Electrolysis system and reduction method for utilising carbon dioxide

Abstract

The invention relates to an electrolysis system (36) for utilising carbon dioxide, comprising an electrolysis cell (10) having at least one anode (12), at least one cathode (14) and at least one aqueous electrolyte (38) which is effectively connected to the anode (12) and / or the cathode (14), wherein at least one inlet for carbon dioxide (CO2) is allocated to the cathode (14) and therefore CO2 can be brought into contact with the cathode (14) for reduction. In a starting state, the electrolyte (38) is at least substantially free of carbonate and hydrogen carbonate and contains at least one dissolved carboxylate. The invention also relates to a reduction method for utilising carbon dioxide by means of an electrolysis system (36).

Description

[0001] Description

[0002] Electrolysis system and reduction process for carbon dioxide utilization

[0003] The present invention relates to an electrolysis system and a reduction process for carbon dioxide utilization.

[0004] State of the art

[0005] Currently, around 80% of the world's energy needs are met by burning fossil fuels, whose combustion processes cause a global emission of around 34,000 million tons of carbon dioxide into the atmosphere every year. The majority of carbon dioxide is disposed of through this release into the atmosphere, which can amount to up to 50,000 tons per day in a lignite-fired power plant, for example. In addition to fossil energy generation, the production and conversion of industrial raw materials makes a significant contribution to the continuous increase in carbon dioxide concentrations in the atmosphere, accounting for around 9.2% of total emissions. Carbon dioxide is one of the so-called greenhouse gases, and its negative effects on the atmosphere and climate are still being discussed, despite its small proportion of around 0.04%.Since carbon dioxide is thermodynamically very low, it is difficult to reduce it to recyclable products, which has practically prevented the actual utilization of carbon dioxide so far.

[0006] In nature, CO2 is converted into carbohydrates through photosynthesis. This process, which is divided into many sub-steps in time and space at the molecular level, is very difficult to replicate on an industrial scale. The electrochemical reduction of CO2 is currently the more efficient method than pure photocatalysis. As with photosynthesis, this process converts CO2 into a product with higher energy value (CO, CH4, C2H4, etc.) with the aid of electrical energy, which can be obtained from renewable energy sources such as wind or solar power. The amount of energy required for this reduction ideally corresponds to the combustion energy of the fuel and should preferably come only from renewable sources or use electricity that is not currently available from the grid.

[0007] However, overproduction of renewable energy is not available continuously, but currently only during periods of strong solar radiation and strong winds. This situation will become even more pronounced in the near future with the further expansion of so-called renewable energy.

[0008] One alternative is the electrochemical reduction of carbon dioxide. Systematic investigations into the electrochemical reduction of carbon dioxide are still a relatively new field of development. Efforts to develop an electrochemical system capable of reducing an acceptable amount of carbon dioxide have only been underway in recent years. Laboratory-scale research has shown that metals are preferable as catalysts for the electrolysis of carbon dioxide.

[0009] Faraday efficiencies at different metal cathodes can be found in the publication "Electrochemical CO2 reduction on metal electrodes" (Y. Hori, published in: C. Vayenas, et al. (Eds.), Modern Aspects of Electrochemistry, Springer, New York, 2008, pp. 89-189). Table 1 from this publication, reproduced below, lists various Faraday efficiencies [%] of products formed during carbon dioxide reduction at various metal electrodes. The values ​​given apply to a 0.1 M potassium bicarbonate solution as electrolyte and current densities below 10 mA / cm 2 . Table 1

[0010] The reactions at the anode and cathode can be described with the following

[0011] Reaction equations are shown:

[0012] Cathode: 2 C02+ 4 e~ + 4 H+ 2 CO + 2 H20

[0013] Anode : 2 H2O 02+ 4 H+ + 4 e~

[0014] As with many catalytic processes, the chemical reactions involved in electrolysis systems do not proceed selectively, but rather result in a spectrum of main and by-products from various compounds formed. As can be seen from Table 1, for example, a copper cathode produces a variety of hydrocarbons as reaction products. Of particular commercial interest, for example, is the electrochemical production of methane and ethanol according to the following reaction equations:

[0015] Methane: C02+ 8 e~ + 4 H20 CH4+ 4 OH“ Ethanol: 2 C02+ 12 e~ + 9 H20 C2H5OH + 12 0H~

[0016] These are products with higher energy value than carbon dioxide.

[0017] The following reaction equations depict the reactions at the anode and cathode for the reduction at a copper cathode. Of particular interest is the formation of the highly expensive ethene. The reductions at other metals are analogous to this:

[0018] Cathode: 2 C02+ 12 e~ + 8 H20 C2H4+ 12 OH-

[0019] Anode: 6 H20 3 02+ 12 H+ + 12 e~

[0020] Sum equation: 2 C02+ 2 H20 C2H4+ 3 O2

[0021] In addition, however, a number of other by-products are produced, such as CO, H2etc.

[0022] However, all reactions do not proceed quantitatively, resulting in product streams consisting of a mixture of unreacted reactants and target or by-products. These product streams, which usually also contain unreacted reactants, are not industrially or commercially viable in this form and must be separated into their components. The separated reactants can be recycled back into the electrolysis process.

[0023] Depending on the requirements, chemical raw materials are usually required in very high purities (>99.9%). Here, it is important to find and combine suitable separation steps or processes so that, on the one hand, high purities and yields are achieved while, on the other hand, the energy required is as low as possible.

[0024] As already mentioned, CO2 electrolysis is carried out in carbonate- or bicarbonate-containing electrolytes in molar concentrations. Gaseous products (CO, C2H4, CH4, H2, etc.) or products that can be converted into the gas phase (ethanol, propanol) can in principle be separated relatively easily from such a salt-containing electrolyte. Depending on the electrode material, larger amounts of acids can also be formed as additional by-products, which cancel out the buffering effect of the carbonate / bicarbonate / CO2 buffer system. As the pH value decreases, however, the undesirable parasitic hydrogen evolution increases and the Faradaic efficiency of the electrolysis for producing the desired, easily separated target products (ethylene, ethanol, etc.) decreases.

[0025] The object of the present invention is to provide an electrolysis system and a reduction process for carbon dioxide utilization, which enable an improved production and recovery of valuable materials.

[0026] These objects underlying the present invention are achieved by an electrolysis system according to patent claim 1 and by a reduction process according to patent claim 11. Advantageous embodiments of the invention are the subject of the dependent claims, wherein advantageous embodiments of one aspect of the invention are to be regarded as advantageous embodiments of the other aspect of the invention.

[0027] Description of the invention

[0028] A first aspect of the invention relates to an electrolysis system for carbon dioxide utilization, comprising an electrolysis cell with at least one anode, with at least one cathode and with at least one aqueous electrolyte which is directly or indirectly operatively connected to the anode and / or the cathode, wherein at least one inlet for carbon dioxide (CO2) is assigned to the cathode, so that CO2 can be brought into contact with the cathode for reduction. Improved production and recovery of valuable materials is made possible according to the invention in that the electrolyte in a starting state is at least substantially free of carbonate and bicarbonate and contains at least one dissolved carboxylate. In other words, the invention provides that the electrolyte used in the electrolysis system in the starting state, that is to say after the preparation of the electrolyte orbefore it has taken part for the first time in the reduction process carried out in the electrolysis system as anolyte and / or catholyte, is at least substantially free of carbonate and bicarbonate. In the context of the present disclosure, the term “at least substantially free” is understood to mean a concentration as low as possible, of at most 0.1 mol / l, in particular of at most 0.01 mol / l or less. A concentration below the technical detection limit, i.e. complete freedom from carbonate and bicarbonate, is generally often not technically achievable or not reasonably achievable due to CO2 from the environment dissolving in the electrolyte, but is also not absolutely necessary. Instead of the carbonate / bicarbonate buffer customary in the prior art, the electrolyte according to the invention instead contains at least one dissolved carboxylate, i.e. a salt of an organic carboxylic acid.

[0029] The invention is based on the finding that during the electrolytic utilization of carbon dioxide using the electrolysis system according to the invention, which can also be referred to as an electrolyzer, carboxylates such as formate and acetate are formed. The following reactions, for example, take place at the cathode:

[0030] 2 CO2+ 5 H2O + 8 e- CH3COO- + 7 OH~ CO2+ H2O + 2 e- HCOO“ + OH~ 2 H2O + 2 e- H2+ 2 OH~ CO2+ H2O + 2 e- CO + 2 OH~

[0031] For neutral molecules such as ethylene, ethanol, or CO, a corresponding number of hydroxide ions is formed based on the required electrons. For singly negatively charged species such as formate and acetate, one less hydroxide ion is formed, as the charge is compensated by the resulting anion. For doubly negatively charged species, two less hydroxide ions are formed, and so on. The formed hydroxide ions react with excess CO2 to form carbonate or bicarbonate, which is released into the electrolyte:

[0032] C02+ OH- HCO3- C02+ 2 OH- CO3 2 - + H20

[0033] Formate, acetate and / or other carboxylates which form over time during a reduction process carried out with the aid of the electrolysis system according to the invention, accumulate in the electrolyte over time. These singly or multiply negatively charged ionic charge carriers then move, for example in a system with a membrane permeable to anions (AEM membrane), to the anode and there release CO2, where they are neutralized by protons formed at the anode to form formic acid, acetic acid and / or corresponding other carboxylic acids. This forms, in combination with the at least one carboxylate originally added, a corresponding buffer system which ensures high Faraday efficiencies for the formation of organic compounds and thus high yields of interesting reaction products. In particular when the carboxylate formed and the existing or...Since the carboxylate originally used is identical, any separation problems can be avoided particularly reliably. Furthermore, the carboxylic acids produce other uncharged valuable substances besides methane and ethylene, which, unlike salts, can be separated relatively easily from the remaining electrolyte.

[0034] As already mentioned, during operation of the electrolysis system, carbonate and / or bicarbonate are formed by reactions at the cathode and introduced into the electrolyte. Using acetate formation as an example, the following partial reactions occur: Desired cathode reaction:

[0035] 2 C02+ 5 H20 + 8 e- CH3COO- + 7 0H~

[0036] Side reaction ions: C02+ OH- HCO3- C02+ 2 OH“ CO3 2 - + H20

[0037] According to the invention, the electrolyte used should, as already mentioned, preferably contain no or at least as little HCO in the unused starting or initial state 3- / CO3 2- contained. Due to the stoichiometric reaction, the HCO 3 - / CO3 2- - Concentration in the electrolyte subsequently also during its use in the electrolysis small, since HCO 3 - / CO3 2" is decomposed back into CO2 immediately after formation at the interface by the protons formed at the anode. In summary, the electrolysis system according to the invention therefore offers numerous advantages. It avoids buffer breakdown by resulting carboxylic acids (e.g. acetic acid) because these can be distilled off as free, uncharged compounds or separated in some other way. It avoids neutralization of the resulting carboxylic acids by carbonate in the electrolyte (RCOOH + KHCO3RCOOK + CO2 + H2O). It offers a broad process window with at least 50 percent enrichment in carboxylic acids, for example acetic acid. The internal CO2 management in the electrolysis cell remains unaffected compared to conventional carbonate buffer systems.The achievable current density is increased because the local pH value in the vicinity of the cathode rises naturally due to the higher production rate of basic substances (hydroxide, bicarbonate, carbonate, etc.), thus allowing lower pH values ​​in the electrolyte. In general, "a" / "an" are to be read as indefinite articles in this disclosure, i.e., unless expressly stated otherwise, they always mean "at least one". Conversely, "a" / "an" can also be understood as "only one". Regardless of the grammatical gender of a particular term, this includes persons of male, female, or other gender identity.

[0038] In an advantageous embodiment, it is provided that the at least one carboxylate is selected from a group comprising salts of linear, branched, cyclic or heterocyclic, aliphatically saturated or aliphatically unsaturated or aromatic, substituted or unsubstituted organic monocarboxylic acids, dicarboxylic acids, tricarboxylic acids or polycarboxylic acids and any mixtures thereof. In other words, generally mixed or pure salts of linear, branched, cyclic, heterocyclic, saturated, unsaturated, aromatic substituted and / or unsubstituted carboxylic acids with one or more carboxyl (ate) group(s) are suitable. This includes, for example

[0039] - aliphatic, linear, saturated monocarboxylic acids

[0040] - aliphatic, branched, saturated monocarboxylic acids

[0041] - aliphatic, linear, unsaturated monocarboxylic acids

[0042] - aliphatic, branched, unsaturated monocarboxylic acids

[0043] - aliphatic, cyclic, saturated monocarboxylic acids

[0044] - aliphatic, cyclic, unsaturated monocarboxylic acids

[0045] - aliphatic, saturated dicarboxylic acids,

[0046] - aliphatic, unsaturated dicarboxylic acids,

[0047] - aliphatic, saturated tricarboxylic acids,

[0048] - aliphatic, unsaturated tricarboxylic acids,

[0049] - aromatic carboxylic acids,

[0050] - polyaromatic carboxylic acids and / or

[0051] - heterocyclic carboxylic acids .

[0052] Aliphatic, linear or branched, saturated monocarboxylic acids are particularly preferred. This allows the properties of the electrolyte to be optimally adapted to the respective intended use and the expected (by-)product spectrum. Alternatively or additionally, it can be provided that the at least one carboxylate is selected depending on a by-product spectrum of the cathode. This means that the carboxylate is selected such that it corresponds to a carboxylic acid that is formed during CO2 electrolysis. This creates an optimal buffer system in situ during the electrolysis.

[0053] In a further advantageous embodiment of the invention, the carboxylate is selected from salts of formic acid, acetic acid, propanoic acid, malonic acid, n-butanoic acid, isobutanoic acid, n-pentanoic acid, isopentanoic acid, sec-pentanoic acid, tert-pentanoic acid, oxalic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, maleic acid, fumaric acid, malic acid, citric acid, isocitric acid, propane-1,2,3-tricarboxylic acid, trimesic acid, ethenebis(oxyethenenitrilo)tetraacetic acid, benzoic acid, terephthalic acid, salicylic acid, or anthranilic acid, as well as any desired mixtures thereof. Particular preference is usually given to salts of formic acid and / or salts of acetic acid, since formic acid and / or acetic acid are generally formed in appreciable amounts during electrolysis.

[0054] Further advantages arise from the fact that the electrolyte in the initial state contains starter cations selected from alkali cations, in particular K+ and / or Cs+, alkaline earth cations and / or ammonium ions and / or ammonium compounds, in particular alkylammonium compounds and / or cyclic ammonium compounds and / or mono-, di-, tri- and / or tetraethanolammonium compounds. In other words, it is intended that the electrolyte contains starter cations after preparation or before it has participated for the first time in the reduction process carried out in the electrolysis system as anolyte and / or catholyte. The starter cations can be alkali and / or alkaline earth ions, with K+ and / or Cs+ being preferred since they have a particularly positive effect on the Faraday efficiency of hydrocarbon formation.In the simplest embodiment, the starting cations can be added to the electrolyte as a counterion of the carboxylate used, for example in the form of CH3COOK, and / or as a separate salt with a correspondingly suitable counteranion. In a further advantageous embodiment of the invention, it is provided that in the initial state, a concentration of the starting cations in the electrolyte is between 50 mol% and 100 mol% based on a concentration of the at least one carboxylate in the electrolyte. For the purposes of the present disclosure, the concentration of the starting cations can be reduced during the electrolysis process to a maximum of 0.01% with respect to the concentration of the carboxylate anion used. Percentages in the context of the present disclosure are generally to be regarded as molar percentages, unless stated otherwise.However, particularly preferred is control and / or regulation of the concentration ratio to between 40 mol% and 60 mol%, since in this range the Faraday efficiency of hydrocarbon formation does not decrease or decreases only slightly. As soon as a predetermined concentration ratio limit, which can be determined continuously or at specific times, is violated, the electrolysis system can interrupt the electrolysis and / or generate a visual, acoustic, and / or haptic indication of the violation of the concentration ratio limit.

[0055] Further advantages arise from the electrolyte being at least substantially free of Ni, Fe, Au and / or Pt ions in the starting state. As has been recognized, these cations usually have a negative effect on the cathode and lead to reaction disturbances and reduced yield. Here, too, it should be noted that the expression “at least substantially free” does not necessarily have to mean 0.0 mol%, but that an initial concentration of these undesirable ions of at most 0.1 mol / l and preferably of at most 0.01 mol / l or less can be present in the electrolyte, which should be kept as low as possible. However, active addition during mixing of the electrolyte, apart from unavoidable impurities and the like, should generally be avoided. In a further advantageous embodiment of the invention, the cathode is a gas diffusion electrode, along which the CO2 can pass at least in some regions.A gas diffusion electrode (GDE) typically consists of three main components: a porous support material, a catalytic material deposited on the surface of the porous support material that catalyzes the desired electrochemical reactions, and a current collector, which usually consists of an electrically conductive layer that establishes electrical contact with the electrochemical cell and enables charge transfer. The porous structure of the GDE distributes the CO2 evenly over the entire surface of the electrode. This enables improved reaction kinetics and reduces possible local overreactions. The porous structure of the GDE also enables efficient use of the catalytic material, leading to improved electrochemical reactions. GDEs also allow the passage of gases and liquids with low pressure drop.This is particularly important for G02 reduction, where smooth gas flow is required. The porous structure of the GDE also enables effective heat dissipation, thus avoiding overheating. Furthermore, GDEs allow high flexibility in the choice of catalyst used, depending on the specific requirements of the application. This enables optimal adaptation to the desired reactions and improves performance. Finally, GDEs can be relatively easily manufactured in various sizes and shapes to meet the requirements of the specific electrolysis system. They can therefore be scaled up for both small laboratory-scale systems and large industrial plants.

[0056] Alternatively or additionally, the cathode is selected from a group comprising tin-, lead-, silver-, or copper-based cathodes. This allows particularly high yields of carboxylic acids (Sn, Pb) or carboxylates (Ag, Cu) to be achieved.

[0057] A particularly long operating life of the electrolyte is achieved in a further embodiment by ensuring that the pH of the electrolyte in the starting state is in an alkaline range, in particular between 10 and 8. The exact pH depends in particular on the carboxylate used and its concentration. This enables a particularly long reaction with high Faraday efficiencies via a buffer effect that is present from the outset or at least sets in in situ, since the pH of the electrolyte can drop very slowly from the alkaline starting range to an acidic lower pH limit, at which hydrogen evolution then dominates. The lower pH limit is typically between 4 and 5. As soon as this is reached, appropriate countermeasures can be taken to increase the pH again. An electrolysis cell with a cation-permeable membrane (CEM membrane) is preferably used.

[0058] Alternatively or additionally, it can be provided that the electrolyte in the starting state comprises a buffer system made up of at least one carboxylate and at least one carboxylic acid, wherein a molar ratio between the carboxylic acid and the carboxylate is preferably between 4:1 and 1:4, in particular approximately 1:1. The carboxylic acid used can in principle be the corresponding acid of the carboxylate used or another carboxylic acid or a mixture of corresponding and non-corresponding carboxylic acid. In general, the carboxylic acid can be selected independently of the carboxylate from the same compounds mentioned above as the carboxylate. In general, the lower the molar proportion of carboxylic acid in the starting state, the longer the electrolyte can be used. The molar proportion of carboxylic acid in the starting state can therefore also be zero.In a further preferred advantageous embodiment of the invention it is provided that the anode and the cathode are separated from one another by an ion-conducting membrane, in particular a cation-conducting membrane, and that the electrolyte is preferably arranged in a gap between the cathode and the membrane, in particular when the cathode is designed as a gas diffusion electrode. In other words it is provided that the electrolysis cell is optionally designed in a 2-gap architecture (two-gap architecture) or a 1-gap architecture (one-gap architecture). The electrolyte according to the invention can be arranged in at least one gap and, if appropriate, flow through the gap in order to continuously remove reaction products and to be able to operate the electrolysis system for a longer period.The electrolysis cell is preferably designed in a single-gap architecture, in which the gap is arranged between the cathode and the membrane, while the anode is directly connected to the side of the membrane facing away from the cathode, as this enables inherent CO2 separation in the gap. In general, some embodiments may provide for the use of a so-called “zero gap” electrolysis cell, in which the anode and the cathode are arranged without a gap on opposite sides of the membrane, while the electrolyte is arranged on the side of the anode opposite the membrane. Other, optionally combined cell arrangements, e.g. with a combination of anion- and cation-conducting or bipolar membranes, can also be provided, but are less preferred due to their complexity or, in some cases, their lower energy efficiency.

[0059] In a further advantageous embodiment of the invention, the electrolysis system comprises a separation device which can be fluidically coupled to the electrolysis cell and by means of which at least one carboxylic acid contained in the electrolyte can be derivatized and / or at least partially separated from the electrolyte, in particular by distillation. In other words, the electrolysis system has a separation device for partially or completely separating the carboxylic acid(s) present in the electrolyte, wherein the separation device can be or is coupled fluidically to the electrolysis cell, so that the electrolyte can be passed continuously or batchwise from the electrolysis cell into the separation device. The separation can be carried out, for example, by distillation.Likewise, particularly in the case of low-volatility compounds, it may be provided that the carboxylic acid(s) are first derivatized in the separation device, for example esterified with ethanol, in order to lower their boiling point and facilitate separation.

[0060] A second aspect of the invention relates to a reduction process for carbon dioxide utilization by means of an electrolysis system which comprises an electrolysis cell with at least one anode, with at least one cathode and with at least one aqueous electrolyte which is in active connection with the anode and / or the cathode, in which carbon dioxide (CO2) is brought into contact with the cathode for reduction. According to the invention, it is provided that in a starting state an electrolyte is used which is at least substantially free of carbonate and bicarbonate and contains at least one dissolved carboxylate. In other words, it is provided according to the invention that the electrolyte used in the reduction process is at least substantially free of carbonate and bicarbonate in the starting state, that is to say after the preparation of the electrolyte or before it takes part in the reduction process for the first time as anolyte and / or catholyte.The term "at least substantially free" in the context of the present disclosure is understood to mean a concentration as low as possible, of at most 0.1 mol / l, in particular of at most 0.01 mol / l or less. A concentration below the technical detection limit, i.e. complete freedom from carbonate and bicarbonate, is generally often technically unattainable or not reasonably achievable due to CO2 from the environment dissolving in the electrolyte. In addition, carbonate / bicarbonate is formed by the reduction process and introduced into the electrolyte, so that absolute freedom from carbonate / bicarbonate in the starting state is not absolutely necessary. Instead of this carbonate / bicarbonate buffer which has been customary in the prior art to date, the electrolyte instead contains at least one dissolved carboxylate, i.e. a salt of an organic carboxylic acid.Carboxylates, which form over time during a reduction process carried out with the aid of the electrolysis system according to the invention, accumulate in the electrolyte over time. These singly or multiply negatively charged ionic charge carriers then move to the anode and there release CO2, where they are neutralized by protons formed at the anode to form the corresponding carboxylic acids. In addition to methane and ethene, the carboxylic acids also produce other uncharged valuable substances which, in contrast to salts, can be separated relatively easily from the remaining electrolyte. This enables the invention to improve the production and extraction of valuable substances during the reduction process.Further features and their advantages can be found in the descriptions of the first aspect of the invention, wherein advantageous embodiments of the first aspect of the invention are to be regarded as advantageous embodiments of the second aspect of the invention, and vice versa. For applications or application situations that may arise during the method and which are not explicitly described here, it can be provided that, according to the method, an error message and / or a request to enter user feedback is output and / or a standard setting and / or a predetermined initial state is set.

[0061] In an advantageous embodiment of the invention, it has been shown to be advantageous that at least one carboxylic acid, in particular formic acid and / or acetic acid, is formed in the electrolyte and forms a buffer with the carboxylate. In other words, in combination with the at least one originally added carboxylate, the formation of carboxylic acids during electrolysis creates a corresponding buffer system, which ensures high Faraday efficiencies for the formation of organic compounds and thus high yields of interesting reaction products.

[0062] Further advantages arise from the fact that the at least one carboxylic acid is at least partially derivatized and / or at least partially removed from the electrolyte, either continuously or batchwise. This allows for simple separation and recovery of valuable materials. Derivatization of the carboxylic acid can be carried out prior to separation in order to lower the boiling point and facilitate separation by distillation. Furthermore, the carboxylic acid is removed from the reaction equilibrium with the carboxylate by derivatization, and is then reformed accordingly.

[0063] In a further advantageous embodiment of the invention, it is provided that an electrolyte is used which has starting cations in the starting state, wherein a concentration of the starting cations in the electrolyte during the reduction process with respect to a concentration of the at least one carboxylate decreases from a starting value between 50 mol% and 100 mol% to a maximum of 0.01 mol% and in particular is between 40 mol% and 60 mol%. The associated advantages were discussed above in connection with the first aspect of the invention.

[0064] Further advantages arise from the fact that the reduction process can be carried out at an average current density of at least 300 mA / cm 2is operated and / or in which an initial concentration of the at least one carboxylate in the electrolyte is at least 0.25 mol / l, in particular at least 0.5 mol / l and preferably at least 1 mol / l and / or wherein a pH value of the electrolyte increases during the reduction process from at least 3 to at most 9, in particular to at most 6 or, depending on the membrane used, vice versa. All of these measures, individually or in any combination, ensure high Faraday ef fi ciencies and correspondingly high product yields.

[0065] Further features of the invention emerge from the claims, the figures and the description of the figures. The features and combinations of features mentioned above in the description, as well as the features and combinations of features mentioned below in the description of the figures and / or shown alone in the figures can be used not only in the respective combination specified, but also in other combinations without departing from the scope of the invention. Embodiments are therefore to be regarded as encompassed and disclosed by the invention which are not explicitly shown and explained in the figures, but which arise and can be produced by separate combinations of features from the explained embodiments. Embodiments and combinations of features are also to be regarded as disclosed which therefore do not have all the features of an originally formulated independent claim.Furthermore, embodiments and combinations of features are to be considered disclosed, in particular by the embodiments presented above, which go beyond or deviate from the combinations of features set out in the claims. This shows:

[0066] FIG 1 shows a schematic representation of the product and reactant streams of an electrolysis process;

[0067] FIG 2 shows a schematic representation of an electrolysis system according to the invention;

[0068] FIG 3 is a schematic sectional view of an electrolysis cell of the electrolysis system shown in Fig . 2 ;

[0069] FIG 4 shows a schematic sectional view of an alternative electrolysis cell with a two-gap architecture; FIG 5 shows a schematic sectional view of an alternative electrolysis cell with a one-gap architecture;

[0070] FIG 6 is a schematic sectional view of an alternative electrolysis cell with a zero-gap architecture;

[0071] FIG 7 a diagram showing the molar concentration ratios of conj ugated acid-base pairs of carbonic acid as a function of the pH value;

[0072] FIG 8 is a diagram showing the molar concentration ratios of conj ugated acid-base pairs of acetic acid as a function of pH;

[0073] FIG 9 a diagram of the pH-value-dependent molar composition of the acetic acid-acetate buffer system;

[0074] FIG 10 is a diagram of the time course of the Faraday efficiency of the formation of several compounds during a first embodiment of a reduction process according to the invention with an electrolyte which contains acetic acid and potassium acetate;

[0075] FIG 11 is a diagram of the time course of the pH value of the electrolyte during the first embodiment of the reduction process;

[0076] FIG 12 is a diagram of the temporal potential profile of the electrolyte during the first embodiment of the reduction process according to the invention;

[0077] FIG 13 is a diagram of the time course of the Faraday efficiency of the formation of several compounds during a second embodiment of the reduction process according to the invention with an electrolyte which contains formic acid and potassium formate;

[0078] FIG 14 is a diagram of the time course of the pH value of the electrolyte during the second embodiment of the reduction process according to the invention;

[0079] FIG 15 is a diagram of the temporal potential profile of the electrolyte during the second embodiment of the reduction process according to the invention;

[0080] FIG 16 is a diagram of the time course of the ratio of the formation of liquid and gaseous products as a function of an electrolyte composition;

[0081] FIG 17 a diagram of the time course of the Faraday effect of the formation of CO with different electrolytes;

[0082] FIG 18 is a diagram of the time course of the Faraday efficiency of the formation of ethene with different electrolytes;

[0083] FIG 19 is a diagram of the time course of the Faraday efficiency of the formation of H2 with different electrolytes; and

[0084] FIG 20 a diagram of the time course of the Faradayef fi ciency of the formation of methane with different electrolytes .

[0085] FIG 1 shows a schematic representation of the possible product and reactant flows of a general electrolysis process. In an electrolysis cell 10 there is an anode 12 and a cathode 14, which are separated by a membrane

[0086] 16 are separated from each other. The membrane 16 is in this case permeable to protons, which migrate from an anolyte 18 into a catholyte 20 according to arrow I for charge equalization. Alternatively, an anion-permeable membrane can also be used. Educts are transported to the anode via a feed 22, while Educts are transported to the cathode via a feed 24. Gaseous and liquid products are formed on the anode and cathode sides. Gaseous anode products 28 are transported from the anolyte 18 into a gas separator 26 and at least partially separated from the remaining anolyte 18. The remaining anolyte 18, which may contain a residue of dissolved gaseous products, can be transported back to the anode 12 and, if necessary, processed or fed into a cyclic process, as long as it meets the required quality criteria. The same applies to the cathode side.Liquid anode products 30 can also be separated from the anolyte 18, after which the remaining anolyte 18 can optionally be reused, processed, or discarded. Analogous to the anode side, gaseous cathode products 32 and liquid and dissolved cathode products 34 are produced on the cathode side, which can in principle be separated accordingly.

[0087] FIG. 2 shows a schematic representation of an electrolysis system 36 according to the invention for carbon dioxide utilization according to an exemplary embodiment. The electrolysis system 36 comprises an electrolysis cell 10, which is shown in FIG. 3 in an enlarged schematic sectional view. The electrolysis cell 10 has an anode 12 and a cathode 14 designed as a gas diffusion electrode (GDE), between which an anion-permeable membrane 16 (anion exchange membrane, AEM) is arranged. Preferably, a cation-permeable membrane 16 (CEM) can be provided instead. Between the electrodes 12, 14 and the membrane 16 there is in each case an aqueous electrolyte 38, which is in operative connection on the one hand between the membrane 16 and the anode 12 and on the other hand between the membrane 16 and the cathode 14.The cathode 14 has a porous carrier material 14a with a catalytic material 14b which is applied to the surface of the porous carrier material and catalyzes the desired electrochemical reactions. A current collector of the cathode 14 is not shown for reasons of clarity. The catalytic material 14b is arranged on the side of the cathode 14 facing the electrolyte 38. The cathode 14 is also assigned an inlet 40 for carbon dioxide (CO2) so that CO2 can be brought into contact with the catalytically active layer of the cathode 14 for reduction. The CO2 flows along the cathode 14 for reduction. Unreacted CO2 is then passed out of the electrolysis cell 10 through an outlet 42 together with the resulting gaseous cathode products 32. Part of the unreacted CO2 passes through the GDE cathodes 14 into the catholyte 38 and leaves the cell together with it.

[0088] The CO2 comes from a storage tank 44 and is fed to the access or inlet 40 via a mass flow controller (MFC) 46 and optionally via a humidifier 48. The outlet 42, via which the gaseous cathode products 32 are discharged, is fluidically connected to an electrolyte tank 50, in which the electrolyte 38 is held and circulated via a corresponding supply 52 and discharge 54. In the exemplary embodiment shown, the electrolyte tank 50 has a pH sensor 56 for determining the pH value of the electrolyte 38. Furthermore, the electrolyte tank 50 is fluidically coupled to a gas chromatograph 58, by means of which an analysis of the gaseous cathode products 32 is carried out. The result of the analysis can be displayed or displayed in a manner known per se, for example using a monitor 60. may be further evaluated using a computing device not shown.

[0089] The electrolyte 38 acting as the anolyte is stored in a second electrolyte container 51 and, analogous to the cathode side, is passed in a circuit through the anode chamber via an inlet 62 and an outlet 64. Gaseous anode products 28 can be discharged from the electrolyte container 51. It should be emphasized that the electrolyte 38 used on the anode and cathode sides can basically have the same or different compositions in their starting or initial state. During operation, its composition can also vary depending on the processing method. In other words, the same or different electrolytes 38 can initially be used as the anolyte and catholyte. During the reduction process, the composition of the electrolytes 38 then inevitably differs due to the different reaction products on the anode and cathode sides.The electrolyte 38 used is, in its starting state, at least substantially free of carbonate and bicarbonate and contains at least one dissolved carboxylate. In other words, instead of a conventional electrolyte containing carbonate and / or bicarbonate, an electrolyte 38 which is free of carbonate and bicarbonate - as far as this is technically possible and sensible - is used, which contains at least one carboxylate, i.e. a salt of an organic carboxylic acid. The carboxylate can be, for example, an alkali salt of formic acid and / or acetic acid. As a result, the electrolyte 38 forms a buffer system during the reduction process, since, among other things, organic carboxylic acids are formed as by-products during the CO2 electrolysis and are thereby introduced into the electrolyte 38, creating a buffer system comprising carboxylate and the carboxylic acid formed.Alternatively, it can be provided that, in addition to the at least one carboxylate, one or more carboxylic acids are also added to the electrolyte 38 in the initial state. Therefore, the previously mentioned buffers of formic acid and acetic acid are particularly preferred, but not restrictive, because they also belong to the product spectrum of CO2 electrolysis.

[0090] Generally, mixed and pure buffers made from linear and branched, substituted and unsubstituted carboxylic acids of alkanes and their salts are suitable. Aromatic or polyaromatic carboxylic acids (benzoic acid, terephthalic acid, salicylic acid, anthranilic acid, etc.) can also be used individually or in any combination. The same applies to di-, tri-, and polycarboxylic acids, which, due to the large number of dissociation constants K S1 . . N form correspondingly more complex buffer systems.

[0091] Preferred cations are the alkali cations Li+, Na+, K+, Rb+, and Cs+, or substituted or unsubstituted ammonium ions. K+ and Cs+ are particularly preferred. Cations that negatively affect the cathode, e.g., Ni, Fe, Au, Pt, etc., should not be used if possible, or should not be contained in the electrolyte. All other cations are possible in principle.

[0092] It should be noted that during operation of the electrolysis system 36, carbonate and / or bicarbonate are introduced into the electrolyte 38 by the reactions at the cathode 14. For this reason, too, a complete absence of carbonate and bicarbonate in the electrolyte 38 in the starting state is not technically mandatory. Due to stoichiometric degradation reactions, the HCO3~ / CO3 2“ Concentration during electrolysis is small, because these compounds are decomposed back to CO2 immediately after their formation at the interface of the cathode 14 by the protons formed at the anode 12. However, the starting concentration of HCO3~ / CO3 2 " should nevertheless be as low as possible. The electrolyte 38 has independent inventive significance and can also be used in known electrolysis systems.

[0093] The CO2 electrolysis cell 10 can generally have various architectures. These are not restrictive, but in turn allow for various sub-variants. FIG. 4 shows a schematic sectional view of an alternative electrolysis cell 10 with a so-called "two-gap architecture", i.e. the anode 12 and the cathode 14 are separated by a cation-permeable membrane (CEM) 16, wherein the electrolyte 38 is arranged in respective "gaps" between the electrodes 12, 14 and the membrane 16 or flows through these gaps ("flow cell" architecture). The CO2, optionally after moistening, flows past the cathode 14 on a side of the cathode 14 facing away from the membrane 16 ("flow-by operation") and is converted into gaseous cathode products 32. On the side of the cathode 14 facing the membrane 16, the liquid cathode products 34 are formed, which pass into the electrolyte 38.Accordingly, O2 and H+ are formed on the anode side and released into the electrolyte 38. The anode 12 can, for example, be an IrO2 / Ti electrode. The cathode 14 is also a gas diffusion electrode (GDE). In general, the anode 12 can also be a gas diffusion electrode (GDE). The "two-gap" cell design is often less attractive for commercial operation due to the localized, equal release of O2 and CO2, as an additional separation problem arises. Furthermore, the operating voltages of the cells are generally much higher than in the zero-gap configuration.

[0094] FIG 5 shows a schematic sectional view of an alternative electrolysis cell 10 with a so-called "one-gap architecture". In contrast to the "two-gap architecture", there is only one gap between the cathode 14 and the cation-permeable membrane 16, in which the electrolyte 38 is arranged as catholyte or through which the electrolyte 38 is pumped. The "one-gap architecture" overcomes a problem of the "two-gap architecture" because the CO2 is released in the gap and O2 on the anode side. The resulting O2 therefore contains only a few percent of CO2. The CO2 released in the gap can in turn be recycled. The "one-gap" design is therefore preferred for commercial operation. An additional electrolyte circuit can optionally be connected to the back of the anode 12 (not shown). This can consist of water or acid.Salts may require additional electrolyte management to control the cation equilibrium in the electrolyte 38. In this configuration (not shown), the anode 12 is then preferably a GDE in order to be able to evacuate the resulting O2 from the system.

[0095] FIG 6 shows a schematic sectional view of an alternative electrolysis cell with a so-called "zero-gap architecture". The "zero-gap architecture" contains an anion-permeable membrane 16 (AEM) which is directly adjacent to the anode 12 and the cathode 14. The electrolyte 38, which acts as the anolyte, is arranged on a side of the anode 12 facing away from the membrane 16. In addition to the hydroxide ion shown, hydrogen carbonate, carbonate, formate and / or acetate are also ionic charge carriers which move to the anode 12. These carboxylic acids in turn release CO2 there and are neutralized to formic acid or acetic acid. Alternatively, formate, formic acid, acetate or acetic acid can be oxidized back to CO2, although this can have a negative impact on the overall efficiency.

[0096] Within the scope of the invention, other electrolysis cell architectures can generally also be provided, e.g. with a combination of anion- and cation-conducting or bipolar membranes 16.

[0097] For clarification, FIG. 7 shows a well-known diagram showing the molar concentration ratios of conjugated acid-base pairs of carbonic acid as a function of the pH of a solution of CO2. In a medium pH range, dissolved carbon dioxide CO2 (carbonic acid) and its salts exist side by side. While in the strongly basic pH range (pH>11), carbon dioxide CO2 exists predominantly as carbonate CO2- and in the medium pH range (about 6.5 <pH<10 , 5 ) überwiegend als Hydrogencarbonat HCOg- vorliegt , kommt es bei niedrigen pH- Werten (pH< 6 ) im sauren Milieu zur Kohlensäurebildung und in Folge zur Austreibung von C02. FIG 8 zeigt ein an sich bekanntes Diagramm mit den molaren Konzentrationsverhältnissen konjugierter Säure-Base-Paare der Essigsäure in Abhängigkeit des vorliegenden pH-Werts. FIG 9 zeigt zur weiteren Verdeutlichung ein Diagramm der pH-Wertabhängigen Stoffmengenzusammensetzung des Essigsäure-Acetat- Puffersystems.It can be seen that the buffering effect of this buffer system is due to the pK. s -value of 4.75 occurs predominantly in a pH range between approximately 4.75 ± 1.

[0098] FIG 10 shows a diagram of the time course of the Faraday efficiency FE [%] of the formation of several compounds during a first embodiment of a reduction process according to the invention with an aqueous electrolyte 38, which in the starting or initial state contained 0.5 M potassium acetate + 0.5 M acetic acid (molar ratio carboxylate : carboxylic acid = 1 : 1), using a "two-gap" electrolysis cell 10. As cathode 14, a Cu gas diffusion electrode was used, which was charged with 100 mg Cu4O3(CPC 9 / 3240) + 1 wt.% sustainion (2 mg / cm 2Catalyst loading on the GDE). An anion exchange membrane (AEM, Fumasep FAB-PK-130) was used as membrane 16. The CO2 flow rate was 135 sccm (standard cubic centimeters per minute), and the current density was 300 mA / cm 2 The products determined by gas chromatography were C2H4 (curve 70), H2 (curve 72), CO (curve 74), and CH4 (curve 76). During the first 20 minutes of electrolysis, GC measurements were taken every 2 minutes, and then every 20 minutes. It can be seen that the Faraday efficiency (FE) increases for all products except CH4 during the first 20 minutes and then remains essentially constant over a longer period.

[0099] FIG 11 shows a diagram of the time course of the pH of the electrolyte 38 during the first embodiment of the reduction process. Here, too, it is clear that the pH increases continuously in the first 20 minutes due to the formation of carboxylic acids, until it remains essentially constant after about 30 minutes. The pH of the electrolyte increases from a starting value of about 4.75 (value of the acetate buffer) to about 9 (pure acetate or beginning carbonate formation). At the beginning of the reaction, the main charge carrier through the membrane 16 is acetate, while the GDE 14 gradually neutralizes the acetic acid with hydroxide or bicarbonate / carbonate. This means that the entire range of the inventive

[0100] Electrolyte composition can be mapped in the first approximately 20 minutes. It is shown that electrolyte 38 can be enriched to a pH of approximately 5.5 without negatively affecting the Faraday efficiencies (FE).

[0101] The mixing ratio according to the Henderson-Hasselbalch equation below can therefore be log (ratio) = 5.5 - 4.75 = 0.75. This means that the ratio of acetic acid to acetate can be 0.18 or acetate to acetic acid 5.6. Obtaining 18% acetic acid (uncharged reaction product) based on acetate is technically feasible.

[0102] (Henderson-Has se Iba Ich- Equation)

[0103] FIG. 12 shows a diagram of the temporal potential profile of the electrolyte 38 during the first exemplary embodiment of the reduction process according to the invention. The negative electrical potential value cp [V] increases during the first approximately 20 minutes of electrolysis and also remains essentially constant for the remainder of the electrolysis. Higher negative potentials require greater energy expenditure.

[0104] FIG. 13 shows a diagram of the time course of the Faraday efficiency FE [%] of the formation of several compounds during a second exemplary embodiment of the reduction process according to the invention using an electrolyte 38 which, in the initial state, contained 0.5 M formate and 0.5 M formic acid (molar ratio of carboxylate to carboxylic acid = 1:1). The other reaction and measurement conditions, as well as the electrode and membrane materials used, corresponded to those of the first exemplary embodiment explained above. It can be seen that consistently high Faraday efficiencies can be achieved in the production of various valuable products even with a formic acid buffer system.

[0105] FIG. 14 shows a diagram of the time course of the pH of the electrolyte 38 during the second exemplary embodiment of the reduction process according to the invention. Since the acid constant (pKa value) of formic acid is 3.77, the pH value in the second exemplary embodiment starts at a lower value than in the first exemplary embodiment. Otherwise, the pH value course is similar to that of the first exemplary embodiment.

[0106] FIG. 15 shows a diagram of the temporal potential profile of the electrolyte 38 during the second exemplary embodiment of the reduction method according to the invention. Due to a technical fault, no usable data for the negative electrical potential cp [V] could be determined in the first 20 minutes. However, it can be seen that, at least after the technical fault has been rectified, the temporal potential profile here also corresponds in principle to that of the previous exemplary embodiment. It can therefore be assumed that the potential profile for the first 20 minutes is also similar to the potential profile in FIG. 12.

[0107] FIG. 16 shows a diagram of the time course of the ratio G / L [%] of the formation of liquid (L) and gaseous (G) products as a function of the electrolyte composition and illustrates the influence of the KOAc concentration in the electrolyte 38 on this ratio. The individual experiments were carried out using the electrolysis system 36 shown in FIG. 2 using a "two-gap" electrolysis cell 10. The other reaction and measurement conditions as well as the electrode and membrane materials used corresponded to those of the first embodiment of the reduction process explained above. The measured curves 80-88 relate to the percentage formation of gaseous products over time using the following aqueous electrolyte compositions: Measurement curve 80 Electrolyte 0.75 M KHCO3 + 0.25 M KOAc

[0108] Curve 82 Electrolyte 0.5 M KHCO3+0.5 M KOAc

[0109] Curve 84 Electrolyte 1 M KHCO3 Curve 86 Electrolyte 0.25 M KHCO3+0.75 M KOAc Curve 88 Electrolyte 1 M KOAc

[0110] The measurement curves 90-98 concern the percentage temporal formation of liquid products using these aqueous electrolyte compositions:

[0111] Measurement curve 90 Electrolyte 1 M KOAc

[0112] Curve 92 Electrolyte 0.25 M KHCO3+0.75 M KOAc

[0113] Measurement curve 94 Electrolyte 1 M KHCO3

[0114] Measurement curve 96 Electrolyte 0.5 M KHCO3+0.5 M KOAc

[0115] Measurement curve 98 Electrolyte 0.75 M KHCO3+0.25 M KOAc

[0116] It can be seen that the ratio of gaseous to liquid products formation can be varied by the initial KOAc content of the electrolyte used.

[0117] FIG 17 shows a diagram of the time course of the Faraday efficiency FE [%] of the formation of CO with different electrolytes. Where: Measurement curve 100 Electrolyte 1 M KHCO3

[0118] Measurement curve 102 Electrolyte 0.75 M KHCO3+0.25 M KOAc

[0119] Measurement curve 104 Electrolyte 0.5 M KHCO3+0.5 M KOAc

[0120] Measurement curve 106 Electrolyte 0.25 M KHCO3+0.75 M KOAc Measurement curve 108 + 1 M KOAc

[0121] FIG 18 shows a diagram of the time course of the Faraday efficiency FE [%] of the formation of ethene with the different electrolytes. Where: Measurement curve 110 Electrolyte 1 M KHCO3 Measurement curve 112 Electrolyte 0.75 M KHCO3 + 0.25 M KOAc Measurement curve 114 Electrolyte 0.5 M KHCO3 + 0.5 M KOAc Measurement curve 116 Electrolyte 0.25 M KHCO3 + 0.75 M KOAc Measurement curve 118 Electrolyte 1 M KOAc

[0122] FIG 19 shows a diagram of the time course of the

[0123] Faradayef f iciency of the formation of H2with the different

[0124] Electrolytes. These include:

[0125] Measurement curve 120 Electrolyte 1 M KOAc

[0126] Curve 122 Electrolyte 0.75 M KHCO3+0.25 M KOAc

[0127] Measurement curve 124 Electrolyte 0.5 M KHCO3+0.5 M KOAc

[0128] Measurement curve 126 Electrolyte 0.25 M KHCO3+0.75 M KOAc

[0129] Measurement curve 128 Electrolyte 1 M KOAc

[0130] FIG 20 shows a diagram of the time course of the Faraday efficiency FE [%] of methane formation with the different electrolytes. Where: Measurement curve 130 Electrolyte 1 M KHCO3 Measurement curve 132 Electrolyte 0.75 M KHCO3 + 0.25 M KOAc Measurement curve 134 Electrolyte 0.5 M KHCO3 + 0.5 M KOAc Measurement curve 136 Electrolyte 0.25 M KHCO3 + 0.75 M KOAc Measurement curve 138 Electrolyte 1 M KOAc

[0131] It can be seen that using electrolytes 38 that are at least initially free of carbonate / bicarbonate and instead contain at least one carboxylate to form a buffer system during electrolysis, high Faradaic efficiencies can be achieved in the production of various valuable materials within an electrolytic CO2 reduction process. The experiments also show that bicarbonate and carbonate are interchangeable with the carboxylates without a significant decrease in the Faradaic efficiency of the CO2 reduction products.

[0132] The parameter values ​​specified in the documents for defining process and measurement conditions for characterizing specific properties of the subject matter of the invention are to be considered as being included within the scope of the invention, even in the event of deviations - for example due to measurement errors, system errors, weighing errors, DIN tolerances and the like.

Claims

Patent claims 1. Electrolysis system (36) for carbon dioxide utilization, comprising an electrolysis cell (10) with at least one anode (12), with at least one cathode (14) and with at least one aqueous electrolyte (38) which is in operative connection with the anode (12) and / or the cathode (14), wherein the cathode (14) is assigned at least one inlet for carbon dioxide (CO2) so that CO2 can be brought into contact with the cathode (14) for reduction, characterized in that the electrolyte (38) in a starting state is at least substantially free of carbonate and bicarbonate and contains at least one dissolved carboxylate.

2. Electrolysis system (36) according to claim 1, characterized in that the at least one carboxylate is selected from a group comprising salts of linear, branched, cyclic or heterocyclic, aliphatically saturated or aliphatically unsaturated or aromatic, substituted or unsubstituted organic monocarboxylic acids, dicarboxylic acids, tricarboxylic acids or polycarboxylic acids and any mixtures thereof and / or that the at least one carboxylate is selected depending on a by-product spectrum of the cathode (14).

3. Electrolysis system (36) according to claim 2, characterized in that the carboxylate is selected from salts of formic acid, acetic acid, propanoic acid, malonic acid, n-butanoic acid, isobutanoic acid, n-pentanoic acid, iso-pentanoic acid, sec-pentanoic acid, tert-pentanoic acid, oxalic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, maleic acid, fumaric acid, malic acid, citric acid, isocitric acid, propane-1, 2, 3-tricarboxylic acid, trimesic acid, Ethenebis (oxyethenenitrilo) tetraacetic acid, benzoic acid, terephthalic acid, salicylic acid or anthranilic acid and any mixtures thereof.

4. Electrolysis system (36) according to one of claims 1 to 3, characterized in that the electrolyte (38) in the starting state contains starting cations which are selected from alkali cations, in particular K+ and / or Cs+, alkaline earth cations, ammonium ions and / or ammonium compounds, in particular alkylammonium compounds and / or cyclic ammonium compounds and / or mono-, di-, tri- and / or tetraethanolammonium compounds.

5. Electrolysis system (36) according to claim 4, characterized in that in the starting state a concentration of the starting cations in the electrolyte (38) is between 50 mol% and 100 mol% based on a concentration of the at least one carboxylate in the electrolyte (38).

6. Electrolysis system (36) according to one of claims 1 to 5, characterized in that the electrolyte (38) in the starting state is at least substantially free of Ni, Fe, Au and / or Pt ions.

7. Electrolysis system (36) according to one of claims 1 to 6, characterized in that the cathode (14) is a gas diffusion electrode along which the CO2 can pass at least in regions, and / or that the cathode (14) is selected from a group comprising tin-, lead-, silver- or copper-based cathodes (14).

8. Electrolysis system (36) according to one of claims 1 to 7, characterized in that a pH value of the electrolyte (38) in the starting state is in an alkaline range, in particular between 10 and 8, and / or that the electrolyte (38) in the starting state comprises a buffer system of the at least one carboxylate and at least one carboxylic acid, wherein a molar ratio between the carboxylic acid and the carboxylate is preferably between 4:1 and 1:4, in particular approximately 1:

1.

9. Electrolysis system (36) according to one of claims 1 to 8, characterized in that the anode (12) and the cathode (14) are separated from one another by an ion-conducting membrane (16), in particular a cation-conducting membrane (16), and that the electrolyte (38) is preferably arranged in a gap between the cathode (14) and the membrane (16), in particular when the cathode is designed as a gas diffusion electrode.

10. Electrolysis system (36) according to one of claims 1 to 9, characterized in that it comprises a separation device which can be fluidically coupled to the electrolysis cell (10), by means of which at least one carboxylic acid contained in the electrolyte (38) can be derivatized and / or at least partially separated from the electrolyte (38), in particular by distillation.

11. Reduction process for carbon dioxide utilization by means of an electrolysis system (36) which comprises an electrolysis cell (10) with at least one anode (12), with at least one cathode (14) and with at least one aqueous electrolyte (38) which is in operative connection with the anode (12) and / or the cathode (14), in which carbon dioxide (CO2) is brought into contact with the cathode (14) for reduction, characterized in that in a starting state an electrolyte (38) is used which is at least substantially free of carbonate and bicarbonate and contains at least one dissolved carboxylate.

12. Reduction process according to claim 11, wherein at least one carboxylic acid, in particular formic acid and / or acetic acid, is formed in the electrolyte (38) and forms a buffer with the carboxylate.

13. Reduction process according to claim 12, wherein the at least one carboxylic acid is continuously or batchwise at least partially derivatized and / or at least partially removed from the electrolyte (38).

14. Reduction process according to one of claims 11 to 13, in which an electrolyte (38) is used which has start cations in the starting state, wherein a concentration of the start cations in the electrolyte (38) during the reduction process with respect to a concentration of the at least one carboxylate is reduced from a starting value between 50 mol% and 100 mol% to a maximum of 0.01 mol% and in particular between 40 mol% and 60 mol%.

15. Reduction process according to one of claims 11 to 14, which is carried out at an average current density of at least 300 mA / cm 2is operated and / or in which an initial concentration of the at least one carboxylate in the electrolyte (38) is at least 0.25 mol / l, in particular at least 0.5 mol / l and preferably at least 1 mol / l and / or wherein a pH value of the electrolyte (38) increases from at least 3 to at most 9, in particular to at most 6, during the reduction process.

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