AU2020369070B2

AU2020369070B2 - Electrolyser device and method for carbon dioxide reduction

Info

Publication number: AU2020369070B2
Application number: AU2020369070A
Authority: AU
Inventors: Nemanja Martic; David Reinisch; Günter Schmid
Original assignee: Siemens Energy Global GmbH and Co KG
Current assignee: Siemens Energy Global GmbH and Co KG
Priority date: 2019-10-25
Filing date: 2020-10-16
Publication date: 2023-06-15
Anticipated expiration: 2040-10-16
Also published as: AU2020369070A1; CN114585773A; WO2021078635A1; EP4010514A1

Abstract

The invention relates to an electrolyser for carbon dioxide reduction, comprising an electrolytic cell (4), with a cathode gas diffusion electrode (6) and an anode gas diffusion electrode (8), in which a first side (12) of the cathode gas diffusion electrode (8) adjoins a cathode gas chamber (10) in a planar manner and likewise a first side (13) of the anode gas diffusion electrode (8) adjoins an anode gas chamber (14), and an electrolyte chamber (16) common to both gas diffusion electrodes (6, 8) is provided, which extends from the cathode gas diffusion electrode (6) to the anode gas diffusion electrode (8) and is at least partially delimited by the two gas diffusion electrodes (6, 8) with their second sides (18, 19) facing away from the respectively associated gas chambers (10, 14).

Description

Electrolyser device and method for carbon dioxide reduction

The invention relates to an electrolyzer for carbon dioxide reduction. Carbon dioxide is transported past a gas diffusion cathode of an electrolytic cell, where it is reduced catalytically to give at least one higher-energy product.

Introduction and Prior Art

The combustion of fossil fuels currently covers about 80% of the worldwide energy demand. As a result, 2011 saw worldwide emissions of around 34 032.7 million metric tons of carbon dioxide into the atmosphere. This releasing of the carbon dioxide is the simplest route for disposal of carbon dioxide even in large quantities. More than 50 000 metric tons arise daily in brown coal power stations. As a result of the debate about the adverse consequences of the greenhouse gas carbon dioxide on the climate, however, the reutilization of carbon dioxide is desired and is the subject of work. Because carbon dioxide occupies a very low position thermodynamically, it is difficult to reduce it to give useful products.

By a natural route, the carbon dioxide is converted by photosynthesis into carbohydrates. On an industrial scale it is very difficult to copy this process, which is broken down into numerous substeps in terms both of time and, at the molecular level, of space. In comparison to pure photocatalysis, the electrochemical reduction of the carbon dioxide is presently the more efficient route. A hybrid form is that of light-assisted electrolysis or electrically assisted photocatalysis. The two terms are used synonymously, depending on the viewpoint of the commentator.

As in the case of photosynthesis as well, this process, with optionally photo-assisted supply of electrical energy, which is obtained preferably from regenerative energy sources such as wind or solar, converts carbon dioxide into a higher-energy product such as carbon monoxide, methane, ethene, or other alcohols. The quantity of energy required for this reduction corresponds ideally to the combustion energy of the fuel and ought to come only from regenerative sources.

Currently under debate are a number of possible routes to the production of energy sources and basic chemicals on the basis of regenerative energies. Considered particularly worthwhile is the direct electrochemical or photochemical conversion of carbon dioxide into hydrocarbons or oxygen derivatives thereof. At present there are still no industry-fit catalysts available for these direct approaches. Consequently, there is debate around multistage approaches, with a prospect of a near-term solution by virtue of the greater technical maturity of the individual steps. The most important intermediate in these multistage value-added chains is the carbon monoxide. It is commonly deemed the most important Cl building block in synthetic chemistry. As a synthesis gas mixture, this being hydrogen and carbon monoxide in a ratio of more than 2:1, it can be used by way of the Fischer-Tropsch process for the synthesis of hydrocarbons and for methanol synthesis. Relatively carbon monoxide-rich gas mixtures or pure carbon monoxide, moreover, are used for carbonylation reactions such as hydroformylation through carboxylic acid synthesis or alcohol carbonylation, in which the primary carbon chain is extended. The possibility of generating carbon monoxide from carbon dioxide, incorporating regenerative energy sources, therefore opens up a host of possibilities for full or partial replacement of fossil raw materials as a carbon source for numerous chemical products.

One of these approaches is the electrochemical decomposition of carbon dioxide into carbon monoxide and oxygen. This is a single-stage process requiring no high temperatures or super atmospheric pressure. It is, however, a relatively complex electrolysis process, where the carbon dioxide substrate has to be supplied in the form of a gaseous substrate. Moreover, the gaseous carbon dioxide can react with the charge carriers generated in the electrolysis, and is therefore bound chemically into the electrolyte used:

CO2 + 2 e- + H20 - CO + 2 OH- Eq 1 C02 + 2 OH- - C032- + H2 0 Eq 2

During the process these carbonates are then further decomposed as a consequence of the proton generation at the anode:

2 H20 - 02 + 4 H+ + 4 e- Eq 3 4 H+ + 2CO23- -- 2 C02 + 2 H 2 0 Eq 4

Depending on reactant composition, and influenced in particular by various catalyst materials in the gas diffusion electrodes, there may also be other reactions, with retention of the C02 reduction.

Depending on the construction of the electrolytic cell, release takes place alternatively in the electrolyte, at a membrane contact face, or directly at the anode. In the first two cases, gas bubbles are released in the ionic flow path, and this may lead to sharply increased cell voltages and hence to massive decreases in the energy efficiency. In the latter case, a mixture of carbon dioxide and oxygen would form at the anode. There are presently no utility options for such mixtures, and fractionation would be necessary but very costly. Conventional methods for removing carbon dioxide, such as amine or methanol scrubbing, cannot be used, for reasons of safety. Furthermore, electrochemical cells of this kind for the decomposition of carbon dioxide to carbon monoxide and oxygen employ purified carbon dioxide. Consequently, there is a significant loss of resources if carbon dioxide is lost by way of a gas mixture of oxygen and carbon dioxide formed at the anode. This loss of resources drives the operating costs significantly upward. Furthermore, given the predominant release of carbon dioxide, the technology loses its green technology character. Returning the entire gas into the reactant gas stream is inefficient, since the electrochemically generated oxygen in the reactant gas stream would be reduced to water again and would therefore diminish the efficiency of the electrolysis system and process.

DE 10 2018 210 303 Al discloses a process for the electrochemical reaction of a gas comprising C02, and an apparatus for the electrochemical reaction of a gas comprising C02, where a gas comprising H2 is reacted at an anode of an electrolytic cell.

DE 10 2018 202 184 Al discloses an electrolytic cell comprising a cathode space comprising a cathode, an anode space comprising an anode, and a salt bridge space disposed between cathode and anode, where the cathode and the anode take the form of a gas diffusion electrode.

US 2012/0 228 147 Al describes processes and apparatuses for the electrochemical preparation of formic acid.

There is a need, therefore, for a carbon dioxide electrolyzer which enables electrochemical decomposition of carbon dioxide into carbon monoxide and oxygen and with which at the same time the carbon dioxide losses via the gas mixture formed at the anode are minimized and also entries of carbon dioxide into the electrolyte can be removed completely or near-completely.

It is an object of the present invention to substantially overcome or at least ameliorate one or more of the above disadvantages.

According to an aspect of the present invention, there is provided an electrolyzer for carbon dioxide reduction, comprising an electrolytic cell, having a cathode gas diffusion electrode and also having an anode gas diffusion electrode, where the cathode gas diffusion electrode at a first side areally adjoins a cathode gas space and the anode gas diffusion electrode likewise with a first side adjoins an anode gas space, and where an electrolyte space common to both gas diffusion electrodes is provided, which reaches from the cathode gas diffusion electrode to the anode gas diffusion electrode and is bounded at least in sections by the two gas diffusion electrodes with their second sides facing away from the respectively assigned gas spaces, wherein the anode gas diffusion electrode has a cation-selective coating, where the anode gas diffusion electrode is coated with an ion-conducting polymer, wherein the cathode gas space has a reactant gas supply apparatus and a product gas outlet apparatus, and wherein molecular oxygen 02 is released at the anode gas diffusion electrode surface and diffuses through the anode gas diffusion electrode into the anode gas space.

According to another aspect of the present invention, there is provided a method for operating an electrolyzer, comprising the following steps: - introducing a C0 2 -containing reactant gas into a cathode gas space, - wherein the C02 is reduced at least partly to CO at a cathode gas diffusion electrode, and the cathode gas diffusion electrode abuts the cathode gas space with a first side and abuts an electrolyte space with the opposite second side, - a liquid electrolyte flows through the electrolyte space, - C02 is dissolved in the electrolyte, and - molecular oxygen 02 is released at the anode gas diffusion electrode surface and diffuses through an anode gas diffusion electrode into an anode gas space, wherein - both the cathode gas diffusion electrode and the anode gas diffusion electrode, each with a second side, adjoin the common electrolyte space, and - the C02 dissolved in the electrolyte is discharged therefrom outside the electrolyte space, wherein the anode gas diffusion electrode has a cation-selective coating, wherein the anode gas diffusion electrode is coated with an ion-conducting polymer.

- 5a

The electrolyzer according to an aspect of the present disclosure for carbon dioxide reduction comprises an electrolysis cell having a cathode gas diffusion electrode and also having an anode gas diffusion electrode. The cathode gas diffusion electrode here (also abbreviated below to GDC) at a first side areally adjoins a cathode gas space. Similarly, the anode gas diffusion electrode (GDA) with a first side areally adjoins an anode gas space. The two gas diffusion electrodes each have a second side, which is opposite the respective first side, and which communicates with a common electrolyte space. The configuration of this electrolyte space is such that it reaches from the cathode gas diffusion electrode to the anode gas diffusion electrode and is bounded at least in sections by the two gas diffusion electrodes with their second sides, facing away from the respectively assigned gas spaces. The anode gas diffusion electrode has a cation-selective coating.

The electrolytic cell of the electrolyzer according to an aspect of the present disclosure has two gas diffusion electrodes, namely one gas diffusion electrode at the anode (GDA) and one at the cathode (GDC). Both gas diffusion electrodes communicate with a dedicated, separate gas space, and they each delimit this separate gas space from a common electrolyte space. The electrolytic cell described in the electrolyzer therefore has only one electrolyte space, which is also not separated by a membrane or a diaphragm. The electrolyte located in the electrolyte space and flowing through it is therefore in communication with both gas diffusion electrodes.

It has emerged experimentally that the construction of an electrolyzer having these two essential features, namely two gas diffusion electrodes both at the anode and at the cathode, and a common, unseparated electrolyte space, means that carbon dioxide which enters the electrolyte space through the GDC is able to dissolve in this space in a supersaturated way and can be taken off from the electrolyte space before mixing at the GDA with the oxygen formed there and hence becoming unusable economically for the further operational supply. The oxygen is formed, as described, at the GDA, and diffuses through the latter and is taken off by the separate gas space of the GDA. Mixing of the generated oxygen with the carbon dioxide is therefore greatly reduced. The reduction can be lowered to 5% of the value customary for a conventional construction with one gas diffusion electrode and two separate electrolyte spaces.

In one embodiment of the invention the electrolyte space is provided with an electrolyte feedline and an electrolyte drain line, which together with a pumping apparatus form an electrolyte circuit. Again, therefore, this is a common electrolyte circuit for the whole electrolyzer, and renders superfluous two separate electrolyte reservoirs and/or else neutralization of the respective reservoirs.

Additionally provided, usefully, are a cathode gas space and a reactant gas supply apparatus for the supply of reactant gases.

It is useful, furthermore, to install a carbon dioxide deposition apparatus in the electrolyte circuit, allowing the carbon dioxide in the electrolyte to be discharged and allowing it to be supplied to the reactant gas again through a corresponding further advantageous connecting line.

In one advantageous embodiment, furthermore, the anode space has an oxygen exhaust apparatus. This allows oxygen which has entered the anode gas space through the anode to be withdrawn from the process.

In the invention the GDA is embodied such that it has a cation selective coating. In this case the GDA is coated with an ion conducting polymer. This polymer is able to conduct the protons formed into the electrolyte, but is impermeable to gases. Consequently, C02 gas bubbles are unable to enter the anode gas space, and molecular gaseous oxygen formed at the anode is unable to enter the electrolyte. The cation-selective coating is preferably located on a side which faces the electrolyte space, enabling effective transport of protons into the electrolyte.

Another aspect of the present disclosure provides a method for operating an electrolyzer. This method comprises the following steps: - the introducing of a carbon dioxide-containing gas into a cathode gas space. Here the carbon dioxide is reduced to carbon monoxide at a cathode gas diffusion electrode,

- where the cathode gas diffusion electrode with a first side abuts the cathode gas space and with the opposite second side abuts an electrolyte space. The cathode gas diffusion electrode here has a flat configuration and has the first and second flat sides, with one side abutting the cathode gas space and the other side abutting the electrolyte space.

- A liquid electrolyte flows through the electrolyte space, and carbon dioxide is dissolved in this electrolyte.

- Furthermore, molecular oxygen is released at an anode gas diffusion electrode surface and diffuses through an anode gas diffusion electrode.

- Both the cathode gas diffusion electrode and the anode gas diffusion electrode with their respective second side adjoin the common electrolyte space.

- The C02 dissolved in the electrolyte is subsequently discharged therefrom outside the electrolyte space.

The anode gas diffusion electrode in the method has a cation selective coating. In particular the electrolyzer of the invention is used to implement the method of the invention. Embodiments described for the electrolyzer may be employed correspondingly in the method of the invention, and vice versa.

The method according to an aspect of the present disclosure as well has the special feature that the electrolyte space is a common electrolyte space both of the cathode and of the anode and accordingly has no corresponding separation such as a membrane or diaphragm, for example. Furthermore, both the anode and the cathode are each configured as gas diffusion electrodes, GDA and GDC. The effect of this is that oxygen released in the electrolyte during the process is able to diffuse as molecular oxygen through the anode gas diffusion electrode and in doing so does not mix with the carbon dioxide likewise produced in the electrolyte. This has the consequence in turn that the carbon dioxide can be in supersaturated form in the electrolyte and can be taken out of the electrolyte space and discharged from the electrolyte outside the electrolyte space. The carbon dioxide thus discharged can be passed back to the process as reactant gas, and this makes the overall process much more economic.

With the method described it is useful for the PH of the electrolyte to be in the acidic range, the aim in this case being for a slightly acidic range between a PH of between 7 and 2. The electrolyte is, in particular, an aqueous electrolyte.

It is also useful for a gas volume flow of the carbon dioxide at the gas diffusion cathode to be at least 5 times as great, more particularly 15 times as great, as at the gas diffusion anode. This leads to further increases in the economic viability of the process.

Further embodiments and further examples and features of the invention are elucidated in more detail in the context of the following description of figures. The embodiments in question are purely exemplary embodiments which do not represent any restriction on the scope of protection.

In the figures:

figure 1 shows an electrolyzer with schematic representation of the individual process apparatuses, figure 2 shows a highly schematic representation of the flow of substances in the electrolyzer according to figure 1, with the representation of the individual chemical components, figure 3 shows a cross section through an anode gas diffusion electrode, and figure 4 shows a diagram representing the gas volume flow at different regions in the electrolyzer.

The electrolyzer 2 of figure 1 is represented there in a highly schematic way in terms of its construction. The electrolyzer 2 comprises an electrolytic cell 4, in which, in turn, two gas diffusion electrodes are arranged. One of these is a cathode gas diffusion electrode 6 (called GDC below). Also provided is an anode gas diffusion electrode 8, which is called GDA below. The two gas diffusion electrodes 6, 8 are configured as flat structures each having two flat sides, and they separate a gas space from an electrolyte space 16. The configuration here in detail is as follows:

The GDC 6 has a first side 12, which is in communication with the cathode gas space 10 and delimits it at least partly from the electrolyte space 16. The electrolyte space 16 in turn is in communication with a second side 18 of the GDC 6. Additionally, the GDA 8 likewise has a second side 19, which bounds the electrolyte space 16 from the other side. The first side of the GDA 13 in turn adjoins a further gas space, namely the anode gas space 14. The two gas diffusion electrodes 6, 8 therefore bound the electrolyte space 16 at least partly from two sides. The particular feature of the construction described is that, in contrast to other electrolyzer constructions and electrolytic cells according to the prior art, the electrolyte space 16 has no separation between the two electrodes. Between the GDC 6 and the GDA 8 there is only a common electrolyte space 16. In this case there is no continuous structure such as a membrane or a diaphragm, for example. A liquid electrolyte 42 located in the electrolyte space 16 in the operating state is in direct communication both with the second side 18 of the GDC 6 and with the second side 19 of the GDA 8.

The electrolytic cell 4 described therefore has 2 essential features. Firstly, instead of the customary one gas diffusion electrode as cathode, two gas diffusion electrodes are used in the case described, and in this case, accordingly, the anode configured as GDA 8 is also a gas diffusion electrode. In addition there is only one, common electrolyte space for both electrodes. The mode of action of this construction will be addressed further.

First of all it may be further elucidated, referring to figure 1, that an electrolyte circuit 26 is provided which comprises an electrolyte feedline 20 and an electrolyte drain line 22 and also a pumping apparatus 24. Additionally provided in the electrolyte circuit 26 is a C02 deposition apparatus 32, which in turn leads via a connecting line 34, optionally via a C02 preparation apparatus 46, to a reactant gas supply apparatus 28. In the electrolyte circuit 26 there is additionally an electrolyte reservoir 44 provided.

For the supplying of the electrolytic cell 4 and the electrolyzer 2 with reactants and for the removal of products, there is, as already mentioned, the reactant supply apparatus 28 provided, in which a reactant gas 40 comprising carbon dioxide is introduced into the cathode gas space 10. The cathode gas space 10 further comprises a product gas outlet apparatus 30, in which excess carbon dioxide and the carbon monoxide formed during the process are passed out. The electrolysis cell, additionally, further comprises the anode gas space 14, which has an oxygen exhaust apparatus 36. The voltage U is applied between the two gas diffusion electrodes 6 and 8. In an embodiment in accordance with the invention, the anode gas diffusion electrode has a cation-selective coating (not shown).

In figure 2, the electrolyzer 2 described in reference to figure 1 is represented once again more schematically, the intention being to illustrate the reaction process and the flow of substances using the chemical symbols. In this case essentially carbon dioxide as reactant gas is introduced into the cathode gas space 10 and is reduced at least partly to carbon monoxide at the first surface 12 of the GDC and is discharged again by the product gas outlet apparatus 30 described. Since in the majority of processes reduction of the entire carbon dioxide (C02) to carbon monoxide (CO) is not complete, the outlet apparatus 30 lets out both carbon dioxide and carbon monoxide, which are separated from one another later. As described by the chemical equations Eq 1 to Eq 4, the carbon dioxide also passes through the GDC 6 into the electrolyte space 16, where it reacts with the water present there to form hydrocarbonate anions (cf. equation 1 and equation 2).

C02 + 2 e- + H20 - CO + 2 OH- Eq 1 C02 + 2 OH- - C032- + H2 0 Eq 2

In contrast to this, both protons and molecular oxygen are formed at the GDA (cf. equation 3), and the protons react with the carbonate ions to form carbon dioxide and water (equation 4).

2 H20 - 02 + 4 H+ + 4 e- Eq 3 4 H+ + 2CO23- -- 2 C02 + 2 H 2 0 Eq 4

The carbon dioxide thus recovered is dissolved in the electrolyte 42, as a possibly highly supersaturated solution, and is passed out with this electrolyte from the electrolytic cell 2 or the electrolyte space 16. The carbon dioxide passed out can be removed again from the electrolyte 42 in the electrolyte circuit 26, in the C02 deposition apparatus 32 described, and can be resupplied to the reactant gas 40. In this case there may optionally also be preparation of the carbon dioxide in a preparation apparatus 46.

As can also already be seen in equation 3, molecular oxygen (02)

is formed at the GDA 8 and is able to diffuse through the GDA 8 and hence to enter the anode gas space 14 and to escape via the oxygen exhaust apparatus 36. Only as a result of the configuration of the anodes in the form of a GDA 8 is it possible for the molecular oxygen automatically formed in the process not to mix with the carbon dioxide in the electrolyte and have to be separated from it again later. As represented in figure 3, the GDA 8 here is provided with a hydrophobic layer 38, allowing the molecular oxygen to diffuse through the GDA 8, but retaining the liquid water by means of the hydrophobic layer 38 as mentioned.

As described, the carbon dioxide is dissolved in a relatively pure form in the electrolyte 42 and can be withdrawn from the electrolyte again and resupplied to the process. In this case there is no need for costly and inconvenient separation of a carbon dioxide-oxygen mixture, and on this basis, therefore, the process design is significantly more efficient.

In the case of a conventional process regime, the assumption is that about half of the carbon dioxide introduced is lost via the process, being lost more particularly at least economically during the process as a result of the mixing with molecular oxygen. Being lost economically means that it is not profitable for the already contaminated carbon dioxide to be separated again from the oxygen profitably on a business-economical scale.

To illustrate this, figure 4 shows the gas volume flows 50 of the key gases involved in the process (02, CO, C02, and H2 ) at the individual electrodes and in the electrolyte. The individual gas volume flows 50 are apparent on the basis of different types of hatching. The gas volume flow 51 of the carbon dioxide is of particular interest here. As at the left-hand bar, which illustrates the gas volume flow 52 at the GDC 6, there is a very high carbon dioxide fraction prevailing here, though the essential carbon monoxide fraction is also present at this point, in other words at the GDC. The gas volume flow 54 in the electrolyte 42 (middle bar) also shows a high carbon dioxide gas volume flow, with only very little accompanying carbon monoxide present. This shows that the electrolyte takes up virtually only carbon dioxide as gas, as already described, in saturated form, which is removed from the electrolyte 42 again in the further course of the method. The gas volume flow 56 at the GDA 8 here has only a very small fraction of the gas volume flow 51 of the carbon dioxide. In the case of a good process regime, this fraction may amount only to one 20th of the gas volume flow 51 at the GDC 6. This means that via the GDA 8 only one 2 0th of the carbon dioxide used is discharged together with the oxygen and therefore lost. As already maintained, with conventional processes this figure is up to 50% of the carbon dioxide used.

Figure 4 therefore illustrates how the measures taken in this case of the C02 electrolyzer 2 present here, namely the use of two gas diffusion electrodes, as GDC 6 and GDA 8, and an electrolyte space 16 enclosed and common to these electrodes, means that the loss of carbon dioxide on electrolysis can be reduced from 50% to about 5% or even lower still. Ultimately significant is the reduction in the loss of carbon dioxide of more than 90%.

The GDA 8 here has a hydrophobic layer 38, which prevents penetration of the electrolyte 42, which exists in particular as an aqueous basis. The molecular oxygen though is able to diffuse through the pores of the GDA 8 into the anode gas space 14.

Since C02 in aqueous solution can also be present in the highly supersaturated concentration, the C02 present as a result of the neutralization (equation 4) need not automatically lead to the formation of gas bubbles in the electrolyte space 16. As a result in particular of the low concentrations of anodically formed H+ and cathodically formed carbonates, the neutralization is distributed over the entire electrolyte space 16. Because of the possible supersaturation, the expulsion of gas is distributed over the entire electrolyte 42 in the electrolyte space 16 and also over the electrolyte circuit 26, which also includes the electrolyte reservoir 44. The C02 bubbles present in the electrolyte 42 can be removed from the electrolyte 42 prior to entry into the electrolytic cell. This takes place in the C02 deposition apparatus 32 described. Accordingly, the release of the carbon dioxide bound chemically by the cathode reaction is distributed, and only a small proportion of the gas bubbles are actually released in the electrolyte space 16.

For the carbon dioxide bubbles released in the electrolyte space 16 there are three possibilities here:

- They come into contact with the GDC 6 and are absorbed by it. In this case the carbon dioxide is made available to the reaction again. This case is advantageous for the process.

- The C02 bubbles come into contact with the GDA and are absorbed by it. In this case the carbon dioxide introduced into the solution as a result of the reaction mixes with the anodically formed oxygen in the anode gas space 14. This proportion corresponds to the gas volume flow 51 of the carbon dioxide in the right-hand bar 56 of figure 4. This carbon dioxide may be regarded as being lost for the process.

- In the third possible case, the C02 gas bubbles do not come into contact with either of the gas diffusion electrodes 6, 8, and are carried with the electrolyte 42 out of the electrolytic cell 4. This proportion of the carbon dioxide may be removed with the residual electrolyte 42, as described, from the liquid electrolyte 42 and after possible preparation (C02 preparation apparatus 46), may be made available to the gas circuit again, in particular to the reactant gas 40 by way of the connecting line 34.

List of reference numerals

2 electrolyzer 4 electrolytic cell 6 cathode gas diffusion electrode (GDC) 8 anode gas diffusion electrode (GDA) cathode gas space 12 first side of GDC 13 first side of GDA 14 anode gas space 16 electrolyte space 18 second side of GDC 19 second side of GDA electrolyte feedline 22 electrolyte drain line 24 pumping apparatus 26 electrolyte circuit 28 reactant gas supply product gas outlet apparatus 32 C02 deposition apparatus 34 connecting line 36 oxygen exhaust apparatus 38 hydrophobic coating reactant gas 42 electrolyte 44 electrolyte reservoir 46 C02 preparation

48 product gas gas volume flow 51 gas volume flow C02 52 gas volume flow GDC 54 gas volume flow electrolyte 56 gas volume flow GDA

Claims

1. An electrolyzer for carbon dioxide reduction, comprising an electrolytic cell, having a cathode gas diffusion electrode and also having an anode gas diffusion electrode, where the cathode gas diffusion electrode at a first side areally adjoins a cathode gas space and the anode gas diffusion electrode likewise with a first side adjoins an anode gas space, and where an electrolyte space common to both gas diffusion electrodes is provided, which reaches from the cathode gas diffusion electrode to the anode gas diffusion electrode and is bounded at least in sections by the two gas diffusion electrodes with their second sides facing away from the respectively assigned gas spaces, wherein the anode gas diffusion electrode has a cation-selective coating, where the anode gas diffusion electrode is coated with an ion-conducting polymer, wherein the cathode gas space has a reactant gas supply apparatus and a product gas outlet apparatus; and wherein molecular oxygen 02 is released at the anode gas diffusion electrode surface and diffuses through the anode gas diffusion electrode into the anode gas space.

2. The electrolyzer as claimed in claim 1, wherein the electrolyte space is provided with an electrolyte feedline and an electrolyte drain line, which together with a pumping apparatus form an electrolyte circuit.

3. The electrolyzer as claimed in claim 1 or 2, wherein a C02 deposition apparatus is provided in the electrolyte circuit.

4. The electrolyzer as claimed in claim 3, wherein there is a connecting line from the C02 deposition apparatus to the reactant gas supply apparatus.

5. The electrolyzer as claimed in any of the preceding claims, wherein an oxygen exhaust apparatus is provided at the anode gas space.

6. A method for operating an electrolyzer, comprising the following steps: - introducing a C0 2 -containing reactant gas into a cathode gas space, - wherein the C02 is reduced at least partly to CO at a cathode gas diffusion electrode, and the cathode gas diffusion electrode abuts the cathode gas space with a first side and abuts an electrolyte space with the opposite second side, - a liquid electrolyte flows through the electrolyte space, - C02 is dissolved in the electrolyte, and - molecular oxygen 02 is released at an anode gas diffusion electrode surface and diffuses through the anode gas diffusion electrode into an anode gas space, wherein - both the cathode gas diffusion electrode and the anode gas diffusion electrode, each with a second side, adjoin the common electrolyte space, and - the C02 dissolved in the electrolyte is discharged therefrom outside the electrolyte space, wherein the anode gas diffusion electrode has a cation-selective coating, where the anode gas diffusion electrode is coated with an ion-conducting polymer.

7. The method as claimed in claim 6, wherein the electrolyte comprises an aqueous salt solution, where the dissolved salt is not based on a salt of carbonic acid.

8. The method as claimed in claim 6 or 7, wherein the pH of the electrolyte is less than 7.

9. The method as claimed in claim 8, wherein pH of the electrolyte is between 2 and 7.

10. The method as claimed in any one of claims 6 to 9, wherein the C02 discharged from the electrolyte is supplied to the C02 containing reactant gas.

11. The method as claimed in any one of claims 6 to 10, wherein a gas volume flow of the C02 at the gas diffusion cathode is at least 5 times as great as at the gas diffusion anode.

12. The method as claimed in claim 11, wherein a gas volume flow of the C02 at the gas diffusion cathode is at least 15 times as great as at the gas diffusion anode.

Siemens Energy Global GmbH & Co. KG Patent Attorneys for the Applicant/Nominated Person SPRUSON&FERGUSON