Nanoinks of Carbon Nanomaterials for Printing and Coating

Abstract

Conductive ink compositions are provided that contain carbon nanomaterials in specific ratios, with one-dimensional, two-dimensional, and quasi-three-dimensional nanostructures and / or combinations thereof, and / or doped with elements such as nitrogen, boron, sulfur, etc. The carbon nanomaterials are selected from the group consisting of graphene and graphene oxide particles, carbon nanotubes, and graphene aerosol gels.

Description

Technical Field

[0001] Related Applications This application claims the benefit of U.S. Provisional Patent Application No. 62 / 866,412, filed Jun. 25, 2019, and U.S. Provisional Patent Application No. 62 / 954,118, filed Dec. 27, 2019, both of which are hereby incorporated by reference in their entirety. Description of Research Funded by the Federal Government This invention was made with government support under Grant No. CBET1935676 awarded by the National Science Foundation (NSF) of the United States. The government has certain rights in this invention.

Background Art

[0002] Field of the Invention The present invention relates to formulations of various electronic grade inks and / or dispersions comprising, consisting of, or essentially consisting of carbon nanomaterials of 1-dimensional, 2-dimensional, and quasi-3-dimensional nanostructures and / or combinations thereof, at specific ratios. These inks can be used in many applications including, but not limited to, printable and flexible electronic devices and functional coatings. Preferred applications of these inks are in the areas of energy storage, electrochemical sensing, field effect transistors, and transparent conductive electrodes. The process is scalable and suitable for large-scale production / manufacturing.

[0003] Description of the Prior Art Rechargeable and microscale energy storage devices such as micro-supercapacitors (micro-SCs) and micro-batteries are in increasing demand for new applications such as the Internet of Things (IoT), autonomous ubiquitous sensors, and small household appliances including wearable devices. Based on the principle of electric double layer capacitance (EDLC) and redox active reactions at the solid-electrolyte interface, the energy storage devices have charging and discharging characteristics. Therefore, reliable charge-discharge cycles constitute an essential feature of any super / micro-supercapacitor technology and battery technology. Graphene has been sought for energy storage applications for a long time due to its large surface area (2630 m 2 / g for an isolated single sheet of graphene). However, it has been difficult to utilize the characteristics in a large-scale and scalable platform along with thickness regulation due to inter-particle resistance, mechanical instability, and their correlations. Modifying the physical structure of graphene at the micro / nano-scale (e.g., by increasing the available surface area exposed to the environment), alternative approaches to combine and mitigate these adverse effects, and alternative approaches to utilize the advantages of higher EDLC would be beneficial for the development of new technical means. Various types of additive manufacturing such as screen printing, inkjet printing, and 3D printing have been increasingly adopted recently due to their cost-effectiveness, manufacturing simplicity, and wide compatibility in the process. Therefore, there is a need in the art to be able to print graphene micro-SCs and micro-batteries with micro / nano-scale processed surfaces having a higher available surface area to achieve the next-generation SCs and batteries described above.

[0004] U.S. Patent Application Publication No. 2017 / 0081537 relates to a rapid and scalable methodology for graphene dispersion and concentration in a polymer organic solvent medium that can be utilized without centrifugation to increase graphene concentration.

[0005] International Patent Publication No. 2014 / 210584 relates to a dispersion of nanoplatelet graphene-like materials such as graphene nanoplatelets in a solid or liquid dispersion, wherein the nanoplatelet graphene-like materials are substantially uniformly dispersed in the dispersion by a graphene-like material dispersant. Such dispersions can be used to prepare articles by three-dimensional (3D) printing and to provide conductive inks and coatings, chemical and biosensors, electrodes, energy storage devices, solar cells, and the like. The liquid dispersion can be prepared, for example, by ultrasonic treatment of a solution of graphite flakes, a dispersant, and the liquid dispersion, while the solid dispersion can be prepared, for example, by combining a molten polymer with the liquid dispersion, dissolving a solid polymer in a miscible solvent and then blending with the liquid dispersion, dissolving a solid polymer in the liquid dispersion, or polymerizing one or more monomers in the liquid dispersion to form a solid polymer.

[0006] U.S. Patent No. 9,440,857 relates to a method for producing pristine graphene particles by one-step, gas-phase, catalyst-free detonation of a mixture of one or more carbon-containing compounds, hydrocarbon compounds, and one or more oxidants. The detonation reaction occurs very rapidly at a relatively high temperature above 3000 K to produce graphene nanosheets that can be recovered from the reaction vessel in the form of an aerosol, etc. The graphene nanosheets may be stacked, for example, in monolayers, bilayers, or trilayers and may have an average particle size of about 35 to about 250 nm.

[0007] Each of the foregoing references is hereby incorporated by reference in its entirety.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Means for Solving the Problems

[0009] In one embodiment, a graphene aerosol gel ink is formulated and used for inkjet printing of micro-supercapacitors. In certain embodiments, the graphene aerosol gel used is synthesized by the detonative synthesis in a controlled environment of hydrocarbons such as methane, ethylene, and acetylene, for example.

[0010] In another embodiment, an ink composition is provided that includes a nanomaterial selected from the group consisting of graphene, carbon nanotubes, and graphene aerosol gel. In a preferred embodiment, the ink composition comprises, consists of, or consists essentially of a graphene aerosol gel as the main or sole graphic carbon or graphene material.

[0011] According to yet another embodiment of the present invention, a method of forming an ink composition is provided that includes providing a mixture comprising a graphene aerosol gel and a surfactant, preferably ethyl cellulose or nitrocellulose. The mixture is dispersed within a liquid vehicle system that preferably includes a mixture of one or more ketones and one or more alcohols, more preferably a mixture of cyclohexanone and terpineol.

[0012] In another embodiment of the present invention, an ink composition is provided that includes graphene or graphene oxide and a low concentration of a second element such as carbon nanotubes or graphene aerosol gel or a combination thereof. The base graphene ink used in the tests described below is a commercially available graphene ink and a self-made graphene ink. The carbon nanotubes used to produce the composite ink are commercially available, and the graphene aerosol gel used in the composite ink is produced according to the teachings of U.S. Patent No. 9,440,857, which is hereby incorporated by reference in its entirety. The carbon nanotubes are multi-walled carbon nanotubes having metallic properties, and the graphene aerosol gel is reduced graphene oxide having a controlled carbon-to-oxygen ratio when synthesized via a hydrocarbon detonation route.

[0013] According to yet another embodiment of the present invention, a method of forming an ink composition is provided that includes providing a graphene or graphene oxide-containing ink precursor that includes a certain amount of graphene or graphene oxide particles dispersed in a liquid vehicle. At least one of a certain amount of carbon nanotubes and graphene aerosol gel is dispersed in the ink.

[0014] After including carbon nanotubes and graphene aerosol gel in the graphene ink, the resulting nanoink exhibits unique characteristics with respect to their structural, electrical, and electrochemical properties. Both the coated surface and the printed pattern were tested on a flexible substrate such as a polyimide film, and results were obtained. Excellent electronic and electrochemical pathways for the technology are useful for applications such as electronic devices, sensing, energy, and IoT.

Brief Description of the Drawings

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[0016] Embodiments of the present invention relate to nanoinks and methods of making such nanoinks, comprising, consisting of, or consisting essentially of a very stable suspension containing a hybrid structure of 2D graphene, 1D nanotubes, and / or 3D aerosol gel, all of which are of microscopic dimensions and consist of carbon as the basic chemical element. In certain embodiments, various energy storage devices in the form of mechanically flexible supercapacitors fabricated (printed) from graphene ink and graphene aerosol gel ink on a polyimide substrate are provided (in the form of interdigitated electrodes (IDE)).

[0017] According to one or more embodiments, an ink composition is provided that includes graphene (including reduced graphene oxide), carbon nanotubes, and / or graphene aerosol gel (GAG), generally referred to as “carbon nanomaterials,” dispersed in a liquid vehicle. The graphene and / or carbon nanotubes can be prepared according to any technique known in the art. In the case of graphene, high shear forces in a particular combination of solvent type and ratio, when used within a solvent medium with a particular concentration of graphene relative to the solvent, enable the exfoliation of graphite. The graphene aerosol gel can be prepared by non-catalytic, electrical spark-initiated detonation of an acetylene precursor (C2H2) using a controlled amount of oxygen within a reaction chamber. An exemplary process for manufacturing GAG is described in U.S. Patent No. 9,440,857. The process described in the ’857 patent can be understood as involving the conversion of acetylene molecules to free carbon atoms or ions, subsequent carbon aerosol formation, graphic carbon formation, and subsequent gelation processes to form the gel.

[0018] In one or more embodiments, the liquid vehicle in which the carbon nanomaterials are dispersed includes one or more organic compounds that are compatible with the carbon nanomaterials. In certain embodiments, the liquid vehicle includes one or more ketones, one or more alcohols, or a mixture of one or more ketones and one or more alcohols. Exemplary ketones that can be used in accordance with the present invention include aliphatic and aromatic cyclic ketones such as cyclohexanone. Exemplary alcohols that can be used in accordance with the present invention include aliphatic and aromatic alcohols such as terpineol. In other embodiments, the liquid vehicle can include an amide such as N,N-dimethylformamide (DMF) and / or a lactam such as N-methyl-2-pyrrolidone (NMP).

[0019] In certain embodiments, the liquid vehicle comprises one or more ketones in an amount of from about 60 wt% to about 99 wt%, from about 70 wt% to about 95 wt%, or from about 80 wt% to about 90 wt%. In certain embodiments, the liquid vehicle comprises one or more alcohols in an amount of from about 1 wt% to about 35 wt%, from about 5 wt% to about 30 wt%, or from about 10 wt% to about 20 wt%.

[0020] In one or more embodiments, when the carbon nanomaterial comprises carbon nanotubes and / or graphene aerosol gel particles, the concentration of these carbon nanomaterials in the ink is from about 0.01 to about 10 mg / ml, from about 0.05 to about 5 mg / ml, or from about 0.1 to about 3.0 mg / ml. In certain embodiments, in addition to CNT and / or GAG particles, the ink composition may further comprise graphene (including reduced graphene oxide) and / or graphene oxide particles, and / or doped carbon nanomaterials by doping elements such as nitrogen, sulfur, and boron (Figure 11). In such embodiments, the concentration of graphene and / or graphene oxide particles is from about 1 to about 30 mg / ml, from about 5 to about 25 mg / ml, or from about 10 to about 15 mg / ml.

[0021] In certain embodiments, the carbon nanomaterial, particularly graphene aerosol gel, has a D90 particle size (90% of the distribution has a smaller particle size and 10% has a larger particle size) of from about 0.1 to about 10 μm, from about 0.25 to about 7.5 μm, or from about 0.45 to about 5 μm.

[0022] The ink composition may further include one or more optional components. In one or more embodiments, the ink composition may include one or more surfactants that help form a stable suspension of carbon nanomaterials in the liquid vehicle. Exemplary surfactants that may be used with the present invention include ethyl cellulose, nitrocellulose, sodium dodecyl sulfate (SDS), ethylenediaminetetraacetic acid (EDTA), and sodium dodecylbenzenesulfonate (SDBS). In certain embodiments, the ink composition includes one or more surfactants in an amount of about 1 to about 500 mg / ml, about 2 to about 300 mg / ml, or about 5 to about 200 mg / ml.

[0023] In certain embodiments, the ink composition may have a viscosity of less than 30 cP, less than 25 cP, or less than 20 cP at room temperature (i.e., about 25 °C) in order to be ejectable from an inkjet printing head.

[0024] In other embodiments of the present invention, a method of forming a composite ink is provided. Exemplary methods are described in the following examples. However, generally, the composite ink composition according to the present invention may be formulated via a number of routes. According to one embodiment, a mixture comprising the graphene aerosol gel described herein and one or more surfactants is provided. In certain embodiments, the mixture may be formed by first dispersing a certain amount of graphene aerosol gel particles in a liquid medium such as an organic solvent. In certain embodiments, the organic solvent used may be an alcohol such as ethyl alcohol. A certain amount of surfactant, such as ethyl cellulose or nitrocellulose, is added to the GAG dispersion. In one or more embodiments, the ratio of GAG to surfactant is by weight, 10:1 to 1000:1, 25:1 to 500:1, or 100:1 to 250:1. Then, stirring or sonication is used to disperse the GAG in the solvent over a period of time until the GAG particles disintegrate into a plurality of GAG flakes.

[0025] Next, GAG flakes of a smaller size are separated from the suspension, for example, by centrifugation. The GAG flakes contained in the supernatant may be aggregated and an excess amount of surfactant may be removed. This can be achieved by preparing an NaCl solution and adding this to the flakes. The resulting suspension can be filtered to recover the aggregated GAG flakes. The obtained flakes can be dried to recover the graphene-surfactant powder. Optionally, the dry powder can be redispersed in an organic solvent and the dispersion can be filtered to remove GAG flakes of a larger size from the powder. The permeate containing the GAG flakes that have passed through the filter can be aggregated and dried as in the previous step to form a powder.

[0026] Finally, the composite ink can be formed by dispersing the GAG flakes and the surfactant powder in a liquid vehicle system as described above. Then, the dispersion can be stirred, for example, by using sonication, to form a homogeneous ink composition.

[0027] According to another embodiment of the present invention, the composite ink can be formulated by first providing a graphene or reduced graphene oxide dispersion. An exemplary reduced graphene oxide dispersion can be formed by mixing reduced graphene oxide microparticles with a liquid vehicle system such as a mixture of cyclohexanone and terpineol as described above. In certain embodiments, the reduced graphene oxide particles are present in the vehicle system at a concentration of about 1 to about 30 mg / ml, about 5 to about 20 mg / ml, or about 10 to about 15 mg / ml. Next, a certain amount of surfactant may be added to the mixture at a level of about 0.5 to about 15 mg / ml, about 1 to about 10 mg / ml, or about 3 to about 5 mg / ml. The resulting mixture can be stirred under high temperature conditions not exceeding 400 °C, such as by sonication, to form an ink suspension. In one or more embodiments, a certain amount of CNT or GAG particles may be added to the ink formulation at a level of about 0.01 to about 5 mg / ml, about 0.05 to about 2.5 mg / ml, or about 0.1 to about 1 mg / ml. The resulting mixture can then be sonicated to form a stable suspension.

[0028] An electronic device including one or more traces printed with the ink can be fabricated using the composite ink. In certain embodiments, the ink was printed onto a substrate using an inkjet printer, although any printing technique capable of creating traces of the desired dimensions could have been used. Exemplary electronic devices that can be fabricated with the composite ink include capacitors, supercapacitors, microcapacitors, ultracapacitors, and pseudocapacitors. In certain embodiments, the ink can be used to print electrodes and, when combined with one or more electrolytes, can be used to fabricate a battery.

[0029] In one or more embodiments, the electronic device may be formed by printing conductive traces on a substrate using any of the composite inks described herein. In certain embodiments, the substrate used is a flexible substrate formed from a synthetic resin material such as a polyimide film. However, other types of substrates including non-flexible substrates such as rigid plastics, glass, and ceramics are within the scope of the present invention. In certain embodiments, once the conductive traces are fabricated, the traces can be annealed. The annealing process can be carried out at a temperature of about 200 °C to about 500 °C, about 300 °C to about 450 °C, or about 350 °C under an inert atmosphere such as a nitrogen atmosphere. The annealing process can be carried out over a period of about 1 to about 5 hours.

[0030] FIG. 11 schematically illustrates various concepts according to the present invention starting from the production of carbon-based nanomaterials through the formulation of nanoinks and composite inks and the fabrication of energy storage devices. The first step involves producing a carbon-based nanomaterial from at least one low-dimensional material 40. Exemplary low-dimensional materials are one or more organic reactants such as graphite or hydrocarbon compounds that can be used to synthesize the nanomaterial. From the low-dimensional material 40, various nanomaterials such as one-dimensional carbon nanotubes 42, two-dimensional graphene and graphene oxide sheets 44, quasi-three-dimensional graphene aerogel 46, or various doped carbon-nanomaterials 48 can be produced. The doped carbon-nanomaterials may include, for example, any of the nanomaterials 42, 44, or 46 that further contain nitrogen, boron, or sulfur atoms as dopants.

[0031] Next, the nanomaterials 42, 44, 46, 48 can be used to formulate nanoinks and composite nanoinks 50. The nanoinks 50 can then be used in the fabrication of various energy storage devices such as printed electrodes 52 that can be used to produce, for example, supercapacitors 54 and pseudocapacitors 56. The printed electrodes 52 can also be used with one or more electrolytes 58 to construct a battery 60.

Examples

[0032] The following examples describe various embodiments of the present invention, particularly ink compositions comprising graphene, reduced graphene oxide, and graphene aerogel. They illustrate specific concepts that can be used with the present invention and should not be construed as limiting its overall scope. In certain embodiments, the formulation of the nanoink can start with commercially available graphene ink, such as the graphene ink available from Sigma Aldrich. However, as described below, carbon-containing nanomaterials can be synthesized and incorporated into the composite ink composition.

[0033] Figure 1 schematically shows a process for making two graphene-containing inks according to an embodiment of the present invention. In both processes, graphene ink 10 is used as the base formulation. In one process, graphene aerogel is added to the graphene ink base to form graphene aerogel ink 12 containing both graphene 14 and graphene aerogel particles 16. In a second process, carbon nanotubes are added to graphene ink 10 to form graphene-carbon nanotube ink 18 containing both graphene 14 and carbon nanotubes 20. It should be noted that the carbon nanotubes can include single-walled carbon nanotubes or multi-walled carbon nanotubes. As shown in Figure 2, the ink can then be used to fabricate electronic devices 22, such as electrodes, by printing ink 24 from inkjet head 26 onto plastic substrate 28 in a desired configuration.

[0034] [Example 1] Reduced Graphene Oxide Ink The reduced graphene oxide ink can be formed using the following steps.

[0035] 1. 85% by volume of cyclohexanone (Sigma Aldrich) was mixed with 15% by volume of terpineol (Sigma Aldrich) to prepare a homogeneous solvent mixture. This solvent mixture forms a vehicle for the ink formulation.

[0036] 2. The solvent was mixed with reduced graphene oxide (manufactured by ACS Materials) at a ratio of 10 - 15 mg / mL. Alternatively, commercially available graphene powder (e.g., available from Graphene Supermarket) can be used.

[0037] 3. 3 - 5 mg / mL of ethyl cellulose (ethyl cellulose manufactured by Sigma Aldrich) was added to the mixture.

[0038] 4. The mixture was sonicated in an ultrasonic bath overnight at a temperature not exceeding 400 °C.

[0039] 5. The mixture was probe sonicated for 60 minutes with a 15 - minute process at 30% output set for probe sonication and a 15 - minute pause in between. A process was carried out to ensure that the temperature did not exceed 400 °C.

[0040] 6. The ink / suspension was sonicated in an ultrasonic bath for an additional 10 minutes and then vortex mixed for about 5 minutes before printing.

[0041] Furthermore, for the composite nano - ink formulation containing carbon nanotubes and / or graphene aerosol gel, the following steps are also carried out.

[0042] 7. 0.3 mg / mL of nanotubes or graphene aerosol gel was mixed with the graphene ink and the resulting ink was sonicated in an ultrasonic bath for 1 hour.

[0043] Next, formulated composite inks having graphene nanosheets and carbon nanotubes (CNTs), and formulated composite inks having graphene nanosheets and graphene aerosol gel (GAG) were used in a material printer (SonoPlot Microplotter II) to fabricate electrodes printed (on a flexible and bendable substrate such as polyimide) for the characterization of their electrochemical properties. After the printing process, the electrodes were annealed at 300 °C for 2 hours in a nitrogen atmosphere. Standard hexamine ruthenium(III) chloride solution was used as an electrochemical probe to test the printed electrodes. To understand the electron transfer (or charge transfer) process between the nanomaterial surface and the solution sample by electrochemical methods, cyclic voltammetry (CV) tests were carried out, sweeping the voltage across the entire electrode with respect to the standard Ag / AgCl reference potential and monitoring the oxidation and reduction currents. The oxidation and reduction active surfaces when the voltage is swept back and forth are indicators of charge transfer and mean the electroactivity of the surface. The electrodes (i.e., printed electrodes) fabricated by the additive manufacturing method of the above composite inks were characterized using this technique, and both of the above composite inks showed significantly active electrochemical signals (one order of magnitude higher), indicating the role of carbon nanotubes and graphene aerosol gel additives in graphene inks for manufacturing sensors and energy storage devices. This property is thought to be due to the porous structure of the added nanomaterials, which can provide a larger effective reaction surface area. The porous structure and network distribution of the GAG and CNT matrix were confirmed from transmission electron microscope (TEM) images.

[0044] Figure 3 schematically shows a drop-on-demand and printing process that can be used to fabricate graphene electronics, such as electrodes, sensors, and circuits. Ink 30 is ejected from inkjet head 32 and deposited on plastic substrate 34 disposed on printer plate 36. The electrochemical and charge transfer properties of strip electrodes 38 formed with each ink were tested.

[0045] Figures 4a-4c show the surface microstructure of the printed sensors measured using scanning electron microscope imaging (SEM). The three sensors comprise, consist of, or consist essentially of (a) graphene, (b) a composite of graphene and graphene aerosol gel, and (c) a composite of graphene and carbon nanotubes. Isolated and / or smaller clusters of graphene aerosol gel and individual carbon nanotubes are shown in close connection with the host graphene flakes and provide additional surface properties evident from electrochemistry, as described below. The density of the aerosol gel and carbon nanotubes was intentionally kept low to avoid aggregation of nanostructures that could adversely affect their functionality.

[0046] Figures 5a-5d are TEM images showing the nanostructures of the components and their mixing at the nanoscale of the three inks. Figures 5a, 5c, and 5e show TEM images of graphene flakes, graphene aerosol gel mixed with and covered by graphene, and carbon nanotubes mixed with and covered by graphene, respectively. Figures 5b, 5d, and 5f show high-resolution images of graphene, graphene aerosol gel in close contact with graphene, and carbon nanotubes (less than 10 nm) in close contact with graphene.

[0047] Raman spectra were obtained on printed sensors printed in a one-pass print using inks containing (a) graphene, (b) composite graphene with graphene aerosol gel, and (c) composite graphene with carbon nanotubes. Characteristic D peaks, G peaks, and 2D peaks were observed at approximately 1350 cm -1 , 1580 cm -1 , and 2700 cm -1 indicating the presence of intact molecules and structural vibration modes of the carbon nanomaterials.

[0048] Figure 6 is a graph of cyclic voltammetry of three printed sensors fabricated using inks containing graphene (a), composite graphene with graphene aerogel (b), and composite graphene with CNT (c). The sensors were tested with a standard Ag / AgCl reference electrode and a standard hexammine ruthenium(III) chloride redox probe. When the graphene ink contains a graphene aerogel or carbon nanotube additive, very distinct and symmetric oxidation and reduction peaks are observed, indicating an improvement in the electroactive properties of the composite ink. The cathodic and anodic currents are orders of magnitude higher in the composite ink compared to the graphene ink. The interaction of the graphene ink with the redox probe is still weak.

[0049] The cyclic voltammetry of printed sensors containing composite graphene with graphene aerogel ink and composite graphene with CNT ink was analyzed at scan rates of 5 mV / s, 10 mV / s, 25 mV / s, 50 mV / s, and 100 mV / s. Graphs showing the linear anodic and cathodic characteristics of these two sensors were also created. Both types show high electrochemical electron transfer, but the sensors containing the carbon nanotube additive showed slightly higher performance than the sensors containing the graphene aerogel additive.

[0050] The cyclic voltammetry of each of the composite graphene - graphene aerogel and composite graphene - carbon nanotube sensors in 5.0 mM glucose in 0.1 M NaOH at various scan rates was also measured. Graphs showing the corresponding linear cathodic and anodic peak current positions were also created, indicating that sensors printed or coated with the composite graphene ink could be used as glucose sensors.

[0051] [Example 2] Formulation of Graphene Ink from Graphite Powder In another embodiment, the graphene aerogel ink can be formulated as follows.

[0052] 1. First, 1.5 mg / ml ethyl cellulose (viscosity 4 cP) (i.e., 0.25 g) was dispersed in 50 ml ethanol and sonicated in an ultrasonic bath. Then, 50 mg / ml graphite (ACS Materials) (i.e., 2.5 g) was added to the mixture.

[0053] 2. The dispersion was probe sonicated in a probe device for 3 hours at 50% of the set energy (set to 5 seconds ON followed by 5 seconds OFF, and the beaker containing the mixture was maintained in an ice bath).

[0054] 3. The dispersion was centrifuged at 10,000 g (11641 rpm) for 15 minutes, and then the supernatant was recovered.

[0055] 4. The supernatant was mixed with 0.04 g / ml NaCl in a volume ratio of 1:2 and then heat-treated at 50 °C on a hot plate with stirring using a magnetizer for 5 minutes.

[0056] 5. To recover the powder without residual salts, the graphene / EC solid was washed with deionized water, isolated by vacuum, and then placed on a hot plate again to dry.

[0057] 6. The graphene / EC powder was redispersed in ethanol and filtered through a 5 μm filter pore.

[0058] 7. The dispersion was mixed again with 0.04 g / ml NaCl in a volume ratio of 1:2 and then stirred with a magnetizer on a hot plate for 5 minutes.

[0059] 8. To recover the final powder without residual salts, the graphene / EC solid was washed with deionized water and isolated by a vacuum filtration process. Finally, the product was placed on a hot plate to dry.

[0060] 9. Finally, this powder is used in a mixture of 85% by volume cyclohexanone (Sigma Aldrich) and 15% by volume terpineol (Sigma Aldrich) at a concentration of 50 - 200 mg / mL to produce a homogeneous solvent mixture.

[0061] [Example 3] Formulation of Graphene Aerosol Gel Ink In another embodiment, the graphene aerosol gel ink can be formulated as follows.

[0062] The following chemicals can be used. a. Solvent: Ethanol b. Surfactant: Ethyl cellulose (product number 200646, Sigma Aldrich, 4 cp, 5% in toluene / ethanol (80:20 (liter))) c. Graphene aerosol gel: (O2 / C2H2) ratio = 0.5

[0063] Step 1. Suspend the graphene aerosol gel in the solvent. a. Using a calibrated volumetric cylinder, pour 50 ml of ethanol into a clean glass container and add 250 mg of graphene aerosol gel (i.e., at a concentration of 5 mg / ml). Gently shake for 10 - 15 minutes. b. Add 1.25 g of ethyl cellulose (i.e., at a concentration of 25 mg / ml). c. Maintain in an ice bath for 1 hour at 50% watt setting, use probe sonication, and use a pulse on time of 5 seconds and a pulse off time of 5 seconds to disperse the graphene aerosol gel in the solvent. After this step, the graphene aerosol gel disintegrates into graphene aerosol gel flakes.

[0064] Step 2. Separate smaller graphene aerosol gel flakes. d. Centrifuge the suspension at a speed corresponding to 11,000 rpm (10,000 rcf) for 15 minutes. Gently pour the liquid from the top of the centrifuge vial and collect it in a beaker.

[0065] Step 3. Aggregation and removal of excessive surfactant (ethyl cellulose). e. Prepare an aqueous solution of NaCl with a concentration of 0.04 g / ml in deionized water. f. Add the NaCl sol to the suspension while maintaining a volume ratio of 1:2 between the suspension and the NaCl aqueous sol. g. Filter the aggregated graphene aerosol gel by vacuum filtration using a filter with a pore size of 0.45 μm. h. Dry the solid powder at 50 °C overnight using a hot plate to obtain graphene-ethyl cellulose powder.

[0066] Step 4. Removal of larger graphene aerosol gel flakes. i. Redisperse the dry powder in an ethyl alcohol solvent and filter it through a 5 μm sieve. j. Re-aggregate after steps (e), (f) and (g). k. Dry it overnight using a hot plate at 50 °C.

[0067] Step 5. Preparation of the final graphene aerosol gel ink. l. Slowly add 60 mg of ethyl cellulose-graphene powder into a mixture of 450 μL of cyclohexanone and 50 μL of terpineol. m. Use ultrasonic bath treatment at room temperature for 15 minutes to mix the powder in the solvent to obtain a homogeneous and suspended ink.

[0068] When a lower concentration of ethyl cellulose was used in the ink formulation, it was observed that the ink showed better conductivity after printing and thermal annealing.

[0069] Figure 7 is a photograph of a ~1 cm x 1 cm supercapacitor device (interdigitated electrodes) printed with an aerosol gel ink having a lower concentration of surfactant. These devices were printed onto a polyimide (KAPTON) film using a one-pass printing with a 20 μm diameter printer tip. After printing, the devices were annealed at 350 °C for 2 hours in nitrogen.

[0070] Nanoscale materials with tunable physical structures gain the unique benefit of a large surface area exposure to their surrounding environment, which leads to numerous applications. However, important bottlenecks such as interparticle electrical resistance and mechanical breakdown, surface sensitivity to undesirable species, and the inability to control their defects limit their potential for scalable and practical applications such as renewable energy in the form of energy storage devices. Graphene has shown great physical properties over the past 15 years due to its atomic dimension (thinness) and special electronic properties due to its successful use in energy and sensing. Although there is high demand for aspects of large-scale material synthesis of graphene, in contrast to the micromechanical exfoliation method, the manufacturing of graphene devices and the exploration of their highly reliable properties have not yet been adopted in the industry. In one embodiment of the present invention, a stable graphene aerosol gel ink is produced by using a graphene aerosol gel to functionalize surfactants and using them to fabricate graphene microsupercapacitors printed on a number of substrate materials including flexible and bendable substrates such as polyimide. Microsupercapacitors with excellent properties have been demonstrated to closely depend on the effective surface area of the printed electrodes exposed to the electrolyte. As shown in more detail below, over 80% capacitance retention was obtained in printed microsupercapacitor electrochemical devices over 10,000 operating cycles with a 1-ethyl-3-methylimidazolium tetrafluoroborate electrolyte and a discharge current density of 5 - 10 μA cm -1 and.

[0071] Characterization and Printing of Ink Graphene aerosol gel ink was characterized by many methods, including stability tests (qualitative observations on sedimentation of the suspension over time), aerosol gel density estimation, electron microscopy measurements, and electrical resistance measurements after printing. In the case of micro-supercapacitors, interdigitated electrodes were printed on a polyimide substrate by using a SonoPlot Microplotter II with a nozzle size of 20 μm at room temperature. Multiple printing passes can be used to optimize the supercapacitor characteristics, but this study focuses on printed supercapacitor devices by single-pass writing, mainly due to the manufacture of micro-devices for small power sources. Post-print heat treatment was performed on the devices at 350 °C for 2 hours in an inert ambient of a 5% hydrogen mixture in nitrogen.

[0072] Catalyst-free electrical spark-initiated detonation of acetylene precursor (C2H2) with a controlled amount of oxygen was carried out in a 4L chamber to synthesize graphene aerosol gel. The summary of the process during detonation can be understood by the conversion of acetylene molecules into free carbon atoms or ions, followed by carbon aerosol formation, and then through a gelation process to form a gel called carbon aerosol gel (CAG). The detailed processes of CAG, graphene aerosol gel, and their characterization have been reported previously, and several important parameters such as the layer thickness, porosity, oxygen-to-carbon atom ratio, and carbon-carbon bond characteristics (hybridization) of the aerosol gel wall have been studied. The surface area of the aerosol gel was tested using Brunauer-Emmett-Teller (BET) measurements before formulating the aerosol gel ink.

[0073] The graphene properties of the ink components were confirmed and the quality of the aerosol gel material was guaranteed by a number of characteristics such as Raman spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy with local lattice spacing, and diffraction patterns from selected area electron diffraction (SAED). The suspension also holds its particle dispersion without sedimentation, ensuring the ink quality for long-term use later. The rheology and particle size of the ink are some of the most important parameters that determine not only the quality of the printed pattern but also its physical and mechanical stability.

[0074] The microsupercapacitor shown in FIG. 7 is printed on a polyimide substrate, which is a mechanically very flexible and bendable sheet with a thickness of 25 μm corresponding to a high level of mechanical flexibility. In these devices, the cells are typically 10 mm x 8 mm and can be further miniaturized.

[0075] FIG. 8 shows the high-resolution transmission electron microscopy (HRTEM) imaging results of graphene aerosol gel ink. The randomly distributed collapsed structure of the aerosol gel particles (left image in FIG. 8) indicates the graphene aerosol gel signature. The right image (FIG. 8) is an enlarged view of the graphene aerosol gel located in the region indicated by the circular region of the left image. The image shows the number of graphene layers present in typical aerosol gel particles used in the ink (7 - 10 parallel lines can be seen from TEM). It is important to understand that this unique structure is an essential component for the development of energy storage devices, along with the reliability (mechanical integrity) in print quality.

[0076] To investigate the carbon-carbon bond properties of the printing device in more detail, Raman spectroscopy measurements were performed on the aerosol gel ink, and FIG. 9 shows the vibration spectrum of the aerosol gel lattice shown in FIG. 8 (the spot corresponding to the inserted image is indicated as the measurement site). The G peak (at about 1580 cm -1 ) and the 2D peak (at about 2700 cm -1) The appearance of means the existence of graphite bonds in the structure. As shown in Figure 10 of the multilayer graphene lattice, the intensity of the 2D peak is expected to be smaller than that of the G peak due to a different band structure (a linear band structure with Dirac points) compared to monolayer graphene. The D peak located around 1340 cm -1 is assigned to the peak originating from defects. Due to its quasi-3D random structure and the number of defects in the structure, this peak is expected to occur in the Raman spectrum of the ink.

[0077] The printed microsupercapacitors were tested for their electrochemical performance using a Gamry Instrument interface 1010E potentiostat / galvanostat device at potential windows of 0 V and 1 V. The electrolyte used in the cell was an ionic liquid electrolyte, namely 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4, an organic electrolyte). This is a room-temperature ionic liquid (IL) with a wide range of applications, including, for example, the calibration of redox probes used to test electrochemical transport mechanisms, the evaluation of single-electron or multi-electron redox processes, charge shuttling in electric double-layer capacitors (supercapacitors), and the investigation of the reliability of the electrode-electrolyte interface.

[0078] Cyclic voltammetry analysis of the printed supercapacitors was performed at various scan rates (100 mV / s to 2,000 mV / s) between polarities of 0 V to 1 V applied to two sets of finger electrodes. The rectangular current-voltage characteristics (especially at some lower scan rates and even at a scan rate of 1,000 mV / s) demonstrated the characteristics of the electrochemical double-layer capacitance. The measurements were reliable over hundreds of cycles of operation at each of the lower scan rates, indicating the stability of the electrode-electrolyte interface. At higher scan rates (2,000 mV / s, etc.), the current-voltage characteristics were somewhat distorted from the ideal rectangular shape that needs to be recognized and improvement is required.

[0079] The areal specific capacitance (C of the microsupercapacitorS ) can be measured from CV curves at different scanning speeds or from galvanostatic charge-discharge characteristics (described later). These are microsupercapacitors that utilize graphene aerosol gel with a randomized structural signature. Therefore, it would be more appropriate to consider the areal specific capacitance instead of deriving the volume specific capacitance or the weight specific capacitance.

[0080] Figures 10a and 10b show the charge-discharge characteristics of a microsupercapacitor using galvanostatic measurements. A galvanostatic density of 6 μA cm -2 was used for the measurement.

[0081] The areal specific capacitance of the microsupercapacitor is obtained as follows,

Equation

[0082] Figure 11 shows the expansion of material discovery into nanocarbon composites and their application to the manufacture of energy devices (supercapacitors and battery devices) by functional inks and additive manufacturing. The composites and composite inks include 1D nanotubes, 2D graphene, quasi-3D graphene aerogel, and carbon nanomaterials doped with elements such as nitrogen, sulfur, and boron.

[0083] [Example 4] Formulation of Graphene Aerogel Ink In this example, graphene aerogel ink was prepared and the physical characteristics and properties of the interdigitated electrodes were tested.

[0084] Material Characterization The Philips CM-100 transmission electron microscope was used to test the microstructure of the graphene aerogel particles at an acceleration voltage of 100 kV. The TEM specimen was prepared directly on a TEM copper grid by directly immersing the copper grid in the synthesized GAG ink. The surface morphology and uniformity of the printed electrodes were measured with a Hitachi field emission scanning electron microscope (FESEM). Raman (Renishaw Invia Raman Microscope, excitation wavelength 532 nm) and XPS spectra (PHI 5000 Versa Probe II, Physical Electronics Inc.) were measured directly on the printed device to determine the phase and elemental analysis. The XPS spectra were achieved with a combination of electron and argon ion flood guns. The X-ray beam size was 100 μm, and the survey spectra were recorded with a pass energy (PE) of 117 eV, a step size of 1 eV, and a dwell time of 20 ms, while the high energy resolution spectra were recorded with a PE of 23.5 eV, a step size of 0.05 eV, and a dwell time of 20 ms.

[0085] Aerogel Ink Preparation The GAG powder synthesized by the detonation method contained graphene nanosheet aggregates in the initial state. Therefore, in order to disperse the aggregates, probe sonication was used for 30 minutes under ice bath conditions by an ultrasonic probe (500 W, 20 kHz, Q500 sonicator, USA). 250 mg of GAG powder was dispersed in 50 ml of ethanol, and 1 w / v% ethyl cellulose (EC, Sigma-Aldrich, 5 wt%, measured at 4 cP grade in 80:20 toluene:ethanol with 48% ethoxy) was used as an emulsifier. Subsequently, the suspension was filtered through a 5 μm glass fiber syringe filter to remove larger GAG particles. Then, the recovered suspension was aggregated by adding an aqueous NaCl solution (0.04 g / ml in deionized water) and subsequently vacuum filtered using a 0.45 μm nylon filter. Then, the obtained GAG / EC paste was dried using a hot plate at 70 °C. The GAG / EC powder was uniformly suspended in cyclohexanone and terpineol (volume ratio 85:15), and then the ink was prepared by sonicating in a bath at a concentration of 70 mg / ml.

[0086] Inkjet printing of interdigitated electrodes (IDE) The interdigitated electrodes (IDE) of MSC were patterned on a flexible substrate using an inkjet printer (SonoPlot, Microplotter II, USA) with a glass tip of nozzle size 20 μm at room temperature. The substrate was thoroughly cleaned by sonicating in a mixture of acetone and methanol and dried by blowing with nitrogen gas before printing. Then, the printed electrodes were heat-treated at 350 °C for 2 hours in an N2 / H2 mixture (5% hydrogen in nitrogen) to burn off the organic binder.

[0087] Electrochemical performance The electrochemical performance of the printed microsupercapacitor was tested using a Gamry interface 1010E potentiostat / galvanostat in a potential window of 0 to 1 V with an organic electrolyte of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4, Sigma Aldrich). The areal specific capacitance (C A ), volumetric specific capacitance (C V ), and equivalent series resistance (R ESR ) were measured from the galvanostatic charge-discharge curves using equations (1), (2), and (3),

Number

Number

[0088] Results and Discussion Thermogravimetric analysis (TGA) was performed to measure the graphene content in the treated GAG / EC powder. The TGA showing the mass change as a function of temperature indicated the decomposition of the surfactant at an initial temperature of approximately 250 °C. A significant mass change (30 wt%) occurred between 250 °C and 350 °C, indicating the complete decomposition of the surfactant into aromatic compounds. Thus, the GAG / EC powder contained 70 wt% graphene. The GAG / EC powder was further suspended in a cyclohexanone / terpineol mixture (volume ratio 85:15) to prepare an ink for inkjet printing.

[0089] The formulated ink exhibited long-term stability and printability. Figures 12a and 12b show the optical images of the inkjet-printed interdigital μ-SC and resistor elements, respectively, on a flexible polyimide (25 μm thick) substrate. The geometric dimensions of the printed devices are shown in Table 1.

[0090] Table 1. Geometric measurements of inkjet-printed microsupercapacitors [Table 1]

[0091] To determine the uniformity of the printed pattern, SEM and AFM images (SI-II) were recorded. As can be seen from the SEM image in Figure 12c, the printed pattern is very uniform and has no coffee-ring effect. Thus, during the printing process, the ink flow was consistent, there was no aggregation, and good rheological properties of the formulated ink were obtained. The printability of the ink was examined by printing a plurality of devices with a double number of fingers. All printed patterns showed good uniformity. Furthermore, high-resolution images of the printed electrode surface were recorded by SEM to test the surface morphology. See Figure 12d. The surface of the printed electrode contains spherical particles and forms a highly porous surface. Such a porous morphology is beneficial for the storage capacitance.

[0092] Microstructure characterization The fine structure of the GAG powder represented by high-resolution TEM measurement is shown in Fig. 13. Fig. 13(a) shows an aggregate of graphene nanosheets (GNS) having a nearly uniform size distribution close to 100 nm. As shown in (b), it is worth noting that the edges of the GNS have a more positive contrast with respect to the center of the sheet. As is clear from the high-magnification image of (b), the boundary of the GNS basically includes a shell-like structure in which the shell is formed by edge-terminated graphene sheets, as shown in the inserted image of (c). The randomly oriented sheets fuse to form a unique nanoporous system, i.e., an aerosol gel-like structure. The abundance of edge-terminated graphene is similar to the form of carbon onions in which graphene sheets are concentrically arranged to form a closed multi-shell structure, as shown in (d). In this case, the shell structure is limited only to the edges of the particles, and thus the unique fine structure inherits porosity and can bring benefits to electrochemical energy storage.

[0093] Raman spectrum of graphene aerogel Raman spectroscopy is used as a convincing method for non-destructive high-throughput characterization of carbon materials. The unique band structure of graphene has led to the generation of strong Raman bands due to resonant phonon scattering. Therefore, careful analysis of the Raman spectrum has revealed important microstructural aspects regarding the defects, stacking order, number of layers, doping, stress, and thermal conductivity of graphene. The Raman spectrum of GAG particles recorded at room temperature fits a Lorentzian function and includes three strong Raman bands centered at 1351 cm 2 −1, 1583 cm -1 −1, and 2700 cm -1 −1. These optically Raman-active phonon modes are typically assigned as the D, G, and 2D bands, which are due to the overtones of the A -1 1, E 1g 2, and A 2g 1 phonon modes, respectively. Furthermore, two weak Raman bands were also present at 1622 cm 1g −1 and 2452 cm -1 −1. -1

[0094] These phonon modes have previously been assigned as the D’ and D+D’’ bands, respectively. The origin of the D’ band is attributed to the intra-valley double resonance (DR) scattering process around the K (or K’) point of the Brillouin zone. However, the D+D’ mode is activated from the combination of the LA-branch phonon at 1100 cm -1 and the D phonon at the K point of the Brillouin zone. The appearance of the strong 2D band is a signature of graphite carbon. Furthermore, the linear shape of the 2D band fits very well with a single Lorentzian function, similar to the 2D band of single-layer graphene (SLG). However, the increase in the wave number by about 20 cm -1 and the higher full width at half maximum (FWHM) are consistent with the turbostatic stacking of graphene layers in GAG.

[0095] Furthermore, the intensity of the D band generally correlates with the defect density present in the form of structural defects and irregular edges due to the loss of translational symmetry. As seen from the high-resolution TEM images, the microstructure of GAG contains a shell-like structure with an average size distribution of 100 nm. The boundaries of the shell are restricted by edge-oriented graphene sheets that substantially persist in the GAG morphology. Thus, intuitively, a large I(D) / I(G) is expected in GAG due to a significant amount of exposed edges, as previously shown for onion-like carbon. Conversely, a smaller I(D) / I(G) of about 0.2 suggests not only a low concentration of structural defects but also edges where translational symmetry is maintained to a good extent. Thus, the edges have either a zigzag or armchair arrangement of carbon atoms with a smaller amount of edge defects. It has previously been shown that zigzag edges do not effectively contribute to the D band intensity due to momentum conservation, suggesting that the stripe-like structure contains a substantial density of edge-oriented graphene sheets with an armchair arrangement of carbon atoms.

[0096] X-ray photoelectron spectroscopy The chemical purity of the GAG was analyzed by XPS spectra recorded from the surface of the printed device. The investigated spectra showed only optical peaks related to C1s carbon and a very low oxygen concentration. The asymmetric shape of the XPS band indicates a deconvolution into three components. The greater intensity of the XPS band is shared by the sp 2 hybrid state (284.05 eV) of the hexagonal network of carbon atoms, further confirming the chemical purity of the GAG. The sp 3 hybrid state (284.7 eV) is also present in the XPS spectrum at a significant concentration (22%) along with a minimum concentration of C-O groups. This can refer to the size and its considerable amount of the chemically more active graphene edge state that reacts with oxygen.

[0097] Electrochemical performance The electrochemical performance of the printed GAG μ-SC investigated by cyclic voltammograms (CVs) at different scan rates showed a potential window of 0.0 - 1.0 volts. The typical rectangular shape of the CV curve indicates the ideal double-layer capacitance characteristics of the printed μ-SC. The rectangular shape of the CV curve persisted linearly for measured high scan rates of up to approximately 2 V / s. However, the rounded corners suggest a significant equivalent series resistance (ESR) present in the printed device. The ESR value was calculated using Equation (2) from the voltage drop at the start of the discharge curve (ΔV = 36 mV at 5 μ-amp / cm 2 ). The magnitude of R ESR was found to be approximately 45 kΩ. The magnitude of the ESR results from contact resistance, electrode-electrolyte interface resistance, and bulk electrode resistance. The high porosity of the GAG electrode can be a major factor contributing to the magnitude of the ESR. Furthermore, charge-discharge (CDC) profiles were measured at different current densities for the MSC. The CDC represents a typical triangular-shaped profile, although an asymmetric shape is well known, especially at low current densities. However, at high current densities, the triangular shape became more symmetric. Areal specific capacitance (C A ) and volumetric specific capacitance (C v) was calculated from the slope of the constant current discharge profile using equations (2) and (3) as a function of current density. The stability of the printed supercapacitor was tested with a large number of CDC cycles at a constant current density of 6 μ-amp / cm 2 . The device showed good capacitance retention of about 80% after 10,000 cycles.

[0098] To increase the output density for practical applications, generally, as shown in FIGS. 14a and 14b, multiple cells are assembled in a combination of series and parallel. The series combination, as expected, showed a decrease in capacitance by one-third when operating in a potential window of 0 to 1 volt, but showed a slight increase in charge and discharge time when operating at up to 3 volts. Similarly, the parallel arrangement showed a three-fold increase in capacitance compared to a single cell. Therefore, the formulated GAG ink can be directly used to print multiple devices in a combination of series and parallel to adjust the output.

[0099] Conclusion All printed patterns showed high uniformity without any obvious dirt or coffee-ring effect. Therefore, the successful ink formulation protocol was confirmed by the good printability and long-term stability of the GAG ink. The printed μ-SC showed good excellent capacitance retention of about 80% over a large number of charge and discharge cycles (10,000 cycles) when operated at 6 μA cm -2 in a potential window of 0 to 1 volt. Therefore, this approach was able to bridge the gap in the mass production of graphene and the manufacturing of energy storage devices.

Claims

1. An ink composition containing a certain amount of graphene aerosol gel and graphene.

2. The ink composition according to Claim 1, wherein the certain amount of graphene aerosol gel and graphene are dispersed in a liquid vehicle.

3. The ink composition according to Claim 2, wherein the liquid vehicle contains a mixture of one or more ketones and one or more alcohols.

4. The ink composition according to Claim 3, wherein the liquid vehicle contains cyclohexanone and terpineol.

5. The ink composition according to Claim 4, wherein the liquid vehicle contains 60% to 99% by weight of cyclohexanone and 1% to 35% by weight of terpineol.

6. The ink composition according to Claim 1, wherein the ink composition contains 1 to 500 mg / ml of a surfactant.

7. The ink composition according to Claim 1, wherein the ink composition contains 0.01 to 10 mg / ml of the graphene aerosol gel.

8. The ink composition according to Claim 1, wherein the ink composition further contains a certain amount of graphene oxide particles.

9. The ink composition according to Claim 1, wherein the ink composition contains 1 to 30 mg / ml of the graphene.

10. The ink composition according to Claim 1, wherein the graphene aerosol gel has a D90 particle size of 0.1 to 10 μm.

11. The ink composition according to Claim 1, wherein the graphene aerosol gel is doped with at least one of nitrogen, sulfur, and boron.

12. An electronic device including one or more traces printed with the ink composition according to any one of Claims 1 to 11.

13. The electronic device according to Claim 12, wherein the electronic device is a capacitor, supercapacitor, microcapacitor, ultracapacitor, or pseudocapacitor.

14. A method for forming a composite ink, comprising: providing a graphene-containing ink precursor containing a certain amount of graphene and optional graphene oxide particles dispersed in a liquid vehicle; dispersing a certain amount of graphene aerosol gel into the ink precursor; A method for forming a composite ink, including the above steps.

15. The step of providing the ink precursor includes: forming the liquid vehicle by mixing at least one ketone and at least one alcohol. The step of adding the predetermined amount of graphene and optional graphene oxide particles to the liquid vehicle; Optionally, the step of adding a surfactant to the liquid vehicle and the graphene and optional graphene oxide particle mixture; A stirring step of stirring the liquid vehicle and the graphene and optional graphene oxide particle mixture, thereby dispersing the graphene and optional graphene oxide particles in the liquid vehicle; The method according to claim 14, comprising:

16. The method according to claim 15, wherein the stirring step comprises subjecting the ink precursor to ultrasonic treatment.

17. The method according to claim 16, wherein the ultrasonic treatment of the ink precursor is performed at a temperature of 400 ° C or lower.

18. The method according to claim 14, wherein the step of dispersing the predetermined amount of graphene aerosol gel in the ink precursor comprises subjecting the ink precursor and the dispersed graphene aerosol gel to ultrasonic treatment.

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