Si Zheng,
Yin Cui,
Jianwei Zhang,
Yuxing Gu,
Xiaowen Shi*,
Chuang Peng* and
Dihua Wang
School of Resource and Environmental Science, Hubei International Scientific and Technological Cooperation Base of Sustainable Resource and Energy, Hubei Biomass-Resource Chemistry and Environmental Biotechnology Key Laboratory, Wuhan University, Wuhan 430079, China. E-mail: Shixw@whu.edu.cn; Chuang.peng@whu.edu.cn
First published on 9th April 2019
N-doped porous carbon nanospheres were fabricated directly by pyrolyzing chitin nanogels, which were facilely prepared by mechanical agitation induced sol–gel transition of chitin solution in NaOH/urea solvent. The resulting carbon nanospheres displayed ordered micropores (centered at ∼0.6 nm) and high BET surface area of up to 1363 m2 g−1, which is substantially larger than that of the carbons from raw chitin (600 m2 g−1). In addition, the carbon nanospheres retained a nitrogen content of 3.2% and excellent conductivity. Consequently, supercapacitor electrodes prepared from the carbon nanospheres pyrolyzed at 800 °C showed a specific capacitance as high as 192 F g−1 at a current density of 0.5 A g−1 and impressive rate capability (81% retention at 10 A g−1). When assembled in a symmetrical two-electrode cell, N-doped porous carbon nanospheres demonstrated excellent cycling stability both in aqueous and organic electrolytes (95% retention after 10000 cycles at 10 A g−1), together with outstanding energy density of 5.1 W h kg−1 at the power density of 2364.9 W kg−1. This work introduces a novel and efficient method to prepared N-doped porous carbon nanospheres directly from chitin and demonstrates the great potential of utilization of abundant polymers from nature in power storage.
Chitin is mainly extracted from aquatic biomass waste such as the shells of shrimp and crab. It is β-(1-4)-linked 2-acetamido-2-deoxy-D-glucopyranose with a nitrogen amount of 6.9 wt%. N-doped porous carbon can be facilely obtained by pyrolyzing chitin and has been developed for high-performance capacitor.21 However, direct carbonization of bulk chitin does not offer sufficient control over porosity and microstructure. Recently, chitin microspheres,19,22 nanofibers and aerogels18,20 were selected as precursors for N-doped carbon, which demonstrated large surface area, hierarchical porosity and high electrochemical performance. Importantly, chitin derived carbon showed favorable retention of morphology after pyrolyzation due to the stiffness of chitin chains.19 Therefore, a preformed chitin precursor with well define nanostructure provides great opportunities for new nanocarbon materials.
Recent work revealed the advantages of carbon nanospheres23–25 as high performance supercapacitor electrode when applied in supercapacitors.26,27 Particularly, carbon nanospheres with hierarchical porous architectures demonstrated maximized specific surface area while minimized electron and ion transport distances.28 The methodology to prepare carbon nanospheres generally includes either a hard-template method or a soft template method.28–30 Organic polymers29,31,32 and silica33,34 were commonly used for the fabrication of carbon nanospheres. Direct carbonization of preformed natural polymeric nanoparticles provides a facile and environmentally friendly method for the preparation of carbon nanospheres, though few reports have been devoted to this effort.
Herein, we report the preparation of carbon nanospheres directly from chitin nanogels for the first time. Chitin nanogels with a diameter of 20–30 nm were prepared by a mechanical agitation induced sol–gel transition method with high efficiency. Interconnected porous framework of N-doped carbon nanospheres (nitrogen content of 3.2%) was obtained by directly pyrolyzing the chitin nanogels. The as-prepared typical carbon had a high BET surface area (up to 1363 m2 g−1) and a regular micropore peaked at 0.6 nm, which benefit the fast transportation and diffusion of electrolyte ions. When used for supercapacitor, the carbon nanospheres showed a specific capacitance of 192 F g−1 at 0.5 A g−1 in 1 M H2SO4 electrolyte. Besides, the chitin nanospheres could be used in organic electrolyte, demonstrating a specific capacitance of 107 F g−1 at 1.0 A g−1 in TEABF4-AN electrolyte and an excellent cycling stability, with a capacitance retention of 95% after 10000 cycles at 10 A g−1. The present study provides a simple and highly efficient route to fabricate N-doped carbon nanospheres from chitin and highlights the great potential of well-structured carbon nanospheres derived from chitin for high-performance supercapacitors.
(1) |
In addition, to further investigate the electrochemical performance, symmetrical two-electrode systems were installed in 1 M H2SO4 aqueous electrolyte and 1 M TEABF4-AN organic electrolyte, respectively. In aqueous electrolyte, CV and GCD were tested with the potential window of 0–1 V. Electrochemical impedance spectroscopy (EIS) was recorded over a frequency range from 0.01 Hz to 100 kHz with an alternating current voltage of 5 mV amplitude. In organic electrolyte, the potential window increased to 0–2.5 V. Under this condition, CV, GCD and cycling stabilities measurements were conducted. The specific capacitance of the symmetric electrodes (Cs, F g−1), the energy density (E, W h kg−1) and power density (P, W kg−1) of the symmetrical electrode system were calculated as follows:37–39
(2) |
(3) |
(4) |
The morphology of chitin nanogels was firstly observed by TEM and AFM (Fig. 2a and b). Well shaped nanospheres with a diameter of 20–30 nm were clearly observed. Freeze drying of the chitin nanogels resulted in a porous sponge, which was further observed by SEM. The chitin sponge exhibited interconnected network with randomly opened pores (Fig. 2c) and the pore walls demonstrated a rough surface consisting of nanoparticles (Fig. 2d). N-doped carbon nanospheres were obtained by direct carbonization of the chitin nanogels. After carbonization, the original porous structure of the chitin nanogels was still maintained as shown by the low magnification images in Fig. 3a and c. The zoom in images demonstrated the carbon nanospheres were assembled into an interconnected framework and nanopores were clearly observed (Fig. 3b and d), especially in CNC-800. By comparison, the SEM images of raw chitin calcined at 800 °C (Fig. 3e) and 900 °C (Fig. 3f) demonstrated more compact structure. TEM and HRTEM images (Fig. 4) further confirmed the spherical morphology of CNC-800 and CNC-900 with the mean size of 3.3 nm and 5.4 nm, respectively. The HRTEM micrographs (Fig. 4c and f) showed the carbon nanosphere had partly graphitized structure with an adjacent interlayer distance of ∼0.3 nm.
Fig. 2 Structural characterization of chitin nanogels: TEM image (a), AFM image (b) and SEM images (c and d). |
Fig. 3 Structural characterization of chitin-derived carbons: the SEM images of CNC-800 (a and b), CNC-900 (c and d), chitin-800 (e), chitin-900 (f). |
Fig. 4 TEM images of CNC-800 (a) and CNC-900 (d), high-resolution TEM images of CNC-800 (b and c), CNC-900 (e and f). |
In order to better understand the detailed pore structure of resultant chitin-derived carbon nanospheres, the nitrogen adsorption/desorption experiments were conducted. According to IUPAC classification, the isotherms of chitin-derived carbon nanospheres corresponded to the Type I, while the isotherms of carbons from raw chitin were the combined Type I and Type IV (Fig. 5a). All the curves exhibited sharp increase in N2 adsorption at relative low pressure (P/P0), which was caused by the enhanced interaction between adsorbent–adsorbate in the narrow micropores.40 The H4 hysteresis loops were observed in the isotherms of raw chitin-derived carbon at relative pressure in the range of 0.4–0.8, which were associated with the capillary condensation in mesopores.41 In addition, according to the corresponding DFT pore size distribution curves (Fig. 5b), most pores in CNC-800 and CNC-900 felt in the ultramicroporous region (pore size < 0.7 nm) and peaked at 0.6 nm, a few pores were in the domain of supermicropores (0.7–2 nm) and narrow mesopores (2–4 nm), whereas the chitin-800 and chitin-900 showed large portion of mesopores. Recent study suggests that the existence of ultramicropores (<0.7 nm) close to the ion size is advantageous for electrochemical performance of carbon.42–44 The hydration sheaths could be removed as the solvated ions squeezed through the ultramicropores.42,45 As a consequence, the distance from the ion center to the electrode surface was closer, leading to enhanced capacitor performance. More structural parameters were listed in Table 1. CNC-800 and CNC-900 showed high BET surface areas of 1031 m2 g−1 and 1363 m2 g−1, much higher than the carbons derived from the direct carbonization of chitin (626 m2 g−1 and 600 m2 g−1 respectively). The high surface area and narrow micropore size distribution of chitin derived carbon nanospheres are beneficial to capacitance performance.
Sample | SBET (m2 g−1) | Smicro (m2 g−1) | Vtotal (cm3 g−1) | Vmicro (cm3 g−1) | Vmeso (cm3 g−1) | Da (nm) |
---|---|---|---|---|---|---|
Chitin-800 | 626 | 312 | 0.46 | 0.16 | 0.30 | 2.95 |
Chitin-900 | 600 | 262 | 0.45 | 0.13 | 0.32 | 3.02 |
CNC-800 | 1031 | 706 | 0.57 | 0.35 | 0.22 | 2.22 |
CNC-900 | 1363 | 877 | 0.78 | 0.44 | 0.34 | 2.29 |
XRD patterns of chitin-derived carbon nanospheres as well as carbons from raw chitin are shown in Fig. 6a. All the XRD patterns showed a broad peak at ∼24° and a weak peak at ∼43°, which corresponded to the (002) diffraction and (100) diffraction,5 indicating a partly graphitized structure and a disorder phase.21,46 This structure feature was also confirmed by Raman spectroscopy (Fig. 6b). The G band at ∼1606 cm−1 and D band at ∼1355 cm−1 are associated with the graphitic layers and disordered carbons or defective graphitic structures, respectively.7,43,47 The intensity ratio of these two peaks partially depends on the degree of graphitization.19 The ID/IG ratio for chitin-800, chitin-900, CNC-800, CNC-900 is 0.83, 0.81, 0.88, 0.87, respectively. The slightly increase of ID/IG ratios for chitin-derived carbon nanospheres suggested the existence of larger amounts of disordered carbon or graphitic defects with a low graphitization degree.
Fig. 6 The structural characterization: XRD patterns (a), Raman spectra (b), full-scale XPS spectra (c), high-resolution N 1s spectra (d). |
Due to the presence of acetyl amino groups on chitin chains, the calcination of chitin nanogels leads to N-doping in the carbon nanospheres. The full survey spectra of XPS confirmed the existence of C, N, and O elements in the resulted carbons (Fig. 6c). High resolution spectrum of N 1s can be divided into four peaks (Fig. 6d), centered at 397.7 eV (pyridinic N), 398.7 eV (pyrrolic N), 400.6 eV (graphitic N) and 403.5 eV (oxidized-N).19 From quantitative analysis in Table 2, we can see the carbons carbonized at 800 °C possessed more pyridinic and pyrrolic types of nitrogen, which can create defects and electrochemically active sites contributing to the pseudocapacitance contents.19 Furthermore, it has been reported that nitrogen doping can enhance the conductivity of the carbon materials and improves the charge transfer property.16 The nitrogen doping also increases the wettability of the carbon surface, resulting in improved ion adsorption and increased capacitance.48
Samples | C% | O% | N% | % of total N 1s | |||
---|---|---|---|---|---|---|---|
N-Q | N-5 | N-6 | Oxidized-N | ||||
Chitin-800 | 88.3 | 7.6 | 4.1 | 57.8 | 9.5 | 20.8 | 11.9 |
Chitin-900 | 93.9 | 4.4 | 1.7 | 58.7 | 4.3 | 9.3 | 27.7 |
CNC-800 | 91.5 | 5.3 | 3.2 | 57 | 11.8 | 21.5 | 9.7 |
CNC-900 | 95.6 | 2.8 | 1.6 | 66.5 | 7.2 | 9.8 | 16.5 |
Cyclic voltammetry (CV) and charge/discharge measurements (GCD) were performed to evaluate the electrochemical characteristics of the obtained chitin-derived carbon nanospheres. Fig. 7a showed the CV curves of different carbons at a scan rate of 20 mV s−1 in 1 M H2SO4 aqueous electrolyte using a three-electrode system. All the CV curves displayed quasi-rectangular shapes, suggesting a typical electrical double layer capacitive behavior. Slight humps occurred due to the pseudocapacitance provide by the doped nitrogen. The integrated area of the rectangles for CNCs was larger than the carbons from raw chitin (chitin-900 < chitin-800 < CNC-900 < CNC-800), indicating higher capacitance of the carbon nanospheres, which may be related to more defects and larger specific surface area. Fig. 7b showed the GCD curves of different carbons at a current density of 1 A g−1. The GCD curves presented almost symmetrical triangles with slight tilts, confirming the good capacitive performance and a small fraction of pseudocapacitance nature. The specific capacitance values from GCD curves were shown in Fig. 7c. CNC-800 exhibited superior specific capacitance of 192 F g−1 at 0.5 A g−1, 179 F g−1 at 1 A g−1, which was comparable to several previously reported bio-carbons but lower than polymer precursor-derived carbons (Table 3).6,39,49–52 This may be caused by a more optimal structure and higher nitrogen content for polymer precursor-derived carbons. When the current density increased to 10 A g−1, the specific capacitance decreased to 155 A g−1 in virtue of the diffusion limitation of ions during fast charging–discharging,53 with capacitance retention of 81%, indicating good rate capability.
Material | Precursor | Capacitance at 1 A g−1 (F g−1) | Electrolyte | Ref. |
---|---|---|---|---|
N-doped carbon nanospheres | Melamine–phenolic–formaldehyde (MPF) resin | 432 | 6 M KOH | 39 |
Hierarchical N-doped porous carbon | Endothelium corneum gigeriae galli | 198 | 6 M KOH | 49 |
Sulfur and nitrogen dual-doping carbon | Shell of broad beans | 174 | 6 M KOH | 6 |
N-doped carbon double-shell nanoparticles | Polydopamine | 184 | 1 M H2SO4 | 50 |
Activated carbon | Coconut kernel | 173 | 1 M H2SO4 | 51 |
N-doped hollow carbon nanospheres | Glucose and glucosamine | 205 | 1 M H2SO4 | 52 |
N-doped microporous carbon nanospheres | Chitin | 179 | 1 M H2SO4 | Our work |
EIS measurements (Fig. 7d) were also conducted to further investigate the conductivity and ion transfer. The Nyquist plots consist of a semicircle in the high frequency range and a straight line in the low frequency range. The intercept of the semicircle with the real axis represents the equivalent series resistance (ESR), including the resistance of the electrolyte (Rs), the internal electrode resistance and the contact resistance between the electrode and current collector,8,42 among which the electrolyte resistance (Rs) is the dominant part.54 The Rs values of chitin-900, chitin-800, CNC-800, CNC-900 were 0.93, 0.86, 0.25 and 0.26 Ω, respectively, indicating that the carbon nanospheres have low resistance and high conductivity. The diameter of semicircle refers to the interfacial charge transfer resistance (Rct),55 the CNC-800 and CNC-900 have smaller semicircle diameters, revealing a rapid charge transference. CNC-800 was further characterized with CV at different scan rates and GCD at various current densities. As shown in Fig. 7e and f, the curves all exhibited ideal shapes, indicating good capacitive characteristic and electrochemical reversibility.
In order to further evaluate the applicability of the chitin derived carbon nanospheres, the electrochemical performance of CNC-800 was tested with a symmetrical two-electrode cell both in aqueous and organic electrolytes. Fig. 8a showed the CV curves of the aqueous device, which presented perfect symmetrical rectangular shapes even under a high scan rate, indicating a high rate capability. The GCD curves (Fig. 8b) also maintained symmetric triangular shapes at all current densities. The specific capacitance at current density of 1 A g−1 was 180 F g−1, being comparable to that in three-electrode system. Ragone plot was recorded in Fig. 8c. The energy density and power density were calculated from the GCD curves. The maximal energy density was 6.4 W h kg−1 at a power density of 125 W kg−1, while the energy density remained as high as 5.1 W h kg−1 with a power density of 2364.9 W kg−1, indicating good energy storage property and fast charge/discharge performance. Fig. 8d illustrated the CV curves of organic device. Interestingly, the CV curves still maintained rectangular-like shapes as the scan rate increased to 700 mV s−1. The specific capacitance from Fig. 8e was 107 F g−1 at current density of 1 A g−1. The energy density in organic electrolyte was 23.4 W h kg−1 with a power density of 313.9 W kg−1, 20.6 W h kg−1 with a power density of 6035.6 W kg−1, which was comparable with other literature.56–58 And as shown in Fig. 8f, the capacitance retention still retained 95% after 10000 cycles at high current density of 10 A g−1, manifesting an outstanding cycling stability. The advanced performance of CNC-800 in both organic and aqueous electrolytes confirms the versatile applicability of chitin derived carbon nanospheres for energy storage.
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