DE102015213947A1 - Reduction process for electrochemical carbon dioxide recovery and electrolysis system with anion exchange membrane - Google Patents

Abstract

Described is a reduction process and an electrolysis system (EA), z. As an electrolyzer, with an anode (A), a cathode (K) and a membrane (M) for the separation of anolyte and catholyte side. By using an anion exchange membrane (AEM), complementary mass transport processes can be carried out in such a way that migrations can be compensated. This increases the possible running time of the system.

Description

The present invention relates to a reduction method and an electrolysis system for electrochemical carbon dioxide utilization. Carbon dioxide is introduced into an electrolytic cell and reduced at a cathode.

State of the art

Currently, about 80% of global energy needs are met by the burning of fossil fuels, whose combustion processes cause a worldwide emission of about 34,000 million tonnes of carbon dioxide into the atmosphere each year. This release into the atmosphere, the majority of carbon dioxide is disposed of what z. B. in a lignite power plant can be up to 50,000 tons per day. Carbon dioxide is one of the so-called greenhouse gases whose negative effects on the atmosphere and the climate are discussed. Since carbon dioxide is thermodynamically very low, it can be difficult to reduce to recyclable products, leaving the actual recycling of carbon dioxide in theory or academia.

Natural carbon dioxide degradation occurs, for example, through photosynthesis. In this process, carbon dioxide is converted into carbohydrates in a temporally and on a molecular level spatially divided into many steps. This process is not easily adaptable on an industrial scale. A copy of the natural photosynthesis process with large-scale photocatalysis is not yet sufficiently efficient.

An alternative is the electrochemical reduction of carbon dioxide. For about 20 years, systematic investigations of the electrochemical reduction of carbon dioxide have been sought, so are still a relatively young field of development. Only for a few years have there been efforts to develop an electrochemical system that can reduce an acceptable amount of carbon dioxide, in line with increasing economic interest. 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 CO2 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 can be seen on different metal cathodes, see Table 1. Carbon dioxide, for example, almost exclusively reduced to carbon monoxide at silver, gold, zinc, palladium and gallium cathodes, produced on a copper cathode, a variety of hydrocarbons as reaction products.

For example, predominantly carbon monoxide and little hydrogen would be produced on a silver cathode. The reactions at the anode and cathode can be represented by the following reaction equations: Cathode: 2CO ₂ + 4e ^- + 4H ⁺ - 2CO + 2H ₂ O Anode: 2H ₂ O → O ₂ + 4H ⁺ + 4e ^-

As can also be seen from Table 1, a large number of hydrocarbons are formed as reaction products, for example, on a copper cathode. Of particular economic interest is, for example, the electrochemical production of carbon monoxide, methane or ethylene, ethanol or monoethylene glycol. These are higher-energy products than carbon dioxide. Ethylene: 2CO ₂ + 12e ^- + 8H ₂ O → C ₂ H ₄ + 12OH Methane: CO ₂ + 8e ^- + 4H ₂ O → CH ₄ + 4OH ^- Ethanol: 2CO ₂ + 12e ^- + 9H ₂ O → C ₂ H ₅ OH + 12OH ^- Monoethylene glycol: 2CO ₂ + 10e ^- + 8H ₂ O → HOC ₂ H ₄ OH + 10OH ^-

With a chloride-containing electrolyte, the following reaction can take place at the anode: 2Cl ^- → Cl ₂ + 2e ^- 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:

The table shows Faraday efficiencies [%] of products produced by carbon dioxide reduction on 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 .

In contrast to basic research, an electrolysis process that is to be operated industrially must fulfill the requirement of being economical. So far, attention has been focused on long-term stable cathode reactions. This challenge was previously met z. Example, that the durability of the catalyst and the cathode have been optimized.

However, there are several complementary mass transport processes in an electrolysis cell that over time lead to undesirable local depletion or disruptive local enrichment of a species. This can have a negative effect on the product selectivity, the conductivity and, associated therewith, the material turnover but also in turn on the longevity of catalyst and / or anode materials, e.g. B. if the mass transfer leads to a local pH change.

From the publication: J. Shi, A novel electrolysis cell for CO2 reduction in ionic liquid / organic solvent electrolyte, 2014 , the problem is known that especially when using ionic liquids as catholyte in an electrolysis cell, which increases the selectivity and the unwanted water reduction to hydrogen is to be avoided at the cathode, this system is not long-term stability. As soon as the water content in the catholyte rises due to the electrophoretically induced water diffusion, the Faraday efficiency for the formation of hydrogen increases while the conversion rate for carbon dioxide increases, for example. B. decreases to carbon monoxide.

More detailed investigations of these mass transfer processes in an electrolysis system for carbon dioxide reduction are not yet known. Therefore, so far no permanent, continuous process could be established.

Consequently, it is technically necessary to propose an improved solution for the electrochemical carbon dioxide utilization, which avoids the disadvantages known from the prior art. In particular, the proposed solution should allow a continuous carbon dioxide conversion. It is an object of the invention to provide an improved reduction process and electrolysis system for carbon dioxide utilization.

These objects underlying the present invention are achieved by a reduction method according to claim 1 and by an electrolysis system according to claim 6. Advantageous embodiments of the invention are the subject of the dependent claims.

Description of the invention

In the reduction method according to the invention for the utilization of carbon dioxide, an electrolysis system is used, in which a catholyte is guided in a catholyte circuit, the catholyte and carbon dioxide are introduced into a cathode compartment and brought into contact with a cathode, where the carbon dioxide to carbon monoxide and / or at least one Hydrocarbon compound is reduced. In this case, an anion exchange membrane is used for the separation of anode space and cathode space. In the reduction process, an ionic liquid is preferably used as the catholyte, for example butyl-trimethylammonium to trifluoromethylsulfonimides ([N ₁₁₁₄ ] TFSA) can be used:

welche mit entsprechenden Anionen kombiniert sein können, z. B. mit Halogeniden (F^–, Cl^–, Br^–, I^–), Tetrafluoroborat (a-1), Hexafluorphosphat (a-2), Carboxylate (a-3), insbesondere Acetat (a-4) oder Trifluoracetat (a-5), Sulfat (a-6) oder organisch substituiertem Sulfat (a-7), Sulfonat (a-8), insbesondere Trifluormethansulfonat (a-9) (Triflat), Bis(trifluormethansulfon)imide (a-10), Carbonat (a-11) oder Hydrogencarbonat (a-12), Prolinat (a-13), Dicyanamide (a-14), Tricyanomethanide (a-15), Imidazolat (a-16, a-17), Pyrrazolat (a-18), Triazolate (a-19) oder Tetrazolate (a-20).

Alternative advantageous ionic liquids for use as catholyte in the reduction process are based, for example, on cations such as imidazolium (k-1), pyridinium (k-2), pyrollidinium (k-3), piperidinium (k-4), organically substituted phosphonium (k). 5), organosubstituted sulfonium (k-6), morpholinium (k-7), guanidinium (k-8), organo-substituted ammonium (k-9), especially butyltrimethylammonium or thiazolium (k-10),

which may be combined with appropriate anions, e.g. With halides (F ^- , Cl ^- , Br ^- , I ^- ), tetrafluoroborate (a-1), hexafluorophosphate (a-2), carboxylates (a-3), especially acetate (a-4) or trifluoroacetate (a -5), sulfate (a-6) or organosubstituted sulfate (a-7), sulfonate (a-8), especially trifluoromethanesulfonate (a-9) (triflate), bis (trifluoromethanesulfone) imide (a-10), carbonate (a-11) or hydrogencarbonate (a-12), prolinate (a-13), dicyanamide (a-14), tricyanomethanide (a-15), imidazolate (a-16, a-17), pyrrazolate (a-18 ), Triazolates (a-19) or tetrazolates (a-20).

The letter R and R ¹ to R ⁶ in the structural formulas, as usual for different substituents.

Anolytes are preferably selected on an aqueous basis. This can be z. As sulfuric acid (H ₂ SO ₄ ), but also many other anolyte based on an aqueous electrolyte solution, eg. As alkali salts, alkali halides, sulfates, phosphates can be used in the sense of the described reduction process.

Also particularly preferred is the use of an anolyte based on an alkali metal bicarbonate. By the entry of carbonate anions through the membrane and the simultaneous prevention of Alkalikationenmigration out of the anolyte out of z. B. a high pH in the reduction process can be maintained. The carbon dioxide, together with the oxygen gas produced at the anode, leaves the anolyte and thus buffers the anolyte system, which reduces the overvoltage at the anode. This works until the carbonate is consumed in the anolyte. Preferred alkali cations for use in the anolyte are, for example, potassium (K ⁺ ) or sodium cations (Na ⁺ ). For an example of a use of an anolyte based on a potassium hydrogencarbonate (KHCO ₃ ), the reaction equation would be as follows: KHCO ₃ + H ⁺ → K ⁺ + [H ₂ CO ₃ ] K ⁺ + CO ₂ + H ₂ O

The longevity of an electrolysis system is therefore significantly related to the pH stability in the electrolysis chambers, which is influenced by the ion migration through the membrane. The targeted use of an anion exchange membrane in an electrolytic cell for carbon dioxide utilization, the use of particularly suitable anolytes in addition to the use of particularly suitable catholyte allows because their mixing or exchange, mutual enrichment or dilution by the anion exchange membrane is sufficiently avoided. When using an alkali hydrogen carbonate-based anolyte, for. For example, potassium bicarbonate, the conductivity in this anolyte can be kept high, since by the anion exchange membrane preferably a continuous entry of additional carbonate anions that arise in the carbon dioxide reduction, or z. B. is ensured from a used ionic liquid as the catholyte. At the same time, alkali cation migration, in particular potassium cation migration out of the anolyte, is avoided and a suitably high pH is maintained at the anode, which lowers the overpotential at the anode and thus increases system efficiency. Also, an undesirable accumulation of cations in the catholyte is effectively prevented by the anion exchange membrane. In addition, thus salt deposits are avoided, thereby enabling an efficient long-term stable system and method.

A suitable catholyte is to be understood in particular to mean an ionic liquid which, due to its low to non-existent water content, ensures a particularly high selectivity for the reduction of carbon dioxide, but has hitherto reacted very sensitively to a water entry through the membrane. By using the anion exchange membrane in the electrolysis system not only the cation transport is effectively suppressed, but at the same time the transport of anions, eg. As hydrogencarbonate anions, promoted, which ensures two opposing ion currents. In theory, if only the anion current is allowed, in practice, however, it is two oppositely directed ion currents, because the transport path for cations, in particular for protons, can never be completely blocked. This means at the same time that two opposite water transport paths through the membrane are opened, which allows the setting of a dynamic equilibrium state. Thus, the desired product selectivity on the cathode side is hardly affected.

The electrolysis system for carbon dioxide utilization according to the invention comprises an electrolyzer with an anode in an anode compartment, a cathode in a cathode compartment, wherein the cathode compartment has at least one access for carbon dioxide and is designed to bring the incoming carbon dioxide in contact with the cathode, where this carbon monoxide and / or at least one hydrocarbon compound can be reduced, characterized in that the electrolyzer comprises an anion exchange membrane, which is arranged between anode space and cathode space. By an anion exchange membrane is meant a polymer exchange membrane, commonly referred to in English as Polymer Exchange Membrane PEM, which has functional groups that fundamentally differentiate the anion exchange membrane from the cation exchange membrane in its transport selectivity of ions. By means of the anion exchange membrane unwanted mass transfer processes can be prevented or compensated in the system. The anion exchange membrane makes it possible to accomplish the necessary charge balance within the cell no longer solely by migration of cations or protons, but it can be targeted an opposite flow of anions z. For example, hydroxide anions or bicarbonate anions can be established. This has the advantage that the system does not "waste" over time.

The formation of carbon monoxide and / or at least one hydrocarbon compound in the reduction of the carbon dioxide depends on the catholyte. Only if protons are contained in the catholyte, z. As in an aqueous catholyte, one or more hydrocarbon compounds may arise. In the case of a pure ionic liquid, for example, complete conversion of the carbon dioxide can only take place to carbon monoxide, while hydrocarbons can also be formed in a water-containing ionic liquid.

In fact, in every electrolysis system, it is already in idle state that mass transport processes occur more frequently during operation. H. for the migration of protons, ions or even water molecules in the case that aqueous systems are used as electrolytes. The predominant mass transport processes are effected by osmosis, electrophoresis and concentration tensions.

By osmosis is meant a directed flow of particles caused by the osmotic pressure. If two substances located in the liquid phase are spatially separated by a semipermeable membrane, an osmotic pressure is produced whenever the chemical potential of the solvent in one phase is greater than in the other, ie, for example. B. when a pure solvent is present through a semipermeable membrane of a solution of a substance in just this solvent. The osmotic pressure then causes diffusion, which continues until the chemical potentials have become equal. Depending on whether the semipermeable membrane for solvent or the solute or both is permeable, diffusion takes place in one or the other direction.

Concentration stresses are used when a concentration gradient of a solute occurs at a phase boundary, such as at the membrane. If diffusion beyond this phase boundary is possible, depending on which membrane is involved, for example, it is analogous to the first Fick's law, see equation 1: j = D · ΔC (equation 1) a directed particle flow j. D denotes the diffusion constant, which in the case of the electrolysis cell is largely determined by the membrane, and ΔC denotes the difference in concentration of a substance in question from the anolyte to the catholyte. The diffusion takes place until the concentrations increase Both sides of the membrane are aligned. In an electrolytic cell, a time-varying particle current density is always to be assumed, because the concentration gradient constantly changes during operation.

By electrophoresis is meant the migration of dissolved ions in an electric field. The ions always migrate solvated, ie the ions are surrounded by a solvation shell of solvent molecules. The electric field E within an electrolytic cell, which forms homogeneously between the cathode and the anode arranged in parallel, depends on their distance d and on the clamping voltage U, see equation 2: E = U / d Eq. 2

The velocity of the solvated ions and, accordingly, the number of migrating ions depend on the applied field E and the electrical attenuation of the field by catholyte, anolyte and membrane as dielectrics.

In the event that an ionic liquid is used as the catholyte and an aqueous electrolyte is used as the anolyte, this means that water migrates into the catholyte by diffusion, osmosis and by electrophoresis. This is undesirable for the operation of an electrolysis system and can be prevented by the presented anion exchange membrane.

Thus, in the reduction process of the invention for carbon dioxide utilization, in which a catholyte is fed into a catholyte, this catholyte and carbon dioxide is introduced into a cathode compartment and brought into contact with a cathode, where the carbon dioxide is reduced to carbon monoxide and / or at least one hydrocarbon compound, and in which an anion exchange membrane is used in the electrolyser for the separation of anode space and cathode space, and ionic liquids are used as catholyte.

The described electrolysis system with an anion exchange membrane can thus generate a flow of anions, so that the migration of cations and water into the catholyte is slowed down or compensated. In addition, the use of the anion exchange membrane has the advantage that no dilution or salt accumulation happens in the respective electrolyte in the anode compartment and cathode compartment. Above all, the dilution of an ionic liquid can be avoided by the use of a suitable anion exchange membrane. By means of the anion exchange membrane, it is thus no longer possible to accomplish the necessary charge balance within the cell solely by the migration of cations or protons, but rather to deliberately set an opposing anion flow of hydroxide ions and / or hydrogen carbonate ions. In the case where an ionic liquid is used as the catholyte, the migration of solvated anions even causes water to migrate out of the ionic liquid.

Particularly preferred is the embodiment of the electrolysis system in which an anion exchange membrane is used which comprises a polyether ketone. A polyetherketone backbone is capable of raising and stabilizing an anion exchange membrane. A further advantageous embodiment describes an anion exchange membrane which has a functionalization by quaternary ammonium groups, for. For example, quaternary ammonium salts and / or positively charged amines. These can be, for example: NH ₄ ⁺ -R ^- NH ₂ -R RNH-R NR ₃ NR ₄ ⁺ (X-)

Here, R and X are as usual for a substituent, in this case, for. B. alkyl or aryl groups.

These act in the anion exchange membrane as a functional group for anion transport across the membrane. These features of an anion exchange membrane are particularly advantageous for the desired transport selectivity of the membrane: In contrast to the cation exchange membrane, an anion exchange membrane has no immobilized anions but immobilized cations. So z. For example, quaternary ammonium ions may be immobilized on the polymer backbone of the membrane. To ensure charge neutrality, the membrane also contains mobile anions. These can migrate through the membrane because of the attractive interaction with the immobilized cations.

In a particularly advantageous embodiment of the invention, the cathode compartment of the electrolysis system comprises an ionic liquid. An ionic liquid is a usually at room temperature, or at least at a temperature below 100 ° C, liquid salt with a high stability and an adjustable water content. Most of these are organic salts whose ions prevent the formation of a (stable) crystal lattice through charge delocalization and steric effects. It is therefore a liquid phase dissociated salt, a conductive liquid that is not based on an uncharged organic or inorganic solvent. By varying the substituents of a given cation and by varying the anion, the physico-chemical properties of an ionic liquid can be varied. So z. B. the melting point and the solubility of catalysts, products or educts can be adjusted in an ionic liquid. As used herein, ionic liquids typically have customizable carbon dioxide solubility and conductivity. When using ionic liquids in an electrolysis system, in particular for the reduction of carbon dioxide, the cation of the ionic liquid can also act catholytically or co-catalytically at the cathode electrolyte interface. For example, by using an ionic liquid, the overpotential for carbon dioxide reduction can be lowered while increasing the overpotential for water reduction, which increases product selectivity and increases system efficiency. Thus, the use of an ionic liquid solves the problem that in aqueous electrolytes the thermodynamically favorable competition reaction of reduction of water to hydrogen favors proceeds, so the problem of reduced longevity of the electrolysis system by the use of an ionic liquid so far from the anode space in the catholyte. As a result, in addition to the processes already described, consumption of the system very quickly occurs here, which does not permit any long-term continuous reduction of carbon dioxide emissions. As soon as the electrophoretically induced diffusion of water from the anode compartment or the anolyte into the catholyte begins, namely the hydrogen generation at the cathode increases as competing reaction z. For carbon monoxide formation.

Of particular advantage is the use of an anion exchange membrane from fumatech GmbH in an electrolysis cell for carbon dioxide reduction, in particular the anion exchange membrane fumasep FAB PK130. This has a thickness of 130 microns, a specific conductivity for chloride anions of 1.8 mScm ^-1 , a proton transfer rate of 363 .mu.molmin ^-1 cm ^-2 , a selectivity for 0.5 mol / kg of potassium chloride at 25 ° C of 97.4 , an increase in H ₂ O of 11 wt% and a pH stability in the range 1-14. This has the advantage of blocking migration of spatially large ionic liquids, as well as ensuring good proton shielding, but allowing for charge equalization by an electrophoretically induced anion flux.

Examples and embodiments of the present invention will be further exemplified with reference to FIGS 1 to 9 the attached drawing:

1 shows the time course of the electrophoretically induced cation concentration in the cathode region,

2 shows the water diffusion in a two-chamber design with an ionic liquid as the catholyte,

3 shows an electrolytic cell EA with anion exchange membrane AEM,

4 shows the mode of operation of the cation transport through a cation exchange membrane KEM,

5 shows the mode of operation of the anion transport through an anion exchange membrane AEM,

6 shows a scheme for the proton hopping mechanism,

7 shows a diagram of the Faraday efficiency F as a function of the voltage applied to the electrolysis cell U,

8th shows a diagram of the time course of Faraday efficiency F for hydrogen production and

9 shows a diagram of the time course of Faraday efficiency F for carbon monoxide production.

In the 1 and 2 First, the described disturbing mass transfer processes are shown in an electrolytic cell E for carbon dioxide reduction. In the 1 For this purpose, four electrolysis cells E are each shown in a two-chamber construction with cathode space KR, anode space AR, cathode K, anode A and membrane M. Arrows illustrate the chronological sequence of operation. Without applied voltage U no carbon dioxide CO _{2 is} consumed, but already the osmotic pressure and the concentration voltage cause a diffusion of cations M ⁺ through the membrane M. If at a time t = 0 a voltage U is applied to cathode K and anode A, so For example, at the cathode K, the reduction process of carbon dioxide CO ₂ , in aqueous electrolyte, for example, to a hydrocarbon C _x H _y O _z , begins to run in an ionic liquid IL to carbon monoxide CO (x = 1, y = 0, Z = 1) , At the same time, however, water H ₂ O is also hydrolyzed, so it is present in the catholyte, wherein at least hydroxide anions OH ^- arise. Alternatively, in a system with ionic liquid IL as an electrolyte formally arise oxygen anions O ^2- . At the anode A, water H ₂ O or oxygen anions O ^{2- are} oxidized to oxygen O ₂ , which can then escape from the anode space AR. The resulting protons H ⁺ and the cations M ^{+ of} the conducting salt can migrate through the membrane M and accumulate in the cathode space KR. At a later point in time t1, the schematic representation of the electrolysis cell E already shows a clear accumulation of the anolyte cations M ⁺ in the cathode space KR and a reduction in the guide salt concentrations in the anode space AR. At a later point in time t ² , for example, a carbonate CO ₃ ^2- or bicarbonate formation HCO ₃ ^- in the cathode space KR, the carbonates CO ₃ ^2- or bicarbonates HCO ₃ ^- arises in the carbon dioxide reduction and binds the migrated anolyte cations M ⁺ per se.

Are z. B. very good soluble salts z. B. potassium chloride KCl used in an electrolytic cell E, the formation of corresponding bicarbonates with the cations M ⁺ in this case, so the formation of potassium bicarbonate KHCO ₃ very disturbing act. The solubility of potassium hydrogen carbonate KHCO ₃ at 337 g / l is only slightly lower than that of potassium chloride KCl at 344 g / l. Therefore, when potassium electrolytes with a concentration above 2.4 moles are used, formation of potassium bicarbonate precipitates with some amount of crystallized potassium chloride KCl occurs.

When using carbonate anolyte, it also leads to a complete deionization of the anolyte. In the case of carbonate CO ₃ ^2- can not such. B. sulfates SO ₄ ^2- sulfuric acid H ₂ SO _{4 are} formed, but there are carbon dioxide CO ₂ and water H ₂ O. The example in the 1 For example, in both compartments, electrolysis cells E shown contain a 0.5 molar potassium sulfate solution K ₂ SO ₄ (aq). Completely without conducting salt, an electrolytic cell E would not be able to work efficiently for carbon dioxide reduction. The conductivity of carbon dioxide CO ₂ saturated water H ₂ O is very low. An increase in the conductivity of the electrolyte by adding a conducting salt lowers the electrolyte resistance, leads to higher current densities at the same applied voltage and increases the system efficiency by lower voltage drop in the electrolyte. In previously known electrolysis cells E in which, for example, membranes M of sulfonated tetrafluoroethylene polymer are used. So now finds at the beginning of the electrolysis process at the cathode K instead of a carbon dioxide reduction reaction of the following form: xCO ₂ + yH ₂ O + (4x + y - 2z) e ^- → C _x H _y O _z + (4x + y - 2z) OH ^- , Thus, in order to ensure charge neutrality, an anode reaction must take place in parallel, in which, for the choice of an aqueous electrolyte, as here, usually a water oxidation takes place: H ₂ O → O ₂ + 2e ^- + 4H ⁺

Since on the anode side of the system electrons e ^- are withdrawn and supplied to the cathode side of the system, there would be a charge but which is degraded by a charge transfer in the electrolyte solution. Would pass through a proton conductive membrane M exclusively to neutralize protons on the cathode side, the hydroxide OH formed there may ^- be neutralized as follows: OH ^- + H ⁺ → H ₂ O

In fact, however, membranes M are at least partially permeable or permeable not only to protons H ⁺ but always to other cations M ⁺ as well. Thus, the disturbing cation migration occurs in the catholyte. In the anode space AR, the migrated cations M ⁺ can not be reproduced and for charge compensation then the protons H ⁺ formed migrate. In the example of the potassium sulfate anolyte, it is converted over time to potassium hydrogen sulfate KHSO ₄ (aq) and finally to dilute sulfuric acid H ₂ SO ₄ . The pH in the anode compartment AR drops massively, the conductivity of the anolyte drops and there is a high voltage drop. At the same time it comes at the cathode K to an enrichment with potassium hydroxide KOH, the pH increases, potassium bicarbonate KHCO ₃ is formed and finally falls: KOH + CO ₂ → KHCO ₃

Only at very high pH potassium carbonate K ₂ CO _{3 is} formed, which, however, is much more soluble than potassium bicarbonate KHCO ₃ , in water 1120 g / l. KOH + KHCO ₃ → K ₂ CO ₃ + H ₂ O

On the catholyte side, finally, potassium sulfate K ₂ SO ₄ , possibly also small amounts of potassium carbonate K ₂ CO ₃ , while the anolyte consists almost of sulfuric acid H ₂ SO ₄ . The increased concentration of potassium ions K ⁺ in the catholyte also lowers the carbon dioxide solubility, which further contributes to the efficiency reduction. In addition, the salt precipitate of the bicarbonate can lead to encrustations on the electrode K. In the example of the initial 0.5 molar potassium sulfate solution, from its maximum solubility in water of about 0.64 mol, a solubility product of about 1.0 mol ³ / l ³ can be estimated. For the initial concentration of sulfate ions and potassium ions of 0.5 mol, no salt precipitation or crusts are expected. However, if the potassium concentration in the catholyte doubles due to cation migration, the solubility product is exceeded and potassium sulfate deposits are formed.

When using differentiated electrolytes, the description of all mass transport processes in the overall system is more complex. At rest, before operation, osmosis and diffusion of ions through the membrane M causes the electrolytes to equalize. The lower salt concentration in the catholyte, which is often chosen because of higher carbon dioxide solubility, increases. In addition, the catholyte level may drop. In operation, the alkali or alkaline earth metal ions of the anolyte will again migrate by electrophoresis into the catholyte. They are not stopped by cation exchange membranes KEMs. It comes again after some time to precipitating salts. The above-described pH gradient as well as the conductivity loss of the anolyte result again. A carbon dioxide electrolysis system can not be operated so long-term stable and is unattended for industrial use.

In the 2 is now once again an electrolytic cell E with an analogous structure as in 1 whose cathode space KR is filled with an ionic liquid IL as a catholyte. For example, a mixture of an ionic liquid IL and water H ₂ O can also be used as the catholyte. This mixture may be heterogeneous or homogeneous, that is without formation of phase boundaries between the water H ₂ O and the ionic liquid IL. Already at rest, water H ₂ O diffuses into the ionic liquid IL on the basis of Fick's law (equation 1). Either individual water molecules H ₂ O migrate through channels of the cation exchange membrane KEM or cations M ^{+ together} with the hydrate shell diffuse. In operation, this mechanism is massively amplified by electrophoresis.

As anolyte z. As an aqueous salt solution, acid or dilute acid used. In this case, further processes should be taken into consideration: Already at rest, ie without an applied voltage U, there is a strong migration of substances into the ionic liquid. On the one hand, the diffusion of water H ₂ O in the mostly hydrophilic ionic liquids happens due to the high concentration gradient through the membrane M. The ionic liquid is diluted over time. On the other hand, solvated alkali metal or alkaline earth metal cations M ⁺ or proton H ⁺ also diffuse into the ionic liquid due to the concentration gradient. Thus, even more H ₂ O-water molecules are introduced into the ionic liquid. During operation, the electrophoretically induced migration processes described above are added. The steady entry of water H ₂ O due to solvated ions migrating by electrophoresis is even more severe in this case: the ionic liquid is being inextricably diluted and its additional co-catalyst benefit is being destroyed. Even when using organic solvents as electrolytes on the anode side, these problems arise in principle.

3 finally shows an example of an embodiment of the described electrolysis system EA with an anion exchange membrane AEM. Highly schematic again is a two-chamber construction of a Electrolysis cell EA shown with a cathode K in a cathode space KR and an anode A in an anode space AR. Cathode K and anode A are electrically connected to each other via a voltage source U, while anolyte and catholyte are separated from each other by an anion exchange membrane AEM. The charges e- which flow in operation from the anode A to the cathode K through the electrical line must be compensated by an opposing charge current into the electrolyte in order to ensure charge neutrality. This compensation charge current must also pass through the anion exchange membrane AEM.

The electrolysis system EA may instead of the two-chamber structure also have a different common structure, which comprises at least one anode A in an anode space AR and a cathode K in a cathode space KR. In each case, the anode space AR and the cathode space KR are separated from each other at least by a membrane AEM. Another defining parameter for the membrane is the so-called bubble point. This describes from which pressure difference ΔP a gas flow through the membrane AEM would take place between the two sides of the membrane AEM. Preferably, therefore, a membrane AEM is used with a high rubble point of 10 mbar or greater. Anode A and cathode K are each electrically connected to a power supply U. The anode compartment AR of the electrolysis cell is equipped with an electrolyte inlet. Likewise, the anode compartment AR comprises an electrolyte outlet via the anolyte, and at the anode A formed electrolysis products, for. B. oxygen gas O ₂ can flow out of the anode compartment AR. The cathode space KR has at least one electrolyte and product outlet. In this case, the total electrolysis product can be composed of a large number of electrolysis products. While in the two-chamber structure anode A and cathode K are separated by the anode space AR and cathode space KR of the membrane M ₁ , the electrodes are in a so-called polymer electrolyte membrane structure (PEM) with porous electrodes directly to the membrane M. Then it is a porous anode A and a porous cathode K. In the two-chamber design and in the PEM structure of the electrolyte and the carbon dioxide CO _{2 is} preferably introduced via a common Edukteinlass in the cathode chamber KR. In contrast to this, in a so-called three-chamber design, in which the cathode space KR has an electrolyte inlet, the carbon dioxide CO ₂ separately flows into the cathode space KR via the cathode K, which in this case is necessarily porous. Preferably, the porous cathode K is designed as a gas diffusion electrode. A gas diffusion electrode is characterized in that a liquid component, for. As an electrolyte, and a gaseous component, for. B. a Elektrolyseedukt, in a pore system of the electrode, for. B. the cathode K, can be brought into contact with each other. The pore system of the electrode is designed so that the liquid and the gaseous phase can equally penetrate into the pore system and can be present in it at the same time. Typically, a reaction catalyst is designed to be porous and takes over the electrode function, or a porous electrode has catalytically active components. To introduce the carbon dioxide CO ₂ into the catholyte cycle, the gas diffusion electrode GDE comprises a carbon dioxide inlet. The invention could thus be implemented, for example, in one of these described electrolysis cell structures, if they are provided with a corresponding anion exchange membrane AEM. These electrolysis cells can also be modified for use in accordance with the invention so that they are combined into mixed variants. For example, an anode space AR can be embodied as a polymer electrolyte membrane half cell, while a cathode space KR consists of a half cell whose cathode space KR is arranged between membrane AEM and cathode K and / or has a gas diffusion electrode as cathode K.

Ion exchange membranes can be considered as conducting salt solutions in which an ionic species has been immobilized on a carrier polymer. If this immobilized charge carrier is the anion, it is called a cation exchange membrane (KEM), and if the cations are stationary, this is called an anion exchange membrane (AEM).

The charge transport through an ion exchange membrane can generally be accomplished by three types of particles: anions, cations, and protons. The latter are formally cations, but to be considered specifically. In the 4 is first shown a greatly simplified representation of a section of the framework of a cation exchange membrane KEM, which is often referred to simply as an ion exchange membrane. A first arrow, which is to indicate the directed electric field E between electrodes arranged in parallel, shows from right to left, as in FIGS 1 to 3 the cell arrangement is selected. The polymer scaffold P is represented by a straight channel through which the ion migration is to take place. The surface of the polymer backbone P has functional groups S ^- , O ⁺ immobilized thereon. In the 4 is a cation exchange membrane KEM based on negatively charged sulfonate groups S ^- , characterized by their attractive interaction with dissolved cations M ⁺ , in the 4 indicated by arrows meeting, transport these cations M + through the membrane KEM. At the same time sulfonate act S ^- over anions A ^- repulsive, as is illustrated by the double arrows. The cation and anion migration directions M _k , M _A are indicated below the polymer channel by arrows.

Like in the 4 can be used as a cation exchange membrane KEM, for example, sulfonated polytetrafluoroethylene (PTFE, also known under the trade name Nafion). Their mechanism of selectivity can then generally be described as follows: Cations M ⁺ , such as metal ions (eg potassium) or organic cations (eg tetra-alkyl-ammonium, imidazolium) have to move on their own. In a Nafion membrane as are the anions A ^- immobilized ^- in the form of negatively charged sulfonate groups S. A migrating cation M ⁺ such as potassium K ⁺ , can freely enter the membrane KEM, since it is attracted by the negatively charged sulfonate groups S ^- . Because the cations M ⁺ which are already present for the charge neutrality in the membrane KEM also migrate in the electric field E, the newly occurring cation M ^{+ can take} its place. By entering further cations M ⁺ in the membrane KEM from the anode compartment AR, the cation M ^{+ is} gradually transported through the membrane KEM.

On the other hand, the migration of anions A ^{- is} largely blocked. An anion A ^- is repelled by the negatively charged sulfonate groups S ^- . In addition, there are no mobile anions A in the resting state ^- in the membrane KEM. An anion A ^- would thus unlikely to enter the membrane KEM, as this would increase the ionic strength in the membrane KEM, but this is thermodynamically unfavorable. It needed other mechanisms, such as osmosis, through which salts could penetrate into the membrane KEM and eventually migrate through it.

In the 5 is analogous to 4 a highly simplified representation of a section of the framework of an anion exchange membrane AEM shown. The polymer scaffold P is again shown greatly simplified by a straight channel through which the ion migration is to take place. The surface of the polymer backbone P has functional groups O ⁺ immobilized thereon. In this case, positively charged ammonium groups O ^{+ are} immobilized on the polymer P, which by their attractive interaction with dissolved anions A ^- in the 5 indicated by arrows, transport them through the membrane AEM. At the same time, the ammonium groups O ^{+ are} repulsive to cations M ⁺ , as illustrated by double arrows. Thus, the anion transport opposite to the direction of the electric field E is favored.

In the overall balance, the proton transport H ⁺ still has to be considered, as shown schematically in the 6 is shown. The charge transport by protons H ⁺ in protic media such as water or alcohols is fundamentally different in its mechanism. In protic media, networks of so-called hydrogen-bond bonds exist, which are formed by the protons H ⁺ bound in the medium. Excess protons H ⁺ , which act as charge carriers, are also integrated into this network. Although hydrogen bonds and chemical bonds in the medium differ in their length, minimal shifts in the atomic distances in the medium already allow the charge to migrate without the actual proton H ⁺ having actually migrated. This has the consequence that the proton current is very fast and can be prevented virtually no membrane M. When a membrane M, also an anion exchange membrane AEM, is brought into contact with a protic medium, e.g. B. with water H ₂ O, so it swells with it and is passable for protons H ⁺ .

In the 7 to 9 diagrams are shown in which the Faraday efficiency F in% for the electrolysis product carbon monoxide CO and hydrogen H _{2 are} plotted.

als Katholyt eingesetzt. Diese ionische Flüssigkeit kann z. B. von IoLiTec GmbH bezogen werden. Der weitere Aufbau des Elektrolysesystems EA umfasst in diesem Beispiel eine Silber-Kathode K und eine Platin-Anode A. Als Anolyt wurde 0,3 M Schwefelsäure H₂SO₄ verwendet. Zwischen die beiden Kompartiments Anodenraum AR und Kathodenraum KR ist, z. B. mit Hilfe eines Flansches, eine Anionentauschermembran AEM eingesetzt.In the 7 the working electrode potential is initially plotted: this was butyltrimethylammonium bis (trifluoromethylsulfonyl) imide ([N ₁₁₁₄ ] TFSA)

used as catholyte. This ionic liquid may, for. B. from IoLiTec GmbH. The further construction of the electrolysis system EA comprises in this example a silver cathode K and a platinum anode A. The anolyte used was 0.3 M sulfuric acid H ₂ SO ₄ . Between the two compartments anode space AR and cathode space KR is, for. B. using a flange, an anion exchange membrane AEM used.

The Faraday efficiency F, that is to say the proportion of electrons in the total current, is plotted against an Ag / AgCl reference electrode as a function of the voltage U applied to the electrodes, for the carbon monoxide CO and for the hydrogen generation H ₂ at the cathode K. So it became experimental a voltage U _opt of -1.95 V vs. Ag / AgCl at which the system has the best selectivity and Faraday efficiency F for carbon monoxide CO. This voltage U _opt was used for in the 8th and 9 adjusted chronoamperometries set.

Due to the very different chemical nature of catholyte and anolyte in the exemplary measurement explained in more detail here, very high concentration gradients are already present in the quiescent state of the electrolytic cell for all substances involved. Since the ionic liquid [N1114] TFSA used here is "spatially large", it will not be able to migrate through the membrane AEM. Thus, it can be assumed that protons migrate H ⁺ , water molecules H ₂ O and sulfate anions SO ₄ ^2- for concentration compensation in the catholyte. With applied cell voltage U _opt then additionally the charge equalization by electrophoresis ensures the migration of the named species.

The anion exchange membrane AEM fumasep FAB-PK-130 from fumatech GmbH, which is advantageously used in this example, has good proton-blocking properties and has a high selectivity for anions A ^- . These properties can be demonstrated particularly well experimentally:
For this purpose, two chronoamperometries were recorded over 3 hours by means of gas chromatography analysis of the gaseous electrolysis products carbon monoxide CO and hydrogen H ₂ . The voltage U _opt of -1.95 V was determined in a series of voltages as described above. Two otherwise identical carbon dioxide electrolysis systems were operated once with a Nafion N117 membrane KEM, identified in the figures with a, and once with a fumasep FAB-PK-130 membrane AEM, marked b in the figures. With the anion exchange membrane AEM, the Faraday efficiency F (H ₂ ) for hydrogen H ₂ can be kept constant at zero, while the Faraday efficiency F (CO) for carbon monoxide CO remains constantly high. In particular, the anion exchange membrane AEM used ensures a nearly constant Faraday efficiency F (CO) for carbon monoxide of more than 80%.

When using a cation exchange membrane KEM, the Faraday efficiency F (H ₂ ) for the production of hydrogen H ₂ increases significantly over time and the Faraday efficiency F (CO) for carbon monoxide CO decreases over time.

After an operating time of the electrolysis cell with cation exchange membrane KEM of 3 hours, 17 mol% water content was measured in the catholyte, in the electrolytic cell with anion exchange membrane AEM only 6 mol% water. Thus, it is shown in this example presented that the steady dilution of the catholyte has been greatly slowed down. This result is interpreted to mean that a dynamic water equilibrium may have set in. The desired gas selectivities were retained. For the measurement results shown, an electrolyzer with very low electrolyte volumes was used, making this system much more sensitive to material changes than large reservoir systems described in the literature, which react much more slowly to an amount of migrated matter.

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 non-patent literature

• Electrochemical CO2 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 [0004]
• J. Shi, A novel electrolysis cell for CO2 reduction in ionic liquid / organic solvent electrolyte, 2014 [0011]

Claims (10)

Reduction method for carbon dioxide utilization by means of an electrolysis system, in which a catholyte is guided in a catholyte (KK), the catholyte and carbon dioxide (CO ₂ ) introduced into a cathode chamber (KR) and brought into contact with a cathode (K), where the carbon dioxide (CO ₂ ) to carbon monoxide (CO) and / or at least one hydrocarbon compound (C _X H _Y O _Z ) is reduced, wherein an anion exchange membrane for the separation of anode space (AR) and cathode space (KR) is used. The reduction process according to claim 1, wherein an ionic liquid is used as the catholyte in the catholyte cycle (KK). The reduction process according to claim 2, wherein the catholyte used is butyltrimethylammonium bis (trifluoromethylsulfonyl) imide ([N ₁₁₁₄ ] TFSA). A reduction process according to claim 1 or 2, wherein the catholyte used is an ionic liquid based on one of these cations: imidazolium (k-1), pyridinium (k-2), pyrollidinium (k-3), piperidinium ( k-4), organosubstituted phosphonium (k-5), organosubstituted sulfonium (k-6), morpholinium (k-7), guanidinium (k-8), organosubstituted ammonium (k-9) , in particular butyltrimethylammonium or thiazolium cations (k-10),

with corresponding anions, where the anions are halides (F ^- , Cl ^- , Br ^- , I ^- ), tetrafluoroborate (a-1), hexafluorophosphate (a-2), carboxylates (a-3), in particular acetate (a-4) or trifluoroacetate (a-5), sulphate (a-6) or organically substituted sulphates (a-7), sulphonates (a-8), in particular trifluoromethanesulphonate (triflate) (a-9), bis (trifluoromethanesulphone) imide (a) 10), carbonate (a-11) or bicarbonate (a-12), prolinate (a-13), dicyanamide (a-14), tricyanomethanide (a-15), imidazolate (a-16, a-17), pyrrazolate (a-18), triazolates (a-19) or tetrazolates (a-20)

could be. The reduction process according to any of the preceding claims 1 to 4, wherein the anolyte comprises an inorganic acid, in particular sulfuric acid (H ₂ SO ₄ ), and / or an alkali metal conducting salt, in particular potassium hydrogencarbonate (KHCO ₃ ). Electrolysis system for carbon dioxide utilization, comprising an electrolyzer (EA) with an anode (A) in an anode compartment (AR), a cathode (K) in a cathode compartment (KR), wherein the cathode compartment (KR) at least one access for carbon dioxide (CO ₂₎ and is designed in the, OBTAINED carbon dioxide (CO ₂₎ (in contact with the cathode K) to bring where C _X H _Y O _Z) can be reduced by this (to carbon monoxide CO) and / or at least one hydrocarbon compound (wherein in that the electrolyzer comprises an anion exchange membrane (AEM) which is arranged between anode space (AR) and cathode space (KR). An electrolysis system according to claim 6, wherein the anion exchange membrane (AEM) comprises a polyether ketone. An electrolysis system according to claim 6 or 7, wherein the anion exchange membrane (AEM) has a functionalization by quaternary ammonium groups. Electrolysis system according to one of the preceding claims 6 to 8, wherein the cathode space (KR) comprises an ionic liquid. Electrolysis system according to one of the preceding claims 6 to 9, wherein the cathode space (KR) butyltrimethylammonium bis (trifluoromethylsulfonyl) imides

([N ₁₁₁₄ ] TFSA).

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