DE102017211930A1 - Membrane-coupled cathode for the reduction of carbon dioxide in acid-based electrolytes without mobile cations - Google Patents

Abstract

The present invention relates to a process for the electrolysis of CO ₂ , the electrolysis cell having a salt bridge space comprising a liquid and / or dissolved acid, an electrolytic cell having a cathode space comprising a cathode, a first ion exchange membrane containing an anion exchanger and / or anion transporter and which adjoins the cathode space, the cathode directly contacting the first ion exchange membrane, an anode space comprising an anode, and a diaphragm adjacent to the anode space, further having a salt bridge space disposed between the first ion exchange membrane and the diaphragm. an electrolysis plant comprising the electrolysis cell, as well as the use of the electrolysis cell or the plant for the electrolysis of CO ₂ .

Description

The present invention relates to a process for the electrolysis of CO ₂ , the electrolysis cell having a salt bridge space comprising a liquid and / or dissolved acid, an electrolytic cell having a cathode space comprising a cathode, a first ion exchange membrane containing an anion exchanger and / or anion transporter and which adjoins the cathode space, the cathode directly contacting the first ion exchange membrane, an anode space comprising an anode, and a diaphragm adjacent to the anode space, further having a salt bridge space disposed between the first ion exchange membrane and the diaphragm. an electrolysis plant comprising the electrolysis cell, as well as the use of the electrolysis cell or the plant for the electrolysis of CO ₂ .

State of the art

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

The discussion about the negative effects of the greenhouse gas CO ₂ on the climate has led to a reflection on the recycling of CO ₂ . Thermodynamically, CO _{2 is} very low and can therefore be reduced back to useful products.

Systematic studies of the electrochemical reduction of carbon dioxide are still a relatively recent field of development. Only for a few years has there been an effort to develop an electrochemical system that can reduce an acceptable amount of carbon dioxide. Research on a laboratory scale has shown that it is preferable to use metals as catalysts for the electrolysis of carbon dioxide. From the publication Electrochemical CO ₂ reduction on metal electrodes by Y. Hori, published in: C. Vayenas, et al. (Eds.), Modern Aspects of Electrochemistry, Springer, New York, 2008, pp. 89-189 , Faraday efficiencies (FE) are exemplified on different metal cathodes, some of which are shown by way of example in Table 1. Table 1: Faraday efficiencies for converting CO ₂ into various products on different metal electrodes electrode CH ₄ C ₂ H ₄ C ₂ H ₅ OH C ₃ H ₇ OH CO HCOO ^- H ₂ Total Cu 33.3 25.5 5.7 3.0 1.3 9.4 20.5 103.5 Au 0.0 0.0 0.0 0.0 87.1 0.7 10.2 98.0 Ag 0.0 0.0 0.0 0.0 81.5 0.8 12.4 94.6 Zn 0.0 0.0 0.0 0.0 79.4 6.1 9.9 95.4 Pd 2.9 0.0 0.0 0.0 28.3 2.8 26.2 60.2 ga 0.0 0.0 0.0 0.0 23.2 0.0 79.0 102.0 pb 0.0 0.0 0.0 0.0 0.0 97.4 5.0 102.4 hg 0.0 0.0 0.0 0.0 0.0 99.5 0.0 99.5 In 0.0 0.0 0.0 0.0 2.1 94.9 3.3 100.3 sn 0.0 0.0 0.0 0.0 7.1 88.4 4.6 100.1 CD 1.3 0.0 0.0 0.0 13.9 78.4 9.4 103.0 tl 0.0 0.0 0.0 0.0 0.0 95.1 6.2 101.3 Ni 1.8 0.1 0.0 0.0 0.0 1.4 88.9 92.4 Fe 0.0 0.0 0.0 0.0 0.0 0.0 94.8 94.8 Pt 0.0 0.0 0.0 0.0 0.0 0.1 95.7 95.8 Ti 0.0 0.0 0.0 0.0 0.0 0.0 99.7 99.7

Table 1 shows Faraday efficiencies FE (in [%]) of products resulting from carbon dioxide reduction on various metal electrodes. The values given here apply to a 0.1 M potassium bicarbonate solution as the electrolyte.

As can be seen from Table 1, the electrochemical reduction of CO ₂ to solid-state electrodes in aqueous electrolyte solutions offers a variety of product options.

Currently, the electrification of the chemical industry is discussed. This means that chemical precursors or fuels are preferably to be prepared from CO ₂ and / or CO and / or H ₂ O while supplying excess electrical energy, preferably from regenerative sources. In the introductory phase of such a technology, the aim is that the economic value of a substance is significantly greater than its calorific value or fuel value in order to achieve economic efficiency at an early stage.

To be able to achieve acceptable conversion rates in the CO ₂ electrolysis, it is preferred to ensure sufficient availability of CO ₂ at the catalytically active centers of the cathode. At current densities above ~50mAcm ^{-2, however} , this supply via the solubility of CO ₂ in an electrolyte is difficult.

Therefore, at high current densities usually the CO _{2 is provided} directly as gas. Here, a so-called three-phase zone is advantageous, at which the reaction gas CO ₂ , the catalytically active electrode and the electrolyte are available. For this purpose, porous electrodes, so-called gas diffusion electrodes, can be used, which can be realized in various ways.

For example, they may be present as polymer bound, electrically conductive catalyst particles, for example as extruded or calendered film, which corresponds to a full catalyst gas diffusion electrode, or as a porous, catalytically inactive but conductive electrode, e.g. in the form of carbon fiber gas diffusion layers impregnated with a small amount of active catalyst particles.

Alternatively, it is also possible to attach a catalyst to a solid electrolyte, which may also be referred to as a catalyst-coated membrane. Here too, a three-phase zone can form between the catalyst, the solid electrolyte and the CO ₂ .

With corresponding structures, an electrochemical reduction of CO ₂ to chemically usable products is possible.

For example, in the US 2017/0037522 A1 discloses a method of producing formic acid in an electrochemical device.

In addition, acids in anode space are quite common, such as in J. Shi, F. Shi, N. Song, JX. Liu, XK Yang, YJ Jia, ZW Xiao, P. Du Journal of Power Sources, 2014, 259, 50-53 described.

However, there is a need for a simple and effective electrolytic process for reducing CO ₂ using high current densities while avoiding the formation of CO ₂ at the anode, for example, by protolysis of carbonate-containing electrolytes in aqueous electrolytes. Electrolysis cells in which the CO ₂ / O ₂ mixtures formed at the anode are, for example, from US 2016 0251755 A1 and the US 9,481,939 known.

Summary of the invention

The inventors have made basic considerations for CO ₂ electrolysis to solve the above problem, and have found that, contrary to previous expectations, CO ₂ is present at sufficiently high current densities in the presence of a liquid and / or dissolved acid on a membrane facing the cathode compartment containing an anion exchanger and / or anion transporter, for example in a salt bridge space and / or in the anolyte, CO ₂ can be effectively converted to economically reusable products and the formation of hydrogen can be surprisingly suppressed.

The results of these basic considerations for the electrolysis of CO ₂ , which form the basis of the present invention, are further elaborated below. The following explanations are valid for example for the above systems. Based on these considerations, a charge transport model for CO ₂ electrolysis can be constructed as follows:

Considerations for salt electrolytes

The cathodic reduction of CO ₂ to CO in the presence of water can be represented in the simplest approximation by the following equation, whereby analogous equations can also be correspondingly stated in the production of hydrocarbons from CO ₂ : CO ₂ + H ₂ O + 2e ^- → CO + 2 OH ^- (1)

As in the 3-phase zone typically CO ₂ is in excess of the available (which may also be present in deficiency, since one can not associate with a certain catalyst center, the CO ₂₎ OH formed may ^- ions with this to HCO ₃ ^- Ablate ions. OH ^- + CO ₂ → HCO ₃ ^- (2)

It follows: 3CO ₂ + H ₂ O + 2e → CO + 2HCO ₃ ^- (1 in 2)

This reaction has far-reaching consequences for charge transport within the porous electrode.

Since HCO ₃ ^- unlike OH ^{- is} not capable of charge transport via the Grotthuß mechanism, its molar conductivity is many times lower.

It should also be noted that the solubility of alkali metal hydrogencarbonates MHCO ₃ (M = Li, Na, K, Rb, Cs) is lower than that of the hydroxides of the alkali metals, which can lead to unwanted crystallization of salts faster.

However, solutions of alkali metal salts are frequently used as electrolytes in CO ₂ -electrolysis. The molar conductivity of HCO ₃ ^- is only about half that of the alkali metal ions (hereinafter M ⁺ ), which is why in a region of the electrode in which both the M ⁺ ions of the electrolyte and the HCO generated at the cathode ₃ ^- ions are present, most of the charge transport is carried by the M ⁺ ions.

Because of their low conductivity, the cathodically produced HCO ₃ occur in this case ^- ions not leak from the electrode into the electrolyte. Instead penetrate the more mobile M ⁺ ions in the electrode and form with the HCO ₃ ^- ion salt. This salt can then emerge as a solution or permeate at the side of the electrode facing away from the electrolyte. However, if no attention is paid to efficient removal of this MHCO ₃ solution, crystallization of these salts may also occur.

Over a longer period of time, this phenomenon leads to increasing penetration of the electrode with the electrolyte solution. As a result, pores can be irreversibly flooded, which can lead to the collapse of the CO ₂ supply to the electrode and thus to their failure.

Considerations for solid electrolytes

Solid electrolytes are in electrolysis cells, for example, membranes from polymers modified with charged functionalities. In particular, the applicability of anion exchange membranes (AEM) for CO ₂ electrolysis is known from the literature, for example from US 2016 0251755 A1 , of the US 9481939 and the US 2017/0037522 A1 ,

In an AEM the cationic functional groups are stationary. In the absence of other ions of the charge transport can only HCO ₃ in this case therefore usually ^- done ions. However, this method can be used in particular only if the anode is connected directly to the membrane.

However, the supply of HCO ₃ ^- ions to the anode is not desirable because there the bound CO _{2 is released} by neutralization again. H ₂ O - 2e ^- → 2H ⁺ + ½O ₂ HCO3 ^- + H ⁺ → H ₂ O + CO ₂

It mixes with the oxygen formed at the anode, and there is a difficult to work up or almost unusable CO ₂ / O ₂ mixture with a CO ₂ content of up to 80 mol%. As a result, up to 67 mol% of the CO _{2 used can be} lost unused for the CO case.

As described above, the considerations given above also apply in a similar way to other CO ₂ reduction products. For products derived from CO ₂ by reduction with more than two electrons, for example, the proportion of CO _{2 used} , which is converted into bicarbonate, correspondingly higher. For example, for methane: 9 CO ₂ + 8e- + 6H ₂ O → CH ₄ + 8HCO ₃ ^- 14 CO ₂ + 12 e + 8 H ₂ O → C ₂ H ₄ + 12 HCO ₃ ^-

In this case, for example, up to 89 mol% in the case of CH ₄ or 86 mol% in the case of C ₂ H _{4 of} the CO ₂ used can be lost via the anode.

On the other hand, if a cathode AEM composite is to be coupled to an anodic half cell terminated against HCO ₃ ^, an electrolyte is again required. The above-mentioned condition of absence of other ions is no longer given, and the charge transport is again of ions other than the HCO ₃ ^- ion worn, for example, in particular M ⁺ ions. Although the fixed cationic functional groups of the AEM can repel the M ⁺ ions, however, M ⁺ ions in the AEM with the HCO ₃ ^- ions, a counter-ion available.

Therefore, even inside an AEM, a formal double salt system may exist in which, for example, the anionic part is completely occupied by HCO ₃ ^- , while the cationic part is occupied partly by M ⁺ ions and partly by the cationic functional groups of the polymer.

Thus, the penetration of M ⁺ in the presence of a salt electrolyte, although limited by an AEM, but not completely prevented. In corresponding laboratory experiments - as indicated in Comparative Example 1 below - a crystallization of MHCO ₃ at the back of the electrode (gas side) could be observed. However, the phenomenon is greatly attenuated over direct contact between the cathode and the electrolyte. Although the proportion of HCO ₃ ^{- in} the charge transport can be significantly increased compared to a mode without AEM, and can be determined for example to ~50 mol%, for example by gas chromatographic CO ₂ measurement, but is still limited. The reason for this is the low mobility of bicarbonate anions, as indicated above and from the following Table 2, which results from Current Separations 18: 3 (1999), Conductance Measurements, Part 1: Theory, Lou Coury, pp. 91-96 is taken. Table 2: Ladunsmobility of different ions cation Λ ⁰ ₊ (S · cm ² / mol) anion Λ ⁰ _ (S · cm ² / mol) H ⁺ 349, 6 OH 199.1 Li ⁺ 38.7 F ^- 55.4 Na ⁺ 50,10 Cl ^- 76.35 K ⁺ 73,50 Br ^- 78.1 Rb ⁺ 77.8 I ^- 76.8 Cs ⁺ 77.2 NO ₂ ^- 71.8 Ag ⁺ 61, 9 NO ₃ ^- 71.46 NH ₄ ⁺ 73.5 ClO ₃ ^- 64, 6 ethylammonium 47.2 ClO ₄ ^- 67, 3 diethylammonium 42, 0 IO ₄ ^- 54.5 triethylammonium 34.3 HCO ₃ ^- 44.5 tetraethylammonium 32, 6 H ₂ PO ₄ ^- 57 Tetra-n-butylammonium 19.5 HSO ₃ ^- 50 dimethyl 51.8 HSO ₄ ^- 50 trimethyl 47.2 HC ₂ O ₄ ^- 40.2 Tetramethylammonium 44.9 HCOO 54.6 piperidinium 37.2 CH ₃ COO ^- 40.9 C ₆ H ₅ COO ^- 32.4 Be ²⁺ 90 Mg ²⁺ 106.0 CO ₃ ^2- 138, 6 Ca ²⁺ 119.0 HPO ₄ ^2- 66 Sr ²⁺ 118.9 SO ₄ ^2- 160, 0 Ba ²⁺ 127.2 C ₂ O ₄ ^2- 148.2 Fe ²⁺ 108.0 Cu ²⁺ 107.2 PO ₄ ^3- 207 Zn ²⁺ 105, 6 Fe (CN) ₆ ^3- 302.7 Pb ²⁺ 142.0 UO ₂ ²⁺ 64 Fe (CN) ₆ ^4- 442.0 Al ³⁺ 183 Fe ³⁺ 204 La ³⁺ 209.1 Ce ³⁺ 209.4

Considerations for acidic electrolytes:

The CO ₂ electrolysis in aqueous electrolytes might not be thermodynamically possible, since the decomposition voltage for the reaction CO ₂ → CO + ½ O ₂ 1.32V (3) so higher is the one for the reaction: H ₂ O → H ₂ + ½ O ₂ 1.23V (4)

Nevertheless, the process is possible under suitable conditions because, on the one hand, suitable catalysts have a high overpotential for water decomposition, and on the other hand, at higher current densities, high local pH values can typically form in the immediate vicinity of the electrode. For the latter effect, a diffusion gradient is usually required, in which the OH ^- 'CO ₃ ^{2 -} and HCO ₃ ^- ions formed at the electrode displace the counterions of M ⁺ ions from the electrolyte. In addition, the M ⁺ concentration in the immediate vicinity of the electrode should be increased by attraction of the M ⁺ ions by the electric field. As a result, the water reduction potential can drop, which suppresses the evolution of hydrogen. In contrast, the initial step of CO ₂ reduction is not pH dependent, which makes it more dominant.

However, if acids are added to the electrolyte, this gradient can not develop in sufficient strength. In acidified electrolytes therefore usually only H ₂ / CO mixtures are obtained with a large H ₂ excess. In pure acid electrolytes the CO ₂ reduction usually takes place only in the trace range.

It should be mentioned at this point, however, that the above-described passage of electrolytes through the electrode when using pure acid electrolytes is not observed, as well as from the Comparative Example 2 will be apparent. The electrolyte passage is thus caused as previously discussed by the penetration of cations into the electrode. Since no cations are present in the present comparative example 2 and in the present exemplary embodiment, as described in more detail below, no more electrolyte penetration takes place. The previously discussed models can be confirmed by this.

Considerations for a combination of an AEM and pure acid electrolyte

An entirely different situation arises when an AEM is introduced between a pure acid electrolyte (eg H ₂ SO ₄ , as in the embodiment 1). In this case, for example, very good selectivities for CO> 95% can also be achieved in a pure acid electrolyte at high current densities> 100mAcm ^-2 .

The reason for this lies in a peculiarity of the carbonate-acid-base system. In contrast to other systems such as the sulfate system, no neutral acid exists in the carbonate equilibrium. CO ₃ ^2- ↔ HCO ₃ ^- ↔ CO ₂ + H ₂ O SO ₄ ^2- ↔ HSO ₄ ^- ↔ H ₂ SO ₄

Consequently, HCO ₃ ^- can not act as a counter ion for the only cations present in the electrolyte "H ⁺ ". A double salt situation as with alkali metal salt electrolytes can not exist. A presence of "H ⁺ " ions in the AEM is therefore only possible if the acid anions (eg SO ₄ ^2- ) of the electrolyte are present in the AEM. If these are displaced from the AEM by a sufficiently high ion current, a high pH value can be established in the cathode AEM composite despite an acid electrolyte. The only other charge transport pathway is the conduction of OH ^- via the Grotthuß mechanism by the swollen in H ₂ O membrane, or a hopping transport of HCO ₃ ^- from localized polymer-bound cation to localized cation.

Since, as already explained, first the acid anions must be displaced from the AEM, a minimum current density is required to exploit this effect. In the following embodiment, this was about 50mAcm ^-2 ; below this current density almost exclusively H ₂ evolution was observed. At higher current densities, the selectivity for CO was> 90% and increased steadily with increasing current density, as shown below in the embodiment.

1 and 2 illustrate this difference in the use of different electrolytes 1 , which abut the anion exchanger membrane AEM, and which ions pass as to the cathode K. In 1 Here is the variant with a salt M ⁺ X ^- as the electrolyte 1 shown as an example, whereas in 2 the variant with an acid H ⁺ X ^- as an electrolyte 1 is shown.

• - einen Kathodenraum umfassend eine Kathode;
• - eine erste Ionenaustauschermembran, welche einen Anionenaustauscher und/oder Anionentransporter enthält und welche an den Kathodenraum angrenzt, wobei die Kathode die erste Ionenaustauschermembran direkt kontaktiert, wobei der Kontakt gemäß bestimmten Ausführungsformen zudem ionischer Natur ist;
• - einen Anodenraum umfassend eine Anode;
• - eine erste Separator-Membran; und
• - einen Salzbrückenraum, wobei der Salzbrückenraum zwischen der ersten Ionenaustauschermembran und der ersten Separator-Membran angeordnet ist,

verwendet wird, wobei CO₂ an der Kathode reduziert wird, wobei der Salzbrückenraum eine flüssige und/oder gelöste Säure aufweist.According to a first aspect, the present invention relates to a method for the electrolysis of CO ₂ , comprising an electrolysis cell

• - a cathode compartment comprising a cathode;
• a first ion exchange membrane containing an anion exchanger and / or anion transporter adjacent to the cathode compartment, wherein the cathode directly contacts the first ion exchange membrane, wherein the contact according to certain embodiments is also ionic in nature;
• an anode compartment comprising an anode;
• a first separator membrane; and
• a salt bridge space, wherein the salt bridge space is arranged between the first ion exchange membrane and the first separator membrane,

is used, wherein CO _{2 is} reduced at the cathode, wherein the salt bridge space having a liquid and / or dissolved acid.

• - einen Kathodenraum umfassend eine Kathode;
• - eine erste Ionenaustauschermembran, welche einen Anionenaustauscher und/oder Anionentransporter enthält und welche an den Kathodenraum angrenzt, wobei die Kathode die erste Ionenaustauschermembran direkt kontaktiert, wobei der Kontakt gemäß bestimmten Ausführungsformen zudem ionischer Natur ist; und
• - einen Anodenraum umfassend eine Anode, wobei der Anodenraum an die erste Ionenaustauschermembran angrenzt; verwendet wird, wobei CO₂ an der Kathode reduziert wird, wobei der Anodenraum eine flüssige und/oder gelöste Säure aufweist.

In addition, in another aspect, the present invention relates to a process for the electrolysis of CO ₂ , comprising an electrolytic cell

• a cathode compartment comprising a cathode;
• a first ion exchange membrane containing an anion exchanger and / or anion transporter adjacent to the cathode compartment, wherein the cathode directly contacts the first ion exchange membrane, wherein the contact according to certain embodiments is also ionic in nature; and
• an anode compartment comprising an anode, wherein the anode compartment adjoins the first ion exchange membrane; is used, wherein CO _{2 is} reduced at the cathode, wherein the anode compartment comprises a liquid and / or dissolved acid.

• - einen Kathodenraum umfassend eine Kathode;
• - eine erste Ionenaustauschermembran, welche einen Anionenaustauscher und/oder Anionentransporter enthält und welche an den Kathodenraum angrenzt, wobei die Kathode die erste Ionenaustauschermembran direkt kontaktiert;
• - einen Anodenraum umfassend eine Anode; und
• - ein Diaphragma, welches an den Anodenraum angrenzt; weiter umfassend einen Salzbrückenraum, wobei der Salzbrückenraum zwischen der erste Ionenaustauschermembran und dem Diaphragma angeordnet ist.

In addition, an electrolysis cell is disclosed comprising

• a cathode compartment comprising a cathode;
• a first ion exchange membrane containing an anion exchanger and / or anion transporter adjacent to the cathode compartment, the cathode directly contacting the first ion exchange membrane;
• an anode compartment comprising an anode; and
• a diaphragm adjacent to the anode compartment; further comprising a salt bridge space, wherein the salt bridge space between the first ion exchange membrane and the diaphragm is arranged.

Furthermore, an electrolysis plant, comprising the electrolysis cell according to the invention, as well as the use of the electrolytic cell according to the invention or the electrolysis plant according to the invention for the electrolysis of CO _{2 is} disclosed.

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

list of figures

• 1 und 2 zeigen eine graphische Repräsentation der kathodischen Halbzelle der zuvor beschriebenen Transportmodelle von Ionen von Salzen und Säuren in einer AEM, welche an einer Kathode anliegt.
• 3 zeigt schematisch ein Beispiel einer Elektrolyseanlage mit einer Elektrolysezelle, wie sie in einem erfindungsgemäßen Verfahren zur Anwendung kommt.
• 4 und 5 zeigen schematisch weitere Beispiele von Elektrolysezellen, mit denen das erfindungsgemäße Verfahren durchgeführt werden kann.
• In 6 und 7 ist schematisch eine graphische Repräsentation der unterschiedlichen CO₂-Freisetzung bei einer Verwendung eines Salzelektrolyten (6) und eines Säureelektrolyten (7) dargestellt.
• 8 zeigt schematisch eine erfindungsgemäße Elektrolyseanlage mit einer erfindungsgemäßen AEM-Diaphragma-Zelle, in der das erfindungsgemäße Verfahren durchgeführt werden kann.
• 9 ist eine schematische Darstellung einer AEM-Bipolar-Doppelmembran-Zelle zu entnehmen, in der ebenfalls das erfindungsgemäße Verfahren durchgeführt werden kann.
• 10 zeigt schematisch den Versuchsaufbau im erfindungsgemäßen Beispiel 1.
• Der 11 sind Versuchsergebnisse des erfindungsgemäßen Beispiels 1 zu entnehmen, wobei dort eine Auftragung der Faraday-Effizienz gegen die angelegte Stromdichte erfolgt ist.
• 12 zeigt schematisch den Versuchsaufbau im vorliegenden Vergleichsbeispiel 1, sowie 13 die damit erhaltenen Versuchsergebnisse, wobei wiederum eine Auftragung der Faraday-Effizienz gegen die angelegte Stromdichte erfolgt ist.
• In 14 sind die Versuchsergebnisse des erfindungsgemäßen Beispiels 1 (durchgezogene Linien) denen des Vergleichsbeispiels 1 (gepunktete Linien) zum Vergleich gegenübergestellt.
• 15 zeigt eine schematische Darstellung des Versuchsaufbaus in Vergleichsbeispiel 2, und 16 die damit erhaltenen Versuchsergebnisse, wobei wiederum eine Auftragung der Faraday-Effizienz gegen die angelegte Stromdichte erfolgt ist.
• In 17 sind die Versuchsergebnisse des erfindungsgemäßen Beispiels 1 (durchgezogene Linien) denen des Vergleichsbeispiels 2 (gepunktete Linien) zum Vergleich gegenübergestellt.
• Darüber hinaus ist in 18 ein Vergleich der in Vergleichsbeispiel 2 (durchgezogen; w/o AEM) und Beispiel 1 (gestrichelt; w/ AEM) bei einer Stromdichte von 150 mAcm^-2 erhaltenen Gaschromatogramme gezeigt.
• 19 und 20 zeigen jeweils schematisch den Versuchsaufbau in Referenzbeispielen 1 und 2, und 21 und 22 die dabei erhaltenen Versuchsergebnisse.

The accompanying drawings are intended to illustrate embodiments of the present invention and to provide a further understanding thereof. In the context of the description, they serve to explain concepts and principles of the invention. Other embodiments and many of the stated advantages will become apparent with reference to the drawings. The elements of the drawings are not necessarily to scale. Identical, functionally identical and identically acting elements, features and components are in the figures of the drawings, unless otherwise stated, each provided with the same reference numerals.

• 1 and 2 show a graphical representation of the cathodic half-cell of the previously described transport models of ions of salts and acids in an AEM, which is applied to a cathode.
• 3 shows schematically an example of an electrolysis plant with an electrolytic cell, as used in a method according to the invention for use.
• 4 and 5 schematically show further examples of electrolysis cells, with which the inventive method can be carried out.
• In 6 and 7 is a schematic representation of the different CO ₂ release when using a salt electrolyte ( 6 ) and an acid electrolyte ( 7 ).
• 8th schematically shows an electrolysis plant according to the invention with an AEM diaphragm cell according to the invention, in which the inventive method can be carried out.
• 9 is a schematic representation of an AEM bipolar double-membrane cell in which also the inventive method can be carried out.
• 10 schematically shows the experimental setup in Example 1 of the invention.
• Of the 11 are test results of Example 1 of the invention, where there is a plot of the Faraday efficiency against the applied current density.
• 12 schematically shows the experimental set-up in the present comparative example 1, and 13 the test results obtained therewith, again with a plot of Faraday efficiency against the applied current density.
• In 14 the experimental results of Inventive Example 1 (solid lines) are compared with those of Comparative Example 1 (dotted lines) for comparison.
• 15 shows a schematic representation of the experimental setup in Comparative Example 2, and 16 the test results obtained therewith, again with a plot of the Faraday efficiency against the applied current density.
• In 17 the experimental results of Inventive Example 1 (solid lines) are compared with those of Comparative Example 2 (dotted lines) for comparison.
• In addition, in 18 a comparison of the gas chromatograms obtained in Comparative Example 2 (solid, w / o AEM) and Example 1 (dashed, w / AEM) at a current density of 150 mAcm ^{-2 is} shown.
• 19 and 20 each show schematically the experimental setup in Reference Examples 1 and 2, and 21 and 22 the test results obtained.

Detailed description of the invention

definitions

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

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

The design may be of a different nature, for example, as a porous "bulk catalyst" with optional adjuvants to adjust the hydrophobicity, then, for example, a membrane-GDE composite, e.g. AEM-GDE composite, can be produced; as a conductive porous support to which a catalyst can be applied in a thin layer, in which case again a membrane-GDE composite, e.g. AEM-GDE composite, can be produced; or as a composite porous catalyst, optionally with additive directly on a membrane, e.g. AEM, can be applied and then form a CCM in the composite.

In the context of the present invention, hydrophobic is understood as meaning water-repellent. Hydrophobic pores and / or channels according to the invention are therefore those which repel water.

In particular, hydrophobic properties are associated according to the invention with substances or molecules with nonpolar groups.

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

In the application quantities are by weight, unless otherwise stated or obvious from the context.

The normal pressure is 101325 Pa = 1.01325 bar.

Electro-osmosis:

Electro-osmosis is an electrodynamic phenomenon in which a force towards the cathode acts on particles in solution with a positive zeta potential, and a force acts on the anode on all particles with a negative zeta potential. If conversion takes place at the electrodes, i. If a galvanic current flows, a stream of the particles with a positive zeta potential also flows to the cathode, irrespective of whether the species participates in the reaction 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. It is also known as an electro-osmotic pump.

The material flows caused by electro-osmosis can also flow in opposite directions to concentration gradients. Diffusion-related currents that balance the concentration gradients can thereby be overcompensated.

• - einen Kathodenraum umfassend eine Kathode;
• - eine erste Ionenaustauschermembran, welche einen Anionenaustauscher und/oder Anionentransporter enthält und welche an den Kathodenraum angrenzt, wobei die Kathode die erste Ionenaustauschermembran direkt kontaktiert;
• - einen Anodenraum umfassend eine Anode;
• - eine erste Separator-Membran; und
• - einen Salzbrückenraum, wobei der Salzbrückenraum zwischen der ersten Ionenaustauschermembran und der ersten Separator-Membran angeordnet ist,

verwendet wird, wobei CO₂ an der Kathode reduziert wird, wobei der Salzbrückenraum eine flüssige Säure und/oder gelöste Säure aufweist. Gemäß bestimmten Ausführungsformen besteht der Elektrolyt im Salzbrückenraum aus der flüssigen Säure und/oder der Lösung einer - z.B. festen oder gasförmigen - Säure, beispielsweise in Wasser, z.B. bidestilliertem oder vollentsalztem Wasser.According to a first aspect, the present invention relates to a method for the electrolysis of CO ₂ , comprising an electrolysis cell

• a cathode compartment comprising a cathode;
• a first ion exchange membrane containing an anion exchanger and / or anion transporter adjacent to the cathode compartment, the cathode directly contacting the first ion exchange membrane;
• an anode compartment comprising an anode;
• a first separator membrane; and
• a salt bridge space, wherein the salt bridge space is arranged between the first ion exchange membrane and the first separator membrane,

is used, wherein CO _{2 is} reduced at the cathode, wherein the salt bridge space having a liquid acid and / or dissolved acid. According to certain embodiments, the electrolyte in the salt bridge space consists of the liquid acid and / or the solution of an acid, for example solid or gaseous acid, for example in water, for example double-distilled or demineralized water.

• - einen Kathodenraum umfassend eine Kathode;
• - eine erste Ionenaustauschermembran, welche einen Anionenaustauscher und/oder Anionentransporter enthält und welche an den Kathodenraum angrenzt, wobei die Kathode die erste Ionenaustauschermembran direkt kontaktiert; und
• - einen Anodenraum umfassend eine Anode, wobei der Anodenraum an die erste Ionenaustauschermembran angrenzt; verwendet wird,

wobei CO₂ an der Kathode reduziert wird, wobei der Anodenraum eine flüssige und/oder gelöste Säure aufweist. Gemäß bestimmten Ausführungsformen besteht der Elektrolyt im Anodenraum aus der flüssigen Säure und/oder der Lösung einer - z.B. festen oder gasförmigen - Säure, beispielsweise in Wasser, z.B. bidestilliertem oder vollentsalztem Wasser.In a second aspect, the present invention relates to a process for the electrolysis of CO ₂ , comprising an electrolytic cell

• a cathode compartment comprising a cathode;
• a first ion exchange membrane containing an anion exchanger and / or anion transporter adjacent to the cathode compartment, the cathode directly contacting the first ion exchange membrane; and
• an anode compartment comprising an anode, wherein the anode compartment adjoins the first ion exchange membrane; is used,

wherein CO _{2 is} reduced at the cathode, wherein the anode compartment comprises a liquid and / or dissolved acid. According to certain embodiments, the electrolyte in the anode compartment consists of the liquid acid and / or the solution of an acid, for example solid or gaseous acid, for example in water, for example double-distilled or demineralized water.

In order to illustrate in advance the inventive similarities and the differences of the methods according to the invention, these will be clarified in advance with reference to figures, wherein the methods are not limited to the embodiments shown in these figures. The individual constituents of the cells used in the method according to the invention as well as the cell according to the invention in which the method according to the invention can be carried out are then disclosed in detail below.

Exemplary different modes of operation of a double-membrane cell and single-membrane cell with which the methods according to the invention can be carried out are disclosed in US Pat 3 to 5 shown - in 3 also in conjunction with other components of an electrolysis system, also with regard to the inventive method. In the figures, a CO ₂ reduction to CO is assumed as an example. In principle, however, the process is not restricted to this reaction but can also be used for any other products, such as hydrocarbons, etc., preferably gaseous and / or liquid.

3 shows by way of example a 2-membrane structure for CO ₂ -Electro reduction with an acidic anodic reaction. Here, in each case, the cathode K in the cathode compartment I and the anode A in the anode compartment III provided, between these spaces a salt bridge room II is formed, from the cathode compartment I through a first ion exchange membrane, here as AEM, and the anode compartment III by a first separator membrane, here as CEM, for example as a cation and / or proton exchange membrane separated. Also shown are the supply of catholyte k for supplying the cathode with substrate, for example H ₂ O saturated, gaseous CO ₂ , electrolyte s in the salt bridge space comprising liquid and / or dissolved acid which couples the half cells to one another, and anolyte a for supplying the anode with substrate, eg HCl and / or H ₂ O, as well as a return R of CO ₂ . The other symbols in the 3 as well as the analog 8th . 9 . 10 . 12 . 15 . 19 and 20 are common fluidic symbols.

In contrast to the use of a salt-to-salt neutral to weak salt electrolyte, cathodically generated HCO ₃ ^- at the interface between the anion exchange membrane (AEM) and the salt bridge electrolyte can be neutralized by the present process according to the first aspect. This can prevent that HCO ₃ ^- can get to the anode and then lost as useless CO ₂ / O ₂ mixture. Thus, according to certain embodiments, almost pure CO _{2 is} released in the salt bridge space with only minimal traces of cathodic products that directly reach the cathode space I can be fed again.

4 and 5 show, moreover, further structures of an electrolytic cell, as it can be used in a method according to the second aspect of the present invention. In the two-chamber design, no salt bridge space is provided so that the anode compartment III connects directly to the AEM, in which case the anode itself, as in 4 and 5 shown at any point in the anode compartment III can be located. Corresponding embodiments of the anode compartment are also in a method with a structure as in 3 shown possible, where therefore the anode A not attached to the CEM. In the 4 and 5 The electrolysis cells shown can also be found in the in 3 shown electrolysis system are used. Also, the different half-cells can look out 3 to 5 , as well as the corresponding arranged components of the electrolysis system, are arbitrarily combined, as well as with other (not shown) Elektrolysehalbzellen. How out 3 to 5 Obviously, the methods according to the invention are characterized in that the cathode K the first ion exchange membrane, which contains an anion exchanger and / or anion transporter, contacted directly, in particular also ionic. In addition, the space adjacent to the first ion exchange membrane contains either the salt bridge space II in 3 or the anode compartment III in 4 and 5 - a liquid and / or dissolved acid.

The processes according to the invention are distinguished, in particular, by the use of a liquid and / or dissolved acid in the salt bridge space or in the anode space, especially in comparison with strongly acidic ion exchanger packages or similar solid devices:

First of all, gas bubbles produced in the salt bridge space or anode space can be transported away unhindered by the fluid through the medium, which enables a simple mode of operation. In addition, higher flow rates can be selected to ensure better cooling of the system. In addition, of course, even with the use of liquid and / or dissolved acids a simple and inexpensive operation is possible, especially in comparison to ion exchangers. Furthermore, with the use of liquid and / or dissolved acids accumulation of metal impurities in parts of the electrolysis cell can be prevented by washing them with the liquid and / or dissolved acid. Accordingly, an external electrolyte treatment, eg with a cation exchanger, is also possible below. In particular, this makes a big difference US 2017/0037522 A1 in which an empty or packed with ion exchangers middle chamber is disclosed.

The salt bridge space in the method of the first aspect and the anode space in the method of the second aspect are not particularly limited insofar as they respectively connect to the first ion exchange membrane. The term salt bridge space is used here in terms of its function to act as a bridge between the anode assembly and cathode assembly and thereby have cations and anions, which, however, do not have to form salts in the present case. Since in the present case a liquid or dissolved acid is present in the salt bridge space, one could also name it as acid bridge space or ion bridge space. However, since this term is not familiar, the space is referred to as salt bridge space according to the invention, even if in the classic sense, no salt must be present.

In the salt bridge space, if present, an electrolyte which can ensure the electrolytic-ionic connection between the cathode arrangement and the anode arrangement is present in the method according to the invention. This electrolyte is also referred to as a salt bridge and has a liquid and / or dissolved acid.

The salt bridge thus serves as an electrolyte, preferably with high ionic conductivity, and serves to establish the contact between anode and cathode. According to certain embodiments, the salt bridge also allows the dissipation of waste heat. In addition, the salt bridge can serve as the reaction medium for anodically and cathodically generated ions such as proton, hydroxide or hydrogen carbonate ions.

The technical teaching consists in the construction and operation of the cathodic half-cell. This consists of a gas-permeable electrically connected catalyst layer, which is in direct contact with an AEM, on the opposite side, an acid-based electrolyte, preferably without alkali metal cations, in particular without metal cations, connects. The acid is in this case not particularly limited insofar as it is present as a liquid and / or in solution, that is, the acid can flow through the salt bridge space and / or the anode space. According to certain embodiments, the acid is water-soluble and / or is present as a solution in a suitable solvent such as water, alcohols, aldehydes, esters, carbonates, etc. and / or mixtures, in particular water, eg bidistilled or demineralized water.

In certain embodiments, an acid of the liquid and / or dissolved acid in salt bridge space has a pK _s value of 6 or less, preferably 5 or less, more preferably 3 or less, more preferably 1 or less, especially preferred 0 or less, preferably wherein the liquid and / or dissolved acid is selected from dilute or undiluted H ₂ SO ₄ , a solution of H ₂ N-SO ₂ -OH, dilute or undiluted HClO ₄ , a solution of H ₃ PO ₄ , dilute or undiluted CF ₃ -COOH, dilute or undiluted CF ₃ -SO ₂ -OH, a solution of (CF ₃ -SO ₂ ) ₂ -NH, a solution of HF, dilute or undiluted HCOOH, dilute or undiluted CH ₃ - COOH, a solution of HCl, a solution of HBr, a solution of HI, and / or mixtures thereof.

The acid electrolyte, according to certain embodiments, is characterized by the absence of mobile cations, in particular metal cations, as defined below, except "H ⁺ " or "D ⁺ ". In the following, instead of H ⁺ or D ^+, only H ⁺ or protons are used. The electrolyte thus preferably contains no mobile cations other than "H ⁺ ", in particular no metal cations. Sulfuric acid, in particular dilute sulfuric acid (H ₂ SO ₄ ), which is particularly preferred as liquid and / or dissolved acid due to its low price and its high conductivity, was used in the exemplary embodiment according to the invention. Alternatively, other acids can be used as set forth above, with strong acids having non-oxidizing anions being preferred, such as H ₂ N-SO ₂ -OH, HClO ₄ , H ₃ PO ₄ , CF ₃ -COOH, CF ₃ -SO ₂ -OH, (CF ₃ -SO ₂ ) ₂ -NH, etc. Even weak acids may, preferably in higher concentrations, for example greater than 10 wt.% Or 20 wt.%, Eg greater than 30 wt.%, Or at their respective Conductivity maximum, can be used, for example, HF, HCOOH, CH ₃ -COOH. This is particularly preferred if this acid is identical to the cathodic product of the CO ₂ electrolysis, for example in the case of formic acid or acetic acid. In particular, the acids can be present in a concentration of up to 30% by weight, preferably up to 50% by weight, more preferably up to 70% by weight, and in particular up to 100% by weight. Also, especially with proven compatibility with the electrode catalysts, other acids can be used, such as dissolved HCl, HBr, HI.

For the purposes of the invention, a salt electrolyte which is usually adjacent to the first ion exchange membrane, for example in the salt bridge space or in the anode space, is replaced in particular by an acid.

In the presence of a salt bridge space, the advantage of the CO ₂ free anode and the partial separation of the CO ₂ surplus in the salt bridge space continue, as in 6 and 7 shown schematically. Unlike using a salt electrolyte 2 in the salt bridge room, as in 6 However, the release of CO ₂ does not take place at the interface between CEM and electrolyte but, as shown in FIG 7 shown at the interface between AEM and acid 3 , In 6 and 7 In this case, an acid is also present in the anode compartment.

Since acids are used in the preferred variant shown both for the anolyte and for the salt bridge, it is also possible to choose these with identical composition. Since no osmotic pressures occur in this case and the release of the CO ₂ s can take place in front of the salt bridge, in particular in the region of the first ion exchange membrane and thus away from the first separator membrane, if present, wherein the HCO ₃ ^- the first separator membrane according to In certain embodiments, it is no longer absolutely necessary to use an ion-selective membrane as the first separator membrane, and, for example, a diaphragm can be used to separate CO ₂ and O ₂ . Accordingly, a diaphragm is also possible as the first separator membrane, as further explained below, and consequently a corresponding electrolysis cell according to the invention, for example an AEM diaphragm cell - as further described below - can also be used in the process according to the invention.

It should be noted that it is in principle possible to pump anolyte and salt bridge from a common reservoir, in which case it is preferable to ensure reliable degassing of the electrolyte is to not carry off any gases. This succeeds particularly well due to the low solubility of CO ₂ in the acid-based electrolyte. According to certain embodiments, the processes according to the invention are therefore preferably carried out at elevated temperatures in the range from 50 to 120 ° C., preferably between 60 and 90 ° C., in order to further minimize gas solubility.

Corresponding considerations also apply analogously when a two-chamber structure according to the method of the second aspect is used.

In the process according to the invention, the acid concentration is preferably chosen such that it lies around the maximum conductivity of the acid. In this case, the conductivity, in particular for sulfuric acid (3 mol / l = ~ 30%), can be almost an order of magnitude higher than that which can be achieved by salt concentrations (1 to 2 mol / l) which are similarly high but at the saturation limit. Exemplary conductivities are shown in Tables 3 and 4 for sulfuric acid and phosphoric acid. Table 3: Electrical Conductivity of Sulfuric Acid (and Oleum) at 25 ° C (from Conductometry - Conductivity Measurement, Peter Bruttel, revised by Dr. Christine Thielen, Dr. Anja Zimmer, Metrohm AG, Switzerland Page 37 % H ₂ SO ₄ Conductivity [mS / cm] % H ₂ SO ₄ (SO ₃ ) Conductivity [mS / cm] 3, 93 177 53.5 555 7.00 308 58.4 471 10, 0 426 63, 1 380 14.6 586 72.3 223 19.8 717 85.9 124 25, 3 796 95, 4 124 29.4 825 98, 0 94, 7 34, 3 819 100, 0 10, 46 39, 1 781 101, 5 32, 05 43.9 714 103.8 34,50 48.7 640 105, 1 28, 84 M (H ₂ SO ₄ ) = 98, 07 g / mol M (SO ₃ ) = 80, 06 g / mol

Table 4: Electrical conductivity of phosphoric acid at 25 ° C (from Conductometry - Conductivity measurement, Peter Bruttel, revised by Dr. Christine Thielen, Dr. Anja Zimmer, Metrohm AG, Switzerland, page 37) % H ₃ PO ₄ Conductivity [mS / cm] % H ₃ PO ₄ Conductivity [mS / cm] 5 31 40 222 10 61 45 232 15 91 50 233 20 722 55 224 25 152 60 210 30 180 70 169 35 204 80 98 M (H ₃ PO ₄ ) = 97.995 g / mol

The individual constituents of an electrolysis cell used in the process according to the invention as well as the electrolysis cell according to the invention will now be further described and disclosed.

According to the invention, the cathode space, the anode space and possibly the salt bridge space present are not particularly limited in terms of shape, material, dimensions, etc., inasmuch as they comprise the cathode, the anode and the first ion exchange membrane and, if necessary, the electrolysis cell according to the invention can accommodate the first separator membrane. The two or three spaces can be formed, for example, within a common cell, in which case they can be correspondingly separated by the first ion exchange membrane and optionally the first separator membrane. Depending on the electrolysis to be carried out, corresponding supply and discharge devices for educts and products, for example in the form of liquid, gas, solution, suspension, etc., may be provided for the individual spaces, and these may possibly also be recycled in each case. Again, there is no limitation, and the individual rooms can be flowed through in parallel streams or in countercurrent. For example, in an electrolysis of CO ₂ - which may still contain CO, that is, for example, at least 20 vol.% CO ₂ contains - this to the cathode in solution, as a gas, etc. are supplied - for example, in countercurrent to an electrolyte flow in the salt bridge space in the three-chamber design or in the anode chamber in a two-chamber design (without first separator membrane). There is no restriction here. Corresponding feed options also exist in the anode compartment and are also described in more detail below. The respective supply can be provided both continuously and discontinuously, for example pulsed, etc., for which pumps, valves, etc. can be provided in an electrolysis plant according to the invention, as well as cooling and / or heating devices, to correspondingly desired reactions at the anode and / or cathode catalyze. The materials of the respective rooms or the electrolysis cell and / or the other constituents of the electrolysis plant can also be appropriately adapted to desired reactions, reactants, products, electrolytes, etc. In addition, of course, at least one power source per electrolytic cell is included. Other device parts which occur in electrolysis cells or electrolysis systems can also be provided in the electrolysis plant or electrolysis cell according to the invention. According to certain embodiments, a stack comprising 2 - 1000, preferably 2 - 200 cells, and whose operating voltage is preferably in the range of 3 - 1500 V, particularly preferably 200 - 600 V, is constructed from these individual cells.

According to certain embodiments, a reactant gas formed in the salt bridge space, for example CO ₂ , which may also contain H ₂ and / or CO, is recycled again in the direction of the cathode space.

According to the invention, the cathode is not particularly limited and may be adapted to a desired half reaction, for example with respect to the reaction products insofar as it contacts the first ion exchange membrane directly, ie it is in direct contact with the first ion exchange membrane at at least one point, preferably with the surface being substantially flat is in direct contact with the first ion exchange membrane. Thus, at least in one area, the cathode directly adjoins the first ion exchange membrane. For example, a cathode such as Cu, Ag, Au, Zn, Pb, Sn, Bi, Pt, Pd, Ir, Os, Fe, Ni, Co, W, Mo, etc. may be a cathode for reducing CO ₂ and possibly CO. , or mixtures and / or alloys thereof, preferably Cu, Ag, Au, Zn, Pb, Sn, or mixtures and / or alloys thereof, and / or a salt thereof, wherein suitable materials can be adapted to a desired product. The catalyst can thus be selected depending on the desired product. In the case of the reduction of CO ₂ to CO, for example, the catalyst is preferably based on Ag, Au, Zn and / or their compounds such as Ag ₂ O, AgO, Au ₂ O, Au ₂ O ₃ , ZnO. For the preparation of hydrocarbons Cu or Cu-containing compounds such as Cu ₂ O, CuO and / or copper-containing mixed oxides with other metals, etc., are preferred. For a production of formic acid, for example, catalysts based on Pb and / or Cu, in particular Cu, are possible. Since hydrogen formation at high current densities can be completely suppressed by the anion transport, it is also possible to use catalysts for CO ₂ reduction which do not have a high overvoltage to hydrogen, for example reduction catalysts such as Pt, Pd, Ir, Os or carbonyl-forming metals such as Fe, Ni , Co, W, Mo. Thus, the mode of operation described in connection with cell design opens up new avenues in CO ₂ reduction chemistry, which do not depend on hydrogen overvoltage.

• - Gasdiffusionselektrode bzw. poröse gebundene Katalysatorstruktur, die gemäß bestimmten Ausführungsformen mittels eines geeigneten Ionomers, beispielsweise eines anionischen Ionomers, mit der ersten Ionenaustauschermembran, beispielsweise einer Anionenaustauschermembran (AEM), z.B. ionenleitend und/oder mechanisch verklebt sein kann;
• - Gasdiffusionselektrode bzw. poröse gebundene Katalysatorstruktur, die gemäß bestimmten Ausführungsformen partiell in die erste Ionenaustauschermembran, beispielsweise eine AEM, gepresst sein kann;
• - poröse, leitfähige, katalytisch inaktiven Struktur, z.B. Kohlenstoff-Papier-GDL bzw. Carbon-Paper-GDL (gas diffusion layer, Gasdiffusionsschicht), Kohlenstoff-Tuch-GDL bzw. Carbon-Cloth-GDL, und/oder polymergebundener Film aus granularem Glaskohlenstoff, der mit dem Katalysator der Kathode und ggf. einem Ionomer, dass die Anbindung an die erste Ionenaustauschermembran, beispielsweise eine AEM, ermöglicht, imprägniert ist, wobei die Elektrode dann mechanisch auf die erste Ionenaustauschermembran, beispielsweise eine AEM, gepresst oder vorher mit der ersten Ionenaustauschermembran, beispielsweise einer AEM, verpresst sein kann, um einen Verbund zu bilden;
• - partikulärer Katalysator, der mittels eines geeigneten Ionomers auf einen geeigneten Träger, beispielsweise einen porösen leitfähigen Träger, aufgebracht ist und gemäß bestimmten Ausführungsformen an der ersten Ionenaustauschermembran, beispielsweise einer AEM, anliegen kann;
• - partikulärer Katalysator, der in die die erste Ionenaustauschermembran, beispielsweise eine AEM, eingepresst ist oder auf diese beschichtet ist und beispielsweise entsprechend leitend verbunden ist, wobei diese Struktur dann beispielsweise als sogenannte CCM (Catalyst-Caoted-Membrane; katalysator-beschichtete Membran) dann auf eine leitfähige, poröse Elektrode gepresst sein kann, wobei eine katalytische Aktivität dieser Elektrode grundsätzlich nicht erforderlich ist und beispielsweise Kohlenstoff basierte GDL's oder Gitter, beispielsweise aus Titan, verwendet werden können, wobei hierbei nicht ausgeschlossen ist, dass diese Elektrode Ionomere enthält und/oder den aktiven Katalysator enthält oder in großen Teilen daraus besteht;
• - nicht geschlossenes Flächengebilde, z.B. ein Netz oder ein Streckmetall, das beispielsweise aus einem Katalysator besteht bzw. diesen umfasst oder mit diesem beschichtet ist und gemäß bestimmten Ausführungsformen an der ersten Ionenaustauschermembran, beispielsweise einer AEM, anliegt;
• - polymergebundene Vollkatalysator-Struktur aus partikulärem Katalysator, die ein Ionomer, das die Anbindung an die erste Ionenaustauschermembran, beispielsweise eine AEM, ermöglicht, enthält oder nachträglich damit imprägniert wurde, wobei die Elektrode dann mechanisch auf die erste Ionenaustauschermembran, beispielsweise eine AEM, gepresst oder vorher mit der ersten Ionenaustauschermembran, beispielsweise einer AEM, verpresst sein kann, um einen Verbund zu bilden;
• - poröser, leitfähiger Träger, der mit einem geeigneten Katalysator und ggf. einem Ionomer imprägniert ist und gemäß bestimmten Ausführungsformen an der ersten Ionenaustauschermembran, beispielsweise einer AEM, anliegt;
• - nicht ionenleitfähige Gasdiffussionselektrode, die nachträglich mit einem geeigneten Ionomer, beispielsweise einem anionleitfähigen Ionomer, imprägniert wurde und gemäß bestimmten Ausführungsformen an der ersten Ionenaustauschermembran, beispielsweise einer AEM, anliegt oder mit dieser, z.B. über ein Ionomer, verbunden ist.

The cathode is the electrode at which the reductive half-reaction takes place. It can be embodied in one or more parts and be formed, for example, as a gas diffusion electrode, porous electrode or directly with the AEM in a composite, etc.
The following embodiments are possible, for example:

• - Gas diffusion electrode or porous bonded catalyst structure, which according to certain embodiments by means of a suitable ionomer, for example an anionic ionomer, with the first ion exchange membrane, such as an anion exchange membrane (AEM), for example ion-conducting and / or mechanically bonded;
• - Gas diffusion electrode or porous bonded catalyst structure, which according to certain embodiments may be partially pressed into the first ion exchange membrane, such as an AEM;
• porous, conductive, catalytically inactive structure, eg carbon-paper-GDL or carbon-paper-GDL (gas diffusion layer), carbon-cloth-GDL or carbon-cloth-GDL, and / or polymer-bonded film of granular Glassy carbon, which is impregnated with the catalyst of the cathode and optionally an ionomer, which allows the connection to the first ion exchange membrane, for example an AEM, the electrode then mechanically pressed onto the first ion exchange membrane, for example an AEM, or beforehand with the first ion exchange membrane, for example an AEM, may be compressed to form a composite;
• particulate catalyst, which is applied by means of a suitable ionomer to a suitable support, for example a porous conductive support, and according to certain embodiments, can abut the first ion exchange membrane, for example an AEM;
• Particulate catalyst, in which the first ion exchange membrane, for example an AEM, is pressed or coated onto it and, for example, correspondingly conductively connected, this structure then being known, for example, as CCM (Catalyst-Caoted membrane, catalyst-coated membrane) may be pressed onto a conductive, porous electrode, wherein a catalytic activity of this electrode is basically not required and, for example, carbon-based GDL's or gratings, for example made of titanium, can be used, whereby it is not excluded that this electrode contains ionomers and / or contains or consists in large part of the active catalyst;
• non-closed sheet material, eg a net or an expanded metal, which for example consists of, comprises or is coated with a catalyst and according to certain embodiments bears against the first ion exchange membrane, for example an AEM;
• polymer-bonded unsupported catalyst structure comprising particulate catalyst containing or subsequently impregnated with an ionomer which allows attachment to the first ion exchange membrane, for example an AEM, the electrode then being mechanically pressed onto the first ion exchange membrane, for example an AEM previously compressed with the first ion exchange membrane, such as an AEM, to form a composite;
• porous conductive carrier impregnated with a suitable catalyst and optionally an ionomer and, according to certain embodiments, being applied to the first ion exchange membrane, for example an AEM;
• non-ion-conductive gas diffusion electrode which has subsequently been impregnated with a suitable ionomer, for example an anionic ionomer, and according to certain embodiments is applied to the first ion exchange membrane, for example an AEM, or is connected thereto, eg via an ionomer.

Also, various combinations of the electrode structures described above are possible as the cathode.

The corresponding cathodes may in this case also contain materials customary in cathodes, such as binders, ionomers, for example anionic ionomers, fillers, hydrophilic additives, etc., which are not particularly limited. Thus, according to certain embodiments, besides the catalyst, the cathode may comprise at least one ionomer, for example an anionic ionomer (eg, anion exchange resin, anion transport resin) which may include, for example, various ion exchange functional groups, which may be the same or different, for example, tertiary amine groups, alkyl ammonium groups and / or phosphonium groups), for example a conductive carrier material (eg a metal such as titanium), and / or at least one non-metal such as carbon, Si, boron nitride (BN), boron-doped diamond, etc., and / or at least one conductive one Oxide such as indium tin oxide (ITO), aluminum zinc oxide (AZO) or fluorinated tin oxide (FTO) - for example as used to make photoelectrodes, and / or at least one polymer based on polyacetylene, polyethoxythiophene, polyaniline or polypyrrole, such as in polymer based electrodes; non-conductive supports such as polymer nets are possible, for example, with sufficient conductivity of the catalyst layer), binders (eg hydrophilic and / or hydrophobic polymers, eg organic binders, eg selected from PTFE (polytetrafluoroethylene), PVDF (polyvinylidifluoride), PFA (perfluoroalkoxy polymers), FEP (fluorinated ethylene-propylene copolymers), PFSA (perfluorosulfonic acid polymers), and mixtures thereof, in particular PTFE), conductive fillers (eg carbon), non-conductive fillers (eg glass) and / or hydrophilic additives (eg Al ₂ O ₃ , MgO ₂ , hydrophilic Materials such as polysulfones, for example polyphenylsulfones, polyimides, polybenzoxazoles or polyether ketones or in the electrolyte generally electrochemically stable polymers, polymerized "ionic liquids", and / or organic conductors such as PEDOT: PSS or PANI (champhersulfonsäuredortiertes polyaniline) included, which are not particularly limited.

The cathode, especially in the form of a gas diffusion electrode, e.g. bonded to the first ion exchange membrane, or contained in the form of a CCM, according to certain embodiments, contains ionically conductive components, in particular an anionic conductive component.

Other cathode forms are possible, for example, cathode structures, as in US20160251755-A1 and US9481939 are described.

Also, the anode according to the invention is not particularly limited and may be adapted to a desired half reaction, for example with respect to the reaction products. At the anode, which is electrically connected to the cathode by means of a current source for providing the voltage for the electrolysis, the oxidation of a substance takes place in the anode space. Moreover, the material of the anode is not particularly limited and depends primarily on the desired reaction. Exemplary anode materials include platinum alloys, palladium and glassy carbon, iron, nickel, etc. Other anode materials are also conductive oxides such as doped TiO ₂ , indium tin oxide (ITO), fluorine doped tin oxide (FTO), aluminum doped zinc oxide (AZO), iridium oxide, etc. If necessary. For example, these catalytically active compounds can also be superficially applied only in thin-film technology, for example on a titanium and / or carbon support. The anode catalyst is not particularly limited. As a catalyst for O ₂ - or Cl ₂ generation, for example, IrO _x (1.5 <x <2) or RUO ₂ are used. These may also be present as a mixed oxide with other metals, eg TiO ₂ , and / or be supported on a conductive material such as C (in the form of carbon black, activated carbon, graphite, etc.). Alternatively, catalysts based on Fe-Ni or Co-Ni can also be used for O ₂ production. For this purpose, for example, the structure described below with bipolar membrane or bipolar membrane is suitable.

• - Gasdiffusionselektrode bzw. poröse gebundene Katalysatorstruktur, die gemäß bestimmten Ausführungsformen mittels eines geeigneten Ionomers, beispielsweise eines kationischen Ionomers, mit der ersten Separator-Membran, so vorhanden, beispielsweise einer Kationenaustauschermembran (CEM) oder einem Diaphragma, z.B. ionenleitend und/oder mechanisch verklebt sein kann;
• - Gasdiffusionselektrode bzw. poröse gebundene Katalysatorstruktur, die gemäß bestimmten Ausführungsformen partiell in die erste Separator-Membran, beispielsweise eine CEM oder ein Diaphragma, gepresst sein kann;
• - partikulärer Katalysator, der mittels eines geeigneten Ionomers auf einen geeigneten Träger, beispielsweise einen porösen leitfähigen Träger, aufgebracht ist und gemäß bestimmten Ausführungsformen an der ersten Separator-Membran, beispielsweise einer CEM oder einem Diaphragma, anliegen kann;
• - partikulärer Katalysator, der in die erste Separator-Membran, beispielsweise eine CEM oder ein Diaphragma, eingepresst ist und beispielsweise entsprechend leitend verbunden ist;
• - nicht geschlossenes Flächengebilde, z.B. ein Netz oder ein Streckmetall, das beispielsweise aus einem Katalysator besteht bzw. diesen umfasst oder mit diesem beschichtet ist und gemäß bestimmten Ausführungsformen an der ersten Separator-Membran, beispielsweise einer CEM oder einem Diaphragma, anliegt;
• - solide Elektrode, wobei in diesem Fall auch ein Spalt zwischen der ersten Separator-Membran, beispielsweise einer CEM oder einem Diaphragma, und der Anode bestehen kann, wobei dies jedoch nicht bevorzugt ist;
• - poröser, leitfähiger Träger, der mit einem geeigneten Katalysator und ggf. einem Ionomer imprägniert ist und gemäß bestimmten Ausführungsformen an der ersten Separator-Membran, beispielsweise einer CEM oder einem Diaphragma, anliegt;
• - nicht ionenleitfähige Gasdiffussionselektrode, die nachträglich mit einem geeigneten Ionomer, beispielsweise einem kationleitfähigen Ionomer, imprägniert wurde und gemäß bestimmten Ausführungsformen an der ersten Separator-Membran, beispielsweise einer CEM oder einem Diaphragma, anliegt
• - beliebige Varianten der diskutierten Ausführungsformen, wobei die Elektrode z.B. ein anodisch stabiles anionenleitfähiges Material enthält und direkt an der anionenleitenden Schicht einer Bipolarmembran anliegt.

The anode is the electrode at which the oxidative half reaction takes place. It can also be designed as a gas diffusion electrode, porous electrode or solid electrode or solid electrode, etc.
The following embodiments are possible:

• - Gas diffusion electrode or porous bonded catalyst structure, which according to certain embodiments by means of a suitable ionomer, for example a cationic ionomer, with the first separator membrane, if present, for example, a cation exchange membrane (CEM) or a diaphragm, for example ion-conducting and / or mechanically bonded can;
• - Gas diffusion electrode or porous bonded catalyst structure, which according to certain embodiments may be partially pressed into the first separator membrane, such as a CEM or a diaphragm;
• particulate catalyst applied to a suitable support, for example a porous conductive support, by means of a suitable ionomer and, according to certain embodiments, abutting the first separator membrane, for example a CEM or a diaphragm;
• particulate catalyst which is pressed into the first separator membrane, for example a CEM or a diaphragm, and is correspondingly conductively connected, for example;
• non-closed sheet material, eg a net or an expanded metal, which for example consists of, comprises or is coated with a catalyst and, according to certain embodiments, bears against the first separator membrane, for example a CEM or a diaphragm;
• solid electrode, in which case a gap may also exist between the first separator membrane, for example a CEM or a diaphragm, and the anode, although this is not preferred;
• porous conductive carrier impregnated with a suitable catalyst and optionally an ionomer and, according to certain embodiments, abutting the first separator membrane, for example a CEM or a diaphragm;
• - non-ionic gas diffusion electrode which has been subsequently impregnated with a suitable ionomer, for example a cationic ionomer, and according to certain embodiments is applied to the first separator membrane, for example a CEM or a diaphragm
• - Any variants of the discussed embodiments, wherein the electrode, for example, contains an anodically stable anion conductive material and is applied directly to the anion-conducting layer of a bipolar membrane.

The anode may be attached to the acid electrolyte or directly on the first ion exchange membrane, e.g. AEM, for example in the form of a sheet (e.g., a fine mesh coated mesh), if no salt bridge space is present, but the latter is not preferred. Again, various combinations of different anode structures are possible. According to certain embodiments, the cathode is passed over the liquid acid, for example in the salt bridge space or in the anode space, e.g. in the salt bridge space, coupled to the anodic half-cell.

The corresponding anodes may also contain materials common in anodes such as binders, ionomers, e.g. also cation-conducting ionomers, for example containing sulfonic acid and / or phosphonic acid groups, fillers, hydrophilic additives, etc., which are not particularly limited, which are described, for example, also above with respect to the cathodes.

In an electrolysis cell according to the invention as well as in the methods according to the invention, the electrodes mentioned above by way of example can be combined with one another as desired.

In addition, electrolytes may also be present in the anode space and / or cathode space, which are also referred to as anolyte or catholyte, but it is not excluded according to the invention that no electrolytes are present in the two spaces and correspondingly, for example, only gases for conversion into this supplied be, for example, only CO ₂ , possibly also as a mixture with, for example CO and / or H ₂ O, which may also be liquid, for example as an aerosol, but preferably gaseous H ₂ O to the cathode and / or water or HCl to the anode. According to certain embodiments, an anolyte is present, which may differ from the salt bridge, ie the electrolyte of the salt bridge space, which has a liquid and / or dissolved acid - if present - or may correspond to it, for example with regard to solvents, acids etc., etc. Salt bridge is present, the anolyte comprises a liquid and / or dissolved acid.

A catholyte in this case is the electrolyte flow around the cathode and, according to certain embodiments, serves to supply the cathode with substrate or educt. The following embodiments are possible, for example. The catholyte can be present, for example, as a solution of the substrate (CO ₂ ) in a liquid carrier phase (eg water) and / or as a mixture of the substrate with other gases (eg CO + CO ₂ , water vapor + CO ₂ ). Also, recycle gases such as CO and / or H ₂ may be present through recycle. Also, as described above, the substrate may be present as a pure phase, for example CO ₂ . If uncharged liquid products are formed during the reaction, they can be washed out by the catholyte and can then be optionally separated afterwards.

An anolyte is an electrolyte stream around the anode or at the anode and, according to certain embodiments, serves to supply the anode with substrate or starting material and, if appropriate, the removal of anode products. The following embodiments are possible, for example. The anolyte can be used as a solution of the substrate (eg hydrochloric acid = HCl _aq ) in a liquid carrier phase (eg water), optionally with conductive salts, which are not limited - in particular when using a bipolar membrane as the first separator membrane, wherein the anolyte here also basic may be and may also contain cations, as described below, or as a mixture of the substrate with other gases (eg hydrochloric acid = HCl _g + H ₂ O, SO ₂ , etc.) are present. As with the catholyte, however, the substrate can also be present as a pure phase, for example in the form of hydrogen chloride gas = HCl _g .

According to certain embodiments, the anode compartment comprises an anolyte which comprises a liquid and / or dissolved acid, preferably wherein the anolyte and / or the acid in the salt bridge space or the electrolyte in the salt bridge space no mobile cations except protons and / or deuterons, in particular no metal ions. Cations, includes. According to certain embodiments, the acid in the salt bridge space does not comprise mobile cations except protons and / or deuterons, especially no metal cations. According to certain embodiments, the anolyte does not comprise mobile cations except protons and / or deuterons, especially no metal cations. Mobile cations are here cations which are not bound by a chemical bond to a carrier, and / or in particular an ion mobility of more than 1 × 10 ^-8 m ² / (s × V), in particular more than 1 × 10 ^-10 m ² / (s × V). According to certain embodiments, in the anodic half-reaction no mobile cations other than "D ⁺ ", H ⁺ ", especially no metal cations, are released or generated. In such a case, therefore, for example, for the special case of O ₂ evolution at the anode water (especially in the case of CCM anode) or acids with non-oxidizable anions as anolyte or reagent in question. For the generation of halogens at the anode are suitable in particular for this case, the halogen-hydrogen acids HCl, HBr or HI, halide salts may not be suitable when using a diaphragm as a first separator membrane, but when using a bipolar membrane as first separator membrane can be used. The use of SO ₂ in the anolyte for the production of sulfuric acid, or H ₂ O for the production of H ₂ O ₂ , etc., is possible.

According to certain embodiments, the anolyte is an aqueous electrolyte, it being possible where appropriate for the corresponding anolyte to be added to the anolyte, which are reacted at the anode. The educt addition is not particularly limited here. Likewise, the educt addition in the supply to the cathode space is not limited. For example, CO ₂ may be added to water outside the cathode compartment, or may also be added through a gas diffusion electrode, or may be supplied only as a gas to the cathode compartment. Corresponding considerations are possible analogously for the anode compartment, depending on the educt used, for example water, HCl, etc., and the desired product.

The first ion exchange membrane containing an anion exchanger and / or anion transporter or an anion transport material and which adjoins the cathode compartment is not particularly limited in the present invention. It separates in the process of the first aspect as well as in the electrolytic cell according to the invention, the cathode of the salt bridge space or in the process of the second aspect separates the cathode from the anode compartment, so from the direction of the cathode compartment comprising CO ₂ in the direction of the electrolyte, the order of cathode / first ion exchange membrane / Salt bridge space (first aspect) or cathode / first ion exchange membrane / anode space result. In particular, according to certain embodiments, it contains or consists of an anion exchanger which, in the electroless state, is present in the form of an acid-anion salt, preferably in accordance with the acid of the salt bridge, and more preferably passes from a minimum current density into the bicarbonate / carbonate form. According to certain embodiments, the first ion exchange membrane is an anion exchange membrane and / or anion transporter membrane. According to certain embodiments, the first ion exchange membrane may comprise a hydrophobic layer, for example, on the cathode side for better gas contacting. Preferably, the anion exchange membrane and / anion transport membrane also acts as cation (albeit in trace), in particular proton, blocker. Specifically, an anion exchanger and / or anion transporter with tightly bound cations can represent a blockade for mobile cations by Coulombabstoßung, which can additionally counteract salt precipitation, in particular within the cathode. The reason for this is probably the previously described formation of hydrogen carbonate ions during electrolysis and the resulting formation of bicarbonate salts from the cations transported through the membrane, if they are present. Without liquid electrolyte or sufficient active anion transport, these or their salts can usually not be removed.

Particularly in the case of a membrane electrode assembly (MEA), the accumulation of the electrolyte cations in the region of the interface is usually due to the electroosmosis. A concentration gradient can not be easily degraded on the electrode side, since a catalyst-based cathode, which is designed as set out above, e.g. a gas diffusion electrode or a CCM, usually has only a very poor Anionenleitfähigkeit. By integrating anions of conductive components, the anion conductivity can be significantly improved. In the method described here, the electrolyte contains only protons. These, like metal cations, are also pulled by the electric field towards the cathode, but they can not pass as such through the AEM in the carbonate / hydrogen carbonate since they react with the hydrogen carbonate ions contained therein. The fundamental difference to the use of salt electrolytes is that the charge transport at the AEM electrolyte boundary is not due to migration of a charge carrier, but by destruction of two oppositely charged charge carriers.

To improve the operational stability, ion transporters, in particular anion transport resins, can be used as binder material or additive in the electrode itself and / or in an anion exchange layer which is applied to the cathode, for example, to rapidly dissipate or partially buffer the resulting OH ^- ions, so that the Reaction with CO ₂ and the associated formation of bicarbonates can be reduced or the Anionentransportharze HCO ₃ ^- direct yourself. In principle, an anion transport can be carried out by anion exchangers. Furthermore, specially designed an integrated Anion exchangers, in turn, constitute a blockade for cations, eg also metal cation traces, which can additionally counteract salt precipitation and contamination of the electrode. In the case of protons, hydrogen formation can be suppressed.

The first ion exchange membrane, for example, from the cathode side adjacent to the salt bridge in the process of the first aspect, can thus contain, for example, an anion exchanger and / or anion and / or anion transporter in the form of an anion exchanger and / or transporter layer, in which case further layers, such as hydrophobicizing layers, are used to improve the Contact with a gas, such as CO ₂ , may be included. According to certain embodiments, the first ion exchange membrane is an anion exchange membrane and / or anion transporter membrane, ie for example an ion-conductive membrane (or in a broader sense a membrane with an anion exchange layer and / or anion transport layer) with positively charged functionalizations, which is not particularly limited. A preferred charge transport takes place in the anion exchanger layer and / or Anionentransporterschicht or a Anionenaustauschermenbran and / or Anionentransportermembran by anions. In particular, the first ion exchange membrane and, in particular, anion exchange layer and / or anion transport layer or an anion exchange membrane and / or anion transport membrane serve to provide an anion transport along fixed fixed positive charges. In particular, the penetration of an electrolyte, for example proton-containing electrolyte, into the cathode promoted by electro-osmotic forces can be reduced or completely avoided. The ion exchanger contained in the membrane can be converted in accordance with certain embodiments, in particular in operation in the carbonate / bicarbonate form and thereby suppress the passage of protons through the membrane to the cathode.

A suitable first ion exchange membrane, for example anion exchange membrane and / or anion transporter membrane, according to certain embodiments, exhibits good wettability by water and / or acids, in particular aqueous acids, high ion conductivity, and / or a tolerance of the functional groups contained therein to high pH values, in particular, shows no Hoffmann elimination. An exemplary AEM according to the present invention is the A201-CE membrane marketed by Tokuyama, the "Sustainion" marketed by Dioxide Materials, or an anion exchange membrane sold by Fumatech, such as, e.g. Fumasep FAS-PET or Fumasep FAD-PET.

Moreover, the first separator membrane is not particularly limited if it is present, that is, for example, in the method according to the first aspect of the present invention.

According to certain embodiments, the first separator membrane (viewed from the anode side adjacent to the salt bridge) is selected from an ion exchange membrane containing a cation exchanger, a bipolar membrane, wherein in the bipolar membrane, preferably the cation conductive layer is oriented toward the cathode and the anions conductive layer towards the anode, and a diaphragm.

A suitable first separator membrane, for example a cation exchange membrane or a bipolar membrane, contains, for example, a cation exchanger which can be in contact with the electrolyte in the salt bridge space. It may, for example, contain a cation exchanger in the form of a cation exchange layer, in which case further layers, such as hydrophobicizing layers, may be present. It may also be designed as a bipolar membrane or as a cation exchange membrane (CEM). The cation exchange membrane or cation exchange layer is, for example, an ion-conducting membrane or ion-conducting layer with negatively charged functionalizations. An exemplary charge transport into the salt bridge takes place in such a first separator membrane by cations. For example, commercially available Nafion® membranes are suitable as CEM, or Fumapem-F membranes sold by Asuma, Asahi Kasei-depleted Aciplex, or Flemion membranes marketed by AGC. In principle, however, it is also possible to use other polymer membranes modified with strongly acidic groups (groups such as sulfonic acid, phosphonic acid). According to certain embodiments, the first separator membrane prevents the passage of anions, in particular HCO ₃ ^- , into the anode space.

In addition, in the present method as well as in the electrolytic cell according to the invention, the first separator membrane may be formed as a diaphragm, whereby the cell can be made less complex and cheaper. According to certain embodiments, the diaphragm essentially separates the anode space and the salt bridge space, for example more than 70%, 80% or 90%, based on the interface between anode space and salt bridge space, or separates the anode space and the salt bridge space, that is to say 100%, based on the interface between anode compartment and salt bridge space. In particular, can be prevented by the use of liquid acid in the salt bridge space that HCO ₃ ^- ions in the Anode room arrive. In this respect, it is thus possible to dispense with a cation exchanger layer in the first separator membrane. The diaphragm is not particularly limited in this case and may for example be based on a ceramic (eg ZrO ₂ or Zr ₃ (PO ₄ ) ₃ ) and / or a swellable functionalized polymer, eg PTFE. Also binders (eg hydrophilic and / or hydrophobic polymers, for example organic binders, for example selected from PTFE (polytetrafluoroethylene), PVDF (polyvinylidifluoride), PFA (perfluoroalkoxy polymers), FEP (fluorinated ethylene-propylene copolymers), PFSA (perfluorosulfonic acid polymers ), and mixtures thereof, in particular PTFE), conductive fillers (eg carbon), non-conductive fillers (eg glass) and / or hydrophilic additives (eg Al ₂ O ₃ , MgO ₂ , hydrophilic materials such as polysulfones, eg polyphenylsulfones (PPSU), Polyimides, polybenzoxazoles, or polyether ketones, or polymers generally electrochemically stable in the electrolyte, may be present According to certain embodiments, the diaphragm is porous and / or hydrophilic, and since it is not itself ionically conductive, it should preferably be capable of reacting in the acid electrolyte used Furthermore, it is a physical barrier to gases and can not be penetrated by gas bubbles It is a porous polymer structure where the base polymer is hydrophilic (eg PPSU). Unlike the CEM or bipolar membrane, the polymer does not contain charged functionalities. Furthermore, the diaphragm may further preferably contain hydrophilic structuring components such as metal oxides (eg ZrO ₂ ) or ceramics, as stated above.

A suitable first separator membrane, such as a cation exchange membrane, a bipolar membrane and / or a diaphragm, exhibits, according to certain embodiments, good wettability by water and / or acids, high ionic conductivity, stability to reactive species generated at the anode may (for example given for perfluorinated polymers), and / or stability in the required pH regimes, in particular to the liquid acid in the salt bridge space.

According to certain embodiments, the first ion exchange membrane and / or the first separator membrane are hydrophobic if they form a CCM with the electrodes, at least on the side facing the electrodes, so that the reactants of the electrodes are gaseous. According to certain embodiments, the anode and / or cathode are at least partially hydrophilic. According to certain embodiments, the first ion exchange membrane and / or the first separator membrane are wettable with water. To ensure good ionic conductivity of the ionomers, preference is given to swelling with water. The experiment has shown that poorly wettable membranes can lead to a significant deterioration of the ionic bonding of the electrodes.

Also, for some of the electrochemical reactions on the catalyst electrodes, the presence of water is advantageous. eg 3 CO ₂ + H ₂ O + 2e ^- → CO + 2 HCO ₃ ^-

Therefore, according to certain embodiments, the anode and / or cathode also have sufficient hydrophilicity. Possibly. This can be adapted by hydrophilic additives such as TiO ₂ , Al ₂ O ₃ , or other electrochemically inert metal oxides, etc.

• - Ein Diaphragma wird bevorzugt verwendet, wenn die Salzbrücke (der Elektrolyt im Salzbrückenraum) und der Anolyt eine identische, bevorzugt inerte, Säure aufweisen oder aus dieser bestehen, wobei hier dann das Diaphragma dazu dient, Gase getrennt zu halten, sodass Kohlendioxid nicht in den Anodenraum übertritt, und/oder wenn an der Anode O₂ produziert wird, insbesondere um Kosten zu sparen.

In particular, according to certain embodiments, at least one of the following first separator membranes is used.

• - A diaphragm is preferably used when the salt bridge (the electrolyte in the salt bridge space) and the anolyte have an identical, preferably inert, acid or consist of this, in which case the diaphragm serves to keep gases separated so that carbon dioxide is not in the Anodenraum exceeds, and / or if at the anode O _{2 is} produced, in particular to save costs.

• - Eine Kationenaustauschermembran oder eine Membran mit einer Kationenaustauscherschicht werden insbesondere verwendet, wenn die Salzbrücke und der Anolyt nicht identisch sind, und/oder insbesondere wenn der Anolyt HCl, HBr und/oder HI enthält, und/oder wenn eine Chlorproduktion an der Anode erfolgt. Da die Kationenaustauschermembran den Übertritt von Anionen aus dem Anolyten in die Salzbrücke verhindert und im Unterschied zum Diaphragma keine offene Porosität aufweist, kann die Anode freier gestaltet werden. Im Prinzip ist die Anodenreaktion in solch einer Ausführungsform nur darin beschränkt, dass sie keine mobilen Kationen, außer Protonen, freisetzt, die durch die CEM in die Salzbrücke übertreten können.
• - Eine bipolare Membran, wobei bevorzugt eine Anionenaustauscherschicht und/oder Anionentransporterschicht der bipolaren Membran zum Anodenraum hin gerichtet ist und eine Kationenaustauscherschicht und/oder Kationentransportschicht der bipolaren Membran zum Salzbrückenraum hin gerichtet ist, wird insbesondere verwendet, wenn die Salzbrücke und der Anolyt nicht identisch sind, und/oder der Anolyt insbesondere Basen und oder Salze enthält, und/oder bei Verwendung wässriger Elektrolyte. Insbesondere bei der Verwendung von bipolaren Membranen als erste Separator-Membran kann der Anodenraum unabhängig von der Salzbrücke und dem Kathodenraum gestaltet werden, was eine Vielzahl an Anodenreaktionen mit gewünschten Produkten erlaubt, und insbesondere bei der Verwendung von Basen können auch billigere Anoden bzw. Anodenkatalysatoren, beispielsweise Nickel basierte Anodenkatalysatoren für eine Sauerstoffentwicklung, verwendet werden.

A corresponding structure of an exemplary electrolysis plant with diaphragm DF is in 8th shown here, where the other system components which the 3 correspond.

• A cation exchange membrane or a membrane with a cation exchange layer are used in particular if the salt bridge and the anolyte are not identical, and / or in particular if the anolyte contains HCl, HBr and / or HI and / or if chlorine production takes place at the anode. Since the cation exchange membrane prevents the passage of anions from the anolyte into the salt bridge and unlike the diaphragm has no open porosity, the anode can be made more free. In principle, in such an embodiment, the anodic reaction is limited only in that it does not release mobile cations, except protons, that can cross into the salt bridge through the CEM.
• - A bipolar membrane, wherein preferably an anion exchanger layer and / or Anionenentransporterschicht the bipolar membrane is directed towards the anode space and a cation exchange layer and / or cation transport layer of the bipolar membrane directed towards the salt bridge space, is particularly used when the salt bridge and the anolyte are not identical, and / or the anolyte contains in particular bases and / or salts, and / or when using aqueous electrolytes. Particularly when bipolar membranes are used as the first separator membrane, the anode compartment can be designed independently of the salt bridge and the cathode compartment, which allows a large number of anode reactions with desired products, and in particular when using bases, cheaper anodes or anode catalysts can also be used. For example, nickel-based anode catalysts for oxygen evolution can be used.

An exemplary special construction with bipolar membrane is in 9 The example shows a 2-membrane structure for CO ₂ -Electro reduction with AEM on the cathode side and bipolar membrane (CEM / AEM) on the anode side, here as well as in 1 to 3 the supply of catholyte k, electrolyte s with liquid and / or dissolved acid (electrolyte for the salt bridge space) and anolyte a, as well as a return R of CO ₂ , and an oxidation of water is exemplified on the anode side. The other reference numerals correspond to those in 3 ,

A bipolar membrane may, for example, be designed as a sandwich of a CEM and an AEM. In this case, but usually not two superimposed membranes are present, but it is a membrane with at least two layers. The representation in 9 with AEM and CEM this serves only to illustrate the preferred orientation of the layers. The AEM or anion exchanger layer shows the anode, the CEM or cation exchanger layer to the cathode. These membranes are almost impassable for both anions and cations. The conductivity of a bipolar membrane is therefore not based on the transportability of ions. Instead, ion transport usually occurs by acid-base dissociation of water in the middle of the membrane. As a result, two oppositely charged charge carriers are generated, which are transported away by the E-field.

The thus-generated OH ^- ions can be passed through the AEM part of the bipolar membrane to the anode, where they are oxidized 40H ^- → O ₂ + 2H ₂ O + 4e ^- and the "H ⁺ " ions through the CEM part of the bipolar membrane into the salt bridge or salt bridge space II where they can be neutralized by the cathodically generated HCO ₃ ^- ions. HCO ₃ ^- + H ⁺ → CO ₂ + H ₂ O

Since the conductivity of the bipolar membrane is based on the separation of charges in the membrane, however, is usually expected with a higher voltage drop.

The advantage of such a structure lies in the decoupling of the electrolyte circuits, since, as already mentioned, the bipolar membrane is almost impermeable to all ions.

As a result, it is also possible to realize a construction for a basic anode reaction which manages without constant tracking and removal of salts or anode products. This is otherwise possible only with the use of anolyte based on acids with electrochemically inactive anions such as H ₂ SO ₄ . When using a bipolar membrane, hydroxide electrolytes such as KOH or NaOH may also be used as the anolyte. High pH values thermodynamically favor the water oxidation and allow the use of much cheaper anode catalysts, for example based on iron-nickel, which would not be stable in acidic form.

Thus, for the purposes of the invention, when using a bipolar membrane as the first separator membrane, the use of bases, e.g. a hydroxide base, as anolyte when an acid is used in the salt bridge. The preferences for certain anolytes listed above do not apply to this special case. The advantage here is that in basic anolyte much cheaper anode catalysts, e.g. Ni / Fe based, can be used.

It is not excluded according to the invention that in addition to the first ion exchange membrane and possibly the first separator membrane further membranes and / or diaphragms are provided.

Besides, according to certain embodiments, the anode and / or cathode have sufficient hydrophilicity. Possibly. This can be adapted by hydrophilic additives such as TiO ₂ , Al ₂ O ₃ , or other electrochemically inert metal oxides, etc.

According to certain embodiments, the cathode and / or the anode is impregnated as a gas diffusion electrode, as a porous bound catalyst structure, as a particulate catalyst on a support, as a coating of a particulate catalyst on the first and / or second ion exchange membrane, as a porous conductive support into which a catalyst is impregnated is, and / or formed as a non-closed sheet. According to certain embodiments, the cathode is a gas diffusion electrode, a porous bonded catalyst structure, a particulate supported catalyst, a particulate catalyst coating on the first and / or second ion exchange membrane, a porous conductive support in which a catalyst is impregnated, and / or is formed as a non-closed sheet containing (r / s) an anion exchange material and / or anion transport material. According to certain embodiments, the anode is a gas diffusion electrode, a porous bound catalyst structure, a particulate supported catalyst, a particulate catalyst coating on the first and / or second ion exchange membrane, a porous conductive support in which a catalyst is impregnated, and / or is formed as a non-closed sheet which contains (r / s) a cation exchange material and / or is coupled and / or bound to a bipolar membrane. The various embodiments of the cathode and anode can be combined with one another as desired.

According to certain embodiments, the anode contacts the first separator membrane, as already described above by way of example. This makes a good connection to the salt bridge space possible. In addition, no charge transport through the anolyte is necessary in this case and the charge transport path is shortened. Also electrical shading effects can be bypassed by supporting structures between the anode and the first separator membrane so.

According to certain embodiments, the anode and / or the cathode are contacted with a conductive structure on the side facing away from the salt bridge space. The conductive structure is not particularly limited here. The anode and / or the cathode are therefore contacted in accordance with certain embodiments of the salt bridge opposite side by conductive structures. These are not particularly limited. These may be, for example, coal flows, metal foams, metal knits, expanded metals, graphite structures or metal structures.

According to certain embodiments, the electrolysis is carried out at a current density of more than 50 mAcm ^-2 , preferably more than 100 mAcm ^-2 , more preferably 150 mAcm ^-2 or more, even more preferably 170 mAcm ^-2 or more or 200 mAcm ^-2 or more, in particular of 250 mAcm ^-2 or more, eg 300 mAcm ^-2 or more, 400 mAcm ^-2 or more, or 600 mAcm ^-2 or more. As stated above, the Faraday yield can be improved - contrary to expectation.

Another advantage of the methods of the invention is the comparatively low requirements for the chemical stability of the first ion exchange membrane, e.g. an AEM. The stability and thus the applicability of particular AEM's is currently limited mainly by two degradation mechanisms, on the one hand by the often lacking stability of the functional groups over concentrated bases, e.g. KOH (Hoffmann-Elimination of Quaternary Ammonium Ions, on the other hand, by the destruction of the polymer backbone by anodic oxidation.) Since only electrolytes in contact with the first ion exchange membrane are used in the electrolysis systems introduced here, the first ion exchange membrane, eg one In addition, the anode is preferably not directly attached to the first ion exchange membrane, eg an AEM, which also precludes the anodic damage of this membrane.

The present electrolysis processes can produce various products from CO ₂ , such as CO and / or hydrocarbons. Formate can also be generated electrochemically from CO ₂ . 2 CO ₂ + H ₂ O + 2 e ^- → HCOO ^- + HCO ₃ ^- (5) HCOO ^- + H ₂ O ⇆ OH ^- + HCOOH

In usually used carbonate-buffered salt electrolytes as salt bridge it comes usually to the deprotonation of the formic acid. The actual product are thus formate salts. HCOOH + MHCO ₃ → HCOOM + H ₂ O + CO ₂ (6)

The cleavage of formate salts in formic acid is technically difficult and costly, which has hitherto limited the applicability of CO ₂ electrolytes to formic acid.

In the system introduced here, this deprotonation does not take place since the electrolyte used is an acidic electrolyte, in particular pure acid. For further simplification, for example, formic acid, for example dilute formic acid, can also be used in the salt bridge, which can be concentrated by the electrolysis, which is favored by a corresponding electrical conductivity of the formic acid, as can be seen from Table 5. Table 5: Conductivity of organic acids at 25 ° C Formic acid [% by weight] Conductivity [mS / cm] Acetic acid [wt.%] Conductivity [mS / cm] 5 6.22 5 1.36 10 8.26 10 1.76 15 9.86 15 1, 82 20 11, 1 20 1.82 25 11.4 25 1.71 30 11.8 30 1.58 40 11.1 40 1.23 50 9.78 50 0, 840 60 7.92 60 0.521 70 5.92 70 0,270 80 3.92 80 0.093 90 1.95 100 0.32 M (HCOOH) = 46.026 g / mol M (CH ₃ COOH) = 60.052 g / mol

In operation, according to Table 5, for example, a 10 wt.% Formic acid is presented, which is concentrated in operation, for example, 60-70 wt.%. The electrolyte is then stripped down to a residue which is used to restore the starting concentration of 10% by weight. Continuously operating systems in a narrower concentration range are also conceivable. For formic acid, electrodes such as those based on or made from tin or lead can preferably be used. Occurring HCO ₃ ^- shows Transport that there is a high pH in the region of the cathode. Since formic acid has a lower pK _s value than CO ₂ , this is present in the region of the cathode as formate. These anions are then, for example, also transported away by the first ion exchange membrane, eg AEM, into the salt bridge (first aspect) or the anolyte (second aspect) and re-protonated by the acid there. The latter is regenerated by the protons which pass from the anodic half-cell or are present in the anolyte. A leakage of formic acid on the side facing away from the salt bridge space of the electrode, if any, is not expected.

For CO ₂ to CO electrolysis, for example, a first ion exchange membrane, such as AEM diaphragm cell advantageous because the components are cheaper and the electrical resistance of the cell is lower.

• - wenn Anolyt und Salzbücke nicht identisch sind;
• - bei der Ko-Elektrolyse von CO₂ und HCl, um gleichzeitig ein CO₂-Reduktionprodukt, z.B. CO, und Cl₂ zu erzeugen;
• - bei der oben beschriebenen Herstellung von Ameisensäure, um eine Re-Oxidation der Ameisensäure an der Anode zu vermeiden;
• - bei der Verwendung von kupferbasierten Kathoden, die Alkene, Alkane, Alkohole und flüssige Oxygenate erzeugen;
• - bei beliebigen Kombinationen dieser Punkte

A double membrane cell with acid-salt bridge is also advantageous for those applications in which an exchange of anions between salt bridge and anolyte should be avoided, eg

• - if anolyte and saline are not identical;
• in the co-electrolysis of CO ₂ and HCl, to simultaneously produce a CO ₂ reduction product, eg CO, and Cl ₂ ;
• in the production of formic acid described above, in order to avoid a re-oxidation of the formic acid at the anode;
• using copper-based cathodes that produce alkenes, alkanes, alcohols, and liquid oxygenates;
• - with any combination of these points

The process according to the invention CO _{2 is} electrolyzed, although it is not excluded that on the cathode side in addition to CO ₂ another reactant such as CO is present, which can also be electrolyzed, ie a mixture is present, the CO ₂ comprises, and for example CO. For example, an educt on the cathode side contains at least 20 vol.% CO ₂ , for example at least 50 or at least 70 vol.% CO ₂ , and in particular the educt on the cathode side to 100 vol.% CO ₂ .

• - einen Kathodenraum umfassend eine Kathode;
• - eine erste Ionenaustauschermembran, welche einen Anionenaustauscher und/oder Anionentransporter enthält und welche an den Kathodenraum angrenzt, wobei die Kathode die erste Ionenaustauschermembran direkt, bevorzugt auch ionisch, kontaktiert;
• - einen Anodenraum umfassend eine Anode; und
• - ein Diaphragma, welches an den Anodenraum angrenzt; weiter umfassend einen Salzbrückenraum, wobei der Salzbrückenraum zwischen der erste Ionenaustauschermembran und dem Diaphragma angeordnet ist.

In a further aspect, the present invention relates to an electrolytic cell comprising

• a cathode compartment comprising a cathode;
• a first ion exchange membrane which contains an anion exchanger and / or anion transporter and which adjoins the cathode space, the cathode contacting the first ion exchange membrane directly, preferably also ionically;
• an anode compartment comprising an anode; and
• a diaphragm adjacent to the anode compartment; further comprising a salt bridge space, wherein the salt bridge space between the first ion exchange membrane and the diaphragm is arranged.

With this electrolysis cell, the method according to the invention can be carried out according to the first aspect of the present invention. Consequently, all features discussed with regard to the inventive method are also applicable to the electrolysis cell according to the invention. In particular, the cathode compartment, the cathode, the first ion exchange membrane, the anode compartment, the anode, the diaphragm and the salt bridge compartment have already been discussed with regard to the methods according to the invention. The corresponding features can thus be embodied in accordance with the above-discussed in the electrolysis cell according to the invention. The electrolysis cell according to the invention or the electrolysis plant according to the invention are therefore used in particular in the process according to the invention for the electrolysis of CO ₂ , which is why aspects which are discussed in connection with these in advance and below also relate to the electrolysis cell or the electrolysis plant according to the invention. Accordingly, in connection with the electrolytic cell and / or electrolysis system according to the invention may relate to the inventive method.

• - einen Kathodenraum umfassend eine Kathode;
• - eine erste Ionenaustauschermembran, welche einen Anionenaustauscher und/oder Anionentransporter enthält und welche an den Kathodenraum angrenzt, wobei die Kathode die erste Ionenaustauschermembran direkt kontaktiert;
• - einen Anodenraum umfassend eine Anode; und
• - eine erste Separator-Membran, welche an den Anodenraum angrenzt;

weiter umfassend einen Salzbrückenraum, wobei der Salzbrückenraum zwischen der erste Ionenaustauschermembran und der ersten Separator-Membran angeordnet ist, wobei der Salzbrückenraum eine flüssige und/oder gelöste Säure umfasst. Auch mit dieser Zelle kann das erfindungsgemäße Verfahren gemäß dem ersten Aspekt durchgeführt werden, sodass auch die dort beschriebenen Merkmale hier entsprechend Anwendung finden können.Also described is an electrolytic cell comprising

• a cathode compartment comprising a cathode;
• a first ion exchange membrane containing an anion exchanger and / or anion transporter adjacent to the cathode compartment, the cathode directly contacting the first ion exchange membrane;
• an anode compartment comprising an anode; and
• a first separator membrane adjacent to the anode compartment;

further comprising a salt bridge space, wherein the salt bridge space between the first ion exchange membrane and the first separator membrane is arranged, wherein the salt bridge space comprises a liquid and / or dissolved acid. The method according to the invention according to the first aspect can also be carried out with this cell, so that the features described therein can also be used correspondingly here.

According to certain embodiments, the anode contacts the diaphragm. According to certain embodiments, the anode and / or the cathode are contacted with a conductive structure on the side facing away from the salt bridge space.

According to certain embodiments, the cathode and / or the anode are impregnated as a gas diffusion electrode, as a porous bound catalyst structure, as a particulate catalyst on a support, as a coating of a particulate catalyst on the first and / or second ion exchange membrane, as a porous conductive support into which a catalyst is impregnated is, and / or formed as a non-closed sheet.

According to certain embodiments, the cathode is a gas diffusion electrode, a porous bonded catalyst structure, a particulate supported catalyst, a particulate catalyst coating on the first and / or second ion exchange membrane, a porous conductive support in which a catalyst is impregnated, and / or is formed as a non-closed sheet containing (r / s) an anion exchange material and / or Anionentransportmaterial, and / or the anode as a gas diffusion electrode, as a porous bound catalyst structure, as a particulate catalyst on a support, as a coating of a particulate catalyst on the first and or second ion exchange membrane, as a porous conductive support in which a catalyst is impregnated, and / or formed as a non-closed sheet containing (r / s) a cation exchange material.

According to certain embodiments, the first ion exchange membrane and / or the diaphragm are hydrophilic.

According to certain embodiments, the salt bridge space comprises a liquid and / or dissolved acid, wherein preferably an acid of the liquid and / or dissolved acid in the salt bridge space has a pK _s value of 6 or less, preferably 5 or less, more preferably 3 or less, still further preferably 1 or less, more preferably 0 or less, more preferably the liquid and / or dissolved acid is selected from dilute or undiluted H ₂ SO ₄ , a solution of H ₂ N-SO ₂ -OH, dilute or undiluted HClO ₄ , a solution of H ₃ PO ₄ , dilute or undiluted CF ₃ -COOH, dilute or undiluted CF ₃ -SO ₂ -OH, a solution of (CF ₃ -SO ₂ ) ₂ -NH, a solution of HF, dilute or undiluted HCOOH, dilute or undiluted CH ₃ -COOH, a solution of HCl, a solution of HBr, a solution of HI, and / or mixtures thereof. According to certain embodiments, the electrolyte in the salt bridge space consists of a liquid and / or dissolved acid and possibly unavoidable impurities.

According to certain embodiments, the anode compartment contains an acid which is preferably identical to the electrolyte in the salt bridge, in particular if the second diaphragm is designed as a diaphragm.

In a further aspect, the present invention relates to an electrolysis plant comprising the electrolysis cell according to the invention. The corresponding embodiments of the electrolysis cell as well as further exemplary components of an electrolysis plant according to the invention have already been discussed above and are therefore also applicable to the electrolysis plant according to the invention. According to certain embodiments, an electrolysis plant according to the invention comprises a multiplicity of electrolysis cells according to the invention, wherein it is not excluded that there are also other electrolysis cells in addition.

According to certain embodiments, the electrolysis plant according to the invention further comprises a recirculation device which is connected to a discharge of the salt bridge space and a supply of the cathode space, which is adapted to a reactant of the cathode reaction, which can be reformed in the salt bridge space, in particular a gaseous or not with the electrolyte miscible educt to lead back into the cathode compartment, such as CO ₂ , which may also contain CO and / or H ₂ ..

According to certain embodiments, the electrolysis plant according to the invention further comprises an external device for electrolyte treatment, in particular a device for removing dissolved gases from an acid, with which in particular the anolyte and / or the electrolyte are treated in the salt bridge space, for example, gases such as CO ₂ or To remove O ₂ , and thus to allow a return of anolyte and / or the electrolyte in the salt bridge space. This is particularly advantageous if both are pumped from a common reservoir, so there is only one common anolyte / electrolyte for salt bridge reservoir, so the anolyte and the electrolyte in the salt bridge space are identical.

According to certain embodiments, the electrolysis plant according to the invention comprises two separate circuits for anolyte and electrolyte in the salt bridge space, which may optionally have separate devices for electrolyte treatment, in particular devices for removing dissolved gases from an acid, or wherein only the circuit for the electrolyte in Salt bridge space has a corresponding device.

In a still further aspect, the present invention relates to the use of an electrolysis cell according to the invention or an electrolysis plant according to the invention for the electrolysis of CO ₂ , which may also comprise a plurality of electrolysis cells according to the invention.

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 which have not been explicitly mentioned above or described below with regard to the exemplary embodiments. 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 further explained in detail with reference to various examples thereof. However, the invention is not limited to these examples.

Examples

Example 1:

The structure of the electrolyzer in Example 1 is similar to the in 3 shown construction and is shown schematically in 10 shown.

In this embodiment, a three-compartment cell was used. The cathode used was a silver particle-coated carbon GDL: Freundenberg HL 23 used. The particles were precipitated by means of NaBH ₄ from AgNO ₃ in ethanol as follows. AgNO ₃ (3.4g, 20mmol) was dissolved in ethanol (250ml). NaBH ₄ (3g, 80mmol) was dissolved in NaOH saturated methanol (100ml) and this solution added dropwise. After all silver was precipitated (no blackening at the drop in site), the addition was stopped. The precipitate was in a frit ( P4 ) and washed 4x with 50 ml of ethanol and 1x with 50 ml of diethyl ether. Subsequently, the powder was dried in vacuo. Yield: 2.88 g borate stabilized particles

From the particles (90 mg), a dispersion with the ionomer AS- 4 (Anion exchanger, Tokuyama) (180 mg 5% solution in n-PrOH (n-propanol)) in n-PrOH (2.8 g). Three layers of this dispersion were applied to a 60 cm ² piece of GDL.

A 10cm ² piece of this cathode was mechanically placed on an AEM A201 -CE (Tokuyama) and contacted the cathode through a titanium frame.

The anode used was a Ti expanded metal with a width of 1 × ₂ mm coated with IrO ₂ . As CEM became a Nafion N115 Membrane used which was pressed directly onto the metal plug.

To ensure sufficient mechanical pressure, five polymer meshes with a mesh size of 0.5mm were integrated into the cell. As electrolyte were in salt bridge space II and in the anode room III 0.1MH ₂ SO ₄ used. CO ₂ would be supplied via a gas humidifier GH with water. The CO ₂ flux was chosen for the current densities 50, 100, & 150 mAcm ^-2 so that a threefold excess is available (λ = 4). For a first measurement at 10mAcm ^-2 , the same gas supply was chosen for metrological reasons as for 50mAcm-2 (λ = 20). Oxygen was produced at the anode, and a product gas from the cathode space K was examined after passing through a bubbler B with a GC gas chromatograph. Similarly, a gas deposited in the salt bridge space was analyzed by GC.

At the beginning of the experiment, the cell was run at 4V for 20 min. Subsequently, the cell was retracted for another 30 minutes at 10mAcm ^-2 . Subsequently, at 10, 50, 100 and 150mAcm ^-2 both the amount and the composition of the gases were split I and split 2 certainly.

observations:

At the lowest current density 10mAcm ^-2 , no gas evolution was observed in salt bridge space II. At the higher current densities of 50, 100 and 150mAcm ^-2 , gas evolution was observed in salt bridge space II. Gas chromatographic analysis of this gas showed that it is pure CO ₂ > 98 vol.%. The proportion of CO in this gas was less than 1 vol. ‰. The highest H ₂ content resulted in about 1.5 vol.%. A direct return of this gas in the cathode feed is therefore possible.

Throughout the experiment, no significant electrolyte penetration was observed on the back of the cathode. When disassembling the cell, neither liquid nor salt crystallites were found on the back of the electrode. A final pH measurement of the bubbler B's fill solution revealed a pH ~ 5, which definitively precludes passage of the acid electrolyte.

The test results of Example 1 are in 11 shown in which the Faraday efficiency FE is plotted against the applied current density J.

The Faraday efficiency for CO increases steadily in this cell with the current density. The reason for this is the transport model described above. Due to the integrated spacer, the electrolyte in the cell warmed up to ~ 60 ° C, which had no negative influence on the selectivity.

Comparative Example 1

The experimental setup used in Comparative Example 1 is in 12 is substantially identical to that of Example 1 and is identical in terms of device components, except that the acid is in the salt bridge space II was replaced by a KHCO ₃ salt electrolyte.

For current densities of 50 and 100mAcm ^-2 , 1M KHCO _{3 was} used as the electrolyte in the salt bridge space. For 150mAcm ^{-2 it} had to be changed to a 2M KHCO ₃ for reasons of instrumentation (maximum voltage of the potentiostat).
At all current densities, a triple CO ₂ excess was used.

observations:

At all current densities, there was gas evolution in the salt bridge space II to observe. There was no observed increased electrolyte penetration. However, when the cell was dissected, liquid and salt crystallites were found on the back of the cathode.

The test results of the comparative example 1 are in 13 shown, in turn, the Faraday efficiency FE against the applied current density J is plotted.

As in 13 can be seen, the selectivity decreases with increasing current density. This is due to the increased passage of alkali metal cations through the electrode and the associated partial flooding of the electrode.

In a comparison between Comparative Example 1 (dotted lines) and the embodiment (solid lines) in FIG 14 show in comparison to the conventional salt electrolyte, the advantages of the method according to the invention at elevated current densities.

Comparative Example 2:

A schematic representation of the experimental setup in Comparative Example 2 is shown in FIG 15 shown. In this comparative example, as compared with Example 1, the AEM was omitted to show that it is essential, with the other experimental setup corresponding to that of Example 1. It should be noted that the cathode still contains an anion exchange ionomer corresponding to the polymer base of the AEM.

observations:

At all measured current densities, gas evolution occurred in the salt bridge space II observed. The analysis of this gas shows, however, that this gas is in contrast to the Embodiment mainly H ₂ acts (81 vol.% H ₂ , 18 vol.% CO ₂ ). In addition, no liquid passage through the cathode was observed. 60 Vol.% Of the hydrogen was observed in the cathode compartment I.

The experimental results of Comparative Example 2 are in 16 shown, in turn, the Faraday efficiency FE against the applied current density J is plotted. This shows the preferred production of hydrogen.

When in 17 Comparison shown between the comparative example 2 (dashed lines) and the embodiment (solid lines) shows the advantageous embodiment in Example 1 with increased selectivity for CO. This is also from the in 18 shown comparison of the gas chromatograms of Comparative Example 2 (solid line) and the embodiment (dashed line) at J = 150mAcm ^-2 , here the measurement without AEM (w / o AEM) as a solid line and with AEM (w / AEM) as dashed line is shown.

In Example 1 and in Comparative Example 2 it is described that with the use of acid electrolyte no liquid penetrates through the cathode on the side facing away from the electrolyte. Over long operating periods, however, leakage of liquid from the GDE would in principle be conceivable. By design, this is not a concentrated carbonate solution, but almost pure water, in particular no salt solutions - as in the case of metal cation-containing electrolytes. This circumstance brings advantages in the construction of the cell as well as in the design of the entire electrolyte system. It has been observed, for example, that titanium contacts can corrode on contact with saline solutions passing through the electrode due to the highly negative potential. As a consequence, the permeate turns blue (Ti3 ⁺ ). The titanium corrosion is confirmed here as the cause of the blue coloration by means of chromotropic acid, and the cathodic corrosion detected in control experiments. The permeate liquids (if any) are not or little electrically conductive in the inventive arrangement presented here or in the inventive method. Although the contacts are still exposed to a strong negative potential, but not contacted ionic. Consequently, such corrosion phenomena do not occur or severely limited. Since any liquid that may be present on the backside of the electrode is water, it contains no ions that need to be returned to the electrolyte. This liquid can therefore simply be discarded. Any occurring corrosion products of the contacts are not washed accordingly in the electrolyte.

Reference Examples 1 and 2

Reference Examples 1 and 2 examined the effects of low anode pH on cell voltage.

According to the Nernst equation, the potential of oxidation of water to oxygen depends on the pH of the electrolyte. $$e=1.2V-2.3\times \frac{RT}{F}\times pH$$

In order to keep the cell voltage as low as possible, it is therefore advisable to have the highest possible pH value in the region of the anode. However, this can be maintained according to the invention under the general condition of a CO ₂ -free anode using only a bipolar membrane.

With a cation exchange membrane or a diaphragm, the cations would be removed from the anode compartment, which would lead to a lowering of the pH. An anion exchange membrane at the anode would lead to the penetration of HCO ₃ ^- into the anode space, which would lead to the undesired mixing of the anodically generated oxygen with CO ₂ .

To allow for constant operation, an acid is chosen as the anolyte (except when using a bipolar membrane). From the point of view of cell voltage, this initially seems less advantageous, since this procedure leads to a high water oxidation potential. However, it has been shown experimentally that the thermodynamic considerations (according to the Nernst equation) apply only to the "onset" region (ie the range of minimum current densities). At higher current densities, the same cell voltage was observed for an acidic anode and a pH-neutral to slightly basic anode.

For this purpose, a simple comparative experiment was carried out. Initially, it was of a simple construction with acid anolyte and neutral buffered salt bridge 19 - With the corresponding components of Example 1 and Comparative Example 1 - added the UI characteristic. The peripherals were connected according to 20 rebuilt so that now the anode was supplied with the neutral-buffered electrolyte. No changes were made to the cell. In addition, the anode is a "zero gap" anode which rests directly against the membrane. The conductivity of the anolyte therefore plays no role for the voltage. The electrolyte in the salt bridge is identical in both cases. All changes in voltage are therefore due to the different pH of the anolyte. Subsequently, a UI characteristic was recorded again. The construction in 19 represents an adaptation of an alkaline electrolysis cell for CO ₂ electrolysis. The replacement of the cation exchange membrane in a diaphragm was omitted for reasons of comparability.

It should be noted here that the anolyte in the structure after 20 contains no anions of stable acids. Therefore, the expression of a locally low pH values, as would be possible, for example in the case of Na ₂ SO ₄ , also excluded here.

The 21 and 22 show the comparison of the UI characteristic curves, with the measured values with the structure 19 with filled squares and the measurements with the structure after 20 with open circles, where 21 the "onset" range of the characteristic curve (especially left and 22 the complete characteristic up to 200 mAcm ^-2 shows.

How out 21 shows that the electrolysis in the case of acid anolyte about 480 mV later. This matches well with the expected value of 460 mV expected for a pH difference of 7. Out 22 However, this effect is only valid in the "onset" area. Above a current density of 100 mAcm ^-2 the characteristics coincide.

This clearly shows that for a productive electrolysis system operated at high current densities, the use of acids as the anolyte does not cause any disadvantages in terms of cell voltage.

Effects of CO ₂ release on cell voltage:

In the two constructions of the reference examples, CO ₂ release from HCO ₃ ^{- occurs} in the cell. In case of construction after 19 this happens in the salt bridge, in the case of the construction after 20 in the anode compartment. In both cases, the fourfold volume of CO _{2 is} released compared to the anodically generated oxygen.

In case of construction after 19 This happens before the CEM in the room of the salt bridge. In case of 20 this happens in the immediate vicinity of the anode. The resulting gas bubbles are, however, transported away behind the anode. They are not in the current path, which is the smoother curve for this construction (filled squares) in 22 explained. From the figure, however, also shows that the total voltage does not increase due to the load of the salt bridge.

For the purposes of the present invention, it is further particularly contemplated not to use a bare cathode, but a cathode AEM composite. For these constructions, a transport number for CO ₂ ≤ 0.55 was observed experimentally. The gas load of a salt bridge space is thus only about half as large as in the present comparative example. Up to a current density of 400mAcm ^-2 is therefore not to be expected in comparable structures with a drastic increase in the voltage through these gas bubbles.
The situation is different with the anode. This is burdened by the transition from an acid anolyte to a carbonate-containing, neutral-buffered electrolyte with five times the amount of gases produced. As a result, parts of the anode can be isolated and cut off from the electrolyte, which simultaneously represents the substrate. In the region of 150-200mAcm ^-2 , the voltage for the acid anolyte is even lower, which is due not least to the high gas load of the anode (in both cases a non-closed area with catalyst coating).

The present invention is characterized by using liquid and / or dissolved acids, in particular pure, acids as electrolytes for CO ₂ electrolysis at high current densities and at the same time high Faraday efficiency. In addition, a new cell type was introduced with a three-chamber design with a first ion exchange membrane and a diaphragm, for example an AEM diaphragm double-separator cell.

• - Keine CO₂-Freisetzung an der Anode, nur O₂ bzw. andere anodische Produkte
• - CO₂ wird in einer separaten Kammer freigesetzt und kann rückgeführt werden.
• - Keine Salzausscheidung
• - Faraday-Effizienz der CO Erzeugung nimmt mit steigender Stromdichte zu
• - Kein oder nur sehr wenig Permeat in den Gasraum des Kathodenraums
• - Bei Verwendung der gleichen Säure im Anodenraum und in der Salzbrücke reicht ein Diaphragma zur Trennung von Anodengas und CO₂.
• - Anwendbar auch auf die Herstellung anderer CO₂ Reduktionsprodukte (z.B. Ameisensäure)

Among other things, there are the following advantages over previous embodiments:

• - No CO ₂ release at the anode, only O ₂ or other anodic products
• - CO ₂ is released in a separate chamber and can be recycled.
• - No salt excretion
• - Faraday efficiency of CO generation increases with increasing current density
• - No or very little permeate into the gas space of the cathode compartment
• - When using the same acid in the anode compartment and in the salt bridge, a diaphragm is sufficient to separate anode gas and CO ₂ .
• - Also applicable to the production of other CO ₂ reduction products (eg formic acid)

In addition, a CO ₂ -free anode is not obtained by the construction of the anodic half-cell but by that of the cathodic half-cell. This result is completely unexpected and is based on the mechanism of anion-based charge transport compensated by fixed positive charges.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely 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.

Cited patent literature

• US 2017/0037522 A1 [0013, 0027, 0069]
• US 20160251755 A1 [0015, 0027, 0090]
• US 9481939 [0015, 0027, 0090]

Cited non-patent literature

• C. Vayenas, et al. (Eds.), Modern Aspects of Electrochemistry, Springer, New York, 2008, pp. 89-189 [0004]
• J. Shi, F. Shi, N. Song, J-X. Liu, X-K Yang, Y-J Jia, Z-W Xiao, P. Du Journal of Power Sources, 2014, 259, 50-53 [0014]
• Theory, Lou Coury, pp. 91-96 [0035]

Claims (16)

A method of electrolysis of CO ₂ , wherein an electrolytic cell comprising - a cathode compartment comprising a cathode; a first ion exchange membrane containing an anion exchanger and / or anion transporter adjacent to the cathode compartment, the cathode directly contacting the first ion exchange membrane; an anode compartment comprising an anode; a first separator membrane; and - a salt bridge space, wherein the salt bridge space between the first ion exchange membrane and the first separator membrane is arranged, is used, wherein CO _{2 is} reduced at the cathode, wherein the salt bridge space having a liquid and / or dissolved acid. A method of electrolysis of CO ₂ , wherein an electrolytic cell comprising - a cathode compartment comprising a cathode; a first ion exchange membrane containing an anion exchanger and / or anion transporter adjacent to the cathode compartment, the cathode directly contacting the first ion exchange membrane; and an anode compartment comprising an anode, wherein the anode compartment adjoins the first ion exchange membrane; is used, wherein CO _{2 is} reduced at the cathode, wherein the anode compartment comprises a liquid and / or dissolved acid. Method according to Claim 1 wherein the second ion exchange membrane is selected from an ion exchange membrane containing a cation exchanger, a bipolar membrane and a diaphragm. Method according to Claim 1 or 3 wherein the anode compartment comprises an anolyte comprising a liquid and / or dissolved acid, preferably wherein the anolyte and / or the acid in the salt bridge space does not comprise mobile cations except protons and / or deuterons, in particular no metal cations. Method according to Claim 2 wherein the anode is applied to the first ion exchange membrane. Method according to one of the preceding claims, wherein the electrolysis with a current density of more than 50 mAcm ^-2 , preferably more than 100 mAcm ^-2 , more preferably from 150 mAcm ^-2 or more, even more preferably 170 mAcm ^-2 or more, in particular of 250 mAcm ^-2 or more, eg 400 mAcm ^-2 or more, or 600 mAcm ^-2 or more. Method according to one of the preceding claims, wherein an acid of the liquid and / or dissolved acid in the salt bridge space a pK _s value of 6 or less, preferably 5 or less, more preferably 3 or less, even more preferably 1 or less, particularly preferably 0 or less, preferably wherein the liquid and / or dissolved acid is selected from dilute or undiluted H ₂ SO ₄ , a solution of H ₂ N-SO ₂ -OH, dilute or undiluted HClO ₄ , a solution of H ₃ PO ₄ , dilute or undiluted CF ₃ -COOH, dilute or undiluted CF ₃ -SO ₂ -OH, a solution of (CF ₃ -SO ₂ ) ₂ -NH, a solution of HF, dilute or undiluted HCOOH, dilute or undiluted CH ₃ - COOH, a solution of HCl, a solution of HBr, a solution of HI, and / or mixtures thereof. Electrolytic cell comprising a cathode compartment comprising a cathode; a first ion exchange membrane containing an anion exchanger and / or anion transporter adjacent to the cathode compartment, the cathode directly contacting the first ion exchange membrane; an anode compartment comprising an anode; and a diaphragm adjacent to the anode compartment; further comprising a salt bridge space, wherein the salt bridge space between the first ion exchange membrane and the diaphragm is arranged. Electrolysis cell after Claim 8 wherein the anode contacts the diaphragm, and / or wherein the anode and / or the cathode are contacted on the side facing away from the salt bridge space with a conductive structure. Electrolysis cell after Claim 8 or 9 in which the cathode and / or the anode are impregnated as a gas diffusion electrode, as a porous bound catalyst structure, as a particulate catalyst on a support, as a coating of a particulate catalyst on the first and / or second ion exchange membrane, as a porous conductive support into which a catalyst is impregnated, and / or is designed as a non-closed sheet. Electrolysis cell after Claim 10 wherein the cathode as a gas diffusion electrode, as a porous bound catalyst structure, as a particulate catalyst on a support, as a coating of a particulate catalyst on the first and / or second ion exchange membrane, as a porous conductive support in which a catalyst is impregnated, and / or as not in which the anode is a gas diffusion electrode, a porous bound catalyst structure, a particulate catalyst supported on a carrier, a coating of a particulate catalyst on the first and / or or a second ion exchange membrane, as a porous conductive support in which a catalyst is impregnated, and / or formed as a non-closed sheet containing (r / s) a cation exchange material. Electrolysis cell according to one of Claims 8 to 11 wherein the first ion exchange membrane and / or the diaphragm are hydrophilic. Electrolysis cell according to one of Claims 8 to 12 wherein the salt bridge space comprises a liquid and / or dissolved acid, preferably wherein an acid of the liquid and / or dissolved acid in the salt bridge space has a pK _s value of 6 or less, preferably 5 or less, more preferably 3 or less, even more preferred 1 or less, more preferably 0 or less, more preferably wherein the liquid and / or dissolved acid is selected from dilute or undiluted H ₂ SO ₄ , a solution of H ₂ N-SO ₂ -OH, dilute or undiluted HClO ₄ , a solution of H ₃ PO ₄ , dilute or undiluted CF ₃ -COOH, dilute or undiluted CF ₃ -SO ₂ -OH, a solution of (CF ₃ -SO ₂ ) ₂ -NH, a solution of HF, dilute or undiluted HCOOH, dilute or undiluted CH ₃ -COOH, a solution of HCl, a solution of HBr, a solution of HI, and / or mixtures thereof. Electrolysis plant, comprising an electrolytic cell according to one of Claims 8 to 13 , Electrolysis system after Claim 14 , further comprising a recirculation device, which is connected to a discharge of the salt bridge space and a supply of the cathode space, which is adapted to lead a reactant of the cathode reaction, which can be formed in the salt bridge space, back into the cathode space. Use of an electrolytic cell according to one of Claims 8 to 13 or an electrolysis system Claim 14 or 15 for the electrolysis of CO ₂ .

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