BIBECHANA Vol. 20, No. 1, April 2023, 10–20 ISSN 2091-0762 (Print), 2382-5340 (Online) Journal homepage: http://nepjol.info/index.php/BIBECHANA Publisher:Dept. of Phys., Mahendra Morang A. M. Campus (Tribhuvan University)Biratnagar Porous activated carbon materials from Triphala seed stones for high-performance supercapacitor applications Chhabi Lal Gnawali1,∗, Sabina Shahi2, Sarita Manandhar3, Ganesh Kumar Shrestha1, Mandira Pradhannanga Adhikari2, Rinita Rajbhandari1, Bhadra P. Pokharel1 1Department of Applied Sciences and Chemical Engineering, Pulchowk Campus, Institute of Engineering (IOE), Tribhuvan University, Lalitpur, Nepal 2Central Department of Chemistry, Tribhuvan University, Kirtipur, Kathmandu, Nepal 3Tri-Chandra Multiple Campus, Kathmandu, Nepal ∗Corresponding author. Email: chhabig123@ioe.edu.np Abstract Porous activated carbon materials derived from biomass could be the suitable materials for high-rate performance electrochemical supercapacitors as it exhibits high surface area due to the development of nanopores. Here, we report the novel porous activated carbon from Triphala seed stones by chemical activation with zinc chloride at different carbonization tem- perature (400-700 0C) under the nitrogen gas atmosphere. The activated carbon was charac- terized by Fourier transform-infrared (FTIR) spectroscopy, Raman scattering and scanning electron microscopy (SEM). Nitrogen adsorption-desorption measurements was used to study the surface properties (effective surface areas, pore volumes and pore size distributions). The electrochemical measurements were performed in an aqueous 1 M sulphuric acid (H2SO4) so- lution in a three-electrode cell set up. The specific surface area of Triphala seed stones-derived porous carbon materials with well-defined micro- and mesopores ranges from 878.7 to 1233.3 m2 g−1 and total pore volume ranges from 0.439 to 0.626 cm3 g−1. The specific capacitance obtained by electrochemical measurement experiment was 208.7 F g−1 at 1 A g−1. These results indicate that the prepared nanoporous activated carbon material from Triphala seed stones would have significant possibility as supercapacitor electrode material for high-energy- storage supercapacitor applications. Keywords Triphala seed, chemical activation, zinc chloride, nanoporous, electrochemical measurement, super- capacitor. Article information Manuscript received: March 21, 2023; Accepted: March 28, 2023 DOI https://doi.org/10.3126/bibechana.v20i1.53432 This work is licensed under the Creative Commons CC BY-NC License. https://creativecommons. org/licenses/by-nc/4.0/ 10 http://nepjol.info/index.php/BIBECHANA chhabig123@ioe.edu.np https://doi.org/10.3126/bibechana.v20i1.53432 https://creativecommons.org/licenses/by-nc/4.0/ https://creativecommons.org/licenses/by-nc/4.0/ Chhabi Lal Gnawali et al./ BIBECHANA 20 (2023) 10-20 11 1 Introduction In 21st century, because of urbanization, indus- trialization as well as the rapid growth of pop- ulation, the energy storage and the environmen- tal pollution are considered thoughtful challenges worldwide. The continuous depletion of fossil fuel, climate change due to excessive greenhouse gases, high energy cost leads the society towards renew- able as well sustainable energy sources so it is es- sential to develop the sustainable energy storage system with high power density and energy den- sity. Nowadays, the electrochemical energy storage systems like lithium-ion batteries, fuel cells, solar cells, supercapacitors are widely used in electron- ics and electrical vehicles [1–4]. So, researchers have keen interest on supercapacitors with ultra- high power density, fast charging discharging rates, superior cycle life, low internal resistance, environ- ment friendly, low cost, safe and easy operations [5–8]. Supercapacitors, also known as electrical double-layer capacitors (EDLCs), store charges at the surface of electrode material by means of electri- cal double layers via electrolyte-ion diffusion from electrolyte solution to the electrode. The perfor- mance of supercapacitors depends upon the elec- trode material. The carbon black, carbon nan- otubes, glassy carbon, activated carbon-the carbon- based materials and the transition metals are com- monly used for electrode materials [9]. In general, the commercial activated carbons prepared from fossil fuel-based precursors which are nonrenew- able, expensive and environmentally non-friendly hence the researchers have great interest to pre- pare the activated carbon from the lignocellulosic materials. The biomass precursors are renewable, cheaper, easily available, environment friendly and highly porous materials. Because of high specific surface area, well-developed pore size distributions, nanoporous activated carbons can be used as the suitable material for the supercapacitors to increase the specific capacitance [10–12]. In recent years, re- searchers have used various biomass materials and industrial wastes like lapsi seed, argan seed, peanut shell, sugarcane bagasse, wheat husk, etc. [13–19] as a raw material for the preparation of nanoporous activated carbon that can be used in the waste wa- ter treatment as well as the energy applications. They are renewable, locally available resources in large quantities as well as cost efficient and eco- friendly. Triphala is well recognized polyherbal medicine comprises Harro (Terminalia Chebula), Barro (Ter- minalia bellirica), and Amala (Phyllanthus em- blica) have found potentially effective in Ayurvedic medicine and even used for treating and prevent- ing from cancer [20,21]. However, no research work observed on Triphala seed stones for the prepara- tion of activated carbon but Phyllanthus emblica seed-derived hierarchically porous carbon materi- als for supercapacitor applications is recently re- ported [22]. Because of being the pillar of Ayurvedic medicine, biomass materials, low cost which are lo- cally available in different regions of Nepal, there is particular interest towards the study of preparing a nanoporous carbon material from Triphala seed stones. In this contribution, we report the production of AC from Triphala seed stones by chemical ac- tivation method using zinc chloride as an activat- ing agent to study the performance of supercapac- itor. The Triphala seed powder was mixed with zinc chloride in 1:1 weight ratio and carbonized at different temperatures (400, 500, 600 and 700 0C) for 4 hours under the constant flow of ul- trapure nitrogen gas. Thus, prepared AC ma- terials were named on the basis of carbonization temperature as MxC-Z400, MxC-Z500, MxC-Z600 and MxC-Z700 respectively. The control sample is prepared by direct carbonization at 500 0C and named as MxP-500. Raman spectroscopy, Fourier- transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and nitrogen adsorp- tion/desorption isotherms are used to characterize the prepared activated carbons. The electrochem- ical energy-storage supercapacitance performance of the novel nanoporus activated carbon prepared from Triphala seed stones by ZnCl2 activation also reported. The obtained AC shows high surface area 1233.3 m2g−1, and pore volume 0.626 cm3g−1 achieving excellent specific capacitance of 208.7 F g−1 at 1 A g−1. These results indicate that Triphala seed, a Bio-waste material, has the potential for the production of porous carbon materials to improve the performance of supercapacitors and may be use- ful to control the energy crisis in Nepal. 2 Materials and Methods 2.1 Preparation of activated carbon The samples acquired from local vendors were peeled and seed stone separation was done. The ob- tained seed stones were cleaned with distilled water several times before drying in an oven for 24 hours at 50 0C. After that, the seed stones were mechani- cally broken into powder and sieved through a mesh size of 300 µm for the precursor. The MxP-500, the control sample refers to directly carbonized carbon that was synthesized in a tube furnace under an in- ert atmosphere of nitrogen gas for four hours at 500 0C. The activated carbon was prepared through chemical activation using ZnCl2 as an activating agent in 1:1 ratio on the powder precursor, and the Chhabi Lal Gnawali et al./ BIBECHANA 20 (2023) 10-20 12 effects of different carbonization temperature (400, 500, 600, and 700 0C) were investigated. The car- bonization process was carried out in a tube fur- nace (Accumax India) with a constant supply of ni- trogen gas. After cooling, the carbonized products were treated with 5 % HCl and then with distilled water until the supernatant liquid reached a pH of 7, and then dried at 1000C for three hours. The activated carbons prepared at carbonization tem- peratures 400, 500, 600, and 700 0C were labeled as MxC-Z400, MxC-Z500, MxC-Z600, and MxC-Z700, respectively. 2.2 Characterizations of Triphala seed- derived carbon materials The surface area and textural properties of the mix- ture precursor powder and the activated porous car- bon materials were investigated by different meth- ods. Precursor powder was subjected to a ther- mogravimetric analysis (TGA) on a SII instrument (Model Exstar 600) from 25 to 10000C with a temperature ramp of 10 0C min−1 in a nitrogen gas atmosphere. Raman spectroscopy (NRS-3100, JASCO, Tokyo, Japan), Fourier-transform infrared (FTIR) spectroscopy (Nicolet 4700, Thermo Elec- tron Corporation, Waltham, MA, USA), scanning electron microscopy (SEM: S-4800, Hitachi Co., Ltd. Tokyo, Japan operated at 10 kV) and nitrogen adsorption/desorption isotherms (Quantachrome Autosorb-iQ2, Boynton Beach, FL, USA) were used for instrumental characterization. The nitrogen ad- sorption/desorption isotherms estimated the total pore volume, pore size distribution, and specific surface area. Using Barrett-Joyner-Halenda (BJH) and density functional theory (DFT) method, pore diameters and volumes were determined. For ni- trogen adsorption isotherm, about 35 mg of sample was placed in a glass cell and degassed at 120 0C for 24 hrs before recordings. 2.3 Electrochemical supercapacitance per- formance studies The electrochemical supercapacitance of Triphala seed-derived activated carbon was studied by using cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical- impedance-spectroscopy (EIS) measurements in a three-electrode-cell set-up in aqueous 1 M H2SO4 electrolyte solution. The glassy-carbon electrode (GCE) was used as the working electrode. The fine carbon powder was added to a mixture of water and ethanol (2 mL: 4:1 v/v ratio) and sonicated for 60 min to obtain a dispersion of carbon (2 mg mL−1. Certain volume of the suspension (3 µL) was dropped on a clean and dry GCE, and dried at 600C for 2 hrs to solvent evaporation. The mass of the active electrode-material for each system was 3 × 10−6 g. Nafion solution (5 µL: 5% in ethanol) was used as the binder, after that the working elec- trode was dried at 80 0C for 2 hrs under vacuum. The CV, GCD, and EIS measurements were per- formed on a CHI 660E workstation (CH Instru- ments, Inc. Austin, TX, USA) by taking Platinum wire as the counter electrode, and Ag/AgCl as the reference electrode. The GCD curves obtained from the chronopotentiometry measurements is used to calculate the specific capacitance (Cs) of the elec- trode materials as Cs = I × td m×∆V (1) where, I (A) indicate the discharge current, td(s) is the discharge time, m (g) is the mass of active ma- terial on the GCE electrode and (V) represent the operating voltage. 3 Results and Discussion 3.1 TGA and FTIR analysis The pyrolytic seed decomposition of mixture seed powder was studied by thermogravimetric analysis. The TGA curve in figure 1a shows the recording from 25 to 1000 0C under nitrogen gas atmosphere that indicates the pyrolysis process of precursor oc- curred in three stages. In the first stage, weight loss happens as a result of evaporation of moisture being locked in the sample at temperatures below 250 0C. At the second stage, there is a significant weight loss between 250 and 370 0C, which is caused by the py- rolytic destruction of the cellulose and hemicellulose components along with partial decomposition of the lignin content. Precursor slowly breaks down into carbon components in the third stage; hence there is no considerable weight loss above 370 0C. FTIR analysis of powder (figure 1b) indicates the presence of several oxygenated surface functional groups of precursors corresponding to cellulose, hemicellulose and lignin. A broad FTIR peak at 3337 cm−1 re- lates to the O–H stretching (str.) of the moisture water and/or alcoholic group of the cellulose. At the same time, bands at 2929 and 2850 cm−1 de- note the cellulose’s aliphatic C-H (str.). The band at 1730 cm−1 corresponds to the unsaturated ester’s C=O (str.) [23]. In addition, the absorption band at 1587 and 1229 cm−1 arise from the aromatic C=C stretching and the C–O stretching in lignin, respec- tively. The band at 1322 cm−1 can be associated with the C-H symmetric deformation that is typical in cellulose and hemicelluloses [24, 25]. The bands in the regions 1028 cm−1 are due to alkoxy C-O (str.) or other C-O (str.) in cellulose, hemicellulose or lignin. Due to the high temperature carboniza- tions, the FTIR bands corresponding to the oxy- genated surface functional group in the ZnCl2 acti- vated carbon samples substantially diminished [26]. Chhabi Lal Gnawali et al./ BIBECHANA 20 (2023) 10-20 13 Figure 1: (a) TGA curve of Triphala seed powder (precursor); (b) corresponding FTIR spectrum. 3.2 Nitrogen adsorption desorption isotherm By using nitrogen adsorption desorption isother- mometry, the surface textural characteristics of pro- duced carbon samples were investigated. The ad- sorption isotherm of carbons activated at various temperatures is shown in Figure 2a. All of the car- bon materials showed higher nitrogen adsorption at low relative pressures and significant hysteresis at high relative pressures. These isotherms combine type I and type IV, which suggests that the micro- and mesopore structures are hierarchical [27]. High nitrogen uptake may be attributed to the filling of micropores at low relative pressures (P/P0), but the hysteresis loop in the region of high relative pressures may be caused by capillary condensation taking place in the mesopores. While nitrogen up- take at low relative pressure appeared to increase, the integral area of the hysteresis loop decreased as carbonization temperature increased from 400 to 700 0C, indicating that there were more microp- ores present at higher temperatures and fewer meso- pores in the carbon framework. Large surface areas offered by these micro- and mesopore structures fa- cilitate dye molecule dispersion on the carbon sur- face during adsorption [28]. The pore size distri- bution curves obtained by DFT and BJH analysis is shown in figure 2b and 2c respectively. Table 1 provides a summary of the surface textural prop- erties of ZnCl2-activated Triphala seed carbon as determined by nitrogen adsorption experiments. 3.3 Raman spectra The image displays two distinct Raman bands in the spectra. The defect or disordered phase of car- bon may be responsible for the first peak (D-band) at 1350 cm−1 and the graphitic phase of carbon may be responsible for the second peak (G-band) at 1598 cm−1 . Although the G-band reflects the stretch- ing vibration of sp2 hybridized carbon atoms in the graphitic layer, the D-band represents the vibra- tion of sp3 hybridized carbon atoms in disordered graphitic structure [29]. Table 1: Surface textural properties of Triphala seed carbon at different carbonization temperature (SSA=total specific surface area, Smic=micropore surface area, Smes=mesopore surface area, Vp=total pore volume, Vmic=micropore volume, Vmes=mesopore volume, WH=average half-pore width of micro- pores, Dp=average pore diameter of mesopores) System SSA(m2/g) Smic(m2/g) Smes(m2/g) Vp(cc/g) Vmic(cc/g) Vmes(cc/g) WH (nm) Dp(nm) MxC- Z400 1137.7 1087.2 50.5 0.6 0.54 0.06 0.286 3.66 MxC- Z500 1233.3 1195.1 38.2 0.626 0.574 0.052 0.286 3.66 MxC- Z600 1143.8 1082.7 61.1 0.617 0.537 0.08 0.286 3.66 MxC- Z700 878.7 836.9 41.8 0.439 0.384 0.055 0.286 3.67 Chhabi Lal Gnawali et al./ BIBECHANA 20 (2023) 10-20 14 Figure 2: (a)Nitrogen adsorption-desorption isotherms of the carbon samples, (b) pore size distributions calculated using the DFT method and (c) pore size distributions calculated from the BJH method The degree of graphitization of carbon materials is estimated by the help of ratio of the intensities of the G and D bands (IG/ID). Decreasing crys- tallinity is indicated by an increase in the IG/ID ratio. The produced carbon samples exhibit IG/ID ratios between 1.05 and 1.29, which point to the development of amorphous graphitic carbon with few structural defects. As the carbonization tem- perature is raised from 400 to 700 C , the IG/ID ratio falls from 1.15 to 1.05, indicating an increase in graphitic carbon. Electrical conductivity, specific area, and pore size are all impacted by the degree of graphitic structure, which impact on increasing the performance of the supercapacitor electrode [22]. 3.4 SEM analysis The surface morphology of the mixture seed car- bons was investigated using SEM images. SEM im- ages show irregularly shaped and sized pore struc- ture with microporous channels on their surfaces. Most carbon particles are between a range of few and several tens of microns in size. The MxP- 500 has very less surface porosity (Figure 4a-b) which in agreement with nitrogen sorption data. The mesopore structure of MxP-500 is hardly visi- ble in the high-resolution SEM image (Figure 4b). The ZnCl2 activation results increases the surface porosity (Figure 4d, f, g, h, i, j). The nitro- gen adsorption isotherm results correspond with the SEM pictures of the ZnCl2− activated samples, which show a large number of macroporous channel- like structures whose frameworks comprise of mi- cro/mesopores, indicating the formation of hierar- chical pore structures. While mesopores are numer- ous, micropores are not easily visible in the ZnCl2− activated samples (Figure 4d, f, g, h, i, j). 3.5 Electrochemical measurements The high surface area and the hierarchically porous structure of the prepared sample motivate to calcu- late the supercapacitance performance. The com- parison of CV profiles of MxP-500, MxC-Z400, MxC-Z500, MxC-Z600 and MxC-Z700 performed at 50 mVs−1 is shown in figure 5a. The quasi- rectangular profiles of all the samples exhibit the EDLC-type energy storage mechanism. The weak redox peaks at 0.2-0.4 V for MxC-Z400 CV curve indicate the partial contribution of pseudocapaci- tance to the EDLC because of presence of the oxy- gen functionalities in the carbon material [22]. The low value of total internal current of CV curve in MxP-500 sample is due to the lack of porosity. The current output of the MxC-Z700 sample is higher which indicates the sample MxC-Z700 stored the Chhabi Lal Gnawali et al./ BIBECHANA 20 (2023) 10-20 15 Figure 3: Raman scattering spectra of MxP-500, MxC-Z400, MxC-Z500, MxC-Z600, and MxC-Z700. Figure 4: SEM images of (a, b) MxP-500; (c, d) MxC-Z400; (e, f) MxC-Z500; (g, h) MxC-Z600; and (i, j) MxC-Z700. highest energy among all the samples. By using Galvano-static charge-discharge (GCD) at different current densities (1, 2, 3, 4, 5, 10, 20, 30, 40, 50 A g−1), the electrochemical charge stor- age mechanism was studied. Figure 6a shows the GCD profiles of all the samples at 1 A g−1, in- cluding the reference carbon, which are of triangu- lar form confirming an EDLC-type charge storage mechanism. From the above result, we see that the reference sample MxP-500 shows the lowest charge discharge time whereas the MxC-Z700 shows the highest charge discharge time which confirms the maximum charge storage capacity because of the nanoporous structure [30–32]. The GCD curves of all the samples have the triangular form and re- mains even at high current density of 50 A g−1), that confirm the effective ion diffusion mechanism at interiors of the nanoporous carbon materials at higher current densities. Figure 6e shows the spe- cific capacitance as the function of current density for all the samples. From this graph, we see that the specific capacitance of direct carbonized sample is very low as expected because of its low porosity while that of MxC-Z700 is superior at all current densities from 1 to 50 A g−1. The specific capaci- tance was calculated by using equation (1). To investigate the diffusion kinetics of the electrolyte ions, electron-transfer resistance and double-layer charging at the electrode-electrolyte interface, the electrochemical impedance spec- troscopy (EIS) was conducted [33]. Figure 7 shows the Nyquist plot of prepared nanoporous activated carbon materials. The plots of ZnCl2 activated car- bon samples shows the lack of semicircular compo- nent indicating ideal EDLC type charge transfer. The steep rise in the low frequency region indicates the rapid ion transfer in the electrode. The series resistance of the electrode is given by the x-axis in- tercept of the imaginary component of impedance, which is close to 4.9 Ω for the activated carbon. This low value of series resistance confirms the im- portance of interconnected nanoporous structure to increase conductivity during ion transfer and al- most similar for all carbon samples, which indicates the surface-textural properties governed the energy- storage capacity of the electrodes. 4 Conclusion In conclusion, we have successfully synthesized nanoporous activated carbon materials from novel material, Triphala (Terminalia chebula, Termina- lia bellirica, and Phyllanthus emblica) by chemi- cal activation with zinc chloride (ZnCl2) at dif- ferent carbonization temperatures (400, 500, 600 and 700 0C). Surface textural properties show that the carbonization temperature play the significant role on porosity. The surface area of the pre- pared novel carbon materials from Triphala ranges from 878.7 to 1233.3 m2 g−1 and the pore vol- ume ranges from 0.439 to 0.626 cm3 g−1. Also, we have studied the electrochemical supercapaci- Chhabi Lal Gnawali et al./ BIBECHANA 20 (2023) 10-20 16 Figure 5: Electrochemical -energy-storage performance by CV measurements: (a) CV curves at fixed potential sweep of 50 mVs−1; and CV curves at various sweep-rates (5, 10, 20, 50, 80, 100, 200, 300, 400 and 500 mVs−1) for (b) MxP-500; (c) MxC-Z400; (d) MxC-Z500; (e) MxC-Z600; (f) MxC-Z700 Chhabi Lal Gnawali et al./ BIBECHANA 20 (2023) 10-20 17 Figure 6: (a) GCD profiles of all the samples at current density of 1 A g-1, CD curve of (b) MxP-500, (c) MxC-Z600, (d), MxC-Z700 at different current densities from 1 to 50 A g−1 and (e) specific capacitance as a function of current density for all samples. Chhabi Lal Gnawali et al./ BIBECHANA 20 (2023) 10-20 18 Figure 7: EIS Results: (a) Nyquist plots of different carbon materials (MxP-500, MxC-Z400, MxC-Z500, MxC-Z600 and MxC-Z700) in 1 M H2SO4 electrolyte from 0.01 Hz to 100 kHz at an amplitude of 5 mV (b) the corresponding magnified plot. tance performance of the prepared nanoporous ac- tivated carbon in aqueous electrolyte (1 M H2SO4) in a three-electrode system. Because of their large surface area, large pore volume and interconnected mesopore structure, the prepared sample exhibit the excellence specific capacitance of 208.7 F g−1 at 1 A g−1. From these results we can conclude that the bio-waste material like Triphala seed stones could be the potential source for the preparation of nanoporous activated carbon that will be suitable for the preparation of electrode material for high performance energy-storage supercapacitors. 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Introduction Materials and Methods Preparation of activated carbon Characterizations of Triphala seed-derived carbon materials Electrochemical supercapacitance performance studies Results and Discussion TGA and FTIR analysis Nitrogen adsorption desorption isotherm Raman spectra SEM analysis Electrochemical measurements Conclusion