METHOD FOR PRODUCING AN ELECTRICALLY CONDUCTIVE CARBON CONDUCTOR WITH CARBON STRUCTURAL SHAPES

Description

State of the art

[0001] The present invention relates to a method for producing an electrically conductive carbon conductor. The carbon conductor has at least one carbon structural form that is an allotropic modification of carbon. The carbon conductor is provided with further compounds to increase its electrical conductivity.

[0002] Carbon conductors are known from the prior art. For example, electrical conductors made of graphite, pyrolytic graphite, carbon nanotubes, or graphene are known from the prior art. To increase their electrical conductivity, it is known to dope the carbon conductor.

[0003] From CN106744888A it is known to produce graphene in a graphene dispersion by adding aluminium fluoride and amines.

[0004] In US 9 306 096 B1, it is stated that the electrical conductivity of the perfluorinated sulfonic acid polymer Nafion-H can be increased by small additions of carbon nanotubes (approx. 0.1 wt%).

[0005] Methods for producing an electrical carbon conductor according to the preamble of claim 1 are known from DE 29 46 414 A1, US 2010 / 206363 A1 and US 2013 / 180581 A1. Disclosure of the invention

[0006] The method according to the invention enables the doping of carbon conductors, particularly with fluorides. The invention avoids the use of chlorine or fluorine as doping agents, thereby preventing undesirable reactions with the carbon conductor. Since fluorination of the carbon conductor would lead to a significant deterioration of its electrical conductivity, the electrical conductivity is thus improved. Furthermore, the dopants according to the invention exhibit high temperature stability and resistance to atmospheric humidity. Simultaneously, the electrical conductivity of the carbon conductor is enhanced.

[0007] The process according to the invention serves to produce an electrical carbon conductor from carbon structures in the sense of allotropic modifications of carbon, in particular graphite, pyrolytic graphite, graphene and / or carbon nanotubes, as well as graphene precursor compounds such as graphene oxide. The carbon structures are doped with an additive to increase the electrical conductivity of the electrical carbon conductor. The additive is in particular aluminum fluoride and / or aluminum chlorofluoride and / or perfluorinated polymeric sulfonic acid.

[0008] The process begins with the preparation of a liquid dispersion. This dispersion contains both the carbon structure forms and a solvent. An additive is then added to the dispersion. The combined components are then mixed.

[0009] The next step involves producing a fiber- or film-shaped conductor strand to form the carbon conductor. This is preferably done by wet spinning the dispersion or by depositing the dispersion onto a support material. The dispersion fluid is then removed from the conductor strand without generating gas or vapor bubbles by diffusion from the liquid phase, achieved in particular through drying, vacuum, and heat treatment. The additive remains in the conductor strand and, in particular, results in the aforementioned doping. This allows for a simple and reliable increase in the electrical conductivity of the carbon conductor.

[0010] According to the invention, the additive is aluminum fluoride and / or aluminum chlorofluoride. These substances enable effective doping. To suppress crystallization of these additives, a crystallization inhibitor is additionally added to the dispersion according to the invention. The crystallization inhibitor is, in particular, a complexing agent with polycyclic aromatic carbon groups. Alternatively or additionally, surfactants are used as crystallization inhibitors, in particular nonionic surfactants with hydrophilic groups, anionic surfactants with hydrophilic groups, or amphoteric surfactants with hydrophilic groups. The crystallization inhibitors advantageously enable a molecular distribution of the aluminum fluoride and / or aluminum chlorofluoride into the molecular spaces between the carbon structures.This monomolecular distribution, compared to the non-monomolecular distribution, causes the additives to become stronger Lewis acids.

[0011] The dependent claims describe preferred embodiments of the invention.

[0012] The manufactured conductor strand is particularly advantageously heat-treated. This heat treatment achieves the thermal decomposition of the crystallization inhibitor within the conductor strand material. Alternatively or additionally, the heat treatment removes oxygen atoms from the conductor strand material. This ensures that no or virtually no impurities remain in the finished carbon conductor. The crystallization inhibitor is only required during the manufacturing of the conductor strand to prevent the additive from crystallizing. Subsequently, the crystallization inhibitor can be removed, with the amorphous additive ensuring high electrical conductivity of the carbon conductor.

[0013] In a further particularly preferred embodiment, aluminum fluoride and / or aluminum chlorofluoride are added such that the conductor strand has at least 80 vol% carbon structure forms and a maximum of 20 vol% aluminum fluoride and / or aluminum chlorofluoride. In particular, the proportion of aluminum fluoride and / or aluminum chlorofluoride is a maximum of 10 vol%.

[0014] Aluminum chlorofluoride preferably has the molecular formula AlCl₃F₃²⁻ with 0.005 ≤ x ≤ 0.01. This minimizes the chlorine content. This results in lower sensitivity to the presence of Lewis bases, such as atmospheric moisture, which reduce the acid strength of the aluminum chlorofluoride by reacting with the chlorine. Since only the chlorine in aluminum chlorofluoride is sensitive to such Lewis bases, aluminum chlorofluorides with the lowest possible chlorine content are advantageous.

[0015] In a non-inventive embodiment, the additive is a precursor compound of aluminum fluoride. This precursor compound is partially decomposable to aluminum fluoride by heat. Thus, aluminum fluoride is again used for doping, but the aluminum fluoride does not need to be directly introduced into the dispersion for manufacturing the conductor strand. Instead, precursor compounds of aluminum fluoride are used, which allow for simpler handling and thus simplify the manufacturing process. Such precursor compounds of aluminum fluoride are, in particular, aluminum trifluoroacetate and / or aluminum difluoroacetate and / or fluorinated aluminum alkoxides, especially aluminum trifluoroethoxide and / or triammonium hexafluoroaluminate and / or aluminum hydroxide fluoride, wherein the aluminum hydroxide fluoride in particular has the molecular formula AlF₆(OH)₃-x with x=2.If such precursor compounds of aluminum fluoride are used, the precursor compounds can be converted to aluminum fluoride after the conductor strand has been manufactured in order to achieve optimal doping of the conductor strand.

[0016] Heat treatment of the manufactured conductor strand is particularly advantageous. In the embodiment not according to the invention, the heat treatment of the conductor strand results in the thermal decomposition of the aluminum fluoride precursor compound in the conductor strand material. This decomposition leads to the in-situ production of aluminum fluoride in the conductor strand and thus to the doping of the conductor strand. Alternatively or additionally, the heat treatment removes oxygen atoms from the conductor strand material.

[0017] For the embodiment not according to the invention, it is also preferred that the additive is perfluorinated polymeric sulfonic acid, wherein the carbon structure comprises only graphite, pyrolytic graphite and / or graphene and / or carbon nanotubes. Perfluorinated polymeric sulfonic acids are non-volatile, temperature-stable up to over 200°C, and resistant to atmospheric humidity. Furthermore, perfluorinated polymeric sulfonic acids are water-soluble and can therefore be incorporated into, for example, an aqueous dispersion of graphite, pyrolytic graphite, graphene, or carbon nanotubes to distribute them molecularly on the surface of the nanocarbon. Since perfluorinated polymeric sulfonic acids are also strong Lewis acids, they exhibit optimal properties as dopants, as high electrical conductivity is achieved in the conductor strand.

[0018] For the embodiment not according to the invention, the perfluorinated polymeric sulfonic acid preferably has the following structure with x < 1, y < 3, m < 7 and n < 1000:

[0019] These perfluorinated polymeric sulfonic acids are ideally suited for use in the manufacturing process and result in high electrical conductivity of the finished carbon conductor. At the same time, they are temperature-stable and resistant to moisture.

[0020] The addition of the perfluorinated polymeric sulfonic acid is carried out in such a way that the conductor strand has at least 85 vol% carbon structural forms and a maximum of 15 vol% perfluorinated polymeric sulfonic acid. Preferably, the proportion of perfluorinated polymeric sulfonic acid is limited to a maximum of 10 vol%.

[0021] The amorphous aluminum fluoride and / or aluminum chlorofluoride are preferentially and uniformly distributed within the conductor strand. Therefore, these additives are primarily present in an amorphous state. This results in a high Lewis acid strength and thus optimal doping concentrations. Consequently, high electrical conductivities of the carbon conductors can be achieved. Embodiments of the invention

[0022] Preferably, the electrical conductivity of carbon conductors based on graphite, pyrolytic graphite, graphene, or carbon nanotubes is to be increased by extrinsic doping. The doping is temperature-stable and resistant to humidity.

[0023] The carbon conductor is manufactured by first producing a liquid dispersion of, in particular undoped, carbon structures, a solvent, and other substances to improve dispersibility. The undoped carbon structures are primarily graphite, pyrolytic graphite, graphene, or carbon nanotubes, as well as their graphene precursor compounds such as graphene oxide. To increase the electrical conductivity of the carbon conductor, it is doped with an additive, which is added to the dispersion and then mixed.

[0024] Finally, a fiber- or film-shaped conductor strand is produced to form the carbon conductor. This is achieved, for example, by wet spinning the dispersion or by depositing the dispersion onto a support material. The dispersion fluid is then removed from the conductor strand by diffusion, which is achieved primarily through drying, vacuum, and heat treatment.

[0025] Various additives can be used to produce the doping, as explained below: Use of precursor compounds of aluminum fluoride (AlF 3 )

[0026] In a non-inventive embodiment, the use of aluminum fluoride precursor compounds for the in-situ production of amorphous aluminum fluoride (AlF₃) is provided within the conductor strand. AlF₃ is produced by thermolysis. Suitable precursor compounds decompose to form AlF₃ without melting or forming a liquid phase. The resulting amorphous AlF₃ causes strong extrinsic doping of the graphite, pyrolytic graphite, graphene, or carbon nanotubes, thereby improving the electrical conductivity by at least one order of magnitude. Due to the high strength of amorphous AlF₃ as a Lewis acid, electrical conductivities of up to 90 MS / m can be achieved.

[0027] The aforementioned precursor compounds of AlF₃ are added to the dispersion as additives. Preferably, aluminum trifluoroacetate, aluminum difluoroacetate, fluorinated aluminum alcoholates such as, in particular, aluminum trifluoroethoxide, triammonium hexafluoroaluminate, and aluminum hydroxide fluoride AlF₆(OH)₃-x, especially with x=2, are used as precursor compounds for amorphous AlF₃. These are dissolved in the solvent in which graphene or graphene oxide can also be dispersed. A suitable solvent is, for example, dimethylformamide.

[0028] After fabrication of the conductor strand, in which at least some of the AlF₃ precursor compounds remain alongside the undoped carbon structures, a heat treatment is performed to decompose the precursor compounds into AlF₃. By utilizing such AlF₃ precursor compounds, any existing clusters of these compounds are destroyed through thermolysis. This results in a near-molecular distribution on the graphite, pyrolytic graphite, graphene, or carbon nanotubes. This leads to a stronger doping effect and thus to high electrical conductivity of the conductor strand.

[0029] The aluminum fluorides formed from the precursor compounds are predominantly amorphous. This can be achieved if the precursor compounds of AlF₃ are distributed at approximately molecular levels on the surface of the graphite, pyrolytic graphite, graphene, or carbon nanotubes. This is achieved particularly by avoiding the use of solvent-dissolved AlF₃, since AlF₃ has a strong tendency to crystallize, thus hindering the formation of amorphous AlF₃ from the solvent. The formation of nanoparticles prior to thermolysis is acceptable because the gases produced during thermolysis cause the particles to break down into even smaller particles.

[0030] The carbon conductor preferably consists of at least 80 vol.% graphite, pyrolytic graphite, graphene or carbon nanotubes, i.e. the conductor strand, and a maximum of 20 vol.% amorphous aluminum fluoride, i.e. the additive.

[0031] It is particularly advantageous if the proportion of amorphous aluminum fluoride is limited to a maximum of 50 vol.%. Use of aluminum fluoride (AlF 3 ) and a crystallization inhibitor

[0032] In one embodiment of the invention, the use of aluminum fluoride (AlF₃) or aluminum chlorofluoride (AlF₃-xCl₃) for extrinsic doping of the conductor strand based on graphite, pyrolytic graphite, graphene, or carbon nanotubes is provided. Due to the high strength of monomolecular AlF₃ or AlF₃-xCl₃ as Lewis acids, optimal doping and thus high electrical conductivity of the carbon conductor can be achieved.

[0033] To suppress or at least reduce the crystallization of AlF3 or AlF3-xClx, the use of a crystallization inhibitor is planned. This allows for a monomolecular distribution of AlF3 or AlF3-xClx on the graphene or carbon nanotubes, respectively.

[0034] The AlF₃ or AlF₃-xCl₃ is preferably dissolved in the solvent in which the undoped carbon structure forms are or can be dispersed. This solvent exhibits solubility for both of the aforementioned fluorides. After formation of the conductor strand as previously described, at least some of the fluorides remain in it as strong Lewis acids.

[0035] Since the chlorine in aluminium chlorofluoride is very sensitive to Lewis bases and therefore also to moisture, the parameter x in the molecular formula AlCl x F 3-x is preferably chosen to be between 0.005 and 0.01, so that only a small proportion of chlorine is present.

[0036] To ensure that the aforementioned fluorides are predominantly amorphous and thus achieve their high strength as Lewis acids, their tendency to form crystals in aqueous solution is preferably suppressed by crystallization inhibitors. These inhibitors bind to the fluorides and sterically impede crystallization. This results in the fluorides being predominantly amorphous and, particularly preferably, monomolecular in their interaction with the conductor material. Such crystallization inhibitors include, in particular, surfactants or complexing agents. Complexing agents with polycyclic aromatic carbon groups are especially suitable.

[0037] The following surfactants are preferred: Nonionic surfactants with hydrophilic groups such as -OH, i.e., multiple alcohols, -O-, i.e., ethers, or the combination -O-CH2-CH2-OH, e.g., ethoxylates; anionic surfactants with hydrophilic groups such as -COO-<, i.e., carboxylates, -SO3-<, i.e., sulfonates, or -OSO3-<, i.e., sulfates, or amphoteric surfactants, i.e., zwitterionic surfactants, with hydrophilic groups such as -COO-<, i.e., carboxylates, and R4N+<, i.e., quaternary ammonium groups.

[0038] The hydrophobic part of particularly suitable surfactants contains polycyclic aromatic carbon groups such as phenyl, napthalin, or anthracene. Due to their structural similarity to graphite, pyrolytic graphite, graphene, and carbon nanotubes, these groups have a high affinity for them and bind accordingly.

[0039] Finally, the conductor strand undergoes heat treatment. The surfactants or complexing agents used decompose at temperatures up to 500°C, leaving behind no inorganic substances other than carbon. Therefore, no impurities are present. This ensures that the highly effective doping with amorphous aluminum chlorofluoride and aluminum fluoride, which increases the electrical conductivity of the conductor strand, is not affected.

[0040] The carbon conductor preferably consists of at least 80 vol.% graphite, pyrolytic graphite, graphene and / or carbon nanotubes, i.e., the conductor strand, and a maximum of 20 vol.% aluminum fluoride and / or aluminum chlorofluoride, i.e., the additive. It is particularly advantageous if the proportion of aluminum fluoride and / or aluminum chlorofluoride is limited to a maximum of 10 vol.%. Use of perfluorinated polymeric sulfonic acids (PFSA)

[0041] In a further embodiment of the invention not according to the invention, the use of perfluorinated polymeric sulfonic acids (PFSA) for the extrinsic doping of the electrical conductor strand based on graphite, pyrolytic graphite, graphene, or carbon nanotubes is provided. PFSA is a high-strength Lewis acid, enabling high electrical conductivity of the conductor strand. Simultaneously, temperature-stable extrinsic doping is enabled, particularly at temperatures above 200°C.

[0042] Industrially produced PFSAs have an acidity, expressed as pKa, in the range of -5.5 to -6. Furthermore, PFSAs are non-volatile, temperature-stable up to over 200°C, and resistant to atmospheric humidity. They are water-soluble and can therefore be incorporated into, for example, aqueous dispersions of graphite, pyrolytic graphite, graphene, or carbon nanotubes to distribute them molecularly on the surface of the carbon nanotubes.

[0043] PSFAs with the following structure with x < 1, y < 3, m < 7 and n < 1000 are particularly advantageous:

[0044] PSFA is therefore added to the dispersion as an additive. After the conductor strand is manufactured as described above, at least some of the PSFA remains in the conductor strand as a strong Lewis acid. The use of PSFA thus enables doping to increase the electrical conductivity of the doped conductor strand, with the doping being particularly temperature- and humidity-stable.

[0045] The carbon conductor preferably consists of at least 85 vol.% graphite, pyrolytic graphite, graphene, or carbon nanotubes (i.e., the conductor strand) and a maximum of 15 vol.% perfluorinated polymeric sulfonic acid (i.e., the additive). It is particularly advantageous if the proportion of perfluorinated polymeric sulfonic acid is limited to a maximum of 10 vol.%. Examples

[0046] Different examples of the invention are described below. Example 1, which is not according to the invention:

[0047] One liter of aqueous graphene dispersion containing 0.5 wt% graphene, with an average lateral extent of 10 µm, is mixed with one milliliter of a compound containing 25 wt% perfluorinated polymeric sulfonic acid (PFSA) in a container using a stirrer. The mixture is then dispersed using an ultrasonic device to achieve a homogeneous distribution of the graphene and PFSA.

[0048] The dispersion is separated onto filter paper using vacuum filtration. After drying, the graphene film is peeled off the filter. Example 2, which is not according to the invention:

[0049] One liter of graphene dispersion with DMF as the dispersion medium (containing 0.5% graphene by mass, with an average lateral extent of 10 µm) is mixed with 0.25 g of aluminum trifluoroacetate (C₆AlF₆O₆) in a container using a stirrer. The mixture is then dispersed using an ultrasonic device to achieve a homogeneous distribution of the graphene and complete dissolution of the aluminum trifluoroacetate.

[0050] The dispersion is separated onto filter paper using vacuum filtration. After drying, the composite film is removed from the filter. Further heat treatments are carried out under argon in a temperature range of 250°C to 1270°C. Example 3, which is not in the invention:

[0051] One liter of graphene dispersion with water as the dispersion medium, containing 0.5 wt% graphene and with an average lateral extent of 10 µm, is mixed with 0.2 g of triammonium hexafluoroaluminate ((NH₄)₃[AlF₆]) in a container using a stirrer. The mixture is then dispersed using an ultrasonic device to achieve a homogeneous distribution of the graphene and complete dissolution of the triammonium hexafluoroaluminate.

[0052] The dispersion is separated onto filter paper using vacuum filtration. After drying, the composite film is removed from the filter. Further heat treatments are carried out under argon in a temperature range of 100°C to 1270°C. Example 4 according to the invention:

[0053] One liter of aqueous graphene dispersion containing 0.5% graphene by mass, with an average lateral extent of 10 µm, is mixed with 0.25 g of aluminum fluoride trihydrate (AlF₃·3H₂O) and 0.25 g of the non-ionic surfactant Octoxinol 9 in a container using a stirrer. The mixture is then dispersed using an ultrasonic device to achieve a homogeneous distribution of the graphene and complete dissolution of the aluminum fluoride and the surfactant.

[0054] The dispersion is separated onto filter paper using vacuum filtration. After drying, the composite film is removed from the filter. Further heat treatments are carried out under argon in a temperature range of 250°C to 1272°C.

Claims

1. Process for producing an electrical carbon conductor composed of at least one structure form of carbon which is an allotrope of carbon, in particular graphite, pyrolytic graphite, graphene and / or carbon nanotubes, and precursor compounds of graphene, for example graphene oxide, and which has been doped in order to increase the electrical conductivity of the electrical carbon conductor with an additive for doping the structure form of carbon, in particular aluminium fluoride and / or aluminium chlorofluoride and / or perfluorinated polymeric sulfonic acid, comprising the steps of: • producing a liquid dispersion from an undoped structure form of carbon and a solvent, • adding the additive to the dispersion and mixing the dispersion, • producing a conductor strand in fibre or film form in order to form the carbon conductor, in particular by wet spinning the dispersion or by depositing the dispersion on a carrier material, and by removing the dispersion fluid from the conductor strand, which is achieved in particular by drying, reduced pressure and heat treatment, characterized in that the additive is aluminium fluoride and / or aluminium chlorofluoride, with additional addition of a crystallization inhibitor to the dispersion, in particular of complexing agent having polycyclic aromatic carbon groups or surfactants, in particular nonionic surfactants having hydrophilic groups, anionic surfactants having hydrophilic groups or amphoteric surfactants having hydrophilic groups.

2. Process according to Claim 1, wherein the process additionally comprises the step of: • heat treating the conductor strand produced, wherein the heat treating achieves a breakdown of the crystallization inhibitor in the material of the conductor strand and / or a removal of oxygen atoms from the material of the conductor strand.

3. Process according to Claim 1 or 2, characterized in that aluminium fluoride and / or aluminium chlorofluoride is added in such a way that the conductor strand has at least 80% by volume of the structure forms of carbon and not more than 20% by volume, preferably not more than 10% by volume, of aluminium fluoride and / or aluminium chlorofluoride.

4. Process according to Claim 1 or 2 or 3, characterized in that the aluminium chlorofluoride has the empirical formula AlClxF3-x with 0.005 ≤ x ≤ 0.01.

5. Process according to any of Claims 1 to 4, characterized in that the amorphous aluminium fluoride and / or the aluminium chlorofluoride are evenly distributed in the conductor strand at the molecular level.