three-dimensional nitrogen-doped garphene hybrid nanoarchitectures for advanced asymmetric supercapacitor

Journal Pre-proofs Self-assembling NiCo2S4 nanorods arrays and T-Nb2O5 nanosheets / three-dimensional nitrogen-doped garphene hybrid nanoarchitectures for advanced asymmetric supercapacitor Mingmei Zhang, Hong Liu, Zixiang Song, Tianjiao Ma, Jimin Xie PII: DOI: Reference:

S1385-8947(19)33084-0 https://doi.org/10.1016/j.cej.2019.123669 CEJ 123669

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

25 July 2019 25 October 2019 29 November 2019

Please cite this article as: M. Zhang, H. Liu, Z. Song, T. Ma, J. Xie, Self-assembling NiCo2S4 nanorods arrays and T-Nb2O5 nanosheets / three-dimensional nitrogen-doped garphene hybrid nanoarchitectures for advanced asymmetric supercapacitor, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123669

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© 2019 Published by Elsevier B.V.

Self-assembling NiCo2S4 nanorods arrays and T-Nb2O5 nanosheets / three-dimensional nitrogen-doped garphene hybrid nanoarchitectures for advanced asymmetric supercapacitor Mingmei Zhang, Hong Liu, Zixiang Song, Tianjiao Ma, Jimin Xie School of Chemistry and Chemical Engineering, Jiangsu University, 301

Xuefu

Road, Zhenjiang 212013, China

Abstract In this study, NiCo2S4 nanorods arrays on nickel foam (NiCo2S4/NF) have been developed using a simple solvothermal method and T-Nb2O5 nanosheets / three-dimensional nitrogen-doped graphene hybrid (T-Nb2O5/3DNG) have been synthesized by hydrothermal accompanying an annealing calcination treatment. In a three-electrode system, NiCo2S4/NF nanorod and T-Nb2O5/3DNG electrodes display excellent specific capacitances of 1709.2 F g-1 and 952.7 F g-1 at the current density of 1 A g-1, respectively, with the capacitance retention of 93.1 % and 90.8% after 5000 cycles. Furthermore, a hybrid asymmetric supercapacitor device consisting of a binder-free NiCo2S4/NF nanorods arrays as a positive electrode and T-Nb2O5/3DNG as a negative electrode in 6 M KOH electrolyte is



Correspondence author

E-mail address: [email protected]. (M. M. Zhang) 1

designed, which exhibits an expanded potential voltage window up to 1.8 V, presents extremely high capacitance of 203.4 F g -1at 0.5A g-1 and a remarkable energy density of 45.4 Wh kg-1 while delivering a power density of 9568.3 Wkg-1. Encouragingly, the device also delivers outstanding cycling stability of 93.6 % and a coulombic efficiency close to 100% after 10000 charge-discharge cycles. This strategy for the reasonable design of the efficient positive and negative electrode materials provides a promising route for asymmetric supercapacitor and opens new avenues with production of high energy and long life capacitor equipment in modern electronic industries. Keywords: T-Nb2O5 nanosheets; NiCo2S4 nanorods; N-doped graphene; high-performance supercapacitors

1.

Introduction

With the current rising oil prices, the energy problem has severely restricted the development of the world economy. Therefore, people pay more attention to all kinds of new energy equipment. Among the available electrochemical energy storage families, supercapacitor has widely prospect in electric vehicles, wearable electronic devices and high-power heavy machinery because of its ultra-high power density, long service life, security and fast charge/discharge advantages. Whereas, with the progress of technology and the improvement of people's needs, the development of supercapacitor technology has been difficult 2

to meet the requirements of consumer, so it has great practical significance to develop a supercapacitor with high performance. As the core of supercapacitors, the electrochemical performances of electrode materials have a decisive impact on the energy storage performance [1-4]. Transition metal oxides/sulfides have been extensively used as electrode materials for supercapacitors due to their outstanding capacitive properties [5-7]. Nickel and cobalt are abundant in the earth, cheap, easy to obtain, and have good cycling and rate electrochemical properties. One metal often has more or less shortcomings, which is difficult to achieve satisfactory results both in structure and performance [8,9]. Recently, a large number of reports have shown that the combination of multi-metal can enhance the electrochemical performance of supercapacitors because of their multivalence crystal structures and rich redox behavior. For example, Hong et al. fabricated Ni@CoNi-MOF core-sheath structured film as electrode material for energy-intensive supercapacitors[10]. Zhou et al. prepared hollow-structured NiCo hydroxide/carbon nanotube composite as an asymmetric supercapacitor electrode exhibiting high coulombic efficiency and along cycle life [11]. As a kind of bimetal based sulfide electrode material, nickel cobalt sulfides have paid significant attentions with the advantages of wider electrochemical window, high conductivity and high energy density. Ning et al. prepared NiCo2S4 nanoparticles on CBC-N fibers as the highly conductive negative electrode material for asymmetric supercapacitor showing excellent electrochemical activity [12]. Various effective strategies have been explored to solve the accumulation of 3

active substances ingeniously and increased the active sites per unit area of nickel cobalt sulfides, such as growing nanorods arrays directly on foam nickel, which is beneficial for efficient electrochemical reactions. Significantly, the nanostructure arrays with a robust contact on high conductive Ni form can not only provide faster pathway for electrons transport, but also avoid the subsequent complex electrode fabrication process and realize the binder-free structure with lower resistance. Liang et al. produced NiFe -LDH nano arrays in foam nickel by hydrothermal method, which exhibits excellent electrochemical stability in alkaline electrolyte [13]. So far, asymmetric capacitors can combine the difference of potential windows between positive and negative electrodes to obtain higher operating voltage, so their energy density is obviously higher than that of symmetric capacitors. Great efforts have been made to produce various asymmetric supercapacitor, such as CuCo2S4/CNT// Activated Carbon[14], Ni/Co-S//Activated carbon[15], NiFeS2 /3DSG//3DSG[16]. At present, carbon materials, such as activated carbon and graphene are widely used as negative materials in asymmetric supercapacitor because they have good conductivity and high power density besides larger specific surface area. Unfortunately, the specific capacitances of carbon materials are low, which seriously hinder the whole performance of supercapacitors, so designing and development of materials matching positive capacitance are also very important in the field of asymmetric supercapacitors.

4

Orthorhombic Nb2O5 (T- Nb2O5) is a potential developer in the field of electrode materials of supercapacitors because of its good capacitance performance, excellent electrochemical stability and acid-alkali media corrosion resistance [17-20]. Therefore, the low conductivity of Nb2O5 has slow electrochemical reaction kinetics, which leads to the fact that the actual specific capacitance is far below their theoretical value. Recently, many projects have been adopted to promote the electrical conductivity of Nb2O5, such as compounded with new carbon materials, which can not only improve the conductivity of Nb2O5, but also solve its capacity attenuation problem. Lee et al. synthetized Nb2O5 @Carbon core–shell nanoparticles and reduced graphene oxide, which showed high energy/power densities and stable cycle life [21]. On the other hand, different nanostructures also improve the active sites per unit area. Luo et al. prepared flake Nb2O5 by hydrothermal method, which reduced the distance of lithium ion in diffusion process and effectively improves its conductivity [22]. Hao et al. prepared Nb2O5@carbon/reduced

graphene

oxide using Nb-based

metal organic frameworks, which delivers high energy density of 71.5 Wh kg-1 and good cyclic stability [23]. At present, electrodes always use metal-based current collectors with heavy mass to transfer current and load active materials, which have no contribution to capacity. Now, we first design a asymmetric supercapacitor utilizing NiCo2S4 hollow nanorods arrays growth on Ni foam as the

positive

electrode

and

T-Nb2O5

nanosheets

/

three-dimensional

nitrogen-doped garphene as the negative electrode without using metal current 5

collectors, which can work in a dual-ion system and successfully improve the specific capacitance and energy density of the system. Herein, N-doped graphene (NG) have been successfully prepared via simple pyrolysis route using 5-HMF as carbon source and urea as nitrogen source and the uniform distribution of Nb2O5 nanosheets on nitrogen-doped graphene were synthesized subsequently, which significantly improved the conductivity of the hybrids and prevents the agglomeration of the nanosheets. At the same time, NiCo2S4 nanorods were in situ grown on the collector surface (NF), which solved the disordered complex mode by constructing NiCo2S4 nanorod arrays and reduced any possible contact resistances, the three-dimensional nanorod array of NiCo2S4/NF can ensure the full contact between the electrode material and the electrolyte and reduce the ion diffusion resistance. Moreover, a novel high-performance asymmetric device with self-assembled NiCo2S4/NF arrays as a positive electrode and T-Nb2O5/3DNG hybrids as a negative electrode (NiCo2S4/NF// T-Nb2O5/3DNG) were designed, which not only obtained a high specific capacitance of 203.4 F g-1 but also got an a maximum energy density of 72.3 Wh kg-1 at power density 400.1 W kg-1, demonstrating potential commercial prospect and practical value. 2. Experimental 2.1. Synthesis of 3DNG Three-dimensional nitrogen-doped graphene (3DNG) was synthesized as follows: 6

first, the 5-hydroxymethylfurfural (5-HMF) was added into saturated urea aqueous solution and was dissolved under magnetic stirring, then was dried in oven at 90-100 ℃. The solid mixture obtained above was put into quartz boats and was transferred to a vacuum tubular furnace under Ar flowing. The sample had been programmed to rise to 600 °C at a heating rate of 1 °C per minute and maintained this temperature for one hour and continued heating to 950 °C with the same heating procedure. Finally, the hybrids were dispersed in distilled water by ultrasonic dispersion for 2 hours and dried in vacuum freeze drier. 2.2. Synthesis of T-Nb2O5/3DNG Briefly, 50 mg 3DNG was dispersed in 60 mL absolute ethanol and sonicated for 1 h to obtain a homogeneous, then, 0.4098 g NbCl5 was added into the above mixture under vigorous stirring. Subsequently, 1 M (NH4)2C2O4 solution (15mL) was dropped into the above solution and continuously stirred overnight. After that, the mixed solution was transferred into a 100ml Teflon-lined stainless steel autoclave and was heated at 180 °C oven for 24 h. Nb2O5 supported on the surface of 3DNG was taken out and subsequently washed with distilled water for several times, and then dried in oven at 60°C. Finally, T-Nb2O5/3DNG was achieved by annealing treated at 650°C for 2h in nitrogen atmospheres. For comparison, T-Nb2O5 nanosheets were also synthetized under the same condition without adding 3DNG. 2.3. Synthesis of NiCo2S4 hollow nanorods on Ni foam (NF) 7

NiCo2S4 nanorods arrays growth on NF was achieved by a facile solvothermal process followed with a simple annealing process. In a typical synthesis, 0.36 g Ni(NO3)2·6H2O, 0.72 g Co(NO3)2 ·6H2O, and 2.4 g thiourea were dispersed into ethylene glycol (50 mL) under vigorous stirring for 1 hour. Then, the homogeneous pink solution and a piece of surface treated NF (2 cm × 2 cm) fixed on the glass slide with raw material belt were transferred into an 100-mL autoclave and kept at 160 °C for 24 hour in an oven. Finally, the nickel foam loaded cobalt-nickel precursor was dried in vacuum oven at 80 °C, and was further crystallized in annealing at 400 °C in Ar for 2 h, finally, NiCo2S4 /NF nanocomposites were obtained. The method of determining the loading mass of active substances was as follows: first, before the reaction, the mass of nickel foam was not taken into the reactor, and the average mass of the nickel foam is m1. Then the mass of nickel foam after reaction was obtained, and the average mass of the nickel foam was m2, then the mass of active substance was Sm = (m2 - m1) /a, where a is the area of foam nickel (cm2). According to the above calculation, the loading mass of NiCo2S4 was 8.5 mg cm-2. To confirm the ratio of Ni to Co in the nanomaterial, the inductively coupled plasma spectroscopy (ICP, Optima2000DV, USA) test was carried out, which showed that the mass ratio of Ni and Co was1.02:2.05, which closed to the theoretical value (0.996:2).

2.4 Characterization 8

The crystalline features of the as-synthesized sample were analyzed by D8 Advance X-ray diffractometer (XRD, Bruker AXS Company, Germany), and the chemical oxidation states of the surface elements of the prepared sample were tested by X-rays photoelectron microscopy (XPS, Thermo ESCALAB 250 XI spectrometer). Field emission scanning electron microscopy (FESEM, Hitachi S-4800 II, Japan) and transmission electron microscopy (TEM, JEOL-JEM-2010 JEOL, Japan) were installed to analyze the microstructural characteristics of the samples. 2.5. Electrochemical measurements The electrochemical performances of the individual electrode were investigated under

a

standard

three-electrode

configuration.

Firstly,

the

prepared

T-Nb2O5/3DNG powder were covered with clean carbon paper using 0.6 wt% Nafion suspension and were pressed between two pieces of copper to form a uniform and soft sheet. The prepared NiCo2S4/NF electrode materials were directly cut into square pieces, because the electrode materials have good electrical conductivity and mechanical properties, the working electrodes did not use metal collectors. Platinum tablets 1 cm × 1 cm was the counter electrode and mercury-mercury oxide electrode (Hg/HgO) was the reference electrode. Then, they are immersed in 6 M of potassium hydroxide solution. The specific capacity (Cs, F g-1) was evaluated using the following equation:

9

Cs 

It mV

(1)

where I refers to the discharge current (mA), ∆t refers to the discharge time (s), m refers to the loading mass of the electrode material (g) and ΔV is the potential window. 2.6. Assembly of asymmetric supercapacitor An asymmetric supercapacitor (ASC) was configured with a positive electrode of NiCo2S4 / NF, a negative electrode of T-Nb2O5/3DNG in a two-electrode system, in which the quality of the active substances on the two electrodes were satisfied with the charge balance equation (q+ = q-). The 6 m KOH aqueous solution is used as the electrolyte, and the positive and negative electrodes are separated by a cellulose separator in the middle, and the electrodes, the separator and the electrolyte are sealed by a thermoplastic self sealing bag.

m C   V  m C   V

(2)

Here, C+ and C- refer to the mass specific capacitances of NiCo2S4 / NF and T-Nb2O5/3DNG electrodes obtained from the three electrode system, respectively. V+ and V- represent the working voltage window range of the NiCo2S4/NF and T-Nb2O5/3DNG electrodes measured in three electrodes, respectively,

m+ and m- are the active mass of the NiCo2S4 and the

T-Nb2O5/3DNG (g), respectively.

10

(3) where the Ccell ( F g-1) is the specific capacitance of the equipment, m' (g) is the total mass of material of both positive and negative electrodes, I' is the discharge current and ∆V' is the potential window of the equipment. The energy density (E, Wh kg-1) and power density (P, kW kg-1) of an asymmetric supercapacitor cells are deduced by the following equation:

(4)

P

3600  E t '

(5)

All electrochemical measurements were measured using an electrochemical workstation (CHI 760E, Shanghai Chenhua Instrument Co., Shanghai, China).

3. Results and discussion 3.1. Negative electrode material The T-Nb2O5 nanosheets were prepared on 3DNG via a two-step route, which was schematically presented in Fig. 1a. First of all, the 3DNG were obtained by pyrolysis of 5-hydroxymethylfural at high temperature. Secondly, 3DNG were uniformly distributed into an aqueous solution including Nb5+ and C2O42-, then Nb2O5 uniformly have distributed on the surfaces of 3DNG in-situ growth under

11

hydrothermal conditions. To acquire exact structure and morphology, SEM and TEM images were presented to survey the as-synthesized hybrids (Fig. 1a-i). Low magnification SEM images of NiCo2S4 /NF were shown in Fig.4 (a), NiCo2S4 was evenly distributed on the surface of nickel foam and they are closely integrated. From Fig.1b, c, it can be seen that T-Nb2O5 was formed by the accumulation of nanosheets of hexagonal crystal types. From Fig.1 d, we can see that the 3DNG were successfully obtained by pyrolysis of 5-hydroxymethylfural at high temperature without any help of metal catalysts, which exhibited porous structure and numerous surface wrinkles. After hydrothermal and calcination crystallization, T-Nb2O5 nanosheets were archored on 3D nitrogen-doped graphene space network formation 3D pore structure (Fig.1 e), which could not only provide more active points and ion diffusion channels, but also increase the wettability of electrolyte, accelerate the electron transfer and enhance the electrochemical performance. TEM images of T-Nb2O5(Fig.1 f) reveal that the thickness of nanosheets are relatively thin and the size of nanosheets are about 100-300 nanometers. Fig.1 g displayed that nitrogen-doped graphene had a transparent thin chiffon structure, the unique wrinkle and crimp morphologies with unique three-dimensional hole structure and a high surface area. Fig.1 h reveals that the T-Nb2O5/3DNG hybrids are composed of many T-Nb2O5 nanosheets suspended or firmly dispersed on 3DNG network surfaces, which provide a large area of electrochemical activity for electrochemical reaction. The HRTEM image of T-Nb2O5 illustrated in Fig.1 i clearly revealed that the highly 12

Nb2O5 crystalline structure with a d lattice fringes spacing of 0.39 nm was indexed to the peak of (001). Two-dimensional nanosheets with poor crystallinity usually have more defects, which will form local states to bind electrons and reduce the electron mobility in nanosheets, resulting in the decline of properties of nanosheets. So, the high crystallinity T-Nb2O5 nanosheets will accelerate the electron transfer on the nanosheet surface, resulting in better conductivity and activity of the composite. (a)

saturated urea

Nb5+

stirring

heating

drying

Ar flowing

C2O4

hydrothermal annealing

2-

T-Nb2O5/3DNG

3DNG

b

c

d pore

Nb2O5

100 nm

200 nm

200 nm

500 nm

f

h

g

i

3DNG

100 nm

180

001

100 nm

e

3DNG

j

Nb2O5 0.39 nm (001) 2 nm

k

10

20

30

40

50

60

382

2161

181

C(100) 002 321 380 202 381 2160

C(002)

Intensity (a.u.)

Nb2O5/3DNG

PDF#30-0873

70

80

2Theata (degree)

Fig.1 (a) Preparation process of T-Nb2O5 nanosheets; (b)(c) the SEM images of T-Nb2O5; (d) the SEM image of 3DNG; (e) the SEM image of T-Nb2O5/3DNG; 13

(f) the TEM image of T-Nb2O5; (g) the TEM image of 3DNG; (h) the TEM image of T-Nb2O5/3DNG; (i) the HRTEM image of T-Nb2O5; (j) the XRD pattern of T-Nb2O5/3DNG; (k) EDS analysis of the hybrid Fig.1 j is the XRD pattern of T-Nb2O5/3DNG composites. The main diffraction peaks of T-Nb2O5/3DNG appeared around 22.5, 28.3, 36.6, 46.1, 49.7, 50.9, 55.1, 56.4, 58.6, 63.7 and 71.2 °(2θ), which corresponded to (001), (180), (181), (002), (321), (380), (202), (381), (2160), (2161) and (382) crystal planes of T-Nb2O5, respectively, which were well indexed as the standard cards of orthorhombic card (JCPDS No.30-0873). In addition, the two typical diffraction peaks located at 24.3° and 42.6° are caused by the (002) and (100) lattice planes of graphene, respectively[24]. Besides, EDS analysis (Fig.1 k) shows that the hybrids are composed of C, Nb, O, N and Si elements, in which Si is in silicon slice. The above results verified that T-Nb2O5 nanosheets were successfully synthesized on 3DNG surface.

14

C 1s

665

ID/IG=1.01

500

1000

1500

2000

2500

-1

Nb 3d Nb 3p

0

3000

200

400

600

800

1000

Binding Energy (eV)

c

206.7 eV

d

400.1 eV

N 1s

Pyrrolic N 398.6 eV

Intensity (a.u.)

Nb 3d5/2

Intensity (a.u.)

N

Nb 4d Nb 4s

Raman Shift (cm )

Nb 3d

b

O 1s

3DNG

Intensity (a.u.)

Intensity (a.u.)

Nb2O5

T-Nb2O5/3DNG

a

3DNG T-Nb2O5/3DNG

ID/IG=1.03

209.5 eV Nb 3d3/2

Pyridinic N

401.5 eV

Graphitic N

202

204

206

208

210

212

214

394

396

398

400

402

404

Binding Energy(eV)

Binding Energy (eV)

Fig.2 (a) Raman spectral image of T-Nb2O5/3DNG and 3DNG; (b) the full spectrum of XPS characterization; (c) the high resolution of Nb3d spectrum; (d) the high resolution of N1s spectrum Fig.2 a is Raman spectral image of T-Nb2O5/3DNG and 3DNG samples. Two remarkable bands of the composites at 1350 and 1583cm-1 are correspond to D band (sp3 hybridized defect-related) and G band (sp2 bonded-related) of graphene materials respectively [25]. The ratio of D peak to G peak intensities (ID/IG) represents disorder and defect degree of carbon materials. In T-Nb2O5/3DNG, the ID/IG value is calculated to be 1.03, which is higher than that of DNG(1.01), , indicating that there are the much higher level of defects in T-Nb2O5/3DNG structure. Moreover, the wider peak located at about 665cm-1 is the characteristic band of the symmetric stretching mode of Nb-O bonds, which is consistent with 15

the reported literature [26,27]. Furthermore, X-ray photoelectron spectroscopy (XPS) was used to evaluate the binding energy and surface chemical state of the bonding elements. Typical XPS of T-Nb2O5/3DNG and 3DNG are shown in Fig. 2 b-d. From Fig. 2 b, a full XPS spectrum of the two samples is characterized. For 3DNG, there are only three elements of C, N, O signals. The four signals of C, O, N and Nb can be observed in T-Nb2O5/3DNG, which further confirmed the formation of T-Nb2O5/3DNG hybrids. Fig. 2 c presents the high resolution of Nb3d spectrum, and the central peaks at 206.7 and 209.5 eV are respectively consistent with Nb 3d5/2 and Nb 3d3/2 states, which are assigned to the typical binding energy for Nb5+ in Nb2O5[28,29]. Analysis of N1s spectra indicate that N exist mainly in the form of pyridinic defects N (398.6 eV), pyrrolic N (400.1 eV) and graphitic N

substitutional defects (401.5eV) [30,31], suggesting that N

have been successfully doped into graphene, which can not only obtain more active sites, but also improve the conductivity of graphene materials.

T-Nb2O5 3DNG

200

T-Nb2O5/3DNG

0.6

3DNG T-Nb2O5

-1

300

0.7

0.5

3

400

0.8

a

T-Nb2O5/3DNG

dV(r)/cm nm g-1

3 -1

Quantity Absorbed (cm g , STP)

500

0.4

b

0.3 0.2 0.1

100

0.0 0 0.0

-0.1 0.2

0.4

0.6

0.8

1.0

-2

0

2

4

6

8

10

12

14

16

18

Pore radius / nm

Relative Pressure (P/P0)

Fig.3 (a) The N2 adsorption desorption isotherm of samples; (b) pore size distribution of samples 16

It is well known that the pore volume, pore size and specific surface area of nanomaterials are very important for their capacitance performance. The T-Nb2O5/3DNG, T-Nb2O5 and 3DNG were tested by physical nitrogen adsorption-desorption method, as shown in Fig. 3 a. The nitrogen adsorption/desorption isotherm curves of all the three materials show the IV-type with the hysteresis loops similar to H1 type, suggesting the presence of micropore-mesoporous with these hybrids. The amount of nitrogen adsorption is very small in the low pressure region, and there is no obvious inflection point in the curve, which indicates that the interaction force between the surface adsorbent and the adsorbate (nitrogen) is quite weak. When the nitrogen partial pressure increases further (P/P0>0.5), the three isothermal curves do not overlap, accompanied by obvious hysteresis loops, further indicating that the material contain some mesoporous structures [32]. After calculation, the specific surface areas of 3DNG, T-Nb2O5/3DNG and T-Nb2O5 are 431.6 m2 g-1, 274.3 m2 g-1, 98.7 m2 g-1, respectively. Moreover, pore size distribution was tested by Barrett-Joyner-Halenda (BJH) method. As can be seen from Fig. 3 b, the pore size of T-Nb2O5/3DNG ranges from 1 to 18.5 nm and the corresponding pore size are two peaks, which are 3.07 nm and 3.85 nm, respectively. The pore size distribution of 3DNG is mainly 2.42 nm (mesoporous), while T-Nb2O5 is mainly 1.45 nm (microporous).

Table1 displays the specific surface area, pore size and

total pore volume of all samples, which demonstrate that microporous and mesoporous are interlaced and synergistic to form a fast diffusion channel of 17

electrolyte and a larger contact area between electrolyte and active material, which are the key to improve the capacitance performance of self-supporting electrodes. Table1 Specific surface area, pore size and pore volume of all samples Pore size

Pore volume

Sample

Specific surface area ( m2 g-1 )

( nm )

Vtotal ( cm3 g-1 )

3DNG

431.6

2.42

0.84

T-Nb2O5/3DN G

274.3

3.29

0.62

T-Nb2O5

98.7

1.45

0.18

3.2. Positive electrode material The NiCo2S4 hollow nanorods arrays are prepared on NF via alcohothermal followed by annealing treatment, which is schematically illustrated in Fig. 4 a. First of all, Ni2+, Co2+ and thiourea were tightly absorbed on the surface of NF. Secondly, transfer the above solution to a 50 mL Teflon sealed reactor, in which foam nickel should be leaned on the inner wall of the tetrafluoroethylene to ensure that the nickel foam does not touch the bottom of the bottle, so as to prevent the precipitation generated during the reaction from covering the surface of the nickel foam and hinder the continuous growth of the nanostructure. Finally, NiCo2S4 /NF nanorod array electrode material was obtained by annealing the sample at 400 °C for 2 h in argon atmosphere. From the low magnification SEM images NiCo2S4/NF in Fig. 4, NiCo2S4 nanorods uniformly grow on the surface 18

of nickel foam and NiCo2S4 and nickel foam are closely linked together. In order to verify the good nanorod array structure, the sample was characterized by SEM and TEM, in which foam nickel was removed. As shown in Fig.4 b and c, it is obvious that the NiCo2S4 nanorods are arranged more orderly and the array structures are already very distinct. The length of single nanowires are about 500-800 nm, and the tips are slightly pointed. Very dense and cluster structure formed by adhesion of many nanorods are beneficial to electron transfer during electrochemical reaction. As can be seen from Transmission electron microscopic (TEM) (as depicted in Fig.4 d), the nanorods have high Ni2+ Co2+

thiourea

NF

alcohothermal

stirring ethylene glycol

(a) annealing

1 hour

NiCo2S4 /NF precursor

b

SEM NiCo2S4 /NF

c

10 um

d

e 0.28 nm (311)

100 nm

200 nm

500 nm

50 nm

g

f Intensity (a.u.)

NiCo2S4 JCPDS:20-0782

(311)

(440) (400)

(111)

20

(220)

30

(422) (511)

40 50 2 Theta (degree)

(731) (642)

60

70

Fig. 4 (a) The fabrication process of NiCo2S4/NF and low magnification SEM images NiCo2S4/NF; (b)(c) the SEM images of the NiCo2S4 nanorods; (d)(e) the 19

TEM images of the NiCo2S4 nanorods; (f) the XRD pattern of NiCo2S4; (g) EDS analysis of the sample length-diameter ratio and obvious needle-like structure. The length of a single nanorod is more than 500 nm, the bottom diameter is about 100 nanometers, and the tip size is less than 20 nanometers. Fig.4 e reveals that vertically aligned nanorods are composed of many connected small nanoparticles, and there is numerous mesopores structure among the particles, which will promote the transfer of electrolyte ions into the nanorods during the energy storage reaction [33]. The high resolution microscopic image (HRTEM) of the nanorods (the insert part of Fig.4 e) indicate that the crystal plane spacing of 0.28 nm corresponds to the (311) crystal plane of cubic phase NiCo2S4 [34]. The XRD test were demonstrated in Fig.4 f, the acquired diffraction peaks of NiCo2S4 appeared around 16.3, 26.8, 31.7, 38.2, 47.4, 50.4, 55.3, 72.9 and 76.3°, corresponding to (111), (220), (311), (400), (422), (511), (440), (642) and (731) crystal planes of NiCo2S4, respectively, which coincide well with the standard cards of orthorhombic form (JCPDS No.20-0782). Fig.4 g depicts the EDS spectra of NiCo2S4 sample, in which Si peak is from Si film of the supporting nanomaterials used in SEM testing, and the Ni, Co, S peaks are from the NiCo2S4 materials and no other elemental peaks are found. Fig.5 shows the survey X-ray photoelectron spectroscopy (XPS) spectrum of the NiCo2S4 sample, from which the peaks of Co, Ni and S can be observed. As shown in Table 2, the

20

contents of Co, Ni and S in NiCo2S4 were 14.27 %, 28.59% and 57.14%, respectively, further indicating the successful synthesis of the NiCo2S4 nanomaterial. High resolution XPS spectra of Co 2p, Ni 2p and S 2p are depicted in Fig. 5(b-d), respectively. In the Co 2p spectrum (Fig.5b), two strong peaks at 781.2 eV for Co 2p3/2 and 797.3 eV for Co 2p1/2 are detected, respectively. The energy difference between Co 2p3/2 and Co 2p1/2 spectrums is 16.1 eV, which indicates the existence of both Co3+ and Co2+ on the surface of the hybrids[35,36], while the concomitant satellite peaks at 786.8 eV and 803.2 eV indicating the majority of Co3+ in cobalt oxidation state[37, 38]. As for Ni 2p, (Fig.5c), the peaks at 857 eV and 875 eV are ascribe to Ni 2p3/2 and Ni 2p1/2,

a

1000

600 400 Binding Energy (eV)

200

Ni 2p

2p3/2

797.3 eV

Sat.

Sat.

803.3 eV

810

0

786.9 eV

800 790 Binding energy (eV)

162.1 eV

c

b

781.2 eV

Co 2p1/2 Intensity (a.u.)

800

Co 2p3/2

Co 2p

S 2p

Co 2p

Ni 2p

Intensity (a.u.)

NiCo2S4

780

2p3/2

S 2p

d

885

856.8 eV

Sat.

2p1/2 873.9 eV

880.1 eV

880

875

Intensity (a.u.)

Intensity (a.u.)

855.4 eV

Sat.

862.1 eV

870 865 860 Binding Energy (eV)

855

2p1/2

163.2 eV

Sat. 169.2 eV

170

850

168

166 164 162 Binding Energy (eV)

160

Fig.5 (a) the full spectrum of XPS characterization of NiCo2S4; (b) the high resolution of Co2p spectrum; (c) the high resolution of Ni2p spectrum; (d) the 21

high resolution of S2p spectrum and their satellite peaks locate at 862.2 and 880.6 eV, respectively, suggesting the existence of both Ni2+ and Ni3+ in the NiCo2S4 nanorods. The binding energies of S2p can be fitted into three peaks located at 161.7, 162.8 and 169.3 eV, respectively (Fig.5d). The former two peaks should be assigned to S 2p3/2 (metal-sulfur bonds) and S 2p1/2 (sulfur-vacancies bonds), the latter one is attributed to the shake-up satellite [39]. Table. 2 The relative surface element content of the NiCo2S4 sample. Elements

Ni

Co

S

At.%

14.27

28.59

57.14

Weight%

19.22

38.69

42.09

3.3. Electrochemical studies The electrochemical behavior of T-Nb2O5/3DNG has been investigated in a three-electrode system in 6M KOH electrolyte. Fig.6a presented the cyclic voltammetry (CV) curves of T-Nb2O5/3DNG at different sweep speeds. Wider redox peaks near -0.6 V can be observed, which are pseudocapacitive behavior induced by N heteroatoms and the Faraday reaction takes place on the surface of of T-Nb2O5, which can be expressed in the following equations: Nb2O5 + OH- = Nb2O5OH + e-

(6) 22

It can be seen that T-Nb2O5/3DNG electrode exhibits the wide potential voltage rang of -1.0-0.2. Though the specific capacitance calculated by CV curve decreases with the scanning rate increasing from 5 m V s-1 to 100 mV s-1 due to the insufficient time of ion diffusion and electron transfer, however, the shape of the figure remains unchanged without obvious polarization phenomenon even at 100 m V s-1.

-1

Current Density (A g )

5 mv s -1 10 mv s -1 20 mv s -1 30 mv s -1 50 mv s -1 100 mv s

30 20 10

a

0.2

Potential (V vs. Hg/HgO)

-1

40

0 -10 -20

-1

b

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

0.0 -0.2 -0.4 -0.6 -0.8 -1.0

-30

-1.0

-0.8

0.0 -0.2 -0.4 -0.6 Potential (V vs. Hg/HgO)

0

0.2

500

1000

1500

Tims (s)

2000

800 20 A g

700

750

500

250

0

5

10 15 -1 Current Density (A g )

20

d

90. 8% Retention

500 0.2

400

st

1 th 5000

0.0

300 200 100

0

T-Nb2O5/3DNG

600

Potential (V vs. Hg/HgO)

-1

Specific Capacity (F g )

-1

Specific Capacity (F g )

-1

c

1000

-0.2 -0.4 -0.6 -0.8 -1.0 0

10

20

30

Tims (s)

40

50

60

0 0

1000

2000

3000

4000

5000

Cycle Number

Fig. 6 (a) CV curves of T-Nb2O5/3DNG at different sweep speeds; (b) GCD curves of T-Nb2O5/3DNG in different current density; (c) the specific capacitance of T-Nb2O5/3DNG; (d) cycling stability of T-Nb2O5/3DNG, the inset are the first and 5000 cycles of GCD at the current density of 20 A g-1 The GCD curves of T-Nb2O5/3DNG hybrids as shown in Fig.6b display a

23

symmetrical line without slight resistance drop and has excellent capacitive behavior in the range of current density from 1 A g-1 to 20 A g-1. Similar to CV curve, the GCD curve of T-Nb2O5/3DNG does not show an ideal triangular shape of double-layer capacitance, but has a certain redox platform, which further illustrates the pseudocapacitance produced by doping nitrogen in graphene and T-Nb2O5. The specific capacitances calculated by the formula (1) are 952.7,911.3, 804.7, 720.1, 673.2, 667.3 F g-1 at the current densities of 1, 2, 5, 10, 15, 20 A g-1, respectively (Fig.6c). This high Coulomb efficiency of GCP is due to the three-dimensional porous structure allows the ion rapidly to diffuse into T-Nb2O5 interlayers, which accelerate the process of oxidation/redox reaction, and the doping of nitrogen adds many electrochemical active sites to the surface of graphene. For T-Nb2O5/3DNG electrode, the initial specific capacitance at 20 A g-1 is about 667.3 F g-1 with the retention rate of capacitance above 90.8% after 5000 cycles (as seen in Fig.6d), which shows excellent cycle stability. The insets display the charge/discharge curves of the first and the 5000th cycle, which further indicate that the electrochemical behavior of the T-Nb2O5/3DNG electrode have a small change. The above electrochemical behavior clearly show that the introduction of T-Nb2O5 nanosheets on the surface of nitrogen-doped graphene improves the overall conductivity of the electrode material, increases the active sites on the surface of the electrode material, shortens the ion transfer distance and reduces the diffusion resistance and accelerates the electrochemical reaction, so that the composite materials have high specific capacitance and good 24

rate characteristics. Fig.7a and b show CV curves at different scanning rates and GCD curves at different charge and discharge current densities. As can be seen from Fig.7a, characteristic redox peaks appear at different scanning rates, revealing typical Faraday capacitance characteristics. According to the reversible Faraday redox processes of Co2+/Co3+/Co4+ and Ni2+/Ni3+, the specific processes are as follows: CoS + OH- = CoSOH + e-

(7)

CoSOH + OH- = CoSO + H2O + e-

(8)

NiS + OH- = NiSOH+ e-

(9)

150

0.7

a

-1

-1

50 0 -50 -100

0.0

0.1

0.2

0.3

0.4

0.5

0.4 0.3 0.2 0.1 0.0

0.6

0

1000

1200

1251.3

1200

800

400

8

-1

12

d

-1

15 A g

Retention 93.1%

0.6

800

st

1 th 5000

0.5

600 400 200

0 4

4000

1000

0 0

3000

-1

-1

c Specific capacity (F g )

NiCo2S4

1600

2000

Time(s)

1709.2

Specific capacity (F g )

0.5

Potential (V vs. Hg/HgO)

2000

-1

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

b

0.6

Potential (V vs. Hg/HgO )

Current Density (A g )

100

Potential (V vs. Hg/HgO)

10 mv s -1 20 mv s -1 30 mv s -1 50 mv s -1 100 mvs

16

Current density (A g )

0.4 0.3 0.2 0.1 0.0

0

0

40

1000

80 Time(s)

120

160

2000 3000 Cycle number

4000

5000

Fig. 7 (a) CV curves of NiCo2S4 at different scanning rates; (b) GCD curves of 25

NiCo2S4 at different current densities; (c) the specific capacitance of NiCo2S4; (d) cycling stability of NiCo2S4, the inset are the first and 5000 cycles of GCD at the current density of 15 A g-1 Obviously, the electrodes are polarized due to the increase of diffusion resistance of electrolyte ions in the electrode material at high sweep speed and the oxidation /reduction peak of NiCo2S4 shift to positive and negative voltage, respectively. When the scanning speed reaches 100 m Vs-1, the shape of the curve has hardly changed and the redox peak is still obvious, suggesting that the NiCo2S4 electrode has excellent conductivity and mass transfer rate and good charge-discharge ability. The better symmetry of GCD curve in Fig.7b indicates that the electrode material has good reversibility. The mass specific capacitance of NiCo2S4 electrodes at 1, 2, 5, 10 and 15 A g-1 current densities are calculated 1709.2, 1620.1, 1399.9, 1288.1, 1251.2 F g-1 on the basis of formula (1), respectively. Additionally, there is very small IR drop (0.015V) at the beginning of discharge when the current density is 1 A g-1, implying fairly minor internal resistance. Although the IR increases with the increase of current density, nevertheless, when the current density reaches 15 A g-1, the IR only increases to 0.049V, indicating that the internal resistance of the material itself does not change much with the change of current density. As presented in Fig. 7c, with the increase of current density, the mass specific capacitance of NiCo2S4 electrodes decrease, which is due to that the time required for charging and

26

discharging is greatly shortened at higher charging and discharging rates limited the diffusion efficiency of electrolyte ions to active substances. Besides, when the current density increases to 15 A g-1, the capacitance retention rate of NiCo2S4 electrode is 73.2% of the initial specific capacitance, demonstrating its excellent rate performance (Fig. 7c). The long-term stability of the NiCo2S4 electrode was tested at a high current density (15 A g-1), as shown in Fig. 7d. After 5000 cycles of testing, the specific capacitance of NiCo2S4 electrode materials decrease to 1164.6 F g-1 and the capacitance retention rate is 93.1% respectively. The excellent electrochemical properties are ascribed to that the nanorod array structures of NiCo2S4 can effectively prevent the dense stacking of active materials, promote the transmission of electrons and ions in the process of charge and discharge, and greatly increase the effective contact area between electrolyte ions and active materials. 40

-1

b=0.829

-1

log ip (A g )

1.5 b=0.648

b

Diffusion-controlled contribution

20 10

30mV s

-1

0

70.54%

-10

1.0

0.5

1.0

1.5 -1 log  (mV s )

2.0

-30

50 30mV s

-1

0

83.89%

-50

-20

Capacitive contribution

Capacitive contribution -1.0

-0.8

-0.6 -0.4 -0.2 Potential (V)

0.0

0.2

c

Diffusion-controlled contribution -1

NiCo2S4

100

30 Current density (A g )

T-Nb2O5/3DNG

Current density (A g )

2.0

a

-100

0.0

0.1

0.2 0.3 0.4 Potential (V)

0.5

0.6

Fig. 8. (a) b-value estimation of cathodic peak currents of NiCo2S4/NF and T-Nb2O5/3DNG, (b, c) capacitive and diffusion contribution ratio of capacitance of NiCo2S4/NF and T-Nb2O5/3DNG To further thorough investigate the mechanism of charge storage and the factors

27

affecting the behavior of capacitance and batteries, the b value is determined by the formula [40,41]: i (V) = k1v + k2v1/2= a vb, where i is the redox peak current (A g-1), v is the scan rate (mV s-1), k1, k2 and a are suitable constants, b is a numerical value changing from 0.5 to 1 (b = 0.5 indicates the electrode material behaves as a battery property and b = 1 demonstrates a capacitive process). As shown in Fig. 8a, the b values of NiCo2S4/NF and T-Nb2O5/3DNG are 0.829 and 0.648 within range of 0.5-1, respectively, indicating both battery and capacitive properties existence in the charge storage process. The capacitive contribution represented capacitive process (k1v) and diffusion-controlled contribution represented battery type process (k2v1/2) are analyzed for these two synthesized electrodes by plotting v1/2 versus 1/v1/2 at the scanning rate of 30 mV s-1. From Fig. 8. (b and c), the capacitive contribution are 70.54 and 83.89 ％

of

T-Nb2O5/3DNG and NiCo2S4/NF respectively, indicating that the surface control contribution is dominant for these two electrodes and the NiCo2S4/NF electrode has better capacitive contribution at the same scanning rate. Accordingly, we matched the prepared NiCo2S4/NF as positive electrode, and T-Nb2O5/3DNG as a negative electrode to form NiCo2S4/NF // T-Nb2O5/3DNG asymmetric supercapacitor (ACS), in which the energy storage capacity of supercapacitors can be described more effectively and truthfully. In order to maximize the energy of an asymmetric supercapacitor assembled from positive and negative electrode materials, the ratio of mass between positive and negative

28

electrode is 1.15:1, which was obtained by calculating equation (2). The cyclic voltammetry curves of T-Nb2O5/3DNG and NiCo2S4/NF with potential window of −1.0 to 0.2V and 0–0.6 V at the sweep speed of 10 m Vs-1 are shown in Fig. 9a. The CV curve of the ACS device at the sweep speed of 5-100 m V/s with the working voltage of 0-1.8 V is displayed in Fig. 9b, which demonstrates no obvious deformity even at a high scanning rate of 100 mV s-1. Fig. 9c displays the GCD curves of the device in different voltage ranges at 50 mV s-1 sweep speed. Even when the voltage is as high as 1.8 V, the curve remains symmetrical and does not deviation, indicating the maximum voltage window up to 1.8V. As shown in Fig. 9d, the GCD curve under different current densities basically keeps a symmetrical shape, which reveals good reversible and fast charge-discharge performance of the cell. The calculated mass specific capacitances of this ACS device according to equation (3) are 203.4, 190.9, 178.8, 161.7, 145.3, 133.4 and 117.3 F g-1 at current densities of 0.5, 0.6, 1, 2, 3, 4 and 8A g-1, respectively (Fig. 9e) , which is greatly improved compared with the results in many related literature on asymmetric supercapacitors [42-45]. The electrochemical impedance spectroscopy (EIS) was measured under open circuit voltage with the frequency range 0.01-100 kHz. Nyquist plot of NiCo2S4/NF // T-Nb2O5/3DNG is displayed in Fig. 9f, where the Nyquist plots from individual negative of T-Nb2O5/3DNG and positive of NiCo2S4/NF electrode are also provided. The Nyquist diagram of NiCo2S4/NF and full cell at low frequencies are almost vertical, which are the characteristic of ideal capacitive behavior. The 29

semicircle diameter in high frequency region represent charge transfer resistance (Rct). 30

60

a

T-Nb2O5/3DNG

-1

Current Density (A g )

NiCo2S4

-1

Current (A g )

40

b

-1

20 0

-20

5 mv s -1 10 mv s -1 20 mv s -1 30 mv s -1 50 mv s -1 100 mv s

20

10

0

-40 -1.0

-0.8

-0.6

-0.4

-0.2

0.2

0.4

0.6

0.0

c

1.6 1.4 1.2 1.0 0.8 0.6

150

1.8

2.0 -1

d

0.5 A g -1 0.6 A g -1 1Ag -1 2 Ag -1 3 Ag -1 4 Ag -1 8 Ag

900

1200

0.6

0

300

600 Tims (s)

T-Nb2O5/3DNG

15

160 -Z''( Ω)

-1

Specific Capacitance (F g )

1.6

20

e

120

1.4

0.8

0.0

200

117.3

1.2

1.0

Time (s) 203.4

1.0

1.2

0.2

0.0 100

0.8

Povential (V)

1.4

0.4

200

0.6

1.6

0.2 50

0.4

1.8

0.4

0

0.2

2.0 0 - 1.8 V 0 - 1.6 V 0 - 1.4 V 0 - 1.1 V 0 - 0.9 V

Potential (V)

2.0 1.8

Potential (V)

0.0

Potential (V)

f

NiCo2S4 full cell

10

80

Rs

CPE

Rs

Wo

Rp

5

40 0

0

2

4

6

8

0 0

-1

Current Density (A g )

Element Rs CPE-T CPE-P 5 Rp Rs Wo-R Wo-T Wo-P

Freedom Free(+) Free(+) Free(+) 10 Free(+) Free(+) Free(+) Free(+) Free(+)

Z' (Ω)

Value 11.72 0.0016244 0.85148 15 5.004 1.096 25.39 1.638 0.39722

Error 1.5353E06 0.00048138 0.06234 20 0.45091 1.5353E06 1.7357 0.15678 0.0031328

Error % 1.31E07 29.634 7.3214 9.011 1.4008E08 6.8362 9.5714 0.78868

Chi-Squared: Weighted Sum of Squares:

0.00060164 0.016846

Data File: Circuit Model File: Mode: Maximum Iterations: Optimization Iterations: Type of Fitting: Type of Weighting:

C:\Users\Administrator\Desktop\1.txt

Fig.9 (a) The CV curves of T-Nb2O5/3DNG and NiCo2S4/NF electrodes; (b) the Run Fitting / Freq. Range (0.001 - 1000000) 100 0 Complex Calc-Modulus

CV curve of the ACS device at the sweep speed of 5-100 mV/s; (c) the GCD

curves of the device in different voltage ranges at 50 mV s-1; (d) the GCD curve under different current densities; (e) the calculated mass specific capacitances of this device; (f) Nyquist plot of NiCo2S4/NF // T-Nb2O5/3DNG. 30

The smaller semi-circular arc in the high frequency region also reflects the smaller charge transfer resistance at the interface between electrolyte ions and active materials. The resistances of the three hybrids are fitted by the equivalent circuit model in the illustration. It can be concluded that the Rct of full cell is 1.25 Ω, which is larger than that of T-Nb2O5/3DNG (0.9 Ω) and NiCo2S4/NF (0.7 Ω). This is due to the whole battery system includes positive, negative,

150

120

100

80 -1

2Ag 50

0

40

0

2500

5000

7500

Coulombic efficiency (%)

Capacitance retention (%)

electrolyte and their contact interface, which all affect the Rct.

0 10000

Cycle number

Fig. 10 The cycling stability test of the NiCo2S4/NF // T-Nb2O5/3DNG Fig. 10 displays the cycling performance of the NiCo2S4/NF // T-Nb2O5/3DNG asymmetric supercapacitor. On the first 2000 charge/discharge cycles, the specific capacitance of the device does not decrease, but increases slightly. After 10,000 cycles, the specific capacitances remain at 93.6% of their initial capacitance and the coulombic efficiency is up to 99.2% at a current density of 2 A g-1 in the potential window 0-1.8V. When the current density is less than 1 A 31

g-1, the coulomb efficiency is about 95% after 10000 cycles. The reason why the coulomb efficiency increases with the increase of current density may be that the current density is large, the charging time is short, the charge loss is small, and the coulomb efficiency is high.

100

T-Nb2O5/3DNG OHSeparator K+ NiCo2S4/NF

+

b

-1

-

Energy density (Wh kg )

a

e-

10

This work T-Nb2O5 //AC CC@T-Nb2O5 @MnO2 // GO

1

rGO/Nb2O5 // N-rGO NiCo2S4/CC // CNF NiCo2S4 /NCF //OMC/NCF PCs/NiCo2S4//AC

0.1 100

e-

1000 -1

10000

Power density (W kg )

Fig. 11 (a) The schematic diagram of the integrated NiCo2S4/NF // T-Nb2O5/3DNG; (b) Ragone plot of the NiCo2S4/NF // T-Nb2O5/3DNG ASC device In addition, the schematic diagram of the integrated NiCo2S4/NF // T-Nb2O5/3DNG asymmetric supercapacitor is illustrated in Fig. 11 a. Energy density and power density are two important factors in practical application to assess the properties of this ACS supercapacitors. Fig. 11 b displays a Ragone plot of the NiCo2S4/NF // T-Nb2O5/3DNG ASC device. The ASC NiCo2S4/NF // T-Nb2O5/3DNG device presents a maximum energy density of 81.37 Wh kg-1 at a power density of 400.1 W kg-1 and still maintains 45.4 Wh kg-1 at a power 32

density of 9568.3Wkg-1, which could ascribe to wide voltage windows and high specific capacitance. Moreover, the obtained energy density of this device is higher than other previously reported T-Nb2O5 and NiCo2S4 based ASCs systems, including T-Nb2O5 // AC (15.80 Wh kg-1, 8750 W kg-1) [46], CC@T-Nb2O5 @MnO2 // GO (31.76Wh kg-1, 2250 W kg-1) [47], rGO/Nb2O5//N-rGO (55Wh kg-1, 2000W kg-1) [48], NiCo2S4/CC // CNF (41.28Wh kg-1, 1564 W kg-1) [49], NiCo2S4/NCF // OMC/NCF (45.5 Wh kg-1 at 5000 W kg-1) [50], PCs/NiCo2S4 // AC (23.2 Wh kg-1 at 335 W kg-1) [51]. Such high electrochemical performance of the as-fabricated NiCo2S4/NF // T-Nb2O5/3DNG ASC device might be explained due to the improved capacitive performance of NiCo2S4/NF as a positive electrode and the expanded potential window range of T-Nb2O5/3DNG as a negative electrode.

4. Conclusions

In summary, an anode of T-Nb2O5/3DNG nano-hybrids was successfully synthesized through two-step method, which shows a good specific capacitance of 952.7 F g-1 at 1.0A g-1. In addition, a cathode material of NiCo2S4 with the rich mesoporous nanorods arrays vertically growing on the surface of foam nickel was also prepared by hydrothermal concomitant calcination, which exhibits a much higher specific capacitance of 1709.2 F g-1 at 1 A g-1. Because of 33

their excellent electrochemical properties and wide voltage window range, they were

assembled

into

asymmetric

supercapacitors

(NiCo2S4/NF//

T-Nb2O5/3DNG), which reveal excellent capacitance of 203.4 F g-1 at 0.5 A g-1 and wider voltage window of 0-1.8 V, as well as a high energy density of 45.4Wh kg-1 at a power density of 9568.3 W kg-1. Furthermore, the NiCo2S4/NF// T-Nb2O5/3DNG equipment shows good specific capacitance retention after 10000 cycles, which can not only provide new ideas for asymmetric capacitors, but also help to promote the practical process of high energy density supercapacitors.

Acknowledgements

This work was supported by the financial supports from China Postdoctoral Science Foundation (2017M621656), Jiangsu Planned Projects for Postdoctoral Research Funds (1701170C), Zhenjiang Key Research and Development Program (GY2019011).

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Highlights: 1. NG have been successfully prepared via simple pyrolysis route using 5-HMF as carbon source and urea as nitrogen source. 2. NiCo2S4 nanorod arrays grow on NF reinforcing capacitance properties of materials. 3. Nb2O5 nanosheets on nitrogen-doped graphene widen the range of voltage window. 4. The NiCo2S4/NF// T-Nb2O5/3DNG asymmetric device delivers long cycling stability and high energy density.

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Declaration of interests

☑ The

authors

declare

that

they

have

no

known

competing

financialinterestsor personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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