Trends Sci. 2025; 22(9): 10377

Among various carbon nanomaterials, such as amorphous carbon, graphite, carbon nanotubes (CNTs), nanodiamonds, and graphene, graphene has garnered significant attention in both research and industry due to its promising scientific applications [1]. Graphene, often referred to as a “wonder material”, is a two-dimensional (2D) material composed of sp²-hybridized carbon atoms arranged in a single layer of hexagonal rings [2]. It exhibits outstanding properties, including high electrical and thermal conductivity, a large specific surface area, excellent carrier mobility, high chemical stability, superior optical transmittance, and exceptional mechanical strength [3]. These remarkable properties make graphene suitable for diverse applications, such as supercapacitors, sensors, photonic devices, transparent conductive electrodes, nanoelectronics, and photocatalysis [4].

Graphene is commonly produced via chemical reactions starting from graphite. This process typically involves the oxidation of graphite powder to graphite oxide, followed by the conversion of graphite oxide to graphene oxide (GO), and its subsequent reduction to reduced graphene oxide (rGO) [5]. rGO has a structure similar to pure graphene [6]. GO is often prepared using the Hummers method, which introduces a large number of oxygen-containing functional groups (e.g., -COOH, -OH, C = O, and -CHO) on the surface and edges of the graphene sheets. These functional groups impede electron movement in GO; thus, reducing them enhances electrical conductivity and improves the materialʼs electronic properties [7].

Various reduction methods, such as chemical reduction [8], thermal reduction [9], microwave-assisted reduction [10], photoactive reduction [11], and hydrothermal reduction [12], have been developed to transform GO into rGO. The resulting rGO serves as a foundation for the production of graphene-metal oxide nanocomposites, which exhibit significantly improved properties compared to pristine rGO. The wrinkled structure of rGO acts as a supporting matrix for the growth of nanoscale metal oxide crystals. Furthermore, integrating metal oxides with graphene introduces new functionalities and enhances the physical, chemical, and mechanical properties of the composite materials [13]. Different metal oxide-graphene composites, including TiO₂ [14], Mn_₃O₄ [15], ZnO [16], CdO [17], MoO₃ [18], CeO₂ [19], NiO [20], Co₃O₄ [21], and MgO [22], have been extensively studied for their unique properties and applications. Among these, tungsten oxide (WO₃) integrated with rGO has gained significant attention due to its excellent optical and electrochemical properties.

The integration of graphene with metal oxides, such as tungsten oxide (WO₃), has demonstrated significant improvements in electrochemical performance. However, to be viable for large-scale applications in energy storage devices, these materials must be further optimized. WO₃ is a non-toxic transition metal oxide and an n-type semiconductor with a narrow bandgap of 2.4 - 2.8 eV, making it highly suitable for visible light applications [23]. It also possesses a high intrinsic carrier density, mechanical stability, and chemical resistance, enabling its use in sensors, electrochromic devices, photocatalysis, batteries, optoelectronics, dye-sensitized solar cells, and supercapacitors [24]. However, the primary limitations of WO₃ are its low energy density and electrical conductivity. To overcome these challenges, rGO sheets are employed to enhance the electrical conductivity and overall performance of WO₃ composites. The 2D structure of rGO facilitates the growth of WO₃ nanoparticles, significantly improving the compositeʼs electrical properties. The synthesis of WO₃-rGO composites has been extensively studied, and various synthetic approaches, including hydrothermal [25], precipitation [26], and oxidation of WS₂ nanoparticles at room temperature [27], have been reported. Researchers have demonstrated that WO₃ nanofibers coated on rGO surfaces exhibit enhanced electrochemical properties [28]. Similarly, WO₃ nanoflowers grown on graphene nanosheets have shown excellent performance as electrode materials for supercapacitors. This research not only aims to enhance the electrical capacity and efficiency of energy storage systems but also seeks to address broader issues related to energy accessibility. By incorperating activated carbon derived from durian peel as a low-cost and sustainable material, the study offers a potential solution for the development of high-performance supercapacitors that are accessible to communities with limited resources. Through this approach, the work aspires to deliver practical benefits and improve quality of life, ensuring that advances in material science lead to real-world impact rather than remaining confined to academic publication.

In this project, a hybrid WO₃/rGO electrode was developed to fabricate a supercapacitor with enhanced electrical capacity and faster charging capability. Additionally, an activated carbon (AC) layer prepared from durian peel was incorporated as a carrier layer in the final step to optimize the electrodeʼs performance further.

Materials and methods

Materials

Graphite powder (99.99 % purity; Lianyungang Jinli Carbon), sodium nitrate (NaNO₃), potassium permanganate (KMnO₄), sulfuric acid (H₂SO₄, 97 wt%), hydrogen peroxide (H₂O₂, 30 wt%), hydrazine monohydrate (NH₂NH₂·H₂O), sodium hydroxide (NaOH), Nafion-117, tungsten (VI) chloride (WCl₆) and distilled water were used in this study. All chemicals were of analytical grade and used without further purification.

Graphene oxide synthesis

Graphene oxide (GO) was synthesized from graphite using the Hummers method. In this process, 3 g of graphite powder were mixed with 69 mL of sulfuric acid (H₂SO₄), 1.5 g of sodium nitrate (NaNO₃), and 9 g of potassium permanganate (KMnO₄) in a container maintained at a temperature below 20 °C. The mixture was stirred for 30 min while ensuring the temperature did not exceed 35 °C.

Next, 138 mL of deionized water was slowly added to the mixture, as the reaction is highly exothermic. The temperature was then adjusted to 80 °C, and the mixture was stirred for an additional 30 min. The resulting mixture turned brown. The container was placed in a cold water bath for 10 s before 3 mL of 30 % v/v hydrogen peroxide (H₂O₂) was added. At this stage, the graphene oxide separated as a light-yellow solution, while manganese dioxide (MnO₂) formed as a brown precipitate at the bottom of the beaker.

The beaker was left overnight to ensure the reaction was completed. The graphene oxide solution (yellow solution) was carefully aspirated using a pipette and filtered under vacuum. The resulting material was washed multiple times with deionized water until the washing water reached a neutral pH of approximately 7. It was then washed sequentially with methanol and acetone. Finally, the material was vacuum-dried for 3 h to obtain graphene oxide [29].

Reduced graphene oxide synthesis

To synthesize reduced graphene oxide (rGO), 0.5 g of graphene oxide powder was dissolved in 250 mL of deionized water. The solution was subjected to ultrasonication at 40 kHz for 20 min to ensure proper dispersion.

Hydrazine monohydrate solution (NH₂NH₂·H₂O) was then added to the mixture in a ratio of GO: NH₂NH₂·H₂O = 3 g:1 % v/v. The reaction was carried out using the reflux method at 90 °C for 12 h.

The resulting product was filtered using a vacuum filtration system with filter paper (Whatman No. 40). The sample was washed several times with deionized water until the filtrate reached a neutral pH of approximately 7. Finally, the sample was sequentially washed with methanol and acetone before drying [30].

Tungsten oxide nanoparticles synthesis

Dissolve 1 g of tungsten chloride in 50 mL of deionized water in a 250-mL glass bottle. Microwave the solution at 400 W for 30 s, pausing for 10 s after each interval, for a total of 5 min. Allow the solution to cool to room temperature. Ultrasonicate at 40 kHz for 20 min, then filter the WO₃ precipitate using a vacuum filter. Collect the white precipitate, which represents pure WO₃, after natural cooling of the autoclave [31].

Tungsten oxide/reduced graphene oxide nanoparticles synthesis

Dissolve 0.5 g of graphene oxide in 50 mL of deionized water and add 200 mg of tungsten chloride powder. Transfer the mixture to a 250-mL glass bottle and microwave at 400 W for 30 s, pausing for 10 s after each interval, for a total of 5 min. Allow the solution to cool to room temperature, ultrasonicate at 40 kHz for 20 min, and filter the WO₃/rGO composite precipitate using a vacuum filter. The resulting composite material has a color ranging from grey to black [32].

Preparation of activated carbon (AC) powder from durian peels

Durian peels were thoroughly washed with filtered water (twice) and rinsed with deionized water. The peels were cut into pieces measuring 2 × 1 cm and burned in a muffle furnace at 650 °C for 2 h under an Ar atmosphere. The resulting material was ground and dried in a 50-mL Teflon bottle using a stainless steel ball mill for 12 h. The reaction involved 6 stages using concentrated sulfuric acid, followed by filtration and washing with filtered and deionized water. Finally, the material was dried, burned in a muffle furnace at 650 °C for 2 h under an Ar atmosphere, and sieved through a steel wire mesh [30].

Characterizations

The composite materials—GO, rGO, AC, and WO₃/rGO—were deposited onto glass slides and dried using a hotplate prior to characterization. Surface morphology was examined using field-emission scanning electron microscope (FESEM; Hitachi, S-4700 model). Raman spectroscopy (InVia, Renishaw) was employed to identify the graphitic structure, using a laser excitation wavelength of 514 nm. The structural properties of the composites were analyzed via X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer with Cu K_α radiation, operated at a low scanning speed of 0.021° s⁻¹.

Fourier-transform infrared spectroscopy (FTIR; Thermo) was conducted to identify functional groups. Samples were prepared by mixing composite powders with 0.02 g of potassium bromide (KBr), then scanning over a wavenumber range of 4,000 - 1,000 cm^-1. Thermogravimetric analysis (TGA) was performed from room temperature to 1,000 °C at a heating rate of 5 °C/min.

The specific surface area (SSA) of the samples was measured using the Brunauer-Emmett-Teller (BET) method based on nitrogen adsorption-desorption isotherms. BET analysis was conducted with a Microceritics volymetric adsorption system at 77 K, over a relative pressure range of 0 ≤ P/P₀ ≤ 0.99. While commonly applied, the BET method may overestimate SSA in microporous materials due to gas condensation phenomena. In graphene-based materials, the formation of multilayer stacks can also limit the accessible surface area. These factors were considered when interpreting the results. While commonly applied, the BET method may overestimate SSA in microporous materials due to gas condensation phenomena. In graphene-based materials, the formation of multilayer stacks can also limit the accessible surface area. These factors were considered when interpreting the results.

Preparation of thin films on three-electrode systems

Samples of GO, WO₃ NPs, AC, WO₃/rGO/AC, and rGO were finely ground in an agate mortar. A 0.1 mg/mL Nafion-117 solution in ethanol was prepared, and the samples were coated onto three-electrode systems (screen-printed electrodes, SPEs) and allowed to dry. A 2 M KOH solution was applied to the electrodes to measure their electrochemical properties [33].

Electrochemical measurements

Electrochemical behavior studies were conducted using a screen-printed electrode system consisting of a platinum working electrode, a platinum counter electrode, and an Ag/AgCl reference electrode. Cyclic voltammetry (CV) and charge/discharge techniques were employed to evaluate the electrical behavior within a potential range of 0 to 0.5 V. The electrochemical performance of various samples, including GO, rGO, WO₃, WO₃/rGO, AC, and WO₃/rGO/AC, was compared. The results were analyzed and interpreted accordingly.

Results and discussion

TGA study

Figure 1 presents the thermogravimetric analysis (TGA) cuvres used to evaluate the thermal stability of the samples. The TGA curve of GO exhibits three distinct weight-loss stages: ~16 % at 100 °C due to the evaporation of adsorbed water; ~43% at 240 °C attributed to the decomposition of oxygen-containing functional groups; and ~69% at 510 °C corresponding to the combustion of carbon framework [34]. Above 510 °C, the weight remained nearly constant, indicating the complete decomposition of GO.

In contrast, the WO₃/rGO composite showed a total weight loss of ~9.83% within the 30 - 800 °C temperature range, with minor decomposition leading to an overall weight loss of ~17%. Below 230 °C, the observed weight loss was primarily due to the pyrolysis of residual oxygen functional groups attached to the 2D graphite structure [35]. Between 300 and 600 °C, further weight loss occurred as a result of the oxidative degradation of the carbon lattice [36]. Based on the residual mass, the WO₃ content on the rGO surface was estimated to be 17 wt%.

Raman study

As shown in Figure 2, the Raman spectra of pure WO₃ and WO₃/rGO nanocomposites, tested in the range of 500 - 3500 cm^-1, exhibit four sharp and well-defined bending vibration bands in the W-O-W bonds at ~272 cm^-1 and ~326 cm^-1. The sharp peak at ~272 cm^-1 is attributed to the bending vibration mode of W⁴⁺-O bonds, while ~326 cm^-1 corresponds to the bending vibration mode of W⁵⁺-O bonds [37]. Additionally, the graphitic peaks at ~717 cm^-1 and ~807 cm^-1 are associated with the elastic vibration of the O-W-O stretching mode in WO₃ [38].

For the WO₃/rGO nanocomposites, the stretching mode and vibration pattern were observed to shift slightly, likely due to strong interactions between WO₃ and the rGO surface. Furthermore, the (d) and (g) bands of GO in the nanocomposites also shifted slightly, potentially caused by crystal lattice vibrations and the introduction of functional groups in the carbon network [39]. This shift indicates charge transfer between the rGO and WO₃ sheets, resulting in crosslinking between the two.

The I_D/I_G ratio for the WO₃/rGO nanocomposites was found to be 0.75, indicating the presence of defects in the rGO crystal lattice. A significant increase in the I_D/I_G ratio was observed, from 0.10 for GH to 0.60 for GO, indicating defect formation in rGO and WO_₃/rGO sheets. This increase is attributed to the nucleation of WO_₃ on the GO layer, forming a strong interface via C-O-W bonds [37]. These results demonstrate the efficient reduction of GO during the synthesis of WO₃/rGO nanocomposites.

X-ray study

Figure 3 shows the X-ray diffraction (XRD) patterns of (a) WO₃/rGO nanocomposites and (b) pure WO₃ nanoparticles, recorded in the 2θ range of 10° - 80°. The XRD pattern of the WO₃ nanoparticles (NPs) revealed a pure monoclinic phase with no observable impurity peaks. The diffraction peaks were indexed to the monoclinic crystal planes at (002), (020), (211), (022), (122), (114), (420), and (242), consistent with the JCPDS card No. 83-0950 [38], confirming high crystallinity and phase purity. The most intense peaks corresponded to the (200), (020), and (002) planes.

In the case of WO₃/rGO nanocomposites, the XRD patterns showed reduced intensity of WO₃ peaks and slight shifts in peak positions with increasing rGO content. These changes indicate interactions between WO₃ nanoparticles and functional groups present on the rGO surface [39]. Notably, a characteristic peak of reduced graphene oxide (rGO) appeared at 2θ ≈ 25.5°, corresponding to the (002) plane, suggesting successful reduction of GO and suppression of restacking due to interlayer spacing effects.

FTIR study

FTIR analysis was conducted to investigate the chemical composition and interaction between WO₃ and GO. The FTIR spectra showed prominent broad absorption peaks around 3,400 cm⁻¹, corresponding to the stretching vibration of hydroxyl groups (-OH) and absorbed water [40]. In GO, the characteristic peak at 1,675 cm^-1 represents C = O stretching, while the peak at 1,623 cm^-1 corresponds to hydroxyl groups (O-H) from intercalated water [41].

In the WO₃ sample, the peaks below 1,000 cm^-1 correspond to W = O and O-W-O stretching vibrations [42]. In the WO₃/rGO nanocomposites, the peak at 1,571 cm^-1 is attributed to the COO⁻ stretching vibration, indicating interactions between carboxyl groups on the graphene surface and WO₃ [43]. A peak at 1,400 cm⁻¹ corresponds to C-O functional groups. In contrast, the GO sample exhibits an absorption peak at 1,631 cm⁻¹, which is associated with the bending vibration mode of O-H groups [34]. These FTIR results confirm the successful interaction between WO₃ and rGO during the synthesis process.

Figure 1 TGA curves of GO, rGO, WO_₃, and WO_₃/rGO samples.

Figure 2 Raman Spectra of (a) WO₃/rGO, (b) rGO, (c) GO, and (d) GH powder.

Figure 3 XRD patterns of (a) WO₃/rGO nanocomposites and (b) WO₃ nanoparticles.

Figure 4 FTIR spectra of GO, rGO, AC, and WO₃/rGO nanocomposites.

Morphological analysis

FESEM study

Field-emission scanning electron microscopy (FESEM) was used to study the morphology and surface texture of the synthesized samples. The FESEM images of GO revealed layered structures with visible folds and wrinkles, typical of its two-dimensional morphology. The wrinkled surface provided a large specific surface area, enabling the attachment of WO₃ nanoparticles.

The WO₃ nanoparticles exhibited a flower-like morphology, with an average diameter of 21.56 ± 0.0834 nm, as calculated using the ImageJ software. In the WO₃/rGO nanocomposites, WO₃ nanoparticles were uniformly distributed on the rGO surface. Elemental analysis using energy-dispersive X-ray spectroscopy (EDS) confirmed the presence of W, O, and C, further verifying the formation of WO₃/rGO nanocomposites.

After synthesizing and characterizing the nanocomposites, the electrochemical performance of the supercapacitor was evaluated using a 3-electrode setup. The results revealed that supercapacitors incorporating WO_₃ as a dopant in a high concentration of KOH have been reported, but many researchers suggest that these supercapacitors are only suitable for certain electrolyte solutions [42]. In contrast, pure reduced graphene oxide (rGO) supercapacitors may exhibit relatively low specific capacitance values, even though these values are theoretically promising. The rGO layers may undergo agglomeration and stacking, which can lead to low capacitance due to reduced interaction between the rGO electrode and the electrolyte [44]. The specific characteristics of activated carbon derived from durian peel, used in this study, were investigated in a previously published journal article [32].

Electrochemical analyses

The integration of electric double-layer capacitance (EDLC) properties from activated carbon (AC) with the pseudocapacitive characteristics of WO₃/rGO enhances both the specific capacitance and overall electrochemical performance of the resulting supercapacitor. This combination addresses the limitations of low-efficiency materials in energy storage applications and contributes to improving system efficiency. Such advancements have the potential to broaden the application of high-performance capacitors in energy storage systems, particularly in high-efficiency electrical infrastructure.

This study focuses on the use of activated carbon (AC) as a conductive carrier layer, leveraging its high specific surface area and excellent electrical conductivity to enhance supercapacitor performance. Five different electrodes were tested: GO, rGO, WO₃, WO₃/rGO, and AC: WO₃/rGO, with Nafion-117 used as a binder to ensure firm adhesion of the active materials to the electrode surface. The charge storage behavior was analyzed using voltammetry [45].

Figure 5 FESEM images of GO, rGO, WO₃/rGO nanocomposites, AC, WO₃ NPs, and EDS spectra.

Figures 6(a) and 6(b) present the cyclic voltammetry (CV) results, which show nearly rectangular shapes for all electrodes—characteristic of ideal capacitive behavior—and are compared against the performance of the pure rGO electrode. Among the tested materials, the WO₃/rGO composite exhibited the highest capacitance, indicating a significant contribution from the pseudocapacitive mechanism. The area under the CV curve for the WO₃/rGO electrode was substantially larger than that of the other samples, confirming enhanced specific capacitance and strong electrochemical interactions between rGO and WO₃. The calculated results align with the cyclic voltammetry (CV) data, indicating a low capacitance of 117.22 F·g^-1 for the rGO electrode, a high specific capacitance of 697.29 F·g^-1 for the WO₃/rGO nanocomposite, and a maximum capacitance of 733.30 F·g^-1 for the WO₃/rGO/AC electrode. The AC electrode has a specific capacitance of 137.25 F·g^-1, which acts as a carrier for the electrons, facilitating better movement of electrons in the WO₃/rGO electrode. Therefore, an increase of 36.01 F·g^-1 was observed when comparing the WO₃/rGO/AC nanocomposite, and an increase of 616.08 F·g^-1 when comparing the WO₃/rGO/AC electrode with the rGO electrode. This suggests an improvement in supercapacitor performance by combining the properties of EDLC supercapacitors with those of pseudocapacitive materials.

Figure 6(b) also illustrates the effect of scan rate on charge storage performance. The scan rate, in this context, refers to the rate of voltage change over time, which directly influences the current response. The specific capacitance (C_sp​) can be calculated using the formula C_sp = I⋅Δt/m⋅ΔV, where I is the current, Δt is the discharge time, m is the electrode mass, and ΔV is the voltage window. As the scan rate increases, the specific capacitance tends to decrease, reflecting limited ion diffusion at higher rates [46]. Figure 6(c) shows the cycling stability of the electrode material, which maintains excellent stability over 5,000 charge-discharge cycles at a current density of 1 A·g⁻¹.

Further comparison of the rate of increase in potential difference and specific capacitance revealed that the WO₃/rGO/AC sample exhibited the highest electrode efficiency, followed by the WO_₃/rGO nanocomposite. The pure rGO electrode demonstrated the lowest specific capacitance, confirming that the WO₃/rGO/AC electrode delivered the best performance. Figures 6(c) and 6(d) show the results of a stability study conducted from cycle 100 to cycle 5,000. The results indicate that the WO₃/rGO/AC electrode is highly stable, as evidenced by a relatively small decrease in the voltammogram between cycle 1 and cycle 5,000. This confirms the excellent stability of the WO₃/rGO/AC electrode.

The charge-discharge performance of the WO₃/rGO/AC samples using a three-electrode system is shown in Figure 6(e). Galvanostatic charge-discharge (GCD) measurements are a common and reliable method for evaluating electrochemical performance. The specific capacitance was calculated within a current range of 1 - 5 A·g⁻¹. It was observed that as the current increased, the charging rate improved while the discharge time decreased, as shown in Figure 7(a). When the equivalent series resistance (R_ESR) was calculated to be 50 Ω, the pure rGO and WO₃ electrodes exhibited lower Δt values compared to the WO₃/rGO nanocomposites.

Figure 6 (a) Cyclic voltammetry data of GO, rGO, WO₃, WO₃/rGO and WO₃/rGO/AC composites with a potential range of 0 - 0.5 V, scan rate of 50 mV s^-1 in 2 M KOH, (b) Specific capacitance vs. scan rate of rGO, WO₃/rGO and WO₃/rGO/AC, (c) Stability from the 100th cycle to the 5,000th cycle; (d) Cyclic voltammogram of the sample for the 5,000th cycle; and (e) Galvanostatic charge-discharge (GCD) curves at 1 A g^-1 for 10 cycles.

Figure 7 (a) Galvanostatic charge-discharge (GCD) curves at various current densities, (b) Electrochemical impedance spectroscopy (EIS) of GO, rGO, WO₃, AC, and WO_₃/rGO/AC in 2 M KOH solution over a frequency range of 0.1 Hz -100 kHz.

Figure 7 presents the electrochemical performance of various electrode materials through (a) galvanostatic charge-discharge (GCD) curves and (b) electrochemical impedance spectroscopy (EIS) plots. In Figure 7(a), the GCD curves at different current densities (1-5 A·g⁻¹) exhibit nearly triangular shapes, indicating capacitive behavior. As the current increases, the discharge time decreases due to higher internal resistance and lower charge storage capacity. The symmetrical curves reflect efficient charge storage and rapid charge transfer kinetics.

Figure 7(b) shows the electrochemical impedance spectroscopy (EIS) values of GO, rGO, WO₃, AC, and WO₃/rGO/AC samples in 2 M KOH solution over the frequency range of 0.1 Hz to 100 kHz, used to study and compare the complex resistivity properties in Nafion-117 solution. It was found that the complex resistivity of the WO_₃ electrode is the highest, followed by GO, rGO, AC, and WO₃/rGO/AC, respectively, with the WO₃ electrode exhibiting the highest electrical insulating properties. The GO electrode has a lower resistance than the WO₃ electrode because it still contains some charge carriers that can conduct electricity, making its complex resistance lower than that of the WO_₃ electrode.

When comparing the AC electrode with the rGO electrode, it was found that at low frequencies (complex resistance values lower than Zʼ < 20 kΩ), the AC electrode has a lower complex resistance value than rGO. However, at high frequencies (complex resistance values higher than Z > 20 kΩ), the AC electrode has a higher complex resistance value than rGO, indicating that the rGO electrode responds well at high frequencies. The electrochemical complex resistance (EIS) value reflects the ionic resistance of the electrolyte and the internal resistance between the electrode materials and current collector [47]. It also accounts for Faradaic reaction and charge transfer resistance in active materials (Rct). The C_pe represents the stationary phase component described by the pseudocapacitance, and (Z'') indicates the Warburg resistance due to ion diffusion on the electrode surface [48]. The steeper vertical line in the EIS plot for WO₃/rGO/AC > WO_₃/rGO > AC > GO > WO₃ suggests excellent electron conductivity from the rGO layer [49]. Additionally, this corresponds to the maximum value of the imaginary part (Z'') and the time constant (τ), calculated from the frequency (f*) as shown in the equation below.

Table 1 presents the nitrogen adsorption-desorption results and calculated electrochemical performance metrics—including specific capacitance (Csp), energy density (E), and power density (P)—for 6 different composite materials: GH, GH (annealed), rGO, AC, WO_₃/rGO, and WO₃/rGO/AC. The specific surface areas, measured from nitrogen adsorption isotherms, were 13.90, 20.25, 645.67, 1,822.76, 102.89 and 834.12 m²·g⁻¹, respectively. The corresponding specific capacitances were 28.33, 52.52, 117.22, 137.25, 697.29 and 733.30 F·g⁻¹, respectively. Among them, GH exhibited the lowest surface area and capacitance. However, after calcination at 300 °C, the surface area of GH increased due to the thermal decomposition of impurities, which created a more porous structure and increased its capacitance.

The rGO sample showed a high specific surface area of 645.67 m²·g⁻¹, consistent with previous reports. For example, rGO synthesized via the hydrothermal method can reach surface areas up to 1,435 m²·g⁻¹ [49]. However, this value remains lower than the theoretical surface area of graphene (2,640 m²·g⁻¹) [50] due to the presence of oxygen-containing functional groups on rGO, which limit its full exfoliation and surface accessibility.

The WO_₃/rGO heterostructure, despite having a lower surface area (102.89 m²·g⁻¹), achieved a very high capacitance (697.29 F·g⁻¹), attributed to the pseudocapacitive behavior of WO₃ and its efficient interaction with the rGO support. The nitrogen adsorption isotherm indicated that rGO effectively supported the immobilization of WO₃ nanoparticles, preventing their agglomeration and thereby improving the electrochemical efficiency of the composite.

The WO_₃/rGO/AC composite further enhanced performance by incorporating activated carbon, which contributed a large surface area (834.12 m²·g⁻¹) and dual capacitive behavior (EDLC and pseudocapacitance). Although the surface area of WO₃/rGO is relatively low, its Faradaic contribution significantly boosts capacitance. The combination of WO_₃/rGO and AC yielded the highest capacitance (733.30 F·g⁻¹), highest energy density (101.85 Wh·kg⁻¹), and highest power density (16,109.40 W·kg⁻¹) in this study, demonstrating a strong synergistic effect.

Table 1 Calculation of specific capacity (Csp), energy density (E), and power (P) of different composite materials.

Conclusions

In summary, this project successfully synthesized graphene oxide (GO), reduced graphene oxide (rGO), activated carbon (AC), tungsten oxide (WO₃), and WO₃/rGO nanocomposites using a microwave-assisted chemical process. The synthesized samples were characterized using various techniques to study their physical structure and electrochemical properties. Raman spectroscopy confirmed the presence of the desired chemical compounds, while X-ray diffraction (XRD) analysis indicated that the samples exhibited monoclinic crystal structures. Morphological analysis showed that the WO₃ nanoparticles had an average diameter of 21.56 ± 0.0834 nm. In the WO₃/rGO nanocomposites, rGO provided thermal stability, with WO₃ content measured at 17 wt%. Electrochemical testing using a screen-printed electrode setup showed that the WO₃/rGO/AC composite exhibited the highest specific capacitance of 733.30 F·g⁻¹. When configured as a supercapacitor, it reached a peak capacitance of 1,742.16 F·g⁻¹ at 250 cycles and demonstrated excellent stability over more than 5,000 cycles. The WO_₃ nanoparticles and activated carbon significantly contributed to the enhanced charge storage capability. These results confirm that the developed synthesis method is both effective and economical, offering strong potential for future applications in energy storage systems, such as power banks.

To make this research more understandable, the project focused on creating a new material that can store electrical energy more efficiently-similar to how batteries or power banks work. We combined several substances, including forms of carbon (graphene oxide and activated carbon) and a metal oxide (tungsten oxide), using a quick and cost-effective heating method. The final product is a composite material that was tested and proven to be stable, efficient, and capable of storing a large amount of energy for a long time. Notably, adding activated carbon further improved its performance. This new material holds promise for improving the quality and lifespan of energy storage devices used in everyday electronics.

Acknowledgements

This study was funded by a research grant from Rajamangala University of Technology Rattanakosin, Thailand.

Declaration of Generative AI in Scientific Writing

Portions of the language in this manuscript were edited with the assistance of a generative AI language model (ChatGPT, developed by OpenAI). The authors have thoroughly reviewed and verified all AI-generated content to ensure accuracy and scientific validity.

CRediT Author Statement

Mr. Santi Rattanaveeranon conceptualized the study, designed and conducted the experiments, synthesized the materials, performed the characterization, analyzed the data, and drafted the manuscript. Asst. Prof. Knavoot Jiamwattanapong contributed to the interpretation of results, provided suggestions for manuscript improvement, and assisted with language editing.

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