System for direct electrochemical reduction of carbon dioxide to carbon

Abstract

The invention relates to a modularly constructed reaction unit, consisting of a modularly constructed electrochemical cell, which comprises an anode, a cathode, and an electrolyte and is operated at higher temperatures, and a suitably modularly constructed electronic device, which provides the current or voltage programs required to operate the electrochemical cell. The aim of the method is to introduce carbon dioxide directly into the modularly constructed reaction unit at the locations where the gas accumulates at so-called point sources in a sufficient concentration (greater than 10% based on the respective total volume of the gas produced in the process) during a technical process or another process, for example a combustion process, and to convert the carbon dioxide in the reaction unit preferably into pure carbon with a different degree of quality. In the context of a sustainable method, the electronic unit and the electrochemical cell are constructed in such a way that the reduction of carbon dioxide by means of alternatively obtained electrical energy can be carried intermittently using special current or voltage profiles. Solar energy or wind energy are therefore preferably usable. However, the electrical energy obtained in a technical process can also be partly used to reduce carbon dioxide. By suitably varying the deposition potentials and the used current densities (by means of pulsed technology), carbon with different grain sizes and a different crystallinity can be sustainably produced directly from carbon dioxide with high faradaic efficiency for various technical applications.

Description

[0001] PLANT AND METHOD FOR CARBON EXTRACTION

[0002] The invention relates to a modularly constructed reaction unit, consisting of a modularly constructed electrochemical cell with anode, cathode and electrolyte, which is operated at higher temperatures, and of a suitably modularly constructed electronic device that provides the current or voltage programs necessary for the operation of the electrochemical cell.

[0003] The aim of the process is to introduce carbon dioxide directly into the modular reaction unit at locations where this gas is produced in sufficient concentration (greater than 10% of the total volume of the gas generated) at so-called point sources during a technical or other process, such as combustion. There, it is preferably converted into pure carbon of varying purity. To ensure sustainability, the electronic unit and the electrochemical cell are designed so that the carbon dioxide reduction can be operated intermittently using alternatively generated electrical energy. The use of solar or wind energy is therefore preferred. However, electrical energy generated during a technical process can also be partially used for carbon dioxide reduction.

[0004] Based on the modular design of the overall electrochemical setup, the process can be scaled up even for high volume flows. The properties of the carbon electrochemically produced at the cathode can subsequently be adapted for a wide variety of industrial applications by adjusting the electrochemical reduction conditions. For example, it is possible to produce carbon with different grain sizes ranging from a few nanometers to several hundred micrometers by appropriately varying the deposition potentials and current densities used (employing pulse techniques), or to modify the crystallinity of the carbon from largely amorphous to crystalline.

[0005] State of the art

[0006] In recent decades, the problem of the globally increasing emissions of so-called greenhouse gases, particularly carbon dioxide, has been extensively discussed in countless scientific publications and conferences (see the climate conferences held to date, the most recent one in Abu Dhabi). Most considerations and technical concepts focus on two main approaches: firstly, the removal of carbon dioxide from the atmosphere using energy-intensive processes, and secondly, the separation of carbon dioxide from exhaust gases using scrubbing processes or membrane technologies, followed by its storage, for example, in underground reservoirs. Both approaches require a relatively high energy input, especially since, from a thermodynamic perspective, all these processes involve endothermic reaction steps.Other proposed processes attempt to directly produce essential chemical building blocks such as methanol or short-chain hydrocarbons from carbon dioxide and hydrogen produced using alternative energy sources via electrochemical processes. These processes utilize carbon dioxide from the air as well as from so-called point sources such as exhaust gases from cement plants or steel production, etc.

[0007] A disadvantage of all these processes lies in their generally low electrochemical efficiency and lack of selectivity in the reaction processes, resulting in a mixture of reaction products that usually requires one or more process steps for separation and purification. Furthermore, many processes necessitate the use of expensive catalyst materials.

[0008] Detailed descriptions of ongoing or completed projects on this topic, including those in Germany, can be found in reports from the Karlsruhe Institute of Technology (KIT) from 2021 and 2022.

[0009] Recent studies describe, among other things, the production of pure carbon by direct electrolysis in carbonate melts (BPThapaliya et al., ACS Appk Mat. Interfaces 2024, 16, 2251). This approach is also pursued in the present invention, whereby the inventive implementation of the process and the reaction parameters used go beyond the process described in the aforementioned work.

[0010] In US 2023366104 (AI) the direct electrochemical production of graphene by reduction of carbon dioxide in carbonate melts is described, with a general discussion of the possibilities of production and in particular the problem of nucleation of transition elements ar? of the cathode.

[0011] CN 11.2030177 (A) reports on the production of spiral carbon nanotubes by constant current or constant voltage from a carbonate melt at a carbonate cathode, with the products being obtained in a largely homogeneous state.

[0012] CN 110923776 (A) describes the production of metal carbides on high-melting-point cathode materials using carbon dioxide directly in an electrochemical cell, where these carbides are said to have good catalytic properties for the electrochemical production of hydrogen.

[0013] WO2023181794 (AI) reports on the “Fixation method” of carbon dioxide using seawater by means of electrolysis and the use of a cation exchanger, whereby the electrolysis preferentially produces carbonates and largely prevents the production of magnesium hydroxide.

[0014] In another variant, reported in WO 2.023087370 (AI), an alkaline solution is used to collect carbon dioxide, which is then electrolytically reacted, forming hydroxides and hydrogen at the cathode and bicarbonates and an oxygen / carbon dioxide mixture at the anode. The hydroxides are recirculated, and pure carbon dioxide can be obtained from the bicarbonates.

[0015] RU2011103157 (A) describes the removal of carbon dioxide from exhaust gas streams via fogging and the electrolytic production of sodium hydroxide from a protonated solution and the production of bicarbonates. The process is thus implemented in aqueous media.

[0016] US Patent 2021108320 (AI) describes an electrochemical arrangement for the formation and purification of solid carbon from carbonate electrolytes, using carbon dioxide to regenerate the carbonate electrolyte. The arrangement largely corresponds to the usual setup of an electrochemical production cell.

[0017] CN117504766 (A) describes the electrochemical production of sodium percarbonate and hydrogen peroxide, wherein the sodium carbonate is continuously supplied from a carbon dioxide source. The process also takes place in an aqueous phase.

[0018] Description of the invention

[0019] The object of the invention is the efficient and sustainable production of carbon in crystalline or amorphous form by direct electrochemical reduction of carbon dioxide, preferably from so-called point sources such as biogas plants or other production plants where carbon dioxide is produced in concentrations of at least 10 vol% based on the total volume of the exhaust gas, and preferably using alternatively obtained electrical energy, in particular by using solar or wind energy.

[0020] Basically, this electrochemical reduction of carbon dioxide to carbon requires carbonate-containing electrolyte©, stable anodes and cathode materials, and, when using molten electrolytes at higher temperatures, a well thermally insulated electrochemical reaction cell made of a corrosion-resistant material such as aluminum oxide or carbides.

[0021] In accordance with the invention, an external cathodic protection system is provided for the reactor vessel using high-alloy, corrosion-optimized stainless steel©. This system uses anodes made of corrosion-resistant material positioned in the immediate vicinity of the reactor wall – the necessary distance of a few millimeters to several centimeters is maintained by spacers – to cathodically polarize the reactor vessel. The detailed design of such a cathodic protection system is familiar to those skilled in the art and is therefore not described in detail here.

[0022] This cathodic protection can be omitted if, in an embodiment according to the invention, the reactor vessel is directly connected as the cathode for the electrochemical reduction of carbon dioxide. In this case, the carbon dioxide-containing reaction gas is supplied via ring mains with openings in the range of 1 mm. 2 up to 25mm 2, arranged in a ring shape along the inner wall in the middle and lower area of ​​the electrochemical reaction vessel, the cathode.

[0023] The reactor vessel can also be configured as a cathode in a double-walled version, formed from two nested concentric vessels with a wall spacing of 3 mm to 50 mm, preferably 10 mm to 20 mm. Vertically arranged gas guides are provided at intervals of 20 mm to 200 mm, preferably 30 mm to 70 mm according to the invention, to guide the reaction gas in the space between the concentric vessels, depending on the actual size of the reactor vessel. The inner vessel has slot-shaped, angular, or round openings in its lower and middle regions for the exit of the reactor gases. These openings can comprise up to 50% of the inner surface area of ​​the reactor vessel. The reactor gas is supplied either from above via a ring main for the electrochemical reaction or from below via lateral inlets and openings.The reactor vessels are shielded from the molten electrolyte by a gas-permeable ceramic membrane ("frit"). In both variants, the reactor vessel is intermittently vibrated by an ultrasonic or vibration sensor to remove the carbon formed on the cathode wall. This carbon can then be collected, for example, via a suction line through a lateral opening located in the upper part of the reactor vessel, or it settles in the electrolyte due to its higher density and is then removed discontinuously from the reactor.

[0024] In the classic version, when an alkali carbonate mixture with a eutectic composition, such as lithium carbonate (43.5%), sodium carbonate (31.5%), and potassium carbonate (25%), is used at temperatures above 450°C, excellent insulation of the cell is necessary, especially for intermittent operation of the electrolysis tents as provided for in the invention. This insulation is preferably made of multiple layers of mineral fiber insulation material with intermediate layers of metallic foils, thus ensuring that the cell temperature can be kept largely stable for at least a period of 24 hours.A temperature gradient of less than 3°C / 24 h is reliably ensured with multi-layer insulation, 'as can be achieved when carried out by a skilled professional.' Therefore, if the cell is switched off for a longer period due to a lack of alternatively generated electrical energy, it can be assumed that the eutectic melt will not solidify even during a prolonged standstill of the electrolysis and that further operation can be resumed without disruption.

[0025] Essential components of the process according to the invention lie in the design of the electrochemical reaction line and the cathode or anode materials (a), the possibility of using electrolytes based on ionic carbonate organic salts with melting points below 350°C (b), and in the use of special current or voltage profiles for operating the electrochemical reaction time (c).

[0026] Ad (a)

[0027] According to the invention, the electrochemical reaction cell can be modularly constructed in four variants A, B, C, and >. A characteristic feature of these variants is that the inventive design enables the continuous production of pure carbon. The modular construction means that the respective variants of the reaction cells can be set up and loaded in parallel, allowing a high volume flow of carbon dioxide-containing gas to be distributed across multiple cells. In an additional variant according to the invention, a cell can also be designed and loaded with bipolar electrodes. Further details regarding this will be discussed in the section on variant B. Variant A utilizes a horizontal arrangement of the anode 1 and the cathode 2 in a thermally insulated reactor vessel 3 with supply lines 4 for the carbon dioxide reaction gas.The holders for anode 1 and cathode 2 are made of corrosion-resistant stainless steel with chromium content above 15% and nickel content above 9%, such as HR3C, 316H, or 304L. Carbon dioxide is introduced via the laterally arranged supply lines 4, which are immersed in the molten electrolyte 5 and are shielded against electrolyte ingress by a ceramic membrane 17 (“frit”).

[0028] The anode 1 can be flat or cut lengthwise as a 120 cGrad-tube section with a diameter of up to 500 mm made of cast carbon, graphite, reactor graphite, or other oxidation-resistant materials such as titanium or titanium alloys, or iridium-coated steel variants, or stainless steel variants, or stainless steel variants with a diamond-coated surface (nanoscale or microscale diamonds deposited by CVD or PVD processes or electrochemically by dispersion processes and embedded in oxidation-resistant metallic support layers) with thicknesses between 2 mm and 15 mm, preferably between 5 mm and 10 mm for sufficient mechanical stability. According to the invention, a perforated structure with holes having an average diameter of 2 to 20 mm is provided. A structure with longitudinal slots with a width between 0.5 mm and 1 cm and a length between 2 mm and 50 mm is also provided. According to the invention, these slots preferably have diameters between 3 mm and 10 mm.A width between 1 mm and 5 mm and a length between 10 mm and 30 mm is also possible.

[0029] Furthermore, the anode 1 is designed such that the gas produced at the anode during electrolysis, namely oxygen, is directed by means of a funnel-shaped gas-conducting structure 6 on the top of the anode 1 to the gas outlet opening 7 located on the top of the reactor vessel 3. This inventive arrangement makes it possible to collect, compress, and store the pure oxygen produced during electrolysis and thus make it available for various applications.

[0030] The cathode 2 can be made of stainless steel variants, nickel alloys or nickel foam structures, or steel variants with a specially manufactured nanoscale carbon-containing surface structure, wherein, as with the anode 1, a structure with circular recesses with diameters between 1 mm and 250 mm is realized, or alternatively, a structure with longitudinal slots with a width between 0.5 mm and 2 cm and a length between 3 mm and 100 mm. Preferably, according to the invention, with diameters between 3 mm and 20 mm, or with a width between 1 mm and 5 mm and a length between 10 mm and 30 mm.

[0031] According to the invention, the surface of the cathode 2 is coated with a nanoscale carbon layer by cathodic pulse loading of the steel variants (preferably steels with high mechanical strength) in the carbonate melt. The carbon particles can have dimensions between 3 nm and 5000 nm, depending on the electrochemical parameters used for electrolysis. Carbon particles with dimensions between 5 nm and 500 nm are preferred. This is achieved by adjusting the pulse parameters of the electrolysis current or potential. According to the invention, current pulses in the millisecond range (10 to 500 milliseconds) with current densities up to 500 A / cm are used, with relaxation times between 1 second and 10 seconds. Potential pulses range between 5 volts and 30 volts, with pulse durations between 0.5 milliseconds and 10 seconds.Pulse lengths between 5 milliseconds and 300 milliseconds are preferred, with pulse amplitudes increasing in steps up to 15 volts. The resulting cathode surface structures, in combination with the electrochemical deposition parameters, have a significant influence on the crystallinity of the carbon produced.

[0032] In another embodiment of the cathode 2 surface, the crystallinity of the cathodically deposited carbon is also controlled. In this embodiment, the cathode 2 is coated with a nanoscale nickel layer in an external electrolyte. A water bath at a temperature of approximately 50°C is used as the electrolyte. The current densities range from 600 A / m². 2and 800 A / nr. The thickness of this Ni layer is between 5 nm and 5000 nm, with the nickel particles being deposited in a dimension between 5 nm and 500 nm. According to the invention, a layer thickness between 20 nm and 200 nm is targeted with a dimension of the Ni particles between 20 nm and 500 nm, preferably between 15 nm and 150 nm.

[0033] If this cathode 2 is used for carbon dioxide reduction, carbon layers and carbon particles with high crystallineity are preferentially formed under specially used current densities and potential programs.

[0034] In a further embodiment of the surface modification of the cathode 2 according to the invention, the surface of the steel or stainless steel used is anodically oxidized in a defined manner in an external alkaline solution using a pulsed process, thereby also achieving a nanoscale surface structure. This structure is characterized by the formation of nanoscale magnetite and hematite particles with dimensions between 10 nm and 150 nm, which exhibit a catalytic effect for the deposition of carbon particles. Depending on the pulse current program, carbon particles with diameters between 100 nm and 10 pm are obtained electrolytically.

[0035] In the embodiment of the electrochemical line according to the invention, the reactor vessel 3, the cathode 2 is designed such that an ultrasonic transducer 8 or a vibration transducer causes the cathode 2 to vibrate discontinuously, and the deposited carbon is continuously removed from the cathode 2 and collects at the end of the reactor vessel. When the electrolysis is switched off, the reactor vessel 3 is tilted slightly to an angle of approximately 15° and the carbon can be drawn off from the electrochemical cell 3 via a lateral opening 10.

[0036] In the Vaoanfe ß, anode 1 and cathode 2 are arranged vertically in the reactor vessel 3. The dimensions of the reactor vessel 3 are adapted to this electrode arrangement. The reactor vessel 3 can be cylindrical or rectangular. The height of the reactor vessel 3 is at least twice the diameter of the vessel or three times the side length in a version with a rectangular or square base. The internal diameters of the reactor vessel range from 200 mm to 1000 mm, depending on the amount of carbon dioxide to be converted. For square or rectangular bases, areas between 300 cm² are used. 2 and 8000 cm 2 used, preferably between 600 cm 2 and 1400 cm 2 Preferred internal diameters of the reactor vessel range between 250 mm and 500 mm.

[0037] As described in variant A of the design, the same basic materials for the anode and cathode can be used for variant 8. The options for surface conditioning the cathode are also identical. However, the geometric arrangement is different.

[0038] The cathode 2 is used as a tube with a diameter approximately half the diameter of the reactor vessel 3. This cathode 2 is surrounded externally at a distance of approximately 10 mm to 20 mm by the tubular anode 11, which can again have the structure described in variant 1. In addition, a further rod-shaped anode 12 is used inside the cathode 2. Gas is discharged along the preferably longitudinally structured anode 12 in the inner region of the cathode 2 and along the also preferably longitudinally structured anode 11, arranged in a ring shape outside the cathode 2, to the surface of the reactor vessel 3. The oxygen produced is collected by a funnel-shaped structure 13 at the top of the reactor vessel 3 and discharged through an opening 14. The carbon dioxide required for the reaction is fed into the reaction vessel 3 via a lateral inlet 15 and combined with...The solution was brought to the lower areas of the cathode 2 by means of two pipes 16.

[0039] Similar to variant A of the design, the cathode 2 is periodically vibrated by an external ultrasonic transducer so that the electrolytically formed carbon is removed from the cathode surface.

[0040] Carbon is discharged upwards from the toe 3 via a suction 20. This discharge can be carried out continuously during cell operation or discontinuously after electrolysis is switched off. A viewing window 30 is provided laterally in the reactor vessel 3 to check whether sufficient carbon has been deposited for discharge. In variant C, the cathode 2 is designed as two concentric tubes 2a and 2b. The inner tube 2b has a diameter approximately two to five centimeters smaller and is gas-tightly connected to the outer tube 2a along its longitudinal axis at a distance of approximately five to twenty centimeters, depending on the overall diameter of the arrangement. The inner tube has a number of inward openings, and the outer tube has a number of outward openings, each with an open area of ​​approximately 0.05 cm². 2 and 5 cm 2, according to the invention preferably between 0.1 cm 2 and 1cm 2 open, whereby up to 50% of the pipe surfaces remain open for gas escape through these openings. The top of this arrangement is also covered gas-tight by a semi-circular pipe guide 2c, through which the carbon dioxide-containing gas can be introduced into the cathode structure.

[0041] This cathode structure is surrounded on both sides by concentrically arranged tubular anodes 21 and 22, the distance of which to the cathode tubes can vary between 0.5 cm and 1 / 2 cm. According to the invention, a distance of approximately 10 mm to 20 nm is preferred, since sufficiently high current densities for the conversion of carbon dioxide can be achieved at this distance. The carbon formed can then be removed from this cathode structure continuously or discontinuously by means of an ultrasonic transducer and subsequently discharged from the electrochemical chamber.

[0042] In variant D, the reaction vessel, the electrochemical cell, is directly connected as the cathode; in this case, no cathodic protection of the vessel is necessary. The anode is arranged concentrically at a defined distance from the inside of the vessel. The oxygen produced is collected via a funnel-shaped ring line at the top of the anode, subsequently compressed, and stored in a suitable medium, such as pressure cylinders. In this case, the carbon dioxide-containing reaction gas is carried out via ring lines with openings in the range of 1 mm. 2 up to 25mm 7 , arranged in a ring shape along the inner wall in the middle and lower area of ​​the electrochemical reaction vessel, the cathode.

[0043] As previously described, the reactor vessel can be constructed in a double-walled version, consisting of two concentric vessels placed one behind the other with a preferred wall spacing of 10 mm to 20 mm. Inside, it features vertically arranged gas baffles spaced between 20 mm and 200 mm apart, preferably between 30 mm and 70 mm apart, depending on the actual size of the reactor vessel. Slit-shaped, angular, or round openings with an area of ​​up to 50% of the inner wall in the lower region of the cathode allow the reactor gases to escape. The reactor gas is supplied either from above via a ring main for the electrochemical reaction or from below via lateral inlets and openings, which are fitted with a gas-permeable ceramic membrane (fritt). 1') are shielded from the molten electrolyte. In both variants, the reactor vessel is intermittently vibrated by an ultrasonic or vibration sensor to remove the carbon formed from the cathode wall. This can then be recovered, for example, via a suction line through a lateral opening located in the upper part of the reactor vessel, or it settles in the electrolyte due to its higher density and is then removed discontinuously from the row.

[0044] Ad b)

[0045] As briefly mentioned at the beginning, in addition to the classic alkali-carbonate electrolytes of eutectic composition LisCOs (43.5%) - NazCCh (31.5%) - K₂CO₃ (25%) with an operating range around 450°C, low-melting-point electrolytes based on ionic liquids with carbonate anions can also be used. The cations in question can be selected from a variety of organic salts, with electrochemical stability being a key selection criterion.Since the reduction of carbon dioxide to carbon in these electrolytes proceeds at significantly higher reduction potentials (cathode potentials up to 5 V are sometimes necessary) due to the lower electrolyte temperatures (between 4 CFC and 80°C), the electrochemical window is the limiting property. According to the invention, imidazole, fluorinated, and choline-based variants are preferred, as these exhibit a sufficiently large electrochemical window and are only minimally decomposed during the electrochemical reduction of carbon dioxide. A person skilled in the art can make a suitable selection from the aforementioned classes of substances based on extensive published results on the stability of such ionic salt melts.

[0046] Ad c)

[0047] For efficient cathodic deposition of carbon directly from carbon dioxide-containing carbonate-based molten salts, electrolysis with constant current or constant potentials can only be used to a limited extent. The aim of the present invention is therefore to ensure a continuous reduction of carbon dioxide to carbon by means of suitable current or potential profiles at cathode 2.

[0048] It should be noted that the quality and crystallinity of the deposited carbon products can be significantly controlled depending on the cathode materials and cathode surfaces used.

[0049] The electronic components required for the pulsed current or voltage waveforms can be obtained from various manufacturers and are, in most cases, freely programmable. The parameters required according to the invention are described below.

[0050] Based on fundamental considerations of deposition kinetics, the times of polarization of cathode 2 and the times of relaxation, in which no current flows through the cell, must be coordinated and adapted to the flow of carbon dioxide according to the invention.

[0051] A high mass input is required: Carbon dioxide generally requires a higher current density at cathode 2 for the reductive conversion to carbon. However, due to the limiting exchange current density for the respective reaction, an approximately complete conversion can only be achieved using pulsed methods. According to the invention, current pulses for the high-temperature electrolytes range from a few milliseconds to seconds at current densities between a few mA / cm². 2 and up to several 100 mA / cm 2with relaxation times between 50 milliseconds and several tens of seconds. On nanoscale structured steel cathodes, current densities in the range between 20 and 100 mA / cm² are preferably used, with pulse times between 100 milliseconds and 2 seconds and relaxation times between 500 milliseconds and 5 to 8 seconds. In a further embodiment of the current program according to the invention, the pulse current density is continuously increased along a linearly rising curve with pauses between 100 milliseconds and 2 seconds up to a maximum value of several hundred mA / cm². 2 The pulse current density is increased, followed by a longer pause (> than two seconds) for the relaxation period, after which the pulse current density is again increased linearly per pulse continuously up to the specified maximum value.

[0052] According to the invention, higher current densities up to a range of 1000 mA / cm² are achieved on the cathode surfaces conditioned with nickel particles on a nanoscale. 2and pulse times of up to 500 milliseconds are used. For this cathode material and the high pulse current densities, the relaxation times range between 2 and 5 seconds. The carbon products obtained exhibit high hydrocarbon character for these parameters. The proportion of amorphous structures drops below 15%, and to below 10% at pulse times below 100 milliseconds and higher electrolyte temperatures (above 500°C).

[0053] Regarding “modularly structured cells”

[0054] Due to the inventive design of the electrochemical cell, modular units can be built.

[0055] It is possible to integrate several electrode assemblies with cathode 2 and anode 1 in parallel into a suitably large reaction vessel 3 and to operate these electrode assemblies via a multi-channel voltage or current source. In this modular design, significantly higher quantities of carbon dioxide can be converted in the same electrolyte at lower investment costs compared to individually constructed electrolysis systems.

[0056] When using electrolytes with a lower melting point, a bipolar arrangement for electrolysis can also be implemented according to the invention. In this variant, corrosion-resistant stainless steel with a nickel coating on one side is used as the electrode material, with this coated side of the electrode serving as the cathode. The electrodes are arranged vertically as in variant S, but in a plate-like structure arranged perpendicular to the longitudinal direction of the electrolyte vessel 3 with a rectangular base, the electrochemical line. The height and number of electrodes can be adjusted to the required carbon dioxide conversion.

[0057] Due to the inventive arrangements of cathodes and anodes and the current and voltage programs used, high faradaic efficiencies are achieved for the electrochemical reduction of carbon dioxide. These efficiencies range between 75% and 96%, depending on the electrode arrangement, current program, and electrolyte temperature, based on the amount of charge used and the amount of carbon dioxide supplied. At higher electrolyte temperatures (above 550°C), the highest faradaic efficiencies are achieved under previously applied conditions. However, for practical operation, especially considering corrosion reactions, the lowest possible electrolyte temperatures are desirable. The inventive use of an electrode arrangement as shown in Figure 3 and an adapted current program enables an electrochemical efficiency of at least 85% for the reduction of carbon dioxide even at electrolyte temperatures around 460°C.These values ​​are also achieved in intermittent operation of the electrochemical cell, although slightly lower efficiencies are measured during the first pulses after the electrochemical cells are switched back on. This can be explained by the fact that in the initial phase of the electrochemical reaction, the overall reaction must be activated and the necessary concentration of carbon dioxide at the cathode surface must be established.

[0058] The produced carbon variants can be adapted to a wide range of technical requirements for industrial applications using the power or potential programs, thereby achieving significant savings in energy and effort for the production of technically relevant carbon modifications. In principle, even when using fossil energy resources, the processes according to the invention can achieve a substantial reduction in carbon dioxide emissions into the atmosphere through the cyclical utilization of the produced carbon.

[0059] Reference symbol list

[0060] 1 anode

[0061] 2 Cathode

[0062] 2a outer cathode tube

[0063] 2b inner cathode tube

[0064] 2c upper semicircular pipe guide

[0065] 3 Reactor vessel

[0066] 4 supply lines

[0067] 5 Sch me ize le kt ro ly t

[0068] 6 gas-conducting structure

[0069] 7 Gas outlet opening on the anode side

[0070] 8 Ultrasonic transducers

[0071] 9 vibration sensors

[0072] 10 Opening for removing the carbon

[0073] 11 tubular anode

[0074] 12 rod-shaped anodes

[0075] 13 funnel-shaped structures

[0076] 14 Oxygen outlet

[0077] 15 lateral carbon dioxide inlets

[0078] 16 pipelines

[0079] 17 Ceramic membrane (“frit”)

[0080] 18 gas guide rails between the Kalhoden pipes 2a and 2b

[0081] 20 Extraction

[0082] 21 inner tube anode

[0083] 22 external pipe anode

[0084] 30 viewing windows

[0085] Description of the characters

[0086] Figure 1 schematically shows the arrangement of the electrochemical cell, the reactor vessel 3, with the anode 1 and the cathode 2, the molten electrolyte 5 and the supply lines 4, the gas-conducting structure 6, the anode-side gas outlet opening 7 with the ceramic membrane 17 and the ultrasonic sensor 8 or vibration sensor 9, the opening for removing the formed carbon 10, and the position of the viewing window 30.

[0087] Figure 2 shows in horizontal cross-section the arrangement of the cathode 2 with the tubular anode 11 and the rod-shaped anode 12, as well as the outlet openings for the oxygen 14 and the lateral feed for carbon dioxide-containing reaction gas 15 with the ceramic membrane 17. Figure 3 schematically shows in horizontal cross-section the arrangement of the cathode 2 with the inner cathode tube 2b and outer cathode tube 2a connected by the gas guide vanes 18, as well as the inner tubular anode 21 and the outer tubular anode 22.

Claims

Rat spray© 1. An arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon, comprising a cathode arrangement, an anode arrangement, a heat-treating electrolyte, and an electronic device for generating suitable current and potential profiles, characterized in that the surface-structured cathode (2), specially designed with defined openings according to the invention, is immersed in an electrolyte (5) and is provided horizontally by the anode (1) or by a tubular anode (11) on the outside and a rod anode (12) on the inside, or is concentrically surrounded by an inner tubular anode (21) and a tubular anode (22) on the outside, and is continuously or discontinuously subjected to vibration by means of an ultrasonic transducer (8) or vibration transducer (9), and is polarized by means of a suitable electronic device with different current and potential programs.

2. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claim 1, characterized in that the cathode (2) and the anode (1) are arranged horizontally in a thermally insulated reactor vessel (3) and the supply lines are made of corrosion-resistant stainless steels.

3. An arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claim 1, characterized in that the anode is made of glassy carbon, graphite, reactor graphite or other oxidation-stable materials such as titanium or titanium alloys, or iridium-coated steel variants or stainless steel variants with a diamond-coated surface (nanoscale or microscale diamonds deposited by CVD or PVD processes or electrochemically by dispersion processes and embedded in oxidation-stable metallic support layers) with thicknesses between 2 mm and 15 mm, preferably between 5 mm and 10 mm, with a hole structure having holes with an average diameter of 2 to 20 mm.

4. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claim 3, characterized in that the anode (1) is designed as a longitudinally cut 120 degree tube section with a diameter of up to 500 mm and has a structure with longitudinal slots with a width between 0.5 mm and 1 cm and a length between 2 mm and 50 mm, preferably with diameters between 3 mm and 10 mm or with a width between 1 mm and 5 mm and a length between 10 mm and 30 mm.

5. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claim 4, characterized in that a gas-conducting funnel-shaped structure (6) is attached to the top of the anode (1) which is connected to a gas outlet opening (7).

6. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claim 1, wherein the cathode (2) is made of stainless steel variants, nickel alloys or nickel foam structures or Steel variants with a specially manufactured nanoscale carbon-containing surface structure are designed with a structure with circular recesses with diameters between 1 mm and 250 mm or with a structure with longitudinal slots with a width between 0.5 mm and 2 cm and a length between 3 mm and 100 mm, preferably with diameters between 3 mm and 20 mm or with a width between 1 mm and 5 mm and a length between 10 mm and 30 mm.

7. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claim 6, characterized in that the cathode (2) consisting of steel or stainless steel variants is provided with a nanoscale coating of carbon, wherein the carbon particles are in a dimension between 3 nm and 5000 nm, preferably in a dimension between 5 nm and 500 nm.

8. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claims 6 and 7, characterized in that the carbon particles on the steel surfaces are directly reduced in the carbonate melt by current pulses in the millisecond range between 10 and 500 milliseconds with current densities up to 500 A / cm² 2with relaxation times between 1 second and 10 seconds or by potential pulses between 5 volts and 30 volts, with pulse lengths between 0.5 milliseconds and 10 seconds, preferably with pulse lengths between 5 milliseconds and 300 milliseconds with stepwise increasing pulse heights up to 15 volts, 8. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claims 1 to 6, characterized in that the cathode (2) is provided with a nanoscale nickel coating having a layer thickness between 5 nm and 5000 nm and a nickel particle size between 5 nm and 500 nm, preferably with a layer thickness between 20 nm and 200 nm and a nickel particle size dimension between 15 nm and 150 nm.

10. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claims 1 to 6, characterized in that the cathode (2) is coated with nanoscale magnetite and / or hematite particles of a dimension between 10 nm and 150 nm.

11. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claims 1 to 10, characterized in that the cathode (2) is designed as a tube with a diameter approximately half the diameter of the reactor vessel (3) and this cathode (2) is surrounded on the outside at a distance of 10 mm to 20 mm from a tubular anode (11) and in the inner region of the cathode (2) an additional rod-shaped anode (12), preferably with diameters between 20 mm and 100 mm, is arranged.

12. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claims 1 to 10, characterized in that the cathode (2) is designed in the form of two concentric tubes (2a) and (2b), wherein the inner tube (2b) has a diameter approximately two to five centimeters smaller and is spaced approximately five to twenty centimeters, depending on the overall diameter of the arrangement, is connected gas-tight to the outer tube (2a) in the longitudinal axis (vertically).

13. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claims 1, 3, 6 to 10 and 12, characterized in that the inner tube (2b) has a number of openings facing inwards and the outer tube (2a) has a number of openings facing outwards, each with an open area of ​​between 0.05 cm² 2 and 5 cm*, preferably between 0.1 cm 2 and 1cm 2so that up to 50% of the pipe surface remains free.

14. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claims 1, 3, 6 to 10, 12 and 13, wherein the top of the cathode arrangement (2a) and (2b) is sealed gas-tight at the top by a semicircular tube guide 2c as a supply line for carbon dioxide. 15 Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claims 1, 3, 6 to 14, characterized in that the cathode structure (2a) and (2b) is surrounded on both sides by concentrically arranged tubular anodes (21) and (22) at a distance from the cathode tubes (2a) and (2b) between 0.5 cm and 10 cm, preferably between 10 mm and 20 mm.

16. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claims 1 to 15, characterized in that current pulse processes are used for the electrochemical reduction of carbon dioxide with pulse times between a few milliseconds and seconds and current densities between a few mA / cm². 2 and up to several hundred mA / cm 2 with relaxation times (pauses) between 50 milliseconds and several tens of seconds.

17. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claims 1 to 16, characterized in that, when using nanoscale structured steel kaihodes, current densities preferably in a range between 20 and 100 mA / cm² are used. 2 with pulse times between 100 milliseconds and 2 seconds and relaxation times between 500 milliseconds and 5 to 8 seconds.

18. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claims 1 to 16, characterized in that when using the cathode surfaces conditioned nanoscale with nickel particles, current densities of up to 1000 mA / cm² are achieved. 2 Pulse times of up to 500 milliseconds and relaxation times (pauses) of up to 5 seconds can be applied.

19. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claims 1 to 18, characterized in that continuously linearly increasing pulsed current lights with up to several 1000 mA / cm are used for the electrochemical reduction of carbon dioxide. 2 each with pauses of up to two seconds between the individual steps of the Pulsed current density reduction is used with a longer pause following the pulse program for relaxation of the system in the range of several seconds. .

20. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claims 1 to 18, characterized in that several electrode arrangements with cathode (2) and anode (1) are arranged in parallel in the reactor vessel (3) and these are supplied via a multi-channel voltage or current source. .

21. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claims 1 to 20, characterized in that when using electrolytes with a lower melting point (“ionic liquids”), a bipolar arrangement for carbon dioxide reduction is used with corrosion-resistant, one-sided nickel-coated stainless steel plates, arranged vertically, and this coated side is switched as the cathode in each case.

22. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claims 1 to 20, characterized in that the reaction vessel is cathodically polarized by a device for cathodic protection with auxiliary electrodes arranged in the electrolyte.

23. Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claims 1 to 20, characterized in that the reaction vessel, made of single-walled or double-walled steel or stainless steel variants, is directly connected as the cathode. 24 Arrangement of an electrochemical reactor for the direct reduction of carbon dioxide to carbon according to claims 1 to 20 and .23, characterized in that the double-walled reaction vessel has vertically arranged gas guide vanes inside and has slot-shaped, angular or round openings on the inside in the lower area of ​​the reaction vessel with a surface area of ​​up to 50% for gas discharge.

Images