Process for the production of carbon allotropes by means of electrochemical deposition

Abstract

The present invention relates to the production of carbon allotropes by means of electrochemical deposition from compressed fluids.

Description

[0001] Process for the production of carbon allotropes by electrochemical deposition

[0002] The present invention relates to the technical field of the production of carbon allotropes by electrochemical deposition.

[0003] In particular, the present invention relates to a process for producing carbon allotropes by electrochemical deposition of a carbon allotrope on a substrate of dense fluids.

[0004] Carbon is one of the essential elements of the living world and, by weight, is the second most important element in all living things after oxygen. Every living thing is composed predominantly of organic carbon compounds.

[0005] The mass fraction of carbon in the Earth's crust, however, is only 0.027%. In inanimate nature, carbon exists both in elemental form and in bound form, for example, as carbonate rock.

[0006] In its elemental form, carbon can exist in a number of different forms—so-called modifications or allotropes. Allotropes of a chemical element exhibit different structural forms or crystal structures, which in turn result in different chemical and physical properties of the respective allotrope. Natural allotropes of carbon include diamond and graphite.

[0007] Other allotropic forms of carbon can be produced synthetically, such as graphene, fullerenes, or even carbon nanotubes. Also of great interest are the carbon allotropes carbyne and graphyne, whose synthesis has so far been achieved with considerable effort (carbyne) and not yet (graphyne).

[0008] Graphene is a two-dimensional single-atom layer of honeycomb-shaped, sp 2 - hybridized carbon six-membered rings. Multiple layers of graphene form graphite. Carbon nanotubes, on the other hand, consist of a single graphene layer rolled up in a tube-like configuration. In addition to single-wall carbon nanotubes (SWCNTs), multi-walled tubular structures (MWCNTs) can also be produced. In the case of fullerenes, the graphene layer is folded into a sphere.

[0009] Carbyne differs from the aforementioned allotropes in that sp- and sp2 -hybridized carbon atoms are arranged in long one-dimensional chains, either alternating between single and triple bonds or linked via double bonds. Graphyne is the two-dimensional variant of carbyne.

[0010] The synthetic allotropes of carbon mentioned above are all characterized by special chemical, mechanical, electronic and physical properties, so that the development of processes for the targeted production of different allotropes is of great interest. In particular, many carbon allotropes exhibit excellent thermal and / or electrical conductivities, which are often direction-dependent.

[0011] The carbon allotropes can be used, in particular, as active materials for electrodes in battery applications, such as lithium-ion batteries or aluminum-ion batteries. Furthermore, they can also be used as fillers to achieve electrical and / or thermal conductivity, for example, as fillers in bipolar plates for battery applications and fuel cells, in supercapacitors, and electrolyzers.

[0012] In addition, carbon allotropes can be used to shield against high-frequency radiation, as biocompatible implants or as filter media, especially for water filtration.

[0013] Different processes have emerged for the production of graphene or carbon nanotubes as well as other synthetic allotropes of carbon.

[0014] The simplest method for producing graphene is by exfoliating graphite and was described by Novoselov et al. (Novoselov et al.: "Electric Field Effect in Atomically Thin Carbon Films", Science 2004, Vol. 306(5696)). By quickly peeling an adhesive tape from a graphite block, thin layers of graphite particles remain on the tape. Repeated peeling of the thin film creates fine graphene films, which can be released after a final chemical dissolution of the adhesive tape. The disadvantage of this method is that it is time-consuming to produce in very small quantities. Furthermore, highly ordered (highly crystalline) graphite is required as the basis for the process.

[0015] Another method for producing graphene is the treatment of graphite with supercritical carbon dioxide (CO2). In this process, the CO2 is deposited between the individual graphite layers and is driven out of the graphite into the gaseous phase by rapid expansion of the CO2. The increase in volume during expansion destroys the graphite and separates individual graphene layers from the graphite structure. The disadvantage of this process is the undefined or indefinable particle size that is obtained. Furthermore, the purest possible graphite must be used as the starting material.

[0016] Another method for producing graphene is ultrasonically treating graphite. This involves vibrating high-purity graphite in such a way that the individual graphene layers peel off each other. The quality of the particles depends on the quality of the starting graphite and the frequency of the ultrasonic treatment, resulting in varying, non-uniform particle sizes.

[0017] For the production of carbon nanotubes and graphene, the deposition of carbon particles by chemical vapor deposition (CVD) has become established. High-purity graphite is evaporated at high temperatures (> 1,000 °C) and deposited onto a metallic support using a carrier gas. In addition to the batch production of graphene or carbon nanotubes, a continuous production method is also known. A disadvantage is that the carrier gas can contribute to undesirable impurities in the particles, which can lead to defects in the graphene lattice and, consequently, to a reduction in the electrical and mechanical properties of the produced carbon allotropes. In addition, deposition in a continuous process requires a long residence time to achieve the desired particle structure and subsequently cool the material.

[0018] A modification of chemical vapor deposition by Takakura et al. involves the use of a plasma (plasma-enhanced chemical vapor deposition, PECVD) (Takakura et al.: "Strength of carbon nanotubes depends on their chemical structures", Nature Communications 2019, Vol. 10(1)). Due to the energy introduced by the plasma, deposition takes place at comparatively lower temperatures of below 700 °C, and a resulting magnetic field also enables a vertical arrangement of the deposited particles. Similar to the carrier gas, however, the high reactivity of the plasma can lead to contamination of the material, which can influence the electrical and mechanical properties of the resulting carbon allotropes.

[0019] In addition, the preparation of graphene oxide by detonation has been described by Wang et al. in "Preparation of graphene nanosheets through detonation," New Carbon Materials 2011, Vol. 26(1). However, the properties of the oxidized material differ significantly from those of graphene or other carbon allotropes.

[0020] In addition, numerous studies have investigated the electrochemical deposition of carbon from organic liquids, but invariably amorphous carbon is obtained. However, the electrochemical deposition of amorphous carbon from organic liquids suggests that the type of deposited carbon is primarily influenced by the substrate and the carbon source.

[0021] Wu et al. report in "Electrochemical reduction of CO2 to carbon films on stainless steel around room temperature", Electrochemistry Communications 2020, Vol. 110, that the electrochemical deposition of carbon from methanolic CCh solutions under elevated pressure is possible. Voltages of more than 3 V are required to deposit carbon on stainless steel electrodes. However, the resulting carbon films are not formed from a specific carbon allotrope, but appear amorphous and disordered, so that the targeted production of defined carbon allotropes has not yet been achieved by this method. The electrochemical deposition of carbon from molten salts further demonstrates that carbon can also be deposited in monolayers, which in principle enables the formation of graphene layers. However, due to the high temperatures and the equipment required, the process is not feasible on a larger scale.

[0022] The state of the art therefore still lacks a process for the targeted production of defined carbon allotropes, which can be carried out reproducibly under comparatively moderate conditions and is also less sensitive to deviations in the process control.

[0023] Furthermore, the state of the art also lacks processes for the production of carbon allotropes that start from relatively uncomplicated and easy-to-handle starting materials.

[0024] It is also not yet possible to produce multiple carbon allotropes using a single process.

[0025] The present invention is therefore based on the object of eliminating the aforementioned disadvantages associated with the prior art, or at least mitigating them.

[0026] In particular, it is an object of the present invention to provide a process for the production of carbon allotropes which allows the targeted production of specific carbon allotropes reproducibly under relatively moderate conditions, in a time- and cost-effective manner.

[0027] Furthermore, it is an object of the present invention to provide a process for the production of carbon allotropes which starts from starting materials which are easy to obtain and is overall less sensitive to possible impurities or deviations in the process procedure.

[0028] The present invention thus relates to a process for producing carbon allotropes by electrochemical deposition according to claim 1; further advantageous embodiments of this aspect of the invention are the subject of the relevant subclaims.

[0029] It goes without saying that with all relative or percentage quantities, especially those based on weight, mentioned below, it should be noted that, within the scope of the present invention, these must be selected by the person skilled in the art in such a way that the sum of the ingredients, additives, auxiliaries, or the like always amounts to 100 percent or 100% by weight. However, this is self-evident to the person skilled in the art.

[0030] In addition, all parameter specifications or similar mentioned below can in principle be determined or ascertained using standardized or explicitly specified determination procedures or using determination methods that are familiar to the person skilled in the art.

[0031] Having said this, the subject matter of the present invention will be explained in more detail below.

[0032] The present invention - according to a first aspect of the present invention - relates to a process for the production of carbon allotropes by electrochemical deposition of a carbon allotrope on a substrate, wherein the electrochemical deposition is carried out from a densified aprotic or proton-poor, preferably aprotic, carbon-containing electrolyte.

[0033] As the applicant has surprisingly discovered, it is possible to obtain carbon allotropes, in particular targeted individual carbon allotropes, cost-effectively from generally available starting materials and on a large scale by electrochemical deposition, in particular electrolysis.

[0034] In particular, it has been shown that during electrochemical deposition or electrolysis, no protons or only a small number of protons, preferably no protons, should be present in the electrolyte. Therefore, the present invention uses aprotic or at least proton-poor, preferably aprotic, carbon-containing electrolytes. The presence of protons always leads to disruptive side reactions, which complicate the targeted deposition of carbon allotropes, especially of individual carbon allotropes, or, if the proton concentration is too high, make it impossible; only amorphous carbon is then obtained. Furthermore, the presence of protons, especially the use of protic electrolytes, leads to the incorporation of hydrogen into the carbon structure.

[0035] Within the scope of the present invention, it is particularly possible to specifically adjust both the morphology and the reaction rate, as well as the carbon allotrope itself, by varying the type of electrode and the applied current density. Furthermore, the optional application of a magnetic field can additionally orient the carbon allotropes and thus control the deposition process.

[0036] In particular, the use of high-purity, dense CO2 allows for the simple, targeted production of various high-quality carbon allotropes, which are characterized by exceptional purity compared to the state of the art. In particular, impurities can be minimized, resulting in optimal material properties. In particular, the use of carbon dioxide as a direct carbon source and also as a solvent allows the influence of foreign substances to be minimized within the scope of the present invention.

[0037] Compared to the production process for graphene from molten salts, as described, for example, in WO 2020 / 243320 A1, the process according to the invention can be carried out at significantly lower temperatures and is therefore more economical. Furthermore, the process according to the invention enables a direct reduction of CO2 or other carbon-containing gases, whereas the process according to WO 2020 / 243320 A1 only allows an indirect reduction of CO2, since the CO2 must first be converted to carbonate before reduction.

[0038] Furthermore, it is possible, particularly by using carbon dioxide as a direct carbon source, to deposit high-quality, defect-free graphene layers over a large area. The magnetic field created by the applied current can manipulate the preferred direction of particle growth and thus specifically modify and optimize the material properties. Furthermore, it is also possible to influence the deposition process using an external magnetic field and specifically adjust the material properties. In contrast to the deposition of carbon using the CVD method, the present invention enables very simple formation of carbon nanotubes through vertical growth, so that the formation of carbon nanotubes can be carried out much more easily and quickly using the method according to the invention. In addition, the reaction kinetics can be specifically controlled using the applied current.Unlike the CVD method, no high temperatures are required for the evaporation of graphite or the pyrolytic decomposition of methane.

[0039] The process according to the invention offers simple and sustainable access to high-purity carbon allotropes and thus ultimately to functional materials which are neither harmful to health nor unacceptable for environmental reasons in their application and disposal.

[0040] Furthermore, within the scope of the present invention, it is also possible to deliberately introduce impurities into the carbons, for example as doping, and thus to specifically adjust the properties of the carbon allotropes.

[0041] In the present invention, the carbon allotropes are preferably deposited electrochemically by cathodic reduction.

[0042] In the context of the present invention, electrochemical deposition refers in particular to an electrolysis process. Electrolysis is generally understood to be a chemical reaction induced by the flow of current. In electrolysis, a chemical reaction is forced in an electrolyte, particularly by applying an external voltage between at least two electrodes.

[0043] Electrolysis, as particularly envisaged within the scope of the present invention, or electrochemical deposition, generally takes place in an electrochemical cell with at least two electrodes. For example, electrochemical deposition can be carried out using a three-electrode setup comprising a working electrode, a reference electrode, and a counter electrode. The electrodes are brought into contact with the fluid. Subsequently, a positive potential is preferably applied to the counter electrode, which simultaneously forms the anode, so that an oxidation reaction can take place there. The electrons generated at the anode can then be transferred through an external circuit, while the positively charged oxidation products can be transferred through the fluid and consumed at the working electrode or cathode during the complementary reduction process.At the same time, a negative potential is applied to the cathode, allowing a reduction reaction to occur by absorbing electrons and, if necessary, positively charged oxidation products. However, the oxidation products can also be reduced by electrons. The reference electrode has a stable, usually known potential (e.g., Ag / AgCl) and is used to control the applied potential of the cathode relative to the reference electrode. However, especially for large-scale processes, a two-electrode configuration is preferred, whereby the cathode and / or the anode can be constructed from several individual electrodes connected in parallel.

[0044] For the purposes of the present invention, an electrolyte is understood to mean, in particular, a substance, i.e., a chemical compound, or a mixture of substances that contains mobile ions. For the purposes of the present invention, the electrolyte is preferably a fluid or a mixture of fluids that contains mobile ions. The ions can originate, for example, from salts or ionic liquids.

[0045] The electrolyte can be present as a solution, i.e., electrolyte solution, or a dispersion. The electrolyte is preferably present as a solution.

[0046] In the context of the present invention, a solution is understood to be a single-phase system in which at least one substance, in particular a compound or its constituents, such as ions, is homogeneously distributed in another substance. In the context of the present invention, a dispersion is understood to be an at least two-phase system, wherein a first phase, namely the dispersed phase, is distributed in a second phase, the continuous phase. The continuous phase is also called a dispersant or dispersion medium. In the context of the present invention, a fluid is in particular a gas or a mixture of gases or a liquid or a mixture of liquids. In the context of the present invention, a compressed fluid is understood to be in particular a fluid in which a pressure prevails which is higher than the standard pressure of 1.013 bar.The compression can be achieved, for example, by applying external pressure or by increasing the temperature or both.

[0047] In the context of the present invention, "aprotic" means that the electrolyte or its components do not contain any functional groups from which hydrogen can be released in the form of protons. However, under real conditions, it is unavoidable that the aprotic electrolyte, in particular an aprotic carbon-containing fluid and / or an aprotic solvent, contains at least traces of compounds whose functional groups can release protons, in particular protic solvents or water, especially water, especially as an impurity, which cannot be removed.

[0048] As previously stated, the carbon-containing electrolyte in the context of the present invention is aprotic or low in protons, preferably aprotic. Best results are obtained in the context of the present invention when the electrolyte is aprotic, i.e., with the exception of unavoidable impurities, it contains no compounds whose functional groups can release protons, in particular protic solvents or water, especially water.

[0049] However, the applicant has found that the process according to the invention can still be carried out with good yields if the carbon-containing electrolyte contains small amounts of compounds whose functional groups can abstract protons, in particular protic solvents or water, in particular water.

[0050] It has been shown that good results can also be achieved with a low-proton carbon-containing electrolyte. In the context of the present invention, a low-proton carbon-containing electrolyte is understood to mean in particular an electrolyte which or whose individual substances, such as carbon-containing fluids and / or solvents, consist of aprotic compounds which are used without prior removal of compounds whose functional groups can split off protons, in particular protic solvents or water, i.e. without drying. In this context, it is particularly possible for the carbon-containing electrolyte to contain up to 0.01 wt.%, in particular 0.001 wt.%, preferably up to 0.0005 wt.%, more preferably 0.0002 wt.%, of compounds whose functional groups can split off protons, in particular protic solvents, in particular alcohols, or water, especially water, based on the electrolyte.

[0051] Likewise, it is possible for the electrolyte to comprise an aprotic solvent which comprises compounds whose functional groups can split off protons, in particular protic solvents, in particular alcohols, or water, preferably water, in amounts of up to 0.01 wt.%, in particular 0.005 wt.%, preferably up to 0.001 wt.%, based on the aprotic solvent.

[0052] For the purposes of the present invention, an allotrope refers to different structural forms of a chemical element in the same aggregate state, which differ from one another physically and in their chemical reactivity. With regard to carbon, it can exist in the same – solid – aggregate state, i.e., in various structural forms or modifications, which also behave differently both chemically and physically. The best-known carbon modifications are diamond and graphite. For the purposes of the present invention, carbon allotropes that exhibit electrical conductivity are preferably considered.

[0053] Within the scope of the present invention, it is possible for the carbon allotrope to be selected from the group consisting of diamond, graphite, graphene, carbon nanotubes, fullerenes, graphyne, carbyne, and mixtures thereof. Typically, within the scope of the present invention, the carbon allotrope is selected from the group consisting of graphite, graphene, carbon nanotubes, fullerenes, graphyne, carbyne, and mixtures thereof. Particularly good results are obtained in this context when the carbon allotrope is selected from the group consisting of graphene, carbon nanotubes, fullerenes, graphyne, carbyne, and mixtures thereof. Graphene is most preferably the carbon allotrope. Graphene is a hexagonal carbon that is arranged in a single layer but can also form fibers.Graphite consists of layers of graphene arranged one above the other, the individual layers of which can be easily moved mechanically against each other. Graphite can occur in both crystalline and amorphous forms. Fullerenes are closed molecules made up of carbon atoms arranged in pentagons and hexagons. The Ceo fullerene, also known as a Buckminster fullerene, is particularly well-known and has the structure of a soccer ball.

[0054] Carbon nanotubes are rolled-up graphene sheets or elongated fullerenes. They occur as single-wall carbon nanotubes or multi-wall carbon nanotubes. The carbon nanotubes can take on zigzag, chiral, or armchair configurations.

[0055] Carbynes are chains of carbon atoms connected by alternating triple and single bonds or continuous double bonds, while graphyne is a multi-chain, sheet-like form of carbyne.

[0056] Furthermore, within the scope of the present invention, it can be provided that the carbon allotrope is doped. Particularly good results are obtained in this context if the doping is carried out with a doping element, in particular selected from the group consisting of magnesium, silicon, yttrium, cobalt, boron, nitrogen, and mixtures thereof. Within the scope of the present invention, it is thus possible to introduce a targeted impurity into a material to be doped, whereby novel materials with novel properties can be produced. For example, doping carbon with nitrogen induces lithophilic behavior. Doping is preferably carried out by introducing an electrically oxidizable or reducible compound, preferably an electrically reducible compound, of the doping element into the electrolyte.

[0057] Within the scope of the present invention, particularly good results are obtained when the densified aprotic or low-proton carbon-containing electrolyte comprises an aprotic carbon-containing fluid and an ionic substance. By using ionic substances, in particular salts or ionic liquids, it is possible to directly reduce relatively inexpensive fluids, especially carbon dioxide, to carbon allotropes.

[0058] In the context of the present invention, an ionic substance is understood to mean a chemical substance which contains ions or is composed of ions.

[0059] Within the scope of the present invention, it has further proven useful if the aprotic carbon-containing fluid is produced from a carbon-containing gas or a carbon-containing liquid, preferably a carbon-containing gas. This means that, for example, within the scope of the present invention, a carbon-containing gas is compressed and liquefied or converted into the supercritical state, preferably converted into the supercritical state, and then admixed with an ionic substance, such as a salt or ionic liquid. Likewise and preferably, however, it can be provided here that the not yet compressed carbon-containing gas is admixed with an ionic substance, in particular a salt or an ionic liquid, and then compressed and in particular liquefied or brought into the supercritical state, preferably converted into the supercritical state.The same applies to carbon-containing liquids, which can be compressed before or after the addition of the ionic substance and preferably brought into the supercritical state.

[0060] In the context of the present invention, particularly good results are obtained in this context if the carbon-containing gas and / or the carbon-containing liquid is selected from the group of short-chain hydrocarbons, in particular alkanes, alkenes, alkynes and mixtures thereof, preferably C 1 -C 4 alkanes, C 1 -C 4 alkenes and / or C 1 -C 4 alkynes, cyclic and / or aromatic C 5 - and C 10 -hydrocarbons, in particular benzene, toluene, phenol and / or naphthalene; carbon oxides, in particular carbon monoxide and / or carbon dioxide; and mixtures thereof. In the context of the present invention, it has proven particularly useful if the carbon-containing gas or the carbon-containing liquid is selected from C1 to C4 alkanes, C1 to C4 alkenes, C1 to C4 alkynes, carbon monoxide, carbon dioxide and mixtures thereof, preferably C1 to C4 alkanes, C1 to C4 alkenes, carbon dioxide and mixtures thereof.Particularly good results are obtained in this context when the carbon-containing gas is carbon dioxide.

[0061] In the context of the present invention, it is particularly preferred if the carbon-containing gas or the carbon-containing liquid is compressed by increasing the temperature and / or pressure, in particular liquefied or converted into the supercritical state, preferably converted into the supercritical state.

[0062] For the purposes of the present invention, a supercritical state is understood to be the state of a substance whose pressure and temperature are above the critical point. The supercritical state is a thermodynamic state of a substance characterized by the equalization of the densities of the liquid and gaseous phases, whereby the differences between the two states of matter cease to exist, and the substance then exists solely as a supercritical fluid.

[0063] In the context of the present invention, it has been found to be advantageous if the aprotic carbon-containing fluid is produced from the carbon-containing gas by compressing, in particular liquefying or converting into the supercritical state, by increasing the temperature and / or pressure, or is produced from the carbon-containing liquid by compressing, in particular converting into the supercritical state, by increasing the temperature and / or pressure.

[0064] Within the scope of the present invention, the procedure is usually such that the aprotic or low-proton carbon-containing electrolyte is compressed, in particular liquefied or converted into the supercritical state, preferably converted into the supercritical state, by increasing the temperature and / or pressure. Likewise, it can also be provided that the compressed aprotic or low-proton carbon-containing electrolyte is obtained by compression, in particular liquefaction or conversion into the supercritical state, preferably conversion into the supercritical state, by increasing the temperature and / or pressure. Conversion of the aprotic or low-proton carbon-containing electrolyte into the supercritical state is to be understood as meaning that at least one constituent or component of the aprotic or low-proton carbon-containing electrolyte is converted into the supercritical state.

[0065] As far as the process implementation, especially the process parameters such as pressure and temperature, are concerned, these can vary widely. However, it has proven effective to carry out the process, especially electrochemical deposition, at elevated temperature and / or pressure.

[0066] In general, the process, in particular the electrochemical deposition, is carried out at a temperature in a range of -60 °C or more, in particular -20 °C or more, preferably 0 °C or more, preferably 20 °C or more, more preferably 30 °C or more, even more preferably 35 °C or more, particularly preferably 40 °C or more, again preferably 50 °C or more, most preferably 60 °C or more.

[0067] Likewise, it can be provided that the process, in particular the electrochemical deposition, is carried out at a temperature in a range of -60 °C to 90 °C, in particular -20 to 90 °C, preferably 0 to 85 °C, preferably 20 to 85 °C, more preferably 30 to 80 °C, even more preferably 35 to 80 °C, particularly preferably 40 to 75 °C, again preferably 50 to 75 °C, most preferably 60 to 70 °C.

[0068] In the context of the present invention, it has also proven useful if the process, in particular the electrochemical deposition, is carried out at a pressure of 1.1 bar or more, in particular 5 bar or more, preferably 10 bar or more, preferably 30 bar or more, more preferably 60 bar or more, particularly preferably 70 bar or more, again preferably 75 bar or more, again more preferably 80 bar or more, even more preferably 85 bar or more, very particularly preferably 90 bar or more.

[0069] Likewise, it can be provided that the process, in particular the electrochemical deposition, is carried out at a pressure in a range of 1.1 to 250 bar, in particular 5 to 230 bar, preferably 10 to 210 bar, preferably 30 to 200 bar, more preferably 60 to 190 bar, particularly preferably 70 to 180 bar, again preferably 75 to 170 bar, again more preferably 80 to 170 bar, even more preferably 85 to 160 bar, very particularly preferably 90 to 150 bar.

[0070] The aforementioned pressure and temperature ranges allow, in particular, the density and solubility properties for gases and, if necessary, other solvents to be increased. In particular, the high density of the carbon-containing fluid also allows for a high deposition rate on the substrate or electrode.

[0071] As already stated above, within the scope of the present invention, it is typically provided that the electrolyte comprises an ionic substance. Particularly good results are obtained in this context when the ionic substance comprises, in particular consists of, a salt, in particular a conducting salt, an ionic liquid, and mixtures thereof, preferably a conducting salt. The ionic substance is preferably selected from salts, in particular conducting salts, ionic liquids, and mixtures thereof. The ionic substance is preferably a salt, in particular a conducting salt.

[0072] In the context of the present invention, an ionic liquid is understood to mean in particular a salt whose melting temperature is less than 100 °C.

[0073] As far as the salt, in particular the conducting salt, is concerned, this can be selected from a variety of possible compounds. However, it has proven useful within the scope of the present invention if the salt, in particular the conducting salt, is selected from tetraalkylammonium salts, in particular C1- to C4-tetraalkylammonium salts, preferably C1- to C4-tetraalkylammonium tetrafluoroborates, C1- to C4-tetraalkylammonium hexafluorophosphates and mixtures thereof, preferably tetrabutylammonium tetrafluoroborate.

[0074] Tetrabutylammonium hexafluorophosphate and mixtures thereof.

[0075] As previously stated, it is also possible for the ionic substance to be an ionic liquid. If the ionic substance is an ionic liquid, it is acceptable if the ionic liquid is selected from the group of liquid imidazolium salts, pyridinium salts, pyrrolidinium salts, guanidinium salts, uronium salts, thiouronium salts, piperidinium salts, morpholinium salts, ammonium salts, phosphonium salts, and mixtures thereof. In particular, the liquid halides, tetrafluoroborates, hexafluorophosphates, sulfonamides, and trifluoroacetamides of the aforementioned salts are preferably used in the context of the present invention. The addition of a conductive salt or an ionic liquid particularly increases the ionic conductivity of the fluid and thus makes it possible to lower the deposition voltage, which makes the targeted deposition of carbon allotropes possible in the first place.

[0076] Within the scope of the present invention, it can further be provided that the aprotic or low-proton, in particular aprotic, carbon-containing electrolyte contains an aprotic solvent. The aprotic solvent is preferably a polar solvent. Polar solvents help dissolve the ionic substance and thus improve the conductivity of the electrolyte.

[0077] As previously stated generally in connection with the carbon-containing fluid, it is also important that any additional solvents added to the fluid are also aprotic, although small amounts of contamination with compounds whose functional groups can release protons can certainly be tolerated. In particular, the release or formation of protons should be excluded as far as possible during the process to avoid the formation of by-products from the reaction between carbon species and protons. Otherwise, the formation of, for example, hydrocarbons, alcohols, especially methanol, or other derivatives such as formic acid would be preferentially favored, rather than the formation of carbon allotropes.The process in aprotic or at least proton-poor, preferably aprotic, systems fundamentally enables the targeted production of different carbon allotropes, in particular those such as those provided for in the invention, ie those which go beyond the mere deposition of amorphous carbon.

[0078] Regarding the aprotic solvent, particularly good results are obtained when the aprotic solvent is selected from the group of ethers, aldehydes, esters, amides, aldehydes, ketones, sulfoxides, nitriles, and mixtures thereof. In this context, it has proven particularly useful when the aprotic solvent is selected from the group of short-chain dialkyl ethers, especially dimethyl ether; C1- to C4-aldehydes; C1- to C4-ketones; methyl and ethyl esters of C1- to C4-carboxylic acids; amides of C1- to C4-carboxylic acids; C1- to C4-sulfoxides; C1- to C4-nitriles, and mixtures thereof. Preferably, the aprotic solvent is selected from the group of acetone, ethyl acetate, N,N-dimethylformamide, dimethyl sulfoxide, propionitrile, acetonitrile and mixtures thereof, preferably propionitrile, acetonitrile and mixtures thereof. Preferably, the aprotic solvent has a pK s -value of at least 20, preferably over 20.

[0079] In this context, it has proven particularly useful when the aprotic solvent is acetonitrile.

[0080] As far as the proportions in which the individual components of the electrolyte are present in the aprotic or proton-poor, especially aprotic, carbon-containing electrolyte, are concerned, these can also vary within wide ranges.

[0081] However, it has proven useful if the compacted aprotic or low-proton, in particular aprotic, carbon-containing electrolyte contains the aprotic carbon-containing fluid in proportions in a range from 10 to 100 mol%, in particular 20 to 95 mol%, preferably 30 to 92 mol%, preferably 40 to 90 mol%, more preferably 50 to 88 mol%, even more preferably 60 to 86 mol%, again preferably 70 to 84 mol%, very particularly preferably 75 to 82 mol%, based on the electrolyte.

[0082] With respect to the electrolyte in the non-compressed state, ie the electrolyte in the initial state before carrying out the process according to the invention, it applies in particular to the same extent that the non-compressed aprotic or low-proton, in particular aprotic, carbon-containing electrolyte has the carbon-containing gas or the carbon-containing liquid in the above-mentioned proportions.

[0083] As far as the ionic substance in the aprotic or proton-poor, in particular aprotic, carbon-containing electrolyte is concerned, it has proven useful if the aprotic or proton-poor, in particular aprotic, carbon-containing electrolyte contains the ionic substance in proportions in a range from 0.1 to 10 mol%, in particular 0.5 to 8 mol%, preferably 0.8 to 6.5 mol%, more preferably 1 to 5.5 mol%, particularly preferably 1.2 to 5 mol%, based on the electrolyte.

[0084] Furthermore, good results are obtained when the aprotic or low-proton, particularly aprotic, carbon-containing electrolyte contains the aprotic, preferably the polar aprotic, solvent in proportions ranging from 1 to 90 mol%, particularly 3 to 80 mol%, preferably 5 to 70 mol%, preferably 7 to 60 mol%, more preferably 9 to 50 mol%, even more preferably 11 to 40 mol%, again preferably 13 to 30 mol%, most preferably 15 to 25 mol%, based on the electrolyte. As for the substrate on which the carbon allotropes are deposited, this can also consist of a variety of materials, particularly conductive ones.

[0085] According to a preferred embodiment of the present invention, the aprotic or low-proton, in particular aprotic, carbon-containing electrolyte has the following composition:

[0086] (i) an aprotic carbon-containing fluid, in particular in proportions ranging from 10 to 100 mol%, based on the electrolyte,

[0087] (ii) an ionic substance in proportions, in particular in a range of 0.1 to 10 mol%, based on the electrolyte, and

[0088] (iii) an aprotic, preferably polar aprotic, solvent, in particular in proportions ranging from 1 to 90 mol%, based on the electrolyte.

[0089] In the context of the present invention, the substrate typically has a metallic surface or is made of a metal. Preferably, the substrate is made of a metal.

[0090] In this context, particularly good results are obtained when the metal is a transition metal. It has proven particularly useful if the transition metal is selected from the group of metals of the iron group (group 8), metals of the cobalt group (group 9), metals of the nickel group (group 10), metals of the copper group (group 11) of the Periodic Table of the Elements and mixtures and alloys thereof. Preferably, the transition element is selected from the group of iron, cobalt, nickel, copper, palladium, silver, platinum, gold and mixtures and alloys thereof, preferably iron, cobalt, nickel, copper, silver, platinum and mixtures and alloys thereof. Particularly good results are obtained within the scope of the present invention if the transition metal is selected from iron, cobalt, nickel, copper and mixtures and alloys thereof, preferably cobalt, nickel, copper and mixtures and alloys thereof.Most preferably, the metal is selected from cobalt, nickel and copper.

[0091] The aforementioned metals, especially cobalt, nickel, and copper, exhibit ideal binding or adsorption properties for the components of the aprotic carbonaceous fluid, achieving efficient conversion to carbon allotropes. Excessive adsorption or binding of the fluid components can lead to deactivation of carbon formation and gradual poisoning of the substrate surface. Insufficient binding or adsorption strength, on the other hand, can result in incomplete conversion and thus the formation of byproducts.

[0092] The nature of the substrate, particularly in conjunction with the applied current density, allows for adjustment of both the reaction rate and the morphology, as well as the nature of the resulting carbon allotrope. By optionally applying an external magnetic field, a forced orientation of the carbon allotropes can also be achieved.

[0093] Within the scope of the present invention, it is typically provided that the substrate is used as an electrode, in particular as a working electrode, preferably in a three-electrode configuration. Typically, the working electrode is the cathode.

[0094] Likewise, within the scope of the present invention, it can be provided that the substrate is an electrode, in particular the working electrode. Preferably, the working electrode is the cathode.

[0095] At the cathode, carbon, especially carbon dioxide, can be effectively reduced. As already mentioned, a silver-silver chloride electrode can serve as the reference electrode. The counter electrode can generally be made of the same metal as the working electrode, in particular, it can be made of the same metal. However, platinum electrodes are particularly preferred as the counter electrode.

[0096] As far as the electrical voltage at which the deposition is carried out is concerned, this can also vary depending on the respective process conditions.

[0097] Typically, the process, in particular the electrochemical deposition, is carried out at an electrical voltage in a range of 10 V or less, in particular 8 V or less, preferably 6 V or less, preferably 5 V or less, preferably 4 V or less.

[0098] Likewise, it can be provided that the process, in particular the electrochemical deposition, is further carried out at an electrical voltage in a range of 1 to 10 V, in particular 1.5 to 8 V, preferably 1.5 to 6 V, preferably 2 to 5 V, more preferably 2 to 4 V. The precise adjustment and control of the voltage allows, in particular, the targeted control of the electrochemical deposition with regard to the speed of the deposition or the deposition rate from the compressed fluid and, in particular, the type of deposited carbon allotrope.

[0099] Furthermore, the magnetic field created by the applied current can control the preferred direction of particle growth, allowing the material properties to be specifically modified and optimized. The influence of an external magnetic field is also possible to specifically influence the deposition behavior.

[0100] As far as the process is concerned, the substrate and the aprotic or proton-poor, in particular aprotic, carbon-containing electrolyte are usually provided in a first process step (a) in a device that can be subjected to pressure and / or temperature. Likewise, within the scope of the present invention, the device can be an autoclave and / or a high-pressure reactor. It is particularly preferred if an electrode assembly, preferably a three-electrode assembly, is arranged in the device. Preferably, the substrate is integrated into the electrode assembly and forms, in particular, the working electrode or cathode. In addition to the working electrode, which is preferably the cathode, the assembly also comprises a counterelectrode, which is then preferably the anode, and, if possible, a reference electrode.

[0101] According to a preferred embodiment of the present invention, it is provided that in a second process step (b) following the first process step (a), the aprotic or proton-poor, in particular aprotic, carbon-containing electrolyte is compressed, in particular liquefied or converted into the supercritical state, preferably converted into the supercritical state, by applying pressure and / or temperature to the device.

[0102] Particularly good results are obtained within the scope of the present invention if, in a third process step (c) following the second process step (b), a voltage is applied in the device, in particular between the working electrode and the counter electrode, in particular as a result of which the electrochemical deposition of carbon allotropes on the substrate takes place.

[0103] As far as the duration of the process according to the invention is concerned, it has proven useful if the process according to the invention, in particular the third process step (c), is carried out over a period of 10 to 150 min, in particular 20 to 130 min, preferably 25 to 120 min, preferably 30 to 110 min, more preferably 35 to 100 min, even more preferably 40 to 90 min, again preferably 45 to 80 min, more preferably 50 to 70 min, very particularly preferably 55 to 65 min.

[0104] In general, after completion of the electrochemical deposition, in particular after completion of process step (c), the voltage is switched off and the pressure and / or temperature are reduced, in particular to ambient values, and the carbon allotropes deposited on the substrate are isolated. According to a preferred embodiment of the present invention, the carbon allotropes deposited on the substrate are isolated in a fourth process step (d) following the third process step (c).

[0105] Furthermore, within the scope of the present invention, it is also possible for the substrate, in particular the electrodes, to be cleaned and dried, in particular degreased, in preparation for the process in order to ensure reproducible good deposition rates.

[0106] According to a preferred embodiment of the present invention, the process is thus a process for producing carbon allotropes, wherein

[0107] (a) in a first process step, a substrate and an aprotic or proton-poor, in particular aprotic, carbon-containing electrolyte are provided in a device which can be subjected to pressure and / or temperature,

[0108] (b) in a second process step following the first process step (a), the aprotic or proton-poor, in particular aprotic, carbon-containing electrolyte is compressed, in particular liquefied or converted into the supercritical state, preferably converted into the supercritical state, by applying pressure and / or temperature to the device,

[0109] (c) in a third process step following the second process step (b), a voltage is applied in the device, in particular between the electrodes, in particular by means of which the electrochemical deposition of carbon allotropes on the substrate takes place, and

[0110] (d) in a fourth process step following the third process step (c), the carbon allotropes deposited on the substrate are isolated.

[0111] For this preferred embodiment of the present invention, all features, special features, and advantages previously mentioned for the other embodiments apply equally. Prior to the process, in particular process step (a), various washing and, if appropriate, drying steps, particularly for cleaning the substrate, can be provided. The resulting carbon allotropes are particularly suitable for use as or in active material for electrodes in battery applications, filler for bipolar plates for battery applications and PEM fuel cells, electrolyzers, supercapacitors, resistance heating elements, heat exchangers, for shielding high-frequency radiation (e.g., in medical technology), as biocompatible implants, or even for water filtration.

[0112] The subject matter of the present invention is explained below with reference to preferred embodiments in a non-limiting manner by means of the exemplary embodiment.

[0113] Examples of implementation: Experimental setup:

[0114] A three-electrode system consisting of a working electrode, a counter electrode, and a reference electrode is installed in a 300 ml high-pressure reactor. The working electrode has a copper surface. A platinum electrode serves as the counter electrode, and a silver electrode serves as the reference electrode. The electrode system is connected to a potentiostat outside the reactor via a high-pressure feedthrough. Electrolysis preparation

[0115] The reactor is filled with an electrolyte solution. For this purpose, 20 mol% acetonitrile and 3 mol% conducting salt are mixed together and placed in the reactor chamber. Subsequently, approximately 80 mol% CO2 is introduced into the reactor chamber.

[0116] The resulting electrolyte solution is then subjected to pressure and temperature and compressed or converted to the supercritical state at 150 bar and 70 °C. Electrochemical deposition

[0117] A constant voltage in the range of 0.5 to 2 V, in the form of a constant potential in the range of -0.5 to -2 V, is applied to the thus compacted electrolyte solution for a period of one hour against the reference electrode. Potential changes occur in 0.5 V increments. Carbon is deposited on the working electrode, particularly in the form of graphene, carbon nanotubes, and fullerenes. Post-treatment and analysis

[0118] After the electrolysis period has elapsed, the pressure is released, the temperature is reduced and the working electrode is finally removed from the reactor.

[0119] The working electrode is then analyzed using energy-dispersive X-ray spectroscopy (EDX, see Fig. 1), including its elemental composition. In addition to copper, the electrode also clearly exhibits high amounts of carbon on its surface.

Claims

Patent claims: A method for producing carbon allotropes by electrochemically depositing a carbon allotrope on a substrate, characterized in that the electrochemical deposition is carried out from a dense aprotic or proton-poor, in particular aprotic, carbon-containing electrolyte. The method according to claim 1, characterized in that the carbon allotrope is selected from the group consisting of diamond, graphite, graphene, carbon nanotubes, fullerenes, graphyne, carbyne, and mixtures thereof, in particular graphene, carbon nanotubes, fullerenes, graphyne, carbyne, and mixtures thereof. The method according to claim 1 or 2, characterized in that the dense aprotic or proton-poor carbon-containing electrolyte comprises an aprotic carbon-containing fluid and an ionic substance.Process according to claim 3, characterized in that the aprotic carbon-containing fluid is a carbon-containing gas or a carbon-containing liquid and / or is produced from a carbon-containing gas or a carbon-containing liquid. Process according to claim 4, characterized in that the carbon-containing gas or the carbon-containing liquid is selected from short-chain hydrocarbons, in particular alkanes, alkenes, alkynes and mixtures thereof, preferably C1- to C4-alkanes, C1- to C4-alkenes and / or C1- to C4-alkynes; cyclic and / or aromatic C5- and C10-hydrocarbons, in particular benzene, toluene, phenol and / or naphthalene; carbon oxides, in particular carbon monoxide and / or carbon dioxide; and mixtures thereof. Process according to claim 4 or 5, characterized in that the carbon-containing gas is carbon dioxide.

7. The method according to any one of claims 4 to 6, characterized in that the carbon-containing gas or the carbon-containing liquid is compressed by increasing the temperature and / or pressure, in particular liquefied or converted into the supercritical state, preferably converted into the supercritical state.

8. The method according to any one of the preceding claims, characterized in that the method, in particular the electrochemical deposition, is carried out at a temperature in a range of -60 °C or more, in particular -20 °C or more, preferably 0 °C or more, preferably 20 °C or more, more preferably 30 °C or more, even more preferably 35 °C or more, particularly preferably 40 °C or more, again preferably 50 °C or more, most preferably 60 °C or more.

9. The method according to any one of the preceding claims, characterized in that the method, in particular the electrochemical deposition, is carried out at a pressure in a range of 1.1 bar or more, in particular 5 bar or more, preferably 10 bar or more, preferably 30 bar or more, more preferably 60 bar or more, particularly preferably 70 bar or more, again preferably 75 bar or more, again more preferably 80 bar or more, even more preferably 85 bar or more, very particularly preferably 90 bar or more.

10. The method according to any one of claims 3 to 9, characterized in that the ionic substance comprises, in particular consists of, a salt, in particular a conducting salt, an ionic liquid and mixtures thereof, preferably a conducting salt.

11. The method according to claim 10, characterized in that the salt, in particular the conducting salt, is selected from tetraalkylammonium salts, in particular C1- to C4-tetraalkylammonium salts, preferably C1- to C4-tetraalkylammonium tetrafluoroborates, C1- to C4-tetraalkylammonium hexafluorophosphates and mixtures thereof, preferably tetrabutylammonium tetrafluoroborate, tetrabutylammonium hexafluorophosphate and mixtures thereof.

12. The method according to claim 10, characterized in that the ionic liquid is selected from the group of liquid imidazolium salts, pyridinium salts, pyrrolidinium salts, guanidinium salts, uronium salts, thiouronium salts, piperidinium salts, morpholinium salts, ammonium salts, phosphonium salts and mixtures thereof, in particular the liquid halides, tetrafluoroborates, hexafluorophosphates, sulfonamides, trifluoroacetamides of the aforementioned salts.

13. The method according to any one of claims 3 to 12, characterized in that the aprotic or low-proton carbon-containing electrolyte comprises an aprotic solvent, in particular a polar aprotic solvent.

14. The process according to claim 13, characterized in that the aprotic solvent is selected from the group consisting of ethers, aldehydes, esters, amides, aldehydes, ketones, sulfoxides, nitriles and mixtures thereof.

15. The process according to claim 13 or 14, characterized in that the aprotic solvent is acetonitrile.

16. The method according to any one of claims 3 to 15, characterized in that the compacted aprotic or low-proton carbon-containing electrolyte comprises the aprotic carbon-containing fluid in proportions in a range from 10 to 100 mol%, in particular 20 to 95 mol%, preferably 30 to 92 mol%, more preferably 40 to 90 mol%, more preferably 50 to 88 mol%, even more preferably 60 to 86 mol%, again preferably 70 to 84 mol%, most preferably 75 to 82 mol%, based on the electrolyte.

17. The method according to claim 3 to 16, characterized in that the aprotic or low-proton carbon-containing electrolyte contains the ionic substance in proportions in a range of 0.1 to 10 mol%, in particular 0.5 to 8 mol%, preferably 0.8 to 6.5 mol%, more preferably 1 to 5.5 mol%, particularly preferably 1.2 to 5 mol%, based on the electrolyte.

18. The method according to claim 3 to 17, characterized in that the aprotic carbon-containing electrolyte contains the solvent in proportions in a range from 1 to 90 mol%, in particular 3 to 80 mol%, preferably 5 to 70 mol%, preferably 7 to 60 mol%, more preferably 9 to 50 mol%, even more preferably 11 to 40 mol%, again preferably 13 to 30 mol%, very particularly preferably 15 to 25 mol%, based on electrolyte.

19. Method according to one of the preceding claims, characterized in that the substrate has a metallic surface or consists of a metal.

20. The method according to claim 19, characterized in that the metal is a transition metal, in particular wherein the transition metal is selected from the group of metals of the iron group, metals of the cobalt group, metals of the nickel group, metals of the copper group of the Periodic Table of the Elements and mixtures and alloys thereof, preferably from the group of iron, cobalt, nickel, copper, palladium, silver, platinum, gold and mixtures and alloys thereof, preferably iron, cobalt, nickel, copper, silver, platinum and mixtures and alloys thereof, particularly preferably iron, cobalt, nickel, copper and mixtures and alloys thereof, very particularly preferably cobalt, nickel, copper and mixtures and alloys thereof.

21. Method according to one of the preceding claims, characterized in that the substrate is used as an electrode, in particular as a working electrode, preferably in a three-electrode structure, in particular wherein the working electrode is the cathode.

22. Method according to one of the preceding claims, characterized in that the method, in particular the electrochemical deposition, is carried out at an electrical voltage in a range of 10 V or less, in particular 8 V or less, more preferably 6 V or less, preferably 5 V or less, further preferably 4 V or less.