Introduction

Green energy is an efficient and effective alternative to energy generated from the combustion of fossil fuels. It is clean, sustainable, safe, and renewable, and many countries have committed to reducing their carbon footprint by 2030 [1] by using green energy as an alternative energy source. Some examples of green energies used around the world are wind, hydrothermal, solar, and geothermal energy.

Energy storage is crucial to balance the supply and demand of energy. To decrease the imbalances between the demand for energy and the production of energy, energy storage systems (ESSs) are used to capture energy produced for use later. Most commonly, electrochemical power sources such as batteries are used for energy storage. Alternatively, supercapacitors, which have lower cost, longer cycle life, higher power density, rapid charge/discharge, and are environmentally friendly, may be used [2]. There are three categories of supercapacitors: Electrochemical Double-Layer Capacitors (EDLC), which are a carbon material that is formed due to the separation of charge at the electrode/electrolyte interface, pseudocapacitors that uses fast charge transfer through a redox process using transition metal oxides to store energy and conducting polymers and hybrid capacitors which combine both the EDLC and pseudocapacitors [3].

Carbon-based material like graphene has been widely used in supercapacitor applications, as graphene has a large specific surface area, making it an ideal supercapacitor electrode material [4]. Conventionally, graphene is synthesized using coal, a non-renewable source, through mechanical exfoliation, chemical exfoliation, and chemical vapor deposition (CVD). However, these techniques have several drawbacks, such as precise synthesis condition requirements, high temperatures, high cost, high energy, environmentally harmful to the environment, and time-consuming.

Laser scribing solves all these drawbacks as it is simple, cost-effective, and requires less precise synthesis conditions [5]. Z. Wan et al. reviewed laser-scribed graphene for supercapacitors [6] and found that laser-scribed graphene has unique properties such as flexible fabrication, large surface area, and high electrical conductivity. A flexible graphene-based supercapacitor electrode can be synthesized from polyimide sheets through laser scribing. However, the fabrication of graphene from polyimide is not environmentally friendly, requiring large resources and polyimide waste after the service lifetime. In that regard, lignin biopolymer is a viable alternative to polyimide and coal in synthesizing graphene [7]. Lignin is a biopolymer that is abundant in plants. It has high carbon content and aromatic subunits. Apart from cellulose and hemicellulose, lignin is naturally found in biomass. Nevertheless, lignin has an amorphous nature that limits electrical conductivity. 135 million tons of oil palm waste is generated in Malaysia annually [8,9]. Lignin extracted from oil palm waste was scribed through a carbon dioxide laser scribing process to fabricate eco-friendly graphene as reported by Naidu Loganathan N et al [7]. The graphene-derived oil palm lignin has limited electrical properties as it is not up to the standard of commercial graphene.

MoS₂ has a unique sandwich structure of triple hexagonal layers of S-Mo-S and is used for the fabrication of electrodes due to its low cost, non-toxicity, abundance, high specific capacitance, high chemical stability, excellent electrocatalytic properties, and good cyclic performance [10–12]. It is a promising transition metal dichalcogenide with wide-ranging potential applications in energy storage devices nanoelectronics, biosensors, and supercapacitors. Various transition metal dichalcogenides nanostructures, such as manganese diselenide, titanium disulfide, and tungsten disulfide nanostructure, have been incorporated in the electrodes of EDLC and hybrid capacitors. A manganese-doped MoS₂ supercapacitor was reported by I.T.Bello et al. with a specific capacitance of 70.37 Fg⁻¹[13]. M.A. Bissett et al. reported that the conjugation of graphene with MoS₂ improves specific capacitance through morphology alteration and resistivity reduction [14]. S.K. Asl et al. synthesized a MoS2-decorated graphene supercapacitor with a specific capacitance of ~ 8 Fcm^-3 through a laser scribing method. Graphene oxide and MoS₂ were chemically mixed, coated on a light-scribed DVD, and lasered [15]. M.M. Baig et al. obtained a specific capacitance of 850 F/g  with MoS₂ nanoflowers on reduced graphene oxide, forming a nanohybrid through a hydrothermal process, [16]. Another study reported that nickel oxide nanoparticles incorporated with MoS₂ and reduced graphene oxide enhance charge transfer behavior and the corresponding capacitance. The MoS₂/reduced graphene oxide/nickel oxide nanoparticles composite has a capacitance of 7.38 mFcm^-2 [17].

In this study, we have synthesized graphene from oil palm lignin similar to the reported article by Naidu Loganathan N et al [7]. Thus, we have grown MoS₂ on modified laser-scribed graphene through a hydrothermal process. The formation of LSG/MoS₂ hybrid overcomes the drawback of lignin-based graphene in terms of limitation in electrical properties and also the severe stacking issue of MoS₂. The severe stacking and restacking problems of MoS₂ were due to the unstable van der Waals interaction, which significantly reduces its active sites and causes poor energy density. In that regard, hybrid material bonding with MoS₂ is necessary.  The coupling of LSG/MoS₂ forming a hybrid performs higher specific capacitance than the reported specific capacitance by Naidu Loganathan N et al. It should be taken note that the current experiment setup is conducted at a different voltage range due to the presence of a composite material. A new result has been given by the current electrode as it can run as both positive and negative electrodes at different voltage ranges. Moreover, the addition of MoS₂ to LSG has increased the capacitance to 3-fold due to higher electrical conductivity and higher surface area of produced MoS₂ nanoflower. Carbon-based materials comprising hybrid capacitors and transition metal dichalcogenides were used as electrode material due to their large specific capacitance, and weak van der Waals force, for improved greater energy density and specific capacitance.

materials and methodology

Materials

Lignin extracted from an oil palm empty fruit bunch was purified [18] for conversion to graphene. Chemicals such as carbon black, sulphuric acid (H₂SO₄), polyvinylidene fluoride (PVDF), N-methyl-2-pyrrolidinone (NMP), sodium molybdate dehydrate, thiourea, and ethanol absolute were obtained from Merck & Co. All these chemicals were used as received, without additional purification. Screen-Printed Carbon Electrodes (SPCE) were purchased from Metrohm (Malaysia) Sdn. Bhd.

Synthesis of lignin-based graphene through laser scribing technique

Firstly, a 20% concentration lignin solution was prepared by adding 20 grams of lignin powder into 100 milliliters of distilled water. The solution was stirred using a magnetic stirrer for 30 minutes on a hot plate until a homogenous lignin solution was formed. Then, the lignin solution was coated on a laboratory glass substrate through the drop coat method and let to dry in an oven at 50°C for 20 minutes. The dried lignin on the glass substrate was allowed to cool for 2 minutes before adding the second layer to the same glass substrate. This step was repeated until five layers of lignin were coated on the glass substrate successfully. Next, the dried lignin-coated glass substrate was placed in a laser scribe machine. The laser scribe parameters were set to 50% laser speed, 50% laser power, and 500% pulse per inch. CO₂ laser penetrated the lignin-coated glass substrate forming graphene after 30 minutes with the brown lignin turned into black graphene. The graphene was collected and placed in a desiccator to avoid contamination. 

Preparation of modified laser-scribed graphene (LSG)

The lignin-based graphene is insoluble in water due to carbon atoms. Therefore, to increase the solubility of the laser-scribed graphene, carbon black was added to improve the electrode conductivity and accessibility of electrolyte-electrode, polyvinylidene fluoride (PVDF). This is to enhance the thermal stability with N-methyl-2-pyrrolidinone (NMP) acting as a homogenizer and aiding dissolution. PVDF is the only polymer that can be used to fabricate an electrochemical device that acts as a binder, separator, electrolyte, and piezoelectricity generator [19]. The polymeric binder properties of PVDF improve the mechanical strength of lignin-based graphene, which directly influences the supercapacitor performance [20]. Carbon black conversely, acts as a conductive additive to improve the electrical properties of lignin-based graphene [21]. Lignin-derived graphene, carbon black, and PVDF were mixed in a ratio of 75:15:10, equivalent to 150 milligrams of lignin-derived graphene, 30 milligrams of carbon black, and 20 milligrams of PVDF. The ratio for LSG preparation was taken from a previously reported work [22] with a slight modification. Next, NMP was added to the mixture to form a semi-fluidic slurry product through sonication.

Fabrication of laser-scribed graphene/molybdenum disulfide (LSG/MoS₂)

LSG/MoS₂ was prepared by dissolving 0.1 grams of modified LSG, 0.6 grams of sodium molybdate dehydrate, and 1.77 grams of thiourea in 60 milliliters of distilled water [23,24]. The mixture was stirred using a magnetic stirrer for 1 hour at room temperature on a hot plate at 1000 rpm. After a homogenous solution of MoS₂ was obtained, the MoS₂ was transferred into 250-millilitre Teflon-lined stainless-steel autoclave. Then, for smooth fabrication, 60% of the total volume of the Teflon-lined stainless-steel autoclave was filled with distilled water, equal to 90 milliliters. The Teflon chamber was closed to form an air-tight seal before placing it in the oven for a hydrothermal process at 200°C for 24 hours. After the hydrothermal process, the Teflon autoclave was cooled for 4 to 5 hours. Black precipitates of LSG/MoS₂ were produced during the hydrothermal process. The precipitates were washed thrice using absolute ethanol and distilled water simultaneously through centrifugation. After centrifugation, the product was dried in an oven at 60°C for 12 hours. The same procedure was repeated with 0.5 grams and 1.0 grams of modified LSG [25]. Also, pure MoS₂ was fabricated without adding complex LSG into the hydrothermal mixture. The LSG/MoS₂ hybrid was classified as LSG/ MoS₂-0.1, LSG/ MoS₂-0.5, and LSG/ MoS₂-1.0, according to the amount of LSG added.

Characterization

Field Emission Scanning (VP-FESEM) (Carl Zeiss SUPRA55 VP, Gemini) and Transmission Electron Microscopy (TEM) (HITACHI HT 7830) imaging, Raman spectroscopy (HORIBA Jobin Yvon HR800), X–ray diffraction (XRD) (X’Pert3 Powder & Empyrean, PANalytical with a Cu Kα radiation (λ=1.54 Ǻ)), X-ray photoelectron spectroscopy (XPS) (Thermo Scientific K-Alpha), and Fourier Transform Infrared Spectroscopy (FT-IR) (Brand: Pelkin Elmer Inc, Spectrum One/ BX) were carried out to determine the morphological, chemical states and optical properties of the fabricated laser scribed materials. Electrochemistry Impedance Spectra (EIS), Cyclic Voltammetry (CV), and Galvanostatic charge and discharge (GCD) tests were carried out through Metrohm Multi Autolab M204 Potentiostat/Galvanostat. CV and GCD tests were performed using 1M H₂SO₄ solution.

RESULTS AND DISCUSSION

Raman Spectroscopy

Raman Spectroscopy of the fabricated laser-scribed graphene’s structure, quality, and crystalline size. revealed 3 bands: D, G, and 2D. The bands were identified and evaluated based on acceptable ranges to validate the presence of laser-scribed graphene. Based on Fig. 1, the first peak with a value of 1327.52 cm^-1 was identified as a D band due to the structural defects or partially disordered graphitic domains and lattice distortions of graphene basal planes [26]. During the laser scribing procedure, uneven laser power and speed may have caused the identified D peak.

Furthermore, the presence of lignin within the graphene sheet might be another defect, causing the D peak to develop. The D peak may be caused by the edge of the laser scribe graphene. The second peak at 1581.32 cm^-1 is considered a G band, confirming the presence of graphitic carbon that corresponds to the first-order scattering of graphite’s sp² mode [26,27]. When high-quality graphene is generated at optimal power and speed, the intensity of the G peak will rise. The last peak found was the 2D band, which confirms the restorations of sp² hybridized carbon-carbon bonds in graphitic structure and the development of stacked multiple graphenes after reduction [4]. The effective thickness of the graphene layer and quality of graphene can be calculated using 𝐼_𝐺 / 𝐼_2𝐷 (0.5845) and 𝐼_𝐷 / 𝐼_𝐺 (0.8521). The crystalline structure’s value can be calculated using the equation below:

The calculated crystalline structure La showed a value of 1.5497 E^-7. The use of a lignin concentration of 20% with 50% laser power and speed is more effective in recovering the sp₂ network graphite structure of the LSG than conventional methods [27,28]. To confirm these findings, FESEM was carried out to examine the surface morphology of LSG. 

Field-Emission Scanning Electron Microscopy (FESEM)

Fig. 2 shows the surface morphology of the fabricated LSG/MoS₂ hybrids, LSG, and pure MoS₂. The LSG/MoS₂ hybrids were synthesized through hydrothermal technique with various amounts of LSG added, classified as LSG/MoS₂-0.1, LSG/MoS₂-0.5, and LSG/MoS₂-1.0, respectively. Fig. 2(a-f) shows uniform MoS₂ nanoflower anchoring around the surface of the fabricated LSG foam-like nanostructure in all three cases. As shown in Fig. 2(a & b), the shape and size of the MoS₂ nanoflower formed around the LSG/MoS₂-0.1 hybrid nanostructure were not uniform and aggregated. As for Fig. 2(c & e), the presence of LSG can be observed only in a few places, with the amount of LSG affecting the morphology structure. LSG is abundant in the LSG/MoS₂-1.0 hybrid nanostructure, as shown in Fig. 2(e & f). The LSG with mesopores has a large surface area which can boost electrochemical performance. The close-up view of the LSG/MoS₂ composite in Fig. 2(f) shows the incorporation of MoS₂ nanoflower onto the LSG nanofoam. The MoS₂ and LSG nanostructure are entwined to form an interconnected conductive network with a relatively smooth surface, which ensures optimum accessibility of electrolyte ions due to the hydrothermal process [29].

The heterogeneous growth of the MoS₂ nanoflower on LSG nanostructure results in a larger diameter, with dense MoS₂ nanoflower with significant active side edge exposure compared to the hybrid of nanoflowers of other concentrations. The 3D nanostructure also enhanced the stability of the LSG/ MoS₂-1.0 composite due to the strength of LSG, reducing the agglomeration [30]. The LSG/MoS₂ nanostructure was fabricated without any binder and conducting agent with abundant mesopores is beneficial to providing a 3D network for enhanced the electron transferred and can shorten the electron transport distance, leading to stable performance of the supercapacitor. Fig. 2(a-f) proves that as the amount of graphene increase, the surface area of the nanomaterial increase, resulting in enhanced electrochemical performance of the material respectively. Fig. 2(g & h) shows the morphology of LSG. At low FESEM magnification, the LSG material has a foam-like appearance, with overlying elongated nanograin-like structures. The foam-like appearance is caused by the release of gas during the laser scribing process, causing chemical bonds to break sublimed atoms and recombine into gaseous products, which find their way out of the structure through the generation of pores [31,32]. LSG becomes highly porous when converting lignin into LSG, mainly due to carburizing. The sp³ carbon atom in the lignin molecule will be converted to the sp² carbon atom due to heat produced by the laser machine. The oxygen will be mainly removed from graphene oxide to form reduced graphene oxide with a 3D structure during carbonization.

 As shown in Fig. 2h, the LSG has a large surface area with porous structures with different diameters. This structure induces the binding of nanoparticles that improves the diffusion of ions to the electrode from electrolyte and mass transportation during the electrochemical performance. The FESEM images of MoS₂ can be observed in Fig. 2(i & j). In Fig. 2(i), the MoS₂ formed in sheets due to its intrinsic lamellar structure. Weak Van der Waals interaction causes these sheets to clump, forming three-dimensional MoS₂ nanoflower, as shown in Fig. 2(j). Furthermore, the growth and size ranging below 200 nm of MoS₂ are similar. These fabricated materials have unique properties, which can induce physiochemical performance for supercapacitor applications.

Transmission Electron Microscopy (TEM)

The structural morphology of the MoS₂-decorated on LSG was further examined under TEM. Fig. 3(a & b) reveals that the MoS₂ nanoflower was homogeneously distributed on the graphene resulting in the strong interaction of MoS₂ on LSG [33]. Furthermore, the graphene sheets show a strong ability to prevent the aggregation of MoS₂ nanosheets, increasing the active area. Fig. 3(c) is a high-resolution TEM image taken along the edge part of these composites. Stripe-like grains with ~20 nm in length and several nanometres in width were found, However, these grains are densely packed and overlapped with each other, preventing an accurate lattice symmetry. Single stripe-like grain reveals 62 individual atomic planes ordered in the S–Mo–S sequence to form each layer. The carbon surface with a lighter color contrast under TEM confirms the hexagonal arrangement of S-Mo-S atoms. Part of the MoS₂ layer is vertically aligned, resulting in exposed molybdenum or sulfur atoms, which is beneficial for the higher catalytic activity of the MoS₂ sheets [34]. The high-resolution TEM image in Fig. 3d shows that the LSG layers covered with MoS₂ nanostructures with lattice spacing of 0.62 and 0.27nm, of the (002) and (100) planes of MoS₂, in agreement with the XRD results. The SAED pattern in Fig. 3d has several diffraction rings that corresponded well to MoS₂ planes. These results show that MoS₂ nanostructures on the LSG layer have good crystallinity [33].

X-ray diffraction (XRD)

An X-ray diffraction analysis was carried out to examine the crystallinity and plane orientation of LSG/MoS₂-1.0, as shown in Fig. 4. Fig. 4(a) displays the peak of Laser Scribe Graphene at 25°, indicating the successful formation of LSG. For pure MoS₂ in Fig. 4(b), the diffraction peaks are well matched with Joint Committee on Powder Diffraction Standards (JCPDS) Card No. 37-1492. The peaks appear at 14.2°, 33.5°, 43.1°, and 59.3°, corresponding to (002), (100), (103), and (110) planes originating from stacking layers and confirming the layered structure of MoS₂ [23]. For LSG/MoS₂-1.0 nanocomposite in Fig. 4(c), all of the characteristic peaks of pure MoS₂ are present, which suggests the same atomic arrangement along the basal planes of MoS₂ in LSG/MoS₂-1.0. There is no obvious peak for the LSG component due to the composite hybrid dominant by MoS₂. Only the broad peak observed at 2°- 24.30° indicates the presence of LSG layers [16]. All these indicative diffraction features could be used to confirm the presence of LSG wrapped with the formation of few-layer MoS₂ structures in the LSG/MoS₂-1.0 hybrid.

Fourier Transform Infrared Spectroscopy (FT-IR)

FT-IR analysis was conducted to determine the functional groups of the prepared materials. Fig. 5 depicts the FT-IR spectra of lignin-based graphene (Fig. 5a), MoS₂ (Fig. 5b) and LSG/MoS₂-1.0 (Fig. 5c). The absorption peaks of LSG/MoS₂-1.0 are distinct at 3450 cm^-1, 1700 cm^-1, 1430 cm^-1, 1125 cm^-1,752 cm^-1 and 650 cm^-1, which assigned to O-H, C=O, C=C, C-O, Mo-S and C=C groups, respectively [23,35,36]. The 1700 cm^-1 peak also contributes to the C=C stretching vibration of carbon black [37]. The broad O-H bond (hydroxyl) at 3450 cm^-1 is almost similar for all fabricated materials and is caused by the adsorption of water molecules. The presence of PVDF confirms the existence of α and β phases. The β phase peaks are 1400 cm^-1 and 840 cm^-1, while the IR vibration modes of α phases are 975 cm^-1 [38]. The FT-IR analysis of LSG/MoS₂-1.0 has validated the successful pairing of LSG and MoS₂ in the fabricated material.

X-ray Photoelectron Spectroscopy (XPS)

XPS was conducted to study the elemental composition and chemical present on the outmost layer of grown MoS₂ nanoflower on the LSG surface. As expected, the survey scan of the LSG/MoS₂-1.0 has photoelectron peaks of carbon (C), oxygen (O), molybdenum (Mo), sulfur (S), and fluorine (F), as shown in Fig. 6a. The carbon, C1s spectrum (Fig. 6b) of LSG/MoS₂-1.0 have peaks at 284.3 eV (C=C and sp² carbon), 285.5 eV (sp³ carbon), 286.5 eV (C-O), 287.8 eV (C=O), 289.2 eV (O-C=O), 291.2 eV (CF₂) and 292.9 eV (CF₃) [39–41]. These peaks correspond to the presence of graphene and the interactions of carbon atoms with oxygen, forming carbonyl and epoxide functional groups found at the graphene edges. The epoxy functional groups determine the defects and disorder of graphene structure in the basal plane. The formation of CF₂ and CF₃ proves that PVDF is merged with carbon bonds forming LSG [42]. The oxygen, O1s spectra displayed in Fig. 6c showed peaks of carbonyl (C=O) at 534.2 eV, carboxylic group (COOH) at 531.4 eV, hydroxyl or ethers (C-O) at 532.7 eV and chemisorbed oxygen in C-OH [22]. The carboxyl and carbonyl groups in LSG/MoS₂-1.0 boosts the electrochemical performance of the sensor by enhancing the wettability of the electrode and electron diffusion rate. The enlarged spectra of Mo 3d are in Fig. 6d and can be deconvoluted to binding energy peaks at 228.6, 231.9, and 235.6 eV, which are assigned to Mo⁴⁺ 3d_5/2, Mo⁴⁺ 3d_3/2, and Mo⁶⁺ 3d_3/2, respectively [43,44]. The formation of MoS₂ is identified from the presence of Mo⁴⁺. However, the Mo⁶⁺ is from the surface oxidized and the unconverted molybdenum residues in the precursor forming the Mo-O group at peaks 233.4 eV. The Mo⁴⁺ peaks confirm the formation of 2H-MoS₂ (semiconductor), whereas a peak at 229.8 eV refers to 1T-MoS2 (metallic). S 2s orbital was also observed at 226 eV [45]. The XPS spectra of three characteristic peaks of S 2p at 161.5 (S 2p_3/2), 162.7 (S 2p_1/2), and 169 (S-O) eV in Fig. 6e reveal the presence of sulfur. The S-O bond appeared during the formation of MoS₂ nanoflower. The XPS analysis confirms the existence and interactions in the LSG/MoS₂-1.0.

Electrochemical Impedance Spectroscopy (EIS)

EIS was carried out to investigate the electrochemical performance of LSG/MoS₂ hybrids on the surface of screen-printed electrode (SPE) chips at room temperature. Fig. 7 shows the Nyquist plot indicating the impedance of the material tested under an initial frequency of 1 MHz and a final frequency of 150Hz. The diameter of the semicircle arc corresponds to the resistance transfer charge (Rct) due to the combination of the double-layer capacitance and Faradic reactions [46]. The lignin-based graphene with an absence of modified materials like carbon black and PVDF has an Rct value of 100K, as shown in Fig. 7a. The unavoidable residue of lignin in scribed graphene and the amorphous nature of lignin causes the resistivity to be high [47]. In Fig. 7b, the values of Rct obtained for LSG/MoS₂-1.0, LSG/MoS₂-0.5, LSG/MoS₂-0.1, MoS₂ and LSG electrodes are 13KΩ, 18KΩ, 27KΩ, 44KΩ, and 77KΩ, respectively. The modified LSG with the presence of carbon black, PVDF, and NMP reduces the resistivity and improves the electron transfer efficiency. As the addition of LSG increases in hybrids, the capacitive behavior improves, with LSG/MoS₂-1.0 having the best capacitive and electrical conductivity. The reduction in the diameter of the semicircle arc in the hybrid material indicates a significant decrease in transfer resistance, enhanced redox reaction of charge and discharge, and higher flexibility of the polymer chain for intercalation [30,48]. The increase in graphene increases the surface area, which enhances the number of charges carried. Furthermore, the anchored MoS₂ nanoflower prevents the zero band gap of Laser Scribed Graphene layers, and the loosely-formed open pores structures promote quick electron transmission between the active material and the charge collector [49]. The EIS results revealed LSG/MoS₂-1.0 hybrid attribute to the improved storage and transport of charges within the electrode compared to other concentrations for potential supercapacitor application [50,51].

Cyclic Voltammetry (CV)

The capacitive properties of fabricated graphene were further investigated through Cyclic Voltammetry analysis at a voltage range of -0.4V to 0.4V. The graph presented in Fig. 8 (a) shows all CV plots obtained at a scan rate of 100 mV/s exhibit a redox peak, attributed to the presence of both MoS₂ nanoflowers and lignin-derived aromatics in the composites [52,53]. Notably, the composite with a greater amount of LSG shows a more pronounced redox peak, indicating a more significant contribution from the lignin aromatics to the redox behavior of the composite. According to the CV reading from SPE chips, LSG/MoS₂-1.0 has the maximum areal capacitance (1.72 mF/cm²), followed by other composite materials with lower and zero concentrations of LSG with MoS₂ nanoflowers (1.25, 0.73, and 0.48 mF/cm²). The facilitation of charge transfer from the edges to the inner core of the material has been improved by the higher surface area and strong electrical conductivity capabilities of MoS₂ nanoflowers [46]. Fig. 8 (b) compares the LSG/MoS₂-1.0 electrode at various scan rates, demonstrating faster and slower charging and discharging processes because of the shift in exposure time.

Galvanostatic Charge and Discharge (GCD)

Fig. 9 (a) shows the charge and discharge curves of fabricated graphene and composite material at a current density of 0.05 mA/cm². The irregular triangle-shaped charge/discharge curves of both ordinary and composite materials were strongly impacted by the pseudocapacitive characteristics of the aromatic compounds present on lignin and from the fabricated MoS₂ nanoflower. The GCD-specific capacitances of LSG and composite electrodes are 2.42, 3.18, 4.17, and 6.7 mF/cm² at increasing LSG mass. The composite containing the largest quantity of LSG (LSG/MoS₂-1.0) has the best capacitive properties. This is attributed to the combination of an electric double-layer capacitance (ELDC) and a pseudocapacitance material, forming a highly conductive composite in the presence of MoS₂ nanoflowers. The cyclic performance of ordinary LSG and LSG/MoS₂-1.0 were examined at 0.1 mA/cm² for 1000 charge/discharge cycles (Fig. 9 (b)). LSG/MoS₂-1.0 recorded the highest cyclic stability of 98.1%, indicating that the addition of MoS₂ nanoflowers has improved the quality of the LSG.

CONCLUSION

Oil palm lignin was successfully converted into Laser Scribed Graphene (LSG) at a lignin concentration of 20%, laser power of 50%, and laser speed of 50%. The converted 3D LSG has high graphene quality. Besides that, the hybrid LSG/MoS₂ nanostructure and 3D MoS₂ nanoflower were successfully fabricated through a finely tuned hydrothermal method. Morphology analyses have been conducted for all the synthesized nanomaterials. The electrochemical characterization of hybrids revealed that LSG/MoS₂-1.0 has the ideal capacitance behavior and high electrical conductivity with low resistivity. The incorporation of the optimized ratio of EDLC (LSG) to Faradaic (MoS₂) material to form a promising hybrid material resulted in LSG/MoS2-1.0 having the highest areal capacitance and capacitance retention of 6.7mF/cm² and 98.1%. The development and enhancement of graphene from green materials such as lignin would create a new path for green electronics, specifically high-performing supercapacitors.

Acknowledgments

This research is supported by the Ministry of Higher Education Malaysia Fundamental Research Grant Scheme (FRGS) (FRGS/1/2020/TKO/UTP/03/7). The Nanotechnology Research Laboratory and Dye Solar Cell Laboratory, Universiti Teknologi PETRONAS (UTP), provided the testing facilities.

CONFLICT OF INTEREST

The authors hereby declare that there is no conflict of interest.