Bo Weia,
Tiantian Weia,
Caifeng Xieabc,
Kai Liabc and
Fangxue Hang*abc
aSchool of Light Industrial and Food Engineering, Guangxi University, Nanning, 530004, China. E-mail: hangfx@163.com
bProvincial and Ministerial Collaborative Innovation Center for Industry, Nanning, 530004, China
cEngineering Research Center for Sugar Industry and Comprehensive Utilization, Ministry of Education, Nanning, 530004, China
First published on 19th August 2021
We present a simple, low-cost method for producing activated-carbon materials from sugarcane tips (ST) via two-step pre-carbonization and KOH activation treatment. After optimizing the amount of KOH, the resulting ST-derived activated carbon prepared with a KOH to PC-ST mass ratio of 2 (ACST-2) contained 17.04 wt% oxygen and had a large surface area of 1206.85 m2 g−1, which could be attributed to the large number of micropores in ACST-2. In a three-electrode system, the ACST-2 electrode exhibited a high specific capacitance of 259 F g−1 at 0.5 A g−1 and good rate capability with 82.66% retention from 0.5 to 10 A g−1. In addition, it displayed a high capacitance retention of 89.6% after 5000 cycles at a current density of 3 A g−1, demonstrating excellent cycling stability. Furthermore, the ACST-2//ACST-2 symmetric supercapacitor could realize a high specific energy density of 7.93 W h kg−1 at a specific power density of 100 W kg−1 in 6 M KOH electrolyte. These results demonstrate that sugarcane tips, which are inexpensive and easily accessible agricultural waste, can be used to create a novel biomass precursor for the production of low-cost activated carbon materials for high-performance supercapacitors.
In principle, the charge storage of EDLCs can be attributed to electrostatic charge accumulated at the electrode/electrolyte interfaces; therefore, it is highly dependent on the effective surface area of the electrode materials that is accessible to the electrolyte ions.14 In general, the electrode materials of supercapacitors are usually processed via an activation process to increase the specific surface area, which can lead to enhanced electrochemical performance. However, previous studies have found that the electrochemical performance and specific surface area are not linearly related. To obtain high electrochemical performance, it is necessary not only to increase the specific surface area but also have a suitable pore structure.15 Normally, chemical activation is a clear pathway to prepare activated carbon. To achieve the above conditions, chemical activating agents such as KOH, NaOH, K2CO3, and H3PO4 have been used to prepare ACs by changing the activation conditions. Among these reagents, KOH is the most effective and commonly used activator for chemical activation.7
The sugarcane tip is formed by the top 2–3 shoots of the sugarcane, accounting for ∼10% of the sugarcane biomass, and is the main lignocellulosic waste after agricultural sugarcane harvest. Although bagasse, a waste product of sugarcane, has been widely used for the preparation of carbon materials, the application of sugarcane tips in this field has not been widely investigated. Because sugarcane tips are cheap, plentiful, and contain 43% cellulose, 27% hemicellulose, and 17% lignin,16 they also serve as an effective activated-carbon precursor. Every year, 7 million tons of sugarcane tip waste is generated in Guangxi, China, of which, only a small portion is used for seed retention and feed, and the majority is left in the field to be burned, causing not only pollution and safety risks but is also a waste of resources. Therefore, the use of sugarcane tips to prepare activated-carbon materials can be one effective way to address these issues.
In this work, we demonstrate a facile strategy to synthesize activated carbon materials with a high specific surface area via a two-step method using ST as the biomass precursor. The effect of the activator (KOH) content on the morphology, crystallinity, and porosity of the produced activated carbon along with their electrochemical performance was investigated. ACST-2 had a high surface area of 1206.85 m2 g−1, a synergism of porosity and graphitization degree. Benefiting from its unique properties, ACST-2 presented a high specific capacitance of 259 F g−1 at 0.5 A g−1 in a three-electrode system with 6 M KOH electrolyte. Furthermore, the ACST-2//ACST-2 symmetric supercapacitor device showed excellent energy storage performance with a high specific energy density of 7.93 W h kg−1 at a specific power density of 100 W kg−1. Hence, this simple synthesis method has significant application prospects to prepare sugarcane-tip-derived activated carbon for use as supercapacitor electrode materials.
(1) |
The specific capacitance from GCD was calculated with the symmetrical-electrode method calculated using equation:
(2) |
The specific energy density (E, W h kg−1) and specific power density (P, W kg−1) of the symmetrical supercapacitor were calculated from the following equations:
(3) |
(4) |
Scheme 1 Schematic illustration of synthesis procedure to obtain activated carbon derived from sugarcane tip (ACST). |
The microstructure of ACST-2 was further characterized via TEM. Fig. 1f shows the presence of plate-like structure with relatively rough edges and abundant pores in ACST-2, which is consistent with the SEM results. Fig. 1g exhibits a high-resolution TEM image, from which a large number of micropores can be clearly observed, demonstrating that the material mainly consists of amorphous carbon. Furthermore, an unclear diffraction ring in the SAED diagram (inset in Fig. 1g) indicates that the sample has a low degree of crystallinity.18
Fig. 2a shows the XRD spectrums of the as-obtained ACST-1, ACST-2, and ACST-3. All products display two broad diffraction peaks at approximately 23° and 43° that correspond to the (002) and (100) planes of graphite,11,19 respectively, suggesting the presence of amorphous carbon;20,21 these results are in good agreement with the TEM and SAED results. Furthermore, it is noteworthy that clear intensity associated with ACST-2 and ACST-3 could be observed at a low-angle scattering peak. The shift of the intensity from high to low at a low angle can be explained by the presence of a certain amount of micropores in the sample.22,23 The results for the degree of graphitization of the as-prepared carbon materials were further confirmed by Raman spectroscopy, as shown in Fig. 2b. The two dominant peaks at 1345 and 1590 cm−1 correspond to the D-band and G-band of carbon materials, respectively. The D-band is associated with defects in the carbon material, while the G-band can be attributed to the vibration of sp2 carbon in the graphite crystallites.24 Generally, the intensity ratio of the D-band and G-band (ID/IG) is used to reflect the graphitic degree of the samples, and a higher value of ID/IG indicates the presence of more defects in the carbon materials.12,25 Here, the measured ID/IG ratios of ACST-1, ACST-2, and ACST-3 were 0.84, 0.87, and 0.88, respectively. It can be observed that as the amount of KOH increased, the ID/IG ratio gradually increased, demonstrating increased disordered portions and defects in ACST-x. The presence of defects is beneficial for promoting the specific surface area and charge transport.26 However, the electrical conductivity of the materials may decline with the existence of the defects.27 Therefore, determining a suitable ID/IG ratio is beneficial for enhancing electrochemical performance.
Fig. 2 (a) XRD pattens; (b) Raman spectra; (c) N2 adsorption–desorption isotherms, and (d) the pore size distribution curves of the ACST-1, ACST-2, and ACST-3. |
To further study the porous texture, the as-obtained carbon materials were investigated via N2 adsorption–desorption measurements. As shown in Fig. 2c, all the samples revealed typical type I isotherms, and a sharp increasing adsorption isotherm could be observed at low relative pressures <0.1. This increase indicated that the ACST-x contained a large number of micropores because the adsorption and desorption curves almost coincided with each other.28,29 Fig. 2d shows the corresponding pore size distribution curves of ACST-x based on the NLDFT method with the desorption curve. It can be seen that all the samples had micropores primarily less than 2 nm, which is suitable for charge accumulation, and thus, increases the electric double-layer capacitance. Details of the porous texture of the ACST-x samples are summarized in Table 1. It can be noted that the specific surface area and total pore volume increased rapidly as the activator content increased. The specific surface area of ACST-1, ACST-2, and ACST-3 were determined to be 496.48, 1206.85, and 1438.01 m2 g−1 and the total pore volumes were 0.28, 0.52, and 0.61 cm3 g−1, respectively. Intriguingly, this trend was similar to that of the ID/IG ratio with increasing amounts of KOH. These results indicate increased defect generation due to the reaction between KOH and the carbon skeleton, which facilitates utilization of the porosity and increases the surface area by generating a large number of micropores and enlarging the pore size.
Sample | SBETa (m2 g−1) | Smicb (m2 g−1) | Vtotalc (cm3 g−1) | Vmicb (cm3 g−1) | Daverd (nm) |
---|---|---|---|---|---|
a Total surface area calculated by the BET method.b The specific surface area and volume of micropore measured by the t-plot method.c Total pore volume calculated at P/P0 = 0.99.d Average pore diameter determined by BJH desorption. | |||||
ACST-1 | 496.48 | 403.74 | 0.28 | 0.21 | 4.42 |
ACST-2 | 1206.85 | 1062.26 | 0.52 | 0.42 | 3.83 |
ACST-3 | 1438.01 | 1266.56 | 0.61 | 0.50 | 3.50 |
Fig. 3 displays the chemical components and functional groups of CST and ACST-2 analyzed via XPS. Three peaks with binding energies of 285.2, 401.1, and 533.0 eV can be observed from the survey of CST and ACST-2, as shown in Fig. 3a, which can be assigned to the C 1s, N 1s, and O 1s peaks, respectively. The C:O:N content was determined to be 84.35 wt%:13.73 wt%:1.91 wt% and 81.28 wt%:17.04 wt%:1.68 wt% for the CST and ACST-2 samples, respectively. Fig. 3b exhibits the high-resolution spectrum of C 1s, which could be deconvoluted into four individual peaks at 284.8, 286.3, 287.2, and 288.4 eV that could be attributed to C–C, C–O, CO, and O–CO, respectively.30 The high-resolution spectrum of O 1s is given in Fig. 3c, and three peaks could be observed and attributed to CO, C–O, and OC–O bonds at the binding energies of 531.8, 532.7, and 533.6 eV, respectively.31,32 Previous literature reported that double and single bonded oxygen could enhance the hydrophilicity of carbon materials and markedly improve the total specific capacity by enhancing the faradaic pseudo-capacitance.33,34 In addition, the N 1s spectra (Fig. 3d) revealed three major peaks that could be attributed as pyridinic N (398.9 eV, N-6), pyrrolic N (400.4 eV, N-5), and graphitic N (402.1 eV, N-Q).35 It has been widely demonstrated that pyridinic N and pyrrolic N display excellent electrochemical capability in an alkaline solution and generate pseudo-capacitance.36,37 Graphitic N can be embedded in the carbon matrix and combined with carbon atoms, contributing to electron diffusion and improving the conductivity of the electrode.38
Fig. 3 XPS (a) survey spectrum of CST and ACST-2; high-resolution spectrum of ACST-2 of (b) C 1s; (c) O 1s; and (d) N 1s. |
Fig. 4c exhibits the GCD profiles of the ST-derived electrodes under a current density of 0.5 A g−1. The profiles display a quasi-triangle shape instead of a symmetric triangle, which indicates an associative feature of electrical double-layer capacitance and pseudocapacitive behaviors for the N and O active species.42 As illustrated in literature, N-5 and N-6 play a vital role in affording pseudo-capacitance and thus leading to curve distortion.43 Notably, the ACST-2 electrode displays the longest discharge time, while ACST-1 and ACST-3 show slightly reduced discharge times, and the CST electrode presents the shortest discharge time. This reveals that the ACST-2 electrode exhibits the largest specific capacitance (259 F g−1), followed by ACST-1 (243.8 F g−1), ACST-3 (211.9 F g−1), and CST (19.5 F g−1), which agreed well with the CV results at 10 mV s−1. Although the specific capacitance of ACST-2 is the largest, the surface area of the carbon materials did not follow the same sequence (ACST-3 > ACST-2 > ACST-1). This demonstrates that the capacitance is not only related to the specific surface area but also associated with other characteristics, such as pore size distribution or functional groups.44 Importantly, the surface area-normalized capacitance of ACST-2 was 0.21 F m−2, which is higher than ACST-1 (0.15 F m−2), exhibiting a higher surface area utilization rate for ACST-2 than ACST-3. The superior specific capacitance for the ACST-2 can be attributed to the relatively high effective surface area with abundant exposed ion absorption sites for charge accumulation in electric double-layer.45
The GCD profiles of the ACST-2 electrode displayed an approximately triangular shape at various current densities, as shown in Fig. 4d. Additionally, the GCD profile could still be maintained as a triangle under a high current density of 10 A g−1, demonstrating desirable capacitive behavior. In addition, only a slight IR drop (200 mV) was detected at this high current density, indicating a high charge–discharge efficiency and low equivalent series resistance,10,46 which can explain the distortion of the CV curves.
The specific capacitances of all the samples gradually decreased with increasing current densities from 0.5 to 10 A g−1, as shown in Fig. 4e. This decrease could be attributed to the slow migration of ions between the electrolyte and surface of electrode at low current density. All hydrated ions could entirely diffuse within the pore, enhancing the ion storage capacity and thus increasing the specific capacitance. However, at high current densities, the electrolyte ions do not have sufficient time to access the microporous surface, and thus, the specific capacitance at high current densities is lower than that at low current densities.40 In addition, at a current density of 10 A g−1, the capacitance of ACST-1, ACST-2, and ACST-3 remains at 79.70% (194.3 F g−1), 82.66% (214.1 F g−1), and 91.65% (194.2 F g−1) of the initial capacitance at 0.5 A g−1. These results demonstrate that ACST-2 is a promising carbon electrode material for supercapacitors. The cycling stability imply that the number of times the electrolyte ions could migrate into the available pores for the repeated number of cycles. Fig. 4f shows the long cycling stability of the ACST-2 electrode as demonstrated via GCD measurement at 3 A g−1 for 5000 cycles. After cycling, the capacitance value decreased, which may be due to the increase of internal resistance. The ACST-2 electrode still retained 89.6% of the initial capacitance after 5000 cycles, demonstrating excellent cycling stability and high reversibility.
The electrochemical impedance spectroscopy (EIS) of ACST-1, ACST-2, and ACST-3 were further studied from 0.01 to 100000 Hz in a 6 M KOH solution. Fig. 5a shows the Nyquist plots of the ACST-1, ACST-2, and ACST-3 electrodes and an equivalent electric circuit to fit the Nyquist plot. All samples were divided into three different sections: a semicircle, a straight line of 45°, and a vertical line in the high, medium, and low frequencies. As shown in the inset of Fig. 5a, the intercept of the semicircle at the real axis (Z′) indicates the equivalent series resistance (Rs), which contains the intrinsic electroactive material resistance, ionic transport resistance of the electrolyte, and the contact resistance at the interface of the electrode materials and current collectors.47 The Rs value of ACST-1, ACST-2, and ACST-3 were 0.32, 0.53, and 0.33 Ω, respectively, indicating negligible contact resistance and good conductivity.48 The diameter of the semicircle is related to the interfacial charge transfer resistance (Rct). The Rct values for ACST-1, ACST-2, and ACST-3 were 1.24, 0.54, and 0.62 Ω, respectively, which demonstrated minimal charge transfer resistance for the ACST-2 electrode. In addition, the short line with the 45° slope is called the Warburg resistance, which is associated with ion diffusion/transport into the active materials.49 At low frequencies, the straight line is nearly vertical—close to 90°—suggesting better capacitive behavior and rapid ion diffusion in the carbon electrode structure.50 Fig. 5b displays the Bode phase plots of ACST-1, ACST-2, and ACST-3, where the phase angle shifts to negative values of −71.77°, −74.04° and −77.80°, respectively, at a low frequency of 0.01 Hz. This indicates that the results are relatively close to that of an ideal supercapacitor (−90°), which suggests a primarily double-layer charge storage contribution with some pseudo-capacitive contribution for the material.51
To study the practical application of the ACST-2 sample, the ACST-2//ACST-2 symmetric supercapacitor was assembled in 6.0 M KOH electrolyte with an operating voltage range of 0–1 V. Fig. 6a exhibits the CV curves of the symmetric capacitor, the electrode exhibits a rectangular shape at a scanning rate of 10 mV s−1, indicating a typical double-layer capacitor behavior at the electrode and electrolyte interface. In addition, the CV curve remains approximately rectangular at a high scan rate of 100 mV s−1, indicating an ideal electrochemical capacitance behavior and excellent rate performance. The GCD curves were tested at different current densities (0.2 to 10 A g−1) and are shown in Fig. 6b. All the curves exhibited an approximately symmetric linear behavior, demonstrating the existence of bilayer behavior with favorable reversibility. From the GCD curves, specific capacitances of 228.3, 215.2, 203.6, 189.6, 162, and 136 F g−1 were obtained at 0.2, 0.5,1, 2, 5, and 10 A g−1, respectively. Fig. 6c shows the relationship between the specific capacitance and the current density, a specific capacitance of 59.6% was still retained even though the current density increased 50 times, indicating good rate performance for the ACST-2 supercapacitor. Fig. 6d shows the Ragone plot of the ACST-2//ACST-2 symmetric supercapacitor. The specific energy density of the symmetry supercapacitor was measured at 7.93 W h kg−1 for a specific power density of 100 W kg−1. For practical applications, it is important to measure the leakage current and self-discharge of the device, the results of our studies in this direction are shown in Fig. S5.† These results demonstrated that ACST-2 is a promising electrode material for supercapacitor applications.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra04143f |
This journal is © The Royal Society of Chemistry 2021 |