TWI860024B - Preparation method of suspension of supercapacitor carbon fiber electrode and preparation method of carbon quantum dot electrolyte - Google Patents

Abstract

A method for preparing a suspension of a supercapacitor carbon fiber electrode and a carbon quantum dot electrolyte comprises preparing a gum arabic aqueous solution with gum arabic and deionized water, adding the gum arabic aqueous solution to carbon fiber powder, and then stirring the solution with a magnet and vibrating the solution with ultrasonic waves to obtain a carbon fiber suspension. The carbon fiber suspension is centrifuged and repeated several times, and the supernatant is retained to obtain a uniformly dispersed carbon fiber suspension. Polyvinyl alcohol and deionized water are heated, mixed and stirred until completely dissolved to form a PVA aqueous solution, and then phosphoric acid is added and mixed evenly, and then placed in a vacuum oven for heating and dehydration for a period of time to obtain a PVA/H ₃ PO ₄ gel electrolyte, so that the prepared carbon fiber suspension and gel electrolyte can be used as a carbon fiber electrode suspension and carbon quantum dot electrolyte of a supercapacitor, respectively. When 0.10% of carbon quantum dots are added, the specific capacitance of the gel electrolyte material can reach 110.57F/g, and the equivalent series resistance in the gel electrolyte can be effectively reduced to 29.98Ω.

Description

Preparation method of suspension of supercapacitor carbon fiber electrode and carbon quantum dot electrolyte

The present invention relates to a method for preparing a carbon fiber electrode suspension and a carbon quantum dot electrolyte for a supercapacitor, and in particular to a carbon fiber electrode suspension and carbon quantum dot electrolyte preparation technology that utilizes a carbon fiber conductive suspension and a gel electrolyte as a carbon fiber electrode suspension and a carbon quantum dot electrolyte for a supercapacitor, respectively.

With the continuous development of economy and society, the reliance on energy has increased exponentially. Therefore, how to save energy and store energy effectively has become an important subject in the development of energy technology. In addition, a large number of wearable electronic products have been launched in recent years, which has greatly improved people's quality of life. However, no matter what kind of electronic products are used, they need a continuous supply of energy to operate smoothly, which indirectly shows that energy storage components are the most important key. On the other hand, with the rapid increase in the number of global population, the demand for food or industrial products has also increased year by year. In terms of agricultural products, the non-edible parts are often discarded at will during the production and processing stage, such as oyster shells, corn stalks, and garlic skins. In terms of industrial production, there are also a lot of scraps derived, such as metal cutting scraps, cotton wool left after making towels, making drones; or carbon fibers left after cutting wind turbine blades.

With the signing and initiative of the Paris Agreement and the Kyoto Protocol, people's awareness of environmental responsibility has gradually increased. Since many agricultural or industrial wastes cannot be decomposed by themselves, problems arising from the production process and the surrounding environment have gradually attracted the attention of the world. For example, in Reference [1], one of the advantages of carbon quantum dots (CQDs) prepared from plants is that they are highly biocompatible. Usually such carbon quantum dots are synthesized from natural products, such as reference [2]. Because semiconductors or metal quantum dots (QDs) do not dissolve each other in aqueous solutions, they usually must be coated with hydrophilic molecules on the surface to be used in biological fields, such as reference [3]. However, this undoubtedly increases the energy cost of the QD production process and generates more waste. When carbon quantum dots are produced using green raw materials, even if they are synthesized by hydrothermal synthesis, such as in references [4,5], The hydrophilic functional groups on the surface of CQD still exist. These hydrophilic functional groups can enable carbon quantum dots to maintain good dispersion properties in water, and if they are utilized, there will be good development.

In the past, there have been many studies on using plants as green carbon sources. For example, starch extracted from corn stalks and other plants can be used as a raw material for carbon quantum dots (CQDs), such as reference [6]. Rice husks, coconut shells, and other seed shells have also been widely studied and discussed, such as reference [7]. Many researchers have even synthesized carbon quantum dots from charcoal made from various plants, such as reference [8]. There are many methods for preparing carbon quantum dots, among which high-pressure hydrothermal synthesis is the most common technology, such as reference [9]. The carbon source is sealed in a container and decomposed by pure water or other aqueous solutions, and heated at a temperature of 150 to 260°C for several hours. This simple production process is sufficient to produce carbon quantum dots. In addition, carbon quantum dots can be prepared using laser processing technology, such as reference [10] or microwave plasma processing technology, such as reference [11].

As far as we know, the technical prior art related to the present invention is shown in the following patents:

1. Patent name: Electrospinning liquid composition for preparing carbon fiber electrode of supercapacitor, manufacturing method of carbon fiber electrode of supercapacitor and carbon fiber electrode of supercapacitor (Patent Announcement No. I583734). The patent is an electrospinning liquid composition for preparing carbon fiber electrode of supercapacitor, which contains solvent, polyacrylonitrile, graphene oxide and carbon nanotubes. The patent uses a mixture of multiple materials to prepare carbon fiber electrodes and electrospinning liquid compositions; in contrast, the present invention uses the prepared carbon fiber suspension as the carbon fiber electrode suspension of the supercapacitor, so the preparation technology is indeed different. Therefore, the technical means adopted by the patent and the functions or effects achieved are indeed different from those of the present invention.

2. Patent name: Carbon quantum dots and their preparation method and use (Patent Announcement No. I773949). The patent uses dopamine and spermine as precursors and performs high-temperature pyrolysis at 250°C to obtain carbon quantum dots, which effectively inhibit the growth of pathogenic bacteria and various symptoms of biofilm formation. It can be seen that the patent uses dopamine as the material of carbon quantum dots; in contrast, the present invention uses the prepared gel electrolyte as the carbon quantum dot electrolyte of the supercapacitor, so the technical means and functions or effects adopted by the patent are indeed different from those of the present invention.

3. Patent name: Carbon quantum dots and their uses (Patent Announcement No. I770489). This patent uses polyamines and polyamine derivatives with halogen-containing compounds as precursors, uses high-temperature pyrolysis, centrifugation and dialysis to make carbon quantum dots, and finally applies the carbon quantum dots to antibacterial tests. This patent uses polyamines and halogen-containing compounds as materials for carbon quantum dots; in contrast, the present invention uses garlic peels to prepare carbon quantum dots and uses the prepared gel electrolyte as the carbon quantum dot electrolyte of the supercapacitor, so the technical means and functions or effects used in this patent are indeed different from those of the present invention.

4. Patent title: Method for producing carbon quantum dots with high luminescence quantum yield by simple process (Patent Announcement No. I800325), which includes the following steps: preparing a mixture by mixing a crystalline compound containing boron, sulfur or phosphorus, which is solid at about 25°C under atmospheric pressure and has crystallinity, and an organic compound having a reactive group; and preparing carbon quantum dots by heating the mixture to a temperature of 100°C to 300°C in the absence of a solvent. The amount of the crystalline compound in the mixture is 45 parts by mass to 1000 parts by mass relative to 100 parts by mass of the organic compound. It can be seen that the patent uses materials such as boron, sulfur or phosphorus as the material of carbon quantum dots; in contrast, the present invention uses garlic peels to prepare carbon quantum dots and uses the prepared gel electrolyte as the carbon quantum dot electrolyte of the supercapacitor, so the technical means adopted by the patent and the functions or effects achieved are indeed different from those of the present invention.

5. Patent name: Preparation method of stretchable capacitor electrode-conductor structure (Patent Announcement No. I710142), the patent is to establish a nano-carbon tube active material composite layer on an elastic substrate. A nano-carbon tube film structure is established on an elastic substrate, and the nano-carbon tube film structure includes a plurality of specially arranged nano-carbon tube films. Subsequently, the substrate under the accumulated multi-layer nano-carbon tube film structure is removed to form a nano-carbon tube active material composite layer with only a plurality of wrinkles. The patent uses nano-carbon tubes to prepare capacitors; in contrast, the carbon fiber electrode and electrolyte materials used in the present invention are different from those of the patent, so the technical means and functions or effects adopted by the patent are indeed different from those of the present invention.

6. Patent name: Graphene framework for supercapacitors (Patent Announcement No. I759278). This patent uses a three-dimensional porous reduced graphene oxide framework on one of the electrodes of the supercapacitor to prepare a complete supercapacitor. This patent uses graphene oxide to prepare the electrode material in the supercapacitor. In contrast, the present invention uses carbon fiber electrode material, which is different from the patent. Therefore, the technical means adopted by the patent and the functions or effects achieved are indeed different from the present invention.

7. Patent name: Composition containing carbon quantum dots and method for producing the composition for easily obtaining the composition (Patent Announcement No. I767345). The properties of the carbon quantum dots of the patent composition, such as the luminescence wavelength, are within the required range, and the carbon quantum dots and layered clay minerals are uniformly dispersed. The composition containing carbon quantum dots contains: carbon quantum dots, which are obtained by carbonizing an organic compound having a reactive group in the presence of a layered clay mineral; and the aforementioned layered clay mineral. The patent does not have the preparation technology of carbon fiber suspension and gel electrolyte, so it is technically different from the present invention, and the effect achieved in the energy field is also different. Therefore, the technical means and the functions or effects achieved in the patent are indeed different from the present invention.

In view of this, the above-mentioned prior art and patents are indeed not perfect, so there is still a need for further improvement. In addition, based on the urgent needs of the relevant industries, the inventors of the present invention have finally developed a set of inventions that are different from the above-mentioned prior art and patents after continuous efforts in research and development.

The first purpose of the present invention is to provide a method for preparing a suspension of a supercapacitor carbon fiber electrode and a carbon quantum dot electrolyte. The prepared carbon fiber suspension and gel electrolyte are used as a carbon fiber electrode suspension and a carbon quantum dot electrolyte of a supercapacitor, respectively. When 0.10% of carbon quantum dots are added, the specific capacitance of the gel electrolyte material can reach 110.57F/g, and the equivalent series resistance in the gel electrolyte can be effectively reduced to 29.98Ω. The technical means for achieving the first purpose of the present invention is to prepare a gum arabic aqueous solution with gum arabic and deionized water, add the gum arabic aqueous solution to carbon fiber powder, and then use magnetic stirring and ultrasonic vibration to form a carbon fiber suspension, centrifuge the carbon fiber suspension, repeat twice, and retain the supernatant to obtain a uniformly dispersed carbon fiber suspension. Polyvinyl alcohol and deionized water are heated, mixed and stirred until completely dissolved to form a PVA aqueous solution, and then phosphoric acid is added and evenly mixed, and then placed in a vacuum oven for heating and dehydration for a period of time to obtain a PVA/H ₃ PO ₄ gel electrolyte, which is used as the electrolyte of carbon quantum dots in supercapacitors.

The second purpose of the present invention is to provide a method for preparing a supercapacitor using a carbon fiber suspension and a gel electrolyte as a carbon fiber electrode suspension and a carbon quantum dot electrolyte for a supercapacitor. The method mainly uses carbon quantum dots and carbon fibers to prepare a conductive electrode, and then uses the gel electrolyte and electrode material doped in the gel electrolyte as a supercapacitor to form a supercapacitor. After cyclic charge and discharge at a constant current, the capacity retention rate is tested. After 2000 cycles of charge and discharge tests at a constant current of 0.20 mA, the capacity still retains 96% of the initial value. The technical means to achieve the second purpose of the present invention is to prepare a gum arabic aqueous solution with gum arabic and deionized water, add the gum arabic aqueous solution to carbon fiber powder, and then use magnetic stirring and ultrasonic vibration to form a carbon fiber suspension, centrifuge the carbon fiber suspension, repeat twice, and retain the supernatant to obtain a uniformly dispersed carbon fiber suspension. Polyvinyl alcohol and deionized water are heated, mixed and stirred until completely dissolved to form a PVA aqueous solution, and then phosphoric acid is added and evenly mixed, and then placed in a vacuum oven for heating and dehydration for a period of time to obtain a PVA/H ₃ PO ₄ gel electrolyte, which is used as the electrolyte of carbon quantum dots in supercapacitors. The carbon quantum dots are mixed with the deionized water to form a uniformly dispersed CQD conductive liquid, and the CQD conductive liquid is added to the PVA/H ₃ PO ₄ gel electrolyte; the surface of the dimethyl cellulose-based electrode is uniformly covered with the PVA/H ₃ PO ₄ gel electrolyte, and the diaphragm is placed between the dimethyl cellulose-based electrodes, and then the methyl cellulose-based electrode, a part of the PVA/H ₃ PO ₄ gel electrolyte, the diaphragm, another part of the PVA/H ₃ PO ₄ gel electrolyte and the second methyl cellulose-based electrode are sequentially placed on the polyethylene terephthalate film in a sandwich structure assembly manner. The capacitor is packaged in PET to form a flexible supercapacitor.

10: Carbon fiber suspension

10a: Gum Arabic

10b,11b: Deionized water

10c: Gum Arabic aqueous solution

10d: Carbon fiber powder

10e: Carbon fiber suspension

11:PVA/H ₃ PO ₄ Gel Electrolyte

11a: Polyvinyl alcohol

11c:PVA aqueous solution

11d: Phosphoric acid

Figure 1 is a schematic diagram of the preparation of the carbon fiber electrode suspension and carbon quantum dot electrolyte of the present invention.

Figure 2 shows (a) particle size analysis of garlic skin carbon quantum dots of the present invention; (b) surface potential analysis; (c) (i) SEM, (ii) TEM photos.

Figure 3 is (a) the ultraviolet-visible spectrum of the garlic skin carbon quantum dots of the present invention, the inset (left) is the CQD aqueous solution under sunlight (right) is the CQD aqueous solution under ultraviolet light; (b) infrared spectrum.

Figure 4 shows the (a) complete measurement high-resolution XPS spectrum of the garlic skin carbon quantum dots of the present invention; (b) XPS spectrum of C1s; (c) XPS spectrum of N1s; (d) XPS spectrum of O1s.

Figure 5 shows (a) cyclic voltammetry curves of adding different concentrations of CQDs to the phosphate gel electrolyte of the present invention; (b) specific capacitance value.

Figure 6 shows the (a) AC impedance analysis spectrum of the present invention when adding different concentrations of CQD to the phosphate gel electrolyte, (i) low frequency region; (ii) high frequency region; (b) ESR change graph.

Figure 7 shows the (a) cyclic voltammetry curve of the methyl cellulose-based recycled carbon fiber electrode placed in phosphoric acid electrolyte at a scanning speed of 0.20~0.02V/s; (b) specific capacitance value; (c) internal resistance analysis and charge-discharge test diagram at a constant current of 0.20mA.

Figure 8 is a schematic diagram showing the results of the methyl cellulose recovery carbon fiber electrode of the present invention after 2000 cycles of charge and discharge testing at a constant current of 0.20 mA.

In order to allow the Honorable Review Committee to further understand the overall technical features of the present invention and the technical means to achieve the purpose of the present invention, a specific embodiment and accompanying drawings are provided for detailed description: Please refer to FIG. 1, the first embodiment to achieve the first purpose of the present invention includes the following steps:

(a) Material preparation step, providing a predetermined amount of gum arabic 10a, a predetermined amount of deionized water 10b, 11b, a predetermined amount of carbon fiber powder 10d, a predetermined amount of polyvinyl alcohol 11a and a predetermined amount of phosphoric acid 11d.

(b) Preparation step of carbon fiber electrode suspension: gum arabic 10a and deionized water 10b are used to prepare gum arabic aqueous solution 10c, and the gum arabic aqueous solution 10c is added to carbon fiber powder 10d, and then the carbon fiber suspension 10e is obtained by magnetic stirring and ultrasonic vibration treatment. The carbon fiber suspension 10e is centrifuged and repeated at least twice, and the supernatant is retained to obtain a uniformly dispersed carbon fiber suspension 10, so that the carbon fiber suspension 10 can be used as the suspension of the carbon fiber electrode of the supercapacitor.

(c) In the step of preparing the carbon quantum dot electrolyte, polyvinyl alcohol 11a and deionized water 11b are heated, mixed and stirred until completely dissolved to form a PVA aqueous solution 11c, and phosphoric acid 11d is added and mixed evenly, and then placed in a vacuum oven for heating and dehydration for a period of time to obtain a PVA/H ₃ PO ₄ gel electrolyte 11. The PVA/H ₃ PO ₄ gel electrolyte 11 can then be used as the electrolyte of the carbon quantum dots of the supercapacitor.

Specifically, in the carbon fiber electrode suspension preparation step according to the first embodiment, Arabic gum 10a (Arabic After preparing about 0.1-50% gum arabic aqueous solution 10c (preferably 0.4-0.6%) with deionized water 10b, add about 0.1-30% carbon fiber powder 10d (preferably 0.4-0.6g), stir with a magnet for about 55-65 minutes (preferably 60 minutes) and ultrasonically vibrate for about 3-5 hours (preferably 4 hours) to form a carbon fiber suspension 10e, and then centrifuge the carbon fiber suspension 10e at a speed of about 6000 rpm for about 20-30 minutes (preferably 25 minutes) with a high-speed centrifuge, repeat at least 2 times, and retain the supernatant to obtain a uniformly dispersed carbon fiber suspension 10.

Specifically, the present embodiment is a first specific embodiment based on the first embodiment, and further includes a step of preparing a carbon fiber powder 10d, wherein the long carbon fiber waste is chopped to shorten the carbon fiber length, and the chopped carbon fiber is degreased with a neutral detergent and then ultrasonically vibrated for about 1 hour to remove excess impurities; then, the carbon fiber is placed in a mortar and about 20 ml of alcohol is added to wet grind the carbon fiber. The carbon fiber was ground for about 1 hour to turn it into small pieces of powder, and then the carbon fiber and alcohol were mixed in a ratio of about 1:10 to prepare a wet grinding solution; then, the carbon fiber was ground for about 24 hours in a planetary ball mill, and the ball-milled carbon fiber powder 10d was centrifuged at about 4000 rpm in a high-speed centrifuge for about 20 minutes, and the solid powder was retained and dried in a vacuum oven to obtain carbon fiber powder 10d.

Specifically, in the preparation step of the carbon quantum dot electrolyte according to the first embodiment, about 10 g of polyvinyl alcohol 11a and about 100 ml of deionized water 10b are heated at about 85°C, mixed and stirred until completely dissolved to form a PVA aqueous solution 11c, and about 20 g of phosphoric acid 11d is added and evenly mixed, and then placed in a vacuum oven at about 60°C for heating and dehydration for about 6 hours to obtain a PVA/H ₃ PO ₄ gel electrolyte 11.

This embodiment is a second embodiment to achieve the second purpose of the present invention, and includes the following steps:

(a) Providing a predetermined amount of carbon quantum dots (not shown in this example), a predetermined amount of mixed deionized water 10b, a predetermined amount of PVA/H ₃ PO ₄ gel electrolyte 11, a dimethyl cellulose-based electrode (not shown in this example), a separator (not shown in this example), and a polyethylene terephthalate membrane (not shown in this example).

(b) Mixing carbon quantum dots with deionized water 10b to form a uniformly dispersed CQD conductive liquid, and then incorporating the CQD conductive liquid into the PVA/H ₃ PO ₄ gel electrolyte 11.

(c) The surfaces of the dimethyl cellulose-based electrodes are uniformly covered with PVA/H ₃ PO ₄ gel electrolyte 11, and a separator is interposed between the dimethyl cellulose-based electrodes. Then, the one methyl cellulose-based electrode, a portion of the PVA/H ₃ PO ₄ gel electrolyte 11, the separator, another portion of the PVA/H ₃ PO ₄ gel electrolyte 11 and the dimethyl cellulose-based electrode are sequentially placed in a polyethylene terephthalate (PET) film in a sandwich structure assembly manner to encapsulate and manufacture a flexible supercapacitor.

Specifically, in a second specific embodiment based on the second embodiment, carbon quantum dots are mixed with deionized water 11b at a ratio of about 1 mg/ml to form a uniformly dispersed CQD conductive liquid, and then about 0.01~50% of the CQD conductive liquid (preferably 0.05~0.20%) is added to the PVA/H ₃ PO ₄ gel electrolyte 11, wherein the concentration of the PVA/H ₃ PO ₄ gel electrolyte 11 is about 0.1M~10M.

Specifically, the third specific embodiment based on the second embodiment further includes a step of preparing a methyl cellulose-based electrode, which is to drop a carbon fiber suspension on the surface of a dimethyl cellulose substrate and then dry it to obtain a dimethyl cellulose-based electrode.

More specifically, an application embodiment based on the third specific embodiment is to use Arabic gum 10a (Arabic After preparing about 0.5% gum arabic aqueous solution 10c with deionized water 10b, about 0.5g carbon fiber powder is added, and the mixture is stirred by a magnet for about 1 hour and ultrasonically vibrated for about 4 hours to form a carbon fiber suspension 10e; then, the carbon fiber suspension is centrifuged at about 6000rpm for about 25 minutes in a high-speed centrifuge, and the process is repeated at least 2 times, and the supernatant is retained to obtain a uniformly dispersed carbon fiber suspension 10e. About 2ml of the carbon fiber suspension 10e is then taken with a micropipette and dropped onto the surface of a methylcellulose substrate with a high roughness, and dried at about 50°C to obtain a methylcellulose-based electrode.

In addition, the methylcellulose substrate is made by a biodegradable substrate preparation step, which is to add about 4g of methylcellulose to about 100ml of anhydrous ethanol (Ethyl Alcohol) solution, heat and stir at about 60℃ until completely dissolved to prepare a 4% methylcellulose solution, and pour about 2ml of the methylcellulose solution on the glass substrate through a micropipette, and place it at room temperature for about 15 minutes in vacuum baking to remove bubbles; then, increase the temperature of the vacuum oven to about 50℃ and dry for about 2 hours, and after drying and demolding, a biodegradable methylcellulose substrate is obtained.

Specifically, based on the fourth specific embodiment of the second embodiment, this embodiment uses a high-temperature pyrolysis method to make garlic peels in agricultural waste into carbon quantum dots, and at the same time prepares waste carbon fibers into electrodes. The high-temperature pyrolysis method can be processed using only a tubular sintering furnace and a quartz boat, and is also one of the simplest methods for preparing carbon quantum dots. From the perspective of sustainable development goals, it is helpful for mitigating and adapting to climate change. Because garlic peels are discarded after garlic is eaten, the problem of recycling carbon fiber waste is also solved. Therefore, this implementation further includes a preparation step of carbon quantum dots, which includes the following steps:

(a) The step of chopping the discarded garlic skins is to chop the discarded garlic skins into small pieces to shorten their fiber length.

(b) CQD precursor preparation step, carbonizing the chopped garlic skin at high temperature to form a CQD precursor;

(c) CQD precursor solution preparation step, immersing the CQD precursor in the impurity dissolving solution, centrifuging it with a centrifuge to obtain the supernatant to form the CQD precursor solution.

(d) CQD precursor solution dialysis separation step, the CQD precursor solution is filtered with filter paper to obtain CQD precursor solution filter liquid, and then the CQD precursor solution filter liquid is treated with ultrasonic vibration and placed in a dark place, and then the CQD precursor solution filter liquid is poured into a dialysis bag for dialysis separation to obtain CQD precursor solution dialyzate.

(e) Freeze drying step, freeze drying the dialyzate of the CQD precursor solution to obtain carbon quantum dots.

Specifically, in the preparation step of the CQD precursor based on the fourth specific embodiment, chopped garlic skin is placed in a quartz boat and then moved into a tubular sintering furnace for high-temperature carbonization at about 240°C for about 2 hours to form a CQD precursor.

Specifically, in the preparation step of the CQD precursor solution based on the fourth specific embodiment, sodium hydrogen phosphate, sodium hydroxide, deionized water, etc. are stirred and mixed to form an impurity dissolving solution, and the CQD precursor is immersed in the impurity dissolving solution, and the centrifuge is performed at a speed of about 9000 rpm for about 30 minutes to obtain the supernatant to form the CQD precursor solution.

In the dialysis separation step of the CQD precursor solution based on the first embodiment, the CQD precursor solution filtrate is vibrated in an ultrasonic vibration tank with a vibration frequency of about 900 Hz for about 2 hours, and then placed in a dark place for about 24 hours. The liquid of the CQD precursor solution filtrate after the placement is poured into a dialysis bag for dialysis separation for about 12 hours to obtain the CQD precursor solution dialyzate.

In the freeze-drying step of the fourth specific embodiment, the CQD precursor solution dialyzate is freeze-dried at about -80°C to obtain carbon quantum dots.

A specific embodiment based on the second embodiment further includes a step of preparing a carbon fiber powder, wherein the long carbon fiber waste is chopped to shorten the carbon fiber fiber length, and the chopped carbon fiber is degreased with a neutral detergent and then ultrasonically vibrated for about 1 hour to remove excess impurities; then, the carbon fiber is placed in a mortar and about 20 ml of alcohol is added to wet-grind the carbon fiber. Grind for about 1 hour to turn the carbon fiber into small pieces of powder, and then prepare a wet grinding solution with carbon fiber and alcohol at a ratio of about 1:10; then grind for about 24 hours in a planetary ball mill, centrifuge the ball-milled carbon fiber powder at about 4000rpm in a high-speed centrifuge for about 20 minutes, retain the solid powder, and dry it in a vacuum oven to obtain carbon fiber powder.

In the carbon quantum dot analysis experiment of the present invention, the average particle size and surface potential of garlic peel carbon quantum dots were measured by particle size analysis and potential analyzer (Zetasizer; 3000HS, Malvern Instruments). The surface morphology was analyzed by SEM and transmission electron microscope (PHILIPS CM-200, TEM). Subsequently, UV-Vis spectrophotometer (Spectrophotometer, Jasco V-730) was used to analyze the carbon quantum dot conductive liquid diluted 1000 times. In addition, in order to determine the possible functional groups on the surface of the prepared carbon quantum dots, the CQDs were evenly distributed on the CaF2 wafer as a carrier, and the sample was scanned 128 times with a resolution of 4cm-1 and a scanning range of 4000-800cm-1 (ThermoFisher Scientific, Nicolet iS5) attenuated total reflection infrared vibration spectrum. Since the vibration modes corresponding to some functional groups will cause the overlap of infrared spectrum characteristic peaks, further analysis is required through X-ray photoelectron spectroscopy (XPS).

In the experimental example of the electrochemical characteristics analysis of the gel electrolyte of the present invention, a multifunctional cyclic voltammetry analyzer (CHI Instruments, CHI6273E) was used to perform cyclic voltammetry (CV) analysis and electrochemical impedance spectroscopy (EIS) analysis on gel electrolytes with different proportions (0.05, 0.10, 0.15, 0.20%) of CQD added to understand the specific capacitance and equivalent series resistance of the sample. The three-electrode system is composed of a working electrode, a reference electrode (RE) and an auxiliary electrode (CE), which correspond to a glassy carbon electrode, a silver-silver chloride electrode (Ag/AgCl electrolye) and a platinum electrode (Pt electrolyte). In addition, the electrochemical impedance spectroscopy (EIS) test can also be used to understand the electrochemical impedance characteristics of the sample. Under an AC voltage of 0.0 to 0.7V, the scanning frequency is set to 100KHz to 0.01Hz as the measurement range of EIS.

In the experimental example of the surface analysis results of garlic skin carbon quantum dots of the present invention, the garlic skin carbon quantum dots prepared by the present invention were respectively analyzed by particle size analysis and potential analysis to obtain an average particle size of about 7nm (see Figure 2(a)) and a surface potential of +0.00364mv (see Figure 2(b)). Since the smaller the Zeta potential value, the easier it is for the powder to agglomerate. Therefore, if a very small amount of carbon quantum dots is added to the electrolyte, it will help improve its electrical properties. However, if an excessive amount of carbon quantum dots is added, it is very likely that nanoparticles will agglomerate. Figure 2(c)-(i) and (ii) are SEM and TEM photos of garlic skin carbon quantum dots, respectively. The SEM photo shows that the carbon quantum dots are aggregated spheres of about 250nm. The aggregated spheres were dispersed in a 1000-fold dilution. Through TEM photos, it was found that the carbon quantum dots could be normally dispersed in water without agglomeration. Through TEM surface morphology analysis (see Figure 2 (c)-(ii)), it was confirmed that the size of the carbon quantum dots we prepared was about 6 to 10nm. The present invention also adds a very small amount of carbon quantum dots to the electrolyte to increase the specific capacitance of the electrolyte without agglomeration.

In the experimental example of the UV-VIS and infrared spectroscopy analysis results of garlic peel carbon quantum dots of the present invention, Figure 3(a) is the UV-visible spectrum of the carbon quantum dot solution diluted 1000 times. A typical n-π× transition peak is generated at the 345nm position. The n-π* transition is the result of the appearance of oxygen-containing functional groups (such as C-O, C=O) at the edge of the CQD [14]. The left inset of Figure 3(a) shows that the CQD aqueous solution appears yellow-brown under sunlight; the right side shows that the CQD aqueous solution will be excited to emit fluorescence under ultraviolet light. The infrared spectrum of carbon quantum dots (Figure 2(b)) has a peak at 1463cm-1, which is its C=O bending vibration mode. This shows that the surface of CQD prepared from garlic peel has oxygen-containing functional groups. In addition, two peaks appear at 2918cm-1 and 2848cm-1, corresponding to the asymmetric stretching mode of CH2 and the symmetric stretching mode of CH2, respectively, indicating that CQD may contain methyl functional groups.

In the experimental example of the X -ray photoelectron spectroscopy (XPS) analysis results of garlic skin carbon quantum dots of the present invention, the present invention analyzes CQD by X-ray photoelectron spectroscopy (XPS) and completes the peak separation process. Figure 4 (a) is the overall XPS spectrum of carbon quantum dots, in which the positions of 284.8, 400.8 and 532.8 eV correspond to the C1s, N1s and O1s peaks respectively, and the atomic proportions of C1s, O1s and N1s are 60.9, 28.4 and 10.7% respectively. Figure 4 (b) is the high-resolution C1s curve of CQD, which confirms the existence of oxygen-containing groups after further decomposition, including CH (285 eV), C=O (288.3 eV), COC (286.7 eV) and OC=O (289.3 eV). The high-resolution XPS spectrum of N1s is shown in Figure 4(c). This shows the positions corresponding to the main nitrogenated functional groups. N-(C=O) (399.7eV), N-(C=O)-O (400.3eV) and N-(C=O)-N (399.9eV). In addition, the peak measured at 399.7eV is likely related to the amide bond, which also corresponds to the NH bending vibration results shown in the infrared spectrum. The high-resolution XPS spectrum of O1s, as shown in Figure 4(d), has a single N1s peak at 531.0eV corresponding to PO ₄ ^3- , and there are no other impurity peaks. It is speculated that the oxygen-containing functional group may contain phosphate ions.

Although there are many carbon quantum dots prepared from agricultural waste, only a few studies so far have been conducted on garlic peels, and their main use is in the application of fluorescent sensors. The carbon quantum dots synthesized from garlic peels in the present invention are synthesized by high-temperature pyrolysis technology rather than by high-pressure hydrothermal reactor. In addition to using thermal field emission scanning electron microscope (SEM) and transmission electron microscope (TEM) to analyze the structure and surface morphology of carbon quantum dots, the present invention also uses a particle size analyzer and a surface potential analyzer to measure the particle size and surface potential of carbon quantum dots. An ultraviolet/visible light analyzer (UV-VIS) is also used to evaluate whether garlic peel carbon quantum dots have hydrophilic oxygen-containing functional groups. The surface functional groups of carbon quantum dots are analyzed by infrared spectrophotometer (FTIR). The types of functional groups on the surface of carbon quantum dots and their main constituent elements were analyzed by X-ray photoelectron spectroscopy (XPS). The present invention doped carbon quantum dots with PVA and H ₃ PO ₄ to form a gel electrolyte, and used a multifunctional cyclic voltammetry analyzer to perform cyclic voltammetry and AC impedance analysis. Carbon fiber waste was prepared into electrodes by grinding and deposition, and cyclic voltammetry and internal resistance analysis were performed. In addition, we used electrodes prepared from waste carbon fibers, gel electrolytes with garlic-peel carbon quantum dots added, and diaphragms to form a supercapacitor through a sandwich structure, and performed 2,000 cycles of charge and discharge tests using a multifunctional cyclic voltammetry analyzer. The present invention recycles two types of waste that would otherwise be directly discarded and applies them to the energy sector. While solving energy problems, we do not forget our environmental responsibilities. This will be a sustainable development trend and long-term goal in the future.

In the experimental example of analyzing the electrochemical characteristics of the electrode of the present invention, cyclic voltammetry (CV) analysis and chronopotentiometry (CP) analysis were performed on the carbon fiber electrode using a multifunctional cyclic voltammetry analyzer (CHI Instruments, CHI6273E). When the scanning voltage range is between 0.0V and 0.7V and the scanning rate (Scan Rate) is 0.2 to 0.02V/s, the working electrode, reference electrode (Reference Electrolyte, RE) and auxiliary electrode (Counter Electrolyte, CE) form a three-electrode system, corresponding to methylcellulose-based carbon fiber electrode, silver-silver chloride electrode (Ag/AgCl electrolye) and platinum gold electrode (Pt electrolyte), and 1M H3PO4 is used as the electrolyte. During the test, the scanning voltage range is set to 0.0V to 0.7V and the scanning rate (ScanRate) is 0.2 to 0.02V/s. Finally, the obtained cyclic voltammetry curve is used to calculate the specific capacitance (CS) of the methylcellulose-based electrode [12]:

In the formula, Cs is the specific capacitance value of the sample (F/g); i is the discharge current during the scanning period (mA); m is the weight of the active material (g); s is the scanning rate (V/s); △V is the difference in the voltage range of the scan (V); t is the discharge time during the scan period. In addition, the energy density (ED) and power density (PD) of the electrode can be calculated using Eq. (2) and Eq. (3) [13]:

In the above formula, ED is energy density and PD is power density, and their units correspond to (Wh/kg) and (W/kg) respectively.

In the experimental example of the electrochemical characteristics analysis of the supercapacitor of the present invention, the timing potential function of the multi-function cyclic electrical analyzer was used to perform 2000 cycles of charge and discharge tests on the supercapacitor assembled in a sandwich structure at a constant current of 0.20mA.

In the experimental example of the effect of adding garlic skin carbon quantum dots on the specific capacitance of gel electrolyte in the present invention, Figure 5 (a) (b) are the cyclic voltammogram and specific capacitance analysis diagrams of garlic skin carbon quantum dots added to 1M H3PO4/PVA gel electrolyte. When the addition amount of quantum dots is 0, 0.05, 0.10, 0.15 and 0.20%, the corresponding cyclic voltammogram areas are 4.84× ^10-6 , 1.15× ^10-5 , 1.24× ^10-5 , 1.10× ^10-5 , 1.08× ^10-5 cm2, as shown in Figure 5 (a). This shows that the CV area will first increase and then decrease with the addition of carbon quantum dots. In addition, it can be seen from Figure 5(b) that when carbon quantum dots are added to the electrolyte from 0, 0.05, 0.10, 0.15 to 0.20%, the specific capacitance will correspond to 0.002, 51.53, 110.57, 32.57, 23.98F/g. When 0.10% carbon quantum dots are added, the maximum specific capacitance value of 110.57F/g can be obtained. This may be because the doping of carbon quantum dots increases the charge conduction capacity of the electrolyte, reduces the internal resistance, and leads to an increase in specific capacitance. However, when carbon quantum dots are added to 0.20%, the specific capacitance is only 23.98F/g. This may be because the excessive carbon quantum dots in the gel electrolyte agglomerate, resulting in a decrease in the transfer efficiency between charge ions, which reduces its specific capacitance. Although adding excess carbon quantum dots will cause a decrease in specific capacitance, the overall cyclic voltammogram curve still shows a stable willow leaf-shaped pattern, which shows that the addition of trace amounts of carbon quantum dots in the gel electrolyte can still maintain the excellent reversibility and ideal capacitance characteristics of the electrolyte. In summary, the addition of trace amounts of carbon quantum dots can significantly improve the specific capacitance of the electrolyte, which will be of great help to the production of supercapacitors in the future.

In the experimental example of the effect of adding carbon quantum dots on the equivalent series resistance of the gel electrolyte of the present invention, the effect of adding carbon quantum dots on the electrolyte can be explored from a more microscopic perspective through EIS measurement. Figure 6 (a) (b) respectively shows the effect of adding different concentrations of carbon quantum dots in the gel electrolyte on the EIS AC impedance and equivalent series resistance (ESR), where (i) and (ii) represent the high-frequency region and low-frequency region in the AC impedance diagram. The Nyquist diagram consists of a semicircle in the high-frequency region and a straight line inclined at a 45-degree angle in the low-frequency region. The charge transfer process controls the size of the semicircle in the high-frequency region. The products or reactants of the electrode reaction control the diffusion impedance curve in the low-frequency region. As mentioned in reference [15], an ideal capacitor has a curve in the low-frequency region that is perpendicular to the Z’ axis and close to the left Z” axis, indicating that its diffusion impedance is reduced, allowing ions to easily migrate to the depths of the holes on the electrode surface, as shown in reference [16]. The intersection of the black line and the red dotted line in (i) of Figure 3(a) represents that the EIS curve of the electrolyte without carbon quantum dots in the low-frequency region presents an angle of 45° with the Z’ axis, indicating that the gel electrolyte has Warburg impedance increases the difficulty of ions moving to the deep holes of the electrode. Figure 3 (ii) shows the EIS high-frequency curves corresponding to the addition of different carbon quantum dots in the gel electrolyte. Among them, the ESR values of carbon quantum dots added at (a) 0%, (b) 0.05%, (c) 0.10%, (d) 0.15%, and (e) 0.20% are 30.18, 30.00, and 2 9.92, 29.98, 30.18Ω. When the concentration of carbon quantum dots is 0.10%, its ESR value is the lowest, indicating that its energy loss is the lowest. This also corresponds to the highest specific capacitance in the CV curve shown in Figure 5(b). When the amount of carbon quantum dots added exceeds 0.10%, its ESR value will increase again, resulting in a decrease in specific capacitance, as shown in Figure 6(b). The high-frequency curve of an ideal capacitor will show a semicircular characteristic with the Z’ axis, and The smaller the diameter of this semicircle, the better. The diameter of the semicircle will affect the transfer rate of ions between the electrolyte and the electrode surface. Figure 6 (ii) shows that there is almost no semicircle curve formed, indicating that the charge transfer impedance inside this electrolyte is extremely small, so that the ions have a good transfer rate between the electrolyte and the electrode surface. As shown in Figure 6 (b), adding 0.10% carbon quantum dots to the gel electrolyte can effectively reduce the equivalent series resistance in the electrolyte and increase its specific capacitance.

In the experimental example of cyclic voltammetry analysis of the methyl cellulose-based recycling carbon fiber electrode of the present invention, in the three-electrode system, a perfect cyclic voltammetry curve should appear close to a rectangular shape. However, due to the contact between the electrode and the electrolyte, contact resistance or internal resistance of the electrolyte will be generated, causing the ideal cyclic voltammetry curve and shape to deviate, as shown in reference [17]. Figure 7 (a) is a cyclic voltammetry curve of a methyl cellulose-based carbon fiber electrode placed in phosphoric acid at a scanning rate of 0.20V/s to 0.02V/s. The curves are symmetrical at different scanning rates, indicating that this electrode has excellent capacitance properties and ideal redox characteristics. The specific capacitance of the carbon fiber electrode at a scanning rate of 0.20 to 0.02 V/s can be calculated by Eq. (1). In addition, the specific capacitance, energy density (ED) and power density (PD) of the carbon fiber electrode at different scanning rates can be calculated by Eqs. (2, 3). The results are shown in Table 1. The recycled carbon fiber electrode at a scanning rate of 0.02 V/s has the highest specific capacitance, energy density and power density, which are 155.58 F/g, 10.59 Wh/kg and 4047 W/kg respectively. This shows that the specific capacitance of the electrode increases with the decrease of the scanning rate. This may be because reducing the scanning rate makes it easier for more electrolyte ions to contact the electrode surface, which increases the charge transfer rate and thus increases its specific capacitance. The present invention also compares the specific capacitance of other types of electrodes. The present invention still has room for improvement in the performance of specific capacitance, but it still increases the application field of waste. Under constant current testing, the change in specific capacitance at different scanning rates is called the capacitance attenuation rate (Attenuation rate, α ). The higher the capacitance attenuation rate, the lower the capacitance retention rate (Ratecapability, β ) will be [18]. The calculation method is as follows:

△Cs=| Cs _l - Cs _h | (4)

β=1-α (6)

In Eq. (4), Csh and Csl represent the specific capacitance values at the highest and lowest scanning rates in the measurement range, respectively; △Cs is the absolute value of the capacitance change. Through the calculation of Figure 7(b) and Eqs. (4)~(6), it can be seen that the capacitance retention rate of the carbon fiber electrode corresponding to different scanning rates is 29.68%. This means that there is still room for improvement in the capacitance retention characteristics of the carbon fiber electrode. Through analysis, we found that as the scanning rate decreases, the corresponding specific capacitance value will gradually increase. The main reason may be that at low scanning rates, the electrolyte ions can more easily penetrate into the interior of the electrode, allowing the electrode surface to adsorb more charges and achieve the purpose of storing more charges. From Eq. (6), we can see that when △Cs is higher, the corresponding capacitance decay rate α will also increase, while the capacitance maintenance rate β will decrease. The constant current charge and discharge analysis of the methyl cellulose-based carbon fiber electrode was tested at 0.20mA using a multi-energy cyclic conductivity analyzer, and the results are shown in Figure 7(c). The perfect constant current charge and discharge shape will appear as an isosceles triangle. However, due to the discharge process, the charged ions will generate impedance when moving back to the electrolyte at both ends of the electrode. This impedance is called internal resistance (iRdrop). By reducing the iRdrop value, the discharge process can avoid unnecessary energy consumption and waste heat generation, and indirectly improve the specific capacitance [19]. The iRdrop value of the carbon fiber electrode is 0.122V, which has a lot of room for improvement in the future. In addition, there is a decreasing peak in the discharge curve. It is speculated that the pseudocapacitor material may be degraded during the reduction process, resulting in a subsequent decrease in specific capacitance. In the future, it may be improved by adding other materials. In summary, the constant current charge and discharge results of the carbon fiber electrode show that its curve is close to an isosceles triangle, indicating that this electrode has excellent fast charge and discharge characteristics and good reversibility.

In Eq. (4), Csh and Csl represent the specific capacitance values at the highest and lowest scanning rates in the measurement range, respectively; △Cs is the absolute value of the capacitance change. Through the calculation of Figure 7(b) and Eqs. (4)~(6), it can be seen that the capacitance retention rate of the carbon fiber electrode corresponding to different scanning rates is 29.68%. This means that there is still room for improvement in the capacitance retention characteristics of the carbon fiber electrode. Through analysis, we found that as the scanning rate decreases, the corresponding specific capacitance value will gradually increase. The main reason may be that at low scanning rates, the electrolyte ions can more easily penetrate into the interior of the electrode, allowing the electrode surface to adsorb more charges and achieve the purpose of storing more charges. From Eq. (6), we can see that when △Cs is higher, the corresponding capacitance decay rate α will also increase, while the capacitance maintenance rate β will decrease. The constant current charge and discharge analysis of the methyl cellulose-based carbon fiber electrode was tested at 0.20mA using a multi-energy cyclic conductivity analyzer, and the results are shown in Figure 8(c). The perfect constant current charge and discharge shape will appear as an isosceles triangle. However, due to the discharge process, the charged ions will generate impedance when moving back to the electrolyte at both ends of the electrode. This impedance is called internal resistance (iRdrop). By reducing the iRdrop value, the discharge process can avoid unnecessary energy consumption and waste heat generation, and indirectly improve the specific capacitance [19]. The iRdrop value of the carbon fiber electrode is 0.122V, which has a lot of room for improvement in the future. In addition, there is a decreasing peak in the discharge curve. It is speculated that the pseudocapacitor material may be degraded during the reduction process, resulting in a subsequent decrease in specific capacitance. In the future, it may be improved by adding other materials. In summary, the constant current charge and discharge results of the carbon fiber electrode show that its curve is close to an isosceles triangle, indicating that this electrode has excellent fast charge and discharge characteristics and good reversibility.

In the experimental example of electrochemical analysis of the supercapacitor of the present invention, the present invention is a supercapacitor assembled by a sandwich structure, as shown in Figure 8, in which 0.1% CQD is added to the gel electrolyte and filter paper is used as a separator. 2000 cycles of charge and discharge test were carried out with a constant current of 0.20mA. Subsequently, 96% of the initial capacity was still retained. It was verified that the energy storage effect of this component is good and it has excellent capacitance stability and cycle life. The reason why the present invention can maintain a good specific capacitance value retention rate may be that the addition of an appropriate amount of carbon quantum dots in the electrolyte reduces the equivalent resistance and the carbon fiber electrode increases the charge transfer rate, which plays a multiplying role.

This invention prepares carbon fiber waste into electrodes by grinding and deposition, and conducts cyclic voltammetry and internal resistance analysis. In addition, we use electrodes prepared from waste carbon fibers, gel electrolytes with garlic-peel carbon quantum dots, and diaphragms to form supercapacitors through a sandwich structure, and conduct 2,000 cycles of charge and discharge tests using a multifunctional cyclic voltammetry analyzer. This invention recycles two types of waste that were originally to be directly discarded and applies them to the energy field. While solving energy problems, we do not forget our responsibility to the environment. This will be a sustainable development trend and long-term goal in the future.

The present invention successfully prepared carbon quantum dots from garlic peels, and used recycled carbon fibers to prepare conductive electrodes. When 0.10% of carbon quantum dots are added to the gel electrolyte, the specific capacitance can reach 110.57F/g, and the equivalent series resistance in the gel electrolyte can be effectively reduced to 29.98Ω. In addition, the specific capacitance, energy density and power density of the carbon fiber electrode at a scanning rate of 0.02V/s can reach 155.58F/g, 10.59Wh/kg and 4047W/kg respectively. In addition, the supercapacitor assembled with a sandwich structure was tested with a constant current at 0.20mA. After 2000 cycles of charge and discharge tests, its capacitance still retained 96% of the initial value. This invention uses the concept of environmental waste reduction to apply agricultural waste and industrial waste to the production of energy storage components. From the perspective of circular economy, it is completely consistent with the United Nations SDGs goal of environmental sustainable development.

The above is only a feasible implementation example of the present invention and is not intended to limit the patent scope of the present invention. Any equivalent implementation based on the content, features and spirit described in the following claims shall be included in the patent scope of the present invention. The structural features of the present invention specifically defined in the claims are not seen in similar articles and are practical and progressive. They have met the requirements for invention patents. Therefore, we hereby submit an application in accordance with the law. We sincerely request the Jun Bureau to grant the patent in accordance with the law to protect the legitimate rights and interests of the present applicant.

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10: Carbon fiber suspension

10a: Gum Arabic

10b,11b: Deionized water

10c: Gum Arabic aqueous solution

10d: Carbon fiber powder

10e: Carbon fiber suspension

11:PVA/H ₃ PO ₄ Gel Electrolyte

11a: Polyvinyl alcohol

11c:PVA aqueous solution

11d: Phosphoric acid

Claims (8)

A method for preparing a suspension of a supercapacitor carbon fiber electrode and a carbon quantum dot electrolyte comprises the following steps: a material preparation step, providing a predetermined amount of gum arabic, a predetermined amount of deionized water, a predetermined amount of carbon fiber powder, a predetermined amount of polyvinyl alcohol and a predetermined amount of phosphoric acid; a carbon fiber electrode suspension preparation step, using the predetermined amount of gum arabic and the predetermined amount of deionized water to prepare a gum arabic aqueous solution. A predetermined amount of gum arabic aqueous solution is added to a predetermined amount of carbon fiber powder, and then the carbon fiber suspension is prepared by magnet stirring and ultrasonic vibration. The carbon fiber suspension is centrifuged, and the process is repeated at least twice, and the supernatant is retained to obtain a uniformly dispersed carbon fiber suspension as a suspension of a carbon fiber electrode of a supercapacitor; wherein the gum arabic is used to prepare a carbon fiber suspension. After preparing an aqueous solution of about 0.1-50% gum arabic with the deionized water, about 0.1-30% of the carbon fiber powder is added, and the mixture is stirred by the magnet for about 55-65 minutes and ultrasonically vibrated for about 3-5 hours to form the carbon fiber suspension, and the carbon fiber suspension is centrifuged at a rotation speed of about 6000 rpm for about 20-30 minutes by a high-speed centrifuge. Repeat at least 2 times and retain the supernatant to obtain a uniformly dispersed carbon fiber suspension; and a carbon quantum dot electrolyte preparation step, heating the predetermined amount of polyvinyl alcohol and the predetermined amount of deionized water, and mixing and stirring until completely dissolved to form a PVA aqueous solution, then adding the predetermined amount of phosphoric acid and mixing evenly, placing in a vacuum oven for a period of time to heat and dehydrate to obtain a PVA/H ₃ PO ₄ gel electrolyte, which is used as the electrolyte of the carbon quantum dots of the supercapacitor. The method for preparing a suspension of a supercapacitor carbon fiber electrode and a carbon quantum dot electrolyte as described in claim 1 further includes a step of preparing a carbon fiber powder, wherein the chopped carbon fiber is degreased with a neutral detergent and then subjected to ultrasonic vibration treatment to remove excess impurities, and the carbon fiber is placed in a container and alcohol is added, and then the carbon fiber is ground by wet grinding to turn the carbon fiber into small pieces, and the carbon fiber and alcohol are mixed in a specific ratio to prepare a wet grinding liquid containing carbon fiber, and then ball milling is performed, and then the carbon fiber powder after ball milling is centrifuged to retain the solid powder, and then dried in a vacuum oven to obtain carbon fiber powder. The method for preparing a suspension of a supercapacitor carbon fiber electrode and a carbon quantum dot electrolyte as described in claim 1, wherein, in the step of preparing the carbon quantum dot electrolyte, about 10 g of polyvinyl alcohol and about 100 ml of deionized water are heated at about 85°C, mixed and stirred until completely dissolved to form the PVA aqueous solution, and about 20 g of phosphoric acid is added and evenly mixed, and then placed in a vacuum oven at about 60°C for heating and dehydration for about 6 hours to obtain the PVA/H ₃ PO ₄ gel electrolyte. The method for preparing a suspension of a supercapacitor carbon fiber electrode and a carbon quantum dot electrolyte as described in claim 3, wherein the concentration of the PVA/H ₃ PO ₄ gel electrolyte is between 0.1M and 10M. A method for preparing a supercapacitor using the method described in claim 1, comprising: providing a predetermined amount of carbon quantum dots, a predetermined amount of mixed deionized water, a predetermined amount of PVA/H ₃ PO ₄ gel electrolyte, a dimethyl cellulose-based electrode, a separator and a polyethylene terephthalate film; mixing the carbon quantum dots with the deionized water to form a uniformly dispersed CQD conductive liquid, and doping the CQD conductive liquid into the PVA/H ₃ PO ₄ gel electrolyte; and uniformly covering the surface of the dimethyl cellulose-based electrode with the PVA/H ₃ PO 4 and the separator. ₄ gel electrolyte, and the separator is interposed between the dimethyl cellulose-based electrodes, and then one of the methyl cellulose-based electrodes, a portion of the PVA/H ₃ PO ₄ gel electrolyte, the separator, another portion of the PVA/H ₃ PO ₄ gel electrolyte and the second methyl cellulose-based electrode are sequentially placed in the polyethylene terephthalate (PET) film in a sandwich structure assembly manner for packaging to form a flexible supercapacitor. The method for preparing a supercapacitor as described in claim 5, wherein the carbon quantum dots are mixed with the deionized water at a ratio of 1 mg/ml to form a uniformly dispersed CQD conductive liquid, and then about 0.01-50% of the CQD conductive liquid is added to the PVA/H ₃ PO ₄ gel electrolyte. The method for preparing a supercapacitor as described in claim 5 further includes a step of preparing a methyl cellulose-based electrode, which is to drop the carbon fiber suspension onto the surface of a dimethyl cellulose substrate and then dry it to obtain the dimethyl cellulose-based electrode. The method for preparing a supercapacitor as described in claim 5 further includes a step for preparing carbon quantum dots, which includes the following steps: a step of crushing discarded garlic skins, chopping the discarded garlic skins to shorten the fiber length; a step of preparing a CQD precursor, carbonizing the chopped garlic skins at a high temperature to form a CQD precursor; a step of preparing a CQD precursor solution, immersing the CQD precursor in an impurity dissolving solution, and centrifuging the solution with a centrifuge to obtain a supernatant to form a CQD precursor solution; The CQD precursor solution dialysis separation step is to filter the CQD precursor solution with filter paper to obtain a CQD precursor solution filter solution, and then the CQD precursor solution filter solution is treated with ultrasonic vibration, and after being kept still in the dark, the CQD precursor solution filter solution is poured into a dialysis bag for dialysis separation to obtain a CQD precursor solution dialysate; and the freeze drying step is to freeze dry the CQD precursor solution dialysate to obtain carbon quantum dots.

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