carbon nanohorns composite as a high-performance supercapacitor electrode

Accepted Manuscript Title: Nitrogen-doped Graphene/Carbon Nanohorns Composite as a High-performance Supercapacitor Electrode Authors: Xiao-Qiang Lin, Wen-Dong Wang, Qiu-Feng Lu, ¨ Yan-Qiao Jin, Qilang Lin, Rui Liu PII: DOI: Reference:

S1005-0302(17)30157-3 http://dx.doi.org/doi:10.1016/j.jmst.2017.06.006 JMST 1001

To appear in: Received date: Revised date: Accepted date:

14-4-2017 11-5-2017 7-6-2017

Please cite this article as: Xiao-Qiang Lin, Wen-Dong Wang, Qiu-Feng Lu, ¨ Yan-Qiao Jin, Qilang Lin, Rui Liu, Nitrogen-doped Graphene/Carbon Nanohorns Composite as a High-performance Supercapacitor Electrode (2010), http://dx.doi.org/10.1016/j.jmst.2017.06.006 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 Graphene/Carbon Nanohorns Composite as a High-performance Supercapacitor Electrode Xiao-Qiang Lin1, Wen-Dong Wang1, Qiu-Feng Lü1,2, * Yan-Qiao Jin1,2, Qilang Lin1, Rui Liu3,*

1

College of Materials Science and Engineering, Fuzhou University, Fuzhou 350116, China

2

Key Laboratory of Eco-materials Advanced Technology, Fuzhou University, Fuzhou 350116, China

3

Ministry of Education Key Laboratory of Advanced Civil Engineering Material, School of Materials

Science and Engineering, and Institute for Advanced Study, Tongji University, Shanghai 201804, China

* Corresponding authors. Ph.D. (Q.-F. Lü); Prof., Ph.D. (R. Liu); Tel.: +86 591 22 866 540; Fax: +86 591 22 866 539.

E-mail addresses: [email protected] (Q.-F. Lü); [email protected] (R. Liu).

[Received 14 April 2017; Received in revised form 11 May 2017; Accepted 7 June 2017] Nitrogen-doped graphene/carbon nanohorns composite (NGLC) was prepared by one-step co-pyrolysis of graphene oxide, carbon nanohorns (CNHs), urea, and lignosulfonate. CNHs as spacers were inserted into graphene nanosheets. The introduction of CNHs and the loosened nano-structure of NGLC make it achieve a high specific capacitance of 363 F g-1 at a discharge current density of 1 A g-1, and NGLC exhibits an ultrahigh stability of 93.5% capacitance retention ratio after 5000 cycles. The outstanding comprehensive electrochemical performance of NGLC could meet the need of the future acted as an efficient supercapacitor electrode material. Keywords:

Nitrogen-doped

graphene;

Carbon

High-performance

1

nanohorns;

Spacer;

Supercapacitor;

1. Introduction Supercapacitors are considered a class of excellent energy storage devices due to their ultrahigh power density and exceptionally long cycling life[1-4]. Because of its large surface area[5,6], supernal chemical stability[7,8] and great electrical conductivity[6,9], graphene is regarded as an ideal supercapacitor electrode material[10-12]. However, under strong Van der Waals interactions among graphene layers, graphene nanosheets are inclined to form nonreversible agglomerates and restacking. As a result, electron and ion transports are limited and lead to a poor specific capacitance. In order to address the restacking challenge, several approaches have been developed. Typical methods include fabrication of highly crumpled and corrugated graphene nanosheets[13,14], template-directed or graphene layers self-assembly to form three-dimensional (3D) porous structures[15,16], or use of guest materials as spacers[17,18]. On the other hand, chemical doping has been widely used to enhance capacitor property of graphene[19,20]. Among various dopants, nitrogen doping is easy to achieve in graphene due to its comparable atomic size and high electronegativity. Usually, N atoms mainly exist in three forms in graphene. Pyridinic-N and pyrrolic-N are beneficial to wettability as well as inducing pseudo-capacitance, while graphitic-N improves the electrical conductivity of graphene. Several methods (such as electron beam deposition[21], arc discharge[22], nitrogen plasma[23,24]) have been reported to produce N-doped graphene when ammonia or pyridine is served as a nitrogen source. However, low nitrogen doping rate, rigorous reaction conditions and special instruments prompt us to search effective methods to obtain N-doped graphene with high quality. Among a variety of novel nitrogen sources, urea has a high N content (46 wt%), and can react with oxygen-containing groups. Hence, urea has been widely used in nitridation of metal oxide or activated carbon[25-27]. Recently, graphene-based nanocomposites have been explored to enhance the utilization of graphene. A typical example is graphene/carbon nanotube composites with excellent electrochemical performances[28,29]. Carbon nanotubes are not only used to effectively avoid stack of graphene nanosheets but also as pathways of electron conduction in graphene/carbon nanotube composites[30]. However, Carbon nanotubes tend to be stacked into bales, resulting that only a few outmost surfaces of carbon nanotubes can absorb ions, and their inner surfaces are all waste [18]. Carbon nanohorns (CNHs) 2

have a length of less than 50 nm, a conical structure with a base diameter of less than 5 nm, and novel physical properties and a porous feature

[31]

. Although similar to graphene structures, CNHs have

unique structure with a horn shape with large surface area, superior porosity, internal nanospace, and great electrical conductivity[32]. Combination of CNHs and graphene can improve ability of electron transfer when CNHs are inserted into graphene layers and as a bridge to connect graphene nanosheets. Meanwhile, CNHs increase the basal spacing between graphene nanosheets to reduce the restacking and agglomerates. Furthermore, the intense  stacking interaction between CNHs and graphene can enhance specific capacitance value of the graphene/carbon nanohorns composites[33]. In this study, nitrogen-doped graphene/carbon nanohorns composite (NGLC), as an electrode material for supercapacitor, was prepared by using a one-step co-pyrolysis method from graphene oxide (GO), CNHs, low-cost lignosulfonate and urea. Urea was served as an expansion agent and nitrogen source for NGLC. Lignosulfonate was used as a dispersant to render graphene nanosheets with loosened nano-structures. CNHs not only performed as spacers adsorbed on the graphene oxide nanosheets but also acted as connectors between graphene players. During the co-pyrolysis process, GO was thermally reduced[34], and nitrogen atoms have been brought into the graphene lattice[35], thus fluffy structure of NGLC was fabricated. Consquently, the obtained NGLC composite is expected to possess superb specific capacitance and ultrahigh cycling stability.

2. Experimental 2.1. Materials Expanded graphite, phosphoric acid, sulfuric acid, hydrogen peroxide, potassium permanganate and urea were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Lignosulfonate (LS) and carbon nanohorns were purchased from Liangfeng Chemical Co., Ltd. (Jinan, China) and Beijing Tsing Da Jinguang Science & Technology Development Co., Ltd. (Beijing, China), respectively. And all of them were used directly as received. 2.2. Preparation of NGLC, GCH and NG composites GO was prepared as previously published literature[36]. Firstly, GO suspension (200 mg GO) and CNHs (10 mg) were added into deionized water, and then the mixture was ultrasonically dispersed for 3

2 h. Secondly, 20 g urea and 20 mg LS were successively added into the above mixture solution in regular sequence and stirred for 0.5 h. The solution was dried at 60 °C for 24 h to prepare a precursor. Finally, NGLC was obtained by pyrolyzing the precursor at 800 °C with heating rate of 3 °C min-1 under nitrogen flow for 2 h. The typical procedure including mixing and co-pyrolysis was illustrated in Scheme 1. For comparison, graphene/carbon nanohorns composite (GCH), and nitrogen-doped graphene (NG) were prepared from a mixture of GO and CNHs, a mixture of GO and urea, respectively, using the identical above-mentioned procedure. 2.3. Characterization Microstructures of samples were characterized by field emission scanning electron microscopy (FE-SEM, Carl Zeiss ULTRA 55), and transmission electron microscopy (TEM, FEI TECNAI F20). Fourier transform infrared (FT-IR) spectra were measured to characterize functional groups by employing a spectrophotometer (Nicolet FT-IR 5700). Raman spectra were obtained from a Raman spectrometer (HORIBA Jobin-Yvon). Wide-angle X-ray diffractometer system (Rigaku, Japan) was applied to the X-ray analysis. Elemental analysis was recorded on an X-ray photoelectron spectrum from an X-ray photoelectron spectrometer (ESCALAB 250). Brunauer-Emmett-Teller (BET) surface area was observed on a Quantachrome NOVA 4200e analytical instrument. 2.4. Electrochemical measurements Electrochemical experiments were carried out by using a CHI660E electrochemical workstation (Chenhua Instruments Co.). Three electrodes corresponded to glassy carbon working electrode, platinum wire counter electrode and Ag/AgCl counter electrode, respectively. Meanwhile, 6 M KOH aqueous solution was used as electrolytic medium. Working electrode was fabricated by a titration method: in brief, 3 mg sample and 10 μL 5 wt% Nafion were directly dispersed into 1 mL ethyl alcohol, and then mixed under ultrasound for 0.5 h; afterwards, 10 μL mixture was dropped on a 3-mm diameter glassy carbon disk, and then dried at 60 °C. Cyclic voltammetry (CV) curves were recorded from -1.0 to 0 V. Galvanostatic charge-discharge (GCD) test was measured at different current densities from 0.2 A g-1 to 5 A g-1. The electrochemical impedance spectroscopy (EIS) was obtained and the range of coverage was 0.01 - 100 kHz. The cycling stability of NGLC was performed by using CV at a scanning speed of 100 mV s-1 for 5000 4

cycles.

3. Results and discussion 3.1. Characterization of NGLC FE-SEM image of GCH in Fig. 1(a) shows a stacked graphene and smooth surface. Carbon nanohorns are only deposited on the stacked graphene surface. However, when urea was added into GO and the mixture of GO, LS and CNHs, the wrinkled and corrugation nanostructures and the few-layer graphene structures of the as-prepared NG and NGLC samples are obvious in images of Fig. 1(b, c). It can be attributed to the formation of gases (NH3 and CO2) during the decomposition process of urea, the gases increase distance between graphene nanosheets and prevent graphene from stacking[37]. Fig. 1(c) clearly shows that CNHs have been inserted between graphene nanosheets. Although the dispersion of CNHs in NGLC is not uniform, CNHs work as spacers to increase the basal spacing between graphene nanosheets. Therefore, the aggregation and uneven dispersion of CNHs have little effect on the electrochemical performance of NGLC. Furthermore, TEM images in Fig. 1(d, e) further confirm the existence of CNHs, and reveal that loosened structures of NGLC were obtained with the introduction of LS into the precursor [38]. As shown in Fig. 2(a), FT-IR spectra of NG and NGLC exhibit broad and intense peaks at 3400 cm-1, which may be attributed to O-H and N-H stretching vibrations[39]. These results reveal that some oxygen functional groups derived from graphene oxide surface still remain after annealing. The absorption peaks located at 1590 cm-1 and 1150 cm-1 are ascribed to C=C stretching deformation of benzenoid rings and C-O stretching vibrations[40]. The peak located at 1350 cm-1 is a characteristic peak of C-N stretching vibrations of benzenoid rings[39]. It can be concluded that N-H groups have been effectively introduced into the NGLC composite. Fig. 2(b) shows the XRD curves of GCH, NG and NGLC. There were two obvious peaks located at 26.3° and 43.4°, corresponding to the (002) and (100) lattice planes of graphene nanosheets, respectively[41]. The existing of the two diffraction peaks of NG and NGLC suggests that nitrogen doping did not destroy the structure of graphene lattice. XRD analyses of NG and NGLC indicate that 5

the characteristic of (002) peaks shifts to lower angle compared with GCH, revealing bigger lattice distances of NG and NGLC[42]. They also demonstrate that the graphene sheets were weaker stacking aggregation. In addition, the full-width half-maximum (FWHM) of NGLC is wider than those of others, illustrating the change of preferred orientation[38]. The reason for this consequence can be ascribed to the introduction of CNHs in NGLC. On the other side, the FWHM of GCH is smallest, indicating that the GCH grains stack more closely. The above results are consistent with observations of FE-SEM images. As presented in Fig. 2(c), Raman spectra of the three samples have two obvious peaks. The peaks located at 1350 cm-1 and 1590 cm-1 are D band and G band[43], respectively. The D band is a typical characteristic of defects and disorder of carbon materials; and the sp2 hybrid carbon atoms are reflected by the G band. The D and G bands intensities comparison values (ID/IG) are a quantitative measure to estimate defects of carbon materials. The ID/IG values of GCH, NG and NGLC are 0.95, 1.06 and 1.02, respectively. Obviously, The ID/IG values of NG and NGLC are higher than that of GCH, which might be attributed to the generation of smaller nanocrystalline graphene domains in NG and NGLC [5]. The surface chemical compositions and state of elements in NGLC are revealed by the X-ray photoelectron spectra (XPS) analysis (Fig. 3(a)). It is found that NGLC displays prominent C 1s (285 eV), N 1s (400 eV) and O 1s (532 eV) peaks and a faint S 2p (164 eV) signal, evidencing the coexistence of C, N, O and S elements in NGLC sample[44]. The atomic concentrations of C, N, O and S elements are up to 89.01, 6.12, 4.57 and 0.30 at%, respectively (Table 1). The high resolution C 1s spectrum of NGLC is divided into six peaks and shown in Fig. 3(b). The peak of C 1s at 284.95 eV with a strong intensity corresponds to C=C bonds; and it is suggested that most carbon atoms of NGLC are sp2 carbon atoms existing in conjugated lattice. Another two peaks located at 285.5 and 287.2 eV are ascribed to the C-C, C=N and C-N bonds, suggesting that nitrogen atoms have been introduced into the NGLC composite[45]. The peaks centered at 286.45, 288.35 and 291.8 eV are corresponding to the C-O, C=O and π-π* interaction, respectively. During the pyrolysis process, some carbon atoms of graphene have been replaced with nitrogen atoms. It is further confirmed by N 1s peak in Fig. 3(c). There are four different states of nitrogen in graphene basal planes, which are pyridinic-N (398.5 eV, 31.52 at.%), pyrrolic-N (399.85 eV, 25.93 6

at.%), pyridinie-N-oxide (404.4 eV, 10.08 at.%) and graphitic-N (401.3 eV, 32.47 at.%)[45], respectively. It is known that, pyridinic-N and pyrrolic-N are beneficial to the wettability and capacitance of NGLC, and graphitic-N can improve the electrical conductivity of NGLC. Therefore, N-doped composites possess excellent electrochemical property. From the O 1s XPS spectra in Fig. 3(d), three peaks at 531.6 eV (C-O bond), 532.55 eV (C=O bond) and 533.85 eV (O-C=O bond), respectively, are observed[46]. In addition, the small amounts of sulfur atoms doping in the NGLC composite are derived from LS, indicating that LS and GO had a reaction during the co-pyrolysis process. From the above results, we could conclude that urea, LS, graphene, and CNHs have reacted during the co-pyrolysis, and that N atoms have successfully been doped in graphene layers. Pore size distributions of NG and NGLC are further determined (Fig. 4) by using N2 adsorption and desorption measurement. NGLC exhibits a narrow meso-pores distribution ranging, whereas NG has a wide range distribution. The average pore diameters of NG and NGLC are of 27.03 and 7.97 nm, respectively. Thus, NGLC has a higher BET surface area of 465.0 m2 g-1 than that of NG (197.6 m2 g-1). This might be due to the introduction of LS and carbon nanohorns in the precursor. LS could be used as a dispersant of graphene during co-pyrolysis process. CNHs could be inserted between graphene layers as spacers[34]. Therefore, LS and CNHs effectively prevented the agglomerate and restack of as-prepared graphene layers, increasing the specific surface area. The loosened nano-structure of NGLC with numerous meso-pores and high specific surface area will facilitate electrolyte ions more easily access to the surface of graphene sheets and improve their specific capacitance. 3.2 Electrochemical performance The CV curves of GCH, NG, and NGLC electrodes at a scanning speed of 100 mV s-1 in a 6 M KOH solution are shown in Fig. 5(a). The CV curves of the electrodes present nearly rectangular shapes, indicating that the electrodes display ideal double layer capacitors nature[47]. There are some small redox peaks at about -0.5 V, which may be attributed to the residual oxygenous groups derived from graphene oxide surface and the nitrogen-doped graphene. Furthermore, the larger surrounded area of a CV curve means a higher specific capacitance[48]. It is observed that the CV curve of NGLC is larger than those of the other materials. So, NGLC is a high specific capacitance electrode material for 7

supercapacitor. The GCD tests of different electrode materials at a discharge current density of 1 A g-1 are shown in Fig. 5(b). The high specific capacitance of NGLC is up to 363 F g-1, and it is the biggest value among the three electrode materials. Accordingly, nitrogen doping can improve the wettability of materials so as to enhance their capacitance [49]. Obviously, residual oxygen containing groups and nitrogen doping greatly improve the capacitances of NGLC and NG due to their pseudo-capacitance during the galvanostatic charge-discharge processes. Compared with NG, carbon nanohorns in NGLC work as spacers preventing the agglomerate of graphene layers, and they also could improve the surface area of NGLC. Besides, compared with GCH, NGLC has a loosened structure of graphene sheets because of the addition of LS and urea in the precursor. The CV curves of NGLC at different scan rates from 5 to 300 mV s-1 are shown in Fig. 6(a), the typical rectangular shapes at different scan rates exhibit excellent electric double layer capacitive features. The CV shapes still remained rectangle even at the high scan rate, meaning an excellent rate capability of NGLC as an electrode material. The GCD curves of NGLC at different current densities are shown in Fig. 6(b). It is obvious that the curves are mainly symmetric between charging plots and discharging plots at different current densities from 0.5 to 5.0 A g-1, implying that NGLC has a good capacitive reversibility [50]. From Fig. 6(c), when the current density is 0.5 A g-1, the capacitance of NGLC is up to 375 F g-1. At a current density of 5.0 A g-1, the capacitance still remained 310 F g-1, and only a 17.3% capacitance loss over the current density range. So, NGLC has a great rate performance as a supercapacitor electrode[51]. Furthermore, the capacitance and rate capability of NGLC are compared with previously published different nitrogen-doped graphene electrode materials (Table 2). Apparently, NGLC possesses a superior specific capacitance and great rate capability as an electrode for supercapacitor. In order to explore the electrochemical information of NGLC, EIS is carried out within a frequency range of 0.01 - 100 kHz. The Nyquist plots of GCH, NG and NGLC are shown in Fig. 7(a). Inner resistance or equivalent series resistance (ESR) represents the sums of internal resistances of the carbon material, the electrolyte resistance, and the contact resistance of the working electrode. Owing to the introduction of CNHs, the ESRs of NGLC and GCH (about 4.0 Ω) are smaller than that of NG of 33 Ω. 8

The CNHs work as bridges to connect graphene nanosheets, and provide the channels of the charge transfer and great conductivity of NGLC. The Nyquist plot shows a semicircle at high frequencies, which is associated with the Faradic charge-transfer resistance of the sample. However, GCH and NGLC possess more inconspicuous semicircles than that of NG, meaning that Faradic charge-transfer resistances of GCH and NGLC are very small[55]. The nano-porous structure is used as an electron transfer pathway, and the CNHs greatly improve the conductivities of GCH and NGLC, leading to lower resistance value[56]. In the case of the ideal double-layer supercapacitor, the Nyquist plot should theoretically be a vertical line in the lower frequencies. However, it could be seen that the phase angle of NGLC is around 75°. NGLC does not follow such capacitive behavior because of the pseudo-capacitive effect caused by nitrogen doping. In short, nitrogen doping and the insertion of CHNs in NGLC effectively decrease its resistance. So, NGLC shows an outstanding capacitive behavior. Cycling stability, one of the most important properties of supercapacitor electrode, is carried out at a scanning speed of 100 mV s-1 for 5000 cycles in 6 M KOH solution. As shown in Fig. 7(b), the specific capacitance of NGLC still maintains 93.5% of the original value after 5000 CV cycles, meaning that the NGLC electrode has a supernal electrochemical stability[50]. On the contrary, the specific capacitance of NG increased 14%, which is probably ascribed to an “electro-activation”[38]. The high stability of NGLC is attributed to the nitrogen doping and porous structure, which could make ions and electrons in the electrolyte be steadily transported to the electrode surface[38]. Although the graphitization degree of NGLC is lower than that of GCH, nitrogen-doping effect and the loosen structures with high specific area could effectively improve the conductivity of NGLC, and its specific capacitance. Therefore, NGLC with a high electrochemical performance can be used as an ideal supercapacitor electrode.

3. 4. Conclusion NGLC has been developed through pyrolysis procedure from the mixture of GO, CNHs, LS and urea. The specific surface area of NGLC is up to 465.4 m2 g-1, resulting from the introduction of carbon 9

nanohorns into graphene nanosheets, and the loosened nano-structure of NGLC. CNHs are inserted between graphene layers, and they are used not only as spacers but also as a bridge to connect graphene sheets. NGLC possesses an outstanding comprehensive electrochemical performance, and the specific capacitance is up to 363 F g-1 at a current density of 1 A g-1, and a high stability of 93.5% capacitance retention ratio after 5000 CV cycles. The results demonstrate a promising prospect for the application of NGLC as a high performance supercapacitor.

Acknowledgments The authors acknowledge support from the National Natural Science Foundation of China (Grant No. 51503041), the Natural Science Foundation of Fujian Province, China (Grant No. 2016J01729), and the Key Program of Youth Natural Science Foundation of Fujian Province University, China (Grant No. JZ160413).

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13

Figure and table captions

Scheme 1. Schematic illustration of fabrication procedure of NGLC.

Fig. 1. FE-SEM images of (a) GCH, (b) NG, (c) NGLC, and (d, e) TEM images of NGLC. Fig. 2. FT-IR spectra (a), XRD diffraction curves (b) and Raman spectra (c) of GCH, NG, and NGLC. Fig. 3. (a) XPS survey spectrum, (b) C 1s, (c) N 1s and (d) O 1s spectra of NGLC. Fig. 4. Pore size distributions of NG and NGLC. Fig. 5. (a) CV curves of GCH, NG, and NGLC at a scan rate of 100 mV s-1, and (b) GCD curves of GCH, NG, and NGLC at a current density of 1 A g-1. Fig. 6. (a) CV curves of NGLC at different scan rates, (b) GCD curves of NGLC and (c) specific capacitances of GCH, NG, and NGLC at different current densities. Fig. 7. (a) Nyquist plots of GCH, NG, and NGLC, and (b) cycling stabilities of GCH, NG, and NGLC at 100 mV s-1 after 5000 cycles.

Figure list:

14

Scheme 1.

15

Fig. 1

Fig. 2

16

Fig. 3

Fig. 4

17

Fig. 5

Fig. 6

Fig. 7

18

Table list: Table 1 Table 1 Element contents (at.%) of NGLC obtained from XPS analysis

C

O

N

S

graphitic-N

pyridinic-N

pyrrolic-N

pyridinie-N-oxide

89.01

6.12

4.57

0.30

32.47

31.52

25.93

10.08

Table 2 Table 2 Comaparisons of capacitances and rate capabilities of different nitrogen-doped graphene electrodes

Cs at 1 A g-1 current

Materials

System

density (F g-1) 2 electrode

soluble

6 M KOH

N-doped

Refs.

within 0.5-5 A g-1

Dried distillers grains with 310 derived

Rate capability

~90

[52]

~91

[53]

~78

[54]

~77

[44]

graphene N-doped graphene hydrogel

217.8

2 electrode 6 M KOH

Hexamethylenetetramine as N 270

3 electrode

source for N-doped graphene

1 M H2SO4

Ethylenediamine as N source 170

3 electrode

for N-doped graphene

6M KOH 19

Urea as N source for N-doped 270

2 electrode

graphene

6 M KOH

Porous N-doped graphene

165.8

3 electrode

~79

[41]

~84

[48]

~89

[41]

~83

This

6 M KOH Porous

N-doped 210

graphene/carbon

nanotubes

3 electrode 6M KOH

composites N-doped

graphene/carbon 363

nanohorns composites

3 electrode 6 M KOH

20

work