Electrolytic method, electrolyzer, electrolysis system, use, and facility

Abstract

The invention relates to a method for obtaining carbon dioxide, having the following steps: a) carrying out an anodic oxidation of hydrogen gas, an acid oxidation product being obtained; b) reacting the acid oxidation product with an alkaline carbonate-containing aqueous solution which has a pH value of > 7 to 9 in particular, an aqueous acid solution being obtained; c) removing carbon dioxide from the aqueous acid solution, carbon dioxide gas and a degassed aqueous acid solution being obtained; and d) cathodically reducing acid components of the degassed aqueous acid solution in order to obtain cathodically generated hydrogen gas, wherein an aqueous alkaline solution is additionally obtained which has a pH value of 9.4 to ≥ 8.

Description

[0001] Electrolytic process, electrolyzer, electrolysis system, use and installation

[0002] Technical field of the invention

[0003] The present invention relates to a process for carbon dioxide production, in particular from seawater or a gas containing carbon dioxide, in particular air or a point source, an electrolyzer for carbon dioxide production, an electrolysis system comprising such an electrolyzer, a use of such an electrolyzer in a plant for the electrolytic carbon dioxide production from seawater or a gas containing carbon dioxide, in particular air or a point source, and a plant for the electrolytic carbon dioxide production from seawater.

[0004] Description

[0005] As the concentration of carbon dioxide in the atmosphere increases and global average temperatures rise, systems for removing carbon dioxide from the atmosphere are becoming increasingly important. In addition to direct air capture, there are also technical options for extracting or recovering carbon dioxide from carbonate-containing aqueous solutions, particularly seawater.

[0006] Using electrochemical processes, water can be split into a basic and an acidic part. In an acidic environment, the chemical equilibrium shifts from CO^~ via HCO^ to CO2 and H2O (see equation (1) below).

[0007] C0 ~ + 2 H + HCOj + H + H2CO ?1 CO2+ H20 (1)

[0008] Common electrochemical processes for splitting water into an acidic and a basic stream are usually based on one or more ion exchange membranes. These selectively allow either cations (CEM) or anions (AEM) to pass through the membrane. Furthermore, bipolar membranes (BPM) can be used, which convert aqueous systems into an acidic (H + ) and a basic (OW) current.

[0009] So-called E-CEM (Electrocatalytic Cation Exchange Module) systems work with two cation exchange membranes between the electrodes. Water is passed through all three separate compartments. By applying a voltage, the proton concentration in the middle compartment is increased by the migration of the H +-ions are increased. Consequently, the pH value is lowered, making it possible to capture CO2 from the solution. The energy consumption of these systems is approximately 20,000 kWh / t-CO2. Compared to E-CEM systems, bipolar membrane electrodialysis systems (BPMED) have a better energy efficiency in the extraction of CO2 from an aqueous solution, with approximately 920 kWh / t-CC>2 and 1400 kWh / t-CC>2, respectively. By using a redox-active electrolyte stream, which is circulated directly at the cathode or anode, redox reactions such as the formation of H2(HER) and O2(OER) from H2O can be suppressed. The preferred electrolyte is a system consisting of Fe(II) and Fe(III) ions, such as potassium hexacyanidoferrate(II) and potassium hexacyanidoferrate(III), but can also be replaced by other compounds. At the bipolar membrane, the water molecules are converted into an acidic (H +) and a basic (OW) stream. The acidic stream can be used to recover CO2, while the basic stream is used to basify the process water.

[0010] The highest energy efficiency to date for extracting CO2 from aqueous systems was achieved using electrochemical hydrogen circulation (EHL). The setup is divided into three compartments. The cathode compartment is flooded with seawater, forming H2 and OH- at the cathode. The resulting hydrogen is fed into the anode compartment, where H2 is converted to H + oxidized and fed into the middle compartment to acidify the incoming seawater.

[0011] Nanofiltration is typically required for electrochemical processes for removing or capturing carbon dioxide from seawater. If a high pH environment prevails in any part of the system, such as the cathode compartment, this leads to the precipitation of minerals containing divalent magnesium or calcium ions. These ions are necessary to maintain a high alkalinity in the seawater and thus the reabsorption capacity of atmospheric carbon dioxide. Furthermore, the precipitated minerals can lead to impurities in the system, such as electrode fouling, which would increase the required energy costs over time.

[0012] Nanofiltration is a very complex and expensive process, resulting in high operating costs, estimated at €0.20 per cubic meter of water processed. To extract one ton of carbon dioxide from seawater with a concentration of 2.2 mM of dissolved carbon at an efficiency of 90%, 1,1477 m³ would be needed. 3 Water. In this example, the cost of nanofiltration alone would be approximately €2,000 for the extraction of one ton of CO2.

[0013] Accordingly, the present invention is based on the object of reducing or eliminating the above disadvantages.

[0014] Summary of the invention

[0015] The above object is achieved by the invention as defined in the independent claims.According to a first aspect, the present invention relates to an electrolytic process, in particular a continuously operated electrolytic process, for carbon dioxide production, comprising the following steps: a) anodic oxidation of hydrogen gas, whereby an acidic oxidation product is obtained; b) reacting the acidic oxidation product with an alkaline carbonate-containing aqueous solution, which in particular has a pH of > 7 to 9, whereby an acidic aqueous solution is obtained; c) removal of carbon dioxide from the acidic aqueous solution, whereby carbon dioxide gas and a degassed acidic aqueous solution are obtained; d) cathodic reduction of acidic components of the degassed acidic aqueous solution in order to obtain cathodically generated hydrogen gas, whereby an alkaline aqueous solution is additionally obtained which has a pH of 10 to > 7.1.

[0016] According to a second aspect, the present invention relates to an electrolyzer for carbon dioxide recovery comprising:

[0017] • an anode chamber,

[0018] • a gap, and

[0019] • a cathode chamber, wherein the intermediate space is arranged between the anode chamber and the cathode chamber; the anode chamber is connected to the intermediate space via a first transport membrane; the cathode chamber is connected to the intermediate space via a second transport membrane; the anode chamber and the cathode chamber are fluidically connected via a hydrogen gas line; the intermediate space has an inlet and an outlet, wherein the outlet is fluidically connected to a carbon dioxide extraction device and the carbon dioxide extraction device is directly fluidically connected to an inlet of the cathode chamber via a liquid line.

[0020] According to a third aspect, the present invention relates to an electrolysis system comprising at least one electrolyzer according to the second aspect of the present invention.

[0021] According to a fourth aspect, the present invention relates to the use of an electrolyzer according to the second aspect of the present invention in a plant for the electrolytic production of carbon dioxide from seawater. According to a fifth aspect, the present invention relates to a plant for the electrolytic production of carbon dioxide from seawater comprising an electrolyzer according to the second aspect of the present invention or an electrolysis system according to the third aspect of the present invention.

[0022] According to a sixth aspect, the present invention relates to a use of an electrolyzer according to the second aspect of the present invention in a plant for the electrolytic production of carbon dioxide from a gas containing carbon dioxide, in particular from air or a point source.

[0023] According to a seventh aspect, the present invention relates to a plant for the electrolytic production of carbon dioxide from a gas containing carbon dioxide, in particular from air or a point source, comprising an electrolyzer according to the second aspect of the invention or an electrolysis system according to the third aspect of the present invention.

[0024] Technical effects of the invention

[0025] The aspects of the invention enable a significantly more energy-efficient process than previous approaches to the electrolytic production of carbon dioxide. Furthermore, these aspects enable a reduction in mineral precipitation from the carbonate-containing solution, especially in the cathode compartment, even though bivalent cations may be present. This results in a more gentle process, as the maintenance intervals of the cathode are extended. This is particularly the case because fouling of the cathode material is reduced by a reduction in the hydroxide precipitation of calcium and / or magnesium. At the same time, bivalent cations, such as Mg 2+ and Ca 2+, be returned to the sea, as there is no need to use a nanofilter. This has ecological advantages, as these cations enable further absorption of CO2 via carbonates, and economic advantages, as there is no need for an expensive and maintenance-intensive nanofilter. Furthermore, the aspects of the invention make it possible to generate a pH value, e.g. approximately 8.1, which does not deviate significantly from that of seawater. This prevents the effluent from the electrolysis, which is usually returned to the sea, from having a lower pH value than seawater. Thus, the present process is also beneficial for the marine ecosystem in light of the pH value, as ocean acidification is avoided.

[0026] In addition, the energy efficiency of carbon dioxide extraction from air can be improved with the aspects of the invention, since lower voltages are achieved.

[0027] Further information on the technical effects achieved by the invention can be found in the detailed description. Definitions

[0028] Unless otherwise stated, all technical terms used herein correspond to the common professional understanding.

[0029] The term "carbon dioxide recovery" is to be understood broadly and is understood here as the removal of carbon dioxide in its gaseous form from a carbonate-containing aqueous solution, formally using H + Cations carbonate / hydrogen carbonate is converted into dihydrogen carbonate, which decomposes into water and carbon dioxide gas, as shown for example in equation (1) above.

[0030] The term "acidic" is to be understood broadly, with an "acidic aqueous solution" having a pH value of < 7. Similarly, "pH neutral" means a pH value of 7.

[0031] The term "alkaline" or "basic" is to be understood equally broadly, with an "alkaline aqueous solution" having a pH value of > 7.

[0032] The term "pH value" corresponds to the common professional understanding and is measurable in this case using a pH meter. The pH values ​​disclosed herein can be measured, for example, with a VOLTCRAFT PHT-200 combination pH and redox (ORP) meter. The following technical specifications for this meter are listed below.

[0033] • Measuring range mV: -1999 to 1999 mV

[0034] • Accuracy mV: ± 0.5%.

[0035] accuracy

[0036] ± (0.02 pH + 2 d)

[0037] Automatic temperature compensation

[0038] Yes

[0039] Working temperature

[0040] 0 ~ 50 °C

[0041] resolution

[0042] 0.01

[0043] Power supply (details)

[0044] 9 V

[0045] Dim.

[0046] (L x W x H) 68 x 30 x 195 mm

[0047] Weight

[0048] 250 g height

[0049] 195 mm

[0050] length

[0051] 68 mm wide

[0052] 30 mm

[0053] Measuring range pH value 0 - 14 pH

[0054] Calibrated

[0055] Factory standard (without certificate)

[0056] Digital display

[0057] Interfaces

[0058] RS-232

[0059] pH measuring function

[0060] Redox (ORP)

[0061] pH measuring range (max.)

[0062] 14 pH

[0063] pH measuring range (min.)

[0064] 0 pH

[0065] Product type

[0066] Combination measuring device

[0067] The named measuring device is calibrated by default using a factory standard and can also be calibrated using the pH buffer solutions (pH = 4 and pH = 7) included in the scope of delivery.

[0068] The term "acidic oxidation product" is to be understood broadly and, without wishing to be bound to a specific theory, refers to the formal proton product of the oxidation of diatomic hydrogen, in particular hydrogen gas, where this formal proton product, for example, solvated in aqueous solution as H3O + <aq), wobei Cl (aq) formal als Gegenanion vorliegen kann, und / oder als Teil eines festen Elektroden- und / oder Membranmaterials, wodurch das formale Protonenprodukt über bekannte Mechanismen transportiert wird, vorliegen kann. Neben Chlorid können in allen vorgenannten Fällen formal auch andere Gegenanionen vorliegen, die sich jeweils von den Säuren H2SO4, HCOs-, H2CO3, H3BO3 und HBr ableiten.

[0069] The term "carbonate-containing aqueous solution" is to be understood broadly here and includes carbonate in any chemical form. A "carbonate-containing aqueous solution" is typically alkaline and in particular has a pH of >7 to 9 or >7 to 9.4 or >7 to 10. This means that the aqueous solution contains dissolved carbonate CO3. 2-and / or bicarbonate. An example of a "carbonate-containing aqueous solution" is seawater. The carbonate-containing aqueous solution also contains, in particular, divalent cations such as magnesium, calcium and / or strontium. Furthermore, the carbonate-containing aqueous solution can contain monovalent cations, such as sodium. The term "seawater" is used here on a par with and equivalent to "salt water" and "saline water" is to be understood broadly and refers to an aqueous solution that has been taken from a saline body of water or a saltwater body of water, in particular a saltwater sea, such as an ocean. Of course, the exact chemical composition can vary depending on the location of the seawater extraction. However, an example is a composition such as that described by Kester, DR, Duedall, IW, Connors, DN and Pytkowicz, RM (1967). Preparation of Artificial Seawater Archived 2008-12-17.Limnology & Oceanography 12, 176-179, the contents of which are incorporated herein by reference. Tables 1 and 2 from this publication are given as approximate examples of the composition of seawater.

[0070] Table 1 : Information on the approximate composition of seawater with regard to gravimetric salts.

[0071] Gravimetric salts

[0072] Table 2: Information on the approximate composition of seawater with regard to volumetric salts.

[0073] Volumetric Salts I |

[0074] The term "degassed acidic aqueous solution" is to be understood broadly here and refers to an aqueous solution with a pH of < 7 from which carbon dioxide gas has been essentially completely removed. In particular, a degassed acidic solution can be produced by removing carbon dioxide gas from an acidic aqueous solution, e.g., by means of a membrane contactor. Such a degassed acidic aqueous solution contains, in particular, divalent cations, such as magnesium and / or calcium. In particular, < 10 mass %, preferably < 5 mass %, of carbon dioxide remains in the degassed acidic aqueous solution, wherein the percentage mass fraction of carbon dioxide refers to the original mass of carbon dioxide that was present in the form of carbonate and / or bicarbonate in the alkaline carbonate-containing aqueous solution. In other words, the separation efficiency is at least 90%, preferably at least 95%.

[0075] The term "acidic components" usually refers to protons in their aqueous form, i.e. H3O + (aq).

[0076] The term "fluidically connected" is to be understood broadly here and refers in particular to a connection, such as a pipeline, between two electrolysis units, which is designed to transport a fluid, e.g., a liquid or a gas, such as hydrogen gas, from a first to a second electrolysis unit. Such an electrolysis unit may comprise a cathode compartment, an anode compartment, or an intermediate compartment, which may be arranged between the anode and cathode compartments.

[0077] The term "gas containing carbon dioxide" is to be understood broadly here. In particular, such a gas can include air. Those skilled in the art know the composition of the air at the respective location and / or know methods for measuring this composition. Furthermore, such a gas containing carbon dioxide can also be a point source. A point source is generally an industrial source of CO2, in which more carbon dioxide gas is produced due to the process than is normally present in the air. Examples of this are flue gases from industrial processes, such as those produced in cement production and / or coal combustion. A point source can have a gas containing carbon dioxide with a carbon dioxide content of approximately 10 mol% to approximately 25 mol%, in particular approximately 14 mol% to approximately 21 mol%, based on the total amount of the moist gas. Such a gas can also have a carbon dioxide content of approximately 10 vol% to approximately 20 vol%.-%. Overall, the mass fraction / volume fraction / molar fraction of carbon dioxide in the gas containing carbon dioxide is fundamentally uncritical for the functioning of the present invention and its aspects.

[0078] An example process setup for capturing carbon dioxide from a gas containing carbon dioxide is shown in Figure 4. Various technical setups can be used to absorb the CO2. For example, for dilute CO2 concentrations, such as air, it may be more advantageous to expose the gas to the alkaline solvent in a cross-flow process and a cooling tower-like setup. At higher CO2 concentrations, such as those commonly found in flue gas in industrial processes, a column that brings the gas and the aqueous alkaline solution into contact according to the countercurrent principle may make more technical and economic sense. Furthermore, the carbonate-containing aqueous solution may contain monovalent cations, such as sodium. A carbonate-containing aqueous solution may also contain exclusively or non-exclusively monovalent cations, such as sodium or potassium.These monovalent cations can, for example, have counteranions selected from the group consisting of sulfate, perchlorate, nitrate, iodide, or combinations thereof. In particular, dissolved salts can be selected from the group consisting of Na2SO4, K2SO4, NaClO4, KClO4, NaNO3, KNO3, NaI, and K1. The term "alkaline carbonate-containing aqueous solution" is used equivalently to "carbonate-containing aqueous solution."

[0079] Brief description of the drawings

[0080] Figure 1 shows an embodiment of an electrolyzer according to the invention.

[0081] Figure 2 shows an embodiment of an electrolysis system according to the invention with a monopolar structure.

[0082] Figure 3 shows an example Pourbaix diagram of water.

[0083] Figure 4 shows an embodiment of the electrolysis system according to the invention with a removal of carbon dioxide from a gas containing carbon dioxide.

[0084] Figure 5 shows an embodiment of an electrolysis system according to the invention with a bipolar structure.

[0085] Detailed description

[0086] The embodiments shown below illustrate advantageous embodiments of the present invention, which are in no way to be understood as a limitation of the present invention. The embodiments listed within the various aspects of the invention can be freely combined with one another, unless otherwise stated. Electrolytic process

[0087] According to a first aspect, the present invention relates to an electrolytic process, in particular a continuously operated electrolytic process, for carbon dioxide production, comprising the following steps: a) anodic oxidation of hydrogen gas, whereby an acidic oxidation product is obtained; b) reacting the acidic oxidation product with an alkaline carbonate-containing aqueous solution, e.g. seawater, which has a pH of > 7 to 9, whereby an acidic aqueous solution is obtained; c) removal of carbon dioxide from the acidic aqueous solution, whereby carbon dioxide gas and a degassed acidic aqueous solution are obtained; d) cathodic reduction of acidic components of the degassed acidic aqueous solution, whereby an alkaline aqueous solution is additionally obtained, whereby an alkaline aqueous solution is additionally obtained which has a pH of 10 to > 7.1 or 9.4 to > 8.In particular, this pH value can be from 9.2 to >8, especially from 8.5 to >8. Optionally, the value can also be between 8.8 to >7.1 or, further optionally, from 8.0 to >7.1. A pH range of from 8.4 to >7.1 is also possible. In particular, this pH value is measured at the outlet of the cathode compartment using the measurement methods disclosed herein.

[0088] Anodic oxidation of hydrogen gas according to step a) can be carried out in an aqueous solution or via a gas diffusion electrode, e.g., a zero-gap electrode. The acidic oxidation product formally corresponds to H + in equation (2) below:

[0089] H22 H + + 2 e~ E° = 0V (2)

[0090] H + can be dissolved in aqueous solution as H3O + <aq) oder als Teil eines festen Elektroden- und / oder Membranmaterials vorliegen. Die tatsächliche Form des sauren Oxidationsproduktes H +is uncritical as long as it is available for step b). In certain embodiments, hydrogen gas can be oxidized in step a) at a gas diffusion electrode without using an aqueous solution, at most with minimal humidification of the hydrogen gas. Oxidation of the hydrogen gas allows for lower energy consumption within the process, since oxygen generation is not necessary. As shown in Fig. 3, the redox potential of hydrogen oxidation is significantly lower than the oxidation of water to produce oxygen.

[0091] In step b), a carbonate-containing aqueous solution is converted by means of the acidic oxidation product. For example, the oxidation product from step a) can be produced in an anode compartment, while the conversion in step b) takes place in an intermediate compartment of an electrolyzer, wherein the intermediate compartment can be arranged between a cathode and an anode compartment. The acidic oxidation product can be brought into contact with the carbonate-containing aqueous solution via a transport membrane for conversion. The carbonate-containing aqueous solution preferably has a pH of approximately > 8 to approximately 8.5. The conversion in step b) particularly results in a chemical reaction according to equation (1) above, producing dissolved carbon dioxide, which—due to the conversion by means of the acidic oxidation product—is present in dissolved form after the conversion according to step b).In the following step c), the carbon dioxide gas is removed, in particular via a membrane contactor, which can be arranged outside the intermediate space and downstream of step c). Thus, carbon dioxide is removed from the acidic aqueous solution to obtain a degassed acidic aqueous solution, which, for example, has a pH value of < 7. In step d), which takes place in particular in the aforementioned cathode compartment, acidic components, e.g., H, are removed. + <aq) dieser entgasten Sauren wässrigen Lösung sowie Wasser reduziert, wobei Wasserstoffgas und ebenso Hydroxidionen, d.h. eine alkalische wässrige Lösung welche einen pH-Wert von etwa 10 bis > 7.1 or 9.4 to > 8 - or one of the above-mentioned values. In particular, the alkaline aqueous solution has a pH of about > 7.1 to 9 or 8 to 9, preferably > 7.1 to 8.5 or 8 to 8.5, e.g., approximately 8.1. Formally, these reactions proceed according to equations (3) and (4) below:

[0092] 2 H + + 2e" 2H2. E° ® 0.247 @ pH=4 (3)

[0093] H2O + 2 Na + + 2e~ -> 2 NaOH + H2, E° = -0.487 @ pH = 8.1 (4)

[0094] In particular, the process according to the first aspect of the invention can be operated with a DC voltage of <1.5 V, in particular <1.3 V. In this case, the sodium cations from equation (4) are transferred, in particular, from the intermediate space into the cathode space, e.g., via a transport membrane. The reactions according to equations (3) and (4) within step d) lead to the following significant technical advantages of the present invention.

[0095] By achieving a pH value between 10 to > 7.1 or 9.4 to > 8 - or one of the above-mentioned value ranges - of the alkaline aqueous solution in step d), the tendency for divalent cations, such as magnesium and / or calcium, which are particularly present in the carbonate-containing aqueous solution, to precipitate as solid hydroxides is reduced. This pH range is made possible not least by the presence of the degassed acidic aqueous solution in step d). In particular, the degassed acidic aqueous solution can contain divalent cations, such as magnesium and / or calcium, as explained above. By reducing them according to equation (3) and further reducing the water according to equation (4), the alkaline aqueous solution with a pH value between 9.4 to > 7 is obtained.Outside this range, the pH would be too high, resulting in the unfavorable precipitation of said hydroxides, which would lead to the significant fouling at the cathode discussed above. By reducing this hydroxide precipitation, the divalent cations, such as calcium, magnesium, and / or strontium, can be retained in the carbonate-containing aqueous solution without the need for a costly and maintenance-intensive nanofilter to remove them. Reducing or even eliminating contamination from the precipitated hydroxides also leads to higher efficiency, as impurities typically result in losses in energy efficiency. Furthermore, excessive precipitation of hydroxide ions would have the disadvantage of adversely lowering the pH of the alkaline aqueous solution.

[0096] Reintroducing the alkaline aqueous solution into saline water, such as the ocean, is also advantageous, as the continued presence of divalent cations allows carbon dioxide to be bound again. Thus, the achieved pH value also has ecological benefits. Another ecological advantage is that no acidic wastewater needs to be discharged into saline water, such as the ocean; instead, the alkaline aqueous solution has a more environmentally friendly pH value. A pH that is too low in seawater would also lead to the release of CO2 into the atmosphere.

[0097] Surprisingly, it was also found that the partially acidic environment created by the introduction of a degassed acidic aqueous solution (see equation (2) above) at the cathode leads to a more energy-efficient generation of hydrogen gas due to kinetic and thermodynamic advantages.

[0098] Before introducing the carbonate-containing aqueous solution in step b), nitrogen and oxygen gas can be removed from the carbonate-containing aqueous solution. This is possible, for example, using a membrane contactor.

[0099] The electrolytic process according to the first aspect can be operated with an electrolyzer according to the second aspect of the present invention. The process can also be used for carbon dioxide recovery from a gas containing carbon dioxide, such as air or a point source.

[0100] In certain embodiments, the hydrogen gas produced cathodically in step d) is transferred to step a) and oxidized. In other words, a hydrogen cycle takes place within the process (oxidation in step a), reduction in step b), and further oxidation in step d). Thus, the process according to the invention can generate hydrogen largely autonomously, and the need for an external hydrogen supply is reduced. Since hydrogen production typically requires high energy consumption, said hydrogen cycle represents efficient savings.

[0101] In certain embodiments, the acidic oxidation product from step a) is transported through a first transport membrane for conversion in step b), wherein this transport membrane is in contact with the alkaline carbonate-containing solution at a point where the acidic oxidation product exits. For example, such a first transport membrane can comprise a gas diffusion electrode, a gas diffusion layer (GDL), and / or an electrode with a zero-gap membrane (CEM). A first transport membrane can also be understood as a gas diffusion layer. In particular, the first transport membrane can comprise a perfluorosulfonic acid membrane. Preferably, such a transport membrane can be based on a perfluorosulfonic acid / polytetrafluoroethylene copolymer.Materials for transport membranes can furthermore or alternatively be selected in particular from the group consisting of: PTFE / PTFE- (polytetrafluoroethylene / Teflon)-based membranes, hydrocarbon membranes, sPPS (sulfonated polyphenylene sulfone) membranes. Examples of these are, in particular, membranes familiar to the person skilled in the art, which are sold under the names Nation, Gore, Fumasep, Fumapem, Aquivion, and / or Xion, with a Nafion membrane or a Gore Select membrane being preferred. In particular, the acidic oxidation product can be produced in the anode compartment in step a) and diffuse through such a first transport membrane, after which the reaction according to step b) takes place in the intermediate space.

[0102] In certain embodiments, metal cations, e.g., sodium cations, are fed from the carbonate-containing aqueous solution during the reaction via a second transport membrane to step d) of cathodic reduction. The second transport membrane can, in particular, have the above-mentioned features of the first transport membrane. In particular, sodium cations can be transported from the carbonate-containing aqueous solution from the intermediate space via a second transport membrane into the cathode space, where step d) can be carried out. These cations formally serve to equalize the charge according to equation (4) above.

[0103] In certain embodiments, the pH of the degassed aqueous solution is <5. In particular, this pH is approximately 2 to approximately <5, further preferably 3 to approximately 4.5. This can be specifically achieved by the anodic oxidation in step 1. The above advantages can be achieved by said acidic pH.

[0104] According to certain embodiments, the degassed acidic aqueous solution in step d) can also be contacted with an alkaline aqueous solution. For example, through the reduction according to equation (4), an alkaline aqueous solution can accumulate in the cathode compartment over time, leading to a pH gradient because the degassed acidic aqueous solution is acidic at the inlet.

[0105] Electrolyzer for carbon dioxide production

[0106] According to a second aspect, the present invention relates to an electrolyzer for carbon dioxide recovery comprising:

[0107] • an anode chamber,

[0108] • a gap, and

[0109] • a cathode chamber, wherein the intermediate space is arranged between the anode chamber and the cathode chamber; the anode chamber is connected to the intermediate space via a first transport membrane; the cathode chamber is connected to the intermediate space via a second transport membrane; the anode chamber and the cathode chamber are fluidically connected via a hydrogen gas line; the intermediate space has an inlet and an outlet, wherein the outlet is fluidically connected to a carbon dioxide extraction device and the carbon dioxide extraction device is directly fluidically connected to an inlet of the cathode chamber via a liquid line.

[0110] Of course, the electrolyzer comprises the features and technical effects of the electrolytic process. In particular, the electrolyzer according to the second aspect of the present invention is configured to carry out the electrolytic process according to the first aspect of the present invention. Accordingly, the electrolyzer can also be used for carbon dioxide extraction from seawater.

[0111] Furthermore, the electrolyzer can be used in a process for carbon dioxide recovery from a gas containing carbon dioxide, such as air or a point source. In this case, a solvent-air contactor as described herein can be used.

[0112] In particular, the anode compartment is configured to carry out step a) of the method according to the first aspect of the present invention. Furthermore, the intermediate compartment is configured to carry out step b) of the method according to the first aspect of the present invention. Accordingly, the cathode compartment is configured to carry out step d) of the method according to the first aspect of the present invention. Furthermore, the carbon dioxide removal device is configured to carry out step c) of the method according to the first aspect of the present invention.

[0113] In certain embodiments, the carbon dioxide removal device is selected from the group consisting of membrane contactors, preferably 3M Liqui-Cel, heat exchangers, and combinations thereof. Preferably, a membrane contactor is used. In the case of a heat exchanger, it is configured to heat the acidic aqueous solution to enable the outgassing of carbon dioxide gas.

[0114] In particular, the first transport membrane is an ion transport membrane designed to transport the acidic oxidation product from the anode compartment to the cathode compartment. Additionally or alternatively, the second transport membrane can be designed as an ion transport membrane designed to transport monovalent cations, such as sodium cations, from the intermediate compartment to the cathode compartment.

[0115] In particular, it follows from the previous description that the electrolyzer does not have a nanofilter.

[0116] Furthermore, in particular, no further operations take place between the outlet of the intermediate space and the inlet of the cathode compartment other than the carbon dioxide removal. In particular, a fluidic overall connection between the outlet of the intermediate space and the inlet of the cathode compartment does not provide for any further mixing device, with the pH of the acidic aqueous solution and the degassed acidic aqueous solution remaining essentially constant.

[0117] In certain embodiments, the anode chamber comprises an anode material that is in direct contact with the first transport membrane. The anode active material is in particular selected from the group consisting of platinum, nickel / iron, nickel / cobalt, nickel, cobalt / platinum, stainless steel, iridium, iridium oxide, ruthenium, ruthenium oxide, and palladium, and combinations thereof. The anode active material is preferably platinum. The anode material can be configured as a zero-gap electrode, with no gap at all between the anode material and the first transport membrane. Furthermore, the anode material can be applied to supports. Such supports can be selected from the group consisting of iron, steel, titanium, and carbon paper, or combinations thereof. Alternatively, the anode can be configured as a gas diffusion electrode. The gas diffusion electrode can, for example, comprise product variants from the Gore Primea series.

[0118] In certain embodiments, a device for removing oxygen and nitrogen from the alkaline carbonate-containing aqueous solution can be provided upstream of the entrance to the intermediate space. Such a device can be designed analogously to the carbon dioxide removal device.

[0119] In certain embodiments, the anode chamber may include an external inlet for hydrogen gas. This allows any hydrogen deficiencies that might occur during a hydrogen cycle to be compensated for.

[0120] In certain embodiments, the cathode compartment may have a side facing away from the second transport membrane, with the entrance of the cathode compartment being located closer to the side facing away from the second transport membrane. This can lead to alkalization of the aqueous solution in the cathode compartment, creating a high pH gradient between the cathode and the second transport membrane. This significantly improves the transport of cations into the cathode compartment.

[0121] In certain embodiments, the inlet of the cathode compartment and the inlet of the intermediate space are arranged such that a liquid inflow entering the inlet of the cathode compartment flows through the electrolyzer in countercurrent or cocurrent to a liquid inflow entering the inlet of the intermediate space. In particular, countercurrent flow has the advantage that the pH gradient has the same sign along the gradient and positively charged cations move into the cathode compartment.

[0122] In certain embodiments, the cathode chamber comprises a cathode material, and the cathode material is preferably selected from the group consisting of platinum, nickel, titanium, carbon paper, or a combination thereof. In particular, platinum is used as the preferred cathode active material. Carbon paper with a deposited Pt / C catalyst is particularly preferred.

[0123] In certain embodiments, the process according to the first aspect of the invention is operated at a total pressure above atmospheric pressure, in particular at a total pressure between 2 bar and 50 bar.

[0124] The electrolyzer according to the first aspect of the present invention can be operated at <100°C. In particular, the electrolyzer can be operated at 60 to 80°C. Furthermore, it is possible for the electrolyzer to be operated at a temperature of at least 95°C and below 100°C.

[0125] Electrolysis system

[0126] According to a third aspect, the present invention relates to an electrolysis system comprising at least one electrolyzer according to the second aspect of the present invention.

[0127] Of course, such an electrolysis system can be configured to carry out the method according to the first aspect of the present invention. The electrolysis system accordingly comprises the method steps according to the first aspect of the invention, as well as the corresponding technical advantages. The same applies to the electrolyzer according to the first aspect of the invention.

[0128] For example, multiple electrolyzers can be used together within the electrolysis system. These multiple electrolyzers can be connected in a stacked configuration. For example, two electrolyzers can be linked together via a shared anode compartment. Another electrolyzer can be linked to these two electrolyzers via a shared cathode. Another electrolyzer can, in turn, be linked to this other electrolyzer via a shared anode compartment, and so on.

[0129] Use of the electrolyzer

[0130] According to a fourth aspect, the present invention relates to a use of an electrolyzer according to the second aspect of the present invention in a plant for the electrolytic production of carbon dioxide from seawater.

[0131] According to a sixth aspect, the present invention relates to the use of an electrolyzer according to the second aspect of the present invention in a plant for the electrolytic production of carbon dioxide from a gas containing carbon dioxide, in particular from air or a point source. In this case, a solvent-air contactor as described herein can be used. The use according to the fourth or sixth aspect comprises the technical features and effects, as well as advantages, of the method according to the first aspect and the electrolyzer according to the second aspect, respectively.

[0132] Plant for the electrolytic production of carbon dioxide from seawater or a gas containing carbon dioxide

[0133] According to a fifth aspect, the present invention relates to a plant for the electrolytic production of carbon dioxide from seawater comprising an electrolyzer according to the second aspect of the present invention or an electrolysis system according to the third aspect of the present invention.

[0134] Such a system can be located near a saline body of water, such as the sea. This system can also be operated with seawater as a carbonate solution.

[0135] According to a seventh aspect, the present invention relates to a plant for the electrolytic production of carbon dioxide from a gas containing carbon dioxide, in particular from air or a point source, comprising an electrolyzer according to the second aspect of the invention or an electrolysis system according to the third aspect of the present invention. In this case, a solvent-air contactor as described herein can be used.

[0136] Furthermore, this system according to the fifth or seventh aspect can also be operated with the method according to the first aspect of the present invention. To avoid repetition, it should be noted that the system has the features as well as technical advantages and effects according to the first, second, third, and fourth aspects of the invention.

[0137] Description of the drawings

[0138] The following figures show exemplary embodiments of the present invention and are therefore in no way to be interpreted as limiting.

[0139] Fig. 1 shows an exemplary embodiment of an electrolyzer 1 according to the second aspect of the present invention for extracting carbon dioxide from seawater. Fresh alkaline seawater with a pH of approximately 8.1 is fed via a first feed line 13 to a first membrane contactor 17a, through which oxygen and nitrogen are removed from the seawater. Via a second feed line 14, the alkaline seawater is introduced into an intermediate space 51, wherein the intermediate space 51 is arranged between an anode space 50 containing an anode 45 and a cathode space 52 containing a cathode 46. The anode space 50 and the intermediate space 51 are separated by a first ion transport membrane 61, which is configured to transport protons from the anode space 50 into the intermediate space 51.The intermediate space 51 and the cathode space 52 are separated by a second ion transport membrane 62, which is designed to transport sodium ions from the intermediate space 51 into the cathode space 52. These electrochemical reactions are driven by an alternating voltage source 44. Within the anode space 50, hydrogen gas is oxidized at the anode 45 to form an acidic oxidation product. This acidic oxidation product is transferred via the first ion transport membrane 61 into the intermediate space 51, where it reacts with the seawater to produce an acidic aqueous solution. The hydrogen gas for the oxidation is transported, for example, via hydrogen line 23 from the cathode space 52, where it is formed at the cathode 46, to the anode space 50. The acidic aqueous solution obtained in the intermediate space contains dissolved carbon dioxide, which is obtained according to equation (1).Via a first outlet line 4, the acidic aqueous solution (pH approximately 4) is fed to second and third membrane contactors 17b, 17c, where carbon dioxide gas is removed. A degassed acidic aqueous solution (pH approximately 4) is also contained, which is transferred via a second outlet line 5 to the cathode compartment 52. Within the cathode compartment, a pH gradient exists (light for lower pH, darker for higher pH), which is created by the degassed acidic aqueous solution being introduced into a cathode inlet 55, while hydroxide ions are produced within at the cathode 46. Depending on the gradient within the cathode compartment 52, the degassed acidic aqueous solution is neutralized via the hydroxide ions. Furthermore, the pH value also increases due to the consumption of formal protons to form hydrogen according to equation (3). This creates a pH value of > 8 within the cathode compartment.1 which can be returned to the sea via a seawater discharge line 11.

[0140] Fig. 2 shows an example of a monopolar structure of an electrolysis system 100a according to the third aspect of the present invention. The arrows indicate the material inputs and outputs already shown in Fig. 1. Accordingly, only the parts relevant to the present discussion are identified by reference numerals. A first electrolyzer 1a and a second electrolyzer 1b are linked via their anode compartment 50. Furthermore, a third electrolyzer 1c is linked to the second electrolyzer 1b via the cathode 46. A further electrolyzer 1n can, in turn, be linked to the third electrolyzer 1c via the anode compartment 50 according to the aforementioned stacking technique, which is indicated by the three dots in Figure 2, and so on.

[0141] Fig. 3 shows a Pourbaix diagram for water for illustration purposes.

[0142] Figure 4 shows an exemplary embodiment of the inventive system 200 according to the third aspect of the invention, with which a method according to the first aspect of the invention can be carried out. Via an air supply system 30, a gas, which can be passed, for example, through an air-liquid contactor 31, is supplied to the system 200 by absorption in an alkaline aqueous solution. The resulting carbonate-containing aqueous solution is then mixed with an acidic aqueous solution in a mixing tank 32, so that, for example, carbon dioxide gas can be removed from the system via one or more membrane contactors 33. The acidic degassed solution is then fed, in a first and a second part, each to the electrolyzer 34 according to the second aspect of the invention.The first portion of the acidic, degassed solution is, for example, fed to an intermediate chamber 52, where an acidic oxidation product from the anode chamber 51 is added to the acidic, degassed aqueous solution. The second portion of the acidic, degassed aqueous solution is reduced in a cathode chamber 53, producing an alkaline aqueous solution. Subsequently, the hydrogen gas can be separated from the alkaline aqueous solution in a gas-water separator 35, and the hydrogen can be fed to the anode chamber 51, where it is oxidized into protons according to the above reaction equation, which are then fed to the intermediate chamber 52, for example, via the first ion transport membrane 61.

[0143] Fig. 5 shows an example of a bipolar structure of an electrolysis system 100b according to the third aspect of the present invention. The arrows indicate the material inputs and outputs already shown in Fig. 1. Accordingly, only the parts relevant to the present discussion are identified by reference numerals. Here, a first electrolyzer 1a and a second electrolyzer 1b are linked via their anode compartment 50. A further electrolyzer 1n can, in turn, be linked to an adjacent electrolyzer via an anode compartment 50 according to the aforementioned stacking technique, which is indicated by the three dots in Figure 2, etc.

[0144] Examples

[0145] Experimental setup

[0146] The experiments were conducted in a system that may correspond to the setup shown in Figure 4. For the experiments, an electrolysis cell consisting of two steel end plates, each with a graphite current collector with an integrated flow field on the anode side, and a titanium grid as a current collector on the cathode side, was used as the electrolyzer according to the second aspect of the invention. The flow field for the intermediate space, with a thickness of 1 mm, is made of PTFE. Gaskets made of PTFE and FKM were used for the seals. Two Nafion cation exchange membranes were used. A carbon fiber diffusion medium was used as the diffusion media in each case. In addition, platinum on carbon was used as the catalyst on both electrodes. On the anode side, the catalyst was applied to the gas diffusion medium and on the cathode side directly to the membrane using the so-called decal process.

[0147] The electrolyte used for the experiment was 0.5 M concentrated sodium chloride in distilled water with a conductivity of 30 mS / cm.

[0148] A Zennium Pro potentiostat / galvanostat from Zahner Elektrik was used for both the power supply and the measurement.

[0149] A Shenchen LabN6l 11 peristaltic pump with two pump heads was used to move the liquid. The electrolyte flow rate was set to 80 mL / min. In addition to the hydrogen separated using the gas-water separator, the external hydrogen supply was controlled by a Bronkhorst F201-CV mass flow controller.

[0150] Experiment description

[0151] The electrolysis cell was constructed from two graphite flow fields at both the anode and cathode. Two types of FKM-based gaskets, each 0.2 mm and 0.3 mm thick, were also used. The 0.3 mm thick gaskets were placed between the graphite flow fields and the membranes. The thinner 0.2 mm thick gaskets were placed between the membrane and the interstitial flow field.

[0152] The flow rate was kept constant at 80 mL / min for all experiments. The active area of ​​the cell is 10.2 cm 2 and the applied current was varied between 0.02 A, 0.05 A and 0.1 A for the experiments.

[0153] Various fluidic flow patterns

[0154] In the two series of experiments conducted, only the fluid flow pattern was modified. In the first experiment, the fluid flow to both the cathode compartment and the intermediate compartment was fed from the same container containing the 0.5 M electrolyte in deionized water. The electrolyte had a pH of 7.3 before being fed into the cell.

[0155] In a second experiment, the outlet of the intermediate space was fluidically connected to the inlet of the cathode compartment. Thus, the pH value at the outlet of the intermediate space is the same as the pH value at the inlet of the cathode compartment, which represents the key difference from the first experiment.

[0156] Results

[0157] In the tests carried out according to the present invention, the voltages were measured for the respective varying, differently applied currents as described above. The test results for the introduction of the same solution with an identical, slightly basic pH value into the gap and into the cathode are shown in Table 3. Table 4 shows the measured values ​​for the system according to the present invention. The pH value at the exit of the gap is identical to the pH value at the entrance to the cathode compartment, since these two are fluidically connected. Thus, pHzwi,ouT = pHKatnode.iN applies here. Table 3: Measured cell voltages and pH values ​​with the introduction of identical solution, whereby the pH value at the entrance to both the cathode and the gap is the same.

[0158] * Comparative example determined according to the data from the experimental setup and execution as described in L. Yan et al., ACS Energy Lett. 2022, 7, 1947-1952.

[0159] Table 4: Measured pH values ​​at the inlets and outlets of the intermediate and cathode compartments according to the present invention.

[0160] The results in Table 3 show that the results for the comparison example are qualitatively consistent. The difference in the strength of the pH swing may be explained by the fact that the flow rate in the comparison example was only half as high at 40 mL / min. Both examples show pH values ​​greater than 10 at a current density of only 10 mA / cm 2This would mean that a significant precipitation of divalent ions can be expected here. Surprisingly, according to Table 4, it was found that by diverting the acidic solution into the cathode compartment according to the invention, the pH value remains significantly lower. So low that a mineral precipitation of divalent cations such as Ca 2+ or Mg 2+ can be avoided. Another notable observation is that the measured cell voltage for all three measured current densities is significantly lower than the voltages of the comparison experiments due to the diversion, which allows the process to be operated significantly more energy-efficiently.

[0161] I Electrolyzer

[0162] 1a First electrolyzer

[0163] 1 b Second electrolyzer

[0164] 1c Third electrolyzer

[0165] 1 n Additional electrolyzer

[0166] II Seawater discharge line

[0167] 13 First entry line

[0168] 14 Second entry line

[0169] 17a First membrane contactor

[0170] 17b Second membrane contactor

[0171] 17c Third membrane contactor

[0172] 23 Hydrogen pipeline

[0173] 30 Air supply

[0174] 31 Air-Solvent Contactor

[0175] 32 mixing containers

[0176] 33 Desorption device

[0177] 34 Electrolyzer

[0178] 35 Gas-water separator

[0179] 44 DC voltage source

[0180] 45 Anode

[0181] 46 Cathode

[0182] 50 Anode compartment

[0183] 51 space

[0184] 52 Cathode compartment

[0185] 55 Cathode input

[0186] 61 First ion transport membrane

[0187] 62 Second ion transport membrane a Electrolysis system according to monopolar design b Electrolysis system according to bipolar design Direct Air Capture system

Claims

Claims 1 . An electrolytic process for carbon dioxide production, comprising the following steps: a) anodic oxidation of hydrogen gas, whereby an acidic oxidation product is obtained; b) reacting the acidic oxidation product with an alkaline carbonate-containing aqueous solution, which in particular has a pH of > 7 to 9, whereby an acidic aqueous solution is obtained; c) removal of carbon dioxide from the acidic aqueous solution, whereby carbon dioxide gas and a degassed acidic aqueous solution are obtained; d) cathodic reduction of acidic components of the degassed acidic aqueous solution in order to obtain cathodically generated hydrogen gas, whereby an alkaline aqueous solution is additionally obtained which has a pH of 10 to > 7.1 or 9.4 to > 8.

2. The process according to claim 1, wherein the hydrogen gas cathodically produced in step d) is transferred to step a) and oxidized.

3. The process according to claim 1 or 2, wherein the acidic oxidation product from step a) is transported through a first transport membrane for reaction in step b), wherein the transport membrane is in contact with the alkaline carbonate-containing aqueous solution at a point of exit of the acidic oxidation product.

4. Process according to one of the preceding claims, wherein metal cations from the carbonate-containing aqueous solution are fed to step d) of cathodic reduction during the reaction via a second transport membrane.

5. Process according to one of the preceding claims, wherein the pH of the degassed acidic aqueous solution is < 5.

6. A process according to any one of the preceding claims, wherein the degassed aqueous acidic solution is contacted with an alkaline aqueous solution in step d).

7. Process according to one of the preceding claims, wherein the process is operated at a total pressure above atmospheric pressure, in particular at a total pressure between 2 bar and 50 bar.

8. Electrolyzer for carbon dioxide production comprising: an anode chamber, a gap, and • a cathode chamber, wherein the intermediate space is arranged between the anode chamber and the cathode chamber; the anode chamber is connected to the intermediate space via a first transport membrane; the cathode chamber is connected to the intermediate space via a second transport membrane; the anode chamber and the cathode chamber are fluidically connected via a hydrogen gas line; the intermediate space has an inlet and an outlet, wherein the outlet is fluidically connected to a carbon dioxide extraction device and the carbon dioxide extraction device is directly fluidically connected to an inlet of the cathode chamber via a liquid line. Electrolyzer according to claim 8, wherein the anode chamber has an anode material that is in direct contact with the first transport membrane, and the anode material in particular comprises platinum.Electrolyzer according to claim 8 or 9, wherein a device for removing oxygen and nitrogen from an alkaline carbonate-containing aqueous solution is arranged upstream of the inlet of the intermediate space. Electrolyzer according to one of claims 8 to 10, wherein the anode space comprises an external inlet for hydrogen gas. Electrolyzer according to one of claims 8 to 11, wherein the cathode space has a side facing away from the second transport membrane and the inlet of the cathode space is arranged closer to the facing away side than to the second transport membrane. Electrolyzer according to one of claims 8 to 12, wherein the inlet of the cathode space and the inlet of the intermediate space are arranged such that a liquid inflow entering the inlet of the cathode space flows through the electrolyzer in countercurrent or cocurrent to a liquid inflow entering the inlet of the intermediate space.The electrolyzer according to any one of claims 8 to 13, wherein the cathode chamber comprises a cathode material, and the cathode material is selected from the group consisting of nickel, titanium, or a combination thereof. The electrolysis system comprises at least one electrolyzer according to any one of claims 8 to 14. Use of an electrolyzer according to one of claims 8 to 14 in a plant for the electrolytic production of carbon dioxide from seawater. Plant for the electrolytic production of carbon dioxide from seawater comprising an electrolyzer according to one of claims 8 to 14 or an electrolysis system according to claim 15. Use of an electrolyzer according to one of claims 8 to 14 in a plant for the electrolytic production of carbon dioxide from a gas containing carbon dioxide, in particular air or a point source. Plant for the electrolytic production of carbon dioxide from a gas containing carbon dioxide comprising an electrolyzer according to one of claims 8 to 14 or a Electrolysis system according to claim 15.

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