High-performance hybrid supercapacitor realized by nitrogen-doped carbon dots modified cobalt sulfide and reduced graphene oxide

High-performance hybrid supercapacitor realized by nitrogen-doped carbon dots modified cobalt sulfide and reduced graphene oxide

Journal Pre-proof High-performance hybrid supercapacitor realized by nitrogen-doped carbon dots modified cobalt sulfide and reduced graphene oxide Zhe...

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Journal Pre-proof High-performance hybrid supercapacitor realized by nitrogen-doped carbon dots modified cobalt sulfide and reduced graphene oxide Zhenyuan Ji, Na Li, Minghua Xie, Xiaoping Shen, Wenyao Dai, Kai Liu, Keqiang Xu, Guoxing Zhu PII:

S0013-4686(20)30023-2

DOI:

https://doi.org/10.1016/j.electacta.2020.135632

Reference:

EA 135632

To appear in:

Electrochimica Acta

Received Date: 17 July 2019 Revised Date:

2 January 2020

Accepted Date: 3 January 2020

Please cite this article as: Z. Ji, N. Li, M. Xie, X. Shen, W. Dai, K. Liu, K. Xu, G. Zhu, High-performance hybrid supercapacitor realized by nitrogen-doped carbon dots modified cobalt sulfide and reduced graphene oxide, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.135632. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.

Authors' contributions

The work presented here was carried out in collaboration between all authors. Zhenyuan Ji designed the study and drafted the manuscript. Na Li carried out the laboratory experiments. Minghua Xie interpreted the results. Xiaoping Shen participated in the design of the study, interpreted the results and revised the manuscript. Wenyao Dai and Kai Liu carried out the electrochemical measurements. Keqiang Xu participated in the analysis of the experimental data. Guoxing Zhu participated in the revision of the manuscript. All authors read and approved the final manuscript.

Graphical Abstract

High-performance hybrid supercapacitor realized by nitrogen-doped carbon dots modified cobalt sulfide and reduced graphene oxide Zhenyuan Ji,a* Na Li,a Minghua Xie,b Xiaoping Shen,a* Wenyao Dai,a Kai Liu,a Keqiang Xua and Guoxing Zhua

a

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang

212013, P. R. China b

Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu

Province, Yancheng Institute of Technology, Yancheng 224051, P. R. China

* Corresponding author. E-mail address: [email protected] (Z.Y. Ji), [email protected] (X.P. Shen)

1

Abstract: Hybrid supercapacitor devices have elicited increasing interest because of their enormous potential for energy storage applications. Herein, a facile hydrothermal strategy was presented for the coordinative preparation of nitrogen-doped carbon dots (N-dopd CDs) modified flower-like cobalt sulfide (CoS) hierarchitectures and reduced graphene oxide nanosheets (rGO) for use as advanced cathode and anode in hybrid supercapacitor. Profiting from the merits of N-dopd CDs, the as-prepared CoS/N-dopd CDs and rGO/N-dopd CDs composites deliver enhanced specific capacitances and excellent cycle stability as compared with bare CoS and rGO. A hybrid supercapacitor device assembled by flower-like CoS/N-dopd CDs cathode and rGO/N-dopd CDs anode achieves an energy density of approximately 36.6 Wh kg-1 at 800 W kg-1 and good electrochemical stability with 85.9% retention after 10000 cycles at 10 A g-1. This study provides a new insight to coordinately design high-performance cathodes and anodes for developing new types hybrid supercapacitor devices.

Keywords: Carbon dots; Cobalt sulfide; Graphene; Hybrid supercapacitor; Energy storage.

2

1. Introduction With the increasing demand for high-performance power sources, sustainable energy supplies have become one of the vital elements throughout the world. Supercapacitors, a potential equipment for energy storage, have gradually become the research focus in scientific communities due to their ideal power density and exceptional cycling stability [1,2]. However, the widespread application of supercapacitor devices has been a challenge because of their relative low energy density [3]. The electrochemical properties of supercapacitors closely rely on the nature of the electrodes. As a result, constructing novel electrode materials with excellent specific capacities is of great importance for high-performance supercapacitor devices [4,5]. Transition metal sulfides, as important candidates for supercapacitor electrodes, have received a wide range of interests for which feature higher electrochemical properties [6]. Because of the lower electronegativity of sulfur than oxygen, transition metal sulfides usually possess higher conductivity than their corresponding oxides, which is beneficial to the rapid transmission of electrons [7]. Among various transition metal sulfides, cobalt-based compounds (CoS, CoS2, Co9S8 and Co4S3) are widely used as active and low-cost electrode materials in energy storage owing to their progressive electronic properties [8,9]. Nevertheless, the performances of cobalt sulfide are still restricted by the low conductivity and poor cyclic stability of them. Up to now, numerous efforts have been made to explore alternative electrode materials with superior electrochemical performance. Carbon-based materials are 3

extensively exploited in supercapacitors due to their ultrahigh specific surface area, splendid electroconductibility and remarkable structural stability [10,11]. On the one hand, the incorporation of carbonaceous materials can signally ameliorate the electrochemical performance of transition metal sulfides, attributing to the synergistic effect derived from them. On the other hand, these carbonaceous materials, such as activated carbon and graphene, can be utilized directly as negative electrode materials for supercapacitors. Unfortunately, the dense carbon layer would hinder the contact between transition metal sulfides and electrolytes, which has a negative effect on the capacities of the electrode materials [12]. Moreover, conventional carbon-based materials have the shortcoming of a poor specific capacitance because of the low charge separation efficiency at the surface [13]. For hybrid supercapacitors, most researches have concentrated on developing positive electrodes with high electrochemical properties, and only a few studies have focused on the construction of novel negative materials. Most recently, the monodisperse zero-dimensional carbon dots have drawn substantial attention due to their indispensable prospects in bioimaging, sensors, catalysis, chemiluminescence and many other fields [14]. Carbon dots possess ultrasmall sizes, which endow them brilliant electron transfer/reservoir properties and stronger edge quantum effects than multidimensional carbonaceous materials [15]. Besides, carbon dots can be readily dispersed in different solvents. This is beneficial for improving the surface wettability of carbon dots-based materials. Furthermore, carbon dots can be well doped by other heteroatoms (such as N and O) and modified 4

with various functional groups. The doping and functionalization could further modulate and optimize their electrochemical properties. All these advantages render carbon dots attractive for energy storage [16]. However, to the best of our knowledge, few study has been reported by using carbon dots as synergistic agents to simultaneously increase the electrochemical performances of cobalt sulfide (CoS) and reduced graphene oxide (rGO). Stimulated by the above considerations, herein, a novel hybrid supercapacitor device assembled with nitrogen-doped carbon dots (N-doped CDs) decorated flower-like CoS hierarchitectures and rGO nanosheets were demonstrated. In three-electrode system, the as-prepared CoS/N-doped CDs and rGO/N-dopd CDs composite electrodes all presented high specific capacities and superior cycle stability. In particularly, the CoS/N-doped CDs//rGO/N-dopd CDs hybrid supercapacitor delivered enhanced energy density of 25.6-36.6 Wh kg-1 at 800-16000 W kg-1, and exceptional electrochemical stability (capacity retention of 85.9% after 10000 cycles at 10 A g-1).

2. Experimental 2.1. Chemicals Polytetrafluoroethylene (PTFE) aqueous solution with the concentration of 60 wt% was acquired from Sigma-Aldrich. All other analytical chemicals were supplied by Sinopharm Chemical Reagent Co., Ltd. The N-doped CDs were synthesized according to our previous report [17]. Graphite oxide was synthesized based on a modified Hummers route [18]. 5

2.2. Preparation of flower-like CoS/N-doped CDs composites Flower-like CoS/N-doped CDs composites were synthesized according to a hydrothermal

strategy.

Typically,

cobalt

chloride

hexahydrate

(1

mmol),

thiocarbamide (2 mmol) and N-doped CDs powder were dispersed in 40 mL of ethylene glycol by ultrasonication. The solution was transferred into a 50 mL of Teflon-lined autoclave by heating at 200 °C for 16 h. The composites were gathered by centrifugation, thoroughly washed with water and ethanol, and finally dried at 45 °C under vacuum. The products obtained with different feeding amount of N-doped CDs (0, 2.5, 5, 7.5 and 10 mg) were designated as CoS, CoS/N-doped CDs-1, CoS/N-doped CDs-2, CoS/N-doped CDs-3 and CoS/N-doped CDs-4, respectively. 2.3. Preparation of rGO/N-doped CDs composites In a typical procedure to prepare rGO/N-doped CDs composites, N-doped CDs powder and graphite oxide (40 mg) were dispersed in 20 mL of deionized water by ultrasonication for 2 h. Subsequently, the pH value of the mixture was tuned to 11 using 3 M KOH aqueous solution. The mixture was then added into a 30 mL of autoclave by heating at 180 °C for 5 h. The products were separated by suction filtration and finally freeze-dried. The composites were named as rGO/N-doped CDs-X, in which X represents the amount of N-doped CDs (mg) added in the reaction. 2.4. Characterization and analysis The phase composition and structure of the samples were traced by X-ray diffraction (XRD, Rigaku Ultimate IV) and Raman spectrometer (DXR) with a laser 6

beam of 532 nm. The surface morphology and composition of the composites were studied by scanning electron microscopy (SEM, JSM-7800F), transmission electron microscopy (TEM, FEI Tecnai G2 F20) and X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI). The Brunauer-Emmett-Teller (BET) surface areas were performed on a Micrometrics ASAP 2020 sorption instrument. 2.5. Electrochemical measurements The electrochemical signals were collected on a CHI 760E analyzer in a 3 M KOH solution. For the electrode preparations of CoS/N-doped CDs and rGO/N-doped CDs, the composites were ground with acetylene black and PTFE binder (8:1:1) in ethanol solvent. After thorough mixing, the obtained homogeneous paste was pressed onto a Ni foam substrate at a pressure of 10 MPa and dried at 60 °C in vacuum oven for 12 h. The mass loading of active material on Ni foam was about 2 mg cm-2. For a three-electrode system test, a platinum foil was chosen as counter electrode and Hg/HgO electrode was used as reference electrode. Electrochemical investigations were performed using cycle voltammetry (CV) and galvanostatic charge-discharge measurements. Specific capacitance (C) was estimated in accordance with equation C = I△t/(m△V), where I represents the current density, △t is the discharge time, △V is the potential window in the curve, and m is the mass of CoS/N-doped CDs and rGO/N-doped CDs deposited on the electrode. The hybrid supercapacitor device was packaged into a 2032 type coin cell by choosing CoS/N-doped CDs as cathode and rGO/N-doped CDs as anode. A non-woven film (NKK MPF30AC, Nippon Kodoshi Corporation) was used as separator. The mass ratio of CoS/N-doped CDs and 7

rGO/N-doped CDs was balanced by the equation of m+△V+C+ = m−△V−C− [19].

3. Results and Discussion 3.1. Characterization of flower-like CoS/N-doped CDs composites (110)

(102)

(101)

Intensity (a.u.)

(100)

(a)

CoS/N-doped CDs CoS JCPDS: 65-3418

20

30

40

50

2θ θ (degree)

60

70

Fig. 1. (a) XRD patterns of flower-like CoS and CoS/N-doped CDs-3 composites; (b) Raman spectrum of CoS/N-doped CDs-3 composites; SEM images of (c) CoS and (d, e) CoS/N-doped CDs-3 composites.

To examine the crystallinity of CoS/N-doped CDs composites, the samples were investigated by XRD technique. As shown in Fig. 1a, the samples of CoS and CoS/N-doped CDs show similar XRD patterns, and all diffraction peaks can be well indexed to hexagonal phase CoS (JCPDS No. 65-3418). The characteristic peaks of NCDs can hardly be observed in CoS/N-doped CDs, this may be due to the relatively poor crystallinity and low mass loading of N-doped CDs in the composites [20,21]. In order to prove the existence of N-doped CDs, Raman spectroscopy analysis was 8

Fig. 2. (a, b) TEM and (c, d) HRTEM images of CoS/N-doped CDs-3 nanocomposites.

carried out for CoS/N-doped CDs composites (Fig. 1b). The two broad bands located at ca. 1357 and 1567 cm-1 can be indexed to the characteristic D and G peaks of carbon. It is known that the D band originates from the well-documented in-plane A1g zone-edge mode and related to local defects and disorders of carbon, while G band is assigned to the doubly degenerate zone center E2g phonon of C sp2 atoms [22]. In addition, the peaks at 463, 507(604) and 665 cm-1 can be assigned to Eg, F2g and A1g vibrational modes of hexagonal CoS [23,24]. The microstructures and morphologies of CoS and CoS/N-doped CDs were characterized by SEM analysis. From Fig. 1c and 1d, it can be seen that the prepared CoS and CoS/N-doped CDs-3 composites show 9

representative three-dimensional flower-like structure. Obviously, the morphology of CoS did not show obvious change after the introduction of NCDs. The higher magnification SEM image (Fig. 1e) show the more detailed microstructure of CoS/N-doped CDs composites, in which the three-dimensional nanoflower structure are assembled in an orderly fashion by many crossed two-dimensional nanosheets. There are many gaps in the three-dimensional structure, which can improve the specific surface area and make the OH- in the electrolyte more easily penetrate into the inner surface of the composites. The elemental composition and distribution of the CoS/N-doped CDs composites were analyzed by energy-dispersive X-ray (EDX) spectrometry. The resulting spectrum demonstrated the containing C, O, N, Co and S atoms in the composites (Fig. S1). In addition, from the element mapping images, it can be seen that the elements show highly uniform distribution in the nanoflowers (Fig. S2), which indicates that N-doped CDs are uniformly dispersed on the product. Further morphological and structural investigations were performed using TEM and high-resolution TEM (HRTEM). As shown in Fig. 2, the surface of CoS/N-doped CDs micro-flowers is uniformly decorated with lots of nanoparticles. The lattice fringe of 0.25 nm correspond to the (101) plane of CoS, while the lattice spacing of 0.21 nm can be ascribed to the (100) plane of N-doped CDs, which indicating the N-doped CDs have been incorporated in the CoS/N-doped CDs composites. The chemical states of different elements in bare CoS and CoS/N-doped CDs composites were investigated by XPS measurements in depth. Fig. 3a presents the high-resolution C 1s spectrum of CoS/N-doped CDs-3 composites, which can be 10

(a)

(b)

(c) N 1s

284

286

288

290

292

Intensity (a.u.)

Intensity (a.u.) 282

396

398

Binding Energy (eV)

400

402

404

780

785

790

795

800

805

785

790

795

800

810

805

810

Binding Energy (eV)

(f) S 2p

2p 1/2

2p 3/2

Intensity (a.u.)

Intensity (a.u.)

Intensity (a.u.)

sat.

sat.

775

780

S 2p

Co 2p 2p 1/2

sat.

sat.

775

406

(e) 2p 3/2

2p 1/2

Binding Energy (eV)

(d)

Co 2p

2p 3/2

Intensity (a.u.)

C 1s

sat.

160

Binding Energy (eV)

162

164

166

168

170

Binding Energy (eV)

172

174

2p 1/2

2p 3/2

sat.

160

162

164

166

168

170

172

174

Binding Energy (eV)

Fig. 3. (a) C 1s, (b) N 1s, (d) Co 2p and (f) S 2p of CoS/N-doped CDs-3 composites; (c) Co 2p and (e) S 2p of bare CoS.

deconvoluted into three different peaks. The peaks at 284.8, 286.0 and 288.0 eV can be classified as C-C/C=C, oxygen-carbon and nitrogen-carbon, respectively [25]. The high resolution spectrum of N 1s is presented in Fig. 3b. The bands at 398.0, 399.6, 400.6 and 402.0 eV correspond to pyridine N, pyrrole N, graphite N and N-H, respectively. Both C and N elements come from N-doped CDs. These results clearly indicate that the CDs are successfully doped by nitrogen. Fig. 3c and 3d show the Co 2p XPS spectra of CoS and CoS/N-doped CDs-3 composites, respectively. The Co 2p1/2 and Co 2p3/2 peaks can be fitted with two spin-orbit doublets, corresponding to Co2+ (781.1 and 797.1 eV) and Co3+ (778.8 and 793.9 eV) [26]. The two satellite peaks (designed as “Sat.”) are centered at 785.1 and 803.0 eV. Notably, the peak intensities of Co2+ in CoS/N-doped CDs-3 increase slightly in comparison with bare CoS, indicating that more Co2+ species exist in the composites. The higher Co2+ 11

content in the composites may be attributed to the reduction of Co3+ by N-doped CDs, and can contribute to the enhanced electrochemical activity [27,28]. Fig. 3e and 3f show the high resolution S 2p spectra of CoS and CoS/N-doped CDs-3. The two peaks at 161.8 and 163.0 eV represent S 2p3/2 and S 2p1/2 bonding, while the characteristic peak at 168.6 eV can be ascribed to the high valent sulfur at oxidized state [29-31]. The specific surface area and porosity of CoS and CoS/N-doped CDs were also characterized by BET analysis (Fig. S3). The specific surface area of CoS/N-doped CDs-3 composites is calculated to be about 27.7 m2 g-1. This value is higher than that of bare CoS (20.3 m2 g-1). In addition, the pore size distributions indicate that the CoS and CoS/N-doped CDs-3 have porous structure. Compared with CoS, the CoS/N-doped CDs-3 composites possess more abundant pores. The enhanced surface area and abundant porous structure will expose more electrochemical active sites and provide larger open channels for electrolytes, contributing to the significantly enhanced electrochemical property [32]. 3.2. Characterization of rGO/N-doped CDs composites The XRD patterns of graphite oxide, rGO and rGO/N-doped CDs composites are shown in Fig. 4a. Graphite oxide exhibits a characteristic peak at 11.2°, which corresponds to the (001) diffraction of graphite oxide [33]. For rGO and rGO/N-doped CDs, the peak at 10.1° completely disappears, while two broad bands appear at about 24.9° and 43.2°, which can be indexed to the (002) and (100) diffractions of rGO nanosheets. This indicates that graphite oxide was successfully reduced. Fig. 4b exhibits the Raman spectra of the resultant samples. Obviously, the 12

(c)

rGO

C 1s rGO graphite oxide

O 1s

rGO/N-doped CDs

N 1s

rGO/N-doped CDs

G

Intensity (a.u.)

(100)

D

Intensity (a.u.)

(002)

(b)

(001)

Intensity (a.u.)

(a)

graphite oxide 10

20

30

40

2θ (degree)

50

60

500

1000

1500

2000

288

600

290

800

(f) O 1s

Intensity (a.u.)

Intensity (a.u.)

Intensity (a.u.)

286

Binding Energy (eV)

400

N 1s

C 1s

284

200

Binding Energy (eV)

(e)

(d)

282

0

2500

-1

Raman shift (cm )

398

400

402

Binding Energy (eV)

404

526

528

530

532

534

536

538

540

Binding Energy (eV)

Fig. 4. (a) XRD patterns and (b) Raman spectra of graphite oxide, rGO and rGO/N-doped CDs-1.0 samples; (c) Survey scan, (d) C 1s, (e) N 1s and (f) O 1s spectra of rGO/N-doped CDs-1.0 composites.

well-defined D and G peaks can be observed in the three samples. The intensity ratio of D band and G band (ID/IG) relates to the amount of structural defects and the degree of disorder in graphene materials [34]. Herein, the ID/IG value of graphite oxide is 1.148, which is significantly smaller than that of rGO (1.854) and rGO/N-doped CDs (1.962). This result suggests that more defects and disorders were produced after the reduction of graphite oxide and the deposition of N-doped CDs on rGO sheets. To prove the existence of N-doped CDs, XPS measurements were carried out for rGO/N-doped CDs composites. The survey scan spectrum of rGO/N-doped CDs suggests the containing of C, N and O atoms in the composites (Fig. 4c). The C 1s spectrum (Fig. 4d) can be deconvoluted into three bands at 284.7, 286.1 and 288.0 eV, attributing to carbon-carbon, oxygen-carbon and nitrogen-carbon, respectively. The 13

four peaks at 398.0, 399.5, 400.5 and 402.2 eV in N 1s spectrum can be attributed to pyridinic N, pyrrolic N, graphitic N and N-H, respectively. Fig. 4f shows the high resolution O 1s spectrum of RGO/NCDs. The peaks at 531.2, 532.2 and 533.5 eV represent O-H, C=O and C-O bonding, respectively.

Fig. 5. (a) SEM and (b, c) TEM images of rGO/N-doped CDs-1.0 composites.

The morphology of rGO/N-doped CDs was further studied by SEM and TEM. The rGO/N-doped CDs composites (Fig.5a) show featured wrinkle structure, which is typical for rGO nanosheets. Fig. 5b and 5c show the TEM images of rGO/N-doped CDs, the as-synthesized rGO/N-doped CDs composites exhibit semitransparent sheet-like morphology with abundant pores on the surface. As mentioned above, the mesoporous architecture is of great significance for optimization the electrochemical characteristics of electrode materials. The BET surface areas of rGO and rGO/N-doped CDs-1.0 composites are calculated to be about 413.4 and 423.4 m2 g-1, respectively, which suggests that the aggregation of rGO sheets can be slightly prevented after N-doped CDs deposition. The pore size distributions of rGO and rGO/N-doped CDs are below 10 nm (Fig. S5). After modification with N-doped CDs, the rGO/N-doped CDs composites possess a relatively larger pore volume, possibly 14

due to the formation of new voids between the anchored N-doped CDs and rGO support [35]. 3.3. Electrochemical properties

18

-1

9 0 -9

10 mv s

90

25 mv s

60

50 mv s 75 mv s

30

(c)

-1 -1 -1 -1

100 mv s

-1

0

0.3 0.2 0.1

-60 0.0

-27 0.2

0.3

0.4

0.5

0.6

0.0

CoS CoS/N-doped CDs-1 CoS/N-doped CDs-2 CoS/N-doped CDs-3 CoS/N-doped CDs-4

0.5 0.4 0.3 0.2 0.1 0.0

100 200 300 400 500 600 700 800 900

-1 Specific capacitance (F g )

(d)

0.1

0.2

0.3

0.4

0.5

0

0.6

100 200 300 400 500 600 700 800 900

Potential (V vs. Hg/HgO)

(e)

Time (s)

CoS CoS/N-doped CDs-1 CoS/N-doped CDs-2 CoS/N-doped CDs-3 CoS/N-doped CDs-4

750 600 450 300 150 0

Time (s)

5

10

15

-1

20

-1

0.1

Potential (V vs. Hg/HgO)

Specific capacitance (F g )

0.0

Potential (V)

0.4

-30

-18

0

-1 1Ag -1 2Ag -1 5Ag -1 10 A g -1 15 A g -1 20 A g

0.5

Potential (V)

(b)

CoS CoS/N-doped CDs-1 CoS/N-doped CDs-2 CoS/N-doped CDs-3 CoS/N-doped CDs-4

27

Current density (A g )

-1

Current density (A g )

(a)

(f) 600 500

CoS/N-doped CDs-3

400 300 200

CoS

100 0

0

2000

4000

6000

8000

10000

Cycle number

Current density (A g )

Fig. 6. (a) CV curves of CoS and CoS/N-doped CDs composites at 25 mV s-1; (b) CV curves

of

CoS/N-doped

CDs-3

composites

at

different

scan

rates;

(c)

Charge-discharge profiles of CoS/N-doped CDs-3 composites at various current densities; (d) Charge-discharge profiles of CoS and CoS/N-doped CDs composites at 1 A g-1; (e) The specific capacitance values of CoS and CoS/N-doped CDs at different current densities; (f) Cycling stability of CoS and CoS/N-doped CDs-3 at 10 A g-1 over 10000 cycles.

The electrochemical characteristics of the electrode materials were firstly evaluated in a three-electrode test system. Fig. 6a presents the CV curves of CoS and CoS/N-doped CDs electrodes at scan speed of 25 mV s-1. Two pairs of redox peaks appear in the CV curves, corresponding to the reversible faraday process between 15

Co2+/Co3+ and Co3+/Co4+ in the presence of OH-. Obviously, the areas of CV curves of flower-like CoS/N-doped CDs composites are larger than that of bare CoS electrode, which suggests their enhanced specific capacitance values. Moreover, the CoS/N-doped CDs-3 electrode exhibits the optimum capacitance characteristics among all these samples. Thus, to further demonstrate the properties of CoS/N-doped CDs-3 electrodes, CV curves were analyzed at various scan rates, as shown in Fig. 6b. With the increase of sweep rate, the oxidation peak of CoS/N-doped CDs-3 shifts to high voltage and the reduction peak shifts to low voltage, which can be attributed to the relatively slow kinetics of the electrochemical reactions at higher scan rates [36]. There was no significant change in the curves even at 100 mV s-l. This indicates that the as-prepared flower-like CoS/N-doped CDs-3 composites possess the ability of fast charge transfer. The charge-discharge curves of CoS/N-doped CDs-3 electrodes at current densities of 1-20 A g-1 are presented in Fig. 6c. The specific capacitance of CoS/N-doped CDs-3 composites is 697 F g-1 at 1 A g-1. With the increase of current density, the specific capacitance decreases, which can be attributed to the insufficient faraday reaction time at high charge/discharge rate. However, the CoS/N-doped CDs-3 electrode still has a specific capacitance of 469 F g-1 even at 20 A g-1, which maintains about 67% of the original capacity. The specific capacitance of CoS/N-doped CDs-3 composites is comparable to or even better than those of previously reported cobalt sulfide materials, such as flower-like CoS nanostructure (348 F g-1 at 1 A g-1) [37], CoS nanospheres (632 F g-1 at 1 A g-1) [38], hollow structured Co1-xS (420 F g-1 at 1 A g-1) [39], the composite of CoS1.097 with N-doped 16

carbon matrix (360.1 F g-1 at 1.5 A g-1) [40], CNTs/Co3S4 (653 F g-1 at 1 A g-1) [41], and RGO/CoS2 (635.8 F g-1 at 1 A g-1) [42]. Galvanostatic charge-discharge curves of CoS and CoS/N-doped CDs composites at 1 A g-1 are displayed in Fig. 6d, from which it can be seen that the CoS and CoS/N-doped CDs electrode materials display different charging/discharging times at the same current density. According to the formula of specific capacitance, it is obvious that the capacities of CoS/N-doped CDs electrodes are much larger than that of bare CoS electrode. Fig. 6e shows the specific capacity values of CoS and CoS/NCDs at various current densities, which further reveals that the N-doped CDs have a substantial influence on the electrochemical properties of CoS. The capacities decrease in the order of CoS/N-doped CDs-3 > CoS/N-doped CDs-4 > CoS/N-doped CDs-2 > CoS/N-doped CDs-1 > CoS, which is consistent with the CV characterizations. The long-term cycling performance of electrodes is also crucial for the practical application of supercapacitor. Herein, the cyclic stability of CoS and CoS/N-doped CDs-3 composites was examined by charge-discharge tests at 10 A g-1 (Fig. 6f). The CoS/N-doped CDs-3 electrode exhibits a superior cyclic stability without undergoing decay after 10000 cycles. In contrast, the capacity retention of bare CoS was only 80.3%. The enhanced electrochemical performance of CoS/N-doped CDs electrodes should be attributed to the higher Co2+ species, improved specific surface area and abundant porous structure in the composites. However, excessive N-doped CDs would hinder the contact between electrolyte and CoS, and thus has a negative effect on the specific capacitance of CoS/N-doped CDs-4. 17

Fig. 7a shows the CV curves of rGO/N-doped CDs-1.0 composites at various sweep rates. Slightly deviation from symmetrical shape can be observed in the curves, which

is

attributed

to

the

pseudocapacitance

caused

by

the

residual

oxygen(nitrogen)-containing groups in the composites [43]. The shapes of the CV curves show almost no change even at 100 mV s-l, which suggests the fast current response ability of our prepared rGO/N-doped CDs composites. The charge-discharge profiles of rGO/N-doped CDs-1.0 at current densities from 1 to 20 A g-1 in Fig.7b are approximately triangular. This indicates that the electrode possesses high reversibility. 0.0

10

-0.2

0 -10

5 mv s-1 10 mv s-1

-20

25 mv s-1 50 mv s-1

-30

100 mv s-1 -1.0

-0.8

-0.6

-0.4

-0.2

Potential (V)

(b)

20

-1 Current density (A g )

(a)

-0.4 -0.6 -0.8 -1.0

0.0

Potential (V vs. Hg/HgO)

0

100

200

300

400

500

Time (s)

(c)

(d)

-1

Specific capacitance (F g )

-1 Specific capacitance (F g )

1 A/g 2 A/g 5 A/g 10 A/g 20 A/g

350

rGO/N-doped CDs-1.0 rGO/N-doped CDs-1.5 rGO/N-doped CDs-0.5 rGO

300 250 200 150

0

5

10

15

-1 Current density (A g )

20

250

200

150

100 0

5000

10000

15000

20000

Cycle number

Fig. 7. (a) CV curves of rGO/N-doped CDs-1.0 at various sweep rates; (b) Charge-discharge profiles of rGO/N-doped CDs-1.0 at various current densities; (c) The capacities of RGO and rGO/N-doped CDs estimated by the discharge curves; (d) Cycling stability of rGO/N-doped CDs-1.0 at 5 A g-1 over 20000 cycles. 18

The specific capacitances of rGO/N-doped CDs-1.0 are calculated to be about 350, 272, 227, 201 and 166 F g-1 at 1, 2, 5, 10 and 20 A g-1, respectively. The specific capacitance of rGO/N-doped CDs-1.0 composites is higher than previously reported graphene-supported carbon dots composites [43,44]. Fig. 7c presents the specific capacitances of rGO and rGO/N-doped CDs electrodes at various current densities, from which it can be seen that the electrochemical property of rGO can also be significantly improved by combination with N-doped CDs. The rGO/N-doped CDs-1.0 electrode displays the highest specific capacities. The cyclic stability of rGO/N-doped CDs-1.0 composites was performed by consecutive charge-discharge tests at 5 A g-1 (Fig. 7d). The rGO/N-doped CDs-1.0 electrode retained 93.4% of its initial capacity after 20000 cycles, suggesting the splendid cycle stability of our prepared electrode materials. Since the highly capacitive N-doped CDs-based composites have been prepared, a hybrid supercapacitor device was then assembled by using CoS/N-doped CDs-3 as the positive electrode and RGO/N-doped CDs-1.0 as the negative electrode. The mass ratio of CoS/N-doped CDs-3 to rGO/N-doped CDs-1.0 is calculated to be about 0.9 using the charge balance formula. Fig. 8a presents the representative CV curves of CoS/N-doped CDs-3//rGO/N-doped CDs-1.0 hybrid supercapacitor device in the range of 0-1.7 V. The CV curves of CoS/N-doped CDs//rGO/N-doped CDs exhibit the contributions from battery-type and capacitor-type capacity. Similar to the three-electrode characterization, the current densities increase with the sweep rates. The charge-discharge curves of CoS/N-doped CDs//rGO/N-doped CDs at current 19

(b)

-1

10 mv s -1 25 mv s -1 50 mv s -1 75 mv s -1 100 mv s

15 10 5

-1

1 Ag -1 2 Ag -1 5 Ag -1 10 A g -1 15 A g -1 20 A g

1.5

Potential (V)

-1 Current density (A g )

(a)

0 -5

1.2 0.9 0.6 0.3

-10 0.0

-15

0.0

0.4

0.8

1.2

0

1.6

50

100

-1 This work CoS//AC CoS2//AC

20

Co9S8//AC Ni-Co-S//AC Co3O4//AC NiCo2S4/Co9S8//AC

10

MnCo2S4//RGO NiCo2S4//Graphene/CS [email protected]//RGO

0 10

200

250

300

350

400

(d)

40

Specific capacitance (F g )

Energy density (Wh Kg-1)

(c) 30

150

Time (s)

Potential (V)

100

1000

-1

90 80 70 60 50 40 30 20 10 0

10000

Power density (W Kg )

0

2000

4000

6000

8000

10000

Cycle number

Fig. 8. (a) CV curves of CoS/N-doped CDs-3//rGO/N-doped CDs-1.0 hybrid supercapacitor device at various sweep rates; (b) Charge-discharge profiles of CoS/N-doped CDs-3//rGO/N-doped CDs-1.0 at various current densities; (c) Ragone plot of the CoS/N-doped CDs-3//rGO/N-doped CDs-1.0 hybrid supercapacitor device; (d) Cycling stability of the device at 10 A g-1.

densities from 1 to 20 A g-1 are presented in Fig. 8b. The nearly symmetrical profile suggests the high coulombic efficiency and reversibility. The energy densities and power densities of CoS/N-doped CDs//rGO/N-doped CDs were calculated from the discharge curves in Fig. 8b. The device delivers an energy density of 36.6 Wh kg-1 at 800 W kg-1, and it can still maintain 25.6 Wh kg-1 even at the high power density of 16 000 W kg-1. The energy densities of the device are comparable to those of 20

previously reported devices (Table S1), such as CoS//AC [37], CoS2//AC [45], Co9S8//AC [46], Ni-Co-S//AC [47], Co3O4//AC [48], NiCo2S4/Co9S8//AC [49], MnCo2S4//RGO [50], NiCo2S4//Graphene/CS [51], and [email protected]//RGO [52]. The cyclic stability of our prepared CoS/N-doped CDs//rGO/N-doped CDs hybrid supercapacitor device was also investigated at 10 A g-1. As shown in Fig. 8d, the capacity of the assembled device was practically unchanged after 6000 cycles, and about 85.9% of the initial capacity was retained after 10000 continuous cycles. These results indicate that the hybrid supercapacitor device composed of CoS/N-doped CDs cathode and rGO/N-doped CDs anode is highly desirable for future energy storage system.

4. Conclusions In summary, N-doped CDs decorated flower-like CoS hierarchitectures and rGO nanosheets were coordinately prepared by a facile hydrothermal strategy. It is shown that the presence of N-doped CDs can significantly increase the specific surface areas and pore volumes of CoS and rGO, resulting in enhanced electrochemical properties of CoS/N-doped CDs and rGO/N-doped CDs with high specific capacitances and excellent cycling stability. More prominently, a hybrid supercapacitor device was assembled by using the CoS/N-doped CDs flowers and rGO/N-doped CDs nanosheets as cathode and anode, respectively. The hybrid supercapacitor device delivers a high energy density of about 36.6 Wh kg-1 at 800 W kg-1, good cycling stability (85.9% retention after 10000 cycles). This study reveals the superiority of N-doped 21

CDs-based materials and provides a novel high performance hybrid supercapacitor device for future energy storage.

Acknowledgements This study was sponsored by the National Natural Science Foundation of China (Nos. 51602129 and 21875091), the Joint Open Fund of Jiangsu Collaborative Innovation Center for Ecological Building Material and Environmental Protection Equipments and Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province (JH201804), and the Young Talents Training Program of Jiangsu University.

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Declaration of Interest Statement

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.