Graphene oxide quantum dot-derived nitrogen-enriched hybrid graphene nanosheets by simple photochemical doping for high-performance supercapacitors

Accepted Manuscript Title: Graphene oxide quantum dot-derived nitrogen-enriched hybrid graphene nanosheets by simple photochemical doping for high-performance supercapacitors Authors: Yongjie Xu, Xinyu Li, Guanghui Hu, Ting Wu, Yi Luo, Lang Sun, Tao Tang, Jianfeng Wen, Heng Wang, Ming Li PII: DOI: Reference:

S0169-4332(17)31545-3 http://dx.doi.org/doi:10.1016/j.apsusc.2017.05.189 APSUSC 36124

To appear in:

APSUSC

Received date: Revised date: Accepted date:

17-3-2017 28-4-2017 22-5-2017

Please cite this article as: Yongjie Xu, Xinyu Li, Guanghui Hu, Ting Wu, Yi Luo, Lang Sun, Tao Tang, Jianfeng Wen, Heng Wang, Ming Li, Graphene oxide quantum dot-derived nitrogen-enriched hybrid graphene nanosheets by simple photochemical doping for high-performance supercapacitors, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.05.189 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.

Graphene oxide quantum dot-derived nitrogen-enriched hybrid graphene nanosheets by simple photochemical doping for high-performance supercapacitors

Yongjie Xu, Xinyu Li*, Guanghui Hu, Ting Wu, Yi Luo, Lang Sun, Tao Tang, Jianfeng Wen, Heng Wang, Ming Li *

College of Science & Ministry-province jointly-constructed cultivation base for state key laboratory of Processing for mom-ferrous metal and featured materials & Key Lab. of Nonferrous Materials and New Processing Technology, Guilin University of Technology, Guilin 541004, China.

* Corresponding authors. Email addresses: [email protected] (Xinyu Li) and [email protected] (Ming Li) Highlights 

Binder-free

electrodes

were

fabricated

with

GQDs

and

GO

for

supercapacitors. 

GQDs as “spacers” insert between GO sheets to increase layer spaces and enlarge, surface areas.



The combination of photoreduction and NH3 not only reduces the GO and GQDs, composites in a shorter time but also induces a high level of nitrogen.



Nitrogen-enriched graphene (denoted as NrGO/GQDs) showed the highest specific, capacitance.

Abstract: Nitrogen-enriched graphene was fabricated via a facile strategy. Graphene oxide (GO) nanosheets and graphene oxide quantum dots (GQDs) were used as a structure-directing agent

and

in

situ

activating

agent,

respectively,

after

photoreduction under NH3 atmosphere. The combination of photoreduction and NH3 not only reduced GO and GQD composites (GO/GQDs) within a shorter duration but also doped a high level of nitrogen on the composites (NrGO/GQDs). The nitrogen content of NrGO/GQDs reached as high as 18.86 at% within 5 min of irradiation. Benefiting from the nitrogen-enriched GO/GQDs hybrid structure, GQDs effectively prevent the agglomeration of GO sheets and increased the numbers of ion channels in the material. Meanwhile, the high levels of nitrogen improved electrical conductivity and strengthened the binding energy between GQD and GO sheets. Compared with reduced GO and low nitrogen-doped reduced GO, NrGO/GQD electrodes exhibited better electrochemical characteristics with a high specific capacitance of 344 F g−1 at a current density of 0.25 A g−1. Moreover, the NrGO/GQD electrodes exhibited 82% capacitance retention after 3,000 cycles at a current density of 0.8 A g−1 in 6 M KOH electrolyte. More importantly, the NrGO/GQD electrodes deliver a high energy density of 43 Wh kg−1 at a power density of 417 W kg−1 in 1 M Li2SO4 electrolyte. The nitrogen-doped graphene and corresponding supercapacitor presented in this study are novel materials with potential applications in advanced energy storage systems. Keywords: Graphene oxide quantum dots, Reduced graphene oxide, Photochemical doping,

Nitrogen-enriched graphene, Supercapacitors

1. Introduction Electrical double-layer capacitors, or supercapacitors, have been accepted as novel energy storage devices given their outstanding advantages, such as high-power density and long cycling life.1-3 Their practical applications, however, remain limited by their relatively low energy densities. The electrochemical behaviors of supercapacitors, including capacitance and

cycling stability, are highly dependent on the compositions and structures of their electrodes.4 Given its high electrical conductivity and theoretical surface area,5 graphene is a promising and attractive electrode material for supercapacitors. Graphene oxide (GO) materials are widely utilized6 due to their processing advantage. However, bare reduced GO (rGO) framework electrodes impart mediocre electrochemical capacitive properties due to their easy aggregation during electrode fabrication, low surface area, limited electrical conductivity, slow electrolyte ion transfer rates, insufficient wettability, and low pore volume. Introducing heteroatoms and/or nanoparticles to the graphene matrix is one approach for enhancing the electrochemical performance and expanding the potential of graphene electrodes.7-10 Some researchers have reported that electrodes based on carbon doped with heteroatoms, such as nitrogen (N), exhibit improved electronic conductivity and surface wettability.11,12 Moreover, these heteroatoms can overcome stacking and form electrochemically active species that undergo reversible faradaic redox reactions during charging/discharging processes, thus increasing specific capacitance.13-16 Nitrogen is easily introduced into the graphene structure and forms strong atomic bonds given its similar atomic size to that of carbon. Doping graphene with high N concentrations is highly desirable for the synthesis of high-performance supercapacitors. For example, doping pristine graphene with 2.51 at% N12 increases the specific capacitance of the material from 69 to 280 F g−1, doping with 10.13 at% N17 increases specific capacitance to 326 F g−1. The nitrogen content of N-doped graphene electrode is a key variable that affects the electrochemical performance of supercapacitors. The amount of nitrogen that is introduced into the graphene lattice typically ranges from 2 at% to 10 at%. Recently, Liu et al.18 doped graphene with a very high level of N (29.82 at%) via fluorination followed by thermal defluorination in the presence of XeF2. The residual contamination from the toxic chemical, however, renders this doping procedure disadvantageous. Despite remarkable progress, the preparation of N-doping graphene via a simple and effective

method remains lacking. Thus, the preparation of highly nitrogen-doped graphene that exhibits low agglomeration and high electrical conductivity for improving supercapacitor performances remains challenging. Dimensionality is an important parameter for evaluating the properties of a material. Graphene exhibits different properties in different dimensions. Unlike two-dimensional (2D) graphene sheets, which readily aggregate during electrode fabrication, graphene oxide quantum dots (GQDs) are 0D carbon nanomaterials with a particle size that is less than 10 nm. GQDs have highly defective oxygen-containing moieties that are randomly distributed at the basal plane and edge sites. GQDs can be strategically manipulated via controlled modification,

functionalization,

assembly,

and

stacking

prevention.19-23

Moreover, surfaces with various groups can be easily achieved in situ and exclusively at basal planes and edge sites. Some researchers have recently reported that GQDs with electrochemical capacitive properties considerably improve

the

performance

of

supercapacitors,

including

GQD

micro-supercapacitors,21 GQD/graphene supercapacitors,24-27 and GQD/CNT supercapacitors.23 Therefore, a novel composite of functionalized GQDs and GO is an ideal electrode material with excellent physicochemical properties and a wide range of potential applications in high-performance supercapacitors. In this work, highly N-doped graphene was prepared via the combination of photoreduction and NH3 gas. First, GO and graphene oxide quantum dots (GQDs) were used as the structure-directing agent and in situ-activating agent, respectively. The obtained GQDs had an average diameter of 5 nm. Dispersed GQDs on GO sheets, which were denoted as GO/GQDs, were easily coated on a nickel foam framework without any binder via van der Waals attractions between GO and nickel foam. Then, the GO/GQDs were irradiated with a high-pressure Hg lamp under NH3 atmosphere to yield NrGO/GQDs. The decoration of GO with GQDs shortened ion transport distance in the nanoscale dimension and offered accessible active sites via N doping, hence considerably increasing nitrogen doping content to high levels. The doping levels obtained

for NrGO/GQDs were 18.86 at%, which is three times higher than the previously reported maximum levels.28 Moreover, the decoration of GO with GQDs effectively mitigated the self-aggregation and stacking of graphene, as well as provided a high specific surface area. The photochemical doping of graphene with nitrogen is chemical reductant-free, eco-friendly, and can be easily scaled up. Given these advantages, photochemical doping has been extensively applied in the fabrication, modification, and manipulation of carbon materials.28-31

Benefiting

from

these

advantageous

characteristics,

the

as-prepared NrGO/GQD electrodes showed a high specific capacitance of 344 F g−1 at a current density of 0.25 A g−1. In addition, the NrGO/GQD electrodes retained 82% capacitance after 3,000 cycles at a current density of 0.8 A g −1. Furthermore, the NrGO/GQD electrodes possessed a good energy density of 43 Wh kg−1 at a power density of 417 W kg−1 in a Li2SO4 electrolyte. These properties

indicate

the

potential

use

of

NrGO/GQD

electrodes

in

supercapacitors. 2. Experimental 2.1 Preparation of materials GO was prepared by a modified Hummers' method. GQDs was purchased from Nanjing JI Cang Nano Technoloyg Co., Ltd., China. The synthesis scheme of NrGO/GQD electrodes was illustrated in Fig. 1. First, Ni foam sheets（with a size of 2.5 cm × 1 cm × 0.1 cm）were treated with hydrochloric acid to etch the surface and remove contaminants, and then washed with deionized water and absolute ethanol. A well-dispersed aqueous solution of GO was prepared by putting 30 mg of GO in 2 ml of deionized water by ultrasonication for 2 hours to open the layers of GO. Then the GO solution was mixed with GQDs (the mass of GO and GQDs in the rate of 6:1) and stirred 5min. After that, the mixture was sonicated for 1 hour for the attachment of GQDs onto GO layers.

2.2 Synthesis of NrGO/GQD electrodes Electrodes were prepared via the following steps. The nickel foam sheets were firstly immersed into a GO/GQDs dispersion to ensure that GO/GQDs were filled into the microspores of Ni foam, and then dried for few hours in room temperature. Then the mixture was coated on nickel foam and dried for few hours in room temperature to remove the water. This step was repeated a number of times to increase the GO/GQDs loading. The nickel foam sheet coated with GO/GQDs was placed inside a quartz tubeand irradiated for 5 min with light from a Hg lamp (1000 W) in NH3 atmosphere. This combination of photoreduction and NH3 method was reported by our previous works.30,31 As a contrast, N-doped rGO (NrGO) electrodes were prepared by using GO dispersion without GQDs, and then irradiated for 5 min with a Hg lamp in NH 3 atmosphere. Moreover, rGO electrodes without GQDs were prepared by irradiating for 5 min with light from a Hg lamp in Ar atmosphere. 2.3 Characterization The

scanning

electron

microscopy

(SEM)

and

transmission

electron

microscopy (TEM) were carried out on all prepared materials using S-4800 and JEM-2100F. The data of XPS were recorded on ESCALAB 250Xi. 2.4 Electrochemical Testing Electrochemical measurements were performed by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impendence spectroscopy (EIS) in a two-electrode system using a CHI760E electrochemical workstation. Two identical electrodes were separated by a separator (NKK TF45, 40 μm). An aqueous solution of 6 M KOH and 1 M Li 2SO4 were used as the electrolytes. Specific capacitances Cs (F g−1) were calculated from the discharge processes according to the following formula:

CS =

4IΔt AΔV

(1)

where I is the discharge current (A), A is the total mass of both electrodes (g), ΔV is the voltage range (V), and Δt is the discharge time (s). The energy density E (Wh kg−1) and power density P (W kg−1) of the symmetric supercapacitors were calculated based on the following formulas:

CS 1 V 2 8 3.6 E P= t

(2)

E=

(3)

Where Δt is the discharge time (h). 3. Results and discussion The morphology and microstructure of GO and GO/GQDs were investigated via scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM and TEM images are shown in Figs. 2 and S1 (Supplementary). The graphene flakes exhibit a prominently wrinkled and a glossy carpet-like morphology (Fig. 2a) because abundant oxygen-containing groups on the surface of GO tightly bound with residual H2O molecules.32 Moreover, oxygen-containing groups are an important factor in the interaction between GO and GQDs. Figure 1b depicts the TEM image of the NrGO/GQDs sample. The GQDs, which are approximately 5 nm in size, are abundantly distributed on the top surface of the GO nanosheets. When GQDs and GO suspensions are mixed together, GQDs effectively attach to the hydrophilic surface of GO sheets through hydrogen bonding. During drying, the attachment of GQDs on GO sheets may be enhanced by van der Waals forces and π–π stacking. In addition, GO nanosheets can prevent the aggregation of GQDs, thus increasing ion channels in the interior of the material.25-33 As shown in Fig. 2c, raman spectroscopy was employed to monitor the synthesis of NrGO/GQDs. Two distinct peaks in the spectrum correspond to the broad disorder D-band at 1,348 cm−1 and an in-plane vibrational G-band at 1,583 cm−1. For GO, the intensity ratio of the D band over the G band (ID/IG) is 0.88, which signifies that GO was graphitized at low levels due to the presence

of oxygen-containing functional groups.25 The estimated ID/IG of rGO is 0.96 and that of NrGO is 1.02. The higher ID/IG of NrGO film than that of rGO film suggests that the nitrogen moiety is substituted in the carbon network of GO and that a new sp2 domain size is formed via photochemical treatment. Furthermore, the ID/IG of NrGO/GQD composite is 1.06, which is slightly higher than that of NrGO. This result indicates that loading GQDs on GO sheets increases the defects or edges and the randomness of graphene sheets. Compared with NrGO, more defects or edges exist in NrGO/GQDs given the increase in unsymmetrical variation near to or at crystalline edges, such as additional defects. Therefore, these N-derived defects in the as-prepared NrGO/GQDs offer electrochemically active and defect sites, thus enhancing electrochemical activity.34,35 The chemical compositions and bonding configurations of GO, NrGO, and NrGO/GQDs were characterized via XPS. The XPS full scan spectrum (Fig. 3a) for GO shows obvious carbon peaks (285.6 eV) and oxygen peaks (533.5 eV) but no N signal. However, N 1s is present in the NrGO and NrGO/GQDs spectra at 397.8 eV, which indicates that the simple photochemical treatment induced the covalent doping of nitrogen into GO and GO/GQDs. Figure 3b shows the high-resolution C 1s spectrum of GO. Four kinds of carbon peaks are located at 284.5, 286.7, 287.5, and 288.4 eV, which can be assigned to sp2-hybridized graphitic carbon (C=C), carbon in C–O bonds, carbonyl carbon (C=O), and carboxyl carbon (O–C=O), respectively.28,31 The peak intensity of the oxygenated carbons significantly decreases after photo-irradiation under NH3 atmosphere. Figure 3c shows a typical single peak of C=C bonds, as well as two small peaks at 285.8 and 287.3 eV that correspond to C=N and C–N bonds, respectively.36 In order to further characterize the incorporation of N into carbon, the N 1s spectrum of NrGO/GQDs was fine-scanned. Each peak can be mainly deconvoluted into four subpeaks (Fig. 3d). The sharp peak at approximately

399 eV corresponds to amino-like nitrogen (N-A). The other peaks at approximately 398.2, 400, and 401 eV are pyridine-like nitrogen (N-6), pyrrolic-like nitrogen (N-5), and graphitic-like nitrogen (N-Q), respectively. The nitrogen content of NrGO/GQDs is approximately 18.86 at%, which is almost 2.5 times higher than that of pristine GO that was directly photoreduced under NH3 atmosphere (7.55 at%). Given that nitrogen is mainly doped in vacancies in graphitic lattices, the high nitrogen levels indicate a high concentration of vacancies in NrGO/GQDs. This result implies that the anchoring process of GQDs to GO generates a high concentration of vacancies or active sites that facilitates N-doping. These combined features make NrGO/GQDs

a

promising

nitrogen-enriched

carbon

nanosheet

for

high-efficiency energy storage/conversion devices. The energy storage effect of three samples were characterized by electrochemical measurements using a symmetrical two-electrode system in 6 M KOH with a potential window of 1 V. The curves of electrochemical properties of samples are discussed in Figs. 4 and 5. The NrGO/GQD electrodes exhibit excellent performance. Figure 4a presents the cyclic voltammetry (CV) curves of NrGO/GQD electrodes. Figure 4b–f shows the comparison of three samples at various scan rates in a 6 M KOH solution. At different scan rates (5–300 mV s−1), the CV curves of NrGO/GQD electrodes are nearly rectangular. The areas enclosed by the CV curves are the largest, thus indicating lower contact resistances and higher specific capacitance than those of rGO and NrGO electrodes (Fig. S2, Supplementary). 37 With increasing scan rates, the shapes of the CV curves deviate from the rectangular, indicating that the electrolyte ions are kinetically limited from entering small pores.38 Galvanostatic charge–discharge (GCD) measurements were also taken to estimate the electrochemical characteristics of three samples. The results are shown in Fig. 5. The GCD curves of NrGO/GQD electrodes at different current densities are shown in Fig. 5a. All the GCD curves exhibit linear and

symmetrical shapes that are attributed to efficient charge transfer and superior electrical conductivity owing to the presence of active nitrogen atoms. The GCD curves, moreover, reveal a highly reversible behavior.21,39 No obvious IR drop occurred at the start of all discharge curves, thus indicating the small equivalent series resistance (ESR) of the supercapacitors. 39 In accordance with the CV curves, NrGO/GQD electrodes show the longest discharge time compared with rGO and NrGO electrodes at the same current densities (Fig. 4b–f). The discharge time of the three electrodes increase in the following order: rGO electrodes
NrGO/GQD electrodes can be attributed to the structure of GQDs that anchored to the graphene nanosheets and the high level of N-doping. The above performance are also supported by the electrochemical impedance spectroscopy (EIS) results that were obtained in 6 M KOH at open-circuit potential. The corresponding Nyquist plots are shown in Fig. 6a. The inset shows the high-frequency region and the equivalent circuit. In the high-frequency region, the first intersection of the curves with the real axis indicates the ESR, including the intrinsic resistance of the electrode materials and of the electrolyte, as well as the contact resistance between the interfaces of electrodes, electrolyte, and current collector substrates.45 All three samples have a low ESR of almost 0.5 Ω that highly correspond with GCD curves. The three samples, however, have completely different charge-transfer resistances, as shown in the high- to medium-frequency semicircle regions. The semicircles of rGO and NrGO (about 45 Ω and 14 Ω) reveal high pore-diffusion impedance and large charge transfer resistance, which likely resulted from high oxygen-containing groups, low reduction degree, and low electrolyte access and diffusion via conductive paths.28,46 In the low-frequency portion of the spectrum, the tails should be vertical lines that are normal to the real axis if there is no diffusion resistance. The Warburg-type line is short and inclines toward a vertical line, which indicates a short ion diffusion pathway and the purely

capacitive

behavior

of

NrGO/GQD

electrodes.25,38,45

N-doping

significantly increases the electrical conductivity and electrochemical activity of graphene in the electrochemical process. 41 The cycling stability of NrGO/GQD electrodes at 0.8 A g−1 is presented in Fig. 6b. After 3,000 cycles, the device maintains 82% of its initial capacitance, which is higher than those of others (Fig. S4, Supplementary) and indicates good cycling stability. These characteristics are mainly due to the very high N content and NH 3 atmosphere. The very high doping of N lead to the high conductively and NH3 atmosphere can greatly improve the reduced degree. According to the equation of energy density E = 1/2CV2, where C is the specific capacitance and V is the potential

window, increasing the working voltage can enhance the energy and power densities of the two-electrode cell. In the present study, 1 M Li2SO4 was selected as the electrolyte for the evaluation of the electrochemical performance of NrGO/GQD electrodes. The evaluation results are shown in Fig. 7.28 Figures 7a and 7b show that the electrodes have a stable voltage window from 0 to 2 V during the CV and GCD tests. There are almost rectangular curves and no obvious distortion of CV curves at different scan rates between 0 and 2 V (Fig. 7c). These results indicate the nearly ideal capacitive behavior of NrGO/GQDs electrode in Li2SO4. The GCD curves at different current densities are shown in Fig. 7d. The specific capacitances are up to 309 F g−1 at 0.42 A g−1. Moreover, all of the curves clearly show acceptable symmetry without an obvious IR drop in the given cell voltage range from 0 to 2 V. This indicates the good charge propagation and low equivalent series resistance in this system. These characteristics are confirmed by EIS, as shown in Fig. 7e. Figure 7f presents the Ragone plot of NrGO/GQDs in 6 M KOH and 1 M Li2SO4. The energy density is 43 Wh kg−1 at a power density of 417 W kg−1 in 1 M Li2SO4 whereas energy density is only 10 Wh kg−1 in 6 M KOH according to Eqs. (2) and (3). Moreover, the energy density remains at 24 Wh kg−1 at a power density of 4 kW kg−1. This result is attributed to the unique porosity and N-doping of NrGO/GQDs. The N-doping structures favor the electrostatic interactions of alkali metal cations (Li+) to accelerate electron transport and the strong solvation of Li+ and sulfate anions. Moreover, N-doping structures lead to the formation of a high over-potential for di-hydrogen evolution and extends the voltage window to 2 V.47-49 Therefore, combining NrGO/GQD electrodes with Li2SO4 electrolytes can provide superior energy and power density for high-performance supercapacitors. 4. Conclusion A new strategy is developed for anchoring GQDs to graphene nanosheets. The GQDs acts as the initial nucleation sites for controllable N-doping at graphene

basal

planes.

Then,

nitrogen-doped

graphene

was

prepared

through

photoreduction with NH3 gas. Finally, nitrogen-doped graphene was deposited on nickel foam to form a supercapacitor electrode. GQDs can prevent the stacking of graphene nanosheets. At the same time, they can offer ample active sites for the fully accessible nitrogen adsorption/desorption. The aqueous solution created more crumples on the graphene nanosheets and achieved high specific surface area. The electrochemical test results demonstrated that NrGO/GQDs with cross-linked nanosheets exhibited a high nitrogen content of 18.86% and a specific capacitance of 344 F g−1 at a current density of 0.25 A g−1 in 6 M KOH. NrGO/GQDs exhibited better electrochemical performance than NrGO and rGO. Compared with synthesis routes for other graphene-based nanocomposites, our synthesis route for NrGO/GQDs is green, simple, rapid, and has a great potential for the synthesis of different GQD-decorated graphene nanocomposites with energy conversion and storage applications. Acknowledgements This work was financially supported by National Natural Science Foundation of China (51662004, 11364010, 11404072), Natural Science Foundation of Guangxi Zhuang Autonomous Region of China (2014GXNSFBA118021, 2014GXNSFBA118014). Authors acknowledge Fuchi Liu for experimental help and valuable discussions.

References

1 J. Yan, Q. Wang, T. Wei, L. L. Jiang, M. L. Zhang, X. Y. Jing and Z. J. Fan, Template-assisted low temperature synthesis of functionalized graphene for ultrahigh volumetric performance supercapacitors, ACS Nano 8 (2014) 4720-4729. 2 C. N. R. Rao, K. Gopalakrishnan and A. Govindaraj. Synthesis, properties and applications of graphene doped with boron, nitrogen and other elements, Nano Today 9 (2014) 324-343. 3 C. Zhang, W. Lv, Y. Tao and Q.H. Yang, Towards superior volumetric performance: design and preparation of novel carbon materials for energy storage, Energy Environ. Sci. 8 (2015) 1390-1403. 4 M.Y. Cho, M.H. Kim, H.K.Kim, K.B. Kim, J.R. Yoon and K.C. Roh, Electrochemical performance of hybrid supercapacitor fabricated using multi-structured activated carbon, Electrochem.Commun. 47 (2014) 5-8. 5 C. G. Liu, Z. N. Yu, D. Neff, A. Zhamu and B. Z. Jang, Graphene-based supercapacitor with an ultrahigh energy density, Nano Lett. 10 (2010) 4863-4868. 6 Y. Y. Shao, J. Wang, M. Engelhard, C. M. Wang and Y. H. Lin, Facile and controllable electrochemical reduction of graphene oxide and its applications, J. Mater. Chem. 20 (2010) 743-748. 7 K. Lee, Y. Yoon, Y. Cho, S. M. Lee, Y. Shin, H. Lee and H. Lee, Tunable sub-nanopores of graphene flake interlayers with conductive molecular linkers for supercapacitors, ACS Nano 10 (2016) 6799-6807. 8 Y. Y. Wen, T. E. Rufford, D. Hulicova-Jurcakova, X. B. Zhu and L. Z. Wang, Structure control of nitrogen-rich graphene nanosheets using hydrothermal

treatment

and

formaldehyde

polymerization

for

supercapacitors, ACS Appl. Mater. Interfaces 8 (2016) 18051-18059. 9 Z. B. Lei, J. T. Zhang, L. L. Zhang, N. A. Kumar and X. S. Zhao, Functionalization of chemically derived grapheme for improving its electrocapacitive energy, Energy Environ. Sci. 9 (2016) 1891-1930.

10 Q. Yang, S.K. Pang and K.C. Yung, Electrochemically reduced graphene oxide/carbon

nanotubes

composites

as

binder-free

supercapacitor

electrodes, J. Power Sources 311 (2016) 144-152. 11 D. Hulicova-Jurcakova, M. Seredych, G. Q. Lu and T. J. Bandosz, Combined effect of nitrogen- and oxygen-containing functional groups of microporous activated carbon on its electrochemical performance in supercapacitors, Adv.Funct. Mater. 19 (2009) 438-447. 12 H. M. Jeong, J. W. Lee, W. H. Shin, Y. J. Choi, H. J. Shin, J. K. Kang and J. W. Choi, Nitrogen-doped graphene for high-performance ultracapacitors and the importance of nitrogen-doped sites at basal planes, NanoLett. 11 (2011) 2472-2477. 13 G. Xin, Y. Wang, X. Liu, J. Zhang, Y. Wang, J. Huang and Y. Zang, Preparation of self-supporting graphene on flexible graphite sheet and electrodeposition of polyaniline for supercapacitor. Electrochim Acta 167 ( 2015) 254–261. 14 X. Fan, C. Yu, J. Yang, Z. Ling and J. Qiu, Hydrothermal synthesis and activation of graphene-incorporated nitrogen-rich carbon composite for high-performance supercapacitors, Carbon 70 (2014) 130–141. 15 Naderi H. R., Norouzi P., Ganjali M. R., Electrochemical study of a novel high performance supercapacitor based on MnO2/nitrogen-doped graphene nanocomposite, Appl. Surf. Sci. 2016, 366, 552-560. 16 Naderi H. R., Norouzi P., Ganjali M. R., Gholipour-Ranjbar H., Synthesis of

a

novel

magnetite/nitrogen-doped

reduced

graphene

oxide

nanocomposite as high performance supercapacitor. Powder Technol. 2016, 302, 298-308. 17 L. Sun, L. Wang, C. G. Tian, T. X. Tan, Y. Xie, K. Y. Shi, M. T. Li and H. G. Fu, Nitrogen-doped graphene with high nitrogen level via a one-step hydrothermal reaction of graphene oxide with urea for superior capacitive energy storage, RSC Adv. 2 (2012) 4498-4506.

18 Y. Liu, Y. T. Shen, L. T. Sun, J. C. Li, C. Liu, W. C. Ren, F. Li, L. B. Gao, J. Chen, F. C. Liu, Y. Y. Sun, N. J. Tang, H. M. Cheng and Y. W. Du, Elemental superdoping of graphene and carbon nanotubes, Nat.Commun., 7 (2016) 10921. 19 L. Wang, Y. L. Wang, T. Xu, H. B. Liao, C. J. Yao, Y. Liu, Z. Li, Z. W. Chen, D. Y. Pan, L. T. Sun and M. H. Wu, Gram-scale synthesis of single-crystalline graphene quantum dots with superior optical properties, Nat.Commun. 5 (2014) 5357. 20 W. W. Liu, X. B. Yan, J. T. Chen, Y. Q. Feng and Q. J. Xue, Novel and high-performance asymmetric micro-supercapacitors based on graphene quantum dots and polyaniline nanofibers, Nanoscale 5 (2013) 6053-6062. 21 W.W. Liu, Y.Q. Feng, X.B. Yan, J.T. Chen and Q.J. Xue, Superior micro-supercapacitors based on graphene quantum dots, Adv. Funct. Mater. 23 (2013) 4111-4122. 22 M. F. El-Kady, Y. Shao and R. B. Kaner, Graphene for batteries, supercapacitors and beyond, Nat. Rev. Mater. 1 (2016) 16033. 23 Y. Hu, Y. Zhao, G. Lu, N. Chen, Z. Zhang, H. Li, H. Shao, L. Qu, Graphene

quantum

dots–carbon

nanotube

hybrid

arrays

for

supercapacitors, Nanotechnology 24 (2013) 195401-195401. 24 Q. Chen, Y. Hu, C. G. Hu, H. Cheng, Z. P. Zhang, H. B. Shao and L. T. Qu, Graphene quantum dots–three-dimensional graphene composites for high-performance supercapacitors, Phys. Chem. Chem. Phys. 16 (2014) 19307-19313. 25 Y. L. Huang, A. M. Gao, X. N. Song, D. Shu, F. Y. Yi, J. Zhong, R. H. Zeng, S. X. Zhao and T. Meng, Supramolecule-inspired fabrication of carbon nanoparticles in situ anchored graphene nanosheets material for high-performance supercapacitors, ACS Appl. Mater. Interfaces 8 (2016) 26775-26782. 26 A. K. Samantara, S. Chandra Sahu, A. Ghosh and B. K. Jena, Sandwiched graphene with nitrogen, sulphur codoped CQDs: an efficient metal-free

material for energy storage and conversion applications, J. Mater. Chem. A 3 (2015) 16961-16970. 27 K. Lee, H. Lee, Y. Shin, Y. Yoon, D. Kim and H. Lee, Highly transparent and flexible supercapacitors using graphene-graphene quantum dots chelate, Nano Energy 26 (2016) 746-754. 28 H. F. Huang, G. S. Luo, L. Q. Xu, C. L. Lei, Y. M. Tang, S. L. Tang and Y. W. Du, NH3 assisted photoreduction and N-doping of graphene oxide for high performance electrode materials in supercapacitors, Nanoscale 7 (2015) 2060-2068. 29 F. C. Liu, N. J. Tang, T. Tang, Y. Liu, Q. Feng, W. Zhong, and Y. W Du, Photochemical

doping

of

graphene

oxide

with

nitrogen

for

photoluminescence enhancement, Appl. Phys. Lett. 103 (2013) 123108. 30 X. Y. Li, T. Tang, M. Li and X. C. He, Photochemical doping of graphene oxide thin films with nitrogen for electrical conductivity improvement, J Mater Sci: Mater. Electron. 26 (2015) 1770–1775. 31 X. Y. Li, T. Tang, M. Li and X. C. He, Nitrogen-doped graphene films from simple photochemical doping for n-type field-effect transistors, Appl. Phys. Lett. 106 (2015) 013110. 32 S.Y. Yang, K.H. Chang, H.W. Tien, Y.F. Lee, S.M. Li, Y.S. Wang, J.Y. Wang, C.C. M. Ma and C.C. Hu, Design and tailoring of a hierarchical graphene-carbon nanotube architecture for supercapacitors, J. Mater. Chem. 21 (2011) 2374-2380. 33 J. H. Liu, G. S. Chen and M. Jiang, Supramolecular hybrid hydrogels from noncovalently

functionalized

graphene

with

block

copolymers,

Macromolecules 44 (2011) 7682-7691. 34 L. Liu, J. W. Lang, P. Zhang, B. Hu and X. B. Yan, Facile synthesis of Fe2O3 nano-dots @nitrogen-doped graphene for supercapacitor electrode with ultralong cycle life in KOH electrolyte, ACS Appl. Mater. Interfaces 8 (2016) 9335-9344.

35 Y. Y. Wen, T. E. Rufford, D. Hulicova-Jurcakova, X. B. Zhu and L. Z. Wang, Structure control of nitrogen-rich graphene nanosheets using hydrothermal

treatment

and

formaldehyde

polymerization

for

supercapacitors, ACS Appl. Mater. Interfaces, 8 (2016) 18051-18059. 36 S. Y. Liu, J. Xie, H. B. Li, Y. Wang, H. Y. Yang, T. J. Zhu, S. C. Zhang, G. S. Cao and X. B. Zhao, Nitrogen-doped reduced graphene oxide for high-performance flexible all-solid-state micro-supercapacitors, J. Mater. Chem. A 2 (2014) 18125-18131. 37 Y. Wang,Z. Q. Shi, Y. Huang, Y. F. Ma, C. Y. Wang, M. M. Chen and Y. S. Chen. Supercapacitor devices based on graphene materials.J. Phys. Chem. C 113 (2009) 13103–13107. 38 Y.Z. Liu, Y.F. Li, F.Y. Su, L.J. Xie, Q.Q. Kong, X.M. Li, J.G. Gao and C.M. Chen. Easy one-step synthesis of N-doped graphene for supercapacitors, Energy Storage Materials 2 (2016) 69-75. 39 L. L. Jiang, L. Z. Sheng, C. L. Long, T. Wei and Z. J. Fan, Functional pillared graphene frameworks for ultrahigh volumetric performance supercapacitors, Adv. Energy Mater. 5 (2015) 1500771. 40 E. Frackowiak, Carbon materials for supercapacitor application, Phys. Chem. Chem. Phys. 9 (2007) 1774-1785. 41 G. Lota, B. Grzyb, H. Machnikowska, J. Machnikowski and E. Frackowiak, Effect of nitrogen in carbon electrode on the supercapacitor performance, Chem. Phys. Lett. 404 (2005) 53-58. 42 S. H. Yang, X. F. Song, P. Zhang and L. Gao, Crumpled nitrogen-doped graphene-ultrafine Mn3O4 nano hybrids and their application in supercapacitors, J. Mater. Chem. A 1(2013) 14162-14169. 43 Dezfuli A. S., Ganjali M. R., Naderi H. R., Anchoring samarium oxide nanoparticles

on

reduced

graphene

oxide

for

high-performance

supercapacitor. Appl. Surf. Sci. 2017, 402, 245-253. 44 Naderi H. R., Ganjali M. R., Dezfuli A. S., Norouzi P., Sonochemical preparation of a ytterbium oxide/reduced graphene oxide nanocomposite

for supercapacitors with enhanced capacitive performance. RSC Adv. 2016, 6 (56), 51211-51220. 45 J. Y. Luo, H. D. Jang, and J. X. Huang, Effect of sheet morphology on the scalability of graphene-based ultracapacitors, ACS Nano. 7 (2013) 1464–1471. 46 S. M. Li, S. Y. Yang, Y. S. Wang, H. P. Tsai, H. W. Tien, S. T. Hsiao, W. H. Liao, C. L. Chang, C. C.M. Ma and C. C. Hu, N-doped structures and surface functional groups of reduced graphene oxide and their effect on the electrochemical performance of supercapacitor with organic electrolyte, J. Power Sources 278 (2015) 218-229. 47 M. P. Bichat, E. Raymundo-Piñero and F. Béguin, High voltage supercapacitor built with seaweed carbons in neutral aqueous electrolyte, Carbon 48 (2010) 4351-4361. 48 K. Fic, G. Lota, M. Meller and E. Frackowiak, Novel insight into neutral medium as electrolyte for high-voltage supercapacitors, Energy Environ. Sci., 2012, 5, 5842-5850. 49 Q. Gao, L. Demarconnay, E. Raymundo-Piñero and F. Béguin, Exploring the large voltage range of carbon/carbon supercapacitors in aqueous lithium sulfate electrolyte, Energy Environ. Sci., 2012, 5, 9611-9617.

Fig. 1 Schematic diagram of the process for fabricating NrGO/GQD electrodes.

Fig. 2 TEM images of (a) GO and (b) NrGO/GQDs. (c) Raman spectra of GO, rGO, NrGO, and NrGO/GQDs.

Fig. 3 (a) XPS full scan spectra of GO, rGO, NrGO, and NrGO/GQDs. (b) and (c) Highresolution C1s XPS spectra of GO and NrGO/GQDs. (d) High resolution N1s XPS spectra of NrGO/GQDs.

Fig. 4 (a) CV curves of NrGO/GQDs at different scan rates. (b)-(f) comparison of CV curves for rGO, NrGO and NrGO/GQDs at scanrates of 5 mV s−1, 10 mV s−1, 50 mV s−1, 100 mV s−1 and 300 mV s−1, respectively in 6 M KOH.

Fig. 5 (a) GCD curves of NrGO/GQDs at different current densities. (b)-(f) comparison of GCD curves for rGO, NrGO, and NrGO/GQDs at current densities of 0.25 A g−1, 0.42 A g−1, 0.58 A g−1,

0.83 A g−1 and 1.67 A g−1, respectively in 6 M KOH.

Fig. 6 (a) Nyquist plots of rGO, NrGO, and NrGO/GQDs at a frequency rang from 100 mHz to 100

kHz.

Cycling

Inset:

stability

High

frequency

region

and

a

equivalent

circuit.

(b)

of NrGO/GQDs obtained from GCD curves at 0.8 A g−1 for 3000 circles.

Fig. 7 (a) CV curves at different working voltages ranging from 1.2 to 2.0 V at a scan rate of 50

mV s−1. (b) GCD curves at different working voltages ranging from 1.2 to 2.0 V at 0.42 A g−1. (c) CV curves in the voltage rang of 0 to 2 V at different rates. (d) GCD curves in the voltage rang of 0 to 2 V at different current densities. (e) Nyquist plots of NrGO/GQDs in 1 M Li2SO4. Inset: High frequency region and a equivalent circuit. (f) Ragone plots of NrGO/GQDs in 6 M KOH and 1 M Li2SO4.