3.1 Morphological and structural analysis
As shown in Fig. 1, there are two diffraction peaks at 21° and 27°, corresponding to the (100) and (011) diffraction peaks of silica[12]. The intensity of both peaks decreases with the increasing of activing temperature, indicating that SiO2 reacts with KOH at high temperature to form soluble K2SiO3. The reduction of silica would be beneficial for improving the electrical conductivity of the prepared activated carbon. Additionally, there are two peaks at 22° and 44° in the figure, corresponding to the (002) and (100) diffraction peaks of graphite[13, 14].
The Raman spectra of the RC and RC-X (X = 110, 130, 150, 170) samples are shown in Fig. 2a. The D-peak at 1380 cm− 1 corresponds to the amorphous structure and the G-peak at 1660 cm− 1 is related to the sp2 hybrid carbon atoms in the graphite[15]. The ID/IG represents the degree of defects in the material[16]. The larger ratio implies the more defects. The graphitization of the actived RC-X (X = 110, 130, 150, 170) was less than that of RC. Furthermore, the graphitization of the RC-X samples fistly decrease and then increases with the increasing of the activation temperature. The reason may be that KOH reacts with the more reactive sp2 carbon in addition to SiO2, and with the improving of the activation temperature, KOH can further react with the sp2- sp3 carbon[17] during the activation process. The peak area of each structure in Fig. 2b corresponds to its relative content.
Table 1
The pore stucture data of RC and RC-X (X = 110,130,150,170)
Samples | SBET(m2/g) | Vtotal(cm3/g) | Vmeso(cm3/g) | Vmeso/Vt(%) | Dava(nm) |
RC | 276 | 0.340 | 0.309 | 73.321 | 4.690 |
RC-110 | 607 | 0.782 | 0.292 | 37.340 | 5.272 |
RC-130 | 716 | 0.563 | 0.425 | 75.488 | 5.351 |
RC-150 | 787 | 0.906 | 0.807 | 89.073 | 5.395 |
RC-170 | 687 | 0.779 | 0.364 | 46.727 | 5.700 |
Figure 3a shows the adsorption and desorption isothermal curves of RC and RC-X (X = 110, 130, 150, 170) in the relative pressure range of 0–1.0 V. By comparison, they all belong to type IV (IUPAC) with slit-like mesoporous pores, which can offer the channels for ion transport[18]. According to the adsorption and desorption curves in (a) combined with the BET theoretical model, the specific surface areas of the RC and RC-X (X = 110,130,150,170) attains 276, 607, 716, 787 and 687 m2/g, respectively, indicating that KOH can activate and etch the RC, furhter creating abundant pores[19, 20]. From the data in Table 1 and Fig. 3b, it can be seen that the total specific surface areas of the RC-X (X = 110, 130, 150, 170) samples show a trend from rise to decline. Further, RC-150 possesses the largest surface area, pore size, pore capacity and meso pore ratio among the RC and actived samples. The pore sturture provides advantageous condition for storing and transparting charges[21]. During the activation process, the carbon and KOH redox reaction form the high surface areas and the porous strucuture[22]. The KOH solution etching carbon material[23] is mainly shown in three aspects: opening previously closed pores, forming new pores, and enlarging the existing pores. Some processes of the three aspects play the main roles at the different temperuture. The high temperature is beneficial to produce more macropores and even cause the carbon skeleton to collapse[24].
Figure 4 shows the weight loss processes of the RC and RC-X (X = 110, 130, 150, 170). It can be seen from the curves that all samples exhibit a slight weight loss ratio of less than 5% below 200°C, which caused by a slight dehydration of the samples[25]. RC has a significant weight loss ratio of 26% between 510–720°C. RC-X (X = 110, 130, 150, 170) have significant weight loss ratios of 72%, 77%, 88% and 83% in the range of 500–720°C, proving that the carbon starts to burn at about 500°C and reacts completely at about 720°C. As can be seen from Fig. 4a, the higher the activation temperature, the lower the final mass of residual material, which proves the better effect of eliminating impurities. Comparing to RC, the increased weight loss of RC-X (X = 110, 130, 150, 170) samples could be due to the exsiting of oxygen-containing functional groups and the defective carbon structure formed during the activation process of KOH. Figure 4b shows the DTG curve, RC-150 exhibits the largest area of weight loss[26], mainly due to producing CO, CO2 and H2 during the high-temperature activation process of KOH solution[27].
As shown in the Fig. 5a and Fig. 5b, it can be found that there are numerous inorganic oxide microspheres and obvious mesopores on the surface of the lamellar RC. Figure 5c and Fig. 5d shows the SEM images of the sample activated at 150℃. It can be observed that many pore channels exist and the inorganic oxide microspheres decrease.The rich pore strucutre could be attributed to these overflowing gases produced from KOH reacting with active carbon[28, 29]. The results indicates the effective activation of KOH.
The X-ray photoelectron spectra of the RC and RC-150 are shown in Fig. 6a. The main elements of RC and RC-150 are C, O and the characteristic peaks include the C1s peak at 284.8 eV, and the O1s peak at 532.3 eV[30]. Figure 6b shows the fine fractions of C1s show C-C/C = C (284.8 eV), C = O (286.4 eV), C-O (285.4 eV), and O = C-OH (289.1 eV)[31]. Figure 6c shows the fine spectrum of O1s of the RC-150. By split-peak fitting, the O1s spectrum can be fitted to four peaks of C-O, O-H, C = O and O = C-O located at 530.1 eV, 532.1 eV, 531.8 eV and 534.1 eV[32], which proves that there are some oxygen-containing functional groups on the carbon surface of RC-150. It can be seen from Fig. 6d that the content of C and O increases, and the content of Si、Al、Ca decreases after KOH activation, which could improve the conductivity and hydrophilicity of the material. The result indicates that the method can effectively remove inorganic impurities and decorate oxygen-containing functional groups[33].
To further verify the change of surface hydrophilicity of the prepared RC-150 material, the contact angle tests have been performed with KOH aqueous solution at room temperature. The smaller the angle, the stronger the hydrophilicity of the material[34]. The results are shown in Fig. 7. The contact angle of RC is 139.6°, and the angle of RC-150 is only 62.9°, indicating that the hydrophilicity of RC-150 is higher than that of RC. The result can be attributed to the increase of surface hydrophilic oxygen-containing functional groups. The better hydrophilicity of RC-150 is conducive to the soaking and permeating of aqueous KOH electrolyte and the improvement of its electrochemical performance[35].
3.2 Electrochemical characterization of the samples
The electrochemical test results of the RC and RC-X (X = 110, 130, 150, 170) in the three-electrode system are shown in Fig. 8. As can be seen in Fig. 8a, the cyclic voltammetric curves of all samples exhibit a rectangular-like shape at a scan rate of 20 mV/s, indicating that the electric capacity of RC is mainly derived from the bilayer capacitance[36]. The CV curves of RC-X (X = 110, 130, 150, 170) are more similar to the standard rectangle after KOH activation, indicating that KOH activation is beneficial to improve the electrons-transporting reversibility of residual carbon[37]. The area circled by the CV curve reflects the energy storage performance of the material[38], and with the increase of activation temperature, the CV curve-circled area of the corresponding activated material shows a trend of first increasing and then decreasing. The specific capacitance of RC-150 is the largest. Figure 8b shows the CV curves of the RC-150 electrode material at different scan rates from 5 to 100 mV/s in the voltage range of -1.0-0 V. From the figure, it can be observed that the CV curves of RC-150 maintain a good rectangular-like shape with the increasing of the scan rate, indicating that the RC-150 carbon material possesses excellent ion response characteristics and capacitive behaviour [39].
To further contrast the specific capacitance of the electrode materials, the constant current charge/discharge curves have been measured. Figure 8c shows the GCD plots of five electrodes, RC and RC-X (110, 130, 150, 170), measured at a current density of 0.5 A/g and a voltage window of -1.0-0 V. As can be seen from the plots, the GCD curves of all five electrodes are isosceles, the slope of each curve is essentially constant. The electric potential of each sample vary approximately linearly with time, showing an ideal bilayer characteristic and a high charging and discharging efficiency[40]. Also, the charge and discharge times in the GCD curves of each sample are almost equal, indicating the good electrochemical reversibility of the material. According to the calculation of Eq. 1–1, the discharge-specific capacities of RC, RC-110, RC-130, RC-150 and RC-170 are 34.0 F/g, 71.0 F/g, 88.0 F/g, 61.8 F/g, 109.0 F/g and 87.0 F/g, respectively, at a current density of 0.5 A/g. The specific capacitance of actived samples first increases and then decreases with the increasing of the activation temperature. RC-150 actived at 150°C exhibits the optimal discharge ratio capacitance. The reasons could be attributed to several structure characteristics, such as the oxygen-containing functional groups conducive to electrolyte penetration, the abundant pore structure beneficial to ion diffusion and transport, and the high carbon content good for the charge storage and transportation. Figure 8d shows the GCD curve of the RC-150 electrode material at a current density of 0.5–20 A/g. The specific capacitance of RC-150 is 109.0 F/g, 103.2 F/g, 101.8 F/g, 97.1 F/g and 96.4 F/g at current densities of 0.5 A/g, 1 A/g, 5 A/g, 10 A/g and 20 A/g, respectively. As the current density increases, the specific capacitance of the electrode starts to decrease, indicating that the ions are more likely to diffuse into the active material at the low current densities. However, at the high current densities, the specific capacitance decreases because the ions can only partially enter the inner part of the active material due to the spatial potential resistance and the rapid charging-discharging[41].
Figure 8e are Nyquist Plots for RC and RC-X (X = 110, 130, 150, 170). The curves in the figure consist of semicircular arcs in the high-frequency region and sloping lines in the low-frequency region[42]. The slope of the line in the low-frequency region is very close to the y-axis for both RC and RC-X (X = 110, 130, 150, 170), indicating that all samples have excellent capacitance characteristics[43]. The diameter of the semi-circular arc in the high-frequency region indicates the charge transfer resistance (Rct), the magnitude of which depends on the structural properties and conductivity of the material. As can be seen from the figure, the semicircles of the RC and RC-X (X = 110, 130, 150, 170) samples are not very obvious, indicating that Rct values are small. The intercept of the Nyquist curve on the x-axis represents the equivalent series resistance (Rs), which is the sum of the carbon material/collector contact resistance, the electrolyte ion resistance, and the internal resistance of the carbon material[44]. Observe the magnified view of the high-frequency region of the RC and RC-X (X = 110, 130, 150, 170) samples, which shows that the RC-150 has the smallest Rs, indicating that the RC-150 possesses an optimal conductivity of about 0.58 Ω. This may be due to the larger average pore size and the high carbon content of the RC-150.
The specific capacitance of RC-150 is higher or slightly lower than recently reported (Table 2). It is worth noting that the electrode material in this paper is made of cheap gasification residue carbon as raw material, and the active temperature is only 150℃, which is far lower than the activation temperature reported in the literature (> 700℃).
Table 2
Comparison of specific capacitance of recently reported carbon electrode materials.
Materials | Activation method | Specific capacitance (F/g) | Vintage | Ref. |
PS-KOH | KOH (850℃) | 56.08 | 2023 | [3] |
RCS-AC | KOH (900℃) | 137 | 2023 | [4] |
K82 | KOH (800℃) | 160.9 | 2023 | [5] |
AC | KOH (800℃) | 106 | 2023 | [7] |
Qui-KOH | KOH (850℃) | 89 | 2022 | [45] |
JP-850 | KOH (850℃) | 85 | 2020 | [46] |
AC | KOH (700℃) | 105.3 | 2019 | [47] |
RC | KOH (150℃) | 109 | 2023 | This work |
From the comparison of the aboved electrochemical performance in the above three electrode systems, it can be concluded that RC-150 has the best electrochemical performance among the activated samples. To further examine the practical application effect of RC-150 in supercapacitor energy storage, a button symmetric (RC-150//RC-150) supercapacitor has been assembled with RC-150 carbon material. Figure 9a shows the CV curves of the RC-150//RC-150 capacitor at different scan rates from 0–1.0 V. It can be seen from the figure that the curves are rectangular-like, indicating that the capacitor is a double-layer capacitor[48]. Figure 9b shows the GCD curves of the RC-150//RC-150 capacitor at different current densities, and the curves are isosceles triangular, indicating a double-layer capacitance. According to the calculation of Eq. 1–2, the specific capacitance of the RC-150//RC-150 capacitor is 28.7 F/g at a current density of 0.5 A/g. When the current densities are 1 A/g, 2 A/g, 5 A/g, 10 A/g and 20 A/g, the specific capacitance of RC-150//RC-150 symmetric capacitor is 26.4 F/g, 24.7 F/g, 23.3 F/g, 23.1 F/g and 22.9 A/g, respectively. Figure 9c shows the EIS spectrum of the RC-150//RC-150 capacitor, from which it can be seen that the supercapacitor has a low resistance. The result is consistent with the results of the three-electrode system[49]. In the low-frequency region, it can be seen that the linear part of the impedance diagram is close to 90°, indicating that the carbon material is typical of the double layer capacitive behaviour[50]. Figure 9d shows the rate performance of the RC-150//RC-150 capacitor. When the current density is increased from 0.5 A/g to 20 A/g, the specific capacitance of the RC-150//RC-150 symmetrical capacitor decreases from 28.7 F/g to 22.9 F/g. The capacitance retention rate reaches 80% after the current density is enlarged by a factor of 40, showing a very good multiplicative performance. The energy density and power density of the RC-150//RC-150 capacitor can be calculated based on Equations 1–3 and Equations 1–4. Figure 9e shows the cycling performance of the RC-150//RC-150 capacitor, which maintains 103.81% of the initial capacitance after 10,000 charge/discharge cycles at a current density of 2 A/g, indicating that the RC-150// RC-150 symmetrical capacitor has very excellent cycling stability[51]. The results are shown in Fig. 9f, and the Ragone plot[52] of this capacitor shows that the maximum energy density of the RC-150 material is 3.78 Wh/kg at a power density of 254 W/kg.