High Surface Area Porous Carbon Flakes Derived from Boat-Fruited

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High Surface Area Porous Carbon Flakes Derived from Boat-Fruited Sterculia Seeds for High Energy Density Aqueous Symmetric Supercapacitors Youcai Ding, Li'e Mo, Chun Gao, Xuepeng Liu, Ting Yu, Wenyong Chen, Shuanghong Chen, Zhao-Qian Li, and Linhua Hu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00967 • Publication Date (Web): 27 Jun 2018 Downloaded from http://pubs.acs.org on June 27, 2018

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ACS Sustainable Chemistry & Engineering

High Surface Area Porous Carbon Flakes Derived from Boat-Fruited Sterculia Seeds for High Energy Density Aqueous Symmetric Supercapacitors Youcai Dinga,b, Li'e Moa, Chun Gaoa,b, Xuepeng Liua,b, Ting Yua,b, Wenyong Chena,b, Shuanghong Chen*a, Zhaoqian Lia, Linhua Hu*a

a

Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, 2221 Changjiangxi Road, Shushan District, Hefei, Anhui, 230088, P.R. China

b

University of Science and Technology of China, 96 Jinzhai Road, Baohe District, Hefei, 230026, P.R. China

Corresponding Authors *E-mail: [email protected] (S. Chen); [email protected] (L. Hu). Abstract: A new kind of N-doped porous carbon flakes with a super high surface area of 3243.92 m2 g-1 was derived from a common biomass waste (boat-fruited sterculia seeds) by a facile low-cost pyrolysis and activation method. This particular biomass has natural net-like structure consisting of interconnected polysaccharide chains rich in hydrophilic groups. We used the sponge-like biomass gel as carbon precursor directly without drying process and added some NaCl as the structure template, surprised to find that it is crucial for high surface area and the formation of two-dimensional materials. It is noted that the obtained materials possess a high specific capacitance (411.5 F g-1 at 1 A g-1) and excellent cycling stability in aqueous electrolytes. Moreover, the assembled symmetric cell with a wide voltage range of 2.0 V in 1 M Na2SO4 aqueous electrolyte, delivering a high energy density of 22.4 W h kg-1 at 500 W kg-1. Therefore, the outstanding electrochemical performance of the developed materials makes them promising candidates for high performance supercapacitors. Keywords: hydrous biomass carbonization; porous carbon; high surface area; ACS Paragon Plus Environment

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NaCl-templating; supercapacitors; INTRODUCTION The current energy demand and environmental crisis have greatly stimulated the related researches on developing advanced energy-storage devices.1-3 Nowadays, supercapacitors are considered as a priority candidate for future electrochemical energy storage (EES) devices, which store charge via electrolyte ions adsorption at the electrode/electrolyte interface or reversible redox reaction (Faradaic reaction) in the near-surface regions of the electrode.4-6 These electrochemical processes occurring in supercapacitors are rapid and reversible, which lead to the super properties of supercapacitor such as high power density, long cycle life, and fast charge-discharge rate, etc. Considering the amount of energy storage in supercapacitor is given by the formula of E=1/2 C U2, where C is the capacitance and U is the operating voltage of device, the supercapacitors with high energy density can be obtained either by developing higher capacitance electrodes or by increasing the operating voltage. Activated carbon materials were widely used in supercapacitors for their low cost, high specific surface area and excellent conductivity.7-9 However, the capacitance of conventional activated carbon was still limited due to its intrinsic characteristics. On the basis of previous works, the incorporation of heteroatoms can introduce additional surface pseudocapacitance in carbon materials for decreasing charge transfer resistance and improving wettability.10-14 Furthermore, it is worth mentioning that the rate performance of activated carbon is poor for its blocked pore texture. Recently, two-dimensional (2D) porous carbon has shown great potential to improve the rate performance of supercapacitors owing to its short diffusion distance and large surface area.15 Nevertheless, 2D carbon materials tend to agglomerate during the wet-chemistry processing, and the preparation are complicated and expensive. Therefore, using renewable biomass or its derivatives to produce 2D porous carbon has attracted increasing attention.16-19 The electrolyte commonly used in activated carbon (AC) based supercapacitors are organic salts, such as tetraethylammonium tetrafluoroborate (TEABF4) in ACS Paragon Plus Environment

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acetonitrile or propylene carbonate and aqueous electrolytes, for instance, KOH and H2SO4 aqueous solution. For organic electrolytes, the supercapacitors may operate at a voltage of about 2.8 V, while in basic and acidic aqueous electrolytes, the operating voltage is limited to 0.6-0.8 V. However, the high price and low conductivity of organic electrolyte limit the large-scale application of supercapacitors. Therefore, it is attractive to study aqueous electrolyte with wide potential window. In this work, we report a cost-effective method to produce a new kind of N-doped porous carbon flakes derived from boat-fruited sterculia seeds (It is a kind of beverage just like tea or coffee in China and Southeast Asian countries). This particular biomass has natural net-like structure consisting of interconnected polysaccharide chains rich in hydrophilic groups, which contribute to form porous structure. The obtained unique N-doped porous carbon possesses a super high surface area of 3243.92 m2 g-1 and interconnected porous texture with convenient ion diffusion paths, resulting in high specific capacitance of 411.5 F g-1 in 1 M Na2SO4 aqueous electrolytes at a current density of 1 A g-1, outstanding rate performance in aqueous electrolytes. To the best of my knowledge, the BET surface area and specific capacitance in 1 M Na2SO4 electrolytes of the obtained carbon are better than that of most of biomass-derived carbon ever reported.

EXPERIMENTAL SECTION Preparation of the NaCl-templating nitrogen-doped porous carbon (T-N-PC) Boat-fruited sterculia seeds were purchased from Tianfang Tea Co. Ltd. (Anhui, China). In a typical synthesis, 10 g of dried boat-fruited sterculia seeds were washed with deionized (DI) water, clearing the surface impurities, and then was impregnated into 200 ml of deionized water for about 2 hours until it becomes a sponge-like gel. Subsequently, removing seeds and skin of the boat-fruited sterculia seeds, and pouring out the excess DI water, here we get carbon precursors. Ultrathin nanosheet structured porous carbon was synthesized by one-step carbonization using the mixture of sponge-like gel, 1 g of urea, and 2 g of NaCl as the precursors. Here, urea was used as ACS Paragon Plus Environment


nitrogen source for N-doped carbon and NaCl was employed as template to obtain porous and nanosheets structure. The mixture was vigorously stirred by glass rod for about 10 minutes, and then transferred to the quartz tube furnace at a temperature of 600 °C for 2 hours in N2 flow. After that, the obtained product was ground into fine powder and washed with abundant DI water, and then dried at 80 °C for 12 h. Finally, the obtained carbon was thoroughly mixed with KOH at a weight ratio of 1:4, and then activated at 800 °C for 1 hour under a N2 flow. The activated carbon was thoroughly washed with 6 M HCl aqueous solution to remove inorganic salts, followed by rinsing with appropriate deionized water until the solution is neutral and then naturally drying at 80 °C for 12 hours. To explain the effect of water on the specific surface area of carbon materials, we set up a control experiment. The hydrous and non-hydrous boat-fruited sterculia seeds were employed as carbon precursors, the following carbonation and activation processes were the same as that of T-N-PC. We denoted the samples that derived from non-hydrous precursors as activated carbon (AC), the other one as porous carbon (PC). In particular, we dried the precursors of T-N-PC and deal them as the same method of T-N-PC, the obtained carbon was denoted as N-doped porous carbon (N-PC).

Material Characterization The microstructures of carbon were conducted with field-emission scanning electron microscope (SEM, Zeiss Sigma). Transmission electron microscopy (TEM) images of the carbon were obtained with a JEOL2010 microscope. X-ray photoelectron spectroscopy (XPS) was carried out in vacuum with a Kratos Axis Ultra X-ray photoelectron spectrometer using monochromatic Al Kα radiation as the excitation source at 15 kV and 150 W. Raman scattering spectra were taken on a Renishaw inVia Reflex spectrometer using the 514.5 nm line of Ar+ for excitation. The surface areas and porosities of the samples were measured by N2 adsorption/desorption experiments on a ASAP 2460 surface area and porosity analyzer (Micromertritics).

Electrochemical Measurements ACS Paragon Plus Environment

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All the electrochemical performances were measured on a Zahner IM6ex electrochemical workstation. The electrochemical measurements of these as-prepared carbon materials were carried out using a three-electrode system in both 1 M Na2SO4 and 6 M KOH aqueous solution. The saturated calomel and Hg/HgO electrodes served as reference electrodes, respectively. The working electrode was fabricated by mixing as-prepared samples (80 wt %), carbon black (15 wt %) and polytetrafluoroethylene (PTFE) binder (5 wt %) with ethanol to obtain the slurry and then coated the homogenous slurry onto a piece of nickel foam (1 cm2), finally pressed into a thin slice at a pressure of 10.0 MPa. The fabricated electrode was then dried at 80 °C for 8 hours in a vacuum oven. The mass loading of electrode materials on the nickel foam was approximately 2.8 mg cm-2. The symmetric supercapacitors were fabricated into a 2032-type coin cell with a glassy fibrous paper as separator, and 1 M Na2SO4 solution are used as the electrolyte. The specific capacitance of an individual electrode was calculated according to the flowing formula (1): ∆ (1) ∆ where I (A) is the discharge current, ∆t (s) is the discharge time, m (g) is the mass of ∁=

active material of single electrode and ∆V (V) is the voltage window. The specific capacitance calculated from CV curves was according to the formula S1 (Support Information). The electrochemical impedance spectroscopy (EIS) was obtained in the frequency ranging from 10 mHz to100 kHz with amplitude of 5 mV. The specific capacitance of the single electrode in the symmetric supercapacitor in a two-electrode system was obtained by the equation (2):

=

2 × × ∆ (2) × ∆

The energy density E (Wh kg-1) and power density P (W kg-1) can be calculated as follows:

E=

∁ × ∆ (3) 8 × 3.6

ACS Paragon Plus Environment


P=

3600 × (4) ∆

RESULTS AND DISCUSSIONS Material characterization Figure 1a illustrates the synthetic process of the NaCl templating N-doped porous carbon (T-N-PC). Boat-fruited sterculia seeds has many fruit bodies with a variety of cellular structure and shows a marked wrinkle in the dried stated. Putting some dried seeds of boat-fruited sterculia seeds into aqueous solution of sodium chloride, water molecules, Na+ and Cl- ions can be automatically inhaled into the tissue, and a huge volume increase of the seeds can be observed. Scanning electron microscopy (SEM) images reveal that the tissue has continuous network (Figure 1a) and cross-linked fibrous structure (Figure 1b), which favorable for ion transport. After carbonizing the gelatinous tissue of boat-fruited sterculia seeds, the black samples with porous structure were obtained, as shown in Figure 1d-f. It is made up of many carbon flakes with numerous uniformly sized mesopores and micropores. It worth noting that the AC exhibits monolithic morphology (Figure S1a, Support Information), the PC shows a small amount of irregular sheets structure with smooth surface (Figure S1b, Support Information) and the N-PC reveals a porous block structure (Figure S1c, Support Information), implying that hydrous precursors with some NaCl helps to form thin sheets of carbon and to create porous structure. To further investigate the detailed microporous structure of the T-N-PC, high-resolution TEM images were carried out. Figure 1g reveal that the T-N-PC consists of many carbon flakes with numerous nanopores of 1-3 nm in sizes, which contributes to a super high surface area. Beside the connected thin layers, there are many amorphous carbon particles about 20 nm in diameter interconnected tightly (Figure 1h), which conductive to a high-rate performance supercapacitor for facilitating the diffusion of electrolyte ions. Additionally, Figure 1i shows the primarily defective micro- and mesoporous morphology surrounded by curved carbon, indicating highly disordered structure. ACS Paragon Plus Environment

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Figure 1. (a) Schematic illustration of the synthesis of NaCl-templating N-doped porous carbon nanosheets derived from boat-fruited sterculia seeds. (b~c) SEM image ACS Paragon Plus Environment


of sponge-like boat-fruited sterculia seeds after water absorption. (d~f) SEM images of the NaCl-templating nitrogen-doped porous carbon (T-N-PC). (g~i) TEM images of the NaCl-templating nitrogen-doped porous carbon (T-N-PC). The X-ray diffraction (XRD) patterns of the obtained samples show two characteristic peaks located at 2-theta of 26° and 44°, corresponding to the (2 0 0) and (1 0 1) planes of graphitic carbon layers, respectively (Figure 2a).20 The broad diffraction peak of carbon indicates that there are abundant micropores exist in the samples.11 Comparing the spectra of PC and AC, we can see that the water residual in biomass tissue might be effective to increasing porosity as the spectra of PC shows a broader peak. As the Raman spectra shown in Figure 2b, two peaks located at about 1350 and 1590 cm-1 correspond to D (disordered carbon) and G bands (graphitic carbon), respectively. The intensity ratio between D and G bands (ID/IG) is summarized in Table 1. The ID/IG ratio obviously decreases from 1.51 for PC to 0.95 for AC, indicating the existence of water might result in more defects, which is consistent with the XRD analysis and TEM images. The porosity parameters of these carbon were further characterized by N2 adsorption/desorption isothermal analysis and the relevant data were listed in Table 1. All the samples exhibit typical type-I sorption isotherm curves with well-defined plateaus (Figure 2c), indicating the existence of abundant micropores.21 The T-N-PC possesses an apparent hysteresis loop at relative pressure P/P0 from 0.4 to 0.9, suggesting a well-ordered mesoporous structure. This result can be further confirmed by the pore size distribution calculated by the BJH model. As shown in Figure 2d, all samples have both micropores (0-2 nm) and mesopores (2-50 nm), and the pore size mainly focuses on ~ 2.6 nm. Compared with PC and AC, T-N-PC has the widest pore size distribution, which contributed to a high nitrogen BET specific surface area of 3243.92 m2 g-1 with a pore volume of 1.64 cm3 g-1 (Table 1). From the above results, it can be seen that the NaCl-templating and N-doping effectively make holes and increase specific surface area. What draws more of our attention is that the surface area of PC is much higher than that of AC (Table 1), further confirmed that the water is crucial for a high surface area carbon. ACS Paragon Plus Environment

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Figure 2. (a, b) XRD and Raman spectra patterns of AC, PC, N-PC and T-N-PC, respectively. (c) N2 adsorption/desorption isotherms and (d) pore-size distributions of AC, PC, N-PC and T-N-PC. Table 1. The porosity parameters and the ratio of IG/ID of the samples Sample

SBET (m2 g-1)

SMicro (m2 g-1)

Vtotal (cm3 g-1)

Daverage (nm)

ID / IG

Sp2 C-C

T-N-PC

3243.92

1512.42

1.64

2.02

1.61

69.16

N-PC

2778.45

774.39

2.03

2.92

1.55

71.89

PC

2450.61

1006.30

1.19

2.22

1.51

72.23

AC

228.89

188.76

0.14

3.80

0.95

82.45

To get more information of elemental chemical states of the heteroatoms in the TN-PC, X-ray photoelectron spectroscopy (XPS) was conducted. The full XPS spectrum of the T-N-PC shows three peaks at about 286.0, 401.0, and 532.8 eV corresponding to the C1s, N1s, O1s peaks, respectively (Figure 3a). Results of elemental analysis reveals that the T-N-PC mainly consist of C, N, O elements, and



their content is 84.22, 4.64, 10.69 at%, respectively. The high resolution C1s spectra have been resolved into four peaks centering at around 284.5, 285.5, 286.0, 288.0 eV, which belong to C-C, C-N, C-O and C=O. respectively. Moreover, the deconvoluted N 1s spectra contains three peaks, pyridinic-N (N-6, 398.2 eV), pyrrolic-N (N-5, 399.5 eV) and quaternary-N (N-Q, 400.8 eV) (Figure 3b and 3c). The relative percentages of the carbon species are given in Table 1. T-N-PC reveals relatively weaker sp2 C = C peaks compared to that of PC, indicating that nitrogen doping of porous carbon can produce defects, which is also confirmed by XRD and Raman spectra analysis. It has been reported that nitrogen doping of porous carbon can not only improve the wettability but also provide a large number of active sites to enhance pseudo-capacitance.22 Furthermore, oxygen-containing functional groups exist on the surface of porous carbon could also contributed to additional pseudo-capacitance by faradaic reactions (Figure 3d). With such characteristics, the carbon materials derived from boat-fruited sterculia seeds are expected to be promising materials for supercapacitors.

Figure 3. (a) General XPS spectra of T-N-PC. (b) The high resolution C1s spectra and


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fitting peaks of T-N-PC. (c) The high resolution N1s spectra and fitting peaks of T-N-PC. (d) Possible locations for N incorporation into T-N-PC. (e) Schematic illustration of the formation process of the carbon nanosheets. Based on the above analysis, we proposed a formation mechanism of porous carbon nanosheets. Figure 3e illustrates the structure evolution of the boat-fruited sterculia seeds aerogel during the carbonization and activation process. As shown in Figure 3e, the boat-fruited sterculia seeds possess net-like structure consisting of a large number of polysaccharide chains. These molecules of polysaccharide have substantial hydrophilic functional groups such as hydroxy and carboxy groups, adsorbing amounts of water molecules. In the carbonizing process, Na+ and Cl- ions vibrate violently in water, preventing the adjacent cell walls from agglomeration, and the atoms decomposed from molecules of polysaccharide connect together in situ to form ultra-thin nanosheets. During the subsequent high temperature activation process, KOH molecules react with carbon atoms on the surface of nanosheets, resulting in porous structure.

Electrochemical behaviors To estimate the electrochemical performances of these obtained carbon materials, cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) were carried out both in 1 M Na2SO4 and 6 M KOH electrolyte with a three-electrode configuration. As shown in Figure 4a and Figure S2a (6 M KOH, Support Information), the CV curves at different scan rates range from 2 to 100 mV s-1 exhibit an approximately rectangular shape with slight faradaic humps, indicating the combination of double-layer capacitive and pseudocapacitive behavior. The current density increases as the scan rate increases, the CV profiles still remain a near rectangle shape at various scan rates, demonstrating its excellent capacitive and behavior rate performance. The CV curves of T-N-PC, N-PC, PC, and AC electrodes in 1 M Na2SO4 electrolyte collected at a scan rate of 50 mV s-1 were shown in Figure 4c. Compared with T-N-PC, N-PC and PC electrodes, the AC shows symmetrical rectangular shape without any faradaic humps and the smallest CV area, ACS Paragon Plus Environment


suggesting that the pseudocapacitance mainly comes from the doped heteroatom (N) and oxygen-containing functional group. The T-N-PC electrode shows a little higher current density more than that of the N-PC and PC electrode with a similar mass loading, indicating that the NaCl templating can improve the capacitance performance of the samples effectively, which resulted from more micropores and higher specific surface area. The result is consistent with the BET analysis. It is remarkable that the CV curves in 6 M KOH electrolyte have the same conclusion as above (Figure S2c). Further studied by the GCD measurements at different current density in 1 M Na2SO4 (Figure 4b) and 6 M KOH electrolyte (Figure S2b), the capacitances of T-N-PC was 412 F g-1 at 1 A g-1 (380 F g-1 in 6 M KOH electrolyte). The GCD curves were symmetrical, nearly linear and almost without voltage drop (IR), indicating a good capacitive behavior, high coulombic efficiency and conductivity. As calculated from the CV curves, the specific capacitances of T-N-PC was 500 F g-1 at 2 mV s-1 (Figure 4c). The T-N-PC electrode exhibits high specific capacitance due to its high heteroatom doping, and suitable pore size distribution. The capacitance and specific surface area of the T-N-PC are better than those of most of biomass-derived carbon in aqueous electrolytes (Table S1). With the increase of current density, the capacitance decreases due to inadequate surface reaction and slow ion diffusion kinetics. At a current of 20 A g-1, the T-N-PC electrode retains a capacitance of 220 F g-1 in 1 M Na2SO4 (53% retention of its initial capacitance, Figure4c) and 326 F g-1 in 6 M KOH (86% retention of its initial capacitance, Figure S2d). The T-N-PC electrode shows excellent rate performance in KOH, while in Na2SO4, the rate performance is general. This result illustrate that the conductivity of the electrode is good, but the pore structure is more beneficial to K+ and OH-1 ions diffusion.


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Figure 4. (a) CV curves of the T-N-PC electrode at various sweep rates. (b) GCD curves of the T-N-PC electrode at different current densities. (c) CV curves of the T-N-PC, N-PC, PC and AC electrodes at a scan rate of 50 mV s-1. (d) Specific capacitances at different charge/discharge current densities and scan rates in 1 M Na2SO4. (e) Nyquist plots of the T-N-PC, N-PC, PC and AC electrodes (equivalent circuit in inset). (f) The enlarged view of the high-frequency region of Nyquist plots. Electrochemical impedance spectroscopy (EIS) is also a critical technique to further study the electrochemical performance of the samples. As the Nyquist plots of T-N-PC in 1 M Na2SO4 shown (Figure 4e), the steep linear curve of T-N-PC in the low frequency region is more close to a vertical line than that of PC, demonstrating that the NaCl templating can improve the capacitive behavior effectively. The small semicircle at high frequency region represents a small charge transfer resistance due to its super conductivity. The impedance data of the T-N-PC electrode are analyzed by fitting to an equivalent electrical circuit which can be expressed by Central Digital Computer code as Rs(Cd(Rct W)). Rs represent the total resistance of electrolyte resistance, electrode resistance and contact resistance. Cd, Rct and W designate the capacitance of the double layer, charge transfer resistance at the electrode/electrolyte interface and Warburg impedance, respectively. These quantitative parameters calculated by ZView software and some values are listed in Table S2. It is noticed that the T-N-PC electrode exhibit smaller charge transfer resistance, indicating a higher ACS Paragon Plus Environment


specific power density.

Figure 5. (a) CV curves of the T-N-PC symmetric supercapacitor at different scan rates. (b) CV curves of T-N-PC symmetric device measured at different potential window at 50 mV s-1. (c) GCD curves of the T-N-PC symmetric device at different current densities. (d) Cycling stability of the T-N-PC device for 10000 cycles at 5 A g-1. (e) Specific capacitance variation with voltage holding time for a period of 100 h. (f) Self-discharge profile for a period of 24 h. (h) Nyquist plots of the T-N-PC symmetric device before and after 10000 cycles. (i) Ragone plots of the T-N-PC device. (j) Ragone plots of the T-N-PC device compared with some reported carbon-based supercapacitors in different aqueous electrolytes. To further investigate practical applications of T-N-PC electrode materials, we assembled symmetric supercapacitors both in 1 M Na2SO4 and 6 M KOH aqueous electrolytes. As shown in Figure 5a (1 M Na2SO4) and Figure S2e (6 M KOH), the CV curves at scan rates from 2 to 100 mV s-1 retain a regular rectangular shape demonstrating its excellent capacitive behavior. Obviously, the CV profiles of the


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symmetric supercapacitors are more regular than that of the three-electrode system, suggesting a better capacitive performance. Figure 5b shows the CV curves of T-N-PC symmetric supercapacitor at 50 mV s-1 in different load voltage, the stable voltage window of the supercapacitor can expand to 2.0 V. The specific capacitance of the T-N-PC symmetric supercapacitor at 0.5 A g-1were 160.9 F g-1 in 1 M Na2SO4 and 190 F g-1 in 6 M KOH (Figure 5c, Figure S2f). Moreover, the T-N-PC symmetric cell exhibits a high energy density of 22.4 Wh kg-1 at a power density of 500 W kg-1 (Figure 5i). The performance of T-N-PC device precede to those of previously reported carbon-based electrodes in aqueous electrolyte (Figure 5f), such as commercially available activated carbons (5-10 W h kg-1), carbon/graphene nanoflakes (11.3 Wh kg-1 at 25 W kg-1), N-doped active carbon fiber (8.3 W h kg-1 at 4700 W kg-1), willow catkin derived carbon nanosheets (21 Wh kg-1 at 180 W kg-1) and fluorine and nitrogen co-doped carbon nanofibers (8.1 Wh kg-1 at 248 W kg-1). The long-term cyclic stability is one of the important factors of supercapacitors for practice use. To evaluate the durability of the T-N-PC device, we taken GCD method at 5 A g-1 for 10000 recycles, as shown in Figure 5d, the capacitance retention can nearly reach to 98%, indicating remarkable electrochemical stability. In order to further investigate the stability of the device, floating and self-discharge tests were carried out on the symmetric device. The device of T-N-PC was held at its maximum voltage of 2.0 V for 100 h and carried out charge-discharge test at every 10 h intervals. The capacitance increased after the first 20 h of voltage holding and then there is slight increase before tending to stabilize (Figure 5e). The self-discharge test is also important to a supercapacitor, which shows the device capability to withhold a continuous maximum voltage for 24 h. As shown in Figure 5f, these devices were charged up to 2.0 V (1 M Na2SO4) and 0.8 V (6 M KOH), the potential of the device (1 M Na2SO4) dropped sharply in a short period of time and down to 0.6 V within 8 h. However, for the other device (6 M KOH), the potential dropped slowly and kept at 0.4 V after 24 h. These results show that the stability of the supercapacitor employed 1 M Na2SO4 as electrolyte was lower than that of the supercapacitor used 6 M KOH as electrolyte. The poor stability of voltage holding is due to a large internal resistance. ACS Paragon Plus Environment


The Nyquist plots of the T-N-PC symmetric device before and after 10000 cycles was conducted and compared. As the Figure 5h shown, the shape of these two Nyquist plots was the same, but the internal resistance (Rs) of the Nyquist plot after 10000 cycles was increased, which is coincide with the self-discharge analysis. CONCLUSION In summary, a nitrogen-doped porous carbon with ultrathin nanosheet structure was derived from boat-fruited sterculia seeds by a one-step pyrolysis and activation process. The as-obtained carbon materials have abundant micropores with a super high surface area of 3243.92 m2 g-1 and high nitrogen content of 4.64 at%, which contributes to a high specific capacitance of 411.5 F g-1 at a current density of 1 A g-1. The symmetric supercapacitor by employing the T-N-PC as the electrodes, displays a remarkable specific energy of 22.35 Wh kg-1 and excellent cycling stability (2% loss after 10000 cycles). Therefore, this work shows a facile strategy to synthesize a porous carbon with super high surface area and excellent electrochemical performance, which supposed to be a promising material for supercapacitors, adsorbents and catalysts.

ACKNOWLEDGEMENTS This study was supported by the National High Technology Research and Development Program of China (No.2015AA050602), the National Natural Science Foundation of China (No. 61404142, No. GJHZ1607). China Scholarship Council International Clean Energy Talent Program Supported by the CASHIPS Director’s Fund (No. YZJJ2018QN21). ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM images of N-PC, PC and AC samples. Comparison of the properties for porous carbon obtained from different biomasses. Some electrochemical properties of these obtained carbon materials in 6 M KOH aqueous electrolytes. EIS parameters of the ACS Paragon Plus Environment

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For Table of Contents Use Only

The N-doped porous carbon nanosheets derived from a biomass waste is promising candidates for high performance supercapacitors.


High Surface Area Porous Carbon Flakes Derived from Boat-Fruited

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