The carbonization of black liquor is carried in Teflon lined hydrothermal reactor and Orange peel has been carbonized directly in the muffle furnace. The carbonization of activated carbon as AC-BL is represented in Figure 2. The carbonized materials are dried to hold less than 2 wt moisture before subjected to activation with 0.5 M H2SO4.
The wavelength of 1614 cm-1 indicates the carbonyl structure either -(CO)H or –(CO)-. The aldehydes or ketones implies at 2923 cm-1 which show a broader spectrum indication. At this broadband wavelength of 2730 and 1710 cm-1 shows the formation of carbonyl acids that was due to the hydroxyl bond over the carbon atom. The FTIR result in Figure 3 and 4shows that wavelength from 3010 to 2800 cm-1 has a broad intensity. This more widespread intensity denotes the presence of a high concentration of carbon.
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 1614,1578and 1514 cm-1 indicates aromatic/cyclic compounds with hydroxyl or carbonyl group. Also, there is arene compounds formation from the coupling of α-carbon radical and carbonyl group 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 β-hydroxyl carbonyl compounds. Table 2 reports the band assignment in AC-BL and AC-OP.
Table 2.Wavenumber observed in activated carbon in FTIR on transmission mode on both AC-BL and AC-OP
Absorption Range (cm-1)
|
Appearance
|
Group
|
Compound Class
|
3430
|
Strong
|
-CH2-
|
Hydrocarbon
|
2923
|
Strong
|
-CH2-
|
Hydrocarbon
|
2853
|
Strong
|
-CH2-
|
Hydrocarbon
|
1614
|
Strong
|
C=O Stretching
|
Aldehyde
|
1578
|
Medium
|
C=O Stretching
|
Aldehyde or Ketone
|
1467
|
Weak
|
C-H Bending
|
Alkane
|
1378
|
Weak
|
C-H Bending
|
Aldehyde
|
1212
|
Strong
|
C-O Stretching
|
Alkyl aryl Ether
|
1133
|
Strong
|
C-O Stretching
|
Aliphatic Ether
|
1090
|
Strong
|
C-O Stretching
|
Aliphatic Ether
|
TGA is an instrument used to conduct the chemical properties like thermal decomposition, chemisorption and solid-gas reaction and physical properties like phase transition, absorption, adsorption and desorption.TGA can be used for materials characterization through analysis of characteristic decomposition patterns.In TGA the composition and thermal stability of the material can be exhibits weight loss due to decomposition, dehydration and oxidation. Figure 6 show the AC-OP in TGA investigation. Here, the sample is heated in an environment. The weight loss in the sample have drastically declined to 90 wt% until 70oC and, further increase in temperature show a linear decrease manner from 90 to 80 wt% from 70 to 700oC. Similarly, Figure 7 shows the multistage decomposition in the sample AC-BL observed at 100 and 150oC in TGA graph.
The weight losses in TGA result indicate that this might be due to different processes such as decomposition, evaporation, reduction and desorption. First phase at 100oC the respective weight loss in TGA can be appeared as water evaporation and desorption of the hydroxyl compounds. Second phase at 150oC the respective of weight loss might be the decomposition of alkaloids and desorption of allyl and cyclic hydrocarbon. By interpretating 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 2 electrode or 3 electrode system is preferred. Here, the steps involved for working electrode preparation is discussed. To estimate the electrochemical performance, electrode should be prepared using the samples, mixed with rubber solution in the proportionate ratio of 60:40. The prepared mixture looked like slurry and it was coated on graphite pencil rod using brush. Finally, it was dried at room temperature for the required duration. The amount of material coated on the lead is 0.1 g, approximately. The electrolyte for the analysis process chosen was 1 M H2SO4. The electrochemical analysis was tested in the potential window of -1 to 1 V. CV, GCD and EIS analysis are carried out in electrochemical workstation (Origalys, France).
From the three electrode system, CV results of the samples using 0.5 M H2SO4 solution as an electrolyte is shown in Fig. with the range of -1 to +1 V. For an ideal supercapacitor, CV curve with rectangular nature is important. AC-BL exhibited an EDLC mechanism and AC-OP showed the pseudo capacitance. There dox 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, electronic conductivity of the sample is getting increased. In general, electrochemical stability of the EDLC is far better than pseudo capacitor. Owing to, electrochemical property of microwave treated material, it exhibited high electrochemical performance.
In GCD, during the charging/discharging process, the presence of meso-pores smoothens the progress of the rapid transport of ions and migration of ions. GCD curves of AC-BL and AC-OP is shown in Figure 8 with a potential range of -0.45 V to + 0.7 V and +0.3 V to 0.45 V at current density of 0.05 A g-1. It is observed that, the specific capacitanceof 89.28 and 133.92F g-1obtained from GCD spectra for AC-BL and AC-OP respectively. From the GCD curves it was observed that, AC-OP exhibited 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 Figure 9. In the high frequency zone, EIS curve is intercepted with the real axis ascribed to equivalent series resistance (ESR). From the EIS analysis, ESR value is very small for AC-BL (60 Ω), when compared to AC-OP (155 Ω). Similarly, the charge transfer resistance of 40 Ω and 19 Ω is observed for AC-BL and AC-OP. Increased Rct for AC-BL is due to the poor conductivity of electrode and an electrolyte used. AC-OP exhibited closer to Warburg resistance in the low frequency zone suggested the level of ion diffusion.