Utilization of biomass wastes: coconut and Pangium edule shells as activated carbon for energy storage device material

. In this research, activated carbon (AC) was synthesized from biomass wastes of coconut and Pangium edule shells and utilized as a material for electrochemical double-layer capacitors (EDLC), which are eco-friendly energy storage devices. This research was intended to bridge the need for greenhouse gas-free energy storage device and the handling of abundant biomass wastes. These efforts would undoubtedly contribute to mitigating climate change. To begin the research, the coconut and Pangium edule shells underwent carbonization at varying temperatures of 600°C and 700°C for 2 hours. Subsequently, they were subjected to chemical activation using KOH and physical activation at varying temperatures of 110°C and 600°C. Some characterization techniques, including SEM, XRD, TGA/DSC, BET, Iodine number, and proximate analysis, were employed to analyze the materials. The capacitive properties of EDLC electrodes were assessed through cyclic voltammetry (CV). After carbonization at 700°C and subsequent physical activation at 600°C, the coconut and Pangium edule shells exhibited the highest active surface area of 548.542 m 2 g -1 and 333.4 m 2 g -1 , respectively. Notably, the EDLC demonstrated a maximum specific capacitance of 364.5 Fg -1 at 2 mVs -1 scan rate. These findings indicate the viability of utilizing AC from biomass waste as a promising material for EDLC applications.


Introduction
Coconut holds a prominent position in Indonesia's agricultural sector.In 2021, the country achieved a substantial output of coconut products, totaling 2.85 million tons from both public and private plantations.This figure saw a further uptick in 2022, reaching 2.86 million tons.Notably, there has been a consistent average annual growth rate of 0.14% in production up to the year 2026 [1].As coconut production increases, it is inevitable that there will also be a rise in biomass waste, including coconut shells and fruit husks Furthermore, Pangium edule, known as 'kluwek' in the Indonesian language, is a fruit product commonly used as an ingredient in traditional dishes.Similar to coconut shells, the shells of Pangium edule also generate biomass waste.The coconut ad Pangium edule shells are organic materials consisting of various components, including cellulose, hemicellulose, and lignin [2].Lignin, a high-molecular-weight polymer, acts as a binder, lending strength to plant cells [3].
Activated carbon, a type of charcoal, is created by subjecting it to activation processes involving CO2 gas, water vapor, or chemicals.These processes create open pores, greatly improving its capacity to absorb substances such as color and odor.Activated carbon typically contains 5 to 15 percent water, 2 to 3 percent ash, with the remaining composition being carbon.It is commonly available in granular or powdered forms [4].
The adsorption capacity of activated carbon is heavily dependent on its pore structure, consisting of micropores (< 20 Å), mesopores (20 -500 Å), and macropores (> 500 Å) [5].Different raw materials and processing methods result in diverse pore distributions.Activated carbon rich in micropores excels in adsorbing small molecules, such as gases with low contamination levels.In contrast, activated carbon with a predominance of macropores is well-suited for adsorbing larger molecules and is especially effective in decolorization processes [6].
In general, the production of activated carbon comprises three primary stages: a. Dehydration: This initial step involves heating raw materials to approximately 170°C to remove moisture.b.Carbonization: Organic matter is transformed into carbon during this phase, typically occurring at temperatures ranging from 400 °C to 900 °C.The resulting carbon is then cooled, washed, and processed to remove activating chemicals.Temperatures exceeding 170°C can lead to the formation of CO, CO 2, and acetic acid.c.Activation: During this stage, pores in the carbon structure expand as tar decomposes.Activation can be achieved using either steam or CO2 as activators.The activation process can be achieved through either chemical or physical (thermal) treatments [7].
The electric double-layer capacitor (EDLC) functions by establishing electrical double layers between activated carbon and the electrolyte, effectively acting as a dielectric.During charging and discharging cycles, ion absorption and desorption processes occur in both activated carbon electrode layers.When voltage is applied to the electrodes, ions migrate to the electrode surfaces during charging and move away during discharging.[8].
The research on activated carbon from biomass wastes have been widely reported with various results.However, the usage of Pangium edule was rarely reported.In addition, the research on the activated carbon-based EDLC have been reported by many papers.For example, Deng et.al (2022) reported EDLC from coconut shell-based activated carbon which resulted in the highest active surface area of 2410 m 2 g -1 and the highest specific capacitance of 395 Fg -1 [9].
In this research, we synthesized activated carbon through both physical and chemical activation processes, employing coconut and Pangium edule shells as raw materials.Subsequently, we utilized this activated carbon to create EDLC electrodes and quantified their specific capacitances.These endeavors aimed to establish a connection between harnessing biomass from coconut and Pangium edule shells and fulfilling the requirements of environmentally sustainable energy storage devices.

Experimental Details
The coconut and pangium edule shells were initially washed and crushed into flakes sized between 0.1-3 mm.Carbonization was conducted in a horizontal furnace at temperatures of 700°C and 800°C, with a 2-hour holding time.The samples were placed in alumina crucible during this process, and nitrogen gas was introduced to ensure uniform pore size in the activated carbon.Following carbonization, the carbon samples were pulverized using a blender until they reached a consistency that could pass through a 120-mesh sieve.
In the chemical activation process, charcoal and KOH were combined in a glass beaker, with distilled water added in a 1:1:4 ratio of water, carbon, and KOH.This mixture was then heated and stirred on a magnetic stirrer hot plate at 80°C for 4 hours, with a stirring speed of 200 rpm.Afterward, sedimentation and washing steps were carried out until the pH approached neutrality.
For physical activation, the carbon deposits resulting from the chemical activation process underwent hydrothermal heating at varying temperatures between 110°C and 600°C, each held for 4 hours.This involved placing the still-moist activated carbon mixture into a crucible, which was then inserted into an autoclave and heated in a furnace.

Result and Discussion
Table 1 presents the proximate analysis results of raw materials from coconut and Pangium edule shells, following ASTM D 1762-84 standards.Notably, both samples exhibited similar fixed carbon contents exceeding 74%, signifying their high carbon content.Fig. 1 illustrates a noticeable distinction between the activated carbon samples produced at different activation temperatures, namely 160°C (a) and 600°C (b).It is evident that the samples activated at 600°C exhibit a more amorphous structure, as indicated by their less intense peaks.These XRD peaks align with the JCPDF 75-1621 pattern for carbon, which is characterized by a hexagonal structure.Specifically, the three peaks observed at approximately 2θ = 26°, 45°, and 81° correspond to the (002), (101), and (112) crystallographic planes, respectively.Fig. 2 displays SEM images of activated carbon obtained from coconut and Pangium edule shells.These materials were activated at two different temperatures, namely 160 °C and 600 °C, and subsequently carbonized at 600 °C and 800 °C.The corresponding pore sizes can be found in Table 2. Notably, carbonization at 800°C yielded a combination of mesopore and macropore structures.In contrast, carbonization at 700°C resulted in a macropore structure.BET Total Volume (cm 3 g -1 ) BET Active Area (m 2 /g) Table 2 provides data on the iodine number, BET total volume, and active surface area for all activated carbon samples.Generally, samples activated at 600°C exhibited higher iodine numbers, BET total volumes, and BET active surface areas compared to those activated at 110°C.Furthermore, activation at 700°C led to higher iodine numbers, BET total volumes, and BET active surface areas compared to activation at 800°C, due to consistently larger pore diameters.The highest BET active surface areas recorded were 548.542 m²/gram and 333.4 m²/gram, observed in the case of coconut shell and Pangium edule shell, respectively, carbonized at 700°C and activated at 600°C.
According to SNI-06-3730-1995, activated carbon must adhere to specific parameters.These include a maximum water content of 15%, a maximum ash content of 10%, a maximum weight loss at 950°C of 25%, and an Iodine number of 750 mg/gram.Additionally, activated carbon is defined by an active surface area ranging from 300 to 3000 m²/gram.Based on the data presented in Table 1 and Table 2, it can be concluded that the activated carbon materials were obtained through a physical activation process at a temperature of 600°C (b, d, f, h in Fig. 2 and Table 2).Figure 3 illustrates the cyclic voltammogram (CV) of the EDLC constructed with activated carbon derived from coconut shells.The presence of a rectangular, mirror-like CV curve is a distinctive hallmark of EDLC behavior.In Table 3, we present specific capacitance values at various scan rates.Notably, an increase in scan rate correlates with a reduction in specific capacitance.Slower scan rates afford more time for charges to be stored and released from the less accessible electrode material sites, resulting in higher specific capacitance values.At a scan rate of 2 mV/s, the highest specific capacitance was achieved, measuring 364.5 F/g.This figure is comparable to the result reported by Deng et al. (2022), which stood at 395 F/g, despite their significantly larger active surface area of 2410 m²/g.[9].

Conclusion
Activated carbon material has been successfully synthesized from both coconut shell and Pangium edule shell, which underwent carbonization at varying temperatures of 700°C and 800°C.Subsequently, they were chemically activated using KOH and physically activated at 600°C.Among these processes, the highest values for key parameters defining activated carbon were achieved by the coconut shell material carbonized at 700°C and physically activated at 600°C, resulting in an active surface area of 458.542 m²/gram.In terms of its electrochemical performance, the highest specific capacitance of the electric double-layer capacitor (EDLC) electrode constructed from coconut shell-activated carbon reached 364.5 F/g at a scan rate of 2 mV/s.While this value may appear moderate, it remains suitable for specific applications.

Fig. 1 .
Fig. 1.The XRD patterns of the activated carbon resulted from (a) activation temperature of 160 C and (b) activation temperature of 600 C for coconut and Pangium edule shells which carbonized at 700 C and 800 C, respectively.

Fig. 2 .
Fig. 2. SEM Results for Coconut Shell Activated Carbon (a, b, c, d) and Pangium edule Shell (e, f, g, h).SEM test results for activated carbon derived from both coconut shell and Pangium edule shell under various carbonization and physical activation conditions: Carbonization Temperature 700°C, Physical Activation Temperature 110°C (a, e) and 600°C (b, f) Carbonization Temperature 800°C, Physical Activation Temperature 110°C (c, g) and 600°C (d, h).

Table 2 .
Pore diameter, Iodine number, BET total volume, and BET active surface area.

Table 3 .
The specific capacitances of EDLC from coconut shell -based activated carbon.