Nilgiri tree peels-derived activated porous carbon for high performance supercapacitor applications

In this study, the activated porous carbon (APC-x) was collected from Nilgiri tree peels, making it a sustainable and cost-effective green synthesis approach. The raw material undergoes carbonization and activating it using KOH as chemical activation agent. The chemical composition, crystallinity and morphology of the surface of ACP-x was examined by Fourier Transform infra-red (FTIR), X-ray diffraction and Field emission scanning electron microscope (FESEM). The cyclic voltammetry technique was used to measure the electrochemical performance of APC-x using PVA/KOH gel electrolyte in two-electrode system. The highest specific capacitance value 140 F/g was obtained for APC-0 at scan rate of 10 mV/s. The use of Nilgiri tree peels as precursor for activated porous carbon production presents a sustainable approach for developing high-performance supercapacitor materials.


Introduction
Activated porous carbon derived from biomass have gained significant attention for their potential use in supercapacitor applications due to its high surface area, regulated pore size distribution, and low cost in comparison to other carbon materials [1].Recently, biomass-derived activated carbon materials are considered a sustainable and cost-effective alternative to traditional carbon materials, which are typically derived from non-renewable sources such as coal or petroleum.The production of activated porous carbon from biomass involves the pyrolysis of biomass precursor at high temperatures, follwed by the activation of the resulting carbon material to create a highly porous structure with a large surface area.The activation process can be achieved through various methods, including chemical activation, physical activation, or a combination of both [2].During the physical process, a significant amount of internal carbon is removed in order to impart well-developed carbon structures.In contrast, during the chemical activation process, dehydrating agents are utilized, which affects pyrolytic decomposition and causes the formation of tar, increasing the carbon yield [3].In this paper, we prepared activated porous carbon by a simple process including carbonization of nilgiri tree peel with a chemical activation by KOH.It shown high-performance electrodes for supercapacitor applications.Nilgiri tree peels-derived carbon source is preferred due to their low cost, extensive availability, eco-friendly and renewable properties.

Preparation of activated porous carbon derived from Nilgiri tree peels (APC-x)
Activated porous carbon was prepared by using Nilgiri tree peels as the carbon precursor, and chemical activation using KOH.The Nilgiri tree peels were first washed by using distilled water (DW) and dried at 90 ℃ for 24 h.Then, the peels were ground into a powder.After that 3 gm of Nilgiri tree peels powder were dissolved in 30 ml of 1 M KOH and heated at 100 ℃ for 15 h.The result was crushed and carbonization in a tube furnace at 450 °C in air atmosphere for 2 h.The product was filtered repeatedly with 1 M HCl and DW and dried in 100 ℃ for 10 h.Samples prepared by different mass ratio of Nilgiri tree peels powder to KOH (1:0.5 & 1:1) were labeled APC-0 and APC-1 respectively.

characterization
The surface morphology and the chemical change of the material were analyzed by Field Emission Scanning Electron Microscopy (Sigma Zeiss FESEM) and Fourier transformation infrared spectrometer (FTIR, ALPHA BURKUE model).The crystallization of the material was tested by the Rigaku Miniflex 500 (XRD).The CHI 660E model was used for electrochemical performance.

X-ray Diffraction Analysis
The XRD spectra of the activated porous carbon APC-0 and APC-1 samples are depicted in figure 1.All the samples show strong broad and weak peaks centered at 2θ values of 23.5° and 43° which ascribed to the reflection of (002) and (100) planes of graphitic carbon, respectively [4,5].These strong peak suggest that the activated porous carbon contains some degree of graphite crystallite , which can contribute to its electrical conductivity and electrochemical properties.Furthermore, the APC-1 sample shows a new sharp peak at 2θ value of 29.5° which belongs to the graphite lattice of (110) plane [6].The presence of sharp peak indicates to enhancement in crystallinity due to increase of KOH concentration.

FTIR Analysis
FTIR spectroscopy is a useful technique for analyzing the functional groups and chemical bonds present in the surface of activated porous carbon.Figure 2. shows the FTIR spectra of the APC-0 and APC-1 samles.It can be seen that the APC-0 and APC-1 samples display the FTIR characteristic bonds slightly similar.The peak around 3728 cm -1 represent the O-H stretching mode of hydroxyl groups.The strong peak centered around 1529 cm -1 associated with C-C or C-O stretching vibration in aromatic compounds [7].The presence of strong band at 2342 cm -1 ascribed to the prsesent of CO2 from the atmosphere [8].Additionally, the intensity of stretching peaks in the APC-1 sample is decreased with increase mass ratios of KOH.Moreover, the APC-0 sample shows new peak in the fingerprint region 1199 cm -1 represent C-O bonds [9].The FTIR spectrum reveals the presence of hydroxyle groups, aromatic carbon, and various oxygen functinalities in the surface of activated porous carbon samples which endow it with suitable characteristics for supercapacitor electrode.

FESEM Analysis
The surface structure and morphology of Nilgiri tree peels-derived activated porous carbon were characterized by FE-SEM.Both the activated porous carbon APC-0 and APC-1 samples displayed slight similar irregular porous structure with a smooth surface morphology as dipected in Fig. 3(a-b).Furthermore, the pore size of the APC-1 sample was increased as result to increase of mass ratio of KOH as shown in Fig. 3(b).The pore size can be controlled during the activation process, allowing for the optimization of the ion transport and charge storage capacity of the material [10].

Cyclic Voltammetry
The electrochemical properties of the activated porous carbon were tested in two-electrode system by using PVA/KOH gel electrolyte.Fig. 4(a&b) display the cyclic voltammetry (CV) curves of the APC-0 and APC-1 electrodes in the potential windows from -0.4 to 0.4 V at various scan rates from 10 to 50 mV/s.Both the electrodes a quasi-rectangular shape with a rapid current response, indicating high performance of electrical double layer capacitor [11].Additionally, the CV curves of the APC-0 demonstrate the highest current density area.The specific capacitance of APC-0 and APC-1 electrodes is 140 F/g and 110 F/g at scan rate of 10 mV/s respectively.The specific capacitance decreases with increasing the activation agent (KOH) ratio from 0.5 to 1.

Conclusions
In this study, we used Nilgiri tree peels as precursor to produce activated porous carbon (APC-x) by activation process using KOH.The characterization techniques used for activated porous carbon include X-ray diffraction (XRD), Fourier Transform infra-red (FTIR), and Field Emission scanning electron  microscopy (FE-SEM).These techniques provide information about the crystallinity, chemical composition and morphology of the APC-x surface.The APC-0 exhibits high specific capacitance of 140 F/g at scan rate of 10 mV/s.The results show that the activated porous carbon derived from Nilgiri tree peels is a promising material for supercapacitor applications due to its high specific capacitance, and environmentally friendly approach that can reduce waste and lower production costs.