Nitrogen-doped graphene and uses thereof

Abstract

To provide a method for producing a nitrogen-doped graphene that is particularly useful as a supercapacitor material.SOLUTION: The method contains the following steps: a) providing a dispersion of fluorinated graphite; b) subjecting the dispersion of fluorinated graphite to sonication and / or mechanical treatment and / or thermal treatment; c) contacting the product from step b) with an azide reagent at a temperature within the range of 40 to 200°C; d) separating the solid product formed in step c) from the mixture; and e) dialyzing the product obtained in step d) against water. The method yields a nitrogen-doped graphene containing at least 8.9 at.% of nitrogen and up to 16.6 at.% of fluorine, wherein the at.% are relative to the total atoms present in the sample and determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα source; and the nitrogen-doped graphene has a density above 1.2 g / cm3 when pressed at 80 kN for 1 min.SELECTED DRAWING: None

Description

Detailed Description of the Invention

[0001] [Technical Field] The present invention relates to a novel method for producing nitrogen-doped graphene, which provides graphene with high nitrogen doping and some residual fluorine content. The resulting material can be used to form electrodes for supercapacitors with high volumetric energy density at high power densities.

[0002] [Background technology] Supercapacitors represent an energy storage technology with remarkably superior qualities, such as rapid charge / discharge capabilities (i.e., high power density) and extended cycle life. Significant efforts have been made to increase the energy density, which is approximately one order of magnitude lower than that of batteries, without sacrificing power density. The adsorption / desorption of ions dissolved in the electrolyte of a supercapacitor cell onto the surface of the electrode material is involved in the energy storage mechanism of supercapacitors. Due to the importance of these interfacial phenomena, efforts have focused on high surface area porous carbons. However, such carbons have very low densities (approximately 0.3–0.5 g / cm). 3 ), resulting in large electrolyte-filled void spaces in the electrode. Thus, the electrolyte-filled pore space, rather than the interface, does not increase capacitance and limits the energy density of the material.

[0003] A further strategy for improving ion adsorption and supercapacitor performance is to make the surface of carbon or graphene-based electrodes more polar by doping with nitrogen or other elements. Nitrogen doping in carbon is beneficial from the standpoint of electronic conductivity and wettability, and also contributes to pseudocapacitance (charge storage via redox fabrication methods). In principle, nitrogen-doped graphene is currently available through two methods: direct synthesis (e.g., chemical vapor deposition, arc discharge, and segregation growth techniques) or post-treatment (e.g., by nitrogen plasma treatment and interaction with nitrogen-containing molecules during thermal, ultrasonic, solvothermal, or microwave treatment). Typical nitrogen contents achieved are between 1 and 8 at%.

[0004] In certain cases, graphene-based materials were superdoped with nitrogen (a term used for doping higher than approximately 10 at%). Here, the graphene precursor was first fluorinated prior to the N-doping reaction to give derivatives containing N and F. Fluorination is used to create vacancies / defects in the graphene sheets to enable higher doping with nitrogen up to 12.5 at% (see, e.g., J. Chem. Tech. Biotech., 2019, vol. 94, 3530-3537) and up to 30 at% (see, e.g., Yuan Liu et al., Nature Comm. 2016, vol. 7, 10921). In the former case (see, e.g., J. Chem. Tech. Biotech., 2019, vol. 94, 3530-3537), a capacitance of 225 F / g was obtained, resulting in an energy density of 7.8 Wh / kg at a power density of 0.12 kW / kg. In the latter case, graphene material containing 30 at% N was tested as a supercapacitor electrode in a three-electrode system (one working electrode). A capacitance of 390 F / g was obtained, but the gravimetric energy density at a power density of 0.5 kW / kg was very limited at 8.6 Wh / kg. A very similar procedure leading to nitrogen contents of up to 15.8 at% on graphene was published by the authors of the above study in Adv. Mater. 2017, vol. 29(36), 1701677. In this case, the material was tested in a full-cell device (two electrodes with active material) and achieved a performance of 280 F / g, with a power density of 0.12 kW / kg at an energy density of 6.2 Wh / kg. Such a gravimetric energy density is too low for a competitive supercapacitor. To reach 90 Wh / L at such a gravimetric density (which has been achieved with pressed or densified graphene-based electrodes (see below)), a 10 g / cm2 solution is required. 3 This requires a density greater than 2.26 g / cm, which is not possible with carbon-based materials. For comparison, the densities of graphite, diamond, and metallic iron are each 2.26 g / cm. 3 , 3.51g / cm3 and 7.87 g / cm 3 Another strategy (see J. Mater. Chem. A, 2019, vol. 7, 3353-3365) used fluorinated graphite to facilitate the covalent modification of graphene with nitrogen-containing molecules (e.g., ethylenediamine), resulting in graphene-based networks containing electroactive nitrogen atoms (15 at%) and small amounts of fluorine (0.5 at% to 2 at%) outside the plane of the graphene backbone. The same study also reported that the addition of sodium amide in the reaction of fluorinated graphite with ethylenediamine increased the final nitrogen content. A supercapacitor device using this product yielded 326 F / g and an energy density of 18 Wh / kg at a power density of 7 kW / kg. By applying capillary compression, the density of the product was reduced to 0.13 g / cm. 3 to 1.33 g / cm 3 at 9 kW / L, resulting in a higher volumetric energy density of 20 Wh / L.

[0005] A strategy to increase the volumetric energy density of supercapacitors related to the present invention is to increase their density by controlling the pore size of the active electrode material to avoid unusable volume but without sacrificing the ability to adsorb ions of the electrolyte. Murali et al., in Nano Energy 2013, vol. 2, 764-768, reported that a carbon (graphene-based) electrode was compressed to a density of 0.34 g / cm. 3 to 0.75 g / cm 3 Shortly thereafter, X. Yang et al. (see Science 2013, vol. 341, 534-537) used capillary densification of chemically reduced graphene gel in the presence of ionic liquids to further increase the density of graphene-based electrodes, achieving a volumetric energy density of 1.3 g / cm. 3The product achieved a density of 90 Wh / L at 1.1 kW / L. It was suggested that capillary concentration aided densification, and the presence of a non-evaporating ionic liquid prevented the graphene sheets from repacking, contributing to the preservation of charge storage and transport properties. A year later, in Nature Commun. 2014, vol. 5, 5554, treatment of chemically reduced graphene oxide with H2O2 produced a porous structure in the sheets, proving an alternative method for facilitating three-dimensional ion storage and transport. This resulted in a density of 0.7 g / cm after mechanical compression. 3 and energy densities as high as 85 Wh / L, resulting in an improved power density of 1.75 kW / L. Other attempts to increase volumetric energy density through high-density heteroatom tri-doped carbon and alternative capillary densification have proven unfeasible, reaching 40 Wh / L and 65 Wh / L, respectively. This value, at approximately 90 Wh / L, is about five times higher than commercially available high-performance activated carbons and is therefore competitive with lead-acid and metal hydride batteries. However, theoretically, graphene could reach as high as 300 Wh / L. Combined with their unparalleled power density and stability, this poses challenges for transforming supercapacitors into a competitive energy storage technology.

[0006] Generally, the use of such nitrogen, or nitrogen and fluorine, containing graphene materials for the fabrication of supercapacitor electrodes having a volumetric energy density greater than about 90 Wh / L at a power density greater than 2 kW / L, or 2.26 g / cm 3 (This is the density of bulk graphite.) Higher mass densities of graphene materials and respective electrode formulations have yet to be achieved.

[0007] DISCLOSURE OF THE INVENTION The present invention provides a method for producing nitrogen-doped graphene, comprising the following steps a) to e): a) providing a dispersion of fluorinated graphite; b) subjecting said dispersion of fluorinated graphite to ultrasonic and / or mechanical and / or thermal treatment; c) contacting the product of step b) with an azide reagent at a temperature between 40°C and 200°C; d) separating the solid product formed in step c) from the mixture; e) Dialyzing the product against water.

[0008] The term "fluorinated graphite" includes fluorographite, graphite fluoride, graphite fluoride, and exfoliated forms of these materials. Fluorinated graphite is sold under the names poly(carbon monofluoride), carbon monofluoride, or poly(carbon fluoride). The initial fluorine content in the starting graphite fluoride is typically at least 40 at% and more preferably at least 45 at% or at least 50 at% based on all atoms present in the sample as determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα radiation source.

[0009] The term "N-doped graphene" or "nitrogen-doped graphene" refers to graphene with N atoms (nitrogen atoms) incorporated into the graphene lattice. This term encompasses both single-layer graphene and materials containing single-layer graphene in mixtures with parts (e.g., flakes) or particulates containing multiple graphene layers. However, this term also encompasses graphene in which a small percentage (e.g., up to 10% or up to 5%) of nitrogen atoms are bonded to carbon atoms as out-of-plane substituents (e.g., amino groups), i.e., the nitrogen atoms are not incorporated into the graphene lattice. This term also encompasses graphene in which a small amount of fluorine is present (up to 16.6 at %, preferably less than 5 at %).

[0010] The mechanical treatment preferably comprises at least one treatment selected from high shear mixing, stirring, vigorous stirring, stirring with a magnetic bar, stirring with a mechanical stirrer.

[0011] The heat treatment preferably involves heating the dispersion in step b) at a temperature in the range of 50° C. to 250° C., or 80° C. to 200° C., or more preferably 100° C. to 150° C. It may also involve treatment in a solvothermal reactor at a pressure higher than normal atmospheric pressure.

[0012] The dispersion prepared in step a) is a dispersion of fluorinated graphite in a solvent. The solvent is preferably a polar solvent or a mixture of a polar solvent and a non-polar solvent. The solvent may be preferably selected from dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMA), glycols (e.g., ethylene glycol, etc.), and mixtures thereof. A less polar or non-polar solvent (e.g., acetonitrile, benzene, toluene, or chlorobenzene, etc.) may be used in combination with a polar organic solvent (e.g., DMF, NMP, DMSO, DMA).

[0013] The present invention also encompasses embodiments in which a different solvent is used for the ultrasonic and / or mechanical and / or thermal treatment than the solvent used for the reaction with the azide reagent.

[0014] The ultrasonic and / or mechanical and / or thermal treatment steps produce a mixture containing fluorinated graphene and / or exfoliated fluorinated graphite particles. Ultrasonic treatment is typically performed at frequencies between 20 kHz and 100 kHz for at least 2 hours, more preferably at least 3 hours, and even more preferably at least 4 hours. Thermal treatment is typically performed at temperatures between 40°C and 200°C for at least 1 hour, preferably at least 6 hours, more preferably at least 24 hours, and even more preferably at least 80 hours. Mechanical treatment is most typically performed by high-shear mixing or magnetic bar stirring.

[0015] The azide reagent is added to the reaction solvent, preferably in the form of a powder or in the form of a suspension in the solvent.

[0016] The solvent is preferably a polar solvent. The solvent may be preferably selected from dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMA), glycols (e.g., ethylene glycol, etc.), and mixtures thereof. A less polar or non-polar solvent (e.g., acetonitrile, benzene, toluene, or chlorobenzene, etc.) may be used in combination with a polar organic solvent (e.g., DMF, NMP, DMSO, DMA). In a particularly preferred embodiment, the solvent is the same as the solvent used to prepare the dispersion of fluorinated graphene prepared in step b).

[0017] The azide reagent may be preferably selected from metal azides and tri(C1-C4) alkylsilyl azides, and more preferably selected from NaN3, KN3, LiN3, Pb(N3)2, and trimethylsilyl azide.

[0018] After contacting the product of step b) containing fluorinated graphene with the azide reagent, the mixture is typically heated at a temperature in the range of 40°C to 200°C, preferably 70°C to 170°C, and even more preferably 100°C to 140°C. The heating step is preferably carried out for at least 4 hours, preferably 4 hours to 20 days, even more preferably at least 8 hours, even more preferably at least 24 hours, and even more preferably at least 2 days (48 hours) or at least 3 days (72 hours). The longer the heating period, the higher the nitrogen doping.

[0019] The step of isolating the product (nitrogen-doped graphene) can be carried out by known techniques (e.g., centrifugation, sedimentation, filtration, etc.).

[0020] The method of the present invention makes it possible to prepare graphene containing nitrogen and fluorine atoms. Although the final nitrogen-doped graphene usually contains residual fluorine atoms, this method makes it possible to reliably achieve a higher nitrogen doping than most methods known in the prior art. The achieved nitrogen doping is at least 8.9 at% if the reaction of step c) is carried out for 4 hours, at least 13.9 at% if the reaction of step c) is carried out for 24 hours, and at least 16.1 at% if the reaction of step c) is carried out for 72 hours. The prepared nitrogen-doped graphene has a density of 1.2 g / cm when pressed at 80 kN for 1 minute. 3 and preferably has a density of 1.4 g / cm when pressed at 80 kN for 1 minute. 3 This manufacturing method achieves a density of 2 g / cm3 when pressed at 80 kN for 1 minute. 3 The highest density in the examples was 2.7 g / cm when pressed at 80 kN for 1 minute. 3 (when the reaction in step c) was carried out for 72 hours). Such a density is higher than that of any graphite, graphene, or graphene derivative prepared to date. The preparation method that makes it possible to achieve these properties is simple and effective, and uses economically effective starting compounds.

[0021] In particular, the method of the present invention is the only wet chemical method capable of achieving such high nitrogen doping, and furthermore, it is the only method that achieves high nitrogen doping at relatively low reaction temperatures.

[0022] Nitrogen-doped graphene exhibits a well-balanced set of parameters that allows its use as a supercapacitor electrode without the drawbacks typical of materials known in the art. In particular, the unprecedented density combined with the maintenance of its ability to absorb ions from the electrolyte results in an ultra-high volumetric energy density, which is higher than any prior art N- and F-containing graphene-based supercapacitor material to date. The highest volumetric energy density achieved, as described in the examples, was approximately 170 Wh / L at a volumetric power density of 5.2 kW / L.

[0023] A further object of the present invention is nitrogen-doped graphene containing at least 8.9 at% (preferably at least 13.9 at%) nitrogen and up to 16.6 at% (preferably up to 5 at%) fluorine, where at% is based on all atoms present in the sample and determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα radiation source, the nitrogen-doped graphene having a density of 1.2 g / cm when pressed at 80 kN for 1 minute. 3 (preferably, the density when pressed at 80 kN for 1 minute is 1.4 g / cm 3 exceeding 100%.

[0024] The high density is achieved primarily through a process of dialysis against water.

[0025] Preferably, the nitrogen-doped graphene comprises at least 16 at.% (more preferably at least 16.1 at.%) nitrogen and / or up to 5 at.% (more preferably up to 2 at.%, even more preferably up to 1.5 at.%) fluorine relative to all atoms present in the sample as determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα radiation source.

[0026] There is a minimum residual content of fluorine, typically at least about 0.1 at.%, or about 0.3 at.%, relative to all atoms present in the sample and determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα source.

[0027] The nitrogen content in nitrogen-doped graphene produced by the manufacturing method of the present invention can reach a maximum value of about 20 at% relative to all atoms present in the sample and determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα radiation source.

[0028] Preferably, the product has a density of 2 g / cm when pressed at 80 kN for 1 minute. 3 Exceeds.

[0029] Density measurements were performed by spreading 4 mg of material dispersed in 20 microliters of solvent (e.g., N-methyl-2-pyrrolidone) onto aluminum foil, allowing the dispersion to dry, and then pressing at 80 kN for 1 minute.

[0030] In some embodiments, the product has a surface area (BET) of 50 m as measured by N adsorption / desorption measurements at 77 K. 2 / g~200m 2 / g, more preferably 55m 2 / g~150m 2 / g.

[0031] Another aspect of the present invention is the use of the above-mentioned nitrogen-doped graphene as a supercapacitor material. The nitrogen-doped graphene of the present invention has high volumetric energy and power density and high capacitance stability during cycling. An inherent property of the material that results in high volumetric capacitance is its high density after pressing.

[0032] High density (typically a density of at least 1.4 g / cm when pressed at 80 kN for 1 minute) 3) results in high volumetric energy and power densities achievable in the product. Volumetric energy densities are typically greater than 30 Wh / L, and volumetric power densities are typically greater than 2.7 kW / L. Gravimetric energy densities are greater than 20 Wh / kg (preferably greater than 50 Wh / kg) at gravimetric power densities of at least 1.6 (preferably at least 1.8) kW / kg. All energy and power densities refer to material pressed at 80 kN for 1 minute, with a current density of 2 A / g.

[0033] The present invention also provides an electric cell comprising at least two electrodes, a separator, and an electrolyte, wherein at least one electrode comprises or consists of the nitrogen-doped graphene described above.

[0034] The electrolyte may be a liquid electrolyte containing a salt or preferably an ionic liquid.

[0035] The electric cell may include at least two electrodes, at least one separator membrane disposed between the electrodes, the separator membrane impregnated with an electrolyte, and current collectors attached to the electrodes, wherein at least one electrode is made from the nitrogen-doped graphene of the present invention coated on an aluminum foil.

[0036] In a specific embodiment, a two-electrode system and a symmetric full-cell supercapacitor device were used to evaluate the performance, rate stability, and cycling stability of the nitrogen-doped graphene obtained from step (d). The nitrogen-doped graphene was uniformly dispersed in N-methyl-2-pyrrolidone with added polytetrafluoroethylene and carbon (TimCal) (preferably, a mass ratio of 85:10:5 between the nitrogen-doped graphene, polytetrafluoroethylene, and carbon) and sonicated for preferably 4 hours to form a uniform paste. The slurry was coated onto aluminum foil. The film was then dried overnight at 120°C in a vacuum oven. Two electrodes (in a specific embodiment, 18 mm in diameter) were then cut and pressed between two metal plates with a force of 80 kN for 1 minute. The mass and thickness of the electrodes were then measured and they were dried again at 120°C under vacuum (40 mbar), preferably for 6 hours. The electrodes in the flask were transferred (under reduced pressure) to a glove box. The two electrodes were placed facing each other with a separator membrane between them. The separator membrane was soaked with the selected electrolyte. The electrodes were sealed in an airtight package, and the current collectors were connected to a test device (battery tester). Prior to actual testing of the supercapacitor cells, they were conditioned by charging them at a voltage and current density (less than 1 A / g) lower than the final voltage to be used.

[0037] BRIEF DESCRIPTION OF THE DRAWINGS Figure 1. a) X-ray photoelectron spectrum of the starting fluorinated graphite material, and b) X-ray photoelectron spectrum of the product of Example 1.

[0038] Figure 2. Combined thermogravimetric analysis and differential scanning calorimetry of the solid product isolated from the reaction of Example 1. The analysis was carried out in normal atmosphere at 5°C / min up to 1000°C. The exothermic process is the rise in the graph.

[0039] Figure 3. (a) Infrared spectrum of the starting material, fluorinated graphite, and (b) infrared spectrum of the product from Example 1.

[0040] Figure 4. X-ray photoelectron spectrum of the product of Example 2 (after 4 hours of reaction in DMF).

[0041] Figure 5. X-ray photoelectron spectrum of the product of Example 3 (after reaction in DMF for 24 hours).

[0042] Figure 6. a) X-ray photoelectron spectrum of the product from Example 4, and b) X-ray photoelectron spectrum of the product from Example 1 (for comparison).

[0043] Figure 7. Scanning electron microscope images of a)-c) a film of electrode material (prepared as described in Example 5) coated on aluminum foil before pressing between two metal plates with a force of 80 kN for 1 minute, and d)-f) after pressing between two metal plates with a force of 80 kN for 1 minute.

[0044] Figure 8. Electrochemical characteristics of the product from Example 1: a)-b) Cyclic voltammetry curves in EMIM-BF4 and TTE (9:1) electrolyte, panel a at a slow scan rate and panel b at a fast scan rate; c) Galvanostatic charge-discharge profiles at various current densities.

[0045] Figure 9. Cycling stability of material produced from Example 1. Shown are galvanostatic charge-discharge profiles during the beginning, middle, and end portions of a 10,000 cycle test.

[0046] Figure 10. Galvanostatic charge-discharge profiles at various current densities in EMIM-BF4 and TTE (9:1) electrolyte for the procedure described in Example 6.

[0047] Figure 11. Galvanostatic charge-discharge profiles at various current densities in EMIM-BF4 and TTE (9:1) electrolyte for the procedure described in Example 7.

[0048] Figure 12. Galvanostatic charge-discharge profiles at various current densities in EMIM-BF4 and TTE (9:1) electrolyte for the procedure described in Example 8.

[0049] The present invention can also be configured as follows.

[0050] [1] A method for producing nitrogen-doped graphene, comprising the following steps a) to e): a) providing a dispersion of fluorinated graphite; b) subjecting said dispersion of fluorinated graphite to ultrasonic and / or mechanical and / or thermal treatment; c) contacting the product of step b) with an azide reagent at a temperature in the range of 40°C to 200°C; d) separating the solid product formed in step c) from the mixture; e) Dialysis of the product obtained in step d) against water.

[0051] [2] The method according to [1], wherein the initial content of fluorine in the starting fluorinated graphite is at least 40 at. % based on all atoms present in the sample and determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα radiation source, more preferably at least 45 at. % or at least 50 at. %.

[0052] [3] The method according to [1] or [2], wherein the dispersion prepared in step a) is a dispersion of fluorinated graphite in a polar organic solvent, and the polar organic solvent is preferably selected from dimethylformamide, dimethyl sulfoxide, N-methylpyrrolidone, glycol (e.g., ethylene glycol), and mixtures thereof.

[0053] [4] The method according to any one of [1] to [3], wherein the azide reagent is added to the mixture from step b) in the form of a powder or in the form of a suspension in a polar solvent, the polar solvent being preferably selected from dimethylformamide, dimethyl sulfoxide, N-methylpyrrolidine, glycols (e.g., ethylene glycol), and mixtures thereof.

[0054] [5] The method according to any one of [1] to [4], wherein the azide reagent is selected from metal azides and tri(C1-C4) alkylsilyl azides, and preferably, the azide reagent is selected from NaN3, KN3, LiN3, Pb(N3)2, and trimethylsilyl azide.

[0055] [6] The method according to any one of [1] to [5], wherein the product of step b) containing fluorinated graphite is contacted with the azide reagent, and then the mixture is heated at a temperature in the range of 70°C to 170°C, even more preferably 100°C to 140°C, and the heating step is carried out for at least 4 hours, preferably 4 hours to 20 days, even more preferably at least 24 hours.

[0056] [7] Nitrogen-doped graphene containing at least 8.9 at% nitrogen and up to 16.6 at% fluorine, where the at% is based on all atoms present in the sample as determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα source; and 1.2 g / cm when pressed at 80 kN for 1 minute. 3 Nitrogen-doped graphene with a density exceeding 1000 .mu.m.

[0057] [8] The nitrogen-doped graphene according to [7], containing at least 13.9 at. % nitrogen and up to 5 at. % fluorine, based on all atoms present in the sample, as determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα radiation source.

[0058] [9] The nitrogen-doped graphene according to [7], containing at least 16 at. % nitrogen and up to 5 at. % fluorine, based on all atoms present in the sample, as determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα radiation source.

[0059]

[10] The nitrogen-doped graphene according to [8] or [9], containing 0.1 at % to 2 at % fluorine relative to all atoms present in the sample as determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα radiation source.

[0060]

[11] Density of at least 1.4 g / cm when pressed at 80 kN for 1 minute 3 The nitrogen-doped graphene according to any one of [7] to

[10] , wherein

[0061]

[12] Use of nitrogen-doped graphene according to any one of [7] to

[11] as a supercapacitor material.

[0062]

[13] An electric cell comprising at least two electrodes, a separator, and an electrolyte, wherein at least one electrode comprises or consists of the nitrogen-doped graphene according to any one of [7] to

[11] .

[0063]

[14] The electric cell according to

[13] , wherein the electrolyte contains an ionic liquid.

[0064] [Examples of carrying out the invention] material and method: Graphite fluoride (>61 wt% F), NaN3 (BioXtra), 1-methyl-2-pyrrolidinone anhydrous (99.5%), and N,N-dimethylformamide (≥98%) were purchased from Sigma-Aldrich. Acetone (pure) and ethanol (absolute) were purchased from Penta (Czech Republic). All chemicals were used without further purification. Ultrapure water was used for the preparation of all aqueous solutions.

[0065] FT-IR spectra were measured on an iS5 FTIR spectrometer (Thermo Nicolet) using a Smart Orbit ATR accessory with a ZnSe crystal. A droplet of a dispersion of the sample in ethanol or water was placed on the ZnSe crystal and allowed to dry, forming a film in the ambient environment. Spectra were recorded by summing 50 scans. Nitrogen gas was allowed to flow through the ATR accessory during measurement and background acquisition. ATR and baseline corrections were used to process the collected spectra.

[0066] X-ray photoelectron spectroscopy (XPS) was performed on a PHI VersaProbe II (Physical Electronics) spectrometer using an Al-Kα source (15 kV, 50 W). The data were deconvoluted using the MultiPak (Ulvac-PHI, Inc.) software package.

[0067] Images from transmission electron microscopy were obtained using a JEOL 2100 TEM equipped with a LaB6 type emission gun operated at 160 kV. Samples were also analyzed by scanning electron microscopy using a Hitachi SU6600 instrument with an accelerating voltage of 5 kV. For these analyses, a small drop of a dispersion of the material in ultrapure water (concentration of approximately 0.1 mg / ml) was placed on a carbon-coated copper grid and left to dry.

[0068] Thermal analysis was carried out using a STA449 C Jupiter Netzsch instrument.

[0069] Surface area analysis was performed by measuring N adsorption / desorption at 77 K using a volumetric gas adsorption analyzer (3Flex, Micromeritics) up to 0.965 P / P. Prior to analysis, high purity (99.999%) N and He gases were used for the measurements, while the samples were placed under high vacuum (10 -4 The mixture was degassed at 130°C (Pa) for 12 hours. The Brunauer-Emmett-Teller area (BET) was calculated with respect to the Rouquerol standard for the N isotherm, giving a molecular cross-sectional area of ​​16.2 Å for N (77 K). 2 It was assumed that:

[0070] The sample membrane was pressed between metal plates using a hydraulic press (Trystom spols.ro, Olomouc).

[0071] Cyclic voltammetry (CV) and galvanostatic discharge (GCD) were performed on a Bio-Logic battery tester (BCS-810) controlled by BT-Lab software (version 1.64).

[0072] The following sections define supercapacitor metrics used herein and generally accepted in the art: Gravimetric capacitance (C s [F / g]) and volumetric capacitance (C v [F / cm 3 ]) is calculated from the galvanostatic charge / discharge curves according to the following formula:

[0073]

number

[0074] Gravimetric energy density (E g ), gravimetric power density (P g ), volumetric energy density (E v ), and volumetric power density (P v ) is calculated according to the following formula:

[0075]

number

[0076] where m (g) is the mass of the active material in one electrode (including the mass of the binder and conductive additive), I (A) is the discharge current, t (s) is the discharge time, V (V) is the potential change during discharge, and V el (g / cm 3 ) is the volume of electrode material on one electrode).

[0077] Example 1: Synthesis of nitrogen-doped graphene (72-hour reaction) In a glass spherical flask, 1 g of graphite fluoride was dispersed in 40 ml of DMF. The flask was covered and stirred for 2 days. It was then sonicated for 4 hours and stirred overnight. In a glass beaker, 2 g of NaN3 was dissolved in 20 ml of DMF. This was then added to the graphite fluoride and / or low-layer fluorographene dispersion. The mixture was heated in a hood using a concentrator at 130 °C for 72 hours while stirring with a Teflon-coated magnetic bar. After heating, the reaction mixture was allowed to cool and transferred to a 50 ml Falcon centrifuge tube. The solid particles (product) were separated from the solvent and the by-product was obtained by centrifugation at 15,000 rcf for approximately 10 minutes. The supernatant was discarded, and the tube was refilled with the next wash solvent. The sample was homogenized by shaking for at least 1 minute to redisperse the precipitate in the new solvent. Washing was performed with different solvents: DMF (3x), acetone (3x), ethanol (3x), hot ethanol (1x), distilled water (3x), and hot distilled water (1x). The container was then refilled with distilled water. Finally, the dispersed solids were inserted into a dialysis bag (molecular weight cutoff 10 kDa) until the conductivity of the surrounding water no longer increased above approximately 10 μS / cm and the conductivity within the dialysis bag was approximately 5 μS / cm. The dispersion was finally transferred from the dialysis bag and either stored for further use or dried.

[0078] X-ray photoelectron spectroscopy (FIG. 1) on the starting graphite fluoride and the product of Example 1 showed that the reaction with NaN3 introduced N atoms into the product, reaching 16.1 at.% after 72 hours of reaction, and a significant loss of fluorine atoms from 50.5 at.% to 1.5 at.% (Table 1).

[0079] Thermogravimetric analysis in normal atmosphere showed a gradual mass loss of the material up to about 450 °C and a rapid decomposition process between about 500 °C and 680 °C (Figure 2).

[0080] The density of the material measured after depositing 4 mg of material on an aluminum foil and pressing it at 80 kN for 1 minute was 2.7 g / cm 3When a portion of the same batch of this product was not dialyzed and pressed under the same conditions, the density was about 1.5 g / cm 3 It was.

[0081] The FT-IR spectrum of the starting material, graphite fluoride (Figure 3a), shows the presence of C—F and C—F bonds (1200 cm -1 and 1310 cm -1 ), whereas the spectrum of the product (Fig. 3b) showed a band from 1560 cm -1 and 1110 cm -1 ~1180cm -1 These vibrations are typical of aromatic carbons and heteroaromatic rings. -1 Additional vibrational modes of the aromatic ring appearing at 1110 cm can be attributed to heteroatom substitution (e.g., in the pyridine configuration). -1 ~1180cm -1 The bands in the graphite fluoride observed are the CF x However, as confirmed by XPS, almost all F atoms (about 1.5 at% remained) were removed. These bands overlapped with the vibrations at 1560 cm -1 vibration and 1400cm -1 3000 cm -1 The broad absorption at 1560 cm can be attributed to the NH stretching vibration of the primary or secondary amino groups (R2N-H, R-NH2) covalently bonded perpendicularly to the graphene backbone. In the same region, -OH vibrations can also appear. -1 The broad vibration at may also include signals from bending vibrations of primary amino groups.

[0082] According to the BET method, the specific surface area at 20 seconds of N2 sorption equilibrium is 59 m 2 / g.

[0083] [Table 1]

[0084] Example 2: Synthesis of nitrogen-doped graphene (4-hour reaction) The same procedure as in Example 1 was followed, except that the mixture was heated at 130° C. for 4 hours instead of 72 hours.

[0085] After dialysis, the density of the material was measured after depositing 4 mg of material on aluminum foil and pressing it at 80 kN for 1 min, and found to be 1.4 g / cm 3 It was.

[0086] X-ray photoelectron spectroscopy (FIG. 4) of the product of this example showed that the reaction with NaN3 introduced N atoms into the product, reaching 8.9 at% after 4 hours of reaction, and the fluorine atoms in the starting fluorinated graphite were significantly reduced from 50.5 at% to 16.6 at% (Table 2).

[0087] The specific surface area is 146m 2 / g.

[0088] [Table 2]

[0089] Example 3: Synthesis of nitrogen-doped graphene (24-hour reaction) The same procedure as in Example 1 was followed, except that the mixture was heated at 130° C. for 24 hours instead of 72 hours.

[0090] After dialysis, the density of the material was measured after depositing 4 mg of material on aluminum foil and pressing it at 80 kN for 1 min, and found to be 1.4 g / cm 3 It was.

[0091] X-ray photoelectron spectroscopy of the product of this example (FIG. 5) showed that the reaction with NaN3 introduced N atoms into the product, reaching 13.9 at% after 24 h of reaction, and the fluorine atoms in the starting fluorinated graphite were significantly reduced from 50.5 at% to 1.6 at% (Table 3).

[0092] The specific surface area is 127m 2 / g.

[0093] [Table 3]

[0094] Example 4: Preparation of nitrogen-doped graphene without using sonication, centrifugation, or dialysis for washing (Comparative Example) In a glass spherical flask, 0.25 g of graphite fluoride was dispersed in 10 ml of DMF. The flask was covered and stirred for 3 days. Then, 0.5 g of NaN3 was added to the flask, and the sonication step described in Example 1 was omitted. The mixture was heated at 130 °C for 72 hours in a hood using a concentrator while stirring with a Teflon-coated magnetic bar. After heating, the reaction mixture was allowed to cool and filtered on S1 sintered glass using filter paper. Washing was performed in a frita with DMF (3x), distilled water (3x), and hot distilled water (1x). The conductivity of the filtrate was measured to check the purity of the product. If the conductivity was above 100 μS / cm, an additional washing step with water was performed. Finally, the solid was redispersed in distilled water, characterized (conductivity, zeta potential, pH, concentration, infrared and X-ray photoelectron spectroscopy), and stored for further use. The dialysis step described in Example 1 was omitted.

[0095] The density of the material measured after depositing 4 mg of material on an aluminum foil and pressing it at 80 kN for 1 minute was 0.7 g / cm 3 It was.

[0096] X-ray photoelectron spectroscopy of the product of this example (FIG. 6) showed that the reaction with NaN3 introduced N atoms into the product, reaching 15 at% after 72 h of reaction, and the fluorine atoms in the starting fluorinated graphite were significantly reduced from 50.5 at% to 4.6 at% (Table 4).

[0097] [Table 4]

[0098] Example 5: Electrochemical testing of the product from Example 1 (72 hour product) in a two-electrode symmetric supercapacitor full cell.

[0099] The active material (nitrogen-doped graphene from Example 1) was uniformly dispersed in N-methyl-2-pyrrolidone (p<0.05, Sigma-Aldrich) with binders PTFE (Sigma-Aldrich) and conductive carbon (TimCal from MTI) in a ratio of 85:10:5 and sonicated for 4 hours to form a uniform paste. The slurry was applied to a carbon-coated aluminum foil (Cambridge Energy Solutions, 15 μm thick) using a doctor blade technique (Erichsen, Quadruple Film Applicator, Model 360). The resulting film containing nitrogen-doped graphene flakes was examined by scanning electron microscopy and showed a thickness of 10 μm to 12 μm (Figures 7a-c). The flakes were randomly oriented (Figure 7b). The film was then dried overnight at 120 °C in a vacuum oven. Two 18 mm diameter electrodes were then cut out and pressed between two metal plates for 1 min with a force of 80 kN (Trystom spols.ro, Olomouc). After pressing, the film thickness decreased to 1.7 μm–1.8 μm. This revealed a lamellar structure, which was highly oriented parallel to the aluminum foil (Figures 7d–f). Two 1.8 cm diameter electrodes (containing 1.4 mg and 1.3 mg of electrode material, respectively) were dried again at 120 °C under vacuum (40 mbar) for 6 h and then transferred (under vacuum) to a glove box (O2 and HO contents <2 ppm, argon atmosphere). The density of the film thus produced was 2.7 g / cm3. 3To assemble the supercapacitor device, two electrodes were placed in a sleeve (El-Cell insulator sleeve with a 0.26 mm thick Whatman® glass microfiber paper separator). The separator membrane was soaked with 90 μl to 100 μl of electrolyte. A 90:10 mixture of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4, Sigma-Aldrich, ≥99.0% (HPLC)) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE, Tokyo Chemical Industry, >95.0%) was used as the electrolyte and dried over molecular sieves before use. The electrodes were sealed inside the sleeve with a stainless steel plunger, and the entire device was clamped and connected to a battery tester for analysis.

[0100] Prior to testing the device, the electrode material was conditioned as follows: The potential was maintained at 1.2 V for 5 min, cycled 20 times to 2 V at a current density of 0.5 A / g, and cycled 20 times to 3.7 V at a current density of 1 A / g.

[0101] Cyclic voltammetry (Figure 8a, b) showed nearly rectangular curves with slight redox peaks, primarily evident at lower scan rates, likely due to nitrogen atoms. Galvanostatic charge-discharge (Figure 8c) measurements showed a substantially linear and symmetric profile (124 s charge and 118 s discharge at 1 A / g, 95% energy efficiency), improving to 100% efficiency at 5 A / g (22 s charge and 22 s discharge) (Table 5). The device performance is listed in Table 5, demonstrating an unprecedented volumetric energy density of 169.8 Wh / L at a power density of 5.2 kW / L. The capacitance retention after 10,000 cycles was 94%. The cell also exhibited excellent stability, maintaining 100% of its capacity after 10,000 cycles (Figure 9).

[0102] [Table 5]

[0103] Example 6: Electrochemical testing in a two-electrode symmetric supercapacitor full cell using the product from Example 4 (comparative example, no dialysis).

[0104] The experiment of Example 5 was repeated using the product obtained from Example 4 (no sonication, no dialysis). The density of the membrane produced was 0.7 g / cm. 3 Galvanostatic charge-discharge measurements showed very good performance stability with increasing current density (from 2 A / g to 20 A / g) (FIG. 10 and Table 6). While the gravimetric energy density was slightly lower than that of the product from Example 1, the volumetric energy density decreased significantly from 169.8 Wh / L to 35.4 Wh / L at a current density of 2 A / g and a power density of 1.3 kW / L. The decrease in volumetric energy density is caused by a much lower packing density, which is particularly due to the omission of the dialysis step in the preparation of doped graphene.

[0105] [Table 6]

[0106] Example 7: Electrochemical testing of the product from Example 2 (4-hour product) in a two-electrode symmetric supercapacitor full cell.

[0107] The experiment of Example 5 was repeated using the product obtained from Example 2 (4 hour reaction). The density of the produced membrane was 1.4 g / cm 3 The constant current charge / discharge measurements showed poor performance (Figure 11 and Table 7). Both the weight and volume data were significantly lower than in Example 1.

[0108] [Table 7]

[0109] Example 8: Electrochemical testing of the product from Example 3 (24-hour product) in a two-electrode symmetric supercapacitor full cell.

[0110] The experiment of Example 5 was repeated using the product obtained from Example 3 (24 hour reaction). The density of the membrane produced was 1.4 g / cm. 3 Galvanostatic charge-discharge measurements showed performance similar to that of Example 1 (72-hour product) (FIG. 12 and Table 8). However, the significantly lower density resulted in a significantly lower volumetric energy density than that measured in Example 5 for the 72-hour product of Example 1.

[0111] [Table 8] [Brief explanation of the drawings]

[0112] [Figure 1] a) X-ray photoelectron spectrum of the starting material, fluorinated graphite, and b) X-ray photoelectron spectrum of the product of Example 1. [Figure 2] Combined thermogravimetric analysis and differential scanning calorimetry of the solid product isolated from the reaction of Example 1. The analysis was carried out in normal atmosphere at 5°C / min up to 1000°C. The exothermic process is the rise in the graph. [Figure 3] (a) Infrared spectrum of the starting material, fluorinated graphite, and (b) infrared spectrum of the product from Example 1. [Figure 4] X-ray photoelectron spectrum of the product of Example 2 (after reacting in DMF for 4 hours). [Figure 5] X-ray photoelectron spectrum of the product of Example 3 (after reacting in DMF for 24 hours). [Figure 6] a) X-ray photoelectron spectrum of the product from Example 4, and b) X-ray photoelectron spectrum of the product from Example 1 (for comparison). [Figure 7]Scanning electron microscope images of a)-c) a film of electrode material (prepared as described in Example 5) coated on aluminum foil before pressing between two metal plates with a force of 80 kN for 1 minute, and d)-f) after pressing between two metal plates with a force of 80 kN for 1 minute. [Figure 8] Electrochemical characterization of the product from Example 1: a)-b) Cyclic voltammetry curves in EMIM-BF4 and TTE (9:1) electrolyte, panel a at a slow scan rate and panel b at a fast scan rate; c) Galvanostatic charge-discharge profiles at various current densities. [Figure 9] Cycle stability of materials produced from Example 1. Galvanostatic charge-discharge profiles are shown during the beginning, middle, and end portions of a 10,000 cycle test. [Figure 10] Galvanostatic charge-discharge profiles at various current densities in EMIM-BF4 and TTE (9:1) electrolyte according to the procedure described in Example 6. [Figure 11] Galvanostatic charge-discharge profiles at various current densities in EMIM-BF4 and TTE (9:1) electrolyte according to the procedure described in Example 7. [Figure 12] Galvanostatic charge-discharge profiles at various current densities in EMIM-BF4 and TTE (9:1) electrolyte according to the procedure described in Example 8.

Claims

1. Nitrogen-doped graphene comprising at least 13.9 at% nitrogen and up to 5 at% fluorine, wherein the at% is based on all atoms present in a sample as determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα radiation source; and a graphene doped with nitrogen having a fluorine content of 1.2 g / cm when 4 mg of the nitrogen-doped graphene is deposited on aluminum foil and pressed at 80 kN for 1 minute. ３ Nitrogen-doped graphene with a density exceeding 1000 .mu.m.

2. 2. The nitrogen-doped graphene of claim 1, comprising at least 16 at. % nitrogen and up to 5 at. % fluorine, relative to all atoms present in the sample, as determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα radiation source.

3. 3. The nitrogen-doped graphene according to claim 1, comprising 0.1 at % to 2 at % fluorine relative to all atoms present in the sample as determined by X-ray photoelectron spectroscopy (XPS) using an Al-Kα radiation source.

4. When 4 mg of the nitrogen-doped graphene is deposited on an aluminum foil and pressed at 80 kN for 1 minute, the density is at least 1.4 g / cm ３ The nitrogen-doped graphene according to any one of claims 1 to 3,

5. 5. Use of nitrogen-doped graphene according to any one of claims 1 to 4 as a supercapacitor material.

Images