Two Steps for Improving Reduced Graphene Oxide/Activated Durian Shell Carbon Composite by Hydrothermal and 3-D Ball Milling Process for Symmetry Supercapacitor Device

Abstract

Durian shell waste was used to fabricate activated carbon (AC) using a hydrothermal process and three-dimensional (3-D) ball milling. Reduced graphene oxide (rGO) was composited with activated durian shell carbon (DC) to enhance the electrochemical properties for fabricating a supercapacitor (SC) device. Scanning electron microscopic (SEM) examination of the AC from hydrothermally processed durian shell carbon (AC–HDC) and AC–HDC that was 3D ball milled for 15 min (rGO/AC–HDC–3D15M) showed compacted and uniformly distributed particles with good porosity. The rGO/AC–HDC–3D15M sample exhibited high specific surface area (SSA) using the Brunauer–Emmett–Teller (BET) methodology, 2311 m²/g, and an average pore size of 1.88 nm. Electrochemical results showed that the rGO/AC–HDC–3D15M sample had the highest specific capacitance (Cs) of 545.78 F/g, power density (Pd) of 260.834 W/kg and energy density (Ed) of 60.834 Wh/kg. A coin cell SC device using an rGO/AC–HDC3D15M electrode with a 3M KOH electrolyte exhibited a high Cs of 65.585 F/g with a high energy density of 5.123 W h/kg and power density of 47.286 W/kg. Thus, the novelty of this manuscript is that (1) the structure of the rGO/AC–HDC–3D15M composite could promote fast ionic and electronic migration during charging and discharging and (2) a rGO/AC–HDC–3D15M composite, which showed electric double-layer capacitor (EDLC) could produce a positive synergistic effect for efficient electrochemical reactions. Moreover, the high surface area of the rGO/AC–HDC–3D15M composite may mitigate the volume expansion of electrodes during cycling. Thus, this work shows that an rGO/AC–HDC–3D15M composite prepared using a hydrothermal process with 3-D ball milling can show enhanced electrochemical performance for the fabrication of an EDLC supercapacitor device.

1. Introduction

Currently, there are developments in many fields of energy storage. Rechargeable metal-ion batteries are gaining attention for their potential to power portable electronic devices and electric vehicles. One major challenge is developing electrodes that are highly reversible, low-cost, stable, and safe. A supercapacitor (SC) or electric double-layer capacitor (EDLC), which is an energy storage device with a high power density (Pd) and a long cycle life, is studied and developed for energy storage applications [1]. A highly porous material is required for the fabrication of electrodes for SC devices. An SC must be charged and discharged quickly [2,3,4,5,6,7,8]. The SC itself is stable and can be used for many cycles. Suitable SC electrode materials must have high electrical conductivity and highly stable electrochemical properties. This behavior is shown by materials such as graphite, graphene, and reduced graphene oxide (rGO) [2,3,4,5,6,7,8]. rGO has excellent and strong electrical conductivity, and it is widely used in SC cathodes [2,3,4,5,6,7,8]. It undergoes a chemical reaction that releases oxygen. The components of an SC electrode are its positive and negative electrodes that contain AC and an electrolyte. The electrolyte consists of positive and negative ions dissolved in a solvent. Energy can be stored in an electric static as generated by positive charges and the negative charge forming at the electrode. When voltage is applied to the electrodes, the ions from the electrolyte can pass to a charge from the opposite electrode. This will cause the development of a two-layer capacitor.

In the preparation of SC electrodes, carbon materials such as AC are of interest in electrode fabrication. AC can be derived from carbon that is found in all living organisms. As such, it can be derived from abundant and low-cost agricultural wastes. Teo et al. [9] reported on a study of SC electrode materials containing AC from rice husks. The AC was prepared from Brønsted or Lewis acid sites, achieving a maximum specific capacitance (Cs) of 147 F/g with a surface area of 2696 m²/g. Guo et al. [10] studied the synthesis of AC from rice husks using the chemical activation of KOH. The resulting electrode had a maximum Cs of 125–210 Fg⁻¹. Additionally, it has been reported that durian shells, which have high porosity, are suitable for SC electrodes [2,3,4,5,6,7,8]. Tey et al. [11] studied the use of durian shells to prepare AC for EDLC-type electrodes using chemical activation with phosphoric acid. These electrodes displayed a high Cs value of 93.1 F/g with Ed and Pd values of 11.3 Wh/kg and 170 kW/kg, respectively. Ukkakimapan et al. [12] investigated using durian husks to prepare AC for SC applications through chemical activation using sodium hydroxide. They achieved a specific capacitance of 145 F/g and a volumetric capacitance of 70 F/cm³ in an organic electrolyte. Ong et al. [13] prepared carbon electrodes using durian shells to produce AC for use in EDLCs. Microwave irradiation and ultrasonication methods were used to control the effects of temperature and heating time. This material had a maximum Cs at 103.6 F/g. Moreover, using durian shells as a raw material is advantageous in fabricating electrodes for SC devices. This is an agricultural waste that is environmentally friendly for electrode fabrication. However, Bo et al. [14] reported the use of rGO in energy storage applications. Rattanaveeranon et al. [15] studied the effect of rGO composites with durian shell ash with added zinc oxide in a chemical sensor for hydrazine detection. However, there have been no reports on the preparation of AC derived from durian shell waste for enhancing electrochemical properties using rGO composited with durian shell via a hydrothermal process and high-speed 3D ball milling to fabricate a coin cell SC device. The hydrothermal process is used to achieve high sample porosity, and 3D ball milling is advantageous in providing highly uniform grinding and mixing within a short period of time [16].

The current work was conducted to improve the electrochemical properties of AC from durian shell waste so that it has a high surface area and to develop high-performance carbon electrodes for symmetrical SCs. The improved method consisted of a carbonization process followed by a hydrothermal technique. It reduced particle sizes through 3D ball milling, which simultaneously operated around the x, y, and z axes to increase the AC surface area for SC electrodes. rGO was composited with durian shell carbon (DC) to enhance the electrochemical properties of electrodes in a coin cell SC device. The electrochemical properties and the coin cell SC containing rGO composited with DC prepared using a hydrothermal technique with reduced particle size and homogeneity from treatment in a high-speed 3D ball mill are reported.

2. Experiment

2.1. Materials

The materials for the synthesis of rGO samples were graphite powder (99%), hydrazine monohydrate (N₂H₄⋅H₂O, 82%), and hydrogen peroxide (H₂O₂, 30%) supplied by Sigma-Aldrich, St. Louis, MO, USA. Sodium nitrate (NaNO₃), potassium permanganate (KMnO₄), hydrochloric acid (HCl), and sulfuric acid (H₂SO₄, 98%) were obtained from Merck (Germany), and an ammonia solution (NH₃, 30%) was supplied by Baker.

The materials for the preparation of the electrodes were polyvinylidene fluoride (PVDF, 99.95% purity) and an acetylene black supplied by Sigma-Aldrich. Ethanol (C₂H₅OH, 99.9% purity) and an N-methyl-2-pyrrolidone (NMP, 99.5% purity) were obtained from RCI Labscan. Potassium hydroxide (KOH, 85% purity) was supplied by Ajax Fine Chem Laboratory Chemicals.

2.2. Preparing of Reduced Graphene Oxide

In this work, rGO was prepared by an in-house laboratory by following the simple chemical reduction method of Reference [15] using graphene oxide (GO) as a starting material and N₂H₄⋅H₂O as a reductant. GO was synthesized in the same way by an in-house laboratory utilizing a modified Hummers method employing graphite as a starting material [15].

Briefly, graphite powder was first mixed with H₂SO₄ in a glass-bottomed flask at a temperature of approximately 3 °C. KMnO₄ was added at a temperature of less than 15 °C and was simultaneously stirred for 30 min until the temperature reached 40 °C. Deionized water (DI water) was added. Then, the solution was heated and stirred under a temperature of 95 °C for 90 min. DI water and H₂O₂ were added to stop the reaction. This solution was centrifuged and washed to reduce sulfate using an HCl solution and filtered DI water to control the pH to about 7. A GO suspension was obtained after the process, which was refiled DI water, followed by ultrasonication and centrifugation for around 90 min.

Then, the obtained GO suspension was used as a precursor to prepare rGO by mixing it with NH₃ (1 mL), N₂H₄⋅H₂O (0.1 mL), and DI water (100 mL). rGO was obtained after the mixed solution was heated at 95 °C and stirred for 45 min.

2.3. Preparation of Waste DC

In preparing the DC, the waste of Monthong durians from Thailand was used in this work. First, the waste durian shell was selected for preparing an AC called DC, durian shell carbon. A 30 g mass of dried waste durian shell was carbonized by loading them into a tube furnace at 600 °C for 3 h under an argon (Ar) atmosphere. Then, the obtained DC was ground into carbon powder having a micron-size particle diameter.

To improve its properties, DC was produced in two steps: a hydrothermal treatment and high-speed 3D ball milling. In the hydrothermal treatment, the DC powder (1 g) was extracted, and its porosity increased by immersing it in 0.2 M KOH solution (1.122 g in 100 mL of water) with stirring for 3 h. The mixed DC solution was placed in a Teflon–lined stainless steel autoclave operating at 200 °C for 2 h. Then, the AC–HDC was separated as sediment and washed using DI water. The AC–HDC powder was obtained after the product was dried in an electric oven at 80 °C for 6 h and ground into a fine powder. In the high-speed 3D ball milling process, the AC–HDC-3D was simultaneously rotated around the x, y, and z axes with 20 g of zirconium oxide balls (0.5 mm diameter) at 300 rpm. Two different samples were produced. AC–HDC-3D powder was ball milled two different times, AC–HDC–3D15M for 15 min and AC–HDC–3D30M for 30 min. The DC production process is shown in Figure 1.

2.4. Preparation of rGO/AC–HDC Materials

A composite of AC–HDC and rGO (rGO/AC–HDC) was prepared using a hydrothermal treatment that was similar to the process for producing AC–HDC. Briefly, 1 g of DC powder was dissolved in an rGO suspension. Next, KOH was added to the mixture with stirring for 3 h. The mixed solution was subjected to hydrothermal treatment in a Teflon–lined stainless steel autoclave at 200 °C for 2 h. The obtained precipitate was separated after sedimentation and polishing using DI water. The obtained product was dried in an electric oven at 80 °C for 6 h, producing a sample powder called rGO/AC–HDC. Finally, the rGO/AC–HDC powders were subjected to high-speed 3D ball milling for 15 min to produce rGO/AC–HDC–3D15M.

2.5. Preparation of Working Electrodes and Fabrication of SC Devices

The active materials were varied in the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M and rGO/AC–HDC–3D15M powder to compare the electrochemical properties of the resulting electrodes. The electrode slurry was prepared using acetylene black as a conductive material, PVDF, and an active material (rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M powder) in a ratio of 10:5:85 by wt% in an NMP (0.5 mL) solvent. The electrode slurry was mechanically mixed at room temperature for 24 h. The working electrode slurry was prepared by dripping the electrode slurry (0.2 mL) onto a nickel foam sheet having an area of 1 cm × 1 cm. It was dried in an oven at 80 °C for 3 h. Uniaxial compression was used to press the working electrodes. The total mass of rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, rGO/AC–HDC–3D15M, and the coin cell was 0.5240, 0.8494, 0.9130, 0.9075, 0.9360, 0.9207 and 0.9910 mg, respectively. Finally, the working electrodes were soaked in a 3 M KOH electrolyte for 24 h, after which they were subjected to electrochemical measurements.

For fabrication of the SC devices, the symmetric SC was comprised of two active electrodes in a CR 2032-coin cell, as shown in Figure 2. In brief, the first active electrodes were prepared using the homogeneous electrode slurry described above, coated on the surface of a thin disk-shaped nickel foil having a diameter of 13 mm, and dried at 80 °C for 3 h. Then, 0.5 g PVA was filled in DI water (5 mL) at 80 °C with stirring for 2 h. The electrolyte was prepared by adding 3 M KOH (5 mL) to the PVA solution and simultaneously stirring at 80 °C for 3 h to produce a PVA-3 KOH electrolyte. Next, the active electrodes were used as coin cell components by stacking three layers: (1) an rGO/AC–HDC–3D15M electrode, (2) a separator, and (3) an rGO/AC–HDC–3D15M electrode, referenced as rGO/AC–HDC–3D15M//rGO/AC–HDC–3D15M with a PVA-3 KOH electrolyte. This was dried at 40 °C for 1 h. Last, the supercapacitor coin component cells were formed using a uniaxial compression of 0.5 tons for 1 min to produce an rGO/AC–HDC–3D15M symmetric SC coin cell device. A schematic diagram of the symmetric SC device for the rGO/AC–HDC–3D15M coin cell is shown in Figure 3.

2.6. Material Characterization

The crystal phase structures of the rGO, DC, AC–HDC, AC–HDC–3D15M, ACHDC–3D30M, and rGO/AC–HDC–3D15M materials were evaluated using X-ray diffraction (XRD) (Miniflex600, Rigaku, Japan) with an X-ray source having λ CuKα = 1.5406 Å. Morphology, grain sizes, and element composition of the samples were evaluated using scanning electron microscopy (SEM) (JEOL SEM JSM-5800 LV). The atomic distribution was observed using energy-dispersive X-ray spectroscopy (EDX) and EDX mapping. Raman spectroscopy was performed (DXR Smart, Thermo Fisher Scientific, Waltham, MA, USA) with an excitation wavelength of 532 nm. A Fourier transform infrared (FTIR) technique was used for analyzing the vibration mode of atomic bonding (Bruker, Senterra, Saint Paul, MN, USA). The pore-size distribution and specific surface area (SSA) of the materials were measured using Barrett–Joyner–Halenda (BJH) and Brunauer–Emmett–Teller (BET) techniques employing a nitrogen adsorption–desorption isotherm (Autosorb1, Quantachrome, Boynton Beach, FL, USA). The oxidation states of the materials were evaluated using X-ray photoelectron spectroscopy (XPS) (Kratos Axis ultra DLD, XPS, PHI5000 Versa Probe II, ULVAC-PHI, Chigasaki, Japan) at the SUT-NANOTEC-SLRI Joint Research Facility, Synchrotron Light Research Institute (SLRI), Thailand. A potentiostat/galvanostat electrochemical workstation (Wuhan Corrtest Instruments Co., Ltd., Wuhan, China) was used for cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). A three-electrode feature was used for working electrode measurements (rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M) in a 3.0 M KOH aqueous electrolyte with a 0.5 mm diameter platinum wire and Ag/AgCl in 3.0 M HCl as a reference and counter electrodes. The electrical conductivity of the electrode slurry was measured using a Hall Effect measurement technique by applying the slurry to a glass substrate followed by drying at 80 °C for 3 h prior to measurements with an HCS 1—Hall Effect Measuring System (Linseis Inc., Selb, Germany).

2.7. Electrochemical Measurements

GCD and CV results were measured over a potential window ranging between −1.0 V and 0.0 V, varying scan rates from 5 to 200 mV/s, and current densities from 0.5 Ag⁻¹ to 5 A/g. The EIS results were obtained by applying an alternating current voltage of 10 mV at frequencies ranging from 100 Hz to 0.01 Hz. The capacity of the working electrode was measured by setting a current density at 5 A/g for 1000 cycles on the GCD measurement device. The Cs results were in units of (F/g) for working electrodes (rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M) and were calculated from the GCD results according to Equation (1),

$$C_s=\frac{I\mathsf{\Delta }t}{m\mathsf{\Delta }V}$$

(1)

where I (A), Δt (s), m(g), and ΔV (V) are the constant discharge current, discharge time, mass of active materials in the electrode, and the potential window, respectively.

The Ed (Wh/kg) and Pd (W/kg) values for the working electrodes were computed from GCD measurements employing Equations (2) and (3) to investigate the performance of the active materials before assembling symmetrical coin cells.

$$E_d=\frac{C_s\times \mathsf{\Delta }{V}^2}{7.2}$$

(2)

$$P_d=\frac{E_d\times 3600}{∆t}$$

(3)

3. Results and Discussion

3.1. XRD and Raman Spectroscopy

Figure 4a presents the XRD patterns of the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M samples. The rGO sample showed a broad diffraction peak at 2θ = 25.14° that was related to the (002) crystalline plane corresponding to the rGO nanosheet with a d-spacing of 0.354 nm [17]. The XRD peaks of the DC, AC–HDC, AC–HDC–3D15M, and AC–HDC–3D30M samples displayed peaks at 2θ = 24.94° and 43.56° that are related to the (002) and (100) diffraction planes [18,19,20,21,22,23,24,25]. The results matched the diffraction peaks for amorphous carbon materials obtained from biomass material after carbonization. The XRD peaks of the rGO/AC–HDC–3D15M samples appeared combined in a small and slightly broader peak at 2θ = 25.14°, confirming the rGO composite with the AC–HDC–3D15M sample. Notably, a broad peak at 2θ= 24–25° is from amorphous carbon found predominantly in natural carbon. Furthermore, the XRD peak at 43–44° indicates a hexagonal graphite lattice present in the structure, corresponding to JCPDS 75-1621.

Figure 4b presents the Raman spectra of the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M samples. The Raman spectra of these samples display two main peaks at positions of about 1598.83 cm⁻¹ and 1346.686 cm⁻¹, corresponding to the G-band, which is described as sp² bonded carbon atoms vibrating in a 2D hexagonal lattice, and the D-band which describes the defects and disorders of the first hexagonal graphitic layers, [14,21,25,26]. The values of the ID/IG intensity ratio of the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M samples were 0.98, 0.83, 0.84, 0.95, 0.95, and 1.03, respectively. The ID/IG ratio of the rGO/AC–HDC–3D15M sample was similar to that of the rGO sample, indicating that the rGO was composited in the rGO/AC–HDC–3D15M sample, in agreement with the XRD results shown in Figure 4a.

3.2. SEM and EDX Analysis

The overall area of the SEM images and elemental analyses of the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M samples is shown in Figure 5a–f. Figure 5a presented an SEM image of the rGO sample, confirming that the rGO morphology was unchanged on the basis of stacking disorder and overlap, indicating the successful reduction of GO materials. In Figure 5b, the SEM image of the DC sample shows non-uniformly distributed particles that are not activated. For the AC–HDC sample, Figure 5c displays flat pores as presenting the samples suffering from steam activation with high porosity properties. The AC–HDC–3D15M sample, which was ball milled for 15 min, presents an SEM image with good porosity, as shown in Figure 5d. Figure 5e shows an SEM image of the AC–HDC–3D30M sample, which was ball milled for 30 min, displaying no overlapping porosity. Figure 5f displays an SEM image of the rGO/AC–HDC–3D15M samples, showing that the sample had stacking disorder with closely agglomerated and randomly grouped particles in an extensive network of interconnected particles with improved porosity. Additionally, the energy dispersive X-ray (EDS) spectra of the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M samples showed a decreased C/O wt% ratio, as presented in

Table 1. D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M samples.

Sample	Element				C/O Ratio
	Carbon (C)	Oxygen (O)	Calcium (Ca)	Magnesium (Mg)
rGO	81.52	18.48	-	-	4.411
DC	82.33	10.70	0.41	0.52	7.694
AC–HDC	87.98	9.60	0.26	0.95	9.164
AC–HDC–3D15M	82.29	12.49	0.85	1.28	6.588
AC–HDC–3D30M	83.35	12.33	1.22	1.30	6.759
rGO/AC–HDC–3D15M	83.74	11.70	0.76	1.44	7.157

. The results showed that a residual oxygen function was grouped with reducing the greater extent, corresponding to the FTIR results in Figure 4b.

3.3. BET Analysis

Figure 6 shows the SSA results for the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M samples. The SSA of the samples was investigated based on nitrogen adsorption–desorption isotherms. The rGO isotherms showed a Type IV hysteresis loop, indicating the presence of macrospores and mesopores [18,27]. The average pore-size distribution of the samples is displayed in Figure 6. The SSA of the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M samples was 109, 62, 1146, 1683, 1065, and 2311 m²/g, respectively. These results show that the SSA values of these samples, which were hydrothermally treated, were higher than those of the DC sample. This implies that the hydrothermal process could enhance the SSA of the DC materials. Figure 6b shows the average pore-size distributions using the BJG technique for the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M samples, which were 4.26, 3.82, 2.63, 1.80, 3.24, and 1.88 nm, respectively. The results showed that the average pore-size distributions of AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M samples treated using a hydrothermal process were smaller than that of the DC sample. This implies that the hydrothermal process could reduce the average pore-size distributions for the DC materials. The results of the pore volume distributions of the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M samples were 0.105, 0.053, 0.611, 0.680, 0.702, and 0.970 cm³/g, respectively. These results show that the average pore-size distribution for the rGO/AC–HDC–3D15M sample was higher than that for the AC–HDC, AC–HDC–3D15M, and AC–HDC–3D30M samples. This implies that the rGO interacted with the rGO/AC–HDC–3D15M sample to enhance the average pore-size distribution of the DC materials.

According to Wang et al. (2018) [18], a novel process for pore generation in a fungus that colonizes roots can potentially produce usable porous carbons with a high specific surface area. A fungi-enabled approach for the fabrication of porous carbons with ultrahigh specific surface area (3439 m² g⁻¹) was created as a result of the mechanisms underlying the direct development of mushrooms toward its lignocellulosic substrate being clarified. Such porous carbons may find use in electrocatalysis, environment treatment, and energy storage. In nature, high SSA and small average pore-size distributions for active electrode materials are advantageous for electrolyte ion transport into electrodes with improved and high specific capacitance of supercapacitor devices [18]. Moreover, the rGO/AC–HDC–3D15M sample was more suitable for active electrode materials in the fabrication of SC devices than other samples.

3.4. XPS Analysis

Figure 7 shows the XPS spectra of the rGO, DC, AC–HDC, and AC–HDC–3D15M samples. The chemical composition and chemical valence of the elements of all samples were analyzed. Figure 7a,c,e,g displays high-resolution C1s spectra of the rGO, DC, AC–HDC, and AC–HDC–3D15M samples, while Figure 7b,d,f,h presents high-resolution O1s spectra of the rGO, DC, AC–HDC, AC–HDC–3D15M samples. Figure 7a shows the C1s spectrum of the rGO sample with main peak positions at 284.60, 286.18, 287.67 and 289.32 eV. The results confirm the C–C bonding contribution from the sp³ aromatic structure and the C–O bonding (carbonyl carbon) at the surface of the rGO sheets [14,26,28]. Figure 7b shows the O1s spectrum of the rGO sample displayed by three main peak positions at 531.22, 533.45, and 535.04 eV. These results confirmed the C–O bonding (carbonyl carbon) for the rGO sheets [26,28,29,30]. Figure 7c shows the C1s spectrum of the DC sample with main peaks at 282.23, 284.60, 285.92, and 287.37 eV. Figure 7d displays the O1s spectrum of the DC sample with three main peak positions at 529.71, 531.31, and 533.03 eV. Figure 7e shows the C1s spectrum of the AC–HDC sample with peaks at 284.67, 285.80, 286.95, and 288.39 eV. Figure 7f shows the O1s spectrum of the AC–HDC sample with peak positions at 531.15, 532.32, and 533.63 eV. Figure 7g shows the C1s spectrum of the AC–HDC–3D15M sample with peaks at 284.60, 285.75, 286.92, and 288.56 eV. Figure 7h displays the O1s spectrum of the AC–HDC–3D15M sample with peak positions at 530.99, 532.13, and 533.49 eV. These results show the C1s spectrum with close peak positions for the DC, AC–HDC, and AC–HDC–3D15M samples. The O (1s) spectrum main peak position of the DC, AC–HDC, and AC–HDC–3D15M samples were similar to the rGO sample. The C/O ratio for the rGO, DC, AC–HDC, and AC–HDC–3D15M samples were about 7.19, 5.21, 7.65, and 7.70, respectively. These results showed a C/O ratio for the rGO (7.19) that was higher than for DC (5.21). The C/O ratio for the AC–HDC (7.65) and AC–HDC–3D15M samples (7.70) was higher than for the DC sample (5.21). This result implies that the increased value of the C/O ratio was affected by the hydrothermal process. The C/O ratio of the AC–HDC–3D15M sample (7.70) was close to that of the AC–HDC sample (7.65). These results imply that the C/O ratio of DC materials can be enhanced by the hydrothermal process. Additionally, rGO acted in conjunction with the AC–HDC–3D15M sample to enhance the C/O ratio for the DC and DC-based samples.

3.5. FT-IR Analysis

Figure 8 shows the FT-IR spectra of the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M samples. The vibration peaks of all samples appeared at positions of about 4000 to 3450 cm⁻¹. These results correspond to the O–H stretching modes, which were present due to the vibration of the hydroxyl group formed by H₂O on the molecular surfaces [21,23,31]. The vibration peaks at about 2450–2160 cm⁻¹, 1460–1260 cm⁻¹, and 1020–1210 cm⁻¹ indicate symmetric and asymmetric stretching of –C=C–, C–H, and C–O–C bonding, respectively. Vibration peaks appeared at 840–940 cm⁻¹ for the DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M samples, but not for the rGO sample. Additionally, the band vibration peaks at 1670–1492 cm⁻¹ for the rGO and rGO/AC–HDC–3D15M samples resulted from symmetric C=O stretching. This shows the presence of sp² characteristics [21,22,23,28] for the rGO structure and confirms that rGO was significantly composited with the rGO/AC–HDC–3D15M materials.

3.6. Electrical Conductivity

Figure 9 shows the electrical conductivity of the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M slurries for working electrodes at room temperature from Hall effect measurements. This figure shows that the electrical conductivities of the AC–HDC, AC–HDC–3D15M, and AC–HDC–3D30M slurries were 0.915, 2.318, and 1.046 (1/Ωcm), respectively. The electrical conductivities of the raw rGO and DC materials, which were bulk stacking powder materials obtained from coating slurry on a glass substrate followed by drying, were 3.1 × 10⁻³ and 6.68 × 10⁻⁴ (1/Ωcm), respectively, as in the insert of Figure 9. The rGO/AC–HDC–3D15M slurry electrode showed the highest electrical conductivity, confirming that the rGO acted in conjunction with the AC–HDC–3D15M sample to enhance its electrical conductivity.

3.7. Electrochemical Properties

The electrochemical properties of each working electrode fabricated from the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M samples were studied. A KOH (3 M) electrolyte was added to the working electrodes of each sample using Ag/AgCl in HCl (3 M) as a reference electrode and platinum wire as a counter electrode.

Figure 10a–f displays the CV curves of the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M active electrodes. The measurements were performed at scan rates from 5 to 200 mV/s using a potential window between −1.0 V and 0.0 V. The results show that these CV curves of all electrodes present an increased current density with greater scan rates. The rectangular shape with small redox peaks shows an EDLC behavior, and the distorted shape indicated pseudo-capacitive behavior resulting from residual oxygen functionalities. The quasi-rectangular form of the CV curves indicated reversible reactions with a capacitive behavior from interactions at the electrode/electrolyte interface. These show the charge storage behavior of the EDLC. All working electrodes containing the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M active materials present EDLC behavior.

Clearly, in Figure 10f, rGO/AC–HDC–3D15M, which is a composite of rGO with AC–HDC–3D15M that was 3D ball milled for 15 min, presented higher CV curve capacitive values compared to other electrodes. These show that preparing DC for composite electrode materials with rGO subjected to hydrothermal treatment and high-speed 3D ball milling for 15 min can enhance the capacitive value of electrodes. These electrical conductivity values were improved in composite materials due to the transfer between quinine/hydroquinone groups via carbon materials.

Figure 11 displays GCD curves of the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M samples evaluated by GCD over a potential window between -1.0 V and 0.0 V at a current density of 0.5 A/g. Figure 11a–f displays data for all samples at current densities from 0.5 A/g to 30 A/g. All samples show GCD curves with a nearly symmetrical triangular shape. This is characteristic of EDLC’s behavior, revealing the high electric conductivity of the working electrodes. Clearly, in Figure 11f, the rGO/AC–HDC–3D15M sample shows a triangle shape for the current and slow discharges, presenting the largest GCD curve with the highest Cs. This implies that the rGO/AC–HDC–3D15M electrode is suitable for use in EDLCs. The rGO/AC–HDC–3D15M electrode displayed good specific capacitance for charge carriers in EDLCs.

Figure 12a presents the Cs values calculated using Formula (1) for the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M electrodes from the GCD measurements at current densities from 0.5 to 5 A/g. The Cs values of the rGO, DC, AC–HDC, and AC–HDC–3D30M electrodes decreased with increasing current density. Cs values of the rGO/AC–HDC–3D15M electrode were the highest of all applied current densities. The Cs values at 0.5 A/g for the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M electrodes were 57.1776, 176.594, 188.936, 219.015, 438.11, and 545.78 F/g, respectively. The rGO/AC–HDC–3D15M electrode presented the highest Cs. Preparing DC composite electrode materials with hydrothermally treated rGO and high-speed 3D ball milling for 15 min enhanced Cs values. From the Ragone plots shown in Figure 12b, Pd versus Ed is plotted according to Equations (2) and (3) at current densities of 0.5–5 A/g. The Pd of the AC–HDC–3D30M electrode increased from 267.091 W/kg to 2400.555 W/kg, while its energy density decreased from 30.418 Wh/kg to 13.453 Wh/kg. Moreover, the Pd in the AC–HDC–3D15M electrode increased from 275.478 W/kg to 2408.15 W/kg, while its Ed decreased from 60.834 W/kg to 13.378 W/kg. The results show that both the Ed and Pd of the rGO/AC–HDC–3D15M electrodes were higher than those of the AC–HDC–3D15M and AC–HDC–3D30M electrodes. The Pd of the rGO/AC–HDC–3D15M electrode increased from 260 W/kg to 5000 W/kg, while its Ed was reduced from 60 W/kg to 13 W/kg. The Ed and Pd of the rGO/AC–HDC–3D15M electrode show good values according to the Ragone plots. Nyquist plots show the internal resistance of the charge-discharge kinetics and electrolyte ion diffusion. EIS data were used for the Nyquist plots, which are plots of the real and imaginary components of impedance. A Nyquist plot has two parts: a semicircle in the high-frequency region and an oblique linear portion at low frequencies. The plot is used as an equivalent circuit with an equivalent series resistance (Rs), charge transfer resistance (Rct), double layer capacitance (CPE), Warburg element (W), and a constant phase element. Rs, which is the value of the intercept of the Nyquist plot on the real component’s axis in the high-frequency region, is a summation of electrolyte resistance, the intrinsic active materials resistance, and contact resistance by the active material and by the current collector. Rct arises from the charge transfer processes and is related to the electrical double layer and Faradaic charge transfer processes. Rct and CPE are presented in a semicircular region in the plot. The Warburg resistance corresponds to ion diffusion at low frequencies at an angle of 45°. The constant phase is described as the spatial distribution of the capacitance value. Nyquist plots of the rGO, DC, AC–HDC, AC–HDC–3D15M, AC–HDC–3D30M, and rGO/AC–HDC–3D15M electrodes at frequencies from 100 kHz to 0.01 Hz are shown in Figure 12c and its insert. At lower frequencies, the Nyquist plot of all samples shows that the imaginary part of impedance sharply increased. These results confirmed the capacitive behavior of the electrode materials. At higher frequencies, the plot of all samples showed a small semicircular curve representing the time constant. This is related to electrical charge transfer in the electrode material from the Faradic process. The curves in this figure show that the AC–HDC–3D15M had the lowest Rs, 1 Ohm. Additionally, the arc in the high-frequency region for the rGO/AC–HDC–3D15M electrode presented the smallest semicircular region. This implies that rGO composted with AC–HDC–3D15M (as rGO/AC–HDC–3D15M) contributed to the charge transfer processes in the electrical double layer and the Faradaic charge transfer process. For the Warburg information, the rGO/AC–HDC–3D15M substrate displayed a tail with an angle of 45° at low frequencies corresponding to ion diffusion dependent on frequency. The low value for the Rct is proposed in the EDLC mechanism. The results show that the rGO/AC–HDC–3D15M electrode displays a high capacitance of the EDLC behavior for SC devices.

3.8. Coin Cell Devices

The rGO/AC–HDC–3D15M sample was used in an SC device to demonstrate a practical application. Coin cell devices were fabricated using the synthesized rGO/AC–HDC–3D15M materials as electrodes for the SC device, as shown in Figure 13. Symmetrical rGO/AC–HDC–3D15M electrodes for coin cells were fabricated using a 3 M KOH electrolyte to increase the Ed of the devices. The CV values were calculated by converting the current response at scan rates from 5 mV/s to 200 mV/s. Figure 13a displays the CV curves of the rGO/AC–HDC–3D15M coin cell device, remaining in the form of an ideal rectangular shape in a window between 0.0 V and 1.5 V. Figure 13b displays the GCD curves of the coin cell device at the current densities from 0.25 A/g to 5.0 A/g. The results show that the Cs value of the rGO/AC–HDC–3D15M coin cell device in a 3 M KOH electrolyte is 65.585 F/g at 0.25 A/g and 48.496 F/g at 1.0 A/g. Figure 13c shows Cs values at various current densities for the rGO/AC–HDC–3D15M//rGO/AC–HDC–3D15M coin cell. Moreover, Figure 13d displays an excellent Cs retention of 98.74% after 1000 cycles at a 5.0 A/g current density. Figure 13e shows Nyquist plots for the rGO/AC–HDC–3D15M//rGO/AC–HDC–3D15M coin cell SC in a frequency range from 100 kHz to 0.01 Hz. The Nyquist plot has a semicircle portion in the high-frequency region and a linear portion in the low-frequency regions. The lowest Rs was 1 Ohm. Figure 13f displays the energy storage performance of the rGO/AC–HDC–3D15M//rGO/AC–HDC–3D15M SC device. According to Miao et al. [21,23,31], carbon materials display appealing physical, chemical, and mechanical properties and have been extensively studied as supercapacitor electrodes. The capacitance of an acid-etched carbon cloth electrode can approach 5310 mF cm⁻² at a current density of 5 mA cm⁻² with remarkable recycling stability. The all-solid-state symmetric supercapacitor delivered a high energy density of 4.27 mWh cm⁻³ at a power density of 1.32 W cm⁻³. Furthermore, this symmetric supercapacitor exhibited outstanding mechanical flexibility, and the capacity remained nearly unchanged after 1000 bending cycles. Interestingly, an integrated eutectic electrolyte was introduced to construct a gradient organic/inorganic hybrid SEI (GHS) layer on the Zn anode through in situ chemical reconstruction, as reported by Meng et al. [21,23,31]. This results in an ultra-stable Zn anode with a substantially improved CE of 99.8% over 1200 cycles and a high cumulative plated capacity of 5.57 A h cm⁻² at 5 mA cm⁻². The effectiveness of this approach is demonstrated by the extremely long lifespan of 22,000 cycles of a Zn//V₂O₅ full cell.

The two-coin cells can supply sufficient power to drive three green light-emitting diodes using two devices connecting in series. The Ragone plots of the rGO/AC–HDC–3D15M coin cell device in a 3 M KOH organic electrolyte compared with various porous carbons from durian shell are shown in

Table 2. Literature reports on carbon-based supercapacitor cells.

Ref.	Material	Electrolytes	Specific Capacitance (F g−1)	Current Density (A g−1) or Scan Rate (mVs−1)	Energy Density (Wh/kg)	Power Density (kW/kg)	Cell (3E/2E)
[32]	Golden chrysanthemum	6.0 M KOH	165 F g−1	0.5 A g−1	25.3 Wh kg−1	225 W kg−1	2E
[33]	Waste wood-dust	6 M KOH	300.1 F g−1	1 A g−1	16.3 Wh kg−1	148.2 W kg−1	3E
[11]	Durian	Na2SO4	93.1 F g−1	50 mV s−1	11.3 Wh kg−1	170 kW kg−1	3E
[11]	Durian	Organic electrolyte	Specific gravimetric 145 F g−1	0.1 A g−1	32 Wh kg−1	316 W kg−1	2E
[33]	Heavy bio-oil	6 M KOH	259 F g−1	0.5 A g−1	12.95 Wh kg−1	12.95 W h kg−1	2E
[33]	Coal tar pitch		113.3 mA h g− 1	0.1 A g−1	64.9 Wh kg−1	1.23 kW kg−1	2E
[33]	Eggplant	6 M KOH	469 F g−1	1 A g−1	38.51 Wh kg−1	687.1 W kg−1	2E
This work	rGO/durian (electrode)	3 M KOH	545.78 F g−1	0.5 A g−1	60.834 Wh kg−1	275.478 W kg−1	3E
	rGO/durian (coin cell)	3 M KOH	65.585 F g−1	0.25 A g−1	5.123 Wh kg−1	47.289 W kg−1	2E

. The comparison shows that the rGO/AC–HDC–3D15M electrode in 3 M KOH has an Ed of 5.123 Wh/kg and a Pd of 47.289 W/kg, which are higher than those of previous works, as shown in Table 2.

4. Conclusions

Activated durian shell carbon (DC) was prepared by drying, carbonization, and hydrothermal treatment with chemical activation by KOH (AC–HDC). It was further treated using high-speed 3D ball milling with a mill speed of 300 rpm (AC–HDC-3D). The rGO was composited with the AC–HDC3D sample (rGO/AC–HDC-3D) to enhance its electrochemical properties and fabricate a symmetric SC device. The experiment results confirm that the carbonization process under argon affected a decrease in d-spacing value related to small and slightly broader XRD peaks. SEM results of the rGO/AC–HDC-3D sample showed the particles compacted and uniformly distributed with good porosity for the sample that had undergone 3D ball milling for 15 min (rGO/AC–HDC–3D15M). The experimental results revealed that hydrothermal treatment can enhance the SSA and the average pore-size distributions of the DC materials. The rGO composite with the AC–HDC–3D15M sample showed enhanced electrical conductivity. The rGO/AC–HDC–3D15M displayed the highest SSA, 2311 m²/g, with the highest average pore-size distributions of 1.88 nm. The rGO/AC–HDC–3D15M sample displayed excellent EDLC properties. It had the highest Cs of 545.78 F/g, Pd of 260.834 W/kg, and Ed of 60.834 Wh/kg from a Ragone plot at a current density of 0.5 A/g. The Ed and Pd values of the coin cell rGO/AC–HDC–3D15^M were 5.123 Wh/kg and 47.289 W/kg. This shows that the rGO/AC–HDC–3D15M sample processed using 3D ball milling for 15 min could be used in a functional SC device.