A METHOD FOR PREPARATION OF CARBON DOT-INFUSED GEL POLYMER ELECTROLYTE FOR SUPERCAPACITOR

Abstract

The present invention is related to a method for preparation of carbon dot-infused gel polymer electrolyte for supercapacitor. This supercapacitor utilizing carbon dots (CDs)-infused gel polymer electrolyte (GPE) for enhanced energy storage performance. The fabrication begins with the synthesis of graphitic carbon nitride (g-C₃N₄) (101), followed by its sonication in H₂SO₄ and HNO₃ for 16 hours to produce g-C₃N₄-derived carbon dots (gCN-CDs) (102, 103). These CDs are then incorporated into a gel polymer matrix composed of hydroxyethyl cellulose (HEC), glycerol, and sodium perchlorate (NaClO₄), which undergoes a gelation process (104, 105) to form the GPE (106). This electrolyte is then integrated between activated carbon and graphene electrodes to fabricate the supercapacitor (107). The fabricated supercapacitor exhibited a maximum specific capacitance of 52 F/g for the activated carbon electrode and 15 F/g for the graphene electrode, using cyclic voltammetry, with excellent energy and power performance, stability, and cycling efficiency.

Specification

Description:TECHNICAL FIELD OF INVENTION

The present invention is related to the field of electrical engineering. More specifically, it relates to a method for preparation of carbon dot-infused gel polymer electrolyte for supercapacitor.

BACKGROUND OF THE INVENTION

The background information herein below relates to the present disclosure but is not necessarily prior art.

The current development of energy storage devices seeks to simultaneously enhance energy and power densities while also prioritizing high safety standards and low costs. Supercapacitors (SCs), called electrical double-layer capacitors, are crucial electrochemical energy storage devices due to their fast charge/discharge capability and ultra-long life. Supercapacitors traditionally employ organic liquid electrolytes, which pose risks like flammability, corrosion, leakage, and instability. Therefore, there is a growing need for sustainable and safer electrolyte technologies that can provide high levels of safety.

Polymer electrolytes have been investigated as promising alternatives to liquid electrolytes. Polymer electrolytes offer several advantages, including noncombustible properties, no risk of internal short circuits, and no leakage issues. However, solid electrolytes devoid of solvents or plasticizers face challenges such as low ionic conductivity and weak electrolyte-electrode interfaces. Gel polymer electrolytes (GPEs) offer distinct advantages over solid electrolytes, particularly in establishing a more robust electrode interface. GPEs possess a unique blend of characteristics, combining the diffusive nature of liquids with the cohesive properties of solids. They comprise a solid polymer matrix that contains or retains liquid electrolytes. This design allows GPEs to function as electrolytes and separators, mitigating the risk of electrolyte leakage. By incorporating ionic electrolytes within the gel matrix, GPEs significantly enhance ionic conductivity at room temperature and strengthen the stability of the electrolyte-electrode interface. The estimated ionic conductivity of GPEs at ambient temperature typically falls within the range of 10-4 to 10-3 S/cm, comparable to commercial liquid electrolytes. Commonly used polymer matrixes for gel polymer electrolytes (GPEs) include Poly (methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP)polyacrylonitrile (PAN), and polyethylene oxide (PEO). The increasing emphasis on eco-friendly materials, often called green materials, is pervasive. Green chemistry focuses on designing chemical products and processes to minimize or eliminate the utilization and creation of harmful substances. Critical concerns regarding green chemistry methods in materials science include using non-toxic chemicals, environmentally friendly solvents, and renewable materials. Therefore, biodegradable polymers sourced from renewable materials significantly contribute to green chemistry efforts.

Cellulose, the most abundant natural polymer, is derived from organic sources such as wood and cotton. It is renewable and manufactured on a large scale, totaling multi-ton production annually for products like paper, tissue paper, and cellophane. Cellulose is economically advantageous, with production costs averaging between 0.5 and 1.5 € per kilogram, depending on yearly output. While natural cellulose is insoluble in water and most organic solvents, it can be dissolved in specific ionic liquids like 1-ethyl-3-methylimidazolium acetate (EMIM Ac). Cellulose membranes offer excellent electrolyte and electrolyte absorption capacities. Their high initial decomposition temperature (over 270 °C) contributes to outstanding thermal stability, renewability, and thermal resistance. Hydroxyethyl cellulose (HEC) consists of β-1,4 glycosidic linkages that connect the glucose rings. (HEC) is utilized as a thickening and gelling agent in industries like cosmetics, paints, and pharmaceuticals. It exhibits excellent thermal stability and electrochemical efficiency, making it suitable for applications as a polymer electrolyte material. Zhang et al. have shown that incorporating HEC in electrochemical devices prevents micro short circuits due to the hydroxyethyl group attached to HEC's cellulose backbone. Given these characteristics, incorporating 2,3-hydroxyethyl cellulose (HEC) as a gel polymer electrolyte (GPE) in supercapacitors would significantly advance towards creating cost-effective and environmentally friendly devices.

Several strategies have been investigated to improve the ionic conductivity of biodegradable polymer electrolytes, such as polymer blending, plasticizer incorporation, dopant salt addition, ionic liquid integration, and filler inclusion. Glycerol was selected as a plasticizer in this study due to its advantageous properties, including its non-volatility at typical operating temperatures and minimal alteration of vapor pressure in glycerol solutions up to 70 °C. Additionally, the hydroxyl groups present in glycerol can help dissociate doped salts and maintain an ion-conducting viscous pathway within the polymer matrices. Sodium perchlorate (NaClO4) is utilized as a dopant to enhance the conductivity and electrochemical performance of the supercapacitor. It helps improve charge transfer kinetics and increase the specific capacitance, thereby strengthening the supercapacitor device's overall energy storage capacity and efficiency.

Carbon dots (CDs) are emerging members of carbon-based nanomaterials that were first reported in 2004. These zero-dimensional nanomaterials are characterized by their exceptional fluorescence capabilities and unique structural properties. CDs consist of ultra-fine, quasi-spherical carbon nanoparticles, typically smaller than 10 nanometers in size.CDs consist of amorphous or nano-crystalline cores with sp2 clusters and the attachment of oxygen functional groups such as carboxyl, hydroxyl, and aldehyde groups on the surface of edge planes. Their small size and amorphous-crystalline structure allow for tunable chemical, physical, optical, and electronic properties. CDs, because of their low toxicity, high water solubility, thermal stability, and chemical inertness, make them ideal for applications such as biolabeling, optical sensing, drug delivery, biosensing, energy storage, and catalysis. Moreover, CDs can be tailored to exhibit specific properties by introducing doping elements like nitrogen, sulfur, and others during their synthesis, expanding their utility and enhancing performance. Graphitic carbon nitride (g-C3N4) is an excellent active supercapacitor material due to its chemical stability, nitrogen-rich structure, abundance, low-cost, eco-friendly nature, and mild synthesis conditions. Its high nitrogen content provides active sites for faradaic reactions, enhances surface polarity for better electrolyte wettability, and improves mass transfer efficiency, leading to superior electrochemical performance. Specifically, incorporating pyrrolic and pyridinic nitrogen groups in g-C3N4 can significantly enhance pseudo-capacitance.

In this invention, synthesized eco-friendly carbon dots (CDs) from graphitic carbon nitride and incorporated them into a bio-based gel polymer electrolyte to develop a sodium-based supercapacitor. The CDs were embedded in a gel polymer matrix made of 2,3-hydroxy ethyl cellulose (HEC) and glycerol (Gly) as a plasticizer, with sodium perchlorate (NaClO4) as the doping salt. This incorporation significantly enhanced the interactions between the electrolyte and electrodes, improving specific capacitance and overall device efficiency. This approach highlights the potential of integrating green chemistry principles into energy storage technologies and paves the way for developing high-performance, eco-friendly supercapacitors.

OBJECTIVE OF THE INVENTION

The primary objective of the present invention is to provide a method for preparation of carbon dot-infused gel polymer electrolyte for supercapacitor.

Yet another objective of the invention is to develop a supercapacitor with improved specific capacitance, energy, and power density by integrating carbon dots (gCN-CDs) into a gel polymer electrolyte (GPE), optimizing ion transport and charge storage efficiency.

Yet another objective of the invention is to create a gel polymer electrolyte using hydroxyethyl cellulose (HEC), glycerol, and sodium perchlorate (NaClO₄), infused with carbon dots, aimed at improving electrolyte conductivity.

Yet another objective of the invention is to explore sustainable, low-cost materials like graphitic carbon nitride-derived carbon dots and gel polymers, providing a scalable and efficient solution for next-generation energy storage devices with long cycle life and durability.

SUMMARY OF THE INVENTION

Accordingly, the following invention provides a method for preparation of gel polymer electrolyte for supercapacitor. This supercapacitor using activated carbon and graphene electrodes combined with a gel polymer electrolyte (GPE) infused with graphitic carbon nitride-derived carbon dots (gCN-CDs). The fabrication process begins with bulk graphitic carbon nitride (101) undergoing acid treatment with H₂SO₄ and HNO₃ followed by 16 hours of sonication (102) to obtain gCN-CDs (103). These carbon dots are then integrated into a hydroxyethyl cellulose (HEC) solution (104), which undergoes gelation (105) to form the gel polymer electrolyte (106). The supercapacitor (107) is assembled using this GPE, aiming to improve energy storage performance.

The gel polymer electrolyte, with its globular cluster-like surface, enhances ion diffusion and charge transfer mechanisms, resulting in higher specific capacitance and improved charge-discharge stability. Cyclic voltammetry tests showed specific capacitance values of 15 F/g for graphene electrodes and 52 F/g for activated carbon electrodes. This approach not only offers improved energy and power densities but also contributes to more sustainable and efficient energy storage solutions, paving the way for further research into advanced supercapacitor technologies.

BRIEF DESCRIPTION OF DRAWING

This invention is described by way of example with reference to the following drawings where,

Figure 1 of sheet 1 illustrated the flow diagram of a method for preparation of carbon dot-infused gel polymer electrolyte for supercapacitor.
Where,
101 denotes bulk g-C₃N₄ (graphitic carbon nitride),
102 denotes H₂SO₄ + HNO₃ sonication for 16 hours.
103 denotes g-C₃N₄ carbon dots (gCN-CDs),
104 denotes HEC (Hydroxyethyl Cellulose) solution,
105 denotes gelation process,
106 denotes gel polymer electrolyte (GPE),
107 denotes supercapacitor.

Figure 2 of sheet 2 illustrated the (A) schematic illustration for synthesis of gCN-CDs from bulk g-CN and (B) schematic representation of the formation of GPE.

Figure 3 of sheet 3 illustrated the FTIR spectrum of gCN and gCN-CD

Figure 4 of sheet 3 illustrated the FTIR spectrum of HEC, HEC-CDs, Na doped HEC-CDs

Figure 5 of sheet 4 illustrated the XRD results of S1, S2, S3, S4 and S5

Figure 6 of sheet 4 illustrated the thermo-gravimetric analysis of S1, S2 and S5.

Figure 7 of sheet 5 illustrated the SEM images of S1, S2 and S5

Figure 8 of sheet 5 illustrated the HRTEM and SAED images of gCN-CDs

Figure 9 of sheet 6 illustrated the CV curves of S2 and S5 for graphene-coated electrode

Figure 10 of sheet 6 illustrated the CV curves of S2 and S5 for AC electrode


Figure 11 of sheet 7 illustrated the EIS plots of S2-S5 for graphene and AC electrode

Figure 12 of sheet 7 illustrated the GCD of S5 for graphene and AC electrode

Figure 13 of sheet 8 illustrated the possible ion transport mechanism inside the supercapacitor during charging condition (SS: stainless steel, AC: activated carbon electrode/graphene electrode).

DETAILED DESCRIPTION OF THE INVENTION

As used in the description herein and throughout the claims that follow, the meaning of "a," "an," and "the" includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of "in" includes "in" and "on" unless the context clearly dictates otherwise.

The present invention is related to a method for preparation of carbon dot-infused gel polymer electrolyte for supercapacitor. a redox-mediated supercapacitor based on a carbon dots (CD) and biodegradable gel polymer electrolyte (GPE) has been fabricated using g-C3N4 as the carbon dot source, 2,3-hydroxy ethyl cellulose as the polymer matrix, glycerol as a plasticizer, sodium perchlorate (NaClO4) as the doping salt, and activated carbon/graphene as electrodes. FTIR, XRD, and TGA characterizations indicated favorable interactions among the components. Scanning electron microscopy (SEM) images of the CD-GPE revealed a small globular cluster-like surface morphology. HRTEM images of synthesized carbon dots exhibited a size of less than 10 nm. The fabricated supercapacitor exhibited a maximum specific capacitance of 52 F/g for the activated carbon electrode and 15 F/g for the graphene electrode, using cyclic voltammetry. AC impedance studies confirmed the electrochemical capacitive behavior of the fabricated supercapacitor. Galvanostatic charge-discharge curves displayed nonlinear behavior characteristics of pseudocapacitors, indicating redox behavior at the electrode/electrolyte interface.

Preparation of carbon dots:
50g of urea from Loba Chemie were placed in a semi-closed ceramic crucible and heated in a muffle furnace at 500°C for two hours, with a slow heating rate of 3-4°C per minute. This process yielded approximately 3 grams of off-whitish graphitic carbon nitride powder (g-C₃N₄). To obtain carbon dots (CDs), 0.1 grams of g-C₃N₄ were sonicated in a mixed solution of concentrated sulfuric acid (H₂SO₄, 20 ml) and nitric acid (HNO₃, 60 ml) for 16 hours. The resulting mixture was washed with 400 ml of deionized water to remove impurities and then centrifuged.

Preparation of polymer solution:
The stock solution preparation involved dissolving 2 g of HEC (Aldrich, average Mw Ꝃ≈250,000) in 100 milliliters of distilled water. It was placed on a magnetic stirrer for half an hour. As a result, a precise and uniform stock solution of 2,3-hydroxy ethyl cellulose was obtained.

Preparation of GPE:
GPE was prepared with HEC and polyvinyl alcohol (PVA) in a 9:1 ratio, 1 ml CD solution, 3-4 drops of Gly(Merck), and 0.01-0.03 wt% NaClO4 (Loba Chemie), with various sample combinations as shown in the Table 1.

Table 1: Various sample combinations of GPE
Polymer + glycerol S1
Polymer + glycerol + CD S2
Polymer + glycerol + CD + Salt (0.01 wt%) S3
Polymer + glycerol + CD + Salt (0.02 wt%) S4
Polymer + glycerol + CD + Salt (0.03 wt%) S5

The prepared combinations were placed separately in clean 10 ml beakers and allowed to undergo initial gelation at room temperature. Subsequently, they were placed in an oven at 40⁰C for 48 hours to facilitate the formation of gel polymer electrolytes (GPEs) before proceeding with further studies.

Characterization techniques:
Fourier transform infrared spectroscopy (FTIR) measurements of HEC, HEC-CDs, and Na-doped HEC-CDs were conducted at room temperature using a PerkinElmer Spectrum Two FTIR spectrometer. The spectra were recorded between 4000 and 400 cm⁻¹ with a resolution of 4 cm⁻¹. Thermal gravimetric analysis (TGA) measurements of the gel polymer electrolytes (GPEs) were conducted using a PerkinElmer TGA 4000 instrument. The measurements were performed under a nitrogen atmosphere with a flow rate of 50 ml min⁻¹, over a temperature range from 30⁰C to 700⁰C, at a heating rate of 10⁰C min⁻¹. Data were recorded from the initial heating cycle. XRD studies were done using the Rigaku MiniFlex XRD instrument, and the diffraction patterns were recorded for 2θ values between 0⁰ and 90⁰. The GPE sample underwent cutting into small cubes followed by freeze-drying in a deep freezer, subsequent storage in a nitrogen atmosphere, and exposure to high vacuum conditions. Microscopic images were captured using a scanning electron microscope (SEM), specifically the ZEISS EVO 18 special edition. High-resolution transmission electron microscopy (HRTEM) images were acquired using a FEI Tecnai G2-30 TEM instrument. Electrochemical characterization, including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge-discharge (GCD) studies, was conducted using a BioLogic SP-150 instrument. The supercapacitor's specific capacitance (C, F g−1) was calculated from the CV curve using the following equation.
(1)

The specific capacitance (C, F g⁻¹) of the supercapacitor was determined from charge-discharge curves using the following equation.

(2)

The energy density (E, Wh kg−1) and power density (P, kWkg−1) of the supercapacitor were obtained from the following equations.

(3)

(4)

where A is the area under the CV curve, k is the scan rate (mV/s), m is the mass of the active material (g), ΔV represents the potential window (V), I is the discharge current (A), and t is the discharge time (s).

Fabrication of supercapacitor cell:
The electrode material for supercapacitor fabrication was prepared using activated carbon (AC) derived from areca fibers and graphene (Merck), with polyvinylidene fluoride (PVDF) binder dissolved in N-methylpyrrolidone at a 3:1 ratio. The PVDF binder solution was mixed with AC at a weight ratio of 0.3:1 and ground using a pestle and mortar to form a slurry. This slurry was coated onto two stainless steel electrodes. The supercapacitor cell was assembled by sandwiching a gel polymer electrolyte (GPE) between two prepared AC and graphene electrodes. The unit cells were then sealed in plastic with two wires extending outside. Electrochemical characterization was performed using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge-discharge (GCD) studies, all conducted using a BioLogic SP-150 instrument.

BEST METHOD FOR PERFORMANCE OF THE INVENTION

A method for the preparation of a carbon dot-infused gel polymer electrolyte supercapacitor involves several key steps. First, carbon dots (CDs) are prepared by placing 50g of urea in a semi-closed ceramic crucible and heating it in a muffle furnace at 500°C for two hours, with a slow heating rate of 3-4°C per minute, yielding approximately 3g of off-whitish graphitic carbon nitride (g-C₃N₄) powder (101). This powder is then sonicated for 16 hours with 20ml of concentrated sulfuric acid (H₂SO₄) and 60ml of nitric acid (HNO₃) (102). The resulting mixture is washed with 400ml of deionized water to remove impurities, followed by centrifugation to isolate the carbon dots (gCN-CDs) (103).

To prepare the hydroxyethyl cellulose (HEC) solution, 2g of HEC (104) is dissolved in 100ml of distilled water and stirred for 30 minutes. The gel polymer electrolyte (GPE) is then prepared by combining the HEC solution with polyvinyl alcohol (PVA) in a 9:1 ratio, along with 1ml of CD solution, 3-4 drops of glycerol (Gly), and NaClO₄ at 0.02 wt% (S4 formulation). This mixture undergoes gelation (105) by being poured into 10ml beakers and allowed to gel at room temperature, followed by placement in an oven at 40°C for 48 hours to form the gel polymer electrolyte (GPE) (106).

For supercapacitor fabrication, the electrode material is prepared by mixing activated carbon (AC) from areca fibers with graphene and polyvinylidene fluoride (PVDF) binder dissolved in N-methylpyrrolidone (NMP) at a 0.3:1 ratio. This mixture is ground into a slurry and coated onto two stainless steel electrodes. The GPE is sandwiched between the two electrodes, and the unit cells are sealed with wires extending outside for electrical connectivity. The fabricated supercapacitor (107), using the activated carbon/graphene electrodes and carbon dot-infused GPE, exhibited a specific capacitance of 15 F/g for the graphene electrode and 52 F/g for the activated carbon electrode, as determined by cyclic voltammetry.

FTIR studies:
Figure 3 shows the FTIR spectra of pure graphitic nitride and graphitic nitride carbon dots. The peak at 807 cm−1 is due to the tri-s-heptazine or tri-s-triazine unit of gCN, whose intensity disappears with increasing sonication time, indicating damage to the basic unit. The stretching vibration of the repeating unit of heptazine causes the peaks at 1641, 1570, 1462, and 1412 cm−1, according to the FTIR study of g-C3N4. The C-N/C-N stretching vibration mode corresponds to the strong band between 1200 and 1650 cm−1. As the sonication period is extended, the peak intensity in this area similarly decreases, suggesting that the sonication damages the C-N bond. With more ultrasound time, the broad peaks observed between 3000 and 3500 cm⁻¹ correspond to stretching vibrations of N-H bonds, gradually lose intensity, and even shift to higher energy levels. The formation of hydrogen bonds between water and carbon dots in the region of 3300cm-1 (-OH stretching) indicates the solvation of carbon dots. The FT-IR research demonstrates that sonication had some impact on the gCN structure.

In Figure 4, S1 shows strong and broad bands at 3334 cm⁻¹ and 1042 cm⁻¹, corresponding to the hydroxyl group in the pyranose unit and the C-O-C skeletal vibration.

Respectively. The band at 2886 cm⁻¹ is associated with the C-H stretching due to the presence of CH and CH₂ groups. The absorption peak at 1643 cm⁻¹ is attributed to the bending mode of naturally absorbed water, while the peak at 1357 cm⁻¹ is linked to O-H bending. The area corresponding to the O-H stretching vibration region at 3294 cm⁻¹ is greater for glycerol. This indicates glycerol's hydrophilic behavior, with the increased area attributed to hydrogen bonds formed by the hydroxyl groups in both the HEC and glycerol structures. For S2 spectra, a new peak emerges at 1200 cm⁻¹ due to C-N stretching in CDs, while the prominent peaks shift due to the strong interactions between HEC and CDs. The Na salt-doped GPE shows peaks at 3400 cm⁻¹, 1650 cm⁻¹, and 1100-1000 cm⁻¹ in S3, S4, and S5, enhancing the interaction between the polymer chain, CD and salt, leading to more pronounced absorption features.

XRD studies:
XRD measurements were performed on the gel polymer electrolyte to study their crystalline/amorphous nature. Figure 5. shows the XRD pattern of S1, S2, S3, S4, and S5. The diffraction pattern of S1 exhibits diffraction peaks at 2θ= 23.2⁰, 31⁰ and 48.5⁰. The strong and sharp peak at 2θ= 23.2⁰ depicts the semi-crystalline nature of HEC, as reported by Gupta and Varshney. In S2, the peak located at 2θ~27.6⁰ corresponds to the (002) plane arising from the stacked graphite-like layers, which are held together by weak Van der Waals forces between porous gCN layers. The intensity of the peaks decreased at 27.6⁰ for S3, S4, and S5 with the addition of salt to the polymer, which reveals that the carbon dot and doping salt enhance the amorphous nature of the electrolytes. XRD proves a correlation between the intensity of the peak and the degree of the crystallinity.

TGA studies:
Figure 6 shows that degradation occurs in two distinct steps. Initially, all samples exhibit minor mass loss (5 to 25%) between 30⁰C and 200⁰C, attributed to moisture loss and glycerol decomposition. The second significant decomposition step occurs between approximately 220⁰C and 340⁰C, attributed to the decomposition of the COO− group within the polymer electrolyte system. Adding carbon dots (CDs) and salt slightly lowers the decomposition temperature, indicating a minor decrease in thermal stability. However, the weight loss over an extended period suggests an interaction between CDs and the gel polymer electrolyte (GPE). who noted decreased thermal stability with increased salt concentration, likely due to salt-polymer complexation. Most energy storage devices operate below 100⁰C, so our CD-GPE meets these thermal stability requirements.

SEM studies:
Figure 7. shows SEM images of GPEs. As seen in the figures, S1 shows a wrinkled surface, indicating the solvent of gel was able to form pools between the polymer matrix during the SEM analysis at higher vacuum conditions. These pools were quickly dried, leaving behind the wrinkles. In the figure of S2, after the addition of CDs, the surface is observed to be smoothened and more uniform; this indicates some interaction between CDs and the gel polymer system. Hence avoiding the wrinkled surface. In Figure S5, adding NaClO4 salt results in a rough surface with small globular clusters, indicating that the salt disrupts intermolecular bonds in the gel matrix. This breakdown enhances the surface area and introduces more active sites on the gel's surface. The rough, globular texture facilitates better ion diffusion and charge transfer, leading to higher energy density, improved capacitance, and faster charge-discharge cycles.

Figure 8 (a) shows the high-resolution transmission electron microscopy (HRTEM) image of gCN-CDs with distinct lattice fringes, indicating a crystalline structure. The scale bar of 5 nm suggests the observed structures are on the nanometer scale, emphasizing the fine structural details of the g-CN dots. In Figure 8 (b). The selected area electron diffraction (SAED) pattern of gCN-CDs exhibits a concentric ring structure, indicating the polycrystalline nature of the composite material. Calculations for gCN-CDs revealed an interplanar d-spacing of 0.32 nm, corresponding to the (002) planes, which signifies the interlayer distance between the graphitic planes within the structure.

CV studies
Figure 9 show the CV curves of S2 and S5 for graphene-coated electrodes at scan rates from 5 mV/s to 100 mV/s. Maximum specific capacitances of 15 F/g for S5 at 5 mV/s were obtained. The observed improvement in the current response, characterized by the shifting of peaks, is attributed to the pseudocapacitance resulting from redox processes occurring at the electrolyte/electrode interfaces.

Figure 10 show the CV curves of S2 and S5 for AC electrodes at scan rates from 5 mV/s to 100 mV/s. Maximum specific capacitances of 52 F/g for S5 at 5 mV/s were obtained. The redox process at the electrode/electrolyte interface generates pseudocapacitance in the electrode, causing bumps in the CV curves of the supercapacitor with HEC-CD-Gly-NaClO4 electrolyte due to Faradaic processes. The total capacitance for the supercapacitor using this gel polymer electrolyte is the sum of double-layer capacitance and Faradaic pseudocapacitance.

EIS studies:
Figure 11 (a) and 11 (b) show the EIS plot of S3 of graphene-coated electrode and carbon-coated electrode in a frequency range from 1MHz to 100 MHz. Nyquist plots obtained for both graphene and AC The electrodes exhibited ideal electrochemical capacitance behavior, characterized by nearly linear imaginary parts of impedance at low frequencies, indicative of Warburg impedance, denoted as "W"; this may be attributed to the low faradaic resistance within the electrolyte, indicating a capacitive behavior. As the salt concentration increases, the Nyquist plots become more linear towards the imaginary parts of impedance, resulting in lower resistance.

GCD studies:
Figures 12a and 12b display the GCD curves of supercapacitors for S5 at varying current densities (0.1 to 1 mA/cm²) within a 1V potential window. The non-ideal shapes of these curves indicate Faradaic contributions to the charge accumulation process, characteristic of a pseudocapacitor. These results align with the CV test data. For the graphene electrode at 1 mA, the specific capacitance (Cs) is 11 F/g, energy density is 3.8 Wh/kg, and power density is 0.318 kW/kg. For the AC electrode, the Cs is 8 F/g, energy density is 2.8 Wh/kg, and power density is 0.315 kW/kg, indicating redox reactions at the electrode/electrolyte interface.

Table 2: summarizes similar gel-based polymers' Cs, Energy density, and Power density values.
Gel electrolyte Specific capacitance (Cs) Energy density Power density
PVA/KOH/K₃[Fe(CN)₆ 430.95 F. g−1 at 500 mA.g−1 57.84 Wh. kg−1 9.84 kW. kg−1
PVA/H2SO₄/H2O 0.125 F. cm−3 At 0.1 mA.cm−3 0.011mWh.cm−3 0.032 W.cm−3
CMC-Na2SO4 145 F.g−1 at 5 mV.s−1 6 Wh. kg−1 2.6 kW. kg−1
(K3[Fe(CN)]6 597 F.g−1 (0-0.1 V) 28.3 Wh. kg−1 7.1 kW.kg−1
PVA- LiClO4 145 F.g−1 at 2 mA.cm−1 41 Wh.kg−1 2.1 kW. kg−1
PVA/H2SO₄/P-benzenediol 474.29 F. g−1 at 0.83 A. g−1 10 Wh. kg−1 1000 W. kg−1
PVA/H3PO₄/H2O 53 F.g−1 at 1 A.g−1 6 Wh. kg−1 19.2 kW. kg−1

Based on characterization studies, the most probable mechanism of charge transport in the fabricated supercapacitor through GPE and AC/graphene electrode is explained in Figure 13. FTIR, XRD, and thermal characterization studies revealed strong interactions between CDs, HEC, glycerol, and salt within the gel system. SEM studies indicated that Na-doped CD-based GPE exhibited a rounded texture, enhancing ion diffusion and charge transfer mechanism.The redox peaks observed in CV studies are attributed to introducing gCN-CDs into the HEC polymer matrix,

facilitating redox processes at the electrolyte|electrode interface and thus contributing to pseudocapacitance. Compared to graphene electrodes, AC electrodes demonstrated higher capacitance and superior electrochemical performance due to their high surface area and porosity. GCD results are also consistent with the data obtained from the CV tests. The EIS results indicated capacitance behavior with lower resistance, suggesting the fabricated supercapacitor exhibits efficient charge storage capabilities and enhanced conductivity.

The supercapacitor using activated carbon/graphene electrodes and a gel polymer (HEC-Gly-NaClO4) infused with carbon dots. With its globular cluster-like surface morphology, the gel polymer enhances ion diffusion and charge transfer mechanisms. The fabricated supercapacitor exhibited a specific capacitance of 15 F/g for the graphene electrode and 52 F/g for the activated carbon electrode, as determined by cyclic voltammetry. It demonstrated good specific energy and power, remaining stable and efficient over numerous charge-discharge cycles. This study also paves the way for further exploration in this field of research.

While various embodiments of the present disclosure have been illustrated and described herein, it will be clear that the disclosure is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions, and equivalents will be apparent to those skilled in the art, without departing from the spirit and scope of the disclosure, as described in the claims.
, Claims:1. A method for preparation of carbon dot-infused gel polymer electrolyte for supercapacitor, comprising the following steps;

preparation of carbon dots (CDs) (103);
place 50g of urea (from loba chemie) in a semi-closed ceramic crucible and heat it in a muffle furnace at a temperature of 500°C for a duration of two hours, with a slow heating rate of 3-4°C per minute, resulting in the production of approximately 3g of off-whitish (101) graphitic carbon nitride (g-C₃N₄) powder;

subsequently, sonicate 0.1g of g-C₃N₄ in a mixture of 20ml of concentrated sulfuric acid (H₂SO₄) and 60ml of nitric acid (HNO₃) for 16 hours;

wash the resulting mixture with 400ml of deionized water to remove impurities, followed by centrifugation to isolate the carbon dots (CDs);

preparation of hydroxyethyl cellulose (HEC) solution (104);
dissolve 2g of hydroxyethyl cellulose (HEC) (Aldrich, average Mw ≈ 250,000) in 100ml of distilled water, placing it on a magnetic stirrer for 30 minutes to obtain a precise and uniform stock solution;

preparation of gel polymer electrolyte (GPE) (106);
combine the HEC solution with polyvinyl alcohol (PVA) in a weight ratio of 9:1;

add 1ml of CD solution, 3-4 drops of glycerol (Gly) (Merck), and varying concentrations of NaClO₄ (Loba Chemie) at 0.02 wt% i.e S4 (Polymer + glycerol + CD + Salt 0.02 wt%);

pour the prepared combinations into clean 10ml beakers and allow them to undergo initial gelation at room temperature;

subsequently, place the beakers in an oven set at 40°C for 48 hours to facilitate the formation of gel polymer electrolytes (GPEs);

fabrication of supercapacitor Cell (107);
prepare the electrode materials for the supercapacitor by mixing activated carbon (AC) derived from areca fibers with graphene (Merck) and polyvinylidene fluoride (PVDF) binder dissolved in N-methylpyrrolidone (NMP) at a weight ratio of 0.3:1;

grind the resulting mixture using a pestle and mortar to form a slurry;

coat the slurry onto two stainless steel electrodes;

assemble the supercapacitor cell by sandwiching the GPE (from the previous step) between two prepared electrodes made from AC and graphene;

seal the unit cells in plastic, ensuring that two wires extend outside for electrical connectivity;

2. The method for preparation of carbon dot-infused gel polymer electrolyte for supercapacitor as claimed in claim 1 wherein the supercapacitor (107) using activated carbon/graphene electrodes and a gel polymer (HEC-Gly-NaClO4) infused with carbon dots.

3. The method for preparation of carbon dot-infused gel polymer electrolyte for supercapacitor as claimed in claim 1 wherein the fabricated supercapacitor (107) exhibited a specific capacitance of 15 F/g for the graphene electrode and 52 F/g for the activated carbon electrode, as determined by cyclic voltammetry.

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