Significant Potential of Cocos nucifera L. Bunches Waste as Carbon Source for Sustainable Electrode Material Supercapacitors

The interest in utilizing biowaste materials to produce carbon source, especially for electrode materials in physicochemical energy storage devices like Li-ion batteries and supercapacitors, is driven by concerns regarding energy cost-effectiveness and environmental pollution. This study specifically concentrates on the utilization of waste from Cocos nucifera L. bunches as an eco-friendly source of carbon source designed for supercapacitor electrodes. The precursors chosen in this study were old Cocos nucifera L. bunches and young Cocos nucifera L. bunches. The preparation involves chemical activation of the waste using a 0.5 M potassium hydroxide solution at 900°C, with pyrolysis occurring in an nitrogen and carbon dioxide gas environment. The resultant activated carbon, configured as monolithic coins, retains its structure by optimizing the bonding characteristics of the precursor, eliminating the necessity for supplementary adhesives. Every coin-shaped monolith displays carbon structures ranging from turbostratic to highly amorphous. Electrochemical testing in a symmetric supercapacitor showed old Cocos nucifera L. bunches a high specific capacitance of 262 F g−1 at a constant density of 1.0 A g−1. Furthermore, the maximum energy density was determined to be 3.54 Wh kg−1 at an optimal power density of 85 W kg−1 in 1 M H2SO4 aqueous electrolyte. Utilizing waste, combined with a straightforward preparation process and remarkable electrochemical properties, positions carbon source derived from aged Cocos nucifera L. bunches as a prospective and sustainable electrode material for energy storage applications in supercapacitors.


Introduction
The coconut (Cocos nucifera L.) is a monocotyledonous plant affiliated with the Palmaceae family, capable of yielding fruit up to 13 times annually.With a lifespan spanning 60 to 70 years, coconut plantations wield significant influence on numerous islands, particularly those lining tropical coastlines.Globally renowned as one of the paramount tree species, the coconut holds immense economic importance due to the indispensability of its fruit products for human survival.Many farmers and countries in the Eastern region depend on coconut as their main source of income in both tropical and subtropical countries, especially because it is used in various forms of products such as food and beverages, building materials, and energy and is exported to other countries around the world [1].At a worldwide production scale of 12 million tons, where 3 million tons are exchanged each year, coconut oil continues to maintain its status as the foremost significant commercial product [2].Innovative food products made from coconut as well as from oil extraction residues, such as meal and flour, have been developed or are already available on the market, which can significantly reduce production costs.It is thought that the coconut plant originates from the coastal areas of the Indian Ocean on the Asian side, but has now spread throughout the tropics, especially in the Asia-Pacific Region.Countries from the Asian region that produce coconuts in large quantities are the world's largest coconut producers.Meanwhile, Indonesia holds the title of being the world's leading producer of coconuts, consistently generating an average of 2,822,600 tons annually.This makes coconut one of the agricultural commodities that contribute most to the country's foreign exchange earnings with an export volume of 2.11 million tonnes in 2020 and an export value of 1.17 billion.Between 2016 and 2020, the average coconut trade volume balance increased by 7.68% per year, and the coconut trade balance surplus in 2020 reached 1.12 billion (Coconut Commodity Trade Performance Analysis, 2021).
The high production of coconut plantations and market demand for primary products and their derivatives tend to be accompanied by environmental impacts from harvest waste and their production waste.Traditionally, this waste is handled by simple methods such as burning in open fields.In addition, the parts of waste that can be recycled are used as by-products, especially the coir, sticks and trees.Furthermore, their biomass-dominated waste rich in lignocellulosic components shows great potential for other quality products or even advanced technologies.A potential approach involves utilizing their biomass residue to transform it into active carbon or porous carbon materials, specifically for applications related to energy storage, such as supercapacitors [3], [4].
Extensive research has been conducted on diverse components of the coconut, serving as a carbon source for supercapacitor applications.Notably, coconut shells have been thoroughly examined and verified for their potential in functioning as supercapacitor devices with remarkable capacitance (403.8F g -1 at 1.0 A g -1 ), and density of energy and power respectively 378.7 Wh kg -1 and 5,299.2W kg -1 [5].Furthermore, studies on coconut water as a supercapacitor material have also been reported by Natake et al., 2021 with a specific capacitance performance of 782.7 F g -1 , energy density of 12.29 Wh kg -1 and power density of 1600 W kg -1 [6].Meanwhile, coconut fiber biomass shows the quality of their carbon for energy storage in two electrode configurations with a capacitance of 184 F g -1 , energy density, and power density are 6.4 W h/kg and 46.17 kg-1 [7].Recently, Rongke et al., 2023 studied the behavior of coconut fiber-based carbon as a supercapacitor material, where they found a remarkably enhanced capacitance of 634 F g -1 at a density of 10.73 kg -1 [8].Several examined studies have revealed that coconut bunch waste remains an unexplored subject.To the best of our knowledge, there is currently no documentation or research paper highlighting the potential of this waste as a carbon material for applications in supercapacitors.
Therefore, this study focuses on the synthesis of coconut bunches as a solid carbon source as an electrode component in supercapacitor applications.Coconut bunch waste was selected using two physical conditions such as young coconut bunches (YCB) and old coconut bunches (OCB).A series of synthetic treatments apply a simple approach with chemical activation and physical activation processes.The porosity behavior of the solid carbon designs was evaluated through their density degradation during the pyrolysis process.Electrochemical performance was studied intensively through cyclic profiles and galvanostatic profiles with the highest capacitance found at 262 F g -1 in the two-electrode system.

Materials
Coconut bunches as biomass was selected in young (green) and old (brown) conditions.The chemical and physical activating ingredients are focused on potassium hydroxide solution and carbon dioxide gas obtained from Merck KGA and Samator Indonesia.Nitrogen gas was chosen as the inert gas in the carbonization process provided by Aneka Gas Tbk.The acid medium is provided in concentrated H2SO4 solution from Quimica S.A.U.Test cells and other necessary components are available at the Materials Physics and Nanotechnology Laboratory, Riau University.

Preparation of activated carbon from coconut bunches
Firstly, coconut bunches were randomly chopped to obtain precursors measuring ≤1.5cm 2 .Next, the biomass is washed until clean using distilled water under running conditions.This process is repeated periodically until the elements of soil, sand and other impurities are completely removed.The drying processes are implemented gradually through exposure to sunlight and oven drying.The hydro content undergoes a substantial reduction by subjecting the precursor to heat within a vacuum container at a temperature ranging from 200 to 250°C.This procedure renders the precursor comparatively fragile and facilitates its transformation into powder.The powder obtained was homogenized by milling for two days.After that, the fine powder was sieved to a 250 mesh size.The sifted powder precursor is immersed in potassium hydroxide solution for 2 hours.Their precipitate is dried in an oven and ground again.In the subsequent phase, a dry powder combined with potassium hydroxide undergoes compression using a hydraulic device at a pressure of 8 metric tons, resulting in the formation of a uniform solid precursor design without the inclusion of external adhesives.This step is carried out periodically to obtain the required precursor design.This homogeneous solid precursor design is inserted into the furnace tube for pyrolysis.The pyrolysis procedure is executed by integrating both carbonization and physical activation processes into a single stage.Carbonization is set at a temperature increase rate of 1-3 deg/min with an optimum temperature of 600°C.Meanwhile, The physical activation procedure takes place within the temperature interval of 601-900°C, with an increment in temperature of 10 deg/min.Finally, the product that has been obtained is neutralized in a solution in water for 4 days.

Characterizations
Assessment of the characteristics of the initial carbon material involves examining the dimensional alterations in the created design.Changes in dimensions of the homogeneous solid carbon design in the pyrolysis treatment are reviewed through measurements of mass, thickness and diameter which are then calculated into precursor density using standard equations.Density (ρ) is calculated by equation ( 1): (1) ρ is the density (g cm -3 ), m is the precursor mass (g) and V is the volume (cm 3 ) of the homogeneous solid carbon design.The designs are correlated to form tubes so that their volumes can be calculated with standard tube equations.
The assessment of electrochemical characteristics was conducted through cyclic voltammetry (CV) measurements in an acidic environment, covering a potential range from 0 to 1 V, and adjusting the scan speed between 1 to 5 mV s -1 .Specific capacitance is obtained through specific equations that have been widely reported previously [9].The test system was chosen in the configuration of two solid electrodes bounded by a thin membrane made of egg shell.The current collector is shaped similarly to the electrode surface from stainless steel.In-depth confirmation of the capacitive properties, energy output and power was carried out using galvanostatic charge discharge (GCD) measurements.The specific capacitance, along with the calculated energy and power, is assessed using established formulas [10].

Result and discussions
The carbon material design is considered one of the important factors in determining the efficiency of the best electrode approach for solid-state supercapacitor applications.In this study, carbon materials based on young and old coconut bunches (YCB and OCB) were systematically designed to be a homogeneous solid resembling a thin tube (coin) without any adhesive [11], [12].This design is made at the beginning of the treatment before the pyrolysis process has been determined.Therefore, it is important to evaluate the dimensional reduction of mass, thickness, and diameter of the carbon material design.Their reduction is accumulated through density calculations which are closely related to mass divided by volume, according to formula (1).Before pyrolysis treatment, the two carbon materials that had been prepared showed the same density of 0.98 g cm -3 .The selected pyrolysis involves a sequentially connected combination of carbonization and physical activation processes.Carbonization at room temperature up to 600°C in a nitrogen atmosphere tends to hydrolyze volatile, hydrous compounds and is followed by the decomposition of lignocellulose from lignin, cellulose, and hemicellulose.This series of evaporation and decomposition significantly reduces the dimensions of the carbon material design through the release of carbon dioxide vapor, carbon monoxide, and water vapor.As a residue of their reactions, the precursor materials produce ash and tar in carbon.The refinement of the process for converting carbon material persists through a physical activation phase within a temperature range of 600°C to 900°C, conducted in a carbon dioxide gas atmosphere.This stage is instrumental in substantially eliminating any remaining tar and ash in the carbon material.Furthermore, the interaction between carbon and CO2 facilitates the etching of carbon chains, leading to the emergence of new pores, enlargement of narrow pores, and expansion of wider pores.Moreover, the chemical reaction of the KOH activating agent also plays a major role in providing empty spaces in carbon materials.Their reaction occurs at 600°C causing KOH to etch the carbon chains in the precursor.The combination of these steps reduces the density of carbon material based on young and old coconut biomass, as shown in Figure 2. It is interesting to discuss that the density degradation of young coconut bunches carbon material (YCB) is relatively different from carbon material based on old coconut bunches (OCB).This event is closely related to the degeneration of the lignocellulose ratio in coconut biomass.Young coconut bunches precursors tend to be rich in strong cellulose with a high density so their decomposition is relatively more difficult.This allows for relatively less density reduction in carbon material designs.Meanwhile, old coconut bunches tend to have lignocellulosic components that are relatively more fragile, the cellulose is significantly degenerated thus that their dimensional reduction is relatively greater.Precursor of OCB showed a density degradation of 44.5% and YCB indicated a density reduction of 25.6%.These findings demonstrate that the carbon material derived from coconut bunches exhibits exceptional porosity, enhancing the efficacy of the electrode as an electrochemical energy storage device.

Figure 2. Density reduction of YCB and OCB precursor
Identification of the potential of young and old coconut bunch-based carbon materials for the application of electrochemical energy storage devices is comprehensively reviewed by cyclic voltammetry measurements, as shown in Figure 3. Cyclic performance is confirmed at 1 mV s -1 with a voltage range of 0 to 1V in the acidic medium of H2SO4 in solution 1 mol/L.both precursors show identical cyclic patterns resembling rectangular leaves adopting normal double-layer electrochemical properties.In more detail, the YCB material illustrates a strength cyclical shape with small loop area widths confirming poor porosity.This analysis correlates with their density being reduced by around 25%. Specifically, marginal rise in current density at lower potentials signifies the existence of a limited number of smaller pores.Moreover, the substantial surge in current density at higher potentials is intricately linked to the prevalence of meso and macro pores.Therefore, the specific capacitance shown is on the tens scale of 44.9 F g -1 in a symmetric two-electrode system.Meanwhile, the OCB material reveals a relatively ideal cyclic shape with large hysteresis loops identifying a smooth and unimpeded infiltration process of ions to form an abundant electric layer.As a consequence, their energy-power storage capacity is relatively much greater than that of YCB materials.In two solid electrodes, the specific capacitance of OCB increases rapidly to about three times higher than that of YBC to 149.4 F g -1 at 1 mV s -1 .Moreover, the cyclic behavior of OBCs is confirmed also with different scanning rates from 1 to 5 mV s -1 (Figure 4), indicating their quite good capacitive retention with a specific capacitance of about 87 Fg -1 at 10 mV s -1 .The ion insertion-desertion process from the acid electrolyte tends to be evenly distributed in the OCB material.In addition, they also display relatively fast ion accessibility compared to YCB materials.

Figure 4. Cyclic profile of OCB in different current density
A more thorough analysis is needed to confirm the superior electrochemical characteristics of OCB materials.Hence, galvanostatic charge-discharge measurements were conducted using a system comprising two solid electrodes, with alterations in current density.The consistent electrolyte medium chosen is the acid solution H2SO4.As depicted in Figure 4, the GCD profiles at modest current densities of 1, 2, and 5 A g -1 displayed a triangular pattern, aligning with notable pseudocapacitance and electrical layer features, in accordance with the outcomes of the CV analysis.Meanwhile, an evaluation of the voltage drop was also carried out, which were 0.138, 0.275, and 0.320 Ω respectively (see Figure 5).At elevated current densities of up to 5 A g -1 , the OCB substance exhibits a consistent curve pattern, indicative of robust stability.This pattern aligns with a substantial pseudocapacitance attributed to the dopant oxidation reaction within the material.A pronounced decrease in voltage (iR drop), at various current densities signifies the resistance arising from the electrolyte and electrode contact.The lowest resistance of OCB material was recorded at 0.138 Ω at 1 A g -1 .The factor that influences this phenomenon is the precursor surface area ratio.The creation of micropores at elevated temperatures impacts the accessibility of charge at higher current densities, which is reflected in Figure 3.In addition, OCB materials with mesoporous pores show lower resistance at high current densities due to smoother ion acceleration.The assessment of the OCB material's coulombic efficiency was extended by considering the insertion and relaxation times of ion charge in the electrode, as illustrated in Figure 5.At a current density of 1 A g -1 , the coulombic efficiency achieved a notable 84.6%.This coulombic efficiency confirms the dominance of EDLC in materials with pseudocapacitance features.The distribution of pores in the material is a key factor in confirming this phenomenon.The accessible pores along with the presence of potential dopants in the material produce an excess ion effect in the solid cylinder that exhibits pseudocapacitance properties, which is reflected in Figure 5.Standard analysis shows the capacitive capacity of the OBC at 1 A g -1 is 262 F g - 1 , Affirming the superior performance of OCB as an electrode material for supercapacitors, particularly in a two-electrode setup.The specific capacitance was also computed at current densities of 2 and 5 A g -1 .Surprisingly OCB materials can maintain their high capacitive capacity of 262 F g -1 with retention reaching 61.45%.This phenomenon is due to the OCB material having a strong pore structure due to high-temperature pyrolysis combined with the chemical activating agent KOH and the amazing growth of porosity, as has been confirmed in the density degradation in Figure 2.
Thorough investigation into the energy storage capability and output power characteristics of solidstate supercapacitor systems was conducted using the Ragone plot method, as illustrated in Figure 6.OCB materials confirm their extraordinary potential as supercapacitor electrode materials with an energy storage capacity of 3.4 Wh kg -1 with a power output of 85 W kg -1 .The obtained results in this investigation demonstrate a level of competitiveness comparable to other studies employing materials derived from biomass, as summarized in Table 1.Finally, this study has clearly confirmed the potential of coconut bunch waste as a carbon material for supercapacitor electrodes with a simple treatment involving pyrolysis and chemical activating agent through solid material design without adding adhesives.

Conclusion
This study thoroughly affirms the recognition of the potential of coconut bunch waste as a carbon source, structured into a uniform solid without requiring an external binder for the electrode material in the context of electrochemical energy storage device applications.Coconut bunch precursor focus on the differences between young bunches and old bunches.A variety of innovative methods have been employed, including the amalgamation of carbonization and activation processes into a singular stage, along with the creation of uniform solid carbon materials without the necessity of external binders.Surprisingly, OCB precursor shows high carbon material quality compared to YCB.Density measurements showed a reduction in their dimensions of 25% and 44%, respectively.Confirmation of the electrochemical properties through cyclic performance shows the specific capacitance of YCB and OCB of 44.9 and 149.9 F g -1 , respectively.Further confirmation via GCD has recorded that the OCB material significantly has an energy output of 3.4 Wh kg -1 with a power output of 85 W kg -1 .Therefore, this research furnishes insights into the viability of coconut bunch biomass as a unified solid carbon source, eliminating the need for external binders in electrode materials for applications in electrochemical energy storage devices.

Figure 1 .
Figure 1.Preparation of porous activated carbon based on Cocos nucifera L

Figure 3 .
Figure 3.The cyclic profile of YCB and OCB in 1 mV s -1

Figure 6 .
Figure 6.internal resistance, coulombic efficiency, and specific capacitance in different current density

Table 1 .
Comparison of electro-chemical performance of supercapacitor derived from different biomass.