, this can relate, for example, to a stream of carbonate and / or hydrogen carbonate ions in the first electrolytic cell.

A separator is a barrier, for example a layer, which in an electrolysis cell has a spatial and at least partially also material separation between different spaces of the electrolysis cell, e.g. Anode space, electrolyte space, salt bridge space, cathode space, etc. can accomplish as well as an electrical separation between anode and cathode, but allows an ion transport between the different spaces. In particular, a separator has no permanently assigned potential, like an electrode. A separator can be, for example, a flat barrier with the same surface area. In particular, membranes and diaphragms are to be regarded as special examples of separators.

A first aspect of the present invention relates to a method for converting a gas comprising CO ₂ , preferably for producing a product gas comprising CO from a gas comprising CO ₂ , wherein
on a cathode of an electrolysis cell which comprises a metal which is selected from the group comprising Ag, Au, Zn, Pd, Cu, and / or alloys and / or mixtures thereof, a gas comprising CO ₂ is preferably converted to a product gas including CO, and
wherein a gas comprising H _{2 is converted} to a product comprising protons at an anode of the electrolytic cell.

Accordingly, in accordance with certain embodiments, a method for producing a product gas comprising CO from a gas comprising CO _{2 is also} disclosed, wherein
on a cathode of an electrolysis cell which comprises a metal which is selected from the group comprising Ag, Au, Zn, Pd, Cu, and / or alloys and / or mixtures thereof, a gas comprising CO ₂ is preferably converted to a product gas including CO, and
wherein a gas comprising H _{2 is converted} to a product comprising protons at an anode of the electrolytic cell.

The method according to the invention can in particular be carried out with the device according to the invention.

In particular place in the process of this invention according to certain embodiments, at the cathode the reduction of CO ₂ to CO and the further reaction of the OH generated thereby ^- ions with additional CO ₂ instead.

Cathode: CO ₂ + 2e- + H ₂ O → CO + 2OH ^- CO ₂ + 2 OH ^- → CO ₃ ^2- + H ₂ O

Net: 2 CO ₂ + 2e ^- → CO + CO ₃ ^2-

The carbonate ions generated in the process can then be transported to the anode in accordance with certain embodiments through an electrolyte and / or a separator, for example a membrane.

In the process according to the invention, the H _{2 is} oxidized to protons at the anode. The protons can decompose the CO ₃ ² ions and / or hydrogen carbonate ions originating from the cathode reaction into water.

Anode: H ₂ → 2 H ⁺ + 2e ^- 2H ⁺ + CO ₃ ^2- → CO ₂ + H ₂ O H ₂ + CO ₃ ^2- → CO ₂ + H ₂ O + 2e ^-

In such embodiments, the overall equation thus corresponds to the rWGS reaction.

Cathode: 2CO ₂ + 2e ^- → CO + CO ₃ ^2-

Anode: H ₂ + CO ₃ ^2- → CO ₂ + H ₂ O

Total: CO ₂ + H ₂ → CO + H ₂ O

However, the process has decisive advantages over the electrolytic cleavage of CO ₂ . First of all, the required cell voltage is lower: CO ₂ → CO + 0.5O ₂ ΔG ⁰ = +257.2 kJ / mol, ΔE ⁰ = 1.33V CO ₂ + H ₂ → CO + H ₂ O ΔG ⁰ = +20.1 kJ / mol, ΔE ⁰ = 0.10V

(Water in liquid phase)

Since no water oxidation has to be carried out in the acidic or neutral pH regime, no Ir-based anode catalysts are required. Since the hydrogen can be generated in an alkaline electrolyzer in a strongly basic medium, no Ir catalysts are necessary in the overall process.

Instead, other transition metal catalysts with better availability can be used. For example, Pt, Pd or Ni would be ideal for this application because they have a high affinity for hydrogen and are not oxidized by protons. Fe, Cu, Ti, Co, Mo, Cr, Rh-based catalysts are also possible. Oxides and alloys thereof as well as organometallic compounds are equally suitable. According to certain embodiments, the anode comprises a metal which is selected from the group comprising Pd, Pt, Ni, Ru, Rh, Fe, Cu, Co, Ti, Cr and / or Mo, and oxides and / or alloys thereof and / or organometallic systems such as Porphyrins, in particular of Pd, Pt, Ni, Ru, Rh, Fe, Cu, Co, Ti, Cr and / or Mo, and / or mixtures thereof.

In addition, H _{2 is} consumed at the anode and CO ₂ can be generated, while a CO ₂ / O ₂ mixture is generated during electrolysis with water. If the H ₂ conversion is incomplete, an electrochemical rWGS can also give an H ₂ / CO ₂ mixture which, in contrast to O ₂ / CO ₂ mixtures, can be separated and recycled.

According to certain embodiments, an essentially complete or complete H ₂ conversion is set at the anode of the electrolytic cell, for example the first electrolytic cell, in particular in order to obtain as pure CO as possible as the target product.

According to certain embodiments, however, an excess of H ₂ is added to the anode with regard to the reaction in the electrolysis cell, in particular in the first electrolysis cell, in particular if synthesis gas is to be generated, for example in at least twice the amount, preferably at least three times the amount, and up to eight times the amount, in particular up to five times the amount, for example up to four times the amount, with regard to the conversion in the electrolysis cell, in particular when converting the CO ₂ to CO. For the generation of synthesis gas mixtures (for example CO / H ₂ 3: 1-1: 4), it is particularly useful to lead and with all the hydrogen through the anode chamber of the electrolysis cell, for example an ErWGS cell - as described further below to lead the mixture into the cathode compartment. This does not lead to H ₂ depletion in the area of the gas outlet, particularly in the anode compartment. The H ₂ in turn cannot be converted at the cathode, so it does not interfere. In this way, at least in principle, no gas separation is required to recycle the CO ₂ s released on the anode side. Only in accordance with certain embodiments can a final CO ₂ separation from the raw synthesis gas follow. Due to the low temperatures, this is also kinetically stable.

In the method according to the invention, the electrolysis cell is not particularly restricted and is in particular the first electrolysis cell of the device according to the invention. According to certain preferred embodiments, the electrolysis cell, for example the first electrolysis cell, is an electrolysis cell in which a reverse water gas shift reaction (rWGS) takes place, so that for such embodiments it is subsequently also referred to as ErWGS (electrolysis cell for a reverse water gas shift Reaction).

In the electrolytic cell, the cathode and the anode are not restricted with regard to the design, the materials used and the structure, provided that the cathode comprises a metal which is selected from the group comprising Ag, Au, Zn, Pd, Cu, and / or alloys and / or mixtures thereof, preferably Ag, Au, Zn and / or alloys and / or mixtures thereof. With these metals, the gas comprising CO ₂ and in particular the CO ₂ can be converted efficiently, in particular to a product gas comprising CO. The cathode can be present in or delimit a cathode space, the cathode space can be provided as a gas space for the supply and discharge of gas - here for the gas comprising CO ₂ , and the anode can be present or delimit in an anode space, the anode space can also be provided as a gas space for the supply and discharge of gas - here for the gas comprising H ₂ .

In the production of a gas comprising CO, in particular a synthesis gas can be provided so that an excess of hydrogen is used, so that corresponding embodiments are preferred. In particular, unreacted carbon dioxide can simply be separated off from a corresponding product gas mixture comprising synthesis gas.

According to certain embodiments, the cathode and / or anode are designed as a gas diffusion electrode (GDE).

The design of the respective gas diffusion electrode is not particularly limited. For example, the respective gas diffusion electrode can separate a gas space and an electrolyte space, lie on a separator, for example a membrane, or can be connected to a separator, for example a membrane.

There are no restrictions on the design of the electrodes. In particular, they are designed to separate a gas and an electrolyte space from one another, to discharge reaction gases into the gas space and / or to identify suitable catalysts for the respective electrochemical reactions.

These can be, for example, porous hydrophobic electrodes, for example GDEs, GDE membrane composites or half-sided catalyst-coated membranes.

• • Katalytisch inaktive, hydrophobe, elektrisch leitfähige Katalysatorschichten beispielsweise auf Kohlenstoffbasis;
• • Elektrochemisch aktive Katalysatorschichten, die beispielsweise neben dem Katalysator noch hydrophobe Binderpolymere, welche nicht beschränkt sind, oder hydrophile ionenleidende Komponenten, welche ebenfalls nicht beschränkt sind, umfassen können; wenn die Elektroden einen Verbund mit einer ionenselektiven Membran bilden, sind diese ionenleitenden Zusätze insbesondere auf die Natur der Membran abgestimmt, sodass beispielsweise bei einer Anionentransportmembran, bevorzugt Anionenaustauschermembran (AEM) die ionenleitenden Zusätze ebenfalls anionenleitend sein können, etc.
• • Nicht geschlossene Deckschichten aus beispielsweise Ionenaustauschermaterialen oder hydrophoben Polymeren, die einen Übergang zu einer Membran oder zum Gasraum verbessern können.

The GDEs can have different layers. These include, for example:

• • Catalytically inactive, hydrophobic, electrically conductive catalyst layers, for example based on carbon;
• • Electrochemically active catalyst layers which, for example, in addition to the catalyst, can also comprise hydrophobic binder polymers, which are not restricted, or hydrophilic, ion-impaired components, which are likewise not restricted; If the electrodes form a composite with an ion-selective membrane, these ion-conducting additives are matched in particular to the nature of the membrane, so that, for example in the case of an anion transport membrane, preferably an anion exchange membrane (AEM), the ion-conducting additives can also be anion-conducting, etc.
• • Non-closed cover layers made of, for example, ion exchange materials or hydrophobic polymers, which can improve a transition to a membrane or to the gas space.

One or more of these layers can be provided in each of the electrodes. According to certain embodiments, however, at least hydrophobic additives or even porous hydrophobic layers are present in the electrodes on the respective side of the gas supply and preferably gas discharge in order to be able to bring the gases efficiently to the respective catalytically active layer and to be able to efficiently remove gaseous products.

The respective electrode can also be a single-layer porous layer made of the catalyst, largely hydrophobic binders, and also fillers and ion-exchange materials. In this case, however, it should preferably be ensured that the electrode as a whole is hydrophobic and only the electrolyte contact area becomes hydrophilic as a result of the electrochemical reaction (and the adsorbed ions formed thereby).

Furthermore, it is of course possible to design one or both electrodes as composite membrane electrodes with ion-selective / ion-conducting membranes (ISM, ion-selective membrane), for example anion-selective or anion-conducting membranes, or as half-sided catalyst-coated membrane electrodes. This can also contribute to improving gas separation and / or improving the erosion resistance of the catalysts.

According to certain embodiments, the electrodes can be supplied with the respective starting gas from the side facing away from the respective counter-electrode, that is to say in the case of the cathode with the gas comprising CO ₂ and in the case of the anode with the gas comprising H ₂ .

The anode and the cathode can be constructed differently with regard to their embodiments (single-layer or multi-layer; adjacent to a separator or preferably a membrane or not; with corresponding ion-conducting additives or not; etc.) and can in particular be adapted to the gases to be reacted and their products his.

Since, for example, anions can be formed on the cathode side, according to certain embodiments, anion-conducting additives are present in the cathode, and the anode can, for example, abut an anion-transporting membrane, for example an AEM, or preferably be at least partially connected to it.

Accordingly, additives for proton transport can also be added on the anode side.

Since the formation of oxygen at the anode of the cell according to the invention, in particular an ErWGS, can be avoided and hydrogen is converted, the overvoltage there is very low.

The gas comprising CO ₂ is not particularly limited, provided that it comprises CO ₂ , for example with a volume fraction of more than 20 vol.%, Preferably more than 50 vol.%, For example more than 80 vol.%, For example more than 90 vol. % or more than 95% by volume or even more than 99% by volume. For example, it can also consist essentially of CO ₂ . The gas comprising CO ₂ can come from any source, for example from fossil fuel combustion, but also from other sources. It is not excluded that the gas flows have a certain volume of water vapor to keep the membrane moist.

Likewise, the gas comprising H _{2 is} not particularly limited if it comprises H ₂ , for example with a volume fraction of more than 20 vol.%, Preferably more than 50 vol.%, For example more than 80 vol.%, For example more than 90 vol .% or more than 95 vol.% or even more than 99 vol.%. For example, it can also consist essentially of H ₂ .

The source of the hydrogen required for operating the electrolysis cell, for example for operating the ErWGS, is not particularly limited, and this can be supplied externally via a suitable, for example the second, feed device from any source, for example H ₂ O electrolysis. eg polymer membrane electrolyzers, alkali electrolyzers and / or high-temperature electrolysers. However, according to certain embodiments, it preferably comes from renewable sources that use the electrolytic splitting of water, for example electrolysis cells or electrolysis systems for the electrochemical conversion of water that are operated with renewable energies, for example polymer membrane electrolyzers, alkali electrolyzers and / or high-temperature electrolysers. The electrolysis of water is not particularly restricted and can be carried out in a suitable manner, for example also in remote locations where renewable energy is more readily available, so that the hydrogen produced can then be suitably transported to the ErWGS.

It is not excluded that the gas streams, ie the gas comprising CO ₂ and / or the gas comprising H ₂ , have a certain volume fraction of water vapor in order to keep the membrane moist.

According to certain embodiments, carbonate ions and / or hydrogen carbonate ions form at the cathode, which react with the protons of the anode to form water and CO ₂ . This happens, for example, depending on the temperature in the condensed phase, and the extent of this can depend, for example, on the selection of the cathode material. The reaction of the carbonate ions and / or bicarbonate ions with hydrogen can in turn produce carbon dioxide, in addition to water, so that the carbonate ions and / or bicarbonate ions can be broken down and no precipitation occurs.

In certain embodiments, the gas comprising H ₂ that is reacted at the anode is produced by water electrolysis. The water electrolysis is not particularly restricted here and can be produced at any location outside the electrolysis cell, in particular the first electrolysis cell, for example in a second electrolysis cell. The corresponding second electrolysis cell is not particularly limited here, and conventional water electrolysers can be used, for example.

According to certain embodiments, the anode and the cathode are separated by a membrane, preferably an anion exchange membrane (AEM). The presence of a membrane enables better separation of products, so that, for example, the product gas comprising CO can be better removed from the cathode compartment. If, in addition, the membrane is designed as an AEM, carbonate ions and / or hydrogencarbonate ions can be suitably removed from the cathode and transported to the anode, so that carbon dioxide and water can form at the anode and the carbon dioxide, possibly with excess H ₂ , is released an anode-side gas space, for example the anode space (if the anode adjoins the membrane or delimits the anode space from an electrolyte space).

According to certain embodiments, the anode and / or the cathode, preferably the anode and the cathode, lie at least partially or also completely on the membrane. This can shorten the transport routes of the carbonate ions and / or hydrogencarbonate ions and make the implementation efficient.

In such an electrolysis cell, in which the two electrodes abut the membrane, in particular directly, they are preferably designed as gas diffusion electrodes, and the membrane is preferably designed as an ion-conducting membrane, preferably an anion-conducting membrane. The electrocatalysts of the electrodes can, for example, either be applied directly to the membrane and touch it directly if it is a physical part of the gas diffusion electrodes are, or can also touch the membrane with the electrode, for example by being pressed together with it. In such embodiments, the electrolytic cell preferably comprises the anode compartment and the cathode compartment as two gas compartments behind the two GDEs. No liquid electrolyte or water is preferably pumped through the two gas spaces. However, it is not excluded that the gas flows have a certain volume of water vapor to keep the membrane moist.

This design of an electrolysis cell, in particular an ErWGS, thus resembles the physical structure of a PEM fuel cell. However, there are significant differences in terms of how it works. On the one hand, there is no O ₂ reduction at the cathode but CO ₂ reduction. Since CO _{2 is} a much weaker oxidizing agent than O ₂ , an adequate voltage is usually applied to the cell for the reaction to take place. Furthermore, an anion-conducting membrane (AEM) is preferably used in such embodiments, as a result of which the water formation can take place above all at the anode. With PEM fuel cells, water is generated at the cathode. The third essential difference is that in the PEM fuel cell only gases are converted into liquids (gas-to-liquid), while gaseous products can be produced on both electrodes. This is particularly advantageous on the anode, since the gases emerging from the anode, preferably an anode GDE, can carry the water formed during the reaction with them from the electrode. This reduces the risk of the electrodes drowning / flooding, as is known from PEM fuel cells.

The electrodes for this type of cell structure are preferably connected directly to the common membrane, for example a polymer electrolyte membrane, e.g. an AEM, compatible. The respective electrodes, e.g. a catalyst-containing layer of the electrodes, which adjoins the membrane, an anion-conducting additive such as an ionomer, which is preferably similar or identical in its chemical nature to the membrane, for example the nature of the membrane polymer. In this way, the anion transport of hydrogen carbonate and / or carbonate ions can be further improved.

The electrodes, in particular catalyst-containing layers of the electrodes, can either be part of the membrane as in a CCM (catalyst coated membrane) or part of the electrode, e.g. the GDE. If the catalyst is part of the membrane, the contact can preferably be made by an inert porous electrode, for example a GDL. Both electrode concepts can be combined as required for the anode and cathode. For example, a catalyst-coated membrane on one side can be contacted from the catalyst-coated side by an inert GDL and from the other side by a catalyst-containing GDE, etc.

The catalyst-containing electrodes can be constructed in one or more layers, provided that they are able to contact the membrane directly. In addition to an ideally ionomer-containing catalyst layer, for example, inert hydrophobic gas diffusion layers can also be present for better gas contact, as described above.

In alternative embodiments, the anode and the cathode are separated by at least one electrolyte space or else two or more electrolyte spaces between the two electrodes. Here, a separator, for example a membrane, can also be provided between the anode and / or cathode space and the one or more electrolyte spaces, and / or preferably the respective electrode can be used to separate the anode and / or cathode space from the one or more electrolyte spaces to avoid power loss through separators. One advantage of such embodiments is that water formed at the anode can be discharged via the electrolyte, a dilution possibly occurring here, which should then be suitably compensated for in accordance with certain embodiments when the electrolyte is recycled, for example by correspondingly concentrating the electrolyte. In such embodiments with at least one electrolyte space, the electrolyte is not particularly limited and is, for example, aqueous. The electrodes can therefore be ionically bound, for example, by means of a pumped-around aqueous electrolyte.

If there is more than one electrolyte space, which can also be referred to as a salt bridge space, a suitable separator, for example a membrane, for example an AEM, can be provided in each case between two electrolyte spaces, the particular separator, for example the respective membrane, not being particularly limited , In some cases, for example, it may be advantageous to use two electrolyte gaps and a separating membrane in order to minimize diffusion of H ₂ through the electrolyte.

The main difference between the two cells is that in the second case the water that is formed can be absorbed by the liquid electrolyte, while in the first case it is preferably carried away with the product gas stream on the anode side.

If the water is transported away in such embodiments, it can be suitably separated, for example by cold traps, separators, etc.

In both configurations, one or more membranes can be provided, preferably at least one AEM.

In particular if the one or more membranes, in particular configurations with electrolyte space, are to be used purely for gas separation and / or for erosion protection, anion-conducting membranes (AEMs) are preferably used as membranes, as a result of which the charge transport mechanism in the cell is not changed. If a cation, for example proton-conducting, membrane (CEM) is used in front of the anode, the carbonates can be neutralized at the membrane-electrolyte interface. Accordingly, CO _{2 is} no longer released at the anode, but only H _{2 is} consumed. However, since gases are released in the electrolyte compartment in such a setup, an increased effort is to be expected in the design of the cell, similar to CO ₂ electrolysis, which is carried out in DE 102017208610.6 , DE102017211930.6, and DE102017223521.7. In principle, installing a bipolar membrane on the anode is not excluded according to the invention, but does not bring any advantages.

The use of CEMs or bipolar membranes on the cathode or cathode side is also not excluded according to the invention, but is not preferred in certain embodiments in the production of CO, since neither a cation flow through the cathode nor a proton flow into the cathode for CO ₂ CO implementation are an advantage.

For CO ₂ electrolysis cells of the same structure, it was also shown in DE102018202184.8) that no separation membranes were required for operation, since the gas-liquid separation can be achieved by adequately implemented GDEs.

According to certain embodiments, an excess of H ₂ compared to the conversion rate of CO ₂ at the cathode (for example determined by the current density) is added to the anode, and CO ₂ formed at the anode is supplied to the cathode with the remaining H ₂ , whereby on a gas comprising synthesis gas is formed on the cathode.

The process according to the invention can in particular be carried out at low temperatures T, for example of 200 ° C or less, preferably of 150 ° C or less, for example <100 ° C, e.g. in a range of 50 ° C ≤ T ≤ 90 ° C, e.g. in a range of 80 ° C ≤ T ≤ 90 ° C.

In a further aspect, the present invention relates to a device for converting a gas comprising CO ₂ to a product gas, preferably for producing a product gas comprising CO from a gas comprising CO ₂ , comprising a first electrolytic cell comprising a first cathode space comprising a first cathode, which a Comprises metal, which is selected from the group comprising Ag, Au, Zn, Pd, Cu, and / or alloys and / or mixtures thereof, and which is designed to convert a gas comprising CO ₂ , preferably to a product gas comprising CO, and a first anode space comprising a first anode which is designed to convert a gas comprising H ₂ to a product comprising protons;
a first supply device for the gas comprising CO ₂ , which is designed to supply the gas comprising CO ₂ to the first cathode compartment of the first electrolytic cell;
a second supply device for the gas comprising H ₂ , which is designed to supply the gas comprising H ₂ to the first anode space of the first electrolytic cell;
a first discharge device for the product gas, preferably the product gas comprising CO, which is designed to discharge the product gas, preferably the product gas comprising CO, from the first cathode compartment; and
a second discharge device for a gas comprising CO ₂ , which is designed to discharge a gas comprising CO ₂ from the first anode space.

In particular, the method according to the invention can be carried out with the device according to the invention. Correspondingly, certain aspects which are described with regard to embodiments of the method according to the invention can also be used in the device according to the invention.

In the device according to the invention, the first electrolysis cell is not particularly limited, provided that it comprises a first cathode space comprising a first cathode which comprises a metal which is selected from the group comprising Ag, Au, Zn, Pd, Cu, and / or alloys and / or mixtures thereof, and which is designed to convert a gas comprising CO ₂ , preferably to a product gas comprising CO, and a first anode space comprising a first anode which is designed to convert a gas comprising H ₂ to a product comprising protons , includes.

According to certain embodiments, the first anode comprises a metal which is selected from the group comprising Pd, Pt, Ni, Ru, Rh, Fe, Cu, Co, Ti, Cr and / or Mo, as well as oxides and / or alloys thereof and / or organometallic systems such as porphyrins, in particular of Pd, Pt, Ni, Ru, Rh, Fe, Cu, Co, Ti, Cr and / or Mo, and / or mixtures thereof.

The electrolysis cell can accordingly comprise a suitable housing, which at least ensures that the gases supplied do not escape to the cathode compartment and / or anode compartment. According to certain embodiments, the first electrolytic cell has a housing which comprises at least the first feed device, the second feed device, the first discharge device and the second discharge device and, for example, only has the first feed device, the second feed device, the first discharge device and the second discharge device, above it beyond that, essentially no gas exchange - apart from unavoidable losses - to the surroundings.

The first cathode space comprises the first cathode, the first cathode at least partially delimiting the first cathode space in accordance with certain embodiments.

The first anode space comprises the first anode, the first anode at least partially delimiting the first anode space in accordance with certain embodiments.

According to certain embodiments, the cathode and / or anode are designed as a gas diffusion electrode (GDE).

The design of the respective gas diffusion electrode is not particularly limited. For example, the respective gas diffusion electrode can separate a gas space and an electrolyte space, lie on a separator, for example a membrane, or can be connected to a separator, for example a membrane.

There are no restrictions on the design of the electrodes. In particular, they are designed to separate a gas and an electrolyte space from one another, to discharge reaction gases into the gas space and / or to identify suitable catalysts for the respective electrochemical reactions.

These can be, for example, porous hydrophobic electrodes, for example GDEs, GDE membrane composites or half-sided catalyst-coated membranes.

• • Katalytisch inaktive, hydrophobe, elektrisch leitfähige Katalysatorschichten beispielsweise auf Kohlenstoffbasis;
• • Elektrochemisch aktive Katalysatorschichten, die beispielsweise neben dem Katalysator noch hydrophobe Binderpolymere, welche nicht beschränkt sind, oder hydrophile ionenleidende Komponenten, welche ebenfalls nicht beschränkt sind, umfassen können; wenn die Elektroden einen Verbund mit einer ionenselektiven Membran bilden, sind diese ionenleitenden Zusätze insbesondere auf die Natur der Membran abgestimmt, sodass beispielsweise bei einer Anionentransportmembran, bevorzugt Anionenaustauschermembran (AEM) die ionenleitenden Zusätze ebenfalls anionenleitend sein können, etc.
• • Nicht geschlossene Deckschichten aus beispielsweise Ionenaustauschermaterialen oder hydrophoben Polymeren, die einen Übergang zu einer Membran oder zum Gasraum verbessern können.

The GDEs can have different layers. These include, for example:

• • Catalytically inactive, hydrophobic, electrically conductive catalyst layers, for example based on carbon;
• • Electrochemically active catalyst layers which, for example, in addition to the catalyst, can also comprise hydrophobic binder polymers, which are not restricted, or hydrophilic, ion-impaired components, which are likewise not restricted; If the electrodes form a composite with an ion-selective membrane, these ion-conducting additives are matched in particular to the nature of the membrane, so that, for example in the case of an anion transport membrane, preferably an anion exchange membrane (AEM), the ion-conducting additives can also be anion-conducting, etc.
• • Non-closed cover layers made of, for example, ion exchange materials or hydrophobic polymers, which can improve a transition to a membrane or to the gas space.

One or more of these layers can be provided in each of the electrodes. According to certain embodiments, however, at least hydrophobic additives or even porous hydrophobic layers are present in the electrodes on the respective side of the gas supply and preferably gas discharge in order to be able to bring the gases efficiently to the respective catalytically active layer and to be able to efficiently remove gaseous products.

The respective electrode can also be a single-layer porous layer made of the catalyst, largely hydrophobic binders, and also fillers and ion-exchange materials. In this case, however, it should preferably be ensured that the electrode as a whole is hydrophobic and only the electrolyte contact area becomes hydrophilic as a result of the electrochemical reaction (and the adsorbed ions formed thereby).

Furthermore, it is of course possible to design one or both electrodes as composite membrane electrodes with ion-selective / ion-conducting membranes (ISM, ion-selective membrane), for example anion-selective or anion-conducting membranes, or as half-sided catalyst-coated membrane electrodes. This can also contribute to improving gas separation and / or improving the erosion resistance of the catalysts.

According to certain embodiments, the electrodes can be supplied with the respective educt gas from the side facing away from the respective counter-electrode, ie in the case of the cathode with the gas comprising CO ₂ and in the case of the anode with the gas comprising H ₂ .

The anode and the cathode can be constructed differently with regard to their embodiments (single-layer or multi-layer; adjoining a separator or preferably a membrane or not; with corresponding ion-conducting additives or not; etc.) and can in particular be adapted to the gases to be reacted and their products his.

Since, for example, anions can be formed on the cathode side, according to certain embodiments, anion-conducting additives are present in the cathode, and the anode can, for example, abut an anion-transporting membrane, for example an AEM, or preferably be at least partially connected to it.

Accordingly, additives for proton transport can also be added on the anode side.

Likewise, the first supply device for the gas comprising CO ₂ , which is designed to supply the gas comprising CO ₂ to the first cathode compartment of the first electrolytic cell, is not particularly limited. This can be designed, for example, as a line, for example a hose, pipe, etc. and is suitable for conducting a gas comprising CO ₂ .

In addition, the second supply device for the gas comprising H ₂ , which is designed to supply the gas comprising H ₂ to the first anode space of the first electrolytic cell, is not particularly limited. This can also be designed, for example, as a line, for example a hose, tube, etc. and is suitable for conducting a gas comprising H ₂ .

There are also no restrictions for the first discharge device for the product gas, preferably the product gas comprising CO, which is designed to discharge the product gas, preferably the product gas comprising CO, from the first cathode compartment. This can also be used, for example, as a line, e.g. Hose, pipe, etc. are designed and is suitable to conduct a product gas, preferably comprising CO.

In addition, the second discharge device for a gas comprising CO ₂ , which is designed to discharge a gas comprising CO ₂ from the first anode space, is not particularly limited, provided that it can discharge a gas comprising CO ₂ from the first anode space. This can be designed, for example, as a line, for example a hose, pipe, etc. and is suitable for conducting a gas comprising CO ₂ .

In addition, as in connection with the method according to the invention, the device according to the invention can have one or more separators, for example one or more membranes, for example one or more AEMs.

According to certain embodiments, the device according to the invention further comprises a second electrolysis cell which is designed to produce the gas comprising H ₂ from water; and a third discharge device which is designed to discharge the gas comprising H ₂ from the second electrolytic cell, the third discharge device being connected to the second supply device. The second electrolytic cell and the third discharge device are not particularly limited here. The second electrolytic cell can be, for example, a suitable water electrolyzer, and the third discharge device can be a line such as a hose or a pipe, which is connected to the second feed device.

According to certain embodiments, the first cathode and / or the first anode are designed as a gas diffusion electrode.

According to certain embodiments, the first anode and the first cathode are separated by a first membrane, preferably an anion exchange membrane.

According to certain embodiments, the first anode and the first cathode abut the first membrane.

In alternative embodiments, the anode and the cathode are separated by at least one electrolyte space between the two electrodes, as described above in connection with the method according to the invention, to which reference is made here. If there is more than one electrolyte compartment, a suitable separator can be provided between two electrolyte compartments, for example a membrane, e.g. an AEM.

According to certain embodiments, the second discharge device is connected to the first feed device. This is particularly advantageous if an excess of H ₂ compared to the rate of conversion of CO ₂ at the cathode is used in the electrolysis cell and excess H ₂ can be mixed into a product gas comprising CO in order to form synthesis gas. Here, water, which can be found in the product gas comprising CO ₂ , can be suitably separated, for example by providing a suitable separation device for the connection between the second discharge device and the first feed device Water is provided, for example at least one separator, a cold trap, etc.

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.

The invention will be explained in more detail below with reference to various examples thereof. However, the invention is not limited to these examples.

Examples

Example 1:

A first exemplary embodiment is shown in FIG 1 shown in which an ErWGS with electrolyte gap is shown.

Here, a feed stream comprising CO _{2 is} fed through the first feed device 1 to the cathode compartment I, which adjoins the cathode GDE-K designed as a gas diffusion electrode. Via the second feed device 3 a gas stream comprising H _{2 is} fed to the anode compartment III, which adjoins the anode GDE-A designed as a gas diffusion electrode. The two electrodes are on the power source U provided. From an electrolyte reservoir 5 Electrolyte is pumped into an electrolyte space II between the two electrodes, which is used to make electrical contact with the electrodes. Via the first discharge device 2 the resulting product gas comprising CO and any remaining CO ₂ (CO ₂ ^R ) is discharged, and via the second discharge device 4 resulting product gas comprising CO ₂ and any remaining H ₂ (H ₂ ^R ). formed H ₂ O can pass into the electrolyte.

In this exemplary embodiment, the electrodes can be ionically bonded by means of a pumped-over aqueous electrolyte.

Example 2

In 2 is an example ErWGS shown with double electrolyte gap. In this embodiment with two electrolyte spaces IIa, IIb, which consist of two electrolyte reservoirs 5k . 5a for the cathode side and anode side are supplied with electrolyte and separated by a membrane M, in particular an AEM, diffusion of H ₂ through the electrolyte can be kept to a minimum. In addition, the embodiment corresponds to that of 1

Example 3

The in 3 shown structure of the ErWGS with membrane composite electrodes corresponds to a large extent to that of 1 , except that the electrodes are designed as membrane composite electrodes, in which the respective GDE adjoin or are connected to an ion-selective membrane ISM on the side of the electrolyte space II.

Example 4

In 4 is a membrane cell for one ErWGS shown, in which the electrolyte chamber II is replaced by an anion-conducting membrane AEM. Here, water formed at the anode is drained off with the product gas comprising CO ₂ and remaining H ₂ ^R. In order to have a sufficient electrical connection through the membrane, it is also preferred in the embodiment shown that the starting gas fed to the cathode is thoroughly moistened with CO ₂ .

The cell also contains two gas diffusion electrodes GDE-K, GDE-A, which directly contact an anion-conducting membrane AEM. Furthermore, the cell comprises two gas spaces I, III behind the two GDEs. No liquid electrolyte or water is usually pumped through the two gas spaces I, III. However, it is not excluded that the gas flows have a certain volume of water vapor to keep the membrane moist.

Example 5:

In 5 is an interconnection of an inventive ErWGS with a H ₂ O Electrolysis for the production of synthesis gas shown. The structure of the ErWGS corresponds to that of the example 1 , however, the product gas stream in the second discharge device 4 to the first feeder 1 is fed for recycling of the CO ₂ . As a result of an excess of H ₂ compared to the conversion rate of CO ₂ at the cathode, such a structure then arises in the first discharge device 2 a synthesis gas which comprises unreacted CO ₂ . In the water electrolysis cell E-H2O there are also gas diffusion electrodes GDE-K 'and GDE-A', which are not particularly restricted and on which water is electrolyzed. The materials of the gas diffusion electrodes of the E-H2O can differ from those of the ErWGS differentiate, since different reactions take place. On the cathode side of the E-H2O is over the there Hydrogen formed in the cathode compartment I via the third discharge device 2 ' to the second feed device 3 and ultimately in the anode compartment III of the ErWGS directed. Via a fourth discharge device 4` oxygen produced from the E-H20 at the anode GDE-A 'is conducted out of the E-H2O. For the electrical connection of the two electrodes GDE-K 'and GDE-A', an electrolyte compartment II is provided, which consists of an electrolyte reservoir 5` is fed with electrolyte, whereby in the E-H2O the electrolyte is also different from that of the ErWGS can distinguish, but can also be the same. The power supply U ' supplies the electricity required for water electrolysis.

In 5 can the interconnection of a water electrolysis and the electrochemical ErWGS for synthesis gas generation (x> y). In this case, both electrochemical cells are shown in the same double GDE design with an electrolyte gap, although other designs of the respective electrolysis cells are also possible. Especially regarding the execution of the H ₂ O Electrolysers have no restrictions. The synthesis gas mixture can be adjusted by regulating the current in the two electrochemical cells. CO / H ₂ = y / (xy) applies. Since both the water reduction to H ₂ and the CO ₂ reduction to CO transfer two electrons and the current strength is proportional to the conversion rate according to δn / δt = I (zF), this relationship also applies to the current strengths in both cells. x is the current in the H ₂ O electrolyzer, and y is the current in ErWGS ,

To set a desired synthesis gas mixture, it is therefore only necessary to adapt the current strengths in both cells.

It is not preferred in this arrangement to use the cathode ErWGS to overload and generate additional H ₂ on this. The CO ₂ reduction catalyst of the ErWGS cathode GDE-K preferably has a high overvoltage for water reduction and should therefore not be used for hydrogen generation for reasons of energy efficiency.

In theory, pure CO can also be generated for the case y = x. In practice, however, this is usually not practical because the anode feed gas is the ErWGS depleted of H ₂ too much, which can lead to an uneven distribution of the current density. In practice, therefore, an excess of H ₂ is usually used compared to the conversion rate of CO ₂ at the cathode. Pure CO can nevertheless be generated by dispensing with direct recirculation of the product gas on the anode side. The H ₂ / CO ₂ containing anode product gas will then ideally be fed into another gas stream with CO ₂ separation in order not to waste CO ₂ and H ₂ . Such a structure makes sense, for example, if both synthesis gas and pure CO are used on the same site. The stream containing H ₂ / CO ₂ can then be fed, for example, as a feed into a synthesis gas system.

Example 6

Another embodiment with ErWGS and E-H2O is in 6 shown which an interconnection of an H ₂ O electrolysis and an ErWGS electrolyser for synthesis gas production with a membrane cell on the part ErWGS shows, analog 4 , The structure of the E-H2O corresponds to that of 5 , and the structure of the ErWGS essentially that of 4 , but with the product gas of ErWGS on the anode side via the second discharge device to the first feed device 1 is supplied and from this water via a water separator 6 is deposited.

Example 7

Yet another embodiment with ErWGS and E-H2O is in 7 schematically shown, which an interconnection of H ₂ O Electrolysis and ErWGS Electrolyser for the production of CO shows. The structure corresponds to that of 5 , however, the second discharge device 4 is not connected to the first feed device. This then corresponds to the case discussed in Example 5, in which a product gas comprising CO, for example CO, and also a product gas comprising H ₂ / CO ₂ , for example a mixture of H ₂ / CO ₂ , can be obtained.

With the method according to the invention, an rWGS reaction in an electrochemical cell at low temperatures of e.g. <200 ° C, in particular <100 ° C, are carried out. This avoids the disadvantages of the thermal processes currently being discussed in a targeted manner.

Furthermore, the CO ₂ released on the anode side can be returned to the cathode feed without interposed gas separation, as a result of which the CO ₂ conversion can be increased. According to certain embodiments, any synthesis gas mixture can be set by setting the current strengths in series-connected H ₂ O electrolyzers and ErWGS cells.

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

• DE 102017208610 [0105]

Claims (15)

Process for converting a gas comprising CO ₂ , preferably for producing a product gas comprising CO from a gas comprising CO ₂ , wherein on a cathode of an electrolysis cell which comprises a metal which is selected from the group comprising Ag, Au, Zn, Pd, Cu, and / or alloys and / or mixtures thereof, a gas comprising CO _{2 is} converted, preferably to a product gas comprising CO, and wherein a gas comprising H _{2 is converted} to a product comprising protons at an anode of the electrolytic cell. Procedure according to Claim 1 , where carbonate ions and / or hydrogen carbonate ions form at the cathode, which react with the protons of the anode to form water and CO ₂ . Procedure according to Claim 1 or 2 , wherein the gas comprising H ₂ , which is reacted at the anode, is produced by water electrolysis. Method according to one of the preceding claims, wherein the cathode and / or anode are designed as a gas diffusion electrode. Method according to one of the preceding claims, wherein the anode and the cathode are separated by a membrane, preferably an anion exchange membrane. Procedure according to Claim 5 , wherein the anode and the cathode rest on the membrane. Method according to one of the preceding claims, wherein an excess of H ₂ compared to the conversion rate of CO ₂ at the cathode is added to the anode and CO ₂ formed at the anode is fed with the remaining H _{2 in} addition to the cathode, whereby at the cathode Gas comprising synthesis gas is formed. Method according to one of the preceding claims, wherein the anode comprises a metal which is selected from the group comprising Pd, Pt, Ni, Ru, Rh, Fe, Cu, Co, Ti, Cr and / or Mo, and oxides and / or Alloys thereof and / or organometallic systems such as Porphyrins, in particular of Pd, Pt, Ni, Ru, Rh, Fe, Cu, Co, Ti, Cr and / or Mo, and / or mixtures thereof. Device for converting a gas comprising CO ₂ to a product gas, preferably for producing a product gas comprising CO from a gas comprising CO ₂ , comprising a first electrolytic cell comprising a first cathode space comprising a first cathode, which comprises a metal which is selected from the group comprising Ag, Au, Zn, Pd, Cu, and / or alloys and / or mixtures thereof, and which is designed to convert a gas comprising CO ₂ , preferably to a product gas comprising CO, and a first anode space comprising a first anode, which is designed to convert a gas comprising H ₂ into a product comprising protons; a first supply device for the gas comprising CO ₂ , which is designed to supply the gas comprising CO ₂ to the first cathode compartment of the first electrolytic cell; a second supply device for the gas comprising H ₂ , which is designed to supply the gas comprising H ₂ to the first anode space of the first electrolytic cell; a first discharge device for the product gas, preferably the product gas comprising CO, which is designed to discharge the product gas, preferably the product gas comprising CO, from the first cathode compartment; and a second discharge device for a gas comprising CO ₂ , which is designed to discharge a gas comprising CO ₂ from the first anode space. Device after Claim 9 , further comprising a second electrolytic cell which is designed to produce the gas comprising H ₂ from water; and a third discharge device which is designed to discharge the gas comprising H ₂ from the second electrolytic cell, the third discharge device being connected to the second supply device. Device after Claim 9 or 10 , wherein the first cathode and / or the first anode are designed as a gas diffusion electrode. Device according to one of the Claims 9 to 11 , wherein the first anode and the first cathode are separated by a first membrane, preferably an anion exchange membrane. Device after Claim 12 , wherein the first anode and the first cathode abut the first membrane. Device according to one of the Claims 9 to 13 , wherein the second discharge device is connected to the first feed device. Device according to one of the Claims 9 to 14 , wherein the first anode comprises a metal which is selected from the group comprising Pd, Pt, Ni, Ru, Rh, Fe, Cu, Co, Ti, Cr and / or Mo, and oxides and / or alloys thereof and / or organometallic systems such as porphyrins, in particular of Pd, Pt, Ni, Ru, Rh, Fe, Cu, Co, Ti, Cr and / or Mo, and / or mixtures thereof.

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