Song Yan,
Jingjing Lin,
Ping Liu,
Zhicheng Zhao,
Jun Lian,
Wei Chang,
Lu Yao,
Yueran Liu,
Hualin Lin* and
Sheng Han*
School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Haiquan Road 100, 201418, Shanghai, P. R. China. E-mail: hansheng654321@sina.com; lhl6534@163.com; Fax: +86-021-60873560; Fax: +86-021-60873228; Tel: +86-13524694909 Tel: +86-17 701878558
First published on 12th February 2018
In this study, advanced nitrogen-doped porous carbon materials for supercapacitor was prepared using low-cost and environmentally friendly waste lotus stems (denoted as LS-NCs). Nitrogen in the surface functionalities of LS-NCs was investigated using X-ray photoelectron spectroscopy analysis. The sum of pyridine nitrogen (N-6) and pyrrolic/pyridinic (N-5) contents accounted for 94.7% of the total nitrogen and significantly contributed to conductivity. Pore structure and surface area of activated carbons were measured using the Brunauer–Emmett–Teller method. A maximum specific surface area of 1322 m2 g−1 was achieved for LS-NCs. The porous carbons exhibited excellent electrochemical properties with a specific capacitance of 360.5 F g−1 at a current density of 0.5 A g−1 and excellent cycling stability (96% specific capacitance retention after 5000 cycles). The above findings indicate that taking advantage of the unique structure of abundant waste lotus stem provides a low-cost and feasible design for high-performance supercapacitors.
In recent years, bio-derived activated carbon materials have been widely investigated because of their good conductivity and low cost.11,12 As a renewable energy resource, biomass can be utilised for preparing porous carbon materials; not only can it reduce the cost of manufacturing, but it also improves waste recycling and development. For example, coconut shells are broadly used in production of commercial activated carbons.13,14 More natural biomass materials, such as leaves,15 pistachio nutshells,16 auricularia,17 shiitake mushroom,18 longan shells,19 potato,20 waste celtuce leaves21 and cherry stone22 are also used in production of commercial activated carbons. As a perennial aquatic herb, lotus stems possess several large pores and abundant longitudinal ventilation holes, and its microstructure benefits improvement of electrochemical performance of samples. Lotus stems are widely distributed in China; some of them are used for traditional Chinese medicine, but the remaining majority are directly abandoned in rural areas, causing environmental pollution and waste of resources. In addition, discarded lotus stem waste draff is rich in cellulose and can be used to prepare carbon materials; such application opens an effective avenue for utilising discarded lotus stem waste draff as resources.
For this research, waste lotus stems were treated as carbon sources to prepare porous carbon by pyrolysis at 600 °C in nitrogen and followed by KOH activation to improve specific area of the material. Urea contains abundant nitrogen, which can improve electronic conductivity of carbon materials. The tube and lamellar of lotus stems provide an excellent platform to further optimise its structure and property and can be directly utilised as high-performance supercapacitors. Synthesis procedures of LS-NCs are shown in Scheme 1.
Carbonised lotus stems were chemically activated. Carbonised lotus stems (1 g) and KOH (2 g) were added to ethanol solution (50 wt%, 10 ml), impregnated for 4 h and dried in an oven at 60 °C for 12 h. The dried mixture was activated under N2 atmosphere at 600 °C, 700 °C, 800 °C and 900 °C for 2 h. After activation, the obtained carbon materials were washed with aqueous HCl solution (2 M) to remove any inorganic salts, then rinsed with DI water and ethanol for several times until a pH of 7 was reached and dried at 60 °C for 3 h. The activated nitrogen-doped porous carbon is denoted as LS-NC-X in further discussions, where X represents activation temperature (600 °C, 700 °C, 800 °C and 900 °C).
Specific capacitance of the electrode was calculated based on GCD measurements and according to the following equation:23
Cm = IΔt/m(ΔV) |
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Fig. 1 (a) XRD patterns and (b) Raman spectra of LS-NC-600, LS-NC-700, LS-NC-800 and LS-NC-900, respectively. |
Raman spectroscopy was utilised to further illuminate LS-NCs structures, and results are shown in Fig. 1b. Two distinctive peaks can be observed in all specimens, and they correspond to D (1350 cm−1) and G (1590 cm−1) band. Integral ratio (ID/IG) is estimated to indicate degree of structural disorder with respect to a perfect graphitic structure.28 In the present study, ratios of LS-NC-600, LS-NC-700, LS-NC-800 and LS-NC-900 reached 0.98, 1.11, 1.15 and 1.17, respectively. With increasing activation temperature, ID/IG ratios gradually increased, indicating that order of graphitic structure was destroyed, and that deeper activation has promoted the presence of defects in LS-NCs.29 The above XRD pattern and Raman spectrum results imply that these LS-NCs are multiaperture-activated carbon material with disordered carbon structure.
N2 adsorption–desorption isotherms of LS-NCs were measured at −196 °C to investigate the surface area and porous structure of LS-NCs, as shown in Fig. 2a and Table. S1.† LS-NCs showed typical IV nitrogen adsorption–desorption isotherms curves, indicating the existence of micropores and mesopores.30,31 Isotherms of the obtained LS-NCs exhibited a relatively broad keen in the low-pressure range, implying the existence of microporous structure. LS-NCs possessed a shot-range hysteresis loop at relative pressure P/P0 from 0.45 to 0.90, exhibiting a well-ordered mesoporous structure.32 Table. S1† summarises porous textural details of these LS-NCs materials. The pore size distribution shown in Fig. 2b was determined through density functional theory. For all LS-NCs, the peaks that centred at about 0.5 nm indicate that LS-NCs possessed micropore and mesopore structures. Along with rising activation temperature from 600 °C to 900 °C, specific surface areas of LS-NCs reached 1322, 2013, 2221 and 1986 m2 g−1 (Table. S1†). Specific surface area of the material first increased and then decreased with increasing activation temperature; this result can be attributed the excessive temperature, resulting in enhancement of KOH activation and collapse of the structure. Furthermore, LS-NC-800 yielded high BET surface area and micropore volume. This hierarchical porous structure with high surface area plays a key role in enhancing ion transport and charge storage.
The nature of nitrogen on surface functionalities of LS-NCs was further investigated using XPS analysis (Fig. 3). As shown in Fig. S2,† the coexistence of C, N and O was confirmed by XPS survey, suggesting that nitrogen was successfully doped into porous carbons. N 1s core level spectra of the LS-NCs are exhibited in Fig. 3 and Table 1. As predicted, the samples featured three peaks at 397.6, 399.6 and 403.2 eV, referring to the three types of nitrogen species, namely, pyridine nitrogen (N-6), pyrrolic/pyridinic nitrogen (N-5) and oxidised nitrogen (N-X), respectively. N-6 percentage increased from 39.1% to 79.1%, whereas N-5 percentage decreased from 43.3% to 15.6% in LS-NC-900, LS-NC-800, LS-NC-700 and LS-NC-600. A careful observation showed that by comparing with the results of LS-NC-600, LS-NC-700 and LS-NC-800, LS-NC-900 summarized in Table 1. The total nitrogen contents in LS-NCs were found to be inversely proportional to the activation temperature, indicating that higher temperature will lead to more loss of nitrogen atoms during activation, this trend is accordance with the reports elsewhere.33,34 In many previous reports, N-6 and N-5 located at the edges of grapheme layers are considered representing the pseudo capacitive effect in aqueous electrolyte, which are of importance to improve the capacitance characteristics for nitrogen-doped carbon materials.35,36 In general, the N-6 and N-5 in the carbons were regarded as electroactive sites, which would benefit the enhancement of electrical conductivity as well as capacitances.37 These results illustrate increasing total contents of N-6 and N-5, similar to previous results indicating that high N-6 and N-5 contents in porous carbons significantly contribute to conductivity.38
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Fig. 3 N 1s spectra of the nitrogen-doped porous carbons (a) LS-NC-600; (b) LS-NC-700; (c) LS-NC-800; (d) LS-NC-900, respectively. |
Sample | Nitrogen content (at%) | ||
---|---|---|---|
N-6 (397.6 eV) | N-5 (399.6 eV) | N-Q (403.2 eV) | |
LS-NC-600 | 79.1 | 15.6 | 5.3 |
LS-NC-700 | 76.3 | 16.2 | 7.5 |
LS-NC-800 | 47.2 | 36.4 | 16.4 |
LS-NC-900 | 39.1 | 43.3 | 17.6 |
Fig. 4a and b present photographic images and SEM images of lotus stems. The lotus stems presented numerous lamellar and thin-slice appearance, contributing to molten KOH permeation for activation. As shown in Fig. 4c and d, numerous microporous structures were observed when temperature was increased to 600 °C. TEM images of pore structure are presented in Fig. 4e and f. These two TEM images confirm that LS-NC-600 comprises turbostratic carbon with a disordered graphitic microstructure. This unique microporous structure benefits improvement of material conductivity. SEM and mapping images of LS-NC-600 are shown in Fig. S3,† which also displays uniform distribution of C, O and N on the surface. Nitrogen content is relatively small, corresponding to XPS results.
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Fig. 4 (a) The photographic images of lotus stem; (b) the SEM images of lotus stem; (c, d) the SEM images of LS-NC-600; (e, f) the SEM images of LS-NC-600. |
GCD test was also conducted to explore capacitance performance of four LS-NCs at a current density of 0.5 A g−1, as shown in Fig. 5c. Fig. 5d presents the GCD curves of LS-NC-600 at various current densities from 2 A g−1 to 10 A g−1. LS-NC-600 electrode manifested the longest discharge time with specific capacitance as high as 360.5 F g−1 at 0.5 A g−1. Specific capacitance values of LS-NC-700, LS-NC-800 and LS-NC-900 reached 269.1, 243.3 and 195.1 F g−1, respectively. The high specific capacitance of LS-NC-600 may be attributed to its well-developed porous structure and abundant N-5 and N-6 contents, which can improve charge transmission capacity and further promote electrochemical capacitive properties. Although the specific surface area of LS-NC-600 was not the highest, its specific capacitance was the highest (360.5 F g−1 at 0.5 A g−1); this result shows that N-5 and N-6 can enhance electrochemical properties of materials. LS-NC-600 featured better electrochemical performance than some biomass carbon materials mentioned in previous literature (Table S2†). Fig. 5a, the CV curves show obviously polarization peak at the voltage about −0.2 V, and from Fig. 5b and c, GCD curves do not exhibit a symmetrical shape as stated. Basically, these due to the following factor: at higher scan rates, the larger the ion transport resistance in the pores, which impedes the formation of the electrical double layer.39 This phenomenon indicates an excellent pseudo-capacitance effect through the nitrogen functional group.40,41
As a contrast, Fig. S4a† shows the CV curves of LS-NC-500 at different scan rates. The CV coves of LS-NC-500 at different scan rates display a typical electric double-layer capacitor characteristic.42 When the scan rate is increased to 100 mV s−1, the CV curves is a little deviate the typical rectangular shape, which is due to that the larger ion transport resistance in the pores impedes the formation of the electrical double layer at higher scan rates.43 The GCD curves of LS-NC-500 at different current density are illustrated in Fig. S4b.† When current density increased from 0.5 A g−1 to 10 A g−1, the GCD curves of LS-NC-500 always remain in a nearly triangular shape. The specific capacitance values of LS-NC-500 is 186.5 F g−1 at 0.5 A g−1, which is lower than LS-NC-600, may be attributed to its undeveloped porous structure and lower nitrogen content synergistic effect.44 This results are correspond to the early study.
Nyquist plots of the LS-NCs are shown in Fig. 5e. The small semicircle in the high-frequency region represents charge-transfer resistance,45 which is controlled by reaction kinetics. LS-NC-600 exhibited an almost vertical line at low frequency, implying its potential as an ideal capacitor.46 Fig. 5f summarises specific capacitance at different current densities in the range of 0.5–10 A g−1. With increasing current density, specific capacitance decreased, which can be attributed to the increase in diffusion limitation. In comparison with LS-NC-700, LS-NC-800 and LS-NC-900, LS-NC-600 electrode showed the highest specific capacitance, thereby implying its excellent rate capability. These results indicate the synergistic effects between nitrogen atoms doped in the electrode material and high specific surface area of the porous structure. Cycling stability is a key parameter for supercapacitor in practical applications. As shown in Fig. 6d, the LS-NC-600 electrode exhibited excellent stability with specific capacitance retention of 96% after 5000 cycles in three electrodes at the current density of 5 A g−1, suggesting its possible use for supercapacitor applications.
A two-electrode symmetric supercapacitor was assembled to evaluate energy storage performance of supercapacitors, as shown in the schematic in Fig. 6c. As shown in Fig. 6a, CVs of LS-NC-600 retained a rectangular shape as the scan rate was increased from 5 mV s−1 to 100 mV s−1, indicating rapid charge–discharge characteristics and good capacitance properties at high scan rates. All charge–discharge curves exhibited a typical double layer capacitance behaviour, as shown in Fig. 6b, which implies small internal resistance. High N content, hierarchical pore structure and good conductivity of the LS-NC-600 electrode led to low mass transfer resistance.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra13013a |
This journal is © The Royal Society of Chemistry 2018 |