Description:Title of the Invention:
A Crotalaria juncea derived biochar electrode (CJbiochar) for energy storage applications and a Solid-State Symmetric Device Thereof

Field of the Invention:
The present invention relates to the field of electrochemical energy storage devices, specifically to the preparation and application of bio-waste derived carbon electrodes for use in supercapacitors, and more particularly to a solid-state symmetric supercapacitor using Crotalaria Juncea biochar.

Background of Invention
There is an increasing demand for sustainable and cost-effective energy storage systems due to the growing emphasis on clean energy solutions. Supercapacitors have emerged as promising candidates due to their high-power density, long life cycles, and fast charging capabilities. Conventional carbon sources are expensive and environmentally taxing. The use of biomass-derived carbon materials has garnered attention, but most require activation agents, increasing cost and environmental burden. Therefore, a need exists for a low-cost, activation-free method for preparing carbon materials from bio-waste that exhibit favorable electrochemical performance.
US20190263083A1 – Biomass-Derived Carbon Electrodes for Energy Storage Devices discloses methods for preparing carbon electrodes from biomass precursors such as coconut shell, sawdust, and corncob. The carbonization process includes chemical activation using KOH or H₃PO₄, requires chemical activation, which increases cost and introduces environmental concerns.
CN105341257A – Preparation Method of Activated Biomass Carbon for Supercapacitors: Describes activated carbon production using agricultural waste, treated with ZnCl₂ as an activation agent followed by high-temperature pyrolysis involves chemical activation and corrosive chemicals.
WO2016072434A1 – Biochar-Based Electrodes for Supercapacitor Devices: Discloses general use of biochar from unspecified biomass for electrodes. Emphasizes the need for activation and surface modification and is Non-specific to biomass source and requires post-treatment.
IN202011035095 – A Method for Preparation of Carbon Electrodes from Banana Stem Waste: Focuses on supercapacitor electrodes made from banana stems, with acid treatment and microwave-assisted activation,requires multiple steps and chemical activation.
R. B. Rakhi et al., Biomass Derived Carbon Materials for Supercapacitor Applications, J. Mater. Chem. A, 2014: Reviews different biomass sources for carbon materials in energy storage, noting that chemical activation is commonly used for porosity enhancement.
Wang et al., Recent Advances in Biomass-Derived Carbon for Supercapacitors, Electrochim. Acta, 2017: Highlights the potential of lignocellulosic biomass but identifies poor electrochemical performance without activation as a challenge.
Patel et al., Waste-Derived Carbon Electrodes for Supercapacitors: A Review, Renewable & Sustainable Energy Reviews, 2021: Provides a survey of waste-derived carbon materials and notes the lack of consistency in electrode behavior without chemical modification.
Das et al., PVA–LiClO₄ Based Gel Electrolyte for Flexible Supercapacitors, J. Electroanalytical Chemistry, 2016: Describes the use of PVA–LiClO₄ gel in flexible energy devices but uses conventional activated carbon.
Thus, the no prior art discloses the use of Crotalaria juncea seed waste as a carbon precursor in activation-free, low-temperature (600°C) pyrolysis for energy storage applications. None demonstrate a solid-state supercapacitor device with such high performance without any chemical treatment or doping.

The present invention designed as a smart, economical, and eco-friendly energy storage device by utilizing plant biowaste from Crotalaria juncea seeds for the fabrication of high-performance carbon-based electrodes suitable for supercapacitor applications signify 3D microflake morphology with graphitic features, verified by XRD and Raman, and the device's ability to power actual electronics after a short charge period, distinguish it from prior technologies.
Objective of Invention
Primary objective of the to develop preparing a Crotalaria juncea-derived biochar electrode (CJbiochar) for use in energy storage applications.

Secondary objective of the invention is to fabricate a solid-state symmetric supercapacitor device using the prepared Crotalaria juncea biochar (CJBiochar) electrodes with PVA–LiClO₄ gel electrolyte, delivering enhanced electrochemical performance in terms of specific capacitance, energy density, power density, and cyclic stability.
Another objective of the invention is to characterize the structural, morphological, and electrochemical properties of the carbon electrode using XRD, RAMAN spectroscopy, and FE-SEM techniques.
Yet another objective of the invention is to demonstrate the practical applicability of the fabricated supercapacitor by powering LED panels, small fan and like devices to indicate the device’s feasibility in real-world low-power applications.

Summary of the Invention
The invention provides a bioactive, film-forming herbal formulation using Tridax procumbens extract, tea tree oil, and lavender oil, with a polymer matrix of HPMC and PVA. The formulation is cast as a thin film and laminated into sanitary pads to offer antifungal, antimicrobial, and soothing effects. The composition also includes emulsifiers, humectants, and natural soothing agents like rose water and witch hazel. The invention is safe, biodegradable, and suitable for intimate hygiene applications.
As per the first aspect of the invention a method for preparing a Crotalaria juncea-derived biochar electrode (CJbiochar) for use in energy storage applications, particularly in solid-state symmetric supercapacitor devices involves the process of collecting dry beans of Crotalaria juncea, removing the seed material, and screening to eliminate external contaminants. The cleaned seeds are ground into fine powder using a ball mill and manual crucible rubbing technique. A 5 g sample of this powder is washed with distilled water and dried for 48 hours. The dried powder is carbonized in a tubular furnace at 600°C under a nitrogen atmosphere for 5 hours, achieved by ramping the temperature over 3 hours and maintaining it for an additional 2 hours. The carbonized material is then cooled, washed multiple times with ethanol and double-distilled water (DDW), and dried at 80°C for 12 hours.
As per the second aspect of the invention a slurry is prepared by mixing 80 wt% of the biochar, 10 wt% polyvinylidene fluoride (PVDF), and 10 wt% acetylene black in N-methyl-2-pyrrolidone (NMP), and the slurry is deposited onto a well-polished stainless steel (SS) substrate using an applicator. The coated substrate is dried overnight at 80°C to obtain the final CJbiochar electrode. The resulting electrode exhibits a high specific capacitance of 213.93 F g⁻¹ at a scan rate of 5 mV s⁻¹ and demonstrates excellent cyclic stability, retaining 106.9% of capacitance after 3000 cycles.
As per the third aspect of the the invention includes a method of fabricating the solid-state device by preparing a PVA–LiClO₄ gel electrolyte through the dissolution of 1 g LiClO₄ in 10 ml DDW at 70°C, followed by gradual addition of 1 g polyvinyl alcohol (PVA) with continued stirring for 3 hours to form a viscous gel. The gel is cooled and coated onto both CJbiochar electrodes, which are then assembled in symmetric fashion under 1-ton uniaxial pressure and dried overnight at room temperature. The resulting device exhibits electrochemical double-layer capacitor (EDLC) behavior with a potential window of up to 1.55 V and specific capacitance of approximately 17.7 F g⁻¹ at 5 mV s⁻¹. The symmetric device configuration (SS/CJBiochar //PVA–LiClO₄// CJBiochar/SS) delivers an energy density of 1.67 Wh kg⁻¹ and a power density of 86.11 W kg⁻¹. The device is capable of powering a 1V DC fan. One of the preferred embodiments of the present invention is to utilize a low-temperature (approx. 550°C) and activation-agent-free carbonization method in a tubular furnace under nitrogen atmosphere for producing biochar from Crotalaria juncea seed powder.
One of the preferred embodiments of the present invention is to deposit a thin film of carbon material on stainless steel substrate using the doctor blade method to produce a uniform electrode surface.
One of the preferred embodiments of the present invention is to characterize the structural, morphological, and electrochemical properties of the carbon electrode using XRD, RAMAN spectroscopy, and FE-SEM techniques.
One of the preferred embodiments of the present invention is to achieve a specific capacitance of 213.92 F g⁻¹ at 5 mV s⁻¹ and cyclic stability of 106.9% over 3000 cycles using the carbon electrode in liquid LiClO₄ electrolyte.
One of the preferred embodiments of the present invention is to demonstrate the practical applicability of the fabricated supercapacitor by powering a 21 red LED panel and a small fan, indicating the device’s feasibility in real-world low-power applications.
Brief Description of Drawings

The following thorough explanation of the various aspects of the invention is taken in conjunction with the corresponding drawing that represents various aspects and other features of the disclosure invention.
Figure 1 A: Flow chart of preparation process of Crotalaria juncea derived biochar electrode
Figure 1 B: Flow chart of method for fabricating the solid-state symmetric supercapacitor device
Figure 1 C : Schematic of synthesis process
Figure 2(a–d): XRD, Raman, SEM, EDX of carbon material
Figure 3(a–d): CV, GCD, SC, and rate performance
Figure 4(a–c): Cyclic stability, EIS Nyquist and Bode plots
Figure 5(a–e): CV, GCD, energy/power graphs of symmetric device
Figure 6(a–c): Stability, EIS and τ of symmetric device
Figure 7(a–b): Device powering LEDs and fan
Detailed Description of the Invention
The following description is of exemplary embodiments only and is not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description provides a convenient illustration through explanation and figures for implementing exemplary embodiments of the invention. Various changes to the described embodiments may be made in the function and arrangement of the ingredients described without departing from the scope of the invention.
The use of “including”, “comprising” or “having” variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. Further, the use of terms “first”, “second”, and “third”, and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another.
Important steps of preparation method of CJbiochar electrode and Solid-State Symmetric Device
A) Crotalaria juncea seed preparartion: A method for preparing a Crotalaria juncea-derived biochar electrode (CJbiochar) for use in energy storage applications, particularly in solid-state symmetric supercapacitor devices involves the process of collecting dry beans of Crotalaria juncea, removing the seed material, and screening to eliminate external contaminants. The cleaned seeds are ground into fine powder using a ball mill and manual crucible rubbing technique. A 5 g sample of this powder is washed with distilled water and dried for 48 hours. The dried powder is carbonized in a tubular furnace at 600°C under a nitrogen atmosphere for 5 hours, achieved by ramping the temperature over 3 hours and maintaining it for an additional 2 hours. The carbonized material is then cooled, washed multiple times with ethanol and double-distilled water (DDW), and dried at 80°C for 12 hours.
B) CJbiochar electrode method: A slurry is prepared by mixing 80 wt% of the biochar, 10 wt% polyvinylidene fluoride (PVDF), and 10 wt% acetylene black in N-methyl-2-pyrrolidone (NMP), and the slurry is deposited onto a well-polished stainless steel (SS) substrate using an applicator. The coated substrate is dried overnight at 80°C to obtain the final CJbiochar electrode. The resulting electrode exhibits a high specific capacitance of 213.93 F g⁻¹ at a scan rate of 5 mV s⁻¹ and demonstrates excellent cyclic stability, retaining 106.9% of capacitance after 3000 cycles.
C) Solid-State Symmetric Device: A method of fabricating the solid-state device by preparing a PVA–LiClO₄ gel electrolyte through the dissolution of 1 g LiClO₄ in 10 ml DDW at 70°C, followed by gradual addition of 1 g polyvinyl alcohol (PVA) with continued stirring for 3 hours to form a viscous gel. The gel is cooled and coated onto both CJbiochar electrodes, which are then assembled in symmetric fashion under 1-ton uniaxial pressure and dried overnight at room temperature. The resulting device exhibits electrochemical double-layer capacitor (EDLC) behavior with a potential window of up to 1.55 V and specific capacitance of approximately 17.7 F g⁻¹ at 5 mV s⁻¹. The symmetric device configuration (SS/CJBiochar //PVA–LiClO₄// CJBiochar/SS) delivers an energy density of 1.67 Wh kg⁻¹ and a power density of 86.11 W kg⁻¹. The device is capable of powering a 1V DC fan.One of the preferred embodiments of the present invention is to utilize a low-temperature (approx. 550°C) and activation-agent-free carbonization method in a tubular furnace under nitrogen atmosphere for producing biochar from Crotalaria juncea seed powder.
Table 1: Technical Specifications of CJbiochar-Based Supercapacitor Device
Name of Parameter Specification
Source Material Crotalaria juncea (dry seed material)
Carbonization Temperature
600°C

Atmosphere for Carbonization
Nitrogen (N₂)

Carbonization Time 5 hours total (3 hrs ramp + 2 hrs hold)
Post-Treatment Washing
Ethanol and double-distilled water (DDW)

Drying Conditions (Post-washing) 80°C for 12 hours
Electrode Composition 80 wt% CJbiochar, 10 wt% PVDF, 10 wt% acetylene black
Slurry Solvent N-Methyl-2-pyrrolidone (NMP)
Current Collector Substrate Stainless Steel (SS), well-polished
Gel Electrolyte Composition 1 g LiClO₄ + 1 g PVA in 10 mL DDW
Gel Electrolyte Processing Temp
70°C for 3 hours

Electrode Assembly
Symmetric configuration, 1-ton uniaxial pressure

Device Drying Condition Room temperature overnight
Specific Capacitance (Electrode) 213.93 F g⁻¹
Capacitance Retention (Electrode) 106.9% after 3000 CV cycles
Potential Window (Device) Up to 1.55 V
Specific Capacitance (Device) 17.7 F g⁻¹ (at 5 mV s⁻¹)
Capacitance Retention (Device) 76.27 % after 3000 CV cycles
Energy Density (Device) 1.67 Wh kg⁻¹
Power Density (Device) 86.11 W kg⁻¹
Demonstration Output Power a 1V DC fan
Example 1
A method for preparing a Crotalaria juncea derived biochar electrode (CJbiochar) for energy storage applications, comprising the steps of:
a) collecting dry beans of Crotalaria juncea and removing seed material;
b) screening the seed material to eliminate external particle contamination;
c) grinding the cleaned seeds into a fine powder using a ball mill and manual crucible rubbing technique;
d) washing 5 grams of the powdered seed material with distilled water and drying the washed powder for 48 hours in a dryer;
e) carbonizing the dried powder in a tubular furnace at 600°C under a nitrogen atmosphere for 5 hours, wherein the temperature is ramped to 600°C over 3 hours and held at 600°C for an additional 2 hours;
f) cooling the carbonized material to room temperature;
g) washing the resulting biochar multiple times with ethanol and double-distilled water (DDW), and drying it in an oven at 80°C for 12 hours;
h) preparing a slurry by mixing 80 wt% of the dried biochar, 10 wt% polyvinylidene fluoride (PVDF), and 10 wt% acetylene black in N-methyl-2-pyrrolidone (NMP);
i) depositing the prepared slurry onto a well-polished stainless steel (SS) substrate using an applicator; and
j) drying the coated substrate overnight in an oven at 80°C to form the final CJbiochar composed electrode.
Example 2
The method for fabricating the solid-state symmetric supercapacitor device as claimed in claim 1, using the Crotalaria juncea-derived biochar (CJBiochar) electrodes, comprising the steps of:
a) preparing a gel electrolyte by dissolving 1 gram of lithium perchlorate (LiClO₄) in 10 milliliters of double-distilled water (DDW) and stirring the solution at 70°C until complete dissolution;
b) adding 1 gram of polyvinyl alcohol (PVA) gradually into the LiClO₄ solution under continuous stirring and maintaining the temperature at 70°C for approximately 3 hours to obtain a viscous PVA–LiClO₄ gel;
c) cooling the viscous gel to room temperature;
d) coating the prepared PVA–LiClO₄ gel electrolyte onto both electrodes made from CJBiochar as defined in claim 1;
e) assembling the coated electrodes in a symmetric configuration and applying a uniaxial pressure of approximately 1 ton normal to the electrode surface;
f) drying the assembled supercapacitor device at room temperature overnight to ensure full gel solidification and electrode adhesion; and
g) obtaining a solid-state symmetric supercapacitor device exhibiting an electrochemical double-layer capacitor (EDLC) behaviour with a potential window of up to 1.55 V and a specific capacitance of approximately 17.7 F g⁻¹ at a scan rate of 5 mV s⁻¹.
Example 3
Structural and Morphological Analysis
The crystallinity of the carbonized CJBiochar powder was further confirmed by X-ray diffraction (XRD). The XRD pattern of the carbonized Juncea seed powder is shown in figure 2 (a). The peaks observed at 24.2 corresponding to the (004) plane (JCPDS NO- 01-0646), indicating the presence of graphitic carbon and confirming the hexagonal phase of carbonized CJBiochar. It is a conducting form of carbon which is advantageous during electrochemical process by reducing resistance.
Raman Spectroscopy is generally used to assess the structural characteristics of carbon materials. The presence of graphite phase in biochar was confirmed by Raman spectroscopy analysis. The Raman spectrum of the carbonized Juncea seed powder in figure 2 (b) shows bands at 1337.5 cm-1 and 1571.4 cm-1 corresponding to diamond (D) and graphite (G) phases of carbon [1]. The D-peak is indicating the structural defects on graphite carbon and G-peak show the presence of graphitic carbon and characteristics of SP2 Bonded structure. The ratio of intensities of D and G bands provides disorder information of carbon. The Spectrum intensity of G band (IG) is higher than D band intensity (ID) indicates a good structural alignment.
FE-SEM has been used to studied the surface morphology of the Juncea seed carbon electrode which exhibit the three-dimensional micro flakes structure like morphology with micron particle size as illustrated in figure 2 (c). The EDX spectra of deposited carbonized CJBiochar thin film electrode which confirm the existence of C and F on deposited thin film as shown in figure 2(d). The atomic and weight percentage of deposited thin film of carbonized CJBiochar electrode are inserted in tabular form along with EDX spectra.
Example 4
Electrochemical Performance
All electrochemical assessment of CJBiochar electrode was performed using three electrode system in 1 M LiClO4 electrolyte solution, with SS/Biochar as working electrode, Platinum as counter electrode and Ag/AgCl as reference electrode. The electrochemical performance was evaluated using CV at different scan rate varying from 5 to 100 mV s-1 at the stable voltage range of 0.2 to -0.6 V and summarized in figure 3 (a). The CV curve shows no redox peaks, indicating that the synthesized CJBiochar exhibits electric double-layer capacitor behaviour. The CJBiochar electrode delivered SC of 213.93 F g-1 at 5 mV s-1 as illustrated in figure 3 (b). Figure 3 (c-d) explain the GCD curve and SC graph of CJBiochar electrode with current density varying from 1.8 to 2.8 mA cm-2. The supercapacitor electrode delivered SC of 116.1 F g-1 at current density of 1.8 mA cm-2.
The Juncea Biochar electrode shows excellent cyclic stability of 106.9% at 3000 cycles (figure 4 a). The EIS investigation of CJBiochar electrode conducted in 1 M LiClO4 electrolyte over a frequency range from 0.1 Hz to 100 kHz shown in figure 4 (b). The R(Q(RW))(QR) circuit was used to fit the EIS data. The solution resistance ( ) and charge transfer resistance ( ) of CJBiochar electrode were found to be 8.102 Ω cm-2 and 21.19 Ω cm-2. Additionally, the phase angle of CJBiochar electrode was approximate 50⁰, as observed from the Bode plot as depicted in figure 4 (c). The relaxation time constant of CJBiochar electrode was calculated using the following formula of at the angle of 45⁰ which was measured 2.5 s.
Example 5
Symmetric solid-state supercapacitor
The solid-state symmetric supercapacitor device of CJBiochar design was prepared using PVA-LiClO4 gel [2]. The desired PVA-LiClO4 gel was made using mixing of LiClO4(1 g) in DDW (10 ml) which was then stirred at 70 ⁰C. After that PVA (1 g) was gradually mixed in above-mentioned solution and after 3 hours viscous gel was obtained. After cooling the gel at room temperature, it was coated on both CJBiochar symmetric electrodes and then subjected 1 ton of pressure to the normal of electrode, which was kept at room temperature for overnight to full drying. The fabricated device was tested through CV with good potential window of 1.55 V. Figure 5 (a) shows CV performance at various scan rate varying from 5 to 100 mV s-1. The CV curve of device depicts the rectangular type shape and there are no redox peaks observed, which show EDLC type mechanism. The solid-state symmetric supercapacitor device was measured 17.7 F g-1 at 5 mV s-1 as shown in figure 5 (b).
Additionally, GCD technique was employed to assess the electrochemical capacitance of the fabricated device at current ranges from 1 to 3 mA as shown in figure 5 (c). GCD shows the SC of 5.01 F g-1 at 1 mA in figure 5 (c). The device delivered the power density (PD) of 86.11 and 258.33 W kg-1 at energy density (ED) of 1.67 and 1.01 Wh kg-1 respectively, shown in figure 5 (d). The Coulombic efficiency graph corresponding to current of device is as shown in figure 5 (e) which demonstrates the maximum efficiency of 77.55 % at 3 mA.
Furthermore, the fabricated device exhibits good cyclic stability of 76.27 % over 3000 cycles as illustrated in figure 6 (a). Additionally, EIS analysis was employed to assess the device performance in the frequency range from 0.1 Hz to 100 kHz shown in figure 6 (b). The and value of device were calculated to be 0.39 Ω 0.85 Ω, respectively. The relaxation time constant of solid-state device was measured to be 0.19 s figure 6 (c). In order to demonstrate the practical ability of the designed symmetric device, device was run 1V DC fan presented in figure 7 (a).
Advantages of the Invention
Sustainable Raw Material Source: Utilizes Crotalaria juncea seed waste—an agricultural by-product—making the process environmentally friendly and economically viable.
Low-Cost Electrode Material: The biochar derived from plant waste reduces the cost of electrode production compared to traditional carbon-based materials.
Simple and Scalable Synthesis Process: The method does not require chemical activation agents or complex post-treatment, making it scalable for industrial applications.
High Electrochemical Performance: The CJbiochar electrodes exhibit: High specific capacitance of 213.93 F g⁻¹.
Exceptional Cycle Stability: The electrode retains 106.9% capacitance after 3000 charge–discharge cycles, indicating excellent electrochemical durability.
Efficient Gel Electrolyte Integration: Use of PVA–LiClO₄ gel electrolyte enhances safety, mechanical flexibility, and solid-state configuration.
High Electrochemical Performance of Solid-State Device: The solid-state symmetric supercapacitor device was measured 17.7 F g-1 at 5 mV s-1. Superior energy density of 1.67 Wh kg⁻¹; and Power density of 86.11 W kg⁻¹.
Practical Demonstration: The device is capable of powering a 1V DC fan showcasing its real-world utility.
3D Microflake Graphitic Morphology: The carbon exhibits favorable morphology and structure, as confirmed by XRD and Raman analysis, enhancing surface area and conductivity.
Scope of the Invention
The present invention relates to the development of an eco-friendly and economical carbon electrode material derived from Crotalaria Juncea seed bio-waste for use in supercapacitor applications can be applicable in in future as follows: Biochar-Based Energy Storage Devices: Applicable to the development of various types of solid-state supercapacitors and hybrid capacitors using plant-based biochar materials; Electrode Design for High-Performance Capacitors: The invention covers electrode formulation techniques using biochar, conductive additives, and polymer binders for optimized electrochemical output; Gel Electrolyte Integration Techniques: Encompasses the preparation and application of polymer–salt gel electrolytes (e.g., PVA–LiClO₄) for enhanced device stability and safety; Material Characterization: Includes the use of structural and morphological analysis (e.g., XRD, Raman) to confirm the quality and electrochemical suitability of biochar materials; Applications in Wearable and Flexible Electronics: The invention can be extended to wearable, portable, and flexible energy storage systems due to the lightweight and solid-state design; Green and Circular Economy Applications: Contributes to waste-to-energy initiatives by converting biomass waste into valuable functional materials for clean energy technologies; and Customizable to Other Biomass Sources: The methodology can potentially be adapted to other plant-derived materials for similar or improved energy storage performance.

It is to be understood that the present invention is not limited to the embodiments described above, it should be clear that various modifications and alterations can be made along with various features of one embodiment included in other embodiments, within the scope of the present invention.

, C , Claims:We claim,
1. A method for preparing a Crotalaria juncea derived biochar electrode (CJbiochar) for energy storage applications, comprising the steps of:
a) collecting dry beans of Crotalaria juncea and removing seed material;
b) screening the seed material to eliminate external particle contamination;
c) grinding the cleaned seeds into a fine powder using a ball mill and manual crucible rubbing technique;
d) washing 5 grams of the powdered seed material with distilled water and drying the washed powder for 48 hours in a dryer;
e) carbonizing the dried powder in a tubular furnace at 600°C under a nitrogen atmosphere for 5 hours, wherein the temperature is ramped to 600°C over 3 hours and held at 600°C for an additional 2 hours;
f) cooling the carbonized material to room temperature;
g) washing the resulting biochar multiple times with ethanol and double-distilled water (DDW), and drying it in an oven at 80°C for 12 hours;
h) preparing a slurry by mixing 80 wt% of the dried biochar, 10 wt% polyvinylidene fluoride (PVDF), and 10 wt% acetylene black in N-methyl-2-pyrrolidone (NMP);
i) depositing the prepared slurry onto a well-polished stainless steel (SS) substrate using an applicator; and
j) drying the coated substrate overnight in an oven at 80°C to form the final CJbiochar composed electrode effectively configured into a solid-state symmetric supercapacitor device, herein the electrode exhibit a specific capacitance of 213.93 F g⁻¹ and retains 106.9% of capacitance over 3000 cycles; the solid-state symmetric supercapacitor device was measured 17.7 F g-1 at 5 mV s-1 and exhibits an energy density of 1.67 Wh kg⁻¹ and power density of 86.11 W kg⁻¹; and said device can power a 1V DC fan.
2. The method for fabricating the solid-state symmetric supercapacitor device as claimed in claim 1, using the Crotalaria juncea-derived biochar (CJBiochar) electrodes, comprising the steps of:
a) preparing a gel electrolyte by dissolving 1 gram of lithium perchlorate (LiClO₄) in 10 milliliters of double-distilled water (DDW) and stirring the solution at 70°C until complete dissolution;
b) adding 1 gram of polyvinyl alcohol (PVA) gradually into the LiClO₄ solution under continuous stirring and maintaining the temperature at 70°C for approximately 3 hours to obtain a viscous PVA–LiClO₄ gel;
c) cooling the viscous gel to room temperature;
d) coating the prepared PVA–LiClO₄ gel electrolyte onto both electrodes made from CJBiochar as defined in claim 1;
e) assembling the coated electrodes in a symmetric configuration and applying a uniaxial pressure of approximately 1 ton normal to the electrode surface;
f) drying the assembled supercapacitor device at room temperature overnight to ensure full gel solidification and electrode adhesion; and
g) obtaining a solid-state symmetric supercapacitor device exhibiting an electrochemical double-layer capacitor (EDLC) behaviour with a potential window of up to 1.55 V and a specific capacitance of approximately 17.7 F g⁻¹ at a scan rate of 5 mV s⁻¹.
3. The biochar-based carbon electrode and the solid-state symmetric supercapacitor device as claimed in claim 1, wherein the carbon material exhibits 3D microflake morphology and graphitic character as confirmed by XRD and Raman.