Nitrogen-doped activated carbon derived from prawn shells for high-performance supercapacitors

Accepted Manuscript Title: Nitrogen-doped activated carbon derived from prawn shells for high-performance supercapacitors Author: Feng Gao Jiangying Qu Zongbin Zhao Zhiyu Wang Jieshan Qiu PII: DOI: Reference:

S0013-4686(16)30018-4 http://dx.doi.org/doi:10.1016/j.electacta.2016.01.005 EA 26383

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

1-11-2015 3-1-2016 3-1-2016

Please cite this article as: Feng Gao, Jiangying Qu, Zongbin Zhao, Zhiyu Wang, Jieshan Qiu, Nitrogen-doped activated carbon derived from prawn shells for high-performance supercapacitors, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.01.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Nitrogen-doped activated carbon derived from prawn shells for high-performance supercapacitors

Feng Gao* a, b, Jiangying Qua, b, Zongbin Zhaob, Zhiyu Wang *b, Jieshan Qiu *b

a

Faculty of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian,

Liaoning, 116029, China. b

Carbon Research Laboratory, Center for Nano Materials and Science, School of

Chemical Engineering, State Key Lab of Fine Chemicals, Dalian University of Technology, Dalian, 116012, China.

Corresponding authors. Tel: +86-411-82158329. E-mail: [email protected] (F. Gao), [email protected] (Z. Y. Wang), [email protected] (J. S. Qiu).

 

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Abstract Nitrogen doped activated carbons have been fabricated by simultaneous carbonization and KOH activation of chitosan biomass regenerated from prawn shell at high temperature. Sufficient porosity with high content of nitrogen heteroatom makes them very appealing for charge storage in supercapacitors. Specifically, the activated carbon obtained at 700 oC shows excellent capacitive performance in both acidic and alkaline electrolyte in virtue of the optimized combination of electronic double-layer capacitance and pseudo-capacitance. It exhibits high specific capacitance of 695 F g−1 in 1 M H2SO4 and 357 F g−1 in 6 M KOH at a current density of 50 mA g−1. Even cycled at high current density of 5 A g−1, high capacitance of over 280 F g−1 can be still retained in both electrolytes. Stable capacitance retention over 95 % for as long as 5000 cycles is also achieved at 1 A g−1 in both cases.

Keywords: Prawn shells, nitrogen-doped, activated carbon, supercapacitors

 

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1. Introduction Supercapacitors have received great attention as next-generation devices for electrical energy storage because of their unique advantages including high efficiency, superior power capability and long life-time. However, their practical applications in many fields such as consumer electronics, hybrid electric vehicles, and energy efficient industrial equipment have been greatly hindered by the intrinsic low energy densities [1-3]. Combining the electronic double layer (EDL) ion electrosorption and reversible redox reactions over the electrodes represents the most effective way to maximize the energy densities of supercapacitors [4-6]. Among available electrode materials, porous carbons offer good performance for EDL charge storage owing to the large surface area, rich porosity, and good electrical conductivity. Structural doping with electron-donated heteroatoms (e.g., N, O, B) could further enhance the capacitance of porous carbon electrode by introducing the pseudo-capacitance via reversible faradic reaction over the heteroatom-doping induced active site. To date, two strategies, including the synthesis via post-doping and in situ doping route, have been proposed to introduce the N heteroatom into carbon framework [7]. The post-doping method is commonly based on multistep treatment of porous carbon with nitrogen-containing precursors such as ammonia, melamine, or urea at high temperatures [7-11] being tedious in procedure with environmental and cost concern. The in situ incorporation of N heteroatom into carbon matrix can be achieved by using nitrogen-containing substance such as polyaniline, pyrrole, or polyacrylonitrile as both carbon and nitrogen precursors [12-14], rendering high efficiency of the process. However, those precursors, which are derived from petroleum, are dependent on dwindling resources and do not offer an environmentally sustainable solution. Alternatively, implementation of biomass precursors for synthesis of energy storage materials is receiving increasing interest. For example, N-doped microporous carbon plates from regenerated silk proteins have displayed specific capacitances of 264 F  

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g-1 in 1 M H2SO4 at 100 mA g-1 [15]. Hierarchically mesoporous nitrogen-rich carbons derived from egg white proteins have exhibited capacitances of 390 F g-1 as supercapacitors in 1 M H2SO4 at 250 mA g-1 [16]. Recently, porous N-doped carbons produced by hydrothermal carbonization of chitosan and glucosamine, followed by chemical activation, have demonstrated excellent charge storage performance [17-19]. These diverse pathways demonstrate the promising possibilities of various nitrogen-rich biomass precursors for supercapacitor applications. Prawn shells are an inexpensive and accessible source of biomass, with million tons of prawn shells annually produced and discarded [20]. They are composed of a nitrogen-rich chitin (poly-β(1→4)-N-acetyl-D-glucosamine) polysaccharide. Titirici et al. pioneered the synthesis of N-doped porous carbon using natural inorganic CaCO3/chitin shell composite as the precursor [20]. In that case, carbon materials with accessible interconnected mesopores (> 10 nm diameter) architecture, specific surface area of 328 m2 g-1, and 5.8 wt % nitrogen content was obtained. However, there were scarcely micro-pores in the obtained carbon materials, which was unfavorable for the improvement of capacitance for the supercapacitor [21]. So it is highly desirable to develop a cost-effective strategy to synthesize microporous nitrogen-rich carbons with good supercapacitor performance using pristine prawn shells as precursors for electrode materials. Herein, we report a simple, cost-effective, and sustainable process to prepare carbon materials with high surface area and moderate nitrogen content by in situ chemical activation of pristine prawn shells. As expected, the resulting N-doped carbons with much micropore and large specific surface area named as activated carbons offer promising energy storage properties in supercapacitor electrodes.

 

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2. Experimental 2.1 Synthesis of prawn shell-derived N-doped activated carbons In this study, shells of “Bohai prawn” were used as the raw precursor. Prior to use, the pristine shells were suspended in 10 wt % HCl solution at room temperature for 24 h to completely remove CaCO3. The demineralized shells were treated with 9 wt % NaOH solution at 95°C for 2 h with a weight ratio of 1:1 (w/w), yielding the chitin. Subsequently, 4.0 g of the chitin was refluxed in 140 mL of 40 wt % KOH solution under stirring for 5 h to get chitosan, followed by washing with deionized H2O to remove 48.8 g KOH and freeze-drying. The products were harvested and then annealed at 600-800 oC for 1h with a ramp rate of 3 oC min-1 in Ar flow. The black products were completely washed with 2 wt % HCl for 2 h to remove residual KOH, eventually giving rise to N-doped activated carbons (denoted as AC-T, where the T referred to annealing temperature). For comparison, the carbon materials without activation (denoted as C-700) were produced by annealing KOH-free chitosan derived from prawn shells at 700 oC for 1 h in Ar flow. 2.2 Material Characterization The morphology of the N-doped activated carbons was characterized with field-emission scanning electron microscopy (FESEM, FEI NovaNano SEM 450) and TEM (JEOL JEM-2100, Philips Tecnai G220). Nitrogen sorption isotherms of the carbon samples were collected at 77 K using a Micromeritics ASAP 2020 sorptometer. The composition of the samples was examined by a vario EL III Elemental Analyzer (Elementar, Germany). X-ray photoelectron spectroscopy (XPS) was carried out with the surface analysis system of ESCALAB250 (Thermo Electron Corporation of America). The peak resolution and fitting were processed by the software of XPSPEAK41.

 

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2.3 Electrochemical Measurement Cyclic voltammetry and galvanostatic charge/discharge were carried out in a one-compartment cell in 1 M H2SO4 or 6 M KOH solution using a three-electrode configuration on a CHI 660C electrochemical workstation. Working electrodes were made by pressing the mixture of active material (e.g., C-700), acetylene black and polytetrafluoroethylene (PTFE) with a weight ratio of 85:10:5 onto nickel foam (used as current collector in KOH solution) or titanium mesh (used as current collector in H2SO4). Platinum foil was used as the counter electrodes. Hg/Hg2SO4 and Hg/HgO electrodes were used as reference electrodes in 1 M H2SO4 and 6 M KOH electrolytes, respectively. The active mass was about 3.0-4.0 mg per electrode. Before testing, the prepared electrodes were soaked overnight in the electrolyte. The galvanostatic charge/discharge was used to estimate the specific capacitances (Cs) according to the formula of Cs = I × △t / (m × △V), where I is the constant discharge current, △t is the discharge time, △V is the potential window during discharge, and m is the active mass [22, 23]. The long-term cycling performance of AC-700 electrode was measured by the consecutive galvanostatic charge-discharge in 1 M H2SO4 and 6 M KOH solution at current density of 1 A g-1 on a Land Battery Tester at room temperature. For the assembly of the supercapacitor with symmetrical two-electrode configuration, the electrodes were fabricated by pressing a mixture of active materials, acetylene black and polytetrafluoroethylene (PTFE) with a weight ratio of 85:10:5 onto nickel foam or titanium mesh with a diameter of 1.0 cm. Two electrodes were assembled together with a non-woven fabric as separator. The mass loading of AC-700 in each electrode was typically 2.5 mg. The electrolyte was 6 M KOH (using nickel foam as collector) or 1 M H2SO4 solution (using titanium mesh as collector). The specific capacitance of the

 

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symmetrical supercapacitor was calculated from the discharge curves according to the equation of Cs = I × t / (ΔV × m), where m is the total mass of the AC-700 on both electrodes [24, 25]. The energy density (E) and power density (P) derived from the charge-discharge curves were calculated by the following equations: E= CsΔV2 / 7.2 (Wh kg-1), P = 3600 × E / t (W kg-1).

3. Results and Discussion Fig. 1 illustrated the strategy for the synthesis of N-doped activated carbon by demineralization-deproteination-deacetylation of prawn shells followed by KOH activation. Initially, prawn shells were demineralized and deproteinated to form chitin by sequential treatment with HCl and NaOH. The chitin was subsequently deacetylated in concentrated KOH to get chitosan through the stoichiometric reaction (1) [26-29], where the KOH was inserted into the layers of chitosan and mixed intensively with the chitosan. The mixture of chitotsan and KOH was annealed at 600-800 oC in Ar flow, eventually giving rise to N-doped activated carbon for capacitive applications by carbonization of chitosan and simultaneous activation.

Fig. 2a and Fig. 2b show field emission SEM and high-resolution TEM (HRTEM) images of AC-700, revealing the presence of flat micro-plates with a rough surface. Fig. 2c shows the N2 sorption isotherms of the carbon samples obtained at various temperatures, indicating the existence of micro-pores in AC-T samples (Fig. 2c and Fig. 2d). Such microporosity may greatly benefit the charge storage, thereby enhancing the capacitance of the electrodes [21]. The surface characteristics of all samples are summarized in Table 1. It is observed that the annealing temperature for KOH activation has great influence on the porous structure of AC-T samples. The specific surface area increases from 554.7, 1606.4 to 1917.6 m2 g-1 for AC-600, AC-700, and AC-800, respectively. The pore volume for the  

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AC-T samples follows similar changes. The pore size distribution of AC-T samples is calculated in terms of density functional theory (DFT) method, as shown in Fig. 2d. The micro-pores with a size of less than 3 nm dominate the porous structure of all the samples. As compared to the AC-T samples, non-activated C-700 shows much lower BET surface area (35.7 m2 g-1) with poor porosity.

(a) SBET is the specific surface area from multiple BET method. (b) Smic is the microporous surface area from t-plot method. (c) Smes is the mesoporous surface area from t-method external surface area (Smes = SBET-Smic). (d) Vpore is the total volume calculated at a relative pressure of 0.99.

The calcination and chemical activation process also influence the nitrogen content in carbon samples. As shown in Table 1, the nitrogen content is inversely correlated with annealing temperature. For example, the element analysis reveals that AC-600 contains 6.2 wt % nitrogen, while AC-700 and AC-800 contain 4.0 and 2.7 wt % nitrogen, respectively. In agreement with prior research reports [30, 31], we observe that oxidation-induced removal of nitrogen heteroatom that is correlated with the activation at high temperatures. The presence of nitrogen heteroatom in the samples was further examined using X-ray Photoelectron Spectroscopy (XPS). Fig. 3 shows high-resolution N 1s XPS spectra of AC-T samples. For all samples, the peaks at binding energies of 398.5, 400.3, and 401.2 eV can be assigned to pyridinic N (398.5±0.2 eV), pyrrolic or pyridone N (400.5±0.2 eV), and quaternary N (401.2±0.2 eV), respectively [32, 33]. The nitrogen content and the ratios of the three different nitrogen species evaluated by the peak separation analysis are listed in Table 2. It shows that the nitrogen content decreases with the increase of the calcination temperature. For example, the content of pyrindinic N in AC-600 sample is 1.44 wt %, while its content decreases to the 0.44 wt % in the AC-800 sample. Moreover, the ratios of  

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pyridinic, pyrrolic and quaternary N atom change when the annealing temperatures increase. It can be seen that as the main form of nitrogen element, the ratio of pyrrolic N reaches 58.6 % in the AC-600 sample, while in the AC-800 sample its ratio decreases to 27.5 % and the quaternary N becomes the main form nitrogen with the ratio of 51.5 %. From the Table 2, it can be indicated that pyrrolic and pyridinic nitrogen chemically transforms into quaternary groups at high temperatures, which is in accordance with the literature [34].

Cyclic voltammetry and galvanostatic charge/discharge are used to characterize the capacitive properties of the resultant carbon materials in both acidic and alkaline medium in three-electrode system. Fig. 4a and Fig. 5a show the typical cyclic voltammograms (CV) of AC-T and C-700 samples at a scan rate of 5 mV s−1 in 1 M H2SO4 and 6 M KOH, respectively. In both cases, the non-activated C-700 sample exhibits a rectangular charge/discharge profile with low capacitance, in agreement with its low specific surface area. After chemical activation with KOH, the AC-T samples present obvious redox peaks in H2SO4 electrolyte (Fig. 4a)and a rectangular-like shape in KOH electrolyte (Fig. 5a) on CV curves at a scan rate of 5 mV s−1. Such observations indicate that the capacitive response comes from the combination of EDL over the porosity and redox reaction over the N heteroatom doping induced active sites. The humps on CV curves are more obvious in acidic than that in basic medium, probably because of the Lewis base behavior of the nitrogen functionalities in the carbons [30, 35, 36].

The redox reactions can be also observed in the galvanostatic charge/discharge curves (Fig. 4b and Fig. 5b). Unlike linear characteristics, a transition can be easily noticed in both acidic and alkaline electrolyte for AC-T samples. Fig. 4c and 5c present the relationship between Cs and charge/discharge current densities. Despite the high nitrogen  

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content (6.7 wt %), the C-700 exhibits very low capacitance in both acidic and alkaline electrolytes (86.7 F g-1 in H2SO4 and 51.3 F g-1 in KOH at 50 mA g-1). After KOH activation, the AC-T samples exhibit dramatically increased capacitances under the same conditions. The results underscore electrosorption as the primary charge storage mechanism, i.e. a high specific surface area and pore volume are essential towards high capacitance. The Cs of the AC-T samples in both acidic and alkaline electrolytes decreases in the following order: AC-700 >AC-800 >AC-600. As known from N2 sorption isotherms, the surface area and pore volume of the carbon samples change in the order  as follow: AC-800>AC-700>AC-600 (Table 1). That is, the Cs for AC-T samples is not directly dependent on the specific surface areas. Such a difference may be ascribed to the pseudocapacitive contributions from nitrogen doping in carbon structures. The pseudocapacitance raises from the redox reactions of electrochemically active nitrogen functionalities on the carbon surface, e.g. >C=NH + 2e- +2H+ + >CH-NH2 [10, 37]. AC-700 shows the best performance in both acidic and alkaline electrolytes because it posesses high specific surface area (1606.4 m2 g−1) and a moderate N-doping level (4.0% obtained by elemental analysis). Its Cs is as high as 695, 507 and 445 F g−1 in 1 M H2SO4 (Fig. 4c) and 357, 328 and 315 F g−1 in 6 M KOH (Fig. 5c) at a current density of 50, 100 and 200 mA g−1, respectively. The specific capacitance of AC-700 is higher than that of AC-800 in both acidic and basic solution because the nitrogen content in AC-700 is twice the value in AC-800 (4.0 wt % vs. 2.0 wt %), although the surface area of AC-700 is lower than that of AC-800. Furthermore, the specific capacitances of AC-700 are significantly higher than those of activated carbon with a specific surface area of more than 3000 m2 g-1 (260 F g−1 in 6 M KOH at 100 mA g-1) [38], N-doped carbon materials (300 F g−1 in 1M H2SO4 and 220 F g−1 in 6 M KOH at 100 mA g-1) [30], and polymer/carbon composites (180-480 F g-1 in 2 M H2SO4 at 100 mA g-1) [39] due to the excellent combination of the electric double-layer capacitance with pseudocapacitance originated from the high specific  

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surface area and a moderate N-doping level. For all the samples, the specific capacitances progressively decrease with the increase of the current density from 0.05 to 15 A g-1, which is correlated with the increasing diffusion limitation. However, the Cs of AC-700 at 5 A g−1 still retain high values of 289 F g−1 (in 1 M H2SO4) and 288 F g−1 (in 6 M KOH), respectively, which means a good rate performance of AC-700 for the supercapacitor. Fig. 4d and Fig. 5d show the cycling performance of AC-700 electrodes in 1 M H2SO4 and 6 M KOH, respectively. The Cs of AC-700 decreases slightly during the first 100 cycles but retains over 95% of its initial value after 5000 cycles. In both acidic and basic electrolytes, the nearly symmetric charge and discharge curves (inset in Fig. 4d and Fig. 5d) confirm the high degree of the reversibility for charge storage in AC-700.

The capacitive performance of AC-700 is further evaluated using a symmetrical two-electrode cell configuration in 1 M H2SO4 and 6 M KOH solution. Fig. 6a-b show typical cyclic voltammograms curves of AC-700 at different scan rates in acid and basic solution. The curves of AC-700 retain relatively rectangular shapes even at a scan rate of 50 mV s−1, indicating good capacitance retention at high charge/discharge rates. Galvanostatic charge-discharge measurements for AC-700 symmetrical supercapacitors are also carried out at different current densities (see supporting information Fig. S1a and Fig. S1b). The charging and discharging curves look mostly symmetrical with a slight curvature, indicating some pseudocapacitive contribution along with the EDL contribution [40]. Specific capacitances for AC-700 sample calculated from discharge curves in acid and basic solution at different current densities are presented in Fig. 6c. The Cs of AC-700 in 1 M H2SO4 is higher than that of the electrode in 6 M KOH at identical current densities. The specific energy density is 10.0 and 7.8 Wh kg−1 at a current density of 50 mA g−1 in 1 M H2SO4 and 6 M KOH solution, respectively (Fig. 6d). With high specific power densities of 1000 W kg −1, the specific energy density of 6.5 Wh kg −1 can be still retained  

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for AC-700 symmetrical supercapacitors at high current densities of 2 A g

−1

in both acid

and basic solutions. These results are comparable to those of porous carbon nanosheets in aqueous electrolyte [41].

4. Conclusions N-doped

activated

carbons

have

been

fabricated

by

demineralization-deproteination-deacetylation-activation process of prawn shell. The resulting activated carbons have large specific area, nanoporosity with narrow pore size distribution, and robust surface chemistries. As an example, the AC-700 sample can deliver high specific capacitances of 695 F g−1 (in 1 M H2SO4) and 357 F g−1 (in 6 M KOH) at the current density of 50 mA g−1, a stable cycle life over 5000 cycles, a specific energy density of 10 Wh kg-1 and a specific power density of 1000 W kg-1. The materials, which are synthesized from inexpensive and readily available materials, offer a promising approach for the development of novel capacitive-based energy storage systems.

Acknowledgements This work is supported by the NSFC (Nos. 51372277, 50902066), China Postdoctoral Science Foundation (2013M530922, 2014T70253), Program for Liaoning Excellent Talents in University (LJQ2014118) and Doctoral Scientific Research Foundation of Liaoning Province. The authors also acknowledge the Dr. Boris Dyatkin from Drexel University to polish the article.

 

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[39] K. Zhang, L.L. Zhang, X.S. Zhao, J. Wu, Graphene/polyaniline nanofiber composites as supercapacitor electrodes, Chem. Mater. , 22 (2010) 1392. [40] B.G. Choi, Y.S. Huh, W.H. Hong, H.J. Kim, H.S. Park, Electrochemical assembly of MnO2 on ionic liquid-graphene films into a hierarchical structure for high rate capability and long cycle stability of pseudocapacitors, Nanoscale, 4 (2012) 5394. [41] X. Fan, C. Yu, J. Yang, Z. Ling, C. Hu, M. Zhang, J. Qiu, A Layered-nanospace-confinement strategy for the synthesis of two-dimensional porous carbon nanosheets for high-rate performance supercapacitors, Adv. Energy Mater., 5 (2014) DOI: 10.1002/aenm.201401761.

                             

18

  Figure captions   

 

1) 40% KOH

1) 10% HCl 2) 9% NaOH 95 oC

2) Freeze‐drying Chitin Chitosan/ KOH 600‐800 oC

Prawn shell

application

Supercapacitor

AC‐T

Fig. 1 Schematic illustration of the synthesis of N-doped activated carbons derived from prawn shells by the demineralization-deproteination-deacetylation-activation process.  

 

19

 

b

AC-800

)

500

8

c

-1

600

dV(d) (cm nm g

-1

AC-700

400

3

3

-1

Adsorbed volume (cm g )

a

300 AC-600

200 100 C-700

0

d

AC-600 AC-700 AC-800

6

4

2

0 0.0

0.2

0.4

0.6

0.8

0.4

1.0

0.8

1.2

1.6

2.0

Pore size (nm)

Relative pressure (P/P0)

2.4

2.8

 

Fig. 2 (a) FE-SEM and (b) HR-TEM images of AC-700. (c) N2 sorption isotherms of activated and non-activated carbon obtained at various temperatures. (d) DFT pore distributions of various activated carbons.

 

20

404

402

400

398

396

Binding Energy (eV)

394

Sum Back ground Pyrrolic N Pyridinic N Quaternary N

b

406

404

402

400

398

396

Binding energy (eV)

394

Intensity (a.u.)

Intensity (a.u.) 406

Intensity (a.u.)

Sum Back ground Pyrrolic N Pyridinic N Quaternary N

a

Sum Back ground Pyrrolic N Pyridinic N Quaternary N

c

406

404

402

400

Fig. 3 N 1s XPS spectra of (a) AC-600; (b) AC-700 and (c) AC-800.

 

21

398

396

Binding energy (eV)

394

5 mv s

1.50

-1

0.2

Potential (V)

-1

Current density (A g )

0.4

1 M H2SO4

a

2.25

0.75 0.00 -0.75 -1.50

AC-600 AC-700 AC-800 C-700

-2.25 -3.00 -0.8

-0.6

-0.4

-0.2

0.0

0.2

b

1 M H2SO4 50 mA g

AC-600 AC-700 AC-800 C-700

0.0 -0.2 -0.4 -0.6 -0.8

0.4

0

5000

-1

0.1 A g -1 1.0 A g -1 10 A g

1 M H2SO4

600

400

200

0 C-700

AC-600

AC-700

15000

20000

25000

-1

0.2 A g -1 2.0 A g -1 15 A g

AC-800

Sample name

100

d

60 40

1 M H2SO4

0.4

80

Potential (V)

c

0.05 A g -1 0.5 A g -1 5.0 A g

10000

Time (second) Capacitance retention (%)

-1

-1

Specific capacitance(F g )

Potential(V vs. Hg/Hg2SO4) 800

-1

20

1Ag

0.2

-1

0.0 -0.2 -0.4 -0.6 0

2000

4000

6000

Time (second) 0

0

1000

2000

3000

4000

5000

Cycle number

Fig. 4 (a) CV curves of C-700, AC-600, AC-700 and AC-800 at a scan rate of 5 mV s-1; (b) Galvanostatic charge-discharge curves of these samples at a current density of 50 mA g-1; (c) Specific capacitance of C-700, AC-600, AC-700 and AC-800 at different current densities; (d) Cycling performance of AC-700 electrode measured at current density of 1 A g-1, and the inset is the last 10 cycles of galvanostatic charge-discharge. All the above tests were conducted in 1 M H2SO4 solution.

 

22

Potential (V)

0.0 -0.5 -1.0

AC-600 AC-700 AC-800 C-700

-1.5 -2.0 -0.8

-0.6

-0.4

-0.2

-0.4

-0.6

-0.8

0.0

0

2000

500

-1

c

0.05 A g -1 0.5 A g -1 5.0 A g

-1

0.1 A g -1 1.0 A g -1 10 A g

-1

0.2 A g -1 2.0 A g -1 15 A g

6 M KOH 300

200 100 0 C-700

AC-600

AC-700

4000

6000

8000 10000 12000

Time (second)

Potential (V vs. Hg/HgO)

AC-800

100

d

80 60 40

0.0

6 M KOH -1 1Ag

-0.2

Potential (V)

-1

AC-600 AC-700 AC-800 C-700

-0.2

0.5

400

6 M KOH -1 50 mA g

b

1.0

-2.5 -1.0

Specific capacitance(F g )

0.0

6 M KOH -1 5 mV s

a

1.5

Capacitance retention (%)

-1

Current density (A g )

2.0

-0.4

-0.6

-0.8

20

0

1000

2000

3000

4000

5000

Time (second)

0

0

1000

2000

3000

4000

5000

Cycle number

Sample name

Fig. 5 (a) CV curves of C-700, AC-600, AC-700 and AC-800 at a scan rate of 5 mV s-1; (b) Galvanostatic charge-discharge curves of these samples at a current density of 50 mA g-1; (c) Specific capacitances of C-700, AC-600, AC-700 and AC-800 at different current densities; (d) Cycle performance of AC-700 electrode measured at a current density of 1 A g-1, and the inset is the last 10 cycles of galvanostatic charge-discharge. All the above tests were conducted in 6 M KOH solution.

 

23

a

3 -1

Current density (A g )

AC-700 1 M H2SO4

-1

Current density (A g )

3 2 1 0 -1

-1

5 mV s -1 10 mV s -1 30 mV s -1 50 mV s

-2 -3 -4

0.0

0.2

0.4

0.6

0.8

1 0 -1

-1

5 mV s -1 10 mV s -1 30 mV s -1 50 mV s

-2 -3 -4

1.0

0.0

Energy density (Wh kg )

AC-700 1 M H2SO4

-1

-1

Specific capacitance (F g )

100 AC-700 6 M KOH

60

40

20

0

500

1000

1500

0.2

0.4

0.6

0.8

1.0

Potential (V vs. Hg/HgO)

Potential (V vs. Hg/Hg2SO4)

c

AC-700 6 M KOH

b

2

2000

d

AC-700 1 M H2SO4 AC-700 6 M KOH

10

1

0.1 10

100

1000 -1

-1

Power density (W kg )

Current density (mA g )

Fig. 6 (a, b) CV curves of AC-700 symmetrical supercapacitors at different scan rate in 1 M H2SO4 and 6 M KOH solution, respectively; (c) Specific capacitance of AC-700 symmetrical supercapacitors at different current densities in 1 M H2SO4 and 6 M KOH solution; (d) Ragone plot of AC-700 symmetrical supercapacitors.

 

24

Table 1. Porous properties and chemical compositions of AC-T Sample

SBET a 2

AC-600 AC-700 AC-800

Smic b

-1

2

-1

[m g ]

[m g ]

554.7 1606.4 1917.6

361.6 1554.2 1790.6

Smes c 2

Vpore d

-1

3

-1

[m g ] [cm g ] 193.1 52.2 127.0

0.17 0.65 0.82

Elemental analysis (wt %) C

H

N

O

C/N

65.6 72.9 84.2

2.9 2.1 1.0

6.2 4.0 2.7

25.3 21.0 12.1

10.6 18.2 31.2

   

Table 2: Contents and ratios of pyridinic, pyrrolic and quaternary N atom in AC-T samples. sample AC-600 AC-700 AC-800

Nitrogen

Pyrindinic

Pyrrolic

Quaternary

content (wt %)

(wt %)

(wt %)

(wt %)

6.0 3.8 2.0

1.44 (24.0%) 3.52 (58.6%) 0.84 (22.1%) 1.26 (33.2%) 0.42 (21.0%) 0.55 (27.5%)

   

 

25

1.04 (17.4%) 1.70 (44.7%) 1.03 (51.5%)