Method of producing graphene and its derivatives from palm kernel shell (PKS) and product thereof

Abstract

The present invention relates to graphene and its derivatives derived from lignocellulosic biomass including palm kernel shell (PKS) and a method of producing thereof without the use of harmful chemicals and at a low temperature. Particularly, the method of producing graphene and its derivatives from palm kernel shell (PKS), comprising the steps of: carbonizing raw palm kernel shell (PKS) in an inert environment to obtain carbonized form of PKS; subjecting the carbonized form of PKS to graphitization in the presence of catalyst and heating to obtain graphite form of PKS, followed by a multi-step washing step; oxidizing the graphite form of PKS at a temperature of less than or equal to 10 °C; exfoliating the oxidized graphite, followed by neutralizing thereof in the presence of water and an alkali to obtain a graphite oxide suspension; and subjecting the graphite oxide suspension to chemical reduction in the presence of a weak acid to obtain the reduced graphene oxide (rGO), followed by another multi-step washing step.

Description

[0001] METHOD OF PRODUCING GRAPHENE AND ITS DERIVATIVES FROM PALM KERNEL SHELL (PKS) AND PRODUCT THEREOF FIELD OF TECHNOLOGY

[0002] The present invention relates generally to graphene and its derivatives derived from lignocellulosic biomass including palm kernel shell (PKS) and a method of producing thereof without the use of harmful chemicals and at a low temperature.

[0003] BACKGROUND OF THE INVENTION

[0004] Graphene is a form of graphite that is made of a single layer of carbon atoms arranged in a hexagonal lattice. Its unique properties, such as high surface area, excellent electrical conductivity, and mechanical strength, renders it a promising material for various applications, including energy storage devices such as lithium-ion batteries. Graphene can be modified and functionalized to alter its physical and chemical properties. Graphene and its derivatives, such as graphene oxide (GO) and reduced graphene oxide (rGO) are further explored for the use in semiconductor materials.

[0005] Reduced graphene oxide (rGO) refers to the graphene-like nanosheet, which is prepared by the reduction of GO via methods such as chemical reduction, electrochemical reduction, and thermal reduction. An overview in applications of rGO as a basis of electroanalytical sensing platforms, as well as the differences of GO and rGO, were summarized by Samuel J.Rowley-Neale el al. in Applied Materials Today, Volume 10, March 2018, Pages 218-226. It was understood that rGO has higher electrical and thermal conductivity than graphene oxide (GO) due to the reduction of oxygen groups. Also, the properties of rGO would be different with different reduction processes. Generally, the preparation of reduced graphene oxide (rGO) involves the reduction of oxidized graphene oxide (GO), which isitself derived from the oxidative exfoliation of graphite sheets. In a chemically-mediated reduction, a variety of chemicals has been used to remove the oxygen groups from the GO. Examples of the chemicals include hydrazine hydrate, lithium aluminum hydride (LiAlH4), sodium borohydrate (NaBH 4). However, the chemicals used are often strong and would cause harm to the environment and human upon exposure. An example involving production of rGO using the aforementioned chemicals can be found in Patent Cooperation Treaty Publication No. WO 2024 / 091109 Al which disclosed the use of biomass waste, particularly oil palm tree waste, such as oil palm trunk, as starting material in producing synthetic graphite, and then rGO.

[0006] According to the preceding description, thermal reduction method can be used to reduce GO. Nonetheless, conditions of thermal reduction may vary' according to the starting material used. An example from United States Patent Application Publication No. US 2023 / 0365411 Al disclosed a method of producing isolated graphene sheets directly from a biomass. According to US 2023 / 0365411 Al, heat treatment involves hydrothermal carbonization and pyrolysis procedures which occur concurrently with a chemical or mechanical means.

[0007] In view of the above, there is a need for a su stainable and safer alternative in production of graphene or it derivatives from sustainable material. The present invention provides such a solution.

[0008] SUMMARY OF THE INVENTION

[0009] One object of the present invention is to provide a method of producing graphene and its derivatives from palm kernel shell (PKS). Preferably, the PKS used are preferably obtained from agricultural waste, such as from oil palm waste. Hence, the method promotes sustainability and adds value by converting waste into graphene and its derivatives.Another object of the present invention is to provide a method of producing graphene or its derivatives that consumes less energy and uses milder chemicals. Conventionally, Hummer’s method has been used to synthesize graphene oxide (GO) from graphite. The Hummer’s method includes treating graphite with oxidizing agents including sulfuric acid (H2SO4), potassium permanganate (KMnO4) and sodium nitrate (NaNOs). This method introduces oxygen-containing functional groups and expands the graphite layers into graphene oxide. On the contrary, the present invention omits the use of NaNOs in oxidization of graphite form of PKS. Further, subsequent quenching step uses hydrochloric acid (HC1) and water instead of hydrogen peroxide (H2O2) to stop the oxidation process. Such changes provide an enhanced structure in resulting reduced graphene oxide (rGO). Advantageously, it also minimizes production of toxic by-products such as nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4).

[0010] Further, strong reducing agents like sodium borohydride or hydrazine hydrate as mentioned in the background of the preceding description are not used in the method of the present invention. The method in the present invention emphasizes a reduction step conducted in the presence of a weak acid and at a temperature of less than 100 °C to obtain reduced graphene oxide (rGO).

[0011] At least one of the preceding aspects is met, in whole or in part, by the present invention, in which one of the embodiments of the present invention is a method of producing graphene and its derivatives from palm kernel shell (PKS), comprising the steps of: carbonizing raw palm kernel shell (PKS) in an inert environment to obtain carbonized form of PKS; subjecting the carbonized form of PKS to graphitization in the presence of catalyst and heating to obtain graphite form of PKS, followed by a multi-step washing step; oxidizing the graphite form of PKS at a temperature of less than or equal to 10 °C; exfoliating the oxidized graphite form of PKS in the presence of stirring and sonication, followed by neutralizing thereof in the presence of water and an alkali to obtain a graphite oxide suspension; and reducing the graphite oxide suspension in the presence of a weakacid, followed by another multi-step washing step, to obtain reduced graphene oxide (rGO) therefrom.

[0012] In the preferred embodiment, the step of carbonizing the raw palm kernel shell (PKS) is conducted by heating the raw PKS at a temperature ranging from about 400 °C to 900 °C in an inert and vacuum environment.

[0013] In one embodiment, the method further comprises a step of ball milling the carbonized form of PKS into powder form.

[0014] In the preferred embodiment, the step of subjecting the carbonized form of PKS to graphitization is conducted in the presence of the catalyst and heating at a temperature ranging from about 1750 °C to 1800 °C.

[0015] Preferably, the catalyst used in graphitization includes iron (III) nitrate nonahydrate.

[0016] In one embodiment, the multi-step washing step involves a multi-step washing process using hydrochloric acid, water and deionized water separately to remove impurities and achieve neutralization.

[0017] In one embodiment, the method further comprises another step of drying and then ball milling the neutralized, graphite form of PKS into powder form.

[0018] In one embodiment, the step of oxidizing the graphite form of PKS is conducted in an ice bath and the presence of an oxidizing agent in order to maintain a temperature of less than or equal to 10 °C.

[0019] Preferably, the oxidizing agent used in the oxidizing step includes sulfuric acid and potassium permanganate.Preferably, the alkali used in neutralizing the graphite oxide suspension includes sodium hydroxide.

[0020] In the preferred embodiment, the step of reducing the graphite oxide suspension is conducted in the presence of heating at a temperature of less than 100 °C in the presence of the weak acid.

[0021] Preferably, the weak acid used in the reducing step includes ascorbic acid.

[0022] In one embodiment, the method further comprises another step of drying and then ball milling the rGO into powder form.

[0023] One embodiment of the present invention includes graphene and derivatives thereof obtained according to the method aforementioned.

[0024] Another embodiment of the present invention includes reduced graphene oxide (rGO) derived from palm kernel shell (PKS), wherein the rGO exhibits a Brunauer-Emmett-Teller (BET) surface area ranging from 60 to 70 m2 / g.

[0025] Another embodiment of the present invention includes reduced graphene oxide (rGO) derived from carbonized palm kernel shell (PKS), wherein the rGO has a carbon content of more than 80% and an oxygen content of less than 20 %.

[0026] BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The invention is illustrated, although not limited, by the following description of embodiments made with reference to the accompanying drawings in which:Figure 1 shows Raman spectroscopy graph displaying the Raman shifts for PKS-graphene and its derivatives, PKS-graphite, and PKS-carbon.

[0028] Figure 2 shows X-ray diffraction (XRD) graph displays the diffraction patterns of PKS-graphene and its derivative, PKS-graphite, and PKS-carbon.

[0029] Figure 3 shows an image of Field Emission Scanning Electron Microscopy (FESEM) of reduced graphene oxide (rGO) obtained according to the preferred embodiment of the present invention.

[0030] Figure 4 shows an image of Scanning Electron Microscopy (SEM) of reduced graphene oxide (rGO) reduced using conventional method involving the use of sodium borohydride.

[0031] Figure 5 shows an Energy Dispersive X-ray (EDX) analysis of the reduced graphene oxide (rGO) obtained according to the preferred embodiment of the present invention.

[0032] DETAILED DESCRIPTION OF THE INVENTION

[0033] Exemplary, non-limiting embodiments of the invention will be disclosed. However, it is to be understood that limiting the description to the preferred embodiments of the invention is merely to facilitate discussion of the present invention and it is envisioned that those skilled in the art may devise various modifications without departing from the scope of the appended claim.

[0034] The method in the present invention mainly involves (1) converting raw palm kernel shell (PKS) into PKS-carbon; (2) converting PKS-carbon into PKS-graphite; and (3) converting PKS-graphite to PKS-graphene and its derivatives.PKS is a lignocellulosic material primarily composed of lignin, cellulose, and hemicellulose. Therefore, it has a relatively high carbon content as compared to other biomass such as oil palm trunks or leaves, as high carbon yield from lignin provides an ample carbon source. Further, PKS can be carbonized to form a stable carbon matrix essential for forming graphitic structures. On the other hand, cellulose is a linear polymer of glucose that decomposes at relatively lower temperature than lignin. The decomposition of cellulose creates porosity as cellulose volatilizes in heat, leaving behind micropores within the carbon structure. The presence of micropores increases surface area in the PKS-graphene and its derivatives, which is a desirable characteristic for their application particularly in the field of energy storage and catalysis.

[0035] In the present invention, the method involves a step to convert raw palm kernel shell (PKS) into PKS-carbon. For the purpose of understanding, the term ‘PKS-carbon’ is also referred to ‘carbonized form of PKS’ hereinafter. In the preferred embodiment, raw PKS is carbonized in an inert and vacuum environment to obtain carbonized form of PKS. Further, it is preferred that the method provides the carbonized form of PKS which is subjected to ball milling in order to ensure uniformity and consistency of the powder used in later processes. Uniformity in material used can prevent aggregation and promote better exfoliation later. Further, it ensures reproducibility in the method hence ensuring the reliability and scalability of the method.

[0036] In the preferred embodiment, the step of carbonizing the raw palm kernel shell (PKS) is conducted by heating the raw PKS at a temperature ranging from about 400 °C to 900 °C in an inert and vacuum environment. More preferably, the raw PKS subjected to carbonization is in a fine powder form. More particularly, the raw PKS is preferably cleaned to remove dust or impurities, followed by drying thereof. The drying can be conducted by air and / or in an oven for thorough dryness. Subsequently, the dried PKS is preferably ground into fine powder before subjected to carbonization. Thereinafter, the step of carbonizing the powdered PKS is conducted in a vacuum furnace operating from about400 °C to about 900 °C, more preferably from about 450 °C to about 850 °C. Alternatively, the step of carbonizing the powdered PKS is conducted in a furnace supplied with nitrogen gas operating from about 400 °C to about 900 °C, more preferably from about 450 °C to about 850 °C. Subsequently, the method further comprises a step of ball milling the carbonized form of PKS. In one embodiment, the ball milling step can be conducted at a rotational speed of 300 to 400 rpm for about 1 hour.

[0037] According to Figure 1, the PKS-carbon obtained exhibits strong and broad D Band (-1350 cm " ’), indicating a high level of structural disorder in the carbon matrix. This suggests that the PKS-carbon has a significant amount of defects or is largely composed of amorphous carbon. Further, the PKS-Carbon exhibits significantly broader and less intense G Band (-1580 cm " ’)• This suggests that the PKS-carbon is less graphitized and more disordered, likely having amorphous or non-crystalline carbon structures. Further, there is no distinct 2D band (-2700 cm " ’) observed in the PKS-carbon obtained, which is consistent with its disordered or amorphous nature. According to Figure 2, PKS-carbon is highly disordered, with characteristics of amorphous carbon.

[0038] Further in the present invention, the method involves a step to convert PKS-carbon into PKS-graphite. For the purpose of understanding, the term ‘PKS -graphite’ is also referred to ‘graphite form of PKS’ hereinafter. In the preferred embodiment, the method comprises a step of subjecting the carbonized form of PKS to graphitization in the presence of catalyst and heating to obtain graphite form of PKS, followed by a first quenching step. More preferably, the step of subjecting the carbonized form of PKS to graphitization is conducted by immersing the PKS-carbon in a catalyst solution until a homogeneous reaction is achieved, followed by heating the PKS-carbon solution in a vacuum environment for about from about 1750 °C to 1800 °C and then followed by the first quenching step. The catalyst used is preferably iron (III) nitrate nonahydrate. The concentration and amount of catalyst used may vary according to the amount of PKS-carbon. For example, 1 M of iron (III) nitrate nonahydrate can be used.The multi-step washing step in the present invention involves a multi-step washing process using hydrochloric acid (HC1), water and deionized water separately to remove impurities and achieve neutralization. In one embodiment, the multi-step washing process includes a first step of immersing the PKS-graphite in hydrochloric acid and allowing it to form a precipitate and a supernatant; a second step of discarding the supernatant and adding deionized water to the precipitate and soaking thereof; and a third step of sieving the precipitate with deionized water to achieve neutralization (pH around 7). Thereinafter, the precipitate is preferably dried. In accordance to the preceding description, the quenching step uses HC1 and water instead of hydrogen peroxide (H2O2) to stop the oxidation process. Such changes provide an enhanced graphite structure in resulting reduced graphene oxide (rGO), as shown in Fig. 2.

[0039] In one embodiment, after the first quenching step, the PKS-graphite is further subjected to another step of drying and then subjected to ball milling. In one embodiment, the ball milling step can be conducted at a rotational speed of 300 to 400 rpm for about 1 hour.

[0040] According to Figure 1, the G band (-1580 cm " ’) is sharp and intense, typical of crystalline graphite. This indicates the presence of highly ordered graphitic domains with stacked layers. The D band (-1350 cm " ’) is relatively weaker, indicating fewer defects in the structure. The low intensity of the D band reflects the ordered crystalline nature of the material. A prominent 2D band (-2700 cm " ’) is clearly observed, characteristic of multilayer graphite. This sharp 2D band suggests that the PKS-graphite has a well-ordered layer stacking. In graphite, the 2D band typically shows as a prominent peak with a complex shape due to the interaction between graphene layers, which is evident here. This is indicative of multilayer graphene or bulk graphite. According to Figure 2, the PKS-graphite shows a well-defined crystalline structure with sharp peaks, confirming its high degree of graphitic order. This result represents a more structured form of carbon with layers arranged in the regular graphitic lattice.Further in the present invention, the method involves a step to convert the PKS-graphite to PKS-graphene and its derivatives. For the purpose of understanding, the term ‘PKS-graphene and its derivatives’ include graphite oxide, graphene oxide, reduced graphene oxide, and other possible intermediaries produced between oxidation, exfoliation and reduction of graphite into graphene, more preferably, reduced graphene oxide (rGO). In accordance to the aforementioned description, the method of producing graphene and its derivatives from the PKS-graphite comprises the steps of oxidizing the graphite form of PKS at a temperature of less than or equal to 10 °C, subsequently exfoliating the oxidized graphite form of PKS in the presence of stirring and sonication, followed by neutralizing thereof in the presence of water and an alkali to obtain a graphite oxide suspension; and then reducing the graphite oxide suspension in the presence of a weak acid, followed by another multi-step washing step, to obtain reduced graphene oxide (rGO) therefrom.

[0041] In one embodiment, the step of oxidizing the graphite form of PKS is conducted in the presence of an oxidizing agent and in an ice bath in order to maintain a temperature of less than or equal to 10 °C, more preferably a temperature between 0 °C to 10 °C. This is necessary to control the reaction's exothermic nature and prevent excessive heat generation, which could compromise the reaction's safety and efficiency. Preferably, the oxidizing agent used in the oxidizing step includes sulfuric acid (H2SO4) and potassium permanganate (KMnCh). In one embodiment, the PKS-graphite is mixed with FhSChin the ice bath at a temperature below 10 °C, followed by gradual addition of KMnCh to form a mixture. In the present invention, the method omits the of NaNCh (conventionally used in Hummer’s method) in oxidization of the graphite form of PKS. Advantageously, it minimizes production of toxic by-products such as nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4).

[0042] Following the oxidation, the method comprises a step of exfoliating the oxidized graphite form of PKS in the presence of stirring and sonication at room temperature. Preferably, theoxidized graphite form of PKS removed from the ice bath from the previous step, followed by the exfoliating step. In the preferred embodiment, the exfoliating step is conducted by stirring and sonicating the mixture in multiple cycles, preferably 8 to 14 cycles, more preferably 10 to 12 cycles, wherein each cycle comprises about 20 to 25 minutes of stirring, followed by about 4 to 8 minutes of sonication at room temperature. Thereinafter, the mixture is neutralized in the presence of water and an alkali to obtain a graphite oxide suspension around pH of 6. Preferably, the alkali used in neutralizing the graphite oxide suspension includes sodium hydroxide, more preferably about 1 M of NaOH. In one embodiment, the neutralization step is further conducted by adding deionized water into the mixture to quench the reaction and conducted in the presence of sonication.

[0043] Following the exfoliation and neutralization, the method comprises a step of reducing the graphite oxide suspension in the presence of heating at a temperature of less than or equal to 100 °C in the presence of the weak acid. As compared to conventional method of using sodium borohydride, the present invention uses weak acid as reducing agent. Distinct differences in BET surface area and FE-SEM morphology of the resulting rGO are observed in Figure 3 and 4, reflecting the impact of the reduction agent on the structural characteristics of the rGO. As a comparison, Figure 4 shows an SEM image of rGO reduced using reducing agent of the prior art (sodium borohydride).

[0044] Preferably, the weak acid used in the reducing step includes ascorbic acid. More preferably, ascorbic acid solution of 0.05 to 10 g / mL can be used. In one embodiment, ascorbic acid solution of 0.1 to 1 g / mL is used. Further, the step of reducing is conducted in the presence of stirring and heating at about 90 to 100 °C.

[0045] In the present invention, the reducing step is further followed by another multi-step washing process using hydrochloric acid (HC1), water and deionized water separately to remove impurities and further achieve neutralization. In one embodiment, the multi-step washing process includes a first step of immersing the PKS-graphene in hydrochloric acidand allowing it to form a precipitate and a supernatant; a second step of discarding the supernatant and adding deionized water to the precipitate and soaking thereof; and a third step of sieving the precipitate with deionized water to achieve neutralization (pH around 7). Thereinafter, the precipitate is preferably dried and subjected to ball milling into powder form. The step of ball milling can be conducted at about 200 rpm to 400 rpm. As a result, a PKS-graphene powder is obtained.

[0046] According to Figure 1, the PKS-graphene obtained according to the preferred embodiment of the present invention exhibits broad but prominent G Band (-1580 cm " ’) which suggested that the PKS-graphene has sp2carbon structure but with significant disorder. The broadness of the G band could indicate reduced graphene oxide or few-layer graphene with defects. Further, D Band (-1350 cm " ’) is visible, indicating the presence of defects or disordered carbon domains. The relatively high intensity of the D band compared to the G band suggests a moderate level of structural disorder, possibly from the exfoliation process or reduction of graphene oxide. There is no distinct or prominent 2D Band (-2700 cm " ’): 2D band in the PKS-graphene obtained, indicating that this material is likely not singlelayer graphene. The absence of a strong 2D band suggests that it may consist of few-layer graphene or highly defective graphene structures. According to Figure 2, PKS-graphene exhibits a broad peak typical of few-layer or disordered graphene, suggesting successful exfoliation and a reduction in layer stacking. The reduced crystallinity is typical for graphene materials produced from reduction methods.

[0047] According to Figure 3, the PKS-graphene (or rGO) reduced using the method of the present invention, particularly using ascorbic acid, has a BET surface area of about 63.7008 m2 / g. This moderate surface area suggests a well-preserved layered structure with some residual oxygen groups, which can contribute to the stability and reactivity of the material. In contrast, rGO reduced with sodium borohydride typically achieves higher BET surface areas, generally 200-600 m2 / g. This difference is likely due to the stronger reductioncapability of sodium borohydride, which removes more oxygen-containing functional groups, potentially creating a more porous structure with a higher surface area.

[0048] According to Figure 3 and 4, the FE-SEM images further highlight the structural distinctions between reduction step using ascorbic acid and sodium borohydride. The FESEM image of rGO synthesized with ascorbic acid shows a layered, sheet-like morphology with smooth, well-defined edges and moderate wrinkling. This indicates a gentle reduction process that preserves the layered structure with minimal porosity and defects. Such morphology is advantageous for applications requiring a balance of conductivity and structural stability, as it enhances surface area while maintaining coherence in the rGO sheets. On the other hand, rGO reduced with sodium borohydride typically shows a more porous and wrinkled morphology in Figure 2. This porous structure arises from the aggressive removal of functional groups, increasing defect density and possibly creating additional active sites. However, this increase in porosity can sometimes compromise mechanical stability due to the introduction of structural irregularities.

[0049] According Figure 5, the EDX analysis confirms the high carbon content (81.6%) expected for the rGO obtained, with a smaller percentage of oxygen (18.4%), indicating that the material is partially reduced. The remaining oxygen content suggests the presence of oxygenated functional groups, which may benefit specific applications.

[0050] The present invention may be embodied in other specific forms without departing from its essential characteristics. The described embodiments are to be considered in all aspects only as illustrative and not restrictive. The scope of the invention is therefore indicated by the appended claims rather than by the foregoing description. All changes, which come within the meaning and range of equivalency of the claims, are to be embraced within their scope.

Claims

CLAIMS1. A method of producing graphene and its derivatives from palm kernel shell (PKS), comprising the steps of:carbonizing raw palm kernel shell (PKS) in an inert environment to obtain carbonized form of PKS;subjecting the carbonized form of PKS to graphitization in the presence of catalyst and heating to obtain graphite form of PKS, followed by a multi-step washing step; oxidizing the graphite form of PKS at a temperature of less than or equal to 10 °C; exfoliating the oxidized graphite form of PKS in the presence of stirring and sonication, followed by neutralizing thereof in the presence of water and an alkali to obtain a graphite oxide suspension; andreducing the graphite oxide suspension in the presence of a weak acid, followed by another multi-step washing step, to obtain reduced graphene oxide (rGO) therefrom.

2. The method according to claim 1, wherein the step of carbonizing the raw palm kernel shell (PKS) is conducted by heating the raw PKS at a temperature ranging from about 400 °C to 900 °C in an inert environment.

3. The method according to claim 2 further comprises a step of ball milling the carbonized form of PKS into powder form.

4. The method according to claim 1, wherein the step of subjecting the carbonized form of PKS to graphitization is conducted in the presence of the catalyst and heating at a temperature ranging from about 1750 °C to 1800 °C.

5. The method according to claim 1 or 4, wherein the catalyst includes iron (III) nitrate nonahydrate.

6. The method according to claim 1, wherein the multi-step washing step involves a multiple washing process using hydrochloric acid, water and deionized water separately to remove impurities and to achieve neutralization.

7. The method according to claim 6 further comprises another step of drying and then ball milling the neutralized, graphite form of PKS into powder form.

8. The method according to claim 1, wherein the step of oxidizing the graphite form of PKS is conducted in an ice bath and the presence of an oxidizing agent in order to maintain a temperature of less than or equal to 10 °C.

9. The method according to claim 9, wherein the oxidizing agent includes sulfuric acid and potassium permanganate.

10. The method according to claim 1, wherein the alkali used includes sodium hydroxide.

11. The method according to claim 1, wherein the step of reducing the graphite oxide suspension is conducted in the presence of heating at a temperature of less than 100 °C in the presence of the weak acid.

12. The method according to claim 1, wherein the weak acid used includes ascorbic acid.

13. The method according to claim 1 or 6 further comprises another step of drying and then ball milling the rGO into powder form.

14. Graphene or derivatives thereof obtained according to the method as claimed in any one of claims 1 to 13.

15. Reduced graphene oxide (rGO) derived from palm kernel shell (PKS), wherein the rGO exhibits a Brunauer-Emmett-Teller (BET) surface area ranging from 60 to 70 m2 / g.

16. Reduced graphene oxide (rGO) derived from carbonized palm kernel shell (PKS), wherein the rGO has a carbon content of more than 80% and an oxygen content of less than 20 %.

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