Patent Application
20240229256
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.
The combustion of fossil fuels currently covers about 80% of the worldwide energy demand. As a result, 2011 saw worldwide emissions of around 34032.7 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 50000 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 advantageously 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 C1 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+2e−+H2O→CO+2OH− Eq 1
CO2+2OH−→CO32−+H2O Eq 2
During the process these carbonates are then further decomposed as a consequence of the proton generation at the anode:
2H2O→O2+4H++4e− Eq 3
4H++2CO23−→2CO2+2H2O 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 CO2 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 A1 discloses a process for the electrochemical reaction of a gas comprising CO2, and an apparatus for the electrochemical reaction of a gas comprising CO2, where a gas comprising H2 is reacted at an anode of an electrolytic cell.
DE 10 2018 202 184 A1 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 A1 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.
The solution to the problem lies in an electrolyzer and also in a method having the features according to the claims.
The electrolyzer of the invention 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 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, CO2 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 advantageously located on a side which faces the electrolyte space, enabling effective transport of protons into the electrolyte.
A further constituent of the invention is 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.
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.
As already mentioned, the claimed method 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:
The electrolyzer 2 of
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
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
CO2+2e−+H2O→CO+2OH− Eq 1
CO2+2OH−→CO32−+H2O 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).
2H2O→O2+4H++4e− Eq 3
4H++2CO23−→2CO2+2H2O 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 CO2 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 (O2) 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
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,
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 CO2 in aqueous solution can also be present in the highly supersaturated concentration, the CO2 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 CO2 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 CO2 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:
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2019 216 500.1 | Oct 2019 | DE | national |
| 10 2019 217 914.2 | Nov 2019 | DE | national |
This application is the US National Stage of International Application No. PCT/EP2020/079144 filed 16 Oct. 2020, and claims the benefit thereof. The International Application claims the benefit of German Application No. DE 10 2019 216 500.1 filed 25 Oct. 2019 and German Application No. DE 10 2019 217 914.2 filed 20 Nov. 2019. All of the applications are incorporated by reference herein in their entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/079144 | 10/16/2020 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20240133057 A1 | Apr 2024 | US |
