Electrolytic System And Reduction Method For Electrochemical Carbon Dioxide Utilization, Alkali Carbonate Preparation And Alkali Hydrogen Carbonate Preparation

Abstract

The present disclosure relates to electrolysis. The teachings thereof may be embodied in a reduction process and/or an electrolysis system for electrochemical carbon dioxide utilization wherein carbon dioxide is introduced into an electrolysis cell and reduced at a cathode. For example, an electrolysis system for carbon dioxide utilization may comprise: an electrolyzer including an anode in an anode space and a cathode in a cathode space. The cathode space has an entrance for carbon dioxide. The cathode space comprises a catholyte including alkali metal cations. The anode space has an entrance for an anolyte. The anode space comprises an anolyte comprising chlorine anions.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of International Application No. PCT/EP2016/065277 filed Jun. 30, 2016, which designates the United States of America, and claims priority to DE Application No. 10 2015 212 504.1 filed Jul. 3, 2015, the contents of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to electrolysis. The teachings thereof may be embodied in a reduction process and/or an electrolysis system for electrochemical carbon dioxide utilization wherein carbon dioxide is introduced into an electrolysis cell and reduced at a cathode.

BACKGROUND

At present, about 80% of the global energy requirement is provided by the combustion of fossil fuels, the combustion processes of which cause global emission of about 34 000 million metric tons of carbon dioxide into the atmosphere per annum. This release into the atmosphere comprises the majority of carbon dioxide released, which can be up to 50 000 metric tons per day in the case of a brown coal power plant, for example. Carbon dioxide is one of the gases known as greenhouse gases, the adverse effects of which on the atmosphere and the climate are a matter of some dispute. Since carbon dioxide exists at a very low thermodynamic level, it can be reduced to reutilizable products only with difficulty, which has left the actual reutilization of carbon dioxide in the realm of theory or in the academic field to date.

Natural carbon dioxide degradation proceeds, for example, via photosynthesis. This involves conversion of carbon dioxide to carbohydrates in a process subdivided into many component steps over time and, at the molecular level, in terms of space. As such, this process cannot easily be adapted to the industrial scale. No copy of the natural photosynthesis process with photocatalysis on the industrial scale to date has had adequate efficiency.

An alternative is the electrochemical reduction of carbon dioxide. Systematic studies of the electrochemical reduction of carbon dioxide are still a relatively new field of development. Only in the last few years have there been efforts to develop an electrochemical system that can reduce an acceptable amount of carbon dioxide. Research on the laboratory scale has shown that electrolysis of carbon dioxide may be accomplished using metals as catalysts. 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, p. 89-189, discloses Faraday efficiencies at different metal cathodes; see table 1. If carbon dioxide is reduced, for example, at silver, gold or zinc cathodes, what is formed is almost exclusively carbon monoxide.

TABLE 1
Electrode	CH4	C2H4	C2H5OH	C3H7OH	CO	HCOO−	H2	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

The table gives Faraday efficiencies [o] of products that form in the carbon dioxide reduction at various metal electrodes. The values reported apply to a 0.1 M potassium hydrogen-carbonate solution as electrolyte and current densities below 10 mA/cm².

At a silver cathode, for example, predominantly carbon monoxide and only a little hydrogen form. The reactions at anode and cathode can be represented by the following reaction equations:

Cathode: 2 CO₂+4 e⁻+4 H⁺→2 CO+2 H₂O

Anode: 2 H₂O→O₂+4 H⁺+4 e⁻

As can also be inferred from table 1, at a copper cathode for instance, a multitude of hydrocarbons are formed as reaction products. One aspect of particular economic interest is, for example, the electrochemical production of methane or ethylene, ethanol or monoethylene glycol. These are higher-energy products than carbon dioxide.

Ethylene: 2CO₂+12e⁻+8H₂O→C₂H₄+12OH⁻

Methane: CO₂+8e⁻+4H₂O→CH₄+4OH⁻

Ethanol: 2CO₂+12e⁻+9H₂O C₂H₅OH+12OH⁻

Monoethylene glycol: 2CO₂+10e⁻+8H₂O→HOC₂H₄OH+10OH⁻

With a chloride-containing electrolyte, the following reaction can proceed at the anode:

2 Cl⁻→Cl₂+2 e⁻

In the electrochemical conversion of matter of carbon dioxide to a higher-energy product, there is an interest in increasing the economic viability, and in improvement with regard to the continuous operability of the electrolysis systems.

SUMMARY

Consequently, an improved solution for the electrochemical utilization of carbon dioxide would avoid the disadvantages known from the prior art. More particularly, the solution may enable continuous carbon dioxide conversion. The teachings of the present disclosure may provide an improved reduction process and electrolysis system for carbon dioxide utilization.

For example, some embodiments may include electrolysis systems for carbon dioxide utilization, comprising an electrolyzer (E1-E5) having an anode (A) in an anode space (AR) and a cathode (K) in a cathode space (KR). The cathode space (KR) has at least one entrance for carbon dioxide (CO₂) and is configured to bring the carbon dioxide (CO₂) that has entered into contact with the cathode (K). The cathode space (KR) comprises or can accommodate a catholyte which can enter the cathode space (KR) through the same entrance or a separate entrance and which includes alkali metal cations. The anode space (AR) has at least one entrance for an anolyte and comprises an anolyte or can accommodate it via this entrance, wherein the anolyte includes chlorine anions.

In some embodiments, there is a deposition tank (AB), wherein the deposition tank (AB) is configured for crystallization of an alkali metal hydrogencarbonate and/or alkali metal carbonate out of the catholyte and has a product outlet (PA3).

In some embodiments, the deposition tank (AB) has a cooling apparatus.

In some embodiments, at least one reservoir (PR) is configured and arranged with connection to the cathode space (KR) and/or the deposition tank (AB) such that it serves to buffer the catholyte.

In some embodiments, the catholyte comprises at least one solvent, especially water.

In some embodiments, the anolyte includes at least one water-soluble alkali metal salt.

In some embodiments, the anode space (AR) is connected to a gas separation unit for separation of chlorine gas from the anolyte.

In some embodiments, anode space (AR) and cathode space (KR) are separated from one another by a cation-conducting membrane (M).

As another example, some embodiments may include a reduction process for carbon dioxide utilization by means of an electrolysis system as described above. In some embodiments, a catholyte and carbon dioxide (CO₂) are introduced into a cathode space (KR) and brought into contact with a cathode (K). Carbon dioxide (CO₂) is reduced at the cathode (K). An anolyte including chloride anions (Cl⁻) is introduced into an anode space (AR) and brought into contact with an anode (A). Chloride anions (Cl⁻) are oxidized at the anode (A) to chlorine (Cl₂) and the latter is separated from the anolyte as chlorine gas by means of a gas separation unit. The anolyte includes alkali metal cations that migrate into the catholyte. At least a portion of the catholyte volume is introduced into a deposition tank, where an alkali metal hydrogencarbonate and/or alkali metal carbonate crystallizes out.

In some embodiments, there is reduction at the cathode (K) of carbon dioxide (CO₂) to carbon monoxide (CO), ethylene (C₂H₄), methane (CH₄), ethanol (C₂H₅OH) and/or monoethylene glycol (OHC₂H₄OH).

In some embodiments, the hydroxide ions (OH⁻) formed in the carbon dioxide reduction are converted to hydrogencarbonate ions (HCO₃⁻) with carbon dioxide (CO₂) present in excess.

In some embodiments, at least a portion of the catholyte volume is introduced into a deposition tank, where it is cooled down by at least 15 kelvin, preferably at least 20 kelvin.

In some embodiments, at least a portion of the catholyte volume is introduced into a deposition tank, where the pH thereof is lowered from above 8 to a pH of 6 or less by blowing in carbon dioxide (CO₂).

In some embodiments, at least a portion of the catholyte volume is introduced into a deposition tank, where an alkali metal hydrogencarbonate is crystallized and is subsequently converted to an alkali metal carbonate by heating.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples and embodiments of teachings of the present disclosure are described in an illustrative manner with reference to FIGS. 1 to 6 of the appended drawing:

FIG. 1 shows a schematic diagram of an electrolysis system with a carbon dioxide reservoir and deposition tank, according to teachings of the present disclosure;

FIG. 2 shows a schematic diagram of an electrolysis system with a gas diffusion electrode, according to teachings of the present disclosure;

FIG. 3 shows a schematic diagram of a PEM setup of an electrolysis cell, according to teachings of the present disclosure;

FIG. 4 shows a schematic diagram of a PEM half-cell coupled to a gas diffusion electrode, according to teachings of the present disclosure;

FIG. 5 shows a schematic diagram of a PEM half-cell coupled to a cathode with backflow, according to teachings of the present disclosure; and

FIG. 6 shows a Hagg diagram.

DETAILED DESCRIPTION

The electrolysis system of the present disclosure may allow improved carbon dioxide utilization. Some embodiments include at least one electrolyzer having an anode in an anode space and a cathode in a cathode space. The cathode space has at least one entrance for carbon dioxide and is configured to bring the carbon dioxide that has entered into contact with the cathode. In addition, the cathode space comprises a catholyte or is configured to be able to accommodate a catholyte. The catholyte can access the cathode space through the same entrance as the carbon dioxide or via a separate second entrance. At least the anode space also includes alkali metal cations in the operation of the cell anode and cathode space.

Catholyte refers to an electrolyte which directly affects the cathode in the electrolysis. Correspondingly, reference is also made hereinafter to anolyte when referring to an electrolyte directly affecting the anode in an electrolysis. Alkali metal cations refer to positively charged ions having at least one element of the first main group of the Periodic Table.

The anode space of the electrolyzer has at least one entrance for an anolyte and comprises an anolyte or is at least configured to accommodate an anolyte via this entrance, said anolyte including chlorine anions.

In some embodiments, in the electrode system, the anode space and the cathode space are separated from one another by a membrane. The membrane here may include at least one mechanically separating layer, for example a diaphragm, which separates the electrolysis products formed in the anode space and cathode space from one another. They could then also be referred to as separator membrane or separation layer.

Since the electrolysis products are in many cases gaseous substances, the membrane may have a high bubble point of 10 mbar or higher. The “bubble point” is a defining parameter for the membrane used, which describes the pressure difference ΔP between the two sides of the membrane from which gas flow through the membrane would set in. The membrane may also be a proton- or cation-conducting or -permeable membrane. While molecules, liquids or gases are being separated, proton or cation flow from the anode space to the cathode space is assured. In some embodiments, the membrane comprises sulfonated polytetrafluoroethylene, e.g. Nafion.

In some embodiments, the electrolysis system further comprises at least one deposition tank for crystallization of an alkali metal hydrogencarbonate and/or alkali metal carbonate out of the catholyte. In some embodiments, this deposition tank has a product outlet. According to the product, whether an alkali metal hydrogencarbonate and/or alkali metal carbonate is to be taken from the catholyte, and according to the alkali metal, a second deposition tank may also be provided for a crystallization process. The latter is then typically arranged downstream of the first deposition tank in catholyte circulation direction.

According to the cathode material used, the reduction of carbon dioxide gives rise to different products: for example, carbon monoxide, ethylene, methane, ethanol, or monoethylene may be formed. In all these cases, hydroxide ions also form, which may be neutralized to hydrogencarbonate by excess carbon dioxide. The source of the alkali metal cations is in the anode space. A cation stream through the membrane compensates for the electrical current resulting from the voltage applied.

For example, the alkali metal cations and the chloride anions may be metered into the anolyte in the form of a chloride salt. While the chloride anions are oxidized at the anode to chlorine and leave the anolyte circuit as chlorine gas, the alkali metal cations migrate through the membrane into the catholyte circuit, where they react in the cathode space with the carbonate or hydrogencarbonate formed there to give an alkali metal carbonate or alkali metal hydrogencarbonate and may leave the catholyte circuit via the separate product outlet of the deposition tank.

The electrolysis systems of the present disclosure may have produce not only chlorine but at least one alkali metal carbonate and/or alkali metal hydrogencarbonate as a chemical material of value. Whether alkali metal carbonate or alkali metal hydrogencarbonate is formed depends, for example, on the alkali metal and the utilization method. In aqueous solution, for instance, the solubility is crucial. The sparingly soluble carbonate or hydrogencarbonate crystallizes out. In the case of sodium and potassium it is the hydrogencarbonate that is more sparingly soluble than the carbonate, and then has to be calcined in a subsequent step. The combustion of sodium in carbon dioxide is an example in which carbon monoxide and, in a direct manner, sodium carbonate Na₂CO₃ are produced.

Furthermore, the electrolysis system may utilize carbon dioxide and it is thus also typically possible to provide at least one third material of value, for example carbon monoxide, ethylene, methane, ethanol or monoethylene glycol. The exploitation of the compensating current of the cations thus creates an electrolysis system which enables continuous hydrogencarbonate production.

As already described, the actual cathode reaction in which the carbon dioxide is reduced is followed by a subsequent reaction, namely the neutralization of the hydroxide ions (OH—). These are especially neutralized by excess carbon dioxide to hydrogencarbonate (HCO₃⁻). This firstly has the effect that the pH in the cathode space is thus buffered within a pH range from 6 to 8. It also has the effect that the electrolyte concentration rises considerably. But if the catholyte is conducted into a catholyte circuit, i.e. pumped into the cathode space and led out of it again, the hydrogencarbonate formed in the cathode space can be taken from the catholyte. For this purpose, more particularly, at least one pump in each case may be arranged in the catholyte circuit, or else, for example, in the anolyte circuit, and this ensures electrolyte circulation.

Subsequent to the neutralization reaction of the hydroxide ions (OH⁻) with excess carbon dioxide to give hydrogencarbonate ions (HCO₃⁻), these may react further with alkali metal cations to give alkali metal hydrogencarbonates. The alkali metal cations present in the cathode space come from the anode space, into which they were initially introduced especially in the form of alkali metal chloride as oxidation reactant or in the form of another alkali metal salt, for increasing the conductivity for example. The alkali metal cations in the anode space may be replenished in the form of alkali metal chloride. The membrane between the anode space and cathode space may be chosen such that the cation flow from the anode space toward the cathode in the electrical field of the electrolyzer is assured. The effect of the temperature and also pH dependence of the solubility of alkali metal hydrogencarbonates is then that different processes for crystallization or for withdrawal from the catholyte are undertaken:

Firstly, it is possible to utilize the temperature dependence of the solubility of the alkali metal hydrogencarbonates desired as electrolysis product. For this purpose, the deposition tank may comprise a cooling apparatus, by means of which the catholyte is cooled down by several degrees Kelvin compared to the temperature range that prevails in the electrolyzer. In some embodiments, the temperature difference set from the deposition tank to the electrolyzer is at least 15 K, especially at least 20 K. According to the electrolyte concentration in the catholyte and according to the alkali metal cations with which the hydrogencarbonate is formed, a temperature difference between 30 K and 50 K may also be particularly suitable. The temperature difference between the electrolyzer and deposition tank may be within a temperature range between 5 K and 70 K.

The lowering of the temperature in the deposition tank may provide cooling in the catholyte circuit precedes the recycling of the catholyte into the cathode space. Thus, excessively high systemic evolution of heat, specifically in the electrolyzer, is avoided. But it is also possible to dispense with any cooling unit provided specially for this purpose.

In the case of the deposition method of crystallization of the alkali metal hydrogencarbonate by means of cooling of the catholyte too, preference is given to using pH buffers provided, for example, in a buffer reservoir to the deposition tank and/or to the catholyte circuit and/or to the cathode space, to correspondingly buffer the catholyte volume.

The pH of the catholyte can also be employed as such for the control of the operation of deposition of the alkali metal hydrogencarbonate out of the electrolyte. For this purpose, more particularly, the pH in the cathode space is at first kept at a higher value, for example 8 or higher. This can shift the equilibrium in favor of the alkali metal carbonate and away from the alkali metal hydrogencarbonate. For crystallization in the deposition tank, the pH is then lowered, e.g., to a value of 6 or less, which leads to formation and crystallization of the alkali metal hydrogencarbonate. The lowering of the pH is typically accomplished by blowing carbon dioxide into the deposition tank.

According to the alkali metal cation with which the hydrogencarbonate reacts, and depending on the pH in the cathode space, it is at first possible to form an alkali metal hydrogencarbonate or an alkali metal carbonate. More particularly, the two procedures described for withdrawal of the desired product from the catholyte can also be combined. In some cases, for example in the case of formation of sodium hydrogencarbonate NaHCO₃, it is also possible, for example, to obtain the sodium carbonate Na₂CO₃ subsequently from the sodium hydrogencarbonate NaHCO₃ that has crystallized out by heating. In that case, hydrogencarbonate may be first produced and deposited, and subsequently the desired proportion thereof is processed further to give carbonate.

The pH dependence of the hydrogencarbonate or carbonate ions is shown, for example, in FIG. 6 in a Hagg diagram for a sodium carbonate solution.

In the electrolysis system, a buffer reservoir may be provided in the anolyte circuit, which can especially also serve for introduction or replenishment of alkali metal chloride into the electrolyte, to maintain the salt content in the anolyte.

In some embodiments, the catholyte includes at least one solvent, especially water. Typically, aqueous electrolytes and correspondingly water-soluble conductive salts may be employed. The conductive salt content can be increased by the addition of further carbonates, hydrogencarbonates, but also sulfates or other conductive salts, to increase the conductivity of the electrolyte in the catholyte circuit and also in the anolyte circuit, which leads to an increase in the conversion of matter in the overall system. According to which and what amounts of additional conductive salts are present in the catholyte circuit, the crystallization process is adjusted correspondingly to extract the desired product with maximum purity. Conductive salts used may be chosen such that the solubility thereof differs significantly from that of the alkali metal hydrogencarbonate or the alkali metal carbonate.

Typically, the electrolysis system has a gas separation unit on the anolyte side, which is configured to undertake the separation of chlorine gas from the anolyte. In the catholyte circuit too, a gas separation unit may be provided, for example when it is directed to carbon monoxide gas production via use of a silver-containing cathode. In the anolyte circuit and in the catholyte circuit, additional units for inlets or outlets from the system or additional buffer reservoirs may be provided.

The nature and quality of the membrane used in the electrolyzer ultimately makes a significant contribution to how pure the crystallized product is. If the membrane used is merely a separator, it is also possible, for example, for chloride anions to diffuse into the cathode space, even counter to the electrical field in the electrolyzer, such that not only hydrogencarbonate but possibly also chlorides are formed. Therefore, in some embodiments, there is a cation-conducting membrane through which virtually exclusively cations can pass. A purely anion-conducting membrane may be less useful.

In some embodiments, the reduction process described for carbon dioxide utilization by means of an electrolysis system as described above comprises the following steps: a catholyte and carbon dioxide are introduced into a cathode space, where they are contacted with a cathode. Within the cathode space, this catholyte includes alkali metal cations which migrate through the membrane that separates anode space and cathode space. At least a portion of the catholyte volume may be introduced into a deposition tank, where an alkali metal hydrogencarbonate and/or an alkali metal carbonate crystallizes out.

In some embodiments, an anolyte including chloride anions, is brought into contact with an anode. The chloride anions are oxidized at the anode to chlorine and the latter is separated from the anolyte as chlorine gas by means of a gas separation unit. Typically, this reduction process is effected such that anolyte and catholyte are each conducted into a separate circuit, meaning that two pumps are provided in the electrolysis system, which bring about transport of the catholyte through the cathode space and transport of the anolyte through the anode space at least at one point in the circuit.

The circuits are separated from one another by the membrane in the electrolyzer, which may permit exclusively transport of cations from the anode space into the cathode space. More particularly, the alkali metal cations required in the cathode space may be obtained from the anode space. For this purpose, the anolyte may include an alkali metal chloride; the latter may be used as conductive salt, or else likewise as electrolysis reactant. In some embodiments, the alkali metal chloride in the anolyte can be used as electrolysis reactant, and an additional conductive salt, for example a sulfate, a phosphate et cetera, e.g, an alkali metal sulfate, can be used. In some embodiments, it is also possible to use ammonium salts or homologs thereof. Imidazolium salts or other ionic liquids can have a positive effect on the selectivity of the electrode, particularly the cathode.

In some embodiments, in the reduction process, the reduction of the carbon dioxide at the cathode produces carbon monoxide, ethylene, methane, ethanol and/or monoethylene glycol. For this purpose, an appropriate cathode may be used as catalyst for these reactions. For this purpose, the cathode may include copper. In some embodiments, this reduction process produces, in addition to carbon dioxide utilization, chemical substances of value.

In some embodiments, the hydroxide ions formed in the carbon dioxide reduction can be converted to hydrogencarbonate ions with carbon dioxide present in excess. Hydrogencarbonate production directly in the cathode space allows these to react further directly with alkali metal cations present in the cathode space to give a further material of value which is of interest, which would otherwise have to be produced in separate production processes. In some embodiments, to withdraw this material of value from the system, at least a portion of the catholyte volume is introduced into a deposition tank, where it is cooled down by at least 15 K, and/or by at least 20 K. Here, the temperature dependence of the carbonate solubility is thus exploited to withdraw the material of value from the catholyte circuit. The temperature differential from deposition tank to electrolyzer may also be more than 30 K, especially also more than 50 K, according to the present alkali metal hydrogencarbonate to be extracted and also depending on which further salts are present in the circuit. The temperature differential between electrolyzer and deposition unit may be between 5 K and 70 K. In some embodiments, for extraction of the hydrogencarbonate product from the catholyte volume, the dependence of the solubility on the pH is exploited. This process can be combined with the temperature-dependent process.

In some embodiments, for this purpose, at least a portion of the catholyte volume is introduced into a deposition tank, where the pH thereof is lowered, especially by means of blowing in carbon dioxide, from above 8 to a pH of 6 or less. Specifically the buffering of the pH to a value of more than 8 in the cathode space prevents the precipitation of the alkali metal hydrogencarbonate in the cathode space itself.

In some embodiments, the reduction process can be undertaken such that the precipitated alkali metal hydrogencarbonate is converted to alkali metal carbonate by heating. This can be effected directly after the crystallization of the hydrogencarbonate in the deposition tank or separately from the electrolysis system described.

In some embodiments, as an alternative to the temperature method of crystallization or to the temperature-assisted crystallization, or else in combination therewith, the process can also be run such that the pH in the cathode space is kept at the upper limit of the reaction of around 8 or higher, such that the equilibrium is at first shifted in favor of sodium carbonate:

2 NaHCO₃→Na₂CO₃+H₂O+CO₂.

For this purpose, the carbon dioxide supply to the system must be very well controlled, to arrive at and maintain this basic regime. In the deposition tank, the pH would then be lowered for optimal deposition of the sodium hydrogencarbonate by blowing in carbon dioxide, and hence the equilibrium reaction would again be shifted in favor of sodium hydrogencarbonate.

However, the process is not restricted to sodium hydrogencarbonate. For example, it is also possible to prepare potassium hydrogencarbonate in this process. Analogously to the deposition process described for sodium hydrogencarbonate, it is also possible to crystallize the potassium hydrogencarbonate out of a pure potassium hydrogencarbonate electrolyte by lowering the temperature in the deposition tank. At 20° C. the solubility of potassium hydrogencarbonate is 337 g/l, and at 60° C. it is 600 g/l.

A somewhat different procedure is necessary if an additional conductive salt, for example potassium sulfate (K₂50₄), is to be used. This has a lower solubility of 111.1 g/l at 20° C. and 250 g/l at 100° C., which means that the potassium sulfate would always precipitate out first in the mixed electrolyte. In order to obtain the potassium hydrogencarbonate (KHCO₃) from an electrolyte containing both potassium sulfate and potassium hydrogencarbonate, it is necessary to proceed as follows: in the deposition tank AB, potassium sulfate K₂50₄ preferentially crystallizes out and can be fed back to the electrolyte subsequently, i.e. downstream of the deposition tank AB in circulation direction. The electrolyte volume from which the potassium sulfate K₂SO₄ has already been removed is then concentrated, preferably in a further deposition tank, meaning that the water is removed from the potassium hydrogencarbonate solution, for example by cooling, to obtain the crystalline material.

In principle, this process is also applicable to other cations or mixtures of cations. The migration of the cations results in concentration of the catholyte to such an extent that the most sparingly soluble salt or double salt separates out. It is important here that the process of concentration and deposition does not proceed in the cathode space, i.e. not in the electrolysis cell itself, but that the catholyte is transported for the purpose into a deposition tank integrated within the electrolysis system. By means of a further additional physical or chemical difference between the electrolysis cell and deposition tank, i.e., for example, by means of a temperature, pH or pressure gradient, the deposition in the deposition tank is achieved or promoted. A suitable pressure differential between electrolysis cell and deposition tank may be up to 100 bar. A pressure differential between 2 bar and 20 bar would preferably be chosen. An elevated pressure in the deposition tank would promote hydrogencarbonate formation.

On the anode side, in principle, alternative anode reactions are also conceivable, but coupling to chlorine production is the most economically viable, since the chlorine market is about 75 million metric tons per year. Current production of sodium hydrogencarbonate (NaHCO₃) is about 50 million metric tons per year, which have to date been produced via the energetically unfavorable Solvay process.

With the electrolysis system and reduction process described, it is possible to electrochemically, continuously and simultaneously produce three materials of value: at the cathode, a material of value such as carbon monoxide, ethylene, methane, ethanol or monoethylene glycol is obtained from the carbon dioxide reduction, sodium hydrogencarbonate and/or sodium carbonate is co-produced as a conversion product of this reduction reaction formed in the cathode space, and on the anode side chlorine is produced.

FIGS. 1 and 2 show, in a schematic representation, examples of electrolysis systems for carbon dioxide reduction, which can equally be read as flow diagrams for the reduction process described. Shown on the left-hand side in each case is the anolyte circuit AK, and on the right-hand side the catholyte circuit KK. These two circuits AK, KK are connected via the electrolyzer E1, E2, the anode space AR and cathode space KR of which are connected to one another and separated from one another by means of a membrane M. The membrane M used may be a cation-conducting membrane M. In the anode space AR is disposed an anode A, and in the cathode space KR a cathode K, which are electrically connected by a voltage source U.

Each of the circuits AK, KK may include a pump P1, P2, which pump the electrolytes through the electrolyzer. In addition, units N1, N2, N3 in the two circuits AK, KK may be present at different points in the flow direction, which may be additional inlets or outlets or in the form of buffer reservoirs. In the anolyte circuit AK, at least one gas separation unit G2 with a product outlet PA2 is provided, by means of which the chlorine gas product Cl₂ can be withdrawn. Likewise provided in the catholyte circuit KK is at least one gas separation unit G1 with a product outlet PA1, by means of which, for example, the carbon monoxide electrolysis product CO, and, for example, hydrogen H₂ as well can be withdrawn. But it is also possible for further electrolysis products, such as ethylene, methane, ethanol, monoethylene glycol, to be withdrawn from the system via this or, for example, via a further product outlet. The electrolyzer E1, E2 has, for example, a gas diffusion electrode GDE for the carbon dioxide inlet.

In the case of the electrolyzer E1 shown in FIG. 1, a two-chamber setup is chosen and the carbon dioxide CO₂ is introduced into the electrolyte via a reservoir CO₂—R and upstream of the cathode space KR in circulation direction. The catholyte circuit KK, in both cases shown, has a deposition tank AB which may be incorporated directly into the circuit or through which just a portion of the catholyte volume is conducted. For this purpose, as shown in FIGS. 1 and 2, a branch in the circuit KK may be provided. The deposition tank AB or a plurality of series-connected deposition tanks may be connected, for example, to a cooling unit or to a buffer reservoir PR, such that the crystallization of the hydrogencarbonate is promoted by establishing a temperature differential, pressure differential or pH differential with respect to the electrolyzer E1, E2. In addition, the deposition tank AB may include a product outlet PA3. Multiple series-connected deposition tanks would each have a product outlet.

FIGS. 1 and 2 thus show electrolysis systems usable for the methods described herein. In this setup, it is ensured that there are separate anolyte circuits AK and catholyte circuits KK. The electrolytes used are then pumped continuously through the electrolysis cell E1, E2, i.e. through the anode space AR and through the cathode space KR. For this purpose, in the setup, one pump P1, P2 is provided in each of the two circuits AK, KK. The setup may include materials made of plastic, plastic-coated metal or glass. Reservoir vessels used may be glass flasks; the cell itself is made, for example, of PTFE, and the hoses of neoprene.

The electrolyzer E1, E2, as constructed in the electrolysis systems shown, may also have a different setup as shown, for example, in FIGS. 3 to 5. An alternative electrolysis cell is that according to the polymer electrolyte membrane setup (PEM setup). In this case, at least one electrode directly adjoins the polymer electrolyte membrane PEM. Correspondingly, the electrolysis cell can be configured as a PEM half-cell, as shown in FIGS. 4 and 5, in which the anode side is configured as a PEM half-cell, i.e. the anode A is arranged in direct contact with the membrane PEM and the anode space AR is arranged on the side of the anode A facing away from the membrane.

In the cases as shown in FIGS. 4 and 5, the cathode K is porous and at least partly gas-permeable and/or electrolyte-permeable. In FIG. 4, the anode PEM half-cell is combined with a gas diffusion electrode GDE for introducing the carbon dioxide CO₂ into the cathode space KR. Also shown in FIG. 5 is a cathode K with backflow, the cathode space KR of which is connected to a gas reservoir via the cathode K. The gas reservoir here, for its part, has at least one gas inlet GE and optionally a gas outlet GA. Such an embodiment has been used to date, for example, as an oxygen-depolarized electrode, for example in the production of sodium hydroxide solution. In that case, there would be oxygen backflow through the cathode K. The oxygen-depolarized cathode can be used, for example, to avoid hydrogen formation H₂ in the cathode space KR in favor of a reaction to give water H₂O. The energy of water formation here lowers the necessary system voltage U and thus brings about lower energy consumption of the electrolysis system. Since the cathode K of an oxygen-depolarized electrode consists primarily of silver, it can also catalyze carbon dioxide reduction. If no oxygen is provided, the oxygen-consuming reaction cannot proceed. Instead, carbon dioxide reduction to carbon monoxide CO takes place with a certain degree of hydrogen formation.

If, for example, sodium is chosen as alkali metal, in the case of use of a copper-containing cathode K, the following reactions proceed in the cathode space KR:

Ethylene: 12 NaCl+14 CO₂+8 H₂O→C₂H₄+12 NaHCO₃+6 Cl₂

Methane: 8 NaCl+9 CO₂+4 H₂O→CH₄+8 NaHCO₃+4 Cl₂

Ethanol: 12 NaCl+14 CO₂+9 H₂O→C₂H₅OH+12 NaHCO₃+6 Cl₂

Monoethylene Glycol:

10 NaCl+12 CO₂+8 H₂O→HOC₂H₄OH+10 NaHCO₃+5 Cl₂.

In the case of a silver-containing cathode K, the following reactions would proceed at the cathode:

Carbon Monoxide:

2 NaCl+3 CO₂+H₂O→CO+2 NaHCO₃+Cl₂.

These equations describe the cumulative process in the electrolysis cell. The chlorine gas Cl₂ forms, as described, through oxidation of the chloride anions Cl⁻ at the anode A; the other electrolysis products form at the cathode K or through conversion reactions in the cathode space KR.

The example of sodium may be suitable since sodium hydrogencarbonate can be deposited very efficiently from the electrolyte. Moreover, sodium hydrogencarbonate and sodium carbonate are important chemical materials of value that are frequently required. Global annual sodium carbonate production is about 50 000 000 metric tons, as can be inferred for example from the Roskill market report “Soda Ash: Market Outlook to 2018”, available from Roskill Information Services Ltd, E-Mail: info@roskill.co.uk, www.roskill.co.uk/soda-ash.

The solubility of sodium hydrogencarbonate NaHCO₃ in water H₂O is comparatively low and also shows strong temperature dependence; see table 2.

	TABLE 2
	Molecular formula
	KHCO3	K2SO4	K3PO4	KI	KBr	KCl	NaHCO3	Na2SO4
Molar mass	100.1	174.3	212.3	166.0	119.0	74.6	84.01	142.04
in g/mol
Solubility in H2O at 20° C.:
In g/l	337	111	900	1400	678	344	96	170
In mol/l	3.37	0.64	4.24	8.43	5.70	4.61	1.19	1.14
Conductivities σ in mS/cm:
At 0.05M	4.8	9.9	17.3	7.2	7.7	7.4	5.8	14.8
At 0.1M	9.1	19.2	30.1	14.0	14.3	13.8	28.1	51.6
At 0.5M	38.9	(69.9)	108	65.2	67.5	62.8

Table 2 lists further salts, potassium hydrogencarbonate KHCO₃, potassium sulfate K₂SO₄, potassium phosphate K₃PO₄, potassium iodide KI, potassium bromide KBr, potassium chloride KCl, sodium hydrogencarbonate NaHCO₃, sodium sulfate Na₂SO₄, which can be used with preference. But other sulfates, phosphates, iodides or bromides can also be used to increase the conductivity in the electrolyte. By constantly supplying the carbon dioxide, it is not necessary to supply carbonates or hydrogencarbonates; instead, they are formed in operation in the cathode space KR.

The solubility of sodium hydrogencarbonate NaHCO₃ in water is 69 g/l at 0° C., 96 g/l at 20° C., 165 g/l at 60° C. and 236 g/l at 100° C. Sodium carbonate NaCO₃, by contrast, has comparatively good solubility; the solubility thereof is 217 g/l at 20° C. With continuing electrolysis, the sodium hydrogencarbonate NaHCO₃ thus has a tendency to crystallize out in the electrolysis cell E1, E2. This can be counteracted via an elevated temperature as arises as a result of the operation of the system, and also via corresponding buffering of the pH.

The sodium hydrogencarbonate NaHCO₃ is not supposed to crystallize out of the electrolyte until within the deposition tank AB. As a result of the pumped circulation of the electrolyte in a circuit KK, the sodium hydrogencarbonate NaHCO₃ formed in the cathode space KR is conducted out of it and the catholyte circuit KK can run through a deposition tank AB, or a part-volume of the catholyte is branched into a deposition tank AB in which, for example, the sodium hydrogencarbonate NaHCO₃ crystallizes out as a result of the cooling of the electrolyte and can thus be recovered. Since the electrolysis cells E1, E2 are in any case heated significantly in operation as a result of process losses, there can be effective crystallization at temperature differentials of up to 70 K between cathode space KR and deposition tank AB. Preference is given to working within a range between temperature differential 30 K and 50 K. Especially with a temperature differential of at least 15 K or even at least 20 K.

If the catholyte also contains further additions for enhancing conductivity and hence increasing the energy efficiency, and an additional conductive salt thus minimizes the ohmic losses in the electrolyte, this has to be taken into account in the crystallization of the sodium hydrogencarbonate NaHCO₃, in order to obtain a product of maximum purity. In some embodiments, a hydrogensulfate HSO₄⁻ or sulfate SO₄²⁻ is included as a conductive additive. This may, for example, be sodium sulfate Na₂SO₄ or sodium hydrogensulfate NaHSO₄. The solubility of sodium hydrogensulfate NaHSO₄ is 1080 g/l at 20° C. and that of sodium sulfate Na₂SO₄ is 170 g/l at 20° C.; see table 2. Given this great difference in solubility from sodium hydrogencarbonate NaHCO₃, it is assured that sodium hydrogencarbonate NaHCO₃ will crystallize out preferentially in the deposition tank.

This variant of the reduction process may basically replace the Solvay process that has been used as standard to date for sodium hydrogencarbonate production. This is because the Solvay process for sodium hydrogencarbonate production has a great disadvantage, namely that it consumes very large amounts of water. Moreover, for every kilogram of soda, i.e. sodium carbonate Na₂CO₃, about one kilogram of unusable calcium chloride CaCl₂ is also produced, which is usually released into the wastewater and hence into rivers and seas. Given an annual production of 50 million metric tons of sodium carbonate Na₂CO₃, this is thus about 50 million metric tons of calcium chloride CaCl₂.

The natural sources for soda Na₂CO₃ that are available aside from the Solvay process are by no means sufficient. Sodium hydrogencarbonate NaHCO₃ occurs as the natural mineral nahcolite in the United States of America. It usually occurs in fine distribution in oil shale and can then be produced as a by-product of oil production. Particularly rich nahcolite horizons are being mined in the state of Colorado. However, annual production in 2007 was only 93 440 metric tons. It also occurs, for example, in soda lakes in Egypt, in Turkey in Lake Van, in East Africa, for example in Lake Natron and other lakes in the East African rift, in Mexico, in California (USA), and as trona (Na(HCO₃).Na₂CO₃.2H₂O) in Wyoming (USA), Mexico, East Africa and in the southern Sahara.

FIG. 6 shows, for illustration of the dependence on the concentration and pH parameters, an example of a Hagg diagram of a 0.05 molar solution of carbon dioxide CO₂. Within a moderate pH range, carbon dioxide CO₂ and salts thereof are present alongside one another. While carbon dioxide CO₂ under strongly basic conditions preferentially takes the form of carbonate CO₃²⁻ and preferentially takes the form of hydrogencarbonate HCO₃⁻ in the moderate pH region, the hydrogencarbonate ions are driven out of the solution in the form of carbon dioxide CO₂ at low pH values in an acidic medium.

Claims

1. An electrolysis system for carbon dioxide utilization, the system comprising:

an electrolyzer including an anode in an anode space and a cathode in a cathode space;

the cathode space has an entrance for carbon dioxide;

the cathode space comprises a catholyte including alkali metal cations;

the anode space has an entrance for an anolyte;

the anode space comprises an anolyte comprising chlorine anions.

2. The electrolysis system as claimed in claim 1, further comprising a deposition tank configured for crystallization of an alkali metal hydrogencarbonate and/or alkali metal carbonate out of the catholyte;

wherein the deposition tank includes a product outlet.

3. The electrolysis system as claimed in claim 2, further comprising a cooling apparatus for the deposition tank.

4. The electrolysis system as claimed in claim 2, further comprising a reservoir connected to the cathode space or the deposition tank to buffer the catholyte.

5. The electrolysis system as claimed in claim 1, wherein the catholyte comprises a solvent.

6. The electrolysis system as claimed in claim 1, wherein the anolyte includes at least one water-soluble alkali metal salt.

7. The electrolysis system as claimed in claim 1, further comprising the anode space connected to a gas separation unit for separation of chlorine gas from the anolyte.

8. The electrolysis system as claimed in claim 1, further comprising a cation-conducting membrane separating the anode space and cathode space from one another.

9. A reduction process for carbon dioxide utilization, the process comprising:

introducing a catholyte and carbon dioxide into a cathode space with a cathode;

reducing carbon dioxide at the cathode;

introducing an anolyte including chloride anions into an anode space and brought into contact with an anode;

wherein the anolyte includes alkali metal cations that migrate into the catholyte;

oxidizing chloride anions at the anode to chlorine;

separating chlorine from the anolyte as chlorine gas using a gas separation unit; and

introducing at least a portion of the catholyte volume into a deposition tank, where an alkali metal hydrogencarbonate and/or alkali metal carbonate crystallizes out.

10. The reduction process as claimed in claim 9, further including reducing, at the cathode, carbon dioxide (CO2) to carbon monoxide (CO), ethylene (C2H4), methane (CH4), ethanol (C2H5OH), and/or monoethylene glycol (OHC2H4OH).

11. The reduction process as claimed in claim 10, further comprising converting the hydroxide ions (OH−) formed in the carbon dioxide reduction to hydrogencarbonate ions (HCO3−) with carbon dioxide (CO2) present in excess.

12. The reduction process as claimed in claim 9, further including introducing at least a portion of the catholyte into a deposition tank, where it is cooled down by at least 15 kelvin.

13. The reduction process as claimed in claim 9, further comprising introducing at least a portion of the catholyte volume into a deposition tank, where the pH thereof is lowered from above 8 to a pH of 6 or less by blowing in carbon dioxide (CO2).

14. The reduction process as claimed in claim 9, further comprising introducing at least a portion of the catholyte volume into a deposition tank, where an alkali metal hydrogencarbonate is crystallized and is subsequently converted to an alkali metal carbonate by heating.