Synthesis of activated carbon from black liquor for the application of supercapacitor

In the present study, black liquor carbonization has been investigated by hydrothermal process. The activated carbon from the carbonization of black liquor (AC-BL) and biomass-based activated carbon from citrus sinensis flavedos (AC-OP) has been investigated for suitability in supercapacitor application. The study has analyzed the electrochemical measurement of both AC-BL and AC-OP in electrochemical stations. The role of stable hydroxyl molecules on the surface of carbon material has been observed and its effective conductivity is studied. The superior performance of AC-OP-derived nanoporous carbon has fast ionic and electronic diffusion of the electrolyte in and out of the pores during charging and discharging due to high surface area. AC-BL exhibited with an EDLC mechanism, but AC-OP shows the pseudocapacitance property. The porous structure and oxygen doping characteristics in AC-BL can influence the potential electrode material for applications in the field of supercapacitors. With the help of this movement, the electronic conductivity of the AC-BL has been increased. In general, the electrochemical stability of the EDLC is far better than the pseudocapacitor. From the GCD analysis, it is observed that the specific capacitance of 17.4 and 148.2 F g−1 is obtained from GCD spectra for AC-BL and AC-OP, respectively. From the EIS analysis, the ESR value is very small for AC-BL (60 Ω), when compared to AC-OP (155 Ω). To conclude that the EIS results of low conductivity by AC-BL have the potential to be future supercapacitors with enhanced treatment in carbonization techniques.


Introduction
Activated carbon, also called activated charcoal, is a type of carbon handled to have little, low-volume pores that expand the surface area and are accessible for adsorption or chemical reactions. Because of its high level of microporosity, carbon has a surface area of higher than 3000 m 2 /g as identified by gas adsorption [1][2][3][4]. Activated carbon (AC) is usually derived from charcoal and is referred to as activated coal [5,6]. From the available carbonaceous materials, AC is the capable candidate for supercapacitor, owing to the high specific surface area and low cost. Although in AC, more carbon atoms cannot be contacted with the electrolyte which directed the ineffectiveness of carbon atoms. Similarly, the electrical conductivity of AC restricted the application in high power density devices [6][7][8][9]. Initially activated carbons from biomass are produced from bamboo, coir, lignite, rice husk, coconut shell carbon, coconut tree sawdust carbon and from agricultural by-products, peels of banana [7] and orange [8]. Black liquor is a by-product of the kraft pulping process which is directly used as a liquid fuel with the substituent, while burning black liquor can emit pollutants such as CO 2 and SO 2 . There is a recent development in the production of activated carbon from black liquor [10]. In this development, the different methods used for the production of activated carbon are hydrothermal, pyrolysis, and hydride oxygenation processes [10][11][12][13][14].
By removal of color and impurities, Lignin and cellulose substitutions of the black liquor can convert into carbonization material, in which the properties of black liquor as represented in Table 1 have high organic compounds that are the main source for carbonization. In the carbonization process, organic compounds are converted into saturated carbons as carbon compounds. The process of carbon bonds at high pressure and temperature under a closed system at 160-300°C can produce activated carbon from the sources of rice husk, cottonseed, and coconut shell [11][12][13][14].
Kyle McGaughy et al. [15] conducted hydrothermal carbonization of food waste at 200, 230, and 260°C for 30 min. He found that fixed carbon content has increased from 18.8 to 22.4 with an increase in temperature. The product hydrochar is mainly used as solid fuel and carbon storage. Suteerawattananonda et al. [16] conducted hydrothermal carbonization on rice husk at 180-250°C and 12-42 bar. He found that the fixed carbon percentage has doubled by increasing the temperature. XRF analysis shows that reduction in alkali and alkali earth materials has decreased. Fernandez ME et al. [17] conducted hydrothermal on an orange peel. The hydrochar was chemically activated with phosphoric acid. The characteristic shows that it adsorbs three pharmaceuticals diclofenac sodium, salicylic acid, and flurbiprofen. Kyle McGaughy et al. [15] conducted hydrothermal on paper mill sludge at 180, 220, and 260°C. The hydrochar produced will have the same combustion indices as that of the coal. It is used to be a solid fuel. Similarly, Toufiq Reza et al. [18] conducted hydrothermal on wheat straw at 180-260°C for 2-8 h. Up to a temperature of about 220°C, the digestate contains primarily cellulose and lignin. After reaching a temperature of 260°C, the cellulose is completely converted to carbon and lignin-rich hydrocarbon.
Hydrothermal carbonization on sugarcane bagasse was carried out by wet torrefaction by Wei-Hsin Chen et al. in which it has been carried by water or dilute sulfuric acid [19]. The bagasse has a higher heating value which raises to 20.3%. Microwaves are used to raise the temperature of the solution and maintain the temperature. Microwave treatment on the samples offered higher number active sites which are further expected to provide more channels for speedy ion transport and enhance the electrolyte diffusion into the electrodes [6][7][8]. Additionally, this microwave approach highly influenced the electrochemical performances of the material [20]. When compared to the normal samples, the microwave annealed samples exhibited a good value of specific capacitance and small equivalent resistances [21]. The calorific value of biomass can be calculated. Lee et al. processed hydrothermal carbonization on leather waste at a temperature of 180-200°C [22]. The temperature gives a good yield, higher heating value, and high quality of solid fuel. The produced hydrochar has lower sulfur content which is a pollutionfree energy source. It reduces the pollution due to leather waste. Elaigwu et al. studied hydrothermal carbonization on rapeseed husk at 150-200°C [23]. The microwave heating is carried out with the help of water. When the residence time and temperature increase, the yield gets decreased. The energy quality of produced hydrochar gets increased. The process is zigzag for a time of about 20 min. For further increase in residence time, the process gets steady.
The treatment of activated carbon is used for the removal of such impurities over surface atoms to wash out. The environmental application of activated carbon is used for groundwater remediation, drinking water filtration, air purification, and medical applications [24][25][26]. However, the majority of activated carbon is used as solid fuel, membrane, and electrodes. Similarly, graphene oxide is one of the preferred choices for EDLC-based supercapacitors [27] and like metal oxide-based graphene composite in recent research publications [28][29][30][31][32][33][34][35]. Activated carbon derived from biowaste exhibited higher specific surface area and longer life than graphenebased supercapacitor [35][36][37][38][39][40] Carbon black is mainly focused on its high specific surface area and EDLC property. A metal oxide is not sufficient to give that much specific surface area. And, also metal oxide supported the pseudocapacitance. Even though the pseudocapacitance is good, still the life cycle is very poor compared to carbon black. For electrochemical supercapacitors, activated carbon obtained from black liquor is still new. In this work, the carbon is extracted biomass (citrus sinensis flavedos) and black liquor, which is the source material for the electrodes used in the supercapacitor. The carbon is then activated using activating agents like sulfuric acid. The stability of carbon of the samples is analyzed using TGA and FTIR. By cyclic voltammetry, galvanostatic charge-discharge measurement (GCD) and electrochemical impedance spectroscopy (EIS), electrochemical measurements for the application of super-capacitor were investigated.

Experimental setup
Black Liquor is base in nature. The pH value of the black liquor is 14. It is above the pH level of 7. Black liquor is neutralized by adding the dil.H 2 SO 4 . Due to the addition of acid, the lignin separation takes place. The black liquor is changed to dark brown color. The color is changed due to the precipitate present in the black liquor. After that, the filtration process is carried out using filter paper. The lignin is separated from the black liquor in the filtration process. Carbonization is the process of conversion of dead or alive organic matter into carbon products. It converts organic matter to carbon residue by destructive distillation. The hydrothermal process takes place mainly at 180 to 300°C. The carbon bonds present in the materials get destructed and residue carbon is obtained as a product. The filtered product is taken in Teflon-lined reactor. The hydrothermal reactor is placed inside the furnace at a temperature of 200°C for about 24 h. In this process, the carbon-carbon bond present in black liquor starts to break due to pressure and temperature developed inside the reactor. The black liquor is taken outside after 24 h from the furnace. The carbonized black liquor converts into a solid-state by a drying process named AC-BL. The AC-BL is kept inside the hot air oven for the drying process at a temperature of 150°C for about 4 h. After completing the process, the AC-BL is converted into activated carbon and it is stable. The sample is characterized and its properties are studied.
Fresh orange peels (citrus sinensis flavedos), named AC-OP, were purchased from the local shop and sliced with appropriate dimensions for simple activation. The pre-carbonized orange peel was grinded and sieved with particles less than 1 mm. These samples were carbonized at 600°C in a muffle furnace in a nitrogen gas environment, at a constant flow rate of 1 L per minute for a period of 3 h. This material is kept overnight under the CO 2 atmosphere. The collected carbon was grounded into powder and rinsed with 1 M of H2SO4 named AC-OP. Then, the samples were dried at 60°C for 24 h. Finally, the chemically activated carbon is neutralized to 7 pH after washing with deionized water.
The hydrothermal reactor consists of Teflon-lined 100 ml capacity stainless steel cylindrical system. It can work under autogenic pressure and with a maximum temperature of 210°C. The Teflon lining has 4 mm thickness. Figure 1 shows the pictorial representation of the hydrothermal reactor.
The Perkin-Elmer spectrum one FTIR was used to analyze the activated carbon for both AC-OP and AC-BL. Drops of liquid samples are placed between two plates of salts made of potassium chloride or sodium hydroxide without any air-lock. The wavelength between 400 and 4000 cm -1 is fixed and data are interpreted for all the liquid samples. This technique is used widely for studying the infrared spectrum for liquids, gas, or solids. During FTIR analyses, mainly four different groups were studied due to the presence in the samples. Thermogravimetric analyses (TGA) were performed for two samples for each AC composition using a model TGA Q 500 V2010 Build 36 TA Instrument System. After drying at 60°C, the AC was placed inside a cylindrical steel mold with 5.5 mm in diameter and 10 mm in length. The mass variation as a function of temperature was carried out in air at a heating rate of 10°C/min from RT to 800°C. Its design integrates a thermobalance engineered for maximum baseline flatness and high sensitivity, with the power and flexibility of an infrared furnace, and a proven horizontal purge gas system. Electrochemical properties were measured by cyclic voltammetry using Origalys electrochemical work station at room temperature (25°C). The graphite rod is considered the working electrode. To prepare the working electrode, 0.025 mg AC is mixed with the rubber solution and made slurry. Then, the slurry is coated on a graphite rod. A Pt wire and Ag/AgCl electrode are used as counter and reference electrodes, respectively. Cyclic voltammetry is recorded in 0.5 M H 2 SO 4 aqueous solution at a scan rate of 30 mVs -1 . For analyzing the performance of the supercapacitor, the electrode is prepared as reported in the previous study. An equivalent series resistance and charge transfer resistance of the AC-BL and AC-OP samples are measured by electrochemical impedance spectroscopy.

Results and discussion
The carbonization of black liquor is carried in Teflonlined hydrothermal reactor and orange peel has been carbonized directly in the muffle furnace. The carbonization of activated carbon as AC-BL is represented in Fig. 2. The carbonized materials are dried to hold less than 2 wt% moisture before being subjected to activation with 0.5 M H 2 SO 4 . The C-O bond of lignocellulose breaks at low enthalpy by leaving aldehyde or acid or alcohol, or by leaving all others. The intensity in the sample at 1614, 1578, and 1514 cm -1 indicates aromatic/cyclic compounds with hydroxyl or carbonyl group. Also, there is arene compounds formation from the coupling of a-carbon radical and carbonyl group which are either formed from ester dissociation or phenolic compounds. The aromatic/cyclic compounds, as indicated in the wavelength of 1058 cm -1 , confirmed the presence of b-hydroxyl carbonyl compounds. Table 2 reports the band assignment in AC-BL and AC-OP.
From the EDS analysis, it was observed that 85.9 wt% of carbon, 7.5 wt% of oxygen, and the rest of the traces of potassium, sodium, and magnesium were in the carbonized sample. The higher carbon content  TGA is an instrument used to conduct chemical properties like thermal decomposition, chemisorption, and solid-gas reaction and physical properties like a phase transition, absorption, adsorption, and desorption. TGA can be used for material characterization through analysis of characteristic decomposition patterns. In TGA, the composition and thermal stability of the material can exhibit weight loss due to decomposition, dehydration, and oxidation. Figure 6 shows the AC-OP in the TGA investigation. Here, the sample is heated in an environment. The weight loss in the sample has drastically declined to 90 wt% until 70°C and, further, increase in temperature shows a linear decrease manner from 90 to 80 wt% from 70 to 700°C. Similarly, Fig. 7 shows the multistage decomposition in the sample AC-BL observed at 100 and 150°C in the TGA graph.
The weight losses in the TGA result indicate that this might be due to different processes such as decomposition, evaporation, reduction, and desorption. In the first phase at 100°C, the respective weight loss in TGA can appear as water evaporation and desorption of the hydroxyl compounds. In the second phase at 150°C, the respective weight loss might be the decomposition of alkaloids and desorption of allyl and cyclic hydrocarbon. By interpreting with FTIR and TGA, it is proved that alkaloids are present over the surface of the AC-BL sample.

Electrochemical measurements
To perform an electrochemical analysis, either two electrodes or three-electrode system is preferred. Here, the steps involved in working electrode preparation are discussed. To estimate the electrochemical performance, the electrode should be prepared using the samples and mixed with rubber solution in the proportionate ratio of 60:40. The prepared mixture looked like slurry and it was coated on a graphite pencil rod using a brush. Finally, it was dried at room temperature for the required duration. The amount of material coated on the lead is 0.025 mg, approximately. The electrolyte for the analysis process chosen was 1 M H 2 SO 4 . The electrochemical analysis was tested in the potential window of -1 to 1 V. CV, GCD, and EIS analyses are carried out in electrochemical workstation (Origalys, France). Here, Ag/AgCl is the reference electrode, Pt is the counter electrode and carbon black is used as the working electrode.
From the three-electrode system, CV results of the samples using 0.5 M H 2 SO 4 solution as an electrolyte are shown in Fig. 8 with the range of -1 to ? 1 V at a scan rate of 30 mV s -1 . For an ideal supercapacitor, a CV curve with rectangular nature is important. AC-BL exhibited an EDLC mechanism and AC-OP showed the pseudocapacitance. The redox peaks movement of the sample AC-OP is observed at the positive voltage, due to the samples obtained from the activation of orange peels. With the help of this movement, the electronic conductivity of the sample is getting increased. In general, the pseudocapacitive material exhibited oxidation and reduction humps. Here, AC-OP exhibited both the EDLC and pseudocapacitive mechanism. The chemical bonding of the samples significantly enhances the current level and generates oxidation and reduction peaks. In general, the electrochemical stability of the EDLC is far better than the pseudocapacitor. Owing to electrochemical property of microwave-treated material, it exhibited high electrochemical performance. In GCD, during the charging/discharging process, the presence of mesopores smoothens the progress of the rapid transport of ions and migration of ions. GCD curves of AC-BL and AC-OP are shown in Fig. 9 with a potential range of -0.45 V to ? 0.7 V and ? 0.3 V to 0.45 V at a current density of 0.05 A g -1 . It is observed that the specific capacitance of 17.4 and 148.2 F g -1 is obtained from GCD spectra for AC-BL and AC-OP, respectively. From the GCD curves, it was observed that AC-OP exhibited a low potential window. At the same time, the voltage drop is very less for AC-OP, when compared to AC-BL. AC-BL exhibited near triangular GCD curves, with EDLC and a little bit of pseudocapacitive effect.
Impedance spectra of AC-BL and AC-OP are shown in Fig. 10. In the high-frequency zone, the EIS curve is intercepted with the real axis ascribed to Equivalent series resistance (ESR). From the EIS analysis, the ESR value is very small for AC-BL (60 X), when compared to AC-OP (155 X). Similarly, the charge transfer resistance of 40 X and 19 X is observed for AC-BL and AC-OP. Increased R ct for AC-BL is due to the poor conductivity of the electrode and an electrolyte used. AC-OP exhibited closer to Warburg resistance in the low-frequency zone suggested the level of ion diffusion.
For the sample AC-BL: Rs = 62.34 X Rct = 32.97 X For the sample AC-OP: Rs = 156.7 X R ct = 8.04 X The specific capacitance of various electrodes was given with R s and R ct in Table 3.

Conclusion
The role of stable hydroxyl molecules on the surface of carbon material has been observed and its effective conductivity is studied. The superior performance of AC-OP-derived nanoporous carbon has fast ionic and electronic diffusion of the electrolyte in and out of the pores during charging and discharging due to high surface area. AC-BL exhibited with an EDLC mechanism, but AC-OP shows the pseudocapacitance property. The porous structure and oxygen doping characteristics in AC-BL can influence the potential electrode material for applications in the field of supercapacitors. With the help of this movement, the electronic conductivity of the AC-BL has been increased. In general, the electrochemical stability of the EDLC is far better than the pseudocapacitor. From the GCD analysis, the specific capacitance of the AC-BL and AC-OP exhibited 17.4 and 148.2 F g -1 , respectively. From the EIS analysis, the ESR value is small for AC-BL (60 X), when compared to AC-OP (155 X). To conclude that the EIS results of low conductivity by AC-BL have the potential to be future supercapacitors with enhanced treatment in carbonization techniques.